Abstract
Numerous small-molecule amines (SMAs) play critical roles in maintaining bone homeostasis and promoting bone regeneration regardless of whether they are applied as drugs or biomaterials. On the one hand, SMAs promote bone formation or inhibit bone resorption through the regulation of key molecular signaling pathways in osteoblasts/osteoclasts; on the other hand, owing to their alkaline properties as well as their antioxidant and anti-inflammatory features, most SMAs create a favorable microenvironment for bone homeostasis. However, due to a lack of information on their structure/bioactivity and underlying mechanisms of action, certain SMAs cannot be developed into drugs or biomaterials for bone disease treatment. In this review, we thoroughly summarize the current understanding of SMA effects on bone homeostasis, including descriptions of their classifications, biochemical features, recent research advances in bone biology and related regulatory mechanisms in bone regeneration. In addition, we discuss the challenges and prospects of SMA translational research.
Introduction
Bone is a vital organ in the body, providing support and protection for soft tissues and internal organs, and is involved in many biofunctions, e.g., hematopoiesis, mineral storage, endocrine regulation, and immunological modulation. Normal bone metabolism maintains bone homeostasis. However, many factors, such as aging, inflammation, malnutrition, or endocrine disorders, may interfere with the balance between bone formation and resorption, causing excessive bone resorption and bone loss.1,2,3,4 In addition, typical bone diseases such as osteoporosis, osteoarthritis (OA), and rheumatoid arthritis (RA) are often related to defects in bone remodeling (Fig. 1).5,6 Therefore, it is essential to restore the normal function of bone and reverse any imbalance in bone homeostasis through effective strategies.
Osteoblasts and osteoclasts are two principal cell types in bone tissue that play fundamental roles in maintaining bone homeostasis (Fig. 1). Osteoblasts (OBs) are the cells critical for bone formation. Mesenchymal stem cells (MSCs) and skeletal stem cells (SSCs) can differentiate into preosteoblasts (preOBs) and ultimately form mature OBs,7 which can be embedded in lacunae of a mineralized matrix and become osteocytes with a stellate shape.8 In contrast, osteoclasts (OCs) are multinucleated cells derived from hematopoietic stem cells (HSCs)9 and are critical for bone resorption. The preosteoclasts (pre-OCs) generated by HSCs migrate to the bone surface and fuse to form multinucleated OCs.10,11,12 The coordination and balance between OBs and OCs strictly control bone homeostasis. For instance, both macrophage-colony stimulating factor (M-CSF) and receptor activator for nuclear factor-κB ligand (RANKL) are key regulators that initiate the differentiation of pre-OCs into mature OCs.13,14,15,16 In addition, OC undergo fission to generate daughter cells named osteomorphs, which can be fused into OCs and thus recycled via the activation of RANKL.17 Furthermore, OBs and OCs can directly interact through membrane-bound mediators or secreted factors (Fig. 1). However, bone homeostasis can be disrupted when OB formation and activity are reduced and/or when bone resorption, such as that induced by inflammation, oxidation stress, or altered pH, is excessive.18,19,20 Therefore, strategies to regulate key molecular signaling pathways or to create favorable microenvironments for promoting OB formation/activity and/or inhibiting OC formation/activity need to maintain bone homeostasis. Recently, two major approaches have been taken for treating bone-related diseases by ameliorating bone homeostasis and bone regeneration: drug therapy and biomaterial-based repair. The former is used to regulate the key molecular signaling pathways for the maintenance of bone homeostasis, and the latter is mainly used to rebuild the local microenvironment to promote bone regeneration in defective bone.
Small-molecule amines (SMAs), nitrogen-containing compounds with a molecular weight below 900 daltons,21 are derived from a wide range of sources, including plants, animals, and microorganisms, or are artificially synthesized. Recently, extensive studies have demonstrated that SMAs, such as deferoxamine,22 dopamine,23 berberine,24 tetramethylpyrazine,25 and SB242784,26 show an excellent ability to maintain bone homeostasis via the regulation of molecular signaling pathway activity. Some SMAs, such as tofacitinib,27,28 baricitinib,27 and bortezomib,29 have been developed as drugs to treat bone diseases. Some SMA drugs that are used to treat nonbone diseases have also been found to exhibit potential efficacy in treating bone diseases. For example, metformin30 and glimepiride,31 used to treat type 2 diabetes mellitus, have shown positive effects on the attenuation of osteoporosis. Furthermore, the hydrogen atoms (one, two, or three) of the ammonia group in SMAs endow these molecules with special characteristics (e.g., alkalinity and antioxidant), which can maintain bone homeostasis via the local reconstruction of the extracellular microenvironment. Some SMAs are alkaloids, belonging to a large and diverse chemical group with alkali-like properties and at least one nitrogen atom in a heterocyclic ring structure.32 Interestingly, it has been reported that most of these alkaloids regulate bone homeostasis by inhibiting OC activity,33,34,35 which may be partially due to their alkalinity. In addition, due to the hydrogen atom in ammonia groups, SMAs with high reactivity can be used as monomers to develop SMA-based biomaterials for repairing bone defects. Upon the degradation of SMA-containing biomaterials, SMAs are released and reconstitute the local microenvironment to maintain bone homeostasis via the regulation of inflammation, oxidation or pH. Therefore, SMAs are suitable for developing novel drugs or biomaterials to treat bone-related diseases. However, the lack of information on SMA structure/bioactivity and underlying mechanisms of action has profoundly restricted the application of SMAs for treating bone diseases. Thus, a comprehensive analysis of the current developments in SMAs that enable bone homeostasis regulation is an extremely urgent need.
Given the aforementioned facts, in this review we thoroughly summarize the current understanding of SMAs in bone homeostasis (Fig. 1). First, we define SMAs, describing in detail their structures, classifications, and biochemical features. Next, SMA-based drugs and biomaterials, two key applications of SMAs to the regulation of bone homeostasis, are specifically discussed. Subsequently, the possible effects and mechanisms of SMA action on bone cells and the microenvironment are extensively elaborated. Finally, we further discuss current challenges and prospects in promoting SMA translational research.
Definition of SMAs
Structure and classification
SMAs, with a molecular weight of less than 900 Daltons, are generally classified as primary, secondary, or tertiary SMAs depending on the number of hydrogen atoms (one, two, or three) in the ammonia group, which is eventually replaced by organic groups (Fig. 2a). In chemical notation, SMAs can be categorized into three classes: RNH2-, R2NH-, and R3N-containing SMAs.36 In addition, some heterocyclic SMAs have been described (Fig. 2a). For instance, pyrrolidine and piperidine are five- and six-numbered heterocyclic secondary amine compounds.36 Pyridine is both an aromatic amine and a tertiary amine.
According to their sources, SMAs can be classified into endogenous SMAs and exogenous SMAs (Fig. 2b). In particular, endogenous SMAs, derived from the human body, usually play important roles in cell metabolism, mainly interacting with neurotransmitters (e.g., dopamine) and metabolic molecules (e.g., epinephrine and adenosine). Exogenous SMAs are derived from animals, plants, or microorganisms or are artificially synthesized amines, and herein, they are further classified into three categories: drug-SMAs, alkaloid-SMAs, and other-SMAs (Fig. 2b). Drug-SMAs are commercially available pharmaceuticals that have been used in clinical applications. Alkaloid-SMAs constitute a large and diverse group of chemicals with alkali-like properties that carry at least one nitrogen atom in a heterocyclic ring structure and are derived from a large variety of organisms, including bacteria, fungi, plants, and animals.
Biochemical features
SMAs are show clear physiological activity, e.g., regulation of molecular signaling pathways or reconstruction of the extracellular microenvironment, due to their special biochemical features. First, SMAs show nucleophilic affinity for unbound electrons in nitrogen atoms, resulting in both the alkylation and acylation of an amine.36 Second, since lone pairs of electrons in nitrogen atoms tend to attract protons (namely, hydrogen ions), amines are often alkaline in nature. SMAs with alkalinity affect the pH of the extracellular microenvironment. In addition, SMAs exhibit antioxidative functions. Unbonded electrons in the nitrogen atoms of SMAs are easily removed and oxidized. Polyamines have been shown to inhibit lipid peroxidation in rat liver microsomes, scavenge free radicals and exert a powerful antioxidant effect in vivo, owing to the combination of anion- and cation-binding properties.37 The binding of polyamines to anions (phospholipid membranes and nucleic acids) contributes to a high local concentration at cellular sites particularly prone to oxidation, whereas the binding to cations efficiently prevents the site-specific generation of “active oxygen” (i.e., hydroxyl radicals and singlet oxygen). Spermidine has also been demonstrated to enhance the antioxidative activity of mung bean sprouts.38 Several previous studies have reported on the relationships between SMA chemical structures and specific biochemical features. However, the mechanisms of SMA actions in the regulation of key molecular signaling pathways and in the reconstruction of extracellular microenvironments (such as inflammation) need to be further clarified.
State-of-the-art research on SMA functions in the regulation of bone homeostasis
Recent progress
To understand the current status of SMAs in the regulation of bone homeostasis, we searched the Web of Science database with the following keywords: osteo- or bone and amine/small molecular/alkaloid/drug (before Jan. 2022). We found 83 SMAs with specific osteotropic activity (Fig. 2b) and then searched again using the keywords osteo- or bone and each name of these SMAs (before Jan. 2022). A total of 79 254 related documents published in the past 50 years were recovered, and they were analyzed in this project (Fig. 2c).
As shown in Fig. 2b, 82 kinds of bioactive SMAs involved in bone homeostasis were categorized into 16 different types of endogenous SMAs (listed in Table 1) and 66 different types of exogenous SMAs. Among the exogenous SMAs, 18 types of drug-SMAs (listed in Table 2), 34 types of alkaloid-SMAs (listed in Table 3), and 14 types of other-SMAs (listed in Table 4) were identified. An analysis of the aforementioned 79 254 publications displays (Fig. 2c) indicated that the number of SMA publications on bone homeostasis did not show an increasing trend until the early 21st century. The role of SMAs in bone homeostasis has attracted considerable attention, and research on SMAs in bone homeostasis is expected to surge in the future. Therefore, investigation into the effect and mechanism underlying SMA actions in bone homeostasis is of great interest.
In view of the beneficial effects of SMAs on bone homeostasis, it is necessary to try to identify the mechanisms of action at the cellular and molecular levels. As shown in Fig. 2d, SMAs may affect cellular function directly or indirectly. Through a direct mechanism, they bind to receptors on the cell surface or receptors inside the nucleus or on the mitochondrial membrane after entering cells through active or passive transport. Through an indirect mechanism, SMAs may regulate cellular behaviors by binding to extracellular matrix receptors, which further affect membrane proteins on the cell surface or influence the extracellular microenvironment, such as its pH and oxidative inflammatory status.
SMA applications
SMA-based drugs
The use of SMAs as drugs (SMA-based drugs) is a vital application for bone homeostasis (Fig. 3a). The primary application of SMAs is for the treatment of osteoporosis, OA, RA, or tumors related to aberrant bone homeostasis. Phase III trials for Odanacatib, an inhibitor of cathepsin K, or the treatment of osteoporosis have been completed.39 Some drug SMAs have been developed to treat RA through the regulation of bone homeostasis. For instance, as Janus kinase (JAK) inhibitors, tofacitinib and baricitinib have been used to treat RA in the clinic. Recent studies have shown that they inhibit OC formation as well as promote OB formation.27 In addition, it has been reported that some drug-SMAs developed for the treatment of osteosarcoma show excellent osteotropic activity. Bortezomib is a proteasome inhibitor used to treat multiple myeloma and has been shown to promote bone formation in vivo.29,40 Similarly, had completed Phase II trials of saracatinib, an inhibitor of Src, for the treatment of osteosarcoma indicated that exhibited osteotropic activity.41,42
Since other diseases (e.g., diabetes and inflammation) often affect bone homeostasis, drug-SMAs used to treat these diseases might exert a positive effect on bone homeostasis43 (Table 2). Diabetes adversely affects bone homeostasis due to impaired glucose metabolism and toxic effects of glucose oxidation derivatives.44 Moreover, it has been reported that patients with diabetes present with lower bone quality and increased fracture risk compared with nondiabetic patients.43 Metformin and glimepiride, drugs used to treat type 2 diabetes mellitus, have been studied for the regulation of bone homeostasis. In particular, metformin has been demonstrated to inhibit OC formation and promote OB differentiation in vitro and in vivo; therefore, it might be used for the treatment of osteoporosis in the future.30 Additionally, numerous studies have indicated that metformin increased bone density and reduced bone turnover and fracture risk in patients with T2DM.30 As previously mentioned, inflammation can disrupt bone homeostasis. Therefore, some anti-inflammatory drug-SMAs such as benzydamine,45 cetirizine,46 and cimetidine47 display beneficial effects on the treatment of inflammation-related bone diseases. For example, benzydamine, a nonsteroidal anti-inflammatory drug, has been shown to prevent OC differentiation and inhibit interleukin-1β production.45 Cetirizine, a histamine 1 receptor antagonist, has been demonstrated to promote bone healing.46 In addition, other drug-SMAs with antibacterial or anti-infection functions, such as doxycycline,48 exert a positive influence on bone homeostasis (Table 2). For instance, Gomes, P. S., et al.48 declared that doxycycline, an antibacterial and anti-infection drug, restored the impaired osteogenic commitment of bone marrow mesenchymal stromal cells (BMSCs) derived from diabetic patients by activating Wnt/β-catenin signaling. Phase III trials to evaluate Lidocaine, a local anesthetic drug, for the treatment of postmenopausal osteoporosis and a Phase IV trial for its use as a treatment of knee and hand OA have been completed.49 Taken together, the usage of drugs used to treat other nonbone diseases may be expanded to applications to treat bone diseases.
SMA-based biomaterials
The use of SMA monomers to develop novel biomaterials (SMA-based biomaterials) is another pivotal way to increase SMA applications. Although most SMAs are still being evaluated through preclinical research or show no precise medicinal effects, many SMAs are used to prepare SMA-based biomaterials with enhanced osteotropic activity. Recently, two main types of SMA application via SMA-based biomaterials have been described (Fig. 3b): a) biomaterials with physically loaded SMAs and b) biomaterials with chemically engrafted SMAs, including chemical surface modifications and structural units of block copolymers.
Biomaterials after physical SMA loading
The method of the physical loading of bioactive SMAs involves SMA adsorption with biomaterials via weak interactions, such as hydrogen bonding, electrostatic attraction, and conjugation. The application of DA as a polydopamine (PDA) coating is the most common physically loaded modification due to its excellent conjugation effect. Li et al.50 reported that a PDA coating enhanced the attachment and proliferation of MC3T3-E1 cells to the surface of 3D-printed porous Ti6Al4V scaffolds and promoted the expression of osteogenic genes and proteins, making its use a great strategy for the orthopedic application of implants. In addition, berberine has become an important molecule for the physical modification of biomaterials via electrostatic attraction. Hu et al.51 fabricated a biomimetic CaP scaffold coating with berberine onto Ag nanoparticles loaded with silk fibroin. According to research by Sang et al., the modification of a polyether ether ketone (PEEK) surface with osthole particles and berberine led to effective osteogenic and antibacterial PEEK functions52 (Fig. 3b). Since the lifespan of deferoxamine (DFO) is extremely short, a sustained release system was needed. To meet demand, Chen et al.53 developed a novel drug delivery system by combining DFO-loaded liposomes carrying photocrosslinked gelatin hydrogel to control the sustained release of DFO from the hydrogel matrix (Fig. 3b). The results showed that DFO simultaneously released from the hydrogel facilitated angiogenesis and osteogenesis, further accelerating new vessel formation and bone regeneration.53 Song et al.54 fabricated titanium implants coated with doxycycline-loaded coaxial nanofibers and found that the implants enhanced osseointegration. In addition, some alkaloid-SMAs with osteotropic activity have been loaded into scaffolds. For instance, Wang et al.55 revealed that berberine-loaded porous calcium phosphate cements with the release of berberine sustained for as long as 9–10 days enhanced BMSC proliferation and differentiation, obviously increased the ALP and mineral deposition levels, and significantly promoted bone regeneration in osteoporotic rats. Physical loading of SMAs into biomaterials is a facile method and can reduce the amount of chemical residue. However, the release SMAs relatively quickly. To better control drug release, methods of chemically grafting SMAs into biomaterials have become increasingly attractive.
Biomaterials with chemical-engrafted SMAs
Two main ways to integrate SMAs with biomaterials independently are modification of the material surface with chemicals or structural units of block copolymers. A previous report disclosed that histidine-conjugated (3-aminopropyl)-triethoxysilane-modified polydimethylsiloxane led to higher ALP activity and greater deposition of mineralized ECM components than a control group of hFOBs56 (Fig. 3b). Compared with surface chemical modification, a greater number of biomaterials with SMAs added as structural units of block copolymers have been reported. Some simple SMAs, such as dopamine, lysine, and piperazine, can be polymerized to form copolymer biomaterials. Cui et al.57 successfully synthesized foamy poly(Nε-benzyl formateoxycarbonyl-L-Lysine) (PZL) and poly(Nε-benzyl formateoxycarbonyl-L-lysine-co-L-phenylalanine) (PZLP) scaffolds, and the results of analysis showed that PZL scaffolds increased the adhesion, proliferation and OB differentiation of MC3T3-E1 cells compared to the effects of PZLP scaffolds. Previous work from our laboratory revealed that a series of piperazine-based polyurethane-urea (P-PUU) modifications enhanced OB differentiation as the number of piperazine units was increased within a certain concentration range both in vitro and in vivo (Fig. 3b).58 Later, we found that piperazine itself regulated OB differentiation in a dose-dependent manner.58 In a recent study, Mao et al.59 synthesized a novel citrate-based biodegradable elastomeric poly(citric acid-1,8-octanediol–1,4-bis(2-hydroxyethyl) piperazine (BHEp)) (POPC) material by incorporating the alkaline fragment BHEp and then fabricated 3D printed POPC/β-tricalcium phosphate porous scaffolds (PTCPs). The results of a subsequent analysis demonstrated that PTCP neutralized the acidic microenvironment to enhance adhesion, proliferation, and bone regeneration owing to the activity of BHEp. Therefore, some SMAs can not only serve as a unit of a base material that enhances the physical properties of the treatment but can also promote bone formation after release. Primary amines in SMAs may be among the reasons for SMAs promotion of the proliferation and osteogenic differentiation of MSCs.
Chemical grafting of SMAs into biomaterials allows SMAs to directly affect the properties of a biomaterial itself. Surface chemical modification affects mainly surface properties, such as hydrophobicity, while the polymerization method affects the physicochemical properties of the whole biomaterial. In contrast, chemical combinations can better achieve slow drug release. However, although some problems with polymerized materials, such as degraded products of the SMA-based biomaterials, may not be evident for all SMA monomers, some chemical fragments can increase the complexity of the material for cellular action.
Mechanisms of SMA action in the regulation of bone homeostasis
Regulation of bone cell behaviors
Promotion of OB formation
OBs, the chief bone-making cells with abundant mitochondria and a huge Golgi apparatus, synthesize a variety of extracellular matrix proteins, such as high levels of type I collagen (COLI), osteocalcin (OCN), alkaline phosphatase (ALP) and osteopontin (OPN), and subsequently promote mineralization through the deposition of calcium phosphate in the form of hydroxyapatite, the major inorganic component of bone.60,61,62,63 Therefore, the promotion of OB formation is crucial for bone homeostasis. Recent investigations have shown that certain SMAs positively regulated OBs through multiple signaling pathways, including the mitogen-activated protein kinase (MAPK) pathway, Wnt pathway, PI3K/Akt pathway, AMPK pathway, and mTOR pathway (Fig. 4a).
The MAPK signaling pathway
MAPKs are important mediators of a cell signaling pathway with activity that is regulated through a three-tiered cascade composed of MAPKs, MAPK kinases (MAPKKs, MKK or MEK) and MAPKK kinases or MEK kinases (MAPKKKs and MEKKs). Extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 are family members of MAPKs that play significant roles in cell proliferation, differentiation, and apoptosis.64 ERK activation promoted osteogenic differentiation and bone formation by upregulating the expression of β-catenin and Runx2.65,66 Wang et al.67 demonstrated that a low concentration (5 nmol·L−1) of DA activated the D1 receptor and further promoted osteogenesis through the activation of the ERK signaling pathway mediated by enhanced Runx2 transcriptional activity, whereas blocking the ERK1/2 signaling pathway inhibited the dopamine-induced osteogenic differentiation of human bone mesenchymal stem cells (hBMSCs). Berberine has been shown to facilitate the proliferation of human dental pulp stem cells (hDPSCs) in a dose-dependent pattern and stimulate osteogenic differentiation partially by activating MAPK pathways68,69 (Fig. 4b). It has also been shown that glutamine promoted the proliferation, differentiation and migration of human dental pulp cells (hDPCs) by activating p38, JNK, and ERK, which were blocked by specific MAPK inhibitors, indicating that the MAPK pathway is one of the important signaling pathways in the induction of glutamine-mediated proliferation and differentiation of hDPCs.70 In addition, evodiamine might prevent osteoporosis by reversing an imbalance in bone formation/bone resorption and activating the MMP3-OPN-MAPK signaling pathway. Compared to those in the control group, the significant decrease in mRNA and protein levels related to MAPK activity and calcium deposition in dexamethasone-induced osteoporosis in zebrafish was reversed by evodiamine treatment.71 In addition, Kim et al.72 demonstrated that salicylideneamino-2-thiophenol enhanced the differentiation of multipotent BMSCs into OBs mediated through the MAPK pathway. In addition, melatonin, betaine, and taurine also promoted OB formation by activating the ERK pathway.73,74,75 It is not difficult to conclude that most endogenous SMAs promote OB formation through the MAPK pathway. Taken together, studies have shown that the MAPK pathway regulates bone formation and that some SMAs exert a positive influence on bone formation by activating MAPKs. However, the overall net effect of the MAPK pathway on bone homeostasis is unclear because OC-mediated bone resorption can also be activated through MAPKs, which is further discussed in a subsequent section on OC pathways.
The Wnt signaling pathway
Wnt signaling is vital to bone homeostasis and affects almost all types of bone cells.76 β-Catenin is a central molecule of canonical Wnt signaling, and it forms a destruction complex with other proteins, including GSK-3β, APC, and Axin1/Axin2. Through stimulation by the Wnt ligand, the destruction complex proteins are dissociated, leading to the accumulation of β-catenin in the cytoplasm that is then translocated to the nucleus to enhance the expression of its downstream target genes (Fig. 4a).77 In contrast, the inhibition of Wnt leads to bone loss.78 Berberine enhanced the expression of β-catenin and further upregulated the expression of OB marker genes such as OPN and OCN, thereby increasing the MSC differentiation rate in vitro.79 Leonurine hydrochloride, a synthetic chemical compound with antioxidant and antiapoptotic activities derived from leonurine, was shown to promote phosphorylation of GSK-3β to enhance the activity of β-catenin, thereby accelerating bone formation.80 Similarly, OB differentiation was increased by stimulating the Wnt/β-catenin signaling pathway via treatment piperine treatment, resulting in elevated bone mineral density in OVX mice.81 In addition to alkaloid-SMAs, many endogenous SMAs (e.g., adenosine, Gln, taurine, glucosamine, and Sphingosine-1-phosphate) facilitated bone formation through the Wnt pathway. Studies have revealed that the promotion of OB formation via the Wnt signaling pathway relies on Gln metabolism. Specifically, Gln synthetase (Gls)-dependent Gln catabolism was necessary for Wnt-induced OB differentiation.82 Moreover, Gln promoted the growth, migration, and differentiation of hDPCs to accelerate pulp repair and regeneration by activating Wnt pathways.70 After treatment with 100 μmol·L-1 glutamate, the deamination of Gln increased ALP activity and extracellular matrix mineralization.83 Taurine, a nonessential amino acid in humans, was synthesized from the sulfur-containing amino acids methionine and cysteine.84 It suppressed the expression of inhibitors of Wnt signaling, such as sclerostin and DKK1 synthesized by OCs.85 In a low dosage regimen (1 μmol·L−1), Doxycycline, a broad-spectrum antibacterial drug, has been demonstrated to enhance the expression of β-catenin, Runx2, and OCN, thereby increasing osteogenic differentiation of MSCs derived from diabetic rats.48 Benidipine, an antihypertensive drug, upregulated the expression of Runx2, ALP, and OCN and activated Wnt/β-catenin signaling in vitro and in vivo, advancing bone formation.86 Metformin, the first-line drug for the treatment of type 2 diabetes, was confirmed to inhibit the phosphorylation of GSK-3β, increasing its activity, and increase the steady-state levels of the β-catenin protein, thus promoting the osteogenic differentiation of hBMSCs.87 In addition, 6-bromoindirubin-3′-oxime, an inhibitor of GSK-3β, also triggered the Wnt/β-catenin signaling pathway and enhanced the osteogenic differentiation of canine BMSCs.88 All kinds of SMAs facilitate OB differentiation by stimulating Wnt/β-catenin. These outcomes may be due to the extensive interactions between the Wnt/β-catenin pathway and other pathways and the multiple functions of Wnt/β-catenin signaling in various life activities of cells.
The PI3K/Akt signaling pathway
Phosphatidylinositol-3-kinases (PI3Ks) constitute a family of lipid kinases.89 The PI3K/Akt signaling pathway mainly affects cell metabolism, proliferation, migration, differentiation, and apoptosis.90 Growth factors or hypoxia, among other factors, can activate PI3K, leading to the phosphorylation of AKT, which triggers the activation of downstream mammalian target of rapamycin (mTOR) pathway, vascular endothelial growth factor (VEGF), etc., increasing MSC survival, proliferation, migration and angiogenesis (Fig. 4a).90 Leonurine, an alkaloid from Herba leonuri, enhanced the proliferation and differentiation of rat BMSCs administered at a 10 μmol·L−1 dose, and the effect was mediated through autophagy, which depended on the PI3K/AKT/mTOR pathway.91 Studying endogenous SMAs, Mirones et al.92 showed that dopamine enhanced the migration of mesenchymal progenitor cells via the PI3K/Akt pathway, which was suppressed by D2-class receptor antagonists or blocking antibodies. Glimepiride, an anti-type 2 diabetes drug, has been shown to facilitate the proliferation and differentiation of OBs through the PI3K/Akt pathway in rats.93 Adenosine, a natural nucleoside, is essential for all cellular life events because it is involved in energy production and utilization in the body. Its role in bone homeostasis has been widely recognized. For example, human osteoprogenitor cells are known to produce adenosine and express four adenosine receptor subtypes, namely, the A1 receptor, A2A receptor, A2B receptor, and A3 receptor.94 Gharibi et al.95 revealed that adenosine receptors, especially A2B, were expressed and activated during the differentiation of MSCs into OBs.95 In addition, both activation and overexpression of the A2B receptor promoted the expression of Runx2 and ALP in OBs, increasing the differentiation and mineralization rates of OBs and the formation of bone in vivo.95,96 The expression of the A2A receptor was upregulated in the late stage of OB differentiation.95 Furthermore, the Akt signaling pathway was activated and the nuclear localization of β-catenin was enhanced in MC3T3C-E1 and primary murine OBs treated with CGS21680, a highly selective A2A receptor agonist, promoting bone regeneration in vivo (Fig. 4c).97 In addition, studies have shown that S1P stimulates OB migration, prolongs their survival, and inhibits their apoptosis via the activation of the PI3K/Akt signaling pathway.98,99,100 Endo et al.101 demonstrated that the phosphorylation of Akt in the PI3K/Akt signaling pathway indirectly activated the Wnt/β-catenin pathway by mediating the inactivation of GSK-3β. Therefore, the PI3K/Akt pathway and Wnt/β-catenin pathway may exert synergistic effects on bone formation. For instance, both adenosine and sphingosine-1-phosphate promote OB formation by activating the PI3K/Akt and Wnt/β-catenin signaling pathways. Collectively, studies have indicated that various SMAs enhance bone formation via the PI3K/Akt signaling pathway.
The AMPK signaling pathway
AMPK controls the osteogenic differentiation of hMSCs through early mTOR inhibition-mediated autophagy and late activation of the Akt/mTOR signaling axis.102 Runx2, a novel substrate of AMPK, directly phosphorylates the serine 118 residue in the DNA-binding domain of Runx2.103 Adil et al.104 demonstrated that the expression of Runx2 increased through activation of the AMPK pathway upon oral administration of berberine (100 mg·kg-1) for 12 weeks in vivo. Similarly, piperine was shown to enhance OB differentiation through AMPK-dependent Runx2 expression in MC3T3-E1 cells.105 In addition, sanguinarine was identified as a candidate for use as an osteoporosis drug due to its induction of OB differentiation mediated via the AMPK/Smad1 signaling pathway and promotion of bone formation in a rat model of ovariectomy osteoporosis (OVX).106 In addition, it has been shown that metformin enhanced the osteogenesis of stem cells from human exfoliated deciduous teeth by activating the AMPK pathway (Fig. 4d).107 In particular, some SMAs have been identified as activators of the AMPK pathway by protecting OBs. For instance, OSU53 attenuated the damage to OBs induced by dexamethasone or glucose.108 Palmitate-induced apoptosis in bone marrow-derived osteoblastic cells has been proven to be impeded by AICAR treatment, which restored the activity of the ERK pathway via the activation of AMPK.109 A-769662 and GSK621 suppressed apoptosis or ameliorated the damage to OBs induced by H2O2 through the activation of AMPK.110,111,112 It seems that the AMPK pathway enhances bone formation mainly through its positive influence on Runx2 and protection of OBs. The SMAs that mainly stimulate the AMPK pathway are alkaloid-SMAs and activators (other-SMAs) in the AMPK pathway. However, activators of the AMPK pathway have not been experimentally confirmed in vivo, which may hinder their further development into drugs.
Other pathways
In addition to the aforementioned pathways, SMAs can ameliorate bone formation through other pathways, such as the HIF, mTOR, and BMP signaling pathways. OBs produce erythropoietin (EPO) in an HIF-dependent manner under physiological and pathophysiological conditions; in other words, OBs express HIF-1 and HIF-2, further activating the expression of EPO via increased transcription, thereby enhancing angiogenesis.113 Stegen et al.114 found that concurrent changes in Gln and glycogen metabolism, which depend on HIF1α, were vital to cell survival and led to increased bone formation. In addition, DFO promoted angiogenesis and osteogenesis by increasing the expression of HIF1α/VEGF.115 Regarding the mechanisms of action, HIF-1 is involved in redox regulation of bone homeostasis, which is explained in detail in a subsequent section.
The mTOR signaling pathway is of great importance to bone homeostasis because it regulates the proliferation of OBs and OCs.116 Glucosamine promoted the proliferation of OBs through the mTOR pathway, thereby promoting bone regeneration.117,118 Tranylcypromine, a small-molecule inhibitor of histone lysine-specific demethylase 1, has been proven to facilitate bone formation both in vitro and in vivo.119 It has also been reported that tranylcypromine affected osteogenesis through the BMP pathway and Wnt7b-mTORC1 signaling since both the mRNA and protein levels of BMP2 and Wnt7b were increased after treatment with 50 μmol·L-1 tranylcypromine for 48 h.119
The BMP pathway is considered a vital pathway in bone formation. Yonezawa et al.120 claimed that harmine-induced OB differentiation of MC3T3-E1 cells, primary calvarial OBs, and the C3H10T1/2 MSC lines by activating the BMP pathway and subsequently upregulating the gene expression of Runx2. Metformin has also been proven to enhance OB differentiation from MSCs obtained from type 2 diabetes mellitus (T2DM) samples through the BMP-4/Smad/Runx2 signaling pathway.121 In addition, γ-aminobutyric acid promoted the osteogenesis of MSCs through the upregulation of TNFAIP3.122 Wu et al.123 found that purmorphamine was a small-molecule agonist of Hedgehog signaling and induced osteogenesis in multipotent mesenchymal progenitor cells. In addition to promoting OB differentiation, some SMAs, such as Gln, increase the bone formation rate through energy metabolism, redox, and other pathways. Gln generates important reducing substances such as glutathione and the nutrient glutamate in the body, all of which participate in nutrient metabolism, redox and energy metabolism via multiple signaling pathways, including the Wnt, mTOR, and reactive oxygen species (ROS) signaling pathways, to promote cell proliferation, lineage distribution and bone formation124; notably, whereas Gln deficiency leads to decreased bone formation.125
Src family kinases are crucial targets in bone homeostasis. On the one hand, c-Src has been shown to increase the bone resorption rate in mice.126 On the other hand, the reduction in Src expression stimulates OB differentiation and bone formation.127,128 Dasatinib, a Src inhibitor, administer in low doses has been shown to promote osteogenic differentiation of MSCs obtained from multiple myeloma patients and healthy donors. Moreover, further experiments showed that it increased trabecular bone formation in vivo, which was primarily attributable to increased OB formation and activity rather than to an inhibitory effect on OC formation.129 In addition, dasatinib stimulated chondrogenic differentiation of MSCs via the Src/Hippo-YAP signaling pathway.130 Therefore, dasatinib may be a potential drug for bone diseases.
In fact, some other SMAs promote OB differentiation, such as tetramethylpyrazine,131 arecoline,132 and cinchonine.133 However, the underlying molecular mechanisms are unclear and need to be further elucidated.
Inhibition of OC activity
OCs are critical for bone resorption; therefore, inhibiting the formation and activity of OCs is beneficial for reducing bone loss and maintaining bone homeostasis. Some possible mechanisms for the influence of SMAs on OCs are described (Fig. 5).
The MAPK signaling pathway
The stimulation of the MAPK signaling pathway activates OC differentiation (Fig. 5). The MAPK signaling pathway can be stimulated by the upstream RANKL-RANK interaction.134 Matsumoto et al.135 discovered that the p38/MAPK signaling pathway was necessary for OC formation induced by RANKL. Many alkaloid-SMAs alleviate bone resorption by inhibiting MAPK signaling pathway activation. Vinpocetine suppressed the phosphorylation of ERK and JNK involved in osteoclastogenesis and attenuated OVX-induced bone loss in vivo.33 Li et al. revealed that sanguinarine treatment impeded OC formation and bone resorption.136 In addition, piperine was shown to hinder OC differentiation by suppressing p38/NFATc1/c-signaling.137 Deferoxamine ameliorated bone loss by suppressing OC differentiation partially through MAPK signaling.138 Moreover, norisoboldine,139 Cytisine,140 and L-tetrahydropalmatine141 inhibited MAPK pathway activation to prevent OC formation. However, as previously mentioned, MAPKs, such as ERK1 and ERK2,142 also exert positive effects on OB differentiation. Herein, the positive or negative effects of SMAs that regulate bone homeostasis through the MAPK pathway need to be verified in vivo under different conditions. Notably, we found that most alkaloid-SMAs regulated OC activity via the MAPK pathway, while most endogenous-SMAs regulated OB differentiation via the MAPK pathway. Thus, alkaloid-SMAs and endogenous-SMAs may effectively alter bone homeostasis through the MAPK pathway in different cell types.
The NF-κB signaling pathway
The NF-κB family consists of five protein monomers, including p50, p52, RelA (p65), c-Rel, and RelB, and they form homodimers or heterodimers that differentially bind DNA.143 The NF-κB signaling pathway is essential for OC formation and bone resorption (Fig. 5). The NF-κB pathway is mediated through an upstream RANKL-RANK combination, which activates downstream signaling such as NF-κB, c-Fos, and NFATc1 signaling,144,145,146,147,148,149 contributing to inhibited osteogenic differentiation of BMSCs and differentiated pre-OCs into OCs. Moreover, Yamashita et al. confirmed that NF-κB p50 and p52 regulated receptor activator of NF-κB ligand (RANKL) by activating c-Fos and NFATc1.150 Additionally, inhibition of NF-κB also hampered inflammation in bone diseases, which is explained in detail in a subsequent section. Hence, the inhibition of NF-κB signaling is conducive to bone regeneration.
Among SMAs, most alkaloid-SMAs have been shown to inhibit OC formation or exert anti-inflammatory effects through the NF-κB pathway, improving bone homeostasis. For instance, nuciferine, derived from lotus, inhibited OC formation by decreasing the expression of OC-specific genes and proteins via the inhibition of MAPK and NF-κB pathway activation.151 Furthermore, it promoted type H vessel formation151 to ameliorate bone loss caused by ovariectomy or breast cancer in vivo.151,152 Neferine, also isolated from Nelumbo nucifera (lotus), exhibits anti-inflammatory and antioxidant properties, and recently, it was verified to suppress osteoclastogenesis and attenuate OVX-induced osteoporosis in vivo by inhibiting NF-κB pathway activation.153 Tetramethylpyrazine, one of the effective ingredients of the traditional Chinese medicine Ligusticum chuanxiong, with anti-inflammatory and antioxidant properties, activated the autophagy of MSCs derived from rats with glucocorticoid-induced osteoporosis (GIOP) to protect cells against apoptosis25 and reduced RANKL and IL-6 levels to inhibit osteoclastogenesis, thereby promoting osteogenesis and increasing bone mass in the GIOP state.131 Hu et al.154 discovered that tomatidine prevented OVX-induced bone loss in vivo. At the molecular level, in the presence of tomatidine, RANK-TRAF6 binding was abrogated, downregulating RANKL-induced JNK, p38, NF-κB, and Akt phosphorylation, resulting in the suppression of osteoclastogenesis.154 In addition, it has been reported that other alkaloid-SMAs, such as neotuberostemonine,155 tetrandrine,156,157,158 sanguinarine,106,136 vinpocetine,33 and norisoboldine,139,159 prevented OC formation via the NF-κB pathway. Notably, alkaloid-SMAs exert anti-inflammatory and antioxidant effects simultaneously and exhibit increased bone regeneration rates through the inhibition of bone resorption, which is discussed in a subsequent section. In addition, benzydamine, an anti-inflammatory drug, retarded the degradation of IκB kinase to inhibit the activation of NF-κB, attenuating bone loss in lipopolysaccharide- and OVX-treated mice.45 In addition, it has been proven that RANKL activation of NF-κB and MAPK pathways in bone marrow-derived macrophages (BMMs) was inhibited after cell treatment with meclizine (20 μmol·L−1).160 For endogenous SMAs, spermidine and spermine exerted negative regulation on the transcriptional activity of NF-κB in OCs in vitro and prevented OVX-induced bone loss.161 The inhibitory effects of SMAs on OC activity mediated through the NF-κB pathway may be related to their alkalinity. However, no studies exploring the relationship between the alkalinity of SMAs and the inhibition of OC activity have been reported, and this connection deserves further study. On the basis of Table 3, we calculated that 97.05% of alkaloid-SMAs inhibited OC activity, and 63.64% of these SMAs suppressed OC activity by inhibiting NF-κB pathway activation.
The PI3K/Akt signaling pathway
The PI3K/Akt signaling pathway positively affects OBs according to the aforementioned studies, and it exerts a similar effect on OCs.162 For example, some SMAs suppress bone resorption by inhibiting the PI3K/Akt pathway and AKT activation by regulating the GSK3β/NFATc1 signaling cascade in pre-OCs or Ocs.163 Zhong et al.158 demonstrated that injecting tetrandrine into mice after OVX markedly reduced bone loss. The effects of four different concentrations, 0.125, 0.25, 0.5, and 1 μmol·L-1 tetrandrine, were analyzed in the study. The results further showed that tetrandrine inhibited OC differentiation by suppressing the NF-κB, Ca2+, PI3K/AKT, and MAPK signaling pathways in BMMs and RAW264.7 cells in a dose-dependent manner. In addition, stachydrine has also been reported to prevent LPS-induced bone loss via NF-κB and Akt signaling. Notably, it inhibited osteoclastogenesis by suppressing RANKL-induced phosphorylation of Akt and GSK3β.164 SC79, an SMA, was found to activate Akt and downstream Nrf2 signaling in OBs, thereby protecting OBs from dexamethasone-induced oxidative stress.165 Another study reported that SC79 released from porous SC79-loaded ZSM-5/chitosan scaffolds enhanced the proliferation and osteogenic differentiation of hBMSCs, and results of an in vivo study further showed that it promoted new bone formation in cranial defects.166 Notably, inhibition of the PI3K/Akt pathway might exert negative effects on OBs. Therefore, although SMA inhibits the PI3K/Akt pathway in OCs, whether it exerts a negative effect on OBs needs to be determined. Cytisine and diaporisoindole E suppressed OC formation by inhibiting activation of the RANKL-induced PI3K-AKT signaling pathways without affecting OB differentiation in vitro.140,167 In contrast, it has been reported that cinchonine not only inhibited osteoclastogenesis through the AKT pathway but also enhanced OB differentiation,133 implying that mechanisms in addition to the PI3K/AKT pathway mechanism may play a major role in promoting OB differentiation. Overall, the role of the PI3K/Akt pathway in bone hemostasis remains unclear. Therefore, in vivo experiments are urgently needed.
The OC ruffled-border vacuolar H+-ATPase
Vacuolar H+-ATPases are vital ATP-dependent proton pumps, known as housekeeping enzymes in eukaryotic cells.168 In addition, the specific isoenzymes, OC ruffled-border H+-ATPases (ORV), emerge at the OC ruffled border and display specific functions in OCs; for example, they solubilize bone mineral by acidifying an extracellular resorption compartment.169,170 Specifically, ORV leads to lacunar acidification through proton pumping and soluble acid protease (e.g., cathepsin K, MMP9) release, which causes bone resorption (Fig. 5).170 Thus, the development of anti-bone-resorption drugs that function by inhibiting ORV has become a new strategy that has attracted attention. There are also several SMAs inhibit ORV and show the potential to be developed into anti-bone-resorption drugs. For instance, SB242784, a selective inhibitor of ORV, has been found to inhibit retinoid-induced hypercalcemia in thyroparathyroidectomized rats and bone loss in ovariectomized rats.26,171 The benzamide derivatives FR167356, FR202126, and FR177995 have also been reported to prohibit ORV in OCs and exert anti-bone-resorption effects.172 Moreover, KM91104, a benzohydrazide derivative, was shown to be s an effective molecule in terms of its inhibition of ORV to impede bone resorption.173 Enoxacin, a fluoroquinolone antibiotic, interfered with OC formation and activity, as evidenced by enoxacin inhibition of the differentiation of primary marrow cells and RAW 264.7 cells into OCs.174 In summary, ORV is a promising target for the treatment of bone loss diseases, and there therefore, its inhibitors show enormous potential to be developed into new drugs; however, the effects of ORV-targeting drugs need to be confirmed because evidence based on in vivo studies is rare.
Other signaling pathways
There are other ways for SMAs to inhibit OC formation. OCs need to take up and store Fe2+ to meet the increased energy demand during OC differentiation and bone resorption. DFO chelates Fe2+, thus inhibiting OC formation.115 In addition, metformin suppresses bone resorption by activating the AMPK pathway.175 It has been shown that dopamine suppressed OC differentiation in a D2-like receptor (D2R)-dependent manner. The binding of dopamine to D2R downregulated the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway during osteoclastogenesis, resulting in decreases in cAMP-response element-binding protein (CREB) phosphorylation, and blocking D2R abolished the inhibitory effects of dopamine.23 Another neurotransmitter, epinephrine, has been proven to promote osteoblast differentiation by enhancing BMP signaling through a cAMP/protein kinase A signaling pathway.176 As mentioned above, Scr signaling inhibited bone resorption and promoted bone regeneration. Studies have shown that saracatinib reduced the formation of active phosphorylated c-Src in OC-like cells and reversibly prevented OC precursor migration from the OB layer to the bone surface and subsequent formation of actin rings and resorption pits.41
Regulation of bone microenvironments
SMAs play significant roles in the regulation of inflammation, oxidative stress, and pH on bone homeostasis (Fig. 6), in addition to their direct effects on OBs and OCs. Some SMAs exhibit anti-inflammatory, antioxidation, and alkaline properties, improving the bone microenvironment and thus contributing to bone homeostasis.
Anti-inflammation
Inflammation influences the progress of bone regeneration in bone diseases. Chronic inflammation can impede bone regeneration and cause other diseases such as RA, OA, diabetes mellitus, and systemic lupus erythematosus.177 Some SMAs exhibit anti-inflammatory functions mediated through the NF-κB and JAK pathways, regulating the behaviors of chondrocytes, OCs, or OBs in bone regeneration.
The NF-κB signaling pathway is not only a vital signaling pathway in OC formation but also a typical inflammation pathway. The majority of proinflammatory factors, including IL-1, TNF and IL-6, can increase the production of RANKL, leading to accelerated OC formation.178,179,180 Moreover, the activation of the NF-κB pathway also causes extracellular matrix (ECM) damage and cartilage erosion in response to the inflammation of chondrocytes in OA.181 Therefore, the regulatory effects of the NF-κB pathway on inflammation make it an important target for the treatment of inflammatory bone diseases.182 Some drug-SMAs with anti-inflammatory functions tend to inhibit OC formation via the NF-κB signaling pathway. For example, benzydamine suppressed IL-1β expression via the inhibition of NF-κB and AP-1, thereby inhibiting OC differentiation and bone resorption, and its inhibitory effects were reversed by IL-1β treatment.45 Furthermore, LPS- or OVX-induced bone destruction was ameliorated by the treatment of benzydamine at a dose of 10 mg·kg-1.45 Evidence showed that TNF was the major factor involved in the response to synovial arthritis in the OA context.183 In TNF-α-induced mouse models of OA, synovial inflammation was inhibited after spermidine treatment.184 Mechanistically, spermidine prevented TNF-induced NF-κB/p65 activation by suppressing RIP1 ubiquitination, thereby ameliorating cartilage degeneration and osteophyte formation.184 In addition, many alkaloid-SMAs exhibit anti-inflammatory effects by inhibiting the NF-κB pathway. For instance, tetramethylpyrazine has been reported to attenuate the senescent phenotype of cells and contribute to an anti-inflammatory and angiogenic microenvironment.185 Subsequently, an investigation performed by Yu et al. revealed that treatment with tetramethylpyrazine downregulated the inflammatory reaction of chondrocytes in the joint effusion of OA patients and alleviated the injury and matrix degradation induced by IL-1β.186 Mechanistically, it blocked NF-κB pathway activation, enhanced the expression of SOX9 and decreased the production of ROS in IL-1β-induced chondrocytes, thereby protecting chondrocytes.186 In addition, Duan et al. discovered that the inflammation and apoptosis of LPS-stimulated human periodontal ligament cells (PDLSCs) were reduced via the downregulation of miR-302b by tetramethylpyrazine.187 Thus, tetramethylpyrazine may be a potential drug for treating OA. Moreover, other alkaloid SMAs, such as tomatidine,154 tetrandrine,157 berberine,188 vinpocetine,33 dauricine,189 and nitensidine A,190 exert anti-inflammatory effects, which may contribute to a reduction in bone loss and an improved bone microenvironment.
Drug-SMAs, baricitinib and tofacitinib have been used for the treatment of inflammatory diseases such as RA.191,192 Recent research revealed that these two drugs, both SMAs, mitigated OVX-induced bone loss and inhibited bone loss in the context of arthritis in vivo.27
Some SMAs exert anti-inflammatory effects mediated via other pathways in addition to those mediated by the NF-κB pathway. For example, when bone was in an inflammatory condition, the inhibition of c-Src downregulated IL-6 expression.128 Therefore, c-Src inhibitors such as saracatinib might show osteotropic activity by suppressing inflammation.42 Moreover, another c-Src inhibitor, dasatinib, has been reported to inhibit inflammation and increase bone mineral density in a mouse model of chronic recurrent multifocal osteomyelitis.193 As mentioned above, adenosine exerts positive effects on bone regeneration. In addition, it is critical to reducing inflammation. However, adenosine has an extremely short half-life, which means it is rapidly metabolized in blood, where it is converted to other molecular forms.194 Adenosine N1-oxide, a product of adenosine oxidation at the N1 position of the adenine base moiety, has been found in royal jelly, and it was more stable than adenosine and maintained anti-inflammatory activity.195 Specifically, it inhibited the secretion of TNF-α and IL-6 in LPS-treated RAW264.7 cells via its action on the PI3K/Akt/GSK-3β pathway, further promoting the osteogenic differentiation of MC3T3-E1 cells.195 Additionally, cetirizine, a histamine 1 receptor antagonist, promoted bone formation after suture expansion, mostly by suppressing osteoclastic activity.46 According to the aforementioned studies, SMAs with anti-inflammatory effects improve bone homeostasis, although their mechanisms of action may differ.
Antioxidation
Oxidative stress, caused by high levels of ROS, increases osteoclastogenesis and inhibits osteogenesis and mineralization by inducing the apoptosis of OBs and OCs, leading to dysfunctional bone.196 Thus, inhibition of oxidative stress can be a feasible strategy for treating bone diseases. It has been reported that certain SMAs inhibit oxidation stress through redox-related signaling factors such as hypoxia-inducible 1 (HIF-1) or the Nrf2 pathway in bone homeostasis. In addition, oxidative stress is associated with inflammation (Fig. 5). ROS can lead to the generation of proinflammatory molecules, inducible NO synthase (iNOS) and cyclooxygenase (COX-2),197 and inflammation also causes the increased production of ROS. Hence, the regulation of oxidation levels in cells or tissues is crucial for bone homeostasis.
HIF or Nrf2 proteins can be rapidly stabilized by hypoxia or oxidative stress, respectively, to respond to induce rapid changes in the redox state of cells,198 thereby protecting cells. HIF-1, composed of the HIF-1α subunit and HIF-1β subunit, among which HIF-1α is the core of the oxygen sensing mechanism,199 regulates the expression of many antioxidants.200,201,202 Jing et al. found that DFO upregulated HIF-1α expression in a dose-dependent manner, and the downregulated expression of HIF-1α induced by dexamethasone was rescued by DFO treatment (100 μmol·L-1).115 Similarly, the activation of Nrf2 promoted the transcription of antioxidant enzymes (e.g., SOD) and the production of antioxidant substances (e.g., GSH).203 Lack of Nrf2 induced oxidative stress and promoted OC differentiation induced by RANKL, resulting in bone loss.204 Chen et al. declared that the inhibition of Nrf2 resulting from aberrant DNA methyltransferase level elevation and subsequent Nrf2 promoter hypermethylation was probably a vital epigenetic mechanism underlying the pathogenesis of osteoporosis.205 In addition, Nrf2 reduced the toxicity of iron-induced oxidative stress.198 For example, Jia et al.206 showed that metformin ameliorated oxidative stress caused by H2O2 in PDLSCs, and pretreatment or cotreatment with metformin reversed the activity of SOD and the concentrations of GSH and ROS, thereby protecting cells from oxidative stress. Moreover, the positive effect of metformin on cell viability was diminished by the knockdown of Nrf2.206
Notably, some agonists of the AMPK pathway, including OSU53,108 AICAR,109 A-769662,110 GSK621,111, and Compound 13,207 are antioxidants and protect OBs from oxidative damage, which might be related to the relationship between AMPK and Nrf2. Joo et al. discovered that a subnetwork integrating neighboring molecules suggested a direct interaction between AMPK and Nrf2.208 They found that AMPK stimulation caused nuclear accumulation of Nrf2 and that AMPK phosphorylated Nrf2 at the Ser558 residue (Ser550 in mice) located in the canonical nuclear export signal peptide.208
Additionally, SMAs can influence other signals by regulating oxidative stress. For instance, it has been reported that dauricine decreased oxidation of serine/threonine-protein phosphatase 2A to block the activation of NF-κB by reducing the ROS levels in OCs, resulting in protection from bone loss.189 Zhan et al.209 demonstrated that treatment with vindoline suppressed intracellular ROS production in a dose-dependent manner, thus inhibiting OC differentiation. Similarly, Zhu et al. revealed that vinpocetine inhibited the RANKL-induced production of ROS and increased the expression of cytoprotective enzymes such as HO-1 and NQO-1.33 In addition, in IL-1β-treated chondrocytes, the level of SOD was significantly decreased, and the production of ROS and MDA was increased, which was attenuated by treatment with ligustrazine.186 Pyrroloquinoline quinone (PQQ), a powerful antioxidant, has been shown to prevent bone loss in mice after orchiectomy (ORX).210 Mechanistically, PQQ reduced the elevated ROS levels in thymus tissues and partially promoted the expression of antioxidant enzymes such as SOD-1 and SOD-2 in the mice after ORX.210 Taken together, the studies show that SMAs with antioxidant effects protect OBs or chondrocytes from oxidative stress or inhibit osteoclastogenesis by interacting with various pathways. In conclusion, SMAs with antioxidant effects are endogenous SMAs, alkaloid-SMAs, anti-inflammatory drug SMAs, and AMPK pathway activators.
Influence of local pH
The acid–base microenvironment, which affects the proliferation, differentiation, and apoptosis of bone tissue-related cells, is being increasingly appreciated. Acidosis not only accelerates bone resorption and bone mineral dissolution but also prevents mineralization and OB formation, leading to severe bone loss.211,212,213 In contrast, a weakly alkaline microenvironment can promote bone formation/mineralization214 and reduce bone resorption.215 Even subtle changes in the extracellular pH can affect the secretion of phenotype-inducing proteins in OBs, including collagen and osteocalcin.216 ALP activity and collagen synthesis were found to be increased by 2–3 fold when the pH value was from 6.6 to 7.8.216 Weak alkalinity increased the proliferation and differentiation of OBs.217 In addition, the acid–base microenvironment can also regulate inflammation and blood vessel formation. It has been reported that an acidic microenvironment increases the expression of certain inflammatory factors, such as IL-6 and cathepsin B, in a time-dependent manner.218 Spector et al.219 discovered that compared with a neutral environment (pH = 7.4), an acidic environment (pH = 7.0) decreased the production of VEGF in OBs. Overall, the acid–base microenvironment contributes to an alkaline microenvironment, which is conducive to bone formation.
Recent findings suggest that creating an alkaline microenvironment by releasing alkaline ions from tissue-engineered materials has great benefits for bone regeneration.220 It has been confirmed that the differentiation potential and pit-formation capability of OCs were greatly suppressed when they were cultured in titrated material extracts with a pH value of 7.8 or higher.221 Moreover, Liu et al. revealed that the expression of OC-related enzyme genes, including cathepsin K, TRAP, MMP-9, and NFATc1, was inhibited under alkaline conditions, with a pH 7.8−8.0, in all tested materials.221 Therefore, providing a relatively alkaline microenvironment on the surface of biodegradable implant material may be a great strategy to inhibit the activity of OCs and thus promote bone regeneration.221 Our groups have performed experiments with variations in the interfacial pH values of poly (D, L-lactide) (PDLLA) and P-PUUs. The results demonstrated the interfacial pH rapidly decreased after the release of degraded products, and this outcome was reversed by the introduction of the alkaline segments of piperazine.222 More intriguingly, OBs also constructed the microenvironment by secreting cellular metabolites, including ALP and extracellular calcium, to upregulate the interfacial pH of the materials, thereby promoting their own proliferation, differentiation, and mineralization.222 Moreover, compared with PDLLA, P-PUU showed a greater ability to promote OB differentiation, which is attributed to the piperazine units in the P-PUU.222 Hence, the addition of proper alkaline molecules or units to biodegradable biomaterials can create a weak alkaline microenvironment, which is beneficial to bone regeneration. SMAs may be considered candidates for loading onto bone tissue-engineered materials due to their alkaline properties. The investigations into the relationship between local pH values and the behaviors of OCs and OBs demonstrated that an alkaline microenvironment inhibited OC activity and enhanced OB formation. However, they mainly focused on the local pH of the biomaterials and did not specifically explore the specific relationship between SMA alkalinity and bone homeostasis.
Current challenges and prospects
Challenges to the development of SMAs to treat bone diseases
In recent years, SMAs have been developed as potential drug molecules or biomaterials with broad applications for the treatment of bone diseases. However, large challenges for SMA development remain, and each needs to be further investigated.
For the development of SMA-based drugs, the main challenges are summarized as follows:
-
(1)
Some studies on SMAs for bone homeostasis lack validation via in vivo experiments. The in vivo effect of SMAs may be very different from that observed under in vitro conditions. Therefore, when only in vitro studies are carried out, the regulatory effects of SMAs on bone homeostasis are still unconvincing.
-
(2)
The mechanisms by which SMAs enter cells and interact with receptor proteins are still unclear. Most studies have led to the identification of only one or two signaling pathways. More comprehensive studies are still needed.
-
(3)
SMAs have been shown to exert antioxidative and anti-inflammatory effects. However, the detailed mechanisms of these effects have not been clearly characterized.
For the development of SMA-based biomaterials, the main challenges are summarized as follows:
-
(1)
The stability and reproducibility of a synthetic process for producing SMA-based biomaterials are insufficient and need to be further optimized. On the one hand, the most recently available biomaterials lack standardized preparation procedures or evaluation criteria. As a result, the reproducibility of biomaterial experiments is not sufficiently high. On the other hand, the successful loading SMAs onto biomaterials heavily depends on the physical and chemical properties of the SMAs. We need to develop a proper design to load SMAs with different chemical structures, such as RNH2, RNH, and R3N SMAs.
-
(2)
There is a lack of in vivo data about the local release of SMAs from SMA-based biomaterials. The release mechanisms and degradation kinetics for most SMA-based biomaterials have not been clearly determined. More work is needed to further improve SMA loading efficiency at effective and safe concentrations.
Prospects
SMA-based drug development
To address the above problems and further promote the development of SMA-based drugs, the following points should be emphasized:
-
(1)
The extensive screening of SMAs with osteotropic activity should be increased. Effective SMAs will be identified from libraries of traditional Chinese medicine by new technologies such as network pharmacology and high-throughput omics, and the efficacy of positive molecules needs to be compared with that of the most commonly used drugs.
-
(2)
In vivo, efficacy should be used as the primary criterion for SMA-drug evaluation before further mechanistic studies are performed.
-
(3)
The underlying mechanisms of SMA effects on the maintenance of bone homeostasis need to be comprehensively and precisely understood. New techniques, such as single-cell RNA sequencing, proteomic profiling, differential expression analysis, and pathway analysis, may be applied to achieve this level of comprehension.223 The SILAC + SM pull-down technique has been used to determine the specific binding of small molecules to proteins,224 and it can be used in future studies.
-
(4)
The relationship between the chemical structure and biochemical activity of SMAs needs to be further explored, which may provide robust guidance for their in vivo application. For example, in-depth structure–activity studies of SMAs anti-inflammation, antioxidation and pH-alter effects may lay a solid foundation for the development of SMA-based drugs and biomaterials.
SMA-based biomaterials development
Some prospects for SMA-based biomaterial development should also be emphasized:
-
(1)
Establishing an SMA-based biomaterial with a stable and reproducible synthesis process is urgent for its scalable production. The synthesis of SMA-based biomaterials that is stable and reproducibility should be identified, and effective loading strategies for similar structures and properties of SMAs need to be classified. Moreover, the selection of a biomaterial with proven preparation methods is crucial for enhancing the stability and reproducibility for SMA-based biomaterial synthesis.
-
(2)
Developing an SMA controlled-release strategy is important for SMA-based biomaterials. Hydrogels, fibrous structure biomaterials, porous microspheres, etc., may be applied to respond to the pH, temperature, and oxidation conditions of a microenvironment. Micro/nanorobots are promising drug-targeted delivery systems, and 3D printing techniques can achieve precise loading and controlled release of SMAs. In addition, a combination of in vitro and in vivo methods should be developed to prepare SMA-based biomaterials with the desired drug-loading procedure and sustained release period.
-
(3)
Selecting SMAs with definitive treatment effects or biomaterials that have received approval for clinical use can reduce the R&D time. Mechanistic studies can further promote the development of SMA-based biomaterials.
References
Moller, A. M. J. et al. Aging and menopause reprogram osteoclast precursors for aggressive bone resorption. Bone Res. 8, 27 (2020).
Modinger, Y., Loffler, B., Huber-Lang, M. & Ignatius, A. Complement involvement in bone homeostasis and bone disorders. Semin. Immunol. 37, 53–65 (2018).
Liao, J., Han, R., Wu, Y. & Qian, Z. Review of a new bone tumor therapy strategy based on bifunctional biomaterials. Bone Res. 9, 18 (2021).
Wiese, A. & Pape, H. C. Bone defects caused by high-energy injuries, bone loss, infected nonunions, and nonunions. Orthop. Clin. North Am. 41, 1–4 (2010). table of contents.
Zhang, Y., Luo, G. & Yu, X. Cellular communication in bone homeostasis and the related anti-osteoporotic drug development. Curr. Med. Chem. 27, 1151–1169 (2020).
Chen, D. et al. Osteoarthritis: toward a comprehensive understanding of pathological mechanism. Bone Res. 5, 16044 (2017).
Salhotra, A., Shah, H. N., Levi, B. & Longaker, M. T. Mechanisms of bone development and repair. Nat. Rev. Mol. Cell Biol. 21, 696–711 (2020).
Chen, H., Senda, T. & Kubo, K. Y. The osteocyte plays multiple roles in bone remodeling and mineral homeostasis. Med. Mol. Morphol. 48, 61–68 (2015).
Feng, X. & Teitelbaum, S. L. Osteoclasts: new insights. Bone Res. 1, 11–26 (2013).
Walker, D. G. Osteopetrosis cured by temporary parabiosis. Science 180, 875 (1973).
Zambonin Zallone, A., Teti, A. & Primavera, M. V. Monocytes from circulating blood fuse in vitro with purified osteoclasts in primary culture. J. Cell Sci. 66, 335–342 (1984).
Søe, K. et al. Coordination of fusion and trafficking of pre-osteoclasts at the marrow-bone interface. Calcif. Tissue Int. 105, 430–445 (2019).
MacDonald, B. R. et al. Effects of human recombinant CSF-GM and highly purified CSF-1 on the formation of multinucleated cells with osteoclast characteristics in long-term bone marrow cultures. J. Bone Min. Res. 1, 227–233 (1986).
Udagawa, N. et al. Origin of osteoclasts: mature monocytes and macrophages are capable of differentiating into osteoclasts under a suitable microenvironment prepared by bone marrow-derived stromal cells. Proc. Natl. Acad. Sci. USA 87, 7260–7264 (1990).
Wiktor-Jedrzejczak, W. et al. Total absence of colony-stimulating factor 1 in the macrophage-deficient osteopetrotic (op/op) mouse. Proc. Natl. Acad. Sci. USA 87, 4828–4832 (1990).
Pixley, F. J. & Stanley, E. R. CSF-1 regulation of the wandering macrophage: complexity in action. Trends Cell Biol. 14, 628–638 (2004).
McDonald, M. M. et al. Osteoclasts recycle via osteomorphs during RANKL-stimulated bone resorption. Cell 184, 1–18 (2021).
Hardy, R. & Cooper, M. S. Bone loss in inflammatory disorders. J. Endocrinol. 201, 309–320 (2009).
Callaway, D. A. & Jiang, J. X. Reactive oxygen species and oxidative stress in osteoclastogenesis, skeletal aging and bone diseases. J. Bone Min. Metab. 33, 359–370 (2015).
Arnett, T. R. Extracellular pH regulates bone cell function. J. Nutr. 138, 415S–418S (2008).
Macielag, M. J. Antibiotic Discovery and Development (eds T. J. Dougherty & M. J. Pucci) 793–819 (Springer New York, NY, 2011).
Momeni, A., Rapp, S., Donneys, A., Buchman, S. R. & Wan, D. C. Clinical use of deferoxamine in distraction osteogenesis of irradiated bone. J. Craniofac. Surg. 27, 880–882 (2016).
Wang, L. et al. Dopamine suppresses osteoclast differentiation via cAMP/PKA/CREB pathway. Cell Signal. 78, 109847 (2021).
Gu, L., Ke, Y., Gan, J. & Li, X. Berberine suppresses bone loss and inflammation in ligature-induced periodontitis through promotion of the G protein-coupled estrogen receptor-mediated inactivation of the p38MAPK/NF-kappaB pathway. Arch. Oral. Biol. 122, 104992–104992 (2020).
Wang, L. et al. Tetramethylpyrazine protects against glucocorticoid-induced apoptosis by promoting autophagy in mesenchymal stem cells and improves bone mass in glucocorticoid-induced osteoporosis rats. Stem Cells Dev. 26, 419–430 (2017).
Price, P. A., June, H. H., Buckley, J. R. & Williamson, M. K. SB 242784, a selective inhibitor of the osteoclastic V-H + ATPase, inhibits arterial calcification in the rat. Circ. Res. 91, 547–552 (2002).
Adam, S. et al. JAK inhibition increases bone mass in steady-state conditions and ameliorates pathological bone loss by stimulating osteoblast function. Sci. Transl. Med. 12, eaay4447 (2020).
Vidal, B. et al. Effects of tofacitinib in early arthritis-induced bone loss in an adjuvant-induced arthritis rat model. Rheumatology 57, 1461–1471 (2018).
Chandra, A. et al. Proteasome inhibitor bortezomib is a novel therapeutic agent for focal radiation-induced osteoporosis. FASEB J. 32, 52–62 (2018).
Shaik, A. R. et al. Metformin: is it the well wisher of bone beyond glycemic control in diabetes mellitus? Calcif. Tissue Int. 108, 693–707 (2021).
Ma, P. et al. Glimepiride promotes osteogenic differentiation in rat osteoblasts via the PI3K/Akt/eNOS pathway in a high glucose microenvironment. PLoS One 9, e112243 (2014).
S. L., Taylor & S. L., Hefle. Foodborne Diseases (eds C. E. R. Dodd et al.) 327–344 (Academic Press, 2017).
Zhu, M. P. et al. Vinpocetine inhibits RANKL-induced osteoclastogenesis and attenuates ovariectomy-induced bone loss. Biomed. Pharmacother. 123, 10 (2020).
Jin, H., Yao, L. & Chen, K. Evodiamine inhibits RANKL-induced osteoclastogenesis and prevents ovariectomy-induced bone loss in mice. J. Cell. Mol. Med. 23, 4850–4850 (2019).
Zeng, X. Z. et al. Aconine inhibits RANKL-induced osteoclast differentiation in RAW264.7 cells by suppressing NF-kappa B and NFATc1 activation and DC-STAMP expression. Acta Pharmacol. Sin. 37, 255–263 (2016).
Ouellette, R. J. & Rawn, J. D. Organic Chemistry (eds R. J. Ouellette & J. David Rawn) 803–842 (Elsevier, 2014).
Lovaas, E. Antioxidative and metal-chelating effects of polyamines. Adv. Pharm. 38, 119–149 (1997).
Zhou, T., Wang, P., Gu, Z., Ma, M. & Yang, R. Spermidine improves antioxidant activity and energy metabolism in mung bean sprouts. Food Chem. 309, 125759 (2020).
Drugbank online. Odanacatib, <https://go.drugbank.com/drugs/DB06670> (2022).
Kim, S. H. et al. Bortezomib prevents ovariectomy-induced osteoporosis in mice by inhibiting osteoclast differentiation. J. Bone Min. Metab. 36, 537–546 (2018).
de Vries, T. J. et al. The Src inhibitor AZD0530 reversibly inhibits the formation and activity of human osteoclasts. Mol. Cancer Res. 7, 476–488 (2009).
Yang, J. C. et al. Effect of the specific Src family kinase inhibitor saracatinib on osteolytic lesions using the PC-3 bone model. Mol. Cancer Ther. 9, 1629–1637 (2010).
Jiao, H., Xiao, E. & Graves, D. T. Diabetes and its effect on bone and fracture healing. Curr. Osteoporos. Rep. 13, 327–335 (2015).
Lecka-Czernik, B. Diabetes, bone and glucose-lowering agents: basic biology. Diabetologia 60, 1163–1169 (2017).
Son, H. S. et al. Benzydamine inhibits osteoclast differentiation and bone resorption via down-regulation of interleukin-1 beta expression. Acta Pharm. Sin. B 10, 462–474 (2020).
Aasarod, K. M. et al. Effects of the histamine 1 receptor antagonist cetirizine on the osteoporotic phenotype in H+/K(+)ATPase beta subunit KO mice. J. Cell. Biochem. 117, 2089–2096 (2016).
Yamaura, K., Yonekawa, T., Nakamura, T., Yano, S. & Ueno, K. The histamine H-2-receptor antagonist, cimetidine, inhibits the articular osteopenia in rats with adjuvant-induced arthritis by suppressing the osteoclast differentiation induced by histamine. J. Pharmacol. Sci. 92, 43–49 (2003).
Gomes, P. S., Resende, M. & Fernandes, M. H. Doxycycline restores the impaired osteogenic commitment of diabetic-derived bone marrow mesenchymal stromal cells by increasing the canonical WNT signaling. Mol. Cell Endocrinol. 518, 110975 (2020).
Drugbank online. Lidocaine, <https://go.drugbank.com/drugs/DB00281>(2021).
Li, L. et al. Polydopamine coating promotes early osteogenesis in 3D printing porous Ti6Al4V scaffolds. Ann. Transl. Med. 7, 14 (2019).
Hu, C. et al. Berberine/Ag nanoparticle embedded biomimetic calcium phosphate scaffolds for enhancing antibacterial function. Nanotechnol. Rev. 9, 568–579 (2020).
Sang, S., Wang, S. J., Yang, C., Geng, Z. & Zhang, X.L. Sponge-inspired sulfonated polyetheretherketone loaded with polydopamine-protected osthole nanoparticles and berberine enhances osteogenic activity and prevents implant-related infections. Chem. Eng. J. 437, 135255 (2022).
Chen, H., Yan, Y. F., Qi, J., Deng, L. F. & Cui, W. G. Sustained delivery of desferrioxamine via liposome carriers in hydrogel for combining angiogenesis and osteogenesis in bone defects reconstruction. J. Control. Release 259, E79–E79 (2017).
Song, W. et al. Doxycycline-loaded coaxial nanofiber coating of titanium implants enhances osseointegration and inhibits Staphylococcus aureus infection. Biomed. Mater. 12, 045008 (2017).
Wang, D., Zhang, P., Mei, X. & Chen, Z. Repair calvarial defect of osteoporotic rats by berberine functionalized porous calcium phosphate scaffold. Regen. Biomater. 8, 10 (2021).
Ozturk-Oncel, M. O., Odabas, S., Uzun, L., Hur, D. & Garipcan, B. A facile surface modification of poly(dimethylsiloxane) with amino acid conjugated self-assembled monolayers for enhanced osteoblast cell behavior. Colloids Surf. B Biointerfaces 196, 111343 (2020).
Cui, N., Han, K., Li, M., Wang, J. & Qian, J. Pure polylysine-based foamy scaffolds and their interaction with MC3T3-E1 cells and osteogenesis. Biomed. Mater. 15, 025004 (2020).
Ma, Y. et al. Three-dimensional printing of biodegradable piperazine-based polyurethane-urea scaffolds with enhanced osteogenesis for bone regeneration. ACS Appl. Mater. Interfaces 11, 9415–9424 (2019).
Mao, L. et al. Regulation of inflammatory response and osteogenesis to citrate-based biomaterials through incorporation of alkaline fragments. Adv. Health. Mater. 11, e2101590 (2022).
Pritchard, J. J. A cytological and histochemical study of bone and cartilage formation in the rat. J. Anat. 86, 259–277 (1952).
Young, M. F., Kerr, J. M., Ibaraki, K., Heegaard, A. M. & Robey, P. G. Structure, expression, and regulation of the major noncollagenous matrix proteins of bone. Clin. Orthop. Relat. Res. 281, 275–294 (1992).
Robey, P. G. et al. Structure and molecular regulation of bone matrix proteins. J. Bone Min. Res. 8, S483–S487 (1993).
Long, F. Building strong bones: molecular regulation of the osteoblast lineage. Nat. Rev. Mol. Cell Biol. 13, 27–38 (2011).
Chang, L. & Karin, M. Mammalian MAP kinase signalling cascades. Nature 410, 37–40 (2001).
Jaiswal, R. K. et al. Adult human mesenchymal stem cell differentiation to the osteogenic or adipogenic lineage is regulated by mitogen-activated protein kinase. J. Biol. Chem. 275, 9645–9652 (2000).
Kim, J. M. et al. The ERK MAPK pathway is essential for skeletal development and homeostasis. Int. J. Mol. Sci. 20, 1803 (2019).
Wang, C. X. et al. Dopamine D1 receptor-mediated activation of the ERK signaling pathway is involved in the osteogenic differentiation of bone mesenchymal stem cells. Stem Cell Res. Ther. 11, 13 (2020).
Lee, H. W. et al. Berberine promotes osteoblast differentiation by Runx2 activation with p38 MAPK. J. Bone Min. Res. 23, 1227–1237 (2008).
Xin, B. C. et al. Berberine promotes osteogenic differentiation of human dental pulp stem cells through activating EGFR-MAPK-Runx2 pathways. Pathol. Oncol. Res. 26, 1677–1685 (2020).
Kim, D. S. et al. Effects of glutamine on proliferation, migration, and differentiation of human dental pulp cells. J. Endod. 40, 1087–1094 (2014).
Yin, H. et al. Preventive effects of evodiamine on dexamethasone-induced osteoporosis in zebrafish. Biomed. Res. Int. 2019, 6 (2019).
Kim, H. K. et al. Salicylideneamino-2-thiophenol enhances osteogenic differentiation through the activation of MAPK pathways in multipotent bone marrow stem cell. J. Cell. Biochem. 113, 1833–1841 (2012).
Chan, Y. H. et al. Melatonin enhances osteogenic differentiation of dental pulp mesenchymal stem cells by regulating MAPK pathways and promotes the efficiency of bone regeneration in calvarial bone defects. Stem Cell Res. Ther. 13, 73 (2022).
Yuan, L. Q. et al. Taurine promotes connective tissue growth factor (CTGF) expression in osteoblasts through the ERK signal pathway. Amino Acids 32, 425–430 (2007).
Villa, I. et al. Betaine promotes cell differentiation of human osteoblasts in primary culture. J. Transl. Med. 15, 132 (2017).
Baron, R. & Kneissel, M. WNT signaling in bone homeostasis and disease: from human mutations to treatments. Nat. Med. 19, 179–192 (2013).
Nusse, R. & Clevers, H. Wnt/beta-catenin signaling, disease, and emerging therapeutic modalities. Cell 169, 985–999 (2017).
Madan, B. et al. Bone loss from Wnt inhibition mitigated by concurrent alendronate therapy. Bone Res. 6, 17 (2018).
Zhang, L. N. et al. Berberine improves advanced glycation end products‑induced osteogenic differentiation responses in human periodontal ligament stem cells through the canonical Wnt/β‑catenin pathway. Mol. Med. Rep. 19, 5440–5452 (2019).
Yang, L. et al. Leonurine hydrochloride promotes osteogenic differentiation and increases osteoblastic bone formation in ovariectomized mice by Wnt/beta-catenin pathway. Biochem. Biophys. Res. Commun. 504, 941–948 (2018).
Li, C. et al. The protective effect of piperine on ovariectomy induced bone loss in female mice and its enhancement effect of osteogenic differentiation via Wnt/β-catenin signaling pathway. J. Funct. Food 58, 138–150 (2019).
Karner, C. M., Esen, E., Okunade, A. L., Patterson, B. W. & Long, F. Increased glutamine catabolism mediates bone anabolism in response to WNT signaling. J. Clin. Investig. 125, 551–562 (2015).
Seidlitz, E. P., Sharma, M. K. & Singh, G. Extracellular glutamate alters mature osteoclast and osteoblast functions. Can. J. Physiol. Pharm. 88, 929–936 (2010).
Schaffer, S. & Kim, H. W. Effects and mechanisms of taurine as a therapeutic agent. Biomol. Ther. 26, 225–241 (2018).
Prideaux, M., Kitase, Y., Kimble, M., O’Connell, T. M. & Bonewald, L. F. Taurine, an osteocyte metabolite, protects against oxidative stress-induced cell death and decreases inhibitors of the Wnt/beta-catenin signaling pathway. Bone 137, 115374 (2020).
Ma, Z. P., Liao, J. C., Zhao, C. & Cai, D. Z. Effects of the 1, 4-dihydropyridine L-type calcium channel blocker benidipine on bone marrow stromal cells. Cell Tissue Res. 361, 467–476 (2015).
Ma, J., Zhang, Z. L., Hu, X. T., Wang, X. T. & Chen, A. M. Metformin promotes differentiation of human bone marrow derived mesenchymal stem cells into osteoblast via GSK3beta inhibition. Eur. Rev. Med. Pharm. Sci. 22, 7962–7968 (2018).
Zhao, X. E. et al. 6-Bromoindirubin-3’-oxime promotes osteogenic differentiation of canine BMSCs through inhibition of GSK3beta activity and activation of the Wnt/beta-catenin signaling pathway. Acad. Bras. Cienc. 91, e20180459 (2019).
Fruman, D. A., Meyers, R. E. & Cantley, L. C. Phosphoinositide kinases. Annu. Rev. Biochem. 67, 481–507 (1998).
Chen, J., Crawford, R., Chen, C. & Xiao, Y. The key regulatory roles of the PI3K/Akt signaling pathway in the functionalities of mesenchymal stem cells and applications in tissue regeneration. Tissue Eng. Part B Rev. 19, 516–528 (2013).
Zhao, B. et al. Leonurine promotes the osteoblast differentiation of Rat BMSCs by activation of autophagy via the PI3K/Akt/mTOR pathway. Front. Bioeng. Biotechnol. 9, 615191 (2021).
Mirones, I. et al. Dopamine mobilizes mesenchymal progenitor cells through D2-class receptors and their PI3K/AKT pathway. Stem Cells 32, 2529–2538 (2014).
Ma, P. et al. Glimepiride induces proliferation and differentiation of rat osteoblasts via the PI3-kinase/Akt pathway. Metabolism 59, 359–366 (2010).
Evans, B. A. J. et al. Human osteoblast precursors produce extracellular adenosine, which modulates their secretion of IL-6 and osteoprotegerin. J. Bone Miner. Res. 21, 228–236 (2006).
Gharibi, B., Abraham, A. A., Ham, J. & Evans, B. A. J. Adenosine receptor subtype expression and activation influence the differentiation of mesenchymal stem cells to osteoblasts and adipocytes. J. Bone Miner. Res. 26, 2112–2124 (2011).
Carroll, S. H. et al. A2B adenosine receptor promotes mesenchymal stem cell differentiation to osteoblasts and bone formation in vivo. J. Biol. Chem. 287, 15718–15727 (2012).
Borhani, S., Corciulo, C., Larranaga-Vera, A. & Cronstein, B. N. Adenosine A(2A) receptor (A2AR) activation triggers Akt signaling and enhances nuclear localization of beta-catenin in osteoblasts. FASEB J. 33, 7555–7562 (2019).
Ryu, J. et al. Sphingosine 1-phosphate as a regulator of osteoclast differentiation and osteoclast-osteoblast coupling. EMBO J. 25, 5840–5851 (2006).
Grey, A. et al. The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase. Endocrinology 143, 4755–4763 (2002).
Matsuzaki, E. et al. Sphingosine-1-phosphate promotes the nuclear translocation of beta-catenin and thereby induces osteoprotegerin gene expression in osteoblast-like cell lines. Bone 55, 315–324 (2013).
Endo, H., Nito, C., Kamada, H., Nishi, T. & Chan, P. H. Activation of the Akt/GSK3beta signaling pathway mediates survival of vulnerable hippocampal neurons after transient global cerebral ischemia in rats. J. Cereb. Blood Flow. Metab. 26, 1479–1489 (2006).
Pantovic, A. et al. Coordinated time-dependent modulation of AMPK/Akt/mTOR signaling and autophagy controls osteogenic differentiation of human mesenchymal stem cells. Bone 52, 524–531 (2013).
Chava, S., Chennakesavulu, S., Gayatri, B. M. & Reddy, A. B. M. A novel phosphorylation by AMP-activated kinase regulates RUNX2 from ubiquitination in osteogenesis over adipogenesis. Cell Death Dis. 9, 754 (2018).
Adil, M., Mansoori, M. N., Singh, D., Kandhare, A. D. & Sharma, M. Pioglitazone-induced bone loss in diabetic rats and its amelioration by berberine: a portrait of molecular crosstalk. Biomed. Pharmacother. 94, 1010–1019 (2017).
Kim, D. Y., Kim, E. J. & Jang, W. G. Piperine induces osteoblast differentiation through AMPK-dependent Runx2 expression. Biochem. Biophys. Res. Commun. 495, 1497–1502 (2018).
Zhang, F. Z., Xie, J. L., Wang, G. L., Zhang, G. & Yang, H. L. Anti-osteoporosis activity of Sanguinarine in preosteoblast MC3T3-E1 cells and an ovariectomized rat model. J. Cell. Physiol. 233, 4626–4633 (2018).
Zhao, X. et al. Metformin enhances osteogenic differentiation of stem cells from human-exfoliated deciduous teeth through AMPK pathway. J. Tissue Eng. Regen. Med. 14, 1869–1879 (2020).
Xu, D. et al. OSU53 rescues human OB-6 osteoblastic cells from dexamethasone through activating AMPK signaling. PLoS One 11, e0162694 (2016).
Kim, J. E. et al. AMPK activator, AICAR, inhibits palmitate-induced apoptosis in osteoblast. Bone 43, 394–404 (2008).
Zhu, Y., Zhou, J., Ao, R. & Yu, B. A-769662 protects osteoblasts from hydrogen dioxide-induced apoptosis through activating of AMP-activated protein kinase (AMPK). Int. J. Mol. Sci. 15, 11190–11203 (2014).
Liu, W. et al. Targeted activation of AMPK by GSK621 ameliorates H2O2-induced damages in osteoblasts. Oncotarget 8, 10543–10552 (2017).
Wu, Y. H., Li, Q., Li, P. & Liu, B. GSK621 activates AMPK signaling to inhibit LPS-induced TNFalpha production. Biochem. Biophys. Res. Commun. 480, 289–295 (2016).
Rankin, E. B. et al. The HIF signaling pathway in osteoblasts directly modulates erythropoiesis through the production of EPO. Cell 149, 63–74 (2012).
Stegen, S. et al. HIF-1 alpha promotes glutamine-mediated redox homeostasis and glycogen-dependent bioenergetics to support postimplantation bone cell survival. Cell Metab. 23, 265–279 (2016).
Jing, X. Z. et al. Desferoxamine protects against glucocorticoid-induced osteonecrosis of the femoral head via activating HIF-1 alpha expression. J. Cell. Physiol. 235, 9864–9875 (2020).
Chen, J. & Long, F. mTOR signaling in skeletal development and disease. Bone Res. 6, 1 (2018).
Lv, C. et al. Glucosamine promotes osteoblast proliferation by modulating autophagy via the mammalian target of rapamycin pathway. Biomed. Pharmacother. 99, 271–277 (2018).
Ma, Y. H. et al. Glucosamine promotes chondrocyte proliferation via the Wnt/beta-catenin signaling pathway. Int. J. Mol. Med. 42, 61–70 (2018).
Sun, J. et al. Histone demethylase LSD1 regulates bone mass by controlling WNT7B and BMP2 signaling in osteoblasts. Bone Res. 6, 14 (2018).
Yonezawa, T. et al. Harmine promotes osteoblast differentiation through bone morphogenetic protein signaling. Biochem. Biophys. Res. Commun. 409, 260–265 (2011).
Liang, C., Sun, R. X., Xu, Y. F., Geng, W. & Li, J. Effect of the abnormal expression of BMP-4 in the blood of diabetic patients on the osteogenic differentiation potential of alveolar BMSCs and the rescue effect of metformin: a bioinformatics-based study. Biomed. Res. Int. 2020, 19 (2020).
Li, H. W. et al. γ-Aminobutyric acid promotes osteogenic differentiation of mesenchymal stem cells by inducing TNFAIP3. Curr. Gene Ther. 20, 152–161 (2020).
Wu, X., Walker, J., Zhang, J., Ding, S. & Schultz, P. G. Purmorphamine induces osteogenesis by activation of the hedgehog signaling pathway. Chem. Biol. 11, 1229–1238 (2004).
Zhou, T., Yang, Y. Q., Chen, Q. M. & Xie, L. Glutamine metabolism is essential for stemness of bone marrow mesenchymal stem cells and bone homeostasis. Stem Cells Int. 2019, 13 (2019).
Yu, Y. L. et al. Glutamine metabolism regulates proliferation and lineage allocation in skeletal stem cells. Cell Metab. 29, 966–978 (2019).
Soriano, P., Montgomery, C., Geske, R. & Bradley, A. Targeted disruption of the c-src proto-oncogene leads to osteopetrosis in mice. Cell 64, 693–702 (1991).
Marzia, M. et al. Decreased c-Src expression enhances osteoblast differentiation and bone formation. J. Cell Biol. 151, 311–320 (2000).
Peruzzi, B. et al. c-Src and IL-6 inhibit osteoblast differentiation and integrate IGFBP5 signalling. Nat. Commun. 3, 630 (2012).
Garcia-Gomez, A. et al. Dasatinib as a bone-modifying agent: anabolic and anti-resorptive effects. PLoS One 7, e34914 (2012).
Nie, P. et al. Dasatinib promotes chondrogenic differentiation of human mesenchymal stem cells via the Src/Hippo-YAP signaling pathway. ACS Biomater. Sci. Eng. 5, 5255–5265 (2019).
Wang, L. et al. Antiosteoporotic effects of tetramethylpyrazine via promoting osteogenic differentiation and inhibiting osteoclast formation. Mol. Med. Rep. 16, 8307–8314 (2017).
Liu, F. L., Chen, C. L., Lai, C. C., Lee, C. C. & Chang, D. M. Arecoline suppresses RANKL-induced osteoclast differentiation in vitro and attenuates LPS-induced bone loss in vivo. Phytomedicine 69, 11 (2020).
Jo, Y. J. et al. Cinchonine inhibits osteoclast differentiation by regulating TAK1 and AKT, and promotes osteogenesis. J. Cell Physiol. 236, 1854–1865 (2021).
Takayanagi, H. RANKL as the master regulator of osteoclast differentiation. J. Bone Min. Metab. 39, 13–18 (2021).
Matsumoto, M., Sudo, T., Saito, T., Osada, H. & Tsujimoto, M. Involvement of p38 mitogen-activated protein kinase signaling pathway in osteoclastogenesis mediated by receptor activator of NF-kappa B ligand (RANKL). J. Biol. Chem. 275, 31155–31161 (2000).
Li, H. W. et al. Sanguinarine inhibits osteoclast formation and bone resorption via suppressing RANKL-induced activation of NF-kappa B and ERK signaling pathways. Biochem. Biophys. Res. Commun. 430, 951–956 (2013).
Deepak, V., Kruger, M. C., Joubert, A. & Coetzee, M. Piperine alleviates osteoclast formation through the p38/c-Fos/NFATc1 signaling axis. Biofactors 41, 403–413 (2015).
Zhang, J. et al. Deferoxamine inhibits iron-uptake stimulated osteoclast differentiation by suppressing electron transport chain and MAPKs signaling. Toxicol. Lett. 313, 50–59 (2019).
Wei, Z. F. et al. Norisoboldine suppresses osteoclast differentiation through preventing the accumulation of TRAF6-TAK1 complexes and activation of MAPKs/NF-kappa B/c-Fos/NFATc1 pathways. PLoS One 8, 16 (2013).
Qian, Z. et al. Cytisine attenuates bone loss of ovariectomy mouse by preventing RANKL-induced osteoclastogenesis. J. Cell Mol. Med. 24, 10112–10127 (2020).
Zhi, X. et al. l-tetrahydropalmatine suppresses osteoclastogenesis in vivo and in vitro via blocking RANK-TRAF6 interactions and inhibiting NF-kappa B and MAPK pathways. J. Cell. Mol. Med. 24, 785–798 (2020).
Matsushita, T. et al. Extracellular signal-regulated kinase 1 (ERK1) and ERK2 play essential roles in osteoblast differentiation and in supporting osteoclastogenesis. Mol. Cell Biol. 29, 5843–5857 (2009).
Karin, M. Nuclear factor-kappaB in cancer development and progression. Nature 441, 431–436 (2006).
Franzoso, G. et al. Requirement for NF-kappaB in osteoclast and B-cell development. Genes Dev. 11, 3482–3496 (1997).
Dougall, W. C. et al. RANK is essential for osteoclast and lymph node development. Genes Dev. 13, 2412–2424 (1999).
Kong, Y. Y. et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature 397, 315–323 (1999).
Kong, Y. Y., Boyle, W. J. & Penninger, J. M. Osteoprotegerin ligand: a common link between osteoclastogenesis, lymph node formation and lymphocyte development. Immunol. Cell Biol. 77, 188–193 (1999).
Takayanagi, H. Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems. Nat. Rev. Immunol. 7, 292–304 (2007).
Chen, X., Zhi, X., Wang, J. & Su, J. RANKL signaling in bone marrow mesenchymal stem cells negatively regulates osteoblastic bone formation. Bone Res. 6, 34 (2018).
Yamashita, T. et al. NF-kappaB p50 and p52 regulate receptor activator of NF-kappaB ligand (RANKL) and tumor necrosis factor-induced osteoclast precursor differentiation by activating c-Fos and NFATc1. J. Biol. Chem. 282, 18245–18253 (2007).
Song, C. et al. Nuciferine prevents bone loss by disrupting multinucleated osteoclast formation and promoting type H vessel formation. FASEB J. 34, 4798–4811 (2020).
Kang, E. J. et al. Liensinine and nuciferine, bioactive components of nelumbo nucifera, inhibit the growth of breast cancer cells and breast cancer-associated bone loss. Evid. Based Complement. Altern. Med. 2017, 1583185 (2017).
Chen, S. et al. Neferine suppresses osteoclast differentiation through suppressing NF-kappaB signal pathway but not MAPKs and promote osteogenesis. J. Cell Physiol. 234, 22960–22971 (2019).
Hu, B. et al. Tomatidine suppresses osteoclastogenesis and mitigates estrogen deficiency-induced bone mass loss by modulating TRAF6-mediated signaling. Faseb J. 33, 2574–2586 (2019).
Yun, J., Lee, K. Y. & Park, B. Neotuberostemonine inhibits osteoclastogenesis via blockade of NF-kappa B pathway. Biochimie 157, 81–91 (2019).
Takahashi, T. et al. Tetrandrine prevents bone loss in sciatic-neurectomized mice and inhibits receptor activator of nuclear factor kappa B ligand-induced osteoclast differentiation. Biol. Pharm. Bull. 35, 1765–1774 (2012).
Liu, Z. et al. Tetrandrine inhibits titanium particle-induced inflammatory osteolysis through the nuclear factor-kappaB pathway. Mediat. Inflamm. 2020, 1926947 (2020).
Zhong, Z. Y. et al. Tetrandrine prevents bone loss in ovariectomized mice by inhibiting RANKL-induced osteoclastogenesis. Front. Pharmacol. 10, 14 (2020).
Wei, Z. F. et al. Norisoboldine, an anti-arthritis alkaloid isolated from radix linderae, attenuates osteoclast differentiation and inflammatory bone erosion in an aryl hydrocarbon receptor-dependent manner. Int. J. Biol. Sci. 11, 1113–1126 (2015).
Guo, J. et al. Meclizine prevents ovariectomy-induced bone loss and inhibits osteoclastogenesis partially by upregulating PXR. Front. Pharm. 8, 693 (2017).
Yamamoto, T. et al. The natural polyamines spermidine and spermine prevent bone loss through preferential disruption of osteoclastic activation in ovariectomized mice. Br. J. Pharm. 166, 1084–1096 (2012).
Zhao, H. et al. Berberine ameliorates cartilage degeneration in interleukin-1beta-stimulated rat chondrocytes and in a rat model of osteoarthritis via Akt signalling. J. Cell Mol. Med. 18, 283–292 (2014).
Moon, J. B. et al. Akt induces osteoclast differentiation through regulating the GSK3beta/NFATc1 signaling cascade. J. Immunol. 188, 163–169 (2012).
Meng, J. H. et al. Stachydrine prevents LPS-induced bone loss by inhibiting osteoclastogenesis via NF-kappa B and Akt signalling. J. Cell. Mol. Med. 23, 6730–6743 (2019).
Li, S. T. et al. SC79 rescues osteoblasts from dexamethasone though activating Akt-Nrf2 signaling. Biochem. Biophys. Res. Commun. 479, 54–60 (2016).
Zhu, R., Chen, Y. X., Ke, Q. F., Gao, Y. S. & Guo, Y. P. SC79-loaded ZSM-5/chitosan porous scaffolds with enhanced stem cell osteogenic differentiation and bone regeneration. J. Mater. Chem. B 5, 5009–5018 (2017).
Hua, P. et al. Diaporisoindole E inhibits RANKL-induced osteoclastogenesis via suppression of PI3K/AKT and MAPK signal pathways. Phytomedicine 75, 8 (2020).
Holliday, L. S. Vacuolar H(+)-ATPases (V-ATPases) as therapeutic targets: a brief review and recent developments. Biotarget 1, 1–14 (2017).
Blair, H. C., Teitelbaum, S. L., Ghiselli, R. & Gluck, S. Osteoclastic bone resorption by a polarized vacuolar proton pump. Science 245, 855–857 (1989).
Kartner, N. & Manolson, M. F. Novel techniques in the development of osteoporosis drug therapy: the osteoclast ruffled-border vacuolar H(+)-ATPase as an emerging target. Expert Opin. Drug Discov. 9, 505–522 (2014).
Visentin, L. et al. A selective inhibitor of the osteoclastic V-H(+)-ATPase prevents bone loss in both thyroparathyroidectomized and ovariectomized rats. J. Clin. Investig. 106, 309–318 (2000).
Duan, X., Yang, S., Zhang, L. & Yang, T. V-ATPases and osteoclasts: ambiguous future of V-ATPases inhibitors in osteoporosis. Theranostics 8, 5379–5399 (2018).
Kartner, N. et al. Inhibition of osteoclast bone resorption by disrupting vacuolar H+-ATPase a3-B2 subunit interaction. J. Biol. Chem. 285, 37476–37490 (2010).
Toro, E. J. et al. Enoxacin directly inhibits osteoclastogenesis without inducing apoptosis. J. Biol. Chem. 287, 17894–17904 (2012).
Yamaguchi, N. et al. Adiponectin inhibits induction of TNF-alpha/RANKL-stimulated NFATc1 via the AMPK signaling. FEBS Lett. 582, 451–456 (2008).
Uemura, T. et al. Epinephrine accelerates osteoblastic differentiation by enhancing bone morphogenetic protein signaling through a cAMP/protein kinase A signaling pathway. Bone 47, 756–765 (2010).
Claes, L., Recknagel, S. & Ignatius, A. Fracture healing under healthy and inflammatory conditions. Nat. Rev. Rheumatol. 8, 133–143 (2012).
Jimi, E. et al. Interleukin 1 induces multinucleation and bone-resorbing activity of osteoclasts in the absence of osteoblasts/stromal cells. Exp. Cell Res. 247, 84–93 (1999).
Azuma, Y., Kaji, K., Katogi, R., Takeshita, S. & Kudo, A. Tumor necrosis factor-alpha induces differentiation of and bone resorption by osteoclasts. J. Biol. Chem. 275, 4858–4864 (2000).
Liu, X. H., Kirschenbaum, A., Yao, S. & Levine, A. C. Cross-talk between the interleukin-6 and prostaglandin E(2) signaling systems results in enhancement of osteoclastogenesis through effects on the osteoprotegerin/receptor activator of nuclear factor-{kappa}B (RANK) ligand/RANK system. Endocrinology 146, 1991–1998 (2005).
Marcu, K. B., Otero, M., Olivotto, E., Borzi, R. M. & Goldring, M. B. NF-kappaB signaling: multiple angles to target OA. Curr. drug targets 11, 599–613 (2010).
Lin, T. H. et al. NF-kappaB as a therapeutic target in inflammatory-associated bone diseases. Adv. Protein Chem. Struct. Biol. 107, 117–154 (2017).
Wojdasiewicz, P., Poniatowski, L. A. & Szukiewicz, D. The role of inflammatory and anti-inflammatory cytokines in the pathogenesis of osteoarthritis. Mediat. Inflamm. 2014, 561459 (2014).
Chen, Z. et al. Spermidine activates RIP1 deubiquitination to inhibit TNF-alpha-induced NF-kappaB/p65 signaling pathway in osteoarthritis. Cell Death Dis. 11, 503 (2020).
Gao, B. et al. Local delivery of tetramethylpyrazine eliminates the senescent phenotype of bone marrow mesenchymal stromal cells and creates an anti-inflammatory and angiogenic environment in aging mice. Aging Cell 17, e12741 (2018).
Yu, T., Qu, J., Wang, Y. & Jin, H. Ligustrazine protects chondrocyte against IL-1beta induced injury by regulation of SOX9/NF-kappaB signaling pathway. J. Cell Biochem. 119, 7419–7430 (2018).
Duan, Y., An, W., Wu, Y. & Wang, J. Tetramethylpyrazine reduces inflammation levels and the apoptosis of LPSstimulated human periodontal ligament cells via the downregulation of miR302b. Int. J. Mol. Med. 45, 1918–1926 (2020).
Wong, S. K., Chin, K. Y. & Ima-Nirwana, S. Berberine and musculoskeletal disorders: the therapeutic potential and underlying molecular mechanisms. Phytomedicine 73, 152892 (2020).
Park, H. J., Zadeh, M. G., Suh, J. H. & Choi, H. S. Dauricine protects from LPS-induced bone loss via the ROS/PP2A/NF-kappa B Axis in osteoclasts. Antioxidants 9, 15 (2020).
Tajima, Y. et al. Nitensidine A, a guanidine alkaloid from Pterogyne nitens, induces osteoclastic cell death. Cytotechnology 67, 585–592 (2015).
Drug Databases. Drugs@FDA: FDA-Approved Drugs, <https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&varApplNo=207924> (2018).
Drug Databases. Drugs@FDA: FDA-Approved Drugs, <https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=203214> (2012).
Yoshikawa, R. & Abe, K. The multi-kinase inhibitor dasatinib suppresses autoinflammation and increases bone density in a mouse model for chronic recurrent multifocal osteomyelitis. Cell Biochem. Funct. 39, 521–527 (2021).
Klabunde, R. E. Dipyridamole inhibition of adenosine metabolism in human blood. Eur. J. Pharm. 93, 21–26 (1983).
Ohashi, E. et al. Adenosine N1-oxide exerts anti-inflammatory effects through the PI3K/Akt/GSK-3beta signaling pathway and promotes osteogenic and adipocyte differentiation. Biol. Pharm. Bull. 42, 968–976 (2019).
Domazetovic, V., Marcucci, G., Iantomasi, T., Brandi, M. L. & Vincenzini, M. T. Oxidative stress in bone remodeling: role of antioxidants. Clin. Cases Min. Bone Metab. 14, 209–216 (2017).
Chung, H. Y. et al. Molecular inflammation: underpinnings of aging and age-related diseases. Ageing Res. Rev. 8, 18–30 (2009).
Duarte, T. L., Talbot, N. P. & Drakesmith, H. NRF2 and hypoxia-inducible factors: key players in the redox control of systemic iron homeostasis. Antioxid. Redox Signal. 35, 433–452 (2021).
Thompson, C. B. Into Thin air: how we sense and respond to hypoxia. Cell 167, 9–11 (2016).
Ishii, T. et al. Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages. J. Biol. Chem. 275, 16023–16029 (2000).
Motohashi, H. & Yamamoto, M. Nrf2-Keap1 defines a physiologically important stress response mechanism. Trends Mol. Med. 10, 549–557 (2004).
Cho, H. Y., Reddy, S. P. & Kleeberger, S. R. Nrf2 defends the lung from oxidative stress. Antioxid. Redox Signal. 8, 76–87 (2006).
Yamamoto, M., Kensler, T. W. & Motohashi, H. The KEAP1-NRF2 system: a thiol-based sensor-effector apparatus for maintaining redox homeostasis. Physiol. Rev. 98, 1169–1203 (2018).
Hyeon, S., Lee, H., Yang, Y. & Jeong, W. Nrf2 deficiency induces oxidative stress and promotes RANKL-induced osteoclast differentiation. Free Radic. Biol. Med. 65, 789–799 (2013).
Chen, X. et al. Nrf2 epigenetic derepression induced by running exercise protects against osteoporosis. Bone Res. 9, 15 (2021).
Jia, L. L., Xiong, Y. X., Zhang, W. J., Ma, X. N. & Xu, X. Metformin promotes osteogenic differentiation and protects against oxidative stress-induced damage in periodontal ligament stem cells via activation of the Akt/Nrf2 signaling pathway. Exp. Cell Res. 386, 12 (2020).
Guo, S. et al. Activating AMP-activated protein kinase by an alpha1 selective activator compound 13 attenuates dexamethasone-induced osteoblast cell death. Biochem. Biophys. Res. Commun. 471, 545–552 (2016).
Joo, M. S. et al. AMPK facilitates nuclear accumulation of Nrf2 by phosphorylating at serine 550. Mol. Cell. Biol. 36, 1931–1942 (2016).
Zhan, Y. F. et al. Vindoline inhibits RANKL-induced osteoclastogenesis and prevents ovariectomy-induced bone loss in mice. Front. Pharmacol. 10, 11 (2020).
Wu, X. et al. Pyrroloquinoline quinone prevents testosterone deficiency-induced osteoporosis by stimulating osteoblastic bone formation and inhibiting osteoclastic bone resorption. Am. J. Transl. Res. 9, 1230–1242 (2017).
Barzel, U. S. & Jowsey, J. The effects of chronic acid and alkali administration on bone turnover in adult rats. Clin. Sci. 36, 517–524 (1969).
Arnett, T. R. & Dempster, D. W. Effect of pH on bone resorption by rat osteoclasts in vitro. Endocrinology 119, 119–124 (1986).
Brandao-Burch, A., Utting, J. C., Orriss, I. R. & Arnett, T. R. Acidosis inhibits bone formation by osteoblasts in vitro by preventing mineralization. Calcif. Tissue Int. 77, 167–174 (2005).
Blair, H. C. et al. Support of bone mineral deposition by regulation of pH. Am. J. Physiol. Cell Physiol. 315, C587–c597 (2018).
Bushinsky, D. A. Metabolic alkalosis decreases bone calcium efflux by suppressing osteoclasts and stimulating osteoblasts. Am. J. Physiol. 271, F216–F222 (1996).
Kohn, D. H., Sarmadi, M., Helman, J. I. & Krebsbach, P. H. Effects of pH on human bone marrow stromal cells in vitro: implications for tissue engineering of bone. J. Biomed. Mater. Res. 60, 292–299 (2002).
Galow, A. M. et al. Increased osteoblast viability at alkaline pH in vitro provides a new perspective on bone regeneration. Biochem. Biophys. Rep. 10, 17–25 (2017).
Han, S. H. et al. Acidic pH environments increase the expression of cathepsin B in osteoblasts: the significance of ER stress in bone physiology. Immunopharmacol. Immunotoxicol. 31, 428–431 (2009).
Spector, J. A. et al. Osteoblast expression of vascular endothelial growth factor is modulated by the extracellular microenvironment. Am. J. Physiol. Cell Physiol. 280, C72–C80 (2001).
Liu, W. et al. Akermanite used as an alkaline biodegradable implants for the treatment of osteoporotic bone defect. Bioact. Mater. 1, 151–159 (2016).
Liu, W. et al. Spatial distribution of biomaterial microenvironment ph and its modulatory effect on osteoclasts at the early stage of bone defect regeneration. ACS Appl. Mater. Interfaces 11, 9557–9572 (2019).
Ruan, C. et al. The interfacial pH of acidic degradable polymeric biomaterials and its effects on osteoblast behavior. Sci. Rep. 7, 6794 (2017).
Sabe, V. T. et al. Current trends in computer aided drug design and a highlight of drugs discovered via computational techniques: a review. Eur. J. Med. Chem. 5, 113705 (2021).
Liao, L. X. et al. Highly selective inhibition of IMPDH2 provides the basis of antineuroinflammation therapy. Proc. Natl. Acad. Sci. USA 114, E5986–E5994 (2017).
Lee, J. S., Yi, J. K., An, S. Y. & Heo, J. S. Increased osteogenic differentiation of periodontal ligament stem cells on polydopamine film occurs via activation of integrin and PI3K signaling pathways. Cell. Physiol. Biochem. 34, 1824–1834 (2014).
Hanami, K. et al. Dopamine D2-like receptor signaling suppresses human osteoclastogenesis. Bone 56, 1–8 (2013).
Lee, D. J. et al. Dopaminergic effects on in vitro osteogenesis. Bone Res. 3, 15020 (2015).
Cao, L. et al. Melatonin mediates osteoblast proliferation through the STIM1/ORAI1 pathway. Front. Pharm. 13, 851663 (2022).
Wang, X. et al. Melatonin promotes bone marrow mesenchymal stem cell osteogenic differentiation and prevents osteoporosis development through modulating circ_0003865 that sponges miR-3653-3p. Stem Cell Res. Ther. 12, 150 (2021).
Han, Y., Kim, Y. M., Kim, H. S. & Lee, K. Y. Melatonin promotes osteoblast differentiation by regulating Osterix protein stability and expression. Sci. Rep. 7, 5716 (2017).
Kim, S.-S., Jeong, S.-P., Park, B.-S. & Kim, I.-R. Melatonin attenuates RANKL-induced osteoclastogenesis via inhibition of Atp6v0d2 and DC-STAMP through MAPK and NFATc1 signaling pathways. Molecules 27, 1–13 (2022).
Xu, L. et al. Melatonin suppresses estrogen deficiency-induced osteoporosis and promotes osteoblastogenesis by inactivating the NLRP3 inflammasome. Calcif. Tissue Int. 103, 400–410 (2018).
Lee, S., Le, N. H. & Kang, D. Melatonin alleviates oxidative stress-inhibited osteogenesis of human bone marrow-derived mesenchymal stem cells through AMPK activation. Int. J. Med. Sci. 15, 1083–1091 (2018).
Du, Z. H. et al. Adenosine A2A receptor mediates inhibition of synovitis and osteoclastogenesis after electroacupuncture in rats with collagen-induced arthritis. Evid.-Based Complement. Altern. Med. 2019, 11 (2019).
Kim, B. H., Oh, J. H. & Lee, N. K. The inactivation of ERK1/2, p38 and NF-kappa B is involved in the down-regulation of osteoclastogenesis and function by A2B adenosine receptor stimulation. Mol. Cells 40, 752–760 (2017).
Borhani, S., Corciulo, C., Larranaga-Vera, A. & Cronstein, B. N. Signaling at adenosine A2A receptors (A2aR); crosstalk with Wnt/beta-catenin signaling pathway in osteoblasts. Purinergic Signal. 14, S55–S55 (2018).
Olkku, A. & Mahonen, A. Wnt and steroid pathways control glutamate signalling by regulating glutamine synthetase activity in osteoblastic cells. Bone 43, 483–493 (2008).
Yuan, L. Q. et al. Taurine transporter is expressed in osteoblasts. Amino Acids 31, 157–163 (2006).
Yuan, L. Q. et al. Taurine inhibits osteoclastogenesis through the taurine transporter. Amino Acids 39, 89–99 (2010).
Kang, I. S. & Kim, C. Taurine 11 Advances in Experimental Medicine and Biology (eds J. Hu et al.) Vol. 1155 p,p. 61–70 (Springer International Publishing Ag, 2019).
Facchini, A. et al. Role of polyamines in hypertrophy and terminal differentiation of osteoarthritic chondrocytes. Amino Acids 42, 667–678 (2012).
D’Adamo, S. et al. Spermidine rescues the deregulated autophagic response to oxidative stress of osteoarthritic chondrocytes. Free Radic. Biol. Med. 153, 159–172 (2020).
Guidotti, S. et al. Enhanced osteoblastogenesis of adipose-derived stem cells on spermine delivery via beta-catenin activation. Stem Cells Dev. 22, 1588–1601 (2013).
Mandl, P. et al. Nicotinic acetylcholine receptor ligands inhibit osteoclastogenesis by blocking Rankl-induced calcium-oscillation and induction of Nfatc1 and Cfos. Ann. Rheum. Dis. 73, A58–A59 (2014).
Yang, Q. et al. Betaine alleviates alcohol-induced osteonecrosis of the femoral head via mTOR signaling pathway regulation. Biomed. Pharmacother. 120, 109486 (2019).
Santos, C. et al. Development of hydroxyapatite nanoparticles loaded with folic acid to induce osteoblastic differentiation. Int. J. Pharm. 516, 185–195 (2017).
Akpinar, A. et al. Comparative effects of riboflavin, nicotinamide and folic acid on alveolar bone loss: a morphometric and histopathologic study in rats. Srp. Arh. Celok. Lek. 144, 273–279 (2016).
Wu, X., Zhou, X., Liang, S., Zhu, X. & Dong, Z. The mechanism of pyrroloquinoline quinone influencing the fracture healing process of estrogen-deficient mice by inhibiting oxidative stress. Biomed. Pharmacother. 139, 111598 (2021).
Hashimoto, Y. et al. Sphingosine-1-phosphate-enhanced Wnt5a promotes osteogenic differentiation in C3H10T1/2 cells. Cell Biol. Int. 40, 1129–1136 (2016).
Ran, Q. C. et al. Deferoxamine loaded titania nanotubes substrates regulate osteogenic and angiogenic differentiation of MSCs via activation of HIF-1 alpha signaling. Mat. Sci. Eng. C-Mater. 91, 44–54 (2018).
Shi, R. et al. Electrospun artificial periosteum loaded with DFO contributes to osteogenesis via the TGF-beta1/Smad2 pathway. Biomater. Sci. 9, 2090–2102 (2021).
Fan, K.-J. et al. Metformin inhibits inflammation and bone destruction in collagen-induced arthritis in rats. Ann. Transl. Med. 8, 1565 (2020).
Gao, X. L. et al. Effects oF Targeted Delivery Of Metformin And Dental Pulp Stem Cells On Osteogenesis Via Demineralized Dentin Matrix Under High Glucose Conditions. ACS Biomater. Sci. Eng. 6, 2346–2356 (2020).
Lin, J. T., Xu, R. Y., Shen, X., Jiang, H. B. & Du, S. H. Metformin promotes the osseointegration of titanium implants under osteoporotic conditions by regulating BMSCs autophagy, and osteogenic differentiation. Biochem. Biophys. Res. Commun. 531, 228–235 (2020).
Gu, Q., Gu, Y., Yang, H. & Shi, Q. Metformin Enhances Osteogenesis And Suppresses Adipogenesis Of Human Chorionic Villous Mesenchymal Stem Cells. Tohoku J. Exp. Med. 241, 13–19 (2017).
Marycz, K. et al. Metformin decreases reactive oxygen species, enhances osteogenic properties of adipose-derived multipotent mesenchymal stem cells in vitro, and increases bone density in vivo. Oxid. Med. Cell Longev. 2016, 9785890 (2016).
Bahrambeigi, S., Yousefi, B., Rahimi, M. & Shafiei-Irannejad, V. Metformin; an old antidiabetic drug with new potentials in bone disorders. Biomed. Pharmacother. 109, 1593–1601 (2019).
Liu, Z. et al. The effects of tranylcypromine on osteoclastogenesis in vitro and in vivo. FASEB J. 33, 9828–9841 (2019).
LaBranche, T. P. et al. JAK inhibition with tofacitinib suppresses arthritic joint structural damage through decreased RANKL production. Arthritis Rheum. 64, 3531–3542 (2012).
Yannaki, E. et al. The proteasome inhibitor bortezomib drastically affects inflammation and bone disease in adjuvant-induced arthritis in rats. Arthritis Rheum. 62, 3277–3288 (2010).
Pennypacker, B. L. et al. Odanacatib increases mineralized callus during fracture healing in a rabbit ulnar osteotomy model. J. Orthop. Res. 34, 72–80 (2016).
Khosla, S. Odanacatib: location and timing are everything. J. Bone Min. Res. 27, 506–508 (2012).
Hao, L. et al. A small molecule, odanacatib, inhibits inflammation and bone loss caused by endodontic disease. Infect. Immun. 83, 1235–1245 (2015).
Wang, Y., Fu, Q. & Zhao, W. Tetramethylpyrazine inhibits osteosarcoma cell proliferation via downregulation of NF-kappaB in vitro and in vivo. Mol. Med. Rep. 8, 984–988 (2013).
Jia, X. et al. Berberine ameliorates periodontal bone loss by regulating gut microbiota. J. Dent. Res. 98, 107–116 (2019).
Liu, M. & Xu, Z. Berberine promotes the proliferation and osteogenic differentiation of alveolar osteoblasts through regulating the expression of miR-214. Pharmacology 106, 70–78 (2021).
Fukuma, Y. et al. Rutaecarpine attenuates osteoclastogenesis by impairing macrophage colony stimulating factor and receptor activator of nuclear factor -B ligand-stimulated signalling pathways. Clin. Exp. Pharmacol. Physiol. 45, 863–865 (2018).
Chen, S. et al. Lycorine suppresses RANKL-induced osteoclastogenesis in vitro and prevents ovariectomy-induced osteoporosis and titanium particle-induced osteolysis in vivo. Sci. Rep. 5, 13 (2015).
Park, H. J., Gholam-Zadeh, M., Suh, J. H. & Choi, H. S. Lycorine attenuates autophagy in osteoclasts via an axis of mROS/TRPML1/TFEB to Reduce LPS-induced bone loss. Oxid. Med. Cell. Longev. 2019, 11 (2019).
He, L. G. et al. Sinomenine induces apoptosis in RAW 264.7 cell-derived osteoclasts in vitro via caspase-3 activation. Acta Pharmacol. Sin. 35, 203–210 (2014).
He, L. G. et al. Sinomenine down-regulates TLR4/TRAF6 expression and attenuates lipopolysaccharide-induced osteoclastogenesis and osteolysis. Eur. J. Pharmacol. 779, 66–79 (2016).
Zhang, Y. Y. et al. Sinomenine inhibits osteolysis in breast cancer by reducing IL-8/CXCR1 and c-Fos/NFATc1 signaling. Pharmacol. Res. 142, 140–150 (2019).
Zheng, T., Noh, A., Park, H. & Yim, M. Aminocoumarins inhibit osteoclast differentiation and bone resorption via downregulation of nuclear factor of activated T cells c1. Biochem. Pharmacol. 85, 417–425 (2013).
Clough, B. H. et al. Theobromine upregulates osteogenesis by human mesenchymal stem cells in vitro and accelerates bone development in rats. Calcif. Tissue Int. 100, 298–310 (2017).
Yuan, F. L. et al. Leonurine hydrochloride inhibits osteoclastogenesis and prevents osteoporosis associated with estrogen deficiency by inhibiting the NF-kappaB and PI3K/Akt signaling pathways. Bone 75, 128–137 (2015).
Yonezawa, T. et al. Harmine, a beta-carboline alkaloid, inhibits osteoclast differentiation and bone resorption in vitro and in vivo. Eur. J. Pharmacol. 650, 511–518 (2011).
Chen, X. et al. Matrine derivate MASM uncovers a novel function for ribosomal protein S5 in osteoclastogenesis and postmenopausal osteoporosis. Cell Death Dis. 8, e3037 (2017).
Xin, Z. et al. A matrine derivative M54 suppresses osteoclastogenesis and prevents ovariectomy-induced bone loss by targeting ribosomal protein S5. Front. Pharm. 9, 22 (2018).
Li, J. et al. Matrine enhances osteogenic differentiation of bone marrow-derived mesenchymal stem cells and promotes bone regeneration in rapid maxillary expansion. Arch. Oral. Biol. 118, 104862 (2020).
Kwak, S. C. et al. Securinine suppresses osteoclastogenesis and ameliorates inflammatory bone loss. Phytother. Res. 34, 3029–3040 (2020).
Sun, Z. et al. Magnoflorine suppresses MAPK and NF-kappaB signaling to prevent inflammatory osteolysis induced by titanium particles in vivo and osteoclastogenesis via RANKL in vitro. Front. Pharm. 11, 389 (2020).
Liu, Q. et al. Nitidine chloride prevents OVX-induced bone loss via suppressing NFATc1-mediated osteoclast differentiation. Sci. Rep. 6, 36662 (2016).
Bowers, A. et al. Total synthesis and biological mode of action of largazole: a potent class I histone deacetylase inhibitor. J. Am. Chem. Soc. 130, 11219–11222 (2008).
Lee, S. U. et al. In vitro and in vivo osteogenic activity of largazole. ACS Med. Chem. Lett. 2, 248–251 (2011).
Farina, C. & Gagliardi, S. Selective inhibition of osteoclast vacuolar H(+)-ATPase. Curr. Pharm. Des. 8, 2033–2048 (2002).
Niikura, K., Takeshita, N. & Takano, M. A vacuolar ATPase inhibitor, FR167356, prevents bone resorption in ovariectomized rats with high potency and specificity: potential for clinical application. J. Bone Min. Res. 20, 1579–1588 (2005).
Niikura, K., Takeshita, N. & Chida, N. A novel inhibitor of vacuolar ATPase, FR202126, prevents alveolar bone destruction in experimental periodontitis in rats. J. Toxicol. Sci. 30, 297–304 (2005).
Niikura, K. Comparative analysis of the effects of a novel vacuolar adenosine 5’-triphosphatase inhibitor, FR202126, and doxycycline on bone loss caused by experimental periodontitis in rats. J. Periodontol. 77, 1211–1216 (2006).
Niikura, K., Nakajima, S., Takano, M. & Yamazaki, H. FR177995, a novel vacuolar ATPase inhibitor, exerts not only an inhibitory effect on bone destruction but also anti-immunoinflammatory effects in adjuvant-induced arthritic rats. Bone 40, 888–894 (2007).
Liou, S. F. et al. KMUP-1 promotes osteoblast differentiation through cAMP and cGMP pathways and signaling of BMP-2/Smad1/5/8 and Wnt/beta-catenin. J. Cell Physiol. 230, 2038–2048 (2015).
Yuan, Y. et al. Fumitremorgin C attenuates osteoclast formation and function via suppressing RANKL-induced signaling pathways. Front. Pharm. 11, 238 (2020).
Acknowledgements
The authors gratefully acknowledge the support for this work from the National Natural Science Foundation of China [Grant Nos. 32122046, 32000959, 82030067, and 82161160342]; the National Key R&D Program [Grant No. 2018YFA0703100]; the Youth Innovation Promotion Association of CAS [Grant No. 2019350]; the Guangdong Natural Science Foundation [Grant No. 2020A1515111190] and the Shenzhen Fundamental Research Foundation [Grant Nos. JCYJ20190812162809131, JCYJ20200109114006014, JCYJ20210324113001005, JCYJ20210324115814040, and JSGGKQTD20210831174330015]; and the Shenzhen Fund for Guangdong Provincial High-level Clinical Key Specialties [Grant No. SZGSP001].
Author information
Authors and Affiliations
Contributions
Conceptualization, preparation, and revision of the manuscript, C.R., D.C.; methodology, Q.Z., J.Y., N.H.; preparation of the manuscript, Q.Z., J. Y, N.H., J.L., H.Y., and H.P. All authors have read and agreed to the published version of the manuscript.
Corresponding authors
Ethics declarations
Competing interests
The authors declare no competing interests.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Zhang, Q., Yang, J., Hu, N. et al. Small-molecule amines: a big role in the regulation of bone homeostasis. Bone Res 11, 40 (2023). https://doi.org/10.1038/s41413-023-00262-z
Received:
Revised:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41413-023-00262-z