Altered Endothelin-1 Signaling in Production of Thromboxane A₂ in Kupffer Cells from Bile Duct Ligated Rats

Abstract

Kupffer cells (KCs), the liver resident macrophages accounting for 80–90% of the total population of fixed tissue macrophages in the body, not only play a key role in host defense via removing particulate materials from the portal circulation, but may also contribute to the pathogenesis of various liver diseases. We have previously demonstrated that KCs play an important role in controlling portal hypertension and hepatocellular injury via releasing thromboxane A₂ (TXA₂) in early fibrosis induced by one-week bile duct ligation (BDL). Production of TXA₂ is controlled by cytosolic phospholipase A₂ (cPLA₂) that is activated by the interaction of entothelin-1 (ET-1) with its G-protein coupled ET receptor B (ETBR). However, the signaling pathways that contribute to the ET-1-induced activation of cPLA₂ and production of TXA₂ in KCs in the normal healthy or injured livers are not yet clear, which are investigated in the present study using isolated KCs from one-week BDL or sham rats. The pharmacological inhibition of cPLA₂ or chelation of intracellular calcium abrogated the ET-1 induction of TXA₂ from KCs. Compared to those from sham rats, KCs from BDL animals displayed a significantly enhanced responsiveness of p38 MAPK to ET-1, increased ETBR and Gαi subunit but decreased Gαq and Gα11 expression. Inhibition of ERK1/2 or Gq signaling abrogated significantly the ET-1 induction of TXA₂ in sham KCs but only slightly in BDL KCs. In contrast, inhibition of p38 MAPK and Gi signaling markedly attenuated the ET-1 induction of TXA₂ in BDL KCs but had no effect in sham KCs. Lastly, inhibition of PLC or PKC abrogated ET-1 induction of TXA₂ in KCs from both sham and BDL groups. The hepatic stress (such as BDL) induces significant modifications in the receptor and intermediates of ET-1 signaling in KC and subsequently alters ET-1 signaling mechanisms, particularly a shift from Gq induced signaling to Gi induced signaling, in the activation of cPLA₂ and production of TXA₂ in response to ET-1.