A walk on the wild side — wild-type kinases promote metastasis

A functional screen of 125 wild-type kinases that have been implicated in prostate cancer progression has identified five that promote visceral and bone metastases in this disease, according to new data published in Proceedings of the National Academy of Sciences.

Mutationally activated kinases are known to have an important role in progression and metastasis in other cancers, but mutations of kinases in prostate cancer are uncommon. However, much evidence indicates nonmutated kinases are involved in prostate cancer progression.

Faltermeier and colleagues used three data sources to identify kinases with evidence of enhanced expression and/or activity in human metastatic prostate cancer and selected 125 kinases for further investigation. Using a gain-of-function screening design, the researchers tested whether increased expression of any of their chosen kinases was enough to drive metastasis in vivo.

Kinases were individually and stably overexpressed in Cap8 cells, which have minimal metastatic capacity in vivo but metastasize when overexpressing a mutationally activated kinase, SRC^Y529F. Five kinases per mouse were tested, to remove bias for inducers of rapid metastasis, and equal numbers of five different Cap8-kinase-expressing cells were pooled and injected into the tail vein of immunocompromised CB17 mice. Of the 125 tested kinases, 20 promoted lung metastases in vivo. Rapid metastasis, occurring within 2 weeks of injection, was caused by NTRK2 and MAP3K8, whereas MAP3K15, MERTK and ARAF, BRAF and CRAF caused lung metastasis within 3 weeks.

To discover which of the 20 kinases drive, rather than enhance, metastasis in human cells, they were overexpressed in the nonmalignant RWPE-1 normal-prostate cell line. Each kinase cell line was individually injected into the tail vein of NOD scid γ mice. Mice injected with cells overexpressing MERTK, ARAF, BRAF, CRAF or NTRK2 developed hind leg weakness, rather than showing signs of lung metastasis. Those mice that received cells overexpressing CRAF, MERTK or NTRK2 showed symptoms first, at 1–2 months after injection. Mice injected with ARAF-expressing or BRAF-expressing cells took up to 6 months to develop symptoms. ¹⁸F-Fluorodeoxyglucose-PET–CT imaging showed that mice injected with cells expressing these five kinases had high radiolabel accumulation in bones, lungs and lymph nodes. Histological examination revealed that RAF family members and NTRK2 drove lung and lymph node metastasis, but not MERTK.

Immunohistochemistry of metastatic and localized prostate cancer and benign tissue showed that four of the five kinases are more highly expressed in prostate cancer metastases than in localized disease, whereas no difference in expression was observed for CRAF.

Together, these data show that overexpression of wild-type kinases can drive prostate cancer metastasis and could be important therapeutic targets in this disease.