Gas6 and the Receptor Tyrosine Kinase Axl in Clear Cell Renal Cell Carcinoma

BACKGROUND: The molecular biology of renal cell carcinoma (RCC) is complex and not fully understood. We have recently found that the expression of the receptor tyrosine kinase Axl in the RCC tumors independently correlates with survival of the patients. PRINCIPAL FINDINGS: Here, we have investigated the role of Axl and its ligand Gas6, the vitamin-K dependent protein product of the growth arrest-specific gene 6, in clear cell RCC (ccRCC) derived cells. The Axl protein was highly expressed in ccRCC cells deficient in functional von Hippel-Lindau (VHL) protein, a tumor suppressor gene often inactivated in ccRCC. VHL reconstituted cells expressed decreased levels of Axl protein, but not Axl mRNA, suggesting VHL to regulate Axl expression. Gas6-mediated activation of Axl in ccRCC cells resulted in Axl phosphorylation, receptor down-regulation, decreased cell-viability and migratory capacity. No effects of the Gas6/Axl system could be detected on invasion. Moreover, in ccRCC tumor tissues, Axl was phosphorylated and Gas6 γ-carboxylated, suggesting these molecules to be active in vivo. SIGNIFICANCE: These results provide novel information regarding the complex function of the Gas6/Axl system in ccRCC.