Expression and role of Gas6 protein and of its receptor Axl in hepatic regeneration from oval cells in the rat

Background& Aims: The growth arrest-specific gene 6 (Gas6) protein is a vitamin K-dependent protein that binds to the Axl subfamily of tyrosine kinase receptors and exerts antiapoptotic and proliferative effects. Because Gas6 plays a role in development and tissue remodelling, we studied its expression as well as that of its high-affinity receptor Axl in a well-characterized model of hepatic regeneration from precursor oval cells. Methods : Hepatic regeneration was induced by treating rats with acetylaminofluorene followed by partial hepatectomy. Results: Oval cell accumulation, which predominated in periportal regions, reached a maximum at days 9 and 14 after hepatectomy and declined thereafter. Oval cells expressed Gas6 protein and messenger RNA (mRNA). Axl mRNA hepatic levels paralleled the number of oval cells, and immunohistochemistry showed Axl expression in these cells. WB-F344 cells, a hepatocytic precursor cell line, also expressed Gas6 and Axl. Addition of Gas6 significantly increased the number of WB-F344 cells cultured with or without serum. Gas6 did not increase cell entry in the S phase of the cell cycle but inhibited 15-d-prostaglandin J(2)-induced WB-F344 cell apoptosis. Conclusions: Our data demonstrate an expression of Gas6 and of its receptor Axl by oval cells during hepatic regeneration. Because the Gas6/Axl couple protects from apoptosis a hepatocytic precursor cell line, these results strongly suggest that the Gas6/Axl couple favors oval cell accumulation in regenerating liver by an autocrine/paracrine mechanism.