The focal adhesion kinase FAK, but not PYK2, plays a central role for growth factor signaling in HepG2-hepatoma cells

Abstract

BKG: The structurally related focal adhesion kinases FAK and PYK2 are involved in key signal transduction pathways in normal and malignant tissues. In spite of their structural homology, they have distinct roles in key cellular events such as cell adhesion, regulation of the cell cycle and apoptosis. In contrast to PYK2, FAK promotes cell-survival and proliferation. Hepatocellular carcinoma is one of the most common cancers. There is rising evidence for FAK to be implicated in hepatic malignancies. AIM: To study the expression of focal adhesion kinases, their activation by growth factors (GF) and their involvement in intracellular pathways. METH: HepG2-cells were cultivated in DMEM standard medium. FAK and PYK2 were detected by immunoprecipitation (IP), their activation assessed by western blotting with phosphotyrosine or phosphospecific antibodies. The involvement with other proteins was assessed by coimmunoprecipitation. RES: FAK, but not PYK2 was expressed in HepG2-cells as shown by WB. The stimulation with the GF IGF (1µM) and HGF (1µM) stimulated the tyrosine phosphorylation (TyrP) of FAK, as well as its autophosphorylation site pY397. Stimulation with HGF (1µM) and IGF (1µM) caused rapid tyrosine phosphorylation, reaching a maximum after 0,5 minutes followed by a rapid decrease. Both, HGF and IGF induced an association between FAK and the Src family kinase pp60Src. The addition of the specific Src-inhibitor PP2, but not of the inactive analogue PP3, inhibited FAK's tyrosine phosphorylation. CONCL: Similar to the expression in other malignant tissues and cell lines, we found FAK expression in HepG2 hepatoma cells. FAK gets activated upon growth factor stimulation as shown by its tyrosine phosphorylation and moreover of its autophosphorylation site. This TyrP is dependent on Src. The association with Src upon stimulation shows FAK's involvement in key signal transduction pathways. These data suggest that FAK mediates growth factor signaling in hepatoma cells, implying a central role in hepatic carcinogenesis for FAK.