The integrin, alpha 6 beta 1, is necessary for the matrix-dependent activation of FAK and MAP kinase and the migration of human hepatocarcinoma cells

Expression of the integrin, (alpha6 beta1, a receptor for laminins, is asso ciated with the progression of hepatocellular carcinoma (HCC). The approach to investigating the alpha6 beta1 integrin signaling in HCC cells was to e xpress a deletion mutant of the beta4 integrin cytoplasmic domain (beta4-De lta cyt) in 2 HCC cell lines, HepG2 and Huh7. Expression of this mutant pre vents formation of the alpha6 beta1 heterodimer. As expected, adhesion of b oth the HepG2/beta4-Delta cyt and Huh7/beta4-Delta cyt transfectants to lam inin, but not to collagen, was reduced compared with the mock transfectants . However, migration of the beta4-Delta cyt transfectants toward both colla gen and laminin was inhibited, suggesting a role for alpha6 beta1 in the si gnaling of migration. Migration of HCC cells requires mitogen-activated pro tein (MAP) kinase. The adhesion of the beta4-Delta cyt transfectants to col lagen resulted in a substantial reduction in MAP kinase activation in compa rison with the mock transfectants, although their ability to activate MAP k inase in response to epidermal growth factor (EGF) stimulation was not impa ired. In addition, matrix adhesion of the beta4-Delta cyt transfectants did not stimulate the tyrosine phosphorylation of focal adhesion kinase (FAK), and this defect correlated with reduced binding of adaptor protein Grb2 to FAK. These results suggest that FAK tyrosine phosphorylation is dependent on alpha6 beta1 expression, and that FAK-Grb2 association plays a central r ole in alpha6 beta1-mediated activation of MAP kinase. Moreover, the expres sion of (alpha6 beta1 in HCC cells is necessary for FAK/MAP kinase-dependen t migration.