p125Fak focal adhesion kinase is a substrate for the insulin and insulin-like growth factor-I tyrosine kinase receptors.

The focal adhesion kinase p125(Fak) is a widely expressed cytosolic tyrosine kinase, which is involved in integrin signaling and in signal transduction of a number of growth factors. In contrast to tyrosine kinase receptors such as the platelet-derived growth factor and the hepatocyte growth factor receptors, which induce p125(Fak) phosphorylation, insulin has been shown to promote its dephosphorylation. In this study, we compared p125(Fak) phosphorylation in insulin-stimulated cells maintained in suspension or in an adhesion state. We found that, in nonattached cells, insulin promotes p125(Fak) phosphorylation, whereas dephosphorylation occurred in attached cells. This was observed in Rat-1 fibroblasts overexpressing the insulin receptor, as well as in Hep G2 hepatocytes and in 3T3-L1 adipocytes expressing more natural levels of insulin receptors. Insulin-induced p125(Fak) phosphorylation correlated with an increase in paxillin phosphorylation, indicating that p125(Fak) kinase activity may be stimulated by insulin. Mixing of purified insulin or insulin-like growth factor-I (IGF-I) receptors with p125(Fak) resulted in an increase in p125(Fak) phosphorylation. Using a kinase-deficient p125(Fak) mutant, we found that this protein is a direct substrate of the insulin and IGF-I receptor tyrosine kinases. This view is supported by two additional findings. (i) A peptide corresponding to p125(Fak) sequence comprising amino acids 568-582, which contains tyrosines 576 and 577 of the kinase domain regulatory loop, is phosphorylated by the insulin receptor; and (ii) p125(Fak) phosphorylation by the insulin receptor is prevented by addition of this peptide. Finally, we observed that p125(Fak) phosphorylation by the receptor results in its activation. Our results show that the nature of the cross-talk between the insulin/IGF-I receptors and p125(Fak) is dependent on the cell architecture, and hence the interaction of the insulin/IGF-I signaling system with the integrin system will vary accordingly.