P125(FAK) 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 ty rosine kinase, which is involved in integrin signaling and in signal t ransduction of a number of growth factors. In contrast to tyrosine kin ase receptors such as the platelet-derived growth factor and the hepat ocyte growth factor receptors, which induce p125(Fak) phosphorylation, insulin has been shown to pro mote its dephosphorylation. In this stu dy, we compared p125(Fak) phosphorylation in insulin-stimulated cells maintained in suspension or in an adhesion state. We found that, in no nattached cells, insulin promotes p125(Fak) phosphorylation, whereas d ephosphorylation occurred in attached cells, This was observed in Rat- 1 fibroblasts overexpressing the insulin receptor, as well as in Hep G 2 hepatocytes and in 3T3-L1 adipocytes expressing more natural levels of insulin receptors. Insulin-induced p125(Fak) phosphorylation correl ated with an increase in paxillin phosphorylation, indicating that p12 5(Fak) kinase activity may be stimulated by insulin. Mixing of purifie d insulin or insulin-like growth factor-I (IGF-I) receptors with p125( Fak) resulted in an increase in p125(Fak) phosphorylation. Using a kin ase-deficient p125(Fak) mutant, we found that this protein is a direct substrate of the insulin and IGF-I receptor tyrosine kinases. This vi ew is supported by two additional findings. (i) A peptide correspondin g to p125(Fak) sequence comprising amino acids 568-582, which contains tyrosines 576 and 577 of the kinase domain regulatory loop, is phosph orylated by the insulin receptor; and (ii) p125(Fak) phosphorylation b y the insulin receptor is prevented by addition of this peptide, Final ly, we observed that p125(Fak) phosphorylation by the receptor results in its activation. Our results show that the nature of the crosstalk between the insulin/IGF-I receptors and p125(Fak) is dependent on the cell architecture, and hence the interaction of the insulin/IGF-I sign aling system with the integrin system will vary accordingly.