INSULIN STIMULATES THE TYROSINE DEPHOSPHORYLATION OF PP125 FOCAL ADHESION KINASE

The phosphorylation state of pp125 focal adhesion kinase in response to insulin was examined in parental and transfected Rat-1 fibroblasts expressing both wildtype (HIRc cells) and mutant human insulin receptor cDNAs lacking the C-terminal twin tyrosine phosphorylation sites (YFB cells) or a deletion mutant lacking the distal 43 amino acids of the beta-subunit (Delta CT cells). In HIRc cells insulin stimulated the tyrosine dephosphorylation of pp125(fak), whereas IGF-I did not, In contrast, the tyrosine phosphorylation state of pp125(fak) was unchanged in the parental Rat-1 fibroblasts and the YF2 or Delta CT mutant cell lines in response to insulin. Analysis of the supernatants revealed that pp125(fak) was only one component of the major M(r) 120-130-kDa phosphotyrosine band seen in HIRc cells. We conclude that: 1) in contrast to other growth factors, insulin stimulates the dephosphorylation of pp125(fak); 2) the presence of the insulin receptor C-terminal tyrosines 1328 and 1334 is required for the insulin-stimulated tyrosine dephosphorylation of pp125(fak), suggesting a possible SH2 domain-dependent interaction; 3) insulin may modulate integrin-mediated signaling through pp125(fak) by altering the phosphorylation state of pp125(fak).