DIVERGENT INSULIN AND PLATELET-DERIVED GROWTH-FACTOR REGULATION OF FOCAL ADHESION KINASE (PP125(FAK)) TYROSINE PHOSPHORYLATION, AND REARRANGEMENT OF ACTIN STRESS FIBERS

Insulin treatment of Chinese hamster ovary cells expressing high levels of the human insulin receptor resulted in the tyrosine dephosphorylation of the 125-kDa focal adhesion kinase (pp125(FAK)). The decrease in pp125(FAK) tyrosine phosphorylation paralleled a decrease in the cellular content of actin stress fibers, and these changes were independent of the extracellular matrix on which the cells were grown. The reduction in both pp125(FAK) tyrosine phosphorylation and actin stress fibers occurred in an insulin concentration-dependent manner, with significant effects at approximately 0.3 nM and a maximal effect at 3 nM. However, in the continuous presence of insulin, the decreases in the tyrosine phosphorylation state of pp125(FAK) and actin stress fiber content were transient, Maximal reduction of pp125(FAK) tyrosine phosphorylation was observed following 15 min of insulin treatment, with a return to unstimulated control levels by 60 min. Similarly, actin stress fiber content was maximally reduced by 15 min of insulin treatment and fully recovered by 60 min. In contrast to insulin, platelet-derived growth factor stimulation increased actin stress fiber content and enhanced pp125(FAK) tyrosine phosphorylation, These data demonstrate a novel signaling role for insulin in inducing the tyrosine dephosphorylation of pp125(FAK) and a concomitant reorganization of actin stress fibers, which underlies at least one aspect of signaling divergence between the insulin and platelet-derived growth factor receptor tyrosine kinases.