Coordinated phosphorylation of insulin receptor substrate-1 by glycogen synthase kinase-3 and protein kinase C betaII in the diabetic fat tissue

Serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) is an important negative modulator of insulin signaling. Previously, we showed that glycogen synthase kinase-3 (GSK-3) phosphorylates IRS-1 at Ser(332). However, the fact that GSK-3 requires prephosphorylation of its substrates suggested that Ser(336) on IRS-1 was the "priming" site phosphorylated by an as yet unknown protein kinase. Here, we sought to identify this "priming kinase" and to examine the phosphorylation of IRS-1 at Ser(336) and Ser(332) in physiologically relevant animal models. Of several stimulators, only the PKC activator phorbol ester PMA enhanced IRS-1 phosphorylation at Ser(336). Treatment with selective PKC inhibitors prevented this PMA effect and suggested that a conventional PKC was the priming kinase. Overexpression of PKCalpha or PKCbetaII isoforms in cells enhanced IRS-1 phosphorylation at Ser(336) and Ser(332), and in vitro kinase assays verified that these two kinases directly phosphorylated IRS-1 at Ser(336). The expression level and activation state of PKCbetaII, but not PKCalpha, were remarkably elevated in the fat tissues of diabetic ob/ob mice and in high-fat diet-fed mice compared with that from lean animals. Elevated levels of PKCbetaII were also associated with enhanced phosphorylation of IRS-1 at Ser(336/332) and elevated activity of GSK-3beta. Finally, adenoviral mediated expression of PKCbetaII in adipocytes enhancedphosphorylation of IRS-1 at Ser(336). Taken together, our results suggest that IRS-1 is sequentially phosphorylated by PKCbetaII and GSK-3 at Ser(336) and Ser(332). Furthermore, these data provide evidence for the physiological relevance of these phosphorylation events in the pathogenesis of insulin resistance in fat tissue.