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This Article Full Text Full Text (PDF) All Versions of this Article: 294/6/E1169    most recent 00050.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Liberman, Z. Articles by Eldar-Finkelman, H. PubMed PubMed Citation Articles by Liberman, Z. Articles by Eldar-Finkelman, H.

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

¹Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv; and ²Faculty of Life Science, Bar Ilan University, Ramat-Gan, Israel

Submitted 24 January 2008 ; accepted in final form 21 April 2008

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 Ser³³². However, the fact that GSK-3 requires prephosphorylation of its substrates suggested that Ser³³⁶ 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 Ser³³⁶ and Ser³³² in physiologically relevant animal models. Of several stimulators, only the PKC activator phorbol ester PMA enhanced IRS-1 phosphorylation at Ser³³⁶. Treatment with selective PKC inhibitors prevented this PMA effect and suggested that a conventional PKC was the priming kinase. Overexpression of PKC or PKCβII isoforms in cells enhanced IRS-1 phosphorylation at Ser³³⁶ and Ser³³², and in vitro kinase assays verified that these two kinases directly phosphorylated IRS-1 at Ser³³⁶. The expression level and activation state of PKCβII, but not PKC, 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 PKCβII were also associated with enhanced phosphorylation of IRS-1 at Ser^336/332 and elevated activity of GSK-3β. Finally, adenoviral mediated expression of PKCβII in adipocytes enhancedphosphorylation of IRS-1 at Ser³³⁶. Taken together, our results suggest that IRS-1 is sequentially phosphorylated by PKCβII and GSK-3 at Ser³³⁶ and Ser³³². Furthermore, these data provide evidence for the physiological relevance of these phosphorylation events in the pathogenesis of insulin resistance in fat tissue.

insulin signaling; insulin resistance; obesity