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J Biol Chem. 2004 Jun 11;279(24):25157-63. Epub 2004 Apr 5.

Protein kinase C-zeta-induced phosphorylation of Ser318 in insulin receptor substrate-1 (IRS-1) attenuates the interaction with the insulin receptor and the tyrosine phosphorylation of IRS-1.

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  • 1Department of Internal Medicine IV, Endocrinology, Metabolism, Pathobiochemistry and Clinical Chemistry, University Hospital Tübingen, D-72076 Tübingen, Germany.

Abstract

Insulin receptor substrate-1 (IRS-1) was recently identified as a novel upstream substrate for the insulin-activated protein kinase C (PKC)-zeta. This interaction down-regulates insulin signal transduction under hyper-insulinemic conditions. To clarify the molecular mechanism of this feedback loop, we sought to identify the PKC-zeta phosphorylation sites of IRS-1 and to investigate their biological significance. Upon incubation of recombinant IRS-1 fragments with PKC-zeta, we identified Ser(318) of rat IRS-1 (Ser(323) in human IRS-1) as the major in vitro phosphorylation site (confirmed by mutation of Ser(318) to alanine). To monitor phosphorylation of Ser(318) in cellular extracts, we prepared a polyclonal phosphosite-specific antibody. The biological significance was studied in baby hamster kidney cells stably expressing the insulin receptor (BHK(IR)). Using the phospho-Ser(318)-specific antibody we observed that insulin stimulates phosphorylation of Ser(318) in IRS-1, which is mediated, at least partially, by PKC-zeta. Moreover, we found that the previously described insulin-stimulated, PKC-zeta-mediated inhibition of the interaction of IRS-1 with the insulin receptor and the reduced tyrosine phosphorylation of IRS-1 was abrogated by mutation of IRS-1 Ser(318) to alanine. These results, generated in BHK(IR) cells, suggest that phosphorylation of Ser(318) by PKC-zeta might contribute to the inhibitory effect of prolonged hyperinsulinemia on IRS-1 function.

PMID:
15069075
[PubMed - indexed for MEDLINE]
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