Serine/Threonine Phosphorylation of IRS-1 Triggers Its Degradation

Possible Regulation by Tyrosine Phosphorylation

  1. Terry M. Pederson,
  2. Deborah L. Kramer and
  3. Cristina M. Rondinone
  1. From the Diabetes Research, Pharmaceutical Products Division, Abbott Laboratories, Abbott Park, Illinois.
  1. Address correspondence and reprint request to Cristina M. Rondinone, PhD, Diabetes Research, Pharmaceutical Products Division, Abbott Laboratories, Dept 47H, AP9A, Abbott Park, IL 60064-3500. E-mail: cristina.rondinone{at}abbott.com .

Abstract

Insulin receptor substrate (IRS)-1 protein expression is markedly reduced in many insulin-resistant states, although the mechanism for this downregulation is unclear. In this study, we have investigated the early events in the insulin pathway that trigger the degradation of IRS-1. Incubation of the adipocytes with insulin induced a fast electrophoretic mobility shift of IRS-1 and a subsequent degradation of the protein. Wortmannin and rapamycin blocked this mobility shift of IRS-1, maintained the insulin-induced tyrosine phosphorylation of IRS-1, and blocked its degradation. In contrast, a glycogen synthase kinase 3 inhibitor, a mitogen-activated protein kinase/extracellular-regulated kinase inhibitor, and various protein kinase C inhibitors had no effect. Incubation with okadaic acid increased the serine/threonine phosphorylation of IRS-1 and its degradation, mimicking insulin, and its effect was prevented by the proteasome inhibitor lactacystin, as well as by rapamycin. Treatment of the cells with the tyrosine phosphatase inhibitor orthovanadate in the presence of insulin or okadaic acid partially inhibited the degradation of IRS-1. We propose that a rapamycin-dependent pathway participates as a negative regulator of IRS-1, increasing its serine/threonine phosphorylation, which triggers degradation. Thus, regulation of serine/threonine versus tyrosine phosphorylation may modulate IRS-1 degradation, affecting insulin sensitivity.

Footnotes

  • T.M.P. and D.L.K. contributed equally to this work.

  • DMEM, Dulbecco's modified Eagle's medium; ERK, extracellular signal-regulated kinase; FBS, fetal bovine serum; GSK3, glycogen synthase kinase 3; IRS, insulin receptor substrate; MAPK, mitogen-activated protein kinase; MEK, MAPK/ERK; mTOR, target of rapamycin; PDGF, platelet-derived growth factor; PI, phosphatidylinositol; PKB, protein kinase B; PKC, protein kinase C; SH2, Src homology 2.

    • Accepted October 2, 2000.
    • Received December 20, 1999.
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