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Retinoic acid mediates degradation of IRS-1 by the ubiquitin-proteasome pathway, via a PKC-dependant mechanism.


Insulin receptor substrate-1 (IRS-1) mediates signaling from the insulin-like growth factor type-I receptor. We found that all-trans retinoic acid (RA) decreases IRS-1 protein levels in MCF-7, T47-D, and ZR75.1 breast cancer cells, which are growth arrested by RA, but not in the RA-resistant MDA-MB-231 and MDA-MB-468 cells. Based on prior reports of ubiquitin-mediated degradation of IRS-1, we investigated the ubiquitination of IRS-1 in RA-treated breast cancer cells. Two proteasome inhibitors, MG-132 and lactacystin, blocked the RA-mediated degradation of IRS-1, and RA increased ubiquitination of IRS-1 in the RA-sensitive breast cancer cells. In addition, we found that RA increases serine phosphorylation of IRS-1. To elucidate the signaling pathway responsible for this phosphorylation event, pharmacologic inhibitors were used. Two PKC inhibitors, but not a MAPK inhibitor, blocked the RA-induced degradation and serine phosphorylation of IRS-1. We demonstrate that RA activates PKC-delta in the sensitive, but not in the resistant cells, with a time course that is consistent with the RA-induced decrease of IRS-1. We also show that: (1) RA-activated PKC-delta phosphorylates IRS-1 in vitro, (2) PKC-delta and IRS-1 interact in RA-treated cells, and (3) mutation of three PKC-delta serine sites in IRS-1 to alanines results in no RA-induced in vitro phosphorylation of IRS-1. Together, these results indicate that RA regulates IRS-1 levels by the ubiquitin-proteasome pathway, involving a PKC-sensitive mechanism.

Pubmed ID: 15516986


  • del Rinc√≥n SV
  • Guo Q
  • Morelli C
  • Shiu HY
  • Surmacz E
  • Miller WH



Publication Data

December 9, 2004

Associated Grants


Mesh Terms

  • Base Sequence
  • Blotting, Northern
  • Blotting, Western
  • Cell Line, Tumor
  • DNA Primers
  • Humans
  • Hydrolysis
  • Immunoprecipitation
  • Insulin Receptor Substrate Proteins
  • Phosphoproteins
  • Phosphorylation
  • Protease Inhibitors
  • Proteasome Endopeptidase Complex
  • Protein Kinase C
  • Protein Processing, Post-Translational
  • Tretinoin
  • Ubiquitin