A novel domain mediates insulin-induced proteasomal degradation of Insulin Receptor Substrate 1 (IRS-1)

Abstract

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signaling, therefore its degradation is exquisitely regulated. Here, we show that insulin-stimulated degradation of IRS-1 requires the presence of a highly conserved Ser/Thr-rich domain that we named domain involved in degradation of IRS-1 (DIDI). DIDI (amino acids 386-430 of IRS-1) was identified by comparing the intracellular degradation rate of several truncated forms of IRS-1 transfected into CHO cells. The isolated DIDI domain underwent insulin-stimulated Ser/Thr phosphorylation, suggesting that it serves as a target for IRS-1 kinases. The effects of deletion of DIDI were studied in Fao rat hepatoma and in CHO cells expressing Myc-IRS-1WT or Myc-IRS-1Δ386-430. Deletion of DIDI maintained the ability of IRS-1Δ386-434 to undergo ubiquitination while rendering it insensitive to insulin-induced proteasomal degradation, which affected IRS-1WT (80% at 8 h). Consequently, IRS-1Δ386-434 mediated insulin signaling (activation of Akt and glycogen synthesis) better than IRS-1WT. IRS-1Δ386-434 exhibited a significant greater preference for nuclear localization, compared with IRS-1WT. Higher nuclear localization was also observed when cells expressing IRS-1 WT were incubated with the proteasome inhibitor MG-132. The sequence of DIDI is conserved more than 93% across species, from fish to mammals, as opposed to approximately 40% homology of the entire IRS-1. These findings implicate DIDI as a novel, highly conserved domain of IRS-1, which mediates its cellular localization, rate of degradation, and biological activity, with a direct impact on insulin signal transduction. Copyright © 2010 by The Endocrine Society.