Insulin-like growth factor I-induced degradation of insulin receptor substrate 1 is mediated by the 26S proteasome and blocked by phosphatidylinositol 3 '-kinase inhibition

Insulin receptor substrate 1 (IRS-1) is a critical adapter protein involved in both insulin and insulin-like growth factor (IGF) signaling. Due to the fact that alteration of IRS-1 levels can affect the sensitivity and respon se to both insulin and IGF-I, we examined the ability of each of these liga nds to affect IRS-1 expression. IGF-I (10 nM) stimulation of MCF-7 breast c ancer cells caused a transient tyrosine phosphorylation of IRS-1 that was m aximal at 15 min and decreased thereafter, The decrease in tyrosine phospho rylation of IRS 1 was paralleled by an apparent decrease in IRS 1 levels. T he IGF mediated decrease in IRS-1 expression was posttranscriptional and du e to a decrease in the half-life of the IRS-1 protein. Insulin (10 nM) caus ed tyrosine phosphorylation of IRS-1 but not degradation, whereas high conc entrations of insulin (10 mu M) resulted in degradation of IRS-1, IGF-I (10 nM) stimulation resulted in transient IRS-1 phosphorylation and extracellu lar signal-related kinase (ERK) activation. In contrast, insulin (10 nM) ca used sustained IRS-1 phosphorylation and ERK activation. Inhibition of 268 proteasome activity by the use of lactacystin or MG132 completely blocked I GF-mediated degradation of IRS-1, Furthermore, coimmunoprecipitation experi ments showed an association between ubiquitin and IRS-1 that was increased by treatment of cells with IGF-I, Finally, IGF-mediated degradation of IRS- 1 was blocked by inhibition of phosphatidylinositol 3'-kinase activity but was not affected by inhibition of ERK, suggesting that this may represent a direct negative-feedback mechanism resulting from downstream IRS-1 signali ng. We conclude that TGF-I can cause ligand-mediated degradation of IRS-1 v ia the ubiquitin-mediated 268 proteasome and a phosphatidylinositol 3'-kina se-dependent mechanism and that control of degradation may have profound ef fects on downstream activation of signaling pathways.