Insulin Receptor Substrate 1 is Required for Insulin-Mediated Mitogenic Signal Transduction

Insulin treatment of mammalian cells immediately stimulates the tyrosine phosphorylation of a cellular protein of 185 kDa referred to as pp185 or IRS-1 (insulin receptor substrate 1). The potential role of the IRS-1 protein in insulin signaling has been examined by microinjecting affinity-purified antibodies into living cells. Stably transfected Rat-1 fibroblasts, which overexpress the human insulin receptor, were microinjected and subsequently stimulated with insulin or other growth factors. Progression through the cell cycle was monitored by using a single-cell assay, which employs bromodeoxyuridine labeling of DNA and analysis with immunofluorescence microscopy. Microinjection of anti-IRS-1 antibody completely inhibited incorporation of bromodeoxyuridine into the nuclei of cells stimulated with insulin or insulin-like growth factor I but did not affect cells stimulated with serum or a variety of purified growth factors. These studies indicate that IRS-1 is a critical component of the insulin and insulin-like growth factor I signaling pathways, which lead to DNA synthesis and cell growth.