Insulin receptor substrate-1 is present in hepatocyte nuclei from intact rats

Insulin receptor substrate-1 (IRS-1), the primary substrate for the insulin receptor tyrosine kinase in most cells and tissues, is a key component of the insulin signaling network. Numerous studies have documented the trafficking of IRS-1 from the cell membrane to intracellular, extranuclear compartments. During the course of our previous studies aimed at defining the ontogeny of insulin signaling in the rat, Western immunoblotting showed minimal IRS-1 content in late gestation fetal liver. Immunohistochemical analyses, aimed at corroborating these Western immunoblotting results, showed hepatocyte nuclear staining for IRS-1 in adult liver but not fetal liver. Further analysis of fixed tissue as well as immunofluorescent staining of liver cryosections confirmed the localization of IRS-1 to the nuclear matrix and nucleoli of adult hepatocytes within intact liver. Tissue fractionation and Western immunoblotting also showed nuclear localization of IRS-1, with this fraction accounting for approximately 25% of total liver IRS-1. Administration of insulin to intact animals did not stimulate nuclear translocation of hepatic IRS-1 or the p85 regulatory subunit of phosphatidylinositol-3 kinase. However, insulin did activate IRS-1-associated phosphatidylinositol-3 kinase in nuclear extracts. Our results indicate that insulin signaling, which terminates in an array of nuclear events, may originate at the immediate postreceptor level with IRS-1 activation within the nucleus of normal hepatocytes.