Nuclear translocation of insulin receptor substrate-1 by oncogenes and Igf-I - Effect on ribosomal RNA synthesis

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-1 receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-1 or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.