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Insulin receptor substrates (IRSs) are signaling adaptors that play a major role in the metabolic and mitogenic actions of insulin and insulin-like growth factors. Reports have recently noted increased levels, or activity, of IRSs in many human cancers, and some have linked this to poor patient prognosis. We found that overexpressed IRS-1 was constitutively phosphorylated in vitro and in vivo and that transgenic mice overexpressing IRS-1 or IRS-2 in the mammary gland showed progressive mammary hyperplasia, tumorigenesis, and metastasis. Tumors showed extensive squamous differentiation, a phenotype commonly seen with activation of the canonical beta-catenin signaling pathway. Consistent with this, IRSs were found to bind beta-catenin in vitro and in vivo. IRS-induced tumorigenesis is unique, given that the IRSs are signaling adaptors with no intrinsic kinase activity, and this supports a growing literature indicating a role for IRSs in cancer. This study defines IRSs as oncogene proteins in vivo and provides new models to develop inhibitors against IRSs for anticancer therapy.

The precise role of circulating IGF-I in somatic growth under normal and GH-deficient conditions remains unclear. To define the contribution of circulating IGF-I to the endocrine regulation of somatic growth and the GH/IGF-I axis, we constructed a transgene with the transthyretin (TTR) enhancer/promoter and the mouse IGF-I cDNA and generated TTR-IGF-I transgenic mice. The transgene was exclusively expressed in the liver, which resulted in a 50-60% increase in serum IGF-I, a decrease in serum GH, and an improved tolerance to glucose challenge. The body weight and lean mass of TTR-IGF-I mice were heavier compared with wild-type (WT) mice. The increase in lean mass was a result of increase in both number and thickness of skeletal muscle fibers. The femur, tibia, and body lengths of TTR-IGF-I mice also were longer. In WT mice, the GH antagonist pegvisomant (Peg) suppressed the transcription of endogenous IGF-I and acid-labile subunit (ALS) genes with no effect on IGF-binding protein 3 (IGFBP-3) mRNA. Consequently, Peg-induced GH deficiency in WT mice severely reduced ALS, IGF-I, and IGFBP-3 in the circulation and caused a severe growth deficit. In TTR-IGF-I mice, Peg reduced the mRNA of the endogenous IGF-I gene with no effect on the TTR-IGF-I transgene expression, leading to a blunted decrease in serum IGF-I levels. Interestingly, IGFBP-3 mRNA was elevated and circulating IGFBP-3 was less reduced in Peg-treated TTR-IGF-I mice. Peg-treated TTR-IGF-I mice also exhibited heavier body weight and longer body length than Peg-treated WT mice. Therefore, liver-expressed IGF-I can stimulate IGFBP-3 mRNA expression and stabilize IGFBP-3 under GH deficiency, leading to a better maintenance of IGF-I levels in the circulation. Higher circulating levels of IGF-I can stimulate somatic growth and lean mass and improve glucose tolerance.