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¹ Department of Pediatrics,
² Department of Molecular and Cellular Biology,
³ Department of Medicine, The Breast Center, Baylor College of Medicine, USDA/ARS Children’s Nutrition Research Center, 1100 Bates Street, Houston, Texas 77030, USA
⁴ Department of Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8655, Japan

(Requests for offprints should be addressed to D L Hadsell; Email: dhadsell{at}mbcr.bcm.tmc.edu)

Expression of insulin receptor substrates (IRS)-1 and -2 within the mammary gland was found to be high at mid-lactation and dramatically decreased with mammary involution. This observation supports the hypothesis that these proteins are induced in the mammary gland with lactogenesis and involved in normal milk synthesis. To test this hypothesis, lactation capacity, along with indices of mammary secretory cell glucose metabolism and cell signaling were compared in normal mice and mice carrying targeted mutations in either the Irs1 or Irs2 genes. Mammary IRS-1 and IRS-2 protein levels were increased within 1 day of parturition and reached maximal levels by 5 days post partum. Dams carrying germline mutations of Irs1 or Irs2 displayed reduced lactation capacity as assessed by weight gain of pup litters. The reduction was more dramatic in Irs1^–/– versus Irs2^–/– dams. Maternal body weight was also reduced in Irs1^–/– dams as well as in Irs1^+/– Irs2^+/– dams. The loss of IRS-1 had little impact on mammary gland expression of milk protein mRNAs, glucose transport, or on the abundance and subcellular localization of hexokinases I and II. The loss of IRS-1 was associated with a compensatory increase in insulin-induced IRS-2 phosphorylation; however, the loss of IRS-1 did also cause a reduction in insulin-dependent mammary gland-specific activation of Akt phosphorylation. These results support the conclusion that IRS-1 is important for insulin-dependent activation of Akt signaling within the lactating mammary gland, but that loss of this protein has only modest impact on normal milk synthesis, since related signaling proteins such as IRS-2 may act in compensatory fashion.

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