Submitted to: Journal of Endocrinology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/2/2005
Publication Date: 6/1/2007
Publication URL: www.endocrinology-journals.org
Citation: Hadsell, D.L., Olea, W., Lawrence, N., George, J., Torres, D., Kadowaki, T., Lee, A.V. 2007. Decreased lactation capacity and altered milk composition in insulin receptor substrate null mice is associated with decreased maternal body mass and reduced insulin-dependent phosphorylation of mammary Akt. Journal of Endocrinology. 194:327-336. Interpretive Summary: Insulin is important to the normal metabolic process required for lactation. This study tested the importance of two proteins that mediate insulin signaling to lactation. The importance of the proteins insulin receptor substrate 1 and 2 to lactation was studied in mouse models that carry defective genes for these two proteins. These mice were found to have a decreased capacity to produce milk in comparison to normal mice. In addition the study found that the loss of insulin receptor 1 decreased the activation of a specific signaling protein in the mammary gland Akt. This result suggested that loss of these signaling proteins reduces the capacity of insulin to support the metabolic processes within mammary cells that are necessary to normal lactation. These results have the potential to be applied to breastfeeding mothers that have difficulty in establishing a normal lactation.
Technical Abstract: 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.