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

Abstract

Expression of insulin receptor substrates (IRS)-1 and -2 within the mammary gland was found to be high at midlactation 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 K/K versus Irs2 K/K dams. Maternal body weight was also reduced in Irs1 K/K dams as well as in Irs1 Irs2C/K 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.

The insulin receptor plays an important role in secretory differentiation in the mammary gland

“…In rodents, withdrawal of progesterone leads to both parturition and activation of copious milk secretion. Lactation itself depends on the increase in PRL and its receptor that occurs at secretory activation (23) as well as corticosterone and insulin (4,16). Although strong evidence for a role for the IGF-IR in both proliferation and differentiation during pregnancy has been presented (36,45), our studies provide the first clear evidence that the IR plays a more important role in late pregnancy.…”

“…We found that selective deletion of IR using Cre-lox technology in the mouse mammary epithelium reduces the rate of growth of pups suckled by IR fl/fl Cre ϩ dams by 74%. Insulin and IR are known to support milk production during lactation (4,16), so our studies cannot definitively attribute the entire reduction in growth rate to impaired mammary gland development. However, the 50% decrease in alveolar number and the 60% decrease in staining for casein and lipid droplets at midpregnancy would suggest that the IR mediates both proliferative and differentiation phases.…”

“…The evidence is quite clear that insulin signaling through IR is mediated by the IRS2 ¡ AKT pathway during lactation (16). There is only one piece of evidence from cells isolated during pregnancy.…”

“…Hadsell and colleagues (16) showed that infusion of insulin restored AKT phosphorylation following starvation of a lactating mouse for 8 h, but an active form of IGF-I did not, implicating insulin, not IGF-I, in recovery from starvation. AKT phosphorylation was decreased in insulin receptor substrate-deficient (IRS1 Ϫ/Ϫ ) mice, providing evidence that insulin acts through its normal signaling pathway in the lactating gland to maintain milk synthesis.…”

Gene expression in the human mammary epithelium during lactation: the milk fat globule transcriptome

“…The ERK/MAPK signaling pathway is thought not to be activated by insulin, IGF-1, or PRL. These hormones are known to influence mammary differentiation and synthesis of milk components primarily via PI3-AKT and STAT signaling (9,21). That the ERK/MAPK pathway is significantly enriched in the MFG gene list is very interesting and may support findings by Finlay et al (6) of its importance in MEC survival in the presence of insulin and cell-extracellular matrix (ECM) association.…”