Developmental and Hormonal Signals Dramatically Alter the Localization and Abundance of Insulin Receptor Substrate Proteins in the Mammary Gland

Abstract

Insulin receptor substrates (IRS) are central integrators of hormone, cytokine, and growth factor signaling. IRS proteins can be phosphorylated by a number of signaling pathways critical to normal mammary gland development. Studies in transgenic mice that overexpress IGF-I in the mammary gland suggested that IRS expression is important in the regulation of normal postlactational mammary involution. The goal of these studies was to examine IRS expression in the mouse mammary gland and determine the importance of IRS-1 to mammary development in the virgin mouse. IRS-1 and -2 show distinct patterns of protein expression in the virgin mouse mammary gland, and protein abundance is dramatically increased during pregnancy and lactation, but rapidly lost during involution. Consistent with hormone regulation, IRS-1 protein levels are reduced by ovariectomy, induced by combined treatment with estrogen and progesterone, and vary considerably throughout the estrous cycle. These changes occur without similar changes in mRNA levels, suggesting posttranscriptional control. Mammary glands from IRS-1 null mice have smaller fat pads than wild-type controls, but this reduction is proportional to the overall reduction in body size. Development of the mammary duct (terminal endbuds and branch points) is not altered by the loss of IRS-1, and pregnancy-induced proliferation is not changed. These data indicate that IRS undergo complex developmental and hormonal regulation in the mammary gland, and that IRS-1 is more likely to regulate mammary function in lactating mice than in virgin or pregnant mice.

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

“…Taken together, the results support our hypothesis that IRS-1 and IRS-2 are important for normal lactation, but they also suggest that compensatory mechanisms within the secretory epithelium may prevent the appearance of a more dramatic phenotype in animals with a single deletion of either IRS-1 or IRS-2. The induction of IRS protein expression during the first few days of lactation is consistent with previous studies published in our laboratory which demonstrated higher levels of IRS protein expression during mid-lactation compared with late pregnancy (Lee et al 2003b). The immunoblot results in the current study extend that observation by demonstrating that this upregulation of IRS-1 and IRS-2 is an early event that is temporally linked with the process of secretory activation.…”

“…Our previous work showed that IRS-1 and IRS-2 levels were developmentally regulated in the mammary gland (Lee et al 2003b). Interestingly, the expression profile of IRSs was suggested to be similar to that of milk proteins such as b-casein and whey acidic protein (Rosen et al 1999), with increased expression during late pregnancy, dramatically elevated levels during lactation, and rapid loss of protein expression during involution.…”

“…Previous studies in our laboratory demonstrated that expression of IRS-1 and IRS-2 within the mammary gland is developmentally regulated and that both proteins are highly expressed during lactation (Lee et al 2003b). The goal of the present study was to determine the importance of IRS-1 and IRS-2 to normal lactation and understand the potential mechanisms through which these two proteins might mediate the processes necessary to milk synthesis.…”

Divergent Roles for IRS-1 and IRS-2 in Breast Cancer Metastasis

“…30,32,[42][43][44][45][46][47] Of the IRS family members, only IRS-1 and IRS-2 are expressed in normal mammary epithelial cells and in breast carcinoma cells. 32,[48][49][50] A limited number of studies have investigated IRS-1 expression in human tumors by immunohistochemistry and the results have been inconsistent. One study reported strong down-regulation of IRS-1 in Grade 3, poorly differentiated breast tumors, suggesting a positive correlation between low IRS-1 and poor prognosis.…”

An association between a common variant (G972R) in the IRS-1 gene and sex hormone levels in post-menopausal breast cancer survivors

“…A role for IRS-1 in the cross-talk between the estrogen, insulin, and IGF-1 pathways has been shown in experimental studies. In mice, IRS-1 plays a role in mammary gland development and this function is regulated by steroid hormones, especially the combination of estrogen and progesterone [65]. In vitro studies [11,66,67] have found that estrogen, especially E 2 , can stimulate and increase the expression of IRS-1 protein levels in breast tumor cells resulting in enhanced insulin or IGF-1 signaling.…”