Expression of a carboxy terminally truncated Stat5 with no transactivation domain in the mammary glands of transgenic mice inhibits cell proliferation during pregnancy, delays onset of milk secretion, and induces apoptosis upon involution

Abstract

Signal transducer and activator of transcription (Stat5) is a transcription factor, which transduces extracellular cytokine and growth-factor signals to the nuclei of mammalian cells. As a major mediator of prolactin action, it is involved in the regulation of the development, function, and survival of mammary epithelial cells. The carboxyl terminal of Stat5 encodes a transactivation domain (TAD), which interacts with coactivators and is crucial for the transcriptional induction of Stat5 target genes. To study the role of the Stat5 TAD in mediating Stat5 functions, a carboxy terminally truncated Stat5 variant (Stat5Delta750) was directed for expression in the mammary glands of transgenic mice by regulatory sequences of the beta-lactoglobulin (BLG) gene. Expression of Stat5Delta750 in mammary tissue reduced the rates of cell proliferation at mid and late pregnancy. Subsequently, morphological signs of milk secretion upon parturition were delayed. In double-transgenic mice, expression of Stat5Delta750 drastically decreased BLG/luciferase activity during lactation, but did not affect the expression and secretion of the endogenous beta-casein or alpha-lactalbumin into the milk. Expression of Stat5Delta750 also caused an increase in the number of apoptotic cells during mammary involution by a factor of 3 relative to control glands. Our data established a role for the Stat5 TAD in mediating the effects of Stat5 on mammary development, regulation of milk protein gene activity, and cell survival. The full effects of Stat5Delta750 may be partially buffered by the expression of endogenous wild-type Stat5 and the formation of truncated and wild-type heterodimers.

An Epigenetic Memory of Pregnancy in the Mouse Mammary Gland

“…Deletion of both genes early in pregnancy allows normal alveolar development but hampers milk production (Cui et al, 2004). Furthermore, inhibition of Stat5a/b function late in pregnancy accelerates involution (Iavnilovitch et al, 2006). These developmental phenotypes may be a consequence of the deregulation of Stat5a/b downstream targets, where it acts either by transcriptional activation or repression.…”

Stat5 in the mammary gland: Controlling normal development and cancer

“…In these mice, mammary involution was delayed, and 3 days after pup removal, mammary architecture was hardly affected. Correspondingly, apoptosis was accelerated in the mammary glands of transgenic mice expressing the C-terminally truncated Stat5 (Iavnilovitch et al, 2006).…”

“…These constructs were directed for expression in the mammary gland by the b-lactoglobulin regulatory sequences. Overexpression of both the constitutively active and native Stat5-induced mammary proliferation during mid and late pregnancy, whereas the truncated Stat5 acted in a dominant-negative fashion (Iavnilovitch et al, 2002(Iavnilovitch et al, , 2006. The increase in cell proliferation during pregnancy in mice expressing the constitutively active form was accompanied by abnormal development of ''shrunken'' alveoli during late pregnancy and early lactation, resembling that found in mice expressing the dominant-negative variants of the erb2 gene (Jones et al, 1999), or to a lesser extent, in Stat5a À/À mice .…”

“…Loss of Stat5a locus repressed WAP expression, but b-casein was hardly affected Cui et al, 2004). Correspondingly, no dominant-negative properties could be related to expression of the C-terminally truncated ovine Stat5 variant in the mammary glands (Iavnilovitch et al, 2006).It was proposed that maximum b-casein gene expression can be accomplished with very little Stat5b, in the absence of Stat5a. In contrast, high expression of the WAP gene requires high levels of Stat5a .…”

Survival Factors from Activated Accessory Cells and their Role in Triggering Autoimmune Diseases

“…1). Recently, data have revealed a critical role of STAT5 in cell survival [100][101][102]. The anti-apoptotic effect of IL-15 is evident in its promoting intracellular expression of antiapoptotic molecules upon binding to its receptor, including the cell-survival related signaling molecules Akt and PI3K [103] and Bcl-2 [61,65,98].…”