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Submitted on September 23, 2005
Accepted on November 10, 2005
Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, England, and MRC-RFCGR, Hinxton, Cambridge, CB10 1SB, England
* To whom correspondence should be addressed. E-mail: cjw53{at}mole.bio.cam.ac.uk.
Prolactin (Prl) and leukemia inhibitory factor (LIF) have different roles in the adult mammary gland which are mediated in part by the signal transducers and activators of transcription, (STAT)5 and STAT3. In vivo studies have shown that STAT5 contributes to Prl-dependent lobuloalveolar development and lactation, while STAT3 mediates LIF-dependent epithelial apoptosis during post-lactational involution. To understand the molecular basis of these STAT-dependent pathways, we demonstrate the ligand-independent effects of STAT5 and STAT3 in mammary epithelial cells in vitro and also identify the genes regulated by these related transcription factors. Thus, using conditionally active STAT3- or STAT5a-GyraseB fusion proteins, we observed that enforced and specific dimerization of STAT3 induced apoptosis while STAT5 induced differentiation of mammary epithelial cells. Furthermore, STAT5 attenuated apoptosis mediated by LIF, the physiological inducer of STAT3. Microarray analysis of STAT3- and STAT5-induced genes using this system demonstrated a marked specificity, which reflected their different physiological effects in vitro and in vivo. STAT5-specific gene targets included the milk protein genes 
-casein and kallikrein-8 and the survival factors prosaposin and Grb10. STAT3-specific genes included the apoptosis regulators c/ebpdelta, PI3K-regulatory subunits, purine nucleoside phosphorylase and c-fos. These data illustrate that specific activation of STAT3 and STAT5 alone is sufficient to induce and suppress apoptosis respectively and that these transcription factors elicit their actions by inducing distinct subsets of target genes in mammary epithelial cells.
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