Stat5 Promotes Survival of Mammary Epithelial Cells through Transcriptional Activation of a Distinct Promoter in Akt1

Abstract

The signal transducer and activator of transcription 5 (Stat5) plays a pivotal role in the proliferation, secretory differentiation, and survival of mammary epithelial cells. However, there is little information about Stat5 target genes that facilitate these biological processes. We provide here experimental evidence that the prolactin-mediated phosphorylation of Stat5 regulates the transcriptional activation of the Akt1 gene. Stat5 binds to consensus sequences within the Akt1 locus in a growth factor-dependent manner to initiate transcription of a unique Akt1 mRNA from a distinct promoter, which is only active in the mammary gland. Elevating the levels of active Akt1 restores the expression of cyclin D1 and proliferation of Jak2-deficient mammary epithelial cells, which provides evidence that Akt1 acts downstream of Jak/Stat signaling. The ligand-inducible expression of Stat5 in transgenic females mediates a sustained upregulation of Akt1 in mammary epithelial cells during the onset of postlactational involution. Stat5-expressing mammary glands exhibit a delay in involution despite induction of proapoptotic signaling events. Collectively, the results of the present study elucidate an underlying mechanism by which active Stat5 mediates evasion from apoptosis and self-sufficiency in growth signals.

This study was supported by the Public Health Service grant CA117930 from the National Cancer Institute. Additional financial support provided to K.-U.W. by the Nebraska Cancer and Smoking Disease Research Program (NE DHHS LB506 2009-45) was imperative to finance the maintenance of the mouse models. R.M. was supported by the Austrian Science Fund (FWF) grant SFB F28. B.A.C. and J.W.S. received graduate fellowships through the UNMC Cancer Research Training Program (CA009476) and a Program of Excellence Graduate Assistantship through the UNMC Graduate Studies Office. K.S. received a postdoctoral fellowship from the Susan G. Komen Breast Cancer Foundation (PDF0600835).

The sorting of viable cells was carried out at the UNMC Cell Analysis Facility (Charles A. Kuszynski), and we thank the members of the UNMC Mouse Genome Engineering Core Facility, Don Harms and Judith Stribley, for the pronuclear injection of the TetO-Stat5 construct. We also thank Anita Jennings from the UNMC Molecular Phenotyping Core Facility for her help labeling apoptotic cells in mammary tissue sections. The myr-Akt1 cDNA was kindly provided by Jorge Martin-Perez (Instituto de Investigaciones Biomedicas A. Sols, Madrid, Spain), and the caPRLR was gift from Isabelle Gourdou (INRA, Jouy-en-Josas Cedex, France).