P4-03-01: Stat5 Regulates the PI3-Kinase/Akt1 Pathway during Mammary Gland Development and Promotes Neoplastic Transformation in a Mouse Model for Cowden Syndrome.
Authors: Schmidt JW , Sakamoto K , Creamer BA , Leone G , Wagner K-U 
Published: June 2012

Abstract
Background: The Jak/Stat pathway responds to extracellular signals (i.e. cytokines) to promote physiological responses at the transcriptional level. In the developing mammary gland, Jak2/Stat5 signaling is critical for transducing prolactin signals by promoting the proliferation, differentiation, and survival of alveolar epithelial cells during pregnancy and lactation. Additionally, hyper-activation of Jak2/Stat5 activity has also been shown to cause mammary tumors in a number of transgenic models and we have previously shown it plays a crucial role in the early neoplastic events of prolactin and ErbB2-induced tumorigenesis. However, the mechanisms and target genes by which Stat5 promotes cell survival and neoplastic transformation remain to be elucidated and are the focus of this current study. Materials and Methods: To assess the functions of Stat5 during mammary development, gain-of-function Stat5 was expressed in a spatial and temporal manner within the mammary epithelial cells of double transgenic females (Wap-rtTA TetO-Stat5S710F model) during lactation and involution in a doxycycline (dox)-inducible manner. Mammary tissues were collected for histological examination along with mRNA, protein, and chromatin immunprecipitation (ChIP) analyses. HC11 mouse mammary epithelial cells were infected to overexpress wildtype or gain-of-function Stat5, a constitutively active prolactin receptor, or combinations of both. Finally, the functional cooperation between Stat5 and PI3-kinase/Akt signaling in promoting mammary cancer was assessed by expressing hyperactive Stat5 specifically within the mammary epithelium of females harboring a knock-in PtenG129E mutation found in patients with Cowden Syndrome (PtenG129E MMTV-tTA TetO-Stat5S710F model).
Results: Dox-induced expression of active Stat5 during involution overrides pro-apoptotic signaling and sustains p-Akt1 levels along with expression of the PI3-kinase and Akt1, which are normally downregulated at the transcriptional and protein levels during involution. ChIP analysis revealed that Stat5 binds two consensus sites in the Akt1 promoter during lactation whereas this is lost by the second day of involution. Furthermore, Stat5 promotes the transcriptional activation of Akt1 through a novel mammary-specific promoter and this suggests a mechanism for Stat5 in cell survival both in mammary development and breast cancer. Preliminary results also suggest that forced expression of active Stat5 in the mammary glands of PtenG129E females accelerates the development of mammary intraepithelial neoplasia (MIN) lesions predisposed by the loss of PI3K/Akt inhibition.
Discussion: In this study we demonstrate that Stat5 regulates PI3-kinase/Akt1 signaling during mammary gland development and this provides a mechanism for enhancing cell survival during lactation and for allowing apoptosis through loss of this regulation during involution. These findings further suggest that Jak2/Stat5 signaling may play an important role in the development of breast cancers induced through aberrant PI3-kinase/Akt signaling (i.e. Cowden Syndrome) and that targeting Jak2/Stat5 activity might be a suitable strategy for prophylactic intervention.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-03-01.