Role of prolactin hormone and Jak2 kinase in mammary epithelial cell polarity

Extensive research indicates that the pregnancy/lactation hormone prolactin (PRL) and its downstream pathway Jak2/Stat5a are required for lobuloalveolar growth/morphogenesis of the mammary gland and terminal differentiation of mammary epithelial cells. The role of PRL in breast cancer development and progression is yet to be fully elucidated. While some studies have indicated that PRL may enhance breast cancer cell viability, these effects may not fully describe the role of PRL in breast carcinogenesis. Importantly, recent evidence including data generated from our laboratory indicates that PRL may suppress certain aspects of breast carcinogenesis. Indeed, our laboratory has previously shown that PRL is an effective suppressor of epithelial-mesenchymal transformation process and the invasive potential of breast cancer cells. Therefore, the aim of my project was to characterize the role of PRL and its signaling mechanisms leading to the establishment and maintenance of mammary epithelial architecture.;In mammals, epithelial cell polarity is defined by tight junctions which separate apical from basolateral membrane domain. These processes are regulated by a conserved protein complex designated as the Par complex consisting of par6-PKCzeta-par3. Using the mouse mammary epithelial cells HC11 we showed that in contrast to EGF, PRL in combination with hydrocortisone and insulin induces formation of mammospheres on Matrigel with complete lumen and apical/basal polarity as determined by apical localization of ZO1 and basal/lateral localization of E-cadherin. Furthermore, our results indicate that Jak2 kinase plays an important role in determining cell polarity. Indeed, specific Jak2 knockdown in HC11 cells led to complete loss of lumen formation and apical/basal polarity of the mammospheres. Moreover, Jak2 knockdown cells showed not only disruption of ZO1 localization but also that of Par3 in 2D and 3D cultures. In addition, our results showed that PRL promoted tyrosine phosphorylation of the Par3 and Par complex formation. Our data together indicate that PRL regulates cell polarity through tyrosine phosphorylation of Par3 via Jak2, thereby promoting the Par complex formation and localization to the membrane. Our results showed that PRL has a critical role in regulating epithelial polarity. Since the establishment and maintenance of cell polarity is required for the development of all metazoans, and disruption of cell polarity is a hallmark of malignant cancer our results also implicate PRL as an invasion suppressor in breast cancer cells.