SEMA7A in postpartum mammary gland development and cellular transformation

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Background: We seek to further our understanding of normal mammary gland biology and determine how pregnancy associated changes in the mammary gland may contribute to development of breast cancers. Though pregnancy is well-known to provide a protective effect against breast cancer risk, all women who give birth experience a transient increase in breast cancer risk following each pregnancy. The magnitude and length of this elevated risk is largely determined by the woman's age at first birth and for women over 30 at the time of first pregnancy the protective effect likely never occurs. Postpartum breast cancers (PPBC), defined as breast cancers diagnosed within 5-10 years of last childbirth, are more than twice as likely to become metastatic and result in death. This devastating diagnosis affects ~12,000 women annually. In 2006, for the first time, the number of women having children in their 30s was greater than the number under 25. Thus, PPBC cases are rising. It is thought that modern childbearing practices contribute to the increased risk for developing PPBC. We have shown that tumor cells and normal adjacent mammary epithelial cells (MECs) in PPBC patients exhibit high levels of SEMA7A expression, and SEMA7A expressing tumors are more metastatic. Thus, we believe that understanding the mechanisms underlying SEMA7A signaling in the mammary gland will lead to novel insights into aggressive PPBC. Our recent preliminary data reveal SEMA7A+ live MECs during postpartum involution suggesting that SEMA7A may promotes cell survival during postpartum glandular regression. Consistent with our hypothesis, SEMA7A promotes cell survival in cultured MECs and our preliminary data suggest a reduction in MECs, as well as lymphatic endothelial cells (LECs) and immune cell populations, in SEMA7A-/- (KO) mice. Additional data from KO mice reveal a decrease in programmed death ligand-1 (PD-L1+), a molecule that we have shown to be important for immunosuppression during involution, on mammary macrophages, LECs and MECs. As the epithelial cell apoptosis that occurs during involution likely results in presentation of self-antigens, an immune-tolerant tissue microenvironment may be necessary to prevent activation of the immune system by self-antigens and to simultaneously support survival of remaining cells in the mammary and stromal compartments. Our overarching hypothesis is that SEMA7A expression promotes cell survival and a transiently tolerant microenvironment in the mammary gland; furthermore, aberrant sustained upregulation of SEMA7A may pre-dispose women to breast malignancy. Aim1 will define the role of SEMA7A in epithelial cell survival during mammary gland involution. Aim2 will decipher SEMA7A mediated mechanisms of immune tolerance in the mammary microenvironment during postpartum involution. Relevance: To establish clinical relevance, we will also perform multi-color immunostaining on normal mammary tissues from recently lactating women.