Semaphorin 7a dependent programs of cell survival, matrix remodeling, and EMT drive breast (postpartum?) tumor progression

ABSTRACT Breast cancer (BC) remains the second leading cause of cancer related deaths in women in the US -- mainly due to metastatic disease. Thus, understanding how BC progresses is of the utmost importance to developing new strategies to block metastasis. Young women diagnosed with BC generally have poor prognosis due to increased rates of metastasis. Additionally, women who are within 5 years of most recent child-birth at diagnosis are ∼3 times more likely to develop metastasis than age and stage matched nulliparous women. We define these cases as postpartum breast cancers (PPBC) and propose that the unique biology of the postpartum mammary gland drives tumor progression and metastasis. Semaphorin 7a (SEMA7A) is a unique member of the Semaphorin family as it is the only GPI-anchored member that can be shed into the extracellular environment. Our published results show that SEMA7A expression is associated with decreased overall survival in BC patient cohorts and revealed roles for SEMA7A in breast tumor cell growth, motility, invasion, and tumor associated-lymphangiogenesis all of which are also increased in pre-clinical models of PPBC. However, whether primary SEMA7A driver PPBC progression remains largely unexplored. We utilized a pre-clinical of PPBC with breast cancer cells where we silenced or overexpressed SEMA7A. Our results show that silencing of SEMA7A decreases tumor growth in postpartum hosts while overexpression increases growth in nulliparous hosts to rates closer to those observed in postpartum hosts. Further, we show that SEMA7A effects multiple pro-metastatic tumor attributes such as cell survival, tumor associated COX-2 expression, fibronectin deposition, and fibroblast-mediated collagen deposition in the tumor microenvironment regardless of whether the host is nulliparous or postpartum. Finally, we show that co-expression of SEMA7A/Collagen/COX2/FN mRNA predicts for increased metastasis in breast and ovarian cancer cohorts. These studies suggest SEMA7A as a key mediator of general BC progression and that targeting SEMA7A alone or in combination may open avenues for novel therapeutic strategies to prevent BC progression to metastasis.