Mammary gland involution as a target for pregnancy-associated breast cancer prevention: Insights from NSAIDs and macrophages

Pregnancy-associated breast cancer (PABC) is diagnosed within 5-10 years postpartum, and has poor prognosis. Involution, the post-partum remodeling of the mammary gland, engages physiologic programs characteristic of wound-healing and inflammation known to be tumor promotional. Our involution-hypothesis predicts cancerous lesions exposed to the involuting microenvironment are likely to metastasize. Further, treatment targeting the inflammatory attributes of involution may decrease progression of PABC. Previous work demonstrates mammary extracellular matrix (ECM) from involuting glands promotes metastasis in a xenograft model, while nulliparous-ECM does not. To address whether systemic non-steroidal anti-inflammatory drugs (NSAIDs) administered during post-partum involution decrease the tumor promotional characteristics of mammary involution-ECM, ECM from NSAID treated rats was used as substrata for breast epithelial cells in 3D-culture assays. The organization of non-transformed and tumorigenic mammary cells in the 'NSAID-ECM' phenocopied the response of cells cultured in tumor suppressive nulliparous-ECM. Importantly, NSAID use during involution did not interrupt mammary epithelial cell regression critical to gland remodeling. In a PABC mouse model, the involution-microenvironment promoted tumor size, tumor cell COX-2 expression, and tumor cell escape to the lung, all of which were suppressed by ibuprofen treatment during involution. These data confirm the tumor promotional microenvironment of mammary involution can be targeted with NSAIDs. One potential specific target in the involuting microenvironment is the macrophage, an immune cell that promotes mammary cancer metastasis in mice and is associated with negative outcomes for breast cancer patients. IHC and cytokine analyses revealed an increase in tumor promotional M2-macrophages during mammary involution in rodents. Involution-specific increases in monocyte chemotactic protein and fibrillar collagen proteolysis were identified as potential mediators of macrophage recruitment. Conditional depletion of macrophages (i.e. cells expressing CSF-1R) in Mafia mice resulted in delayed involution characterized by decreases in mammary epithelial cell apoptosis, alveolar regression, and adipocyte repopulation. These data suggest macrophages are necessary for involution. Investigation into the effect of directing involution M2 macrophages to the tumoricidal M1-type remains intriguing for a prevention strategy. Altogether this work reveals plausible targets for prevention of PABC progression in the mammary involution microenvironment, with implications for improving outcomes for thousands of young women with breast cancer.