Estrogen exacerbates mammary involution through neutrophil dependent and independent mechanism

There is strong evidence that the pro-inflammatory microenvironment during post-partum mammary involution promotes parity-associated breast cancer. Estrogen exposure during mammary involution drives tumour growth through the activity of neutrophils. However, how estrogen and neutrophils influence mammary involution are unknown. Combined analysis of transcriptomic, protein, and immunohistochemical data in Balb/c mice with and without neutrophil depletion showed that estrogen promotes involution by exacerbating inflammation, cell death and adipocytes repopulation through neutrophil-dependent and neutrophil-independent mechanisms. Remarkably, 88% of estrogen-regulated genes in mammary tissue were mediated through neutrophils, which were recruited through estrogen-induced CXCL2-CXCR2 signalling. While neutrophils mediate estrogen-induced inflammation and adipocytes repopulation, estrogen-induced mammary cell death was mediated by neutrophils-independent upsurges of cathepsins and their lysosomal leakages that are critical for lysosome-mediated cell death. Notably, these multifaceted effects of estrogen are unique to the phase of mammary involution. These findings are important for the development of intervention strategies for parity-associated breast cancer.