Estrogen regulation of mammary involution and its implication in Parity Associated Breast Cancer

Abstract

With the onset of pregnancy, the mammary gland undergoes a cycle of mammary alveolar development, lactation, followed by post-partum mammary involution to return back to pre-pregnancy state. Epidemiological studies indicated that each pregnancy increases the risk of breast cancer within 10 years of child birth, known as Parity-Associated Breast Cancer (PABC). Previous study has suggested that the tissue microenvironment during mammary involution promotes mammary tumorigenesis which is further accelerated by estrogen. Despite the extensive studies of estrogen effect on postnatal mammary development, limited knowledge was available in its regulation of post-partum mammary involution. In this study, I seek to characterize the effect of estrogen in post-partum mammary involution and evaluate the potential role of mammary neutrophils during this process. I also investigated the effect of estrogen on circulating neutrophils in mice with or without tumour and determined how it can bring about a pro-tumoral effect contributing to the development of PABC. Using RNA-seq analysis, this study revealed that estrogen plays a significant role in the regulation of many aspects of mammary involution. Estrogen treatment enhanced the progression of mammary involution by stimulating increased expression of pro-inflammatory factors, cell death markers, adipogenesis modulator, and tissue remodelling genes. It was also demonstrated that some of these processes such as inflammation, adipocyte repopulation, and tissue remodelling was mediated through the estrogen-regulated mammary neutrophil activities. Using a mice tumour model inoculated with ER-negative 4T1-Luc2 breast cancer cells, this study showed that estrogen significantly increased the tumour growth during mammary involution as compared to the age-matched nulliparous mice. In addition to stimulating tumour growth via tissue neutrophil activity, estrogen also exerts strong effect on circulating neutrophils. Interestingly, whilst estrogen induced the expression of pro-tumoral factors in circulating neutrophils, it reduced markedly the number of the putatively pro-tumoral low-density neutrophils by decreasing its production from the bone marrow and increasing apoptosis in both nulliparous and involuting tumour bearing mice. In summary, this study demonstrated that estrogen strongly promotes mammary involution and much of the estrogenic effect was mediated through the regulation of mammary neutrophil activity. This study also showed that estrogen exerts strong effect on circulating neutrophil activity in mice bearing mammary tumours and stimulates the expression of pro-tumoral factors. Together, these data indicate the importance of estrogen regulation of mammary involution and in PABC. Furthermore, estrogen plays a pivotal role in the regulation of activity of both mammary and circulating neutrophils that likely contribute to the pro-tumoral effect of estrogen via the tumour microenvironment.