Natural and Synthetic Estrogens in Chronic Inflammation and Breast Cancer

SIMPLE SUMMARY: Estrogens regulate key physiological functions in the human body, including the development of female reproductive organs. However, dysregulated estrogen signaling—mainly mediated by estrogen receptors (ER)—is associated with many developmental, mental, and other diseases, including cancer. An increasing number of studies have demonstrated that estrogen modulates inflammatory processes within many tissues. In addition to estrogens made within the body, humans are also constantly exposed to many outside estrogens, naturally occurring in plants and those artificially synthesized in industries. Here, we will also discuss the link between chronic exposure to environmental estrogens with tissue inflammation and breast cancer development. ABSTRACT: The oncogenic role of estrogen receptor (ER) signaling in breast cancer has long been established. Interaction of estrogen with estrogen receptor (ER) in the nucleus activates genomic pathways of estrogen signaling. In contrast, estrogen interaction with the cell membrane-bound G-protein-coupled estrogen receptor (GPER) activates the rapid receptor-mediated signaling transduction cascades. Aberrant estrogen signaling enhances mammary epithelial cell proliferation, survival, and angiogenesis, hence is an important step towards breast cancer initiation and progression. Meanwhile, a growing number of studies also provide evidence for estrogen’s pro- or anti-inflammatory roles. As other articles in this issue cover classic ER and GPER signaling mediated by estrogen, this review will discuss the crucial mechanisms by which estrogen signaling influences chronic inflammation and how that is involved in breast cancer. Xenoestrogens acquired from plant diet or exposure to industrial products constantly interact with and alter innate estrogen signaling at various levels. As such, they can modulate chronic inflammation and breast cancer development. Natural xenoestrogens generally have anti-inflammatory properties, which is consistent with their chemoprotective role in breast cancer. In contrast, synthetic xenoestrogens are proinflammatory and carcinogenic compounds that can increase the risk of breast cancer. This article also highlights important xenoestrogens with a particular focus on their role in inflammation and breast cancer. Improved understanding of the complex relationship between estrogens, inflammation, and breast cancer will guide clinical research on agents that could advance breast cancer prevention and therapy.