Exogenous Estrogens as Breast Cancer Risk Factors: A Perspective

Background: The human body’s exposure to high levels of endogenous estrogens and their metabolites, such as estradiol, estriol, 2-hydroxyestradiol, and 4-hydroxyestradiol, is implicated in the development and complications of breast cancers (BCs). Besides endogenous estrogen production, the human body is also exposed to environmental sources of estrogen and estrogen-like compounds, which include pharmaceutical estrogens, xenoestrogens, and phytoestrogens. Females consume pharmaceutical estrogens as a constituent of postmenopausal hormone replacement therapy (HRT) and oral contraceptive pills, either alone or in combination with progestins. Additionally, humans, including females, are exposed to estrogen-resembling non-native compounds called xenoestrogens, prevailing in pesticides, plastics, and personal care items via inhalation, dermal contact, and oral consumption. Several phytoestrogens, such as isoflavones and lignans, are consumed by humans as food ingredients. Methods and Results: Emerging cellular and molecular experimental evidence indicates that when binding to estrogen receptors (ERs), various pharmaceutical estrogens, including equine/synthetic forms, progestin combinations, and xenoestrogens, promote BC development and complications by triggering survival, proliferation, angiogenesis, and invasion of these cells. Conversely, other experimental observations reveal the protective and beneficial effects of phytoestrogens like genistein from soy products on BC development and complications. Conclusions: This comprehensive review article describes the implications of exposure to exogenous estrogens, such as pharmaceutical estrogens, xenoestrogens, and phytoestrogens, as risk factors in the prevention or development of BC and its complications.