The Dual Faces of Oestrogen: The Impact of Exogenous Oestrogen on the Physiological and Pathophysiological Functions of Tissues and Organs.

Oestrogen plays a crucial physiological role in both women and men. It regulates reproductive functions and maintains various non-reproductive tissues through its receptors, such as oestrogen receptor 1/oestrogen receptor α (ESR1/Erα), oestrogen receptor 2/oestrogen receptor β (ESR2/Erβ), and G protein-coupled oestrogen receptor 1 (GPER). This hormone is essential for the proper functioning of women's ovaries and uterus. Oestrogen supports testicular function and spermatogenesis in men and contributes to bone density, cardiovascular health, and metabolic processes in both sexes. Nuclear receptors Er-α and Er-β belong to the group of transcription activators that stimulate cell proliferation. In the environment, compounds similar in structure to the oestrogens compete with endogenous hormones for binding sites to receptors and to disrupt homeostasis. The lack of balance in oestrogen levels can lead to infertility, cancer, immunological disorders, and other conditions. Exogenous endocrine-active compounds, such as bisphenol A (BPA), phthalates, and organic phosphoric acid esters, can disrupt signalling pathways responsible for cell division and apoptosis processes. The metabolism of oestrogen and its structurally similar compounds can produce carcinogenic substances. It can also stimulate the growth of cancer cells by regulating genes crucial for cell proliferation and cell cycle progression, with long-term elevated levels linked to hormone-dependent cancers such as breast cancer. Oestrogens can also affect markers of immunological activation and contribute to the development of autoimmune diseases. Hormone replacement therapy, oral contraception, in vitro fertilisation stimulation, and hormonal stimulation of transgender people can increase the risk of breast cancer. Cortisol, similar in structure to oestrogen, can serve as a biomarker associated with the risk of developing breast cancer. The aim of this review is to analyse the sources of oestrogens and their effects on the endogenous and exogenous process of homeostasis.