Estrogen receptor signaling and targets: Bones, breasts and brain (Review).

Estrogens are involved in a number of physiological functions, including in the development of the brain, growth, reproduction and metabolism. The biological actions of estrogens are achieved by binding to estrogen receptors (ERs) in numerous types of tissues. ERα and ERβ belong to the nuclear receptor superfamily and the G‑protein coupled ER1 (GPER1) is a membrane receptor. The primary biologically active estrogen, 17β‑estradiol demonstrates a high affinity for ERs. Mechanistically, estrogens bind to the ERs in the nucleus, and the complex then dimerize and bind to estrogen response elements (EREs) located in the promoter regions of the target genes. This is referred to as the genomic mechanism of ERs' function. Furthermore, ERs can also act through kinases and other molecular interactions leading to specific gene expression and functions, referred to as the non‑genomic mechanism. While ERα and ERβ exert their functions via both genomic and non‑genomic pathways, GPER1 exerts its function primarily via the non‑genomic pathways. Any aberrations in ER signaling can lead to one of a number of diseases such as disorders of growth and puberty, fertility and reproduction abnormalities, cancer, metabolic diseases or osteoporosis. In the present review, a focus is placed on three target tissues of estrogens, namely the bones, the breasts and the brain, as paradigms of the multiple facets of the ERs. The increasing prevalence of breast cancer, particularly hormone receptor‑positive breast cancer, is a challenge for the development of novel antihormonal therapies other than tamoxifen and aromatase inhibitors, to minimize toxicity from the long treatment regimens in patients with breast cancer. A complete understanding of the mechanism of action of ERs in bones may highlight options for novel targeted treatments for osteoporosis. Likewise, the aging of the brain and related diseases, such as dementia and depression, are associated with a lack of estrogen, particularly in women following menopause. Furthermore, gender dysphoria, a discordance between experienced gender and biological sex, is commonly hypothesized to emerge due to discrepancies in cerebral and genital sexual differentiation. The exact role of ERs in gender dysphoria requires further research.