Estradiol (E2), is also known as "estrogen" and "courtship hormone". Estradiol is a steroid hormone and a natural estrogen, mainly from the follicle, corpus luteum, and placenta of the female ovary, and to a lesser extent from the adrenal glands and testes of the male. There are two types: α and β. The α type has a strong physiological effect. It is an important hormone that promotes and regulates the normal development of female sex organs and secondary sex characteristics.
E2 Biosynthesis
In humans and other higher animals, steroid hormones are derived from cholesterol. Cholesterol undergoes oxidation and side-chain breaks to produce pregnenolone, which in turn produces progesterone, 17α-hydroxy-pregnanolone, to a key intermediate, androstenedione. A portion of androstenedione is converted to testosterone, which is converted to estradiol by aromatase. Another pathway, androstenedione, is aromatized to estrone, which is subsequently converted to estradiol.
Estradiol is produced by granulosa cells, follicular membrane cells, placenta, adrenal cortex, and testicular mesenchymal cells. In women of childbearing age, estradiol is produced mainly from androstenedione produced in follicular cells of the follicular membrane and metabolized by ovarian granulosa cells. Organs and tissues other than the gonads, such as adipocytes, also produce estradiol, even in women after menopause. Although estradiol does not readily cross the blood-brain barrier from the circulatory system to the brain, it is one of the two active metabolites of testosterone (the other being dihydrotestosterone), and can therefore be produced in the brain.
For plants, estradiol synthesis has been found in Phaseolus vulgaris.
E2 Receptor Binding
Estradiol can bind to two estrogen receptors, estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). These two receptors are expressed in different tissues and cells and trigger different biological effects.
When estradiol binds to the estrogen receptor, it triggers a series of intracellular signaling pathways. These signaling pathways can regulate biological processes such as gene expression, cell proliferation, differentiation, and apoptosis.
ERα is highly expressed in the female reproductive system, such as the uterus, breasts, and ovaries. When combined with estradiol, it promotes the growth and proliferation of the uterine lining, as well as breast development.
ERβ is widely expressed in several tissues, including bone, brain, and cardiovascular systems. When combined with estradiol, it can have effects on bone metabolism, neuroprotection, and cardiovascular function.
It is important to note that estrogen receptor expression and activity are regulated by a variety of factors, including hormone levels, cell type, and environmental factors.
Estradiol signaling pathway
E2 Metabolism
In plasma, estradiol is predominantly bound to sex hormone-binding globulin or albumin, with only a small free fraction (2.21% ± 0.04%) biologically active, and this percentage remains constant throughout the menstrual cycle. Estradiol metabolism involves conversion to the relatively inactive estrone and estriol. It is bound to sulfate and glucuronide in the liver and excreted by the kidneys. Some of the water-soluble conjugates are excreted by the bile ducts and partially hydrolyzed and reabsorbed from the intestine. This hepatic-intestinal cycle contributes to the maintenance of estradiol levels.
E2 Mechanism of Action
Estradiol is the most potent naturally occurring estrogen and its mechanism of action is broadly based on crossing cell membranes and binding to specialized receptors in the cytoplasm to form a complex. Upon binding, the conformation of estradiol changes, and its affinity for DNA is greatly increased. This complex moves from the cytoplasm to the nucleus, where it binds to chromatin and activates the transcriptional activity of the corresponding genes, resulting in a large amount of mRNA synthesis, which in turn synthesizes a large number of specific proteins. An example is the mechanism by which estradiol promotes uterine growth. Estradiol binds to two receptors, ERα and ERβ. Drugs that also bind to these receptors are called selective estrogen receptor modulators (SERM) and are used to treat osteoporosis, among other things.
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