ESTROGEN RECEPTORS
Estrogens act on target tissues by binding to parts of cells called estrogen receptors. An estrogen receptor is a protein molecule found inside those cells that are targets for estrogen action. Estrogen receptors contain a specific site to which only estrogens (or closely related molecules) can bind.
The target tissues affected by estrogen molecules all contain estrogen receptors; other organs and tissues in the body do not. Therefore, when estrogen molecules circulate in the bloodstream and move throughout the body, they exert effects only on cells that contain estrogen receptors. Estrogen receptors normally reside in the cell's nucleus, along with DNA molecules.
In the absence of estrogen molecules, these estrogen receptors are inactive and have no influence on DNA (which contains the cell's genes). But when an estrogen molecule enters a cell and passes into the nucleus, the estrogen binds to its receptor, thereby causing the shape of the receptor to change. This estrogen-receptor complex then binds to specific DNA sites, called estrogen response elements, which are located near genes that are controlled by estrogen.
After it has become attached to estrogen response elements in DNA, this estrogen-receptor complex binds to coactivator proteins and more nearby genes become active. The active genes produce molecules of messenger RNA, which guide the synthesis of specific proteins. These proteins can then influence cell behavior in different ways, depending on the cell type involved.
Paradoxically, estrogen can be both a beneficial and a harmful molecule.
The main beneficial effects of estrogen include its roles in
- Programming the breast and uterus for sexual reproduction,
- Controlling cholesterol production in ways that limit the buildup of plaque in the coronary arteries, and
- Preserving bone strength by helping to maintain the proper balance between bone buildup and breakdown.
Unfortunately, in addition to these important beneficial effects, estrogen can also be harmful. The most serious problem arises from the ability of estrogen to promote the proliferation of cells in the breast and uterus. Although this ability to stimulate cell proliferation is one of estrogen's normal roles, it can also increase a woman's chance of developing breast or uterine cancer.
(Source: NIH - MedlinePlus Drug Information and National Cancer Insstitute)
