Receptors are protein molecules that regulate a cell’s responses to particular substances in the body. Due to their ability to regulate cell responses, hormone receptors often are thought of as “on-off switches.” The analogy is that when we turn on an electrical switch connected to a light, the light goes on. Similarly, when the cell responds to particular substances, certain activities in the cell are activated.
Estrogen receptors are one of the body’s most important receptors. Estrogen in the body seeks out estrogen receptor sites on the surfaces of healthy breast cells and breast cancer cells. When the estrogen connects with the estrogen receptor, the binding of the hormone and its receptor sends a message to the cell to create new cells. Breast cancers that are estrogen-receptor positive (ER-positive), have estrogen receptors present on the surface of many of the cancer cells.
Many breast cancers are hormone-dependent, which means they depend on a hormone to bind to a specific hormone receptor in order to stimulate further cell growth. About 60% of breast cancers are ER-positive.
Estrogen is, in a sense, a major component of nourishment of these breast cancer cells. Take away the estrogen, and these breast cancer cells can no longer grow. Deprive these breast cancer cells of estrogen long enough, and the breast cancer cells eventually will weaken and die.
This is the goal of anti-estrogen therapy -- take away the estrogen source and “starve” the breast cancer cells. The greater the number of estrogen receptors on a breast cancer cell, the more likely these breast cancers are to respond to anti-estrogen therapies. Tamoxifen, an example of an anti-estrogen therapy, belongs to a family of drugs called selective estrogen receptor modulators (SERMs).