Tamoxifen Is a Drug That Is Used to Treat Breast

Tamoxifen is a drug that is used to treat breast cancer. It inhibits the activities of the estrogen receptor in breast epithelial cells, which are the cells that are relevant to breast cancer. One of the concerns that scientists had during the development of Tamoxifen as a therapy for breast cancer was that other cell types require estrogen signaling for their function. For example, it is known that cells in bone tissue require estrogen signaling. As a result, there was a concern that use of Tamoxifen would lead to bone loss in patients. In reality, however, researchers were surprised to find that Tamoxifen actually led to an increase in bone density in breast cancer patients. At the same time, Tamoxifen did inhibit the activity of the estrogen receptor in breast epithelial cells in these patients. What is the best explanation for this finding?

A) The estrogen receptor is bound to different coactivators in different tissues.
B) Tamoxifen binds different parts of the estrogen receptor in different tissues.
C) Tamoxifen can only enter certain cell types.
D) Bone cells make use of a different type of estrogen receptor that does not bind to Tamoxifen.