Antiestrogens are estrogen receptor antagonists, ie. they block or interfere with the estrogen receptor. Tamoxifen is a synthetic antiestrogen (although with agonist properties as well) used to treat breast cancer because of its ability to inhibit the breast cancer cell proliferation.

Because estrogen is critical to not only the regulation of the reproductive system, but to growth, bone development, and metabolism, the suspected effects of antiestrogens are numerous. Most endocrine disruption research has focused on the reproductive impacts of antiestrogens, which are typically identified by their ability to inhibit the proliferation of the human breast cancer cell.

TCDD and some hydroxy-PCB congeners have reported anti-estrogenic effects. It is important to note, however, that dioxin is not an antiestrogen at the receptor level in that it does not bind to the estrogen receptor. Rather, dioxin modulates estrogen's effect, and depending on the the time of exposure, dose and targeted tissue, can have a variety of effects. Antiestrogenicity is one of these effects. Dioxin has been shown to suppress the 17-ß estradiol dependent-growth of human breast cancer cells. Dioxin's estrogen antagonism is believed to be the result of 'cross talk' interaction between the Ah receptor, for which dioxin has a high affinity, and the estrogen receptor(s). Ah receptor agonists including dioxin, PCBs and hexachlorobenzene all have anti-estrogenic effects. In carp, dioxin has been shown to inhibit the production of vitellogenin, the yolk protein precursor, in female fish. More ...

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