Phytoestrogens - Mechanism of Action

Mechanism of Action

Phytoestrogens exert their effects primarily through binding to estrogen receptors (ER). There are two variants of the estrogen receptor, alpha (ER-α) and beta (ER-β) and many phytoestrogens display somewhat higher affinity for ER-β compared to ER-α.

The key structural elements that enable phytoestrogens to bind with high affinity to estrogen receptors and display estradiol-like effects are:

• The phenolic ring that is indispensable for binding to estrogen receptor
• The ring of isoflavones mimicking a ring of estrogens at the receptors binding site
• Low molecular weight similar to estrogens (MW=272)
• Distance between two hydroxyl groups at the isoflavones nucleus similar to that occurring in estradiol
• Optimal hydroxylation pattern

In addition to interaction with ERs, phytoestrogens may also modulate the concentration of endogenous estrogens by binding or inactivating some enzymes, and may affect the bioavailability of sex hormones by binding or stimulating the synthesis of sex hormone binding globuline (SHBG).

Emerging evidence shows that some phytoestrogens bind to and transactivate peroxisome proliferator-activated receptors (PPARs). In vitro studies show an activation of PPARs at concentrations above 1 μM, which is higher than the activation level of ERs. At the concentration below 1 μM, activation of ERs may play a dominant role. At higher concentrations (>1 μM), both ERs and PPARs are activated. Studies have shown that both ERs and PPARs influence each other and therefore induce differential effects in a dose-dependent way. The final biological effects of genistein are determined by the balance among these pleiotrophic actions.