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Estrogen Acts Through at Least Two ReceptorsThat different estrogens can have different actions in the same tissue is partly explained by the recent discovery of a second estrogen receptor (ER beta).(26) Both receptors (ER alpha and ER beta) have virtually identical mechanisms for binding with DNA, but their structure is different in the binding regions. What this means is that even though estradiol, with its high affinity, can interact equally with these two receptors, there may be other compounds with preferential affinity for one receptor over another. Thus, an estrogen compound may have different activities on each of the receptors. In addition, ER alpha and ER beta receptors are not equally distributed in all tissues. Some cells have ER alpha receptors exclusively, others have ER beta, and still others have both.(32,33)Many mysteries that have surrounded estrogen action may be unraveled now that a second estrogen receptor has been identified. Ongoing research is attempting to determine which tissues are most responsive to which receptors, and there may be other receptors discovered in the process. As research uncovers specific information about receptor distribution in other tissues, the role of estrogen, or antiestrogens, can be better defined. Another reason that estrogens can exhibit diverse activities in different cells pertains to the interaction between an estrogen or an estrogen-like compound and the receptor itself. When a compound binds with an estrogen receptor, it forms a new unique shape (Figure 9) that influences its interaction with the DNA in the cell. Given the existence of a number of estrogen-like compounds and at least 2 known receptors along with variable receptor distribution, the possible combinations are numerous. Different compounds produce different biological activities (Figure 10). For example, a pure agonist like estrogen binds to a receptor creating one type of active configuration. The fact that SERMs bind with receptors to create a shape slightly different from that of estrogen means they will have different biological consequences in different tissues.(25,34,35) Thus, the potential for clinical efficacy varies among compounds. In summary, the field of estrogen receptor activity is in its infancy, and discoveries are continuously changing old concepts. Research has found that estrogens can exhibit different activities in different tissues. Such new findings about the science of estrogen action support the concept that different estrogens acting through the same receptor can induce different biological activity. And, the implications for clinical efficacy are greatly expanded.
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