Sex hormone receptors in breast cancer.

The dependency of certain breast cancers on estrogen is undeniably one of the most important observations in oncology. Since this early observation, there has been a tremendous effort to define the precise roles of the estrogen receptor (ER) in the pathogenesis of breast cancer. Estrogen signaling pathways can also be exploited as effective targets for cancer treatment. Both ligand-dependent and ligand-independent receptor activation pathways have been successfully blocked by hormonal therapies including selective ER modulators such as tamoxifen, by blocking and accelerating the degradation of ER (fulvestrant), and by depleting tissue levels of estrogen (aromatase inhibitors). Because of the immense prognostic and predictive value of the ER and PR receptor, accurately defining hormone dependency is also of paramount importance. Despite this avalanche of discovery and development resulting in improved outcome for the patient, resistance to these therapies, both intrinsic and acquired, is well known. Uncovering the various mechanisms of resistance has deepened scientific understanding of posttranslational modifications of these receptors, as well as their cross talk with other receptor families such as the HER-2/neu receptor. The recent discovery that orphan estrogen-related receptors may also play an important role in breast cancer is just starting to be appreciated. A clear understanding of the historical perspective and the intricacies of ER structure and function is required to improve current therapeutic strategies for breast cancer.