Estrogen receptor β in the breast: Role in estrogen responsiveness and development of breast cancer

Breast cancer is one of the most common forms of cancer observed in women. Endogenous estrogen is thought to play a major role in its development and estrogen receptor blockers are the most important drugs in its treatment. It has long been thought that any conditions or exposures, which enhance estrogenic responses, would result in an increased risk for breast cancer. The discovery of the second estrogen receptor, ERβ, which can have effects opposite to those of the well-known 'original' estrogen receptor (now called ERα) challenges this simplistic view. In order to understand breast cancer one must first understand how the normal breast is maintained. The functions of ERβ in the breast remain to be defined but from what we have learnt about its activities in in vitro systems, this estrogen receptor may have a protective role in the breast. Studies in human and rodent breasts as well as in human breast cancer biopsies reveal that ERβ is by far the more abundant of the two ERs. Despite the role of estrogen in proliferation of the breast, neither of the two ERs appears to located in epithelial cells which divide in response to estrogen. In order to define the functions of ERβ in the normal and malignant breast, we have created mice in which the ERβ gene has been inactivated. Studies of the breasts of ERβ knock out mice (BERKO) revealed abnormal epithelial growth, overexpression of Ki67 and severe cystic breast disease as mice age.