Sources of estrogen and their importance

Abstract

In premenopausal women, the ovaries are the principle source of estradiol, which functions as a circulating hormone to act on distal target tissues. However, in postmenopausal women when the ovaries cease to produce estrogen, and in men, this is no longer the case, because estradiol is no longer solely an endocrine factor. Instead, it is produced in a number of extragonadal sites and acts locally at these sites as a paracrine or even intracrine factor. These sites include the mesenchymal cells of adipose tissue including that of the breast, osteoblasts and chondrocytes of bone, the vascular endothelium and aortic smooth muscle cells, and numerous sites in the brain. Thus, circulating levels of estrogens in postmenopausal women and in men are not the drivers of estrogen action, they are reactive rather than proactive. This is because in these cases circulating estrogen originates in the extragonadal sites where it acts locally, and if it escapes local metabolism then it enters the circulation. Therefore, circulating levels reflect rather than direct estrogen action in postmenopausal women and in men. Tissue-specific regulation of CYP19 expression is achieved through the use of distinct promoters, each of which is regulated by different hormonal factors and second messenger signaling pathways. Thus, in the ovary, CYP19 expression is regulated by FSH which acts through cyclic AMP via the proximal promoter II, whereas in placenta the distal promoter I.1 regulates CYP19 expression in response to retinoids. In adipose tissue and bone by contrast, another distal promoter--promoter I.4--drives CYP19 expression under the control of glucocorticoids, class 1 cytokines and TNFalpha. The importance of this unique aspect of the tissue-specific regulation of aromatase expression lies in the fact that the low circulating levels of estrogens which are observed in postmenopausal women have little bearing on the concentrations of estrogen in, for example, a breast tumor, which can reach levels at least one order of magnitude greater than those present in the circulation, due to local synthesis within the breast. Thus, the estrogen which is responsible for breast cancer development, for the maintenance of bone mineralization and for the maintenance of cognitive function is not circulating estrogen but rather that which is produced locally at these specific sites within the breast, bone and brain. In breast adipose of breast cancer patients, aromatase activity and CYP19 expression are elevated. This occurs in response to tumor-derived factors such as prostaglandin E2 produced by breast tumor fibroblasts and epithelium as well as infiltrating macrophages. This increased CYP19 expression is associated with a switch in promoter usage from the normal adipose-specific promoter I.4 to the cyclic AMP responsive promoter, promoter II. Since these two promoters are regulated by different cohorts of transcription factors and coactivators, it follows that the differential regulation of CYP19 expression via alternative promoters in disease-free and cancerous breast adipose tissue may permit the development of selective aromatase modulators (SAMs) that target the aberrant overexpression of aromatase in cancerous breast, whilst sparing estrogen synthesis in other sites such as normal adipose tissue, bone and brain.