Steroid Hormone Ligands for the Estrogen Receptor
Circulating levels of estrogens are variable, but tightly regulated in normal human physiology (Hammond 1993). e primary estrogenic agonist, estradiol, comes from two sources: (1) synthesis in the ovaries in premenopausal women and (2) conversion from adrenal steroids, largely through aromatization (and aromatase enzymes) (Figure 16.2) present in a variety of tissues, most notably fat, breast tissue, and breast cancers (Kuerer et al. 2001, Reed and Purohit 2001). Premenopausal levels of estradiol vary depending upon the time of day and the phase of the menstrual cycle, reaching levels as high as 500pg/mL (1.7nM) midcycle (Hammond 1993). During pregnancy, levels can be considerably higher. In postmenopausal women and men, levels are generally <30pg/ mL (0.1nM). Because estradiol is lipophilic, there is nonspecic cellular uptake and estradiol is generally present in slightly higher concentration in tissues with greater fat content.