Comparison of estrogen receptors in human premenopausal and postmenopausal uteri using isoelectric focusing

Previous studies have suggested that the estrogen receptor in the postmenopausal uterus, although capable of binding estrogen, may be inactive, that is, incapable of translocating and binding to nuclear acceptor sites. The present study uses isoelectric focusing to characterize differences in the estrogen receptor between premenopausal and postmenopausal uterine tissue. Human myometrium (15 premenopausal and ten postmenopausal samples) was examined by isoelectric focusing in the presence and absence of diisopropylfluorophosphate (DFP) and molybdate, which inhibit proteolysis and stabilize the receptor. As demonstrated previously, receptor forms could be identified in two regions of the pH gradient, one in the pH 4 to 5 region and another at pH 6 to 7. When prepared without DFP and molybdate, the percentage of receptors focusing in the pH 4 to 5 range was significantly greater for premenopausal than postmenopausal tissue (P less than .01). The addition of DFP and molybdate significantly altered the focusing pattern of postmenopausal tissue (P less than .01), increasing the percentage of binding in the pH 4 to 5 range. In the presence of DFP and molybdate, the focusing patterns of receptor from premenopausal and postmenopausal tissue were not significantly different. These data suggest that endogenous proteases are more prevalent in postmenopausal tissue and can degrade the receptor to forms that focus at a higher pH. With the inhibition of these proteases, the focusing patterns of receptors in premenopausal and postmenopausal tissue become very similar, representing the undegraded and perhaps native form of the receptor.