Estren Behaves as a Weak Estrogen Rather than a Nongenomic Selective Activator in the Mouse Uterus (original) (raw)

A proposed membrane-mediated mechanism of rapid nongenomic response to estrogen has been the intense focus of recent research. Estren, a synthetic steroid, is reported to act selectively through a rapid membrane-mediated pathway, rather than through the classical nuclear estrogen receptor (ER)-mediated pathway, to maintain bone density in ovariectomized mice without uterotropic effects. To evaluate the mechanism and physiological effects of estren, we studied responses in adult ovariectomized mice. In a 3-d uterine bioassay, we found that 300 g estren significantly increased uterine weight; in comparison, a more maximal response was seen with 1 g estradiol (E2). The estren response was partly ER␣ independent, because ER␣ knockout (␣ERKO) uteri also exhibited a more moderate weight increase. Estren induced epithelial cell proliferation in wild-type, but not ␣ERKO, mice, indicating ER␣ dependence of the epithelial growth response. Examination of estren-regulated uterine genes by microarray indicated that early (2 h) changes in gene expression are similar to the early responses to E2. These gene responses are ER␣ dependent, because they are not seen in ␣ERKO mice. Later estren-induced changes in gene expression (24 h) are blunted compared with those seen 24 h after E2. In contrast to early genes, these later estren responses are independent of ER␣, because the ␣ERKO shows a similar response to estren at 24 h. We found that E2 or estren treatments lead to depletion of ER␣ in the uterine cytosol fraction and accumulation in the nuclear fraction within 30 -60 min, consistent with the ability of estren to regulate genes through a nuclear ER␣ rather than a nongenomic mechanism. Interestingly, estren, but not E2, induces accumulation of androgen receptor (AR) in the nuclear fraction of both wild-type and ␣ERKO samples, suggesting that AR might be involved in the later ER␣-independent genomic responses to estren. In conclusion, our studies suggest that estren is weakly estrogenic in the mouse uterus and might induce nuclear ER␣-and AR-mediated responses. Given its activity in our uterine model, the use of estren as a bone-selective clinical compound needs to be reconsidered.