4-Estren-3alpha,17beta-diol (estren), a synthetic ligand of the estrogen (ER) or androgen (AR) receptor represents a novel class of activators of nongenotropic estrogen-like signaling (ANGELS), compounds that faithfully reproduce the nongenotropic actions of estradiol on osteoblast and osteoclast apoptosis in vitro and in vivo but lack the genotropic effects of classical estrogen. Estren has been shown to have distinct biologic effects compared to estradiol. Unlike estradiol, estren has no effect on female or male reproductive organs but increases serum osteocalcin, cortical width, and bone mineral density of ovariectomized females above the level of the estrogen-replete controls. In view of estren's distinct skeletal profile, versus classical estrogen or other anti-remodeling agents, the postulate that the superior effects of estren must result not only from its favorable effect on bone cell apoptosis, but also from additional mechanisms, will be tested. In studies leading directly to this application, the hypothesis that, unlike estradiol, estren may promote the commitment and/or differentiation of osteoblast progenitors, was explored. Estren induced lineage commitment and differentiation toward osteoblasts via ER-/AR-dependent, kinase-mediated potentiation of BMP-2 and Wnt signaling. Estradiol and DHT are three to four orders of magnitude less potent than estren in these effects. Unlike purported pro-differentiating effects of estradiol, which could be only shown in cells in which the expression of ERalpha was artificially increased by transfection of an ERalpha cDNA, the pro-differentiating effects of estren are demonstrable in bone cells, which express low levels of endogenous ER and AR. Thus, the molecular mechanism of the induction of osteoblast lineage commitment and differentiation by estren will be elucidated by (1) determining the specificity of the ligand/receptor interaction and (2) the involvement of BMP and Wnt signaling in these effects in murine and human osteoblast progenitors and in bone in vivo. A mechanistic explanation will also be sought for why sex steroids, although capable of nongenotropic signaling, differ from estren in their ability to induce lineage commitment and promote osteoblast differentiation, by (3) searching for genotropic counter-regulatory actions on cytokines, kinases, and the BMP and Wnt and signaling pathways. Results of these studies could provide essential understanding for mechanisms to control bone anabolism.
