A DNA binding mutation in estrogen receptor-α leads to suppression of Wnt signaling via ß-catenin destabilization in osteoblasts.

Estrogen receptors (ERs) play vital roles in the function and remodeling of bone. Their cellular mechanisms can broadly be categorized into those involving direct DNA binding (classical) or indirect DNA binding (non-classical). The generation of non-classical ER knock-in (ERα(-/NERKI) ) mice provides a unique opportunity to define these pathways in bone. We previously demonstrated that ERα(-/NERKI) mice exhibit an osteoporotic phenotype; however, the mechanism(s) for this remain unresolved. Gene expression analyses of cortical bone from ERα(-/NERKI) mice revealed suppression of lymphoid enhancer factor-1 (Lef1), a classic Wnt-responsive transcription factor that associates with ß-catenin. Since Wnt signaling is generally considered bone anabolic, this observation leads to the hypothesis that NERKI-induced suppression of Wnt signaling may contribute to the low bone mass phenotype. We generated ERα(-/NERKI) mice crossed with the Wnt-responsive TOPGAL transgenic mouse model and observed significantly less ß-galactosidase activity in ERα(-/NERKI) mice, confirming suppression of Wnt activity in vivo. Adenoviral expression of the NERKI receptor using an in vitro cell system resulted in the induction of several secreted antagonists of Wnt signaling. Furthermore, expression of NERKI abrogated Wnt10b-dependent Wnt activation using a lentiviral-mediated reporter assay. Finally, expression of NERKI destabilized ß-catenin cellular protein levels and disrupted ER/ß-catenin interactions. Collectively, these data suggest the osteoporotic phenotype of ERα(-/NERKI) mice may involve the suppression of Lef1-mediated Wnt signaling through both the stimulation of secreted Wnt inhibitors and/or disruption of normal ß-catenin function.

Loss of ERE binding activity by estrogen receptor-alpha alters basal and estrogen-stimulated bone-related gene expression by osteoblastic cells.

Estrogen receptor (ER)-alpha can signal either via estrogen response element (ERE)-mediated pathways or via alternate pathways involving protein-protein or membrane signaling. We previously demonstrated that, as compared to wild type (WT) controls, mice expressing a mutant ER-alpha lacking the ability to bind EREs (non-classical estrogen receptor knock-in (NERKI)) display significant impairments in the skeletal response to estrogen. To elucidate the mechanism(s) underlying these in vivo deficits, we generated U2OS cells stably expressing either WT ER-alpha or the NERKI receptor. Compared to cells transfected with the control vector, stable expression of ER-alpha, even in the absence of E2, resulted in an increase in mRNA levels for alkaline phosphatase (AP, by 400%, P < 0.01) and a decrease in mRNA levels for insulin growth factor-I (IGF-I) (by 65%, P < 0.001), with no effects on collagen I (col I) or osteocalcin (OCN) mRNA levels. By contrast, stable expression of the NERKI receptor resulted in the suppression of mRNA levels for AP, col I, OCN, and IGF-I (by 62, 89, 60, and 70%, P < 0.001). While E2 increased mRNA levels of AP, OCN, col I, and IGF-I in ER-alpha cells, E2 effects in the NERKI cells on AP and OCN mRNA levels were attenuated, with a trend for E2 to inhibit col I mRNA levels. In addition, E2 had no effects on IGF-I mRNA levels in NERKI cells. Collectively, these findings indicate that ERE signaling plays a significant role in mediating effects of estrogen on osteoblastic differentiation markers and on IGF-I mRNA levels.