Estrogen receptor-β regulates mechanical signaling in primary osteoblasts

Mechanical loading is an important regulator in skeletal growth, maintenance, and aging. Estrogen receptors have a regulatory role in mechanically induced bone adaptation. Estrogen receptor-alpha (ER alpha) is known to enhance load-induced bone formation, whereas ER beta negatively regulates this process. We hypothesized that ER beta regulates mechanical signaling in osteoblasts. We tested this hypothesis by subjecting primary calvarial cells isolated from wild-type and ER beta-knockout mice (BERKO) to oscillatory fluid flow in the absence or presence of estradiol (E-2). We found that the known responses to fluid shear stress, i.e., phosphorylation of the mitogen-activated protein kinase ERK and upregulation of COX-2 expression, were inhibited in BERKO cells in the absence of E-2. Flow-induced increase in prostaglandin E-2 (PGE(2)) release was not altered in BERKO cells in the absence of E-2, but was increased when E-2 was present. Additionally, immunofluorescence analysis and estrogen response element luciferase assays revealed increased ER alpha expression and flow-and ligand-induced nuclear translocation as well as transcriptional activity in BERKO cells in both the presence and absence of E-2. Taken together, these data suggest that ER beta plays both ligand-dependent and ligand-independent roles in mechanical signaling in osteoblasts. Furthermore, our data suggest that one mechanism by which ER beta regulates mechanotransduction in osteoblasts may result from its inhibitory effect on ER alpha expression and function. Targeting estrogen receptors (e. g., inhibiting ER beta) may represent an effective approach for prevention and treatment of age-related bone loss.