ERβ compensates for the absence of ERα function to promote osteoblast viability by inhibition of SOST signaling

Estrogen receptors alpha and beta (ER alpha and ER beta) serve key functions in bone development and maintenance, and in the metabolism of bone mineral. ER beta and ER alpha form heterodimers, and ER beta negatively regulates the transactivation of ER alpha. ER beta also inhibits recruitment of ER alpha to the estrogen-responsive promoters. However, the relationship of ER alpha and ER beta in the regulation of osteoblast viability and differentiation remains unclear. The present study aimed to investigate whether ER beta plays a role in balancing ER alpha activity in osteoblast cells. Downregulation of ER alpha by short hairpin RNA (shRNA) was found to significantly increase cell cycle arrest at G1 phase (P<0.01). In addition, this effect was found to be significantly enhanced by downregulation of ER beta (P<0.05). Inversely, ER alpha-knocked down osteoblasts were treated with ER beta agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN) to activate ER beta. It was found that activation of ER beta significantly rescued the arrest of cell cycle induced by the downregulation of ER alpha (P<0.05). Furthermore, downregulation of ER alpha was found to significantly inhibit cell viability (P<0.01), and knockdown of ER beta was found to have a significant synergic effect with ER alpha downregulation on the inhibition of cell viability (P<0.01). Treatment with ER beta agonist DPN significantly rescued the effects of downregulation of ER alpha on cell viability (P<0.01). It was also demonstrated that the synergic effects of ER alpha and ER beta deletion was via upregulation of SOST gene expression, and the subsequent inhibition of OPG and Runx2 gene expression. Thus, ER beta may serve a function in balancing osteoblast viability and differentiation induced by ER alpha.