Estrogen Receptor α Mediates Proliferation of Osteoblastic Cells Stimulated by Estrogen and Mechanical Strain, but Their Acute Down-regulation of the Wnt Antagonist Sost Is Mediated by Estrogen Receptor β

Mechanical strain and estrogens both stimulate osteoblast proliferation through estrogen receptor (ER)-mediated effects, and both down-regulate the Wnt antagonist Sost/sclerostin. Here, we investigate the differential effects of ER alpha and -beta in these processes in mouse long bone-derived osteoblastic cells and human Saos-2 cells. Recruitment to the cell cycle following strain or 17 beta-estradiol occurs within 30 min, as determined by Ki-67 staining, and is prevented by the ER alpha antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride. ER beta inhibition with 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-beta]pyrimidin-3-yl]phenol (PTHPP) increases basal proliferation similarly to strain or estradiol. Both strain and estradiol down-regulate Sost expression, as does in vitro inhibition or in vivo deletion of ER alpha. The ER beta agonists 2,3-bis(4-hydroxyphenyl)-propionitrile and ERB041 also down-regulated Sost expression in vitro, whereas the ER alpha agonist 4,4',4 ''-[4-propyl-(1H)-pyrazol-1,3,5-triyl]tris-phenol or the ER beta antagonist PTHPP has no effect. Tamoxifen, a nongenomic ER beta agonist, down-regulates Sost expression in vitro and in bones in vivo. Inhibition of both ERs with fulvestrant or selective antagonism of ER beta, but not ER alpha, prevents Sost down-regulation by strain or estradiol. Sost down-regulation by strain or ER beta activation is prevented by MEK/ERK blockade. Exogenous sclerostin has no effect on estradiol-induced proliferation but prevents that following strain. Thus, in osteoblastic cells the acute proliferative effects of both estradiol and strain are ER alpha-mediated. Basal Sost down-regulation follows decreased activity of ER alpha and increased activity of ER beta. Sost down-regulation by strain or increased estrogens is mediated by ER beta, not ER alpha. ER-targeting therapy may facilitate structurally appropriate bone formation by enhancing the distinct ligand-independent, strain-related contributions to proliferation of both ER alpha and ER beta.