Induction of Osteoblast Differentiation by Selective Activation of Kinase-Mediated Actions of the Estrogen Receptor

Abstract

Estrogens control gene transcription by cis or trans interactions of the estrogen receptor (ER) with target DNA or via the activation of cytoplasmic kinases. We report that selective activation of kinase-mediated actions of the ER with 4-estren-3␣,17␤-diol (estren) or an estradiol-dendrimer conjugate, each a synthetic compound that stimulates kinase-mediated ER actions 1,000 to 10,000 times more potently than direct DNA interactions, induced osteoblastic differentiation in established cell lines of uncommitted osteoblast precursors and primary cultures of osteoblast progenitors by stimulating Wnt and BMP-2 signaling in a kinase-dependent manner. In sharp contrast, 17␤-estradiol (E 2 ) suppressed BMP-2-induced osteoblast progenitor commitment and differentiation. Consistent with the in vitro findings, estren, but not E 2 , stimulated Wnt/␤-catenin-mediated transcription in T-cell factor-lacZ transgenic mice. Moreover, E 2 stimulated BMP signaling in mice in which ER␣ lacks DNA binding activity and classical estrogen response element-mediated transcription (ER␣ NERKI/؊ ) but not in wild-type controls. This evidence reveals for the first time the existence of a large signalosome in which inputs from the ER, kinases, bone morphogenetic proteins, and Wnt signaling converge to induce differentiation of osteoblast precursors. ER can either induce it or repress it, depending on whether the activating ligand (and presumably the resulting conformation of the receptor protein) precludes or accommodates EREmediated transcription.Steroid hormones, including estrogens and androgens, can induce cellular responses not only through direct activation of gene transcription resulting from cis or trans interactions of their receptors with DNA binding sequences on target gene promoters but also indirectly. The latter responses result from extranuclear actions of the same hormone receptors, causing activation of various cytoplasmic protein kinase cascades, which in turn alter the activities of other transcription factors or coactivators of the receptors themselves (5,6,27,40,43,46,53,67). During the last few years, we and others have identified several synthetic compounds that, unlike estradiol, can selectively activate kinase-initiated routes by which the estrogen receptor (ER) controls gene transcription (28,38,60). By use of these compounds, substantial evidence that kinase-mediated effects on gene transcription are dissociable from direct cis or trans effects on transcription has been obtained (28,38,39,54). Moreover, the use of selective activators of kinases, as well as compounds that dissociate the classical genomic actions of the estrogen receptor from cross talk with other transcription factors (8, 56), has provided extensive proof of the general concept that it is possible to eliminate the uterotropic activity of estrogens while retaining other nonreproductive actions. Together with cell and murine models expressing mutant receptors that cannot enter the nucleus and interact with DNA directly, these tools have become increas...

Naringin improves bone properties in ovariectomized mice and exerts oestrogen‐like activities in rat osteoblast‐like (UMR‐106) cells

“…In contrast, 10 nM naringin could induce ERa phosphorylation at serine 118 in UMR-106 cells in a ligand-independent manner. Recent studies by Kousteni's group suggested that ligand, which activates kinase-mediated actions of ER, could reverse the loss of bone mass and strength in OVX mice without significant stimulatory effects on reproductive organs (Kousteni et al, 2002), and that kinase-mediated actions of the ER are important for inducing osteoblast differentiation (Kousteni et al, 2007). The fact that naringin could reverse bone loss in OVX mice without any uterotrophic effects suggests that it might induce osteoblast differentiation via kinase-mediated actions of the ER.…”

Glycitein decreases the generation of murine osteoclasts and increases apoptosis

“…Estrogen has been shown to increase osteoclast apoptosis and inhibit their generation through a decreased ratio of receptor for RANKL to OPG [4][5][6][7]. The effects that estrogen exert on osteoblast proliferation and differentiation are less clear but its apoptosis-reducing effects on osteoblasts are well supported [8][9][10][11]. Hormone replacement therapy (HRT) has been shown to be efficacious in maintaining bone mass and reducing fracture risk in post-menopausal women [12,13].…”

Estrogen-related receptor α regulates osteoblast differentiation via Wnt/β-catenin signaling

“…This strongly suggests that ERRa regulation of WNT target genes is independent of b-catenin translocation. DNA-binding-independent activities through kinase interactions have been demonstrated for the estrogen receptor (ER) in osteoblast differentiation (Kousteni et al 2007). To test whether ERRa DNA binding was required for activation of the TCF reporter, we generated point mutants in the DNA-binding domain of ERRa.…”