Estradiol Protects against Ethanol-Induced Bone Loss by Inhibiting Up-Regulation of Receptor Activator of Nuclear Factor-κB Ligand in Osteoblasts

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Abstract
To investigate the effects of sex hormones on ethanol (EtOH)-induced bone loss, female Sprague-Dawley rats were fed control or EtOH-containing diets (12 g/kg/day) by intragastric infusion. After 3 weeks, rats receiving EtOH had significant decreases in tibial trabecular and total bone mineral density, induction of receptor activator of nuclear factor-κB ligand (RANKL) mRNA expression, and enhanced bone resorption, all of which were prevented by treatment with 17β-estradiol (E2). The addition of progesterone did not enhance the beneficial effect of E2 alone. Consistent with our in vivo findings, EtOH stimulated RANKL mRNA expression in cultured primary osteoblasts, and this expression was blocked by 4-methylpyrazole. Acetaldehyde also induced RANKL expression. Class 1 alcohol dehydrogenase was found to be expressed and EtOH-inducible in cultured osteoblasts, whereas CYP2E1 was undetectable. We found that EtOH induced phosphorylation of extracellular signal-regulated kinase (ERK) and signal transducers and activators of transcription 3 (STAT3). E2 and the mitogenactivated protein kinase kinase inhibitor 2′-amino-3′-methoxyflavone (PD98059) blocked ERK and STAT3 phosphorylation and blocked RANKL induction. Moreover, E2 completely blocked EtOH-induced osteoclastogenesis in a primary osteoblast and osteoclast precursor coculture system. The E2 effects were estrogen receptor-mediated. Therefore, E2 prevents EtOH-induced bone loss by opposing the induction of RANKL mRNA in osteoblasts and ethanol-induced osteoclastogenesis, through opposing effects on sustained ERK signaling.

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