Phytoestrogen genistein stimulates the production of osteoprotegerin by human trabecular osteoblasts.

Abstract

The anti-resorptive effects of estrogen on bone metabolism are thought to be mediated through modulation of paracrine factors produced by osteoblastic lineage cells that act on osteoclastic lineage cells. Receptor activator of nuclear factor-kappaB ligand (RANKL) is the essential factor for osteoclast formation and activation and enhances bone resorption. By contrast, osteoprotegerin (OPG), which is produced by osteoblastic lineage cells acts as a decoy receptor that neutralizes RANKL and prevents bone loss. Recently, 17 beta-estradiol was found to stimulate OPG mRNA levels and protein secretion in a human osteoblastic cell line through activation of the estrogen receptor (ER)-alpha. In this study, we assessed the effects of the phytoestrogen genistein on OPG mRNA steady state levels (by semiquantitative RT-PCR and Northern analysis) and protein production (by ELISA) in primary human trabecular osteoblasts (hOB) obtained from healthy donors. Genistein increased OPG mRNA levels and protein secretion by hOB cells by up to two- to six-fold in a dose- (P<0.0001) and time-dependent (P<0.0001) fashion with a maximum effect at 10(-7) M. Co-treatment with the pure ER antagonist ICI 182,780 completely abrogated the stimulatory effects of genistein on OPG protein secretion, indicating that these effects were specific and directly mediated through the ER. Pre-treatment with genistein partially prevented the inhibitory effects of the glucocorticoid dexamethasone on OPG mRNA and protein production. The stimulation of OPG mRNA levels by genistein was not affected by the protein synthesis inhibitor, cycloheximide and was shown to be due to enhancement of OPG gene transcription. In conclusion, our data suggest that the phytoestrogen genistein is capable of upregulating the production of OPG by human osteoblasts. Thus, dietary sources of phytoestrogens may help to prevent bone resorption and bone loss by enhanced osteoblastic production of OPG.