The effects of estrogen on osteoprotegerin, RANKL, and estrogen receptor expression in human osteoblasts

Estrogen is essential for bone growth and development and for the maintenance of bone health in adulthood. The cellular responses of osteoblasts and osteoclasts to estrogen are initiated via two high-affinity receptors (ERs). Osteoblasts synthesize RANKL (receptor activator of NF-kappaB ligand), necessary for osteoclast formation and function, and osteoprotegerin (OPG), its decoy receptor. To investigate the effects of estrogen on the expression of OPG, RANKL, and ERs in human osteoblasts, cells were cultured with physiological (10(-10) M) and high-dose (10(-7) M) 17beta-estradiol for 24 and 48 h. Proteins and corresponding mRNA levels were quantitatively determined by immunocytochemistry and RT-PCR. OPG expression was significantly increased three- and sevenfold at 24 h with 10(-10) M (P < 0.05) and 10(-7) M (P < 0.01) estradiol, respectively, compared to untreated cells. Similar but smaller increases were seen at 48 h (P < 0.05). Osteoblasts treated with estradiol demonstrated increased RANKL protein expression at 24 h (P < 0.05), but this was not maintained at 48 h. ERalpha expression was significantly increased by high-dose estradiol (P < 0.01) at 24 h and dose-dependently increased at 48 h (P < 0.01), while ERbeta was only increased at 24 h (P < 0.01). The estrogen-induced protein expression of ER, OPG, and RANKL was abrogated when cells were cultured in the presence of the estrogen antagonist ICI 182780. mRNA levels at 24 h demonstrated a significant suppression of RANKL with the low-dose but not the high dose. ERalpha mRNA but not ERbeta expression was up-regulated by estrogen. Our results suggest that estrogen may exert its anti-resorptive effects on bone, at least in part, by stimulating ER and OPG expression in osteoblasts.