Estrogens (E2) regulate expression and response of 1,25-dihydroxyvitamin D3 receptors in bone cells:: changes with aging and hormone deprivation

Studies on the effect of estrogens (E-2) on the expression of vitamin D receptor (VDR) and its bioresponse in bone have demonstrated that E-2 modulate activity and increase the number of VDRs in vitro; however, no in vivo studies have been pursued to assess this interaction. Our study identifies the changes in the number of VDR-expressing cells in bone of C57BL/6J young and old oophorectomized mice (4 and 24 months) with and without 17beta estradiol (E-2) replacement. A total of 36 mice were sacrificed; both tibiae and femora were isolated and VDR expression was quantified by Northern blot, immunohistochemistry, immunofluorescence, and flow cytometry. Among the intact mice there was a significant difference in the number of VDR-expressing osteoblasts between young (68%) and old (56%) (p < 0.04). In young oophorectornized mice the number of VDR-expressing osteoblasts decreased from 68% to 46% after oophorectomy and recovered to 72% after E-2 administration (p < 0.02), while in the group of old mice, the number of VDR-expressing osteoblasts decreased from 56% to 48% after oophorectomy (p < 0.01) and recovered to 85% after E-2 administration (p < 0.001). Our results show that VDR expression in bone decreases with aging and estrogen deprivation but recovers after E-2 supplementation in both young and old mice with a more significant level of response in older bone. To evaluate the level of VDR bioresponse to E-2 we assessed the effect of E-2 supplementation to human osteoblasts (N-976) in vitro. Northern blot showed a significant up-regulation of VDR expression in E-2 treated cells as compared to non-treated cells (p < 0.05). We also assessed the previously known anti-apoptotic effect of vitamin D in osteoblasts in vitro after serum deprivation by using either E-2, E-2 + 1,25(OH)(2)D-3 or 1,25(OH)(2)D-3 alone. We found a lower number of apoptotic cells and longer cell survival after 48 h of treatment with 1,25(OH)(2)D-3 + E-2 as compared to 1,25(OH)(2)D-3 or E-2 alone (p < 0.002). In summary, our results demonstrate that E-2 increases VDR expression in bone in vivo and potentiate the bioresponse of VDR in ostcoblasts in vitro. (C) 2002 Elsevier Science (USA). All rights reserved.