Estrogen Enhances Vitamin D_3-mediated Expression of Osteocalcin mRNA by Increasing Vitamin D_3 Receptor Expression in Osteoblasts

Regulation of osteocalcin (OC) gene expression in osteoblasts by estrogen has been controversial. We thus investigated the effect of 17β-estradiol (E2) on la,25-dihydroxyvitamin D3 (VD 3 )-mediated increase in the transcription of OC gene in rat osteoblastic ROS 17/2.8 cells. Treatment of the cells with E2 or VD 3 alone did not increase OC mRNA. However, combined treatment with E2 and VD3 significantly increased the OC mRNA levels, suggesting that VD3-dependent up-regulation of OC expression requires E2 action. Since it was demonstrated that treatment with E2 alone increased the expression of VD3 receptor mRNA, it was suggested that E 2 -mediated increase in the receptor number is contributing to VD3-mediated increase in OC gene transcription.