AbstractMost of the biological effects of 1α,25‐dihydroxyvitamin D3 (1,25(OH)2D3) are elicited by the binding to vitamin D receptor (VDR), which regulates gene expression. Earlier studies reported no correlation between the VDR protein and mRNA levels, suggesting the involvement of posttranscriptional regulation. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression through translational repression or mRNA degradation. A potential miR‐125b recognition element (MRE125b) was identified in the 3′‐untranslated region of human VDR mRNA. We investigated whether VDR is regulated by miR‐125b. In luciferase assays using a plasmid containing the MRE125b, the antisense oligonucleotide for miR‐125b significantly increased (130% of control) the reporter activity in KGN cells, whereas the precursor for miR‐125b significantly decreased (40% of control) the reporter activity in MCF‐7 cells, suggesting that miR‐125b functionally recognized the MRE125b. By electrophoretic mobility shift assays, it was demonstrated that the overexpression of miR‐125b significantly decreased the endogenous VDR protein level in MCF‐7 cells to 40% of control. 1,25(OH)2D3 drastically induced the CYP24 mRNA level in MCF‐7 cells, but the induction was markedly attenuated by the overexpression of miR‐125b. In addition, the antiproliferative effects of 1,25(OH)2D3 in MCF‐7 cells were significantly abolished by the overexpression of miR‐125b. These results suggest that the endogenous VDR level was repressed by miR‐125b. In conclusion, we found that miR‐125b posttranscriptionally regulated human VDR. Since the miR‐125b level is known to be downregulated in cancer, such a decrease may result in the upregulation of VDR in cancer and augmentation of the antitumor effects of 1,25(OH)2D3. © 2009 UICC