The 3-epi- and 24-oxo-derivatives of 1 alpha,25 dihydroxyvitamin D-3 stimulate transcription through the vitamin D receptor

Vitamin D is enzymatically modified to more than 35 metabolites. While many of these are thought to represent degradation products, some have been shown to exhibit biological activity. We tested whether 3-epi-1 alpha,25-dihydroxyvitamin D-3 (3-epi-1 alpha,25(OH)(2)D-3), 1 alpha,25-dihydroxy-24-oxo-vitamin D-3 (1 alpha,25(OH)(2)-24-oxo-D-3), and 1 alpha,25(OH)(2)D-3-26,23-lactone can stimulate transcription of vitamin D responsive genes. MC3T3-E1 cells transfected with a 25-hydroxyvitamin D 24-hydroxylase (CYP24) promoter construct displayed a 6 fold response when treated with either 1 alpha,25(OH)(2)D-3 or 3-epi-1 alpha,25(OH)(2)D-3. Caco-2 cells were transfected with the wild type CYP24 promoter construct, or a Vitamin D Response Element (VDRE)-mutated form. Cells acquiring the wild type reporter responded to 1 alpha,25(OH)(2)D-3 and 3-epi-1 alpha,25(OH)(2)D-3 but not cells which acquired the mutated reporter. Additionally, VDR-negative COS-7 cells transfected with the wild type promoter responded (approximately 13 fold) to 1 alpha,25(OH)(2)D-3 and 3-epi-1 alpha,25(OH)(2)D-3, only when co-transfected with the VDR. These results were confirmed using shorter incubation times and serum-free conditions. This strongly suggested that 3-epi-1 alpha,25(OH)(2)D-3 mediates its effects through the VDR and its cognate binding site. Similar results were obtained with 1 alpha,25(OH)(2)-24-oxo-D-3 using VDR-negative P19 cells. We could never detect activity from 1 alpha,25(OH)(2)D-3-26,23-lactone on vitamin D-responsive target promoters. Our results firmly conclude that both 3-epi-1 alpha,25(OH)(2)D-3 and the 1 alpha,25(OH)(2)-24-oxo-D-3 elicit their biological effects by acting through the VDR/VDRE. (C) 2000 Elsevier Science Ltd. All rights reserved.