Metabolism and Action of 25-Hydroxy-19-nor-Vitamin D3 in Human Prostate Cells

Since the discovery of 1 alpha,25(OH)(2)D-3 in the early 1970s, it has been widely accepted that this metabolite is responsible for the biological actions of vitamin D. Likewise, we have assumed that 25(OH)-19-nor-D-3-dependent growth inhibition of human prostate PZ-HPV-7 cells was the result of its subsequent conversion to 1 alpha,25(OH)(2)-19-nor-D-3, catalyzed by CYP27B1 within the prostate cells. However, further in vitro studies in a reconstituted system using recombinant CYP27B1 revealed that 25(OH)-19-nor-D-3 was hardly converted to 1 alpha,25(OH)(2)-19-nor-D-3 by the enzyme. The kinetic analysis of 1 alpha-hydroxylation of 25(OH)D-3 and 25(OH)-19-nor-D-3 demonstrated that the k(cat)/K-m for 25(OH)-19-nor-D-3 is less than 0.1% of that for 25(OH)D-3. When 25(OH)-19-nor-D-3 was added to cultured PZ-HPV-7 cells, eight metabolites were detected, while no 1 alpha,25(OH) 2-19-nor-D-3 was found. In addition, the time course of VDR translocation into the nucleus induced by 100 nM 25(OH)-19-nor-D-3, and the subsequent transactivation of CYP24A1 gene were almost identical to those induced by 1 nM 1 alpha,25(OH) 2-19-nor-D-3. These results strongly suggest that 25(OH)-19-nor-D-3 binds directly to VDR as a ligand to transport VDR into the nucleus to induce CYP24A1 gene transactivation. Furthermore, knockdown of CYP27B1 gene did not affect the antiproliferative activity of 25(OH)-19-nor-D-3, whereas VDR knockdown attenuated the effect, suggesting that the antiproliferative activity of 25(OH)-19-nor-D-3 is VDR dependent but CYP27B1 independent. Finally, our recent studies using the same cell line demonstrate that 25(OH)D-3 can act as a VDR agonist to induce gene transactivation. These findings suggest that vitamin D analogs without 1 alpha-hydroxyl group could be developed as drugs for osteoporosis or cancer treatment.