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

Abstract Since the discovery of 1α,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 1α,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 1α,25(OH) 2 -19-nor-D 3 by the enzyme. The kinetic analysis of 1α-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 1α,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 1α,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 1α-hydroxyl group could be developed as drugs for osteoporosis or cancer treatment.