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

Since the discovery of 1α,25(OH)₂D₃ 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-D₃-dependent growth inhibition of human prostate PZ-HPV-7 cells was the result of its subsequent conversion to 1α,25(OH)₂-19-nor-D₃, 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-D₃ was hardly converted to 1α,25(OH)₂-19-nor-D₃ by the enzyme. The kinetic analysis of 1α-hydroxylation of 25(OH)D₃ and 25(OH)-19-nor-D₃ demonstrated that the k_cat/K_m for 25(OH)-19-nor-D₃ is less than 0.1% of that for 25(OH)D₃. When 25(OH)-19-nor-D₃ was added to cultured PZ-HPV-7 cells, eight metabolites were detected, while no 1α,25(OH)₂-19-nor-D₃ was found. In addition, the time course of VDR translocation into the nucleus induced by 100 nM 25(OH)-19-nor-D₃, and the subsequent transactivation of CYP24A1 gene were almost identical to those induced by 1 nM 1α,25(OH)₂-19-nor-D₃. These results strongly suggest that 25(OH)-19-nor-D₃ 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-D₃, whereas VDR knockdown attenuated the effect, suggesting that the antiproliferative activity of 25(OH)-19-nor-D₃ is VDR dependent but CYP27B1 independent. Finally, our recent studies using the same cell line demonstrate that 25(OH)D₃ 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.