Tissue specific metabolism of 1α,25‐dihydroxy‐20‐epi‐vitamin D3 into new metabolites with significant biological activity: Studies in rat osteosarcoma cells (UMR 106 and ROS 17/2.8)

In a recent study, we investigated the metabolism of 1α,25‐dihydroxy‐20‐epi‐vitamin D3 (1α,25(OH)2‐20‐epi‐D3), a potent synthetic vitamin D3 analog in the isolated perfused rat kidney and proposed that the enhanced biological activity of 1α,25(OH)2‐20‐epi‐D3 is in part due to its metabolism into stable bioactive intermediary metabolites derived via the C‐24 oxidation pathway (Siu‐Caldera et al. [ 1999 ] J. Steroid. Biochem. Mol. Biol. 71:111–121). It is now well established that 1α,25(OH)2D3 and its analogs are metabolized in target tissues not only via the C‐24 oxidation pathway but also via the C‐3 epimerization pathway. As the perfused rat kidney does not express the C‐3 epimerization pathway, we could not identify other possible bioactive metabolites of 1α,25(OH)2‐20‐epi‐D3 such as 1α,25(OH)2‐20‐epi‐3‐epi‐D3, derived via the C‐3 epimerization pathway. Therefore, we studied the metabolism of 1α,25(OH)2‐20‐epi‐D3 in rat osteosarcoma cells (UMR 106) which express both the C‐24 oxidation and the C‐3 epimerization pathways. Our results indicate that 1α,25(OH)2‐20‐epi‐D3 is metabolized in UMR 106 cells into several metabolites which included not only the previously known metabolites of the C‐24 oxidation pathway but also three new metabolites which were labeled as metabolites X, Y1, and Y2. Metabolite X was unequivocally identified as 1α,25(OH)2‐20‐epi‐3‐epi‐D3. Eventhough definite structure identification of the metabolites, Y1 and Y2 was not achieved in our present study, we determined that the metabolite Y1 is produced from 1α,25(OH)2‐20‐epi‐D3 and the metabolite Y2 is produced from 1α,25(OH)2‐20‐epi‐3‐epi‐D3. We also noted the production of both 1α,25(OH)2‐20‐epi‐3‐epi‐D3 and the two metabolites Y1 and Y2 in different rat osteosarcoma cells (ROS 17/2.8) which express only the C‐3 epimerization pathway but not the C‐24 oxidation pathway. Furthermore, we investigated the metabolism of 1α,25(OH)2‐20‐epi‐D3 in the isolated perfused rat kidney in an earlier study. The results of this study indicated that the rat kidney unlike rat osteosarcoma cells did not produce either 1α,25(OH)2‐20‐epi‐3‐epi‐D3 or the metabolites Y1 and Y2. Thus, it appears that the metabolites Y1 and Y2, like 1α,25(OH)2‐20‐epi‐3‐epi‐D3, are produced only in specific tissues. Preliminary biological activity of each new metabolite is assessed by measuring its ability to generate VDR‐mediated gene transcription. 1α,25(OH)2‐20‐epi‐3‐epi‐D3 was found to be almost equipotent to 1α,25(OH)2‐20‐epi‐D3 while the metabolites, Y1 and Y2 were found to be less active. The metabolite Y1 when compared to the metabolite Y2 has higher biological activity and its potency is almost equal to 1α,25(OH)2D3. In summary, we report for the first time tissue specific metabolism of 1α,25(OH)2‐20‐epi‐D3 into several bioactive metabolites which are derived not only via the previously established C‐24 oxidation and C‐3 epimerization pathways but also via a new pathway. J. Cell. Biochem. 82: 599–609, 2001. © 2001 Wiley‐Liss, Inc.