In Vitro Metabolism of 19-Nor-1α,25-(OH)2D2 in Cultured Cell Lines: Inducible Synthesis of Lipid- and Water-Soluble Metabolites

Abstract The active vitamin D analog, 19-nor-1α,25-dihydroxyvitamin D 2 (19-nor-1α,25-(OH) 2 D 2 ), has a similar structure to the natural vitamin D hormone, 1α,25-dihydroxyvitamin D 3 (1α,25-(OH) 2 D 3 ), but lacks the C10-19 methylene group and possesses an ergosterol/vitamin D 2 rather than a cholesterol/vitamin D 3 side chain. We have used this analog to investigate whether any of these structural features has any effect upon the type and rate of in vitro metabolism observed. Using a vitamin D-target cell, the human keratinocyte, HPK1A-ras, we observed formation of a number of metabolites, three of which were purified by extensive HPLC and conclusively identified by a combination of GC-MS and chemical derivatization as 19-nor-1α,24,25-(OH) 3 D 2 , 19-nor-1α,24,25,26-(OH) 4 D 2 , and 19-nor-1α,24,25,28-(OH) 4 D 2 . The first metabolite is probably a product of the vitamin D-inducible cytochrome P450, P450cc24 (CYP24), while the latter two metabolites are likely to be further metabolic products of 19-nor-1α,24,25-(OH) 3 D 2 . These hydroxylated metabolites resemble those identified by other workers as products of the metabolism of 1α,25-(OH) 2 D 2 in the perfused rat kidney. It therefore appears from the similar metabolic fate of 19-nor-1α,25-(OH) 2 D 2 and 1α,25-(OH) 2 D 2 that the lack of the C10–19 methylene group has little effect upon the nature of the lipid-soluble metabolic products and the rate of formation of these products seems to be comparable to that of products of 1α,25-(OH) 2 D 3 in vitamin D-target cells. We also found extensive metabolism of 19-nor-1α,25-(OH) 2 D 2 to water-soluble metabolites in HPK1A-ras, metabolites which remain unidentified at this time. When we incubated 19-nor-1α,25-(OH) 2 D 2 with the liver cell line HepG2, we obtained only 19-nor-1α,24,25-(OH) 3 D 2 . We conclude that 19-nor-1α,25-(OH) 2 D 2 is efficiently metabolized by both vitamin D-target cells and liver cells.