The secosteroid hormone, 1 alpha ,25-dihydroxyvitamin D-3 [1 alpha ,25(OH)(2)D-3], induces differentiation of the human promyelocytic leukemia (HL-60) cells into monocytes/macrophages. At present, the metabolic pathways of 1 alpha ,25(OH)(2)D-3 and the biologic activity of its various natural intermediary metabolites in HL-60 cells are not fully understood. 1 alpha ,25(OH)(2)D-3 is metabolized in its target tissues via modifications of both the side chain and the A-ring. The C-24 oxidation pathway, the main side chain modification pathway initiated by hydroxylation at C-24 leads to the formation of the end product, calcitroic acid. The C-23 and C-26 oxidation pathways, the minor side chain modification pathways initiated by hydroxylations at C-23 and C-26 respectively together lead to the formation of the end product, 1 alpha ,25(OH)(2)D-3-lactone. The C-3 epimerization pathway, the newly discovered A-ring modification pathway is initiated by epimerization of the hydroxyl group at C-3 to form 1 alpha ,25-dihydroxy-3-epi-vitamin-D-3. We performed the present study first to examine in detail the metabolism of 1 alpha ,25(OH)(2)D-3 in HL-60 cells and then to assess the ability of the various natural intermediary metabolites of 1 alpha ,25(OH)(2)D-3 in inducing differentiation and in inhibiting clonal growth of HL-60 cells. We incubated HL-60 cells with [1 beta-H-3] 1 alpha ,25(OH)(2)D-3 and demonstrated that these cells metabolize 1 alpha ,25(OH)(2)D-3 mainly via the C-24 oxidation pathway and to a lesser extent via the C-23 oxidation pathway, but not via the C-3-epimerization pathway. Three of the natural intermediary metabolites of 1 alpha ,25(OH)(2)D-3 derived via the C-24 oxidation pathway namely, 1 alpha ,24(R),25-trihydroxyvitamin D-3, 1 alpha ,25-dihydroxy-24-oxovitamin D-3 and 1 alpha ,23(S),25-trihydroxy-24-oxovitamin D-3 [1 alpha ,23(S),25(OH)(3)-24-oxo-D-3] were almost as potent as 1 alpha ,25(OH)(2)D-3 in terms of their ability to differentiate HL-60 cells into monocytes/macrophages. We then selected 1 alpha ,23(S),25(OH)(3)-24-oxo-D-3 which has the least calcemic activity among all the three aforementioned natural intermediary metabolites of 1 alpha ,25(OH)(2)D-3 to examine further its effects on these cells. Our results indicated that 1 alpha ,23(S),25(OH)(3)-24-oxo-D-3 was also equipotent to it!; parent in inhibiting clonal growth of HL-60 cells and in inducing expression of CD11b protein. In summary, we report that 1 alpha ,25(OH)(2)D-3 is metabolized in HL-60 cells into several intermediary metabolites derived via both the C-24 and C-23 oxidation pathways but not via the C-3 epimerization pathway. Some of the intermediary metabolites derived via the C-24 oxidation pathway are found to be almost equipotent to 1 alpha ,25(OH)(2)D-3 in modulating growth and differentiation of HL-60 cells. In a previous study, the same metabolites when compared to 1 alpha ,25(OH)(2)D-3 were found to be less calcemic. Thus, the findings of our study suggest that some of the natural metabolites of 1 alpha ,25(OH)(2)D-3 may be responsible for the final expression of the noncalcemic actions that are presently being attributed to their parent, 1 alpha ,25(OH)(2)D-3. (C) 2001 Elsevier Science Inc. All rights reserved.
