title: Ketoconazole inhibits self-induced metabolism of 1,25-dihydroxyvitamin D3 and amplifies 1,25-dihydroxyvitamin D3 receptor up-regulation in rat osteosarcoma cells.
abstract: Ketoconazole (an inhibitor of vitamin D-24 hydroxylase) was used to study the role of self-induced 1,25-dihydroxyvitamin D3 (1,25-D3) metabolism on cellular responsiveness to 1,25-D3. Eighteen hours of treatment with 1,25-dihydroxy-[26,27-methyl-3H]vitamin D3 (1,25-[3H]D3) increased total 1,25-D3 receptors (VDR) from 60 to 170 fmol mg/protein. In cells treated with both 1,25-[3H]D3 and ketoconazole, up-regulation of VDR was increased by 40% over that observed with cells receiving 1,25-[3H]D3 alone. Ketoconazole alone had no agonistic activity. Treatment of cells with 1 nM 1,25-[3H]D3 plus increasing doses of ketoconazole (0-30 microM) resulted in a dose-dependent increase in occupied VDR and total VDR. This up-regulation was associated with reduced 1,25-[3H]D3 catabolism. 1,25-[3H]D3-induced up-regulation of VDR typically peaked at 14 h and declined thereafter. Ketoconazole lengthened the time to reach peak VDR up-regulation to 20 h. The ability of ketoconazole to increase cell responsiveness (VDR up-regulation) was the result of both increased and prolonged occupancy of VDR by 1,25-[3H]D3. The t1/2 of occupied VDR was 2 h in the absence of ketoconazole and greater than 7 h when ketoconazole was present. Collectively, these results suggested that self-induced catabolism of 1,25-D3 is an important regulator of VDR occupancy and therefore cellular responsiveness to hormone. These data also demonstrate the usefulness of ketoconazole as an inhibitor of vitamin D hydroxylases in intact cells.
authors.affiliation: National Animal Disease Center, U.S. Department of Agriculture, Ames, Iowa 50010.
authors.forename: T A
authors.initials: TA
authors.lastname: Reinhardt
authors.forename: R L
authors.initials: RL
authors.lastname: Horst
mesh_terms: D000818:Animals; D002117:Calcitriol / Q000378:metabolism*; D007654:Ketoconazole / Q000494:pharmacology*; D012516:Osteosarcoma / Q000378:metabolism*; D051381:Rats; D018167:Receptors, Calcitriol; D011987:Receptors, Steroid / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D018167:Receptors, Calcitriol; D011987:Receptors, Steroid; D002117:Calcitriol; D007654:Ketoconazole