The retinoid X receptor agonist 9-cis-retinoic acid and the 24-hydroxylase inhibitor ketoconazole increase activity of 1,25-dihydroxyvitamin D₃ in human skin in vivo

1,25-Dihydroxyvitamin D₃ [1,25(OH)₂D₃] transactivates its target genes via the vitamin D receptor (VDR). VDR functions in physiology as a dimer complexed with retinoid X receptor (RXR), whose natural ligand is 9-cis-retinoic acid (9-c-RA). Inactivation of 1,25(OH)₂D₃ occurs through a cytochrome p-450 24-hydroxylase (OHase). The promoter of the human 24-OHase gene contains a 1,25(OH)₂D₃-responsive enhancer element (VDRE). We have used this VDRE containing gene as an endogenous reporter for vitamin D₃-mediated gene activation in vivo. Normal adult human skin was keratomed after a 2-d exposure to 1,25(OH)₂D₃, 9-c-RA, all-trans-RA, and ketoconazole. 1,25(OH)₂D₃ caused a concentration-dependent increase in 24-OHase mRNA expression as determined by northern blot analysis. The activity of epidermal 24-OHase was also induced by 1,25(OH)₂D₃. Compared with vehicle, neither of the RA isomers nor ketoconazole alone induced 24-OHase mRNA. Addition of 9-c-RA or t-RA to 1,25(OH)₂D₃, however, caused a synergistic increase in 24-OHase mRNA. Similarly, 1,25(OH)₂D₃ plus ketoconazole increased 24-OHase mRNA synergistically. Ketoconazole inhibited ex vivo 1,25(OH)₂D₃-induced epidermal 24-OHase activity. Thus, 24-OHase mRNA induction is a sensitive reporter of 1,25(OH)₂D₃ activity in vivo; RXR bound to VDR is not a silent partner in vivo, because 9-c-RA enhances 1,25(OH)₂D₃-liganded RXR/VDR stimulation of the VDRE containing 24-OHase gene; ketoconazole inhibition of 24-OHase enhances 1,25(OH)₂D₃ activity by impeding its breakdown. Thus, the synergistic response of human skin to topical 1,25(OH)₂D₃ and/or 1,25(OH)₂D₃ analogs plus RXR retinoids and/or ketoconazole may be exploited to give a desired biologic/therapeutic response with less 1,25(OH)₂D₃, minimizing the potential calcemic risk from systemic absorption of 1,25(OH)₂D₃.