title: 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.
abstract: 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.
authors.affiliation: Department of Dermatology, University of Michigan Medical Center, Ann Arbor 48109-0314, U.S.A.
authors.forename: S
authors.initials: S
authors.lastname: Kang
authors.forename: X Y
authors.initials: XY
authors.lastname: Li
authors.forename: E A
authors.initials: EA
authors.lastname: Duell
authors.forename: J J
authors.initials: JJ
authors.lastname: Voorhees
mesh_terms: D000328:Adult; D000077556:Alitretinoin; D002117:Calcitriol / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System; D004357:Drug Synergism; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D007654:Ketoconazole / Q000494:pharmacology*; D018168:Receptors, Retinoic Acid / Q000819:agonists*; D047488:Retinoid X Receptors; D012867:Skin / Q000737:chemistry* / Q000187:drug effects / Q000201:enzymology; D013250:Steroid Hydroxylases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000819:agonists*; D014158:Transcription, Genetic; D014212:Tretinoin / Q000494:pharmacology*; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D018168:Receptors, Retinoic Acid; D047488:Retinoid X Receptors; D014157:Transcription Factors; D000077556:Alitretinoin; D014212:Tretinoin; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D002117:Calcitriol; D007654:Ketoconazole