Vitamin D analogs affect the uptake and metabolism of retinol by human epidermal keratinocytes in culture.

Human epidermis utilizes retinol as precursor for local production of a range of bioactive vitamin A metabolites including 3,4-didehydroretinol, retinoic acid, and 3,4-didehydroretinoic acid. These endogenously formed retinoids bind to nuclear retinoic acid receptors (RARs), thereby altering gene transcription. Because 9-cis-retinoic acid receptors (RXRs) form heterodimers both with RARs and the vitamin D3 receptor (VDR), it is plausible that vitamin D3 may affect retinol metabolism if altered transcription is involved in the regulation of vitamin A-metabolizing enzymes. To investigate the potential effect of vitamin D on retinol metabolism in human skin keratinocytes, HaCaT cells were preincubated with various vitamin D3-analogs at 10(-7)M for 24 h followed by the addition of [3H]retinol for another 24 h period. The uptake and metabolism of the radioactive tracer was monitored by HPLC-radiochromatography. It was found that all synthetic vitamin D-analogs tested (MC903, KH1060, EB1089, and EB1213) reduced the amount of cell-associated [3H]retinoid activity by 35-50% as compared to the vehicle. More specifically, the appearance of the parent substrate and two of its main metabolites, e.g., 3,4-didehydroretinol (ddROH) and 3,4-didehydroretinoic acid (ddRA), was inhibited by the synthetic vitamin D-analogs. The effects on retinol metabolism were not potentiated by coincubation of cells with vitamin D-analogs plus retinoic acid (RA) or 9-cis-RA. This study demonstrates that synthetic vitamin D3 interferes with both the uptake and the metabolism of retinol by human epidermal keratinocytes. Whether the effects are due to direct inhibition of cellular retinol uptake and metabolism or involve VDR-mediated transcriptional alteration of vitamin A metabolizing enzymes remains to be clarified.