The mechanism of 1,25-dihydroxyvitamin D3 autoregulation in keratinocytes

The synthesis of 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) from its precursor, 25-dihydroxyvitamin D-3 (25(OH)D-3), is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D-3-1alpha-hydroxylase (1alpha-hydroxylase). It has been generally assumed that 1,25(OH)(2)D-3 inhibits the activity of this enzyme by regulating its expression at the genomic level. We confirmed that 1,25(OH)(2)D-3 reduced the apparent conversion of 1,25(OH)D-3 to 1,25(OH)(2)D-3 while stimulating the conversion of 1,25(OH)(2)D-3 and 25(OH)D-3 to 1,24,25(OH)(3)D-3 and 24,25(OH)(2)D-3, respectively. However, 1,25(OH)(2)D-3 failed to reduce the abundance of its mRNA or its encoded protein in human keratinocytes. Instead, when catabolism of 1,25(OH)(2)D-3 was blocked with a specific inhibitor of the 25-hydroxyvitamin D-3-24-hydroxylase (24-hydroxylase) all apparent inhibition of la-hydroxylase activity by 1,25(OH)(2)D-3 was reversed. Thus, the apparent reduction in la-hydroxylase activity induced by 1,25(OH)(2)D-3 is due to increased catabolism of both substrate and product by the 24-hydroxylase. We believe this to be a unique mechanism for autoregulation of steroid hormone synthesis.