The Mechanism of 1,25-Dihydroxyvitamin D3Autoregulation in Keratinocytes

The synthesis of 1,25-dihydroxyvitamin D3(1,25(OH)2D3) from its precursor, 25-dihydroxyvitamin D3 (25(OH)D3), is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D3-1α-hydroxylase (1α-hydroxylase). It has been generally assumed that 1,25(OH)2D3inhibits the activity of this enzyme by regulating its expression at the genomic level. We confirmed that 1,25(OH)2D3 reduced the apparent conversion of 25(OH)D3 to 1,25(OH)2D3 while stimulating the conversion of 1,25(OH)2D3 and 25(OH)D3 to 1,24,25(OH)3D3 and 24,25(OH)2D3, respectively. However, 1,25(OH)2D3 failed to reduce the abundance of its mRNA or its encoded protein in human keratinocytes. Instead, when catabolism of 1,25(OH)2D3 was blocked with a specific inhibitor of the 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) all apparent inhibition of 1α-hydroxylase activity by 1,25(OH)2D3 was reversed. Thus, the apparent reduction in 1α-hydroxylase activity induced by 1,25(OH)2D3 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.