Title: The Mechanism of 1,25-Dihydroxyvitamin D3Autoregulation in Keratinocytes
Abstract: The synthesis of 1,25-dihydroxyvitamin D<sub>3</sub>(1,25(OH)<sub>2</sub>D<sub>3</sub>) from its precursor, 25-dihydroxyvitamin D<sub>3</sub> (25(OH)D<sub>3</sub>), is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D<sub>3</sub>-1α-hydroxylase (1α-hydroxylase). It has been generally assumed that 1,25(OH)<sub>2</sub>D<sub>3</sub>inhibits the activity of this enzyme by regulating its expression at the genomic level. We confirmed that 1,25(OH)<sub>2</sub>D<sub>3</sub> reduced the apparent conversion of 25(OH)D<sub>3</sub> to 1,25(OH)<sub>2</sub>D<sub>3</sub> while stimulating the conversion of 1,25(OH)<sub>2</sub>D<sub>3</sub> and 25(OH)D<sub>3</sub> to 1,24,25(OH)<sub>3</sub>D<sub>3</sub> and 24,25(OH)<sub>2</sub>D<sub>3</sub>, respectively. However, 1,25(OH)<sub>2</sub>D<sub>3</sub> failed to reduce the abundance of its mRNA or its encoded protein in human keratinocytes. Instead, when catabolism of 1,25(OH)<sub>2</sub>D<sub>3</sub> was blocked with a specific inhibitor of the 25-hydroxyvitamin D<sub>3</sub>-24-hydroxylase (24-hydroxylase) all apparent inhibition of 1α-hydroxylase activity by 1,25(OH)<sub>2</sub>D<sub>3</sub> was reversed. Thus, the apparent reduction in 1α-hydroxylase activity induced by 1,25(OH)<sub>2</sub>D<sub>3</sub> 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.