Altered pharmacokinetics of 1α,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 in the blood and tissues of the 25-hydroxyvitamin D-24-hydroxylase (Cyp24a1) null mouse

The 25-hydroxyvitamin D-24-hydroxylase (CYP24A1) plays an important role in regulating concentrations of both the precursor 25-hydroxyvitamin D-3 [25(OH)D-3] and the hormone 1alpha,25-dihydroxyvitamin D-3 [1alpha,25(OH)(2)D-3]. Previous studies suggest that Cyp24a1-null mice cannot clear exogenous 1alpha,25(OH)(2)D-3 efficiently. Here, we examined the metabolic clearance in Cyp24a1-null mice in vivo and in vitro using a physiological dose of [1beta-H-3]1alpha,25(OH)(2)D-3 or [26,27-methyl-H-3] 25(OH)D-3. Cyp24a1-null mice showed difficulty in eliminating [1beta-H-3]1alpha,25(OH)(2)D-3 from the bloodstream and tissues over a 96-h time course, whereas heterozygotic mice eliminated the hormone within 6-12 h, although there was clearance of labeled hormone into water-soluble products involving liver in both genotypes. RT-PCR showed that Cyp24a1-null mice have decreased expression of 25-hydroxyvitamin D-1alpha-hydroxylase that must play a role in their survival. After the administration of [26,27-methyl-H-3]25(OH)D-3, Cyp24a1-null mice showed higher [26,27-methyl-H-3]25(OH)D-3 levels and no [26,27-methyl-H-3]24,25(OH)(2)D-3 formation, whereas heterozygotic mice showed significant [26,27-methyl-H-3]24,25(OH)(2)D-3 production. Based upon in vitro experiments, keratinocytes from Cyp24a1-null mice fail to synthesize [1beta-H-3] calcitroic acid from [1beta-H-3] 1alpha,25(OH)(2)D-3 or [26,27-methyl-H-3]24,25(OH)(2)D-3 from [26,27-methyl-H-3]25(OH)D-3 as do control mice, confirming the target cell catabolic role of CYP24A1 in these processes. Finally, the role of vitamin D receptor (VDR) in the vitamin D catabolic cascade was examined using VDR-null mice. Keratinocytes from VDR-null mice failed to metabolize [1beta-H-3]1alpha,25(OH)(2)D-3 confirming the importance of vitamin D-inducible, VDR-mediated, C24 oxidation pathway in target cells. These results suggest that the absence of CYP24A1 or VDR retards catabolism of 1alpha,25(OH)(2)D-3 and 25(OH)D-3, reinforcing the physiological importance of CYP24A1 in vitamin D homeostasis.