The impact of inducible-whole body or intestine-specific Cyp24a1 gene knockout on vitamin D metabolism in mice

Expression of 25 hydroxyvitamin D 24 hydroxylase from the Cyp24a1 gene mediates 1,25 dihydroxyvitamin D (1,25(OH)2D) catabolism but gaps exist in our understanding of this enzyme's physiologic importance. Here, we used tamoxifen to induce Cyp24a1 gene knockout (KO) in adult mice (50 mg Tamoxifen /g BW, ip, 5 d, at 11 wks of age) or intestinal-epithelial-cell-specific knock-out mice (IEC KO) to evaluate the role of CYP24A1 in adult mice and the contribution of the intestine to vitamin D (Vit D) metabolism. At 12-wks mice were euthanized and serum was analyzed for Vit D metabolites by LC MS/MS while duodenal (Dd) and kidney (Kd) mRNA levels were quantified using qPCR. Adult Cyp24a1 KO mice had higher 25 hydroxyvitamin D (25(OH)D, + 185 %) and 1,25 (OH)2D (+41.4 %) levels and reduced levels of 1,24,25(OH)3D (-53.7 %). No changes in serum Vit D metabolites were seen in adult mice lacking one Cyp24a1 allele (HT). In kidney, compensatory changes in Cyp27b1 mRNA (-85.3 % in KO,- 36.5 % in HT) and Cyp24a1 mRNA (+147 % in KO, +43 % in HT) were observed. No changes in Dd Trpv6 or S100g mRNA were observed and Dd Cyp3a13 mRNA did not compensate for Cyp24a1 gene loss. Neither serum Vit D metabolites nor Dd Trpv6 and S100g mRNA were changed in IEC KO mice but there was a trend towards elevated renal Cyp24a1 mRNA (+61 %, p = 0.06). Our data in adult KO mice indicate that CYP24A1 has an important physiologic impact on Vit D metabolism while IEC KO data suggests that local degradation of the hormone by CYP24A1 is not a strong regulator of intestinal Vit D action or systemic vitamin D metabolism.