Targeted deletion of Cyp24a1 in the intestine reduces mucosal injury and preserves epithelial proliferation after 5-fluorouracil treatment.

Vitamin D has been proposed to attenuate chemotherapy-induced gastrointestinal mucositis (GM). In the intestine, local catabolism of active vitamin D [1,25-dihydroxyvitamin D₃] is mediated by the enzyme Cyp24a1. This study assessed whether deletion of Cyp24a1 specifically in intestinal epithelial cells can protect against 5-fluorouracil (5-FU)-induced intestinal injury and microbiome disruption in mice. Using the Cre-loxP system, Cyp24a1 was selectively ablated in the intestinal epithelium (IEC-KO mice). Male IEC-KO and Cyp24a1^fl/fl^ littermate control mice received a single intraperitoneal injection of 5-FU (450mg/kg) or saline and were euthanised 48hours later. In control mice, 5-FU markedly reduced duodenal villous height and crypt area (p < 0.01), whereas IEC-KO mice retained intestinal architecture. Proliferation, measured by Ki-67 immunostaining, was preserved in both the small and large intestine of IEC-KO mice following 5-FU treatment (p < 0.05). Notably, colonic Tlr4 mRNA was significantly upregulated in IEC-KO mice (p < 0.001), with no corresponding increase in inflammatory cytokines. 16S rRNA sequencing revealed no change in overall microbial diversity; however, there were notable differences in the relative abundance of key taxa, such as Bifidobacteriaceae and Alistipes. These findings suggest that intestinal Cyp24a1 contributes to susceptibility to chemotherapy-induced intestinal injury and microbial dysbiosis, and that its deletion enhances epithelial regeneration, potentially via innate immune pathways.