Background: We recently reported that the gut epithelial vitamin D receptor (VDR) signaling inhibits colitis through inhibition of intestinal epithelial cell apoptosis, and the level of colonic epithelial VDR is markedly reduced in patients with inflammatory bowel diseases (IBD). VDR downregulation promotes colitis, but the mechanism underlying VDR downregulation in IBD is unknown.
Methods: VDR expression was analyzed in colon cancer cells under proinflammatory cytokine treatment. VDR as a target of miR-346 was confirmed using colon cancer cell culture. The relationship among inflammation, miR-346, and VDR was assessed in human IBD biopsies and experimental colitis models.
Results: We showed that tumor necrosis factor α (TNF-α) suppresses VDR expression while simultaneously upregulating miR-346 in human colon cancer cells. Further studies demonstrated that miR-346 inhibits VDR by a specific target sequence in the 3' untranslated region of the human VDR transcript, and blockade of miR-346 with a hairpin inhibitor abrogates the ability of TNF-α to inhibit VDR, confirming that TNF-α downregulates VDR by inducing miR-346. Consistently, in human IBD biopsies, the reduction of epithelial VDR is associated with increased immune cell infiltration and elevation of TNF-α and miR-346. In an experimental model of colitis, mucosal VDR expression is reduced over time with the progression of colitis, inversely correlated with the induction of TNF-α and miR-346 in the mucosa.
Conclusions: These data suggest that during mucosal inflammation TNF-α induces miR-346, which downregulates epithelial VDR. Mucosal VDR reduction in turn compromises the integrity of the mucosal epithelial barrier, further driving mucosal inflammation and colitis development.