Intestinal tight junction CLDN2 gene is a direct target of the vitamin D receptor

Abstract

The breakdown of intestinal barrier is a common manifestation of GI disorders. Claudin‐2 is a tight junction protein that mediates paracellular water transport in leaky epithelium. Recent evidence suggests that vitamin D and vitamin D receptor (VDR) may regulate functions of tight junction proteins. However, it is unknown how Claudin‐2 is regulated by VDR signaling in normal and inflamed intestine. Using whole body VDR‐/‐ mice, intestinal epithelial VDR conditional knockout (VDRΔIEC) mice, human samples, and cultured human intestinal epithelial cells (IEC), we performed a series of molecular and biochemical experiments in vivo and in vitro. We provide evidence that the CLDN2 gene is a direct target of the transcription factor VDR. VDR‐inducing Claudin‐2 promoter activity required the Cdx binding site on the promoter of the CLDN2 gene. We further identify a functional vitamin D response element in the Claudin‐2 promoter. In vivo, VDR deletion in intestinal epithelial cells (IEC) led to decreased Claudin‐2 at both mRNA and protein levels. Functionally, we found a robust increase of Claudin‐2 in inflammatory IECs which lacked VDR regulation and allowed the inflammatory cytokines to take over. Furthermore, in inflamed intestine of ulcerative colitis patients, VDR expression is low and Claudin‐2 is enhanced. A lack of intestinal VDR regulation leads to dysfunction of Claudin‐2 in inflammatory responses. This study reveals a complex role and novel mechanism for intestinal VDR by regulation of epithelial barriers and inflammation.