Interleukin-33 delays recovery of mucosal inflammation via downregulation of homeostatic ABCG5/8 in the colon

Previous studies have suggested that interleukin-33 (IL-33) is involved in the pathogenesis of ulcerative colitis (UC), though the detailed mechanisms are not fully known. We investigated IL-33-mediated colonic homeostasis using a mechanistic method. Il33(-/-) mice were more tolerant to dextran sulfate sodium-induced acute colitis than the wild type and also showed delayed recovery from colitis with recombinant IL-33 (rIL-33) administration. Unexpectedly, microarray analysis identified significant downregulation of the Abcg5/8 genes in mouse colons following rIL-33 treatment. ABCG5/8 are known cholesterol transporters in the small intestine and liver, though their colon activities have not been elucidated, thus their role in IL-33-mediated inflammation was investigated. In vitro, toll-like receptor (TLR) stimulation upregulated ABCG5/8 mRNA expression in Caco2 and HCT-15 cells, with subsequent downregulation by rIL-33, while inhibition of ABCG5/8 along with their siRNA increased TLR-stimulated IL-8 production. Together, these results indicated that colonic ABCG5/8 play a regulatory role in TLR-induced inflammation, while histological inflammation in human UC was correlated positively with the level of mucosal IL-33 and inversely with that of colonic ABCG5/8. This is the first report of IL-33-mediated downregulation of colonic ABCG5/8 in a colitis recovery phase, indicating their involvement in UC pathogenesis and potential as a therapeutic target. The authors investigated the potential mechanisms of interleukin-33 (IL-33) in the pathogenesis of ulcerative colitis. IL-33 was found to sustain mucosal inflammation by down-regulation of protective ABCG5/8 during recovery from colitis. These results may help to increase understanding regarding the IL-33-mediated pathogenesis of ulcerative colitis and potentially contribute to creation of a novel therapeutic strategy.