Background:Intestinal epithelial cells (IECs) express low levels of TLR4 and are hyporesponsive to commensal bacteria. Results: TLR4 gene is methylated in IECs, and this process is dependent on commensal bacteria in the large intestine. Conclusion: Commensal bacteria control epigenetic modification of the host gene. Significance: This study shows a novel mechanism underlying the maintenance of intestinal symbiosis.

α-Defensin 5 is important to both maintenance of a gut microbiota and host immunity. While previous reports have shown that gut bacteria are able to upregulate α-defensin 5 through Toll-like receptor signaling, we demonstrate here that α-defensin 5 expression can also be regulated by microbial metabolites. Among these, lactate appeared to significantly suppress α-defensin 5 gene transcription. Actually, fractions of <3 kD compounds obtained from the ceca of SPF mice were suppressed α-defensin 5 gene transcription at specific concentrations. Our results also suggest that cecal content may include as yet unidentified factors that can enhance α-defensin 5 expression. Our data point to a novel function for the gut microbial metabolites in controlling the expression of antimicrobial peptides in the intestine.

Immune responses against gut microbiota should be minimized to avoid unnecessary inflammation at mucosal surface. In this study, we analyzed the expression patterns of Toll-interacting protein (Tollip), an inhibitor of TLRs and IL-1 family cytokine-related intracellular signaling, in intestinal epithelial cells (IECs). Comparable mRNA expression was observed in murine small and large IECs (S-IECs and L-IECs). However, Tollip protein was only detected in L-IECs, but not in S-IECs. Similar results were obtained in germ-free mice, indicating that L-IEC-specific TOLLIP expression does not depend on bacterial colonization. Next, to understand the mechanisms underlying the post-transcriptional repression of Tollip, 3´-UTR-mediated translational regulation was evaluated. The region +1876/+2398 was responsible for the repression of Tollip expression. This region included the target sequence of miR-31. The inhibition of miR-31 restored the 3´-UTR-meditaed translational repression. In addition, miR-31 expression was significantly higher in S-IECs than in L-IECs, suggesting that miR-31 represses the translation of Tollip mRNA in S-IECs. Collectively, we conclude that the translation of Tollip is inhibited in S-IECs, at least in part, by miR-31 to yield L-IEC-specific high-level expression of the Tollip protein, which may contribute to the maintenance of intestinal homeostasis.

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