Physiological TLR5 expression in the intestine is regulated by differential DNA binding of Sp1/Sp3 through simultaneous Sp1 dephosphorylation and Sp3 phosphorylation by two different PKC isoforms.

Toll-like receptor 5 (TLR5) expression in the intestinal epithelial cells (IECs) is critical to maintain health, as underscored by multiple intestinal and extra-intestinal diseases in mice genetically engineered for IEC-specific TLR5 knockout. A gradient of expression exists in the colonic epithelial cells from the cecum to the distal colon. Intriguingly, an identical gradient for the dietary metabolite, butyrate also exists in the luminal contents. However, both being critical for intestinal homeostasis and immune response, no studies examined the role of butyrate in the regulation of TLR5 expression. We showed that butyrate transcriptionally upregulates TLR5 in the IECs and augments flagellin-induced immune responses. Both basal and butyrate-induced transcription is regulated by differential binding of Sp-family transcription factors to the GC-box sequences over the TLR5 promoter. Butyrate activates two different protein kinase C isoforms to dephosphorylate/acetylate Sp1 by serine/threonine phosphatases and phosphorylate Sp3 by ERK-MAPK, respectively. This resulted in Sp1 displacement from the promoter and binding of Sp3 to it, leading to p300 recruitment and histone acetylation, activating transcription. This is the first study addressing the mechanisms of physiological TLR5 expression in the intestine. Additionally, a novel insight is gained into Sp1/Sp3-mediated gene regulation that may apply to other genes.