Lipopolysaccharide Upregulated Intestinal Epithelial Cell Expression of Fn14 and Activation of Fn14 Signaling Amplify Intestinal TLR4-Mediated Inflammation.

Xuefeng Qi,Lijuan Qin,Ruijing Du,Yungang Chen,Meiyu Deng,Mingzhu Lei,Jingyu Wang

doi:10.3389/fcimb.2017.00315

Abstract

TLR4 in intestinal epithelial cells has been shown both inflammatory and homeostatic roles following binding of its cognate ligand lipopolysaccharide (LPS). TWEAK-Fn14 axis plays an important role in pathologies caused by excessive or abnormal inflammatory responses. This study aimed to evaluate potential cross-talk between TLR4 and TWEAK/Fn14 system in porcine small intestinal epithelial cells. Our in vivo results showed that, compared with the age-matched normal control piglets, increased expression of Fn14 in epithelium and decreased TWEAK expression in lamina propria were detected in the small intestinal of piglets stimulated with LPS. Consistent with this finding, treatment with LPS increased the expression of Fn14 and TLR4 while decreased TWEAK expression in porcine small intestinal epithelial cell lines SIEC02. Interestingly, modulating Fn14 activation using agonistic anti-Fn14 decreased TLR4-mediated TNF-α production by SIEC02. In addition, pretreatment of LPS-stimulated SIEC02 with recombinant TWEAK protein suppresses the expression of Fn14 and TNF-α and inhibits the negative impact of LPS on the tight junctional protein occludin expression. In conclusion, this study demonstrates that the TWEAK-independent Fn14 activation augments TLR4-mediated inflammatory responses in the intestine of piglets. Furthermore, the TWEAK-dependent suppression of Fn14 signaling may play a role in intestinal homeostasis.

Highlights

Intestinal epithelial cells play a critical role in the activation and regulation of intestinal inflammation
Very weak tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) staining in the central lacteals and strong factor-inducible molecule 14 (Fn14) staining in the intestinal epithelial cells was observed in the LPS stimulated piglets compared with the age matched untreated control (Figure 1A)
Based on the use of multiple blocking antibodies and siRNAmediated Fn14 knockdown experiments, our principal findings are that stimulation through TLR4 by its cognate ligand LPS, increases the expression of Fn14 in vivo and in vitro, and that activation of Fn14 in intestinal epithelial cells augments TLR4mediated inflammation

Summary

Introduction

Intestinal epithelial cells play a critical role in the activation and regulation of intestinal inflammation. Toll-like receptors (TLRs) are critical components of the innate immune response that detect microorganisms through the recognition of pathogen-associated molecular patterns (PAMPs) (Magalhaes et al, 2007). Studies performed to characterize signal transduction pathways of TLRs in intestinal epithelial cells have shown both inflammatory and homeostatic roles for these receptors following of their cognate ligands binding (Kamdar et al, 2013). TLR4 expressed on intestinal epithelial cells have shown pro-inflammatory role by recognizing lipopolysaccharide (LPS). The intracellular signaling of TLR4 has been mainly divided into TIRAPMyD88 and TRIF-TRAM pathways (Akira and Takeda, 2004; Kawai and Akira, 2010).

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