TAK1 is a Central Mediator of NOD2 Signaling and is Essential for Intestinal Epithelial Cell Protection against Chemical-induced Colitis.

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Title: TAK1 is a Central Mediator of NOD2 Signaling and is Essential for Intestinal Epithelial Cell Protection against Chemical-induced Colitis.
Author: Kim, Jae-Young
Advisors: Robert C. Smart, Committee Member
Yoshiaki Tsuji, Committee Member
Jun Ninomiya-Tsuji, Committee Chair
Christian Jobin, Committee Member
Abstract: Innate immunity is the first line of defense against invasive microbial pathogens. It is triggered by several families of pattern-recognition receptors (PRRs), which activate cellular responses including activation of NF-κB and mitogen-activated protein kinases (MAPKs) signaling, and subsequently the induction of proinflammatory genes. There are two groups of PRRs, toll-like receptors (TLRs) and nod-like receptors (NLRs). In contrast to TLRs that recognize microbes via plasma membrane or endosomal receptors, NLRs induce innate immune response by detecting bacterial components released into the cytosol. Nucleotide oligomerization domain 2 (NOD2) is the best characterized member of the NLR family. Upon NOD2 stimulation by muramyl dipeptide (MDP), a NOD2-specific ligand originating from the bacterial cell wall, it associates with an adaptor molecule, RIP-like interacting CLARP kinase (RICK, also called Rip2) and activates both NF-κB and MAPKs signaling pathways, which lead to inflammatory gene expression. However, the molecular mechanism by which NOD2-RICK complex activates its downstream signals remains elusive. Here, we report that TGF β-activated kinase 1 (TAK1) is a central mediator of NOD2 signaling in epidermal cells. TAK1 belongs to MAPKKK family and plays an essential role in activating NF-κB and MAPKs signaling in tumor necrosis factor (TNF), interleukin-1 (IL-1), and TLR signaling pathways. In this study, we found that MDP-induced proinflammatory gene expression as well as the activation of NOD2 downstream signals including NF-κB, c-Jun N-terminal kinase (JNK) and p38 were completely abolished in TAK1-deficient keratinocytes, indicating that TAK1 is essential for eliciting NOD2-induced innate immune responses. NOD2 and its downstream adaptor, RICK, associated with and activated TAK1. Endogenous TAK1-RICK interaction was enhanced by MDP, suggesting that NOD2-RICK-TAK1 complex is induced in an MDP-dependent manner. We have previously reported that intestinal epithelium-specific TAK1 deletion mice showed severe inflammation and mortality at postnatal day 1 due to TNF-induced epithelial cell death. In this study, we investigated TNF-independent role of TAK1 by utilizing mice harboring double deletion of TNF receptor 1 (TNFR1) and intestinal epithelium-specific deletion of TAK1 (TNFR1KO/TAK1IEKO). To study the role of TAK1 in the intestinal epithelial barrier, the mice were subjected to acute colitis by administration of dextran sulfate sodium (DSS). We found that loss of TAK1 significantly augments DSS-induced experimental colitis. DSS-induced weight loss, intestinal damage and inflammatory markers were significantly increased in TNFR1KO/TAK1IEKO mice compared to the TNFR1KO control mice. Following DSS exposure, apoptosis was strongly induced and epithelial cell proliferation was decreased in the TAK1-deficient intestinal epithelium. These results suggest that epithelial-derived TAK1 signaling is important for cytoprotection and tissue repair after injury. Finally, we showed that TAK1 is essential for IL-1- and bacterial components-induced expression of cytoprotective factors including IL-6 and cycloxygenase 2 (COX2). Collectively, we propose that homeostatic cytokines and microbes-mediated intestinal epithelial TAK1 signaling is pivotal for protecting the intestinal epithelium against injury by promoting cell survival and maintaining proliferation. Taken together, this study demonstrates that TAK1 is an essential mediator of NOD2 signaling in epidermal cells, and also suggests that bacterial components- and cytokine-induced TAK1 signaling both play a pivotal role in preventing intestinal inflammation by maintaining the integrity of the intestinal epithelium.
Date: 2009-12-19
Degree: PhD
Discipline: Toxicology
URI: http://www.lib.ncsu.edu/resolver/1840.16/5847

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