Transforming growth factor ␤-activated kinase 1 (TAK1) functions downstream of inflammatory cytokines to activate c-Jun N-terminal kinase (JNK) as well as NF-B in several cell types. However, the functional role of TAK1 in an in vivo setting has not been determined. Here we have demonstrated that TAK1 is the major regulator of skin inflammation as well as keratinocyte death in vivo. Epidermal-specific deletion of TAK1 causes a severe inflammatory skin condition by postnatal day 6 -8. The mutant skin also exhibits massive keratinocyte death. Analysis of keratinocytes isolated from the mutant skin revealed that TAK1 deficiency results in a striking increase in apoptosis in response to tumor necrosis factor (TNF). TAK1-deficient keratinocytes cannot activate NF-B or JNK upon TNF treatment. These results suggest that TNF induces TAK1-deficient keratinocyte death because of the lack of NF-B (and possibly JNK)-mediated cell survival signaling. Finally, we have shown that deletion of the TNF receptor can largely rescue keratinocyte death as well as inflammatory skin condition in epidermal-specific TAK1-deficient mice. Our results demonstrate that TAK1 is a master regulator of TNF signaling in skin and regulates skin inflammation and keratinocyte death. TAK12 (transforming growth factor ␤-activated kinase 1) is a member of the mitogen-activated protein kinase kinase kinase family and is activated by inflammatory cytokines interleukin 1 (IL-1) and tumor necrosis factor (TNF) and Toll-like receptor ligands (1, 2). In IL-1, TNF, and Toll-like receptor ligand signaling pathways, TAK1 has been shown to be an essential signaling intermediate that functions upstream of IB kinase (IKK)-NF-B and c-Jun N-terminal kinase (JNK) in B cells and some culture cells (3-5). However, the role of TAK1 has not been established in an in vivo context due to embryonic death of TAK1 germ line knock out (3, 4).Skin homeostasis is maintained through a well balanced interplay of cytokines and growth factors (6). Several cytokines, including TNF, activate JNK and NF-B pathways (7, 8) that play critical roles in epidermal homeostasis involving skin inflammation and cancer development (9 -16). Inactivation of IKK␤ or IKK␥, which blocks the so-called canonical NF-B pathway, produces a severe inflammatory skin condition. Furthermore, NF-B hypofunction is implicated in epidermal squamous cell carcinoma. In contrast, activation of JNK pathway is involved in epidermal hyperplasia and subsequent cancer development (14, 15). Roles of IKK-NF-B and JNK in skin have been demonstrated by using genetic and pharmacological inhibitory approaches (12,14,15). However, the upstream regulators of NF-B and JNK pathways in skin have not yet been determined. In this study, we generated and characterized mice with epidermal-specific deletion of TAK1. We found that TAK1 is an essential intermediate in TNF signaling to activate both IKK and JNK in keratinocytes. TAK1 deficiency causes severe dysregulation of skin homeostasis. Our results suggest that the dysregulation in TA...

Mice with a keratinocyte-specific deletion of Tak1 exhibit severe skin inflammation due to hypersensitivity to tumor necrosis factor (TNF) killing. Here we have examined the mechanisms underlying this hypersensitivity. We found that TAK1 deficiency up-regulates reactive oxygen species (ROS) resulting in cell death upon TNF or oxidative stress challenge. Because blockade of NF-B did not increase ROS or did not sensitize cells to oxidative stress in keratinocytes TAK1 regulates ROS mainly through the mechanisms other than those mediated by NF-B. We found that c-Jun was decreased in TAK1-deficient keratinocytes and that ectopic expression of c-Jun could partially inhibit TNF-induced increase of ROS and cell death. Finally, we show that, in an in vivo setting, the antioxidant treatment could reduce an inflammatory condition in keratinocyte-specific Tak1 deletion mice. Thus, TAK1 regulates ROS partially through c-Jun, which is important for preventing ROS-induced skin inflammation. Tumor necrosis factor (TNF)2 plays a central role in inflammation, and also regulates cell death and survival (1-3). TNF initiates intracellular signaling by binding to its receptor, initiating the formation of the TNF receptor complex, which consists of several proteins including RIP1 kinase, Fas-associated death domain (FADD), and pro-caspase-8 (also called FLICE). The TNF receptor complex in turn activates two opposing intracellular signaling pathways; one leads to up-regulation of the expression of anti-apoptotic genes such as cellular FLICEinhibitory protein (c-FLIP) (4) and caspase inhibitor IAPs (inhibitor of apoptosis proteins) (5) (anti-cell death pathway); another activates the caspase cascade to execute apoptotic cell death (pro-cell death pathway). TNF-induced activation of the transcription factor NF-B is one of the major pathways of antiapoptotic gene up-regulation and cell death inhibition. NF-B is activated upon degradation of IB (inhibitor of NF-B), which is induced by its phosphorylation by IB kinases (6).The pro-cell death pathway is activated through FADD (1-3). FADD recruits the apoptosis-initiating protease caspase-8, which is in turn autoactivated by proteolysis. Caspase-8 cleaves and activates executor caspases such as caspase-3. c-FLIP is a specific inhibitor of caspase-8 (7). Additionally, the TNF receptor complex activates NADPH oxidase 1 and increases ROS production (8). ROS causes prolonged activation of JNK (9, 10). Prolonged JNK activation down-regulates the E3 ubiquitin ligase Itch, which degrades c-FLIP (11,12). This ROS-JNK-mediated c-FLIP degradation facilitates the activation of caspase-8 (12). The ROS-facilitated caspase pathway is believed to be the major pathway of TNF-induced cell death.TAK1 kinase is a member of the mitogen-activated protein kinase kinase kinase family and is activated by innate immune stimuli including bacterial components and proinflammatory cytokines such as interleukin-1 and TNF (13,14). TAK1 is an ubiquitin-dependent kinase and plays an essential role in innate immune signaling...

Activation of the TAK1 kinase drives RIPK3-dependent necrosis and inhibits apoptosis downstream of TNF-α stimulation.

Muramyl dipeptide (MDP) is a peptidoglycan moiety derived from commensal and pathogenic bacteria, and a ligand of its intracellular sensor NOD2. Mutations in NOD2 are highly associated with Crohn disease, which is characterized by dysregulated inflammation in the intestine. However, the mechanism linking abnormality of NOD2 signaling and inflammation has yet to be elucidated. Here we show that transforming growth factor ␤-activated kinase 1 (TAK1) is an essential intermediate of NOD2 signaling. We found that TAK1 deletion completely abolished MDP-NOD2 signaling, activation of NF-B and MAPKs, and subsequent induction of cytokines/chemokines in keratinocytes. NOD2 and its downstream effector RICK associated with and activated TAK1. TAK1 deficiency also abolished MDP-induced NOD2 expression. Because mice with epidermisspecific deletion of TAK1 develop severe inflammatory conditions, we propose that TAK1 and NOD2 signaling are important for maintaining normal homeostasis of the skin, and its ablation may impair the skin barrier function leading to inflammation.