Mitogen-activated protein kinase kinase kinase 7 (MAP3K7), known as TAK1, is an intracellular signaling intermediate of inflammatory responses. However, a series of mouse Tak1 gene deletion analyses have revealed that ablation of TAK1 does not prevent but rather elicits inflammation, which is accompanied by elevation of reactive oxygen species (ROS). This has been considered a consequence of impaired TAK1-dependent maintenance of tissue integrity. Contrary to this view, here we propose that TAK1 inhibition–induced ROS are an active cellular process that targets intracellular bacteria. Intracellular bacterial effector proteins such as Yersinia’s outer membrane protein YopJ are known to inhibit TAK1 to circumvent the inflammatory host responses. We found that such TAK1 inhibition induces mitochondrial-derived ROS, which effectively destroys intracellular bacteria. Two cell death–signaling molecules, caspase 8 and RIPK3, cooperatively participate in TAK1 inhibition–induced ROS and blockade of intracellular bacterial growth. Our results reveal a previously unrecognized host defense mechanism, which is initiated by host recognition of pathogen-induced impairment in a host protein, TAK1, but not directly of pathogens.

TAK1 is an intracellular signaling intermediate of TNF signaling pathway, in which it mediates inflammatory responses and plays a critical role in blocking TNF‐induced cell death. Tak1‐deficiency makes most cell types hyper‐sensitive to TNF‐induced apoptosis, which is mediated by caspase 8 with typical apoptotic features, e.g. caspase 3 activation and membrane blebbing. On the contrary, Tak1‐deficienct macrophages die with features closer to necrosis accompanied with marginal activation of caspases. TNF‐induced necrosis, known as necroptosis, is mediated by a protein kinase complex within which receptor interacted protein kinase 3 (RIPK3) plays a critical role. However, Tak1‐deficient macrophages only marginally activates RIPK3 pathway, and their cell death was not rescued by deletion of Ripk3. In the current study, we attempted to define the mechanism of this atypical Tak1‐deficient macrophage death. We found that mitochondria‐derived reactive oxygen species (ROS) were highly upregulated by ablation of TAK1 in macrophages. Unexpectedly, even though they lack typical apoptosis and necroptosis features, we found that double but not any single deletion of caspase 8 and RIPK3 largely abolished elevation of ROS and cell death in Tak1‐deficient macrophages. Interestingly, blockade of ROS diminished Tak1 deficiency‐induced marginal activation of the caspase and RIPK3 pathways. These demonstrate that ablation of TAK1 elicits a feedforward loop involving caspase 8, RIPK3 and mitochondrial ROS leading to atypical macrophage death. We also characterized mice having Casp8, Ripk3 and Tak1 gene deletion in the in vivo settings. Tak1 deletion caused animal mortality within several days with tissue injuries in the liver and the intestine. Compound double deletion of Casp8 and Ripk3 prolonged Tak1‐deficient animal life for an additional several days, but was insufficient for complete rescue. Thus, caspase 8 and RIPK3‐independent pathway(s) are also involved in Tak1‐deficient animal mortality.Support or Funding InformationThis work was supported by National Institutes of Health Grant GM068812 and GM112986 (J.N‐T). Y.S. and W.L‐P were partly supported by National Institutes of Health Training Grant T32 ES007046.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mitogen activated protein kinase kinase kinase 7 (MAP3K7), known as TAK1, is an intracellular signaling intermediate of inflammatory responses. However, a series of mouse Tak1 gene deletion analysis has revealed that ablation of TAK1 does not prevent but rather elicits inflammation, which is accompanied by elevation of reactive oxygen species (ROS). These have been considered as consequences of non‐physiological loss of TAK1. Contrary to this view, here we propose that TAK1 inhibition‐induced ROS are an active cellular process that targets intracellular bacteria. Intracellular bacterial effector proteins such as Yersinia's outer membrane protein YopJ are known to inhibit TAK1 to circumvent the inflammatory host responses. We found that such TAK1 inhibition particularly induces mitochondrial‐derived ROS, which effectively destroys intracellular bacteria. Our results reveal a previously unrecognized host defense mechanism linking inhibition of inflammatory signaling with mitochondrial ROS.