A20 prevents chronic liver inflammation and cancer by protecting hepatocytes from death.

An important regulator of inflammatory signalling is the ubiquitin-editing protein A20 that acts as a break on nuclear factor-κB (NF-κB) activation, but also exerts important cytoprotective functions. A20 knockout mice are cachectic and die prematurely due to excessive multi-organ inflammation. To establish the importance of A20 in liver homeostasis and pathology, we developed a novel mouse line lacking A20 specifically in liver parenchymal cells. These mice spontaneously develop chronic liver inflammation but no fibrosis or hepatocellular carcinomas, illustrating an important role for A20 in normal liver tissue homeostasis. Hepatocyte-specific A20 knockout mice show sustained NF-κB-dependent gene expression in the liver upon tumor necrosis factor (TNF) or lipopolysaccharide injection, as well as hepatocyte apoptosis and lethality upon challenge with sublethal doses of TNF, demonstrating an essential role for A20 in the protection of mice against acute liver failure. Finally, chronic liver inflammation and enhanced hepatocyte apoptosis in hepatocyte-specific A20 knockout mice was associated with increased susceptibility to chemically or high fat-diet-induced hepatocellular carcinoma development. Together, these studies establish A20 as a crucial hepatoprotective factor.

Keratinocyte-specific ablation of the NF-κB regulatory protein A20 (TNFAIP3) reveals a role in the control of epidermal homeostasis.

The ubiquitin-editing enzyme A20 (tumor necrosis factor-α-induced protein 3) serves as a critical brake on nuclear factor κB (NF-κB) signaling. In humans, polymorphisms in or near the A20 gene are associated with several inflammatory disorders, including psoriasis. We show here that epidermis-specific A20-knockout mice (A20(EKO)) develop keratinocyte hyperproliferation, but no signs of skin inflammation, such as immune cell infiltration. However, A20(EKO) mice clearly developed ectodermal organ abnormalities, including disheveled hair, longer nails and sebocyte hyperplasia. This phenotype resembles that of mice overexpressing ectodysplasin-A1 (EDA-A1) or the ectodysplasin receptor (EDAR), suggesting that A20 negatively controls EDAR signaling. We found that A20 inhibited EDAR-induced NF-κB signaling independent from its de-ubiquitinating activity. In addition, A20 expression was induced by EDA-A1 in embryonic skin explants, in which its expression was confined to the hair placodes, known to be the site of EDAR expression. In summary, our data indicate that EDAR-induced NF-κB levels are controlled by A20, which functions as a negative feedback regulator, to assure proper skin homeostasis and epidermal appendage development.

Prolonged exposure to IL-1beta and IFNgamma induces necrosis of L929 tumor cells via a p38MAPK/NF-kappaB/NO-dependent mechanism.

Interleukin-1beta (IL-1beta) is a cytokine that shares with tumor necrosis factor (TNF) the ability to initiate largely similar signaling pathways, leading to proinflammatory gene expression. In contrast to TNF, however, IL-1beta is not believed to induce tumor cell death. Here we demonstrate that prolonged treatment with IL-1beta, in combination with interferon-gamma (IFNgamma), is cytotoxic for L929 tumor cells. IL-1beta/IFNgamma-induced cytotoxicity requires only minimal amounts of IL-1beta and shows morphological features of necrosis. Although TNF induces a similar response, we could exclude a contribution of endogenous TNF production in the effect of IL-1beta/IFNgamma. Cell death in response to IL-1beta/IFNgamma is independent of caspases, but requires the IL-1beta/IFNgamma-induced production of inducible nitric oxide synthase (iNOS) and NO. Moreover, necrosis and iNOS/NO production could be prevented by treatment of the cells with a p38 mitogen activated protein kinase (p38MAPK) or IkappaB kinase beta inhibitor. Altogether, these findings demonstrate that prolonged exposure to IL-1beta plus IFNgamma induces L929 tumor cell necrosis, via a p38MAPK and nuclear factor-kappaB (NF-kappaB)-dependent signaling pathway, leading to the expression of iNOS and the production of toxic NO levels.