A20--an omnipotent protein in the liver: prometheus myth resolved?

Contribution of NF-kappaB inhibitory and ubiquitin-editing A20 (tnfaip3) to the liver's protective response to injury, particularly to its anti-inflammatory armamentarium, is exemplified by the dramatic phenotype of A20 knockout mice that die prematurely of unfettered inflammation predominantly in the liver. A number of additional studies originating from our laboratory and others clearly demonstrate that A20 is part of the liver response to injury and resection. Upregulation of A20 in hepatocytes serves a broad hepatoprotective goal through combined anti-inflammatory, anti-apoptotic, anti-oxidant and pro-regenerative functions. The molecular basis for A20's hepatoprotective functions were partially resolved and include blockade of NF-kappaB activation in support of its anti-inflammatory function, inhibition of pro-caspase 8 cleavage in support of its anti-apoptotic function, increasing Peroxisome Proliferator Activated Receptor alpha (PPARalpha) expression in support of its anti-oxidant function, and decreasing Cyclin Dependent Kinase Inhibitor p21 while boosting IL-6/STAT3 proliferative signals as part of its pro-regenerative function. In experimental animal models, overexpression of A20 in the liver protects from radical acute fulminant toxic hepatitis, lethal hepatectomy, and severe liver ischemia reperfusion injury (IRI), and allows successful engraftment of marginal liver grafts. Conversely, partial loss of A20, as in A20 heterozygote mice, significantly impairs liver regeneration and damage, which confers high lethality to an otherwise safe procedure i.e., 2/3 partial hepatectomy. This is the ultimate proof of the physiologic role of A20 in liver regeneration and repair. In recent work, A20's functions in the liver have expanded to encompass regulation of lipid and glucose metabolism, unlocking a whole new set of metabolic diseases that could be affected by A20. In this chapter we review all available data regarding A20's physiologic role in the liver, and Reflect on the clinical implication of these findings with regard to A20-based therapies in the context of liver transplantation, resection of large liver tumors, liver fibrosis, and metabolic liver diseases.

Single nucleotide polymorphisms at the TNFAIP3/A20 locus and susceptibility/resistance to inflammatory and autoimmune diseases.

The anti-inflammatory and immune regulatory functions of the ubiquitin-editing and NF-kappaB inhibitory protein A20 are well documented in vitro, and in multiple animal models. The high rank held by A20 in the cell's physiologic anti-inflammatory defense mechanisms is highlighted by the striking phenotype of A20 knockout mice, characterized by cachexia, multi-organ failure, and premature death. Even partial depletion of A20, as in A20 heterozygous mice, significantly alters NF-kappaB activation in response to pro-inflammatory activators, even though these mice are phenotypically unremarkable at baseline. A recent burst of genome wide association studies (GWAS), fueled by advances in genomic technologies and analysis tools, uncovered associations between single nucleotide polymorphisms (SNPs) at the TNFAIP3/A20 gene locus and multiple autoimmune and inflammatory diseases in humans. Interestingly, some of these studies emphasized significant associations between TNFAIP3/A20 SNPs imparting decreased expression or loss of NF-kappaB inhibitory function, and susceptibility to systemic lupus erythematosus (SLE) and coronary artery disease (CAD). These clinical data phenocopy partial loss of A20 in mouse models of inflammatory diseases, thereby incriminating TNFAIP3/A20 deficiency as a pathogenic culprit in autoimmune and inflammatory diseases. In this chapter, we undertook a thorough review of studies that explored association between TNFAIP3/A20 SNPs and human autoimmune and inflammatory diseases. Beyond the prognostic value of TNFAIP3/ A20 SNPs for assessing disease risk, their implication in the pathogenic processes of these maladies prompts the pursuit of A20-targeted therapies for disease prevention/treatment in patients harboring susceptibility haplotypes.