Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease resulting from apoptotic destruction of β cells in the islets of Langerhans. Low expression of antioxidants and a predilection to produce nitric oxide (NO) have been shown to underscore β cell apoptosis. With this perspective in mind, we questioned whether β cells could mount an induced protective response to inflammation. Here we show that human and rat islets can be induced to rapidly express the antiapoptotic gene A20 after interleukin (IL)-1β activation. Overexpression of A20 by means of adenovirus-mediated gene transfer protects islets from IL-1β and interferon γ–induced apoptosis. The cytoprotective effect of A20 against apoptosis correlates with and is dependent on the abrogation of cytokine-induced NO production. The inhibitory effect of A20 on cytokine-stimulated NO production is due to transcriptional blockade of inducible NO synthase (iNOS) induction; A20 inhibits the activation of the transcription factor nuclear factor κB at a level upstream of IκBα degradation. These data demonstrate a dual antiapoptotic and antiinflammatory function for A20 in β cells. This qualifies A20 as part of the physiological cytoprotective response of islets. We propose that A20 may have therapeutic potential as a gene therapy candidate to achieve successful islet transplantation and the cure of IDDM.

Apoptosis of hepatocytes is a seminal feature of fulminant hepatic failure. We show that the anti-apoptotic protein A20 is upregulated in hepatocytes by pro-inflammatory stimuli and functions to protect from apoptosis and limit inflammation by inhibiting NF-B. Adenoviral mediated hepatic expression of A20 in BALB/c mice yields an 85% survival rate in the D-galactosamine (D-gal)/lipolysaccharide (LPS) model of acute toxic hepatitis compared with 15% to 20 % in control mice. Expression of A20 preserves normal liver function as assessed by prothrombin time. The protective effect of A20 is independent of tumor necrosis factor (TNF) inhibition. Maintaining high circulating TNF levels may be advantageous for liver regeneration. Our data supports this hypothesis as evidenced by increased proliferating cell nuclear antigen (PCNA) expression in the livers of mice expressing A20 compared with a dominant negative mutant of the TNF receptor (TNF-R), 6 hours following D-gal/LPS administration. In conclusion, these results qualify A20 as part of a physiologic, protective response of hepatocytes to injury and a promising gene therapy candidate for clinical applications aimed at preventing and treating viral and toxic fulminant hepatic failure. I n normal livers, apoptosis is a rare event (2-4 apoptotic cells/ 10,000) reflecting physiologic cell turnover. 1 Necrosis and apoptosis of hepatocytes are critical pathologic features of liver injury. 1 Hepatotoxic substances such as ethanol, acetaminophen, and cytostatic drugs result in hepatocyte apoptosis. 1 Apoptosis of hepatocytes is also a feature of viral hepatitis, ischemic liver injury, sepsis, and cholestasis. 2,3 Hepatocyte apoptosis and necrosis, when massive, result in fulminant hepatic failure (FHF). 4 Only 14% of patients diagnosed with FHF recover with medical therapy. Orthotopic liver transplantation (OLT) has improved the fate of these patients (49% undergo OLT), yet 37% die while awaiting OLT. 5 OLT carries high surgical, infectious and oncogenic risks associated with immunosuppression. 5 Identifying and increasing the anti-apoptotic armamentarium of hepatocytes is one approach to treating FHF. Hepatocytes express lower constitutive levels of Bcl-2 and Bcl-x L (the prototypic bcl family members) than other organs, which could account for their particular sensitivity to Fas-mediated apoptosis. 6 Expression of a bcl-2 or a bcl-x L transgene in hepatocytes protects mice from anti-Fas-induced FHF but not against acute toxic hepatitis induced by D-galactosamine (D-gal) and lipopolysaccharide (LPS) or TNF, prompting us to seek other protective genes. 7,8 In this study, we sought to determine whether the anti-apoptotic protein A20 is expressed in hepatocytes and fills a cytoprotective function. We were particularly interested in whether A20 could protect mice from D-gal/LPS acute toxic hepatitis, a model in which Bcl family members are ineffective. 9 Combined administration of LPS and D-gal show biochemical and metabolic changes akin to FHF. 10 A20 was originally identified...

A20 is a stress response gene in endothelial cells (ECs). A20 serves a dual cytoprotective function, protecting from tumor necrosis factor (TNF)-mediated apoptosis and inhibiting inflammation via blockade of the transcription factor nuclear factor-B (NF-B). In this study, we evaluated the molecular basis of the cytoprotective function of A20 in EC cultures and questioned whether its protective effect extends beyond TNF to other apoptotic and necrotic stimuli. Our data demonstrate that A20 targets the TNF apoptotic pathway by inhibiting proteolytic cleavage of apical caspases 8 and 2, executioner caspases 3 and 6, Bid cleavage, and release of cytochrome c, thus preserving mitochondrion integrity. A20 also protects from Fas/CD95 and significantly blunts natural killer cell-mediated EC apoptosis by inhibiting caspase 8 activation. In addition to protecting ECs from apoptotic stimuli, A20 safeguards ECs from complement-mediated necrosis. These data demonstrate, for the first time, that the cytoprotective effect of A20 in ECs is not limited to TNF-triggered apoptosis. Rather, A20 affords broad EC protective functions by effectively shutting down cell death pathways initiated by inflammatory and immune offenders. IntroductionA20 is a zinc finger protein originally identified as a tumor necrosis factor (TNF)-responsive gene in endothelial cells (ECs). 1 A20 is expressed in multiple cell types in response to a variety of stimuli that activate the transcription factor nuclear factor-B (NF-B), including interleukin 1 (IL-1), lipopolysaccharide (LPS), phorbol 12-myristate 13-acetate (PMA), H 2 O 2 , and CD40 ligand. [2][3][4][5][6][7] We and others have demonstrated that A20, initially described as an antiapoptotic gene, is also a potent inhibitor of the transcription factor NF-B. 7-9 A20-null mice fail to terminate TNF-induced NF-B activation, develop severe inflammation and cachexia, and die prematurely, indicating the importance of A20 in the hierarchy of anti-inflammatory defense processes. 6,10 Elucidating the molecular basis and binding partner(s) that determine the inhibitory effect of A20 upon NF-B activation is the focus of ongoing research. 8 A20 has been shown to interact with components of the NF-B signaling cascade upstream of inhibitor of NF-B (IB), including TNF receptor-associated factors (TRAFs) 11,12 TRAF-1, TRAF-2, and TRAF-6; A20 binding inhibitors of NF-B; and the signalosome IB kinase-␥ (IKK␥)/NF-B essential modulator (NEMO) unit. [13][14][15] Further work is required to test the relevance of these interactions in blocking NF-B activation in response to stimuli other than TNF. 16 In contrast to its so far universal inhibitory effect on NF-B activation, the antiapoptotic activity of A20 remains controversial and appears to be specific to cell type and stimulus. Overexpression of A20 protects human breast carcinoma MCF-7 cells, murine fibrosarcoma WEHI 164, and murine embryonic NIH3T3, but not Hela and lung epithelial A459 cells from TNF-mediated apoptosis. 9,[17][18][19] In cultures derived from prima...