Transcriptional and non-transcriptional functions of IRF3 in ALD

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ABSTRACT Alcohol consumption is a leading cause of preventable morbidity and mortality worldwide. Chronic alcohol abuse underlies the pathogenesis of alcoholic liver disease (ALD), encompassed by a spectrum of pathologies, ranging from steatosis, to more severe forms of liver injury, including alcoholic hepatitis (AH), fibrosis, and cirrhosis. There is a growing appreciation of a dynamic and complex role of the innate immune system in the progression of ALD, as well as the resolution of hepatocellular injury and fibrosis. The controlled and appropriate resolution of inflammation is an essential feature of the innate immune response. This dynamic interplay between injury and repair is mediated, at least in part, by the tremendous plasticity of resident tissue macrophages and infiltrating monocytes as they respond to signals within their microenvironment. Signaling via IRF3 is an important element in the dynamic regulation of pro-survival vs pro-death pathways in immune cells. While several studies have implicated IRF3 in the progression of ALD, the mechanisms by which IRF3 contributes to liver injury are not well understood. Classically, IRF3 is known as a transcription factor essential for the induction of interferon- (IFN-) and antiviral genes. Recently, members of our team discovered that, in addition to its transcriptional functions, IRF3 directly triggers a pro-apoptotic pathway, termed RIG-I- like receptors (RLR)-induced IRF-3-mediated pathway of apoptosis (RIPA) via a non-transcriptional mechanism. In IRF3-mediated apoptosis, IRF3 is activated by linear ubiquitination on two lysine residues, resulting in the interaction of IRF3 with the pro-apoptotic protein Bax. The IRF3/BAX complex then translocates to the mitochondria where it triggers apoptosis. In order to study this pathway, we have generated a novel knock-in mouse that expresses an IRF3 protein that only exhibits non-transcriptional functions. Importantly, in preliminary experiments, we discovered that IRF3-mediated apoptosis of innate immune cells recruited to the liver in response to Gao-binge ethanol exposure dynamically exacerbates inflammation and contributes to murine alcoholic hepatitis (mAH). Here we propose to investigate the mechanisms by which the non-transcriptional activity of IRF3 contributes to both hepatic inflammation and fibrosis in models of mAH and fibrosis in 2 specific aims: SA1) Determine the contribution of the non-transcriptional function of IRF3 in mediating ethanol-induced liver injury in the Gao-binge model of mAH and a murine model of acceleration of fibrosis by ethanol and SA2) Investigate the mechanisms of IRF3 activation in immune cells during mAH or fibrosis. In summary, our studies will reveal novel roles for the non-transcriptional activity of IRF3 in exacerbating innate immune responses to alcohol, as well as identify potential therapeutic targets that would prevent and/or improve the resolution of inflammation and fibrosis in patients with ALD.