High-mobility group box-1 and alcoholic liver disease

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ABSTRACT A critical barrier to progress in the field of alcoholic liver disease (ALD) is the lack of knowledge on the key proinflammatory mediators and the mechanisms whereby they drive liver injury. High-mobility group box-1 (HMGB1) is a damage-associated molecular pattern that communicates and amplifies inflammation to neighboring cells. Preliminary studies supporting this application reveal that HMGB1 increases, undergoes post- translational modifications and is secreted in alcoholic patients and mouse models of ALD. We identified that both hepatocytes and Kupffer cells produce fully reduced and acetylated HMGB1 whereas hepatocytes are the main source of oxidized HMGB1. We show that conditional ablation of Hmgb1 in hepatocytes or myeloid cells partially protects while deletion in both prevents inflammation, interleukin-1β (IL1β) production and ALD. Likewise, knockdown of the HMGB1 receptor for advanced glycation end-products (RAGE) in myeloid cells protects from ALD. We have identified that oxidized HMGB1 forms a complex with IL1β in alcoholic patients and in mice. Overall, HMGB1 drives immune cell infiltration, activates NFκB and increases the proinflammatory cytokine IL1β, all central events for the onset and progression of ALD. While the HMGB1 isoforms appear to have distinct effects; yet, the precise contribution of each one of them to alcohol-induced inflammation and IL1β production remains undefined. We believe that the levels of acetylated and oxidized HMGB1 regulate inflammatory cell infiltration upon alcohol exposure. These isoforms may also drive the expression of the key NFκB target proinflammatory cytokine IL1β. Since oxidized HMGB1 forms a complex with IL1β, it could be immunostimulatory and enhance RAGE and/or IL1R signaling. This may be particularly relevant as both molecules are central to the pathogenesis of ALD. Yet, further understanding is needed on how the isoforms lead to immune cell infiltration, the key receptor involved, their binding affinity and if they signal per se or via immunostimulatory complexes with IL1β to drive NFκB induction of Il1β mRNA and ultimately maturation of IL1β protein in ALD. Our central hypothesis is that the levels of acetylated and oxidized HMGB1 regulate inflammatory cell infiltration and IL1β production in ALD. Two specific aims are planned to prove this hypothesis. In Aim 1, we will dissect how the alcohol-mediated increase in the HMGB1 isoforms regulates inflammatory cell infiltration into the liver. In Aim 2, we will determine the receptor binding affinity of the HMGB1 isoforms and if they signal per se or form immunostimulatory complexes with IL1β to drive NFκB induction of Il1β mRNA and maturation of IL1β protein in ALD. Our long-term goal is to dissect the pathogenic role of the HMGB1 isoforms as potential therapeutic targets to prevent ALD.