Chronic Alcohol Consumption Leads to Hepatic Lysine Hyperacetylation: Mechanisms and Consequences
Although the clinical manifestations of alcoholic liver disease are well-described, little is known about the molecular basis for liver damage. We have been using hepatic WIF-B cells to examine ethanol-induced liver injury. These cells polarize in culture and maintain liver-specific activities including the ability to metabolize alcohol. Ethanol metabolism leads to the formation of highly reactive metabolites that covalently modify DNA, lipids and proteins. More recently, it is apparent that chronic ethanol consumption leads to increased post-translational protein modifications of the natural repertoire including acetylation. This reversible modification on lysine residues modulates multiple cellular processes. These studies were aimed at further characterizing ethanol-induced protein acetylation. Previously, we observed that ethanol induces microtubule hyperacetylation and stability and this requires ethanol metabolism. To determine the mechanism for increased microtubule acetylation, we examined the microtubule deacetylase, HDAC6. While ethanol does not alter its distribution or activity, HDAC6 protein levels are decreased 25%. Furthermore, ethanol impairs HDAC6-microtubule binding, likely due to ethanol-induced tubulin modifications. Therefore, lower HDAC6 levels combined with decreased microtubule binding leads to increased tubulin acetylation. Ethanol consumption has been shown to impair clathrin-mediated internalization and addition of trichostatin A (TSA), an inhibitor of HDAC6 that leads to hyperacetylation, mimicked this defect. Using a morphological approach, it was determined that ethanol and TSA impairs clathrin internalization at a late stage of vesicle budding. We further determined that ethanol impairs dynamin-membrane binding and association with members of the clathrin machinery. Dynamin is the GTPase responsible for vesicle fission and decreased membrane association likely contributes to impaired internalization. A proteomics screen to identify novel ethanol-induced hyperacetylated proteins revealed that both actin and cortactin were hyperacetylated. Cortactin is thought to promote actin polymerization and mediate dynamin assembly on the necks of invaginated coated pits. Since cortactin acetylation disrupts its actin association, it is possible that ethanol-induced actin and cortactin hyperacetylation prevent proper dynamin recruitment and vesicle fission. Recently, a specific deacetylase activator, resveratrol, was shown to attenuate fatty liver in alcohol-exposed mice indicating that lysine acetylation plays a dominant role in regulating hepatic function and reducing acetylation is a promising novel therapeutic strategy.
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