Pathogenesis of Alcohol-Associated Fatty Liver: Lessons From Transgenic Mice

Acetylation of proximal cysteine-lysine pairs by alcohol metabolism

Alcohol consumption induces hepatocyte damage through complex processes involving oxidative stress and disrupted metabolism. These factors alter proteomic and epigenetic marks, including alcohol-induced protein acetylation, which is a key post-translational modification (PTM) that regulates hepatic metabolism and is associated with the pathogenesis of alcohol-associated liver disease (ALD). Recent evidence suggests lysine acetylation occurs when a proximal cysteine residue is within ∼15 Å of a lysine residue, referred to as a cysteine-lysine (Cys-Lys) pair. Here, acetylation can occur through the transfer of an acetyl moiety via an S → N transfer reaction. Alcohol-mediated redox stress is known to occur coincidentally with lysine acetylation, yet the biochemical mechanisms related to cysteine and lysine crosstalk within ALD remain unexplored. A murine model of ALD was employed to quantify hepatic cysteine redox changes and lysine acetylation, revealing that alcohol metabolism significantly reduced the cysteine thiol proteome and increased protein acetylation. Interrogating both cysteine redox and lysine acetylation datasets, 1280 protein structures generated by AlphaFold2 represented by a 3D spatial matrix were used to quantify the distances between 557,815 cysteine and lysine residues. Our analysis revealed that alcohol metabolism induces redox changes and acetylation selectively on proximal Cys-Lys pairs with an odds ratio of 1.88 (p < 0.0001). Key Cys-Lys redox signaling hubs were impacted in metabolic pathways associated with ALD, including lipid metabolism and the electron transport chain. Proximal Cys-Lys pairs exist as sets with four major motifs represented by the number of Cys and Lys residues that are pairing (Cys1:Lys1, Cysx:Lys1, Cys1:Lysx and Cysx:Lysx) each with a unique microenvironment. The motifs are composed of functionally relevant Cys-Ly altered within ALD, identifying potential therapeutic targets. Furthermore, these unique Cys-Lys redox signatures are translationally relevant as revealed by orthologous comparison with severe alcohol-associated hepatitis (SAH) explants, revealing numerous pathogenic thiol redox signals in these patients.

Modest effect of differential dietary vitamin A intake on the pathogenesis of alcohol-associated liver disease

BACKGROUND

Chronic alcohol consumption is a major public health issue. The primary organ damaged by alcohol abuse is the liver, leading to alcohol-associated liver disease (ALD). ALD begins with hepatic steatosis and can progress to fibrosis and cirrhosis; however, we have an incomplete understanding of ALD pathogenesis. Interestingly, the liver is also the major organ for vitamin A metabolism and storage, and ALD has previously been linked with altered hepatic vitamin A homeostasis. We hypothesize that alcohol-induced vitamin A depletion disrupts its normal function in the liver, contributing to the pathogenesis of ALD. To test this hypothesis, we postulated that adding copious vitamin A to the diet might alleviate ALD, and conversely, that a vitamin A deficient diet would worsen ALD.

METHODS

We conducted two dietary intervention studies in mice comparing deficient (0 IU/g diet) and copious (25 IU/g diet) dietary vitamin A intake versus control (4 IU/g diet), using the NIAAA chronic-binge model of ALD. Hepatic steatosis was assessed using histopathological and biochemical approaches. Tissue Vitamin A levels were measured using high-performance liquid chromatography. Markers of ALD, hepatic inflammation and lipid metabolism were analyzed by the quantitative polymerase chain reaction and western blotting.

RESULTS

As expected, a 0 IU/g Vitamin A diet decreased, and a 25 IU/g Vitamin A diet increased hepatic Vitamin A stores. However, alcohol induced changes in hepatic triglyceride levels, markers of hepatic lipid metabolism, inflammation and fibrosis were not significantly different in mice consuming a copious or deficient vitamin A diet compared to control.

CONCLUSIONS

Altered vitamin A intake and hepatic vitamin A storage have a minor effect on the pathogenesis of ALD. Thus, given the known link between altered retinoic acid signaling and ALD, future studies that further explore this linkage are warranted.

Piperine alleviates nonalcoholic steatohepatitis by inhibiting NF-κB-mediated hepatocyte pyroptosis

PURPOSE

Nonalcoholic steatohepatitis (NASH) is the progressive form of nonalcoholic fatty liver disease (NAFLD), which has a high risk of cirrhosis, liver failure, and hepatocellular carcinoma. Piperine (Pip) is an extract of plants with powerful anti-inflammatory effects, however, the function of Pip in NASH remains elusive. Here, we aim to explore the role of Pip in NASH and to find the possible mechanisms.

METHODS

Methionine and choline-deficient (MCD) diets were used to induce steatohepatitis, methionine- and choline-sufficient (MCS) diets were used as the control. After Pip treatment, H&E staining, Oil Red O staining, hepatic triglyceride (TG) content and F4/80 expression were performed to analysis liver steatosis and inflammation; Masson's staining, COL1A1 and α-SMA were detected liver fibrosis. Lipopolysaccharide (LPS) -treated AML12 cells were used to as the cell model to induce pyroptosis. Then, pyroptosis-related proteins, IL-1β and LDH release were detected in vivo and in vitro. Finally, NF-κB inhibitor, BAY11-7082, was used to further demonstrate the mechanism of Pip in NASH.

RESULTS

The study found that Pip alleviated liver steatosis, inflammation, hepatocyte injury, and fibrosis in mice fed with MCD diets. Moreover, the pyroptosis markers (NLRP3, ASC, caspase-1 p20, and GSDMD), IL-1β and LDH release were decreased by Pip treatment. NF-κB activation was suppressed by Pip treatment and pyroptosis-related proteins were down regulated by BAY11-7082.

CONCLUSION

Pip ameliorates NASH progression, and the therapeutical effect was associated with inhibition of hepatocyte pyroptosis induced by NF-κB.

BI-3231, an enzymatic inhibitor of HSD17B13, reduces lipotoxic effects induced by palmitic acid in murine and human hepatocytes

17-β-hydroxysteroid dehydrogenase 13 (HSD17B13), a lipid droplet-associated enzyme, is primarily expressed in the liver and plays an important role in lipid metabolism. Targeted inhibition of enzymatic function is a potential therapeutic strategy for treating steatotic liver disease (SLD). The present study is aimed at investigating the effects of the first selective HSD17B13 inhibitor, BI-3231, in a model of hepatocellular lipotoxicity using human cell lines and primary mouse hepatocytes in vitro. Lipotoxicity was induced with palmitic acid in HepG2 cells and freshly isolated mouse hepatocytes and the cells were coincubated with BI-3231 to assess the protective effects. Under lipotoxic stress, triglyceride (TG) accumulation was significantly decreased in the BI-3231-treated cells compared with that of the control untreated human and mouse hepatocytes. In addition, treatment with BI-3231 led to considerable improvement in hepatocyte proliferation, cell differentiation, and lipid homeostasis. Mechanistically, BI-3231 increased the mitochondrial respiratory function without affecting β-oxidation. BI-3231 inhibited the lipotoxic effects of palmitic acid in hepatocytes, highlighting the potential of targeting HSD17B13 as a specific therapeutic approach in steatotic liver disease.NEW & NOTEWORTHY 17-β-Hydroxysteroid dehydrogenase 13 (HSD17B13) is a lipid droplet protein primarily expressed in the liver hepatocytes. HSD17B13 is associated with the clinical outcome of chronic liver diseases and is therefore a target for the development of drugs. Here, we demonstrate the promising therapeutic effect of BI-3231 as a potent inhibitor of HSD17B13 based on its ability to inhibit triglyceride accumulation in lipid droplets (LDs), restore lipid metabolism and homeostasis, and increase mitochondrial activity in vitro.

The intersection between alcohol-related liver disease and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) and alcohol-related liver disease (ALD) are the leading causes of chronic liver disease worldwide. NAFLD and ALD share pathophysiological, histological and genetic features and both alcohol and metabolic dysfunction coexist as aetiological factors in many patients with hepatic steatosis. A diagnosis of NAFLD requires the exclusion of significant alcohol consumption and other causes of liver disease. However, data suggest that significant alcohol consumption is often under-reported in patients classified as having NAFLD and that alcohol and metabolic factors interact to exacerbate the progression of liver disease. In this Review, we analyse existing data on the interaction between alcohol consumption and metabolic syndrome as well as the overlapping features and differences in the pathogenesis of ALD and NAFLD. We also discuss the clinical implications of the coexistence of alcohol consumption, of any degree, in patients with evidence of metabolic derangement as well as the use of alcohol biomarkers to detect alcohol intake. Finally, we summarize the evolving nomenclature of fatty liver disease and describe a recent proposal to classify patients at the intersection of NAFLD and ALD. We propose that, regardless of the presumed aetiology, patients with fatty liver disease should be evaluated for both metabolic syndrome and alcohol consumption to enable better prognostication and a personalized medicine approach.

Chronic-binge ethanol feeding aggravates systemic dyslipidemia in Ldlr-/- mice, thereby accelerating hepatic fibrosis

Objective

Chronic ethanol consumption is known to cause alcohol-associated liver disease, which poses a global health concern as almost a quarter of heavy drinkers develop severe liver damage. Alcohol-induced liver disease ranges from a mild, reversible steatotic liver to alcoholic steatohepatitis and irreversible liver fibrosis and cirrhosis, ultimately requiring liver transplantation. While ethanol consumption is associated with dysregulated lipid metabolism and altered cholesterol homeostasis, the impact of dyslipidemia and pre-existing hypercholesterolemia on the development of alcohol-associated liver disease remains to be elucidated.

Design

To address the influence of systemic dyslipidemia on ethanol-induced liver disease, chronic-binge ethanol feeding was applied to female C57BL/6J (wild type) mice and mice deficient for the low-density lipoprotein receptor (Ldlr-/-), which display a human-like lipoprotein profile with elevated cholesterol and triglyceride levels in circulation. Respective control groups were pair-fed an isocaloric diet.

Results

Chronic-binge ethanol feeding did not alter systemic lipid levels in wild type mice. While increased systemic cholesterol levels in Ldlr-/- mice were not affected by ethanol feeding, chronic-binge ethanol diet aggravated elevated plasma triglyceride levels in Ldlr-/- mice. Despite higher circulatory triglyceride levels in Ldlr-/- mice, hepatic lipid levels and the development of hepatic steatosis were not different from wild type mice after ethanol diet, while hepatic expression of genes related to lipid metabolism (Lpl) and transport (Cd36) showed minor changes. Immunohistochemical assessment indicated a lower induction of infiltrating neutrophils in the livers of ethanol-fed Ldlr-/- mice compared to wild type mice. In line, hepatic mRNA levels of the pro-inflammatory genes Ly6g, Cd11b, Ccr2, Cxcl1 and F4/80 were reduced, indicating less inflammation in the livers of Ldlr-/- mice which was associated with reduced Tlr9 induction. While systemic ALT and hepatic MDA levels were not different, Ldlr-deficient mice showed accelerated liver fibrosis development after chronic-binge ethanol diet than wild type mice, as indicated by increased levels of Sirius Red staining and higher expression of pro-fibrotic genes Tgfb, Col1a1 and Col3a1. Ldlr-/- and wild type mice had similar plasma ethanol levels and did not show differences in the hepatic mRNA levels of Adh1 and Cyp2e1, important for ethanol metabolism.

Conclusion

Our results highlight that chronic-binge ethanol feeding enhances systemic dyslipidemia in Ldlr-/- mice which might accelerate the development of hepatic fibrosis, independent of hepatic lipid levels.

Antioxidant and anti-inflammatory agents in chronic liver diseases: Molecular mechanisms and therapy

Chronic liver disease (CLD) is a continuous process that causes a reduction of liver function lasting more than six months. CLD includes alcoholic liver disease (ALD), non-alcoholic fatty liver disease (NAFLD), chronic viral infection, and autoimmune hepatitis, which can lead to liver fibrosis, cirrhosis, and cancer. Liver inflammation and oxidative stress are commonly associated with the development and progression of CLD. Molecular signaling pathways such as AMP-activated protein kinase (AMPK), C-Jun N-terminal kinase, and peroxisome proliferator-activated receptors (PPARs) are implicated in the pathogenesis of CLD. Therefore, antioxidant and anti-inflammatory agents from natural products are new potent therapies for ALD, NAFLD, and hepatocellular carcinoma (HCC). In this review, we summarize some powerful products that can be potential applied in all the stages of CLD, from ALD/NAFLD to HCC. The selected agents such as β-sitosterol, curcumin, genistein, and silymarin can regulate the activation of several important molecules, including AMPK, Farnesoid X receptor, nuclear factor erythroid 2-related factor-2, PPARs, phosphatidylinositol-3-kinase, and lysyl oxidase-like proteins. In addition, clinical trials are undergoing to evaluate their efficacy and safety.

Alcohol use disorder and liver injury related to the COVID-19 pandemic

Alcohol use disorder is a complex and heterogeneous phenomenon that can be studied from several points of view by focusing on its different components. Alcohol is a hepatotoxin whose metabolism creates profound alterations within the hepatocyte. The liver is the central organ in the metabolism of alcohol, a process that also involves other organs and tissues such as the brain, heart and muscles, but the most relevant organ is the liver. The anatomopathological alterations in the liver associated with the prolonged use of alcohol range from the simple accumulation of neutral fats in the hepatocytes, to cirrhosis and hepatocellular carcinoma. Alcohol abuse frequently leads to liver disease such as steatosis, steatohepatitis, fibrosis, cirrhosis, and tumors. Following the spread of coronavirus disease 2019 (COVID-19), there was an increase in alcohol consumption, probably linked to the months of lockdown and smart working. It is known that social isolation leads to a considerable increase in stress, and it is also recognized that high levels of stress can result in an increase in alcohol intake. Cirrhotic patients or subjects with liver cancer are immunocompromised, so they may be more exposed to COVID-19 infection with a worse prognosis. This review focuses on the fact that the COVID-19 pandemic has made the emergence of alcohol-induced liver damage a major medical and social problem.