Recent Advances in Understanding of Pathogenesis of Alcohol-Associated Liver Disease

Abstract

Alcohol-associated liver disease (ALD) is one of the major diseases arising from chronic alcohol consumption and is one of the most common causes of liver-related morbidity and mortality. ALD includes asymptomatic liver steatosis, fibrosis, cirrhosis, and alcohol-associated hepatitis and its complications. The progression of ALD involves complex cell-cell and organ-organ interactions. We focus on the impact of alcohol on dysregulation of homeostatic mechanisms and regulation of injury and repair in the liver. In particular, we discuss recent advances in understanding the disruption of balance between programmed cell death and prosurvival pathways, such as autophagy and membrane trafficking, in the pathogenesis of ALD. We also summarize current understanding of innate immune responses, liver sinusoidal endothelial cell dysfunction and hepatic stellate cell activation, and gut-liver and adipose-liver cross talk in response to ethanol. In addition,we describe the current potential therapeutic targets and clinical trials aimed at alleviating hepatocyte injury, reducing inflammatory responses, and targeting gut microbiota, for the treatment of ALD.

Keywords: ALD; PCD; PRR; alcohol-associated liver disease; complement; gut-liver axis; pattern recognition receptors; programmed cell death.

Figure 1

Multiple formats of programmed cell death in alcohol-associated liver disease. (a) Ethanol exposure induces Fas ligand and DR5-mediated extrinsic apoptotic pathways through miRNA-21. Activation of IRF3 initiates alcohol-induced hepatocyte apoptosis. cGAS-driven IRF3 signaling spreads through hepatic gap junction communication between hepatocytes via Cx32, thereby amplifying inflammation and accelerating hepatocyte apoptosis; ethanol also induces IRF3-mediated apoptosis in the Ly6C^low population. (b) Necroptosis classically depends on the death receptor action and RIP3-mediated phosphorylation of MLKL. In addition, TLRs and interferon receptors can induce activation of MLKL. Ethanol feeding induces phosphorylation of RIP1 and RIP3. In an acute-on-chronic model, ethanol impairs TFEB-mediated lysosome biogenesis through activation of mTORC1, resulting in insufficient autophagy. MLKL also plays a critical role in the regulation of endosomal trafficking and generation of EVs. (c) Ethanol can induce caspase-1-mediated pyroptosis via miRNA-148a-targeted overexpression of TXNIP in hepatocytes. The NLRP3 inflammasome pathway is activated in hepatocytes in response to LPS-induced ER stress. After the transition from chronic alcoholic steatohepatitis to alcohol-associated hepatitis, noncanonical caspase-112013GSDMD signaling is activated. (d) Ethanol feeding results in iron-dependent ferroptotic cell death, which is characterized by excessive accumulation of intracellular lipid ROS and lipid peroxidation. Adipose-specific overexpression of lipin-1 exacerbates steatosis, hepatobiliary damage, and mild fibrotic injury by a GPX4-independent induction of hepatic iron overload lipid peroxidation. Further, intestinal SIRT1 could mediate ethanol-induced hepatic iron metabolism dysfunction and ferroptosis through the circulating LCN2-SAA1 axis in a GPX4-independent mechanism, ultimately contributing to ethanol-induced liver injury. Solid arrows indicate direct promotion, dashed arrows indicate indirect promotion, and blunt-ended arrows indicate direct inhibition. Thin arrows pointing up or down next to items indicate increase or decrease. Abbreviations: Cx32, connexin 32; DR5, death receptor 5; ER, endoplasmic reticulum; EtOH, ethanol; EV, extracellular vesicle; GSDM, gasdermin; IFN, interferon; IFNR, interferon receptor; IRF3, interferon regulatory factor 3; LPS, lipopolysaccharide; miRNA-21, microRNA-21; NLRP3, NLR family pyrin domain containing 3; RIP3, receptor interacting protein kinase 3; ROS, reactive oxygen species; TFEB, transcription factor EB; TLR, Toll-like receptor. Adapted with permission, Cleveland Clinic Foundation ©2022. All rights reserved.

Figure 2

PRR family in ALD. PAMPs and DAMPs signal through PRRs to activate the immune system. Multiple PRRs have been implicated in progression of ALD, including extracellular receptors, such as TLR2/4, CLRs, and RAGE, as well as intracellular receptors including TLR3/7/8/9, AIM2, NLR3, and RLRs. The CLR family senses a broad repertoire of PAMPs, including many diverse fungi, viruses, commensal bacteria, eukaryotic pathogens, and DAMPs that originate from distinct cells and tissues. Large blue arrows indicate the signal transduction cascade. Abbreviations: AIM2, absent in melanoma 2; ALD, alcohol-associated liver disease; CLR, C-type lectin receptor; DAMP, damage-associated molecular pattern; NLR3, nucleotide oligomerization domain (NOD)-like receptor 3; PAMP, pathogen-associated molecular pattern; PRR, pattern recognition receptor; RAGE, receptor for advanced glycation end products; RLR, retinoic acid-inducible gene I (RIG-I)-like receptor; TLR2/4, Toll-like receptor 2/4. Adapted with permission, Cleveland Clinic Foundation ©2022. All rights reserved.

Figure 3

Organ-organ interactions in the pathogenesis of ALD. The gut communicates with the liver through the portal vein, biliary system, and other circulating soluble mediators. Chronic alcohol consumption results in enteric dysbiosis and bacterial overgrowth. ALD-associated dysbiosis is characterized by a reduction in Lactobacillus and Candida overgrowth. Bacterial overgrowth causes an increase in secondary BAs, resulting in an overall increase in hepatic BA synthesis. A reduction in hepatic phosphatidylcholine is also seen in ALD, causing triglyceride accumulation in the liver. Alcohol-associated dysbiosis in mice was further linked to reduced LCFA biosynthesis and short-chain fatty acids, including butyrate. Ethanol and its metabolite acetaldehyde have been implicated in weakening the intestinal tight junctions and inflammatory response by downregulating AMPs in the intestine. Consequently, increased translocation of PAMPs and gut metabolites elicits intestinal and hepatic inflammatory responses, leading to progressive liver damage. Ethanol also causes adipocyte death and metabolic and immune dysfunctions of adipose tissue. Adipose-liver cross talk is mediated by the release of mediators, including neurotransmitters, cytokines, chemokines, adipocytokines, miRNAs, extracellular vesicles, and metabolites. Abbreviations: ALD, alcohol-associated liver disease; AMP, antimicrobial peptide; BA, bile acid; ER, endoplasmic reticulum; KC, Kupffer cell; LCFA, long-chain fatty acid; LPS, lipopolysaccharide; miRNA, microRNA; Neut, neutrophil; NKT, natural killer T cell; PAMP, pathogen-associated molecular pattern. Adapted with permission, Cleveland Clinic Foundation ©2022. All rights reserved.

Figure 4

Therapeutic targets and clinical trials for alcohol-associated liver disease. Three approaches are currently being investigated: (a) strategies to reduce hepatic injury by decreasing oxidative stress, decreasing death of hepatocytes, and improving hepatic regeneration; (b) use of agents to reduce inflammation, including the classic agent prednisolone, as well as more specific targets including TNF and IL-1; and (c) targeting gut health via treatment with probiotics or fecal transplants and FXR agonists. Abbreviations: FMT, fecal microbiota transplantation; FXR, farnesoid X receptor; G-CSF, granulocyte colony stimulating factor; IL, interleukin; TNF, tumor necrosis factor. Adapted with permission, Cleveland Clinic Foundation ©2022. All rights reserved.