Pathogenesis of alcohol-associated liver disease

INTRODUCTION — The pathogenesis of liver disease associated with alcohol ingestion is incompletely understood. What is known is that some patients who chronically abuse alcohol develop liver disease, primarily because the liver metabolizes the majority of ingested ethanol. Furthermore, the metabolism of ethanol is required for hepatic injury to occur, although variations in ethanol metabolism do not completely explain the variable susceptibility to alcohol-associated liver disease.

This topic review will focus on three issues related to the development of alcoholic liver disease:

●The basic aspects of alcohol metabolism

●The mechanisms that may be responsible for the development of hepatic disease

●The factors that determine the frequency with which alcohol abuse leads to liver disease

The different pathologic types of alcohol-associated liver disease are discussed separately. (See "Clinical manifestations and diagnosis of alcohol-associated fatty liver disease and cirrhosis".)

ALCOHOL METABOLISM — Ethanol is metabolized via several pathways, each of which can contribute to toxicity. The primary hepatic pathway generates acetaldehyde and reduced nicotinamide adenine dinucleotide (NAD) (figure 1). In addition, accessory inducible microsomal pathways may be responsible for the generation of toxic metabolites from several drugs, while a possible role for gastric ethanol metabolism remains controversial.

Ethanol is metabolized first in the liver to acetaldehyde; this process occurs in the hepatocyte cytosol via a reaction catalyzed by the enzyme alcohol dehydrogenase (ADH) [1]. There are multiple forms of the dimeric ADH molecule, each comprised of different combinations of subunits encoded by separate gene loci [2]. Polymorphisms of ADH do not predict the susceptibility to alcohol-associated liver disease but could explain variations in blood alcohol levels among individuals ingesting the same amount of ethanol.

The acetaldehyde that is generated is subsequently metabolized to acetate, a process that occurs in the mitochondria and is catalyzed by a different enzyme, acetaldehyde dehydrogenase (ALDH) [3]. This enzyme, like ADH, has multiple isoforms with differing activities. One of the consequences of excess acetaldehyde is impaired macrophage function, which could contribute to the immune defects associated with alcohol-associated liver disease [4].

Genetic variability in ALDH accounts for the well described flushing reaction among some Asian individuals who drink alcohol; this reaction resembles the response to alcohol in patients taking disulfiram. (See "Approach to flushing in adults", section on 'Alcohol'.)

The flushing reaction in Asian individuals is due to relative deficiency of ALDH2 isozyme, which results in the accumulation of acetaldehyde [5]. Although acetaldehyde may be a potential accelerant to liver injury, this risk is generally offset by an aversion to further alcohol-induced flushing. As an example, the rate of alcohol use disorder among Chinese individuals with this genotype is much lower than in those without it; however, a separate reduced genetic risk for alcohol use disorder in this subpopulation cannot be excluded [6].

GASTRIC METABOLISM OF ALCOHOL — The stomach has sufficient ADH activity to metabolize a significant amount of ingested alcohol. A study in 1990 aroused great interest by demonstrating that the metabolism of ingested ethanol in the stomach by gastric ADH was lower in women than men [7]. This was associated with increased levels of ethanol reaching the liver via the portal vein in women. This observation provides a potential explanation for why women seem to have an increased risk of liver injury, compared to men, for a given amount of chronic ethanol consumption. Infection with Helicobacter pylori and gastritis are other factors that can reduce the activity of gastric ADH [8].

PATHOGENESIS OF ALCOHOL-INDUCED LIVER INJURY — The strongest evidence for alcohol as a cause of liver disease comes from epidemiologic data, not biochemical or clinical studies. As an example, a marked reduction in the number of liver-related deaths was documented in the period after prohibition began in 1916 in the United States [9]. Other data suggest that risk of alcohol-associated liver disease is related to the amount and duration of alcohol abuse [10].

ALCOHOL-ASSOCIATED STEATOSIS — Several factors contribute to the development of alcohol-associated fatty liver including reduced oxidation of hepatic fatty acids and increased lipogenesis [11]. The altered redox state in the liver induced by alcohol impairs beta-oxidation and tricarboxylic acid cycle activity [12]. Chronic ethanol consumption also increases expression of key lipogenic enzyme regulators, such as sterol regulatory element binding proteins (SREBPs) and SREBP-1 target genes, such as fatty acid synthase, acetyl-coenzyme A carboxylase, and stearoyl-CoA desaturase [13]. Genetic determinants may also drive fat accumulation in those who drink alcohol through the gene encoding PNPLA3, a patatin-like phospholipase [14].

ALCOHOL-ASSOCIATED STEATOHEPATITIS — Not all patients with steatosis develop steatohepatitis. Several factors may contribute to the development of steatohepatitis including cytokines, oxidative stress, and toxic metabolic products of alcohol.

Immunologic defects — The innate immune system is increasingly implicated in the liver's response to alcohol [15]. As an example, dendritic cell function may be altered in alcoholic liver disease, an effect that could impair the cell-mediated response to chronic hepatitis C virus infection [16-18]. Altered dendritic function may also lead to increased release of proinflammatory cytokines [19]. Another study identified reduced expression of a glucocorticoid-induced leucine zipper (GILZ) protein, which amplifies macrophage responses and sensitivity to lipopolysaccharide [20]. Inflammasome activation has emerged as a major pathway mediating signals associated with alcohol-induced liver injury. (See 'Endotoxin' below.)

More broadly, activation of innate immunity is thought to amplify inflammation, with engagement of several inflammatory cell types, including neutrophils, macrophages, natural killer cells, natural killer T cells, dendritic cells, as well as T lymphocytes [21]. As macrophages respond to alcohol-associated liver injury, they undergo profound metabolic reprogramming, including phagocytosis, apoptotic regulation, and arachidonic acid metabolism, among others [22]. At the same time, as disease progresses the antigen-presenting capacity of liver immune cells is diminished, rendering patients more susceptible to infections. While these important observations advance our understanding of alcohol-associated liver disease pathogenesis, they have not yet been directly translated into novel diagnostics or therapeutics for patients.

Cytokines and inflammation

Neutrophil infiltration and activation — One of the most characteristic pathologic hallmarks of alcohol-associated hepatitis is infiltration of neutrophils, which are generally absent from other forms of inflammatory liver disease, generally in association with an increase in the peripheral neutrophil count [23] (see "Clinical manifestations and diagnosis of alcohol-associated fatty liver disease and cirrhosis"). This observation has spawned a vigorous effort to determine if neutrophil infiltration with release of reactive oxygen species and proteases is at least in part responsible for the hepatic disease. Most attention has focused on characterization of mediators, which stimulate chemotaxis, and which might direct migration of neutrophils into regions of alcohol metabolism.

Studies have implicated both the IL-33 and IL-8 pathways in neutrophil infiltration [24]. Defects in IL-33 activity in patients with alcohol-associated hepatitis could lead to enhanced risk of infection [24]. IL-8 is a member of the chemokine family, and a known neutrophil chemoattractant. Three studies have documented elevated plasma IL-8 levels in patients with alcoholic hepatitis [23,25,26]. The plasma IL-8 levels correlated with the severity of hepatic injury [23], and IL-8 has been identified immunohistochemically in the liver in patients with alcohol use disorder (AUD). Whether IL-8 is produced locally in the liver and is required for injury to occur are unproven.

In addition to IL-8, increasing attention focuses on other chemokines as significant drivers of alcohol-associated disease. Macrophage migration inhibitory factor (MIF) has been identified as a central coordinator that orchestrates a broad chemokine response in alcohol-associated hepatitis, based on both human and animal studies [27]. Moreover, this response is unique to alcohol-associated liver disease and not seen in hepatitis C virus infection. Regulation of chemokine and cytokine gene expression in this response is complex and involves genetic as well as epigenetic levels of control [28,29].

An animal study suggested that hepatic stellate cells may promote steatohepatitis through their membrane receptor neuropilin-1 (NRP-1), which leads to enhanced lgfbp3 (an insulin-binding growth factor) [30]. Lgfbp3 promotes lipid accumulation in hepatocytes, which is antagonized by the protease inhibitor SerpinA12. Interestingly, in patients with alcohol-induced steatohepatitis, serum levels of lgfbp3 are increased while those of SerpinA12 are decreased [30].

Antigenic adduct formation — Acetaldehyde and hydroxyethyl radicals are both derived from the oxidation of ethanol; they bind covalently to proteins, thereby forming adducts that are antigenic. Acetaldehyde, for example, is a highly reactive molecule that can bind avidly to the epsilon amino group of internal lysine residues and the alpha amino N-terminal amino acids of proteins, creating acetaldehyde-protein adducts via a Schiff base mechanism [31]. A number of potential intracellular protein targets have been proposed, including tubulin (a cytoskeletal protein) and collagen, a major component of the hepatic extracellular matrix. There are at least two potentially important consequences of adduct formation.

●Adducts could form with intracellular proteins that are critical to cellular function, leading to dysfunction [32].

●Acetaldehyde protein adducts may serve as neoantigens, provoking both cell-mediated and humoral immune responses to attack cells bearing these compounds [33]. This possibility could explain the large number of immunologic features long recognized in many patients with alcohol-associated liver injury [34]. Several studies, for example, have identified antibodies in the serum of patients with AUD which are reactive to acetaldehyde adducts; however, this observation does not easily explain how the antibodies might attack an intracellular protein. In addition, acetaldehyde-protein adducts have also been detected in patients with nonalcohol-associated liver disease.

●Acetaldehyde can also impair macrophage function [4]. (See 'Alcohol metabolism' above.)

Hydroxyethyl radical adducts also may play an important role in the pathogenesis of alcohol-induced liver injury [35]. In one report, animals given ethanol in vivo generated hydroxyethyl adducts on the external surface of hepatocytes that were recognized by antihydroxyethyl radical adduct antibodies that were present in the sera of patients with cirrhosis. In the presence of monocytes, this antigen-antibody complex was able to trigger a cell-mediated cytotoxic reaction that killed the hepatocytes.

A toxic metabolite of lipid peroxidation, acrolein, can form covalent protein adducts if not cleared through conjugation to glutathione, in turn provoking endoplasmic reticulum stress that leads to cell damage and death [36]. Scavenging of acrolein by hydralazine can attenuate experimental alcohol-induced liver injury.

Available data does not definitively establish if adducts have a pathogenic role or are simply an epiphenomenon of alcohol ingestion. Nevertheless, some studies suggest a potentially important mechanism whereby alcohol can induce liver injury that may be amenable to therapeutic intervention.

Endotoxin — There is a growing appreciation for the contribution of the gut microbiome to the pathogenesis of alcohol-associated liver disease [37-39]. Elevated levels of endotoxin (lipopolysaccharide [LPS]) have been detected in the blood of patients with AUD and in animal models exposed to alcohol [40], and toll-like receptors, endoplasmic reticulum stress, and the inflammasome are implicated in these responses [41-43]. Endotoxin interacts with LPS-binding protein and binds to CD14, a receptor on the surface of Kupffer cells (liver macrophages). CD14 interacts with toll-like receptor type 4 (TLR4), which is involved in cytokine activation. It has been hypothesized that the increased levels of LPS contribute to the development of alcohol-associated liver disease by stimulating inflammatory cytokines. In support of this hypothesis are experiments in mice in which the LPS-binding protein, CD14 receptor, or TLR4 have been knocked out; all of these models are less likely to develop hepatic injury than wild-type controls when challenged with alcohol [44-46]. In contrast to TLR4, toll-like receptor 3 (TLR3) may attenuate alcohol-induced liver injury [47].

On the other hand, contradictory data have also been published. One experimental model demonstrated that ethanol-fed animals given infusions of LPS developed increased expression of LPS binding protein [48]. While both pro- and anti-inflammatory cytokines responses were observed, the animals did not develop the net pro-inflammatory cytokine responses observed after acute LPS challenge [48]. This suggests that individuals with AUD might develop tolerance to the chronic exposure to LPS and that LPS exposure may not be a central process in the development of alcohol-associated liver disease.

Injurious cytokines — In addition to chemokines, several studies have documented increased levels of the proinflammatory cytokines tumor necrosis factor, interleukin-1 [43] and interleukin-6 [23,49]. These factors have been implicated in the hemodynamic alterations found in patients with cirrhosis. A study has implicated the p90RSK kinase pathway as a mediator of both injury and fibrosis [50]. (See "Pathogenesis of ascites in patients with cirrhosis".)

Inflammatory cytokines also correlate with disease severity and histologic findings [51]. While their role in alcohol-induced liver injury is unproven, evidence is mounting [52]. In one report, the hepatic inflammation and necrosis observed in ethanol-fed rats were significantly decreased by the administration of neutralizing antibodies to tumor necrosis factor-alpha (TNFa), suggesting a role for this cytokine in the pathogenesis of alcohol-induced liver injury [53]. In another study, mice lacking the TNF receptor 1 gene (but not the TNF receptor 2 gene) were protected from the development of liver disease following ethanol ingestion [54]. A related molecule, TRAIL (tumor necrosis factor-related apoptosis-inducing ligand), may also contribute to the pathogenesis of steatosis and liver injury [55].

Interleukin-17A (IL-17) is a pro-inflammatory cytokine that signals through its receptor IL-17RA to activate macrophages and regulate steatosis in the liver, and injury in a number of organs including liver, lung and colon. IL-17 signaling is activated in experimental injury due to alcohol plus the chemical carcinogen diethylnitrosamine and parallels increases in patients with alcohol-induced liver injury. In these ethanol-fed mice, abrogation of IL-17 signaling through genetic depletion or antagonism of IL-17RA prevents the development of hepatocellular carcinomas [56]. The findings are especially relevant because antagonism of IL-17 is already an approved therapy for psoriatic arthritis and is being explored in other chronic inflammatory diseases, including asthma and inflammatory bowel disease. IL-17 may also promote gut inflammation that drives hepatic injury as Th17 cells are increased in the intestine following alcohol intake; moreover, reduction of Th17 cells in the gut through antagonism of sphingosine-1-phosphate signaling can reduce gut and liver inflammation in experimental mouse models [57].

COX-2 enzyme — In addition to TNF-alpha, a role has emerged for cyclooxygenase 2 (COX-2). This enzyme, which produces prostaglandins, is elevated in experimental alcohol-induced liver injury, raising the possibility that inhibitors may have a therapeutic benefit [58]. Ethanol-fed rats show increased expression by Kupffer cells of cyclooxygenase 2 mRNA, a key enzyme regulating the production of thromboxanes [59,60]. Furthermore, thromboxane inhibitors attenuate some of the pathologic changes in these animals [60]. It is uncertain whether this approach will be clinically useful. (See "Pathogenesis of hepatic fibrosis".)

Dietary fat — Inflammation may also be influenced by dietary fat. As an example, saturated fat is capable of down-regulating liver injury in experimental alcohol feeding [58]. The identification of a single nucleotide polymorphism in the PNPLA3 gene (also called adiponutrin) that heightens the risk of both nonalcoholic [61] and alcohol-associated liver disease [62] has brought attention to the importance of fat metabolism in the pathogenesis of this disease. While the exact mechanism of how this protein, which is a phospholipase, amplifies liver injury is unknown, the findings nonetheless focus attention on lipid homeostasis as a pathogenic component of disease.

Oxidative stress — Oxidative stress, damage to cells by reactive oxygen species, remains a major factor in alcohol-induced liver injury and fibrosis [63-65]. Diversion of ethanol to minor pathways of oxidation in heavy drinkers may be a critical determinant of liver injury because these pathways are more likely to generate injurious reactive oxygen intermediates than ADH. The main sources of reactive oxygen species are the mitochondria and CYP2E1 in hepatocytes and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in activated Kuppfer cells [12].

Chronic alcohol consumption leads to the induction of microsomes (intracellular membrane compartments, such as smooth endoplasmic reticulum) due to spillover from the ADH pathway. The microsomal pathway has been referred to as the microsomal ethanol oxidation system (MEOS) [66]. Induction of specific components of the MEOS, the membrane associated enzyme, cytochrome CYP2E1 has been documented in individuals with AUD [67] and contributes to the generation of oxidative stress [68,69].

Polymorphisms of CYP2E1 correlate with differences in susceptibility to alcohol-associated liver disease, although a definitive role for polymorphisms in accounting for variable susceptibility to alcohol-associated liver disease remains uncertain [70]. Both ADH and CYP2E1 are more concentrated in the centrizonal region of the hepatic lobule, the area in which alcohol-induced liver injury is most prominent. (See "Clinical manifestations and diagnosis of alcohol-associated fatty liver disease and cirrhosis".)

Chronic alcohol ingestion also leads to increased inducible nitric oxide synthase (iNOS) levels, which increase nitric oxide production [71]. Nitric oxide reacts with reactive oxygen species to produce toxic compounds within the liver [12]. One reactive species (superoxide) is produced by NADPH oxidase from Kuppfer cells.

An epigenetic modulator, sirtuin 6 (SIRT6), has been implicated in protecting liver from oxidant stress during alcohol-induced liver injury [72]. In mice with genetic loss of SIRT6, alcohol induces severe inflammation, whereas transgenic animals overexpressing this molecule were protected. The molecule metal regulatory transcription factor (Mtf1) is a functional co-activator of SIRT6 that enhances its activity.

Another sirtuin, SIRT1, decreases in the livers of aging mice, which may render the livers of these aging animals more sensitive to oxidative stress and injury as well as amplified fibrosis [73].

MicroRNAs — MicroRNAs (noncoding RNA species) are increasingly recognized as modulators of disease in a variety of clinical conditions. Profiling of microRNAs in the liver and serum from patients with alcoholic hepatitis identified several upregulated microRNAs that regulate a range of biologic functions, with the greatest increase seen in microRNA-182, which correlated with disease severity, short-term mortality, and the ductular reaction [74], and in miR-155, which promotes steatosis in hepatocytes and increased TNF-alpha by macrophages [75]. In hepatic stellate cells, increased miR-21, especially through release in extracellular vesicles, can enhance macrophage infiltration [75]. Studies to manipulate microRNA-182 levels in an animal model implicate this molecule in liver inflammation involving biliary ductal cells but not as a direct result of alcohol metabolism. Another down-regulated microRNA is microRNA-122 [76]. MicroRNAs are also increasingly implicated in hepatic fibrosis, especially miR-29 and miR-214 [75].

Impaired regeneration — As liver injury and inflammation persist over years of chronic ethanol ingestion, the liver slowly loses the capacity to regenerate. A comprehensive study has identified the hepatocyte transcription factor, HNF4-alpha, as a key determinant of this response [76]. The liver in alcoholic hepatitis undergoes profound changes in the expression of many liver-enriched transcription factors and a range of liver-specific genes as detailed in a study using single cell RNA sequencing [77]. In particular, upregulation of the Wnt signaling pathway receptor, FZD6, and unique changes in macrophage subsets provide clues to how alcohol affects liver homeostasis. Another key finding in humans in alcohol-associated hepatitis is the activation of the Hippo/Yes-associated protein (YAP) growth regulatory pathway, which leads to increased conversion of hepatocytes to biliary cells and loss of hepatocyte identity, which may impair regeneration [78]. The findings suggest that inhibition of YAP may be a possible therapeutic option; however, such an inhibitor would need to be carefully titrated because the Hippo/YAP signaling pathway also regulates other cell types and tissues.

An elegant study that sequenced the livers of patients with progressive stages of alcohol-associated liver disease has demonstrated loss of activity of downstream HNF4-alpha gene targets through DNA hypermethylation, which is an epigenetic modification [76]. Deregulation of HNF4-alpha has been ascribed to increased activity of the fibrogenic cytokine transforming growth factor beta 1 (TGFb1). In aggregate, these findings establish a paradigm wherein broad changes in the transcription landscape involving HNF4-alpha account for the progressive functional deterioration of patients with severe alcohol-associated liver disease. The findings nicely complement an earlier study in which forced expression of HNF4-alpha in the liver of rescued rats with severe fibrosis and irreversible liver failure [79]. These findings establish the potential to reprogram the failing liver by re-expressing differentiation factors, but the translation to clinical implementation of such an approach will require that therapy does not increase the risk of liver cancer.

Microbiome — There has been an increase in our understanding and focus on the microbiome as a determinant of alcohol-associated liver disease, as well as many other non-liver disorders. It makes sense to implicate the microbiome in liver disease because changes in gut bacterial homeostasis and bacterial products affect the liver before any other solid organ through drainage of gut contents into the portal vein. Moreover, advanced liver disease has long been associated with progressively reduced intestinal barrier function, leading to increased permeability that facilitates translocation of intestinal products to the liver. However, methodologies for characterizing the microbiome have yielded major insights into its role in modulating liver disease. For several years, animal models have demonstrated that manipulation of the fecal microbiome in murine alcohol-induced liver injury can alter the extent of injury [80], but this had not yet translated into therapies. In humans, susceptibility to alcohol-associated liver disease has been attributed to the microbiome, in particular through a "cirrhosis dysbiosis ratio" reflecting an imbalance of different bacterial species [37]. In addition to changes in the bacterial composition, studies have also implicated reduced fungal diversity and Candida overgrowth, with production of a pathogenic endotoxin [81,82]. Bacterial dysbiosis is also reflected in specific circulating microbiome signatures in alcoholic hepatitis, raising the possibility of developing diagnostic blood markers to interrogate the microbiome as a therapeutic target [83].

Another study has linked a specific constituent of the microbiome, Enterococcus faecalis, as a cause of the hepatocyte death and liver injury through the production of a cytolysin [84]. Investigators identified a predominance of this organism in the microbiome of patients with alcoholic hepatitis, but not in patients with no alcohol use or with alcohol use disorder alone, without liver injury. In animal model studies, the investigators ascribed alcohol-induced liver injury specifically to a cytolysin secreted by this bacterium, by demonstrating that clearance of this bacterium using a specific bacteriophage abrogated alcohol-induced liver injury in mice. The findings, if validated, raise the possibility of a new approach to defining the pathogenesis of alcoholic hepatitis with direct therapeutic potential since bacteriophage therapy has already been used for refractory bacterial infections in burn patients [85]. These findings may represent a breakthrough in the understanding, diagnosis, and treatment of alcohol-associated liver disease, and likely other liver disorders as well. Moreover, knowledge in this field is likely to accelerate as technologies to characterize the microbiome continue to advance, which will identify other targets in the microbiome.

A related study further links Enterococcus faecalis to the complement pathway in alcohol-associated liver disease. Specifically, the complement receptor of immunoglobulin superfamily (CRIg) is expressed on macrophages and binds both complement C3 protein and gram-positive bacteria. CRIg is reduced in livers of patients with alcohol-associated liver disease, and mice deficient in CRIg develop more severe alcohol-induced liver damage while failing to effectively clear Enterococcus faecalis that had translocated from the gut to the liver [86].

MECHANISM OF FIBROGENESIS — Great progress has been made in the past several years in identifying the cellular sources of extracellular matrix (ECM), or scar protein in alcohol-induced liver injury and in elucidating mechanisms of fibrogenesis. The ECM in fibrotic liver is comprised of several groups of molecules which together gradually form an insoluble scar, remarkably similar to the scar of cutaneous wounds. These components include "interstitial" collagens (types I and III), glycoproteins such as laminin and fibronectin, and proteoglycans such as dermatan and chondroitin sulfate. The composition of the hepatic scar is similar in all forms of fibrosing liver injury, supporting the concept that the fundamental response of the liver to injury is similar whether due to alcohol, drugs, viral infection, or metabolic abnormality (such as hemochromatosis).

The hepatic stellate cell (also known as Ito cell or lipocyte) is the principal source of the ECM in hepatic fibrosis, including alcohol-associated liver disease [87-89]. In a normal liver, stellate cells are perisinusoidal cells distributed throughout the liver that are important for storing the bulk of hepatic vitamin A. With liver injury, these cells undergo a characteristic "activation" in which they lose vitamin A, proliferate, and become fibrogenic [87]. Stellate cell activation has been documented in both experimental alcohol-induced injury [90] and human alcohol-associated fibrosis [91].

Development of methods to isolate stellate cells from rat and human liver has greatly advanced our ability to study these cells and understand how they become activated in vivo. In particular, the release of cytokines from neighboring cells (paracrine activation) and alterations in the surrounding cellular microenvironment contribute to activation [87,92]. The major cytokines involved in this process include platelet-derived growth factor (PDGF), transforming growth factor-1, and endothelin-1.

Mediators unique to alcohol-associated liver injury may also have a role. Acetaldehyde, a metabolite of alcohol, has minor fibrogenic activity towards cultured stellate cells [93], but is unlikely to be a significant stimulant in vivo. Alcohol itself has no effect. However, increasing evidence supports a role for lipid aldehydes, which are unstable products of the interaction between reactive oxygen intermediates and cellular proteins. Free radicals responsible for this action could be a product of ethanol oxidation by the non-ADH microsomal enzyme oxidation system. Two lipid aldehydes in particular, 4-hydroxynonenal and malondialdehyde, are increased in experimental models of alcohol feeding to rodents [94-97], and may have fibrogenic activity. In addition, there may be other less stable intermediates, which act locally to activate stellate cells and stimulate matrix production. As methods for generating and measuring these unstable compounds improve, more information is likely to emerge about their pathogenic role. A more detailed review of hepatic fibrosis appears elsewhere [87]. (See "Pathogenesis of hepatic fibrosis".)

In addition to mechanistic studies, investigators increasingly focus on genetic determinants of the progression of alcohol-associated liver disease to cirrhosis. A large, multicenter genome-wide association study has identified genetic polymorphisms that influence the risk of progression within the genes PNPLA3, HSD17B13, and Fas-associated factor family member 2 (FAF2); all three of their gene products are localized to fat droplets in hepatocytes, suggesting that they may influence both hepatic fat accumulation and injury in individuals with alcohol-associated liver disease [98].

SUMMARY AND RECOMMENDATIONS

●The pathogenesis of liver disease associated with alcohol ingestion is incompletely understood. What is known is that some patients with alcohol use disorder develop liver disease, primarily because the liver metabolizes the majority of ingested ethanol. Furthermore, the metabolism of ethanol is required for hepatic injury to occur, although variations in ethanol metabolism do not completely explain the variable susceptibility to alcohol-associated liver disease. Increasing evidence points to the microbiome and pathways of immune dysregulation as important determinants of liver injury and fibrosis due to alcohol.

●Ethanol is metabolized via several pathways, each of which can contribute to toxicity. The primary hepatic pathway generates acetaldehyde and reduced nicotinamide adenine dinucleotide (NAD) (figure 1). (See 'Alcohol metabolism' above.)

●Several factors contribute to the development of alcohol-associated steatosis including reduced oxidation of hepatic fatty acids and increased lipogenesis. (See 'Alcohol-associated steatosis' above.)

●Not all patients with steatosis develop steatohepatitis. Several factors may contribute to the development of steatohepatitis including genetic determinants (eg, PNPLA3), cytokines, chemokines, oxidative stress, the microbiome and its products, and toxic metabolic products of alcohol. (See 'Alcohol-associated steatohepatitis' above.)

●Exciting studies have begun to define the contribution of the microbiome and its dysregulation as a risk factor for alcoholic hepatitis, and further demonstrate the potential of manipulating the microbiome as a therapeutic approach. (See 'Microbiome' above.)