Keywords
ADH - ALDH - CYP2E1 - liver disease - liver cancer
Alcohol consumption has severe impacts on human health. According to the World Health
Organization (WHO), approximately 2.6 million people died due to harmful alcohol consumption,
accounting for 4.7% of all deaths worldwide, in 2019.[1] The metabolism of alcohol involves the actions of various enzymes, mainly alcohol
dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), cytochrome P450 2E1 (CYP2E1),
and catalase (CAT).[2]
[3] These enzymes play key roles in the oxidation of alcohol, affecting the rate of
alcohol clearance in the body and the generation of its metabolites.
Alcohol consumption has been linked to more than 200 diseases,[1] with the liver being the primary target due to its central role in alcohol metabolism.
Alcohol-associated liver disease (ALD) is widely recognized to be caused by long-term
heavy drinking and is a major cause of chronic liver disease (CLD) worldwide.[4] Moreover, the active metabolites produced during alcohol metabolism, especially
acetaldehyde, are significantly associated with carcinogenesis.[5] Against this backdrop, investigating the roles of alcohol-metabolizing enzymes in
liver diseases and cancer is of particular importance. It not only aids the understanding
of alcohol's effects on liver health, but also provides new perspectives for clinical
prevention and treatment strategies. The present article focuses on the roles of these
enzymes in the occurrence and progression of liver diseases and cancer, aiming to
advance research and clinical interventions. We also discuss the potential effects
of alcohol-metabolizing enzymes on endogenous ethanol and the contribution of gut
ethanol metabolism to liver diseases.
Alcohol Metabolism and the Enzymes Involved
Alcohol Metabolism and the Enzymes Involved
Alcohol Metabolism
After its ingestion, alcohol is absorbed into the bloodstream primarily through the
stomach and the upper part of the duodenum,[6] with more than 90% of it being transported to the liver via the portal vein system.[7]
[8] The liver is the main organ for alcohol metabolism in the body; however, this traditional
notion is challenged by recent studies showing that the gut–liver axis rather liver
alone plays an important role in systemic acetaldehyde clearance.[9] Gut ethanol metabolism likely contributes significantly to alcohol-induced bowel
disease.[10] In the liver, alcohol is metabolized through oxidative and non-oxidative pathways
(see [Fig. 1] for details).[7]
[11]
[12]
Fig. 1
Schematic of alcohol metabolism in the liver. In the liver, alcohol is metabolized to acetaldehyde primarily via oxidative pathways
by ADHs, CYP2E1, and CAT. Acetaldehyde is further metabolized by ALDH to acetate,
which is subsequently released into the circulation and metabolized peripherally into
carbon dioxide and water. Ethanol can also be metabolized through non-oxidative pathways,
resulting in the formation of non-oxidative metabolites such as PEth, EtG, EtS, and
FAEEs through interactions with phosphatidylcholine, glucuronic acid, sulfate, and
fatty acids. ADH, alcohol dehydrogenase; ALDH, aldehyde dehydrogenase; CAT, catalase;
CYP2E1, cytochrome P450 2E1; EtG, ethyl glucuronide; EtS, ethyl sulfate; FAEEs, fatty-acid
ethyl esters; PEth, phosphatidylethanol. (Created in BioRender.)
Acetaldehyde, a highly reactive and toxic substance generated during alcohol metabolism,
primarily causes DNA mutations, chromosomal damage, structural and functional impairments
of organ, and the development of tumors by forming various adducts with DNA and proteins.[13]
[14] These protein adducts also upregulate CYP2E1 expression and enhance oxidative stress.[15]
[16] Non-oxidative pathway products also mediate ethanol-induced organ damage; for example,
fatty acid ethyl esters induce endoplasmic reticulum stress (ERs), thereby promoting
the occurrence of acute liver injury[17] and inducing mitochondrial dysfunction in hepatocytes and intestinal epithelial
cells.[18] Non-oxidative metabolites are cleared more slowly from the body and can thus be
used to assess recent alcohol consumption.[12]
[19]
Alcohol Dehydrogenase
ADH, a zinc-containing dimer enzyme located in the cytoplasm, is the most important
enzyme in the metabolism of ethanol to acetaldehyde.[20] Based on its structural and kinetic characteristics, human ADH is classified into
five types, with classes I, II, and IV being involved primarily in ethanol metabolism
under physiological conditions.[21]
Class I ADH, primarily expressed in the liver, stomach, colon, kidneys, and blood
vessels, may be related closely to alcohol-induced flushing.[22] It is encoded by the genes ADH1A, ADH1B, and ADH1C, and genetic polymorphisms in ADH1B and ADH1C loci affect ethanol metabolism rate. Compared to ADH1B*1 carriers, ADH1B*3 carriers metabolize alcohol faster, which can reduce the risk of alcohol abuse.[23] Relative to carriers of other alleles, ADH1B*1 and ADH1C*2 carriers exhibit significantly slower ethanol metabolism, increasing the risk of
alcohol use disorder (AUD) development.[24]
Class II ADH is expressed mainly in the liver and has less affinity for ethanol than
does class I ADH, with a correspondingly lesser role in ethanol metabolism.[25] Among ADH family members, only class IV ADH is not expressed in the liver, and it
is most active in the presence of high ethanol concentrations.[26] Class IV ADH was recently detected in the esophagus and stomach and was found to
be responsible primarily for first-pass ethanol clearance.[27] Approximately one-third of Asian individuals do not express it.[27]
Aldehyde Dehydrogenase
The human ALDH family has 19 members, among which mitochondrial ALDH2, ALDH1B1, and
cytosolic ALDH1A1 are related closely to ethanol metabolism.[28] ALDH1B1 and ALDH1A1 have functions similar to those of ALDH2 but are expressed at
much lower levels and have weaker affinities for acetaldehyde; thus, they typically
play minor roles in acetaldehyde oxidation.[29]
[30] Low ALDH1A1 activity is associated with a mild alcohol flush reaction in European
individuals but has little impact on drinking behavior.[26]
[31]
ALDH2, the primary enzyme for metabolizing acetaldehyde, is expressed strongly in
the liver and adipose tissue, and more weakly in the kidneys, lungs, stomach, and
skin.[3]
[32] A genetic variant, ALDH2*2 (rs671), prevalent in East Asian populations, disrupts the ALDH2 tetramer and significantly
reduces its ability to metabolize acetaldehyde.[33]
[34] Carriers experience discomfort (e.g., nausea and headache) after ethanol intake,
which reduces their risk of AUD and acute alcohol-related disease development. Therapeutic
approaches involving the inhibition of ALDH2's function have been explored for the
treatment of AUD.[35]
[36]
Cytochrome P450 2E1
CYP2E1 oxidizes ethanol,[37] with CYP1A2 and CYP3A4 contributing to lesser extents. With low ethanol concentrations,
CYP2E1 has low catalytic efficiency (Km = 10 mM) and accounts for about 10% of all
ethanol metabolism; it may play a greater role in ethanol oxidation at higher alcohol
concentrations.[38] Chronic alcohol consumption can induce the expression of CYP2E1, which may be associated
with faster alcohol clearance in heavy drinkers. CYP2E1 is also involved in the oxidation
of compounds such as benzene and acetone, and it exacerbates oxidative stress in hepatocytes
by generating reactive oxygen species (ROS).[39] The expression of CYP2E1 is regulated by the ethanol concentration and can be modulated
by cytokines such as interleukin-4 and miRNAs (e.g., hsa-miR-214-3p).[39]
[40]
[41] Due to the increased activity of CYP2E1 in heavy drinkers, the metabolism of certain
drugs may be accelerated, increasing the risk of adverse reactions.[42]
Catalase
CAT is a tetrameric enzyme containing heme that metabolizes hydrogen peroxide (H2O2) into oxygen and water and is distributed widely in various tissues.[43] It promotes ethanol oxidation by degrading H2O2 in peroxisomes, but this pathway is not critical in hepatic ethanol metabolism.[11] CAT expression can be induced by long-term ethanol intake, especially under oxidative
stress, with the ability of this enzyme to metabolize ethanol increasing with H2O2 production.[16]
[44] CAT plays an important role in alcohol metabolism in the brain, and its metabolite
acetaldehyde is considered to be a key factor in alcohol reinforcing effects, tolerance,
and voluntary ethanol intake. These effects are likely related closely to the interaction
of acetaldehyde with catecholamines to produce various condensation products.[38]
[45]
[46]
Alcohol-Metabolizing Enzymes and Liver Diseases
Alcohol-Metabolizing Enzymes and Liver Diseases
Potential Effects of Gut Ethanol Metabolism on Liver Disease and Cancer
Ethanol in the intestine is primarily absorbed through the mucosa diffusion and metabolized
by gut cells and microbiota.[11] The gut microbiota and enterocytes express alcohol-metabolizing enzymes, including
ADH and ALDH, which co-metabolize ethanol into acetaldehyde and acetate.[47] ADH expression in the intestine exhibits a gradient, with higher levels in the proximal
region and gradual reduction toward the distal intestine. Recent studies have challenged
the view that the liver is the sole site of acetaldehyde metabolism, suggesting that
gut–liver synergy is the primary mechanism for the clearance of acetaldehyde from
the circulation.[9] Animal experiments have shown that dual ALDH2 knockout in the gut and liver synergistically
reduces alcohol preference and intake relative to knockout in one of these organs.[9] Moreover, liver-specific ALDH2 inhibition alleviates heavy, but not moderate, drinking.[3] This finding provides a new perspective for the treatment of AUD and moderate drinking,
which are common in patients with CLD.
Acetaldehyde, the primary metabolite of ethanol, is reintroduced into the intestinal
lumen via bile.[9] Together with residual ethanol and acetate, it synergistically damages the intestinal
barrier, disrupting tight junction integrity and increasing permeability.[48] Simultaneously, ethanol suppresses the expression of antimicrobial proteins, weakening
microbial homeostasis.[49]
[50] These alterations promote the translocation of endotoxins (e.g., lipopolysaccharide
[LPS]) into the portal circulation via the compromised barrier.[51] Following this translocation, microbial-associated molecular patterns and damage-associated
molecular patterns from stressed and damaged cells can enter the liver through the
portal vein, activating immune and parenchymal cells on the surfaces of toll-like
and nucleotide-binding oligomerization domain-like receptors, triggering inflammatory
responses, promoting hepatic fat accumulation, and accelerating liver fibrosis.[4]
[52] Notably, gut-derived ethanol (e.g., from high ethanol-producing Klebsiella pneumoniae strains) induces intestinal barrier damage and hepatic inflammation via mechanisms
analogous to exogenous ethanol.[53] The bidirectional gut–liver axis interactions are experimentally validated in both
ALD and metabolic dysfunction-associated steatotic liver disease (MASLD).[4]
[52]
[53]
Approximately 30% of the acetaldehyde produced in the liver is secreted into the bile,
where it is concentrated in the gallbladder, leading to further increase in acetaldehyde
levels after alcohol consumption.[9] This process may partly explain the reported link between alcohol intake and an
increased risk of gallbladder cancer (GBC).[54] However, a recent meta-analysis showed that the risk of GBC is slightly elevated
in regular drinkers without a clear dose–response relationship.[55] This discrepancy may arise from bile dynamics: short-term alcohol consumption transiently
raises acetaldehyde levels (later excreted), while chronic use prolongs gallbladder
exposure, possibly increasing GBC risk.
Alcohol consumption slightly increases the risk of colorectal cancer (CRC).[54]
[56] Studies of ALDH2*2 and CRC have yielded inconsistent results.[56]
[57]
[58] Fu et al[9] demonstrated that acetaldehyde entering the gut via bile is metabolized primarily
by intestinal ALDH2, with minimal reabsorption, while gut microbiota contributes secondarily.[9] Thus, the intestinal acetaldehyde exposure time and concentration in ALDH2*2 allele carriers may change with alcohol consumption. In addition to hosts' alcohol-metabolizing
enzymes, the gut microbiota plays a role in alcohol metabolism. Although animal studies
suggest chronic alcohol promotes rectal carcinogenesis through bacterial ethanol and
acetaldehyde metabolism,[59] recent findings argue gut microbiota does not directly metabolize ethanol but responds
to elevated circulating acetate. These interactions require further clarification.[60]
Alcohol-Metabolizing Enzymes and Associated Liver Diseases
Alcohol-Associated Liver Disease
ALD is the leading cause of CLD worldwide. Alcohol-related cirrhosis accounts for
nearly 60% of cirrhosis cases in Europe, North America, and Latin America, and the
number of deaths due to ALD has increased in developed countries in the past decade.[61]
[62]
[63] The promoter region (c-262, C > T) of the CAT-encoding gene not only affects CAT
activity, but is also related closely to the susceptibility to and severity of alcohol
dependence. However, evidence for an association between CAT gene polymorphism and ALD is currently lacking.[64] Epidemiological studies have shown that the frequency of the CYP2E1 rs2031920 c2 allele is higher than that of the c1 allele in patients with ALD, but its correlation with ALD remains uncertain.[65] Existing evidence suggests that the rate of alcohol metabolism by ADH is inversely
proportional to the risk of AUD development in carriers.[2]
[20] This association may be related to the discomfort caused by acetaldehyde accumulation.[2]
[20] Yokoyama et al[66] found that the presence of slow-metabolizing ADH1B*1 increases the susceptibility to fatty liver in alcohol-dependent men in Japan. However,
data on ADH mutations and ALD remain inconsistent, requiring further research.[67]
[68]
The ALDH2*2 allele can reduce the risk of AUD and ALD development in carriers. A study conducted
in Taiwan showed that this allele was significantly less frequent in patients with
alcohol-associated cirrhosis and alcohol dependence than in controls (9 and 6% vs.
30%).[68] Tanaka et al[69] found that the frequency of the ALDH2*1/*1 genotype was significantly higher in Japanese patients with ALD than in controls
(80.6% vs. 39.4%). Yokoyama et al[66] reported that the presence of the ALDH2*1/*2 genotype in alcohol-dependent Japanese men increased susceptibility to fatty liver.
Targeted resequencing of genes in the ethanol metabolism pathway identified ALDH1L2 (c.337, C > G) heterozygote was distributed significantly in patients with ALD,[70] but further research is needed to confirm its association with ALD.
Chronic alcohol consumption upregulates CYP2E1,[40] promoting ROS production in hepatocytes and nicotinamide adenine dinucleotide phosphate
oxidase 4 overexpression. This process activates nuclear factor-kappa B (NF-κB), increases
the production of tumor necrosis factor-α (TNF-α), and enhances the sensitivity of
Kupffer cells to LPS, driving ALD development.[71] ROS generated from alcohol metabolism in Kupffer cells activates inflammatory cytokines
(e.g., TNF-α, interleukin [IL]-1β, and IL-6), triggering immune responses involving
macrophages, neutrophils, and natural killer cells and thereby promoting liver damage.
Additionally, ROS contributes to ALD progression by inducing mitochondrial damage,
lipid peroxidation, ERs, and apoptosis.[18]
[72]
[73] With chronic alcohol consumption, the ADH/ALDH alcohol metabolism pathway becomes
saturated, decreasing the oxidized/reduced nicotinamide adenine dinucleotide phosphate
ratio, enhancing fatty acid synthesis (by increasing the expression of lipogenesis
genes), reducing fatty acid oxidation, and deacetylating the transcription factor
EB, which leads to reduced autophagy and increased oxidative stress. Furthermore,
Haseba et al[74] found that ADH5 acts as an S-nitrosoglutathione reductase in mice fed alcohol for
a long time, activating peroxisome proliferator-activated receptor γ (PPARγ) in the
liver to promote lipid accumulation and facilitate ALD development ([Fig. 2]).
Fig. 2
The mechanisms by which alcohol-metabolizing enzymes are involved in alcohol-associated
liver disease (ALD). Alcohol abuse promotes the production of reactive oxygen species (ROS) through ALDH1A1
and CYP2E1. It induces endoplasmic reticulum stress (ERs), autophagy, lipid peroxidation,
and mitochondrial damage, contributing to liver injury. FAEEs, as non-oxidative metabolites,
also induce mitochondrial damage. In conjunction with acetaldehyde adducts such as
MMA, they promote the release of pro-inflammatory cytokines, activating various immune
cells and leading to inflammation. LPS translocation further stimulates the inflammatory
response. Chronic alcohol consumption not only reduces the NAD +/NADH ratio, but also
activates PPARγ through ADH5, increasing lipid synthesis and decreasing lipid catabolism,
ultimately resulting in lipid accumulation. In ALDH2 knockout (ALDH2–/–) models, IL-6
levels are elevated, which reduces fat accumulation by inhibiting the expression of
lipogenesis-related genes and stimulates the activation and proliferation of HSCs,
promoting fibrosis. FAEE, fatty-acid ethyl esters; FXR, farnesoid X receptor; HSCs,
hepatic stellate cells; IL-6, interleukin-6; LPS, lipopolysaccharide; MMA, malondialdehyde-acetaldehyde
adduct; NAD +/NADH, oxidized/reduced nicotinamide adenine dinucleotide; NF-κB, nuclear
factor-kappa B; NOX4, NADPH oxidase 4; PPARγ, peroxisome proliferator-activated receptor
γ; TNF-α, tumor necrosis factor-α. (Created in BioRender.)
ALDH2*2/*2 mice are more susceptible to alcohol-induced liver inflammation and fibrosis, but
exhibit stronger resistance to alcohol-induced steatosis, which may be related to
the activation of IL-6 expression in Kupffer cells by the malondialdehyde-acetaldehyde
adduct. This activation, in turn, leads to the activation of signal transducer and
activator of transcription 3 (STAT3) in hepatocytes, thereby inhibiting the expression
of lipogenesis-related genes and promoting hepatic stellate cell activation and proliferation.[75] Guo et al[76] found that ALDH2 overexpression protected against cell apoptosis induced by long-term
alcohol consumption. Morel et al[77] found that the farnesoid X receptor knockout mice had elevated oxidative stress,
increased expression of ALDH1A1 and CYP2E1, and aggravated liver damage relative to
controls. Their in-vitro experiments confirmed that ALDH1A1 activity is a key factor
in alcohol-induced ROS generation.[77]
Wang et al[78] found that CYP2E1-targeting RNAi delivered via lipid nanoparticles alleviated hepatic
lipid accumulation, inflammation, and fibrosis in ALD mice, suggesting its therapeutic
potential for ALD. After short-term exposure to large amounts of alcohol, the ADH
metabolic capacity in the body becomes saturated. The targeting of ADH activation
to accelerate alcohol metabolism is a promising therapeutic strategy for ALD. In recent
years, various food proteins, traditional Chinese medicine components, and cell extracts
have been found to enhance antioxidant, ADH, and ALDH2 activity, reducing liver steatosis,
cell apoptosis, and alcohol-induced liver toxicity.[79]
[80]
[81]
[82] Given the important role of ALDH2 and the large population of mutation carriers,
the targeting of ALDH2 is a promising therapeutic strategy for ALD. Selective small
molecule ALDH2 activators have been reported to accelerate acetaldehyde clearance
in mice and to alleviate liver steatosis and cell apoptosis.[83]
[84] However, the potential effects of ALDH2 activation on alcohol consumption limit
the application of ALDH2 activators in ALD treatment.[85] Currently, no drug intended specifically for ALD treatment is available, and abstinence
from alcohol remains the foundation of this treatment.
Metabolic Dysfunction–Associated Steatotic Liver Disease
MASLD (formerly known as non-alcoholic fatty liver disease [NAFLD]) has become the
most common cause of CLD worldwide.[62] According to a recent meta-analysis, the global prevalence of MASLD has surpassed
30% and continues to increase.[86] Approximately 99% of patients diagnosed with NAFLD meet the criteria for MASLD.
To avoid confusion, we use “MASLD” henceforth.
In addition to metabolizing alcohol, CYP2E1 is involved in ketone gluconeogenesis,
fatty acid oxidation, and the detoxification of exogenous substances.[87] Studies of CYP2E1 mRNA and protein expression levels in the livers of patients with
MASLD have yielded inconsistent results.[87]
[88] However, with the increasing prevalence of MASLD,[89] the changes in drug metabolism in this population are worth noting. Compared with
those in healthy livers, the CYP2E1, CAT, ADH, and ALDH transcription levels are upregulated in the livers of patients with MASLD,[90] and these levels in patients with severe MASLD resemble those in patients with alcohol-associated
hepatitis. Baker et al[91] observed significantly increased ADH1 and ADH4 protein levels in liver tissues from
patients with metabolic dysfunction–associated steatohepatitis (MASH). However, Li
et al[92] found that in MASH patients (stratified by liver fat content >5% or ≤5%), mRNA levels
of alcohol-metabolizing enzymes remained unchanged compared to controls (except reduced
ALDH4A1). Moreover, protein analysis revealed decreased CAT, ADH1A, ADH1B, and ADH4
and increased ALDH2 in both groups, while ALDH1A1 and ALDH1B1 reduction occurred only
in the >5% fat group.[92] Moreover, in a population of mainly non-Hispanic white patients, compared with ADH1B*1, ADH1B*2 was associated with a lower incidence of MASH and fibrosis.[93]
In the absence of alcohol consumption, obese mice and obese patients with MASLD have
significantly higher blood and exhaled ethanol concentrations than do controls,[94]
[95] which further rise with MASLD worsening and are reversible by antibiotic treatment.[96]
[97] Burger et al[98] recently found reduced ADH activity in the blood and livers of mice and patients
with MASLD, potentially linking to TNF-α–c-Jun N-terminal kinase (JNK) pathway activation
and subsequent ADH serine hyperphosphorylation. Treatment with TNF-α antibodies alleviates
these changes in mice.[98] Current findings suggest complex associations between alcohol-metabolizing enzymes
and MASLD. Further research is required to elucidate the relationships among alcohol
consumption, alcohol-metabolizing enzymes, and MASLD.
Metabolic Dysfunction and Alcohol–Associated Liver Disease
Recently, a new nomenclature for fatty liver disease (SLD) introducing a new category
of liver disease that involves alcohol and metabolic factors—metabolic dysfunction
and alcohol–associated liver disease (MetALD)—was defined.[99] MetALD manifests as a spectrum from MASLD-dominant to ALD-dominant phenotypes. A
recent US nationally representative data indicates a MetALD prevalence of 2.56%, with
worse survival than MASLD.[100]
[101] Additionally, given the high underreporting of alcohol consumption in SLD populations,
the true incidence of MetALD cases may exceed current estimates.[99] Although research remains limited, existing evidence underscores the critical role
of alcohol-metabolizing enzymes in disease progression.
Numerous studies have revealed independent and complex relationships between metabolic
risk factors included in the diagnostic criteria for MASLD and alcohol consumption.
Alcohol intake is known to contribute to weight gain,[99] and 98% of heavy drinkers participating in one study had at least one cardiovascular
metabolic risk factor, with more than 40% having four or more such factors.[102] Recent studies report that ADH1B*2 allele reduces ethanol-derived energy utilization, leading to lower weight gain in
moderate drinkers compared to ADH1B*1 carriers. Notably, this effect is absent in non-drinkers, highlighting alcohol consumption
as a prerequisite for the allele's metabolic impact.[93] A Scottish study linked BMI and alcohol consumption to liver disease, with the highest
risk from BMI–heavy drinking (>120 g/week) interactions.[103] Separately, Japanese research found ALDH2*1/*2 genotype carriers with drinking habits exhibited elevated energy intake compared
with ALDH2*1/*1, potentially tied to acetaldehyde metabolism.[104]
The relationship between alcohol consumption and type 2 diabetes (T2DM) is complex,
with meta-analyses of Eastern and Western ethnic groups yielding conflicting results
for moderate drinking.[105]
[106] This discrepancy may be related to genetic factors affecting alcohol-metabolizing
enzymes. A recent meta-analysis of data from genome-wide association studies indicated
that ALDH2*1/*1 is a susceptibility variant for T2DM in East Asian male populations.[107] Moreover, compared to ALDH2*2 carriers, ALDH2*1/*1 carriers increase alcohol intake, reduce fasting blood glucose clearance, and promote
hepatic insulin resistance, elevating fasting glucose levels and T2DM's susceptibility.[108]
Alcohol consumption correlates positively with hypertension,[109] which in turn is related closely to the ADH1B and ALDH2 genotypes in Chinese individuals who consume alcohol, with the greatest hypertension
risk seen in males carrying the ADH1B*1/*1 genotype.[110] Japanese men with the ADH1B *2/*2 genotype exhibited a stronger positive correlation between serum triglyceride levels
and alcohol intake compared to ADH1B*1 allele carriers.[111] Additionally, compared to the ALDH2*1/*1 genotype, the ALDH2*2 allele is associated with lower high-density lipoprotein levels in alcohol-dependent
and -independent manners.[112]
[113]
Viral Hepatitis
The WHO estimates that 296 million and 58 million people worldwide were living with
chronic hepatitis B (CHB) and C in 2019, respectively, with the highest disease burden
being in Asia and Africa.[62] Alcohol use exacerbates liver injury in both infections: 26.5% of CHB patients report
excessive alcohol consumption, rising to 35.6 and 41.8% in hepatitis B virus (HBV)-related
cirrhosis and liver cancer patients, respectively,[114] while alcohol consumption synergistically exacerbates hepatitis C virus (HCV)-related
liver damage.[115]
[116]
Mechanistically, alcohol-induced CYP2E1 overexpression enhances HBV replication by
upregulating hepatocyte nuclear factor-4α, the key transcription factor for the HBV
core promoter,[117] and increases HCV-related mitochondrial ROS, reducing antioxidant capacity and depleting
mitochondrial glutathione, which heightens oxidative damage and cell death.[118] However, both ethanol (via CYP2E1) and HBV can induce oxidative stress, complicating
differentiation of their roles in liver damage.[119]
[120] Both the viruses interact with alcohol metabolism pathways: Patients with CHB carrying
the ALDH2*2/*2 genotype are at greater risk of persistent HBV infection and high viral loads, and
this genotype can predict the incidence of hepatocellular carcinoma (HCC) to some
degree.[121] Whereas in HCV, the ADH/ALDH pathway may mediate the expression of interferon-stimulated
genes via retinol (ROL) and retinoic acid (RA), thereby exerting antiviral effects.
Alcohol may compete metabolically with ROL, weakening this antiviral function.[116] Despite these insights, the synergistic mechanisms between alcohol, metabolic enzymes,
and viral hepatitis remain incompletely elucidated, necessitating further research.
Alcohol-Metabolizing Enzymes and Liver Cancer
Alcohol-Metabolizing Enzymes and Liver Cancer
Recent data show that about 4.1% of cancers are alcohol related, causing significant
health, economic, and social impacts.[122] Multiple studies have established dose–response relationships between alcohol consumption
and the incidence of various cancers, including oral cancer, pharyngeal cancer, laryngeal
cancer, CRC, HCC, and breast cancer.[5]
[123]
[124] The molecular mechanisms by which ethanol metabolism is linked to various cancer
types, including liver cancer, have been recently reviewed.[125] Here, we focus on correlations between alcohol-metabolizing enzymes and tumors ([Table 1]).
Table 1
Summary of alcohol-metabolizing enzymes and their associations with liver diseases
and tumors
|
Enzyme name
|
Encoding gene
|
Primary function
|
Associated liver diseases/Tumors
|
|
ADH
|
ADH1A
|
Metabolizes ethanol, ROL, and other short-chain alcohols (e.g., methanol, ethylene
glycol)[24]
|
MASLD[91]
[92]; HCC[146]
[147]
[148]
[149]
|
|
ADH1B
|
Metabolizes ethanol, fatty acids, acetone, epinephrine, glucose, ROL, tyrosine, tryptophan,
ifosfamide, cyclophosphamide, abacavir, celecoxib, neurotransmitters serotonin, and
norepinephrine[143]
|
ALD,[66]
[67]
[68] MASLD[91]
[92]
[93]; MetALD[93]
[110]
[111]; HCC[141]
[146]
[147]
[149]
|
|
ADH1C
|
Metabolizes ethanol, ROL and other aliphatic alcohols, hydroxysteroids, and LPO products[150]
|
MASLD[91]; HCC[144]
[146]
[147]
[149]
|
|
ADH4
|
Oxidizes ethanol at higher concentrations; metabolizes ROL, tyrosine, fatty acid,
drugs, and environmental toxicants[152]
|
MASLD[91]
[92]; HCC[149]
[151]
[152]
[153]
[155]
|
|
ADH5
|
Metabolizes formaldehyde and low affinity for ethanol; acts as a denitrating GSH reductase
in ALD; exacerbates tetrachloride-induced liver fibrosis by increasing RA levels;
protects MASLD by maintaining cellular GSH levels[74]
|
ALD[74]
|
|
ADH6
|
Metabolizes ROL, ethanol, other aliphatic alcohols, LPO products, and hydroxysteroids[124]
|
HCC[149]
|
|
ALDH
|
ALDH1A1
|
Metabolizes ROL; oxidizes acetaldehyde, LPO-derived aldehydes, DOPAL; protects against
ultraviolet-induced damage as lens and corneal crystallins; mediates a GABA synthesis
pathway[36]
|
ALD[77]; MASLD[92]; HCC[36]
[159]
[171]
[174]
|
|
ALDH1B1
|
Oxidizes acetaldehyde and LPO-derived aldehydes[36]
|
MASLD[92]; HCC[36]
[148]
[159]
[160]
|
|
ALDH1L1
|
Converts 10-fTHF to tetrahydrofolate[36]
|
HCC[160]
|
|
ALDH1L2
|
Converts 10-fTHF to tetrahydrofolate[36]
|
ALD[70]
|
|
ALDH2
|
Metabolizes acetaldehyde, DOPAL, and LPO-derived aldehydes; acts as a nitrate reductase[36]
|
ALD[66]
[68]
[69]
[74]
[75]
[76]; MASLD[92]; MetALD[104]
[107]
[108]
[110]
[112]
[113]; viral hepatitis[121]; HCC[36]
[58]
[114]
[121]
[141]
[148]
[156]
[157]
[158]
[159]
[161]
|
|
ALDH3A1
|
Oxidizes aromatic, aliphatic aldehydes, and LPO-derived aldehydes; protects the cornea
and lens against ultraviolet-induced oxidative stress[36]
|
HCC[165]
[166]
|
|
ALDH3B1
|
Oxidizes LPO-derived aldehydes; involved in an alteration of dopamine metabolism[36]
|
HCC[148]
|
|
ALDH4A1
|
Oxidizes glutamate γ-semialdehyde; oxidizes short- and medium-chain aliphatic LPO-derived
aldehydes[36]
|
MASLD[92]
|
|
CYP2E1
|
CYP2E1
|
Metabolizes ethanol, catalyzes acetone, glycerol, different fatty acids, drugs, and
environmental toxicants[39]
|
ALD[65]
[71]
[77]; MASLD[87]
[88]
[90]; viral hepatitis[117]
[118]; HCC[126]
[127]
[128]
[129]
[130]
[131]
[132]
[133]
[134]
[135]
[136]
[137]
[138]
[139]
|
|
CAT
|
CAT
|
Dismutate of H2O2; decomposes peroxynitrite; oxidizes nitric oxide to nitrogen dioxide; metabolizes
reactive sulfide species[43]
|
MASLD[90]
[92]
|
Abbreviations: ALD, alcohol-associated liver disease; DOPAL, 3,4-dihydroxyphenylacetaldehyde;
fTHF, 10-formyltetrahydrofolate; GABA, gamma-aminobutyric acid; GSH, glutathione;
HCC, hepatocellular carcinoma; LPO, lipid peroxidation; MASLD, metabolic dysfunction-associated
steatotic liver disease; MetALD, metabolic dysfunction and alcohol-associated liver
disease; RA, retinoic acid; ROL, retinol.
Cytochrome P450 2E1
Chronic alcohol intake accelerates CYP450-mediated (notably CYP2E1) chemically induced
HCC in rodents, likely via enhanced activation of procarcinogenic toxins.[126] Studies on the relationship between the CYP2E1 rs2031920 genetic polymorphism and HCC have yielded inconsistent results. Korean research identified
higher prevalence of CYP2E1 c1/c2 and c2/c2 genotypes among drinkers, although no direct hepatocarcinogenic correlation emerged.[127] Contrastingly, another study has suggested the CYP2E1 c1/c1 genotype in familial HCC susceptibility.[128] However, a Japanese study reported no association between CYP2E1 (c1/c2) genotype distribution and HCV-related HCC in patients versus controls.[129] The combined ALDH2 (*2/*2) CYP2E1 (c1/c1) genotype was associated significantly with an increased risk of HCC.[129] Notably, Yu et al[130] demonstrated that the CYP2E1 c1/c1 genotype amplifies the risk of HCC in smokers, further exacerbated by alcohol intake,
highlighting the significant influence of gene–gene and gene–environment interactions.
However, a recent meta-analysis revealed no association between the CYP2E1 rs2031920 polymorphism and HCC susceptibility in East Asian populations.[131] Further large, well-designed studies are needed to clarify the inconsistent relationship
between CYP2E1 polymorphisms and HCC. In another CYP2E1 polymorphic site, the A1/A1 genotype of the variable number tandem repeat is associated with non-drinking and
may reduce the risk of alcohol-related cancers in carriers.[132]
The expression and activity of CYP2E1 are decreased in HCC tissue relative to those
in adjacent normal liver tissue.[133] In real-time polymerase chain reaction analyses, Kinoshita and Miyata[134] and Ho et al[135] observed weak CYP2E1 mRNA expression in liver tissue from patients with HCC, which
may be related closely to tumor invasiveness and poor prognosis. Additionally, patients
with liver fibrosis and HCC showed more CYP2E1 activity than did controls. In rats,
CYP2E1 activity significantly declined with liver fibrosis progression to HCC, yet
higher activity correlated with more severe HCC, suggesting CYP2E1 hyperactivity is
a risk factor for the progression of liver fibrosis to HCC.[136] Thus, CYP2E1 may play different roles in various stages of HCC development, and
further in-depth research is needed.
CYP2E1 is involved in multiple signaling pathways. Abnormal activation of the Wnt/β-catenin
signaling cascade is closely associated with the occurrence and development of HCC.[137] CYP2E1 overexpression triggers ROS accumulation, enhancing the interaction between
disheveled segment polarity protein 2 (Dvl2) and Kelch-like protein 12 and thereby
inducing the ubiquitination and degradation of Dvl2 and suppressing tumor growth via
Wnt/β-catenin signaling.[133] Bile acids (BAs) are considered to be carcinogens that promote HCC.[138] Cholic acid (CA), one of the primary BAs synthesized in the liver, induces autophagy
through the upregulation of protein kinase B (Akt) phosphorylation and downregulation
of mammalian target of rapamycin (mTOR) phosphorylation and CYP2E1, promoting HCC
cell growth and metastasis.[139] The upregulation of CYP2E1 in HCC cells inhibits this CA-induced autophagy and HCC
cell growth via the Akt/mTOR pathway.[139] However, Ishteyaque et al[126] found that increased CYP2E1 levels stimulates the unfolded protein response and
the upregulation of ERs-related proteins, suppresses the expression of B-cell lymphoma
2 (BCL2), and promotes HCC development via glucose-related protein 78/activating transcription
factor 6 and CCAAT/enhancer-binding protein homologous protein signaling. These findings
suggest that CYP2E1 plays a dynamic role in the pathogenesis of HCC and is a potential
target for liver therapy.
Alcohol Dehydrogenase
Epidemiological studies have revealed no independent correlation between HCC and ADH gene polymorphism.[58]
[140]
[141]
[142]
[143] However, the ADH1C*1/*1 genotype was found to increase the risk of HCC by 2.56 times in a population of 818
individuals with AUD (85 of whom had HCC).[144]
ADH and ALDH interact synergistically.[145] A study conducted in Japan showed that ADH1B*2/*2 and ALDH2*1/*2 combined genotypes conferred a 2 to 4 times greater risk of HCC relative to other
genotypes among light to moderate drinkers.[141] These findings indicate alcohol interacts with ADH/ALDH, urging medical professionals to closely monitor drinking habits, especially in East
Asians.
Total and class-I ADH activities in cells are significantly elevated in primary and
metastatic liver cancers, but activity in serum is elevated only in metastatic liver
cancer.[146]
[147] In a proteomics analysis, Gao et al[148] classified patients with HBV-related HCC into three subgroups and found that ADH1A
was associated significantly with differentiation and survival of this subgroup. Further
validation in an independent cohort confirmed the association with survival, suggesting
that ADH1A could serve as a robust prognostic marker.[148]
Liu et al[149] found that high ADH1A, ADH1B, ADH1C, ADH4, and ADH6 expression levels were independent
factors for improved HCC survival and prognosis. Gene enrichment analysis showed that
low expression of these ADH genes correlated positively with pro-cancer signaling
pathways.[149]
[150] Furthermore, compared with that in healthy liver tissue, the expression of ADH4
mRNA and protein in HCC tissue was found to be significantly reduced and to correlate
positively with survival.[151]
[152] Tumor microenvironment (TME) immune cell infiltration and immune checkpoint expression
affect the outcomes of immunotherapy for HCC. Li et al[153] recently found that ADH4 expression in HCC correlated negatively with the infiltration
of immune cells (e.g., CD19+ B cells, CD68+ macrophages, and CD4+ T cells) and the
expression of immune checkpoint proteins (e.g., cytotoxic T lymphocyte–associated
antigen-4, programmed cell death protein 1, lymphocyte activation gene-3, and vascular
endothelial growth factor B), suggesting that ADH4 is a potential target for immunotherapeutic
intervention for HCC. Abnormal histone deacetylation is one of the mechanisms of immune
evasion in HCC,[154] and the inhibitor trichostatin A (TSA) shows therapeutic potential. In a mouse model
of c-Myc–induced HCC, ADH4 expression was regulated via the Akt–mTOR pathway, with
the inhibition of adenosine triphosphate production and tumor growth, after TSA treatment.[155] Thus, ADH4 can serve as an independent prognostic marker and a potential therapeutic
target in this context.
Aldehyde Dehydrogenase
ALDH has been associated with the occurrence, development, and prognosis of HCC.[36] A Japanese study revealed that light to moderate drinkers with the ALDH2*1/*2 genotype had a significantly higher HCC risk compared with ALDH2*1/*1 carriers,[141] and this genotype is an independent risk factor for HCC development in patients
with alcohol-associated liver cirrhosis.[156] A Taiwan study showed that heavy drinking and ALDH2*2 allele significantly increased HCC risk in HBV-related cirrhosis.[157] However, a large prospective study revealed no significant association between the
ALDH2 genotype and the HCC risk.[58] Moreover, in CHB patients, the ALDH2 allele was not associated with HCC, but the ALDH2*2 allele combined with alcohol consumption was associated with reduced odds of HCC
development.[158]
Some studies have shown that ALDH1A1 expression is elevated and ALDH1B1 and ALDH2
expression is reduced in HCC tissues.[36]
[159] A recent proteomics and genomics analysis revealed that the protein expression levels
of ALDH1B1, ALDH2, and ALDH3B1 are reduced, indicating the loss of liver-specific
metabolic functions in patients with HBV-related HCC, and that this reduction may
promote tumor progression by increasing the accumulation of exogenous compounds.[148] Stronger ALDH1B1 and ALDH1L1 expression has been associated with better clinical
outcomes in patients with HBV-related HCC.[160] ALDH2 expression correlates negatively with HCC metastasis and invasion.[159] In addition, decreased ALDH2 levels and phosphorylation at S9 and S276 were associated
with poor prognosis in patients with HCC.[161]
Certain ALDH isoforms, such as ALDH1, ALDH3A1, and ALDH18A1, are overexpressed in
tumor cells.[162] Such overexpression reduces ROS production in these cells, inhibits lipid peroxidation,
and prevents the toxic aldehyde accumulation that triggers cell apoptosis. ALDHs also
promote RA generation by immune cells (e.g., macrophages and dendritic cells), inducing,
maintaining, and enhancing the function and stability of tumor-infiltrating regulatory
T cells (Tregs),[163] while reducing ERs and immune cell death,[164] thereby weakening the immune response against tumors. The combined effects of these
mechanisms provide survival advantages to tumor cells, promoting tumor progression,
treatment resistance, and immune evasion. Significant ALDH3A1 upregulation in HCC
and hepatocellular adenomas serves as a biomarker of Wnt/β-catenin signaling pathway
activation, although no correlation has been found between ALDH3A1 expression and
patient prognosis.[165]
[166]
Our previous study showed that simple ALDH2 deficiency doesn't lead to liver disease
progression, but increases HCC risk in patients with excessive alcohol consumption.[114] Prolonged exposure to alcohol leads to the release of oxidized mitochondrial DNA
via extracellular vesicles in ALDH2-deficient mouse liver cells, and this DNA is transferred
to nearby HCC cells and activates carcinogenic (JNK, STAT3, BCL2, and transcriptional
coactivator with PDZ-binding motif) pathways, promoting the development of alcohol-related
HCC.[114] AMP-activated protein kinase (AMPK) is associated with the occurrence and invasion
of various tumors. Hou et al[159] reported that ALDH2 regulates acetaldehyde levels, alters the intracellular redox
status, and activates AMPK and downstream signaling pathways, influencing the metastatic
behavior of HCC. Tregs mediate immune tolerance in the TME and are related closely
to poor prognosis. A recent multi-omics analysis found reduced ALDH2 levels and activity
in HCC tissues and that ALDH2 overexpression inhibited Treg differentiation via the
β-catenin/transforming growth factor-β1 signaling pathway, thereby suppressing HCC
development.[161]
Cancer stem cells (CSCs) have cancerous and hepatocyte-like characteristics and strong
drug resistance, and are considered to be the root cause of HCC recurrence.[167] ALDH in various tumors has been identified as a CSC marker. High ALDH expression
in CSCs contributes to chemotherapy and radiotherapy resistance.[168] CD133 +/ALDH(high) HCC cells are believed to have high tumorigenic potential,[169] and ALDH1 isoforms have been identified as stem cell markers associated with HCC
tumorigenesis, metastasis, and chemotherapy resistance.[170] Previous study showed that ALDH1A1 overexpression in HCC-derived CSCs promotes sorafenib
resistance,[171] while reducing the expression of several stem cell markers, including ALDH, could
reverse the sorafenib-resistant HCC cell lines.[172] Zhang et al[173] recently reported that disulfiram/Cu reduced ALDH activity in HCC cells, thereby
decreasing cell stemness and sorafenib resistance, which enhanced the cytotoxic effect
of sorafenib. However, Suzuki et al [174] found that strong ALDH1A1 expression was associated with better tumor differentiation
and had little correlation with stem cell characteristics in HCC cells. Tanaka et
al[175] reached similar conclusions. Given the limited research on the relationship between
ALDH and liver CSCs and the inconsistency of findings, further studies are needed
to clarify this relationship and the potential application of ALDH inhibitors in liver
cancer treatment.
Potential Effects of Alcohol-Metabolizing Enzymes on Endogenous Ethanol Production:
Controversial Data
Potential Effects of Alcohol-Metabolizing Enzymes on Endogenous Ethanol Production:
Controversial Data
Endogenous ethanol is generally believed to be produced by gut microbiota and to interact
with hosts' health in complex ways.[97] In healthy individuals, common gut bacteria produce certain amounts of ethanol;
those with metabolic diseases (e.g., T2DM, MASLD) or auto-brewery syndrome show significantly
elevated endogenous ethanol.[53]
[176]
[177] However, due to the liver's first-pass metabolism, the amount of endogenous ethanol
produced is difficult to infer from the peripheral circulation. Blood ethanol levels
are elevated in MASLD patients.[96]
[178] The chronic overproduction of endogenous ethanol and its metabolites not only induces
hepatic metabolic dysregulation and the development of liver disease, but may also
exacerbate neurodegenerative diseases through brain inflammation.[97]
Mechanisms of endogenous ethanol production in MASLD remain controversial. Alcohol-producing
Klebsiella pneumoniae (found in up to 60% of MASLD patients) can induce MASLD in mice.[53] In contrast, Engstler et al[178] suggested that the elevated plasma ethanol in children with MASLD relative to healthy
controls are associated closely with obesity indices and insulin resistance, not gut
bacterial overgrowth. Furthermore, genetically obese (ob/ob) mice show similar portal
vein and intestinal ethanol levels to lean mice, higher systemic ethanol, and reduced
hepatic ADH and CYP2E1 activity.[178] Based on these results, the insulin-dependent impairment of ADH activity in the
liver, rather than endogenous ethanol production, elevates blood ethanol in MASLD,
a view also supported by Burger et al.[98]
Studies report inconsistent data on alcohol-metabolizing enzyme expression (ADH, CYP2E1,
CAT) in MASLD,[90]
[91]
[92]
[178] highlighting a complex relationship between alcohol-metabolizing enzymes and MASLD.
Overall, the elevation of ethanol levels in the circulation under pathological conditions
may stem not only from endogenous production but also liver metabolic dysfunction
and insulin dysregulation.
Conclusion
Although a substantial amount of research has focused on the roles of alcohol-metabolizing
enzymes in liver diseases and tumors, many questions remain unanswered. Due to genetic
factors, the expression and function of these enzymes differ among individuals. The
impacts of these differences on alcohol's effects on the liver and tumorigenesis are
not yet fully understood. Further research can help clarify these enzymes' functional
variations in different populations and their interactions with alcohol, genes, and
environmental factors. In addition, the regulatory mechanisms of alcohol-metabolizing
enzymes, their relationships to oxidative stress and immune responses, and their potential
as therapeutic targets for liver diseases and tumors are important areas for future
research.
