Keywords

Introduction

Long-term chronic heavy alcohol (ethanol) intake is known to increase the incidences of cancer in many tissues, including the liver, mouth, esophagus, gastrointestinal tract, pancreas, prostate, and female breast [1,2,3,4,5,6]. The alcohol-mediated cancer rates are significantly increased in the presence of co-morbidity factors such as smoking, viral and bacterial infections, carcinogens, and potentially harmful diets, such as poor nutrition, western-style high fat diets, and soft drinks containing high fructose corn syrup. We have previously reviewed that the rates of alcohol-mediated cancer in experimental rodent models and alcoholic people are increased by the one or combinations of the following risk factors: (1) formation of etheno-(or acetaldehyde)-DNA adducts; (2) elevated production of reactive oxygen species (ROS), reactive nitrogen species (RNS), lipid peroxides, and metabolic conversion of pro-carcinogens to carcinogens via ethanol-inducible cytochrome P450-2E1 (CYP2E1); (3) accumulation of iron leading to increased ROS generation, lipid peroxidation, mutation of p53 gene or its covalent modifications of its protein; (4) decreased cellular levels of antioxidant glutathione (GSH) and S-adenosylmethionine (SAMe), resulting in oxidative stress and DNA hypomethylation of oncogenes and epigenetics changes; (5) depletion of retinoic acid with consequent cell proliferation through activation of activator protein-1 (AP-1); (6) activation of an inflammatory cascade via increased intestinal barrier dysfunction, resulting in endotoxemia, activation of tissue macrophages, including hepatic Kupffer cells via Toll-like receptor-4 (TLR-4), oxidative stress, the nuclear factor-KappaB (NF-kB) or early growth response-1 (Egr-1) activation, and production of inflammatory cytokines and chemokines; (7) reduced number and/or function of Natural Killer cells; (8) decreased activities of antioxidant enzymes and DNA repair enzymes. Since these areas have been covered in our previous review [1] and others [2,3,4,5,6], we specifically focus on the updated roles of CYP2E1-related oxidative stress, leaky gut, and metabolic activation of potentially toxic substrates in DNA adduct formation and cancer development in this chapter. The important roles of the ethanol-induced CYP2E1 and other cytochrome P450 isozymes in the metabolisms of various substrates and consequent cytotoxicities, including acetaldehyde and other carcinogenic substances in many tissues, are briefly summarized. Additive and/or synergistic interactions between ethanol and other risk factors toward increased levels of DNA adducts and carcinogenesis are also described. In addition, the potential roles of the endogenous ethanol and acetaldehyde produced by bacteria in certain tissues, as frequently observed in rodent models and patients with nonalcoholic fatty liver disease (NAFLD) and/or steatohepatitis (NASH), in promoting inflammatory tissue injury and cancer development are briefly discussed. Finally, based on the mechanistic study results, we have described the translational opportunities against alcohol-associated injury and carcinogenesis in many tissues.

Updated Mechanisms of Ethanol-Mediated Carcinogenesis

The International Agency for Research on Cancer (IARC) has concluded that both ethanol and its oxidative metabolite acetaldehyde are human carcinogens [7]. One of the major factors for the increased cancer development in alcoholic individuals and ethanol-exposed animal models could be increased oxidative and nitrative stress, through activation of many pro-oxidant enzymes with decreased contents of small molecule antioxidants and suppressed antioxidant enzymes. In fact, chronic excessive alcohol intake is known to increase oxidative and nitrative stress which can be produced through impaired mitochondrial electron transport chain (i.e., mitochondrial dysfunction), elevated levels of the CYP2E1, NADPH oxidases, the inducible form of nitric oxide synthase (iNOS), xanthine oxidase, etc. [8,9,10,11,12,13,14,15,16,17]. In addition, chronic alcohol ingestion is known to decrease the levels of many small molecule antioxidants glutathione (GSH), S-adenosylmethionine (SAMe), folic acid, many vitamins, including retinol (vitamin A), thiamine (vitamin B1), ascorbic acid (vitamin C), vitamin D (ergocalciferol and cholecalciferol), α-tocopherol (vitamin E), menadione (vitamin K3) through insufficient absorption in the GI tract, suppression of biosynthesis, and increased metabolic degradation [1, 8, 18,19,20,21]. The depletion of these enzyme cofactors and co-enzymes can exert dramatic influences on major metabolic pathways and genetic/epigenetics changes. Moreover, the activities of antioxidant enzymes such as mitochondrial low-Km aldehyde dehydrogenase-2 (ALDH2), glutathione peroxidase (Gpx), superoxide dismutase (SOD), methionine adenosyltransferase-1 (MAT1), and catalase can be significantly suppressed through oxidative modifications in alcohol-exposed tissues [22,23,24,25]. It is likely that the decreased levels of antioxidants and suppressed antioxidant enzymes or proteins render the host more susceptible to inflammatory tissue injury and carcinogenesis [1,2,3,4].

Alcohol (ethanol) is not a strong carcinogen compared to its oxidative metabolite acetaldehyde. However, alcohol-induced carcinogenesis is significantly increased in the presence of a risk factor such as smoking, western-style high fat fast food, and viral infection. Elevated levels of highly reactive acetaldehyde and lipid aldehydes, through suppressed ALDH2 activity or genetic mutation in the ALDH2 gene, as observed in many people in East Asian countries [26,27,28] or in various tissues of Aldh2-null mice exposed to alcohol gavages [29] can increase the amounts of DNA adducts and cancer. Alternatively, increased amounts of acetaldehyde produced from activated ADH through mutation of its gene [30] can interact with the amine groups of deoxyguanosine (dG), deoxyadenosine (dA), and deoxycytosine (dC) to generate N2-ethylidene-DNA and more stable N2-etheno-DNA adducts [31,32,33], contributing to increased mutagenesis and cancer development, as recently reviewed [34]. In the presence of reducing agents, such as polyamines, acetaldehyde dimer crotonaldehyde can interact with DNA and produce a stronger mutagenic propano-DNA adduct [35]. Furthermore, under increased oxidative stress following alcohol exposure, the amounts of lipid peroxides are significantly elevated and some of the highly reactive lipid aldehydes such as acrolein (ACR), malonaldehyde (MDA), MDA-Acetaldehyde (MDA-AA), and 4-hydroxynonenal (4-HNE) can interact with DNA, producing mutagenic DNA adducts, leading to increased carcinogenesis [36,37,38]. Analyses of human specimens revealed that the levels of the lipid-aldehyde DNA adducts in the liver and mucosa of the esophagus and colon in alcoholic people appear to depend on the levels of CYP2E1. In contrast, these adducts in some patients with NASH do not correlate with the CYP2E1 levels and are likely derived from inflammation-driven oxidative stress, as reviewed [39]. Furthermore, the rates of carcinogenesis could be markedly increased when p53 and DNA repair enzymes, such as oxoguanine DNA glycosylase (Ogg1), are inactivated in alcohol-exposed rodents [40, 41]. Although the molecular mechanisms for the inactivation of DNA repair enzymes in alcohol-exposed rodents have not been studied in detail, it is likely that these enzymes could be oxidatively modified and thus inactivated under increased oxidative and nitrative stress.

Contributing Roles of CYP2E1 and Other P450 Isoforms in Tissue Injury and Cancer Development

Multiple Regulations of CYP2E1 and Alcohol-Related Tissue Injury and Carcinogenesis

It is well-established that ingested ethanol is primarily metabolized by alcohol dehydrogenase (ADH , Km for ethanol 0.8–1 mM) expressed in the liver, esophagus, stomach, and intestine. However, after chronic alcohol exposure or intake of large amounts of ethanol, a significant amount of alcohol is also metabolized by another enzyme system so-called the microsomal ethanol oxidizing system (MEOS), consisting of CYP2E1, CYP1A2, and CYP3A with CYP2E1 being a major component [8, 9, 42, 43]. In fact, under higher blood alcohol concentration (BAC) up to ~100 mM, as observed in some alcoholics [44], ethanol-induced CYP2E1 (Km for ethanol 8–10 mM) becomes important in the oxidative metabolism of ethanol, producing acetaldehyde, which can impair intestinal barrier function and produce DNA adducts, contributing to inflammatory tissue injury [45] and carcinogenesis [34], respectively. Unlike other P450 enzymes, CYP2E1, a loosely bound enzyme to the ER membrane, exhibits NADPH oxidase activity, thus producing ROS during its catalytic cycle or even in the absence of its substrate, as reviewed earlier [8, 25, 42, 43]. The ROS include superoxide anion, hydrogen peroxide, and hydroxyethyl radical, depending on the local environment, including the presence of iron, which is known to be accumulated by alcohol exposure [46], and other preexisting conditions, contributing to DNA damage and carcinogenesis. In addition, CYP2E1 is known to produce RNS in certain conditions despite little induction of iNOS [47]. CYP2E1, present in both endoplasmic reticulum (ER) and mitochondria [48, 49] in the liver and extra-hepatic tissues such as kidney, colon, and brain [50], is induced and activated by acute or chronic exposure to alcohol and other small molecules such as acetone and isoniazid or pathophysiological conditions such as fasting and diabetes through different regulatory mechanisms [8, 18, 51,52,53]. Moreover, its level and activity are increased in obese and/or hyperglycemic diabetic rodents and in humans [8, 18, 52,53,54,55]. Because of different induction mechanisms of CYP2E1 (e.g., protein stabilization by ethanol or acetone [50, 56,57,58]), increased mRNA translation by isoniazid and pyridine, and mRNA increase by fasting, western-style high fat diet, over-feeding obesity, or diabetes [52,53,54,55], the overall levels and activities of CYP2E1 are expected to be increased in an additive or synergistic manner [8, 18, 59, 60]. For example, alcohol exposure in diabetic rodents and people would markedly elevate CYP2E1 activity, thus producing greater levels of oxidative stress and tissue injury [15, 18, 59]. Another example of additive or synergistic effect is interactions between alcohol drinking and other risk factors such as western-style high fat diet [52], nicotine [61], infection with hepatitis viruses [62], and certain chemicals or carcinogens, which are CYP2E1 substrates, such as dimethylnitrosamine (DMN) [56], diethylnitrosamine (DEN) [63], urethane, and benzene [64, 65], promoting acute toxicity or inflammatory tissue injury, as reviewed [15, 18, 59, 66]. Consequently, the degree of these interactions with cellular macromolecules, including DNA, is significantly increased in fasting or other pathological conditions with lower GSH levels [67], making the host more susceptible to oxidative DNA damage and mutations, contributing to inflammatory tissue injury and carcinogenesis.

In addition to the oxidative ethanol metabolism, CYP2E1 is known to metabolize many small molecule environmental toxicants and potential carcinogens, some of which are the inducers of CYP2E1 [51, 65]. The exogenous CYP2E1 substrate compounds are thioacetamide (TAA), acetaminophen (APAP), isoniazid, cisplatin, halothane, isoflurane, salicylic acid, solvents (e.g., ethylene, carbon tetrachloride, chloroform, dichloromethane, benzene, pyridine, and toluene), various long-chain fatty acids, DMN, DEN, bromodichloromethane, Vitamin A derivatives (retinoic acid), and others [51, 63,64,65]. Endogenous substrates of CYP2E1 can be acetone, long-chain fatty acids, glycerol, 4-HNE, and others, including ethanol and acetaldehyde produced by oral or gut bacteria [18, 68,69,70,71,72]. Metabolism of these substrates by CYP2E1 and consequent organ damage appear to positively correlate with the levels of CYP2E1 activity, with a few exceptions of APAP- or carbon tetrachloride (CCL4)-exposed models, as reviewed [14, 18, 24, 66]. For instance, clinically relevant doses of APAP, halothane, thioacetamide, or CCL4 can cause acute drug-induced liver injury (DILI) or toxicity via alcohol and drug interactions, especially in alcohol-exposed or fasted individuals or rodents with increased CYP2E1. The APAP- or CCL4-mediated hepatic (and/or kidney) injury is initiated through their metabolism by CYP2E1, since pretreatment with CYP2E1 inhibitors or Cyp2e1-null mice was fully protected from these types of DILI or acute toxicity [24, 47, 73]. The decreased levels of retinoic acid by CYP2E1-mediated metabolism [74] and substrate competition with ethanol may also contribute to elevated hepatocyte proliferation and liver tumor progression in alcohol-exposed rodents and alcoholic individuals [74, 75]. By using knockout mice deficient of a specific pro-oxidant enzyme, Bradford and colleagues demonstrated that CYP2E1 but not NADPH oxidase is important in promoting alcohol-mediated DNA damage [76]. In this model, the levels of etheno-DNA adduct were significantly decreased in ethanol-exposed Cyp2e1-null mice compared to those of the wild-type mice. In contrast, the elevated levels of ethanol-related DNA adducts were unchanged and still observed in the corresponding NADPH-oxidase-null mice. In addition, the levels of exocyclic ethanol-DNA adduct were significantly increased in CYP2E1-overexpressing HepG2 cells upon ethanol exposure [77] and some patients with alcoholic fatty liver and fibrosis [78]. The levels of these DNA adducts can be significantly decreased in the presence of chlormethiazole (CMZ), a specific CYP2E1 inhibitor [75, 77]. Furthermore, the elevated levels of exocyclic ethanol-DNA adduct observed in experimental rodents were also observed in the biopsied esophagus specimens of human alcoholic patients with esophagus cancer [79]. The levels of etheno-DNA adduct significantly correlated with cell proliferation, which was markedly increased in people who both drank and smoked [80]. All these results strongly indicate an important role of CYP2E1 in producing carcinogenic etheno-DNA lesions in the experimental model and alcoholic individuals [75,76,77, 81].

Chronic inflammation plays an important role in cancer development and progression of malignant states [82,83,84]. A recent long-term epidemiological study with more than 121,000 health professional men and women revealed that consumption of pro-inflammatory diet is associated with colon cancer, underscoring the important role of inflammation in carcinogenesis [85]. Cancer-associated inflammation and inflammation-derived DNA lesions and malignancies seem to be genetically stable [36] and can be affected by the extrinsic and intrinsic factors. For instance, extrinsic factors, such as alcohol intake, smoking, viral and bacterial infections, exposure to environmental toxicants and pro-carcinogens, and pathophysiological conditions, can increase inflammation and cancer risk. Additionally, cancer-causing mutations can stimulate inflammatory reactions by activating and recruiting inflammatory cells through various pro-inflammatory cytokines and chemokines or complementary factors [82,83,84]. Additionally, it is well-established that excessive chronic alcohol intake can cause chronic inflammation through increased intestinal barrier dysfunction and endotoxemia [86,87,88,89]. Elevated plasma levels of bacterial endotoxin lipopolysaccharide (LPS) can interact with TLR-4 in the Kupffer cells, leading to inflammatory liver injury and carcinogenesis [82, 90]. It is possible that plasma LPS, a potent inducer of iNOS and nitration of many cellular proteins [89, 91, 92], can further stimulate inflammation and injury to the GI tract and other tissues. Recent data suggest that binge alcohol can stimulate gut leakiness and inflammatory liver injury in a CYP2E1-dependent manner [89, 93, 94]. The elevated CYP2E1 was responsible for increased oxidative and nitrative stress, causing nitration of several intestinal tight and adherent junction proteins [93]. Nitrated junctional complex proteins were degraded by proteolytic degradation following ubiquitin conjugation. The markedly decreased amounts of the intestinal junctional complex proteins in binge ethanol-exposed rats compared to control counterparts were confirmed by quantitative mass-spectral analysis [93]. Subsequently, the levels of the gut junctional complex proteins were significantly decreased and contributed to leaky gut and endotoxemia in ethanol-exposed rodents and people who died suddenly due to heavy alcohol intoxication compared to people who died from nonalcoholic causes [93]. These events of ethanol-mediated leaky gut and inflammatory liver injury were not observed in the corresponding Cyp2e1-null mice or were significantly attenuated in the ethanol-exposed wild-type mice co-treated with CMZ, a specific inhibitor of CYP2E1 [93] or an antioxidant N-acetylcysteine (NAC) [89]. Consequently, elevated levels of serum LPS can upregulate TLR4 in the liver, stimulating inflammation and hepatic injury, including fibrosis, potentially leading to carcinogenesis [82, 95, 96]. Furthermore, both extrinsic and intrinsic inflammation are known to modulate or suppress immune responses, which can provide a suitable environment for alcohol-induced carcinogenesis and tumor progression, as reviewed [83, 84].

Distribution of CYP2E1 and Carcinogenesis in Extra-Hepatic Tissues

The majority of CYP2E1 is expressed in the liver. However, it is also expressed in many extra-hepatic tissues such as kidney [50, 97], brain [50, 98, 99], lymphocytes [100], lung [101, 102], pancreas [103], nasal mucosa [104], esophagus [105], stomach [105], intestine [50, 93, 94], and female breast [106]. Induction of CYP2E1 (and other P450 isozymes) following exposure to ethanol, potentially environmental toxicants, or under pathophysiological conditions such as fasting and diabetes, is likely to result in production of ROS and RNS which can lead to increased levels of DNA adducts, inflammatory tissue injury, and carcinogenesis [75,76,77,78,79,80,81, 93, 94]. Furthermore, CYP2E1 was shown to stimulate post-translational modifications followed by inactivation of various proteins in different subcellular organelles, resulting in ER stress, mitochondrial dysfunction, and inflammatory cell death of many tissues, as reviewed [14, 18, 66, 92]. In many cases, the levels of tissue injury, DNA adducts, or cancer positively correlated with those of CYP2E1 [30,31,32, 61, 73,74,75,76,77,78,79, 93, 94, 107,108,109]. In contrast, no or little correlation between the severity of tissue injury and the CYP2E1 level was observed in some other cases [95, 97, 110]. However, the lack of correlation between the levels of CYP2E1 and DNA adduct or tissue injury does not necessarily rule out the important role of CYP2E1 because of its permissive role to allow other proteins or genes to exhibit their damaging effects, as described in some experimental models [95, 97, 110, 111].

Contribution of Other P450 Isoforms in Alcohol-Related Tissue Injury and Carcinogenesis

Most of the studies on alcohol-related DNA mutations and carcinogenesis appear to focus on the correlative roles of ADH, CYP2E1, and ALDH2 involved in the oxidative metabolism of ethanol and acetaldehyde [75,76,77,78,79,80,81, 96, 112]. However, it has been demonstrated that chronic ethanol exposure can induce CYP2E1 as well as other cytochrome P-450 isoforms, such as CYP1A1 [104], CYP2A5 [111, 113], and CYP3A [114, 115]. The levels of these P450 isoforms induced by ethanol exposure may be small compared to those of CYP2E1 induction. However, we also need to pay attention to their contributing roles in promoting DNA mutation, tissue injury, and caner in both liver and extra-hepatic tissues such as esophagus, gastrointestinal tract, nasal cavity, and lung in alcoholic individuals and/or alcohol-exposed rodents especially in the presence of another co-risk factors such as tobacco smoking and potentially harmful drugs, toxicants, or solvents. For instance, CYP1A1 is known to metabolize carcinogens aryl hydrocarbons [AHs] or polyaromatic hydrocarbons (PAHs) [116], such as benzo[a]pyrene and 3-methyl-cholanthrene, contained in charred foods and cigarette tars or smokes. Consequently, ethanol-mediated induction of CYP1A would lead to increased levels of DNA adducts and carcinogenesis in the liver and other extra-hepatic tissues such as nose, esophagus, and lung. Similarly, ethanol-mediated inductions of CYP2A5 can increase DNA adducts in the lung since it is known to metabolize a tobacco carcinogen, the tobacco-related nitrosamine-related carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) [117]. Likewise, alcohol-mediated induction of CYP3A4 [114, 115] and CYP2A5 [118] may render the host with increased levels of DNA adducts and carcinogenesis caused by a mycotoxin aflatoxin B1 (AFB1) [119]. In addition, elevated CYP3A isozymes are likely to metabolize many drugs, including tamoxifen, leading to their activation and production of DNA adducts and carcinogenesis in certain endometrial tissues such as uterus [120]. Furthermore, elevated CYP3A and other P450 isoforms may accelerate metabolic clearance of many drugs, including anti-retroviral agents, leading to oxidative stress and cellular injury [121, 122] in HIV-1-infected people, who exhibit higher rates of hepatic cirrhosis and cancer [123]. In fact, alcohol-mediated elevation/activation of other P450 isoforms in the esophagus, GI tract, nasal cavity, and lung is likely involved in increased DNA mutation, inflammatory injury, and cancer in these tissues in the presence of another risk factor, like smoking, and/or exposure to other potentially toxic substances, such as benzene and toluene [63,64,65], or western-style high fat fast foods [95].

Increased DNA Adducts, Inflammatory Tissue Injury, and Carcinogenesis in NAFLD/NASH Through the Production of Endogenous Ethanol and Acetaldehyde

In the previous section, we have focused on DNA mutations, inflammatory tissue injury, and carcinogenesis in alcoholic individuals and alcohol-exposed rodents. However, it is now known that people with NAFLD/NASH are more susceptible to DNA damage and cancer in the liver [124] and many extra-hepatic tissues, including the GI tract [125]. Some main reasons for increased DNA damage and cancer could be overgrowth of gut bacteria, increased alteration of gut microflora (dysbiosis), mucosal inflammation, oxidative/nitrative stress, and leaky gut after exposure to western-style high fat fast foods, fructose-rich soft drinks, and metabolic syndromes [124,125,126,127,128]. In addition, it was shown that ethanol and acetaldehyde can be endogenously produced in obese rodents with NAFLD [70, 129] and some people, including children with NAFLD/NASH [71, 129,130,131] without exogenous ethanol intake. Moreover, production of acetaldehyde was demonstrated in various bacteria present in the mouth [68, 69, 132], lung [132, 133], and GI tract, including colon [70, 129, 134]. Consequently, the levels of etheno-DNA adduct, inflammatory injury, and carcinogenesis could be increased in these tissues in rodents and people with NAFLD/NASH, as reviewed [124, 125]. For instance, gut dysbiosis with the increased population of the ethanol-producing bacterial family Enterobacteriaceae, including Escherichia coli, can lead to increased production of ethanol, local inflammation, and leaky gut, contributing to endotoxemia and inflammatory tissue injury, as demonstrated with pediatric patients with NASH [129, 131]. Although not studied, it would be of interest to know whether gut CYP2E1 is induced by the endogenously produced ethanol, albeit small amounts compared to those of alcohol intake. If gut CYP2E1 is induced in people or rodents with NAFLD/NASH, it may cause oxidative stress and oxidatively modify the intestinal junctional complex proteins, resulting in decreased amounts of the intestinal junctional complex proteins in alcohol-exposed rats and mice, as recently demonstrated [93, 94]. These events caused by the endogenous ethanol in NAFLD/NASH [135] may contribute to leaky gut, endotoxemia, and inflammatory tissue injury accompanied with DNA damage. Although the mechanisms for increased DNA mutation and carcinogenesis remain to be further studied, it is likely that CYP2E1 and other P450 isoforms, which can be induced by the endogenously produced ethanol, may be involved in the oxidative metabolisms of ethanol and acetaldehyde. In addition, these P450 isoforms can metabolize pro-inflammatory substances n-6 long-chain fatty acids and/or other environmental toxicants or potential pro-carcinogens such as PAHs contained in charred western-style fast foods, contributing to elevated DNA adducts and carcinogenesis in the GI tract and other organs. Based on the damaging roles of the endogenously produced ethanol and acetaldehyde with potentially elevated CYP2E1 and other P450 isoforms, it is expected that people with diabetes or NAFLD/NASH could be more susceptible to DNA damage, inflammatory tissue injury, and cancer especially when they drink even small amounts of alcohol through additive or synergistic interactions [136, 137].

Translational Research Opportunities

Alcohol and acetaldehyde are human carcinogens [7]. The incidences of alcohol-related inflammatory tissue injury, DNA mutation, and carcinogenesis are significantly increased in the presence of another risk factor(s). As mentioned earlier, these risk factors are smoking, viral and bacterial infections, pro-inflammatory western-style high fat fast foods with fructose-containing soft drinks, poor nutrition, and preexisting pathophysiological conditions such as fasting and diabetes [18, 59, 66, 92]. Simultaneous exposure to these risk factors is likely to decrease cellular antioxidants, such as GSH and SAMe, and inactivate many antioxidant enzymes. Consequently, the rates of oxidative stress, lipid peroxidation, gut leakiness, endotoxemia, inflammatory tissue injury, DNA damage, and carcinogenesis would be increased [8, 18, 66, 92]. Based on these mechanisms, prevention or moderation of alcohol drinking would be the best remedy for alcohol-related tissue injury and carcinogenesis. Unfortunately, it would be difficult to decrease alcohol intake in many addicted alcoholic individuals. If alcohol drinking is not prevented, we may consider using adequately balanced diets with antioxidants (such as NAC) [89] or dietary supplements such as n-3 docosahexaenoic acid (DHA) [23], garlic compounds, including diallyl sulfide [37], resveratrol [138], walnut [139], indole-3-carbinol [140], ellagic acid [141], and pomegranate [94], many of which were shown to reduce or suppress the amount or activity of CYP2E1. As reported earlier [89, 93, 94, 140, 142], decreased CYP2E1 would lead to prevention of oxidative stress, leaky gut, and inflammatory tissue injury. Administration of soy protein isolate was also shown to protect from alcohol-mediated tumor promotion in DEN-exposed mice [143]. In addition, eubiosis by administering probiotics Lactobacillus [144] and Bifidobacterium strains [145] may be considered. In fact, a recent study showed that supplementation with Akkermansia muciniphila prevented alcohol-mediated intestinal barrier dysfunction and inflammatory liver injury through the gut–liver axis [146]. Furthermore, treatment with synthetic chemical inhibitors of CYP2E1, such as CMZ [75, 77, 89] and YH439 [147], can be considered to mitigate alcohol- and acetaldehyde-mediated inflammatory tissue injury, DNA mutation, and carcinogenesis.

Conclusion

As reviewed previously, both chronic and acute alcohol intake can change many different metabolic pathways and immunological dysregulations along with genetic and epigenetic changes. In the liver, alcohol drinking stimulates fatty liver, inflammation, fibrosis, cirrhosis, and cancer [148]. The development and progression of chronic liver disease usually depend on the amounts and duration of alcohol intake as well as the presence of another co-morbidity risk factor(s). Alcohol or acetaldehyde-mediated cancer in extra-hepatic tissues may also depend on the amounts of DNA adducts of the pro-carcinogens by CYP2E1 and other P450 isoforms-mediated metabolisms that can be increased by exposure to alcohol and/or another environmental toxicant(s). In this chapter, we have briefly summarized the biochemical properties of CYP2E1 and its roles in ethanol and acetaldehyde metabolism. We have also described its multiple regulations, tissue distribution, and causal roles in alcohol-mediated gut leakiness, inflammation, apoptosis, tissue injury, DNA mutation, and carcinogenesis. In addition, we have mentioned the potential roles of other P450 isoforms, which are also induced or activated by alcohol or another environmental toxicant, in metabolizing potentially harmful substances, contributing to increased carcinogenesis in the liver and extra-hepatic tissues. The causal roles of CYP2E1 and other P450 isoforms in stimulating inflammatory tissue injury, DNA adducts, and carcinogenesis would be significantly increased in the presence of another risk factor such as smoking and/or western-style high fat fast foods. We have also briefly described the newly emerging roles of the gut microbiome changes and the endogenously produced ethanol in promoting DNA adduct formation and disease progression despite the absence of exogenous alcohol intake. Based on the understanding of the mechanisms of increased carcinogenesis, we have described potential methods of preclinical translational opportunities by preventing alcohol drinking or using dietary supplements, including various naturally occurring antioxidants and probiotics, and chemical inhibitors of CYP2E1. In fact, many drug candidates are being evaluated for preventing or treating liver disease through targeting the gut–liver axis [143,144,146, 149, 150]. Based on the important roles of gut microbiome changes in promoting leaky gut, endotoxemia, inflammatory tissue injury, some of these drug candidates may become a good candidate as an anti-cancer agent.