28.2.1 Increased Generation of Acetaldehyde
Acetaldehyde is probably the most important single mechanism through which alcohol contributes to increased cancer risk.4 Acetaldehyde is produced as a metabolite from ethanol by the action of cellular alcohol dehydrogenase (ADH), mainly in the liver, but also in peripheral tissues and through the action of bacterial ADH in the colon and saliva.5 Acetaldehyde is highly reactive and binds rapidly to cellular proteins as well as to DNA, resulting in protein malfunction and formation of stable DNA adducts.6 It can induce cross-links between DNA molecules and between DNA and proteins. Alterations to DNA by acetaldehyde increase the risk of replication errors and mutations, and trigger replication errors and/or mutations of oncogenes and tumor suppressor genes.6 Acetaldehyde also exerts direct mutagenic effects on mammalian DNA by causing point mutations, sister chromatid exchanges, and chromosomal aberrations, as demonstrated by in vitro experiments.78 Furthermore, acetaldehyde interferes with DNA repair machinery by inhibiting 06-methyl-guanyltransferase, an enzyme responsible for the removal of DNA adducts.9
High levels of acetaldehyde in saliva have been proposed to contribute to carcinogenesis.1012 Even in volunteers who consumed moderate amounts of alcohol, a substantial production of acetaldehyde was detected in their saliva at concentrations (19 to 143 |jM which could cause mutagenic damage.1012 In addition, acetaldehyde contributes significantly to the development of autoimmu-nity in alcoholic patients through the formation of protein adducts to enzymes, collagen, albumin, hemoglobin, and microtubules.13 These acetaldehyde-protein adducts can act as autoantigens and mediate inflammatory responses with increases immunoglobulin levels and cellular cytotoxicity, resulting in tissue damage.
The important role for acetaldehyde as a mediator of increased cancer risk in alcoholic individuals is further highlighted by the fact that patients with deficient acetaldehyde metabolism (e.g., through mutations in acetaldehyde
Clone of mutated cells
Activation of procarcinogens
(ADH and ALDH)
Production of acetaldehyde
Induction CYP450 enzymes
Interaction with growth factors
Interaction with growth factors
Impairment of immune function
Production of free radicals
Inhibition of Oxidative DNA repair DNA damage
Dysregulation of apoptosis
Production of free radicals
Inhibition of Oxidative DNA repair DNA damage
Induction of inflammation
(Folate, Vitamin B6, Vitamin A, Vitamin E, Zinc, Selenium, etc.)
Biochemical and Molecular Alterations (Aberrant DNA methylation, diminished antioxidant capacity, malfunction of retinoid receptors and other nuclear receptors, etc.)
FIGURE 28.1 Simplified schematic illustration of possible mechanisms for excessive ethanol effects on carcinogenesis. (See the text for more detail.)
dehydrogenase 2, ALDH2, gene) have significantly elevated acetaldehyde levels and increased risk of cancer.14-16 Inactivating polymorphisms of ALDH2 lead to significantly slowed acetaldehyde metabolism and are strongly associated with esophageal squamous cell carcinoma in Asian drinkers.14 Patients with ALDH2 polymorphisms also have a higher risk for multiple cancers, especially of the oropharynx and stomach. When the ALDH2 polymorphism occurs in combination with a less active form of ADH2, this risk is further increased.17 For Caucasians, ADH1 polymorphisms (ADH1C*1) resulting in increased enzyme activity and increased acetaldehyde production lead to higher risk of upper aerodigestive tract and liver cancer in heavy alcohol drinkers.18 It should be mentioned that ADH plays a role in the oxidation of vitamin A (retinol) to retinoic acid.19 In the presence of alcohol, the reaction velocity for retinal formation from retinol, the rate-limiting step in the synthesis of retinoic acid, is dramatically reduced through competitive inhibition.20 Han et al.21 showed that the retinol-oxidizing activity of ADH1 was 90% inhibited by 5 mM ethanol (blood ethanol levels of 5 to 20 mM are usually reached after social drinking), and the retinol-oxidizing activity of some forms of ADH2 and ADH3 was 60 to 80% inhibited by 20 or 50 mM ethanol (only seen in heavy drinking). Kedishvili et al.22 showed that the contribution of ADH isozymes to retinoic acid biosynthesis depends on the amount of free retinol in cells, and that physiological levels of ethanol can substantially inhibit the oxidation of retinol by human ADHs. These earlier observations have been substantiated by the demonstration that biosynthesis of retinoic acid following a dose of retinol was reduced by 82% in ADH null mutant mice (ADH1-/-).23 This reduction was similar in magnitude to the inhibition in retinoic acid biosynthesis seen in wild-type mice treated with ethanol (87% decrease). In addition, it has been reported that ethanol inhibits the oxidation of retinol into retinoic acid in the human gastric and esophageal mucosa and rat colon mucosa24 and the acetaldehyde inhibits the generation of retinoic acid in human prenatal tissue.25 The importance of the ADH system for the oxidation of retinol to retinoic acid is supported by the observation that retinol oxidation is inhibited to a similar degree in ADH-/- mice as it is after ethanol pretreatment.20 These studies clearly demonstrate that retinoic acid biosynthesis can be impaired by ethanol via competition for ADH and ALDH, which may contribute to the increased risk of developing certain alcohol-related cancers. This has been shown for the liver and colon, two primary organs affected by cancer in alcoholic patients.26-28 It has been reported that the presence of an inactivating ADH2 polymorphism leads to slower metabolism of alcohol to acetaldehyde;16 however, whether the presence of inactivating ADH and ALDH polymorphisms lead to slower conversion of retinol to retinal and, further, to retinoic acid is currently unknown.
28.2.2 Induction of Cytochrome P4502E1 and Activation of Chemical Procarcinogens
Chronic alcohol consumption induces cytochrome P450 isoform 2E1 (CYP2E1) in the liver and, to a lesser degree, in other target organs such as the lungs and the mucosal layers of the esophagus and lower gastrointestinal tract.1 This enzyme, a microsomal cytochrome oxidase, catalyses the conversion of ethanol into acetaldehyde, and is able to metabolize a wide variety of xenobiotics.29 30 After ethanol consumption, the activity of CYP2E1 is increased to four- to tenfold levels in the liver, with a high rate of individual variation.31 Activation of CYP2E1 activity can be observed after 1 week at relatively low levels of alcohol intake (40 g/day).31 It is assumed that CYP2E1 induction after ethanol consumption occurs through two distinct pathways, post-translational mechanisms at low eth-anol concentrations, and increased mRNA transcription at high ethanol concentrations.32 The activation of CYP2E1 may be modulated by hormones and growth factors, but the details regarding this regulation are not yet understood.
The important role for CYP2E1 induction as a link between alcohol abuse and cancer is highlighted by the fact that high CYP2E1 expression has been found in the liver and in peripheral tissues (oral cavity, esophagus, colon, and rectum), which are known from epidemiological studies to have increased risk of cancer formation associated with chronic alcohol consumption.130 In the presence of alcohol, the hepatic first-pass metabolism is reduced for many carcinogens, including nitrosamines. This leads to higher peripheral tissue levels of nitro-samines, which are then activated by mucosal CYP2E1. Inducible expression of CYP2E1 exposes the liver and peripheral tissues to a range of pathogenic and potentially carcinogenic substances due to this enzyme's low substrate specificity.29 Inducible expression of CYP2E1 leads to generation of reactive and harmful acetaldehyde in peripheral tissues, along with increased oxidative stress.1 It also exposes the peripheral epithelium to activated carcinogenic metabolites from procarcinogens such as nitrosamines, alkanes, aflatoxin, vinylchloride, and aromatic hydrocarbons, which would otherwise be metabolized in the liver. CYP2E1 induction by alcohol leads to accelerated metabolism of a multitude of therapeutic drugs, thereby leading to increased toxicity in some (e.g., acetaminophen) and decreased therapeutic effect in others (e.g., inhalation anesthetics), a fact that should be considered when drugs are prescribed.31 For example, chronic ethanol intake increases catabolism of vitamin A (retinol and retinoic acid) into more polar metabolites in the liver.33-35 Recent studies have shown that the enhanced catabolism of retinol and retinoic acid in ethanol-fed rats can be inhibited by chlormethiazole (an inhibitor of CYP2E1) in vitro and in vivo,34,35 indicating that CYP2E1 is the major enzyme responsible for the ethanol-enhanced catabolism of retinoic acid in hepatic tissue after exposure to alcohol. It is possible that CYP2E1 enzyme induction in chronic intermittent drinking could continue to be a factor mediating oxidative stress and destroying retinol and retinoic acid, even after alcohol is cleared. This may provide one explanation for why chronic and excessive alcohol intake is a risk not only for hepatic, but also for extra-hepatic cell proliferation and carcinogenesis, as it has been reported that CYP2E1 is also present and inducible by alcohol in the esophagus, forestomach, and surface epithelium of the proximal colon.36 It has been shown that treatment with CYP2E1 inhibitors (e.g., chlormethiazole) protects against ethanol-induced liver injury.37-39 The restoration of hepatic vitamin A status in ethanol-fed rats by chlormethiazole may provide a possible mechanism for the protective effect of chlormethiazole on ethanol-induced liver injury.35
28.2.3 Generation of Reactive Free Radicals That Cause DNA Damage
Oxidative stress is caused by an imbalance in protective cellular antioxidant systems and generation of free radicals and their metabolites. Reactive oxygen species can interact with membrane lipids to form lipid peroxides and reactive aldehydes, such as hydroxynonenal and malondialdehyde, and form DNA adducts, which increase the risk of mutations during mitosis. The formation of DNA adducts is one of the earliest events in the multistage development of cancer. Chronic alcohol consumption has been shown to increase oxidative stress on cellular and systemic levels.40 Markers of oxidative stress are increased in heavy drinkers and experimental animals, and reduced levels of plasma and tissue antioxidants have been reported.40
Several cellular mechanisms contribute to the increased generation of free radicals after acute or chronic alcohol intake. C ellular induction of the m icrosomal cytochrome P450 enzymes, especially the inducible isoform CYP2E1, results in a surplus formation of oxygen-derived free radicals and hydrogen peroxide (H2O2) within the microsomes. This enzyme has a high redox potential and was found to produce H2O2 even in the absence of hydroxylable substrates. Under in vitro conditions, cells overexpressing CYP2E1 can only survive when adequate levels of antioxidant substances (e.g., glutathione) are provided, highlighting the potential of this enzyme to increase oxidative stress.41 Independent of CYP2E1 induction, other alcohol-induced changes can result in a net increase in oxidative stress. For example, in the presence of ethanol, hydroxyethyl radicals can be formed, which have a more toxic potential than hydroxyl radicals. They interact freely with lipids and proteins, have a longer lifespan compared to hydroxyl radicals, and are able to diffuse through membrane barriers more easily based on their more hydrophobic structure. Chronic alcohol consumption also leads to alterations in mitochondrial respiratory chain enzymes, with resulting disturbances in the electron transfer from flavinmononucleotide of complex I to complex III.42 The subsequent accumulation of semiquinones is thought to contribute to increased formation of reactive oxygen species.42
In the early stages of alcohol-induced liver damage, proinflammatory cyto-kines such as tumor necrosis factor-alpha (TNF-a), interleukin 1 beta (IL-10), and IL-6 are released by neutrophils and macrophages and mediate a pronounced inflammatory reaction within the hepatic parenchyma. Hepatocytes react to these inflammatory signals by increased intracellular formation of reactive oxygen species and reactive nitrogen species. On the other hand, chronic alcohol treatment decreases glutathione peroxidase-1 levels and leads to a disturbance of the glu-tathione-based intracellular antioxidative system. This results in decreased clearance of free radicals and increased oxidative stress. Further mechanisms include vitamin E (tocopherol) depletion and alterations in the mitochondrial respiratory chain enzymes. Vitamin E is one of the most important intracellular antioxidants within the lipophilic compartment of the cell. Chronic alcohol consumption leads to lower tocopherol levels in experimental animals, independent of the level of dietary vitamin E, and increases oxidative stress. Eskelson43 has demonstrated that free radicals produced during ethanol metabolism promote tumor formation in the esophagus, whereas diets supplemented with high levels of vitamin E inhibit ethanol-induced free radical activity and suppress the promotion of cancer by ethanol. A recent study also showed that alcohol-associated colorectal hyperpro-liferation can be prevented by supplementation with a-tocopherol.44
Both chronic and acute alcohol consumption lead to a change in the overall immune system function, thereby reducing the individual's defenses against pathogenic stimuli.45 Clinically, this compromised immune system is reflected in higher rates of pneumonia and other bacterial infections. It is plausible that a compromised immune defense also leaves the organism more susceptible to cancer development. Several mechanisms contribute to the reduction of the individual's immune function, indicating direct effects of ethanol and its metabolites and indirect effects through deficient nutrient supply in alcoholic individuals.46 General changes to the immune system that can be observed in alcoholic individuals and experimental animals include atrophy of lymphoid organs, loss or redistribution of peripheral blood leukocytes, diminished hormonal and cellmediated immune response, and impaired epithelial barrier function, especially in the gastrointestinal tract.47 Alcohol displays an inhibitory effect on the function of natural killer (NK) cells in alcohol-fed animals as well as in alcohol consuming patients.48-52 It is assumed that ethanol or its metabolites has an inhibiting effect on NK cell calcium-dependent programming and signal transduction. After long-term alcohol exposure, numerical reduction in NK cells and circulating lymphocytes also contributes to a loss in NK cell function.53 Neutrophils, circulating blood cells with a key function in the defense against bacteria, show functional changes in the presence of alcohol, such as impaired migration to inflammatory foci and reduced capability to kill bacteria. Alcohol interferes with the cell-cell interactions of different immune cells, including interactions between monocytes and T lymphocytes.45 After pretreatment with alcohol, human monocytes are less able to present a pathogenic antigen to T lymphocytes, leading to reduced antigen-specific T-cell response and proliferation.54
Chronic alcoholism leads to a shift in immune cytokine signaling resulting in a reduction of cellular and increase of hormonal responses. Frequently, increases in proinflammatory cytokines IL-1, IL-6, IL-8, and TNF-a have been observed and are attributed to oversecretion by monocytes. These changes are also likely to contribute to the inflammatory response within the liver, resulting in the formation of fibrosis. Frequently an increased level of immunoglobulins can be found in alcoholic individuals, even in a situation of immunodeficiency. Some of this immunoglobulin increase is attributable to autoimmunity arising from antibody formation against protein adducts formed under the influence of highly reactive acetaldehyde.
28.2.5 Induction of Cell Hyperproliferation That Promotes Genomic Instability
Chronic alcohol intake leads to increased cellular proliferation in various tissues, such as liver, colon, and rectum.262755 Such hyperproliferation predisposes development of genetic instability and cancer development by increasing the number of cellular divisions. Several mechanisms contribute to increased cellular turnover after acute and chronic ethanol intake.56 One of the effects of ethanol on prolif-erative signaling pathways within the cells includes alteration of the mitogen-activated protein kinase (MAPK including Jun N-terminal kinase, JNK, extracellular signal-regulated kinase, ERK, and p38 kinase) pathway and its downstream cascades (e.g., C-jun is phosphorylated by JNKs, resulting in increased AP-1 transcriptional activity; Figure 28.2). Products of the two proto-oncogenes, c-Jun and c-Fos, form a complex in the nucleus, termed AP-1, that binds to a DNA sequence motif referred to as the AP-1 response element (AP-1 RE). Recent evidence has accumulated supporting a role for ethanol in the regulation of AP-1 gene expression. It has been shown that components of AP-1 are important in modulating carcinogenesis, and transactivation of AP-1-dependent genes is required for tumor promotion.57
We have observed that chronic ethanol intake in rats significantly increases hepatic c-Jun and c-Fos protein levels, as compared with control animals.58 AP-1 plays a key role in regulating proliferative target gene expression. It mediates signals from a variety of sources of proliferative stimuli, including growth factors, cytokines, oxidative stress, and others. One of its key target genes is the cell cycle regulating gene, cyclin D1, which controls progression from Gj to S phase during mitosis. In the livers of chronically ethanol fed rats, phosphorylation of c-jun and also of JNKs is significantly increased, resulting in increased AP-1-mediated transcription and cellular proliferation.2759 In transformed hepatocytes, alcohol administration leads to activation of ERK and increased DNA synthesis, and enhances the MAPK activation after G protein signaling.60 Increased ERK activation is also found in human cancers related to alcohol, such as hepatocellular carcinoma and breast cancer.
Antiproliferative and antioncogenic effects of retinoids may be mediated by inhibiting AP-1 activity.61 We have shown that the retinoic acid treatment in ethanol-fed rats dramatically inhibited the ethanol-induced overexpression of c-Jun and cyclin D1, AP-1 DNA binding activities, as well as the ethanol-induced proliferating cellular nuclear antigen-positive hepatocytes.27 Because transactivation of AP-1-dependent genes is required for tumor promotion57 and cyclin D1 plays an important role in tumorigenesis and tumor progression in hepatocellular carcinoma,62 the identification of c-Jun and cyclin D1 as two potential targets of retinoic acid action in ethanol-fed rats indicates that retinoids play an important role in preventing certain types of ethanol-promoted cancer. Furthermore, we have
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Alcoholism is something that can't be formed in easy terms. Alcoholism as a whole refers to the circumstance whereby there's an obsession in man to keep ingesting beverages with alcohol content which is injurious to health. The circumstance of alcoholism doesn't let the person addicted have any command over ingestion despite being cognizant of the damaging consequences ensuing from it.