Enhancement of ethanol induced rectal mucosal hyper regeneration with age in F344 rats

Associations between Folate and Alcohol Consumption with Colorectal Tumor Ki67 Expression in the Southern Community Cohort Study

Folate is hypothesized to accelerate cell proliferation in colorectal cancer (CRC) by supporting DNA synthesis, while alcohol is also linked to gastrointestinal epithelial proliferation, despite biological antagonism of folate. We report associations between folate and alcohol consumption with the proliferation marker Ki67 in CRC tumors from the Southern Community Cohort Study. Tumor samples were obtained from formalin-fixed paraffin-embedded tissue blocks. The percentage of cells expressing Ki67 was measured immunohistochemically. Exposures were assessed via questionnaire pre-diagnosis. Associations were assessed via linear regression. In 248 cases (40-78 years), neither dietary folate, folic acid supplements, nor total folate intake were associated with Ki67. Folic acid supplement use was associated with Ki67 in distal/rectal tumors (β [95% confidence interval]: 7.5 [1.2-13.8], p = .02) but not proximal tumors (-1.4 [-7.1-4.3], p=.62). A positive trend for total folate was observed for distal/rectal tumors (1.6 [0.0-3.3] per 200 μcg, p-trend=.05). Heavy drinking (women: ≥1 drink/day, men: ≥2 drinks/day) was associated with higher Ki67 (6.4 [1.0-11.9], vs. nondrinkers, p=.02), especially for distal/rectal tumors (10.4 [1.6-19.1], p=.02). Negative interaction between alcohol, total folate was observed for distal/rectal tumors (p-interaction=.06). Modest associations between folate, alcohol consumption and distal/rectal tumor Ki67 expression suggest accelerated proliferation, consistent with folate's role in DNA synthesis.

Alteration of oxidative-stress and related marker levels in mouse colonic tissues and fecal microbiota structures with chronic ethanol administration: Implications for the pathogenesis of ethanol-related colorectal cancer

Chronic ethanol consumption is a risk factor for colorectal cancer, and ethanol-induced reactive oxygen species have been suggested to play important roles in the pathogenesis of ethanol-related colorectal cancer (ER-CRC). In this study, the effects of 10-week chronic administration of ethanol on the colonic levels of oxidative stress and advance glycation end product (AGE) levels, as well as fecal microbiota structures, were examined in a mouse model. Chronic oral administration of ethanol in mice (1.0 mL of 1.5% or 5.0% ethanol (v/v) per day per mouse, up to 10 weeks) resulted in the elevation of colonic levels of oxidative stress markers (such as 8-hydroxy-2'-deoxyguanosine and 4-hydroxynonenal) compared to control mice, and this was consistently accompanied by elevated levels of inflammation-associated cytokines and immune cells (Th17 and macrophages) and a decreased level of regulatory T (Treg) cells to produce colonic lesions. It also resulted in an alteration of mouse fecal microbiota structures, reminiscent of the alterations observed in human inflammatory bowel disease, and this appeared to be consistent with the proposed sustained generation of oxidative stress in the colonic environment during chronic ethanol consumption. Moreover, the first experimental evidence that chronic ethanol administration results in elevated levels of advanced glycation end products (AGEs) and their receptors (RAGE) in the colonic tissues in mice is also shown, implying enhanced RAGE-mediated signaling with chronic ethanol administration. The RAGE-mediated signaling pathway has thus far been implicated as a link between the accumulation of AGEs and the development of many types of chronic colitis and cancers. Thus, enhancement of this pathway likely exacerbates the ethanol-induced inflammatory states of colonic tissues and might at least partly contribute to the pathogenesis of ER-CRC.

Alcoholic liver disease is a strong predictor of colorectal polyps in liver transplant recipients

BACKGROUND AND AIMS

 Colorectal cancer (CRC) is associated with a significantly reduced survival rate in transplant recipients. The prevalence and risk factors of CRC and of colorectal polyps after orthotopic liver transplant (OLT) remain unclear. The study aim was to determine the prevalence of colorectal polyps in OLT recipients. A secondary objective was to explore possible risk factors of polyps.

PATIENTS AND MATERIALS

 This was a retrospective single center study of all OLT recipients transplanted between 2007 and 2009. All patients who underwent a colonoscopy 5 ± 5 years after OLT were included. The outcome was colorectal polyps, as identified on colonoscopy. A logistic regression model was performed to identify potential predictors of polyps.

RESULTS

 Of 164 OLT recipients, 80 were included in this study. Polyps were diagnosed in 37 % of patients before transplant and in 33 % afterwards. With regard to post-transplant lesions, 22 % were advanced adenomas or cancerous. In the regression analysis, the odds of post-transplant polyps were 11 times higher in patients with alcoholic liver disease (OR 11.3, 95 %CI 3.2 - 39.4; P  < 0.001).

CONCLUSION

 Patients with end-stage liver disease may be at high risk of colorectal polyps before and after liver transplant, and screening should be continued in both contexts. Those with alcoholic liver disease are particularly at risk for post-OLT polyps and may benefit from more intensive screening.

Effects of moderate alcohol consumption on gene expression related to colonic inflammation and antioxidant enzymes in rats

Excessive alcohol consumption is a risk factor associated with colorectal cancer; however, some studies have reported that moderate alcohol consumption may not contribute additional risk for developing colorectal cancer while others suggest that moderate alcohol consumption provides a protective effect that reduces colorectal cancer risk. The purpose of this study was to determine the effects of moderate voluntary alcohol (20% ethanol) intake on alternate days for 3 months in outbred Wistar rats on risk factors associated with colorectal cancer development. Colonic gene expression of cyclooxygenase-2, RelA, 8-oxoguanine DNA glycosylase 1, superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glutathione-S-transferase M1, and aldehyde dehydrogenase 2 were determined. Blood alcohol content, liver function enzyme activities, and 8-oxo-deoxyguanosine DNA adducts were also assessed. Alcohol-treated rats were found to have significantly lower 8-oxo-deoxyguanosine levels in blood, a marker of DNA damage. Alanine aminotransferase and lactate dehydrogenase were both significantly lower in the alcohol group. Moderate alcohol significantly decreased cyclooxygenase-2 gene expression, an inflammatory marker associated with colorectal cancer risk. The alcohol group had significantly increased glutathione-S-transferase M1 expression, an antioxidant enzyme that helps detoxify carcinogens, such as acetaldehyde, and significantly increased aldehyde dehydrogenase 2 expression, which allows for greater acetaldehyde clearance. Increased expression of glutathione-S-transferase M1 and aldehyde dehydrogenase 2 likely contributed to reduce mucosal damage that is caused by acetaldehyde accumulation. These results indicate that moderate alcohol may reduce the risk for colorectal cancer development, which was evidenced by reduced inflammation activity and lower DNA damage after alcohol exposure.

Effects of long-term ethanol consumption and Aldh1b1 depletion on intestinal tumourigenesis in mice

Ethanol and its metabolite acetaldehyde have been classified as carcinogens for the upper aerodigestive tract, liver, breast, and colorectum. Whereas mechanisms related to oxidative stress and Cyp2e1 induction seem to prevail in the liver, and acetaldehyde has been proposed to play a crucial role in the upper aerodigestive tract, pathological mechanisms in the colorectum have not yet been clarified. Moreover, all evidence for a pro-carcinogenic role of ethanol in colorectal cancer is derived from correlations observed in epidemiological studies or from rodent studies with additional carcinogen application or tumour suppressor gene inactivation. In the current study, wild-type mice and mice with depletion of aldehyde dehydrogenase 1b1 (Aldh1b1), an enzyme which has been proposed to play an important role in acetaldehyde detoxification in the intestines, received ethanol in drinking water for 1 year. Long-term ethanol consumption led to intestinal tumour development in wild-type and Aldh1b1-depleted mice, but no intestinal tumours were observed in water-treated controls. Moreover, a significant increase in DNA damage was detected in the large intestinal epithelium of ethanol-treated mice of both genotypes compared with the respective water-treated groups, along with increased proliferation of the small and large intestinal epithelium. Aldh1b1 depletion led to increased plasma acetaldehyde levels in ethanol-treated mice, to a significant aggravation of ethanol-induced intestinal hyperproliferation, and to more advanced features of intestinal tumours, but it did not affect intestinal tumour incidence. These data indicate that ethanol consumption can initiate intestinal tumourigenesis without any additional carcinogen treatment or tumour suppressor gene inactivation, and we provide evidence for a role of Aldh1b1 in protection of the intestines from ethanol-induced damage, as well as for both carcinogenic and tumour-promoting functions of acetaldehyde, including increased progression of ethanol-induced tumours. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Possible Mechanisms of Ethanol-Mediated Colorectal Carcinogenesis: The Role of Cytochrome P4502E1, Etheno-DNA Adducts, and the Anti-Apoptotic Protein Mcl-1

BACKGROUND

Chronic alcohol consumption is a risk factor for colorectal cancer. The mechanisms by which ethanol (EtOH) exerts its carcinogenic effect on the colorectal mucosa are not clear and may include oxidative stress with the action of reactive oxygen species (ROS) generated through EtOH metabolism via cytochrome P4502E1 (CYP2E1) leading to carcinogenic etheno-DNA adducts. ROS may also induce apoptosis. However, the effect of chronic EtOH consumption on CYP2E1, etheno-DNA adducts as well as anti-apoptotic proteins in the colorectal mucosa of heavy drinkers without colorectal inflammation is still not known.

METHODS

Rectal biopsies from 32 alcoholics (>60 g EtOH/d) and from 12 controls (<20 g EtOH/d) were histologically examined, and immunohistochemistry for CYP2E1 and etheno-DNA adducts was performed. Apoptosis (cleaved PARP) as well as anti-apoptotic proteins including Bcl-x_L , Bcl-2, and Mcl-1 were immunohistochemically determined.

RESULTS

No significant difference in mucosal CYP2E1 or etheno-DNA adducts was observed between alcoholics and control patients. However, CYP2E1 and etheno-DNA adducts correlated significantly when both groups were combined (p < 0.001). In addition, although apoptosis was found not to be significantly affected by EtOH, the anti-apoptotic protein Mcl-1, but neither Bcl-x_L nor Bcl-2, was found to be significantly increased in heavy drinkers as compared to controls (p = 0.014).

CONCLUSIONS

Although colorectal CYP2E1 was not found to be significantly increased in alcoholics, CYP2E1 correlated overall with the level of etheno-DNA adducts in the colorectal mucosa, which identifies CYP2E1 as an important factor in colorectal carcinogenesis. Most importantly, however, is the up-regulation of the anti-apoptotic protein Mcl-1 in heavy drinkers counteracting apoptosis and possibly stimulating cancer development.

Mechanisms of alcohol-induced hepatocellular carcinoma

Chronic alcohol abuse is a major risk factor for hepatocellular carcinoma (HCC), the third leading cause of cancer deaths worldwide. Alcohol can also function synergistically with other risk factors to cause HCC. Hence, alcohol consumption is a major factor affecting hepatic carcinogenesis in millions and the cause of a substantial public health burden. Chronic alcohol consumption interferes with several host anti-tumor mechanisms, thereby facilitating hepatocyte proliferation and tumorigenesis. This review summarizes the major mechanisms of alcohol-induced HCC. These include pathways of ethanol metabolism, alcohol-induced oxidative stress and hypomethylation of DNA, and interplay of alcohol with iron elevation, retinoid metabolism, the immune system, inflammatory pathways, and neoangiogenesis. The relevance of each pathway in affecting HCC transformation is a topic of intense investigation. Ongoing research will enhance our insight into the alcohol-induced occurrence of HCC and offer hope in developing better therapeutics.

Acetaldehyde and retinaldehyde-metabolizing enzymes in colon and pancreatic cancers

Colorectal cancer (CRC) and pancreatic cancer are two very significant contributors to cancer-related deaths. Chronic alcohol consumption is an important risk factor for these cancers. Ethanol is oxidized primarily by alcohol dehydrogenases to acetaldehyde, an agent capable of initiating tumors by forming adducts with proteins and DNA. Acetaldehyde is metabolized by ALDH2, ALDH1B1, and ALDH1A1 to acetate. Retinoic acid (RA) is required for cellular differentiation and is known to arrest tumor development. RA is synthesized from retinaldehyde by the retinaldehyde dehydrogenases, specifically ALDH1A1, ALDH1A2, ALDH1A3, and ALDH8A1. By eliminating acetaldehyde and generating RA, ALDHs can play a crucial regulatory role in the initiation and progression of cancers. ALDH1 catalytic activity has been used as a biomarker to identify and isolate normal and cancer stem cells; its presence in a tumor is associated with poor prognosis in colon and pancreatic cancer. In summary, these ALDHs are not only biomarkers for CRC and pancreatic cancer but also play important mechanistic role in cancer initiation, progression, and eventual prognosis.

Alcoholic disease: liver and beyond

The harmful use of alcohol is a worldwide problem. It has been estimated that alcohol abuse represents the world's third largest risk factor for disease and disability; it is a causal factor of 60 types of diseases and injuries and a concurrent cause of at least 200 others. Liver is the main organ responsible for metabolizing ethanol, thus it has been considered for long time the major victim of the harmful use of alcohol. Ethanol and its bioactive products, acetaldehyde-acetate, fatty acid ethanol esters, ethanol-protein adducts, have been regarded as hepatotoxins that directly and indirectly exert their toxic effect on the liver. A similar mechanism has been postulated for the alcohol-related pancreatic damage. Alcohol and its metabolites directly injure acinar cells and elicit stellate cells to produce and deposit extracellular matrix thus triggering the "necrosis-fibrosis" sequence that finally leads to atrophy and fibrosis, morphological hallmarks of alcoholic chronic pancreatitis. Even if less attention has been paid to the upper and lower gastrointestinal tract, ethanol produces harmful effects by inducing: (1) direct damaging of the mucosa of the esophagus and stomach; (2) modification of the sphincterial pressure and impairment of motility; and (3) alteration of gastric acid output. In the intestine, ethanol can damage the intestinal mucosa directly or indirectly by altering the resident microflora and impairing the mucosal immune system. Notably, disruption of the intestinal mucosal barrier of the small and large intestine contribute to liver damage. This review summarizes the most clinically relevant alcohol-related diseases of the digestive tract focusing on the pathogenic mechanisms by which ethanol damages liver, pancreas and gastrointestinal tract.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

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Cytochrome P-450 2E1 is induced following chronic ethanol ingestion.

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CYP2E1 correlates with carcinogenic etheno-DNA formation.

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CYP2E1 and oxidative stress are important mechanisms in alcohol mediated carcinogenesis in the liver, undefined and colon.

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In NASH hepatic etheno-DNA adducts occur but possible due to inflammation.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

Acute alcohol-induced pancreatic injury is similar with intravenous and intragastric routes of alcohol administration

OBJECTIVES

Five percent of alcoholics develop an acute pancreatitis (AP). The mechanism leading to pancreatic injury is not yet understood. Microcirculatory disorders seem to play a pivotal role. The objective of this study was to compare alcoholic pancreatic injury in response to intravenous and intragastric routes of alcohol administration.

METHODS

Alcohol was applied in rats intravenously (IV) or gastric via a surgical implanted feeding tube (IG). Serum alcohol concentration was maintained between 1.5‰ and 2.5‰. Four subgroups (n = 6/group) were examined in the IV/IG arm and compared with healthy controls. Pancreatic microcirculation, enzyme levels, and morphological damage were assessed after 3, 6, 12, and 24 hours.

RESULTS

Microcirculatory analysis showed significantly disturbed pancreatic perfusion and increased adherent leukocytes in IV and IG animals. In IV and IG groups, serum amylase was increased without morphological signs of AP compared with healthy controls.

CONCLUSIONS

Alcohol application does not induce AP in rodents, but impairs pancreatic microcirculation irrespectively of the application route. Intravenous application is commonly used and shows no disadvantages compared with the physiological intragastric application form. Therefore, the intravenous route offers a valid model, which mimics the physiological process for further studies of the influence of acute alcohol intoxication on the pancreas.

Commentary: acetaldehyde and epithelial-to-mesenchymal transition in colon

Elamin and colleagues in this issue report that acetaldehyde activates Snail, a transcription factor involved in epithelial-to-mesenchymal transition, in an intestinal epithelium. Snail mediates acetaldehyde-induced tight junction disruption and increase in paracellular permeability. Results of this study and other previous studies raise several important questions. This commentary addresses these questions by discussing the acetaldehyde concentration in colon, disruption of epical junctional complexes in the intestinal epithelium by acetaldehyde, and the consequence of long-term exposure to acetaldehyde on colonic epithelial regeneration, carcinogenesis, and metastases. The precise role of acetaldehyde in colonic epithelial modifications and promotion of colorectal cancers still remains to be understood.

Acetaldehyde as a common denominator and cumulative carcinogen in digestive tract cancers

The key issue in cancer prevention is the identification of specific aetiologic factors. Acetaldehyde, the first metabolite of ethanol oxidation, is carcinogenic in animals. ADH and ALDH2 gene mutations provide an exceptional human model to estimate the long-term effects of acetaldehyde exposure in man. These models provide strong evidence for the local carcinogenic potential of acetaldehyde also in humans. Ethanol is metabolized to acetaldehyde by both mucosal and microbial enzymes. Many microbes produce acetaldehyde from ethanol, but their capacity to eliminate acetaldehyde is low, which leads to the accumulation of acetaldehyde in saliva during an alcohol challenge. Acetaldehyde is the most abundant carcinogen in tobacco smoke, and it readily dissolves into saliva during smoking. Fermented food and many alcoholic beverages can also contain significant amounts of acetaldehyde. Thus acetaldehyde, derived from mucosal or microbial oxidation of ethanol, tobacco smoke, and/or diet, appears to act as a cumulative carcinogen in the upper digestive tract of humans. The evidence strongly suggests the importance of world-wide screening of acetaldehyde and ethanol levels in many beverages and foodstuffs, as well as an urgent need for regulatory measures and consumer guidance. Screening of the risk groups with enhanced acetaldehyde exposure, e.g. people with ADH and ALDH2 gene polymorphisms and hypochlorhydric atrophic gastritis, should also be seriously considered. Most importantly, the GRAS (generally regarded as safe) status of acetaldehyde, which allows it to be used as a food additive, should be re-evaluated, and the classification of acetaldehyde as a carcinogen should be upgraded.

Acetaldehyde as an underestimated risk factor for cancer development: role of genetics in ethanol metabolism

Chronic ethanol consumption is a strong risk factor for the development of certain types of cancer including those of the upper aerodigestive tract, the liver, the large intestine and the female breast. Multiple mechanisms are involved in alcohol-mediated carcinogenesis. Among those the action of acetaldehyde (AA), the first metabolite of ethanol oxidation is of particular interest. AA is toxic, mutagenic and carcinogenic in animal experiments. AA binds to DNA and forms carcinogenic adducts. Direct evidence of the role of AA in alcohol-associated carcinogenesis derived from genetic linkage studies in alcoholics. Polymorphisms or mutations of genes coding for AA generation or detoxifying enzymes resulting in elevated AA concentrations are associated with increased cancer risk. Approximately 40% of Japanese, Koreans or Chinese carry the AA dehydrogenase 2*2 (ALDH2*2) allele in its heterozygous form. This allele codes for an ALDH2 enzyme with little activity leading to high AA concentrations after the consumption of even small amounts of alcohol. When individuals with this allele consume ethanol chronically, a significant increased risk for upper alimentary tract and colorectal cancer is noted. In Caucasians, alcohol dehydrogenase 1C*1 (ADH1C*1) allele encodes for an ADH isoenzyme which produces 2.5 times more AA than the corresponding allele ADH1C*2. In studies with moderate to high alcohol intake, ADH1C*1 allele frequency and rate of homozygosity was found to be significantly associated with an increased risk for cancer of the upper aerodigestive tract, the liver, the colon and the female breast. These studies underline the important role of acetaldehyde in ethanol-mediated carcinogenesis.

Lifetime history of alcohol consumption and K-ras mutations in pancreatic ductal adenocarcinoma

BACKGROUND

In pancreatic ductal adenocarcinoma (PDA), evidence on the etiopathogenic role of alcohol consumption in the occurrence of K-ras mutations is scant, and the role of alcohol in pancreatic carcinogenesis is not well established. We analyzed the relation between lifetime consumption of alcohol and mutations in codon 12 of the K-ras oncogene in patients with PDA.

METHODS

Incident cases of PDA were prospectively identified and interviewed face-to-face during hospital admission about lifetime alcohol consumption and other lifestyle factors. Logistic regression was used to compare PDA cases (N = 107) with mutated and wild-type K-ras tumors (case-case study).

RESULTS

Mutated cases were moderate or heavy drinkers more frequently than wild-type cases: the odds ratio adjusted by age, sex, smoking, and history of pancreatitis (ORa) was 3.18 (95% confidence interval: 1.02-9.93; P = 0.046). Total grams of alcohol and years of consumption were higher in mutated than in wild-type cases: the ORa for lifetime alcohol consumption over 507,499 g was 3.35 (95% CI: 0.81-13.88); and for more than 40 years of alcohol consumption it was 4.47 (95% CI: 1.05-19.02). Age at onset of alcohol consumption and years of abstinence were also associated with the presence of K-ras mutations. There were no significant differences in alcohol dependency.

CONCLUSIONS

Alcohol consumption is weakly associated with an increased risk of having a K-ras mutated PDA. To confirm or to refute the hypothesis that ethanol, acetaldehyde or other alcohol-related substances might influence the acquisition or persistence of K-ras mutations in the pancreatic epithelium, large and unselected studies are warranted.

Contribution of alcohol and tobacco use in gastrointestinal cancer development

Tobacco smoke and alcohol are major risk factors for a variety of cancer sites, including those of the gastrointestinal tract. Tobacco smoke contains a great number of mutagenic and carcinogenic compounds, including polycyclic carbohydrates, nitrosamines, and nicotine, while ethanol per se has only weak carcinogenic potential, but its first metabolite, acetaldehyde, is a mutagen and carcinogen, since it forms stable adducts with DNA. The possibility of proto-oncogene mutation in gastrointestinal mucosa cells may be associated with tobacco smoking-induced cancers through the formation of unfavorable DNA adducts. Individuals with defective DNA repair mechanisms and unfavorable genetic make-up for carcinogen metabolism may be at increased risk for gastrointestinal cancers. Individuals with a high production rate of acetaldehyde from ethanol also have an increased cancer risk when they drink chronically. These include individuals with a genetically determined increased acetaldehyde production due to alcohol dehydrogenase polymorphism and those with a decreased detoxification of acetaldehyde due to acetaldehyde dehydrogenase mutation. In addition, oral bacterial overgrowth due to poor oral hygiene also increases salivary acetaldehyde. Dietary deficiencies such as a lack of folate, riboflavine, and zinc may also contribute to the increase cancer risk in the alcoholic patient. It is of considerable importance that smoking and drinking act synergistically. Smoking increases the acetaldehyde burden following alcohol consumption and drinking enhances the activation of various procarcinogens present in tobacco smoke due to increased metabolic activation by an induced cytochrome P450-2E1-dependent microsomal biotransformation system in the mucosa of the upper digestive tract and the liver.

Molecular mechanisms of alcohol-mediated carcinogenesis

Approximately 3.6% of cancers worldwide derive from chronic alcohol drinking, including those of the upper aerodigestive tract, the liver, the colorectum and the breast. Although the mechanisms for alcohol-associated carcinogenesis are not completely understood, most recent research has focused on acetaldehyde, the first and most toxic ethanol metabolite, as a cancer-causing agent. Ethanol may also stimulate carcinogenesis by inhibiting DNA methylation and by interacting with retinoid metabolism. Alcohol-related carcinogenesis may interact with other factors such as smoking, diet and comorbidities, and depends on genetic susceptibility.

The role of acetaldehyde in upper digestive tract cancer in alcoholics

Chronic excessive alcohol consumption is the strongest risk factor for upper aerodigestive tract (UADT) cancer. Multiple mechanisms are involved in alcohol-associated cancer development of the UADT, including acetaldehyde (AA) effects. AA is toxic, mutagenic, and carcinogenic. Evidence of the role of AA in alcohol-associated carcinogenesis derived from genetic linkage studies in alcoholics. Polymorphism or mutation in genes coding for AA generation or detoxification enzymes are associated with increased cancer risk. It has been clearly shown in Asians that individuals carrying the acetaldehyde dehydrogenase 2*2 (ALDH2*2) allele have a significantly increased cancer risk when they consume alcohol. In Caucasians, alcohol dehydrogenase 1*1 (ADH1C*1) allele encodes for an alcohol dehydrogenase (ADH) isoenzyme, which produces 2.5 times more AA than the corresponding allele ADH1C*2. The authors found that the ADH1C*1 allele frequency and rate of homozygosity was significantly associated with an increased risk for alcohol-related cancer. AA seems to be an important factor in alcohol-associated carcinogenesis of the UADT.

The activity of class I, III, and IV of alcohol dehydrogenase isoenzymes and aldehyde dehydrogenase in gastric cancer

The metabolism of cancer is in many way different than in healthy cells. Increased metabolism of several carcinogenic substances may take place in cancer cells. The one of them was ethanol, that is oxidized by alcohol dehydrogenase (ADH) to high concentration of acetaldehyde, a toxic and carcinogenic compound. The enzyme responsible for oxidation of acetaldehyde is aldehyde dehydrogenase (ALDH). The aim of this study was to compare the capacity for ethanol metabolism measured by ADH isoenzymes and ALDH activity between gastric cancer and normal gastric mucosa. Total ADH activity was measured by photometric method with p-nitrosodimethylaniline (NDMA) as a substrate and ALDH activity by the fluorometric method with 6-methoxy-2-naphtaldehyde as a substrate. For the measurement of the activity of class I isoenzymes, we used fluorometric methods, with class-specific fluorogenic substrates. The activity of class III ADH was measured by the photometric method with n-octanol and class IV with m-nitrobenzaldehyde as a substrate. The samples were taken surgically during routine operations of gastric carcinomas from 55 patients. The activities of total ADH, and the most important in gastric mucosa, class IV ADH were significantly higher in cancer cells than in healthy tissues. The other tested classes of ADH and ALDH showed a tendency toward higher activity in cancer than in healthy mucosa. The activities of all tested enzymes and isoenzymes were not significantly higher in men than in women in wither gastric cancer tissues or normal mucosa. The increased ADH IV activity may be 1 of the factors intensifying carcinogenesis by the increased ability to acetaldehyde formation from ethanol.

Update on pancreatic cancer and alcohol-associated risk

Ductal adenocarcinoma of the pancreas is characterized by extremely aggressive behavior, with an overall 5-year survival of <4%. Because conventional and specifically tailored therapeutic regimens have little impact on patient survival, epidemiological and molecular research aims at identifying and reducing risk factors. Cigarette smoking, obesity, diabetes mellitus, and chronic pancreatitis are amenable to medical prevention or therapy. Heavy alcohol consumption is an inconsistent single risk factor for pancreatic cancer but may promote carcinogenesis by increasing the risk of diabetes mellitus or chronic pancreatitis. For various agents, the key carcinogenic effect is probably an inflammatory response in the pancreatic tissue. On the molecular level, mutations of oncogenes and tumor suppressor genes, as well as various epigenetic alterations, such as overexpression of growth factors and their receptors, are important in tumorigenesis. Complete and safe surgical resection, together with adjuvant therapy, offers prolonged survival, with 5-year survival rates of approximately 25%. However, for unresectable or disseminated disease, which constitutes the vast majority of cases, treatment is palliative. Despite increasing knowledge about the molecular pathology of pancreatic cancer and despite advances in treatment, the overall course of the disease is dismal, and reinforced efforts to reduce incidence and improve outcome are needed desperately.

Risk factors and mechanisms of hepatocarcinogenesis with special emphasis on alcohol and oxidative stress

Hepatocellular cancer is the fifth most frequent cancer in men and the eighth in women worldwide. Established risk factors are chronic hepatitis B and C infection, chronic heavy alcohol consumption, obesity and type 2 diabetes, tobacco use, use of oral contraceptives, and aflatoxin-contaminated food. Almost 90% of all hepatocellular carcinomas develop in cirrhotic livers. In Western countries, attributable risks are highest for cirrhosis due to chronic alcohol abuse and viral hepatitis B and C infection. Among those with alcoholic cirrhosis, the annual incidence of hepatocellular cancer is 1-2%. An important mechanism implicated in alcohol-related hepatocarcinogenesis is oxidative stress from alcohol metabolism, inflammation, and increased iron storage. Ethanol-induced cytochrome P-450 2E1 produces various reactive oxygen species, leading to the formation of lipid peroxides such as 4-hydroxy-nonenal. Furthermore, alcohol impairs the antioxidant defense system, resulting in mitochondrial damage and apoptosis. Chronic alcohol exposure elicits hepatocyte hyperregeneration due to the activation of survival factors and interference with retinoid metabolism. Direct DNA damage results from acetaldehyde, which can bind to DNA, inhibit DNA repair systems, and lead to the formation of carcinogenic exocyclic DNA etheno adducts. Finally, chronic alcohol abuse interferes with methyl group transfer and may thereby alter gene expression.

Alcohol consumption and risk of colon cancer: evidence from the national health and nutrition examination survey I epidemiologic follow-up study

The epidemiologic findings on the relationship between alcohol consumption and colon cancer are inconsistent. The National Health and Nutrition Examination Survey (NHANES) I Epidemiologic Follow-Up Study (NHEFS) included a prospective cohort population representative of the general U.S. population, which had not been fully utilized for examining the risk between colon cancer and alcohol drinking. The NHEFS consisted of 10,220 participants prospectively followed over a decade. Alcohol consumption, amount and type of beverage, and drinking patterns at baseline were considered in examination of the effect of alcohol consumption on the risk of colon cancer. The consumption of one or more alcoholic beverages a day at baseline was associated with approximately a 70% greater risk of colon cancer [relative risk (RR)=1.69; 95% confidence interval (CI)=1.03, 2.79], with a strong positive dose-response relationship (P=0.04). This association appeared to be exclusively related to daily drinking of one or more drinks of liquor (RR=2.48; 95% CI=1.66, 4.53). Additionally, more than a 70% increased risk of colon cancer was observed for more than 34 yr of alcohol drinking history compared with nondrinkers (RR=1.73; 95% CI=1.08, 2.78). Overall, alcohol consumption was significantly associated with increased risk of colon cancer. The most important factor for colon cancer seems to be liquor consumption.

Animal models and their results in gastrointestinal alcohol research

Alcohol-induced diseases of the gastrointestinal tract play an important role in clinical gastroenterology. However, the precise pathophysiological mechanisms are still largely unknown. Alcohol research depends essentially on animal models due to the fact that controlled experimental studies of ethanol-induced diseases in humans are unethical. Animal models have already been successfully applied to disclose and analyze molecular mechanisms in alcohol-induced diseases, partially by using knockout technology. Because of a lack of transferability of some animal models to the human condition, results have to be interpreted cautiously. For some alcohol-related diseases like chronic alcoholic pancreatitis, the ideal animal model does not yet exist. Here we provide an overview of the most commonly used animal models in gastrointestinal alcohol research. We will also briefly discuss the findings based on animal models as well as the current concepts of pathophysiological mechanisms involved in acute and chronic alcoholic damage of the esophagus, stomach, small and large intestine, pancreas and liver.

Effect of alcohol consumption on the gut

Both acute and chronic alcohol consumption have severe effects on the structure and function of the entire gastrointestinal tract (GIT) which result in a vicious cycle. The healthy person who begins to drink heavily, first experiences the toxic effects of high concentrations of ethanol. Mucosal damage compromises the basic functions of the GIT. Suppression of the gastrointestinal immune system and increased transport of toxins across the mucosa result in increased susceptibility to infections. Inhibition of digestion, absorption and secretion cause diarrhea and reduce the transfer of nutrients to the rest of the body. As the individual becomes more dependent on alcohol, the functional reserve and regenerative capacity of the GIT are overwhelmed and malnutrition increases. The rate of progression of this cycle depends on several factors including nutritional intake. Whilst the clinical effects of alcohol are well recognized, more research is required to fully elucidate the underlying mechanisms.

Alcohol consumption and cancer of the gastrointestinal tract

Epidemiological data have identified chronic alcohol consumption as a significant risk factor for upper gastrointestinal cancer (oropharynx, hypopharynx, esophagus) and colorectal cancer. Pathophysiological mechanisms include generation of acetaldehyde (AA) and reactive oxygen species (ROS), induction of cytochrome P 4502E1 (CYP2E1), and local and nutritional factors. Genetic polymorphisms of alcohol-metabolizing enzymes may individually influence the risk of carcinogenesis. AA, the first and major metabolite of ethanol, has proven to be the most carcinogenic and mutagenic agent in alcohol-associated cancer. Gastrointestinal bacteria as well as various isozymes of alcohol dehydrogenase (ADH) are capable of metabolizing ethanol to AA thus leading to an increased cell turnover of the gastrointestinal mucosa after chronic alcohol consumption. In Caucasians, ADH1C polymorphism is most important, for the ADH1C*1 transcription results in an ADH isoenzyme 2.5 times more active than that from ADH1C*2, which is associated with an increase in AA production. Additionally, oxidative stress due to an induction of CYP2E1 in the gastrointestinal mucosa of alcoholics should be considered as another key factor in alcohol-induced carcinogenesis. Nutritional deficiencies, i.e. lack of folic and retinoic acid, as well as malnutrition itself may also contribute to the development of gastrointestinal cancer.

The activity of class I, II, III, and IV alcohol dehydrogenase isoenzymes and aldehyde dehydrogenase in colorectal cancer

Chronic ethanol consumption is associated with an increased risk for cancer of the colorectum. The highly toxic and carcinogenic compound is acetaldehyde, the product of ethanol metabolism. Ethanol is metabolized to acetaldehyde by alcohol dehydrogenase (ADH) in colorectal mucosa and bacteria. The enzyme responsible for oxidation of acetaldehyde is aldehyde dehydrogenase. The aim of this study was to compare ADH isoenzymes and ALDH activity in colorectal cancer with the activity in normal colonic mucosa. Total ADH activity was measured by a photometric method with p-nitrosodimethylaniline (NDMA) as substrate, and ALDH activity by a fluorometric method with 6-methoxy-2-naphthaldehyde as a substrate. For measurement of the activity of class I and II isoenzymes we employed fluorometric methods, with class-specific fluorogenic substrates. The activity of class III ADH was measured by the photometric method with n-octanol as substrate, and class IV with m-nitrobenzaldehyde as substrate. Samples were taken surgically during routine operations of colorectal carcinomas from 32 patients. The activities of total ADH and, the most important in colon mucosa, class I ADH were significantly higher in cancer than in healthy tissues. The other tested classes of ADH had a tendency to higher-level activity in cancer cells than in healthy mucosa. ALDH activity was not significantly lower in the cancer cells. The activities of all tested enzymes and isoenzymes were not significantly higher in drinkers than in nondrinkers both in colorectal cancer and in normal mucosa. The differences in activities of total ADH and class I isoenzyme between cancer tissues and normal colon mucosa might be a factor for metabolic changes and disturbances in low-mature cancer cells and, additionally, might be a reason for the higher level of acetaldehyde, which can intensify carcinogenesis.

Increased cancer risk in heavy drinkers with the alcohol dehydrogenase 1C*1 allele, possibly due to salivary acetaldehyde

BACKGROUND

Chronic ethanol consumption is associated with an increased risk of upper aerodigestive tract cancer. As acetaldehyde seems to be a carcinogenic factor associated with chronic alcohol consumption, alcoholics with the alcohol dehydrogenase (ADH) 1C*1 allele seem to be particularly at risk as this allele encodes for a rapidly ethanol metabolising enzyme leading to increased acetaldehyde levels. Recent epidemiological studies resulted in contradictory results and therefore we have investigated ADH1C genotypes in heavy alcohol consumers only.

METHODS

We analysed the ADH1C genotype in 107 heavy drinkers with upper aerodigestive tract cancer and in 103 age matched alcoholic controls without cancer who consumed similar amounts of alcohol. Genotyping of the ADH1C locus was performed using polymerase chain reaction based on restriction fragment length polymorphism methods on leucocyte DNA. In addition, ethanol was administered orally (0.3 g/kg body weight) to 21 healthy volunteers with the ADH1C*1,1, ADH1C*1,2, and ADH1C*2,2 genotypes, and 12 volunteers with various ADH genotypes consumed ethanol ad libitum (mean 211 (29) g). Subsequently, salivary acetaldehyde concentrations were measured by gas chromatography or high performance liquid chromatography.

RESULTS

The allele frequency of the ADH1C*1 allele was found to be significantly increased in heavy drinkers with upper aerodigestive tract cancer compared with age matched alcoholic controls without cancer (61.7% v 49.0%; p = 0.011). The unadjusted and adjusted odds ratios for all cancer cases versus all alcoholic controls were 1.67 and 1.69, respectively. Healthy volunteers homozygous for the ADH1C*1 allele had higher salivary acetaldehyde concentrations following alcohol ingestion than volunteers heterozygous for ADH1C (p = 0.056) or homozygous for ADH1C*2 (p = 0.011).

CONCLUSIONS

These data demonstrate that heavy drinkers homozygous for the ADH1C*1 allele have a predisposition to develop upper aerodigestive tract cancer, possibly due to elevated salivary acetaldehyde levels following alcohol consumption.

Effect of alcohol consumption on the gut

Consumption of large quantities of alcoholic beverages leads to disturbances in the intestinal absorption of nutrients including several vitamins. The inhibition of the absorption of sodium and water caused by alcohol contributes to the tendency in alcoholics to develop diarrhoea. Excessive alcohol consumption (even a single episode) can result in duodenal erosions and bleeding and mucosal injury in the upper jejunum. An increased prevalence for bacterial overgrowth in the small intestine may contribute to functional and/or morphological abnormalities of this part of the gut and also to non-specific abdominal complaints in alcoholics. The mucosal damage caused by alcohol increases the permeability of the gut to macromolecules. This facilitates the translocation of endotoxin and other bacterial toxins from the gut lumen to the portal blood, thereby increasing the liver's exposure to these toxins and, consequently, the risk of liver injury. The results of recent experimental studies support the assumption that alcohol significantly modulates the mucosal immune system of the gut.

Animal models in gastrointestinal alcohol research-a short appraisal of the different models and their results

Alcohol-related diseases of the gastrointestinal tract play an important role in clinical gastroenterology. However, the mechanisms and pathophysiology underlying the effects of ethanol on the organs of the digestive tract are not yet completely understood. Animal models represent an essential tool for investigating alcohol-related diseases because they give researchers the opportunity to use methods that cannot be used in humans, such as knockout technology. However, there is still a need for new animal models resembling the human condition, since for some alcohol-related diseases such as chronic alcoholic pancreatitis, the ideal animal model does not yet exist. In this chapter, we provide an overview of the most commonly used animal models in gastrointestinal alcohol research. We will also briefly discuss the current concepts of the pathophysiological mechanisms involved in acute and chronic alcoholic damage of the oesophagus, stomach, small and large intestine, pancreas and liver.

Alcohol metabolism: role in toxicity and carcinogenesis

This article contains the proceedings of a symposium at the 2002 RSA Meeting in San Francisco, organized and co-chaired by Thomas M. Badger, Paul Shih-Jiun Yin, and Helmut Seitz. The presentations were (1) First-pass metabolism of ethanol: Basic and clinical aspects, by Charles Lieber; (2) Intracellular CYP2E1 transport, oxidative stress, cytokine release, and ALD, by Magnus Ingelman-Sundberg; (3) Pulsatile ethanol metabolism in intragastric infusion models: Potential role in toxic outcomes, by Thomas M. Badger and Martin J.J. Ronis; (4) Free radicals, adducts, and autoantibodies resulting from ethanol metabolism: Role in ethanol-associated toxicity, by Emanuele Albano; and (5) Gastrointestinal metabolism of ethanol and its possible role in carcinogenesis, by Helmut Seitz.

Ethanol decreases negative cell-cycle-regulating proteins in a head and neck squamous cell carcinoma cell line

Epidemiologic studies have provided evidence of an alcohol-associated increased risk of upper aerodigestive tract cancers. Recently we reported ethanol-induced proliferation in a squamous cell carcinoma of the head and neck (SCCHN) cell line, but the underlying mechanisms are unknown. In order to further clarify these findings, major G0/G1-regulating proteins were investigated in the present study. Synchronized cells of a SCCHN line (JP-PA) and a human immortalized keratinocyte line (HaCaT)-used as a control-were cultured with or without 10(-3) M ethanol for up to 96 h. At distinct time intervals the expression of cyclin D1 and the inhibitors p16, p18, p19 and p21 were determined by Western blot analyses. In both lines ethanol had no influence on the protein expression of cyclin D1. In contrast, distinct downregulations of p21, p18 and p19 were detectable at the protein level. The p16 protein was not expressed in the SCCHN line and was unchanged in the control line after the addition of ethanol. In these in vitro experiments the marked downregulation of important cell-cycle inhibitors may accelerate progression from the G1 to the S phase of the cell cycle. The relevance of our findings to in vivo conditions remains speculative, but the observed mechanisms of significantly reduced expression of cell-cycle inhibitor proteins may be involved in the carcinogenesis of head and neck malignancies.

Aldehyde dehydrogenase 2 (ALDH2) genotype affects rectal cancer susceptibility due to alcohol consumption

BACKGROUND

Epidemiologic studies have shown the association between alcohol consumption and colorectal cancer, especially for rectal cancer. The alcohol related enzyme encoding gene ALDH2 has polymorphism Glu487Lys, and 487Lys allele is closely linked with phenotypic loss of enzyme activity.

MATERIALS AND METHODS

A hospital-based case-control study was conducted with 72 colon and 70 rectal cancer cases and 241 non-cancer controls to evaluate the alcohol consumption and ALDH2 Glu487Lys polymorphism. The logistic regression model was applied to estimate the odds ratios (ORs).

RESULT

The crude ORs for Glu/Lys and Lys/Lys genotype relative to Glu/Glu for colon and rectal cancer were not statistically significant. However, with the rectal cancer analysis, the ORs for high alcohol consumption were greater with 487Glu/Lys genotype compared with Glu/Glu, albeit not.

CONCLUSIONS

These observations suggested rectal cancer risk might be influenced by ALDH2 gene polymorphism. The prevention effect by alcohol reduction might differ by ALDH2 genotype.

Removal of acetaldehyde from saliva by a slow-release buccal tablet of L-cysteine

High alcohol intake is an independent risk factor for upper gastrointestinal (GI)-tract cancers. There is increasing evidence that acetaldehyde, the first metabolite of ethanol, might be responsible for ethanol-associated carcinogenesis. Especially among Asian heavy drinkers with the ALDH2-deficiency gene, i.e., a genetic inability to remove acetaldehyde, the risk of digestive tract cancers is markedly increased. Local acetaldehyde production from ethanol either by oral microbes, mucosal cells or salivary glands is a plausible carcinogenic agent in the saliva. The aim of our study was to examine whether is it possible to bind carcinogenic acetaldehyde from saliva with L-cysteine, which is slowly released from a special buccal tablet. Nine healthy male volunteers took part in our study, and each subject served as his own control. A placebo or L-cysteine-containing tablet was fastened under the upper lip. Thereafter the volunteers ingested 0.8 g/kg of body weight of 10% (v/v) ethanol, and saliva samples were collected at 20 min intervals for 320 min. Salivary acetaldehyde and ethanol levels were analysed by headspace gas chromatography. The mean reduction of acetaldehyde concentration of the saliva with the L-cysteine tablet compared to placebo was 59% (CL(95%) 43%, 76%). Area under the curve (AUC(0-320min)) with the L-cysteine and placebo tablet were 54.3 +/- 11 microM x hr and 162 +/- 34.2 microM x hr (mean +/- SEM), respectively (p = 0.003). After alcohol intake, up to two-thirds of carcinogenic acetaldehyde can be removed from saliva with a slow-releasing buccal L-cysteine drug formulation. Thus, a buccal cysteine tablet could potentially be used to prevent upper GI-tract cancers, especially among high-risk individuals.

Dynamics of cytochrome P4502E1 activity in man: induction by ethanol and disappearance during withdrawal phase

BACKGROUND/AIMS

Chronic ethanol consumption results in the induction of hepatic cytochrome P4502E1 (CYP2E1) in man, which is believed to play an important role in the pathogenesis of alcoholic liver disease. However, the amount and duration of alcohol intake associated with CYP2E1 induction is not known but limited information is available on the disappearance of CYP2E1 following alcohol withdrawal.

METHODS

To study these questions, five healthy male volunteers received ethanol daily (40 g/day) over 4 weeks. CYP2E1 induction was monitored by using the chlorzoxazone test before and every week following the start of alcohol ingestion. In addition, CYP2E1 was also determined in five alcoholics 1, 3, 8 and 15 days following ethanol withdrawal and in five patients with non-alcoholic liver disease.

RESULTS

A significant CYP2E1 induction occurred 1 week following the ingestion of 40 g ethanol per day and increased further after 4 weeks. The disappearance of CYP2E1 was found to be significant 3 days following ethanol withdrawal and further decreased up to day 8. Thereafter, no significant change occurred and CYP2E1 activities were comparable with those in patients with non-alcoholic liver disease.

CONCLUSIONS

These data show a significant and quick induction of CYP2E1 activity, already at moderate alcohol consumption, which may be of importance in the pathogenesis of alcoholic liver disease, of ethanol, drug and vitamin A interactions and in alcohol associated carcinogenesis.

Increased rectal cell proliferation following alcohol abuse

BACKGROUND

Epidemiological data indicate an increased risk for rectal cancer following chronic alcohol consumption. As chronic ethanol ingestion leads to rectal hyperregeneration in experimental animals, indicating a state of increased susceptibility to carcinogens, we studied cell proliferation in alcohol abusers.

METHODS

Rectal biopsies were taken from 44 heavy drinkers and 26 controls. Cell proliferation, including proliferative compartment size, was measured by immunohistological staining for proliferative cell nuclear antigen (PCNA) and Ki67, and by in situ hybridisation for histone H3. Quantification of cell proliferation using PCNA staining was evaluated in 27 alcohol abusers and 12 controls. In addition, immunohistology was performed for cytokeratins and gene products of Rb1, bcl-2, and p53.

RESULTS

Heavy drinking resulted in increased cell proliferation of the rectal mucosa, as shown by increased detection of different proliferation markers. However, cell differentiation regarding cytokeratin expression patterns was unchanged as well as regulatory factors involved in carcinogenesis and/or apoptosis.

CONCLUSION

Chronic alcohol abuse leads to rectal mucosal hyperproliferation in humans, a condition associated with an increased cancer risk.

Alcohol and cancer

A great number of epidemiological data have identified chronic alcohol consumption as a significant risk factor for upper alimentary tract cancer, including cancer of the oropharynx, larynx, and the esophagus, and for the liver. In contrast to those organs, the risk by which alcohol consumption increases cancer in the large intestine and in the breast is much smaller. However, although the risk is lower, carcinogenesis can be enhanced with relatively low daily doses of ethanol. Considering the high prevalence of these tumors, even a small increase in cancer risk is of great importance, especially in those individuals who exhibit a higher risk for other reasons. The epidemiological data on alcohol and other organ cancers are controversial and there is at present not enough evidence for a significant association. Although the exact mechanisms by which chronic alcohol ingestion stimulates carcinogenesis are not known, experimental studies in animals support the concept that ethanol is not a carcinogen, but under certain experimental conditions is a cocarcinogen and/or (especially in the liver) a tumor promoter. The metabolism of ethanol leads to the generation of acetaldehyde and free radicals. These highly reactive compounds bind rapidly to cell constituents and possibly to DNA. Acetaldehyde decreases DNA repair mechanisms and the methylation of cytosine in DNA. It also traps glutathione, an important peptide in detoxification. Furthermore, it leads to chromosomal aberrations and seems to be associated with tissue damage and secondary compensatory hyperregeneration. More recently, the finding of considerable production of acetaldehyde by gastrointestinal bacteria was reported. Other mechanisms by which alcohol stimulates carcinogenesis include the induction of cytochrome P4502E1, associated with an enhanced activation of various procarcinogens present in alcoholic beverages, in association with tobacco smoke and in diets, a change in the metabolism and distribution of carcinogens, alterations in cell cycle behavior such as cell cycle duration leading to hyperregeneration, nutritional deficiencies such as methyl, vitamin A, folate, pyrridoxalphosphate, zinc and selenium deficiency, and alterations of the immune system, eventually resulting in an increased susceptibility to certain viral infections such as hepatitis B virus and hepatitis C virus. In addition, local mechanisms in the upper gastrointestinal tract and in the rectum may be of particular importance. Such mechanisms lead to tissue injury such as cirrhosis of the liver, a major prerequisite for hepatocellular carcinoma. Thus, all these mechanisms, functioning in concert, actively modulate carcinogenesis, leading to its stimulation.

Characteristics of aldehyde dehydrogenases of certain aerobic bacteria representing human colonic flora

We have proposed the existence of a bacteriocolonic pathway for ethanol oxidation resulting in high intracolonic levels of toxic and carcinogenic acetaldehyde. This study was aimed at determining the ability of the aldehyde dehydrogenases (ALDH) of aerobic bacteria representing human colonic flora to metabolize intracolonically derived acetaldehyde. The apparent Michaelis constant (Km) values for acetaldehyde were determined in crude extracts of five aerobic bacterial strains, alcohol dehydrogenase (ADH) and ALDH activities of these bacteria at conditions prevailing in the human large intestine after moderate drinking were then compared. The effect of cyanamide, a potent inhibitor of mammalian ALDH, on bacterial ALDH activity was also studied. The apparent Km for acetaldehyde varied from 6.8 (NADP+-linked ALDH of Escherichia coli IH 13369) to 205 microM (NAD+-linked ALDH of Pseudomonas aeruginosa IH 35342), and maximal velocity varied from 6 nmol/min/mg (NAD+-linked ALDH of Klebsiella pneumoniae IH 35385) to 39 nmol/min/mg (NAD+-linked ALDH of Pseudomonas aeruginosa IH 35342). At pH 7.4, and at ethanol and acetaldehyde concentrations that may be prevalent in the human colon after moderate drinking, ADH activity in four out of five bacterial strains were 10-50 times higher than their ALDH activity. Cyanamide inhibited only NAD+-linked ALDH activity of Pseudomonas aeruginosa IH 35342 at concentrations starting from 0.1 nmM. We conclude that ALDHs of the colonic aerobic bacteria are able to metabolize endogenic acetaldehyde. However, the ability of ALDHs to metabolize intracolonic acetaldehyde levels associated with alcohol drinking is rather low. Large differences between ADH and ALDH activities of the bacteria found in this study may contribute to the accumulation of acetaldehyde in the large intestine after moderate drinking. ALDH activities of colonic bacteria were poorly inhibited by cyanamide. This study supports the crucial role of intestinal bacteria in the accumulation of intracolonic acetaldehyde after drinking alcohol. Individual variations in human colonic flora may contribute to the risk of alcohol-related gastrointestinal morbidity.

Alcohol-stimulated promotion of tumors in the gastrointestinal tract

Alcohol is a major risk factor for cancers of the upper gastrointestinal tract but the association with cancers of the large bowel is not as clearly established. In recent studies, we have provided experimental support for the associations in the esophagus and oral cavity. Our studies also indicate that the tumor promotion ability of ethanol is related to its ability to generate oxygen free radicals as measured by an increase in indices of lipid peroxidation. This increase in lipid peroxidation was evident in the liver as well as the tissues targeted by the site-specific carcinogens and promoted by ethanol. Studies in mice showed that the increased lipid peroxidation as well as tumor incidence was inhibited by the administration of vitamin E, the potent antioxidant. Determination of fatty acid profiles showed significant alterations when ethanol was used as a tumor promoter after treatment with the carcinogen. Ethanol as a promoter caused an increase in esophageal polyunsaturated fatty acids (PUFA). Ethanol promotion was also evident in increased arachidonate and an exaggeration in PUFA that are involved in eicosanoid production. Thus, these results suggest that ethanol-related promotion may be the result of excessive cell proliferation induced by disordered lipid and eicosanoid metabolism that may cause a selective outgrowth of the carcinogen-initiated cells. Supporting evidence for ethanol-induced hyper-regeneration is also reviewed.

Gastrointestinal alcohol dehydrogenase

Alcohol dehydrogenase (ADH) consists of a family of isozymes that convert alcohols to their corresponding aldehydes using NAD+ as a cofactor. The metabolism of ethanol by gastrointestinal ADH isozymes results in the production of acetaldehyde, a highly toxic compound that binds to cellular protein and DNA if not further metabolized to acetate by acetaldehyde dehydrogenase isozymes. Acetaldehyde seems to be involved in ethanol-associated cocarcinogenesis. The metabolism of retinol and the generation of retinoic acid is a function of class I and class IV ADH, and its inhibition by alcohol may lead to an alteration of epithelial cell differentiation and cell growth and may also be involved in ethanol-associated gastrointestinal cocarcinogenesis.

Microbial metabolism of ethanol and acetaldehyde and clinical consequences

Many bacteria possess marked alcohol dehydrogenase activity and in the presence of ethanol they produce reactive and toxic acetaldehyde. Acetaldehyde production mediated by microbial alcohol dehydrogenases has been demonstrated in the oropharynx and bronchopulmonary washings. Also the most important gastric pathogen, Helicobacter pylori, and many skin bacteria associating with pathological dermatological conditions, possess alcohol dehydrogenase activity and produce acetaldehyde from ethanol. The most richly colonized site of the human body, however, is the large intestine, and therefore bacterial acetaldehyde production is most important in this organ. Alcohol ingested orally is transported to the colon by blood circulation and, after the distribution phase, intracolonic ethanol levels are equal to those in the blood. In the large bowel ethanol is oxidized by a bacteriocolonic pathway. In this pathway intracolonic ethanol is at first oxidized by bacterial alcohol dehydrogenase to acetaldehyde. Then acetaldehyde is oxidized either by colonic mucosal or bacterial aldehyde dehydrogenase to acetate. Part of intracolonic acetaldehyde may also be absorbed via the portal vein and metabolized in the liver. Bacteriocolonic pathway offers a new explanation for the disappearance of a part of ethanol calories. Due to the low aldehyde dehydrogenase activity of colonic mucosa acetaldehyde accumulates in the colon. Accordingly, during ethanol oxidation highest acetaldehyde levels of the body are found in the colon and not in the liver. High intracolonic acetaldehyde may contribute to the pathogenesis of alcohol-induced diarrhoea. Acetaldehyde has been proven to be a carcinogen in experimental animals. It may therefore contribute to the increased risk of colon polyps and colon cancer found to be associated with heavy alcohol consumption in man. Intracolonic acetaldehyde may also be an important determinant of blood acetaldehyde level and a possible hepatotoxin. In addition to acetaldehyde, gut-derived endotoxin is another potential candidate in the pathogenesis of alcohol-related liver injury.