Jagadeesan Nair

Chronic inflammation and oxidative stress in the genesis and perpetuation of cancer: role of lipid peroxidation, DNA damage, and repair

Background and aimsChronic inflammation, induced by biological, chemical, and physical factors, was associated with increased risk of human cancer at various sites. Chronic inflammatory processes induce oxidative/nitrosative stress and lipid peroxidation (LPO), thereby generating excess reactive oxygen species (ROS), reactive nitrogen species (RNS), and DNA-reactive aldehydes. Miscoding etheno- and propano-modified DNA bases are generated inter alia by reaction of DNA with these major LPO products. Steady-state levels of LPO-derived (etheno-) DNA adducts in organs affected by persistent inflammatory processes were investigated as potential lead markers for assessing progression of inflammatory cancer-prone diseases.ResultsUsing ultrasensitive and specific detection methods for the analysis of human tissues, cells, and urine, etheno-DNA adduct levels were found to be significantly elevated in the affected organs of subjects with chronic pancreatitis, ulcerative colitis, and Crohn’s disease. Patients with alcohol-related liver diseases showed excess hepatic DNA damage progressively increasing from hepatitis, fatty liver, to liver cirrhosis. Ethenodeoxyadenosine excreted after DNA repair in urine of hepatitis B virus-related chronic hepatitis and liver cirrhosis patients was increased up to 90-fold. Putative mechanisms that may control DNA damage in inflamed tissues including impaired or imbalanced DNA repair pathways are reviewed.ConclusionPersistent oxidative/nitrosative stress and excess LPO are induced by inflammatory processes in a self-perpetuating process and cause progressive accumulation of DNA damage in target organs. Together with deregulation of cell homeostasis, the resulting genetic changes act as driving force in chronic inflammation-associated human disease pathogenesis. Thus steady-state levels of DNA damage caused by ROS, RNS, and LPO end products provide promising molecular signatures for risk prediction and potential targets and biomarkers for preventive measures.

Etheno DNA-base adducts from endogenous reactive species

Promutagenic etheno (epsilon) adducts in DNA are generated through reactions of DNA bases with LPO products derived from endogenous sources or from exposure to several xenobiotics. The availability of sensitive methods has made it possible to detect three epsilon-adducts in vivo, namely epsilon dA, epsilon dC and N2,3-epsilon dG. One probable endogenous source for the formation of these adducts arises from LPO products such as trans-4-hydroxy-2-nonenal (HNE), resulting in highly variable background epsilon-adduct levels in tissues from unexposed humans and rodents. The range of background levels of epsilon dAx10-8dA detected inhuman tissues was <0.05 to 25 and in rodent tissues 0.02 to 10; the corresponding values for epsilon dCx10-8dC were 0.01 to 11 and 0.03 to 24, respectively. Part of this variability may be associated with different dietary intake of antioxidants and/or omega-6 PUFAs which oxidize readily to form 4-hydroxyalkenals, as epsilon dA and epsilon dC levels in WBC-DNA of female volunteers on a high omega-6 PUFA diet were drastically elevated. Increased levels of etheno adducts were also found in the liver of cancer-prone patients suffering from hereditary metal storage diseases, i.e., Wilsons disease (WD) and primary hemochromatosis (PH) as well as in Long-Evans Cinnamon rats, an animal model for WD. Increased metal-induced oxidative stress and LPO-derive epsilon-adducts, along with other oxidative damage, may trigger this hereditary liver cancer. Epsilon-Adducts could hence be explored as biomarkers (i) to ascertain the role of LPO mediated DNA damage in human cancers associated with oxidative stress imposed by certain lifestyle patterns, chronic infections and inflammations, and (ii) to verify the reduction of these epsilon-adducts by cancer chemopreventive agents. This article summarizes recent results on the formation, occurrence and possible role of epsilon-DNA adducts in carcinogenesis and mutagenesis.

DNA adducts and chronic degenerative diseases. Pathogenetic relevance and implications in preventive medicine

Chronic degenerative diseases are the leading causes of death in developed countries. Their control is exceedingly difficult due to their multiplicity and diversity, the interconnection with a network of multiple risk factors and protective factors, the long latency and multistep pathogenesis, and the multifocal localization. Adducts to nuclear DNA are biomarkers evaluating the biologically effective dose, reflecting an enhanced risk of developing a mutation-related disease more realistically than the external exposure dose. The localization and accumulation of these promutagenic lesions in different organs are the composite result of several factors, including (a) toxicokinetics (first-pass effect); (b) local and distant metabolism; (c) efficiency and fidelity of DNA repair; and (d) cell proliferation rate. The last factor will affect not only the dilution of DNA adducts but also the possible evolution towards either destructive processes, such as emphysema or cardiomyopathies, or proliferative processes, such as benign or malignant tumors at various sites. They also include heart tumors affecting fetal myocytes after transplacental exposure to DNA-binding agents, blood vessel tumors, and atherosclerotic plaques. In this article, particular emphasis is given to molecular alterations in the heart, which is the preferential target for the formation of DNA adducts in smokers, and in human aorta, where an extensive molecular epidemiology project is documenting the systematic presence of adducts to the nuclear DNA of smooth muscle cells from atherosclerotic lesions, and their significant correlation with known atherogenic risk factors. Exocyclic DNA adducts resulting from lipid peroxidation, and age-related indigenous adducts (I-compounds) may also originate from endogenous sources, chronic infections and infestations, and inflammatory processes. Type II I-compounds are bulky DNA lesions resulting from oxidative stress, whereas type II-compounds are presumably normal DNA modifications, which display positive correlations with median life span and are decreased in cancer and other pathological conditions. Profiles of type II-compounds strongly depend on diet and are related to the antidegenerative effects of caloric/ dietary restriction. Even broader is the possible meaning of adducts to mitochondrial DNA, which have been detected in rodents exposed to genotoxic agents and complex mixtures, as well as in untreated rodents, in larger amounts when compared to the nuclear DNA of the same cells. Mutations in mitochondrial DNA increase the number of oxidative phosphorylation-defective cells, especially in energy-requiring postmitotic tissues such as brain, heart and skeletal muscle, thereby playing an important role in aging and a variety of chronic degenerative diseases. A decreased formation of DNA adducts is an indicator of reduced risk of developing the associated disease. Therefore, these molecular dosimeters can be used as biomarkers in the prevention of chronic degenerative diseases, pursued either by avoiding exposure to adduct-forming agents or by using chemopreventive agents. Interventions addressed to the human organism by means of dietary measures or pharmacological agents have encountered a broad consensus in the area of cardiovascular diseases, and are deserving a growing interest also in cancer prevention. The efficacy of chemopreventive agents can be assessed by evaluating inhibition of nuclear DNA or mitochondrial DNA adduct formation in vitro, in animal models, and in phase II clinical trials in high-risk individuals.

Ultrasensitive and specific detection methods for exocylic DNA adducts : Markers for lipid peroxidation and oxidative stress

Among exocyclic DNA adducts, etheno (epsilon) bases (epsilond A, epsilond C, N(2),3-epsilond G) are generated by reactions of DNA bases with lipid peroxidation (LPO) products derived from endogenous sources and from the carcinogens vinyl chloride or urethane. The recent development of ultrasensitive methods has made it possible to detect these epsilon-adducts in vivo and to study their formation and role in experimental and human carcinogenesis. The promutagenic epsilon-DNA modifications can be detected by immunoaffinity/32P-postlabelling or by immunohistochemistry. When epsilon-adducts are excised from tissue DNA, the modified nucleosides can be quantified in urine by an immunoaffinity-HPLC-fluorescence method. Highly variable background levels of epsilon-adducts were detected in tissues from unexposed humans and rodents, suggesting an endogenous pathway of formation from reaction of trans-4-hydroxy-2-nonenal (via its 2,3-epoxide) with DNA bases. Several known cancer risk factors increased the level of these DNA lesions: Elevated epsilon-adducts were found in hepatic DNA from patients with excess metal storage (haemochromatosis, Wilsons disease), resulting in oxidative stress and high risk of liver cancer. Reactive O/N-intermediates generated during inflammatory processes, for example in patients with inflammatory bowel disease (IBD) and familial adenomatous polyposis (FAP) led to the formation of epsilon-adducts likely through peroxynitrite-mediated LPO and/or increased oxidative arachidonic acid metabolism. A high omega-6-polyunsaturated fatty acid (PUFA) diet increased epsilon-DNA adducts in white blood cells (WBC), particularly in female subjects (about 40-fold), while the level of adducted malondialdehyde in deoxyguanosine of WBC-DNA was only moderately elevated. In conclusion, there is now growing evidence that epsilon-adducts were elevated in cancer-prone patients and in rodents (liver, pancreas, colon, skin), suggesting that promutagenic epsilon-adducts, when formed as a consequence of persistent oxidative stress, can drive cells to malignancy. Therefore, biomonitoring of exocyclic DNA adducts offers useful tools: (i) to evaluate the etiological contributions of dietary fats, oxidative stress, and chronic inflammatory/infectious processes; (ii) to verify the efficacy of chemopreventive agents on endogenous DNA damage and cancer risk; and (iii) to gain mechanistic insights into the role of oxidative stress/LPO-derived lesions in the initiation and progression of human cancer.

Ethanol‐induced cytochrome P4502E1 causes carcinogenic etheno‐DNA lesions in alcoholic liver disease

Oxidative stress is thought to play a major role in the pathogenesis of hepatocellular cancer (HCC), a frequent complication of alcoholic liver disease (ALD). However, the underlying mechanisms are poorly understood. In hepatocytes of ALD patients, we recently detected by immunohistochemistry significantly increased levels of carcinogenic etheno‐DNA adducts that are formed by the reaction of the major lipid peroxidation product, 4‐hydroxynonenal (4‐HNE) with nucleobases. In the current study, we show that protein‐bound 4‐HNE and etheno‐DNA adducts both strongly correlate with cytochrome P450 2E1 (CYP2E1) expression in patients with ALD (r = 0.9, P < 0.01). Increased levels of etheno‐DNA adducts were also detected in the liver of alcohol‐fed lean (Fa/?) and obese (fa/fa) Zucker rats. The number of nuclei in hepatocytes stained positively for etheno‐DNA adducts correlated significantly with CYP2E1 expression (r = 0.6, P = 0.03). To further assess the role of CYP2E1 in the formation of etheno‐DNA adducts, HepG2 cells stably transfected with human CYP2E1 were exposed to ethanol with or without chlormethiazole (CMZ), a specific CYP2E1 inhibitor. Ethanol increased etheno‐DNA adducts in the nuclei of CYP2E1‐transfected HepG2 cells in a concentration‐dependent and time‐dependent manner, but not in vector mock‐transfected control cells. CMZ blocked the generation of etheno‐DNA adducts by 70%‐90% (P < 0.01). Conclusion: Our data support the assumption that ethanol‐mediated induction of hepatic CYP2E1 leading inter alia to highly miscoding lipid peroxidation–derived DNA lesions may play a central role in hepatocarcinogenesis in patients with ALD. (HEPATOLOGY 2009.)

Potential role of lipid peroxidation derived DNA damage in human colon carcinogenesis: studies on exocyclic base adducts as stable oxidative stress markers

Molecular pathways to colorectal cancer involve multiple genetic changes that may be caused by overproduction of reactive oxygen species in cancer-related genes. Our aim was to investigate, whether besides direct oxidative DNA damage, reactive oxygen and nitrogen species induce lipid peroxidation (LPO) that could yield etheno-DNA adducts via trans-4-hydroxy-2-nonenal, a major aldehyde generated by LPO, in colon tissue. We analyzed the etheno-DNA adducts by a highly specific, ultrasensitive method involving immunoaffinity chromatography coupled with 32P-postlabelling [Carcinogenesis 16 (1995) 613] in affected colon epithelium from ulcerative colitis, Crohns disease and familial adenomatous polyposis (FAP) and compared them with asymptomatic colon tissue. In all these cancer prone colon tissues, the formation of markedly enhanced etheno adduct levels was demonstrated for the first time. Etheno-DNA adducts are promutagenic and cause genomic instability that could drive the inflamed colonic epithelia to malignancy. Etheno-DNA adducts appear promising biomarkers for (i) quantifying increased DNA damage in early stages of colon carcinogenesis and for (ii) verifying the efficacy of new antioxidants (e.g. [Lancet Oncol. 1 (2000) 107]) and chemopreventive agents in lowering oxidative stress and related cancer risk.

Exocyclic DNA Adducts as Oxidative Stress Markers in Colon Carcinogenesis: Potential Role of Lipid Peroxidation, Dietary Fat and Antioxidants

Abstract Molecular pathways to colorectal cancer involve multiple genetic changes, whereby extensive oxyradical damage causes mutations in cancerrelated genes and leads to a cycle of cell death and regeneration. Besides direct oxidative DNAdamage, reactive oxygen and nitrogen species can induce etheno ()DNA adducts mainly via trans-4-hydroxy-2-nonenal, generated as the major aldehyde by lipid peroxidation (LPO) of ω-6 PUFAs. Patients with familial adenomatous polyposis (FAP) develop multiple colorectal adenomas. In affected tissues increased LPO could be triggered due to increased arachidonic acid metabolism as a result of elevated cyclooxygenases. Our studies demonstrated an increased DNA adduct level in affected colon epithelia of FAP patients. DNA adducts are promutagenic and can cause genomic instability that drives colorectal adenoma to malignancy. We have further investigated the potential chemopreventive properties of olive oil and its polyphenolic components. Mediterranean diet, of which olive oil is a major fatty acid source, has protective effects against human breast and colorectal cancers. Olive oil extracts and the newly identified lignan fractions showed high antioxidant capacity in vitro. As DNA adducts are biomarkers for oxidative stress and LPO induced DNA damage, they can verify the efficacy of newly identified antioxidants, e.g. from olive oil, as chemopreventive agents against colon carcinogenesis.

Increased levels of promutagenic etheno-DNA adducts in colonic polyps of FAP patients.

Nonsteroidal anti‐inflammatory drugs (NSAIDs) can regress adenomas in patients with familial adenomatous polyposis (FAP), and the mechanism involves inhibition of cyclooxygenases (COX). Reactive intermediates formed during the arachidonic acid cascade, notably by COX‐2, which is upregulated in polyps of FAP patients, may promote various stages of the polyp → adenoma → carcinoma sequence. Etheno‐DNA adducts can be derived from reactive intermediates generated during arachidonic acid metabolism and lipid peroxidation. We tested this hypothesis in colonic polyps from FAP patients and colorectal tissue from cancer patients to see whether increased formation of etheno‐DNA adducts occurs. Using an ultra‐sensitive and specific immunoaffinity/32P‐postlabelling method, 1,N6‐ethenodeoxyadenosine (ϵdA) and 3,N4‐ethenodeoxycytidine (ϵdC) were quantitated in epithelial cell DNA from asymptomatic colon, FAP polyps and colon tumor tissues. Mean adduct levels in FAP polyps were 65 ϵdA/109 and 59 ϵdC/109 parent nucleotides, being 2 to 3 times higher than in unaffected colon tissue (p < 0.02 for ϵdA; p < 0.05 for ϵdC). Adduct levels in colonic epithelia decreased in the order: FAP polyps > tumor‐adjacent tissue > tumor, normal and tumor‐distal tissue. Based on this study, requiring confirmation in a larger number of patients and in experimental models, we have demonstrated the formation of promutagenic etheno‐DNA adducts in adenomatous polyps of FAP patients that may contribute to genetic instability and cancer progression. Int. J. Cancer 87:1–4, 2000.

No prevention of liver and kidney tumors in Long–Evans Cinnamon rats by dietary curcumin, but inhibition at other sites and of metastases

Long-Evans Cinnamon (LEC) rats, an inbred mutant strain which accumulates copper due to an aberrant copper-transporting ATPase gene, develop acute hepatitis, chronic liver injury and liver tumors as a result of copper-induced oxidative stress, lipid peroxidation and DNA damage. Curcumin, an antioxidant and anti-inflammatory agent, has shown anticancer properties in many rodent models. We investigated the modulating role of curcumin in liver and kidney carcinogenesis in LEC rats. Two groups of 4-week-old LEC rats (n = 60 each) were fed either a standard diet (control) or received 0.5% curcumin in the diet for life. In untreated LEC rats, the rate of acute liver failure, the incidence of liver tumors and of kidney tumors were 32, 100 and 10% respectively, which was not altered by curcumin treatment. However, curcumin reduced tumor incidence at other organ sites (15% versus 0%; P = 0.025) and suppressed formation of metastases (18% versus 0%; P = 0.01). Median survival time was decreased from 88.7 to 78.1 weeks in curcumin-treated rats (P = 0.002). The lack of chemoprevention of liver and kidney tumors in LEC rats by curcumin may be caused by enhanced toxicity and oxidative stress due to excess copper. We conclude that curcumin should be contra-indicated for patients suffering from inherited and acquired metal storage diseases that include patients with hepatitis C virus infection.

DNA damage, and repair in mutagenesis and carcinogenesis: implications of structure-activity relationships for cross-species extrapolation

Previous studies on structure-activity relationships (SARs) between types of DNA modifications and tumour incidence revealed linear positive relationships between the log TD50 estimates and s-values for a series of mostly monofunctional alkylating agents. The overall objective of this STEP project was to further elucidate the mechanistic principles underlying these correlations, because detailed knowledge on mechanisms underlying the formation of genotoxic damage is an absolute necessity for establishing guidance values for exposures to genotoxic agents. The analysis included: (1) the re-calculation and further extension of TD50 values in mmol/kg body weight for chemicals carcinogenic in rodents. This part further included the checking up data for Swain-Scott s-values and the use of the covalent binding index (CBI); (2) the elaboration of genetic toxicity including an analysis of induced mutation spectra in specific genes at the DNA level, i.e., the vermilion gene of Drosophila, a plasmid system (pX2 assay) and the HPRT gene in cultured mammalian cells (CHO-9); and (3) the measurement of specific DNA alkylation adducts in animal models (mouse, rat, hamster) and mammalian cells in culture. The analysis of mechanisms controlling the expression of mammalian DNA repair genes (alkyltransferases, glycosylases) as a function of the cell type, differentiation stage, and cellular microenvironment in mammalian cells. The 3 classes of genotoxic carcinogens selected for the project were: (1) chemicals forming monoalkyl adducts upon interaction with DNA; (2) genotoxins capable of forming DNA etheno-adducts; and (3) N-substituted aryl compounds forming covalent adducts at the C8 position of guanine in DNA. In general, clear SARs and AARs (activity-activity relationships) between physiochemical parameters (s-values, O6/N7-alkylguanine ratios, CBI), carcinogenic potency in rodents and several descriptors of genotoxic activity in germ cells (mouse, Drosophila) became apparent when the following descriptors were used: TD50 estimates (lifetime doses expressed in mg/kg b.wt. or mmol/kg b.wt.) from cancer bioassays in rodents; the degree of germ-cell specificity, i.e., the ability of a genotoxic agent to induce mutations in practically all cell stages of the male germ-cell cycle of Drosophila (this project) and the mouse (literature search), as opposed to a more specific response in postmeiotic stages of both species; the Mexr-/Mexr+ hypermutability ratio, determined in a repair assay utilizing Drosophila germ cells; mutation spectra induced at single loci (the 7 loci used in the specific-locus test of the mouse (published data), and the vermilion gene of Drosophila); and doubling doses (DD) in mg/kg (mmol/kg) for specific locus test results on mice. By and large, the TD50 values, the inverse of which can be considered as measures of carcinogenic potency, were shown to be predictable from knowledge of the in vivo doses associated with the absorbed amounts of the investigated alkylators and with the second-order constant, kc, reaction at a critical nucleophilic strength, nc. For alkylating agents kc can be expressed as the second-order rate constant for hydrolysis, kH2O, and the substrate constant s:kH2OTD50 is a function of a certain accumulated degree of alkylation, here given as the (average) daily increment, ac, for 2 years exposure of the rodents. The TD*50 in mmol/kg x day) could then be written: [formula: see text] This expression would be valid for monofunctional alkylators provided the reactive species are uncharged. This is the case for most SN2 reagents. Although it appears possible to predict carcinogenic potency from measured in vivo doses and from detailed knowledge of reaction-kinetic parameter values, it is at present not possible to quantify the uncertainty of such predictions. One main reason for this is the complication due to uneven distribution in the body, with effects on the dose in target tissues. The estimation can be impro