Inhibition of 8-hydroxyguanine repair in testes after administration of cadmium chloride to GSH-depleted rats

Underlying mechanisms of cytotoxicity in HepG2 hepatocarcinoma cells exposed to arsenic, cadmium and mercury individually and in combination

BACKGROUND

Toxicity data regarding combinational exposure of humans to arsenic, cadmium and mercury is scarce. Although hepatotoxicity has been reported, limited information is available on their mechanistic underpinnings. The cytotoxic mechanisms of these metals were determined in HepG2 hepatocarcinoma cell lines after individual and combinational exposure.

METHODS

HepG2 cells were exposed to heavy metals (sodium arsenite, cadmium chloride, and mercury chloride) individually or in combination for 24 h, after which cell density, mitochondrial membrane potential (ΔΨm), reactive oxygen species (ROS), reduced glutathione (GSH), adenosine triphosphate (ATP) and caspase-3/7 activity was assessed.

RESULTS AND DISCUSSION

Cadmium (IC₅₀ = 0.43 mg/L) and the combination (0.45 mg/L, arsenic reference) were most cytotoxic, followed by arsenic (6.71 mg/L) and mercury (28.23 mg/L). Depolarisation of the ΔΨm and reductions in ROS, GSH and ATP levels occurred. Arsenic, cadmium and the combination increased caspase-3/7 activity, while mercury reduced it.

CONCLUSION

The combination produced a greater, albeit mechanistically similar, cytotoxicity compared to individual metals. Cytotoxicity was dependent on altered mitochondrial integrity, redox-status, and bioenergetics. Although the combination's cytotoxicity was associated with caspase-3/7 activity, this was not true for mercury. Heavy metal interactions should be assessed to elucidate molecular underpinnings of cytotoxicity.

Oxidative DNA damage in the rat lung induced by intratracheal instillation and inhalation of nanoparticles

Nanoparticles are widely used as useful industrial materials. Therefore, their possible adverse health effects must be appraised. We assessed and compared the oxidative DNA damage caused by four different nanoparticles (TiO₂, NiO, ZnO and CeO₂). The effects of the administration methods, intratracheal instillation and inhalation, were also evaluated. Rats were subjected to intratracheal instillations or 4 weeks of inhalation exposure to the nanoparticles, and the 8-hydroxydeoxyguanosine (8-OHdG) levels in the lung were analyzed by an HPLC-EC detector method. The 8-OHdG levels were increased in a dose-dependent manner with the inhalation of NiO. ZnO also increased the 8-OHdG levels with inhalation. In comparison with the control, the 8-OHdG levels were significantly and persistently higher with the CeO₂ nanoparticle administration, by both intratracheal instillation and inhalation. In contrast, there were no significant differences in the 8-OHdG levels between the control and TiO₂ nanoparticle-treated groups, with either intratracheal instillation or inhalation during the observation period. These results indicated that NiO, ZnO and CeO₂ nanoparticles generate significant amounts of free radicals, and oxidative stress may be responsible for the lung injury caused by these nanoparticles. In addition, both intratracheal instillation and inhalation exposure induced similar tendencies of oxidative DNA damage with these nanoparticles.

Programmed Effects in Neurobehavior and Antioxidative Physiology in Zebrafish Embryonically Exposed to Cadmium: Observations and Hypothesized Adverse Outcome Pathway Framework

Non-communicable diseases (NCDs) are a major cause of premature mortality. Recent studies show that predispositions for NCDs may arise from early-life exposure to low concentrations of environmental contaminants. This developmental origins of health and disease (DOHaD) paradigm suggests that programming of an embryo can be disrupted, changing the homeostatic set point of biological functions. Epigenetic alterations are a possible underlying mechanism. Here, we investigated the DOHaD paradigm by exposing zebrafish to subtoxic concentrations of the ubiquitous contaminant cadmium during embryogenesis, followed by growth under normal conditions. Prolonged behavioral responses to physical stress and altered antioxidative physiology were observed approximately ten weeks after termination of embryonal exposure, at concentrations that were 50–3200-fold below the direct embryotoxic concentration, and interpreted as altered developmental programming. Literature was explored for possible mechanistic pathways that link embryonic subtoxic cadmium to the observed apical phenotypes, more specifically, the probability of molecular mechanisms induced by cadmium exposure leading to altered DNA methylation and subsequently to the observed apical phenotypes. This was done using the adverse outcome pathway model framework, and assessing key event relationship plausibility by tailored Bradford-Hill analysis. Thus, cadmium interaction with thiols appeared to be the major contributor to late-life effects. Cadmium-thiol interactions may lead to depletion of the methyl donor S-adenosyl-methionine, resulting in methylome alterations, and may, additionally, result in oxidative stress, which may lead to DNA oxidation, and subsequently altered DNA methyltransferase activity. In this way, DNA methylation may be affected at a critical developmental stage, causing the observed apical phenotypes.

The mammary gland carcinogens: the role of metal compounds and organic solvents

The increased rate of breast cancer incidences especially among postmenopausal women has been reported in recent decades. Despite the fact that women who inherited mutations in the BRCA1 and BRCA2 genes have a high risk of developing breast cancer, studies have also shown that significant exposure to certain metal compounds and organic solvents also increases the risks of mammary gland carcinogenesis. While physiological properties govern the uptake, intracellular distribution, and binding of metal compounds, their interaction with proteins seems to be the most relevant process for metal carcinogenicity than biding to DNA. The four most predominant mechanisms for metal carcinogenicity include (1) interference with cellular redox regulation and induction of oxidative stress, (2) inhibition of major DNA repair, (3) deregulation of cell proliferation, and (4) epigenetic inactivation of genes by DNA hypermethylation. On the other hand, most organic solvents are highly lipophilic and are biotransformed mainly in the liver and the kidney through a series of oxidative and reductive reactions, some of which result in bioactivation. The breast physiology, notably the parenchyma, is embedded in a fat depot capable of storing lipophilic xenobiotics. This paper reviews the role of metal compounds and organic solvents in breast cancer development.

Earthworms and soil pollutants

Although the toxicity of metal contaminated soils has been assessed with various bioassays, more information is needed about the biochemical responses, which may help to elucidate the mechanisms involved in metal toxicity. We previously reported that the earthworm, Eisenia fetida, accumulates cadmium in its seminal vesicles. The bio-accumulative ability of earthworms is well known, and thus the earthworm could be a useful living organism for the bio-monitoring of soil pollution. In this short review, we describe recent studies concerning the relationship between earthworms and soil pollutants, and discuss the possibility of using the earthworm as a bio-monitoring organism for soil pollution.

Alcohol consumption and oxidative DNA damage

To examine the effects of alcohol consumption on cancer risk, we measured oxidative DNA damage and its repair activity in the livers and esophagi of rats fed with ethanol. Using our previously designed protocol for feeding rats with a high concentration of ethanol, we examined the effects of ethanol consumption on 8-oxo-Gua generation and repair activity in the livers and esophagi of rats. We found that a high concentration of ethanol accompanied with a vitamin-depleted diet increased 8-oxo-Gua and its repair activity. 8-Oxo-Gua is known to induce point mutations, leading to carcinogenesis. Therefore, these results suggested that a high concentration of ethanol and an irregular diet increased liver and esophageal cancer risk. On the other hand, we showed that a low concentration of ethanol decreased 8-oxo-Gua and its repair activity in the livers of mice treated with a carcinogen. Taken together, the effects of ethanol consumption on cancer risk depend on the ethanol concentration and the diet pattern.

8-Hydroxyguanine levels and repair capacity during mouse embryonic stem cell differentiation

To evaluate the defence capacities of embryonic stem (ES) cells against gene impairment, this study measured the levels of 8-hydroxyguanine (8-OH-Gua), a well-known marker of oxidative stress in DNA, and its repair capacity during differentiation. Undifferentiated ES cells (EB3) were cultured without leukaemia inhibitory factor (LIF) for 0, 4 and 7 days and are referred to as ES-D0, ES-D4 and ES-D7, respectively. These three cell lines were treated with 300 μM hydrogen peroxide (H(2)O(2)) for 48 and 72 h. After treatment, the amounts of 8-OH-Gua in the cells were determined by the high-performance liquid chromatography (HPLC)-electrochemical detector (ECD) method. The levels of 8-OH-Gua in ES-D7 treated with H(2)O(2) were higher than those in ES-D0 and ES-D4, suggesting that the DNA in the undifferentiated cells was protected against gene impairment, as compared to that in the differentiated cells. To examine the repair capacity for 8-OH-Gua, this study analysed the expression of 8-OH-Gua repair-associated genes, 8-oxoguanine DNA glycosylase 1 (OGG1), MutY homolog (MUTYH) and Mut T homolog 1 (MTH1), in ES-D0, ES-D4 and ES-D7. The mRNA levels of MUTYH and MTH1 showed no significant change, whereas OGG1 mRNA was significantly decreased in ES-D7 treated with H(2)O(2). Moreover, it was observed that ES-D7 treated with H(2)O(2) readily underwent apoptosis, in comparison to its undifferentiated counterparts, ES-D0 and ES-D4. Taken together, ES cells are more resistant to DNA oxidative stresses than differentiated cells.

Chronic alcohol consumption prevents 8-hydroxyguanine accumulation in 3'-methyl-4-dimethylaminoazobenzene-treated mouse liver

Alcohol consumption is known to have opposing effects on carcinogenesis: promotion and prevention. In this study, we examined the effects of 12% ethanol on oxidative DNA damage accumulation and its repair in mouse livers treated with 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB), a well-known hepatic carcinogen. We previously reported that 3'-MeDAB increased 8-hydroxyguanine (8-OH-Gua) accumulation and its repair activity, accompanied by the fragmentation of 8-oxoguanine DNA glycosylase 1 (OGG1), the main repair enzyme of 8-OH-Gua. The present results showed that 12% ethanol intake attenuated the 8-OH-Gua accumulation, but not the fragmentation of OGG1 induced by 3'-MeDAB. Additionally, no significant changes in oxidative status, as monitored by lipid peroxidation (LPO), were observed among the 3'-MeDAB-treated mouse livers with/without alcohol administration. These findings suggested that 12% ethanol consumption may reduce the risk of 3'-MeDAB-induced carcinogenesis by decreasing 8-OH-Gua accumulation.

Long duration exposure to cadmium leads to increased cell survival, decreased DNA repair capacity, and genomic instability in mouse testicular Leydig cells

Epidemiological and experimental studies have shown that cadmium is carcinogenic to human and experimental animals, however, the mechanism of cadmium-induced carcinogenesis is not clear. The aberrant expression of cell cycle and DNA repair genes resulting in increased cell proliferation and genomic instability are the characteristic features of cancer cells. The purpose of this study was to determine if exposure to cadmium can perturb cell proliferation/survival and causes genomic instability in TM3 cells, a mouse testicular Leydig cell line. The results of this study revealed that short-duration exposure to lower doses of cadmium significantly increase the growth of TM3 cells, whereas, higher doses are toxic and cause cell death. The long duration exposure to higher doses of cadmium, however, results in increased cell survival and acquisition of apoptotic resistance. Gene expression analysis by real-time PCR revealed increased expression of the anti-apoptotic gene Bcl-2, whereas decreased expression of pro-apoptotic gene Bax. Decreased expression of genes for maintenance of DNA methylation, DNMT1, and DNA repair, OGG1 and MYH, was also observed in cells exposed to cadmium for 24h. The random amplified polymorphic DNA (RAPD) assay revealed genomic instability in cells with chronic exposure to cadmium. The findings of this study indicate that mouse testicular Leydig cells adapt to chronic cadmium exposure by increasing cell survival through increased expression of Bcl-2, and decreased expression of Bax. The increased proliferation of cells with genomic instability may result in malignant transformation, and therefore, could be a viable mechanism for cadmium-induced cancers.

Effect of glutathione (GSH) depletion on DNA damage and blood chemistry in aged and young rats

DNA is damaged by reactive oxygen species (ROS) and such damage is age-dependent. Blood chemical parameters also change age-dependently. Glutathione (GSH) plays an important role as an antioxidant. However, the effects of GSH on DNA damage and blood chemistry are unclear. Therefore, this study was aimed to evaluate GSH contribution to DNA damage and changes of blood chemical parameters in aged and young rats. The GSH content in the livers and kidneys of aged rats (20 months) were lower than that in young rats (9 weeks of age) with higher DNA damage detected by a comet assay. There was a negative correlation between the GSH content and the DNA damage in the liver and kidney. L-buthionine (S,R)-sulfoximine (BSO; 0, 5, 20 mM), which inhibits GSH synthesis, was administered in drinking water for 28 days to young and aged rats (8 weeks and 19 months of age at the start of the administration). The treatment significantly decreased GSH levels in the heart, liver, lung and kidney of either the young or aged rats without causing DNA damage in those organs. When compared with young rats, aged rats showed higher levels in aspartate aminotransferase, alanine aminotransferase, total bilirubin, total cholesterol, globulin, creatinine, sodium and chloride and lower levels in alkaline phosphatase, triglyceride, albumin/globulin and inorganic phosphorus. However, BSO did not change these parameters in young or aged rats. These results showed that there was a negative correlation between GSH and DNA damage during aging, but the BSO-induced GSH depletion did not affect DNA damage or blood chemistry levels in young and aged rats under these study conditions.

Repair system of 7, 8-dihydro-8-oxoguanine as a defense line against carcinogenesis

Reactive oxygen species (ROS) are essentially harmful for living organisms, including human beings. It is well known that ROS-induced damage of cellular components may lead to human diseases, such as inflammatory diseases, degenerative diseases, or cancer. In particular, oxidative DNA damage is premutagenic, and thus, the generation of DNA damage and the failure of its removal are critical events for tumorigenesis or carcinogenesis. To prevent this disadvantage, living organisms have defense mechanisms against ROS-induced gene instability. Studies of 8-oxo-Gua and its main repair enzyme, 8-oxoguanine DNA glycosylase 1 (OGG1), are informative and useful, because 8-oxo-Gua is commonly observed in DNA, and OGG1 enzymes exist in a wide variety of living organisms. The importance of OGG1 was confirmed by polymorphism analyses and studies using knockout mice. Moreover, analyses of the influences of environmental factors on DNA damage and repair systems have confirmed the effects of heavy metals on 8-oxo-Gua formation and OGG1 expression. These studies revealed that the 8-oxo-Gua repair system is crucial for the prevention of mutation-related diseases, such as cancer. In this review, the advances in this field during the last two decades are described.

DNA repair systems as targets of cadmium toxicity

Cadmium (Cd) is a heavy metal and a potent carcinogen implicated in tumor development through occupational and environmental exposure. Recent evidence suggests that proteins participating in the DNA repair systems, especially in excision and mismatch repair, are sensitive targets of Cd toxicity. Cd by interfering and inhibiting these DNA repair processes might contribute to increased risk for tumor formation in humans. In the present review, the information available on the interference of Cd with DNA repair systems and their inhibition is summarized. These actions could possibly explain the indirect contribution of Cd to mutagenic effects and/or carcinogenicity.

Urinary 8-hydoxydeoxyguanosine (8-OHdG) and plasma malondialdehyde (MDA) levels in Aldh2 knock-out mice under acetaldehyde exposure

To clarify the carcinogenicity of acetaldehyde when associated with ALDH (aldehyde dehydrogenase) 2 polymorphism, Aldh2 knock-out (Aldh2-/-) mice and their wild type (Aldh2+/+) mice were exposed to two different concentrations of acetaldehyde (125 ppm and 500 ppm) for two weeks. Aldh2-/- mice, which have the same genetic background as C57BL/6J (wild mice) except for the Aldh2 gene, were used as models of humans who lack ALDH2 activity. Urinary 8-hydroxydeoxyguanosine (8-OHdG) and plasma malondialdehyde (MDA) levels were measured as indicators of oxidative DNA damage and lipid peroxidation, respectively. At 125 ppm acetaldehyde exposure for 12 d, urinary 8-OHdG levels in Aldh2+/+ mice did not increase. However, urinary 8-OHdG levels in Aldh2-/- mice were slightly increased by the end of the exposure. On the other hand, plasma MDA levels did not increase in either Aldh2-/- orAldh2+/+ mice. At 500 ppm, urinary 8-OHdG levels in both Aldh2-/- and Aldh2+/+ mice significantly increased after 6 and 12 d, but there was no genetic difference. On the other hand, plasma MDA levels in Aldh2+/+ and Aldh2-/- mice did not increase at either 125 ppm or 500 ppm after two weeks of exposure. In conclusion, it is suspected that DNA was damaged by acetaldehyde inhalation, and that susceptibility to acetaldehyde varies according to Aldh2 genotype.

Cadmium Down-regulates Human OGG1 through Suppression of Sp1 Activity

Cadmium is a well known human and animal carcinogen and is a ubiquitous contaminant in the environment. Although the carcinogenic mechanism of cadmium is a multifactorial process, oxidative DNA damage is believed to be of prime importance. In particular, cadmium suppresses the capacity of cells to repair oxidative DNA damage. In this study, cadmium treatment led to a significant increase in gamma-ray-induced 8-oxoguanine (8-oxoG) formation. Western blotting and semiquantitative reverse transcription-PCR revealed that cadmium treatment caused a decrease in the expression level of human OGG1 (8-oxoguanine-DNA glycosylase-1; hOGG1) in human fibroblast GM00637 and HeLa S3 cells. In addition, the cadmium-mediated decrease in hOGG1 transcription was the result of decreased binding of the transcription factor Sp1 to the hOGG1 promoter. Finally, we show that an increase in the functional hOGG1 expression level could inhibit the cadmium-mediated increase in gamma-ray-induced 8-oxoG accumulation as well as in gamma-radiation-induced mutation frequency at the HPRT (hypoxanthine-guanine phosphoribosyltransferase) gene locus. These results suggest that cadmium attenuates removal of gamma-ray-induced 8-oxoG adducts, which in turn increases the mutation frequency, and that this effect might, at least in part, result from suppression of hOGG1 transcription via inactivation of Sp1 as a result of cadmium treatment.

Detection of a mouse OGG1 fragment during caspase-dependent apoptosis: oxidative DNA damage and apoptosis

We investigated the expression of mouse 8-oxoguanine DNA glycosylase 1 (mOGG1) in mouse non-parenchymal hepatocytes (NCTC) during etoposide- or mitomycin C (MMC)-induced apoptosis. We observed mOGG1 fragmentation in apoptotic cells. The apoptosis accompanying the fragmentation of mOGG1 was caspase-dependent. The mOGG1 fragment existed in both the cytoplasm and nucleus of the etoposide-treated NCTC, indicating that the mOGG1 fragment could be transferred into the nucleus. In addition, 8-hydroxyguanine (8-OH-Gua, 7,8-dihydro-8-oxoguanine) accumulated in the DNA of NCTC treated with etoposide, suggesting that the mOGG1 fragment might not function as a normal repair enzyme in etoposide-treated NCTC. Although we have not clarified in detail the mechanism and the significance of the mOGG1 fragmentation, further study of the fragmentation of DNA repair enzymes might provide insights into the relationship between oxidative DNA damage and apoptosis.

Antioxidants and radical scavenging properties of vegetable extracts in rats fed aflatoxin-contaminated diet

The present study evaluated the protection role of garlic, cabbage, and onion extracts against the toxic effects of aflatoxin. One hundred and twenty mature male Sprague-Dawley rats were randomly assigned to eight experimental groups and treated for 15 days with extracts with or without aflatoxin. Blood samples were collected from all animals from the retro-orbital venous plexus at the end of the experimentation period for biochemical analysis. Livers and kidneys were removed at the end of the treatment period for determination of glutathione, malondialdehyde, and superoxide dismutase. The results indicated that animals treated with aflatoxin showed significant signs of aflatoxicosis. Extracts alone had insignificant effects on all parameters tested, whereas cotreatment with aflatoxin and extracts resulted in a significant improvement in all parameters; moreover, garlic extract was found to be the most effective in the prevention of aflatoxin-induced toxicity and free radical generation in rats.

Cadmium exposure down-regulates 8-oxoguanine DNA glycosylase expression in rat lung and alveolar epithelial cells

The current study tested the hypothesis that the pulmonary carcinogenic potential of cadmium (Cd) is related to its ability to inhibit the expression (mRNA and protein) and activity of 8-oxoguanine-DNA glycosylase (OGG1), a base excision repair (BER) enzyme that functions to preferentially excise pre-mutagenic 7,8-dihydro-8-oxoguanine (8-oxoG) from DNA. We demonstrate that a single Cd aerosol exposure of adult male Lewis rats causes time- and dose-dependent down-regulation in the pulmonary levels of rOGG1 mRNA and OGG1 protein, quantified by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assays and western analyses, respectively. Immunohistochemical studies confirmed that Cd inhalation reduces the relative amount of OGG1 in lungs of exposed animals without altering its over-all distribution within the lung, which appears to be more prominent within the alveolar epithelium. In agreement with our in vivo studies, we show that OGG1 expression is also attenuated in alveolar epithelial cell cultures exposed to CdCl(2) either acutely or by repeated passaging in Cd-containing medium. The effects caused by Cd were observed in cells that show no loss in viability, as assessed by colony forming ability, the MTT assay, and propidium iodide membrane permeability studies. Nuclear extracts prepared from Cd-treated cells also exhibit a reduction in the ability to nick a synthetic oligonucleotide containing 8-oxoG. We conclude from these studies that Cd causes suppression of OGG1 in the lung and that this mechanism may, in part, play a role in the Cd carcinogenic process.

Acute arsenite-induced 8-hydroxyguanine is associated with inhibition of repair activity in cultured human cells

8-Hydroxyguanine (8-OH-Gua) is one of the major modified bases in DNA produced by oxidative damage. Human lung carcinoma cells (A549) were treated with 0.5-2mM sodium arsenite for 4h. By an immunohistochemical type procedure, 8-OH-Gua was clearly detected in A549 cells using a fluorescence microscope and an increase in the percentage of A549 cells with oxidative DNA damage was observed using flow cytometry. The formation of 8-OH-Gua in DNA was also detected by a HPLC-ECD. A dose-dependent increase in oxidative DNA damage in A549 cells with increasing arsenite concentrations was obtained. Therefore, oxidative stress is induced after arsenite treatment. Furthermore, we also found that arsenite decreased the activity of the 8-OH-Gua repair enzyme, hOGG1 (8-oxoguanine-DNA glycosylase 1) as well as its gene and protein expression. We conclude that the 8-OH-Gua level in cultured human cells increases partly by the generation of reactive oxygen species (ROS) and partly by the influence on hOGG1 expression, followed by the inhibition of the repair activity for 8-OH-Gua.

Inactivation of mammalian 8-oxoguanine-DNA glycosylase by cadmium(II): implications for cadmium genotoxicity

Cadmium(II) is a toxic, mutagenic and carcinogenic metal (IARC Class 1 human carcinogen). It causes damage to eukaryotic cells both in acute and chronic modes of exposure via multiple biochemical mechanisms. In particular, Cd diminishes the capacity of cells to repair oxidative DNA damage. Oxidative DNA lesions are important precursors to mutations and ultimately may lead to neoplastic transformation of human cells. We investigated interactions of Cd with murine Ogg1 (mOgg1), an enzyme that removes 8-oxoguanine (8-oxoG), an abundant oxidative lesion, from DNA. Cd(2+) and Zn(2+), but not other divalent cations tested, suppressed mOgg1-catalyzed reactions. The apparent inhibition by Cd consisted of at least two independent processes: irreversible, DNA-independent first-order inactivation of mOgg1 and DNA-dependent inhibition. Irreversibly inactivated mOgg1 has nearly normal affinity for damaged DNA and a normal catalytic rate constant but is defective in formation of the covalent reaction intermediate. When both modes of inhibition are in effect, the catalytic rate constant is dramatically lowered, while affinity to damaged DNA is decreased moderately. Potential sites for Cd binding in mOgg1 and mOgg1-DNA complex are identified. Inactivation of Ogg1 may play a role in the mutagenic and carcinogenic action of Cd.

The role of metals in site-specific DNA damage with reference to carcinogenesis

We reviewed the mechanism of oxidative DNA damage with reference to metal carcinogenesis and metal-mediated chemical carcinogenesis. On the basis of the finding that chromium (VI) induced oxidative DNA damage in the presence of hydrogen peroxide (H2O2), we proposed the hypothesis that endogenous reactive oxygen species play a role in metal carcinogenesis. Since then, we have reported that various metal compounds, such as cobalt, nickel, and ferric nitrilotriacetate, directly cause site-specific DNA damage in the presence of H2O2. We also found that carcinogenic metals could cause DNA damage through indirect mechanisms. Certain nickel compounds induced oxidative DNA damage in rat lungs through inflammation. Endogenous metals, copper and iron, catalyzed ROS generation from various organic carcinogens, resulting in oxidative DNA damage. Polynuclear compounds, such as 4-aminobiphenyl and heterocyclic amines, appear to induce cancer mainly through DNA adduct formation, although their N-hydroxy and nitroso metabolites can also cause oxidative DNA damage. On the other hand, mononuclear compounds, such as benzene metabolites, caffeic acid, and o-toluidine, should express their carcionogenicity through oxidative DNA damage. Metabolites of certain carcinogens efficiently caused oxidative DNA damage by forming NADH-dependent redox cycles. These findings suggest that metal-mediated oxidative DNA damage plays important roles in chemical carcinogenesis.

Increasing the DNA damage threshold in breast cancer cells

The biochemical role of estrogens in the development of estrogen-dependent breast cancer remains to be elucidated, and the involvement of estrogens in tumor initiation remains controversial. Reports regarding estrogen-mediated DNA damage include the induction of 8-oxo-2'-deoxyguanosine (8-oxo-dG) in vitro and in vivo, indicating a role for oxidative stress in tumor initiation and/or progression. However, DNA isolation, cellular DNA repair, and high antioxidant status have made the measurement of 8-oxo-dG in vivo and in cell culture somewhat challenging. In this regard, a potentiation in DNA damage can be achieved by depleting cellular stores of glutathione. We chose to deplete glutathione in the estrogen receptor (ER)-positive MCF-7 breast cancer cell line with a gamma-glutamylcysteine transpeptidase enzyme inhibitor buthionine sulphoximine (BSO) for the purpose of studying estrogen-induced DNA damage. Treatment of GSH-depleted MCF-7 cells with 10 microM 2-OH-E2 or 4-OH-E2 for 30 min resulted in a statistically significant increase in 8-oxo-dG/10(5) dG of 127 and 160%, respectively. A potentiation in catechol estrogen-induced DNA damage was observed with the addition of copper(II) chloride for both 2-OH-E2 and 4-OH-E2 by 165 and 200%, respectively. In addition, 100 nM and 1.0 microM estradiol increased DNA damage in a dose-response-like fashion by 145 and 189%, respectively. The depletion of GSH by BSO may prove to be an advantageous technique for the study of DNA damage in cells otherwise resistant to oxidative stress and/or alkylating agents and has proven useful in the study of estrogen-induced oxidative DNA damage in a highly reproducible and sensitive manner.

Intracellular distribution of the antimutagenic enzyme MTH1 in the liver, kidney and testis of F344 rats and its modulation by cadmium

Cellular distribution of the antimutagenic MTH1protein in the liver, kidney, and testis of Fischer rat was evaluated using the immunohistochemical staining with anti-MTH1 polyclonal antibody. The present investigation revealed a non-uniform distribution of MTH1 among cells and among the cytoplasmic, nuclear, and membranal structures of cells within a given tissue. A particularly strong expression of MTH1 was observed for the first time in the perinuclear acrosomic bodies of spermatocytes and in the acrosomic vesicles of sperm heads. Treatment of rats with a single sc dose of 20 micromol Cd(II)/kg body wt. produced histopathologic changes in these organs accompanied by redistribution of the cellular MTH1 protein between the cytoplasm and nuclei. The acute phase of Cd(II) toxicity, that in the liver and especially in the testes (but not in kidneys) led to cell necrosis, was accompanied by a characteristic decrease in the abundance of MTH1-expressing nuclei. Chronic toxicity without necrosis, persisting in the kidney over the entire 14-day study, as well as the survival and proliferation of cells, observed in the liver and testis after the necrotizing phase, were signified by increased number of nuclei expressing MTH1. Thus, unlike previous biochemical studies, immunohistochemistry managed to reveal alterations in the patterns of inter- and intracellular distribution of MTH1, associated apparently with the conditional changes in the dynamics of synthesis of nucleic acids, assisted by this protein.

Increase in 8-hydroxyguanine and its repair activity in the esophagi of rats given long-term ethanol and nutrition-deficient diet

Epidemiological studies have shown that an increased risk of esophageal cancer is associated with the chronic consumption of alcoholic beverages, although alcohol itself is not a carcinogen in animal models. Reactive oxygen species produced by the metabolism of ethanol or by chronic inflammation may play an important role in the carcinogenic process. In this study, we analyzed one type of oxidative DNA damage, 8-hydroxyguanine (8-OH-Gua), and its repair activity in the esophagus as indicators of cellular oxidative stress in rats given long-term ethanol and an autoclaved diet (nutrition-deficient diet). Three-week-old male Sprague-Dawley rats were fed an ethanol beverage whose concentration was increased from 12 to 70% over 20 weeks. When the concentration reached 50%, the diet of one group was changed from the regular diet to an autoclaved diet. At the feeding periods of 20, 25, 30, and 35 weeks, the rats were sacrificed and the 8-OH-Gua levels and repair activities within the esophagi were measured. After 30 weeks of ethanol- and autoclaved diet-feeding, significant increases of 8-OH-Gua and its repair activity were observed in the esophagi, but not in those of the ethanol- and normal diet-fed rats. This result indicates that the combined effects of long-term ethanol consumption and nutritional deficiency may be involved in inducing oxidative stress in the rat esophagus.

Inhibition of antimutagenic enzymes, 8-oxo-dGTPases, by carcinogenic metals. Recent developments

Nickel, cadmium, cobalt, and copper are carcinogenic to humans and/or animals, but the underlying mechanisms are poorly understood. Our studies have been focused on one such mechanism involving mediation by the metals of promutagenic oxidative damage to DNA bases. The damage may be inflicted directly in DNA or in the deoxynucleotide pool, from which the damaged bases are incorporated into DNA. Such incorporation is prevented in cells by 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphatases (8-oxo-dGTPases). Thus, inhibition of these enzymes should enhance carcinogenesis. We have studied effects of Cd(II), Cu(II), Co(II), and Ni(II) on the activity of isolated bacterial and human 8-oxo-dGTPases. Cd(II) and Cu(II) were strongly inhibitory, while Ni(II) and Co(II) were much less suppressive. After developing an assay for 8-oxo-dGTPase activity, we confirmed the inhibition by Cd(II) in cultured cells and in the rat testis, the target organ for cadmium carcinogenesis. 8-Oxo-dGTPase inhibition was accompanied by an increase in the 8-oxo-dG level in testicular DNA.

Iron-induced oxidative DNA damage in rat sperm cells in vivo and in vitro

We investigated whether acute iron intoxication causes oxidative DNA damage, measured in terms of 7-hydro-8-oxo-2'-deoxyguanosine, 8-oxodG, in nuclear DNA in testes and epididymal sperm cells in vivo and in vitro in rats. In addition, we investigated levels of the modified nucleoside in liver and kidney and measured its urinary excretion. Sperm cells were isolated from the epididymides and the testes cells were isolated after homogenisation. In vitro, the sperm and testes cells were incubated with increasing concentrations of FeCl2 ranging from 0 to 600 microM. The median (range) levels of 8-oxodG/10(5) dG in the epididymal sperm cells increased from 0.48 (0.42-0.90) to 15.1 (11.4-17.6) (p < 0.05), whereas the level rose from 0.63 (0.22-0.81) to 8.8 (4.5-11.6) (p < 0.05) at 0 and 600 microM, respectively, in the testicular cells. In vivo groups of 7-8 rats received 0, 200 or 400 mg iron/kg as dextran i.p. After 24 h, epididymal sperm cells, testes, kidneys and liver were collected for analysis. Kidney and sperm DNA showed a significant increase in 8-oxodG in the iron-treated animals. The median (range) values of the 8-oxodG/10(5) dG in the epididymal sperm cells rose from 0.66 (0.38-1.09) to 1.12 (0.84-5.88) (p < 0.05) at 0 and 400 mg iron/kg, respectively, whereas the values in the testes and liver showed no significant change. In the kidneys the 8-oxodG/10(5) dG median (range) values were 0.98 (0.73-1.24), 1.21 (1.13-1.69) and 1.34 (1.12-1.66) after 0, 200 and 400 mg iron/kg, respectively (p < 0.05). The 8-oxodG-excretion rate was measured in 24h urine before and after iron treatment. The rate of urinary 8-oxodG excretion increased from 129 (104-179) pmol/24 h before treatment to 147 (110-239) pmol/24 h after treatment in the group receiving 400 mg iron/kg (p < 0.05). The results indicate that acute iron intoxication may increase oxidative damage to sperm and kidney DNA.

Distinct responses of a recA::luxCDABE Escherichia coli strain to direct and indirect DNA damaging agents

The recombinant Escherichia coli strain DPD2794 containing a recA::luxCDABE fusion is used to detect genotoxicity of various chemicals. Genotoxic agents were previously categorized into two groups, Direct DNA Damaging (DDD) agents and Indirect DNA Damaging (IDD) agents; these two groups have been distinguished with this strain. Minimum detectable concentrations of the DDD agents were about one to five orders of magnitude lower than those of the IDD agents. The response patterns of this strain to DDD agents differed from those to IDD agents in terms of kinetics and the forms of the dose-dependent response.

Experimental study of oxidative DNA damage

Animal experiments allow the study of oxidative DNA damage in target organs and the elucidation of dose-response relationships of carcinogenic and other harmful chemicals and conditions as well as the study of interactions of several factors. So far the effects of more than 50 different chemical compounds have been studied in animal experiments mainly in rats and mice, and generally with measurement of 8-oxodG with HPLC-EC. A large number of well-known carcinogens induce 8-oxodG formation in liver and/or kidneys. Moreover several animal studies have shown a close relationship between induction of dative DNA damage and tumour formation. In principle the level of oxidative DNA damage in an organ or cell may be studied by measurement of modified bases in extracted DNA by immunohistochemical visualisation, and from assays of strand breakage before and after treatment with repair enzymes. However, this level is a balance between the rates of damage and repair. Until the repair rates and capacity can be adequately assessed the rate of damage can only be estimated from the urinary excretion of repair products albeit only as an average of the entire body. A number of model compounds have been used to induce oxidative DNA damage in experimental animals. The hepatocarcinogen 2-nitropropane induces up to 10-fold increases in 8-oxodG levels in rat liver DNA. The level of 8-oxodG is also increased in kidneys and bone marrow but not in the testis. By means of 2-nitropropane we have shown correspondence between the increases in 8-oxodG in target organs and the urinary excretion of 8-oxodG and between 8-oxodG formation and the comet assay in bone marrow as well potent preventive effects of extracts of Brussels sprouts. Others have shown similar effects of green tea extracts and its components. Drawbacks of the use of 2-nitropropane as a model for oxidative DNA damage relate particularly to formation of 8-aminoguanine derivatives that may interfere with HPLC-EC assays and have unknown consequences. Other model compounds for induction of oxidative DNA damage, such as ferric nitriloacetate, iron dextran, potassium bromate and paraquat, are less potent and/or more organ specific. Inflammation and activation of an inflammatory response by phorbol esters or E. coli lipopolysaccharide (LPS) induce oxidative DNA damage in many target cells and enhance benzene-induced DNA damage in mouse bone marrow. Experimental studies provide powerful tools to investigate agents inducing and preventing oxidative damage to DNA and its role in carcinogenesis. So far, most animal experiments have concerned 8-oxodG and determination of additional damaged bases should be employed. An ideal animal model for prevention of oxidative DNA damage has yet to he developed.