Mode of action of butoxyethanol-induced mouse liver hemangiosarcomas and hepatocellular carcinomas

Chronic exposure to 2-butoxyethanol resulted in an increase in liver hemangiosarcomas and hepatic carcinomas in male mouse liver. No increase in liver neoplasia was observed in similarly exposed male and female rats or female mice. We have proposed that the production of liver neoplasia in the male mouse is the result of oxidative damage secondary to the hemolytic deposition of iron in the liver. Our working hypothesis is that the mode of action of butoxyethanol-induced mouse liver hemangiosarcomas and hepatic neoplasia involves the metabolism of 2-butoxyethanol to butoxyacetic acid which results in the induction of RBC hemolysis. This hemolytic response is translated into the accumulation of iron in both liver hepatocytes and Kupffer cells. The Kupffer cell response to this insult is two-fold: (1) the production of oxidative species-through both Kupffer cell activation and through the Fenton reaction involving iron and (2) the production of cytokines (for example TNF alpha). The induction of reactive oxygen species can, if not scavenged, produce oxidative DNA damage (the formation of OH8dG), as well as increase cell growth through modulation of gene expression. While the reactive oxygen species generation would occur in the both rats and mice, the ability of the rat to detoxify the reactive oxygen species would preclude the remaining steps from occurring. In contrast, in the mouse, the reactive oxygen species would override antioxidant defense mechanisms and allow the proposed mode of action to move forward. Our results to date in male B6C3F1 mice and male F344 rats treated with 2-butoxyethanol (via daily gavage; five times per week) at doses of 0, 225, 450, and 900 mg/kg/day (mice) and 0, 225, 450 mg/kg/day (rats), respectively, showed: an increase in hemolysis in 2-butoxyethanol treated rats and mice in a dose-dependent manner, in addition, an increase in the percent of iron stained Kupffer cells in the liver was observed following treatment with 450 and 900 mg/kg of 2-butoxyethanol in mice and 225 and 450 mg/kg of 2-butoxyethanol in rat. With the iron deposition, a biphasic increase in oxidative damage (OH8dG and malondialdehyde) was seen in mouse liver after treatment with 2-butoxyethanol. In contrast, no increase in oxidative damage was observed in the rat liver at any of the doses examined. Concomitant with the increase in oxidative damage, Vitamin E levels were similarly reduced by 2-butoxyethanol in both mice and rat liver. However, the basal level of Vitamin E in rat liver was 2.5-fold greater than in mouse liver. A biphasic induction of DNA synthesis was seen following 2-butoxyethanol in the mouse. In mouse liver, increased DNA synthesis was observed in hepatocytes at 90 days and in endothelial cells at 7 and 14 days at all doses. No change in DNA synthesis was seen in 2-butoxyethanol treated rat liver. No apparent differences in apoptosis and mitosis in the liver were observed in mouse and rat liver between 2-butoxyethanol treatment groups and untreated controls. These results suggest that the induction of DNA synthesis, possibly from oxidative stress and/or Kupffer cell activation, occurs selectively in the mouse liver, in endothelial cells and in hepatocytes following exposure to 2-butoxyethanol, and support the hypothesis proposed above.

Reversibility and persistence of di-2-ethylhexyl phthalate (DEHP)- and phenobarbital-induced hepatocellular changes in rodents

The tumor promotion stage of chemical carcinogenesis has been shown to exhibit a persistence of cellular effects during treatment and the reversibility of these changes upon cessation of treatment. Inhibition of gap-junctional intercellular communication and increased replicative DNA synthesis appear to be important in this process. The present study assessed the persistence and reversibility of gap-junctional intercellular communication inhibition, peroxisomal proliferation, and replicative DNA synthesis in livers from male F344 rats and B6C3F1 mice. Dietary administration of 20,000 mg/kg DEHP to male rats for 2 weeks decreased intercellular communication (67% of control) and enhanced replicative DNA synthesis (4.8-fold over control). Elevation of the relative liver weight and the induction of peroxisomal beta oxidation were also observed following treatment with 20,000 mg/Kg DEHP for 2 weeks. Following DEHP administration at a dose of 6000 mg/kg for 18 months, inhibition of gap-junctional intercellular communication persisted, and the relative liver weight and induction of peroxisomal beta oxidation remained elevated in both rats and male B6C3F1 mice. Treatment of rats and mice with phenobarbital for 18 months (500-mg/kg diet) also produced an increase in relative liver weight and a decrease in cell-to-cell communication. In recovery studies in which DEHP was administered to male F344 rats for 2 weeks and then withdrawn, the relative liver weight, rate of peroxisomal beta oxidation, increase in replicative DNA synthesis, and inhibition of gap-junctional intercellular communication returned to control values within 2 to 4 weeks after DEHP treatment ceased. Recovery studies with phenobarbital produced similar results. The primary active metabolite of DEHP, mono-2-ethylhexyl phthalate (MEHP), was detected in the livers of animals treated with DEHP for greater than 2 weeks. However, it could not be detected after removal of DEHP from the diet for 2 weeks. This study demonstrated that inhibition of gap-junctional intercellular communication, along with indicators of peroxisomal proliferation, including increased relative liver weight and enhanced peroxisomal beta oxidation, persist while DEHP treatment continues but reverses when treatment is stopped. Studies with phenobarbital produced a similar pattern of response.

Inhibition of cellular transformation by berry extracts

Recent studies have examined and demonstrated the potential cancer chemopreventive activity of freeze-dried berries including strawberries and black raspberries. Although ellagic acid, an abundant component in these berries, has been shown to inhibit carcinogenesis both in vivo and in vitro, several studies have reported that other compounds in the berries may also contribute to the observed inhibitory effect. In the present study, freeze-dried strawberries (Fragara ananassa, FA) or black raspberries (Rubus ursinus, RU) were extracted, partitioned and chromatographed into several fractions (FA-F001, FA-F003, FA-F004, FA-F005, FA-DM, FA-ME from strawberries and RU-F001, RU-F003, RU-F004, RU-F005, RU-DM, RU-ME from black raspberries). These extracts, along with ellagic acid, were analyzed for anti-transformation activity in the Syrian hamster embryo (SHE) cell transformation model. None of the extracts nor ellagic acid by themselves produced an increase in morphological transformation. For assessment of chemopreventive activity, SHE cells were treated with each agent and benzo[a]pyrene (B[a]P) for 7 days. Ellagic acid, FA-ME and RU-ME fractions produced a dose-dependent decrease in transformation compared with B[a]P treatment only, while other fractions failed to induce a significant decrease. Ellagic acid, FA-ME and RU-ME were further examined using a 24 h co-treatment with B[a]P or a 6 day treatment following 24 h with B[a]P. Ellagic acid showed inhibitory ability in both protocols. FA-ME and RU-ME significantly reduced B[a]P-induced transformation only when co-treated with B[a]P for 24 h. These results suggest that a methanol extract from strawberries and black raspberries may display chemopreventive activity. The possible mechanism by which these methanol fractions (FA-ME, RU-ME) inhibited cell transformation appear to involve interference of uptake, activation, detoxification of B[a]P and/or intervention of DNA binding and DNA repair.

Comparative effects of dieldrin on hepatic ploidy, cell proliferation, and apoptosis in rodent liver

Dieldrin-induced hepatocarcinogenesis, which is seen only in the mouse, apparently occurs through a nongenotoxic mechanism. Previous studies have demonstrated that dieldrin induces hepatic DNA synthesis in mouse, but not rat liver. A number of nongenotoxic hepatocarcinogens have been shown to increase hepatocyte nuclear ploidy following acute and subchronic treatment in rodents, suggesting that an induction of hepatocyte DNA synthesis may occur without a concomitant increase in cell division. The current study examined the effects of dieldrin on changes in hepatocyte DNA synthesis, mitosis, apoptosis, and ploidy in mouse liver (the sensitive strain and target tissue for dieldrin-induced carcinogenicity) and the rat liver (an insensitive species). Male F344 rats and B6C3F1 mice were treated with 0, 1, 3, or 10 mg dieldrin/kg diet and were sampled after 7, 14, 28, or 90 d on diet. Liver from mice fed 10 mg dieldrin/kg diet exhibited significantly increased DNA synthesis and mitosis at 14, 28, or 90 d on diet. In rats, no increase in DNA synthesis or mitotic index was observed. The apoptotic index in liver of mice and rats did not change over the 90-d study period. Exposure of mice to only the highest dose of dieldrin produced a significant increase in octaploid (8N) hepatocytes and a decrease in diploid (2N) hepatocytes, which were restricted primarily to centrilobular hepatocytes, with the periportal region showing little or no change from control. No changes in hepatocyte nuclear ploidy were observed in the rat. This study demonstrates that exposure to high concentrations of dieldrin is accompanied by increased nuclear ploidy and mitosis in mouse, but not rat, liver. It is proposed that the observed increase in nuclear ploidy in the mouse may reflect an adaptive response to dieldrin exposure.

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Morphological transformation by 8-hydroxy-2'-deoxyguanosine in Syrian hamster embryo (SHE) cells

8-Hydroxy-2'-deoxyguanosine (OH8dG) is one of the most prevalent oxidative DNA modifications found in eukaryotic cells. Previous studies have suggested an association between OH8dG formation and carcinogenesis. However, it is unclear whether OH8dG formation results in the necessary genotoxic events for cancer development. In the present study, the formation of OH8dG and its ability to transform Syrian hamster embryo (SHE) cells was examined. Methylene blue, a photosensitizer that in the presence of light can generate singlet oxygen by a type II mechanism, was used to produce oxidative DNA damage (predominantly OH8dG) in SHE cells. Photoactivated methylene blue produced a dose-dependent increase in OH8dG as well as a dose-dependent increase in morphological transformation in SHE cells. SHE cells transfected with DNA that contained increasing concentrations of OH8dG displayed a dose-dependent increase in morphological transformation. Treatment with beta-carotene (a singlet oxygen quencher) inhibited both the formation of OH8dG and the induction of morphological transformation in photoactivated methylene blue-treated SHE cells. These results suggest that formation of OH8dG can induce morphological transformation and provide further support for a role of OH8dG formation in the carcinogenesis process.

Effects of di-isononyl phthalate, di-2-ethylhexyl phthalate, and clofibrate in cynomolgus monkeys

The effects of the peroxisome proliferators di-isononyl phthalate (DINP) and di-2-ethylhexyl phthalate (DEHP) were evaluated in young adult male cynomolgus monkeys after 14 days of treatment, with emphasis on detecting hepatic and other effects seen in rats and mice after treatment with high doses of phthalates. Groups of 4 monkeys received DINP (500 mg/kg/day), DEHP (500 mg/kg/day), or vehicle (0.5% methyl cellulose, 10 ml/kg) by intragastric intubation for 14 consecutive days. Clofibrate (250 mg/kg/day), a hypolipidemic drug used for cholesterol reduction in human patients was used as a reference substance. None of the test substances had any effect on body weight or liver weights. Histopathological examination of tissues from these animals revealed no distinctive treatment-related effects in the liver, kidney, or testes. There were also no changes in any of the hepatic markers for peroxisomal proliferation, including peroxisomal beta-oxidation (PBOX) or replicative DNA synthesis. Additionally, in situ dye transfer studies using fresh liver slices revealed that DINP, DEHP, and clofibrate had no effect on gap junctional intercellular communication (GJIC). None of the test substances produced any toxicologically important changes in urinalysis, hematology, or clinical chemistry; however, clofibrate produced some emesis, small increases in serum triglyceride, decreased calcium, and decreased weights of testes/epididymides and thyroid/parathyroid. The toxicological significance of these small changes is questionable. The absence of observable hepatic effects in monkeys at doses that produce hepatic effects in rodents suggests that DINP, DEHP, and clofibrate would also not elicit in primates other effects such as liver cancer. These data, along with results from in vitro hepatocyte studies, indicate that rodents are not good animal models for predicting the hepatic effects of phthalates in primates, including humans.

Effects of Di-2-ethylhexyl phthalate (DEHP) on gap-junctional intercellular communication (GJIC), DNA synthesis, and peroxisomal beta oxidation (PBOX) in rat, mouse, and hamster liver

The present study evaluated the effect of di-2-ethylhexyl phthalate (DEHP) on gap-junctional intercellular communication (GJIC), peroxisomal beta-oxidation (PBOX) activity, and replicative DNA synthesis in several rodent species with differing susceptibilities to peroxisome proliferator-induced hepatic tumorigenesis. A low (non-tumorigenic) and high (tumorigenic) dietary concentration of DEHP was administered to male F344 rats for 1, 2, 4, and 6 weeks. Additionally, a previously non-tumorigenic dose (1000 ppm) and tumorigenic dose of DEHP (12,000 ppm), as determined by chronic bioassay data, were examined following 2 weeks dietary administration. Male B6C3F1 mice were fed the non-tumorigenic concentration, 500 ppm, and the tumorigenic concentration, 6000 ppm, of DEHP for two and four weeks. The hepatic effects of low and high concentrations of DEHP, 1000 and 6000 ppm, were also examined in male Syrian Golden hamsters (refractory to peroxisome proliferator-induced tumorigenicity). In rat and mouse liver, a concentration-dependent increase in the relative liver weight, PBOX activity, and replicative DNA synthesis was observed at the earliest time point examined. Concurrent to these observations was an inhibition of GJIC. In hamster liver, a slight increase in the relative liver weight, PBOX activity, and replicative DNA synthesis was observed. However, these effects were not of the same magnitude or consistency as those observed in rats or mice. Furthermore, DEHP had no effect on GJIC in hamster liver at any of the time points examined (2 and 4 weeks). HPLC analysis of DEHP and its primary metabolites, mono-2-ethylhexyl phthalate (MEHP), and phthalate acid (PA), indicated a time- and concentration-dependent increase in the hepatic concentration of MEHP. At equivalent dietary concentrations and time points, the presence of MEHP, the primary metabolite responsible for the hepatic effects of DEHP, demonstrated a species-specific response. The largest increase in the hepatic concentration of MEHP was observed in mice, which was greater than the concentration observed in rats. The hepatic concentration of MEHP was lowest in hamsters. Hepatic concentrations of DEHP and phthalic acid were minimal and did not correlate with concentration and time. Collectively, these data demonstrate the inhibition of hepatic GJIC and increased replicative DNA synthesis correlated with the observed dose- and species-specific tumorigenicity of DEHP and may be predictive indicators of the nongenotoxic carcinogenic potential of phthalate esters.

Comparative in vivo hepatic effects of Di-isononyl phthalate (DINP) and related C7-C11 dialkyl phthalates on gap junctional intercellular communication (GJIC), peroxisomal beta-oxidation (PBOX), and DNA synthesis in rat and mouse liver

The short-term hepatic effects of DINP (CAS 68515-48-0, designated DINP-1) in rats and mice were evaluated at tumorigenic and nontumorigenic doses from previous chronic studies. Groups of male F344 rats were fed diets with DINP-1 at concentrations of 0, 1000, or 12,000 ppm and male B6C3F1 mice at 0, 500, or 6000 ppm DINP-1. After 2 or 4 weeks of treatment, changes in liver weight, gap junctional intercellular communication (GJIC), peroxisomal beta-oxidation (PBOX), and replicative DNA synthesis were examined. In addition, hepatic and serum concentrations of the parent compound and major metabolites were determined. Relative to controls in both species, increased liver weight and PBOX at the high dose of DINP-1 were consistent with peroxisomal proliferation. Hepatic GJIC was inhibited and DNA synthesis was increased at the high dose of DINP-1, which is also consistent with the tumorigenic response in rats and mice reported in other chronic studies at these doses. These hepatic effects were not observed at the low doses of DINP-1. At comparable low doses of DINP-1 in other chronic studies, no liver tumors were observed in rats and mice. The monoester metabolite (MINP-1) was detected in the liver at greater concentrations in mice than rats. This result is also consistent with the dose-response observations in rat and mouse chronic studies. Additionally, other structurally similar dialkyl phthalate esters ranging from C7 to C11 were evaluated using a similar protocol for comparison to DINP-1; these included an alternative isomeric form of DINP (DINP-A), di-isodecyl phthalate (DIDP), di-isoheptyl phthalate (DIHP), di-heptyl, nonyl undecyl phthalate (D711P), and di-n-octyl phthalate (DNOP). Collectively, these data indicate that in rats and mice, DINP-1 and other C7-C11 phthalates exhibit a threshold for inducing hepatic cellular events. Further, where previous chronic data were available for these compounds, these phthalates elicited hepatic effects at doses that correlated with the tumorigenic response. Overall, these studies suggest a good correlation between the inhibition of GJIC when compared with the data on production of liver tumors in chronic studies.

Acrylonitrile-induced morphological transformation in Syrian hamster embryo cells

Acrylonitrile (ACN) is a monomer used in the synthesis of rubber, fibers and plastics. Previous studies demonstrated that ACN induces brain neoplasms (predominately astrocytomas) in rats following chronic treatment. While the mechanisms of ACN-induced glial cell carcinogenicity have not been completely elucidated, investigations by our group and others have suggested a role for the induction of oxidative stress and the resultant oxidative damage in this process. In vitro cell transformation models are useful for detecting and studying the mechanisms of chemical carcinogenesis. Cell transformation by chemical carcinogens in Syrian hamster embryo (SHE) cells exhibits a multistage process similar to that observed in vivo, for both non-genotoxic and genotoxic carcinogens. In the present study, the ability of ACN to induce morphological transformation and oxidative damage was examined in SHE cells. ACN induced an increase in morphological transformation at doses of 50, 62.5 and 75 microg/ml (maximum sub-toxic dose tested) following 7 days of continuous treatment. SHE cells exposed to ACN for 24 h failed to increase morphological transformation. Morphological transformation by ACN was inhibited by co-treatment with the antioxidants alpha-tocopherol and (-)-epigallocathechin-3 gallate (EGCG) for 7 days. Treatment of SHE cells with 75 microg/ml ACN produced a significant increase in 8-hydroxy-2'-deoxyguanosine that was also inhibited by co-treatment with alpha-tocopherol or EGCG. These results support the proposal that oxidative stress and the resulting oxidative damage is involved in ACN-induced carcinogenicity.

Mechanisms of cancer chemoprevention in hepatic carcinogenesis: modulation of focal lesion growth in mice

Studies in our laboratory have concentrated on further understanding the mechanism by which chemicals induce cancer and the means to prevent or retard this process. Recent investigations have revolved around the role of oxidative stress and oxidative damage in the induction of cancer by nongenotoxic carcinogens. Hepatocarcinogenic compounds including selective chlorinated hydrocarbons appeared to induce oxidative stress in the liver. This oxidative stress and oxidative damage in turn may be responsible for the tumor-promoting activity of these compounds. Reduction of oxidative damage by antioxidants, or dietary-restriction, results in an ablation of the induction of selective cell growth by these agents. The oxidative stress induced by nongenotoxic agents may influence cell proliferation and/or apoptosis in the preneoplastic cells. Our studies with nongenotoxic hepatic carcinogens showed a dose-dependent increase in oxidative stress and an increase in hepatic focal lesion growth. Antioxidant dietary supplementation or caloric restriction prevented the lesion growth. This appeared to be through an increase in apoptosis in the hepatic lesions.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Induction of oxidative stress and oxidative damage in rat glial cells by acrylonitrile

Chronic treatment of rats with acrylonitrile (ACN) resulted in a dose-related increase in glial cell tumors (astrocytomas). While the exact mechanism(s) for ACN-induced carcinogenicity remains unresolved, non-genotoxic and possibly tumor promotion modes of action appear to be involved in the induction of glial tumors. Recent studies have shown that ACN induced oxidative stress selectively in rat brain in a dose-responsive manner. The present study examined the ability of ACN to induce oxidative stress in a rat glial cell line, a target tissue, and in cultured rat hepatocytes, a non-target tissue of ACN carcinogenicity. Glial cells and hepatocytes were treated for 1, 4 and 24 h with sublethal concentrations of ACN. ACN induced an increase in oxidative DNA damage, as evidenced by increased production of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) in glial cells but not in rat hepatocytes. Hydroxyl radical formation following ACN treatment was also selectively increased in glial cells. Following 1 and 4 h of ACN exposure, the levels of the non-enzymatic antioxidant glutathione, as well as the activities of the enzymatic antioxidants catalase and superoxide dismutase were significantly decreased in the rat glial cells. Lipid peroxidation and the activity of glutathione peroxidase were not affected by ACN treatment in rat glial cells. No changes in any of these biomarkers of oxidative stress were observed in hepatocytes treated with ACN. These data indicate that ACN selectively induced oxidative stress in rat glial cells.

The effect of acrylonitrile on gap junctional intercellular communication in rat astrocytes

Rats chronically exposed to acrylonitrile (ACN) have shown a dose-dependent increase in the incidence of astrocytomas in the brain. The mechanism(s) by which ACN induces cancer in rodents has not been established. ACN does not appear to be directly genotoxic in the brain and thus a nongenotoxic mode of action has been proposed. Inhibition of gap junctional intercellular communication (GJIC) has been shown to be a property of many nongenotoxic carcinogens. The present study examined the effects of ACN on GJIC in a rat astrocyte transformed cell line, DI TNC1 cells (a target cell for ACN carcinogenicity) and primary cultured hepatocytes (a nontarget cell for ACN carcinogenicity). ACN inhibited GJIC in rat astrocytes in a dose-dependent manner. Inhibition of GJIC was observed following 2 h treatment with 0.10 mmol/L and 1.00 mmol/L ACN. However, in primary cultured hepatocytes, ACN exposed did not result in inhibition of GJIC even after 48 h of continued treatment. In the astrocytes, GJIC inhibition plateaued after 4 h of treatment and remained blocked throughout the entire experimental period examined. Inhibition of GJIC in DI TNC1 cells was reversed by removal of ACN from the culture medium after 4 or 24 h of treatment. Cotreatment of astrocytes with vitamin E reduced the effect of ACN-induced inhibition of GJIC. Similarly, inhibition of GJIC was prevented by treatment with 2-oxothiazolidine-4-carboxylic acid (OTC), a precursor of glutathione synthesis. Decreasing cellular glutathione by treatment with buthionine sulfoxamine alone (without ACN) did not affect GJIC in astrocytes. Collectively, these results demonstrate that treatment with ACN caused a selective inhibition of GJIC in rat DI TNC1 astrocytes (the target cell type), but not in rat hepatocytes (a nontarget tissue). Inhibition of GJIC in astrocytes was reversed by treatment with antioxidants and suggests a potential role for oxidative stress in ACN-induced carcinogenesis.

The effect of tea consumption on oxidative stress in smokers and nonsmokers

While the anticarcinogenic effects of tea in animal models have been reported by several groups, human epidemiological studies examining tea consumption and cancer prevention have produced equivocal results. The beneficial properties of tea to human health may be related to the antioxidant properties of tea components. However, little evidence has been provided that tea consumption can either increase the antioxidant capacity or decrease oxidative stress in humans. In the present study, the effects of tea treatment (green tea) on biomarkers of oxidative stress were investigated in smokers and nonsmokers in two volunteer study groups (one in China and the other in United States). Green tea consumption in both study groups decreased oxidative DNA damage (8-OHdG in white blood cells and urine), lipid peroxidation (MDA in urine), and free radical generation (2, 3-DHBA in urine) in smokers. Nonsmokers (US study group) also exhibited a decrease in overall oxidative stress.

Metabolism of cocaine and heroin is catalyzed by the same human liver carboxylesterases

Concomitant i.v. use of cocaine and heroin ("speedballing") is prevalent among drug-abusing populations. Heroin is rapidly metabolized by sequential deacetylation of two separate ester bonds to yield 6-monoacetylmorphine and morphine. Hydrolysis of heroin to 6-monoacetylmorphine is catalyzed by pseudocholinesterase. The pathway for hydrolysis of 6-monoacetylmorphine to morphine in vivo has yet to be established. Pseudocholinesterase and two human liver carboxylesterases [human liver carboxylesterase form 1 (hCE-1) and human liver carboxylesterase form 2 (hCE-2)] catalyze the rapid hydrolysis of ester linkages in cocaine. This investigation examined the relative catalytic efficiencies of hCE-1, hCE-2 and pseudocholinesterase for heroin metabolism and compared them with cocaine hydrolysis. Enzymatic formation of 6-monoacetylmorphine and morphine was determined by reverse-phase high-performance liquid chromatography. All three enzymes rapidly catalyzed hydrolysis of heroin to 6-monoacetylmorphine (hCE-1 kcat = 439 min-1, hCE-2 kcat = 2186 min-1 and pseudocholinesterase kcat = 13 min-1). The catalytic efficiency, under first-order conditions, for hCE-2-catalyzed formation of 6-monoacetylmorphine (314 min-1 mM-1) was much greater than that for either hCE-1 or pseudocholinesterase (69 and 4 min-1 mM-1, respectively). Similarly, the catalytic efficiency for hydrolysis of 6-monoacetylmorphine to morphine by hCE-2 (22 min-1 mM-1) was substantially greater than that for hCE-1 (0.024 min-1 mM-1). Cocaine competitively inhibited hCE-1-, hCE-2- and pseudocholinesterase-catalyzed hydrolysis of heroin to 6-monoacetylmorphine (Ki = 530, 460 and 130 microM, respectively) and 6-monoacetylmorphine hydrolysis to morphine (Ki = 710, 220 and 830 microM, respectively). These data demonstrate that metabolism of cocaine and heroin in humans is mediated by common metabolic pathways. The role of hepatic hCE-2 is particularly important for the hydrolysis of heroin to 6-monoacetylmorphine and of 6-monoacetylmorphine to morphine.

Dimethylnitrosamine-induced DNA damage and toxic cell death in cultured mouse hepatocytes

Chronic exposure to dimethylnitrosamine produces hepatic tumors through recurrent DNA alkylation, whereas acute exposure can cause liver necrosis through mechanisms that remain largely unknown. Our laboratory recently demonstrated that DNA fragmentation occurs early on and may be a causal event in dimethylnitrosamine-induced necrosis in liver. A challenge to interpreting these results is that up to 30% of liver cells are non-parenchymal and could account for the observed DNA fragmentation. In the present study, we have examined whether dimethylnitrosamine induces early genomic DNA fragmentation in cultured mouse hepatocytes. Hepatic parenchymal cells isolated from male ICR mice were cultured in Williams E medium. DNA damage was assessed quantitatively as a fragmented fraction that was not sedimented at 27,000 x g, and qualitatively from agarose gel electrophoresis. Cellular response to DNA damage was assessed by measuring induction of the DNA repair enzyme DNA ligase. Toxic cell death was estimated from release of lactate dehydrogenase (LDH) or adenine nucleotides from cells prelabeled with [3H]adenine. Dimethylnitrosamine produced a twofold increase in [3H]adenine release by 6 h and LDH release at 36 h. DNA fragmentation and DNA ligase activity increased by as early as 1 h. The Ca(2+)-endonuclease inhibitor aurintricarboxylic acid and the Ca2+ chelator ethylenediamine tetraacetic acid (EDTA) prevented DNA fragmentation through 6 h and virtually abolished cytotoxicity through 30 h. DNA ligase induction was strongly associated with DNA fragmentation. Early increases in DNA fragmentation and DNA ligase were highly correlated with later toxic cell death. Such results strongly suggest that dimethylnitrosamine-induced fragmentation of DNA in target parenchymal cells is a causal factor in the toxic death of these liver cells.

Menadione-induced DNA fragmentation without 8-oxo-2'-deoxyguanosine formation in isolated rat hepatocytes

Menadione (2-methyl-1,4-naphthoquinone) induces oxidative stress in cells causing perturbations in the cytoplasm as well as nicking of DNA. The mechanisms by which DNA damage occurs are still unclear, but a widely discussed issue is whether menadione-generated reactive oxygen species (ROS) directly damage DNA. In the present study, we measured the effect of menadione on formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG), an index of oxidative DNA base modifications, and on DNA fragmentation. Isolated hepatocytes from phenobarbital-pretreated rats were exposed to menadione, 25-400 microM, for 15, 90 or 180 min with or without prior depletion of reduced glutathione (GSH) by diethyl maleate. Menadione caused profound GSH depletion and internucleosomal DNA fragmentation, which was demonstrated by a prominent fragmentation ladder on agarose gel electrophoresis. We found no oxidative modification of DNA in terms of increased 8-oxodG formation. In contrast, the positive control of sunlamp light increased 8-oxodG 5-fold in rat hepatocytes. We conclude that oxidative modification of DNA bases is unlikely to be important in menadione-induced DNA damage.

DNA as a critical target in toxic cell death: enhancement of dimethylnitrosamine cytotoxicity by DNA repair inhibitors

Our working hypothesis states that DNA damage is a critical step in toxic cell death. The DNA hypothesis was tested in cultured mouse hepatocytes by examining whether inhibitors of DNA repair would increase dimethylnitrosamine toxicity and DNA damage in parallel. Inhibitors were chosen for selectivity toward DNA polymerase alpha (aphidicolin, myricetin), DNA ligase (ethidium bromide), or multiple repair enzymes (ara-C, doxorubicin). Dimethylnitrosamine caused concentration-dependent DNA damage at 6 hr and cell death at 24 hr (35% ALT release vs. 8.8% in control cultured hepatocytes). Each repair inhibitor increased dimethylnitrosamine-induced DNA damage and toxic cell death in parallel. Doxorubicin maximally elevated DNA fragmentation and toxicity (57% ALT release). Repair inhibitors alone failed to damage DNA or cause cell death in this model system. These data support the hypothesis that DNA damage is an early causal event in toxic cell death caused by alkylating hepatotoxicants.

Ca2+ antagonists inhibit DNA fragmentation and toxic cell death induced by acetaminophen

Ca2+ accumulates in the nucleus and DNA undergoes enzymatic cleavage into internucleosome-length fragments before acetaminophen and dimethylnitrosamine produce hepatic necrosis in vivo and toxic cell death in vitro. However, Ca(2+)-endonuclease fragmentation of DNA is characteristic of apoptosis, a type of cell death considered biochemically and functionally distinct from toxic cell death. The present studies investigate DNA fragmentation as a critical event in toxic cell death by testing whether the Ca(2+)-calmodulin antagonist chlorpromazine and the Ca2+ channel blocker verapamil prevent acetaminophen-induced hepatic necrosis by inhibiting Ca2+ deregulation and DNA damage. Acetaminophen overdose in mice produced accumulation of Ca2+ in the nucleus (358% of control) and fragmentation of DNA (250% of control) by 6 h, with peak release of ALT occurring at 12-24 h (38,000 U/l). Pretreatment with chlorpromazine prevented increases in nuclear Ca2+ and DNA fragmentation and nearly abolished biochemical evidence of toxic cell death. Verapamil pretreatment also decreased Ca2+ accumulation and DNA damage while attenuating liver injury. The Ca2+ antagonists did not protect against toxic cell death through hypothermia because neither produced the delay in toxicity that is customarily associated with hypothermia. Nor did chlorpromazine or verapamil protect through inhibiting acetaminophen bioactivation. Chlorpromazine failed to diminish glutathione depletion in whole liver and isolated nuclei. Verapamil (250 microM) also failed to alter glutathione depletion in whole liver and had no effect on acetaminophen-glutathione adduct formation by mouse liver microsomes and by cultured mouse hepatocytes. Collectively, these results support the hypothesis that Ca(2+)-induced DNA fragmentation plays a significant role in cell necrosis produced by acetaminophen and may contribute to toxic cell death caused by other alkylating hepatotoxins.

Independence and additivity of cultured hepatocyte killing by Ca2+ overload and ATP depletion

In two competing models of toxic cell death, hepatocyte killing by chemical hypoxia (CN/IAA) is attributed to ATP depletion and killing by A23187 is attributed to Ca(2+)-induced damage. The independence of these models can be questioned because CN/IAA elevates Ca2+ before killing 1c1c7 hepatoma cells and because the ATP source fructose prevents hepatocyte killing by Br-A23187. In the present studies, cultured mouse hepatocytes were exposed to CN/IAA, A23187, or treatments in combination. A23187 produced toxicity proportional to Ca(2+)-activated DNA fragmentation. CN/IAA caused comparable toxicity but no fragmentation of DNA. Treatments in combination were more toxic than either treatment alone. Aurintricarboxylic acid, a Ca(2+)-endonuclease inhibitor, decreased DNA fragmentation and the toxicity of A23187 and combination treatment without affecting CN/IAA toxicity. ATP plus oligomycin decreased CN/IAA and combination treatment toxicity but not that of A23187. These findings indicate that cultured mouse hepatocytes are killed through mechanisms that are independent and additive in their toxicities.

Ca(++)-activated DNA fragmentation and dimethylnitrosamine-induced hepatic necrosis: effects of Ca(++)-endonuclease and poly(ADP-ribose) polymerase inhibitors in mice

Several hepatotoxic agents damage Ca++ regulation and produce toxic cell death in a manner consistent with a cause-and-effect relationship; however, vital targets of Ca++ remain unidentified. Recent results show that DNA may be the chief Ca++ target during apoptosis, a form of cell death considered distinct from toxic cell death or necrosis. The present studies explored whether nuclear Ca++ regulation is lost before dimethylnitrosamine-induced necrosis, whether DNA is attacked by Ca(++)-dependent endonucleases and whether inhibitors of Ca(++)-endonuclease activity and the DNA repair enzyme poly(ADP-ribose)polymerase affect necrosis. Adult male ICR mice received 100 mg/kg of dimethylnitrosamine i.p. By 2 to 4 hr, total nuclear Ca++ reached 150 to 180% of control and DNA fragmentation was 140 to 170% of control. Electrophoresis of DNA revealed a sharp decline in genomic DNA with the appearance of DNA fragments in a ladder-like pattern. Ca++ elevation and DNA fragmentation preceded toxic cell death by 4 hr or more and reached peak values at 18 to 24 hr, coincident with maximal alanine aminotransferase leakage. Aurintricarboxylic acid, a Ca(++)-endonuclease inhibitor, reduced toxicity 67%. 3-Aminobenzamide, nicotinamide adenine dinucleotide and theophylline, inhibitors of poly(ADP-ribose)polymerase-mediated DNA repair, potentiated liver damage 2-fold. These results support the hypothesis that DNA fragmentation plays a contributing role in toxic cell death induced by dimethylnitrosamine. Furthermore, the findings suggest that new opportunities may exist to moderate the toxicity of alkylating hepatotoxins by altering DNA regulation.

Acetaminophen-induced cytotoxicity in cultured mouse hepatocytes: effects of Ca(2+)-endonuclease, DNA repair, and glutathione depletion inhibitors on DNA fragmentation and cell death

Hepatotoxic alkylation of mouse liver cells by acetaminophen is characterized by an early loss of ion regulation, accumulation of Ca2+ in the nucleus, and fragmentation of DNA in vitro and in vivo. Acetaminophen-induced DNA cleavage is accompanied by the formation of a "ladder" of DNA fragments characteristic of Ca(2+)-mediated endonuclease activation. These events unfold well in advance of cytotoxicity and the development of necrosis. The present study utilized cultured mouse hepatocytes and mechanistic probes to test whether DNA fragmentation and cell death might be related in a "cause-and-effect" manner. Cells were isolated by collagenase perfusion, cultured in Williams' E medium for 22-26 hr, and exposed to acetaminophen. Aurintricarboxylic acid, a general Ca(2+)-endonuclease inhibitor, and EGTA, a chelator of Ca2+ required for endonuclease activation, significantly decreased DNA fragmentation at 6 and 12 hr and virtually abolished cytotoxicity. N-Acetylcysteine also eliminated DNA fragmentation and cytotoxicity. 3-Aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase-stimulated DNA repair, failed to alter the amount of DNA fragmentation at 6 hr but substantially increased acetaminophen cytotoxicity in hepatocytes at 12 hr. With the exception of when DNA repair was inhibited by 3-aminobenzamide, Ca2+ accumulation in the nucleus, DNA fragmentation, and hepatocyte death varied consistently and predictably with one another. Collectively, these findings suggest that unrepaired damage to DNA contributes to acetaminophen-induced cell death in vivo and may play a role in necrosis in vivo.

Acetaminophen-induced cytotoxicity in cultured mouse hepatocytes: correlation of nuclear Ca2+ accumulation and early DNA fragmentation with cell death

Hepatotoxic doses of acetaminophen cause widespread alkylation of liver and early loss of cytosolic Ca2+ regulation. Although the precise location and target of lethal alkylation are not known, Ca2+ accumulation is viewed as a possible link between cell alkylation and cell death. We have recently shown that Ca2+ accumulates in the nucleus and that DNA fragments in vivo before the development of acetaminophen-induced necrosis in mice. The present study examined cultured hepatocytes for nuclear damage and its association with cell death in vitro. Positive results would argue for two key points. (1) Nonparenchymal cell damage does not explain DNA fragmentation induced by acetaminophen in vivo. (2) A chemical that causes necrosis can produce DNA damage considered characteristic of apoptosis. Hepatocytes from NIH Swiss mice were isolated by collagenase perfusion, cultured in Williams' E medium for 24 hr, and exposed to acetaminophen. Cytotoxicity was assessed by lactate dehydrogenase leakage and release of [3H]adenine from a prelabeled nucleotide pool. Genomic DNA fragmentation was assessed quantitatively by colorimetric analysis and qualitatively by agarose gel electrophoresis. Acetaminophen caused DNA damage from 1-4 hr onward and produced significant release of lactate dehydrogenase and [3H]adenine nucleotides at later times. Agarose gel electrophoresis revealed a "ladder" of DNA fragments characteristic of Ca(2+)-mediated endonuclease activation. Cytotoxicity correlated with nuclear Ca2+ accumulation (r greater than 0.895, p less than 0.05) and with percentage DNA fragmentation (r greater than 0.835, p less than 0.05). Nuclear changes in vitro generally reproduced those observed in vivo. Collectively, these findings demonstrate that nuclear Ca2+ accumulation and DNA fragmentation appear as early events that correlate directly with later cytotoxicity. These changes may contribute to acetaminophen-induced injury leading to cell death in vitro and necrosis in vivo.