Lack of genotoxic activity of di(2-ethylhexyl)phthalate (DEHP) in rat and human hepatocytes

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Structure-Dependent Effects of Phthalates on Intercellular and Intracellular Communication in Liver Oval Cells

Humans are exposed to phthalates released from plastics, cosmetics, or food on a daily basis. Phthalates have low acute liver toxicity, but their chronic exposures could induce molecular and cellular effects linked to adverse health outcomes, such as liver tumor promotion or chronic liver diseases. The alternation of gap junctional intercellular communication (GJIC) and MAPK-Erk1/2 pathways in liver progenitor or oval cells can disrupt liver tissue homeostatic mechanisms and affect the development and severity of these adverse outcomes. Our study with 20 different phthalates revealed their structurally dependent effects on liver GJIC and MAPK-Erk1/2 signaling in rat liver WB-F344 cell line with characteristics of liver oval cells. The phthalates with a medium-length side chain (3–6 C) were the most potent dysregulators of GJIC and activators of MAPK-Erk1/2. The effects occurred rapidly, suggesting the activation of non-genomic (non-transcriptional) mechanisms directly by the parental compounds. Short-chain phthalates (1–2 C) did not dysregulate GJIC even after longer exposures and did not activate MAPK-Erk1/2. Longer chain (≥7 C) phthalates, such as DEHP or DINP, moderately activated MAPK-Erk1/2, but inhibited GJIC only after prolonged exposures (>12 h), suggesting that GJIC dysregulation occurs via genomic mechanisms, or (bio)transformation. Overall, medium-chain phthalates rapidly affected the key tissue homeostatic mechanisms in the liver oval cell population via non-genomic pathways, which might contribute to the development of chronic liver toxicity and diseases.

Bonding of Butylparaben, Bis(2-ethylhexyl)-phthalate, and Perfluorooctanesulfonic Acid to DNA: Comparison with Benzo[a]pyrene Shows Low Probability for Strong Noncovalent DNA Intercalation

Parabens, phthalates, and perfluorinated compounds are pollutant compounds used in cosmetics, plastics, and fire-fighting foams. All three compounds have been studied over several years for toxicity mechanism; however, a clear view of their ability to bind to DNA has not been supplied empirically. In this work, a simulation study is done to reveal the interaction of three of these pollutants, bis(2-ethylhexyl)-phthalate (DEHP), butylparaben (BPRB), and the protonated form of perfluorooctanesulfonic acid (PFOS(H)), with DNA. The results show that the DEHP, PFOS(H), and BPRB bind with a probability of 1/5 to DNA, with respective bonding energies -23.96 kJ/mol (PFOS(H)), -94.92 kJ/mol (BPRB), and -216.52 kJ/mol (DEHP). The positive control, benzo[a]pyrene diol epoxide (BAP), which is known for its notorious DNA intercalation, binds at a rate of 3/5 simulations, with bonding energies of -6544.52, -7034.66, and -7578.67 kJ/mol. The results are compared to empirical studies and conclusively show that all these pollutants can interfere with transcription and DNA related mechanisms by forming noncovalent interactions with DNA. The results show also that these pollutants are unlikely to undergo strong noncovalent intercalation to DNA, such as BAP, and do not possess the frontier orbital profiles to undergo adduct formation. After many years of research and several unanswered questions on the action of these pollutants on DNA, a calculation on their properties hence to the DNA confirms that there is a low probability for these to undergo a strong intercalation with DNA. Literature shows however that the pollutants are strongly interfering with the protein machinery and receptors on the cell surface and are therefore still priority pollutants for ecotoxicity research.

Hepatoprotective effect of silymarin on di(2-ethylhexyl)phthalate (DEHP) induced injury in liver FL83B cells

Di(2-ethylhexyl)phthalate (DEHP), is a toxic environmental pollutant in our life which can contaminate air, water, and soil. The hepatoprotective effect of silymarin on DEHP-induced injury in FL83B mouse liver cells was investigated by analyzing the cell viability, lactate dehydrogenase (LDH), alanine aminotransferase (ALT), cell cycle arrest, and cell morphology. The results revealed that cell viability decreased while released LDH and ALT increased with the increase of DEHP concentrations. Moreover, cell population of sub-G1 and S phase increased as the concentrations of DEHP increased. Silymarin at 25 μM achieved the highest hepatoprotective effect and exhibited 79% cell viability while only 46% cell viability was found in DEHP injured control. It was also found to reduce LDH release and cell populations of sub-G1 and S phase. Therefore, silymarin could ameliorate DEHP-induced injury and have potential to be further developed as a natural ingredient of health food against phthalate plasticizers induced liver injury.

Induction of oxidative stress and histological changes in liver by subacute doses of butyl cyclohexyl phthalate

Phthalates are esters of phthalic acid and are mainly used as plasticizers in a wide variety of products and applications. There is no information on butyl cyclohexyl phthalate (BCP) toxicity. This study was performed to evaluate the histopathological effects and to determine oxidative stress inducing potential in liver by subacute exposure of BCP. The animals of the treatment groups were orally administered 100, 200, and 400 mg/kg/day BCP for 5 consecutive days per week during 28 days. As a result, no significant changes were observed in body weight gains, and absolute and relative liver weights of liver of BCP treated mice, when compared with control group. Although the degree of lipid peroxidation in the liver tissue of all BCP exposure groups were significantly higher than those of the control (p < 0.01), SOD and CAT activities in liver tissue of mice of 200 and 400 mg/kg exposure groups were significantly lower than those of the controls (p < 0.01). Moreover, BCP caused dose-dependent histological changes in the liver of mice such as congestions in vena centralis, an enlargement of the sinusoids, degeneration in hepatocytes, vacuole formations and presence of lipid droplets in hepatocytes, eosinophilic cytoplasm. While iNOS immunoreactivity was increased in all treatment groups, Type IV collagen and Connexin 43 immunoreactivities were decreased in all treatment groups compared with the control group. Significant decrease was observed in the number of TUNEL-positive liver cells of BCP treated mice. These results suggested that BCP exposure induces oxidative stress in liver and exposure of BCP during long time period could lead to hepatocarcinogenesis.

Fibrates induce hepatic peroxisome and mitochondrial proliferation without overt evidence of cellular proliferation and oxidative stress in cynomolgus monkeys

There is little primate risk factor data in the literature evaluating the relationship between proposed mechanisms of PPAR agonist-induced hepatocarcinogenesis at clinically relevant therapeutic exposures. These studies were conducted to characterize the hepatic effects of fenofibrate and ciprofibrate in the cynomolgus monkey. Male cynomolgus monkeys were given fenofibrate (250, 1250 or 2500 mg/kg/day) or ciprofibrate (3, 30, 150 or 400 mg/kg/day) for up to 15 days. The highest doses used were approximately 4 times (fenofibrate) and 9.4 times (ciprofibrate) the human therapeutic exposure for these agents based on AUC (area under the curve). For both compounds, there was a treatment-related increase in liver weight and periportal hepatocellular hypertrophy, which was related to increases in peroxisomes (up to 2.8 times controls) and mitochondria (up to 2.5 times controls). An increase in smooth endoplasmic reticulum probably contributed to the hypertrophy. There was no indication of cell proliferation as determined by the number of mitotic figures and this was confirmed by evaluating cell proliferation by immunohistochemical staining for the Ki-67 antigen. Consistent with the findings by light microscopy, there was no treatment-related effect on the level of mRNA for proteins known to be involved in the control of hepatocyte cell division or apoptosis (e.g. P21, Cyclin D1, PCNA, CDKN1A). Furthermore, there was minimal indication of oxidative stress. Thus, there was no evidence of lipofuscin accumulation, and there was no remarkable increase in the mRNA levels for most proteins known to respond to oxidative stress (e.g. catalase, glutathione peroxidase). A mild induction in the mRNA levels of cellular beta-oxidation and detoxification enzymes (e.g. acyl CoA oxidase, thioredoxin reductase) was observed. Collectively, the data from these studies suggest that the primate responds to PPARalpha agonists in a manner that is different from the rodent suggesting that the primate may be refractory to PPAR-induced hepatocarcinogenesis.

Comparison of oxidative stress and changes of xenobiotic metabolizing enzymes induced by phthalates in rats

Phthalates are widely used as a plasticizer and cause a peroxisome proliferation. Peroxisome proliferators (PPs), such as di-2-ethylhexyl phthalate (DEHP) and clofibrate (CF) are known to have a hepatocarcinogenic potential in rodents. It has been proposed that these PPs may cause hepatocellular cancer by an oxidative damage-mediated mechanism(s). The primary purpose of this study is to find whether there is a difference between the oxidative damage by hepatocarcinogenic PPs (DEHP and CF) and the oxidative damage by weak PPs [di-n-butyl phthalate (DBP) and n-butylbenzyl phthalate (BBP)]. The second purpose is to investigate if phthalates can affect the phase I/phase II enzymes, and if the effect of PPs on metabolizing enzymes correlates with peroxisome proliferation or not. After rats were treated with PPs (DEHP, DBP and BBP; 50, 200, 1000 mg/kg, CF; 100 mg/kg, p.o., for 14 days), the activities of metabolizing enzymes and peroxisomal enzymes were investigated, and the oxidative damage was measured using 8-hydroxydeoxyguanosine (8-OHdG) in the DNA and malonedialdehyde (MDA) in the livers. These four PPs significantly increased the relative liver weights, palmitoyl-CoA oxidation and activity of carnitine acetyltransferase. DEHP was found to be the most potent PP among three phthalates. A dramatic and dose-dependent increase in hepatic MDA levels was observed in CF (100 mg/kg), DEHP (>or=50 mg/kg), DBP and BBP (>or=200 mg/kg) groups. However, the 8-OHdG in hepatic DNA was increased only in DEHP (1000 mg/kg) and CF groups. Activities of cytochrome p4501A1, 1A2, 3A4, UDP-glucuronosyl transferase and glutathione S-transferase were decreased overall by PPs, but there is no correlation between the inhibitory effect on metabolizing enzymes and the peroxisome proliferation. These results indicate that 8-OHdG positively correlates with carcinogenic potential of PPs, but other factors as well as peroxisomal H(2)O(2) could be involved in the generation of 8-OHdG and the carcinogenesis of PPs. The present findings also demonstrate that the effect of PPs on xenobiotic metabolizing enzymes may be independent of the peroxisome proliferation and the oxidative stress. Thus it is possible that the PPs affect the hepatic toxification/detoxification capacity even in humans.

In vitro models to study hepatotoxicity

Drug discovery and development consists of a series of processes starting with the demonstration of pharmacological effects in experimental cell and animal models and ending with drug safety and efficacy studies in patients. A main limitation is often the unacceptable level of toxicity with the liver as the primary target organ. Therefore, approaches to study hepatic toxicity in the early phase of drug discovery represent an important step towards rational drug development. A variety of in vitro liver models have been developed in the past years. Next to their use in drug development, they can also be applied to study environmental toxins and their hepatotoxicity. The 3 main approaches are ex vivo isolated and perfused organ models, precision-cut liver slices and cell culture models. Although the advantage of whole organ perfusions is based on the assessment of physiologic parameters such as bile production and morphologic parameters such as tissue histology, cell culture models can be efficiently used to assess cellular metabolism, cytotoxicity and genotoxicity. The advantage of precision-cut liver slices is based on the juxtaposition of cellular assays and tissue morphology. None of these models can be compared as they all focus on different fields of hepatoxicology. For the future, the ideal setup for testing the hepatic toxicity of a new compound could of primary studies in cell or slice cultures to assess cellular effects and secondary studies using ex vivo perfused organs to examine gross organ function parameters and histology.

Lack of effect of furfural on unscheduled DNA synthesis in the in vivo rat and mouse hepatocyte DNA repair assays and in precision-cut human liver slices

The ability of furfural to induce unscheduled DNA synthesis (UDS) in hepatocytes of male and female B6C3F(1) mice and male F344 rats after in vivo administration and in vitro in precision-cut human liver slices has been studied. Preliminary toxicity studies established the maximum tolerated dose (MTD) of furfural to be 320 and 50 mg/kg in the mouse and rat, respectively. Furfural was dosed by gavage at levels of 0 (control), 50, 175 and 320 mg/kg to male and female mice and 0, 5, 16.7 and 50 mg/kg to male rats. Hepatocytes were isolated by liver perfusion either 2-4 h or 12-16 h after treatment, cultured in medium containing [3H]thymidine for 4 h and assessed for UDS by grain counting of autoradiographs. Furfural treatment did not produce any statistically significant increase or any dose-related effects on UDS in mouse and rat hepatocytes either 2-4 h or 12-16 h after dosing. In contrast, UDS was markedly induced in mice and rats 2-4 h after treatment with 20 mg/kg dimethylnitrosamine and 12-16 h after treatment of mice and rats with 200 mg/kg o-aminoazotoluene and 50 mg/kg 2-acetylaminofluorene (2-AAF), respectively. Precision-cut human liver slices from four donors were cultured for 24 h in medium containing [3H]thymidine and 0-10 mM furfural. Small increases in the net grain count (i.e. nuclear grain count less mean cytoplasmic grain count) observed with 2-10 mM furfural were not due to any increase in the nuclear grain count. Rather, it was the result of concentration-dependent decreases in the mean cytoplasmic grain counts and to a lesser extent in nuclear grain counts, due to furfural-induced cytotoxicity. In contrast, marked increases in UDS (both net grain and nuclear grain counts) were observed in human liver slices treated with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM aflatoxin B(1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. This study demonstrates that furfural does not induce UDS in the hepatocytes of male and female B6C3F(1) mice and male F344 rats after oral treatment at doses up to the MTDs. Moreover, human liver slice studies suggest that furfural is also not a genotoxic agent in human liver.

Lack of effect of coumarin on unscheduled DNA synthesis in the in vivo rat hepatocyte DNA repair assay

The ability of coumarin to induce UDS in male Sprague-Dawley CD rat hepatocytes in vivo was assessed using the unscheduled DNA synthesis (UDS) assay. From a preliminary toxicity study the oral maximum tolerated dose (MTD) of coumarin was determined to be 320 mg/kg body weight. For the UDS studies, rats were treated with 0 (corn oil control), 32 (one-tenth the MTD), 107 (one-third the MTD) and 320 (MTD) mg/kg coumarin via oral gavage. Rats were also treated with 20mg/kg body weight dimethylnitrosamine (DMN) or 50mg/kg body weight 2-acetylaminofluorene (2-AAF) as positive controls for the 2-4 hr and 12-16 hr expression of UDS, respectively. Hepatocytes were isolated by liver perfusion either 2-4 hr or 12-16 hr after treatment and cultured in medium containing [methyl-(3)H]thymidine for 4 hr and assessed for UDS by grain counting of autoradiographs. Coumarin treatment at doses of 32-320 mg/kg body weight had no statistically significant or dose-related effect on UDS in rat hepatocytes either 2-4 hr or 12-16 hr after dosing. In contrast, both DMN 2-4 hr after dosing and 2-AAF 12-16 hr after dosing produced significant increases in UDS assessed as the net nuclear grain count. Both genotoxins also increased the percentage of hepatocyte nuclei with greater than 5 net grains. Treatment with coumarin, DMN and 2-AAF had no statistically significant effect on the proportion of rat hepatocytes undergoing replicative DNA synthesis. In summary, this study demonstrates that coumarin does not induce UDS in hepatocytes of male Sprague-Dawley CD rats after oral administration at doses up to the MTD of 320 mg/kg. The responsiveness of the animals used in this study to genotoxic agents was demonstrated by the clear induction of DNA repair after treatment with DMN and 2-AAF.

Inhalation toxicity and genotoxicity of hydrochlorofluorocarbon (HCFC)-225ca and HCFC-225cb

The acute, subchronic and genetic toxicity of the hydrochlorofluorocarbons HCFC-225ca and HCFC-225cb were evaluated to assist in establishing proper handling guides. In acute inhalation studies, rats were exposed for 4 h to various concentrations of each isomer. Based on the mortality incidence, the LC50 value for HCFC-225cb for males and females (combined) was 36800 ppm. For HCFC-225ca, the LC50 for males and females (combined) was 37300 ppm. Narcotic-like effects, e.g. prostration, incoordination and reduced motor activity, were observed during exposure to either isomer, but these signs were not evident 15 min after termination of exposure. Histopathological examination of the liver revealed an increase in mitotic figures with vacuolation of hepatocytes and fluid-filled, congested hepatic sinusoids. In cardiac sensitization studies, HCFC-225cb induced a cardiac sensitization response at 20000 ppm, with one fatal response, whereas a blend of the two isomers (45% HCFC-225ca/55% HCFC-225cb) produced a cardiac sensitization response at 15000 ppm. In 4-week subchronic inhalation studies, male and female rats were whole-body exposed to HCFC-225cb at concentrations of 0, 1000, 5000 or 15000 ppm for 6 h a day, 5 days per week. Similarly, male and female rats were whole-body exposed to HCFC-225ca concentrations of 0, 50, 500 or 5000 ppm for 6 h a day, 5 days per week. During exposure, narcotic-like and irritant effects were observed. A dose-related decrease in cholesterol and triglycerides was observed in the treated rats, with males being affected more than females. Increases in liver weight were observed in most male and female rats exposed to either isomer. The increase in liver weight was consistent in male rats with microscopic evidence of hepatocyte hypertrophy. Although liver weight was increased in female rats, no hepatocyte enlargement was observed in treated female rats. Increases in cytochrome P-450 and beta-oxidation activities were also observed in male and female rats exposed to either isomer. Neither of the HCFC-225 isomers was mutagenic in the Ames reverse mutation assay, or clastogenic in the chromosomal aberration assay with Chinese hamster lung cells. Also, neither isomer induced unscheduled DNA synthesis in liver cells. However, both isomers were clastogenic in the chromosomal aberration assay with human lymphocytes in the absence of S-9. No increases in aberrant cells were observed in activated cells exposed to either isomer.

Lack of effect of coumarin on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of coumarin on unscheduled DNA synthesis (UDS) in precision-cut human liver slices has been examined. Liver slices from tissue samples from four donors were cultured for 24 hr in medium containing [3H]thymidine and 0-5.0 mM coumarin using a dynamic organ culture system and processed for autoradiographic evaluation of UDS. As positive controls liver slices were also cultured with three known genotoxic agents, namely 0.02 and 0.05 mM 2-acetylaminofluorene (2-AAF), 0.002 and 0.02 mM aflatoxin B1 (AFB1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with more than five net grains. Compared with control liver slice cultures, treatment with 0.05-5.0 mM coumarin had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in the net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced significant increases in the number of centrilobular hepatocyte nuclei with more than five net grains. At the concentrations examined neither coumarin. 2-AAF, AFB1 nor PhIP had any significant effect on replicative DNA synthesis in 24 hr cultured human liver slices. These results demonstrate that coumarin does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. The results of this study suggest that coumarin is not a genotoxic agent in human liver.

Effect of some cooked food mutagens on unscheduled DNA synthesis in cultured precision-cut rat, mouse and human liver slices

Precision-cut liver slices were prepared from male Fischer 344 rats, female CDF1 mice and humans (both male and female subjects). Liver slices were cultured for 24 hr in medium containing [3H]thymidine and either PhIP, IQ, MeIQ, MeIQx, Glu-P-1 or Trp-P-1, and then processed for auto-radiographic evaluation of unscheduled DNA synthesis (UDS). All six cooked food mutagens examined produced concentration-dependent increases in UDS in human liver slices. PhIP was the most potent compound examined, followed by MeIQx, IQ and then MeIQ, Glu-P-1 and Trp-P-1. Significant increases in UDS were observed with PhIP, IQ and MeIQx at concentrations as low as 5 microM in the culture medium. The same rank order of potency was not apparent in either rat or mouse liver slices. In rat liver slices only MeIQ significantly induced UDS, although positive results were obtained with two other genotoxins, namely 2-acetylaminofluorene and aflatoxin B1. Apart from MeIQx, all the cooked food mutagens produced significant increases in UDS in mouse liver slices. This study demonstrates the usefulness of precision-cut liver slices to evaluate species differences in xenobiotic-induced genotoxicity. Both marked compound and species differences in induction of UDS were observed. The data provide further evidence that dietary cooked food mutagens are potential human carcinogens.

Evidence against peroxisome proliferation-induced hepatic oxidative damage

It has been proposed that nongenotoxic peroxisome proliferators may cause hepatocellular cancer by an oxidative damage-mediated mechanism(s). The argument for this hypothesis is based mainly on the noted ability of peroxisome proliferators to induce significantly H2O2-producing peroxisomal beta-oxidation while causing a minimal induction of H2O2-degrading catalase. The recent discovery, accurate determination, and use of isoprostanes as a sensitive indicator of oxidative damage prompted us to investigate whether induction of hepatic peroxisomal beta-oxidation in male B6C3F1 mice is accompanied by elevated levels of isoprostanes in those livers. The data show that while 7 days of feeding mice a diet containing 100 ppm [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid (WY-14,643) increased peroxisomal beta-oxidation by 16-fold and catalase activity by only 2-fold, hepatic levels of esterified F2-isoprostanes were not altered. These levels were 2.8 +/- 0.5 ng/g liver in control mice and 2.4 +/- 0.1 ng/g liver in mice fed the experimental diet for 7 days. Consequently, it is concluded that oxidative stress does not appear to occur in response to peroxisome proliferation, as evidenced by the lack of increase in hepatic levels of F2-isoprostanes in livers of mice treated with the potent peroxisome proliferator WY-14,643.

Hepatocarcinogenic potential of di(2-ethylhexyl)phthalate in rodents and its implications on human risk

The plasticizer di(2-ethylhexyl) phthalate (DEHP), to which humans are extensively exposed, was found to be hepatocarcinogenic in rats and mice. DEHP is potentially set free from objects made of synthetic materials (e.g., those used in medicine). Chronically, the greatest amounts are transferred to persons undergoing hemodialysis (up to 3.1 mg/kg b.w. per day) who would thus be considered the individuals most endangered by tumorigenesis. Although toxicokinetics seem to play a certain unclear role in the course of DEHP-related toxicity, toxicodynamic factors appear more decisive. DEHP is a representative of "peroxisome proliferators" (PP), a distinct group of substances that, in rodents, do not only induce peroxisomes but also specific enzymes in other organelles, organ growth, and DNA synthesis. The cluster of the characteristic effects of PP is generally, although perhaps not quite appropriately summarized as "peroxisome proliferation," and is strongest in the liver. The lowest observed effect level (LOEL) and the no observed effect level (NOEL) of peroxisome proliferation in the rat, as determined by the induction of specific enzymes (peroxisomal beta-oxidation, carnitine-acetyl-transferase, cytochrome P-452), DNA synthesis, and hepatomegaly, may be assumed as 50 and 25 mg/kg b.w. per day, respectively. DEHP and other carcinogenic PP are neither genotoxic nor tumor initiators, but they appear to be tumor promoters, also implicating a threshold level for the carcinogenic effect. Although a causal relationship between a particular effect of peroxisome proliferation and hepatocarcinogenesis is as yet unknown, peroxisome proliferation as a whole phenomenon appears to be associated with the potential of tumor induction, as shown by comparison of the relative strength of individual PP and by comparison of species and organ specificities. Likewise, LOEL and NOEL of rodent carcinogenesis, that is, 300 and 50 to 100 mg/kg b.w. per day, respectively, are above but not too far from the corresponding values for the investigated parameters of peroxisome proliferation. Thus, with respect to dose alone, worst-case exposure in hemodialysis patients is at least 16-fold below the LOEL of any characterized PP-specific effect of DEHP and approximately 100-fold below that of DEHP-related tumorigenesis. Also, primates are less responsive to PP than rats with respect to the investigated biochemical and morphological parameters. If this lower primate responsiveness is extrapolated to estimate carcinogenicity in humans, we might thus arrive at an even larger safety margin than when based on exposure alone. Doses of PP hypolipidemics that had clearly induced several indicators of peroxisome proliferation in rats did not cause any clear-cut enhancements in the peroxisomes of patients, even though most of these hypolipidemics were considerably stronger PP than DEHP. Thus, an actual threat to humans by DEHP seems rather unlikely. Accordingly, hepatocarcinogenesis was neither enhanced in workers exposed to DEHP nor in patients treated with hypolipidemics.

Induction of peroxisomal beta-oxidation and P-450 4A-dependent activities by pivalic and trichloroacetic acid in rat liver and kidney

The influence of pivalic acid (PIV), a compound often used to make pro-drugs, and of the structurally related trichloroacetic acid (TCA), on several hepatic and renal enzymes was investigated in Sprague-Dawley rats, following a 4-day treatment period. The PIV and TCA treatments resulted in a similar and selective induction (2-3 times) of peroxisomal palmitoyl-CoA oxidase and the cytochrome P-450 4A dependent microsomal (omega)- and (omega-1)-lauric acid activities, both in liver and kidney. Western blot analysis of liver and kidney microsomes from PIV- and TCA-treated rats, using antibody to the P-450 4A1, revealed induction of members of the P-450 4A subfamily. These results suggest that PIV, like TCA, is a renal and hepatic peroxisome proliferator in rats, and further support the previously indicated close association between the peroxisomal fatty acid beta-oxidation enzymes and microsomal P-450 4A sub-family enzymes.

The in vivo-in vitro replicative DNA synthesis (RDS) test with hepatocytes prepared from male B6C3F1 mice as an early prediction assay for putative nongenotoxic (Ames-negative) mouse hepatocarcinogens

To assess the efficacy of the in vivo-in vitro hepatocyte replicative DNA synthesis (RDS) test as a short-term assay, 41 putative nongenotoxic (Ames-negative) mouse hepatocarcinogens, as well as 31 noncarcinogens, were examined using male 8-week-old B6C3F1 mice and an in vitro [methyl-3]thymidine-incorporation technique. Animals were exposed to the maximum tolerated dose (MTD) and 1/2 MTD of each chemical by gavage and after 24, 39 or 48 h, hepatocytes were prepared with a collagenase-perfusion technique. Assessment of the distribution of spontaneous RDS in a total of 337 control mice gave an average incidence of 0.15 +/- 0.08% within the range of 0 to 0.39% (mean +/- 3 x SE) with a 99.7% probability. Values of 0.4% or more for RDS incidences induced by test samples were therefore judged as indicating a positive response in our RDS test. Under the experimental conditions applied, 32 of 41 putative nongenotoxic hepatocarcinogens gave clear positive responses (positive sensitivity: 78%), and of 31 noncarcinogens 25 samples gave negative responses (negative specificity: 81%), thus giving an overall concordance for the RDS test with long-term findings of 79%.

Mechanistically-based human hazard assessment of peroxisome proliferator-induced hepatocarcinogenesis

In this review we have evaluated the relationship between peroxisome proliferation and hepatocarcinogenesis. To do so, we identified all chemicals known to produce peroxisome proliferation and selected those for which there are data (on peroxisome proliferation and hepatocarcinogenesis) which meet certain criteria chosen to facilitate comparison of these phenomena. The summarised data and definition of the methodology used has been collected in appendices. These comparisons enabled us to evaluate the relationship between these phenomena using reliable data. As there is a good correlation between them, we further explored the mechanisms of action that have been proposed (direct genotoxic activity, production of hydrogen peroxide, cell proliferation and receptor activation). The relationship between these events in other species, including humans, was also reviewed and finally an overview of the assessment of human hazard is presented in section IX. Some of the first chemicals which were shown to produce peroxisome proliferation were also hepatocarcinogens whose carcinogenicity could not be readily explained by genotoxic activity. This raised the suggestion that the unusual phenomenon of peroxisome proliferation was intricately linked to the carcinogenic activity of these agents. Three questions have exercised the attention of regulatory, industrial and academic toxicology since then; are chemicals which elicit peroxisome proliferation in the liver actually a coherent class of chemical carcinogens?; does the early biological phenomenon of peroxisome proliferation have real predictive value for and mechanistic association with rodent carcinogenesis?; and what hazard/risk do these agents pose to humans that may be exposed to them? Whether peroxisome proliferators are indeed a discrete class of rodent carcinogens would appear to be the single, most important question. If so, then the assumptions and procedures relevant to human hazard and risk assessment should be applied to the class and should be essentially generic; if not, each chemical should be considered independently. Our critical analysis of the published data for over 70 agents which have been shown to possess intrinsic ability to induce peroxisome proliferation in the livers of rodents has led to the conclusion that there exists a strong correlation between peroxisome proliferation as n early effect in the liver and hepatocarcinogenicity in chronic exposure studies. An almost perfect correlation was observed between the induction of peroxisomes in the rodent liver and the eventual appearance of tumours following chronic exposure The few exceptions to this were largely explainable (section II).(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Use of precision-cut liver slices for studies of unscheduled DNA synthesis

Precision-cut liver slices were prepared from untreated and Aroclor 1254 (ARO)-treated male Sprague-Dawley rats with a Krumdieck tissue slicer. Liver slices were cultured for 24 hr in medium containing [3H]thymidine and 0-0.1 mM 2-acetylaminofluorene (2-AAF) using a dynamic organ culture system and processed for autoradiographic evaluation of unscheduled DNA synthesis (UDS). Compared with control (i.e. 0 mM 2-AAF) liver slice cultures, 2-AAF produced a concentration-dependent increase in UDS, the effect being more marked in liver slices from ARO-treated than from untreated rats. With liver slices from untreated rats, 2-AAF produced the greatest increase in UDS in centrilobular hepatocytes. 2-AAF-induced UDS in liver slices from ARO-treated rats was most marked in centrilobular hepatocytes but the effect also extended to other areas of the liver lobule. These results demonstrate that precision-cut liver slices may be a valuable alternative in vitro system to hepatocyte cultures for screening chemicals for potential genotoxicity. Unlike hepatocyte cultures, liver slices permit the study of zonal differences in UDS. Moreover, this technique could be applied to other tissues and the study of species differences in response.

Di(2-ethylhexyl)phthalate-induced changes in liver estrogen metabolism and hyperplasia

Exposure to a common phthalate, di(2-ethylhexyl)phthalate (DEHP), is associated with liver hyperplasia prior to the development of hepatocellular carcinoma in rodents. The exact mechanism of liver hyperplasia as well as tumorigenesis by this agent is not known. Since other lines of evidence point to estrogens as mediators of liver hyperplastic changes, we investigated whether DEHP exposure might alter hepatic estrogen metabolism and induce hyperplasia. Male Fischer 344 rats were fed either control or 1.2% DEHP-containing diets and sacrificed after 4, 8 and 16 weeks of exposure; activities of several sex hormone-responsive markers were measured. Rats fed DEHP had significantly increased serum estradiol levels, but hepatic activity of both cytosolic and nuclear estrogen receptor (ER) was significantly reduced. The serum content of ceruloplasmin, an estrogen-responsive protein synthesized by the liver, was also reduced, perhaps as a consequence of loss of ER activity. The rise in serum estradiol in DEHP-treated rats may be explained by the observation that these rats showed significant losses in hepatic activity of both a major male estrogen-metabolizing enzyme, estrogen 2-hydroxylase, and a male-specific estrogen-sequestering protein. In contrast to reductions in these activities, the expression of proliferating cell nuclear antigen and mRNAs for both ER and fos increased significantly as a result of exposure to DEHP. Our results suggest that changes in estrogen metabolism, receptor activity and activation of genes for cell proliferation are among the earliest metabolic alterations induced by DEHP. These changes together with the induced hyperplasia could play a crucial role in hepatocellular carcinoma development as a result of continuous exposure to DEHP.

Recommendations for the performance of UDS tests in vitro and in vivo

The Working Group (WG) dealt with the harmonization of routine methodologies of tests for unscheduled DNA synthesis (UDS) both in vitro and in vivo. In contrast to the existing guidelines from OECD, EPA and EC on in vitro UDS tests (there is no Japanese UDS guideline), the Working Group recommends that in general in vitro UDS tests should be performed with primary hepatocytes. For routine applications any other cell types would need special justification. Hepatocytes from male rats are preferable, unless there are contra-indications on the basis of e.g. toxicokinetic data. According to the OECD, EPA and EC guidelines, UDS may be analysed by means of autoradiography (AR) or liquid scintillation counting (LSC). The WG recommends use of AR. LSC is less suitable due to the problem of differentiation between UDS activity and replicative DNA synthesis, and the disadvantage that cells cannot be analysed individually. Since a specific cell type was recommended by the WG, methodological aspects could be described in more detail than in the present guidelines. For in vitro tests, it was agreed that the initial viability of freshly isolated hepatocytes should be at least 70%. With regard to the need for confirmatory experiments in the event of a clear-cut negative result, the majority view was that confirmation by a second (normally not identical) experiment is still needed; this is in line with the present OECD and EC guidelines. Evaluation of results from UDS tests should be based primarily on net nuclear grain (NNG) values, although it is recognised that nuclear and cytoplasmic grains result from different biological processes. Since grain counts are influenced by a number of methodological parameters, no global threshold NNG value can be recommended for discrimination of positive and negative UDS results. For in vitro assays, the criteria for positive findings go beyond those of the present guidelines and two alternative approaches are given which are based on (1) dose-dependent increases in NNG values and (2) reproducibility, dose-effect relationship and cytotoxicity. At present there is no official guideline on the performance of in vivo UDS tests. Some fundamental recommendations given for in vitro methodology also apply to the in vivo assay. For routine testing with the in vivo UDS test, again the general use of hepatocytes from male rats is recommended. However, concerning the requirement to use one or two sexes, consistency with other in vivo genotoxicity assays (e.g. the micronucleus assay) would be preferable. As for the in vitro methodology, AR is preferred rather than LSC.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of testosterone in the induction of hepatic peroxisome proliferation by clofibrate

Hepatic peroxisome proliferation is induced by a number of agents, including clofibrate. Sustained proliferation of peroxisomes is associated with the development of hepatocellular carcinoma. In the present study, we have investigated the role of testosterone in peroxisome proliferation induced by clofibrate. Three groups of male rats (intact, castrated, and castrated replaced with testosterone) were studied. Proliferation of peroxisomes was induced by feeding clofibrate (0.25%, 0.50%, and 1.0% of diet) for 2 weeks. Peroxisome proliferation was monitored by measuring total peroxisomal beta-oxidation activity. In intact rats, the peroxisomal beta-oxidation activity (nmol/min/mg protein) increased in a dose-dependent manner and was 7.2 +/- 0.4, 52.6 +/- 7.5, 63.2 +/- 3.7, and 92.4 +/- 4.0 at clofibrate doses of 0%, 0.25%, 0.50%, and 1.0%, respectively. In contrast, in castrated rats, the total peroxisomal beta-oxidation activity was significantly (P < .01) lower at clofibrate levels of 0.25% and 0.50% (25.8 +/- 2.7 and 42.5 +/- 2.2, respectively), but not at the clofibrate level of 1.0% (85.0 +/- 6.3). Testosterone replacement of castrated rats restored the peroxisomal beta-oxidation activity. To determine whether the above results were related to the metabolism of clofibrate in the absence or presence of testosterone, we measured serum clofibrate levels. These levels were 50% lower in castrated rats than in intact rats or in testosterone-treated castrated rats. The activity of hepatic uridine diphosphate (UDP)-glucuronyltransferase, the enzyme catalyzing the glucuronidation of clofibrate, was measured using either bilirubin or 4-methylumbelliferone as substrates and was found to be unaffected by castration or testosterone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

An in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes as an early prediction assay for nongenotoxic hepatocarcinogens screening of 22 known positives and 25 noncarcinogens

To evaluate the applicability of an in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes, we conducted the RDS test with 22 nongenotoxic (Ames-negative) hepatocarcinogens and 25 noncarcinogens under our standardized conditions and judgement criteria. Compared to controls (RDS incidence of under 1.0%), the RDS test gave positive results for 18 hepatocarcinogens (positive sensitivity: 82%), and negative results for 20 noncarcinogens (negative specificity: 80%), and thus the overall concordance was 81%. These findings strongly suggest that the RDS test is an extremely useful method for early detection of nongenotoxic hepatocarcinogens.

Hepatic peroxisome proliferation in rodents and its significance for humans

Peroxisomes are subcellular organelles found in all eukaryotic cells. In the liver they are usually round and measure about 0.5-1.0 microns; in rodents they contain a prominent crystalloid core, but this may be absent in newly formed rodent peroxisomes as well as in human peroxisomes. A major role of the peroxisomes is the breakdown of long-chain fatty acids, thereby complementing mitochondrial fatty-acid metabolism. Many chemicals are known to increase the number of peroxisomes in rat and mouse hepatocytes. This peroxisome proliferation is accompanied by replicative DNA synthesis and liver growth. No clear structure-activity relationships are apparent. Many of these peroxisome proliferators contain acid functions that can modulate fatty acid metabolism. Two mechanisms have been proposed for the induction of peroxisome proliferation. One is based on the existence of one or several specific cytosolic receptors that bind the peroxisome proliferator, facilitating its translocation to the cell nucleus and the activation of the expression of specific genes. The second, perhaps more general, hypothesis involves chemically mediated perturbation of lipid metabolism. These two hypotheses are not mutually exclusive. Many peroxisome proliferators have been shown to induce hepatocellular tumours, despite being uniformly non-genotoxic, when administered at high dose levels to rats and mice for long periods. Three mechanisms have been proposed to explain the induction of tumours. One is based on increased production of active oxygen species due to imbalanced production of peroxisomal enzymes; it has been proposed that these reactive oxygen species cause indirect DNA damage with subsequent tumour formation. In rodents, an alternative mechanism is the promotion of endogenous lesions by sustained DNA synthesis and hyperplasia. Thirdly, it is conceivable that sustained growth stimulation may be sufficient for tumour formation. Marked species differences are apparent in response to peroxisome proliferations. Rats and mice are extremely sensitive, and hamsters show an intermediate response while guinea pigs, monkeys and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. These species differences may be reproduced in vitro using primary culture hepatocytes isolated from a variety of species including humans. The available experimental evidence suggests a strong association and a probable casual link between peroxisome-proliferator-elicited liver growth and the subsequent development of liver tumours in rats and mice. Since humans are insensitive or unresponsive, at therapeutic dose levels, to peroxisome-proliferator-induced hepatic effects, it is reasonable to conclude that the encountered levels of exposure to these non-genotoxic agents do not present a hepatocarcinogenic hazard to humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Genotoxic effects of selected peroxisome proliferators

Peroxisome proliferators, a class of structurally dissimilar chemicals including hypolipidemic drugs and industrial plasticizers, have been shown to be associated with hepatocarcinogenesis although an initiating effect could not yet be demonstrated in the cell systems utilized. For this reason the genotoxic potential of the peroxisome proliferators nafenopin, ciprofibrate and di(2-ethylhexyl)adipate (DEHA) was determined in primary cultures of adult rat hepatocytes. To further test if these compounds are genotoxic per se or the genotoxic effect is due to peroxisome proliferation, the cultures were exposed for 3 and 51 h. Treatment for 3 h with the hypolipidemic drugs nafenopin and ciprofibrate induced statistically significant increases of SCE at concentrations > or = 30 and 100 microM respectively. At higher concentrations statistically significant increases of chromosomal aberrations (nafenopin: 100 microM; ciprofibrate: > or = 100 microM) and micronuclei (ciprofibrate: > or = 250 microM) were also found. The presence of peroxisome proliferators in the media until harvesting (51 h) did not significantly alter the dose response of SCE, micronuclei and chromosomal aberration induction by ciprofibrate, while long-term exposure to nafenopin resulted in statistically significant increases of chromosomal aberrations and micronuclei at concentrations > or = 30 microM. The differences were statistically significant at 30 and 100 microM for micronuclei, and at 30 microM for chromosomal aberrations. Neither short- nor long-term exposure to DEHA produced a significant genotoxic effect up to 200 microM. The peroxisome proliferators tested were not cytotoxic at any concentration, as determined by mitotic index. These results clearly demonstrate that the peroxisome proliferators nafenopin and ciprofibrate can cause genotoxic effects in primary cultures of adult rat hepatocytes. The comparison of short- and long-term exposure does not suggest a strong correlation between the induction of peroxisome proliferation and genotoxicity, since long-term exposure did not significantly alter the dose response and--except for nafenopin--the extent of the genotoxic effects.

Induction of sister chromatid exchange and micronuclei in primary cultures of rat and human hepatocytes by the peroxisome proliferator, Wy-14,643

The ability of peroxisome proliferators to induce hepatocellular carcinomas in rodents has been known since the mid 1970's, but the mechanism of tumor formation is still poorly understood. In this study, we have used primary cultures of both rat and human hepatocytes to address the question of whether the peroxisome proliferator, [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid (Wy-14,643), causes genotoxic damage in hepatocytes as measured by sister chromatid exchange (SCE), micronuclei formation, and chromosomal aberrations. We have found that in rat hepatocytes the number of SCEs per chromosome increased in a dose-dependent manner from a background level of 0.7 to a maximum of 1.1 in cells exposed for 48 h to 100 microM of Wy-14,643. In contrast, no increase in SCE frequency was observed in rat hepatocytes exposed to Wy-14,643 for 3 h. A dose-dependent increase in micronuclei formation was also seen in the 48 h but not in the 3 h cultures. The maximum frequency of micronuclei formation after a 48 h exposure occurred at 20 microM Wy-14,643 and was 2.3 times that for control cells. At this concentration of Wy-14,643, the frequency of chromosomal aberrations was increased by more than 10-fold. A 48 h exposure to Wy-14,643 also significantly increased micronuclei formation in human hepatocytes, but it was less effective than in rat hepatocytes. To investigate the potential role of peroxisome proliferation in these genotoxic responses, we measured the activities of palmitoyl-CoA beta-oxidase in hepatocytes exposed for 48 h to Wy-14,643. A dose-dependent increase in palmitoyl-CoA beta-oxidase activity was observed in rat hepatocytes, but not in human hepatocytes. The SCE frequency in rat hepatocytes correlated well with the degree of peroxisome proliferation, however, the increased formation of micronuclei in both rat and human hepatocytes occurred by a mechanism that appeared to be independent of peroxisome induction. In summary, these results demonstrate that the peroxisome proliferator, Wy-14,643, causes genotoxic damage in primary cultures of both rat and human hepatocytes.

In vivo-in vitro replicative DNA synthesis (RDS) test using perfused rat livers as an early prediction assay for nongenotoxic hepatocarcinogens: I. Establishment of a standard protocol

To establish a standard protocol for an in vivo-in vitro hepatocyte replicative DNA synthesis (RDS) test using male F344 rats for screening nongenotoxic (the Ames-negative) hepatocarcinogens, experimental conditions were examined. After treatment with three model hepatocarcinogens, isolated hepatocytes showed highest RDS incidences when plated at a density of 5 x 10(4) cells/ml. Spontaneous RDS incidences in hepatocytes from rats aged 9 weeks or older showed a constant value. The use of hepatocytes from 9-week-old rats at the 5 x 10(4) cells/ml plating density was therefore determined as the standard. Based on the distribution of mean spontaneous RDS incidences over 105 additional experiments (0.4 +/- 0.18%, with SEM), an RDS incidence of over 1% was adopted as the criterion for a positive response in our rat liver RDS test.

Relationship of hepatocarcinogenicity and hepatocellular proliferation induced by mutagenic noncarcinogens vs carcinogens. II. 1- vs 2-nitropropane

2-Nitropropane (2-NP) is mutagenic in a number of short-term mutagenicity assays in vitro and in vivo, and is a potent hepatocarcinogen in rats. A structural isomer, 1-nitropropane (1-NP), is mutagenic in V79 cells and can induce unscheduled DNA synthesis in rat hepatocytes, yet did not induce tumors in rats following chronic exposure. We examined the correlation of cell proliferation and hepatocarcinogenesis induced by this mutagenic noncarcinogen-carcinogen pair in a rat liver proliferation model. Rats were exposed to gavage doses of 0.5, 1, or 2 mmol/kg of 1-NP or 2-NP daily for 10 days; the highest two dose groups were similar to the doses used in the carcinogenesis bioassay. Cell proliferation was quantitated by incorporation of bromodeoxyuridine, detected immunohistochemically, into newly synthesized DNA. Animals exposed to the vehicle exhibited a labeling index (LI) of approximately 1.9% and animals exposed to CCL4 had a LI of approximately 30%. Rats exposed to the hepatocarcinogen 2-NP exhibited a dose-related increase in LI to 6.3 and 11% at the 1 and 2 mmol/kg doses, respectively, and no increase above control at the 0.5 mmol/kg exposure level. Animals exposed to the noncarcinogenic isomer 1-NP showed no statistically significant increase in LI above controls at any dose level tested. Serum chemistries were consistent with mild to moderate decreases in hepatocellular function, cholestasis, and necrosis following 2-NP exposure, but only minimal effects were observed, probably due to slight dehydration resulting from 1-NP exposure. These data indicate a positive association between increased cell proliferation and hepatocarcinogenesis induced by these two nitropropane isomers.

Correlation of hepatocellular proliferation with hepatocarcinogenicity induced by the mutagenic noncarcinogen:carcinogen pair--2,6- and 2,4-diaminotoluene

2,4-Diaminotoluene (2,4-DAT) and 2,6-diaminotoluene (2,6-DAT) are equally genotoxic in the Ames/Salmonella assay and are both readily absorbed, metabolized, and excreted and metabolites of both compounds are mutagenic with metabolic activation. However, there are marked differences in the results of chronic rodent bioassays with these two compounds. 2,4-DAT is a potent hepatocarcinogen whereas 2,6-DAT failed to produce an increased incidence of tumors in any tissue even when administered at a dose higher than that of 2,4-DAT. In an effort to elucidate the source of these apparently discordant results, the present studies were designed to determine the effects of these two chemicals on cell proliferation in the liver when administered at the dose levels comparable to those used in the original bioassays. This study utilized repeated oral dosing, osmotic minipumps to deliver bromodeoxyuridine (BrDU) for 8 days, and immunohistochemistry to quantitate BrDU incorporation into hepatic DNA, CCl4 (0.4 ml/rat, single ip dose) or vehicle control groups were included as positive and negative controls, respectively. The degree of cell proliferation was quantified by the labeling index from at least 1000 hepatocytes. Results from the control studies indicate that approximately 1.1% of the hepatocytes from vehicle-treated animals replicated during the exposure period whereas approximately 50% replicated in the positive controls. The carcinogen 2,4-DAT produced a dose-dependent increase in cell proliferation of approximately 10% and 20% in livers of animals exposed to 12.5 and 25.0 mg/kg/day, respectively, whereas the noncarcinogen 2,6-DAT produced no increase in cell turnover compared to vehicle control following treatment with 25.0 or 50.0 mg/kg/day. These results indicate a positive correlation between increased cell proliferation and hepatocarcinogenesis induced by these two isomers of diaminetoluene.

Induction of peroxisomal acyl CoA oxidase activity and lipid peroxidation in primary rat hepatocyte cultures

Peroxisome proliferators have been suggested to induce liver carcinogenesis as a result of increased peroxisomal hydrogen peroxide production and cellular oxidative stress. Primary monolayer cultures of hepatocytes isolated from male F344 rats were incubated in medium containing one of three different peroxisome proliferators and examined for the induction of peroxisomal CoA oxidase activity and lipid peroxidation. The latter parameter was determined by measuring levels of conjugated dienes in lipid fractions extracted from harvested cells. The peroxisome proliferators used in these studies were nafenopin and clofibric acid (two hypolipidemic drugs) and mono(2-ethylhexyl)phthalate (MEHP), the primary metabolite of the industrial plasticizer, di(2-ethylhexyl)phthalate (DEHP). The relative specific activity of peroxisomal acyl CoA oxidase was increased by about 300% after incubation for 44 h with 200 microM nafenopin; lower levels of induction were observed with clofibric acid or MEHP. Relative to controls, the level of conjugated dienes was increased approximately 2-fold after incubation with 200 microM nafenopin; there was no apparent increase in conjugated dienes after incubation with up to 200 microM MEHP or 400 microM clofibric acid. The increase in conjugated dienes with 200 microM nafenopin was inhibited by co-incubation with the antioxidant, N,N'-diphenyl-p-phenylenediamine. Thus, peroxisomal enzyme induction by nafenopin can result in membrane lipid peroxidation and monolayer cultures of rat hepatocytes may provide a useful model system for studying relationships between peroxisome proliferation, enhanced hydrogen peroxide production and cellular changes due to hepatic oxidative stress.

Toxicological studies on a benzofuran derivative. III. Comparison of peroxisome proliferation in rat and human hepatocytes in primary culture

Primary cultures of rat and human hepatocytes were used in our in vitro studies for investigating species differences in the response to a peroxisome proliferating benzofuran derivative, benzbromarone. Cyanide-insensitive palmitoyl coenzyme A oxidation (a marker of peroxisome fatty acid beta-oxidation) and electron microscopy were used to assess peroxisome proliferation. Hepatocytes were cultured essentially as described by Mitchell et al. (1984, Arch. Toxicol. 55, 239-246); clofibric acid and mono(2-ethylhexyl) phthalate (MEHP) were used as reference compounds, as they are well known to cause peroxisome proliferation in rat hepatocytes in primary culture. The benzofuran derivative, tested at drug concentrations ranging from 2.37 to 59.20 microM in rat hepatocyte primary cultures, induced, after 96 hr, a dose-related increase of the peroxisomal beta-oxidase activity correlated with an increased number of peroxisomes; this increase was much less marked than that obtained with clofibric acid or MEHP. By contrast, using the same range of concentrations, human hepatocytes in primary culture treated with benzbromarone revealed no enhancement of enzymatic activity and no concomitant statistically significant increase in the number of peroxisomes; the same observations were reported with clofibric acid and MEHP. These results demonstrate clearly that species differences in sensitivity to peroxisome proliferation with the benzofuran derivative do exist.

Long-term effects of peroxisome proliferators on the balance between hydrogen peroxide-generating and scavenging capacities in the liver of Fischer-344 rats

In order to clarify whether peroxisomal hydrogen peroxide (H2O2) plays an important role in peroxisome proliferator-induced hepatocarcinogenesis, we examined the change in metabolism of peroxisomal H2O2 in vivo and in vitro using male Fischer-344 rats fed clofibrate, bezafibrate and di(2-ethylhexyl)phthalate (DEHP) for up to 78 weeks. Hepatic peroxisomal fatty acyl-CoA oxidase activity increased 12-20-fold after 2 or 4 weeks treatment; later this level gradually decreased toward controls, and at 78 weeks activity was 3-10-times of control. Although hepatic H2O2 levels were increased slightly by clofibrate, bezafibrate and DEHP, the changes did not correlate with the changes in peroxisomal fatty acyl-CoA oxidase activity. In isolated hepatocytes, the rate of leakage of peroxisomal H2O2 from peroxisomes into the cytosol and the hepatocellular H2O2 content was measured. The rate of leakage of peroxisomal H2O2 into cytosol increased 2.5-4-fold when peroxisomal beta-oxidation activity was induced by peroxisome proliferators, and the increases in this rate corresponded with changes in the peroxisomal beta-oxidation activity. In contrast, the hepatocellular H2O2 contents were not affected by induced peroxisomal beta-oxidation. These data show that H2O2 leaking from peroxisome into cytosol would be quickly decomposed, and thus peroxisomal H2O2 does not appear to play an important role in hepatocarcinogenesis by such an oxidative stress mechanism after the long-term treatment with peroxisome proliferators.

Inductions of ornithine decarboxylase and replicative DNA synthesis but not DNA single strand scission or unscheduled DNA synthesis in the pyloric mucosa of rat stomach by catechol

The possible tumor-promoting and genotoxic activities of catechol were examined. Administration of catechol by gastric intubation at doses of 10 to 90 mg/kg body weight to male F344 rats induced up to 19-fold increase in ornithine decarboxylase activity with a maximum after 8 h and up to 8-fold increase in replicative DNA synthesis with a maximum after 24 h in the pyloric mucosa of the stomach. These results suggest that catechol has tumor-promoting activity in the pyloric mucosa of rat stomach. However, its administration at doses of 37.5 to 90 mg/kg body weight did not induce DNA single strand scission in the pyloric mucosa as determined by the alkaline elution method after 2 and 6 h or unscheduled DNA synthesis examined after 2 and 12 h.

Evaluation of liver cell proliferation during ciprofibrate-induced hepatocarcinogenesis

To determine if the carcinogenic potential of peroxisome proliferators is dependent upon their ability to induce cell proliferation, we have investigated the extent of cell proliferation in the livers of rats fed ciprofibrate, a peroxisome proliferator. Male rats were maintained on a diet containing ciprofibrate (0.025% w/w) and killed at selected intervals following 1 week of continuous [3H]thymidine labeling. Evaluation of labeling indices demonstrated a significant increase in cell proliferation during the first week but not in rats killed at the end of 5 and 20 weeks of treatment. Increases in hepatocyte nuclear labeling were found at 40 and 70 weeks of ciprofibrate administration which coincided with the appearance in livers of putative preneoplastic and neoplastic lesions. In a short-term feeding study, ciprofibrate and ethoxyquin were fed to rats at a dietary concentration of 0.025% and 0.5%, respectively, either alone or in combination for 7 days. Ciprofibrate and ethoxyquin either alone or in combination produced marked hepatomegaly and a significant increase in DNA synthesis as demonstrated by [3H]thymidine incorporation and autoradiographic studies. DNA synthesis in the group receiving ciprofibrate and ethoxyquin simultaneously, was slightly more than in animals that received either compound alone, suggesting a synergistic effect, although chronic feeding of these agents together resulted in inhibition of liver carcinogenesis (Rao, M. S. et al. (1984) Cancer Res., 44, 1072-1076). The results of this study further suggest that cell proliferation induced by peroxisome proliferators may be less important in carcinogenesis than peroxisome proliferation induced by these compounds.

Genetic toxicology of 1,1,2-trichloroethylene

1,1,2-Trichloroethylene (TCE) is a widely used halogenated solvent, produced in hundreds of millions of kg each year for industrial purposes. Occupational and environmental exposure of human populations to TCE has been reported in industrialized areas. Long-term carcinogenicity studies in rodents demonstrate that exposure to high doses of TCE results in the induction of liver and lung tumors in the mouse, and tumors of the kidney and the testis in the rat. An indirect mechanism, based on the stimulation of liver peroxisome proliferation by TCE metabolites, was proposed to explain species differences in TCE hepatocarcinogenicity. Mutagenicity studies indicate that TCE is weakly active both in vitro, where liver microsomes produce electrophilic TCE metabolites, and also in vivo in mouse bone marrow, where high rates of micronuclei, but no structural chromosome aberrations, are found. Among TCE metabolites, trichloroacetic acid was reported to be carcinogenic to mouse liver. Furthermore, both trichloroacetic acid and chloral hydrate were found to be genotoxic in vivo, inducing structural and numerical chromosome abnormalities, respectively.

Inhibition of gap-junctional intercellular communication between Chinese hamster lung fibroblasts by di(2-ethylhexyl) phthalate (DEHP) and trisodium nitrilotriacetate monohydrate (NTA)

Di(2-ethylhexyl)phthalate and trisodium nitrilotriacetate monohydrate, two apparently nongenotoxic carcinogens, were tested for effects on gap-junctional communication between Chinese hamster V79 lung fibroblasts. Both compounds inhibited gap-junctional communication in a concentration-dependent manner. The inhibiting effects of these chemicals on gap-junctional communication in vitro correlate with their tumor-promoting activity. Such results further support the hypothesis that inhibition of gap-junctional communication is an in vitro biomarker for some tumor-promoting chemicals.

Failure of the peroxisome proliferator WY-14,643 to initiate growth-selectable foci in rat liver

The ability of the peroxisome proliferator WY-14,643 to act as an initiator of hepatocarcinogenesis was examined using a modified growth-selection protocol in rats. Feeding rats 0.1% WY-14,643 in the diet for 3 weeks, coupled with partial hepatectomy after 10 days, failed to initiate the development of growth-selectable foci identifiable by 3 enzyme histochemical stains. Elevation of palmitoyl CoA oxidase activity in WY-14,643 rats from 1 to 11 days after partial hepatectomy suggested tht peroxisome proliferation, even when coupled with hepatocellular DNA replication, did not result in initiation. The failure of WY-14,643 to initiate growth-selectable foci may indicate that promotional activity is important in the hepatocarcinogenicity of the peroxisome proliferators.

Effects of commercial chlorophenolate, 2,3,7,8-TCDD, and pure phenoxyacetic acids on hepatic peroxisome proliferation, xenobiotic metabolism and sister chromatid exchange in the rat

The induction of hepatic peroxisome proliferation and drug metabolizing enzymes and of sister chromatid exchange (SCE) in lymphocytes was studied in male Han/Wistar rats after exposing them for 2 weeks to a commercial chlorophenolate formulation (Ky-5) (100 mg/kg/day), to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD; 0.05-5 micrograms/kg/wk) and to the pure phenoxyacetic acids, 2,4-dichlorophenoxyacetic acid (2,4-D; 100 mg/kg/day) and 2-chloro-4-methylphenoxyacetic acid (MCPA; 100 mg/kg/day). The chlorophenolate formulation and pure 2,4-D and MCPA caused significant increases in the number of peroxisomes in liver cells, although the average size of peroxisomes was not affected, whereas the effect of even the highest dose of 2,3,7,8-TCDD remained small. This finding indicates that dioxin impurities do not account for the peroxisome proliferation induced by chlorophenolate. The relative weight of the liver increased significantly in rats treated with the chlorophenolate formulation and with 2,3,7,8-TCDD (5.0 and 0.5 micrograms/kg). The pattern of induction of xenobiotic metabolizing enzymes showed some differences between chlorophenolate treatment and 2,3,7,8-TCDD treatment. Furthermore, the effects of pure phenoxyacetic acids were different from that seen with chlorophenolate and 2,3,7,8-TCDD. The highest dose of 2,3,7,8-TCDD increased the frequency of SCE in circulating lymphocytes slightly, but significantly.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper No. 5. Genotoxicity tests as predictors of carcinogens: an analysis

Differences between the results of numerical validation studies comparing in vitro and in vivo genotoxicity tests with the rodent cancer bioassay are leading to the perception that short-term tests predict carcinogenicity only with uncertainty. Consideration of factors such as the pharmacokinetic distribution of chemicals, the systems available for metabolic activation and detoxification, the ability of the active metabolite to move from the site of production to the target DNA, and the potential for expression of the induced lesions, strongly suggests that the disparate sensitivity of the different test systems is a major reason why numerical validation is not more successful. Furthermore, genotoxicity tests should be expected to detect only a subset of carcinogens, namely genotoxic carcinogens, rather than those carcinogens that appear to act by non-genetic mechanisms. Instead of relying primarily on short-term in vitro genotoxicity tests to predict carcinogenic activity, these tests should be used in a manner that emphasizes the accurate determination of mutagenicity or clastogenicity. It must then be determined whether the mutagenic activity is further expressed as carcinogenicity in the appropriate studies using test animals. The prospects for quantitative extrapolation of in vitro or in vivo genotoxicity test results to carcinogenicity requires a much more precise understanding of the critical molecular events in both processes.

Induction of peroxisome proliferation and hepatic tumours in C57BL/6N mice by ciprofibrate, a hypolipidaemic compound

The hepatic effects of ciprofibrate, a potent peroxisome proliferator, were evaluated in male C57BL/6N mice, a mouse strain with very low incidence of spontaneous liver tumour development. Dietary feeding of ciprofibrate (0.0125% or 0.025% w/w) for 2 weeks resulted in a marked proliferation of peroxisomes (9-fold increase) and several-fold increase (8- to 10-fold) in the activity of peroxisomal beta-oxidation enzymes. Feeding ciprofibrate at 0.025% concentration for 15 months followed by a 0.0125% for 6 months led to the development of hepatic adenomas in 8/14 (57%) and hepatocellular carcinomas (HCC) in 3/14 (21%) mice. In mice given 0.0125% ciprofibrate for 18 months 5 of 8 (62%) and 3 of 8 (37%) developed adenomas and HCC respectively. Similar to the findings observed in rats, both the adenomas and HCC were negative for gamma-glutamyltranspeptidase. These results in C57BL/6N mice of hepatocarcinogenic effect of ciprofibrate, a non-genotoxic chemical, indicate that peroxisome proliferation can be used as a reliable parameter to evaluate the carcinogenicity of hypolipidaemic compounds.