Possible mechanisms in hepatocarcinogenesis by the peroxisome proliferator di(2-ethylhexyl)phthalate

The effects of di(2-ethylhexyl)phthalate on rat liver in relation to selenium status

This study was performed to determine the hepatotoxicity of di(2-ethylhexyl)phthalate (DEHP) in relation to selenium status. In 3-week-old Sprague-Dawley rats, selenium deficiency was induced by a ≤0.05 selenium mg/kg. A selenium supplementation group was given 1 mg selenium/kg diet for 5 weeks. Di(2-ethylhexyl)phthalate-treated groups received 1000 mg/kg dose by gavage during the last 10 days of the experiment. Histopathology, peroxisome proliferation, catalase (CAT) immunoreactivity and activity and apoptosis were assessed. Activities of antioxidant selenoenzymes [glutathione peroxidase 1 (GPx1), glutathione peroxidase 4 (GPx4), thioredoxin reductase (TrxR1)], superoxide dismutase (SOD), and glutathione S-transferase (GST); aminotransferase, total glutathione (tGSH), and lipid peroxidation (LP) levels were measured. Di(2-ethylhexyl)phthalate caused cellular disorganization while necrosis and inflammatory cell infiltration were observed in Se-deficient DEHP group (DEHP/SeD). Catalase activity and immunoreactivity were increased in all DEHP-treated groups. Glutathione peroxidase 1 and GPx4 activities decreased significantly in DEHP and DEHP/SeD groups, while GST activities decreased in all DEHP-exposed groups. Thioredoxin reductase activity increased in DEHP and DEHP/SeS, while total SOD activities increased in all DEHP-treated groups. Lipid peroxidation levels increased significantly in SeD (26%), DEHP (38%) and DEHP/SeD (71%) groups. Selenium supplementation partially ameliorated DEHP-induced hepatotoxicity; while in DEHP/SeD group, drastic changes in hepatic histopathology and oxidative stress parameters were observed.

Effects of the peroxisome proliferator di(2-ethylhexyl)phthalate on cell turnover and peroxisome proliferation in primary chick embryo hepatocytes

The peroxisome proliferator (PP) di(2-ethylhexyl)phthalate (DEHP) is widely used as a plasticizer and can contaminate air, water, and soil. As yet, no data have been published on its potential to induce changes in cell growth of nonmammalian hepatocytes. In the present study, the effects of DEHP on cell turnover and induction of peroxisome proliferation were evaluated in primary hepatocyte cultures from chick embryos. Cells were treated after attachment with 0, 25, 50, 75, and 100 µM DEHP for up to 96 h. S-phase increased significantly (p < 0.01) from a background level of 5.5 ± 0.1% in solvent-control hepatocytes to a maximum level of 7.1 ± 0.1% in cells exposed for 48 h to 100 µM DEHP and decreased to near 6% by 96 h. Lower (p < 0.05) levels of induction were seen at 50 and 75 µM DEHP. Spontaneous apoptosis showed a slight (p < 0.05) decrease in hepatocytes treated with ≥75 µM dosages, as measured at 72 to 96 h. Induction of peroxisome proliferation was observed for cultures treated with ≥75 µM dosages at 48 h onwards. The results of the present study indicate that avian species may be responsive to the effects of PPs and may thus be affected by the presence of DEHP in the environment, but that this species is less sensitive than rodents.

Interactive effects of benzo(a)pyrene and cadmium and effects of di(2-ethylhexyl) phthalate on antioxidant and peroxisomal enzymes and peroxisomal volume density in the digestive gland of mussel Mytilus galloprovincialis Lmk

Exposure of marine animals to certain organic and metal pollutants is thought to enhance reactive oxygen species (ROS) production with concomitant alterations of antioxidant defence mechanisms. Some of these organic pollutants cause peroxisome proliferation, a process resulting also in possible enhanced production of ROS. The aim of this study was to investigate the effects of two organic xenobiotics, benzo(a)pyrene (B(a)P) and di(2-ethylhexyl)phthalate (DEHP), as well as the effects of cadmium (Cd), on antioxidant and peroxisomal enzymes and on peroxisomal volume density in the digestive gland of mussel, Mytilus galloprovincialis Lmk., experimentally exposed for 21 days. Special attention was paid to the interactive effects of organic and metal compounds by exposing one group of mussels to a mixture of B(a)P and Cd. Exposure of mussels to Cd caused a decrease in superoxide dismutase (SOD) activity, in Mn-SOD protein levels and in volume density of peroxisomes. B(a)P exposure significantly increased catalase and glutathione peroxidase (GPX) and inhibited Mn-SOD after 21 days of exposure. B(a)P also caused a slight increase in acyl-CoA oxidase (AOX) activity and peroxisomal volume density after 21 days of exposure. Cd tended to inhibit changes provoked by B(a)P, indicating that responses to organic xenobiotics can be modulated by concomitant exposure to metal contaminants. Exposure to DEHP increased catalase and AOX and inhibited SOD activity and Mn-SOD protein levels. In conclusion, peroxisome proliferation, measured as an increase of the peroxisomal enzymes catalase and AOX (up to 1.53-fold for AOX), is a specific response to organic contaminants such as B(a)P and DEHP, whereas Cd does not cause peroxisome proliferation. Thus, peroxisome proliferation may be a specific biomarker of organic pollutants in mussels. Both organic and metal pollutants inhibited SOD activity and protein levels (up to 0.21-fold for Mn-SOD protein levels), the latter offering potential as general marker of pollution.

Peroxisome proliferator perfluorodecanoic acid alters glutathione and related enzymes

Previously we have shown that treatment with the peroxisome proliferator perfluorodecanoic acid (PFDA) significantly increased hepatic reduced glutathione (GSH) content without altering the activity of selenium-glutathione peroxidase. In this study we examined some potential mechanisms by which PFDA treatment increases GSH levels. Male Sprague-Dawley rats were given a single injection of 0, 8.8, 17.5, and 35 mg PFDA in corn oil per kg body weight. Twelve days later the effects of PFDA on the activities of enzymes associated with GSH synthesis, utilization, and regeneration were assessed. The results showed that in a dose-dependent manner, PFDA treatment significantly decreased the activity of gamma-glutamylcysteine synthetase, while the activities of NADPH-generating enzymes, malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase were increased. PFDA treatment also dose dependently decreased cytosolic, but not microsomal, glutathione S-transferase activity, and the activity of glutathione reductase was decreased by the highest dose of PFDA. The data obtained suggest that increased hepatic GSH levels following PFDA treatment may result from increased regeneration and/or decreased utilization.

Mechanisms of nongenotoxic carcinogenesis and assessment of the human hazard

Regulatory toxicologists in the pharmaceutical area are faced with many chemical entities to be classified as rodent carcinogens, in most cases on the basis of a nongenotoxic mechanism. The purpose of this paper is to describe some mechanisms for nongenotoxic tumorigenicity and to indicate which type of testing should be done to substantiate why in those cases such a mechanism is not relevant to humans. The increasing attention being given to epigenetic carcinogenesis points at the need for a thorough evaluation during the toxicological program for safety assessment, enabling adequate assessment of the human hazard posed by such compounds. Data to support the nongenotoxic carcinogenesis may be obtained by collecting specific information from current safety assessment programs or from future, separate studies.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.

Effects of the rodent peroxisome proliferator and hepatocarcinogen, perfluorooctanoic acid, on apoptosis in human hepatoma HepG2 cells

The effects of perfluorooctanoic acid (PFOA), a potent hepatocarcinogen and peroxisome proliferator in rodents, on human cells have not yet been examined. In the present study we demonstrate that treatment of human hepatoblastoma HepG2 cells with PFOA induces apoptosis, as well as perturbs the cell cycle. This apoptosis was characterized by electron microscopy, which revealed typical nucleosomal fragmentation (also observed as a 'DNA ladder' upon electrophoresis on agarose) and was quantitated using propidium iodide staining of cellular DNA and the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay. This process was dose- and time-dependent: apoptosis became manifest with 200 microM and maximal (45% of the cells) upon exposure to 450 microM PFOA for 24 h. Electrophoresis of the DNA from HepG2 cells exposed to 500 microM PFOA for 24 h or to 400 microM PFOA for 48 h revealed a smear typical of non-specific degradation. These findings indicate that in the presence of high concentrations of PFOA for long times, HepG2 cells undergo primary and secondary necrosis. Quantitation of trypan blue exclusion supported this conclusion. Flow cytometric analysis revealed that the cell cycle of HepG2 cells was perturbed by exposure to 50-150 microM PFOA. A 50 microM concentration resulted in a significant increase in the proportion of G(2)/M cells and, simultaneously, a decrease in the number of cells in the S phase, whereas treatment with 100 or 150 microM PFOA increased the proportion of cells in the G(0)/G(1) phase and decreased the number of cells in the G(2)/M and S phases. Simultaneous flow cytometric analysis of apoptosis-associated DNA strand breaks using the TUNEL procedure and of propidium iodide staining of cellular DNA revealed DNA breaks in HepG2 cells exposed to 150 microM PFOA, prior to nuclear fragmentation.

Captan impairs CYP-catalyzed drug metabolism in the mouse

To investigate whether the fungicide captan impairs CYP-catalyzed drug metabolism in murine liver, kidney and lung, the modulation of the regio- and stereo-selective hydroxylation of testosterone, including 6beta-(CYP3A), 6alpha-(CYP2A1 and CYP2B1) and 16alpha-(CYP2B9) oxidations was studied. Specific substrates as probes for different CYP isoforms such as p-nitrophenol (CYP2E1), pentoxyresorufin (CYP2B1), ethoxyresorufin (CYP1A1), aminopyrine (CYP3A), phenacetin and methoxyresorufin (CYP1A2), and ethoxycoumarin (mixed) were also considered. Daily doses of captan (7.5 or 15 mg/kg b.w., i.p.) were administered to different groups of Swiss Albino CD1 mice of both sexes for 1 or 3 consecutive days. While a single dose of this fungicide did not affect CYP-machinery, repeated treatment significantly impaired the microsomal metabolism; in the liver, for example, a general inactivating effect was observed, with the sole exception of testosterone 2alpha-hydroxylase activity which was induced up to 8.6-fold in males. In vitro studies showed that the mechanism-based inhibition was related to captan metabolites rather than the parental compound. In the kidney, both CYP3A- and CYP1A2-linked monooxygenases were significantly induced (2-fold) by this pesticide. Accelerated phenacetin and methoxyresorufin metabolism (CYP1A2) was also observed in the lung. Data on CYP3A (kidney) and CYP1A2 (kidney and lung) induction were corroborated by Western immunoblotting using rabbit polyclonal anti-CYP3A1/2 and CYP1A1/2 antibodies. By means of electron spin resonance (EPR) spectrometry coupled to a spin-trapping technique, it was found that the recorded induction generates a large amounts of the anion radical superoxide (O*2-) either in kidney or lung microsomes. These findings suggest that alterations in CYP-associated activities by captan exposure may result in impaired (endogenous) metabolism as well as of coadministered drugs with significant implications for their disposition. The adverse outcomes associated to CYP changes (e.g. cotoxicity, comutagenicity and promotion) may also have harmful consequences.

Induction and suppression of murine CYP-mediated biotransformation by dithianon: organ- and sex-related differences

With the aim of evaluating the co-carcinogenic properties of dithianon, the regio- and stereo-selective hydroxylation of testosterone was used as a multibiomarker of effect for cytochrome P450 (CYP) changes. CYP-catalysed reactions have been studied in liver, kidney and lung microsomes from male and female Swiss albino CD1 mice treated i.p. with single (3 or 6 mg/kg body wt.) or repeated (3 mg/kg body wt. daily for 3 days) doses of this fungicide. Induction or suppression was recorded under various situations in different organs and sexes. In liver, all testosterone hydroxylase (TH) activities were increased in the single treatment from 2.8- (6beta-, 16alpha- and 16beta-TH activities) to 16-fold (2beta-TH activity) in males at the lower dose. In contrast, activities were reduced from 33.3% (16beta- and 17-TH activities, lower dose) to 66.4% (16beta-TH activity, higher dose) in females. In kidney, a similar pattern of modulation was achieved: induction from 2.9- to 5-fold (6beta- and 2alpha-TH activities, higher and lower doses, respectively) in males; suppression from 47.4 to 50.2% (2alpha- and 2beta-TH activities, either at lower or higher doses) in females. In lung, a significant induction ranging from 7.1- to 29.3-fold (16alpha- and 2alpha-TH activities, respectively, lower dose) in males, and up to a 7-fold increase (2beta-TH activity, higher dose) in females was obtained. After repeated treatment, hepatic 6beta-, 16beta-, 2alpha- and 2beta-TH activities were reduced up to approximately 60% in males, whereas no effect was seen in females. In extrahepatic tissues, a generalized increase of different THs was observed. The increase of 6beta-TH activity (CYP3A-linked), one of the most representative isoforms in humans, was sustained in liver and kidney by means of Western immunoblotting, using rabbit polyclonal antibodies anti CYP3A1/2. On the whole, a complex pattern of induction/suppression of CYP-dependent reactions was achieved depending on sex and tissue. The data are consistent with co-toxic, co-carcinogenic and promoting potentials of this fungicide and provide information of interest in evaluating the risk associated with human exposure.

Exposure of hemodialysis patients to di-2-ethylhexyl phthalate

The migration of di-2-ethylhexyl phthalate (DEHP) from dialyzers was studied in 21 patients with chronic renal failure undergoing maintenance hemodialysis. The circulating concentrations of DEHP were measured by high performance liquid chromatography in blood of patients obtained from the inlet and the outlet of the dialyzer during a 4-h dialysis session. During treatment of renal failure using plasticized tubing, the plasma level of DEHP increased. On average, an estimated 75.2 mg of DEHP was extracted from the dialyzer during a single dialysis session, with a range of 44.3-197. 1 mg. On the other hand, the total amount of DEHP retained by the patient during the dialysis session was evaluated by the difference between the AUCout and the AUCin and ranged from 3.6 to 59.6 mg. The rate of extraction of DEHP from the dialyzer was correlated (r=0.705, P<0.05) with serum lipid content (cholesterol and triglyceride).So, we confirmed that patients on hemodialysis are always regularly exposed to considerable amounts of DEHP. However, several metabolic effects have been reported in various animal species following treatment with DEHP, such as changes in lipid metabolism and in hepatic microsomal drug-metabolizing enzyme activities. DEHP is now a well-known hepatic peroxisomal proliferator in rodents and an inducer of many peroxisomal and non-peroxisomal enzymes. So, lipid metabolism modifications and hepatic changes observed in hemodialysis patients could be explained from chronic exposition to DEHP. In the coming years, it seems necessary to reconsider the use of DEHP as a plasticizer in medical devices. Highly unacceptable amounts of DEHP leached during the dialysis session could be easily avoided by careful selection of hemodialysis tubing.

Molecular non-genetic biomarkers of effect related to methyl thiophanate cocarcinogenesis: organ- and sex-specific cytochrome P450 induction in the rat

We used selective biochemical markers of effect to evaluate some non-genotoxic cocarcinogenic properties of methyl thiophanate (MTH) associated with cytochrome P450 (CYP) changes. Several CYP-dependent reactions were monitored in the liver, kidney and lung microsomes of male and female Sprague-Dawley rats treated (i.p.) with a single (285 or 570 mg/kg body weight) or repeated (daily 285 or 570 mg/kg body weight for three consecutive days) doses of this pesticide. No significant changes in absolute or relative liver, kidney and lung weights were observed after MTH injection. Highly specific substrates were used as probes of different isoforms, such as CYP1A1, 1A2, 2B1, 2E1 and 3A. A complex pattern of CYP induction, including organ- and sex-related differences, was observed, particularly in the liver (CYP3A, 2B1), kidney (CYP1A1, 2E1) and lung (CYP3A, 1A1). In the liver, an increase up to 29-fold in the 2B1-like activity, probed by the O-dealkylation of pentoxyresorufin, was observed at lower dose in both sexes, and the induction of CYP 1A2-mediated methoxyresorufin O-demethylase activity (up to 3.6-fold) was recorded at the higher dose in males. In the kidney, the O-deethylation of ethoxyresorufin (CYP1A1-linked) was increased up to 28.2-fold and the CYP2E1-dependent p-nitrophenol hydroxylases were enhanced up to 6.3-fold in females receiving higher multiple MTH administration. In the lung, the CYP3A-associated activity was the most induced oxidases, as exemplified by the marked increase in the O-demethylation of aminopyrine (up to 3.6-fold) in males. A weak, although significant, reduction of CYP2B1-linked oxidases was also observed in repeated treatment in the kidney (males) and lung (females). These results suggest that the induction of CYP-catalyzed drug metabolism by prolonged exposure to MTH may result in accelerated metabolism of coadministered drugs with important implications for their disposition Together with an alteration of endogenous metabolism, the adverse effects associated with CYP changes such as toxicity/cotoxicity, cocarcinogenicity and promotion may also have clinical consequences.

Mechanism of action of the nongenotoxic peroxisome proliferators: role of the peroxisome proliferator-activator receptor alpha

Peroxisome proliferators are a diverse group of chemicals that include several therapeutically used drugs (e.g., hypolipidemic agents), plasticizers and organic solvents used in the chemical industry, herbicides, and naturally occurring hormones. As the name implies, peroxisome proliferators cause an increase in the number and size of peroxisomes in the liver, kidney, and heart tissue of susceptible species, such as rats and mice. Long-term administration of peroxisome proliferators can cause liver cancer in these animals, a response that has been the central issue of research on peroxisome proliferators for many years. Peroxisome proliferators are representative of the class of nongenotoxic carcinogens that cause cancer through mechanisms that do not involve direct DNA damage. The fact that humans are frequently exposed to these agents makes them of particular concern to government regulatory agencies responsible for assuring human safety. Whether frequent exposure to peroxisome proliferators represents a hazard to humans is unknown; however, increased cancer risk has not been shown to be associated with long-term therapeutic administration of the hypolipidemic drugs gemfibrozil, fenofibrate, and clofibrate. To make sound judgments regarding the safety of peroxisome proliferators, the validity of extrapolating results from rodent bioassays to humans must be based on the agents' mechanism of action and species differences in biologic activity and carcinogenicity. The peroxisome proliferator-activated receptor alpha (PPARalpha), a member of the nuclear receptor superfamily, has been found to mediate the activity of peroxisome proliferators in mice. Gene-knockout mice lacking PPARalpha are refractory to peroxisome proliferation and peroxisome proliferator-induced changes in gene expression. Furthermore, PPARalpha-null mice are resistant to hepatocarcinogenesis when fed a diet containing a potent nongenotoxic carcinogen WY-14,643. Recent studies have revealed that humans have considerably lower levels of PPARalpha in liver than rodents, and this difference may, in part, explain the species differences in the carcinogenic response to peroxisome proliferators.

Effect of the peroxisome proliferator ciprofibrate on hepatic cyclooxygenase and phospholipase A2 in rats

Peroxisome proliferators, which include several hypolipidemic drugs, plasticizers and other chemicals, induce hepatic tumors in rodents. These chemicals alter the expression of enzymes involved in lipid metabolism, such as the cytochrome P450 4A family and peroxisomal beta-oxidation enzymes. Previous studies have shown that the peroxisome proliferator ciprofibrate reduces eicosanoid concentrations in rat livers and primary hepatocyte cultures, yet the mechanism is still unclear. In this study we examined cyclooxygenases 1 and 2 (COX-1 and COX-2) and cytosolic phospholipase A2 (cPLA2) to determine whether the rate-limiting enzymes in the eicosanoid synthetic pathway are altered by ciprofibrate. Rats were fed 0.01% ciprofibrate for 3, 6, or 10 days. Western analysis revealed that COX-2 protein was induced by ciprofibrate (up to 13-fold at day 10), but that calcium-dependent (Ca-D) cPLA2 protein was not different from controls. The enzyme activity of calcium-independent (Ca-I) cPLA2 in ciprofibrate-treated rats was increased 2-fold, whereas Ca-D cPLA2 and total COX activities were not affected. Using enzyme kinetics, we found that COX-1 (Ki = 143 microM) and Ca-I cPLA2 (Ki = 121 microM) were competitively inhibited by ciprofibrate, but the inhibition was not physiologically significant. COX-2 and Ca-D cPLA2 were not inhibited by ciprofibrate. These results show that ciprofibrate increases Ca-I cPLA2 enzyme activity and COX-2 protein expression.

Perfluorodecanoic acid as a useful pharmacologic tool for the study of peroxisome proliferation

The phenomena of peroxisome proliferation in rodent liver has received considerable attention due to its association with hepatocellular carcinoma. Chemicals that cause peroxisome proliferation include several structurally unrelated hypolipidemic drugs, phthalate esters and halogenated solvents. The mechanism by which peroxisome proliferators exert their beneficial (hypolipidemia) as well as their toxic (cancer) effects is still largely unknown. Perfluorodecanoic acid (PFDA) is a potent peroxisome proliferator in rodent liver that resembles other members of this chemical class in many aspects, including its effects on gene expression and fatty acid metabolism. However, there are many dissimilarities between PFDA and hypolipidemic peroxisome proliferators that have not been extensively explored. PFDA is unlike other peroxisome proliferators in parent compound metabolism, hypolipidemia, and tumor promotion. The present review article will discuss what is currently known about PFDA and how it may be utilized to dissect the mechanism of action of an important group of hypolipidemic drug and environmental pollutant, the peroxisome proliferators.

Reduction of the concentrations of prostaglandins E2 and F2alpha, and thromboxane B2 in cultured rat hepatocytes treated with the peroxisome proliferator ciprofibrate

Several hypolipidemic drugs, plasticizers and other chemicals induce peroxisome proliferation and hepatic tumors in rodents, but the mechanism by which they induce tumors is not fully understood. Their carcinogenic activity may be related to alterations in gene expression, such as induction of peroxisomal beta-oxidation enzymes or of the cytochrome P450 4A family. These enzymes metabolize lipids, including eicosanoids and their precursor fatty acids. Because eicosanoids likely play a role in the carcinogenic process, alterations in their concentration by xenobiotics may be important in their carcinogenic or promoting activities. In this study we used isolated hepatocytes to study if peroxisome proliferators alter the metabolism of prostaglandins (PG) and thromboxanes (Tx). Isolated rate hepatocytes were cultured for 4 days with 2 concentrations of ciprofibrate (CIP): 100 and 400 microM. Fatty acyl CoA oxidase activities of the 100 and 400 microM CIP treatment groups at the end of the experiment were increased 5.3 and 9.6 times, respectively. TxB2 and PGF2alpha concentrations in cultures treated with CIP were significantly lower than the control at days 3 and 4, whereas a lower concentration of PGE2 was seen at day 4 only. These studies show that PG and Tx concentrations in cultured hepatocytes are lowered by the peroxisome proliferator CIP.

Peroxisomes in mice fed a diet supplemented with low doses of fish oil

The influence of low dietary doses (0.1 and 0.8% w/w) of a commercial fish oil preparation on peroxisomes in normal mice was studied and compared to the known strong inductive effects of high (10%) fish oil diets. Low fish oil doses were chosen to supply the mice with a concentration of docosahexaenoic acid, which was beneficial to patients with a peroxisomal disease. Peroxisomes were evaluated by cytochemical, morphometric, and enzymological techniques. The 0.1% fish oil diet had no effect on peroxisomes in liver, heart, and kidney even after prolonged treatment. The 0.8% diet did not change the peroxisomal number nor the catalase (EC 1.11.1.6) activity in the liver. Hepatic peroxisomal beta-oxidation, however, was increased by 50% after 14 d. This was accompanied by reduced peroxisomal size. The 0.8% diet also caused a small increase (+25%) in myocardial catalase activity. No effect was observed in kidneys. Our results indicate that in mice a low (< 0.8%) dietary fish oil dose has no or only a slight effect on hepatic peroxisomal beta-oxidation. This may be of particular interest to patients with a peroxisomal fatty acid beta-oxidation defect and who display a severe deficiency of docosahexaenoic acid--diets supplemented with low fish oil doses will improve the docosahexaenoic acid level without adding a strong load to the disturbed fatty acid metabolism.

Trichloroethylene--a review of the literature from a health effects perspective

This report reviews the literature on the impact of exposure to trichloroethylene (TCE) on human health. Special emphasis is given to the health effects reported in excess of national norms by participants in the TCE Subregistry of the Volatile Organic Compounds Registry of the National Exposure Registries--persons with documented exposure to TCE through drinking and use of contaminated water. The health effects reported in excess by some or all of the sex and age groups studied were speech and hearing impairments, effects of stroke, liver problems, anemia and other blood disorders, diabetes, kidney disease, urinary tract disorders, and skin rashes.

Secondary alterations of human hepatocellular peroxisomes

The morphological and morphometric characteristics of peroxisomes in normal human liver and the peroxisomal alterations in the liver of patients with acquired or congenital non-peroxisomal diseases are reviewed. Secondary peroxisomal changes are observed in steatosis, hepatitis and cirrhosis induced by various agents (viruses, alcohol, drugs, etc.), in cholestasis, in hepatomas, in extra-hepatic cancer with or without liver metastasis, in extrahepatic inflammatory processes, in metabolic disorders affecting metabolism of carbohydrates, lipids and lipoproteins, glycoproteins, amino acids, bilirubin or copper, and in altered thyroid hormone levels. They are recognized as a proliferation of peroxisomes (increased in number and to a lesser extent in surface density and volume density) often accompanied by a minor reduction in size (at most to 68% of the mean diameter in control livers) but very rarely by an increase in mean peroxisomal diameter, and as proliferation-related changes in shape (tails, gastruloid cisternae, funnel-like constrictions, elongation, protrusions) in at least a few of the peroxisomes. These secondary alterations of the peroxisomes are clearly distinguishable from the primary changes in peroxisomes observed in the liver of patients with congenital peroxisomal disorders.

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)

Phenotypic alteration of hepatocellular foci in rats treated with clofibrate and phenobarbital

In male F344 rats pretreated with diethylnitrosamine (DEN), subsequent administration of clofibrate increased the proportion of eosinophilic foci, to become the most abundant type, and reduced numbers of basophilic, clear and vacuolated foci, the total not being changed. A similar shift towards eosinophilia was also observed in phenobarbital-treated animals, but in this case clear increases in total number and area were apparent. Expression of the glutathione S-transferase placental form (GST-P) in foci was much lowered by clofibrate treatment, while the proportion of positive foci was very high in both phenobarbital and control groups. A marked contrast was found with eosinophilic foci, with 74% positive after phenobarbital as compared to only 15% for clofibrate. Thus, the decrease in GST-P positive foci by clofibrate was mainly due to increased negativity in the most abundant eosinophilic type foci. In a long-term feeding study without DEN initiation, similar negativity of foci was observed and, furthermore, only minimal effects of clofibrate on foci development was revealed in both young and old animals.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Fatty-acid metabolism and the pathogenesis of hepatocellular carcinoma: review and hypothesis

Despite increasing understanding of the genetic control of cell growth and the identification of several involved chemical and infectious factors, the pathogenesis of clinical and experimental hepatocellular carcinoma remains unknown. Available evidence is consistent with the possibility that selected changes in the hepatocellular metabolism of long-chain fatty acids may contribute significantly to this, process. Specifically, studies of the peroxisome proliferators, a diverse group of xenobiotics that includes the fibrate class of hypolipidemic drugs, suggest that increased fatty acid oxidation by way of extramitochondrial pathways (i.e., omega-oxidation in the smooth endoplasmic reticulum and beta-oxidation in the peroxisomes) results in a corresponding increase in the generation of hydrogen peroxide and, thus, oxidative stress. This in turn leads to alterations in gene expression and in DNA itself. We also review evidence supporting a potentially decisive influence of particular aspects of hepatocellular fatty acid metabolism in determining the activity of the extramitochondrial pathways. Moreover, certain intermediates of extramitochondrial fatty acid oxidation (e.g., the long-chain dicarboxylic fatty acids) impair mitochondrial function and are implicated as modulators of gene expression through their interaction with the peroxisome proliferator-activated receptor. Finally, the occurrence of hepatic tumors in type I glycogen storage disease (glucose-6-phosphatase deficiency) may exemplify this general mechanism, which may also contribute to nonneoplastic liver injury and to tumorigenesis in other tissues.

Effect of the peroxisome proliferator perfluorodecanoic acid on the promotion of two-stage hepatocarcinogenesis in rats

This study was conducted to determine if the peroxisome proliferator perfluorodecanoic acid (PFDA) has promoting activity in two-stage hepatocarcinogenesis. Because PFDA is a non-competitive inhibitor of the peroxisomal bifunctional enzyme and thus inhibits the peroxisomal beta pathway, we hypothesized that PFDA may not have promoting activity as do other peroxisome proliferators, because hydrogen peroxide production is inhibited. Twenty-four hours after partial hepatectomy, female Sprague-Dawley rats were given an initiating dose of 10 mg/kg diethylnitrosamine by gavage. The rats were divided into five groups that received monthly i.p. injections of 0.0, 0.05, 0.50 or 5.0 mg/kg PFDA in corn oil or were placed on diets that contained either 0.01% ciprofibrate or 0.05% phenobarbital for 9 or 18 months. Both ciprofibrate and the highest dose of PFDA increased the activity of the peroxisomal enzyme fatty acyl CoA oxidase. PFDA treatment did not increase the tumor incidence or the number of altered hepatic foci at 9 or 18 months, although the mean volume of foci was increased at 9 months. Ciprofibrate increased the incidence of hepatocellular carcinomas at 18 months but did not increase the number or volume of altered hepatic foci at 9 or 18 months. Phenobarbital increased the number and volume of foci but did not influence the tumor incidence. The results of this investigation indicate that PFDA is not a promoter of hepatocarcinogenesis.

Effects of the peroxisome proliferator mono(2-ethylhexyl)phthalate in primary hepatocyte cultures derived from rat, guinea pig, rabbit and monkey. Relationship between interspecies differences in biotransformation and peroxisome proliferating potencies

Primary hepatocyte cultures derived from rat, rabbit, guinea pig and monkey have been treated in vitro with metabolites of di(2-ethylhexyl)phthalate, i.e. mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate (metabolite V) and mono(2-ethyl-5-oxohexyl)phthalate (metabolite VI). In rat hepatocyte cultures MEHP and metabolite VI were equally potent in inducing peroxisome proliferation, while metabolite V was much less potent. In rat hepatocytes a 50% increase in both peroxisomal palmitoyl-CoA oxidase activity and microsomal lauric acid omega-hydroxylation activity was found after treatment with 5-15 microM MEHP. In guinea pig, rabbit and monkey hepatocyte cultures, a 50% increase in peroxisomal palmitoyl-CoA oxidase activity was found after treatment with 408-485 microM MEHP. No induction of lauric acid omega-hydroxylation activity was found. These results indicate that peroxisome proliferation can be induced by MEHP in rabbit, guinea pig and monkey hepatocytes, but that these species are at least 30-fold less sensitive to peroxisome proliferation induction than rats. The proposed mechanistic inter-relationship between induction of lauric acid omega-hydroxylation activity and peroxisome proliferation is found in rat hepatocytes, but not in hepatocytes of the other three species. Treatment of guinea pig hepatocyte cultures with MEHP resulted in an increase in triglyceride concentrations in the hepatocytes. In rat and rabbit hepatocyte cultures, triglyceride concentrations were much less altered by MEHP. In monkey hepatocytes a decrease in hepatic triglyceride concentration was found after treatment with MEHP. These effects are in agreement with in vivo effects observed before. After treatment of primary hepatocyte cultures with MEHP, high concentrations of omega- and (omega-1)-hydroxylated metabolites of MEHP were found in media from rat, rabbit and guinea pig cultures. The formation of these metabolites did not decline in time. During treatment the metabolite profile in media from rat hepatocyte cultures moved towards omega-hydroxy metabolites of MEHP. In media from monkey hepatocyte cultures the lowest concentrations of hydroxylated metabolites were determined. No major species differences were found in the potency to form oxidized MEHP metabolites, and thus no unique metabolite differences were found, which could explain the species differences in sensitivity for peroxisome proliferation.

The role of dichloroacetate in the hepatocarcinogenicity of trichloroethylene

The induction of hepatic tumors in B6C3F1 mice treated with trichloroethylene (TRI) has been attributed to its metabolism to trichloroacetate (TCA). Trichloroacetate is an effective peroxisome proliferator in mice at blood concentrations that are readily achieved with carcinogenic doses of TRI. Recent data has demonstrated that both TCA and dichloroacetate (DCA) are capable of inducing liver tumors in B6C3F1 mice. Although long recognized as a metabolite of TRI, little attention has focussed on the role DCA might play in the hepatocarcinogenic effects of TRI. There are significant differences in the effects of DCA and TCA on the liver of B6C3F1 mice. Trichloroacetate treatment induces peroxisome proliferation, increases lipid deposition, and results in a marked accumulation of lipofuscin in the liver with long-term exposures. Dichloroacetate induces a markedly enlarged liver associated with a cytomegaly and large accumulations of glycogen. The cytomegaly is associated with the development of focal areas of recurrent liver necrosis which in turn lead to high levels of cell proliferation in the area surrounding these lesions. Induction of peroxisomes with DCA is transitory and the accumulation of lipofuscin is much less evident than with TCA treatment. Studies of TRI metabolism demonstrate that blood levels of DCA produced are sufficient to account for the hepatocarcinogenic effects of TRI. The rather low concentrations of DCA found in the urine of mice treated with TRI relative to TCA concentrations are due to the much more rapid and complete metabolism of DCA. These data do not support the conclusion that the hepatocarcinogenic effects of TRI are simply related to peroxisome proliferation.

Metabolites of the plasticizer di(2-ethylhexyl)phthalate in urine samples of workers in polyvinylchloride processing industries

Little is known about occupational exposure to the plasticizer di(2-ethylhexyl)phthalate (CAS number 117-81-7), a compound widely used in polyvinylchloride (PVC) plastics. We have studied the uptake of DEHP in workers by determining the concentrations of four metabolites of DEHP in urine samples, i.e., mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate, mono(2-ethyl-5-oxohexyl)phthalate, and mono(2-ethyl-5-hydroxyhexyl)phthalate. In addition DEHP concentrations in the air were determined by personal air sampling. Nine workers in a PVC boot factory exposed to a maximum of 1.2 mg/m3 DEHP showed an increase in the urinary concentrations of all four metabolites over the workshift. These results were obtained on both the first and the last day of the workweek. With the exception of MEHP, the increases in the concentrations of the metabolites during a workday were statistically significant. Six workers from a PVC cable factory exposed to a maximum of 1.2 mg/m3 DEHP showed a one- to fourfold increase in the concentrations of the four metabolites over the workshift, but these increases were not statistically significant. These results indicate that measurement of DEHP metabolites in urine samples may be of use for monitoring the occupational exposure to DEHP.

Determination of four metabolites of the plasticizer di(2-ethylhexyl)phthalate in human urine samples

A method for biological monitoring of exposure to the plasticizer di(2-ethylhexyl)phthalate (DEHP) is described. In this method the four main metabolites of DEHP [i.e., mono(2-ethylhexyl)phthalate (MEHP), mono(5-carboxy-2-ethylpentyl)phthalate, mono(2-ethyl-5-oxohexyl)phthalate, and mono(2-ethyl-5-hydroxyhexyl)-phthalate] are determined in urine samples. The procedure includes enzymatic hydrolysis, ether extraction, and derivatization with triethyloxonium tetrafluoroborate. Analysis is performed by gas chromatography-electron impact mass spectrometry. The detection limit for all four metabolites is less than 25 micrograms/l urine. The coefficient of variation based on duplicate determinations of urine samples of workers occupationally exposed to DEHP was 16% for MEHP (mean concentration 0.157 mg/l) and 6%-9% for the other three metabolites (mean concentrations 0.130-0.175 mg/l). The method described here was used to study DEHP metabolism in man. Most persons excrete mono(2-ethyl-5-oxohexyl)-phthalate and mono(2-ethyl-5-hydroxyhexyl)phthalate as a (glucuronide) conjugate. Mono(5-carboxy-2-ethylpentyl)phthalate is mainly excreted in free form, while for MEHP a large interindividual variation in conjugation status was observed. Of the four metabolites quantified, 52% are products of a (omega-1)-hydroxylation reaction of MEHP [i.e., mono(2-ethyl-5-oxohexyl)phthalate and mono(2-ethyl-5-hydroxylation reaction of MEHP [i.e., mono(5-carboxy-2-ethylpentyl)phthalate], and 26% is not oxidized further (i.e., MEHP). A good correlation is obtained when the amount of MEHP omega-hydroxylation products is compared with the amount of MEHP (omega-1)-hydroxylation products in urine samples. When the internal dose of DEHP has to be established we recommend that the levels of all four metabolites of DEHP be studied in urine samples.

An in vitro model of rodent nongenotoxic hepatocarcinogenesis

An in vitro model of liver in which rat hepatocytes are maintained as cocultures with nonparenchymal epithelial cells (NPC) derived from liver has been developed and characterized with respect to maintenance of hepatocyte viability and differentiated function. The system was then evaluated as a model for studying peroxisome proliferator-induced rodent liver nongenotoxic carcinogenesis. Within the coculture model, hepatocyte viability and morphology were maintained for 1 month or more within a system that is both easily accessible for microscopic examination and is free of any additives that may lead to artifacts. Even after 1 month or more, hepatocyte cocultures retained expression of the constitutive liver marker albumin. In addition, they maintained the ability to show induction of the peroxisome proliferator-inducible enzymes peroxisomal bifunctional enzyme (PBE) and cytochrome P450IVA1 in response to the peroxisome proliferator nafenopin. After 4 weeks, NPC cocultures showed a six- and a fourfold induction of PBE and cytochrome P450IVA1 expression, respectively, which compared well with the three- and fivefold induction seen in freshly isolated cells. This was paralleled by an increase in the cytoplasmic volume fraction of peroxisomes averaging eightfold. Interestingly, great heterogeneity was exhibited between adjacent hepatocytes in terms of the degree of peroxisome proliferation, a finding reflected by immunocytochemical staining which indicated heterogeneity in the level of expression of the peroxisome proliferator-inducible enzymes. Other cell lines representing different tissue types, morphologies, and species were also examined for their ability to support hepatocyte survival but were found to be ineffective, with the exception of a bovine corneal endothelial cell line. This line supported hepatocyte survival and maintenance of differentiated function but to a lesser extent than that observed with NPC. Ultrastructural examination of NPC cocultures revealed extensive interhepatocyte junctional complexes and interdigitation of adjacent membranes together with the presence of bile canalicular structures. There were no junctional complexes between the hepatocytes and the supporting feeder cells with any contact being limited to a close association of the hepatocytes with the extracellular matrix presumably produced by the NPC. The data demonstrate that hepatocytes maintained in vitro within an NPC coculture system retain differentiated function and the ability to respond to the peroxisome proliferator class of nongenotoxic carcinogens. Cocultures will provide us with a model system for the study of changes in hepatocyte growth regulation during rodent liver nongenotoxic carcinogenesis.

Enantioselective induction of peroxisomal proliferation in CD-1 mice by leukotriene antagonists

The effects of a racemic leukotriene antagonist (MK-0571) and its component enantiomers (L-668,018 and L-668,019) on hepatic peroxisome proliferation were examined in mice, rats, and rhesus monkeys. Administration of racemic MK-0571 to mice resulted in increased liver weights, increased peroxisomal volume density, and a pleiotropic induction of characteristic peroxisomal and nonperoxisomal enzyme activities associated with peroxisomal proliferation. When the individual enantiomers of MK-0571 were administered to mice, a pronounced enantioselective induction of peroxisome proliferation was observed. Toxicokinetic studies showed that the levels of each enantiomer in the liver or plasma after separate administration were similar. Thus, the enantioselectivity in the induction of peroxisome proliferation could not be explained on the basis of pharmacokinetic differences between the enantiomers. The hepatic peroxisomal response of the rat to MK-0571 was greatly attenuated compared to the mouse. As has been seen with other peroxisome-proliferating agents, MK-0571 had no effect on either peroxisomal volume density or peroxisomal enzyme activity in monkeys. Due to the high degree of enantiomeric discrimination toward the induction of peroxisomal proliferation by these enantiomers, compounds of this type may prove useful as probes to examine the mechanisms by which peroxisomal proliferating agents induce their effects.

Microsomal lauric acid hydroxylase activities after treatment of rats with three classical cytochrome P450 inducers and peroxisome proliferating compounds

In order to investigate a proposed relationship between induction of hepatic microsomal lauric acid hydroxylase activity and peroxisome proliferation in the liver, male Wistar rats were treated with peroxisome proliferating compounds, and the lauric acid hydroxylase activity, the immunochemical detectable levels of cytochrome P450 4A1 and the activities of peroxisomal enzymes were determined. In addition, the levels of cytochrome P450 4A1 and lauric acid hydroxylase activities were studied after treatment of rats with three cytochrome P450 inducers. After treatment with aroclor-1254, phenobarbital or 3-methylcholanthrene total cytochrome P450 was 1.7-2.7 times induced. However, no induction of lauric acid omega-hydroxylase activities or P450 4A1 levels were found. After treatment of rats with di(2-ethylhexyl)phthalate (DEHP) a dose-dependent induction of lauric acid omega-hydroxylase activities, levels of cytochrome P450 4A1 and peroxisomal fatty acid beta-oxidation was found. Even at a dose-level of 100 mg DEPH/kg body weight per day a significant induction of these activities was observed. The main metabolites of DEHP, mono(2-ethylhexyl)phthalate and 2-ethyl-1-hexanol, also caused an induction of levels of P450 4A1, lauric acid omega-hydroxylase activities and the activity of peroxisomal palmitoyl-CoA oxidase. 2-Ethyl-1-hexanoic acid did not influence lauric acid omega-hydroxylase activities, but did induce levels of P450 4A1 and palmitoyl-CoA oxidase activities. Three other compounds (perfluoro-octanoic acid, valproate and nafenopin) induced both lauric acid omega-hydroxylase activity and peroxisomal palmitoyl-CoA oxidase activity. The plasticizer, di(2-ethylhexyl)adipate, did not induce levels of P450 4A1, lauric acid omega-hydroxylase activities or palmitoyl-CoA oxidase activities. With the compounds tested a close association between the induction of lauric acid omega-hydroxylase activities and peroxisomal palmitoyl-CoA oxidase activity was found. These data support the theory that peroxisome proliferating compounds do induce lauric acid omega-hydroxylase activities and that there might be a mechanistic inter-relationship between peroxisome proliferation and induction of lauric acid omega-hydroxylase activities.

Covalent binding of perfluorinated fatty acids to proteins in the plasma, liver and testes of rats

Perfluorinated fatty acids alter hepatic lipid metabolism and are potent peroxisome proliferators in rodents. Two such perfluorinated acids, perfluorodecanoic acid (PFDA) and perfluorooctanoic acid (PFOA), were examined to determine if they covalently bind cellular proteins. PFDA and PFOA were found to covalently bind proteins when administered to rats in vivo. The liver, plasma and testes of male rats treated with [1-14C]PFDA or PFOA (9.4 mumol/kg) contained detectable levels of covalently bound 14C (0.1-0.5% of the tissue 14C content). Characterization of PFDA covalent binding to albumin in vitro showed that cysteine significantly decreased binding with no effect of methionine, suggesting protein sulfhydryl groups are involved. In cytosolic and microsomal incubation there was no effect of the addition of CoA, ATP or NADPH on the magnitude of the covalent binding of PFDA. Therefore PFDA need not be metabolically activated to form covalent adducts. Despite demonstration of covalent binding of PFDA and PFOA to proteins both in vivo and in vitro, the role of this macromolecular binding in perfluorinated fatty acid toxicity is not known.

Effects of the peroxisome proliferators ciprofibrate and perfluorodecanoic acid on hepatic cellular antioxidants and lipid peroxidation in rats

The purpose of this study was to determine if hepatic cellular antioxidants and indices of oxidative damage are altered by administration of the peroxisome proliferators ciprofibrate and perfluorodecanoic acid (PFDA). Rats were fed 0.01% ciprofibrate in the diet or were injected with PFDA (0.5 or 5.0 mg/kg, i.p.) every 4 weeks for 6, 14, 30, 54, and 78 weeks. Peroxisomal fatty acyl-CoA oxidase and catalase activities were increased by both ciprofibrate and PFDA throughout the study. Neither ciprofibrate nor PFDA increased the levels of malonaldehyde or conjugated dienes, but ciprofibrate decreased these indices at early time points. Ciprofibrate decreased the following cellular antioxidants or antioxidant enzymes: vitamin C, vitamin D, DT-diaphorase, glutathione peroxidase, glutathione-S-transferase, and glutathione reductase; superoxide dismutase and glutathione were not affected. PFDA decreased DT-diaphorase and increased superoxide dismutase, but did not affect other cellular antioxidants. This study shows that administration of the peroxisome proliferators ciprofibrate and PFDA did not increase indices of lipid peroxidation, but that cellular antioxidant defenses were inhibited for a prolonged period of time by the peroxisome proliferator ciprofibrate.

Hepatocellular peroxisomes in human alcoholic and drug-induced hepatitis: a quantitative study

The peroxisomes in the liver of four patients with alcoholic hepatitis and in six patients with drug-induced hepatitis are compared to eight control livers by catalase cytochemistry and morphometry. A decrease of catalase activity is observed in alcoholic, amitriptyline, aprindine, clomipramine and methiomazole hepatitis. Peroxisomes with a heterogeneous distribution of the catalase reaction product are found in most hepatitis livers. The number of organelles is increased 1.5 to 4.2 times in alcoholic, aprindine, methimazole and phenytoin hepatitis livers. In the last case, peroxisomes are also smaller. Changes in shape are seen in all hepatitis livers; they include invaginations, tails, funnel-like constrictions and gastruloid cisternae. In aprindine, phenytoin, methimazole and two alcoholic hepatitis livers, surface density exceeds the upper control value. These data indicate a loss of catalase activity in most hepatitis livers but also peroxisomal proliferation and shape modifications. It has been proposed that the latter changes are favorable for metabolic activity.

Non-mutagenicity of 4 metabolites of di(2-ethylhexyl)phthalate (DEHP) and 3 structurally related derivatives of di(2-ethylhexyl)adipate (DEHA) in the Salmonella mutagenicity assay

Four metabolites of the rat liver carcinogen di(2-ethylhexyl)phthalate (DEHP) (mono-(2-ethylhexyl)phthalate, mono-(2-ethyl-5-hydroxyhexyl)phthalate, mono-(2-ethyl-5-oxohexyl)phthalate, and mono-(5-carboxy-2-ethylpentyl)phthalate) and 3 structurally related derivatives of di(2-ethylhexyl)adipate (DEHA) (mono-(2-ethylhexyl)adipate, mono-(2-ethyl-5-hydroxyhexyl)adipate, and mono-(2-ethyl-5-oxohexyl)adipate) were tested for mutagenicity in the Ames assay using Salmonella typhimurium strains TA97, TA98, TA100, and TA102, with and without a metabolic activation preparation. Aroclor 1254-induced rat liver S9 and DEHP-induced rat liver S9 were used. Concentrations of these compounds up to 1000 micrograms/plate were negative with all tester strains in the presence or absence of metabolic activation.

Examination of potential mechanisms of carcinogenicity of 1,4-dioxane in rat nasal epithelial cells and hepatocytes

Several long-term studies with 1,4-dioxane (dioxane) have shown it to induce liver tumors in mice, and nasal and liver tumors in rats when administered in amounts from 0.5 to 1.8% in the drinking water (Argus et al. 1965; Kociba et al. 1974; National Cancer Institute, 1978). In order to examine potential mechanisms of action, chemically-induced DNA repair (as an indicator of DNA reactivity) and cell proliferation (as an indicator of promotional activity) were examined in nasal turbinate epithelial cells and hepatocytes of male Fischer-344 rats treated with dioxane. Neither dioxane nor 1,4-dioxane-2-one, one of the proposed metabolites, exhibited activity in the in vitro primary rat hepatocyte DNA repair assay, even from cells that had been isolated from animals given either 1 or 2% dioxane in the drinking water for 1 week to induce enzymes that might be responsible for producing genotoxic metabolites. No activity was seen in the in vivo hepatocyte DNA repair assay in animals given a single dose of up to 1000 mg/kg dioxane or up to 2% dioxane in the drinking water for 1 week. Treatment of rats with 1.0% dioxane in the drinking water for 5 days yielded no increase in liver/body weight nor induction of palmitoyl CoA oxidase, indicating that dioxane does not fit into the class of peroxisomal proliferating carcinogens. The percentage of cells in DNA synthesis phase (S-phase) was determined by administration of 3H-thymidine and subsequent quantitative histoautoradiography. The hepatic labeling index (LI) did not increase at either 24 or 48 h following a single dose of 1000 mg/kg dioxane.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxisomal oxidases and catalase in liver and kidney homogenates of normal and di(ethylhexyl)phthalate-fed rats

1. Activities of peroxisomal oxidases and catalase were assayed at neutral and alkaline pH in liver and kidney homogenates from male rats fed a diet with or without 2% di(2-ethylhexyl)phthalate (DEHP) for 12 days. 2. All enzyme activities were higher at alkaline than at neutral pH in both groups. 3. The effect of the DEHP-diet on the peroxisomal enzymes was different in kidney and liver. Acyl-CoA oxidase activity was raised three- and sixfold in kidney and liver homogenates, respectively. The activity of D-amino acid oxidase decrease in liver, but increased in kidney homogenates. In liver homogenates, urate oxidase activity was not affected by the DEHP diet. The catalase activity was twofold induced in liver, but not in kidney. 4. The differences suggest that the changes of peroxisomal enzyme activities by DEHP treatment are not directly related to peroxisome proliferation. 5. DEHP treatment caused a marked increase of total and peroxisomal fatty acid oxidation in rat liver homogenates. 6. In the control group the rate of peroxisomal fatty acid oxidation was higher at alkaline pH than at neutral pH. 7. This rate was equal at both pH values in the DEHP-fed group, in contrast to the acyl-CoA oxidase activity. These results indicate that after DEHP treatment other parameters than acyl-CoA oxidase activity become limiting for peroxisomal beta-oxidation.

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.

Toxicological studies on a benzofurane derivative. I. A comparative study with phenobarbital on rat liver

The benzofurane derivative benzbromarone (BBR) previously has led to liver tumor formation after long-term treatment of rats, but no indications of genotoxicity were detected. The present studies were designed to elucidate the mechanism(s) possibly involved in liver tumor formation by BBR. Female Wistar rats were used. Phenobarbital (PB) served as a positive control. (1) Short-term treatment (7 days) with daily doses of 2 to 100 mg/kg BBR led to adaptive responses in the liver, i.e., growth (increases in DNA, RNA, and protein) and induction of monooxygenases. These changes were also observed after feeding BBR for 8, 33, 77, and 102 weeks at doses of 2, 10, and 50 mg/kg/day but tended to weaken with time. Similar effects were obtained with PB fed at 2, 10, or 50 mg/kg/day. However, unlike PB, BBR did not enhance the expression of cytochrome P450-PB as demonstrated by immunostaining of histological liver sections. (2) BBR feeding for 102 weeks, but not for 77 weeks, produced some neoplastic liver nodules and at 50 mg/kg produced one hepatocellular carcinoma (HCC). Thus, BBR was tumorigenic in the present study, but was clearly weaker than PB which had induced liver nodules and HCCs at 77 weeks and even more markedly at 102 weeks. (3) To check for tumor-initiating activity 100 mg/kg BBR was given 14 hr after a two-thirds hepatectomy followed by promotion with PB (50 mg/kg) for 15 weeks. No phenotypically altered liver foci were detected. (4) To test for tumor-promoting activity rats received a single dose of N-nitrosomorpholine (250 mg/kg), and subsequently BBR or PB at doses of 2, 10, and 50 mg/kg/day. While PB markedly enhanced the development of neoplastic nodules and HCCs, BBR had only a weak enhancing effect on the induction of HCC, which was not dose related. gamma-glutamyl transpeptidase-positive foci dramatically increased in PB-treated animals, in contrast they showed no response after 2 and 10 mg/kg BBR and even decreased after 50 mg/kg BBR. (5) With PB changes in liver growth, monooxygenase activity, foci expansion, and tumor promotion all correlating with tumorigenesis in a quantitative manner, apparent no-observed-effect-levels are somewhat below 2 mg/kg (or 10 mg/kg for liver enlargement). (6) These studies suggest that BBR belongs to a group of nongenotoxic, growth-stimulating drugs with tumorigenic potential in rat liver. Its effects on the liver are different from those of PB, but seemed to resemble those of peroxisome proliferators, a hypothesis studied in the subsequent papers.

Consideration of both genotoxic and nongenotoxic mechanisms in predicting carcinogenic potential

Bacterial and cell culture genotoxicity assays have proven to be valuable in the identification of DNA reactive carcinogens because mutational events that alter the activity or expression of growth control genes are a key step in carcinogenesis. The addition of metabolizing enzymes to these assays have expanded the ability to identify agents that require metabolic activation. However, chemical carcinogenesis is a complex process dependent on toxicokinetics and involving at least steps of initiation, promotion and progression. Identification of those carcinogens that are activated in a manner unique to the whole animal, such as 2,6-dinitrotoluene, require in vivo genotoxicity assays. There are many different classes of non-DNA reactive carcinogens ranging from the potent promoter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) that acts through a specific receptor, to compounds that alter growth control, such as phenobarbital. Many compounds, such as saccharin, appear to exhibit initiating, promotional and/or carcinogenic activity as events secondary to induced cytotoxicity and cell proliferation seen only at the chronic lifetime maximum tolerated doses mandated in rodent bioassays. Simple plus/minus vs. carcinogen/noncarcinogen comparisons used to validate the predictivity of bacterial and cell culture genotoxicity assays have revealed that a more comprehensive analysis will be required to account for the carcinogenicity of so many diverse chemical agents. Predictive assays and risk assessments for the numerous types of nongenotoxic carcinogens will require understanding of their mechanism of action, reasons for target organ and species specificity, and the quantitative dose-response relationships between endpoints such as induced cell proliferation and carcinogenic potential.

Nongenotoxic carcinogens in the regulatory environment

The biological activity of many carcinogens is to directly induce mutational events, thereby altering the information encoded in the DNA. Short-term tests for potential carcinogens and risk assessment models generally rely on the assumption that the agent in question will operate through a genotoxic mechanism. However, carcinogenesis is a multistep process, and it is increasingly clear that the primary biological effect for many carcinogenic chemicals involves events other than direct DNA reactivity. For many experimental rodent models as well as human cancers, nongenotoxic mechanisms appear to be the driving force in the formation of tumors. Many of these nongenotoxic mechanisms are highly species-specific. Thus, it is increasingly important to ask if the rodent model applies to the human situation at all, in addition to the examination of appropriate, hypothetical, mathematical risk assessment models. More research is now being focused to better define the mechanisms by which the many distinctly different classes of nongenotoxic carcinogens are acting. This understanding will become the basis for new predictive assays and more realistic risk assessment models. If specific conditions are met, then a no observed effect level with a safety factor may be the most appropriate risk model for some carcinogens.