Purpose and guidelines for toxicokinetic studies within the National Toxicology Program

Toxicokinetic studies undertaken within the National Toxicology Program are intended to aid the design of toxicology and carcinogenicity studies, help interpret the results of toxicology and carcinogenicity studies with respect to the relationship between toxic effects and external exposure, and define the parameters of dose, distribution, metabolism, and elimination that can be used in human risk assessment. Descriptions of two study designs presented here represent the possible extremes in approaches to toxicokinetic studies. The comprehensive approach is geared toward the development of physiology based pharmacokinetic models that relate external exposure to target organ dosimetry and addresses the questions: Is the chemical absorbed? How is the chemical metabolized? Where are the chemical and/or metabolites distributed in the body? What are the elimination rate and route of the chemical? What is the effect of dose on absorption, distribution, metabolism, and elimination? The minimal study design is more limited in scope than the comprehensive design and addresses primarily the issues of absorption, distribution, and elimination of the parent chemical. Study protocols for most chemicals lie somewhere between these two extreme approaches. An increased understanding of the relationships between external exposure, target organ dosimetry, and adverse effects should provide greater confidence in making low-dose extrapolations of human risk. This paper focuses on the collection of data from animal toxicokinetic studies. The construction of comparable models to characterize target organ dosimetry in exposed humans would certainly require the use of human parameter values obtained from human tissue samples and volunteers.

Both K-ras and H-ras protooncogene mutations are associated with Harderian gland tumorigenesis in B6C3F1 mice exposed to isoprene for 26 weeks

Isoprene is the 2-methyl analog of 1,3-butadiene, a genotoxic and carcinogenic compound in rats and mice. Male B6C3F1 mice were exposed to 0, 2200 or 7000 ppm isoprene by inhalation (6 h/day; 5 days/week) for 26 weeks. Following a 26-week recovery period, an increased incidence of Harderian gland (HG) neoplasms was observed at both concentrations. The present study was designed to characterize genetic alterations in the K-ras and H-ras protooncogenes in HG neoplasms. Mutations in K-ras and H-ras were identified by single-strand conformational analysis and direct sequencing of polymerase chain reaction (PCR) amplified DNA, isolated from paraffin-embedded sections of HG neoplasms. A higher frequency of ras mutations, in particular K-ras mutations, was detected in isoprene-induced neoplasms than in 1,3-butadiene-induced or control HG neoplasms. All of the isoprene-induced HG neoplasms exhibited activated K-ras (60%) or H-ras (40%) mutations. In contrast, ras mutations were detected in 69% of HG neoplasms from 1,3-butadiene exposed mice (14% K-ras and 55% H-ras) and in 56% of HG neoplasms obtained from control B6C3F1 mice (8% K-ras and 48% H-ras). The predominant mutations in isoprene-induced HG neoplasms, but not in previously or newly analysed 1,3-butadiene-induced HG neoplasms, consisted of A-->T transversions (CAA-->CTA) at K-ras codon 61 (15/30) and C-->A transversions (CAA-->AAA) at H-ras codon 61 (8/30). Two-thirds of the K-ras CTA mutations were detected in HG neoplasms from the 2200 ppm exposure group while one-third was present in the 7000 ppm group. Isoprene-induced HG neoplasms with K-ras or H-ras mutations had an elevated proliferating cell nuclear antigen (PCNA) index, compared to spontaneous HG neoplasms without ras mutations. The high frequency and specificity of the ras mutation profile suggest that ras protooncogene activation contributes to isoprene-induced HG tumorigenesis.

Inhalation toxicity and carcinogenicity of isoprene in rats and mice: comparisons with 1,3-butadiene

As with 1,3-butadiene (BD), inhalation exposure of B6C3F1 mice to isoprene (2-methyl-1,3-butadiene) caused a macrocytic anemia; induced increases in sister chromatid exchanges in bone marrow cells and in levels of micronucleated erythrocytes in peripheral blood; and produced degeneration of the olfactory epithelium, forestomach epithelial hyperplasia, and testicular atrophy. Most notable was the finding that like BD, isoprene induced neoplasms in the liver, lung, Harderian gland, and forestomach of mice. The carcinogenic effects of isoprene were observed after a 26-week exposure (6 h/day, 5 days/week) of male mice to 700 ppm or higher concentrations of isoprene followed by a 26-week recovery period. Unlike BD, isoprene did not induce lymphomas or hemangiosarcomas of the heart in mice under these conditions nor did it induce chromosomal aberrations in mouse bone marrow cells. No toxicological effects were evident in rats exposed for 13 weeks to either isoprene or BD at concentrations up to 7000 ppm or 8000 ppm, respectively. Interstitial cell hyperplasia of the testis was observed in male F344 rats exposed to 7000 ppm isoprene for 26 weeks, and following a 26-week recovery period, there was a marginal increase in benign testicular interstitial cell tumors.

Effects of the structure of a toxicokinetic model of butadiene inhalation exposure on computed production of carcinogenic intermediates

A flow-limited physiologically based toxicokinetic model was constructed for uptake, metabolism, and clearance of butadiene (BD) and its principal metabolite 1,2-epoxy-3-butene (EB), using physiological and biochemical parameters from the literature where available. The model includes compartments for blood, liver, lung, fat, GI tract, other rapidly perfused tissues, and slowly perfused tissues. The blood was distributed among compartments for arterial plus venous blood and subcompartments for vascular spaces associated with each of the tissue compartments. The lung contained a subcompartment for the alveolar space. Metabolic activation of BD by cytochrome P450-catalyzed epoxidation was modeled as occurring in liver, lung, and the rapidly perfused tissue compartments. The detoxication of EB catalyzed by epoxide hydrolase and glutathione S-transferase (GST) was modeled as occurring in liver, lung, and the rapidly perfused tissues compartments and by blood GST activity. The model also includes depletion of glutathione (GSH) by GST-catalyzed conjugation of EB and 3-butene-1,2-diol and resynthesis of GSH from cysteine. Values of biochemical parameters that were unavailable in the literature were estimated by iteratively reweighted least squares optimization to reproduce data for uptake of BD and EB by rats and mice in closed chambers. The resulting model also reproduced the depletion of GSH in liver and lung in flow-through systems. It reproduced the concentrations of expired EB produced from BD in closed chambers but overpredicted separately measured blood EB concentrations in flow-through systems, indicating an inconsistency between these two experiments that cannot be resolved by this model or an inadequacy in the model. Equilibration of chamber gases with the alveolar space and alveolar gas with lung capillary blood results in much less dilution of the inhaled gas in the blood compared with the predictions of models in which chamber gas equilibrates directly with the total circulation. The production of EB predicted by the present model was found to be sensitive to a number of physiological and biochemical parameters. A valid and useful toxicokinetic model must have reliable physiological and enzymological data for BD biotransformation before it can be credibly used for human risk assessment.

Toxicity of inhaled chloroprene (2-chloro-1,3-butadiene) in F344 rats and B6C3F(1) mice

Chloroprene (2-chloro-1,3-butadiene) is a high production chemical used almost exclusively in the production of polychloroprene (neoprene) elastomer. Because of its structural similarity to isoprene (2-methyl-1,3-butadiene) and to 1,3-butadiene, a potent trans-species carcinogen, inhalation studies were performed on chloroprene to characterize its toxicological potential and to provide a basis for selecting exposure concentrations for chronic toxicity and carcinogenicity studies. Thirteen-week inhalation toxicology studies were conducted in male and female F344 rats and B6C3F(1) mice at exposure concentrations of 0, 5, 12, 32 or 80 ppm (6 h/day; 5 days/week). A 200 ppm exposure group was also included for rats only, because a previous study showed that this concentration of chloroprene is lethal to mice. In mice, exposure to 80 ppm chloroprene caused a marginal decrease in body weight gain in males and epithelial hyperplasia of the forestomach in males and females. This lesion has been observed in mice exposed to isoprene or 1,3-butadiene. In rats, exposure to 80 ppm chloroprene or higher concentrations caused degeneration and metaplasia of the olfactory epithelium and exposure to 200 ppm caused anemia, hepatocellular necrosis and reduced sperm motility. These lesions have not been observed in rats exposed to isoprene or 1,3-butadiene. The profile of toxic effects of chloroprene is considerably different from that of isoprene or 1,3-butadiene; this may be due to differences in exposure concentrations that were used in toxicology studies of these compounds and /or to the influence of the chlorine substitution on the toxicokinetics of these compounds, on their biotransformation, or on the reactivity of metabolic intermediates with tissue macromolecules.

Implications for risk assessment of suggested nongenotoxic mechanisms of chemical carcinogenesis

Nongenotoxic carcinogens are chemicals that induce neoplasia without it or its metabolites reacting directly with DNA. Chemicals classified as nongenotoxic carcinogens have been assumed to act as tumor promoters and exhibit threshold tumor dose-responses. This is in contrast to genotoxic carcinogens that are DNA reactive, act as tumor initiators, and are assumed to exhibit proportional responses at low doses. In this perspective, we examine the basic tenets and utility of this classification for evaluating human cancer risk. Two classes of so-called nongenotoxic chemical carcinogens selected for review include cytotoxic agents that induce regenerative hyperplasia (trihalomethanes and inducers of alpha 2-microglobulin nephropathy) and agents that act via receptor-mediated mechanisms (peroxisome proliferators and dioxin). Major conclusions of this review include: a) many chemicals considered to be nongenotoxic carcinogens actually possess certain genotoxic activities, and limiting evaluations of carcinogenicity to their nongenotoxic effects can be misleading; b) some nongenotoxic activities may cause oxidative DNA damage and thereby initiate carcinogenesis; c) although cell replication is involved in tumor development, cytotoxicity and mitogenesis do not reliably predict carcinogenesis; d) a threshold tumor response is not an inevitable result of a receptor-mediated mechanism. There are insufficient data on the chemicals reviewed here to justify treating their carcinogenic effects in animals as irrelevant for evaluating human risk. Research findings that characterize the multiple mechanisms of chemical carcinogenesis should be used quantitatively to clarify human dose-response relationships, leading to improved scientifically based public health decisions. Excessive reliance on oversimplified classification schemes that do not consider all potential contributing effects of a toxicant can obscure the actual causal relationships between exposure and cancer outcome.

Mechanistic data indicate that 1,3-butadiene is a human carcinogen

A review of the epidemiological and mechanistic data on 1,3-butadiene indicates that this chemical is a human carcinogen for which the mouse is an appropriate model for assessing human cancer risk. Butadiene is carcinogenic at multiple organ sites in laboratory animals, including the induction of lymphomas in mice, while epidemiological studies have consistently found associations between occupational exposure to butadiene and increased mortality from lymphatic and hematopoietic cancers. Activated oncogenes and inactivated tumor suppressor genes in butadiene-induced tumors in mice are analogous to genetic alterations frequently observed in human cancers. Butadiene is metabolized to mutagenic and carcinogenic epoxides in all mammalian species studied, including humans. These metabolites form N7-alkylguanine adducts which have been detected in liver DNA of mice exposed to butadiene and in urine of exposed workers. Increases in hprt mutations were observed in lymphocytes from mice exposed to butadiene and in occupationally exposed humans. The mutational spectra for butadiene and its epoxide metabolites at the hprt locus in mouse lymphocytes are similar to the mutational spectrum of ethylene oxide; all of these chemicals exhibit a high percentage of frameshift mutations. Ethylene oxide, an alkylating agent that also forms an N7-alkylguanine adduct, was recently classified by the International Agency for Research on Cancer as a human carcinogen. Based on these data, we suggest that cancer induction by ethylene oxide and butadiene involve similar molecular mechanisms.

Isoprene, an endogenous hydrocarbon and industrial chemical, induces multiple organ neoplasia in rodents after 26 weeks of inhalation exposure

Isoprene, the 2-methyl analogue of 1,3-butadiene, is a high production chemical used largely in the manufacture of synthetic rubber and is the major endogenous hydrocarbon exhaled in human breath. Thirteen-week inhalation toxicology studies of isoprene were conducted in male and female F344 rats and B6C3F1 mice at exposure concentrations of 0, 70, 220, 700, 2200, and 7000 ppm (6 h/day; 5 days/week). In addition, 26-week inhalation studies at the same exposure levels, followed by a 26-week recovery period, were conducted in male rats and mice. The 13-week exposures produced no discernible exposure-related toxic effects in rats. Interstitial cell hyperplasia of the testis was observed in all male rats in the 7000 ppm group after 26 weeks of exposure; following the 26-week recovery period the only effect in rats was a marginal increase in benign testicular interstitial cell tumors. In mice, isoprene induced toxic and carcinogenic effects at multiple organ sites. Following the 26-week exposure and 26-week recovery periods, incidences of neoplastic lesions in the liver, lung, forestomach, and harderian gland were significantly increased. Neoplastic effects were observed at 700 ppm and higher exposures. Non-neoplastic lesions in mice exposed to isoprene included spinal cord degeneration, testicular atrophy, degeneration of the olfactory epithelium, and epithelial hyperplasia of the forestomach. A partial hindlimb paralysis and a nonresponsive macrocytic anemia were also seen in mice. Most of the toxic and carcinogenic effects caused by isoprene, as well as the species' difference in response, had been observed after inhalation exposures to 1,3-butadiene.

Trihalomethanes and Other Environmental Factors That Contribute to Colorectal Cancer

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In vitro evidence for involvement of CoA thioesters in peroxisome proliferation and hypolipidaemia

The mechanisms of peroxisomal induction and hypolipidaemia caused by treatment with peroxisome proliferators, such as nafenopin and clofibrate, remain to be elucidated. Proposed mechanisms include receptor-mediated processes or adaptations resulting from disruption of hepatic lipid metabolism. The latter mechanism was investigated in a series of in vitro studies. Incubation of primary rat hepatocytes with various carboxyl-containing compounds revealed no clear common factor which imparted potency as a peroxisomal inducer. Inhibitors of fatty acyl-CoA synthetase, norepinephrine and desulpho-CoA, however, decreased the level of peroxisomal induction by nafenopin in rat hepatocytes, suggesting that activation of carboxyl-containing compounds to their CoA thioesters may be a necessary step in initiating peroxisome proliferation. Coenzyme A thioesters of nafenopin, clofibric acid and other carboxyl-containing chemicals were synthesised and found to inhibit the activity of acetyl-CoA carboxylase to varying degrees. The CoA thioester of nafenopin was the most potent inhibitor among this group (Ki = 1.45 x 10(-5) M), but weaker than palmitoyl-CoA (Ki = 2.22 x 10(-6) M), the feedback inhibitor of acetyl-CoA carboxylase. Hypolipidaemia caused by treatment with peroxisome proliferators may, therefore, be related to inhibition of fatty-acid synthesis by the corresponding CoA thioester derivative.

Toxicity of diethanolamine. 1. Drinking water and topical application exposures in F344 rats

Toxicology studies of diethanolamine were conducted in male and female F344 rats for 13 weeks' duration to characterize and compare effects of exposure in the drinking water with those caused by topical application. Doses of diethanolamine ranged from 160 to 5000 ppm in the drinking water study (equivalent to daily doses of 25-440 mg kg-1 in males and 15-240 mg kg-1 in females) and from 32 to 500 mg kg-1 in the topical application study. Dose-dependent toxic effects due to exposure to diethanolamine included hematological changes (a poorly regenerative, microcytic anemia), as well as toxic responses in the kidney (increased weight, tubular necrosis, decreased renal function, and/or tubular mineralization), brain and spinal cord (demyelination), testis (degeneration of the seminiferous tubules) and skin (site of application: ulceration, inflammation, hyperkeratosis and acanthosis). A no-observed-adverse-effect level was not achieved for hematological changes, nephropathy or hyperkeratosis of the skin. Differences in dose-response between the drinking water and topical application exposures were attributed largely to the limited dermal absorption of this chemical.

Toxicity of diethanolamine. 2. Drinking water and topical application exposures in B6C3F1 mice

Toxicology studies of diethanolamine were conducted in male and female B6C3F1 mice to characterize and compare effects of exposure in the drinking water with those caused by topical application and to compare responses in mice to those observed in rats. Each study consisted of five dose groups plus controls and the size of each group was 10 animals per sex. Doses of diethanolamine ranged from 630 to 10,000 ppm in the drinking water study (approximately equivalent to daily doses of 100-1700 mg kg-1 in males and 140-1100 mg kg-1 in females) and from 80 to 1250 mg kg-1 in the topical application study. Exposure to diethanolamine caused dose-dependent toxic effects in the liver (hepatocellular cytological alterations and necrosis), kidney (nephropathy and tubular epithelial necrosis in males), heart (cardiac myocyte degeneration) and skin (site of application: ulceration, inflammation, hyperkeratosis, and acanthosis). Cytological alterations in the liver consisted of multiple hepatocyte changes, including enlarged cells that were frequently multinucleated, increased nuclear pleomorphism, increased eosinophilia and disruption of hepatic cords. A no-observed-adverse-effect level (NOAEL) was not achieved for hepatocellular cytological alterations or for acanthosis in the skin.

Cell proliferation and chemical carcinogenesis: symposium overview

Cancer, by definition, is a proliferative disease. The fundamental scientific issue explored at the international symposium "Cell Proliferation and Chemical Carcinogenesis" was the impact of chemically enhanced cell proliferation on the dynamic carcinogenic processes. This conference, held at the National Institute of Environmental Health Sciences January 14-16, 1992, provided an open forum for the exchange of new results, information, and ideas in four areas: a) general principles of cell division and carcinogenesis, b) critical evaluation of cell proliferation methodologies, c) cell proliferation and modeling of organ-specific carcinogenesis, and d) cell proliferation and human carcinogenesis. This overview summarizes key findings from that symposium. The general view expressed was that although cell proliferation is involved inextricably in the development of cancers, chemically enhanced cell division does not reliably predict carcinogenicity. Our knowledge of the multistep nature of carcinogenesis has advanced substantially during recent years; however, much still needs to be learned. A greater understanding of the cellular and molecular events in chemical carcinogenesis should improve all aspects of the overall risk assessment process, including extrapolations based on dose, species, and interindividual differences.

Assessment of the carcinogenic potential of chlorinated water: experimental studies of chlorine, chloramine, and trihalomethanes

BACKGROUND

Water chlorination has been one of the major disease prevention treatments of this century. While epidemiologic studies suggest an association between cancer in humans and consumption of chlorination byproducts in drinking water, these studies have not been adequate to draw definite conclusions about the carcinogenic potential of the individual byproducts.

PURPOSE

The purpose of this study was to investigate the carcinogenic potential of chlorinated or chloraminated drinking water and of four organic trihalomethane byproducts of chlorination (chloroform, bromodichloromethane, chlorodibromomethane, and bromoform) in rats and mice.

METHODS

Bromodichloromethane, chlorodibromomethane, bromoform, chlorine, or chloramine was administered to both sexes of F344/N rats and (C57BL/6 x C3H)F1 mice (hereafter called B6C3F1 mice). Chloroform was given to both sexes of Osborne-Mendel rats and B6C3F1 mice. Chlorine or chloramine was administered daily in the drinking water for 2 years at doses ranging from 0.05 to 0.3 mmol/kg per day. The trihalomethanes were administered by gavage in corn oil at doses ranging from 0.15 to 4.0 mmol/kg per day for 2 years, with the exception of chloroform, which was given for 78 weeks.

RESULTS

The trihalomethanes were carcinogenic in the liver, kidney, and/or intestine of rodents. There was equivocal evidence for carcinogenicity in female rats that received chlorinated or chloraminated drinking water; this evidence was based on a marginal increase in the incidence of mononuclear cell leukemia. Rodents were generally exposed to lower doses of chlorine and chloramine than to the trihalomethanes, but the doses in these studies were the maximum that the animals would consume in the drinking water. The highest doses used in the chlorine and chloramine studies were equivalent to a daily gavage dose of bromodichloromethane that induced neoplasms of the large intestine in rats. In contrast to the results with the trihalomethanes, administration of chlorine or chloramine did not cause a clear carcinogenic response in rats or mice after long-term exposure.

CONCLUSION

These results suggest that organic byproducts of chlorination are the chemicals of greatest concern in assessment of the carcinogenic potential of chlorinated drinking water.

Liver carcinogenesis is not a predicted outcome of chemically induced hepatocyte proliferation

Cell proliferation has long been recognized as a basic component of multistage carcinogenesis. Based largely on the finding that certain nongenotoxic chemical carcinogens induce cell proliferation in the same organ that develops tumors after long-term exposure, some suggest that the increased rates of cell division account for the carcinogenicity of these chemicals. This paper examines relationships between chemically induced liver toxicity, cell proliferation, and liver carcinogenesis; major factors include consistency, transient vs. sustained dose-response correspondence, and scientific plausibility. For a presumed mechanism to be valid, a sustained proliferative response is critical, largely because transient increases in hepatocyte proliferation are not sufficient to induce cancer or promote liver tumor development. A consistent association between liver toxicity and carcinogenicity has not been established. Our evaluation of studies on purported relationships between chemically induced cell proliferation and liver carcinogenesis shows: 1) that inconsistencies in sex and species specificity exist, 2) that a large percentage of proliferative responses are transient, 3) that inconsistencies in response to various hepatic peroxisome proliferators are common, and 4) that dose-response and duration relationships have not been sufficiently examined. Studies of proliferative responses of putative preneoplastic cells in the liver indicate that these cells divide faster than normal hepatocytes and also have higher death rates. Chemicals that induce cell proliferation in preneoplastic foci do not always provide a persistent increase in replication rates, even with continuous exposure. A selective growth advantage to preneoplastic cells in the liver may be provided either by an enhancement of the replication rates of these cells compared to the surrounding normal hepatocytes, by inhibition of cell loss, or by inhibition of the growth rate of normal cells. More work is needed to understand how chemical carcinogens and noncarcinogens affect cell division and cell loss of normal hepatocytes and of preneoplastic cells; measurements of hepatocyte proliferation alone are not sufficient to elucidate mechanisms of liver tumor development or to predict liver carcinogenesis. Because of our limited knowledge of the complex molecular changes occurring during liver cancer, it would be inappropriate and far too premature to amend scientific risk assessment procedures for nongenotoxic chemical carcinogens based on oversimplified or incompletely tested speculations.

Carcinogenicity of 1,3-butadiene

1,3-Butadiene, a high-production volume chemical used largely in the manufacture of synthetic rubber, is a multiple organ carcinogen in rats and mice. In inhalation studies conducted in mice by the National Toxicology Program, high rates of early lethal lymphomas occurring at exposure levels of 625 ppm or higher reduced the development and expression of later developing tumors at other sites. Use of survival-adjusted tumor rates to account for competing risk factors provided a clearer indication of the dose responses for 1,3-butadiene-induced neoplasms. An increase in lung tumors in female mice was observed at exposure concentrations as low as 6.25 ppm, the lowest concentration ever used in a long-term carcinogenicity study of this gas. Human exposures to 1,3-butadiene by workers employed at facilities that produce this chemical and at facilities that produce styrene-butadiene rubber have been measured at levels higher than those that cause cancer in animals. Furthermore, epidemiology studies have consistently revealed associations between occupational exposure to 1,3-butadiene and excess mortality due to lymphatic and hematopoietic cancers. In response to the carcinogenicity findings for 1,3-butadiene in animals and in humans, the Occupational Safety and Health Administration has proposed lowering the occupational exposure standard for this chemical from 1000 ppm to 2 ppm. Future work is needed to understand the mechanisms of tumor induction by 1,3-butadiene; however, the pursuit of this research should not delay the reduction of human exposure to this chemical.

Species differences in the production and clearance of 1,3-butadiene metabolites: a mechanistic model indicates predominantly physiological, not biochemical, control

Inhaled 1,3-butadiene, a monomer used in the production of synthetic rubber and other resins, is metabolized to mutagenic and carcinogenic epoxide intermediates. A physiologically based pharmacokinetic model of the uptake, tissue distribution, and metabolism of butadiene was constructed to determine if the biochemical kinetic constants obtained from in vitro studies are consistent with the observed in vivo uptake and metabolism. The model includes compartments for lung, blood, fat, liver, other rapidly perfused tissues ('viscera') and slowly perfused tissues. Metabolism of butadiene was assumed to occur in viscera in addition to lung and liver. Enzymatic reaction rate equations for the formation of 1,2-epoxy-3-butene, for hydrolysis of this epoxide, and for its conjugation with glutathione were also included. Physiological and biochemical parameters for the mouse, rat and human were obtained from the literature; they were not adjusted to produce a fit to experimental data. The model was used to test the hypothesis that differences in uptake and clearance of butadiene by the three species are due to differences in the activities of the metabolizing enzymes. The model reproduces whole-body observations for the mouse and rat. It predicts that inhalation uptake of butadiene and formation and retention of epoxybutene are controlled to a much greater extent by physiological parameters than by biochemical parameters and that storage in the fat represents a significant fraction of the retained butadiene. Accumulation of epoxybutene in the blood is predicted to be higher in mice than in rats or humans, but accumulation of the epoxide intermediate in the liver is predicted to be highest in humans. The epoxide tissue concentrations predicted by the model do not, by themselves, correlate with tumor incidence in mice and rats, indicating that other factors are crucial for carcinogenesis induced by butadiene.

An alternative hypothesis on the role of chemically induced protein droplet (alpha 2u-globulin) nephropathy in renal carcinogenesis

Based on associations between the accumulation of protein droplets containing alpha 2u-globulin in proximal tubular epithelial cells and increased incidences of renal tubular neoplasms in male rats, it has been suggested that the carcinogenicity of chemicals that cause alpha 2u-globulin nephropathy is unique to animals that synthesize this protein. Chemicals that caused alpha 2u-globulin nephropathy and renal carcinogenicity in male rats have not been shown to produce renal tumors in animals that lack the capability for hepatic alpha 2u-globulin synthesis, including female rats, male NBR rats, or mice of either sex. Because humans do not synthesize alpha 2u-globulin it has been suggested that chemicals which cause renal toxicity associated with alpha 2u-globulin accumulation do not pose an increased cancer risk to humans. In this review on the association between alpha 2u-globulin nephropathy and renal carcinogenesis, it is apparent that (a) there are data inconsistent with the hypothesis linking these occurrences, (b) alternative mechanisms of renal toxicity and carcinogenicity are plausible, (c) data on quantitative dose-response correspondences between the various stages of alpha 2u-globulin nephropathy and renal carcinogenicity are limited, and (d) a greater understanding of the molecular changes occurring during renal carcinogenesis is needed before assuming that the current hypothesis is correct. Future research aimed at resolving issues raised in this paper should help determine whether or not the association between alpha 2u-globulin nephropathy and renal carcinogenesis represents a cause-and-effect relationship.

Does chemically induced hepatocyte proliferation predict liver carcinogenesis?

Cell proliferation has long been recognized as having an important role in chemically induced carcinogenesis. Based on findings that certain nongenotoxic chemical carcinogens induced cell proliferation in the same organ that had an increased incidence of tumors, it has been hypothesized that a chemically induced response of enhanced DNA synthesis and cellular division causes cancer by increasing the rate of spontaneous mutations. It was further suggested that there would be no increased human risk of cancer by non-DNA-reactive compounds at doses that do not cause a proliferative response. An evaluation of the literature on the relationship between chemically induced cell proliferation and liver carcinogenesis reveals that very few systematic cell proliferation studies have been conducted over periods of extended exposure, and in many cases the exposure concentrations were not similar to those used in the cancer studies. The proliferative response resulting from exposure to many nongenotoxic carcinogens is not well sustained, whereas the carcinogenic response by these chemicals often requires prolonged exposure. The available literature leads to the conclusion that quantitative correspondences between cellular proliferation and carcinogenic responses have not been demonstrated and do not support the hypothesis that chemically induced cell proliferation is the primary mechanism by which nongenotoxic chemicals cause liver cancer. Studies of liver carcinogenesis in two-stage models point out the need to better understand chemical effects on cell loss as well as on cell replication, and demonstrate that measurements of cell proliferation alone are not sufficient to elucidate mechanisms of tumor development.

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.

Carcinogenicity of 1,3-butadiene in C57BL/6 x C3H F1 mice at low exposure concentrations

The carcinogenicity of inhaled 1,3-butadiene was evaluated in C57BL/6 x C3H F1 mice exposed to concentrations of this gas ranging from 6.25 to 625 ppm. Butadiene is a high production volume chemical, used mainly in the manufacture of synthetic rubber. In these 2-yr inhalation studies, a potent multisite carcinogenic response was observed, including neoplasms of the lung at concentrations as low as 6.25 ppm. Early occurrence and extensive development of lethal lymphocytic lymphomas in mice exposed to 625 ppm of butadiene reduced the number of animals at risk for the expression of later developing neoplasms at other sites; at lower exposure concentrations, dose responses were demonstrated for hemangiosarcomas of the heart and neoplasms of the lung, forestomach, Harderian gland, preputial gland, liver, mammary gland, and ovary. So far, no long-term studies on butadiene have been conducted at exposure concentrations that have not shown a carcinogenic response. In separate experiments with reduced exposure durations, butadiene induced neoplastic responses at multiple organ sites even after only 13 wk of exposure. Because of the correspondence between these animal data and recent epidemiology findings, there is a worldwide public health need to reevaluate current workplace exposure standards for 1,3-butadiene.

Inhalation toxicology and carcinogenicity of 1,3-butadiene in B6C3F1 mice following 65 weeks of exposure

1,3-Butadiene, a large-production volume chemical used mainly in the manufacture of synthetic rubber, was found to induce multiple-organ carcinogenicity in male and female B6C3F1 mice at exposure concentrations (625 and 1250 ppm) equivalent to and below the OSHA standard of 1000 ppm. Since this study was terminated after 60 weeks of exposure because of reduced survival due to fatal tumors, and because dose-response relationships for 1,3-butadiene-induced neoplastic and nonneoplastic lesions were not clearly established, a second long-term inhalation study of 1,3-butadiene in B6C3F1 mice was conducted at lower exposure concentrations, ranging from 6.25 to 625 ppm. Both the histopathological findings from animals dying through week 65 and the results of evaluations of animals exposed for 40 and 65 weeks are presented in this report. Exposure to 1,3-butadiene caused a regenerative anemia at concentrations of 62.5 ppm and higher. Testicular atrophy was induced at 625 ppm, and ovarian atrophy was observed at 20 ppm and higher. During the first 50 weeks of the study, lymphocytic lymphoma was the major cause of death of mice exposed to 625 ppm 1,3-butadiene. Neoplasms of the heart, forestomach, lung, Harderian gland, mammary gland, ovary, and liver were frequently observed in 1,3-butadiene-exposed mice that died between week 40 and week 65 of the study. Studies in which exposure to 1,3-butadiene was stopped after limited periods were also included to assess the relationship between exposure levels and duration of exposures on the outcome of 1,3-butadiene-induced carcinogenicity. In these studies, lymphocytic lymphomas were induced in male mice exposed to 625 ppm 1,3-butadiene for only 13 weeks. The incidence of lymphocytic lymphoma in male mice exposed to 625 ppm 1,3-butadiene for 26 weeks was two times that in mice exposed to 625 ppm for 13 weeks. However, when the exposure concentration was reduced by half to 312 ppm and the exposure duration extended to 52 weeks, the incidence of lymphocytic lymphoma was reduced by 90%. Thus, the multiple of the exposure concentration times the exposure duration did not predict the incidence of lymphocytic lymphoma in mice. The early mortalities resulting from lymphocytic lymphomas in male mice exposed to 625 ppm 1,3-butadiene limited the expression of tumors at other sites. A clearer dose-response for 1,3-butadiene-induced neoplasia should be apparent from experiments in mice exposed to lower concentrations of this chemical for 2 years.

Inhalation toxicology of isoprene in F344 rats and B6C3F1 mice following two-week exposures

Isoprene (2-methyl-1,3-butadiene) was selected for toxicologic evaluations because of its structural similarity to 1,3-butadiene, a potent rodent carcinogen. Two-week inhalation toxicology studies of isoprene were conducted in F344 rats and B6C3F1 mice at exposure concentrations of 0, 438, 875, 1750, 3500, or 7000 ppm. For rats, there were no chemically related changes in survival, body weight gain, clinical signs, hematologic or clinical chemistry parameters, or gross or microscopic lesions. Exposure of mice to isoprene did not produce mortalities and only caused a decrease in body weight gain for male mice in the 7000 ppm exposure group; however, hematologic changes and microscopic lesions including testicular atrophy, olfactory epithelial degeneration, and forestomach epithelial hyperplasia were observed in isoprene-exposed mice. Similar toxicologic effects have been previously observed in B6C3F1 mice exposed to 1,3-butadiene. A species difference in susceptibility between rats and mice exposed to isoprene was evident in these short-term exposure studies.

Metabolic processes in isolated rat small intestine villus cells: effects of cis-diamminedichloroplatinum (II)

Intestinal cells were isolated from male Fischer 344 rats by the collagenase portal vein perfusion procedure and evaluated for direct effects of cis-diamminedichloroplatinum (CDDP) and ethylacrylate (EtAc) on metabolic activities. Specific activities of marker enzymes of intestinal crypt and villus cells indicated that the preparations contained predominantly villus cells. Cell viability was generally greater than 90%, and was maintained longest when the cells were suspended in M-199 medium supplemented with 1% BSA. EtAc, an industrial intermediate which is toxic to tissues which are directly exposed to this chemical, had no apparent effect on rates of glucose metabolism or protein synthesis in suspensions of the isolated intestinal cells. These metabolic processes, however, were inhibited by the anticancer agent, CDDP; the mechanism of cytotoxicity of CDDP may therefore be due to interference with intermediary metabolism. The present studies indicate that isolated intestinal cell suspensions may be useful in examining direct and immediate effects of chemicals which are toxic to the intestinal epithelium, and in evaluating potential cytotoxic effects of CDDP analogs which have been developed.

Neoplastic lesions induced by 1,3-butadiene in B6C3F1 mice

1,3-butadiene (CAS No. 106-99-0) was evaluated for carcinogenicity and chronic toxicity by inhalation exposure in B6C3F1 mice because of its high production volume, widespread exposure of workers, and the lack of carcinogenicity and toxicity data (NTP Report #228, 1984). Butadiene (BD) had long been considered to have low toxicity. The long-term studies established that BD is a potent mouse carcinogen with multiple organ carcinogenicity (Huff et al. 1985; Melnick et al. 1988). This paper presents morphological descriptions and illustrations of the neoplastic lesions induced by 1,3-butadiene in B6C3F1 mice.

Metabolism of 1,3-butadiene by lung and liver microsomes of rats and mice repeatedly exposed by inhalation to 1,3-butadiene

1,3-Butadiene, a colorless gas widely used as an intermediate in the production of synthetic rubber, is carcinogenic in rats and mice. Species differences exist in the sensitivity to inhaled 1,3-butadiene and the target tissue specificity for tumor formation. We examined whether repeated inhalation exposure of rats and mice to 1,3-butadiene would affect the rate of metabolism of 1,3-butadiene by lung and liver microsomes in these species. Male Sprague-Dawley rats and B6C3F1 mice were exposed nose-only to air (control) or 7600 +/- 170 ppm 1,3-butadiene (13,600 +/- 300 micrograms/l) and 740 +/- 10 ppm 1,3-butadiene (1300 +/- 20 micrograms/l), respectively, for 6 h/day for 5 days. After the last exposure, nasal tissue (rats only), lungs and livers were removed from the animals and microsomes were prepared. Microsomes from the different tissues were incubated with 6 mumol 1,3-butadiene and 10 mumol NADPH for 30 min and the rate of disappearance of 1,3-butadiene from the reaction flasks was quantitated. There was a statistically significant (P less than 0.05) depression in the rate of 1,3-butadiene metabolism (50%) in microsomes from lungs of both rats and mice that were exposed repeatedly to 1,3-butadiene compared to control animals. There was no effect of repeated 1,3-butadiene exposure on liver or nasal tissue (rats only) metabolism of 1,3-butadiene in rats or mice. The data from these studies indicate that it is unlikely that species differences in sensitivity or tissue susceptibility are due to an inductive or inhibitory effect of 1,3-butadiene on its own metabolism in the tissues examined.

Chloroprene and isoprene: cytogenetic studies in mice

Groups of male B6C3F1 mice (n = 15) were exposed for 6 h per day to ambient air, to chloroprene (12, 32, 80, 200 p.p.m.) or to isoprene (438, 1750 and 7000 p.p.m.) on 12 days. These compounds are the 2-chloro and the 2-methyl analogues, respectively, of 1,3-butadiene, a genotoxic and carcinogenic chemical in B6C3F1 mice. Exposure to chloroprene resulted in a 100% incidence of mortality among the mice exposed to 200 p.p.m. At concentrations of 80 p.p.m. and below, chloroprene neither induced a significant increase in chromosomal aberrations (CA), sister chromatid exchanges (SCE) or micronucleated erythrocytes, nor significantly altered the rate of erythropoiesis or of bone marrow cellular proliferation kinetics. However, the mitotic index (MI) in the bone marrow of chloroprene-exposed mice was significantly increased. Under similar conditions, exposure to isoprene induced significant increases at all concentrations in the frequency of SCE in bone marrow cells and in the levels of micronucleated polychromatic erythrocytes (PCE) and of micronucleated normochromatic erythrocytes in peripheral blood. In addition, a significant lengthening of the bone marrow average generation time and a significant decrease in the percentage of circulating PCE was detected. However, exposure to isoprene did not induce in bone marrow a significant increase in the frequency of CA nor did the exposure significantly alter the MI. The dose-response curves for SCE and micronuclei induction were non-linear, appearing to saturate at 438 and 1750 p.p.m., respectively. These results suggest that, similarly to butadiene, inhaled isoprene can be expected to induce tumors at multiple sites in B6C3F1 mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in F344 rats by di(2-ethylhexyl)phthalate

The effects of cotreatment with a hyperlipidemic chemical, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and a hypolipidemic agent, di(2-ethylhexyl)-phthalate (DEHP), on lipid metabolism and toxicologic responses were studied in F344 rats. Treatment with TCDD alone (160 micrograms/kg) caused an increase in serum triglycerides and cholesterol while treatment with DEHP alone (2 g/kg/day) caused a decrease in triglycerides and cholesterol versus untreated controls. When administered before or after TCDD, DEHP caused a decrease in TCDD-induced hyperlipidemia. This change was attributed to enhanced hepatic peroxisomal beta-oxidation and decreased hepatic lipid synthesis resulting from treatment with DEHP. TCDD treatment produced a fatty liver, as determined by gravimetric analysis of extracted lipid and microscopic examination of liver sections which revealed extensive cytoplasmic vacuolization that stained positive with Oil Red 0, but did not induce peroxisomal beta-oxidation. Thus, an increase in hepatic or serum lipid levels is not sufficient for induction of peroxisome proliferation. Neither TCDD nor DEHP treatment affected mitochondrial beta-oxidation. Pretreatment of rats with DEHP, followed by daily exposure to this hypolipidemic agent after treatment with TCDD, had a partial protective effect against TCDD-induced fatty liver, body weight loss and mortality. Microscopic examination of liver sections confirmed the suppression of TCDD-induced fatty liver by pretreatment with DEHP. When DEHP treatment was initiated after the TCDD dose, there was less protection against the above parameters of TCDD toxicity. This study demonstrates that TCDD-induced fatty liver, hyperlipidemia and mortality can be antagonized by treatment with a hypolipidemic agent such as DEHP.

A biochemical basis for 1,2-dibromo-3-chloropropane-induced male infertility: inhibition of sperm mitochondrial electron transport activity

A rapid decrease in male fertility in laboratory animals exposed to 1,2-dibromo-3-chloropropane (DBCP) has been suggested to be due, in part, to a postglycolytic inhibition of sperm carbohydrate metabolism. The present studies were performed to identify the specific site of DBCP-induced inhibition of intermediary metabolism. 14CO2 generation by epididymal sperm, isolated from Fischer 344 rats, was measured using radiolabeled tricarboxylic acid (TCA) cycle intermediates: acetyl CoA, citrate, alpha-ketoglutarate, and succinate. There was 0-28% inhibition of CO2 generation after addition of 0.5 mM DBCP and 81-98% inhibition with 3 mM DBCP, with all four substrates. The activities of alpha-ketoglutarate dehydrogenase, pyruvate dehydrogenase, malate dehydrogenase, and lactate dehydrogenase were not inhibited by DBCP. Since the DBCP-induced inhibition of metabolism of different substrates to CO2 was similar, and since DBCP did not inhibit enzyme activities of glycolysis or the TCA cycle, a common site of inhibition was suspected. In evaluations of mitochondrial electron transport chain activity, DBCP (3 mM) inhibited oxygen consumption resulting from metabolism of endogenous substrates plus alpha-ketoglutarate or malate by about 80%. When succinate, an FAD-dependent oxidation, was used as a substrate, oxygen consumption was not inhibited by DBCP. It is concluded that DBCP inhibits sperm carbohydrate metabolism at the NADH dehydrogenase step in the mitochondrial electron transport chain.

Acyl CoA oxidase is the most suitable marker for hepatic peroxisomal changes caused by treatment of F344 rats with di(2-ethylhexyl)phthalate

Hepatic peroxisomal changes, caused by treating male Fischer 344 rats with di(2-ethylhexyl)phthalate, were examined by measuring activities of enzymes involved in peroxisomal beta-oxidation, catalase and levels of the 80,000 Da peroxisome proliferation-associated polypeptide. Acyl CoA oxidase activity was increased 2.5-fold after 1 day, and 8-fold after 14 days. Enoyl CoA hydratase activity increased 2-fold after 2 days and 6-fold after 14 days. There were no significant increases in hydroxyacyl CoA dehydrogenase or catalase activities after 3 days of treatment. Thus acyl CoA oxidase activity was the most sensitive marker of early peroxisomal changes. The apparent no-observable-effect level for this change was 0.06 g/kg/day.

Studies by the National Toxicology Program on di(2-ethylhexyl)phthalate

In a 2-year feed study previously reported by the National Toxicology Program (NTP), the plasticizer di(2-ethylhexyl)phthalate (DEHP) was found to produce increased incidences of hepatocellular neoplasms in both sexes of Fischer 344 (F344) rats and B6C3F1 mice. Further studies by the NTP on this chemical have investigated its genotoxicity, dermal absorption, reproductive and developmental toxicity, and biochemical mechanism of action. DEHP was not mutagenic in Salmonella typhimurium (strains TA98, TA100, TA1535 or TA1537), in L5178Y mouse lymphoma cells, or in Drosophila melanogaster. DEHP did not induce chromosomal aberrations, but did cause a marginal dose-related increase in sister chromatid exchanges in CHO cells. In a dermal absorption study, DEHP was not absorbed well through the skin of F344 rats. In a fertility assessment study, DEHP was shown to be a reproductive toxicant in both male and female CD-1 mice. The teratogenic potential of DEHP was evaluated in F344 rats and CD-1 mice. In the rat study, there were no significant differences in percent fetuses malformed between control and treatment groups, even at dose levels (1.0, 1.5 and 2.0%) which produced significant maternal and fetal toxicity. In the mouse study, the incidence of fetuses with malformations was significantly increased at dose levels which produced maternal and/or fetal toxicity (0.10 and 0.15%), and at a dose level (0.05%) which did not cause maternal or fetal toxicity. The no-observed effect level for developmental toxicity in mice was 0.025% DEHP. Kinetic data on the rates of formation of H2O2 by peroxisomal palmitoyl CoA oxidase, and of degradation of H2O2 by catalase, was used to estimate in vitro steady-state H2O2 concentrations during peroxisomal oxidation of palmitoyl CoA. Increases in steady-state H2O2 in liver homogenates of rats treated with DEHP, di(2-ethylhexyl)adipate, or nafenopin, a hypolipidemic drug, correlated well with the carcinogenic potential of these chemicals determined in previous carcinogenicity studies, and are consistent with but not definitive evidence for the involvement of peroxisome proliferation in the hepatocarcinogenesis of these compounds.

Application of microencapsulation for toxicology studies. I. Principles and stabilization of trichloroethylene in gelatin-sorbitol microcapsules

Microencapsulation is an innovative, alternative means of incorporating volatile, reactive, and/or unpalatable chemicals into animal feed for toxicologic studies. For such usage, the materials in the microcapsule shell must not adversely affect the laboratory animals, and the encapsulation process must not alter the chemical under study. Trichloroethylene (TCE), a volatile chemical identified in drinking water, was encapsulated in gelatin-sorbitol microcapsules. The concentration of TCE ranged from 40 to 43% (w/w) and most particles (approximately 85%) were between 300 and 420 micron in diameter. Under optimum storage conditions, loss of TCE from the microcapsules was less than 1% per month. Less than 2% of the TCE was lost from microcapsules held in uncovered petri dishes at ambient temperature and humidity for 14 days. Microencapsulated TCE was mixed with NIH-07 rodent feed at a level of 50 mg microcapsules/g feed (equivalent to 20.6 mg TCE/g feed) and stored at room temperature for 7 days in an open container or for 21 days in a sealed container. There was no detectable loss of TCE from the feed blends stored under these conditions. Thus, the stability of TCE in gelatin-sorbitol microcapsules is adequate for dosed-feed toxicity studies of this chemical.

Application of microencapsulation for toxicology studies. II. Toxicity of microencapsulated trichloroethylene in Fischer 344 rats

Gelatin-sorbitol microcapsules containing 44.1% trichloroethylene (TCE) were prepared and mixed in NIH-07 rodent meal diet and provided at microcapsule concentrations of 0 (untreated control group), 1.25, 2.5, 5.0, or 10% (equivalent to 0, 0.55, 1.10, 2.21, or 4.41% TCE, respectively) to groups of 10 male F344 rats for 14 days. An additional control group received diets containing 5% empty capsules. For comparisons, TCE dissolved in corn oil was administered by gavage to different groups of 10 male rats for 14 consecutive days at dose levels adjusted to correspond to those in the feed study. Treatment-related deaths occurred only in the highest dose group of the gavage study. Body weight gain and feed consumption were reduced in high-dose groups of both the feed and gavage studies. There was no measurable loss of TCE in feed sampled from the cages during the study. Dose-related increases in organ (liver and kidney) weight/body weight ratios, individual cell necrosis in the liver, and hepatic microsomal NADPH cytochrome c reductase and peroxisomal palmitoyl-CoA oxidase and catalase activities were found in both the dosed-fed and gavage groups. Induction of cytochrome P-450 occurred only in the dosed-feed study. There were no significant compound-related pathologic lesions observed in the kidneys, the only other organ examined microscopically. Differences in lethality, cytochrome P-450 levels, and induction of microsomal or peroxisomal enzyme activities were attributed to differences in the method of dosing (gavage versus dosed-feed). The demonstration of no significant loss of TCE from the feed and of similar toxic effects produced by microencapsulated TCE via feed and TCE in corn oil via gavage indicate that microencapsulation can provide an excellent alternative exposure route for studying the oral toxicological properties of volatile chemicals, such as TCE, in rats.

In vitro steady-state levels of hydrogen peroxide after exposure of male F344 rats and female B6C3F1 mice to hepatic peroxisome proliferators

The hypothesis that hepatocarcinogenesis resulting from treatment of rats and mice with peroxisome proliferators is linked to increased cellular levels of hydrogen peroxide from peroxisomal beta-oxidation was investigated. Male F344 rats and female B6C3F1 mice were treated for 14 days with di(2-ethylhexyl)phthalate (DEHP) or di(2-ethylhexyl)adipate (DEHA), industrial plasticizers, or nafenopin, a hypolipidemic drug. Activities of enzymes responsible for the production [peroxisomal palmitoyl CoA oxidase (PCO)] and degradation [catalase (Cat) and glutathione peroxidase (GSHPx)] of H2O2 were assayed in liver homogenates prepared from treated animals. The activities of the peroxisomal enzymes PCO and Cat were enhanced 5- to 25-fold and 1.5- to 3-fold respectively by treatment with the peroxisome proliferators. The activity of GSHPx, a cytoplasmic enzyme, was decreased 40-60% in liver homogenates prepared from treated animals compared to control animals. A kinetic treatment of the rates of formation of hydrogen peroxide by PCO, and of degradation of hydrogen peroxide by catalase was used to estimate steady-state hydrogen peroxide concentrations ([H2O2]) during peroxisomal oxidation of palmitoyl CoA. Increases in peroxisomal steady-state [H2O2] for the F344 rat liver homogenates correlated well with the carcinogenic potential of these chemicals, determined in previous carcinogenicity studies. Increases in the steady-state [H2O2] were also calculated for liver homogenates prepared from mice treated with these compounds. Decreases in liver lipid peroxidation were observed after treatment with each chemical in both species. The results of these studies are consistent with an involvement of increased peroxisomal hydrogen peroxide in the hepatocarcinogenesis of these compounds.

Multiple organ carcinogenicity of 1,3-butadiene in B6C3F1 mice after 60 weeks of inhalation exposure

Groups of 50 male and 50 female B6C3F1 mice were exposed 6 hours per day, 5 days per week, for 60 to 61 weeks to air containing 0, 625, or 1250 parts per million 1,3-butadiene. These concentrations are somewhat below and slightly above the Occupational Safety and Health Administration standard of 1000 parts per million for butadiene. The study was designed for 104-week exposures but had to be ended early due to cancer-related mortality in both sexes at both exposure concentrations. There were early induction and significantly increased incidences of hemangiosarcomas of the heart, malignant lymphomas, alveolar-bronchiolar neoplasms, squamous cell neoplasms of the forestomach in males and females and acinar cell carcinomas of the mammary gland, granulosa cell neoplasms of the ovary, and hepatocellular neoplasms in females. Current workplace standards for exposure to butadiene should be reexamined in view of these findings.

Effect of phthalate esters on energy coupling and succinate oxidation in rat liver mitochondria

Isolated rat liver mitochondria were exposed to mono- and di-n-butyl phthalate (MBP and DBP) and mono- and di(2-ethylhexyl)phthalate (MEHP and DEHP) and examined for effects on mitochondrial energy-dependent processes, including oxidative phosphorylation and active K+ uptake. Additional studies on the effects of these phthalate esters on succinate oxidation and on mitochondrial membrane integrity are also included. DBP and MEHP stimulated succinate state 4 respiration, impaired K+-valinomycin induced swelling with succinate, ascorbate, or ATP as the energy sources, and inhibited succinate state 3 respiration and succinate cytochrome c reductase activity. MEHP was found to act as a non-competitive inhibitor of succinate dehydrogenase activity, with an apparent Ki = 2.4 X 10(-4) M. At concentrations which uncouple energy linked reactions, MEHP and DBP produced only slight energy-independent swelling and release of soluble proteins from isolated mitochondria. MBP caused only slight stimulation of state 4 respiration and impairment of K+-valinomycin induced swelling with each of the 3 energy sources, however, of the 4 phthalate esters, it produced the greatest energy-independent swelling and led to the greatest release of soluble mitochondrial proteins. DEHP had no apparent effect on any of these processes except for slight impairment of ATP-dependent K+-valinomycin induced swelling. It is concluded that phthalate ester toxicity in liver mitochondria is due to uncoupling of energy linked reactions and/or inhibition of succinate dehydrogenase activity. Uncoupling by MBP may involve disruption of mitochondrial membrane integrity, while uncoupling by DBP and MEHP is probably due to an increase in membrane permeability to H+ and other small ions.

Toxicities of ethylene glycol and ethylene glycol monoethyl ether in Fischer 344/N rats and B6C3F1 mice

The toxicities of ethylene glycol (EG) and ethylene glycol monoethyl ether (EGEE) were studied in Fischer 344/N rats and B6C3F1 mice. In a 13-week study, EG was administered in feed to groups of 10 rats and 10 mice of both sexes at dose levels of 0 (control), 0.32, 0.63, 1.25, 2.5 and 5.0%. Kidney/body weight ratios were elevated in the 2.5 and 5.0% dose groups of male and female rats relative to controls, while serum urea nitrogen and serum creatinine levels were elevated in the two highest dose groups of male rats. Toxic nephrosis and crystal deposits in renal tubules were observed in the 2.5 and 5.0% dose groups of male rats. Crystals were also observed in brains of male rats in the 5.0% dose group. Nephrosis was the only lesion observed in female rats (5.0% dose group). Mild, compound-related lesions were seen in kidneys (nephrosis) and livers (centrilobular degeneration) of male mice in the 2.5 and 5.0% dose groups. There were no adverse effects observed in female mice. Groups of 50 rats and 50 mice of both sexes were administered EGEE by gavage in a 2-year study at dose levels of 0 (control), 0.5, 1.0 and 2.0 g/kg body weight. Testicular atrophy was observed in male rats that died early in this study and in the medium- and high-dose male mouse groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Urolithiasis and bladder carcinogenicity of melamine in rodents

Melamine (2,4,6-triamino-s-triazine) was administered in the diet to F344 rats or B6C3F1 mice for 13 weeks (subchronic) or for 103 weeks (chronic) to determine its toxicologic profile, including carcinogenic potential in the chronic study. The dose levels of melamine in the subchronic studies ranged from 750 to 18,000 ppm for rats, and 6000 to 18,000 ppm for mice. In the chronic studies the dose levels of melamine were 2250 or 4500 ppm for male rats and mice of each sex, and 4500 or 9000 ppm for female rats. In these studies, compound-related lesions were observed in the urinary tract. Most noticeable was the development of uroliths (urinary bladder stones), which occurred at a greater frequency in males than females of either species. Increased incidences of urinary bladder stones and hyperplasia of the bladder epithelium were observed at 13 weeks in male rats fed diets containing melamine. In the chronic study, transitional-cell carcinomas in the urinary bladder of male rats occurred at a significantly (p less than or equal to 0.016) higher incidence in the 4500 ppm (high dose) group (8/49) than in the controls (0/45). Seven of the eight male rats with transitional-cell carcinomas of the urinary bladder also had bladder stones. There was a statistically significant association (p less than or equal to 0.001) between bladder stones and bladder tumors in male rats fed melamine (4500 ppm). Urinary bladder tumors were not observed in the low-dose (2250 ppm) male rat group, while bladder stones were observed in one rat in this group. In the female rat chronic study, chronic inflammation of the kidney was observed at an increased incidence (relative to controls) in both the low (4500 ppm) and high (9000 ppm) dose groups. Ulceration of the bladder epithelium was observed in male and female mice in the 13-week study. The distribution of these toxic lesions was not correlated statistically with the distribution of urinary bladder stones. Acute and chronic inflammation and epithelial hyperplasia of the urinary bladder were found in increased incidence in dosed male mice (2250 and 4500 ppm) in the chronic study. In addition, a high incidence of urinary bladder stones was observed in dosed male mice relative to controls. However, there was no evidence of bladder tumor development in this species.

Chronic effects of agar, guar gum, gum arabic, locust-bean gum, or tara gum in F344 rats and B6C3F1 mice

Diets containing 25,000 (2.5%) or 50,000 ppm (5.0%) agar, guar gum, gum arabic, locust-bean gum or tara gum were fed to groups of 50 male and 50 female F344 rats and B6C3F1 mice for 103 wk. Separate groups of 50 rats and 50 mice of each sex served as controls for each study. There were no significant differences in survival between any of the dosed groups of rats or mice and their respective control groups. Depressions in body-weight gain greater than 10% for dosed groups relative to their respective control groups were observed for male (low dose only) and female mice fed diets containing agar, female mice fed diets containing guar gum (high dose only), male mice fed diets containing locust-bean gum (high dose only) and male and female mice fed diets containing tara gum (high dose only). Depressions in body-weight gain greater than 5% were observed for female rats fed diets containing agar, guar gum or gum arabic. There were no histopathological effects associated with the administration of the test materials. Under the conditions of these bioassays, none of the five polysaccharides was carcinogenic for F344 rats or B6C3F1 mice of either sex.

Mitochondrial toxicity of phthalate esters

The effects of mono- and dibutyl phthalate and mono- and di(2-ethylhexyl) phthalate on energy-dependent K+ uptake, respiration rates, and succinate cytochrome c reductase activities of isolated rat liver mitochondria were evaluated. The energy-coupling processes, active K+ transport and oxidative phosphorylation, were affected most by di-n-butyl phthalate and mono(2-ethylhexyl) phthalate. Mono-n-butyl phthalate had a moderate effect on energy coupling and di(2-ethylhexyl) phthalate had no apparent effect. The potency of inhibition of succinate cytochrome c reductase activity was mono(2-ethylhexyl) phthalate greater than di-n-butyl phthalate greater than mono-n-butyl phthalate = di(2-ethylhexyl) phthalate. It is concluded that phthalate esters affect mitochondrial activities by altering the permeability properties of the inner membrane and by inhibiting succinate dehydrogenase activity.

Use of fluorescent probes that form intramolecular excimers to monitor structural changes in model and biological membranes

1,3-dipyrenylpropane (PC3P) and bis(4-biphenylmethyl)ether, two molecules that form intramolecular excimers, were embedded in phospholipid vesicles and biological membranes to monitor dynamic properties of membrane lipids. Excimer formation was evaluated from determinations of excimer to monomer emission intensity ratios (ID/IM). ID/IM values of PC3P and bis(4-biphenylmethyl)ether were reduced when cholesterol was added to egg lecithin vesicles. PC3P was sensitive to the temperature-induced crystalline to liquid-crystalline phase transition in dimyristoyl phosphatidylcholine vesicles. For studies of cellular membranes, membranes, PC3P was used exclusively, because of the fluorescence of tryptophan residues of membrane proteins interferes with the responses bis(4-biphenylmethyl)ether. Microviscosities of membrane interiors were calculated from standard curves of IM/ID plotted against solvent viscosity. Microviscosity values of egg lecithin vesicles and biological membranes, especially those obtained with PC3P, were more than an order of magnitude lower than values obtained by other techniques. We concluded that the intramolecular process leading to the formation of the excimer is influenced differently in isotropic solvents than in anisotropic environments, such as lipid bilayers. Although distinguishable ID/IM ratios can be obtained for different biological membranes (mitochondrial, microsomal, and plasma membranes were studied), this parameter may be phenomenological and not simply related to membrane microviscosity. As such, fluorescent probes that form intramolecular excimers are of value in making qualitative comparisons of different membranes and in studying the relative effects of physical changes and chemical agents on membrane structure. These probes may also be valuable for studying structural anisotropy of biological membranes.