Enhancement by Wy-14,643, a hepatic peroxisome proliferator, of diethylnitrosamine-initiated hepatic tumorigenesis in the rat

Assessment of Possible Carcinogenic Risk to Humans Resulting from Exposure to Di(2-ethylhexyl)phthalate (DEHP)

The purpose of this work was to estimate the degree of risk that might be associated with human exposure to low levels of the plasticizer di(2-ethylhexyl)phthalate (DEHP). DEHP is a common component, sometimes at high concentrations, of polyvinyl chloride (PVC) plastics and was recently reported by the National Toxicology Program (NTP) to be carcinogenic in rats and mice, inducing hepatocellular tumors in both species. This work was also designed to illustrate an approach to risk assessment that attempts to incorporate all available biological data. Based on the dose-response data generated by the NTP bioassays, we have performed extrapolations of risk to low dose levels using several procedures, including some that incorporate inferences from the available data that shed light on the likely relationship between dose level and risk at low dose levels. In drawing these inferences, consideration was given to such factors as genotoxicity, metabolism and pharmacokinetics, and physiological and biochemical effects of DEHP that might reveal its mechanism of action. The relative merits of each of the various risk estimates are described, based on current understanding of DEHP's mode of biological action. It is concluded that DEHP's mechanism of carcinogenicity in rodents most likely involves its ability to induce peroxisome proliferation and related enzymatic changes, although other mechanisms cannot be excluded. If humans and rodents are assumed to be at the same risk at the same daily dose level of DEHP, application of the various low dose extrapolation models leads to the prediction that the daily dose resulting in a lifetime risk of no more than 1 in 1 million would be between 1.5 and 791 mg/kg per day, with the most likely figure being 116 mg/kg per day. If the carcinogenicity of DEHP is dependent upon its pattern of metabolism, however, it would be inappropriate to extrapolate from rodents to man without qualification because of the major quantitative differences in metabolism in rats, mice, and primates, including man. One of the major differences in metabolism of DEHP between rats and mice and primates is in production of a metabolite whose level may be an indicator of the level of peroxisomal activity and, hence, if the peroxisome proliferation theory of DEHP carcinogenicity is correct, of carcinogenic risk. However, the substantial doubt that exists regarding the applicability of rodent carcinogenicity data to humans must be expressed in qualitative terms.

Hypolipidemics Carcinogenicity and Extrapolation of Experimental Results for Human Safety Assessments

Dyslipoproteinemias represent a group of disorders closely related to alterations of cholesterol and triglycerides. The alterations of these lipids are considered important risk factors in coronary heart disease and indicate the need for clinically effective and safe drugs.

Hypolipidemic agent therapy, however, does not appear without risk since the administration of these agents is by necessity, on a long-term basis. In the conduct of animal safety studies with some hypolipidemics, hyperplastic nodules or tumors developed in the liver of rodents. Data from the literature seem to indicate that the tumor response in rodents varies with the type of hypolipidemic drug administered. This paper summarizes the studies with the new lipid-regulating agent gemfibrozil. Aside from conventional long-term studies in rodents, the ultrastructural aspects of the liver were analyzed in several species and genotoxicity assays and short-term tests for hepatocarcinogenicity were conducted. Thus, it was possible to obtain an overview of these biological phenomena in order to allow for safety extrapolations. The biological behavior of these liver nodules showed that gemfibrozil and clofibrate-induced hepatocytes had not undergone malignant transformation. Further, the phenomenon of peroxisome proliferation, a characteristic event that follows hypolipidemic administration in rodents, was not confirmed in primate or human liver. Peroxisome proliferation has been linked to the process of hepatocarcinogenesis in rodents, although genotoxicity assays were negative and initiation/promotion tests failed to elicit tumors or nodules in a system where hepatocarcinogens manifest their activity.

Thus, hypolipidemics such as gemfibrozil or Clofibrate may possess low tumorigenic potential with low risk due to the lack of correlation between these tests. Nevertheless, these agents are indicated for specific lipoprotein phenotype alteration with the resulting clinical benefits.

Hypolipidemics Carcinogenicity and Extrapolation of Experimental Results for Human Safety Assessments

Dyslipoproteinemias represent a group of disorders closely related to alterations of cholesterol and triglycerides. The alterations of these lipids are considered important risk factors in coronary heart disease and indicate the need for clinically effective and safe drugs.

Hypolipidemic agent therapy, however, does not appear without risk since the administration of these agents is by necessity, on a long-term basis. In the conduct of animal safety studies with some hypolipidemics, hyperplastic nodules or tumors developed in the liver of rodents. Data from the literature seem to indicate that the tumor response in rodents varies with the type of hypolipidemic drug administered. This paper summarizes the studies with the new lipid-regulating agent gemfibrozil. Aside from conventional long-term studies in rodents, the ultrastructural aspects of the liver were analyzed in several species and genotoxicity assays and short-term tests for hepatocarcinogenicity were conducted. Thus, it was possible to obtain an overview of these biological phenomena in order to allow for safety extrapolations. The biological behavior of these liver nodules showed that gemfibrozil and clofibrate-induced hepatocytes had not undergone malignant transformation. Further, the phenomenon of peroxisome proliferation, a characteristic event that follows hypolipidemic administration in rodents, was not confirmed in primate or human liver. Peroxisome proliferation has been linked to the process of hepatocarcinogenesis in rodents, although genotoxicity assays were negative and initiation/promotion tests failed to elicit tumors or nodules in a system where hepatocarcinogens manifest their activity.

Thus, hypolipidemics such as gemfibrozil or clofibrate may possess low tumorigenic potential with low risk due to the lack of correlation between these tests. Nevertheless, these agents are indicated for specific lipoprotein phenotype alteration with the resulting clinical benefits.

Evidence that the capacity of nongenotoxic carcinogens to induce oxidative stress is subject to marked variability

Many drugs and environmental chemicals which are not directly mutagenic have the capacity to increase the incidence of tumors in the liver and other tissues. For this reason, such compounds are known as nongenotoxic carcinogens. The mechanisms underlying their effects remain unclear; however, their capacity to induce oxidative stress is considered to be a critical step in the carcinogenic process, although the evidence that this is actually the case remains equivocal and sparse. We have exploited a novel heme oxygenase-1 reporter mouse to evaluate the capacity of nongenotoxic carcinogens with different mechanisms of action to induce oxidative stress in the liver in vivo. When these compounds were administered at doses reported to cause liver tumors, marked differences in activation of the reporter were observed. 1,4-Dichlorobenzene and nafenopin were strong inducers of oxidative stress, whereas phenobarbital, piperonyl butoxide, cyproterone acetate, and WY14,643 were, at best, only very weak inducers. In the case of phenobarbital and thioacetamide, the number of LacZ-positive hepatocytes increased with time, and for the latter also with dose. The data obtained demonstrate that although some nongenotoxic carcinogens can induce oxidative stress, it is not a dominant feature of the response to these compounds. Therefore in contrast to the current models, these data suggest that oxidative stress is not a key determinant in the mechanism of nongenotoxic carcinogenesis but may contribute to the effects in a compound-specific manner.

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

Inhibition of hepatocarcinogenesis by the deletion of the p50 subunit of NF-kappaB in mice administered the peroxisome proliferator Wy-14,643

Wy-14,643 (WY) is a hypolipidemic drug that induces hepatic peroxisome proliferation and tumors in rodents. We previously showed that peroxisome proliferators increase NF-kappaB DNA binding activity in rats, mice, and hepatoma cell lines, and that mice deficient in the p50 subunit of NF-kappaB had much lower cell proliferation in response to the peroxisome proliferator ciprofibrate. In this study we examined the promotion of hepatocarcinogenesis by WY in the p50 knockout (-/-) mice. The p50 -/- and wild type mice were first administered diethylnitrosamine (DEN) as an initiating agent. Mice were then fed a control diet or a diet containing 0.05% WY for 38 weeks. Wild-type mice receiving DEN only developed a low incidence of tumors, and the majority of wild-type mice receiving both DEN and WY developed tumors. However, no tumors were seen in any of the p50 -/- mice. Cell proliferation and apoptosis were measured in hepatocytes by BrdU labeling and the TUNEL assay, respectively. Treatment with DEN + WY increased both cell proliferation and apoptosis in both the wild-type and p50 -/- mice; DEN treatment alone has no effect. In the DEN/WY-treated mice, cell proliferation and apoptosis were slightly lower in the p50 -/- mice than in the wild-type mice. These data demonstrate that NF-kappaB is involved in the promotion of hepatic tumors by the peroxisome proliferator WY; however, the difference in tumor incidence could not be attributed to alterations in either cell proliferation or apoptosis.

Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk

A wide variety of chemicals, both naturally occurring and synthetic, have exhibited carcinogenic activity in rodent liver. Some are clearly DNA reactive whereas others produce only epigenetic effects. Hepatocarcinogens are categorized according to these properties and the characteristics of examples of both types are reviewed. DNA-reactive rodent hepatocarcinogens represent human cancer risk even at non-toxic exposures, whereas epigenetic agents pose either no risk because their effects are specific to rodents, or a risk only at high exposures at which they produce the same cellular effects in humans that are the basis for their carcinogenic activity in rodents.

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

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

Hepatic peroxisome proliferation in rodents and its significance for humans

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

The molecular biology of carcinogenesis

Carcinogenesis may result from the action of any one or a combination of chemical, physical, biologic, and/or genetic insults to cells. The process of carcinogenesis may be divided into at least three stages: initiation, promotion, and progression. The first stage of carcinogenesis, initiation, results from an irreversible genetic alteration, most likely one or more simple mutations, transversions, transitions, and/or small deletions in DNA. The reversible stage of promotion does not involve changes in the structure of DNA but rather in the expression of the genome mediated through promoter-receptor interactions. The final irreversible stage of progression is characterized by karyotypic instability and malignant growth. Critical molecular targets during the stages of carcinogenesis include proto-oncogenes, cellular oncogenes, and tumor suppressor genes, alterations in both alleles of the latter being found only in the stage of progression. Although many of these critical target genes have been identified, the ultimate number and characteristics of molecular alterations that define neoplasia have not been elucidated.

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.

The molecular biology of carcinogenesis

Carcinogenesis may result from the action of any one or a combination of chemical, physical, biologic, and/or genetic insults to cells. The process of carcinogenesis may be divided into at least three stages: initiation, promotion, and progression. The first stage of carcinogenesis, initiation, results from an irreversible genetic alteration, most likely one or more simple mutations, transversions, transitions, and/or small deletions in DNA. The reversible stage of promotion does not involve changes in the structure of DNA but rather in the expression of the genome mediated through promoter-receptor interactions. The final irreversible stage of progression is characterized by karyotypic instability and malignant growth. Critical molecular targets during the stages of carcinogenesis include proto-oncogenes, cellular oncogenes, and tumor suppressor genes, alterations in both alleles of the latter being found only in the stage of progression. Although many of these critical target genes have been identified, the ultimate number and characteristics of molecular alterations that define neoplasia have not been elucidated.

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

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

Dose-response relationships of hepatic acyl-CoA oxidase and catalase activity and liver mitogenesis induced by the peroxisome proliferator ciprofibrate in C57BL/6N and BALB/c mice

The dose-response for key hepatic effects of the peroxisome proliferator ciprofibrate, 2-[4-(2,2-dichlorocyclopropyl)phenoxy]-2- methylpropanoic acid, was delineated in mice and strain differences in response were demonstrated. Ciprofibrate was fed at concentrations ranging from 0.1 to 250 ppm to male C57BL/6N and BALB/c mice and the induction of hepatic acyl-CoA oxidase and catalase, peroxisomal enzymes involved in the formation and degradation of hydrogen peroxide, and liver hepatomegaly and mitogenesis were measured. No effect was found for enzyme induction at 5.0 ppm or less in either strain. Likewise, hepatomegaly was not found at 5.0 ppm, but mitogenesis was observed in BALB/c mice at 1.0 ppm. C57BL/6N mice demonstrated greater basal and postexposure acyl-CoA oxidase activity than BALB/c mice, while BALB/c mice demonstrated greater catalase activity and induction of liver mitogenesis. The threshold exposure level for induction of acyl-CoA oxidase activity was approximately the same as that for induction of mitogenesis in C57BL/6N mice; in contrast, the threshold exposure level for induction of acyl-CoA oxidase activity was at least one order of magnitude greater than that required for induction of mitogenesis in BALB/c mice. Thus, the induction of the peroxisomal enzyme involved in the formation of hydrogen peroxide and increased mitogenesis are not mechanistically linked. The differential effects observed in the two mouse strains provide the basis for development of a quantitative model of peroxisome proliferator-induced carcinogenicity in which cellular effects can be related to carcinogenicity.

An overview of peroxisome proliferator-induced hepatocarcinogenesis

Peroxisome proliferators are hepatocarcinogens in rats and mice. Chronic administration of these compounds results in the development of altered areas and neoplastic nodules followed by hepatocellular carcinomas. All three types of hepatic lesions do not express gamma-glutamyltranspeptidase, glutathione 8-transferase-P, and alpha-fetoprotein and are resistant to iron accumulation after overload. The mechanism by which nongenotoxic peroxisome proliferators induce hepatic tumors is not well understood. It has been proposed that with continuous administration of peroxisome proliferators, liver cells are subjected to persistent oxidative stress resulting from marked proliferation of peroxisomes and a differential increase in the levels of H2O2 producing (20- to 30-fold) and degrading (2-fold) enzymes. Free oxygen radicals lead to DNA damage (both directly and through lipid peroxidation) and thus may cause initiation and promotion of the carcinogenic process.

Lack of induction of hepatic DNA damage on long-term administration of peroxisome proliferators in male F-344 rats

In order to evaluate the relationship between hydrogen peroxide (H2O2) generation and subsequent DNA damage caused by peroxisome proliferation, we examined DNA damage and changes in peroxisomal beta-oxidation activity in rat liver. Male F-344 rats were given orally clofibrate, bezafibrate or di(2-ethylhexyl)phthalate (DEHP) for up to 78 weeks. In rats fed DEHP for 52 or 78 weeks hepatocarcinomas or neoplastic nodules were found. In rats treated for 2 weeks with peroxisome proliferators, peroxisomal beta-oxidation activity was increased 10-17 times over control levels. After long-term treatment (20-78 weeks), the level of peroxisomal beta-oxidation activity remained 3-13-times higher in each group. When single strand DNA breaks were measured by a DNA-alkaline elution technique, no increase in DNA damage was observed in livers from rats fed peroxisome proliferators for 2, 40 or 78 weeks. In rats bearing hepatocarcinomas induced by DEHP, the hepatic DNA showed significant breaks; the rate of DNA-alkaline elution was found to increase approximately 5-fold. No significant increase in hepatic lipid peroxide level was observed in each group. These results show that although prolonged treatment with peroxisome proliferators induces markedly peroxisomal beta-oxidation activity, the active oxygen species from peroxisomal beta-oxidation are not enough to give rise to significant DNA damage. Moreover, the change in the activity of peroxisomal beta-oxidation may not relate to hepatocarcinogenesis induced by peroxisome proliferators.

On the mechanism of the hepatocarcinogenicity of peroxisome proliferators

The absence of a genotoxic action in the rat of several peroxisome proliferators (PP) has been confirmed by measuring gross degradation, unscheduled DNA-synthesis (UDS), as well as by measurement of single strand breaks using alkali unwinding in absence and presence of inhibitors of DNA-repair. Similar results were obtained even after drastically lowering the glutathione content of liver. Further, after oral administration of ciprofibrate, no potentiating effect was found in vivo on the generation of micronuclei in hepatocytes by ionizing radiation. The metabolically inert PP, perfluorooctanoic acid, was found to act as a promoter of liver tumors in the rat induced by diethylnitrosamine in an initiation-selection-promotion protocol. The results are discussed in light of available information concerning the mechanism of action of PPs.

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.

Effect of the peroxisome proliferator ciprofibrate on hepatic DNA synthesis and hepatic composition following partial hepatectomy in rats

The peroxisome proliferator ciprofibrate was examined for its ability to alter liver regrowth following partial hepatectomy in rats. Ciprofibrate was fed to female Sprague-Dawley rats at concentrations of 0, 0.01% and 0.025% in the diet for 2 weeks. All rats were then subjected to partial hepatectomy and were killed at 0, 12, 24, 36, 48, 72, and 168 h afterwards. The increase in liver weight after partial hepatectomy occurred at a similar rate in control and ciprofibrate-fed rats, although liver weights were always higher in ciprofibrate-fed rats. The marked increase in DNA synthesis normally seen after partial hepatectomy, however, was partially inhibited in rats fed 0.025% ciprofibrate, as compared to control rats or rats fed 0.01% ciprofibrate. An increase in the ratio of protein to DNA in the liver was observed in rats fed either level of ciprofibrate. The marked increase in total lipid content normally seen after partial hepatectomy was inhibited by ciprofibrate treatment. Vitamin E levels were also reduced in ciprofibrate-fed rats. The activity of the peroxisomal enzyme fatty acyl CoA oxidase was increased in rats fed ciprofibrate at all time points, verifying the induction of peroxisomes by ciprofibrate. This study shows that the administration of 0.025% ciprofibrate before partial hepatectomy inhibits the peak of DNA synthesis normally seen shortly after partial hepatectomy but does not affect the regrowth of the liver. The regrowth of the liver in rats fed 0.025% ciprofibrate may be caused by cellular hypertrophy, as evidenced by the enhanced protein content of the liver.

Effects of the peroxisome proliferator di(2-ethylhexyl)phthalate on enzymes in rat liver and on carcinogen-induced liver altered foci in comparison to the promoter phenobarbital

The livers of rats given either the peroxisome proliferating hepatocarcinogen di(2-ethylhexyl)phthalate (DEHP) following initiation by 2-acetylaminofluorene (AAF) or the neoplasm promoter phenobarbital (PB) were studied for changes in 8 histochemical properties. Male F344 rats were fed 200 ppm AAF for 7 weeks to induce hepatocellular altered foci, and were then fed diets containing either no chemical, 12,000 ppm DEHP or 500 ppm PB for 24 weeks. In hepatocytes, DEHP increased alkaline phosphatase activity throughout the lobule, but reduced gamma-glutamyltransferase (GGT) activity in periportal hepatocytes. PB, in contrast, increased GGT activity in periportal hepatocytes. In foci that were induced by AAF, DEHP reduced the histochemical activity of GGT and did not increase the number, mean volume or volume % of foci detected by deficiencies in iron storage, glucose-6-phosphatase, adenosine triphosphatase or fibronectin. PB enhanced the expression of all 8 phenotypic abnormalities in foci such that either more profiles were detected or the area of foci was increased.

Species and strain sensitivity to the induction of peroxisome proliferation by chloroacetic acids

B6C3F1 mice and Sprague-Dawley rats were provided drinking water containing 6-31 mM (1-5 g/liter) trichloroacetic acid (TCA), 8-39 mM (1-5 g/liter) dichloroacetic acid (DCA), or 11-32 mM (1-3 g/liter) monochloroacetic acid (MCA) for 14 days. TCA and DCA, but not MCA, increased the mouse relative liver weight in a dose-dependent manner. Rat liver weights were not altered by TCA or DCA treatment, but were depressed by MCA. Hepatic peroxisome proliferation was demonstrated by (1) increased palmitoyl-CoA oxidase and carnitine acetyl transferase activities, (2) appearance of a peroxisome proliferation-associated protein, and (3) morphometric analysis of electron micrographs. Mouse peroxisome proliferation was enhanced in a dose-dependent manner by both TCA and DCA, but only the high DCA concentration (39 mM) increased rat liver peroxisome proliferation. MCA was ineffective in both species. Three other mouse strains (Swiss-Webster, C3H, and C57BL/6) and two strains of rat (F344 and Osborne-Mendel) were examined for sensitivity to TCA. TCA (12 and 31 mM) effectively enhanced peroxisome proliferation in all mouse strains, especially the C57BL/6. A more modest enhancement in the Osborne-Mendel (288%) and F344 rat (167%) was seen. Dosing F344 rats with 200 mg/kg TCA in water or corn oil for 10 days increased peroxisome proliferation 179 and 278%, respectively, above the vehicle controls. These studies demonstrate that the mouse is more sensitive than the rat with respect to the enhancement of liver peroxisome proliferation by TCA and DCA and suggest that if peroxisome proliferation is critical for the induction of hepatic cancer by TCA and DCA, then the rat should be less sensitive or refractory to tumor induction.

Lack of initiating activity of the peroxisome proliferator ciprofibrate in two-stage hepatocarcinogenesis

The peroxisome proliferator ciprofibrate was examined for its ability to initiate hepatocarcinogenesis in rats. Ciprofibrate was fed in the diet at levels of 0%, 0.01% and 0.025% for 2 weeks in order to induce steady-state peroxisome proliferation. Rats were then subjected to partial hepatectomy and then maintained for 6 months on a basal diet or one containing 0.05% phenobarbital. Ciprofibrate treatment did not increase the number or volume of altered hepatic foci (putative preneoplastic lesions). However, ciprofibrate treatment increased liver weights in a dose-dependent manner in rats which did not receive phenobarbital. It is concluded that ciprofibrate-induced peroxisome proliferation is not sufficient to induce initiation, but that a permanent change is produced which results in an increased liver weight.

Chemically induced proliferation of peroxisomes: implications for risk assessment

An increasing number of beneficial and economically important drugs, industrial chemicals, and agrichemicals are being found to cause a dose-related hepatomegaly in rodent species which is associated with the proliferation of the subcellular organelle, the peroxisome. The prolonged proliferation of hepatocellular peroxisomes and the enhanced production of the normal peroxisomal metabolic byproduct, hydrogen peroxide, in these animals during chronic bioassays has been hypothesized to account for the tumorigenicity of several of these compounds, most of which lack any measurable genotoxicity in in vitro and in vivo assays. This paper briefly reviews the basic morphology and enzymology of the peroxisome and its relationship to specific pathologic changes in animals. The potential impact of the mechanism of action of peroxisome proliferators upon the design of toxicity studies and, in conjunction with interspecies sensitivity data, upon risk assessment is discussed.

In vivo studies on the mechanism of di(2-ethylhexyl)phthalate carcinogenesis

In a study sponsored by the National Toxicology Program, di(2-ethylhexyl)phthalate (DEHP) fed in the diet at 1.2% significantly increased the incidence of female rats with hepatocellular carcinomas. Extensive evaluation of DEHP for carcinogenicity has yielded negative results. The present investigations were designed to elucidate the mechanism of DEHP hepatocarcinogenesis under the conditions of the original bioassay. Short-term studies designed to evaluate the promoting capability of DEHP, when administered after initiation, were negative when livers of female Fischer-344 rats were evaluated using multiple histochemical stains to identify foci of cellular alteration. Two different protocols were used to evaluate the initiating potential of DEHP in the liver using histochemically defined foci as the endpoint. In both experiments the results were negative. Chronic exposure to DEHP at 1.2% in the diet for 2 years resulted in elevation of hepatic peroxisomal enzymes while DNA replication, an indication of cell proliferation, was not affected in hepatocytes. The number of foci was not elevated in the DEHP group compared to the controls, even though a low incidence of rats with liver tumors occurred in the treated group. The results of this series, as well as other published results, suggest that DEHP and other peroxisomal proliferating chemicals have unique effects on the development of hepatic neoplasms. The absence of altered foci after chronic administration or in initiation-promotion protocols distinguishes DEHP and perhaps other peroxisomal proliferating chemicals from both classic liver carcinogens and promoters.

Cellular events during hepatocarcinogenesis in rats and the question of premalignancy

The cellular, biochemical, and genetic changes that occur in the liver of rats exposed to chemical hepatocarcinogens are reviewed. Multiple new cell types appear in the liver of carcinogen-treated rats including foci, nodules, ducts, oval cells, and atypical hyperplastic areas. The application of phenotypic markers for these cell types suggests that hepatocellular carcinomas may arise from more than one cell type, including a putative liver stem cell that proliferates following carcinogen exposure. Study of DNA, RNA, and proteins produced by hepatocellular carcinomas and putative premalignant cells has so far failed to identify a gene or gene product clearly associated with the malignant or premalignant phenotype. Understanding the cellular lineage from normal cell through putative premalignant cell to cancer is critical to understanding the process of carcinogenesis. Application of new immunological (monoclonal antibody, transplantation) and molecular biological (gene cloning, oncogene identification) approaches to this problem holds promise that the process of hepatocarcinogenesis will be better known in the near future.

Di-n-octyl phthalate (DOP), a relatively ineffective peroxisome inducing straight chain isomer of the environmental contaminant di(2-ethylhexyl)phthalate (DEHP), enhances the development of putative preneoplastic lesions in rat liver

Di-n-octyl phthalate (DOP) is the straight chain isomer of di(2-ethylhexyl) phthalate (DEHP) which is a widely used plasticizer and an environmental contaminant. DEHP is a strong inducer of peroxisome proliferation in rat liver. This is significant since other compounds which are strong inducers of peroxisome proliferation have been reported to be weak carcinogens (Reddy, J.K. and Lalwani, N.D., CRC Crit. Rev. Toxicol., 12 (1983) 1). In contrast to DEHP, DOP causes little or no induction of liver peroxisomes (Mann, A.H. et al., Toxicol. Appl. Pharmacol., 77 (1985) 116, and Gray, T.J.B. et al., Toxicology, 28 (1983) 167). In the current study the ability of 1% DOP to promote the development of putative preneoplastic lesions was evaluated. The effect of feeding 0.5% DEHP as well as equimolar amounts of its 2 major metabolites, mono(2-ethylhexyl)phthalate (MEHP) and 2-ethylhexanol (2-EH) were also investigated. GGT+ foci were initiated in the livers of Sprague--Dawley male rats with a single dose of diethylnitrosamine (DEN) following partial hepatectomy. The control group of rats was fed a semipurified diet (Co) for 10 weeks while the experimental groups received the semipurified diet containing the respective compounds. Induction of peroxisome proliferation was monitored by carnitine acetyltransferase (CAT) levels. DOP treatment resulted in a 6-fold increase in the number of GGT+ foci (20.8 +/- 4.0 vs. 3.5 +/- 1.3; P less than 0.05). This was accompanied by no change in liver weight and only a slight increase in CAT activity when compared with control animals. In contrast to DOP, 2-EH produced essentially no effect with regard to number of foci, peroxisome proliferation or liver weight. DEHP and MEHP induced significant peroxisome proliferation and hepatomegaly but the number of foci were significantly lower than in 2-EH-treated rats. The mechanism for the promoting ability of DOP is not clear but would not appear to be related to peroxisome proliferation. Because of the close similarity of chemical structure and metabolism between DOP and DEHP, it is possible that studies to define the mechanism of DOP induced promotion might also serve to further clarify the mechanism of DEHP induced carcinogenesis.

Peroxisome proliferators show tumor-promoting but no direct transforming activity in vitro

The chemically unrelated hypolipidemic drugs, tiadenol, niadenate, and clofibrate have been tested for carcinogenic and tumor-promoting potential in the C3H/10TI/2 C18 cell test system. None of these chemicals were carcinogenic, while both niadenate and clofibrate were active tumor promoters at micromolar concentrations. All 3 drugs induced the differentiation of C3H/10T1/2 C18 cells to adipocytes. This latter finding confirms previously observed effects of the tumor promoter TPA.

Concentration-dependent inhibition of development of GGT positive foci in rat liver by the environmental contaminant di(2-ethylhexyl) phthalate

The ability of di(2-ethylhexyl) phthalate (DEHP), a widely used plasticizer and environmental contaminant, to suppress development of putative preneoplastic lesions in rat liver was evaluated. gamma-Glutamyl transpeptidase-positive (GGT+) foci were initiated in the livers of Sprague-Dawley male rats with a single dose of diethylnitrosamine (DEN) following partial hepatectomy. Promotion of foci was commenced by feeding a choline-deficient diet (CD). A group of control rats was fed a choline-supplemented diet (CS). The ability of DEHP to suppress the emergence of GGT+ foci was evaluated by feeding additional groups of rats the CD diet containing either 0.1%, 0.5%, 1.0% or 2.0% DEHP. The CD diet promoted the number of GGT+ foci above levels in control livers. Inclusion of the plasticizer to the levels of 0.5%, 1.0% and 2.0% in the CD diet effectively inhibited the appearance of the foci. However, DEHP was unable to inhibit the promoting effect of the CD diet at a concentration of 0.1%. DEHP's ability to block development of GGT+ foci correlated with its ability to increase liver weight and to induce carnitine acetyltransferase (EC 2.3.1.7), a marker of peroxisome proliferation.

Absence of a promoting or sequential syncarcinogenic effect in rat liver by the carcinogenic hypolipidemic drug nafenopin given after N-2-fluorenylacetamide

The hypolipidemic agent nafenopin, (NF), has been reported to be carcinogenic to rat liver. To determine whether nafenopin exerts a promoting or syncarcinogenic effect in rat liver, its effect on liver carcinogenesis induced by N-2-fluorenylacetamide (FAA) was studied. In two separate experiments, male F344 rats were fed 0.02% FAA for either 10 or 8 weeks to induce preneoplastic liver lesions. Following a recovery period of 1 week, rats were given 0.01 or 0.02% NF in the diet for 23 weeks in one experiment and 0.05 or 0.1% for 24 weeks in the other. The final incidence of neoplasms, and their numbers, size distribution, and degrees of differentiation were not significantly different in groups given NF after FAA compared to those maintained on a basal diet after FAA. In the group treated with the highest dose level of NF following FAA, however, there was a decrease in the number of grossly visible small neoplasms. In contrast, the liver neoplasm promoter phenobarbital increased the multiplicity, although not the incidence, of liver neoplasms when given after FAA. Thus, four different dose levels of NF showed no promoting or syncarcinogenic effect on FAA-induced hepatocarcinogenesis.

Inhibition of phenobarbital and dietary choline deficiency promoted preneoplastic lesions in rat liver by environmental contaminant di(2-ethylhexyl)phthalate

The effect of di(2-ethylhexyl)phthalate (DEHP), a widely used plasticizer and environmental contaminant, on the emergence of gamma-glutamyltranspeptidase positive (GGT+) preneoplastic foci in the liver of rats fed promoting diets was studied. GGT+ foci were initiated in the liver of Sprague--Dawley male rats with a single dose of diethylnitrosamine (DEN) following partial hepatectomy. One series of control rats received saline vehicle alone. Promotion of foci was commenced by feeding: (1) a choline-deficient diet (CD); (2) a choline-supplemented diet (CS) containing 0.06% phenobarbital (CS + PHB); or (3) a CD diet containing 0.06% phenobarbital (CD + PHB). In the absence of initiation by DEN, dietary treatments did not increase the number of GGT+ foci. In rats receiving DEN, each promoting regimen effectively increased the number of GGT+ foci above levels in control rats fed only the choline-supplemented diet. Inclusion of the plasticizer at a level of 2% in each of the dietary promotion treatments, however, effectively inhibited the appearance of the foci.

Role of oxygen radicals in tumor promotion

Tumor promoters provoke the elaboration of oxygen radicals by direct chemical generation and through the indirect activation or alteration of cellular sources including membrane oxidases, peroxisomes, and electron transport chains in mitochondria and endoplasmic reticulum. Although direct measurement of amplified oxygen radical production in response to tumor promoters in target tissues remains problematic, studies with scavengers of reactive oxygen species demonstrate inhibition of biochemical and biological sequelae of tumor promoter exposure and provide strong presumptive evidence for oxygen radical involvement in this late stage of carcinogenesis. The critical macromolecular targets for these oxygen radicals remain undefined; however, they may include lipids, DNA, DNA repair systems, and other enzymes.

Effect of di(2-ethylhexyl) phthalate on DNA repair and lipid peroxidation in rat hepatocytes and on metabolic cooperation in Chinese hamster V-79 cells

Experiments were conducted to test the hypothesis that the hepatocarcinogenicity of di(2-ethylhexyl) phthalate (DEHP) is due to its ability to produce DNA damage, either directly or as a result of the proliferation of peroxisomes and accompanying increased production of H2O2 and other DNA--damaging oxygen radicals induced by sustained exposure to the plasticizer. DNA repair, as assessed by the autoradiographic measurement of unscheduled DNA synthesis (UDS), was not observed in primary rat hepatocytes exposed in vitro to 10(-5)-10(-2) M DEHP or in vivo by a single gavage dose of 5 g DEHP/kg body weight administered 2, 15, or 24 h prior to the isolation of hepatocytes. Thus, DEHP does not appear to directly produce repairable DNA damage in rat hepatocytes. Sustained feeding of DEHP at a dietary concentration of 2% led to a marked proliferation of peroxisomes in the liver after 4 wk. Additional administration of a single gavage dose of 5 g DEHP/kg body weight to animals fed the 2% diet for 4 or 8 wk, as well as to 4-wk-fed animals that were also pretreated with 3-amino-1,2,4-triazole to inhibit endogenous catalase activity, did not induce any detectable DNA repair in hepatocytes isolated 15 h following the single gavage dose of DEHP. Lipid peroxidation measured in the 9000 X g supernatant of livers from animals treated with a single dose of 5 g DEHP/kg body weight or the 2% DEHP diet for 6 wk plus a single dose of 5 g/kg body weight did not differ from controls. These findings suggest that DEHP does not elicit DNA damage or lipid peroxidation in liver consequent to the proliferation of peroxisomes resulting from prolonged administration. In addition, at noncytotoxic concentrations DEHP failed to produce a positive response in the Chinese hamster V-79 metabolic cooperation assay for tumor promoters.

Inhibition of development of preneoplastic lesions in the livers of rats fed a weakly carcinogenic environmental contaminant

The effects of di(2-ethylhexyl)phthalate (DEHP), a widely used plasticizer and environmental contaminant, on the emergence of gamma-glutamyltrans-peptidase positive (GGT+) preneoplastic foci in the liver was investigated. Sprague--Dawley male rats initiated with diethylnitrosamine (DEN) following partial hepatectomy were placed on: (1) a choline supplemented diet (CS); (2) a CS diet containing 2% DEHP (CS + DEHP); (3) a choline deficient diet (CD); (4) a CD diet containing a 0.06% phenobarbital (CD + PHB); or (5) a CD diet containing 2% DEHP (CD + DEHP). Rats maintained on the CS + DEHP diet for 5 and 10 weeks showed no increase in GGT + foci. The plasticizer effectively inhibited the appearance of the preneoplastic foci when it was included with the CD diet.

Carcinogenesis by hepatic peroxisome proliferators: evaluation of the risk of hypolipidemic drugs and industrial plasticizers to humans

In this critical review, I would like to provide a brief outline of the morphology, biochemical composition, distribution, and functions of peroxisomes. The induction of peroxisome proliferation and peroxisome-associated enzymes in the rodent liver by two classes of chemicals (hypolipidemic drugs and the industrial plasticizers) will be considered. The role of peroxisomes in lipid metabolism will be discussed. Carcinogenicity studies in rats and mice with these peroxisome proliferators will be evaluated critically. Careful consideration will be given to the hypothesis that "potent hepatic peroxisome proliferators as a class are carcinogenic." The possible mechanism(s) by which peroxisome proliferators induce liver tumors will be outlined. Particular attention will be paid to the possible role of peroxisome proliferation-mediated radical toxicity and generation of endogenous initiators of carcinogenesis.

Phthalate esters as peroxisome proliferator carcinogens

The phthalate ester di(2-ethylhexyl) phthalate is both a peroxisome proliferator and a hepatic carcinogen. Peroxisome proliferators as a class are hepatocarcinogenic in rodent species. However, none of the peroxisome proliferators tested to date including the phthalate esters and related alcohol and acid analogs have demonstrated mutagenic or DNA-damaging activity in the in vitro Salmonella typhimurium/microsomal or the lymphocyte 3H-thymidine assays. A working hypothesis is proposed that peroxisome proliferation itself initiates neoplastic transformation of hepatic parenchymal cells by increasing intracellular rates of DNA-damaging reactive oxygen production. Evidence which supports such a hypothesis includes increased fatty acid beta-oxidation, elevated H2O2 levels, accumulation of peroxidized lipofuscin, disproportionately small increase in catalase, and elevated peroxisomal uricase activity which accompany peroxisome proliferation in hepatocytes. Direct testing of this hypothesis will provide insight into mechanisms of phthalate ester carcinogenicity and cytotoxicity.

Adaptive responses of rat liver to the gestagen and anti-androgen cyproterone acetate and other inducers. II. Induction of growth

Treatment of female Wistar rats with cyproterone acetate (CPA) leads to considerable enlargement of the liver. The organ content of water, dry mass, protein, RNA, and DNA increased in parallel with the enlargement; only lipid accumulation showed a slight excess. The changes were maximal after treatment for 3 days and increased in a dose-dependent manner, the threshold dose being 5--10 mg CPA/kg DNA synthesis was strongly enhanced after a lag phase of 12--14 h; a maximal rate of synthesis was attained after 18--24 h. The number of parenchymal cells involved in DNA synthesis and mitosis were increased up to 20-fold. Sinusoidal cells participated only slightly in the growth process, and their number decreased relative to the number of parenchymal cells. These results indicate that CPA induces (presumably adaptive) liver growth essentially by parenchymal hyperplasia; of the model inducers used for comparison only pregenolone-16 alpha-carbonitrile (PCN) produced a similarly strong hyperplastic response while liver enlargement elicited by a alpha-hexachlorocyclohexane (alpha-HCH), phenobarbital (PB) and 3-methylcholanthrene (3-MC) was partly or exclusively due to hypertrophy. Liver growth was also observed in male rats treated with CPA, but was less pronounced in this sex. After discontinuation of treatment, liver enlargement and the increase of DNA regressed partially within 1--3 weeks; this regression seemed to be due to sequestration of old cells which were not involved in replication after CPA treatment. The relationship between induction of liver growth by CPA and hepatoma formation in rats is discussed.

Effect of host-cell interactions on clonogenic carcinoma cells in human malignant effusions

We have studied the clonogenic capacity of tumour cells in agar from 38 malignant effusions from 31 patients with epithelial tumours. Colony formation of unfractionated cells, varies considerably from patient to patient, and is positively correlated with the percentage of tumour cells in the sample. Clonogenicity was shown to be reduced in 8/9 cases by removal of plastic-adherent and iron-phagocytic cells. In the ninth case, increased clonogenicity occurred after this procedure. When the autologous adherent cells were removed from the effusion and used in reconstitution experiments as an underlayer in a two-layer agar system, they were able to reverse the effect of the initial fractionation in a dose-dependent fashion. This indicates cellular communication based on release of a diffusible product of plastic-adherent cells. Morphological, phagocytic and prostaglandin-synthetic analysis of the fractions involved in the reconstitution experiments implicate the macrophage as the operative cell in this interaction. However, an accessory role for lymphoid cells or tumour cells themselves cannot be excluded.

Tumorigenicity of the hypolipidaemic peroxisome proliferator ethyl-alpha-p-chlorophenoxyisobutyrate (clofibrate) in rats

Ethyl-alpha-p-chlorophenoxyisobutyrate), a hypolipidaemic drug which induces hepatomegaly and proliferation of peroxisomes in liver cells of rats and mice, was fed to 15 male F344 rats at a dietary concentration of 0.5% (v/w) for up to 28 months. Hepatocellular carcinomas developed in 10/11 (91%) rats killed between 24 and 28 months. Other tumours included carcinoma of the pancreas (2 rats), leiomyoma of the small intestine (1 rat) and a large dermatofibrosarcoma (1 rat). Clofibrate is the third hypolipidaemic peroxisome proliferator demonstrated to be hepatocarcinogenic in rats. These studies suggest that hypolipidaemic agents which are capable of producing a sustained hepatomegalic and peroxisome-proliferative effect also induce liver tumours.