The potential role of chemically induced hyperplasia in the carcinogenic activity of the hypolipidemic carcinogens

Reduced camptothecin sensitivity of estrogen receptor-positive human breast cancer cells following exposure to di(2-ethylhexyl)phthalate (DEHP) is associated with DNA methylation changes

Di(2-ethylhexyl)phthalate (DEHP) has been considered as an estrogen receptor alpha (ERα) agonist due to its ability to interact with ERα and promote the cell proliferation of ERα-positive breast cancer cells. The impact of DEHP on the chemical therapy in breast cancer is little known. Two breast cancer cell lines, MCF-7 (ERα-dependent) and MDA-MB-231 (ERα-independent) were examined. We found that DEHP impaired the effectiveness of camptothecin (CPT) and alleviated the CPT-induced formation of reactive oxygen species in ERα-positive MCF-7 cells, but not in ERα-negative MDA-MB-231 cells. DEHP also significantly protected MCF-7 cells against the genotoxicity of CPT. Genome-wide DNA methylation profiling revealed that after 48 hours of exposure to 100 μM DEHP, MCF-7 cells exhibited a significant change in their DNA methylation pattern, including hypermethylation of 700 genes and hypomethylation of 221 genes. The impaired therapeutic response to CPT in DEHP-exposed MCF-7 cells is probably mediated by epigenetic changes, especially through Wnt/β-catenin signaling. A zebrafish xenograft model confirmed the disruptive effect of DEHP on CPT-induced anti-growth of MCF-7 cells. In summary, DEHP exposure induces acquired CPT-resistance in breast cancer cells and epigenetic changes associated with Wnt/β-catenin signaling activation are probably depending on an ER-positive status.

Experimental Model for Studying the Effects of 2-Ethylhexyl-Phthalate and Dialysate on Connective Tissue

In order to have a model for studying the possible implications of 2-ethylhexyl-phthalate and dialysate on connective tissue, we evaluated their direct effects on the air pouch lining tissue and on fibroblast cultures. Air pouches were formed on the backs of 60 ten-week-old Wistar rats by subcutaneous injections of 10 ml sterile air. On the tenth day 2 ml sterile air, or 2 ml 5μg/L or 2 ml 10 μg/L 2-ethylhexyl-phthalate in olive oil, or 2 ml olive oil alone, or 2 ml 5 mg/ml or 12 mg/ml lyophilized dialysate were injected into the air pouches. After sampling at seven or twenty-one days, the rats were killed. The biochemical data showed an increase in sulphated glycosaminoglycans with 2-ethylhexyl-phthalate and dialysate. Electron microscopy findings revealed cellular alterations such as vacuolation and cell remnants with 2-ethylhexyl-phthalate, while the cells of the air pouches treated with dialysate showed regular organelles with increased and dilated cisternae of rough endoplasmic reticulum. Moreover, an increase in collagen fibres surrounding the damaged zones was noticed in 2-ethylhexyl-phthalate and dialysate treated rats. The glycosaminoglycan modifications and collagen fibre increase seem to suggest that the morfological changes, with the features of fibrosis, could be the result of 2-ethylhexyl-phthalate and dialysate action on connective tissue. Moreover, the air pouch technique can be considered a good model for studying the direct effects of 2-ethylhexyl-phthalate and other substances, such as uremic toxins, on connective tissue.

A Comprehensive, Integrated Review and Evaluation of the Scientific Evidence Relating to the Safety of the Herbicide 2,4-D

The safety of 2,4-D to farm and forestry workers, commercial applicators and the general public has been of continuing concern because certain epidemiological studies of groups potentially exposed to 2,4-D have suggested a relationship between 2,4-D use and increased risk of soft tissue sarcoma, Hodgkin's disease or non-Hodgkin's lymphoma. This review on 2,4-D is unique in that the approach taken was to integrate data from worker exposure studies, whole animals, metabolic and other relevant laboratory studies with the epidemiological findings to assess the extent to which there is scientific support for the hypothesis that 2,4-D exposure is associated with any increased risk of human cancer.

The case-control epidemiological studies that have been the source of the cancer risk hypothesis are inconclusive. Problems in assessing exposure based on patients' memories make these studies difficult to interpret. Cohort studies of exposed workers do not generally support the specific hypothesis that 2,4-D causes cancer. Taken together, the epidemiological studies provide, at best, only weak evidence of an association between 2,4-D and the risk of cancer.

Importantly, the cancer hypothesis is not supported by other data. A critical evaluation of the exposure data indicates that exposure to 2,4-D in user groups is intermittent and much lower than the doses tested chronically in long-term animal studies that have not shown evidence of tumor induction. Moreover, the structure of 2,4-D does not suggest it would be a carcinogen. 2,4-D is a simple organic acid, that is largely excreted unchanged, and there is no evidence that it is metabolized to critically reactive metabolites or accumulates in tissues. This evidence is supported by a large body of negative studies on genotoxicity, which taken together with the metabolic studies, clearly indicates that 2,4-D is highly unlikely to be a genotoxic carcinogen. Furthermore, 2,4-D has no known hormonal activity and does not induce proliferative changes in any tissue or organ, indicating that it does not possess any of the characteristics of non-genotoxic animal carcinogens. Thus the available mechanistic studies provide no plausible basis for a hypothesis of carcinogenicity.

In this review, data relating to potential neurotoxicity, immunotoxicity and reproductive toxicity also were evaluated. There is no evidence that 2,4-D adversely affects the immune system and neurotoxic and reproductive effects only have been associated with high toxic doses that would not be encountered by 2,4-D users.

Historical exposures to 2,4-D by user groups, particularly farmers, forestry workers and commercial applicators, would be higher than those sustained under present rigorous standards for application which involve the use of protective clothing and other measures to reduce exposure. Proposed label changes indicate that in the future exposures will be even further reduced. Viewed in this context, the available data indicate that the potential public health impact of 2,4-D, including the risk of human cancer, was negligible in the past and would be expected to be even smaller in the present and future.

Effect of peroxisome proliferators on Leydig cell peripheral-type benzodiazepine receptor gene expression, hormone-stimulated cholesterol transport, and steroidogenesis: role of the peroxisome proliferator-activator receptor alpha

In this study, we hypothesized that many of the reported effects of phthalate esters and other peroxisome proliferators (PPs) in the testis are mediated by members of the PP- activated receptor (PPAR) family of transcription factors through alterations in proteins involved in steroidogenesis. Exposure of Leydig cells to PPs prevented cholesterol transport into the mitochondria after hormonal stimulation and inhibited steroid synthesis, without altering total cell protein synthesis or mitochondrial and DNA integrity. PPs also reduced the levels of the cholesterol-binding protein peripheral-type benzodiazepine receptor (PBR) because of a direct transcriptional inhibition of PBR gene expression in MA-10 Leydig cells. MA-10 cells contain mRNAs for PPARalpha and PPARbeta/delta, but not for PPARgamma. In vivo treatment of mice with PPs resulted in the reduction of both testis PBR mRNA and circulating testosterone levels, in agreement with the proposed role of PBR in steroidogenesis. By contrast, liver PBR mRNA levels were increased, in agreement with the proposed role of PBR in cell growth/tumor formation in nonsteroidogenic tissues. However, PPs did not inhibit testosterone production and testis PBR expression in PPARalpha-null mice. These results suggest that the antiandrogenic effect of PPs is mediated by a PPARalpha-dependent inhibition of Leydig cell PBR gene expression.

Induction of DNA synthesis in mouse liver following increases of DNA adduct levels elicited by very low cumulative doses of the genotoxic hepatocarcinogen 7H-dibenzo[c,g]carbazole

The purpose of this work was to investigate the administration of very low but repeated doses of a genotoxic carcinogen and an eventual correlation with cellular DNA synthesis. The compound 7H-dibenzo[c,g]carbazole is a genotoxic carcinogen in the mouse liver and was administered topically at the dose of 13.35 microg per animal every 2 days to give a total of 13 applications. Animals were sacrificed 48 hours after every 2 applications until the 10th treatment, then 48 hours after every treatment. Postulated genotoxic effects such as DNA adduct formation were detected by the 32P-post labeling assay. Liver sections were examined for microscopic changes and DNA synthesis. Results showed an increase of the total DNA adduct level in the liver throughout the study with a slowing down in the level after the sixth application of the compound. This change could correspond to the onset of DNA synthesis and to the moderate hepatocellular apoptosis which was observed. The DNA synthesis, which was considered to be secondary to the cytotoxicity induced by the high level of DNA adducts altering normal cellular activity, could also be the opportunity to fix the DNA adducts into heritable mutations. These results raise the question regarding the risk assessment in humans exposed regularly to very low doses of chemicals in the environment: for non-proliferating tissue, the regular accumulation of DNA adducts could remain silent until a "threshold level" is reached from which stimulation of the DNA synthesis may fix the DNA adducts into heritable mutations, eventually leading to tumors.

Weight-of-evidence versus strength-of-evidence in toxicologic hazard identification: Di(2-ethylhexyl)phthalate (DEHP)

Toxicokinetic and mode of action data for DEHP reduce the concern for its potential carcinogenic hazard to human health. Chronic, high dose ingestion of DEHP and related peroxisome proliferators (PP) by mice and rats precipitate the following: activation of peroxisome proliferator activated receptor (PPARalpha) and its binding to peroxisome proliferator response elements (PPREs) within promoters of PP-responsive genes, peroxisome proliferation, increased microsomal fatty acid oxidation, increased hepatic hydrogen peroxide, hepatomegaly, hyperplasia and subsequent neoplasia. Neither peroxisome proliferation nor increased liver cancer occur in patients treated with pharmacologic doses of PP. Species differences in endogenous PPARalpha expression and differential activity of the peroxisome proliferator response element (PPRE) contribute to the failure of humans to respond in a manner qualitatively similar to that of rats or mice. Where it can be demonstrated that a mechanism for rodent tumor formation has no relevance for humans, then a substance which elicits a carcinogenic response in the test species via that mechanism should not be classified as anything other than an animal carcinogen. Systemic noncarcinogenic endpoints are available for definition of a DEHP reference dose. Considerable difficulty is encountered in the revision of promulgated regulations and in public risk communication when a material is no longer considered a carcinogenic hazard to humans.

Cloning genes responsive to a hepatocarcinogenic peroxisome proliferator chemical reveals novel targets of regulation

To better understand the molecular basis of the hepatocyte proliferation and induction of hepatocellular adenomas by exposure to peroxisome proliferator chemicals (PPC), a systematic search for genes modulated by a PPC (WY-14643) in rat liver was carried out using the differential display technique. The fragments fell into two classes based on the time of initial and maximal induction by WY-14643. The class I genes (clones 5 and 30) were induced 3 h after a gavage exposure to WY-14643 with maximal expression at 24 h. The class II genes (clones 13 and 16) were induced after 24 h with maximal expression at 78 weeks. Expression of the class II genes was also increased after other treatments that cause cell proliferation. Clone 30 was identified as CYP4A2, previously shown to be regulated by PPC. Clone 13 was homologous to the mouse protein H gene, a component of the heterogeneous nuclear ribonucleoprotein particle important in mRNA splicing. Clone 16 was identified as cyclophilin-A, the receptor for the immunosuppressant drug cyclosporin A. The sequence of clone 5 was unique. These data demonstrate that WY-14643 increases the levels of a number of novel genes that are coordinately regulated with increases in chronic cell proliferation and fatty acid metabolism.

Induction of hepatic poly(ADP-ribose) polymerase by peroxisome proliferators, non-genotoxic hepatocarcinogens

Two peroxisome proliferators, [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid (Wy-14,643) or di(2-ethylhexyl) phthalate (DEHP), were given orally to male F-344 rats for up to 78 or 97 weeks. At 1 week, the activity of poly(ADP-ribose) polymerase (pADPRP) was increased 2- and 1.8-fold in the liver of rats treated with Wy-14,643 and DEHP, respectively. The induction of the activity was maintained at 2.5- or 2-fold for up to 52 weeks. The immunoblot and Northern blot analyses revealed that the induction of pADPRP activity would be responsible for the increase in the amount of mRNA. In addition, in the liver tumor induced by Wy-14,643 and DEHP, the pADPRP mRNA level increased 3.6- or 3.7-fold. The magnitude of the increase in the mRNA level was higher than that in the non-tumor portion. These findings suggest that the induction of pADPRP may play an important role in the hepatocarcinogenesis induced by peroxisome proliferators.

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.

Incidence of pancreatic and testicular tumors in rats treated with ciprofibrate, a peroxisome proliferator

It is hypothesized that hepatic tumors in rats induced by peroxisome proliferators is dependent on peroxisome proliferative effect of these compounds and the resulting oxidative stress. However, it is argued that since these compounds also induce tumors in pancreas and testes, the two organs in which there is no proliferation of peroxisomes, the carcinogenic effect is unlikely to be related to oxidative stress. To clarify this controversy we have systematically analyzed the incidence of pancreatic acinar cell foci and nodules, and testicular Leydig cell tumors in ciprofibrate treated and control rats. In animals treated with 0.025% ciprofibrate for 22 months the incidence of Leydig cell tumors and acinar cell lesions was 100% and 66%, respectively. In age-matched controls the incidence of testicular and pancreatic lesions was 93% and 66%, respectively. These findings clearly demonstrate that the Leydig cell tumors and pancreatic lesions develop spontaneously and are not induced by ciprofibrate.

Sequential appearance and ultrastructure of amphophilic cell foci, adenomas, and carcinomas in the liver of male and female rats treated with dehydroepiandrosterone

Dehydroepiandrosterone (DHEA), a hormone of the adrenal cortex, acts as a peroxisome proliferator and hepatocarcinogen in rats upon long-term treatment with high doses in the diet. The aim of the present study was to identify the site of origin of hepatocellular neoplasms and the sequence of preneoplastic lesions. Twenty-five female and 25 male rats were given 0.6% DHEA in the diet; 25 animals of each sex were controls. Groups of 5 treated and untreated animals were sacrificed after 4, 20, 32, 70, and 84 wk. Amphophilic cell foci were detected after 32 wk of treatment; they developed from the liver parenchyma almost exclusively in the vicinity of portal tracts. Adenomas of the amphophilic or amphophilic/tigroid cell phenotype were observed at 70 wk of treatment. Highly differentiated hepatocellular carcinomas presenting a similar cellular phenotype occurred after 70-84 wk. The incidence of hepatocellular carcinomas was 44% in female and 11% in male rats. Ultrastructural studies of the amphophilic cell foci and tumors revealed a marked proliferation of mitochondria and a moderate proliferation of peroxisomes in all lesions. In addition, a very strong peroxisome proliferation was observed in perivenular hepatocytes in the liver of female rats. Peroxisomes usually lacked core and showed flocculent matrices. In male rats, weak peroxisomal proliferation was observed. Typical morphological abnormalities of these peroxisomes were paracrystalline inclusions of striated appearance. Although the most prominent peroxisome proliferation was observed in perivenular hepatocytes, these cells did not seem to be involved in tumor development. In contrast, the morphological similarity of the amphophilic cell foci and the amphophilic/tigroid cell adenomas and carcinomas, their coincident localization near portal tracts, and the sequential appearance of these lesions suggest that the amphophilic cell foci represent an early stage in DHEA-induced hepatocellular neoplasia. Mitochondrial proliferation as the most prominent feature in all stages of this model of hepatocarcinogenesis may offer a new approach for analysis of hepatocarcinogenesis induced by DHEA and possibly other peroxisomal proliferators.

Induction of hepatic microsomal CYP4A activity and of peroxisomal beta-oxidation by two non-steroidal anti-inflammatory drugs

The effects of the non-steroidal anti-inflammatory drugs fenbufen and ibuprofen on hepatic cytochrome P450 activities and peroxisomal proliferation were investigated in the rat, following intraperitoneal administration at three dose levels. At the two highest doses, 30 and 150 mg/kg, ibuprofen stimulated lauric acid hydroxylase activity but no other dose-dependent effects on cytochrome P450 activities were evident. Fenbufen, at the highest dose of 150 mg/kg, decreased cytochrome P450 content and related activities, and this effect was attributed to the toxicity of the drug at this dose. Immunoblot studies employing solubilized microsomes from ibuprofen-treated rats revealed that ibuprofen increased the apoprotein levels of CYP4A1, at the two higher doses. The same treatment with ibuprofen, at the highest dose only, increased the beta-oxidation of palmitoyl CoA, determined in liver homogenates, and immunoblott analysis showed an increase in the apoprotein levels of the trans-2-enoyl CoA hydratase trifunctional protein. Fenbufen did not influence palmitoyl beta-oxidation. Computer graphic overlays with clofibric acid showed that ibuprofen, when compared with fenbufen, displayed a better overall fit to clofibric acid. Finally, interaction energies between the two drugs and the putative peroxisome proliferator-activated receptor ligand domain revealed that ibuprofen had a higher affinity for the receptor than fenbufen, but the difference was modest. It is concluded that ibuprofen, at doses far exceeding those employed clinically, is a weak inducer of both CYP4A1 activity and peroxisomal proliferation and these effects may be attributed to the presence of an aryl propionic acid moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic peroxisome proliferation in rodents and its significance for humans

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

Genotoxic effects of selected peroxisome proliferators

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

The influence of fenarimol on DNA synthesis and mitotic activity in rat liver

Fenarimol administered in one single oral dose of 125 or 62.5 mg kg-1 body wt. day-1, respectively, stimulated rat liver enlargement at a dose-independent rate. Three single doses of fenarimol produced dose-dependent liver growth, whereas five single doses caused no further increase in liver weight. This increase was accompanied by an increase in hepatic DNA synthesis and mitotic activity, with a peak on the first day after the beginning of the experiments. The increase in binuclear hepatocytes and signs of necrosis suggested that the hepatomitogenic effect reflected a compensatory hyperplasia. After both three and five single doses the hepatomitogenic effect was suppressed, as a result of tolerance development.

Inhibition of mitochondrial respiration and oxygen-dependent hepatotoxicity by six structurally dissimilar peroxisomal proliferating agents

The purpose of this study was to test the hypothesis that a variety of structurally dissimilar peroxisomal proliferators inhibited O2 uptake and caused O2-dependent hepatotoxicity in the perfused rat liver. Aspirin, valproate, ethylhexanol, clofibric acid, ciprofibrate and perfluorooctanoate were selected as a representative group of weak, moderate, and potent peroxisomal proliferators, respectively. All compounds studied inhibited state 3 but not state 4 rates of oxygen uptake in isolated mitochondria (perfluorooctanoate greater than ciprofibrate greater than ethylhexanol greater than clofibric acid greater than aspirin greater than valproate; half maximal inhibition occurred at concentrations ranging from 0.6 to 3.2 mM depending on the compound). Clofibric acid, ethylhexanol and aspirin inhibited oxygen uptake only in upstream, oxygen-rich periportal regions of the perfused liver lobule by 30-40%. Perfusion with the six agents studied caused release of lactate dehydrogenase into the effluent perfusate in a dose-dependent manner and caused damage predominantly in periportal regions of the lobule as reflected by trypan blue uptake. A strong correlation between the concentration of compound needed to inhibit respiration in isolated mitochondria and cause hepatotoxicity in the perfused liver was observed. We propose that peroxisomal proliferators accumulate in the liver due to their lipophilicity where they inhibit actively respiring mitochondria in periportal regions of the liver lobule and cause local toxicity.

Changes in expression of cellular oncogenes and endogenous retrovirus-like sequences during hepatocarcinogenesis induced by a peroxisome proliferator

Previous studies have demonstrated that BR-931, a hepatic peroxisome proliferator, can induce liver tumours in mice and rats. Since alterations in gene expression may play a critical role in multistage hepatocarcinogenesis, the present studies examined the expression of the c-myc, c-H-ras, epidermal growth factor (EGF) receptor and ODC (ornithine decarboxylase) genes, as well as endogenous retrovirus-like sequences, in F344 rat liver during the first 8 weeks of feeding a 0.16% Br931 diet and in liver tumours induced by chronic feeding of this diet. Northern blot analysis of poly A + liver RNA samples showed an increase in the level of RNAs homologous to rat leukaemia virus (RaLV) but no significant change in the level of 30S-retrovirus related RNAs in the liver RNA samples obtained from rats during the first 8 weeks of feeding the diet containing BR931. An increase in the levels of c-myc, c-H-ras and ODC transcripts was also seen in the liver RNA samples from the treated rats. Of particular interest was a decrease in the abundance of EGF receptor transcripts in the liver RNA samples from rats fed the BR931 diet. Increased levels of RaLV, c-myc, and ODC RNAs were also seen in the tumours induced by BR931, but this was not the case for 30S and c-H-ras. The liver tumour samples also showed a decrease in EGF receptor RNA. These changes in cellular levels of specific RNAs resemble, in several respect, those we previously described in rodent liver during regeneration and tumour promotion, and also those seen in rodent hepatomas induced by other agents. Therefore, they may reflect a common profile of gene expression relevant to liver proliferation and carcinogenesis.

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.

Quantitative analysis of hepatocellular lesions induced by di(2-ethylhexyl)phthalate in F-344 rats

Di(2-ethylhexyl)phthalate (DEHP), a peroxisome proliferator, has been shown to be a weak hepatocarcinogen in rats and mice. However, in previous studies no quantitative analysis of tumors was carried out. In the present study, F-344 male rats were given a diet containing 2% DEHP ad libitum for 108 wk. At necropsy livers were quantitatively analyzed for total tumor incidence and the number of lesions per liver after slicing the entire organ at 1- to 2-mm intervals. Neoplastic nodules and/or hepatocellular carcinomas were observed in 11 of 14 rats (78.5%). When evaluated according to the size, 57, 16, and 36% rats contained nodules ranging from 1 to 3, 3 to 5, and greater than 5 mm in size, respectively. The number of nodules per liver ranged from zero to four. These results indicate that DEHP induces tumors in a large number of animals at 2% dose levels. It is clear from this study that when a weak peroxisome proliferator is evaluated for carcinogenic effects, a complete and thorough gross examination of the liver is essential to obtain accurate tumor incidence.

Renal tubular cell or hepatocyte hyperplasia is not associated with tumor promotion by di(2-ethylhexyl)phthalate in B6C3F1 mice after transplacental initiation with N-nitrosoethylurea

B6C3F1 mice of both sexes that had been exposed transplacentally on day 18 of gestation to 0.5 mmole N-nitrosoethylurea (NEU) were fed either normal diets or diets containing di(2-ethylhexyl)phthalate (DEHP) at 6,000 ppm beginning at 6 wk of age and continuing to 78 wk of age. At 52 and 78 wk of age, 6-26 mice from each group received a single injection of 5-bromo-2'-deoxyuridine (Brdu) at 200 mg/kg i.p. and were sacrificed 1 h later for determination of the levels of renal and hepatic DNA synthesis by the Brdu immunohistochemical technique. No differences occurred in incidences of gross or microscopic renal tubular cell tumors between the NEU (males 15%, females 21%) and NEU-DEHP groups (males 10%, females 15%) at 78 wk. The labelling index (LI) of renal cortical tubular cells was significantly increased at 78 wk (22.3 +/- 3.7/mm2 for males, 21.8 +/- 1.2 for females) in mice given NEU and DEHP as compared with NEU alone (9.7 +/- 1.0 for males, 6.9 +/- 0.7 for females). The number and sizes of focal hepatocellular proliferative lesions (FHPL), including hyperplastic foci, hepatocellular adenomas and carcinomas, were quantified by image analysis and stereology. DEHP significantly enhanced the mean volume and volume % of FHPL, including liver tumors, but not numbers of FHPL/liver. Hepatocyte LI was also not affected, at least as detected by the technique used, while FHPL had significantly increased LI (14.5-48.3) as compared with normal hepatocytes (0.5-2.4). This study provides some evidence that enhanced chronic cell replication in the kidney may not always be associated with renal carcinogenesis of tumor promotion, while tumor promotion in liver may be a consequence of increased DNA synthesis in initiated or focus cells rather than in nonproliferative parenchymal hepatocytes, which may not be target cells of some tumor promoters.

Evaluation of liver cell proliferation during ciprofibrate-induced hepatocarcinogenesis

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

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 17. Can a mechanistic rationale be provided for non-genotoxic carcinogens identified in rodent bioassays?

In a recent survey of the results of the National Cancer Institute/National Toxicology Program's Carcinogenesis Bioassay Program, Ashby and Tennant (1988) drew attention to the high proportion of carcinogens that were non-genotoxic insofar as their response to the Salmonella-microsome test was concerned. The present review contrasts these findings with what is known mechanistically about non-genotoxic carcinogens that affect the tissues which are considered to be particularly prone to non-genotoxic tumor induction. Excessive and often thresholded increases in cellular proliferation in the affected tissues appear to be one common feature in tumor induction by these agents, which act either through cytotoxicity followed by regeneration or through hormone-mimetic action. It is suggested that a weight of the evidence approach on a chemical by chemical basis is necessary to decide the relevance of these agents to the human situation.

The chronic hepatic or renal toxicity of di(2-ethylhexyl) phthalate, acetaminophen, sodium barbital, and phenobarbital in male B6C3F1 mice: autoradiographic, immunohistochemical, and biochemical evidence for levels of DNA synthesis not associated with carcinogenesis or tumor promotion

Male B6C3F1 mice, 6 weeks of age, were fed diets or water containing di(2-ethylhexyl) phthalate (DEHP) at 12,000 or 6000 ppm, acetaminophen (ACT) at 10,000 or 5000 ppm, sodium barbital (BBS) at 1000 ppm, or phenobarbital (PB) at 500 ppm for 40 weeks. Groups of six mice were terminated at 2, 8, 24, and 40 weeks for evaluation of liver and kidney weights, histopathology, and thymidine kinase (TK) activity in liver and kidney and levels of DNA synthesis, measured by tritiated thymidine [( 3H]T) autoradiography or bromodeoxyuridine (BrdU) immunohistochemistry. Liver weights, as percentage of body weight, were significantly elevated at most time intervals for mice exposed to all chemicals at each dose. The hepatocyte labeling indices (LI) with [3H]T autoradiography or BrdU immunocytochemistry were significantly elevated in mice fed DEHP at 12,000 ppm at 24 and 40 weeks or BBS and ACT at 2 weeks. LI were not elevated in mice fed PB. Hepatic TK activity was significantly elevated in mice fed DEHP, BBS, or ACT at Weeks 2 and 8. Histopathologic hepatic lesions were associated with these elevations, while hepatic lesions were not associated with changes in TK activity in PB-treated mice. In contrast, only DEHP and BBS induced toxic renal lesions. Persistent or transient elevation of the renal LI and TK activity accompanied renal toxicity. Thus, the hepatic toxin DEHP induced chronic renal hyperplasia without evidence of renal carcinogenicity or tumor promotion in previous studies at the doses used. ACT, a hepatotoxin, produced transient chronic hepatic hyperplasia without evidence of carcinogenicity in B6C3F1 mice in earlier studies at the same doses used. Thus, persistent or transient hepatic or renal hyperplasia was associated with carcinogenic or tumor promoting activity of these chemicals in some cases but not in others.

Leupeptin-mediated alteration of renal phagolysosomes: similarity to hyaline droplet nephropathy of male rats exposed to unleaded gasoline

alpha 2u-Globulin, a protein of hepatic origin found in the urine of male rats, is accumulated in the kidney cortex during exposure to unleaded gasoline and has been implicated in the development of fuel hydrocarbon-induced nephropathy and renal neoplasia. The principal morphological feature of gasoline-induced nephropathy is accumulation of hyaline droplets (enlarged secondary lysosomes or phagolysosomes) in epithelial cells of the proximal convoluted tubule S1 and S2 segments. Inhibition of cathepsin B (a major lysosomal peptidase) by treatment of male rats with leupeptin causes rapid accumulation of phagolysosomes and alpha 2u-globulin in the kidney very similar to gasoline exposure. Further, the renal cortical subcellular distribution of alpha 2u-globulin, determined with an electron microscopic immunochemical method, is almost totally confined to phagolysosomes following administration of either gasoline or leupeptin. These results, taken together, indicate that the mechanism of nephrotoxicity of gasoline involves inhibition of renal phagolysosomal proteolysis.