Non-genotoxic hepatocarcinogenesis: suppression of apoptosis by peroxisome proliferators

PPARα Signaling is Activated by Cocoa in Mouse Liver

In this study we evaluated in mouse liver the effects of cocoa on PPARα signaling. To this aim, mouse diet was supplemented with 10%, w/w, cocoa for one and two weeks. We quantified the expression of PPARα target genes and PPARα gene level and some parameters related to PPARα activation (hepato-somatic index, peroxisomal β-oxidation system and catalase activity). Moreover, we evaluated antioxidant capacity of cocoa by detecting the expression of CAT and SOD1 genes (known to be involved in oxidative balance) and hypolipidemic properties on serum triglycerides. We made a parallel treatment with 0.025%, w/w, ciprofibrate, a well-known PPARα activator, to quantify signal modulation by cocoa. It is known that PPARα activation by ciprofibrate is mediated by direct binding to the receptor and strongly induces expression of target genes. Our results show that cocoa weakly up-regulates PPARα target genes as a consequence of the modulation of the PPARα gene level and does not improve the triglyceride profile in blood. Finally, cocoa increased SOD1 gene expression suggesting an antioxidant effect.

PPARs: Interference with Warburg' Effect and Clinical Anticancer Trials

The metabolic/cell signaling basis of Warburg's effect ("aerobic glycolysis") and the general metabolic phenotype adopted by cancer cells are first reviewed. Several bypasses are adopted to provide a panoramic integrated view of tumoral metabolism, by attributing a central signaling role to hypoxia-induced factor (HIF-1) in the expression of aerobic glycolysis. The cancer metabolic phenotype also results from alterations of other routes involving ras, myc, p53, and Akt signaling and the propensity of cancer cells to develop signaling aberrances (notably aberrant surface receptor expression) which, when present, offer unique opportunities for therapeutic interventions. The rationale for various emerging strategies for cancer treatment is presented along with mechanisms by which PPAR ligands might interfere directly with tumoral metabolism and promote anticancer activity. Clinical trials using PPAR ligands are reviewed and followed by concluding remarks and perspectives for future studies. A therapeutic need to associate PPAR ligands with other anticancer agents is perhaps an important lesson to be learned from the results of the clinical trials conducted to date.

Applications of metabolomics for understanding the action of peroxisome proliferator-activated receptors (PPARs) in diabetes, obesity and cancer

The peroxisome proliferator-activated receptors (PPARs) are a set of three nuclear hormone receptors that together play a key role in regulating metabolism, particularly the switch between the fed and fasted state and the metabolic pathways involving fatty-acid oxidation and lipid metabolism. In addition, they have a number of important developmental and regulatory roles outside metabolism. The PPARs are also potent targets for treating type II diabetes, dyslipidemia and obesity, although a number of individual agonists have also been linked to unwanted side effects, and there is a complex relationship between the PPARs and the development of cancer. This review examines the part that metabolomics, including lipidomics, has played in elucidating the roles PPARs have in regulating systemic metabolism, as well as their role in aspects of drug-induced cancer and xenobiotic metabolism. These studies have defined the role PPARδ plays in regulating fatty-acid oxidation in adipose tissue and the interaction between aging and PPARα in the liver. The potential translational benefits of these approaches include widening the role of PPAR agonists and improved monitoring of drug efficacy.

Peroxisome proliferator-activated receptors in lung cancer

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily. Their discovery in the 1990s provided insights into the cellular mechanisms involved in the control of energy homeostasis; the regulation of cell differentiation, proliferation, and apoptosis; and the modulation of important biological and pathological processes related to inflammation, among others. Since then, PPARs have become an exciting therapeutic target for several diseases. PPARs are expressed by many tumors including lung carcinoma cells, and their function has been linked to the process of carcinogenesis in lung. Consequently, intense research is being conducted in this area with the hope of discovering new PPAR-related therapeutic targets for the treatment of lung cancer. This review summarizes the research being conducted in this area and focuses on the mechanisms by which PPARs are believed to affect lung tumor cell biology.

Origins of injection-site sarcomas in cats: the possible role of chronic inflammation-a review

The etiology of feline injection-site sarcomas remains obscure. Sarcomas and other tumors are known to be associated with viral infections in humans and other animals, including cats. However, the available evidence suggests that this is not the case with feline injection-site sarcomas. These tumors have more in common with sarcomas noted in experimental studies with laboratory animals where foreign materials such as glass, plastics, and metal are the causal agent. Tumors arising with these agents are associated with chronic inflammation at the injection or implantation sites. Similar tumors have been observed, albeit infrequently, at microchip implantation sites, and these also are associated with chronic inflammation. It is suggested that injection-site sarcomas in cats may arise at the administration site as a result of chronic inflammation, possibly provoked by adjuvant materials, with subsequent DNA damage, cellular transformation, and clonal expansion. However, more fundamental research is required to elucidate the mechanisms involved.

Low dose rotenone treatment causes selective transcriptional activation of cell death related pathways in dopaminergic neurons in vivo

Mitochondrial complex I inhibition has been implicated in the degeneration of midbrain dopaminergic (DA) neurons in Parkinson's disease. However, the mechanisms and pathways that determine the cellular fate of DA neurons downstream of the mitochondrial dysfunction have not been fully identified. We conducted cell-type specific gene array experiments with nigral DA neurons from rats treated with the complex I inhibitor, rotenone, at a dose that does not induce cell death. The genome wide screen identified transcriptional changes in multiple cell death related pathways that are indicative of a simultaneous activation of both degenerative and protective mechanisms. Quantitative PCR analyses of a subset of these genes in different neuronal populations of the basal ganglia revealed that some of the changes are specific for DA neurons, suggesting that these neurons are highly sensitive to rotenone. Our data provide insight into potentially defensive strategies of DA neurons against disease relevant insults.

Administration of ciprofibrate to lactating mothers induces PPARalpha-signaling pathway in the liver and kidney of suckling rats

It is well known that the hypolipidemic drug ciprofibrate induces peroxisome proliferation in rodent liver, which in turn leads to the oxidative stress, and modifies some parameters related to cell proliferation and apoptosis. The administration of ciprofibrate to rats during the lactating period determined in their pups significant modifications in hepatic peroxisome enzyme activities, induction of the PPARalpha-target gene, Cyp4a10, and perturbation in cell proliferation and apoptosis, which affected the size of the liver. Moreover, this modification was associated to about two-fold induction of mRNA-PPARalpha. On the contrary, in the kidney, although a similar two-fold up-regulation of PPARalpha was detected, the induction of both peroxisomal enzyme activities and Cyp4a10 were weak, and no alterations were detected, neither in cell cycle nor in the size of the tissue. Our results indicate that the response to ciprofibrate is stronger in the liver than in the kidney of newborn rats.

Present concepts and future outlook: function of peroxisome proliferator-activated receptors (PPARs) for pathogenesis, progression, and therapy of cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of transcriptional regulators that regulate lipid, glucose, and amino acid metabolism. In recent studies it also has been shown that these receptors are implicated in tumor progression, cellular differentiation, and apoptosis and modulation of their function is therefore considered as a potential target for cancer prevention and treatment. PPAR ligands and other agents influencing PPAR signalling pathways have been shown to reveal chemopreventive potential by mediating tumor suppressive activities in a variety of human cancers and could represent a potential novel strategy to inhibit tumor carcinogenesis and progression. This review summarizes the currently available data on the roles of PPARs in relation to the processes of cell differentiation and carcinogenesis as well as their role as promising future therapeutic targets.

Clofibrate treatment in pigs: effects on parameters critical with respect to peroxisome proliferator-induced hepatocarcinogenesis in rodents

Background

In rodents treatment with fibrates causes hepatocarcinogenesis, probably as a result of oxidative stress and an impaired balance between apoptosis and cell proliferation in the liver. There is some debate whether fibrates could also induce liver cancer in species not responsive to peroxisome proliferation. In this study the effect of clofibrate treatment on peroxisome proliferation, production of oxidative stress, gene expression of pro- and anti-apoptotic genes and proto-oncogenes was investigated in the liver of pigs, a non-proliferating species.

Results

Pigs treated with clofibrate had heavier livers (+16%), higher peroxisome counts (+61%), higher mRNA concentration of acyl-CoA oxidase (+66%), a higher activity of catalase (+41%) but lower concentrations of hydrogen peroxide (-32%) in the liver than control pigs (P < 0.05); concentrations of lipid peroxidation products (thiobarbituric acid-reactive substances, conjugated dienes) and total and reduced glutathione in the liver did not differ between both groups. Clofibrate treated pigs also had higher hepatic mRNA concentrations of bax and the proto-oncogenes c-myc and c-jun and a lower mRNA concentration of bcl-X_L than control pigs (P < 0.05).

Conclusion

The data of this study show that clofibrate treatment induces moderate peroxisome proliferation but does not cause oxidative stress in the liver of pigs. Gene expression analysis indicates that clofibrate treatment did not inhibit but rather stimulated apoptosis in the liver of these animals. It is also shown that clofibrate increases the expression of the proto-oncogenes c-myc and c-jun in the liver, an event which could be critical with respect to carcinogenesis. As the extent of peroxisome proliferation by clofibrate was similar to that observed in humans, the pig can be regarded as a useful model for investigating the effects of peroxisome proliferators on liver function and hepatocarcinogenesis.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

Dissociation of oxidant production by peroxisome proliferator-activated receptor ligands from cell death in human cell lines

Ligands of peroxisome proliferator-activated receptors (PPARs) come from a diverse group of chemicals that include pharmaceutical drugs, phthalate plasticizers, steroids, and pesticides. PPAR ligands exhibit a number of effects, including an ability to induce apoptosis in some systems. The mechanism(s) underlying the induction of apoptosis is not known. The current study examined the ability of Wy14643, a fibrate and PPARalpha agonist, and ciglitazone, a thiazolidinedione and PPARgamma agonist, to induce apoptosis as well as the production of oxidants in human Jurkat T cells that express all PPAR isoforms. Treatment with increasing doses of Wy14643 caused a substantial time-dependent increase in the overall oxidant status (as reflected by increased dichlorofluorescein fluorescence) of Jurkat cells without any change in viability except at the highest dose and longest time. Ciglitazone also caused a dose- and time-dependent increase in oxidant production. However, although the extent of this production was less than that seen with Wy14643, ciglitazone caused a dose- and time-dependent increase in apoptosis that could not be inhibited by antioxidants. Confocal micrographs of Jurkat cells loaded with dichlorofluorescein diacetate or dihydrorhodamine 123 and treated with Wy14643 or ciglitazone revealed a punctate pattern of fluorescence at early time points suggestive of a mitochondrial origin for these oxidants. Rotenone and antimycin A prevented Wy14643- but not ciglitazone-induced oxidant production. Other relatively specific PPARgamma agonists (15delta-PGJ2, and troglitazone), but not nonspecific agonists (bezafibrate and conjugated linoleic acid), were also able to induce oxidant production in Jurkat cells. These data, as well as the findings that oxidant production could be induced by Wy14643 in A549 cells that lack PPARalpha, and could not be blocked in Jurkat cells by the PPARalpha inhibitor MK886, indicate oxidant formation is unrelated to PPARalpha. These data also suggest that oxidant production induced by PPARalpha ligands originates in the mitochondria.

Chromatin sphingomyelin changes in cell proliferation and/or apoptosis induced by ciprofibrate

It has been shown that neutral-sphingomyelinase and sphingomyelin-synthase activities are present in chromatin and they modify the sphingomyelin (SM) content. The activity of the first enzyme is stimulated and the second inhibited, when the hepatocytes enter into the S-phase after partial hepatectomy, thus suggesting that ceramide may have a pivotal role in cell proliferation. An opposite function was attributed to ceramide in hepatocytes which undergo apoptosis after lobular ligature. In order to clarify this point, a model was developed in which the same liver cells undergo proliferation followed by induced apoptosis. To this purpose, the rats were treated for 7 days with ciprofibrate and then left without treatment for 4 days. During the treatment, the peroxisome enzyme markers increase their activity and the number of proliferating cells increases, reaching a maximum after 3 days of treatment, as shown by the number of cells positive for the proliferating cell nuclear antigen. At the same time, the chromatin sphingomyelinase activity reaches the maximum, while a similar increase is not found in the cytoplasm or in the isolated nuclei. On the contrary, SM-synthase activity is depressed in chromatin, but not in the nuclei in which a peak is shown after 3 days of ciprofibrate treatment. After drug withdrawal, the hepatocytes undergo apoptosis as confirmed by the increase of Bax and tissue transglutaminase (tTGase) expression; the chromatin SM increases as a consequence of an increase of SM-synthase activity. It can be hypothesised that chromatin SM may have a role in cell duplication by influencing the chromatin structure stability.

Inhibition of peroxisome-proliferator-activated receptor (PPAR)alpha by MK886

Although MK886 was originally identified as an inhibitor of 5-lipoxygenase activating protein (FLAP), recent data demonstrate that this activity does not underlie its ability to induce apoptosis [Datta, Biswal and Kehrer (1999) Biochem. J. 340, 371--375]. Since FLAP is a fatty-acid binding protein, it is conceivable that MK886 may affect other such proteins. A family of nuclear receptors that are activated by fatty acids and their metabolites, the peroxisome-proliferator-activated receptors (PPARs), have been implicated in apoptosis and may represent a target for MK886. The ability of MK886 to inhibit PPAR-alpha, -beta and -gamma activity was assessed using reporter assay systems (peroxisome-proliferator response element--luciferase). Using a transient transfection system in monkey kidney fibroblast CV-1 cells, mouse keratinocyte 308 cells and human lung adenocarcinoma A549 cells, 10--20 microM MK886 inhibited Wy14,643 activation of PPAR alpha by approximately 80%. Similar inhibition of PPAR alpha by MK886 was observed with a stable transfection reporter system in CV-1 cells. Only minimal inhibitory effects were seen on PPAR beta and PPAR gamma. MK886 inhibited PPAR alpha by a non-competitive mechanism as shown by its effects on the binding of arachidonic acid to PPAR alpha protein, and a dose-response study using a transient transfection reporter assay in COS-1 cells. An assay assessing PPAR ligand-receptor interactions showed that MK886 prevents the conformational change necessary for active-complex formation. The expression of keratin-1, a protein encoded by a PPAR alpha-responsive gene, was reduced by MK886 in a culture of mouse primary keratinocytes, suggesting that PPAR inhibition has functional consequences in normal cells. Although Jurkat cells express all PPAR isoforms, various PPAR alpha and PPAR gamma agonists were unable to prevent MK886-induced apoptosis. This is consistent with MK886 functioning as a non-competitive inhibitor of PPAR alpha, but may also indicate that PPAR alpha is not directly involved in MK886-induced apoptosis. Although numerous PPAR activators have been identified, the results show that MK886 can inhibit PPAR alpha, making it the first compound identified to have such an effect.

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.

The use of structure-activity relationships and markers of cell toxicity to detect non-genotoxic carcinogens

In contrast to the situation for genotoxic carcinogens, few in vitro tests exist that can detect early markers of the events thought to be associated with non-genotoxic carcinogenesis. Also, comparatively little is known about the quantitative structure-activity relationships (Q)SARs of these agents. This review discusses published SAR studies conducted on non-genotoxic carcinogens, in relation to the use of several markers of in vitro cell toxicity (inhibition of gap-junctional intercellular communication, inhibition of tubulin polymerization, modulation of apoptosis and induction of cell proliferation), which are used as endpoints for screening this class of carcinogen. Much of the work has involved the identification of new biophores (substructural features of molecules associated with toxicity), as well as other structural features, which are thought to predispose the chemicals to ligand binding with specific target molecules acting as possible receptors (e.g. protein kinase C, the oestrogen, peroxisome-proliferator and tubulin protein receptors), implicated in the mechanism of toxicity involved. It is concluded that (a) there is an urgent need for more information on (Q)SARs for non-genotoxic carcinogens; (b) this information should be acquired by using several different approaches in a variety of laboratories; and (c) such research should proceed together with more studies on the mechanisms of cell toxicity caused by these chemicals, including the identification and characterisation of further specific receptors involved in mediating the various types of cell toxicity associated with this type of carcinogenesis.

Central role of PPARalpha in the mechanism of action of hepatocarcinogenic peroxisome proliferators

Peroxisome proliferators (PP) are a large class of structurally dissimilar chemicals. These chemicals have diverse effects in rodents and humans, including regulation of lipid metabolism, growth promotion, and induction of hepatocarcinogenesis. Most, if not all, effects of PP are mediated by three members of the nuclear receptor superfamily called PP-activated receptors (PPAR). In this review, we discuss the evidence that PPARalpha, the predominant PPAR in the, liver is involved in the growth promoting and hepatocarcinogenic effects of PP.

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

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

A cancer risk assessment of di(2-ethylhexyl)phthalate: application of the new U.S. EPA Risk Assessment Guidelines

The current United States Environmental Protection Agency (EPA) classification of di(2-ethylhexyl)phthalate (DEHP) as a B2 "probable human" carcinogen is based on outdated information. New toxicology data and a considerable amount of new mechanistic evidence were used to reconsider the cancer classification of DEHP under EPA's proposed new cancer risk assessment guidelines. The total weight-of-evidence clearly indicates that DEHP is not genotoxic. In vivo administration of DEHP to rats and mice results in peroxisome proliferation in the liver, and there is strong evidence and scientific consensus that, in rodents, peroxisome proliferation is directly associated with the onset of liver cancer. Peroxisome proliferation is a transcription-mediated process that involves activation by the peroxisome proliferator of a nuclear receptor in rodent liver called the peroxisome proliferator-activated receptor (PPARalpha). The critical role of PPARalpha in peroxisomal proliferation and carcinogenicity in mice is clearly established by the lack of either response in mice genetically modified to remove the PPARalpha. Several mechanisms have been proposed to explain how, in rodents, peroxisome proliferation can lead to the formation of hepatocellular tumors. The general consensus of scientific opinion is that PPARalpha-induced mitogenesis and cell proliferation are probably the major mechanisms responsible for peroxisome proliferator-induced hepatocarcinogenesis in rodents. Oxidative stress appears to play a significant role in this increased cell proliferation. It triggers the release of TNFalpha by Kupffer cells, which in turn acts as a potent mitogen in hepatocytes. Rats and mice are uniquely responsive to the morphological, biochemical, and chronic carcinogenic effects of peroxisome proliferators, while guinea pigs, dogs, nonhuman primates, and humans are essentially nonresponsive or refractory; Syrian hamsters exhibit intermediate responsiveness. These differences are explained, in part, by marked interspecies variations in the expression of PPARalpha, with levels of expression in humans being only 1-10% of the levels found in rat and mouse liver. Recent studies of DEHP clearly indicate a nonlinear dose-response curve that strongly suggests the existence of a dose threshold below which tumors in rodents are not induced. Thus, the hepatocarcinogenic effects of DEHP in rodents result directly from the receptor-mediated, threshold-based mechanism of peroxisome proliferation, a well-understood process associated uniquely with rodents. Since humans are quite refractory to peroxisomal proliferation, even following exposure to potent proliferators such as hypolipidemic drugs, it is concluded that the hepatocarcinogenic response of rodents to DEHP is not relevant to human cancer risk at any anticipated exposure level. DEHP should be classified an unlikely human carcinogen with a margin of exposure (MOE) approach to risk assessment. The most appropriate and conservative point of reference for assessing MOEs should be 20 mg/kg/day, which is the mouse NOEL for peroxisome proliferation and increased liver weight. Exposure of the general human population to DEHP is approximately 30 microg/kg body wt/day, the major source being from residues in food. Higher exposures occur occupationally [up to about 700 microg/kg body wt/day (mainly by inhalation) based on current workplace standards] and through use of certain medical devices [e.g., up to 457 microg/kg body wt/day for hemodialysis patients (intravenous)], although these have little relevance because the routes of exposure bypass critical activation enzymes in the gastrointestinal tract.

The perturbation of apoptosis and mitosis by drugs and xenobiotics

Drugs such as the barbiturate phenobarbitone and fibrate hypolipidaemic agents, in addition to a range of chemicals of environmental and industrial significance, are able to perturb rodent tissue homeostasis, leading to tissue enlargement. Many of these xenobiotics are rodent nongenotoxic carcinogens since they do not damage DNA, yet cause tumours in the rat and mouse. These nongenotoxic carcinogens display both species and tissue specificity; for example, rat and mouse hepatocytes display S-phase induction and a suppression of apoptosis in response to drugs such as phenobarbitone or the hypolipidaemic peroxisome proliferators (PPs). In contrast, human hepatocytes or other types of rodent cells are refractory to these effects. However, in the absence of a discrete mechanism of action, the clear species differences preclude extrapolation of rodent data to provide an accurate human risk assessment. Recent data have demonstrated that PPs activate the PP-activated receptor alpha in rodent liver, leading to enzyme induction, stimulation of S-phase, and a suppression of apoptosis. How these acute effects may lead to hepatocarcinogenesis and the relevance of this for humans will be discussed.

Peroxisome proliferator-activated receptors: a critical link among fatty acids, gene expression and carcinogenesis

It has been known for many years that long-chain fatty acids derived from endogenous metabolism and/or nutrition can act as second messengers and regulators of cell signaling pathways. For example, fatty acids regulate the activity of protein kinase C (PKC) in a mechanism distinct from activation by diacylglycerol. Like PKC activators such as phorbol esters, essential fatty acids activate PKC and in doing so modulate the activity of growth factor receptors such as epidermal growth factor receptor (EGFR). Unsaturated fatty acids can inhibit GTPase activating protein, thereby quenching signals from p21-ras. These studies have shown that fatty acids can influence numerous signaling pathways and that these small lipophilic substances may be ancient second messengers. Fatty acids are also known modulators of the carcinogenic process, showing distinct tissue-specific pro- or anticancer effects. However, the reason for such a dichotomous effect on cellular processes has not been adequately described. In this article, the inclusion of a steroid hormone receptor-signaling pathway in mediating fatty acids' effects will be summarized. This signaling molecule has been deemed the peroxisome proliferator-activated receptor (PPAR) and has been extensively examined in regard to its response to xenobiotic, fatty acid-like chemicals (peroxisome proliferators, PP). PP, like fatty acids, activate PPAR and modulate tissue-specific responses. The goal of this review is to describe a potential role for PPAR in mediating the effects of fatty acids on gene expression, cell growth, differentiation and apoptosis.

Peroxisome proliferator-activated receptor (PPAR) alpha-regulated growth responses and their importance to hepatocarcinogenesis

Peroxisome proliferators (PPs) are a class of non-genotoxic rodent hepatocarcinogens that act by perturbing liver growth regulation. We have demonstrated previously that PPs suppress both spontaneous rat hepatocyte apoptosis and that induced by exogenous stimuli such as transforming growth factor-beta1 (TGF beta1). More recently, we have demonstrated that PPs can suppress apoptosis induced by more diverse stimuli such as DNA damage or ligation of Fas, a receptor related to the tumour necrosis factor alpha (TNF alpha) family of cell surface receptors. PPs transcriptionally activate the peroxisome proliferator activated receptor-alpha, PPAR alpha, a member of the nuclear hormone receptor superfamily. We investigated whether activation of PPAR alpha mediates the suppression of rat hepatocyte apoptosis induced by PPs. We isolated a naturally occurring variant form of PPAR alpha (hPPAR alpha-6/29) from human liver by PCR cloning. hPPAR alpha-6/29 shared the ability of mPPAR alpha to bind to DNA but, unlike mPPAR alpha, could not be activated by PPs. Furthermore, hPPAR alpha-6/29 could act as a dominant negative regulator of PPAR-mediated gene transcription. When introduced into primary rat liver cell cultures by transient transfection, hPPAR alpha-6/29 prevented the suppression of hepatocyte apoptosis by the PP nafenopin, but not that seen in response to phenobarbitone (PB), a non-genotoxic carcinogen whose action does not involve PPAR alpha. The suppression of hepatocyte apoptosis was abrogated completely even though only 30% of hepatocytes were transfected, suggesting the involvement of a soluble factor. Recent data have suggested that TNF alpha, perhaps released by liver Kupffer cells in response to PPs, may play a key role in mediating the effects of PPs on hepatocyte growth regulation.

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

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

Effect of rodent hepatocarcinogenic peroxisome proliferators on fatty acyl-CoA oxidase, DNA synthesis, and apoptosis in cultured human and rat hepatocytes

The effects of the rodent hepatocarcinogens clofibric acid and diprofibrate on the activity of the peroxisomal fatty acyl-CoA oxidase, DNA synthesis, and apoptosis were compared in cultured rat and human hepatocytes. Rat hepatocytes expressed a 10-fold greater level of the peroxisomal fatty acyl-CoA oxidase compared to human hepatocytes. At the highest concentration (1.0 mM), both drugs induced a two- to threefold increase in this enzyme activity in both rat and human hepatocytes. Ciprofibrate (0.1 and 0.2 mM) caused a twofold increase in DNA synthesis in rat hepatocytes, whereas clofibric acid had no effect on DNA synthesis in these cells. In contrast, increasing concentrations of both clofibric acid and ciprofibrate produced inhibition of DNA synthesis in human hepatocytes. By using the terminal transferase dUTP-biotin nick end labeling technique, it was observed that 0.1 and 0.2 mM clofibric acid and ciprofibrate suppressed transforming growth factor-beta (TGF beta)-induced apoptosis by 50% in rat hepatocytes, but they had no effect on TGF beta-induced apoptosis in human hepatocytes. Although clofibric acid and ciprofibrate diminished TGF beta-induced apoptosis, they had no effect on the basal apoptotic levels in the rat hepatocyte cultures. However, both drugs significantly increased the percent of apoptotic cells in the human hepatocyte cultures. It is concluded that primary rat and human hepatocyte cultures respond differently to peroxisome proliferators. The differences in effects on DNA synthesis and apoptosis support the hypothesis that human liver cells are refractory to peroxisome proliferator-induced hepatocarcinogenesis.