Comparison of the effects of nafenopin on hepatic peroxisome proliferation and replicative DNA synthesis in the rat and Syrian hamster

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Tesaglitazar, a dual PPAR-α/γ agonist, hamster carcinogenicity, investigative animal and clinical studies

Tesaglitazar was developed as a dual peroxisome proliferator-activated receptor (PPARα/γ). To support the clinical program, a hamster carcinogenicity study was performed. The only neoplastic findings possibly related to treatment with tesaglitazar were low incidences of hemangioma and hemangiosarcoma in the liver of male animals. A high-power, two-year investigative study with interim necropsies was performed to further elucidate these findings. Treatment with tesaglitazar resulted in changes typical for exaggerated PPARα pharmacology in rodents, such as hepatocellular hypertrophy and hepatocellular carcinoma, but not an increased frequency of hemangiosarcomas. At the highest dose level, there was an increased incidence of sinusoidal dilatation and hemangiomas. No increased endothelial cell (EC) proliferation was detected in vivo, which was confirmed by in vitro administration to ECs. Immunohistochemistry and gene expression analyses indicated increased cellular stress and vascular endothelial growth factor (VEGF) expression in the liver, which may have contributed to the sinusoidal dilatation. A two-fold increase in the level of circulating VEGF was detected in the hamster at all dose levels, whereas no effect on VEGF was observed in patients treated with tesaglitazar. In conclusion, investigations have demonstrated that tesaglitazar does not produce hemangiosarcomas in hamster despite a slight effect on vascular morphology in the liver.

Regulation of cell proliferation and cell death by peroxisome proliferators

Peroxisome proliferators cause increases in liver mass in rodents, linked to changes in cell proliferation and cell death of hepatocytes. These effects are reversible upon cessation of treatment. The underlying mechanism of the response in rodent liver is complex, but clearly dependent on activation of the nuclear receptor PPARalpha. Other signaling pathways have been implicated in this response, but evidence is mixed. Differing sensitivity among various species to effects of peroxisome proliferators has been associated with differences in PPARalpha expression and function. Changes in cell proliferation and cell death in neoplastic hepatocytes also have been found in liver tumors caused by long-term treatment with peroxisome proliferators.

The functions of cytokines and their uses in toxicology

Cytokines are critical controllers of cell, and hence tissue, growth, migration, development and differentiation. The family includes the inflammatory cytokines such as the interleukins and interferons, growth factors such as epidermal and hepatocyte growth factor and chemokines such as the macrophage inflammatory proteins, MIP-1alpha and MIP-1beta. They do not include the peptide and steroid hormones of the endocrine system. Cytokines have important roles in chemically induced tissue damage repair, in cancer development and progression, in the control of cell replication and apoptosis, and in the modulation of immune reactions such as sensitization. They have the potential for being sensitive markers of chemically induced perturbations in function but from a toxicological point of view, the detection of cytokine changes in the whole animal is limited by the fact that they are locally released, with plasma measures being generally unreliable or irrelevant, and they have short half lives which require precise timing to detect. Even where methodology is adequate the interpretation of the downstream effects of high, local concentrations of a particular cytokine is problematic because of their interdependence and the pleiotropism of their action. A range of techniques exist for their measurement including those dependent upon antibodies specific for the respective cytokines, but with the introduction of genomic and proteomic technology, a more complete study of cytokine changes occurring under the influence of chemical toxicity should be possible. Their further study, as markers of chemical toxicity, will undoubtedly lead to a greater understanding of how synthetic molecules perturb normal cell biology and if, and how, this can be avoided by more intuitive molecular design in the future.

Species differences in hepatic peroxisome proliferation, cell replication and transforming growth factor-beta1 gene expression in the rat, Syrian hamster and guinea pig

The objective of this study was to evaluate species differences in the hepatic effects of three potent rodent peroxisome proliferators, namely methylclofenapate (MCP), ciprofibrate (CIP) and Wy-14,643 (WY), particularly with respect to effects on replicative DNA synthesis and transforming growth factor-beta1 (TGF-beta1) gene expression. Male Sprague-Dawley rats, Syrian hamsters and Dunkin-Hartley guinea pigs were given daily oral doses of 0 (corn oil) and 75 mg/kg MCP for periods of 6 and 21 days. Syrian hamsters and guinea pigs were also treated with 25 mg/kg CIP and 25 mg/kg WY. Relative liver weights were significantly increased in peroxisome proliferator-treated rats and Syrian hamsters, but not in guinea pigs. Hepatic peroxisomal (palmitoyl-CoA oxidation) and microsomal (lauric acid 12-hydroxylase) fatty acid oxidising enzyme activities and CYP4A isoform mRNA levels were significantly increased in rats and Syrian hamsters, whereas only minor effects were observed in the guinea pig. Replicative DNA synthesis was studied by implanting 7-day osmotic pumps containing 5-bromo-2'-deoxyuridine during study days -1 to 6 and 14 to 21. Hepatocyte labelling index values were increased by MCP in the rat, but neither MCP, CIP nor WY produced any significant effect on replicative DNA synthesis in the Syrian hamster and guinea pig. MCP treatment increased TGF-beta1 and insulin-like growth factor II/mannose-6-phosphate (IGFII/Man6P) receptor gene expression in the rat. In the Syrian hamster, effects on TGF-beta1 and IGFII/Man6P receptor gene expression were also observed in some instances, whereas TGF-beta1 mRNA levels were essentially unchanged in the guinea pig. These results provide further evidence for marked species differences in response to rodent peroxisome proliferators. While peroxisome proliferators produce a wide spectrum of effects in rat liver, other species such as the Syrian hamster and guinea pig are less responsive and in the case of some endpoints (e.g., cell replication) may be refractory.

Species differences in peroxisome proliferation; mechanisms and relevance

Peroxisome proliferators are a class of structurally diverse chemicals, which induce liver carcinogenesis in rodents through interaction and activation of the Peroxisome Proliferator-Activated Receptor alpha (PPARalpha). PPARalpha agonists elicit a powerful pleiotropic response, which include hypolipidaemia. We have examined the response of species that are classically unresponsive to peroxisome proliferators. Whereas hamster responds to PPARalpha agonists by hepatomegaly and induction of marker genes, the guinea pig does not undergo hepatomegaly or induction of marker genes, such as CYP4A13. Both the hamster and the guinea pig have PPARalpha, and the guinea pig receptor has been characterised to be fully functional, as demonstrated in reporter gene expression assays. However, the guinea pig PPARalpha is expressed at low levels in liver, and the currently favoured hypothesis to explain species differences in hepatic peroxisome proliferation invokes the low level of PPARalpha as the principal determinant of species responsiveness. However, the demonstration that guinea pigs and humans undergo hypolipidaemia induced by PPARalpha-agonists calls into question the mode of action of PPARalpha agonists in "non-responsive" species.

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.

Rodent hepatocarcinogenic peroxisome proliferators induce proliferation of rat hepatocytes in primary mixed cultures with rat liver epithelial cells

The effect of two members of the hypolipidemic medicine class of hepatocarcinogenic peroxisome proliferators on proliferation of hepatocytes in primary mixed cultures with liver epithelial cells was studied. Rat hepatocytes present in primary mixed cultures with rat liver epithelial cells were maintained for 3 months retaining their differentiated characteristics and proliferative potential. Hepatocyte clusters in mixed cultures stained positive for albumin, indicating that they retained some metabolic functions. Furthermore, in mixed cultures exposed to 0.2 mM clofibric acid or ciprofibrate for 3 months, hepatocytes were engaged in proliferation as shown by the expression of proliferating cell nuclear antigen (PCNA) and the presence of mitotic figures. This in vitro system could be useful to obtain more information about responses of liver cells during prolonged exposure to peroxisome proliferators.