Peroxisome proliferator-activated receptor-α activation and excess energy burning in hepatocarcinogenesis.

Peroxisome proliferator-activated receptor-α (PPARα) modulates the activities of all three interlinked hepatic fatty acid oxidation systems, namely mitochondrial and peroxisomal ß-oxidation and microsomal ω-oxidation pathways. Hyperactivation of PPARα, by both exogenous and endogenous activators up-regulates hepatic fatty acid oxidation resulting in excess energy burning in liver contributing to the development of liver cancer in rodents. Sustained PPARα signaling disproportionately increases H2O2-generating fatty acid metabolizing enzymes as compared to H2O2-degrading enzymes in liver leading to enhanced generation of DNA damaging reactive oxygen species, progressive endoplasmic reticulum stress and inflammation. These alterations also contribute to increased liver cell proliferation with changes in apoptosis. Thus, reactive oxygen species, oxidative stress and hepatocellular proliferation are likely the main contributing factors in the pathogenesis of hepatocarcinogenesis, mediated by sustained PPARα activation-related energy burning in liver. Furthermore, the transcriptional co-activator Med1, a key subunit of the Mediator complex, is essential for PPARα signaling in that both PPARα-null and Med1-null hepatocytes are unresponsive to PPARα activators and fail to give rise to liver tumors when chronically exposed to PPARα activators.

Cytotoxic properties of clofibrate and other peroxisome proliferators: relevance to cancer progression.

The biological activity of peroxisome proliferators (PPs) is mediated by a class of receptors, known as PPARs (PP-Activated Receptor), belonging to the nuclear receptor superfamily. Upon ligand binding, PPARs dimerize with retinoid receptors, translocate to the nucleus, recognize specific PP-responsive elements on DNA and transactivate a number of genes. Several processes are regulated by PPARs, such as mitochondrial and peroxisomal fatty acid uptake and beta-oxidation, inflammation, intracellular lipid trafficking, cell proliferation and death. In addition, PPARs have been proposed to act as tumor suppressors or as tumor promoters, depending on the circumstances. In particular, PPs have been extensively studied for their hepatocarcinogenic action in rodents, most often ascribed to their antiapoptotic action. Recent evidence, however, has been provided about the antiproliferative, proapoptotic, and differentiation-promoting activities displayed by PPAR ligands. The present review will focus on the cytotoxic effects exerted by several PPs, among which clofibrate, on different types of tumor cells, with particular reference to the mechanisms of cell death and to their relevance to cancer induction and progression.

Human health risk assessment for peroxisome proliferators: more than 30 years of research.

Substances like gemfibrozil, clofibrate and fenofibrate, widely used in human care for their hypolipidemic effects, belong to a larger class of chemicals called peroxisome proliferators (PPs). PPs, by binding and activing the peroxisome proliferator-activated receptor alpha (PPARalpha), modulate gene involved in lipid homeostasis both in human and in rodent. In a different way, long term administration of PPs results in hepatocarcinogenesis only in rodent. Although the phenomenon is known since more than 30 years, the exact mechanism is not well understood and the human health risks are not established. In this mini-review is inspected the major findings done in the different species and illustrates the possible doubts for human health by the use of PPs.

Aging and enhanced hepatocarcinogenicity by peroxisome proliferator-activated receptor alpha agonists.

The hepatocarcinogenic effect of PPARalpha agonists is enhanced by aging. Exposure to these chemicals produces a five- to seven-fold higher yield of grossly visible hepatic tumors in old relative to young animals. This review presents current experimental evidence, which supports a mechanism involving enhanced exposure to oxidative stress, and diminished apoptosis in this age-related difference in sensitivity. In the aged liver, a decrease in hepatic antioxidant activity, coupled with a PPARalpha agonist-induced increase in the activities of various oxidases, may expose these livers to oxidative stress. Additionally, livers of senescent animals appeared more sensitive to the anti-apoptotic effect of PPARalpha agonists. Since apoptosis safeguards cells with damaged DNA from progressing to the point of tumor formation, inhibition of hepatocellular apoptosis by PPARalpha agonists could well lead to the formation of focal lesions in the aged liver. Although PPARalpha-dependent alterations in cell cycle regulatory proteins have been reported, the correlation between hepatocellular DNA replication and liver cancer caused by PPARalpha agonists is a weak one. These findings have implications for human susceptibility to these chemicals.

Bezafibrate induces a mitochondrial derangement in human cell lines: a PPAR-independent mechanism for a peroxisome proliferator.

Bezafibrate is a hypolipidemic drug that belongs to the group of peroxisome proliferators because it binds to peroxisome proliferator-activated receptors type alpha (PPARs). Peroxisome proliferators produce a myriad of extraperoxisomal effects, which are not necessarily dependent on their interaction with PPARs. An investigation on the peculiar activities of bezafibrate could clarify some of the molecular events and the relationship with the biochemical and pharmacological properties of this class of compounds. In this view, the human acute promyelocytic leukemia HL-60 cell line and human rabdomiosarcoma TE-671 cell line were cultured in media containing bezafibrate and a number of observations such as spectrophotometric analysis of mitochondrial respiratory chain enzymes, NMR metabolite determinations, phosphofructokinase enzymatic analysis, and differentiation assays were carried on. Bezafibrate induced a derangement of NADH cytochrome c reductase activity accompanied by metabolic alterations, mainly a shift to anaerobic glycolysis and an increase of fatty acid oxidation, as shown by NMR analysis of culture supernatants where acetate, lactate, and alanine levels increased. On the whole, the present results suggest a biochemical profile and a therapeutic role of this class of PPARs ligands more complex than those previously proposed.

The peroxisome proliferator-activated receptor alpha (PPARalpha): role in hepatocarcinogenesis.

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear receptor superfamily and mediates most of the known biological effects of peroxisome proliferators. The latter represents a large group of chemicals that include the fibrate hyperlipidemic drugs, the pthalate plasticizers, various solvents and degreasing agents, and endogenous hormones and fatty acids. Peroxisome proliferators are classical members of the nongenotoxic group of chemical carcinogens that do not require metabolic activation to electrophiles in order to exert their harmful effects. These chemicals are of particular concern to regulatory agencies since they can only be detected by long-term carcinogen bioassays using rodents. The mechanism of the carcinogenic action of peroxisome proliferators is beginning to emerge. PPARalpha-null mice are resistant to hepatocarcinogenesis indicating that this receptor is necessary for cancer. However, recent studies indicate that Kupffer cells, in a PPARalpha independent manor, are required for the major effects of peroxisome proliferators on cell proliferation. An interaction between PPARalpha and estrogen carcinogenesis has also been elucidated.

Carnitine protects mitochondria and removes toxic acyls from xenobiotics.

The carnitine system is altered by several xenobiotics (drugs and chemicals). These alterations are responsible for most toxic effects, which can be reverted or minimized by L-carnitine administration. Formation of nonmetabolizable acyl coenzyme A (CoA) is a typical step in the biotransformation of pivaloyl antibiotics, valproate and ifosfamide. The elevated levels of acylcarnitine occurring in human urine due to impaired metabolism of specific acyl CoA support the role of L-carnitine as an acceptor of specific, nonmetabolizable acyl CoA. The consequence of this process is a secondary carnitine deficiency. The formation of stable complexes with an essential component of mitochondrial membrane, cardiolipin, and the inhibition of myocardial specific isoform of carnitine-palmitoyl transferase are presumably the basis of adriamycin cardiotoxicity. L-carnitine interacts with cardiolipin, modifying membrane permeability and protecting the functions of the mitochondria. This mechanism can be proposed to explain the protective effects of L-carnitine against adriamycin cardiotoxicity, ammonium acetate and zidovudine-induced mitochondrial ultrastructural and functional alterations. Cisplatin, cephalosporin and carbapenem antibiotics inhibit carnitine reabsorption in renal tubules and cause proximal tubular damage. The study of peroxisomal producing agents belonging to largely different chemical classes showed that these agents caused carnitine system perturbations which may have the potential to be highly relevant biomarkers of exposure to the nongenotoxic peroxisomal proliferating agent class of hepatic tumorigens.

Plasma cholecystokinin and hepatic enzymes, cholesterol and lipoproteins in ammonium perfluorooctanoate production workers.

Ammonium perfluorooctanoate is a potent synthetic surfactant used in industrial applications. It rapidly dissociates in biologic media to perfluorooctanoate [CF3(CF2)6CO2-], which is the anion of perfluorooctanoic acid [PFOA, CF3(CF2)6COOH]. PFOA is a peroxisome proliferator known to increase the incidence of hepatic, pancreas and Leydig cell adenomas in rats. The pancreas acinar cell adenomas may be the consequence of a mild but sustained increase of cholecystokinin as a result of hepatic cholestasis. Although no significant clinical hepatic toxicity was observed, PFOA was reported to have modulated hepatic responses to obesity and alcohol consumption among production workers. To further assess these hypotheses, we examined medical surveillance data of male workers involved in ammonium perfluorooctanoate production in 1993 (n=111), 1995 (n=80) and 1997 (n=74). Serum PFOA was measured by high-performance liquid chromatography mass spectrometry methods. Plasma cholecystokinin was measured (only in 1997) by the use of direct radioimmunoassay. Serum biochemical tests included hepatic enzymes, cholesterol and lipoproteins. Serum PFOA levels, by year, were: 1993 (mean 5.0 ppm, SD 12.2, median 1.1 ppm, range 0.0-80.0 ppm); 1995 (mean 6.8 ppm, SD 16.0, median 1.2 ppm, range 0.0-114.1 ppm); and 1997 (mean 6.4 ppm, SD 14.3, median 1.3 ppm, range 0.1-81.3 ppm). Cholecystokinin values (mean 28.5 pg/ml, SD 17.1, median 22.7 pg/ml, range 8.8-86.7 pg/ml) approximated the assay's reference range (up to 80 pg/ml) for a 12 hour fast and were negatively, not positively, associated with employees' serum PFOA levels. Our findings continue to suggest there is no significant clinical hepatic toxicity associated with PFOA levels as measured in this workforce. Unlike a previously reported observation, PFOA did not appear to modulate hepatic responses to either obesity or alcohol consumption. Limitations of these findings include: 1) the cross-sectional design as only 17 subjects were common for the three surveillance years; 2) the voluntary participation that ranged between 50 and 70 percent; and 3) the few subjects with serum levels > or = 10 ppm.

Effects of di-isononyl phthalate, di-2-ethylhexyl phthalate, and clofibrate in cynomolgus monkeys.

The effects of the peroxisome proliferators di-isononyl phthalate (DINP) and di-2-ethylhexyl phthalate (DEHP) were evaluated in young adult male cynomolgus monkeys after 14 days of treatment, with emphasis on detecting hepatic and other effects seen in rats and mice after treatment with high doses of phthalates. Groups of 4 monkeys received DINP (500 mg/kg/day), DEHP (500 mg/kg/day), or vehicle (0.5% methyl cellulose, 10 ml/kg) by intragastric intubation for 14 consecutive days. Clofibrate (250 mg/kg/day), a hypolipidemic drug used for cholesterol reduction in human patients was used as a reference substance. None of the test substances had any effect on body weight or liver weights. Histopathological examination of tissues from these animals revealed no distinctive treatment-related effects in the liver, kidney, or testes. There were also no changes in any of the hepatic markers for peroxisomal proliferation, including peroxisomal beta-oxidation (PBOX) or replicative DNA synthesis. Additionally, in situ dye transfer studies using fresh liver slices revealed that DINP, DEHP, and clofibrate had no effect on gap junctional intercellular communication (GJIC). None of the test substances produced any toxicologically important changes in urinalysis, hematology, or clinical chemistry; however, clofibrate produced some emesis, small increases in serum triglyceride, decreased calcium, and decreased weights of testes/epididymides and thyroid/parathyroid. The toxicological significance of these small changes is questionable. The absence of observable hepatic effects in monkeys at doses that produce hepatic effects in rodents suggests that DINP, DEHP, and clofibrate would also not elicit in primates other effects such as liver cancer. These data, along with results from in vitro hepatocyte studies, indicate that rodents are not good animal models for predicting the hepatic effects of phthalates in primates, including humans.

An update on the mechanisms of action of the peroxisome proliferator-activated receptors (PPARs) and their roles in inflammation and cancer.

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors and have been initially described as molecular targets for compounds which induce peroxisome proliferation. The interest of researchers for PPARs increased dramatically when these receptors were shown to be directly activated by a number of medically relevant compounds. These compounds include: the fibrate class of hypolidemic drugs, the thiazolidinediones, which are insulin sensitizers used as orally active antidiabetic agents, certain non-steroidal anti-inflammatory drugs (NSAIDs), and naturally occurring fatty acid-derived molecules. Rapidly, it was demonstrated that PPARs are key regulators of lipid homeostasis and provide a molecular link between nutrition and gene regulation. Recently, detailed studies of PPAR expression profiles in different tissues pointed to the roles these receptors play in inflammation control and cell proliferation. In this review we will focus on the new insights gained into these two areas and we will also discuss our current knowledge of the regulation of PPAR transcriptional activity by cofactors.

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.