Apoptosis and proliferation in nongenotoxic carcinogenesis: species differences and role of PPARalpha

Nuclear Receptors in Liver Fibrosis

Nuclear receptors are ligand-activated transcription factors that regulate gene expression of a variety of key molecular signals involved in liver fibrosis. The primary cellular driver of liver fibrogenesis are activated hepatic stellate cells. Different NRs regulate the hepatic expression of pro-inflammatory and pro-fibrogenic cytokines that promote the transformation of hepatic stellate cells into fibrogenic myofibroblasts. Importantly, nuclear receptors regulate gene expression circuits that promote hepatic fibrogenesis and/or allow liver fibrosis regression. In this review, we highlight the direct and indirect influence of nuclear receptors on liver fibrosis, with a focus on hepatic stellate cells, and discuss potential therapeutic effects of nuclear receptor modulation in regard to anti-fibrotic and anti-inflammatory effects. Further research on nuclear receptors-related signaling may lead to the clinical development of effective anti-fibrotic therapies for patients with liver disease.

Nonalcoholic steatohepatitis: the role of peroxisome proliferator-activated receptors

The increasing epidemic of obesity worldwide is linked to serious health effects, including increased prevalence of type 2 diabetes mellitus, cardiovascular disease and nonalcoholic fatty liver disease (NAFLD). NAFLD is the liver manifestation of the metabolic syndrome and includes the spectrum of liver steatosis (known as nonalcoholic fatty liver) and steatohepatitis (known as nonalcoholic steatohepatitis), which can evolve into progressive liver fibrosis and eventually cause cirrhosis. Although NAFLD is becoming the number one cause of chronic liver diseases, it is part of a systemic disease that affects many other parts of the body, including adipose tissue, pancreatic β-cells and the cardiovascular system. The pathomechanism of NAFLD is multifactorial across a spectrum of metabolic derangements and changes in the host microbiome that trigger low-grade inflammation in the liver and other organs. Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear regulatory factors that provide fine tuning for key elements of glucose and fat metabolism and regulate inflammatory cell activation and fibrotic processes. This Review summarizes and discusses the current literature on NAFLD as the liver manifestation of the systemic metabolic syndrome and focuses on the role of PPARs in the pathomechanisms as well as in the potential targeting of disease.

Peroxisome proliferator-activated receptor activity correlates with poor survival in patients resected for hepatocellular carcinoma

BACKGROUND/PURPOSE

Few clinically useful biomarkers are known to predict prognosis in patients with hepatocellular carcinoma (HCC). The aim of this study was to investigate the correlation between PPAR activity and ALDH7A1 expression and their prognostic significance using RNA sequencing in patients undergoing liver resection for HCC.

METHODS

We included patients undergoing liver resection for HCC at a tertiary referral center for hepato-pancreato-biliary surgery between May 2014 and January 2018. PPAR activity and ALDH7A1 expression were evaluated by RNA sequencing and correlated with overall survival, recurrence and histological features.

RESULTS

We included 52 patients with a median follow-up of 20.9 months, predominantly males (88.5%) with a single tumor (84.6%) in a non-cirrhotic liver (73.1%). Three-year overall survival was 48.6% in patients with a specific PPAR target gene expression profile (cancer cluster 3) compared with 81.7% in controls (P = .04, Log-rank test). This remained significant (odds ratio 14.02, 95% confidence interval 1.92-102.22, P = .009) when adjusted for age, cirrhosis, microvascular invasion, number of tumors and free resection margins. ALDH7A1 expression was not correlated with PPAR or any outcomes.

CONCLUSION

PPAR activity in a subset of tumor samples was associated with reduced overall survival indicating that PPAR may be a valuable prognostic biomarker.

Toxic Effects of the Mixture of Phthalates and Bisphenol A-Subacute Oral Toxicity Study in Wistar Rats

Phthalates and bisphenol A, classified as endocrine disruptors, have weak estrogenic, anti-androgenic properties, and affect thyroid hormone regulation. The aim of this study on male rats was to compare the subacute toxic effects of low doses of single compounds (bis (2 -ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), and bisphenol A (BPA)) with the effects of their mixture through different biochemical, hormonal, and hematological parameters. Rats were divided into five experimental groups: Control (corn oil), DEHP (50 mg/kg b.w./day), DBP (50 mg/kg b.w./day), BPA (25 mg/kg b.w./day), and MIX (50 mg/kg b.w./day DEHP + 50 mg/kg b.w/day DBP + 25 mg/kg b.w./day BPA). Animals were sacrificed after 28 days of oral treatment and blood was collected for further analysis. The results demonstrated that the mixture produced significant changes in lipid profile, liver-related biochemical parameters, and glucose level. Furthermore, the opposite effects of single substances on the thyroxine level have been shown in comparison with the mixture, as well as a more pronounced effect of the mixture on testosterone level. This study contributes to the body of knowledge on the toxicology of mixtures and gives one more evidence of the paramount importance of mixture toxicity studies, especially in assessing the endocrine disruptive effects of chemicals.

Effects of apoptosis on liver aging

As an irreversible and perennial process, aging is accompanied by functional and morphological declines in organs. Generally, aging liver exhibits a decline in volume and hepatic blood flow. Even with a preeminent regenerative capacity to restore its functions after liver cell loss, its biosynthesis and metabolism abilities decline, and these are difficult to restore to previous standards. Apoptosis is a programmed death process via intrinsic and extrinsic pathways, in which Bcl-2 family proteins and apoptosis-related genes, such as p21 and p53, are involved. Apoptosis inflicts both favorable and adverse influences on liver aging. Apoptosis eliminates transformed abnormal cells but promotes age-related liver diseases, such as nonalcoholic fatty liver disease, liver fibrosis, cirrhosis, and liver cancer. We summarize the roles of apoptosis in liver aging and age-related liver diseases.

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.

Fenofibrate in cancer: mechanisms involved in anticancer activity

Objective: To review the mechanisms of anti-cancer activity of fenofibrate (FF) and other Peroxisome Proliferator Activator Receptor α (PPARα) agonists based on evidences reported in the published literature.Methods: We extensively reviewed the literature concerning FF as an off target anti-cancer drug. Controversies regarding conflicting findings were also addressed.Results: The main mechanism involved in anti-cancer activity is anti-angiogenesis through down-regulation of  Vascular Endothelial Growth Factor (VEGF), Vascular Endothelial Growth Factor Receptor (VEGFR) and Hypoxia Inducible factor-1 α (HIF-1α), inhibition of endothelial cell migration, up-regulation of endostatin and thrombospondin-1, but there are many other contributing mechanisms like apoptosis and cell cycle arrest, down-regulation of Nuclear Factor Kappa B (NF-kB) and Protein kinase B (Akt) and decrease of cellular energy by impairing mitochondrial function. Growth impairment is related to down-regulation of Phospho-Inositol 3 Kinase (PI3K)/Akt axis and down-regulation of the p38 map kinase (MAPK) cascade. A possible role should be assigned to FF stimulated over-expression of Tribbles Homolog-3 (TRIB3) which inhibits Akt phosphorylation. Important anti-cancer and anti-metastatic activities are due to down-regulation of MCP-1 (monocyte chemotactic protein-1), decreased Metalloprotease-9 (MMP-9) production, weak down-regulation of adhesion molecules like E selectin, intercellular adhesion molecules (ICAM) and Vascular Endothelial Adhesion Molecules (VCAM), and decreased secretion of chemokines like Interleukin-6 (IL-6), and down-regulation of cyclin D-1. There is no direct link between FF activity in lipid metabolism and anticancer activity, except for the fact that many anticancer actions are dependent from PPARα agonism. FF exhibits also PPARα independent anti-cancer activities.Conclusions: There are strong evidences indicating that FF can disrupt growth-related activities in many different cancers, due to anti-angiogenesis and anti-inflammatory effects. Therefore FF may be useful as a complementary adjunct treatment of cancer, particularly included in anti-angiogenic protocols like those currently increasingly used in glioblastoma. There are sound reasons to initiate well planned phase II clinical trials for FF as a complementary adjunct treatment of cancer.

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.

The effects of di(2-ethylhexyl)phthalate on rat liver in relation to selenium status

This study was performed to determine the hepatotoxicity of di(2-ethylhexyl)phthalate (DEHP) in relation to selenium status. In 3-week-old Sprague-Dawley rats, selenium deficiency was induced by a ≤0.05 selenium mg/kg. A selenium supplementation group was given 1 mg selenium/kg diet for 5 weeks. Di(2-ethylhexyl)phthalate-treated groups received 1000 mg/kg dose by gavage during the last 10 days of the experiment. Histopathology, peroxisome proliferation, catalase (CAT) immunoreactivity and activity and apoptosis were assessed. Activities of antioxidant selenoenzymes [glutathione peroxidase 1 (GPx1), glutathione peroxidase 4 (GPx4), thioredoxin reductase (TrxR1)], superoxide dismutase (SOD), and glutathione S-transferase (GST); aminotransferase, total glutathione (tGSH), and lipid peroxidation (LP) levels were measured. Di(2-ethylhexyl)phthalate caused cellular disorganization while necrosis and inflammatory cell infiltration were observed in Se-deficient DEHP group (DEHP/SeD). Catalase activity and immunoreactivity were increased in all DEHP-treated groups. Glutathione peroxidase 1 and GPx4 activities decreased significantly in DEHP and DEHP/SeD groups, while GST activities decreased in all DEHP-exposed groups. Thioredoxin reductase activity increased in DEHP and DEHP/SeS, while total SOD activities increased in all DEHP-treated groups. Lipid peroxidation levels increased significantly in SeD (26%), DEHP (38%) and DEHP/SeD (71%) groups. Selenium supplementation partially ameliorated DEHP-induced hepatotoxicity; while in DEHP/SeD group, drastic changes in hepatic histopathology and oxidative stress parameters were observed.

PPARs and HCV-Related Hepatocarcinoma: A Mitochondrial Point of View

Hepatitis-C-virus-related infective diseases are worldwide spread pathologies affecting primarily liver. The infection is often asymptomatic, but when chronically persisting can lead to liver scarring and ultimately to cirrhosis, which is generally apparent after decades. In some cases, cirrhosis will progress to develop liver failure, liver cancer, or life-threatening esophageal and gastric varices. HCV-infected cells undergo profound metabolic dysregulation whose mechanisms are yet not well understood. An emerging feature in the pathogenesis of the HCV-related disease is the setting of a pro-oxidative condition caused by dysfunctions of mitochondria which proved to be targets of viral proteins. This causes deregulation of mitochondria-dependent catabolic pathway including fatty acid oxidation. Nuclear receptors and their ligands are fundamental regulators of the liver metabolic homeostasis, which are disrupted following HCV infection. In this contest, specific attention has been focused on the peroxisome proliferator activated receptors given their role in controlling liver lipid metabolism and the availability of specific pharmacological drugs of potential therapeutic utilization. However, the reported role of PPARs in HCV infection provides conflicting results likely due to different species-specific contests. In this paper we summarize the current knowledge on this issue and offer a reconciling model based on mitochondria-related features.

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.

Transcriptomic effects of di-(2-ethylhexyl)-phthalate in Syrian hamster embryo cells: an important role of early cytoskeleton disturbances in carcinogenesis?

Background

Di-(2-ethylhexyl)-phthalate (DEHP) is a commonly used plasticizer in polyvinylchloride (PVC) formulations and a potentially non-genotoxic carcinogen. The aim of this study was to identify genes whose level of expression is altered by DEHP by using a global wide-genome approach in Syrian hamster embryo (SHE) cells, a model similar to human cells regarding their responses to this type of carcinogen. With mRNA Differential Display (DD), we analysed the transcriptional regulation of SHE cells exposed to 0, 12.5, 25 and 50 μM of DEHP for 24 hrs, conditions which induced neoplastic transformation of these cells. A real-time quantitative polymerase chain reaction (qPCR) was used to confirm differential expression of genes identified by DD.

Results

Gene expression profiling showed 178 differentially-expressed fragments corresponding to 122 genes after tblastx comparisons, 79 up-regulated and 43 down-regulated. The genes of interest were involved in many biological pathways, including signal transduction, regulation of the cytoskeleton, xenobiotic metabolism, apoptosis, lipidogenesis, protein conformation, transport and cell cycle. We then focused particularly on genes involved in the regulation of the cytoskeleton, one of the processes occurring during carcinogenesis and in the early steps of neoplastic transformation. Twenty one cytoskeleton-related genes were studied by qPCR. The down-regulated genes were involved in focal adhesion or cell junction. The up-regulated genes were involved in the regulation of the actin cytoskeleton and this would suggest a role of cellular plasticity in the mechanism of chemical carcinogenesis. The gene expression changes identified in the present study were PPAR-independent.

Conclusion

This study identified a set of genes whose expression is altered by DEHP exposure in mammalian embryo cells. This is the first study that elucidates the genomic changes of DEHP involved in the organization of the cytoskeleton. The latter genes may be candidates as biomarkers predictive of early events in the multistep carcinogenic process.

Effect of a Large Dose of Di (2-ethylhexyl) phthalate (DEHP) on Hepatic Peroxisome in Cynomolgus Monkeys (Macaca Fascicularis)

To elucidate the effect of a large dose of di (2-ethylhexyl) phthalate (DEHP), a plasticizer and peroxisome proliferator-activated receptor-α (PPARα) agonist, on hepatic peroxisomes, we orally administered 1,000 mg/kg/day, once daily, to 3 male and 4 female cynomolgus monkeys for 28 days consecutively. Light-microscopic and electron microscopic examinations of the liver were carried out in conjunction with measurement of the hepatic fatty acid β-oxidation system (FAOS), carnitine acetyltransferase (CAT) and carnitine palmitoyltransferase (CPT) activities, which are peroxisomal and/or mitochondrial enzyme activities. Electron microscopically, enlargement of the mitochondria was observed with lamellar orientation of the cristae along the major axis. Although the number of peroxisomes showed a tendency to increase when compared with those in a biopsied specimen before treatment, no abnormality in morphology was observed. A slight increase in CPT activity was noted at termination. No changes were noted in hepatic FAOS or CAT activity. In conclusion, although repeated oral treatment of cynomolgus monkeys with a large dose of DEHP induced a subtle increase in the numbers of peroxisomes with slight enlargements of the mitochondria, this low-sensitivity response to peroxisome proliferators in cynomolgus monkeys was considered to be closer to the response in humans than that in rodents.

Mechanisms of resistance of hepatocyte retinoid X receptor alpha-null mice to WY-14,643-induced hepatocyte proliferation and cholestasis

Peroxisome proliferators, such as the lipid-lowering fibrates that function as agonists for peroxisome proliferator-activated receptor alpha (PPARalpha), induce liver tumors in rodents and may produce cholestasis in humans. Considerable attention has focused on peroxisome proliferator-induced hepatocellular carcinoma, a phenomenon not noted in man, whereas limited studies examine fibrates and other therapeutic drugs that induce cholestasis, a common finding in humans. Moreover, the mechanisms by which fibrates induce hepatocyte proliferation and cholestasis are still not fully understood. We have examined the role of hepatocyte retinoid X receptor alpha (RXRalpha), an essential partner of PPARalpha, in modulating WY-14,643-induced hepatocyte proliferation and cholestasis. WY-14,643 treatment induced hepatomegaly in wild type (WT) mice that was also accompanied by induction of the expression of cyclins D1, D3, A2, and B1 and Cdc2 as well as inhibition of Wee 1. Such changes were either absent or greatly reduced in hepatocyte RXRalpha-null mice. Furthermore, neither WY-14,643 treatment nor RXRalpha deficiency affected apoptosis, indicating the importance of PPARalpha/RXRalpha in regulating Wee 1-mediated Cdc2/cyclin B1 expression for cells to enter into mitosis. WY-14,643 treatment also induced cholestasis and liver injury, which is evidenced by induction of alanine aminotransferase, alkaline phosphatase, and hepatic bile acid levels in WT mice. Hepatocyte RXRalpha deficiency protected the mice from WY-14,643-induced liver injury. WY-14,643-mediated induction of the small heterodimer partner, Mrp3, and Cyp3a11 levels was greater in hepatocyte RXRalpha-null than in WT mouse livers suggesting enhanced repression of bile acid synthesis and increased efflux of bile acids into blood for renal excretion as well as hydroxylation of bile acids because of hepatocyte RXRalpha deficiency. These data establish a crucial role of hepatocyte RXRalpha in regulating WY-14,643-mediated cell cycle progression as well as bile acid homeostasis.

Evaluation of the role of peroxisome proliferator-activated receptor alpha (PPARalpha) in mouse liver tumor induction by trichloroethylene and metabolites

Trichloroethylene (TCE) is an industrial solvent and a widespread environmental contaminant. Induction of liver cancer in mice by TCE is thought to be mediated by two metabolites, dichloroacetate (DCA) and trichloroacetate (TCA), both of which are themselves mouse liver carcinogens. TCE, TCA, and DCA are relatively weak peroxisome proliferators (PP), a group of rodent hepatocarcinogens that activate a nuclear receptor, PP-activated receptor alpha (PPARalpha. The objective of this review is to assess the weight of evidence (WOE) that PPARalpha is or is not mechanistically involved in mouse liver tumor induction by TCE and metabolites. Based on similarities of TCE and TCA to typical PP, including dose-response characteristics showing PPARalpha-dependent responses coincident with liver tumor induction and abolishment of TCE and TCA effects in PPARalpha-null mice, the WOE supports the hypothesis that PPARalpha plays a dominant role in TCE- and TCA-induced hepatocarcinogenesis. Data indicates that the MOA for DCA tumor induction is PPARalpha-independent. Uncertainties remain regarding the genesis of the TCE-induced tumors. In contrast to the TCA-induced tumors, which have molecular features similar to those induced by typical PP, there is evidence, albeit weak, that TCE tumors arise by a mode of action (MOA) different from that of TCA tumors, based largely on dissimilarities in molecular markers found in TCE versus TCA-induced tumors. In summary, the WOE indicates that TCA-induced liver tumors arise by a PPARalpha-dependent MOA. Although the TCE MOA is likely dominated by a PPARalpha-dependent contribution from TCA, the contribution of a PPARalpha-independent MOA from DCA cannot be ruled out.

Role of the p50 subunit of NF-kappaB in vitamin E-induced changes in mice treated with the peroxisome proliferator, ciprofibrate

Peroxisome proliferators (PPs) are a diverse class of chemicals, which cause a dramatic increase in the size and number of hepatic peroxisomes in rodents and eventually lead to the development of hepatic tumors. Nuclear factor-kappaB (NF-kappaB) is a transcription factor activated by reactive oxygen and is involved in cell proliferation and apoptosis. Previously we found that the peroxisome proliferator ciprofibrate (CIP) activates NF-kappaB and that dietary vitamin E decreases CIP-induced NF-kappaB DNA binding. We, therefore, hypothesized that inhibition of NF-kappaB by vitamin E is necessary for effects of vitamin E on CIP-induced cell proliferation and the inhibition of apoptosis by CIP. Sixteen B6129 female mice (p50+/+) and twenty mice deficient in the p50 subunit of NF-kappaB (p50-/-) were fed a purified diet containing 10 or 250mg/kg vitamin E (alpha-tocopherol acetate) for 28 days. At that time, half of the mice were placed on the same diet with 0.01% CIP for 10 days. CIP treatment increased the DNA binding activity of NF-kappaB and cell proliferation, but had no significant effect on apoptosis. Compared to wild-type mice, the p50-/- mice had lower NF-kappaB activation, higher basal levels of cell proliferation and apoptosis, and a lower ratio of reduced glutathione to oxidized glutathione (GSH/GSSG). There was approximately a 60% reduction in cell proliferation in the CIP-treated p50-/- mice fed higher vitamin E in comparison to the p50-/- mice fed lower vitamin E. Dietary vitamin E also inhibited the DNA binding activity of NF-kappaB, increased apoptosis, and increased the GSH/GSSG ratio. This study shows the effects of vitamin E on cell growth parameters do not appear to be solely through decreased NF-kappaB activation, suggesting that vitamin E is acting by other molecular mechanisms.

Possible involvement of oxidative stress in fenofibrate-induced hepatocarcinogenesis in rats

To clarify whether oxidative stress is involved in the development of hepatocellular preneoplastic foci induced by fenofibrate (FF), a peroxisome proliferator-activated receptor alpha agonist, male F344/N rats were fed a diet containing 6,000, 3,000, or 0 ppm of FF for 13 weeks after N-diethylnitrosamine initiation. Two-third partial hepatectomy was performed 1 week after the FF treatment. Histopathologically, the number of hepatocellular altered foci significantly increased in the FF-treated groups with a concomitant increase in the number of hepatocytes positive for anti-Ki-67 antibody, but the number and area of glutathione S-transferase placental form (GST-P)-positive foci decreased in these groups, as compared to those in the controls. Microarray analysis or quantitative real-time reverse transcription-polymerase chine reaction demonstrated the significant up-regulations of Aco and Cyp4a1 (genes related to lipid metabolism); Gpx2, Yc2, Cat, Cyp2b15, and Ugt1a6 (metabolic oxidative stress-related genes); Apex1, Mgmt, Xrcc5, Nbn, and Gadd45a (DNA repair-related genes); and Ccnd1 (cell cycle-related genes) in the FF-treated groups, and the significant down-regulations of Cyp1a2, Gsta2, Gstm2, and Gstm3 (phase I or II metabolism-related genes); Mlh1 and Top1 (DNA repair-related genes); and Cdkn1a, Cdkn1b, Chek2, and Gadd45b (cell cycle/apoptosis-related genes) in these rats. FF-treatment increased the activity of enzymes such as carnitine acetyltransferase, carnitine palmitoyltransferase, fatty acyl-CoA oxidizing system, and catalase in the liver, but not superoxide dismutase in the liver. In addition, 8-OHdG level in liver DNA, lipofuscin deposition in hepatocytes, and in vitro reactive oxygen species production in microsomes significantly increased due to FF treatment. These results suggest that oxidative stress is involved in the development of FF-induced hepatocellular preneoplastic foci in rats.

Genomic profiling in nuclear receptor-mediated toxicity

Nuclear receptors (NRs) are attractive drug targets due to their role in regulation of a wide range of physiologic responses. In addition to providing therapeutic value, many pharmaceutical agents along with environmental chemicals are ligands for NRs and can cause adverse health effects that are directly related to activation of NRs. Identifying the molecular events that produce a toxic response may be confounded by the fact that there is a significant overlap in the biological processes that NRs regulate. Microarrays and other methods for gene expression profiling have served as useful, sensitive tools for discerning the mechanisms by which therapeutics and environmental chemicals invoke toxic effects. The capability to probe thousands of genes simultaneously has made genomics a prime technology for identifying drug targets, biomarkers of exposure/toxicity and key players in the mechanisms of disease. The complex intertwining networks regulated by NRs are hard to probe comprehensively without global approaches and genomics has become a key technology that facilitates our understanding of NR-dependent and -independent events. The future of drug discovery, design and optimization, and risk assessment of chemical toxicants that activate NRs will inevitably involve genomic profiling. This review will focus on genomics studies related to PPAR, CAR, PXR, RXR, LXR, FXR, and AHR.

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.

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

Differential gene expression in mouse primary hepatocytes exposed to the peroxisome proliferator-activated receptor alpha agonists

Background

Fibrates are a unique hypolipidemic drugs that lower plasma triglyceride and cholesterol levels through their action as peroxisome proliferator-activated receptor alpha (PPARα) agonists. The activation of PPARα leads to a cascade of events that result in the pharmacological (hypolipidemic) and adverse (carcinogenic) effects in rodent liver.

Results

To understand the molecular mechanisms responsible for the pleiotropic effects of PPARα agonists, we treated mouse primary hepatocytes with three PPARα agonists (bezafibrate, fenofibrate, and WY-14,643) at multiple concentrations (0, 10, 30, and 100 μM) for 24 hours. When primary hepatocytes were exposed to these agents, transactivation of PPARα was elevated as measured by luciferase assay. Global gene expression profiles in response to PPARα agonists were obtained by microarray analysis. Among differentially expressed genes (DEGs), there were 4, 8, and 21 genes commonly regulated by bezafibrate, fenofibrate, and WY-14,643 treatments across 3 doses, respectively, in a dose-dependent manner. Treatments with 100 μM of bezafibrate, fenofibrate, and WY-14,643 resulted in 151, 149, and 145 genes altered, respectively. Among them, 121 genes were commonly regulated by at least two drugs. Many genes are involved in fatty acid metabolism including oxidative reaction. Some of the gene changes were associated with production of reactive oxygen species, cell proliferation of peroxisomes, and hepatic disorders. In addition, 11 genes related to the development of liver cancer were observed.

Conclusion

Our results suggest that treatment of PPARα agonists results in the production of oxidative stress and increased peroxisome proliferation, thus providing a better understanding of mechanisms underlying PPARα agonist-induced hepatic disorders and hepatocarcinomas.

HMG-CoA reductase and PPARalpha are involved in clofibrate-induced apoptosis in human keratinocytes

Contrasting data have been reported on the effects of clofibrate, a PPARalpha agonist and hypolipidemic drug. The carcinogenic and anti-apoptotic effects have been demonstrated especially in rodents in both "in vivo" and "in vitro" experiments. In contrast, in rat and human hepatoma cell lines, several reports have shown its concentration-dependent pro-apoptotic effect. No epidemiological data exist about its carcinogenetic effect in man. This study shows that clofibrate also induced apoptosis in a human non-tumour cell line, NCTC 2544, which shares the characteristic of proliferation with tumour cells. Both HMG-CoA reductase and PPARalpha were found to be involved in the signal transduction pathway inducing apoptosis, the former being the principal target: HMG-CoA reductase decreased and PPARalpha increased. Changes in HMG-CoA reductase expression caused activation of parameters leading to apoptosis via the mitochondria pathway. Clofibrate must be considered a pro-apoptotic molecule at concentrations of 0.25 mM and above: the effect is exercised not only on tumour cells but also on normal human proliferating cells. Clofibrate should thus be regarded as a potential drug to reduce the number of proliferating cells in pathological conditions.

In vitro and in vivo effects of the PPAR-alpha agonists fenofibrate and retinoic acid in endometrial cancer

Fenofibrate, an agonist of PPAR-alpha, in doses above 25 μM, inhibits proliferation and induces apoptosis in Ishikawa endometrial cancer cells. We show that these effects are potentiated by retinoic acid, an agonist of the retinoid-X-receptor. DNA content analysis shows that G₁/S phase progression through the cell cycle is inhibited. Independent Component Analysis of gene microarray experiments demonstrated downregulation of Cyclin D1 (CCND1) and associated changes in cell cycle gene expression. Expression of PPAR-alpha mRNA was reduced by >75% using RNA-interference but this resulted in only minor changes in biological effects. A nude mouse model of endometrial carcinoma was used to investigate the effect of fenofibrate in vivo but failed to show consistent inhibition of tumour growth.

The combination of fenofibrate and retinoic acid is a potent inhibitor of Ishikawa endometrial cancer cell growth in vitro.

Cyclooxygenase-2 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third leading cause of cancer related mortality worldwide. The incidence of HCC is rising worldwide, especially in the United States. The overall survival of patients with HCC is grim and currently no efficient secondary prevention or systemic treatments are available. Recent evidence suggests that COX-2 signaling is implicated in hepatocarcinogenesis and COX-2 inhibitors prevent HCC cell growth in vitro and in animal models. However, given the recently reported side effect associated with some of the COX-2 inhibitors, it is imperative to develop chemotherapeutic strategy that simultaneously targets COX-2 and other related key molecules in hepatocarcinogenesis or to utilize agents inhibiting COX-2 signaling in conjunction with other standard chemotherapy or radiation therapy. Such combinational therapeutic approaches are expected to provide synergistic anti-tumor effect with lesser side effect. In this regard, the recently delineated interplay between COX-2-derived PG signaling and other growth-regulatory pathways such as EGFR, Met, iNOS, VEGF and n-3 polyunsaturated fatty acids is expected to provide important therapeutic implications. This review summarizes the recent advances in understanding the mechanisms for COX-2-derived PG signaling in hepatocarcinogenesis and focuses on the newly unveiled interactions between PG cascade and other key signaling pathways that coordinately regulate HCC growth. Understanding these mechanisms and interplays will facilitate the development of more effective chemopreventive and therapeutic strategies.

Di-(2-ethylhexyl)phthalate (DEHP) increases Bcl-2/Bax ratio and modifies c-myc expression in Syrian hamster embryo (SHE) cells

The objective of this work was to study the anti-apoptotic properties of the non-genotoxic rodent carcinogen, di(2-ethylhexyl)phthalate (DEHP) in Syrian hamster embryo (SHE) cells. We demonstrated that a 24 h pre-treatment of SHE cells with 50 microM DEHP inhibited apoptosis triggered by growth factors deprivation. The RNA expression levels of the regulator genes involved in the apoptotic pathway, bcl-2, bax and of c-myc were measured using Western blotting and RT-PCR. We showed that a 24 h treatment of SHE cells with 50 microM DEHP increased (P < 0.05) the bcl-2 expression, while c-myc expression was decreased. No effect on bax expression was observed in the range of 10-50 microM. The defective regulation of apoptosis caused by DEHP treatment could contribute to its carcinogenicity.

Peroxisome proliferator-activated receptor-α and liver cancer: where do we stand?

The peroxisome proliferator-activated receptor-alpha (PPARalpha), first identified in 1990 as a member of the nuclear receptor superfamily, has a central role in the regulation of numerous target genes encoding proteins that modulate fatty acid transport and catabolism. PPARalpha is the molecular target for the widely prescribed lipid-lowering fibrate drugs and the diverse class of chemicals collectively referred to as peroxisome proliferators. The lipid-lowering function of PPARalpha occurs across a number of mammalian species, thus demonstrating the essential role of this nuclear receptor in lipid homeostasis. In contrast, prolonged administration of PPARalpha agonists causes hepatocarcinogenesis, specifically in rats and mice, indicating that PPARalpha also mediates this effect. There is no strong evidence that the low-affinity fibrate ligands are associated with cancer in humans, but it still remains a possibility that chronic activation with high-affinity ligands could be carcinogenic in humans. It is now established that the species difference between rodents and humans in response to peroxisome proliferators is due in part to PPARalpha. The cascade of molecular events leading to liver cancer in rodents involves hepatocyte proliferation and oxidative stress, but the PPARalpha target genes that mediate this response are unknown. This review focuses on the current understanding of the role of PPARalpha in hepatocarcinogenesis and identifies future research directions that should be taken to delineate the mechanisms underlying PPARalpha agonist-induced hepatocarcinogenesis.

PPARalpha agonists stimulate progastrin production in human colorectal carcinoma cells

The three subtypes of peroxisome proliferator activated-receptors (PPARalpha, delta and gamma) control the storage and metabolism of fatty acids. Treatment of rats with the PPARalpha ligand ciprofibrate increases serum gastrin concentrations, and several lines of evidence suggest that non-amidated gastrins act as growth factors for the colonic mucosa. The aim of the present study was to investigate the expression of PPARs and the effect of PPAR ligands on gastrin production and cell proliferation in human colorectal carcinoma (CRC) cell lines. mRNAs for all three PPAR subtypes were detected by PCR in all CRC cell lines tested. The concentrations of progastrin, but not of glycine-extended or amidated gastrin, measured by radioimmunoassay in LIM 1899 conditioned media and cell extracts were significantly increased by treatment with the PPARalpha ligand clofibrate. Similar increases in progastrin were seen following treatment with the PPARalpha ligands ciprofibrate and fenofibrate, but not with bezafibrate, gemfibrozil or Wy 14643. The PPARgamma agonist rosiglitazone had no significant effect on progastrin production. The PPARalpha ligand clofibrate also stimulated proliferation of the LIM 1899 cell line. We conclude that some PPARalpha ligands increase progastrin production by the human CRC cell line LIM 1899, and that clofibrate increases proliferation of LIM 1899 cells. These studies have revealed a relationship between PPARs and gastrin, two regulatory molecules implicated in the pathogenesis of CRC.

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.

Biochemical and morphological effects of K-111, a peroxisome proliferator-activated receptor (PPAR)alpha activator, in non-human primates

K-111 has been characterized as a potent peroxisome proliferator-activated receptor (PPAR)alpha activator. Antidiabetic potency and amelioration of disturbed lipid metabolism were demonstrated in rodents, which were accompanied by elevations of peroxisomal enzymes and liver weight. To examine the possible therapeutic application of K-111 we have now assessed its efficacy in non-human primates with high transferability to humans. For this purpose obese, hypertriglyceridaemic, hyperinsulinaemic prediabetic rhesus monkeys were dosed sequentially with 0, 1, 3 and 10mg/kg per day orally over a period of 4 weeks each. In addition, the effect of K-111 on the peroxisome compartment was analyzed in cynomolgus monkeys using liver samples obtained following a 13-week oral toxicity study. In prediabetic monkeys, the reduction of hyperinsulinaemia and improvement of insulin-stimulated glucose uptake rate indicated amelioration of insulin resistance. These effects were nearly maximal at a dose of 3mg/kg per day, while triglycerides and body weight were lowered significantly in a dose-dependent manner. This reduction of body weight contrasts sharply with the adipogenic response observed with thiazolidinediones, another family of insulin-sensitizing agents. In young cynomolgus monkeys at a dosage of 5mg/kg per day and more, K-111 induced an up to three-fold increase in lipid beta-oxidation enzymes with an 1.5- to 2-fold increase in peroxisome volume density. This moderate increase in peroxisomal activity by K-111 in monkeys is consistent with its role as an PPARalpha activator and corresponds to the observations with fibrates in other low responder mammalian species. The increase in beta-oxidation may explain, at least in part, the lipid modulating effect as well as the antidiabetic potency of K-111. This pharmacological profile makes K-111 a highly promising drug candidate for clinical applications in the treatment of type 2 diabetes, dyslipidaemia, obesity and the metabolic syndrome.

Effects of vitamin E on the NF-κB pathway in rats treated with the peroxisome proliferator, ciprofibrate

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic compounds, which induce hepatic tumors in rodents. The mechanisms leading to hepatic tumors have not been elucidated, but oxidative stress may play a role in the process. Previous studies in our laboratory have shown that peroxisome proliferators activate the transcription factor nuclear factor-kappa B (NF-kappaB) and that this activation is mediated at least in part by oxidative stress. We therefore hypothesized that increased dietary vitamin E would decrease NF-kappaB DNA binding in rodents treated with ciprofibrate (CIP). In this study, 36 male Sprague-Dawley rats were fed a purified diet containing varying levels of vitamin E (10, 50, 250 ppm alpha-tocopherol acetate). After 28 days on the purified diet, seven animals per vitamin E group received 0.01% CIP in the diet for 10 days. Electrophoretic mobility shift assays (EMSAs) showed that CIP treatment increased DNA binding of NF-kappaB. Increased dietary alpha-tocopherol acetate inhibited CIP-induced NF-kappaB DNA binding. Because NF-kappaB translocates to the nucleus upon the phosphorylation and degradation of inhibitor of IkappaB, we also used Western blots to measure cytosolic protein levels of IkappaBalpha and IkappaBbeta, and the IkappaB kinases, IKKalpha and IKKbeta. IkappaBalpha protein levels were decreased in all three CIP-treated groups, with the 10 ppm vitamin E diet also decreasing IkappaBalpha levels in control rats. No difference in IkappaBbeta protein levels was observed among any of the groups. The CIP-treated rats generally had lower protein levels of IKKalpha and IKKbeta. This study supports our working hypothesis that an increased antioxidant environment can inhibit CIP-mediated NF-kappaB induction.

Transcription profiling distinguishes dose-dependent effects in the livers of rats treated with clofibrate

Peroxisome proliferators such as the fibrates act via the peroxisome proliferator activated receptor (PPAR)-alpha as hypolipidemic agents. Many peroxisome proliferators are also nongenotoxic hepatic carcinogens and hepatotoxicants in rodents. We performed transcription profiling using cDNA microarrays on livers of rats treated for 5 days with 3 doses of the peroxisome proliferator clofibrate. All 3 doses had hepatic effects as assessed by liver to body weight ratio, alanine aminotransferase (ALT) increases and histopathology examination. Analysis of the transcription profiling data identified changes in the expression of many genes within several mechanistic pathways that support existing hypotheses regarding peroxisome proliferator mediated carcinogenicity. Additionally, the transcription profiling, histopathology, and clinical chemistry results suggested a biphasic response to clofibrate. These findings provide insight into the pathogenesis of toxic and carcinogenic effects of clofibrate in rodents and demonstrate the ability of cDNA microarrays to provide information regarding mechanisms of toxicity identified during the drug development process.

Increased expression of the lipocalin 24p3 as an apoptotic mechanism for MK886

MK886, a strong proapoptotic agent, is an inhibitor of 5-lipoxygenase (LOX) through binding to the 5-LOX-activating protein (FLAP). Although MK886-induced apoptosis is through a FLAP-independent pathway, the precise mechanisms are not understood. In the present study, a possible role of 24p3, a lipocalin, in MK886-induced apoptosis was investigated. Exposure of murine prolymphoid progenitor cells (FL5.12) to 20 microM MK886 for 16 h dramatically increased 24p3 mRNA and protein expression. Induction could also be achieved with another FLAP inhibitor, MK591. The induction of 24p3 by MK886 was dose- and time-dependent. The up-regulated 24p3 mRNA expression by MK886 was enhanced a further 3.1-fold by WY14643, an activator of peroxisome-proliferator-activated receptor alpha, whereas ciglitazone, an activator of peroxisome-proliferator-activated receptor gamma attenuated the MK886-induced 24p3 expression by more than 50%. Neither WY14643 nor ciglitazone alone had any effect on the expression of 24p3. The induction of 24p3 by MK886 was dependent on the synthesis of new protein(s), since cycloheximide, an inhibitor of protein synthesis, prevented this effect. In all cases, including the inhibition of MK886-induced 24p3 protein expression by stable transfection with antisense cDNA of 24p3, the extent of apoptosis closely paralleled 24p3 levels. Apoptosis induced by MK886, or enhanced by WY14643, was accompanied by the cleavage and activation of caspase-3. The overexpression of bcl-2 or bcl-x(L) in FL5.12 cells inhibited apoptosis induced by MK886 as well as the enhancement of apoptosis by WY14643. Thus 24p3 is an MK886-inducible gene and may play an important role in MK886-induced apoptosis.

Comprehensive analysis of gene expression in rat and human hepatoma cells exposed to the peroxisome proliferator WY14,643

Peroxisome proliferators (PPs) are an important class of chemicals that act as hepatic tumor promoters in laboratory rodents. The key target for PPs is the nuclear receptor peroxisome proliferator-activated receptor-alpha (PPARalpha) and these chemicals cause cancer by altering the expression of a subset of genes involved in cell growth regulation. The purpose of the present study was to utilize high-density gene expression arrays to examine the genes regulated by the potent PP Wy14,643 (50 microM, 6 h) in both rat (FaO) and human (HepG2) hepatoma cells. Treatment of FaO cells, but not HepG2, revealed the expected fatty acid catabolism genes. However, a larger than expected number of protein kinases, phosphatases, and signaling molecules were also affected exclusively in the FaO cells, including MAPK-phosphatase 1 (MKP-1), Janus-activated kinases 1 and 2 (JAK1 and 2), and glycogen synthetase kinase alpha and beta (GSKalpha and beta). The mRNA accumulation of these genes as well as the protein level for GSK3alpha, JAK1, and JAK2 and MKP-1 activity was corroborated. Due to the importance of MKP-1 in cell signaling, this induction was examined further and was found to be controlled, at least in part, at the level of the gene's promoter. Interestingly, overexpression of MKP-1 in turn affected the constitutive activity of PPARalpha. Taken together, the gene expression arrays revealed an important subset of PP-regulated genes to be kinases and phosphatases. These enzymes not only would affect growth factor signaling and cell cycle control but also could represent feedback control mechanisms and modulate the activity of PPARalpha.

RXR activators molecular signalling: involvement of a PPAR alpha-dependent pathway in the liver and kidney, evidence for an alternative pathway in the heart

(1) In this study we compared the molecular signalling elicited by rexinoids, selective retinoid X receptor (RXR)-activators, in several organs (i.e. liver, kidney, heart) and in hepatocytes of various species. (2) RXR plays the pivotal role of a hetero-dimerization partner for the members of the class II subset of nuclear receptors which regulate the transcription of numerous target genes, following chemical activation. Several of these selective activators are currently used to treat hyperlipidaemia (fibrates), type II diabetes (glitazones), or skin disorders (retinoic acid). Although these therapeutic pathways are not fully elucidated, receptor activation is considered a pre-requisite for efficacy. Therefore RXR, which accepts numerous dimeric partners, is considered a worthwhile pharmacological target. (3) We analysed a number of biochemical and molecular responses to rexinoids which were given orally to mice. Our results showed a prominent involvement of the peroxisome proliferator-activated receptor (PPARalpha) as a majority of the observed hepatic and renal regulations were abolished in PPARalpha-knockout animals. Therefore we documented the species-specificity of these rexinoid actions which were reproduced in rat primary hepatocyte cultures but not in cultures of rabbit or human origin. Conversely, we established that the regulation of the pyruvate dehydrogenase kinase (PDK4) gene in the heart, by rexinoids, is independent of PPARalpha expression. (4) Our results support the obligatory expression of the active, although quiescent, PPARalpha to sustain a subset of relevant regulations attributable to rexinoids in the liver and kidney. Their cardiac molecular signalling unveiled an alternate transduction pathway and therefore opens new prospects in the therapeutic potential of rexinoids.

Hepatocarcinogenic potential of the glucocorticoid antagonist RU486 in B6C3F1 mice: effect on apoptosis, expression of oncogenes and the tumor suppressor gene p53

BACKGROUND

Glucocorticoids inhibit hepatocellular proliferation and modulate the expression of oncogenes and tumor suppressor genes via mechanisms involving the glucocorticoid receptor. Glucocorticoids also produce a receptor-mediated inhibitory effect on both basal and hormone-stimulated expression of a newly discovered family of molecules important for shutting off cytokine action. We therefore hypothesized that inhibiting glucocorticoid receptors may disturb hepatocellular growth and apoptosis. Consequently, we investigated the effect of RU486, a potent antagonist of the glucocorticoid receptor, on basal levels of hepatocellular proliferation and apoptosis in male B6C3F1 mice. Furthermore, we evaluated the effect of this compound on cellular genes involved in the regulation of these important processes.

RESULTS

Data show that treatment of male B6F3C1 mice with RU486 (2 mg/kg/d, ip) for 7 days dramatically inhibited liver cell proliferation by about 45% and programmed hepatocellular death by approximately 66%. RU 486 also significantly increased hepatic expression of the oncogenes mdm2 and JunB, while reducing that of the tumor suppressor gene p53.

CONCLUSION

Exposure to RU486 may ultimately enhance the susceptibility of the liver to cancer risk by diminishing its ability to purge itself of pre-cancerous cells via apoptosis. This effect may be mediated through increases in the hepatic expression of the oncogene mdm2, coupled with decreases in that of the tumor suppressor gene p53. The decrease in hepatocellular proliferation caused by RU 486 may be related to effects other than its anti-glucocorticoid activity.

PPAR alpha and the regulation of cell division and apoptosis

Peroxisome proliferators (PPs) such as the hypolipidaemic drug, nafenopin and the phthalate plasticiser 2-diethylhexylphthalate induce rodent hepatocyte cell proliferation and suppress apoptosis leading to tumours. PPs act via the nuclear hormone receptor peroxisome proliferator activated receptor alpha (PPAR alpha) which directly regulates genes implicated in the response to PPs such as the peroxisomal gene acyl CoA oxidase. As expected for xenobiotics that perturb proliferation, PPs alter expression of cell cycle regulatory proteins. However, the ability to alter expression of cyclins and cyclin-dependent kinases is shared by physiological hepatic mitogens such as epidermal growth factor and is thus unlikely to be specific to the PP-induced aberrant growth associated with hepatocarcinogenesis. Recent evidence suggests that the response of hepatocytes to PPs is not only dependent upon PPAR alpha but also on the trophic environment provided by nonparenchymal cells and by cytokines such as tumour necrosis factor alpha. Additionally, the ability of PPs to suppress apoptosis and induce proliferation depends upon survival signalling mediated by p38 mitogen activated protein kinase. The cross talk between PPAR alpha-mediated transcription, survival signalling and cell cycle will be discussed with particular emphasis on relevance to toxicology.

Clofibric acid down-regulation of metallothionein IIA in HepG2 human hepatoma cells

Among the different hypotheses advanced to explain the peroxisome proliferator (PP)-induced hepatocarcinogenicity in rodents, one is based on the development of an oxidative stress due to an imbalance in the production of reactive oxygen species that leads to DNA damages and lipid peroxidation. On the other hand, human cells appear to be nonresponsive to PPs. As metallothionein proteins play an important antioxidant role, the aim of the present study was to investigate the expression of metallothionein IA (MTIA) and IIA (MTIIA) in HepG2 human hepatoma cells exposed to clofibric acid. When HepG2 cells were treated for 24 hr with 0.50 or 0.75 mM CA, a significant decrease was observed in MT protein-level determined by Western blotting and in the MTIIA mRNA content analyzed by RT-PCR and Northern blotting. No significant change was observed in the MTIA mRNA amount whatever the CA concentration and the duration of treatment. The decrease in MTIIA mRNA-level was not mediated via peroxisome proliferator-activated receptor alpha as attested by our data from gel mobility shift DNA binding assays, Dot blotting and cotransfection experiments with MTIIA promoter-driven luciferase reporter gene and PPARalpha expression vector. These results provide new insights about the pleiotropic effects of PPs on human cells.

Differential gene regulation in human versus rodent hepatocytes by peroxisome proliferator-activated receptor (PPAR) alpha. PPAR alpha fails to induce peroxisome proliferation-associated genes in human cells independently of the level of receptor expresson

We compared the ability of rat and human hepatocytes to respond to fenofibric acid and a novel potent phenylacetic acid peroxisome proliferator-activated receptor (PPAR) alpha agonist (compound 1). Fatty acyl-CoA oxidase (FACO) activity and mRNA were increased after treatment with either fenofibric acid or compound 1 in rat hepatocytes. In addition, apolipoprotein CIII mRNA was decreased by both fenofibric acid and compound 1 in rat hepatocytes. Both agonists decreased apolipoprotein CIII mRNA in human hepatocytes; however, very little change in FACO activity or mRNA was observed. Furthermore, other peroxisome proliferation (PP)-associated genes including peroxisomal 3-oxoacyl-CoA thiolase (THIO), peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (HD), peroxisomal membrane protein-70 (PMP-70) were not regulated by PPAR alpha agonists in human hepatocytes. Moreover, other genes that are regulated by PPAR alpha ligands in human hepatocytes such as mitochondrial HMG-CoA synthase and carnitine palmitoyl transferase-1 (CPT-1) were also regulated in HepG2 cells by PPAR alpha agonists. Several stably transfected HepG2 cell lines were established that overexpressed human PPAR alpha to levels between 6- and 26-fold over normal human hepatocytes. These PPAR alpha-overexpressing cells had higher basal mRNA levels of mitochondrial HMG-CoA synthase and CPT-1; however, basal FACO mRNA levels and other PP-associated genes including THIO, HD, or PMP-70 mRNA were not substantially affected. In addition, FACO, THIO, HD, and PMP-70 mRNA levels did not increase in response to PPAR alpha agonist treatment in the PPAR alpha-overexpressing cells, although mitochondrial HMG-CoA synthase and CPT-1 mRNAs were both induced. These results suggest that other factors besides PPAR alpha levels determine the species-specific response of human and rat hepatocytes to the induction of PP.

Peroxisome proliferator-activated receptors in tumorigenesis: targets of tumour promotion and treatment

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors. There are three genes that code for the PPAR isoforms: PPARalpha, PPARbeta and PPARgamma. In the present review, studies characterizing the various PPAR isoforms are discussed. Peroxisome proliferator-activated receptor alpha has been implicated in the lipid-lowering effects of the fibrate drugs. Peroxisome proliferator-activated receptor gamma has a clear role in adipocyte differentiation and is therapeutically targeted by the thiazolidinedione drugs for the treatment of type II diabetes. The physiological role of PPARbeta is less well understood but, as described in the present review, recent studies have implicated it with a role in colon cancer. In the present review, particular attention is focused on the role of PPAR in the regulation of expression of proteins associated with cell cycle control and tumorigenesis.