Peroxisome proliferator-activated receptors: structures and function

Peroxisome proliferator-activated receptors: A key link between lipid metabolism and cancer progression

Lipids represent the essential components of membranes, serve as fuels for high-energy processes, and play crucial roles in signaling and cellular function. One of the key hallmarks of cancer is the reprogramming of metabolic pathways, especially abnormal lipid metabolism. Alterations in lipid uptake, lipid desaturation, de novo lipogenesis, lipid droplets, and fatty acid oxidation in cancer cells all contribute to cell survival in a changing microenvironment by regulating feedforward oncogenic signals, key oncogenic functions, oxidative and other stresses, immune responses, or intercellular communication. Peroxisome proliferator-activated receptors (PPARs) are transcription factors activated by fatty acids and act as core lipid sensors involved in the regulation of lipid homeostasis and cell fate. In addition to regulating whole-body energy homeostasis in physiological states, PPARs play a key role in lipid metabolism in cancer, which is receiving increasing research attention, especially the fundamental molecular mechanisms and cancer therapies targeting PPARs. In this review, we discuss how cancer cells alter metabolic patterns and regulate lipid metabolism to promote their own survival and progression through PPARs. Finally, we discuss potential therapeutic strategies for targeting PPARs in cancer based on recent studies from the last five years.

Pathogenesis of Alcohol-Associated Fatty Liver: Lessons From Transgenic Mice

Alcohol-associated liver disease (ALD) is a major public health issue that significantly contributes to human morbidity and mortality, with no FDA-approved therapeutic intervention available. The health burden of ALD has worsened during the COVID-19 pandemic, which has been associated with a spike in alcohol abuse, and a subsequent increase in hospitalization rates for ALD. A key knowledge gap that underlies the lack of novel therapies for ALD is a need to better understand the pathogenic mechanisms that contribute to ALD initiation, particularly with respect to hepatic lipid accumulation and the development of fatty liver, which is the first step in the ALD spectrum. The goal of this review is to evaluate the existing literature to gain insight into the pathogenesis of alcohol-associated fatty liver, and to synthesize alcohol’s known effects on hepatic lipid metabolism. To achieve this goal, we specifically focus on studies from transgenic mouse models of ALD, allowing for a genetic dissection of alcohol’s effects, and integrate these findings with our current understanding of ALD pathogenesis. Existing studies using transgenic mouse models of ALD have revealed roles for specific genes involved in hepatic lipid metabolic pathways including fatty acid uptake, mitochondrial β-oxidation, de novo lipogenesis, triglyceride metabolism, and lipid droplet formation. In addition to reviewing this literature, we conclude by identifying current gaps in our understanding of how alcohol abuse impairs hepatic lipid metabolism and identify future directions to address these gaps. In summary, transgenic mice provide a powerful tool to understand alcohol’s effect on hepatic lipid metabolism and highlight that alcohol abuse has diverse effects that contribute to the development of alcohol-associated fatty liver disease.

Systemic PFOS and PFOA exposure and disturbed lipid homeostasis in humans: what do we know and what not?

Associations between per- and polyfluoroalkyl substances (PFASs) and increased blood lipids have been repeatedly observed in humans, but a causal relation has been debated. Rodent studies show reverse effects, i.e. decreased blood cholesterol and triglycerides, occurring however at PFAS serum levels at least 100-fold higher than those in humans. This paper aims to present the main issues regarding the modulation of lipid homeostasis by the two most common PFASs, PFOS and PFOA, with emphasis on the underlying mechanisms relevant for humans. Overall, the apparent contrast between human and animal data may be an artifact of dose, with different molecular pathways coming into play upon exposure to PFASs at very low versus high levels. Altogether, the interpretation of existing rodent data on PFOS/PFOA-induced lipid perturbations with respect to the human situation is complex. From a mechanistic perspective, research on human liver cells shows that PFOS/PFOA activate the PPARα pathway, whereas studies on the involvement of other nuclear receptors, like PXR, are less conclusive. Other data indicate that suppression of the nuclear receptor HNF4α signaling pathway, as well as perturbations of bile acid metabolism and transport might be important cellular events that require further investigation. Future studies with human-relevant test systems would help to obtain more insight into the mechanistic pathways pertinent for humans. These studies shall be designed with a careful consideration of appropriate dosing and toxicokinetics, so as to enable biologically plausible quantitative extrapolations. Such research will increase the understanding of possible perturbed lipid homeostasis related to PFOS/ PFOA exposure and the potential implications for human health.

Pyruvate dehydrogenase kinases (PDKs): an overview toward clinical applications

Pyruvate dehydrogenase kinase (PDK) can regulate the catalytic activity of pyruvate decarboxylation oxidation via the mitochondrial pyruvate dehydrogenase complex, and it further links glycolysis with the tricarboxylic acid cycle and ATP generation. This review seeks to elucidate the regulation of PDK activity in different species, mainly mammals, and the role of PDK inhibitors in preventing increased blood glucose, reducing injury caused by myocardial ischemia, and inducing apoptosis of tumor cells. Regulations of PDKs expression or activity represent a very promising approach for treatment of metabolic diseases including diabetes, heart failure, and cancer. The future research and development could be more focused on the biochemical understanding of the diseases, which would help understand the cellular energy metabolism and its regulation by pharmacological effectors of PDKs.

Beneficial effects of elafibranor on NASH in E3L.CETP mice and differences between mice and men

Non-alcoholic steatohepatitis (NASH) is the most rapidly growing liver disease that is nevertheless without approved pharmacological treatment. Despite great effort in developing novel NASH therapeutics, many have failed in clinical trials. This has raised questions on the adequacy of preclinical models. Elafibranor is one of the drugs currently in late stage development which had mixed results for phase 2/interim phase 3 trials. In the current study we investigated the response of elafibranor in APOE*3Leiden.CETP mice, a translational animal model that displays histopathological characteristics of NASH in the context of obesity, insulin resistance and hyperlipidemia. To induce NASH, mice were fed a high fat and cholesterol (HFC) diet for 15 weeks (HFC reference group) or 25 weeks (HFC control group) or the HFC diet supplemented with elafibranor (15 mg/kg/d) from week 15–25 (elafibranor group). The effects on plasma parameters and NASH histopathology were assessed and hepatic transcriptome analysis was used to investigate the underlying pathways affected by elafibranor. Elafibranor treatment significantly reduced steatosis and hepatic inflammation and precluded the progression of fibrosis. The underlying disease pathways of the model were compared with those of NASH patients and illustrated substantial similarity with molecular pathways involved, with 87% recapitulation of human pathways in mice. We compared the response of elafibranor in the mice to the response in human patients and discuss potential pitfalls when translating preclinical results of novel NASH therapeutics to human patients. When taking into account that due to species differences the response to some targets, like PPAR-α, may be overrepresented in animal models, we conclude that elafibranor may be particularly useful to reduce hepatic inflammation and could be a pharmacologically useful agent for human NASH, but probably in combination with other agents.

Endocannabinoid System in Hepatic Glucose Metabolism, Fatty Liver Disease, and Cirrhosis

There is growing evidence that glucose metabolism in the liver is in part under the control of the endocannabinoid system (ECS) which is also supported by its presence in this organ. The ECS consists of its cannabinoid receptors (CBRs) and enzymes that are responsible for endocannabinoid production and metabolism. ECS is known to be differentially influenced by the hepatic glucose metabolism and insulin resistance, e.g., cannabinoid receptor type 1(CB₁) antagonist can improve the glucose tolerance and insulin resistance. Interestingly, our own study shows that expression patterns of CBRs are influenced by the light/dark cycle, which is of significant physiological and clinical interest. The ECS system is highly upregulated during chronic liver disease and a growing number of studies suggest a mechanistic and therapeutic impact of ECS on the development of liver fibrosis, especially putting its receptors into focus. An opposing effect of the CBRs was exerted via the CB₁ or CB₂ receptor stimulation. An activation of CB₁ promoted fibrogenesis, while CB₂ activation improved antifibrogenic responses. However, underlying mechanisms are not yet clear. In the context of liver diseases, the ECS is considered as a possible mediator, which seems to be involved in the synthesis of fibrotic tissue, increase of intrahepatic vascular resistance and subsequently development of portal hypertension. Portal hypertension is the main event that leads to complications of the disease. The main complication is the development of variceal bleeding and ascites, which have prognostic relevance for the patients. The present review summarizes the current understanding and impact of the ECS on glucose metabolism in the liver, in association with the development of liver cirrhosis and hemodynamics in cirrhosis and its complication, to give perspectives for development of new therapeutic strategies.

Insights into the Role of PPARβ/δ in NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a major health issue in developed countries. Although usually associated with obesity, NAFLD is also diagnosed in individuals with low body mass index (BMI) values, especially in Asia. NAFLD can progress from steatosis to non-alcoholic steatohepatitis (NASH), which is characterized by liver damage and inflammation, leading to cirrhosis and hepatocellular carcinoma (HCC). NAFLD development can be induced by lipid metabolism alterations; imbalances of pro- and anti-inflammatory molecules; and changes in various other factors, such as gut nutrient-derived signals and adipokines. Obesity-related metabolic disorders may be improved by activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)β/δ, which is involved in metabolic processes and other functions. This review is focused on research findings related to PPARβ/δ-mediated regulation of hepatic lipid and glucose metabolism and NAFLD development. It also discusses the potential use of pharmacological PPARβ/δ activation for NAFLD treatment.

The Impact of Di(2-ethylhexyl)phthalate on Cancer Progression

Di(2-ethylhexyl)phthalate (DEHP), a widely used plasticizer, mainly serves as an additive to render polyvinyl chloride (PVC) soft and flexible. PVC plastics have become ubiquitous in our modern society. Yet, the leaching of DEHP from PVC-based consumables ultimately results in the deposition in certain tissues via inadvertent applications. Health risks for human populations exposed to DEHP has been assumed by studies on rodents and other species, including the DEHP-induced developmental dysregulation, reproductive impairments, tumorigenesis, and diseases in a transgenerational manner. In this review, we comprehensively summarize the accumulated literature regarding the multifaceted roles of DEHP in the activation of the nuclear receptors, the alteration of the redox homeostasis, epigenetic modifications and the acquisition of chemoresistance.

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.

A combined genotype of three SNPs in the bovine gene is related to growth performance in Chinese cattle

Abstract. PPARD is involved in multiple biological processes, especially for those associated with energy metabolism. PPARD regulates lipid metabolism through up-regulate expression of genes associating with adipogenesis. This makes PPARD a significant candidate gene for production traits of livestock animals. Association studies between PPARD polymorphisms and production traits have been reported in pigs but are limited for other animals, including cattle. Here, we investigated the expression profile and polymorphism of bovine PPARD as well as their association with growth traits in Chinese cattle. Our results showed that the highest expression of PPARD was detected in kidney, following by adipose, which is consistent with its involvement in energy metabolism. Three SNPs of PPARD were detected and used to undergo selection pressure according the result of Hardy–Weinberg equilibrium analysis (P < 0.05). Moreover, all of these SNPs showed moderate diversity (0.25 < PIC < 0.5), indicating their relatively high selection potential. Association analysis suggested that individuals with the GAAGTT combined genotype of three SNPs detected showed optimal values in all of the growth traits analyzed. These results revealed that the GAAGTT combined genotype of three SNPs detected in the bovine PPARD gene was a significant potential genetic marker for marker-assisted selection in Chinese cattle. However, this should be further verified in larger populations before being applied to breeding.

Species-specific differences in peroxisome proliferation, catalase, and SOD2 upregulation as well as toxicity in human, mouse, and rat hepatoma cells induced by the explosive and environmental pollutant 2,4,6-trinitrotoluene

2,4,6-Trinitrotoluene (TNT) has been widely used as an explosive substance and its toxicity is still of interest as it persisted in polluted areas. TNT is metabolized in hepatocytes which are prone to its toxicity. Since analysis of the human liver or hepatocytes is restricted due to ethical reasons, we investigated the effects of TNT on cell viability, reactive oxygen species (ROS) production, peroxisome proliferation, and antioxidative enzymes in human (HepG2), mouse (Hepa 1-6), and rat (H4IIEC3) hepatoma cell lines. Under control conditions, hepatoma cells of all three species were highly comparable exhibiting identical proliferation rates and distribution of their cell cycle phases. However, we found strong differences in TNT toxicity with the lowest IC₅₀ values (highest cell death rate) for rat cells, whereas human and mouse cells were three to sevenfold less sensitive. Moreover, a strong decrease in cellular dehydrogenase activity (MTT assay) and increased ROS levels were noted. TNT caused peroxisome proliferation with rat hepatoma cells being most responsive followed by those from mouse and human. Under control conditions, rat cells contained fivefold higher peroxisomal catalase and mitochondrial SOD2 activities and a twofold higher capacity to reduce MTT than human and mouse cells. TNT treatment caused an increase in catalase and SOD2 mRNA and protein levels in human and mouse, but not in rat cells. Similarly, human and mouse cells upregulated SOD2 activity, whereas rat cells failed therein. We conclude that TNT induced oxidative stress, peroxisome proliferation and mitochondrial damage which are highest in rat cells rendering them most susceptible toward TNT. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 989-1006, 2017.

Maternal PPARG Pro12Ala polymorphism is associated with infant's neurodevelopmental outcomes at 18 months of age

BACKGROUND

Peroxisome proliferator activated receptors (PPARs) are ligand activated transcription factors with crucial functions in lipid homeostasis, glucose metabolism, anti-inflammatory processes, placental development, and are involved in cognitive functions and neurodegenerative diseases. Polymorphisms in PPAR genes are shown to influence the activity of these receptors.

AIMS

1) To examine the association of PPARG Pro12Ala polymorphism in pregnant women and their offspring on infant's neurodevelopmental outcomes during the first 18 months of life; 2) to determine the influence of Pro12Ala polymorphism on fatty acid concentrations in plasma phospholipids and placental tissue.

STUDY DESIGN

138 mother-infant pairs from the PREOBE observational study were genotyped for PPARG Pro12Ala. Plasma phospholipids and placental fatty acid concentrations were measured at delivery. Infants' neuropsychological assessment at 6 and 18 months of age was performed using Bayley III.

RESULTS

The effect of Pro12Ala on infant's neurodevelopmental outcomes was detected at 18 months, but not at 6 months of age. 18 months old infants born to mothers with wild-type Pro12 genotype had better cognitive (OR=5.11, 95% CI: 1.379-18.96, p=0.015), language (OR=3.41, 95% CI: 1.35-11.24, p=0.044), and motor development scores (OR=4.77, 95% CI: 1.243-18.33, p=0.023) than the Ala allele carriers. Pro12Ala variants did not seem to affect fatty acids concentrations in blood nor in placenta at delivery.

CONCLUSIONS

Infants born to mothers with Pro12 genotype have better neurodevelopmental outcomes at 18 months of age than Ala allele carriers, indicating a long-term transplacental action of PPARγ variants on foetal brain development.

Pterostilbene on metabolic parameters: a randomized, double-blind, and placebo-controlled trial

Introduction. The purpose of this trial was to evaluate the effect of pterostilbene on metabolic parameters. Methods. A prospective, randomized, double-blind, and placebo-controlled study that enrolled 80 patients with a total cholesterol ≥200 mg/dL and/or LDL ≥ 100 mg/dL. Subjects were divided into four groups: (1) pterostilbene 125 mg twice daily; (2) pterostilbene 50 mg twice daily; (3) pterostilbene 50 mg + grape extract (GE) 100 mg twice daily; (4) matching placebo twice daily for 6-8 weeks. Endpoints included lipids, blood pressure, and weight. Linear mixed models were used to examine and compare changes in parameters over time. Models were adjusted for age, gender, and race. Results. LDL increased with pterostilbene monotherapy (17.1 mg/dL; P = 0.001) which was not seen with GE combination (P = 0.47). Presence of a baseline cholesterol medication appeared to attenuate LDL effects. Both systolic (-7.8 mmHg; P < 0.01) and diastolic blood pressure (-7.3 mmHg; P < 0.001) were reduced with high dose pterostilbene. Patients not on cholesterol medication (n = 51) exhibited minor weight loss with pterostilbene (-0.62 kg/m(2); P = 0.012). Conclusion. Pterostilbene increases LDL and reduces blood pressure in adults. This trial is registered with Clinicaltrials.gov NCT01267227.

Cross-species gene expression analysis of species specific differences in the preclinical assessment of pharmaceutical compounds

Animals are frequently used as model systems for determination of safety and efficacy in pharmaceutical research and development. However, significant quantitative and qualitative differences exist between humans and the animal models used in research. This is as a result of genetic variation between human and the laboratory animal. Therefore the development of a system that would allow the assessment of all molecular differences between species after drug exposure would have a significant impact on drug evaluation for toxicity and efficacy. Here we describe a cross-species microarray methodology that identifies and selects orthologous probes after cross-species sequence comparison to develop an orthologous cross-species gene expression analysis tool. The assumptions made by the use of this orthologous gene expression strategy for cross-species extrapolation is that; conserved changes in gene expression equate to conserved pharmacodynamic endpoints. This assumption is supported by the fact that evolution and selection have maintained the structure and function of many biochemical pathways over time, resulting in the conservation of many important processes. We demonstrate this cross-species methodology by investigating species specific differences of the peroxisome proliferator-activator receptor (PPAR) α response in rat and human.

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.

PPARα-mediated responses in human adult liver stem cells: In vivo/in vitro and cross-species comparisons

The peroxisome proliferator-activated receptor α (PPARα) is a ligand-activated transcription factor that regulates a variety of biological processes including lipid metabolism and energy homeostasis. Peroxisome proliferators (PPs) are carcinogens in rodents, while humans are resistant to peroxisome proliferation and carcinogenesis. In this study, we examined the differential gene expression elicited by clofibrate (CLO) and WY-14,643 (WY) in C57BL/6 mouse liver compared to responses in human HepG2 hepatoma and HL1-1 adult stem cells. Mice were gavaged with sesame oil, 300mg/kg CLO or WY for 2, 4, 8, 12, 18 or 24h, or daily for 4 or 14 days. Although no significant changes in body weight gain were observed, WY induced relative liver weight at 4 and 14 days. Genome-wide hepatic gene expression analysis identified 719 and 1443 differentially expressed unique genes elicited by CLO and WY, respectively (|fold change|>1.5, P1(t)>0.99). Functional analysis associated the gene expression changes with lipid metabolism, transport, cell cycle and immune response. Most differentially expressed genes were in common to both treatments and clustered together only at early time points (2-8h). Complementary QRT-PCR studies in human HL1-1 and HepG2 cells treated with 50μM WY or DMSO for 1, 2, 4, 8, 12, 24 or 48h identified a minimal number of conserved orthologous responses (e.g., Pdk4, Adfp and Angptl4) while some genes (i.e., Bmf, a tumor suppressor) exhibited induction in human cells but repression in mice. These data suggest that PPs elicit species-specific PPARα-mediated gene expression.

Expression patterns of peroxisome proliferator-activated receptor gamma 1 versus gamma 2, and their association with intramuscular fat in goat tissues

Intramuscular fat (IMF) shortage causes the lack of juiciness and tenderness of goat meat, while peroxisome proliferator-activated receptor gamma 1 (PPARγ1) and gamma 2 (PPARγ2) play key roles in lipid metabolism. Nevertheless, their expression patterns and the relationship with IMF have been poorly exposed. Using quantitative polymerase chain reaction (qPCR), classical Soxhlet extraction, and in situ hybridization, we demonstrated that among 13 goat tissues, expression of PPARγ1 was dramatically higher than that of PPARγ2 except for lung. We further demonstrated the expression patterns of PPARγ1 and PPARγ2 and their negative association with intramuscular fat content in three goat muscles with kids growing. Meanwhile, PPARγ expression was located in the connective tissues. These results suggest that PPARγ1 is rather active for most tissues of goat, and closely related with the muscular fat metabolism during early postnatal life, but a more direct proof remains to be provided.

Mechanistic considerations for human relevance of cancer hazard of di(2-ethylhexyl) phthalate

Di(2-ethylhexyl) phthalate (DEHP) is a peroxisome proliferator agent that is widely used as a plasticizer to soften polyvinylchloride plastics and non-polymers. Both occupational (e.g., by inhalation during its manufacture and use as a plasticizer of polyvinylchloride) and environmental (medical devices, contamination of food, or intake from air, water and soil) routes of exposure to DEHP are of concern for human health. There is sufficient evidence for carcinogenicity of DEHP in the liver in both rats and mice; however, there is little epidemiological evidence on possible associations between exposure to DEHP and liver cancer in humans. Data are available to suggest that liver is not the only target tissue for DEHP-associated toxicity and carcinogenicity in both humans and rodents. The debate regarding human relevance of the findings in rats or mice has been informed by studies on the mechanisms of carcinogenesis of the peroxisome proliferator class of chemicals, including DEHP. Important additional mechanistic information became available in the past decade, including, but not limited to, sub-acute, sub-chronic and chronic studies with DEHP in peroxisome proliferator-activated receptor (PPAR) α-null mice, as well as experiments utilizing several transgenic mouse lines. Activation of PPARα and the subsequent downstream events mediated by this transcription factor represent an important mechanism of action for DEHP in rats and mice. However, additional data from animal models and studies in humans exposed to DEHP from the environment suggest that multiple molecular signals and pathways in several cell types in the liver, rather than a single molecular event, contribute to the cancer in rats and mice. In addition, the toxic and carcinogenic effects of DEHP are not limited to liver. The International Agency for Research on Cancer working group concluded that the human relevance of the molecular events leading to cancer elicited by DEHP in several target tissues (e.g., liver and testis) in rats and mice can not be ruled out and DEHP was classified as possibly carcinogenic to humans (Group 2B).

Orphan nuclear receptors as targets for drug development

Orphan nuclear receptors regulate diverse biological processes. These important molecules are ligand-activated transcription factors that act as natural sensors for a wide range of steroid hormones and xenobiotic ligands. Because of their importance in regulating various novel signaling pathways, recent research has focused on identifying xenobiotics targeting these receptors for the treatment of multiple human diseases. In this review, we will highlight these receptors in several physiologic and pathophysiologic actions and demonstrate how their functions can be exploited for the successful development of newer drugs.

Interaction of oxazaphosphorines with multidrug resistance-associated protein 4 (MRP4)

Multidrug resistance-associated protein 4 (MRP4) is an organic anion efflux pump capable of transporting nucleoside, nucleotide analogs, and cyclic nucleotide. MRP4 could have an influence on the resistance and transport of the two oxazaphosphorines, cyclophosphamide (CP) and ifosfamide (IF). V/HepG2 (HepG2, hepatoma cells stably transfected with an empty vehicle plasmid) and MRP4/HepG2 (HepG2 cells stably expressing MRP4) were exposed to CP and IF in the absence or presence of various MRP4 inhibitors. HepG2 and HEK293 human kidney cells were also used to investigate the inducing potency of oxazaphosphorines on the MRP4 expression. In this study, insertion of MRP4 gene in HepG2 cells was found to confer significant resistance to CP and IF in the 48-h drug-exposure assays. In the presence of various MRP4 inhibitors, the resistance to CP and IF was then partially reversed. These indicate that CP and IF are highly possible substrates of MRP4. In addition, CP and clofibrate (CFB), a reported MRP4 inducer, in vivo significantly increased the MRP4 expression at both protein level and mRNA level in HEK293 cells at higher concentrations, while IF significantly decreased the MRP4 expression at mRNA level at lower concentration and had no effect at higher concentrations. However, all tested compounds (CP, IF, and CFB) did not change the MRP4 protein expression in HepG2 cells. CP and CFB are cell-specific and concentration-dependent MRP4 inducers. The finding may have implications in the CP- or IF-based chemotherapy.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

A reexamination of the PPAR-alpha activation mode of action as a basis for assessing human cancer risks of environmental contaminants

BACKGROUND

Diverse environmental contaminants, including the plasticizer di(2-ethylhexyl)phthalate (DEHP), are hepatocarcinogenic peroxisome proliferators in rodents. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) activation and its sequelae have been proposed to constitute a mode of action (MOA) for hepatocarcinogenesis by such agents as a sole causative factor. Further, based on a hypothesized lower sensitivity of humans to this MOA, prior reviews have concluded that rodent hepatocarcinogenesis by PPAR-alpha agonists is irrelevant to human carcinogenic risk.

DATA SYNTHESIS

Herein, we review recent studies that experimentally challenge the PPAR-alpha activation MOA hypothesis, providing evidence that DEHP is hepatocarcinogenic in PPAR-alpha-null mice and that the MOA but not hepatocarcinogenesis is evoked by PPAR-alpha activation in a transgenic mouse model. We further examine whether relative potency for PPAR-alpha activation or other steps in the MOA correlates with tumorigenic potency. In addition, for most PPAR-alpha agonists of environmental concern, available data are insufficient to characterize relative human sensitivity to this rodent MOA or to induction of hepatocarcinogenesis.

CONCLUSIONS

Our review and analyses raise questions about the hypothesized PPAR-alpha activation MOA as a sole explanation for rodent hepatocarcinogenesis by PPAR-alpha agonists and therefore its utility as a primary basis for assessing human carcinogenic risk from the diverse compounds that activate PPAR-alpha. These findings have broad implications for how MOA hypotheses are developed, tested, and applied in human health risk assessment. We discuss alternatives to the current approaches to these key aspects of mechanistic data evaluation.

Microgram-order ammonium perfluorooctanoate may activate mouse peroxisome proliferator-activated receptor alpha, but not human PPARalpha

Perfluorooctanoic acid (PFOA) is a ligand for peroxisome proliferator-activated receptor (PPAR) alpha, which exhibits marked species differences in expression and function, especially between rodents and humans. We investigated the functional difference in PFOA response between mice and humans, using a humanized PPARalpha transgenic mouse line. Three genotyped mice, 129/Sv wild-type (mPPARalpha), Pparalpha-null mice and humanized PPARalpha (hPPARalpha) mice (8-week-old males) were divided into three groups: the first was treated with water daily for 2 weeks by gavage (control group), and the remaining two groups were treated with 0.1 and 0.3mg/kg ammonium perflurooctanate (APFO), respectively, for 2 weeks by gavage. The APFO dosages used did not influence the plasma triglyceride or total cholesterol levels in any mouse line, but the high dose increased both hepatic lipid levels only in mPPARalpha mice. APFO increased mRNA and/or protein levels of PPARalpha target genes cytochrome P450 Cyp4a10, peroxisomal thiolase and bifunctional protein only in the liver of mPPARalpha mice, but not in Pparalpha-null or hPPARalpha mice. This chemical also increased expression of mitochondrial very long chain acyl-CoA dehydrogenase only in the liver of mPPARalpha mice. Taken together, human PPARalpha may be less responsive to PFOA than that of mice when a relatively low dose is applied. This information may be very valuable in considering whether PFOA influences the lipid metabolism in humans.

Structure-activity relationships and human relevance for perfluoroalkyl acid-induced transcriptional activation of peroxisome proliferation in liver cell cultures

Perfluoroalkyl acids (PFAAs) are widely distributed and environmentally persistent agents whose potential toxicity is not yet fully characterized. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid elicit a number of potential toxicities in rodents, the most prevalent of which are governed by activation of the peroxisome proliferator-activated receptor alpha (PPARalpha). The purpose of this investigation was twofold: (1) To conduct a structure-activity relationship study of the transcriptional activation of peroxisome proliferation in primary rat liver cell cultures for PFAA-related carboxylic and sulfonic acids of varying carbon chain length and (2) to explore whether this activity can be translated to human liver cells in culture. Exposure to PFOA caused a dose-dependent stimulation of the expression of acyl-CoA oxidase (Acox), Cte/Acot1, and Cyp4a1/11 transcripts that are indicative of peroxisome proliferation in primary rat hepatocytes. PFOA concentrations of 30 microM and above caused cell injury characterized by the expression of Ddit3. Perfluorobutanoic acid (PFBA), on the other hand, stimulated Acox, Cte/Acot1, and Cyp4a1/11 gene expression in primary rat hepatocytes only at concentrations of 100 microM and above. Neither PFOA nor PFBA at concentrations up to 200 microM stimulated PPARalpha-related gene expression in either primary or HepG2 human liver cells. These data demonstrate that (1) PFFAs cause a concentration- and chain length-dependent increase in expression of gene targets related to cell injury and PPARalpha activation in primary rat hepatocytes, (2) the sulfonates are less potent than the corresponding carboxylates in stimulating PPARalpha-related gene expression in rat hepatocytes, and (3) stimulation of PPARalpha-mediated gene transcription is a mechanism that is not shared by human liver cells, adding further substantiation that PPARalpha-dependent liver toxicity in rodents does not extrapolate to assessing human health concerns.

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.

Review of the expression of peroxisome proliferator-activated receptors alpha (PPAR alpha), beta (PPAR beta), and gamma (PPAR gamma) in rodent and human development

The peroxisome proliferator-activated receptors (PPAR) belong to the nuclear hormone receptor superfamily and there are three primary subtypes, PPARalpha, beta, and gamma. These receptors regulate important physiological processes that impact lipid homeostasis, inflammation, adipogenesis, reproduction, wound healing, and carcinogenesis. These nuclear receptors have important roles in reproduction and development and their expression may influence the responses of an embryo exposed to PPAR agonists. PPARs are relevant to the study of the biological effects of the perfluorinated alkyl acids as these compounds, including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), activate PPARalpha. Exposure of the rodent to PFOA or PFOS during gestation results in neonatal deaths, developmental delay and growth deficits. Studies in PPARalpha knockout mice demonstrate that the developmental effects of PFOA, but not PFOS, depend on expression of PPARalpha. This review provides an overview of PPARalpha, beta, and gamma protein and mRNA expression during mouse, rat, and human development. The review presents the results from many published studies and the information is organized by organ system and collated to show patterns of expression at comparable developmental stages for human, mouse, and rat. The features of the PPAR nuclear receptor family are introduced and what is known or inferred about their roles in development is discussed relative to insights from genetically modified mice and studies in the adult.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Conflicting views on chemical carcinogenesis arising from the design and evaluation of rodent carcinogenicity studies

Conflicting views have been expressed frequently on assessments of human cancer risk of environmental agents based on animal carcinogenicity data; this is primarily because of uncertainties associated with extrapolations of toxicologic findings from studies in experimental animals to human circumstances. Underlying these uncertainties are issues related to how experiments are designed, how rigorously hypotheses are tested, and to what extent assertions extend beyond actual findings. National and international health agencies regard carcinogenicity findings in well-conducted experimental animal studies as evidence of potential carcinogenic risk to humans. Controversies arise when both positive and negative carcinogenicity data exist for a specific agent or when incomplete mechanistic data suggest a possible species difference in response. Issues of experimental design and evaluation that might contribute to disparate results are addressed in this article. To serve as reliable sources of data for the evaluation of the carcinogenic potential of environmental agents, experimental studies must include a) animal models that are sensitive to the end points under investigation; b) detailed characterization of the agent and the administered doses; c) challenging doses and durations of exposure (at least 2 years for rats and mice); d) sufficient numbers of animals per dose group to be capable of detecting a true effect; e) multiple dose groups to allow characterization of dose-response relationships, f) complete and peer-reviewed histopathologic evaluations; and g) pairwise comparisons and analyses of trends based on survival-adjusted tumor incidence. Pharmacokinetic models and mechanistic hypotheses may provide insights into the biological behavior of the agent; however, they must be adequately tested before being used to evaluate human cancer risk.

PPARalpha L162V underlies variation in serum triglycerides and subcutaneous fat volume in young males

BACKGROUND

Of the five sub-phenotypes defining metabolic syndrome, all are known to have strong genetic components (typically 50-80% of population variation). Studies defining genetic predispositions have typically focused on older populations with metabolic syndrome and/or type 2 diabetes. We hypothesized that the study of younger populations would mitigate many confounding variables, and allow us to better define genetic predisposition loci for metabolic syndrome.

METHODS

We studied 610 young adult volunteers (average age 24 yrs) for metabolic syndrome markers, and volumetric MRI of upper arm muscle, bone, and fat pre- and post-unilateral resistance training.

RESULTS

We found the PPARalpha L162V polymorphism to be a strong determinant of serum triglyceride levels in young White males, where carriers of the V allele showed 78% increase in triglycerides relative to L homozygotes (LL = 116 +/- 11 mg/dL, LV = 208 +/- 30 mg/dL; p = 0.004). Men with the V allele showed lower HDL (LL = 42 +/- 1 mg/dL, LV = 34 +/- 2 mg/dL; p = 0.001), but women did not. Subcutaneous fat volume was higher in males carrying the V allele, however, exercise training increased fat volume of the untrained arm in V carriers, while LL genotypes significantly decreased in fat volume (LL = -1,707 +/- 21 mm3, LV = 17,617 +/- 58 mm3 ; p = 0.002), indicating a systemic effect of the V allele on adiposity after unilateral training. Our study suggests that the primary effect of PPARalpha L162V is on serum triglycerides, with downstream effects on adiposity and response to training.

CONCLUSION

Our results on association of PPARalpha and triglycerides in males showed a much larger effect of the V allele than previously reported in older and less healthy populations. Specifically, we showed the V allele to increase triglycerides by 78% (p = 0.004), and this single polymorphism accounted for 3.8% of all variation in serum triglycerides in males (p = 0.0037).

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

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

Bezafibrate induces multidrug-resistance P-Glycoprotein 3 expression in cultured human hepatocytes and humanized livers of chimeric mice

AIM AND METHODS

A decreased function of multidrug-resistance 3 P-glycoprotein (MDR3), limiting the rate of biliary phospholipid secretion, predisposes individuals to cholestasis and/or cholangitis. Fibrates induce the expression of mdr2 (homolog of human MDR3) in rodents. To investigate the effects of bezafibrate (BF) on the expression levels of MDR3 in cultured human hepatocytes and human livers, the amount of protein and subcellular localization of MDR3 was assessed in HepG2 cells treated with BF and humanized livers of BF-treated chimeric mice.

RESULTS

In HepG2 cells, while treatment with BF did not increase the protein levels of MDR3, the treatment caused a redistribution of MDR3 in the bile canaliculi. In humanized livers of chimeric mice, oral administration of BF induced a large increase in the protein amount of MDR3 and its redistribution in the bile canaliculi. Moreover, the modulatory effects of BF on key factors involved in hepatic cholesterol and bile acid metabolism in human subjects were traced in the humanized livers of BF-treated chimeric mice.

CONCLUSION

BF causes an induction of MDR3 expression in human livers. This provides a rationale for the beneficial role of BF in improving cholestasis and/or cholangitis associated with defective MDR3 expression and function in various types of cholestatic hepatobiliary diseases.

Induction of peroxisome proliferator-activated receptor alpha (PPARalpha)-related enzymes by di(2-ethylhexyl) phthalate (DEHP) treatment in mice and rats, but not marmosets

To clarify species differences in the induction of peroxisome proliferator-activated receptor alpha (PPARalpha)-related enzymes by di(2-ethylhexyl)phthalate (DEHP) exposure, we investigated the inductions of PPARalpha and its target genes (mitochondrial medium-chain acyl-CoA dehydrogenase (MCAD) and peroxisomal keto-acyl-CoA thiolase (PT) in liver from mice (CD-1), rats (Sprague-Dawley), and marmosets (Callithrix jacchus) exposed to DEHP. Male mice and rats were treated with 0, 1.25 and 2.5 mmol/kg DEHP for 2 weeks, and marmosets with 0, 0.25, 1.25 and 6.25 mmol/kg DEHP for 15 months by gavage. Hepatic mono(2-ethylhexyl)phthalate (MEHP) levels were significantly higher in mice and rats than in marmosets. The constitutive expression of hepatic PPARalpha was 5-7 times greater in rats and mice than in marmosets, but DEHP treatment did not induce PPARalpha-mRNA in all animals. The treatment-induced PT expression detected either by anti-PT antibody or PT-mRNA levels in the liver only from mice and rats, and the induction of the mRNA was greater in the latter than in the former. Thus, DEHP used in this experiment influenced the peroxisomal enzymes in mice and rats, but did not affect the mitochondrial enzymes in any animals or the peroxisomal enzymes in marmosets. These results suggest that there are species differences in the induction of PPARalpha-related enzymes, especially in peroxisomal enzymes by DEHP treatment, and their underlying mechanism may in part reside in the different constitutive levels of PPARalpha and different forming levels of MEHP.

Association of V227A PPARα polymorphism with altered serum biochemistry and alcohol drinking in Japanese men

OBJECTIVES

Peroxisome proliferator-activated receptor (PPAR) alpha plays a major role in alcoholic liver disease in rodents. The two-fold objective of our study was to determine the presence of PPARalpha polymorphisms and their frequencies in Japanese populations and then to evaluate the effects of any alleles on metabolic parameters and alcohol drinking.

METHODS

Analysis of coding SNP in PPARalpha was performed in 706 Japanese men; from these subjects 655 men were further studied after exclusion criteria were applied.

RESULTS

PPARalpha-V227A, which has not been reported in Europe and North America as a major polymorphism, was discovered with the frequency of 0.05. PPARalpha-L162V was found in European and North American populations, but not in Japanese, thus confirming the ethnic differences in PPARalpha allele frequencies. The A227 allele was associated with increased serum concentrations of gamma glutamyltranspeptidase. In non-drinkers, the total cholesterol (TC) levels were significantly lower in those having the PPARalpha-V227A polymorphism. In drinkers, however, it was comparable among V227A polymorphisms, and conversely higher in those having both A227 and aldehyde dehydrogenase 2 (ALDH2) variants when further divided according to the ALDH2 polymorphism. Significant interactions between PPARalpha-V227A polymorphism and drinking were also found for TC, triglyceride levels and AST/ALT ratios. These results suggest that the activity of the A227 allele without drinking may be higher than in wild-type allele, but its activity may become lower during drinking habits.

CONCLUSION

PPARalpha-V227A is a major polymorphism in the Japanese population, and its activity may be greater compared to wild-type, but decreased by alcohol drinking.

Induction of phase I, II and III drug metabolism/transport by xenobiotics

Drug metabolizing enzymes (DMEs) play central roles in the metabolism, elimination and detoxification of xenobiotics and drugs introduced into the human body. Most of the tissues and organs in our body are well equipped with diverse and various DMEs including phase I, phase II metabolizing enzymes and phase III transporters, which are present in abundance either at the basal unstimulated level, and/or are inducible at elevated level after exposure to xenobiotics. Recently, many important advances have been made in the mechanisms that regulate the expression of these drug metabolism genes. Various nuclear receptors including the aryl hydrocarbon receptor (AhR), orphan nuclear receptors, and nuclear factor-erythoroid 2 p45-related factor 2 (Nrf2) have been shown to be the key mediators of drug-induced changes in phase I, phase II metabolizing enzymes as well as phase III transporters involved in efflux mechanisms. For instance, the expression of CYP1 genes can be induced by AhR, which dimerizes with the AhR nuclear translocator (Arnt), in response to many polycyclic aromatic hydrocarbon (PAHs). Similarly, the steroid family of orphan nuclear receptors, the constitutive androstane receptor (CAR) and pregnane X receptor (PXR), both heterodimerize with the retinoid X receptor (RXR), are shown to transcriptionally activate the promoters of CYP2B and CYP3A gene expression by xenobiotics such as phenobarbital-like compounds (CAR) and dexamethasone and rifampin-type of agents (PXR). The peroxisome proliferator activated receptor (PPAR), which is one of the first characterized members of the nuclear hormone receptor, also dimerizes with RXR and has been shown to be activated by lipid lowering agent fibrate-type of compounds leading to transcriptional activation of the promoters on CYP4A gene. CYP7A was recognized as the first target gene of the liver X receptor (LXR), in which the elimination of cholesterol depends on CYP7A. Farnesoid X receptor (FXR) was identified as a bile acid receptor, and its activation results in the inhibition of hepatic acid biosynthesis and increased transport of bile acids from intestinal lumen to the liver, and CYP7A is one of its target genes. The transcriptional activation by these receptors upon binding to the promoters located at the 5-flanking region of these CYP genes generally leads to the induction of their mRNA gene expression. The physiological and the pharmacological implications of common partner of RXR for CAR, PXR, PPAR, LXR and FXR receptors largely remain unknown and are under intense investigations. For the phase II DMEs, phase II gene inducers such as the phenolic compounds butylated hydroxyanisol (BHA), tert-butylhydroquinone (tBHQ), green tea polyphenol (GTP), (-)-epigallocatechin-3-gallate (EGCG) and the isothiocyanates (PEITC, sulforaphane) generally appear to be electrophiles. They generally possess electrophilic-mediated stress response, resulting in the activation of bZIP transcription factors Nrf2 which dimerizes with Mafs and binds to the antioxidant/electrophile response element (ARE/EpRE) promoter, which is located in many phase II DMEs as well as many cellular defensive enzymes such as heme oxygenase-1 (HO-1), with the subsequent induction of the expression of these genes. Phase III transporters, for example, P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRPs), and organic anion transporting polypeptide 2 (OATP2) are expressed in many tissues such as the liver, intestine, kidney, and brain, and play crucial roles in drug absorption, distribution, and excretion. The orphan nuclear receptors PXR and CAR have been shown to be involved in the regulation of these transporters. Along with phase I and phase II enzyme induction, pretreatment with several kinds of inducers has been shown to alter the expression of phase III transporters, and alter the excretion of xenobiotics, which implies that phase III transporters may also be similarly regulated in a coordinated fashion, and provides an important mean to protect the body from xenobiotics insults. It appears that in general, exposure to phase I, phase II and phase III gene inducers may trigger cellular "stress" response leading to the increase in their gene expression, which ultimately enhance the elimination and clearance of these xenobiotics and/or other "cellular stresses" including harmful reactive intermediates such as reactive oxygen species (ROS), so that the body will remove the "stress" expeditiously. Consequently, this homeostatic response of the body plays a central role in the protection of the body against "environmental" insults such as those elicited by exposure to xenobiotics.

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.

Pterostilbene, a new agonist for the peroxisome proliferator-activated receptor alpha-isoform, lowers plasma lipoproteins and cholesterol in hypercholesterolemic hamsters

Resveratrol, a stilbenoid antioxidant found in grapes, wine, peanuts and other berries, has been reported to have hypolipidemic properties. We investigated whether resveratrol and its three analogues (pterostilbene, piceatannol, and resveratrol trimethyl ether) would activate the peroxisome proliferator-activated receptor alpha (PPARalpha) isoform. This nuclear receptor is proposed to mediate the activity of lipid-lowering drugs such as the fibrates. The four stilbenes were evaluated at 1, 10, 100, and 300 microM along with ciprofibrate (positive control), for the activation of endogenous PPARalpha in H4IIEC3 cells. Cells were transfected with a peroxisome proliferator response element-AB (rat fatty acyl CoA beta-oxidase response element)-luciferase gene reporter construct. Pterostilbene demonstrated the highest induction of PPARalpha showing 8- and 14-fold increases in luciferase activity at 100 and 300 microM, respectively, relative to the control. The maximal luciferase activity responses to pterostilbene were higher than those obtained with the hypolipidemic drug, ciprofibrate (33910 and 19460 relative luciferase units, respectively), at 100 microM. Hypercholesterolemic hamsters fed with pterostilbene at 25 ppm of the diet showed 29% lower plasma low density lipoprotein (LDL) cholesterol, 7% higher plasma high density lipoprotein (HDL) cholesterol, and 14% lower plasma glucose as compared to the control group. The LDL/HDL ratio was also statistically significantly lower for pterostilbene, as compared to results for the control animals, at this diet concentration. Results from in vitro studies showed that pterostilbene acts as a PPARalpha agonist and may be a more effective PPARalpha agonist and hypolipidemic agent than resveratrol. In vivo studies demonstrate that pterostilbene possesses lipid and glucose lowering effects.

Stress- (and diet-) related regulation of hepatic nuclear receptors and its relevance for ABC-transporter functions

Nuclear receptors (NRs) play an important role in maintaining cellular homeostasis. With clearly established roles in fatty acid metabolism and inflammation, peroxisome proliferator activated receptors (PPARs) and other nuclear receptors are essential in liver functioning. However, much less is known about the regulation of NRs themselves during inflammatory processes in the liver. Interestingly PPARs and other NRs are negative acute phase proteins because they become rapidly downregulated during the acute phase response. However, PPARs have important roles in modulating inflammatory responses. One of the mechanisms by which dietary or inflammatory stress is relieved involves the hepatic adenosine triphosphate-binding cassette (ABC) transporter proteins, which import and export a wide variety of substrates. These ABC transporters are under close control of several NRs. Because NRs play important roles in fatty acid metabolism and inflammation as well as in the regulation of bile production, they are reviewed here with respect to their role in dietary and stress-related responses of the liver and their impact on the regulation and function of hepatic ABC transporters.

Species differences in the metabolism of di(2-ethylhexyl) phthalate (DEHP) in several organs of mice, rats, and marmosets

To clarify species differences in the metabolism of di(2-ethylhexyl) phthalate (DEHP) we measured the activity of four DEHP-metabolizing enzymes (lipase, UDP-glucuronyltransferase (UGT), alcohol dehydrogenase (ADH), and aldehyde dehydrogenase (ALDH)) in several organs (the liver, lungs, kidneys, and small intestine) of mice (CD-1), rats (Sprague-Dawley), and marmosets (Callithrix jacchus). Lipase activity, measured by the rate of formation of mono(2-ethylhexyl) phthalate (MEHP) from DEHP, differed by 27- to 357-fold among species; the activity was highest in the small intestines of mice and lowest in the lungs of marmosets. This might be because of the significant differences between Vmax/Km values of lipase for DEHP among the species. UGT activity for MEHP in the liver microsomes was highest in mice, followed by rats and marmosets. These differences, however, were only marginal compared with those for lipase activity. ADH and ALDH activity also differed among species; the activity of the former in the livers of marmosets was 1.6-3.9 times greater than in those of rats or mice; the activity of the latter was higher in rats and marmosets (2-14 times) than in mice. These results were quite different from those for lipase or UGT activity. Because MEHP is considered to be the more potent ligand to peroxisome proliferator-activated receptor alpha involved in different toxic processes, a possibly major difference in MEHP-formation capacity could be also considered on extrapolation from rodents to humans.

Peroxisome proliferator-activated receptor alpha protects against alcohol-induced liver damage

The mechanisms underlying alcoholic liver disease are not completely understood, but lipid accumulation seems to be central to the cause of this disease. The peroxisome proliferator-activated receptor alpha (PPARalpha) plays an important role in the control of lipid homeostasis, metabolism of bioactive molecules, and modulation of inflammatory responses. To investigate the roles of PPARalpha in alcoholic liver injury, wild-type and PPARalpha-null mice were continuously fed a diet containing 4% ethanol, and liver injury was analyzed. PPARalpha-null mice fed ethanol exhibited marked hepatomegaly, hepatic inflammation, cell toxicity, fibrosis, apoptosis, and mitochondrial swelling. Some of these hepatic abnormalities were consistent with those of patients with alcoholic liver injury and were not found in wild-type mice. Next, the molecular mechanisms of ethanol-induced liver injury in PPARalpha-null mice were investigated, and changes related to ethanol and acetaldehyde metabolism, oxidative stress, inflammation, hepatocyte proliferation, fibrosis, and mitochondrial permeability transition activation occurred specifically in PPARalpha-null mice as compared with wild-type mice. In conclusion, these studies suggest a protective role for PPARalpha in alcoholic liver disease. Humans may be more susceptible to liver toxicity induced by ethanol as PPARalpha expression in human liver is considerably lower compared to that of rodents.

Peroxisome proliferator-activated receptor alpha protects against alcohol-induced liver damage

The mechanisms underlying alcoholic liver disease are not completely understood, but lipid accumulation seems to be central to the cause of this disease. The peroxisome proliferator-activated receptor alpha (PPARalpha) plays an important role in the control of lipid homeostasis, metabolism of bioactive molecules, and modulation of inflammatory responses. To investigate the roles of PPARalpha in alcoholic liver injury, wild-type and PPARalpha-null mice were continuously fed a diet containing 4% ethanol, and liver injury was analyzed. PPARalpha-null mice fed ethanol exhibited marked hepatomegaly, hepatic inflammation, cell toxicity, fibrosis, apoptosis, and mitochondrial swelling. Some of these hepatic abnormalities were consistent with those of patients with alcoholic liver injury and were not found in wild-type mice. Next, the molecular mechanisms of ethanol-induced liver injury in PPARalpha-null mice were investigated, and changes related to ethanol and acetaldehyde metabolism, oxidative stress, inflammation, hepatocyte proliferation, fibrosis, and mitochondrial permeability transition activation occurred specifically in PPARalpha-null mice as compared with wild-type mice. In conclusion, these studies suggest a protective role for PPARalpha in alcoholic liver disease. Humans may be more susceptible to liver toxicity induced by ethanol as PPARalpha expression in human liver is considerably lower compared to that of rodents.

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.

Bezafibrate stimulates canalicular localization of NBD-labeled PC in HepG2 cells by PPARalpha-mediated redistribution of ABCB4

Fibrates, including bezafibrate (BF), upregulate the expression of ATP binding cassette protein B4 (ABCB4) through gene transcription in mice. To determine the effects of BF on the expression levels of ABCB4 and on the stimulation of biliary phosphatidylcholine (PC) transport in human HepG2 hepatoblastoma cells, mRNA and protein levels as well as subcellular localization were investigated in the cells treated with BF. The canalicular accumulation of a fluorescent PC was assessed by confocal laser scanning microscopy. Treatment with 300 micromol/l BF for 24 h increased levels of ABCB4 mRNA but not protein by up to 151%. BF caused redistribution of ABCB4 into pseudocanaliculi formed between cells. In association with this redistribution, BF accelerated the accumulation of fluorescent PC in bile canaliculi (up to 163% of that in nontreated cells). Suppression of peroxisome proliferator-activated receptor alpha (PPARalpha) expression by either a small interfering RNA duplex or morpholino antisense oligonucleotide attenuated the BF-induced redistribution of ABCB4. These findings suggest that BF may enhance the capacity of human hepatocytes to direct PC into bile canaliculi via PPARalpha-mediated redistribution of ABCB4 to the canalicular membrane. This provides a rationale for the use of BF to improve cholestasis and/or cholangitis that is attributable to hypofunction of ABCB4.

Role of orphan nuclear receptors in the regulation of drug-metabolising enzymes

During the past several years, important advances have been made in our understanding of the mechanisms that regulate the expression of genes that determine drug clearance, including phase I and phase II drug-metabolising enzymes and drug transporters. Orphan nuclear receptors have been recognised as key mediators of drug-induced changes in both metabolism and efflux mechanisms. In this review, we summarise recent findings regarding the function of nuclear receptors in regulating drug-metabolising and transport systems, and the relevance of these receptors to clinical drug-drug interactions and the development of new drugs. Emphasis is given to two newly recognised 'orphan' receptors (the pregnane X receptor [PXR] and the constitutive androstane receptor [CAR]) and their regulation of cytochrome P450 enzymes, such as CYP3A4, CYP2Cs and CYP2B6; and transporters, such as P-glycoprotein (MDR1), multidrug resistance-associated proteins (MRPs) and organic anion transporter peptide 2 (OATP2). Although 'cross-talk' occurs between these two receptors and their target sequences, significant species differences exist between ligand-binding and activation profiles for both receptors, and PXR appears to be the predominant or 'master' regulator of hepatic drug disposition in humans. Several important physiological processes, such as cholesterol synthesis and bile acid metabolism, are also tightly controlled by certain ligand-activated orphan nuclear receptors (farnesoid X receptor [FXR] and liver X receptor [LXR]). In general, their ability to bind a broad range of ligands and regulate an extensive array of genes that are involved in drug clearance and disposition makes these orphan receptors attractive targets for drug development. Drugs have the capacity to alter nuclear receptor expression (modulators) and/or serve as ligands for the receptors (agonists or antagonists), and thus can have synergistic or antagonistic effects on the expression of drug-metabolising enzymes and transporters. Coadministration of drugs that are nuclear receptor agonists or antagonists can lead to severe toxicity, a loss of therapeutic efficacy or an imbalance in physiological substrates, providing a novel molecular mechanism for drug-drug interactions.