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

Rodent carcinogenicity of peroxisome proliferators and issues on human relevance

A variety of substances such as hypolipidemic drugs, phthalate ester plasticizers, pesticides, and industrial solvents have been shown to increase the size and number of peroxisomes in rats and mice. They are grouped under the generic term peroxisome proliferators (PP) because of their unique property of inducing peroxisome proliferation. There are marked species differences in response to PP. Rats and mice are most sensitive, and hamsters show an intermediate response while guinea pigs, monkeys, and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. Out of over 100 PP identified to date, about 30 have been adequately tested and shown to be carcinogenic, inducing tumors (primarily in the liver) upon chronic administration to rats and/or mice; hence, chemicals which induce the proliferations of peroxisomes have formed a unique class of chemical carcinogens. It is not well documented that activation of the "peroxisome proliferator-activated receptor alpha" (PPARalpha) is involved in PP-induced liver growth and carcinogenesis in rodents. PPARalpha is also present in human cells; however, the levels reported are about 10% of those found in the liver of rodents. The human relevance of rodent tumors induced by PP has been the subject of debate over the last decade. Review of the existing evidence on PPAR-alpha agonists by a recent International Life Science Institute (ILSI) workgroup following a human relevance mode of action (MOA) framework has concluded that despite the presence of similar pathways in humans, it is unlikely that the proposed MOA for rodent tumors is plausible in humans, taking into account kinetic and dynamic factors. The data, however, did not permit a definitive conclusion that the animal MOA is not plausible in humans. While these agents appear unlikely to be hepatocarcinogens in humans at expected levels of human exposure, it remains uncertain to some experts in the field whether there is no possibility of carcinogenic potential under any circumstances of PP exposure, and if the potential human carcinogenicity of these chemicals can be summarily ignored. A number of remaining issues on human relevance of rodent tumors induced by PP are discussed.

Alterations of Activities of Cytosolic Phospholipase A2 and Arachidonic Acid-Metabolizing Enzymes in Di-(2-Ethylhexyl)Phthalate-Induced Testicular Atrophy

Di-(2-ethylhexyl) phthalate (DEHP), a peroxisome proliferator-activated receptor alpha (PPARalpha) ligand, alters the lipid composition of rat testis, yet the mechanism is unclear. In this study, we investigated the effect of DEHP on the synthesis and metabolism of arachidonic acid (AA), a precursor of eicosanoids, in the testis of prepubertal rats. DEHP (100 and 1,000 mg/kg, 5 days) administration caused a significant reduction in activity of cytosolic phospholipase A2 (cPLA2), the rate-limiting enzyme in the AA and eicosanoid synthesis pathways. DEHP increased the expression of 12-lipoxygenase (12-LOX) in rat testis, whereas cyclooxygenase-2 (COX-2) expression was not altered. Cytochrome P450 4A1 (CYP4A1), a product of a PPARalpha-regulated gene, was markedly increased in the testis by DEHP administration. Taken together, DEHP suppresses cPLA2 activity and induces the AA metabolizing enzymes such as 12-LOX and CYP4A1, resulting in the reduction of AA level. These data suggest that altered AA metabolic cascades may be related to the decrease of testosterone concentration in DEHP-induced testicular atrophy.

Liver gene expression in rats in response to the peroxisome proliferator-activated receptor-alpha agonist ciprofibrate

Fibrate class hypolipidemic drugs such as ciprofibrate activate the peroxisome proliferator-activated receptor-alpha (PPARalpha), which is involved in processes including lipid metabolism and hepatocyte proliferation in rodents. We examined the effects of ciprofibrate (50 mg/kg body wt per day for 60 days) on liver gene expression in rats using cDNA microarrays. The 60-day dosing period was chosen to elucidate both the metabolic and proliferative actions of this substance, while avoiding confounding effects from the hepatic carcinogenesis seen during more long-term stimulation. Ciprofibrate changed the expression of many genes including previously known PPARalpha agonist-responsive genes involved in processes such as lipid metabolism and inflammatory responses. In addition, many novel candidate genes involved in sugar metabolism, transcription, signal transduction, cell proliferation, and stress responses appeared to be differentially regulated in ciprofibrate-dosed rats. Ciprofibrate also resulted in significant increases in liver weight and hepatocyte proliferation. The cDNA microarray results were confirmed by Northern blot analysis for selected genes. This study thus identifies many genes that appear to be differentially regulated in ciprofibrate-dosed rats, and some of these are potential targets of PPARalpha. The functional diversity of these candidate genes suggests that most of them are likely to be differentially regulated as indirect consequence of the many processes affected by ciprofibrate in rodent liver. Although caution is advisable in the interpretation of genome-wide expression data, the genes identified in the present study provide candidates for further studies that may give new insight into the mechanisms of action of peroxisome proliferators.

Using Argumentation for Absolute Reasoning about the Potential Toxicity of Chemicals

The application of a new argumentation model is illustrated by reference to DEREK for Windows, a knowledge-based expert system for the prediction of the toxicity of chemicals. Examples demonstrate various aspects of the model such as the undercutting of arguments, the resolution of multiple arguments about the same proposition, and the propagation of arguments along a chain of reasoning.

The relationship between liver peroxisome proliferation and adipose tissue atrophy induced by peroxisome proliferator exposure and withdrawal in mice

We have previously demonstrated that severe adipose tissue atrophy occurs upon dietary treatment of mice with potent peroxisome proliferators (PPs). This atrophy occurs subsequent to peroxisome proliferation in the liver and may represent a novel addition to the pleiotropic effects exerted by PPs. In the present study we have characterized the recovery of mice from such atrophy following cessation of exposure. Following termination of treatment with perfluorooctanoic acid (PFOA) for 7 days, the adipose tissue atrophy was rapidly reversed, beginning on 2-5 days of recovery and being complete within 10 days. In contrast, hepatic peroxisome proliferation recovered much more slowly, indicating that these processes are not strictly coordinated. Analysis of lipoprotein lipase and hormone-sensitive lipase activities in adipose tissue revealed that the decrease and increase in these activities, respectively, caused by PFOA were both reversed within 10 days of recovery. Overall, these data provide further support for our previous conclusion that the adipose tissue atrophy induced by PFOA is caused, at least in part, by changes in the activities of lipoprotein lipase and hormone-sensitive lipase. The serum level of cholesterol, which increased after termination of PFOA treatment, returned to normal with a time-course similar to the recovery of adipose tissue weight, although hepatic peroxisome proliferation was still present. The possible relationship between the reduction in serum cholesterol and/or in its availability to peripheral tissues and the associated atrophy of adipose tissues caused by PPs is discussed.

Evidence for increased oxidative stress in peroxisomal D-bifunctional protein deficiency

Peroxisome biogenesis disorders (PBDs) and D-bifunctional protein (D-BP) deficiency are two types of inherited peroxisomal disorders. Patients with a PBD lack functional peroxisomes and patients with D-BP deficiency lack the enzyme, which is responsible for the second and third step of the peroxisomal beta-oxidation. The clinical presentation of these peroxisomal disorders is severe and includes several neurological abnormalities. The pathological mechanisms underlying these disorders are not understood and no therapies are available. Because peroxisomes have been associated with oxidative stress, as oxygen radicals are both produced and scavenged in peroxisomes, we have investigated whether oxidative stress is involved in the pathogenesis of PBDs and D-BP deficiency. We found in D-BP-deficient patients increased levels of thiobarbituric acid-reactive substances (TBARS) and 8-hydroxydeoxyguanosine (8-OHdG), which are markers for lipid peroxidation and oxidative DNA damage, respectively, whereas the levels of the lipophilic antioxidants alpha-tocopherol and coenzyme Q(10) were decreased. In addition, we found in skin fibroblasts from D-BP-deficient patients an imbalance between the activities of the peroxisomal H(2)O(2)-generating straight-chain acyl-CoA oxidase (SCOX) and the peroxisomal H(2)O(2)-degrading enzyme catalase. In conclusion, we have found clear evidence for the presence of increased oxidative stress in patients with D-BP deficiency, but not in patients with a PBD.

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

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

Down-regulation of STAT5b transcriptional activity by ligand-activated peroxisome proliferator-activated receptor (PPAR) alpha and PPARgamma

The nuclear receptor peroxisome proliferator-activated receptor (PPAR) is activated by a diverse group of acidic ligands, including many peroxisome proliferator chemicals present in the environment. Janus tyrosine kinase-signal transducer and activator of transcription (JAK-STAT) signaling is activated by multiple cytokines and hormones and leads to the translocation of dimerized STAT proteins to the nucleus where they activate transcription of target genes. Previous studies have shown that growth hormone (GH)-activated STAT5b can inhibit PPAR-regulated transcription. Here, we show that this inhibitory cross-talk is mutual, and that GH-induced, STAT5b-dependent beta-casein promoter-luciferase reporter gene transcription can be inhibited up to approximately 80% by ligand-activated PPARalpha or PPARgamma. Dose-response experiments showed a direct relationship between the extent of PPAR activation and the degree of inhibition of STAT5-regulated transcription. PPAR did not block STAT5b tyrosine phosphorylation or inhibit DNA-binding activity. Both PPARs inhibited the transcriptional activity of a constitutively active STAT5b mutant, indicating that inhibition occurs downstream of the GH-stimulated STAT5 activation step. Transcriptionally inactive, dominant-negative PPAR mutants did not block STAT5b inhibition by wild-type PPAR, indicating that PPAR target gene transcription is not required. PPARalpha retained its STAT5b inhibitory activity in the presence of the histone deacetylase inhibitor trichostatin, indicating that enhanced histone deacetylase recruitment does not contribute to STAT5b inhibition. PPARalpha lacking the ligand-independent AF-1 trans-activation domain failed to inhibit STAT5b, highlighting the importance of the AF-1 region in STAT5-PPAR inhibitory cross-talk. These findings demonstrate the bidirectionality of cross-talk between the PPAR and STAT pathways and provide a mechanism whereby exposure to environmental chemical activators of PPAR can suppress expression of GH target genes.

Induction of the expression of the peroxisome proliferator-activated receptor alpha (PPARalpha) by clofibrate in jerboa tissues

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear hormone receptor superfamily that can be activated by natural fatty acids and various xenobiotics, including clofibrate. This transcription factor primarily regulates genes involved in lipid metabolism and homeostasis. We present the expression pattern of the PPARalpha subtype in the adult jerboa Jaculus orientalis, determined by RT-PCR and Western blotting using specific probes and a polyclonal antibody for PPARalpha, respectively. PPARalpha is highly expressed in liver and kidney, and to a lesser extent in duodenum and colon. PPARalpha expression is increased at the mRNA and protein levels in liver and duodenum of jerboa treated for 2 weeks with the peroxisome proliferator (PP) clofibrate. The induction is tissue-specific as no significant changes are observed in kidney and colon. The present data indicate that the PP-induced PPARalpha gene expression is not dependent on the PPARalpha content in target cells.

Dual activation of PPARalpha and PPARgamma by mono-(2-ethylhexyl) phthalate in rat ovarian granulosa cells

Peroxisome proliferator-activated receptors (PPARs) are key regulators of lipid metabolism and cell differentiation. The plasticizer di-(2-ethylhexyl) phthalate is a peroxisome proliferator, and its active metabolite mono-(2-ethylhexyl) phthalate (MEHP) activates PPARalpha and PPARgamma in cell transactivation assays. MEHP is a female reproductive toxicant and decreases activity, mRNA, and protein levels of aromatase, the rate-limiting enzyme that converts testosterone to estradiol in ovarian granulosa cells. To test the hypothesis that MEHP suppresses aromatase through PPAR pathways, granulosa cells were cultured with MEHP (50 microM) or selective activators of PPARgamma or PPARalpha for 48 h and gene expression was analyzed by real time RT-PCR. Both PPARalpha and PPARgamma activators significantly decreased aromatase mRNA and estradiol production like MEHP. The PPARgamma-selective antagonist GR 259662 partially blocked the suppression of aromatase by MEHP, suggesting that MEHP acts through PPARgamma, but not exclusively. MEHP and the PPARalpha-selective agonist GW 327647 induced expression of 17beta-hydroxysteroid dehydrogenase IV, a known PPARalpha-regulated gene, and induction was maintained with addition of the PPARgamma-selective antagonist. PPARalpha-selective activation also induced expression of aryl hydrocarbon receptor (AhR), CYP1B1, and epoxide hydrolase in the granulosa cell. These data support a model in which MEHP activates both PPARalpha and PPARgamma to suppress aromatase and alter other genes related to metabolism and differentiation in the granulosa cell.

Effects of HCFC-123 exposure to maternal and infant rhesus monkeys on hepatic biochemistry, lactational parameters and postnatal growth

Peroxisome proliferators are a class of nongenotoxic rodent hepatocarcinogens that cause peroxisome proliferation and liver tumors when administered to rats and mice; but other species, including guinea pigs, dogs, and primates are less sensitive or refractory to the induction of peroxisome proliferation. Therefore, rodent peroxisome proliferators are not believed to pose a hepatocarcinogenic hazard to humans. Some peroxisome proliferators produce developmental toxicity in rats that is expressed as suppressed postnatal growth. To evaluate the relevance of the rat developmental effect to primates, groups of 4 lactating female Rhesus monkeys and their infants were exposed for 6 h/day, 7 days/week for 3 weeks to air or 1000 ppm HCFC-123. Animals were evaluated for clinical signs, body weights, clinical pathology parameters, and biochemical and pathological evaluations of liver biopsy samples. The effect of HCFC-123 exposure on milk quality (protein and fat concentration) was evaluated. The concentrations of HCFC-123 and the major metabolite, trifluoroacetic acid (TFA), were measured in the blood of the mothers and infants and in the milk. Exposure of monkeys to 1000 ppm HCFC-123 did not result in exposure-related clinical observations, or changes in body weight, appetence and behavior. There were no exposure-related effects on serum triglycerides, cholesterol, or glucose levels. HCFC-123 and TFA were present in milk, although maternal HCFC-123 exposure did not affect milk protein and fat content. In general, HCFC-123 was not detected in maternal or infant blood. TFA was detected in the majority of the mothers and TFA levels in infants ranged from 2 to 6 times higher than levels in the corresponding maternal blood. A pharmacokinetic analysis in a maternal animal indicated a peak concentration of TFA at approximately 1 h post-exposure, with a half-life of approximately 20 h. Liver microsomal P450 and peroxisome oxidase activities showed exposure-related decreases in CYP4A1 and CYP2E1 and acyl-CoA oxidase for animals exposed to HCFC-123. Microscopic evaluation of maternal liver from HCFC-123 exposed animals revealed mild to moderate centrilobular hepatocyte vacuolation, trace to mild centrilobular necrosis, and trace to mild subacute inflammation. The histopathological damage and altered hepatic biochemical activities produced by HCFC-123 in monkeys are not consistent with the HCFC-123 peroxisome proliferation response observed in rat livers. These findings demonstrate that HCFC-123 is not a peroxisome proliferator in adult Rhesus monkeys and postnatal exposure to HCFC-123 does not affect body weight of nursing infant monkeys.

PPAR alpha stimulates the rat gastrin-producing cell

The peroxisome proliferator (PP) ciprofibrate stimulates gastrin-producing cells (G-cells) in the rat stomach by an unknown mechanism, inducing hypergastrinemia and secondary enterochromaffin-like (ECL) cell hyperplasia. Ciprofibrate is a specific ligand for the nuclear peroxisome proliferator-activated receptor alpha (PPAR alpha). To see whether the effects of ciprofibrate could be imitated, rats were given another PPAR alpha ligand WY-14643 or the PPAR gamma ligand troglitazone by gastric intubations daily for 28 and 56 days. Troglitazone failed to raise gastrin levels. WY-14643 increased gastrin mRNA abundance, G-cell density and induced hypergastrinemia, but to a lesser extent than ciprofibrate. ECL cell parameters increased in proportion with the relative hypergastrinemia. Ciprofibrate and WY-14643 altered the levels of acyl CoA-oxidase mRNA and PPAR alpha mRNA in antrum, but had no effect in corpus. The PPAR alpha receptor was found in at least some G-cells by immunostaining. This study supports the hypothesis that PPAR alpha specific ligands could stimulate the G-cells by acting locally from the stomach lumen through antral PPAR alpha.

Effects of the rat hepatic peroxisome proliferator, Wyeth 14,643, on the lactating goat

Some peroxisome proliferators have been reported to reduce body weight gain in suckling rats, possibly through a lactational effect. Decreases in milk production or nutritional quality, either as a result of peroxisome proliferator-induced reductions in lipid content or alterations in the hormonal milieu necessary for milk production, could result in pup growth retardation. Wyeth-14,643 (WY) is hypolipidemic agent and a potent inducer of hepatic peroxisome proliferation in rats and mice. As is commonly seen with rodent hepatic peroxisome proliferators, WY produces minimal or no peroxisome induction in guinea pigs or non-human primates. Goats are an excellent model for studying lactation, however, their sensitivity to peroxisome proliferating chemicals is not known. The present study was performed to assess the sensitivity of goats to the hypolipidemic and peroxisome proliferator properties of WY and to determine the effects of WY on milk quantity and quality. Six lactating adult female goats were assigned to either control or treated groups. Goats in the treated group were administered WY (40 mg/kg/day) for 14 consecutive days. The goats were milked twice daily in order to maintain lactation and the quantity of milk collected was recorded. Milk quality was evaluated by determining the content of total fat, protein, and carbohydrate in milk samples collected following 7 and 14 days of treatment. WY administration had no effects on final body weight, liver weight or, gross and histopathological findings. Milk quantity and quality were unaffected by treatment. Serum cholesterol and triglyceride levels were reduced by 25% compared to controls, although only the difference in cholesterol was statistically significant. Hepatic beta-oxidation (3 x control) and aromatase (1.5 x control) activities were significantly greater in the treatment group; however, there was no treatment-related effect in the total content of hepatic cytochrome P450. There was no difference in aromatase activity in a pooled ovarian microsome sample. Milk estradiol and prolactin concentrations were not affected by treatment. These findings indicate that goats are weak responders to the hepatic peroxisome proliferator effects of WY. Additionally, the slight serum hypolipidemic effect does not impact milk production or nutritional value.

Dose-related effects of the peroxisome proliferator methylclofenapate in rat liver

Male Sprague-Dawley rats were fed diets containing 0 (control) and 2.5-750 ppm of the peroxisome proliferator methylclofenapate (MCP) for 1, 4 and 13 weeks. In other studies MCP has been shown to produce liver tumors at dietary levels of 50 and 250, but not 10 ppm. MCP treatment produced increases in relative liver weight and activities of peroxisomal and microsomal fatty acid oxidising enzymes at all time points at doses as low as 10 and 2.5 ppm, respectively. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during study weeks 0-1, 3-4 and 12-13. Hepatocyte labelling index values were significantly increased by treatment with 10-750 ppm MCP for 1 week and 150-750 ppm MCP for 13 weeks. Treatment with 50-750 ppm MCP for 13 weeks increased hepatic peroxisome proliferator-activated receptor alpha and transforming growth factor-β1 gene expression to 150-165 and 150-170% of control, respectively. These results demonstrate that while low doses of MCP produce sustained hepatomegaly and peroxisome proliferation in rat liver, higher doses are required to produce a sustained stimulation of replicative DNA synthesis.

Elevated basal expression of liver peroxisomal beta-oxidation enzymes and CYP4A microsomal fatty acid omega-hydroxylase in STAT5b(-/-) mice: cross-talk in vivo between peroxisome proliferator-activated receptor and signal transducer and activator of transcription signaling pathways

Long-term treatment of rodents with peroxisome proliferator chemicals, a group of structurally diverse nongenotoxic carcinogens, leads to liver cancer in a process dependent on the nuclear receptor peroxisome proliferator-activated receptor-alpha (PPARalpha). Previous in vitro studies have shown that growth hormone (GH) can inhibit PPARalpha-dependent gene expression by down-regulation of PPARalpha expression and by a novel inhibitory cross-talk involving the GH-activated transcription factor STAT5b. Presently, we evaluate the role of STAT5b in mediating these inhibitory actions of GH on PPAR function using a STATb-deficient mouse model. Protein levels of three PPARalpha-responsive peroxisomal beta-oxidation pathway enzymes (fatty acyl-CoA oxidase, 3-ketoacyl-CoA thiolase, and L-bifunctional enzyme) were increased up to two- to threefold in STAT5b(-/-) relative to wild-type control mouse liver, as was the basal expression of two PPARalpha-regulated cytochrome P450 4A proteins. In contrast, protein levels of two PPARalpha-unresponsive peroxisomal enzymes, catalase and urate oxidase, were not affected by the loss of STAT5b. A corresponding increase in expression of fatty acyl-CoA oxidase and L-bifunctional enzyme mRNA, as well as PPARalpha mRNA, was observed in the STAT5b-deficient mice, suggesting a transcriptional mechanism for the observed increases. Although basal liver expression of PPARalpha and its target genes was thus elevated in STAT5b(-/-) mice, the clofibrate-induced level of enzyme expression was unaffected, suggesting that the inhibitory effects of STAT5b are overcome at high concentrations of PPARalpha activators. These findings support the hypothesis that GH and potentially other endogenous activators of STAT5b help to maintain liver PPARalpha function at a low basal level and may thereby moderate PPARalpha-dependent hepatocarcinogenesis and other responses stimulated by exposure to low levels of environmental chemicals of the peroxisome proliferator class.

Molecular modelling of the peroxisome proliferator-activated receptor alpha (PPAR alpha) from human, rat and mouse, based on homology with the human PPAR gamma crystal structure

The generation of homology models of human, rat and mouse peroxisome proliferator-activated receptor alpha (PPAR alpha) are reported, based on the recently published crystal structure of the human PPAR gamma ligand-binding domain (LBD) with bound ligand, rosiglitazone. It is found that a template of peroxisome proliferating fibrate drugs and related compounds can fit within the putative ligand-binding site of rat PPAR alpha, via contacts with amino acid residues which are consistent with their biological potency for peroxisome proliferation, site-directed mutagenesis experiments and with quantitative structure-activity relationship (QSAR) analysis studies. The experimental binding affinity of leukotriene B(4) (LTB(4)) for the mouse PPAR alpha agrees closely with the calculated value based on the modelled interactions, whereas selective PPAR alpha ligands such as clofibric acid are able to fit the human PPAR alpha binding site in agreement with reported site-directed mutagenesis information.

Human carcinogens: an evaluation study via the COMPACT and HazardExpert procedures

The results of computer-optimized molecular parametric analysis of chemical toxicity (COMPACT) and HazardExpert evaluations on 14 established human carcinogens are reported. The concordances between COMPACT and carcinogenicity (71%) and between HazardExpert and carcinogenicity (57%) are significantly improved when taken in combination, where all 14 carcinogens are correctly identified by the two systems used in conjunction. However, if a negative energy of the highest occupied molecular orbital (E(HOMO)) value is regarded as evidence of electrophilic reactivity likely to give rise to mutagenicity and carcinogenicity, then 13/14 (93%) of the carcinogens are correctly identified by combination with the COMPACT procedure alone. It is possible, therefore, to establish likely carcinogenicity arising from either P450 mediation (CYP1 and CYP2E) or compound electrophilicity via the employment of a straightforward approach to molecular and electronic structure calculation, a process that can be performed in a relatively short time frame (i.e., less than 1 hour per chemical) and at a low cost.

The use of in vitro methods for hazard characterisation of chemicals

The usual starting points for hazard characterisation are No Observed Adverse Effect Levels (NOAELs)/benchmark doses for threshold effects and risk-specific doses/unit risks for non-threshold effects. In vitro studies are in general of no use in identifying these doses. However, based in part on in vitro investigations toxic equivalency factors have been developed for selected halogenated organic PCDD/PCDF/PCB congeners. Such factors can be used to determine the total toxic equivalent doses of mixtures of these contaminants. Studies with paracetamol illustrate that in vitro systems may help in the identification of the most sensitive species and strain. In vitro methods have been successfully used to studying qualitative and quantitative species differences in the toxicity of agents such as peroxisome proliferators and dichloromethane. Investigations with a number of chemicals show that in vitro systems are excellent models for characterisation of the mode of action of chemicals, but in vitro findings need to be validated in vivo. Experiments with bis(tri-n-butyltin)oxide illustrate that in vitro systems may aid in the extrapolation from high to low dose and from experimental animals to humans. In addition, in vitro approaches can be used to obtain useful information on the disposition of xenobiotics. It is concluded that if sufficient in vivo mechanistic information is available, in vitro studies using sub-cellular fractions/cells/tissue from animals and humans may significantly aid in the hazard characterisation of chemicals.

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.

Induction profile of rat organic anion transporting polypeptide 2 (oatp2) by prototypical drug-metabolizing enzyme inducers that activate gene expression through ligand-activated transcription factor pathways

Knowledge of regulation of transporters would aid in predicting pharmacokinetics and drug-drug interactions. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN) and phenobarbital increases hepatic uptake of cardiac glycosides. Rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) transports cardiac glycosides with high affinity. Levels of rat hepatic oatp2 protein and mRNA are regulated by PCN and phenobarbital treatment; however, the effects of other microsomal enzyme inducers on oatp2 have not been investigated. Therefore, the purpose of this study was to further determine whether oatp2 is regulated by a broader scale of drug-metabolizing enzyme inducers that are ligands or activators for the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor (PPAR), and antioxidant/electrophile response element (ARE/EpRE). Oatp2 protein levels determined by Western blot were decreased 56 to 72% by the AhR ligands, increased 84 to 132% by the CAR ligands, and increased 230 to 360% by PXR ligands. The PPAR ligands and ARE/EpRE activators generally had minimal effects on oatp2 protein levels. Oatp2 mRNA levels, determined by the bDNA technique, generally did not show a correlation with the altered oatp2 protein levels, e.g., among PXR ligands, only PCN increased oatp2 mRNA levels, but spironolactone and dexamethasone did not. Furthermore, only PCN, but not spironolactone and dexamethasone, increased the transcription of the oatp2 gene as the amount of hnRNA was increased when determined by reverse transcription-polymerase chain reaction. In conclusion, some drug-metabolizing enzyme inducers regulate oatp2 protein levels, especially the CYP3A inducers. However, there is no correlation between their ability to increase levels of oatp2 protein and mRNA, suggesting that regulation of oatp2 by drug-metabolizing enzyme inducers occurs at both the transcriptional and post-translational levels.

Antagonism of the actions of peroxisome proliferator-activated receptor-alpha by bile acids

The peroxisome proliferator-activated receptor-alpha (PPARalpha) is a ligand-activated transcription factor that regulates the expression of a number of genes critical for fatty acid beta-oxidation. Because a number of substrates and intermediates of this metabolic pathway serve as ligand activators of this receptor, homeostatic control of fatty acid metabolism is achieved. Evidence also exists for PPARalpha-dependent regulation of genes encoding critical enzymes of bile acid biosynthesis. To determine whether the primary products of bile acid biosynthesis, cholic acid and chenodeoxycholic acid, were capable of modulating PPARalpha function, a variety of in vivo and in vitro approaches were utilized. Feeding a bile acid-enriched diet significantly reduced the degree of hepatomegaly and induction of target genes encoding enzymes of fatty acid beta-oxidation caused by treatment with the potent PPARalpha ligand Wyeth-14,643. Convergent data from mechanistic studies indicate that bile acids interfere with transactivation by PPARalpha at least in part by impairing the recruitment of transcriptional coactivators. The results of this study provide the first evidence in favor of the existence of compounds, normally found within the body, that are capable of antagonizing the physiological actions of PPARalpha. The impact of PPARalpha antagonism by endogenous bile acids is likely to be limited under normal conditions and to have only minimal effects on bile acid homeostasis. However, during certain pathophysiological states where intracellular bile acid concentrations are elevated, meaningful effects on PPARalpha-dependent target gene regulation are possible.

Peroxisome proliferator-activated receptor (PPAR) modulators: diabetes and beyond

Peroxisome proliferator-activated receptors (PPARs) are a class of ligand modulated transcription factors with a prominent role in the regulation of metabolic processes. This report is intended to provide a limited introduction to the PPAR field, sketched with reference to one early series of PPAR ligands.

Peroxisome proliferator-activated receptors (PPARs): novel therapeutic targets in renal disease

Peroxisome proliferator-activated receptors (PPARs): Novel therapeutic targets in renal disease. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of ligand-dependent transcription factors. PPARs play an important role in the general transcriptional control of numerous cellular processes, including lipid metabolism, glucose homeostasis, cell cycle progression, cell differentiation, inflammation and extracellular matrix remodeling. Three PPAR isoforms, designated PPARalpha, PPARbeta and PPARgamma, have been cloned and are differentially expressed in several tissues including the kidney. PPARalpha primary regulates lipid metabolism and modulates inflammation. PPARalpha is the molecular target of the hypolipidemic fibrates including bezafibrate and clofibrate. PPARbeta participates in embryonic development, implantation and bone formation. PPARgamma is a key factor in adipogenesis and also plays an important role in insulin sensitivity, cell cycle regulation and cell differentiation. Antidiabetic thiazolidinediones (TZDs) such as troglitazone and rosiglitazone are specific ligands of PPARgamma, and this interaction is responsible for the insulin-sensitizing and hypoglycemic effect of these drugs. The kidney has been shown to differentially express all PPAR isoforms. PPARalpha is predominantly expressed in proximal tubules and medullary thick ascending limbs, while PPARgamma is expressed in medullary collecting ducts, pelvic urothelium and glomerular mesangial cells. PPARbeta is ubiquitously expressed at low levels in all segments of nephron. Accumulating data has begun to emerge suggesting physiological and pathophysiological roles of PPARs in several tissues including the kidney. The availability of PPAR-selective agonists and antagonists may provide a new approach to modulate the renal response to diseases including glomerulonephritis, glomerulosclerosis and diabetic nephropathy.

Weight-of-evidence versus strength-of-evidence in toxicologic hazard identification: Di(2-ethylhexyl)phthalate (DEHP)

Toxicokinetic and mode of action data for DEHP reduce the concern for its potential carcinogenic hazard to human health. Chronic, high dose ingestion of DEHP and related peroxisome proliferators (PP) by mice and rats precipitate the following: activation of peroxisome proliferator activated receptor (PPARalpha) and its binding to peroxisome proliferator response elements (PPREs) within promoters of PP-responsive genes, peroxisome proliferation, increased microsomal fatty acid oxidation, increased hepatic hydrogen peroxide, hepatomegaly, hyperplasia and subsequent neoplasia. Neither peroxisome proliferation nor increased liver cancer occur in patients treated with pharmacologic doses of PP. Species differences in endogenous PPARalpha expression and differential activity of the peroxisome proliferator response element (PPRE) contribute to the failure of humans to respond in a manner qualitatively similar to that of rats or mice. Where it can be demonstrated that a mechanism for rodent tumor formation has no relevance for humans, then a substance which elicits a carcinogenic response in the test species via that mechanism should not be classified as anything other than an animal carcinogen. Systemic noncarcinogenic endpoints are available for definition of a DEHP reference dose. Considerable difficulty is encountered in the revision of promulgated regulations and in public risk communication when a material is no longer considered a carcinogenic hazard to humans.