Interaction of some peroxisome proliferators with the mouse liver peroxisome proliferator-activated receptor (PPAR): a molecular modelling and quantitative structure-activity relationship (QSAR) study

Bezafibrate decreases growth stimulatory action of the sphingomyelin signaling pathway in regenerating rat liver

Liver regeneration after partial hepatectomy (PH) is achieved through proliferation of hepatocytes and non-parenchymal cells. The nuclear peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in regulation of lipid metabolism and proliferation of hepatic cells. The sphingomyelin signal transduction pathway is involved in the regulation of the cell cycle in eukaryotic organisms. Sphingosine-1-phosphate (S1P) and ceramide (CER)-- the intermediates of the pathway--are known to stimulate and to inhibit cellular proliferation. The aim of the present study was to investigate the effect of PPARalpha activation by bezafibrate on the sphingomyelin signaling pathway during the first 24h of liver regeneration after PH in the rat. The content of sphingomyelin, ceramide, sphingosine, sphinganine, sphingosine-1-phosphate and the activity of sphingomyelinases and ceramidases were determined at various time points after PH. It has been found that the activity of neutral Mg(2+)-dependent sphingomyelinase (nSMase) increased, whereas the activity of acidic sphingomyelinase (aSMase) decreased in the regenerating liver. Activation of PPARalpha by bezafibrate lower the activity of nSMase and increased the activity of aSMase in the regenerating rat liver. The content of ceramide was higher in bezafibrate-treated rats, whereas the content of sphingosine-1-phosphate was markedly lower as compared to the untreated rats. Therefore, it is concluded that activation of PPARalpha by bezafibrate decreases the growth-stimulatory activity of the sphingomyelin pathway in regenerating rat liver.

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 proliferators: their biological and toxicological effects

One of the most rapidly developing areas of organelle biology, which has a major involvement in biochemical pharmacology, is the research into the peroxisomal function. There is a large group of compounds that are capable of inducing liver enlargement, proliferation of peroxisomal structures, and induction of peroxisomal and extraperoxisomal fatty acid-oxidizing enzymes in rodent liver, called peroxisome proliferators. This list includes hypolipidemic drugs, analgesics, uricosuric drugs, environmental pollutants, phthalates, etc. Some peroxisome proliferators have also been shown to increase the incidence of liver tumors. This review describes the characteristics of peroxisome proliferation in rodent liver and gives examples of different classes of chemicals that produce this effect. Mechanisms of initiation of peroxisome proliferation in rodent hepatocytes, including peroxisome proliferator-activated receptors, are also described. Rodent peroxisome proliferators are not considered to be genotoxic agents. Proposed mechanisms of liver tumor formation include induction of sustained oxidative stress, enhanced cell replication, promotion of spontaneous preneoplastic lesions, and inhibition of apoptosis. In addition, the absence of effects of peroxisome proliferators on peroxisome proliferator-associated parameters supports the hypothesis that human liver cells are refractory to peroxisome proliferator-induced hepatic carcinogenesis.

Further evaluation of COMPACT, the molecular orbital approach for the prospective safety evaluation of chemicals

The molecular dimensions and electronic structures of the first group of 100 US NCI/NTP miscellaneous chemicals, evaluated for potential carcinogenicity by computer-optimized molecular parametric analysis for chemical toxicity (COMPACT) have been re-determined. Using improved criteria for cytochrome P450 (CYP) substrate specificity, re-defined for CYP1 as having a COMPACT radius [square root of (deltaE - 9.5)2 + (a/d(2) - 7.8)2] of < 6.5, and for CYP2E as having a collision diameter of 6.5 angstroms or less and deltaE < 15.5, the likely substrates of CYP1 and CYP2E, which are regarded as potential carcinogens, have been identified. In addition, log P values have been taken into account; those chemicals with log P < 0 are non-lipophilic substrates unlikely to reach the activating cytochrome enzymes, and have been regarded as non-carcinogens. The second group of 100 US NCI/NTP chemicals have also now been categorized by COMPACT into CYP1 and CYP2E substrates, and their potential carcinogenicities evaluated. Of the 203 chemicals in the 2 groups, those positive in the rodent two-species life-span carcinogenicity study (rodent assay) were 53%, those positive in the Ames test (mutagenicity) were 48%, and those positive in the COMPACT programme (carcinogenicity, mutagenicity, cytotoxicity) were 54%. Concordance between the COMPACT prediction of carcinogenicity/cytotoxicity and rodent two species life-span carcinogenicity data for the 203 chemicals is 69%, and correlation of COMPACT with Ames test data is 61%. The sensitivity of COMPACT for predicting rodent carcinogenicity is 72%, whereas the sensitivity of the Ames test for predicting carcinogenicity for the 203 chemicals was only 57%. The degree (severity) of rodent carcinogenicity also showed correlation with the COMPACT predictive evaluations of the chemicals.

Ligand-induced peroxisome proliferator-activated receptor alpha conformational change

Structurally diverse peroxisome proliferators and related compounds that have been demonstrated to induce the ligand-dependent transcriptional activation function of mouse peroxisome proliferator-activated receptor alpha (mPPARalpha) in transfection experiments were tested for the ability to induce conformational changes within mPPARalpha in vitro. WY-14,643, 5,8,11,14-eicosatetraynoic acid, LY-171883, and clofibric acid all directly induced mPPARalpha conformational changes as evidenced by a differential protease sensitivity assay. Carboxyl-terminal truncation mutagenesis of mPPARalpha differentially affected the ability of these ligands to induce conformational changes suggesting that PPAR ligands may make distinct contacts with the receptor. Direct interaction of peroxisome proliferators and related compounds with, and the resulting conformational alteration(s) in, mPPARalpha may facilitate interaction of the receptor with transcriptional intermediary factors and/or the general transcription machinery and, thus, may underlie the molecular basis of ligand-dependent transcriptional activation mediated by mPPARalpha.

Pharmacological activity of the second generation leukotriene B4 receptor antagonist, SC-53228: effects on acute colonic inflammation and hepatic function in rodents

Inflammatory bowel disease is a chronic inflammatory disorder of the gastrointestinal tract that includes ulcerative colitis and Crohn's disease. Leukotriene B4 is thought to be a prominent proinflammatory mediator in these diseases, in that leukotriene B4 levels are increased in the colonic mucosa of inflammatory bowel disease patients and there is increased polymorphonuclear leukocyte infiltration of these tissues. SC-53228 [(+)-(S)-7-[3-[2(-cyclopropylmethyl)-3-methoxy-4- [(methylamino)carbonyl]phenoxy]propoxy]-3,4-dihydro-8-propyl-2H-1- benzopyran-2-propanoic acid], a second generation LTB4 receptor antagonist, was evaluated for therapeutic efficacy in a rodent model of acute colonic inflammation induced by short chain organic acids, as well as for effects on rodent liver. When given intracolonically to mice, SC-53228 inhibited neutrophil infiltration, assessed by myeloperoxidase (MPO) levels, with an ED50 value of 9 +/- 1.2 mg/kg. When given by gavage, SC-53228 inhibited neutrophil influx in colitic mice with an ED50 value of 30 mg/kg. These results were also confirmed histologically. Furthermore, high dose oral SC-53228 treatment had no effect on liver cytochrome P-450 content, fatty acyl CoA oxidase or liver weight in rats and mice. Together, these data suggest that SC-53228 may be efficacious orally and locally, as well as safe for use in trials for the medical management of IBD.

Comparative induction of cytochrome P4504A in rat hepatocyte culture by the peroxisome proliferators, bifonazole and clofibrate

1. The influence of imidazole and triazole antifungal drugs on cytochrome P450 levels in male Wistar primary rat hepatocyte culture for 70 h has been investigated and compared with clofibrate. 2. Bifonazole, clotrimazole, geniconazole clofibrate induced total P450 in hepatocytes, whereas itraconazole, miconazole and UK-47,265 did not. 3. When the CYP4A subfamily was examined, only bifonazole and clofibrate induced CYP4A as assessed by both Western blot analysis and the 11- and 12-hydroxylation of lauric acid. 4. By analysis of concentration-response curves in hepatocyte culture, bifonazole was 160 and 40 times more potent than clofibrate for induction of the 11- and 12-hydroxylation of lauric acid respectively. 5. Taken collectively, our data have identified bifonazole as a relatively potent, non-carboxylate inducer of CYP4A and the mechanism of induction and specificity of this azole is discussed.

A retrospective evaluation of COMPACT predictions of the outcome of NTP rodent carcinogenicity testing

The carcinogenic potentials of 40 National Toxicology Program chemicals previously predicted by Computer Optimised Molecular Parametric Analysis for Chemical Toxicity (COMPACT), based on the identification of potential substrates of cytochromes P4501A and 2E (CYP1A and CYP2E), have been compared with new rodent carcinogenicity results. The COMPACT predictions have also been compared with published Ames mutagenicity data and with our own Hazardexpert predictions for carcinogenicity. Concordance evaluations between rodent carcinogenicity (1/4 segments positive) and predictions by COMPACT or Hazardexpert were 64% for COMPACT (CYP1A only), 72% for COMPACT (CYP1A plus CYP2E), 70% for Hazardexpert alone, and 86% for COMPACT (CYP1A plus CYP2E) plus Hazardexpert. Sensitivities of the predictions were for COMPACT, 75%; Hazardexpert, 60%; and Ames, 54%. Positive predictivities were for COMPACT, 75%; Hazardexpert, 78%; and Ames 81%. Negative predictivites were for COMPACT, 62%; Hazardexpert, 52%; and Ames, 42%.

Molecular modelling of the human estrogen receptor and ligand interactions based on site-directed mutagenesis and amino acid sequence homology

A molecular model of the human estrogen receptor is reported based on a new alignment with the alpha 1-antitrypsin sequence, a homologous protein of known crystal structure. The putative ligand binding site is situated roughly equidistant between the DNA binding and dimerization regions. This binding site contains a number of amino acid residues shown by site-directed mutagenesis to be associated with the binding of agonists and antagonists. This putative ligand binding pocket is well-defined within a loop of peptide, containing complementary amino acids for binding interactions with agonists and antagonists. A leucine-rich region, common to most steroid-binding proteins, is in an optimum position for dimerization leading to DNA interaction. It is likely that ligand binding influences dimerization and DNA interaction by a conformational change in the receptor via the transcriptional activation residues. This model suggests that ligand binding may affect the hydrogen bonding pattern such that transpeptide signalling is initiated. The model accommodates steroidal estrogens and antiestrogens as well as the non-steroidal partial antagonist, hydroxytamoxifen.

Molecular organization of hepatocyte peroxisomes

The matrix of peroxisomes has been considered to be homogeneous. However, a fine network of tubules is visible in electron micrographs at very high magnification. This substructure becomes more positive in a high-contrast photocopy and with an imaging-plate method. Clofibrate, bezafibrate, and aspirin increase peroxisomes. In proliferated peroxisomes, the density of matrix is low and the fine network is more visible. The effect of proliferators is more significant in males than in females. This sex difference may involve the action of estrogen, growth hormone, cytochrome P-450 and thyroxine. Mg-ATPase is localized on the limiting membrane of peroxisomes. Even on the membrane of irregular projections of proliferated peroxisomes, Mg-ATPase is evident cytochemically. Carnitine acetyltransferase is detectable in the matrix of proliferated peroxisomes. Withdrawal of proliferators results in a rapid decrease of peroxisomes. This may indicate the existence of peroxisome suppressors. Alternatively, dynamic transformation of vesicular to tubular types in peroxisome reticulum may occur. Such transformation has been described in lysosomes and mitochondria.

Mechanistically-based human hazard assessment of peroxisome proliferator-induced hepatocarcinogenesis

In this review we have evaluated the relationship between peroxisome proliferation and hepatocarcinogenesis. To do so, we identified all chemicals known to produce peroxisome proliferation and selected those for which there are data (on peroxisome proliferation and hepatocarcinogenesis) which meet certain criteria chosen to facilitate comparison of these phenomena. The summarised data and definition of the methodology used has been collected in appendices. These comparisons enabled us to evaluate the relationship between these phenomena using reliable data. As there is a good correlation between them, we further explored the mechanisms of action that have been proposed (direct genotoxic activity, production of hydrogen peroxide, cell proliferation and receptor activation). The relationship between these events in other species, including humans, was also reviewed and finally an overview of the assessment of human hazard is presented in section IX. Some of the first chemicals which were shown to produce peroxisome proliferation were also hepatocarcinogens whose carcinogenicity could not be readily explained by genotoxic activity. This raised the suggestion that the unusual phenomenon of peroxisome proliferation was intricately linked to the carcinogenic activity of these agents. Three questions have exercised the attention of regulatory, industrial and academic toxicology since then; are chemicals which elicit peroxisome proliferation in the liver actually a coherent class of chemical carcinogens?; does the early biological phenomenon of peroxisome proliferation have real predictive value for and mechanistic association with rodent carcinogenesis?; and what hazard/risk do these agents pose to humans that may be exposed to them? Whether peroxisome proliferators are indeed a discrete class of rodent carcinogens would appear to be the single, most important question. If so, then the assumptions and procedures relevant to human hazard and risk assessment should be applied to the class and should be essentially generic; if not, each chemical should be considered independently. Our critical analysis of the published data for over 70 agents which have been shown to possess intrinsic ability to induce peroxisome proliferation in the livers of rodents has led to the conclusion that there exists a strong correlation between peroxisome proliferation as n early effect in the liver and hepatocarcinogenicity in chronic exposure studies. An almost perfect correlation was observed between the induction of peroxisomes in the rodent liver and the eventual appearance of tumours following chronic exposure The few exceptions to this were largely explainable (section II).(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of the CYP4A subfamily by perfluorodecanoic acid: the rat and the guinea pig as susceptible and non-susceptible species

Male Wistar rats and male Duncan Hartley guinea pigs were treated with one i.p. dose of perfluorodecanoic acid (PFDA) resulting in pronounced hepatomegaly in the rat but not the guinea pig. PFDA treatment also resulted in a 4-fold induction of lauric acid 12-hydroxylase activity in the rat but not the guinea pig, indicating induction of the CYP4A subfamily of isoenzymes. Consistent with this latter conclusion, Western blot analysis of rat liver microsomes using an antibody to CYP4A1 and Northern blot analysis of RNA extracts using a CYP4A1 cDNA probe, revealed PFDA-dependent induction of the CYP4A subfamily in the rat but not the guinea pig. Taken collectively, our data has demonstrated that PFDA, like other peroxisome proliferators, is also a CYP4A inducer and conforms to the well-documented species specificity in induction for this class of compound.

Induction of hepatic microsomal CYP4A activity and of peroxisomal beta-oxidation by two non-steroidal anti-inflammatory drugs

The effects of the non-steroidal anti-inflammatory drugs fenbufen and ibuprofen on hepatic cytochrome P450 activities and peroxisomal proliferation were investigated in the rat, following intraperitoneal administration at three dose levels. At the two highest doses, 30 and 150 mg/kg, ibuprofen stimulated lauric acid hydroxylase activity but no other dose-dependent effects on cytochrome P450 activities were evident. Fenbufen, at the highest dose of 150 mg/kg, decreased cytochrome P450 content and related activities, and this effect was attributed to the toxicity of the drug at this dose. Immunoblot studies employing solubilized microsomes from ibuprofen-treated rats revealed that ibuprofen increased the apoprotein levels of CYP4A1, at the two higher doses. The same treatment with ibuprofen, at the highest dose only, increased the beta-oxidation of palmitoyl CoA, determined in liver homogenates, and immunoblott analysis showed an increase in the apoprotein levels of the trans-2-enoyl CoA hydratase trifunctional protein. Fenbufen did not influence palmitoyl beta-oxidation. Computer graphic overlays with clofibric acid showed that ibuprofen, when compared with fenbufen, displayed a better overall fit to clofibric acid. Finally, interaction energies between the two drugs and the putative peroxisome proliferator-activated receptor ligand domain revealed that ibuprofen had a higher affinity for the receptor than fenbufen, but the difference was modest. It is concluded that ibuprofen, at doses far exceeding those employed clinically, is a weak inducer of both CYP4A1 activity and peroxisomal proliferation and these effects may be attributed to the presence of an aryl propionic acid moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorotrifluoroethylene trimer and tetramer are inducers of the CYP4A subfamily

Male Wistar albino rats were treated for a 7 day period with equimolar doses of the trimer and tetramer oligomers of chlorotrifluoroethylene (CTFE), resulting in significant hepatomegaly for both compounds. In addition, both trimer and tetramer significantly induced the peroxisomal beta-oxidation of fatty acids as assessed by increases in palmitoyl-coenzyme A (CoA) oxidation, thus confirming these oligomers as peroxisome proliferators. Consistent with these conclusions, both trimer and tetramer increased the hydroxylation of lauric acid indicating that the CTFEs were inducers of the CYP4A subfamily, a conclusion further supported by substantial increases in the steady-state levels of the cognate CYP4A1 mRNA as determined by northern blotting. The liver appeared to be more susceptible to induction than the kidney and the CTFE tetramer was more potent than the trimer. These results are discussed with respect to both the differential hepatotoxicity, and biotransformation/disposition of the two polyhalogenated oligomers.