The short- and long-term effects of bezafibrate in the rat

Mechanisms and Functions of Pexophagy in Mammalian Cells

Peroxisomes play essential roles in diverse cellular metabolism functions, and their dynamic homeostasis is maintained through the coordination of peroxisome biogenesis and turnover. Pexophagy, selective autophagic degradation of peroxisomes, is a major mechanism for removing damaged and/or superfluous peroxisomes. Dysregulation of pexophagy impairs the physiological functions of peroxisomes and contributes to the progression of many human diseases. However, the mechanisms and functions of pexophagy in mammalian cells remain largely unknown compared to those in yeast. This review focuses on mammalian pexophagy and aims to advance the understanding of the roles of pexophagy in human health and diseases. Increasing evidence shows that ubiquitination can serve as a signal for pexophagy, and ubiquitin-binding receptors, substrates, and E3 ligases/deubiquitinases involved in pexophagy have been described. Alternatively, pexophagy can be achieved in a ubiquitin-independent manner. We discuss the mechanisms of these ubiquitin-dependent and ubiquitin-independent pexophagy pathways and summarize several inducible conditions currently used to study pexophagy. We highlight several roles of pexophagy in human health and how its dysregulation may contribute to diseases.

Peroxisomal Membrane Contact Sites in Mammalian Cells

Peroxisomes participate in essential cellular metabolic processes, such as oxidation of fatty acids (FAs) and maintenance of reactive oxygen species (ROS) homeostasis. Peroxisomes must communicate with surrounding organelles to exchange information and metabolites. The formation of membrane contact sites (MCSs), where protein-protein or protein-lipid complexes tether the opposing membranes of two organelles, represents an essential means of organelle crosstalk. Peroxisomal MCS (PO-MCS) studies are emerging but are still in the early stages. In this review, we summarize the identified PO-MCSs with the ER, mitochondria, lipid droplets, and lysosomes in mammalian cells and discuss their tethering mechanisms and physiological roles. We also highlight several features of PO-MCSs that may help future studies.

Role of PEX5 ubiquitination in maintaining peroxisome dynamics and homeostasis

Peroxisomes are essential and dynamic organelles that allow cells to rapidly adapt and cope with changing environments and/or physiological conditions by modulation of both peroxisome biogenesis and turnover. Peroxisome biogenesis involves the assembly of peroxisome membranes and the import of peroxisomal matrix proteins. The latter depends on the receptor, PEX5, which recognizes peroxisomal matrix proteins in the cytosol directly or indirectly, and transports them to the peroxisomal lumen. In this review, we discuss the role of PEX5 ubiquitination in both peroxisome biogenesis and turnover, specifically in PEX5 receptor recycling, stability and abundance, as well as its role in pexophagy (autophagic degradation of peroxisomes).

Assays to Monitor Pexophagy in Yeast

Pexophagy is a selective autophagy process that degrades damaged and/or superfluous peroxisomes in the yeast vacuole or in mammalian lysosomes. The molecular mechanisms of pexophagy are well studied in yeast. Peroxisomes can be rapidly induced by oleate in the budding yeast, Saccharomyces cerevisiae, and by oleate or methanol in the methylotrophic yeast, Pichia pastoris. A number of peroxisomal matrix enzymes, such as 3-ketoacyl CoA thiolase (thiolase) and alcohol oxidase (AOX), are upregulated correspondingly to meet metabolic demands of the cells. Removal of these peroxisome-inducing carbon sources creates conditions wherein peroxisomes are superfluous and results in pexophagy and the degradation of these peroxisomal matrix enzymes. In this chapter, we discuss different assays to monitor pexophagy in yeast. These assays rely on tracking the localization of the BFP-SKL protein (a peroxisomally targeted version of the blue fluorescent protein) by microscopy, biochemical analysis of the degradation of peroxisomal matrix proteins, thiolase and AOX, and/or measuring the reduction of AOX activity during pexophagy.

Catabolic Functions of Peroxisomes: Modification by Hypoupidemic Drugs

The peroxisome is a subcellular organelle that is widely distributed in nature and which carries out both catabolic and anabolic functions (Ann. NY Acad. Sch 386:1-550, 1982). The catabolic functions include respiration (based on the formation and decomposition of H2O2) and the ß-oxidation of fatty acids. A number of drugs share the attributes of beingi) hypo-lipidemic, (2) inducers of the peroxisomal ß-oxidation enzyme system, (Lazarow, Science 197: 580-581, 1977), 3) peroxisome proliferators, and 4) carcinogens in rodents. Reddy et al. (Nature 283: 397-398, 1980) have hypothesized that peroxisome proliferators as a class may be carcinogenic Data is presented showing that bezafibrate, at a suitable hypolipidemic dose in rats, induces peroxisomal ß-oxidation but does not cause the striking organelle proliferation commonly observed with hypolipidemic drugs. Similar results have been obtained with clofibrate treatment of female rats. Christiansen et al. (Eur.). Cell Biol. 26: 77-20, 7987) have shown that feeding rats a diet rich in partially hydrogenated marine oils produces changes in the peroxisomes similar to those caused by bezafibrate. Aspirin, which is weakly hypolipidemic and a weak peroxisome proliferator, is apparently not carcinogenic in humans. The evidence indicates that the hypolipidemic effects and the peroxisome proliferative effects of these drugs are largely (although incompletely) dissociable. It suggests the need for considerable caution in evaluating the relationship, if any, between hypolipidemic and carcinogenic effects.

Assessment of Possible Carcinogenic Risk to Humans Resulting from Exposure to Di(2-ethylhexyl)phthalate (DEHP)

The purpose of this work was to estimate the degree of risk that might be associated with human exposure to low levels of the plasticizer di(2-ethylhexyl)phthalate (DEHP). DEHP is a common component, sometimes at high concentrations, of polyvinyl chloride (PVC) plastics and was recently reported by the National Toxicology Program (NTP) to be carcinogenic in rats and mice, inducing hepatocellular tumors in both species. This work was also designed to illustrate an approach to risk assessment that attempts to incorporate all available biological data. Based on the dose-response data generated by the NTP bioassays, we have performed extrapolations of risk to low dose levels using several procedures, including some that incorporate inferences from the available data that shed light on the likely relationship between dose level and risk at low dose levels. In drawing these inferences, consideration was given to such factors as genotoxicity, metabolism and pharmacokinetics, and physiological and biochemical effects of DEHP that might reveal its mechanism of action. The relative merits of each of the various risk estimates are described, based on current understanding of DEHP's mode of biological action. It is concluded that DEHP's mechanism of carcinogenicity in rodents most likely involves its ability to induce peroxisome proliferation and related enzymatic changes, although other mechanisms cannot be excluded. If humans and rodents are assumed to be at the same risk at the same daily dose level of DEHP, application of the various low dose extrapolation models leads to the prediction that the daily dose resulting in a lifetime risk of no more than 1 in 1 million would be between 1.5 and 791 mg/kg per day, with the most likely figure being 116 mg/kg per day. If the carcinogenicity of DEHP is dependent upon its pattern of metabolism, however, it would be inappropriate to extrapolate from rodents to man without qualification because of the major quantitative differences in metabolism in rats, mice, and primates, including man. One of the major differences in metabolism of DEHP between rats and mice and primates is in production of a metabolite whose level may be an indicator of the level of peroxisomal activity and, hence, if the peroxisome proliferation theory of DEHP carcinogenicity is correct, of carcinogenic risk. However, the substantial doubt that exists regarding the applicability of rodent carcinogenicity data to humans must be expressed in qualitative terms.

Autophagic degradation of peroxisomes in mammals

Peroxisomes are essential organelles required for proper cell function in all eukaryotic organisms. They participate in a wide range of cellular processes including the metabolism of lipids and generation, as well as detoxification, of hydrogen peroxide (H2O2). Therefore, peroxisome homoeostasis, manifested by the precise and efficient control of peroxisome number and functionality, must be tightly regulated in response to environmental changes. Due to the existence of many physiological disorders and diseases associated with peroxisome homoeostasis imbalance, the dynamics of peroxisomes have been widely examined. The increasing volume of reports demonstrating significant involvement of the autophagy machinery in peroxisome removal leads us to summarize current knowledge of peroxisome degradation in mammalian cells. In this review we present current models of peroxisome degradation. We particularly focus on pexophagy-the selective clearance of peroxisomes through autophagy. We also critically discuss concepts of peroxisome recognition for pexophagy, including signalling and selectivity factors. Finally, we present examples of the pathological effects of pexophagy dysfunction and suggest promising future directions.

Fenofibrate decreases radiation sensitivity via peroxisome proliferator-activated receptor α-mediated superoxide dismutase induction in HeLa cells

Purpose

The fibrates are ligands for peroxisome proliferator-activated receptor (PPAR) α and used clinically as hypolipidemic drugs. The fibrates are known to cause peroxisome proliferation, enhance superoxide dismutase (SOD) expression and catalase activity. The antioxidant actions of the fibrates may modify radiation sensitivity. Here, we investigated the change of the radiation sensitivity in two cervix cancer cell lines in combination with fenofibrate (FF).

Materials and Methods

Activity and protein expression of SOD were measured according to the concentration of FF. The mRNA expressions were measured by using real time reverse-transcription polymerase chain reaction. Combined cytotoxic effect of FF and radiation was measured by using clonogenic assay.

Results

In HeLa cells total SOD activity was increased with increasing FF doses up to 30 µM. In the other hand, the catalase activity was increased a little. As with activity the protein expression of SOD1 and SOD2 was increased with increasing doses of FF. The mRNAs of SOD1, SOD2, PPARα and PPARγ were increased with increasing doses of FF. The reactive oxygen species (ROS) produced by radiation was decreased by preincubation with FF. The surviving fractions (SF) by combining FF and radiation was higher than those of radiation alone. In Me180 cells SOD and catalase activity were not increased with FF. Also, the mRNAs of SOD1, SOD2, and PPARα were not increased with FF. However, the mRNA of PPARγ was increased with FF.

Conclusion

FF can reduce radiation sensitivity by ROS scavenging via SOD induction in HeLa. SOD induction by FF is related with PPARα.

Type 2 transglutaminase is involved in the autophagy-dependent clearance of ubiquitinated proteins

Eukaryotic cells are equipped with an efficient quality control system to selectively eliminate misfolded and damaged proteins, and organelles. Abnormal polypeptides that escape from proteasome-dependent degradation and aggregate in the cytosol can be transported via microtubules to inclusion bodies called 'aggresomes', where misfolded proteins are confined and degraded by autophagy. Here, we show that Type 2 transglutaminase (TG2) knockout mice display impaired autophagy and accumulate ubiquitinated protein aggregates upon starvation. Furthermore, p62-dependent peroxisome degradation is also impaired in the absence of TG2. We also demonstrate that, under cellular stressful conditions, TG2 physically interacts with p62 and they are localized in cytosolic protein aggregates, which are then recruited into autophagosomes, where TG2 is degraded. Interestingly, the enzyme's crosslinking activity is activated during autophagy and its inhibition leads to the accumulation of ubiquitinated proteins. Taken together, these data indicate that the TG2 transamidating activity has an important role in the assembly of protein aggregates, as well as in the clearance of damaged organelles by macroautophagy.

Mitochondria and redox signaling in steatohepatitis

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

Ultrastructural changes in the rat kidney after single dose of cyclophosphamide—Possible roles for peroxisome proliferation and lysosomal dysfunction in cyclophosphamide-induced renal damage

Electron microscopy was used to examine changes in the subcellular organelles of the rat kidney at different time intervals after a single exposure to cyclophosphamide (CP). The morphological changes were studied at different time points (6 hrs, 16 hrs and 24 hrs) after a single-dose administration of CP. Six rats were killed at each time intervals after the administration of CP. Saline-treated rats served as controls. CP administration resulted in alterations in various subcellular organelles including peroxisomes, lysosomes, mitochondria, and the endoplasmic reticulum (ER) of the renal tubular epithelium as well as damage to the glomerulus. The basement membrane of the glomerulus was thickened. Many podocytes were destroyed. The nucleoplasm of the endothelial cell showed fewer granularities. The tubules were distorted and the brush border was destroyed. Two striking features in the renal tubular cells are increase in number and size of the peroxisomes (peroxisome proliferation) and decrease in the number of lysosomes. The mitochondria were elongated and the number was increased in the tubules of CP-treated rats. The ER was dilated. Cell necrosis was also seen. This study is an evidence of changes in morphology of rat kidney after induction of renal damage by a single dose of CP. Since transmission electron microscopy is the highest magnification tool at present, it can be useful in estimating the degree of injury and outcome of alternative treatment strategies in the management of CP-induced renal damage after establishing a scoring system.

Degradation of excess peroxisomes in mammalian liver cells by autophagy and other mechanisms

Here we discuss the mechanisms for the degradation of excess peroxisomes in mammalian hepatocytes which include (a) autophagy, (b) the action of peroxisomal Lon protease and (c) the membrane disrupting effect of 15-lipoxygenase. A recent study using Atg7 conditional-knock-out mice revealed that 70-80% of excess peroxisomes are degraded by the autophagic process. The remaining 20-30% of excess peroxisomes is most probably degraded by the action of peroxisomal Lon protease. Finally, a selective disruption of the peroxisomal membrane has been shown to be mediated by 15-lipoxygenase activity which is followed by diffusion of matrix proteins into the cytoplasm and cytoplasmic proteolysis.

Method for monitoring pexophagy in mammalian cells

The abundance of peroxisomes within a cell is rapidly controlled depending on environmental changes and physiological conditions. It is well established that phthalate esters can cause a marked proliferation of peroxisomes (Yokota, 1986). Following induction of peroxisomes by a 2-week treatment with phthalate esters in mouse livers, peroxisomal degradation via autophagy can be induced for the subsequent week after discontinuation of the phthalate esters. Autophagic degradation of peroxisomes can be monitored by electron microscopy as well as biochemical assay for some peroxisome markers. Although most of the excess peroxisomes in the liver are selectively degraded within one week, this rapid removal is exclusively impaired in the autophagy-deficient liver.

The peroxisome: still a mysterious organelle

More than half a century of research on peroxisomes has revealed unique features of this ubiquitous subcellular organelle, which have often been in disagreement with existing dogmas in cell biology. About 50 peroxisomal enzymes have so far been identified, which contribute to several crucial metabolic processes such as β-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, and render peroxisomes indispensable for human health and development. It became obvious that peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. However, many aspects of peroxisome biology are still mysterious. This review addresses recent exciting discoveries on the biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross talk of peroxisomes with other subcellular compartments. Furthermore, recent advances on the role of peroxisomes in medicine and in the identification of novel peroxisomal proteins are discussed.

Rat Liver Peroxisomes after Fibrate Treatment

Fibrates are known to induce peroxisome proliferation and the expression of peroxisomal beta-oxidation enzymes. To analyze fibrate-induced changes of complex metabolic networks, we have compared the proteome of rat liver peroxisomes from control and bezafibrate-treated rats. Highly purified peroxisomes were subfractionated, and the proteins of the matrix, peripheral, and integral membrane subfractions thus obtained were analyzed by matrix-assisted laser desorption ionization time-of-flight/time-of-flight mass spectrometry after labeling of tryptic peptides with the iTRAQ reagent. By means of this quantitative technique, we were able to identify 134 individual proteins, covering most of the known peroxisomal proteome. Ten predicted new open reading frames were verified by cDNA cloning, and seven of them could be localized to peroxisomes by immunocytochemistry. Moreover, quantitative mass spectrometry substantiated the induction of most of the known peroxisome proliferator-activated receptor alpha-regulated peroxisomal proteins upon treatment with bezafibrate, documenting the suitability of the iTRAQ procedure in larger scale experiments. However, not all proteins reacted to a similar extent but exerted a fibrate-specific induction scheme showing the variability of peroxisome proliferator-activated receptoralpha-transmitted responses to specific ligands. In view of our data, rat hepatic peroxisomes are apparently not specialized to sequester very long chain fatty acids (C22-C26) but rather metabolize preferentially long chain fatty acids (C16-18).

Opinion: alternative views of AMP-activated protein kinase

Genes most closely related to adenosine monophosphate (AMP)-activated protein kinase, including SAD kinases and Par-1 regulate cell polarity, although AMP-activated protein kinase (AMPK) modulates cellular energy status. LKB1 (Par-4) is required for normal activation of AMPK in the liver and also regulates cell polarity. AMPK is proposed to inhibit energy consuming activity while initiating energy producing activity during energy limitation. Demonstration that metformin, a common drug for Type 2 diabetes, requires LKB1 for full therapeutic benefit has increased interest in AMPK signaling. Despite the potential importance of AMPK signaling for diabetes, metabolic syndrome and even cancer, the developmental processes regulated by AMPK in genetically mutant animals require further elucidation. Mouse conditional null mutants for AMPK activity will allow genetic elucidation of AMPK function in vivo. This perspective focuses on sequence and structural moieties of AMPK and genetic analysis of AMPK mutations. Interestingly, the predicted protein structure of the carboxy-terminus of AMPKalpha resembles the carboxy-terminal KA-1 domain of MARK3, a Par-1 orthologue.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Shared components of mitochondrial and peroxisomal division

Mitochondria and peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity. Recent studies have led to the surprising finding that both organelles share components of their division machinery, namely the dynamin-related protein DLP1/Drp1 and hFis1, which recruits DLP1/Drp1 to the organelle membranes. This review addresses the current state of knowledge concerning the dynamics and fission of peroxisomes, especially in relation to mitochondrial morphology and division in mammalian cells.

Growth and Division of Peroxisomes

Peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity in response to cellular and environmental conditions. Novel proteins and pathways that mediate and control peroxisome formation, growth, and division continue to be discovered, and the cellular machineries that act together to regulate peroxisome number and size are under active investigation. Here, advances in the field of peroxisomal dynamics and proliferation in mammals and yeast are reviewed. The authors address the signals, conditions, and proteins that affect, regulate, and control the number and size of this essential organelle, especially the components involved in the division of peroxisomes. Special emphasis is on the function of dynamin-related proteins (DRPs), on Fis1, a putative adaptor for DRPs, on the role of the Pex11 family of peroxisomal membrane proteins, and the cytoskeleton.

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

Degradation of normal and proliferated peroxisomes in rat hepatocytes: regulation of peroxisomes quantity in cells

Degradation and turnover of peroxisomes is reviewed. First, we describe the historical aspects of peroxisome degradation research and the two major concepts for breakdown of peroxisomes, i.e., autophagy and autolysis. Next, the comprehensive knowledge on autophagy of peroxisomes in mammalian and yeast cells is reviewed. It has been shown that proliferated peroxisomes are degraded by selective autophagy, and studies using yeast cells have been especially helpful in shedding light on the molecular mechanisms of this process. The degradation of extraperoxisomal urate oxidase crystalloid is noted. Overexpressed wild-type urate oxidase in cultured cells has been shown to be degraded through an unknown proteolytic pathway distinct from the lysosomal system including autophagy or the ubiquitin-proteasome system. Finally, peroxisome autolysis mediated by 15-lipoxygenase (15-LOX) is described. 15-LOX is integrated into the peroxisome membrane causing focal membrane disruptions. The content of the peroxisomes is then exposed to cytosol proteases and seems to be digested quickly. In conclusion, the number of peroxisomes appears to be regulated by two selective pathways, autophagy, including macro- and microautophagy, and 15-LOX-mediated autolysis.

Peroxisome proliferation as a biomarker in environmental pollution assessment

Peroxisome proliferators comprise a heterogeneous group of compounds known for their ability to cause massive proliferation of peroxisomes and liver carcinogenesis in rodents. In recent years it has become evident that other animals may be threatened by peroxisome proliferators, in particular aquatic organisms living in coastal and estuarine areas. These animals are exposed to a variety of pollutants of industrial, agricultural and urban origin which are potential peroxisome proliferators. Both laboratory and field studies have shown that phthalate ester plasticizers, PAHs and oil derivatives, PCBs, certain pesticides, bleached kraft pulp and paper mill effluents, alkylphenols and estrogens provoke peroxisome proliferation in different fish or bivalve mollusc species. The response appears to be mediated by peroxisome-proliferator activated receptors, members of the nuclear receptor family, recently cloned in fish. Based on these results it is proposed that peroxisome proliferation could be used as a biomarker of exposure to a variety of pollutants in environmental pollution assessment. This is illustrated by a case study in which mussels, used worldwide as sentinels of environmental pollution, were transplanted from reference to contaminated areas and vice versa. In mussels native to an area polluted with PAHs and PCBs, peroxisomal acyl-CoA oxidase (AOX) activity and peroxisomal volume density were 2-3 fold and 5-fold higher, respectively, compared to the reference site. When animals were transplanted to the polluted station, with increased concentration of organic xenobiotics, a concomitant significant increase of AOX was recorded. Conversely, in animals transplanted to the cleaner station, AOX activity and peroxisomal volume density decreased significantly. These results indicate that peroxisome proliferation is a rapid (i.e., two days) and reversible response to pollution in mussels. Before peroxisome proliferation can be implemented as a biomarker in biomonitoring programs, a well-defined protocol should be established and validated in intercalibration and quality assurance programmes. Furthermore, the influence of biotic and abiotic factors, some of which are known to affect peroxisome proliferation (season, tide level, interpopulation and interindividual variability), should be taken into consideration. The possible hepatocarcinogenic effects as well as the potential adverse effects on reproduction, development, and growth of peroxisome proliferators are unknown in aquatic organisms, thus providing a challenge for future investigations.

The role of 15-lipoxygenase in disruption of the peroxisomal membrane and in programmed degradation of peroxisomes in normal rat liver

Our earlier electron microscopic observations revealed that prolonged exposure of glutaraldehyde-fixed rat liver sections to buffer solutions induced focal membrane disruptions of peroxisomes with catalase diffusion as shown cytochemically. Recently, it was suggested that 15-lipoxygenase (15-LOX) might be involved in natural degradation of membrane-bound organelles in reticulocytes by integrating into and permeabilizing the organelle membranes, leading to the release of matrix proteins. We have now investigated the localization of 15-LOX and its role in degradation of peroxisomal membranes in rat liver. Aldehyde-fixed liver slices were incubated in a medium that conserved the 15-LOX activity, consisting of 50 mM HEPES-KOH buffer (pH 7.4), 5 mM mercaptoethanol, 1 mM MgCl(2), 15 mM NaN(3), and 0.2 M sucrose, in presence or absence of 0.5-0.05 mM propyl gallate or esculetin, two inhibitors of 15-LOX. The exposure of aldehyde-fixed liver sections to this medium induced focal disruptions of peroxisome membranes and catalase diffusion around some but not all peroxisomes. This was significantly reduced by both 15-LOX inhibitors, propyl gallate and esculetin, with the latter being more effective. Double immunofluorescent staining for 15-LOX and catalase revealed that 15-LOX was co-localized with catalase in some but not all peroxisomes in rat hepatocytes. By postembedding immunoelectron microscopy, gold labeling was localized on membranes of some peroxisomes. These observations suggest that 15-LOX is involved in degradation of peroxisomal membranes and might have a physiological role in programmed degradation and turnover of peroxisomes in hepatocytes. (J Histochem Cytochem 49:613-621, 2001)

Inhibition effects of di(2-ethylhexyl)phthalate on mouse-liver lysosomal vacuolar H(+)-ATPase

We investigated the effects of di(2-ethylhexyl)phthalate (DEHP) on mouse-liver lysosomes. After 2 weeks of oral administration in mice, a reduction in vacuolar H(+)-ATPase (V-ATPase) was observed, and after 3 weeks, the liver lysosomal compartment was completely negative for V-ATPase, as determined by immunocytochemical analysis. When the mice were subsequently fed a normal diet for 1 week, V-ATPase levels recovered to normal values. According to Northern blot analysis, V-ATPase subunit A mRNA decreased gradually with DEHP treatment. Enzyme cytochemical staining showed acid phosphatase (AcPase) to be present in lysosomes and late autophagosomes (autolysosomes) in normal animals as well as in DEHP-treated animals. But the number of late autophagosomes containing AcPase increased clearly after DEHP treatment. These results suggest that: (1) DEHP causes marked V-ATPase reduction in the liver lysosomal compartment and the effect of DEHP is reversible; and (2) the effect of DEHP on protein expression is likely to be exerted at the transcriptional level.

Effect of mono-ethylhexyl phthalate on MA-10 Leydig tumor cells

We examined the effect of mono-ethylhexyl phthalate (MEHP) on MA-10 Leydig tumor cell structure and function. Cells were exposed to various concentrations of MEHP for 24 h and then stimulated with saturating concentrations of hCG for 2.5 h. Progesterone production, cell viability, and protein content were moderately inhibited by low concentrations and severely inhibited by high concentrations of MEHP. Electron microscopy showed a variety of alterations in the MEHP-treated cells, increasing in severity with increasing concentrations of MEHP. Lipid droplets were profoundly affected in the cells treated with MEHP and morphologic evidence that metabolism of lipid storage droplets ceases at approximately the same time progesterone synthesis stops was seen. Morphometric studies indicated that the number of lipid droplets appeared to be increased 2.5-fold over control levels at MEHP concentrations of 10(-6) to 10(-3) M whereas mitochondrial volume fraction decreased. These results suggest that MEHP in Leydig cells may act as a mitochondrial toxicant and lipid metabolism disrupter.

Influence of peroxisome proliferator-activated receptor alpha on ubiquinone biosynthesis

The control of ubiquinone biosynthesis by peroxisome proliferators was investigated using peroxisome proliferator activated receptor alpha (PPARalpha)-null mice. Administration of 2-(diethylhexyl)phthalate to control mice resulted in elevated ubiquinone levels in the liver, while dolichol, dolichyl-P and cholesterol concentrations remained unchanged. In PPARalpha-null mice, the level of these lipids were similar to control levels and administration of the peroxisome proliferator did not increase the levels of ubiquinone. The increase in ubiquinone levels was the result of increased synthesis. Induction was most pronounced in liver, kidney and heart, which have relatively high levels of PPARalpha. When the tissue concentration of hydrogen peroxide was elevated by inhibition of catalase activity with aminotriazole, the amount of ubiquinone was not increased, suggesting that the induction of ubiquinone synthesis occured through a direct mechanism. The activities of branch-point enzymes FPP-synthase, squalene synthase, cis-prenyltransferase, trans-prenyltransferase and NPHB-transferase were substantially increased in control but not in PPARalpha-null mice after treatment with peroxisome proliferators. These data suggest that the induction of ubiquinone biosynthesis after administration of peroxisome proliferators is dependent on the PPARalpha through regulation of some of the mevalonate pathway enzymes.

Current cytochemical techniques for the investigation of peroxisomes. A review

The past decade has witnessed unprecedented progress in elucidation of the complex problems of the biogenesis of peroxisomes and related human disorders, with further deepening of our understanding of the metabolic role of this ubiquitous cell organelle. There have been many recent reviews on biochemical and molecular biological aspects of peroxisomes, with the morphology and cytochemistry receiving little attention. This review focuses on the state-of-the-art cytochemical techniques available for investigation of peroxisomes. After a brief introduction into the use of the 3,3'-diaminobenzidine method for localization of catalase, which is still most commonly used for identification of peroxisomes, the cerium technique for detection of peroxisomal oxidases is discussed. The influence of the buffer used in the incubation medium on the ultrastructural pattern obtained in rat liver peroxisomes in conjunction with the localization of urate oxidase in their crystalline cores is discussed, particularly since Tris-maleate buffer inhibits the enzyme activity. In immunocytochemistry, quantitation of immunogold labeling by automatic image analysis enables quantitative assessment of alterations of proteins in the matrix of peroxisomes. This provides a highly sensitive approach for analysis of peroxisomal responses to metabolic alterations or to xenobiotics. The recent evidence suggesting the involvement of ER in the biogenesis of "preperoxisomes" is mentioned and the potential role of preembedding immunocytochemistry for identification of ER-derived early peroxisomes is emphasized. The use of GFP expressed with a peroxisomal targeting signal for the investigation of peroxisomes in living cells is briefly discussed. Finally, the application of in situ hybridization for detection of peroxisomal mRNAs is reviewed, with emphasis on a recent protocol using perfusion-fixation, paraffin embedding, and digoxigenin-labeled cRNA probes, which provides a highly sensitive method for detection of both high- and low-abundance mRNAs encoding peroxisomal proteins. (J Histochem Cytochem 47:1219-1232, 1999)

Morphometry of abnormal peroxisomes induced by withdrawal of bezafibrate, a hypolipidemic drug in male rat hepatocytes

Peroxisomes containing fibrillar structures were induced after 1 week withdrawal of bezafibrate, a peroxisome proliferator. In this report, the relation between the duration of bezafibrate treatment and the induction of abnormal peroxisomes in rat hepatocytes was studied morphometrically. The abnormal peroxisomes did not appear during 3 to 90 days of treatment with bezafibrate, but they appeared after 1 week withdrawal of it. The number and frequency of abnormal peroxisomes were prominent in 3, 7, and 14 days of treatment followed by 1 week of withdrawal of bezafibrate. It was evident that the frequency of abnormal peroxisomes decreased with 30-90 days administration of bezafibrate. This means that long-term (30-90 days) treatment with bezafibrate suppresses the induction of abnormal peroxisomes.

Peroxisome proliferator-activated receptor (PPAR)-dependent and -independent transcriptional modulation of several non-peroxisomal genes by peroxisome proliferators

To better characterize peroxisome proliferator-induced non-peroxisomal responses, we searched the mRNAs of which the levels were modulated by proliferators. We used the PCR-based methods including differential display. The mRNAs were divided into at least four groups by their time-courses of induction and repression: group 1 very rapidly increased then decreased; group 2 increased after a time lag (well-characterized peroxisomal mRNAs belonged to this group); group 3 decreased reciprocally compared with group 2 mRNAs; group 4 increased after group 2 mRNAs, with a much longer lag period. All of these modulations cannot be explained by peroxisome proliferator action through PPAR and RXR dimerization on the target genes to activate transcription. Another unidentified transcription factor may be involved in some of these modulations. It will also be important to consider PPAR-independent pathways when studying the diverse effects of peroxisome proliferators.

Thyromimetic mode of action of peroxisome proliferators: activation of 'malic' enzyme gene transcription

Peroxisome proliferators induce thyroid-hormone-dependent liver activities, e.g. 'malic' enzyme, mitochondrial glycerol-3-phosphate dehydrogenase, glucose-6-phosphate dehydrogenase, S14[Hertz, Aurbach, Hashimoto and Bar-Tana (1991) Biochem. J. 274, 745-751]. Here we report that the thyromimetic effect of peroxisome proliferators with respect to 'malic' enzyme result from transcriptional activation of the 'malic' enzyme gene, mediated by binding of the peroxisome proliferator activated receptor (PPAR alpha)/retinoid X receptor (RXR alpha) heterodimer to a 5'-flanking enhancer of the 'malic' enzyme promoter. The enhancer involved is distinct from the thyroid hormone response element of the 'malic' enzyme promoter and is partly homologous with that which mediates transcriptional activation of peroxisomal acyl-CoA oxidase by peroxisome proliferators. Hence transcriptional activation of thyroid-hormone-dependent liver genes by xenobiotic or endogenous amphipathic carboxylates collectively defined as peroxisome proliferators is mediated by a transduction pathway similar to that involved in transcriptional activation of peroxisomal beta-oxidative genes and distinct from that which mediates thyroid hormone action.

Dieldrin induces peroxisomal enzymes in fish (Sparus aurata) liver

We have previously described the increase of microsomal lipid peroxidation and the appearance of new oxidized forms of Cu,Zn-superoxide dismutase in the liver of gilthead seabrams (Sparus aurata) injected with model xenobiotics, due to the increased production of reactive oxygen species (ROS) (Pedrajas et al., Chem. Biol. Interact., 1995). The effects of dieldrin and copper(II) on subcellular organelles directly related with ROS production are now studied. Immature fish were injected with dieldrin and copper, 0.15 and 1.0 mg/ kg, respectively. After 2 and 7 days, the livers were homogenized and the catalase and superoxide dismutase activities were determined in subcellular fractions isolated by differential centrifugation: nucleus, mitochondria, light mitochondrial fraction (LMF), microsomes and cytosolic fraction. Peroxisomes were isolated from LMF by discontinuous gradient centrifugation using Nycodenz. Changes in catalase and superoxide dismutase depended on the xenobiotic and affected to different subcellular fractions. Thus, the effects of copper(II) were mainly in nucleus and cytosol, whereas dieldrin induced catalase and superoxide dismutase (up to 2.8-fold) preferentially in nucleus and LMF fractions. Dieldrin-injected fish showed also highly increased activity of palmitoyl-CoA-oxidase (9.3-fold) and a nearly 2-fold increase in the protein concentration of the peroxisomal fraction. The results described above suggest that the oxidative stress previously detected for dieldrin in fish liver affects also to the peroxisomal enzymes.

Contributions of the immunogold technique to investigation of the biology of peroxisomes

The immunogold labeling technique has been extremely useful in investigation of the structure and function of peroxisomes. In this report a few examples of the application of this technique with significant implications in the field are briefly reviewed. The problem of extra-peroxisomal catalase, the subject of long controversy between the biochemists and cytochemists, was settled with the immunogold technique, which unequivocally revealed the presence of that enzyme not only in the cytoplasm, but also in the euchromatin region of nucleus, in addition to peroxisomes. On the other hand, lactate dehydrogenase, a typical cytoplasmic protein, has also been shown recently to be present in peroxisomes and to be involved in the reoxidation of NADH produced by the peroxisomal beta-oxidation system. The immunogold technique has revealed several distinct compartments in the matrix of mammalian peroxisomes: urate oxidase in the crystalline cores, alpha-hydroxy acid oxidase B in the marginal plates and D-amino acid oxidase in a non-crystalline condensed region of matrix. The specific alterations of peroxisomal proteins are reflected in their immunolabeling density with gold particles. Quantitation of gold-label by automatic image analysis has revealed that the induction of lipid beta-oxidation enzyme proteins by diverse hypolipidemic drugs is initiated and more pronounced in the pericentral regions of the liver lobule. Finally, immunogold labeling with an antibody to 70 kDa peroxisomal membrane protein has identified a novel class of small peroxisomes that initially incorporate radioactive amino acids more efficiently than regular peroxisomes and thus may represent early stages in the biogenesis of peroxisomes.

In situ heterogeneity of peroxisomal oxidase activities: an update

Oxidases are a widespread group of enzymes. They are present in numerous organisms and organs and in various tissues, cells, and subcellular compartments, such as mitochondria. An important source of oxidases, which is investigated and discussed in this study, are the (micro)peroxisomes. Oxidases share the ability to reduce molecular oxygen during oxidation of their substrate, yielding an oxidized product and hydrogen peroxide. Besides the hydrogen peroxide-catabolizing enzyme catalase, peroxisomes contain one or more hydrogen peroxide-generating oxidases, which participate in different metabolic pathways. During the last four decades, various methods have been developed and elaborated for the histochemical localization of the activities of these oxidases. These methods are based either on the reduction of soluble electron acceptors by oxidase activity or on the capture of hydrogen peroxide. Both methods yield a coloured and/or electron dense precipitate. The most reliable technique in peroxisomal oxidase histochemistry is the cerium salt capture method. This method is based on the direct capture of hydrogen peroxide by cerium ions to form a fine crystalline, insoluble, electron dense reaction product, cerium perhydroxide, which can be visualized for light microscopy with diaminobenzidine. With the use of this technique, it became clear that oxidase activities not only vary between different organisms, organs, and tissues, but that heterogeneity also exists between different cells and within cells, i.e. between individual peroxisomes. A literature review, and recent studies performed in our laboratory, show that peroxisomes are highly differentiated organelles with respect to the presence of active enzymes. This study gives an overview of the in situ distribution and heterogeneity of peroxisomal enzyme activities as detected by histochemical assays of the activities of catalase, and the peroxisomal oxidases D-amino acid oxidase, L-alpha-hydroxy acid oxidase, polyamine oxidase and uric acid oxidase.

Long-term effect of tetradecylthioacetic acid: a study on plasma lipid profile and fatty acid composition and oxidation in different rat organs

Administration of tetradecylthioacetic acid (a 3-thia fatty acid) increases mitochondrial and peroxisomal beta-oxidative capacity and carnitine palmitoyltransferase activity, but reduces free fatty acid and triacylglycerol levels in plasma compared to palmitic acid-treated rats and controls. The decrease in plasma triacylglycerol was accompanied by a reduction (56%) in VLDL-triacylglycerol. Prolonged supplementation of tetradecylthioacetic acid caused a significant increase in lipogenic enzyme activities (ATP-citrate lyase and acetyl-CoA carboxylase) and diacylglycerol acyltansferase, but did not affect phosphatidate phosphohydrolase. Plasma cholesterol, LDL- and HDL-cholesterol levels were reduced. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity was, however, stimulated in 3-thia fatty acid-treated rats compared to controls. In addition. the mRNAs of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and LDL-receptor were increased. Tetradecylthioacetic acid administration affected the fatty acid composition in plasma and liver by increasing the amount of monoenes, especially 18:1(n-9), mostly at the expense of omega-3 fatty acids. Compared to liver a large amount of tetradecylthioacetic acid accumulated in the heart, and this accumulation was accompanied by an increase in omega-3 fatty acids, particularly 22:6(n-3) and a decrease in omega-6 fatty acids, mainly 20:4(n-6). The results show that the hypolipidemic effect of tetradecylthioacetic acid is sustained after prolonged administration and may, at least in part, be due to increased fatty acid oxidation and upregulated LDL-receptor gene expression. The increase in lipogenic enzyme activities as well as increased 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, may be compensatory mechanisms to maintain cellular integrity. Decreased level of 20:4(n-6) combined with increased omega-3/omega-6 ratio in cardiac tissue after tetradecylthioacetic acid treatment may have influence on membrane dynamics and function.

Fibrates induce mdr2 gene expression and biliary phospholipid secretion in the mouse

Disruption of the murine mdr2 gene leads to the complete absence of biliary phospholipids. We tested the hypothesis that the increase in biliary phospholipid output induced by fibrates is mediated via induction of the hepatic mdr2 gene and its encoded product, the P-glucoprotein canalicular flippase. Increased levels of mdr2 mRNA were observed in the liver of mice treated with different fibrates: ciprofibrate, 660+/-155% (as compared with control group); clofibrate, 611+/-77%; bezafibrate, 410+/-47%; fenofibrate, 310+/-52%; gemfibrozil, 190+/-25% (P <0.05 compared with control group). Induction of expression of the mdr gene family was specific to the mdr2 gene. Two- to three-fold increases in P-glycoprotein immunodetection were evident on the canalicular plasma-membrane domain of clofibrate- and ciprofibrate-treated mice. Biliary phospholipid output increased from 4.2+/-1.2 nmol/min per g of liver in the control group to 8.5+/-0.6, 7.1+/-2.9 and 5.8+/-2.5 in ciprofibrate-, clofibrate- and bezafibrate-treated mice respectively (P <0.05 compared with control group). Moreover, a significant correlation between biliary phospholipid output and the relative levels of mdr2 mRNA was found (r=0.86; P <0.05). In treated animals, bile flow as well as cholesterol and bile acid outputs remained unchanged. Our findings constitute the first evidence that pharmacological modulation of biliary lipid secretion mediated by fibrates can be related to the overexpression of a specific liver gene product, the mdr2 P-glycoprotein, and are consistent with the hypothesis that the mdr2 P-glycoprotein isoform plays a crucial role in the secretion of biliary phospholipid.

L-lactate dehydrogenase A4- and A3B isoforms are bona fide peroxisomal enzymes in rat liver. Evidence for involvement in intraperoxisomal NADH reoxidation

The subcellular localization of l-lactate dehydrogenase (LDH) in rat hepatocytes has been studied by analytical subcellular fractionation combined with the immunodetection of LDH in isolated subcellular fractions and liver sections by immunoblotting and immunoelectron microscopy. The results clearly demonstrate the presence of LDH in the matrix of peroxisomes in addition to the cytosol. Both cytosolic and peroxisomal LDH subunits have the same molecular mass (35.0 kDa) and show comparable cross-reactivity with an anti-cytosolic LDH antibody. As revealed by activity staining or immunoblotting after isoelectric focussing, both intracellular compartments contain the same liver-specific LDH-isoforms (LDH-A4 > LDH-A3B) with the peroxisomes comprising relatively more LDH-A3B than the cytosol. Selective KCl extraction as well as resistance to proteinase K and immunoelectron microscopy revealed that at least 80% of the LDH activity measured in highly purified peroxisomal fractions is due to LDH as a bona fide peroxisomal matrix enzyme. In combination with the data of cell fractionation, this implies that at least 0.5% of the total LDH activity in hepatocytes is present in peroxisomes. Since no other enzymes of the glycolytic pathway (such as phosphoglucomutase, phosphoglucoisomerase, and glyceraldehyde-3-phosphate dehydrogenase) were found in highly purified peroxisomal fractions, it does not seem that LDH in peroxisomes participates in glycolysis. Instead, the marked elevation of LDH in peroxisomes of rats treated with the hypolipidemic drug bezafibrate, concomitantly to the induction of the peroxisomal beta-oxidation enzymes, strongly suggests that intraperoxisomal LDH may be involved in the reoxidation of NADH generated by the beta-oxidation pathway. The interaction of LDH and the peroxisomal palmitoyl-CoA beta-oxidation system could be verified in a modified beta-oxidation assay by adding increasing amounts of pyruvate to the standard assay mixture and recording the change of NADH production rates. A dose-dependent decrease of NADH produced was simulated with the lowest NADH value found at maximal LDH activity. The addition of oxamic acid, a specific inhibitor of LDH, to the system or inhibition of LDH by high pyruvate levels (up to 20 mm) restored the NADH values to control levels. A direct effect of pyruvate on palmitoyl-CoA oxidase and enoyl-CoA hydratase was excluded by measuring those enzymes individually in separate assays. An LDH-based shuttle across the peroxisomal membrane should provide an efficient system to regulate intraperoxisomal NAD+/NADH levels and maintain the flux of fatty acids through the peroxisomal beta-oxidation spiral.

Sequential appearance and ultrastructure of amphophilic cell foci, adenomas, and carcinomas in the liver of male and female rats treated with dehydroepiandrosterone

Dehydroepiandrosterone (DHEA), a hormone of the adrenal cortex, acts as a peroxisome proliferator and hepatocarcinogen in rats upon long-term treatment with high doses in the diet. The aim of the present study was to identify the site of origin of hepatocellular neoplasms and the sequence of preneoplastic lesions. Twenty-five female and 25 male rats were given 0.6% DHEA in the diet; 25 animals of each sex were controls. Groups of 5 treated and untreated animals were sacrificed after 4, 20, 32, 70, and 84 wk. Amphophilic cell foci were detected after 32 wk of treatment; they developed from the liver parenchyma almost exclusively in the vicinity of portal tracts. Adenomas of the amphophilic or amphophilic/tigroid cell phenotype were observed at 70 wk of treatment. Highly differentiated hepatocellular carcinomas presenting a similar cellular phenotype occurred after 70-84 wk. The incidence of hepatocellular carcinomas was 44% in female and 11% in male rats. Ultrastructural studies of the amphophilic cell foci and tumors revealed a marked proliferation of mitochondria and a moderate proliferation of peroxisomes in all lesions. In addition, a very strong peroxisome proliferation was observed in perivenular hepatocytes in the liver of female rats. Peroxisomes usually lacked core and showed flocculent matrices. In male rats, weak peroxisomal proliferation was observed. Typical morphological abnormalities of these peroxisomes were paracrystalline inclusions of striated appearance. Although the most prominent peroxisome proliferation was observed in perivenular hepatocytes, these cells did not seem to be involved in tumor development. In contrast, the morphological similarity of the amphophilic cell foci and the amphophilic/tigroid cell adenomas and carcinomas, their coincident localization near portal tracts, and the sequential appearance of these lesions suggest that the amphophilic cell foci represent an early stage in DHEA-induced hepatocellular neoplasia. Mitochondrial proliferation as the most prominent feature in all stages of this model of hepatocarcinogenesis may offer a new approach for analysis of hepatocarcinogenesis induced by DHEA and possibly other peroxisomal proliferators.

Suppression of clofibrate-induction of peroxisomal and microsomal fatty acid-oxidizing enzymes by growth hormone and thyroid hormone in primary cultures of rat hepatocytes

Using primary cultures of rat hepatocytes on a matri-gel, effects of peroxisome proliferator and omega-hydroxydodecanoic acid on cellular levels of acyl-CoA oxidase and CYP4A have been studied to determine the hormonal influence in serum-free media. Peroxisomal acyl-CoA oxidation, microsomal CYP4A content and laurate omega-hydroxylation were increased in rat hepatocytes by the addition of 100 microM clofibrate or Wy14,643 for two days. omega-Hydroxydodecanoic acid (100 microM) also increased peroxisomal acyl-CoA oxidation, but had no clear effect on microsomal CYP4A level and laurate omega-hydroxylation. CYP4A-mediated laurate omega-hydroxylation in hepatocytes was suppressed by the addition of pituitary growth hormone (0.05 mU/ml), but was not altered by the addition of triiodothyronine (30 nM). In contrast, clofibrate-mediated induction of acyl-CoA oxidase activity was decreased by the addition of either one of the hormones in hepatocytes. Suppression by those hormones was also observed with omega-hydroxydodecanoic acid-mediated induction of acyl-CoA oxidase activity. These results indicate the possibility that GH and T3 exert the suppressive effects on peroxisomal acyl-CoA oxidation through plural mechanisms with and without the alteration of CYP4A levels in livers.

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.

Improvement of glucose tolerance by bezafibrate in non-obese patients with hyperlipidemia and impaired glucose tolerance

Glucose intolerance or diabetes mellitus, hyperlipidemia, obesity and hypertension may have a close interrelation based on insulin resistance. We selected 28 impaired glucose tolerance (IGT) patients with hyperlipidemia. The IGT patients demonstrated hypertriglyceridemia associated with hyperinsulinemia, a typical manifestation of insulin resistance. Administration of bezafibrate at 400 mg/day for 4 weeks to the IGT patients with hypertriglyceridemia resulted in an improvement of the plasma glucose level and insulin response to 75 g oral glucose loading associated with a concomitant decrease in non-esterified fatty acids. The ratio of the level of serum C-peptide to that of insulin after a 75 g oral glucose tolerance test (OGTT) was augmented after 4 weeks of bezafibrate administration. However, reduction of the cholesterol level with pravastatin did not alter these parameters. These results suggest that treatment to reduce the level of serum triglycerides, but not that of cholesterol, may have a beneficial effect for improving insulin resistance even in the non-obese subjects with IGT and decreasing the risk of coronary heart disease.

Peroxisome proliferation of hepatocytes in rats by a microbial degradation product of cholic acid, 4-(decahydro-6-methyl-3-oxocyclopenta(f)quinoline-7-yl)valeric acid

Three-week oral administration of 4-(decahydro-6-methyl-3-oxo-cyclopenta(f)quinoline-7-yl)valeric acid (32-1328) in the diet supplemented at concentrations of 0.1% or 0.3% was associated with hepatomegaly and hypotriglyceridemia in male F344 rats. Electron microscopic examination of the liver revealed a remarkable increase of peroxisomes in hepatocytes both in number and size. Biochemically, there were increased activities of peroxisomal marker enzymes including the heat-labile enoyl-CoA hydratase and catalase while the mitochondrial enoyl-CoA hydratase activity was unchanged after feeding of 32-1328. These findings indicate that 32-1328 can exert peroxisome-proliferating activity to rat liver in a manner similar to typical peroxisome proliferators such as clofibrate or di(2-ethylhexyl)phthalate.

The impact of aging on enzyme proteins of rat liver peroxisomes: quantitative analysis by immunoblotting and immunoelectron microscopy

The alterations of hepatic peroxisomes and their enzymes during aging were investigated in male rats. Peroxisomes in the livers of young (2 months) and old (39 months) male Wistar rats were analyzed by morphometry and quantitative immunocytochemistry, as well as by immunoblotting of highly purified peroxisomal fractions. Immunoblots showed that catalase and acyl-CoA oxidase were decreased in peroxisomes of old animals but the trifunctional enzyme, thiolase, and urate oxidase were increased. The morphometrical analysis revealed a heterogeneous distribution of peroxisomes in the liver lobule of the old animals, with a significant elevation of peroxisomal volume density in pericentral over periportal hepatocytes, in contrast to the uniform pattern in the young rats. Furthermore, age-related lobular gradients were also observed by quantitative immunocytochemistry in the peroxisomal concentrations of trifunctional enzyme (central > portal) and, inversely, for catalase (portal > central). Whereas acyl-CoA oxidase was diminished across the liver lobule, the enzyme 3-ketoacyl-CoA thiolase was elevated. These observations show that peroxisomes are significantly altered in aged animals and suggest that these alterations may contribute to the disturbance of lipid metabolism in aged animals. Moreover, the diminution in catalase and the elevation of urate oxidase could contribute to the oxidative stress which is considered to be of fundamental importance in the aging process.