Proliferation of peroxisomes without simultaneous induction of the peroxisomal fatty acid beta-oxidation

Proliferation and fission of peroxisomes - An update

In mammals, peroxisomes perform crucial functions in cellular metabolism, signalling and viral defense which are essential to the health and viability of the organism. In order to achieve this functional versatility peroxisomes dynamically respond to molecular cues triggered by changes in the cellular environment. Such changes elicit a corresponding response in peroxisomes, which manifests itself as a change in peroxisome number, altered enzyme levels and adaptations to the peroxisomal structure. In mammals the generation of new peroxisomes is a complex process which has clear analogies to mitochondria, with both sharing the same division machinery and undergoing a similar division process. How the regulation of this division process is integrated into the cell's response to different stimuli, the signalling pathways and factors involved, remains somewhat unclear. Here, we discuss the mechanism of peroxisomal fission, the contributions of the various division factors and examine the potential impact of post-translational modifications, such as phosphorylation, on the proliferation process. We also summarize the signalling process and highlight the most recent data linking signalling pathways with peroxisome proliferation.

Fission and proliferation of peroxisomes

Peroxisomes are remarkably dynamic, multifunctional organelles, which react to physiological changes in their cellular environment and adopt their morphology, number, enzyme content and metabolic functions accordingly. At the organelle level, the key molecular machinery controlling peroxisomal membrane elongation and remodeling as well as membrane fission is becoming increasingly established and defined. Key players in peroxisome division are conserved in animals, plants and fungi, and key fission components are shared with mitochondria. However, the physiological stimuli and corresponding signal transduction pathways regulating and modulating peroxisome maintenance and proliferation are, despite a few exceptions, largely unexplored. There is emerging evidence that peroxisomal dynamics and proper regulation of peroxisome number and morphology are crucial for the physiology of the cell, as well as for the pathology of the organism. Here, we discuss several key aspects of peroxisomal fission and proliferation and highlight their association with certain diseases. We address signaling and transcriptional events resulting in peroxisome proliferation, and focus on novel findings concerning the key division components and their interplay. Finally, we present an updated model of peroxisomal growth and division. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.

A specific inhibitor of cholesterol biosynthesis, BM15.766, reduces the expression of ?-secretase and the production of amyloid-?in vitro

We have previously shown that statins reduce the production of amyloid-beta (Abeta) by both isoprenoid- and cholesterol-dependent mechanisms. These pathways contribute to the regulation of the dimerisation of BACE into its physiologically active form. Statins reduce cellular cholesterol levels by 20-40%; therefore, it is possible that the remaining cholesterol within the cell may play a significant role in the production of Abeta. Incubation of cells with the specific cholesterol biosynthesis inhibitor BM15.766 together with 50 micromol/L simvastatin and 400 micromol/L mevalonate reduced cellular cholesterol levels in a dose-dependent manner with increasing BM15.766 concentration (r = -0.9736, p = 0.0264). Furthermore, decreases in cellular cholesterol levels correlated with reductions in total Abeta production (r = 0.9683, p = 0.0317). A total of 2.5 micromol/L BM15.766 inhibited the dimerisation of BACE, whilst the expression of BACE monomer was reduced by 5 micromol/L BM15.766. BM15.766 treatment localised BACE predominantly within the Golgi, and reduced total BACE expression per cell. Similar changes were observed in the expression of the Golgi marker golgin-97, suggesting that reduced BACE expression may arise from a decrease in protein trafficking and an increase in degradation. By targeting cholesterol synthesis using specific cholesterol biosynthesis inhibitors, it is possible to reduce Abeta production without reducing protein isoprenylation.

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.

A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARalpha-dependent and -independent pathways

Branched-chain fatty acids (such as phytanic and pristanic acid) are ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in vitro. To investigate the effects of these physiological compounds in vivo, wild-type and PPARalpha-deficient (PPARalpha-/-) mice were fed a phytol-enriched diet. This resulted in increased plasma and liver levels of the phytol metabolites phytanic and pristanic acid. In wild-type mice, plasma fatty acid levels decreased after phytol feeding, whereas in PPARalpha-/- mice, the already elevated fatty acid levels increased. In addition, PPARalpha-/- mice were found to be carnitine deficient in both plasma and liver. Dietary phytol increased liver free carnitine in wild-type animals but not in PPARalpha-/- mice. Investigation of carnitine biosynthesis revealed that PPARalpha is likely involved in the regulation of carnitine homeostasis. Furthermore, phytol feeding resulted in a PPARalpha-dependent induction of various peroxisomal and mitochondrial beta-oxidation enzymes. In addition, a PPARalpha-independent induction of catalase, phytanoyl-CoA hydroxylase, carnitine octanoyltransferase, peroxisomal 3-ketoacyl-CoA thiolase, and straight-chain acyl-CoA oxidase was observed. In conclusion, branched-chain fatty acids are physiologically relevant ligands of PPARalpha in mice. These findings are especially relevant for disorders in which branched-chain fatty acids accumulate, such as Refsum disease and peroxisome biogenesis disorders.

Fibrates induce hepatic peroxisome and mitochondrial proliferation without overt evidence of cellular proliferation and oxidative stress in cynomolgus monkeys

There is little primate risk factor data in the literature evaluating the relationship between proposed mechanisms of PPAR agonist-induced hepatocarcinogenesis at clinically relevant therapeutic exposures. These studies were conducted to characterize the hepatic effects of fenofibrate and ciprofibrate in the cynomolgus monkey. Male cynomolgus monkeys were given fenofibrate (250, 1250 or 2500 mg/kg/day) or ciprofibrate (3, 30, 150 or 400 mg/kg/day) for up to 15 days. The highest doses used were approximately 4 times (fenofibrate) and 9.4 times (ciprofibrate) the human therapeutic exposure for these agents based on AUC (area under the curve). For both compounds, there was a treatment-related increase in liver weight and periportal hepatocellular hypertrophy, which was related to increases in peroxisomes (up to 2.8 times controls) and mitochondria (up to 2.5 times controls). An increase in smooth endoplasmic reticulum probably contributed to the hypertrophy. There was no indication of cell proliferation as determined by the number of mitotic figures and this was confirmed by evaluating cell proliferation by immunohistochemical staining for the Ki-67 antigen. Consistent with the findings by light microscopy, there was no treatment-related effect on the level of mRNA for proteins known to be involved in the control of hepatocyte cell division or apoptosis (e.g. P21, Cyclin D1, PCNA, CDKN1A). Furthermore, there was minimal indication of oxidative stress. Thus, there was no evidence of lipofuscin accumulation, and there was no remarkable increase in the mRNA levels for most proteins known to respond to oxidative stress (e.g. catalase, glutathione peroxidase). A mild induction in the mRNA levels of cellular beta-oxidation and detoxification enzymes (e.g. acyl CoA oxidase, thioredoxin reductase) was observed. Collectively, the data from these studies suggest that the primate responds to PPARalpha agonists in a manner that is different from the rodent suggesting that the primate may be refractory to PPAR-induced hepatocarcinogenesis.

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.

Differential induction of peroxisomal populations in subcellular fractions of rat liver

In rat liver, peroxisome proliferators induce profound changes in the number and protein composition of peroxisomes, which upon subcellular fractionation is reflected in heterogeneity in sedimentation properties of peroxisome populations. In this study we have investigated the time course of induction of the peroxisomal proteins catalase, acyl-CoA oxidase (ACO) and the 70 kDa peroxisomal membrane protein (PMP70) in different subcellular fractions. Rats were fed a di(2-ethylhexyl)phthalate (DEHP) containing diet for 8 days and livers were removed at different time-points, fractionated by differential centrifugation into nuclear, heavy and light mitochondrial, microsomal and soluble fractions, and organelle marker enzymes were measured. Catalase was enriched mainly in the light mitochondrial and soluble fractions, while ACO was enriched in the nuclear fraction (about 30%) and in the soluble fraction. PMP70 was found in all fractions except the soluble fraction. DEHP treatment induced ACO, catalase and PMP70 activity and immunoreactive protein, but the time course and extent of induction was markedly different in the various subcellular fractions. All three proteins were induced more rapidly in the nuclear fraction than in the light mitochondrial or microsomal fractions, with catalase and PMP70 being maximally induced in the nuclear fraction already at 2 days of treatment. Refeeding a normal diet quickly normalized most parameters. These results suggest that induction of a heavy peroxisomal compartment is an early event and that induction of 'small peroxisomes', containing PMP70 and ACO, is a late event. These data are compatible with a model where peroxisomes initially proliferate by growth of a heavy, possibly reticular-like, structure rather than formation of peroxisomes by division of pre-existing organelles into small peroxisomes that subsequently grow. The various peroxisome populations that can be separated by subcellular fractionation may represent peroxisomes at different stages of biogenesis.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

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)

Mechanisms of peroxisome proliferation by perfluorooctanoic acid and endogenous fatty acids

1. The effects of endogenous fatty acids and perfluorooctanoic acid (PFOA) and its analogs on peroxisomal acyl CoA oxidase (ACO) and microsomal laurate hydroxylase (LH) activities were evaluated in primary cultures of rat hepatocytes and activation of peroxisome proliferator-activated receptor alpha (PPARalpha) in CV-1 cells. The rank order for the stimulation of ACO activity in hepatocytes for selected compounds was PFOA >> octanoic acid>octanedioic acid, perfluorooctanol (inactive). Increases in ACO activity by PFOA, like those of ciprofibrate, were associated with a marked increase in peroxisome number and cytosolic occupancy volume. Maximal effects of ciprofibrate and PFOA on the stimulation of ACO activity were not additive, suggesting that these two compounds share a common pathway of peroxisome proliferation. 2. Saturated monocarboxylic acids of C4 to C18 chain length were inactive, and, among dicarboxylic acids, only small elevations (40-45%) in ACO activity were observed with the long-chain C12 and C16 dioic acids. Of the C18 fatty acids tested, only oleic and linoleic acids, at 1 mM, produced a two- to three-fold elevation in ACO and LH activities. In comparison with endogenous fatty acids, PFOA was more potent and exhibited a different time course and greater magnitude of stimulation of ACO and LH activities in cultured hepatocytes. 3. Addition of mitochondrial beta-oxidation inhibitors (3-mercaptopropionic and 2-bromooctanoic acids) did not alter ACO activity in the presence of octanoic acid or octanedioic acid; nor did they modify the stimulation of ACO activity by PFOA. The carnitine palmitoyltransferase I inhibitor 2-bromopalmitic acid produced a 2.5-fold increase in ACO stimulatory activity and reduced both ciprofibrate- and PFOA-mediated stimulations of ACO activity. 4. Cycloheximide treatment reduced PFOA- and ciprofibrate-induced ACO activities; however, the response to oleic acid was not blocked and increased slightly. 5. In rat and human PPARalpha transactivation assays, the rank order of activation was ciprofibrate > PFOA > oleic acid > or = octanoic acid > octanedioic acid or perfluorooctanol (inactive). PFOA, ciprofibrate and oleic acid were activators of rPPARalpha at concentrations that correlated favorably with the changes in ACO activity in cell culture. Octanoic acid did not increase ACO activity and was a weak activator of PPARalpha. 6. Our findings suggest that fatty acids such as oleic acid (endogenous fatty acids) and PFOA (a stable fatty acid) act through more than one pathway to increase ACO activity in rat hepatocytes. We conclude that the potent effects of PFOA are primarily mediated by a mechanism that includes the activation of liver PPARalpha.

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.

Clofibrate and other peroxisomal proliferating agents relatively specifically inhibit synthesis of ethanolamine phosphoglycerides in cultured human fibroblasts

Effects of several classes of peroxisomal proliferators on peroxisomal functions, hepatomegaly, hepatocarcinogenesis and lipid metabolism have been extensively investigated in rodents. Less is known about influences of these agents, some used as hypolipidemic drugs, on various metabolic parameters in humans. We examined effects of clofibrate, di(2-ethyl-hexyl)phthalate (DEHP) and pirinixic acid (WY-14,643) on phospholipid metabolism in human fibroblasts in culture. Clofibrate inhibited incorporation of [1-14C]hexadecanol and [1-14C]linolenic acid into ethanolamine phosphoglycerides in a time- and concentration-dependent manner; labeling of plasmalogens and non-plasmalogen ethanolamine phosphoglycerides was reduced by 40-80% compared to a generalized 10-30% inhibition of labeling of other phospholipids, including phosphatidylcholine. In pulse and pulse-chase experiments, selective inhibition of incorporation of [1,2-14C]ethanolamine, compared to [methyl-3H]choline, confirmed relative specificity of inhibition of ethanolamine phosphoglycerides. Similar concentration dependence and specificity for inhibition of phospholipid turnover was observed for DEHP and WY-14,643, in both control and mutant (Zellweger and adrenoleukodystrophy) fibroblasts, in the absence of major effects on peroxisomal markers. These observations that peroxisomal proliferators specifically inhibit ethanolamine phosphoglyceride turnover in human fibroblasts should be considered when assessing the efficacy and safety of such agents as hypolipidemic drugs or when evaluating mechanisms of proliferator action at the cellular level.

Comparative studies of effects of dehydroepiandrosterone on rat and chicken liver

1. An attempt to identify the cause of decrease of gain in body weight during dehydroepiandrosterone (DHEA) treatment was made comparing the effects of hormone treatment on chickens and rats. 2. Chickens treated with DHEA for 7-10 days do not change their weight gain with respect to controls although their mitochondrial respiration and peroxisomal catalase (index of peroxisomal mass) were increased. 3. Liver cytosolic malic enzyme and sn-glycerol-3-phosphate dehydrogenase were depressed in chickens treated with DHEA in comparison with activities in untreated controls. DHEA treatment did not increase the activity of mitochondrial sn-glycerol 3-phosphate dehydrogenase. 4. In contrast to rat liver cytosolic sn-glycerol-3-phosphate dehydrogenase this enzyme in chicken liver was inactive with NADPH.

Induction of the major integral membrane protein of mouse liver peroxisomes by peroxisome proliferators

Peroxisome proliferators are known to increase the volume of the peroxisomal compartment in rodent liver. We have examined the induction of the major integral membrane protein of mouse liver peroxisomes (PMP68) by a number of these agents, and compared this with their effect on the peroxisomal bifunctional protein (PBP), an enzyme of the beta-oxidation pathway which is located in the peroxisome matrix. Dietary clofibrate, di-2-(ethylhexyl)phthalate and Wy-14,643, three structurally unrelated proliferators, all increased the mRNA and protein content of PMP68 approx. 2-fold, whereas PBP was induced 8-13-fold. The kinetics and sequence of induction of PMP68 and PBP following a single dose of Wy-14,643 were compared and shown to be similar, and the effects were reversible. Another proliferator, BM 15766, caused maximal induction of PMP68 but only a low induction of PBP; further PBP induction was achieved by the administration of BM 15766 in combination with Wy-14,643. Similarly, BM 15766 and Wy-14,643 increased transcription of the PMP68 gene in vitro, whereas PBP gene transcription was increased by Wy-14,643 but not by BM 15766. Thus peroxisome proliferators enhance the expression of the genes for both the membrane protein PMP68 and the matrix protein PBP, but the regulation of this expression appears to be mediated by different mechanisms.