Zonal heterogeneity of peroxisome proliferation and morphology in rat liver after gemfibrozil treatment

Peroxisomes form intralumenal vesicles with roles in fatty acid catabolism and protein compartmentalization in Arabidopsis

Peroxisomes are vital organelles that compartmentalize critical metabolic reactions, such as the breakdown of fats, in eukaryotic cells. Although peroxisomes typically are considered to consist of a single membrane enclosing a protein lumen, more complex peroxisomal membrane structure has occasionally been observed in yeast, mammals, and plants. However, technical challenges have limited the recognition and understanding of this complexity. Here we exploit the unusually large size of Arabidopsis peroxisomes to demonstrate that peroxisomes have extensive internal membranes. These internal vesicles accumulate over time, use ESCRT (endosomal sorting complexes required for transport) machinery for formation, and appear to derive from the outer peroxisomal membrane. Moreover, these vesicles can harbor distinct proteins and do not form normally when fatty acid β-oxidation, a core function of peroxisomes, is impaired. Our findings suggest a mechanism for lipid mobilization that circumvents challenges in processing insoluble metabolites. This revision of the classical view of peroxisomes as single-membrane organelles has implications for all aspects of peroxisome biogenesis and function and may help address fundamental questions in peroxisome evolution.

Peroxisomes are organelles compartmentalising metabolic reactions such as the breakdown of fats, and are commonly thought of as single membrane-bound compartments. Here the authors show that Arabidopsis peroxisomes contain extensive internal vesicles that form from the bounding membrane in an ESCRT-dependent process.

Phytol-induced pathology in 2-hydroxyacyl-CoA lyase (HACL1) deficient mice. Evidence for a second non-HACL1-related lyase

2-Hydroxyacyl-CoA lyase (HACL1) is a key enzyme of the peroxisomal α-oxidation of phytanic acid. To better understand its role in health and disease, a mouse model lacking HACL1 was investigated. Under normal conditions, these mice did not display a particular phenotype. However, upon dietary administration of phytol, phytanic acid accumulated in tissues, mainly in liver and serum of KO mice. As a consequence of phytanic acid (or a metabolite) toxicity, KO mice displayed a significant weight loss, absence of abdominal white adipose tissue, enlarged and mottled liver and reduced hepatic glycogen and triglycerides. In addition, hepatic PPARα was activated. The central nervous system of the phytol-treated mice was apparently not affected. In addition, 2OH-FA did not accumulate in the central nervous system of HACL1 deficient mice, likely due to the presence in the endoplasmic reticulum of an alternate HACL1-unrelated lyase. The latter may serve as a backup system in certain tissues and account for the formation of pristanic acid in the phytol-fed KO mice. As the degradation of pristanic acid is also impaired, both phytanoyl- and pristanoyl-CoA levels are increased in liver, and the ω-oxidized metabolites are excreted in urine. In conclusion, HACL1 deficiency is not associated with a severe phenotype, but in combination with phytanic acid intake, the normal situation in man, it might present with phytanic acid elevation and resemble a Refsum like disorder.

Assessing heterogeneity of peroxisomes: isolation of two subpopulations from rat liver

Peroxisomes exhibit a heterogeneous morphological appearance in rat liver tissue. In this respect, the isolation and subsequent biochemical characterization of peroxisome species from different subcellular prefractions should help to solve the question of whether peroxisomes indeed diverge into functionally specialized subgroups in one tissue. As a means to address this question, we provide a detailed separation protocol for the isolation of peroxisomes from both the light (LM-Po) and the heavy (HM-Po) mitochondrial prefraction for their subsequent comparative analysis. Both isolation strategies rely on centrifugation in individually adapted Optiprep gradients. In case of the heavy mitochondrial fraction, free flow electrophoresis is appended as an additional separation step to yield peroxisomes of sufficient purity. In view of their morphology, peroxisomes isolated from both fractions are surrounded by a continuous single membrane and contain a gray-opaque inner matrix. However, beyond this overall similar appearance, HM-Po exhibit a smaller average diameter, float at lower density, and show a more negative average membrane charge when compared to LM-Po.

RhoA regulates peroxisome association to microtubules and the actin cytoskeleton

The current view of peroxisome inheritance provides for the formation of new peroxisomes by both budding from the endoplasmic reticulum and autonomous division. Here we investigate peroxisome-cytoskeleton interactions and show by proteomics, biochemical and immunofluorescence analyses that actin, non-muscle myosin IIA (NMM IIA), RhoA, Rho kinase II (ROCKII) and Rab8 associate with peroxisomes. Our data provide evidence that (i) RhoA in its inactive state, maintained for example by C. botulinum toxin exoenzyme C3, dissociates from peroxisomes enabling microtubule-based peroxisomal movements and (ii) dominant-active RhoA targets to peroxisomes, uncouples the organelles from microtubules and favors Rho kinase recruitment to peroxisomes. We suggest that ROCKII activates NMM IIA mediating local peroxisomal constrictions. Although our understanding of peroxisome-cytoskeleton interactions is still incomplete, a picture is emerging demonstrating alternate RhoA-dependent association of peroxisomes to the microtubular and actin cytoskeleton. Whereas association of peroxisomes to microtubules clearly serves bidirectional, long-range saltatory movements, peroxisome-acto-myosin interactions may support biogenetic functions balancing peroxisome size, shape, number, and clustering.

Male germ cells require polyenoic sphingolipids with complex glycosylation for completion of meiosis: a link to ceramide synthase-3

Previously, it was found that a novel class of neutral fucosylated glycosphingolipids (GSLs) is required for male fertility. These lipids contain very long-chain (C26-C32) polyunsaturated (4-6 double bonds) fatty acid residues (VLC-PUFAs). To assess the role of these complex GSLs in spermatogenesis, we have now investigated with which of the testicular cell types these lipids are associated. During postnatal development, complex glycosylated and simple VLC-PUFA sphingolipids were first detectable at day 15, when the most advanced germ cells are pachytene spermatocytes. Their synthesis is most likely driven by ceramide synthase-3. This enzyme is encoded by the Cers3/Lass3 gene (longevity assurance genes), and out of six members of this gene family, only Cers3 mRNA expression was limited to germ cells, where it was up-regulated more than 700-fold during postnatal testicular maturation. Increasing levels of neutral complex VLC-PUFA GSLs also correlated with the progression of spermatogenesis in a series of male sterile mutants with arrests at different stages of spermatogenesis. Remarkably, fucosylation of the complex VLC-PUFA GSLs was not essential for spermatogenesis, as fucosylation-deficient mice produced nonfucosylated versions of the complex testicular VLC-PUFA GSLs, had complete spermatogenesis, and were fertile. Nevertheless, sterile Galgt1(-/-) mice, with a defective meiotic cytokinesis and a subsequent block in spermiogenesis, lacked complex but contained simple VLC-PUFA GSLs, as well as VLC-PUFA ceramides and sphingomyelins, indicating that the latter lipids are not sufficient for completion of spermatogenesis. Thus, our data imply that both glycans and the particular acyl chains of germinal sphingolipids are relevant for proper completion of meiosis.

Quantitative proteomic comparison of mouse peroxisomes from liver and kidney

The peroxisome plays a central role in the catabolic and anabolic pathways that contribute to the lipid homeostasis. Besides this main function, this organelle has gained functional diversity. Although several approaches have been used for peroxisomal proteome analysis, a quantitative protein expression analysis of peroxisomes from different tissues has not been elucidated yet. Here, we applied a 2-DE-based method on mouse liver and kidney peroxisomal enriched fractions to study the tissue-dependent protein expression. Ninety-one spots were identified from the 2-DE maps from pH 3.0-10.0 and 51 spots from the basic range corresponding to 31 peroxisomal proteins, 10 putative peroxisomal, 6 cytosolic, 17 mitochondrial and 1 protein from endoplasmic reticulum. Based on the identification and on the equivalent quality of both tissue preparations, the differences emerging from the comparison could be quantified. In liver, proteins involved in pathways such as alpha- and beta-oxidation, isoprenoid biosynthesis, amino acid metabolism and purine and pyrimidine metabolism were more abundant whereas in kidney, proteins from the straight-chain fatty acid beta-oxidation were highly expressed. These results indicate that tissue-specific functional classes of peroxisomal proteins could be relevant to study peroxisomal cellular responses or pathologies. Finally, a web-based peroxisomal proteomic database was built.

Phenylbutyrate up-regulates the adrenoleukodystrophy-related gene as a nonclassical peroxisome proliferator

X-linked adrenoleukodystrophy (X-ALD) is a demyelinating disease due to mutations in the ABCD1 (ALD) gene, encoding a peroxisomal ATP-binding cassette transporter (ALDP). Overexpression of adrenoleukodystrophy-related protein, an ALDP homologue encoded by the ABCD2 (adrenoleukodystrophy-related) gene, can compensate for ALDP deficiency. 4-Phenylbutyrate (PBA) has been shown to induce both ABCD2 expression and peroxisome proliferation in human fibroblasts. We show that peroxisome proliferation with unusual shapes and clusters occurred in liver of PBA-treated rodents in a PPARα-independent way. PBA activated Abcd2 in cultured glial cells, making PBA a candidate drug for therapy of X-ALD. The Abcd2 induction observed was partially PPARα independent in hepatocytes and totally independent in fibroblasts. We demonstrate that a GC box and a CCAAT box of the Abcd2 promoter are the key elements of the PBA-dependent Abcd2 induction, histone deacetylase (HDAC)1 being recruited by the GC box. Thus, PBA is a nonclassical peroxisome proliferator inducing pleiotropic effects, including effects at the peroxisomal level mainly through HDAC inhibition.

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.

Quantitative analysis of the lobular distribution of S-phase in rat liver following dietary administration of di(2-ethylhexyl)phthalate

A simple image-analysis method is described, whereby the distribution of hepatocytes that have entered S-phase, as distinguished by the incorporation of bromodeoxyuridine, can be related to the position of the central and portal veins of the hepatic lobule. Hepatocyte S-phase was induced in the livers of male and female F344 rats by administration of di(2-ethylhexyl)phthalate (DEHP) in the diet for 7 days at 2 dose levels, and these livers were used to develop the procedure. The distributions of the S-phase between control and DEHP-treated livers were compared using statistical techniques. The results showed notable differences in the distribution of S-phase between male and female rats as well as limited dose-related effects.

Peroxisome proliferator nafenopin potentiated cytotoxicity and genotoxicity of cyclophosphamide in the liver and bone marrow cells

Peroxisome proliferators are ubiquitous rodent hepatocarcinogens, known to modulate the activities of xenobiotic-metabolising enzymes such as glutathione S-transferases (GST) and mixed-function oxidase (cytochrome P-450). In addition these compounds induce pleiotropic changes in the liver of rodents even after a short-term treatment. It has been hypothesised that the enzymatic and cellular changes induced by peroxisome proliferators may alter the toxicity of other compounds activated by cytochrome P-450 and detoxified by GST isoenzymes. The effect of nafenopin-induced changes in the liver of rats on the toxicity of an anti-cancer drug cyclophosphamide was studied using cyto- and geno-toxicity parameters in the liver and bone marrow cells. The administration of cyclophosphamide (10 or 20 mg/kg bw) to the rats pre-treated with 80 mg/kg bw of nafenopin for 2 days resulted in significantly increased cytotoxic response in bone marrow cells. However, genotoxicity of cyclophosphamide was increased only in the liver of nafenopin pre-treated rats. Low level of genotoxicity in bone marrow could be accounted for potentiated cytotoxicity of cyclophosphamide. These events coincided with a significant, up to 5-fold, increase in indirect activation-detoxication index for cyclophosphamide, determined as a ratio of ECOD and GST activities, in nafenopin treated rats. This resulted from the induction of ECOD responsible for the formation of reactive metabolites of cyclophosphamide and reduced activity of GST responsible for their detoxication. In addition, mitotic activity of hepatocytes was increased in nafenopin treated rats that might also have an impact on the genotoxicity of cyclophosphamide in this organ.

A review of drug-induced lysosomal disorders of the liver in man and laboratory animals

Lysosomotropic agents are selectively taken up into lysosomes following their administration to man and animals [de Duve et al. (1974) Biochem. Pharmacol. 23:2494-2531] The effects of lysosomotropic drugs studied in vivo and in vitro can be used as models of lysosomal storage diseases. These agents include many drugs still used in clinical medicine: aminoglycosides used in antibiotics [Tulkens (1988)]; phenothiazine derivatives; such antiparasitic drugs as chloroquine and suramin; antiinflammatory drugs like gold sodium thiomalate; and cardiotonic drugs like sulmazol [Schneider (1992) Arch. Toxicol. 66:23-33]. Side-effects to these drugs can be caused by their lysosomotropic properties. In addition to drugs, other compounds to which man and animals are exposed (e.g., metals, cytostatics, vitamins, hormones) are also lysosomotropic. Liver cells, especially Kuppfer cells, are known to accumulate lysosomotropic agents. Here we review studies which evaluate lysosomal changes in the liver following administration of lysosomotropic agents to experimental animals, and relate them to toxic side-effects or pharmacological action, as was suggested by de Duve et al. (1974). Common features of lysosomal changes include, the overload of liver lysosomes by non-digestible material; increased size and number of liver lysosomes; inhibition of several lysosomal enzymes; secondary increase in the activity of some lysosomal enzymes; increased autophagy, and fusion disturbances. There was no significant change in endocytosis, except for an increase in the Triton WR 1339 model.

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.

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.

Reduction of rat liver endoplasmic reticulum Ca(2+)-ATPase activity and mobilization of hepatic intracellular calcium by ciprofibrate, a peroxisome proliferator

Ciprofibrate, a peroxisome proliferating agent, induces cell proliferation in rodent liver during the early periods of exposure. Since Ca2+ plays an important role in mitogenesis, we have investigated the effects of ciprofibrate on hepatic endoplasmic reticulum (ER) Ca(2+)-ATPase, which in part regulates Ca2+ homeostasis. A single oral dose of 200 mg/kg ciprofibrate to male F344 rats produced a transient decrease in liver microsomal Ca(2+)-ATPase activity to 48% of control levels at 24 hr post-exposure. Activity had returned to control levels by 48 and 72 hr after exposure. The decrease in Ca(2+)-ATPase activity was not a function of non-specific enzymatic inhibition, since activity of another microsomal enzyme, glucose-6-phosphatase, was not altered in ciprofibrate-exposed rats. Using an ATP-driven 45Ca2+ accumulation assay, rats exposed to 25, 100 and 200 mg/kg ciprofibrate exhibited a dose-dependent inhibition of liver microsomal Ca2+ accumulation at 24 hr post-exposure. Analysis of Western immunoblots using a polyclonal antibody to the liver ER Ca(2+)-ATPase revealed a marginal increase in Ca(2+)-ATPase protein content in microsomes prepared from ciprofibrate-exposed rats compared to controls 24 hr post-exposure. These data indicate that the reduction of Ca(2+)-ATPase activity is not attributable to diminished Ca(2+)-ATPase protein content in vivo and, therefore, is due to a functional inhibition of the enzyme. Ciprofibrate also produced a concentration-dependent inhibition of rat liver ER Ca(2+)-ATPase activity in vitro (IC50 approximately 170 microM). In freshly isolated rat hepatocytes, ciprofibrate elevated the free intracellular calcium concentration ([Ca2+]i) in the presence and absence of extracellular calcium. Collectively, these results suggest that ciprofibrate mobilizes hepatic [Ca2+]i via inhibition of the ER Ca(2+)-ATPase. These events may lead to an environment of elevated [Ca2+]i during the early stages of ciprofibrate exposure and may serve to augment Ca(2+)-dependent processes, thus playing a pivotal role in the acute mitogenic response.

Regulation of perfluorooctanoic acid--induced peroxisomal enzyme activities and hepatocellular growth by adrenal hormones

A wide variety of compounds, including hypolipidemic drugs, plasticizers and other industrial chemicals, have been found to cause liver enlargement and hepatic peroxisome proliferation by mechanisms that are unclear. Although thyroid and sex hormones have been shown to modulate the hepatic response to these chemicals, the role of adrenal hormones in these phenomena is not clear, and a few studies have produced conflicting data. Therefore this study was undertaken to investigate the role of adrenal hormones in hepatomegaly and peroxisomal enzyme induction caused by peroxisomal proliferators and to further delineate the interrelationship between these parameters. Because adrenalectomy alters hepatic drug metabolism, we have used the nonmetabolizable proliferator perfluorooctanoic acid. Our data show that hepatomegaly caused by perfluorooctanoic acid depends on corticosterone, the major glucocorticoid in rodents. Liver growth caused by perfluorooctanoic acid appears to be predominantly hypertrophic in nature, and DNA synthesis in response to perfluorooctanoic acid predominates in periportal regions of the liver lobule. Data also show that although induction of peroxisomal beta-oxidation by perfluorooctanoic acid is independent of adrenal hormones, induction of catalase is dependent on the presence of these hormones. This study supports the contention that induction of activities of various peroxisomal enzymes is controlled by different regulatory mechanisms.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Zonal distribution of peroxisomal 3-oxoacyl-CoA thiolase mRNA in liver from rats treated with di-(2-ethylhexyl)phthalate

Treatment of rats with di-(2-ethylhexyl)phthalate leads to a dramatic increase in peroxisomal 3-oxoacyl-CoA thiolase RNA, the concentration being higher in the pericentral than in periportal hepatocytes. These findings indicate that the production of peroxisomal thiolase and the zonal distribution of the enzyme are regulated at a pretranslational level.