Ultrastructural observation of changes in (hepatic) peroxisomes by 3 to 90 days administration of bezafibrate in male rats

Although many reports are available to indicate the increase in number and size of peroxisomes by oral administration of hypolipidemic reagents such as clofibrate and bezafibrate, the decrease in number and size of proliferated peroxisomes even during administration of these chemicals has not been well documented. The present study was aimed to detect when the decrease in number and size of peroxisomes would occur. In the present study, ultrastructural changes in hepatocytes of male Wistar rats during 3 to 90 days of oral administration of bezafibrate were analyzed by morphometric and cytochemical methods. By 3-day administration, the weight of the liver and the number and size of peroxisomes were significantly increased. These parameters began to decline from the time 14 days of administration when the weight of the liver and the size of peroxisomes attained the peak. Heterogeneous catalase staining was most remarkable by 7-day administration. Mitochondria were another target cell organelle by bezafibrate. The total area of mitochondria was decreased by 3-day administration. However, by subsequent 7 to 90-day administration, it recovered almost the level of controls. By 90-day administration, many mitochondria had expanded cristae which contained filamentous substances.

The peroxisome proliferator activated receptors (PPARS) and their effects on lipid metabolism and adipocyte differentiation

The three types of peroxisome proliferator activated receptor (PPAR), alpha, beta (or delta), and gamma, each with a specific tissue distribution, compose a subfamily of the nuclear hormone receptor gene family. Although peroxisome proliferators, including fibrates and fatty acids, activate the transcriptional activity of these receptors, only prostaglandin J2 derivatives have been identified as natural ligands of the PPAR gamma subtype, which also binds thiazolidinedione antidiabetic agents with high affinity. Activated PPARs heterodimerize with RXR and alter the transcription of target genes after binding to specific response elements or PPREs, consisting of a direct repeat of the nuclear receptor hexameric DNA core recognition motif spaced by one nucleotide. The different PPARs can be considered key messengers responsible for the translation of nutritional, pharmacological and metabolic stimuli into changes in the expression of genes, more specifically those genes involved in lipid metabolism. PPAR alpha is involved in stimulating beta-oxidation of fatty acids. In rodents, a PPAR alpha-mediated change in the expression of genes involved in fatty acid metabolism lies at the basis of the phenomenon of peroxisome proliferation, a pleiotropic cellular response, mainly limited to liver and kidney and which can lead to hepatocarcinogenesis. In addition to their role in peroxisome proliferation in rodents, PPAR is also involved in the control of HDL cholesterol levels by fibrates and fatty acids in rodents and humans. This effect is, at least partially, based on a PPAR-mediated transcriptional regulation of the major HDL apolipoproteins, apo A-I and apo A-II. The hypotriglyceridemic action of fibrates and fatty acids also involves PPARs and can be summarized as follows: (1) an increased lipolysis and clearance of remnant particles, due to changes in LPL and apo C-III levels, (2) a stimulation of cellular fatty acid uptake and their conversion to acyl-CoA derivatives by the induction of FAT, FATP and ACS activity, (3) an induction of fatty acid beta-oxidation pathways, (4) a reduction in fatty acid and triglyceride synthesis, and finally (5) a decrease in VLDL production. Hence, both enhanced catabolism of triglyceride-rich particles as well as reduced secretion of VLDL particles are mechanisms that contribute to the hypolipidemic effect of fibrates and FFAs. Whereas for PPAR beta no function so far has been identified, PPAR gamma triggers adipocyte differentiation by inducing the expression of several genes critical for adipogenesis.

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.

Comparative hypolipidaemic and peroxisomal effects of ciprofibrate, clofibric acid, and their respective difluorocyclopropyl and 4-fluoro-substituted analogues in rat

1. We investigated the biological activity of the difluoro analogue (WIN 36117) of ciprofibrate, a potent peroxisome proliferator, and re-examined the relative activity of clofibric acid and its 4-fluoro analogue (fluorofibric acid) in the rat. 2. Twenty-four hours after a single dose, ciprofibrate and WIN 36117 produced dosage-related reductions in plasma cholesterol (16-42 and 9-34% respectively) and triglycerides (14-32 and 9-22% respectively). However, a single dose of clofibric acid or fluorofibric acid produced hypocholesterolaemia only (32-58 and 9-29% reductions respectively). 3. After treatment for 7 days reductions in cholesterol were similar at all dosages of ciprofibrate (45% reduction, mean across groups) whereas the effects of WIN 36117, clofibric acid and fluorofibric acid were still dosage related (reductions of 21-44, 37-43 and 2-28% respectively). Hypotriglyceridaemia was produced by all compounds (ciprofibrate 36-50%, WIN 36117 14-36%, clofibric acid 18-48%, fluorofibric acid 6-28%). 4. After treatment for 14 days all compounds produced dosage-related decreases in plasma fibrinogen (ciprofibrate 18-33%, WIN 36117 7-11%, clofibric acid 13-26%, fluorofibric acid 7-15%). 5. Peroxisomal beta-oxidation activity was increased by WIN 36117 (4.8-fold) and fluorofibric acid (4.2-fold) although these increases were less than those produced by ciprofibrate (13.6-fold) and clofibric acid (7.0-fold). WIN 36117 and fluorofibric acid also produced smaller increases in peroxisome numbers, liver weight, and carnitine acetyl transferase activity and smaller decreases in glutathione S-transferase and glutathione peroxidase activities. 6. Maximal increases in peroxisomal beta-oxidation activity produced in cultured rat hepatocytes by WIN 36117 and fluorofibric acid were 58 and 72% of those produced by ciprofibrate and clofibric acid respectively. 7. These results indicate the difluoro and 4-fluoro analogues of ciprofibrate and clofibric acid are hypolipidaemic agents and peroxisome proliferators but with reduced potencies relative to the parent molecules.

Labeling of peroxisomes with green fluorescent protein in living P. pastoris cells

We exploited the light-activated fluorescent properties of the green fluorescent protein (GFP) of the jellyfish Aequorea victoria for studies on the peroxisomal sorting of polypeptides. GFP and GFP-SKL (containing a C-terminal, tripeptide peroxisomal targeting signal, SKL) were expressed from a methanol-inducible, alcohol oxidase (AOX1) promoter in the methylotrophic yeast Pichia pastoris. GFP was cytosolic, whereas the GFP-SKL fusion protein was targeted to peroxisomes, as demonstrated by biochemical fractionation of organelles on Nycodenz gradients. Neither GFP nor GFP-SKL affected the viability of yeast cells but both were fluorescent on excitation with 395-nm UV light. The subcellular locations of GFP and GFP-SKL in living yeast cells were monitored by fluorescence microscopy and their fluorescence was coupled to photo-oxidation of diaminobenzidine (DAB), resulting in the deposition of electron-dense oxidized DAB at intracellular locations of GFP derivatives. This photooxidation procedure permitted facile ultrastructural localization of GFP in cells by electron microscopy, and provided further evidence that GFP produced in P. pastoris is cytosolic, whereas GFP-SKL is peroxisomal. The GFP-SKL fusion protein is therefore a versatile reporter for the peroxisomal compartment, with many applications for studies involving peroxisomal import and biogenesis.

The Pichia pastoris PER6 gene product is a peroxisomal integral membrane protein essential for peroxisome biogenesis and has sequence similarity to the Zellweger syndrome protein PAF-1

We report the cloning of PER6, a gene essential for peroxisome biogenesis in the methylotrophic yeast Pichia pastoris. The PER6 sequence predicts that its product Per6p is a 52-kDa polypeptide with the cysteine-rich C3HC4 motif. Per6p has significant overall sequence similarity with the human peroxisome assembly factor PAF-1, a protein that is defective in certain patients suffering from the peroxisomal disorder Zellweger syndrome, and with car1, a protein required for peroxisome biogenesis and caryogamy in the filamentous fungus Podospora anserina. In addition, the C3HC4 motif and two of the three membrane-spanning segments predicted for Per6p align with the C3HC4 motifs and the two membrane-spanning segments predicted for PAF-1 and car1. Like PAF-1, Per6p is a peroxisomal integral membrane protein. In methanol- or oleic acid-induced cells of per6 mutants, morphologically recognizable peroxisomes are absent. Instead, peroxisomal remnants are observed. In addition, peroxisomal matrix proteins are synthesized but located in the cytosol. The similarities between Per6p and PAF-1 in amino acid sequence and biochemical properties, and between mutants defective in their respective genes, suggest that Per6p is the putative yeast homolog of PAF-1.

Microtubule-based peroxisome movement

The association of peroxisomes with cytoskeletal structures was investigated both by electron microscopy and by kinetic analysis of peroxisome movement. The morphological studies indicated distinct interactions of peroxisomes with microtubules and frequently revealed multiple contact sites. The kinetic approach utilised microinjection and import of fluorescein-labeled luciferase in order to mark and track peroxisomes in vivo. Peroxisomal motility was analysed by time-lapse imaging and fluorescence microscopy. According to their movement peroxisomes were classified into two groups. Group 1 peroxisomes comprising the majority of organelles at 37 degrees C moved slowly with an average velocity of 0.024 +/- 0.012 micron/second whereas the movement of group 2 peroxisomes, 10–15% of the total population, was saltatory exhibiting an average velocity of 0.26 +/- 0.17 micron/second with maximal values of more than 2 microns/second. Saltations were completely abolished by the microtubule-depolymerising drug nocodazole and were slightly reduced by about 25% by cytochalasin D which disrupts the actin microfilament system. Double fluorescence labeling of both peroxisomes and microtubules revealed peroxisome saltations linked to distinct microtubule tracks. Cellular depletion of endogenous levels of NTPs as well as the use of 5′-adenylylimidodiphosphate, a nonhydrolysable ATP analog, applied to a permeabilised cell preparation both completely blocked peroxisomal movement. These data suggest an ATPase dependent, microtubule-based mechanism of peroxisome movement. Both the intact and the permeabilised cell system presented in this paper for the first time allow kinetic measurements on peroxisomal motility and thus will be extremely helpful in the biochemical characterisation of the motor proteins involved.

Peroxisomal remnants in peroxisome-deficient mutants of the yeast Hansenula polymorpha [corrected]

We have analyzed the presence of peroxisomal remnants ('ghosts') in three peroxisome-deficient (per) mutants of the yeast Hansenula polymorpha, namely delta per4, delta per5 and delta per10. Under peroxisome-inducing growth conditions peroxisomal membrane proteins (PMPs) were normally synthesized in cells of these mutants. In addition, these cells contained clusters of small membraneous vesicles, which were absent in cells grown under peroxisome-repressing growth conditions. These structures displayed typical peroxisomal properties in that they proliferated upon overproduction of Per8p, the H. polymorpha peroxisome proliferation factor. Moreover, in delta per4 and delta per5 these vesicles were susceptible to glucose-induced proteolytic degradation.

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.

Peroxisomes and peroxisomal enzymes in the human fetal small intestine

The appearance and development of peroxisomes and the expression of their enzymes in the human fetal intestine have been investigated between 11 and 22 weeks of gestation. In the youngest samples (11-16 weeks of age), cytochemistry at the ultrastructural level revealed the presence of rare, mostly circular peroxisomes. From 16 weeks of gestation onwards, an increase was noted in the number of peroxisomes. Two peroxisomal types were distinguished: round to oval forms and elongated and/or tailed organelles. Biochemical assays revealed that total and specific intestinal catalase activities increased gradually between 11 and 20 weeks of gestation. The activity of fatty acylCoA oxidase, the first enzyme of the peroxisomal beta-oxidation system, was detectable as early as 11 weeks of gestation. Thereafter, total and specific activities of the enzyme increased steadily. Activities of other peroxisomal oxidases (D-amino acid oxidase, L-alpha-hydroxyacid oxidase) appeared more slowly in the fetal intestine during the period studied. This investigation establishes the presence and the morphological changes that occur in intestinal peroxisomes during human fetal development as well as the developmental patterns of associated enzymes.

Interaction of microtubules with peroxisomes. Tubular and spherical peroxisomes in HepG2 cells and their alterations induced by microtubule-active drugs

We have studied the interaction of microtubules with peroxisomes and the influence of changes in the microtubular network on the peroxisomal compartment. From the several cell lines analyzed for this purpose, HepG2 cells proved to be the best candidate exhibiting both a well-developed cytoskeleton and a peroxisomal compartment with great plasticity. Three distinct types of peroxisomes: small spherical (0.1-0.3 micron), rod-shaped (0.5 micron) and elongated tubular (up to 5 microns) ones were identified in this cell line. A shift of the elongated tubular forms to spherical particles was noted by increasing the density of cells in culture, whereas no correlation between the distinct peroxisomal forms and the cellular proliferation could be observed. At time points when the elongated tubular peroxisomes were disappearing, many spherical peroxisomes arranged like 'chains of beads on a string' were observed, suggesting that the fission of elongated tubular forms may give rise to newly developing spherical peroxisomes. A clear association of spherical peroxisomes with microtubules was visualized by double immunofluorescence in combination with confocal laser scanning microscopy (CLSM). Treatment with a variety of microtubule-depolymerizing drugs (colcemid, nocodazole, vinblastine) induced a significant increase in the frequency of tubular peroxisomes and led to the formation of peroxisomal clusters. These effects were reversible since already 1 to 2 h after removal of the drugs from the culture medium, a uniform distribution of spherical peroxisomes was reestablished. Taxol, a microtubule-stabilizing drug, on the other hand exerted no significant effects on the peroxisomal compartment. The direct interaction of microtubules with peroxisomes in vitro was demonstrated using highly purified rat liver peroxisomes and taxol-stabilized microtubules from bovine or pig brain. The binding of peroxisomes to microtubules was visualized by video-enhanced contrast microscopy (VECM) and was abolished by pretreatment of peroxisomes with 100 mM KCl ('stripping'), proteinase K or trypsin. Incubation with cytosol restored the binding capacity of KCl-treated peroxisomes, but did not complement the protease treatment. The data presented provide for the first time evidence for a direct interaction of microtubules with the peroxisomal compartment indicating that this cytoskeletal system plays an important role in the morphogenesis and intracellular distribution of peroxisomes.

Heterologous complementation of peroxisome function in yeast: the Saccharomyces cerevisiae PAS3 gene restores peroxisome biogenesis in a Hansenula polymorpha per9 disruption mutant

PER genes are essential for the biogenesis of peroxisomes in the yeast Hansenula polymorpha. Here we describe the functional complementation of a H. polymorpha per9 disruption strain (delta per9) by a heterologous gene. The Saccharomyces cerevisiae Pas3p, a homologue of per9p, restored peroxisome biogenesis and peroxisomal protein import in the delta per9 mutant, allowing it to grow again on methanol as sole carbon and energy source. This result shows that heterologous complementation of peroxisome function in yeast is indeed feasible and furthermore suggests that H. polymorpha delta per9 may be the candidate of choice to attempt the isolation of Per9p homologues from higher eukaryotes by functional complementation.

Localization of peroxisomal 3-oxoacyl-CoA thiolase in particles of varied density in rat liver: implications for peroxisome biogenesis

In this paper we report on the subcellular localization of peroxisomal thiolase in rat liver using density-gradient centrifugation and immunoelectron microscopy. The results obtained show that peroxisomes display great biochemical heterogeneity and can not be regarded as one homogeneous population of particles. We conclude that rat liver contains at least three distinct populations of peroxisomes, which are present both in normal-fed rats as well in rats treated with a plasticizer, di-(2-ethylhexyl)phthalate, known to induce peroxisomes. The following types of peroxisomes could be discerned: (1) Low-density peroxisomal particles containing 69-kDa peroxisomal membrane protein (PMP), dihydroxyacetonephosphate acyltransferase (DHAPAT) and the precursor form of peroxisomal thiolase (44-kDa). (2) Intermediate-density peroxisomal particles containing 69-kDa peroxisomal membrane protein, dihydroxyacetonephosphate acyltransferase, both 41-kDa (mature) and 44-kDa (immature) peroxisomal thiolase, catalase and D-aminoacid oxidase. (3) High-density peroxisomes containing 69-kDa peroxisomal membrane protein, dihydroxyacetonephosphate acyltransferase, 41-kDa thiolase, catalase and D-aminoacid oxidase.

Bovine latent transforming growth factor beta 1-binding protein 2: molecular cloning, identification of tissue isoforms, and immunolocalization to elastin-associated microfibrils

Monoclonal antibodies to fibrillin 1 (MP340), a component of elastin-associated microfibrils, were used to screen cDNA libraries made from bovine nuchal ligament mRNA. One of the selected clones (cL9; 1.2 kb) hybridized on Northern (RNA) blotting with nuchal ligament mRNA to two abundant mRNAs of 9.0 and 7.5 kb, which were clearly distinct from fibrillin mRNA (10 kb). Further library screening and later reverse transcription PCR by the rapid amplification of cDNA ends (RACE) technique resulted in the isolation of additional overlapping cDNAs corresponding to about 6.7 kb of the mRNA. The encoded protein exhibited sequence similarity of around 80% with a recently identified human protein named latent transforming growth factor beta 1 (TGF-beta 1)-binding protein 2 (LTBP-2), indicating that the new protein was bovine LTBP-2. This was confirmed by the specific localization of bovine LTBP-2 cDNA probes to human chromosome 14q24.3, which is the locus of the human LTBP-2 gene. The domain structure of bovine LTBP-2 is very similar to that of the human LTBP-2, containing 20 examples of 6-cysteine epidermal growth factor-like repeats, 16 of which have the consensus sequence for calcium binding, together with 4 examples of 8-cysteine motifs characteristic of fibrillins and LTBP-1. A 4-cysteine sequence which is unique to bovine LTBP-2 and which has similarity to the 8-cysteine motifs was also present. Antibodies raised to two unique bovine LTBP-2 peptides specifically localized in tissue sections to the elastin-associated microfibrils, indicating that LTBP-2 is closely associated with these structures. Immunoblotting experiments identified putative LTBP-2 isoforms as a 260-kDa species released into the medium by cultured elastic tissue cells and as larger 290- and 310-kDa species in tissue extracts. A major proportion of tissue-derived LTBP-2 required treatment with 6 M guanidine for solubilization, indicating that the protein was strongly bound to the microfibrils. Most of the guanidine-solubilized LTBP-2 appeared to be monomeric, indicating that it was not involved in disulfide-bonded aggregation either with itself or with latent TGF-beta. Additional LTBP-2 was resistant to solubilization with 6 M guanidine but was readily extracted with a reductive saline solution. This treatment is relatively specific for solubilization of microfibrillar constituents including fibrillin 1 and microfibril-associated glycoprotein. Therefore, it can be inferred that some LTBP-2 is bound covalently to the microfibrils by reducible disulfide linkages. The evidence suggests that LTBP-2 has a direct role in elastic fiber structure and assembly which may be independent of its growth factor-binding properties. Thus, LTBP-2 appears to share functional characteristics with both LTBP-1 and fibrillins.

The Candida boidinii peroxisomal membrane protein Pmp30 has a role in peroxisomal proliferation and is functionally homologous to Pmp27 from Saccharomyces cerevisiae

The mechanism of peroxisome proliferation is poorly understood. Candida boidinii is a methylotrophic yeast that undergoes rapid and massive peroxisome proliferation and serves as a good model system for this process. Pmp30A and Pmp30B (formerly designated Pmp31 and Pmp32, respectively) are two closely related proteins in a polyploid strain of this yeast that are strongly induced by diverse peroxisome proliferators such as methanol, oleate, and D-alanine. The function of these proteins is not understood. To study this issue, we used a recently described haploid strain (S2) of C. boidinii that can be manipulated genetically. We now report that strain S2 contains a single PMP30 gene very similar in sequence (greater than 93% identity at the DNA level) to PMP30A and PMP30B. When PMP30 was disrupted, cell growth on methanol was greatly inhibited, and cells grown in both methanol and oleate had fewer, larger, and more spherical peroxisomes than wild-type cells. A similar phenotype was recently described for Saccharomyces cerevisiae cultured on oleate in which PMP27, which encodes a protein of related sequence that is important for peroxisome proliferation, was disrupted. To determine whether Pmp27 is a functional homolog of Pmp30, gentle complementation was performed. PMP30A was expressed in the PMP27 disruptant of S. cerevisiae, and PMP27 was expressed in the PMP30 disruptant of C. boidinii S2. Complementation, in terms of both cell growth and organelle size, shape, and number, was successful in both directions, although reversion to a wild-type phenotype was only partial for the PMP30 disruptant. We conclude that these proteins are functional homologs and that both Pmp30 and Pmp27 have a direct role in proliferation and organelle size rather than a role in a specific peroxisomal metabolic pathway of substrate utilization.

A C-terminal region of the Saccharomyces cerevisiae transcription factor ADR1 plays an important role in the regulation of peroxisome proliferation by fatty acids

The Saccharomyces cerevisiae transcriptional activator ADR1, which controls ADH2 gene expression, was shown to be involved in the regulation of peroxisome proliferation. To study the mode of action of ADR1, we compared strains carrying the adr1-1 mutation, high or low copy numbers of the ADR1 gene, the constitutive allele ADR1-5c, and 3'-deletions of ADR1. High ADR1 gene dosage increased the transcription of genes encoding peroxisomal proteins as compared to one copy of the ADR1 gene. Furthermore, overexpression of ADR1 under ethanol growth conditions induced the proliferation of peroxisomal structures. The organelles were observed to be localized in clusters, a typical feature of peroxisomes induced by oleic acid. In contrast, the ADR1-5c allele, which induces ADH2 expression to a level comparable to that of high ADR1 gene dosage was found to have only a small effect. An analysis of functional domains of the ADR1 protein revealed that the N-terminal 220 amino acids of ADR1 were sufficient for wild-type levels of transcription of the FOX2, FOX3, and PAS1 genes, but the entire ADR1 protein was required for complete induction of the CTA1 gene and for growth oleic acid medium. Our data suggest that a functional domain of the ADR1 protein localized between residues 643 and 1323 is required for the induction of peroxisomal structures and for the utilization of oleic acid.

Differential induction of peroxisomal enzymes by hypolipidaemics in human (HepG2) and rat (MH1C1) hepatoma cell lines

Human (HepG2) and rat (MH1C1) hepatoblastoma cells were incubated with different concentrations of the hypolipidaemics cetaben, clofibrate and thyroxine. The enzymatic activities of catalase, peroxisomal bifunctional enzyme, succinate dehydrogenase, and 3-oxoacyl-CoA thiolase were measured. In order to determine the point of regulation of the enzymatic activities Northern and Slot blot experiments with probes for peroxisomal bifunctional enzyme, catalase and fatty acyl CoA oxidase were performed on total RNA. Catalase activity was enhanced in HepG2 cells treated with 3 mmol/l clofibric acid to 135% of control and the mRNA value to 2.6 fold, whereas in cetaben treated cells the enhancement (up to 119% of control) was less pronounced. In MH1C1 cells catalase activity was not changed by any of the drugs. The activity of the peroxisomal bifunctional enzyme was not affected in HepG2 cells by clofibric acid and cetaben, whereas the mRNA level was elevated to 2.3 fold by 10 micromol/l cetaben. At high concentrations of cetaben all enzyme activities were decreased in both cell lines due to its high cytotoxicity. Our data show that, due to the differences in the genomic organisation, the regulation of the enzyme activities is different in human and rat, but the results from the human and rat hepatoblastoma cells correlate with the findings in whole man and rat, so that a human in vitro system is more suitable for pharmacological tests. These results suggest that the human hepatoma cell line HepG2 may be a useful model system for studies of the influence of hypolipidaemics on the peroxisomal enzyme system.

Formation of the peroxisome lumen is abolished by loss of Pichia pastoris Pas7p, a zinc-binding integral membrane protein of the peroxisome

We have cloned and sequenced PAS7, a gene required for peroxisome assembly in the yeast Pichia pastoris. The product of this gene, Pas7p, is a member of the C3HC4 superfamily of zinc-binding proteins. Point mutations that alter conserved residues of the C3HC4 motif abolish PAS7 activity and reduce zinc binding, suggesting that Pas7p binds zinc in vivo and that zinc binding is essential for PAS7 function. As with most pas mutants, pas7 cells exhibit a pronounced deficiency in import of peroxisomal matrix proteins that contain either the type 1 peroxisomal targeting signal (PTS1) or the type 2 PTS (PTS2). However, while other yeast and mammalian pas mutants accumulate ovoid, vesicular peroxisomal intermediates, loss of Pas7p leads to accumulation of membrane sheets and vesicles which lack a recognizable lumen. Thus, Pas7p appears to be essential for protein translocation into peroxisomes as well as formation of the lumen of the organelle. Consistent with these data, we find that Pas7p is an integral peroxisomal membrane protein which is entirely resistant to exogenous protease and thus appears to reside completely within the peroxisome. Our observations suggest that the function of Pas7p defines a previously unrecognized step in peroxisome assembly: formation of the peroxisome lumen. Furthermore, because the peroxisomal intermediates in the pas7 delta mutant proliferate in response to peroxisome-inducing environmental conditions, we conclude that Pas7p is not required for peroxisome proliferation.

Quantitative microscopy of hepatic changes induced by phenethyl isothiocyanate in Fischer-344 rats fed either a cereal-based diet or a purified diet

Hepatic changes induced by phenethyl isothiocyanate (PEITC) in the liver of rats were determined by quantitative microscopy. Groups of male Fischer-344 rats were fed either a standard, cereal-based diet (Wayne rodent meal) or a purified diet (AIN-76A) containing PEITC at concentrations of 0.75 and 6.0 mmol/kg for 13 wk. Severe hepatic lipidosis was observed in control rats fed the purified diet. Addition of PEITC to the purified diet significantly reduced lipid content in hepatocytes. In contrast, lipid content in the liver of the rats fed the cereal-based diet containing PEITC was greater than in control rats maintained on the same diet. In addition, dose-related reductions in hepatocyte, lipid droplet, peroxisome, and mitochondrial volumes were observed in PEITC-treated rats fed the cereal-based diet. These results indicate that PEITC exerts differential effects on the liver of rats fed either the cereal-based or purified diet.

Species-dependent induction of peroxisome proliferation by haloxyfop, an aryloxyphenoxy herbicide

The potential of haloxyfop [2-(4-((3-chloro-5-(trifluoromethyl)-2- pyridinyl)oxy)phenoxy)propanoic acid; HAL] to induce the proliferation of hepatocellular peroxisomes (PP) was examined in rats, mice, dogs, and monkeys. Chemically induced PP is associated with the development of liver tumors in rodents via an apparent species-dependent, nongenotoxic mechanism of action. HAL is nongenotoxic yet has been shown to cause liver tumors in female B6C3F1 mice. Ingestion of HAL by rats and/or mice (0.1-14 mg/kg/day for 2 to 4 weeks) resulted in significant dose-related PP as evidenced by hepatocellular hypertrophy, increased peroxisome volume density (VD), and induction of peroxisomal enzymes and CYP4A1. Only a relatively weak induction of PP was noted at a carcinogenic dosage in female mice. In contrast to rodent species, ingestion of up to 20 mg/kg/day HAL by male and female Beagle dogs for 13 weeks failed to increase peroxisomal VD while causing only a slight increase in peroxisomal enzyme activity at the highest dosages. Oral administration of up to 30 mg/kg/day HAL by male and female Cynomolgus monkeys for 13 weeks failed to induce PP. While a direct relationship of PP with tumor formation, at least in mice, was not demonstrated, these data still support the concept that PP represents a potential marker of nongenotoxic tumorigenic activity, at some dosage, in rodents.

In situ substrate specificity and ultrastructural localization of polyamine oxidase activity in unfixed rat tissues

Data concerning the substrate specificity and the exact intracellular localization of the polyamine-catabolizing enzyme polyamine oxidase are conflicting. Biochemical studies have shown that N1-acetylation of spermine and spermidine dramatically increases the specificity of these compounds for peroxisomal polyamine oxidase to produce spermidine and putrescine, respectively. On the other hand, polyamine oxidase activity was demonstrated histochemically both in peroxisomes and in cytoplasm of several tissues, using spermidine and/or spermine as substrate. To elucidate the in situ substrate specificity of polyamine oxidase and the localization of its activity, enzyme activity was detected in rat liver, kidney, and duodenum at the light and electron microscopic levels. For this purpose, unfixed cryostat sections were applied to avoid changes in enzyme activity owing to chemical fixation. Spermine, spermidine, their N1-acetylated forms, and putrescine were used as substrates, and cerium ions as capturing agent for H2O2. Control reactions were performed in the absence of substrate or in the presence of substrate and specific oxidase inhibitors. At the light microscopic level, final reaction product specifically generated by polyamine oxidase activity was found exclusively in a granular form in hepatocytes, epithelial cells of proximal tubules of the kidney, and epithelial cells of duodenal villi with N1-acetylspermidine or N1-acetylspermine as substrates. Final reaction product was not observed in any of the tissues after incubation in the presence of putrescine, spermidine, or spermine. Formation of specific final reaction product was prevented by incubation in the presence of a specific polyamine oxidase inhibitor, but it was not affected by a diamine oxidase inhibitor. Ultrastructural studies revealed that polyamine oxidase activity is localized exclusively to the matrix of peroxisomes of kidney and liver and to microperoxisomes of the duodenum. The localization patterns obtained with unfixed tissues are in agreement with biochemical data. Strong intraperoxisomal, interperoxisomal, and intercellular heterogeneity in polyamine oxidase activity was found in all tissues investigated.

Peroxisome proliferation and associated effects caused by perfluorooctanoic acid in vitamin A-deficient mice

Vitamin A-deficient male mice were treated for 10 days with 0.02% perfluorooctanoic acid (PFOA) in their diet. The effects of this highly potent peroxisome proliferator on peroxisomal palmitoyl-CoA oxidation and lauroyl-CoA oxidase activities were reduced by 3.1-7.5 times in comparison to the values obtained with mice receiving a vitamin A-adequate diet. The activity of peroxisomal catalase was virtually unaffected by vitamin A deficiency and treatment with PFOA had no significant effect on this activity in vitamin A-adequate or vitamin A-deficient mice. However, vitamin A deficiency itself caused a more than 800% increase in cytosolic catalase activity. Thus, the percentage increase caused by PFOA on cytosolic catalase activity was reduced 12.6 times in vitamin A-deficient mice compared to the increase in vitamin A-adequate mice, although the total absolute activities were similar. These findings suggest that the peroxisome proliferation caused by this peroxisome proliferator is highly dependent on the vitamin A status of the mouse.

Enrichment of Penicillium chrysogenum microbodies by isopycnic centrifugation in nycodenz as visualized with immuno-electron microscopy

A procedure to enrich microbodies from Penicillium chrysogenum and a method to evaluate the purity and integrity of the microbodies are described. As a P. chrysogenum microbody marker acyltransferase (AT) was used. The P. chrysogenum hyphae were converted into protoplasts with Novozym 234. In Percoll-sucrose buffer the protoplasts were separated from mycelial debris after 10,000 x g centrifugation. Purified protoplasts were lysed, and the cell homogenate was centrifuged to form a 14,000 x g pellet. After 2 h, 45,000 x g isopycnic centrifugation of the 14,000 x g pellet on a continuous 20-60% nycodenz gradient, ten fractions were collected. The fractions were analyzed for AT containing microbodies by immuno-blotting and immuno-electron microscopy. The results showed that AT-microbodies are enriched in the 38% nycodenz fraction. The microbodies had a diameter of 400 to 500 nm, revealed an intact single membrane and confined AT. The estimated equilibrium density of the P. chyrsogenum microbodies was 1.20 g ml-1 as deduced from the 38% (w/v) nycodenz concentration.

Peroxisomes and sterol carrier protein-2 in luteal cell steroidogenesis: a possible role in cholesterol transport from lipid droplets to mitochondria

In the present investigation, we have studied peroxisomes and sterol carrier protein-2 (SCP2) in control and luteinizing hormone stimulated rat luteal cells. Superovulated immature rats in mid-luteal phase (8 days after ovulation) were divided into two groups (n = 4/group) and treated with vehicle (0.2 ml saline), or luteinizing hormone (LH, 20 micrograms/rat). In this animal model, LH acutely stimulates steroidogenesis. Thirty minutes later, corpora lutea were fixed by whole body perfusion and processed for (1) electron microscopic immunocytochemistry to localize SCP2 via the protein A gold immunolabeling technique, and for (2) electron microscopic histochemistry to stain peroxisomal catalase via the alkaline 3,3'-diaminobenzidine tetrahydrochloride method. In the steroidogenic, mid-phase luteal cells of vehicle treated rats (controls), SCP2 was highly concentrated in peroxisomes and sparsely scattered on mitochondria, but no labeling was observed in lipid droplets. In the luteal cells of rats acutely stimulated with LH, peroxisomes immunolabeled for SCP2 were observed within the luteal cell lipid droplets and mitochondria, and in union with lipid droplets and mitochondria. Moreover, in contrast to control luteal cells, significant immunolabeling for SCP2 was detected within the lipid droplets and mitochondria in luteal cells of LH-treated rats. As SCP2 binds cholesterol to 1:1 molar ratio and is known to be involved in the intracellular movement of cholesterol, these findings suggest that peroxisomes and SCP2 may possibly be involved in delivering cholesterol from lipid droplets to the mitochondria when luteal cell steroidogenesis is acutely stimulated by LH.