[The coiled bodies and satellite microbodies of the oocyte nuclei in hibernating Rana temporaria frogs contain actin]

Immunocytochemical analysis of preparation of dispersed nuclei content in oocytes of III-IV stages of oogenesis, in terms of Dumont (1972), from hibernating grass frogs using monoclonal antibodies against actin, revealed two types of intranuclear structures containing this protein: coiled bodies (CB) and satellite microbodies (SM). Staining of these preparations with Rhodamin-phalloidin, known specifically to interact with fibrillar actin, did not reveal it in these structures. Results of our biochemical studies, using protease ESP32 specifically cutting only globular actin, are suggesting that both CB and SM contain globular actin. Gall et al. (1975) proposed that CB may be involved in assembling and sorting of small nuclear RNA for the three main RNA processing pathways: pre-mRNA splicing, pre-rRNA processing, and histone pre-mRNA 3'-end formation. Our finding of actin in CB allows a suggestion on actin involvement in the transport of RNA processing complexes from CB to some actual places where processing of RNA takes place. According to our previous data (Tsvetkov, Parfenov., 1994), SM participate in the karyosphere capsule formation. This process is preceded by SM fusion triggered presumably by actin.

Effect of dexamethasone on ciprofibrate-induced cell proliferation and peroxisome proliferation

Peroxisome proliferators cause liver cell proliferation in addition to other pleiotropic effects such as peroxisome proliferation and induction of certain peroxisomal and cytosolic enzymes in liver. Since dexamethasone has been shown to inhibit mitogen-induced liver cell hyperplasia, we examined whether dexamethasone inhibits only cell proliferation without affecting peroxisome proliferation induced by peroxisome proliferators such as ciprofibrate. Livers of rats fed a diet containing ciprofibrate (0.025%) with or without added dexamethasone (0.5 mg or 1 mg/kg diet) for 1 week were evaluated for hepatocyte proliferation and peroxisome proliferation. Dexamethasone administration resulted in abrogation of ciprofibrate-induced cell proliferation as shown by bromodeoxyuridine (BrdU) labeling and mitoses counts. The hepatocyte proliferative index measured after administration of a single dose of BrdU was 18.3 +/- 1.1 and 2.3 +/- 0.7% (p < 0.01) in ciprofibrate and ciprofibrate + dexamethasone treated rats, respectively. With multiple injections of BrdU (daily injections for 7 days) the proliferative index was 225 +/- 10 and 183 +/- 2% (p < 0.02), respectively, in these two groups. Interestingly, whereas the levels of peroxisome proliferator-induced Mr 80,000 polypeptide and catalase and peroxisomal bifunctional enzyme, and the corresponding mRNAs and peroxisome volume density were unaffected. These results show that dexamethasone selectively inhibits only cell proliferation without inhibiting the peroxisome proliferation caused by ciprofibrate. This model should be useful for examining the role of cell proliferation versus oxidative stress in peroxisome proliferator-induced hepatocarcinogenesis.

A complex containing two transcription factors regulates peroxisome proliferation and the coordinate induction of beta-oxidation enzymes in Saccharomyces cerevisiae

Expression of the POX1 gene, which encodes peroxisomal acyl coenzyme A oxidase in the yeast Saccharomyces cerevisiae, is tightly regulated and can be induced by fatty acids such as oleate. Previously we have shown that this regulation is brought about by interactions between trans-acting factor(s) and an upstream activating sequence (UAS1) in the POX1 promoter. We recently identified and isolated a transcription factor, Oaf1p, that binds to the UAS1 of POX1 and mediates its induction. A screening strategy has been developed and used to identify eight S. cerevisiae mutants, from three complementation groups, that are defective in the oleate induction of POX1. Characterization of one such mutant led to the identification of Oaf2p, a protein that is 39% identical to Oaf1p. Oaf1p and Oaf2p form a protein complex that is required for the activation of POX1 and FOX3 and for proliferation of peroxisomes. We propose a model in which these two transcription factors heterodimerize and mediate this activation process. The mutants that we have isolated, and further identification of the corresponding defective genes, provide us with an opportunity to characterize the mechanisms involved in the coordinate regulation of peroxisomal beta-oxidation enzymes.

Proliferation of myocardial peroxisomes caused by several agents and conditions

In view of considerable gaps in our knowledge of myocardial peroxisomes. the aim of the present study is, on the basis of extensive electron-microscopic investigations, to provide reliable results on the inducibility of a proliferation (increase in the number) of these organelles in rodents' heart by several agents and conditions. As far as possible, we compared the response of heart and liver peroxisomes. Morphometric investigations were performed to assess the effectiveness of the hypolipidemic agent HL 41, erucic acid, ethanol, nifedipine, chlorpromazine, two cardiotonic drugs, isoprenaline, adriamycin, and physical exercise. The study also included spontaneously hypertensive rats (SHR). A further objective was to determine synergistic or additive effects that might occur when two or three peroxisome-proliferating stimuli act simultaneously. In every case we observed a clear peroxisome proliferation, which was found to increase by between 10 and 97% under the influence of an additional inducer. The observed increase in peroxisome number ranged from almost 200% to nearly 400%. Our results suggest that very different agents and conditions can induce myocardial peroxisome proliferation when they lead to metabolic alterations associated with an increased need for a peroxisomal beta-oxidation of fatty acids as an energy source and/or for preventing toxic effects. Regulatory mechanisms of these adaptive processes are apparently also present in the heart via peroxisome proliferator-activated receptors (PPARs) and their activation by fatty acids, which can also stimulate the PPARs gene expression. The assumption that stimulated catalase gene expression might be responsible for the induction of peroxisome proliferation as a cellular response to an extraperoxisomal oxidative stress situation (isoprenaline, adriamycin, or physical exercise) poses some critical questions. These questions pertain especially to: (a) quantitative aspects with regard to the possible effectiveness of an increase in catalase activity by two-, three-, or four-fold enhanced peroxisome numbers; (b) the role of cytoplasmic catalase; (c) the existence and importance of a myocardial mitochondrial catalase; and (d) the co-operation between the two H2O2-destroying enzymes catalase and glutathione peroxidase.

Peroxisome biogenesis

Peroxisomes are eukaryotic organelles that are the subcellular location of important metabolic reactions. In humans, defects in the organelle's function are often lethal. Yet, relative to other organelles, little is known about how cells maintain and propagate peroxisomes or how they direct specific sets of newly synthesized proteins to these organelles (peroxisome biogenesis/assembly). In recent years, substantial progress has been made in elucidating aspects of peroxisome biogenesis and in identifying PEX genes whose products, peroxins, are essential for one or more of these processes. The most progress has been made in understanding the mechanism by which peroxisome matrix proteins are imported into the organelles. Signal sequences responsible for targeting proteins to the organelle have been defined. Potential signal receptor proteins, a receptor docking protein and other components of the import machinery have been identified, along with insights into how they operate. These studies indicate that multiple peroxisomal protein-import mechanisms exist and that these mechanisms are novel, not simply variations of those described for other organelles.

Castor bean isocitrate lyase lacking the putative peroxisomal targeting signal 1 ARM is imported into plant peroxisomes both in vitro and in vivo

To understand and manipulate plant peroxisomal protein targeting, it is important to establish the universality or otherwise of targeting signals. Contradictory results have been published concerning the nature and location of the glyoxysomal/peroxisomal targeting signal of isocitrate lyase (ICL). L.J. Olsen, W.F. Ettinger, B. Damsz, K. Matsudaira, A. Webb, and J.J. Harada ([1993] Plant Cell 5: 941-952) concluded that the last 5 amino acids (AKSRM) of Brassica napus ICL were sufficient and the last 37 amino acids were necessary for targeting to Arabidopsis leaf peroxisomes. In contrast, R. Behari and A. Baker ([1993]) J Biol Chem 268: 7315-7322) could find no requirement for the almost identical carboxy-terminal sequence AKARM for import of Ricinus communis ICL into isolated sunflower cotyledon glyoxysomes. To resolve this discrepancy, the import characteristics of a mutant R. communis ICL lacking the last 19 amino acids of the carboxy terminus was studied. ICL delta 19 was able to be imported by isolated sunflower glyoxysomes and by tobacco leaf peroxisomes when expressed transgenically. These results demonstrate that the in vitro import system faithfully reflects targeting in vivo, and that the source of the organelles (Arabidopsis versus sunflower, leaf peroxisomes versus seed glyoxysomes) is not responsible for observed differences between B. napus and R. communis ICL. The R. communis enzyme would therefore appear to possess an additional glyoxysome/peroxisome targeting signal that is lacking in the B. napus protein.

Isolated dihydroxyacetonephosphate-acyl-transferase deficiency in rhizomelic chondrodysplasia punctata: clinical presentation, metabolic and histological findings

Rhizomelic chondrodysplasia punctata (RCDP) is clinically characterized by symmetrical shortening of the proximal limbs, contractures of joints, a characteristic dysmorphic face, and cataracts. In the classical form an impairment of several peroxisomal functions and enzymes (plasmalogen synthesis, phytanic acid oxidation, 3-oxoacyl-CoA thiolase) has been repeatedly shown. Recently a variant involving only the peroxisomal dihydroxyacetonephosphate acyltransferase (DHAP-AT) has been described. We present a patient with isolated DHAP-AT deficiency and all clinical, radiological and pathological features of classical RCDP. For the first time, microscopy and immunocytochemistry of hepatocytes could be performed.

CONCLUSION

In contrast to studies on classical rhizomelic chondrodysplasia punctata which have shown enlarged peroxisomes in numbers varying from hepatocyte to hepatocyte, the peroxisomes in our patient seem to be normal in size, number and shape.

Induction of beta-oxidation enzymes and microbody proliferation in Aspergillus nidulans

Aspergillus nidulans is able to grow on oleic acid as sole carbon source. Characterization of the oleate-induced beta-oxidation pathway showed the presence of the two enzyme activities involved in the first step of this catabolic system: acyl-CoA oxidase and acyl-CoA dehydrogenase. After isopicnic centrifugation in a linear sucrose gradient, microbodies (peroxisomes) housing the beta-oxidation enzymes, isocitrate lyase and catalase were clearly resolved from the mitochondrial fraction, which contained fumarase. Growth on oleic acid was associated with the development of many microbodies that were scattered throughout the cytoplasm of the cells. These microbodies (peroxisomes) were round to elongated, made up 6% of the cytoplasmic volume, and were characterized by the presence of catalase. The beta-oxidation pathway was also induced in acetate-grown cells, although at lower levels; these cells lacked acyl-CoA oxidase activity. Nevertheless, growth on acetate did not cause a massive proliferation of microbodies in A. nidulans.

Dietary docosahexaenoic acid has little effect on peroxisomes in healthy mice

NMRI mice were fed diets supplemented with 0.05, 0.2, or 2% (w/w) docosahexaenoic acid (DHA), a polyunsaturated fatty acid present in fish oil, for 3 d, 3 wk, or 3 mon. The doses of DHA were chosen to supply the mice with concentrations of DHA which approximate those that have been reported to be beneficial to patients with peroxisomal disease. Diets containing 0.05 or 0.2% DHA did not change hepatic, myocardial, and renal catalase (EC 1.11.1.6) activity except for a slight but significant increase (to 120%) in myocardial catalase activity in mice treated with the 0.05% DHA diet for 3 mon. A diet with 2% DHA induced myocardial catalase activity to 150% after both 3 d and 3 wk of administration. In the liver of mice fed this diet for 3 wk, hepatic catalase activity was increased to 140% while no induction of palmitoyl-CoA oxidase (EC 1.3.99.3), urate oxidase (EC 1.7.3.3), and L-alpha-hydroxyisovalerate oxidase (EC 1.1.3.a) was observed. With the light microscope, no changes in peroxisomal morphology were visually evaluated in catalase stained sections of liver, myocardium, and kidney of mice fed either diet. Our results show that in healthy mice a low dietary DHA dose (< 0.2%; this corresponds to a dose prescribed to peroxisomal patients) has no effect on several hepatic peroxisomal H2O2-producing enzymes, including the rate-limiting enzyme of the peroxisomal fatty acid beta-oxidation. This may indicate that such a DHA dose will not add a strong load on the often disturbed fatty acid metabolism in the liver of patients with peroxisomal disorders.

Changes in cell morphology and carnitine acetyltransferase activity in Candida albicans following growth on lipids and serum and after in vivo incubation in mice

Candida albicans C316, maintained in the yeast form, showed a proliferation of peroxisomes when grown on triolein or serum as sole carbon source but these structures were absent from glucose-grown cells. Peroxisomes were also apparent in C. albicans obtained after injection into mice and recovery from intraperitoneal washings and kidneys; they may therefore be useful markers to assess a potential in vivo response in cells that are growing in vitro. Transcell-wall structures also occurred in C. albicans grown on triolein or serum, and in cells cultured in vivo, but were not seen in cells grown on glucose. These structures consisted of electron-dense fibrillar material penetrating through the cell wall from the plasmalemma side and protruded out to the exterior of the cell. Endoplasmic reticulum, located at the periphery of the cell, was found to be in close proximity with these cell wall structures. Carnitine acetyltransferase (CAT; EC 2.3.1.7), the key enzyme for the translocation of acetyl units between intracellular compartments, was present in low activities in glucose-grown cells; its activity was increased some 100-fold in triolein-grown cells but only 4-fold in serum-grown cells. It was not possible to assess this activity in the in vivo-cultured cells. Two separate CAT proteins, partially purifed from isolated microchondria and peroxisomes, respectively, were identified, with different specificities and kinetic properties.

Tissue-specific distribution of a peroxisomal 46-kDa protein related to the 58-kDa protein (sterol carrier protein x; sterol carrier protein 2/3-oxoacyl-CoA thiolase)

The complete sequence of the nonspecific lipid-transfer protein (nsL-TP; sterol carrier protein 2) including the presequence is present at the C-terminus (residues 405-547) of a 58-kDa protein. To be able to study this 58-kDa protein without the interference of nsL-TP, antibodies were raised against predicted epitope regions in the N-terminal part (peptide I, residues 23-43; peptide II, residues 130-149). Using these antibodies, rat tissues were analyzed by immunoblotting. In rat liver, in addition to the 58-kDa protein the antibody against peptide I (alpha-58K23) as well as the antibody against peptide II (alpha-58K130) detected a 46-kDa protein. This suggests that both peptide sequences are present in this 46-kDa protein. Both the 46- and the 58-kDa-proteins were abundantly present in liver and adrenals, but could also be detected in brain, kidney, heart, lung, testes, and ovary. This distribution was observed in tissues from both male and female rats. Immunogold labeling of cryosections of liver showed that alpha-58K23 labels the peroxisomes. From double-labeling experiments using alpha-nsL-TP and alpha-58K23 we conclude that the 46-kDa protein is peroxisomal. We propose that in the peroxisomes the protease that processes pre-nsL-TP also cleaves the 58-kDa protein giving rise to the 46-kDa protein and nsL-TP. In addition to the 58- and 46-kDa proteins, an immunoreactive 44-kDa protein was prominently present in rat heart and at low levels also in small intestine and brain. Immunogold labeling of cryosections of heart and Western blotting of purified mitochondria showed that the 44-kDa protein is localized in the mitochondria. The 44-kDa protein was shown to be identical to mitochondrial sarcomeric creatine kinase, which has a peptide segment of five amino acid residues in common with peptide I.

Hepatocellular and hepatic peroxisomal alterations in mice with a disrupted peroxisomal fatty acyl-coenzyme A oxidase gene

Peroxisomal genetic disorders, such as Zellweger syndrome, are characterized by defects in one or more enzymes involved in the peroxisomal beta-oxidation of very long chain fatty acids and are associated with defective peroxisomal biogenesis. The biologic role of peroxisomal beta-oxidation system, which consists of three enzymes: fatty acyl-CoA oxidase (ACOX), enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (HD), and thiolase, has been examined in mice by disrupting ACOX gene, which encodes the first and rate-limiting enzyme of this system. Homozygous (ACOX -/-) mice lacked the expression of ACOX protein and accumulate very long chain fatty acids in blood. However, these homozygous mice are viable, but growth-retarded and infertile. During the first 3-4 months of age, the livers of ACOX -/- mice reveal severe microvesicular fatty metamorphosis of hepatocytes. In such steatotic cells, peroxisome assembly is markedly defective; as a result, they contain few or no peroxisomes. Few hepatocytes in 1-3-month-old ACOX -/- mice contain numerous peroxisomes, and these peroxisome-rich hepatocytes show no fatty change. At this stage, the basal mRNA levels of HD, thiolase, and other peroxisome proliferator-induced target genes were elevated in ACOX -/- mouse liver, but these mice, when treated with a peroxisome proliferator, showed no increases in the number of hepatic peroxisomes and in the mRNAs levels of these target genes. Between 4 and 5 months of age, severe steatosis resulted in scattered cell death, steatohepatitis, formation of lipogranulomas, and focal hepatocellular regeneration. In 6-7-month-old animals, the newly emerging hepatocytes, which progressively replaced steatotic cells, revealed spontaneous peroxisome proliferation. These livers showed marked increases in the mRNA levels of the remaining two genes of the beta-oxidation system, suggesting that ACOX gene disruption leads to increased endogenous ligand-mediated transcription levels. These observations demonstrate links among peroxisomal beta-oxidation, development of severe microvesicular fatty liver, peroxisome assembly, cell death, and cell proliferation in liver.

The Yarrowia lipolytica gene PAY5 encodes a peroxisomal integral membrane protein homologous to the mammalian peroxisome assembly factor PAF-1

Pay mutants of the yeast Yarrowia lipolytica fail to assemble functional peroxisomes. One mutant strain, pay5-1, lacks normal peroxisomes and instead contains irregular vesicular structures surrounded by multiple unit membranes. The pay5-1 mutant is not totally deficient in peroxisomal matrix protein targeting, as a subset of matrix proteins continues to localize to a subcellular fraction enriched for peroxisomes. The functionally complementing gene PAY5 encodes a protein, Pay5p, of 380 amino acids (41,720 Da). Pay5p is a peroxisomal integral membrane protein homologous to mammalian PAF-1 proteins, which are essential for peroxisome assembly and whose mutation in humans results in Zellweger syndrome. Pay5p is targeted to mammalian peroxisomes, demonstrating the evolutionary conservation of the targeting mechanism for peroxisomal membrane proteins. Our results suggest that in pay5 mutants, normal peroxisome assembly is blocked, which leads to the accumulation of the membranous vesicular structures observed.

Mutations in the PAY5 gene of the yeast Yarrowia lipolytica cause the accumulation of multiple subpopulations of peroxisomes

We previously reported the cloning of the PAY5 gene of the yeast Yarrowia lipolytica by complementation of the peroxisome assembly mutant pay5-1 (Eitzen, G. A., Titorenko, V. I., Smith, J. J., Veenhuis, M., Szilard, R. K., and Rachubinski, R. A. (1996) J. Biol. Chem. 271, 20300-20306). The peroxisomal integral membrane protein Pay5p is a homologue of mammalian PAF-1 proteins, which are essential for peroxisome assembly and whose mutation in humans results in peroxisome biogenesis disorders. Mutations in the PAY5 gene result in the accumulation of three distinct peroxisomal subpopulations. These subpopulations are characterized by differences in 1) buoyant density, 2) the relative distribution of peroxisomal matrix and membrane proteins, 3) the efficiency of import of several peroxisomal matrix proteins, and 4) the phospholipid levels of peroxisomal membranes. These data, together with the analysis of temporal changes in the relative abundance of individual peroxisomal subpopulations in pay5 mutants, suggest that these subpopulations represent intermediates in a multistep peroxisome assembly pathway normally operating in yeast cells.