The effect of clofibrate on the phospholipid composition of the peroxisomal membranes in mouse liver

Docosahexaenoic acid mediates peroxisomal elongation, a prerequisite for peroxisome division

Peroxisome division is regulated by several factors, termed fission factors, as well as the conditions of the cellular environment. Over the past decade, the idea of metabolic control of peroxisomal morphogenesis has been postulated, but remains largely undefined to date. In the current study, docosahexaenoic acid (DHA, C22:6n-3) was identified as an inducer of peroxisome division. In fibroblasts isolated from patients that carry defects in peroxisomal fatty acid β-oxidation, peroxisomes are much less abundant than normal cells. Treatment of these patient fibroblasts with DHA induced the proliferation of peroxisomes to the level seen in normal fibroblasts. DHA-induced peroxisomal proliferation was abrogated by treatment with a small inhibitory RNA (siRNA) targeting dynamin-like protein 1 and with dynasore, an inhibitor of dynamin-like protein 1, which suggested that DHA stimulates peroxisome division. DHA augmented the hyper-oligomerization of Pex11pβ and the formation of Pex11pβ-enriched regions on elongated peroxisomes. Time-lapse imaging analysis of peroxisomal morphogenesis revealed a sequence of steps involved in peroxisome division, including elongation in one direction followed by peroxisomal fission. DHA enhanced peroxisomal division in a microtubule-independent manner. These results suggest that DHA is a crucial signal for peroxisomal elongation, a prerequisite for subsequent fission and peroxisome division.

Peroxisomal and microsomal lipid pathways associated with resistance to hepatic steatosis and reduced pro-inflammatory state

Accumulation of fat in the liver increases the risk to develop fibrosis and cirrhosis and is associated with development of the metabolic syndrome. Here, to identify genes or gene pathways that may underlie the genetic susceptibility to fat accumulation in liver, we studied A/J and C57Bl/6 mice that are resistant and sensitive to diet-induced hepatosteatosis and obesity, respectively. We performed comparative transcriptomic and lipidomic analysis of the livers of both strains of mice fed a high fat diet for 2, 10, and 30 days. We found that resistance to steatosis in A/J mice was associated with the following: (i) a coordinated up-regulation of 10 genes controlling peroxisome biogenesis and β-oxidation; (ii) an increased expression of the elongase Elovl5 and desaturases Fads1 and Fads2. In agreement with these observations, peroxisomal β-oxidation was increased in livers of A/J mice, and lipidomic analysis showed increased concentrations of long chain fatty acid-containing triglycerides, arachidonic acid-containing lysophosphatidylcholine, and 2-arachidonylglycerol, a cannabinoid receptor agonist. We found that the anti-inflammatory CB2 receptor was the main hepatic cannabinoid receptor, which was highly expressed in Kupffer cells. We further found that A/J mice had a lower pro-inflammatory state as determined by lower plasma levels and IL-1β and granulocyte-CSF and reduced hepatic expression of their mRNAs, which were found only in Kupffer cells. This suggests that increased 2-arachidonylglycerol production may limit Kupffer cell activity. Collectively, our data suggest that genetic variations in the expression of peroxisomal β-oxidation genes and of genes controlling the production of an anti-inflammatory lipid may underlie the differential susceptibility to diet-induced hepatic steatosis and pro-inflammatory state.

Fatty acid alterations in liver peroxisomes from n-3-deficient mice

A diet deficient in n-3 fatty acids dramatically reduces docosahexaenoic acid (4.8-fold) and 20:5n-3 content in murine total peroxisomal phospholipids, and conversely increases 22:5n-6 (17.1-fold) and also, to a lesser extent, 20:4n-6. This was also found in purified phosphatidylethanolamine and phosphatidylcholine. After changing the non-deficient diet (containing alpha-linolenic acid, ALA) to a deficient one (deficient in ALA), it took a very long time for docosahexaenoic acid concentration in peroxisomes to decline (>5 months). In contrast, after changing the deficient to a non-deficient diet, time to complete recovery was more rapid (3 weeks). Changes in 20:5n-3, 22:6n-3 and 20:4n-6 were generally stabilized within 2-4 weeks. Dietary n-3 fatty acids control the fatty acid composition of peroxisomal membranes, and thus possibly affect some of their functions.

Phosphatidylethanolamine-N-methyltransferase activity and dietary choline regulate liver-plasma lipid flux and essential fatty acid metabolism in mice

Phosphatidylethanolamine-N-methyltransferase (PEMT) catalyzes the methylation of phosphatidylethanolamine to form phosphatidylcholine (PC) and represents one of the two major pathways for PC biosynthesis. Mice with a homozygous disruption of the PEMT gene are dependent on the 1,2-diacylglycerol cholinephosphotransferase (CDP-choline) pathway for the synthesis of PC and develop severe liver steatosis when fed a diet deficient in choline. The present study used quantitative lipid metabolite profiling to characterize lipid metabolism in PEMT-deficient mice fed diets containing varying concentrations of choline. Choline supplementation restored liver, but not plasma PC concentrations of PEMT-deficient mice to levels commensurate with control mice. Choline supplementation also restored plasma triglyceride concentrations to normal levels, but did not restore plasma cholesterol ester concentrations in the PEMT-deficient mice to those equal to control mice. PEMT-deficient mice also had substantially diminished concentrations of docosahexaenoic acid [22:6(n-3)] and arachidonic acid [20:4(n-6)] in plasma, independent of choline status. Thus, choline supplementation rescued some but not all of the phenotypes induced by the knockout. These findings indicate that PEMT activity functions beyond its recognized role as a compensatory pathway for PC biosynthesis and that, in contrast, PEMT activity is involved in many physiologic processes including the flux of lipid between liver and plasma and the delivery of essential fatty acids to blood and peripheral tissues via the liver-derived lipoproteins.

Endotoxin-induced alterations of lipid and fatty acid compositions in rat liver peroxisomes

The structure/function of peroxisomal lipids in rat liver treated with a sublethal dose of endotoxin, a lipopolysaccharide (LPS), was investigated. Peroxisomes isolated from LPS-treated rat liver had remarkable alterations in lipid content compared with saline treated control liver peroxisomes. Cholesterol and phospholipids (PL) decreased significantly by 28.7% and 50.8%, respectively, leading to the change in the ratio of cholesterol/phospholipids (control 0.081 versus LPS 0.118, P < 0.001). A quantitative analysis from LPS-treated rat liver peroxisomes showed a general decrease in all classes of PL. No such alterations were observed in lipid content of other subcellular organelles. The peroxisomal fatty acid composition in LPS-treated animals was also altered. An analysis of fatty acid composition in PL and phosphatidylcholine from LPS-treated peroxisomes showed an increase in arachidonic acid (C20:4) and docosahexaenoic acid (C22:6). Very long chain (VLC) fatty acids (> C22:0) were also found increased in all classes of lipids in LPS-treated peroxisomes. Gadolinium chloride (GAD) mediated inactivation of Kupffer cells (KC) normalized cholesterol/PL ratio in LPS-treated peroxisomes. Collectively, the results indicate that the peroxisome metabolism of lipids and fatty acids is specifically altered in endotoxin-treated rat liver and at least part of the alterations may be mediated by factors released by KC.

Quality control: from molecules to organelles

There is a vast body of literature on the quality control of protein folding and assembly into multisubunit complexes. Such control takes place everywhere in the cell. The correcting mechanisms involve cytosolic and organellar proteases; the result of such control is individual molecules with proper structure and individual complexes both with proper stoichiometry and proper structure. Obviously, the formation of organelles as such requires some additional criteria of correctness and some new mechanisms of their implementation. It is proposed in this article that the ability to carry out an integral (key) function may serve as a criterion of correct organelle assembly and that autophagy can be accepted as a mechanism eliminating the assembly mistakes.

Mechanisms of signal transduction in the stress response of hepatocytes

Adaptation of animals to stress is a unique property of life which allows the survival of the species. The stress response of hepatocytes is a very complex phenomenon, sometimes involving a cascade of events. The general stress signals are elucidated by mobilization of carbohydrate stores and akin to the insulin mediators. Oxidative signals are generated by pesticides, heavy metals, drugs, and alcohol which may or may not be under the purview of peroxisomes. Peroxisomal responses are well-defined involving specific receptors, whereas nonperoxisomal responses may be signaled by calcium, the Ah receptor, or built-in antioxidant systems. The intoxication signals are generally thought to be membrane defects induced by xenobiotics which then lead to highly nonspecific responses of hepatocytes. Detoxication signals, on the other hand, are specific responses of hepatocytes triggering de novo syntheses of detoxifier proteins or enzymes. Evidence reveals the existence of two distinct mechanisms of signal transduction in stressed hepatocytes--one involving the peroxisome and the other the plasma membrane.

Cellular signalling: the role of the peroxisome

This article reviews the role of the peroxisome in cellular signalling, with particular emphasis on the unique contributions of this organelle to the complex regulatory inter-relationships of cellular processes within the mammalian organism. Among the topics covered are the close alignments between the signalling systems governing peroxisome proliferation and those of the steroid hormone/thyroid hormone/vitamin D nuclear-receptor superfamily; the regulation of the permeability of the peroxisomal membrane; the involvements of lysophosphatidic acid as an intra- and inter-cellular messenger; the special role of the phosphatidylcholine cycle and its derivative messengers in relation to peroxisomal metabolism; peroxisomal contributions to the regulation of oxygen free radical levels in tissues and the significance of these radicals as second messengers; the evidence of peroxisomal influences on inter-cellular signalling from metabolic turnover studies; modifications of the regulatory significance of fatty acids by the peroxisome; the commonalities in metabolic relationships between the peroxisome and other cellular organelles; and regulatory shuttles associated with peroxisomal function. It is concluded that the peroxisome displays several significant interconnections with the cellular-signalling apparatus, that it is capable of imprinting a characteristic influence on the regulatory network in the cell, and that the contributions of this organelle deserve greater consideration in future investigations of cell-signalling phenomena.

Recent developments in peroxisome biology

Peroxisomes are subcellular organelles that are present in all eukaryotic organisms. These organelles are the focus of much contemporary interest among cellular and medical biologists--an interest which coincides with the realization of their vital role in higher organisms, their unique metabolic and biogenetic characteristics, and their widespread involvement in genetic and degenerative disease. This article reviews some of the major recent developments in peroxisome biology.

Molecular organization of hepatocyte peroxisomes

The matrix of peroxisomes has been considered to be homogeneous. However, a fine network of tubules is visible in electron micrographs at very high magnification. This substructure becomes more positive in a high-contrast photocopy and with an imaging-plate method. Clofibrate, bezafibrate, and aspirin increase peroxisomes. In proliferated peroxisomes, the density of matrix is low and the fine network is more visible. The effect of proliferators is more significant in males than in females. This sex difference may involve the action of estrogen, growth hormone, cytochrome P-450 and thyroxine. Mg-ATPase is localized on the limiting membrane of peroxisomes. Even on the membrane of irregular projections of proliferated peroxisomes, Mg-ATPase is evident cytochemically. Carnitine acetyltransferase is detectable in the matrix of proliferated peroxisomes. Withdrawal of proliferators results in a rapid decrease of peroxisomes. This may indicate the existence of peroxisome suppressors. Alternatively, dynamic transformation of vesicular to tubular types in peroxisome reticulum may occur. Such transformation has been described in lysosomes and mitochondria.

Lipid composition of liver peroxisomes isolated from untreated and clofibrate-treated mice and rats

Peroxisomes were isolated from liver tissue of control and clofibrate-treated adult male NMRI mice and Sprague-Dawley rats. Phospholipids, cholesterol, triglycerides and free fatty acids were measured in the peroxisomes. The fatty acid profiles of the phosphatidylethanolamine, the phosphatidylcholine, the triglyceride and the free fatty acid fractions were also analyzed. Phosphatidylethanolamine was the dominating phospholipid in peroxisomes from untreated animals. The fatty acid profiles of phosphatidylethanolamine, free fatty acids and triglycerides were similar for untreated mice and rats but differences between the species were observed in the pattern derived from phosphatidylcholine. Phosphatidylcholine was the most abundant phospholipid after clofibrate treatment. Clofibrate treatment caused an increase in the concentrations of phospholipids and unsaturated long-chain fatty acids and a decrease in the concentrations of triglycerides, free fatty acids, cholesterol and shorter saturated fatty acids.

Identification of a catalase-negative sub-population of peroxisomes induced in mouse liver by clofibrate

The peroxisomal compartment in mouse liver was investigated using rate sedimentation of liver subfractions on sucrose density gradients. Treatment of mice with clofibrate, a hypolipidemic agent and peroxisome proliferator, resulted in the formation of small particles which were devoid of catalase and urate oxidase, but which were identified as peroxisomal on the basis of content of the clofibrate-induced peroxisomal beta-oxidation enzymes (fatty acyl-CoA oxidase, hydratase/dehydrogenase bifunctional protein, and thiolase) and the 68 kDa peroxisomal integral membrane protein. Immunoelectron microscopy confirmed the membrane-bound organellar nature and enzyme composition of these particles. These particles were absent in normal mice, and were increased to a maximal level within 2 days of clofibrate treatment. These data have been taken as indicative of a role of these particles in the mechanism of drug-induced peroxisome proliferation.

The involvement of carnitine intermediates in peroxisomal fatty acid oxidation: a study with 2-bromofatty acids

Metabolism-dependent inactivators of 3-ketothiolase I and carnitine acyltransferase I (CAT I) have been used to study the oxidation of fatty acids in intact hepatocytes. 2-Bromooctanoate inactivates mitochondrial and peroxisomal 3-ketothiolases I in a time-dependent manner. During the first 5 min of incubation, inactivation of 3-ketothiolase in mitochondria is five times faster than its inactivation in peroxisomes. Almost complete inactivation of 3-ketothiolase I in both types of organelle is achieved after incubation with 1 mM 2-bromooctanoate for 40 min. The inactivation is not affected by preincubating hepatocytes with 20 microM tetradecylglycidate (TDGA), an inactivator of CAT I, under conditions which cause greater than 95% inactivation of CAT I. 2-Bromododecanoate (1 mM) causes 60% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min. These inactivations are greatly reduced by preincubating hepatocytes with 20 microM TDGA, demonstrating that 2-bromododecanoate enters both mitochondria and peroxisomes via its carnitine ester. 2-Bromopalmitate (1 mM) causes less than 5% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min, but causes 95% inactivation of CAT I during this time. Incubation of hepatocytes with 10-200 microM 2-bromopalmitoyl-L-carnitine causes inactivation of mitochondrial and peroxisomal 3-ketothiolases I at similar rates. This inactivation is decreased by palmitoyl-D-carnitine during the first 5 min of incubation. Pretreating hepatocytes with 20 microM TDGA does not affect the inactivation of mitochondrial or peroxisomal 3-ketothiolase I by 2-bromopalmitoyl-L-carnitine. These results demonstrate that in intact hepatocytes, peroxisomes oxidize fatty acids of medium-chain length by a carnitine-independent mechanism, whereas they oxidize long-chain fatty acids by a carnitine-dependent mechanism.

Studies on peroxisomal membranes

The phospholipid/protein ratios of rat liver peroxisomes, mitochondria and microsomes were determined and found to be 257 +/- 26, 232 +/- 20 and 575 +/- 20 nmol.mg-1, respectively. After correction for the loss of soluble protein, a peroxisomal ratio of 153 nmol.mg-1 was calculated. Organelle fractions were treated with sodium carbonate, whereafter membrane fragments containing integral membrane proteins were pelleted. For the membrane fractions of peroxisomes, mitochondria and microsomes phospholipid/protein ratios of 1054 +/- 103, 1180 +/- 90 and 1050 +/- 50 nmol.mg-1 were found, whereas 26 +/- 2, 20 +/- 2 and 49 +/- 2% of the organelle protein was recovered in these membrane fractions, respectively. The phospholipid composition of the different organelle fractions were determined, but no large differences were obtained, except for cardiolipin that was found only in the mitochondrial fraction. After sodium carbonate treatment virtually all enzymatic activity of the enzymes tested was lost. Therefore Triton X-114 phase separation was used to obtain the peroxisomal membrane components. In this fraction 42.9 +/- 3.5% of the protein and 90.2 +/- 3.7% of the phospholipid was found. Enzymatic activity of two integral membrane proteins was recovered for over 90% in the membrane fraction, whereas activity of two matrix proteins was mainly found in the soluble fraction. Urate oxidase, the peroxisomal core protein, behaved differently and was recovered mainly with the membrane components. Recoveries of enzymatic activities after the Triton X-114 phase separation varied from 45 to 116%, and together with the good separation that was obtained between soluble proteins and integral membrane proteins this method provides a useful alternative for the isolation of membrane components.

Alterations in the integrity of peroxisomal membranes in livers of mice treated with peroxisome proliferators

Catalase leakage from its particulate compartment within the light mitochondrial fraction of liver was used as an index of the integrity of peroxisomes in untreated mice and in mice treated with the peroxisome proliferators clofibrate(ethyl-p-chlorophenoxyisobutyrate), Wy-14,643(4-chloro-6[2,3-xylidino)-2-pyrimidinylthio]acetic acid) and DEHP(di-(2-ethylhexyl)phthalate). Catalase leakage represented about 2% of the total catalase activity when fractions from untreated mice were incubated at 4 degrees C, increasing to about 5% during 60 min incubation at 37 degrees C. In fractions from livers of mice treated with peroxisome proliferators, catalase leakage was significantly higher, being 7-11% at 4 degrees C and increasing to approximately 20% after 60 min incubation at 37 degrees C. The pattern of release was similar for all proliferators. Parallel data were obtained for catalase latency in these fractions, i.e. following 60 min incubation at 37 degrees C, free (non-latent) catalase activity was 18% in control mice and 65, 67, and 83% in fractions from clofibrate-, Wy-14,643- and DEHP-treated mice, respectively. Differences in catalase leakage from peroxisomes in fractions from untreated mice and clofibrate-treated mice were also apparent following treatments designed to effect membrane permeabilization, as in freeze-thawing, osmotic rupture, and extraction with Triton X-100 and lysophosphatidylcholine. These data are consistent with a significant alteration in the integrity of the membranes of peroxisomes in livers of mice which have been treated with peroxisome proliferators, and furthermore indicate a commonality of effect of these agents.

Effects of hepatic peroxisome proliferators and 12-O-tetradecanoyl phorbol-13-acetate on catalase and other enzyme activities of embryonic cells in vitro

The effects of the hepatic peroxisome proliferators (HPPs) clofibrate, di-(2-ethylhexyl)-phthalate (DEHP), mono-(2-ethylhexyl)phthalate (MEHP) and 2,4-dichlorophenoxy acetic acid (2,4-D) on the activities of some peroxisome-associated enzymes and marker enzymes for other organelles, have been studied in primary Syrian hamster embryo (SHE) cells and Wistar rat embryo (WRE) cells. The majority of the cells are fibroblast-like. 12-O-Tetradecanoyl phorbol-13-acetate (TPA) was included as it has been suggested that it may act as a peroxisome proliferator. The specific activities of catalase, fatty acyl-CoA oxidase (FAO) and peroxisomal beta-oxidation were approximately 100-fold lower in the embryonic cells than in rat hepatocytes. Other peroxisome-associated oxidases were not detected. The dihydroxyacetone-phosphate acyltransferase (DHAPAT) activity was comparable to that in rat liver. Marker enzymes for other organelles had specific activities comparable to rat hepatocytes. Catalase was shown by digitonin titration to be contained in a peroxisome-like compartment in both SHE and WRE cells. Clofibrate, DEHP and MEHP increased the catalase activity, which might suggest peroxisome proliferation. However, the findings that FAO and peroxisomal beta-oxidation did not increase or only very slightly, argue against peroxisome proliferation. 2,4-D and TPA induced no or only a very slight increase in the catalase activity.

Biogenesis of peroxisomes: immunocytochemical investigation of peroxisomal membrane proteins in proliferating rat liver peroxisomes and in catalase-negative membrane loops

Treatment of rats with a new hypocholesterolemic drug BM 15766 induces proliferation of peroxisomes in pericentral regions of the liver lobule with distinct alterations of the peroxisomal membrane (Baumgart, E., K. Stegmeier, F. H. Schmidt, and H. D. Fahimi. 1987. Lab. Invest. 56:554-564). We have used ultrastructural cytochemistry in conjunction with immunoblotting and immunoelectron microscopy to investigate the effects of this drug on peroxisomal membranes. Highly purified peroxisomal fractions were obtained by Metrizamide gradient centrifugation from control and treated rats. Immunoblots prepared from such peroxisomal fractions incubated with antibodies to 22-, 26-, and 70-kD peroxisomal membrane proteins revealed that the treatment with BM 15766 induced only the 70-kD protein. In sections of normal liver embedded in Lowicryl K4M, all three membrane proteins of peroxisomes could be localized by the postembedding technique. The strongest labeling was obtained with the 22-kD antibody followed by the 70-kD and 26-kD antibodies. In treated animals, double-membraned loops with negative catalase reaction in their lumen, resembling smooth endoplasmic reticulum segments as well as myelin-like figures, were noted in the proximity of some peroxisomes. Serial sectioning revealed that the loops seen at some distance from peroxisomes in the cytoplasm were always continuous with the peroxisomal membranes. The double-membraned loops were consistently negative for glucose-6-phosphatase, a marker for endoplasmic reticulum, but were distinctly labeled with antibodies to peroxisomal membrane proteins. Our observations indicate that these membranous structures are part of the peroxisomal membrane system. They could provide a membrane reservoir for the proliferation of peroxisomes and the expansion of this intracellular compartment.

The hepatocellular transport of sulfobromophthalein-glutathione by clofibrate treated, perfused rat liver

The hypolipidemic drug clofibrate is known to affect the hepatic transport of various organic anions including bilirubin, fatty acids and sulfobromophthalein. Changes in the rate of metabolism and/or intracellular transport have been claimed responsible for the effect. To evaluate these possibilities, the transport of sulfobromophthalein-glutathione, a model compound that does not require metabolism for biliary excretion, was studied in perfused livers isolated from clofibrate-treated and control rats. Cytosolic fatty acid binding protein and glutathione S-transferase activity were also measured. Clofibrate treatment significantly increased liver weight; as a result glutathione S-transferase activity (toward 1-chloro-2,4-dinitrobenzene) fell if expressed per gram of liver (4560 +/- 420 (SE) vs 7010 +/- 260 nmoles/min for clofibrate treated and controls respectively, p less than 0.002), but was unchanged when expressed per total liver (60.8 +/- 6.5 vs 64.6 +/- 3.5 mumoles/min for clofibrate and controls p greater than 0.5). Irrespective of how it was expressed fatty acid binding protein was significantly increased by the drug treatment. Steady state sulfobromophthalein-glutathione removal velocity was saturable with increasing concentrations of sulfobromophthalein-glutathione in both control and clofibrate-treated livers. Steady state extraction ratio, as well as Vmax and Km for removal, did not differ between the two groups. In keeping with other observations, these data collectively indicate that the hepatic steady state removal of nonmetabolized compounds is not affected by clofibrate.(ABSTRACT TRUNCATED AT 250 WORDS)