Alkylthioacetic acid (3-thia fatty acids)--a new group of non-beta-oxidizable, peroxisome-inducing fatty acid analogues. I. A study on the structural requirements for proliferation of peroxisomes and mitochondria in rat liver

Recent advance of ATP citrate lyase inhibitors for the treatment of cancer and related diseases

ATP citrate lyase (ACLY), a strategic metabolic enzyme that catalyzes the glycolytic to lipidic metabolism, has gained increasing attention as an attractive therapeutic target for hyperlipidemia, cancers and other human diseases. Despite of continual research efforts, targeting ACLY has been very challenging. In this field, most reported ACLY inhibitors are "substrate-like" analogues, which occupied with the same active pockets. Besides, some ACLY inhibitors have been disclosed through biochemical screening or high throughput virtual screening. In this review, we briefly summarized the cancer-related functions and the recent advance of ACLY inhibitors with a particular focus on the SAR studies and their modes of action. We hope to provide a timely and updated overview of ACLY and the discovery of new ACLY inhibitors.

Plasma 3-hydroxyisobutyrate (3-HIB) and methylmalonic acid (MMA) are markers of hepatic mitochondrial fatty acid oxidation in male Wistar rats

OBJECTIVE

Discovery of specific markers that reflect altered hepatic fatty acid oxidation could help to detect an individual's risk of fatty liver, type 2 diabetes and cardiovascular disease at an early stage. Lipid and protein metabolism are intimately linked, but our understanding of this crosstalk remains limited.

METHODS

In male Wistar rats, we used synthetic fatty acid analogues (3-thia fatty acids) as a tool to induce hepatic fatty acid oxidation and mitochondrial biogenesis, to gain new insight into the link between fatty acid oxidation, amino acid metabolism and TCA cycle-related intermediate metabolites in liver and plasma.

RESULTS

Rats treated with 3-thia fatty acids had 3-fold higher hepatic, but not adipose and skeletal muscle, expression of the thioesterase 3-hydroxyisobutyryl-CoA hydrolase (Hibch), which controls the formation of 3-hydroxyisobutyrate (3-HIB) in the valine degradation pathway. Consequently, 3-thia fatty acid-stimulated hepatic fatty acid oxidation and ketogenesis was accompanied by decreased plasma 3-HIB and increased methylmalonic acid (MMA) concentrations further downstream in BCAA catabolism. The higher plasma MMA corresponded to higher MMA-CoA hydrolase activity and hepatic expression of GTP-specific succinyl-CoA synthase (Suclg2) and succinate dehydrogenase (Sdhb), and lower MMA-CoA mutase activity. Plasma 3-HIB correlated positively to plasma and hepatic concentrations of TAG, plasma total fatty acids, plasma NEFA and insulin/glucose ratio, while the reverse correlations were seen for MMA.

CONCLUSION

Our study provides new insight into TCA cycle-related metabolic changes associated with altered hepatic fatty acid flux, and identifies 3-HIB and MMA as novel circulating markers reflective of mitochondrial β-oxidation in male Wistar rats.

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.

Increased fatty acid oxidation and mitochondrial proliferation in liver are associated with increased plasma kynurenine metabolites and nicotinamide levels in normolipidemic and carnitine-depleted rats

Dysregulation of the tryptophan (Trp)-NAD⁺ pathway has been related to several pathological conditions, and the metabolites in this pathway are known to influence mitochondrial respiration and redox status. The aim of this project was to investigate if stimulation of beta-oxidation and mitochondrial proliferation by the mitochondrial-targeted compound 2-(tridec-12-yn-1-ylthio)acetic acid (1-triple TTA) would influence metabolites of the Trp-Kyn-NAD⁺ pathway. We wished to investigate how carnitine depletion by meldonium-treatment influenced these metabolites. After dietary treatment of male Wistar rats with 1-triple TTA for three weeks, increased hepatic mitochondrial- and peroxisomal fatty acid oxidation resulted. The plasma content of total carnitines decreased compared to control animals, whereas hepatic genes involved in CoA biosynthesis were upregulated by 1-triple TTA treatment. The plasma Trp level and individual metabolites in the kynurenine pathway were increased by 1-triple TTA, associated with decreased hepatic gene expression of indoleamine2,3-dioxygenase. 1-triple TTA treatment increased conversion of Trp to nicotinamide (Nam) as the plasma content of quinolinic acid, Nam and N1-methylnicotinamide (mNam) increased, accompanied with suppression of hepatic gene expression of α-amino-α-carboxymuconate-ε-semialdehyde decarboxylase. A positive correlation between mitochondrial fatty acid oxidation and Trp-derivatives was found. Almost identical results were obtained by 1-triple TTA in the presence of meldonium, which alone exerted minor effects. Moreover, the plasma Kyn:Trp ratio (KTR) correlated negatively to mitochondrial function. Whether increased flux through the Trp-NAD⁺ pathway increased redox status and lowered inflammation locally and systemically should be considered.

ATP-citrate lyase (ACLY) in lipid metabolism and atherosclerosis: An updated review

ATP citrate lyase (ACLY) is an important enzyme linking carbohydrate to lipid metabolism by generating acetyl-CoA from citrate for fatty acid and cholesterol biosynthesis. Mendelian randomization of large human cohorts has validated ACLY as a promising target for low-density-lipoprotein-cholesterol (LDL-C) lowering and cardiovascular protection. Among current ACLY inhibitors, Bempedoic acid (ETC-1002) is a first-in-class, prodrug-based direct competitive inhibitor of ACLY which regulates lipid metabolism by upregulating hepatic LDL receptor (LDLr) expression and activity. ACLY deficiency in hepatocytes protects from hepatic steatosis and dyslipidemia. In addition, pharmacological inhibition of ACLY by bempedoic acid, prevents dyslipidemia and attenuates atherosclerosis in hypercholesterolemic ApoE^-/- mice, LDLr^-/- mice, and LDLr^-/- miniature pigs. Convincing data from clinical trials have revealed that bempedoic acid significantly lowers LDL-C as monotherapy, combination therapy, and add-on with statin therapy in statin-intolerant patients. More recently, a phase 3 CLEAR Harmony clinical trial ("Safety and Efficacy of Bempedoic Acid to Reduce LDL Cholesterol") has shown that bempedoic acid reduces the level of LDL-C in hypercholesterolemic patients receiving guideline-recommended statin therapy with a good safety profile. Hereby, we provide a updated review of the expression, regulation, genetics, functions of ACLY in lipid metabolism and atherosclerosis, and highlight the therapeutic potential of ACLY inhibitors (such as bempedoic acid, SB-204990, and other naturally-occuring inhibitors) to treat atherosclerotic cardiovascular diseases. It must be pointed out that long-term large-scale clinical trials in high-risk patients, are warranted to validate whether ACLY represent a promising therapeutic target for pharmaceutic intervention of dyslipidemia and atherosclerosis; and assess the safety and efficacy profile of ACLY inhibitors in improving cardiovascular outcome of patients.

The Roles of β-Oxidation and Cofactor Homeostasis in Peroxisome Distribution and Function in Arabidopsis thaliana

Key steps of essential metabolic pathways are housed in plant peroxisomes. We conducted a microscopy-based screen for anomalous distribution of peroxisomally targeted fluorescence in Arabidopsis thaliana This screen uncovered 34 novel alleles in 15 genes affecting oil body mobilization, fatty acid β-oxidation, the glyoxylate cycle, peroxisome fission, and pexophagy. Partial loss-of-function of lipid-mobilization enzymes conferred peroxisomes clustered around retained oil bodies without other notable defects, suggesting that this microscopy-based approach was sensitive to minor perturbations, and that fatty acid β-oxidation rates in wild type are higher than required for normal growth. We recovered three mutants defective in PECTIN METHYLESTERASE31, revealing an unanticipated role in lipid mobilization for this cytosolic enzyme. Whereas mutations reducing fatty acid import had peroxisomes of wild-type size, mutations impairing fatty acid β-oxidation displayed enlarged peroxisomes, possibly caused by excess fatty acid β-oxidation intermediates in the peroxisome. Several fatty acid β-oxidation mutants also displayed defects in peroxisomal matrix protein import. Impairing fatty acid import reduced the large size of peroxisomes in a mutant defective in the PEROXISOMAL NAD⁺ TRANSPORTER (PXN), supporting the hypothesis that fatty acid accumulation causes pxn peroxisome enlargement. The diverse mutants isolated in this screen will aid future investigations of the roles of β-oxidation and peroxisomal cofactor homeostasis in plant development.

Probing fatty acid metabolism in bacteria, cyanobacteria, green microalgae and diatoms with natural and unnatural fatty acids

In both eukaryotes and prokaryotes, fatty acid synthases are responsible for the biosynthesis of fatty acids in an iterative process, extending the fatty acid by two carbon units every cycle. Thus, odd numbered fatty acids are rarely found in nature. We tested whether representatives of diverse microbial phyla have the ability to incorporate odd-chain fatty acids as substrates for their fatty acid synthases and their downstream enzymes. We fed various odd and short chain fatty acids to the bacterium Escherichia coli, cyanobacterium Synechocystis sp. PCC 6803, green microalga Chlamydomonas reinhardtii and diatom Thalassiosira pseudonana. Major differences were observed, specifically in the ability among species to incorporate and elongate short chain fatty acids. We demonstrate that E. coli, C. reinhardtii, and T. pseudonana can produce longer fatty acid products from short chain precursors (C3 and C5), while Synechocystis sp. PCC 6803 lacks this ability. However, Synechocystis can incorporate and elongate longer chain fatty acids due to acyl-acyl carrier protein synthetase (AasS) activity, and knockout of this protein eliminates the ability to incorporate these fatty acids. In addition, expression of a characterized AasS from Vibrio harveyii confers a similar capability to E. coli. The ability to desaturate exogenously added fatty acids was only observed in Synechocystis and C. reinhardtii. We further probed fatty acid metabolism of these organisms by feeding desaturase inhibitors to test the specificity of long-chain fatty acid desaturases. In particular, supplementation with thia fatty acids can alter fatty acid profiles based on the location of the sulfur in the chain. We show that coupling sensitive gas chromatography mass spectrometry to supplementation of unnatural fatty acids can reveal major differences between fatty acid metabolism in various organisms. Often unnatural fatty acids have antibacterial or even therapeutic properties. Feeding of short precursors now gives us easy access to these extended molecules.

Phospholipid molecular species, beta-oxidation, desaturation and elongation of fatty acids in Atlantic salmon hepatocytes: effects of temperature and 3-thia fatty acids

We have investigated the effects of a 3-thia fatty acid (TTA) and of temperature on the fatty acid (FA) metabolism of Atlantic salmon (Salmo salar). One experiment investigated the activity of the peroxisomal beta-oxidation enzyme, acyl-CoA oxidase (ACO), and the incorporation of TTA into phospholipid (PL) molecular species. Salmon hepatocytes in culture were incubated either without TTA (control(spades)) or with 0.8 mM TTA (TTA(spades)) in a short term (48 h) temperature study at 5 degrees C and at 12 degrees C. TTA was incorporated into the four PL classes studied: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS). TTA was preferentially esterified with 18:1, 16:1, 20:4 and 22:6 in the PLs. Hepatocytes incubated with TTA had higher ACO activity at 5 degrees C than at 12 degrees C. In a second experiment salmon were fed a diet based on fish meal-fish oil without any TTA added (control) or a fish meal-fish oil diet supplemented with 0.6% TTA for 8 weeks at 12 degrees C and 20 weeks at 5 degrees C. At the end of the feeding trial, hepatocytes from fish acclimated to high or low temperatures were isolated from both dietary groups and incubated with either [1-(14)C]18:1 n-9 or [1-(14)C]20:4 n-3 at 5 degrees C or 12 degrees C. Radiolabelled 18:1 n-9 was mainly esterified into neutral lipids (NL), whereas [1-(14)C]20:4 n-3 was mainly esterified into PL at both temperatures. The rate of elongation of [1-(14)C]18:1 n-9 to 20:1 n-9 was twice as high in hepatocytes from fish fed the control diet than it was in hepatocytes from fish fed the TTA diet, at both temperatures. The amount of [1-(14)C]20:4 n-3 converted to 22:6 n-3 was approximately the same in hepatocytes from the two dietary groups, but there was a tendency to higher production of 22:6 n-3 at the lower temperature. Oxidation of [1-(14)C]18:1 n-9 to acid soluble products (ASP) and CO(2) was approximately 10-fold greater in hepatocytes kept at 5 degrees C than in those kept at 12 degrees C and the main oxidation products formed were acetate, oxaloacetate and malate.

Bacterial biodegradation of aliphatic sulfides under aerobic carbon- or sulfur-limited growth conditions

AIMS

To isolate bacteria capable of cleaving aliphatic carbon-sulfur bonds as potential biological upgrading catalysts for the reduction of molecular weight and viscosity in heavy crude oil.

METHODS AND RESULTS

Thirty-one bacterial strains isolated from enrichment cultures were able to biotransform model compounds representing the aliphatic sulfide bridges found in asphaltenes. Using gas chromatography and mass spectrometry, three types of attack were identified: alkyl chain degradation, allowing use as a carbon source; nonspecific sulfur oxidation; and sulfur-specific oxidation and carbon-sulfur bond cleavage, allowing use as a sulfur source. Di-n-octyl sulfide degradation produced octylthio- and octylsulfonyl-alkanoic acids, consistent with terminal oxidation followed by beta-oxidation reactions. Utilization of dibenzyl sulfide or 1,4-dithiane as a sulfur source was regulated by sulfate, indicating a sulfur-specific activity rather than nonspecific oxidation. Finally, several isolates were also able to use dibenzothiophene as a sulfur source, and this was the preferred organic sulfur substrate for one isolate.

CONCLUSIONS

The use of commercially available alkyl sulfides in enrichment cultures gave isolates that followed a range of metabolic pathways, not just sulfur-specific attack.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results give new insight into biodegradation of organosulfur compounds from petroleum and for biotreatment of such compounds in chemical munitions.

Sulfur-substituted and alpha-methylated fatty acids as peroxisome proliferator-activated receptor activators

FA with varying chain lengths and an alpha-methyl group and/or a sulfur in the beta-position were tested as peroxisome proliferator-activated receptor (PPAR)alpha, -delta(beta), and -gamma ligands by transient transfection in COS-1 cells using chimeric receptor expression plasmids, containing cDNAs encoding the ligand-binding domain of PPARalpha, -delta, and -gamma. For PPARalpha, an increasing activation was found with increasing chain length of the sulfur-substituted FA up to C14-S acetic acid (tetradecylthioacetic acid = TTA). The derivatives were poor, and nonsignificant, activators of PPARdelta. For PPARgamma, activation increased with increasing chain length up to C16-S acetic acid. A methyl group was introduced in the alpha-position of palmitic acid, TTA, EPA, DHA, cis9,trans11 CLA, and trans10,cis12 CLA. An increased activation of PPARalpha was obtained for the alpha-methyl derivatives compared with the unmethylated FA. This increase also resulted in increased expression of the two PPARalpha target genes acyl-CoA oxidase and liver FA-binding protein for alpha-methyl TTA, alpha-methyl EPA, and alpha-methyl DHA. Decreased or altered metabolism of these derivatives in the cells cannot be excluded. In conclusion, saturated FA with sulfur in the beta-position and increasing carbon chain length from C9-S acetic acid to C14-S acetic acid have increasing effects as activators of PPARalpha and -gamma in transfection assays. Furthermore, alpha-methyl FA derivatives of a saturated natural FA (palmitic acid), a sulfur-substituted FA (TTA), and PUFA (EPA, DHA, c9,t11 CLA, and t10,c12 CLA) are stronger PPARalpha activators than the unmethylated compounds.

Effects of 3-thia fatty acids on feed intake, growth, tissue fatty acid composition, beta-oxidation and Na+,K+-ATPase activity in Atlantic salmon

Atlantic salmon (Salmo salar) with an initial mass of 86 g were reared in 12 degrees C seawater for 8 weeks to a final average mass of 250 g. The fish were fed fish meal and fish oil-based diet supplemented with either 0%, 0.3% or 0.6% of tetradecylthioacetic acid (TTA), a 3-thia fatty acid. The specific growth rate (SGR) decreased with increasing dietary dose of TTA. The SGR of the group fed 0% of TTA (Control) was 1.8; that of the group fed 0.3% of TTA (TTA-L) was 1.7, and that of the group fed 0.6% of TTA (TTA-H) was 1.5. The mortality increased with increased dietary dose of TTA. The mitochondrial beta-oxidation capacity in the liver of fish fed the TTA diets was 1.5 to 2 times higher than that of the Control fish. TTA supplementation caused substantial changes in the fatty acid compositions of the phospholipids (PL), triacylglycerols (TAG) and free fatty acids (FFA) of gills, heart and liver. The percentages of n-3 fatty acids, particularly 22:6 n-3, increased in fish fed diets containing TTA, while the percentage of the saturated FAs 14:0 and 16:0 in the PL fractions of the gills and heart decreased. The sum of monounsaturated FAs in the PL and TAG fractions from liver was significantly higher in fish fed diets containing TTA. TTA itself was primarily incorporated into PL. Two catabolic products of TTA (sulphoxides of TTA) were identified, and these products were particularly abundant in the kidney. TTA supplementation had no significant effect on the activity of the membrane-bound enzyme Na(+),K(+)-ATPase.

Mitochondrial-targeted fatty acid analog induces apoptosis with selective loss of mitochondrial glutathione in promyelocytic leukemia cells

Some fatty acids and derivatives are known to induce cell death in cancer cells. Mitochondria may have important roles in the death process. Therefore, we investigated the mitochondrial contribution in cell death induced by a modified fatty acid, tetradecylthioacetic acid (TTA), which cannot be beta-oxidized. TTA treatment induced apoptosis in IPC-81 leukemia cells via depolarization of the mitochondrial membrane potential (deltapsi) and early release of cytochrome c, accompanied by depletion of mitochondrial glutathione. Caspase-3 activation and cleavage of poly (ADP-ribose) polymerase (PARP) occurred at a late stage, but the broad-spectra caspase inhibitor zVAD-fmk did not block TTA-induced apoptosis. Overexpression of Bcl-2 partially prevented TTA-induced apoptosis, whereas cAMP-induced cell death was completely blocked. In conclusion, TTA seems to trigger apoptosis through mitochondrial-mediated mechanisms and selective modulation of the mitochondrial redox equilibrium.

Apo A-IV: an update on regulation and physiologic functions

Apolipoprotein (apo) A-IV, first identified 28 years ago as a plasma lipoprotein moiety, is now known to participate in the regulation of various metabolic pathways. It is synthesized primarily in the enterocytes of the small intestine during fat absorption. After entry into the bloodstream, the 46-kDa glycoprotein apo A-IV appears associated with chylomicrons, high-density lipoproteins, and in the lipoprotein-free fraction. It has a role in lipid absorption, transport and metabolism, and may act as a post-prandial satiety signal, an anti-oxidant and a major factor in the prevention of atherosclerosis. After summarizing and discussing these functions for reader's comprehension, the current review focuses on the regulation of apo A-IV by nutrients, biliary components, drugs, hormones and gastrointestinal peptides. The understanding of the involved mechanisms that underline apo A-IV regulation may in the long run allow us to switch on its gene, which may confer multiple beneficial effects, including the protection from atherosclerosis.

Antiproliferative effects of a non-beta-oxidizable fatty acid, tetradecylthioacetic acid, in native human acute myelogenous leukemia blast cultures

The lipid metabolism is important in the regulation of cell proliferation. We have examined effects of a fatty acid analogue, tetradecylthioacetic acid (TTA), on the functional phenotype of native, human AML cells. TTA inhibited AML blast proliferation in the presence of single cytokines (GM-CSF and SCF: P > 0.05, 35 patients with detectable proliferation) and a combination of cytokines (P < 0.005, n = 21). This antiproliferative effect was generally stronger than for the normal fatty acid palmitic acid (PA). Both TTA and PA increased the secretion of tumor necrosis factor alpha (TNFalpha) (P < 0.05, 27 patients with detectable cytokine release), but only PA increased interleukein 1beta (IL-1beta) release (P < 0.005, n = 34). AML blast populations varied significantly in their levels and activities of metabolites and enzymes characterizing oxidative status and fatty acid metabolism, and there was no significant correlation between the intrinsic oxidative status and the effects of PA and TTA on blast proliferation. Although TTA reduced the proliferation of mitogen-stimulated normal T cells derived from healthy individuals (P < 0.05, n = 8), no adverse effects were seen on peripheral blood cell counts (reticulocytes, platelets, total white blood cells, differential leukocyte counts) for healthy volunteers receiving TTA (oral administration of 1000 mg/day for 7 consecutive days). Our results suggest that TTA can inhibit AML blast proliferation through pathways that are unrelated to autocrine cytokine secretion and intrinsic oxidative status.

Interactions of fluorochemicals with rat liver fatty acid-binding protein

Liver-fatty acid binding protein (L-FABP) is an abundant intracellular lipid-carrier protein. The hypothesis that perfluorooctanesulfonate (PFOS), perfluorooctanoate (PFOA), and certain related perfluorooctanesulfonamide-based fluorochemicals (PFOSAs) can interfere with the binding affinity of L-FABP for fatty acids was tested. The relative effectiveness of PFOA, PFOS, N-ethylperfluorooctanesulfonamide (N-EtFOSA), N-ethylperfluorooctanesulfonamido ethanol (N-EtFOSE), and of the strong peroxisome proliferator Wyeth-14643 (WY) to inhibit 11-(5-dimethylaminonapthalenesulphonyl)-undecanoic acid (DAUDA) binding to-L-FABP was determined. The dissociation constant (Kd) of the DAUDA-L-FABP complex was 0.47 nM. PFOS exhibited the highest level of inhibition of DAUDA-L-FABP binding in the competitive binding assays, followed by N-EtFOSA, WY, and, with equal IC(50)s, N-EtFOSE and PFOA. The in vitro data presented in this study support the hypothesis that these fluorochemicals may interfere with the binding of fatty acids or other endogenous ligands to L-FABP. Furthermore, this work provides evidence to support the hypothesis that displacement of endogenous ligands from L-FABP may contribute to toxicity in rodents fed these fluorochemicals.

Perfluorooctanoate, perflourooctanesulfonate, and N-ethyl perfluorooctanesulfonamido ethanol; peroxisome proliferation and mitochondrial biogenesis

Compounds that cause peroxisome proliferation in rats and mice have been reported to interfere with mitochondrial (mt) bioenergetics and possibly biogenesis. The purpose of this investigation was to establish whether proliferation of peroxisomes and mitochondria are necessarily related. Perfluorooctanesulfonate (PFOS) and N-ethyl perfluorooctanesulfonamido ethanol (N-EtFOSE) were investigated as peroxisome proliferators in comparison to perfluorooctanoic acid (PFOA). Three parameters were chosen to assess peroxisome proliferation, stimulation of lauroyl CoA oxidase activity, reduction of serum cholesterol concentration, and hepatomegaly. mt Biogenesis was assessed through cytochrome oxidase activity, cytochrome content and mitochondrial DNA (mtDNA) copy number. PFOA, PFOS, or N-EtFOSE was administered via a single i.p. injection at 100 mg/kg in male rats, and measurements were made 3 days later. In this model, PFOS and PFOA share similar potencies as peroxisome proliferators, whereas N-EtFOSE showed no activity. mt Endpoints were altered only in the PFOA treatment group, which consisted of a decrease cytochrome oxidase activity in liver tissue and an increase in the mtDNA copy number. None of the perfluorooctanoates significantly altered mt cytochrome content following acute in vivo treatment. These data demonstrate that acute administration of PFOS or PFOA causes hepatic peroxisome proliferation in rats. However, stimulation of mt biogenesis is not a characteristic response of all peroxisome proliferators.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase and carnitine palmitoyltransferase II as potential control sites for ketogenesis during mitochondrion and peroxisome proliferation

3-Thia fatty acids are potent hypolipidemic fatty acid derivatives and mitochondrion and peroxisome proliferators. Administration of 3-thia fatty acids to rats was followed by significantly increased levels of plasma ketone bodies, whereas the levels of plasma non-esterified fatty acids decreased. The hepatic mRNA levels of fatty acid binding protein and formation of acid-soluble products, using both palmitoyl-CoA and palmitoyl-L-carnitine as substrates, were increased. Hepatic mitochondrial carnitine palmitoyltransferase (CPT) -II and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase activities, immunodetectable proteins, and mRNA levels increased in parallel. In contrast, the mitochondrial CPT-I mRNA levels were unchanged and CPT-I enzyme activity was slightly reduced in the liver. The CoA ester of the monocarboxylic 3-thia fatty acid, tetradecylthioacetic acid, which accumulates in the liver after administration, inhibited the CPT-I activity in vitro, but not that of CPT-II. Acetoacetyl-CoA thiolase and HMG-CoA lyase activities involved in ketogenesis were increased, whereas the citrate synthase activity was decreased. The present data suggest that 3-thia fatty acids increase both the transport of fatty acids into the mitochondria and the capacity of the beta-oxidation process. Under these conditions, the regulation of ketogenesis may be shifted to step(s) beyond CPT-I. This opens the possibility that mitochondrial HMG-CoA synthase and CPT-II retain some control of ketone body formation.

Tetradecylthioacetic acid (a 3-thia fatty acid) impairs secretion of oleic acid-induced triacylglycerol-rich lipoproteins in CaCo-2 cells

The fatty acid analogue tetradecylthioacetic acid (TTA) has previously been shown to decrease triacylglycerol secretion in CaCo-2 cells (Gedde-Dahl et al., J. Lipid Res. 36 (1995) 535-543). The present study was designed to further elucidate the effect of TTA on lipoprotein production in CaCo-2 cells. TTA did not affect oleic acid-induced triacylglycerol synthesis, but it significantly decreased secretion of newly synthesized triacylglycerol when compared to cells incubated with oleic acid alone or oleic acid in combination with palmitic acid. In contrast, pulse-chase experiments showed no difference in the amount of labeled triacylglycerol secreted from cells exposed to either fatty acid combination during the chase period, indicating that TTA did not affect the secretory process in general. Cells incubated with TTA alone secreted triacylglycerol present at 1.025<rho<1.073 g/ml, corresponding to the low density lipoprotein/intermediate density lipoprotein density range. In contrast, cells supplemented with oleic acid or oleic acid in combination with TTA secreted triacylglycerol mainly in the very low density lipoprotein/chylomicron density range (rho<1.006 g/ml). Despite a marked decrease in triacylglycerol secretion, TTA treatment did not change secretion of apolipoprotein B nor the activity of microsomal triacylglycerol transfer protein (MTP) in the cells. Furthermore, the presence of TTA in cellular triacylglycerol had no effect on the ability of purified MTP to transfer triacylglycerol from donor to acceptor vesicles. Together, the above observations suggest that TTA interferes with other MTP-independent factors that regulate the intestinal lipoprotein secretion.

Early modulation of genes encoding peroxisomal and mitochondrial beta-oxidation enzymes by 3-thia fatty acids

The aim of the present study was to elucidate the effects of a single dose of 3-thia fatty acids (tetradecylthioacetic acid and 3-thiadicarboxylic acid) over a 24-hr study period on the expression of genes related to peroxisomal and mitochondrial beta-oxidation in liver of rats. The plasma triglyceride level decreased at 2-4 hr, 4-8 hr, and 8-24 hr, respectively, after a single dose of 150, 300, or 500 mg of 3-thia fatty acids/kg body weight. Four to eight hours after administration of 3-thia fatty acids, a several-fold-induced gene expression of peroxisomal multifunctional protein, fatty acyl-CoA oxidase (EC 1.3.3.6), fatty acid binding protein, and 2,4-dienoyl-CoA reductase (EC 1.3.1.43) resulted, concomitant with increased activity of 2,4-dienoyl-CoA reductase and fatty acyl-CoA oxidase. The expression of carnitine palmitoyltransferase-I and carnitine palmitoyltransferase-II increased at 2 and 4 hr, respectively, although at a smaller scale. In cultured hepatocytes, 3-thia fatty acids stimulated fatty acid oxidation after 4 hr, and this was both L-carnitine- and L-aminocarnitine-sensitive. The hepatic content of eicosapentaenoic acid and docosahexaenoic acid decreased throughout the study period. In contrast, the hepatic content of oleic acid tended to increase after 24 hr and was significantly increased after repeated administration of 3-thia fatty acids. Similarly, the expression of delta9-desaturase was unchanged during the 24-hr study, but increased after feeding for 5 days. To conclude, carnitine palmitoyltransferase-I expression seemed to be induced earlier than 2,4-dienoyl-CoA reductase and fatty acid binding protein, and not later than the peroxisomal fatty acyl-CoA oxidase. The expression of delta9-desaturase showed a more delayed response.

Mechanisms of peroxisome proliferation by perfluorooctanoic acid and endogenous fatty acids

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

On the effect of 2-deuterium- and 2-methyl-eicosapentaenoic acid derivatives on triglycerides, peroxisomal beta-oxidation and platelet aggregation in rats

A series of 2-substituted eicosapentaenoic acid (EPA) derivatives (as ethyl esters) have been synthesized and evaluated as hypolipidemic and antithrombotic agents in feeding experiments in rats. Repeated administration of purified 2-methyl-eicosapentaenoic acid and its deuterium analogues (all as ethyl esters) to rats resulted in a decrease in plasma triglycerides and high density lipoprotein cholesterol. The 2-methyl-EPA analogues were, apparently, four times more potent than EPA in inducing the triglyceride lowering effect. The 2-deuterium-2-methyl-EPA decreased plasma cholesterol level to approximately 40%. A moderate enlargement of the liver was observed in 2-methyl-EPA treated rats. This was accompanied with an acute reduction in the liver content of triglycerides and a stimulation of peroxisomal beta-oxidation and fatty acyl-CoA oxidase activity. The results suggest that the triglyceride-lowering effect of 2-methyl-EPA may be due to a reduced supply of fatty acids for hepatic triglyceride biosynthesis because of increased fatty acid oxidation. Platelet aggregation with ADP and A23187 was performed ex vivo in platelet-rich plasma, after administration of different doses of the EPA-derivatives for five days. EPA and 2,2-dideuterium EPA had no effect on ADP-induced aggregation, while 2-deuterium-, 2-methyl- and 2-deuterium-2-methyl EPA produced a biphasic effect, i.e. potentiation and inhibition at low (250 mg/day kg body weight) and higher doses (600-1300 mg/day kg body weight), respectively. A23187-induced platelet aggregation was affected in a similar way by feeding the 2-substituted EPA derivatives, except that 2-deuterium-2-methyl EPA had no effect relative to EPA itself and that the inhibition was far greater than that for ADP-induced aggregation (approximately 100% inhibition with 600 mg 2-methyl-EPA/day kg body weight). The ranking order of the EPA-derivatives to affect platelet aggregation and to cause hypolipidemia was different, suggesting different mechanisms. Our observations suggest that the effects of the EPA derivatives on platelet aggregation could be related to the degree of bulkiness around C2 and that an asymmetric substitution at C2 caused inhibition of platelet aggregation while a symmetric substitution did not. It is suggested that the bulky, asymmetric derivatives inhibit platelet aggregation by altering platelet membrane phospholipid packing.

Alpha- and beta- alkyl-substituted eicosapentaenoic acids: incorporation into phospholipids and effects on prostaglandin H synthase and 5-lipoxygenase

Alpha-ethyl-, alpha-methyl- and beta-methyl eicosapentaenoic acid (EPA) were prepared and their incorporation into cell lipids and effects on eicosanoid synthesis compared with EPA and docosahexaenoic acid (DHA). alpha- and beta-methyl EPA were incorporated into hepatocyte triacylglycerols as efficiently as EPA, whereas lesser amounts were found in phospholipids. alpha-ethyl EPA was not incorporated into phospholipids but small amounts were detected in triacylglycerol. All derivatives inhibited the synthesis of arachidonic acid, although less efficiently than EPA and DHA. The derivatives were poor substrates of prostaglandin H (PGH) synthase and 5-lipoxygenase, and they all inactivated PGH synthase. In isolated platelets, alpha-methyl EPA was a stronger inhibitor of TxB2 production than EPA, alpha-ethyl- and beta-methyl EPA. All derivatives were stronger inducers of peroxisomal beta-oxidation than EPA and DHA. This increased induction probably is a consequence of the blocked mitochondrial beta-oxidation of the derivatives.

Tetradecylthioacetic acid inhibits the oxidative modification of low density lipoprotein and 8-hydroxydeoxyguanosine formation in vitro

Oxidative modification of low-density lipoprotein (LDL) is thought to play a key role in the formation of foam cells and in initiation and progression of atherosclerotic plaque. The hypolipidemic 3-thia fatty acids contain a sulfur atom and might therefore possess reducing (antioxidant) properties. Consequently, the effects of 3-thia fatty acids on the susceptibility of LDL particles to undergo oxidative modification in vitro were studied. Tetradecylthioacetic acid (TTA), incorporated into the LDL particle and increased the lag time of copper ion induced LDL oxidation in a dose-dependent manner, 80 mumol/L TTA reduced the generation of lipid peroxides during copper ion induced LDL oxidation (for 2 hours) by 100%, 2,2'-azobis-(2,4-dimethylvaleronitrile) induced LDL oxidation by 64%, and 2,2'-azobis-(2-amidinopropane hydrochloride) induced LDL oxidation (for 6 hours) by 21%. The electrophoretic mobility of the oxidized LDL was reduced by TTA in both copper ion and azo-compounds initiated oxidation. This fatty acid analogue was effectively able to reduce in a dose dependent manner the formation of 8-hydroxydeoxyguanosine from 2-deoxyguanosine with ascorbic acid as the radical producer. TTA bound copper(II) ions and did not reduce copper(II) to copper(I). It failed to scavenge the 1,1-diphenyl-2-picrylhydrazyl radicals. The results suggest that the modification of LDL in the lipid and protein moieties can be significantly reduced by TTA. This acid may exert its antioxidant effect partially through metal ion binding and through free radical scavenging.

On the interrelationship between hepatic carnitine, fatty acid oxidation, and triglyceride biosynthesis in nephrosis

The nephrotic syndrome is associated with disturbances in plasma lipid pattern and metabolism. However, the reason for these perturbations is poorly understood. In the present study, we have investigated hepatic triglyceride metabolism in puromycin aminonucleoside-induced nephrotic syndrome in rats. Nephrotic rats displayed a 70% increase in hepatic triglyceride levels compared to controls (16.9 +/- 1.6 vs. 9.8 +/- 0.6 mumol/g liver; means +/- SEM, P < 0.01). The capacity for hepatic mitochondrial beta-oxidation of fatty acids was substantially elevated (80%). This was associated with a rise in the liver content of the fatty acid carrier carnitine (1.24 +/- 0.06 vs. 0.85 +/- 0.07 mumol/g dry weight, P < 0.05). A positive correlation between the levels of acetylcarnitine and acetyl-CoA was found in normal as well as in nephrotic rats, implying that carnitine plays an important role as an acetyl group acceptor in the liver under normo- and hyperlipidemic conditions. Changes in carnitine levels seem to be tightly coupled to the rate of fatty acid oxidation. There was a significant elevation in the activity of phosphatidate phosphohydrolase (E.C. 3.1.3.4) in liver microsomes from nephrotic rats (1.07 +/- 0.09 vs. 0.81 +/- 0.04 nmol/min.mg protein, P < 0.02). Hepatic very low density lipoprotein (VLDL)-triglyceride secretion rate was 18% higher in nephrotic rats than in controls. The results demonstrate a deranged hepatic triglyceride metabolism in nephrosis, with an increased hepatic triglyceride biosynthesis, a sizable accumulation of hepatic triglycerides, and only a modest increase in VLDL triglyceride secretion. In addition, mitochondrial beta-oxidation of fatty acids was enhanced, associated with an increased availability of carnitine.

Thia fatty acids, metabolism and metabolic effects

(1) The chemical properties of thia fatty acids are similar to normal fatty acids, but their metabolism (see below: points 2-6) and metabolic effects (see below: points 7-15) differ greatly from these and are dependent upon the position of the sulfur atom. (2) Long-chain thia fatty acids and alkylthioacrylic acids are activated to their CoA esters in endoplasmatic reticulum. (3) 3-Thia fatty acids cannot be beta-oxidized. They are metabolized by extramitochondrial omega-oxidation and sulfur oxidation in the endoplasmatic reticulum followed by peroxisomal beta-oxidation to short sulfoxy dicarboxylic acids. (4) 4-Thia fatty acids are beta-oxidized mainly in mitochondria to alkylthioacryloyl-CoA esters which accumulate and are slowly converted to 2-hydroxy-4-thia acyl-CoA which splits spontaneously to an alkylthiol and malonic acid semialdehyde-CoA ester. The latter presumably is hydrolyzed and metabolized to acetyl-CoA and CO2. (5) Both 3- and 4-thiastearic acid are desaturated to the corresponding thia oleic acids. (6) Long-chain 3- and 4-thia fatty acids are incorporated into phospholipids in vivo, particularly in heart, and in hepatocytes and other cells in culture. (7) Long-chain 3-thia fatty acids change the fatty acid composition of the phospholipids: in heart, the content of n-3 fatty acids increases and n-6 fatty acids decreases. (8) 3-Thia fatty acids increase fatty acid oxidation in liver through inhibition of malonyl-CoA synthesis, activation of CPT I, and induction of CPT-II and enzymes of peroxisomal beta-oxidation. Activation of fatty acid oxidation is the key to the hypolipidemic effect of 3-thia fatty acids. Also other lipid metabolizing enzymes are induced. (9) Fatty acid- and cholesterol synthesis is inhibited in hepatocytes. (10) The nuclear receptors PPAR alpha and RXR alpha are induced by 3-thia fatty acids. (11) The induction of enzymes and of PPAR alpha and RXR alpha are increased by dexamethasone and counteracted by insulin. (12) 4-Thia fatty acids inhibit fatty acid oxidation and induce fatty liver in vivo. The inhibition presumably is explained by accumulation of alkylthioacryloyl-CoA in the mitochondria. This metabolite is a strong inhibitor of CPT-II. (13) Alkylthioacrylic acids inhibits both fatty acid oxidation and esterification. Inhibition of esterification presumably follows accumulation of extramitochondrial alkylthioacryloyl-CoA, an inhibitor of microsomal glycerophosphate acyltransferase. (14) 9-Thia stearate is a strong inhibitor of the delta 9-desaturase in liver and 10-thia stearate of dihydrosterculic acid synthesis in trypanosomes. (15) Some attempts to develop thia fatty acids as drugs are also reviewed.

Long-term effect of tetradecylthioacetic acid: a study on plasma lipid profile and fatty acid composition and oxidation in different rat organs

Administration of tetradecylthioacetic acid (a 3-thia fatty acid) increases mitochondrial and peroxisomal beta-oxidative capacity and carnitine palmitoyltransferase activity, but reduces free fatty acid and triacylglycerol levels in plasma compared to palmitic acid-treated rats and controls. The decrease in plasma triacylglycerol was accompanied by a reduction (56%) in VLDL-triacylglycerol. Prolonged supplementation of tetradecylthioacetic acid caused a significant increase in lipogenic enzyme activities (ATP-citrate lyase and acetyl-CoA carboxylase) and diacylglycerol acyltansferase, but did not affect phosphatidate phosphohydrolase. Plasma cholesterol, LDL- and HDL-cholesterol levels were reduced. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity was, however, stimulated in 3-thia fatty acid-treated rats compared to controls. In addition. the mRNAs of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and LDL-receptor were increased. Tetradecylthioacetic acid administration affected the fatty acid composition in plasma and liver by increasing the amount of monoenes, especially 18:1(n-9), mostly at the expense of omega-3 fatty acids. Compared to liver a large amount of tetradecylthioacetic acid accumulated in the heart, and this accumulation was accompanied by an increase in omega-3 fatty acids, particularly 22:6(n-3) and a decrease in omega-6 fatty acids, mainly 20:4(n-6). The results show that the hypolipidemic effect of tetradecylthioacetic acid is sustained after prolonged administration and may, at least in part, be due to increased fatty acid oxidation and upregulated LDL-receptor gene expression. The increase in lipogenic enzyme activities as well as increased 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, may be compensatory mechanisms to maintain cellular integrity. Decreased level of 20:4(n-6) combined with increased omega-3/omega-6 ratio in cardiac tissue after tetradecylthioacetic acid treatment may have influence on membrane dynamics and function.

Fatty acyl-CoA oxidase activity is induced before long-chain acyl-CoA hydrolase activity and acyl-CoA binding protein in liver of rat treated with peroxisome proliferating 3-thia fatty acids

1. In this study we explored the relationship between specific acyl-CoA esters and induction of acyl-CoA binding protein (ACBP) and enzymes related to the proliferation of peroxisomes. Male Wistar rats were administered a single dose (150 mg/day/kg) of sulphur-substituted fatty acid analogues, and the effects of tetradecylthioacetic acid and 3-thiadicarboxylic acid, which both act as peroxisome proliferators, were compared with the effects of tetradecylthiopropionic acid and palmitic acid which do not induce peroxisome proliferation. 2. The hepatic level of total long-chain acyl-CoA was significantly increased within 12 h of feeding these fatty acids, except in rat fed tetradecylthioacetic acid. Hplc chromatograms of liver extracts prepared from rat fed tetradecylthioacetic acid showed that tetradecylthioacetyl-CoA ester accumulated in the liver 4 h after feeding and had disappeared after 24 h. In liver extracts of the tetradecylthiopropionic acid-treated rat tetradecylthiopropionyl-CoA was not observed, but the appearance of a new long-chain acyl-CoA ester, probably a metabolite of tetradecylthiopropionic acid, was detected. This new peak reached a maximum 4h after feeding. In rat fed tetradecylthioacetic acid and 3-thiadicarboxylic acid the hepatic level of fatty acyl-CoA oxidase mRNA increased 8 h after feeding, while the acyl-CoA oxidase activity had increased after 12 h. 3. The early accumulation of specific tetradecylthioacetyl-CoA suggests that this ester may be a possible mediator of the induction of fatty acyl-CoA oxidase. The level of hepatic acyl-CoA binding protein, long-chain acyl-CoA hydrolase activity and long-chain acyl-CoA synthetase activity did not change after a single dose of all four fatty acids. Prolonged administration of 3-thia fatty acids resulted, however, in a dose- and time-dependent increase in hepatic ACBP content and ACBP mRNA level. The amount of ACBP increased in parallel to the long-chain acyl-CoA hydrolase activity. The correlated induction of fatty acyl-CoA binding protein and long-chain acyl-CoA hydrolase seems to be dependent on a sustained accumulation of total long-chain acyl-CoA esters.

Fatty acid activation of peroxisome proliferator-activated receptor (PPAR)

Peroxisome proliferators such as clofibric acid, nafenopin, and WY-14,643 have been shown to activate peroxisome proliferator-activated receptor (PPAR), a member of the steroid nuclear receptor superfamily. We have cloned the cDNA from rat that is homologous to that from mouse, which encodes a 97% similar protein. To search for physiologically occurring activators, we established a transcriptional transactivation assay by stably expressing in CHO cells a chimera of rat PPAR and the human glucocorticoid receptor that activates expression of the placental alkaline phosphatase reporter gene under the control of the mouse mammary tumor virus promoter. 150 microM concentrations of arachidonic or linoleic acid but not of dehydroepiandrosterone, cholesterol, or 25-hydroxy-cholesterol, activated the receptor chimera. In addition, saturated fatty acids induced the reporter gene. Shortening the chain length to n = 6 or introduction of an omega-terminal carboxylic group abolished the activation potential of the fatty acid. To test whether a common PPAR binding metabolite might be formed from free fatty acids we tested the effects of differentially beta-oxidizable fatty acids and inhibitors of fatty acid metabolism. The peroxisomal proliferation-inducing, non-beta-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY-14,643, whereas the homologous beta-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, beta-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to beta-oxidation of the acyl-CoA ester.

Acute modulation of rat hepatic lipid metabolism by sulphur-substituted fatty acid analogues

A single oral dose of two 3-thia (3-thiadicarboxylic and tetradecylthioacetic acids) and of 4-thia (tetradecylthiopropionic acid) fatty acids were administered to normolipidemic rats and their effects on lipid metabolism over a 24 hr period were studied. All three thia fatty acids could be detected in plasma 2 hr after treatment. Tetradecylthioacetic and tetradecylthiopropionic acids were detected in different hepatic lipid fractions but were incorporated mainly into hepatic phospholipids. Two hours after administration hepatic mitochondrial beta-oxidation and the total liver level of long-chain fatty acyl-CoA increased with a concomitant decrease in saturated fatty acids, total hepatic malonyl-CoA and plasma triacylglycerol levels in the 3-thia fatty acid groups. Tetradecylthiopropionic acid administration caused a decrease in mitochondrial beta-oxidation and an increase in plasma triacylglycerol at 24 hr. The activities of key lipogenic enzymes were unaffected in all treatment groups. Plasma cholesterol level was reduced only at 8 hr in 3-thiadicarboxylic acid treated rats although 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase was suppressed already at 2, 4, 8 and 12 hr. The results show that thia fatty acids are rapidly absorbed and are systemically available after oral administration but the 3-thia fatty acids reached systemic circulation more slowly and less completely than the 4-thia fatty acid. Very low levels of the thia fatty acids are detected in plasma 24 hr after a single administration. They are incorporated into all hepatic lipid classes, especially phospholipids. Rapid incorporation of a non beta-oxidizable thia fatty acid into hepatic lipids may cause a diversion of other fatty acids from glycerolipid biosynthesis to mitochondrial beta-oxidation. Stimulation of mitochondrial beta-oxidation and suppression of HMG-CoA reductase are primary events, occurring within hours, after 3-thia fatty acid administration. The hypotriglyceridemic effect of the 3-thia fatty acids observed at 2-4 hr is independent of the activities of key lipogenic and triacylglycerol synthesising enzymes.

Hepatic fatty acid metabolism as a determinant of plasma and liver triacylglycerol levels. Studies on tetradecylthioacetic and tetradecylthiopropionic acids

To investigate the importance of factors influencing substrate availability for triacylglycerol biosynthesis on lipoprotein metabolism, the effects of two opposite-acting sulphur-substituted fatty acid analogues, tetradecylthioacetic acid and tetradecylthiopropionic acid were studied. Administration of tetradecylthioacetic acid to rats resulted in a reduction of plasma levels of triacylglycerols (44%) and cholesterol (26%). This was accompanied by a reduction in very-low-density lipoprotein (VLDL) triacylglycerols (48%), VLDL cholesterol (36%), low-density lipoprotein (LDL) cholesterol (36%) and high-density lipoprotein (HDL) triacylglycerols (50%), whereas HDL cholesterol levels did not change. Subsequently, the HDL/LDL-cholesterol ratio increased by 40%. The cholesterol-lowering effect was accompanied by a reduction in hydroxymethylglutaryl CoA (HMG-CoA) reductase activity (37%). Both mitochondrial and peroxisomal fatty acid oxidation increased (1.7-fold and 5.3-fold, respectively). Furthermore, there was a significant negative correlation between plasma triacylglycerols and mitochondrial fatty acid oxidation. Hepatic triacylglycerol synthesis was retarded, as indicated by a decrease in VLDL triacylglycerol secretion (40%), and by a reduced liver triacylglycerol content (29%). The activities of lipoprotein lipase and hepatic lipase in post-heparin plasma were not affected. Microsomal and cytosolic phosphatidate phosphohydrolase activities were inhibited (28% and 70%, respectively). Hepatic malonyl-CoA levels decreased by 29% and the total activity of acetyl-CoA carboxylase was reduced (23%). In hepatocytes treated with tetradecylthioacetic acid, mitochondrial fatty acid oxidation increased markedly (100%) and triacylglycerol secretion was reduced (40%). In tetradecylthiopropionic-acid-treated rats, a significant increase in both plasma and VLDL triacylglycerols was found (46% and 72%, respectively) but VLDL triacylglycerol secretion was unaffected. However, no effect on either plasma or lipoprotein cholesterol levels was seen. Mitochondrial fatty acid oxidation was decreased by 50% and hepatic triacylglycerol levels increased by 33%. In hepatocytes exposed to tetradecylthiopropionic acid, triacylglycerol synthesis increased (100%) while triacylglycerol secretion and fatty acid oxidation remained unaltered. The results illustrate that lipoprotein triacylglycerol levels can be modulated by changes in the availability of fatty acid substrate for triacylglycerol biosynthesis, mainly by affecting mitochondrial fatty acid oxidation. In addition, we demonstrate that suppression of rat hepatic HMG-CoA reductase activity during treatment with tetradecylthioacetic acid may contribute to a cholesterol-lowering effect.

Coordinate induction of hepatic fatty acyl-CoA oxidase and P4504A1 in rat after activation of the peroxisome proliferator-activated receptor (PPAR) by sulphur-substituted fatty acid analogues

1. In the liver of rat fed a single dose of 3-thia fatty acids, 3-dithiahexadecanedioic acid (3-thiadicarboxylic acid) and tetradecylthioacetic acid, steady-state levels of P4504A1 and fatty acyl-CoA oxidase mRNAs increased in parallel. The increases were significant 8 h after administration, reaching a maximum after 12 h and decreased from 12 to 24 h after administration. 2. The corresponding enzyme activities of P4504A1 and fatty acyl-CoA oxidase were also induced in a parallel manner by the 3-thia fatty acids. The enzyme activities were significantly increased 12 h after administration and increased further after 24 h. This may reflect a possible effect of the 3-thia fatty acids not only on mRNA levels, but also on the translation and degradation rate of the two enzymes. 3. Repeated administration of 3-thia fatty acids resulted in an increase of the specific P4504A1 protein accompanied with an increased lauric acid hydroxylase activity. The correlation between induction of P4504A1 and fatty acyl-CoA oxidase mRNAs and their enzyme activities may reflect a coordinated rather than a causative induction mechanism, and that these genes respond to a common signal. This suggests that the increased P450 activity may not be responsible or be a prerequisite for fatty acyl-CoA oxidase induction. 4. Since the peroxisome proliferator-activated receptor (PPAR) plays a role in mediating the induction of fatty acyl-CoA oxidase, we analysed the activation of PPAR by fatty acids and sulphur-substituted analogues utilizing a chimera between the N-terminal and DNA-binding domain of the glucocorticoid receptor and the putative ligand-binding domain of PPAR. Arachidonic acid activated this chimeric receptor in Chinese hamster ovary cells. Inhibitors of P450 did not affect the activation of PPAR by arachidonic acid. Furthermore, dicarboxylic acids including 1,12-dodecanedioic acid or 1,16-hexadecanedioic acid only weakly activated the chimera. 3-Thidicarboxylic acid, however, was a much more effective activator than the non-sulphur-substituted analogues. In conclusion, the data suggest that the most likely mechanism of the induction process is fatty acid-induced activation of PPAR, which then leads to a coordinated induction of P4504A1 and fatty acyl-CoA oxidase.

Gas chromatographic measurement of 3- and 4-thia fatty acids incorporated into various classes of rat liver lipids during feeding experiments

A practical procedure is described for the quantitative measurement of the amount of acyl units derived from tetradecylthioacetic acid (effecting hypolipemia in rats) and tetradecylthiopropionic acid (effecting hyperlipidemia). The procedure involves three main successive steps: (1) extraction; (2) solid-phase lipid class separation yielding free fatty acids, phospholipids, triacylglycerides, cholesterol esters, and diacylglycerides without crosscontamination; and (3) gas chromatography of hydrolyzed lipids derivatized to picolinyl esters, combined with unambiguous identification by gas chromatography-mass spectrometry. The overall recoveries of heptadecanoyl lipids added as internal standards during extraction were 94-96%, except for cholesteryl heptadecanoate where the recovery was 60% owing to incomplete hydrolysis. Recoveries of thia fatty acids from samples spiked with these compounds were 95%. Flame-ionization response factors were found to be 0.92 and 0.81 for the tetradecylthioacetic acid and tetradecylthiopropionic acid picolinyl esters, respectively, compared to that of heptadecanoic acid. The lower limit of quantitation was 25 pmol as injected. Measurement of the amount of thia fatty acyl units in rat plasma and in liver lipids 4 h after administration of single doses by gastric intubation indicated efficient absorbtion and rapid incorporation into liver lipids, particularly in the phospholipid fraction. Both plasma clearance and channelling into lipids was slower for the 4-thia fatty acid.

Effects of perfluorooctanoic acid--a potent peroxisome proliferator in rat--on Morris hepatoma 7800C1 cells, a rat cell line

In this study, Morris hepatoma 7800C1 cells (from rat) were exposed to 500 microM perfluorooctanoic acid (PFOA) in the culture medium for 7 days. This treatment resulted in inductions of catalase, lauroyl-CoA oxidase (which catalyzes the first step in peroxisomal beta-oxidation) and of cytochrome P-450IVA (specialized for omega- and omega-1 hydroxylation of fatty acids). Northern blot analysis revealed that the level of mRNA for peroxisomal fatty acyl-CoA oxidase was enhanced in cells treated with PFOA. Inductions of the enzymes mentioned above are generally connected with peroxisome proliferation in vivo. This work also includes a comparison between the activities of catalase, lauroyl-CoA oxidase, DT-diaphorase and glutathione transferase in rat liver homogenate and 7800C1 cells in order to investigate to what extent this cell line differs from the situation in vivo. The findings suggest that the cells selectively lost most of their peroxisomes during transformation into a cell line and subsequent propagation. The control activities of catalase and lauroyl-CoA oxidase (marker enzymes for peroxisomes) were only about 2% of the corresponding enzyme activities in rat liver. In addition, a morphological study revealed that the frequency of peroxisomes in 7800C1 cells is very low. The control activity of glutathione transferase in 7800C1 cells was 11% of the corresponding activity in rat liver homogenate, whereas the level of DT-diaphorase was virtually the same in 7800C1 cells as in rat liver. Electron microscopic investigation of the control cultures revealed all signs of viable cells, with well-developed cell organelles. Treatment of 7800C1 cells with 500 microM PFOA has little effect on cellular morphology.

Overview of coenzyme A metabolism and its role in cellular toxicity

Coenzyme A (CoASH) has a clearly defined role as a cofactor for a number of oxidative and biosynthetic reactions in intermediary metabolism. Formation of acyl-CoA thioesters from organic carboxylic acids activates the acid for further biotransformation reactions and facilitates enzyme recognition. Xenobiotic carboxylic acids can also form CoA-thioesters, and the resulting acyl-CoA may contribute to the compound's toxicity. Generation of an unusual or poorly-metabolized acyl-CoA from a xenobiotic may lead to cellular metabolic dysfunction through several types of mechanisms including: (1) inhibition of key metabolic enzymes by the acyl-CoA; (2) sequestration of the total cellular CoA pool as the unusual acyl-CoA; (3) physical-chemical effects of the acyl-CoA; and (4) sequestration and depletion of carnitine as the acyl group is transformed from the acyl-CoA to form the corresponding acylcarnitine. Many of these toxicities are similar to sequelae observed in the inherited organic acidurias in which endogenously-generated acyl-CoAs accumulate secondary to an enzymopathy. Insights into the cellular mechanisms of xenobiotic acyl-CoA accumulation have been derived from model systems developed to understand organic acidemias, such as the methylmalonyl-CoA accumulation of the methylmalonic acidurias. The relevance of acyl-CoA accretion to human pathophysiology has now been well established, and identification of the relevant mechanism of toxicity can allow implementation of strategies to minimize the metabolic injury. Additionally, recognition of the potential for acyl-CoA mediated xenobiotic injury should result in improved rational drug design and earlier recognition of such toxicity when it develops.

In vitro evidence for involvement of CoA thioesters in peroxisome proliferation and hypolipidaemia

The mechanisms of peroxisomal induction and hypolipidaemia caused by treatment with peroxisome proliferators, such as nafenopin and clofibrate, remain to be elucidated. Proposed mechanisms include receptor-mediated processes or adaptations resulting from disruption of hepatic lipid metabolism. The latter mechanism was investigated in a series of in vitro studies. Incubation of primary rat hepatocytes with various carboxyl-containing compounds revealed no clear common factor which imparted potency as a peroxisomal inducer. Inhibitors of fatty acyl-CoA synthetase, norepinephrine and desulpho-CoA, however, decreased the level of peroxisomal induction by nafenopin in rat hepatocytes, suggesting that activation of carboxyl-containing compounds to their CoA thioesters may be a necessary step in initiating peroxisome proliferation. Coenzyme A thioesters of nafenopin, clofibric acid and other carboxyl-containing chemicals were synthesised and found to inhibit the activity of acetyl-CoA carboxylase to varying degrees. The CoA thioester of nafenopin was the most potent inhibitor among this group (Ki = 1.45 x 10(-5) M), but weaker than palmitoyl-CoA (Ki = 2.22 x 10(-6) M), the feedback inhibitor of acetyl-CoA carboxylase. Hypolipidaemia caused by treatment with peroxisome proliferators may, therefore, be related to inhibition of fatty-acid synthesis by the corresponding CoA thioester derivative.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Structural and metabolic requirements for activators of the peroxisome proliferator-activated receptor

Fatty acids have recently been demonstrated to activate peroxisome proliferator-activated receptors (PPARs) but specific structural requirements of fatty acids to produce this response have not yet been determined. Importantly, it has hitherto not been possible to show specific binding of these compounds to PPAR. To test whether a common PPAR binding metabolite might be formed, we tested the effects of long-chain omega-3 polyunsaturated fatty acids, differentially beta-oxidizable fatty acids and inhibitors of fatty acid metabolism. We determined the activation of a reporter gene by a chimaeric receptor encompassing the DNA binding domain of the glucocorticoid receptor and the ligand binding domain of PPAR. The omega-3 unsaturated fatty acids were slightly more potent PPAR activators in vitro than saturated fatty acids. The peroxisomal proliferation-inducing, non-beta-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY 14,643, whereas the homologous beta-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, beta-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to beta-oxidation of the acyl-CoA ester. These data should aid understanding of signal transduction via PPAR and the identification of a receptor ligand.