Regulation of fatty acid oxidation and triglyceride and phospholipid metabolism by hypolipidemic sulfur-substituted fatty acid analogues

Changes in Plasma Pyruvate and TCA Cycle Metabolites upon Increased Hepatic Fatty Acid Oxidation and Ketogenesis in Male Wistar Rats

Altered hepatic mitochondrial fatty acid β-oxidation and associated tricarboxylic acid (TCA) cycle activity contributes to lifestyle-related diseases, and circulating biomarkers reflecting these changes could have disease prognostic value. This study aimed to determine hepatic and systemic changes in TCA-cycle-related metabolites upon the selective pharmacologic enhancement of mitochondrial fatty acid β-oxidation in the liver, and to elucidate the mechanisms and potential markers of hepatic mitochondrial activity. Male Wistar rats were treated with 3-thia fatty acids (e.g., tetradecylthioacetic acid (TTA)), which target mitochondrial biogenesis, mitochondrial fatty acid β-oxidation, and ketogenesis predominantly in the liver. Hepatic and plasma concentrations of TCA cycle intermediates and anaplerotic substrates (LC-MS/MS), plasma ketones (colorimetric assay), and acylcarnitines (HPLC-MS/MS), along with associated TCA-cycle-related gene expression (qPCR) and enzyme activities, were determined. TTA-induced hepatic fatty acid β-oxidation resulted in an increased ratio of plasma ketone bodies/nonesterified fatty acid (NEFA), lower plasma malonyl-CoA levels, and a higher ratio of plasma acetylcarnitine/palmitoylcarnitine (C2/C16). These changes were associated with decreased hepatic and increased plasma pyruvate concentrations, and increased plasma concentrations of succinate, malate, and 2-hydroxyglutarate. Expression of several genes encoding TCA cycle enzymes and the malate-oxoglutarate carrier (Slc25a11), glutamate dehydrogenase (Gdh), and malic enzyme (Mdh1 and Mdh2) were significantly increased. In conclusion, the induction of hepatic mitochondrial fatty acid β-oxidation by 3-thia fatty acids lowered hepatic pyruvate while increasing plasma pyruvate, as well as succinate, malate, and 2-hydroxyglutarate.

Hepatic steatosis induced in C57BL/6 mice by a non-ß oxidizable fatty acid analogue is associated with reduced plasma kynurenine metabolites and a modified hepatic NAD+/NADH ratio

Background

Non-alcoholic fatty liver disease is often associated with obesity, insulin resistance, dyslipidemia, and the metabolic syndrome in addition to mitochondrial dysfunction and nicotinamide adenine dinucleotide (NAD⁺) deficiency. The aim of this study was to investigate how inhibition of mitochondrial fatty acid oxidation using the compound tetradecylthiopropionic acid (TTP) would affect hepatic triacylglycerol level and plasma levels of kynurenine (Kyn) metabolites and nicotinamide.

Methods

12 C57BL/6 mice were fed a control diet, or an intervention diet supplemented with 0.9% (w/w) tetradecylthiopropionic acid for 14 days. Blood and liver samples were collected, enzyme activities and gene expression were analyzed in liver, in addition to fatty acid composition. Metabolites in the tryptophan/kynurenine pathway and total antioxidant status were measured in plasma.

Results

Dietary treatment with tetradecylthiopropionic acid for 2 weeks induced fatty liver accompanied by decreased mitochondrial fatty acid oxidation. The liver content of the oxidized form of NAD⁺ was increased, as well as the ratio of NAD⁺/NADH, and these changes were associated by increased hepatic mRNA levels of NAD synthetase and nicotinamide mononucleotide adenyltransferase-3. The downstream metabolites of kynurenine were reduced in plasma whereas the plasma nicotinamide content was increased. Some effects on inflammation and oxidative stress was observed in the liver, while the plasma antioxidant capacity was increased. This was accompanied by a reduced plasma ratio of kynurenine/tryptophan. In addition, a significant decrease in the inflammation-related arachidonic fatty acid in liver was observed.

Conclusion

Fatty liver induced by short-time treatment with tetradecylthiopropionic acid decreased the levels of kynurenine metabolites but increased the plasma levels of NAD⁺ and nicotinamide. These changes are most likely not associated with increased inflammation and oxidative stress. Most probably the increase of NAD⁺ and nicotinamide are generated through the Preiss Handler pathway and/or salvage pathway and not through the de novo pathway.

The take home message is that non-alcoholic fatty liver disease is associated with the metabolic syndrome in addition to mitochondrial dysfunction and nicotinamide adenine dinucleotide (NAD⁺) deficiency. Inducing fatty liver in mice by inhibition of fatty acid oxidation resulted in a concomitant change in kynurenine metabolites increasing the plasma levels of nicotinamides and the hepatic NAD⁺/NADH ratio, probably without affecting the de novo pathway of kynurenines.

Enhanced phosphorylation of AMPK by lutein and oxidised lutein that lead to mitochondrial biogenesis in hyperglycemic HepG2 cells

The stimulation of adenosine monophosphate-activated protein kinase (AMPK) is a prime target to decrease the hyperglycemic condition, hence it is a lutein (L) and oxidised lutein (OXL) is a target molecule for the treatment of type II diabetes. In the current study, a plausible interaction of L and OXL with AMPK was investigated by molecular docking. In addition, the effect of L and OXL for the activation of AMPK that triggers the downstream regulator peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), TFAM expression, mitochondrial DNA (mtDNA), mitochondrial biogenesis and superoxide dismutase 2 (SOD2) in high glucose treated HepG2 cells were investigated by quantitative polymerase chain reaction and Western blot analysis. Molecular docking reveals higher binding affinity of L (ΔG = -6.3 kcal/mol) and OXL (ΔG = -15.5 kcal/mol) with AMPK, compared with metformin (ΔG = -5.0 kcal/mol). The phosphorylation of AMPK increased by 1.3- and 1.5-fold with L and OXL treatment, respectively, in high glucose induced HepG2 cells. The activation of PGC-1α is significant (P < 0.05) in OXL group than L. Similarly, TFAM expression is increased with L and OXL compared with the high glucose group. Further increase in SOD2 and mtDNA, confirms the efficacy of L and OXL in restoring the mitochondrial biogenesis in high glucose induced cells through AMPK, PGC-1α, and TFAM.

Associations between fatty acid oxidation, hepatic mitochondrial function, and plasma acylcarnitine levels in mice

Background

The 4-thia fatty acid tetradecylthiopropionic acid (TTP) is known to inhibit mitochondrial β-oxidation, and can be used as chemically induced hepatic steatosis-model in rodents, while 3-thia fatty acid tetradecylthioacetic acid (TTA) stimulates fatty acid oxidation through activation of peroxisome proliferator activated receptor alpha (PPARα). We wished to determine how these two compounds affected in vivo respiration and mitochondrial efficiency, with an additional goal to elucidate whether mitochondrial function is reflected in plasma acylcarnitine levels.

Methods

C57BL/6 mice were divided in 4 groups of 10 mice and fed a control low-fat diet, low-fat diets with 0.4% (w/w) TTP, 0.4% TTA or a combination of these two fatty acids for three weeks (n = 10). At sacrifice, β-oxidation and oxidative phosphorylation (OXPHOS) capacity was analysed in fresh liver samples. Hepatic mitochondria were studied using transmission electron microscopy. Lipid classes were measured in plasma, heart and liver, acylcarnitines were measured in plasma, and gene expression was measured in liver.

Results

The TTP diet resulted in hepatic lipid accumulation, plasma L-carnitine and acetylcarnitine depletion and elevated palmitoylcarnitine and non-esterified fatty acid levels. No significant lipid accumulation was observed in heart. The TTA supplement resulted in enhanced hepatic β-oxidation, accompanied by an increased level of acetylcarnitine and palmitoylcarnitine in plasma. Analysis of mitochondrial respiration showed that TTP reduced oxidative phosphorylation, while TTA increased the maximum respiratory capacity of the electron transport system. Combined treatment with TTP and TTA resulted in a profound stimulation of genes involved in the PPAR-response and L-carnitine metabolism, and partly prevented triacylglycerol accumulation in the liver concomitant with increased peroxisomal β-oxidation and depletion of plasma acetylcarnitines. Despite an increased number of mitochondria in the liver of TTA + TTP fed mice, the OXPHOS capacity was significantly reduced.

Conclusion

This study indicates that fatty acid β-oxidation directly affects mitochondrial respiratory capacity in liver. As plasma acylcarnitines reflected the reduced mitochondrial β-oxidation in TTP-fed mice, they could be useful tools to monitor mitochondrial function. As mitochondrial dysfunction is a major determinant of metabolic disease, this supports their use as plasma markers of cardiovascular risk in humans. Results however indicate that high PPAR activation obscures the interpretation of plasma acylcarnitine levels.

Electronic supplementary material

The online version of this article (10.1186/s12986-018-0241-7) contains supplementary material, which is available to authorized users.

Effect of Dietary Processed Sulfur Supplementation on Water-holding Capacity, Color, and Lipid Profiles of Pork

This study was performed to investigate the effect of dietary processed sulfur supplementation on water-holding capacity, color, and lipid profiles of pork according to the level of dietary processed sulfur (0%, CON; 0.3%, S). The pigs were slaughtered at an average final weight of 120 kg, and the longissimus dorsi muscles were collected from the carcasses. As results, pork processed with sulfur had significantly higher moisture and ash contents compared to those of CON but lower crude fat, pH, expressible drip, lower redness and yellowness, and greater lightness. Pork processed with sulfur showed significantly lower total lipid content, triglycerides, and atherosclerosis index but significantly higher high-density lipoprotein cholesterol. Feeding processed sulfur significantly lowered myristic acid, heptadecanoic acid, and stearic acid contents, whereas monounsaturated fatty acids and oleic acids were significantly higher compared to those in the CON. Higher amounts of polyunsaturated fatty acids and n-6 fatty acids were observed in the pork processed with sulfur than that of the CON. Therefore, supplementing pigs with dietary sulfur improved nutrient and meat quality.

Tetradecylthiopropionic acid induces hepatic mitochondrial dysfunction and steatosis, accompanied by increased plasma homocysteine in mice

BACKGROUND

Hepatic mitochondrial dysfunction plays an important role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Methyl donor supplementation has been shown to alleviate NAFLD, connecting the condition to the one-carbon metabolism. Thus, the objective was to investigate regulation of homocysteine (Hcy) and metabolites along the choline oxidation pathway during induction of hepatic steatosis by the fatty acid analogue tetradecylthiopropionic acid (TTP), an inhibitor of mitochondrial fatty acid oxidation.

METHODS

Mice were fed a control diet, or diets containing 0.3 %, 0.6 %, or 0.9 % (w/w) TTP for 14 days. Blood and liver samples were collected, enzyme activities and gene expression were analyzed in liver, lipid and fatty acid composition in liver and plasma, one-carbon metabolites, B-vitamin status, carnitine and acylcarnitines were analyzed in plasma.

RESULTS

Liver mitochondrial fatty acid oxidation decreased by 40 % and steatosis was induced in a dose dependent manner; total lipids increased 1.6-fold in animals treated with 0.3 % TTP, 2-fold with 0.6 % TTP and 2.1 fold with 0.9 % TTP compared to control. The higher hepatic concentration of fatty acids was associated with shortening of carbon-length. Furthermore, the inhibited fatty acid oxidation led to a 30-fold decrease in plasma carnitine and 9.3-fold decrease in acetylcarnitine at the highest dose of TTP, whereas an accumulation of palmitoylcarnitine resulted. Compared to the control diet, TTP administration was associated with elevated plasma total Hcy (control: 7.2 ± 0.3 umol/L, 0.9 % TTP: 30.5 ± 5.9 umol/L) and 1.4-1.6 fold increase in the one-carbon metabolites betaine, dimethylglycine, sarcosine and glycine, accompanied by changes in gene expression of the different B-vitamin dependent pathways of Hcy and choline metabolism. A positive correlation between total Hcy and hepatic triacylglycerol resulted.

CONCLUSIONS

The TTP-induced inhibition of mitochondrial fatty acid oxidation was not associated with increased hepatic oxidative stress or inflammation. Our data suggest a link between mitochondrial dysfunction and the methylation processes within the one-carbon metabolism in mice.

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.

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.

The metabolic effects of thia fatty acids in rat liver depend on the position of the sulfur atom

The effects on oxidation and composition of fatty acids in rat liver were compared after administration of fatty acids with sulfur substituted in different positions. It has been hypothesized that drugs with hydrophobic backbone have lipid-lowering effects because they are not easily catabolized by mitochondrial beta-oxidation. Thia fatty acids cannot be beta-oxidized when sulfur is in 3-position, but beta-oxidation is possible when sulfur is positioned further from the carboxyl group. To investigate whether catabolism of thia fatty acids would affect their ability to influence lipid metabolism, a series of thia fatty acids were synthesized and administered by oral gavage to male Wistar rats (300 mg/kg bodyweight/day for 7 days). Depending on the position of the sulfur atom and the chain length, the thia fatty acids were beta-oxidized, desaturated and/or elongated, and the accumulated amounts were lower as the sulfur atom were positioned further from the carboxyl group. All thia fatty acids led to high peroxisomal beta-oxidation of endogenous fatty acids, whereas the mitochondrial beta-oxidation was high when sulfur was in 3-position, low when sulfur was in 4-position and similar to controls when sulfur was in 5- or 7-position. The changes in hepatic fatty acid composition were more pronounced when sulfur was positioned close to the carboxyl group. In conclusion, both the position of the sulfur atom and the chain length appear to determine the catabolic fate of thia fatty acids, and the non-beta-oxidizable thia fatty acids were most potent in regulating oxidation and composition of endogenous fatty acids in rat liver.

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.

Thermally oxidized dietary fat upregulates the expression of target genes of PPAR alpha in rat liver

Oxidized fats affect animal metabolism in several ways. To gain a comprehensive understanding of the molecular mechanisms underlying the effects of dietary oxidized fats in rats at varying dietary vitamin E concentrations, the gene expression profile of the liver was monitored with an array containing 1176 binding sites for cDNAs. Rats were fed diets with a fresh fat and vitamin E concentrations of 25 or 250 mg alpha-tocopherol/kg (FF25, FF250 rats) or a fat heated at 50 degrees C for 38 d, with vitamin E concentrations of 25 or 250 mg alpha-tocopherol/kg (OF25, OF250 rats) for 63 d. Differences in gene expression were considered to be significant at a ratio of at least 1.4. In the OF25 rats, the expression of 47 genes was altered; in the OF250 rats, the expression of 37 genes was altered, and in the FF250 rats, the expression of 21 genes was altered compared with FF25 rats. In both OF25 and OF250 rats, a series of target genes of the peroxisome proliferator-activated receptor alpha (PPAR alpha) was upregulated. Determination of gene expression of acyl CoA oxidase and activity of catalase confirmed that oxidized fats caused peroxisome proliferation in the liver. In OF25 and OF250 rats, there was also upregulation of 12 and 5 genes involved in xenobiotic metabolism and stress response, of 7 and 7 genes involved in protein metabolism, of 5 and 2 genes encoding intracellular effectors or modulators and of 5 and 6 genes, respectively, encoding activators or repressors of transcription or translation. In conclusion, this study provides indirect evidence that dietary oxidized fats cause an activation of the PPAR alpha, irrespective of the dietary vitamin E concentration. Identification of several other differentially regulated genes may be helpful to understand the effects of oxidized fats on animal metabolism.

Evidence for selective induction of phosphoenolpyruvate carboxykinase gene expression by unsaturated and nonmetabolized fatty acids in adipocytes

Polyunsaturated fatty acids (PUFAs) and 3-thia fatty acids are hypolipidemic and decrease insulin resistance in Type II diabetic animals. To exert such an action, these FAs could decrease adipose tissue lipolysis or increase esterification. Glyceroneogenesis is an important metabolic pathway in adipocytes for re-esterification of FAs originating from lipolysis and in hepatocytes for triacylglycerol synthesis during fasting. Cytosolic phosphoenolpyruvate carboxykinase (PEPCK) plays a key role in this pathway. Here we show that the PUFA docosahexaenoic acid (DHA) stimulates PEPCK mRNA in glucose-deprived adipose tissue explants from fed rats and in 3T3-F442A differentiated adipocytes. This effect is maximum at 3 h, stable up to at least 11 h of treatment, and affects the transcription of the gene. PEPCK mRNA half-life is not affected. Among a series of adipocyte transcripts, only the adipocyte lipid binding protein mRNA is also increased by DHA, although later than the PEPCK mRNA and at a much lower extent. DHA has no effect on PEPCK gene expression in the H4IIE hepatoma cells in which this gene is responsive to other inducers like cAMP. This lack of effect is not due to a failure of DHA to act in H4IIE cells since it induces the carnitine palmitoyltransferase 1 (CPT-1) mRNA. Therefore, the DHA effect appears to be cell-selective. Results of experiments using either tetradecylthio acetic acid and alpha-bromopalmitate, two nonmetabolized Fas, or a series of inhibitors of FA metabolism show that the FA effect on PEPCK mRNA is not due to a product of its metabolism. Hence, polyunsaturated and nonmetabolized FAs stimulate adipose PEPCK, therefore potentially enhancing glyceroneogenesis and reducing FA output. This mechanism could participate in the hypolipidemic action of PUFAs.

Nuclear import of factors involved in signaling is inhibited in C3H/10T1/2 cells treated with tetradecylthioacetic acid

The non-beta-oxidisable tetradecylthioacetic acid (TTA) is incorporated into cellular membranes when C3H/10T1/2 cells are cultured in TTA-containing medium. We here demonstrate that this alteration in cellular membranes affect the nuclear translocation of proteins involved in signal transduction. Analysis of cellular fatty acid composition shows that TTA and TTA:1n-8 constitute approximately 40 mol% of total fatty acids in cellular/nuclear membranes. Activation of c-fos expression is significantly inhibited in TTA-treated cells but the enzymatic activation of mitogen activated protein kinase (ERK) is not affected. Immunofluorescence and confocal microscopy studies demonstrate that in mitogene-stimulated TTA-treated cells, the translocation of phosphorylated ERK1/2, protein kinase C alpha (PKC alpha), and PKC beta(1) from the cytoplasm into the nucleus is considerably decreased and delayed. Concomitant with a decreased nuclear import, ERK1/2 dephosphorylation is decreased in TTA-treated cells. There is no TTA-induced inhibition of nuclear import of proteins with a classical nuclear localization signal (NLS), as seen by in vitro nuclear import experiments of BSA fused to the NLS from SV40 large T, or in vivo studies of hnRNP A1 nuclear import. The expression levels of Importin alpha, Importin beta, Importin 7, and NTF2 are not altered in the TTA-treated cells. Taken together, our data indicate that TTA treatment causes changes in cellular fatty acid composition that negatively affect NLS-independent mechanisms of protein translocation through the nuclear pore complex.

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.

Cyclic fatty acid monomers from heated oil modify the activities of lipid synthesizing and oxidizing enzymes in rat liver

Cyclic fatty acid monomers purified from a heated linseed oil were given for 2 wk to adult rats as triacylglycerol at two dose levels, i.e., 0.1 and 1 g/100 g diet, to determine their effect on some aspects of lipid metabolism. Indirect evidence of a peroxisome proliferator-like effect was observed, as determined by an elevation of some characteristic enzyme activities, such as peroxisomal acyl-CoA oxidase, and the microsomal omega- but also (omega-1)-laurate hydroxylase (CYP4A1 and CYP2E1, respectively). The dietary cyclic fatty acids induced a coordinated regulation between the activities of the lipogenic enzymes studied (Delta9-desaturase, phosphatidate phosphohydrolase) and peroxisomal oxidation, but not with mitochondrial beta-oxidation. The dose-dependent decrease of Delta9-desaturase activity (P < 0.05) with cyclic fatty acid monomer intake was accompanied by a similar decrease of the monounsaturated fatty acid level in liver. The increase in the gamma-linolenic acid level also suggested an increase in Delta6-desaturase activity with cyclic fatty acid intake (P < 0.05). In addition, our results strongly suggested that the altered liver levels of eicosapentaenoic and arachidonic acids were due to the peroxisomal retroconversion process in rats fed cyclic acids. Finally, an effect of these cyclic compounds on the carbohydrate metabolism cannot be disregarded because they decreased liver glycogen concentration. We conclude that cyclic fatty acid monomers affect different aspects of lipid metabolism, including a phenotypic peroxisome proliferator response. This provides the ground for further studies investigating the biochemical pathways that underlie the nutritional effect of such molecules.

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.

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.

Polyunsaturated thia- and oxa-fatty acids: incorporation into cell-lipids and their effects on arachidonic acid- and eicosanoid synthesis

EPA, DHA, C15SCH2COOH (n-3), C15SCH2COOH (n-6) and C18SCH2COOH (n-3) are extensively incorporated into phospholipids and triacylglycerol in rat hepatocytes after 24 h incubation with 80 microM fatty acid/derivative. Only traces of polyunsaturated 3-oxa fatty acids (C15OCH2COOH, C18OCH2COOH) were incorporated. C15-S-butyric acid (n-3) is a stronger inhibitor of delta6-desaturase in rat liver-microsomes than C15SCH2COOH (n-3), C15-S-propionic acid (n-3), EPA and DHA. It inhibits delta5-desaturase in a similar manner to EPA and DHA. Arachidonic acid and C15SCH2COOH, (n-6) are better substrates for PGH-synthase than EPA and C15SCH2COOH, (n-3), showing the inhibitory effect of the n-3 bond. The n-3 polyunsaturated fatty acids, including the sulfur-substituted fatty acid derivatives, are poor substrates for PGH-synthase. However, they inactivate the PGH-synthase activity at least as efficiently as arachidonic acid. C15SCH2COOH (n-3), C15S(CH2)2COOH (n-3) and C18SCH2COOH (n-3) induce peroxisomal beta-oxidation more than EPA and DHA.

Eicosapentaenoic acid and sulphur substituted fatty acid analogues inhibit the proliferation of human breast cancer cells in culture

Numerous studies have shown dietary fatty acids to influence the progression of several types of cancers. The purpose of the present investigation was to examine the influence of various types of fatty acids, including omega-3 fatty acids and a new class of hypolipidemic peroxisome proliferating fatty acid analogues, namely the 3-thia fatty acids, on MCF-7 human breast cancer cell growth. 3-thia fatty acids represent non-beta-oxidizable fatty acid analogues in which a sulphur atom substitutes for the beta-methylene group (3-position) in the saturated and unsaturated fatty acids. The effects of increasing concentrations of palmitic acid, tetradecylthioacetic acid (a 3-thia fatty acid), eicosapentaenoic acid, docosahexaenoic acid, and two 3-thia polyunsaturated fatty acids on the proliferation of MCF-7 cells, maintained in serum-free culture, were studied. At the highest concentration of fatty acid used (64 microM) tetradecylthioacetic acid was found to be the most effective of all fatty acids tested in inhibiting cell growth, whilst palmitic acid and docosahexaenoic acid had no significant effect on cell growth. Thus, of the two dietary polyunsaturated omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid, only eicosapentaenoic acid possesses an inhibitory effect on the proliferation of MCF-7 cells. In all cases the inhibitory effect of the fatty acid was found to be reversible. Tetradecylthioacetic acid has been shown to be a potent peroxisome proliferator. It was, therefore, hypothesized that tetradecylthioacetic acid may inhibit the human MCF-7 cell growth by increasing the level of oxidative stress within the cell. However, use of agents which modify the cell's protective apparatus against oxidative stress had no influence on the inhibitory effect of tetradecylthioacetic acid. These experiments indicate that tetradecylthioacetic acid inhibits cell growth by mechanisms which may be independent of oxidative status.

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.

Eicosapentaenoic acid, but not docosahexaenoic acid, increases mitochondrial fatty acid oxidation and upregulates 2,4-dienoyl-CoA reductase gene expression in rats

The aim of the present study was to investigate whether eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) was responsible for the triglyceride-lowering effect of fish oil. In rats fed a single dose of EPA as ethyl ester (EPA-EE), the plasma concentration of triglycerides was decreased at 8 h after acute administration. This was accompanied by an increased hepatic fatty acid oxidation and mitochondrial 2,4-dienoyl-CoA reductase activity. The steady-state level of 2,4-dienoyl-CoA reductase mRNA increased in parallel with the enzyme activity. An increased hepatic long-chain acyl-CoA content, but a reduced amount of hepatic malonyl-CoA, was obtained at 8 h after acute EPA-EE treatment. On EPA-EE supplementation, both EPA (20:5n-3) and docosapentaenoic acid (DPA, 22:5n-3) increased in the liver, whereas the hepatic DHA (22:6n-3) concentration was unchanged. On DHA-EE supplementation retroconversion to EPA occurred. No statistically significant differences were found, however, for mitochondrial enzyme activities, malonyl-CoA, long-chain acyl-CoA, plasma lipid levels, and the amount of cellular fatty acids between DHA-EE treated rats and their controls at any time point studied. In cultured rat hepatocytes, the oxidation of [1-14C]palmitic acid was reduced by DHA, whereas it was stimulated by EPA. In the in vivo studies, the activities of phosphatidate phosphohydrolase and acetyl-CoA carboxylase were unaffected after acute EPA-EE and DHA-EE administration, but the fatty acyl-CoA oxidase, the rate-limiting enzyme in peroxisomal fatty acid oxidation, was increased after feeding these n-3 fatty acids. The hypocholesterolemic properties of EPA-EE may be due to decreased 3-hydroxy-3-methylglutaryl-CoA reductase activity. Furthermore, replacement of the ordinary fatty acids, i.e., the monoenes (16:1n-7, 18:1n-7, and 18:1n-9) with EPA and some conversion to DPA concomitant with increased fatty acid oxidation is probably the mechanism leading to changed fatty acid composition. In contrast, DHA does not stimulate fatty acid oxidation and, consequently, no such displacement mechanism operates. In conclusion, we have obtained evidence that EPA, and not DHA, is the fatty acid primarily responsible for the triglyceride-lowering effect of fish oil in rats.

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.

Effects of non-beta-oxidizable sulfur-substituted fatty acid analogues on synthesis and secretion of triacylglycerol and cholesterol in cultured rat hepatocytes

The mechanisms behind the hypolipidemic effect of two sulfur-substituted fatty acid analogues, 3-thiadicarboxylic acid and tetradecylthioacetic acid, have been investigated in cultured hepatocytes. There was a dose-dependent reduction in incorporation of [3H] water into triacylglycerol and diacylglycerol when tetradecylthioacetic acid was added to rat hepatocytes cultured in the presence of 200 muM oleic acid. Tetradecylthioacetic acid also increased the oxidation of [14C]palmitic acid compared to oleic acid, inhibited the incorporation of radiolabeled precursors into diacylglycerol to a greater extent than into triacylglycerol, and reduced the secretion of triacylglycerol more than its synthesis. A stimulation, rather than a reduction, in glycerolipid synthesis and secretion by oxidation of fatty acids and reduces the synthesis and secretion of glycerolipids. 3-Thiadicarboxylic acid reduces the synthesis and secretion of both glycerolipids and cholesterol to approximately the same extent without a concomitant increase in the oxidation of fatty acids.

Effect of long-chain mono-unsaturated and n-3 polyunsaturated fatty acids on postprandial blood and liver lipids in rats

The effects on blood and liver lipids after feeding rats with concentrated fractions from fish oil consisting of mono-unsaturated fatty acids (80% C20:1 and 22:1) or n-3 polyunsaturated fatty acids (85% C20:5 and 22:6 n-3) were examined. Mono-unsaturated fat had no effects on plasma triacylglycerol, total cholesterol phospholipids or unesterified fatty acid as compared to controls (lard). However, n-3 polyunsaturated fatty acid-fed animals showed a significant decrease in plasma triacylglycerol (74%), phospholipids (40%) and unesterified fatty acids (52%). The concentrated fractions had no effects on liver lipids. While the n-3 diet increased peroxisomal beta-oxidation 2.5-fold, there was only a slight increase with the mono-unsaturated diet. The fatty acid composition in plasma and liver phospholipids was changed with the various diets; 20:4 n-6 was significantly reduced in plasma and liver with the mono-unsaturated diet, and with the n-3 diet in liver. The mono-unsaturated diet, and especially the n-3 diet, increased the 20:5 n-3 level in both plasma and liver. Our results indicate that long-chain mono-unsaturated fatty acids in fish oil do not change the levels of plasma lipids. The beneficial role of fish oil on the level of blood lipids, may therefore be mostly attributed to the effects of long-chain n-3 fatty acids. However, the low 20:4 n-6 and high 20:5 n-3 levels in plasma and liver phospholipids with the concentrated mono-unsaturated fatty acid diet may be of importance for a favourable haemostatic balance with regard to cardiovascular diseases.

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.

A study on lipid metabolism in heart and liver of cholesterol- and pectin-fed rats

Pectin is known as a cholesterol-reducing dietary fibre, and in the present study we addressed the question whether pectin affected the quantity of lipid in droplets in the myocardial cells and of lipid in the liver cells. Male Wistar rats received either a diet containing cholesterol or a standard diet without cholesterol with 0, 50 or 100 g pectin/kg incorporated for 10 d. The fractional volume of lipid droplets in the myocardial cells decreased as a function of pectin dose in both the standard-fed and the cholesterol-fed rats. Serum cholesterol was significantly reduced in both groups after addition of 100 g pectin/kg diet. The cholesterol diet increased the liver cholesterol level, and 100 g pectin/kg diet resulted in a lower concentration of liver cholesterol in the cholesterol-fed animals, but the influence on standard-fed rats was modest. Hydroxymethylglutaryl-CoA reductase (EC 1.1.1.88; HMG-CoA reductase) activity increased when pectin was given in the standard diet. Liver triacylglycerol level increased after cholesterol and pectin feeding. Mitochondrial fatty acid oxidation and phosphatidate phosphohydrolase (EC 3.1.3.4) activity tended to decrease, whereas the peroxisomal fatty acid oxidation and acyl-CoA oxidase activity were unchanged. Increased hepatic triacylglycerol content by cholesterol and pectin treatment may be due to inhibited mitochondrial fatty acid oxidation along with increased availability of fatty acid for esterification and triacylglycerol synthesis. The presence of pectin in the diets of cholesterol-fed rats resulted in increased hepatic concentration of triacylglycerols and increased mitochondrial fatty acid oxidation. In this case the hepatic accumulation of triacylglycerol may be mediated by a reduced efflux of triacylglycerols from the liver.

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.

Effect of 3-thiadicarboxylic acid on lipid metabolism in experimental nephrosis

The effect of the sulfur-substituted fatty acid analogue 1,10 bis(carboxymethylthio)decane, also known as 3-thiadicarboxylic acid, on puromycin aminonucleoside-induced nephrotic hyperlipidemia was studied in rats. Treatment with 3-thiadicarboxylic acid (250 mg/kg) for 5 days reduced plasma levels of triglycerides from 5.8 to 2.7 mmol/L and cholesterol from 11.0 to 7.7 mmol/L. This was accounted for by decreases in very-low-density lipoprotein triglycerides, very-low-density lipoprotein cholesterol, and low-density lipoprotein cholesterol, without any major changes in the composition of plasma lipoproteins. The activities of two enzymes involved in fatty acid synthesis (ATP:citrate lyase and fatty acid synthetase) were inhibited by 3-thiadicarboxylic acid treatment, whereas acetyl-coenzyme A carboxylase activity was unchanged. In contrast, treatment with the sulfur-substituted fatty acid analogue induced the peroxisomal beta-oxidation of fatty acids ninefold and the mitochondrial beta-oxidation by 54% to 73%, depending on the substrate used. This was accompanied by a 26% reduction in hepatic triglyceride secretion rate. The hepatic phosphatidate phosphohydrolase activity was unchanged. 3-Thiadicarboxylic acid treatment suppressed the activity of the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, by 58%, whereas hepatic LDL receptor expression was unaltered. The activities of lipoprotein lipase and hepatic lipase were unchanged by treatment. These results demonstrated that treatment with 3-thiadicarboxylic acid ameliorates hyperlipidemia in experimental nephrosis primarily by decreasing the overproduction of very-low-density lipoprotein present. The data also indicate that hepatic very-low-density lipoprotein synthesis and secretion is strongly influenced by the availability of the fatty acid substrate under the same hyperlipidemic conditions.

Modulation of phosphatidylcholine biosynthesis by peroxisome proliferating fatty acid analogues

The modulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthesis by sulfur-substituted fatty acid analogues has been investigated in rats. We have compared the effects of two non-beta-oxidizable fatty acid analogues, 3-thiadicarboxylic acid and tetradecylthioacetic acid, which induce proliferation of peroxisomes, with those of the analogue tetradecylthiopropionic acid, which is a weak peroxisome proliferator. Repeated administration of 3-thiadicarboxylic acid for seven days resulted in increased hepatic concentrations of both PC and PE, but the PC/PE ratio was decreased. PC synthesis was increased, as evidenced by increased incorporation of [3H]choline into PC and an increased activity of cytidinetriphosphate (CTP): phosphocholine cytidylyltransferase. This was accompanied by a reduction in the pool sizes of choline and phosphocholine. The S-adenosylmethione/S-adenosylhomocysteine ratio (AdoMet/AdoHcy) was marginally affected, indicating no increase in the rate of methylation of PE to PC. Administration of tetradecylthioacetic acid also resulted in increased hepatic phospholipid levels, increased AdoMet/AdoHcy ratios and in slightly elevated activity of CTP:phosphocholine cytidylyltransferase. The most striking effect observed after tetradecylthiopropionic acid treatment was the development of fatty liver. The activity of CTP:phosphocholine cytidylyltransferase and the incorporation of [3H]choline into PC was reduced compared to 3-thiadicarboxylic acid treatment. Although the rate of methylation of PE seemed to be increased at an elevated AdoMet/AdoHcy ratio, this resulted in only minor changes in the hepatic PC and PE levels, and the PC/PE ratio remained unchanged. Furthermore, the hepatic levels of choline and phosphocholine were reduced in these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on triglyceride metabolism: phosphatidate phosphohydrolase from guinea pig harderian gland

The guinea pig Harderian gland, located in the orbit, is characterized by a high production of lipids. However, little is known about the regulation of the metabolic pathways involved. In the present paper the properties of guinea pig Harderian gland phosphatidate phosphohydrolase, a key enzyme in triglyceride biosynthesis, was investigated. The enzyme was present both in the cytosolic and the microsomal fraction from the gland. Cytosolic phosphatidate phosphohydrolase was purified by hydroxylapatite chromatography. The enzyme was dependent on magnesium ions and inhibited in the presence of fluoride. The dependence on the substrate phosphatidate was investigated and the apparent Km for phosphatidate was about 0.6 mM. Phosphatidate phosphohydrolase activity was influenced by different phospholipids. As is the case for the rat liver enzyme, phosphatidylethanolamine was found to stimulate the enzyme activity. The results indicate that Harderian gland phosphatidate phosphohydrolase has similar properties as the corresponding enzyme from rat liver, suggesting that it may be of regulatory importance.

The hypotriglyceridemic effect of eicosapentaenoic acid in rats is reflected in increased mitochondrial fatty acid oxidation followed by diminished lipogenesis

The effect of eicosapentaenoic acid (EPA) on fatty acid oxidation and on key enzymes of triglyceride metabolism and lipogenesis was investigated in the liver of rats. Repeated administration of EPA to normolipidemic rats resulted in a time-dependent decrease in plasma triglycerides, phospholipids and cholesterol. The triglyceride-lowering effect was observed after one day of feeding whereas lowering of plasma cholesterol and phospholipids was observed after five days of treatment. The triglyceride content of liver was reduced after two-day treatment. At that time, increased mitochondrial fatty acid oxidation occurred whereas mitochondrial and microsomal glycerophosphate acyltransferase was inhibited. The phosphatidate phosphohydrolase activity was unchanged. Adenosine triphosphate:citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase and glucose-6-phosphate dehydrogenase were inhibited during the 15 d of EPA treatment whereas peroxisomal beta-oxidation was increased. At one day of feeding, however, when the hypotriglyceridemic effect was established, the lipogenic enzyme activities were reduced to the same extent in palmitic acid-treated animals as in EPA-treated rats. In cultured rat hepatocytes, the oxidation of [14C]palmitic acid to carbon dioxide and acid-soluble products was stimulated in the presence of EPA. These results suggest that the instant hypolipidemia in rats given EPA could be explained at least in part by a sudden increase in mitochondrial fatty acid oxidation, thereby reducing the availability of fatty acids for lipid synthesis in the liver for export, e.g., in the form of very low density lipoproteins, even before EPA induced peroxisomal fatty acid oxidation, reduced triglyceride biosynthesis and diminished lipogenesis.

The hypocholesterolemic effect of sulfur-substituted fatty acid analogues in rats fed a high carbohydrate diet

Sulfur-substituted fatty acid analogues have been administered to rats fed a high carbohydrate diet, and the effect on plasma and hepatic lipid metabolism was investigated. Two of the analogues studied, 3-thiadicarboxylic acid and tetradecylthioacetic acid, reduced the plasma cholesterol level significantly, whereas the effect on plasma triacylglycerol level was only marginal. 3-Thiadicarboxylic acid was the most potent, decreasing the cholesterol level faster and at a lower dose than tetradecylthioacetic acid. The relative effects on plasma cholesterol and triacylglycerol levels were different from what have been observed in rats fed a conventional pellet diet. Tetradecylthiopropionic acid had no hypocholesterolemic effect. The activities of three lipogenic enzymes: ATP-citrate lyase, acetyl-CoA carboxylase and fatty acid synthase was measured. The two hypocholesterolemic analogues reduced the activities of these enzymes in a coordinated manner. The enzyme activities was found to correlate with the the plasma cholesterol level, indicating a coordinated regulation of these enzymes and cholesterol synthesis or secretion. The effect on two enzymes involved in cholesterol metabolism was also studied. The activity of acyl-CoA:cholesterol acyltransferase (ACAT) was reduced by the two hypocholesterolemic analogues, in contrast to the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase, which tended to increase. The cholesterol lowering effect of 3-thiadicarboxylic acid and tetradecylthioacetic acid can probably be ascribed to diminished cholesterol synthesis due to a reduced availability of acetyl-CoA. A reduction in the esterification of hepatic cholesterol may be a contributing factor.

Early effects on mitochondrial and peroxisomal beta-oxidation by the hypolipidemic 3-thia-fatty acids in rat livers

A single administration of 3-thiadicarboxylic and tetradecylthioacetic acids stimulates both mitochondrial and peroxisomal beta-oxidation and lowers plasma triacylglycerol levels. An increased rate of mitochondrial beta-oxidation and carnitine palmitoyl-transferase activity was established after 3 h and this was accompanied by a lowering of plasma triacylglycerol. Peroxisomal beta-oxidation, however, remained unchanged up to 8 h and was significantly increased after 12 h. These results suggest that after a single administration of 3-thia fatty acids mitochondrial beta-oxidation precedes peroxisomal beta-oxidation. Furthermore, they show that the observed tricylglycerol-lowering effect, which is established early (3-4 h) after the administration of 3-thia fatty acids, is initially due to an increased mitochondrial beta-oxidation.

Evidence for mechanisms of the hypotriglyceridemic effect of n-3 polyunsaturated fatty acids

Ingestion of modest amounts of n-3 polyunsaturated fatty acids (PUFA) (2.8% w/w) decreased plasma triglyceride levels in Syrian hamsters by 49%. This was associated with a 45% increase in hepatic carnitine palmitoyl transferase activity. Significantly, at this low level of n-3 PUFA intake, hepatic peroxisomal oxidation measured as CN- insensitive palmitoyl-CoA dependent NAD reduction was unaffected. Consumption of increasing amounts of dietary n-3 PUFA up to 2% (w/w) in hamster diets containing 15% fat, gradually decreased plasma triglycerides to 56% of the control levels. The diet induced changes in plasma triglyceride levels were highly correlated (r = -0.97, P less than 0.01) with changes in hepatic carnitine palmitoyl transferase activity. A gradual decrease up to 59% in hepatic phosphatidate phosphohydrolase activity with n-3 fatty acid consumption was also observed. The hypotriglyceridemic effects of moderate intakes of n-3 fatty acids are, therefore, associated with changes in key enzymes in hepatic triglyceride synthesis and mitochondrial oxidation, but not peroxisomal oxidation.

Relationship between translocation of long-chain acyl-CoA hydrolase, phosphatidate phosphohydrolase and CTP:Phosphocholine cytidylyltransferase and the synthesis of triglycerides and phosphatidylcholine in rat liver

Translocation of long-chain acyl-coenzyme A hydrolase from the microsomal fraction to the cytosolic fraction was promoted in cell-free extracts of rat liver by palmitic acid, oleic acid, tetradecylthioacetic acid, and tetradecylthiopropionic acid, and by their CoA esters. The CoA esters were more effective than the non-esterified acids in the translocation of the enzyme. Treatment of normolipidemic rats with sulfur-substituted non-beta-oxidizable fatty acid analogues resulted in a transitory increase in hepatic concentration of long-chain acyl-CoA. Longer feeding times almost normalized the hepatic long-chain acyl-CoA content. Microsomal long-chain acyl-CoA hydrolase activity was inhibited, whereas the activity of the cytosolic form was stimulated. The rise in enzyme activity coincided with a reduction in liver content of triglyceride and an increase in hepatic phospholipid content. The results suggest that the activity of long-chain acyl-CoA hydrolase in the cytosol may control the amount of acyl-CoA thioesters in the liver. Esterified and non-esterified fatty acids caused in vitro translocation of phosphatidate phosphohydrolase and cytidine 5'-triphosphate (CTP):phosphocholine cytidylyltransferase from the cytosolic fraction to the microsomal fraction. However, the translocation of these two enzyme systems was not obtained in vivo. The activity of phosphatidate phosphohydrolase decreased in microsomal and cytosolic fractions while the activity of cytidylyltransferase in these fractions increased. The activities of soluble phosphatidate phosphohydrolase and long-chain acyl-CoA hydrolase appeared to be inversely correlated.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of oxygen and sulfur-substituted fatty acids on serum lipids and mitochondrial and peroxisomal fatty acid oxidation in rat

Feeding tetradecyloxyacetic acid (a 3-oxa fatty acid) to rats led to decreased serum cholesterol and decreased serum triacylglycerol, resembling the effects of the corresponding 3-thia fatty acid (tetradecylthioacetic acid). The 3-oxa fatty acid inhibited strongly the mitochondrial fatty acid oxidation and led to the development of fatty liver, while the 3-thia fatty acid stimulated the mitochondrial fatty acid oxidation. Feeding tetradecyloxypropionic acid (a 4-oxa fatty acid) had less effect on the serum lipids. It stimulated fatty acid oxidation in the mitochondria and lowered the hepatic level of triacylglycerol. The corresponding 4-thia fatty acid (tetradecylthiopropionic acid) inhibited mitochondrial fatty acid oxidation and induced development of fatty liver. All these compounds, both the oxa and the thia fatty acids, induced some increase in the activity of the peroxisomal acyl-CoA oxidase. Repeated administration of 3-oxadicarboxylic acid to rats resulted in no lipid lowering effects, and marginal changes of fatty acyl-CoA oxidase activity. Oxidation of the S-atom of the 3-thia fatty acid to the corresponding sulfoxide or sulfone eliminated the metabolic effects of the thia fatty acid. The study has shown that the effects of 3- and 4-oxa fatty acids are in some ways opposite to those of the 3- and 4-thia fatty acids. The possibility that the lipophilicity of the fatty acid analogues may be an important factor behind the differences observed are discussed. It is suggested that these oxa- and thia-analogues of fatty acids may be useful in studies on the regulation of fatty acid metabolism.

Effect of n-3 fatty acids on the key enzymes involved in cholesterol and triglyceride turnover in rat liver

The effect of long-chain n-3 fatty acids on hepatic key enzymes of cholesterol metabolism and triglyceride biosynthesis was investigated in two rat models. In the first model, rats were intravenously infused for two weeks with a fat emulsion containing 20% of triglycerides in which either n-6 or n-3 fatty acids predominated. The treatment with n-3 fatty acids led to a reduction primarily of serum cholesterol (45%), but also of serum triglycerides (18%). HMG-CoA reductase activity and cholesterol 7 alpha-hydroxylase activity were reduced by 45% and 36%, respectively. There were no significant effects on diacylglycerol acyltransferase (DGAT) or phosphatidate phosphohydrolase (PAP) activities. In the second model, rats were fed a diet enriched with sucrose, coconut oil and either sunflower oil (n-6 fatty acids) or fish oil (long-chain n-3 fatty acid ethyl esters). The treatment with n-3 fatty acids decreased serum triglycerides (41%) and, to a lesser extent, serum cholesterol (17%). Neither glycerol 3-phosphate acyltransferase (GPAT) or DGAT were affected by n-3 fatty acids. In contrast, PAP activity was reduced by 26%. HMG-CoA reductase was not significantly affected, whereas cholesterol 7 alpha-hydroxylase activity was reduced by 36%. The results indicate that part of the TG-lowering effect of long-chain n-3 fatty acids may be mediated by inhibition of the soluble phosphatidate phosphohydrolase. The effect on serum cholesterol may be partly due to inhibition of HMG-CoA reductase.