Stimulation of fatty acid oxidation by a 3-thia fatty acid reduces triacylglycerol secretion in cultured rat hepatocytes

Reduced lipid and glucose oxidation and reduced lipid synthesis in AMPKα2-/- myotubes

Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) plays a crucial role in regulation of metabolic homeostasis. To understand the role of the catalytic α2 subunit of AMPK in skeletal muscle energy metabolism, myotube cultures were established from AMPKα2+/+ and AMPKα2-/- mice. Myotubes from AMPKα2-/- mice had lower basal oleic acid and glucose oxidation compared to myotubes from AMPKα2+/+ mice. However, the relative response to mitochondrial uncoupling was increased for oleic acid oxidation. Incorporation of acetate into lipids was also lower in myotubes from AMPKα2-/- mice. Proteomics analysis revealed that AMPKα2-/- myotubes had upregulated pathways related to mitochondrial function and fatty acid oxidation, and decreased pathways related to fatty acid biosynthesis. In conclusion, ablation of AMPKα2 catalytic subunit in skeletal muscle cells resulted in reduced basal oxidation of glucose and fatty acids, however upregulated pathways related to mitochondrial function and fatty acid oxidation and reduced lipid formation.

Human HDL subclasses modulate energy metabolism in skeletal muscle cells

In addition to its antiatherogenic role, HDL reportedly modulates energy metabolism at the whole-body level. HDL functionality is associated with its structure and composition, and functional activities can differ between HDL subclasses. Therefore, we studied if HDL2 and HDL3, the two major HDL subclasses, are able to modulate energy metabolism of skeletal muscle cells. Differentiated mouse and primary human skeletal muscle myotubes were used to investigate the influences of human HDL2 and HDL3 on glucose and fatty uptake and oxidation. HDL-induced changes in lipid distribution and mRNA expression of genes related to energy substrate metabolism, mitochondrial function, and HDL receptors were studied with human myotubes. Additionally, we examined the effects of apoA-I and discoidal, reconstituted HDL particles on substrate metabolism. In mouse myotubes, HDL subclasses strongly enhanced glycolysis upon high and low glucose concentrations. HDL3 caused a minor increase in ATP-linked respiration upon glucose conditioning but HDL2 improved complex I-mediated mitochondrial respiration upon fatty acid treatment. In human myotubes, glucose metabolism was attenuated but fatty acid uptake and oxidation were markedly increased by both HDL subclasses, which also increased mRNA expression of genes related to fatty acid metabolism and HDL receptors. Finally, both HDL subclasses induced incorporation of oleic acid into different lipid classes. These results, demonstrating that HDL subclasses enhance fatty acid oxidation in human myotubes but improve anaerobic metabolism in mouse myotubes, support the role of HDL as a circulating modulator of energy metabolism. Exact mechanisms and components of HDL causing the change, require further investigation.

The roles of DGAT1 and DGAT2 in human myotubes are dependent on donor patho-physiological background

The roles of DGAT1 and DGAT2 in lipid metabolism and insulin responsiveness of human skeletal muscle were studied using cryosections and myotubes prepared from muscle biopsies from control, athlete, and impaired glucose regulation (IGR) cohorts of men. The previously observed increases in intramuscular triacylglycerol (IMTG) in athletes and IGR were shown to be related to an increase in lipid droplet (LD) area in type I fibers in athletes but, conversely, in type II fibers in IGR subjects. Specific inhibition of both diacylglycerol acyltransferase (DGAT) 1 and 2 decreased fatty acid (FA) uptake by myotubes, whereas only DGAT2 inhibition also decreased fatty acid oxidation. Fatty acid uptake in myotubes was negatively correlated with the lactate thresholds of the respective donors. DGAT2 inhibition lowered acetate uptake and oxidation in myotubes from all cohorts whereas DGAT1 inhibition had no effect. A positive correlation between acetate oxidation in myotubes and resting metabolic rate (RMR) from fatty acid oxidation in vivo was observed. Myotubes from athletes and IGR had higher rates of de novo lipogenesis from acetate that were normalized by DGAT2 inhibition. Moreover, DGAT2 inhibition in myotubes also resulted in increased insulin-induced Akt phosphorylation. The differential effects of DGAT1 and DGAT2 inhibition suggest that the specialized role of DGAT2 in esterifying nascent diacylglycerols and de novo synthesized FA is associated with synthesis of a pool of triacylglycerol, which upon hydrolysis results in effectors that promote mitochondrial fatty acid oxidation but decrease insulin signaling in skeletal muscle cells.

Knockdown of sarcolipin (SLN) impairs substrate utilization in human skeletal muscle cells

BACKGROUND

Recent studies have highlighted that uncoupling of sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) by sarcolipin (SLN) increases ATP consumption and contributes to heat liberation. Exploiting this thermogenic mechanism in skeletal muscle may provide an attractive strategy to counteract obesity and associated metabolic disorders. In the present study, we have investigated the role of SLN on substrate metabolism in human skeletal muscle cells.

METHODS AND RESULTS

After generation of skeletal muscle cells with stable SLN knockdown (SLN-KD), cell viability, glucose and oleic acid (OA) metabolism, mitochondrial function, as well as gene expressions were determined. Depletion of SLN did not influence cell viability. However, glucose and OA oxidation were diminished in SLN-KD cells compared to control myotubes. Basal respiration measured by respirometry was also observed to be reduced in cells with SLN-KD. The metabolic perturbation in SLN-KD cells was reflected by reduced gene expression levels of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) and forkhead box O1 (FOXO1). Furthermore, accumulation of OA was increased in cells with SLN-KD compared to control cells. These effects were accompanied by increased lipid formation and incorporation of OA into complex lipids. Additionally, formation of complex lipids and free fatty acid from de novo lipogenesis with acetate as substrate was enhanced in SLN-KD cells. Detection of lipid droplets using Oil red O staining also showed increased lipid accumulation in SLN-KD cells.

CONCLUSIONS

Overall, our study sheds light on the importance of SLN in maintaining metabolic homeostasis in human skeletal muscle. Findings from the current study suggest that therapeutic strategies involving SLN-mediated futile cycling of SERCA might have significant implications in the treatment of obesity and associated metabolic disorders.

The small molecule SERCA activator CDN1163 increases energy metabolism in human skeletal muscle cells

Background and objective

A number of studies have highlighted muscle-specific mechanisms of thermogenesis involving futile cycling of Ca²⁺ driven by sarco (endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and generating heat from ATP hydrolysis to be a promising strategy to counteract obesity and metabolic dysfunction. However, to the best of our knowledge, no experimental studies concerning the metabolic effects of pharmacologically targeting SERCA in human skeletal muscle cells have been reported. Thus, in the present study, we aimed to explore the effects of SERCA-activating compound, CDN1163, on energy metabolism in differentiated human skeletal muscle cells (myotubes).

Methods

In this study, we used primary myotube cultures derived from muscle biopsies of the musculus vastus lateralis and musculi interspinales from lean, healthy male donors. Energy metabolism in myotubes was studied using radioactive substrates. Oxygen consumption rate was assessed with the Seahorse XF24 bioanalyzer, whereas metabolic genes and protein expressions were determined by qPCR and immunoblotting, respectively.

Results

Both acute (4 ​h) and chronic (5 days) treatment of myotubes with CDN1163 showed increased uptake and oxidation of glucose, as well as complete fatty acid oxidation in the presence of carbonyl cyanide 4-(trifluromethoxy)phenylhydrazone (FCCP). These effects were supported by measurement of oxygen consumption rate, in which the oxidative spare capacity and maximal respiration were enhanced after CDN1163-treatment. In addition, chronic treatment with CDN1163 improved cellular uptake of oleic acid (OA) and fatty acid β-oxidation. The increased OA metabolism was accompanied by enhanced mRNA-expression of carnitine palmitoyl transferase (CPT) 1B, pyruvate dehydrogenase kinase (PDK) 4, as well as increased AMP-activated protein kinase (AMPK)^Thr172 phosphorylation. Moreover, following chronic CDN1163 treatment, the expression levels of stearoyl-CoA desaturase (SCD) 1 was decreased together with de novo lipogenesis from acetic acid and formation of diacylglycerol (DAG) from OA.

Conclusion

Altogether, these results suggest that SERCA activation by CDN1163 enhances energy metabolism in human myotubes, which might be favourable in relation to disorders that are related to metabolic dysfunction such as obesity and type 2 diabetes mellitus.

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CDN1163 induced an increase in glucose and fatty acid metabolism in primary human myotubes.

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Myotubes treated with CDN1163 showed lower intramyocellular lipid accumulation and higher rate of β-oxidation.

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AMPK activity was upregulated in CDN1163-treated myotubes.

SENP2 is vital for optimal insulin signaling and insulin-stimulated glycogen synthesis in human skeletal muscle cells

Sentrin-specific protease (SENP) 2 has been suggested as a possible novel drug target for the treatment of obesity and type 2 diabetes mellitus after observations of a palmitate-induced increase in SENP2 that lead to increased fatty acid oxidation and improved insulin sensitivity in skeletal muscle cells from mice. However, no precedent research has examined the role of SENP2 in human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism as well as insulin sensitivity in human skeletal muscle using cultured primary human myotubes. Acute (4 ​h) oleic acid oxidation was reduced in SENP2-knockdown (SENP2-KD) cells compared to control cells, with no difference in uptake. After prelabeling (24 ​h) with oleic acid, total lipid content and incorporation into triacylglycerol was decreased, while incorporation into other lipids, as well as complete oxidation and β-oxidation was increased in SENP2-KD cells. Basal glucose uptake (i.e., not under insulin-stimulated conditions) was higher in SENP2-KD cells, whereas oxidation was similar to control myotubes. Further, basal glycogen synthesis was not different in SENP2-KD myotubes, but both insulin-stimulated glycogen synthesis and Akt^Ser473 phosphorylation was completely blunted in SENP2-KD cells. In conclusion, SENP2 plays an important role in fatty acid and glucose metabolism in human myotubes. Interestingly, it also appears to have a pivotal role in regulating myotube insulin sensitivity. Future studies should examine the role of SENP2 in regulation of insulin sensitivity in other tissues and in vivo, defining the potential for SENP2 as a drug target.

Impaired Metabolic Flexibility in the Osteoarthritis Process: A Study on Transmitochondrial Cybrids

Osteoarthritis (OA) is the most frequent joint disease; however, the etiopathogenesis is still unclear. Chondrocytes rely primarily on glycolysis to meet cellular energy demand, but studies implicate impaired mitochondrial function in OA pathogenesis. The relationship between mitochondrial dysfunction and OA has been established. The aim of the study was to examine the differences in glucose and Fatty Acids (FA) metabolism, especially with regards to metabolic flexibility, in cybrids from healthy (N) or OA donors. Glucose and FA metabolism were studied using D-[¹⁴C(U)]glucose and [1-¹⁴C]oleic acid, respectively. There were no differences in glucose metabolism among the cybrids. Osteoarthritis cybrids had lower acid-soluble metabolites, reflecting incomplete FA β-oxidation but higher incorporation of oleic acid into triacylglycerol. Co-incubation with glucose and oleic acid showed that N but not OA cybrids increased their glucose metabolism. When treating with the mitochondrial inhibitor etomoxir, N cybrids still maintained higher glucose oxidation. Furthermore, OA cybrids had higher oxidative stress response. Combined, this indicated that N cybrids had higher metabolic flexibility than OA cybrids. Healthy donors maintained the glycolytic phenotype, whereas OA donors showed a preference towards oleic acid metabolism. Interestingly, the results indicated that cybrids from OA patients had mitochondrial impairments and reduced metabolic flexibility compared to N cybrids.

Treatment of human skeletal muscle cells with inhibitors of diacylglycerol acyltransferases 1 and 2 to explore isozyme-specific roles on lipid metabolism

Diacylglycerol acyltransferases (DGAT) 1 and 2 catalyse the final step in triacylglycerol (TAG) synthesis, the esterification of fatty acyl-CoA to diacylglycerol. Despite catalysing the same reaction and being present in the same cell types, they exhibit different functions on lipid metabolism in various tissues. Yet, their roles in skeletal muscle remain poorly defined. In this study, we investigated how selective inhibitors of DGAT1 and DGAT2 affected lipid metabolism in human primary skeletal muscle cells. The results showed that DGAT1 was dominant in human skeletal muscle cells utilizing fatty acids (FAs) derived from various sources, both exogenously supplied FA, de novo synthesised FA, or FA derived from lipolysis, to generate TAG, as well as being involved in de novo synthesis of TAG. On the other hand, DGAT2 seemed to be specialised for de novo synthesis of TAG from glycerol-3-posphate only. Interestingly, DGAT activities were also important for regulating FA oxidation, indicating a key role in balancing FAs between storage in TAG and efficient utilization through oxidation. Finally, we observed that inhibition of DGAT enzymes could potentially alter glucose-FA interactions in skeletal muscle. In summary, treatment with DGAT1 or DGAT2 specific inhibitors resulted in different responses on lipid metabolism in human myotubes, indicating that the two enzymes play distinct roles in TAG metabolism in skeletal muscle.

Higher lipid turnover and oxidation in cultured human myotubes from athletic versus sedentary young male subjects

In this study we compared fatty acid (FA) metabolism in myotubes established from athletic and sedentary young subjects. Six healthy sedentary (maximal oxygen uptake (VO_2max) ≤ 46 ml/kg/min) and six healthy athletic (VO_2max > 60 ml/kg/min) young men were included. Myoblasts were cultured and differentiated to myotubes from satellite cells isolated from biopsy of musculus vastus lateralis. FA metabolism was studied in myotubes using [¹⁴C]oleic acid. Lipid distribution was assessed by thin layer chromatography, and FA accumulation, lipolysis and re-esterification were measured by scintillation proximity assay. Gene and protein expressions were studied. Myotubes from athletic subjects showed lower FA accumulation, lower incorporation of FA into total lipids, triacylglycerol (TAG), diacylglycerol and cholesteryl ester, higher TAG-related lipolysis and re-esterification, and higher complete oxidation and incomplete β-oxidation of FA compared to myotubes from sedentary subjects. mRNA expression of the mitochondrial electron transport chain complex III gene UQCRB was higher in cells from athletic compared to sedentary. Myotubes established from athletic subjects have higher lipid turnover and oxidation compared to myotubes from sedentary subjects. Our findings suggest that cultured myotubes retain some of the phenotypic traits of their donors.

Increased triacylglycerol - Fatty acid substrate cycling in human skeletal muscle cells exposed to eicosapentaenoic acid

It has previously been shown that pretreatment of differentiated human skeletal muscle cells (myotubes) with eicosapentaenoic acid (EPA) promoted increased uptake of fatty acids and increased triacylglycerol accumulation, compared to pretreatment with oleic acid (OA) and palmitic acid (PA). The aim of the present study was to examine whether EPA could affect substrate cycling in human skeletal muscle cells by altering lipolysis rate of intracellular TAG and re-esterification of fatty acids. Fatty acid metabolism was studied in human myotubes using a mixture of fatty acids, consisting of radiolabelled oleic acid as tracer (14C-OA) together with EPA or PA. Co-incubation of myotubes with EPA increased cell-accumulation and incomplete fatty acid oxidation of 14C-OA compared to co-incubation with PA. Lipid distribution showed higher incorporation of 14C-OA into all cellular lipids after co-incubation with EPA relative to PA, with most markedly increases (3 to 4-fold) for diacylglycerol and triacylglycerol. Further, the increases in cellular lipids after co-incubation with EPA were accompanied by higher lipolysis and fatty acid re-esterification rate. Correspondingly, basal respiration, proton leak and maximal respiration were significantly increased in cells exposed to EPA compared to PA. Microarray and Gene Ontology (GO) enrichment analysis showed that EPA, related to PA, significantly changed i.e. the GO terms "Neutral lipid metabolic process" and "Regulation of lipid storage". Finally, an inhibitor of diacylglycerol acyltransferase 1 decreased the effect of EPA to promote fatty acid accumulation. In conclusion, incubation of human myotubes with EPA, compared to PA, increased processes of fatty acid turnover and oxidation suggesting that EPA may activate futile substrate cycling of fatty acids in human myotubes. Increased TAG-FA cycling may be involved in the potentially favourable effects of long-chain polyunsaturated n-3 fatty acids on skeletal muscle and whole-body energy metabolism.

Human myotubes from myoblast cultures undergoing senescence exhibit defects in glucose and lipid metabolism

Adult stem cells are known to have a finite replication potential. Muscle biopsy-derived human satellite cells (SCs) were grown at different passages and differentiated to human myotubes in culture to analyze the functional state of various carbohydrate and lipid metabolic pathways. As the proliferative potential of myoblasts decreased dramatically with passage number, a number of cellular functions were altered: the capacity of myoblasts to fuse and differentiate into myotubes was reduced, and metabolic processes in myotubes such as glucose uptake, glycogen synthesis, glucose oxidation and fatty acid β-oxidation became gradually impaired. Upon insulin stimulation, glucose uptake and glycogen synthesis increased but as the cellular proliferative capacity became gradually exhausted, the response dropped concomitantly. Palmitic acid incorporation into lipids in myotubes decreased with passage number and could be explained by reduced incorporation into diacyl- and triacylglycerols. The levels of long-chain acyl-CoA esters decreased with increased passage number. Late-passage, non-proliferating, myoblast cultures showed strong senescence-associated β-galactosidase activity indicating that the observed metabolic defects accompany the induction of a senescent state. The main function of SCs is regeneration and skeletal muscle-build up. Thus, the metabolic defects observed during aging of SC-derived myotubes could have a role in sarcopenia, the gradual age-related loss of muscle mass and strength.

Mitochondrial mass is inversely correlated to complete lipid oxidation in human myotubes

Exercise increases while physical inactivity decrease mitochondrial content and oxidative capacity of skeletal muscles in vivo. It is unknown whether mitochondrial mass and substrate oxidation are related in non-contracting skeletal muscle. Mitochondrial mass, ATP, ADP, AMP, glucose and lipid oxidation (complete and incomplete) were determined in non-contracting myotubes established from 10 lean, 10 obese and 10 subjects with type 2 diabetes precultured under normophysiological conditions. ATP, ADP, AMP, mitochondrial mass and energy charge were not different between groups. In diabetic myotubes, basal glucose oxidation and incomplete lipid oxidation were significantly increased while complete lipid oxidation was lower. Mitochondrial mass was not correlated to glucose oxidation or incomplete lipid oxidation in human myotubes but inversely correlated to complete lipid oxidation. Thus within a stable energetic background, an increased mitochondrial mass in human myotubes was not positive correlated to an increased substrate oxidation as expected from skeletal muscles in vivo but surprisingly with a reduced complete lipid oxidation.

PPARα in Obesity: Sex Difference and Estrogen Involvement

Peroxisome proliferator-activated receptor α (PPARα) is a member of the steroid hormone receptor superfamily and is well known to act as the molecular target for lipid-lowering drugs of the fibrate family. At the molecular level, PPARα regulates the transcription of a number of genes critical for lipid and lipoprotein metabolism. PPARα activators are further shown to reduce body weight gain and adiposity, at least in part, due to the increase of hepatic fatty acid oxidation and the decrease in levels of circulating triglycerides responsible for adipose cell hypertrophy and hyperplasia. However, these effects of the PPARα ligand fenofibrate on obesity are regulated with sexual dimorphism and seem to be influenced by the presence of functioning ovaries, suggesting the involvement of ovarian steroids in the control of obesity by PPARα. In female ovariectomized mice, 17β-estradiol inhibits the actions of fenofibrate on obesity through its suppressive effects on the expression of PPARα target genes, and these processes may be mediated by inhibiting the coactivator recruitment of PPARα. Thus, it is likely that PPARα functions on obesity may be enhanced in estrogen-deficient states.

Metabolic switching of human myotubes is improved by n-3 fatty acids

The aim of the present study was to examine whether pretreatment with different fatty acids, as well as the liver X receptor (LXR) agonist T0901317, could modify metabolic switching of human myotubes. The n-3 FA eicosapentaenoic acid (EPA) increased suppressibility, the ability of glucose to suppress FA oxidation. Substrate-regulated flexibility, the ability to increase FA oxidation when changing from a high glucose, low fatty acid condition ("fed") to a high fatty acid, low glucose ("fasted") condition, was increased by EPA and other n-3 FAs. Adaptability, the capacity to increase FA oxidation with increasing FA availability, was enhanced after pretreatment with EPA, linoleic acid (LA), and palmitic acid (PA). T0901317 counteracted the effect of EPA on suppressibility and adaptability, but it did not affect these parameters alone. EPA per se accumulated less, however, EPA, LA, oleic acid, and T0901317 treatment increased the number of lipid droplets (LD) in myotubes. LD volume and intensity, as well as mitochondrial mass, were independent of FA pretreatment. Microarray analysis showed that EPA regulated more genes than the other FAs and that specific pathways involved in carbohydrate metabolism were induced only by EPA. The present study suggests a favorable effect of n-3 FAs on skeletal muscle metabolic switching and glucose utilization.

LXR{beta} is the dominant LXR subtype in skeletal muscle regulating lipogenesis and cholesterol efflux

Liver X receptors (LXRs) are important regulators of cholesterol, lipid, and glucose metabolism and have been extensively studied in liver, macrophages, and adipose tissue. However, their role in skeletal muscle is poorly studied and the functional role of each of the LXRalpha and LXRbeta subtypes in skeletal muscle is at present unknown. To study the importance of each of the receptor subtypes, myotube cultures derived from wild-type (WT) and LXRalpha and LXRbeta knockout (KO) mice were established. The present study showed that treatment with the LXR agonist T0901317 increased lipogenesis and apoA1-dependent cholesterol efflux in LXRalpha KO and WT myotubes but not in LXRbeta KO cells. The functional studies were confirmed by T0901317-induced increase in mRNA levels of LXR target genes involved in lipid and cholesterol metabolism in myotubes established from WT and LXRalpha KO mice, whereas only minor changes were observed for these genes in myotubes from LXRbeta KO mice. Gene expression analysis using microarrays showed that very few genes other than the classical, well-known LXR target genes were regulated by LXR in skeletal muscle. The present study also showed that basal glucose uptake was increased in LXRbeta KO myotubes compared with WT myotubes, suggesting a role for LXRbeta in glucose metabolism in skeletal muscle. In conclusion, LXRbeta seems to be the main LXR subtype regulating lipogenesis and cholesterol efflux in skeletal muscle.

Fatty acid incubation of myotubes from humans with type 2 diabetes leads to enhanced release of beta-oxidation products because of impaired fatty acid oxidation: effects of tetradecylthioacetic acid and eicosapentaenoic acid

OBJECTIVE

Increased availability of fatty acids is important for accumulation of intracellular lipids and development of insulin resistance in human myotubes. It is unknown whether different types of fatty acids like eicosapentaenoic acid (EPA) or tetradecylthioacetic acid (TTA) influence these processes.

RESEARCH DESIGN AND METHODS

We examined fatty acid and glucose metabolism and gene expression in cultured human skeletal muscle cells from control and type 2 diabetic individuals after 4 days of preincubation with EPA or TTA.

RESULTS

Type 2 diabetes myotubes exhibited reduced formation of CO(2) from palmitic acid (PA), whereas release of beta-oxidation products was unchanged at baseline but significantly increased with respect to control myotubes after preincubation with TTA and EPA. Preincubation with TTA enhanced both complete (CO2) and beta-oxidation of palmitic acid, whereas EPA increased only beta-oxidation significantly. EPA markedly enhanced triacylglycerol (TAG) accumulation in myotubes, more pronounced in type 2 diabetes cells. TAG accumulation and fatty acid oxidation were inversely correlated only after EPA preincubation, and total level of acyl-CoA was reduced. Glucose oxidation (CO2 formation) was enhanced and lactate production decreased after chronic exposure to EPA and TTA, whereas glucose uptake and storage were unchanged. EPA and especially TTA increased the expression of genes involved in fatty acid uptake, activation, accumulation, and oxidation.

CONCLUSIONS

Our results suggest that 1) mitochondrial dysfunction in diabetic myotubes is caused by disturbances downstream of fatty acid beta-oxidation; 2) EPA promoted accumulation of TAG, enhanced beta-oxidation, and increased glucose oxidation; and 3) TTA improved complete palmitic acid oxidation in diabetic myotubes, opposed increased lipid accumulation, and increased glucose oxidation.

Insulin resistance and the mitochondrial link. Lessons from cultured human myotubes

In order to better understand the impact of reduced mitochondrial function for the development of insulin resistance and cellular metabolism, human myotubes were established from lean, obese, and T2D subjects and exposed to mitochondrial inhibitors, either affecting the electron transport chain (Antimycin A), the ATP synthase (oligomycin) or respiratory uncoupling (2,4-dinitrophenol). Direct inhibition of the electron transport chain or the ATP synthase was followed by increased glucose uptake and lactate production, reduced glycogen synthesis, reduced lipid and glucose oxidation and unchanged lipid uptake. The metabolic phenotype during respiratory uncoupling resembled the above picture, except for an increase in glucose and palmitate oxidation. Antimycin A and oligomycin treatment induced insulin resistance at the level of glucose and palmitate uptake in all three study groups while, at the level of glycogen synthesis, insulin resistance was only seen in lean myotubes. Primary insulin resistance in diabetic myotubes was significantly worsened at the level of glucose and lipid uptake. The present study is the first convincing data linking functional mitochondrial impairment per se and insulin resistance. Taken together functional mitochondrial impairment could be part of the pathophysiology of insulin resistance in vivo.

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.

Eicosapentaenoic acid (20:5 n-3) increases fatty acid and glucose uptake in cultured human skeletal muscle cells

This study was conducted to evaluate the chronic effects of eicosapentaenoic acid (EPA) on fatty acid and glucose metabolism in human skeletal muscle cells. Uptake of [14C]oleate was increased >2-fold after preincubation of myotubes with 0.6 mM EPA for 24 h, and incorporation into various lipid classes showed that cellular triacylgycerol (TAG) and phospholipids were increased 2- to 3-fold compared with control cells. After exposure to oleic acid (OA), TAG was increased 2-fold. Insulin (100 nM) further increased the incorporation of [14C]oleate into all lipid classes for EPA-treated myotubes. Fatty acid beta-oxidation was unchanged, and complete oxidation (CO2) decreased in EPA-treated cells. Basal glucose transport and oxidation (CO2) were increased 2-fold after EPA, and insulin (100 nM) stimulated glucose transport and oxidation similarly in control and EPA-treated myotubes, whereas these responses to insulin were abolished after OA treatment. Lower concentrations of EPA (0.1 mM) also increased fatty acid and glucose uptake. CD36/FAT (fatty acid transporter) mRNA expression was increased after EPA and OA treatment compared with control cells. Moreover, GLUT1 expression was increased 2.5-fold by EPA, whereas GLUT4 expression was unchanged, and activities of the mitogen-activated protein kinase p38 and extracellular signal-regulated kinase were decreased after treatment with OA compared with EPA. Together, our data show that chronic exposure of myotubes to EPA promotes increased uptake and oxidation of glucose despite a markedly increased fatty acid uptake and synthesis of complex lipids.

Triacylglycerol accumulation is not primarily affected in myotubes established from type 2 diabetic subjects

In the present study, we investigated triacylglycerol (TAG) accumulation, glucose and fatty acid (FA) uptake, and glycogen synthesis (GS) in human myotubes from healthy, lean, and obese subjects with and without type 2 diabetes (T2D), exposed to increasing palmitate (PA) and oleate (OA) concentrations with/without high glucose and/or high insulin concentrations for 4 days. We showed that these myotubes expressed an increased TAG accumulation (P<0.001) without differences between groups. Chronically high insulin, but not high glucose concentrations, increases TAG accumulation by 25% (P<0.001). Inhibition of oxidative phosphorylation by antimycin A and oligomyin was followed by a reduced lipid oxidation (P<0.05) and increased TAG accumulation (P<0.05), but only in the presence of FAs. Both chronic PA and OA exposure reduced the insulin-mediated PA and OA uptake (fold change) (P<0.001), but could not induce insulin resistance at the level of glucose uptake, whereas high insulin concentrations induced insulin resistance (P<0.001). Chronic, high PA, but not OA, induced insulin resistance at the GS level in control subjects (P<0.05). The TAG content correlated negatively with insulin-stimulated FA uptake (P<0.001), but did not correlate with insulin-stimulated glucose uptake for PA or OA (P>0.05). These results indicate that (1) TAG accumulation is not primarily affected in skeletal muscle tissue of obese and T2D; (2) induced inhibition of oxidative phosphorylation is followed by TAG accumulation; (3) increasing FA and insulin availability, and reduced oxidative phosphorylation, and to a lesser extent glucose, are determinants for differences in intramyocellular TAG accumulation; (4) quantitative TAG content may not be the best marker for insulin resistance. Thus, increased TAG content in skeletal muscle of obese and T2D subjects is adaptive.

Insulin and leptin do not affect fatty acid uptake and metabolism in human placental choriocarcinoma (BeWo) cells

Placental transport of long chain polyunsaturated fatty acids is important for fetal growth and development. In order to examine the effects of leptin and insulin on fatty acid uptake by the placenta, placental choriocarcinoma (BeWo) cells were used. BeWo cells were incubated for 5h at 37 degrees C in the absence or presence of different concentrations of insulin (0.6, 60, and 100 ng) or leptin (10 ng) with 200 microM of various radiolabeled fatty acids (docosahexaenoic acid, arachidonic acid, eicosapentaenoic acid, and oleic acid, mixed with 1:1 bovine serum albumin (fat free). After incubation, the uptake and distribution of these fatty acids into different cellular lipid fractions were determined. The uptakes of oleic, eicosapentaenoic, arachidonic, and docosahexaenoic acids were 15.36+/-4.1, 19.95+/-3.6, 28.56+/-8.1, and 62.25+/-9.5 nmol/mg of protein, respectively, in BeWo cells. Incubation of these cells with insulin (0.6 or 60 ng/ml) or leptin (10 ng/ml) did not significantly alter uptake of any of these fatty acids (P>0.5). Insulin or leptin also did not affect beta oxidation of fatty acids in these cells. In contrast, leptin (10 ng/ml) and insulin (0.60 ng/ml)) stimulated the uptake of oleic acid (7.4+/-2.3 nmol/mg protein) in human adipose cells, SGBS cells by 1.28- and 2.48-fold (P<0.05), respectively. The distribution of fatty acids in different cellular lipid fractions was also not affected by these hormones. Our data indicate that unlike adipose tissue, fatty acid uptake and metabolism in placental trophoblasts is not regulated by insulin or leptin.

Effect of 18∶1n−9, 20∶5n−3, and 22∶6n−3 on lipid accumulation and secretion by atlantic salmon hepatocytes

We have studied the effects of dietary FA on the accumulation and secretion of [3H]glycerolipids by salmon hepatocytes in culture. Atlantic salmon were fed diets supplemented with either 100% soybean oil (SO) or 100% fish oil (FO), and grew from an initial weight of 113 +/- 5 g to a final weight of 338 +/- 19 g. Hepatocytes were isolated from both dietary groups and incubated with [3H]glycerol in an FA-free medium; a medium supplemented with 0.75 mM of one of three FA-18:1 n-9, 20:5n-3, or 22:6n-3--or a medium supplemented with 0.75 mM of the sulfur-substituted FA analog tetradecylthioacetic acid (TTA), which cannot undergo beta-oxidation. Incubations were allowed to proceed for 1, 2, 6, or 24 h. The rate of the secretion of radioactive glycerolipids with no FA added was 36% lower from hepatocytes isolated from fish fed the FO diet than it was from hepatocytes isolated from fish fed the SO diet. Hepatocytes incubated with 18:1 n-9 secreted more [3H]TAG than when incubated with no FA, whereas hepatocytes incubated with 20:5n-3 or TTA secreted less labeled TAG than when incubated with no FA. This observation was independent of the feeding group. Hepatocytes incubated with 22:6n-3 secreted the highest amounts of total [3H]glycerolipids compared with the other treatments, owing to increased secretion of phospholipids and mono- and diacylglycerols (MDG). In contrast, the same amounts of [3H]TAG were secreted from these cells as from cells incubated in an FA-free medium. The lipid-lowering effect of FO is thus independent of 22:6n-3, showing that 20:5n-3 is the FA that is responsible for the lipid-lowering effect. The ratio of TAG to MDG in lipids secreted from hepatocytes to which 20:5n-3 or TTA had been added was lower than that in lipids secreted from hepatocytes incubated with 18:1 n-9 or 22:6n-3, suggesting that the last step in TAG synthesis was inhibited. Morphometric measurements revealed that hepatocytes incubated with 20:5n-3 accumulated significantly more cellular lipid than cells treated with 18:1n-9, 22:6n-3, TTA, or no treatment. The area occupied by mitochondria was also greater in these cells. The present study shows that dietary FO reduces TAG secretion from salmon hepatocytes and that 20:5n-3 mediates this effect.

Skeletal muscle lipid accumulation in type 2 diabetes may involve the liver X receptor pathway

Liver X receptors (LXRs) are important regulators of cholesterol and lipid metabolism and are also involved in glucose metabolism. However, the functional role of LXRs in human skeletal muscle is at present unknown. This study demonstrates that chronic ligand activation of LXRs by a synthetic LXR agonist increases the uptake, distribution into complex cellular lipids, and oxidation of palmitate as well as the uptake and oxidation of glucose in cultured human skeletal muscle cells. Furthermore, the effect of the LXR agonist was additive to acute effects of insulin on palmitate uptake and metabolism. Consistently, activation of LXRs induced the expression of relevant genes: fatty acid translocase (CD36/FAT), glucose transporters (GLUT1 and -4), sterol regulatory element-binding protein-1c, peroxisome proliferator-activated receptor-gamma, carnitine palmitoyltransferase-1, and uncoupling protein 2 and 3. Interestingly, in response to activation of LXRs, myotubes from patients with type 2 diabetes showed an elevated uptake and incorporation of palmitate into complex lipids but an absence of palmitate oxidation to CO(2). These results provide evidence for a functional role of LXRs in both lipid and glucose metabolism and energy uncoupling in human myotubes. Furthermore, these data suggest that increased intramyocellular lipid content in type 2 diabetic patients may involve an altered response to activation of components in the LXR pathway.

Differential utilization of saturated palmitate and unsaturated oleate: evidence from cultured myotubes

We recently described a primarily reduced palmitate oxidation in myotubes established from type 2 diabetic subjects, whereas triacylglycerol (TAG) accumulation seemed to be adaptive. However, it is still uncertain whether these changes are similar for saturated and unsaturated fatty acids and whether high concentrations of glucose and/or insulin may change this picture. Studies of palmitic acid and oleic acid metabolism in human myotubes established from control and type 2 diabetic subjects under conditions of acute high concentrations of insulin and/or glucose may solve these questions. Total oleic acid and palmitic acid uptake in myotubes was increased during acute insulin stimulation (P < 0.01) but not under acute, high-glucose concentrations, and no differences were found between the groups. Type 2 diabetic myotubes expressed a reduced palmitic acid oxidation to carbon dioxide (P </= 0.04), whereas oleic acid oxidation showed no differences between myotubes from both groups. High glucose concentrations decreased oleic acid oxidation (P </= 0.03). Lipid distribution was not different in diabetic and control myotubes when palmitic acid and oleic acid incorporation into cellular lipids was compared. Myotubes that were exposed to palmitic acid showed an increased palmitic acid incorporation into diacylglycerol (DAG) and TAG compared with myotubes that were exposed to oleic acid (P < 0.05) expressing an increased intracellular free fatty acid (FFA) level (P < 0.05). Lipid distribution was not affected by high glucose, whereas insulin increased FFAs, DAG, and TAG (P < 0.05). De novo lipid synthesis from glucose in both diabetic and control myotubes was of the same magnitude independent of glucose and insulin concentrations. These results indicate that palmitic acid and oleic acid are utilized in the same pattern in diabetic and control myotubes even though palmitic acid oxidation is primarily reduced in diabetic cells. Palmitic acid and oleic acid are handled differently by myotubes: Palmitic acid seems to accumulate as DAG and TAG, whereas oleic acid accumulates as intracellular FFAs. These observations indicate that oleic acid is preferable as fatty acid as it accumulates to a lesser extent as DAG and TAG than palmitic acid. Neither acute hyperglycemia nor de novo lipid synthesis from glucose seems central to the TAG accumulation in obesity or type 2 diabetes.

Reduced lipid oxidation in skeletal muscle from type 2 diabetic subjects may be of genetic origin: evidence from cultured myotubes

Insulin resistance in skeletal muscle in vivo is associated with reduced lipid oxidation and lipid accumulation. It is still uncertain whether changes in lipid metabolism represent an adaptive compensation at the cellular level or a direct expression of a genetic trait. Studies of palmitate metabolism in human myotubes established from control and type 2 diabetic subjects may solve this problem, as genetic defects are preserved and expressed in vitro. In this study, total uptake of palmitic acid was similar in myotubes established from both control and type 2 diabetic subjects under basal conditions and acute insulin stimulation. Myotubes established from diabetic subjects expressed a primary reduced palmitic acid oxidation to carbon dioxide with a concomitantly increased esterification of palmitic acid into phospholipids compared with control myotubes under basal conditions. Triacylglycerol (TAG) content and the incorporation of palmitic acid into diacylglycerol (DAG) and TAG at basal conditions did not vary between the groups. Acute insulin treatment significantly increased palmitate uptake and incorporation of palmitic acid into DAG and TAG in myotubes established from both study groups, but no difference was found in myotubes established from control and diabetic subjects. These results indicate that the reduced lipid oxidation in diabetic skeletal muscle in vivo may be of genetic origin; it also appears that TAG metabolism is not primarily affected in diabetic muscles under basal physiological conditions.

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.

3-Hydroxy-3-methylglutaryl CoA reductase inhibitors reduce serum triglyceride levels through modulation of apolipoprotein C-III and lipoprotein lipase

Statins are hypolipidemic drugs which not only improve cholesterol but also triglyceride levels. Whereas their cholesterol-reducing effect involves inhibition of de novo biosynthesis of cellular cholesterol through competitive inhibition of its rate-limiting enzyme 3-hydroxy-3-methylglutaryl CoA reductase, the mechanism by which they lower triglycerides remains unknown and forms the subject of the current study. Treatment of normal rats for 4 days with simvastatin decreased serum triglycerides significantly, whereas it increased high density lipoprotein cholesterol moderately. The decrease in triglyceride concentrations after simvastatin was caused by a reduction in the amount of very low density lipoprotein particles which were of an unchanged lipid composition. Simvastatin administration increased the lipoprotein lipase mRNA and activity in adipose tissue and heart. This effect on lipoprotein lipase was accompanied by decreased mRNA as well as plasma levels of the lipoprotein lipase inhibitor apolipoprotein C-III. These results suggest that the triglyceride-lowering effect of statins involves a stimulation of lipoprotein lipase-mediated clearance of triglyceride-rich lipoproteins.

3-Thia fatty acid treatment, in contrast to eicosapentaenoic acid and starvation, induces gene expression of carnitine palmitoyltransferase-II in rat liver

The aim of the present study was to investigate the hepatic regulation and beta-oxidation of long-chain fatty acids in peroxisomes and mitochondria, after 3-thia- tetradecylthioacetic acid (C14-S-acetic acid) treatment. When palmitoyl-CoA and palmitoyl-L-carnitine were used as substrates, hepatic formation of acid-soluble products was significantly increased in C14-S-acetic acid treated rats. Administration of C14-S-acetic acid resulted in increased enzyme activity and mRNA levels of hepatic mitochondrial carnitine palmitoyltransferase (CPT)-II. CPT-II activity correlated with both palmitoyl-CoA and palmitoyl-L-carnitine oxidation in rats treated with different chain-length 3-thia fatty acids. CPT-I activity and mRNA levels were, however, marginally affected. The hepatic CPT-II activity was mainly localized in the mitochondrial fraction, whereas the CPT-I activity was enriched in the mitochondrial, peroxisomal, and microsomal fractions. In C14-S-acetic acid-treated rats, the specific activity of peroxisomal and microsomal CPT-I increased, whereas the mitochondrial activity tended to decrease. C14-S-Acetyl-CoA inhibited CPT-I activity in vitro. The sensitivity of CPT-I to malonyl-CoA was unchanged, and the hepatic malonyl-CoA concentration increased after C14-S-acetic acid treatment. The mRNA levels of acetyl-CoA carboxylase increased. In hepatocytes cultured from palmitic acid- and C14-S-acetic acid-treated rats, the CPT-I inhibitor etomoxir inhibited the formation of acid-soluble products 91 and 21%, respectively. In contrast to 3-thia fatty acid treatment, eicosapentaenoic acid treatment and starvation increased the mitochondrial CPT-I activity and reduced its malonyl-CoA sensitivity. Palmitoyl-L-carnitine oxidation and CPT-II activity were, however, unchanged after either EPA treatment or starvation. The results from this study open the possibility that the rate control of mitochondrial beta-oxidation under mitochondrion and peroxisome proliferation is distributed between an enzyme or enzymes of the pathway beyond the CPT-I site after 3-thia fatty acid treatment. It is suggested that fatty acids are partly oxidized in the peroxisomes before entering the mitochondria as acylcarnitines for further oxidation.

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.

Short-term regulation of carnitine palmitoyltransferase I in cultured rat hepatocytes: spontaneous inactivation and reactivation by fatty acids

Liver carnitine palmitoyltransferase I (CPT I), the rate-limiting enzyme of mitochondrial beta-oxidation, rapidly loses its activity when hepatocytes are put in culture. 3-Thia fatty acids reactivate the enzyme and can increase its activity 3-4-fold in 5-10 min. Normal fatty acids are also able to stimulate CPT I, but to a limited extent, compared to 3-thia fatty acid. This activation does not affect malonyl-CoA sensitivity. CPT I in hepatocytes from both fasted and fasted-carbohydrate refed rats is inactivated and reactivated to a similar extent. Free dodecylthioacetic acid (DTA) is at least as efficient as DTA-CoA as activator. CPT I activity in isolated mitochondria is not influenced by incubation with DTA, suggesting that the regulation of CPT I depends on an extramitochondrial component(s) in the cell. It is concluded that fatty acids activate pre-existing, inactive CPT I without involvement of gene transcription and independently of malonyl-CoA.

Regulation of lipoprotein metabolism by thiazolidinediones occurs through a distinct but complementary mechanism relative to fibrates

Thiazolidinediones are antidiabetic agents, which not only improve glucose metabolism but also reduce blood triglyceride concentrations. These compounds are synthetic ligands for PPAR gamma, a transcription factor belonging to the nuclear receptor subfamily of PPARs, which are important transcriptional regulators of lipid and lipoprotein metabolism. The goal of this study was to evaluate the influence of a potent thiazolidinedione, BRL49653, on serum lipoproteins and to determine whether its lipid-lowering effects are mediated by changes in the expression of key genes implicated in lipoprotein metabolism. Treatment of normal rats for 7 days with BRL49653 decreased serum triglycerides in a dose-dependent fashion without affecting serum total and HDL cholesterol and apolipoprotein (apo) A-I and apo A-II concentrations. The decrease in triglyceride concentrations after BRL49653 was mainly due to a reduction of the amount of VLDL particles of unchanged lipid and apo composition. BRL49653 treatment did not change triglyceride production in vivo as analyzed by injection of Triton WR-1339, indicating a primary action on triglyceride catabolism. Analysis of the influence of BRL49653 on the expression of LPL and apo C-III, two key players in triglyceride catabolism, showed a dose-dependent increase in mRNA levels and activity of LPL in epididymal adipose tissue, whereas liver apo C-III mRNA levels remained constant. Furthermore, addition of BRL49653 to primary cultures of differentiated adipocytes increased LPL mRNA levels, indicating a direct action of the drug on the adipocyte. Simultaneous administration of BRL49653 and fenofibrate, a hypolipidemic drug that acts primarily on liver through activation of PPAR alpha both decreased liver apo C-III and increased adipose tissue LPL mRNA levels, resulting in a more pronounced lowering of serum triglycerides than each drug alone. In conclusion, both fibrates and thiazolidinediones exert a hypotriglyceridemic effect. While fibrates act primarily on the liver by decreasing apo C-III production, BRL49653 acts primarily on adipose tissue by increasing lipolysis through the induction of LPL expression. Drugs combining both PPAR alpha and gamma activation potential should therefore display a more efficient hypotriglyceridemic activity than either compound alone and may provide a rationale for improved therapy for elevated triglycerides.

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.

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

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

Effects of chain length and sulphur position of thia fatty acids on their incorporation into phospholipids in 7800 C1 hepatoma cells and isolated rat hepatocytes, and their effects on fatty acid composition of phospholipids

Incorporation of thia fatty acids and their effects on the fatty acid composition in phospholipids has been investigated in 7800 C1 hepatoma cells and cultured hepatocytes. 3-Thia fatty acids of chain lengths from dodecyl-to hexadecyl-thioacetic acid were incorporated into phospholipids during a 3-day incubation. Longer and shorter 3-thia fatty acids were barely detectable. Tetradecylthioacetic acid, 3-thia stearate, and their delta9- desaturated derivatives were maximally incorporated into whole-cell phospholipids. The amount of tetradecylthioacetic acid incorporated into phospholipids of hepatoma cells remained almost identical in cells cultured for 3 days or adapted over a period of 1 year. Delta9-desaturated metabolites of long chain thia fatty acids (C13-to C16-S-acetic acid) were identified by GC-MS in phospholipids. 3-Thia stearate appeared to be the best substrated for delta9 desaturase. Incubation of hepatoma cells with thia fatty acids led to alterations in the amount of normal fatty acids in total phospholipids. The amounts of 16:0 and 18:1 decreased and 18:2 (n-6) and 20:5 (n-3) increased. Changes in the normal fatty acid composition of phospholipids were seen both with thia acids incorporated into phospholipids and those not incorporated. These effects, therefore, may be only partially dependent on displacement of normal fatty acids by thia fatty acids. Morris 7800 C1 hepatoma cell acyl-CoA synthetase (ACS) and peroxisomal acyl-CpA oxidase (ACO) were induced by thia fatty acids of all chain lengths, and with the sulphur atom(s) in different positions. Control experiments with hepatocytes revealed a similar incorporation of thia fatty acids in these physiologically more normal cells.

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.

Evidence that dietary arachidonic acid increases circulating triglycerides

Male Syrian hamsters and male CD-1 mice were fed diets supplemented with ethyl esters of oleic, linoleic, arachidonic, and eicosapentaenoic acids (1.1-5%, w/w) for 3-4 wk. Plasma and serum triglycerides were significantly higher in the arachidonic acid-supplemented animals compared to those in the other supplementation groups. Changes in serum insulin and glucose levels did not appear to be related to the changes in circulating triglycerides observed in the arachidonic acid-supplemented group. These data indicate that dietary arachidonic acid elevates circulating triglyceride levels compared to other unsaturated fatty acids in hamsters and mice by unknown mechanisms.