Effect of etomoxiryl-CoA on different carnitine acyltransferases

Acetyl-CoA carboxylase inhibition increases retinal pigment epithelial cell fatty acid flux and restricts apolipoprotein efflux

Lipid-rich deposits called drusen accumulate under the retinal pigment epithelium (RPE) in the eyes of patients with age-related macular degeneration and Sorsby's fundus dystrophy (SFD). Drusen may contribute to photoreceptor degeneration in these blinding diseases. Stimulating β-oxidation of fatty acids could decrease the availability of lipid with which RPE cells generate drusen. Inhibitors of acetyl-CoA carboxylase (ACC) stimulate β-oxidation and diminish lipid accumulation in fatty liver disease. In this report, we test the hypothesis that an ACC inhibitor, Firsocostat, can diminish lipid deposition by RPE cells. We probed metabolism and cellular function in mouse RPE-choroid tissue and human RPE cells. We used 13C6-glucose, 13C16-palmitate, and gas chromatography-linked mass spectrometry to monitor effects of Firsocostat on glycolytic, Krebs cycle, and fatty acid metabolism. We quantified lipid abundance, apolipoprotein E levels, and vascular endothelial growth factor release using liquid chromatography-mass spectrometry, ELISAs, and immunostaining. RPE barrier function was assessed by trans-epithelial electrical resistance (TEER). Firsocostat-mediated ACC inhibition increases β-oxidation, decreases intracellular lipid levels, diminishes lipoprotein release, and increases TEER. When human serum or outer segments are used to stimulate lipoprotein release, fewer lipoproteins are released in the presence of Firsocostat. In a culture model of SFD, Firsocostat stimulates fatty acid oxidation, increases TEER, and decreases apolipoprotein E release. We conclude that Firsocostat remodels RPE metabolism and can limit lipid deposition. This suggests that ACC inhibition could be an effective strategy for diminishing pathologic drusen in the eyes of patients with age-related macular degeneration or SFD.

Etomoxir-carnitine, a novel pharmaco-metabolite of etomoxir, inhibits phospholipases A2 and mitochondrial respiration

Mitochondrial fatty acid oxidation serves as an essential process for cellular survival, differentiation, proliferation, and energy metabolism. Numerous studies have utilized etomoxir (ETO) for the irreversible inhibition of carnitine palmitoylcarnitine transferase 1 (CPT1), which catalyzes the rate-limiting step for mitochondrial long-chain fatty acid β-oxidation to examine the bioenergetic roles of mitochondrial fatty acid metabolism in many tissues in multiple diverse disease states. Herein, we demonstrate that intact mitochondria robustly metabolize ETO to etomoxir-carnitine (ETO-carnitine) prior to nearly complete ETO-mediated inhibition of CPT1. The novel pharmaco-metabolite, ETO-carnitine, was conclusively identified by accurate mass, fragmentation patterns, and isotopic fine structure. On the basis of these data, ETO-carnitine was successfully differentiated from isobaric structures (e.g., 3-hydroxy-C18:0 carnitine and 3-hydroxy-C18:1 carnitine). Mechanistically, generation of ETO-carnitine from mitochondria required exogenous Mg2+, ATP or ADP, CoASH, and L-carnitine, indicating that thioesterification by long-chain acyl-CoA synthetase to form ETO-CoA precedes its conversion to ETO-carnitine by CPT1. CPT1-dependent generation of ETO-carnitine was substantiated by an orthogonal approach using ST1326 (a CPT1 inhibitor), which effectively inhibits mitochondrial ETO-carnitine production. Surprisingly, purified ETO-carnitine potently inhibited calcium-independent PLA2γ and PLA2β as well as mitochondrial respiration independent of CPT1. Robust production and release of ETO-carnitine from HepG2 cells incubated in the presence of ETO was also demonstrated. Collectively, this study identifies the chemical mechanism for the biosynthesis of a novel pharmaco-metabolite of ETO, ETO-carnitine, that is generated by CPT1 in mitochondria and likely impacts multiple downstream (non-CPT1 related) enzymes and processes in multiple subcellular compartments.

Dietary dicarboxylic acids provide a non-storable alternative fat source that protects mice against obesity

Dicarboxylic fatty acids are generated in the liver and kidney in a minor pathway called fatty acid ω-oxidation. The effects of consuming dicarboxylic fatty acids as an alternative source of dietary fat have not been explored. Here, we fed dodecanedioic acid, a 12-carbon dicarboxylic (DC12), to mice at 20% of daily caloric intake for nine weeks. DC12 increased metabolic rate, reduced body fat, reduced liver fat, and improved glucose tolerance. We observed DC12-specific breakdown products in liver, kidney, muscle, heart, and brain, indicating that oral DC12 escaped first-pass liver metabolism and was utilized by many tissues. In tissues expressing the "a" isoform of acyl-CoA oxidase-1 (ACOX1), a key peroxisomal fatty acid oxidation enzyme, DC12 was chain shortened to the TCA cycle intermediate succinyl-CoA. In tissues with low peroxisomal fatty acid oxidation capacity, DC12 was oxidized by mitochondria. In vitro, DC12 was catabolized even by adipose tissue and was not stored intracellularly. We conclude that DC12 and other dicarboxylic acids may be useful for combatting obesity and for treating metabolic disorders.

Acetyl-CoA carboxylase Inhibition increases RPE cell fatty acid oxidation and limits apolipoprotein efflux

Purpose

In age-related macular degeneration (AMD) and Sorsby's fundus dystrophy (SFD), lipid-rich deposits known as drusen accumulate under the retinal pigment epithelium (RPE). Drusen may contribute to photoreceptor and RPE degeneration in AMD and SFD. We hypothesize that stimulating β-oxidation in RPE will reduce drusen accumulation. Inhibitors of acetyl-CoA carboxylase (ACC) stimulate β-oxidation and diminish lipid accumulation in fatty liver disease. In this report we test the hypothesis that an ACC inhibitor, Firsocostat, limits the accumulation of lipid deposits in cultured RPE cells.

Methods

We probed metabolism and cellular function in mouse RPE-choroid, human fetal- derived RPE cells, and induced pluripotent stem cell-derived RPE cells. We used 13 C6-glucose and 13 C16-palmitate to determine the effects of Firsocostat on glycolytic, Krebs cycle, and fatty acid metabolism. 13 C labeling of metabolites in these pathways were analyzed using gas chromatography-linked mass spectrometry. We quantified ApoE and VEGF release using enzyme-linked immunosorbent assays. Immunostaining of sectioned RPE was used to visualize ApoE deposits. RPE function was assessed by measuring the trans-epithelial electrical resistance (TEER).

Results

ACC inhibition with Firsocostat increases fatty acid oxidation and remodels lipid composition, glycolytic metabolism, lipoprotein release, and enhances TEER. When human serum is used to induce sub-RPE lipoprotein accumulation, fewer lipoproteins accumulate with Firsocostat. In a culture model of Sorsby's fundus dystrophy, Firsocostat also stimulates fatty acid oxidation, improves morphology, and increases TEER.

Conclusions

Firsocostat remodels intracellular metabolism and improves RPE resilience to serum-induced lipid deposition. This effect of ACC inhibition suggests that it could be an effective strategy for diminishing drusen accumulation in the eyes of patients with AMD.

Valproic acid affects fatty acid and triglyceride metabolism in HepaRG cells exposed to fatty acids by different mechanisms

Treatment with valproate is associated with hepatic steatosis, but the mechanisms are not fully elucidated in human cell systems. We therefore investigated the effects of valproate on fatty acid and triglyceride metabolism in HepaRG cells, a human hepatoma cell line. In previously fatty acid loaded HepaRG cells, valproate impaired lipid droplet disposal starting at 1 mM after incubation for 3 or 7 days. Valproate increased the expression of genes associated with fatty acid import and triglyceride synthesis, but did not relevantly affect expression of genes engaged in fatty acid activation. Valproate impaired mitochondrial fatty acid metabolism by inhibiting β-ketothiolase and the function of the electron transport chain, which was associated with increased mitochondrial reactive oxygen species production. Valproate increased the mitochondrial DNA copy number per HepaRG cell, possibly as a consequence of impaired mitochondrial function. Valproate decreased the hepatocellular mRNA and protein expression of the fatty acid binding protein 1 (FABP1) and of the microsomal triglyceride transfer protein (MTTP) at 1 mM and increased the hepatocellular concentration of free fatty acids. Furthermore, valproate decreased protein expression and excretion of ApoB100 in HepaRG cells at 1 mM, reflecting impaired formation and excretion of very low-density lipoprotein (VLDL). In conclusion, valproate increased the hepatocellular triglyceride content by multiple mechanisms, whereby impaired expression of FABP1 and MTTP as well as impaired VLDL formation and excretion appeared to be dominant. Valproate caused cell death mainly by apoptosis, which may be a consequence of mitochondrial oxidative stress and increased hepatocellular concentration of free fatty acids.

Glucometabolic consequences of acute and prolonged inhibition of fatty acid oxidation

Excessive circulating FAs have been proposed to promote insulin resistance (IR) of glucose metabolism by increasing the oxidation of FAs over glucose. Therefore, inhibition of FA oxidation (FAOX) has been suggested to ameliorate IR. However, prolonged inhibition of FAOX would presumably cause lipid accumulation and thereby promote lipotoxicity. To understand the glycemic consequences of acute and prolonged FAOX inhibition, we treated mice with the carnitine palmitoyltransferase 1 (CPT-1) inhibitor, etomoxir (eto), in combination with short-term 45% high fat diet feeding to increase FA availability. Eto acutely increased glucose oxidation and peripheral glucose disposal, and lowered circulating glucose, but this was associated with increased circulating FAs and triacylglycerol accumulation in the liver and heart within hours. Several days of FAOX inhibition by daily eto administration induced hepatic steatosis and glucose intolerance, specific to CPT-1 inhibition by eto. Lower whole-body insulin sensitivity was accompanied by reduction in brown adipose tissue (BAT) uncoupling protein 1 (UCP1) protein content, diminished BAT glucose clearance, and increased hepatic glucose production. Collectively, these data suggest that pharmacological inhibition of FAOX is not a viable strategy to treat IR, and that sufficient rates of FAOX are required for maintaining liver and BAT metabolic function.

CDK9 Inhibition Induces a Metabolic Switch that Renders Prostate Cancer Cells Dependent on Fatty Acid Oxidation

Cyclin-dependent kinase 9 (CDK9), a key regulator of RNA-polymerase II, is a candidate drug target for cancers driven by transcriptional deregulation. Here we report a multi-omics-profiling of prostate cancer cell responses to CDK9 inhibition to identify synthetic lethal interactions. These interactions were validated using live-cell imaging, mitochondrial flux-, viability- and cell death activation assays. We show that CDK9 inhibition induces acute metabolic stress in prostate cancer cells. This is manifested by a drastic down-regulation of mitochondrial oxidative phosphorylation, ATP depletion and induction of a rapid and sustained phosphorylation of AMP-activated protein kinase (AMPK), the key sensor of cellular energy homeostasis. We used metabolomics to demonstrate that inhibition of CDK9 leads to accumulation of acyl-carnitines, metabolic intermediates in fatty acid oxidation (FAO). Acyl-carnitines are produced by carnitine palmitoyltransferase enzymes 1 and 2 (CPT), and we used both genetic and pharmacological tools to show that inhibition of CPT-activity is synthetically lethal with CDK9 inhibition. To our knowledge this is the first report to show that CDK9 inhibition dramatically alters cancer cell metabolism.

Fatty acid metabolism in CD8+ T cell memory: Challenging current concepts

CD8⁺ T cells are key members of the adaptive immune response against infections and cancer. As we discuss in this review, these cells can present diverse metabolic requirements, which have been intensely studied during the past few years. Our current understanding suggests that aerobic glycolysis is a hallmark of activated CD8⁺ T cells, while naive and memory (T_mem ) cells often rely on oxidative phosphorylation, and thus mitochondrial metabolism is a crucial determinant of CD8⁺ T_mem cell development. Moreover, it has been proposed that CD8⁺ T_mem cells have a specific requirement for the oxidation of long-chain fatty acids (LC-FAO), a process modulated in lymphocytes by the enzyme CPT1A. However, this notion relies heavily on the metabolic analysis of in vitro cultures and on chemical inhibition of CPT1A. Therefore, we introduce more recent studies using genetic models to demonstrate that CPT1A-mediated LC-FAO is dispensable for the development of CD8⁺ T cell memory and protective immunity, and question the use of chemical inhibitors to target this enzyme. We discuss insights obtained from those and other studies analyzing the metabolic characteristics of CD8⁺ T_mem cells, and emphasize how T cells exhibit flexibility in their choice of metabolic fuel.

Effect of Toxicants on Fatty Acid Metabolism in HepG2 Cells

Impairment of hepatic fatty acid metabolism can lead to liver steatosis and injury. Testing drugs for interference with hepatic fatty acid metabolism is therefore important. To find out whether HepG2 cells are suitable for this purpose, we investigated the effect of three established fatty acid metabolism inhibitors and of three test compounds on triglyceride accumulation, palmitate metabolism, the acylcarnitine pool and dicarboxylic acid accumulation in the cell supernatant and on ApoB-100 excretion in HepG2 cells. The three established inhibitors [etomoxir, methylenecyclopropylacetic acid (MCPA), and 4-bromocrotonic acid (4-BCA)] depleted mitochondrial ATP at lower concentrations than cytotoxicity occurred, suggesting mitochondrial toxicity. They inhibited palmitate metabolism at similar or lower concentrations than ATP depletion, and 4-BCA was associated with cellular fat accumulation. They caused specific changes in the acylcarnitine pattern and etomoxir an increase of thapsic (C18 dicarboxylic) acid in the cell supernatant, and did not interfere with ApoB-100 excretion (marker of VLDL export). The three test compounds (amiodarone, tamoxifen, and the cannabinoid WIN 55,212-2) depleted the cellular ATP content at lower concentrations than cytotoxicity occurred. They all caused cellular fat accumulation and inhibited palmitate metabolism at similar or higher concentrations than ATP depletion. They suppressed medium-chain acylcarnitines in the cell supernatant and amiodarone and tamoxifen impaired thapsic acid production. Tamoxifen and WIN 55,212-2 decreased cellular ApoB-100 excretion. In conclusion, the established inhibitors of fatty acid metabolism caused the expected effects in HepG2 cells. HepG cells proved to be useful for the detection of drug-associated toxicities on hepatocellular fatty acid metabolism.

Leptin regulates energy metabolism in MCF-7 breast cancer cells

Obesity is known to be a poorer prognosis factor for breast cancer in postmenopausal women. Among the diverse endocrine factors associated to obesity, leptin has received special attention since it promotes breast cancer cell growth and invasiveness, processes which force cells to adapt their metabolism to satisfy the increased demands of energy and biosynthetic intermediates. Taking this into account, our aim was to explore the effects of leptin in the metabolism of MCF-7 breast cancer cells. Polarographic analysis revealed that leptin increased oxygen consumption rate and cellular ATP levels were more dependent on mitochondrial oxidative metabolism in leptin-treated cells compared to the more glycolytic control cells. Experiments with selective inhibitors of glycolysis (2-DG), fatty acid oxidation (etomoxir) or aminoacid deprivation showed that ATP levels were more reliant on fatty acid oxidation. In agreement, levels of key proteins involved in lipid catabolism (FAT/CD36, CPT1, PPARα) and phosphorylation of the energy sensor AMPK were increased by leptin. Regarding glucose, cellular uptake was not affected by leptin, but lactate release was deeply repressed. Analysis of pyruvate dehydrogenase (PDH), lactate dehydrogenase (LDH) and pyruvate carboxylase (PC) together with the pentose-phosphate pathway enzyme glucose-6 phosphate dehydrogenase (G6PDH) revealed that leptin favors the use of glucose for biosynthesis. These results point towards a role of leptin in metabolic reprogramming, consisting of an enhanced use of glucose for biosynthesis and lipids for energy production. This metabolic adaptations induced by leptin may provide benefits for MCF-7 growth and give support to the reverse Warburg effect described in breast cancer.

Steroidogenesis in MA-10 mouse Leydig cells is altered via fatty acid import into the mitochondria

Mitochondria are home to many cellular processes, including oxidative phosphorylation and fatty acid metabolism, and in steroid-synthesizing cells, they are involved in cholesterol import and metabolism, which is the initiating step in steroidogenesis. The formation of macromolecular protein complexes aids in the regulation and efficiency of these mitochondrial functions, though because of their dynamic nature, they are hard to identify. To overcome this problem, we used Blue-Native PAGE with whole-gel mass spectrometry on isolated mitochondria from control and hormone-treated MA-10 mouse tumor Leydig cells. The presence of multiple mitochondrial protein complexes was shown. Although these were qualitatively similar under control and human chorionic gonadotropin (hCG)-stimulated conditions, quantitative differences in the components of the complexes emerged after hCG treatment. A prominent decrease was observed with proteins involved in fatty acid import into the mitochondria, implying that mitochondrial beta-oxidation is not essential for steroidogenesis. To confirm this observation, we inhibited fatty acid import utilizing the CPT1a inhibitor etomoxir, resulting in increased steroid production. Conversely, stimulation of mitochondrial beta-oxidation with metformin resulted in a dose-dependent reduction in steroidogenesis. These changes were accompanied by changes in mitochondrial respiration and in the lactic acid formed during glycolysis. Taken together, these results suggest that upon hormonal stimulation, mitochondria efficiently import cholesterol for steroid production at the expense of other lipids necessary for energy production, specifically fatty acids required for beta-oxidation.

Palmitate-induced skeletal muscle insulin resistance does not require NF-κB activation

Palmitate activates the NF-κB pathway, and induces accumulation of lipid metabolites and insulin resistance in skeletal muscle cells. Little information is available whether and how these processes are causally related. Therefore, the objectives were to investigate whether intra-cellular lipid metabolites are involved in FA-induced NF-κB activation and/or insulin resistance in skeletal muscle and to investigate whether FA-induced insulin resistance and NF-κB activation are causally related. Inhibiting DGAT or CPT-1 by using, respectively, amidepsine or etomoxir increased DAG accumulation and sensitized myotubes to palmitate-induced insulin resistance. While co-incubation of palmitate with etomoxir increased NF-κB transactivation, co-incubation with amidepsine did not, indicating that DAG accumulation is associated with insulin resistance but not with NF-κB activation. Furthermore, pharmacological or genetic inhibition of the NF-κB pathway could not prevent palmitate-induced insulin resistance. In conclusion, we have demonstrated that activation of the NF-κB pathway is not required for palmitate-induced insulin resistance in skeletal muscle cells.

Systemic treatment with the antidiabetic drug metformin selectively impairs p53-deficient tumor cell growth

The effect of the antidiabetic drug metformin on tumor growth was investigated using the paired isogenic colon cancer cell lines HCT116 p53(+/+) and HCT116 p53(-/-). Treatment with metformin selectively suppressed the tumor growth of HCT116 p53(-/-) xenografts. Following treatment with metformin, we detected increased apoptosis in p53(-/-) tumor sections and an enhanced susceptibility of p53(-/-) cells to undergo apoptosis in vitro when subject to nutrient deprivation. Metformin is proposed to function in diabetes treatment as an indirect activator of AMP-activated protein kinase (AMPK). Treatment with AICAR, another AMPK activator, also showed a selective ability to inhibit p53(-/-) tumor growth in vivo. In the presence of either of the two drugs, HCT116 p53(+/+) cells, but not HCT116 p53(-/-) cells, activated autophagy. A similar p53-dependent induction of autophagy was observed when nontransformed mouse embryo fibroblasts were treated. Treatment with either metformin or AICAR also led to enhanced fatty acid beta-oxidation in p53(+/+) MEFs, but not in p53(-/-) MEFs. However, the magnitude of induction was significantly lower in metformin-treated cells, as metformin treatment also suppressed mitochondrial electron transport. Metformin-treated cells compensated for this suppression of oxidative phosphorylation by increasing their rate of glycolysis in a p53-dependent manner. Together, these data suggest that metformin treatment forces a metabolic conversion that p53(-/-) cells are unable to execute. Thus, metformin is selectively toxic to p53-deficient cells and provides a potential mechanism for the reduced incidence of tumors observed in patients being treated with metformin.

Cardiac substrate uptake and metabolism in obesity and type-2 diabetes: role of sarcolemmal substrate transporters

Cardiovascular disease is the primary cause of death in obesity and type-2 diabetes mellitus (T2DM). Alterations in substrate metabolism are believed to be involved in the development of both cardiac dysfunction and insulin resistance in these conditions. Under physiological circumstances the heart utilizes predominantly long-chain fatty acids (LCFAs) (60-70%), with the remainder covered by carbohydrates, i.e., glucose (20%) and lactate (10%). The cellular uptake of both LCFA and glucose is regulated by the sarcolemmal amount of specific transport proteins, i.e., fatty acid translocase (FAT)/CD36 and GLUT4, respectively. These transport proteins are not only present at the sarcolemma, but also in intracellular storage compartments. Both an increased workload and the hormone insulin induce translocation of FAT/CD36 and GLUT4 to the sarcolemma. In this review, recent findings on the insulin and contraction signalling pathways involved in substrate uptake and utilization by cardiac myocytes under physiological conditions are discussed. New insights in alterations in substrate uptake and utilization during insulin resistance and its progression towards T2DM suggest a pivotal role for substrate transporters. During the development of obesity towards T2DM alterations in cardiac lipid homeostasis were found to precede alterations in glucose homeostasis. In the early stages of T2DM, relocation of FAT/CD36 to the sarcolemma is associated with the myocardial accumulation of triacylglycerols (TAGs) eventually leading to an impaired insulin-stimulated GLUT4-translocation. These novel insights may result in new strategies for the prevention of development of cardiac dysfunction and insulin resistance in obesity and T2DM.

The glucose dependence of Akt-transformed cells can be reversed by pharmacologic activation of fatty acid beta-oxidation

Activation of the oncogenic kinase Akt stimulates glucose uptake and metabolism in cancer cells and renders these cells susceptible to death in response to glucose withdrawal. Here we show that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) reverses the sensitivity of Akt-expressing glioblastoma cells to glucose deprivation. AICAR's protection depends on the activation of AMPK, as expression of a dominant-negative form of AMPK abolished this effect. AMPK is a cellular energy sensor whose activation can both block anabolic pathways such as protein synthesis and activate catabolic reactions such as fatty acid oxidation to maintain cellular bioenergetics. While rapamycin treatment mimicked the effect of AICAR on inhibiting markers of cap-dependent translation, it failed to protect Akt-expressing cells from death upon glucose withdrawal. Compared to control cells, Akt-expressing cells were impaired in the ability to induce fatty acid oxidation in response to glucose deprivation unless stimulated with AICAR. Stimulation of fatty acid oxidation was sufficient to maintain cell survival as activation of fatty acid oxidation with bezafibrate also protected Akt-expressing cells from glucose withdrawal-induced death. Conversely, treatment with a CPT-1 inhibitor to block fatty acid import into mitochondria prevented AICAR from stimulating fatty acid oxidation and promoting cell survival in the absence of glucose. Finally, cell survival did not require reversal of Akt's effects on either protein translation or lipid synthesis as the addition of the cell penetrant oxidizable substrate methyl-pyruvate was sufficient to maintain survival of Akt-expressing cells deprived of glucose. Together, these data suggest that activation of Akt blocks the ability of cancer cells to metabolize nonglycolytic bioenergetic substrates, leading to glucose addiction.

Inhibition of glycogen synthesis by fatty acid in C(2)C(12) muscle cells is independent of PKC-alpha, -epsilon, and -theta

We have previously shown that glycogen synthesis is reduced in lipid-treated C(2)C(12) skeletal muscle myotubes and that this is independent of changes in glucose uptake. Here, we tested whether mitochondrial metabolism of these lipids is necessary for this inhibition and whether the activation of specific protein kinase C (PKC) isoforms is involved. C(2)C(12) myotubes were pretreated with fatty acids and subsequently stimulated with insulin for the determination of glycogen synthesis. The carnitine palmitoyltransferase-1 inhibitor etomoxir, an inhibitor of beta-oxidation of acyl-CoA, did not protect against the inhibition of glycogen synthesis caused by the unsaturated fatty acid oleate. In addition, although oleate caused translocation, indicating activation, of individual PKC isoforms, inhibition of PKC by pharmacological agents or adenovirus-mediated overexpression of dominant negative PKC-alpha, -epsilon, or -theta mutants was unable to prevent the inhibitory effects of oleate on glycogen synthesis. We conclude that neither mitochondrial lipid metabolism nor activation of PKC-alpha, -epsilon, or -theta plays a role in the direct inhibition of glycogen synthesis by unsaturated fatty acids.

Inhibition of mitochondrial carnitine palmitoyltransferase-1 by a trimetazidine derivative, S-15176

The purpose of this study was to investigate the possible effect of the trimetazidine derivative S-15176 on carnitine palmitoyltransferase1 (CPT-1) activity in rat heart and liver mitochondria. S-15176 was compared with the other antianginal agents amiodarone, perhexiline and trimetazidine, which do not show any hemodynamic effects and which are believed to exert their effects by switching the cellular metabolism towards glucose utilization at the expense of lipid metabolism, increasing the yield of oxygen utilization. S-15176 inhibited CPT-1 in vitro and was more effective in heart (IC(50)= 16.8 micro M) than in liver ( 50.8 +/- 3.0 micro M). In the heart, its was less effective than the physiological inhibitor malonyl-CoA (IC(50)= 2.1 micro M), but it was more potent than amiodarone (IC(50)= 140 micro M). Kinetic experiments demonstrated a non-competitive inhibition of CPT-1 by S-15176 indicating that the two compounds did not share the same site of action. CPT-1 inhibition was also obvious ex vivo, in heart and liver tissues, after a 2 week treatment with S-15176. This inhibitory effect may shift heart and liver metabolism from fatty acid to glucose oxidation and contribute to the anti-ischemic effects of the drug.

Carnitine palmitoyl transferase I and the control of beta-oxidation in heart mitochondria

Mitochondrial beta-oxidation provides much of the fuel requirements of heart and skeletal muscle despite the malonyl-CoA concentration greatly exceeding the IC(50) of carnitine palmitoyl transferase for malonyl-CoA. To try to explore the relationship between inhibition of carnitine palmitoyl transferase I activity and beta-oxidation flux, we measured the flux control coefficient of carnitine palmitoyl transferase I over beta-oxidation carbon flux in suckling rat heart mitochondria. The flux control coefficient was found to be 0.08 +/- 0.05 and 50% of carnitine palmitoyl transferase I activity could be inhibited before beta-oxidation flux was affected. These observations may help to explain the presence of high rates of beta-oxidation despite the high concentration of malonyl-CoA in rat heart; we hypothesize that although not rate-limiting in vitro, carnitine palmitoyl transferase is rate-limiting in vivo because of the high malonyl-CoA concentration in heart and muscle.

Fatty acid synthase inhibition in human breast cancer cells leads to malonyl-CoA-induced inhibition of fatty acid oxidation and cytotoxicity

Inhibition of fatty acid synthase (FAS) induces apoptosis in human breast cancer cells in vitro and in vivo without toxicity to proliferating normal cells. We have previously shown that FAS inhibition causes a rapid increase in malonyl-CoA levels identifying malonyl-CoA as a potential trigger of apoptosis. In this study we further investigated the role of malonyl-CoA during FAS inhibition. We have found that: [i] inhibition of FAS with cerulenin causes carnitine palmitoyltransferase-1 (CPT-1) inhibition and fatty acid oxidation inhibition in MCF-7 human breast cancer cells likely mediated by elevation of malonyl-CoA; [ii] cerulenin cytotoxicity is due to the nonphysiological state of increased malonyl-CoA, decreased fatty acid oxidation, and decreased fatty acid synthesis; and [iii] the cytotoxic effect of cerulenin can be mimicked by simultaneous inhibition of CPT-1, with etomoxir, and fatty acid synthesis with TOFA, an acetyl-CoA carboxylase (ACC) inhibitor. This study identifies CPT-1 and ACC as two new potential targets for cancer chemotherapy.

Regulation of the activity of caspases by L-carnitine and palmitoylcarnitine

L-Carnitine facilitates the transport of fatty acids into the mitochondrial matrix where they are used for energy production. Recent studies have shown that L-carnitine is capable of protecting the heart against ischemia/reperfusion injury and has beneficial effects against Alzheimer's disease and AIDS. The mechanism of action, however, is not yet understood. In the present study, we found that in Jurkat cells, L-carnitine inhibited apoptosis induced by Fas ligation. In addition, 5 mM carnitine potently inhibited the activity of recombinant caspases 3, 7 and 8, whereas its long-chain fatty acid derivative palmitoylcarnitine stimulated the activity of all the caspases. Palmitoylcarnitine reversed the inhibition mediated by carnitine. Levels of carnitine and palmitoyl-CoA decreased significantly during Fas-mediated apoptosis, while palmitoylcarnitine formation increased. These alterations may be due to inactivation of beta-oxidation or to an increase in the activity of the enzyme that converts carnitine to palmitoylcarnitine, carnitine palmitoyltransferase I (CPT I). In support of the latter possibility, fibroblasts deficient in CPT I activity were relatively resistant to staurosporine-induced apoptosis. These observations suggest that caspase activity may be regulated in part by the balance of carnitine and palmitoylcarnitine.

Stereoselective synthesis of a conformationally defined cyclohexyl carnitine analogue that binds CPT-1 with high affinity

Carnitine (1, 3-hydroxy-4-trimethylammoniobutyrate) is important in mammalian tissue as a carrier of acyl groups. In order to explore the binding requirements of the carnitine acyltransferases for carnitine, we designed conformationally defined cyclohexyl carnitine analogues. These diastereomers contain the required gauche conformation between the trimethylammonium and hydroxy groups but vary the conformation between the hydroxy and carboxylic acid groups. Here we describe the synthesis and biological activity of the all-trans diastereomer (2), which was prepared by the ring opening of trans-methyl 2,3-epoxycylohexanecarboxylate with NaN3. Racemic 2 was a competitive inhibitor of neonatal rat cardiac myocyte CPT-1 (K(i) 0.5 mM for racemic 2; K(m) 0.2 mM for L-carnitine) and a noncompetitive inhibitor of neonatal rat cardiac myocyte CPT-2 (K(i) 0.67 mM). These results suggest that 2 represents the bound conformation of carnitine for CPT-1.

Evidence that carnitine palmitoyltransferase I (CPT I) is expressed in microsomes and peroxisomes of rat liver. Distinct immunoreactivity of the N-terminal domain of the microsomal protein

Mitochondria, microsomes and peroxisomes all express overt (cytosol-facing) carnitine palmitoyltransferase activity that is inhibitable by malonyl-CoA. The overt carnitine palmitoyltransferase activity (CPTo) associated with the different fractions was measured. Mitochondria accounted for 65% of total cellular CPTo activity, with the microsomal and peroxisomal contributions accounting for the remaining 25% and 10%, respectively. In parallel experiments, rat livers were perfused in situ with medium containing dinitrophenyl (DNP)-etomoxir in order to inhibit quantitatively and label covalently (with DNP-etomoxiryl-CoA) the molecular species responsible for CPTo activity in each of the membrane systems under near-physiological conditions. In all three membrane fractions, a single protein with an identical molecular mass of approximately 88,000 kDa (p88) was labelled after DNP-etomoxir perfusion of the liver. The abundance of labelled p88 was quantitatively related to the respective specific activities of CPTo in each fraction. On Western blots the same protein was immunoreactive with three anti-peptide antibodies raised against linear epitopes of the cytosolic N- and C-domains and of the inter-membrane space loop (L) domain of the mitochondrial enzyme (L-CPT I). However, the reaction of the microsomal protein with the anti-N peptide antibody (raised against epitope Val-14-Lys-29 of CPT I) was an order of magnitude stronger than expected from either microsomal CPTo activity or its DNP-etomoxiryl-CoA labelling. This suggests that the N-terminal domain of the microsomal protein differs from that in the mitochondrial or peroxisomal protein. This conclusion was confirmed using antibody back-titration experiments, in which the binding of anti-N and anti-C antibodies by mitochondria and microsomes was quantified.

Synthesis of 2-[6-(2,4-dinitrophenoxy)hexyl]oxiranecarboxylic acid: a selective carnitine palmitoyltransferase-1 inhibitor

Carnitine palmitoyltransferases 1 and 2 (CPT-1 and CPT-2) catalyze the transfer of long chain fatty acids between carnitine and coenzyme A. Unlike CPT-2, CPT-1 exists in at least two isoforms with different physical and kinetic properties. Liver and skeletal muscle each contain a different isoform of CPT-1. Cardiac muscle contains both isoforms, and the minor component is identical to the isoform found in the liver. 2-[6-(2,4-Dinitrophenoxy)hexyl]oxiranecarboxylic acid (2) was reported to be a selective inhibitor for the liver isoform of CPT-1. A synthesis of 2 is described here which involves the reaction of diethyl malonate with 1-bromo-6-phenoxyhexane.

The flux control coefficient of carnitine palmitoyltransferase I on palmitate beta-oxidation in rat hepatocyte cultures

Two important factors that determine the flux of hepatic beta-oxidation of long-chain fatty acids are the availability of fatty acid and the activity of carnitine palmitoyltransferase I (CPT I). Using Metabolic Control Analysis, the flux control coefficient of CPT I in rat hepatocyte monolayers was determined by titration with 2-[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylate (Etomoxir), which is converted to Etomoxir-CoA, an irreversible inhibitor of CPT I. We measured CPT I activity and flux through beta-oxidation at 0.2 mM and 1.0 mM palmitate to simulate substrate concentrations in fed and fasted states. Rates of beta-oxidation were 4.5-fold higher at 1. 0 mM palmitate compared with 0.2 mM palmitate. Flux control coefficients of CPT I, estimated by two independent methods, were similar: 0.67 and 0.79 for 0.2 mM palmitate, and 0.68 and 0.77 for 1 mM palmitate. It is concluded that the regulatory potential of CPT I is similar at low and high physiological concentrations of palmitate.

Effect of pH and acyl-CoA chain length on the conversion of heart mitochondrial CPT-I/CPTo to a high affinity, malonyl-CoA-inhibited state

The effect of pH and acyl-CoA chain length on the conversion of the malonyl-CoA-sensitive carnitine palmitoyltransferase (CPT-I/CPTo) to a high-affinity, malonyl-CoA-inhibited state using a particle derived from rat heart mitochondria was determined. Preincubation with malonyl-CoA for one minute in the absence of acyl-CoA substrate lowers the IC50 for malonyl-CoA from 2 microM, 14 microM, and 15 microM at pH 7.4 to 15 nM, 14 nM, and 14 nM for decanyl-, lauryl-, and palmitoyl-CoA, respectively. Reducing the pH to 7.1 and 6.8 had little effect on the transition to the high affinity, malonyl-CoA-inhibited state. Preincubation of malonyl-CoA with the acyl-CoA, but not with L-carnitine, prevented the transition to the high affinity, malonyl-CoA-inhibited state.

Over-expression and characterization of active recombinant rat liver carnitine palmitoyltransferase II using baculovirus

The cDNA encoding rat liver carnitine palmitoyltransferase II (CPT-II) was heterologously expressed using a recombinant baculovirus/insect cell system. Unlike Escherichia coli, the baculovirus-infected insect cells expressed mostly soluble active recombinant CPT-II (rCPT-II). CPT activity from crude lysates of recombinant baculovirus-infected insect cells was maximal between 50 and 72 h post-infection, with a peak specific activity of 100-200 times that found in the mock- or wild-type-infected control lysates. Milligram quantities (up to 1.8 mg/l of culture) of active rCPT-II were chromatographically purified from large-scale cultures of insect cells infected with the recombinant baculovirus. The rCPT-II was found to be: (1) similar in size to the native rat liver enzyme (approximately 70 kDa) as judged by SDS/PAGE; (2) immunoreactive with a polyclonal serum raised against rat liver CPT-II; and (3) not glycosylated. Kinetic analysis of soluble rCPT-II revealed Km values for carnitine and palmitoyl-CoA of 950 +/- 27 microM and 34 +/- 5.6 microM respectively.

The effect of etomoxir on the mRNA levels of enzymes involved in ketogenesis and cholesterogenesis in rat liver

The effects of acute treatment with 2-[6-(4-chlorophenoxy)hexyl]-oxirane-2-carboxylate (etomoxir), an antiketonaemic and antidiabetic drug, on the mRNA levels of several regulatory enzymes of ketogenesis, cholesterogenesis, and fatty acid synthesis in rats were determined. In rats treated with etomoxir, mRNA levels for mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase and carnitine palmitoyl transferase I (CPT I) remained unchanged, while mRNA levels for carnitine palmitoyl transferase II (CPT II) significantly increased 2-fold. Injection of etomoxir produced no effect on the mRNA levels of cytosolic HMG-CoA synthase but increased the mRNA levels of HMG-CoA reductase 2.5-fold. Etomoxir led to a 3-fold increase in the mRNA levels of fatty acid synthase of rats under acute treatment. Rats fed with a fat diet significantly increased the expression of mitochondrial HMG-CoA synthase, CPT I and CPT II 3-fold in all cases, while 2-(diethylhexyl)phthalate (DEHP) produced increases in the expression of these genes (5-, 4- and 12-fold, respectively). The mRNA levels of HMG-CoA reductase were not changed by either DEHP or fat diet, while DEHP increased cytosolic HMG-CoA synthase 2.5-fold. DEHP did not change the mRNA levels for fatty acid synthase. It was concluded that etomoxir does not produce its hypoketonaemic, hypocholesteraemic or hypolipogenic effects through changes in the genetic expression of the regulatory enzymes of these pathways, but probably due to the shortage of their common substrate, acetyl-CoA, because of the inhibitory action on CPT I.

Evidence for the involvement of carnitine-dependent long-chain acyltransferases in neuronal triglyceride and phospholipid fatty acid turnover

This study focuses on the potential involvement of carnitine palmitoyltransferase (CPT) on the phospholipid and triglyceride fatty acid turnover in neurons. This category of enzymes, which has been identified in several rat brain tissues, is well known for its role in modulating cellular fatty acid oxidation. Neuronal cell cultures from rat brain cortex incorporated radioactive palmitate or oleate into phospholipids and triglycerides. The largest fraction of radioactive fatty acids was recovered in phosphatidylcholine followed by triglycerides and, to a lesser extent, phosphatidylethanolamine. CPT activity measured in neuronal lysates obtained from neurons treated with 40 microM 2-tetradecylglycidic acid (TDGA) was almost completely abolished. Furthermore, between 2 and 10 microM TDGA CPT activity dropped more rapidly than between 10 and 40 microM. When the cells were pretreated with TDGA, the incorporation process of either radioactive fatty acid into triglycerides was dose-dependently suppressed. Radioactive fatty acid incorporation into phosphatidylcholine was significantly decreased in cells treated with TDGA. In contrast, phosphatidylethanolamine reacylation was essentially not affected by the CPT inhibitor. Similar results on the fatty acid incorporation into triglycerides and phospholipids were observed with neurons treated with palmitoyl-DL-aminocarnitine (PAC), a reversible CPT inhibitor, which does not consume free CoA. These effects do not seem to be the result of an inhibitory activity toward one of the steps involved in the acylation-deacylation process of triglycerides or phospholipids, as cellular lysates from TDGA-treated cells or lysates containing PAC incorporated radioactive fatty acids at rates comparable to controls. Our results suggest that CPT may be an important partner in the pathway of phospholipid and triglyceride fatty acid turnover in neurons.

Beta-oxidation of fatty acids in cultured human skin fibroblasts devoid of the capacity for oxidative phosphorylation

Prolonged treatment of cultured cells with ethidium bromide results in loss of the capacity for oxidative phosphorylation. Because of the tight coupling between mitochondrial beta-oxidation of fatty acids and the activity of the respiratory chain, such cells may be used to study the contribution of mitochondria and peroxisomes to fatty acid beta-oxidation. To investigate this, human skin fibroblasts were cultured in the presence of ethidium bromide for at least 10 cell generations, resulting in a virtually complete absence of oxidative phosphorylation as demonstrated directly in digitonin-permeabilized fibroblasts. The cells showed a lowered ATP/ADP ratio, most likely as the consequence of the inability to generate ATP via oxidative phosphorylation. The loss of the capacity for oxidative phosphorylation was also reflected in an increased cytosolic NADH/NAD+ ratio: the cells showed a highly elevated lactate/pyruvate ratio in the suspending medium when incubated with glucose. The beta-oxidation of octanoic and palmitic acid was dramatically decreased, suggesting that the beta-oxidation of these fatty acids takes place predominantly (> 90%) in mitochondria, at least in the cells studied. In contrast, the rates of pristanic and cerotic acid beta-oxidation were only slightly decreased, suggesting that this is mainly a peroxisomal process. The reduction of beta-oxidation of cerotic and pristanic acid, 27% and 15%, respectively, is most likely due to a lowered ATP level and an increased NADH/NAD(+)-redoxstate in these cells. We conclude that fibroblasts subjected to prolonged treatment with ethidium bromide can be used as a model system to study the substrate specificity and functional characteristics of the peroxisomal beta-oxidation system.