Carnitine o-octanoyltransferase is a p53 target that promotes oxidative metabolism and cell survival following nutrient starvation

Whereas it is known that p53 broadly regulates cell metabolism, the specific activities that mediate this regulation remain partially understood. Here, we identified carnitine o-octanoyltransferase (CROT) as a p53 transactivation target that is upregulated by cellular stresses in a p53-dependent manner. CROT is a peroxisomal enzyme catalyzing very long-chain fatty acids conversion to medium chain fatty acids that can be absorbed by mitochondria during β-oxidation. p53 induces CROT transcription through binding to consensus response elements in the 5'-UTR of CROT mRNA. Overexpression of WT but not enzymatically inactive mutant CROT promotes mitochondrial oxidative respiration, while downregulation of CROT inhibits mitochondrial oxidative respiration. Nutrient depletion induces p53-dependent CROT expression that facilitates cell growth and survival; in contrast, cells deficient in CROT have blunted cell growth and reduced survival during nutrient depletion. Together, these data are consistent with a model where p53-regulated CROT expression allows cells to be more efficiently utilizing stored very long-chain fatty acids to survive nutrient depletion stresses.

Carnitine octanoyltransferase is important for the assimilation of exogenous acetyl-L-carnitine into acetyl-CoA in mammalian cells

In eukaryotes, carnitine is best known for its ability to shuttle esterified fatty acids across mitochondrial membranes for β-oxidation. It also returns to the cytoplasm, in the form of acetyl-L-carnitine (LAC), some of the resulting acetyl groups for posttranslational protein modification and lipid biosynthesis. While dietary LAC supplementation has been clinically investigated, its effects on cellular metabolism are not well understood. To explain how exogenous LAC influences mammalian cell metabolism, we synthesized isotope-labeled forms of LAC and its analogs. In cultures of glucose-limited U87MG glioma cells, exogenous LAC contributed more robustly to intracellular acetyl-CoA pools than did β-hydroxybutyrate, the predominant circulating ketone body in mammals. The fact that most LAC-derived acetyl-CoA is cytosolic is evident from strong labeling of fatty acids in U87MG cells by exogenous 13C2-acetyl-L-carnitine. We found that the addition of d3-acetyl-L-carnitine increases the supply of acetyl-CoA for cytosolic posttranslational modifications due to its strong kinetic isotope effect on acetyl-CoA carboxylase, the first committed step in fatty acid biosynthesis. Surprisingly, whereas cytosolic carnitine acetyltransferase is believed to catalyze acetyl group transfer from LAC to coenzyme A, CRAT-/- U87MG cells were unimpaired in their ability to assimilate exogenous LAC into acetyl-CoA. We identified carnitine octanoyltransferase as the key enzyme in this process, implicating a role for peroxisomes in efficient LAC utilization. Our work has opened the door to further biochemical investigations of a new pathway for supplying acetyl-CoA to certain glucose-starved cells.

Proline/Glycine residues of the PG-levels guide conformational changes along the transport cycle in the mitochondrial carnitine/acylcarnitine carrier (SLC25A20)

Mitochondrial carnitine/acylcarnitine carrier (CAC) is a member of the mitochondrial carrier (MC) family and imports acylcarnitine into the mitochondrial matrix in exchange for carnitine, playing a pivotal role in carnitine shuttle, crucial for fatty acid oxidation. The crystallized structure of CAC has not been solved yet, however, the availability of several in vitro/in silico studies, also based on the crystallized structures of the ADP/ATP carrier in the cytosolic-conformation and in the matrix-conformation, has made possible to confirm the hypothesis of the single-binding centered-gated pore mechanism for all the members of the MC family. In addition, our recent bioinformatics analyses allowed quantifying in silico the importance of protein residues of MC substrate binding region, of those involved in the formation of the matrix and cytosolic gates, and of those belonging to the Pro/Gly (PG) levels, proposed to be crucial for the tilting/kinking/bending of the six MC transmembrane helices, funneling the substrate translocation pathway. Here we present a combined in silico/in vitro analysis employed for investigating the role played by a group of 6 proline residues and 6 glycine residues, highly conserved in CAC, belonging to MC PG-levels. Residues of the PG-levels surround the similarly located MC common substrate binding region, and were proposed to lead conformational changes and substrate translocation, following substrate binding. For our analysis, we employed 3D molecular modeling approaches, alanine scanning site-directed mutagenesis and in vitro transport assays. Our analysis reveals that P130 (H3), G268 (H6) and G220 (H5), mutated in alanine, affect severely CAC transport activity (mutant catalytic efficiency lower than 5 % compared to the wild type CAC), most likely due to their major role in triggering CAC conformational changes, following carnitine binding. Notably, P30A (H1) and G121A (H3) CAC mutants, increase the carnitine uptake up to 217 % and 112 %, respectively, compared to the wild type CAC.

NMR Characterization of Long-Chain Fatty Acylcarnitine Binding to the Mitochondrial Carnitine/Acylcarnitine Carrier

The mitochondrial carnitine/acylcarnitine carrier (CAC) transports short-, medium- and long-carbon chain acylcarnitines across the mitochondrial inner membrane in exchange for carnitine. How CAC recognizes the substrates with various fatty acyl groups, especially long-chain fatty acyl groups, remains unclear. Here, using nuclear magnetic resonance (NMR) technology, we have shown that the CAC protein reconstituted into a micelle system exhibits a typical six transmembrane structure of the mitochondrial carrier family. The chemical shift perturbation patterns of different fatty acylcarnitines suggested that the segment A76–G81 in CAC specifically responds to the long-chain fatty acylcarnitine. Molecular dynamics (MD) simulations of palmitoyl-L-carnitine inside the CAC channel showed the respective interaction and motion of the long-chain acylcarnitine in CAC at the cytosol-open state and matrix-open state. Our data provided a molecular-based understanding of CAC structure and transport mechanism.

CROT (Carnitine O-Octanoyltransferase) Is a Novel Contributing Factor in Vascular Calcification via Promoting Fatty Acid Metabolism and Mitochondrial Dysfunction

OBJECTIVE

Vascular calcification is a critical pathology associated with increased cardiovascular event risk, but there are no Food and Drug Administration-approved anticalcific therapies. We hypothesized and validated that an unbiased screening approach would identify novel mediators of human vascular calcification. Approach and Results: We performed an unbiased quantitative proteomics and pathway network analysis that identified increased CROT (carnitine O-octanoyltransferase) in calcifying primary human coronary artery smooth muscle cells (SMCs). Additionally, human carotid artery atherosclerotic plaques contained increased immunoreactive CROT near calcified regions. CROT siRNA reduced fibrocalcific response in calcifying SMCs. In agreement, histidine 327 to alanine point mutation inactivated human CROT fatty acid metabolism enzymatic activity and suppressed SMC calcification. CROT siRNA suppressed type 1 collagen secretion, and restored mitochondrial proteome alterations, and suppressed mitochondrial fragmentation in calcifying SMCs. Lipidomics analysis of SMCs incubated with CROT siRNA revealed increased eicosapentaenoic acid, a vascular calcification inhibitor. CRISPR/Cas9-mediated Crot deficiency in LDL (low-density lipoprotein) receptor-deficient mice reduced aortic and carotid artery calcification without altering bone density or liver and plasma cholesterol and triglyceride concentrations.

CONCLUSIONS

CROT is a novel contributing factor in vascular calcification via promoting fatty acid metabolism and mitochondrial dysfunction, as such CROT inhibition has strong potential as an antifibrocalcific therapy.

Single Cell Gene Co-Expression Network Reveals FECH/CROT Signature as a Prognostic Marker

Aberrant activation of signaling pathways is frequently observed and reported to be associated with the progression and poor prognosis of prostate cancer (PCa). We aimed to identify key biological processes regulated by androgen receptor (AR) using gene co-expression network from single cell resolution. The bimodal index was used to evaluate whether two subpopulations exist among the single cells. Gene expression among single cells revealed averaging pitfalls and bimodality pattern. Weighted gene co-expression network analysis (WGCNA) was used to identify modules of highly correlated genes. Twenty-nine gene modules were identified and AR-regulated modules were screened by significantly overlapping reported androgen induced differentially expressed genes. The biological function "generation of precursor metabolites and energy" was significantly enriched by AR-regulated modules with bimodality, presenting differential androgen response among subpopulations. Integrating with public ChIP-seq data, two genes FECH, and CROT has AR binding sites. Public in vitro studies also show that androgen regulates FECH and CROT. After receiving androgen deprivation therapy, patients lowly express FECH and CROT. Further survival analysis indicates that FECH/CROT signature can predict PCa recurrence. We reveal the heterogeneous function of "generation of precursor metabolites and energy" upon androgen stimulation from the perspective of single cells. Inhibitors targeting this biological process will facilitate to prevent prostate cancer progression.

Physiology of L-carnitine in plants in light of the knowledge in animals and microorganisms

L-carnitine is present in all living kingdoms where it acts in diverse physiological processes. It is involved in lipid metabolism in animals and yeasts, notably as an essential cofactor of fatty acid intracellular trafficking. Its physiological significance is poorly understood in plants, but L-carnitine may be linked to fatty acid metabolism among other roles. Indeed, carnitine transferases activities and acylcarnitines are measured in plant tissues. Current knowledge of fatty acid trafficking in plants rules out acylcarnitines as intermediates of the peroxisomal and mitochondrial fatty acid metabolism, unlike in animals and yeasts. Instead, acylcarnitines could be involved in plastidial exportation of de novo fatty acid, or importation of fatty acids into the ER, for synthesis of specific glycerolipids. L-carnitine also contributes to cellular maintenance though antioxidant and osmolyte properties in animals and microbes. Recent data indicate similar features in plants, together with modulation of signaling pathways. The biosynthesis of L-carnitine in the plant cell shares similar precursors as in the animal and yeast cells. The elucidation of the biosynthesis pathway of L-carnitine, and the identification of the enzymes involved, is today essential to progress further in the comprehension of its biological significance in plants.

The carnitine system and cancer metabolic plasticity

Metabolic flexibility describes the ability of cells to respond or adapt its metabolism to support and enable rapid proliferation, continuous growth, and survival in hostile conditions. This dynamic character of the cellular metabolic network appears enhanced in cancer cells, in order to increase the adaptive phenotype and to maintain both viability and uncontrolled proliferation. Cancer cells can reprogram their metabolism to satisfy the energy as well as the biosynthetic intermediate request and to preserve their integrity from the harsh and hypoxic environment. Although several studies now recognize these reprogrammed activities as hallmarks of cancer, it remains unclear which are the pathways involved in regulating metabolic plasticity. Recent findings have suggested that carnitine system (CS) could be considered as a gridlock to finely trigger the metabolic flexibility of cancer cells. Indeed, the components of this system are involved in the bi-directional transport of acyl moieties from cytosol to mitochondria and vice versa, thus playing a fundamental role in tuning the switch between the glucose and fatty acid metabolism. Therefore, the CS regulation, at both enzymatic and epigenetic levels, plays a pivotal role in tumors, suggesting new druggable pathways for prevention and treatment of human cancer.

Molecular Regulation of Fatty Acid Oxidation in Skeletal Muscle during Aerobic Exercise

This review summarizes how fatty acid (FA) oxidation is regulated in skeletal muscle during exercise. From the available evidence it seems that acetyl-CoA availability in the mitochondrial matrix adjusts FA oxidation to exercise intensity and duration. This is executed at the step of mitochondrial fatty acyl import, as the extent of acetyl group sequestration by carnitine determines the availability of carnitine for the carnitine palmitoyltransferase 1 (CPT1) reaction. The rate of glycolysis seems therefore to be central to the amount of β-oxidation-derived acetyl-CoA that is oxidized in the tricarboxylic acid (TCA) cycle. FA oxidation during exercise is also determined by FA availability to mitochondria, dependent on trans-sarcolemmal FA uptake via cluster of differentiation 36/SR-B2 (CD36) and FAs mobilized from myocellular lipid droplets.

Nitric oxide inhibits the mitochondrial carnitine/acylcarnitine carrier through reversible S-nitrosylation of cysteine 136

S-nitrosylation of the mitochondrial carnitine/acylcarnitine transporter (CACT) has been investigated on the native and the recombinant proteins reconstituted in proteoliposomes, and on intact mitochondria. The widely-used NO-releasing compound, GSNO, strongly inhibited the antiport measured in proteoliposomes reconstituted with the native CACT from rat liver mitochondria or the recombinant rat CACT over-expressed in E. coli. Inhibition was reversed by the reducing agent dithioerythritol, indicating a reaction mechanism based on nitrosylation of Cys residues of the CACT. The half inhibition constant (IC50) was very similar for the native and recombinant proteins, i.e., 74 and 71μM, respectively. The inhibition resulted to be competitive with respect the substrate, carnitine. NO competed also with NEM, correlating well with previous data showing interference of NEM with the substrate transport path. Using a site-directed mutagenesis approach on Cys residues of the recombinant CACT, the target of NO was identified. C136 plays a major role in the reaction mechanism. The occurrence of S-nitrosylation was demonstrated in intact mitochondria after treatment with GSNO, immunoprecipitation and immunostaining of CACT with a specific anti NO-Cys antibody. In parallel samples, transport activity of CACT measured in intact mitochondria, was strongly inhibited after GSNO treatment. The possible physiological and pathological implications of the post-translational modification of CACT are discussed.

A novel method for determining peroxisomal fatty acid β-oxidation

The purpose of this study is to establish an assay method to screen for chemical compounds that stimulate peroxisomal fatty acid β-oxidation activity in X-linked adrenoleukodystropy (X-ALD) fibroblasts. In this investigation, we used 12-(1-pyrene)dodecanoic acid (pyrene-C12:0), a fluorescent fatty acid analog, as a substrate for fatty acid β-oxidation. When human skin fibroblasts were incubated with pyrene-C12:0, β-oxidation products such as pyrene-C10:0 and pyrene-C8:0 were generated time-dependently. These β-oxidation products were scarcely detected in the fibroblasts from patients with Zellweger syndrome, a peroxisomal biogenesis disorder. In contrast, in fibroblasts with mitochondrial carnitine-acylcarnitine translocase deficiency, the β-oxidation products were detected at a level similar to control fibroblasts. These results indicate that the β-oxidation of pyrene-C12:0 takes place in peroxisomes, but not mitochondria, so pyrene-C12:0 is useful for measuring peroxisomal fatty acid β-oxidation activity. In X-ALD fibroblasts, the β-oxidation activity for pyrene-C12:0 was approximately 40 % of control fibroblasts, which is consistent with previous results using [1-(14)C]lignoceric acid as the substrate. The present study provides a convenient procedure for screening chemical compounds that stimulate the peroxisomal fatty acid β-oxidation in X-ALD fibroblasts.

Nutritional and Hormonal Regulation of Citrate and Carnitine/Acylcarnitine Transporters: Two Mitochondrial Carriers Involved in Fatty Acid Metabolism

The transport of solutes across the inner mitochondrial membrane is catalyzed by a family of nuclear-encoded membrane-embedded proteins called mitochondrial carriers (MCs). The citrate carrier (CiC) and the carnitine/acylcarnitine transporter (CACT) are two members of the MCs family involved in fatty acid metabolism. By conveying acetyl-coenzyme A, in the form of citrate, from the mitochondria to the cytosol, CiC contributes to fatty acid and cholesterol synthesis; CACT allows fatty acid oxidation, transporting cytosolic fatty acids, in the form of acylcarnitines, into the mitochondrial matrix. Fatty acid synthesis and oxidation are inversely regulated so that when fatty acid synthesis is activated, the catabolism of fatty acids is turned-off. Malonyl-CoA, produced by acetyl-coenzyme A carboxylase, a key enzyme of cytosolic fatty acid synthesis, represents a regulator of both metabolic pathways. CiC and CACT activity and expression are regulated by different nutritional and hormonal conditions. Defects in the corresponding genes have been directly linked to various human diseases. This review will assess the current understanding of CiC and CACT regulation; underlining their roles in physio-pathological conditions. Emphasis will be placed on the molecular basis of the regulation of CiC and CACT associated with fatty acid metabolism.

Increased missense mutation burden of Fatty Acid metabolism related genes in nunavik inuit population

Background

Nunavik Inuit (northern Quebec, Canada) reside along the arctic coastline where for generations their daily energy intake has mainly been derived from animal fat. Given this particular diet it has been hypothesized that natural selection would lead to population specific allele frequency differences and unique variants in genes related to fatty acid metabolism. A group of genes, namely CPT1A, CPT1B, CPT1C, CPT2, CRAT and CROT, encode for three carnitine acyltransferases that are important for the oxidation of fatty acids, a critical step in their metabolism.

Methods

Exome sequencing and SNP array genotyping were used to examine the genetic variations in the six genes encoding for the carnitine acyltransferases in 113 Nunavik Inuit individuals.

Results

Altogether ten missense variants were found in genes CPT1A, CPT1B, CPT1C, CPT2 and CRAT, including three novel variants and one Inuit specific variant CPT1A p.P479L (rs80356779). The latter has the highest frequency (0.955) compared to other Inuit populations. We found that by comparison to Asians or Europeans, the Nunavik Inuit have an increased mutation burden in CPT1A, CPT2 and CRAT; there is also a high level of population differentiation based on carnitine acyltransferase gene variations between Nunavik Inuit and Asians.

Conclusion

The increased number and frequency of deleterious variants in these fatty acid metabolism genes in Nunavik Inuit may be the result of genetic adaptation to their diet and/or the extremely cold climate. In addition, the identification of these variants may help to understand some of the specific health risks of Nunavik Inuit.

Downregulation of carnitine acyl-carnitine translocase by miRNAs 132 and 212 amplifies glucose-stimulated insulin secretion

We previously demonstrated that micro-RNAs (miRNAs) 132 and 212 are differentially upregulated in response to obesity in two mouse strains that differ in their susceptibility to obesity-induced diabetes. Here we show the overexpression of miRNAs 132 and 212 enhances insulin secretion (IS) in response to glucose and other secretagogues including nonfuel stimuli. We determined that carnitine acyl-carnitine translocase (CACT; Slc25a20) is a direct target of these miRNAs. CACT is responsible for transporting long-chain acyl-carnitines into the mitochondria for β-oxidation. Small interfering RNA-mediated knockdown of CACT in β-cells led to the accumulation of fatty acyl-carnitines and enhanced IS. The addition of long-chain fatty acyl-carnitines promoted IS from rat insulinoma β-cells (INS-1) as well as primary mouse islets. The effect on INS-1 cells was augmented in response to suppression of CACT. A nonhydrolyzable ether analog of palmitoyl-carnitine stimulated IS, showing that β-oxidation of palmitoyl-carnitine is not required for its stimulation of IS. These studies establish a link between miRNA-dependent regulation of CACT and fatty acyl-carnitine-mediated regulation of IS.

Engineering the Saccharomyces cerevisiae β-oxidation pathway to increase medium chain fatty acid production as potential biofuel

Fatty acid-derived biofuels and biochemicals can be produced in microbes using β-oxidation pathway engineering. In this study, the β-oxidation pathway of Saccharomyces cerevisiae was engineered to accumulate a higher ratio of medium chain fatty acids (MCFAs) when cells were grown on fatty acid-rich feedstock. For this purpose, the haploid deletion strain Δpox1 was obtained, in which the sole acyl-CoA oxidase encoded by POX1 was deleted. Next, the POX2 gene from Yarrowia lipolytica, which encodes an acyl-CoA oxidase with a preference for long chain acyl-CoAs, was expressed in the Δpox1 strain. The resulting Δpox1 [pox2+] strain exhibited a growth defect because the β-oxidation pathway was blocked in peroxisomes. To unblock the β-oxidation pathway, the gene CROT, which encodes carnitine O-octanoyltransferase, was expressed in the Δpox1 [pox2+] strain to transport the accumulated medium chain acyl-coAs out of the peroxisomes. The obtained Δpox1 [pox2+, crot+] strain grew at a normal rate. The effect of these genetic modifications on fatty acid accumulation and profile was investigated when the strains were grown on oleic acids-containing medium. It was determined that the engineered strains Δpox1 [pox2+] and Δpox1 [pox2+, crot+] had increased fatty acid accumulation and an increased ratio of MCFAs. Compared to the wild-type (WT) strain, the total fatty acid production of the strains Δpox1 [pox2+] and Δpox1 [pox2+, crot+] were increased 29.5% and 15.6%, respectively. The intracellular level of MCFAs in Δpox1 [pox2+] and Δpox1 [pox2+, crot+] increased 2.26- and 1.87-fold compared to the WT strain, respectively. In addition, MCFAs in the culture medium increased 3.29-fold and 3.34-fold compared to the WT strain. These results suggested that fatty acids with an increased MCFAs ratio accumulate in the engineered strains with a modified β-oxidation pathway. Our approach exhibits great potential for transforming low value fatty acid-rich feedstock into high value fatty acid-derived products.

Mitochondrial carnitine-dependent acetyl coenzyme A transport is required for normal sexual and asexual development of the ascomycete Gibberella zeae

Fungi have evolved efficient metabolic mechanisms for the exact temporal (developmental stages) and spatial (organelles) production of acetyl coenzyme A (acetyl-CoA). We previously demonstrated mechanistic roles of several acetyl-CoA synthetic enzymes, namely, ATP citrate lyase and acetyl-CoA synthetases (ACSs), in the plant-pathogenic fungus Gibberella zeae. In this study, we characterized two carnitine acetyltransferases (CATs; CAT1 and CAT2) to obtain a better understanding of the metabolic processes occurring in G. zeae. We found that CAT1 functioned as an alternative source of acetyl-CoA required for lipid accumulation in an ACS1 deletion mutant. Moreover, deletion of CAT1 and/or CAT2 resulted in various defects, including changes to vegetative growth, asexual/sexual development, trichothecene production, and virulence. Although CAT1 is associated primarily with peroxisomal CAT function, mislocalization experiments showed that the role of CAT1 in acetyl-CoA transport between the mitochondria and cytosol is important for sexual and asexual development in G. zeae. Taking these data together, we concluded that G. zeae CATs are responsible for facilitating the exchange of acetyl-CoA across intracellular membranes, particularly between the mitochondria and the cytosol, during various developmental stages.

Expanded molecular features of carnitine acyl-carnitine translocase (CACT) deficiency by comprehensive molecular analysis

Carnitine-acylcarnitine translocase (CACT) deficiency is a rare autosomal recessive disease of fatty acid oxidation, mainly affecting long chain fatty acid utilization. The disease usually presents at neonatal period with severe hypoketotic hypoglycemia, hyperammonemia, cardiomyopathy and/or arrhythmia, hepatic dysfunction, skeletal muscle weakness, and encephalopathy. Definitive diagnosis of CACT deficiency by molecular analysis of the SLC25A20 gene has recently become clinically available. In contrast to biochemical analysis, sequence analysis is a more rapid and reliable method for diagnosis of CACT deficiency. In this study, we used Sanger sequencing and target array CGH to identify molecular defects in the SLC25A20 gene of patients with clinical features and an acylcarnitine profile consistent with CACT deficiency. Eight novel mutations, including a large 25.9 kb deletion encompassing exons 5 to 9 of SLC25A20 were found. Review of the published cases revealed that CACT deficiency is a pan-ethnic disorder with a broad mutation spectrum. Mutations are distributed along the entire gene without a hot spot. Two thirds of them are nonsense, frame-shift, or splice site mutations resulting in premature stop codons. This study underscores the importance of comprehensive molecular analysis, including sequencing and targeted array CGH of the SLC25A20 gene when CACT deficiency is suspected.

[Effect of prolonged infusion of propofol on the liver mitochondria respiratory function in rabbits]

OBJECTIVE

To investigate the changes of respiratory function of liver mitochondria in rabbits induced by the general anesthetic propofol.

METHODS

Eighteen New Zealand rabbits weighted 1.5-2.5 kg were randomly divided into three groups: control group, emulsion group and propofol group. The control group received continuous infusion of 0.9% sodium chloride solution. The propofol group received continuous infusion of 1% propofol. The emulsion group received continuous infusion of 10% emulsion. The liver mitochondri of the rabbits were isolated. The carnitine acyl transferase (CPT) activity, H+ -ATPase hydrolysis activity and the content of ATP in the mitochondria were analysed.

RESULTS

The rabbits in the propofol group had lower activity of CPT than the controls (P < 0.05), while no difference was found between the control group and the emulsion group (P > 0.05). The rabbits in the propofol group had higher H+ -ATPase hydrolysis activity than the controls (P < 0.05), while no difference was found between the control group and the emulsion group (P > 0.05). No significant differences were found in the content of ATP in mitochondria between the three groups (P > 0.05).

CONCLUSION

Propofol inhibits CPT activity, which disturbs fatty acid beta-oxidation. Emulsion acted as vehicle of propofol seems to have no significant impact on mitochondria respiratory function.

Carnitine, mitochondrial function and therapy

Carnitine is important for cell function and survival primarily because of its involvement in the multiple equilibria between acylcarnitine and acyl-CoA esters established through the enzymatic activities of the family of carnitine acyltransferases. These have different acyl chain-length specificities and intracellular compartment distributions, and act in synchrony to regulate multiple aspects of metabolism, ranging from fuel-selection and -sensing, to the modulation of the signal transduction mechanisms involved in many homeostatic systems. This review aims to rationalise the extensive range of experimental and clinical data that have been obtained through the pharmacological use of L-carnitine and its short-chain acylesters, over the past two decades, in terms of the basic biochemical mechanisms involved in the effects of carnitine on the various cellular acyl-CoA pools in health and disease.

Mouse carnitine-acylcarnitine translocase (CACT) is transcriptionally regulated by PPARalpha and PPARdelta in liver cells

BACKGROUND

Hepatic PPARalpha acts as the primary mediator of the adaptive response to fasting by upregulation of a number of genes involved in fatty acid catabolism. Whether carnitine-acylcarnitine translocase (CACT), which mediates the import of acylcarnitines into the mitochondrial matrix for subsequent beta-oxidation of fatty acid moieties, is also regulated by PPARalpha in the liver has not yet been investigated.

METHODS AND RESULTS

Herein, we observed that hepatic mRNA abundance of CACT was increased by both, fasting and treatment with PPARalpha agonist WY-14,643 in wild-type mice but not PPARalpha-knockout mice (P<0.05). Cell culture experiments revealed that CACT mRNA abundance was higher in liver cells treated with either WY-14,643 or PPARdelta agonist GW0742, but not with PPARgamma agonist troglitazone (TGZ) than in control cells (P<0.05). In addition, reporter assays revealed activation of mouse CACT promoter by WY-14,643 and GW0742, but not TGZ. Moreover, deletion and mutation analyses of CACT promoter and 5'-UTR revealed one functional PPRE in the 5'-UTR of mouse CACT.

GENERAL SIGNIFICANCE

CACT is upregulated by PPARalpha and PPARdelta, probably by binding to a functional PPRE at position +45 to +57 relative to the transcription start site. The upregulation of CACT by PPARalpha and PPARdelta, which are both important for the regulation of fatty acid oxidation in tissues during fasting, may increase the import of acylcarnitine into the mitochondrial matrix during fasting.

The regulation of ketogenesis

Ketone bodies accumulate in the plasma in conditions of fasting and uncontrolled diabetes. The initiating event is a change in the molar ratio of glucagon:insulin. Insulin deficiency triggers the lipolytic process in adipose tissue with the result that free fatty acids pass into the plasma for uptake by liver and other tissues. Glucagon appears to be the primary hormone involved in the induction of fatty acid oxidation and ketogenesis in the liver. It acts by acutely dropping hepatic malonyl-CoA concentrations as a consequence of inhibitory effects exerted in the glycolytic pathway and on acetyl-CoA carboxylase (EC 6.4.1.2). The fall in malonyl-CoA concentration activates carnitine acyltransferase I (EC 2.3.1.21) such that long-chain fatty acids can be transported through the inner mitochondrial membrane to the enzymes of fatty acid oxidation and ketogenesis. The latter are high-capacity systems assuring that fatty acids entering the mitochondria are rapidly oxidized to ketone bodies. Thus, the rate-controlling step for ketogenesis is carnitine acyltransferase I. Administration of food after a fast, or of insulin to the diabetic subject, reduces plasma free fatty acid concentrations, increases the liver concentration of malonyl-CoA, inhibits carnitine acyltransferase I and reverses the ketogenic process.

Suppression of the acuH13 and acuH31 nonsense mutations in the carnitine/acylcarnitine translocase (acuH) gene of Aspergillus nidulans by the G265S substitution in the domain 2 of the release factor eRF1

A search for suppressors of the carnitine/acylcarnitine translocase (CACT) deficiency in Aspergillus nidulans permitted the identification of the suaE7 mutation, mapping at a new translational suppressor (suaE) gene. The suaE gene is essential in A. nidulans and encodes the eukaryotic release factor 1 (eRF1). The suaE7 mutation suppresses two acuH alleles (acuH13 and acuH31), both carrying nonsense mutations in the CACT encoding gene that involve the replacement of a CAG (Gln) codon with a premature TAG stop codon. In contrast, the suaE7 gene does not suppress the acuH20 amber nonsense mutation involving a TGG-->TAG change. The phenotype associated to the suaE7 mutation strictly resembles that of mutants at the suaA and suaC genes, two translational suppressor genes previously identified, suggesting that their gene products might functionally interact in translation termination. Sequencing of the suaE7 gene allowed the identification of a mutation in the domain 2 of the omnipotent class-1 eukaryotic release factor involving the Gly265Ser substitution in the A. nidulans eRF1. This mutation creates a structural context unfavourable for normal eRF binding that allows the misreading of stop codons by natural suppressor tRNAs, such as the tRNAs(Gln). Structural analysis using molecular modelling of A. nidulans eRF1 domain 2 bearing the G265S substitution and computer simulation results suggest that this mutation might impair the necessary conformational changes in the eRF1 to optimally recognize the stop codon and simultaneously interact with the peptidyl transferase centre of the 60S ribosomal subunit.

The brain-specific carnitine palmitoyltransferase-1c regulates energy homeostasis

Fatty acid synthesis in the central nervous system is implicated in the control of food intake and energy expenditure. An intermediate in this pathway, malonyl-CoA, mediates these effects. Malonyl-CoA is an established inhibitor of carnitine palmitoyltransferase-1 (CPT1), an outer mitochondrial membrane enzyme that controls entry of fatty acids into mitochondria and, thereby, fatty acid oxidation. CPT1c, a brain-specific enzyme with high sequence similarity to CPT1a (liver) and CPT1b (muscle) was recently discovered. All three CPTs bind malonyl-CoA, and CPT1a and CPT1b catalyze acyl transfer from various fatty acyl-CoAs to carnitine, whereas CPT1c does not. These findings suggest that CPT1c has a unique function or activation mechanism. We produced a targeted mouse knockout (KO) of CPT1c to investigate its role in energy homeostasis. CPT1c KO mice have lower body weight and food intake, which is consistent with a role as an energy-sensing malonyl-CoA target. Paradoxically, CPT1c KO mice fed a high-fat diet are more susceptible to obesity, suggesting that CPT1c is protective against the effects of fat feeding. CPT1c KO mice also exhibit decreased rates of fatty acid oxidation, which may contribute to their increased susceptibility to diet-induced obesity. These findings indicate that CPT1c is necessary for the regulation of energy homeostasis.

Effects of doxorubicin-containing chemotherapy and a combination with l-carnitine on oxidative metabolism in patients with non-Hodgkin lymphoma

PURPOSE

Chemotherapy regimens based on anthracycline (doxorubicin) are well established in lymphoma therapy. The purpose of this study was to examine the effects of L-carnitine with a view to reducing cytotoxic side-effects.

METHODS

20 patients were scheduled to receive 3 g L-carnitine before each chemotherapy cycle, followed by 1 g L-carnitine/day during the following 21 days, while 20 patients received a placebo (randomized controlled trial). The plasma lipid profile and relative mRNA levels of key enzymes of oxidative metabolism (carnitine acyltransferases) were measured at three points of time. In addition to the clinical parameters we used the mRNA of white blood cells to evaluate the toxic effects on cardiomyocytes.

RESULTS

In the present study no cardiotoxicity of anthracycline therapy was detected. Carnitine treated patients showed a rise in plasma carnitine which led to an increase of relative mRNA levels from CPT1A (liver isoform of carnitine palmitoyltransferase) and OCTN2 (carnitine transporter). Following chemotherapy, an activation of carnitine acyltransferases was associated with a stimulation of OCTN2 in both groups.

CONCLUSION

Biochemical and molecular analyses indicated a stimulation of oxidative metabolism in white blood cells through carnitine uptake.

Carnitine-acylcarnitine translocase deficiency, clinical, biochemical and genetic aspects

The carnitine-acylcarnitine translocase (CACT) is one of the components of the carnitine cycle. The carnitine cycle is necessary to shuttle long-chain fatty acids from the cytosol into the intramitochondrial space where mitochondrial beta-oxidation of fatty acids takes place. The oxidation of fatty acids yields acetyl-coenzyme A (CoA) units, which may either be degraded to CO(2) and H(2)O in the citric acid cycle to produce ATP or converted into ketone bodies which occurs in liver and kidneys. Metabolic consequences of a defective CACT are hypoketotic hypoglycaemia under fasting conditions, hyperammonemia, elevated creatine kinase and transaminases, dicarboxylic aciduria, very low free carnitine and an abnormal acylcarnitine profile with marked elevation of the long-chain acylcarnitines. Clinical signs and symptoms in CACT deficient patients, are a combination of energy depletion and endogenous toxicity. The predominantly affected organs are brain, heart and skeletal muscle, and liver, leading to neurological abnormalities, cardiomyopathy and arrythmias, skeletal muscle damage and liver dysfunction. Most patients become symptomatic in the neonatal period with a rapidly progressive deterioration and a high mortality rate. However, presentations at a later age with a milder phenotype have also been reported. The therapeutic approach is the same as in other long-chain fatty acid disorders and includes intravenous glucose (+/- insulin) administration to maximally inhibit lipolysis and subsequent fatty acid oxidation during the acute deterioration, along with other measures such as ammonia detoxification, depending on the clinical features. Long-term strategy consists of avoidance of fasting with frequent meals and a special diet with restriction of long-chain fatty acids. Due to the extremely low free carnitine concentrations, carnitine supplementation is often needed. Acylcarnitine profiling in plasma is the assay of choice for the diagnosis at a metabolite level. However, since the acylcarnitine profile observed in CACT-deficient patients is identical to that in CPT2-deficient patients, definitive identification of CACT-deficiency in a certain patient requires determination of the activity of CACT. Subsequently, mutational analysis of the CACT gene can be performed. So far, 9 different mutations have been identified in the CACT gene.

Carnitine acyltransferases and their influence on CoA pools in health and disease

Cells contain limited and sequestered pools of Coenzyme A (CoA) that are essential for activating carboxylate metabolites. Some acyl-CoA esters have high metabolic and signalling impact, so control of CoA ester concentrations is important. This and transfer of the activated acyl moieties between cell compartments without wasting energy on futile cycles of hydrolysis and resynthesis is achieved through the carnitine system. The location, properties of and deficiencies in the carnitine acyltransferases are described in relation to their influence on the CoA pools in the cell and, hence, on metabolism. The protection of free CoA pools in disease states is achieved by excretion of acyl-carnitine so that carnitine supplementation is required where unwanted acyl groups build up, such as in some inherited disorders of fatty acid oxidation. Acetyl-carnitine improves cognition in the brain and propionyl-carnitine improves cardiac performance in heart disease and diabetes. The therapeutic effects of carnitine and its esters are discussed in relation to the integrative influence of the carnitine system across CoA pools. Recent evidence for sequestered pools of activated acetate for synthesis of malonyl-CoA, for the synthesis of polyunsaturated fatty acids and for the inhibition of carnitine palmitoyltransferase 1 to regulate fatty acid oxidation is reviewed.

Acetylcarnitine and cellular stress response: roles in nutritional redox homeostasis and regulation of longevity genes

Aging is associated with a reduced ability to cope with physiological challenges. Although the mechanisms underlying age-related alterations in stress tolerance are not well defined, many studies support the validity of the oxidative stress hypothesis, which suggests that lowered functional capacity in aged organisms is the result of an increased generation of reactive oxygen and nitrogen species. Increased production of oxidants in vivo can cause damage to intracellular macromolecules, which can translate into oxidative injury, impaired function and cell death in vulnerable tissues such as the brain. To survive different types of injuries, brain cells have evolved networks of responses, which detect and control diverse forms of stress. This is accomplished by a complex network of the so-called longevity assurance processes, which are composed of several genes termed vitagenes. Among these, heat shock proteins form a highly conserved system responsible for the preservation and repair of the correct protein conformation. The heat shock response contributes to establishing a cytoprotective state in a wide variety of human diseases, including inflammation, cancer, aging and neurodegenerative disorders. Given the broad cytoprotective properties of the heat shock response, there is now a strong interest in discovering and developing pharmacological agents capable of inducing the heat shock response. Acetylcarnitine is proposed as a therapeutic agent for several neurodegenerative disorders, and there is now evidence that it may play a critical role as modulator of cellular stress response in health and disease states. In the present review, we first discuss the role of nutrition in carnitine metabolism, followed by a discussion of carnitine and acetyl-l-carnitine in mitochondrial dysfunction, in aging, and in age-related disorders. We then review the evidence for the role of acetylcarnitine in modulating redox-dependent mechanisms leading to up-regulation of vitagenes in brain, and we also discuss new approaches for investigating the mechanisms of lifetime survival and longevity.

Differential carnitine/acylcarnitine translocase expression defines distinct metabolic signatures in skeletal muscle cells

Import of acylcarnitine into mitochondrial matrix through carnitine/acylcarnitine-translocase (CACT) is fundamental for lipid catabolism. To probe the effect of CACT down-expression on lipid metabolism in muscle, human myocytes were stably transfected with CACT-antisense construct. In presence of low concentration of palmitate, transfected cells showed decreased palmitate oxidation and acetyl-carnitine content, increased palmitoyl-carnitine level, and reduced insulin-dependent decrease of fatty acylcarnitine-to-fatty acyl-CoA ratio. The augmented palmitoyl-carnitine synthesis, also in the presence of insulin, could be related to an altered regulation of carnitine-palmitoyl-transferase 1 (CPT 1) by malonyl-CoA, whose synthesis is dependent by the availability of cytosolic acetyl-groups. Indeed, all the described effects were completely overcome by CACT neo-expression by recombinant adenovirus vector or by addition of acetyl-carnitine to cultures. Acetyl-carnitine effect was related to an increase of malonyl-CoA and was abolished by down-expression, via antisense RNA strategy, of acetyl-CoA carboxylase-beta, the mitochondrial membrane enzyme involved in the direct CPT 1 inhibition via malonyl-CoA synthesis. Thus, in our experimental model the modulation of CACT expression has consequences for CPT 1 activity, while the biologic effects of acetyl-carnitine are not associated with a generic supply of energy compounds but to the anaplerotic property of the molecule.

dif-1 and colt, both implicated in early embryonic development, encode carnitine acylcarnitine translocase

It has always been assumed that during development the embryo and fetus depend only on glycolysis for energy generation and that they do not oxidize fatty acids. Recently, however, we found abundant expression and activity of fatty acid oxidation (FAO) enzymes in the human embryo and fetus. In a search for FAO gene expression during development we came across two embryonic differentiation genes: differentiation defective (dif-1) and congested-like trachea (colt) of Caenorhabditis elegans and Drosophila melanogaster, respectively. Earlier studies showed that expression of these two genes is essential during developmental stages with high energy requirements. Both dif-1 and colt encode proteins with sequence similarity to the mitochondrial carnitine acylcarnitine carrier (CACT), which suggests that the DIF-1 and COLT proteins might be functional orthologues of CACT. To investigate this, we expressed both dif-1 and colt in Saccharomyces cerevisiae. Our results show that DIF-1 and COLT can functionally complement a yeast CACT deletion strain and thus function as carnitine acylcarnitine transporters. This finding is well in line with the recent observation that embryos are capable of oxidizing fatty acids and furthermore implies that FAO is essential during early embryonic development when the energy demand is high.

Identification by Site-directed Mutagenesis and Chemical Modification of Three Vicinal Cysteine Residues in Rat Mitochondrial Carnitine/Acylcarnitine Transporter

The proximity of the Cys residues present in the mitochondrial rat carnitine/acylcarnitine carrier (CAC) primary structure was studied by using site-directed mutagenesis in combination with chemical modification. CAC mutants, in which one or more Cys residues had been replaced with Ser, were overexpressed in Escherichia coli and reconstituted into liposomes. The effect of SH oxidizing, cross-linking, and coordinating reagents was evaluated on the carnitine/carnitine exchange catalyzed by the recombinant reconstituted CAC proteins. All the tested reagents efficiently inhibited the wild-type CAC. The inhibitory effect of diamide, Cu(2+)-phenanthroline, or phenylarsine oxide was largely reduced or abolished by the double substitutions C136S/C155S, C58S/C136S, and C58S/C155S. The decrease in sensitivity to these reagents was much lower in double mutants in which Cys(23) was substituted with Cys(136) or Cys(155). No decrease in inhibition was found when Cys(89) and/or Cys(283) were replaced with Ser. Sb(3+), which coordinates three cysteines, inhibited only the Cys replacement mutants containing cysteines 58, 136, and 155 of the six native cysteines. In addition, the mutant C23S/C89S/C155S/C283S, in which double tandem fXa recognition sites were inserted in positions 65-72, i.e. between Cys(58) and Cys(136), was not cleaved into two fragments by fXa protease after treatment with diamide. These results are interpreted in light of the homology model of CAC based on the available x-ray structure of the ADP/ATP carrier. They indicate that Cys(58), Cys(136), and Cys(155) become close in the tertiary structure of the CAC during its catalytic cycle.

Crystal structure of mouse carnitine octanoyltransferase and molecular determinants of substrate selectivity

Carnitine acyltransferases have crucial functions in fatty acid metabolism. Members of this enzyme family show distinctive substrate preferences for short-, medium- or long-chain fatty acids. The molecular mechanism for this substrate selectivity is not clear as so far only the structure of carnitine acetyltransferase has been determined. To further our understanding of these important enzymes, we report here the crystal structures at up to 2.0-A resolution of mouse carnitine octanoyltransferase alone and in complex with the substrate octanoylcarnitine. The structures reveal significant differences in the acyl group binding pocket between carnitine octanoyltransferase and carnitine acetyltransferase. Amino acid substitutions and structural changes produce a larger hydrophobic pocket that binds the octanoyl group in an extended conformation. Mutation of a single residue (Gly-553) in this pocket can change the substrate preference between short- and medium-chain acyl groups. The side chains of Cys-323 and Met-335 at the bottom of this pocket assume dual conformations in the substrate complex, and mutagenesis studies suggest that the Met-335 residue is important for catalysis.

Redesign of carnitine acetyltransferase specificity by protein engineering

In eukaryotes, L-carnitine is involved in energy metabolism by facilitating beta-oxidation of fatty acids. Carnitine acetyltransferases (CrAT) catalyze the reversible conversion of acetyl-CoA and carnitine to acetylcarnitine and free CoA. To redesign the specificity of rat CrAT toward its substrates, we mutated Met564. The M564G mutated CrAT showed higher activity toward longer chain acyl-CoAs: activity toward myristoyl-CoA was 1250-fold higher than that of the wild-type CrAT, and lower activity toward its natural substrate, acetyl-CoA. Kinetic constants of the mutant CrAT showed modification in favor of longer acyl-CoAs as substrates. In the reverse case, mutation of the orthologous glycine (Gly553) to methionine in carnitine octanoyltransferase (COT) decreased activity toward its natural substrates, medium- and long-chain acyl-CoAs, and increased activity toward short-chain acyl-CoAs. Another CrAT mutant, M564A, was prepared and tested in the same way, with similar results. We conclude that Met564 blocks the entry of medium- and long-chain acyl-CoAs to the catalytic site of CrAT. Three-dimensional models of wild-type and mutated CrAT and COT support this hypothesis. We show for the first time that a single amino acid is able to determine the substrate specificity of CrAT and COT.

Characterization of the species-specificity of peroxisome proliferators in rat and human hepatocytes

Peroxisome proliferation is a well-defined pleiotropic effect that is mediated by the ligand inducible transcription factor peroxisome proliferator-activated receptor (PPAR) alpha. Because marked peroxisome proliferation occurs in rodents but not in humans, we aimed to elucidate the molecular and cellular determinants of this species-specificity in hepatocytes. Analysis of peroxisomal marker enzyme activities confirmed that peroxisome proliferators induced acyl-CoA oxidase (ACOX) and to a lesser extent catalase in rat hepatocytes, but not in human hepatoma HepG2 cells. Transient transfection assays revealed that ciprofibrate and Wy 14,643 induced rat but not human PPARalpha-mediated reporter gene activity in rat FAO and primary hepatocytes on rat but not on human PPARalpha response elements (PPREs). In contrast, in human HepG2 and primary human hepatocytes, peroxisome proliferators did not induce either human or rat PPARalpha activity regardless of rat or human PPRE sequences. In addition, no induction of ACOX gene expression was observed in human hepatocytes independent of the expression level of human PPARalpha. Remarkably, no distinct peroxisome proliferation related responses were observed in human hepatocytes when rat PPARalpha was transfected, although human hepatocytes were responsive to PPARalpha-mediated induction of carnitine palmitoyl transferase-1A and 3-hydroxy-3-methylglutaryl-CoA synthase. These results confirmed that PPARalpha and PPREs are important determinants for the species-specificity of peroxisome proliferation. Nevertheless, our results showed that human hepatocytes limit the extent of peroxisome proliferation regardless of PPARalpha expression.

Carnitine-acylcarnitine translocase deficiency: identification of a novel molecular defect in a Bedouin patient

Carnitine-acylcarnitine translocase CACT deficiency is a very rare autosomal recessive disease. The neonatal phenotype of CACT deficiency is characterized by hypoketotic hypoglycaemia, hyperammonaemia, cardiomyopathy and skeletal muscle weakness culminating in early death. The disease is caused by mutations in the CACT gene, which encodes a protein transporting long-chain fatty acid carnitine esters into the mitochondrial matrix. In this report, we describe the first case of CACT deficiency in the Bedouin population in Israel. The patient, the first son of consanguineous parents, was born at term after uneventful delivery. During the second day of life, he developed clinical signs of an acute metabolic crisis with severe hypoglycaemia and hyperammonaemia. Biochemical investigation suggested the diagnosis of CACT deficiency. Genetic molecular analysis confirmed this diagnosis by demonstrating that the affected child was homozygous for a novel missense mutation 793A>G, substituting glutamine by arginine (Q238R) in exon 7 of the CACT gene. Despite medical treatment and adequate nutrition, the patient died at 6 months of age.

Regulation of CPT I activity in intermyofibrillar and subsarcolemmal mitochondria from human and rat skeletal muscle

Carnitine palmitoyltransferase I (CPT I) is considered the rate-limiting enzyme in the transfer of long-chain fatty acids (LCFA) into the mitochondria and is reversibly inhibited by malonyl-CoA (M-CoA) in vitro. In rat skeletal muscle, M-CoA levels decrease during exercise, releasing the inhibition of CPT I and increasing LCFA oxidation. However, in human skeletal muscle, M-CoA levels do not change during moderate-intensity exercise despite large increases in fat oxidation, suggesting that M-CoA is not the sole regulator of increased CPT I activity during exercise. In the present study, we measured CPT I activity in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondria isolated from human vastus lateralis (VL), rat soleus (Sol), and red gastrocnemius (RG) muscles. We tested whether exercise-related levels ( approximately 65% maximal O2 uptake) of calcium and adenylate charge metabolites (free AMP, ADP, and Pi) could override the M-CoA-induced inhibition of CPT I activity and explain the increased CPT I flux during exercise. Protein content was approximately 25-40% higher in IMF than in SS mitochondria in all muscles. Maximal CPT I activity was similar in IMF and SS mitochondria in all muscles (VL: 282 +/- 46 vs. 280 +/- 51; Sol: 390 +/- 81 vs. 368 +/- 82; RG: 252 +/- 71 vs. 278 +/- 44 nmol.min-1.mg protein-1). Sensitivity to M-CoA did not differ between IMF and SS mitochondria in all muscles (25-31% inhibition in VL, 52-70% in Sol and RG). Calcium and adenylate charge metabolites did not override the M-CoA-induced inhibition of CPT I activity in mitochondria isolated from VL, Sol, and RG muscles. Decreasing pH from 7.1 to 6.8 reduced CPT I activity by approximately 34-40% in both VL mitochondrial fractions. In summary, this study reports no differences in CPT I activity or sensitivity to M-CoA between IMF and SS mitochondria isolated from human and rat skeletal muscles. Exercise-induced increases in calcium and adenylate charge metabolites do not appear responsible for upregulating CPT I activity in human or rat skeletal muscle during moderate aerobic exercise.

A novel mitochondrial carnitine-acylcarnitine translocase induced by partial hepatectomy and fasting

The carnitine-dependent transport of long-chain fatty acids is essential for fatty acid catabolism. In this system, the fatty acid moiety of acyl-CoA is transferred enzymatically to carnitine, and the resultant product, acylcarnitine, is imported into the mitochondrial matrix through a transporter named carnitine-acylcarnitine translocase (CACT). Here we report a novel mammalian protein homologous to CACT. The protein, designated as CACL (CACT-like), is localized to the mitochondria and has palmitoylcarnitine transporting activity. The tissue distribution of CACL is similar to that of CACT; both are expressed at a higher level in tissues using fatty acids as fuels, except in the brain, where only CACL is expressed. In addition, CACL is induced by partial hepatectomy or fasting. Thus, CACL may play an important role cooperatively with its homologue CACT in a stress-induced change of lipid metabolism, and may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.

Functional analysis of mutations in the human carnitine/acylcarnitine translocase in Aspergillus nidulans

Deficiency of the carnitine/acylcarnitine translocase (CACT), the most severe disorder of fatty acid beta-oxidation, is usually lethal in both humans and animals, precluding the development of animal models of the disease. In contrast, CACT deficiency is conditionally lethal in the fungus Aspergillus nidulans, since loss-of-function mutations in acuH, the translocase structural gene, do not prevent growth on carbon sources other than ketogenic compounds, such as fatty acids. Here, we describe the molecular characterization of extant acuH alleles and the development of a fungal model for CACT deficiency based on the ability of human CACT to fully complement, when expressed at physiological levels, the growth defect of an A. nidulans DeltaacuH strain on acetate and long-chain fatty acids. By using growth tests and in vitro assays this model enabled us to carry out a functional characterization of human CACT mutations showing that it may be useful for distinguishing potentially pathogenic human CACT missense mutations from neutral, single residue substitution-causing polymorphisms.

Downregulation of carnitine acyltransferases and organic cation transporter OCTN2 in mononuclear cells in healthy elderly and patients with myelodysplastic syndromes

Changes in key enzymes of oxidative metabolism at the mitochondrial level are known to be associated with the aging process, apoptosis, and many diseases. Considering the risk of acquiring a myelodysplastic syndrome (MDS) with age, the aim of this study was to quantify mRNA synthesis of the carnitine palmitoyltransferases (CPT1 and CPT2), carnitine acetyltransferase (CRAT), human specific microsomal CPT, and OCTN2 (organic cation transporter) in mononuclear cells of healthy humans of different age groups and MDS patients. Using quantitative reverse transcriptase real-time PCR we compared mRNA synthesis of the above mentioned enzymes in mononuclear cells from peripheral blood of 23 healthy persons (mean age 45 years), 9 blood and 22 bone marrow samples of 31 MDS patients with varying proportions of apoptotic cells (mean age 78 years), and blood samples of 30 age-matched controls. In addition, plasma carnitine levels were determined. Compared to younger adults, there was a 50% downregulation of CPT1 in elderly persons and in MDS patients. Reduction in CRAT, CPT 2, and OCTN2 was more than 85%. Reduction in microsomal CPT was more pronounced in MDS patients than in age-matched controls (96% vs. 43%). In MDS bone marrow cells there was a negative correlation of CPT1 and CRAT with the relative proportion of apoptotic cells. Plasma carnitine values were similar in all groups. The described reduction in transcription of different genes in blood cells which is well known in different tissues may reflect a systemic signaling process, associated with aging, apoptosis, and MDS.

Identification of conserved amino acid residues in rat liver carnitine palmitoyltransferase I critical for malonyl-CoA inhibition. Mutation of methionine 593 abolishes malonyl-CoA inhibition

Carnitine palmitoyltransferase (CPT) I, which catalyzes the conversion of palmitoyl-CoA to palmitoylcarnitine facilitating its transport through the mitochondrial membranes, is inhibited by malonyl-CoA. By using the SequenceSpace algorithm program to identify amino acids that participate in malonyl-CoA inhibition in all carnitine acyltransferases, we found 5 conserved amino acids (Thr(314), Asn(464), Ala(478), Met(593), and Cys(608), rat liver CPT I coordinates) common to inhibitable malonyl-CoA acyltransferases (carnitine octanoyltransferase and CPT I), and absent in noninhibitable malonyl-CoA acyltransferases (CPT II, carnitine acetyltransferase (CAT) and choline acetyltransferase (ChAT)). To determine the role of these amino acid residues in malonyl-CoA inhibition, we prepared the quintuple mutant CPT I T314S/N464D/A478G/M593S/C608A as well as five single mutants CPT I T314S, N464D, A478G, M593S, and C608A. In each case the CPT I amino acid selected was mutated to that present in the same homologous position in CPT II, CAT, and ChAT. Because mutant M593S nearly abolished the sensitivity to malonyl-CoA, two other Met(593) mutants were prepared: M593A and M593E. The catalytic efficiency (V(max)/K(m)) of CPT I in mutants A478G and C608A and all Met(593) mutants toward carnitine as substrate was clearly increased. In those CPT I proteins in which Met(593) had been mutated, the malonyl-CoA sensitivity was nearly abolished. Mutations in Ala(478), Cys(608), and Thr(314) to their homologous amino acid residues in CPT II, CAT, and ChAT caused various decreases in malonyl-CoA sensitivity. Ala(478) is located in the structural model of CPT I near the catalytic site and participates in the binding of malonyl-CoA in the low affinity site (Morillas, M., Gómez-Puertas, P., Rubi, B., Clotet, J., Ariño, J., Valencia, A., Hegardt, F. G., Serra, D., and Asins, G. (2002) J. Biol. Chem. 277, 11473-11480). Met(593) may participate in the interaction of malonyl-CoA in the second affinity site, whose location has not been reported.

Effect of Dietary Fat Level and Sesamin on the Polyunsaturated Fatty Acid Metabolism in Rats

In this study, we examined the effects of sesamin and vegetable oil on the concentrations of polyunsaturated fatty acid (PUFA) and lipids (triacylglycerol, free cholesterol, and phospholipid), and beta-oxidation enzyme activities in the rat liver. Rats were fed a diet containing 5% (low-fat diet) or 20% (high-fat diet) salad oil (rapeseed oil: soybean oil, 7:3) with or without sesamin (0.5% w/w) for 4 wk. As a result, the concentrations of linoleic acid (LA, n-6), alpha-linolenic acid (ALA, n-3), and total PUFA in the liver increased significantly as the result of the high-fat diet. In the high-fat diet groups, sesamin administration decreased the concentrations of LA, ALA, and total PUFA to almost the same level as the low-fat diet group, while it increased the concentrations of dihomo-gamma-linolenic acid (DGLA, n-6) and arachidonic acid (AA, n-6). The activities of carnitine acyltransferase and acyl-CoA dehydrogenase in liver mitochondria were enhanced by the intake of the high-fat diet, and were further enhanced by the administration of sesamin. Peroxisomal acyl-CoA oxidase activity was also enhanced by sesamin, while it was not affected by the dietary fat level. These results suggest that sesamin suppressed the increase of hepatic PUFA concentration caused by feeding the high-fat diet through enhancing the enzyme activities of fatty acid beta-oxidation and PUFA metabolism from LA and ALA.

Mutational spectrum and DNA-based prenatal diagnosis in carnitine-acylcarnitine translocase deficiency

Carnitine-acylcarnitine translocase (CAC) deficiency is a rare autosomal recessive disorder of long-chain fatty acid oxidation with a severe outcome. We report mutation analysis in a cohort of 12 patients. Twelve mutations were identified of which 9 have not been reported so far (G28C, D32N, R178Q, P230R, D231H, 179delG, 802delG, 69-70insTGTGC, and 609-1g>a). Altogether, including our results, 22 mutations of the CAC gene have been published to date in 23 patients demonstrating the allelic heterogeneity of CAC deficiency. DNA-based prenatal diagnosis was performed for the first time in pregnancies at risk for CAC deficiency. Two fetuses were affected and one pregnancy was terminated by family decision. Two other fetuses had normal genotype and five others were heterozygotes. All the offspring of these seven pregnancies are alive and apparently healthy.

Conjugated linoleic acid isomers in mitochondria: evidence for an alteration of fatty acid oxidation

The beneficial effects exerted by low amounts of conjugated linoleic acids (CLA) suggest that CLA are maximally conserved and raise the question about their mitochondrial oxidizability. Cis-9,trans-11-C(18:2) (CLA1) and trans-10,cis-12-C(18:2) (CLA2) were compared to cis-9,cis-12-C(18:2) (linoleic acid; LA) and cis-9-C(16:1) (palmitoleic acid; PA), as substrates for total fatty acid (FA) oxidation and for the enzymatic steps required for the entry of FA into rat liver mitochondria. Oxygen consumption rate was lowest when CLA1 was used as a substrate with that on CLA2 being intermediate between it and the respiration on LA and PA. The order of the radiolabeled FA oxidation rate was PA >> LA > CLA2 > CLA1. Transesterification to acylcarnitines of the octadecadienoic acids were similar, while uptake across inner membranes of CLA1 and, to a lesser extent, of CLA2 was greater than that of LA or PA. Prior oxidation of CLA1 or CLA2 made re-isolated mitochondria much less capable of oxidising PA or LA under carnitine-dependent conditions, but without altering the carnitine-independent oxidation of octanoic acid. Therefore, the CLA studied appeared to be both poorly oxidizable and capable of interfering with the oxidation of usual FA at a step close to the beginning of the beta-oxidative cycle.

Decreased mitochondrial carnitine translocase in skeletal muscles impairs utilization of fatty acids in insulin-resistant patients

Insulin resistance (IR) and its health consequences (diabetes, hypertension, cardiovascular disease, obesity etc.) affect between 25 and 35% of Westernized populations. Decreased fatty acid (FA) oxidation in skeletal muscle is implicated in obesity-related IR. Carnitine-acylcarnitine translocase (CACT) transports long-chain FAs both into mitochondria (as carnitine esters for energy-generating processes) and out of mitochondria. To determine whether CACT activity correlates with decreased FA oxidation we measured CACT concentrations in cellular and mitochondrial extracts from the skeletal muscle of 19 obese IR individuals and of 19 lean controls. We also evaluated carnitine transport in skeletal muscle mitochondria in both groups. Mitochondrial CACT was decreased at translational and transductional level, and carnitine-carnitine and acylcarnitine-carnitine exchange rates were significantly lower in IR subjects. Aberrant acylcarnitine flux into mitochondria was not correlated with decreased activity of other components of the mitochondrial carnitine system (i.e., carnitine palmitoyl transferase-I and II). Our data suggest that by restraining entry of FA-coenzyme A into mitochondria, low CACT levels increase cytosolic FA levels and their incorporation into glycerolipids. The low level of CACT in IR muscle may contribute to the elevated muscle concentrations of triglycerides, diacylglycerol, and FA-coenzyme A characteristic of IR muscle.

Long- and medium-chain fatty acid oxidation is increased in exercise-trained human skeletal muscle

The purpose of this study was to test the hypothesis that skeletal muscle fatty acid oxidation is enhanced by increased entry into the mitochondria with exercise training. Muscle was obtained from young ( approximately 24 years) sedentary (n = 13) and endurance-trained (n = 10) volunteers and oxidation studied by measuring (14)CO(2) production from labeled medium-chain (MCFA) or long-chain (LCFA) fatty acids in muscle homogenate preparations. LCFA (palmitate) oxidation was (P <.05) approximately 34% higher in the trained than sedentary subjects (26.9 +/- 3.0 v 17.8 +/- 1.3 nmol CO(2)/g x h). MCFA (octanoate) oxidation was also about 26% higher (P <.05) in the trained subjects (21.7 +/- 2.1 v 16.1 +/- 2.0 nmol CO(2)/g x h). To examine the roles of carnitine-mediated transport and mitochondrial content, we also measured carnitine palmitoyltransferase I (CPT1), carnitine octanoyl transferase (COT), and citrate synthase (CS) activities. CPT1 and CS activity were significantly (P <.05) higher (approximately 25%) in the endurance-trained subjects; there was no difference in COT activity. These data suggest that adaptations at the level of CPT1 and processes distal to this step may contribute to increases in LCFA or MFCA oxidation with exercise training. In contrast, carnitine-mediated transport (COT) does not appear to contribute to an enhancement in MCFA oxidation with exercise training.

Structural model of a malonyl-CoA-binding site of carnitine octanoyltransferase and carnitine palmitoyltransferase I: mutational analysis of a malonyl-CoA affinity domain

Carnitine octanoyltransferase (COT) and carnitine palmitoyltransferase (CPT) I, which facilitate the transport of medium- and long-chain fatty acids through the peroxisomal and mitochondrial membranes, are physiologically inhibited by malonyl-CoA. Using an "in silico" macromolecular docking approach, we built a model in which malonyl-CoA could be attached near the catalytic core. This disrupts the positioning of the acyl-CoA substrate in the channel in the model reported for both proteins (Morillas, M., Gómez-Puertas, P., Roca, R., Serra, D., Asins, G., Valencia, A., and Hegardt, F. G. (2001) J. Biol. Chem. 276, 45001-45008). The putative malonyl-CoA domain contained His(340), implicated together with His(131) in COT malonyl-CoA sensitivity (Morillas, M., Clotet, J., Rubi, B., Serra, D., Asins, G., Ariño, J., and Hegardt F. G. (2000) FEBS Lett. 466, 183-186). When we mutated COT His(131) the IC(50) increased, and malonyl-CoA competed with the substrate decanoyl-CoA. Mutation of COT Ala(332), present in the domain 8 amino acids away from His(340), decreased the malonyl-CoA sensitivity of COT. The homologous histidine and alanine residues of L-CPT I, His(277), His(483), and Ala(478) were also mutated, which decreased malonyl-CoA sensitivity. Natural mutation of Pro(479), which is also located in the malonyl-CoA predicted site, to Leu in a patient with human L-CPT I hereditary deficiency, modified malonyl-CoA sensitivity. We conclude that this malonyl-CoA domain is present in both COT and L-CPT I proteins and might be the site at which malonyl-CoA interacts with the substrate acyl-CoA. Other malonyl-CoA non-inhibitable members of the family, CPT II and carnitine acetyltransferase, do not contain this domain.

Post-transcriptional regulation of rat carnitine octanoyltransferase

Carnitine octanoyltransferase (COT) produces three different transcripts in rat through cis- and trans-splicing reactions, which can lead to the synthesis of two proteins. The occurrence of the three COT transcripts in rat has been found in all tissues examined and does not depend on sex, fat feeding, peroxisome proliferators or hyperinsulinaemia. Rat COT exon 2 contains a putative exonic splicing enhancer (ESE) sequence. Mutation of this ESE (GAAGAAG) to AAAAAAA decreased trans-splicing in vitro, from which it is deduced that this ESE sequence is partly responsible for the formation of the three transcripts. The protein encoded by cis-spliced mRNA of rat COT is inhibited by malonyl-CoA and etomoxir. cDNA species encoding full-length wild-type COT and one double mutant COT were expressed in Saccharomyces cerevisiae. The recombinant enzymes showed full activity towards both substrates, carnitine and decanoyl-CoA. The activity of the doubly mutated H131A/H340A enzyme was similar to that of the rat peroxisomal enzyme but was completely insensitive to malonyl-CoA and etomoxir. These results indicate that the histidine residues His-131 and His-340 are the sites responsible for the interaction of these two inhibitors, which inhibit COT by interacting with the same sites.

Mitochondrial respiration on rumenic and linoleic acids

Rumenic acid (cis-9, trans-11-C(18:2)) represents approx. 80% of conjugated linoleic acid (CLA) in dairy products. CLA has been shown to exert beneficial effects on health, but little work has been devoted to the ability to oxidize CLA isomers and the role of these isomers in the modulation of beta-oxidation flux. In the present study, respiration on rumenic acid was compared with that on linoleic acid (cis-9, cis-12-C(18:2)) with the use of rat liver mitochondria. In state-3, respiration was decreased by half with rumenic acid in comparison with linoleic acid. In the uncoupled state, respiration on CLA remained 30% lower. The lower ability to oxidize CLA was investigated through characterization of the enzymic steps. Rumenic acid was 33% less activated by acyl-CoA synthase than was linoleic acid. However, after such activation, the transfer of both acyl moieties to carnitine by carnitine acyltransferase I (CAT I) was of the same order. Moreover, CAT II activity was comparable with either isomer. After prior incubation with rumenic acid, oxidation of octanoic acid by re-isolated mitochondria was unimpaired, but that of palmitoleic acid was impaired unless linoleic acid was used in the prior incubation. The slower respiration on cis-9, trans-11-C(18:2) is suggested to arise from lower carnitine-acylcarnitine translocase activity towards the acylcarnitine form, causing an upstream increase in the corresponding acyl-CoA.

Microsomal malonyl-CoA-sensitive carnitine acyltransferase

Liver microsomes contain two carnitine acyltransferase activities. One of these has properties closely corresponding to those of 88 kDa mitochondrial carnitine palmitoyltransferase-1 (CPT-1). Antisera against CPT-1 cross-react with an 88 kDa microsomal protein, suggesting that CPT-1 may be targeted to both microsomal and mitochondrial membranes. However, no experiments using cDNAs corresponding to CPT-1 involving in vitro translation with microsomes or involving in vivo COS-1 cell transfection provided any evidence to support this hypothesis.

Carnitine-dependent metabolic activities in Saccharomyces cerevisiae: three carnitine acetyltransferases are essential in a carnitine-dependent strain

L-carnitine is required for the transfer of activated acyl-groups across intracellular membranes in eukaryotic organisms. In Saccharomyces cerevisiae, peroxisomal membranes are impermeable to acetyl-CoA, which is produced in the peroxisome when cells are grown on fatty acids as carbon source. In a reversible reaction catalysed by carnitine acetyltransferases (CATs), activated acetyl groups are transferred to carnitine to form acetylcarnitine which can be shuttled across membranes. Here we describe a mutant selection strategy that specifically selects for mutants affected in carnitine-dependent metabolic activities. Complementation of three of these mutants resulted in the cloning of three CAT encoding genes: CAT2, coding for the carnitine acetyltransferase associated with the peroxisomes and the mitochondria; YAT1, coding for the carnitine acetyltransferase, which is presumably associated with the outer mitochondrial membrane, and YER024w (YAT2), which encodes a third, previously unidentified carnitine acetyltransferase. The data also show that (a) L-carnitine and all three CATs are essential for growth on non-fermentable carbon sources in a strain with a disrupted CIT2 gene; (b) Yat2p contributes significantly to total CAT activity when cells are grown on ethanol; and that (c) the carnitine-dependent transfer of activated acetyl groups plays a more important role in cellular processes than previously realised.