Cloning and sequencing of the bovine cDNA encoding the mitochondrial tricarboxylate carrier protein

Extracellular Citrate Fuels Cancer Cell Metabolism and Growth

Cancer cells need excess energy and essential nutrients/metabolites not only to divide and proliferate but also to migrate and invade distant organs for metastasis. Fatty acid and cholesterol synthesis, considered a hallmark of cancer for anabolism and membrane biogenesis, requires citrate. We review here potential pathways in which citrate is synthesized and/or supplied to cancer cells and the impact of extracellular citrate on cancer cell metabolism and growth. Cancer cells employ different mechanisms to support mitochondrial activity and citrate synthesis when some of the necessary substrates are missing in the extracellular space. We also discuss the different transport mechanisms available for the entry of extracellular citrate into cancer cells and how citrate as a master metabolite enhances ATP production and fuels anabolic pathways. The available literature suggests that cancer cells show an increased metabolic flexibility with which they tackle changing environmental conditions, a phenomenon crucial for cancer cell proliferation and metastasis.

A plant-like mitochondrial carrier family protein facilitates mitochondrial transport of di- and tricarboxylates in Trypanosoma brucei

The procyclic form of the human parasite Trypanosoma brucei harbors one single, large mitochondrion containing all tricarboxylic acid (TCA) cycle enzymes and respiratory chain complexes present also in higher eukaryotes. Metabolite exchange among subcellular compartments such as the cytoplasm, the mitochondrion, and the peroxisomes is crucial for redox homeostasis and for metabolic pathways whose enzymes are dispersed among different organelles. In higher eukaryotes, mitochondrial carrier family (MCF) proteins transport TCA-cycle intermediates across the inner mitochondrial membrane. Previously, we identified several MCF members that are essential for T. brucei survival. Among these, only one MCF protein, TbMCP12, potentially could transport dicarboxylates and tricarboxylates. Here, we conducted phylogenetic and sequence analyses and functionally characterised TbMCP12 in vivo. Our results suggested that similarly to its homologues in plants, TbMCP12 transports both dicarboxylates and tricarboxylates across the mitochondrial inner membrane. Deleting this carrier in T. brucei was not lethal, while its overexpression was deleterious. Our results suggest that the intracellular abundance of TbMCP12 is an important regulatory element for the NADPH balance and mitochondrial ATP-production.

Mitochondrial tricarboxylate and dicarboxylate-tricarboxylate carriers: from animals to plants

The citrate carrier (CiC), characteristic of animals, and the dicarboxylate-tricarboxylate carrier (DTC), characteristic of plants and protozoa, belong to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors, and nucleotides between the cytoplasm and the mitochondrial matrix. Most of the functional data on these transporters are obtained from the studies performed with the protein purified from rat, eel yeast, and maize mitochondria or recombinant proteins from different sources incorporated into phospholipid vesicles (liposomes). The functional data indicate that CiC is responsible for the efflux of acetyl-CoA from the mitochondria to the cytosol in the form of citrate, the primer for fatty acid, cholesterol synthesis, and histone acetylation. Like the CiC, the citrate exported by DTC from the mitochondria to the cytosol in exchange for oxaloacetate can be cleaved by citrate lyase to acetyl-CoA and oxaloacetate and used for fatty acid elongation and isoprenoid synthesis. In addition to its role in fatty acid synthesis, CiC is involved in other processes such as gluconeogenesis, insulin secretion, inflammation, and cancer progression, whereas DTC is involved in the production of glycerate, nitrogen assimilation, ripening of fruits, ATP synthesis, and sustaining of respiratory flux in fruit cells. This review provides an assessment of the current understanding of CiC and DTC structural and biochemical characteristics, underlying the structure-function relationship of these carriers. Furthermore, a phylogenetic relationship between CiC and DTC is proposed.

The mitochondrial citrate carrier: metabolic role and regulation of its activity and expression

The citrate carrier (CiC), a nuclear-encoded protein located in the mitochondrial inner membrane, is a member of the mitochondrial carrier family. CiC plays an important role in hepatic lipogenesis, which is responsible for the efflux of acetyl-CoA from the mitochondria to the cytosol in the form of citrate, the primer for fatty acid and cholesterol synthesis. In addition, CiC is a key component of the isocitrate-oxoglutarate and the citrate-malate shuttles. CiC has been purified from various species and its reconstituted function characterized as well as its cDNA isolated and sequenced. CiC mRNA and/or CiC protein levels are high in liver, pancreas, and kidney, but are low or absent in brain, heart, skeletal muscle, placenta, and lungs. A reduction of CiC activity was found in diabetic, hypothyroid, starved rats, and in rats fed on a polyunsaturated fatty acid (PUFA)-enriched diet. Molecular analysis suggested that the regulation of CiC activity occurs mainly through transcriptional and post-transcriptional mechanisms. This review begins with an assessment of the current understanding of CiC structural and biochemical characteristics, underlying the structure-function relationship. Emphasis will be placed on the molecular basis of the regulation of CiC activity in coordination with fatty acid synthesis.

n-6 PUFAs downregulate expression of the tricarboxylate carrier in rat liver by transcriptional and posttranscriptional mechanisms

The tricarboxylate (citrate) carrier (TCC), a protein of the mitochondrial inner membrane, is an obligatory component of the shuttle system by which mitochondrial acetyl-CoA is transported into the cytosol, where lipogenesis occurs. The aim of this study was to investigate the molecular basis for the regulation of TCC gene expression by a high-fat, n-6 PUFA-enriched diet. Rats received for up to 4 weeks a diet enriched with 15% safflower oil (SO), which is high in linoleic acid (70.4%). We found a gradual decrease of TCC activity and a parallel decline in the abundance of TCC mRNA, the maximum effect occurring after 4 weeks of treatment. At this time, the estimated half-life of TCC mRNA was the same in the hepatocytes from rats on both diets, whereas the transcriptional rate of TCC mRNA, tested by nuclear run-on assay, was reduced by approximately 38% in the rats on the SO-enriched diet. The RNase protection assay showed that the ratio of mature to precursor RNA, measured in the nuclei, decreased with the change to the n-6 PUFA diet. These results suggest that administration of n-6 PUFAs to rats leads to changes not only in the transcriptional rate of the TCC gene but also in the processing of the nuclear precursor for TCC RNA.

The role and structure of mitochondrial carriers

Members of the mitochondrial carrier family transport compounds over the inner mitochondrial membrane to link the biochemical pathways in the cytosol with those in the mitochondrial matrix. X-ray crystallography has recently provided us with the first atomic model of the bovine ADP/ATP carrier, which is a member of this family. The structure explains the typical three-fold sequence repeats and signature motif of mitochondrial carriers. However, the carrier was crystallised as a monomer in detergent, which is inconsistent with the consensus that mitochondrial carriers exist as homo-dimers. The projection structure of the yeast ADP/ATP carrier by electron crystallography shows that carriers could form homo-dimers in the membrane.

Differential effects of coconut oil- and fish oil-enriched diets on tricarboxylate carrier in rat liver mitochondria

The mitochondrial tricarboxylate carrier (TCC) plays an important role in lipogenesis being TCC-responsible for the efflux from the mitochondria to the cytosol of acetyl-CoA, the primer for fatty acid synthesis. In this study, we investigated the effects of two high-fat diets with different fatty acid composition on the hepatic TCC activity. Rats were fed for 3 weeks on a basal diet supplemented with 15% of either coconut oil (CO), abundant in medium-chain saturated fatty acids, or fish oil (FO), rich in n-3 polyunsaturated fatty acids. Mitochondrial fatty acid composition was differently influenced by the dietary treatments, while no appreciable change in phospholipid composition and cholesterol level was observed. Compared with CO, the TCC activity was markedly decreased in liver mitochondria from FO-fed rats; kinetic analysis of the carrier revealed a decrease of the Vmax, with no change of the Km. No difference in the Arrhenius plot between the two groups was observed. Interestingly, the carrier protein level and the corresponding mRNA abundance decreased following FO treatment. These data indicate that FO administration markedly decreased the TCC activity as compared with CO. This effect is most likely due to a reduced gene expression of the carrier protein.

Biogenesis of rat mitochondrial citrate carrier (CIC): the N-terminal presequence facilitates the solubility of the preprotein but does not act as a targeting signal

Most mitochondrial preproteins carry a cleavable N-terminal presequence that mediates targeting to mitochondria and translocation across the mitochondrial membranes. In this study, we characterized the presequence of the citrate carrier (CIC, tricarboxylate carrier) of rat liver mitochondria. The CIC presequence was found to be dispensable both for targeting to mitochondria and insertion into the inner membrane. Unlike the presequence of the related phosphate carrier, fusion of the CIC presequence to the cytosolic enzyme dihydrofolate reductase did not confer mitochondrial targeting, indicating that the CIC presequence does not act as a targeting signal. However, the presequence was required to keep the CIC in a soluble state. Mature CIC lacking the presequence was prone to aggregation. We conclude that mitochondrial presequences do not necessarily act as mediators of targeting. In the case of the CIC, the presequence appears to determine the folding state of the preprotein.

Starvation-induced posttranscriptional control of rat liver mitochondrial citrate carrier expression

Starvation has been associated with a reduced citrate carrier (CTP) activity in rat liver mitochondria. In the present study the molecular mechanism responsible for this reduction was investigated. Northern blot analysis performed with hepatic total RNA showed a decrease of about 40% in the CTP mRNA abundance in starved rats, when compared to fed animals. Nuclear run-on assay did not reveal any appreciable difference in the rate of CTP mRNA synthesis between the two groups of animals, while the apparent half-life of CTP mRNA in hepatocytes from fed and starved rats was 11 and 6h, respectively. Therefore, these results suggest that in starved rats the regulation of hepatic CTP expression occurs at posttranscriptional level. Moreover, the reduced CTP activity in starved animals gradually increased by refeeding. The carrier activity reached fed rat values 6-9h following refeeding. Interestingly, the accumulation of CTP mRNA raised in parallel with the transport activity.

The mitochondrial tricarboxylate carrier of silver eel: dimeric structure and cytosolic exposure of both N- and C-termini

The mitochondrial tricarboxylate carrier plays a fundamental role in the hepatic fatty acid synthesis. In this study, we investigated the transmembrane organization of this protein in the inner membrane of eel liver mitochondria using anti-N-terminal and anti-C-terminal antibodies. These antibodies recognized the N- and C-termini of the tricarboxylate carrier in intact mitoplasts, thus suggesting a cytosolic exposure of these regions in the membrane-bound protein. This structural arrangement of the tricarboxylate carrier was further confirmed by protease treatment of intact mitoplasts. Moreover, the oligomeric state of the native tricarboxylate carrier was investigated by blue native electrophoresis. A dimeric form of the carrier protein was found when eel liver mitochondria were solubilized with the mild detergent digitonin. These findings suggest an arrangement of the dimeric tricarboxylate carrier into an even number of membrane-spanning domains, with the N-terminal and C-terminal regions oriented toward the intermembrane space of fish mitochondria.

Protection of NRK-52E cells, a rat renal proximal tubular cell line, from chemical-induced apoptosis by overexpression of a mitochondrial glutathione transporter

The dicarboxylate carrier (DCC) is one of two carriers responsible for glutathione (GSH) transport into rat kidney mitochondria. The central hypothesis of the present study was that overexpression of this carrier in renal proximal tubular cells increases content of mitochondrial GSH, which in turn can protect these cells from chemical-induced injury. We first cloned the carrier protein and verified its properties. This was accomplished by reverse transcribing total rat kidney RNA and polymerase chain reaction amplification with primers based on the complete cDNA sequence for the mitochondrial DCC protein. DCC was expressed as a His(6)-tagged protein, purified from Escherichia coli inclusion bodies, and reconstituted into proteoliposomes for transport assays. Time- and concentration-dependent uptake of both L-[(3)H-glycyl]GSH and [2-(14)C]malonate was observed with kinetics, substrate specificity, and inhibitor sensitivities similar to those observed in rat kidney proximal tubules. We next transiently transfected NRK-52E cells with the cDNA for rat kidney DCC to overexpress the protein. The presence of the recombinant DCC-His(6) protein was confirmed by immunoblots. Transport of both GSH and malonate into the mitochondrial fraction of transfected cells was enhanced 2.45- to 11.3-fold, compared with that in wild-type cells. Transfected cells exhibited markedly less apoptosis from tert-butyl hydroperoxide or S-(1,2-dichlorovinyl)-L-cysteine than did wild-type cells, validating the central hypothesis and providing us with a valuable and novel tool with which to further study GSH and thiol redox status in renal mitochondria, and the function of GSH transport in regulation of processes such as apoptosis and oxidative phosphorylation.

Malonyl-CoA, fuel sensing, and insulin resistance

Malonyl-CoA is an allosteric inhibitor of carnitine palmitoyltransferase (CPT) I, the enzyme that controls the transfer of long-chain fatty acyl (LCFA)-CoAs into the mitochondria where they are oxidized. In rat skeletal muscle, the formation of malonyl-CoA is regulated acutely (in minutes) by changes in the activity of the beta-isoform of acetyl-CoA carboxylase (ACCbeta). This can occur by at least two mechanisms: one involving cytosolic citrate, an allosteric activator of ACCbeta and a precursor of its substrate cytosolic acetyl-CoA, and the other involving changes in ACCbeta phosphorylation. Increases in cytosolic citrate leading to an increase in the concentration of malonyl-CoA occur when muscle is presented with insulin and glucose, or when it is made inactive by denervation, in keeping with a diminished need for fatty acid oxidation in these situations. Conversely, during exercise, when the need of the muscle cell for fatty acid oxidation is increased, decreases in the ATP/AMP and/or creatine phosphate-to-creatine ratios activate an isoform of an AMP-activated protein kinase (AMPK), which phosphorylates ACCbeta and inhibits both its basal activity and activation by citrate. The central role of cytosolic citrate links this malonyl-CoA regulatory mechanism to the glucose-fatty acid cycle concept of Randle et al. (P. J. Randle, P. B. Garland. C. N. Hales, and E. A. Newsholme. Lancet 1: 785-789, 1963) and to a mechanism by which glucose might autoregulate its own use. A similar citrate-mediated malonyl-CoA regulatory mechanism appears to exist in other tissues, including the pancreatic beta-cell, the heart, and probably the central nervous system. It is our hypothesis that by altering the cytosolic concentrations of LCFA-CoA and diacylglycerol, and secondarily the activity of one or more protein kinase C isoforms, changes in malonyl-CoA provide a link between fuel metabolism and signal transduction in these cells. It is also our hypothesis that dysregulation of the malonyl-CoA regulatory mechanism, if it leads to sustained increases in the concentrations of malonyl-CoA and cytosolic LCFA-CoA, could play a key role in the pathogenesis of insulin resistance in muscle. That it may contribute to abnormalities associated with the insulin resistance syndrome in other tissues and the development of obesity has also been suggested. Studies are clearly needed to test these hypotheses and to explore the notion that exercise and some pharmacological agents that increase insulin sensitivity act via effects on malonyl-CoA and/or cytosolic LCFA-CoA.

Kinetics of the reconstituted tricarboxylate carrier from eel liver mitochondria

The tricarboxylate carrier from eel liver mitochondria was purified by chromatography on hydroxyapatite and Matrix Gel Blue B and reconstituted into liposomes by removal of the detergent with Amberlite. Optimal transport activity was obtained by using a phospholipid concentration of 11.5 mg/ml, a Triton X- 114/phospholipid ratio of 0.9, and ten passages through the same Amberlite column. The activity of the carrier was influenced by the phospholipid composition of the liposomes, being increased by cardiolipin and phosphatidylethanolamine and decreased by phosphatidylinositol. The reconstituted tricarboxylate carrier catalyzed a first-order reaction of citrate/citrate or citrate/malate exchange. The maximum transport rate of external [14C]citrate was 9.0 mmol/min per g of tricarboxylate carrier protein at 25 degrees C and this value was virtually independent of the type of substrate present in the external or internal space of the liposomes. The half-saturation constant (Km) was 62 microM for citrate and 541 microM for malate. The activation energy of the citrate/citrate exchange reaction was 74 kJ/mol from 5 to 19 degrees C and 31 kJ/mol from 19 to 35 degrees C. The rate of the exchange had an external pH optimum of 8.