The tricarboxylate carrier from rat liver mitochondria. Purification, reconstitution and kinetic characterization

Overexpression of Citrate-Malate Carrier Promoted Lipid Accumulation in Oleaginous Filamentous Fungus Mortierella alpina

The mitochondrial citrate-malate carrier is responsible for the transport of citrate and malate between the mitochondria and cytosol, ensuring citrate supply substrate for fatty acid synthesis. In this study, we investigated the overexpression of the citrate-malate carrier coded by three genes (MaCT1/MaCT2/MaTCT) in Mortierella alpina to enhance lipid accumulation. Our results showed that the overexpression of MaCT1, MaCT2, and MaTCT increased the fatty acid content by up to 21.7, 29.5, and 12.8%, respectively, compared with the control strain, but had no effect on the growth. Among them, the MaCT2-overexpressing strain performed the best, and its total fatty acid yield was increased by 51.6% compared to the control. Furthermore, the relative transcription level of MaCT2 indeed increased significantly in the recombinant strains. These findings are beneficial to understanding the citrate transport system and improve the industrial applications of the oleaginous filamentous fungus M. alpina.

Amino acid abundance and composition in cell culture medium affects trace metal tolerance and cholesterol synthesis

Amino acid compositions of cell culture media are empirically designed to enhance cell growth and productivity and vary both across media formulations and over the course of culture due to imbalance in supply and consumption. The interconnected nature of the amino acid transporters and metabolism suggests that changes in amino acid composition can affect cell physiology. In this study, we explore the effect of a step change in amino acid composition from a DMEM: F12-based medium to a formulation varying in relative abundances of all amino acids, evaluated at two amino acid concentrations (lean LAA vs. rich HAA). Cell growth was inhibited in LAA but not HAA. In addition to the expected effects on expression of the cell cycle, amino acid response and mTOR pathway genes in LAA, we observed an unanticipated effect on zinc uptake and efflux genes. This was accompanied by a lower tolerance to zinc supplementation in LAA but not in the other formulations. Histidine was sufficient but not necessary to prevent such zinc toxicity. Additionally, an unanticipated downregulation of genes in the cholesterol synthesis pathway was observed in HAA, accompanied by an increase in cellular cholesterol content, which may depend on the relative abundances of glutamine and other amino acids. This study shows that changes in the amino acid composition without any evident effect on growth may have profound effects on metabolism. Such analyses can help rationalize the designing of medium and feed formulations for bioprocess applications beyond replenishment of consumed components.

Antioxidant Versus Pro-Apoptotic Effects of Mushroom-Enriched Diets on Mitochondria in Liver Disease

Mitochondria play a central role in non-alcoholic fatty liver disease (NAFLD) progression and in the control of cell death signalling during the progression to hepatocellular carcinoma (HCC). Associated with the metabolic syndrome, NAFLD is mostly driven by insulin-resistant white adipose tissue lipolysis that results in an increased hepatic fatty acid influx and the ectopic accumulation of fat in the liver. Upregulation of beta-oxidation as one compensatory mechanism leads to an increase in mitochondrial tricarboxylic acid cycle flux and ATP generation. The progression of NAFLD is associated with alterations in the mitochondrial molecular composition and respiratory capacity, which increases their vulnerability to different stressors, including calcium and pro-inflammatory molecules, which result in an increased generation of reactive oxygen species (ROS) that, altogether, may ultimately lead to mitochondrial dysfunction. This may activate further pro-inflammatory pathways involved in the progression from steatosis to steatohepatitis (NASH). Mushroom-enriched diets, or the administration of their isolated bioactive compounds, have been shown to display beneficial effects on insulin resistance, hepatic steatosis, oxidative stress, and inflammation by regulating nutrient uptake and lipid metabolism as well as modulating the antioxidant activity of the cell. In addition, the gut microbiota has also been described to be modulated by mushroom bioactive molecules, with implications in reducing liver inflammation during NAFLD progression. Dietary mushroom extracts have been reported to have anti-tumorigenic properties and to induce cell-death via the mitochondrial apoptosis pathway. This calls for particular attention to the potential therapeutic properties of these natural compounds which may push the development of novel pharmacological options to treat NASH and HCC. We here review the diverse effects of mushroom-enriched diets in liver disease, emphasizing those effects that are dependent on mitochondria.

The multifaceted contributions of mitochondria to cellular metabolism

Although classically appreciated for their role as the powerhouse of the cell, the metabolic functions of mitochondria reach far beyond bioenergetics. In this Review, we discuss how mitochondria catabolize nutrients for energy, generate biosynthetic precursors for macromolecules, compartmentalize metabolites for the maintenance of redox homeostasis and function as hubs for metabolic waste management. We address the importance of these roles in both normal physiology and in disease.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

Reduced activity and expression of mitochondrial citrate carrier in streptozotocin-induced diabetic rats

Citrate carrier (CiC), an integral protein of the mitochondrial inner membrane, plays an important role in hepatic intermediary metabolism, supplying the cytosol with acetyl-coenzyme A for fatty acid and cholesterol synthesis. Here, the effect of streptozotocin-induced diabetes on CiC activity and expression in rat liver was investigated. The rate of citrate transport was reduced by about 35% in mitochondria from diabetic vs. control rats. Kinetic studies in mitochondria from diabetic rats showed a reduction in maximum velocity and almost unchanged Michaelis-Menten constant of the CiC protein. Mitochondrial phospholipid amount was not significantly affected, whereas an increase in the cholesterol content and in the cholesterol/phospholipid ratio was observed. To thoroughly investigate the mechanism responsible for the reduced CiC activity in the diabetic state, molecular studies were performed. Ribonuclease protection assays and Western blotting analysis indicated that both hepatic CiC mRNA accumulation and protein level decreased similarly to the CiC activity. The reduced mRNA level and the lower content of the mitochondrial CiC protein, might account for the decline of CiC activity in diabetic animals. To discriminate between the role played by hyperglycemia from that of hypoinsulinemia in the reduction of CiC activity and expression, studies were conducted administrating phlorizin or insulin to streptozotocin-diabetic rats. Our data indicated that both insulin and glucose affect CiC activity and expression in diabetic rats, although they act at different regulatory steps.

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.

Identification and characterization of a novel mitochondrial tricarboxylate carrier

Here we report the identification and functional characterization of a novel mitochondrial tricarboxylate carrier protein, designated BBG-TCC, in rat brain. The cDNA encodes the predicted protein of 342-amino acid residues with five putative membrane-spanning domains. The protein has apparent similarity with a mitochondrial tricarboxylate carrier TCC, but is distinct from the other mitochondria anion transporters. BBG-TCC shows a citrate transport activity. It is specifically expressed in the brain and localizes in the mitochondria of Bergmann glial cells. In contrast, the expression of TCC is rather ubiquitous and strong in neuronal cells in the brain. This new family of proteins may contribute to biosynthesis and bioenergetics in the brain.

Expression of mitochondrial tricarboxylate carrier TCC mRNA and protein in the rat brain

The tricarboxylate carrier protein catalyzes an electroneutral exchange across the mitochondrial inner membrane of tricarboxylate, dicarboxylate or phosphoenolpyruvate. We examined expression and localization of mitochondrial tricarboxylate carrier TCC mRNA and protein in the rat brain. TCC mRNA was ubiquitously expressed in all rat tissues examined and was abundant in brain, liver and kidney. TCC protein as well as mRNA was widely expressed in brain, and the protein expression was strong in neuronal cells in the hippocampus, the olfactory bulb, the corpus mamillare and the cerebellum. Our results suggest that this tricarboxylate carrier protein may contribute to biosynthesis and bioenergetics in neuronal cells in brain.

High level expression and characterization of the mitochondrial citrate transport protein from the yeast Saccharomyces cerevisiae

The gene encoding the mitochondrial citrate transport protein (CTP) in the yeast Saccharomyces cerevisiae has been identified, and its protein product has been overexpressed in Escherichia coli. The expressed CTP accumulates in inclusion bodies and can be solubilized with sarkosyl. Approximately 25 mg of solubilized CTP at a purity of 75% is obtained per liter of E. coli culture. The function of the solubilized CTP has been reconstituted in a liposomal system where both its kinetic parameters (i.e. Km = 0.36 mM and Vmax = 2.5 mumol/min/mg protein) and its substrate specificity have been determined. Notably, the yeast CTP displays a stricter specificity for tricarboxylates than do CTPs from higher eukaryotic organisms. Dot matrix analysis of the yeast CTP sequence indicates the presence of three homologous sequence domains (each approximately 100 residues in length), which are also related to domains in other CTPs. Thus, the yeast CTP displays the tripartite structure characteristic of other mitochondrial transporters. Alignment of the yeast CTP sequence with CTPs from other sources defines a consensus sequence that displays 89 positions of amino acid identity, as well as the more generalized mitochondrial transporter-associated sequence motif. Based on hydropathy analysis, the yeast CTP contains six putative membrane-spanning alpha-helices. Finally, Southern blot analysis indicates that the yeast genome contains a single gene encoding the mitochondrial CTP. Our data indicate that, based on both its structural and functional properties, the expressed yeast CTP can be assigned membership in the mitochondrial carrier family. The identification of the yeast CTP gene, and the expression and purification of large quantities of its protein product, pave the way for investigations into the roles of specific amino acids in the CTP translocation mechanism, as well as for the initiation of crystallization trials.

The mitochondrial tricarboxylate carrier

The tricarboxylate carrier has recently been purified from rat liver mitochondria by three distinct scientific groups using different methods. A 37-38-kDa protein has been prepared by silca gel 60 chromatography by our group (Claeys and Azzi, 1989; Glerum et al., 1990). The specific citrate transport activity of this preparation is not significantly different from that measured in mitochondria and it is inhibitable by 1,2,3-benzenetricarboxylic acid. Bisaccia et al. (1990) have reported the isolation of a 30-kDa protein by Celite 535 chromatography, and Kaplan's group (Kaplan et al., 1990) have isolated a 32.5-kDa protein by Matrex Orange, Matrex Blue, and Affi-Gel chromatography. Peptide mapping has failed to support any structural homologies between the 37-38-kDa and the 30-32.5-kD proteins. The 38-kD protein is N-terminally blocked. The peptides obtained by several cleavage procedures have been partially sequenced. Their sequence information has been used to obtain different cDNA clones by a dual approach, the polymerase chain reaction and screening of a lambda ZAP cDNA library. The largest cDNA which could be isolated is 2,986 bp in length and contains a 1071-bp-long open reading frame and an unusually long 3' untranslated region, both of which have been completely sequenced. The protein sequence of the carrier from the first in-frame methionine is 322 amino acids in length and exhibits a molecular mass of 35,546. Comparison of the protein sequence to the sequences of the four members of the mitochondrial carrier protein family (ADP/ATP carrier, phosphate carrier, 2-oxoglutarate/malate carrier, and uncoupling protein) does not reveal significant similarity (cf. Walker et al., 1987). A tripartite internal homology, which is a characteristic of these proteins, is not present in the sequence of the tricarboxylate carrier protein. The mRNA for the tricarboxylate carrier is expressed in rat liver and brain, but not in rat heart.

Structure, function and regulation of the tricarboxylate transport protein from rat liver mitochondria

Recent progress is summarized on the structure, function, and regulation of the tricarboxylate (i.e., citrate) transport protein (CTP) from the rat liver mitochondrial inner membrane. The transporter has been purified and its reconstituted function characterized. A cDNA clone encoding the CTP has been isolated and sequenced, thus enabling a deduction of the complete amino acid sequence of this 32.6 kDa transport protein. Dot matrix analysis and sequence alignment indicate that based on structural considerations the CTP can be assigned to the mitochondrial carrier family. Hydropathy analysis of the transporter sequence indicates six putative membrane-spanning alpha-helices and has permitted the development of an initial model for the topography of the CTP within the inner membrane. The questions as to whether more than one gene encodes the CTP and whether more than one isoform is expressed remain unanswered at this time. Studies documenting a diabetes-induced alteration in the function of several mitochondrial anion transporters, which can be reversed by treatment with insulin, provide a physiologically/pathologically relevant experimental system for studying the molecular mechanism(s) by which mitochondrial transporters are regulated. Potential future research directions are discussed.

Competition for electron transfer between cytochromes P450scc and P45011 beta in rat adrenal mitochondria

Rat adrenal mitochondria contain approximately equal levels of P450scc and P45011 beta, each reduced by NADPH through adrenodoxin reductase (ADX-reductase) and adrenodoxin (ADX). Constitutive cholesterol side-chain cleavage (SCC) can be increased over 20-fold through a combination of hormonal activation and inhibition of cholesterol metabolism in vivo prior to isolation of the mitochondria. This stimulation, which results from accumulated reactive cholesterol, does not significantly affect either the dependence of activities on the concentration of isocitrate (IC) and succinate (SU) or the ratio of maximum activities [3:1] supported by these reductants. Thus, the rate of cholesterol SCC is determined independently by electron transfer and the amount of reactive cholesterol. Hydroxylation of deoxycorticosterone (11 beta and 18 positions) required much higher levels of each reductant, indicating less effective reductant transfer to P45011 beta. Reactions at P450scc and P45011 beta, mediated by IC, are enhanced by low concentrations of various dicarboxylates anions (fumarate, SU). The actions of SU dehydrogenase inhibitors and the activity of fumarate, a poor direct reductant, suggest that higher production of NADPH results from malate-enhanced uptake of isocitrate. Only synergistic combinations of reductants are sufficient to sustain maximum rates of 11-deoxycorticosterone (DOC) metabolism, whereas IC is fully effective for P450scc. Increased reaction at P450scc (cholesterol loading or addition of 20 alpha-hydroxycholesterol) decreased simultaneous DOC metabolism at P45011 beta in inverse proportion to the estimated intramitochondrial generation of NADPH (1 mM or 10 mM SU > 1 mM IC > 10 mM IC). These decreases were reversed by inhibition of P450scc. Crossover inhibition caused by maximum DOC metabolism was less pronounced. EGTA/albumin treatment, which enhanced activities at both P450scc and P45011 beta, presumably via increased NADPH, diminished this cross-competition. The differential dependence on reductants and the characteristics of crossover competition are consistent with a roughly three-fold more favorable partitioning of electron transfer to P450scc, possibly caused by preferential interaction of reduced adrenodoxin with P450scc.

Reaction mechanism of the reconstituted tricarboxylate carrier from rat liver mitochondria

Transport of citrate and malate by the tricarboxylate carrier from rat liver mitochondria has been studied in a reconstituted system. Homologous citrate/citrate antiport and heterologous (electroneutral) citrate/malate antiport was kinetically analyzed. The maximal rates of the two exchange modes did not vary significantly within pH 7.0 to 7.8 which is the optimum pH-range for transport activity. On the other hand, the apparent transport affinity varied considerably within this range. Calculations on the basis of the different pK values for citrate and malate indicate that only H-citrate2- and malate2- are accepted as transport species by the tricarboxylate carrier. A complete set of half-saturation constants was established for citrate and malate on both the external and the internal side of the membrane. Both the Km and Vmax for citrate and malate were independent of the nature of the countersubstrate at the other side of the membrane. Bisubstrate initial velocity analyses of the exchange reaction resulted in a kinetic pattern which is consistent with a sequential antiport mechanism. This type of mechanism implies formation of a ternary complex of the carrier with two substrate molecules before the transport reaction occurs. Thus the tricarboxylate carrier falls into the functional family of mitochondrial carrier proteins showing sequential transport mechanisms.