Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology

The citrate carrier (CIC) is an integral protein of the inner mitochondrial membrane which catalyzes the efflux of mitochondrial citrate (or other tricarboxylates) in exchange with a cytosolic anion represented by a tricarboxylate or a dicarboxylate or phosphoenolpyruvate. In this way, the CIC provides the cytosol with citrate which is involved in many metabolic reactions. Several studies have been carried out over the years on the structure, function and regulation of this metabolite carrier protein both in mammals and in many other organisms. A lot of data on the characteristics of this protein have therefore accumulated over time thereby leading to a complex framework of metabolic and physiological implications connected to the CIC function. In this review, we critically analyze these data starting from the multiple roles played by the mitochondrial CIC in many cellular processes and then examining the regulation of its activity in different nutritional and hormonal states. Finally, the metabolic significance of the citrate flux, mediated by the CIC, across distinct subcellular compartments is also discussed.

Citrate transporter inhibitors: possible new anticancer agents

The culmination of 80+ years of cancer research implicates the aberrant metabolism in tumor cells as a root cause of pathogenesis. Citrate is an essential molecule in intermediary metabolism, and its amplified availability to critical pathways in cancer cells via citrate transporters confers a high rate of cancer cell growth and proliferation. Inhibiting the plasma membrane and mitochondrial citrate transporters - whether individually, in combination, or partnered with complementary metabolic targets - in order to combat cancer may prove to be a consequential chemotherapeutic strategy. This review aims to summarize the use of different classes of citrate transporter inhibitors for anticancer activity, either individually or as part of a cocktail.

Genome wide analysis of gene expression changes in skin from patients with type 2 diabetes

Non-healing chronic ulcers are a serious complication of diabetes and are a major healthcare problem. While a host of treatments have been explored to heal or prevent these ulcers from forming, these treatments have not been found to be consistently effective in clinical trials. An understanding of the changes in gene expression in the skin of diabetic patients may provide insight into the processes and mechanisms that precede the formation of non-healing ulcers. In this study, we investigated genome wide changes in gene expression in skin between patients with type 2 diabetes and non-diabetic patients using next generation sequencing. We compared the gene expression in skin samples taken from 27 patients (13 with type 2 diabetes and 14 non-diabetic). This information may be useful in identifying the causal factors and potential therapeutic targets for the prevention and treatment of diabetic related diseases.

Cellular citrate levels establish a regulatory link between energy metabolism and the hepatic iron hormone hepcidin

Expression of the hepatic peptide hormone hepcidin responds to iron levels via BMP/SMAD signaling, to inflammatory cues via JAK/STAT signaling, to the nutrient-sensing mTOR pathway, as well as to proliferative signals and gluconeogenesis. Here, we asked the question whether hepcidin expression is altered by metabolites generated by intermediary metabolism. To identify such metabolites, we took advantage of a comprehensive RNAi screen, which revealed effectors involved in citrate metabolism. We show that the inhibition of citrate-consuming enzymes increases hepcidin mRNA expression in primary murine hepatocytes. Consistently, citrate treatment of primary murine hepatocytes or intravenous injection of citrate in mice increases cellular citrate concentrations and hepcidin expression. We further demonstrate that the hepcidin response to citrate involves the SMAD signaling pathway. These results reveal links between iron homeostasis and energy metabolism that may help to explain why iron levels are frequently altered in metabolic disorders.

KEY MESSAGES

• Elevated citrate levels increase hepcidin mRNA expression in primary hepatocytes. • Citrate treatment in primary hepatocytes activates hepcidin expression. • Intravenous injection of citrate in mice increases hepcidin mRNA levels. • The hepcidin response to citrate involves the BMP/SMAD signaling pathway.

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.

Mitochondrial pyruvate transport: a historical perspective and future research directions

Pyruvate is the end-product of glycolysis, a major substrate for oxidative metabolism, and a branching point for glucose, lactate, fatty acid and amino acid synthesis. The mitochondrial enzymes that metabolize pyruvate are physically separated from cytosolic pyruvate pools and rely on a membrane transport system to shuttle pyruvate across the impermeable inner mitochondrial membrane (IMM). Despite long-standing acceptance that transport of pyruvate into the mitochondrial matrix by a carrier-mediated process is required for the bulk of its metabolism, it has taken almost 40 years to determine the molecular identity of an IMM pyruvate carrier. Our current understanding is that two proteins, mitochondrial pyruvate carriers MPC1 and MPC2, form a hetero-oligomeric complex in the IMM to facilitate pyruvate transport. This step is required for mitochondrial pyruvate oxidation and carboxylation-critical reactions in intermediary metabolism that are dysregulated in several common diseases. The identification of these transporter constituents opens the door to the identification of novel compounds that modulate MPC activity, with potential utility for treating diabetes, cardiovascular disease, cancer, neurodegenerative diseases, and other common causes of morbidity and mortality. The purpose of the present review is to detail the historical, current and future research investigations concerning mitochondrial pyruvate transport, and discuss the possible consequences of altered pyruvate transport in various metabolic tissues.

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.

Transcriptional Regulation of the Mitochondrial Citrate and Carnitine/Acylcarnitine Transporters: Two Genes Involved in Fatty Acid Biosynthesis and β-oxidation

Transcriptional regulation of genes involved in fatty acid metabolism is considered the major long-term regulatory mechanism controlling lipid homeostasis. By means of this mechanism, transcription factors, nutrients, hormones and epigenetics control not only fatty acid metabolism, but also many metabolic pathways and cellular functions at the molecular level. The regulation of the expression of many genes at the level of their transcription has already been analyzed. This review focuses on the transcriptional control of two genes involved in fatty acid biosynthesis and oxidation: the citrate carrier (CIC) and the carnitine/ acylcarnitine/carrier (CAC), which are members of the mitochondrial carrier gene family, SLC25. The contribution of tissue-specific and less tissue-specific transcription factors in activating or repressing CIC and CAC gene expression is discussed. The interaction with drugs of some transcription factors, such as PPAR and FOXA1, and how this interaction can be an attractive therapeutic approach, has also been evaluated. Moreover, the mechanism by which the expression of the CIC and CAC genes is modulated by coordinated responses to hormonal and nutritional changes and to epigenetics is highlighted.

Streptozotocin-induced diabetes affects in rat liver citrate carrier gene expression by transcriptional and posttranscriptional mechanisms

Citrate carrier (CiC), also known as tricarboxylate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. Here, we report the effect of streptozotocin-induced diabetes on the activity and expression of CiC in rat liver mitochondria. A significant reduction of CiC activity and a parallel decline in the abundance of CiC mRNA were found in liver from diabetic rats. Diabetes did not influence CiC mRNA stability, whereas nuclear run-on assay revealed that the transcriptional rate of CiC mRNA decreased, when compared to control, in the nuclei from diabetic rats. The ratio of mature to precursor CiC RNA decreased in diabetic animals, indicating that the splicing of CiC RNA was also affected. The 3'-end processing rate of CiC mRNA was not altered in diabetes. These results suggest that diabetes affects CiC expression at both transcriptional and posttranscriptional levels. In addition, by in vitro transfection experiments in rat hepatocytes cultured in the absence of insulin, a reduction of CiC promoter activity was observed, and this was ascribed to a decreased expression of sterol regulatory element-binding protein-1 transcriptional factor. Furthermore, the binding of sterol regulatory element-binding protein-1 to the CiC promoter was reduced in STZ-diabetic rats with respect to control ones, and it was restored to the control values after insulin treatment.

The yeast mitochondrial citrate transport protein: molecular determinants of its substrate specificity

The objective of this study was to identify the role of individual amino acid residues in determining the substrate specificity of the yeast mitochondrial citrate transport protein (CTP). Previously, we showed that the CTP contains at least two substrate-binding sites. In this study, utilizing the overexpressed, single-Cys CTP-binding site variants that were functionally reconstituted in liposomes, we examined CTP specificity from both its external and internal surfaces. Upon mutation of residues comprising the more external site, the CTP becomes less selective for citrate with numerous external anions able to effectively inhibit [(14)C]citrate/citrate exchange. Thus, the site 1 variants assume the binding characteristics of a nonspecific anion carrier. Comparison of [(14)C]citrate uptake in the presence of various internal anions versus water revealed that, with the exception of the R189C mutant, the other site 1 variants showed substantial uniport activity relative to exchange. Upon mutation of residues comprising site 2, we observed two types of effects. The K37C mutant displayed a markedly enhanced selectivity for external citrate. In contrast, the other site 2 mutants displayed varying degrees of relaxed selectivity for external citrate. Examination of internal substrates revealed that, in contrast to the control transporter, the R181C variant exclusively functioned as a uniporter. This study provides the first functional information on the role of specific binding site residues in determining mitochondrial transporter substrate selectivity. We interpret our findings in the context of our homology-modeled CTP as it cycles between the outward-facing, occluded, and inward-facing states.

Renal membrane transport of glutathione in toxicology and disease

Membrane transport processes, at both the plasma membranes and intracellular membranes, play critical roles in renal function and are a determining factor in the susceptibility of renal epithelial cells to blood-borne drugs and toxic chemicals. Proximal tubular epithelial cells possess a large array of transport proteins for organic anions, organic cations, and peptides on both basolateral and brush-border plasma membranes. Although these transporters function in excretion of waste products and reabsorption of nutrients, they also play a role in the susceptibility of the kidneys to drugs and other toxicants in the blood. The proximal tubules are typically the primary target cells because they are the first epithelial cell population exposed to such chemicals in either the renal plasma or glomerular filtrate and because of their large array of membrane transporters. Besides transport across the basolateral and brush-border plasma membranes, transport across intracellular membranes such as the mitochondrial inner membrane is a critical determinant of metabolite distribution. To illustrate the function of these transporters, carrier-mediated processes for transport of the tripeptide and antioxidant glutathione across the basolateral, brush-border, and mitochondrial inner membranes of the renal proximal tubule are reviewed. Studies are summarized that have identified the involvement of specific carrier proteins and characterized the role of these transporters in glutathione metabolism and turnover, susceptibility of the proximal tubules to oxidative and other stresses, and modulation in disease and other pathological processes.

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.

Probing the effect of transport inhibitors on the conformation of the mitochondrial citrate transport protein via a site-directed spin labeling approach

The present investigation utilized the site-directed spin labeling method of electron paramagnetic resonance (EPR) spectroscopy to identify the effect of citrate, the natural ligand, and transport inhibitors on the conformation of the yeast mitochondrial citrate transport protein (CTP) reconstituted in liposomal vesicles. Spin label was placed at six different locations within the CTP in order to monitor conformational changes that occurred near each of the transporter's two substrate binding sites, as well as at more distant domains within the CTP architecture. We observed that citrate caused little change in the EPR spectra. In contrast the transport inhibitors 1,2,3-benzenetricarboxylate (BTC), pyridoxal 5'-phosphate (PLP), and compound 792949 resulted in spectral changes that indicated a decrease in the flexibility of the attached spin label at each of the six locations tested. The rank order of the immobilizing effect was compound 792949 > PLP > BTC. The four spin-label locations that report on the CTP substrate binding sites displayed the greatest changes in the EPR spectra upon addition of inhibitor. Furthermore, we found that when compound 792949 was added vectorially (i.e., extra- and/or intra-liposomally), the immobilizing effect was mediated nearly exclusively by external reagent. In contrast, upon addition of PLP vectorially, the effect was mediated to a similar extent from both the external and the internal compartments. In combination our data indicate that: i) citrate binding to the CTP substrate binding sites does not alter side-chain and/or backbone mobility in a global manner and is consistent with our expectation that both in the absence and presence of substrate the CTP displays the flexibility required of a membrane transporter; and ii) binding of each of the transport inhibitors tested locked multiple CTP domains into more rigid conformations, thereby exhibiting long-range inter-domain conformational communication. The differential vectorial effects of compound 792949 and PLP are discussed in the context of the CTP homology-modeled structure and potential mechanistic molecular explanations are given.

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.

Role of FOXA in mitochondrial citrate carrier gene expression and insulin secretion

In this study, we have investigated the transcriptional role of the FOXA site present in the promoter of the mitochondrial citrate carrier (CIC) gene. We have shown that wild-type (but not mutated) CIC FOXA site cloned in front of the luciferase promoter confers transcriptional activation of the gene reporter, particularly in cells overexpressing FOXA1. We have also demonstrated that overexpression and silencing of FOXA increases and reduces CIC transcript and protein levels, respectively. In addition, FOXA1 silencing in INS-1 cells decreases not only CIC mRNA and protein but also the amount of citrate in the cytosol and glucose-stimulated insulin secretion. These results show that FOXA plays a role in the transcriptional regulation of CIC and in insulin secretion.

The yeast mitochondrial citrate transport protein: identification of the Lysine residues responsible for inhibition mediated by Pyridoxal 5'-phosphate

The present investigation identifies the molecular basis for the well-documented inhibition of the mitochondrial inner membrane citrate transport protein (CTP) function by the lysine-selective reagent pyridoxal 5'-phosphate. Kinetic analysis indicates that PLP is a linear mixed inhibitor of the Cys-less CTP, with a predominantly competitive component. We have previously concluded that the CTP contains at least two substrate binding sites which are located at increasing depths within the substrate translocation pathway and which contain key lysine residues. In the present investigation, the roles of Lys-83 in substrate binding site one, Lys-37 and Lys-239 in substrate binding site two, and four other off-pathway lysines in conferring PLP-inhibition of transport was determined by functional characterization of seven lysine to cysteine substitution mutants. We observed that replacement of Lys-83 with cysteine resulted in a 78% loss of the PLP-mediated inhibition of CTP function. In contrast, replacement of either Lys-37 or Lys-239 with cysteine caused a modest reduction in the inhibition caused by PLP (i.e., 31% and 20% loss of inhibition, respectively). Interestingly, these losses of PLP-mediated inhibition could be rescued by covalent modification of each cysteine with MTSEA, a reagent that adds a lysine-like moiety (i.e. SCH(2)CH(2)NH(3) (+)) to the cysteine sulfhydryl group. Importantly, the replacement of non-binding site lysines (i.e., Lys-45, Lys-48, Lys-134, Lys-141) with cysteine resulted in little change in the PLP inhibition. Based upon these results, we conducted docking calculations with the CTP structural model leading to the development of a physical binding model for PLP. In combination, our data support the conclusion that PLP exerts its main inhibitory effect by binding to residues located within the two substrate binding sites of the CTP, with Lys-83 being the primary determinant of the total PLP effect since the replacement of this single lysine abolishes nearly all of the observed inhibition by PLP.

Transcription of the mitochondrial citrate carrier gene: role of SREBP-1, upregulation by insulin and downregulation by PUFA

In this study we investigated the transcriptional role of the sterol regulatory element (SRE) present in the promoter of the mitochondrial citrate carrier (CIC). We show that wild-type (but not mutated) CIC SRE cloned in front of the luciferase promoter confers transcriptional activation of the gene reporter. We also demonstrate that insulin activates, and polyunsaturated fatty acids (PUFA) inhibit, the gene reporter activity driven by the CIC promoter containing wild-type (but not mutated) SRE. Finally, both insulin treatment and overexpression of SRE binding protein (SREBP-1) increase the CIC transcript and protein levels, whereas PUFA have an opposite effect. These results show that SRE/SREBP-1 play a role in the transcriptional regulation of CIC by insulin and PUFA.

Genes involved in oxidative phosphorylation are coordinately upregulated with fasting hyperglycaemia in livers of patients with type 2 diabetes

AIMS/HYPOTHESIS

Mitochondrial oxidative phosphorylation (OXPHOS) plays an important role in the pathophysiology of type 2 diabetes. Genes involved in OXPHOS have been reported to be down-regulated in skeletal muscle from patients with type 2 diabetes; however, hepatic regulation is unknown.

MATERIALS AND METHODS

We analysed expression of genes involved in OXPHOS from the livers of 14 patients with type 2 diabetes and 14 subjects with NGT using serial analysis of gene expression (SAGE) and DNA chip analysis. We evaluated the correlation between expression levels of genes involved in OXPHOS and the clinical parameters of individuals with type 2 diabetes and NGT.

RESULTS

Both gene analyses showed that genes involved in OXPHOS were significantly upregulated in the type 2 diabetic liver. In the SAGE analysis, tag count comparisons of mitochondrial transcripts showed that ribosomal RNAs (rRNA) were 3.5-fold over-expressed, and mRNAs were 1.2-fold over-expressed in the type 2 diabetes library. DNA chip analysis revealed that expression of genes involved in OXPHOS, which correlated with several nuclear factors, including estrogen-related receptor-alpha or peroxisome proliferator-activated receptor-gamma, was a predictor of fasting plasma glucose levels, independently of age, BMI, insulin resistance and fasting insulin levels (p = 0.04). Surprisingly, genes involved in OXPHOS did not correlate with peroxisome proliferator-activated receptor-gamma coactivator-1alpha or nuclear respiratory factor 1.

CONCLUSIONS/INTERPRETATION

Our results indicate that upregulation of genes involved in OXPHOS in the liver, which are regulated by different mechanisms from genes in the skeletal muscle, is associated with fasting hyperglycaemia in patients with type 2 diabetes.

Metabolic regulation of citrate and iron by aconitases: role of iron–sulfur cluster biogenesis

Iron and citrate are essential for the metabolism of most organisms, and regulation of iron and citrate biology at both the cellular and systemic levels is critical for normal physiology and survival. Mitochondrial and cytosolic aconitases catalyze the interconversion of citrate and isocitrate, and aconitase activities are affected by iron levels, oxidative stress and by the status of the Fe-S cluster biogenesis apparatus. Assembly and disassembly of Fe-S clusters is a key process not only in regulating the enzymatic activity of mitochondrial aconitase in the citric acid cycle, but also in controlling the iron sensing and RNA binding activities of cytosolic aconitase (also known as iron regulatory protein IRP1). This review discusses the central role of aconitases in intermediary metabolism and explores how iron homeostasis and Fe-S cluster biogenesis regulate the Fe-S cluster switch and modulate intracellular citrate flux.

Hypothyroidism Reduces Tricarboxylate Carrier Activity and Expression in Rat Liver Mitochondria by Reducing Nuclear Transcription Rate and Splicing Efficiency

The tricarboxylate carrier (TCC), also known as citrate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. The effect of hypothyroidism on the activity and expression of the hepatic mitochondrial TCC was investigated in this study. TCC activity was significantly decreased in hypothyroid rats as compared with euthyroid animals. This hormone deficiency effect was due to a reduction in the amount of carrier protein, which resulted from a proportionate decrease of the specific mRNA. Hypothyroidism did not influence TCC mRNA stability. On the other hand, nuclear run-on assay revealed that the transcriptional rate of TCC mRNA decreased by approximately 40% in the nuclei from hypothyroid versus euthyroid rats. In addition, the ribonuclease protection assay showed that, in the nuclei of hypothyroid rats, the ratio of mature to precursor RNA decreased, indicating that the splicing of TCC RNA is affected. Furthermore, we found that the ratio of polyadenylated/unpolyadenylated TCC RNA as well as the length of the TCC RNA poly(A) tail were similar in both euthyroid and hypothyroid rats. Thus, the rate of formation of the TCC 3'-end is not altered in hypothyroidism. These results suggest that hypothyroidism affects TCC expression at both the transcriptional and post-transcriptional levels.

Conjugated linoleic acid and hepatic lipogenesis in mouse: role of the mitochondrial citrate carrier

Conjugated linoleic acid (CLA) is able to reduce adiposity by affecting lipid metabolism. In particular, CLA administration to mice reduces body fat mass with a concomitant lipid accumulation in the liver. We investigated the effects of CLA on the activity of the mitochondrial citrate carrier (CIC), which is implicated in hepatic lipogenesis. The transport activity of the CIC, measured both in intact mitochondria and in the proteoliposomes, progressively increased with the duration of CLA feeding. An increase in the CIC activity of approximately 1.7-fold was found in 16 week CLA-treated mice with respect to control animals. A kinetic analysis showed a 1.6-fold increase in the V(max) of citrate transport but no change in the K(m) value. Western blot experiments revealed an increase of approximately 1.7-fold in the expression of CIC after CLA treatment. A strict correlation between the increase in CIC activity and the stimulation of the cytosolic lipogenic enzymes was also found. These data indicate that the CIC may play a role in the onset of hepatic steatosis in CLA-fed mice by supplying the carbon source for de novo fatty acid synthesis.

Heme oxygenase-1 enhances renal mitochondrial transport carriers and cytochrome C oxidase activity in experimental diabetes

Up-regulation of heme oxygenase (HO-1) by either cobalt protoporphyrin (CoPP) or human gene transfer improves vascular and renal function by several mechanisms, including increases in antioxidant levels and decreases in reactive oxygen species (ROS) in vascular and renal tissue. The purpose of the present study was to determine the effect of HO-1 overexpression on mitochondrial transporters, cytochrome c oxidase, and anti-apoptotic proteins in diabetic rats (streptozotocin, (STZ)-induced type 1 diabetes). Renal mitochondrial carnitine, deoxynucleotide, and ADP/ATP carriers were significantly reduced in diabetic compared with nondiabetic rats (p < 0.05). The citrate carrier was not significantly decreased in diabetic tissue. CoPP administration produced a robust increase in carnitine, citrate, deoxynucleotide, dicarboxylate, and ADP/ATP carriers and no significant change in oxoglutarate and aspartate/glutamate carriers. The increase in mitochondrial carriers (MCs) was associated with a significant increase in cytochrome c oxidase activity. The administration of tin mesoporphyrin (SnMP), an inhibitor of HO-1 activity, prevented the restoration of MCs in diabetic rats. Human HO-1 cDNA transfer into diabetic rats increased both HO-1 protein and activity, and restored mitochondrial ADP/ATP and deoxynucleotide carriers. The increase in HO-1 by CoPP administration was associated with a significant increase in the phosphorylation of AKT and levels of BcL-XL proteins. These observations in experimental diabetes suggest that the cytoprotective mechanism of HO-1 against oxidative stress involves an increase in the levels of MCs and anti-apoptotic proteins as well as in cytochrome c oxidase activity.

Hypothyroidism down-regulates mitochondrial citrate carrier activity and expression in rat liver

The effect of hypothyroidism on citrate carrier (CiC) activity has been investigated in rat-liver mitochondria. The rate of citrate transport was reduced by approximately 50% in mitochondria from hypothyroid as compared with euthyroid rats. In parallel, a decrease in the rate of de novo fatty acid synthesis was observed in the cytosol of the former animals. Kinetic analysis of citrate transport revealed that only the Vmax was reduced by hypothyroidism, while Km was almost unaffected. Hypothyroidism increased the mitochondrial percentage of phosphatidylcholine while decreased that of phosphatidylethanolamine; an altered fatty acid pattern but no significant difference in the sum of saturated and unsaturated fatty acids as well as in the unsaturation index was observed. The CiC Arrhenius plot did not show appreciable difference between the two groups of rats. However, Western blot analysis associated with mRNA quantitation indicated that both protein level and mRNA accumulation of hepatic CiC were noticeably decreased in hypothyroid state. Therefore, a reduced content of the carrier protein can represent a plausible mechanism to explain the decline in the CiC activity observed in rat liver mitochondria of hypothyroid rats.

Roles of CTPL/Sfxn3 and Sfxn family members in pancreatic islet

Pancreatic AR42J cells have the feature of pluripotency of the precursor cells of the gut endoderm. Betacellulin (BTC) and activin A (Act) convert them into insulin-secreting cells. Using mRNA differential display techniques, we have identified a novel mitochondrial transporter, which is highly expressed during the course of differentiation, and have designated it citrate transporter protein-like protein (CTPL). Recently sideroflexin 1 (Sfxn1) was shown to be a susceptible gene of flexed-tail (f/f) mice, and CTPL has turned out to be a rat orthologous protein of Sfxn3, a member of sideroflexin family. CTPL/Sfxn3 was targeted to mitochondrial membrane like Sfxn1. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were upregulated in the early phase of differentiation into insulin-secreting cells but the expression levels of Sfxn1 and Sfxn3 did not change. All Sfxn family members were expressed in rat pancreatic islet. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were also upregulated in islets of streptozotocin-induced diabetic rats compared to normal rats. The downregulation of CTPL/Sfxn3 in a rat insulinoma cell line, INS-1, with the antisense oligonucleotide did not affect the insulin secretion. Taken together, CTPL/Sfxn3 and some other family members might be important in the differentiation of pancreatic beta-cells as a channel or a carrier molecule and be related to the regeneration of pancreatic endocrine cells.

Analysis of the secondary structure of the cys-less yeast mitochondrial citrate transport protein and four single-cys variants by circular dichroism

Utilizing cysteine scanning mutagenesis, with functional Cys-less citrate transport protein (CTP) serving as the starting template, we previously demonstrated that four single-Cys mutants located in transmembrane domains III and IV, rendered the CTP nonfunctional. The present investigations assess and quantify the secondary structure of the Cys-less CTP and the four single-Cys mutants, both in the absence and presence of citrate, via circular dichroism (CD) spectroscopy. In detergent micelles, highly purified Cys-less CTP contained approximately 50% alpha-helix and approximately 20% beta-sheet. The CD spectra of the G119C, E122C, R181C, and R189C mutants in detergent micelles were virtually superimposable with that of the functional Cys-less CTP, thereby suggesting that the wild-type residues, rather than affecting structure, may assume important mechanistic roles. Exogenously added citrate caused a significant change in the CD spectra of all solubilized CTP samples. Analyses of the spectra of the Cys-less CTP indicated an approximately 10% increase in its alpha-helical content in the presence of citrate. The conformational changes effected by the addition of substrate were less pronounced with the single-Cys mutants. Studies of the Cys-less CTP reconstituted in liposomes indicated that while the CD spectra was red-shifted, the net secondary structure of the reconstituted carrier is approximately equivalent to that of the transporter in detergent micelles, and displayed a response to added citrate. In combination, the above studies indicate that purified Cys-less CTP in either sarkosyl micelles or in liposomes, and the four inactive single-Cys mutants in sarkosyl micelles, retain native-like structure, and thus represent ideal material for detailed structural characterization.

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.

Citrate carrier activity and cardiolipin level in eel (Anguilla anguilla) liver mitochondria

The activity of the tricarboxylate (citrate) carrier has been assayed in intact liver mitochondria from yellow eel (Anguilla anguilla) and compared to that from rat. The eel-citrate carrier specific activity was approximately 1.7-fold higher than that assayed in rat-liver mitochondria. The content of the main mitochondrial phospholipids, phosphatidylethanolamine and phosphatidylcholine, did not show a significant difference between the two species, while in eel a higher cardiolipin level was observed. Fatty acid composition of eel-liver mitochondrial phospholipids was characterised by a large amount of unsaturated fatty acids, dominated by octadecaenoic acid (C(18:1) (n-9)) and docosahexaenoic acid (C(22:6) (n-3)). The cardiolipin fatty acid pattern of eel-liver mitochondria showed, with respect to the rat, a higher C(20:5) (n-3) and C(22:6) (n-3) content and a lower amount of C(18:2) (n-6) and C(20:4) (n-6). A noticeable activity of lipogenic enzymes was also detected in eel liver cytosol. The results of this study suggest that the remarkable activity of the citrate carrier in eel-liver mitochondria can most likely be ascribed to a considerable cardiolipin level. A covariance of citrate carrier and lipogenic enzyme activities was observed.

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.

Covariance of tricarboxylate carrier activity and lipogenesis in liver of polyunsaturated fatty acid (n-6) fed rats

The mitochondrial tricarboxylate (citrate) carrier plays an important role in hepatic intermediary metabolism because, among other functions, it supplies the cytosol with acetyl units for fatty-acid synthesis. In this study, the effect of polyunsaturated fatty acids (PUFA, n-6) on the function of this mitochondrial transporter and on lipogenic enzyme activities was investigated by feeding rats for 4 weeks with a 15%-fat diet composed of high linoleic safflower oil. Citrate transport was strongly reduced in liver mitochondria isolated from PUFA-treated rats. A reduced transport activity was also observed when solubilized mitochondrial citrate carrier from PUFA-treated rats was reconstituted into liposomes. In the same animals, a decrease of cytosolic lipogenic enzyme activities was observed. These results indicate a coordinated modulation of citrate carrier and of lipogenic enzyme activities by PUFA feeding. Kinetic analysis of the carrier activity showed that only V(max) decreased, whereas K(m) was almost virtually unaffected. The PUFA-mediated effect is most likely due to the reduced mRNA level and lower content of the citrate carrier protein observed in the safflower oil-fed rats.

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.

Bacterial overexpression of putative yeast mitochondrial transport proteins

Thirty-two genes have been identified within the genome of the yeast Saccharomyces cerevisiae which putatively encode mitochondrial transport proteins. We have attempted to overexpress a subset of these genes, namely those which encode mitochondrial transporters of unknown function, and have succeeded in overexpressing 19 of these genes. The overexpressed proteins were then isolated and tested for five well-characterized reconstituted transport activities (i.e., the transport of citrate, dicarboxylates, pyruvate, camitine, and aspartate). Utilizing this approach, we have clearly identified the yeast mitochondrial dicarboxylate transport protein, as well as two additional lower-magnitude transport functions (i.e., tricarboxylate and dicarboxylate transport activities). The implications of these results and the considerations relevant to this approach are discussed.

Identification of a novel gene encoding the yeast mitochondrial dicarboxylate transport protein via overexpression, purification, and characterization of its protein product

A gene encoding the mitochondrial dicarboxylate transport protein (DTP) has been identified for the first time from any organism. Our strategy involved overexpression of putative mitochondrial transporter genes, selected based on analysis of the yeast genome, followed by purification and functional reconstitution of the resulting protein products. The DTP gene from the yeast Saccharomyces cerevisiae encodes a 298-residue basic protein which, in common with other mitochondrial anion transporters of known sequence and function, displays the mitochondrial transporter signature motif, three homologous 100-amino acid sequence domains, and six predicted membrane-spanning regions. The product of this gene has been abundantly expressed in Escherichia coli where it accumulates in inclusion bodies. Upon solubilization of the overexpressed DTP from isolated inclusion bodies with Sarkosyl, 28 mg of DTP was obtained per liter of E. coli culture at a purity of 75%. The purified, overexpressed DTP was then reconstituted in phospholipid vesicles where both its kinetic properties (i.e. Km = 1. 55 mM and Vmax = 3.0 micro;mol/min/mg protein) and its substrate specificity were determined. The intraliposomal substrates malonate, malate, succinate, and phosphate effectively supported [14C]malonate uptake, whereas other anions tested did not. External substrate competition studies revealed a similar specificity profile. Inhibitor studies indicated that the reconstituted transporter was sensitive to inhibition by n-butylmalonate, p-chloromercuribenzoate, mersalyl, and to a lesser extent pyridoxal 5'-phosphate but was insensitive to N-ethylmaleimide and selective inhibitors of other mitochondrial anion transporters. In combination, the above findings indicate that the identified gene encodes a mitochondrial transport protein which upon overexpression and reconstitution displays functional properties that are virtually identical to those of the native mitochondrial dicarboxylate transport system. In conclusion, the present investigation has resulted in identification of a gene encoding the mitochondrial DTP and thus eliminates a major impediment to molecular studies with this metabolically important transporter. Based on both structural and functional considerations, the yeast DTP is assignable to the mitochondrial carrier family. Additionally, the development of a procedure that enables the expression and isolation of large quantities of functional DTP provides the foundation for comprehensive investigations into the structure/function relationships within this transporter via site-directed mutagenesis, as well as for the initiation of crystallization trials.

Organization and sequence of the human gene for the mitochondrial citrate transport protein

The citrate (tricarboxylate) carrier transports citrate (or other tricarboxylates) across the inner membranes of mitochondria in an electroneutral exchange for malate (or other dicarboxylic acids). We have determined the sequence of the human citrate transporter gene from overlapping genomic clones generated by polymerase chain reactions by use of primers and probes based on the rat cDNA sequence and on emerging sequences. The gene is spread over 2.8 kb of human DNA and is divided into eight exons. All the introns are located at the level of the sequences coding for the extramembranous loops (and not for the transmembrane segments) of the mature protein. The open reading frame of the human gene encodes the mature protein consisting of 298 amino acids, preceded by a presequence of 13 amino acids to help to target it into mitochondria. 84 identities and 106 highly conservative substitutions are present in CTPs from man to yeast. In addition, we have determined the sequences of two human pseudogenes related to the citrate carrier gene encompassing the coding sequence of the gene between nucleotides 260 and 720.

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

The tricarboxylate or citrate transporter protein (CTP) catalyzes the transport of citrate across the inner mitochondrial membrane by an exchange for malate or some other anionic metabolite. Using primers based on the rat liver cDNA sequence, overlapping cDNA clones encoding the bovine CTP were isolated from bovine liver poly(A+) cDNA. The entire bovine cDNA is 1151 bp in length with 5' and 3' untranslated regions of 7 and 204 bp, respectively. The open reading frame encodes the mature protein consisting of 298 amino acids, preceded by a presequence of 13 amino acids. The amino acid sequence of the mature bovine CTP is 95.6, 94.9, 32.2% identical to that of the citrate carrier from man, rat and yeast, respectively.

Effect of starvation on the activity of the mitochondrial tricarboxylate carrier

The effect of starvation on the activity of the tricarboxylate carrier has been investigated in intact rat liver mitochondria and in a reconstituted system. In both experimental conditions, the rate of citrate transport, when compared to control, is greatly reduced (35-40%) in starved rats. Similar behaviour is shown by the cytosolic lipogenic enzymes. Kinetic analysis of the carrier activity in intact mitochondria and in the proteoliposomal system has showed that during starvation only the Vmax of this process decreases while there is no change in the Km. No difference in the Arrhenius plot and in the lipid composition has been detected, which indicates that the reduced transport activity in fasted animals is not due to a change in the carrier lipid microenvironment. In starved rats, a reduction of the carrier activity has occurred even after the addition of increasing cardiolipin concentrations to proteoliposomes. These findings thus suggest that starvation-induced decrease of citrate carrier activity could be due to a change of the intrinsic properties of the transport protein.

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.

Probing the active center of the mitochondrial dicarboxylate transporter

2-n-Alkylmalonates with various length of the alkyl residue have been used to study the topography of the active center of the dicarboxylate transporter in intact rat liver mitochondria. Measurements of the Ki values of these competitive inhibitors suggest that in the transporter there is a large hydrophobic region at least 1.7 nm in size, containing a polar domain (ca. 0.5 nm) and situated close to a substrate-binding site. These zones are assumed to be involved in the mechanism of dicarboxylate transport.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional levels of mitochondrial anion transport proteins in non-insulin-dependent diabetes mellitus

The effect of non-insulin-dependent diabetes mellitus (i.e., NIDDM; type 2 diabetes) on the levels of functional mitochondrial anion transport proteins has been determined utilizing a chemically-induced neonatal model of NIDDM. We hypothesized that moderate insulin deficiency exacerbated by the insulin resistance, which is characteristic of NIDDM, would cause changes in mitochondrial anion transporter function that were similar to those we have previously shown to occur in insulin-dependent diabetes mellitus (i.e., IDDM; type 1 diabetes) (Arch. Biochem. Biophys. 280: 181-191, 1990). Our experimental approach consisted of the extraction of the pyruvate, dicarboxylate and citrate transport proteins from the mitochondrial inner membrane with Triton X-114 using rat liver mitoplasts (prepared from diabetic and control animals) as the starting material, followed by the functional reconstitution of each transporter in a proteoliposomal system. This strategy permitted the quantification of the functional levels of these three transporters in the absence of the complications that arise when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental NIDDM did not cause significant changes in the extractable and reconstitutable specific (and total) transport activities of the pyruvate, dicarboxylate, and citrate transporters. These results are in marked contrast to our previous findings obtained using rats with IDDM and negated our hypothesis. The present results, in combination with our earlier findings, allow us to conclude that insulin plays an important role in the regulation of mitochondrial anion transporter function.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of insulin supplementation on diabetes-induced alterations in the extractable levels of functional mitochondrial anion transport proteins

The effect of insulin supplementation on diabetes-induced alterations in the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach consisted of the extraction of the pyruvate, dicarboxylate, and citrate transport proteins from the mitochondrial inner membrane with Triton X-114 using rat liver mitoplasts (prepared from control, diabetic, or insulin-supplemented diabetic animals) as the starting material, followed by the reconstitution of the function of each transporter in a proteoliposomal system. This experimental strategy permitted the quantification of the functional levels of these three transporters in the absence of the complications that arise when such measurements are carried out with intact mitochondria (or mitoplasts). We found that treatment of diabetic rats (i.e., animals that were injected with streptozotocin 3 weeks earlier) on a daily basis with insulin for 3 weeks resulted in a reversal of the diabetes-induced (a) increase in the extractable and reconstitutable total (and specific) transport activities of the pyruvate and dicarboxylate transporters and (b) decrease in the activity of the citrate transporter. These findings indicate that diabetes-induced alterations in the functional levels of mitochondrial anion transport proteins are a direct consequence of the insulin insufficiency that characterizes this disease. Furthermore, this study provides the first demonstration that insulin participates in the regulation of the functional levels of liver mitochondrial anion transport proteins.