Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Genetic studies of paired metabolomes reveal enzymatic and transport processes at the interface of plasma and urine

The kidneys operate at the interface of plasma and urine by clearing molecular waste products while retaining valuable solutes. Genetic studies of paired plasma and urine metabolomes may identify underlying processes. We conducted genome-wide studies of 1,916 plasma and urine metabolites and detected 1,299 significant associations. Associations with 40% of implicated metabolites would have been missed by studying plasma alone. We detected urine-specific findings that provide information about metabolite reabsorption in the kidney, such as aquaporin (AQP)-7-mediated glycerol transport, and different metabolomic footprints of kidney-expressed proteins in plasma and urine that are consistent with their localization and function, including the transporters NaDC3 (SLC13A3) and ASBT (SLC10A2). Shared genetic determinants of 7,073 metabolite-disease combinations represent a resource to better understand metabolic diseases and revealed connections of dipeptidase 1 with circulating digestive enzymes and with hypertension. Extending genetic studies of the metabolome beyond plasma yields unique insights into processes at the interface of body compartments.

Citric Acid: A Multifunctional Pharmaceutical Excipient

Citric acid, a tricarboxylic acid, has found wide application in the chemical and pharmaceutical industry due to its biocompatibility, versatility, and green, environmentally friendly chemistry. This review emphasizes the pharmaceutical uses of citric acid as a strategic ingredient in drug formulation while focusing on the impact of its physicochemical properties. The functionality of citric acid is due to its three carboxylic groups and one hydroxyl group. These allow it to be used in many ways, including its ability to be used as a crosslinker to form biodegradable polymers and as a co-former in co-amorphous and co-crystal applications. This paper also analyzes the effect of citric acid in physiological processes and how this effect can be used to enhance the attributes of pharmaceutical preparations, as well as providing a critical discussion on the issues that may arise out of the presence of citric acid in formulations.

Dietary citrate supplementation enhances longevity, metabolic health, and memory performance through promoting ketogenesis

Citrate is an essential substrate for energy metabolism that plays critical roles in regulating cell growth and survival. However, the action of citrate in regulating metabolism, cognition, and aging at the organismal level remains poorly understood. Here, we report that dietary supplementation with citrate significantly reduces energy status and extends lifespan in Drosophila melanogaster. Our genetic studies in fruit flies implicate a molecular mechanism associated with AMP‐activated protein kinase (AMPK), target of rapamycin (TOR), and ketogenesis. Mice fed a high‐fat diet that supplemented with citrate or the ketone body β‐hydroxybutyrate (βOHB) also display improved metabolic health and memory. These results suggest that dietary citrate supplementation may prove to be a useful intervention in the future treatment of age‐related dysfunction.

Solute Carrier Transporters as Potential Targets for the Treatment of Metabolic Disease

The solute carrier (SLC) superfamily comprises more than 400 transport proteins mediating the influx and efflux of substances such as ions, nucleotides, and sugars across biological membranes. Over 80 SLC transporters have been linked to human diseases, including obesity and type 2 diabetes (T2D). This observation highlights the importance of SLCs for human (patho)physiology. Yet, only a small number of SLC proteins are validated drug targets. The most recent drug class approved for the treatment of T2D targets sodium-glucose cotransporter 2, product of the SLC5A2 gene. There is great interest in identifying other SLC transporters as potential targets for the treatment of metabolic diseases. Finding better treatments will prove essential in future years, given the enormous personal and socioeconomic burden posed by more than 500 million patients with T2D by 2040 worldwide. In this review, we summarize the evidence for SLC transporters as target structures in metabolic disease. To this end, we identified SLC13A5/sodium-coupled citrate transporter, and recent proof-of-concept studies confirm its therapeutic potential in T2D and nonalcoholic fatty liver disease. Further SLC transporters were linked in multiple genome-wide association studies to T2D or related metabolic disorders. In addition to presenting better-characterized potential therapeutic targets, we discuss the likely unnoticed link between other SLC transporters and metabolic disease. Recognition of their potential may promote research on these proteins for future medical management of human metabolic diseases such as obesity, fatty liver disease, and T2D. SIGNIFICANCE STATEMENT: Given the fact that the prevalence of human metabolic diseases such as obesity and type 2 diabetes has dramatically risen, pharmacological intervention will be a key future approach to managing their burden and reducing mortality. In this review, we present the evidence for solute carrier (SLC) genes associated with human metabolic diseases and discuss the potential of SLC transporters as therapeutic target structures.

Compartmentalised acyl-CoA metabolism and roles in chromatin regulation

BACKGROUND

Many metabolites serve as important signalling molecules to adjust cellular activities and functions based on nutrient availability. Links between acetyl-CoA metabolism, histone lysine acetylation, and gene expression have been documented and studied over the past decade. In recent years, several additional acyl modifications to histone lysine residues have been identified, which depend on acyl-coenzyme A thioesters (acyl-CoAs) as acyl donors. Acyl-CoAs are intermediates of multiple distinct metabolic pathways, and substantial evidence has emerged that histone acylation is metabolically sensitive. Nevertheless, the metabolic sources of acyl-CoAs used for chromatin modification in most cases remain poorly understood. Elucidating how these diverse chemical modifications are coupled to and regulated by cellular metabolism is important in deciphering their functional significance.

SCOPE OF REVIEW

In this article, we review the metabolic pathways that produce acyl-CoAs, as well as emerging evidence for functional roles of diverse acyl-CoAs in chromatin regulation. Because acetyl-CoA has been extensively reviewed elsewhere, we will focus on four other acyl-CoA metabolites integral to major metabolic pathways that are also known to modify histones: succinyl-CoA, propionyl-CoA, crotonoyl-CoA, and butyryl-CoA. We also briefly mention several other acyl-CoA species, which present opportunities for further research; malonyl-CoA, glutaryl-CoA, 3-hydroxybutyryl-CoA, 2-hydroxyisobutyryl-CoA, and lactyl-CoA. Each acyl-CoA species has distinct roles in metabolism, indicating the potential to report shifts in the metabolic status of the cell. For each metabolite, we consider the metabolic pathways in which it participates and the nutrient sources from which it is derived, the compartmentalisation of its metabolism, and the factors reported to influence its abundance and potential nuclear availability. We also highlight reported biological functions of these metabolically-linked acylation marks. Finally, we aim to illuminate key questions in acyl-CoA metabolism as they relate to the control of chromatin modification.

MAJOR CONCLUSIONS

A majority of acyl-CoA species are annotated to mitochondrial metabolic processes. Since acyl-CoAs are not known to be directly transported across mitochondrial membranes, they must be synthesized outside of mitochondria and potentially within the nucleus to participate in chromatin regulation. Thus, subcellular metabolic compartmentalisation likely plays a key role in the regulation of histone acylation. Metabolite tracing in combination with targeting of relevant enzymes and transporters will help to map the metabolic pathways that connect acyl-CoA metabolism to chromatin modification. The specific function of each acyl-CoA may be determined in part by biochemical properties that affect its propensity for enzymatic versus non-enzymatic protein modification, as well as the various enzymes that can add, remove and bind each modification. Further, competitive and inhibitory effects of different acyl-CoA species on these enzymes make determining the relative abundance of acyl-CoA species in specific contexts important to understand the regulation of chromatin acylation. An improved and more nuanced understanding of metabolic regulation of chromatin and its roles in physiological and disease-related processes will emerge as these questions are answered.

Inhibition of Slug effectively targets leukemia stem cells via the Slc13a3/ROS signaling pathway

Leukemia stem cells (LSCs) are the rare populations of acute myeloid leukemia (AML) cells that are able to initiate, maintain, and propagate AML. Targeting LSCs is a promising approach for preventing AML relapse and improving long-term outcomes. While Slug, a zinc-finger transcription repressor, negatively regulates the self-renewal of normal hematopoietic stem cells, its functions in AML are still unknown. We report here that Slug promotes leukemogenesis and its loss impairs LSC self-renewal and delays leukemia progression. Mechanistically, Slc13a3, a direct target of Slug in LSCs, restricts the self-renewal of LSCs and markedly prolongs recipient survival. Genetic or pharmacological inhibition of SLUG or forced expression of Slc13a3 suppresses the growth of human AML cells. In conclusion, our studies demonstrate that Slug differentially regulates self-renewal of LSCs and normal HSCs, and both Slug and Slc13a3 are potential therapeutic targets of LSCs.

Unraveling the Potential Role of Glutathione in Multiple Forms of Cell Death in Cancer Therapy

Glutathione is the principal intracellular antioxidant buffer against oxidative stress and mainly exists in the forms of reduced glutathione (GSH) and oxidized glutathione (GSSG). The processes of glutathione synthesis, transport, utilization, and metabolism are tightly controlled to maintain intracellular glutathione homeostasis and redox balance. As for cancer cells, they exhibit a greater ROS level than normal cells in order to meet the enhanced metabolism and vicious proliferation; meanwhile, they also have to develop an increased antioxidant defense system to cope with the higher oxidant state. Growing numbers of studies have implicated that altering the glutathione antioxidant system is associated with multiple forms of programmed cell death in cancer cells. In this review, we firstly focus on glutathione homeostasis from the perspectives of glutathione synthesis, distribution, transportation, and metabolism. Then, we discuss the function of glutathione in the antioxidant process. Afterwards, we also summarize the recent advance in the understanding of the mechanism by which glutathione plays a key role in multiple forms of programmed cell death, including apoptosis, necroptosis, ferroptosis, and autophagy. Finally, we highlight the glutathione-targeting therapeutic approaches toward cancers. A comprehensive review on the glutathione homeostasis and the role of glutathione depletion in programmed cell death provide insight into the redox-based research concerning cancer therapeutics.

Succinate Accumulation Is Associated with a Shift of Mitochondrial Respiratory Control and HIF-1α Upregulation in PTEN Negative Prostate Cancer Cells

The idea of using metabolic aberrations as targets for diagnosis or therapeutic intervention has recently gained increasing interest. In a previous study, our group discovered intriguing differences in the oxidative mitochondrial respiration capacity of benign and prostate cancer (PCa) cells. In particular, we found that PCa cells had a higher total respiratory activity than benign cells. Moreover, PCa cells showed a substantial shift towards succinate-supported mitochondrial respiration compared to benign cells, indicating a re-programming of respiratory control. This study aimed to investigate the role of succinate and its main plasma membrane transporter NaDC3 (sodium-dependent dicarboxylate transporter member 3) in PCa cells and to determine whether targeting succinate metabolism can be potentially used to inhibit PCa cell growth. Using high-resolution respirometry analysis, we observed that ROUTINE respiration in viable cells and succinate-supported respiration in permeabilized cells was higher in cells lacking the tumor suppressor phosphatase and tensin-homolog deleted on chromosome 10 (PTEN), which is frequently lost in PCa. In addition, loss of PTEN was associated with increased intracellular succinate accumulation and higher expression of NaDC3. However, siRNA-mediated knockdown of NaDC3 only moderately influenced succinate metabolism and did not affect PCa cell growth. By contrast, mersalyl acid—a broad acting inhibitor of dicarboxylic acid carriers—strongly interfered with intracellular succinate levels and resulted in reduced numbers of PCa cells. These findings suggest that blocking NaDC3 alone is insufficient to intervene with altered succinate metabolism associated with PCa. In conclusion, our data provide evidence that loss of PTEN is associated with increased succinate accumulation and enhanced succinate-supported respiration, which cannot be overcome by inhibiting the succinate transporter NaDC3 alone.

Single-nephron proteomes connect morphology and function in proteinuric kidney disease

In diseases of many parenchymatous organs, heterogeneous deterioration of individual functional units determines the clinical prognosis. However, the molecular characterization at the level of such individual subunits remains a technological challenge that needs to be addressed in order to better understand pathological mechanisms. Proteinuric glomerular kidney diseases are frequent and assorted diseases affecting a fraction of glomeruli and their draining tubules to variable extents, and for which no specific treatment exists. Here, we developed and applied a mass spectrometry-based methodology to investigate heterogeneity of proteomes from individually isolated nephron segments from mice with proteinuric kidney disease. In single glomeruli from two different mouse models of sclerotic glomerular disease, we identified a coherent protein expression module consisting of extracellular matrix protein deposition (reflecting glomerular sclerosis), glomerular albumin (reflecting proteinuria) and LAMP1, a lysosomal protein. This module was associated with a loss of podocyte marker proteins while genetic ablation of LAMP1-correlated lysosomal proteases could ameliorate glomerular damage in vivo. Furthermore, proteomic analyses of individual glomeruli from patients with genetic sclerotic and non-sclerotic proteinuric diseases revealed increased abundance of lysosomal proteins, in combination with a decreased abundance of mutated gene products. Thus, altered protein homeostasis (proteostasis) is a conserved key mechanism in proteinuric kidney diseases. Moreover, our technology can capture intra-individual variability in diseases of the kidney and other tissues at a sub-biopsy scale.

Distribution of organic anion transporters NaDC3 and OAT1-3 along the human nephron

The initial step in renal secretion of organic anions (OAs) is mediated by transporters in the basolateral membrane (BLM). Contributors to this process are primary active Na(+)-K(+)-ATPase (EC 3.6.3.9), secondary active Na(+)-dicarboxylate cotransporter 3 (NaDC3/SLC13A3), and tertiary active OA transporters (OATs) OAT1/SLC22A6, OAT2/SLC22A7, and OAT3/SLC22A8. In human kidneys, we analyzed the localization of these transporters by immunochemical methods in tissue cryosections and isolated membranes. The specificity of antibodies was validated with human embryonic kidney-293 cells stably transfected with functional OATs. Na(+)-K(+)-ATPase was immunolocalized to the BLM along the entire human nephron. NaDC3-related immunostaining was detected in the BLM of proximal tubules and in the BLM and/or luminal membrane of principal cells in connecting segments and collecting ducts. The thin and thick ascending limbs, macula densa, and distal tubules exhibited no reactivity with the anti-NaDC3 antibody. OAT1-OAT3-related immunostaining in human kidneys was detected only in the BLM of cortical proximal tubules; all three OATs were stained more intensely in S1/S2 segments compared with S3 segment in medullary rays, whereas the S3 segment in the outer stripe remained unstained. Expression of NaDC3, OAT1, OAT2, and OAT3 proteins exhibited considerable interindividual variability in both male and female kidneys, and sex differences in their expression could not be detected. Our experiments provide a side-by-side comparison of basolateral transporters cooperating in renal OA secretion in the human kidney.

Itaconic Acid: The Surprising Role of an Industrial Compound as a Mammalian Antimicrobial Metabolite

Itaconic acid is well known as a precursor for polymer synthesis and has been involved in industrial processes for decades. In a recent surprising discovery, itaconic acid was found to play a role as an immune-supportive metabolite in mammalian immune cells, where it is synthesized as an antimicrobial compound from the citric acid cycle intermediate cis-aconitic acid. Although the immune-responsive gene 1 protein (IRG1) has been associated to immune response without a mechanistic function, the critical link to itaconic acid production through an enzymatic function of this protein was only recently revealed. In this review, we highlight the history of itaconic acid as an industrial and antimicrobial compound, starting with its biotechnological synthesis and ending with its antimicrobial function in mammalian immune cells.

NaDC3 Induces Premature Cellular Senescence by Promoting Transport of Krebs Cycle Intermediates, Increasing NADH, and Exacerbating Oxidative Damage

High-affinity sodium-dependent dicarboxylate cotransporter 3 (NaDC3) is a key metabolism-regulating membrane protein responsible for transport of Krebs cycle intermediates. NaDC3 is upregulated as organs age, but knowledge regarding the underlying mechanisms by which NaDC3 modulates mammalian aging is limited. In this study, we showed that NaDC3 overexpression accelerated cellular senescence in young human diploid cells (MRC-5 and WI-38) and primary renal tubular cells, leading to cell cycle arrest in G1 phase and increased expression of senescent biomarkers, senescence-associated β-galactosidase and p16. Intracellular levels of reactive oxygen species, 8-hydroxy-2'-deoxyguanosine, malondialdehyde, and carbonyl were significantly enhanced, and activities of respiratory complexes I and III and ATP level were significantly decreased in NaDC3-infected cells. Stressful premature senescent phenotypes induced by NaDC3 were markedly ameliorated via treatment with the antioxidants Tiron and Tempol. High expression of NaDC3 caused a prominent increase in intracellular levels of Krebs cycle intermediates and NADH. Exogenous NADH and NAD(+) may aggravate and attenuate the aging phenotypes induced by NaDC3, respectively. These results suggest that NaDC3 can induce premature cellular senescence by promoting the transport of Krebs cycle intermediates, increasing generation of NADH and reactive oxygen species and leading to oxidative damage. Our results clarify the aging signaling pathway regulated by NaDC3.

Transporters involved in renal excretion of N-carbamoylglutamate, an orphan drug to treat inborn n-acetylglutamate synthase deficiency

Inborn defects in N-acetylglutamate (NAG) synthase (NAGS) cause a reduction of NAG, an essential cofactor for the initiation of the urea cycle. As a consequence, blood ammonium concentrations are elevated, leading to severe neurological disorders. The orphan drug N-carbamoylglutamate (NCG; Carbaglu), efficiently overcomes NAGS deficiency. However, not much is known about the transporters involved in the uptake, distribution, and elimination of the divalent organic anion NCG. Organic anion-transporting polypeptides (OATPs) as well as organic anion transporters (OATs) working in cooperation with sodium dicarboxylate cotransporter 3 (NaDC3) accept a wide variety of structurally unrelated drugs. To test for possible interactions with OATPs and OATs, the impact of NCG on these transporters in stably transfected human embryonic kidney-293 cells was measured. The two-electrode voltage-clamp technique was used to monitor NCG-mediated currents in Xenopus laevis oocytes that expressed NaDC3. Neither OATPs nor OAT2 and OAT3 interacted with NCG, but OAT1 transported NCG. In addition, NCG was identified as a high-affinity substrate of NaDC3. Preincubation of OAT4-transfected human embryonic kidney-293 cells with NCG showed an increased uptake of estrone sulfate, the reference substrate of OAT4, indicating efflux of NCG by OAT4. In summary, NaDC3 and, to a lesser extent, OAT1 are likely to be responsible for the uptake of NCG from the blood. Efflux of NCG across the luminal membrane into the tubular lumen probably occurs by OAT4 completing renal secretion of this drug.

Localization of the calcium-regulated citrate transport process in proximal tubule cells

Urinary citrate is an important inhibitor of calcium-stone formation. Most of the citrate reabsorption in the proximal tubule is thought to occur via a dicarboxylate transporter NaDC1 located in the apical membrane. OK cells, an established opossum kidney proximal tubule cell line, transport citrate but the characteristics change with extracellular calcium such that low calcium solutions stimulate total citrate transport as well as increase the apparent affinity for transport. The present studies address several fundamental properties of this novel process: the polarity of the transport process, the location of the calcium-sensitivity and whether NaDC1 is present in OK cells. OK cells grown on permeable supports exhibited apical >basolateral citrate transport. Apical transport of both citrate and succinate was sensitive to extracellular calcium whereas basolateral transport was not. Apical calcium, rather than basolateral, was the predominant determinant of changes in transport. Also 2,3-dimethylsuccinate, previously identified as an inhibitor of basolateral dicarboxylate transport, inhibited apical citrate uptake. Although the calcium-sensitive transport process in OK cells is functionally not typical NaDC1, NaDC1 is present in OK cells by Western blot and PCR. By immunolocalization studies, NaDC1 was predominantly located in discrete apical membrane or subapical areas. However, by biotinylation, apical NaDC1 decreases in the apical membrane with lowering calcium. In sum, OK cells express a calcium-sensitive/regulated dicarboxylate process at the apical membrane which responds to variations in apical calcium. Despite the functional differences of this process compared to NaDC1, NaDC1 is present in these cells, but predominantly in subapical vesicles.

The Role of INDY in Metabolic Regulation

Reduced expression of the Indy (I'm Not Dead Yet) gene in D. melanogaster and C. elegans extends longevity. Indy and its mammalian homolog mINDY (Slc13a5, NaCT) are transporters of TCA cycle intermediates, mainly handling the uptake of citrate via the plasma membrane into the cytosol. Deletion of mINDY in mice leads to significant metabolic changes akin to caloric restriction, likely caused by reducing the effects of mINDY-imported citrate on fatty acid and cholesterol synthesis, glucose metabolism and ß-oxidation. This review will provide an overview on different mammalian SLC1 3 family members with a focus on mINDY (SLCl3A5) in glucose and energy metabolism and will highlight the role of mINDY as a putative therapeutic target for the treatment of obesity, non-alcoholic fatty liver disease and type 2 diabetes.

Proteome analysis and conditional deletion of the EAAT2 glutamate transporter provide evidence against a role of EAAT2 in pancreatic insulin secretion in mice

Islet function is incompletely understood in part because key steps in glutamate handling remain undetermined. The glutamate (excitatory amino acid) transporter 2 (EAAT2; Slc1a2) has been hypothesized to (a) provide islet cells with glutamate, (b) protect islet cells against high extracellular glutamate concentrations, (c) mediate glutamate release, or (d) control the pH inside insulin secretory granules. Here we floxed the EAAT2 gene to produce the first conditional EAAT2 knock-out mice. Crossing with Nestin-cyclization recombinase (Cre) eliminated EAAT2 from the brain, resulting in epilepsy and premature death, confirming the importance of EAAT2 for brain function and validating the genetic construction. Crossing with insulin-Cre lines (RIP-Cre and IPF1-Cre) to obtain pancreas-selective deletion did not appear to affect survival, growth, glucose tolerance, or β-cell number. We found (using TaqMan RT-PCR, immunoblotting, immunocytochemistry, and proteome analysis) that the EAAT2 levels were too low to support any of the four hypothesized functions. The proteome analysis detected more than 7,000 islet proteins of which more than 100 were transporters. Although mitochondrial glutamate transporters and transporters for neutral amino acids were present at high levels, all other transporters with known ability to transport glutamate were strikingly absent. Glutamate-metabolizing enzymes were abundant. The level of glutamine synthetase was 2 orders of magnitude higher than that of glutaminase. Taken together this suggests that the uptake of glutamate by islets from the extracellular fluid is insignificant and that glutamate is intracellularly produced. Glutamine synthetase may be more important for islets than assumed previously.

Sodium-coupled dicarboxylate and citrate transporters from the SLC13 family

The SLC13 family in humans and other mammals consists of sodium-coupled transporters for anionic substrates: three transporters for dicarboxylates/citrate and two transporters for sulfate. This review will focus on the di- and tricarboxylate transporters: NaDC1 (SLC13A2), NaDC3 (SLC13A3), and NaCT (SLC13A5). The substrates of these transporters are metabolic intermediates of the citric acid cycle, including citrate, succinate, and α-ketoglutarate, which can exert signaling effects through specific receptors or can affect metabolic enzymes directly. The SLC13 transporters are important for regulating plasma, urinary and tissue levels of these metabolites. NaDC1, primarily found on the apical membranes of renal proximal tubule and small intestinal cells, is involved in regulating urinary levels of citrate and plays a role in kidney stone development. NaDC3 has a wider tissue distribution and high substrate affinity compared with NaDC1. NaDC3 participates in drug and xenobiotic excretion through interactions with organic anion transporters. NaCT is primarily a citrate transporter located in the liver and brain, and its activity may regulate metabolic processes. The recent crystal structure of the Vibrio cholerae homolog, VcINDY, provides a new framework for understanding the mechanism of transport in this family. This review summarizes current knowledge of the structure, function, and regulation of the di- and tricarboxylate transporters of the SLC13 family.

Sodium-sulfate/carboxylate cotransporters (SLC13)

The SLC13 gene family is comprised of five sequence related proteins that are found in animals, plants, yeast and bacteria. Proteins encoded by the SLC13 genes are divided into the following two groups of transporters with distinct anion specificities: the Na(+)-sulfate (NaS) cotransporters and the Na(+)-carboxylate (NaC) cotransporters. Members of this gene family (in ascending order) are: SLC13A1 (NaS1), SLC13A2 (NaC1), SLC13A3 (NaC3), SLC13A4 (NaS2) and SLC13A5 (NaC2). SLC13 proteins encode plasma membrane polypeptides with 8-13 putative transmembrane domains, and are expressed in a variety of tissues. They are all Na(+)-coupled symporters with strong cation preference for Na(+), and insensitive to the stilbene 4, 4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). Their Na(+):anion coupling ratio is 3:1, indicative of electrogenic properties. They have a substrate preference for divalent anions, which include tetra-oxyanions for the NaS cotransporters or Krebs cycle intermediates (including mono-, di- and tricarboxylates) for the NaC cotransporters. This review will describe the molecular and cellular mechanisms underlying the biochemical, physiological and structural properties of the SLC13 gene family.

Differential interaction of dicarboxylates with human sodium-dicarboxylate cotransporter 3 and organic anion transporters 1 and 3

Organic anions are taken up from the blood into proximal tubule cells by organic anion transporters 1 and 3 (OAT1 and OAT3) in exchange for dicarboxylates. The released dicarboxylates are recycled by the sodium dicarboxylate cotransporter 3 (NaDC3). In this study, we tested the substrate specificities of human NaDC3, OAT1, and OAT3 to identify those dicarboxylates for which the three cooperating transporters have common high affinities. All transporters were stably expressed in HEK293 cells, and extracellularly added dicarboxylates were used as inhibitors of [14C]succinate (NaDC3), p-[3H]aminohippurate (OAT1), or [3H]estrone-3-sulfate (OAT3) uptake. Human NaDC3 was stably expressed as proven by immunochemical methods and by sodium-dependent uptake of succinate ( K0.5 for sodium activation, 44.6 mM; Hill coefficient, 2.1; Km for succinate, 18 μM). NaDC3 was best inhibited by succinate (IC50 25.5 μM) and less by α-ketoglutarate (IC50 69.2 μM) and fumarate (IC50 95.2 μM). Dicarboxylates with longer carbon backbones (adipate, pimelate, suberate) had low or no affinity for NaDC3. OAT1 exhibited the highest affinity for glutarate, α-ketoglutarate, and adipate (IC50 between 3.3 and 6.2 μM), followed by pimelate (18.6 μM) and suberate (19.3 μM). The affinity of OAT1 to succinate and fumarate was low. OAT3 showed the same dicarboxylate selectivity with ∼13-fold higher IC50 values compared with OAT1. The data 1) reveal α-ketoglutarate as a common high-affinity substrate of NaDC3, OAT1, and OAT3 and 2) suggest potentially similar molecular structures of the binding sites in OAT1 and OAT3 for dicarboxylates.

PITX2 is involved in stress response in cultured human trabecular meshwork cells through regulation of SLC13A3

PURPOSE

Mutations of the PITX2 gene cause Axenfeld-Rieger syndrome (ARS) and glaucoma. In this study, the authors investigated genes directly regulated by the PITX2 transcription factor to gain insight into the mechanisms underlying these disorders.

METHODS

RNA from nonpigmented ciliary epithelium cells transfected with hormone-inducible PITX2 and activated by mifepristone was subjected to microarray analyses. Data were analyzed using dCHIP algorithms to detect significant differences in expression. Genes with significantly altered expression in multiple microarray experiments in the presence of activated PITX2 were subjected to in silico and biochemical analyses to validate them as direct regulatory targets. One target gene was further characterized by studying the effect of its knockdown in a cell model of oxidative stress, and its expression in zebrafish embryos was analyzed by in situ hybridization.

RESULTS

Solute carrier family 13 sodium-dependent dicarboxylate transporter member 3 (SLC13A3) was identified as 1 of 47 potential PITX2 target genes in ocular cells. PITX2 directly regulates SLC13A3 expression, as demonstrated by luciferase reporter and chromatin immunoprecipitation assays. Reduction of PITX2 or SLC13A3 levels by small interfering RNA (siRNA)-mediated knockdown augmented the death of transformed human trabecular meshwork cells exposed to hydrogen peroxide. Zebrafish slc13a3 is expressed in anterior ocular regions in a pattern similar to that of pitx2.

CONCLUSIONS

The results indicate that SLC13A3 is a direct downstream target of PITX2 transcriptional regulation and that levels of PITX2 and SLC13A3 modulate responses to oxidative stress in ocular cells.

High-affinity Na(+)-dependent dicarboxylate cotransporter promotes cellular senescence by inhibiting SIRT1

High-affinity Na⁺-dependent dicarboxylate cotransporter (NaDC3) can transport Krebs cycle intermediates into cells. Our previous study has shown that NaDC3 promotes cellular senescence, but its mechanism is not clear. It is known that when the concentration of intermediates in Krebs cycle is increased, NAD⁺/NADH ratio will be decreased. NAD⁺-dependent histone deacetylase sirtuin1 (SIRT1) prolongs mammalian cellular lifespan. Therefore, we propose that NaDC3 accelerates cellular aging by inhibiting SIRT1. After NaDC3 was overexpressed in two human embryo lung fibroblastic cell lines, WI38 and MRC-5, we found that the cells displayed aging-related phenotypes in advance. Meanwhile, the level of SIRT1 activity was down-regulated. In WI38/hNaDC3 cells treated with the activators of SIRT1, aging-related phenotypes induced by NaDC3 were obviously improved. The NAD⁺/NADH ratio in WI38/hNaDC3 cells was also decreased. Further study found that enhanced intracellular NAD⁺ level could attenuate the aging phenotypes induced by NaDC3. Thus, NaDC3 promotes cellular senescence probably by inhibiting NAD⁺-dependent SIRT1.

Succinate modulates Ca(2+) transient and cardiomyocyte viability through PKA-dependent pathway

GPR91 is an orphan G-protein-coupled receptor (GPCR) that has been characterized as a receptor for succinate, a citric acid cycle intermediate, in several tissues. In the heart, the role of succinate is unknown. We now report that rat ventricular cardiomyocytes express GPR91. We found that succinate, through GPR91, increases the amplitude and the rate of decline of global Ca(2+) transient, by increasing the phosphorylation levels of ryanodine receptor and phospholamban, two well known Ca(2+) handling proteins. The effects of succinate on Ca(2+) transient were abolished by pre-treatment with adenylyl cyclase and cAMP-dependent protein kinase (PKA) inhibitors. Direct PKA activation by succinate was further confirmed using a FRET-based A-kinase activity reporter. Additionally, succinate decreases cardiomyocyte viability through a caspase-3 activation pathway, effect also prevented by PKA inhibition. Taken together, these observations show that succinate acts as a signaling molecule in cardiomyocytes, modulating global Ca(2+) transient and cell viability through a PKA-dependent pathway.

Hypocitraturia: pathophysiology and medical management

Low urinary citrate excretion is a known risk factor for the development of kidney stones. Citrate inhibits stone formation by complexing with calcium in the urine, inhibiting spontaneous nucleation, and preventing growth and agglomeration of crystals. Hypocitraturia is a common metabolic abnormality found in 20% to 60% of stone formers. It is most commonly idiopathic in origin but may be caused by distal renal tubular acidosis, hypokalemia, bowel dysfunction, and a high-protein, low-alkali diet. Genetic factors, medications, and other comorbid disorders also play a role. Hypocitraturia should be managed through a combination of dietary modifications, oral alkali, and possibly lemonade or other citrus juice-based therapy. This review concerns the pathophysiology of hypocitraturia and the management of stone formers afflicted with this abnormality.

Oxaloacetate restores the long-term potentiation impaired in rat hippocampus CA1 region by 2-vessel occlusion

Various acute brain pathological conditions are characterized by the presence of elevated glutamate concentrations in the brain interstitial fluids. It has been established that a decrease in the blood glutamate level enhances the brain-to-blood efflux of glutamate, removal of which from the brain may prevent glutamate excitotoxicity and its contribution to the long-lasting neurological deficits seen in stroke. A decrease in blood glutamate level can be achieved by exploiting the glutamate-scavenging properties of the blood-resident enzyme glutamate-oxaloacetate transaminase, which transforms glutamate into 2-ketoglutarate in the presence of the glutamate co-substrate oxaloacetate. The present study had the aim of an evaluation of the effects of the blood glutamate scavenger oxaloacetate on the impaired long-term potentiation (LTP) induced in the 2-vessel occlusion ischaemic model in rat. Transient (30-min) incomplete forebrain ischaemia was produced 72 h before LTP induction. Although the short transient brain hypoperfusion did not induce histologically identifiable injuries in the CA1 region (Fluoro-Jade B, S-100 and cresyl violet), it resulted in an impaired LTP function in the hippocampal CA1 region without damaging the basal synaptic transmission between the Schaffer collaterals and the pyramidal neurons. This impairment could be fended off in a dose-dependent manner by the intravenous administration of oxaloacetate in saline (at doses between 1.5 mmol and 0.1 mumol) immediately after the transient hypoperfusion. Our results suggest that oxaloacetate-mediated blood and brain glutamate scavenging contributes to the restoration of the LTP after its impairment by brain ischaemia.

Use of genetic immunization to generate a high-level antibody against rat dicarboxylate transporter

BACKGROUND

Rat dicarboxylate transporter (SDCT1), expressed in renal tubular epithelial cells, plays a key role in regulating blood and urinary citrate level by reabsorbing citrate from the lumen. Antibodies against this transporter are very important for investigating its expression and function. With the cytokine gene as a molecular adjuvant, genetic immunization-based antibody production offers several advantages compared with current methods. This study aimed, by genetic immunization, to produce a high-specificity antibody against SDCT1.

METHODS

We fused a high-antigenicity fragment of SDCT1 to the plasmid pBQAP-TT containing T-cell epitopes and flanking regions from tetanus toxin. Mice were immunized by gene-gun immunization with recombinant plasmid and two other adjuvant plasmids that express granulocyte/macrophage colony-stimulating factor and FMS-like tyrosine kinase 3 ligand, respectively. The titer of the antibody was detected by enzyme-linked immunosorbent assay (ELISA). Specificity of the antibody was identified with SDCT1 native protein in rat kidney by Western blot analysis and immunohistochemistry, and with SDCT1 protein expressed on Xenopus oocytes plasma membranes by immunofluorescence.

RESULTS

ELISA measurements showed that the antibody titer was 1:32,000. The native protein of SDCT1 in rat kidney can be recognized by this antibody with Western blot analysis and immunohistochemistry. Immunofluorescence showed that this antibody also recognized SDCT1 protein targeted to Xenopus oocytes plasma membranes into which SDCT1 full-length cRNA was injected.

CONCLUSION

Generation of a high-specificity immunoglobulin G antibody against SDCT1 by genetic immunization has provided an important tool for the study of citrate transport.

The cdx genes and retinoic acid control the positioning and segmentation of the zebrafish pronephros

Kidney function depends on the nephron, which comprises a blood filter, a tubule that is subdivided into functionally distinct segments, and a collecting duct. How these regions arise during development is poorly understood. The zebrafish pronephros consists of two linear nephrons that develop from the intermediate mesoderm along the length of the trunk. Here we show that, contrary to current dogma, these nephrons possess multiple proximal and distal tubule domains that resemble the organization of the mammalian nephron. We examined whether pronephric segmentation is mediated by retinoic acid (RA) and the caudal (cdx) transcription factors, which are known regulators of segmental identity during development. Inhibition of RA signaling resulted in a loss of the proximal segments and an expansion of the distal segments, while exogenous RA treatment induced proximal segment fates at the expense of distal fates. Loss of cdx function caused abrogation of distal segments, a posterior shift in the position of the pronephros, and alterations in the expression boundaries of raldh2 and cyp26a1, which encode enzymes that synthesize and degrade RA, respectively. These results suggest that the cdx genes act to localize the activity of RA along the axis, thereby determining where the pronephros forms. Consistent with this, the pronephric-positioning defect and the loss of distal tubule fate were rescued in embryos doubly-deficient for cdx and RA. These findings reveal a novel link between the RA and cdx pathways and provide a model for how pronephric nephrons are segmented and positioned along the embryonic axis.

Associations between renal sodium-citrate cotransporter (hNaDC-1) gene polymorphism and urinary citrate excretion in recurrent renal calcium stone formers and normal controls

PURPOSE

Urinary citrate is a potent inhibitor of renal stone formation. Its excretion is regulated by Na(+)/dicarboxylate cotransporter-1 (NaDC-1), which is expressed on the apical membrane of renal proximal tubules. Many patients with calcium urolithiasis exhibit hypocitraturia, however, the mechanisms are not perfectly understood. We examined whether or not the I550V polymorphism in human NaDC-1 gene (hNaDC-1) influenced urinary citrate excretion.

MATERIALS AND METHODS

I550V polymorphism was investigated in 105 patients with recurrent renal calcium stone formation (RSF) and 107 age-matched healthy volunteers with non-renal stone formation (NSF), using polymerase chain reaction (PCR) restriction fragment length polymorphism analysis and two 24-h urine samples.

RESULTS

Overall and in the RSF groups, subjects with a BB (homozygous for the digested Bcl-I allele) genotype exhibited a significantly lower urinary citrate excretion level than subjects with a bb (homozygous for the undigested allele) genotype. Genotype distributions between subjects with hypocitraturia and normocitraturia were significantly different, with the BB genotype being more frequently observed in subjects with hypocitraturia - both overall and in each of the RSF and NSF groups. Although the BB genotype was observed more frequently in the RSF group than in the NSF group, no statistical differences among the distributions of the three genotypes (BB, Bb [heterozygous] and bb) were observed between the RSF and NSF groups.

CONCLUSION

These results suggest that the B allele of I550V polymorphism of hNaDC-1 may be associated with a reduction in urinary citrate excretion and contribute to hypocitraturia in recurrent renal stone formers.

Membrane topology structure of human high-affinity, sodium-dependent dicarboxylate transporter

High-affinity, sodium-dependent dicarboxylate transporter (NaDC3) is responsible for transport of Krebs cycle intermediates and may involve in regulation of aging and life span. Hydropathy analysis predicts that NaDC3 contains 11 or 12 hydrophobic transmembrane (TM) domains. However, the actual membrane topological structure of NaDC3 remains unknown. In this study, confocal immunofluorescence microscopy and membrane biotinylation of epitope-tagged N and C termini of NaDC3 provide evidence of an extracellular C terminus and an intracellular N terminus, indicating an odd number of transmembrane regions. The position of hydrophilic loops within NaDC3 was identified with antibodies against the loops domains combined with cysteine accessibility methods. A confocal image of membrane localization and transport activity assay of the cysteine insertion mutants show behavior similar to that of wild-type NaDC3 in transfected HEK293 cells, suggesting that these mutants retain a native protein configuration. We find that NaDC3 contains 11 transmembrane helices. The loops 1, 3, 5, 7, and 9 face the extracellular side, and loops 2, 4, 6, and 10 face the cytoplasmic side. A re-entrant loop-like structure between TM8 and TM9 may protrude into the membrane. Our results support the topography of 11 transmembrane domains with an extracellular C terminus and an intracellular N terminus of NaDC3, and for the first time provide experimental evidence for a novel topological model for NaDC3.

Generation and characterization of sodium-dicarboxylate cotransporter-deficient mice

The sodium-dependent dicarboxylate cotransporter (NaDC1) has a proposed function of reabsorbing various Krebs cycle intermediates in the kidney and the small intestine. Since Krebs cycle intermediates have been suggested to be important for renal cell survival and recovery after hypoxia and reoxygenation, the transporter may play a role in the recovery of the kidney. Additionally, mutations in the transporter homolog in Drosophila led to fly longevity which was thought to be similar to that induced by caloric restriction (CR). To clarify the role of the sodium dicarboxylate cotransporter in vivo we generated cotransporter-deficient mice. These knockout mice excreted significantly higher amounts of various Krebs cycle intermediates in their urine; thus confirming the proposed function to reabsorb these metabolic intermediates in the kidney. No other phenotypic change was identified in these mice, however. Transporter deficiency did not affect renal function under normal physiological conditions, nor did it have an effect on renal damage and recovery from ischemic injury. Additionally, the absence of the transporter did not lead to metabolic or physiological changes associated with CR. Our results suggest that although the sodium dicarboxylate cotransporter is involved in regulating levels of various Krebs cycle intermediates in the kidney, impaired uptake of these intermediates does not significantly affect renal function under normal or ischemic stress.

Modulation of succinate transport in Hep G2 cell line by PKC

The cellular uptake of the tricarboxylic acid cycle (TCA) intermediates is very important for cellular metabolism. However, the transport pathways for these intermediates in liver cells are not well characterized. We have examined the transport of succinate and citrate in the human hepatoma cell line Hep G2 and found that it exhibited a higher rate of succinate compared to citrate transport, which was sodium dependent. Comparison of the transport properties of Hep G2 to that of human retinal pigment epithelial (HRPE) cells transfected with human sodium dicarboxylate transporters, hNaDC-1, hNaDC-3, and hNaCT indicated that Hep G2 cells express a combination of hNaDC-3 and hNaCT. Short period activation of protein kinase C (PKC) by phorbol 12-myristate, 13-acetate (PMA) and alpha-adrenergic receptor agonist, phenylephrine (PE), downregulated sodium-dependent succinate transport presumably via hNaDC-3. The inhibition by PMA was partially prevented by cytochalasin D, suggesting that PKC reduces the hNaDC-3 activity, at least in part, by increased endocytosis. In contrast, activation of PKA by both forskolin and epidermal growth factor (EGF) had no effect on succinate transport. Our results suggest that Hep G2 cells provide a useful model for studies of di- and tricarboxylate regulation of human liver.

Functional roles of cationic amino acid residues in the sodium-dicarboxylate cotransporter 3 (NaDC-3) from winter flounder

In the present study, we determined the functional role of 15 positively charged amino acid residues at or within 1 of the predicted 11 transmembrane helixes of the flounder renal sodium-dicarboxylate cotransporter fNaDC-3. Using site-directed mutagenesis, histidine (H), lysine (K), and arginine (R) residues of fNaDC-3 were replaced by alanine (A), isoleucine (I), or leucine (L). Most mutants showed sodium-dependent, lithium-inhibitable [14C]succinate uptake and, in two-electrode voltage-clamp (TEVC) experiments, Km and Δ Imax values comparable to wild-type (WT) fNaDC-3. The replacement of R109 and R110 by alanine and isoleucine (RR109/110AI) prevented the expression of fNaDC-3 at the plasma membrane. When the lysines at positions 232 and 235 were replaced by isoleucine (KK232/235II), the transporter was expressed but showed small transport rates and succinate-induced currents. K114I, located within transmembrane helix 4, showed [14C]succinate uptake similar to WT but relatively small inward currents. When K114 was replaced by arginine, glutamic acid (E), or glutamine (Q), all mutants were expressed at the cell surface. In [14C]succinate uptake and TEVC experiments performed simultaneously on the same oocytes, uptake was similar to or higher than WT, whereas succinate-induced currents were either comparable (K114R) to, or considerably smaller (K114E, K114I, K114Q) than, those evoked by WT. These results suggest that a positively charged residue at position 114 is required for electrogenic sodium-dicarboxylate cotransport.

Functional reconstitution of SdcS, a Na+-coupled dicarboxylate carrier protein from Staphylococcus aureus

In Staphylococcus aureus, the transport of dicarboxylates is mediated in part by the Na+-linked carrier protein SdcS. This transporter is a member of the divalent-anion/Na+ symporter (DASS) family, a group that includes the mammalian Na+/dicarboxylate cotransporters NaDC1 and NaDC3. In earlier work, we cloned and expressed SdcS in Escherichia coli and found it to have transport properties similar to those of its eukaryotic counterparts (J. A. Hall and A. M. Pajor, J. Bacteriol. 187:5189-5194, 2005). Here, we report the partial purification and subsequent reconstitution of functional SdcS into liposomes. These proteoliposomes exhibited succinate counterflow activity, as well as Na+ electrochemical-gradient-driven transport. Examination of substrate specificity indicated that the minimal requirement necessary for transport was a four-carbon terminal dicarboxylate backbone and that productive substrate-transporter interaction was sensitive to substitutions at the substrate C-2 and C-3 positions. Further analysis established that SdcS facilitates an electroneutral symport reaction having a 2:1 cation/dicarboxylate ratio. This study represents the first characterization of a reconstituted Na+-coupled DASS family member, thus providing an effective method to evaluate functional, as well as structural, aspects of DASS transporters in a system free of the complexities and constraints associated with native membrane environments.

Ala-504 is a determinant of substrate binding affinity in the mouse Na(+)/dicarboxylate cotransporter

The Na(+)/dicarboxylate cotransporters from mouse (mNaDC1) and rabbit (rbNaDC1) differ in their ability to handle adipate, a six-carbon terminal dicarboxylic acid. The mNaDC1 and rbNaDC1 amino acid sequences are 75% identical. The rbNaDC1 does not transport adipate and only succinate produced inward currents under two-electrode voltage clamp. In contrast, oocytes expressing mNaDC1 had adipate-dependent inward currents that were about 60% of those induced by succinate. In order to identify domains involved in adipate transport, we examined the functional properties of a series of chimeric transporters made between mouse and rabbit NaDC1. We find that multiple transmembrane helices (TM), particularly TM 8, 9, and 10, are involved in adipate transport. In TM 10 there is only one amino acid difference between the two proteins, corresponding to Ala-504 in mouse and Ser-512 in rabbit NaDC1. The mNaDC1-A504S mutant had decreased adipate-dependent currents relative to succinate-dependent currents and an increase in the K(0.5) for both succinate and glutarate. We conclude that multiple amino acids from TM 8, 9 and 10 contribute to the transport of adipate in NaDC1. Furthermore, Ala-504 in TM 10 is an important determinant of K(0.5) for both adipate and succinate.

Identification of basolateral membrane targeting signal of human sodium-dependent dicarboxylate transporter 3

Sodium-dependent dicarboxylate transporters (NaDC) include low-affinity NaDC1 and high-affinity NaDC3. Despite high similarities structurally and functionally, both are localized to opposite surfaces of renal tubular cells. The molecular mechanisms and localization signals leading to this polarized distribution remain unknown. In this study, distribution of NaDC3 in human kidney tissue was firstly observed by immunohistochemistry and immunofluorescence. Then, EGFP-fused wild-type, NH2- and COOH-terminal deletion and point mutants of NaDC3, and chimera between NaDC3 and NaDC1, were generated and transfected into polarized renal cells lines, LLC-PK1 and MDCK. Their subcellular localizations were analyzed by laser confocal microscopy. Immunolocalization results revealed that NaDC3 was expressed at basolateral membrane of human renal proximal tubular epithelia. Confocal examinations showed that wild-type NaDC3 was targeted to the basolateral membrane of MDCK and LLC-PK1. Deletion mutations indicated that the basolateral targeting signal of NaDC3 located within a short sequence AKKVWSARR of its amino-terminal cytoplasmic domain. Addition of this sequence could redirect apical NaDC1 to the basolateral membrane of LLC-PK1. Point mutagenesis revealed that mutation of either of two hydrophobic amino acids V and W in this short sequence largely redirected NaDC3 to both apical and basolateral surfaces of LLC-PK, indicating that the two hydrophobic amino acids are critical for the basolateral targeting of NaDC3. Our studies provide direct evidence of the localization of NaDC3 at the basolateral membrane of human renal proximal tubule cells and identify a di-hydrophobic amino acid motif VW as basolateral localization signal in the N-terminal cytoplasmic domain of NaDC3.

D-β-hydroxybutyrate protects dopaminergic SH-SY5Y cells in a rotenone model of Parkinson's disease

It has been postulated that the pathogenesis of Parkinson's disease (PD) is associated with mitochondrial dysfunction. Rotenone, an inhibitor of mitochondrial complex I, provides models of PD both in vivo and in vitro. We investigated the neuroprotective effect of D-beta-hydroxybutyrate (bHB), a ketone body, against rotenone toxicity by using SH-SY5Y dopaminergic neuroblastoma cells. SH-SY5Y cells, differentiated by all-trans-retinoic acid, were exposed to rotenone at concentrations ranging from 0 to 1,000 nM. We evaluated cellular oxidation reduction by the alamarBlue assay, viability by lactate dehydrogenase (LDH) assay, and survival/death ratio by live/dead assays. Exposure to rotenone for 48 hr oxidized cells and decreased their viability and survival rate in a concentration-dependent manner. Pretreatment of cells with 8 mM bHB provided significant protection to SH-SY5Y cells. Whereas rotenone caused the loss of mitochondrial membrane potential, released cytochrome c into the cytosol, and reduced cytochrome c content in mitochondria, addition of bHB blocked this toxic effect. bHB also attenuated the rotenone-induced activation of caspase-9 and caspase-3. Administration of 0-10 mM 3-nitropropionic acid, a complex II inhibitor, also decreased the reducing power of SH-SY5Y cells measured by alamarBlue assay. Pretreatment with 8 mM bHB attenuated the decrease of alamarBlue fluorescence. These data demonstrated that bHB had a neuroprotective effect that supported the mitochondrial respiration system by reversing the inhibition of complex I or II. Ketone bodies, the alternative energy source in the mammalian brain, appear to have therapeutic potential in PD.

Molecular properties of the SLC13 family of dicarboxylate and sulfate transporters

The SLC13 gene family consists of five members in humans, with corresponding orthologs from different vertebrate species. All five genes code for sodium-coupled transporters that are found on the plasma membrane. Two of the transporters, NaS1 and NaS2, carry substrates such as sulfate, selenate and thiosulfate. The other members of the family (NaDC1, NaDC3, and NaCT) are transporters for di- and tri-carboxylates including succinate, citrate and alpha-ketoglutarate. The SLC13 transporters from vertebrates are electrogenic and they produce inward currents in the presence of sodium and substrate. Substrate-independent leak currents have also been described. Structure-function studies have identified the carboxy terminal half of these proteins as the most important for determining function. Transmembrane helices 9 and 10 may form part of the substrate permeation pathway and participate in conformational changes during the transport cycle. This review also discusses new members of the SLC13 superfamily that exhibit both sodium-dependent and sodium-independent transport mechanisms. The Indy protein from Drosophila, involved in determining lifespan, and the plant vacuolar malate transporter are both sodium-independent dicarboxylate transporters, possibly acting as exchangers. The purpose of this review is to provide an update on new advances in this gene family, particularly on structure-function studies and new members of the family.

Functional characterization of a Na(+)-coupled dicarboxylate carrier protein from Staphylococcus aureus

We have cloned and functionally characterized a Na(+)-coupled dicarboxylate transporter, SdcS, from Staphylococcus aureus. This carrier protein is a member of the divalent anion/Na(+) symporter (DASS) family and shares significant sequence homology with the mammalian Na(+)/dicarboxylate cotransporters NaDC-1 and NaDC-3. Analysis of SdcS function indicates transport properties consistent with those of its eukaryotic counterparts. Thus, SdcS facilitates the transport of the dicarboxylates fumarate, malate, and succinate across the cytoplasmic membrane in a Na(+)-dependent manner. Furthermore, kinetic work predicts an ordered reaction sequence with Na(+) (K(0.5) of 2.7 mM) binding before dicarboxylate (K(m) of 4.5 microM). Because this transporter and its mammalian homologs are functionally similar, we suggest that SdcS may serve as a useful model for DASS family structural analysis.

Role of glutathione transport processes in kidney function

The kidneys are highly dependent on an adequate supply of glutathione (GSH) to maintain normal function. This is due, in part, to high rates of aerobic metabolism, particularly in the proximal tubules. Additionally, the kidneys are potentially exposed to high concentrations of oxidants and reactive electrophiles. Renal cellular concentrations of GSH are maintained by both intracellular synthesis and transport from outside the cell. Although function of specific carriers has not been definitively demonstrated, it is likely that multiple carriers are responsible for plasma membrane transport of GSH. Data suggest that the organic anion transporters OAT1 and OAT3 and the sodium-dicarboxylate 2 exchanger (SDCT2 or NaDC3) mediate uptake across the basolateral plasma membrane (BLM) and that the organic anion transporting polypeptide OATP1 and at least one of the multidrug resistance proteins mediate efflux across the brush-border plasma membrane (BBM). BLM transport may be used pharmacologically to provide renal proximal tubular cells with exogenous GSH to protect against oxidative stress whereas BBM transport functions physiologically in turnover of cellular GSH. The mitochondrial GSH pool is derived from cytoplasmic GSH by transport into the mitochondrial matrix and is mediated by the dicarboxylate and 2-oxoglutarate exchangers. Maintenance of the mitochondrial GSH pool is critical for cellular and mitochondrial redox homeostasis and is important in determining susceptibility to chemically induced apoptosis. Hence, membrane transport processes are critical to regulation of renal cellular and subcellular GSH pools and are determinants of susceptibility to cytotoxicity induced by oxidants and electrophiles.

Functional characterization of high-affinity Na(+)/dicarboxylate cotransporter found in Xenopus laevis kidney and heart

The SLC13 gene family includes sodium-coupled transporters for citric acid cycle intermediates and sulfate. The present study describes the sequence and functional characterization of a SLC13 family member from Xenopus laevis, the high-affinity Na(+)/dicarboxylate cotransporter xNaDC-3. The cDNA sequence of xNaDC-3 codes for a protein of 602 amino acids that is approximately 70% identical to the sequences of mammalian NaDC-3 orthologs. The message for xNaDC-3 is found in the kidney, liver, intestine, and heart. The xNaDC-3 has a high affinity for substrate, including a K(m) for succinate of 4 muM, and it is inhibited by the NaDC-3 test substrates 2,3-dimethylsuccinate and adipate. The transport of succinate by xNaDC-3 is dependent on sodium, with sigmoidal activation kinetics, and lithium can partially substitute for sodium. As with other members of the family, xNaDC-3 is electrogenic and exhibits inward substrate-dependent currents in the presence of sodium. However, other electrophysiological properties of xNaDC-3 are unique and involve large leak currents, possibly mediated by anions, that are activated by binding of sodium or lithium to a single site.

Effects of Human Na+/Dicarboxylate Cotransporter 3 on the Replicative Senescence of Human Embryonic Lung Diploid Fibroblasts

To investigate the role of human Na(+)/dicarboxylate cotransporter 3 (hNaDC3) in the replicative senescence of normal human embryonic lung diploid fibroblasts (WI-38), a retroviral vector containing hNaDC3 was constructed. hNaDC3 was introduced into normal WI-38 cells through infection with the retroviral virus. Monoclones were selected with G418. The integration and expression of exotic genes were confirmed by Northern blot and Western blot. When compared with the control cells, WI-38 cells transfected with hNaDC3 cDNA showed significant suppression of growth rate (by 40%), increase of positive rate of SA-beta-gal staining, decrease of mitochondrial membrane potential, shortening of telomere length, and increase of P16 and P21 expression. The morphology characteristics of senescent fibroblasts appeared earlier. Our results have, for the first time, demonstrated that high expression of hNaDC3 may be able to, at least partly, promote the cellular senescence of human diploid fibroblasts.

Itaconate reduces visceral fat by inhibiting fructose 2,6-bisphosphate synthesis in rat liver

OBJECTIVE

Itaconate is an analog of phosphoenolpyruvate, which is an inhibitor of fructose-6-phosphate 2-kinase (F6P2Kinase), an enzyme that synthesizes fructose 2,6-bisphosphate (F26BP). Carbohydrates ingested are preferentially used for glycogen synthesis in the liver and muscles, and excess carbohydrates are metabolized by glycolysis in the liver and used for fatty acid synthesis. We hypothesized that itaconate is incorporated into liver cells and suppresses fat synthesis by inhibiting liver glycolysis at the step of phosphofructokinase, which is activated by F26BP.

METHODS

Rats were allowed to eat ad libitum for 3 wk or, in separate experiments, to limit food intake by pair feeding. One group was given drinking water (control group) and the other group was given a 10 g/L itaconate solution (itaconate group). We measured body weight gain, visceral fat accumulation, and F6P2Kinase activity.

RESULTS

Body weight gain in the itaconate group was lower than that in the control group (P < 0.05). In the dietary-controlled rats, there was no difference in body weight increase between groups, but visceral fat content (P < 0.01), plasma free fatty acid, and triacylglycerol levels (P < 0.05) were lower in the itaconate group than in the control group. Further, itaconate decreased the F26BP level (P < 0.05) in vivo and partly inhibited rat liver-type F6P2Kinase in vitro.

CONCLUSIONS

These results indicate that itaconate, which is a decarboxylate and resembles phosphoenolpyruvate, is incorporated into liver cells and suppresses glycolysis by decreasing the level of F26BP, resulting in decreased visceral fat.

Transport characteristics of N ‐acetyl‐ l ‐aspartate in rat astrocytes: involvement of sodium‐coupled high‐affinity carboxylate transporter NaC3/NaDC3‐mediated transport system

We investigated in the present study the transport characteristics of N-acetyl-L-aspartate in primary cultures of astrocytes from rat cerebral cortex and the involvement of NA+-coupled high-affinity carboxylate transporter NaC3 (formerly known as NaDC3) responsible for N-acetyl-L-aspartate transport. N-acetyl-L-aspartate transport was NA+-dependent and saturable with a Michaelis-Menten constant (Km) of approximately 110 microm. NA+-activation kinetics revealed that the NA+ to-N-acetyl-L-aspartate stoichiometry was 3 : 1 and concentration of Na+ necessary for half-maximal transport (KNA m) was 70 mm. NA+-dependent N-acetyl-L-aspartate transport was competitively inhibited by succinate with an inhibitory constant (Ki) of 14.7 microm, which was comparable to the Km value of NA+-dependent succinate transport (29.4 microm). L-aspartate also inhibited NA+-dependent [14C]N-acetyl-L-aspartate transport with relatively low affinity (Ki = 2.2 mm), whereas N-acetyl-L-aspartate was not able to inhibit NA+-dependent aspartate transport in astrocytes. In addition, Li+ was found to have a significant inhibitory effect on the NA+-dependent N-acetyl-L-aspartate transport in a concentration-dependent manner. Furthermore, RT-PCR and western blot analyses revealed that NaC3 is expressed in primary cultures of astrocytes. Taken collectively, these results indicate that NaC3 expressed in rat cerebrocortical astrocytes is responsible for NA+-dependent N-acetyl-L-aspartate transport. This transporter is likely to be an essential prerequisite for the metabolic role of N-acetyl-L-aspartate in the process of myelination.

Substrate specificity of the human renal sodium dicarboxylate cotransporter, hNaDC-3, under voltage-clamp conditions

Proximal tubule cells extract dicarboxylates from filtrate and blood, using cotransporters located in the brush border [sodium dicarboxylate cotransporter (NaDC-1)] and basolateral cell membrane (NaDC-3). We expressed the human NaDC-3 (hNaDC-3) in Xenopus laevis oocytes and characterized it by the two-electrode voltage-clamp technique. At −60 mV, succinate (4 carbons) and glutarate (5 carbons) generated inward currents due to translocation of three sodium ions and one divalent dicarboxylate, whereas oxalate (2 carbons) and malonate (3 carbons) did not. The cis-dicarboxylate maleate produced currents smaller in magnitude, whereas the trans-dicarboxylate fumarate generated currents similar to succinate. The substituted succinate derivatives, malate, 2,2- and 2,3-dimethylsuccinate, and 2,3-dimercaptosuccinate elicited inward currents, whereas aspartate and guanidinosuccinate showed hardly detectable currents. The C-5 dicarboxylates glutarate and α-ketoglutarate produced larger currents than succinate; glutamate and folate failed to cause inward currents. Kinetic analysis revealed, at −60 mV, K0.5 values of 25 ± 12 μM for succinate and 45 ± 13 μM for α-ketoglutarate, values close to the plasma concentration of these compounds. For both compounds, the K0.5 was independent of voltage, whereas the maximal current increased with hyperpolarization. As opposed to the rat and flounder orthologs, hNaDC-3 was hardly inhibited by lithium concentrations up to 5 mM. In the absence of sodium, however, lithium can mediate succinate-dependent currents. The narrow substrate specificity prevents interaction of drugs with dicarboxylate-like structure with hNaDC-3 and ensures sufficient support of the proximal tubule cells with α-ketoglutarate for anion secretion via organic anion transporter 1 or 3.