Cloning of the monocarboxylate transporter isoform MCT2 from rat testis provides evidence that expression in tissues is species-specific and may involve post-transcriptional regulation

Blood-brain barrier alterations and their impact on Parkinson's disease pathogenesis and therapy

There is increasing evidence for blood-brain barrier (BBB) alterations in Parkinson's disease (PD), the second most common neurodegenerative disorder with rapidly rising prevalence. Altered tight junction and transporter protein levels, accumulation of α-synuclein and increase in inflammatory processes lead to extravasation of blood molecules and vessel degeneration. This could result in a self-perpetuating pathophysiology of inflammation and BBB alteration, which contribute to neurodegeneration. Toxin exposure or α-synuclein over-expression in animal models has been shown to initiate similar pathologies, providing a platform to study underlying mechanisms and therapeutic interventions. Here we provide a comprehensive review of the current knowledge on BBB alterations in PD patients and how rodent models that replicate some of these changes can be used to study disease mechanisms. Specific challenges in assessing the BBB in patients and in healthy controls are discussed. Finally, a potential role of BBB alterations in disease pathogenesis and possible implications for therapy are explored. The interference of BBB alterations with current and novel therapeutic strategies requires more attention. Brain region-specific BBB alterations could also open up novel opportunities to target specifically vulnerable neuronal subpopulations.

Lactate transported by MCT1 plays an active role in promoting mitochondrial biogenesis and enhancing TCA flux in skeletal muscle

Once considered as a "metabolic waste," lactate is now recognized as a major fuel for tricarboxylic acid (TCA) cycle. Our metabolic flux analysis reveals that skeletal muscle mainly uses lactate to fuel TCA cycle. Lactate is transported through the cell membrane via monocarboxylate transporters (MCTs) in which MCT1 is highly expressed in the muscle. We analyzed how MCT1 affects muscle functions using mice with specific deletion of MCT1 in skeletal muscle. MCT1 deletion enhances running performance, increases oxidative fibers while decreasing glycolytic fibers, and enhances flux of glucose to TCA cycle. MCT1 deficiency increases the expression of mitochondrial proteins, augments cell respiration rate, and elevates mitochondrial activity in the muscle. Mechanistically, the protein level of PGC-1α, a master regulator of mitochondrial biogenesis, is elevated upon loss of MCT1 via increases in cellular NAD+ level and SIRT1 activity. Collectively, these results demonstrate that MCT1-mediated lactate shuttle plays a key role in regulating muscle functions by modulating mitochondrial biogenesis and TCA flux.

Chemical Biology Approaches Confirm MCT4 as the Therapeutic Target of a Cellular Optimized Hit

Lactic acid transport is a key process maintaining glycolytic flux in tumors. Inhibition of this process will result in glycolytic shutdown, impacting on cell growth and survival and thus has been pursued as a therapeutic approach for cancers. Using a cell-based screen in a MCT4-dependent cell line, we identified and optimized compounds for their ability to inhibit the efflux of intracellular lactic acid with good physical and pharmacokinetic properties. To deconvolute the mechanism of lactic acid efflux inhibition, we have developed three assays to measure cellular target engagement. Specifically, we synthesized a biologically active photoaffinity probe (IC₅₀ < 10 nM), and using this probe, we demonstrated selective engagement of MCT4 of our parent molecule through a combination of confocal microscopy and in-cell chemoproteomics. As an orthogonal assay, the cellular thermal shift assay (CETSA) confirmed binding to MCT4 in the cellular system. Comparisons of lactic acid efflux potencies in cells with differential expression of MCT family members further confirmed that the optimized compounds inhibit the efflux of lactic acid through the inhibition of MCT4. Taken together, these data demonstrate the power of orthogonal chemical biology methods to determine cellular target engagement, particularly for proteins not readily amenable to traditional biophysical methods.

Cost of transport, but not gluteus medius and red blood cells monocarboxylate-transporters density differentiated Brazilian Sport Horses at two performance levels

Cost of transport (COT) and monocarboxylate transporters (MCTs) could affect the ability to perform fast actions during a jumping discipline. This study aimed to compare the COT and evaluate the MCT1, MCT4, and their auxiliary protein CD147 content in the gluteus medius and RBCs of Brazilian sport horses (BH), a breed developed for jumping competitions, with low-level (LL) or intermediate-level (IL) jumping capacities. The physiological difference between the horses was assessed by an incremental jump test (IJT), in which the cost of lactate (COT_LAC) and heart rate (COT_HR) of running were determined for each animal by the ratio between each variable and the running speed. Western blotting was performed on muscle and RBC membranes to quantify MCT1, MCT4, and CD147. IL showed lower COT_LAC and COT_HR than LL at all jumping heights. The amount of MCT1, MCT4, and CD147 found in muscle and RBCs were not dependent on performance level. Muscle MCT4 and MCT1 were correlated positively with CD147. We conclude that the relatively small differences between performances did not relevantly influence MCT expression in BH. While MCT analyses are inaccessible for most trainers and veterinarians, the cost of transport measurements is a feasible and sensitive tool to distinguish intermediate and low-level jumping horses.

Synthesis and anticancer activity of new coumarin-3-carboxylic acid derivatives as potential lactatetransportinhibitors

As an oncometabolite, lactate plays a very important role in tumor proliferation, metastasis, angiogenesis, immune escape and other tumor biological functions. Pharmacological inhibition oflactate transport has been viewed as a promising therapeutic strategy to target a range of human cancers. In this study, a series of new coumarin-3-carboxylic acid derivatives 5a-t and 9a-b were synthesized and evaluated as lactate transport inhibitors. Their cytotoxic activity has been tested against three cell lines high-expressing and low-expressing monocarboxylate transporter 1 (MCT1) which acts as the main carrier for lactate. Compound 5c-e, 5g-i and 5m-o showed significant cytotoxicity and good selectivity against Hela and HCT116 cell lines with high MCT1 expression. Notably, coumarin-3-hydrazide 5o, the lead molecule with the most potent cytotoxic activity, exhibitedsignificant anti-proliferationandapoptosisinductioneffects. Further studies revealed that compound 5o decreased the expression level of target MCT1, and suppressed the energetic metabolism of Hela and HCT116 cells byremarkably reducing glucoseconsumptionandlactate production. Additionally, compound 5o induced intracellular lactate accumulation and inhibited lactate uptake, which implied that it blocked lactate transport via MCT1. These results indicate a good start point for the development of lactate transport inhibitors as new anticancer agents.

Membrane-anchored carbonic anhydrase IV interacts with monocarboxylate transporters via their chaperones CD147 and GP70

Monocarboxylate transporters (MCTs) mediate the proton-coupled exchange of high-energy metabolites, including lactate and pyruvate, between cells and tissues. The transport activity of MCT1, MCT2, and MCT4 can be facilitated by the extracellular carbonic anhydrase IV (CAIV) via a noncatalytic mechanism. Combining physiological measurements in HEK-293 cells and Xenopus oocytes with pulldown experiments, we analyzed the direct interaction between CAIV and the two MCT chaperones basigin (CD147) and embigin (GP70). Our results show that facilitation of MCT transport activity requires direct binding of CAIV to the transporters chaperones. We found that this binding is mediated by the highly conserved His-88 residue in CAIV, which is also the central residue of the enzyme's intramolecular proton shuttle, and a charged amino acid residue in the Ig1 domain of the chaperone. Although the position of the CAIV-binding site in the chaperone was conserved, the amino acid residue itself varied among different species. In human CD147, binding of CAIV was mediated by the negatively charged Glu-73 and in rat CD147 by the positively charged Lys-73. In rat GP70, we identified the positively charged Arg-130 as the binding site. Further analysis of the CAIV-binding site revealed that the His-88 in CAIV can either act as H donor or H acceptor for the hydrogen bond, depending on the charge of the binding residue in the chaperone. Our results suggest that the CAIV-mediated increase in MCT transport activity requires direct binding between CAIV-His-88 and a charged amino acid in the extracellular domain of the transporter's chaperone.

Insights in pathogenesis of multiple sclerosis: nitric oxide may induce mitochondrial dysfunction of oligodendrocytes

Demyelinating diseases, such as multiple sclerosis (MS), are kinds of common diseases in the central nervous system (CNS), and originated from myelin loss and axonal damage. Oligodendrocyte dysfunction is the direct reason of demyelinating lesions in the CNS. Nitric oxide (NO) plays an important role in the pathological process of demyelinating diseases. Although the neurotoxicity of NO is more likely mediated by peroxynitrite rather than NO itself, NO can impair oligodendrocyte energy metabolism through mediating the damaging of mitochondrial DNA, mitochondrial membrane and mitochondrial respiratory chain complexes. In the progression of MS, NO can mainly mediate demyelination, axonal degeneration and cell death. Hence, in this review, we extensively discuss endangerments of NO in oligodendrocytes (OLs), which is suggested to be the main mediator in demyelinating diseases, e.g. MS. We hypothesize that NO takes part in MS through impairing the function of monocarboxylate transporter 1, especially causing axonal degeneration. Then, it further provides a new insight that NO for OLs may be a reliable therapeutic target to ameliorate the course of demyelinating diseases.

Chronic activation of AMP-activated protein kinase increases monocarboxylate transporter 2 and 4 expression in skeletal muscle

Acute activation of AMP-activated protein kinase (AMPK) increases monocarboxylate transporter (MCT) expression in skeletal muscle. However, the impact of chronic activation of AMPK on MCT expression in skeletal muscle is unknown. To investigate, MCT1, MCT2, and MCT4 mRNA expression and protein abundance were measured in the longissimus lumborum (glycolytic), masseter (oxidative), and heart from wild-type (control) and AMPK γ3 pigs. The AMPK γ3 gain in function mutation results in AMPK being constitutively active in glycolytic skeletal muscle and increases energy producing pathways. The MCT1 and MCT2 mRNA expression in muscle was lower ( < 0.05) from both wild-type and AMPK γ3 animals compared to other tissues. However, in both genotypes, MCT1 and MCT2 mRNA expression was greater ( < 0.05) in the masseter than the longissimus lumborum. The MCT1 protein was not detected in skeletal muscle, but MCT2 was greater ( < 0.05) in muscles with an oxidative muscle phenotype. Monocarboxylate transporter 2 was also detected in muscle mitochondria and may explain the differences between muscles. The MCT4 mRNA expression was intermediate among all tissues tested and greater ( < 0.05) in the longissimus lumborum than the masseter. Furthermore, MCT4 protein expression in the longissimus lumborum from AMPK γ3 animals was greater ( < 0.05) than in the longissimus lumborum from wild-type animals. In totality, these data indicate that chronic AMPK activation simultaneously increases MCT2 and MCT4 expression in skeletal muscle.

Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice

The purpose of this study was to investigate the effects of a partial suppression of monocarboxylate transporter (MCT)-1 on skeletal muscle pH, energetics, and function (MCT1^+/- mice). Twenty-four MCT1^+/- and 13 wild-type (WT) mice were subjected to a rest-exercise-recovery protocol, allowing assessment of muscle energetics (by magnetic resonance spectroscopy) and function. The study included analysis of enzyme activities and content of protein involved in pH regulation. Skeletal muscle of MCT1^+/- mice had lower MCT1 (-61%; P < 0.05) and carbonic anhydrase (CA)-II (-54%; P < 0.05) contents. Although intramuscular pH was higher in MCT1^+/- mice at rest (P < 0.001), the mice showed higher acidosis during the first minute of exercise (P < 0.01). Then, the pH time course was similar among groups until exercise completion. MCT1^+/- mice had higher specific peak (P < 0.05) and maximum tetanic (P < 0.01) forces and lower fatigability (P < 0.001) when compared to WT mice. We conclude that both MCT1 and CAII are involved in the homeostatic control of pH in skeletal muscle, both at rest and at the onset of exercise. The improved muscle function and resistance to fatigue in MCT1^+/- mice remain unexplained.-Chatel, B., Bendahan, D., Hourdé, C., Pellerin, L., Lengacher, S., Magistretti, P., Fur, Y. L., Vilmen, C., Bernard, M., Messonnier, L. A. Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice.

Regulation of colonic epithelial butyrate transport: Focus on colorectal cancer

Highlights

Fermentation of the dietary fiber by intestinal microflora results in production of butyrate.Butyrate possesses anticarcinogenic effect at the colonic level.Three transporters (MCT1, SMCT1 and BCRP) regulate the intracellular concentration of BT in colonic epithelial cells.Changes in the expression of these transporters occur in colorectal cancer.

Abstract

Colorectal cancer (CRC) is one of the most common solid tumors worldwide. Consumption of dietary fiber is associated with a low risk of developing CRC. The fermentation of the dietary fiber by intestinal microflora results in production of butyrate (BT). This short-chain fatty acid is an important metabolic substrate in normal colonic epithelial cells and has important homeostatic functions at the colonic level. Because the cellular effects of BT (e.g. inhibition of histone deacetylases) are dependent on its intracellular concentration, knowledge on the mechanisms involved in BT membrane transport and its regulation seems particularly relevant. In this review, we will present the carrier-mediated mechanisms involved in BT membrane transport at the colonic epithelial level and their regulation, with an emphasis on CRC. Several xenobiotics known to modulate the risk for developing CRC are able to interfere with BT transport at the intestinal level. Thus, interference with BT transport certainly contributes to the anticarcinogenic or procarcinogenic effect of these compounds and these compounds may interfere with the anticarcinogenic effect of BT. Finally, we suggest that differences in BT transport between normal colonocytes and tumoral cells contribute to the "BT paradox" (the apparent opposing effect of BT in CRC cells and normal colonocytes).

Analysis of the binding moiety mediating the interaction between monocarboxylate transporters and carbonic anhydrase II

Proton-coupled monocarboxylate transporters (MCTs) mediate the exchange of high energy metabolites like lactate between different cells and tissues. We have reported previously that carbonic anhydrase II augments transport activity of MCT1 and MCT4 by a noncatalytic mechanism, while leaving transport activity of MCT2 unaltered. In the present study, we combined electrophysiological measurements in Xenopus oocytes and pulldown experiments to analyze the direct interaction between carbonic anhydrase II (CAII) and MCT1, MCT2, and MCT4, respectively. Transport activity of MCT2-WT, which lacks a putative CAII-binding site, is not augmented by CAII. However, introduction of a CAII-binding site into the C terminus of MCT2 resulted in CAII-mediated facilitation of MCT2 transport activity. Interestingly, introduction of three glutamic acid residues alone was not sufficient to establish a direct interaction between MCT2 and CAII, but the cluster had to be arranged in a fashion that allowed access to the binding moiety in CAII. We further demonstrate that functional interaction between MCT4 and CAII requires direct binding of the enzyme to the acidic cluster (431)EEE in the C terminus of MCT4 in a similar fashion as previously shown for binding of CAII to the cluster (489)EEE in the C terminus of MCT1. In CAII, binding to MCT1 and MCT4 is mediated by a histidine residue at position 64. Taken together, our results suggest that facilitation of MCT transport activity by CAII requires direct binding between histidine 64 in CAII and a cluster of glutamic acid residues in the C terminus of the transporter that has to be positioned in surroundings that allow access to CAII.

Lactate transporters in the context of prostate cancer metabolism: what do we know?

Metabolic changes during malignant transformation have been noted for many years in tumours. Otto Warburg first reported that cancer cells preferentially rely on glycolysis for energy production, even in the presence of oxygen, leading to the production of high levels of lactate. The crucial role of lactate efflux and exchange within the tumour microenvironment drew attention to monocarboxylate transporters (MCTs). MCTs have been recognized as promising targets in cancer therapy, and their expression was described in a large variety of tumours; however, studies showing how these isoforms contribute to the acquisition of the malignant phenotype are scarce and still unclear regarding prostate cancer. In this review, we focus on the role for MCTs in cell metabolism, supporting the development and progression of prostate cancer, and discuss the exploitation of the metabolic nature of prostate cancer for therapeutic and diagnostic purposes.

Functional interaction between bicarbonate transporters and carbonic anhydrase modulates lactate uptake into mouse cardiomyocytes

Blood-derived lactate is a precious energy substrate for the heart muscle. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCTs) together with H(+), which couples lactate uptake to cellular pH regulation. In this study, we have investigated how the interplay between different acid/base transporters and carbonic anhydrases (CA), which catalyze the reversible hydration of CO2, modulates the uptake of lactate into isolated mouse cardiomyocytes. Lactate transport was estimated both as lactate-induced acidification and as changes in intracellular lactate levels measured with a newly developed Förster resonance energy transfer (FRET) nanosensor. Recordings of intracellular pH showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated a decrease in MCT transport activity, when CA was inhibited. The data indicate that catalytic activity of extracellular CA increases lactate uptake and counteracts intracellular lactate-induced acidification. We propose a hypothetical model, in which HCO3 (-), formed from cell-derived CO2 at the outer surface of the cardiomyocyte plasma membrane by membrane-anchored, extracellular CA, is transported into the cell via Na(+)/HCO3 (-) cotransport to counteract intracellular acidification, while the remaining H(+) stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into cardiomyocytes.

Carbonic anhydrases and their interplay with acid/base-coupled membrane transporters

Carbonic anhydrases (CAs) have not only been identified as ubiquitous enzymes catalyzing the fast reversible hydration of carbon dioxide to generate or consume protons and bicarbonate, but also as intra- and extracellular proteins, which facilitate transport function of many acid/base transporting membrane proteins, coined 'transport metabolon'. Functional interaction between CAs and acid/base transporters, such as chloride/bicarbonate exchanger (AE), sodium-bicarbonate cotransporter (NBC) and sodium/hydrogen exchanger (NHE) has been shown to require both catalytic CA activity as well as direct binding of the enzyme to specific sites on the transporter. In contrast, functional interaction between different CA isoforms and lactate-proton-cotransporting monocarboxylate transporters (MCT) has been found to be isoform-specific and independent of CA catalytic activity, but seems to require an intramolecular proton shuttle within the enzyme. In this chapter, we review the various types of interactions between acid/base-coupled membrane carriers and different CA isoforms, as studied in vitro, in intact Xenopus oocytes, and in various mammalian cell types. Furthermore, we discuss recent findings that indicate the significance of these 'transport metabolons' for normal cell functions.

Monocarboxylate transporters in temporal lobe epilepsy: roles of lactate and ketogenic diet

Epilepsy is a serious neurological disorder that affects approximately 1 % of the general population, making it one of the most common disorders of the central nervous system. Furthermore, up to 40 % of all patients with epilepsy cannot control their seizures with current medications. More efficacious treatments for medication refractory epilepsy are therefore needed. A better understanding of the mechanisms that cause this disorder is likely to facilitate the discovery of such treatments. Impairment in cerebral energy metabolism has been proposed as a possible causative factor in the pathogenesis of temporal lobe epilepsy (TLE), which is one of the most common types of medication-refractory epilepsies in adults. In this review, we will discuss some of the current hypotheses regarding the possible causal relationship between brain energy metabolism and TLE. Emphasis will be placed on the role of energy substrates (lactate and ketone bodies) and their transporter molecules, particularly monocarboxylate transporters 1 and 2 (MCT1 and MCT2). We recently reported that the cellular distribution of MCT1 and MCT2 is perturbed in the hippocampus in patients with TLE. The changes may be an adaptive response aimed at keeping high levels of lactate in the epileptic tissue, which may serve to counteract epileptic activity by downregulating cAMP levels through the lactate receptor GPR81, newly discovered in hippocampus. We propose that the perturbation of MCTs may be further involved in the pathophysiology of TLE by influencing brain energy homeostasis, mitochondrial function, GABA-ergic and glutamatergic neurotransmission, and flux of lactate through the brain.

Role of monocarboxylate transporters in drug delivery to the brain

Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. Currently, fourteen members of this transporter family have been identified by sequence homology, of which only the first four members (MCT1- MCT4) have been shown to mediate the proton-linked transport of monocarboxylates. Another transporter family involved in the transport of endogenous monocarboxylates is the sodium coupled MCTs (SMCTs). These act as a symporter and are dependent on a sodium gradient for their functional activity. MCT1 is the predominant transporter among the MCT isoforms and is present in almost all tissues including kidney, intestine, liver, heart, skeletal muscle and brain. The various isoforms differ in terms of their substrate specificity and tissue localization. Due to the expression of these transporters in the kidney, intestine, and brain, they may play an important role in influencing drug disposition. Apart from endogenous short chain monocarboxylates, they also mediate the transport of exogenous drugs such as salicylic acid, valproic acid, and simvastatin acid. The influence of MCTs on drug pharmacokinetics has been extensively studied for γ-hydroxybutyrate (GHB) including distribution of this drug of abuse into the brain and the results will be summarized in this review. The physiological role of these transporters in the brain and their specific cellular localization within the brain will also be discussed. This review will also focus on utilization of MCTs as potential targets for drug delivery into the brain including their role in the treatment of malignant brain tumors.

Spatial expression and functional flexibility of monocarboxylate transporter isoforms in the zebrafish brain

The present study provides in vivo evidence to prove the functional plasticity of monocarboxylate transporters (MCTs) in brains of vertebrates using zebrafish (Danio rerio) as a model. In the mammalian central nervous system (CNS), energy demands are largely met by oxidation of glucose. In recent studies, in addition to glucose, lactate is also considered an energy substrate for the CNS. Astrocytes were demonstrated to play an important role in transporting lactate as metabolic substrate from capillaries to neurons through monocarboxylate transporters (MCTs). The present study was to use zebrafish as an in vivo model to test the hypothesis of whether the various MCT homologs play differential roles in the development and functioning of the CNS. Using RT-PCR and double in situ hybridization coupling with immunocytochemical staining experiments, zebrafish MCTs1-4 were all found to be expressed in brains of embryos, and were further elucidated to be localized in both neurons and astrocytes. Loss-of-functions by morpholino knockdown further provided in vivo evidences to infer that zMCTs1, -2, and -4 may be involved in metabolite transport and functioning in the developing brain. Subsequent rescue experiments with capped mRNAs of specific isoforms further indicated that zMCT2 is an indispensable monocarboxylate-transporting route for CNS development and function in zebrafish. This information is essential for identifying proper candidates of MCT isoforms that are involved in the development and functioning of the CNS.

Role of monocarboxylate transporters in human cancers: state of the art

Monocarboxylate transporters (MCTs) belong to the SLC16 gene family, presently composed by 14 members. MCT1-MCT4 are proton symporters, which mediate the transmembrane transport of pyruvate, lactate and ketone bodies. The role of MCTs in cell homeostasis has been characterized in detail in normal tissues, however, their role in cancer is still far from understood. Most solid tumors are known to rely on glycolysis for energy production and this activity leads to production of important amounts of lactate, which are exported into the extracellular milieu, contributing to the acidic microenvironment. In this context, MCTs will play a dual role in the maintenance of the hyper-glycolytic acid-resistant phenotype of cancer, allowing the maintenance of the high glycolytic rates by performing lactate efflux, and pH regulation by the co-transport of protons. Thus, they constitute attractive targets for cancer therapy, which have been little explored. Here we review the literature on the role of MCTs in solid tumors in different locations, such as colon, central nervous system, breast, lung, gynecologic tract, prostate, stomach, however, there are many conflicting results and in most cases there are no functional studies showing the dependence of the tumors on MCT expression and activity. Additional studies on MCT expression in other tumor types, confirmation of the results already published as well as additional functional studies are needed to deeply understand the role of MCTs in cancer maintenance and aggressiveness.

Effect of exercise on mouse liver and brain bioenergetic infrastructures

To assess the effects of exercise on liver and brain bioenergetic infrastructures, we exposed C57BL/6 mice to 6 weeks of moderate-intensity treadmill exercise. During the training period, fasting blood glucose was lower in exercised mice than in sedentary mice, but serum insulin levels were not reduced. At week 6, trained mice showed a paradoxical decrease in plasma lactate during exercise, which was accompanied by an increase in the liver monocarboxylate transporter 2 protein level (∼30%, P < 0.05). Exercise increased liver peroxisomal proliferator-activated receptor-γ coactivator 1α expression (approximately twofold, P < 0.001), NAD-dependent deacetylase sirtuin-1 protein (∼30%, P < 0.05), p38 protein (∼15%, P < 0.05), cytochrome c oxidase subunit 4 isoform 1 protein (∼50%, P < 0.05) and AMP-activated protein kinase phosphorylation (∼40%, P < 0.05). Despite this, liver mitochondrial DNA copy number (∼30%, P = 0.05), mitochondrial transcription factor A expression (∼15%, P < 0.05), cytochrome c oxidase subunit 2 expression (∼10%, P < 0.05), cAMP-response element binding protein phosphorylation (∼60%, P < 0.05) and brain-derived neurotrophic factor expression (∼40%, P < 0.05) were all reduced, while cytochrome oxidase and citrate synthase activities were unchanged. The only altered brain parameter observed was a reduction in tumour necrosis factor α expression (∼35%, P < 0.05); tumour necrosis factor α expression was unchanged in liver. Our data suggest that lactate produced by exercising muscle modifies the liver bioenergetic infrastructure, and enhanced liver uptake may in turn limit the ability of exercise-generated lactate to modify brain bioenergetics. Also, it appears that, at least in the liver, a dissociated mitochondrial biogenesis, in which some components are strategically enhanced while others are minimized, can occur.

Redistribution of monocarboxylate transporter 2 on the surface of astrocytes in the human epileptogenic hippocampus

Emerging evidence points to monocarboxylates as key players in the pathophysiology of temporal lobe epilepsy (TLE) with hippocampal sclerosis (mesial temporal lobe epilepsy, MTLE). Monocarboxylate transporters (MCTs) 1 and 2, which are abundantly present on brain endothelial cells and perivascular astrocyte endfeet, respectively, facilitate the transport of monocarboxylates and protons across cell membranes. Recently, we reported that the density of MCT1 protein is reduced on endothelial cells and increased on astrocyte plasma membranes in the hippocampal formation in patients with MTLE and in several animal models of the disorder. Because the perivascular astrocyte endfeet comprise an important part of the neurovascular unit, we now assessed the distribution of the MCT2 in hippocampal formations in TLE patients with (MTLE) or without hippocampal sclerosis (non-MTLE). Light microscopic immunohistochemistry revealed significantly less perivascular MCT2 immunoreactivity in the hippocampal formation in MTLE (n = 6) than in non-MTLE (n = 6) patients, and to a lesser degree in non-MTLE than in nonepilepsy patients (n = 4). Immunogold electron microscopy indicated that the loss of MCT2 protein occurred on perivascular astrocyte endfeet. Interestingly, the loss of MCT2 on astrocyte endfeet in MTLE (n = 3) was accompanied by an upregulation of the protein on astrocyte membranes facing synapses in the neuropil, when compared with non-MTLE (n = 3). We propose that the altered distribution of MCT1 and MCT2 in TLE (especially MTLE) limits the flux of monocarboxylates across the blood-brain barrier and enhances the exchange of monocarboxylates within the brain parenchyma.

The monocarboxylate transporter family--role and regulation

Monocarboxylate transporter (MCT) isoforms 1-4 catalyze the proton-linked transport of monocarboxylates such as L-lactate across the plasma membrane, whereas MCT8 and MCT10 are thyroid hormone and aromatic amino acid transporters, respectively. The importance of MCTs is becoming increasingly evident as their extensive physiological and pathological roles are revealed. MCTs 1-4 play essential metabolic roles in most tissues with their distinct properties, expression profile, and subcellular localization matching the particular metabolic needs of a tissue. Important metabolic roles include energy metabolism in the brain, skeletal muscle, heart, tumor cells, and T-lymphocyte activation, gluconeogenesis in the liver and kidney, spermatogenesis, bowel metabolism of short-chain fatty acids, and drug transport. MCT8 is essential for thyroid hormone transport across the blood-brain barrier. Genetic perturbation of MCT function may be involved in disease states such as pancreatic β-cell malfunction (inappropriate MCT1 expression), chronic fatigue syndromes (impairment of muscle MCT function), and psychomotor retardation (MCT8 mutation). MCT expression can be regulated at both the transcriptional and post-transcriptional levels. Of particular importance is the upregulation of muscle MCT1 expression in response to training and MCT4 expression in response to hypoxia. The latter is mediated by hypoxia inducible factor 1α and often observed in tumor cells that rely almost entirely on glycolysis for their energy provision. The recent discovery of potent and specific MCT1 inhibitors that prevent proliferation of T-lymphocytes confirms that MCTs may be promising pharmacological targets including for cancer chemotherapy.

The monocarboxylate transporter family--Structure and functional characterization

Monocarboxylate transporters (MCTs) catalyze the proton-linked transport of monocarboxylates such as L-lactate, pyruvate, and the ketone bodies across the plasma membrane. There are four isoforms, MCTs 1-4, which are known to perform this function in mammals, each with distinct substrate and inhibitor affinities. They are part of the larger SLC16 family of solute carriers, also known as the MCT family, which has 14 members in total, all sharing conserved sequence motifs. The family includes a high-affinity thyroid hormone transporter (MCT8), an aromatic amino acid transporter (T-type amino acid transporter 1/MCT10), and eight orphan members yet to be characterized. MCTs were predicted to have 12 transmembrane helices (TMs) with intracellular C- and N-termini and a large intracellular loop between TMs 6 and 7, and this was confirmed by labeling studies and proteolytic digestion. Site-directed mutagenesis has identified key residues required for catalysis and inhibitor binding and enabled the development of a molecular model of MCT1 in both inward and outward facing conformations. This suggests a likely mechanism for the translocation cycle. Although MCT family members are not themselves glycosylated, MCTs1-4 require association with a glycosylated ancillary protein, either basigin or embigin, for their correct translocation to the plasma membrane. These ancillary proteins have a single transmembrane domain and two to three extracellular immunoglobulin domains. They must remain closely associated with MCTs1-4 to maintain transporter activity. MCT1, MCT3, and MCT4 bind preferentially to basigin and MCT2 to embigin. The choice of binding partner does not affect substrate specificity or kinetics but can influence inhibitor specificity.

Expression of monocarboxylate transporter (MCT) 1 and MCT4 in overloaded mice plantaris muscle

A number of studies have shown that changes in muscle contractile activity regulate the expression of monocarboxylate transporters (MCTs) in the skeletal muscle. The aim of this study was to investigate the effect of functional overload on MCT1 and MCT4 protein expression. Plantaris muscles were functionally overloaded for 15 days by ablation of the synergistic muscles. MCT1 and MCT4 mRNA abundance increased by 160-161% (p < 0.01) and 265-325% (p < 0.05), respectively, after 1-3 days of functional overload. MCT1 and MCT4 protein expression increased by 92 and 61%, respectively, after 12 days of functional overload (p < 0.05). AMP-activated protein kinase (AMPK) phosphorylation status [phospho-AMPK (Thr172)/total AMPK] was significantly elevated after 3-9 days of functional overload. Plasma testosterone concentration was elevated after 12 days of functional overload, while blood lactate concentration was not altered. Thus, the current study demonstrated that heavy mechanical loading induces increase in MCT1 and MCT4 protein expression in the muscles with increase in AMPK phosphorylation status and plasma testosterone concentration.

Transport activity of the high-affinity monocarboxylate transporter MCT2 is enhanced by extracellular carbonic anhydrase IV but not by intracellular carbonic anhydrase II

The ubiquitous enzyme carbonic anhydrase isoform II (CAII) has been shown to enhance transport activity of the proton-coupled monocarboxylate transporters MCT1 and MCT4 in a non-catalytic manner. In this study, we investigated the role of cytosolic CAII and of the extracellular, membrane-bound CA isoform IV (CAIV) on the lactate transport activity of the high-affinity monocarboxylate transporter MCT2, heterologously expressed in Xenopus oocytes. In contrast to MCT1 and MCT4, transport activity of MCT2 was not altered by CAII. However, coexpression of CAIV with MCT2 resulted in a significant increase in MCT2 transport activity when the transporter was coexpressed with its associated ancillary protein GP70 (embigin). The CAIV-mediated augmentation of MCT2 activity was independent of the catalytic activity of the enzyme, as application of the CA-inhibitor ethoxyzolamide or coexpressing the catalytically inactive mutant CAIV-V165Y did not suppress CAIV-mediated augmentation of MCT2 transport activity. Furthermore, exchange of His-88, mediating an intramolecular H(+)-shuttle in CAIV, to alanine resulted only in a slight decrease in CAIV-mediated augmentation of MCT2 activity. The data suggest that extracellular membrane-bound CAIV, but not cytosolic CAII, augments transport activity of MCT2 in a non-catalytic manner, possibly by facilitating a proton pathway other than His-88.

Regulation of oligodendrocyte development and myelination by glucose and lactate

In the gray matter of the brain, astrocytes have been suggested to export lactate (derived from glucose or glycogen) to neurons to power their mitochondria. In the white matter, lactate can support axon function in conditions of energy deprivation, but it is not known whether lactate acts by preserving energy levels in axons or in oligodendrocytes, the myelinating processes of which are damaged rapidly in low energy conditions. Studies of cultured cells suggest that oligodendrocytes are the cell type in the brain that consumes lactate at the highest rate, in part to produce membrane lipids presumably for myelin. Here, we use pH imaging to show that oligodendrocytes in the white matter of the rat cerebellum and corpus callosum take up lactate via monocarboxylate transporters (MCTs), which we identify as MCT1 by confocal immunofluorescence and electron microscopy. Using cultured slices of developing cerebral cortex from mice in which oligodendrocyte lineage cells express GFP (green fluorescent protein) under the control of the Sox10 promoter, we show that a low glucose concentration reduces the number of oligodendrocyte lineage cells and myelination. Myelination is rescued when exogenous l-lactate is supplied. Thus, lactate can support oligodendrocyte development and myelination. In CNS diseases involving energy deprivation at times of myelination or remyelination, such as periventricular leukomalacia leading to cerebral palsy, stroke, and secondary ischemia after spinal cord injury, lactate transporters in oligodendrocytes may play an important role in minimizing the inhibition of myelination that occurs.

The inhibition of monocarboxylate transporter 2 (MCT2) by AR-C155858 is modulated by the associated ancillary protein

In mammalian cells, MCTs (monocarboxylate transporters) require association with an ancillary protein to enable plasma membrane expression of the active transporter. Basigin is the preferred binding partner for MCT1, MCT3 and MCT4, and embigin for MCT2. In rat and rabbit erythrocytes, MCT1 is associated with embigin and basigin respectively, but its sensitivity to inhibition by AR-C155858 was found to be identical. Using RT (reverse transcription)–PCR, we have shown that Xenopus laevis oocytes contain endogenous basigin, but not embigin. Co-expression of exogenous embigin was without effect on either the expression of MCT1 or its inhibition by AR-C155858. In contrast, expression of active MCT2 at the plasma membrane of oocytes was significantly enhanced by co-expression of exogenous embigin. This additional transport activity was insensitive to inhibition by AR-C155858 unlike that by MCT2 expressed with endogenous basigin that was potently inhibited by AR-C155858. Chimaeras and C-terminal truncations of MCT1 and MCT2 were also expressed in oocytes in the presence and absence of exogenous embigin. L-Lactate K_m values for these constructs were determined and revealed that the TM (transmembrane) domains of an MCT, most probably TM7–TM12, but not the C-terminus, are the major determinants of L-lactate affinity, whereas the associated ancillary protein has little or no effect. Inhibitor titrations of lactate transport by these constructs indicated that embigin modulates MCT2 sensitivity to AR-C155858 through interactions with both the intracellular C-terminus and TMs 3 and 6 of MCT2. The C-terminus of MCT2 was found to be essential for its expression with endogenous basigin.

Monocarboxylate transporters (MCT) as lactate carriers in equine muscle and red blood cells

REASONS FOR PERFORMING STUDY

Monocarboxylate transporters (MCT) facilitate the transport of lactate across membranes. In red blood cells (RBC) the transport activity varies interindividually due to differences in the amount of an ancillary protein CD147. Similar variations in muscles could have a great influence on lactate accumulation during exercise.

OBJECTIVES

To study the expression of MCT isoforms and CD147 in the middle gluteal muscle.

METHODS

Venous blood and muscle biopsy samples were taken from 14 Standardbred horses. Lactate transport activity in RBC and the amounts of MCT1, 2, 4 and CD147 were measured.

RESULTS

In muscle MCT1, MCT4 and CD147 were found. Amount of MCT1 was variable and not dependent on age or training. Expression of MCT4 increased with age and correlated positively with CD147. CD147 in muscle correlated with that in RBC. MCT4 in muscle and CD147/MCT1 in RBC were higher in race fit than in moderately trained horses.

CONCLUSIONS

MCT isoform profile in equine muscle is similar to that in man. The correlation between CD147 in muscle and RBC supports the view that lactate transport activity in muscles may vary interindividually as with RBC.

POTENTIAL RELEVANCE

A larger number of horses need to be analysed to confirm the relationship of CD147 in muscle and RBC; and to allow the use the lactate transport activity in RBC as an indicator of the respective activity in muscles.

AR-C155858 is a potent inhibitor of monocarboxylate transporters MCT1 and MCT2 that binds to an intracellular site involving transmembrane helices 7-10

In the present study we characterize the properties of the potent MCT1 (monocarboxylate transporter 1) inhibitor AR-C155858. Inhibitor titrations of L-lactate transport by MCT1 in rat erythrocytes were used to determine the K_i value and number of AR-C155858-binding sites (Et) on MCT1 and the turnover number of the transporter (k_cat). Derived values were 2.3±1.4 nM, 1.29±0.09 nmol per ml of packed cells and 12.2±1.1 s⁻¹ respectively. When expressed in Xenopus laevis oocytes, MCT1 and MCT2 were potently inhibited by AR-C155858, whereas MCT4 was not. Inhibition of MCT1 was shown to be time-dependent, and the compound was also active when microinjected, suggesting that AR-C155858 probably enters the cell before binding to an intracellular site on MCT1. Measurement of the inhibitor sensitivity of several chimaeric transporters combining different domains of MCT1 and MCT4 revealed that the binding site for AR-C155858 is contained within the C-terminal half of MCT1, and involves TM (transmembrane) domains 7–10. This is consistent with previous data identifying Phe³⁶⁰ (in TM10) and Asp³⁰² plus Arg³⁰⁶ (TM8) as key residues in substrate binding and translocation by MCT1. Measurement of the K_m values of the chimaeras for L-lactate and pyruvate demonstrate that both the C- and N-terminal halves of the molecule influence transport kinetics consistent with our proposed molecular model of MCT1 and its translocation mechanism that requires Lys³⁸ in TM1 in addition to Asp³⁰² and Arg³⁰⁶ in TM8 [Wilson, Meredith, Bunnun, Sessions and Halestrap (2009) J. Biol. Chem. 284, 20011–20021].

Histochemical demonstration of monocarboxylate transporters in mouse brown adipose tissue

Proton-coupled monocarboxylate transporters (MCTs) are essential for the transport of lactate, ketone bodies, and other monocarboxylates through the plasma membrane. The present immunohistochemical study aimed to examine the expression of MCTs in the brown adipose tissue (BAT) of mice. An intense immunoreactivity for MCT1 was found in the plasma membrane of brown adipose cells at light and electron microscopic levels but not in white adipose cells. The expression of MCT1 in BAT was confirmed by Western blot and in situ hybridization analyses. In fetuses (E17.5) and neonates, the MCT1 mRNA expression of BAT was abundant and appeared more intense than that in adult animals. These results, together with the intense expression of CD147 (a functional partner of MCTs) and acetyl-CoA carboxylase-2 (a component of fatty acid oxidation) in perinatal periods, suggest the involvement of MCT1 in the uptake of monocarboxylates from the circulation for thermogenesis rather than lipogenesis.

C-myc amplification altered the gene expression of ABC- and SLC-transporters in human breast epithelial cells

The present study aimed to investigate the effect of c-Myc overexpression on the gene expression and functional activity of major drug transporters in breast epithelial cells. In order to reflect c-Myc overexpressed epithelial cells, MCF10A cells (human mammary epithelial cells) stably expressing c-Myc (c-Myc-MCF10A cells) have been established by retroviral infection and then the effect of c-Myc activation on the gene expression and functional activity of 16 influx and efflux transporters was examined in c-Myc-MCF10A cells. In RT-PCR analysis, the quantitative difference in transporters' gene expression profiles was observed after c-Myc activation. Western blot analysis also indicated that the protein expression of some transporters (MCT1, OCT1, MRP1, MRP2 and BCRP) increased by c-Myc overexpression. In parallel to the alteration in gene expression, functional activity studies indicated that the cellular uptake of benzoic acid, MPP+, calcein-AM, H33342, representative substrates of MCT1, OCT1, MRPs and BCRP, respectively, was significantly (p < 0.05) altered by c-Myc overexpression. In conclusion, c-Myc amplification appeared to be coupled with the modulation of gene expression of certain drug transporters in human breast epithelial cells.

The SLC16 monocaboxylate transporter family

1. The monocarboxylate transporter (MCT, SLC16) family comprises 14 members, of which to date only MCT1-4 have been shown to carry monocarboxylates, transporting important metabolic compounds such as lactate, pyruvate and ketone bodies in a proton-coupled manner. The transport of such compounds is fundamental for metabolism, and the tissue locations, properties and regulation of these isoforms is discussed. 2. Of the other members of the MCT family, MCT8 (a thyroid hormone transporter) and TAT1 (an aromatic amino acid transporter) have been characterized more recently, and their physiological roles are reviewed herein. The endogenous substrates and functions of the remaining members of the MCT family await elucidation. 3. The MCT proteins have the typical twelve transmembrane-spanning domain (TMD) topology of membrane transporter proteins, and their structure-function relationship is discussed, especially in relation to the future impact of the single nucleotide polymorphism (SNP) databases and, given their ability to transport pharmacologically relevant compounds, the potential impact for pharmacogenomics.

Distribution of the monocarboxylate transporter MCT2 in human cerebral cortex: an immunohistochemical study

The monocarboxylate transporter MCT2 belongs to a large family of membrane proteins involved in the transport of lactate, pyruvate and ketone bodies. Although its expression in rodent brain has been well documented, the presence of MCT2 in the human brain has been questioned on the basis of low mRNA abundance. In this study, the distribution of the monocarboxylate transporter MCT2 has been investigated in the cortex of normal adult human brain using an immunohistochemical approach. Widespread neuropil staining in all cortical layers was observed by light microscopy. Such a distribution was very similar in three different cortical areas investigated. At the cellular level, the expression of MCT2 could be observed in a large number of neurons, in fibers both in grey and white matter, as well as in some astrocytes, mostly localized in layer I and in the white matter. Double staining experiments combined with confocal microscopy confirmed the neuronal expression but also suggested a preferential postsynaptic localization of synaptic MCT2 expression. A few astrocytes in the grey matter appeared to exhibit MCT2 labelling but at low levels. Electron microscopy revealed strong MCT2 expression at asymmetric synapses in the postsynaptic density and also within the spine head but not in the presynaptic terminal. These data not only demonstrate neuronal MCT2 expression in human, but since a portion of it exhibits a distinct synaptic localization, it further supports a putative role for MCT2 in adjustment of energy supply to levels of activity.

Overview of the proton-coupled MCT (SLC16A) family of transporters: characterization, function and role in the transport of the drug of abuse gamma-hydroxybutyric acid

The transport of monocarboxylates, such as lactate and pyruvate, is mediated by the SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). Fourteen MCT-related genes have been identified in mammals and of these seven MCTs have been functionally characterized. Despite their sequence homology, only MCT1-4 have been demonstrated to be proton-dependent transporters of monocarboxylic acids. MCT6, MCT8 and MCT10 have been demonstrated to transport diuretics, thyroid hormones and aromatic amino acids, respectively. MCT1-4 vary in their regulation, tissue distribution and substrate/inhibitor specificity with MCT1 being the most extensively characterized isoform. Emerging evidence suggests that in addition to endogenous substrates, MCTs are involved in the transport of pharmaceutical agents, including gamma-hydroxybuytrate (GHB), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors (statins), salicylic acid, and bumetanide. MCTs are expressed in a wide range of tissues including the liver, intestine, kidney and brain, and as such they have the potential to impact a number of processes contributing to the disposition of xenobiotic substrates. GHB has been extensively studied as a pharmaceutical substrate of MCTs; the renal clearance of GHB is dose-dependent with saturation of MCT-mediated reabsorption at high doses. Concomitant administration of GHB and L: -lactate to rats results in an approximately two-fold increase in GHB renal clearance suggesting that inhibition of MCT1-mediated reabsorption of GHB may be an effective strategy for increasing renal and total GHB elimination in overdose situations. Further studies are required to more clearly define the role of MCTs on drug disposition and the potential for MCT-mediated detoxification strategies in GHB overdose.

Characterization and regulation of monocarboxylate cotransporters Slc16a7 and Slc16a3 in preimplantation mouse embryos

Concurrent with compaction, preimplantation mouse embryos switch from the high pyruvate consumption that prevailed during cleavage stages to glucose consumption against a constant background of pyruvate uptake. However, zygotes exposed to and subsequently deprived of glucose can form blastocysts by increasing pyruvate uptake. This metabolic switch requires cleavage-stage exposure to glucose and is one aspect of metabolic differentiation that normally occurs in vivo. Monocarboxylates, such as pyruvate and lactate, are transported across membranes via the SLC16 family of H(+)-monocarboxylate cotransporter (MCT) proteins. Thus, the increase in pyruvate uptake in embryos developing without glucose must involve changes in activity and localization of MCT. In mouse embryos, continued expression of Slc16a1 (MCT1) requires glucose supply. Messenger RNA for Slc17a7 (MCT2) and Slc16a3 (MCT4) has been detected in mouse preimplantation embryos; however, protein function, localization, and regulation of expression at the basis of these net pyruvate uptake changes remain unclear. The expression and localization of SLC16A7 and SLC16A3 have therefore been examined to clarify their respective roles in embryos derived from the reproductive tract and cultured under varied conditions. SLC16A3 appears localized to the plasma membrane until the morula stage and also maintains a nuclear distribution throughout preimplantation development. However, continued Slc16a3 mRNA expression is dependent on prior exposure to glucose. SLC16A7 localizes to apical cortical regions with punctate, vesicular expression throughout blastomeres, partially colocalizing in peroxisomes with peroxisomal catalase (CAT). In contrast to SLC16A3 and SLC16A1, SLC16A7 and CAT demonstrate upregulation in the absence of glucose. These striking differences between the two isoforms in expression localization and regulation suggest unique roles for each in monocarboxylate transport and pH regulation during preimplantation development, and implicate peroxisomal SLC16A7 as an important redox regulator in the absence of glucose.

Alternative promoter usage and alternative splicing contribute to mRNA heterogeneity of mouse monocarboxylate transporter 2

Expression patterns of monocarboxylate transporter 2 (MCT2) display mRNA diversity in a tissue-specific fashion. We cloned and characterized multiple mct2 5'-cDNA ends from the mouse and determined the structural organization of the mct2 gene. We found that transcription of this gene was initiated from five independent genomic regions that spanned >80 kb on chromosome 10, resulting in five unique exon 1 variants (exons 1a, 1b, 1c, 1d, and 1e) that were then spliced to the common exon 2. Alternative splicing of four internal exons (exons AS1, AS2, AS3, and exon 3) greatly increased the complexity of mRNA diversity. While exon 1c was relatively commonly used for transcription initiation in various tissues, other exon 1 variants were used in a tissue-specific fashion, especially exons 1b and 1d that were used exclusively for testis-specific expression. Sequence analysis of 5'-flanking regions upstream of exons 1a, 1b, and 1c revealed the presence of numerous potential binding sites for ubiquitous transcription factors in all three regions and for transcription factors implicated in testis-specific or hypoxia-induced gene expression in the 1b region. Transient transfection assays demonstrated that each of the three regions contained a functional promoter and that the in vitro, cell type-specific activities of these promoters were consistent with the tissue-specific expression pattern of the mct2 gene in vivo. These results indicate that tissue-specific expression of the mct2 gene is controlled by multiple alternative promoters and that both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the gene.

Expression of CD147 and monocarboxylate transporters MCT1, MCT2 and MCT4 in porcine small intestine and colon

Lactate, formed mainly in the stomach and small intestines, and short-chain fatty acids (SCFAs) formed in the colon, are ionised and require transporter proteins such as monocarboxylate transporters (MCTs) for absorption. The amounts of MCT1, MCT2, MCT4 and CD147, an ancillary protein for MCT1 and MCT4, were measured by immunoblotting the small intestine and colon of 40 pigs (Landrace, Yorkshire and LandracexYorkshire). MCT1 and MCT4 were found in both small intestine and colon, but MCT2 only in the small intestine. In both small intestine and colon, Yorkshire pigs had more CD147 than Landrace pigs, while no interbreed differences were found in MCT isoforms. Since CD147 is essential for the activity of MCT1 and MCT4, the breed difference suggests that MCT activity is higher in Yorkshire than in Landrace pigs. The absence of MCT2 in the colon suggests that it is mainly a lactate transporter, while MCT1 and MCT4 facilitate the transport of both lactate and SCFA.

Is lactate food for neurons? Comparison of monocarboxylate transporter subtypes in brain and muscle

Intercellular monocarboxylate transport is important, particularly in tissues with high energy demands, such as brain and muscle. In skeletal muscle, it is well established that glycolytic fast twitch muscle fibers produce lactate, which is transported out of the cell through the monocarboxylate transporter (MCT) 4. Lactate is then taken up and oxidized by the oxidative slow twitch muscle fibers, which express MCT1. In the brain it is still questioned whether lactate produced in astrocytes is taken up and oxidized by neurons upon activation. Several studies have reported that astrocytes express MCT4, whereas neurons express MCT2. By comparing the localizations of MCTs in oxidative and glycolytic compartments I here give support to the idea that there is a lactate shuttle in the brain similar to that in muscle. This conclusion is based on studies in rodents using high resolution immunocytochemical methods at the light and electron microscopical levels.

Glucose affects monocarboxylate cotransporter (MCT) 1 expression during mouse preimplantation development

Cleavage-stage embryos have an absolute requirement for pyruvate and lactate, but as the morula compacts, it switches to glucose as the preferred energy source to fuel glycolysis. Substrates such as glucose, amino acids, and lactate are moved into and out of cells by facilitated diffusion. In the case of lactate and pyruvate, this occurs via H+-monocarboxylate cotransporter (MCT) proteins. To clarify the role of MCT in development, transport characteristics for DL-lactate were examined, as were mRNA expression and protein localisation for MCT1 and MCT3, using confocal laser scanning immunofluorescence in freshly collected and cultured embryos. Blastocysts demonstrated significantly higher affinity for DL-lactate than zygotes (Km 20 +/- 10 vs 87 +/- 35 mmol lactate/l; P = 0.03 by linear regression) but was similar for all stages. For embryos derived in vivo and those cultured with glucose, MCT1 mRNA was present throughout preimplantation development, protein immunoreactivity appearing diffuse throughout the cytoplasm with brightest intensity in the outer cortical region of blastomeres. In expanding blastocysts, MCT1 became more prominent in the cytoplasmic cortex of blastomeres, with brightest intensity in the polar trophectoderm. Without glucose, MCT1 mRNA was not expressed, and immunoreactivity dramatically reduced in intensity as morulae died. MCT3 mRNA and immunoreactivity were not detected in early embryos. The differential expression of MCT1 in the presence or absence of glucose demonstrates that it is important in the critical regulation of pH and monocarboxylate transport during preimplantation development, and implies a role for glucose in the control of MCT1, but not MCT3, expression.

Distribution of monocarboxylate transporters MCT1-MCT8 in rat tissues and human skeletal muscle

In the past decade, a family of monocarboxylate transporters (MCTs) have been identified that can potentially transport lactate, pyruvate, ketone bodies, and branched-chain ketoacids. Currently, 14 such MCTs are known. However, many orphan transporters exist that have transport capacities that remain to be determined. In addition, the tissue distribution of many of these MCTs is not well defined. Such a cataloging can, at times, begin to suggest the metabolic role of a particular MCT. Recently, a number of antibodies against selected MCTs (MCT1, -2, -4, and -5 to -8) have become commercially available. Therefore, we examined the protein expression of these MCTs in a large number of rat tissues (heart, skeletal muscle, skin, brain, testes, vas deferens, adipose tissue, liver, kidney, spleen, and pancreas), as well as in human skeletal muscle. Unexpectedly, many tissues coexpressed 4-5 MCTs. In particular, in rat skeletal muscle MCT1, MCT2, MCT4, MCT5, and MCT6 were observed. In human muscle, these same MCTs were present. We also observed a pronounced MCT7 signal in human muscle, whereas a very faint signal occurred for MCT8. In rat heart, which is an important metabolic sink for lactate, we confirmed that MCT1 and -2 were expressed. In addition, MCT6 and -8 were also prominently expressed in this tissue, although it is known that MCT8 does not transport aromatic amino acids or lactate. This catalog of MCTs in skeletal muscle and other tissues has revealed an unexpected complexity of coexpression, which makes it difficult to associate changes in monocarboxylate transport with the expression of a particular MCT. The differences in transport kinetics for lactate and pyruvate are only known for MCT1, -2 and -4. Transport kinetics remain to be established for many other MCTs. In conclusion, this study suggests that in skeletal muscle, as well as other tissues, lactate and pyruvate transport rates may not only involve MCT1 and -4, as other monocarboxylate transporters are also expressed in rat (MCT2, -5, -6) and human skeletal muscle (MCT2, -5, -6, -7).

Monocarboxylate transporter 1 gene expression in the ovine gastrointestinal tract

In this study, we investigated the tissue distribution and expression of monocarboxylate transporter 1 (MCT1) along the gastrointestinal tract of sheep. Western blot analysis suggested the presence of MCT1 as a 43-kDa protein in immunoblots of membranes from the various tissues examined. The results of Western blotting were further confirmed by immunohistochemical studies, which revealed intense immunoreactivity for the MCT1 protein in the forestomach (rumen, reticulum and omasum) and large intestine (caecum, proximal and distal colon). Moderate reactivity, however, was detected in the abomasum, while no immunoreactivity could be seen in any regions of the small intestine examined. Furthermore, MCT1 was expressed at the mRNA level as determined by reverse transcriptase polymerase chain reaction (RT-PCR), which showed a band of the expected size (300 bp) in all tissues examined. From these results we concluded that MCT1 protein is highly expressed and distributed in the stomach and large intestine of sheep suggesting that MCT1 may play a significant role in the transport of short chain fatty acids and their metabolites in the gastrointestinal tract of ruminants.

The role of monocarboxylate transporters in uptake of lactic acid in HeLa cells

This study was aimed to identify the monocarboxylate transporters (MCTs) in HeLa cells and to delineate their role in transportation of L-lactic acid. The functional role of MCTs in lactic acid transport was evaluated at various mucosal pHs (4.5-7.4) or in the presence of various loading doses (0.2-2mM) of lactic acid, MCT substrates (nicotinic acid, n-butyric acid, etc.) and inhibitors (alpha-cyano-4-hydroxycinnamate and para-chloromercuribenzoic acid). The molecular properties of MCTs were characterized using reverse transcription-polymerase chain reaction (RT-PCR). The uptake rate of lactic acid by HeLa cells significantly increased from 0.353+/-0.052 to 1.103+/-0.196 micromol/mg protein as the extra-cellular pH changed from 7.4 to 4.5, indicating that activities of MCT were mediated through H(+)-linked mechanism. The uptake profile of lactic acid followed the saturable process with the K(m) value of 0.53 mM. The uptake rate of lactic acid is concentration dependent and is reduced in the presence of MCT inhibitors. MCT isoforms 1, 5 and 6 in HeLa cells were identified by RT-PCR. HeLa cell line can be used as an effective screening tool for intravaginally administered drugs targeted toward MCT.