Cellular and Subcellular Expression of the Monocarboxylate Transporter MCT1 in Rat Heart

Lithium: A Promising Anticancer Agent

Lithium is a therapeutic cation used to treat bipolar disorders but also has some important features as an anti-cancer agent. In this review, we provide a general overview of lithium, from its transport into cells, to its innovative administration forms, and based on genomic, transcriptomic, and proteomic data. Lithium formulations such as lithium acetoacetate (LiAcAc), lithium chloride (LiCl), lithium citrate (Li3C6H5O7), and lithium carbonate (Li2CO3) induce apoptosis, autophagy, and inhibition of tumor growth and also participate in the regulation of tumor proliferation, tumor invasion, and metastasis and cell cycle arrest. Moreover, lithium is synergistic with standard cancer therapies, enhancing their anti-tumor effects. In addition, lithium has a neuroprotective role in cancer patients, by improving their quality of life. Interestingly, nano-sized lithium enhances its anti-tumor activities and protects vital organs from the damage caused by lipid peroxidation during tumor development. However, these potential therapeutic activities of lithium depend on various factors, such as the nature and aggressiveness of the tumor, the type of lithium salt, and its form of administration and dosage. Since lithium has been used to treat bipolar disorder, the current study provides an overview of its role in medicine and how this has changed. This review also highlights the importance of this repurposed drug, which appears to have therapeutic cancer potential, and underlines its molecular mechanisms.

Caveolin3 Stabilizes McT1-Mediated Lactate/Proton Transport in Cardiomyocytes

[Figure: see text].

Wheat germ agglutinin-conjugated fluorescent pH sensors for visualizing proton fluxes

Small-molecule fluorescent wheat germ agglutinin (WGA) conjugates are routinely used to demarcate mammalian plasma membranes, because they bind to the cell’s glycocalyx. Here, we describe the derivatization of WGA with a pH-sensitive rhodamine fluorophore (pHRho; pKa = 7) to detect proton channel fluxes and extracellular proton accumulation and depletion from primary cells. We found that WGA-pHRho labeling was uniform and did not appreciably alter the voltage gating of glycosylated ion channels, and the extracellular changes in pH correlated with proton channel activity. Using single-plane illumination techniques, WGA-pHRho was used to detect spatiotemporal differences in proton accumulation and depletion over the extracellular surface of cardiomyocytes, astrocytes, and neurons. Because WGA can be derivatized with any small-molecule fluorescent ion sensor, WGA conjugates should prove useful to visualize most electrogenic and nonelectrogenic events on the extracellular side of the plasma membrane.

Pivotal role of membrane substrate transporters on the metabolic alterations in the pressure-overloaded heart

Cardiac pressure overload (PO), such as caused by aortic stenosis and systemic hypertension, commonly results in cardiac hypertrophy and may lead to the development of heart failure. PO-induced heart failure is among the leading causes of death worldwide, but its pathological origin remains poorly understood. Metabolic alterations are proposed to be an important contributor to PO-induced cardiac hypertrophy and failure. While the healthy adult heart mainly uses long-chain fatty acids (FAs) and glucose as substrates for energy metabolism and to a lesser extent alternative substrates, i.e. lactate, ketone bodies, and amino acids (AAs), the pressure-overloaded heart is characterized by a shift in energy metabolism towards a greater reliance on glycolysis and alternative substrates. A key-governing kinetic step of both FA and glucose fluxes is at the level of their substrate-specific membrane transporters. The relative presence of these transporters in the sarcolemma determines the cardiac substrate preference. Whether the cardiac utilization of alternative substrates is also governed by membrane transporters is not yet known. In this review, we discuss current insight into the role of membrane substrate transporters in the metabolic alterations occurring in the pressure-overloaded heart. Given the increasing evidence of a role for alternative substrates in these metabolic alterations, there is an urgent need to disclose the key-governing kinetic steps in their utilization as well. Taken together, membrane substrate transporters emerge as novel targets for metabolic interventions to prevent or treat PO-induced heart failure.

Intestinal OCTN2- and MCT1-targeted drug delivery to improve oral bioavailability

Various drug transporters are widely expressed throughout the intestine and play important roles in absorbing nutrients and drugs, thus providing high quality targets for the design of prodrugs or nanoparticles to facilitate oral drug delivery. In particular, intestinal carnitine/organic cation transporter 2 (OCTN2) and mono-carboxylate transporter protein 1 (MCT1) possess high transport capacities and complementary distributions. Therefore, we outline recent developments in transporter-targeted oral drug delivery with regard to the OCTN2 and MCT1 proteins in this review. First, basic information of the two transporters is reviewed, including their topological structures, characteristics and functions, expression and key features of their substrates. Furthermore, progress in transporter-targeting prodrugs and nanoparticles to increase oral drug delivery is discussed, including improvements in the oral absorption of anti-inflammatory drugs, antiepileptic drugs and anticancer drugs. Finally, the potential of a dual transporter-targeting strategy is discussed.

The Role of Sodium Hydrogen Exchanger 1 in Dysregulation of Proton Dynamics and Reprogramming of Cancer Metabolism as a Sequela

Cancer cells have an unusual regulation of hydrogen ion dynamics that are driven by poor vascularity perfusion, regional hypoxia, and increased glycolysis. All these forces synergize/orchestrate together to create extracellular acidity and intracellular alkalinity. Precisely, they lead to extracellular pH (pH_e) values as low as 6.2 and intracellular pH values as high as 8. This unique pH gradient (∆pH_i to ∆pH_e) across the cell membrane increases as the tumor progresses, and is markedly displaced from the electrochemical equilibrium of protons. These unusual pH dynamics influence cancer cell biology, including proliferation, metastasis, and metabolic adaptation. Warburg metabolism with increased glycolysis, even in the presence of Oxygen with the subsequent reduction in Krebs’ cycle, is a common feature of most cancers. This metabolic reprogramming confers evolutionary advantages to cancer cells by enhancing their resistance to hypoxia, to chemotherapy or radiotherapy, allowing rapid production of biological building blocks that support cellular proliferation, and shielding against damaging mitochondrial free radicals. In this article, we highlight the interconnected roles of dysregulated pH dynamics in cancer initiation, progression, adaptation, and in determining the programming and re-programming of tumor cell metabolism.

Monocarboxylate transporters in the brain and in cancer

Monocarboxylate transporters (MCTs) constitute a family of 14 members among which MCT1-4 facilitate the passive transport of monocarboxylates such as lactate, pyruvate and ketone bodies together with protons across cell membranes. Their anchorage and activity at the plasma membrane requires interaction with chaperon protein such as basigin/CD147 and embigin/gp70. MCT1-4 are expressed in different tissues where they play important roles in physiological and pathological processes. This review focuses on the brain and on cancer. In the brain, MCTs control the delivery of lactate, produced by astrocytes, to neurons, where it is used as an oxidative fuel. Consequently, MCT dysfunctions are associated with pathologies of the central nervous system encompassing neurodegeneration and cognitive defects, epilepsy and metabolic disorders. In tumors, MCTs control the exchange of lactate and other monocarboxylates between glycolytic and oxidative cancer cells, between stromal and cancer cells and between glycolytic cells and endothelial cells. Lactate is not only a metabolic waste for glycolytic cells and a metabolic fuel for oxidative cells, but it also behaves as a signaling agent that promotes angiogenesis and as an immunosuppressive metabolite. Because MCTs gate the activities of lactate, drugs targeting these transporters have been developed that could constitute new anticancer treatments. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Tumor metabolism, cancer cell transporters, and microenvironmental resistance

Cancer cells reprogram their metabolic machineries to enter into permanent glycolytic pathways. The full reason for such reprogramming takes place is unclear. However, this metabolic switch is not made in vain for the lactate that is generated and exported outside cells is reused by other cells. This results in the generation of a pH gradient between the low extracellular pH that is acidic (pHe) and the higher cytosolic alkaline or near neutral pH (pHi) environments that are tightly regulated by the overexpression of several pumps and ion channels (e.g. NHE-1, MCT-1, V-ATPase, CA9, and CA12). The generation of this unique pH gradient serves as a determining factor in defining "tumor fitness". Tumor fitness is the capacity of the tumor to invade and metastasize due to its ability to reduce the efficiency of the immune system and confer resistance to chemotherapy. In this article, we highlight the importance of tumor microenvironment in mediating the failure of chemotherapeutic agents.

Negligible direct lactate oxidation in subsarcolemmal and intermyofibrillar mitochondria obtained from red and white rat skeletal muscle

We examined the controversial notion of whether lactate is directly oxidized by subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria obtained from red and white rat skeletal muscle. Respiratory control ratios were normal in SS and IMF mitochondria. At all concentrations (0.18-10 mm), and in all mitochondria, pyruvate oxidation greatly exceeded lactate oxidation, by 31- to 186-fold. Pyruvate and lactate oxidation were inhibited by alpha-cyano-4-hydroxycinnamate, while lactate oxidation was inhibited by oxamate. Excess pyruvate (10 mm) inhibited the oxidation of palmitate (1.8 mm) as well as lactate (1.8 mm). In contrast, excess lactate (10 mm) failed to inhibit the oxidation of either palmitate (1.8 mm) or pyruvate (1.8 mm). The cell-permeant adenosine analogue, AICAR, increased pyruvate oxidation; in contrast, lactate oxidation was not altered. The monocarboxylate transporters MCT1 and 4 were present on SS mitochondria, but not on IMF mitochondria, whereas, MCT2, a high-affinity pyruvate transporter, was present in both SS and IMF mitochondria. The lactate dehydrogenase (LDH) activity associated with SS and IMF mitochondria was 200- to 240-fold lower than in whole muscle. Addition of LDH increased the rate of lactate oxidation, but not pyruvate oxidation, in a dose-dependent manner, such that lactate oxidation approached the rates of pyruvate oxidation. Collectively, these studies indicate that direct mitochondrial oxidation of lactate (i.e. an intracellular lactate shuttle) does not occur within the matrix in either IMF or SS mitochondria obtained from red or white rat skeletal muscle, because of the very limited quantity of LDH within mitochondria.

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.

Expression of membrane-bound carbonic anhydrases IV, IX, and XIV in the mouse heart

Expression of membrane-bound carbonic anhydrases (CAs) of CA IV, CA IX, CA XII, and CA XIV has been investigated in the mouse heart. Western blots using microsomal membranes of wild-type hearts demonstrate a 39-, 43-, and 54-kDa band representing CA IV, CA IX, and CA XIV, respectively, but CA XII could not be detected. Expression of CA IX in the CA IV/CA XIV knockout animals was further confirmed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cardiac cells were immunostained using anti-CA/FITC and anti-alpha-actinin/TRITC, as well as anti-CA/FITC and anti-SERCA2/TRITC. Subcellular CA localization was investigated by confocal laser scanning microscopy. CA localization in the sarcolemmal (SL) membrane was examined by double immunostaining using anti-CA/FITC and anti-MCT-1/TRITC. CAs showed a distinct distribution pattern in the sarcoplasmic reticulum (SR) membrane. CA XIV is predominantly localized in the longitudinal SR, whereas CA IX is mainly expressed in the terminal SR/t-tubular region. CA IV is present in both SR regions, whereas CA XII is not found in the SR. In the SL membrane, only CA IV and CA XIV are present. We conclude that CA IV and CA XIV are associated with the SR as well as with the SL membrane, CA IX is located in the terminal SR/t-tubular region, and CA XII is not present in the mouse heart. Therefore, the unique subcellular localization of CA IX and CA XIV in cardiac myocytes suggests different functions of both enzymes in excitation-contraction coupling.

Cross-reinnervation changes the expression patterns of the monocarboxylate transporters 1 and 4: An experimental study in slow and fast rat skeletal muscle

The monocarboxylate transporters 1 and 4 are expressed in brain as well as in skeletal muscle and play important roles in the energy metabolism of both tissues. In brain, monocarboxylate transporter 1 occurs in astrocytes, ependymocytes, and endothelial cells while monocarboxylate transporter 4 appears to be restricted to astrocytes. In muscle, monocarboxylate transporter 1 is enriched in oxidative muscle fibers whereas monocarboxylate transporter 4 is expressed in all fibers, with the lowest levels in oxidative fiber types. The mechanisms regulating monocarboxylate transporter 1 and monocarboxylate transporter 4 expression are not known. We hypothesized that the expression of these transporters would be sensitive to long term changes in metabolic activity level. This hypothesis can be tested in rat skeletal muscle, where permanent changes in activity level can be induced by cross-reinnervation. We transplanted motor axons originally innervating the fast-twitch extensor digitorum longus muscle to the slow-twitch soleus muscle and vice versa. Four months later, microscopic analysis revealed transformation of muscle fiber types in the cross-reinnervated muscles. Western blot analysis showed that monocarboxylate transporter 1 was increased by 140% in extensor digitorum longus muscle and decreased by 30% in soleus muscle after cross-reinnervation. In contrast, cross-reinnervation induced a 62% decrease of monocarboxylate transporter 4 in extensor digitorum longus muscle and a 1300% increase in soleus muscle. Our findings show that cross-reinnervation causes pronounced changes in the expression levels of monocarboxylate transporter 1 and monocarboxylate transporter 4, probably as a direct consequence of the new pattern of nerve impulses. The data indicate that the mode of innervation dictates the expression of monocarboxylate transporter proteins in the target cells and that the change in monocarboxylate transporter isoform profile is an integral part of the muscle fiber transformation that occurs after cross-reinnervation. Our findings support the hypothesis that the expression of monocarboxylate transporter 1 and monocarboxylate transporter 4 in excitable tissues is regulated by activity.

Recent advances in the understanding of the role of nitric oxide in cardiovascular homeostasis

Nitric oxide synthases (NOS) are the enzymes responsible for nitric oxide (NO) generation. To date, 3 distinct NOS isoforms have been identified: neuronal NOS (NOS1), inducible NOS (NOS2), and endothelial NOS (NOS3). Biochemically, NOS consists of a flavin-containing reductase domain, a heme-containing oxygenase domain, and regulatory sites. NOS catalyse an overall 5-electron oxidation of one Nomega-atom of the guanidino group of L-arginine to form NO and L-citrulline. NO exerts a plethora of biological effects in the cardiovascular system. The basal formation of NO in mitochondria by a mitochondrial NOS seems to be one of the main regulators of cellular respiration, mitochondrial transmembrane potential, and transmembrane proton gradient. This review focuses on recent advances in the understanding of the role of enzyme and enzyme-independent NO formation, regulation of NO bioactivity, new aspects of NO on cardiac function and morphology, and the clinical impact and perspectives of these recent advances in our knowledge on NO-related pathways.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

The subcellular compartmentation of fatty acid transporters is regulated differently by insulin and by AICAR

Cellular fatty acid uptake is facilitated by a number of fatty acid transporters, FAT/CD36, FABPpm and FATP1. It had been presumed that FABPpm, was confined to the plasma membrane and was not regulated. Here, we demonstrate for the first time that FABPpm and FATP1 are also present in intracellular depots in cardiac myocytes. While we confirmed previous work that insulin and AICAR each induced the translocation of FAT/CD36 from an intracellular depot to the PM, only AICAR, but not insulin, induced the translocation of FABPpm. Moreover, neither insulin nor AICAR induced the translocation of FATP1. Importantly, the increased plasmalemmal content of these LCFA transporters was associated with a concomitant increase in the initial rate of palmitate uptake into cardiac myocytes. Specifically, the insulin-stimulated increase in the rate of palmitate uptake (+60%) paralleled the insulin-stimulated increase in plasmalemmal FAT/CD36 (+34%). Similarly, the greater AICAR-stimulated increase in the rate of palmitate uptake (+90%) paralleled the AICAR-induced increase in both plasmalemmal proteins (FAT/CD36 (+40%)+FABPpm (+36%)). Inhibition of palmitate uptake with the specific FAT/CD36 inhibitor SSO indicated that FABPpm interacts with FAT/CD36 at the plasma membrane to facilitate the uptake of palmitate. In conclusion, (1) there appears to be tissue-specific sensitivity to insulin-induced FATP1 translocation, as it has been shown elsewhere that insulin induces FATP1 translocation in 3T3-L1 adipocytes, and (2) clearly, the subcellular distribution of FABPpm, as well as FAT/CD36, is acutely regulated in cardiac myocytes, although FABPpm and FAT/CD36 do not necessarily respond identically to the same stimuli.

Relationship between skeletal muscle MCT1 and accumulated exercise during voluntary wheel running

We examined whether the quantity of exercise performed influences the expression of monocarboxylate transporter (MCT) 1 and MCT4 in mouse skeletal muscles (plantaris, tibialis anterior, soleus) and heart. Wheel running exercise (1, 3, and 6 wk) was used, which results in marked variations in self-selected running activity. Differences in muscle MCT1 and MCT4 among animals, before the initiation of running, were not related to the quantity of exercise performed on the first day of wheel running. No changes in MCT4 were observed over the course of the study (P > 0.05). After 6 wk of running, were there significant increases in heart (50%; P < 0.05) and muscle MCT1 (31-60%; P < 0.05) but not after 1 and 3 wk (P > 0.05). Because skeletal muscle MCT1 and running distances varied considerably, we examined the relationship between these two parameters. Within the first week of training, MCT1 was negatively correlated with the accumulated running distance (r = -0.70, P < 0.05). On further analysis, it appears that, in the first week, excessive running (>20 km/wk) represses MCT1 (-16.1%; P < 0.05), whereas more modest amounts of running (<20 km/wk) increase MCT1 (+37%; P < 0.05). After 3 wk of running, a positive relationship was observed between MCT1 and running distance (r = +0.76), although there is a threshold that must be exceeded before an increase over the control animals occurs. Finally, in week 6, when MCT1 was increased in the tibialis anterior and plantaris muscles, there were no correlations with the accumulated running distances. These studies have shown that mild exercise training fails to increase MCT4 and that changes in MCT1 are complex, depending not only the accumulated exercise but also on the stage of training.

Ischemia-induced changes in monocarboxylate transporter 1 reactive cells in rat hippocampus

This study aims to demonstrate the responses of monocarboxylate transporter 1 (MCT1) immunoreactive cells to transient global ischemia in rat hippocampus using confocal and electron microscopy. The MCT1 staining in CA1 pyramidal cells of the sham-operated controls appeared evenly distributed. Most of the MCT1 immunoreactive products were associated with the cell surface; however, some intracellular reaction products are also found. This pattern of stain was not altered in the first three days after an ischemic episode. As the neuronal demise progressed, the MCT1 immunoreactive cells became patchy in the 21-day post-ischemic rats. Besides the neuronal labeling, MCT1 immunoreactivity was found in astroglia, in endothelial cells and in the adjacent ependymal lining. The latter exhibited intense labeling both in the acute and long-term surviving rats. These data suggest that MCT1 plays a role in the initial and long-term neuronal survival in the hippocampus.

Tissue-specific and isoform-specific changes in MCT1 and MCT4 in heart and soleus muscle during a 1-yr period

We examined the postnatal changes (days 10, 36, 84, 160, 365) of monocarboxylate transporters (MCT)1 and MCT4 in rat heart and soleus muscle. In the heart, MCT1 protein and mRNA remained unaltered from day 10 until 1 yr of age. Both MCT4 protein and mRNA in the heart were detected at 10 days of age, but the MCT4 protein and transcript were not detected thereafter. In the soleus muscle, MCT1 protein (+38%) and mRNA (+136%) increased during the first 84 days and remained stable until 1 yr of age. In contrast, soleus MCT4 protein decreased by 90% over the course of 1 yr, with the most rapid decrease (-60%) occurring by day 84 (P < 0.05). At the same time, MCT4 mRNA was increased by 74% from days 10 to 84 (P < 0.05), remaining stable thereafter. In conclusion, developmental changes in MCT transport proteins are tissue specific and isoform specific. Furthermore, it appears that MCT1 expression in the heart and MCT1 and MCT4 expression in the soleus are regulated by pretranslational processes, whereas posttranscriptional processes regulate MCT4 expression in the soleus muscle.

Evidence of separate pathways for lactate uptake and release by the perfused rat heart

The simultaneous release and uptake of lactate by the heart has been observed both in vivo and ex vivo; however, the pathways underlying these observations have not been satisfactorily explained. Consequently, the purpose of this study was to test the hypothesis that hearts release lactate from glycolysis while simultaneously taking up exogenous lactate. Therefore, we determined the effects of fatty acids and diabetes on the regulation of lactate uptake and release. Hearts from control and 1-wk diabetic animals were perfused with 5 mM glucose, 0.5 mM [3-(13)C]lactate, and 0, 0.1, 0.32, or 1.0 mM palmitate. Parameters measured include perfusate lactate concentrations, fractional enrichment, and coronary flow rates, which enabled the simultaneous, but independent, measurements of the rates of 1) uptake of exogenous [(13)C]lactate and 2) efflux of unlabeled lactate from metabolism of glucose. Although the rates of lactate uptake and efflux were both similarly inhibited by the addition of palmitate, (i.e., the ratio of lactate uptake to efflux remained constant), the ratio of lactate uptake to efflux was significantly higher in the controls compared with the diabetic group (1.00 +/- 0.14 vs. 0.50 +/- 0.07, P < 0.002). These data, combined with heterogeneous (13)C enrichment of tissue lactate, pyruvate, and alanine, suggest that glycolytically derived lactate production and oxidation of exogenous lactate operate as functionally separate metabolic pathways. These results are consistent with the concept of an intracellular lactate shuttle.

Upregulation of the cardiac monocarboxylate transporter MCT1 in a rat model of congestive heart failure

BACKGROUND

Cardiac metabolism becomes more dependent on carbohydrates in congestive heart failure (CHF), and lactate may be used as an important respiratory substrate. Monocarboxylate transporter 1 (MCT1) promotes cotransport of lactate and protons into and out of heart cells and conceivably flux of lactate between cells, because it is abundantly present in the intercalated disk.

METHODS AND RESULTS

Six weeks after induction of myocardial infarction (MI) in Wistar rats, left ventricular end-diastolic pressures were >15 mm Hg, signifying CHF. MCT1 and connexin43 protein levels in CHF were 260% and 20%, respectively, of those in sham-operated animals (Sham), and the corresponding mRNA signals were 181% and not significantly changed, respectively. Confocal laserscan immunohistochemistry and quantitative immunogold cytochemistry showed that MCT1 density was much higher in CHF than in Sham both at the surface membrane and in the intercalated disk. In CHF, a novel intracellular pool of MCT1 appeared to be associated with cisternae, some close to the T tubules. In contrast, connexin43 particles, seen exclusively at gap junctions, were substantially fewer. Maximum lactate uptake was 107+/-15 mmol. L(-1). min(-1) in CHF and 42+/-6 mmol. L(-1). min(-1) in Sham cells (P<0.05). The K(m) values were between 7 and 9 mmol/L (P=NS).

CONCLUSIONS

In cardiomyocytes from CHF rats, (1) the amount of functional MCT1 in the sarcolemma, including in the intercalated disk, is increased several-fold; (2) a new intracellular pool of MCT1 appears; (3) another disk protein, connexin43, is much reduced; and (4) increased reliance on lactate and other monocarboxylates (eg, pyruvate) could provide tight metabolic control of high-energy phosphates.

Isoform-specific regulation of the lactate transporters MCT1 and MCT4 by contractile activity

We examined the isoform-specific regulation of monocarboxylate transporter (MCT)1 and MCT4 expression by contractile activity in red and white tibialis anterior muscles. After 1 and 3 wk of chronic muscle stimulation (24 h/day), MCT1 protein expression was increased in the red muscles (+78%, P < 0.05). In the white muscles, MCT1 was increased after 1 wk (+191%) and then was decreased after 3 wk. In the red muscle, MCT1 mRNA accumulation was increased only after 3 wk (+21%; P < 0.05). In the white muscle, MCT1 mRNA was increased after 1 wk (+30%; P < 0.05) and 3 wk (+15%; P < 0.05). MCT4 protein was not altered in either the red or white muscles after 1 or 3 wk. MCT4 mRNA was transiently lowered (approximately 15%) in both muscles in the 1st wk, but MCT4 mRNA levels were back to control levels after 3 wk. In conclusion, chronic contractile activity induces the expression of MCT1 but not MCT4. This increase in MCT1 alone was sufficient to increase lactate uptake from the circulation.

Dystrophin associates with caveolae of rat cardiac myocytes: relationship to dystroglycan

The possibility of an interaction between the cytoskeletal protein dystrophin and cell surface caveolae in the mammalian myocardium was investigated by several techniques. Caveolin (cav)-3-enriched, detergent-insoluble membranes isolated from purified ventricular sarcolemma by density-gradient fractionation were found to contain dystrophin and dystroglycan. Further purification of cav-3-containing membranes by immunoprecipitation using anti-cav-3-coated magnetic beads yielded dystrophin but not always dystroglycan. Electron microscopic analysis of precipitated material revealed caveola-sized vesicular profiles that could be double-labeled with anti-dystrophin and anti-cav-3 antibodies. In contrast, immunoprecipitation of membranes with anti-dystrophin-coated beads yielded both cav-3 and dystroglycan. Electron microscopic analysis of this material showed heterogeneous membrane profiles, some of which could be decorated with anti-cav-3 antibodies. To confirm that dystrophin and cav-3 were closely associated in cardiac myocytes, we verified that dystrophin was also present in immunoprecipitated cav-3-containing membranes from detergent extracts, as well as in sonicated extracts of purified ventricular myocytes. Confocal immunofluorescence microscopy of ventricular and atrial cardiac myocytes showed that the cellular distributions of cav-3 and dystrophin partially overlapped. Immuno-electron micrographs of thin sections of rat atrial myocytes revealed a fraction of dystrophin molecules that are in apparently close apposition to caveolae. These results suggest that a subpopulation of dystrophin molecules interacts with cardiac myocyte caveolae in vivo and that some of the dystrophin is engaged in linking cav-3 with the dystroglycan complex.

Vacuole formation in fatigued skeletal muscle fibres from frog and mouse: effects of extracellular lactate

Isolated, living muscle fibres from either Xenopus or mouse were observed in a confocal microscope and t-tubules were visualized with sulforhodamine B. Observations were made before and after fatiguing stimulation. In addition, experiments were performed on fibres observed in an ordinary light microscope with dark-field illumination. In Xenopus fibres, recovering after fatigue, t-tubules started to show dilatations 2-5 min post-fatigue. These swellings increased in size over the next 10-20 min to form vacuoles. After 2-3 h of recovery the appearance of the fibres was again normal and force production, which had been markedly depressed 10-40 min post-fatigue, was close to control. Vacuoles were not observed in mouse fibres, fatigued with the same protocol and allowed to recover. In Xenopus fibres, fatigued in normal Ringer solution and allowed to recover in Ringer solution with 30-50 mM L-lactate substituting for chloride (lactate-Ringer), the number and size of vacuoles were markedly reduced. Also, force recovery was significantly faster. Replacement of chloride by methyl sulphate or glucuronate had no effect on vacuolation. Resting Xenopus fibres exposed to 50 mM lactate-Ringer and transferred to normal Ringer solution displayed vacuoles within 5-10 min, but to a smaller extent than after fatigue. Vacuolation was not associated with marked force reduction. Mouse fibres, fatigued in 50 mM lactate-Tyrode (L-lactate substituting for chloride in Tyrode solution) and recovering in normal Tyrode solution, displayed vacuoles for a limited period post-fatigue. Vacuolation had no effect on force production. The results are consistent with the view that lactate, formed during fatigue, is transported into the t-tubules where it attracts water and causes t-tubule swelling and vacuolation. This vacuolation may be counteracted in vivo due to a gradual extracellular accumulation of lactate during fatigue.

Abundance and subcellular distribution of MCT1 and MCT4 in heart and fast-twitch skeletal muscles

The expression of two monocarboxylate transporters (MCTs) was examined in muscle and heart. MCT1 and MCT4 proteins are coexpressed in rat skeletal muscles, but only MCT1 is expressed in rat hearts. Among six rat fast-twitch muscles (red and white gastrocnemius, plantaris, extensor digitorum longus, red and white tibialis anterior) there was an inverse relationship between MCT1 and MCT4 (r = -0.94). MCT1 protein was correlated with MCT1 mRNA (r = 0.94). There was no relationship between MCT4 mRNA and MCT4 protein. MCT1 (r = -0.97) and MCT4 (r = 0.88) protein contents were correlated with percent fast-twitch glycolytic fiber. When normalized for their mRNAs, MCT1 but not MCT4 was still correlated with the percent fast-twitch glycolytic fiber composition of rat muscles (r = -0.98). MCT1 and MCT4 were also measured in plasma membranes (PM), triads (TR), T tubules (TT), sarcoplasmic reticulum (SR), and intracellular membranes (IM). There was an intracellular pool of MCT4 but not of MCT1. The MCT1 subcellular distribution was as follows: PM (100%) > TR (31.6%) > SR (15%) = TT (14%) > IM (1.7%). The MCT4 subcellular distribution was considerably different [PM (100%) > TR (66.5%) > TT (36%) = SR (43%) > IM (24%)]. These studies have shown that 1) the mechanisms regulating the expression of MCT1 (transcriptional and posttranscriptional) and MCT4 (posttranscriptional) are different and 2) differences in MCT1 and MCT4 expression among muscles, as well as in their subcellular locations, suggest that they may have different roles in muscle.

Lactate transporters (MCT proteins) in heart and skeletal muscles

Lactate traverses the cell membranes of many tissues, including the heart and skeletal muscle via a facilitated monocarboxylate transport system that functions as a proton symport and is stereoselective for L-lactate. In the past few years, seven monocarboxylate transporters have been cloned. Monocarboxylate transporters are ubiquitously distributed among many tissues, and the transcripts of several monocarboxylate transporters are present within many of the same tissues. This complicates the identification of their metabolic function. There is also evidence that that there is some species specificity, with differences in MCT tissue distributions in hamsters, rats, and humans. MCT1 and MCT3-M/MCT4 are present in rat and human muscles, and MCT1 expression is highly correlated with the oxidative capacity of skeletal muscles and with their capacity to take up lactate from the circulation. MCT1 is also present in heart and is located on the plasma membrane (in subdomains), T-tubules, and in caveolae. With training, MCT1 is increased in rat and human muscle, and in rat hearts, resulting in an increased uptake of lactate from the buffers perfused through these tissues and an increase in lactate efflux out of purified vesicles. In humans, the training-induced increases in MCT1 are associated with an increased lactate efflux out of muscle. MCT3-M/MCT4 is not correlated with the muscles' oxidative capacities but is equally abundant in Type IIa and IIb muscles, whereas it is markedly lower in slow-twitch (Type I) muscles. Clearly, we are at the threshold of a new era in understanding the regulation of lactate movement into and out of skeletal muscle and cardiac cells.

Intra- and extra-cellular lactate shuttles

The "lactate shuttle hypothesis" holds that lactate plays a key role in the distribution of carbohydrate potential energy that occurs among various tissue and cellular compartments such as between: cytosol and mitochondria, muscle and blood, blood and muscle, active and inactive muscles, white and red muscles, blood and heart, arterial blood and liver, liver and other tissues such as exercising muscle, intestine and portal blood, portal blood and liver, zones of the liver, and skin and blood. Studies on resting and exercising humans indicate that most lactate (75-80%) is disposed of through oxidation, with much of the remainder converted to glucose and glycogen. Lactate transport across cellular membranes occurs by means of facilitated exchange along pH and concentration gradients involving a family of lactate transport proteins, now called monocarboxylate transporters (MCTs). Current evidence is that muscle and other cell membrane lactate transporters are abundant with characteristics of high Km and Vmax. There appears to be long-term plasticity in the number of cell membrane transporters, but short-term regulation by allosteric modulation or phosphorylation is not known. In addition to cell membranes, mitochondria also contain monocarboxylate transporters (mMCT) and lactic dehydrogenase (mLDH). Therefore, mitochondrial monocarboxylate uptake and oxidation, rather than translocation of transporters to the cell surfaces, probably regulate lactate flux in vivo. Accordingly, the "lactate shuttle" hypothesis has been modified to include a new, intracellular component involving cytosolic to mitochondrial exchange. The intracellular lactate shuttle emphasizes the role of mitochondrial redox in the oxidation and disposal of lactate during exercise and other conditions.

Cardiac and skeletal muscle mitochondria have a monocarboxylate transporter MCT1

To evaluate the potential role of monocarboxylate transporter-1 (MCT1) in tissue lactate oxidation, isolated rat subsarcolemmal and interfibrillar cardiac and skeletal muscle mitochondria were probed with an antibody to MCT1. Western blots indicated presence of MCT1 in sarcolemmal membranes and in subsarcolemmal and interfibrillar mitochondria. Minimal cross-contamination of mitochondria by cell membrane fragments was verified by probing for the sarcolemmal protein GLUT-1. In agreement, immunolabeling and electron microscopy showed mitochondrial MCT1 in situ. Along with lactic dehydrogenase, the presence of MCT1 in striated muscle mitochondria permits mitochondrial lactate oxidation and facilitates function of the "intracellular lactate shuttle."

Blood-brain barrier permeability to small and large molecules

The objective of this article is to provide the reader with an update of some of the BBB research highlights which have occurred in recent times, and to review the impact and contributions of immunogold electron microscopic studies on our understanding of the brain capillary endothelium. Glucose and monocarboxylic acids are two small molecules which this review will focus upon; and advances in immunogold characterization of the GLUT1 glucose transporter and the MCT1 and MCT2 monocarboxylic acid nutrient transporters will be discussed. Human serum albumin is chosen as a representative large molecule, and it has recently been shown that immunogold identification of this protein can serve as an indicator of compromised BBB function in a variety of pathophysiological conditions.

Cellular and subcellular expression of monocarboxylate transporters in the pigment epithelium and retina of the rat

The cellular and subcellular expression of the monocarboxylate transporters MCT1, MCT2 and MCT4 [corresponding to MCT3 of Price N. T. et al. (1998) Biochem. J. 329, 321-328] were investigated in the pigment epithelium and outer retina of rats. Immunofluorescence and postembedding immunogold analyses revealed strong MCT1 labelling in the apical membrane of the pigment epithelial and no detectable signal in the basolateral membrane. In contrast, antibodies to the glucose transporter GLUT1 produced intense labelling in both membranes. Neither MCT1 nor GLUT1 was enriched in intracellular compartments. The monocarboxylate transporter MCT4 was very weakly expressed in the retinal pigment epithelium of adult animals, but occurred at higher concentrations at this site in 14-day-old rats. However, even at the latter stage, the immunolabelling of MCT4 was weak compared to that of MCT1. In the neural retina, the data were consistent with a predominant glial localization of MCT1. Specifically, immunogold particles signalling MCT1 occurred in Müller cell microvilli and in the velate processes between the photoreceptors. No labelling was obtained with antibodies to MCT2. Taken together with previous biochemical analyses, the present findings indicate that MCT1 is involved in the outward transport of lactate through the retinal pigment epithelial cells, and in the transfer of lactate between Müller cells and photoreceptors.

PALIREL, a computer program for analyzing particle-to-membrane relations, with emphasis on electron micrographs of immunocytochemical preparations and gold labeled molecules

Many vital substances, such as receptors, transporters, and ion channels, in cells occur associated with membranes. To an increasing extent their precise localization is demonstrated by immunocytochemical methods including labeling with gold particles followed by electron microscopy. PALIREL has primarily been developed to facilitate such research, enabling rapid analysis of topographic relations of particles (gold or others) to neighboring linear interfaces (membranes). After digitization of membranes and particles, the program particularly allows computation of (1) the particle number and number per unit length of membrane, in individual bins (membrane lengths) interactively defined along the membrane; (2) the distance of each particle from the membrane; (3) the particle number, and the density (number per micron2), in zones defined along (over and under) the membrane; and (4) the particle number and density in "zonebins" resulting from zones and bins being defined simultaneously. If there occurs, somewhere in the membrane, a segment of different nature, such as a synapse, the quantitative data may be had separately for that and the adjoining parts of the membrane. PALIREL allows interactive redefinition of bins, zones, or objects (particle-line files) while other definitions are retained. The results can be presented on the screen as tables and histograms and be printed on request. A dedicated graphic routine permits inspection on screen of lines, particles, zones, and bins. PALIREL is equally applicable to biological investigations of other kinds, in which the topographic relations of points (structures represented as points) to lines (boundaries) are to be examined. PALIREL is available from the authors on a noncommercial basis.

Lactic acid efflux from white skeletal muscle is catalyzed by the monocarboxylate transporter isoform MCT3

The newly cloned proton-linked monocarboxylate transporter MCT3 was shown by Western blotting and immunofluorescence confocal microscopy to be expressed in all muscle fibers. In contrast, MCT1 is expressed most abundantly in oxidative fibers but is almost totally absent in fast-twitch glycolytic fibers. Thus MCT3 appears to be the major MCT isoform responsible for efflux of glycolytically derived lactic acid from white skeletal muscle. MCT3 is also expressed in several other tissues requiring rapid lactic acid efflux. The expression of both MCT3 and MCT1 was decreased by 40-60% 3 weeks after denervation of rat hind limb muscles, whereas chronic stimulation of the muscles for 7 days increased expression of MCT1 2-3-fold but had no effect on MCT3 expression. The kinetics and substrate and inhibitor specificities of monocarboxylate transport into cell lines expressing only MCT3 or MCT1 have been determined. Differences in the properties of MCT1 and MCT3 are relatively modest, suggesting that the significance of the two isoforms may be related to their regulation rather than their intrinsic properties.