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`
`Current Pharmaceutical Design, 2014, 20, 1487-1498
`1487
`Role of Monocarboxylate Transporters in Drug Delivery to the Brain
`
`Nisha Vijay and Marilyn E. Morris*
`
`Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, University at Buffalo, State University of
`New York, Buffalo, New York 14214-8033, USA
`
`Abstract: Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, py-
`ruvate 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 trans-
`porter 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 -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 includ-
`ing their role in the treatment of malignant brain tumors.
`Keywords: Monocarboxylate transporters, -hydroxybutyrate, brain, lactate.
`
`INTRODUCTION
` Monocarboxylic acids play an important role in energy metabo-
`lism in various tissues such as skeletal muscle, heart, brain and red
`blood cells. Among these monocarboxylates, lactate which is the
`end product of glycolysis is particularly important. This pathway
`leads to intracellular accumulation of lactate which must be ex-
`ported out as high levels of lactate result in inhibition of glycolysis.
`In addition, some of the tissues such as brain, heart and red skeletal
`muscle utilize lactate as a fuel for respiration, thus requiring its
`import into the cell [1, 2]. Monocarboxylate transporters facilitate
`the transport of lactate and other monocarboxylates and therefore
`play an important role in cellular metabolism. Proton dependent
`monocarboxylate transporters (MCTs; SLC16A) are a family of
`transport proteins that contain 14 members which were identified
`based on sequence homology [3]. Only 4 members of this trans-
`porter family (MCT1-4) have been identified as proton dependent
`MCTs which catalyze the transport of important monocarboxylates
`such as lactate, pyruvate, and ketone bodies [4]. Another transporter
`family that has been demonstrated to be involved in monocarboxy-
`late transport is known as sodium coupled monocarboxylate trans-
`porters (SMCTs) which contains only two members, SLC5A8 and
`SLC5A12 [5-7]. MCTs have a ubiquitous distribution in the body
`when compared to SMCTs which are more limited in their distribu-
`tion [7, 8]. Apart from endogenous moncarboxylates, MCTs are
`also involved in the transport of some exogenous drugs such as
`salicylate, valproic acid and atorvastatin [8]. Monocarboxylate
`transporters can significantly influence drug pharmacokinetics due
`to their presence in the kidney, intestine and brain. MCT1, MCT2
`and MCT4 are expressed in the brain and play an important role in
`transport of endogenous monocarboxylates into and out of brain
`cells [9]. The present review summarizes the function and distribu-
`tion of monocarboxylate transporters in the brain. The potential role
`of these transporters in drug delivery to the central nervous system
`will also be discussed with specific emphasis on -hydroxybutyrate
`
`*Address correspondence to this author at the University at Buffalo, 352
`Kapoor Hall, Buffalo, NY 14214-8033; Tel: (716) 645-4839;
`Fax: (716) 829-6569; E-mail: memorris@buffalo.edu
`
`(GHB) which has been shown to be a substrate for both MCTs and
`SMCTs [10-13].
`
`MONOCARBOXYLATE TRANSPORTERS
`
`The presence of proton coupled MCTs was first recognized by
`lactate and pyruvate transport into human red blood cells with
`transport being significantly inhibited by -cyano-4-hydroxy-
`cinnamate (CHC) [14-16]. Currently, this family of transporters
`contains 14 members out of which only 4 members (MCT1-MCT4)
`have been demonstrated to mediate the proton dependent transport
`of monocarboxylates such as lactate, pyruvate, and ketone bodies
`[3, 8]. They provide electroneutral co-transport of monocarboxy-
`lates along with protons in a stoichiometric ratio of 1:1. MCT8 is a
`thyroid hormone transporter and MCT10 is an aromatic amino acid
`transporter and is also known as T-type amino acid transporter1
`(TAT1). The functional characterization of other members of this
`family has not been done and they are known as orphan transport-
`ers. MCTs have 12 transmembrane domains with C- and N-termini
`within the cytoplasm and an intracellular loop between TMDs 6 and
`7 [17]. The conservation of sequence between different isoforms of
`the mammalian MCTs is the greatest for MCT1-4 whereas se-
`quence is least conserved between other members of the family.
`The TMDs are highly conserved between the family members with
`high variations in the C- and N- termini including the intracellular
`loop [3]. The variations in the sequences of different isoforms may
`lead to differences in substrate specificity and regulation of MCTs
`[18]. The regulation of MCTs has been shown to occur both by
`transcriptional as well as post-transcriptional mechanisms [19, 20].
`Although these proteins are not glycosylated, they require associa-
`tion with glycosylated protein, for their functional activity. This
`ancillary protein is called basigin or CD147 for MCT1 and MCT4
`whereas MCT2 differs from its isoforms as it requires embigin
`instead of basigin for its functional activity [21]. The tissue distri-
`bution and substrate specificity of each MCT isoform has been
`outlined in Table 1. The key features of each functionally character-
`ized MCT isoform will be further discussed in detail in this section.
`
`
`
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`
`© 2014 Bentham Science Publishers
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`1488 Current Pharmaceutical Design, 2014, Vol. 20, No. 10
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`Vijay and Morris
`
`Table 1. Tissue distribution and substrate specificity of various MCT and SMCT isoforms.
`
`Protein
`name
`
`Unigene name
`
`Tissue distribution
`
`MCT1
`
`SLC16A1
`
`Ubiquitous
`
`Cellular localiza-
`tion in brain
`
`Brain endothelial
`cells, astrocytes,
`some neurons in rats
`
`Predominant substrates
`
`Lactate, pyruvate, bu-
`tyrate, acetoacetate, -
`hydroxybutyrate,
`XP13512, GHB
`
`Transport
`mechanism
`
`H+ cotransporter or
`monocarboxylate
`exchanger
`
`Reference
`
`[8, 9, 87]
`
`MCT2
`
`SLC16A7
`
`Liver, kidney, brain,
`testis, heart, spleen,
`pancreas
`
`Neurons
`
`Pyruvate, lactate
`
`H+ cotransporter
`
`[8, 9, 34]
`
`Lactate
`
`H+ cotransporter
`
`[8, 40, 43]
`
`Astrocytes
`
`Lactate, pyruvate, ace-
`toacetate, -
`hydroxybutyrate
`
`H+ cotransporter
`
`[8, 9, 44, 87]
`
`MCT3
`
`SLC16A8
`
`MCT4
`
`SLC16A3
`
`MCT6
`
`SLC16A5
`
`MCT8
`
`SLC16A2
`
`MCT10
`(TAT1)
`
`SLC16A10
`
`Retinal pigment
`epithelium, choroid
`plexus
`
`Skeletal muscle,
`brain, kidney, pla-
`centa, leukocytes,
`heart, lung, chondro-
`cytes
`
`Kidney, muscle,
`brain, heart, pla-
`centa, intestine,
`prostate, lung, pan-
`creas
`
`Liver, brain, heart,
`kidney, placenta,
`ovary, prosatate,
`thymus, pancreas
`
`Skeletal muscle,
`intestine, kidney,
`heart, liver, placenta
`
`SMCT1
`
`SLC5A8
`
`Intestine, kidney,
`brain, retina
`
`Neurons
`
`Bumetanide, nateglinide
`
`Orphan
`
`[8, 46]
`
`T3, T4
`
`Orphan
`
`[8, 48, 49]
`
`Facilitated diffusion/
`exchanger
`
`[8, 50]
`
`Na+ cotransporter
`
`[5, 54, 88]
`
`Aromatic amino acids
`(L-tryptophan, L-
`tyrosine, L-
`phenylalanine, L-DOPA
`
`Lactate, pyruvate, bu-
`tyrate, nicotinate, ace-
`toacetate, -
`hydroxybutyrate, -
`ketoisocaproate, salicy-
`lates, benzoate, GHB
`
`SMCT2
`
`SLC5A12
`
`Intestine, kidney,
`brain, retina
`
`Astrocytes and glia
`
`Lactate, pyruvate
`
`Na+ cotransporter
`
`[7, 54, 57]
`
`MCT1 (SLC16A1)
` MCT1 was first identified as a mutation of the wild type protein
`which enhanced the uptake of mevalonate into Chinese-hamster
`
`ovary cells [22]. This protein has been shown to mediate inhibitor
`sensitive transport of monocarboxylates. MCT1 has now been
`cloned from mice, rats and humans and shows 95% sequence ho-
`mology to Chinese-hamster ovary MCT1 [23-26]. The functional
`
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`Monocarboxylate Transporters in Brain
`
`Current Pharmaceutical Design, 2014, Vol. 20, No. 10 1489
`
`activity of MCT1 is dependent on a proton gradient and it acts as a
`proton dependent cotransporter/exchanger [27]. Transport was de-
`termined to follow an ordered, sequential mechanism through ki-
`netic studies of lactate into red blood cells [16, 28]. A proton first
`binds to the transporter followed by binding of lactate. The proton
`and lactate are further translocated across the membrane with their
`sequential release on the other side. The return of the free trans-
`porter binding site across the membrane determines the net flux of
`lactate and thus forms the rate limiting step of transport. Transport
`can be stimulated by a pH gradient (low to high). The predominant
`role of MCT1 is to facilitate the unidirectional proton-linked trans-
`port of monocarboxylates across the plasma membrane. This may
`represent either influx or efflux of substrate depending of the intra-
`cellular and extracellular substrate concentrations and the existing
`pH gradient across the plasma membrane. However, MCT1 can
`also function as an exchanger, with transport occurring bidirection-
`ally with the exchange of one monocarboxylate for another without
`the net movement of protons [3].
`
`The substrate specificity of MCT1 has been extensively studied
`in red blood cells by measuring the inhibition of uptake of 14C-
`lactate [14]. It has been shown that MCT1 is responsible for the
`transport of a broad range of monocarboxylates including lactate,
`pyruvate, acetoacetate, -hydroxybutyrate and GHB [1, 29]. These
`substrates exist as a monocarboxylate anion under physiological
`conditions, which is required for a MCT substrate. The Km value for
`transport decreases with increasing chain lengths of various mono-
`carboxylates. Monocarboxylates that are substituted in the C-2 and
`C-3 positions with halides, hydroxyl, and carbonyl groups represent
`good substrates. The C-2 substitution is preferred over C-3, with the
`carbonyl group being especially favored. Monocarboxylates with
`longer branched aliphatic or aromatic side chains have also been
`found to bind to the transporter, but these are not easily released
`following translocation and may act as potent inhibitors [3]. Lactate
`transport has been found to be stereospecific with higher affinity for
`L-lactate when compared to D-lactate [27].
`
`The inhibitors of MCT1 can be classified into three categories:
`(1) bulky or aromatic monocarboxylates such as 2-oxo-4-methyl-
`pentanoate, phenyl-pyruvate and -cyano-4-hydroxycinnamate
`(CHC) which act as competitive inhibitors and are blockers of
`transport function of MCTs [30,31]; (2) amphiphilic compounds
`with divergent structures which include bioflavanoids such as quer-
`cetin and phloretin and anion transport inhibitors such as 5-nitro-2-
`(3-phenylpropylamino)-benzoate and niflumic acid; and (3) 4,40-
`substituted stilbene 2,20-disulphonates such as 4,40-diisothio-
`cyanostilbene-2,20-disulphonate
`(DIDS) and 4,40-dibenzami-
`dostilbene-2,20-disulphonate (DBDS) which act as reversible in-
`hibitors of MCT1 in erythrocytes [32, 33]. It is important to note
`that CHC is not a specific MCT1 inhibitor and may inhibit one or
`more isoforms of MCTs. One of the important roles of MCT1 is the
`unidirectional transport of L-lactate (influx or efflux) which de-
`pends on the intracellular and extracellular lactate concentrations as
`well as the proton gradient across the membrane.
`
`MCT2 (SLC16A7)
` A second MCT isoform was cloned from a hamster liver cDNA
`library and was shown to have higher affinity for monocarboxylates
`than MCT1 [34-36]. This isoform was named MCT2 and was fur-
`ther characterized following the expression of rat isoform in
`Xenopus oocytes [37]. MCT2 shares 60% identity with MCT1.
`MCT2 has similar substrate specificity when compared to MCT1. It
`has also been shown to be inhibited by similar inhibitors such as
`CHC, DBDS and DIDS but it has been reported to be insensitive to
`the organomercurial reagent pCMBS [8, 34]. It has been shown that
`pCMBS inhibits MCT1 by binding to its associated ancillary pro-
`tein basigin. This may be the reason for insensitivity to pCMBS as
`MCT2 has been shown to associate with embigin and not basigin
`[21, 37, 38]. MCT2 has also been cloned from rat, mouse and hu-
`
`man tissues [35, 36]. The sequence of MCT2 is conserved to a
`lesser extent than MCT1 among these species which results in con-
`siderable species differences in the tissue distribution of this iso-
`form [8]. MCT2 expression is limited in major human tissues
`whereas northern and western blot analysis have shown that this
`isoform is expressed in liver, kidney, brain and sperm tails in rat,
`mouse and hamster [8].
`
`MCT3 (SLC16A8)
` MCT3 has a very limited distribution and is found only in the
`basolateral membrane of the retinal pigment epithelium and the
`choroid plexus in humans, rodents and chickens [39]. The Km value
`of chicken MCT3 for lactate has been found to be around 6 mM in
`a yeast expression system [40]. It has also been found to be resistant
`against typical MCT inhibitors such as phloretin, CHC and
`pCMBS. Further information on substrate kinetics of this MCT
`isoform is not available and further studies are needed. Based on its
`localization, it is thought to be responsible for the export of lactate
`produced as a result of glycolysis from the retina [41, 42].
`
`MCT4 (SLC16A3)
`
`This isoform was initially named MCT3 based on sequence
`homology to chicken MCT3 but later was renamed as MCT4 [43].
`It is mainly found in glycolytic tissues such as white skeletal mus-
`cle fibres, astrocytes, white blood cells, and chondrocytes [3, 8]. It
`has lower affinity for lactate and pyruvate than MCT1 and is be-
`lieved to be involved in efflux of lactate from these tissues to pre-
`vent intracellular accumulation of lactate which would otherwise
`inhibit glycolysis [44]. This has been studied by expression of this
`transport protein in Xenopus oocytes [45]. It has a very high Km
`value for pyruvate (150 mM) which helps in preventing its loss
`from the cell.
`
`MCT 6 (SLC16A5)
` MCT6 was first identified by genomic and EST database
`screening and is predominantly expressed in the kidney and intes-
`tine [43]. It is known to transport pharmaceutical drugs such as
`bumetanide and nateglinide and does not transport short chain
`monocarboxylates like the other isoforms [46]. This isoform has
`also been shown to be present in the intestine implicating its role in
`drug absorption.
`
`MCT 8 AND MCT 10 (SLC16A2 AND SLC16A10)
` MCT8 was earlier known as XPCT (X-linked PEST containing
`transporter) because it contains a PEST domain in its N-terminal
`[47]. This isoform is also known as the thyroid hormone trans-
`porter. Substrate kinetic studies through expression in Xenopus
`oocytes demonstrated that MCT8 transports both the thyroid hor-
`mones (T3 and T4) with high affinity with Km values of 2-5 M
`[48]. MCT8 is distributed in many tissues including liver, kidney,
`skeletal muscle, heart, brain, pituitary, and thyroid [49]. MCT10 is
`also known as TAT1 and was found to transport aromatic amino
`acids such as phenylalanine and tryptophan. It has also been ex-
`pressed in Xenopus oocytes which demonstrated Km values of
`around 5 mM for aromatic amino acid substrates such as trypto-
`phan, tyrosine, and phenylalanine [50]. MCT10 is expressed in a
`variety of tissues including intestine, kidney, liver, skeletal muscle,
`heart, and placenta [51]. Both MCT8 and MCT10 are known to
`mediate proton and sodium independent transport of their sub-
`strates. Delayed brain myelination which results in variable degrees
`of mental retardation, hypotonia, spasticity, ataxia and involuntary
`movements has been attributed to MCT8 deficiency in the brain
`[52]. Various tyrosine kinase inhibitors have been shown to non-
`competitively inhibit MCT8 leading to reduced thyroid hormone
`uptake in brain. Hence tyrosine kinase inhibitors can lead to phar-
`macokinetic drug interactions leading to increased levothyroxine
`requirement of thyroidectomized patients [53].
`
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`1490 Current Pharmaceutical Design, 2014, Vol. 20, No. 10
`
`Vijay and Morris
`
` Other isoforms of MCTs, MCT5, MCT7, MCT9, and MCT 11-
`14 have also been identified but their functional characterization
`has not been performed.
`
`SMCT
`
`The second transport family that is involved in the transport of
`monocarboxylates is the sodium coupled monocarboxylate trans-
`porters (SMCT), part of the solute carrier gene family SLC5. Only
`two members of this family have been identified as sodium-
`dependent monocarboxylate transporters so far, namely SLC5A8
`and SLC5A12 [54]. Characterization of SLC5A8 was done by its
`expression in Xenopus laevis oocytes and it has been shown to
`transport short chain monocarboxylates [5]. This transporter is de-
`pendent on the sodium gradient and typically transports multiple
`sodium ions along with monocarboxylates in a stoichiometric ratio
`of 3:1 making it electrogenic. SLC5A8 is expressed in normal colon
`tissue, and it functions as a tumor suppressor in human colon with
`silencing of this gene occurring in colon carcinoma. This trans-
`porter is involved in the concentrative uptake of butyrate and pyru-
`vate produced as a product of fermentation by colonic bacteria.
`These are known to act as inhibitors of histone deacetylases, which
`supports its suppression in tumor cells [55]. SLC5A8 is also ex-
`pressed in the brush border membrane of renal tubular cells where it
`has been suggested to mediate the active reabsorption of lactate and
`pyruvate to minimize their renal elimination and in the brain [56].
`SLC5A8 is a higher affinity transporter when compared to MCT1
`with Km values for lactate of 159 M determined in Xenopus oo-
`cytes with heterologous expression of SLC5A8 [5]. The second
`member of this family, SLC5A12, has been found to be expressed
`in kidney and intestine with limited distribution in the brain. It is
`also found to mediate the sodium dependent transport of monocar-
`boxylates but the transport is electroneutral, in contrast to SLC5A8.
`The affinity of this transporter is lower when compared with
`SLC5A8, but it exhibits very similar substrate specificity [7, 57].
`
`FUNCTION OF MONOCARBOXYLATE TRANSPORTERS
`IN THE BRAIN
`
`Transport of lactate across the plasma membrane is important
`under hypoxic conditions when glycolysis becomes the predomi-
`nant pathway and also for tissues that rely on glycolysis to meet
`their normal energy demands [3]. Under hypoxic conditions, glyco-
`lysis results in the formation of lactate which needs to be exported
`out of the cell for continued glycolysis to occur [58, 59]. The trans-
`porters have lower affinity for pyruvate thus ensuring that it is not
`lost from the cell and further converted to lactate which results in
`regeneration of NAD+ and continued glycolysis. In the brain, glu-
`cose serves as the major energy source under normal conditions, but
`during prolonged starvation and diabetic ketoacidosis as observed
`in diabetes, other monocarboxylates such as lactate and ketone
`bodies ( -hydroxybutyrate and acetoacetate) become an important
`energy substrate and their transport into the brain is required [60-
`62]. The endothelial cells of the blood vessels in the brain have
`been reported to express MCT1 which probably mediates the trans-
`port of lactate and ketone bodies across the blood brain barrier
`(BBB) [63, 64]. The capacity of the brain to use ketone bodies such
`as -hydroxybutyrate was found to increase in starvation and diabe-
`tes by 50-60% in rats [62]. This study also showed that BBB per-
`meability to ketone bodies increased by both starvation and diabe-
`tes.
` Under certain conditions such as hypoxia or ischemia, glycoly-
`sis is the only pathway for the production of ATP resulting in in-
`creased brain concentrations of lactate [3]. There are different iso-
`forms of MCTs that are expressed in different subcellular regions of
`the brain with MCT1 and MCT4 being predominantly found in the
`astrocytes and MCT2 being the major isoform in the neurons [65].
`This ensures export of lactate from astrocytes formed as a product
`of rapid glycolysis which is then taken up by the neurons to be used
`
`as a respiratory fuel for further oxidation [9]. Glucose is considered
`to be the predominant energy fuel for neurons. However, several
`studies have shown that neurons can efficiently utilize monocar-
`boxylates, especially lactate as oxidative energy substrates in addi-
`tion to glucose [66]. In contrast, astroglial cells are a major source
`of lactate and they predominantly metabolize glucose into lactate in
`the brain followed by lactate efflux [67]. In some cases, it has been
`shown that astrocytes can use lactate as an energy substrate, but to a
`very limited extent when compared to neurons [67]. The export of
`lactate along with a proton also helps in maintaining the intracellu-
`lar pH by preventing cellular acidification. This has been demon-
`strated by disrupting the expression of MCT1 or MCT4 in astro-
`cytes in the hippocampus of rats which resulted in loss of memory
`of learned tasks [68]. This loss in memory could be reversed by
`injecting L-lactate locally whereas the injection of glucose was not
`able to reverse this. Similar loss in memory in rats was obtained by
`disrupting MCT2 in neurons but this could not be reversed by injec-
`tion of either L-lactate or glucose demonstrating that MCT2 is re-
`quired for the uptake of these respiratory fuels into the neurons for
`proper functioning of the brain [68]. This is commonly known as
`the astrocyte-neuron lactate shuttle hypothesis. Exposure to gluta-
`mate has been shown to stimulate glucose utilization and the release
`of lactate by astrocytes [69]. This provides a coupling mechanism
`between neuronal activity and glucose utilization. It has also been
`demonstrated that certain neurotransmitters such as noradrenaline,
`vasoactive intestinal peptide and adenosine that activate glyco-
`genolysis also increase lactate release [70].
` MCTs are also involved in the uptake of ketone bodies in the
`neurons in conditions with low glucose utilization [8]. Neurons
`have the ability to oxidize lactate under both physiological and
`hypoxic conditions similar to heart and red skeletal muscle and they
`contain the same isoform of lactate dehydrogenase (LDH) as pre-
`sent in heart (LDH-1 subunit) [71]. The LDH-5 subunit (muscle
`type) is present in glycolytic tissues, favoring the formation of lac-
`tate from pyruvate whereas the LDH-l subunit (heart type) preferen-
`tially drives the reaction toward the production of pyruvate. It has
`been shown that LDH-1 subunits are present in neurons. However,
`LDH-5 subunit is predominantly present in the astrocytes [72]. This
`selective distribution of lactate dehydrogenase isoenzymes in astro-
`cytes and neurons is consistent with the proposed astrocyte-neuron
`lactate shuttle.
`
`The utilization of lactate and ketone bodies as energy substrates
`has been found to be higher in neonates when compared to adults
`and this is consistent with higher expression of MCT1 in neonates
`[59, 73, 74]. MCT1 expression in the membrane of capillary endo-
`thelium was found to be 25 times higher in 17-day suckling rat pups
`than adults using electron microscopic immunogold methods. This
`transporter was found to be equally distributed in both luminal and
`abluminal membranes [75]. These results were further confirmed by
`a report of high mRNA and protein expression of MCT1 in the
`BBB during suckling and reduction in expression with maturation
`[76]. This also explains the switch in fuel utilization from a
`combination of glucose, lactate and ketone bodies in the neonatal
`brain to complete dependence on glucose in adults. It has been
`shown in rodents that increased susceptibility of the neurons to
`acute severe hypoxia, which mimics the disorder of sleep apnea, is
`mediated by decreased expression of MCT2 in the neurons [77].
`MCT1 and MCT4 have also been associated with the transport of
`short chain fatty acids such as acetate and formate which are then
`metabolized in the astrocytes [78].
`
`LOCALIZATION OF MCTs IN THE BRAIN
` MCTs are widely expressed in rat, mouse and human brain,
`both at the cellular and sub-cellular levels. MCT1 has a ubiquitous
`distribution in the body and is found in the liver, kidney, heart,
`muscle and brain [3]. Of all the identified isoforms of MCTs, it has
`been demonstrated that MCT1, MCT2 and MCT4 are expressed in
`
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`Monocarboxylate Transporters in Brain
`
`Current Pharmaceutical Design, 2014, Vol. 20, No. 10 1491
`
`the brain as depicted in (Fig. 1) [9]. The different subcellular re-
`gions of the brain express different MCT isoforms. The mRNA of
`MCT1 has been found in the cortex, hippocampus and cerebellum
`of adult rat brain [59, 76]. Earlier studies have shown that MCT1 is
`significantly expressed in cerebral blood vessels with specific local-
`ization on the endothelial cells on both luminal and abluminal
`membranes and ependymocytes lining the four brain ventricles in
`rats [73]. MCT1 was also found in the glial end feet surrounding
`capillaries [73, 75] and in brain parenchymal cells [73]. Confocal
`microscopy studies have also identified the expression of MCT1 in
`astrocytic processes both in vitro and in vivo [64, 79, 80]. Low ex-
`pression of MCT1 has also been identified in specific subpopula-
`tions of neurons in adult rat brain such as those in the cerebral cor-
`tex, hippocampus, and hypothalamus [75]. However, MCT1 ex-
`pression was not observed in the adult mouse brain neuron [64].
`Recently, the absolute protein quantities of MCT1 have been de-
`termined in freshly isolated human brain microvessels from patients
`with epilepsy or glioma using quantitative RT-PCR and
`LC/MS/MS. The results of this study demonstrated the expression
`of MCT1 in these samples [81].
`
`Regional distribution of MCT2 in the mouse brain includes
`cortex, hippocampus and cerebellum [59, 65, 80]. MCT2 is the
`major neuronal isoform as demonstrated by immunohistochemistry
`results with major localization in the postsynaptic densities of the
`neurons [80, 82, 83]. There was no co-localization of MCT2 im-
`munoreactivity with presynaptic elements in the neuron. MCT2 has
`also been found in immunoreactivity in the postsynaptic membrane
`of parallel fibre-Purkinje cell synapses in the rat cerebellum and in
`the postsynaptic 2-glutamate receptors as demonstrated by elec-
`tron microscopy [63, 84]. In addition, its presence has also been
`demonstrated at both mRNA and protein levels in cultured neurons
`[80]. The expression of MCT2 was also observed in some popula-
`tions of astrocytes in the white matter and glia but such presence
`was only detected in rat brain and cultured rat brain astrocytes [79,
`85]. The mouse brain or the cultured mouse brain astrocytes failed
`to show such expression suggesting that there could be species
`
`differences in the distribution of MCT2 in the brain [64, 80, 83].
`MCT2 has also been found in the Purkinje fibers of the cerebellum
`as demonstrated by immunohistochemistry [84]. In brain endothe-
`lial cells, the presence of MCT2 was only observed in a few studies
`and thus this still needs to be further examined [82, 86]. Although
`MCT2 expression has been demonstrated in rodent brain, very little
`MCT2 expression was observed in human brain as shown by
`Northern blotting results [43]. It is important to understand that
`there are some discrepancies in results obtained in different studies.
`This could be due to the differences in specificity of the antibodies
`used to identify the MCT isoforms which has been discussed in
`Bergersen et al. [84]. Species differences in MCT expression could
`also contribute to some of these differences. These discrepancies
`remain to be further evaluated in future studies.
` MCT4 expression has been demonstrated in the astrocytes of
`adult rat and mouse brain in the cerebral cortex, striatum, hippo-
`campus, paraventricular nucleus in the hypothalamus and capsula
`internalis [87]. MCT4 has been found to be exclusively expressed
`in the astrocytes [63, 84]. This is consistent with the high rate of
`glycolysis in astrocytes, thus requiring continuous efflux of lactate.
`
`Studies have shown that a developmental switch exists in the
`expression of different MCT isoforms in various regions of the rat
`brain [76]. The mRNA and protein expression of MCT1 in the BBB
`has been found to be maximum during suckling followed by a de-
`cline with maturation in rats [75]. However, MCT2 found predomi-
`nantly in the neurons shows constant expression during maturation,
`thus demonstrating that they play an important role in energy me-
`tabolism in the adult brain. In contrast, Pellerin et al have observed
`a decline in expression of both MCT1 and 2 during maturation by
`Northern blot analysis [87].
`
`SMCT1 has recently been shown to be expressed exclusively in
`the neurons of mouse brain through immunofluorescence studies
`and it was reported to co-localize with MCT2 [88]. Studies in
`mixed cultures of rat brain neurons and astrocytes have also dem-
`onstrated its localization in the neurons. This suggests that SMCT1
`
`Fig. (1). Cellular localization of different MCT isoforms in brain (adapted from Simpson et al. 2007) [125].
`
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`1492 Current Pharmaceutical Design, 2014, Vol. 20, No. 10
`
`Vijay and Morris
`
`can also play a role in the entry of lactate and other monocarboxy-
`lates into the neurons thus maintaining their energy status.
`
`MCTs IN DRUG DISPOSITION
` Apart from their role in the transport of endogenous short chain
`monocarboxylates, MCTs also play a role in the transport of drugs
`such as valproic acid, salicylate, bumetanide, nateglinide, simvas-
`tatin and atorvastatin [8, 46]. The presence of these transporters in
`major organs such as kidney, liver, brain and intestine suggests that
`they may have a potential impact on the pharmacokinetics of sub-
`strate drug molecules. This may be due to the influence of these
`transporters on intestinal absorption, blood-brain and tissue trans-
`port, and the renal reabsorption of these drugs. In addition, due to
`the widespread distribution of MCT1 in various tissues, it may be
`targeted for drug delivery into specific tissues. Presence of MCTs at
`the BBB implies that they can serve as potential targets in order to
`achieve optimum delivery of their substrates into the brain. Earlier
`studies in rats have shown that acidic drugs such as valproic acid,
`benzoic acid, nicotinic acid or beta-lactam antibiotics including
`benzylpenicillin, propicillin and cefazolin could be transported into
`the brain utilizing a carrier mediated transport system in the BBB in
`a pH dependent manner with transport being significantly reduced
`in the presence of their respective unlabeled compounds [89]. The
`uptake of acetic acid was studied in primary cultured bovine brain
`capillary endothelial cells and was found to be significantly inhib-
`ited by a number of monocarboxylates including nicotinic acid
`further suggesting a role of MCTs in the transport of these mono-
`carboxylates into the brain [90]. The uptake of nicotinate was also
`studied in primary cultures of astrocytes from rat cerebral cortex
`[91]. The nicotinate uptake was found to be saturable and pH de-
`