Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Monocarboxylate Transporter (SLC16A)

  • Marilyn E. Morris
  • Rutwij A. Dave
  • Kristin E. Follman
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101724

Synonyms

Historical Background

Until 1974, transport of free acids across plasma membranes of mammalian cells and tissues was thought to occur via nonionic diffusion. Two independent laboratories first reported that facilitated and carrier-mediated transport of lactate and pyruvate into human erythrocytes was strongly inhibited by α-cyano-4-hydroxycinnamate (CHC) (Halestrap and Denton 1974; Poole and Halestrap 1993) and organomercurials, which are potent and nonspecific inhibitors of a monocarboxylate transporter (MCT) (Deuticke 1982). However, subsequent extensive characterization of MCT in various tissues such as erythrocytes, cardiac myocytes, and hepatocytes led to the conclusion that a family of MCTs might exist, which is currently known as the solute carrier 16A (SLC16A) family with 14 known isoforms to date (Poole and Halestrap 1993). The molecular identity and characterization of the first member of this family, MCT1 (SLC16A1), was established by parallel studies in two laboratories:
  1. 1.

    Kim et al. (1992) demonstrated that the N-terminus of MCT1 protein is identical to a putative 12-transmemberane domain transporter (MEV) of unknown function previously cloned by this group from a mutated Chinese hamster ovary cell line that exhibited enhanced mevalonate uptake (Kim et al. 1992). Subsequently, Garcia et al. (1994) reported that the wild-type protein catalyzed proton-dependent pyruvate and lactate transport activity and named the protein monocarboxylate transporter-1 (MCT1) (Garcia et al. 1994).

     
  2. 2.

    Poole and Halestrap (1993) demonstrated this proton-dependent pyruvate and lactate transport through specific labeling studies in rat and rabbit erythrocytes followed by purification and N-terminal sequencing and identified the MCT1 transporter, a 40–50 kDa protein (Halestrap and Meredith 2004; Poole and Halestrap 1993). MCT2 (SLC16A7) was shortly after identified by screening a rat liver cDNA library; MCT2 is highly expressed in the liver (Halestrap and Meredith 2004). During investigations on X-chromosome inactivation, gene sequencing revealed another MCT family member, originally called XPCT, which is currently known as MCT8 (SLC16A2) (Halestrap and Meredith 2004). It should be noted that because of early confusion in the nomenclature, the MCT and SLC16A numbering of the family do not coincide but are correctly annotated in Table 1.

     
Monocarboxylate Transporter (SLC16A), Table 1

Overview of SLC16A family – monocarboxylate transporters (Halestrap and Meredith 2004; Halestrap 2013)

Human gene name

Protein name

Alias

Human gene locus

Sequence accession ID

Tissue distribution

SLC16A1

MCT1

MOT1

1p12

NM_003051

Ubiquitous except β cell of endocrine pancreas

SLC16A2

MCT8

MOT8, XPCT, MCT7

Xq13.2

NM_006517

Most tissues including liver, heart, brain, thymus, intestine, ovary, prostate, pancreas, placenta, lung, kidney, skeletal muscle

SLC16A3

MCT4

MOT4, MCT3

17q25.3

NM_004207

Skeletal muscle, chondrocytes, leucocytes, testis, lung, ovary, placenta, heart

SLC16A4

MCT5

MOT5, MCT4

1p13.3

NM_004696

Brain, muscle, liver, kidney, lung, ovary, placenta, heart

SLC16A5

MCT6

MOT6, MCT5

17q25.1

NM_004695

Kidney, muscle, brain, heart, pancreas, prostate, lung, placenta

SLC16A6

MCT7

MOT7, MCT6

17q24.2

NM_004694

Brain, pancreas, muscle, prostate

SLC16A7

MCT2

MOT2

12q13

NM_004731

High expression in testis, moderate to low in spleen, heart, kidney, pancreas, skeletal muscle, brain and leucocyte

SLC16A8

MCT3

MOT3, REMP

22q112, 3q13.2

NM_013356

Retinal pigment epithelium, choroid plexus

SLC16A9

MCT9

MOT9

10q21.1

NM_194298

Endometrium, testis, ovary, breast, brain, kidney, spleen adrenal, retina

SLC16A10

TAT1, MCT10

MOT10

6q21-q22

NM_01859

Kidney (basolateral), intestine, muscle, placenta, heart

SLC16A11

MCT11

MOT11

17p13.1

NM_153357

Skin, lung, ovary, breast, lung, pancreas, retinal pigment epithelium, choroid plexus

SLC16A12

MCT12

MOT12

10q23.31

NM_213606

Kidney, retina, lung, testis

SLC16A13

MCT13

MOT13

17p13.1

NM_201566

Breast, bone marrow stem cells

SLC16A14

MCT14

MOT14

2q36.3

NM_152257

Brain, heart, muscle, ovary, prostate, breast, lung, pancreas liver, spleen, thymus

MCT3 was identified in the chicken retinal pigment epithelium (Halestrap and Meredith 2004). MCTs 4–7 were identified in the Halestrap laboratory (Halestrap and Meredith 2004). MCTs 9 and 11–14 were identified solely from human genomic expressed sequence tag (EST) database analysis. MCT10, previously known as TAT1, is a sodium- and proton-independent aromatic amino acid transporter (Halestrap and Meredith 2004). The topology of the MCT family and phylogenetic tree are illustrated in Figs. 1 and 2. Tissue distribution of MCT isoforms is described in Table 1.
Monocarboxylate Transporter (SLC16A), Fig. 1

The proposed topology of the monocarboxylate transporter (MCT) family members. CD147, the ancillary protein that associates with MCT1 and MCT4, is also shown. The N- and C-termini and the large loop between TMDs 6 and 7 show the greatest variation between family members, while the TMDs themselves are highly conserved. Critical residues identified in MCT1 and two highly conserved motifs characteristic of the MCT family are included and further discussed in the section “Molecular Structure and Function” (Reproduced with permission from Halestrap and Meredith (2004))

Monocarboxylate Transporter (SLC16A), Fig. 2

A phylogenetic tree of the human SLC16 family in relation to their known functions (Reproduced with permission from Halestrap (2013))

MCTs 1–4 have overlapping substrate specificity with another family of transporters known as sodium-coupled monocarboxylate transporters (SMCTs) which are members of the SLC5 gene family (Halestrap 2013). The first SMCT identified was SMCT1 (SLC5A8) in 2003; later SMCT2 (SLC5A12), a lower affinity SMCT, was identified (Halestrap 2013). These transporters are located apically in the kidney and intestinal epithelial cells; additionally SMCT1 is present in the brain and retina (Halestrap 2013). This review will be restricted to members of the SLC16A family.

Molecular Structure and Function

Monocarboxylate transporters (MCTs) form a 14-member subfamily of the solute carrier family SLC16A. These 14 transporters are related through sequence homology and share several structural characteristics. MCTs have 6 helix N- and C-terminal domains, for a total of 12 transmembrane helices (TMs), connected by an intracellular loop of about 30 amino acids. The N- and C- termini are also located inside the cell (Halestrap 2012). These transporters have not been shown to have sites for glycosylation (Halestrap 2012). Of the 14 members of the family, 8 remain orphan transporters. MCTs 1–4 are the most extensively characterized, followed by MCTs 8 and 10. MCT8 is a thyroid hormone transporter with high affinity for both T3 and T4 (Km 2–4 μM) (Halestrap 2012). MCT10 transports the aromatic amino acids, phenylalanine, tyrosine, and tryptophan with a Km of 1 mM. MCTs 1–4 have different expression patterns and transport monocarboxylates such as l-lactate, pyruvate, and butyrate with different affinities (Halestrap 2012). MCT2 is a higher affinity transporter than MCT1, but MCT1, unlike MCT2, is ubiquitously expressed and can be found in most human tissues. MCTs 3 and 4 have lower affinities for most substrates compared to MCT1 (Halestrap 2012). MCT3 is primarily expressed in retinal pigment and choroid plexus epithelium. MCT4 is expressed in many tissues and has particular abundance in glycolytic tissues, such as skeletal muscle (Halestrap 2012). Table 2 shows substrates and inhibitors of the various MCT isoforms.
Monocarboxylate Transporter (SLC16A), Table 2

Substrates and inhibitors of various MCT isoforms in human, rat, and mouse (Adapted from Morris and Felmlee (2008) and Halestrap (2013))

Species

Isoform

Expression system

Substrate

Km (mM)

Inhibitor

Kia, IC50b, or K0.5c (μM)

Human

MCT1

Xenopus oocytes

Lactate

3.5–6

Phloretin

28a

Pyruvate

1.8–2.5

Quercetin

n.a.

Acetoacetate

5.5

CHC

425a

α-Ketoisovalerate

1.3

pCMBS

n.a.

α-Oxoisohexanoate

0.67

XP13512

0.620b

α-Oxoisovalerate

1.25

  

Butyrate

9

XP13512

0.22

MCT2

Xenopus oocytes

Pyruvate

0.025

CHC

n.a.

l-Lactate

n.a.

GHB

n.a.

MCT3

ARPE-19 cells

Lactate

n.a.

  

MCT4

Xenopus oocytes

l-Lactate

28

pCMBS

21a

d-Lactate

519

CHC

991a

Pyruvate

153

Phloretin

41a

d-β-Hydroxybutyrate

130

NPPB

240a

Acetoacetate

216

Fluvastatin

32b

α-Ketobutyrate

57

Atorvastatin

32b

α-Ketoisocaproate

95

Lovastatin

44b

α-Ketoisovalerate

113

Simvastatin

79b

  

AR-C155858

>10 (nM)c

AR-C117977

>1 (nM)c

3-isobutyl-1-methylxanthine

970c

MCT6

Xenopus oocytes

Butetanide

0.084

Furosemide

46b

Nateglinide

n.a.

Azosemide

21b

Prostaglandin F2α

n.a.

  

MCT8

COS1 and JEG3 cells

T3

n.a.

T4

n.a.

Rat

MCT1

Xenopus oocytes

l-Lactate

3.5

Phloretin

28b

 

γ-Hydroxybutyrate

4.6

Quercetin

14b

Pyruvate

1

Benzbromaron

22b

α-Ketoisocaproateg

0.7

CHC

425b

α-Ketoisovalerateg

1.3

  

MDA-MB231

γ-Hydroxybutyrate

4.6

Erythrocytes

Acetate

3.5

 

Propionate

1.5

MCT2

Xenopus oocytes

l-Lactate

0.74

Phloretin

14b

Pyruvate

0.08

Quercetin

5b

d-β-hydroxybutyrate

1.2h

Benzbromaron

9b

Acetoacetate

0.8

CHC

24b

α-Ketoisocaproateg

0.1

AR-C155858

<10e (nM)c

α-Ketoisovalerateg

0.3

AR-C117977

21 (nM)c

  

DBDS

44%f

  

NPPB

25%f

  

Niflumic acid

14%f

 

MCT4

Xenopus oocytes

l-Lactate

34

CHC

350b

Pyruvate

36.3

  

2-Oxoisohexanoate

13

  

Acetoacetate

31

  

β-Hydroxybutyrate

65

  

MCT8

Xenopus oocytes

T3

n.a.

N-bromoacetyl-T3

n.a.

  

T4

n.a.

Bromosulfophthalein

n.a.

MCT10

Xenopus oocytes

l-Tryptophan

3.8

  

l-Tyrosine

2.6

  

l-Phenylalanine

7

  

l-DOPA

6.4

  

Mouse

MCT1

Ehrlich-Lettre ascites cells

Acetate

3.73

α-Cyanocinnamate

1.7 (mM)c

l-lactate

4.5

CHC

166c

d-lactate

27.5

α-Fluorocinnamate

724c

Pyruvate

0.7

Phenylcinnamate

61c

d-β-Hydroxybutyrate

10.1

UK5099

8.1c

γ-Hydroxybutyrate

7.7

  

Acetoacetate

5.5

DIDS

434c

α-Ketobutyrate

0.2

SITS

1.18 (mM)c

DBDS

215c

DNDS

>5(mM)c

TBenzDS

6.7c

NBDS

397c

AR-C155858

2 (nM)c

AR-C117977

2 (nM)c

Phloretin

5.1c

Quercetin

2c

NPPB

9.3c

Niflumic acid

6.1c

3-Isobutyl-1-methylxanthine

288c

Mersalyld

50c

n.a. transporter kinetic parameters were not determined

CHC α-Cyano-4-hydroxycinnamate, NPPB 5-nitro-2-(3-phenylpropyl-amino)benzoate, DIDS 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, DBDS 4, 4L-dibenzamidostilbene-2, 2L-disulfonate, DNDS 4,4′-dinitro-stilbene-2,2′-disulfonic acid, TBenzDS N,N,N′,N′-tetrabenzyl- 4,4′-diaminostilbene-2,2′-disulphonate, NBDS 4-chloro-7-nitrobenzofurazan, pCMBS 4-chloromercuribenzenesulfonate, SITS 4-acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid

aKi (μM), inhibition constant

bIC50 (μM), inhibitor concentration producing 50% inhibition

cK0.5 (μM), substrate concentration giving v= Vmax/2

dInhibition by organomercurials is only observed when MCT1 associates with basigin and not embigin

eInhibition of MCT2 by AR-C155858 is only observed when MCT1 is associated with basigin and not embigin

fAgent only tested at a single concentration (0.1 mmol/L) and data are presented as percent inhibition

gThese substrates are transported slowly and acted better as inhibitors

hD,L racemic mix used in these studies

Transport mediated by MCTs 1–4 is proton dependent, while the transport by MCTs 8 and 10 is proton independent. It has been shown that MCTs operate through a series of conformation changes that are induced by the ordered binding of a proton and the substrate (Halestrap 2012). The kinetics of this ordered transport are depicted in Fig. 3 with lactate as the substrate. The translocation of the bound protein (k1) is more rapid than that of the transporter alone (k2); for this reason, the transport of a monocarboxylic acid is slower than the rate of monocarboxylate exchange (Halestrap 2013).
Monocarboxylate Transporter (SLC16A), Fig. 3

MCTs transport substrates in a kinetically ordered manner. Translocation of the bound transporter is more rapid than that of the transporter alone (Reproduced with permission from Halestrap (2013))

The proposed mechanism of transport for MCT1 begins with the protein in the open conformation that then binds a proton through an interaction involving a key lysine residue (K38) (Halestrap 2012). The incorporation of the proton changes the binding pocket from a neutral area to one that is charged and able to bind the negatively charged monocarboxylate substrate (Halestrap 2012). Once the second binding event takes place, MCT1 undergoes a conformational change to the closed state as the proton and monocarboxylate are transferred through the channel. This passage is mediated by other key residues in the structure of MCT1 including aspartate 302 (D302) and arginine 306 (R306) (Halestrap 2012). This conformational change results in the opening of the transporter on the opposite side of the membrane, and the release of the monocarboxylate followed by the proton (Halestrap 2012). This mechanism is illustrated in Fig. 4 where lactic acid is the model substrate, providing both the proton and monocarboxylate, lactate, for transport (Halestrap 2013).
Monocarboxylate Transporter (SLC16A), Fig. 4

Proposed mechanism of proton-dependent monocarboxylate transport by MCT1 (Reproduced with permission from Halestrap (2013))

Figure 5 provides a more detailed look at the structure of MCT1 and how this structure relates to the function of the transporter. The structure was obtained through molecular modeling and highlights K38 which has been discussed, as well as F360, which is a phenylalanine that is critical in the translocation cycle of MCT1. D302 and R306 are not highlighted in this figure, but lie along the substrate channel near F360.
Monocarboxylate Transporter (SLC16A), Fig. 5

Molecular modeling derived structure of MCT1. Key residues for transport function (K38 and F360) as well as residues involved in DIDS binding (K45, 282, and 413) are highlighted (Reproduced with permission from Halestrap (2013))

There are two ancillary proteins that are also involved in the function of MCTs. These proteins, known as basigin and embigin, contain a single TM domain with a conserved glutamate residue, an intracellular C-terminus, and an extracellular domain that contains multiple glycosylation sites (Halestrap 2012). The general structure of the conserved elements of these two proteins is shown in Fig. 6. These proteins allow for the plasma membrane expression of MCTs and interact differently with particular MCT isoforms depending on their expression. MCT1 preferentially interacts with basigin, but can bind embigin when basigin is not present; the opposite is true of MCT2. The requirement of an ancillary protein has been highlighted by the ability of certain compounds, such as AR-C155858, to differentially inhibit transport by MCTs based on the involvement of basigin and embigin (Halestrap 2012). MCT1 is inhibited by AR-C155858 regardless of which ancillary protein is available. However, the activity of MCT2 is mostly retained in the presence of AR-C155858 when embigin is present but is lost when only basigin is available (Halestrap 2012).
Monocarboxylate Transporter (SLC16A), Fig. 6

The generalized structure of basigin and embigin showing the conserved elements of the two ancillary proteins (Reproduced with permission from Halestrap (2013))

Physiological Function of MCTs

MCTs 1–4

The predominant role of MCTs 1–4 is the transport of l-lactate, pyruvate, and ketone bodies such as d-β-hydroxybutyrate and acetoacetate across the plasma membrane of mammalian cells and to maintain intracellular pH (Halestrap 2013). Since lactic acid (not l-lactate) is produced and used in cellular respiration, the ability of MCTs to transport monocarboxylic acids with a proton is best suited to its metabolic role. Transport of lactate, pyruvate, and other monocarboxylates into the mitochondria is mediated by a distinct proton-linked carrier known as the mitochondrial pyruvate carrier (MPC). MPCs and MCTs mediate the transport of most monocarboxylic acids (Halestrap 2013). Key metabolic pathways requiring monocarboxylate transport are illustrated in Fig. 7. MCTs 1–4 can facilitate either the influx and/or the efflux of monocarboxylates, with the net direction of transport dependent on the concentration gradient of protons across the plasma membrane (Halestrap 2013).
Monocarboxylate Transporter (SLC16A), Fig. 7

Key metabolic pathways requiring monocarboxylate transport across the plasma and inner mitochondrial membranes (Reproduced with permission from Halestrap (2013))

Lactic Acid Efflux Out of Cells

Lactic acid is a byproduct of anaerobic glycolysis, which is stimulated when the oxygen supply is compromised. MCT1 predominantly mediates efflux of lactic acid. However, red blood cells, lymphocytes, astrocytes, tumor cells, and white muscle fibers rely on glycolysis for their energy supply even under aerobic conditions (aerobic glycolysis). With the exception of red blood cells, such tissues/cells have high expression of MCT1 (uptake) and MCT4 (efflux). MCT4 has a very low affinity for pyruvate (KM = 150 mM) compared to MCT1 (KM = 1 mM), which prevents efflux of pyruvate from the cell (Halestrap 2013). This is crucial since glycolysis requires lactate dehydrogenase-mediated conversion of pyruvate to l-lactate to regenerate cytosolic concentrations of NADH from NAD+. Moreover, MCT4 has a high KM (low affinity) for l-lactate (20 mM), which causes fatigue in white skeletal muscle fibers during high endurance exercise. This is due to the slow rate of lactic acid efflux causing lactic acid accumulation with a consequent decrease in intracellular pH (Halestrap 2013).

Lactic Acid Uptake into Cells

In contrast to glycolytic cells that produce lactic acid, some tissues/cells use lactic acid. Since lactic acid is a substrate for lipogenesis and gluconeogenesis, tissues such as the liver, proximal tubule cells of kidney, and adipose tissue that carry out these processes express both MCT1 and MCT2 depending on the species (Halestrap 2013). Lactic acid and ketone bodies are also important respiratory substrates in tissues such as the heart, red skeletal muscle, and brain. The uptake of lactic acid in the heart and skeletal muscle is mediated primarily by MCT1 and by MCT2 in neurons (Halestrap 2013).

There is increasing evidence that, within a tissue, lactic acid produced by glycolysis in one cell type can be exported for use in other cells as a respiratory fuel (Halestrap 2013). The best documented examples of such “lactate shuttles” are in the brain and skeletal muscle which are illustrated in Fig. 8. Table 3 describes the tissue-specific lactic acid transport mediated by MCT 1–4.
Monocarboxylate Transporter (SLC16A), Fig. 8

In the brain and muscle, MCTs are used to transport lactic and ketone bodies from the blood into the tissue as to shuttle lactic acid between the glycolytic astrocytes and white muscle fibers to the neurons and red fibers that oxidize it. A similar lactic acid shuttle may operate in some tumors where the hypoxic center of the tumor produces lactic acid that is oxidized by the normoxic peripheral cells (Reproduced with permission from Halestrap (2013))

Monocarboxylate Transporter (SLC16A), Table 3

Tissue-specific lactic acid transport mediated by MCTs 1–4 (Halestrap and Meredith 2004; Halestrap 2013)

Tissue

Major monocarboxylate transporters expressed

Major roles of plasma membrane lactic acid transport

Liver

MCT1, MCT2, MCT7

Entry of lactic acid for gluconeogenesis and lipogenesis. Export of ketone bodies

Heart

MCT1

Entry of lactic acid and ketone bodies for oxidation as respiratory fuels

Red skeletal muscle

MCT1

Entry of lactic acid and ketone bodies for oxidation as respiratory fuels

White skeletal muscle

MCT4

Export of lactic acid produced by glycolysis

Kidney cortex

MCT1, MCT2

Lactic acid uptake for gluconeogenesis

Kidney tubule epithelial cells

MCT1, MCT2

Reabsorption of lactate, pyruvate, and ketone bodies

Intestinal epithelial cells

MCT1, MCT2

Absorption of lactate, pyruvate, and ketone bodies

Adipose tissue

MCT1

Efflux of lactic acid produced by glycolysis

Blood-brain barrier

MCT1

Transport of lactic acid and ketone bodies into the brain

Neurons

MCT1, MCT2

Uptake of lactic acid and ketone bodies as respiratory fuels

Glial cells and astrocytes

MCT1, MCT4

Efflux of lactic acid produced by glycolysis for subsequent use as a respiratory fuel by neurons

Retina

MCT1, MCT3, MCT4

Rapid export of lactic acid produced by glycolysis is important to maintain osmotic balance in the retina

Red blood cells

MCT1

Efflux of lactic acid produced by glycolysis

T lymphocytes

MCT1

Efflux of lactic acid produced by glycolysis especially during activation and proliferation

Tumor cells

MCT1, MCT4

Efflux of lactic acid produced by glycolysis in tumor cells

Testis

MCT1, MCT2, MCT4

Uptake of lactic acid, which is essential for spermatogenesis, in Sertoli cells

Sperm

MCT2, MCT4

Efflux of lactic acid produced by glycolytic metabolism

Ketone Body Metabolism

The ketone bodies, acetoacetate and d-β-hydroxybutyrate, are produced by the liver under conditions of high fatty acid oxidation (such as during endurance exercise and starvation), which are taken up by the heart, red skeletal muscle, and brain to be used in cellular respiration. Export of the ketone bodies from the liver involves either MCT1 or MCT2 (depending on the species). Recently, MCT7 has also been implicated (Halestrap 2013).

pH Regulation and Redox Communication Across Cells

While intracellular pH is tightly controlled by a range of facilitated transport mechanisms that regulate acid efflux and uptake such as the Na+/H+ exchanger and bicarbonate/CO2 exchanger, in highly glycolytic cells, the efflux of lactic acid mediated by MCT1 or MCT4 is quantitatively the greatest proton efflux mechanism. This ensures that there is no accumulation of excess lactic acid within the cells, which would cause intracellular acidosis during conditions of high glycolytic flux. Lactic acid efflux by MCT1 also plays an important role in the restoration of intracellular pH following the refractory period after ischemia or hypoxia, whereas its inhibition slows the return of the pH to the homeostatic range. Intracellular carbonic anhydrase II has been shown to interact with MCT1 and MCT4 to enhance their transport activity, while MCT2 is stimulated by an interaction with extracellular carbonic anhydrase IV. Interactions between sodium bicarbonate cotransporter (NBC) and MCT1 have shown to increase the rate of lactic acid transport in Xenopus oocytes co-expressing NBC with MCT1 (Halestrap 2013).

Monocarboxylate transport also regulates the redox state of NADH/NAD+ in one tissue to influence that of another through regulating blood concentrations of l-lactate, pyruvate, d-β-hydroxybutyrate, and acetoacetate. This occurs because lactate dehydrogenase is exclusively cytosolic and uses cytosolic NADH/NAD+ to interconvert pyruvate and l-lactate, while β-hydroxybutyrate dehydrogenase is exclusively intra-mitochondrial and uses mitochondrial NADH/NAD+ to interconvert acetoacetate and d-β-hydroxybutyrate (Halestrap 2013).

MCT8 and MCT10

MCT8 is expressed in most (if not all) tissues. Its expression in the brain endothelial microvessels suggests that it is important for the uptake of T3 (thyroid hormone) across the blood-brain barrier, which is confirmed by the neurological defects seen in patients with mutations in MCT8 (Halestrap 2013). MCT10 is also ubiquitously expressed and is localized predominantly in the basolateral membrane. Apart from being an aromatic amino acid transporter, its major physiological role(s) remains uncertain (Halestrap 2013).

Drug Transport

Given their extensive tissue distribution and broad substrate range, MCTs have the potential to be influential in the pharmacokinetics (PK) of many pharmaceutical compounds. Of particular note, MCTs are expressed in critical tissues for elimination and absorption including the liver, kidney, and intestines (Morris and Felmlee 2008). In the kidney, many MCTs are expressed, including MCT1 on both the apical and basolateral membrane (Morris and Felmlee 2008). This has been shown to be influential in the disposition of a drug of abuse, γ-hydroxybutyric acid (GHB). GHB exhibits nonlinear kinetics that are caused by saturable absorption, capacity-limited metabolism, and nonlinear renal clearance (Morris and Felmlee 2008). GHB is actively reabsorbed from the proximal tubule by MCT1 after it is filtered in the glomerulus. This reabsorption limits the renal clearance (CLR) of GHB at lower doses, but as the dose increases, MCT1 becomes saturated, and the CLR of GHB increases nonlinearly. This increase in CLR has been targeted as a mechanism by which to treat GHB overdose through the use of an MCT1 inhibitor. Vijay et al. used this approach in studies with rats, which showed that the administration of a potent MCT1 inhibitor, AR-C155858, did limit the exposure of the animal to GHB through an increase in CLR and total clearance (CL) (Vijay and Morris 2014).

MCT1 is also expressed in the intestine where it can act as an absorptive transporter and increase the bioavailability of compounds. This property is of interest because it can be exploited to enhance the bioavailability of compounds. This was done in the case of gabapentin, an anticonvulsant with low bioavailability (Morris and Felmlee 2008). A prodrug of this compound was created, XP13512, which was designed to be a substrate of MCT1. XP13512 showed superior oral absorption and bioavailability in rats and monkeys compared to gabapentin (Morris and Felmlee 2008). These examples demonstrate the potential for MCTs to influence the PK of exogenous compounds, as well as their potential to be exploited to achieve a desired outcome in disposition of a compound. MCTs have been shown to transport the β-lactam antibiotics cefdinir and carindacillin, salicylic acid, pravastatin and atorvastatin, and probenecid (Morris and Felmlee 2008). The more recently characterized MCT6 has been shown to transport loop diuretics including bumetanide (Morris and Felmlee 2008).

Regulation

Transcriptional Regulation

Elevated calcium and adenosine monophosphate (AMP), particularly as a result of activity in skeletal muscle, has been shown to upregulate the expression of MCT1 (Halestrap and Wilson 2012). This is likely due to the respective activation of calcineurin and AMP-activated protein kinase (AMPK) by these two agents (Halestrap and Wilson 2012). The effects of calcineurin, a calcium-dependent protein phosphatase, are thought to be mediated through the transcriptional factor nuclear factor of activated T-cells (NFAT) (Halestrap and Wilson 2012). This is supported by the presence of several NFAT binding sites in the promoter region of MCT1, as well as the modulation of MCT1 expression by immunosuppressive agents (Halestrap and Wilson 2012). The transcriptional coactivator, PGC1α, is activated by AMPK and elevated calcium levels and may play a role in the regulation of MCT1. Activation of PGC1α by 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleoside (AICAR) was shown to stimulate MCT1 promotor activity in L6 myoblasts and HepG2 hepatoma cells; however, MCT4 was downregulated in the same study (Halestrap and Wilson 2012). AICAR has also been shown to downregulate MCT1 in rat Sertoli cells and upregulate MCT4 in skeletal muscle (Halestrap and Wilson 2012). Conversely, thyroid hormone (T3) has been shown to upregulate MCT1 and MCT4 mRNA in skeletal muscle, but only protein levels of MCT4 were increased (Halestrap and Wilson 2012). The growth factor, brain-derived neurotrophic factor (BDNF), increases MCT2 expression in mouse-cultured cortical neurons (Halestrap and Wilson 2012).

Both food deprivation and obesity have been shown to affect the expression of MCTs based on the isoform and tissue being investigated. In rats, food depravation and obesity resulted in an increase in expression of MCT2 in the brain; the latter condition has been shown to increase MCT1 and MCT4 as well (Halestrap and Wilson 2012). Hypoxia also plays a major role in the expression of MCTs. In human adipocytes, hypoxia reduces the expression of MCT2, but enhances the expression of MCT1 and MCT4 (Halestrap and Wilson 2012). For MCT4, this effect appears to be regulated by the transcription factor HIF-1α as cells without this factor did not overproduce MCT4 in hypoxic conditions. MCT4, and not MCT1 or MCT2, has four potential hypoxia response elements in its promoter region that could be responsible for this effect (Halestrap and Wilson 2012).

Posttranscriptional Regulation

There is evidence that MCT1 and MCT2 are subject to translational regulation. Noradrenaline, insulin, and IGF-1 have all been shown to increase the expression of MCT2 in the brain following transcription (Halestrap and Wilson 2012). This is due to translational activation and is facilitated through the stimulation of phosphoinositide 3-kinase/Akt/mammalian target of the rapamycin pathway. MCT1 protein expression has been shown to vary with the phases of the cell cycle, with a severalfold surge in expression during the postmitotic and G1 phases (Halestrap and Wilson 2012). This is consistent with the involvement of polyadenylation for several reasons. First, MCT1 has potential cytosolic polyadenylation and hexanucleotide elements, which are implicated in regulation of the exit of the mitotic phase in the cell cycle (Halestrap and Wilson 2012). Additionally, the phosphorylation states of the initiation factors eIF4E and 4EBPI change in parallel with the rate of translation of the proteins (Halestrap and Wilson 2012).

Short-Term Regulation

Trafficking of MCT1 has been implicated in the heart, where protein expression increases following surgery, but little change in mRNA is observed (Halestrap and Wilson 2012). This may be a result of translocation from an intracellular pool of MCT1. This is supported by the presence of motifs in the C-terminus of MCT1 that are involved with endosomal targeting of GLUT 4. The actual transport activity of MCTs has been shown to be modulated by another protein, carbonic anhydrase. The interaction between carbonic anhydrase II and MCT1/4 results in enhanced transport activity, while MCT2 interacts with carbonic anhydrase IV and not II (Halestrap and Wilson 2012). This interaction may be a result of the carbonic anhydrase acting as a “proton-collecting antenna” and facilitating the activity of the MCTs (Halestrap and Wilson 2012). Finally, stimulation of MCT1 activity has been demonstrated via cyclic AMP in rat brain endothelial cells; this, however, has not been confirmed (Halestrap and Wilson 2012).

Role in Disease

Known Mutations

Exercise-Induced Hyperinsulinemia

Mutations in the MCT1 promoter regions have shown to cause the exercise-induced hyperinsulinemia (EIHI) with hypoglycemia, as a result of an increase in MCT1 activity. This leads to the expression of MCT1 in insulin-secreting β cells of the islets of Langerhans that do not normally express MCT1. The activity of MCT1 in β cells enables lactate to be oxidized during exercise and thus provide increased ATP levels, while depleting ADP, to signal insulin secretion. This, in turn, causes the observed hypoglycemia in patients (Halestrap and Wilson 2012; Halestrap 2013).

Severe X-Linked Psychomotor Retardation

A mutation in the MCT8 (SLC16A2) gene (located on the X chromosome) has been established to cause severe X-linked psychomotor retardation, in which the lack of MCT8 activity in the brain prevents thyroid hormone uptake and hence the normal brain development (Halestrap and Wilson 2012; Halestrap 2013).

Fatigue

Any reduction in the expression and activity of MCTs 1–4 impairs the homeostatic lactic acid efflux leading to decrease in intracellular pH and reduction in the rates of glycolysis. It has been proposed that MCT1 deficiency in muscle could cause a rare condition known as cryptic exercise intolerance, which is associated with exercise induced muscle cramping; however, the genotype-phenotype relationships need be confirmed in humans. Alternatively, expression of basigin or embigin and autoantibodies against extracellular epitopes of basigin are currently an area of research for the MCT-induced fatigue (Halestrap and Wilson 2012; Halestrap 2013).

Cancer

Tumor cells heavily depend on glycolysis for their energy metabolism and proliferation. Therefore, cancer cells have high levels of MCT expression to maintain an appropriate pH environment for tumor growth (55, 56). However, there is considerable variation in the MCT isoforms expressed in different tumors and in their associated ancillary proteins. Aggressive metastatic tumors frequently have upregulation of HIF-1α and MCT4 expression and activity, as well as of basigin (55, 56). This helps establish ideal conditions for proliferation, migration, and metastasis of tumor cells (Halestrap and Wilson 2012; Pinheiro et al. 2012; Halestrap 2013). However, this also presents MCTs as therapeutically viable targets for cancer therapy. The metabolic pathways leading to lactate production in cancer cells and strategies for lactate transport inhibition to treat cancer are illustrated in Fig. 9.
Monocarboxylate Transporter (SLC16A), Fig. 9

Overview on the metabolic pathways leading to lactate production (continuous lines) and transport across the plasma membrane, as well as strategies of lactate transport inhibition. Discontinuous arrows represent lactate uptake and flow inside oxidative cancer cells. Abbreviations: CHC α -cyano-4-hydroxycinnamic acid, LDH lactate dehydrogenase, MCT monocarboxylate transporter, PDH pyruvate dehydrogenase, PDK1 pyruvate dehydrogenase kinase 1 (Reproduced with permission from Pinheiro et al. (2012))

Summary

The importance of members of the SLC16 family, and especially MCTs 1–4, in a wide range of physiological and pathological processes is now well established and recognized. Growing research in elucidating the role of other MCT isoforms will add significantly to the current knowledge of this 14-member transporter family. The recent discovery of potent MCT1-specific inhibitors will provide a valuable tool for investigating the metabolic roles of MCT1, and their ability to act as immunosuppressive drugs illustrates the promise of MCTs as pharmacological targets. Additionally, MCTs are in the forefront of cancer research as it has already been shown that inhibiting MCT activity in tumor cells reduces cell proliferation and induces cell death. MCTs represent promising novel therapeutic targets.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Marilyn E. Morris
    • 1
  • Rutwij A. Dave
    • 2
  • Kristin E. Follman
    • 1
  1. 1.Department of Pharmaceutical SciencesUniversity at Buffalo, State University of New YorkBuffaloUSA
  2. 2.Preclinical and Translational Pharmacokinetics, Genentech Inc.South San FransiscoUSA