The SLC16A Family of Monocarboxylate Transporters (MCTs)—Physiology and Function in Cellular Metabolism, pH Homeostasis, and Fluid Transport

Abstract

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1–4) are H⁺-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1–4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1–4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate “lactate-shuttling” between glycolytic and oxidative cells within tissues and across blood–tissue barriers. However, the role of MCTs 1–4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1–4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

Introduction

The SLC16 family of monocarboxylate transporters, MCTs, is composed of 14 members. They share similarities in sequence and protein structure, but each member differs from others in the substrates that they transport. Lactate, among other monocarboxylates, is the primary substrate for MCTs 1–4. MCT6 was shown to transport synthetic drugs such as probenecid and bumetanide. MCT8 transports thyroid hormones (T₃ and T₄) and mutations in this transporter (Allan–Herndon–Dudley Syndrome) cause crippling defects in brain and motor functions throughout development because of the lack of T3 uptake by neurons (Friesema et al., 2004; Schwartz et al., 2005). MCT10 (or T-type amino acid transporter-1; TAT1) is also capable of transporting thyroid hormones (Friesema et al., 2008), but unlike MCT8, its transport substrate includes other aromatic amino acids and that its gene expression is found primarily in the kidney, skeletal muscle, and small intestine (Kim et al., 2002). MCT9 was recently identified as a carnitine transporter through a meta-analysis of 14 GWAS studies of patients with elevated serum uric acid (Kolz et al., 2009). This finding was also independently verified in a separate study that evaluated the genetic links to human metabolism by coupling GWAS analysis with a high-throughput screening of blood metabolites (Suhre et al., 2011). Beyond the abovementioned MCTs, however, the transport substrates of the other MCTs (MCTs 5, 7, 11–14) are unknown. Of all MCTs, MCTs 1–4 are the most widely studied and understood. This is not surprising, as lactate has well-established roles in acid–base balance, with the earliest studies dating back to almost half a century ago. The advent of a noninvasive fluorescence-based intracellular pH indicator, BCECF (Rink, Tsien, & Pozzan, 1982), allowed physiologists to easily demonstrate that transmembrane lactate transport is accompanied by intracellular acidification. This observation gave rise to an explosion of studies that demonstrated the existence and function of H⁺-coupled lactate transporters in various biological systems (Giasson & Bonanno, 1994; Kenyon, Yu, La Cour, & Miller, 1994; Poole & Halestrap, 1991; Roth & Brooks, 1990; Wang, Levi, & Halestrap, 1994).

For many decades, lactate was regarded as nothing more than a metabolic waste product of glycolysis. The accumulation of lactate within cells and their microenvironment, such as in muscle fibers during exercise, was thought to cause lactic acidosis with detrimental consequences to normal physiological function. However, with accumulating evidence demonstrating that lactate can be utilized as a metabolic substrate for oxidative respiration, there came a realization that lactate is more than simply a waste product (Gladden, 2004)—a radical idea at the time that raised much interest in the roles of MCTs 1–4 in metabolism in many parts of the human body, including the muscle, brain, kidney, liver, retina, and testis, all of which will be discussed in detail below (Section 2). Although MCTs are important for normal metabolic function, they are also implicated in disease. In cancer cells, for example, MCT1 and MCT4 are commonly upregulated to adapt to a change in metabolism and to maintain pH homeostasis. In addition to regulating cellular metabolism, MCTs also participate in fluid transport in corneal endothelium of the eye (Section 4.2) and acid–base balance in the proximal convoluted tubule (PCT) (Section 4.3) of the kidney. These processes are achieved by functional interaction between MCTs and HCO₃⁻ transport mechanisms (Section 3). With most of the functions of MCTs 1–4 revolving around lactate, it is easy to overlook the fact that MCTs 1–4 are just as capable of transporting other substrates, such as acetate, propionate, and butyrate—all of which are absorbed from the colon lumen via Na⁺-coupled MCTs (SMCTs) and MCT1 (Section 4.4).

Although the crystal structure of MCTs has not been determined, hydrophobicity plots of MCTs 1–14 predict that these proteins consist of 12 transmembrane domains (TMs) with cytoplasmic N and C termini (Halestrap, 2012). The coupling of H⁺ to monocarboxylate transport is unique to MCTs 1–4, and the structural property that is responsible for this feature, a lysine residue in TM1 (Lys38; Fig. 1A), has only been recently identified in rat MCT1 (Wilson, Meredith, Bunnun, Sessions, & Halestrap, 2009). In their model, Lys38 is normally uncharged in the hydrophobic environment of the ion pore, but it can bind to an extracellular H⁺ to trigger the open conformation, in which the same lysine residue could subsequently bind a lactate ion. Upon binding, both the H⁺ and lactate ions are transferred to an aspartate–arginine ion pair (Asp302–Arg306; TM8) located deeper in the channel pore (Manoharan, Wilson, Sessions, & Halestrap, 2006). These residues were identified in rat MCT1, but they are also found in MCTs 1–4 of both rat and human (Fig. 1B). Although MCTs 1–4 transport lactate and other similar substrates, they differ in substrate binding affinity and selectivity. An early study by Garcia et al. identified an amino acid residue that regulates the substrate selectivity of MCT1—a phenylalanine residue in TM10, Phe360 (Garcia, Brown, Pathak, & Goldstein, 1995). It is interesting to note that in MCTs 3 and 4, however, a tyrosine, rather than phenylalanine, is found in this position (Fig. 1C). This switch may account for the differences in lactate-binding affinities among MCTs 1–4, which seem to correlate with their function when expressed in specific cell types within a biological system. For example, MCT1 and MCT2, with their high affinity for lactate (K_m ≈ 1–3 mM), possess the ability to mediate rapid lactate uptake into cells (Halestrap & Price, 1999). As such, MCT1 and MCT2 are commonly expressed in tissues that utilize lactate as a substrate for oxidative metabolism, such as slow-twitch type I muscle fibers (MCT1) and neurons (MCT2). On the other hand, MCT3 and MCT4 have lower affinities for lactate (MCT3, K_m ≈ 6 mM; MCT4, K_m ≈ 30 mM), but they possess a wide range of transport rates and are, therefore, useful for maintaining lactate equilibrium in cells. MCT4 is commonly expressed in glycolytic cell types, such as type II fast-twitch skeletal muscle cells and astrocytes, where it mediates lactic acid extrusion. MCT3 is preferentially expressed in the retinal pigment epithelium, where it also mediates lactic acid extrusion.

With specific MCTs being differentially expressed in different cell types, or even in different membranes of a polarized epithelium, it is obvious that MCTs, similar to many other transporters, are under strict regulatory control. SLC16A1 (MCT1) and SLC16A3 (MCT4) carry multiple splice variants and differ only at the 5′-UTR region, suggesting that MCT1 and MCT4 transcripts can be subject to both transcriptional and post-transcriptional regulation. This level of control may be necessary because MCT1 and MCT4 are widely expressed in many different cell types, thus requiring additional mechanisms to regulate their relative abundance in a cell-specific manner. Furthermore, MCT1 transcripts can also be regulated by microRNAs, which target the mRNA at its 3′-UTR for translational repression or mRNA degradation. This mechanism is particularly important in beta-cells, where MCT1 is not normally expressed (Pullen, da Silva Xavier, Kelsey, & Rutter, 2011). On the other hand, no other transcript variants for SLC16A7 (MCT2) and SLC16A8 (MCT3) have been found, consistent with the more limited tissue distributions of MCT2 and MCT3.

Although the protein abundance of MCTs 1–4 can be regulated at the genetic level, the proper folding and trafficking of the MCTs 1–4 proteins to the plasma membrane requires each MCT to be assembled with either CD147/basigin (encoded by Bsg) or embigin (encoded by Emb), both of which are highly glycosylated single-pass membrane proteins that serve as chaperones for proper targeting of MCTs to the plasma membrane (Kirk et al., 2000; Philp, Ochrietor, Rudoy, Muramatsu, & Linser, 2003; Wilson et al., 2005). CD147 is widely expressed in tissues and forms a heterodimer with MCT1, 3, or 4. Embigin, on the other hand, associates primarily with MCT2, but it can also interact with MCT1 in some tissues. The dependence of MCTs 1–4 on their accessory proteins has been demonstrated in in vitro studies, but the physiological importance of this interaction in MCT maturation and trafficking is perhaps best demonstrated in the Bsg-null mouse (Nakai, Chen, & Nowak, 2006; Philp et al., 2003), which present with blindness, sterility, immunodeficiency, and problems with learning and memory (Hori et al., 2000; Igakura et al., 1998; Naruhashi et al., 1997). These phenotypes, as is discussed in the following sections (Sections 2 and 4), are direct pathological manifestations caused by the absence of MCTs. The underlying mechanisms behind these conditions will become readily apparent when these MCTs are placed in the context of their physiological functions in various cell types within their native biological systems.