Amino acid transporters revisited: New views in health and disease

AATs are transmembrane proteins that play vital roles by regulating energy metabolism, protein synthesis, gene expression, redox balance, signal transduction pathways and growth at the cellular and whole body levels. Because of their central biological importance, alterations in the expression and function of AATs are linked to a wide range of pathologies, including neurodegenerative diseases, inborn errors of metabolism and chronic kidney diseases. AATs also participate in the secretion and release of hormones (e.g., insulin and glucagon), and their altered function is likely associated with pathogenesis of diabetes. Dysregulation of AATs is implicated in autophagy and tumor cell proliferation via metabolic reprogramming, including the biosynthesis of proteins and nucleotides and the production of ATP and NADPH. Several AATs have been proposed as promising anticancer drug targets, e.g. SLC1A5, SLC7A5, SLC16A4, and SLC38A2. Tumor cell AAT expression can be exploited for molecular imaging of tumors using transporter-specific positron-emission tomography (PET) probes. Recent PET studies have indicated a close relationship between the activation of various oncogenes and alterations of cellular metabolism via AATs. This novel approach will likely help in the future to optimize patient therapy management by improving monitoring of tumor grade staging, planning of best possible chemotherapeutic regimes and monitoring tumor response to therapy. Many of the AATs are still understudied and uncovering their function and pathophysiological roles may unveil novel therapeutic strategies and drug discovery opportunities. Not too surprisingly, given that cancer therapeutics often represent amino acid mimetics, numerous AATs turn out to interact directly with these agents. Thus, AAT expression levels in tumor cells can determine drug efficacy by altering their delivery. Such knowledge will be helpful in the future to predict the outcome of cancer therapies. Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically. Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically. these systems have acronyms that denote the substrate specificity of the amino acid transporter (AAT). Uppercase letters denote Na+-dependent transporters (with the exception of system L, system T, and the proton-coupled amino acid transporters); lower case is used for Na+-independent transporters (for example, asc y+ and x−c). X− or x− indicate transporters for anionic amino acids (as in X−AG and x−c). The subscript ‘AG’ indicates that the transporter accepts aspartate and glutamate, and the subscript ‘c’ indicates that the transporter also accepts cystine. Y+ or y+ refer to transporters of cationic amino acids (an Na+-dependent cationic amino acid transporter has not been unambiguously defined and as a result Y+ is not used). B or b refers to a transporter accepting NAAs, and superscript + indicates a transporter for cationic amino acids. T stands for a transporter for aromatic amino acids and system N indicates selectivity for amino acids with nitrogen atoms in the side chain. In the remaining cases, the preferred substrate is indicated by the one-letter code for amino acids. For example, system L refers to a leucine-preferring transporter and system ASC to a transporter preferring alanine, serine, and cysteine. Proline and hydroxyproline are referred to as imino acids. Description of the specific systems: based on the regulation and the functional properties, the Na+-coupled neutral amino acid (NAA) transporters (SNATs) of the SLC38 superfamily members are subdivided into two groups, namely system A and system N amino acid transporters. System A transporters (SNAT1/SLC38A1, SNAT2/SLC38A2, and SNAT4/SLC38A4) are Na+-dependent cotransporters that facilitate the transport of a broad range of amino acids such as L-Gly, L-Ala, L-Cys, and L-Gln and they are pH sensitive. System N transporters (SNAT3/SLC38A3, SNAT5/SLC38A5, and SNAT8/SLC38A8) mediate the transport of amino acids containing nitrogen in the side chain, such as L-Gln, L-His, and L-Asn. Na+-dependent transporters that function as obligatory exchangers. The SLC1 family members, SLC1A4 (also called ASCT1) and SLC1A5 (also called ASCT2), have been shown to transport short-chain amino acids such as L-Ala, L-Ser, and L-Cys, and also L-Thr and L-Gln. These are amino acid transporters of broad specificity. Upper case letter denotes Na+-coupled active transport. Superscript 0 indicates a transporter accepting neutral amino acids (NAAs) and superscript + indicates a transporter specific for cationic amino acids (CAAs). For example, SLC6A19, also known as B0AT1, facilitates Na+-dependent transport of a broad range of NAAs into epithelial cells of small intestine and kidney proximal tubules. SLC7A5/LAT1 is a member of the SLC7 family and forms a heterodimer with the heavy subunit CD98hc/SLC3A2 via a disulfide bond. CD98hc/SLC3A2 is a type II glycoprotein that functions as an integral subunit of SLC7A5, stabilizing and facilitating its translocation to the plasma membrane. SLC7A5 itself is the functional unit of the heterodimeric transporter complex and its substrate range includes large neutral amino acids (LNAAs) such as L-Tyr, L-Leu, L-Ile, L-Val, and L-Phe as well as pharmaceutical compounds, such L-DOPA, the chemotherapy drug melphalan, and the epilepsy drug gabapentin. An additional system L transporter is SLC7A8/LAT2, also with SLC3A2 as the heavy chain. SLC43A1/LAT3 and SLC43A2/LAT4 are part of the system L family as well and they transport branched chain amino acids (BCCAs) in an Na+-independent manner, although they are structurally and functionally distinct. LAT3 is expressed in human pancreas, liver, skeletal muscle, and fetal liver, whereas LAT4 is expressed in placenta, kidney, and peripheral blood leukocytes. this system corresponds to TAT1 (SLC16A10), a transporter of the aromatic amino acids L-Tyr and L-Trp and, to a lower extent, L-Phe. It is expressed in the basolateral membrane of intestine and kidney proximal tubule epithelial cells. X− or x− indicates transporters for anionic amino acids and ct stands for cystine, the oxidized form of L-Cys. This system is represented by the heterodimeric amino acid exchanger SLC7A11 (also known as xCT), with SLC3A2 being the heavy subunit. It mediates cellular uptake of L-cystine, with exit of L-Glu. This exchanger is essential for glutathione (GSH) synthesis and maintenance of the cellular redox balance (see Figure 4). y+ indicates the capacity of the transporters to mediate the transfer of cationic amino acids (CAAs) in a Na+-independent manner. ‘LAT’ refers to the ability to mediate transport of large neutral amino acids (large NAAs) of system L but in a Na+-coupled manner. For example, SLC7A6 (y+LAT2) and SLC7A7 (y+LAT1) mediate the cellular uptake of large NAAs such as L-Leu in an Na+-coupled manner, but the process is obligatorily coupled to the efflux of cationic amino acids (CAAs). These transporters are expressed in the basolateral membrane of absorptive epithelial tissues of intestine and kidney proximal tubules, where they play an essential role, mediating basolateral exit of CAAs into the blood circulation (see Figure 2). this is an autosomal recessive disorder found primarily in the Japanese population. It is caused by loss of function mutations of the SLC25A13/citrin mitochondrial L-Asp and L-Glu transporter gene. L-Asp is required in the urea cycle during the conversion of L-Orn to L-argininosuccinate. Thus, SLC25A13 dysfunction blocks the urea cycle, with buildup of ammonia (hyperammonemia) and other toxic substances, leading to the characteristic symptoms of the disease, including life-threatening central nervous system dysfunctions (confusion, abnormal behaviors, seizures, etc.) and cholestasis. a tripeptide (γ-L-glutamyl–L-cysteinylglycine) that acts as an antioxidant. The reduced form (GSH) can react with H2O2 to form the oxidized form (GSSG). GSH plays important roles in antioxidant defense, nutrient metabolism and regulation of cellular processes including cell differentiation, proliferation, and apoptosis. the growth and proliferation of many cancer cell types is L-Gln-dependent, and they fail to survive in L-Gln-deprived cell culture medium. This phenomenon is known as glutamine addiction. Tumor cells use L-Gln to generate the building blocks and energy for anabolic purposes through the glutaminolysis pathway (Figure 4). L-Gln is also a key factor that influences protein translation in tumor cells. an amino acid exchanger, formed by a light and heavy subunit through a disulfide bond, that facilitates the transport of one amino acid from outside the plasma membrane in exchange for another amino acid from the cytosol. For example, LAT1, a heterodimer composed of SLC7A5 and SLC3A2, mediates the efflux of L-Gln and the influx of L-Leu and other essential amino acids (EAAs) (Figure 4). hyperornithinemia–hyperammonemia–homocitrullinuria syndrome is an inherited urea cycle disorder caused by deficiency of mitochondrial L-Orn transporter (SLC25A15/ORNT1) that transfers L-Orn from the cytosol to hepatic mitochondria for the L-Orn transcarbamylase reaction. The ornithine transport dysfunction causes a urea cycle blockage and accumulation of ammonia, a condition called hyperammonemia, whereby the nervous system is particularly sensitive toward excess ammonia levels. a chronic and long-lasting neuropsychiatric disorder in which individuals have recurring, unwanted thoughts, ideas or sensations (obsessions) that make them feel driven to do something repetitively (compulsions). In one meta-analysis, it was found that there is an association between a polymorphism in SLC1A1 and OCD (in addition to acidic aminoaciduria). However, mutations in other genes such as the human serotonin transporter gene hSERT (SLC6A4) are also linked to OCD. SLCs make up the largest class of transport proteins, far greater than that of ion channels, ABC Transporters and ATPases. The SLC superfamily include the classical ion-coupled transporters, facilitated transporters and exchangers as well as transport proteins of any substrate not falling into the classical description of ion channels, water channels or pumps. SLCs are integral transmembrane proteins that are composed of 6 to 14 α-helices connected to each other by hydrophilic intracellular and extracellular loops. They facilitate the transport of solutes such as sugars, amino acids, vitamins, trace minerals, or electrolytes into and out of cells and cellular organelles, either against the electrochemical concentration gradient, or in a facilitative manner. The SLC superfamily consists of 65 different SLC families, many of which are structurally and functionally distinct. Over 400 SLC members have been identified within the SLC superfamily. SLCs are gatekeepers in membranes of cells and organelles and, therefore, they control the uptake and exit of nutrients, signaling molecules and drugs into and out of cells and organelles. Thus, they are of great interest for the development of novel therapeutic drugs. Of the 65 SLC families, 11 contain AATs, giving rise to at least 66 amino acid transporters within the SLC superfamily. VGLUT1–3 are responsible for the uptake of the neurotransmitter L-Glu into synaptic vesicles. VGLUT1 (SLC17A7), VGLUT2 (SLC17A6), and VGLUT3 (SLC17A8) are specific markers of canonical glutamatergic neurons (Figure 5). Expression of VGLUTs also determines the kinetics of synaptic vesicle recycling. In both pancreatic α- and β-cells, L-Glu is transported into secretory granules (SGs) and synaptic-like microvesicles (SLMVs) via the VGLUTs (SLC17A7, SLC17A6 and SLC17A8), as shown in Figure 6.