LAT1

LAT1 (L-type / large neutral amino acid transporter 1)

Aliases: SLC7A5, 4F2lc, CD98lc, D16S469E, E16, MPE16

Gene name: solute carrier family 7 member 5 (SLC7A5)

Summary

LAT1 is a heterodimeric, sodium- and pH-independent amino acid transporter. ‘L-type’ in its traditional name refers to its classification with ‘System L’, i.e. the leucine-preferring group of amino acid transporters. Its dimerization partner, 4F2hc (SLC3A2) heavy chain, is indispensable for its translocation to the plasma membrane and stability but not for the intrinsic transport function. LAT1 preferentially transports large branched and aromatic neutral amino acids, mostly to proliferating cells and through barriers like the placenta and the blood-brain barrier (BBB). Although some controversy exists whether LAT1 carries out uniport or antiport activity, it is generally modelled as an obligatory exchanger that imports neutral amino acids, including some essential ones such as leucine and tryptophan, in exchange for intracellular nonessential amino acids, mainly glutamine. Therefore, LAT1 and the glutamine-preferring amino acid exchanger ASCT2 are thought to be functionally coupled in some cancer types. LAT1 also transports thyroid hormones, and it is the major importer of amino acid-like CNS drugs across the BBB. Despite its known role in drug transport, the current regulatory guidelines contain no recommendation on investigating interactions of NCEs with LAT1.

Localization

LAT1 is ubiquitously expressed, with highest levels observed in the brain, spleen, bone marrow, testis, and placenta. In the blood-brain barrier, LAT1 is localized on both the apical and basolateral membranes. In other polarized epithelia, it is mainly localized in basolateral membranes. In the placenta, LAT1 is present on both the maternal and fetal surfaces of the syncytiotrophoblast [1, 2].
According to the Human Protein Atlas, mRNA coding for the heavy chain CD98 is ubiquitously detected, with the highest expression level in kidney, placenta, testis and bone marrow. The expression of CD98 correlates with that of SLC7A5 in terms of localization, as expected from the interaction between the two proteins. However, CD98 is even more broadly expressed since it works as an escort protein for other SLC7 members as well [1].

Function, physiology, and clinically significant polymorphisms

Like all members of the heteromeric amino acid transporter (HAT, SLC7) family, LAT1 resides in the plasma membrane in a heterodimeric form. The LAT1 holotransporter consists of a 55-kDa light chain, SCL7A5 (LAT1 proper), and an escort protein called the heavy chain covalently linked to the light chain via a disulfide bond. LAT1 heterodimerizes with the 4F2hc (SLC3A2) heavy chain, an N-glycosylated ~68-kDa transmembrane protein with 1 membrane-spanning domain. While transport carried out by either chain alone is negligible, the heavy chain is only needed to stabilize the dimer and facilitate its translocation to the plasma membrane, and the actual transport is carried out by the light chain [3].
LAT1 takes part in the transport of a wide range of neutral amino acids, especially ones with large branched or aromatic side chains. Tryptophan, phenylalanine, leucine, and histidine are transported with high affinity (Km: 5 to 50 μM). Glutamine as an uptake substrate has a low affinity toward LAT1 (Km in the mM range). Histidine and tyrosine are transported bidirectionally, whereas the others are preferentially transported in the inward direction only. LAT1 displays asymmetrical affinity towards bidirectionally transported substrates, with extracellular versus intracellular Km values being in the micromolar versus millimolar range [4]. The generally accepted mode of function of LAT1 is obligatory antiport, i.e. the exchange of a large and neutral extracellular substrate for an abundant intracellular amino acid such as glutamine[5], albeit a mixed transport model without obligate exchange has also been proposed [6]. 
In addition to amino acids, LAT1 catalyzes the transport of the thyroid hormones T3 and T4, the nitric oxide-derived S-nitrosothiols, and it is the main point of entry for a number of CNS-acting drugs including the dopamine precursor L-DOPA, the antispastic baclofen, as well as the anticonvulsants gabapentin and pregabalin, to the brain [2, 7, 8]. The organometallic environmental toxicant methylmercury, complexed with L-cysteine, mimics natural System L substrates, and efficient transport of the methylmercury-L-cysteine complex by LAT1 explains its facile access to the brain [9]. The chemotherapy drug melphalan is also a LAT1 substrate [10]. 
The tyrosine-derived compound JPH203, formerly known as KYT-0353, is the most potent and selective inhibitor of LAT1 developed to date [11]. Prior to the discovery of JPH203, the classical generic System L inhibitor 2‐aminobicyclo‐(2,2,1)‐heptane‐2‐carboxylic acid (BCH) was most commonly used to block LAT1 in in vitro experiments. The thyroid hormone T3 can also act as a competitive inhibitor of LAT1 due to its high affinity, although T3 has low transportability and low selectivity toward LAT1 [11]. As opposed to methylmercury, inorganic mercury in the form of HgCl2 is a strong inhibitor, rather than a substrate, of LAT1 [12].
The principal physiological function of LAT1 is in the transport of essential amino acids across biological barriers such as the intestinal epithelium, the placenta, and the blood-brain barrier, as well as the provision of all cell types with these amino acids for metabolism and signaling [2]. LAT1 gains importance under conditions of increased metabolic demand and cell proliferation such as T cell activation [13] and cancer growth (see Clinical significance).
Of more than 13.000 SNPs of the SLC7A5 gene recorded in the dbSNP database, none is marked as clearly or potentially pathogenic. However, likely pathogenic variants of LAT1, LAT2, and the 4F2hc genes have been described in autism spectrum disorder [14]. The SNP rs4240803, on the other hand, has been linked to positive therapeutic response in multiple myeloma patients. When investigated ex vivo in patient-derived PBMCs, this SNP was associated with elevated expression of SLC7A5 mRNA and higher sensitivity to melphalan [15].

Clinical significance

LAT1 is overexpressed in many types of human cancer and plays an important role in cancer metabolism. In addition, some cancer-associated mutations of LAT1 are rated as possibly damaging in the BioMuta database [16]. In cancer cells, LAT1 collaborates with other amino acid transporters such as ASCT2, SNAT1 and SNAT2 in the ‘harmonization’ of extracellular and intracellular amino acid pools, and its action is required to maintain adequate levels of essential amino acids including leucine [17]. In a proposed functional coupling between ASCT2 and LAT1, glutamine imported by ASCT2 into cancer cells is utilized by LAT1 as an exchange substrate [18]. Selective inhibition of LAT1 with JPH203, probably by reducing the availability of leucine, contributes to the suppression of the mTOR signaling pathway and the halt of cancer cell proliferation. Inhibition of LAT1 blocked cell cycle progression from G0/G1 to the S phase, and arrested the growth of thyroid cancer in both xenografted and immunocompetent mouse models [19, 20]. 
As a major transporter of amino acids across the blood-brain barrier, LAT1 is indispensable for normal brain growth and function. Tryptophan, for example, is a LAT1 substrate and key to healthy neurological development. Reduced amounts of the dopamine precursor L-DOPA and essential amino acids in the brain due to decreased expression and inadequate transport capacity of LAT1 have been linked to the onset and development of Parkinson’s disease [21]. Mutations that alter the function of LAT1 have been identified among the molecular determinants of autism spectrum disorder, and of the substrates of LAT1 the concentration of histidine showed the greatest pathology-associated variation [14, 22]. 
Given its diverse roles in physiology and disease, LAT1 emerges as an important pharmacological target. Its inhibition may curb supply of amino acids to cancer or T cells, while it may also be exploited for targeted drug delivery. Recently developed structural models of LAT1 empower the discovery of novel potent inhibitors [23] and facilitate the rational design of prodrugs to be delivered to cancer cells or the brain [24].

Regulatory requirements

Despite its known role in drug transport, the current regulatory guidelines contain no recommendation on investigating interactions of NCEs with LAT1.

Summary information for LAT1

References

1.    Fotiadis, D., Y. Kanai, and M. Palacin, The SLC3 and SLC7 families of amino acid transporters. Mol Aspects Med, 2013. 34(2-3): p. 139-58.
2.    Scalise, M., et al., The Human SLC7A5 (LAT1): The Intriguing Histidine/Large Neutral Amino Acid Transporter and Its Relevance to Human Health. Front Chem, 2018. 6: p. 243.
3.    Yanagida, O., et al., Human L-type amino acid transporter 1 (LAT1): characterization of function and expression in tumor cell lines. Biochim Biophys Acta, 2001. 1514(2): p. 291-302.
4.    Meier, C., et al., Activation of system L heterodimeric amino acid exchangers by intracellular substrates. EMBO J, 2002. 21(4): p. 580-9.
5.    Singh, N. and G.F. Ecker, Insights into the Structure, Function, and Ligand Discovery of the Large Neutral Amino Acid Transporter 1, LAT1. Int J Mol Sci, 2018. 19(5).
6.    Widdows, K.L., et al., Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms. FASEB J, 2015. 29(6): p. 2583-94.
7.    Li, S. and A.R. Whorton, Identification of stereoselective transporters for S-nitroso-L-cysteine: role of LAT1 and LAT2 in biological activity of S-nitrosothiols. J Biol Chem, 2005. 280(20): p. 20102-10.
8.    Takahashi, Y., et al., Transport of Pregabalin Via L-Type Amino Acid Transporter 1 (SLC7A5) in Human Brain Capillary Endothelial Cell Line. Pharm Res, 2018. 35(12): p. 246.
9.    Simmons-Willis, T.A., et al., Transport of a neurotoxicant by molecular mimicry: the methylmercury-L-cysteine complex is a substrate for human L-type large neutral amino acid transporter (LAT) 1 and LAT2. Biochem J, 2002. 367(Pt 1): p. 239-46.
10.    Goldenberg, G.J., H.Y. Lam, and A. Begleiter, Active carrier-mediated transport of melphalan by two separate amino acid transport systems in LPC-1 plasmacytoma cells in vitro. J Biol Chem, 1979. 254(4): p. 1057-64.
11.    Oda, K., et al., L-type amino acid transporter 1 inhibitors inhibit tumor cell growth. Cancer Sci, 2010. 101(1): p. 173-9.
12.    Boado, R.J., et al., Site-directed mutagenesis of cysteine residues of large neutral amino acid transporter LAT1. Biochim Biophys Acta, 2005. 1715(2): p. 104-10.
13.    Hayashi, K., et al., LAT1 is a critical transporter of essential amino acids for immune reactions in activated human T cells. J Immunol, 2013. 191(8): p. 4080-5.
14.    Cascio, L., et al., Abnormalities in the genes that encode Large Amino Acid Transporters increase the risk of Autism Spectrum Disorder. Mol Genet Genomic Med, 2020. 8(1): p. e1036.
15.    Poi, M.J., et al., A Single Nucleotide Polymorphism in SLC7A5 Was Associated With Clinical Response in Multiple Myeloma Patients. Anticancer Res, 2019. 39(1): p. 67-72.
16.    Dingerdissen, H.M., et al., BioMuta and BioXpress: mutation and expression knowledgebases for cancer biomarker discovery. Nucleic Acids Res, 2018. 46(D1): p. D1128-D1136.
17.    Broer, A., F. Rahimi, and S. Broer, Deletion of Amino Acid Transporter ASCT2 (SLC1A5) Reveals an Essential Role for Transporters SNAT1 (SLC38A1) and SNAT2 (SLC38A2) to Sustain Glutaminolysis in Cancer Cells. J Biol Chem, 2016. 291(25): p. 13194-205.
18.    Nicklin, P., et al., Bidirectional transport of amino acids regulates mTOR and autophagy. Cell, 2009. 136(3): p. 521-34.
19.    Hafliger, P., et al., The LAT1 inhibitor JPH203 reduces growth of thyroid carcinoma in a fully immunocompetent mouse model. J Exp Clin Cancer Res, 2018. 37(1): p. 234.
20.    Enomoto, K., et al., A novel therapeutic approach for anaplastic thyroid cancer through inhibition of LAT1. Sci Rep, 2019. 9(1): p. 14616.
21.    Kageyama, T., et al., The 4F2hc/LAT1 complex transports L-DOPA across the blood-brain barrier. Brain Res, 2000. 879(1-2): p. 115-21.
22.    Tarlungeanu, D.C., et al., Impaired Amino Acid Transport at the Blood Brain Barrier Is a Cause of Autism Spectrum Disorder. Cell, 2016. 167(6): p. 1481-1494 e18.
23.    Singh, N., et al., Discovery of Potent Inhibitors for the Large Neutral Amino Acid Transporter 1 (LAT1) by Structure-Based Methods. Int J Mol Sci, 2018. 20(1).
24.    Chien, H.C., et al., Reevaluating the Substrate Specificity of the L-Type Amino Acid Transporter (LAT1). J Med Chem, 2018. 61(16): p. 7358-7373.