The Human SLC1A5 (ASCT2) Amino Acid Transporter: From Function to Structure and Role in Cell Biology

Abstract

SLC1A5, known as ASCT2, is a neutral amino acid transporter belonging to the SLC1 family and localized in the plasma membrane of several body districts. ASCT2 is an acronym standing for Alanine, Serine, Cysteine Transporter 2 even if the preferred substrate is the conditionally essential amino acid glutamine, with cysteine being a modulator and not a substrate. The studies around amino acid transport in cells and tissues began in the ‘60s by using radiolabeled compounds and competition assays. After identification of murine and human genes, the function of the coded protein has been studied in cell system and in proteoliposomes revealing that this transporter is a Na+ dependent antiporter of neutral amino acids, some of which are only inwardly transported and others are bi-directionally exchanged. The functional asymmetry merged with the kinetic asymmetry in line with the physiological role of amino acid pool harmonization. An intriguing function has been described for ASCT2 that is exploited as a receptor by a group of retroviruses to infect human cells. Interactions with scaffold proteins and post-translational modifications regulate ASCT2 stability, trafficking and transport activity. Two asparagine residues, namely N163 and N212, are the sites of glycosylation that is responsible for the definitive localization into the plasma membrane. ASCT2 expression increases in highly proliferative cells such as inflammatory and stem cells to fulfill the augmented glutamine demand. Interestingly, for the same reason, the expression of ASCT2 is greatly enhanced in many human cancers. This finding has generated interest in its candidacy as a pharmacological target for new anticancer drugs. The recently solved 3D structure of ASCT2 will aid in the rational design of such therapeutic compounds.

High Levels of Glutaminase II Pathway Enzymes in Normal and Cancerous Prostate Suggest a Role in ‘Glutamine Addiction’

“…GTK and ω-amidase are also present in normal and cancerous human bladder epithelial cells and in normal and cancerous human prostate epithelial cells [1,7] (present work). We suggest that the relatively high concentrations of GTK and ω-amidase in epithelial tissues, the high concentration of glutamine in the circulation (~0.5-0.8 mM [40]) and the presence of high capacity glutamine transporters (e.g., [41,42]) are important for maintaining cellular nitrogen and energy homeostasis. The glutaminase II pathway may be especially important in rapidly dividing cells that are known to consume glutamine as an important energy source, such as enterocytes [43], lymphocytes [44], and bone cells [45].…”

“…Our findings [1,7] (present work) together with the findings of Udupa et al [9] suggest that the glutaminase II pathway is also important in rapidly dividing cancer cells. Interestingly, the glutamine transporter ASCT2 is strongly expressed in highly proliferative cells, including cancer cells [41,42].…”

The role of glutaminase in cancer

“…In contrast, Ser95 phosphorylation at the GLS N-terminal region leads to decreased GLS activity. 33 The expression levels of GLS2 variants are markedly increased in tumour cells that are well differentiated, and this is associated with a significantly prolonged survival time 24,25,36,37 (Table 1). GLS2 negatively regulates the activity of the phosphoinositide 3-kinase-AKT signalling pathway 38 and Rac1 by mediating p53 function in hepatocellular carcinoma, resulting in the inhibition of migration, invasion and metastasis of cancer cells.…”

Structural basis for stereospecific inhibition of ASCT2 from rational design

“…The four synthesized compounds were tested for inhibitory activity in rat ASCT2 (rASCT2) with electrophysiology. rASCT2 was used because it exhibits a similar pharmacological profile to that of the human ASCT2, while having better expression in HEK293 cells 15,22,23,29 .…”

“…Because ASCT2 is a pharmacologically important target, multiple small molecule competitive inhibitors have been developed for this protein, using both structure-based rational design and ligand-based approaches (reviewed in reference 22). For example, guided by homology modeling, we discovered and optimized a series of proline-based derivatives as well as other chemical scaffolds that inhibit ASCT2 in low µM potencies 16,23,24 .…”