The Concise Guide to PHARMACOLOGY 2023/24: Transporters

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and over 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (https://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.16182. Transporters are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, catalytic receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Amino acid metabolism, transport and signalling in the liver revisited

The liver controls the systemic exposure of amino acids entering via the gastro-intestinal tract. For most amino acids except branched chain amino acids, hepatic uptake is very efficient. This implies that the liver orchestrates amino acid metabolism and also controls systemic amino acid exposure. Although many amino acid transporters have been identified, cloned and investigated with respect to substrate specificity, transport mechanism, and zonal distribution, which of these players are involved in hepatocellular amino acid transport remains unclear. Here, we aim to provide a review of current insight into the molecular machinery of hepatic amino acid transport. Furthermore, we place this information in a comprehensive overview of amino acid transport, signalling and metabolism.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: Transporters

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15543. Transporters are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, catalytic receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Abstract 1007: The amino acid transporter SNAT4: Potential role as a tumor suppressor in melanoma

Cancer cells utilize amino acids to satisfy their need for nutrients and fuel their accelerated growth. The amino acid glutamine has been identified as one of the key building blocks in cancer cells, utilized for macromolecular synthesis and energy production. Due to the increased requirement for glutamine and other amino acids, cancer cells commonly increase expression of amino acid transporters, such as SLC1A5 (ASCT2). Amino acid transporters are membrane transport proteins that are used by cells to move important amino acids in and out of the cell. Interestingly, most amino acid transporters are upregulated in cancer to ensure continued access to nutrients.

We have recently shown that ASCT2 plays a critical role in regulating glutamine uptake in melanoma, prostate and breast cancer. Many other glutamine transporters are upregulated in different cancers, including SLC38 family members SNAT1, SNAT2 and SNAT3. The role of SLC38A4 (SNAT4) in amino acid uptake and cancer growth, however, has not been examined. Like the other SNATs, SNAT4 is a sodium-dependent amino acid transporter that transports neutral amino acids, such as alanine, across the plasma membrane, although it has low affinity for glutamine. Interestingly, unlike the other SNAT family members, our analysis showed that SNAT4 expression is downregulated in 879/917 cancer cell lines in Oncomine. Analysis of SNAT4 mutations in patient samples using cBioPortal showed infrequent mutations in the majority of cancers. However, the TCGA melanoma data (cBioPortal) showed that 32 of 278 melanoma cases (11.5%) have a point mutation in SNAT4 suggesting an important role in melanoma. These mutations included 9 SNAT4 hotspot mutations, present in 2-4 patients each. Using a SNAT4 homology model to predict loss-of-function mutants, we selected five of these mutations to assess in a SNAT4 transport assay in Xenopus oocytes. Two of the five hotspot mutations (G31E, S76F, G78E, G150E and S371F) were able to completely inactivate SNAT4 alanine transport in oocytes. Further analysis of 52 melanoma patient samples in Oncomine showed SNAT4 downregulation in all patients, suggesting that, in contrast to most amino acid transporters, SNAT4 plays a tumor suppressor role in melanoma.

We are currently examining these mutations in melanoma cell lines to determine their effects on cell growth. We are also examining how SNAT4 is involved in either the import or export of amino acids and the downstream metabolic consequences that may directly inhibit melanoma cell growth. Through this research we will gain a better understanding of the role of SNAT4- mediated amino acid metabolism in preventing melanoma cell growth.

Citation Format: Nicholas J. Otte, Angelika Broer, Patrick O’Young, Michelle van Geldermalsen, Qian Wang, Charles G. Bailey, Stefan Broer, Jeff Holst. The amino acid transporter SNAT4: Potential role as a tumor suppressor in melanoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1007.

Diseases associated with general amino acid transporters of the solute carrier 6 family (SLC6)

Amino acid transporters of the SLC6 family mediate the Na(+)-dependent uptake of neutral amino acids into neurons and epithelial cells of the intestine, kidney and other organs. They are integral parts of amino acid homeostasis in the whole body and the brain. In the intestine they are involved in protein absorption, while in the kidney they regulate plasma amino acid concentrations through reabsorption. The metabolic role of SLC6 amino acid transporters in the brain is less clear and most likely related to anaplerosis of the TCA cycle. Mutations in these transporters cause rare inherited disorders such as Hartnup disorder and iminoglycinuria. They may also play a role in complex traits such as depression, anxiety, obesity, diabetes and cancer. The review does not cover the transport of neurotransmitter amino acids.

Stimulation of the EAAT4 glutamate transporter by SGK protein kinase isoforms and PKB

The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in brain tissue and upregulated by ischemia, neuronal excitation, and dehydration. The present study has been performed to elucidate the expression of SGK1 in cerebellar Purkinje cells and to explore whether it influences the colocalized glutamate transporter EAAT4. Intense SGK1 staining was observed in Purkinje cells following 48h of water deprivation. The kinase activates glutamate induced current (I(GLU)) in Xenopus oocytes heterologously expressing EAAT4, an effect mimicked by its isoforms SGK2, 3 and PKB. I(GLU) was decreased by the ubiquitin ligase Nedd4-2, an effect partially but not completely reversed by additional coexpression of the SGK kinase isoforms or PKB. According to immunohistochemistry EAAT4 protein abundance in the cell membrane was enhanced by SGK1 and decreased by Nedd4-2. In conclusion, SGK1 expression is upregulated by ischemia, excitation, and dehydration in cerebellar Purkinje cells. The upregulation of SGK1 may serve to stimulate EAAT4 and thus to reduce neuroexcitotoxicity.

Expression of a renal type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic anions

Two distinct molecular types (I and II) of renal proximal tubular brush border Na+/Pi cotransporters have been identified by expression cloning on the basis of their capacity to induce Na+-dependent Pi influx in tracer experiments. Whereas the type II transporters (e.g., NaPi-2 and NaPi-3) resemble well known characteristics of brush border Na+/Pi cotransport, little is known about the properties of the type I transporter (NaPi-1). In contrast to type II, type I transporters produced electrogenic transport only at high extracellular Pi concentrations (> or =3 mM). On the other hand, expression of NaPi-1 induced a Cl- conductance in Xenopus laevis oocytes, which was inhibited by Cl- channel blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid >> 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. Further, the Cl- conductance was inhibited by the organic anions phenol red, benzylpenicillin (penicillin G), and probenecid. These organic anions induced outwardly directed currents in the absence of Cl-. In tracer studies, we observed uptake of benzylpenicillin with a Km of 0.22 mM; benzylpenicillin uptake was inhibited by NPPB and niflumic acid. These findings suggest that the type I Na+/Pi cotransporter functions also as a novel type of anion channel permeable not only for Cl- but also for organic anions. Such an apical anion channel could serve an important role in the transport of Cl- and the excretion of anionic xenobiotics.

Interaction of Serratia marcescens hemolysin (ShlA) with artificial and erythrocyte membranes. Demonstration of the formation of aqueous multistate channels

Pore formation by hemolysin (ShlA) of Serratia marcescens was studied in erythrocytes and in artificial lipid bilayer membranes. The results with erythrocytes demonstrated that hemolysin pores varied in size. In erythrocyte membranes with reduced fluidity (0 degrees C), the toxin formed small pores with diameter 1-1.5 nm. In fluid membranes (above 20 degrees C), hemolysin pores with larger diameters (approximately 2.5-3.0 nm) were observed, which may be caused by association of ShlA monomers into oligomers. Comparison of the channels formed by Staphylococcus aureus alpha-toxin with channels formed by ShlA indicated a slightly smaller pore diameter of ShlA pores. Analysis of ShlA in artificial lipid bilayers showed the formation of pores with a broad distribution of single channel conductances, suggesting variable sizes of the ShlA pore. The lower limit for the pore diameter was approximately 1.0 nm. The ShlA pores did not exhibit pronounced ion selectivity nor voltage dependence, supporting the presence of a large water-filled pore.