Interaction of Cholesterol With the Human SLC1A5 (ASCT2): Insights Into Structure/Function Relationships

TFEB inhibition induces melanoma shut-down by blocking the cell cycle and rewiring metabolism

Melanomas are characterised by accelerated cell proliferation and metabolic reprogramming resulting from the contemporary dysregulation of the MAPK pathway, glycolysis and the tricarboxylic acid (TCA) cycle. Here, we suggest that the oncogenic transcription factor EB (TFEB), a key regulator of lysosomal biogenesis and function, controls melanoma tumour growth through a transcriptional programme targeting ERK1/2 activity and glucose, glutamine and cholesterol metabolism. Mechanistically, TFEB binds and negatively regulates the promoter of DUSP-1, which dephosphorylates ERK1/2. In melanoma cells, TFEB silencing correlates with ERK1/2 dephosphorylation at the activation-related p-Thr185 and p-Tyr187 residues. The decreased ERK1/2 activity synergises with TFEB control of CDK4 expression, resulting in cell proliferation blockade. Simultaneously, TFEB rewires metabolism, influencing glycolysis, glucose and glutamine uptake, and cholesterol synthesis. In TFEB-silenced melanoma cells, cholesterol synthesis is impaired, and the uptake of glucose and glutamine is inhibited, leading to a reduction in glycolysis, glutaminolysis and oxidative phosphorylation. Moreover, the reduction in TFEB level induces reverses TCA cycle, leading to fatty acid production. A syngeneic BRAFV600E melanoma model recapitulated the in vitro study results, showing that TFEB silencing sustains the reduction in tumour growth, increase in DUSP-1 level and inhibition of ERK1/2 action, suggesting a pivotal role for TFEB in maintaining proliferative melanoma cell behaviour and the operational metabolic pathways necessary for meeting the high energy demands of melanoma cells.

Recent progress of amino acid transporters as a novel antitumor target

Glutamine transporters transport different amino acids for cell growth and metabolism. In tumor cells, glutamine transporters are often highly expressed and play a crucial role in their growth. By inhibiting the amino acid transport of these transporters, the growth of cancer cells can be inhibited. In recent years, more and more attention has been paid to the study of glutamine transporter. In this article, the differences between the ASC system amino acid transporter 2 (ASCT2), L-type amino acid transporter 1 (LAT1), and the cystine–glutamate exchange (xCT) transporters research progress on the mechanism of action and corresponding small molecule inhibitors are summarized. This article introduces 62 related small molecule inhibitors of different transporters of ASCT2, LAT1, and xCT. These novel chemical structures provide ideas for the research and design of targeted inhibitors of glutamine transporters, as well as important references and clues for the design of new anti-tumor drugs.

Structural insights into inhibitory mechanism of human excitatory amino acid transporter EAAT2

Glutamate is a pivotal excitatory neurotransmitter in mammalian brains, but excessive glutamate causes numerous neural disorders. Almost all extracellular glutamate is retrieved by the glial transporter, Excitatory Amino Acid Transporter 2 (EAAT2), belonging to the SLC1A family. However, in some cancers, EAAT2 expression is enhanced and causes resistance to therapies by metabolic disturbance. Despite its crucial roles, the detailed structural information about EAAT2 has not been available. Here, we report cryo-EM structures of human EAAT2 in substrate-free and selective inhibitor WAY213613-bound states at 3.2 Å and 2.8 Å, respectively. EAAT2 forms a trimer, with each protomer consisting of transport and scaffold domains. Along with a glutamate-binding site, the transport domain possesses a cavity that could be disrupted during the transport cycle. WAY213613 occupies both the glutamate-binding site and cavity of EAAT2 to interfere with its alternating access, where the sensitivity is defined by the inner environment of the cavity. We provide the characterization of the molecular features of EAAT2 and its selective inhibition mechanism that may facilitate structure-based drug design for EAAT2.

EAAT2 is an amino acid transporter implicated in glutamate homeostasis in brain and therapy resistance of cancer cells. Here, the authors report cryo-EM structures and reveal inhibitory mechanisms via selective inhibitor WAY213613.

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

Flood risk public perception in flash flood-prone areas of Punjab, Pakistan

Floods due to higher severity of destruction are considered the most destructive natural hazards in the world. Initiating the appropriate strategies of disaster risk reduction is necessary to understand risk perception. This study attempted to examine the flood risk public perception in flash flood-prone areas of Punjab, Pakistan. A simple random sampling technique was used for collecting the data of 560 household respondents, and a logistic regression model was employed to make out the factors of flood risk perception. In classification of low and high perceived risk of relevant indicators, flood risk perception index was constructed. Risk perception is significantly influenced by socioeconomic factors which have a direct impact on disaster preparedness and potential adaptive capacities. After that, potential correlation of risk perception with the demographic status of respondents was investigated in this study. Empirical estimates indicated as respondents' schooling, ownership of house, size of household, employment status and past flood experience significantly influence flood risk perception. Risk perception determinants also diverse among both communities portray spatial differences. Inadequate protection measures from public authorities and institutions, limited preparedness regarding actions of private mitigation, reduced intensity of reliance in institutions and authorities are major reasons for high risk and lower mitigation in these flash flood-prone areas. The outcomes of this research can facilitate to understand flood risk perception and its factors for conniving appropriate management plan of flood risk and communication strategies. Furthermore, this research can help consider multidimensional flood risks and its spatial vibrancy from the perspective of social science.

Cysteine 467 of the ASCT2 Amino Acid Transporter Is a Molecular Determinant of the Antiport Mechanism

The plasma membrane transporter ASCT2 is a well-known Na⁺-dependent obligatory antiporter of neutral amino acids. The crucial role of the residue C467 in the recognition and binding of the ASCT2 substrate glutamine, has been highlighted by structure/function relationship studies. The reconstitution in proteoliposomes of the human ASCT2 produced in P. pastoris is here employed to unveil another role of the C467 residue in the transport reaction. Indeed, the site-directed mutant C467A displayed a novel property of the transporter, i.e., the ability of mediating a low but measurable unidirectional transport of [³H]-glutamine. This reaction conforms to the main features of the ASCT2-mediated transport, namely the Na⁺-dependence, the pH dependence, the stimulation by cholesterol included in the proteoliposome membrane, and the specific inhibition by other common substrates of the reconstituted human ASCT2. Interestingly, the WT protein cannot catalyze the unidirectional transport of [³H]-glutamine, demonstrating an unspecific phenomenon. This difference is in favor of a structural conformational change between a WT and C467A mutant that triggers the appearance of the unidirectional flux; this feature has been investigated by comparing the available 3D structures in two different conformations, and two homology models built on the basis of hEAAT1 and GLT_Ph.

Surface modification with cholesteryl acetyl carnitine, a novel cationic agent, elevates cancer cell uptake of the PEGylated liposomes

The present study aimed to synthesize cholesteryl acetyl carnitine (CAC), and surface modify the PEGylated liposomes with the intention of enhanced cancer cell uptake. For this, CAC synthesis was performed in amine-free esterification conditions and then four liposomal formulations of unmodified, CAC/PEG, and CAC + PEG-modified were prepared by ethanol injection method. Cytotoxicity of the liposomes was investigated in A549 cells, followed by cellular uptake assessments of coumarin 6 (C6)-loaded liposomes. The results of ATR-FTIR, ¹HNMR, and ¹³CNMR demonstrated successful formation of CAC. A molecular docking study showed efficient binding affinities rather than carnitine to the active site of four carnitine transporters. Liposomal formulations possessed spherical morphology with a mean particle size range of 112-138 nm, narrow size distribution, and negative surface charge. All formulations had low cytotoxicity at 0.5 mg/ml, but high cytotoxicity at around 2.5 mg/ml. The lowest IC50 was obtained for CAC modified liposomes. CAC + PEG-modified liposomes had the highest cellular uptake. In conclusion, CAC + PEG modification of liposomes is an effective approach for increasing A549 cellular uptake, with low cytotoxicity at commonly applied liposome concentrations. The elevated uptake may be due to the involvement of the organic cation transporter, cationic structure, and the metabolic preference of CAC in cancer cells.

The involvement of sodium in the function of the human amino acid transporter ASCT2

Alanine, serine, cysteine transporter 2 (ASCT2) is a membrane amino acid transporter with relevance to human physiology and pathology, such as cancer. Notwithstanding, the study on the ASCT2 transport cycle still has unknown aspects, such as the role of Na⁺ in this process. We investigate this issue using recombinant hASCT2 reconstituted in proteoliposomes. Changes in the composition of purification buffers show the crucial role of Na⁺ in ASCT2 functionality. The transport activity is abolished when Na⁺ is absent or substituted by Li⁺ or K⁺ in purification buffers. By employing a Na⁺ fluorometric probe, we measured an inwardly directed flux of Na⁺ and, by combining fluorometric and radiometric assays, determined a 2Na⁺  : 1Gln stoichiometry. Kinetics of Na⁺ transport suggest that pH-sensitive residues are involved in Na⁺ binding/transport. Our results clarify the role of Na⁺ on human ASCT2 transporter activity.

ASCT1 and ASCT2: Brother and Sister?

The SLC1 family includes seven members divided into two groups, namely, EAATs and ASCTs, that share similar 3D architecture; the first one includes high-affinity glutamate transporters, and the second one includes SLC1A4 and SLC1A5, known as ASCT1 and ASCT2, respectively, responsible for the traffic of neutral amino acids across the cell plasma membrane. The physiological role of ASCT1 and ASCT2 has been investigated over the years, revealing different properties in terms of substrate specificities, affinities, and regulation by physiological effectors and posttranslational modifications. Furthermore, ASCT1 and ASCT2 are involved in pathological conditions, such as neurodegenerative disorders and cancer. This has driven research in the pharmaceutical field aimed to find drugs able to target the two proteins.This review focuses on structural, functional, and regulatory aspects of ASCT1 and ASCT2, highlighting similarities and differences.

Anticancer Properties of Lobetyolin, an Essential Component of Radix Codonopsis (Dangshen)

Lobetyolin (LBT) is a polyacetylene glycoside found in diverse medicinal plants but mainly isolated from the roots of Codonopsis pilosula, known as Radix Codonopsis or Dangshen. Twelve traditional Chinese medicinal preparations containing Radix Codonopsis were identified; they are generally used to tonify spleen and lung Qi and occasionally to treat cancer. Here we have reviewed the anticancer properties of Codonopsis extracts, LBT and structural analogs. Lobetyolin and lobetyolinin are the mono- and bis-glucosylated forms of the polyacetylenic compound lobetyol. Lobetyol and LBT have shown activities against several types of cancer (notably gastric cancer) and we examined the molecular basis of their activity. A down-regulation of glutamine metabolism by LBT has been evidenced, contributing to drug-induced apoptosis and tumor growth inhibition. LBT markedly reduces both mRNA and protein expression of the amino acid transporter Alanine-Serine-Cysteine Transporter 2 (ASCT2). Other potential targets are proposed here, based on the structural analogy with other anticancer compounds. LBT and related polyacetylene glycosides should be further considered as potential anticancer agents, but more work is needed to evaluate their efficacy, toxicity, and risk-benefit ratio.

Cholesterol stimulates the cellular uptake of L-carnitine by the carnitine/organic cation transporter novel 2 (OCTN2)

The carnitine/organic cation transporter novel 2 (OCTN2) is responsible for the cellular uptake of carnitine in most tissues. Being a transmembrane protein OCTN2 must interact with the surrounding lipid microenvironment to function. Among the main lipid species that constitute eukaryotic cells, cholesterol has highly dynamic levels under a number of physiopathological conditions. This work describes how plasma membrane cholesterol modulates OCTN2 transport of L-carnitine in human embryonic kidney 293 cells overexpressing OCTN2 (OCTN2-HEK293) and in proteoliposomes harboring human OCTN2. We manipulated the cholesterol content of intact cells, assessed by thin layer chromatography, through short exposures to empty and/or cholesterol-saturated methyl-β-cyclodextrin (mβcd), whereas free cholesterol was used to enrich reconstituted proteoliposomes. We measured OCTN2 transport using [³H]L-carnitine, and expression levels and localization by surface biotinylation and Western blotting. A 20-min preincubation with mβcd reduced the cellular cholesterol content and inhibited L-carnitine influx by 50% in comparison with controls. Analogously, the insertion of cholesterol in OCTN2-proteoliposomes stimulated L-carnitine uptake in a dose-dependent manner. Carnitine uptake in cells incubated with empty mβcd and cholesterol-saturated mβcd to preserve the cholesterol content was comparable with controls, suggesting that the mβcd effect on OCTN2 was cholesterol dependent. Cholesterol stimulated L-carnitine influx in cells by markedly increasing the affinity for L-carnitine and in proteoliposomes by significantly enhancing the affinity for Na⁺ and, in turn, the L-carnitine maximal transport capacity. Because of the antilipogenic and antioxidant features of L-carnitine, the stimulatory effect of cholesterol on L-carnitine uptake might represent a novel protective effect against lipid-induced toxicity and oxidative stress.

ATP modulates SLC7A5 (LAT1) synergistically with cholesterol

The plasma membrane transporter hLAT1 is responsible for providing cells with essential amino acids. hLAT1 is over-expressed in virtually all human cancers making the protein a hot-spot in the fields of cancer and pharmacology research. However, regulatory aspects of hLAT1 biology are still poorly understood. A remarkable stimulation of transport activity was observed in the presence of physiological levels of cholesterol together with a selective increase of the affinity for the substrate on the internal site, suggesting a stabilization of the inward open conformation of hLAT1. A synergistic effect by ATP was also observed only in the presence of cholesterol. The same phenomenon was detected with the native protein. Altogether, the biochemical assays suggested that cholesterol and ATP binding sites are close to each other. The computational analysis identified two neighboring regions, one hydrophobic and one hydrophilic, to which cholesterol and ATP were docked, respectively. The computational data predicted interaction of the ϒ-phosphate of ATP with Lys 204, which was confirmed by site-directed mutagenesis. The hLAT1-K204Q mutant showed an impaired function and response to ATP. Interestingly, this residue is conserved in several members of the SLC7 family.

The Human SLC1A5 Neutral Amino Acid Transporter Catalyzes a pH-Dependent Glutamate/Glutamine Antiport, as Well

ASCT2 is a neutral amino acid transporter, which catalyzes a sodium-dependent obligatory antiport among glutamine and other neutral amino acids. The human ASCT2 over-expressed in Pichia pastoris and reconstituted in proteoliposomes has been employed for identifying alternative substrates of the transporter. The experimental data highlighted that hASCT2 also catalyzes a sodium-dependent antiport of glutamate with glutamine. This unconventional antiport shows a preferred sidedness: glutamate is inwardly transported in exchange for glutamine transported in the counter direction. The orientation of the transport protein in proteoliposomes is the same as in the cell membrane; then, the observed sidedness corresponds to the transport of glutamate from the extracellular to the intracellular compartment. The competitive inhibition exerted by glutamate on the glutamine transport together with the docking analysis indicates that the glutamate binding site is the same as that of glutamine. The affinity for glutamate is lower than that for neutral amino acids, while the transport rate is comparable to that measured for the asparagine/glutamine antiport. Differently from the neutral amino acid antiport that is insensitive to pH, the glutamate/glutamine antiport is pH-dependent with optimal activity at acidic pH on the external (extracellular) side. The stimulation of glutamate transport by a pH gradient suggests the occurrence of a proton flux coupled to the glutamate transport. The proton transport has been detected by a spectrofluorometric method. The rate of proton transport correlates well with the rate of glutamate transport indicating a 1:1 stoichiometry H⁺: glutamate. The glutamate/glutamine antiport is also active in intact HeLa cells. On a physiological point of view, the described antiport could have relevance in some districts in which a glutamate/glutamine cycling is necessary, such as in placenta.

Effect of Cholesterol on the Organic Cation Transporter OCTN1 (SLC22A4)

The effect of cholesterol was investigated on the OCTN1 transport activity measured as [¹⁴C]-tetraethylamonium or [³H]-acetylcholine uptake in proteoliposomes reconstituted with native transporter extracted from HeLa cells or the human recombinant OCTN1 over-expressed in E. coli. Removal of cholesterol from the native transporter by MβCD before reconstitution led to impairment of transport activity. A similar activity impairment was observed after treatment of proteoliposomes harboring the recombinant (cholesterol-free) protein by MβCD, suggesting that the lipid mixture used for reconstitution contained some cholesterol. An enzymatic assay revealed the presence of 10 µg cholesterol/mg total lipids corresponding to 1% cholesterol in the phospholipid mixture used for the proteoliposome preparation. On the other way around, the activity of the recombinant OCTN1 was stimulated by adding the cholesterol analogue, CHS to the proteoliposome preparation. Optimal transport activity was detected in the presence of 83 µg CHS/ mg total lipids for both [¹⁴C]-tetraethylamonium or [³H]-acetylcholine uptake. Kinetic analysis of transport demonstrated that the stimulation of transport activity by CHS consisted in an increase of the Vmax of transport with no changes of the Km. Altogether, the data suggests a direct interaction of cholesterol with the protein. A further support to this interpretation was given by a docking analysis indicating the interaction of cholesterol with some protein sites corresponding to CARC-CRAC motifs. The observed direct interaction of cholesterol with OCTN1 points to a possible direct influence of cholesterol on tumor cells or on acetylcholine transport in neuronal and non-neuronal cells via OCTN1.

From Inert Storage to Biological Activity-In Search of Identity for Oxidized Cholesteryl Esters

Esterification of cholesterol is a universal mechanism to store and transport large quantities of cholesterol between organs and tissues and to avoid toxicity of the excess of cellular cholesterol. Intended for transport and storage and thus to be inert, cholesteryl esters (CEs) reside in hydrophobic cores of circulating lipoproteins and intracellular lipid droplets. However, the inert identity of CEs is dramatically changed if cholesterol is esterified to a polyunsaturated fatty acid and subjected to oxidative modification. Post-synthetic, or epilipidomic, oxidative modifications of CEs are mediated by specialized enzymes, chief among them are lipoxygenases, and by free radical oxidation. The complex repertoire of oxidized CE (OxCE) products exhibit various, context-dependent biological activities, surveyed in this review. Oxidized fatty acyl chains in OxCE can be hydrolyzed and re-esterified, thus seeding oxidized moieties into phospholipids (PLs), with OxPLs having different from OxCEs biological activities. Technological advances in mass spectrometry and the development of new anti-OxCE antibodies make it possible to validate the presence and quantify the levels of OxCEs in human atherosclerotic lesions and plasma. The article discusses the prospects of measuring OxCE levels in plasma as a novel biomarker assay to evaluate risk of developing cardiovascular disease and efficacy of treatment.