Discovery of Potent Inhibitors for the Large Neutral Amino Acid Transporter 1 (LAT1) by Structure-Based Methods

Inhibition of amino acid transporter LAT1 in cancer cells suppresses G0/G1-S transition by downregulating cyclin D1 via p38 MAPK activation

L-type amino acid transporter 1 (LAT1, SLC7A5) is upregulated in various cancers and associated with disease progression. Nanvuranlat (Nanv; JPH203, KYT-0353), a selective LAT1 inhibitor, suppresses the uptake of large neutral amino acids required for rapid growth and proliferation of cancer cells. Previous studies have suggested that the inhibition of LAT1 by Nanv induces the cell cycle arrest at G0/G1 phase, although the underlying mechanisms remain unclear. Using pancreatic cancer cells arrested at the restriction check point (R) by serum deprivation, we found that the Nanv drastically suppresses the G0/G1-S transition after release. This blockade of the cell cycle progression was accompanied by a sustained activation of p38 mitogen-activated protein kinase (MAPK) and subsequent phosphorylation-dependent proteasomal degradation of cyclin D1. Isoform-specific knockdown of p38 MAPK revealed the predominant contribution of p38α. Proteasome inhibitors restored the cyclin D1 amount and released the cell cycle arrest caused by Nanv. The increased phosphorylation of p38 MAPK and the decrease of cyclin D1 were recapitulated in xenograft tumor models treated with Nanv. This study contributes to delineating the pharmacological activities of LAT1 inhibitors as anti-cancer agents and provides significant insights into the molecular basis of the amino acid-dependent cell cycle checkpoint at G0/G1 phase.

Photoredox-Catalyzed Generation of Tertiary Anions from Primary Amines via a Radical Polar Crossover

A method for the generation of tertiary carbanions via a deaminative radical-polar crossover is reported using redox active imines from α-tertiary primary amines. A variety of benzylic amines and amino esters can be used in this approach, with the latter engaging in a novel "aza-Reformatsky" reaction. Electronic trends correlate the stability of the resulting carbanion with reaction efficiency. The anions can be trapped with different electrophiles including aldehydes, ketones, imines, Michael acceptors, and H2 O/D2 O. Selective anion formation can be achieved in the presence of another equivalent or more acidic C-H bond in both an inter- and intramolecular fashion. Mechanistic studies suggest the intermediacy of a discrete carbanion intermediate.

Exploring Amino Acid Transporters as Therapeutic Targets for Cancer: An Examination of Inhibitor Structures, Selectivity Issues, and Discovery Approaches

Amino acid transporters are abundant amongst the solute carrier family and have an important role in facilitating the transfer of amino acids across cell membranes. Because of their impact on cell nutrient distribution, they also appear to have an important role in the growth and development of cancer. Naturally, this has made amino acid transporters a novel target of interest for the development of new anticancer drugs. Many attempts have been made to develop inhibitors of amino acid transporters to slow down cancer cell growth, and some have even reached clinical trials. The purpose of this review is to help organize the available information on the efforts to discover amino acid transporter inhibitors by focusing on the amino acid transporters ASCT2 (SLC1A5), LAT1 (SLC7A5), xCT (SLC7A11), SNAT1 (SLC38A1), SNAT2 (SLC38A2), and PAT1 (SLC36A1). We discuss the function of the transporters, their implication in cancer, their known inhibitors, issues regarding selective inhibitors, and the efforts and strategies of discovering inhibitors. The goal is to encourage researchers to continue the search and development within the field of cancer treatment research targeting amino acid transporters.

Analysis of a new therapeutic target and construction of a prognostic model for breast cancer based on ferroptosis genes

Breast cancer, which is the most common malignant tumor among women worldwide and an important cause of death in women. The existing prognostic model for patients with breast cancer is not accurate as breast cancer is resistant to commonly used antitumor drugs. Ferroptosis is a novel mechanism of programmed cell death that depends on iron accumulation and lipid peroxidation. Various studies have confirmed the role of ferroptosis in tumor regulation and ferroptosis is now considered to play an important role in breast cancer development. At present, the association between breast cancer prognosis and ferroptosis-related gene expression remains unclear. Further exploration of this research area may optimize the evaluation and prediction of prognosis of patients with breast cancer and finding of new therapeutic targets. In this study, clinical factors and the expression of multiple genes were evaluated in breast cancer samples from the Cancer Genome Atlas (TCGA) database and Gene Expression Omnibus (GEO) database database. Eleven prognostication-related genes (TP63, IFNG, MT3, ANO6, FLT3, PTGS2, SLC1A4, JUN, SLC7A5, CHAC1, and TF) were identified from differentially expressed genes to construct a survival prediction model, which showed a good prediction ability. KEGG pathway analysis revealed that immune-related pathways were the primary pathways. ssGSEA analysis showed significant differences in the distribution of certain immune-related cell subsets, such as CD8+T cells and B cells, and in the expression of multiple immune genes, including type II IFN response and APC coinhibition. In addition, 10 immune targets related to ferroptosis in breast cancer were found: CD276, CD80, HHLA2, LILRA2, NCR3LG1, NECTIN3, PVR, SLAMF9,TNFSF4, and BTN1A1. Using TCGA, new ferroptosis genes related to breast cancer prognosis were identified, a new reliable and accurate prognosis model was developed, and 10 new potential therapeutic targets different from the traditional targeted drugs were identified to provide a reference for improving the poor prognosis of patients with breast cancer.

Regulation of Hippo/YAP signaling pathway ameliorates cochlear hair cell injury by regulating ferroptosis

Cisplatin, which is effective for the treatment of solid tumors, also can induce cochlear hair cell damage. Therefore, this study was intended to explore how Hippo/YAP signaling pathway affects the cochlear hair cell injury by regulating ferroptosis. After cisplatin induction, or LAT1-IN-1 (YAP activator) and verteporfin (YAP inhibitor) treatment or transfection, the viability of HEI-OC1 cells was detected by cell counting kit-8 (CCK-8) assay. The iron level and the levels of oxidative stress markers (ROS, MDA and 4-HNE) were analyzed by iron assay kit, reactive oxygen species (ROS), malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) assay kits, respectively. The expression of ferritin light chain (FTL) in HEI-OC1 cells was detected by immunofluorescence and protein expressions of yes associated protein (YAP,) phosphorylated YAP (p-YAP), transferrin receptor (TFRC), glutathione peroxidase 4 (GPX4), acyl-CoA synthetase long-chain family member 4 (ACSL4) and solute carrier family 7 member 11 (SLC7A11) in HEI-OC1 cells were detected by western blot. The transcription of FTL and TFRC by YAP1 was verified by dual-luciferase reporter assay. The transfection efficiency of small interfering RNA (si-RNA) specific to FTL (siRNA-FTL) and TFRC (siRNA-TFRC) was confirmed by reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR). As a result, cisplatin inhibited the viability of HEI-OC1 cells by increasing free Fe²⁺ level and decreasing FTL level. LAT1-IN-1 promoted the viability of cisplatin-induced HEI-OC1 cells by suppressing oxidative stress level, free Fe²⁺ level, ferroptosis and increasing FTL level, while the effect of verteporfin was the opposite. YAP1 transcriptionally regulated the expression of FTL and TFRC. Inhibition of FTL suppressed the viability of cisplatin-induced HEI-OC1 cells by increasing oxidative stress level, free Fe²⁺ level, ferroptosis and decreasing FTL level, while the effect of TFRC inhibition was the opposite. In conclusion, YAP1 ameliorated cochlear hair cell injury by upregulating FTL and TFRC to suppress ferroptosis.

Characterization of substrates and inhibitors of the human heterodimeric transporter 4F2hc-LAT1 using purified protein and the scintillation proximity radioligand binding assay

Amino acids have diverse and essential roles in many cellular functions such as in protein synthesis, metabolism and as precursors of different hormones. Translocation of amino acids and derivatives thereof across biological membranes is mediated by amino acid transporters. 4F2hc-LAT1 is a heterodimeric amino acid transporter that is composed of two subunits belonging to the SLC3 (4F2hc) and SLC7 (LAT1) solute carrier families. The ancillary protein 4F2hc is responsible for the correct trafficking and regulation of the transporter LAT1. Preclinical studies have identified 4F2hc-LAT1 as a valid anticancer target due to its importance in tumor progression. The scintillation proximity assay (SPA) is a valuable radioligand binding assay that allows the identification and characterization of ligands of membrane proteins. Here, we present a SPA ligand binding study using purified recombinant human 4F2hc-LAT1 protein and the radioligand [³H]L-leucine as tracer. Binding affinities of different 4F2hc-LAT1 substrates and inhibitors determined by SPA are comparable with previously reported K _m and IC ₅₀ values from 4F2hc-LAT1 cell-based uptake assays. In summary, the SPA is a valuable method for the identification and characterization of ligands of membrane transporters including inhibitors. In contrast to cell-based assays, where the potential interference with other proteins such as endogenous transporters persists, the SPA uses purified protein making target engagement and characterization of ligands highly reliable.

Describing inhibitor specificity for the amino acid transporter LAT1 from metainference simulations

The human L-type amino acid transporter 1 (LAT1; SLC7A5) is a membrane transporter of amino acids, thyroid hormones, and drugs such as the Parkinson's disease drug levodopa (L-Dopa). LAT1 is found in the blood-brain barrier, testis, bone marrow, and placenta, and its dysregulation has been associated with various neurological diseases, such as autism and epilepsy, as well as cancer. In this study, we combine metainference molecular dynamics simulations, molecular docking, and experimental testing, to characterize LAT1-inhibitor interactions. We first conducted a series of molecular docking experiments to identify the most relevant interactions between LAT1's substrate-binding site and ligands, including both inhibitors and substrates. We then performed metainference molecular dynamics simulations using cryoelectron microscopy structures in different conformations of LAT1 with the electron density map as a spatial restraint, to explore the inherent heterogeneity in the structures. We analyzed the LAT1 substrate-binding site to map important LAT1-ligand interactions as well as newly described druggable pockets. Finally, this analysis guided the discovery of previously unknown LAT1 ligands using virtual screening and cellular uptake experiments. Our results improve our understanding of LAT1-inhibitor recognition, providing a framework for rational design of future lead compounds targeting this key drug target.

Pharmacologic inhibition of LAT1 predominantly suppresses transport of large neutral amino acids and downregulates global translation in cancer cells

L‐type amino acid transporter 1 (LAT1; SLC7A5), which preferentially transports large neutral amino acids, is highly upregulated in various cancers. LAT1 supplies cancer cells with amino acids as substrates for enhanced biosynthetic and bioenergetic reactions and stimulates signalling networks involved in the regulation of survival, growth and proliferation. LAT1 inhibitors show anti‐cancer effects and a representative compound, JPH203, is under clinical evaluation. However, pharmacological impacts of LAT1 inhibition on the cellular amino acid transport and the translational activity in cancer cells that are conceptually pivotal for its anti‐proliferative effect have not been elucidated yet. Here, we demonstrated that JPH203 drastically inhibits the transport of all the large neutral amino acids in pancreatic ductal adenocarcinoma cells. The inhibitory effects of JPH203 were observed even in competition with high concentrations of amino acids in a cell culture medium. The analyses of the nutrient‐sensing mTORC1 and GAAC pathways and the protein synthesis activity revealed that JPH203 downregulates the global translation. This study demonstrates a predominant contribution of LAT1 to the transport of large neutral amino acids in cancer cells and the suppression of protein synthesis by JPH203 supposed to underly its broad anti‐proliferative effects across various types of cancer cells.

Impact of Inhibition of Glutamine and Alanine Transport on Cerebellar Glial and Neuronal Metabolism

The cerebellum, or “little brain”, is often overlooked in studies of brain metabolism in favour of the cortex. Despite this, anomalies in cerebellar amino acid homeostasis in a range of disorders have been reported. Amino acid homeostasis is central to metabolism, providing recycling of carbon backbones and ammonia between cell types. Here, we examined the role of cerebellar amino acid transporters in the cycling of glutamine and alanine in guinea pig cerebellar slices by inhibiting amino acid transporters and examining the resultant metabolism of [1-¹³C]d-glucose and [1,2-¹³C]acetate by NMR spectroscopy and LCMS. While the lack of specific inhibitors of each transporter makes interpretation difficult, by viewing results from experiments with multiple inhibitors we can draw inferences about the major cell types and transporters involved. In cerebellum, glutamine and alanine transfer is dominated by system A, blockade of which has maximum effect on metabolism, with contributions from System N. Inhibition of neural system A isoform SNAT1 by MeAIB resulted in greatly decreased metabolite pools and reduced net fluxes but showed little effect on fluxes from [1,2-¹³C]acetate unlike inhibition of SNAT3 and other glutamine transporters by histidine where net fluxes from [1,2-¹³C]acetate are reduced by ~50%. We interpret the data as further evidence of not one but several glutamate/glutamine exchange pools. The impact of amino acid transport inhibition demonstrates that the cerebellum has tightly coupled cells and that glutamate/glutamine, as well as alanine cycling, play a major role in that part of the brain.

The Evaluation of L-Tryptophan Derivatives as Inhibitors of the LType Amino Acid Transporter LAT1 (SLC7A5)

A series of derivatives of the substrate amino acid l‐tryptophan have been investigated for inhibition of the L‐type amino acid transporter LAT1 (SLC7A5), which is an emerging target in anticancer drug discovery. Of the four isomeric 4‐, 5‐, 6‐, or 7‐benzyloxy‐l‐tryptophans, the 5‐substituted derivative was the most potent, with an IC₅₀ of 19 μM for inhibition of [³H]‐l‐leucine uptake into HT‐29 human colon carcinoma cells. The replacement of the carboxy group in 5‐benzyloxy‐l‐tryptophan by a bioisosteric tetrazole moiety led to a complete loss in potency. Likewise, the corresponding tetrazolide derived from l‐tryptophan itself was found to be neither a substrate nor an inhibitor of the transporter. Increasing the steric bulk at the 5‐position, while reasonably well tolerated in some cases, did not result in an improvement in potency. At the same time, none of these derivatives was found to be a substrate for LAT1‐mediated transport.

Transport of Biologically Active Ultrashort Peptides Using POT and LAT Carriers

Ultrashort peptides (USPs), consisting of 2–7 amino-acid residues, are a group of signaling molecules that regulate gene expression and protein synthesis under normal conditions in various diseases and ageing. USPs serve as a basis for the development of drugs with a targeted mechanism of action. The purpose of this review is to systematize the available data on USP transport involving POT and LAT transporters in various organs and tissues under normal, pathological and ageing conditions. The carriers of the POT family (PEPT1, PEPT2, PHT1, PHT2) transport predominantly di- and tripeptides into the cell. Methods of molecular modeling and physicochemistry have demonstrated the ability of LAT1 to transfer not only amino acids but also some di- and tripeptides into the cell and out of it. LAT1 and 2 are involved in the regulation of the antioxidant, endocrine, immune and nervous systems’ functions. Analysis of the above data allows us to conclude that, depending on their structure, di- and tripeptides can be transported into the cells of various tissues by POT and LAT transporters. This mechanism is likely to underlie the tissue specificity of peptides, their geroprotective action and effectiveness in the case of neuroimmunoendocrine system disorders.

Impact of concurrent gabapentin or pregabalin with high‐dose melphalan in patients with multiple myeloma undergoing autologous hematopoietic stem cell transplant

BACKGROUND

Melphalan is an alkylating agent used in both autologous (ASCT) and allogeneic stem cell transplantation. It is a substrate of L-type amino acid transporter-1 (LAT-1) and LAT-2, which are involved in its tissue penetration and elimination. Gabapentin and pregabalin, common concomitant medications in patients with multiple myeloma undergoing ASCT, are also substrates of LAT transporters, raising concern for potential competitive inhibition of melphalan transport. We evaluated whether concurrent use of gabapentin or pregabalin in patients receiving high-dose melphalan (≥140 mg/m² ) prior to ASCT impacted frequency and severity of melphalan-related adverse events.

OBJECTIVE

We aimed to determine if concurrent administration of gabapentin or pregabalin and melphalan increased melphalan toxicity.

METHODS

This was a single-center, retrospective evaluation including patients ≥18 years of age who received high-dose melphalan as part of a conditioning regimen at the Winship Cancer Institute of Emory University between August 1, 2010 and April 1, 2020 and were followed through their transplant admission. After identification and inclusion of patients who received melphalan in combination with gabapentin or pregabalin, patient matching based on age (±5 years), sex, and melphalan dose (140 mg/m² or 200 mg/m² ) was utilized to generate a comparable cohort of patients who received melphalan alone. The primary outcome was hospital length of stay (LOS); secondary outcomes included supportive care requirements between days +4 and day +14 and time to neutrophil and platelet-20 engraftment.

RESULTS

Among 176 patients evaluated in each group, median hospital LOS was 16 days, median time to neutrophil engraftment was 14 days, and median time to platelet-20 engraftment was 16 days in both groups. In addition, there were no significant differences in supportive care requirements between groups.

CONCLUSION

In this study, use of gabapentin or pregabalin in combination with melphalan did not impact safety of the conditioning regimen in patients undergoing ASCT.

Cysteine Donor-Based Brain-Targeting Prodrug: Opportunities and Challenges

Overcoming blood-brain barrier (BBB) to improve brain bioavailability of therapeutic drug remains an ongoing concern. Prodrug is one of the most reliable approaches for delivering agents with low-level BBB permeability into the brain. The well-known antioxidant capacities of cysteine (Cys) and its vital role in glutathione (GSH) synthesis indicate that Cys-based prodrug could potentiate therapeutic drugs against oxidative stress-related neurodegenerative disorders. Moreover, prodrug with Cys moiety could be recognized by the excitatory amino acid transporter 3 (EAAT3) that is highly expressed at the BBB and transports drug into the brain. In this review, we summarized the strategies of crossing BBB, properties of EAAT3 and its natural substrates, Cys and its donors, and Cys donor-based brain-targeting prodrugs by referring to recent investigations. Moreover, the challenges that we are faced with and future research orientations were also addressed and proposed. It is hoped that present review will provide evidence for the pursuit of novel Cys donor-based brain-targeting prodrug.

Rush Hour of LATs towards Their Transport Cycle

The mammalian SLC7 family comprises the L-amino acid transporters (LATs) and the cationic amino acid transporters (CATs). The relevance of these transporters is highlighted by their involvement in several human pathologies, including inherited rare diseases and acquired diseases, such as cancer. In the last four years, several crystal or cryo-EM structures of LATs and CATs have been solved. These structures have started to fill our knowledge gap that previously was based on the structural biology of remote homologs of the amino acid-polyamine-organocation (APC) transporters. This review recovers this structural and functional information to start generating the molecular bases of the transport cycle of LATs. Special attention is given to the known transporter conformations within the transport cycle and the molecular bases for substrate interaction and translocation, including the asymmetric interaction of substrates at both sides of the plasma membrane.

Highly Specific L-Type Amino Acid Transporter 1 Inhibition by JPH203 as a Potential Pan-Cancer Treatment

Accelerated cancer cell growth requires a massive intake of amino acids. Overexpression of L-type (large) amino acid transporter 1 (LAT1) on the cancer cell membrane facilitates such a demand, which is limited in normal organs. Therefore, LAT1 overexpression is ideal as a molecular cancer therapeutic target. JPH203, a LAT1-selective non-transportable blocker, had demonstrated LAT1 inhibition in <10 µM IC50 values and effectively suppressed cancer cell growth in studies involving several types of cancer cell lines and tumor xenograft models. A limited phase I clinical trial was performed on five different solid tumors and showed that JPH203 is well-tolerated and has a promising activity for the treatment of bile duct cancer. This review details the development and prospect of JPH203 as a LAT1-targeting cancer therapy.

[Drug Discovery Targeting an Amino Acid Transporter for Diagnosis and Therapy]

Nutrients are essential for all living organisms. Because growing cancer cells have strong metabolic demands, nutrient transporters are constitutively increased to facilitate the nutrient uptake. Among these nutrient transporters, L-type amino acid transporter 1 (LAT1), which transports large neutral amino acids including essential amino acids, is critical for cancer growth. Therefore, LAT1 has been considered as an attractive target for diagnosis and therapy of cancers. We have developed several lines of compounds for cancer diagnosis and therapy. To diagnose cancer by using positron emission tomography (PET) probes, we have created amino acid derivatives which are selectively transported by LAT1 and accumulated in cancer cells. In addition to amino acid derivatives as the LAT1 inhibitors, we also have made non-amino acid small compounds as anti-cancer drugs which inhibit LAT1 function and suppress tumor growth. The LAT1 targeting anti-cancer drug showed low toxicity but strong effects on various types of cancer cells in animal models. The novel PET probe is approved for clinical research and the new anti-cancer drug has been under clinical trial. Small compounds targeting the amino acid transporter bring us new tools for cancer diagnosis and therapy.

Selective mono-de-O-acetylation of the per-O-acetylated brasilicardin carbohydrate side chain

Methanol dried over powdered 4 Å molecular sieves can be used for a selective mono-de-O-acetylation of the phenolic acetyl group of the per-O-acetyl protected brasilicardin A carbohydrate side chain. This reaction opens a practical procedure for a synthetic access to derivates of the immunosuppressive and cytotoxic natural product brasilicardin A.

SLC7A5 serves as a prognostic factor of breast cancer and promotes cell proliferation through activating AKT/mTORC1 signaling pathway

Background

The purpose of our research was to determine if the clinical, pathological, and prognostic functions of SLC7A5 are the same as those of other molecular breast cancer (BC) subgroups.

Methods

We used the Oncomine and The Cancer Genome Atlas (TCGA) online databases to examine the expression and genetic changes of SLC7A5 in BC tissues. Immunohistochemical analysis was used to validate the SLC7A5 protein expression in subtypes of BC, while Kaplan-Meier figures and log-rank tests were used to evaluate the prognostic relevance of SLC7A5. Uni- and multivariate Cox regression models were adapted to analyze hazard ratios (HRs) and the independent prognostic factors. We analyzed the alterations of different malignancies of luminal cells by up-regulation of SLC7A5 in human luminal cell lines MCF-7. SLC7A5 was overexpressed in luminal cells, and then the AKT, mTOR, and p70-S6K phosphorylation and expression were analyzed by western blot analysis and real-time quantitative polymerase chain reaction (qPCR).

Results

Our results suggested that SLC7A5 was overexpressed in BC cell lines and in patients’ tissues. Elevated SLC7A5 messenger RNA (mRNA) and SLC7A5 protein expression was correlated to a worse clinical prognosis (P<0.001) in luminal subtypes of BC. The multivariate analysis suggested that high level of SLC7A5 expression could be an independent prognostic factor for decreased overall survival (OS). The study also demonstrated that SLC7A5 overexpression increased proliferation of MCF-7 cells by reducing the cell cycle arrest in G1 phase. Our mechanistic study further indicates that SLC7A5 enhances the proliferation of the MCF-7 cell by activation of AKT/mTORC1 pathway through phosphorylation.

Conclusions

Our study demonstrated that SLC7A5 may have a vital function in the biology of BC cells, indicating that SLC7A5 is a potential prognostic biomarker and may be a valuable therapeutic target in BC patients.

Mechanism of substrate transport and inhibition of the human LAT1-4F2hc amino acid transporter

LAT1 (SLC7A5) is one of the representative light chain proteins of heteromeric amino acid transporters, forming a heterodimer with its heavy chain partner 4F2hc (SLC3A2). LAT1 is overexpressed in many types of tumors and mediates the transfer of drugs and hormones across the blood-brain barrier. Thus, LAT1 is considered as a drug target for cancer treatment and may be exploited for drug delivery into the brain. Here, we synthesized three potent inhibitors of human LAT1, which inhibit transport of leucine with IC₅₀ values between 100 and 250 nM, and solved the cryo-EM structures of the corresponding LAT1-4F2hc complexes with these inhibitors bound at resolution of up to 2.7 or 2.8 Å. The protein assumes an outward-facing occluded conformation, with the inhibitors bound in the classical substrate binding pocket, but with their tails wedged between the substrate binding site and TM10 of LAT1. We also solved the complex structure of LAT1-4F2hc with 3,5-diiodo-l-tyrosine (Diiodo-Tyr) at 3.4 Å overall resolution, which revealed a different inhibition mechanism and might represent an intermediate conformation between the outward-facing occluded state mentioned above and the outward-open state. To our knowledge, this is the first time that the outward-facing conformation is revealed for the HAT family. Our results unveil more important insights into the working mechanisms of HATs and provide a structural basis for future drug design.

Recent developments in ligands and chemical probes targeting solute carrier transporters

Solute carrier (SLC) membrane transporters remain a largely unexploited target class, despite their central roles in cell identity and metabolism. This gap is reflected in the lack of high-quality chemical ligands or probes and in the small number of compounds that have progressed toward clinical development. In this review, we discuss recent advancements in SLC ligand discovery as well as new candidates that have been added to the investigational toolkit, with a particular focus on first-in-class ligands and the cognate discovery strategies. The availability of new probes expands the opportunity to elucidate the functions of SLCs and their relevance in physiology and explores any future potential of SLC druggability.

Heteromeric Amino Acid Transporters in Brain: from Physiology to Pathology

In humans, more than 50 transporters are responsible for the traffic and balance of amino acids within and between cells and tissues, and half of them have been associated with disease [1]. Covering all common amino acids, Heteromeric Amino acid Transporters (HATs) are one class of such transporters. This review first highlights structural and functional studies that solved the atomic structure of HATs and revealed molecular clues on substrate interaction. Moreover, this review focuses on HATs that have a role in the central nervous system (CNS) and that are related to neurological diseases, including: (i) LAT1/CD98hc and its role in the uptake of branched chain amino acids trough the blood brain barrier and autism. (ii) LAT2/CD98hc and its potential role in the transport of glutamine between plasma and cerebrospinal fluid. (iii) y⁺LAT2/CD98hc that is emerging as a key player in hepatic encephalopathy. xCT/CD98hc as a potential therapeutic target in glioblastoma, and (iv) Asc-1/CD98hc as a potential therapeutic target in pathologies with alterations in NMDA glutamate receptors.

Amino Acid Transporters on the Guard of Cell Genome and Epigenome

Tumorigenesis is driven by metabolic reprogramming. Oncogenic mutations and epigenetic alterations that cause metabolic rewiring may also upregulate the reactive oxygen species (ROS). Precise regulation of the intracellular ROS levels is critical for tumor cell growth and survival. High ROS production leads to the damage of vital macromolecules, such as DNA, proteins, and lipids, causing genomic instability and further tumor evolution. One of the hallmarks of cancer metabolism is deregulated amino acid uptake. In fast-growing tumors, amino acids are not only the source of energy and building intermediates but also critical regulators of redox homeostasis. Amino acid uptake regulates the intracellular glutathione (GSH) levels, endoplasmic reticulum stress, unfolded protein response signaling, mTOR-mediated antioxidant defense, and epigenetic adaptations of tumor cells to oxidative stress. This review summarizes the role of amino acid transporters as the defender of tumor antioxidant system and genome integrity and discusses them as promising therapeutic targets and tumor imaging tools.

GRK3 deficiency elicits brain immune activation and psychosis

The G protein-coupled receptor kinase (GRK) family member protein GRK3 has been linked to the pathophysiology of schizophrenia and bipolar disorder. Expression, as well as protein levels, of GRK3 are reduced in post-mortem prefrontal cortex of schizophrenia subjects. Here, we investigate functional behavior and neurotransmission related to immune activation and psychosis using mice lacking functional Grk3 and utilizing a variety of methods, including behavioral, biochemical, electrophysiological, molecular, and imaging methods. Compared to wildtype controls, the Grk3^-/- mice show a number of aberrations linked to psychosis, including elevated brain levels of IL-1β, increased turnover of kynurenic acid (KYNA), hyper-responsiveness to D-amphetamine, elevated spontaneous firing of midbrain dopamine neurons, and disruption in prepulse inhibition. Analyzing human genetic data, we observe a link between psychotic features in bipolar disorder, decreased GRK expression, and increased concentration of CSF KYNA. Taken together, our data suggest that Grk3^-/- mice show face and construct validity relating to the psychosis phenotype with glial activation and would be suitable for translational studies of novel immunomodulatory agents in psychotic disorders.

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.

Small molecule inhibitors provide insights into the relevance of LAT1 and LAT2 in materno-foetal amino acid transport

The placenta supplies the foetus with critical nutrients such as essential amino acids (AA, eg leucine) for development and growth. It also represents a cellular barrier which is formed by a polarized, differentiated syncytiotrophoblast (STB) monolayer. Active Na⁺‐independent leucine transport across the placenta is mainly attributed to the System L transporters LAT1/SLC7A5 and LAT2/SLC7A8. This study explored the influence of trophoblast differentiation on the activity of LAT1/LAT2 and the relevance of LAT1/LAT2 in leucine uptake and transfer in trophoblasts by applying specific small molecule inhibitors (JPH203/JG336/JX009). L‐leucine uptake (total dose = 167 μmol/L) was sensitive to LAT1‐specific inhibition by JPH203 (EC₅₀ = 2.55 µmol/L). The inhibition efficiency of JPH203 was increased by an additional methoxy group in the JPH203‐derivate JG336 (EC₅₀ = 1.99 µmol/L). Interestingly, JX009 showed efficient System L inhibition (EC₅₀ = 2.35 µmol/L) and was the most potent inhibitor of leucine uptake in trophoblasts. The application of JPH203 and JX009 in Transwell^®‐based leucine transfer revealed LAT1 as the major accumulative transporter at the apical membrane, but other System L transporters such as LAT2 as rate‐limiting for leucine efflux across the basal membrane. Therefore, differential specificity of the applied inhibitors allowed for estimation of the contribution of LAT1 and LAT2 in materno‐foetal AA transfer and their potential impact in pregnancy diseases associated with impaired foetal growth.

Demystifying the Molecular Basis of Pyrazoloquinolinones Recognition at the Extracellular α1+/β3- Interface of the GABAA Receptor by Molecular Modeling

GABA_A receptors are pentameric ligand-gated ion channels that serve as major inhibitory neurotransmitter receptors in the mammalian brain and the target of numerous clinically relevant drugs interacting with different ligand binding sites. Here, we report an in silico approach to investigate the binding of pyrazoloquinolinones (PQs) that mediate allosteric effects through the extracellular α+/β- interface of GABA_A receptors. First, we docked a potent prototype of PQs into the α1+/β3- site of a homology model of the human α1β3γ2 subtype of the GABA_A receptor. Next, for each docking pose, we computationally derived protein-ligand complexes for 18 PQ analogs with known experimental potency. Subsequently, binding energy was calculated for all complexes using the molecular mechanics-generalized Born surface area method. Finally, docking poses were quantitatively assessed in the light of experimental data to derive a binding hypothesis. Collectively, the results indicate that PQs at the α1+/β3- site likely exhibit a common binding mode that can be characterized by a hydrogen bond interaction with β3Q64 and hydrophobic interactions involving residues α1F99, β3Y62, β3M115, α1Y159, and α1Y209. Importantly, our results are in good agreement with the recently resolved cryo-Electron Microscopy structures of the human α1β3γ2 and α1β2γ2 subtypes of GABA_A receptors.

Synthesis and Preclinical Evaluation of 6-[18F]Fluorine-α-methyl-l-tryptophan, a Novel PET Tracer for Measuring Tryptophan Uptake

The positron emission tomography (PET) radioligand α-[¹¹C]methyl-l-tryptophan ([¹¹C]AMT) has been used to assess tryptophan metabolism in cancer, epilepsy, migraine, and autism. Despite its extensive application, the utility of this tracer is currently hampered by the short half-life of the radionuclide used for its labeling (¹¹C, t_1/2 = 20.4 min). We herein report the design, synthesis, radiolabeling, and initial in vivo evaluation of a fluorine-18 (¹⁸F, t_1/2 = 109.7 min) labeled analogue that is fluorinated in the 6-position of the aromatic ring ([¹⁸F]6-F-AMTr). In a head-to-head comparison between [¹⁸F]6-F-AMTr and [¹¹C]AMT in mice using PET, peak brain radioactivity, regional brain distribution, and kinetic profiles were similar between the two tracers. [¹⁸F]6-F-AMTr was however not a substrate for IDO1 or TPH as determined in in vitro enzymatic assays. The brain uptake of the tracer is thus more likely related to LAT1 transport over the blood-brain barrier than metabolism along the serotonin or kynurenine pathways.

LAT1 (SLC7A5) Overexpression in Negative Her2 Group of Breast Cancer: A Potential Therapy Target

Objective:

HER2 negative carcinomas of the breast pose a challenge for treatment due to redundancies in potential drug targets and poor patient outcomes. Our aim was to investigate the role of L-type amino acid transporter – LAT1 as a potential prognosticator and a drug target.

Methods:

In this retrospective work, we have studied the expression of LAT1 in 145 breast cancer tissues via immunohistochemistry. Overall survival analysis was used to evaluate patient outcome in various groups of our cohort.

Results:

Positive LAT1 expression was found in 27 (84.4%) luminal A subtype, 27 (64.3%) luminal B/triple positive subtype, 29 (82.9%) triple negative subtype, and 24 (66.7%) HER2-only positive subtype (p=0.1). Interestingly, negative correlation was found between LAT1 and HER2; where positive expression of LAT1 was found in 56 (83.6%) cases in negative HER2 group and 51 (65.4%) cases from positive HER2 group (p=0.01). Unfortunately, we were unable to report significant survival differences when LAT1 expression was studied in the negative HER2 group. Nevertheless, five incidents of mortality (out of 55) were reported in LAT1+/HER2- group compared to none in the LAT1-/HER2- group (N=11).

Conclusion:

Our findings of overexpression of LAT1 in negative HER2 group suggest a role of this protein as prognosticator and drug target in a challenging therapeutic cohort.

Tyrosine-Chlorambucil Conjugates Facilitate Cellular Uptake through L-Type Amino Acid Transporter 1 (LAT1) in Human Breast Cancer Cell Line MCF-7

l-type amino acid transporter 1 (LAT1) is an amino acid transporter that is overexpressed in several types of cancer and, thus, it can be a potential target for chemotherapy. The objectives of this study were to (a) synthesize LAT1-targeted chlorambucil derivatives and (b) evaluate their LAT1-mediated cellular uptake as well as antiproliferative activity in vitro in the human breast cancer MCF-7 cell line. Chlorambucil was conjugated to l-tyrosine—an endogenous LAT1 substrate—via either ester or amide linkage (compounds 1 and 2, respectively). While chlorambucil itself did not bind to LAT1, its derivatives 1 and 2 bound to LAT1 with a similar affinity as with l-tyrosine and their respective cellular uptake was significantly higher than that of chlorambucil in MCF-7. The results of our cellular uptake study are indicative of antiproliferative activity, as a higher intracellular uptake of chlorambucil derivatives resulted in greater cytotoxicity than chlorambucil by itself. LAT1 thus contributes to intracellular uptake of chlorambucil derivatives and, therefore, increases antiproliferative activity. The understanding gained from our research can be used in the development of LAT1-targeted anticancer drugs and prodrugs for site-selective and enhanced chemotherapeutic activity.

Glutamine transporters as pharmacological targets: From function to drug design

Among the different targets of administered drugs, there are membrane transporters that play also a role in drug delivery and disposition. Moreover, drug-transporter interactions are responsible for off-target effects of drugs underlying their toxicity. The improvement of the drug design process is subjected to the identification of those membrane transporters mostly relevant for drug absorption, delivery and side effect production. A peculiar group of proteins with great relevance to pharmacology is constituted by the membrane transporters responsible for managing glutamine traffic in different body districts. The interest around glutamine metabolism lies in its physio-pathological role; glutamine is considered a conditionally essential amino acid because highly proliferative cells have an increased request of glutamine that cannot be satisfied only by endogenous synthesis. Then, glutamine transporters provide cells with this special nutrient. Among the glutamine transporters, SLC1A5, SLC6A14, SLC6A19, SLC7A5, SLC7A8 and some members of SLC38 family are the best characterized, so far, in both physiological and pathological conditions. Few 3D structures have been solved by CryoEM; other structural data on these transporters have been obtained by computational analysis. Interactions with drugs have been described for several transporters of this group. For some of them, the studies are at an advanced stage, for others, the studies are still in nuce and novel biochemical findings open intriguing perspectives.

Amino acids in cancer

Over 90 years ago, Otto Warburg's seminal discovery of aerobic glycolysis established metabolic reprogramming as one of the first distinguishing characteristics of cancer¹. The field of cancer metabolism subsequently revealed additional metabolic alterations in cancer by focusing on central carbon metabolism, including the citric acid cycle and pentose phosphate pathway. Recent reports have, however, uncovered substantial non-carbon metabolism contributions to cancer cell viability and growth. Amino acids, nutrients vital to the survival of all cell types, experience reprogrammed metabolism in cancer. This review outlines the diverse roles of amino acids within the tumor and in the tumor microenvironment. Beyond their role in biosynthesis, they serve as energy sources and help maintain redox balance. In addition, amino acid derivatives contribute to epigenetic regulation and immune responses linked to tumorigenesis and metastasis. Furthermore, in discussing the transporters and transaminases that mediate amino acid uptake and synthesis, we identify potential metabolic liabilities as targets for therapeutic intervention.

Rigorous sampling of docking poses unveils binding hypothesis for the halogenated ligands of L-type Amino acid Transporter 1 (LAT1)

L-type Amino acid Transporter 1 (LAT1) plays a significant role in the growth and propagation of cancer cells by facilitating the cross-membrane transport of essential nutrients, and is an attractive drug target. Several halogen-containing L-phenylalanine-based ligands display high affinity and high selectivity for LAT1; nonetheless, their molecular mechanism of binding remains unclear. In this study, a combined in silico strategy consisting of homology modeling, molecular docking, and Quantum Mechanics-Molecular Mechanics (QM-MM) simulation was applied to elucidate the molecular basis of ligand binding in LAT1. First, a homology model of LAT1 based on the atomic structure of a prokaryotic homolog was constructed. Docking studies using a set of halogenated ligands allowed for deriving a binding hypothesis. Selected docking poses were subjected to QM-MM calculations to investigate the halogen interactions. Collectively, the results highlight the dual nature of the ligand-protein binding mode characterized by backbone hydrogen bond interactions of the amino acid moiety of the ligands and residues I63, S66, G67, F252, G255, as well as hydrophobic interactions of the ligand’s side chains with residues I139, I140, F252, G255, F402, W405. QM-MM optimizations indicated that the electrostatic interactions involving halogens contribute to the binding free energy. Importantly, our results are in good agreement with the recently unraveled cryo-Electron Microscopy structures of LAT1.

The L-Type Amino Acid Transporter LAT1-An Emerging Target in Cancer

Chronic proliferation is a major hallmark of tumor cells. Rapidly proliferating cancer cells are highly dependent on nutrients in order to duplicate their cell mass during each cell division. In particular, essential amino acids are indispensable for proliferating cancer cells. Their uptake across the cell membrane is tightly controlled by membrane transporters. Among those, the L-type amino acid transporter LAT1 (SLC7A5) has been repeatedly found overexpressed in a vast variety of cancers. In this review, we summarize the most recent advances in our understanding of the role of LAT1 in cancer and highlight preclinical studies and drug developments underlying the potential of LAT1 as therapeutic target.