Establishment and characterization of mammalian cell lines stably expressing human L-type amino acid transporters

Structure-activity characteristics of phenylalanine analogs selectively transported by L-type amino acid transporter 1 (LAT1)

L-type amino acid transporter 1 (LAT1) is a transmembrane protein responsible for transporting large neutral amino acids. While numerous LAT1-targeted compound delivery for the brain and tumors have been investigated, their LAT1 selectivity often remains ambiguous despite high LAT1 affinity. This study assessed the LAT1 selectivity of phenylalanine (Phe) analogs, focusing on their structure-activity characteristics. We discovered that 2-iodo-L-phenylalanine (2-I-Phe), with an iodine substituent at position 2 in the benzene ring, markedly improves LAT1 affinity and selectivity compared to parent amino acid Phe, albeit at the cost of reduced transport velocity. L-Phenylglycine (Phg), one carbon shorter than Phe, was found to be a substrate for LAT1 with a lower affinity, exhibiting a low level of selectivity for LAT1 equivalent to Phe. Notably, (R)-2-amino-1,2,3,4-tetrahydro-2-naphthoic acid (bicyclic-Phe), with an α-methylene moiety akin to the α-methyl group in α-methyl-L-phenylalanine (α-methyl-Phe), a known LAT1-selective compound, showed similar LAT1 transport maximal velocity to α-methyl-Phe, but with higher LAT1 affinity and selectivity. In vivo studies revealed tumor-specific accumulation of bicyclic-Phe, underscoring the importance of LAT1-selectivity in targeted delivery. These findings emphasize the potential of bicyclic-Phe as a promising LAT1-selective component, providing a basis for the development of LAT1-targeting compounds based on its structural framework.

Contribution of the L-Type Amino Acid Transporter Family in the Diagnosis and Treatment of Prostate Cancer

The L-type amino acid transporter (LAT) family contains four members, LAT1~4, which are important amino acid transporters. They mainly transport specific amino acids through cell membranes, provide nutrients to cells, and are involved in a variety of metabolic pathways. They regulate the mTOR signaling pathway which has been found to be strongly linked to cancer in recent years. However, in the field of prostate cancer (PCa), the LAT family is still in the nascent stage of research, and the importance of LATs in the diagnosis and treatment of prostate cancer is still unknown. Therefore, this article aims to report the role of LATs in prostate cancer and their clinical significance and application. LATs promote the progression of prostate cancer by increasing amino acid uptake, activating the mammalian target of rapamycin (mTOR) pathway and downstream signals, mediating castration-resistance, promoting tumor angiogenesis, and enhancing chemotherapy resistance. The importance of LATs as diagnostic and therapeutic targets for prostate cancer was emphasized and the latest research results were introduced. In addition, we introduced selective LAT1 inhibitors, including JPH203 and OKY034, which showed excellent inhibitory effects on the proliferation of various tumor cells. This is the future direction of amino acid transporter targeting therapy drugs.

Thyroid Hormone Transporters in a Human Placental Cell Model

Background: Fetal brain development in the first half of pregnancy is dependent on maternal thyroid hormone (TH), highlighting the importance of trans-placental TH transport. It is yet unclear which transporters are involved in this process. We aimed to identify the major TH transporters in a human placental cell model (BeWo cells). Methods: Messenger RNA expression of the known TH transporters (the monocarboxylate transporter [MCT]8, MCT10, the L-type amino acid transporter [LAT]1, LAT2, the organic anion transporting peptide [OATP]1A2 and OATP4A1) in BeWo cells and human placenta were determined by quantitative PCR. To determine the specificity and efficacy of transporter inhibitors, we first determined TH uptake at different inhibitor concentrations in African green monkey kidney fibroblast-like cells (COS1 cells) overexpressing TH transporters. We then tested TH uptake in BeWo cells in the presence or absence of the optimal inhibitor concentrations. Results: All tested TH transporters were expressed in human term placentas, whereas MCT8 was absent in BeWo cells. Both 2-amino-2-norbornanecarboxylic acid (BCH) and L-tryptophan at 1 mM inhibited LATs, whereas at the highest concentration (10 mM) L-tryptophan also inhibited MCT10. Verapamil inhibited OATP1A2 and less efficiently both MCTs, but not LATs. Both rifampicin and naringin reduced OATP1A2 activity. Finally, silychristin inhibited MCT8 at submicromolar concentrations and OATP1A2 partially only at the highest concentration tested (10 μM). In BeWo cells, verapamil reduced triiodothyronine (T3) uptake by 24%, BCH by 31%, and 1 mM L-tryptophan by 41%. The combination of BCH and verapamil additively decreased T3 uptake by 53% and the combination of BCH and 10 mM L-tryptophan by 60%, suggesting a major role for MCT10 and LATs in placental T3 uptake. Indeed, transfection of BeWo cells with MCT10-specific small interfering RNA significantly reduced T3 uptake. Only the combination of BCH and verapamil significantly reduced thyroxine (T4) uptake in BeWo cells, by 32%. Conclusions: Using pharmacological inhibitors, we show that MCT10 and LATs play a major role in T3 uptake in BeWo cells. T4 uptake appears independent of known TH transporters, suggesting the presence of, currently unknown, alternative transporter(s).

[Possible therapeutic use of L-type amino acid transporter 1 (LAT1)-specific inhibitor for intractable asthma treatment]

Bronchial asthma (asthma) is characterized by chronic airway inflammation, reversible obstruction, and hyperresponsive conditions. Although most asthma patients have been becoming controllable by virtue of inhaled corticosteroid (ICS), substantial number of patients still do not respond to the steroid-based therapy. Mast cells, eosinophils, and helper T (Th) 2 cells have been considered as key players in asthma pathogenesis. However, emerging studies have revealed that Th subsets other than Th2, as well as various other immune cells, significantly contribute to the development of steroid-resistant intractable asthma. T cells and other inflammatory cells require incorporating a large amount of nutrients such as amino acids and glucose to exhibit their full function following activation. Based on this remarkable character, it has recently been suggested that the pharmacological inhibition of amino acid transporters is promising for treating immunological and inflammatory disorders through the suppression of inflammatory cell activation. In this review, we explore the possible management of intractable asthma by developing a selective inhibitor for L-type amino acid transporter (LAT) 1.

L-type amino acid transporter 1 as a target for inflammatory disease and cancer immunotherapy

Ingestion of amino acids is fundamental for cellular activity. Amino acids are important components for protein synthesis but are also crucial for intracellular metabolic reactions and signal transduction. Following activation, immune cells induce metabolic reprogramming to generate adequate energy and constitutive substances. Hence, the delivery of amino acids by transporters is necessary for the progression of metabolic rewiring. In this review, we discuss how amino acids and their transporters regulate immune cell functions, with emphasis on LAT1, a transporter of large neutral amino acids. Furthermore, we explore the possibility of targeting amino acid transporters to improve immune disorders and cancer immune therapies.

Amino acid transporter LAT1 (SLC7A5) as a molecular target for cancer diagnosis and therapeutics

Cancer cells require a massive supply of nutrients, including sugars and amino acids-the upregulation of transporters for each nutrient contributes to meet the demand. Distinct from glucose transporters, amino acid transporters include ones whose expression is specific to cancer cells. For example, LAT1 (SLC7A5) displays protein expression mostly limited to the plasma membrane of cancer cells. The exceptions are the placental barrier and the blood-brain barrier, where immunohistochemical and mass spectrometric studies have shown LAT1 expression, although their levels are supposed to be lower than those in cancers. The expression of LAT1 has been reported in cancers from various tissue origins, where high LAT1 expression is related to the poor prognosis of patients. LAT1 is essential for cancer cell growth because the pharmacologic inhibition and knockdown/knockout of LAT1 suppress the proliferation of cancer cells and the growth of xenograft tumors. The inhibition of LAT1 suppresses protein synthesis by downregulating the mTORC1 signaling pathway and mobilizing the general amino acid control (GAAC) pathway in cancer cells. LAT1 is, thus, a candidate molecular target for the diagnosis and therapeutics of cancers. ¹⁸F-labeled 3-fluoro-l-α-methyl-tyrosine (FAMT) is used as a LAT1-specific PET probe for cancer detection due to the LAT1 specificity of α-methyl aromatic amino acids. FAMT accumulation is cancer-specific and avoids non-cancer lesions, including inflammation, confirming the cancer-specific expression of LAT1 in humans. Due to the cancer-specific nature, LAT1 can also be used for cancer-specific delivery of anti-tumor agents such as l-para-boronophenylalanine used for boron neutron capture therapy and α-emitting nuclide-labeled LAT1 substrates developed for nuclear medicine treatment. Based on the importance of LAT1 in cancer progression, high-affinity LAT1-specific inhibitors have been developed for anti-tumor drugs. JPH203 (KYT0353) is such a compound designed based on the structure-activity relationship of LAT1 ligands. It is one of the highest-affinity inhibitors with less affecting other transporters. It suppresses tumor growth in vivo without significant toxicity in preclinical studies at doses enough to suppress tumor growth. In the phase-I clinical trial, JPH203 appeared to provide promising activity. Because the mechanisms of action of LAT1 inhibitors are novel, with or without combination with other anti-tumor drugs, they could contribute to the treatment of cancers that do not respond to current therapy. The LAT1-specific PET probe could also be used as companion diagnostics of the LAT1-targeting therapies to select patients to whom therapeutic benefits could be expected. Recently, the cryo-EM structure of LAT1 has been solved, which would facilitate the understanding of the mechanisms of the dynamic interaction of ligands and the binding site, and further designing new compounds with higher activity.

[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.

Yeast Cell-Based Transport Assay for the Functional Characterization of Human 4F2hc-LAT1 and -LAT2, and LAT1 and LAT2 Substrates and Inhibitors

In mammalian cells, the L-type amino acid transporters (LATs) LAT1 (SLC7A5) and LAT2 (SLC7A8) form heterodimeric amino acid transporters (HATs) with the ancillary protein 4F2hc and are involved in the cellular uptake of specific amino acids. The HAT 4F2hc-LAT1 is found upregulated in various cancer cell types, while 4F2hc-LAT2 is a transporter for non-cancer cells. Preclinical studies have highlighted that 4F2hc-LAT1 plays an important role in tumor progression representing a valid anticancer target. Consequently, current research is focusing on the development of potent and specific human 4F2hc-LAT1 inhibitors. On the other hand, 4F2hc-LAT2 is emerging as target of other diseases, thus also gaining clinical interest. To determine affinity and specificity of substrates and inhibitors for 4F2hc-LAT1 or 4F2hc-LAT2, robust transport cell assays are indispensable. We have optimized and validated a transport assay using cells of the methylotrophic yeast Pichia pastoris stably overexpressing the human HATs 4F2hc-LAT1 or -LAT2, and the LATs LAT1 or LAT2 alone. The radioligand [³H]L-leucine was used as reporter and the substrates L-leucine, triiodothyronine (T3) and thyroxine (T4) as well as the inhibitors BCH and JPH203 (KYT-0353) for assay validation. Obtained half-maximal inhibitory concentrations also provided new insights, e.g., into the LAT specificity of the potent inhibitor JPH203 and on the potency of the thyroid hormones T3 and T4 to inhibit transport through human 4F2hc-LAT2. The LAT1 and LAT2 assays are of particular interest to determine possible implications and influences of 4F2hc in ligand binding and transport. In summary, the presented assays are valuable for characterization of ligands, e.g., towards 4F2hc-LAT1 specificity, and can also be applied for compound screening. Finally, our established approach and assay would also be applicable to other HATs and LATs of interest.

A GC-MS/Single-Cell Method to Evaluate Membrane Transporter Substrate Specificity and Signaling

Amino acid transporters play a vital role in metabolism and nutrient signaling pathways. Typically, transport activity is investigated using single substrates and competing amounts of other amino acids. We used GC-MS and LC-MS for metabolic screening of Xenopus laevis oocytes expressing various human amino acid transporters incubated in complex media to establish their comprehensive substrate profiles. For most transporters, amino acid selectivity matched reported substrate profiles. However, we could not detect substantial accumulation of cationic amino acids by SNAT4 and ATB0,+ in contrast to previous reports. In addition, comparative substrate profiles of two related sodium neutral amino acid transporters known as SNAT1 and SNAT2, revealed the latter as a significant leucine accumulator. As a consequence, SNAT2, but not SNAT1, was shown to be an effective activator of the eukaryotic cellular growth regulator mTORC1. We propose, that metabolomic profiling of membrane transporters in Xe nopus laevis oocytes can be used to test their substrate specificity and role in intracellular signaling pathways.

Protein kinase C activation upregulates human L-type amino acid transporter 2 function

L-type amino acid transporter 2 (LAT2) is a Na+-independent neutral amino acid transporter, whose function regulation system remains unclarified. Since protein kinase C (PKC) is known to regulate the functions of various transporters, we investigated whether human LAT2 (hLAT2) function is regulated by PKC. In mouse proximal tubule S2 cells, hLAT2 transport activity was upregulated by PKC activation. However, we found that the mRNA and protein expression of hLAT2 was not affected by PKC activation and that the upregulation was independent of the three potential PKC consensus sites in the hLAT2 amino acid sequence. Moreover, we found that PKC activation upregulated the Vmax value for hLAT2-mediated alanine transport, which was not accompanied by the induction of hLAT2 membrane insertion. In conclusion, we showed that hLAT2 function is upregulated by PKC activation, which is not related to either the de novo synthesis, the phosphorylation or the membrane insertion of hLAT2.

L-type amino acid transporter 1 is associated with chemoresistance in breast cancer via the promotion of amino acid metabolism

¹⁸F-FDG PET/CT has been used as an indicator of chemotherapy effects, but cancer cells can remain even when no FDG uptake is detected, indicating the importance of exploring other metabolomic pathways. Therefore, we explored the amino acid metabolism, including L-type amino acid transporter-1 (LAT1), in breast cancer tissues and clarified the role of LAT1 in therapeutic resistance and clinical outcomes of patients. We evaluated LAT1 expression before and after neoadjuvant chemotherapy and examined the correlation of glucose uptake using FDG-PET with the pathological response of patients. It revealed that LAT1 levels correlated with proliferation after chemotherapy, and amino acid and glucose metabolism were closely correlated. In addition, LAT1 was considered to be involved in treatment resistance and sensitivity only in luminal type breast cancer. Results of in vitro analyses revealed that LAT1 promoted amino acid uptake, which contributed to energy production by supplying amino acids to the TCA cycle. However, in MCF-7 cells treated with chemotherapeutic agents, oncometabolites and branched-chain amino acids also played a pivotal role in energy production and drug resistance, despite decreased glucose metabolism. In conclusion, LAT1 was involved in drug resistance and could be a novel therapeutic target against chemotherapy resistance in luminal type breast cancer.

Preclinical evaluation of new α-radionuclide therapy targeting LAT1: 2-[211At]astato-α-methyl-L-phenylalanine in tumor-bearing model

INTRODUCTION

Targeted α-radionuclide therapy has attracted attention as a promising therapy for refractory cancers. However, the application is limited to certain types of cancer. Since L-type amino acid transporter 1 (LAT1) is highly expressed in various human cancers, we prepared an LAT1-selective α-radionuclide-labeled amino acid analog, 2-[211At]astato-α-methyl-L-phenylalanine (2-[211At]AAMP), and evaluated its potential as a therapeutic agent.

METHODS

2-[211At]AAMP was prepared from the stannyl precursor. Stability of 2-[211At]AAMP was evaluated both in vitro and in vivo. In vitro studies using an LAT1-expressing human ovarian cancer cell line, SKOV3, were performed to evaluate cellular uptake and cytotoxicity of 2-[211At]AAMP. Biodistribution and therapeutic studies in SKOV3-bearing mice were performed after intravenous injection of 2-[211At]AAMP.

RESULTS

2-[211At]AAMP was stable in murine plasma in vitro and excreted intact into urine. Cellular uptake of 2-[211At]AAMP was inhibited by treatment with an LAT1-selective inhibitor. After 24 h incubation, 2-[211At]AAMP suppressed clonogenic growth at 10 kBq/ml, and induced cell death and DNA double-strand breaks at 25 kBq/ml. When injected into mice, 2-[211At]AAMP exhibited peak accumulation in the tumor at 30 min postinjection, and radioactivity levels in the tumor were retained up to 60 min. The majority of the radioactivity was rapidly eliminated from the body into urine in an intact form immediately after injection. 2-[211At]AAMP significantly improved the survival of mice (P < 0.05) without serious side effects.

CONCLUSION

2-[211At]AAMP showed α-radiation-dependent cellular growth inhibition after it was taken up via LAT1. In addition, 2-[211At]AAMP had a beneficial effect on survival in vivo. These findings suggest that 2-[211At]AAMP would be useful for the treatment of LAT1-positive cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This is the first report of an LAT1-targeting radiopharmaceutical for α-radionuclide therapy; this agent would be applicable for the treatment of various types of cancer.

Review of the Correlation of LAT1 With Diseases: Mechanism and Treatment

LAT1 is a member of the system L transporter family. The main role of the LAT1 is to transport specific amino acids through cell membranes to provide nutrients to cells and participate in several metabolic pathways. It also contributes to the transport of hormones and some drugs, which are essential for the development and treatment of some diseases. In recent years, many studies have shown that LAT1 is related to cancer, obesity, diabetes, and other diseases. However, the specific mechanism underlying the influence of LAT1 on such conditions remains unclear. Through the increasing number of studies on LAT1, we have obtained a preliminary understanding on the function of LAT1 in diseases. These studies also provide a theoretical basis for finding treatments for LAT1-related diseases, such as cancer. This review summarizes the function and mechanism of LAT1 in different diseases and the treatment of LAT1-related diseases. It also provides support for the development of novel and reliable disease treatments.

Albumin is a secret factor involved in multidirectional interactions among the serotoninergic, immune and endocrine systems that supervises the mechanism of CYP1A and CYP3A regulation in the liver

This review focuses on albumin, which is involved in multidirectional interactions among the immune, endocrine and serotoninergic systems and supervises the regulation of cytochrome P450 (CYP) isoforms under conditions of both normal liver function and liver insufficiency. Special attention is paid to albumin, thyroid hormones, testosterone and tryptophan hydroxylase in these interactions as well as their potential roles in liver regeneration. The association of these factors with inflammation and the modification of the mechanism of hepatic drug-metabolizing CYP isoform regulation are also presented because changes in the expression of CYP isoforms in the liver may result in subsequent changes to a marker substance used for testing CYP activity, thus providing a simple way to control the liver regeneration process or the dangerous stimulation of hepatocarcinogenesis.

JPH203, a newly developed anti-cancer drug, shows a preincubation inhibitory effect on L-type amino acid transporter 1 function

JPH203 is a novel anti-cancer drug targeting L-type amino acid transporter 1 (LAT1), which plays a primary role in the uptake of essential amino acids in tumor cells. Although a co-incubation inhibitory effect of JPH203 has been shown in a conventional uptake assay, its preincubation inhibitory effects have remained undetermined. Therefore, we aimed to characterize the preincubation inhibitory effects of JPH203 on LAT1 function using leucine uptake assays in LAT1-positive human colon cancer HT-29 cells. Preincubation of the cells with JPH203 (0.3 μM for 120 min) decreased the activity level to 30% of that in dimethylsulfoxide-treated cells. Similarly, in time-dependency analysis, preincubation of HT-29 cells with 10 μM JPH203 for 30, 60, and 120 min decreased the leucine uptake activity (42%, 32%, and 28% of that in control cells, respectively). Furthermore, the IC₅₀ value of the combination of preincubation and co-incubation effects was lower than that of co-incubation inhibition alone (34.2 ± 3.6 nM vs. 99.2 ± 11.0 nM). In conclusion, we revealed that JPH203 has the capability to inhibit LAT1 function through preincubation effects. Moreover, preincubation synergistically enhances the co-incubation inhibitory effects. These findings provide a novel insight into the anti-cancer effects of JPH203 in cancer therapy.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Characterization of the expression of LAT1 as a prognostic indicator and a therapeutic target in renal cell carcinoma

Large neutral amino acid transporter 1 (LAT1, SLC7A5) is abundantly expressed in various types of cancer, and it has been thought to assist cancer progression through its activity for uptake of neutral amino acids. However, the roles of LAT1 in renal cell carcinoma (RCC) prognosis and treatment remain uncharacterized. Therefore, we first retrospectively examined the LAT1 expression profile and its associations with clinical factors in RCC tissues (n = 92). The results of immunohistochemistry showed that most of the tissues examined (92%) had cancer-associated LAT1 expression. Furthermore, the overall survival (OS) and progression-free survival (PFS) were shorter in patients with high LAT1 expression levels than in those with low LAT1 expression levels (P = 0.018 and 0.014, respectively), and these associations were further strengthened by the results of univariate and multivariate analyses. Next, we tested the effects of JPH203, which is a selective LAT1 inhibitor, on RCC-derived Caki-1 and ACHN cells. It was found that JPH203 inhibited the growth of these cell types in a dose-dependent manner. Moreover, JPH203 clearly suppressed their migration and invasion activities. Thus, our results show that LAT1 has a great potential to become not only a prognosis biomarker but also a therapeutic target in RCC clinical settings.

Molecular Mechanisms of Thyroid Hormone Transport by l-Type Amino Acid Transporter

Thyroid hormones (TH) pass through the plasma membrane into the target cells via transporter proteins. Thyroid hormone transporters that have been identified until now belong to two different solute carrier (SLC) subfamilies i) the major facilitator superfamily (MFS) and ii) the amino acid polyamine-organocation (APC) superfamily. Both are comprised by 12 transmembrane helices, however with different structural topology. The TH transporter MCT8, MCT10 and OATP1C1 are members of the MSF. The l-type amino acid transporters (LATs) are transporting neutral amino acids across the membrane. Two LAT subtypes, LAT1 and LAT2 are members of the APC superfamily, need the escort protein 4F2hc and facilitate uptake but no efflux of TH-subtypes. Homology models of LAT2 that are based on crystal structures of APC transporters guided mutagenesis, revealed molecular structure-function determinants for recognition and transition for import and export of TH-subtypes. The recently solved cryo-EM structure of LAT1 confirmed the structural input. Two other LAT subtypes, LAT3 and LAT4 are members of the MFS. From previous observed negative effect of LAT3 and LAT4 on 3,3'-T2 uptake by LAT1 and LAT2 it was indirectly concluded that LAT3 might export 3,3'-T2. There are still open questions that need to be addressed in order to fully understand the molecular recognition pattern and traversing mechanism of import and export of particular TH-subtypes by LAT1 and LAT2. Moreover, clarification is needed whether LAT3 and LAT4 are exporting TH. Recent new data could not verify the initial hypothesis of TH export by LAT3. Therefore, further investigations are necessary to explain the negative effect of LAT3 on the TH import by LAT2.

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.

Amino acid transporters revisited: New views in health and disease

Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically.

JhI-21 plays a role in Drosophila insulin-like peptide release from larval IPCs via leucine transport

Insulin is present all across the animal kingdom. Its proper release after feeding is of extraordinary importance for nutrient uptake, regulation of metabolism, and growth. We used Drosophila melanogaster to shed light on the processes linking dietary leucine intake to insulin secretion. The Drosophila genome encodes 8 insulin-like peptides (“Dilps”). Of these, Dilp2 is secreted after the ingestion of a leucine-containing diet. We previously demonstrated that Minidiscs, related to mammalian system-L transporters, acts as a leucine sensor within the Dilp2-secreting insulin-producing cells (“IPCs”) of the brain. Here, we show that a second leucine transporter, JhI-21, of the same family is additionally necessary for proper leucine sensing in the IPCs. Using calcium imaging and ex-vivo cultured brains we show that knockdown of JhI-21 in IPCs causes malfunction of these cells: they are no longer able to sense dietary leucine or to release Dilp2 in a leucine dependent manner. JhI-21 knockdown in IPCs further causes systemic metabolic defects including defective sugar uptake and altered growth. Finally, we showed that JhI-21 and Minidiscs have no cumulative effect on Dilp2 release. Since system-L transporters are expressed by mammalian β-cells our results could help to better understand the role of these proteins in insulin signaling.

Thyroid hormone transport across L-type amino acid transporters: What can molecular modelling tell us?

Thyroid hormones (THs) and their derivatives require transmembrane transporters (TTs) to mediate their translocation across the cell membrane. Among these TTs, the L-type amino acid transporters (LAT) not only transport amino acids (AAs) but also certain THs and their derivatives. This review summarizes available knowledge concerning structure function patterns of the TH transport by LAT1 and LAT2. For example, LAT2 imports 3,3'-T₂ and T₃, but not rT₃ and T₄. In contrast to amino acids, THs are not at all exported by LAT2. Homology modelling of LAT1 and LAT2 is based on available crystal structures from the same superfamily the amino acid/polyamine/organocation transporter (APC). Molecular model guided mutagenesis has been used to predict substrate interaction sites. A common recognition feature for amino acid- and TH-derivatives has been suggested in an interior cavity of LAT1 and LAT2. Therein additional distinct molecular determinants that are responsible for the bidirectional AA transport but allowing only unidirectional import of particular THs have been confirmed for LAT2 by mutagenesis. Characterized substrate features that are needed for TH translocation and distinct LAT2 properties will be highlighted to understand the molecular import and export mechanisms of this transporter in more detail.

Antagonists of the system L neutral amino acid transporter (LAT) promote endothelial adhesivity of human red blood cells

The system L neutral amino acid transporter (LAT; LAT1, LAT2, LAT3, or LAT4) has multiple functions in human biology, including the cellular import of S-nitrosothiols (SNOs), biologically active derivatives of nitric oxide (NO). SNO formation by haemoglobin within red blood cells (RBC) has been studied, but the conduit whereby a SNO leaves the RBC remains unidentified. Here we hypothesised that SNO export by RBCs may also depend on LAT activity, and investigated the role of RBC LAT in modulating SNO-sensitive RBC-endothelial cell (EC) adhesion. We used multiple pharmacologic inhibitors of LAT in vitro and in vivo to test the role of LAT in SNO export from RBCs and in thereby modulating RBC-EC adhesion. Inhibition of human RBC LAT by type-1-specific or nonspecific LAT antagonists increased RBC-endothelial adhesivity in vitro, and LAT inhibitors tended to increase post-transfusion RBC sequestration in the lung and decreased oxygenation in vivo. A LAT1-specific inhibitor attenuated SNO export from RBCs, and we demonstrated LAT1 in RBC membranes and LAT1 mRNA in reticulocytes. The proadhesive effects of inhibiting LAT1 could be overcome by supplemental L-CSNO (S-nitroso-L-cysteine), but not D-CSNO or L-Cys, and suggest a basal anti-adhesive role for stereospecific intercellular SNO transport. This study reveals for the first time a novel role of LAT1 in the export of SNOs from RBCs to prevent their adhesion to ECs. The findings have implications for the mechanisms of intercellular SNO signalling, and for thrombosis, sickle cell disease, and post-storage RBC transfusion, when RBC adhesivity is increased.

Cancer progression by reprogrammed BCAA metabolism in myeloid leukaemia

Reprogrammed cellular metabolism is a common characteristic observed in various cancers. However, whether metabolic changes directly regulate cancer development and progression remains poorly understood. Here we show that BCAT1, a cytosolic aminotransferase for branched-chain amino acids (BCAAs), is aberrantly activated and functionally required for chronic myeloid leukaemia (CML) in humans and in mouse models of CML. BCAT1 is upregulated during progression of CML and promotes BCAA production in leukaemia cells by aminating the branched-chain keto acids. Blocking BCAT1 gene expression or enzymatic activity induces cellular differentiation and impairs the propagation of blast crisis CML both in vitro and in vivo. Stable-isotope tracer experiments combined with nuclear magnetic resonance-based metabolic analysis demonstrate the intracellular production of BCAAs by BCAT1. Direct supplementation with BCAAs ameliorates the defects caused by BCAT1 knockdown, indicating that BCAT1 exerts its oncogenic function through BCAA production in blast crisis CML cells. Importantly, BCAT1 expression not only is activated in human blast crisis CML and de novo acute myeloid leukaemia, but also predicts disease outcome in patients. As an upstream regulator of BCAT1 expression, we identified Musashi2 (MSI2), an oncogenic RNA binding protein that is required for blast crisis CML. MSI2 is physically associated with the BCAT1 transcript and positively regulates its protein expression in leukaemia. Taken together, this work reveals that altered BCAA metabolism activated through the MSI2-BCAT1 axis drives cancer progression in myeloid leukaemia.

Structure-activity relationship of a novel series of inhibitors for cancer type transporter L-type amino acid transporter 1 (LAT1)

L-type amino acid transporter 1 (LAT1) is known as a cancer-type amino acid transporter. In cancer cells, LAT1 is responsible for the cellular uptake of many essential amino acids including leucine that activates mechanistic/mammalian target of rapamycin (mTOR), regulating cancer cell growth. In this study, we designed a novel series of LAT1 inhibitors, SKN101-105, based on the structure of triiodothyronine (T3), a known LAT1 blocker. The compounds consist of core structure of 2-amino-3-[3,5-dichloro-4-(naphthalene-1-methoxy)-phenyl]-propanoic acid and different modifications on the naphthalene. Among them, the compounds including SKN103 with a modified phenyl group at C-7 position of naphthalene inhibited LAT1-mediated leucine transport, whereas SKN102 with a phenyl group at C-6 position did not, indicating the importance of the position of substituents on the naphthalene for the interaction with LAT1. SKN103 was suggested to be a non-transportable blocker rather than a substrate of LAT1 and inhibited LAT1 in a competitive manner with the K_i value of 2.1 μM. SKN103 suppressed mTOR activity and the growth of cancer cells. Moreover, SKN103 in combination with cisplatin additively enhanced the growth inhibition in cancer cells. This study provides an additional insight into the structure-activity relationship of LAT1 ligands, which could lead to designing desirable LAT1 inhibitors.

Efficacy of system l amino acid transporter 1 inhibition as a therapeutic target in esophageal squamous cell carcinoma

System l amino acid transporter 1 (LAT1) is highly expressed in various types of human cancer, and contributes to cancer growth and survival. Recently, we have shown that LAT1 expression is closely related to the growth and aggressiveness of esophageal cancer, and is an independent marker of poor prognosis. However, it remains unclear whether LAT1 inhibition could suppress esophageal cancer growth. In this study, we investigated the tumor‐suppressive effects of the inhibition of LAT1. Both LAT1 and CD98, which covalently associates to LAT1 on the membrane, were expressed in human esophageal cancer cell lines KYSE30 and KYSE150. Quantitative PCR analysis showed that the expression of LAT1 was much higher than other subtypes of LAT. A selective inhibitor of LAT, 2‐aminobicyclo‐(2,2,1)‐heptane‐2‐carboxylic acid (BCH), suppressed cellular uptake of l‐¹⁴C‐leucine and cell proliferation in a dose‐dependent manner. It also suppressed phosphorylation of mammalian target of rapamycin, 4E‐BP1, and p70S6K protein, and induced cell cycle arrest at G₁ phase. These results suggest that suppression of both mammalian target of rapamycin signaling and cell cycle progression is involved in BCH‐induced growth inhibition. In tumor‐bearing mice, daily treatment with BCH significantly delayed tumor growth and decreased glucose metabolism, indicating that LAT1 inhibition potentially suppresses esophageal cancer growth in vivo. Thus, our results suggest that LAT1 inhibition could be a promising molecular target for the esophageal cancer therapy.

Effect of α-Methyl versus α-Hydrogen Substitution on Brain Availability and Tumor Imaging Properties of Heptanoic [F-18]Fluoroalkyl Amino Acids for Positron Emission Tomography (PET)

Two [(18)F]fluoroalkyl substituted amino acids differing only by the presence or absence of a methyl group on the α-carbon, (S)-2-amino-7-[(18)F]fluoro-2-methylheptanoic acid ((S)-[(18)F]FAMHep, (S)-[(18)F]14) and (S)-2-amino-7-[(18)F]fluoroheptanoic acid ((S)-[(18)F]FAHep, (S)-[(18)F]15), were developed for brain tumor imaging and compared to the well-established system L amino acid tracer, O-(2-[(18)F]fluoroethyl)-l-tyrosine ([(18)F]FET), in the delayed brain tumor (DBT) mouse model of high-grade glioma. Cell uptake, biodistribution, and PET/CT imaging studies showed differences in amino acid transport of these tracer by DBT cells. Recognition of (S)-[(18)F]15 but not (S)-[(18)F]14 by system L amino acid transporters led to approximately 8-10-fold higher uptake of the α-hydrogen substituted analogue (S)-[(18)F]15 in normal brain. (S)-[(18)F]15 had imaging properties similar to those of (S)-[(18)F]FET in the DBT tumor model while (S)-[(18)F]14 afforded higher tumor to brain ratios due to much lower uptake by normal brain. These results have important implications for the future development of α-alkyl and α,α-dialkyl substituted amino acids for brain tumor imaging.

HOXB9 acts as a negative regulator of activated human T cells in response to amino acid deficiency

T-cell activation is an energy expenditure process and should be properly controlled in accordance with the availability of nutrients such as amino acids to eliminate wasteful energy consumption. However, the details of response to amino acids insufficiency in activated T cells remain largely unknown. Here we show that homeobox B9 (HOXB9), a member of the homeobox gene family that is known as a morphogenesis regulator, acts as a suppressor of activated human T cells to address amino acid starvation. The expression of HOXB9 was triggered by amino acid deprivation as well as functional inhibition of L-type amino acid transporter 1 (also known as SLC7A5) via activating transcription factor 4 in activated T cells. HOXB9 interfered the activities of NF-κB, nuclear factor of activated T-cells (NFAT) and AP-1 but not retinoic acid receptor-related orphan receptor, resulting in attenuation of the production of selective cytokines in activated T cells. Thus, the morphogenetic gene plays an unexpected role in the regulation of cellular metabolism with changes in the nutrition status in human T cells.

Transport of Iodothyronines by Human L-Type Amino Acid Transporters

Thyroid hormone (TH) transporters facilitate cellular TH influx and efflux, which is paramount for normal physiology. The L-type amino acid transporters LAT1 and LAT2 are known to facilitate TH transport. However, the role of LAT3, LAT4, and LAT5 is still unclear. Therefore, the aim of this study was to further characterize TH transport by LAT1 and LAT2 and to explore possible TH transport by LAT3, LAT4, and LAT5. FLAG-LAT1-5 constructs were transiently expressed in COS1 cells. LAT1 and LAT2 were cotransfected with the CD98 heavy chain. Cellular transport was measured using 10 nM (125)I-labeled T4, T3, rT3, 3,3'-T2, and 10 μM [(125)I]3'-iodotyrosine (MIT) as substrates. Intracellular metabolism of these substrates was determined in cells cotransfected with either of the LATs with type 1 or type 3 deiodinase. LAT1 facilitated cellular uptake of all substrates and LAT2 showed a net uptake of T3, 3,3'-T2, and MIT. Expression of LAT3 or LAT4 did not affect transport of T4 and T3 but resulted in the decreased cellular accumulation of 3,3'-T2 and MIT. LAT5 did not facilitate the transport of any substrate. Cotransfection with LAT3 or LAT4 strongly diminished the cellular accumulation of 3,3'-T2 and MIT by LAT1 and LAT2. These data were confirmed by metabolism studies. LAT1 and LAT2 show distinct preferences for the uptake of the different iodocompounds, whereas LAT3 and LAT4 specifically facilitate the 3,3'-T2 and MIT efflux. Together our findings suggest that different sets of transporters with specific influx or efflux capacities may cooperate to regulate the cellular thyroid state.

Preclinical evaluation of 2-amino-2-[11C]methyl-butanoic acid as a potential tumor-imaging agent in a mouse model

OBJECTIVE

C-labeled 2-amino-2-methyl-butanoic acid (Iva) was previously reported to provide high tumor uptake; however, the pharmacokinetic properties of C-labeled Iva have not been characterized. In the present study, we evaluated the potential of [C]Iva as a PET probe for tumor imaging.

METHODS

[C]Iva was incubated in mouse serum for 60 min at 37°C and then analyzed by high-performance liquid chromatography and thin-layer chromatography. In-vitro cellular uptake of [C]Iva was determined in PBS and sodium-free buffer at 37°C using SY human small-cell lung cancer cells. The effects of inhibitors of amino acid transporters on [C]Iva uptake were also determined in PBS. In-vivo distribution and dynamic PET studies were conducted in SY tumor-bearing mice.

RESULTS

[C]Iva was stable in mouse serum in vitro for 60 min. The cellular uptake of [C]Iva was linearly increased for 20 min in both PBS and sodium-free buffer and almost completely inhibited by an inhibitor of system L amino acid transporters and another of LAT1, a transporter of system L. In-vivo distribution and dynamic PET studies showed that [C]Iva was highly accumulated in tumor, but not in normal tissues, except for the pancreas and kidneys. The [C]Iva uptake ratio of tumor to several normal tissues, such as the lung, muscle, and brain, was high.

CONCLUSION

[C]Iva was stable in mouse serum and transported through system L amino acid transporters including LAT1, which is highly expressed in several tumors. [C]Iva is a promising PET probe for noninvasive tumor imaging.

Thyroid hormone transporters--functions and clinical implications

The cellular influx and efflux of thyroid hormones are facilitated by transmembrane protein transporters. Of these transporters, monocarboxylate transporter 8 (MCT8) is the only one specific for the transport of thyroid hormones and some of their derivatives. Mutations in SLC16A2, the gene that encodes MCT8, lead to an X-linked syndrome with severe neurological impairment and altered concentrations of thyroid hormones. Histopathological analysis of brain tissue from patients who have impaired MCT8 function indicates that brain lesions start prenatally, and are most probably the result of cerebral hypothyroidism. A Slc16a2 knockout mouse model has revealed that Mct8 is an important mediator of thyroid hormone transport, especially T3, through the blood-brain barrier. However, unlike humans with an MCT8 deficiency, these mice do not have neurological impairment. One explanation for this discrepancy could be differences in expression of the T4 transporter OATP1C1 in the blood-brain barrier; OATP1C1 is more abundant in rodents than in primates and permits the passage of T4 in the absence of T3 transport, thus preventing full cerebral hypothyroidism. In this Review, we discuss the relevance of thyroid hormone transporters in health and disease, with a particular focus on the pathophysiology of MCT8 mutations.

Structural insights into thyroid hormone transport mechanisms of the L-type amino acid transporter 2

Thyroid hormones (THs) are transported across cell membranes by different transmembrane transporter proteins. In previous studies, we showed marked 3,3'-diiodothyronine (3,3'-T2) but moderate T3 uptake by the L-type amino acid transporter 2 (Lat2). We have now studied the structure-function relationships of this transporter and TH-like molecules. Our Lat2 homology model is based on 2 crystal structures of the homologous 12-transmembrane helix transporters arginine/agmatine antiporter and amino acid/polyamine/organocation transporter. Model-driven mutagenesis of residues lining an extracellular recognition site and a TH-traversing channel identified 9 sensitive residues. Using Xenopus laevis oocytes as expression system, we found that side chain shortening (N51S, N133S, N248S, and Y130A) expanded the channel and increased 3,3'-T2 transport. Side chain enlargements (T140F, Y130R, and I137M) decreased 3,3'-T2 uptake, indicating channel obstructions. The opposite results with mutations maintaining (F242W) or impairing (F242V) uptake suggest that F242 may have a gating function. Competitive inhibition studies of 14 TH-like compounds revealed that recognition by Lat2 requires amino and carboxylic acid groups. The size of the adjacent hydrophobic group is restricted. Bulky substituents in positions 3 and 5 of the tyrosine ring are allowed. The phenolic ring may be enlarged, provided that the whole molecule is flexible enough to fit into the distinctly shaped TH-traversing channel of Lat2. Taken together, the next Lat2 features were identified 1) TH recognition site; 2) TH-traversing channel in the center of Lat2; and 3) switch site that potentially facilitates intracellular substrate release. Together with identified substrate features, these data help to elucidate the molecular mechanisms and role of Lat2 in T2 transport.

Biological significance of fluorine-18-α-methyltyrosine (FAMT) uptake on PET in patients with oesophageal cancer

PURPOSE

(18)F-FAMT as an amino-acid tracer for positron emission tomography (PET) is useful for detecting human neoplasms. (18)F-FAMT is accumulated in tumour cells solely via L-type amino-acid transporter 1 (LAT1). This study was conducted to investigate the biological significance of (18)F-FAMT uptake in patients with oesophageal cancer.

METHODS

From April 2008 to December 2011, 42 patients with oesophageal cancer underwent both (18)F-FAMT PET/CT and (18)F-FDG PET/CT before surgical treatment. The immunohistochemical analysis of LAT1, CD98, Ki-67, CD34, p53, p-Akt and p-mTOR was performed on the primary lesions. In vitro experiments were performed to examine the mechanism of (18)F-FAMT uptake.

RESULTS

High uptake of (18)F-FAMT was significantly associated with advanced stage, lymph node metastasis and the expression of LAT1, CD98, Ki-67 and CD34. LAT1 expression yielded a statistically significant correlation with CD98 expression, cell proliferation, angiogenesis and glucose metabolism. In vitro experiments revealed that (18)F-FAMT was specifically transported by LAT1.

CONCLUSIONS

The uptake of (18)F-FAMT within tumour cells is determined by the LAT1 expression and correlated with cell proliferation and angiogenesis in oesophageal cancer. The present experiments also confirmed the presence of LAT1 as an underlying mechanism of (18)F-FAMT accumulation.

JPH203, an L-type amino acid transporter 1-selective compound, induces apoptosis of YD-38 human oral cancer cells

Compared to most normal cells that express L-type amino acid transporter 2, L-type amino acid transporter 1 is highly expressed in cancer cells and presumed to support their elevated growth and proliferation. This study examined JPH203, a potent and selective L-type amino acid transporter 1 inhibitor, and its ability to suppress YD-38 human oral cancer cell growth. The YD-38 cells express L-type amino acid transporter 1 with its associating protein 4F2 heavy chain, but not L-type amino acid transporter 2. JPH203 and BCH, a non-selective L-type amino acid transporter inhibitor, completely inhibited l-leucine uptake in YD-38 cells. As expected, the intrinsic affinity of JPH203 to inhibit l-leucine uptake was far more efficient than BCH. Likewise, JPH203 and BCH inhibited YD-38 cell growth, with JPH203 being superior to BCH. JPH203 up-regulated the population of apoptotic YD-38 cells through the activation of apoptotic factors, including caspases and PARP. These results suggest that the inhibition of L-type amino acid transporter 1 activity via JPH203, which may act as a potential novel anti-oral-cancer agent, leads to apoptosis by inducing the intracellular depletion of the neutral amino acids essential for cancer cell growth in YD-38 human oral cancer cells.

Clinical significance of L-type amino acid transporter 1 expression as a prognostic marker and potential of new targeting therapy in biliary tract cancer

BACKGROUND

The expression of L-type amino acid transporter 1 (LAT1) has been described to play essential roles in tumor cell growth and survival. However, it remains unclear about the clinicopathological significance of LAT1 expression in biliary tract cancer. This study was conducted to determine biological significance of LAT1 expression and investigate whether LAT1 could be a prognostic biomarker for biliary tract cancer.

METHODS

A total of 139 consecutive patients with resected pathologic stage I-IV biliary tract adenocarcinoma were retrospectively reviewed. Tumor specimens were stained by immunohistochemistry for LAT1, Ki-67, microvessel density determined by CD34, and p53; and prognosis of patients was correlated. Biological significance of LAT1 expression was investigated by in vitro and in vivo experiments with LAT inhibitor, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) using cholangiocarcinoma cell line.

RESULTS

In total patients, high LAT1 expressions were recognized in 64.0%. The expression of LAT1 was closely correlated with lymphatic metastases, cell proliferation and angiogenesis, and was a significant indicator for predicting poor outcome after surgery. LAT1 expression was a significant independent predictor by multivariate analysis. Both in vitro and in vivo preliminary experiments indicated that BCH significantly suppressed growth of the tumor and yielded an additive therapeutic efficacy to gemcitabine and 5-FU.

CONCLUSIONS

High expression of LAT1 is a promising pathological marker to predict the outcome in patients with biliary tract adenocarcinoma. Inhibition of LAT1 may be an effective targeted therapy for this distressing disease.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Biological evaluation of 3-[(18)F]fluoro-α-methyl-D-tyrosine (D-[(18)F]FAMT) as a novel amino acid tracer for positron emission tomography

OBJECTIVE

3-[(18)F]Fluoro-α-methyl-L-tyrosine (L-[(18)F]FAMT) is a useful amino acid tracer for positron emission tomography (PET) imaging of malignant tumors. Because D-amino acids are not well distributed in non-target organs and are rapidly excreted in urine, the D-isomer of [(18)F]FAMT could allow clear PET imaging of tumors early after administration. In this study, we prepared 3-[(18)F]fluoro-α-methyl-D-tyrosine (D-[(18)F]FAMT) and evaluated its usefulness.

METHODS

D-[(18)F]FAMT was synthesized according to the method for preparation of L-[(18)F]FAMT. The in vitro and in vivo stability of [(18)F]FAMT were evaluated by high-performance liquid chromatography. Cellular uptake of [(18)F]FAMT was evaluated using LS180 colon adenocarcinoma cells. Biodistribution studies were performed in LS180 tumor-bearing mice, and the tumors were imaged using a small-animal PET scanner.

RESULTS

The radiolabeling yield of D-[(18)F]FAMT was approximately 10 %, similar to that of L-[(18)F]FAMT. Over 95 % of D-[(18)F]FAMT remained intact in mice until 60 min after administration. D-[(18)F]FAMT was gradually taken up by the LS180 cells. Tumor uptake of D-[(18)F]FAMT was competitively inhibited by pretreatment with α-methyl-L-tyrosine, a selective substrate for the system L-amino acid transporter 1 (LAT1), suggesting the involvement of LAT1 in tumor uptake of D-[(18)F]FAMT. In biodistribution studies, D-[(18)F]FAMT showed rapid clearance from the blood, marked accumulation and retention in the tumor, and lower accumulation in non-target organs, especially kidney and pancreas, compared to L-[(18)F]FAMT. The amount of D-[(18)F]FAMT in the tumor was also reduced, and tumor-to-blood ratio and tumor-to-muscle ratio of D-[(18)F]FAMT were similar to those of L-[(18)F]FAMT at every time point. PET imaging with D-[(18)F]FAMT did not provide a clear image of the tumor early after administration. However, D-[(18)F]FAMT provided higher tumor-to-background contrast than L-[(18)F]FAMT.

CONCLUSIONS

D-[(18)F]FAMT showed rapid blood clearance, low accumulation in non-target organs, and tumor-selective imaging compared with L-[(18)F]FAMT. Thus, D-[(18)F]FAMT could potentially serve as a novel PET tracer for imaging malignant tumors.

The role of LAT1 in (18)F-DOPA uptake in malignant gliomas

Positron emission tomography (PET) imaging with the amino acid tracer 6-(18)F-fluoro-L-3,4-dihydroxy-phenylalanine ((18)F-DOPA) may provide better spatial and functional information in human gliomas than CT or MRI alone. The L-type amino acid transporter 1 (LAT1) is responsible for membrane transport of large neutral amino acids in normal cells. This study assessed the relationship between LAT1 expression and (18)F-DOPA uptake in human astrocytomas. Endogenous LAT1 expression was measured in established glioblastoma (GBM) cell lines and primary GBM xenografts using Western blotting and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Uptake of (18)F-DOPA was approximated in vitro using (3)H-L-DOPA as an analog. Uptake of (3)H-L-DOPA was assessed in cells expressing LAT1 shRNA or LAT1 siRNA and compared to non-targeted (NT) control shRNA or siRNA sequences, respectively. To demonstrate the clinical relevance of these findings, LAT1 immunofluorescence staining was compared with corresponding regions of (18)F-DOPA PET uptake in patients with newly diagnosed astrocytomas. LAT1 mRNA and protein expression varies in GBM, and the extent of (3)H-L-DOPA uptake was positively correlated with endogenous LAT1 expression. Stable shRNA-mediated LAT1 knockdown in T98 and GBM28 reduced (3)H-L-DOPA uptake relative to NT shRNA by 57 (P < 0.0001) and 52 % (P < 0.001), respectively. Transient siRNA-mediated LAT1 knockdown in T98 reduced (3)H-L-DOPA uptake relative to NT siRNA up to 68 % (P < 0.01). In clinical samples, LAT1 expression positively correlated with (18)F-DOPA PET uptake (P = 0.04). Expression of LAT1 is strongly associated with (3)H-L-DOPA uptake in vitro and (18)F-DOPA uptake in patient biopsy samples. These results define LAT1 as a key determinant of (18)F-DOPA accumulation in GBM.

Functional characterization and molecular expression of large neutral amino acid transporter (LAT1) in human prostate cancer cells

The primary objective of this study is to functionally characterize and provide molecular evidence of large neutral amino acid transporter (LAT1) in human derived prostate cancer cells (PC-3). We carried out the uptake of [3H]-tyrosine to assess the functional activity of LAT1. Reverse transcription-polymerase chain reaction (RT-PCR) analysis is carried out to confirm the molecular expression of LAT1. [3H]-tyrosine uptake is found to be time dependent and linear up to 60 min. The uptake process does not exhibit any dependence on sodium ions, pH and energy. However, it is temperature dependent and found maximal at physiological temperature. The uptake of [3H]-tyrosine demonstrates saturable kinetics with K(m) and V(max) values of 34 ± 3 μM and 0.70 ± 0.02 nanomoles/min/mg protein, respectively. It is strongly inhibited by large neutral (phenylalanine, tryptophan, leucine, isoleucine) and small neutral (alanine, serine, cysteine) but not by basic (lysine and arginine) and acidic (aspartic and glutamic acid) amino acids. Isoleucine-quinidine (Ile-quinidine) prodrug generates a significant inhibitory effect on [3H]-tyrosine uptake suggesting that it is recognized by LAT1. RT-PCR analysis provided a product band at 658 and 840 bp, specific to LAT1 and LAT2, respectively. For the first time, this study demonstrates that LAT1, primarily responsible for the uptake of large neutral amino acids, is functionally active in PC-3 cells. Significant increase in the uptake generated by Ile-quinidine relative to quinidine suggests that LAT1 can be utilized for enhancing the cellular permeation of poor cell permeable anticancer drugs. Furthermore, this cell line can be utilized as an excellent in vitro model for studying the interaction of large neutral amino acid conjugated drugs with LAT1 transporter.

Monocarboxylate transporter 10 functions as a thyroid hormone transporter in chondrocytes

Thyroid hormone is essential for normal proliferation and differentiation of chondrocytes. Thus, untreated congenital hypothyroidism is marked by severe short stature. The monocarboxylate transporter 8 (MCT8) is a highly specific transporter for thyroid hormone. The hallmarks of Allan-Herndon-Dudley syndrome, caused by MCT8 mutations, are severe psychomotor retardation and elevated T(3) levels. However, growth is mostly normal. We therefore hypothesized that growth plate chondrocytes use transporters other than MCT8 for thyroid hormone uptake. Extensive analysis of thyroid hormone transporter mRNA expression in mouse chondrogenic ATDC5 cells revealed that monocarboxylate transporter 10 (Mct10) was most abundantly expressed among the transporters known to be highly specific for thyroid hormone, namely Mct8, Mct10, and organic anion transporter 1c1. Expression levels of Mct10 mRNA diminished with chondrocyte differentiation in these cells. Accordingly, Mct10 mRNA was expressed most abundantly in the growth plate resting zone chondrocytes in vivo. Small interfering RNA-mediated knockdown of Mct10 mRNA in ATDC5 cells decreased [(125)I]T(3) uptake up to 44% compared with negative control (P < 0.05). Moreover, silencing Mct10 mRNA expression abolished the known effects of T(3), i.e. suppression of proliferation and enhancement of differentiation, in ATDC5 cells. These results suggest that Mct10 functions as a thyroid hormone transporter in chondrocytes and can explain at least in part why Allan-Herndon-Dudley syndrome patients do not exhibit significant growth impairment.

Establishment of stable cell lines with high expression of heterodimers of human 4F2hc and human amino acid transporter LAT1 or LAT2 and delineation of their differential interaction with α-alkyl moieties

System L is a major transport system for cellular uptake of neutral amino acids. Among system L transporters, L-type amino acid transporter 1 (LAT1) is responsible for the nutrient uptake in cancer cells, whereas L-type amino acid transporter 2 (LAT2) is a transporter for non-cancer cells. In this study, we have established HEK293 cell lines stably expressing high levels of human LAT1 and LAT2 forming heterodimers with native human 4F2hc of the cells. We have found that l-[¹⁴C]alanine is an appropriate substrate to examine the function of LAT2, whereas l-[¹⁴C]leucine is used for LAT1. By using l-[¹⁴C]alanine on LAT2, we have for the first time directly evaluated the function of human LAT2 expressed in mammalian cells and obtained its reliable kinetics. Using α-alkyl amino acids including α-methyl-alanine and α-ethyl-l-alanine, we have demonstrated that α-alkyl groups interfere with the interaction with LAT2. These cell lines with higher practical advantages would be useful for screening and analyzing compounds to develop LAT1-specific drugs that can be used for cancer diagnosis and therapeutics. The strategy that we took to establish the cell lines would also be applicable to the other heterodimeric transporters with important therapeutic implications.

Transport of 3-fluoro-L-α-methyl-tyrosine by tumor-upregulated L-type amino acid transporter 1: a cause of the tumor uptake in PET

l-3-(18)F-α-methyl tyrosine ((18)F-FAMT) has been developed as a PET radiotracer for tumor imaging. Clinical studies have demonstrated the usefulness of (18)F-FAMT PET for the prediction of prognosis and the differentiation of malignant tumors and benign lesions. (18)F-FAMT exhibits higher cancer specificity in peripheral organs than other amino acid PET tracers and (18)F-FDG. The accumulation of (18)F-FAMT is strongly correlated with the expression of L-type amino acid transporter 1 (LAT1), an isoform of system L highly upregulated in cancers. In this study, we examined the interaction of 3-fluoro-l-α-methyl-tyrosine (FAMT) with amino acid transporters to assess the mechanisms of (18)F-FAMT uptake in PET.

METHODS

We applied in vitro assays using established mammalian cell lines stably expressing LAT1 or a non-cancer-type system L isoform LAT2. The inhibitory effect on l-(14)C-leucine uptake and the induction effect on efflux of preloaded l-(14)C-leucine were examined for FAMT and other amino acid tracers. FAMT transport was compared among cell lines with varied LAT1 expression level.

RESULTS

FAMT prominently inhibited LAT1-mediated l-(14)C-leucine uptake in a competitive manner but had less of an effect on LAT2. In the efflux experiments, FAMT induced the efflux of preloaded l-(14)C-leucine through LAT1, indicating that FAMT is transported by LAT1 and not by LAT2. Among amino acid-related compounds examined in this study, including those used for PET tracers, the compounds with an α-methyl group such as FAMT, 2-fluoro-l-α-methyl-tyrosine, 3-iodo-l-α-methyl-tyrosine, and l-α-methyl-tyrosine were well transported by LAT1 but not by LAT2. However, l-methionine, l-tyrosine, 3-fluoro-l-tyrosine, 2-fluoro-l-tyrosine, and O-(2-fluoroethyl)-l-tyrosine were transported by both LAT1 and LAT2, suggesting that the α-methyl moiety is responsible for the LAT1 selectivity of FAMT. FAMT transport rate and LAT1 protein level were well correlated, supporting the importance of LAT1 for the cellular uptake of FAMT.

CONCLUSION

Distinct from other amino acid PET tracers, because of its α-methyl moiety, FAMT is selective to LAT1 and not transported by LAT2. This property of FAMT is proposed to contribute to highly tumor-specific accumulation of (18)F-FAMT in PET.