Upregulation of ATF4 mediates the cellular adaptation to pharmacologic inhibition of amino acid transporter LAT1 in pancreatic ductal adenocarcinoma cells.

L-type amino acid transporter 1 (LAT1) is recognized as a promising target for cancer therapy; however, the cellular adaptive response to its pharmacological inhibition remains largely unexplored. This study examined the adaptive response to LAT1 inhibition using nanvuranlat, a high-affinity LAT1 inhibitor. Proteomic analysis revealed the activation of a stress-induced transcription factor ATF4 following LAT1 inhibition, aligning with the known cellular responses to amino acid deprivation. This activation was linked to the GCN2-eIF2α pathway which regulates translation initiation. Our results show that ATF4 upregulation counteracts the suppressive effect of nanvuranlat on cell proliferation in pancreatic ductal adenocarcinoma cell lines, suggesting a role for ATF4 in cellular adaptation to LAT1 inhibition. Importantly, dual targeting of LAT1 and ATF4 exhibited more substantial anti-proliferative effects in vitro than individual treatments. This study underscores the potential of combining LAT1 and ATF4 inhibition as a therapeutic strategy in cancer treatment.

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.

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.

The NERP-4-SNAT2 axis regulates pancreatic ß-cell maintenance and function.

Insulin secretion from pancreatic ß cells is regulated by multiple stimuli, including nutrients, hormones, neuronal inputs, and local signalling. Amino acids modulate insulin secretion via amino acid transporters expressed on ß cells. The granin protein VGF has dual roles in ß cells: regulating secretory granule formation and functioning as a multiple peptide precursor. A VGF-derived peptide, neuroendocrine regulatory peptide-4 (NERP-4), increases Ca2+ influx in the pancreata of transgenic mice expressing apoaequorin, a Ca2+-induced bioluminescent protein complex. NERP-4 enhances glucose-stimulated insulin secretion from isolated human and mouse islets and ß-cell-derived MIN6-K8 cells. NERP-4 administration reverses the impairment of ß-cell maintenance and function in db/db mice by enhancing mitochondrial function and reducing metabolic stress. NERP-4 acts on sodium-coupled neutral amino acid transporter 2 (SNAT2), thereby increasing glutamine, alanine, and proline uptake into ß cells and stimulating insulin secretion. SNAT2 deletion and inhibition abolish the protective effects of NERP-4 on ß-cell maintenance. These findings demonstrate a novel autocrine mechanism of ß-cell maintenance and function that is mediated by the peptide-amino acid transporter axis.

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

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

Functional coupling of organic anion transporter OAT10 (SLC22A13) and monocarboxylate transporter MCT1 (SLC16A1) influencing the transport function of OAT10.

OAT10 (SLC22A13) is a transporter highly expressed in renal tubules and transporting organic anions including nicotinate, ß-hydroxybutyrate, p-aminohippurate, and orotate. In transport assays using Xenopus oocytes and HEK293 cells, we found that apparent substrate selectivity of OAT10 was different between the expression systems, particularly less pronounced uptake of ß-hydroxybutyrate in HEK293 cells. Because functional coupling between transporters may interfere with functional properties of the transporter, we searched for endogenous transporters in HEK293 cells that could affect OAT10. By means of comprehensive approach with co-immunoprecipitation followed by LC-MS/MS analysis, we identified monocarboxylate transporter MCT1 (SLC16A1) as physically coupled with OAT10. The knockdown of MCT1 in OAT10-expressing HEK293 cells increased the uptake of ß-hydroxybutyrate and nicotinate, common substrates of OAT10 and MCT1, whereas the uptake of orotate, a substrate of only OAT10, was not affected. MCT1 is supposed to act as an escape route and mediate the efflux of nicotinate and ß-hydroxybutyrate taken up by OAT10 localized nearby MCT1, as suggested by co-immunoprecipitation. This functional coupling would explain altered apparent substrate selectivity in HEK293 cells compared with Xenopus oocytes. The findings in this study warn in transporter studies that the expression system can interfere with assessing correct transport properties due to unexpected interactions with endogenous transporters.

Interaction of Halogenated Tyrosine/Phenylalanine Derivatives with Organic Anion Transporter 1 in the Renal Handling of Tumor Imaging Probes.

Halogenated tyrosine/phenylalanine derivatives have been developed for use in tumor imaging and targeted alpha therapy. 3-Fluoro-α-methyl-l-tyrosine (FAMT), targeting amino acid transporter LAT1 (SLC7A5), is a cancer-specific positron emission tomography probe that exhibits high renal accumulation, which is supposed to be mediated by organic anion transporter OAT1 (SLC22A6). In the present study, we investigated the structural requirements of FAMT essential for interaction with OAT1. OAT1 transported FAMT with a K m of 171.9 µM. In structure-activity relationship analyses, removal of either the 3-halogen or 4-hydroxyl group from FAMT or its structural analog 3-iodo-α-methyl-l-tyrosine greatly decreased the interaction with OAT1, reducing the [14C]p-aminohippurate uptake inhibition and the efflux induction. By contrast, the α-methyl group, which is essential for LAT1 specificity, contributed to a lesser degree. In fluorinated tyrosine derivatives, fluorine at any position was accepted by OAT1 when there was a hydroxyl group at the ortho-position, whereas ortho-fluorine was less interactive when a hydroxyl group was at meta- or para-positions. The replacement of the ortho-fluorine with a bulky iodine atom greatly increased the interaction. In in vivo studies, probenecid decreased the renal accumulation (P < 0.001) and urinary excretion (P = 0.0012) of FAMT, whereas the plasma concentration was increased, suggesting the involvement of OAT1-mediated transepithelial organic anion excretion. LAT1-specific 2-fluoro-α-methyltyrosine, which had lower affinity for OAT1, exhibited lower renal accumulation (P = 0.0142) and higher tumor uptake (P = 0.0192) compared with FAMT. These results would provide a basis to design tumor-specific compounds that can avoid renal accumulation for tumor imaging and targeted alpha therapy. SIGNIFICANCE STATEMENT: We revealed the structural characteristics of halogenated tyrosine derivatives essential for interaction with the organic anion transporter responsible for their renal accumulation. We have confirmed that such interactions are important for renal handling and tumor uptake. The critical contribution of hydroxyl and halogen groups and their positions as well as the role of α-methyl group found in the present study may facilitate the development of tumor-specific compounds while avoiding renal accumulation for use in tumor imaging and targeted alpha therapy.

Cryo-EM structure of the human L-type amino acid transporter 1 in complex with glycoprotein CD98hc.

The L-type amino acid transporter 1 (LAT1 or SLC7A5) transports large neutral amino acids across the membrane and is crucial for brain drug delivery and tumor growth. LAT1 forms a disulfide-linked heterodimer with CD98 heavy chain (CD98hc, 4F2hc or SLC3A2), but the mechanism of assembly and amino acid transport are poorly understood. Here we report the cryo-EM structure of the human LAT1-CD98hc heterodimer at 3.3-Å resolution. LAT1 features a canonical Leu T-fold and exhibits an unusual loop structure on transmembrane helix 6, creating an extended cavity that might accommodate bulky amino acids and drugs. CD98hc engages with LAT1 through the extracellular, transmembrane and putative cholesterol-mediated interactions. We also show that two anti-CD98 antibodies recognize distinct, multiple epitopes on CD98hc but not its glycans, explaining their robust reactivities. These results reveal the principles of glycoprotein-solute carrier assembly and provide templates for improving preclinical drugs and antibodies targeting LAT1 or CD98hc.

CYP450s-Activity Relations of Celastrol to Interact with Triptolide Reveal the Reasons of Hepatotoxicity of Tripterygium wilfordii.

Celastrol and triptolide, as the two main bio-activity ingredients in Tripterygium wilfordii, have wide anticancer pharmacological potency, as well as anti-inflammatory and immunosuppression effects. However, they have potential hepatotoxicity and underlying mechanisms of them-induced toxicity mediated by hepatic CYP450s have not been well delineated. In the present study, we accessed the toxic effects and possible mechanism of celastrol and triptolide on primary rat hepatocytes. Models of subdued/enhanced activity of CYP450 enzymes in primary rat hepatocytes were also constructed to evaluate the relationship between the two ingredients and CYP450s. LC-MS/MS was used to establish a detection method of the amount of triptolide in rat hepatocytes. As the results, cell viability, biochemical index, and mitochondrial membrane potential indicated that celastrol and triptolide had toxic potencies on hepatocytes. Moreover, the toxic effects were enhanced when the compounds combined with 1-aminobenzotriazole (enzyme inhibitor) while they were mitigated when combined with phenobarbital (an enzyme inducer). Meanwhile, celastrol could affect the amount of triptolide in the cell. We therefore put forward that increase of triptolide in the cell might be one of the main causes of hepatotoxicity caused by Tripterygium wilfordii.

Boron delivery for boron neutron capture therapy targeting a cancer-upregulated oligopeptide transporter.

Boron neutron capture therapy (BNCT) is a radiotherapy utilizing the neutron capture and nuclear fission reaction of 10B taken up into tumor cells. The most commonly used boron agent in BNCT, p-borono-l-phenylalanine (BPA), is accumulated in tumors by amino acid transporters upregulated in tumor cells. Here, by using dipeptides of BPA and tyrosine (BPA-Tyr and Tyr-BPA), we propose a novel strategy of selective boron delivery into tumor cells via oligopeptide transporter PEPT1 upregulated in various cancers. Kinetic analyses indicated that BPA-Tyr and Tyr-BPA are transported by oligopeptide transporters, PEPT1 and PEPT2. The intrinsic oligopeptide transport activity in tumor cells clearly correlated with PEPT1 protein expression level but not with PEPT2, suggesting that PEPT1 is the predominant oligopeptide transporter at least in tumor cell lines. Furthermore, using BPA-Tyr and Tyr-BPA, boron was successfully delivered into PEPT1-expressing pancreatic cancer AsPC-1 cells via a PEPT1-mediated mechanism. Intravenous administration of BPA-Tyr into the mice bearing AsPC-1 xenograft tumors resulted in significant boron accumulation in the tumors. It is proposed that the oligopeptide transporters, especially PEPT1, are promising candidates for molecular targets of boron delivery in BNCT. The BPA-containing dipeptides would have a potential for the development of novel boron carriers targeting PEPT1.

Identification and characterization of reactive metabolites in myristicin-mediated mechanism-based inhibition of CYP1A2.

Myristicin belongs to the methylenedioxyphenyl or allyl-benzene family of compounds, which are found widely in plants of the Umbelliferae family, such as parsley and carrot. Myristicin is also the major active component in the essential oils of mace and nutmeg. However, this compound can cause adverse reactions, particularly when taken inappropriately or in overdoses. One important source of toxicity of natural products arises from their metabolic biotransformations into reactive metabolites. Myristicin contains a methylenedioxyphenyl substructure, and this specific structural feature may allow compounds to cause a mechanism-based inhibition of cytochrome P450 enzymes and produce reactive metabolites. Therefore, the aim of this work was to identify whether the role of myristicin in CYP enzyme inhibition is mechanism-based inhibition and to gain further information regarding the structure of the resulting reactive metabolites. CYP cocktail assays showed that myristicin most significantly inhibits CYP1A2 among five CYP enzymes (CYP1A2, CYP2D6, CYP2E1, CYP3A4 and CYP2C19) from human liver microsomes. The 3.21-fold IC50 shift value of CYP1A2 indicates that myristicin may be a mechanism-based inhibitor of CYP1A2. Next, reduced glutathione was shown to block the inhibition of CYP1A2, indicating that myristicin utilized a mechanism-based inhibition. Phase I metabolism assays identified two metabolites, 5-allyl-1-methoxy-2,3-dihydroxybenzene (M1) and 1'-hydroxymyristicin or 2',3'-epoxy-myristicin (M2). Reduced glutathione capturing assays captured the glutathione-M1 adduct, and the reactive metabolites were identified using UPLC-MS(2) as a quinone and its tautomer. Thus, it was concluded that myristicin is a mechanism-based inhibitor of CYP1A2, and the reactive metabolites are quinone tautomers. Additionally, the cleavage process of the glutathione-M1 adduct was analyzed in further detail. This study provides additional information on the metabolic mechanism of myristicin inhibition and improves risk evaluation for this compound.

Reactive metabolite activation by CYP2C19-mediated rhein hepatotoxicity.

1. Rhein, an active ingredient in the root of rhubarb, is used for its beneficial effects in a variety of clinical applications including the treatment of osteoarthritis and diabetic nephropathy. However, its hepatotoxicity has been reported in recent years. Rhein belongs to the conjugate structure which could be activated to reactive metabolites (RMs) inducing side-effects. This study is to explore the relationship between RMs and hepatotoxicity. 2. Based on the early detection of RMs, we have established a series of key technologies to research rhein hepatotoxicity mechanism: IC50 shift experiments and reduced glutathione (GSH) trapping experiment are adopted to identify RMs. The model of low activity of CYP450 enzymes (CYPs) in primary rat hepatocyte is constructed to analyze the relationship between the primary metabolic enzyme and hepatotoxicity of rhein better. 3. The IC50 shift value for CYP2C19 is 1.989, it suggests that CYP2C19 could activate rhein to RM. The structure of RM is epoxide intermediate. Besides, it is found that CYP2C19 is a primary metabolic enzyme for rhein. In the cytotoxicity assay, it is reported that rhein could cause mitochondrial dysfunction. Furthermore, mitochondrial membrane potential (Δψm) and AST levels could be restored by adding inhibitor of CYP2C19 together with rhein, which further shows that CYP2C19 could mediate the hepatotoxicity of rhein. 4. We put forward the possible mechanism that reactive metabolite activation by CYP2C19 mediated rhein hepatotoxicity, it provides important information on predicting in vivo drug-induced liver injury (DILI).