Human LAT1, a subunit of system L amino acid transporter: molecular cloning and transport function

The Multifaceted Role of L-Type Amino Acid Transporter 1 at the Blood-Brain Barrier: Structural Implications and Therapeutic Potential

L-type amino acid transporter 1 (LAT1) is integral to the transport of large neutral amino acids across the blood-brain barrier (BBB), playing a crucial role in brain homeostasis and the delivery of therapeutic agents. This review explores the multifaceted role of LAT1 in neurological disorders, including its structural and functional aspects at the BBB. Studies using advanced BBB models, such as induced pluripotent stem cell (iPSC)-derived systems and quantitative proteomic analyses, have demonstrated LAT1's significant impact on drug permeability and transport efficiency. In Alzheimer's disease, LAT1-mediated delivery of anti-inflammatory and neuroprotective agents shows promise in overcoming BBB limitations. In Parkinson's disease, LAT1's role in transporting L-DOPA and other therapeutic agents highlights its potential in enhancing treatment efficacy. In phenylketonuria, studies have revealed polymorphisms and genetic variations of LAT1, which could be correlated to disease severity. Prodrugs of valproic acid, pregabalin, and gabapentin help use LAT1-mediated transport to increase the therapeutic activity and bioavailability of the prodrug in the brain. LAT1 has also been studied in neurodevelopment disorders like autism spectrum disorders and Rett syndrome, along with neuropsychiatric implications in depression. Its implications in neuro-oncology, especially in transporting therapeutic agents into cancer cells, show immense future potential. Phenotypes of LAT1 have also shown variations in the general population affecting their ability to respond to painkillers and anti-inflammatory drugs. Furthermore, LAT1-targeted approaches, such as functionalized nanoparticles and prodrugs, show promise in overcoming chemoresistance and enhancing drug delivery to the brain. The ongoing exploration of LAT1's structural characteristics and therapeutic applications reiterates its critical role in advancing treatments for neurological disorders.

Lysine 204 is crucial for the antiport function of the human LAT1 transporter

LAT1 (SLC7A5) catalyzes an antiport reaction of amino acids with specificity towards the essential ones. It is mainly expressed at the Blood Brain Barrier and placenta barriers, but it becomes over-expressed in virtually all human cancers even if originating from tissues with lower expression levels. The antiport reaction of LAT1 is crucial at the BBB since its inherited loss causes Autism Spectrum Disorder. We have investigated the possible molecular determinant of the antiport by site-directed mutagenesis, in vitro transport assay and computational analysis. Previous data indicated that mutation of K204 impairs, but does not knock-out LAT1 functionality. We have investigated the activity changes in the K204Q mutant by following the transport of [3H]-histidine, one of the major substrates, in proteoliposomes harbouring the WT or K204Q. In the mutant, the [3H]-histidine uptake and efflux are not more stimulated by the counter-substrate as they occur in the WT. Moreover, the mutation strongly decreases the substrate affinity and alters the pH dependence of K204Q. Molecular Dynamics analysis correlates well with the experimental data since it shows that substrate prematurely escapes the binding site. In addition, the K204Q shows a strongly increased mobility in those regions, transmembrane domains and random coils, involved in the transport cycle. The identified Lys residue could be responsible of the same phenomenon in those members of the SLC7 family, described as antiporters, in which it is conserved.

Prolactin upregulates amino acid uptake in dairy cow mammary epithelial cells via LAT1

The uptake of AA in mammary tissues is affected by prolactin (PRL). To investigate whether PRL-induced AA uptake is involved in l-type AA transporter 1 (LAT1), we analyzed the changes of AA in the medium of dairy cow mammary epithelial cells in the presence of PRL or PRL plus heptanecarboxylic acid (BCH), an inhibitor of LAT1. Then Western blot and luciferase assay were used to detect the regulation mechanism of PRL on LAT1 expression and function. Our results showed that Thr, Val, Met, Ile, Leu, Tyr, Lys, Phe, and His are LAT1 substrates and could be transported into mammary epithelial cells via LAT1. PRL stimulation increased the uptake of most AA into mammary epithelial cells of dairy cows, however, inhibition of LAT1 transport activity reduced PRL-induced AA uptake, suggesting that the effect of PRL on AA transport depends on LAT1 expression and function. PRL stimulation upregulated LAT1 expression and plasma membrane location not only in dairy cow mammary epithelial cells, but also in mouse mammary epithelial cell line HC11. Western blot showed that PI3K-AKT-mTOR signaling could be activated in PRL-stimulated mammary epithelial cells. Treatment of cells with LY294002 decreased PI3K-AKT-mTOR activation, as well as LAT1 expression, that in turn decreased milk protein synthesis. Luciferase assay showed PRL treatment increased the promoter activity of LAT1 promoter fragment -419 to -86 bp. Treatment of cells with LY294002, an inhibitor of PI3K, or SC79, an activator of AKT abolished or promoted the transcriptional activity of this promoter fragment in the presence of PRL. These results suggested that the -419 to -86 bp fragment of LAT1 promoter mediates the action of PI3K-AKT-mTOR signaling on LAT1 transcription in mammary epithelial cells of dairy cows, which in turn increased LAT1 expression and AA uptake.

Transport of aromatic amino acids l-tryptophan, l-tyrosine, and l-phenylalanine by the organic anion transporting polypeptide (OATP) 3A1

Amino acids are important for cellular metabolism. Their uptake across the plasma membrane is mediated by transport proteins. Despite the fact that the organic anion transporting polypeptide 4C1 (OATP4C1, Uniprot: Q6ZQN7) mediates transport of l-arginine and l-arginine derivatives, other members of the OATP family have not been characterized as amino acid transporters. The OATP family member OATP3A1 (gene symbol SLCO3A1, Uniprot: Q9UIG8) is ubiquitously expressed in human cells and highly expressed in many cancer tissues and cell lines. However, only a few substrates are known for OATP3A1. Accordingly, knowledge about its biological relevance is restricted. Our aim was to identify new substrates of OATP3A1 to gain insights into its (patho-)physiological function. In an LC-MS-based untargeted metabolomics assay using untreated OATP3A1-overexpressing HEK293 cells and control cells, we identified several amino acids as potential substrates of OATP3A1. Subsequent uptake experiments using exogenously added substrates revealed OATP3A1-mediated transport of l-tryptophan, l-tyrosine, and l-phenylalanine with 194.8 ± 28.7% (P < 0.05), 226.2 ± 18.7% (P < 0.001), and 235.2 ± 13.5% (P < 0.001), respectively, in OATP3A1-overexpressing cells compared to control cells. Furthermore, kinetic transport parameters (Km values) were determined (Trp = 61.5 ± 14.2 μm, Tyr = 220.8 ± 54.5 μm, Phe = 234.7 ± 20.6 μm). In summary, we identified the amino acids l-tryptophan, l-tyrosine, and l-phenylalanine as new substrates of OATP3A1. These findings could be used for a better understanding of (patho-)physiological processes involving increased demand of amino acids, where OATP3A1 should be considered as an important uptake transporter of l-tryptophan, l-tyrosine, and l-phenylalanine.

SLC13A3 is a major effector downstream of activated β-catenin in liver cancer pathogenesis

Activated Wnt/β-catenin pathway is a key genetic event in liver cancer development. Solute carrier (SLC) transporters are promising drug targets. Here, we identify SLC13A3 as a drug-targetable effector downstream of β-catenin in liver cancer. SLC13A3 expression is elevated in human liver cancer samples with gain of function (GOF) mutant CTNNB1, the gene encoding β-catenin. Activation of β-catenin up-regulates SLC13A3, leading to intracellular accumulation of endogenous SLC13A3 substrates. SLC13A3 is identified as a low-affinity transporter for glutathione (GSH). Silencing of SLC13A3 downregulates the leucine transporter SLC7A5 via c-MYC signaling, leading to leucine depletion and mTOR inactivation. Furthermore, silencing of SLC13A3 depletes GSH and induces autophagic ferroptosis in β-catenin-activated liver cancer cells. Importantly, both genetic inhibition of SLC13A3 and a small molecule SLC13A3 inhibitor suppress β-catenin-driven hepatocarcinogenesis in mice. Altogether, our study suggests that SLC13A3 could be a promising therapeutic target for treating human liver cancers with GOF CTNNB1 mutations.

Investigations Towards Tryptophan Uptake and Transport Across an In Vitro Model of the Oral Mucosa Epithelium

Tryptophan is an essential amino acid and plays an important role in several metabolic processes relevant for the human health. As the main metabolic pathway for tryptophan along the kynurenine axis is involved in inflammatory responses, changed metabolite levels can be used to monitor inflammatory diseases such as ulcerative colitis. As a progenitor of serotonin, altered tryptophan levels have been related to several neurogenerative diseases as well as depression or anxiety. While tryptophan concentrations are commonly evaluated in serum, a non-invasive detection approach using saliva might offer significant advantages, especially during long-term treatments of patients or elderly. In order to estimate whether active transport processes for tryptophan might contribute to a potential correlation between blood and saliva tryptophan concentrations, we investigated tryptophan's transport across an established oral mucosa in vitro model. Interestingly, treatment with tryptophan revealed a concentration dependent secretion of tryptophan and the presence of a saturable transporter while transport studies with deuterated tryptophan displayed increased permeability from the saliva to the blood compartment. Protein analysis demonstrated a distinct expression of L-type amino acid transporter 1 (LAT1), the major transporter for tryptophan, and exposure to inhibitors (2 -amino-2-norbornanecarboxylic acid (BCH), L-leucine) led to increased tryptophan levels on the saliva side. Additionally, exposure to tryptophan in equilibrium studies resulted in a regulation of LAT1 at the mRNA level. The data collected in this study suggest the participation of active transport mechanisms for tryptophan across the oral mucosa epithelium. Future studies should investigate the transport of tryptophan across salivary gland epithelia in order to enable a comprehensive understanding of tryptophan exchange at the blood-saliva barrier.

Protein expression of the amino acid transporter SLC7A5 in tumor tissue is prognostic in early-stage colorectal cancer

Proteins overexpressed in early-stage cancers may serve as early diagnosis and prognosis markers as well as targets for cancer therapies. In this study, we examined the expression of an essential amino acid carrier SLC7A5 (LAT1, CD98, or 4F2 light chain) in cancer tissue from two well-annotated cohorts of 575 cases of early-stage and 106 cases of late-stage colorectal cancer patients. Immunohistochemistry showed SLC7A5 overexpression in 72.0% of early-stage and 56.6% of late-stage cases. SLC7A5 expression was not influenced by patient gender, age, location, or mismatch repair status, although it appeared to be slightly less prevalent in tumors of mucinous differentiation or with lymphovascular invasion. Statistical analyses revealed a positive correlation between SLC7A5 overexpression and both overall survival and disease-free survival in early-stage but not late-stage cancers. Co-expression analyses of the TCGA and CPTAC colorectal cancer cohorts identified a network of gene transcripts positively related to SLC7A5, with its heterodimer partner SLC3A2 having the highest co-expression score. Network analysis uncovered the SLC7A network to be significantly associated with ncRNA such as tRNA processing and the mitotic cell cycle. Since SLC7A5 is also a marker of activated lymphocytes such as NK, T, and B lymphocytes, SLC7A5 overexpression in early colorectal cancers might trigger a strong anti-tumor immune response which could results in better clinical outcome. Overall, our study provides clear evidence of differential SLC7A5 expression and its prognostic value for early-stage colorectal cancer, although the understanding of its functions in colorectal tumorigenesis and cancer immunity is currently rather limited and awaits further characterization.

Specific transport of temozolomide does not override DNA repair-mediated chemoresistance

Temozolomide (TMZ) a DNA alkylating agent, is the standard-of-care for brain tumors, such as glioblastoma multiforme (GBM). Although the physicochemical and pharmacokinetic properties of TMZ, such as chemical stability and the ability to cross the blood-brain barrier (BBB), have been questioned in the past, the acquired chemoresistance has been the main limiting factor of long-term clinical use of TMZ. In the present study, an L-type amino acid transporter 1 (LAT1)-utilizing prodrug of TMZ (TMZ-AA, 6) was prepared and studied for its cellular accumulation and cytotoxic properties in human squamous cell carcinoma, UT-SCC-28 and UT-SCC-42B cells, and TMZ-sensitive human glioma, U-87MG cells that expressed functional LAT1. TMZ-AA 6 accumulated more effectively than TMZ itself into those cancer cells that expressed LAT1 (UT-SCC-42B). However, this did not correlate with decreased viability of treated cells. Indeed, TMZ-AA 6, similarly to TMZ itself, required adjuvant inhibitor(s) of DNA-repair systems, O6-methylguanine-DNA methyl transferase (MGMT) and base excision repair (BER), as well as active DNA mismatch repair (MMR), for maximal growth inhibition. The present study shows that improving the delivery of this widely-used methylating agent is not the main barrier to improved chemotherapy, although utilizing a specific transporter overexpressed at the BBB or glioma cells can have targeting advantages. To obtain a more effective anticancer prodrug, the compound design focus should shift to altering the major DNA alkylation site or inhibiting DNA repair systems.

Macromolecular crowding in human tenocyte and skin fibroblast cultures: A comparative analysis

Although human tenocytes and dermal fibroblasts have shown promise in tendon engineering, no tissue engineered medicine has been developed due to the prolonged ex vivo time required to develop an implantable device. Considering that macromolecular crowding has the potential to substantially accelerate the development of functional tissue facsimiles, herein we compared human tenocyte and dermal fibroblast behaviour under standard and macromolecular crowding conditions to inform future studies in tendon engineering. Basic cell function analysis made apparent the innocuousness of macromolecular crowding for both cell types. Gene expression analysis of the without macromolecular crowding groups revealed expression of tendon related molecules in human dermal fibroblasts and tenocytes. Protein electrophoresis and immunocytochemistry analyses showed significantly increased and similar deposition of collagen fibres by macromolecular crowding in the two cell types. Proteomics analysis demonstrated great similarities between human tenocyte and dermal fibroblast cultures, as well as the induction of haemostatic, anti-microbial and tissue-protective proteins by macromolecular crowding in both cell populations. Collectively, these data rationalise the use of either human dermal fibroblasts or tenocytes in combination with macromolecular crowding in tendon engineering.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

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.

Amino acid derivative of probenecid potentiates apoptosis-inducing effects of vinblastine by increasing oxidative stress in a cancer cell-specific manner

Many chemotherapeutic drugs suffer from multidrug resistance (MDR). Efflux transporters, namely ATP-binding cassettes (ABCs), that pump the drugs out of the cancer cells comprise one major reason behind MDR. Therefore, ABC inhibitors have been under development for ages, but unfortunately, without clinical success. In the present study, an l-type amino acid transporter 1 (LAT1)-utilizing derivative of probenecid (PRB) was developed as a cancer cell-targeted efflux inhibitor for P-glycoprotein (P-gp), breast cancer resistant protein (BCRP) and/or several multidrug resistant proteins (MRPs), and its ability to increase vinblastine (VBL) cellular accumulation and apoptosis-inducing effects were explored. The novel amino acid derivative of PRB (2) increased the VBL exposure in triple-negative human breast cancer cells (MDA-MB-231) and human glioma cells (U-87MG) by 10-68 -times and 2-5-times, respectively, but not in estrogen receptor-positive human breast cancer cells (MCF-7). However, the combination therapy had greater cytotoxic effects in MCF-7 compared to MDA-MB-231 cells due to the increased oxidative stress recorded in MCF-7 cells. The metabolomic study also revealed that compound 2, together with VBL, decreased the transport of those amino acids essential for the biosynthesis of endogenous anti-oxidant glutathione (GSH). Moreover, the metabolic differences between the outcomes of the studied breast cancer cell lines were explained by the distinct expression profiles of solute carriers (SLCs) that can be concomitantly inhibited. Therefore, attacking several SLCs simultaneously to change the nutrient environment of cancer cells can serve as an adjuvant therapy to other chemotherapeutics, offering an alternative to ABC inhibitors.

Methionine supply during mid-gestation modulates the bovine placental mTOR pathway, nutrient transporters, and offspring birth weight in a sex-specific manner

The mechanistic target of rapamycin (mTOR) predominantly regulates the expression and activity of placental nutrient transporters. The mTOR pathway can be activated by several nutrients, including the essential amino acid methionine. Additionally, previous research in nonruminant animals suggests that mTOR is influenced in a sexually dimorphic manner. In bovine, there is limited understanding of how maternal nutrition and offspring sexual dimorphism affect the placental transfer of nutrients. Thus, we investigated the effects of increasing the supply of dietary methionine to beef cattle heifers during mid-gestation on mTOR signaling, placental nutrient transporters, and fetal growth in male and female offspring. Forty purebred Angus heifers were used in a randomized complete block design experiment. From days 90 to 180 of gestation, heifers received a basal diet with no added methionine (CON, n = 20), or the basal diet plus 8.3 g of rumen-protected methionine (MET, n = 20) per animal daily. All animals received a basal diet in the first and third trimesters of gestation. Cotyledonary tissue samples were collected at parturition and utilized to examine the mTOR pathway and nutrient transporters through protein and gene expression analysis. The offspring's body weight was measured at birth. Data were analyzed using a mixed model that included the fixed effect of treatment, offspring sex, their interactions, and the random effect of block. At day 170 of gestation, MET-supplemented heifers showed higher plasma concentrations of methionine and glutamate (P < 0.01) and lower glycine and proline levels (P ≤ 0.01) compared to the CON group. A treatment × sex interaction was observed for calf birth weight (P = 0.03). In heifers that delivered male calves, MET supplementation increased the birth weight of the calves (P < 0.01). However, the dietary treatments had no effect on the birth weight of female calves (P = 0.32). The increase in birth weight of male calves from MET-fed heifers resembles the upregulation of placental mTOR and phosphorylated mTOR (P ≤ 0.03), as well as the amino acid transporters SLC1A5, SLC7A5, SLC38A6, and SLC38A11, and the glucose transporters SLC2A1 and SLC2A8 (P ≤ 0.05). Our findings suggest that increasing the supply of methionine to beef heifers during mid-gestation can modulate placental nutrient transport and fetal growth in a sex-dependent manner and that these effects are mediated, at least in part, by the mTOR pathway.

Excessive Substitution of Fish Meal with Fermented Soybean Meal Induces Oxidative Stress by Impairing Glutathione Metabolism in Largemouth Bass (Micropterus salmoides)

The application of fermented soybean meal (FSBM) is an effective strategy to alleviate the shortage of fish meal (FM) in aquaculture. However, an excessive substitution ratio often reduces fish growth and induces liver oxidative stress, while the mechanism remains poorly understood. Here, an 8-week feeding trial was conducted in largemouth bass (initial weight: 6.82 ± 0.09 g) to establish an oxidative stress model by replacing 50% of FM with FSBM (fermented by Bacillus subtilis). The results showed that FSBM substitution significantly reduced the growth performance of largemouth bass, including the weight gain rate and specific growth rate. Moreover, FSBM significantly reduced the contents of essential amino acids and total free amino acids in muscle, along with the mRNA expression of amino acids and small peptide transporters. Enzyme activity detection and liver sections showed that FSBM substitution caused liver oxidative stress, indicating the successful construction of an oxidative stress model. An integrated analysis of transcriptomic and metabolomic data revealed that FSBM substitution impaired glycine, serine and threonine metabolism, as well as glutathione metabolism. In addition, the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) was decreased in the FSBM group, which may explain the mechanism of oxidative stress caused by FSBM substitution. Considering that glycine is an important component of glutathione synthesis, key genes involved in glycine metabolism (glya, gnmt and agxt) and dietary glycine supplementation should be valued to improve the availability of FSBM. This study reveals for the first time the importance of non-essential amino acids in improving the utilization of plant-based protein sources and provides original insight for the optimization of aquatic feeds.

The human LAT1-4F2hc (SLC7A5-SLC3A2) transporter complex: Physiological and pathophysiological implications

LAT1 and 4F2hc form a heterodimeric membrane protein complex, which functions as one of the best characterized amino acid transporters. Since LAT1-4F2hc is required for the efficient uptake of essential amino acids and hormones, it promotes cellular growth, in part, by stimulating mTORC1 (mechanistic target of rapamycin complex 1) signalling and by repressing the integrated stress response (ISR). Gain or loss of LAT1-4F2hc function is associated with cancer, diabetes, and immunological and neurological diseases. Hence, LAT1-4F2hc represents an attractive drug target for disease treatment. Specific targeting of LAT1-4F2hc will be facilitated by the increasingly detailed understanding of its molecular architecture, which provides important concepts for its function and regulation. Here, we summarize (i) structural insights that help to explain how LAT1 and 4F2hc assemble to transport amino acids across membranes, (ii) the role of LAT1-4F2hc in key metabolic signalling pathways, and (iii) how derailing these processes could contribute to diseases.

The effects of environmental aryl hydrocarbon receptor ligands on signaling and cell metabolism in cancer

Dioxin and dioxin-like compounds are chlorinated organic pollutants formed during the manufacturing of other chemicals. Dioxins are ligands of the aryl hydrocarbon receptor (AHR), that induce AHR-mediated biochemical and toxic responses and are persistent in the environment. 2,3,7,8- tetrachlorodibenzo para dioxin (TCDD) is the prototypical AHR ligand and its effects represent dioxins. TCDD induces toxicity, immunosuppression and is a suspected tumor promoter. The role of TCDD in cancer however is debated and context-dependent. Environmental particulate matter, polycyclic aromatic hydrocarbons, perfluorooctane sulfonamide, endogenous AHR ligands, and cAMP signaling activate AHR through TCDD-independent pathways. The effect of activated AHR in cancer is context-dependent. The ability of FDA-approved drugs to modulate AHR activity has sparked interest in their repurposing for cancer therapy. TCDD by interfering with endogenous pathways, and overstimulating other endogenous pathways influences all stages of cancer. Herein we review signaling mechanisms that activate AHR and mechanisms by which activated AHR modulates signaling in cancer including affected metabolic pathways.

Human Placental Adaptive Changes in Response to Maternal Obesity: Sex Specificities

Maternal obesity is increasingly prevalent and is associated with elevated morbidity and mortality rates in both mothers and children. At the interface between the mother and the fetus, the placenta mediates the impact of the maternal environment on fetal development. Most of the literature presents data on the effects of maternal obesity on placental functions and does not exclude potentially confounding factors such as metabolic diseases (e.g., gestational diabetes). In this context, the focus of this review mainly lies on the impact of maternal obesity (in the absence of gestational diabetes) on (i) endocrine function, (ii) morphological characteristics, (iii) nutrient exchanges and metabolism, (iv) inflammatory/immune status, (v) oxidative stress, and (vi) transcriptome. Moreover, some of those placental changes in response to maternal obesity could be supported by fetal sex. A better understanding of sex-specific placental responses to maternal obesity seems to be crucial for improving pregnancy outcomes and the health of mothers and children.

Abundance of Amino Acid Transporters and mTOR Pathway Components in the Gastrointestinal Tract of Lactating Holstein Cows

Data from non-ruminants indicate that amino acid (AA) transport into cells can regulate mTOR pathway activity and protein synthesis. Whether mTOR is expressed in the ruminant gastrointestinal tract (GIT) and how it may be related to AA transporters and the AA concentrations in the tissue is unknown. Ruminal papillae and the epithelia of the duodenum, jejunum, and ileum collected at slaughter from eight clinically healthy Holstein in mid-lactation were used. Metabolites and RNA were extracted from tissue for liquid chromatography–mass spectrometry and RT-qPCR analysis. The glycine and asparagine concentrations in the rumen were greater than those in the intestine (p < 0.05), but the concentrations of other AAs were greater in the small intestine than those in the rumen. Among the 20 AAs identified, the concentrations of glutamate, alanine, and glycine were the greatest. The mRNA abundances of AKT1 and MTOR were greater in the small intestine than those in the rumen (p < 0.05). Similarly, the SLC1A1, SLC6A6, SLC7A8, SLC38A1, SLC38A7, and SLC43A2 mRNA abundances were greater (p < 0.05) in the small intestine than those in the rumen. The mRNA abundances of SLC1A5, SLC3A2, and SLC7A5 were greater in the rumen than those in the small intestine (p < 0.05). Overall, the present study provides fundamental data on the relationship between mTOR pathway components and the transport of AAs in different sections of the gastrointestinal tract.

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.

Glucose supplementation improves intestinal amino acid transport and muscle amino acid pool in pigs during chronic cold exposure

Mammals in northern regions chronically suffer from low temperatures during autumn-winter seasons. The aim of this study was to investigate the response of intestinal amino acid transport and the amino acid pool in muscle to chronic cold exposure via Min pig models (cold adaptation) and Yorkshire pig models (non-cold adaptation). Furthermore, this study explored the beneficial effects of glucose supplementation on small intestinal amino acid transport and amino acid pool in muscle of cold-exposed Yorkshire pigs. Min pigs (Exp. 1) and Yorkshire pigs (Exp. 2) were divided into a control group (17 °C, n = 6) and chronic cold exposure group (7 °C, n = 6), respectively. Twelve Yorkshire pigs (Exp. 3) were divided into a cold control group and cold glucose supplementation group (8 °C). The results showed that chronic cold exposure inhibited peptide transporter protein 1 (PepT1) and excitatory amino acid transporter 3 (EAAT3) expression in ileal mucosa and cationic amino acid transporter-1 (CAT-1) in the jejunal mucosa of Yorkshire pigs (P < 0.05). In contrast, CAT-1, PepT1 and EAAT3 expression was enhanced in the duodenal mucosa of Min pigs (P < 0.05). Branched amino acids (BCAA) in the muscle of Yorkshire pigs were consumed by chronic cold exposure, accompanied by increased muscle RING-finger protein-1 (MuRF1) and muscle atrophy F-box (atrogin-1) expression (P < 0.05). More importantly, reduced concentrations of dystrophin were detected in the muscle of Yorkshire pigs (P < 0.05). However, glycine concentration in the muscle of Min pigs was raised (P < 0.05). In the absence of interaction between chronic cold exposure and glucose supplementation, glucose supplementation improved CAT-1 expression in the jejunal mucosa and PepT1 expression in the ileal mucosa of cold-exposed Yorkshire pigs (P < 0.05). It also improved BCAA and inhibited MuRF1 and atrogin-1 expression in muscle (P < 0.05). Moreover, dystrophin concentration was improved by glucose supplementation (P < 0.05). In summary, chronic cold exposure inhibits amino acid absorption in the small intestine, depletes BCAA and promotes protein degradation in muscle. Glucose supplementation ameliorates the negative effects of chronic cold exposure on amino acid transport and the amino acid pool in muscle.

Absorption of methionine sources in animals-is there more to know?

This literature review evaluates the absorption of methionine (Met) sources such as 2-hydroxy-4-methylthiobutyric acid (HMTBa), its calcium salts (HMTBa-Ca), and DL-methionine (DL-Met) by focusing on the state of knowledge regarding the absorption mechanism, experimental methodology, and factors affecting their absorption. The 2 Met sources differ in mechanism and site of absorption due to differences in their chemical characteristics and enzymatic conversion. This review addresses diffusion- and transport-mediated absorption systems for amino acids and carboxylic compounds, best elucidated by in vitro, ex vivo, and in vivo experimental models. Opportunities and limitations in the use of radioisotopes to depict absorption sites as well as host and microbial metabolism are described. Physiological and environmental conditions that lead to changes in gut absorptive capacity and the impact of Met source absorption are also evaluated. This review concludes that any comparison between HMTBa and DL-Met should consider their different behaviors during the absorption phase. Hence, the chemical characteristics of these 2 molecules entail different absorption sites and mechanisms, from passive absorption in the case of HMTBa and HMTBa-Ca to active transporters for DL-Met, HMTBa, and HMTBa-Ca. In addition, the different conversion modes of these 2 molecules further differentiate their absorption modes. Considering these important differences, it is easier to understand the apparent divergence between the conclusions of existing publications. When comparing these 2 molecules, it is recommended to properly adapt to the conditions under which the absorption of Met sources is evaluated.

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.

The Role of L-type Amino Acid Transporter 1 (Slc7a5) During In Vitro Myogenesis

Satellite cells are required for muscle regeneration, remodeling, and repair through their activation, proliferation, and differentiation; however, how dietary factors regulate this process remains poorly understood. The L-Type amino acid transporter 1 (LAT1) transports amino acids, such as leucine, into mature myofibers, which then stimulates protein synthesis and anabolic signaling. However, whether LAT1 is expressed on myoblasts and is involved in regulating myogenesis is unknown. The aim of this study was to characterize the expression and functional relevance of LAT1 during different stages of myogenesis and in response to growth and atrophic conditions in vitro. We determined that LAT1 is expressed by C2C12 and human primary myoblasts, and its gene expression is lower during differentiation (p<0.05). Pharmacological inhibition and genetic knockdown of LAT1 impaired myoblast viability, differentiation, and fusion (all p<0.05). LAT1 protein content in C2C12 myoblasts was not significantly altered in response to different leucine concentrations in cell culture media or in two in vitro atrophy models. However, LAT1 content was decreased in myotubes under atrophic conditions in vitro (p<0.05). These findings indicate that LAT1 is stable throughout myogenesis and in response to several in vitro conditions that induce muscle remodeling. Further, amino acid transport through LAT1 is required for normal myogenesis in vitro.

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.

Expression of L-type amino acid transporter 1 in canine and feline intracranial tumors

L-type amino acid transporter 1 (LAT1) is upregulated in various malignant tumors in humans. LAT1 expression correlates with the grade of cancer and prognosis. LAT1 is responsible for the supply of many essential amino acids to cancer cells. Inhibition of LAT1 reduces the amino acids that enter the cell and inhibits cancer cell growth. Therefore, novel anticancer drugs targeting LAT1 have attracted much attention in recent years. In this study, to explore the applicability of using LAT1 expression in intracranial tumors as a prognostic factor and therapeutic target, we investigated the expression of LAT1 in surgically resected primary and secondary intracranial tumor tissues from dogs and cats. Immunohistochemical analysis of LAT1 was performed on intracranial tumor tissue from 14 dogs and 3 cats. Primary intracranial tumors were seen in 10 dogs and included meningiomas, histiocytic sarcomas, pituitary tumors, and gliomas, and 9 out of 10 cases were positive for LAT1. Primary intracranial tumors were seen in 2 cats and included meningioma and lymphoma; both cases were positive for LAT1. Secondary intracranial tumors were positive for LAT1 in 3 out of 4 cases in dogs and 1 out of 1 in cats. Since the majority of intracranial tumors in dogs and cats were positive for LAT1, immunostaining for LAT1 is expected to be a prognostic indicator and therapeutic target in the future.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

Uptake of antiepileptic drugs in forskolin-induced differentiated BeWo cells: Alteration of gabapentin transport

Previous studies have indicated that the expression levels of several transporters are altered during placental trophoblast differentiation. However, changes in the transport activities of therapeutic agents during differentiation must be comprehensively characterised. Antiepileptic drugs, including gabapentin (GBP), lamotrigine (LTG), topiramate, and levetiracetam, are increasingly prescribed during pregnancy. The objective of this study was to elucidate differences in the uptake of antiepileptic drugs during the differentiation process.Human placental choriocarcinoma BeWo cells were used as trophoblast models. For differentiation into syncytiotrophoblast-like cells, cells were treated with forskolin.The uptake of GBP and LTG was lower in differentiated BeWo cells than in undifferentiated cells. In particular, the maximum uptake rate of GBP transport was decreased in differentiated BeWo cells. Furthermore, GBP transport was trans-stimulated by the amino acids His and Met. We investigated the profiles of amino acids in undifferentiated and differentiated BeWo cells. Supplementation with His and Met, which demonstrated trans-stimulatory effects on GBP uptake, restored GBP uptake in differentiated cells. The findings of this study suggest that drug transport in BeWo cells can be altered before and after differentiation, and that the altered GBP uptake could be mediated by the intracellular amino acid status.

Pharmacoproteomics of Brain Barrier Transporters and Substrate Design for the Brain Targeted Drug Delivery

One of the major reasons why central nervous system (CNS)-drug development has been challenging in the past, is the barriers that prevent substances entering from the blood circulation into the brain. These barriers include the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB), blood-cerebrospinal fluid barrier (BCSFB), and blood-arachnoid barrier (BAB), and they differ from each other in their transporter protein expression and function as well as among the species. The quantitative expression profiles of the transporters in the CNS-barriers have been recently revealed, and in this review, it is described how they affect the pharmacokinetics of compounds and how these expression differences can be taken into account in the prediction of brain drug disposition in humans, an approach called pharmacoproteomics. In recent years, also structural biology and computational resources have progressed remarkably, enabling a detailed understanding of the dynamic processes of transporters. Molecular dynamics simulations (MDS) are currently used commonly to reveal the conformational changes of the transporters and to find the interactions between the substrates and the protein during the binding, translocation in the transporter cavity, and release of the substrate on the other side of the membrane. The computational advancements have also aided in the rational design of transporter-utilizing compounds, including prodrugs that can be actively transported without losing potency towards the pharmacological target. In this review, the state-of-art of these approaches will be also discussed to give insights into the transporter-mediated drug delivery to the CNS.

The role of SLC transporters for brain health and disease

The brain exchanges nutrients and small molecules with blood via the blood-brain barrier (BBB). Approximately 20% energy intake for the body is consumed by the brain. Glucose is known for its critical roles for energy production and provides substrates for biogenesis in neurons. The brain takes up glucose via glucose transporters GLUT1 and 3, which are expressed in several neural cell types. The brain is also equipped with various transport systems for acquiring amino acids, lactate, ketone bodies, lipids, and cofactors for neuronal functions. Unraveling the mechanisms by which the brain takes up and metabolizes these nutrients will be key in understanding the nutritional requirements in the brain. This could also offer opportunities for therapeutic interventions in several neurological disorders. For instance, emerging evidence suggests a critical role of lactate as an alternative energy source for neurons. Neuronal cells express monocarboxylic transporters to acquire lactate. As such, treatment of GLUT1-deficient patients with ketogenic diets to provide the brain with alternative sources of energy has been shown to improve the health of the patients. Many transporters are present in the brain, but only a small number has been characterized. In this review, we will discuss about the roles of solute carrier (SLC) transporters at the blood brain barrier (BBB) and neural cells, in transport of nutrients and metabolites in the brain.

Comparison of Experimental Strategies to Study l-Type Amino Acid Transporter 1 (LAT1) Utilization by Ligands

l-Type amino acid transporter 1 (LAT1), expressed abundantly in the brain and placenta and overexpressed in several cancer cell types, has gained a lot of interest in drug research and development, as it can be utilized for brain-targeted drug delivery, as well as inhibiting the essential amino acid supply to cancer cells. The structure of LAT1 is today very well-known and the interactions of ligands at the binding site of LAT1 can be modeled and explained. However, less is known of LAT1′s life cycle within the cells. Moreover, the functionality of LAT1 can be measured by several different methods, which may vary between the laboratories and make the comparison of the results challenging. In the present study, the usefulness of indirect cis-inhibition methods and direct cellular uptake methods and their variations to interpret the interactions of LAT1-ligands were evaluated. Moreover, this study also highlights the importance of understanding the intracellular kinetics of LAT1-ligands, and how they can affect the regular function of LAT1 in critical tissues, such as the brain. Hence, it is discussed herein how the selected methodology influences the outcome and created knowledge of LAT1-utilizing compounds.

Chemical Approaches for Studying the Biology and Pharmacology of Membrane Transporters: The Histidine/Large Amino Acid Transporter SLC7A5 as a Benchmark

The localization of membrane transporters at the forefront of natural barriers makes these proteins very interesting due to their involvement in the absorption and distribution of nutrients and xenobiotics, including drugs. Over the years, structure/function relationship studies have been performed employing several strategies, including chemical modification of exposed amino acid residues. These approaches are very meaningful when applied to membrane transporters, given that these proteins are characterized by both hydrophobic and hydrophilic domains with a different degree of accessibility to employed chemicals. Besides basic features, the chemical targeting approaches can disclose information useful for pharmacological applications as well. An eminent example of this picture is the histidine/large amino acid transporter SLC7A5, known as LAT1 (Large Amino Acid Transporter 1). This protein is crucial in cell life because it is responsible for mediating the absorption and distribution of essential amino acids in peculiar body districts, such as the blood brain barrier and placenta. Furthermore, LAT1 can recognize a large variety of molecules of pharmacological interest and is also considered a hot target for drugs due to its over-expression in virtually all human cancers. Therefore, it is not surprising that the chemical targeting approach, coupled with bioinformatics, site-directed mutagenesis and transport assays, proved fundamental in describing features of LAT1 such as the substrate binding site, regulatory domains and interactions with drugs that will be discussed in this review. The results on LAT1 can be considered to have general applicability to other transporters linked with human diseases.

Placenta-specific Slc38a2/SNAT2 knockdown causes fetal growth restriction in mice

Fetal growth restriction (FGR) is a complication of pregnancy that reduces birth weight, markedly increases infant mortality and morbidity and is associated with later-life cardiometabolic disease. No specific treatment is available for FGR. Placentas of human FGR infants have low abundance of sodium-coupled neutral amino acid transporter 2 (Slc38a2/SNAT2), which supplies the fetus with amino acids required for growth. We determined the mechanistic role of placental Slc38a2/SNAT2 deficiency in the development of restricted fetal growth, hypothesizing that placenta-specific Slc38a2 knockdown causes FGR in mice. Using lentiviral transduction of blastocysts with a small hairpin RNA (shRNA), we achieved 59% knockdown of placental Slc38a2, without altering fetal Slc38a2 expression. Placenta-specific Slc38a2 knockdown reduced near-term fetal and placental weight, fetal viability, trophoblast plasma membrane (TPM) SNAT2 protein abundance, and both absolute and weight-specific placental uptake of the amino acid transport System A tracer, 14C-methylaminoisobutyric acid (MeAIB). We also measured human placental SLC38A2 gene expression in a well-defined term clinical cohort and found that SLC38A2 expression was decreased in late-onset, but not early-onset FGR, compared with appropriate for gestational age (AGA) control placentas. The results demonstrate that low placental Slc38a2/SNAT2 causes FGR and could be a target for clinical therapies for late-onset FGR.

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.

SLFN5 Regulates LAT1-Mediated mTOR Activation in Castration-Resistant Prostate Cancer

Androgen deprivation therapy (ADT) is the standard of care for treatment of nonresectable prostate cancer. Despite high treatment efficiency, most patients ultimately develop lethal castration-resistant prostate cancer (CRPC). In this study, we performed a comparative proteomic analysis of three in vivo, androgen receptor (AR)-responsive orthograft models of matched hormone-naïve prostate cancer and CRPC. Differential proteomic analysis revealed that distinct molecular mechanisms, including amino acid (AA) and fatty acid metabolism, are involved in the response to ADT in the different models. Despite this heterogeneity, Schlafen family member 5 (SLFN5) was identified as an AR-regulated protein in CRPC. SLFN5 expression was high in CRPC tumors and correlated with poor patient outcome. In vivo, SLFN5 depletion strongly impaired tumor growth in castrated conditions. Mechanistically, SLFN5 interacted with ATF4 and regulated the expression of LAT1, an essential AA transporter. Consequently, SLFN5 depletion in CRPC cells decreased intracellular levels of essential AA and impaired mTORC1 signaling in a LAT1-dependent manner. These results confirm that these orthograft models recapitulate the high degree of heterogeneity observed in patients with CRPC and further highlight SLFN5 as a clinically relevant target for CRPC. SIGNIFICANCE: This study identifies SLFN5 as a novel regulator of the LAT1 amino acid transporter and an essential contributor to mTORC1 activity in castration-resistant prostate cancer.

The Adipose Tissue-Derived Secretome (ADS) in Obesity Uniquely Induces L-Type Amino Acid Transporter 1 (LAT1) and mTOR Signaling in Estrogen-Receptor-Positive Breast Cancer Cells

Obesity increases the risk of postmenopausal breast cancer (BC). This risk is mediated by obesity-induced changes in the adipose-derived secretome (ADS). The pathogenesis of BC in obesity is stimulated by mTOR hyperactivity. In obesity, leucine might support mTOR hyperactivity. Leucine uptake by BC cells is through L-Type Amino Acid Transporter 1 (LAT1). Our objective was to link obesity-ADS induction of LAT1 to the induction of mTOR signaling. Lean- and obese-ADS were obtained from lean and obese mice, respectively. Breast ADS was obtained from BC patients. Estrogen-receptor-positive BC cells were stimulated with ADS. LAT1 activity was determined by uptake of 3H-leucine. The LAT1/CD98 complex, and mTOR signaling were assayed by Western blot. The LAT1 antagonists, BCH and JPH203, were used to inhibit LAT1. Cell migration and invasion were measured by Transwell assays. The results showed obese-ADS-induced LAT1 activity by increasing transporter affinity for leucine. Consistent with this mechanism, LAT1 and CD98 expression were unchanged. Induction of mTOR by obese-ADS was inhibited by LAT1 antagonists. Breast ADS from patients with BMIs > 30 stimulated BC cell migration and invasiveness. Collectively, our findings show that obese-ADS induction of LAT1 supports mTOR hyperactivity in luminal BC cells.

Interactive effects of dietary amino acid density and environmental temperature on growth performance and expression of selected amino acid transporters, water channels, and stress-related transcripts

Exposure to heat stress (HS) is one of the challenges facing the broiler industry worldwide. Various nutritional strategies have been suggested, such as altering dietary concentrations of some nutrients. Thus, we evaluated feeding different amino acid (AA) densities on live performance, Pectoralis (P.) muscles, and expression of selected AA transporters, water channels, and stress-related transcripts in a fast-growing broiler strain. Ross 308 chicks (n = 576) were randomly assigned to 4 dietary treatments (24 reps, 6 chicks per rep), differing in AA density (110, 100, 90, and 80% of a breeder's AA specifications). During 24 to 36 days of age, half of the birds were kept at a thermoneutral (TN) temperature of 20°C, whereas the other half were subjected to HS at 32° C for 8 h daily, making the treatment design a 4 × 2. The results revealed no interaction between housing temperature and AA density on growth performance or P. muscles weights. Feeding 80% AAs depressed BWG, FCR, and P. muscles at 36 d (P < 0.001). There was an interaction (P < 0.001) between AA density and temperature on the expression of all examined genes. Reducing the AA density beyond 100% upregulated the expression of AA transporter (CAT1, B⁰AT, b^0,+AT, SNAT1, LAT1), HSP70, HSP90, glucocorticoid receptor (GR), and AQP3 in the TN birds’ jejunum. Whereas in the HS birds, inconsistent expressions were observed in the jejunum, of which CAT1, B⁰AT, and LAT1 were markedly downregulated as AA density was reduced. In P. major of TN birds, reducing AA density resulted in upregulating the expression of all AA transporters, HSP70, GR, and AQP1, while downregulating HSP90 and AQP9. In contrast, AA reduction markedly downregulated CAT1, B⁰AT, and LAT1 in the P. major of HS birds. These findings indicate that the dietary AA level alters the expression of various genes involved in AA uptake, protein folding, and water transport. The magnitude of alteration is also dependent on the housing temperature. Furthermore, the results highlight the importance of adequate AA nutrition for fast-growing chickens under HS.

Hemocompatible LAT1-inhibitor can induce apoptosis in cancer cells without affecting brain amino acid homeostasis

Increased amounts of amino acids are essential for cancer cells to support their sustained growth and survival. Therefore, inhibitors of amino acid transporters, such as l-type amino acid transporter 1 (LAT1) have been developed. In this study, a previously reported LAT1-inhibitor (KMH-233) was studied for its hemocompatibility and toxicity towards human umbilical vein endothelial cells (HUVEC) and human aortic smooth muscle cells (AoSMCs). Furthermore, the cytotoxic effects against human breast adenocarcinoma cells (MCF-7) and its ability to affect mammalian (or mechanistic) target of rapamycin (mTOR) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling were evaluated. Moreover, the effects of this inhibitor to modulate LAT1 function on the cell surface and the brain amino acid homeostasis were evaluated after intraperitoneal (i.p.) administration of LAT1-inhibitor (23 µmol/kg) in mice. The results showed that LAT1-inhibitor (KMH-233) is hemocompatible at concentrations below 25 µM and it does not affect coagulation in plasma. However, it can reduce the total protein amount of mTOR and NF-κB, resulting in increased apoptosis in LAT1-expressing cancer cells. Most importantly, the inhibitor did not affect mouse brain levels of l-Leu, l-Tyr or l-Trp or modulate the function of LAT1 on the MCF-7 cell surface. Therefore, this inhibitor can be considered as a safe but effective anti-cancer agent. However, due to the compensative mechanism of cancer cells for their increased amino acid demand, this compound is most effective inducing apoptosis when used in combinations with other chemotherapeutics, such as protease inhibitor, bestatin, as demonstrated in this study.

Improved l-Type amino acid transporter 1 (LAT1)-mediated delivery of anti-inflammatory drugs into astrocytes and microglia with reduced prostaglandin production

Non-steroidal anti-inflammatory drugs (NSAIDs) can have protective effects in the brain by inhibition of cyclooxygenases (COX). However, the delivery into the brain across the blood-brain barrier (BBB) and particularly into the brain parenchymal cells is hindered. Therefore, in the present study, we developed four l-type amino acid transporter 1 (LAT1)-utilizing prodrugs of flurbiprofen, ibuprofen, naproxen, and ketoprofen, since LAT1 is expressed on both, the BBB endothelial cells as well as parenchymal cells. The cellular uptake and utilization of LAT1 by novel prodrugs were studied in mouse cortical primary astrocytes and immortalized microglia (BV2), and the release of the parent NSAID in several tissue and cell homogenates. Finally, the effects of the studied prodrugs on prostaglandin E₂ (PGE₂) production and cell viability were explored. The gained results showed that all four prodrugs were carried into their target cells via LAT1. They also released their parent NSAIDs via carboxylesterases (CES) and most likely also other un-identified enzymes, which need to be carefully considered when administrating these compounds orally or intravenously. Most importantly, all the studied prodrugs reduced the PGE₂ production in astrocytes and microglia after lipopolysaccharide (LPS)-induced inflammation by 29-94% and without affecting the cell viability with the studied concentration (20 µM).

Maternal Obesity Influences Placental Nutrient Transport, Inflammatory Status, and Morphology in Human Term Placenta

CONTEXT

Maternal obesity has a significant impact on placental development. However, this impact on the placenta's structure and function (ie, nutrient transport and hormone and cytokine production) is a controversial subject.

OBJECTIVE

We hypothesized that maternal obesity is associated with morphologic, secretory, and nutrient-related changes and elevated levels of inflammation in the placenta.

DESIGN

We collected samples of placental tissue from 2 well-defined groups of pregnant women from 2017 to 2019. We compared the 2 groups regarding placental cytokine and hormone secretion, immune cell content, morphology, and placental nutrient transporter expressions.

SETTING

Placenta were collected after caesarean section performed by experienced clinicians at Centre Hospitalier Intercommunal (CHI) of Poissy-Saint-Germain-en-Laye.

PATIENTS

The main inclusion criteria were an age between 27 and 37 years old, no complications of pregnancy, and a first-trimester body mass index of 18-25 kg/m2 for the nonobese (control) group and 30-40 kg/m2 for the obese group.

RESULTS

In contrast to our starting hypothesis, we observed that maternal obesity was associated with (1) lower placental IL-6 expression and macrophage/leukocyte infiltration, (2) lower placental expression of GLUT1 and SNAT1-2, (3) a lower placental vessel density, and (4) lower levels of placental leptin and human chorionic gonadotropin production.

CONCLUSION

These results suggest that the placenta is a plastic organ and could optimize fetal growth. A better understanding of placental adaptation is required because these changes may partly determine the fetal outcome in cases of maternal obesity.

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.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Amino Acid Transporter LAT1 (SLC7A5) Mediates MeHg-Induced Oxidative Stress Defense in the Human Placental Cell Line HTR-8/SVneo

The placental barrier can protect the fetus from contact with harmful substances. The potent neurotoxin methylmercury (MeHg), however, is very efficiently transported across the placenta. Our previous data suggested that L-type amino acid transporter (LAT)1 is involved in placental MeHg uptake, accepting MeHg-L-cysteine conjugates as substrate due to structural similarity to methionine. The aim of the present study was to investigate the antioxidant defense of placental cells to MeHg exposure and the role of LAT1 in this response. When trophoblast-derived HTR-8/SVneo cells were LAT1 depleted by siRNA-mediated knockdown, they accumulated less MeHg. However, they were more susceptible to MeHg-induced toxicity. This was evidenced in decreased cell viability at a usually noncytotoxic concentration of 0.03 µM MeHg (~6 µg/L). Treatment with ≥0.3 µM MeHg increased cytotoxicity, apoptosis rate, and oxidative stress of HTR-8/SVneo cells. These effects were enhanced under LAT1 knockdown. Reduced cell number was seen when MeHg-exposed cells were cultured in medium low in cysteine, a constituent of the tripeptide glutathione (GSH). Because LAT1-deficient HTR-8/SVneo cells have lower GSH levels than control cells (independent of MeHg treatment), we conclude that LAT1 is essential for de novo synthesis of GSH, required to counteract oxidative stress. Genetic predisposition to decreased LAT1 function combined with MeHg exposure could increase the risk of placental damage.

The amino acid transporter Slc7a5 regulates the mTOR pathway and is required for granule cell development

Pathogenic mutations in the solute carrier family 7 member 5 (SLC7A5) gene, which encodes an amino acid transporter cause microcephaly and seizures, yet the mechanisms responsible for these phenotypes are unclear. Models have demonstrated that Slc7a5 deletion is embryonic lethal and that these embryos lack a fully formed telencephalon. This phenotype is similar to that of mammalian target of rapamycin (mTOR) protein kinase deletion or mTOR inhibition. Notably, in many cells, Slc7a5 import of amino acids is required to maintain mTOR activity. Slc7a5 is present within neurogenic regions during embryogenesis, is found in cultured neurons and can modulate neuronal electrophysiological properties. However, Slc7a5 is also highly expressed within endothelial cells of the blood-brain barrier where removal in conditional mice leads to severe behavioral defects and non-cell autonomous changes in neurons. Therefore, the extent that neural Slc7a5 is required for development is unclear. Here, subventricular zone neural stem cells that generate olfactory bulb granule cell neurons were electroporated with SLC7A5 or Slc7a5 short hairpin RNA encoding plasmids. Although early phases of neural development were unaltered, Slc7a5 knockdown effected late phases of GC dendrite maturation and survival. Slc7a5 knockdown also decreased mTOR pathway activity. Ras homolog enriched in brain, an mTOR activator, rescued the effect of Slc7a5 knockdown on mTOR pathway activity and dendrite arbors. The data presented here demonstrate that Slc7a5 is required for GC mTOR pathway activity, maturation and survival, which may help explain why Slc7a5 mutations prevent normal brain development and function.

Maternal hypercortisolemia alters placental metabolism: a multiomics view

Previous studies have suggested that increases in maternal cortisol or maternal stress in late pregnancy increase the risk of stillbirth at term. In an ovine model with increased maternal cortisol over the last 0.20 of gestation, we have previously found evidence of disruption of fetal serum and cardiac metabolomics and altered expression of genes related to mitochondrial function and metabolism in biceps femoris, diaphragm, and cardiac muscle. The present studies were designed to test for effects of chronically increased maternal cortisol on gene expression and metabolomics in placentomes near term. We hypothesized that changes in placenta might underlie or contribute to the alterations in fetal serum metabolomics and thereby contribute to changes in striated muscle metabolism. Placentomes were collected from pregnancies in early labor (143 ± 1 days gestation) of control ewes (n = 7) or ewes treated with cortisol (1 mg·kg^-1·day^-1 iv; n = 5) starting at day 115 of gestation. Transcriptomics and metabolomics were performed using an ovine gene expression microarray (Agilent 019921) and HR-MAS NMR, respectively. Multiomic analysis indicates that amino acid metabolism, particularly of branched-chain amino acids and glutamate, occur in placenta; changes in amino acid metabolism, degradation, or biosynthesis in placenta were consistent with changes in valine, isoleucine, leucine, and glycine in fetal serum. The analysis also indicates changes in glycerophospholipid metabolism and suggests changes in endoplasmic reticulum stress and antioxidant status in the placenta. These findings suggest that changes in placental function occurring with excess maternal cortisol in late gestation may contribute to metabolic dysfunction at birth.

Divergence of Iodine and Thyroid Hormones in the Fetal and Maternal Parts of Human-Term Placenta

The human placenta is an important organ that forms a barrier where maternal and fetal exchange takes place. The placenta transport iodine to the fetal circulation by transfer of maternal iodine and deiodination of thyroid hormones (THs). The aim of the study was to examine the distribution of iodine and thyroid hormone transporters in the maternal and fetal sides of human-term placenta. A cross-sectional study was performed at the First Affiliated Hospital of China Medical University. Placental samples (maternal and fetal surfaces) were collected from 113 healthy-term pregnant women. The iodine content; the concentration of thyroxine (T4), triiodothyronine (T3), and reverse T3 (rT3); and the enzyme activity of placental type 2 iodothyronine deiodinase (D2) and D3 were examined. The mRNA and protein localization/expression of iodine and thyroid hormone transporters in the placenta were also studied. We also analyzed the association between expression level of Na+/I- symporter (NIS), thyroid hormone transporter protein, D3 activity in maternal and fetal surfaces of placenta with iodine content, and thyroid hormone levels. Iodine levels in placental samples from the maternal side were significantly higher than those in samples from the fetal side. T3 and T4 expression in fetal placenta was significantly lower than in maternal placenta. D3 activity in the fetal side of the placentas was significantly higher than that in the maternal side. The mRNA and protein expression of monocarboxylate transporters 8 (MCT8), L-amino acid transporters 1 (LAT1), organic anion transporting polypeptides 4A1 (OATP4A1), and TH binding protein transthyretin (TTR) were significantly increased in maternal side, while the NIS expression was higher in fetal side of human-term placenta. In conclusion, the enzymatic deiodination of thyroid hormones forms a barrier which reduces transplacental passage of the hormones and that the maternal part of the placenta is the primary factor in the mechanism regulating the hormonal transfer.

The Disposal of Reactive Carbonyl Species through Carnosine Conjugation: What We Know Now

:

Reactive Carbonyl Species are electrophiles generated by the oxidative cleavage of lipids and sugars. Such compounds have been described as important molecules for cellular signaling, whilst their accumulation has been found to be cytotoxic as they may trigger aberrant modifications of proteins (a process often referred to as carbonylation).

:

A correlation between carbonylation of proteins and human disease progression has been shown in ageing, diabetes, obesity, chronic renal failure, neurodegeneration and cardiovascular disease. However, the fate of reactive carbonyl species is still far from being understood, especially concerning the mechanisms responsible for their disposal as well as the importance of this in disease progression.

:

In this context, some data have been published on phase I and phase II deactivation of reactive carbonyl species. In the case of phase II mechanisms, the route involving glutathione conjugation and subsequent disposal of the adducts has been extensively studied both in vitro and in vivo for some of the more representative compounds, e.g. 4-hydroxynonenal.

:

There is also emerging evidence of an involvement of carnosine as an endogenous alternative to glutathione for phase II conjugation. However, the fate of carnosine conjugates is still poorly investigated and, unlike glutathione, there is little evidence of the formation of carnosine adducts in vivo. The acquisition of such data could be of importance for the development of new drugs, since carnosine and its derivatives have been proposed as potential therapeutic agents for the mitigation of carbonylation associated with disease progression.

:

Herein, we wish to review our current knowledge of the binding of reactive carbonyl species with carnosine together with the disposal of carnosine conjugates, emphasizing those aspects still requiring investigation such as conjugation reversibility and enzyme assisted catalysis of the reactions.

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.

Dietary methionine level alters growth, digestibility, and gene expression of amino acid transporters in meat-type chickens

Imbalance in nutrients can affect digestibility of amino acids by altering gene expression of amino acid transporters. We investigated digestibility and molecular transporters of essential amino acids in chickens fed a methionine-deficient diet. A total of 40 chicks (23 D old) were randomly assigned to either a control (0.49% methionine) or a deficient (0.28%) diet until 41 D when they were sampled for Pectoralis (P.) major, kidney, ileum, and hypothalamus for mRNA expression analysis. The ileal content was collected for apparent ileal digestibility (AID) analysis. Birds fed the deficient diet had reduced growth and worse feed efficiency compared to control. The AID of methionine was similar between both groups. The AID of other essential amino acids was higher in the deficient group than control. mRNA expression of b0,+ AT and LAT4 were upregulated in the ileum and kidney but LAT1 was downregulated only in kidney of the deficient group compared to control. In the P. major, SNAT1, SNAT2, and CAT1 were upregulated in the deficient group compared to control. A diet deficiency in methionine affects digestibility of essential amino acids and cysteine, but not the digestibility of methionine. The change in digestibility is reflected in the mRNA expression of amino acid transporters across different tissues.