Large Neutral Amino Acid Transporter Research Articles

CD98hc (SLC3A2) loss protects against ras-driven tumorigenesis by modulating integrin-mediated mechanotransduction.

CD98hc (SLC3A2) is the heavy chain component of the dimeric transmembrane glycoprotein CD98, which comprises the large neutral amino acid transporter LAT1 (SLC7A5) in cells. Overexpression of CD98hc occurs widely in cancer cells and is associated with poor prognosis clinically, but its exact contributions to tumorigenesis are uncertain. In this study, we showed that genetic deficiency of CD98hc protects against Ras-driven skin carcinogenesis. Deleting CD98hc after tumor induction was also sufficient to cause regression of existing tumors. Investigations into the basis for these effects defined two new functions of CD98hc that contribute to epithelial cancer beyond an intrinsic effect of CD98hc on tumor cell proliferation. First, CD98hc increased the stiffness of the tumor microenvironment. Second, CD98hc amplified the capacity of cells to respond to matrix rigidity, an essential factor in tumor development. Mechanistically, CD98hc mediated this stiffness sensing by increasing Rho kinase (ROCK) activity, resulting in increased transcription mediated by YAP/TAZ, a nuclear relay for mechanical signals. Our results suggest that CD98hc contributes to carcinogenesis by amplifying a positive feedback loop, which increases both extracellular matrix stiffness and resulting cellular responses. This work supports a rationale to explore the use of CD98hc inhibitors as cancer therapeutics.

84. Targeting the kynurenine blood-to-brain transport system to treat inflammation-induced fatigue and depression

Inflammation is associated with activation of the tryptophan-kynurenine metabolism pathway. Circulating kynurenine is transported into the brain at the vascular blood–brain barrier (BBB) by the large neutral amino-acid transporter LAT-1. LAT-1 transports tryptophan and kynurenine. To determine how LAT-1 responds to inflammation, we used mouse microvascular endothelial cells that form tight junctions in vitro and constitutively express the tryptophan degrading enzyme indoleamine 2,3-dioxygenase (IDO) and LAT-1. We first showed that addition of lipopolysaccharide and interferon-γ to the culture significantly increased endothelial cell IDO mRNA and the transport capacity of LAT-1 as measured with tryptophan from the luminal to the abluminal side of an in vitro model of the BBB. Before using leucine as a competitive inhibitor of LAT-1-mediated transport of kynurenine, we determined whether leucine can block the transport of tryptophan across BBB endothelial cells. Whereas leucine had no effect on tryptophan in the untreated BBB, it inhibited a portion of the enhanced transport of tryptophan conferred by pretreatment with lipopolysaccharide and interferon-γ. Consistent with this ability to block LPS-mediated brain uptake of tryptophan, leucine (administered 500 mg/kg IP before and 6 h after lipopolysaccharide) was also effective in abrogating lipopolysaccharide-induced decreases in locomotor activity and sucrose preference. Experiments are under way to assess the effect of leucine on kynurenine transport into the brain both in vivo and in vitro .

Synthesis and biological evaluation of (18)F-labeled Fluoroethoxy tryptophan analogues as potential PET tumor imaging agents.

As a continuation of our research efforts toward the development of tryptophan-based radiotracers for tumor imaging with positron emission tomography (PET), three new fluoroethoxy tryptophan analogues were synthesized and evaluated in vivo. These new tracers (namely, 4-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]4-FEHTP), 6-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]6-FEHTP), and 7-(2-[(18)F]fluoroethoxy)-dl-tryptophan ([(18)F]7-FEHTP) carry the fluoroethoxy side chain either at positions 4-, 6-, or 7- of the indole core. Reference compounds and precursors were synthesized by multistep approaches. Radiosynthesis was accomplished by no-carrier-added nucleophilic (18)F-fluorination following either an indirect approach (O-alkylation of the corresponding hydroxytryptophan with [(18)F]fluoroethyltosylate) or a direct approach (nucleophilic [(18)F] fluorination using a protected mesyl precursor). Radiochemical yields (decay corrected) for both methods were in the range of 10-18%. Small animal PET imaging with xenograft-bearing mice revealed the highest tumor/background ratio for [(18)F]6-FEHTP which, in a direct comparison, outperformed the other two tryptophan tracers and also the well-established tyrosine analogue O-(2-[(18)F]fluoroethyl)-l-tyrosine ([(18)F]l-FET). Investigation of the transport mechanism of [(18)F]6-FEHTP in small cell lung cancer cells (NCI-H69) revealed that it is most probably taken up exclusively via the large neutral amino acid transporter(s) (LAT).

Diagnostic Potential of miR-126, miR-143, miR-145, and miR-652 in Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is difficult to distinguish from reactive mesothelial proliferations (RMPs). It is uncertain whether miRNAs are useful biomarkers for differentiating MPM from RMPs. Thus, we screened with a quantitative RT-PCR (RT-qPCR)-based platform the expression of 742 miRNAs in formalin-fixed, paraffin-embedded, preoperative diagnostic biopsy samples, surgically resected MPM specimens previously treated with chemotherapy, and corresponding non-neoplastic pleura (NNP), from five patients. miR-126, miR-143, miR-145, and miR-652 were significantly down-regulated (≥twofold) in resected MPM and/or chemotherapy-naïve diagnostic tumor biopsy samples. The miRNA expression pattern was validated by RT-qPCR in a cohort of 40 independent MPMs. By performing binary logistic regression on the RT-qPCR data for the four miRNAs, the established four-miRNA classifier differentiated MPM from NNP with high sensitivity and specificity (area under the curve, 0.96; 95% CI, 0.92-1.00). The classifier's optimal logit(P) value of 0.62 separated NNP and MPM samples with a sensitivity of 0.95 (95% CI, 0.89-1.00), a specificity of 0.93 (95% CI, 0.87-0.99), and an overall accuracy of 0.94 (95% CI, 0.88-1.00). The level of miR-126 in MPM was inversely correlated with that of the known target, the large neutral amino acid transporter, small subunit 1 (r = -0.38; 95% CI, -0.63 to -0.06). Overall, these results indicate that these four miRNAs may be suitable biomarkers for distinguishing MPM from RMPs.

Immunohistochemical expression of carcinoembryonic antigen-related cell adhesion molecules 5, CEACAM6, and SLC7A5: Do they aid in predicting the response to neo-adjuvant chemotherapy in locally advanced breast cancer?

Context: Neo-adjuvant chemotherapy (NACT) has become an integral part of multimodality treatment for locally advanced breast cancer (LABC) worldwide. Predictors of therapeutic response to NACT are lacking. Whether carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) like CEACAM5 and CEACAM6 can act as a predictor of response to therapy is unclear. SLC7A5 gene in humans encodes a large neutral amino acid transporter protein, which has an essential role in tumor cell growth and survival. Materials and Methods: Thirty histopathologically proven cases of LABC, being given NACT, were included in the study. Immunohistochemical examination of the tumor sections was performed for CEACAM5, CEACAM6, and SLC7A5. Response to chemotherapy was assessed using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria. A total of three cycles were given at 3 weekly intervals. After 3 weeks of the last cycle of NACT, the patients were taken up for modified radical mastectomy. The specimen was subjected to histopathological examination. The immunohistochemical results were correlated with response to NACT based on RECIST criteria and histopathology. Results: 12/30 (40%) of the patients had objective clinical response of which 4 (13.33%) patients had pathological complete response. The relationship between CEACAM5 and CEACAM6 and response to NACT was found to be statistically significant, P = 0.004 and P = 0.020, respectively. Furthermore, relationship between response to NACT and node-positive tumors with SLC7A5 immunoreactivity was found to be highly significant (P = 0.009). Conclusion: Biomarkers (CEACAM5, CEACAM6, and SLC7A5) showed promise as predictors of poor response to NACT and can help plan an alternative regime in likely nonresponders to prevent the toxicity of chemotherapy and also in tailoring the therapy in a patient with LABC.

Down‐regulation of placental mTOR, insulin/IGF‐I signaling, and nutrient transporters in response to maternal nutrient restriction in the baboon

The mechanisms by which maternal nutrient restriction (MNR) causes reduced fetal growth are poorly understood. We hypothesized that MNR inhibits placental mechanistic target of rapamycin (mTOR) and insulin/IGF-I signaling, down-regulates placental nutrient transporters, and decreases fetal amino acid levels. Pregnant baboons were fed control (ad libitum, n=11) or an MNR diet (70% of controls, n=11) from gestational day (GD) 30. Placenta and umbilical blood were collected at GD 165. Western blot was used to determine the phosphorylation of proteins in the mTOR, insulin/IGF-I, ERK1/2, and GSK-3 signaling pathways in placental homogenates and expression of glucose transporter 1 (GLUT-1), taurine transporter (TAUT), sodium-dependent neutral amino acid transporter (SNAT), and large neutral amino acid transporter (LAT) isoforms in syncytiotrophoblast microvillous membranes (MVMs). MNR reduced fetal weights by 13%, lowered fetal plasma concentrations of essential amino acids, and decreased the phosphorylation of placental S6K, S6 ribosomal protein, 4E-BP1, IRS-1, Akt, ERK-1/2, and GSK-3. MVM protein expression of GLUT-1, TAUT, SNAT-2 and LAT-1/2 was reduced in MNR. This is the first study in primates exploring placental responses to maternal undernutrition. Inhibition of placental mTOR and insulin/IGF-I signaling resulting in down-regulation of placental nutrient transporters may link maternal undernutrition to restricted fetal growth.

Synthesis and biological evaluation of 18F-labeled fluoropropyl tryptophan analogs as potential PET probes for tumor imaging

In the search for an efficient, fluorine-18 labeled amino acid based radiotracer for tumor imaging with positron emission tomography (PET), two new tryptophan analogs were synthesized and characterized in vitro and in vivo. Both are tryptophan alkyl-derivatives, namely 2-(3-[18F]fluoropropyl)-dl-tryptophan ([18F]2-FPTRP) and 5-(3-[18F]fluoro-propyl)-dl-tryptophan ([18F]5-FPTRP). Standard reference compounds and precursors were prepared by multi step approaches. Radiosynthesis was achieved by no-carrier-added nucleophilic [18F]fluorination in 29–34% decay corrected yields with radiochemical purity over 99%. In vitro cell uptake assays showed that both compounds are substrates for amino acid transport and enter small cell lung cancer cells (NCI-H69) most probably almost exclusively via large neutral amino acids transporter(s) (LAT). Small animal PET imaging with xenograft bearing mice revealed high tumor/background ratios for [18F]2-FPTRP comparable to the well established tyrosine analog O-(2-[18F]fluroethyl)-l-tyrosine ([18F]FET). Radiometabolite studies showed no evidence of involvement of a biotransformation step in tumor accumulation.

Functional characterization and molecular expression of large neutral amino acid transporter (LAT1) in human prostate cancer cells [Int. J. Pharm. 443 (2013) 245–253

Functional characterization and molecular expression of large neutral amino acid transporter (LAT1) in human prostate cancer cells [Int. J. Pharm. 443 (2013) 245–253

Involvement of AAT transporters in methylmercury toxicity in Caenorhabditis elegans

Methylmercury (MeHg) is a potent neurotoxin that enters mammalian cells as a conjugate with l-cysteine through L-type large neutral amino acid transporter, LAT1, by a molecular mimicry mechanism by structurally resembling l-methionine. Caenorhabditis elegans (C. elegans) has been increasingly used to study the neurotoxic effects of MeHg, but little is known about uptake and transport of MeHg in the worm. This study examined whether MeHg uptake through LAT1 is evolutionarily conserved in nematodes. MeHg toxicity in C. elegans was blocked by pre-treatment of worms with l-methionine, suggesting a role for amino acid transporters in MeHg transport. Knockdown of aat-1, aat-2, and aat-3, worm homologues to LAT1, increased the survival of C. elegans following MeHg treatment and significantly attenuated MeHg content following exposure. These results indicate that MeHg is transported in the worm by a conserved mechanism dependent on functioning amino acid transporters.

4F2hc-silencing impairs tumorigenicity of HeLa cells via modulation of galectin-3 and β-catenin signaling, and MMP-2 expression

4F2hc is a type-II glycoprotein whose covalent-bound association with one of several described light chains yields a heterodimer mainly involved in large neutral amino acid transport. Likewise, it is well known that the heavy chain interacts with β-integrins mediating integrin-dependent events such as survival, proliferation, migration and even transformation. 4F2hc is a ubiquitous protein whose overexpression has been related to tumor development and progression. Stable silencing of 4F2hc in HeLa cells using an artificial miRNA impairs in vivo tumorigenicity and leads to an ineffective proliferation response to mitogens. 4F2hc colocalizes with β1-integrins and CD147, but this interaction does not occur in lipid rafts in HeLa cells. Moreover, silenced cells present defects in integrin- (FAK, Akt and ERK1/2) and hypoxia-dependent signaling, and reduced expression/activity of MMP-2. These alterations seem to be dependent on the inappropriate formation of CD147/4F2hc/β1-integrin heterocomplexes on the cell surface, arising when CD147 cannot interact with 4F2hc. Although extracellular galectin-3 accumulates due to the decrease in MMP-2 activity, galectin-3 signaling events are blocked due to an impaired interaction with 4F2hc, inducing an increased degradation of β-catenin. Furthermore, cell motility is compromised after protein silencing, suggesting that 4F2hc is related to tumor invasion by facilitating cell motility. Therefore, here we propose a molecular mechanism by which 4F2hc participates in tumor progression, favoring first steps of epithelial–mesenchymal transition by inhibition of β-catenin proteasomal degradation through Akt/GSK-3β signaling and enabling cell motility.

Structure-based ligand discovery for the Large-neutral Amino Acid Transporter 1, LAT-1

The Large-neutral Amino Acid Transporter 1 (LAT-1)--a sodium-independent exchanger of amino acids, thyroid hormones, and prescription drugs--is highly expressed in the blood-brain barrier and various types of cancer. LAT-1 plays an important role in cancer development as well as in mediating drug and nutrient delivery across the blood-brain barrier, making it a key drug target. Here, we identify four LAT-1 ligands, including one chemically novel substrate, by comparative modeling, virtual screening, and experimental validation. These results may rationalize the enhanced brain permeability of two drugs, including the anticancer agent acivicin. Finally, two of our hits inhibited proliferation of a cancer cell line by distinct mechanisms, providing useful chemical tools to characterize the role of LAT-1 in cancer metabolism.

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

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

Structure–activity relationship study of compounds binding to large amino acid transporter 1 (LAT1) based on pharmacophore modeling and in situ rat brain perfusion

Large neutral amino acid transporter 1 (LAT1) is predominantly expressed at the blood–brain barrier and it has a major role in transporting neutral amino acids into the brain. LAT1 has the potential to function as a drug carrier for improved drug brain delivery which makes it an intriguing target protein for central nervous system disorders, e.g., Alzheimer’s disease, Parkinson’s disease and brain tumors. In this study, a 3D pharmacophore was generated for a set of LAT1 substrates whose binding affinities were studied using competitive inhibition of the brain uptake of [14C]-l-leucine with an in situ rat brain perfusion method. The pharmacophore highlights the most important molecular features shared by efficient LAT1-binding compounds and elucidates their 3D-arrangement in detail. This clarifies the structure–activity relationships of LAT1 substrates and provides insights for making a binding hypothesis. The results can be further applied in the design of novel efficient LAT1 substrates.

An Immortalized Human Blood-Nerve Barrier Endothelial Cell Line for In Vitro Permeability Studies

Solute and macromolecular transport studies may elucidate nutritional requirements and drug effects in healthy and diseased peripheral nerves. Endoneurial endothelial cells are specialized microvascular cells that form the restrictive blood-nerve barrier (BNB). Primary human endoneurial endothelial cells (pHEndECs) are difficult to isolate, limiting their widespread availability for biomedical research. We developed a simian virus-40 large T-antigen (SV40-LTA) immortalized human BNB cell line via stable transfection of low passage pHEndECs and observed continuous growth in culture for >45 population doublings. As observed with pHEndECs, the immortalized BNB endothelial cells were Ulex Europaeus agglutinin-1-positive and endocytosed low density lipoprotein, but lost von Willebrand factor expression. Glucose transporter-1, P-glycoprotein (P-gp), γ-glutamyl transpeptidase (γ-GT), large neutral amino acid transporter-1 (LAT-1), creatine transporter (CRT), and monocarboxylate transporter-1 (MCT-1) mRNA expression were retained at all passages with loss of alkaline phosphatase (AP) expression after passages 16-20. Compared with an SV40-LTA immortalized human blood-brain barrier endothelial cell line, there was increased γ-GT protein expression, equivalent expression of organic anion transporting polypeptide-C (OATP-C), organic anion transporter 3 (OAT-3), MCT-1, and LAT-1, and reduced expression of AP, CRT, and P-gp by the BNB cell line at passage 20. Further studies demonstrated lower transendothelial electrical resistance (~181 vs. 191 Ω cm(2)), equivalent permeability to fluoresceinated sodium (4.84 vs. 4.39 %), and lower permeability to fluoresceinated high molecular weight (70 kDa) dextran (0.39 vs. 0.52 %) by the BNB cell line. This cell line retained essential molecular and biophysical properties suitable for in vitro peripheral nerve permeability studies.

Abstract 4543: Uptake of 18F-FDOPA analogue 3H-L-DOPA is mediated by large neutral amino acid transporter LAT1 in human glioblastoma

Abstract INTRODUCTION: The L-Type amino acid transporter 1 (LAT1) transports large neutral amino acids across the plasma membrane in a variety of cells. Positron emission tomography (PET) imaging with the amino acid tracer 6-18F-fluoro-L-dopa (18F-FDOPA) provides useful histological information in human gliomas in addition to providing better localizing information then CT or MRI alone. This study is the first to correlate LAT1 expression with uptake of the 18F-FDOPA analog, 3H-L-DOPA in human glioblastomas. METHODS: Endogenous expression of LAT1 was evaluated in two established glioblastoma multiforme (GBM) cell lines and three primary GBM xenografts using Western blotting and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Uptake of FDOPA was assessed in vitro using 3H-L-DOPA as an analog to 18F-FDOPA. Knockdown of LAT1 expression was achieved by lentiviral expression of anti-LAT1 shRNA in GBM lines with high LAT1 expression. The effect of LAT1 knockdown on 3H-L-DOPA uptake was then quantified. RESULTS: Results from Western blotting and qRT-PCR demonstrate that LAT1 is variably expressed in GBM. Levels of 3H-L-DOPA uptake were directly related to LAT1 expression (p<0.001) across the 5 lines. Knockdown of LAT1 was achieved in T98 cells (knockdown efficiency = 82.8%) and xenograft GBM 28 (knockdown efficiency = 73.7%) as confirmed by qRT-PCR and Western blotting. Knockdown of LAT1 reduced 3H-L-DOPA uptake by 57.2% (p<0.0001) in T98 and 52.1% in GBM 28 (p<0.001). CONCLUSIONS: This is the first report of 3H-L-DOPA uptake being driven by LAT1 in human GBM. Our findings suggest LAT1 as a transporter of 18F-FDOPA in GBM. Additional studies are underway to correlate LAT1 expression with PET 18F-FDOPA uptake in patients with GBM. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4543. doi:1538-7445.AM2012-4543

Primary and secondary thyroid hormone transporters

Thyroid hormones (TH) are essential for the development of the human brain, growth and cellular metabolism. Investigation of TH transporters became one of the emerging fields in thyroid research after the discovery of inactivating mutations in the Monocarboxylate transporter 8 (MCT8), which was found to be highly specific for TH transport. However, additional transmembrane transporters are also very important for TH uptake and efflux in different cell types. They transport TH as secondary substrates and include the aromatic amino acid transporting MCT10, the organic anion transporting polypeptides (e.g. OATP1C1, OATP1A2, OPTP1A4) and the large neutral amino acid transporters (LAT1 and LAT2). These TH transporters characteristically possess 12 transmembrane spanners but due to the strong differing sequences between the three transporter families we assume an identical conformation is not very likely. In contrast to the others, the LAT family members form a heterodimer with the escort protein 4F2hc/CD98. A comparison of sequence proportions, locations and types of functional sensitive features for TH transport discovered by mutations, revealed that transport sensitive charged residues occur as conserved amino acids only within each family of the transporter types but not in all putative TH transporters. Based on the lack of highly conserved sensitive charged residues throughout the three transporter families as a common counterpart for the amino acid moiety of the substrates, we conclude that the molecular transport mechanism is likely organized either a) by different molecular determinants in the divergent transporter types or b) the counterparts for the substrates` amino acid moiety at the transporter are not any charged side chains but other proton acceptors or donators. However, positions of transport sensitive residues coincide at transmembrane helix 8 in the TH transporter MCT8, OATP1C1 and another amino acid transporter, the L-cystine and L-glutamate exchanger xCT, which is highly homologous to LAT1 and LAT2. Here we review the data available and compare similarities and differences between these primary and secondary TH transporters regarding sequences, topology, potential structures, trafficking to the plasma membrane, molecular features and locations of transport sensitive functionalities. Thereby, we focus on TH transporters occurring in the blood-brain barrier.

Maternal Protein Restriction in the Rat Inhibits Placental Insulin, mTOR, and STAT3 Signaling and Down-Regulates Placental Amino Acid Transporters

The mechanisms underlying reduced fetal growth in response to maternal protein restriction are not well established. Maternal levels of insulin, IGF-I, and leptin are decreased in rats fed a low protein (LP) diet. Because these hormones stimulate placental amino acid transporters in vitro, we hypothesized that maternal protein restriction inhibits placental leptin, insulin/IGF-I, and mammalian target of rapamycin signaling and down-regulates the expression and activity of placental amino acid transporters. Pregnant rats were fed either an isocaloric low protein (LP, 4% protein) or control diet (18% protein) and studied at gestational day (GD)15, GD19, or GD21 (term 23). At GD19 and GD21, placental expression of phosphorylated eukaryotic initiation factor 4E binding protein 1 (Thr-36/46 or Thr-70) and phosphorylated S6 ribosomal protein (Ser-235/236) was decreased in the LP group. In addition, placental expression of phosphorylated S6 kinase 1 (Thr-389), phosphorylated Akt (Thr-308), and phosphorylated signal transducer and activator of transcription 3 (Tyr-705) was reduced at GD21. In microvillous plasma membranes (MVM) isolated from placentas of LP animals, protein expression of the sodium-coupled neutral amino acid transporter (SNAT)2 and the large neutral amino acid transporters 1 and 2 was reduced at GD19 and GD21. MVM SNAT1 protein expression was reduced at GD21 in LP rats. SNAT4 and 4F2 heavy chain expression in MVM was unaltered. System A and L amino acid transporter activity was decreased in MVM from LP animals at GD19 and GD21. In conclusion, maternal protein restriction inhibits placental insulin, mammalian target of rapamycin signaling, and signal transducer and activator of transcription 3 signaling, which is associated with a down-regulation of placental amino acid transporters. We speculate that maternal endocrine and metabolic control of placental nutrient transport reduces fetal growth in response to protein restriction.

Brain uptake of ketoprofen–lysine prodrug in rats

The blood–brain barrier (BBB) controls the entry of xenobiotics into the brain. Often the development of central nervous system drugs needs to be terminated because of their poor brain uptake. We describe a way to achieve large neutral amino acid transporter (LAT1)-mediated drug transport into the rat brain. We conjugated ketoprofen to an amino acid l-lysine so that the prodrug could access LAT1. The LAT1-mediated brain uptake of the prodrug was demonstrated with in situ rat brain perfusion technique. The ability of the prodrug to deliver ketoprofen into the site of action, the brain intracellular fluid, was determined combining in vivo and in vitro experiments. A rapid brain uptake from blood and cell uptake was seen both in in situ and in vivo experiments. Therefore, our results show that a prodrug approach can achieve uptake of drugs via LAT1 into the brain intracellular fluid. The distribution of the prodrug in the brain parenchyma and the site of parent drug release in the brain were shown with in vivo and in vitro studies. In addition, our results show that although lysine or ketoprofen are not LAT1-substrates themselves, by combining these molecules, the formed prodrug has affinity for LAT1.

Small Neutral Amino Acid Ester Prodrugs of Acyclovir Targeting Amino Acid Transporters on the Cornea: Possible Antiviral Agents Against Ocular HSV-1 Infections.

The aim of this study was to characterize the affinity and permeability patterns of the amino acid ester prodrugs of acyclovir (ACV), L-alanine-ACV (AACV), L-serine-ACV (SACV), L-serine-succinate-ACV (SSACV) and L-cysteine-ACV (CACV) on rabbit primary corneal epithelial cell culture (rPCEC) and on rabbit cornea. Amino acid prodrugs of acyclovir, AACV, SACV, SSACV and CACV were synthesized in our laboratory. Chemical hydrolysis in aqueous buffer, enzymatic hydrolysis in corneal homogenates and transport across freshly excised rabbit cornea of these prodrugs were studied. SSACV inhibited the uptake of [(3)H] L-alanine on rPCEC and across the intact rabbit cornea. Lineweaver-Burk plot transformation revealed competitive inhibition between L-alanine and SSACV. In corneal tissue homogenate, the half lives of SSACV, SACV and CACV (t1/2) were observed to be 3.5 ± 0.4, 9.2 ± 0.6 and 1.8 ± 0.1 hr respectively, whereas AACV was readily converted to the active parent drug acyclovir exhibiting complete degradation before 5 min. Interestingly translocation of SACV across cornea was inhibited in the presence of 5 mM arginine (~51%), a specific substrate for cationic transport system and in presence of BCH (~38%), a substrate specific for large neutral amino acid transport system (LAT) or cationic and neutral amino acid transport system (B(0,+)). SACV exhibited higher permeability across cornea along with excellent antiviral activity against herpes simplex virus (HSV-1) and varicella-zoster virus (VZV) in comparison to ACV. Recognition by multiple transporters, stability in corneal homogenate and changes in physico-chemical properties contributed to the increased permeability of SACV across cornea.

Regiospecific and conformationally restrained analogs of melphalan and dl-2-NAM-7 and their affinities for the large neutral amino acid transporter (system LAT1) of the blood–brain barrier

Regiospecific and conformationally restrained analogs of melphalan and dl-2-NAM-7 have been synthesized and their affinities for the large neutral amino acid transporter (LAT1) of the blood–brain barrier have been determined to assess their potential for accessing the CNS via facilitated transport. Several analogs had Ki values in the range 2.1–8.5μM with greater affinities than that of either l-phenylalanine (Ki=11μM) or melphalan (Ki=55μM), but lower than dl-2-NAM-7 (Ki=0.08μM). The results indicate that regiospecific positioning of the mustard moiety on the aromatic ring in these analogs is very important for optimal affinity for the large neutral amino acid transporter, and that conformational restriction of the dl-2-NAM-7 molecule in benzonorbornane and indane analogs leads to 25- to 60-fold loss, respectively, in affinity.