Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime?

Functional analyse of GLUT1 and GLUT12 in glucose uptake in goat mammary gland epithelial cells

Glucose transport, mediated by glucose transporters, is necessary for mammary gland development and lactation. GLUT1 and GLUT12 could both be expressed in the pregnant and lactating mammary gland to participate in the glucose uptake process. In this study, the goat GLUT1 and GLUT12 genes were cloned from Saanen dairy goats and transfected into goat mammary gland epithelial cells to assess their biological functions and distributions. The results showed that both goat GLUT1 and GLUT12 had 12 predicted membrane-spanning helices. Goat GLUT1 and GLUT12 each influenced the mRNA expression of the other transporter and increased the glucose consumption and lactose yield in GLUT1- and GLUT12-transfected goat mammary gland epithelial cells, respectively. The overexpression of GLUT1 or GLUT12 also increased the expression of amino acid transporters SLC1A5, SLC3A2 and SLC7A5 and affected genes expressions in GMGE cells. Using immunofluorescence staining, GLUT1 was detected throughout the cytoplasm and localized to the Golgi apparatus around the nuclear membrane, whereas GLUT12 was mainly distributed in the perinuclear region and cytoplasm. This study contributes to the understanding of how GLUT1 and GLUT12 cooperate in the incorporation of nutrient uptake into mammary gland epithelial cells and the promotion of milk synthesis in the goat mammary gland during lactation.

Tumor VEGF:VEGFR2 autocrine feed-forward loop triggers angiogenesis in lung cancer

The molecular mechanisms that control the balance between antiangiogenic and proangiogenic factors and initiate the angiogenic switch in tumors remain poorly defined. By combining chemical genetics with multimodal imaging, we have identified an autocrine feed-forward loop in tumor cells in which tumor-derived VEGF stimulates VEGF production via VEGFR2-dependent activation of mTOR, substantially amplifying the initial proangiogenic signal. Disruption of this feed-forward loop by chemical perturbation or knockdown of VEGFR2 in tumor cells dramatically inhibited production of VEGF in vitro and in vivo. This disruption was sufficient to prevent tumor growth in vivo. In patients with lung cancer, we found that this VEGF:VEGFR2 feed-forward loop was active, as the level of VEGF/VEGFR2 binding in tumor cells was highly correlated to tumor angiogenesis. We further demonstrated that inhibition of tumor cell VEGFR2 induces feedback activation of the IRS/MAPK signaling cascade. Most strikingly, combined pharmacological inhibition of VEGFR2 (ZD6474) and MEK (PD0325901) in tumor cells resulted in dramatic tumor shrinkage, whereas monotherapy only modestly slowed tumor growth. Thus, a tumor cell-autonomous VEGF:VEGFR2 feed-forward loop provides signal amplification required for the establishment of fully angiogenic tumors in lung cancer. Interrupting this feed-forward loop switches tumor cells from an angiogenic to a proliferative phenotype that sensitizes tumor cells to MAPK inhibition.

Voltage-dependent processes in the electroneutral amino acid exchanger ASCT2

Neutral amino acid exchange by the alanine serine cysteine transporter (ASCT)2 was reported to be electroneutral and coupled to the cotransport of one Na⁺ ion. The cotransported sodium ion carries positive charge. Therefore, it is possible that amino acid exchange is voltage dependent. However, little information is available on the electrical properties of the ASCT2 amino acid transport process. Here, we have used a combination of experimental and computational approaches to determine the details of the amino acid exchange mechanism of ASCT2. The [Na⁺] dependence of ASCT2-associated currents indicates that the Na⁺/amino acid stoichiometry is at least 2:1, with at least one sodium ion binding to the amino acid–free apo form of the transporter. When the substrate and two Na⁺ ions are bound, the valence of the transport domain is +0.81. Consistently, voltage steps applied to ASCT2 in the fully loaded configuration elicit transient currents that decay on a millisecond time scale. Alanine concentration jumps at the extracellular side of the membrane are followed by inwardly directed transient currents, indicative of translocation of net positive charge during exchange. Molecular dynamics simulations are consistent with these results and point to a sequential binding process in which one or two modulatory Na⁺ ions bind with high affinity to the empty transporter, followed by binding of the amino acid substrate and the subsequent binding of a final Na⁺ ion. Overall, our results are consistent with voltage-dependent amino acid exchange occurring on a millisecond time scale, the kinetics of which we predict with simulations. Despite some differences, transport mechanism and interaction with Na⁺ appear to be highly conserved between ASCT2 and the other members of the solute carrier 1 family, which transport acidic amino acids.

18F-AFETP, 18F-FET, and 18F-FDG imaging of mouse DBT gliomas

The goal of this study was to evaluate the (18)F-labeled nonnatural amino acid (S)-2-amino-3-[1-(2-(18)F-fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid ((18)F-AFETP) as a PET imaging agent for brain tumors and to compare its effectiveness with the more-established tracers O-(2-(18)F-fluoroethyl)-l-tyrosine ((18)F-FET) and (18)F-FDG in a murine model of glioblastoma. The tracer (18)F-AFETP is a structural analog of histidine and is a lead compound for imaging cationic amino acid transport, a relatively unexplored target for oncologic imaging.

METHODS

(18)F-AFETP was prepared using the click reaction. BALB/c mice with intracranially implanted delayed brain tumor (DBT) gliomas (n = 4) underwent biodistribution and dynamic small-animal PET imaging for 60 min after intravenous injection of (18)F-AFETP. Tumor and brain uptake of (18)F-AFETP were compared with those of (18)F-FDG and (18)F-FET through small-animal PET analyses.

RESULTS

(18)F-AFETP demonstrated focally increased uptake in tumors with good visualization. Peak tumor uptake occurred within 10 min of injection, with stable or gradual decrease over time. All 3 tracers demonstrated relatively high uptake in the DBTs throughout the study. At late time points (47.5-57.5 min after injection), the average standardized uptake value with (18)F-FDG (1.9 ± 0.1) was significantly greater than with (18)F-FET (1.1 ± 0.1) and (18)F-AFETP (0.7 ± 0.2). The uptake also differed substantially in normal brain, with significant differences in the standardized uptake values at late times among (18)F-FDG (1.5 ± 0.2), (18)F-FET (0.5 ± 0.05), and (18)F-AFETP (0.1 ± 0.04). The resulting average tumor-to-brain ratio at the late time points was significantly higher for (18)F-AFETP (7.5 ± 0.1) than for (18)F-FDG (1.3 ± 0.1) and (18)F-FET (2.0 ± 0.3).

CONCLUSION

(18)F-AFETP is a promising brain tumor imaging agent, providing rapid and persistent tumor visualization, with good tumor-to-normal-brain ratios in the DBT glioma model. High tumor-to-brain, tumor-to-muscle, and tumor-to-blood ratios were observed at 30 and 60 min after injection, with higher tumor-to-brain ratios than obtained with (18)F-FET or (18)F-FDG. These results support further development and evaluation of (18)F-AFETP and its derivatives for tumor imaging.

Renal amino acid transport systems and essential hypertension

Several clinical and animal studies suggest that "blood pressure goes with the kidney," that is, a normotensive recipient of a kidney genetically programmed for hypertension will develop hypertension. Intrarenal dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport. The candidate transport systems for L-DOPA, the source for dopamine, include the sodium-dependent systems B(0), B(0,+), and y(+)L, and the sodium-independent systems L (LAT1 and LAT2) and b(0,+). Renal LAT2 is overexpressed in the prehypertensive spontaneously hypertensive rat (SHR), which might contribute to enhanced L-DOPA uptake in the proximal tubule and increased dopamine production, as an attempt to overcome the defect in D1 receptor function. On the other hand, it has been recently reported that impaired arginine transport contributes to low renal nitric oxide bioavailability observed in the SHR renal medulla. Here we review the importance of renal amino acid transporters in the kidney and highlight pathophysiological changes in the expression and regulation of these transporters in essential hypertension. The study of the regulation of renal amino acid transporters may help to define the underlying mechanisms predisposing individuals to an increased risk for development of hypertension.

Predominant contribution of L-type amino acid transporter to 4-borono-2-(18)F-fluoro-phenylalanine uptake in human glioblastoma cells

INTRODUCTION

4-Borono-2-(18)F-fluoro-phenylalanine ((18)F-FBPA) has been used to anticipate the therapeutic effects of boron neutron capture therapy (BNCT) with 4-borono-L-phenylalanine (BPA). Similarly, L-[methyl-(11)C]-methionine ((11)C-MET), the most popular amino acid PET tracer, is a possible candidate for this purpose. We investigated the transport mechanism of (18)F-FBPA and compared it with that of (14)C-MET in human glioblastoma cell lines.

METHODS

Uptake of (18)F-FBPA and (14)C-MET was examined in A172, T98G, and U-87MG cells using 2-aminobicyclo-(2.2.1)-heptane-2-carboxylic acid (a system L-specific substrate), 2-(methylamino)-isobutyric acid (a system A-specific substrate), and BPA. Gene expression was analyzed by quantitative real time polymerase chain reaction.

RESULTS

System L was mainly involved in the uptake of (18)F-FBPA (74.5%-81.1% of total uptake) and (14)C-MET (48.3%-59.4%). System A and ASC also contributed to the uptake of (14)C-MET. Inhibition experiments revealed that BPA significantly decreased the uptake of (18)F-FBPA, whereas 31%-42% of total (14)C-MET uptake was transported by BPA non-sensitive transporters. In addition, (18)F-FBPA uptake correlated with LAT1 and total LAT expressions.

CONCLUSION

This study demonstrated that (18)F-FBPA was predominantly transported by system L in human glioblastoma cells compared to (14)C-MET. Although further studies are needed to elucidate the correlation between (18)F-FBPA uptake and BPA content in tumor tissues, (18)F-FBPA is suitable for the selection of patients who benefit from BNCT with BPA.

Metabolomic profiling of lung and prostate tumor tissues by capillary electrophoresis time-of-flight mass spectrometry

Metabolic microenvironment of tumor cells is influenced by oncogenic signaling and tissue-specific metabolic demands, blood supply, and enzyme expression. To elucidate tumor-specific metabolism, we compared the metabolomics of normal and tumor tissues surgically resected pairwise from nine lung and seven prostate cancer patients, using capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS). Phosphorylation levels of enzymes involved in central carbon metabolism were also quantified. Metabolomic profiles of lung and prostate tissues comprised 114 and 86 metabolites, respectively, and the profiles not only well distinguished tumor from normal tissues, but also squamous cell carcinoma from the other tumor types in lung cancer and poorly differentiated tumors from moderately differentiated tumors in prostate cancer. Concentrations of most amino acids, especially branched-chain amino acids, were significantly higher in tumor tissues, independent of organ type, but of essential amino acids were particularly higher in poorly differentiated than moderately differentiated prostate cancers. Organ-dependent differences were prominent at the levels of glycolytic and tricarboxylic acid cycle intermediates and associated energy status. Significantly high lactate concentrations and elevated activating phosphorylation levels of phosphofructokinase and pyruvate kinase in lung tumors confirmed hyperactive glycolysis. We highlighted the potential of CE-TOFMS-based metabolomics combined with phosphorylated enzyme analysis for understanding tissue-specific tumor microenvironments, which may lead to the development of more effective and specific anticancer therapeutics.

Nutrient transporters: the Achilles' heel of anabolism

Highly proliferative cells, including cancer cells, require a constant supply of molecular building blocks to support their growth. To acquire substrates such as glucose and amino acids from the extracellular space, dividing cells rely on transporter proteins in the plasma membrane. Numerous studies link transcriptional and post-translational control of nutrient transporter expression with proliferation, highlighting the importance of nutrient transporters in both physiologic and pathologic growth. Here we review recent work that spotlights the crucial role of nutrient transporters in cell growth and proliferation, discuss post-translational mechanisms for coordinating expression of different transporters, and consider the therapeutic potential of targeting these proteins in cancer and other diseases characterized by inappropriate cell division.

Radiosynthesis and biological evaluation of alpha-[F-18]fluoromethyl phenylalanine for brain tumor imaging

OBJECTIVES

Radiolabeled amino acids have proven utility for imaging brain tumors in humans, particularly those that target system L amino acid transport. We have prepared the novel phenylalanine analogue, (FMePhe, 9), as part of an effort to develop new system L tracers that can be prepared in high radiochemical yield through nucleophilic [(18)F]fluorination. The tumor imaging properties of both enantiomers of this new tracer were evaluated through cell uptake, biodistribution and microPET studies in the mouse DBT model of high grade glioma.

METHODS

The non-radioactive form of 9 and the cyclic sulfamidate labeling precursor were prepared from commercially available racemic α-benzylserine. Racemic [(18)F]9 was prepared from the labeling precursor in two steps using standard[(18)F]fluoride nucleophilic reaction conditions followed by acidic deprotection. The individual enantiomers [(18)F]9a and [(18)F]9b were isolated using preparative chiral HPLC. In vitro uptake inhibition assays were performed with each enantiomer using DBT cells. Biodistribution and microPET/CT studies were performed with each enantiomer in male BALB/c mice at approximately 2 weeks after implantation of DBT tumor cells.

RESULTS

Radiolabeling of the cyclic sulfamidate precursor 5 provides racemic [(18)F]9 in high radiochemical yield (60%-70%, n=4) and high radiochemical purity (>96%, n=4). In vitro uptake assays demonstrate that both [(18)F]9a and [(18)F]9b undergo tumor cell uptake through system L transport. The biodistribution studies using the single enantiomers [(18)F]9a and [(18)F]9b demonstrated good tumor uptake with lower uptake in most normal tissues, and [(18)F]9a had higher tumor uptake than [(18)F]9b. MicroPET imaging demonstrated good tumor visualization within 10 min of injection, rapid uptake of radioactivity, and tumor to brain ratios of approximately 6:1 at 60 min postinjection.

CONCLUSIONS

The novel PET tracer, [(18)F]FMePhe, is readily synthesized in good yield from a cyclic sulfamidate precursor. Biodistribution and microPET studies in the DBT model demonstrate good tumor to tissue ratios and tumor visualization, with enantiomer [(18)F]9a having higher tumor uptake. However, the brain availability of both enantiomers was lower than expected for system L substrates, suggesting the [(18)F]fluorine group in the β-position affects uptake of these compounds by system L transporters.

Molecular determinants for functional differences between alanine-serine-cysteine transporter 1 and other glutamate transporter family members

The ASCTs (alanine, serine, and cysteine transporters) belong to the solute carrier family 1 (SLC1), which also includes the human glutamate transporters (excitatory amino acid transporters, EAATs) and the prokaryotic aspartate transporter GltPh. Despite the high degree of amino acid sequence identity between family members, ASCTs function quite differently from the EAATs and GltPh. The aim of this study was to mutate ASCT1 to generate a transporter with functional properties of the EAATs and GltPh, to further our understanding of the structural basis for the different transport mechanisms of the SLC1 family. We have identified three key residues involved in determining differences between ASCT1, the EAATs and GltPh. ASCT1 transporters containing the mutations A382T, T459R, and Q386E were expressed in Xenopus laevis oocytes, and their transport and anion channel functions were investigated. A382T and T459R altered the substrate selectivity of ASCT1 to allow the transport of acidic amino acids, particularly l-aspartate. The combination of A382T and T459R within ASCT1 generates a transporter with a similar profile to that of GltPh, with preference for l-aspartate over l-glutamate. Interestingly, the amplitude of the anion conductance activated by the acidic amino acids does not correlate with rates of transport, highlighting the distinction between these two processes. Q386E impaired the ability of ASCT1 to bind acidic amino acids at pH 5.5; however, this was reversed by the additional mutation A382T. We propose that these residues differences in TM7 and TM8 combine to determine differences in substrate selectivity between members of the SLC1 family.

Control of glutamine metabolism by the tumor suppressor Rb

Retinoblastoma (Rb) protein is a tumor suppressor that is dysregulated in a majority of human cancers. Rb functions to inhibit cell cycle progression in part by directly disabling the E2F family of cell cycle-promoting transcription factors. Because the de novo synthesis of multiple glutamine-derived anabolic precursors is required for cell cycle progression, we hypothesized that Rb also may directly regulate proteins involved in glutamine metabolism. We examined glutamine metabolism in mouse embryonic fibroblasts (MEFs) isolated from mice that have triple knock-outs (TKO) of all three Rb family members (Rb-1, Rbl1, and Rbl2) and found that loss of global Rb function caused a marked increase in ¹³C-glutamine uptake and incorporation into glutamate and TCA cycle intermediates in part via upregulated expression of the glutamine transporter ASCT2 and the activity of glutaminase 1 (GLS1). The Rb-controlled transcription factor E2F-3 altered glutamine uptake by direct regulation of ASCT2 mRNA and protein expression, and E2F-3 was observed to associate with the ASCT2 promoter. We next examined the functional consequences of the observed increase in glutamine uptake and utilization and found that glutamine exposure potently increased oxygen consumption whereas glutamine deprivation selectively decreased ATP concentration in the Rb TKO MEFs but not the WT MEFs. In addition, TKO MEFs exhibited elevated production of glutathione from exogenous glutamine, and had increased expression of gamma-glutamylcysteine ligase relative to WT MEFs. Importantly, this metabolic shift towards glutamine utilization was required for the proliferation of Rb TKO MEFs but not for the proliferation of the WT MEFs. Last, addition of the TCA cycle intermediate α-ketoglutarate to the Rb TKO MEFs reversed the inhibitory effects of glutamine deprivation on ATP, GSH levels, and viability. Taken together, these studies demonstrate that the Rb/E2F cascade directly regulates a major energetic and anabolic pathway that is required for neoplastic growth.

L-type amino acid transporter 1 expression is highly correlated with Gleason score in prostate cancer

Upregulation of L-type amino acid transporter 1 (LAT1), a member of the system L amino acid transporter family, may be detected by immunohistochemical methods. Immunoreactive LAT1 expression in prostate cancer is considered to be a promising biomarker for high-grade malignancy. However, the mutual association between LAT1 and Gleason score, the most fixed indicator for grading the malignancy of prostate cancers, remains to be elucidated. The aim of this study was to clarify the correlations between LAT1 and other factors in prostate cancer, including the Gleason score. We evaluated 54 cases of primary prostate cancer, surgically resected without any neoadjuvant therapies and performed immunohistochemistry for LAT1, Ki-67, CD34 and vascular endothelial growth factor on the tissue sections. The Gleason score as well as the age, pathological stage (pStage) of prostate cancer and serum concentration of prostate-specific antigen (PSA) of each case were also assessed. Statistical analysis for the correlations between LAT1 expression and Gleason score and each of the other characteristics studied was performed. As a result, a strong significant correlation between immuno-reactive LAT1 expression and Gleason score was identified (P<0.01). We concluded that immunoreactive LAT1 expression in tissue sections of prostate cancer may be useful as a biomarker for high-grade malignancy.

Up-regulation of amino acid transporter SLC6A19 activity and surface protein abundance by PKB/Akt and PIKfyve

BACKGROUND

The amino acid transporter B0AT1 (SLC6A19) accomplishes concentrative cellular uptake of neutral amino acids. SLC6A19 is stimulated by serum- &amp; glucocorticoid-inducible kinase (SGK) isoforms. SGKs are related to PKB/Akt isoforms, which also stimulate several amino acid transporters. PKB/Akt modulates glucose transport in part by phosphorylating and thus activating phosphatidylinositol-3-phosphate-5-kinase (PIKfyve), which fosters carrier protein insertion into the cell membrane. The present study explored whether PKB/Akt and/or PIKfyve stimulate SLC6A19.

METHODS

SLC6A19 was expressed in Xenopus oocytes with or without wild-type PKB/Akt or inactive (T308A/S473A)PKB/Akt without or with additional expression of wild-type PIKfyve or PKB/Akt-resistant (S318A)PIKfyve. Electrogenic amino acid transport was determined by dual electrode voltage clamping.

RESULTS

In SLC6A19-expressing oocytes but not in water-injected oocytes, the addition of the neutral amino acid L-leucine (2 mM) to the bath generated a current (I(le)), which was significantly increased following coexpression of PKB/Akt, but not by coexpression of (T308A/S473A)PKB/Akt. The effect of PKB/Akt was augmented by additional coexpression of PIKfyve but not of (S318A)PIKfyve. Coexpression of PKB/Akt enhanced the maximal transport rate without significantly modifying the affinity of the carrier. The decline of I(le) following inhibition of carrier insertion by brefeldin A (5 µM) was similar in the absence and presence of PKB/Akt indicating that PKB/Akt stimulated carrier insertion into rather than inhibiting carrier retrieval from the cell membrane.

CONCLUSION

PKB/Akt up-regulates SLC6A19 activity, which may foster amino acid uptake into PKB/Akt-expressing epithelial and tumor cells.

SLC1A5 mediates glutamine transport required for lung cancer cell growth and survival

PURPOSE

We have previously identified solute-linked carrier family A1 member 5 (SLC1A5) as an overexpressed protein in a shotgun proteomic analysis of stage I non-small cell lung cancer (NSCLC) when compared with matched controls. We hypothesized that overexpression of SLC1A5 occurs to meet the metabolic demand for lung cancer cell growth and survival.

EXPERIMENTAL DESIGN

To test our hypothesis, we first analyzed the protein expression of SLC1A5 in archival lung cancer tissues by immunohistochemistry and immunoblotting (N = 98) and in cell lines (N = 36). To examine SLC1A5 involvement in amino acid transportation, we conducted kinetic analysis of l-glutamine (Gln) uptake in lung cancer cell lines in the presence and absence of a pharmacologic inhibitor of SLC1A5, gamma-l-Glutamyl-p-Nitroanilide (GPNA). Finally, we examined the effect of Gln deprivation and uptake inhibition on cell growth, cell-cycle progression, and growth signaling pathways of five lung cancer cell lines.

RESULTS

Our results show that (i) SLC1A5 protein is expressed in 95% of squamous cell carcinomas (SCC), 74% of adenocarcinomas (ADC), and 50% of neuroendocrine tumors; (ii) SLC1A5 is located at the cytoplasmic membrane and is significantly associated with SCC histology and male gender; (iii) 68% of Gln is transported in a Na(+)-dependent manner, 50% of which is attributed to SLC1A5 activity; and (iv) pharmacologic and genetic targeting of SLC1A5 decreased cell growth and viability in lung cancer cells, an effect mediated in part by mTOR signaling.

CONCLUSIONS

These results suggest that SLC1A5 plays a key role in Gln transport controlling lung cancer cells' metabolism, growth, and survival.

High levels of aromatic amino acids in gastric juice during the early stages of gastric cancer progression

Background

Early-stage gastric cancer is mostly asymptomatic and can easily be missed easily by conventional gastroscopy. Currently, there are no useful biomarkers for the early detection of gastric cancer, and their identification of biomarkers is urgently needed.

Methods

Gastric juice was obtained from 185 subjects that were divided into three groups: non-neoplastic gastric disease (NGD), advanced gastric cancer and early gastric cancer (EGC). The levels of aromatic amino acids in the gastric juice were quantitated using high-performance liquid chromatography.

Results

The median values (25th to 75th percentile) of tyrosine, phenylalanine and tryptophan in the gastric juice were 3.8 (1.7–7.5) µg/ml, 5.3 (2.3–9.9) µg/ml and 1.0 (0.4–2.8) µg/ml in NGD; 19.4 (5.8–72.4) µg/ml, 24.6 (11.5–73.7) µg/ml and 8.3 (2.1–28.0) µg/ml in EGC. Higher levels of tyrosine, phenylalanine and tryptophan in the gastric juice were observed in individuals of EGC groups compared those of the NGD group (NGD vs. EGC, P<0.0001). For the detection of EGC, the areas under the receiver operating characteristic curves (AUCs) of each biomarker were as follows: tyrosine, 0.790 [95% confidence interval (CI), 0.703–0.877]; phenylalanine, 0.831 (95% CI, 0.750–0.911); and tryptophan, 0.819 (95% CI, 0.739–0.900). The sensitivity and specificity of phenylalanine were 75.5% and 81.4%, respectively, for detection of EGC. A multiple logistic regression analysis showed that high levels of aromatic amino acids in the gastric juice were associated with gastric cancer (adjusted β coefficients ranged from 1.801 to 4.414, P<0.001).

Conclusion

Increased levels of tyrosine, phenylalanine and tryptophan in the gastric juice samples were detected in the early phase of gastric carcinogenesis. Thus, tyrosine, phenylalanine and tryptophan in gastric juice could be used as biomarkers for the early detection of gastric cancer. A gastric juice analysis is an efficient, economical and convenient method for screening early gastric cancer development in the general population.

HIF2α acts as an mTORC1 activator through the amino acid carrier SLC7A5

The mammalian target of rapamycin (mTOR) pathway, which is essential for cell proliferation, is repressed in certain cell types in hypoxia. However, hypoxia-inducible factor 2α (HIF2α) can act as a proliferation-promoting factor in some biological settings. This paradoxical situation led us to study whether HIF2α has a specific effect on mTORC1 regulation. Here we show that activation of the HIF2α pathway increases mTORC1 activity by upregulating expression of the amino acid carrier SLC7A5. At the molecular level we also show that HIF2α binds to the Slc7a5 proximal promoter. Our findings identify a link between the oxygen-sensing HIF2α pathway and mTORC1 regulation, revealing the molecular basis of the tumor-promoting properties of HIF2α in von Hippel-Lindau-deficient cells. We also describe relevant physiological scenarios, including those that occur in liver and lung tissue, wherein HIF2α or low-oxygen tension drive mTORC1 activity and SLC7A5 expression.

Metabolic alterations in cancer cells and therapeutic implications

Cancer metabolism has emerged as an important area of research in recent years. Elucidation of the metabolic differences between cancer and normal cells and the underlying mechanisms will not only advance our understanding of fundamental cancer cell biology but also provide an important basis for the development of new therapeutic strategies and novel compounds to selectively eliminate cancer cells by targeting their unique metabolism. This article reviews several important metabolic alterations in cancer cells, with an emphasis on increased aerobic glycolysis (the Warburg effect) and glutamine addiction, and discusses the mechanisms that may contribute to such metabolic changes. In addition, metabolic alterations in cancer stem cells, mitochondrial metabolism and its influence on drug sensitivity, and potential therapeutic strategies and agents that target cancer metabolism are also discussed.

Inactivation of the glutamine/amino acid transporter ASCT2 by 1,2,3-dithiazoles: proteoliposomes as a tool to gain insights in the molecular mechanism of action and of antitumor activity

The ASCT2 transport system catalyses a sodium-dependent antiport of glutamine and other neutral amino acids which is involved in amino acid metabolism. A library of 1,2,3-dithiazoles was designed, synthesized and evaluated as inhibitors of the glutamine/amino acid ASCT2 transporter in the model system of proteoliposomes reconstituted with the rat liver transporter. Fifteen of the tested compounds at concentration of 20μM or below, inhibited more than 50% the glutamine/glutamine antiport catalysed by the reconstituted transporter. These good inhibitors bear a phenyl ring with electron withdrawing substituents. The inhibition was reversed by 1,4-dithioerythritol indicating that the effect was likely owed to the formation of mixed sulfides with the protein's Cys residue(s). A dose-response analysis of the most active compounds gave IC(50) values in the range of 3-30μM. Kinetic inhibition studies indicated a non-competitive inhibition, presumably because of a potential covalent interaction of the dithiazoles with cysteine thiol groups that are not located at the substrate binding site. Indeed, computational studies using a homology structural model of ASCT2 transporter, suggested as possible binding targets, Cys-207 or Cys-210, that belong to the CXXC motif of the protein.

Activation of the mTOR pathway by the amino acid (L)-leucine in the 5q- syndrome and other ribosomopathies

Patients with the 5q- syndrome and Diamond-Blackfan anemia (DBA) suffer from a severe macrocytic anemia. The 5q- syndrome and DBA are disorders of aberrant ribosome biogenesis (ribosomopathies) and haploinsufficiency of the ribosomal protein genes RPS14 and RPS19, respectively, underlies the anemia found in these disorders. Erythroblasts obtained from patients with the 5q- syndrome and DBA show impaired mRNA translation and this defect in translation may represent a potential therapeutic target in these ribosomopathies. There are some indications that the amino acid l-leucine, a translation enhancer, may have some efficacy in this group of disorders. Recent studies have shown that l-leucine treatment of zebrafish and murine models of the 5q- syndrome and DBA results in a marked improvement in the anemia. l-leucine treatment of RPS14-deficient and RPS19-deficient erythroblasts and erythroblasts from patients with the 5q- syndrome has been shown to result in an increase in cell proliferation, erythroid differentiation and mRNA translation in culture. l-leucine has been shown to improve hemoglobin levels and transfusion independence in a patient with DBA. l-leucine activates the mTOR (mammalian target of rapamycin) signaling pathway that controls cell growth and mRNA translation. There is evidence to suggest that the promotion of translation via the mTOR pathway by l-leucine is the mechanism that underlies the enhanced erythroid progenitor cell growth and differentiation observed in animal and cellular models of the 5q- syndrome and DBA treated with this amino acid. These data support the rationale for clinical trials of l-leucine as a therapeutic agent for the 5q- syndrome and DBA.

Comparative evaluation of 18F-labeled glutamic acid and glutamine as tumor metabolic imaging agents

(18)F-labeled (2S,4R)-4-fluoro-l-glutamine (4F-GLN) has demonstrated high uptake in tumor cells that undergo high growth and proliferation. Similar tumor targeting properties have also been observed for (18)F-labeled (2S,4R)-4-fluoro-l-glutamate (4F-GLU), suggesting that both are useful imaging agents. A new labeling procedure facilitates the preparation of (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU with confirmed radiochemical and enantiomeric purity. Here, we report the preparation and comparative evaluation of (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU as tumor metabolic imaging agents.

METHODS

Uptake of enantiomerically pure (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU was determined in 3 tumor cell lines (9L, SF188, and PC-3) at selected time points. The in vitro cell uptake mechanism was evaluated by inhibition studies in 9L cells. In vivo biodistribution and PET studies were performed on male F344 rats bearing 9L tumor xenografts.

RESULTS

In vitro cell uptake studies showed that (18)F-(2S,4R)4F-GLN displayed higher uptake than (18)F-(2S,4R)4F-GLU. Amino acid transport system ASC (alanine-serine-cysteine-preferring; in particular, its subtype ASCT2 [SLC1A5 gene]) and system X(c)(-) (SLC7A11 gene) played an important role in transporting (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU, respectively, across the membrane. After being transported into cells, a large percentage of (18)F-(2S,4R)4F-GLN was incorporated into protein, whereas (18)F-(2S,4R)4F-GLU mainly remained as the free amino acid in its original form. In vivo studies of (18)F-(2S,4R)4F-GLN in the 9L tumor model showed a higher tumor uptake than (18)F-(2S,4R)4F-GLU, whereas (18)F-(2S,4R)4F-GLU had a slightly higher tumor-to-background ratio than (18)F-(2S,4R)4F-GLN. Imaging studies showed that both tracers had fast accumulation in 9L tumors. Compared with (18)F-(2S,4R)4F-GLU, (18)F-(2S,4R)4F-GLN exhibited prolonged tumor retention reflecting its incorporation into intracellular macromolecules.

CONCLUSION

Differences in uptake and metabolism in tumor cells were found between (18)F-(2S,4R)4F-GLN and (18)F-(2S,4R)4F-GLU. Both agents are potentially useful as metabolic tracers for tumor imaging.

Cellular signaling of amino acids towards mTORC1 activation in impaired human leucine catabolism

The regulation of cell growth and protein biosynthesis is triggered by the mammalian target of rapamycin complex 1 (mTORC1) responding to amino acids, especially leucine. The molecular mechanisms linking leucine to mTORC1 activation are not well understood. We analyzed whether the free intracellular leucine availability, a metabolite of leucine catabolism or the process of leucine oxidation activates mTORC1 signaling. We further investigated whether mTORC1 signaling is subject to altered regulation in disturbed leucine metabolism. Human fibroblasts with deficiencies in leucine catabolic steps and those from healthy control subjects were utilized. In all cells, leucine-induced mTORC1 signaling was significantly related to leucine pool size and leucine repletion capacity. The leucine/glutamine antiporter SLC7A5/SLC3A2 and the amino acid sensor MAP4K3 were identified as crucial determinants of signaling leucine availability to downstream targets. In cells with defective leucine catabolism, mTORC1 signaling towards phosphorylation of ribosomal protein S6 kinase 1 (S6K1) was significantly increased, whereas transcriptional down-regulation of MAP4K3 upon reduced amino acid supply was abrogated. Remarkably, these effects were observed irrespective of the localization of the enzymatic blockage. Our data provide evidence that mechanisms determining intracellular leucine availability and the amino acid sensor MAP4K3 are key upstream modulators of nutrient-sensitive mTORC1 signaling, whereas specific leucine metabolites or leucine oxidation rates do not play a role. In human fibroblasts deficient in leucine catabolic steps, we observed regulation consistent with sustaining a more efficient MAP4K3 and mTOR-S6K1 signaling. Such regulatory circuit might serve to protect cells against detrimental consequences of reduced nutrient utilization in human conditions associated with disturbed leucine metabolism.

Spherical cell shape of FLC-4 cell, a human hepatoma cell, enhances hepatocyte-specific function and suppresses tumor phenotype through the integration of mRNA-microRNA interaction

The induction mechanism of HNF-4α by spherical cell shape in human hepatoma cells, FLC-4, was investigated. To get insight into the induction mechanism of HNF-4α in three-dimensional FLC-4 cells, mRNA microarray analysis was performed. The gene expression related to drug metabolism and nuclear receptors, such as LXRα, was elevated in spherical FLC-4 cells. We found the first time that the expressions of genes related to malignancy of hepatoma cells, such as HIF-1α, c-Myc and VEGFC, were downregulated by spherical cell shape. Network analysis revealed that HNF-4α would elicit both the enhancement of hepatocyte-specific gene expression and suppression of malignancy. Since HNF-4α gene expression was known to be regulated by microRNA, we inferred that spherical cell shape would induce HNF-4α gene expression through microRNA. To investigate the possibility of such a mechanism, mRNA–microRNA interactions were examined using microRNA microarray and bioinformatics analysis. The level of miR-24, a microRNA targeting HNF-4α, was reduced in spherical FLC-4 cells. On the other hand, spherical cell shape-induced miR-194 and miR-320c would directly downregulate SLC7A5 and E2F1 gene expression, respectively, which are both related to malignancy. Our study suggested that spherical cell shape would induce HNF-4α gene expression and consequent enhancement hepatocyte-specific functions. Spherical cell shape itself would suppress malignancy in FLC-4 cells through microRNA, such as miR-194 and miR-320c.

Francisella tularensis regulates the expression of the amino acid transporter SLC1A5 in infected THP-1 human monocytes

Francisella tularensis, a Gram-negative bacterium that causes the disease tularemia in a large number of animal species, is thought to reside preferentially within macrophages in vivo. F. tularensis has developed mechanisms to rapidly escape from the phagosome into the cytoplasm of infected cells, a habitat with a rich supply of nutrients, ideal for multiplication. SLC1A5 is a neutral amino acid transporter expressed by human cells, which serves, along with SLC7A5 to equilibrate cytoplasmic amino acid pools. We herein analysed whether SLC1A5 was involved in F. tularensis intracellular multiplication. We demonstrate that expression of SLC1A5 is specifically upregulated by F. tularensis in infected THP-1 human monocytes. Furthermore, we show that SLC1A5 downregulation decreases intracellular bacterial multiplication, supporting the involvement of SLC1A5 in F. tularensis infection. Notably, after entry of F. tularensis into cells and during the whole infection, the highly glycosylated form of SLC1A5 was deglycosylated only by bacteria capable of cytosolic multiplication. These data suggest that intracellular replication of F. tularensis depends on the function of host cell SLC1A5. Our results are the first, which show that Francisella intracellular multiplication in human monocyte cytoplasm is associated with a post-translational modification of a eukaryotic amino acid transporter.

Stimulation of MC38 tumor growth by insulin analog X10 involves the serine synthesis pathway

Recent evidence suggests that type II diabetes is associated with increased risk and/or aggressive behavior of several cancers, including those arising from the colon. Concerns have been raised that endogenous hyperinsulinemia and/or exogenous insulin and insulin analogs might stimulate proliferation of neoplastic cells. However, the mechanisms underlying possible growth-promoting effects of insulin and insulin analogs in cancer cells in vivo, such as changes in gene expression, are incompletely described. We observed that administration of the insulin analog X10 significantly increased tumor growth and proliferation in a murine colon cancer model (MC38 cell allografts). Insulin and X10 altered gene expression in MC38 tumors in a similar fashion, but X10 was more potent in terms of the number of genes influenced and the magnitude of changes in gene expression. Many of the affected genes were annotated to metabolism, nutrient uptake, and protein synthesis. Strikingly, expression of genes encoding enzymes in the serine synthesis pathway, recently shown to be critical for neoplastic proliferation, was increased following treatment with insulin and X10. Using stable isotopic tracers and mass spectrometry, we confirmed that insulin and X10 increased glucose contribution to serine synthesis in MC38 cells. The data demonstrate that the tumor growth-promoting effects of insulin and X10 are associated with changes in expression of genes involved in cellular energy metabolism and reveal previously unrecognized effects of insulin and X10 on serine synthesis.

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

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

Prognostic significance of L-type amino-acid transporter 1 expression in surgically resected pancreatic cancer

BACKGROUND

The expression of L-type amino-acid transporter 1 (LAT1) is tumour-specific and has been shown to have essential roles in cell growth and survival. However, little is known regarding the clinical significance of LAT1 expression in pancreatic cancer. This study was conducted to determine the prognostic significance of LAT1 expression.

METHODS

A total of 97 consecutive patients with surgically resected pathological stage I-IV pancreatic ductal adenocarcinoma were retrospectively reviewed. Tumour sections were stained by immunohistochemistry for LAT1, CD98, Ki-67 and vascular endothelial growth factor (VEGF), and microvessel density was determined by CD34 and p53.

RESULTS

L-type amino-acid transporter 1 and CD98 were highly expressed in 52.6% (51/97) and 56.7% (55/97) of cases, respectively (P=0.568). The expression of LAT1 within pancreatic cancer cells was significantly associated with disease stage, tumour size, Ki-67, VEGF, CD34, p53 and CD98. L-type amino-acid transporter 1 expression was confirmed to be a significant prognostic factor for predicting poor outcome by multivariate analysis.

CONCLUSION

L-type amino-acid transporter 1 expression is a promising pathological marker for the prediction of outcome in patients with pancreatic cancer.

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

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

METHODS

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

RESULTS

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

CONCLUSION

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

Croix B. Selective blockade of tumor angiogenesis

Blocking tumor angiogenesis is an important goal of cancer therapy, but clinically approved anti-angiogenic agents suffer from limited efficacy and adverse side effects, fueling the need to identify alternative angiogenesis regulators. Tumor endothelial marker 8 (TEM8) is a highly conserved cell surface receptor overexpressed on human tumor vasculature. Genetic disruption of Tem8 in mice revealed that TEM8 is important for promoting tumor angiogenesis and tumor growth but dispensable for normal development and wound healing. The induction of TEM8 in cultured endothelial cells by nutrient or growth factor deprivation suggests that TEM8 may be part of a survival response pathway that is activated by tumor microenvironmental stress. In preclinical studies, antibodies targeted against the extracellular domain of TEM8 inhibited tumor angiogenesis and blocked the growth of multiple human tumor xenografts. Anti-TEM8 antibodies augmented the activity of other anti-angiogenic agents, vascular targeting agents and conventional chemotherapeutic agents and displayed no detectable toxicity. Thus, anti-TEM8 antibodies provide a promising new tool for selective blockade of neovascularization associated with cancer and possibly other angiogenesis-dependent diseases.

Visualization of glutamine transporter activities in living cells using genetically encoded glutamine sensors

Glutamine plays a central role in the metabolism of critical biological molecules such as amino acids, proteins, neurotransmitters, and glutathione. Since glutamine metabolism is regulated through multiple enzymes and transporters, the cellular glutamine concentration is expected to be temporally dynamic. Moreover, differentiation in glutamine metabolism between cell types in the same tissue (e.g. neuronal and glial cells) is often crucial for the proper function of the tissue as a whole, yet assessing cell-type specific activities of transporters and enzymes in such heterogenic tissue by physical fractionation is extremely challenging. Therefore, a method of reporting glutamine dynamics at the cellular level is highly desirable. Genetically encoded sensors can be targeted to a specific cell type, hence addressing this knowledge gap. Here we report the development of Föster Resonance Energy Transfer (FRET) glutamine sensors based on improved cyan and yellow fluorescent proteins, monomeric Teal Fluorescent Protein (mTFP)1 and venus. These sensors were found to be specific to glutamine, and stable to pH-changes within a physiological range. Using cos7 cells expressing the human glutamine transporter ASCT2 as a model, we demonstrate that the properties of the glutamine transporter can easily be analyzed with these sensors. The range of glutamine concentration change in a given cell can also be estimated using sensors with different affinities. Moreover, the mTFP1-venus FRET pair can be duplexed with another FRET pair, mAmetrine and tdTomato, opening up the possibility for real-time imaging of another molecule. These novel glutamine sensors will be useful tools to analyze specificities of glutamine metabolism at the single-cell level.

Abundance of amino acid transporters involved in mTORC1 activation in skeletal muscle of neonatal pigs is developmentally regulated

Previously we demonstrated that the insulin- and amino acid-induced activation of the mammalian target of rapamycin complex 1 (mTORC1) is developmentally regulated in neonatal pigs. Recent studies have indicated that members of the System A transporter (SNAT2), the System N transporter (SNAT3), the System L transporters (LAT1 and LAT2), and the proton-assisted amino acid transporters (PAT1 and PAT2) have crucial roles in the activation of mTORC1 and that the abundance of amino acid transporters is positively correlated with their activation. This study aimed to determine the effect of the post-prandial rise in insulin and amino acids on the abundance or activation of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 and whether the response is modified by development. Overnight fasted 6- and 26-day-old pigs were infused for 2 h with saline (Control) or with insulin or amino acids to achieve fed levels while amino acids or insulin, respectively, as well as glucose were maintained at fasting levels. The abundance of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 was higher in muscle of 6- compared with 26-day-old pigs. The abundance of the PAT2-mTOR complex was greater in 6- than in 26-day-old pigs, consistent with the higher activation of mTORC1. Neither insulin nor amino acids altered amino acid transporter or PAT2-mTOR complex abundance. In conclusion, the amino acid transporters, SNAT 2/3, LAT 1/2, and PAT1/2, likely have important roles in the enhanced amino acid-induced activation of mTORC1 in skeletal muscle of the neonate.

The role of amino acid transporters in GSH synthesis in the blood-brain barrier and central nervous system

Glutathione (GSH) plays a critical role in protecting cells from oxidative stress and xenobiotics, as well as maintaining the thiol redox state, most notably in the central nervous system (CNS). GSH concentration and synthesis are highly regulated within the CNS and are limited by availability of the sulfhydryl amino acid (AA) l-cys, which is mainly transported from the blood, through the blood-brain barrier (BBB), and into neurons. Several antiporter transport systems (e.g., x(c)(-), x(-)(AG), and L) with clearly different luminal and abluminal distribution, Na(+), and pH dependency have been described in brain endothelial cells (BEC) of the BBB, as well as in neurons, astrocytes, microglia and oligodendrocytes from different brain structures. The purpose of this review is to summarize information regarding the different AA transport systems for l-cys and its oxidized form l-cys(2) in the CNS, such as expression and activity in blood-brain barrier endothelial cells, astrocytes and neurons and environmental factors that modulate transport kinetics.

Synthesis and evaluation of 18F labeled alanine derivatives as potential tumor imaging agents

INTRODUCTION

This paper reports the synthesis and labeling of (18)F alanine derivatives. We also investigate their biological characteristics as potential tumor imaging agents mediated by alanine-serine-cysteine preferring (ASC) transporter system.

METHODS

Three new (18)F alanine derivatives were prepared from corresponding tosylate-precursors through a two-step labeling reaction. In vitro uptake studies to evaluate and to compare these three analogs were carried out in 9L glioma and PC-3 prostate cancer cell lines. Potential transport mechanisms, protein incorporation and stability of 3-(1-[(18)F]fluoromethyl)-L-alanine (L-[(18)F]FMA) were investigated in 9L glioma cells. Its biodistribution was determined in a rat-bearing 9L tumor model. PET imaging studies were performed on rat bearing 9L glioma tumors and transgenic mouse carrying spontaneous generated M/tomND tumor (mammary gland adenocarcinoma).

RESULTS

New (18)F alanine derivatives were prepared with 7%-34% uncorrected radiochemical yields, excellent enantiomeric purity (>99%) and good radiochemical purity (>99%). In vitro uptake of the L-[(18)F]FMA in 9L glioma and PC-3 prostate cancer cells was higher than that observed for the other two alanine derivatives and [(18)F]FDG in the first 1h. Inhibition of cell uptake studies suggested that L-[(18)F]FMA uptake in 9L glioma was predominantly via transport system ASC. After entering into cells, L-[(18)F]FMA remained stable and was not incorporated into protein within 2h. In vivo biodistribution studies demonstrated that L-[(18)F]FMA had relatively high uptake in liver and kidney. Tumor uptake was fast, reaching a maximum within 30 min. The tumor-to-muscle, tumor-to-blood and tumor-to-brain ratios at 60 min post injection were 2.2, 1.9 and 3.0, respectively. In PET imaging studies, tumors were visualized with L-[(18)F]FMA in both 9L rat and transgenic mouse.

CONCLUSION

L-[(18)F]FMA showed promising properties as a PET imaging agent for up-regulated ASC transporter associated with tumor proliferation.

Tryptophan metabolism in breast cancers: molecular imaging and immunohistochemistry studies

INTRODUCTION

Tryptophan oxidation via the kynurenine pathway is an important mechanism of tumoral immunoresistance. Increased tryptophan metabolism via the serotonin pathway has been linked to malignant progression in breast cancer. In this study, we combined quantitative positron emission tomography (PET) with tumor immunohistochemistry to analyze tryptophan transport and metabolism in breast cancer.

METHODS

Dynamic α-[(11)C]methyl-l-tryptophan (AMT) PET was performed in nine women with stage II-IV breast cancer. PET tracer kinetic modeling was performed in all tumors. Expression of L-type amino acid transporter 1 (LAT1), indoleamine 2,3-dioxygenase (IDO; the initial and rate-limiting enzyme of the kynurenine pathway) and tryptophan hydroxylase 1 (TPH1; the initial enzyme of the serotonin pathway) was assessed by immunostaining of resected tumor specimens.

RESULTS

Tumor AMT uptake peaked at 5-20 min postinjection in seven tumors; the other two cases showed protracted tracer accumulation. Tumor standardized uptake values (SUVs) varied widely (2.6-9.8) and showed a strong positive correlation with volume of distribution values derived from kinetic analysis (P<.01). Invasive ductal carcinomas (n=6) showed particularly high AMT SUVs (range, 4.7-9.8). Moderate to strong immunostaining for LAT1, IDO and TPH1 was detected in most tumor cells.

CONCLUSIONS

Breast cancers show differential tryptophan kinetics on dynamic PET. SUVs measured 5-20 min postinjection reflect reasonably the tracer's volume of distribution. Further studies are warranted to determine if in vivo AMT accumulation in these tumors is related to tryptophan metabolism via the kynurenine and serotonin pathways.

Excessive Leucine-mTORC1-Signalling of Cow Milk-Based Infant Formula: The Missing Link to Understand Early Childhood Obesity

Increased protein supply by feeding cow-milk-based infant formula in comparison to lower protein content of human milk is a well-recognized major risk factor of childhood obesity. However, there is yet no conclusive biochemical concept explaining the mechanisms of formula-induced childhood obesity. It is the intention of this article to provide the biochemical link between leucine-mediated signalling of mammalian milk proteins and adipogenesis as well as early adipogenic programming. Leucine has been identified as the predominant signal transducer of mammalian milk, which stimulates the nutrient-sensitive kinase mammalian target of rapamycin complex 1 (mTORC1). Leucine thus functions as a maternal-neonatal relay for mTORC1-dependent neonatal β-cell proliferation and insulin secretion. The mTORC1 target S6K1 plays a pivotal role in stimulation of mesenchymal stem cells to differentiate into adipocytes and to induce insulin resistance. It is of most critical concern that infant formulas provide higher amounts of leucine in comparison to human milk. Exaggerated leucine-mediated mTORC1-S6K1 signalling induced by infant formulas may thus explain increased adipogenesis and generation of lifelong elevated adipocyte numbers. Attenuation of mTORC1 signalling of infant formula by leucine restriction to physiologic lower levels of human milk offers a great chance for the prevention of childhood obesity and obesity-related metabolic diseases.

Defining substrate and blocker activity of alanine-serine-cysteine transporter 2 (ASCT2) Ligands with Novel Serine Analogs

The neutral amino acid transporter alanine-serine-cysteine transporter 2 (ASCT2) belongs to the solute carrier 1 (SLC1) family of solute transporters and transports small, neutral amino acids across the membrane, including the physiologically important and ubiquitous amino acid glutamine. Our understanding of the involvement of ASCT2 in the physiological processes involving glutamine is hampered by a lack of understanding of its pharmacology and the absence of high-affinity inhibitors. In this study, we combined an in silico docking approach with experimental investigation of binding parameters to develop new ASCT2 inhibitors and substrates, a series of serine esters, and to determine structural parameters that govern their functional effects. The series of compounds was synthesized using standard methods and exhibited a range of properties, from inhibitors to partial substrates and full substrates. Our results suggest that amino acid derivatives with small side-chain volume and low side-chain hydrophobicity interact strongly with the closed-loop form of the binding site, in which re-entrant loop 2, the presumed extracellular gate for the substrate binding site, is closed off. However, these derivatives bind weakly to the open-loop form (external gate open to the extracellular side), acting as transported substrates. In contrast, inhibitors bind preferentially to the open-loop form. An aromatic residue in the side chain is required for high-affinity interaction. One of the compounds, the l-serine ester serine biphenyl-4-carboxylate reversibly inhibits ASCT2 function with an apparent affinity of 30 μM.

Relation of LAT1/4F2hc expression with pathological grade, proliferation and angiogenesis in human gliomas

Background

LAT1/4F2hc heterodimeric complex is a major route for the transport of large neutral essential amino acids through the plasma membrane. Although it has been shown that LAT1/4F2hc is highly expressed in a variety of human tumors including gliomas, and LAT1 over-expression is associated with glioma grade and poor prognosis of glioma patients, the precise tissue location of LAT1/4F2hc in gliomas and the precise role of LAT1/4F2hc in glioma biological features remain unclear.

Methods

In the current study, the expressions of LAT1, 4F2hc, CD34 and Ki-67 were investigated by immunohistochemistry in 62 cases of human brain glioma; LAT1/4F2hc expression level, Ki-67 labeling index (Ki-67 LI) and microvessel density (MVD) were measured semi-quantitatively; and the correlation of LAT1/4F2hc expression with histopathological features, Ki-67 LI and MVD in gliomas was further analyzed.

Results

The results showed that both LAT1 and 4F2hc were expressed in all examined specimens. LAT1 but 4F2hc expression levels significantly correlated with the pathological grade and both expression levels significantly correlated with Ki-67 LI of gliomas. We also demonstrated that both LAT1 and 4F2hc immunoreactivity were observed in tumor cells as well as vascular endothelia; furthermore, the LAT1 expression level was markedly associated with glioma MVD as well.

Conclusion

LAT1/4F2hc over-expression is closely correlates with the malignant phenotype and proliferation of gliomas, and LAT1 was associates with glioma angiogenesis. LAT1/4F2hc, especially LAT1, may become a novel potential molecular target for glioma biological therapy.

Metabolic reprogramming in cancer: unraveling the role of glutamine in tumorigenesis

Increased glutaminolysis is now recognized as a key feature of the metabolic profile of cancer cells, along with increased aerobic glycolysis (the Warburg effect). In this review, we discuss the roles of glutamine in contributing to the core metabolism of proliferating cells by supporting energy production and biosynthesis. We address how oncogenes and tumor suppressors regulate glutamine metabolism and how cells coordinate glucose and glutamine as nutrient sources. Finally, we highlight the novel therapeutic and imaging applications that are emerging as a result of our improved understanding of the role of glutamine metabolism in cancer.

Canine Lat1: molecular structure, distribution and its expression in cancer samples

A full-length cDNA sequence of canine L-type amino acid transporter 1 (Lat1) was determined from a canine brain. The sequence was 1828 bp long and was predicted to encode 485 amino acid polypeptides. The deduced amino acid sequence of canine Lat1 showed 93.2% and 91.1% similarities to those of humans and rats, respectively. Northern blot analysis detected Lat1 expression in the cerebellum at 4 kb, and Western blot analysis showed a single band at 40 kDa. RT-PCR analysis revealed a distinct expression of Lat1 in the pancreas and testis in addition to the cerebrum and cerebellum. Notably, Lat1 expression was observed in the tissues of thyroid cancer, melanoma and hemangiopericytoma. Although the cancer samples examined were not enough, Lat1 may serve as a useful biomarker of cancer cells in veterinary clinic.

Differential expression of folate receptor-alpha, sodium-dependent multivitamin transporter, and amino acid transporter (B (0, +)) in human retinoblastoma (Y-79) and retinal pigment epithelial (ARPE-19) cell lines

PURPOSE

The overall objective of this study was to investigate the differential expression of folate receptor-alpha (FR-α), sodium-dependent multivitamin transporter (SMVT), and amino acid transporter [B ((0, +))] in retinoblastoma (Y-79) and retinal pigment epithelial (ARPE-19) cells.

METHODS

Polymerase chain reaction (PCR) analysis was performed to confirm the existence of FR-α, SMVT, and B ((0, +)) in Y-79 and ARPE-19 cell lines. Quantitative real-time PCR was also performed to determine the relative expression of FR-α, SMVT, and B ((0, +)) at mRNA level in these cell lines. Quantitative uptake of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was studied in Y-79 and ARPE-19 cells. Further, saturation kinetics of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was performed in the presence of various concentrations of respective cold substrates to determine the kinetic parameters (K(m) and V(max)) in Y-79 and ARPE-19 cells.

RESULTS

PCR analysis had confirmed the existence of FR-α, SMVT, and B ((0, +)) in Y-79 and ARPE-19 cells. Quantitative real-time PCR analysis had shown significantly higher expression of FR-α, SMVT, and B ((0, +)) mRNA levels in Y-79 cells compared with ARPE-19 cells. Quantitative uptake of [(3)H] Folic acid, [(3)H] Biotin, and [(14)C] Arginine was found to be significantly higher in Y-79 cells relative to ARPE-19 cells. [(3)H] Folic acid uptake process followed saturation kinetics with apparent K(m) of 8.29 nM and V(max) of 393.47 fmol/min/mg protein in Y-79 cells and K(m) of 80.55 nM and V(max) of 491.86 fmol/min/mg protein in ARPE-19 cells. [(3)H] Biotin uptake process also displayed saturation kinetics with K(m) of 8.53 μM and V(max) of 14.12 pmol/min/mg protein in Y-79 cells and K(m) of 138.25 μM and V(max) of 38.85 pmol/min/mg protein in ARPE-19 cells. [(14)C] Arginine uptake process followed saturation kinetics with K(m) of 16.77 μM and V(max) of 348.27 pmol/min/mg protein in Y-79 cells and K(m) of 52.03 μM and V(max) of 379.21 pmol/min/mg protein in ARPE-19 cells.

CONCLUSIONS

This work demonstrated for the first time the higher expression and affinity of FR-α, SMVT, and B ((0, +)) mRNA levels in retinoblastoma (Y-79) cells compared with retinal pigment epithelial (ARPE-19) cells.

Correlation of L-type amino acid transporter 1 and CD98 expression with triple negative breast cancer prognosis

Triple negative breast cancer (TNBC) is a heterogeneous, aggressive cancer for which there is no effective chemotherapy or targeted therapy. We aimed to evaluate L-type amino acid transporter (LAT) 1 and CD98 expression immunohistochemically in patients with breast cancer, especially TNBC. Out of 129 patients, LAT1 was positive in 56 patients (43.4%), and CD98 was positive in 41 patients (31.8%). The positive ratio of LAT1 expression in luminal A cases was 7.9%, 30.0% in luminal B cases, 71.4% in HER2 cases and 64.0% in TN cases. HER2 and TN subtypes expressed LAT1 and CD98 at higher levels than luminal A and B subtypes (both P < 0.001). LAT1 and CD98 expression correlated with tumor size (LAT1, P = 0.010; CD98, P = 0.007), nuclear grade (LAT1, P < 0.001; CD98, P < 0.001) and Ki67 labeling index (LAT1, P < 0.001; CD98, P = 0.001). LAT1 and CD98 expression was negatively associated with ER and PgR (both P < 0.001). In TNBC, the 5-year disease-free rate of CD98+ (63.6%) or LAT1+/CD98+ (61.9%) patients was significantly worse than that of CD98- (89.3%) patients or those with no co-expression of LAT1 and CD98 (89.7%), respectively (P = 0.014, P = 0.009). The 5-year survival rates of CD98 positive/negative patients were 77.3% and 100% (P = 0.050), respectively, whereas that of patients with LAT1+/CD98+ (76.2%) was significantly worse (100%) (P = 0.040). Multivariate analysis confirmed that CD98+ or LAT1+/CD98+ expression were risk factors for relapse in TNBC (P = 0.023, P = 0.019). Thus, in the present study we show that LAT1 and CD98 expression are prognostic factors. Inhibition of these proteins might provide a new therapeutic strategy in TNBC.

PET imaging of glutaminolysis in tumors by 18F-(2S,4R)4-fluoroglutamine

Changes in gene expression, metabolism, and energy requirements are hallmarks of cancer growth and self-sufficiency. Upregulation of the PI3K/Akt/mTor pathway in tumor cells has been shown to stimulate aerobic glycolysis, which has enabled (18)F-FDG PET tumor imaging. However, of the millions of (18)F-FDG PET scans conducted per year, a significant number of malignant tumors are (18)F-FDG PET-negative. Recent studies suggest that several tumors may use glutamine as the key nutrient for survival. As an alternative metabolic tracer for tumors, (18)F-(2S,4R)4-fluoroglutamine was developed as a PET tracer for mapping glutaminolytic tumors.

METHODS

A series of in vitro cell uptake and in vivo animal studies were performed to demonstrate tumor cell addiction to glutamine. Cell uptake studies of this tracer were performed in SF188 and 9L glioblastoma tumor cells. Dynamic small-animal PET studies of (18)F-(2S,4R)4-fluoroglutamine were conducted in 2 animal models: xenografts produced in F344 rats by subcutaneous injection of 9L tumor cells and transgenic mice with M/tomND spontaneous mammary gland tumors.

RESULTS

In vitro studies showed that both transformed 9L and SF188 tumor cells displayed a high rate of glutamine uptake (maximum uptake, ≈ 16% dose/100 μg of protein). The cell uptake of (18)F-(2S,4R)4-fluoroglutamine by SF188 cells is comparable to that of (3)H-L-glutamine but higher than that of (18)F-FDG. The tumor cell uptake can be selectively blocked. Biodistribution and PET studies showed that (18)F-(2S,4R)4-fluoroglutamine localized in tumors with a higher uptake than in surrounding muscle and liver tissues. Data suggest that certain tumor cells may use glutamine for energy production.

CONCLUSION

The results support that (18)F-(2S,4R)4-fluoroglutamine is selectively taken up and trapped by tumor cells. It may be useful as a novel metabolic tracer for tumor imaging.

Na-glutamine co-transporters B(0)AT1 in villus and SN2 in crypts are differentially altered in chronically inflamed rabbit intestine

Glutamine is a major nutrient utilized by the intestinal epithelium and is primarily assimilated via Na-glutamine co-transport (NGcT) on the brush border membrane (BBM) of enterocytes. Recently we reported that B(0)AT1 (SLC6A19) mediates glutamine absorption in villus while SN2 (SLC38A5) does the same in crypt cells. However, how B(0)AT1 and SN2 are affected during intestinal inflammation is unknown. In the present study it was shown that during chronic enteritis NGcT was inhibited in villus cells, however, it was stimulated in crypt cells. Our studies also demonstrated that the mechanism of inhibition of NGcT during chronic enteritis was secondary to a reduction in the number of B(0)AT1 co-transporters in the villus cell BBM without a change in the affinity of the co-transporter. In contrast, stimulation of NGcT in crypt cells was secondary to an increase in the affinity of SN2 for glutamine without an alteration in the number of co-transporters. Thus, glutamine assimilation which occurs via distinct transporters in crypt and villus cells is altered in the chronically inflamed intestine.

Androgen receptor and nutrient signaling pathways coordinate the demand for increased amino acid transport during prostate cancer progression

L-Type amino acid transporters such as LAT1 and LAT3 mediate the uptake of essential amino acids. Here, we report that prostate cancer cells coordinate the expression of LAT1 and LAT3 to maintain sufficient levels of leucine needed for mTORC1 signaling and cell growth. Inhibiting LAT function was sufficient to decrease cell growth and mTORC1 signaling in prostate cancer cells. These cells maintained levels of amino acid influx through androgen receptor-mediated regulation of LAT3 expression and ATF4 regulation of LAT1 expression after amino acid deprivation. These responses remained intact in primary prostate cancer, as indicated by high levels of LAT3 in primary disease, and by increased levels of LAT1 after hormone ablation and in metastatic lesions. Taken together, our results show how prostate cancer cells respond to demands for increased essential amino acids by coordinately activating amino acid transporter pathways vital for tumor outgrowth.

Amino acid signaling in TOR activation

The target of rapamycin (TOR) is a central cell growth regulator conserved from yeast to mammals. Uncontrolled TOR activation is commonly observed in human cancers. TOR forms two distinct structural and functional complexes, TORC1 and TORC2. TORC1 promotes cell growth and cell size by stimulating protein synthesis. A wide range of signals, including nutrients, energy levels, and growth factors, are known to control TORC1 activity. Among them, amino acids (AA) not only potently activate TORC1 but are also required for TORC1 activation by other stimuli, such as growth factors. The mechanisms of growth factors and cellular energy status in activating TORC1 have been well elucidated, whereas the molecular basis of AA signaling is just emerging. Recent advances in the role of AA signaling on TORC1 activation have revealed key components, including the Rag GTPases, protein kinases, nutrient transporters, and the intracellular trafficking machinery, in relaying AA signals to TORC1 activation.