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

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

Prognostic value of LAT-1 status in solid cancer: A systematic review and meta-analysis.

BACKGROUND: The expression of the L-type amino acid transporter 1 (LAT1) plays a significant role in tumor progression. However, it remains unclear whether high LAT1 expression correlates with poor prognosis of solid tumor patients. Here, we conducted a meta-analysis to assess the potential of LAT1 in predicting the prognosis of tumor patients. METHODS AND FINDINGS: A total of 4,579 cases were analyzed from 35 qualified studies. In patients with solid tumors, elevated expression of LAT1 is associated with poor prognosis (overall survival [OS]: pooled hazard ratio (HR) = 1.848, 95% confidence interval (CI) = 1.620-2.108, P < 0.001; disease free survival [DFS]: pooled HR = 1.923, 95% CI = 1.585-2.333, P < 0.001; progression free survival [PFS]: pooled HR = 1.345, 95% CI = 1.133-1.597, P = 0.001). Furthermore, in subgroup analysis, we found an association between high LAT1 expression and poor OS in non-small cell lung cancer (HR = 1.554, 95% CI = 1.345-1.794, P < 0.001), pancreatic cancer (HR = 2.052, 95% CI = 1.613-2.724, P < 0.001) and biliary tract cancer (HR = 2.253, 95% CI = 1.562-3.227, P < 0.001). CONCLUSION: The results of this meta-analysis indicate the reliability and potential of using LAT1 expression as a predictive biomarker in solid cancers prior to treatment. However, further studies with larger sample sizes would be beneficial for fully evaluating the predictive value of LAT1 expression for clinical applications.

The expression of the aminoacid transporters ASCT2 (SLC1A5) and LAT1 (SLC7A5) in astrocytomas / Expressão dos transportadores de aminoácidos ASCT2 (SLC1A5) e LAT1 (SLC7A5) em astrocitomas

OBJECTIVE: ASCT2 and LAT1 are aminoacid transporters involved in glutamine transport and play a role in tumor growth. Previous studies have shown an association of ASCT2 to cell proliferation through the mechanistic Target of Rapamycin (mTOR) translational machinery; LAT1 has been shown as a prognostic marker due to its relation to tumor invasion, microscopic vascular invasion and metastasis. This study analyzed the gene expression of ASCT2 and LAT1 in astrocytomas of different grades and how this correlates to clinical outcome in glioblastoma patients. METHOD: This is an observational study with ASCT2 and LAT1 mRNA expression analysis in 153 samples of human astrocytomas, distributed in different World Health Organization (WHO) grades of malignancy (23 at grade I or pilocytic astrocytoma, 26 at grade II or low-grade astrocytoma, 18 at grade III or anaplastic astrocytoma, 86 at grade IV astrocytoma or glioblastoma (AGIV or GBM)); these were compared to 22 non-neoplastic brain samples. RESULTS: Significant hyperexpression of both genes was observed particularly in malignant astrocytomas (GIII & GBM). Moreover, LAT1 hyperexpression impacted negatively in the overall survival in a subset of GBM patients. CONCLUSION: LAT1 is more expressed in higher grade astrocytomas. It leads to a poorer prognosis among GBM patients and may be a potential therapeutical target.

OBJETIVO: ASCT2 e LAT1 são transportadores de aminoácidos envolvidos no transporte de glutamina e desempenham um papel no crescimento tumoral. Estudos prévios mostraram uma associação de ASCT2 com proliferação celular através da maquinaria de tradução do mTOR; tem sido mostrado que o LAT1 é um marcador prognóstico devido à sua relação com invasão tumoral, invasão vascular microscópica e metástase. Este estudo analisou a expressão gênica de ASCT2 e LAT1 em astrocitomas de diferentes graus e sua correlação com desfecho clínico em pacientes com glioblastoma. METODO: Este é um estudo observacional com análise de expressão de RNAm de ASCT2 e LAT1 em 153 amostras de astrocitomas humanos, distribuídas em diferentes graus de malignidade segundo a OMS (23 astrocitomas de grau I ou astrocitoma pilocítico, 26 de astrocitoma de grau II ou astrocitoma de baixo grau, 18 de astrocitoma de grau III ou astrocitoma anaplásico, 86 de astrocitoma de grau IV ou glioblastoma (AGIV ou GBM); estes foram comparados com 22 amostras cerebrais não neoplásicas. RESULTADOS: Foi observada uma hiperexpressão de ambos os genes, particularmente nos astrocitomas malignos (GIII & GBM). Além disso, a hiperexpressão LAT1 impactou negativamente na sobrevida global em um grupo de pacientes com GBM. CONCLUSÃO: LAT1 é mais expresso em astrocitomas de grau maior. Isso leva a um pior prognóstico entre os pacientes com GBM e pode ser um potencial alvo terapêutico.

L-Type amino acid transporter 1 (LAT1) expression in lymph node metastasis of gastric carcinoma: Its correlation with size of metastatic lesion and Ki-67 labeling.

L-Type amino acid transporter 1 (LAT1) is one of the major amino acid transporters. High levels of LAT1 expression have been reported in various tumors, which can act as a novel prognostic marker. Previously, we demonstrated that LAT1 is highly expressed in advanced gastric carcinoma with lymph node metastasis, and proposed that LAT1 is an independent prognostic factor in non-scirrhous gastric carcinoma. The aim of the present study was to investigate the relationship between LAT1 expression and the size of lymph node metastatic lesions in gastric carcinoma. LAT1 and Ki-67 expression was immunohistochemically analyzed in 64 cases of advanced gastric carcinoma with lymph node metastasis. LAT1 expression in the metastatic lymph nodes was correlated with that in the primary lesions. The high LAT1 expression group showed a larger size of metastatic lesion and a higher Ki-67 labeling index than the low LAT1 expression group. LAT1 expression had a weak association with Ki-67 labeling index and tumor diameter of lymph nodes. These results suggest that LAT1 expression is associated with disease progression in gastric carcinoma. We proposed that LAT1 could be a potential therapeutic target for gastric carcinoma cases with large lymph node metastasis.

CD98 is a promising prognostic biomarker in biliary tract cancer.

CD98 has been described to play a crucial role in tumor progression and survival. However, the role of CD98 in biliary tract cancer remains unclear. We found that 36.7% of all patients with biliary tract cancer had a high CD98 expression. Statistical analysis using Spearman's rank correlation showed that CD98 was significantly correlated with L-type amino acid transporter 1 (LAT1, r=0.562, P<0.001), Ki-67 (r=0.230, P=0.006) and CD34 (r=0.290, P=0.005). Multivariate analysis confirmed that a high CD98 expression was an independent prognostic factor for predicting poor outcome. CD98 is closely associated with tumor growth, biological aggressiveness, and survival of patients. With these data we proposed that CD98 expression is necessary for the development and pathogenesis of biliary tract cancer.

Prognostic significance of amino-acid transporter expression (LAT1, ASCT2, and xCT) in surgically resected tongue cancer.

BACKGROUND: Amino-acid transporters are necessary for the tumour cell growth and survival, and have a crucial role in the development and invasiveness of cancer cells. But, it remains unclear about the prognostic significance of L-type amino-acid transporter 1 (LAT1), system ASC amino-acid transporter-2 (ASCT2), and xCT expression in patients with tongue cancer. We conducted the clinicopathological study to investigate the protein expression of these amino-acid transporters in tongue cancer. METHODS: Eighty-five patients with surgically resected tongue cancer were evaluated. Tumour sections were stained by immunohistochemistry for LAT1, ASCT2, xCT, 4F2hc/CD98hc (4F2hc), Ki-67, and microvessel density (MVD) determined by CD34, and p53. RESULTS: L-type amino-acid transporter 1 and 4F2hc were highly expressed in 61% (52 out of 85) and 45% (38 out of 47), respectively. ASC amino-acid transporter-2 and xCT were positively expressed in 59% (50 out of 85) and 21% (18 out of 85), respectively. The expression of both LAT1 and ASCT2 was significantly associated with disease staging, lymph-node metastasis, lymphatic permeation, 4F2hc expression and cell proliferation (Ki-67). xCT expression indicated a significant association with advanced stage and tumour factor. By univariate analysis, disease staging, lymphatic permeation, vascular invasion, LAT1, ASCT2, 4F2hc, and Ki-67 had a significant relationship with overall survival. Multivariate analysis confirmed that LAT1 was an independent prognostic factor for predicting poor prognosis. CONCLUSIONS: L-type amino-acid transporter 1 and ASCT2 can serve as a significant prognostic factor for predicting worse outcome after surgical treatment and may have an important role in the development and aggressiveness of tongue cancer.

L-amino acid transporter-1 and boronophenylalanine-based boron neutron capture therapy of human brain tumors.

The system l-amino acid transporter-1 (LAT-1) imports p-boronophenylalanine (BPA) into cells and may play a major role in the effectiveness of BPA-based boron neutron capture therapy. The functional status of LAT-1 and its relationship to cell proliferation were simultaneously examined in the same section of human tumor material using a dual-labeling technique. The uptake of BPA (boron inductively coupled plasma mass spectrometry) was profiled in the presence of agonists and antagonists in fresh tumor explants. The number of LAT-1-expressing cells (mean +/- SD) was three times higher than that of proliferating cell nuclear antigen (PCNA)-expressing cells (71.5 +/- 17.02% versus 23.8 +/- 16.5%; P < 0.0001; n = 38 glioblastoma and metastatic tumors). There was no correlation between PCNA cells and the number of LAT-1/PCNA double-stained cells, and not all PCNA-expressing cells coexpressed LAT-1. Boron uptake reached 30 +/- 15 mug/g of wet weight of tissue by 4 hours both in tumor and brain around tumor tissue containing tumor cells compared with time 0 (P < 0.005; n = 4 glioblastoma tumors). This uptake was inhibited by both phenylalanine and 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. These LAT-1 data indicate that BPA-based boron neutron capture therapy might affect up to 70% of tumor cells, representing a three times higher proportion of tumor cells than their cell cycle status might suggest. Cells expressing PCNA, but not LAT-1, will require a different therapeutic strategy.

Evaluation of thoracic tumors with (18)F-FMT and (18)F-FDG PET-CT: a clinicopathological study.

L-[3-(18)F]-alpha-methyltyrosine ((18)F-FMT) is an aminoacid tracer for positron emission tomography (PET). The aim of this study was to determine whether PET-CT with (18)F-FMT provides additional information for the preoperative diagnostic workup as compared with (18)F-FDG PET. PET-CT studies with (18)F-FMT and (18)F-FDG were performed as a part of the preoperative workup in 36 patients with histologically confirmed bronchial carcinoma, 6 patients with benign lesions and a patient with atypical carcinoid. Expression of L-type amino acid transporter 1 (LAT1), CD98, Ki-67 labeling index, VEGF, CD31 and CD34 of the resected tumors were analyzed by immunohistochemical staining, and correlated with the uptake of PET tracers. For the detection of pulmonary malignant tumors, (18)F-FMT PET exhibited a sensitivity of 84% whereas the sensitivity for (18)F-FDG PET was 89% (p = 0.736). (18)F-FMT PET-CT and (18)F-FDG PET-CT agreed with pathological staging in 85 and 68%, respectively (p = 0.151). (18)F-FMT uptake was closely correlated with LAT1, CD98, cell proliferation and angiogenesis. The specificity of (18)F-FMT PET for diagnosing thoracic tumors was higher than that of (18)F-FDG PET. Our results suggest that coexpression of LAT1 and CD98 in addition to cell proliferation and angiogenesis is relavant for the progression and metastasis of lung cancer.

Expression of L-type amino acid transporter 1 (LAT1) in neuroendocrine tumors of the lung.

Amino acid transport systems play an important role in cellular proliferation. L-type amino acid transporter 1 (LAT1) has been associated with tumor growth, and is highly expressed in the established tumor cell lines and primary human neoplasms. In this study, we investigated the expression of LAT1 to evaluate the malignant potential and prognostic significance in neuroendocrine (NE) tumors of the lung. Twenty-one surgically resected, large cell neuroendocrine carcinomas (LCNEC), 13 small cell lung cancers (SCLC), five atypical carcinoids (AC), and 10 typical carcinoids (TC) were enrolled in the study. LAT1 expression and Ki-67 labeling index of the NE tumors were analyzed by immunohistochemical staining. LAT1 was overexpressed in 52.4% of the LCNEC, in 46.2% of the SCLC, and in 25% of the AC. LAT1 expression in LCNEC was significantly associated with lymph node metastasis and poor outcome. Moreover, a significant correlation was found between LAT1 expression and Ki-67 in both LCNEC and SCLC. Expression of LAT1 tended to increase from low-grade to high-grade NE tumors. The present results suggest that LAT1 may play a significant role in cellular proliferation, lymph node metastasis, and poor outcome in patients with NE tumors of the lung.

Expression of LAT1 predicts risk of progression of transitional cell carcinoma of the upper urinary tract.

L-type amino acid transporter 1 (LAT1), a neutral amino acid transporter, requires covalent association with the heavy chain of 4F2 cell surface antigen (4F2hc) for its functional form. We investigated the importance of LAT1 and 4F2hc expressions to progression in upper urinary tract cancer. We examined their expressions and their relationships to clinicopathologic parameters and clinical outcome in 124 cases. Positive expressions of LAT1 (protein and messenger ribonucleic acid) and 4F2hc (protein) were recognized in 79.8, 89.5, and 87.9% of tumor samples, respectively. In tumor cells, LAT1 protein was detected either as nodular granules within the cytoplasm or diffusely within the cytoplasm and/or on plasma membrane. In the normal urothelium, its expression was detected as nodular granules within the cytoplasm. A correlation with stage was shown for LAT1 protein expression and for a cooperative expression of LAT1 protein with 4F2hc protein (active form of LAT1 protein). Further, in all tumors, a cooperative expression of LAT1 protein and 4F2hc protein was significantly correlated with both overall and disease-free survival rates in the univariate analysis but not in the multivariate analysis. In conclusion, the detection of the active form of LAT1 protein would appear to be of value in informing the risk of progression in transitional cell carcinoma of the upper urinary tract.

L-type amino acid transporter 1 as a potential molecular target in human astrocytic tumors.

L-type amino acid transporter 1 (LAT1) is a Na+-independent neutral amino acid transport agency and essential for the transport of large neutral amino acids. LAT1 has been identified as a light chain of the CD98 heterodimer from C6 glioma cells. LAT1 also corresponds to TA1, an oncofetal antigen that is expressed primarily in fetal tissues and cancer cells. We have investigated for the first time, the expression of the transporter in the human primary astrocytic tumor tissue from 60 patients. LAT1 is unique because it requires an additional single membrane spanning protein, the heavy chain of 4F2 cell surface antigen (4F2hc), for its functional expression. 4F2hc expression was also determined by immunohistochemistry. Kaplan-Meier analyses demonstrated that high LAT1 expression correlated with poor survival for the study group as a whole (p<0.0001) and for those with glioblastoma multiforme in particular (p=0.0001). Cox regression analyses demonstrated that LAT1 expression was one of significant predictors of outcome, independent of all other variables. On the basis of these findings, we also investigated the effect of the specific inhibitor to LAT1, 2-aminobicyclo-2 (2,2,1)-heptane-2-carboxylic acid (BCH), on the survival of C6 glioma cells in vitro and in vivo using a rat C6 glioma model. BCH inhibited the growth of C6 glioma cells in vitro and in vivo in a dose-dependent manner. Kaplan-Meier survival data of rats treated with BCH were significant. These findings suggest that LAT1 could be one of the molecular targets in glioma therapy.

LAT1 expression in normal lung and in atypical adenomatous hyperplasia and adenocarcinoma of the lung.

No previous study has investigated neutral large amino acid transporter type 1 (LAT1) in normal lung cells, or in atypical adenomatous hyperplasia(s) (AAH) and nonmucinous bronchioloalveolar carcinoma(s) (NMBAC) of the lung. The authors examined: (1) the levels of LAT1 mRNA/glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA in 41 normal lung tissues and 34 NMBAC using semiquantitative reverse transcription-polymerase chain reaction; (2) LAT1 mRNA and protein expressions in 35 normal lung tissues, 34 AAH (11 lesions were interpreted as low-grade AAH and 23 as high-grade AAH), and 43 NMBAC using in situ hybridization and immunohistochemistry; and (2) the association of the incidences of LAT1 mRNA and protein expressions with cell proliferation in these lesions. The level of LAT1 mRNA/GAPDH mRNA (1) tended to be higher in NMBAC (12.0+/-8.1) than in normal lung tissues (1.0+/-0.2), and (2) covered a much wider range (from 0 to 276) in NMBAC than in normal lung tissues (from 0 to 5.8), with six NMBAC having values higher than 7.0, while 5.8 was the highest value detected in normal lung tissues. In peripheral normal lung tissues, LAT1 mRNA and protein were detected in bronchial surface epithelial cells and alveolar macrophages (but not in nonciliated bronchiolar epithelial cells, or in alveolar type I or type II cells). In bronchial surface epithelial cells, LAT1 protein appeared to be of a nodular type, which was considered to be a nonfunctional protein pattern. The incidences of positive expressions for LAT1 mRNA and protein were 54.5 and 27.3% in low-grade AAH, 65.2 and 52.2% in high-grade AAH, and 65.1 and 79.1% in NMBAC, respectively. In the case of LAT1 protein expression, significant differences could be shown between total (low-grade plus high-grade) AAH and NMBAC, and between low-grade AAH and NMBAC. Thus, in terms of the incidence of LAT1 protein expression, high-grade AAH appeared intermediate between low-grade AAH and NMBAC. The Ki-67 labeling index (a cell proliferation score) was significantly higher in those AAH and NMBAC that were LTA1-protein-positive than in their LAT1-protein-negative counterparts. In conclusion, LAT1 expression may increase with the upregulation of metabolic activity and cell proliferation in high-grade AAH and NMBAC.

High expression of L-type amino acid transporter 1 in infiltrating glioma cells.

L-type amino acid transporter 1 (LAT1), a neutral amino acid transport agent, is essential for the transport of large neutral amino acids. LAT1 also corresponds to tumor-associated gene-1 (TA1), an oncofetal antigen that is expressed primarily in fetal tissues and cancer cells such as glioma cells. We have investigated the expression of the transporter in the human primary glioma tissue from 68 patients. Among these patients, we could see the border zone between tumors and normal bain tissues in 10 patients. By WHO criteria, two of the specimens were diagnosed as grade 2, three as grade 3, and five as grade 4 [glioblastoma multiforme (GBM)]. In 9 of 10 cases, we could identify the infiltrating glioma cells associated with stronger immunoreactivity for LAT1. These tumor cells aggregated around the neurons in the border zone and were often found in the perivascular space. In one GBM case, the tumors seemed to develop expansively and separated from the normal brain with a border of arachnoid membrane. The expression of LAT1 was always higher in infiltrating glioma cells than in cells located in the center of the tumor. These findings suggest that LAT1 is one of the molecular targets for glioma therapy.

Expression and functional characterization of the system L amino acid transporter in KB human oral epidermoid carcinoma cells.

We have examined the expression and function of system l amino acid transporter in KB human oral epidermoid carcinoma cells. The KB cells express l-type amino acid transporter 1 (LAT1) in plasma membrane, but not l-type amino acid transporter 2 (LAT2). The [14C]l-leucine uptake by KB cells is inhibited by system l selective inhibitor BCH. The majority of [14C]l-leucine uptake is, therefore, mediated by LAT1. These results suggest that the transport of neutral amino acids including several essential amino acids into the KB cells mediated by LAT1 and the specific inhibition of LAT1 in oral cancer cells will be a new rationale for anti-cancer therapy.

System L: heteromeric exchangers of large, neutral amino acids involved in directional transport.

The plasma membrane transport system L is in many cells the only (efficient) pathway for the import of large branched and aromatic neutral amino acids. The corresponding transporters are hetero(di)mers composed of a catalytic subunit (LAT1 or LAT2=light chain=glycoprotein-associated amino acid transporter) associated covalently with the glycoprotein 4F2hc/CD98 (heavy chain). The tissue distribution of LAT1 suggests that it is involved mainly in transporting amino acids into growing cells and across some endothelial/epithelial secretory barriers, whereas the localization of LAT2 indicates that it is mainly involved in the basolateral efflux step of transepithelial (re)absorptive amino acid transport. However, system L transporters are obligatory amino acid exchangers with 1:1 stoichiometry, with similar (but not identical) intra- and extracellular substrate selectivities and with highly asymmetrical apparent affinities (low affinity inside). Therefore, net directional transport of large, neutral amino acids by system L depends on the parallel expression of a unidirectional transporter with overlapping selectivity (for instance systems A or N) that provides/recycles amino acids that drive system L exchange function. By mediating the regulated flux of these exchange substrates, unidirectional transporters control the activity of system L.

Functional cooperation of epithelial heteromeric amino acid transporters expressed in madin-darby canine kidney cells.

The heteromeric amino acid transporters b(0,+)AT-rBAT (apical), y(+)LAT1-4F2hc, and possibly LAT2-4F2hc (basolateral) participate to the (re)absorption of cationic and neutral amino acids in the small intestine and kidney proximal tubule. We show now by immunofluorescence that their expression levels follow the same axial gradient along the kidney proximal tubule (S1>S2S3). We reconstituted their co-expression in MDCK cell epithelia and verified their polarized localization by immunofluorescence. Expression of b(0,+)AT-rBAT alone led to a net reabsorption of l-Arg (given together with l-Leu). Coexpression of basolateral y(+)LAT1-4F2hc increased l-Arg reabsorption and reversed l-Leu transport from (re)absorption to secretion. Similarly, l-cystine was (re)absorbed when b(0,+)AT-rBAT was expressed alone. This net transport was further increased by the coexpression of 4F2hc, due to the mobilization of LAT2 (exogenous and/or endogenous) to the basolateral membrane. In summary, apical b(0,+)AT-rBAT cooperates with y(+)LAT1-4F2hc or LAT2-4F2hc for the transepithelial reabsorption of cationic amino acids and cystine, respectively. The fact that the reabsorption of l-Arg led to the secretion of l-Leu demonstrates that the implicated heteromeric amino acid transporters function in epithelia as exchangers coupled in series and supports the notion that the parallel activity of unidirectional neutral amino acid transporters is required to drive net amino acid reabsorption.

Characterization of the system L amino acid transporter in T24 human bladder carcinoma cells.

System L is a major nutrient transport system responsible for the Na(+)-independent transport of large neutral amino acids including several essential amino acids. In malignant tumors, a system L transporter L-type amino acid transporter 1 (LAT1) is up-regulated to support tumor cell growth. LAT1 is also essential for the permeation of amino acids and amino acid-related drugs through the blood-brain barrier. To search for in vitro assay systems to examine the interaction of chemical compounds with LAT1, we have investigated the expression of system L transporters and the properties of [14C]L-leucine transport in T24 human bladder carcinoma cells. Northern blot, real-time quantitative PCR and immunofluorescence analyses have reveled that T24 cells express LAT1 in the plasma membrane together with its associating protein 4F2hc, whereas T24 cells do not express the other system L isoform LAT2. The uptake of [14C]L-leucine by T24 cells is Na(+)-independent and almost completely inhibited by system L selective inhibitor BCH. The profiles of the inhibition of [14C]L-leucine uptake by amino acids and amino acid-related compounds in T24 cells are comparable with those for the LAT1 expressed in Xenopus oocytes. The majority of [14C]L-leucine uptake is, therefore, mediated by LAT1 in T24 cells. Consistent with LAT1 in Xenopus oocytes, the efflux of preloaded [14C]L-leucine is induced by extracellularly applied substrates of LAT1 in T24 cells. This efflux measurement has been proven to be more sensitive than that in Xenopus oocytes, because triiodothyronine, thyroxine and melphalan were able to induce the efflux of preloaded [14C]L-leucine in T24 cells, which was not detected for Xenopus oocyte expression system. T24 cell is, therefore, proposed to be an excellent tool to examine the interaction of chemical compounds with LAT1.

Expression and regulation of 4F2hc and hLAT1 in human trophoblasts.

The neutral amino acid transport system L is a sodium-independent transport system in human placenta and choriocarcinoma cells. Recently, it was found that the heterodimer composed of hLAT1 (a light-chain protein) and 4F2 heavy chain (4F2hc), a type II transmembrane glycoprotein, is responsible for system L amino acid transport. We found that the mRNAs of 4F2hc and hLAT1 were expressed in the human placenta and a human choriocarcinoma cell line. The levels of the 4F2hc and hLAT1 proteins in the human placenta increased at full term compared with those at midtrimester. Immunohistochemical data showed that these proteins were localized mainly in the placental apical membrane. Data from leucine uptake experiments, Northern blot analysis, and immunoblot analysis showed that this transport system was partially regulated by protein kinase C and calcium ionophore in the human choriocarcinoma cell line. Our results suggest that the heterodimer of 4F2hc and hLAT1 may play an important role in placental amino acid transport system L.