Expression of L-type amino acid transporter 1 as a molecular target for prognostic and therapeutic indicators in bladder carcinoma

L-type amino acid transporter 1 (LAT1) plays a role in transporting essential amino acids including leucine, which regulates the mTOR signaling pathway. Here, we studied the expression profile and functional role of LAT1 in bladder cancer. Furthermore, the pharmacological activity of JPH203, a specific inhibitor of LAT1, was studied in bladder cancer. LAT1 expression in bladder cancer cells was higher than that in normal cells. SiLAT1 and JPH203 suppressed cell proliferative and migratory and invasive abilities in bladder cancer cells. JPH203 inhibited leucine uptake by > 90%. RNA-seq analysis identified insulin-like growth factor-binding protein-5 (IGFBP-5) as a downstream target of JPH203. JPH203 inhibited phosphorylation of MAPK / Erk, AKT, p70S6K and 4EBP-1. Multivariate analysis revealed that high LAT1 expression was found as an independent prognostic factor for overall survival (HR3.46 P = 0.0204). Patients with high LAT1 and IGFBP-5 expression had significantly shorter overall survival periods than those with low expression (P = 0.0005). High LAT1 was related to the high Grade, pathological T stage, LDH, and NLR. Collectively, LAT1 significantly contributed to bladder cancer progression. Targeting LAT1 by JPH203 may represent a novel therapeutic option in bladder cancer treatment.

The Regulation and Function of the L-Type Amino Acid Transporter 1 (LAT1) in Cancer

The progression of cancer is associated with increases in amino acid uptake by cancer cells. Upon their entry into cells through specific transporters, exogenous amino acids are used to synthesize proteins, nucleic acids and lipids and to generate ATP. The essential amino acid leucine is also important for maintaining cancer-associated signaling pathways. By upregulating amino acid transporters, cancer cells gain greater access to exogenous amino acids to support chronic proliferation, maintain metabolic pathways, and to enhance certain signal transduction pathways. Suppressing cancer growth by targeting amino acid transporters will require an in-depth understanding of how cancer cells acquire amino acids, in particular, the transporters involved and which cancer pathways are most sensitive to amino acid deprivation. L-Type Amino Acid Transporter 1 (LAT1) mediates the uptake of essential amino acids and its expression is upregulated during the progression of several cancers. We will review the upstream regulators of LAT1 and the downstream effects caused by the overexpression of LAT1 in cancer cells.

Overexpression of L-Type Amino Acid Transporter 1 (LAT1) and 2 (LAT2): Novel Markers of Neuroendocrine Tumors

Background

6-18F-fluoro-L-3,4-dihydroxyphenylalanine (¹⁸F-FDOPA) PET is a useful tool in the clinical management of pheochromocytoma (PHEO) and medullary thyroid carcinoma (MTC). ¹⁸F-FDOPA is a large neutral amino acid biochemically resembling endogenous L-DOPA and taken up by the L-type amino acid transporters (LAT1 and LAT2). This study was conducted to examine the expression of the LAT system in PHEO and MTC.

Methods

Real-time PCR and Western blot analyses were used to assess LAT1 and LAT2 gene and protein expression in 32 PHEO, 38 MTC, 16 normal adrenal medulla and 15 normal thyroid tissue samples. Immunohistochemistry method was applied to identify the proteins’ subcellular localization.

Results

LAT1 and LAT2 were overexpressed in both PHEO and MTC by comparison with normal tissues. LAT1 presented a stronger induction than LAT2, and their greater expression was more evident in PHEO (15.1- and 4.1-fold increases, respectively) than in MTC (9.9- and 4.1-fold increases, respectively). Furthermore we found a good correlation between LAT1/2 and GLUT1 expression levels. A positive correlation was also found between urinary noradrenaline and adrenaline levels and LAT1 gene expression in PHEO. The increased expression of LAT1 is also confirmed at the protein level, in both PHEO and MTC, with a strong cytoplasmic localization.

Conclusions

The present study is the first to provide experimental evidence of the overexpression in some NET cancers (such as PHEO or MTC) of L-type amino acid transporters, and the LAT1 isoform in particular, giving the molecular basis to explain the increase of the DOPA uptake seen in such tumor cells.

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

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

L-type amino acid transporter 1 (LAT1) expression in malignant pleural mesothelioma

L-Type amino acid transporter 1 (LAT1) is known to be highly expressed in various human neoplasms. However, little is known about how LAT1 is expressed in malignant pleural mesothelioma (MPM). Twenty-one patients were included in this study. Tumor sections were stained by immunohistochemistry for LAT1, glucose transporter 1 (GLUT1), GLUT3, hypoxia inducible factor-1α (HIF-1α), hexokinase I, vascular endothelial growth factor (VEGF), microvessel density (MVD) by determination of CD34, epidermal growth factor receptor (EGFR), phosphatase and tensin analog (PTEN), p-AKT, p-manmalian target of rapamycin (mTOR), p-S6K, p53 and BCL-2. LAT1 was overexpressed in approximately 50% of the patients with MPM. LAT1 expression was closely correlated with CD98, hypoxic markers, the mTOR pathway, Ki-67 and p53. The overexpression of LAT1 was closely associated with poor outcome in patients with MPM. LAT1 is closely associated with tumor development and progression in patients with MPM.

Inhibition of L-type amino acid transporter 1 has antitumor activity in non-small cell lung cancer

BACKGROUND

L-type amino acid transporter 1 (LAT1) is highly expressed in various human neoplasms. Antitumor activity of inhibiting LAT1 was analyzed in non-small cell lung cancer (NSCLC).

MATERIALS AND METHODS

Expression of LAT1 mRNA in 54 lung cancer cell lines was examined by RT-PCR. An inhibitor of LAT1, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), was administered to H1395 cell. LAT1 expression was examined in correlation with clinical features and outcome in 51 NSCLC patients.

RESULTS

Inhibition of LAT1 by BCH reduced cell viability in H1395 cells. Furthermore, co-administration of gefitinib with BCH reduced the viability of the cells more than either agent alone. Inhibition of LAT1 reduced the level of phosphorylation of mTOR, p70S6K and 4EBP1. LAT1 protein expression was closely associated with wild type EGFR, and was an independent significant factor to predict a poor prognosis.

CONCLUSION

Inhibition of LAT1 may be a new rationale to the effective therapy of NSCLC without EGFR mutation.

L-type amino-acid transporter 1 expression predicts the response to preoperative hyperthermo-chemoradiotherapy for advanced rectal cancer

AIM

To evaluate whether expression of L-type amino acid transporter 1 (LAT1) in pretreatment rectal cancer biopsies is predictive of tumour response to neoadjuvant hyperthermo-chemoradiotherapy (HCRT).

PATIENTS AND METHODS

Forty-four patients with rectal adenocarcinoma who received neoadjuvant HCRT were investigated. LAT1 expression was immunohistochemically evaluated using pretreatment biopsies. The operation was performed after 2-3 months following HCRT and each resected specimen was graded by the histological criteria of the Japanese Classification of Colorectal Carcinoma.

RESULTS

A positive LAT1 expression was recognized in 50.0% (22/44) of patients. Resected specimens were divided into 2 groups according to the histological grading criteria: good response (n=29) and poor response (n=15). LAT1-negative tumours had an 81.8% probability of good response and 18.2% probability of poor response. LAT1 expression showed marginally significant association with response to HCRT (p=0.05).

CONCLUSION

LAT1 may be a useful predictive marker of response to HCRT in rectal cancer.

Polyamines upregulate the mRNA expression of cationic amino acid transporter-1 in human retinal pigment epithelial cells

We previously showed that ornithine was mainly transported via cationic amino acid transporter (CAT)-1 in human retinal pigment epithelial (RPE) cell line, human telomerase RT (hTERT)-RPE, and that CAT-1 was involved in ornithine cytotoxicity in ornithine-δ-aminotransferase (OAT)-deficient cell produced by a OAT specific inhibitor, 5-fluoromethylornithine (5-FMO). We showed here that CAT-1 mRNA expression was increased by ornithne in OAT-deficient RPE cells, which was reversed by an inhibitor of ornithine decarboxylase (ODC), α-difluoromethylornithine (DFMO). Polyamines, especially spermine, one of the metabolites of ODC, also enhanced the expression of CAT-1 mRNA. ODC mRNA expression was also increased by ornithine and polyamines, and gene silencing of ODC by siRNA decreased ornithine transport activity and its cytotoxicity. In addition, the mRNA of nuclear protein c-myc was also increased in 5-FMO- and ornithine-treated hTERT-RPE cells, and gene silencing of c-myc prevented the induction of CAT-1 and ODC. Increases in expression of CAT-1, ODC, and c-myc, and the inhibition of these stimulated expression by DFMO were also observed in primary porcine RPE cells. These results suggest that spermine plays an important role in stimulation of mRNA expression of CAT-1, which is a crucial role in ornithine cytotoxicity in OAT-deficient hTERT-RPE cells.

The RNA interference of amino acid transporter LAT1 inhibits the growth of KB human oral cancer cells

BACKGROUND

Amino acid transporters are essential for growth and proliferation in all living cells. Among the amino acid transporters, the system L amino acid transporters are the major nutrient transport system responsible for the Na+-independent transport of neutral amino acids, including several essential amino acids. The L-type amino acid transporter 1 (LAT1) is overexpressed to support cell growth in malignant tumors. Double-stranded RNA-mediated RNA interference (RNAi) analysis can be used in a wide variety of eukaryotes to induce the sequence-specific inhibition of gene expression. The current study attempted to investigate the effects of silencing LAT1 expression with small interfering RNA (siRNA) on cell growth in the KB human oral squamous cell carcinoma.

MATERIALS AND METHODS

The effects of silencing LAT1 expression with siRNA KB on cell growth were examined using RT-PCR, Western blot analysis, amino acid transport measurement and the MTT assay.

RESULTS

In the RT-PCR and Western blot analyses, the siRNA of LAT1 inhibited the expressions of LAT1 mRNA and protein. The uptake of [14C]L-leucine was also inhibited by the siRNA of LAT1. In the MTT assay, the siRNA of LAT1 inhibited the growth of the KB cells in a time-dependent manner, indicating that this growth inhibition was induced by the LAT1-mediated blocking of neutral amino acid transport.

CONCLUSION

The transport of neutral amino acids, including several essential amino acids, into the KB human oral squamous cell carcinoma is mainly mediated by LAT1. Furthermore, LAT1 could be a new target for the inhibition of cancer cell growth.

Differential expression and functional characterization of system L amino acid transporters in human normal osteoblast cells and osteogenic sarcoma cells

BACKGROUND

The amino acid transport system L is a major nutrient transport system responsible for Na(+)-independent transport of neutral amino acids, including several essential amino acids. The system L is divided into two major subgroups, the L-type amino acid transporter 1 (LAT1) and the L-type amino acid transporter 2 (LAT2). In malignant tumors, the LAT1 is highly expressed to support tumor cell growth. In the present study, the expressions and functions of the system L amino acid transporters were examined and compared in both FOB human osteoblast cells and Saos2 human osteogenic sarcoma cells.

MATERIALS AND METHODS

The expressions and functions of the system L amino acid transporters in both FOB and Saos2 cells were examined using RT-PCR, Western blot analysis and amino acid transport measurement.

RESULTS

RT-PCR and Western blot analysis revealed that the FOB and Saos2 cells expressed LAT1 and LAT2, together with their associated protein 4F2hc, but the expression of LAT2 in the Saos2 cells was very weak. The uptakes of [14C]L-leucine by FOB and Saos2 cells were Na(+)-independent and were completely inhibited by the system L selective inhibitor, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH). The affinity and the inhibition profiles of [14C]L-leucine uptake by various amino acids in the FOB and Saos2 cells were comparable with those for the LAT2 and LAT1 expressed in Xenopus oocytes, respectively. The majority of [14C]L-leucine uptake is, therefore, mediated by LAT2 and LAT1 in FOB and Saos2 cells, respectively.

CONCLUSION

These results suggest that the transport of neutral amino acids, including several essential amino acids into the FOB and Saos2 cells, are mainly mediated by LAT2 and LAT1, respectively. Moreover, the specific inhibition of LAT1 in tumor cells might be a new rationale for antitumor therapy.

Expression of LAT1 and LAT2 amino acid transporters in human and rat intestinal epithelial cells

The present study evaluated the presence of LAT1 and LAT2 amino acid transporters in human Caco-2 cells and rat IEC-6 cells along the mucosa of the rat digestive tract. The LAT1 cDNA was amplified by PCR using two sets of primers (one specific for rat LAT1 and another simultaneously specific for human, rat and mice). The LAT2 cDNA was amplified by PCR using one set of primers simultaneously specific for human, rat and mice LAT2. The presence of LAT1 and LAT2 protein was examined by means of immunoblotting using an antibody raised against the rat LAT1 and mouse LAT2. Caco-2 and IEC-6 cells, as well as the rat intestinal mucosa, are endowed with both LAT1 and LAT2 transporter transcripts and protein. LAT1 protein is most abundant in IEC-6 cells, which is in agreement with functional data previously reported. The findings in the rat intestinal mucosa indicate that LAT1 protein is most abundant in the colon and its abundance markedly decreases at the level of jejunum and ileum, which contrast with relative homogeneous presence of LAT2 across the digestive tract. In conclusion, Caco-2 and IEC-6 cells, as well as the rat intestinal mucosa, are endowed with both LAT1 and LAT2 amino acid transporter transcripts and protein.

Expression of L-type amino acid transporter 1 (LAT1) in esophageal carcinoma

BACKGROUND AND OBJECTIVES

It has been reported that amino acid transport systems play an important role in cell proliferation. Their activity is increased in malignant cells compared to benign cells. In this study, we investigated whether L-type amino acid transporter 1 (LAT1) is expressed in human non-cancerous esophageal mucosa and esophageal squamous cell carcinoma. We also examined whether LAT1 expression is correlated with histopathological features.

METHODS

From January 1999 to December 2001, sections of formalin-fixed, paraffin-embedded tissue from 11 cases of early esophageal carcinoma (T1) and 19 cases of advanced esophageal carcinoma (T2, T3) were entered in the study. Histopathologically, all 30 cases were squamous cell carcinoma. Immunohistochemical staining was performed using rabbit anti-LAT1 IgG, with the standard avidin-streptavidin immuno-peroxidase method. Measurement was performed by means of computer-assisted image analysis. The ratio of cells with LAT1 expression in esophageal squamous cell carcinoma and non-cancerous esophageal mucosa was used for analysis in this study.

RESULTS

Non-cancerous esophageal mucosa expressed LAT1 only in the basal layer of the esophageal wall. Esophageal squamous cell carcinoma expressed LAT1 throughout the tumor. LAT1 expression in esophageal squamous cell carcinoma was significantly higher than that in non-cancerous esophageal mucosa. LAT1 expression in esophageal squamous cell carcinoma increased as the depth of invasion progressed (T1 < T2 (P = 0.0477), T2 < T3 (P = 0.0415), T1 < T3 (P = 0.0044)), and as the tumor size increased. Also, high LAT1 expression was significantly associated with well-differentiated carcinoma.

CONCLUSION

These results suggest that LAT1 plays a significant role in cell proliferation, differentiation, and invasion in esophageal squamous cell carcinoma.

Lysophosphatidylcholine enhances cytokine production of endothelial cells via induction of L-type amino acid transporter 1 and cell surface antigen 4F2

OBJECTIVE

A diverse range of lipid oxidation products detected in oxidized low-density lipoprotein (oxLDL) and atherosclerotic lesions are capable of eliciting biological responses in vascular cells. We performed DNA microarray experiments to explore novel responses of human umbilical vein endothelial cells (HUVECs) to oxLDL and its components.

METHODS AND RESULTS

cDNA microarray analysis showed that oxLDL, lysophosphatidylcholine (LysoPC), 4-hydroxy-2-nonenal, and oxysterols altered gene expression specifically, but some genes were commonly induced in HUVECs. Solute carrier family 3 member 2 and family 7 member 5, encoding the heavy chain of the cell surface antigen 4F2 (4F2hc) and the L-type amino acid transporter 1 (LAT1), respectively, were induced by oxLDL and many oxidation products. LAT1 requires 4F2hc to form a heterodimeric functional complex to transport neutral amino acids into the cell. LysoPC increased membrane protein levels of LAT1 confirmed by Western blot analysis and also uptake of L-[(14)C]leucine, which was inhibited by a competitive inhibitor for LAT1. The release of interleukin 6 (IL-6) and IL-8 was increased in LysoPC-treated cells and was attenuated by the LAT1 inhibitor.

CONCLUSIONS

These findings suggest that an increase in uptake of neutral amino acids induced by LysoPC results in enhancement of inflammatory responses of endothelial cells.

Expression of L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (4F2hc) in oral squamous cell carcinoma and its precusor lesions

BACKGROUND

Amino acid transporters play an important role in supplying organic nutrients to cells. The expression of L-type amino acid transporter 1 (LAT1) and its subunit 4F2 heavy chain (4F2hc) was evaluated to determine the alterations of these transporters in oral normal mucosa (ONM), oral precancerous lesion (OPL) and oral squamous cell carcinoma (OSCC).

MATERIALS AND METHODS

Sections from formalin-fixed, paraffin-embedded samples of ONM, OPL or OSCC were examined using immunohistochemical staining to detect L4T1 and 4F2hc proteins.

RESULTS

The LAT1 and 4F2hc expression increased progressively from ONM to hyperplastic and to dysplastic lesions and OSCC. In particular, LAT1 may be a more specific indicator of tumor progression than 4F2hc.

CONCLUSION

LAT1 and 4F2hc may have an important role in the early stages of multistep oral carcinogenesis. In addition, the specific inhibition of LAT1 and 4F2hc might be a new rationale to suppress oral cancer progression.

Platelet-derived growth factor stimulates LAT1 gene expression in vascular smooth muscle: role in cell growth

Platelet-derived growth factor (PDGF) contributes to vascular disease by stimulating the growth of vascular smooth muscle cells (SMCs). Since amino acids are required for cell growth, the present study examined the effect of PDGF on system L amino acid transport, which is the predominant cellular pathway for the uptake of essential amino acids. System L amino acid transport was monitored by measuring the uptake of L-leucine. Treatment of SMCs with PDGF stimulated L-leucine transport in a concentration- and time-dependent manner, and this was associated with a selective increase in LAT1 mRNA and protein. PDGF failed to induce the expression of the other system L transport proteins, LAT2 and the heavy chain of the 4F2 cell surface antigen. The induction of LAT1 by PDGF was dependent on de novo RNA and protein synthesis and on mTOR activity. Serum, thrombin, and angiotensin II likewise stimulated L-leucine transport by inducing LAT1 expression. Inhibition of system L amino acid transport by the model substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid blocked growth factor-mediated SMC proliferation and induced SMC apoptosis, whereas it had no effect on quiescent cells. These results demonstrate that growth factors stimulate system L amino acid transport by inducing LAT1 gene expression and that system L amino acid transport is essential for SMC proliferation and survival. The capacity of vascular mitogens to induce LAT1 expression may represent a basic mechanism by which tho acid transport * apoptosis

Functional and molecular characteristics of system L in human breast cancer cells

The functional and molecular properties of system L in human mammary cancer cells (MDA-MB-231 and MCF-7) have been examined. All transport experiments were conducted under Na(+)-free conditions. alpha-Aminoisobutyric acid (AIB) uptake by MDA-MB-231 and MCF-7 cells was almost abolished by BCH (2-amino-2-norbornane-carboxylic acid). AIB uptake by MDA-MB-231 cells was also inhibited by L-alanine (83.6%), L-lysine (75.6%) but not by L-proline. Similarly, L-lysine and L-alanine, respectively, reduced AIB influx into MCF-7 cells by 45.3% and 63.7%. The K(m) of AIB uptake into MDA-MB-231 and MCF-7 cells was, respectively, 1.6 and 8.8 mM, whereas the V(max) was, respectively, 9.7 and 110.0 nmol/mg protein/10 min. AIB efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH, L-glutamine, L-alanine, L-leucine, L-lysine and AIB (all at 2 mM). In contrast, L-glutamate, L-proline, L-arginine and MeAIB had no effect. The interaction between L-lysine and AIB efflux was one of low affinity. The fractional release of AIB from MDA-MB-231 cells was trans-accelerated by D-leucine and D-tryptophan but not by D-alanine. MDA-MB-231 and MCF-7 cells expressed LAT1 and CD98 mRNA. MCF-7 cells also expressed LAT2 mRNA. The results suggest that AIB transport in mammary cancer cells under Na(+)-free conditions is predominantly via system L which acts as an exchange mechanism. The differences in the kinetics of AIB transport between MDA-MB-231 and MCF-7 cells may be due to the differential expression of LAT2.

Expression of L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (4F2hc) in liver tumor lesions of rat models

BACKGROUND AND OBJECTIVES

It has been said that amino acid transporters play an important role in supplying nutrition to cells and for cell proliferation. In this study, we examined whether LAT1 and 4F2hc are closely related to tumor growth.

METHODS

Rat colon cancer cells (RCN-9) were injected into the spleen of 12 male rats (inbred F344/DuCrj). In each rat, liver samples including tumor lesions were immunostained with anti-LAT1 and anti-4F2hc antibodies. The staining area of LAT1 and 4F2hc tumor lesions was calculated by computer analysis.

RESULTS

Sixty-eight tumor nodules were observed in 12 livers. Out of the 68 tumor nodules, 36 nodules (52.9%) indicated a positive staining of LAT1 and 32 (47.1%) had a negative staining of LAT1. However, the LAT1 expression was scarcely detected in non-tumor areas. In terms of the 4F2hc expression, there were 56 nodules (82.4%) with 4F2hc positive and 12 (17.6) with 4F2hc negative. In addition, the expression of 4F2hc in non-tumor areas was almost the same as the expression of 4F2hc in tumor lesions. The average tumor size of the group with LAT1 positive and 4F2hc positive (n = 31) was 0.845 +/- 0.232 mm(2), which was significantly larger than that of the group with LAT1 negative and 4F2hc negative group (n = 7) (0.090 +/- 0.028 mm(2)) or the group with LAT1 positive and 4F2hc negative (n = 5) (0.097 +/- 0.025 mm(2)), respectively (P = 0.0017, P = 0.007).

CONCLUSION

LAT1 was related to tumor growth. We think that LAT1 can possibly enhance its ability to promote tumor growth in cooperation with 4F2hc.