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.

The monocarboxylate transporter 8 and L-type amino acid transporters 1 and 2 are expressed in mouse skeletons and in osteoblastic MC3T3-E1 cells

BACKGROUND

Several plasma membrane transporters have been shown to mediate the cellular influx and/or efflux of iodothyronines, including the sodium-independent organic anion co-transporting polypeptide 1 (OATP1), the sodium taurocholate co-transporting polypeptide (NTCP), the L-type amino acid transporter 1 (LAT1) and 2 (LAT2), and the monocarboxylate transporter 8 (MCT8). The aim of this study was to investigate if the mRNAs of these transporters were expressed and regulated by thyroid hormone (TH) in mouse calvaria-derived osteoblastic MC3T3-E1 cells and in the fetal and postnatal bones of mice.

METHODS

The mRNA expression of the iodothyronine transporters was investigated with real-time polymerase chain reaction analysis in euthyroid and hypothyroid fetuses and litters of mice and in MC3T3-E1 cells treated with increasing doses of triiodothyronine (T(3); 10(-10) to 10(-6) M) or with 10(-8) M T(3) for 1-9 days.

RESULTS

MCT8, LAT1, and LAT2 mRNAs were detected in fetal and postnatal femurs and in MC3T3-E1 cells, while OATP1 and NTCP mRNAs were not. LAT1 and LAT2 mRNAs were not affected by TH status in vivo or in vitro or by the stage of bone development or osteoblast maturation (analyzed by the expression of osteocalcin and alkaline phosphatase, which are key markers of osteoblastic differentiation). In contrast, the femoral mRNA expression of MCT8 decreased significantly during post-natal development, whereas MCT8 mRNA expression increased as MC3T3-E1 cells differentiated. We also showed that MCT8 mRNA was up-regulated in the femur of hypothyroid animals, and that it was down-regulated by treatment with T(3) in MC3T3-E1 cells.

CONCLUSIONS

This is the first study to demonstrate the mRNA expression of LAT1, LAT2, and MCT8 in the bone tissue of mice and in osteoblast-like cells. In addition, the pattern of MCT8 expression observed in vivo and in vitro suggests that MCT8 may be important to modulate TH effects on osteoblast differentiation and on bone development and metabolism.

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.

Reabsorption of neutral amino acids mediated by amino acid transporter LAT2 and TAT1 in the basolateral membrane of proximal tubule

In order to understand the renal reabsorption mechanism of neutral amino acids via amino acid transporters, we have isolated human L-type amino acid transporter 2 (hLAT2) and human T-type amino acid transporter 1 (hTAT1) in human, then, we have examined and compared the gene structures, the functional characterizations and the localization in human kidney. Northern blot analysis showed that hLAT2 mRNA was expressed at high levels in the heart, brain, placenta, kidney, spleen, prostate, testis, ovary, lymph node and the fetal liver. The hTAT1 mRNA was detected at high levels in the heart, placenta, liver, skeletal muscle, kidney, pancreas, spleen, thymus and prostate. Immunohistochemical analysis on the human kidney revealed that the hLAT2 and hTAT1 proteins coexist in the basolateral membrane of the renal proximal tubules. The hLAT2 transports all neutral amino acids and hTAT1 transports aromatic amino acids. The basolateral location of the hLAT2 and hTAT1 proteins in the renal proximal tubule as well as the amino acid transport activity of hLAT2 and hTAT1 suggests that these transporters contribute to the renal reabsorption of neutral and aromatic amino acids in the basolateral domain of epithelial proximal tubule cells, respectively. Therefore, LAT2 and TAT1 play essential roles in the reabsorption of neutral amino acids from the epithelial cells to the blood stream in the kidney. Because LAT2 and TAT1 are essential to the efficient absorption of neutral amino acids from the kidney, their defects might be involved in the pathogenesis of disorders caused by a disruption in amino acid absorption such as blue diaper syndrome.

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.

Transcriptional regulation of the LAT-1/CD98 light chain

LAT-1/CD98 amino acid transporter expression and activity are induced in hepatic cells deprived of arginine. The promoter dependency of this regulation was investigated. LAT-1 expression, in contrast to that of CD98 heavy chain 4F2, was actinomycin D sensitive in cells cultured without arginine. Transient transfection analysis with promoter reporter constructs including the 2 kbp LAT-1 promoter or a sub-sequence containing multiple potential amino acid response elements failed to show significant amino acid sensitivity in various cell types. Chromatin-dependency did not appear to account for this result as hepatic cell clones stably transfected with the promoter constructs showed little or no arginine or leucine responsive promoter activity. These studies suggest that while amino acid sensitivity of LAT-1 expression is transcriptionally regulated, cis elements within the proximal promoter do not directly mediate this regulation. Understanding mechanisms by which this gene responds to amino acid availability will contribute to our knowledge of how eukaryotic cells sense and respond to their environment.

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.

Five novel SLC7A7 variants and y+L gene-expression pattern in cultured lymphoblasts from Japanese patients with lysinuric protein intolerance

Two distinct human light subunits of the heteromeric amino acid transporter, y+LAT-1 coded by SLC7A7 and y+LAT-2 coded by SLC7A6, are both known to induce transport system y+L activity. SLC7A7 has already been identified as the gene responsible for lysinuric protein intolerance (LPI). We successfully identified five novel SLC7A7 variants (S238F, S489P, 1630delC, 1673delG, and IVS3-IVS5del9.7kb) in Japanese patients with LPI by PCR amplification and direct DNA sequencing. In addition, we performed a semi-quantitative expression analysis of SLC7A7 and SLC7A6 in human tissue. In normal tissue, the gene-expression ratio of SLC7A6 to SLC7A7 was high in the brain, muscle, and cultured skin fibroblasts; low in the kidneys and small intestine; and at an intermediate level in peripheral blood leukocytes, the lungs, and cultured lymphoblasts. The gene-expression ratio of SLC7A6 to SLC7A7 in cultured lymphoblasts was significantly different between normal subjects and LPI patients with R410X and/or S238F, where the relative amount of SLC7A7 mRNA was significantly lower and the relative amount of SLC7A6 mRNA was statistically higher in affected lymphoblasts than in normal cells. Expression of SLC7A7 and SLC7A6 may thus be interrelated in cultured lymphoblasts.

Expression of normal and mutant GFP-tagged y(+)L amino acid transporter-1 in mammalian cells

Lysinuric protein intolerance (LPI; MIM 222700) is an autosomal recessive disorder characterized by defective transport of cationic amino acids lysine, arginine and ornithine. The defect is localized in the basolateral membrane of polar epithelial cells of the renal tubules and intestine. The SLC7A7 (solute carrier family 7, member 7) gene that encodes y(+)LAT-1 (y(+)L amino acid transporter-1) is mutated in LPI, and leads to the malfunction of the heterodimer composed of y(+)LAT-1 and 4F2hc (4F2 heavy chain) responsible for the system y(+)L amino acid transport activity at the membrane. In this study, the intracellular trafficking and membrane expression of wild type and four mutant y(+)LAT-1 proteins (LPI(Fin), G54V, 1548delC, W242X) was studied in two human cell lines by expressing green fluorescent protein (GFP) tagged proteins. Different SLC7A7 mutations influenced the trafficking of y(+)LAT-1 in the cells differently, as the wild type and missense mutant fusion proteins localized to the plasma membrane, while the frameshift and nonsense mutants sequestered to the cytoplasmic membranes, never reaching the target areas of the cell.