Investigation of biological significance of neutral amino acid transporter expression in human brain tumors and development of novel therapeutic strategies

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Objectives. Malignant tumor cells show an elevated proliferative activity which would require increased intracellular metabolism. LAT1, in the presence of its co-factor 4F2hc, constitutes the system L neurtral amino acid transporter which is responsible for cellular transport of most essential amino acids. Recent reports demonstrating increased expression of LAT1 in cancer cells prompted us to investigate its expression and biological roles in human glioma cells.Methods. Expression of LAT1 and 4F2hc was determined in 13 human glioma cell lines using Western blot and RT-PCR analyses. LAT1-positive glioma cells were treated with a system L inhibitor BCH, and their proliferation and apoptosis rates were examined by BrdU staining and TUNEL assay, respectively. LAT1-negative glioma cells were transfected with the plasmid encoding human LAT1 cDNA and the cells expressing high levels of LAT1 were subjected to in vitro growth assays, and were injected into nude mice subcutaneously as well. Ami … More no acid uptake activity was measured using ^<14>C-Leu.Results. Majority of glioma cell lines expressed LAT1 protein. All cell lines were positive for 4F2hc mRNA expression. BCH treatment resulted in significant reduction of proliferation in LAT1-positive LNZ308 cells, whereas it was ineffective in LAT1-negative LN319 cells. LNZ308 cells showed reduced proliferation and increased apoptosis upon BCH treatment, which were accompanied with changes of expression levels of cell-cycle regulators and caspase activation. Introduction of LAT1 did not affect growth rates in LN319 cells in vitro. However, LAT1 overexpression leading to 3.4-fold increase in amino acid uptake activity in U87MG cells resulted in enhanced tumorigenicity in vivo.Conclusions. LAT1 and its partner 4F2hc are expressed in majority of human glioma cells. Our results suggest that LAT1, the major component of the amino acid transport system, may play an important role in gliomagenesis and may be a novel potential therapeutic target for malignant gliomas. Less