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.