사람 골육종 세포 Saos2에서 아미노산 수송계 L의 발현 및 기능적 특성

EXPRESSION AND FUNCTIONAL CHARACTERIZATION OF AMINO ACID TRANSPORT SYSTEM L IN SAOS2 HUMAN OSTEOGENIC SARCOMA CELLS

Amino acids are required for protein synthesis and energy sources in all living cells. The amino acid transport system L is a major nutrient transport system that is responsible for Na+-independent transport of neutral amino acids including several essential amino acids. In malignant tumors, the Ltype amino acid transporter 1 (LAT1), the first isoform of system L, is highly expressed to support tumor cell growth. In the present study, the expression and functional characterization of amino acid transport system L were, therefore, investigated in Saos2 human osteogenic sarcoma cells. RT-PCR and western blot analyses have revealed that the Saos2 cells expressed the LAT1 and the L-type amino acid transporter 2 (LAT2), the second isoform of system L, together with their associating protein heavy chain of 4F2 antigen (4F2hc) in the plasma membrane, but the expression of LAT2 was very weak. The uptakes of [14C]L-leucine by 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 of [14C]L-leucine uptake and the inhibition profiles of [14C]L-leucine uptake by various amino acids in the Saos2 cells were comparable with those for the LAT1 expressed in Xenopus oocytes. The majority of [14C]Lleucine uptake is, therefore, mediated by LAT1 in the Saos2 cells. These results suggest that the transports of neutral amino acids including several essential amino acids into Saos2 human osteogenic sarcoma cells are for the most part mediated by LAT1. Therefore, the Saos2 human osteogenic sarcoma cells are excellent tools for examine the properties of LAT1. Moreover, the specific inhibition of LAT1 in tumor cells might be a new rationale for anti-tumor therapy.