Cell growth inhibition by LAT1 siRNA in KB human oral cancer cell lines

Abstract

Amino acids are indispensable for the protein synthesis, which are essential for cell growth and proliferation in both normal and transformed cells. Amino acid transport across the plasma membrane is mediated via amino acid transporters located on the plasma membrane. The continuous growing and proliferating cells must require the continuous protein synthesis. It is known that the supply of amino acids for the continuous protein synthesis in these cells is mediated by the overexpression of amino acid transporters. Among the amino acid transport systems, the amino acid transport system L, which is a Na+-independent neutral amino acid transport system, is a major route for providing living cells including tumor cells with neutral amino acids including several essential amino acids. Recently, the system L-type amino acid transporter 1 (LAT1) was isolated. It was predicted to be 12-membrane-spanning proteins that mediate Na+-independent amino acid exchange. It requires an additional single-membrane-spanning protein, a heavy chain of 4F2 (4F2hc), for its functional expression in the plasma membrane. The LAT1 is highly expressed in the continuous growing and proliferating cells such as malignant tumor cells presumably to support their continuous growth and proliferation. The double stranded RNA-mediated RNA interference (RNAi) analysis can be in a wide variety of eukaryotes to induce the sequence-specific inhibition of gene expression. In this study, the effect of LAT1 short interfering RNA (siRNA) on cell growth was examined in the KB human oral cancer cell lines. In the RT-PCR analysis and Western blot analysis, the siRNA of LAT1 inhibited expression of LAT1 mRNA and protein. The uptake of [14C]L-leucine was inhibited by siRNA of LAT1. In the MTT assay, the siRNA of LAT1 inhibited the growth of the KB cells in time-dependent manner, indicating that the growth inhibition of KB cell by the siRNA of LAT1 is induced by the blocking of neutral amino acid transport mediated by LAT1. These results suggest that the transport of neutral amino acids including several essential amino acids into the KB human oral cancer cell lines was mediated mainly by LAT1. Further, the specific inhibition of LAT1 in oral cancer cell lines could be a new rationale for anti-cancer therapy.