Human L-type amino acid transporter 1 (LAT1): characterization of functionand expression in tumor cell lines

System L is a major nutrient transport system responsible for the transport of large neutral amino acids including several essential amino acids. We p reviously identified a transporter (L-type amino acid transporter 1: LAT1) subserving system L in C6 rat glioma cells and demonstrated that LAT1 requi res 4F2 heavy chain (4F2hc) for its functional expression. Since its oncofe tal expression was suggested in the rat liver, it has been proposed that LA T1 plays a critical role in cell growth and proliferation. In the present s tudy, we have examined the function of human LAT1 (hLAT1) and its expressio n in human tissues and tumor cell lines. When expressed in Xenopus oocytes with human 4F2hc (h4F2hc), hLAT1 transports large neutral amino acids with high affinity (K-m = similar to 15-similar to 50 muM) and L-glutamine and L -asparagine with low affinity (K-m = similar to1.5-similar to2 mM). hLAT1 a lso transports D-amino acids such as D-leucine and D-phenylalanine. In addi tion, we show that hLAT1 accepts an amino acid-related anti-cancer agent me lphalan. When loaded intracellularly, L-leucine and L-glutamine but not L-a lanine are effluxed by extracellular substrates, confirming that hLAT1 medi ates an amino acid exchange. hLAT1 mRNA is highly expressed in the human fe tal liver, bone marrow, placenta, testis and brain. We have found that, whi le all the tumor cell lines examined express hLAT1 messages, the expression of h4F2hc is varied particularly in leukemia cell lines. In Western blot a nalysis, hLAT1 and h4F2hc have been confirmed to be linked to each other vi a a disulfide bond in T24 human bladder carcinoma cells. Finally, in in vit ro translation, we show that hLAT1 is not a glycosylated protein even thoug h an N-glycosylation site has been predicted in its extracellular loop, con sistent with the property of the classical 4F2 light chain. The properties of the hLAT1/h4F2hc complex would support the roles of this transporter in providing cells with essential amino acids for cell growth and cellular res ponses, and in distributing amino acid-related compounds. (C) 2001 Elsevier Science B.V. All rights reserved.