In vitro characterization of the influx of 3-[125I]iodo-L-alpha-methyltyrosine and 2-[125I]iodo-L-tyrosine into U266 human myeloma cells: evidence for system T transport.
The aim of this study was to investigate the cellular uptake mechanisms responsible for the accumulation of 3-[(125)I]iodo-L-alpha-methyltyrosine ((125)I-3-IMT) and 2-[(125)I]iodo-L-tyrosine ((125)I-2-IT), two radiotracers for metabolic tumor imaging, using single-photon emission tomography, into U266 human myeloma cancer cells. Time course and concentration dependency of (125)I-3-IMT uptake was assessed. Kinetic parameters were calculated using an Eadie Hofstee plot. A set of competitive inhibitors of the main amino acid transport systems was used for the discrimination of the transporters responsible for the uptake of (125)I-3-IMT and (125)I-2-IT. Protein incorporation of both tracers was determined using acid precipitation. The measured maximum velocity for (125)I-3-IMT transport was 4.199 nmol per mg protein 20 s(-1), and the Michaelis constant was 107.9 microM. Addition of 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH), a competitive inhibitor of System L, reduced the influx by 39.0+/-3.3% for (125)I-3-IMT and 66.3+/-0.9% for (125)I-2-IT. The BCH-insensitive influx was further reduced by Tryptophan (Trp) by 43.8+/-3.5% for (125)I-3-IMT and 15.3+/-1.3% for (125)I-2-IT. This suggests involvement of System T transport. We measured <2% of radioactivity in the acid precipitable fractions of both tracers with no increase in time. We conclude that the influx of (125)I-3-IMT and (125)I-2-IT into U266 human myeloma cells is mediated by both System L and System T amino acid transporters. The kinetic parameters suggest that elevated plasma levels of aromatic amino acids will reduce (123)I-3-IMT uptake in myeloma patients. Both tracers do not enter protein synthesis significantly.
Lahoutte
T
T
Department of Nuclear Medicine, Academic Hospital Free University, Laarbeeklaan 101, Brussels (AZ-VUB), 1090 Jette, Belgium. nucglet@az.vub.ac.be
Caveliers
V
V
Dierickx
L
L
Vekeman
M
M
Everaert
H
H
Mertens
J
J
Bossuyt
A
A
D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D005260:Female; D015902:Gamma Cameras; D006801:Humans; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics*; D007700:Kinetics; D008297:Male; D008781:Methyltyrosines / Q000493:pharmacokinetics*; D007470:Monoiodotyrosine / Q000493:pharmacokinetics*; D009101:Multiple Myeloma / Q000000981:diagnostic imaging* / Q000378:metabolism; D011877:Radionuclide Imaging; D051381:Rats; D017208:Rats, Wistar; D014018:Tissue Distribution; D014364:Tryptophan / Q000378:metabolism; D014407:Tumor Cells, Cultured
D000596:Amino Acids; D007457:Iodine Radioisotopes; D008781:Methyltyrosines; D014364:Tryptophan; C030855:3-iodo-alpha-methyltyrosine; D007470:Monoiodotyrosine