Lahoutte T, Caveliers V, Dierickx L, Vekeman M, Everaert H, Mertens J, Bossuyt A
Department of Nuclear Medicine, Academic Hospital Free University, Laarbeeklaan 101, Brussels (AZ-VUB), 1090 Jette, Belgium.
Nucl Med Biol. 2001 Feb;28(2):129-34. doi: 10.1016/s0969-8051(00)00184-0.
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.
本研究的目的是利用单光子发射断层扫描技术，研究负责代谢肿瘤成像的两种放射性示踪剂3-[(125)I]碘-L-α-甲基酪氨酸((125)I-3-IMT)和2-[(125)I]碘-L-酪氨酸((125)I-2-IT)在U266人骨髓瘤癌细胞中积累的细胞摄取机制。评估了(125)I-3-IMT摄取的时间进程和浓度依赖性。使用伊迪-霍夫斯泰图计算动力学参数。使用一组主要氨基酸转运系统的竞争性抑制剂来区分负责摄取(125)I-3-IMT和(125)I-2-IT的转运体。使用酸沉淀法测定两种示踪剂的蛋白质掺入情况。(125)I-3-IMT转运的测得最大速度为每毫克蛋白质每秒4.199纳摩尔，米氏常数为107.9微摩尔。添加L系统的竞争性抑制剂2-氨基双环[2,2,1]庚烷-2-羧酸(BCH)，使(125)I-3-IMT的流入量减少39.0±3.3%，使(125)I-2-IT的流入量减少66.3±0.9%。色氨酸(Trp)进一步降低了对BCH不敏感的流入量，(125)I-3-IMT降低了43.8±3.5%，(125)I-2-IT降低了15.3±1.3%。这表明T系统转运参与其中。我们测得两种示踪剂的酸沉淀部分中的放射性均<2%，且随时间没有增加。我们得出结论，(125)I-3-IMT和(125)I-2-IT进入U266人骨髓瘤细胞是由L系统和T系统氨基酸转运体介导的。动力学参数表明，骨髓瘤患者血浆中芳香族氨基酸水平升高会降低(123)I-3-IMT的摄取。两种示踪剂均未显著进入蛋白质合成过程。
