Fotoohi Alan Kambiz, Wrabel Anna, Moshfegh Ali, Peterson Curt, Albertioni Freidoun
Department of Oncology and Pathology, Cancer Center Karolinska (CCK), Karolinska Hospital and Institute, 171 76 Stockholm, Sweden.
Biochem Pharmacol. 2006 Sep 28;72(7):816-23. doi: 10.1016/j.bcp.2006.06.019. Epub 2006 Jul 20.
Methylmercaptopurine riboside (meMPR), a cellular metabolite of 6-mercaptopurine (6-MP), is a potent inhibitor of de novo purine synthesis (DNPS). Human MOLT4 T-lymphoblastic leukaemia cells that have acquired resistance to 6-MP or 6-thioguanine (6-TG) as a consequence of defective transport exhibit enhanced sensitivity to meMPR. HPLC-based analysis of the transport of meMPR revealed normal uptake of this compound by our thiopurine-resistant cell sublines, suggesting a route of transport distinct from that for 6-MP and 6-TG. Studies on the wild-type parental leukemic cells showed that adenosine, dipyridamole and nitrobenzylthioinosine inhibit uptake of meMPR to a significant extent, whereas Na+ ions have no influence on this process. Transfection of these leukemic cells with small interference RNA molecules targeting the gene encoding the first member of the family of equiliberative nucleoside transporters (ENT1) strongly reduced the initial rate of meMPR transport. Our resistant cell lines exhibited 30-52% reductions (p < 0.005) in their levels of mRNA encoding several proteins involved in de novo purine synthesis, i.e., aminoimidazole carboxamide ribonucleotide formyltransferase, glycinamide ribonucleotide transformylase and guanine monophosphate synthetase. Consequently, the rate of de novo purine synthesis in these resistant sublines was decreased by 50%. Furthermore, the levels of ribonucleoside triphosphates in these cells were significantly lower than in the non-resistant parental cells. In combination, a reduced rate of de novo purine synthesis together with low levels of ribonucleoside triphosphates can explain the enhanced sensitivity of our thiopurine-resistant cell lines to meMPR. In this manner, meMPR bypasses the mechanisms of resistance to thiopurines and is even more cytotoxic towards resistant than towards wild-type cells.
甲基巯基嘌呤核苷（meMPR）是6-巯基嘌呤（6-MP）的一种细胞代谢产物，是从头嘌呤合成（DNPS）的强效抑制剂。由于转运缺陷而对6-MP或6-硫鸟嘌呤（6-TG）产生耐药性的人MOLT4 T淋巴细胞白血病细胞对meMPR表现出增强的敏感性。基于高效液相色谱法对meMPR转运的分析表明，我们的硫嘌呤耐药细胞亚系对该化合物的摄取正常，这表明其转运途径与6-MP和6-TG不同。对野生型亲本白血病细胞的研究表明，腺苷、双嘧达莫和硝基苄基硫代肌苷在很大程度上抑制meMPR的摄取，而Na+离子对这一过程没有影响。用靶向编码平衡核苷转运体家族（ENT1）第一个成员的基因的小干扰RNA分子转染这些白血病细胞，强烈降低了meMPR转运的初始速率。我们的耐药细胞系在编码参与从头嘌呤合成的几种蛋白质（即氨基咪唑甲酰胺核糖核苷酸甲酰基转移酶、甘氨酰胺核糖核苷酸转甲酰酶和鸟嘌呤单磷酸合成酶）的mRNA水平上降低了30-52%（p<0.005）。因此，这些耐药亚系中从头嘌呤合成的速率降低了50%。此外，这些细胞中核糖核苷三磷酸的水平明显低于非耐药亲本细胞。综合起来，从头嘌呤合成速率降低以及核糖核苷三磷酸水平较低可以解释我们的硫嘌呤耐药细胞系对meMPR的敏感性增强。通过这种方式，meMPR绕过了对硫嘌呤的耐药机制，并且对耐药细胞的细胞毒性甚至比对野生型细胞更大。
