Giagkousiklidis Stavros, Vogler Meike, Westhoff Mike-Andrew, Kasperczyk Hubert, Debatin Klaus-Michael, Fulda Simone
University Children's Hospital, Ulm, Germany.
Cancer Res. 2005 Nov 15;65(22):10502-13. doi: 10.1158/0008-5472.CAN-05-0866.
Resistance to current treatment regimens, such as radiation therapy, remains a major concern in oncology and may be caused by defects in apoptosis programs. Because inhibitor of apoptosis proteins (IAPs), which are expressed at high levels in many tumors, block apoptosis at the core of the apoptotic machinery by inhibiting caspases, therapeutic modulation of IAPs could target a key control point in resistance. Here, we report for the first time that full-length or mature second mitochondria-derived activator of caspase (Smac), an inhibitor of IAPs, significantly enhanced gamma-irradiation-induced apoptosis and reduced clonogenic survival in neuroblastoma, glioblastoma, or pancreatic carcinoma cells. Notably, Smac had no effect on DNA damage/DNA repair, activation of nuclear factor-kappaB, up-regulation of p53 and p21 proteins, or cell cycle arrest following gamma-irradiation, indicating that Smac did not alter the initial damage and/or cellular stress response. Smac enhanced activation of caspase-2, caspase-3, caspase-8, and caspase-9, loss of mitochondrial membrane potential, and cytochrome c release on gamma-irradiation. Inhibition of caspases also blocked gamma-irradiation-induced mitochondrial perturbations, indicating that Smac facilitated caspase activation, which in turn triggered a mitochondrial amplification loop. Interestingly, mitochondrial perturbations were completely blocked by the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone or the relatively selective caspase-2 inhibitor N-benzyloxycarbonyl-Val-Asp-Val-Ala-Asp-fluoromethylketone, whereas caspase-8 or caspase-3 inhibitors only inhibited the increased drop of mitochondrial membrane potential provided by Smac, suggesting that caspase-2 was acting upstream of mitochondria after gamma-irradiation. In conclusion, our findings provide evidence that targeting IAPs (e.g., by Smac agonists) is a promising strategy to enhance radiosensitivity in human cancers.
对当前治疗方案（如放射治疗）产生耐药性仍是肿瘤学领域的一个主要问题，其可能由凋亡程序缺陷引起。由于凋亡抑制蛋白（IAPs）在许多肿瘤中高表达，通过抑制半胱天冬酶在凋亡机制的核心部位阻断凋亡，因此对IAPs进行治疗性调控可能针对耐药性的一个关键控制点。在此，我们首次报道全长或成熟的第二线粒体来源的半胱天冬酶激活剂（Smac），一种IAPs的抑制剂，能显著增强γ射线诱导的神经母细胞瘤、胶质母细胞瘤或胰腺癌细胞的凋亡并降低其克隆形成存活率。值得注意的是，Smac对γ射线照射后的DNA损伤/DNA修复、核因子κB的激活、p53和p21蛋白的上调或细胞周期阻滞均无影响，这表明Smac并未改变初始损伤和/或细胞应激反应。Smac增强了γ射线照射后半胱天冬酶-2、半胱天冬酶-3、半胱天冬酶-8和半胱天冬酶-9的激活、线粒体膜电位的丧失以及细胞色素c的释放。抑制半胱天冬酶也阻断了γ射线诱导的线粒体扰动，表明Smac促进了半胱天冬酶的激活，进而触发了线粒体放大环。有趣的是，广泛的半胱天冬酶抑制剂N-苄氧羰基-Val-Ala-Asp-氟甲基酮或相对选择性的半胱天冬酶-2抑制剂N-苄氧羰基-Val-Asp-Val-Ala-Asp-氟甲基酮能完全阻断线粒体扰动，而半胱天冬酶-8或半胱天冬酶-3抑制剂仅抑制Smac提供的线粒体膜电位的增加下降，这表明γ射线照射后半胱天冬酶-2在线粒体上游起作用。总之，我们的研究结果提供了证据，表明靶向IAPs（如通过Smac激动剂）是提高人类癌症放射敏感性的一种有前景的策略。
