Fulda S, Friesen C, Los M, Scaffidi C, Mier W, Benedict M, Nuñez G, Krammer P H, Peter M E, Debatin K M
Division of Hematology/Oncology, University Children's Hospital, German Cancer Research Center, Heidelberg.
Cancer Res. 1997 Nov 1;57(21):4956-64.
Betulinic acid (BA), a melanoma-specific cytotoxic agent, induced apoptosis in neuroectodermal tumors, such as neuroblastoma, medulloblastoma, and Ewing's sarcoma, representing the most common solid tumors of childhood. BA triggered an apoptosis pathway different from the one previously identified for standard chemotherapeutic drugs. BA-induced apoptosis was independent of CD95-ligand/receptor interaction and accumulation of wild-type p53 protein, but it critically depended on activation of caspases (interleukin 1beta-converting enzyme/Ced-3-like proteases). FLICE/MACH (caspase-8), considered to be an upstream protease in the caspase cascade, and the downstream caspase CPP32/YAMA/Apopain (caspase-3) were activated, resulting in cleavage of the prototype substrate of caspases PARP. The broad-spectrum peptide inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, which blocked cleavage of FLICE and PARP, also completely abrogated BA-triggered apoptosis. Cleavage of caspases was preceded by disturbance of mitochondrial membrane potential and by generation of reactive oxygen species. Overexpression of Bcl-2 and Bcl-XL conferred resistance to BA at the level of mitochondrial dysfunction, protease activation, and nuclear fragmentation. This suggested that mitochondrial alterations were involved in BA-induced activation of caspases. Furthermore, Bax and Bcl-xs, two death-promoting proteins of the Bcl-2 family, were up-regulated following BA treatment. Most importantly, neuroblastoma cells resistant to CD95- and doxorubicin-mediated apoptosis were sensitive to treatment with BA, suggesting that BA may bypass some forms of drug resistance. Because BA exhibited significant antitumor activity on patients' derived neuroblastoma cells ex vivo, BA may be a promising new agent for the treatment of neuroectodermal tumors in vivo.
桦木酸（BA）是一种黑色素瘤特异性细胞毒剂，可诱导神经外胚层肿瘤（如神经母细胞瘤、髓母细胞瘤和尤因肉瘤，这些是儿童最常见的实体瘤）发生凋亡。BA引发的凋亡途径不同于先前确定的标准化疗药物所引发的途径。BA诱导的凋亡不依赖于CD95配体/受体相互作用和野生型p53蛋白的积累，但它关键依赖于半胱天冬酶（白细胞介素1β转换酶/Ced-3样蛋白酶）的激活。被认为是半胱天冬酶级联反应上游蛋白酶的FLICE/MACH（半胱天冬酶-8）和下游半胱天冬酶CPP32/YAMA/Apopain（半胱天冬酶-3）被激活，导致半胱天冬酶原型底物PARP的裂解。阻断FLICE和PARP裂解的广谱肽抑制剂苄氧羰基-Val-Ala-Asp-氟甲基酮也完全消除了BA引发的凋亡。半胱天冬酶的裂解之前有线粒体膜电位的紊乱和活性氧的产生。Bcl-2和Bcl-XL的过表达在线粒体功能障碍、蛋白酶激活和核碎裂水平赋予对BA的抗性。这表明线粒体改变参与了BA诱导的半胱天冬酶激活。此外，BA处理后，Bcl-2家族的两种促死亡蛋白Bax和Bcl-xs上调。最重要的是，对CD95和阿霉素介导的凋亡具有抗性的神经母细胞瘤细胞对BA处理敏感，这表明BA可能绕过某些形式的耐药性。由于BA在体外对患者来源的神经母细胞瘤细胞表现出显著的抗肿瘤活性，BA可能是一种有前途的体内治疗神经外胚层肿瘤的新药物。
