Yan Huajun, Frost Patrick, Shi Yijiang, Hoang Bao, Sharma Sanjai, Fisher Myrna, Gera Joseph, Lichtenstein Alan
Department of Medicine, Greater Los Angeles VA Healthcare Center, University of California at Los Angeles School of Medicine, Los Angeles, California, USA.
Cancer Res. 2006 Feb 15;66(4):2305-13. doi: 10.1158/0008-5472.CAN-05-2447.
Mammalian target of rapamycin (mTOR) inhibitors curtail cap-dependent translation. However, they can also induce post-translational modifications of proteins. We assessed both effects to understand the mechanism by which mTOR inhibitors like rapamycin sensitize multiple myeloma cells to dexamethasone-induced apoptosis. Sensitization was achieved in multiple myeloma cells irrespective of their PTEN or p53 status, enhanced by activation of AKT, and associated with stimulation of both intrinsic and extrinsic pathways of apoptosis. The sensitizing effect was not due to post-translational modifications of the RAFTK kinase, Jun kinase, p38 mitogen-activated protein kinase, or BAD. Sensitization was also not associated with a rapamycin-mediated increase in glucocorticoid receptor reporter expression. However, when cap-dependent translation was prevented by transfection with a mutant 4E-BP1 construct, which is resistant to mTOR-induced phosphorylation, cells responded to dexamethasone with enhanced apoptosis, mirroring the effect of coexposure to rapamycin. Thus, sensitization is mediated by inhibition of cap-dependent translation. A high-throughput screening for translational efficiency identified several antiapoptotic proteins whose translation was inhibited by rapamycin. Immunoblot assay confirmed rapamycin-induced down-regulated expressions of XIAP, CIAP1, HSP-27, and BAG-3, which may play a role in the sensitization to apoptosis. Studies in a xenograft model showed synergistic in vivo antimyeloma effects when dexamethasone was combined with the mTOR inhibitor CCI-779. Synergistic effects were associated with an enhanced multiple myeloma cell apoptosis in vivo. This study supports the strategy of combining dexamethasone with mTOR inhibitors in multiple myeloma and identifies a mechanism by which the synergistic effect is achieved.
雷帕霉素的哺乳动物靶点（mTOR）抑制剂可抑制帽依赖性翻译。然而，它们也能诱导蛋白质的翻译后修饰。我们评估了这两种效应，以了解雷帕霉素等mTOR抑制剂使多发性骨髓瘤细胞对地塞米松诱导的凋亡敏感的机制。无论多发性骨髓瘤细胞的PTEN或p53状态如何，均可实现致敏作用，AKT激活可增强致敏作用，且与细胞凋亡的内在和外在途径的刺激有关。致敏作用并非由于RAFTK激酶、Jun激酶、p38丝裂原活化蛋白激酶或BAD的翻译后修饰所致。致敏作用也与雷帕霉素介导的糖皮质激素受体报告基因表达增加无关。然而，当用对mTOR诱导的磷酸化具有抗性的突变型4E-BP1构建体转染来阻止帽依赖性翻译时，细胞对地塞米松的反应是凋亡增强，这与同时暴露于雷帕霉素的效果相似。因此，致敏作用是由帽依赖性翻译的抑制介导的。一项针对翻译效率的高通量筛选鉴定出了几种抗凋亡蛋白，其翻译受到雷帕霉素的抑制。免疫印迹分析证实雷帕霉素诱导XIAP、CIAP1、HSP-27和BAG-3的表达下调，这可能在凋亡致敏中发挥作用。在异种移植模型中的研究表明，当地塞米松与mTOR抑制剂CCI-779联合使用时，在体内具有协同抗骨髓瘤作用。协同作用与体内多发性骨髓瘤细胞凋亡增强有关。本研究支持在多发性骨髓瘤中将地塞米松与mTOR抑制剂联合使用的策略，并确定了实现协同效应的机制。
