Bockbrader Katrina M, Tan Mingjia, Sun Yi
Department of Radiation Oncology, Division of Cancer Biology, University of Michigan Comprehensive Cancer Center, 4304 CCGC, 1500 East Medical Center Drive, Ann Arbor, MI 48109-0936, USA.
Oncogene. 2005 Nov 10;24(49):7381-8. doi: 10.1038/sj.onc.1208888.
Inhibitor of apoptosis protein (IAP) suppresses apoptosis through binding and inhibiting active caspases-3, -7 and -9 via its baculoviral IAP repeat (BIR) domains. During apoptosis the caspase inhibition by IAPs can be negatively regulated by a mitochondrial protein second mitochondrial-derived activator of caspase (Smac). Smac physically interacts with multiple IAPs and relieves their inhibitory effect on caspases-3, -7 and -9. Recently, a small molecule Smac-mimic compound (Smac-mimic), which potentiates TNF-related apoptosis-inducing ligand (TRAIL) and tumor necrosis factor (TNF)-alpha mediated cell death in glioblastoma T98G cells and HeLa cells, was identified and characterized. To determine the efficacy of this compound in breast cancer cells, we first measured protein expression of three IAPs: XIAP, cIAP-1, and cIAP-2 in nine independent breast cancer cell lines. Three cell lines were chosen: a high IAPs expressing line MDA-MB-231, and two low IAPs expressing lines, T47D and MDA-MB-453. The cell lines were tested for their sensitivity to Smac-mimic alone or in combination with TRAIL or etoposide. Acting alone, Smac-mimic was quite potent with a cytotoxic IC50 of 3.8 nM in high IAPs expressing MDA-MB-231 cells, but was inactive at a much higher concentration in low IAPs expressing T47D and MDA-MB-453 cells. In fact, as low as 2.5 nM of Smac-mimic alone was sufficient to activate caspase-3 and induce apoptosis in MDA-MB-231 cells. In combinational treatments with TRAIL or etoposide, Smac-mimic significantly sensitized cells to growth suppression in MDA-MB-231 cells, but to a lesser extent in T47D and MDA-MB-453 cells. Furthermore, it significantly synergized MDA-MB-231, but not T47D cells to apoptosis induced by either TRAIL or etoposide. Thus, in these cell lines, Smac-mimic acts in an apparent IAPs dependent manner to induce apoptosis alone as well as sensitizes breast cancer cells to TRAIL or etoposide induced apoptosis via caspase-3 activation.
凋亡抑制蛋白（IAP）通过其杆状病毒IAP重复序列（BIR）结构域结合并抑制活性半胱天冬酶-3、-7和-9来抑制细胞凋亡。在细胞凋亡过程中，IAP对半胱天冬酶的抑制作用可被线粒体蛋白——第二线粒体衍生的半胱天冬酶激活剂（Smac）负调控。Smac与多种IAP发生物理相互作用，并解除它们对半胱天冬酶-3、-7和-9的抑制作用。最近，一种小分子Smac模拟化合物（Smac模拟物）被鉴定和表征，它能增强肿瘤坏死因子相关凋亡诱导配体（TRAIL）和肿瘤坏死因子-α（TNF-α）介导的胶质母细胞瘤T98G细胞和HeLa细胞的细胞死亡。为了确定该化合物在乳腺癌细胞中的疗效，我们首先检测了9个独立乳腺癌细胞系中三种IAP的蛋白表达：X连锁凋亡抑制蛋白（XIAP）、细胞凋亡抑制蛋白-1（cIAP-1）和细胞凋亡抑制蛋白-2（cIAP-2）。选择了三个细胞系：高表达IAP的MDA-MB-231细胞系，以及两个低表达IAP的细胞系T47D和MDA-MB-453。检测了这些细胞系对单独的Smac模拟物或与TRAIL或依托泊苷联合使用时的敏感性。单独作用时，Smac模拟物在高表达IAP的MDA-MB-231细胞中具有很强的细胞毒性，IC50为3.8 nM，但在低表达IAP的T47D和MDA-MB-453细胞中，浓度高得多时仍无活性。事实上，仅2.5 nM的Smac模拟物就足以激活MDA-MB-231细胞中的半胱天冬酶-3并诱导细胞凋亡。在与TRAIL或依托泊苷联合处理时，Smac模拟物能显著增强MDA-MB-231细胞对生长抑制的敏感性，但在T47D和MDA-MB-细胞中程度较小。此外，它能显著协同MDA-MB-231细胞，但不能协同T47D细胞对TRAIL或依托泊苷诱导的细胞凋亡。因此，在这些细胞系中，Smac模拟物以明显依赖IAP的方式单独诱导细胞凋亡，并通过激活半胱天冬酶-3使乳腺癌细胞对TRAIL或依托泊苷诱导的细胞凋亡敏感。
