Investigation of radiosensitizers with radiation-induced apoptosis

电离辐射激活半胱天冬酶，诱导不同类型细胞凋亡。然而，电离辐射如何激活半胱天冬酶并随后诱导细胞死亡仍有待明确阐明。在这里，我们提供的证据表明，电离辐射激活caspase-8和Bid，并通过两种不同的途径破坏淋巴细胞白血病细胞的线粒体膜电位（ΔΨm）。辐射诱导的caspase-8、-3和Bid的切割以及随后的DNA片段被四肽z-IETD-FMK预处理基本上阻断，但它不能抑制ΔΨm的分解，这表明ΔΨm的丢失与caspase的激活无关。此外，仅抑制caspase的激活不足以防止辐射诱导的细胞死亡。相比之下，12- o - tetradecanoylphorbol3 -acetate （TPA）通过阻断caspase-8/Bid激活和ΔΨm损失，显著抑制辐射诱导的细胞死亡。tpa介导的抗凋亡活性被MEK抑制剂PD98059破坏，而PKC抑制剂19-27伪底物不破坏，反而增强了抗凋亡活性。这些数据强烈表明，至少有两种不同的途径促进辐射诱导的细胞凋亡，即caspase激活和caspase非依赖性ΔΨm分解，后者是细胞凋亡的关键过程。这两种途径受到MEK/ erk介导信号的负调控，表明其激活可能是辐射耐药的一种机制。这些研究表明，利用辐射诱导的细胞凋亡作为新的放射增敏剂调节放射敏感性的新方法。

Ionizing radiation activates caspases and induces apoptosis in different cell types. However, how ionizing radiation activates caspases and subsequently induces cell death remains to be definitively elucidated. Here we provide evidence that ionizing radiation activates caspase-8 and Bid and disrupts mitochondrial membrane potential (ΔΨm) by two distinct pathways in lymphocytic leukemia cells. Radiation-induced cleavage of caspase-8, -3 and Bid, and subsequent DNA fragmentation were substantially blocked by pre-treatment with the tetrapeptide z-IETD-FMK, however it could not suppress the breakdown of ΔΨm, indicating that the ΔΨm loss occurred independently of the caspase activation. Moreover, suppression of caspase activation alone was insufficient to prevent the radiation-induced cell death. In contrast, 12-O-tetradecanoylphorbol-3-acetate (TPA) substantially inhibited the radiation-induced cell death through blockage of both caspase-8/Bid activation and the ΔΨm loss. TPA-mediated anti-apoptotic activity was abrogated by a MEK inhibitor PD98059, but not by a PKC inhibitor 19-27 pseudosubstrate, which rather augmented the anti-apoptotic activity. These data strongly suggest that there are at least two distinct pathways to facilitate radiation-induced apoptosis, i.e., caspase activation and caspase-independent ΔΨm reakdown, the latter being a crucial process in the apoptosis. These two pathways are negatively regulated by the MEK/ERK-mediated signal, signifying that its activation is a possible mechanism for radioresistance.These studies demonstrate that the new approach using radiation-induced apoptosis modulates the radiosensitivity as new radiosensitizers.