Division of Immunology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
Oncogene. 2011 Aug 18;30(33):3636-47. doi: 10.1038/onc.2011.75. Epub 2011 Mar 21.
BH3-only protein Bid is a key player in death receptor-induced apoptosis, because it provides the link with the mitochondrial route for caspase activation. In this pathway, Bid is activated upon cleavage by caspase-8. Its BH3 domain-containing carboxy-terminal fragment subsequently provokes mitochondrial outer membrane permeabilization by Bak/Bax activation. Bid has also been implicated in the apoptotic response to ionizing radiation (IR) and the topoisomerase inhibitor etoposide, anti-cancer regimens that cause double-strand (ds)DNA breaks. We confirm the existence of this pathway and show that it is p53-independent. However, the degree of Bid participation in the apoptotic response to dsDNA breaks depends on the nature of cell transformation. We used Bid-deficient mouse embryonic fibroblast (MEF) lines that were reconstituted with Bid to control the cellular background and demonstrated that the Bid-dependent apoptotic pathway induced by IR and etoposide operates in MEFs that are transformed by SV40, but is not evident in E1A/Ras-transformed MEFs. The Bid-dependent apoptotic response in p53-deficient SV40-transformed MEFs contributed to clonogenic execution of the cells, implying relevance for treatment outcome. In these cells, Bid acted in a conventional manner in that it required its BH3 domain to mediate apoptosis in response to IR and etoposide, and triggered apoptotic execution by indirect activation of Bak/Bax, mitochondrial permeabilization and caspase-9 activation. However, the mechanism of Bid activation was unconventional, because elimination of all known or suspected cleavage sites for caspases or other proteolytic enzymes and even complete elimination of its unstructured cleavage loop left Bid's pro-apoptotic role in the response to IR and etoposide unaffected.
BH3-仅蛋白 Bid 是死亡受体诱导细胞凋亡的关键因子，因为它提供了与 caspase 激活的线粒体途径的联系。在这个途径中，Bid 通过 caspase-8 的切割而被激活。其 BH3 结构域包含的羧基末端片段随后通过 Bak/Bax 的激活引发线粒体外膜通透性。Bid 还与电离辐射（IR）和拓扑异构酶抑制剂依托泊苷的凋亡反应有关，这两种药物都会导致双链（ds）DNA 断裂。我们证实了这条途径的存在，并表明它与 p53 无关。然而，Bid 参与双链 DNA 断裂的凋亡反应的程度取决于细胞转化的性质。我们使用 Bid 缺陷型小鼠胚胎成纤维细胞（MEF）系，并用 Bid 重建这些细胞以控制细胞背景，并证明由 IR 和依托泊苷诱导的 Bid 依赖性凋亡途径在 SV40 转化的 MEF 中起作用，但在 E1A/Ras 转化的 MEF 中不明显。p53 缺陷型 SV40 转化的 MEF 中 Bid 依赖性凋亡反应有助于细胞的集落执行，暗示与治疗结果相关。在这些细胞中，Bid 以传统方式发挥作用，即需要其 BH3 结构域来介导对 IR 和依托泊苷的凋亡反应，并通过间接激活 Bak/Bax、线粒体通透性和 caspase-9 激活来触发凋亡执行。然而，Bid 激活的机制是非传统的，因为消除所有已知或可疑的 caspase 或其他蛋白酶的切割位点，甚至完全消除其无结构的切割环，都不会影响 Bid 在 IR 和依托泊苷反应中的促凋亡作用。
