Deficiency in the mitochondrial apoptotic pathway reveals the toxic potential of autophagy under ER stress conditions

Endoplasmic reticulum (ER) stress-induced cell death is normally associated with activation of the mitochondrial apoptotic pathway, which is characterized by CYCS (cytochrome c, somatic) release, apoptosome formation, and caspase activation, resulting in cell death. In this study, we demonstrate that under conditions of ER stress cells devoid of CASP9/caspase-9 or BAX and BAK1, and therefore defective in the mitochondrial apoptotic pathway, still undergo a delayed form of cell death associated with the activation of caspases, therefore revealing the existence of an alternative stress-induced caspase activation pathway. We identified CASP8/caspase-8 as the apical protease in this caspase cascade, and found that knockdown of either of the key autophagic genes, ATG5 or ATG7, impacted on CASP8 activation and cell death induction, highlighting the crucial role of autophagy in the activation of this novel ER stress-induced death pathway. In line with this, we identified a protein complex composed of ATG5, FADD, and pro-CASP8 whose assembly coincides with caspase activation and cell death induction. Together, our results reveal the toxic potential of autophagy in cells undergoing ER stress that are defective in the mitochondrial apoptotic pathway, and suggest a model in which the autophagosome functions as a platform facilitating pro-CASP8 activation. Chemoresistance, a common problem in the treatment of cancer, is frequently caused by the downregulation of key mitochondrial death effector proteins. Alternate stress-induced apoptotic pathways, such as the one described here, may become of particular relevance for tackling the problem of chemoresistance in cancer cells.

内质网（Endoplasmic reticulum, ER）应激诱导的细胞死亡通常与线粒体凋亡通路的激活相关，该通路以细胞色素c，体细胞型（CYCS, cytochrome c, somatic）释放、凋亡小体（apoptosome）形成以及半胱天冬酶（caspase）激活为特征，最终引发细胞死亡。本研究证实，在ER应激条件下，缺失半胱天冬酶9（CASP9/caspase-9）或BAX与BAK1、因而线粒体凋亡通路存在功能缺陷的细胞，仍会发生一种延迟型细胞死亡，且该过程伴随半胱天冬酶的激活，这揭示了一类替代性应激诱导的半胱天冬酶激活通路的存在。我们鉴定出半胱天冬酶8（CASP8/caspase-8）是该半胱天冬酶级联反应中的起始蛋白酶，同时发现敲低关键自噬相关基因ATG5或ATG7会削弱CASP8的激活与细胞死亡的诱导，这凸显了自噬在这一新型ER应激诱导死亡通路激活过程中的关键作用。与此相一致，我们鉴定出一种由ATG5、Fas相关死亡结构域蛋白（FADD）以及半胱天冬酶原8（pro-CASP8）组成的蛋白复合物，其组装进程与半胱天冬酶激活及细胞死亡诱导同步发生。综上，本研究结果揭示了线粒体凋亡通路缺陷的ER应激细胞中自噬的毒性潜能，并提出了一个模型：自噬体（autophagosome）可作为平台促进半胱天冬酶原的激活。化疗耐药是癌症治疗中的常见难题，其常由关键线粒体死亡效应蛋白的表达下调所导致。诸如本研究所述的替代性应激诱导凋亡通路，或在解决癌细胞化疗耐药问题中具备特殊应用价值。