Regulation of caspase-12 during endoplasmic reticulum stress-induced

内质网（ER）应激激活小鼠细胞中的caspase12，触发内质网应激特异性级联实现细胞凋亡。在C2C12小鼠成肌细胞中，级联反应的激活发生在线粒体不释放细胞色素c的情况下，这表明级联反应与线粒体损伤无关。Bcl-xL在C2C12细胞中稳定过表达可抑制caspase-12的活化和细胞凋亡。在ER应激细胞中，而在正常细胞中，Bcl-xL与Bcl-2家族的促凋亡成员Bim共免疫沉淀，表明Bcl-xL与Bim隔离，从而抑制凋亡信号传导。C2C12细胞的分离表明，内质网应激导致Bim从富含动力蛋白的腔室转移到内质网，而Bcl-xL的稳定过表达抑制了Bim在内质网上的积累。虽然先前只在线粒体外膜观察到Bim的毒性作用，但针对内质网表面的Bim衍生物Bim（ER）的过表达导致细胞凋亡。过表达caspase- se12抑制蛋白的C2C12转染物对Bim （ER）具有抗性，表明Bim对ER的毒性作用依赖于caspase-12的激活。通过RNAi敲低Bim使细胞能够抵抗内质网应激。这些结果表明，响应内质网应激的Bim易位到内质网是激活caspase-12和启动内质网应激特异性caspase级联的重要一步。最近，Bax和Bak参与内质网应激诱导的细胞凋亡。根据该模型，Bax/Bak的构象变化和寡聚化可能导致caspase-12的激活，尽管尚未确定Bax/Bak激活的触发因素。我们的数据表明，Bim是内质网应激和Bax/Bak激活之间的分子联系的候选分子。

Endoplasmic reticulum (ER) stress activates caspase12 in murine cells, triggering the ER stress-specific cascade for implementation of apoptosis. In C2C12 murine myoblast cells, activation of the cascade occurs without release of cytochrome c from mitochondria, suggesting that the cascade is independent of mitochondrial damage. Stable overexpression of Bcl-xL in C2C12 cells suppressed activation of caspase-12 and apoptosis. In ER stressed cells, but not in normal cells, Bcl-xL was co-immunoprecipitated with Bim, a proapoptotic member of the Bcl-2 family, suggesting that Bcl-xL sequesters Bim, thereby inhibiting the apoptotic signaling. Fractionation of C2C12 cells revealed that ER stress led to translocation of Bim from a dynein-rich compartment to the ER, while stable overexpression of Bcl-xL suppressed accumulation of Bim on the ER. Although the toxic effect of Bim had been previously observed only at the mitochondrial outer membrane, overexpression of a Bim derivative, Bim(ER), targeted at the surface of the ER led to apoptosis. A C2C12 transfectant overexpressing the caspa-se12 suppressor protein was resistant to Bim (ER), suggesting that the toxic effect of Bim on the ER is dependent on activation of caspase-12. Knockdown of Bim by RNAi provided cells resistant to ER stress. These results suggest that translocation of Bim to the ER in response to ER stress is an important step towards activation of caspase-12 and initiation of the ER stress-specific caspase cascade. Recently, the involvement of Bax and Bak in ER stress-induced apoptosis has been suggested. According to the model, conformational changes and oligomerization of Bax/Bak probably causes activation of caspase-12, although a trigger for Bax/Bak activation has not been identified. Our data demonstrate that Bim is a candidate for the molecular link between ER stress and Bax/Bak activation.