Translocation of Bim to the Endoplasmic Reticulum (ER) Mediates ER Stress Signaling for Activation of Caspase-12 during ER Stress-induced Apoptosis

Abstract

Endoplasmic reticulum (ER) stress activates caspase-12 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 pro-apoptotic 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 caspase-12 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 RNA interference 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 toward activation of caspase-12 and initiation of the ER stress-specific caspase cascade.

Increased survival of muscle stem cells lacking the MyoD gene after transplantation into regenerating skeletal muscle

“…In addition, genes involved in mitochondrial function and redox potential also are markedly altered in MyoD Ϫ/Ϫ myoblasts. MyoD deficiency may protect myoblasts from generating reactive oxygen species and stimulating mitochondria, which can trigger release of cytochrome C, a mitochondrial initiator of major apoptosis cascades (34,35). In any case, the evidence for substantially increased survival of engrafted MyoD Ϫ/Ϫ myoblasts after transplantation into damaged muscle provides reason to believe that they may effectively improve muscle function compared with wild-type myoblasts.…”

Cellular response to endoplasmic reticulum stress: a matter of life or death

“…143,144 For example, variants of the antiapoptotic family members Bcl-2 or Bcl-X L targeted specifically to the ER membrane can block apoptosis induced by pharmacological kinase inhibition or by proapoptotic Bcl-2 family members. 145,146 Conversely, ER stress itself can upregulate or otherwise activate several 'BH3-only' proapoptotic members of the Bcl-2 family, including Bim, 147 BIK, [148][149][150][151] and PUMA. 152,153 Therefore, efferent signaling from the stressed ER can engage the Bcl-2 death machinery directly.…”

“…Caspase-12, a murine protein associated with the ER membrane, is activated by ER stress-induced apoptosis but not by other non-ER stimuli 162 and is required for cell death in response to both pharmacological ER stress 162 and ER-targeted Bim. 147 However, caspase-12 can be activated by ER stress in several ways. For example, the cytoplasmic calcium-activated protease calpain can cleave and activate caspase-12 in response to calcium flux from the ER, which is often triggered by ER stress.…”

The endoplasmic reticulum in apoptosis and autophagy: role of the BCL-2 protein family

“…Both BIM translocation and apoptosis were inhibited by BCL-xL, and a direct BIM/BCL-xL interaction was demonstrated. Ectopic expression of ER-targeted BIM also induced apoptosis, which was inhibited by suppression of caspase-12 (Morishima et al, 2004). Finally, PUMA and NOXA are also upregulated during ER stress, both have been found at the ER, and both play a role in ER Ca 2 þ release (Reimertz et al, 2003;Luo et al, 2005;Rao et al, 2006;Shibue et al, 2006;Kieran et al, 2007;Nickson et al, 2007;Hassan et al, 2008).…”