| Budget Amount *help |
¥3,700,000 (Direct Cost: ¥3,700,000)
Fiscal Year 2005: ¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 2004: ¥1,900,000 (Direct Cost: ¥1,900,000)
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| Research Abstract |
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.
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