Activation of caspase-12 from procaspase-12 is specifically induced by insult to the
endoplasmic reticulum (ER) (Nakagawa, T., Zhu, H., Morishima, N., Li, E., Xu, J.,
Yankner, B. A., and Yuan, J. (2000) Nature 403, 98-103), yet the functional consequences
of caspase-12 activation have been unclear. We have shown that recombinant caspase-12
specifically cleaves and activates procaspase-9 in cytosolic extracts. The activated
caspase-9 catalyzes cleavage of procaspase-3, which is inhibitable by a caspase-9-specific
inhibitor. Although cytochrome c released from mitochondria has been believed to be
required for caspase-9 activation during apoptosis (Zou, H., Henzel, W. J., Liu, X.,
Lutschg, A., and Wang, X. (1997) Cell 90, 405-413, Li, P., Nijhawan, D., Budihardjo,
I., Srinivasula, S. M., Ahmad, M., Alnemri, E. S., and Wang, X. (1997) Cell 91, 479-489),
caspase-9 as well as caspase-12 and -3 are activated in cytochrome c-free cytosols
in murine myoblast cells under ER stress. These results suggest that caspase-12 can
activate caspase-9 without involvement of cytochrome c. To examine the role of caspase-12
in the activation of downstream caspases, we used a caspase-12-binding protein, which
we identified in a yeast two-hybrid screen, for regulation of caspase-12 activation.
The binding protein protects procaspase-12 from processing in vitro. Stable expression
of the binding protein renders procaspase-12 insensitive to ER stress, thereby suppressing
apoptosis and the activation of caspase-9 and -3. These data suggest that procaspase-9
is a substrate of caspase-12 and that ER stress triggers a specific cascade involving
caspase-12, -9, and -3 in a cytochrome c-independent manner.