Antiapoptotic Signaling Generated by Caspase-Induced Cleavage of RasGAP

Abstract

Activation of caspases 3 and 9 is thought to commit a cell irreversibly to apoptosis. There are, however, several documented situations (e.g., during erythroblast differentiation) in which caspases are activated and caspase substrates are cleaved with no associated apoptotic response. Why the cleavage of caspase substrates leads to cell death in certain cases but not in others is unclear. One possibility is that some caspase substrates generate antiapoptotic signals when cleaved. Here we show that RasGAP is one such protein. Caspases cleave RasGAP into a C-terminal fragment (fragment C) and an N-terminal fragment (fragment N). Fragment C expressed alone induces apoptosis, but this effect could be totally blocked by fragment N. Fragment N could also block apoptosis induced by low levels of caspase 9. As caspase activity increases, fragment N is further cleaved into fragments N1 and N2. Apoptosis induced by high levels of caspase 9 or by cisplatin was strongly potentiated by fragment N1 or N2 but not by fragment N. The present study supports a model in which RasGAP functions as a sensor of caspase activity to determine whether or not a cell should survive. When caspases are mildly activated, the partial cleavage of RasGAP protects cells from apoptosis. When caspase activity reaches levels that allow completion of RasGAP cleavage, the resulting RasGAP fragments turn into potent proapoptotic molecules.

ACKNOWLEDGMENTS

We thank Christelle Bonvin for expert technical assistance. We thank Fabio Martinon and Jürg Tschopp for the gift of the human caspase 9 expression plasmid. We thank Mathias Peter, Mark Epping-Jordan, Romano Regazzi, Jean-René Cardinaux, Peter Clarke, Peter Vollenweider, and Christophe Bonny for critical reading of the manuscript. We also thank the reviewers for their helpful comments and suggestions.

This work is supported by grant 3100-055606 from the Swiss National Science Foundation and grants from the Botnar foundation (Lausanne, Switzerland).