Identification of a caspase-2 isoform that behaves as an endogenous inhibitor of the caspase cascade

Procaspase-2 is one of the aspartate-specific cysteine proteases that are a ctivated in response to various apoptotic stimuli. Two isoforms of human pr ocaspase-2 have been described initially. Overexpression of the long isofor m (caspase-2L) promotes cell death whereas the short isoform (caspase-2S) a ntagonizes some apoptotic pathways. In the present study, we Identified two additional CASP-2 mRNAs, designated CASP-2L-Pro and CASP-2S-Pro. The prote ins encoded by these isoforms corresponded to the prodomain of procaspase-2 L and -2S, in which the last alpha -helix of their caspase recruitment doma ins was deleted. Caspase-2L-Pro mRNA and protein were detected in a series of human tissues and cell lines. Yeast 2-hybrid assays and immunoprecipitat ion studies indicated that caspase-2L-Pro can interact with procaspase-2L a nd the adaptor protein RAIDD/CRADD, but not with FADD/MORT1 or APAF-1 adapt or proteins. The addition of recombinant caspase-2L-Pro negatively interfer ed with cytochrome c/dATP-mediated activation of the caspase cascade in a c ell-free system. In transient expression studies of human B lymphoma Namalw a cells, overexpression of caspase-2L-Pro weakly induced apoptosis, which w as prevented by a D83A/E87A double mutation. In stable selected CASP-2L-Pro -transfected Namalwa cells, overexpression of caspase-2L-Pro delayed apopto tic DNA fragmentation induced by death receptor agonists (anti-Fas antibodi es, tumor necrosis factor-alpha) and DNA topoisomerase I- (camptothecin) an d II- (etoposide) inhibitors, and prevented etoposide-induced activation of the caspase cascade. These inhibitory effects were not observed in stable transfected cells expressing the D83A/E87A double mutant. Altogether, these data indicated that the caspase-2L-Pro isoform functions as an endogenous apoptosis inhibitory protein that antagonizes caspase activation and cell d eath.