Presenilin 1 (PS1) gene mutations are the major causes of early-onset familial Alzheimer's disease. Acceleration of apoptosis is one of the major pathogenic mechanisms of PS1 mutants, and PS1 mutants have also been reported to induce overproduction of amyloid-beta protein 42. Here, we investigated aberrancy in activation of initiator caspases related to two PS1 gene mutations, I143T and G384A. Acceleration of apoptosis, elevation of caspase-3/7 activity, and significant increases in caspase-4, -8 and -9 activities during apoptosis induced by several agents were found in these mutant PS1-transfected cells. Interestingly, thapsigargin treatment enhanced caspase-4 and -9 activities in I143T-mutant PS1-transfected cells, while hydrogen peroxide treatment enhanced caspase-4, -8 and -9 activities in G384A-mutant PS1-transfected cells, indicating diverse apoptosis-promoting effects of PS1 gene mutations. In addition, treatment with a beta-secretase inhibitor or gamma-secretase inhibitor significantly attenuated the effects of the PS1 mutants on caspase-3/7 activation and recovered cell viability. Our present data suggest that these PS1 mutants accelerate the activation of initiator caspases and promote apoptosis, which may be associated, at least in part, with amyloid-beta production.