Caspases that are activated during generation of nuclear polyglutamine aggregates are necessary for DNA fragmentation but not sufficient for cell death.
Truncated polypeptides containing expanded polyglutamine (polyQ) stretches tend to form cytoplasmic or nuclear aggregates in cultured cells, leading to cell death. Although it has been shown recently that caspase-8 coaggregates with polyQ and is activated during polyQ-mediated cell death, little is known of the location and timing of caspase-8 activation by nuclear polyQ aggregates. Also, the relationship between nuclear polyQ aggregate-mediated cell death and activation of other caspases is unclear. In P19 embryonal carcinoma (EC) cells, which can be made to differentiate into neuronal cells, polyQ72 repeats preferentially aggregate in the nucleus. Nuclear aggregates of polyQ72 induced P19 EC cell death, with a high frequency of cells exhibiting morphology characteristic of apoptosis (i.e., roundness, cell shrinkage, chromatin condensation) and DNA fragmentation. In the present study, we used antisera that specifically recognized the active forms of caspase-8, -3, and -9 but not their proforms, and showed that only caspase-8 and -3 were activated during the generation of polyQ72 aggregates in P19 EC cell nuclei. Furthermore, we showed that the caspase inhibitor z-VAD-fmk inhibited DNA fragmentation, but only partially inhibited the appearance of apoptotic morphology. Thus, caspase activation, including caspase-8 and -3, is necessary for polyQ-mediated DNA fragmentation but not sufficient for polyQ-mediated cell death in P19 EC cells.