Caspase-independent cell death and mitochondrial disruptions observed in the Apaf1-deficient cells

Apaf1 is a critical molecule in the mitochondria-dependent apoptotic pathwa y. Here we show that Apaf1-deficient embryonic fibroblasts died at a later phase of apoptotic induction, although these cells were resistant to variou s apoptotic stimulants at an early phase. Neither caspase 3 activation nor nuclear condensation was observed during this cell death of Apaf1-deficient cells, Electron microscopic examination revealed that death in response to apoptotic stimulation resembled necrosis ill that nuclei were round and sw ollen with low electron density. Necrosis-like cell death was also observed in wildtype cells treated with z-VAD-fmk. Mitochondria were not only morph ologically abnormal but functionally affected, since mitochondrial transmem brane potential (Delta Psim) was lost even in cells with intact plasma memb rane integrity, These mitochondrial alterations were also observed in the w ild-type cells dying of apoptosis, Combined, these data suggest that cells without caspase activation, such as Apaf1-deficient cells or cells treated with caspase inhibitors, die of necrosis-like cell death with mitochondrial damage in response to "apoptotic stimulation."