Cathepsin D was translocated from lysosomal structures to the cytosol in primary cultures of neonatal rat cardiomyocytes exposed to oxidative stress, and these cells underwent apoptotic death during subsequent incubation. Temporal aspects of cathepsin D relocalization, cytochrome c release, and decrease in mitochondrial transmembrane potential (Δψm) were studied in myocytes exposed to the redox-cycling xenobiotic naphthazarin (5,8-dihydroxy-1,4-naphthoquinone). Immunofluorescence labeling revealed that cathepsin D was translocated to the cytosol after 30 minutes of naphthazarin treatment, and cytochrome c was released from mitochondria to the cytosol after 2 hours. Western blotting and immunoelectron microscopy indicated a minor release of cytochrome c after only 30 minutes and 1 hour, respectively. Thereafter, a decrease in Δψm was detected using the Δψm-sensitive dye JC-1 and confocal microscopy, and ultrastructural analysis indicated apoptotic morphology. Pretreatment of the cultures with the cathepsin D inhibitor pepstatin A prevented release of cytochrome c from mitochondria and maintained the Δψm. Moreover, ultrastructural examination showed no apoptotic morphology. These findings suggest that lysosomal destabilization (detected as the release of cathepsin D) and release of cytochrome c from mitochondria take place early in apoptosis. Also, the former event probably occurs before the latter during apoptosis induced by oxidative stress because pretreatment with pepstatin A prevented release of cytochrome c and loss of Δψm in cardiomyocytes exposed to naphthazarin.
