The lysosomal protease cathepsin D mediates apoptosis induced by oxidative stress

ABSTRACT It has been suggested that lysosomes and the lysosomal proteases cathepsin D and B act as proapoptotic mediators of apoptosis, in addition to mitochondrial release of cytochrome c and the activation of the caspase family of proteases. We found that cathepsin D was implicated in the onset of apoptosis in fibroblasts exposed to oxidative stress generated by redox cycling of naphthazarin (NZ)(5,8‐dihydroxy‐1,4‐naphthoquinon). At the start of NZ treatment, the intracellular reduced glutathione concentration was diminished and cathepsins D, B, and L were all translocated from lysosomes to the cytosol before any biochemical or morphological signs of apoptosis were detected. Increase in cathepsin D activity and in the level of p53 protein, a transcription factor for cathepsin D, was observed before activation of caspase‐3. Moreover, pretreatment with the cathepsin D inhibitor pepstatin A or the caspase‐3 inhibitor Ac‐DEV‐DCHO prevented apoptosis, although the increase of cathepsin D activity was still detected when caspase‐3 was inhibited. Cathepsin B activity decreased following oxidative stress, and inhibition of the protease did not affect the apoptotic process. We suggest that translocation of lysosomal proteases is an early event in NZ‐induced apoptosis and that the release and increased activity of cathepsin D allow this protease to exert an apoptosis‐mediating effect upstream of the caspase cascade.