The lysosomal protease cathepsin D mediates apoptosis induced by oxidative stress

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-DEVD-CHO 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.