Cystein cathepsin and Hsp90 activities determine the balance between apoptotic and necrotic cell death pathways in caspase-compromised U937 cells

Caspase-inhibited cells induced to die may exhibit the traits of either apoptosis or necrosis or both, simultaneously. However, mechanisms regulating the commitment to these distinct forms of cell death are barely identified. We found that staurosporine induced both apoptotic and necrotic traits in U937 cells exposed to the caspase inhibitor benzyloxycarbonyl-Val-Ala-dl-Asp(OMe)-fluoromethylketone. Morphology and flow cytometry revealed that individual cells exhibited either apoptotic or necrotic traits, but not the mixed phenotype. Inhibition of cathepsin activity by benzyloxycarbonyl-Phe-Ala-fluoromethylketone rendered caspase-compromised cells resistant to staurosporine-induced apoptosis, but switched the cell death form to necrosis. Inhibition of heat shock protein 90 kDa (Hsp90) chaperon activity by geldanamycin conferred resistance to necrosis in caspase-compromised cells but switched the cell death form to apoptosis. Combination of benzyloxycarbonyl-Phe-Ala-fluoromethylketone and geldanamycin halted the onset of both forms of cell death by saving mitochondrial trans-membrane potential and preventing acidic volume (lysosomes) loss. These effects of benzyloxycarbonyl-Phe-Ala-fluoromethylketone and/or geldanamycin on cell death were restricted to caspase-inhibited cells exposed to staurosporine but influenced neither only the staurosporine-provoked apoptosis nor hydrogen peroxide (H2O2)-generated necrosis. Our results demonstrate that the staurosporine-induced death pathway bifurcates in caspase-compromised cells and commitment to apoptotic or necrotic phenotypes depends on cathepsin protease or Hsp90 chaperon activities.