Cleavage of cytoskeletal proteins by caspases during ovarian cell death: evidence that cell-free systems do not always mimic apoptotic events in intact cells

Several lines of evidence support a role for protease activation during apoptosis. Herein, we investigated the involvement of several members of the CASP (cysteine aspartic acid-specific protease; CED-3- or ICE-like protease) gene family in fodrin and actin cleavage using mouse ovarian cells and HeLa cells combined with immunoblot analysis. Hormone deprivation-induced apo-ptosis in granulosa cells of mouse antral follicles incubated for 24 h was attenuated by two specific peptide inhibitors of caspases, zVAD-FMK and zDEVD-FMK (50-500 microM), confirming that these enzymes are involved in this paradigm of cell death. Proteolysis of actin was not observed in follicles incubated in vitro while fodrin was cleaved to the 120 kDa fragment that accompanies apoptosis. Fodrin, but not actin, cleavage was also detected in HeLa cells treated with various apoptotic stimuli. These findings suggest that, in contrast to recent data, proteolysis of cytoplasmic actin may not be a component of the cell death cascade. To confirm and extend these data, total cell proteins collected from mouse ovaries or non-apoptotic HeLa cells were incubated without and with recombinant caspase-1 (ICE), caspase-2 (ICH-1) or caspase-3 (CPP32). Immunoblot analysis revealed that caspase-3, but not caspase-1 nor caspase-2, cleaved fodrin to a 120 kDa fragment, wheres both caspases-1 and -3 (but not caspase-2) cleaved actin. We conclude that CASP gene family members participate in granulosa cell apoptosis during ovarian follicular atresia, and that collapse of the granulosa cell cytoskeleton may result from caspase-3-catalyzed fodrin proteolysis. However, the discrepancy in the data obtained using intact cells (actin not cleaved) versus the cell-free extract assays (actin cleaved) raises concern over previous conclusions drawn related to the role of actin cleavage in apoptosis.