ANTI-APOPTOTIC ONCOGENES PREVENT CASPASE-DEPENDENT AND INDEPENDENT COMMITMENT FOR CELL-DEATH

Apoptosis is a morphologically defined type of cell death associated w ith the activation of certain proteases belonging to the ICE/CED-3 fam ily, known as caspases. Resistance to apoptosis has been implicated as one of the mechanisms that participates in oncogenesis. We found that the broad-spectrum peptide inhibitor of the caspases, zVAD-fmk, inter feres in a dose-dependent way with all the morphological and biochemic al changes associated with apoptosis induced by anti-CD95 mAb, stauros porine, VP-16 and Act-D. However, with the exception of anti-CD95-trig gered apoptosis, the insulted cells lost their clonogenic potential, e ven when pretreated with a high dose of zVAD-fmk. Under these circumst ances, the dying cells displayed no signs of apoptosis, including acti vation of caspases, externalization of phosphatidylserine, nuclear con densation, or DNA fragmentation. Instead, this cell death was characte rized by cytoplasmic and nuclear vacuolization followed by the loss of plasma membrane integrity, Thus, preventing the onset of apoptosis by blocking caspase activity did not rescue cells from dying in response to drugs such as staurosporine, VP-16 and Act-D, In comparison, ectop ic expression of anti-apoptotic oncogenes such as bcl-2 and bcr-abl no t only inhibited apoptosis but also preserved the clonogenic potential of the cells, Therefore, oncogenesis is promoted not by simply interf ering with caspase-mediated apoptosis, but by preventing an upstream e vent which we define as the commitment point for cell death.