Bcl-2 regulates a caspase-3/caspase-2 apoptotic cascade in cytosolic extracts

Apoptosis is accompanied by the activation of a number of apoptotic proteas es (caspases) which selectively cleave specific cellular substrates. Caspas es themselves are zymogens which are activated by proteolysis. It is widely believed that 'initiator' caspases are recruited to and activated within a poptotic signalling complexes, and then cleave and activate downstream 'eff ector' caspases, While activation of the effector caspase, caspase-3, has i ndeed been observed as distal to activation of several different initiator caspases, evidence for a further downstream proteolytic cascade is limited. In particular, there is little evidence that cellular levels of caspase-3 that are activated via one pathway are sufficient to cleave and activate ot her initiator caspases, To address this issue, the ability of caspase-3, ac tivated upon addition to cytosolic extracts of cytochrome c, to cause cleav age of caspase-2 was investigated. It was demonstrated that cleavage of cas pase-2 follows, and is dependent upon, activation of caspase-3, Moreover, t he activation of both caspases was inhibited by Bcl-2, Together, these data indicate that Bcl-2 can protect cells from apoptosis by acting at a point downstream from release of mitochondrial cytochrome c, thereby preventing a caspase-3 dependent proteolytic cascade.