Swanton E, Savory P, Cosulich S, Clarke P, Woodman P
School of Biological Sciences, University of Manchester, England, UK.
Oncogene. 1999 Mar 11;18(10):1781-7. doi: 10.1038/sj.onc.1202490.
Apoptosis is accompanied by the activation of a number of apoptotic proteases (caspases) which selectively cleave specific cellular substrates. Caspases themselves are zymogens which are activated by proteolysis. It is widely believed that 'initiator' caspases are recruited to and activated within apoptotic signalling complexes, and then cleave and activate downstream 'effector' caspases. While activation of the effector caspase, caspase-3, has indeed 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 other initiator caspases. To address this issue, the ability of caspase-3, activated upon addition to cytosolic extracts of cytochrome c, to cause cleavage of caspase-2 was investigated. It was demonstrated that cleavage of caspase-2 follows, and is dependent upon, activation of caspase-3. Moreover, the 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.
细胞凋亡伴随着多种凋亡蛋白酶（半胱天冬酶）的激活，这些蛋白酶可选择性地切割特定的细胞底物。半胱天冬酶本身是通过蛋白水解作用被激活的酶原。人们普遍认为，“起始”半胱天冬酶被招募到凋亡信号复合物中并在其中被激活，然后切割并激活下游的“效应”半胱天冬酶。虽然确实观察到效应半胱天冬酶caspase-3的激活发生在几种不同起始半胱天冬酶激活之后，但关于进一步下游蛋白水解级联反应的证据有限。特别是，几乎没有证据表明通过一条途径激活的caspase-3的细胞水平足以切割并激活其他起始半胱天冬酶。为了解决这个问题，研究了在向细胞色素c的胞质提取物中添加后被激活的caspase-3切割caspase-2的能力。结果表明，caspase-2的切割发生在caspase-3激活之后，并且依赖于caspase-3的激活。此外，两种半胱天冬酶的激活均受到Bcl-2的抑制。这些数据共同表明，Bcl-2可以通过在线粒体细胞色素c释放的下游发挥作用来保护细胞免受凋亡，从而防止依赖caspase-3的蛋白水解级联反应。
