Kuwana T, Smith J J, Muzio M, Dixit V, Newmeyer D D, Kornbluth S
La Jolla Institute for Allergy and Immunology, San Diego, California 92121, USA.
J Biol Chem. 1998 Jun 26;273(26):16589-94. doi: 10.1074/jbc.273.26.16589.
Apoptosis often involves the release of cytochrome c from mitochondria, leading to caspase activation. However, in apoptosis mediated by CD95 (Fas/APO-1), caspase-8 (FLICE/MACH/Mch5) is immediately activated and, in principle, could process other caspases directly. To investigate whether caspase-8 could also act through mitochondria, we added active caspase-8 to a Xenopus cell-free system requiring these organelles. Caspase-8 rapidly promoted the apoptotic program, culminating in fragmentation of chromatin and the nuclear membrane. In extracts devoid of mitochondria, caspase-8 produced DNA degradation, but left nuclear membranes intact. Thus, mitochondria were required for complete engagement of the apoptotic machinery. In the absence of mitochondria, high concentrations of caspase-8 were required to activate downstream caspases. However, when mitochondria were present, the effects of low concentrations of caspase-8 were vastly amplified through cytochrome c-dependent caspase activation. Caspase-8 promoted cytochrome c release indirectly, by cleaving at least one cytosolic substrate. Bcl-2 blocked apoptosis only at the lowest caspase-8 concentrations, potentially explaining why CD95-induced apoptosis can often evade inhibition by Bcl-2.
细胞凋亡通常涉及细胞色素c从线粒体的释放，从而导致半胱天冬酶的激活。然而，在由CD95（Fas/APO-1）介导的细胞凋亡中，半胱天冬酶-8（FLICE/MACH/Mch5）会立即被激活，原则上，它可以直接加工其他半胱天冬酶。为了研究半胱天冬酶-8是否也能通过线粒体发挥作用，我们将活性半胱天冬酶-8添加到一个需要这些细胞器的非洲爪蟾无细胞系统中。半胱天冬酶-8迅速促进凋亡程序，最终导致染色质和核膜碎片化。在没有线粒体的提取物中，半胱天冬酶-8会导致DNA降解，但核膜保持完整。因此，线粒体是凋亡机制完全启动所必需的。在没有线粒体的情况下，需要高浓度的半胱天冬酶-8来激活下游半胱天冬酶。然而，当存在线粒体时，低浓度半胱天冬酶-8的作用会通过细胞色素c依赖性半胱天冬酶激活而被极大地放大。半胱天冬酶-8通过切割至少一种胞质底物间接促进细胞色素c的释放。仅在最低浓度的半胱天冬酶-8时，Bcl-2才会阻断细胞凋亡，这可能解释了为什么CD95诱导的细胞凋亡常常能够逃避Bcl-2的抑制作用。
