Hand S C
Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309-0334, USA.
J Exp Biol. 1998 Apr;201(Pt 8):1233-42. doi: 10.1242/jeb.201.8.1233.
Depression of the production and consumption of cellular energy appears to be a prerequisite for the survival of prolonged bouts of anoxia. A correlation exists between the degree of metabolic depression under anoxia and the duration of anoxia tolerance. In the case of brine shrimp (Artemia franciscana) embryos, oxygen deprivation induces a reversible quiescent state that can be tolerated for several years with substantial survivorship. A global arrest of cytoplasmic translation accompanies the transition into anoxia, and rates of protein synthesis in mitochondria from these embryos appears to be markedly reduced in response to anoxia. Previous evidence suggests that the acute acidification of intracellular pH (pHi) by over 1.0 unit during the transition into anoxia contributes to the depression of biosynthesis, but message limitation does not appear to play a role in the down-regulation in either cellular compartment. The ontogenetic increase in mRNA levels for a mitochondrial-encoded subunit of cytochrome c oxidase (COX I) and for nuclear-encoded actin is blocked by anoxia and aerobic acidosis (artificial quiescence imposed by intracellular acidification under aerobic conditions). Further, the levels of COX I and actin mRNA do not decline appreciably during 6 h bouts of quiescence, even though protein synthesis is acutely arrested across this same period. Thus, the constancy of mRNA levels during quiescence indicates that reduced protein synthesis is not caused by message limitation but, instead, is probably controlled at the translational level. This apparent stabilization of mRNA under anoxia is mirrored in an extension of protein half-life. The ubiquitin-dependent pathway for protein degradation is depressed under anoxia and aerobic acidosis, as judged by the acute drop in levels of ubiquitin-conjugated proteins. Mitochondrial protein synthesis is responsive to both acidification of pHi and removal of oxygen per se. Matrix pH declines in parallel with pHi, and evidence from experiments with nigericin indicates that mitochondrial protein synthesis is depressed directly by acidification of matrix pH. The oxygen dependency of organellar protein synthesis is not explained by blockage of the electron transport chain or by the increased redox state. Rather, this cyanide- and antimycin-insensitive, but hypoxia-sensitive, inhibitory signature for the arrest of protein synthesis suggests the presence of a molecular oxygen sensor within the mitochondrion.
细胞能量产生和消耗的抑制似乎是长时间缺氧存活的先决条件。缺氧条件下代谢抑制的程度与缺氧耐受的持续时间之间存在相关性。就卤虫（Artemia franciscana）胚胎而言，缺氧会诱导一种可逆的静止状态，这种状态可持续数年且具有较高的存活率。进入缺氧状态时会伴随细胞质翻译的全面停滞，这些胚胎线粒体中的蛋白质合成速率似乎会因缺氧而显著降低。先前的证据表明，在进入缺氧状态期间细胞内pH（pHi）急性酸化超过1.0个单位会导致生物合成的抑制，但信息限制似乎在两个细胞区室的下调过程中均不起作用。细胞色素c氧化酶（COX I）的线粒体编码亚基和核编码肌动蛋白的mRNA水平在个体发育过程中的增加被缺氧和好氧酸中毒（在有氧条件下通过细胞内酸化施加的人工静止）所阻断。此外，在6小时的静止期内，COX I和肌动蛋白mRNA的水平并没有明显下降，尽管在同一时期蛋白质合成会急性停滞。因此，静止期mRNA水平的恒定表明蛋白质合成减少不是由信息限制引起的，而是可能在翻译水平受到控制。缺氧条件下mRNA的这种明显稳定反映在蛋白质半衰期的延长上。根据泛素结合蛋白水平的急剧下降判断，缺氧和好氧酸中毒会抑制泛素依赖性蛋白质降解途径。线粒体蛋白质合成对pHi的酸化和氧气本身的去除都有反应。线粒体基质pH与pHi平行下降，尼日利亚菌素实验的证据表明线粒体蛋白质合成直接受到基质pH酸化的抑制。细胞器蛋白质合成对氧气的依赖性不能用电子传递链的阻断或氧化还原状态的增加来解释。相反，这种对蛋白质合成停滞的对氰化物和抗霉素不敏感但对缺氧敏感的抑制特征表明线粒体中存在分子氧传感器。
