Heck M M, Hittelman W N, Earnshaw W C
Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
Proc Natl Acad Sci U S A. 1988 Feb;85(4):1086-90. doi: 10.1073/pnas.85.4.1086.
We have utilized antibody probes to examine the expression of DNA topoisomerases I and II and chromosome scaffold protein Sc-2 in normal and transformed cells. Neither topoisomerase I nor Sc-2 shows significant fluctuations in content or stability across the cell cycle. In contrast, topoisomerase II undergoes significant cell cycle-dependent alterations in both amount and stability. As cells progress from mitosis into G1, much of the topoisomerase II is degraded. During the first 2 hr of G1, the half life of topoisomerase II is decreased from that measured in asynchronous cell populations by a factor of 7. This suggests that the chromosome condensation/decondensation cycle is coupled to a parallel cycle of synthesis and degradation of topoisomerase II. In control experiments, we also found that the half-life of topoisomerase II is shorter in normal cells than in transformed cells by a factor of 4. Since the number of copies of topoisomerase II per cell is also lower in normal cells, this suggests that control of topoisomerase II stability is altered upon transformation. The stability of topoisomerase I and Sc-2 does not differ significantly between normal and transformed cells.
我们利用抗体探针检测了正常细胞和转化细胞中DNA拓扑异构酶I和II以及染色体支架蛋白Sc-2的表达。在整个细胞周期中，拓扑异构酶I和Sc-2的含量或稳定性均未表现出显著波动。相比之下，拓扑异构酶II在数量和稳定性上均经历了显著的细胞周期依赖性变化。随着细胞从有丝分裂进入G1期，大部分拓扑异构酶II会被降解。在G1期的前2小时内，拓扑异构酶II的半衰期比在非同步细胞群体中测得的半衰期缩短了7倍。这表明染色体凝聚/解凝聚周期与拓扑异构酶II的合成和降解的平行周期相关联。在对照实验中，我们还发现正常细胞中拓扑异构酶II的半衰期比转化细胞中的半衰期短4倍。由于正常细胞中每个细胞的拓扑异构酶II拷贝数也较低，这表明转化后拓扑异构酶II稳定性的控制发生了改变。正常细胞和转化细胞之间拓扑异构酶I和Sc-2的稳定性没有显著差异。
