The cell cycle-coupled expression of topoisomerase IIα during S phase is regulated by mRNA stability and is disrupted by heat shock or ionizing radiation

Topoisomerase II is a multifunctional protein required during DNA replication, chromosome disjunction at mitosis, and other DNA-related activities by virtue of its ability to alter DNA supercoiling. The enzyme is encoded by two similar but nonidentical genes: the topoisomerase IIα and IIβ genes. In HeLa cells synchronized by mitotic shake-off, topoisomerase IIα mRNA levels were found to vary as a function of cell cycle position, being 15-fold higher in late S phase (14 to 18 h postmitosis) than during G₁ phase. Also detected was a corresponding increase in topoisomerase IIα protein synthesis at 14 to 18 h postmitosis which resulted in significantly higher accumulation of the protein during S and G₂ phases. Topoisomerase IIα expression was not dependent on DNA synthesis during S phase, which could be inhibited without effect on the timing or level of mRNA expression. Mechanistically, topoisomerase IIα expression appears to be coupled to cell cycle position mainly through associated changes in mRNA stability. When cells are in S phase and mRNA levels are maximal, a half-life of greater than 4 h was observed. However, during G₁ phase, when cellular levels are lowest, the half-life of topoisomerase IIα mRNA was determined to be approximately 30 min. A similar decrease in mRNA stability was also induced by two external factors known to delay cell cycle progression. Treatment of S-phase cells, at the time of maximum topoisomerase IIα mRNA stability, with either ionizing radiation (5 Gy) or heat shock (45°C for 15 min) caused the accumulated topoisomerase IIα mRNA to decay. This finding suggests a potential relationship between stress-induced decreases in topoisomerase IIα expression and cell cycle progression delays in late S/G₂.