THE CELL CYCLE-COUPLED EXPRESSION OF TOPOISOMERASE II-ALPHA DURING S-PHASE IS REGULATED BY MESSENGER-RNA STABILITY AND IS DISRUPTED BY HEAT-SHOCK OR IONIZING-RADIATION

Topoisomerase II is a multifunctional protein required during DNA repl ication, chromosome disjunction at mitosis, and other DNA-related acti vities by virtue of its ability to alter DNA supercoiling, The enzyme is encoded by two similar but nonidentical genes: the topoisomerase II alpha and II beta genes. In HeLa cells synchronized by mitotic shake- off, topoisomerase II alpha mRNA levels were found to vary as a functi on of cell cycle position, being 15-fold higher in late S phase (14 to 18 h postmitosis) than during G(1) phase. Also detected was a corresp onding increase in topoisomerase II alpha protein synthesis at 14 to 1 8 h postmitosis which resulted in significantly higher accumulation of the protein during S and G, phases, Topoisomerase II alpha expression was not dependent on DNA synthesis during S phase, which could be inh ibited without effect on the timing or level of mRNA expression, Mecha nistically, topoisomerase II alpha expression appears to be coupled to cell cycle position mainly through associated changes in mRNA stabili ty, When cells are in S phase and mRNA levels are maximal, a hair-life of greater than 4 h was observed, However, during G(1) phase, when ce llular levels are lowest, the half-life of topoisomerase II alpha mRNA was determined to be approximately 30 min, A similar decrease in mRNA stability was also induced by two external factors known to delay cel l cycle progression. Treatment of S-phase cells, at the time of maximu m topoisomerase II alpha mRNA stability, with either ionizing radiatio n (5 Gy) or heat shock (45 degrees C for 15 min) caused the accumulate d topoisomerase II alpha mRNA to decay, This finding suggests a potent ial relationship between stress-induced decreases in topoisomerase II alpha expression and cell cycle progression delays in late S/G(2).