The effects of hyperthermia on DNA replication in HeLa cells

The extent of heat-induced inhibition of DNA replication in HeLa cells was assayed at temperatures between 43 and 48 degrees C. During hyperthermic exposure replicon initiation, as well as elongation of replicons into larger replicative fragment sizes, was rapidly inhibited. Elongation of nascent DNA into replicons continued at a normal rate for up to 45 min at 45 degrees C. Heated cells, replaced at 37 degrees C, elongated nascent DNA at a reduced rate and elongation was incomplete for up to 36 hr. Nascent DNA, not fully elongated 24 hr after hyperthermic exposure, was observed in replicative fragment sizes as small as replicons. The extent of heat-induced inhibition of DNA elongation increased with increasing time-temperature exposure with an activation energy of 122 kcal/mole of DNA. When pulsed cells were incubated at 37 degrees C for various times prior to heating, the extent of heat-induced inhibition of DNA elongation decreased with a half-time of 20-25 min, suggesting that the heat-sensitive structure is associated with replicative fragments having sizes less than 140-150S.