Pretreatment with UV light renders the chromatin in human fibroblasts more susceptible to the DNA-damaging agents bleomycin, gamma radiation and 8-methoxypsoralen.

Confluent human fibroblast cultures were pretreated with either 254 nm UV light (UV) or methyl methanesulphonate (MMS), incubated at 37 degrees C and subsequently challenged on ice with bleomycin (BLM), gamma-radiation or 8-methoxypsoralen (MOP). The resulting number of challenge-induced DNA damages (measured as DNA strand breaks or cross-links) were compared with the numbers induced in similarly challenged but non-pretreated control cells. It was found that the timing of the subsequent challenge of cells pretreated with UV did significantly affect the amount of induced DNA damage. When the challenging agents were administered after a 10-20 min incubation period following UV pretreatment, the amount of induced DNA damage was increased 50% over control cells. In contrast, the timing of the subsequent challenge of cells pretreated with MMS had no influence on the level of challenge-induced damage. It is hypothesized that UV-irradiated chromatin undergoes a time-dependent decondensation that renders it more susceptible to the induction of strand breaks and cross-links by BLM, gamma-radiation and MOP. A possible role for chromatin decondensation in UV-induced excision repair is discussed.