PRECULTURING OF CHINESE-HAMSTER V79 CELLS WITH SUBLETHAL CONCENTRATION OF THEOPHYLLINE SENSITIZES CELLS TO CYTOTOXIC EFFECTS OF MNNG

In our previous work concerning the biologic effects of theophylline, we found that cells incubated during 48 h at low concentrations of the ophylline (0.3 mg/ml of medium) manifested short-term deviations in th e rate of DNA replication; however, this short-term inhibition of DNA replication did not reduce either the growth rate or the colony-formin g ability of cells. In the present study, we concentrated on cytotoxic and DNA-damaging effects of MNNG on V79 cells precultured with sublet hal concentration of methylxanthine theophylline. Cytotoxicity was eva luated on the basis of growth rate of treated cells as well as by colo ny-forming ability (plating efficiency) test and by trypan blue exclus ion test. The level of DNA lesions (strand breaks) induced by MNNG was measured by alkaline DNA unwinding and by the comet assay. In an effo rt to explain higher cytotoxic effects of MNNG on precultured cells, w e studied rejoining of damaged parental DNA after 4 h incubation post- MNNG-treatment as well. We found differences as against the controls i n theophylline-precultured cells after treatment with the mutagen and carcinogen MNNG. The higher cytotoxic effect of MNNG in precultured ce lls was accompanied by a higher level of ss breaks of DNA and by more unrepaired lesions which remained after 4 h in parental DNA. Our resul ts demonstrate that theophylline belongs to the group of agents inhibi ting repair of potentially lethal DNA lesions. (C) 1998 Elsevier Scien ce B.V. All rights reserved.