Mutagenic effects of brief exposure to bromodeoxyuridine on mouse FM3A cells

The time- and dose-dependency of the mutagenic effects of bromodeoxyuridine (BrdU), a thymidine analogue used for cell kinetics studies in vivo and in vitro, were investigated in FM3A cells. Cells incubated with 50-1000 microM BrdU for 72 h showed some inhibition of growth. Cells cultured in BrdU-free medium for 3 d after a 30 min or 2 h exposure to BrdU showed no growth inhibition, while those previously exposed for 24 h to BrdU showed retarded growth. After a 30 min exposure, 60% of cells were labelled with BrdU; after 2 h 70%; and after 24 h almost 100%. After incubation in BrdU-free medium for 3 d (the time required for this cell line to express mutation), cells previously treated for 30 min or 2 h showed reduced BrdU positivity, whereas almost 100% of those treated for 24 h remained BrdU positive. The mutation rate, determined by the number of colonies resistant to ouabain (2 mM) and 6-thioguanine (10 microM) 3 d after exposure to BrdU, was not affected by a 30 min treatment with up to 1000 microM BrdU. Cells treated for 1 or 2 h showed increased resistance to ouabain after exposure to BrdU at concentrations above 100 microM; cells treated for 12 or 24 h showed an increased mutation rate at BrdU concentrations above 50 microM. The number of colonies resistant to 6-thioguanine did not increase in cells treated with BrdU at concentrations up to 1000 microM for 1, 12 or 24 h. We cannot conclude with certainty that brief exposure to BrdU does not modulate DNA to the point of mutation. This study may serve as a guideline for limiting the dose and time of exposure to BrdU for cell kinetics studies in vivo and in vitro.