Unscheduled DNA synthesis in rat pleural mesothelial cells treated with mineral fibres

Abstract

Unscheduled DNA synthesis (UDS) was studied in confluent rat pleural mesothelial cells (RPMCs) arrested in G 0/G 1 with hydroxyurea (HU) and treated with various fibre types, i.e., chrysotile, crocidolite or attapulgite. In addition, the effects of UV light and of benzo[a]pyrene were determined as references. Using autoradiography after [ 3H]thymidine incorporation([ 3H]dThd), RPMCs treated with 4 μg/cm 2 of chrysotile fibres exhibited a low but significant enhancement of net grains compared to untreated cells. Treatment with higher doses of chrysotile was not possible because of the impairment of microsopic observation due to the presence of the fibres. Using liquid scintillation counting, RPMCs treated with chrysotile or crocidolite showed a significant dose-dependent increase in [ 3H]dThd incorporation compared to untreated cells. In contrast, attapulgite did not enhance [ 3H]dThd incorporation compared to untreated cells. Treatment of RPMCs with 1, 2 or 4 μg/ml of benzo[ a]pyrene resulted in a significant increase in[ 3H]dThd incorporation. In order to discount a possible role of S cells in the augmentation of [ 3H]dThd incorporation, despite the presence of 5 mM HU, S cells were counted by autoradiography. Results indicated that the percentage of S cells was similar in asbestos-treated and untreated cultures. Stimulation of the S phase also seems unlikely because treatment of RPMCs with asbestos fibers in the absence of HU resulted in a reduction of [ 3H]dThd incorporation attributed to an impairment of the S phase by the fibers. 1–4 μg/ml benzo[ a]pyrene or 10–50 J/m 2 UV light resulted in an approximate doubling of [ 3H]dThd incorporation. The effects of inhibitors of DNA repair were determined in chrysotile-treated RPMCs. [ 3H]dThd incorporation was inhibited by cytosine arabinoside and nalidixic acid. These results show that asbestos produces UDS and RPMCs.