Effect of benzo[a]pyrene-diol-epoxide-I on growth of nascent DNA in synchronized human fibroblasts.

In logarithmically growing cultures of human fibroblasts, benzo[a]pyrene-diol-epoxide-I (BPDE-I) treatment inhibits both replicon initiation and DNA chain elongation. In this study we have focused on the effect of BPDE-I treatment on the growth of nascent DNA strands in synchronized human fibroblasts. Cells were collected at the beginning of the S phase by replating confluent cultures in the presence of aphidicolin for 24 h. At 1 h after release from aphidicolin, cells were incubated with [3H]thymidine for 10-15 min and treated with BPDE-I for 10 min. Following different periods of incubation with unlabeled thymidine, cells were lysed on top of alkaline sucrose gradients for analysis of the size distribution of nascent DNA and its average molecular weight. After a brief pulse with the radiolabeled precursor, a unimodal distribution of nascent DNA strands was observed in control cells. Upon chase this distribution was displaced to higher average molecular weights at an approximate rate of 1 X 10(6) daltons/min. Treatment with low doses of BPDE-I (less than 0.2 microM) did not decrease this rate. Indeed the nascent DNA in BPDE-I-treated cells appeared to grow faster than in control cells. This higher rate of DNA strand growth may be related to the inhibition of replicon initiation. Treatment with 0.3 microM or 0.7 microM BPDE-I, which are doses that interfere with DNA synthesis in operating replicons in asynchronous cells, also inhibited the growth of nascent DNA strands in synchronized cells by 22 and 64%, respectively. Because of the simultaneous inhibition of replicon initiation, these values are probably underestimations of the total inhibition of DNA strand growth. The use of synchronized cells in this type of study should aid in the elucidation of mechanisms of replication of carcinogen-damaged DNA.