Effects of activated aflatoxin B1 and caffeine on DNA replicon initiation in HeLa cells

Aflatoxin B1 (AFB1) is activated by a rat microsomal extract (S-9) to form a product that inhibits DNA synthesis in HeLa cells. At 10(-7) M, AFB1 inhibited initiation of replicons, as shown in alkaline sucrose gradient profiles 30 min after incubation with the drug. Ninety minutes later, the profile of treated cells was similar to that of control, but 4 h later there was another effect on replicon initiation. At 10(-6) M, the inhibition of initiation was greater than at 10(-7) M and increased progressively. Four hours after removal of the drug, the gradient profile showed low amounts of radioactivity in all size classes of DNA. When cells were incubated in medium containing caffeine (2mM) even as late as 60 min after incubation with AFB1, the inhibition of replicon initiation was prevented. If caffeine was later removed from the medium, replicon initiation was then inhibited. At 10(-7) M or 10(-6) M, AFB1 had little immediate effect on chain elongation, but at 10(-5) M, the gradient profiles showed an accumulation of low molecular weight DNA molecules, with no radioactivity in the region of high molecular weight DNA, owing to a block to chain elongation; this was not affected by caffeine. These results suggest that AFB1 induces damage that changes the conformation of chromatin so that initiation of new replicons cannot occur; in the presence of caffeine this change does not occur and DNA replication is not inhibited.