Production of protein-associated DNA breaks by 8-methoxycaffeine, caffeine and 8-chlorocaffeine in isolated nuclei from L1210 cells: comparison with those produced by topoisomerase II inhibitors.

8-Methoxycaffeine (8-MOC) is a caffeine derivative, more potent than the parent compound, but very similar to caffeine in terms of induction of DNA single-strand breaks (SSBs), DNA double-strand breaks (DSBs) and DNA-protein crosslinks (DPCs). We have studied the capability of 8-MOC, caffeine and 8-chlorocaffeine (8-CC) of inducing SSBs, DSBs and DPCs, and we have compared 8-MOC with ellipticine, a typical inhibitor of DNA topoisomerase II. The DNA effects of 8-MOC appeared similar to those of ellipticine. In both cases SSBs, DSBs and DPCs were present in a similar ratio, and they were rapidly reversible after removal of the drug. The dose-response curve was bell-shaped for both compounds. In addition, 8-MOC, caffeine and 8-CC were capable of inhibiting DSBs induced by ellipticine. These results were obtained at the level of L1210 cell nuclei. In spite of these functional similarities, 8-MOC, caffeine and 8-CC were unable to stimulate the formation of a cleavable complex by purified L1210 topoisomerase II (p170 form) when SV40 DNA and human c-myc DNA were used as substrate. These methylated oxypurines could be active on a different form of topoisomerase II, or, alternatively, they could be active only in the natural chromatin 'milieu' within the nucleus.