Stimulation by γ-carboline derivatives (simplified analogues of antitumor ellipticines) of site specific DNA cleavage by calf DNA topoisomerase II

γ-Carbolines are tricyclic aromatic compounds which intercalate into DNA base pairs and exhibit significant cytotoxic and antitumor activities. These compounds which are structurally related to ellipticine by deletion of an aromatic ring, induce DNA breaks in cultured L1210 cells. Since the mechanism of cytotoxic activity of ellipticines involves DNA topoisomerase II, this enzyme might also be a target for γ-Carbolines. We have tested this hypothesis using an in vitro system containing purified enzyme and pBR322 DNA. The ability of nine derivatives to stabilize the DNA-enzyme covalent complex was studied and compared to their cytotoxicity. The four less cytotoxic compounds do not induce cleavable complex to a significant extent. In contrast, the two most cytotoxic γ-Carbolines are the most efficient stabilizers of the cleavable complex. The last three compounds exhibit an intermediate cytotoxicity and cleavage activity. In the presence of γ-Carbolines, cleavage occurs predominantly at a single site in pBR322 which is one of the cleavage sites observed with ellipticines. The cleavage position was determined at the nucleotide level. The increased DNA cleavage specificity observed with γ-Carbolines suggests that a tricyclic system is as efficient as ellipticines for DNA topoisomerase II cleavage at DNA sequences involved specifically in cytotoxic response. The data presented support the hypothesis that DNA topoisomerase II is a target involved in the mechanisms of action of antitumor γ-Carbolines.