Enhancement of the recombinagenic and mutagenic activities of bleomycin in yeast by intercalation of acridine compounds into DNA

Abstract

Strain D7 of Saccharomyces cerevisiae was used to measure the induction by bleomycin (BLM) of mitotic recombination at the trp5 locus and point mutations at ilv1 in the presence and absence of acridine compounds. BLM is a potent mutagen and recombinagen in the D7 assay. The acridines vary, some being mutagenic or recombinagenic and others not. Combined treatments were used to distinguish whether a genetically inactive acridine has no effect on the genetic activity of BLM or modulates its action. When an acridine is itself genetically active, combined treatments were used to determine whether its effects are additive with those of BLM or whether there is interaction between the two compounds. Acridine compounds that share the ability to intercalate between the base pairs of DNA but differ in their mutagenic specificity owing to the presence of different substituent groups were analysed. Clear potentiation and synergistic interactions were detected in combined treatments with BLM and aminoacridines, nitroacridines or an acridine mustard. Potentiation and synergy were also observed in sequential exposures in which the yeast were grown in the presence of acridine compounds and then treated with BLM in the absence of free acridine. The results are consistent with an increase in BLM susceptibility conferred by acridine intercalation. It is likely that the intercalating agents increase the access of BLM to the minor groove of DNA, where it abstracts a hydrogen from the 4' position of deoxyribose, creating a free radical that is processed into strand breaks.

Potentiation of the mutagenicity and recombinagenicity of bleomycin in yeast by unconventional intercalating agents

“…In contrast, the monoamine oxidase inhibitor pargyline enhances the genotoxicity of BLM under hypoxic conditions but not under euoxic conditions, and its mechanism of potentiating BLM has been ascribed to its preventing the depletion of oxygen by monoamine and polyamine oxidase . Among the amines, aminoacridines consistently enhance the activity of BLM [Hoffmann et al, 2009].…”

“…D7 occurred both with nonreactive acridines and acridine derivatives with reactive nitro or mustard substituents that lead to adduct formation in DNA [Hoffmann et al, 2009]. When the acridine compound was itself genetically active, combined treatments exhibited synergy, as defined by the combined treatment having genotoxic effects significantly greater than the sum of the separate treatments [Hoffmann et al, 2009].…”

“…Higher concentrations of BLM (6.25 and 25 lg/ml) were used for the separate treatments than for the cotreatments (0.2 and 0.4 lg/ml) because they were short-term exposures (2 hr) of stationary-phase cells to BLM in buffer rather than extended treatments (18 hr) of growing cells through exponential phase to stationary phase. Dose-response curves show that both the extended treatments in growth medium [Hoffmann et al, 2009] and short-term treatments in buffer [Hoffmann et al, 1995;Hoffmann et al, 1999] at these concentrations are genetically active but not so strongly toxic as to impede measurement of the enhancement of the genotoxic responses by other agents.…”

“…To quantify the enhancement of BLM by acridines in yeast [Hoffmann et al, 2009], we calculated an Index of Interaction (I i ) for combined treatments. This index is the ratio of the effect of a combined treatment to that of the sum of separate treatments, calculated as follows:…”

“…In tests of 11 compounds that induce frameshift mutations in the Ames assay, 10 were found to be BLM amplifiers in the V79 assay [Snyder and Diehl, 2000]. Acridines, which are classical intercalating agents and frameshift mutagens, also potentiate the genotoxicity of BLM in yeast [Hoffmann et al, 2009].…”