Multiple mechanisms of resistance to cisplatin toxicity in an Escherichia coli K12 mutant

Escherichia coli RecA promotes strand invasion with cisplatin-damaged DNA

The antitumor drug cisplatin causes intrastrand cross-linking of adjacent guanine residues that severely distorts the DNA backbone. These DNA adducts impede the progress of the replisome and may result in replication fork arrest. In Escherichia coli, the response to cisplatin involves the action of the prototypic recombinase RecA. Here we show that RecA can utilize, albeit at reduced levels, oligonucleotides that bear site-specific cisplatin-induced 1,2 d(GpG) intrastrand cross-links in strand invasion reactions. Binding of RecA to cisplatin-damaged oligonucleotides was not affected, indicating that the impediment was in the pairing step. The cognate E. coli single-strand DNA-binding protein specifically stimulated strand invasion particularly with cisplatin-damaged DNA. These results indicate that RecA is capable of processing the major cisplatin-induced lesion via a recombination mechanism.

Elevated homologous recombination activity in fanconi anemia fibroblasts

It is widely believed that Fanconi anemia cells possess a reduced ability to repair inter-strand DNA cross-links. While the mechanism through which inter-strand DNA cross-links are removed from mammalian chromosomes is unknown, these lesions are repaired via homologous recombination in lower eukaryotes and bacteria. Based on the hypothesis that a similar mechanism of DNA repair functions in mammalian somatic cells, we measured homologous recombination activity in diploid fibroblasts from healthy donors, and Fanconi anemia patients. Somewhat surprisingly, homologous recombination levels in nuclear protein extracts prepared from Fanconi anemia cells were nearly 100-fold higher than in extracts prepared from control cells. We observed a similar increase in the activity of a 100-kDa homologous DNA pairing protein in extracts from Fanconi anemia cells. Transfection studies confirmed that plasmid homologous recombination levels in intact Fanconi anemia cells were substantially elevated, compared with control cells. These results suggest that inappropriately elevated levels of homologous recombination activity may contribute to the genomic instability and cancer predisposition that characterize Fanconi anemia.

The antitumor agent cisplatin inhibits DNA gyrase and preferentially induces gyrB gene expression in Escherichia coli

Cisplatin is a widely used anticancer agent that exerts its biological activity principally by damaging DNA. Although detailed knowledge exists concerning mechanisms that lead to cisplatin adducts in DNA, there are few insights into the processes that result in its antitumor action. To explore some of the cellular responses elicited by cisplatin treatment, we studied its influence on DNA supercoiling and DNA gyrase gene expression in E. coli. We found that cisplatin inhibits DNA gyrase in a concentration-dependent manner leading to a transient alteration of DNA supercoiling and to an induction of gyrase gene expression. The induction effect was asymmetrical, affecting gyrB stronger than gyrA. Furthermore, we studied the influence of cisplatin on the supercoiling activity of purified DNA gyrase in vitro and found that cisplatin was an efficient inhibitor of DNA gyrase in the standard assay. However, cisplatin was an excellent inhibitor when added to DNA gyrase before it could interact with its substrate. In this assay GyrB was also more affected by cisplatin than GyrA. This strongly suggests that cisplatin inhibits DNA gyrase primarily by direct interaction with the enzyme. The data from this work present evidence that further cellular responses following cisplatin treatment include DNA gyrase inhibition, altered DNA supercolling and enhanced DNA gyrase gene expression. This suggests an important role of DNA topology in the induction of defense mechanisms against the action of cisplatin in addition to the processes related to DNA damage and repair.

Influence of DNA supercoiling on cisplatin toxicity in Escherichia coli K-12

DNA supercoiling is known to modulate the activity of numerous promoters in vitro and in vivo. Moreover, it has been reported to modulate the rate of formation of cisplatin/DNA crosslinks in vitro. In order to address the question of how the topology influences CDDP toxicity in E. coli, three mutants with altered gyrase activity which led to a decrease of about 25% in superhelical density were studied. Mutant strains gyrA224 and gyrB225 showed similar sensitivity to CDDP as the parental strain while the gyrB226 mutant was resistant. This resistance was abolished in uvrA (excision-repair) and recA (recombination and SOS processes) mutant derivatives. Thus supercoiling might play a role as an indirect modulator of CDDP toxicity in bacteria by interfering with repair processes.