Reduction of ENU-induced transversion mutations by the isoflavone genistein in Escherichia coli

In studies of mutagenesis induced by the carcinogen N-ethyl-N-nitrosourea (ENU) in the bacterium Escherichia coli FX-11, it was observed that G:C to A:T transitions did not require the inducible umuDC gene products, while a portion of the A:T to G:C transitions and all transversion mutations were dependent on a functional umuC gene. This observation suggested that the different base substitutions may result from differential processing of specific DNA adducts produced by ENU. To further understand these processes, we have investigated the effect of the soybean isoflavone genistein on the production of ENU-induced mutations. This compound, in particular, has been shown to exhibit numerous effects including the inhibition of the growth or proliferation of a variety of cancers, inhibition of angiogenesis, inhibition of tyrosine protein kinases and anti-oxidant properties. In our experiments, tyrosine defective (TyrA(-)) E. coli were exposed to ENU and a portion of the ENU-treated cells were exposed to genistein. The results showed a three-fold reduction in the overall mutation frequency when cells were treated with genistein subsequent to ENU-exposure and this anti-mutagenic effect was dependent on the dose of genistein employed. However, only certain types of base substitution mutagenesis were affected. In particular, transversion mutations were reduced an average of about 8.5-fold, while transitions were not greatly affected. In addition, UV-mutagenesis was reduced about three-fold and induction of the SOS response (as monitored with a sulA-lacZ fusion) was decreased. These results suggest that genistein may interfere with expression of the SOS response, including the UmuC-mediated lesion bypass mechanism that is necessary for UV-mutagenesis and the generation of transversions by ENU in E. coli.