Impact of Oxidative Metabolism on the Cytotoxic and Genotoxic Potential of Genistein in Human Colon Cancer Cells

Scope Genistein (GEN) is known to be genotoxic via targeting topoisomerase‐II (TOPII). Oxidative metabolism of GEN is shown to generate hydroxylated metabolites with catecholic structures. The present study focuses on the impact of oxidative metabolism of GEN, exemplified for 3′‐hydroxygenistein (3′‐OH‐GEN) and 6‐hydroxygenistein (6‐OH‐GEN), on topoisomerase interference and the resulting genotoxic potential in HT‐29 human colon carcinoma cells. Methods and results In a cell‐free decatenation assay, 3′‐OH‐GEN slightly exceeds the TOPII‐inhibiting potential of GEN. In HT‐29 cells, its inhibitory action on TOPII does not differ from GEN, but it has greater activity with respect to causing DNA damage (measured by the comet assay), p53 activation (Western blot), apoptosis induction (ELISA), and cytotoxicity (WST‐1 assay). This may to some extent be related to a stronger pro‐oxidative potential of 3′‐OH‐GEN in comparison to GEN, as observed for the highest concentrations (DCF assay). 6‐OH‐GEN exerts much weaker toxic effects than GEN in cell‐based assays, including TOPII poisoning, DNA strand‐breaking potential, and ROS generation. This might in part arise from decreased cellular uptake of the metabolite, as measured by HPLC–DAD. Conclusion Oxidative metabolism alters the toxicological potential of GEN. Depending on the site of oxidation, the toxicity of the parent compound is exceeded (3′‐OH‐GEN) or attenuated (6‐OH‐GEN). Genistein can induce DNA damage via poisoning of TOP II and oxidative stress. Two human phase I oxidative metabolites are found to modulate this toxicity: While 3′‐OH‐GEN produced higher intracellular ROS levels and thus genotoxic effects as compared to GEN, 6‐OH‐GEN is found to act as a much less potent TOP II poison and inducer of oxidative stress in the HT29 colon carcinoma cell model. These differences might in part be related to the cellular uptake of the compounds.