Abstract
Nevadensin, an abundant polyphenol of basil, is reported to reduce alkenylbenzene DNA adduct formation. Furthermore, it has a wide spectrum of further pharmacological properties. The presented study focuses the impact of nevadensin on topoisomerases (TOPO) in vitro. Considering the DNA-intercalating properties of flavonoids, first, minor groove binding properties (IC50 = 31.63 µM), as well as DNA intercalation (IC50 = 296.91 µM) of nevadensin, was found. To determine potential in vitro effects on TOPO I and TOPO IIα, the relaxation and decatenation assay was performed in a concentration range of 1-500 µM nevadensin. A partial inhibition was detected for TOPO I at concentrations ≥ 100 µM, whereas TOPO IIα activity is only inhibited at concentrations ≥ 250 µM. To clarify the mode of action, the isolating in vivo complex of enzyme assay was carried out using human colon carcinoma HT29 cells. After 1 h of incubation, the amount of TOPO I linked to DNA was significantly increased by nevadensin (500 µM), why nevadensin was characterized as TOPO I poison. However, no effects on TOPO IIα were detected in the cellular test system. As a subsequent cellular response to TOPO I poisoning, a highly significant increase of DNA damage after 2 h and a decrease of cell viability after 48 h at the same concentration range were found. Furthermore, after 24 h of incubation a G2/M arrest was observed at concentrations ≥ 100 µM by flow cytometry. The analysis of cell death revealed that nevadensin induces the intrinsic apoptotic pathway via activation of caspase-9 and caspase-3. The results suggest that cell cycle disruption and apoptotic events play key roles in the cellular response to TOPO I poisoning caused by nevadensin in HT29 cells.