A flavonoid, 5-hydroxy-3,7-dimethoxyflavone,from Kaempferia parviflora Wall. Ex. Baker as an inhibitor of Ca2+ signal-mediated cell-cycle regulation in yeast

Calcium (Ca²⁺) signal transduction pathways play important roles in the regulation of diverse biological processes in eukaryotes ranging from unicellular (e.g., yeasts) to complex multicellular (e.g., humans) organisms. Small-molecule inhibitors of Ca²⁺-signaling pathways in humans can be of great medical importance, as represented by the immunosuppressants FK506 and cyclosporine A. A high-throughput drug screening assay for inhibitors of Ca²⁺-signaling has been developed on the basis of the ability of test compounds to restore the severe growth defect of a Ca²⁺-sensitive zds1 null-mutant strain YNS17 of Saccharomyces cerevisiae in a medium containing a high concentration of calcium ions. A previous screening of Thai medicinal plants using this yeast-based assay indicated that the crude extract of Kaempferia parviflora Wall. Ex. Baker contains a potent inhibitory activity. The aim of this study was to isolate and characterize the pure compound(s) responsible for this inhibitory activity against Ca²⁺-mediated cell-cycle regulation in yeast. Dichloromethane and methanol extracts of K. parviflora rhizomes were subjected to bioassay-mediated chromatographic fractionation using this yeast YNS17 (Δzds1) strain]-based assay to screen for and select positive fractions. From the dichloromethane extract, four known flavonoid compounds with significant inhibitory bioactivity were obtained: compounds 1 (5-hydroxy-3,7-dimethoxyflavone), 2 (5-hydroxy-7-methoxyflavone), 3 (5-hydroxy-3,7,4’-trimethoxyflavone) and 4 (5,7-dimethoxyflavone). The inhibitory activity of all four compounds was dose-dependent. Compound 1 exhibited the highest activity and with no observed cytotoxic activity against the yeast. The Ca²⁺ induced severe growth defect, abnormal budding morphology, and G2 cell-cycle delay of the Δzds1 yeast strain were all alleviated or abrogated by 200 μM compound 1. Therefore, we conclude that 5-hydroxy-3,7-dimethoxyflavone possesses a potent inhibitory activity against the Ca²⁺-mediated cell-cycle regulation.