Propolin C from propolis induces apoptosis through activating caspases, Bid and cytochrome c release in human melanoma cells

We had demonstrated that two prenylflavanones, propolin A and propolin B, isolated and characterized from Taiwanese propolis, induced apoptosis in human melanoma cells and significantly inhibited xanthine oxidase activity. Here, we have isolated a third compound called propolin C. The chemical structure of propolin C has been characterized by NMR and HRMS spectra, and was identical to nymphaeol-A. However, no biological activities of this compound have ever been reported. In the present study, propolin C effectively induced a cytotoxic effect on human melanoma cells, with an ic 50 of about 8.5 μM. DNA flow cytometric analysis indicated that propolin C actively induced apoptosis in human melanoma cells and there is a marked loss of cells from the G2/M phase of the cell cycle. To address the mechanism of the apoptosis effect of propolin C, we evaluated the effect of propolin C on induction of apoptosis-related proteins in human melanoma cells. The levels of procaspase-8, Bid, procaspase-3, and poly(ADP-ribose) polymerase were decreased in dose- or time course-dependent manners. Moreover, propolin C was capable of releasing cytochrome c from mitochondria to cytosol. The findings suggest that propolin C may activate a mitochondria-mediated apoptosis pathway. On other hand, propolin C is a potential antioxidant agent and shows a strong capability to scavenge free radicals and inhibit on xanthine oxidase activity with ic 50 of about 17.0 μM. In conclusion, the isolation and characterization of propolin C from bee propolis are described for the first time, and this compound is a powerful inducer of apoptosis in human melanoma cells.