Propolis, a resinous substance that honeybees collect to protect their beehive from enemies, is reported to have various
biological activities. In our screening program to search for antiangiogenic compounds from
propolis, the
ethanol extracts of Okinawan
propolis (EEOP) showed significant antiangiogenic activities in a tube formation assay with human umbilical vein endothelial cells (HUVECs) in vitro at 3.13 μ g/mL and chorioallantoic membrane (CAM) assay in vivo at 25 μ g/egg. To elucidate the active compounds of EEOP and their mode of action, we isolated some prenylated
flavonoids from EEOP and found that
nymphaeol-A had the strongest antiangiogenic activity among them.
Nymphaeol-A significantly reduced in vivo neovessel formation in the CAM assay at 25 μ g/egg. At the molecular level,
nymphaeol-A markedly inactivated
mitogen-activated protein kinase/ERK
kinase 1/2 (MEK1/2) and
extracellular signal-regulated kinase 1/2 (ERK1/2), whose molecular activations signal new vessel formation in HUVECs. In addition,
nymphaeol-A dose- and time-dependently induced caspase-dependent apoptosis in tube-forming HUVECs. Taken together,
nymphaeol-A was shown to inhibit angiogenesis at least in part via inactivation of MEK1/2-ERK1/2 signaling and induction of caspase-dependent apoptosis. Okinawan
propolis and its major component,
nymphaeol-A, may be useful agents for preventing
tumor-induced angiogenesis.