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Apigenin (4’,5,7-trihydroxyflavone), which is widely distributed in the plant kingdom, is a low-toxicity and non-mutagenic phytopolyphenol and protein kinase inhibitor. It exhibits anti-proliferating effects on human breast cancer cells. We have shown that exposure of the HER2/neu-overexpressing breast cancer cells to apigenin resulting in induction of apoptosis by depleting HER2/neu protein and, in turn, suppressing the signaling of the HER2/HER3-PI3K/Akt pathway. Here, we examined whether inhibition of this pathway played a role in the anti-tumor effect. The results revealed that treatment with apigenin induced apoptosis through cytochrome c release and caused a rapid induction of caspase-3 activity and stimulated proteolytic cleavage of DFF-45. Furthermore, apigenin down-regulated cyclin D1, D3 and Cdk4, and increased p27 protein levels. Colony formation in the soft agar assay, a hallmark of the transformation phenotype, was preferentially suppressed in HER2/neu-overexpressing breast cancer cells in the presence of apigenin. In addition, a structure-activity relationship study indicated that (1) the position of B ring; and (2) the existence of the 3’,4’-hydroxyl group on the 2-phenyl group were important for the depletion of HER2/neu protein by flavonoids. These results provided new insights into the structure-activity relationship of flavonoids.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]