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Deguelin, a natural component derived from leguminous plants, has been used as pesticide in some regions. Accumulating evidence show that deguelin has promising chemopreventive and therapeutic activities against cancer cells. This study shows that low concentrations of deguelin can lead to significant delay in zebrafish embryonic development through growth inhibition and induction of apoptosis. Furthermore, we identified fibroblast growth factor receptor 4 ( FGFR4) as the putative target of deguelin. The candidate was initially identified by a microarray approach and then validated through in vitro experiments using hormone-responsive ( MCF-7) and nonresponsive ( MDA- MB-231) human breast cancer cell lines. The results show that deguelin suppressed cell proliferation and induced apoptosis in both cancer cell lines, but not in Hs 578Bst cells, by blocking PI3K/ AKT and mitogen-activated protein kinases ( MAPK) signaling. The FGFR4 mRNA and protein level also diminished in a dose-dependent manner. Interestingly, we found that forced FGFR4 overexpression attenuated deguelin-induced proliferative suppression and apoptotic cell death in both zebrafish and MCF-7 cell lines, p- AKT and p- ERK levels were restored upon FGFR4 overexpression. Taken together, our results strongly suggest that deguelin inhibition of PI3K/ AKT and MAPK signaling in zebrafish and breast cancer cell lines is partially mediated through down-regulation of FGFR4 activity. [ABSTRACT FROM AUTHOR]