Genistein, biochanin-A, and daidzein, the predominant soy isoflavones, have been reported to lower the risk of cancer, but it is not known whether they protect against human hepatoma cancer. This study was designed to investigate their effects on cell growth, the cell cycle, and apoptosis induction in the human hepatoma cell lines, HepG2, Hep3B, Huh7, PLC, and HA22T. Genistein, biochanin-A, and daidzein inhibited growth of all five lines in a dose-dependent manner. DNA fragmentation studies and the TUNEL assay demonstrated that isoflavones caused tumor cell death by induction of apoptosis. Activation of caspase-3 and cleavage of the caspase-3 substrate, poly(ADP-ribose)polymerase, was seen in hepatoma cells after 24 hours' exposure to isoflavones. In addition, isoflavone cytotoxicity correlated with downregulation of Bcl-2 and Bcl-XL expression. Synergistic effects of the three isoflavones were observed on cell growth inhibition, apoptosis induction, and anti-apoptotic protein expression. Flow cytometry s...