The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) can trigger growth inhibition, epithelial-mesenchymal transition (EMT)-like cell scattering, and migration of hepatoma cells HepG2 in a protein kinase C-alpha (PKC-alpha)-dependent manner. Saikosaponin a, an ingredient of antitumorigenic Chinese herb Sho-Saiko-to, inhibited cell growth but did not induce EMT-like cell scattering and cell migration of HepG2. Saikosaponin a and TPA induced transient (for 30 minutes) and sustained (until 6 hours) phosphorylation of extracellular signal-regulated kinase (ERK), respectively. Generation of the reactive oxygen species (ROS) was induced by TPA, but not saikosaponin a, for 3 hours. As expected, scavengers of ROS, such as superoxide dismutase, catalase, and mannitol, and the thiol-containing antioxidant N-acetylcystein dramatically suppressed the TPA-triggered cell migration but not growth inhibition of HepG2. The generation of ROS induced by TPA was PKC, but not ERK, dependent. On the other hand, scavengers of ROS and N-acetylcystein also prevented PKC activation and ERK phosphorylation induced by TPA. On the transcriptional level, TPA can induce gene expression of integrins alpha5, alpha6, and beta1 and reduce gene expression of E-cahedrin in a PKC- and ROS-dependent manner. In conclusion, ROS play a central role in mediating TPA-triggered sustained PKC and ERK signaling for regulation of gene expression of integrins and E-cahedrin that are responsible for EMT and migration of HepG2.