Role of Calmodulin in the Generation of Reactive Oxygen Species and Apoptosis Induced by Tamoxifen in HepG2 Human Hepatoma Cells

Role of Calmodulin in the Generation of Reactive Oxygen Species and Apoptosis Induced by Tamoxifen in HepG2 Human Hepatoma Cells

<P> Tamoxifen, an antiestrogen, has previously been shown to induce apoptosis in HepG2 human hepatoblastoma cells through activation of the pathways independent of estrogen receptors, i.e., intracellular Ca<SUP>2&#8290;</SUP> increase and generation of reactive oxygen species (ROS). However, the mechanism of tamoxifen to link increased intracellular Ca<SUP>2&#8290;</SUP> to ROS generation is currently unknown. Thus, in this study we investigated the possible involvement of calmodulin, a Ca<SUP>2&#8290;</SUP> activated protein, and Ca<SUP>2&#8290;</SUP>/ calmodulin-dependent protein kinase II in the above tamoxifen-induced events. Treatment with calmodulin antagonists (calmidazolium and trifluoroperazine) or specific inhibitors of Ca<SUP>2&#8290;</SUP>/calmodulin-dependent protein kinase II (KN-93 and KN-62) inhibited the tamoxifen-induced apoptosis in a dose-dependent manner. In addition, these agents blocked the tamoxifen-induced ROS generation in a concentration-dependent fashion, which was completely suppressed by intracellular Ca<SUP>2&#8290;</SUP> chelation. These results demonstrate for the first time that, despite of its well-known direct calmodulin-inhibitory activity, tamoxifen may generate ROS and induce apoptosis through indirect activation of calmodulin and Ca<SUP>2&#8290;</SUP>/calmodulin-dependent protein kinase II in HepG2 cells.