Critical Role of Reactive Oxygen Species and Mitochondrial Membrane Potential in Korean Mistletoe Lectin-Induced Apoptosis in Human Hepatocarcinoma Cells

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Abstract
Viscum album L. coloratum agglutinin (VCA), isolated from Korean mistletoe, is a strong inducer of apoptosis in a variety of tumor cells; however, the underlying molecular mechanisms responsible are not clear. Here, we show that VCA induces apoptotic killing, as demonstrated by DNA fragmentation, Hoechst 33258 staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and flow cytometry analysis in hepatocarcinoma Hep3B cells. VCA treatment results in a significant increase in reactive oxygen species (ROS) and loss of mitochondrial membrane potential (Δψm). Furthermore, treatment with the antioxidant N-acetyl-l-cysteine reduces ROS induction by VCA, preventing apoptosis in Hep3B cells, indicating that oxidative stress is involved in VCA-mediated cell death. Our results also show rapid changes in mitochondrial transition permeability, Bax translocation, cytochrome c release, caspase-3 activity, and poly(ADP-ribose) polymerase degradation in Hep3B cells occurring in VCA-induced apoptosis. There is much evidence that implicates c-Jun NH2-terminal kinase (JNK) activation with apoptosis in a variety of cellular and animal models. In this study, we show that VCA induces JNK phosphorylation, which is abolished with pretreatment with a JNK inhibitor. Moreover, Hep3B cells overexpressing JNK1 or stress-activated protein kinase kinase (SEK1) seem to be more susceptible to cell death from ROS and loss of Δψm induced by VCA, whereas expression of dominant-negative JNK1 or SEK1 in Hep3B cells do not. These data suggest that JNK phosphorylation may be a major regulator involved in VCA-induced apoptosis. Together, these results suggest that VCA induces apoptosis by inducing ROS production and a loss of Δψm, in which JNK phosphorylation plays a critical role in these events.