Abstract
Norcantharidin (NCTD) is an anticancer drug routinely used against hepatoma in China. Previously, we reported that NCTD could induce mitotic arrest and apoptosis in human hepatoma HepG2 cells. However, the intracellular signaling pathways involved in NCTD-induced apoptotic cell death are still obscure. Caspase inhibitors were used to clarify the role of specific caspase in NCTD-triggered apoptotic process. Results showed that activation of caspase-9/caspase-3 cascade is required for NCTD-induced apoptotic death. To decipher the upstream signals for NCTD-induced apoptosis, we characterized the involvement of mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK), and p38MAPK. The role of their downstream targets, transcription factors activating protein-1 (AP-1), and nuclear factor kappaB (NF-kappaB) in NCTD-induced apoptosis was also analyzed. Immunoblot analyses and in vitro kinase assay demonstrated that NCTD-induced apoptosis was accompanied by the elevations of the levels of phosphorylated form and kinase activity of ERK and JNK, but not p38MAPK. The inhibitor of ERK pathway (U0126 or PD98059) or JNK pathway (SP600125) markedly prevented kinase activation, and also greatly reduced NCTD-induced apoptotic cell death. Increased DNA-binding activity of AP-1 and NF-kappaB was also observed after NCTD treatment. Inhibition of NF-kappaB activation by PDTC or inhibition of AP-1 activation by curcumin drastically blocked NCTD-induced cell death. These results imply that activation of ERK and JNK, and modulation of downstream transcription factors NF-kappaB and AP-1, may be involved in NCTD-induced apoptosis.