Inhibition of EGFR signaling augments oridonin-induced apoptosis in human laryngeal cancer cells via enhancing oxidative stress coincident with activation of both the intrinsic and extrinsic apoptotic pathways

Oridonin, a bioactive diterpenoid isolated from Rabdosia rubescens, has been reported to have anti-tumor effects, while the epidermal growth factor receptor (EGFR) signal pathway has been reported to play a vital role in the biological progression of several tumors and to be a target for therapeutic intervention. In this work, we show that inhibition of EGFR with tyrphostin AG1478 enhances oridonin-induced cell death in human laryngeal cancer cells HEp-2, a cell line characterized by EGFR gene amplification. The enhanced apoptotic effect correlates with high expression and activation of Bax, FADD, caspase-8 as well as caspase-3 and decreased protein levels of Bcl2 and SIRT1, suggesting that both the extrinsic and intrinsic apoptosis pathways are involved in the apoptotic processes. However, treatment with oridonin and AG1478 greatly enhances nuclear translocation of apoptosis inducing factor (AIF) without caspase-9 activation, indicating that the apoptosis occurs via a caspase-9-independent mitochondrial pathway. Here, it is the active form of caspase-8 but not caspase-9 that activates downstream effector caspase-3, resulting in the cleavage of critical cellular proteins and apoptosis. Furthermore, the combined use of AG1478 and oridonin augments the production of reactive oxygen species (ROS). Incubation of cells with N-Acetylcysteine (NAC) attenuates the apoptosis and the mitochondrial membrane potential (Δψm) disruption induced by the combination of oridonin and AG1478, which indicates that ROS plays a pivotal role in cell death. In conclusion, targeting EGFR combined with other conventional pro-apoptotic drugs should be a potentially very effective anti-neoplastic therapy for laryngeal cancer.