Inhibition of caspase-9 by oridonin, a diterpenoid isolated from Rabdosia rubescens, augments apoptosis in human laryngeal cancer cells

Rabdosia rubescens, a commonly used traditional Chinese medicine, has increasingly gained attention for its use as an antitumor herb. Oridonin, a bioactive diterpenoid isolated from Rabdosia rubescens, has been reported to induce apoptosis in human laryngeal cancer HEp-2 cells by our group. Here, we made unexpected observations that the caspase-9 inhibitor (C9i) enhanced apoptosis in response to selected stimuli, and HEp-2 cells which were made deficient in caspase-9 using siRNA exhibited no resistance to apoptotic signals and actually demonstrated increased apoptotic sensitivity to oridonin. The results were reversed by the transfection of an exogenous caspase-9 expression vector. Caspase-9 reduced sensitivity to apoptotic stimuli through reactive oxygen species (ROS)-suppressing and autophagy-promoting methods. ROS triggered the progression of apoptosis through activation of both the caspase-9-independent mitochondrial pathway and death receptor pathways, and the autophagy had an anti-apoptotic function in oridonin-treated HEp-2 cells. These collective results suggest that oridonin targets caspase-9 to alter ROS production and autophagy situation to promote HEp-2 cell apoptosis. Therefore, oridonin has the potential to be developed as an anticancer agent, and the combination of oridonin with those agents leading to reduction of caspase-9 expression in tumor cells could represent a novel approach to human laryngeal cancer treatment.