Dual targets of lethal apoptosis and protective autophagy in liver cancer with periplocymarin elicit a limited therapeutic effect

Cardiac glycosides (CGs) are candidate anticancer agents that function by increasing [Ca(2+)]i to induce apoptotic cell death in several types of cancer cells. However, new findings have shown that the anti-cancer effects of CGs involve complex cell-signal transduction mechanisms. Hence, exploring the potential mechanisms of action of CGs may provide insight into their anti-cancer effects and thus aid in the selection of the appropriate CG. Periplocymarin (PPM), which is a cardiac glycoside, is an active ingredient extracted from Cortex periplocae. The role of PPM was evaluated in HepG2 cells and xenografted nude mice. Cell proliferation, real-time ATP rate assays, western blotting, cell apoptosis assays, short interfering RNA transfection, the patch clamp technique, electron microscopy, JC-1 staining, immunofluorescence staining and autophagic flux assays were performed to evaluate the function and regulatory mechanisms of PPM in vitro. The in vivo activity of the PPM was assessed using a mouse xenograft model. The present study demonstrated that PPM synchronously activated lethal apoptosis and protective autophagy in liver cancer, and the initiation of autophagy counteracted the inherent pro-apoptotic capacity and impaired the anti-cancer effects. Specifically, PPM exerted a proapoptotic effect in HepG2 cells and activated macroautophagy by initiation of the AMPK/ULK1 and mTOR signaling pathways. Activation of macroautophagy counteracted the pro-apoptotic effects of PPM, but when it was combined with an autophagy inhibitor, the anti-cancer effects of PPM in mice bearing HepG2 xenografts were observed. Collectively, these results indicated that a self-limiting effect impaired the pro-apoptotic effects of PPM in liver cancer, but when combined with an autophagy inhibitor, it may serve as a novel therapeutic option for the management of liver cancer.