OP16, a novel ent-kaurene diterpenoid, potentiates the antitumor effect of rapamycin by inhibiting rapamycin-induced feedback activation of Akt signaling in esophageal squamous cell carcinoma.

Hyperactivation of mTOR signaling pathway has been viewed as a significant molecular pathogenesis of cancer. However, inhibition of mTOR by rapamycin and its analogs could induce numerous negative feedback loops to attenuate their therapeutic efficacy. As a traditional Chinese herbal medicine, Rabdosia rubescens has been used to treat esophageal squamous cell carcinoma (ESCC) for hundreds of years, and its major effective component is oridonin. Here we reported that OP16, a novel analog of oridonin, showed potent inhibition of cell proliferation and Akt phosphorylation in ESCC cells. The combination of OP16 and rapamycin possesses synergistic anti-proliferative and pro-apoptotic effects both in ESCC cells and ESCC xenografts, and no obvious adverse effect was observed in vivo. Mechanistic analysis revealed that OP16 could inhibit rapamycin-induced Akt activation through the p70S6K-mediated negative feedback loops, and the combination of OP16 and rapamycin was more effective in activating caspase-dependent apoptotic signaling cascade. This study supports the combined use of OP16 with rapamycin as a feasible and effective therapeutic approach for future treatment of ESCC.