Osteosarcoma is the most common high-grade human primary malignant bone sarcoma with lower survival in the past decades. Oridonin, a bioactive diterpenoid isolated from Rabdosia rubescens, has been proved to possess potent anti-cancer effects. However, its potential mechanism still remains not fully clear nowadays. In this study, we investigated the anticancer effect of oridonin on human osteosarcoma and illuminated the underlying mechanisms. In vitro, oridonin inhibited the cell viability of various osteosarcoma cells. We demonstrated that oridonin induced mitochondrial-mediated apoptosis by increasing Bax/Bcl-2 ratio, loss of mitochondrial membrane potential (MMP), triggering reactive oxygen species (ROS) generation and activating caspase-3 and caspase-9 cleavage in MG-63 and HOS cells. Moreover, we found that oridonin triggered ROS by inhibiting NF-E2-related factor 2 (Nrf2) pathway and induced mitochondrial apoptosis via inhibiting nuclear factor-κB (NF-κB) activation by activating Peroxisome Proliferator-Activated Receptor γ (PPAR-γ) in MG-63 and HOS cells. We further confirmed the results by PPAR-γ inhibitor GW9662, PPAR-γ siRNA as well as overexpression of PPAR-γ and Nrf2 in vitro. Furthermore, our in vivo study showed that oridonin inhibited tumor growth with high safety via inducing apoptosis through activating PPAR-γ and inhibiting Nrf2 activation in xenograft model inoculated HOS tumor. Taken together, oridonin exerted a dramatic pro-apoptotic effect by activating PPAR-γ and inhibiting Nrf2 pathway in vitro and in vivo. Therefore, oridonin may be a promising and effective agent for human osteosarcoma in the future clinical applications.

Osteosarcoma, the most common malignant bone tumor with recurring disease or lung metastases, has become one of the leading causes of death in humans. In the current study, we made an investigation on the anticancer effect of glaucocalyxin A, a bioactive ent-kauranoid diterpenoid isolated from Rabdosia japonica var., and unraveled the underlying mechanisms. Here, we found that Glaucocalyxin A inhibited the cell viability of numerous osteosarcoma cells. Our results showed that Glaucocalyxin A exerted the pro-apoptotic effect on human osteosarcoma cells, MG-63 and HOS cells. Glaucocalyxin A induced apoptosis by mitochondrial apoptotic pathway through several steps including increasing the Bax/Bcl-2 ratio, triggering the intracellular reactive oxygen species (ROS) generation, reducing mitochondrial membrane potential (MMP), and inducing cleavage of caspase-9 and caspase-3. We demonstrated that Glaucocalyxin A induced apoptosis via inhibiting Five-zinc finger Glis 1 (GLI1) activation by overexpression and knockdown of GLI1 in vitro. We also found that Glaucocalyxin A inhibited GLI1 activation via regulating phosphatidylinositol 3 kinase/protein kinase B (PI3K/Akt) signaling pathway. We further confirmed our findings by using PI3K activator and inhibitor to verify the inhibitory effect of Glaucocalyxin A on PI3K/Akt/GLI1 pathway. Moreover, our in vivo study revealed that glaucocalyxin A possessed a remarkable antitumor effect with no toxicity in the xenograft model inoculated with HOS tumor through the same mechanisms as in vitro. In conclusion, our results suggested that Glaucocalyxin A induced apoptosis in osteosarcoma by inhibiting nuclear translocation of GLI1 via regulating PI3K/Akt signaling pathway. Thus, Glaucocalyxin A might be a potential candidate for human osteosarcoma in the future.