Cinobufagin Induces Apoptosis in Osteosarcoma Cells Via the Mitochondria-Mediated Apoptotic Pathway

Osteosarcoma is a common primary malignant bone tumor that mainly occurs in childhood and adolescence. Despite developments in the diagnosis and treatment of osteosarcoma, the prognosis is still very poor. Cinobufagin is an active component in the anti-tumor Chinese medicine called "Chan Su", and we previously revealed that cinobufagin induced apoptosis and reduced the viability of osteosarcoma cells; however, the underlying mechanism remains to be elucidated. Herein, the present study was undertaken to illuminate the molecular mechanism of cinobufagin-induced apoptosis of osteosarcoma cell.

U2OS and 143B cells were treated with different concentrations of cinobufagin. Cell viability, colony formation ability and morphological changes were assessed by a CCK-8 assay, a clonogenic assay and light microscopy, respectively. Cell apoptosis was detected by Hoechst 33258 and Annexin V-FITC/PI staining. Reactive oxygen species (ROS) and mitochondrial membrane potential (ΔΨm) were determined by flow cytometry. Glutathione (GSH) levels were detected by a GSH and GSSG assay kit. The levels of apoptosis-related proteins were determined by western blotting, and 143B cells were introduced to establish a xenograft tumor model. The effect of cinobufagin on osteosarcoma was further investigated in vivo.

Our results showed that cinobufagin significantly reduced the viability of U2OS and 143B cells in vitro in a dose-and time-dependent manner. In addition, cinobufagin-induced apoptosis in U2OS and 143B cells was concentration-dependent. Moreover, we found that cinobufagin treatment increased the level of intracellular ROS, decreased ΔΨm, reduced GSH and inhibited GSH reductase (GR). The effects of cinobufagin on cell proliferation, apoptosis, ROS generation and ΔΨm loss were dramatically reversed when the cells were pretreated with the thiol-antioxidants NAC or GSH. Moreover, cinobufagin treatment increased the expression of the pro-apoptotic protein Bax and decreased the expression of the anti-apoptitic protein Bcl-2, thus altering the ratio of Bax to Bcl-2. Furthermore, Cinobufagin treatment caused cytochrome c release from the mitochondria to cytoplasm, thus increasing the protein levels of cleaved-caspase family members to induce apoptosis. Ac-DEVD-CHO or Z-LEHD-FMK significantly reduced cinobufagin-induced apoptosis. Finally, a subcutaneous xenograft animal study verified that cinobufagin also significantly suppressed osteosarcoma growth in vivo.

Our present data demonstrated that cinobufagin triggered cell apoptosis in osteosarcoma cells via the intrinsic mitochondria-dependent apoptosis pathway by the accumulation of ROS and the loss of ΔΨm. In an in vivo subcutaneous xenograft model, cinobufagin exhibited excellent tumor inhibitory effects. These results suggest that cinobufagin might potentially be further developed as an anti-tumor candidate for treating osteosarcoma patients in the clinic.