Deoxypodophyllotoxin Mediates Autophagy Death through Inhibition of GRP78 in Human Osteosarcoma

Background: Glucose-regulated protein 78 (GRP78), as a chaperone protein, can protect the endoplasmic reticulum of cells and is expressed to influence chemoresistance and prognosis in cancer. Deoxypodophyllotoxin (DPT) is a compound with antitumor effects on cancers. DPT inhibits the proliferation of osteosarcoma by inducing apoptosis, necrosis, or cell cycle arrest.

Object: This study was performed to demonstrate the molecular mechanism by which DPT attenuates osteosarcoma progression through GRP78.

Methods: Natural compound libraries and western blot (WB) were used to screen the inhibitors of osteosarcoma GRP78. The expression of mitochondria-related genes in cancer cells of the treatment group was detected by quantitative real-time PCR (qPCR) and WB. 3-(4,5)- Dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) and 5-ethynyl-2'- deoxyuridine (EDU) were used to discover the activity and proliferation of osteosarcoma cells treated with DPT. We constructed an in vivo mouse model of DPT drug therapy and carried out immunohistochemical detection of xenografts. The treated osteosarcoma cells were analyzed using bioinformatics and electron microscopy. The data were analyzed finally.

Results: DPT inhibited osteosarcoma cell survival and the growth of tumor xenografts. It promoted up-regulation of BCL2-associated X protein (Bax) and B-cell CLL/lymphoma 2 (Bcl-2), which serves to mediate and attenuate, respectively, the killing activities of DPT through mitochondria dysfunction. The effect of DPT against cancer cells could be attenuated by the overexpression of GRP78, characterized by the inactivation of the caspase cascade. The loss of GRP78 in osteosarcoma cells negatively mediated the basal level of autophagyassociated genes. DPT stimulated autophagy via the phosphoinositide 3-kinase (PI3K)-v-akt murine thymoma viral oncogene homolog (AKT), a mechanistic target of rapamycin (mTOR) axis. The autophagy caused by DPT played an active role in the osteosarcoma of humans and blocked the apoptotic cascade.

Conclusion: Combination treatment with the GRP78 inhibitor DPT and pharmacological autophagy inhibitors will be a meaningful method of obviating osteosarcoma cells.