Background: Autophagy is a lysosomal degradation pathway that can provide energy through its recycling mechanism to act as a cytoprotective adaptive response mediating treatment resistance in cancer cells. We investigated the autophagy-inducing effects of ZD6474, a small-molecule inhibitor that blocks activities of vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), and RET tyrosine kinases.
Methods: We investigated the effects of ZD6474 on autophagy in glioblastomas cells. The ZD6474 mechanism of action was determined by western blot. We then examined the impacts of the inhibition of autophagy in combination with ZD6474 on cell apoptosis in vitro. Furthermore, we evaluated the synergistic anticancer activity of combination treatment with an autophagy inhibitor (chloroquine) and ZD6474 in U251 glioblastoma cells xenograft model.
Results: ZD6474-induced autophagy was dependent on signalling through the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway. ZD6474-induced autophagy was inhibited by both knockdown of the ATG7 and Beclin 1 gene, essential autophagy genes, and pharmacologic agents (chloroquine and 3-methyalanine) treatment. Both treatments also dramatically sensitised glioblastoma cells to ZD6474-induced apoptosis, decreasing cell viability in vitro. Furthermore, in a xenograft mouse model, combined treatment with ZD6474 and chloroquine significantly inhibited U251 tumour growth, and increased the numbers of apoptotic cells compared with treatment with either agent alone.
Conclusion: Autophagy protects glioblastoma cells from the proapoptotic effects of ZD6474, which might contribute to tumour resistance against ZD6474 treatment.