Date Published: December 01, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Hong‐Ye Wan, Jian‐Li Chen, Xingzhong Zhu, Liang Liu, Jianfang Wang, Xiao‐Ming Zhu.
Targeting protein degradation is recognized as a valid approach to cancer therapy. The ubiquitin–proteasome system (UPS) and the autophagy–lysosome pathway are two major pathways for intracellular protein degradation. Proteasome inhibitors such as bortezomib are clinically approved for treating malignancies, but to date, they are still unsatisfactory for cancer therapy. This study identifies titania‐coated gold nano‐bipyramid (NBP/TiO2) nanostructures as an autophagic flux inhibitor, as the smallest NBP/TiO2 nanostructures induce significant autophagosome accumulation in human glioblastoma U‐87 MG cells via blocking the autophagosome–lysosome fusion process and inhibiting lysosomal degradation. Further study indicates that NBP/TiO2 nanostructures reduce the intracellular level of mature cathepsin B and directly inhibit the proteolytic activity of cathepsin B, thereby further inhibiting trypsin‐like proteolytic activity, which is a potential cotarget for UPS inhibition. NBP/TiO2 nanostructures interact synergistically with bortezomib to suppress the viability of U‐87 MG cells, as the combined treatment synergistically induces the intracellular accumulation of ubiquitinated protein and endoplasmic reticulum stress. In addition, photothermal therapy further synergistically reduces the cell viability. In summary, this study suggests that NBP/TiO2 nanostructures function as a promising anticancer agent in combination with proteasome inhibitors.
The ubiquitin–proteasome system (UPS) and the autophagy–lysosome pathway are two major routes for intracellular protein degradation, which is strongly implicated in cancer pathogenesis and therapy. UPS degrades more than 80% of cellular proteins, especially short‐lived proteins. UPS‐mediated proteolysis consists of two steps: ubiquitination and proteasome‐mediated degradation.1 On the other hand, autophagy serves as the primary degradation route of long‐lived proteins, especially misfolded or aggregated proteins, and of damaged organelles. Intracellular proteins and organelles are engulfed in autophagosomes, which then fuse with lysosomes to form autolysosomes for degradation.2 Increasing evidence suggests that autophagy acts as a prosurvival mechanism in cancer cells under therapeutic stress, and it is associated with chemoresistance.3
In summary, we have identified NBP/TiO2 nanostructures as an autophagy flux inhibitor for sensitizing cancer cells to bortezomib. The autophagy inhibitory effect of NBP/TiO2 nanostructures is highly dependent on the TiO2 surface coating and the particle size. The target intracellular organelle for the NBP/TiO2 nanostructures is the lysosome, and they induce significant autophagosome accumulation in human glioblastoma U‐87 MG cells via blocking the autophagosome–lysosome fusion process. NBP/TiO2 nanostructures show a dramatic inhibitory effect against CTSB activity. They inhibit the maturation of CTSB and directly inhibit the proteolytic activity of CTSB by binding to mature CTSB. More importantly, NBP/TiO2 nanostructures also inhibit trypsin‐like proteolytic activity, and they synergistically enhance the anticancer effect of bortezomib. In addition, NBP/TiO2 nanostructure‐based photothermal therapy further enhances the anticancer effect. We believe that the combination of NBP/TiO2 nanostructures with bortezomib‐based chemotherapy will be useful for improving its efficacy.
Cell Culture: U‐87 MG cells were cultured in alpha‐modified minimum essential medium (α‐MEM) containing 10% fetal bovine serum, 100 U mL−1 penicillin, and 100 µg mL−1 streptomycin at 37 °C in a humidified 5% CO2 atmosphere. GFP‐LC3 plasmid was introduced into U‐87 MG cells using the transfection reagent Hilymax, and a stable cell line was established and maintained in medium containing geneticin (500 µg mL−1).
The authors declare no conflict of interest.