After the indicated treatments, PC12 cells were lysed in RIPA buffer and centrifuged at 12,000× g for 10 min at 4°C. The supernatant were incubated overnight at 4°C with 4 μg of anti-USP8 (Santa Cruz Biotech, Chicago, IL, USA), followed by precipitation with 50 μl of Dynabeads protein A (Pierce Biotechnology, Rockford, IL, USA) for 10 min at room temperature. The protein A were then washed extensively with binding buffer, resuspended in SDS-PAGE buffer, and boiled for 5 min. Samples of 30 μg total cell lysate were used as an input control. The precipitated complexes were separated on SDS-PAGE gels, and transferred to nitrocellulose membranes, and immunoblotted with anti-Nrdp1 (BETHYL Laboratories, Montgomery, TX, USA), K48-linkage-specific anti-ubiquitin antibody (Abcam, Cambridge, MA, USA) or anti-HIF-1α antibody (Novus Biological, Littleton, CO, USA) to detect the presence of these proteins in the complex. Normal rabbit IgG (Santa Cruz Biotech) was used as a loading control.