Protein samples were prepared from cultured neurons or PC12 cells using extraction buffer as described previously (Liu et al., 2012). The protein samples were electrophoresed on SDS-PAGE gels and transferred to a nitrocellulose membrane. After blocking with 5% non-fat milk for 1 h at room temperature, the membranes were incubated with the indicated primary antibodies overnight and then with horseradish peroxidase-conjugated secondary antibody for 1 h. The blots were developed using a chemiluminescent system, and the bands were scanned, and densitometry analysis was performed with Gel-pro 4.5 Analyzer (Media Cybernetics, Silver Spring, MD, USA). The primary antibodies were anti-cleaved-PARP, anti-PARP, anti-Bcl-2, anti-Bax, anti-USP8 and anti-β-actin. Anti-Nrdp1 and anti-HIF-1α antibodies were purchased from BETHYL Laboratories and Novus Biologicals, respectively, and the rest primary antibodies were purchased from Santa Cruz Biotech. Relative protein levels were quantified after normalization to the loading control β-actin.