Abstract Previous studies have reported a neuroprotective role for cellular prion protein (PrP C) against apoptosis induced by serum deprivation in an immortalized prion protein gene ( Prnp)-deficient neuronal cell line, but the mechanisms remain unclear. In this study, to investigate the mechanisms by which PrP C prevents apoptosis, the authors compared apoptosis of Prnp −/− cells with that of Prnp −/− cells expressing the wild-type PrP C or PrP C lacking N-terminal octapeptide repeat region under serum-free conditions. Re-introduction of Prnp rescued cells from apoptosis, upregulated superoxide dismutase (SOD) activity, enhanced superoxide anion elimination, and inhibited caspase-3/9 activation. On the other hand, N-terminally truncated PrP C enhanced apoptosis accompanied by potentiation of superoxide production and caspase-3/9 activation due to inhibition of SOD. These results suggest that PrP C protects Prnp −/− cells from apoptosis via superoxide- and caspase-3/9-dependent pathways by upregulating SOD activity. Furthermore, the octapeptide repeat region of PrP C plays an essential role in regulating apoptosis and SOD activity.