The effect of normal cellular prion protein (PrPC) on abnormal protein aggregation was examined by transfecting huntingtin fragments (Htt) into SN56 neuronal-derived cells depleted of PrPC by RNA interference. PrPC depletion caused an increase in both the number of cells containing granules and the number of apoptotic cells. Consistent with the increase in Htt aggregation, PrPC depletion caused an decrease in proteasome activity and a decrease in the activities of cellular defense enzymes compared with control cells whereas reactive oxygen species (ROS) increased more than threefold. Therefore, PrPC may protect against Htt toxicity in neuronal cells by increasing cellular defense proteins, decreasing ROS and increasing proteasome activity thereby increasing Htt degradation. Depletion of endogenous PrPC in non-neuronal Caco-2 and HT-29 cells did not affect ROS levels or proteasome activity suggesting that only in neuronal cells does PrPC confer protection against Htt toxicity. The protective effect of PrPC was further evident in that overexpression of mouse PrPC in SN56 cells transfected with Htt caused a decrease in both the number of cells with Htt granules and the number of apoptotic cells, whereas there was no effect of PrPC expression in non-neuronal NIH3T3 or CHO cells. Finally, in chronically scrapie (PrPSc)-infected cells, ROS increased more than twofold while proteasome activity was decreased compared to control cells. Although this could be a direct effect of PrPSc, it is also possible that, since PrPC specifically prevents pathological protein aggregation in neuronal cells, partial loss of PrPC itself increases PrPSc aggregation.