Opposing roles of prion protein in oxidative stress- and ER stress-induced apoptotic signaling

Although the prion protein is abundantly expressed in the CNS, its biological functions remain unclear. To determine the endogenous function of the cellular prion protein (PrPc), we compared the effects of oxidative stress and endoplasmic reticulum (ER) stress inducers on apoptotic signaling in PrPc-expressing and PrPko (knockout) neural cells. H2O2, brefeldin A (BFA), and tunicamycin (TUN) induced increases in caspase-9 and caspase-3, PKCδ proteolytic activation, and DNA fragmentation in PrPc and PrPko cells. Interestingly, ER stress-induced activation of caspases, PKCδ, and apoptosis was significantly exacerbated in PrPc cells, whereas H2O2-induced proapoptotic changes were suppressed in PrPc compared to PrPko cells. Additionally, caspase-12 and caspase-8 were activated only in the BFA and TUN treatments. Inhibitors of caspase-9, caspase-3, and PKCδ significantly blocked H2O2-, BFA-, and TUN-induced apoptosis, whereas the caspase-8 inhibitor attenuated only BFA- and TUN-induced cell death, and the antioxidant MnTBAP blocked only H2O2-induced apoptosis. Overexpression of the kinase-inactive PKCδK376R or the cleavage site-resistant PKCδD327A mutant suppressed both ER and oxidative stress-induced apoptosis. Thus, PrPc plays a proapoptotic role during ER stress and an antiapoptotic role during oxidative stress-induced cell death. Together, these results suggest that cellular PrP enhances the susceptibility of neural cells to impairment of protein processing and trafficking, but decreases the vulnerability to oxidative insults, and that PKCδ is a key downstream mediator of cellular stress-induced neuronal apoptosis.