Proteasome inhibitor-induced apoptosis is mediated by positive feedback amplification of PKCδ proteolytic activation and mitochondrial translocation

Emerging evidence implicates impaired protein degradation by the ubiquitin proteasome system (UPS) in Parkinson's disease; however cellular mechanisms underlying dopaminergic degeneration during proteasomal dysfunction are yet to be characterized. In the present study, we identified that the novel PKC isoform PKC delta plays a central role in mediating apoptotic cell death following UPS dysfunction in dopaminergic neuronal cells. Inhibition of proteasome function by MG-132 in dopaminergic neuronal cell model (N27 cells) rapidly depolarized mitochondria independent of ROS generation to activate the apoptotic cascade involving cytochrome c release, and caspase-9 and caspase-3 activation. PKC delta was a key downstream effector of caspase-3 because the kinase was proteolytically cleaved by caspase-3 following exposure to proteasome inhibitors MG-132 or lactacystin, resulting in a persistent increase in the kinase activity. Notably MG-132 treatment resulted in translocation of proteolytically cleaved PKC delta fragments to mitochondria in a time-dependent fashion, and the PKC delta inhibition effectively blocked the activation of caspase-9 and caspase-3, indicating that the accumulation of the PKC delta catalytic fragment in the mitochondrial fraction possibly amplifies mitochondria-mediated apoptosis. Overexpression of the kinase active catalytic fragment of PKC delta (PKC delta-CF) but not the regulatory fragment (RF), or mitochondria-targeted expression of PKC delta-CF triggers caspase-3 activation and apoptosis. Furthermore, inhibition of PKC delta proteolytic cleavage by a caspase-3 cleavage-resistant mutant (PKC delta-CRM) or suppression of PKC delta expression by siRNA significantly attenuated MG-132-induced caspase-9 and -3 activation and DNA fragmentation. Collectively, these results demonstrate that proteolytically activated PKC delta has a significant feedback regulatory role in amplification of the mitochondria-mediated apoptotic cascade during proteasome dysfunction in dopaminergic neuronal cells.