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 PKCδ 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. PKCδ 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 PKCδ fragments to mitochondria in a time‐dependent fashion, and the PKCδ inhibition effectively blocked the activation of caspase‐9 and caspase‐3, indicating that the accumulation of the PKCδ catalytic fragment in the mitochondrial fraction possibly amplifies mitochondria‐mediated apoptosis. Overexpression of the kinase active catalytic fragment of PKCδ (PKCδ‐CF) but not the regulatory fragment (RF), or mitochondria‐targeted expression of PKCδ‐CF triggers caspase‐3 activation and apoptosis. Furthermore, inhibition of PKCδ proteolytic cleavage by a caspase‐3 cleavage‐resistant mutant (PKCδ‐CRM) or suppression of PKCδ expression by siRNA significantly attenuated MG‐132‐induced caspase‐9 and ‐3 activation and DNA fragmentation. Collectively, these results demonstrate that proteolytically activated PKCδ has a significant feedback regulatory role in amplification of the mitochondria‐mediated apoptotic cascade during proteasome dysfunction in dopaminergic neuronal cells.