Activation of protein kinase Cδ by proteolytic cleavage contributes to manganese-induced apoptosis in dopaminergic cells: protective role of Bcl-2

Abstract

Chronic inorganic manganese exposure causes selective toxicity to the nigrostriatal dopaminergic system, resulting in a Parkinsonian-like neurological condition known as Manganism. Apoptosis has been shown to occur in manganese-induced neurotoxicity; however, the down-stream cellular target of caspase-3 that contributes to DNA fragmentation is not established. Herein, we demonstrate that proteolytic activation of protein kinase Cδ (PKCδ) by caspase-3 plays a critical role in manganese-induced apoptotic cell death. Treatment of PC12 cells with manganese caused a sequential activation of mitochondrial-dependent pro-apoptotic events, including mitochondrial membrane depolarization, cytochrome c release, caspase-3 activation, and DNA fragmentation. Overexpression of Bcl-2 in PC12 cells remarkably attenuated each of these events, indicating that the mitochondrial-dependent apoptotic cascade contributes to manganese-induced apoptosis. Furthermore, PKCδ was proteolytically cleaved by caspase-3, causing a persistent activation of the kinase. The manganese-induced proteolytic cleavage of PKCδ was significantly blocked by Bcl-2-overexpression. Administration of active recombinant PKCδ induced DNA fragmentation in PC12 cells, suggesting a pro-apoptotic role of PKCδ. Furthermore, expression of catalytically inactive mutant PKCδ K376R via a lentiviral gene delivery system effectively attenuated manganese-induced apoptosis. Together, these results suggest that the mitochondrial-dependent caspase cascade mediates apoptosis via proteolytic activation of PKCδ in manganese-induced neurotoxicity.