Oxidative stress -mediated cell signaling mechanisms in Parkinson's disease models: Roles of the novel protein kinase C isoform delta (PKCδ)
This study investigated the oxidative stress-induced apoptotic cell signaling mechanisms in in-vitro models of Parkinson's disease (PD). In particular, we examined roles of the novel isoforms of serine-threonine kinase family—Protein Kinase C delta (PKCδ) in 1-Methyl-4-phenylpyridinium (MPP+) induced cellular apoptosis in a rat mesencephalic dopaminergic cell line (N27). MPP+, the neurotoxic metabolite of MPTP a known dopaminergic toxicant, induces apoptosis in dopaminergic neurons. The cellular mechanisms underlying the MPTP induced dopaminergic degenerative process are not well understood. In the present study, we demonstrate that caspase-3 mediated proteolytic activation of PKCδ is critical in MPP +-induced oxidative stress and apoptosis. MPP+ exposure in N27 cells resulted in time-dependent increases in reactive oxygen species generation, cytochrome-c release, and caspase-9 and -3 activation. Interestingly, MPP+ induced proteolytic cleavage of PKCδ (72–74 kDa) into a 41-kDa catalytic and a 38-kDa regulatory subunit, resulting in persistently increased kinase activity. The caspase-3 inhibitor Z-DEVD-fmk effectively blocked MPP+-induced PKCδ cleavage and kinase activity, suggesting that the proteolytic activation is caspase-3 mediated. Intracellular delivery of catalytically active recombinant PKCδ significantly increased caspase-3 activity, indicating that PKCδ regulates caspase-3 activity. Finally, over-expression of a kinase inactive PKCδ K376R mutant prevented MPP+-induced caspase activation and DNA fragmentation, confirming the pro-apoptotic function of PKCδ in dopaminergic cell death.
α-Synuclein is a pre-synaptic neuronal protein of unknown function that has been implicated in the pathogenesis of Parkinson's disease (PD). Over-expression of wild-type human α-synuclein attenuated MPP +-induced (300 μM) cytotoxicity, release of mitochondrial cytochrome c, and subsequent caspase-3 activation, without affecting ROS generation. α-Synuclein over-expression also dramatically reduced MPP+-induced caspase-3-mediated proteolytic cleavage of PKCδ whereas the over-expression of the mutant α-synuclein A53T did not offer any protection under similar conditions.
Together these results, for the first time to our knowledge, demonstrate a unique apoptotic cascade in MPP+ induced cellular model of dopaminergic degeneration involving oxidative stress mediated proteolytic cleavage of PKCδ modulated by tyrosine phosphorylation. We also depicted novel neuroprotective mechanism of the human α-synuclein protein via the attenuation of this unique PKCδ mediated apoptotic process in the dopaminergic cells suggesting possible avenues for development of therapeutic measures for PD.