Dieldrin induces apoptosis by promoting caspase-3-dependent proteolytic cleavage of protein kinase Cδ in dopaminergic cells: relevance to oxidative stress and dopaminergic degeneration

Abstract

We previously reported that dieldrin, one of the potential environmental risk factors for development of Parkinson's disease, induces apoptosis in dopaminergic cells by generating oxidative stress. Here, we demonstrate that the caspase-3-dependent proteolytic activation of protein kinase Cδ (PKCδ) mediates as well as regulates the dieldrin-induced apoptotic cascade in dopaminergic cells. Exposure of PC12 cells to dieldrin (100–300 μM) results in the rapid release of cytochrome C, followed by the activation of caspase-9 and caspase-3 in a time- and dose-dependent manner. The superoxide dismutase mimetic Mn(III)tetrakis(4-benzoic acid)porphyrin chloride significantly attenuates dieldrin-induced cytochrome C release, indicating that reactive oxygen species may contribute to the activation of pro-apoptotic factors. Interestingly, dieldrin proteolytically cleaves native PKCδ into a 41 kDa catalytic subunit and a 38 kDa regulatory subunit to activate the kinase. The dieldrin-induced proteolytic cleavage of PKCδ and induction of kinase activity are completely inhibited by pretreatment with 50–100 μM concentrations of the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-FMK) and benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD-FMK), indicating that the proteolytic activation of PKCδ is caspase-3-dependent. Additionally, Z-VAD-FMK, Z-DEVD-FMK or the PKCδ specific inhibitor rottlerin almost completely block dieldrin-induced DNA fragmentation. Because dieldrin dramatically increases (40–80-fold) caspase-3 activity, we examined whether proteolytically activated PKCδ amplifies caspase-3 via positive feedback activation. The PKCδ inhibitor rottlerin (3–20 μM) dose-dependently attenuates dieldrin-induced caspase-3 activity, suggesting positive feedback activation of caspase-3 by PKCδ. Indeed, delivery of catalytically active recombinant PKCδ via a protein delivery system significantly activates caspase-3 in PC12 cells. Finally, overexpression of the kinase-inactive PKCδ K376R mutant in rat mesencephalic dopaminergic neuronal cells attenuates dieldrin-induced caspase-3 activity and DNA fragmentation, further confirming the pro-apoptotic function of PKCδ in dopaminergic cells. Together, we conclude that caspase-3-dependent proteolytic activation of PKCδ is a critical event in dieldrin-induced apoptotic cell death in dopaminergic cells.