Regulatory role of cytosolic phospholipase A₂α in NADPH oxidase activity and in inducible nitric oxide synthase induction by aggregated Aβ_1-42 in microglia

In Alzheimer's disease, extracellular deposits of amyloid β_1-42 (Aβ_1-42) may induce activation of microglial cells by releasing proinflammatory factors that contribute to the neurodegeneration process. Since the activation of cytosolic phospholipase A₂α (cPLA₂α) has been reported in inflammatory conditions, its role in primary rat microglial cell activated by aggregated Aβ_1-42 was elucidated. The results of the present study show that activation of microglia by 5 μM aggregated Aβ_1-42 (as evident by the amoeboid morphology and increased CD68 immunofluorescence reactivity) caused an immediate activation of cPLA₂α, measured by its phosphorylated form and its specific activity, followed by a gradual elevation of its expression and activity during 24 h. Inhibition of cPLA ₂α expression and activity by the presence of 1 μM specific antisense resulted in a significant decrease in NADPH oxidase activity that releases superoxides, PGE₂ formation, iNOS expression, and NO production, indicating a major role for cPLA₂α in the regulation of these inflammatory processes. NADPH oxidase activity, which is under cPLA₂α regulation, was found to upregulate cPLA ₂α and COX-2 protein expression through the redoxsensitive NFκB activation as evident by its phosphorylation on Ser-536, resulting in increased PGE₂ formation. The secreted PGE₂ induced the synthesis of iNOS and the production of NO through the PKA-CREB pathway. Taken together, our results suggest that the response of cPLA₂α to aggregated Aβ_1-42 is probably a key player in the oxidative stress present in AD, regulating potent oxidative agents: the production of superoxides by NADPH oxidase and NO formation by iNOS.