β-Amyloid Stimulation of Inducible Nitric-oxide Synthase in Astrocytes Is Interleukin-1β- and Tumor Necrosis Factor-α (TNFα)-dependent, and Involves a TNFα Receptor-associated Factor- and NFκB-inducing Kinase-dependent Signaling Mechanism

Abstract

In Alzheimer's disease, beta-amyloid (Abeta) plaques are surrounded by activated astrocytes and microglia. A growing body of evidence suggests that these activated glia contribute to neurotoxicity through the induction of inflammatory cytokines such as interleukin (IL)-1beta and tumor necrosis factor-alpha (TNFalpha) and the production of neurotoxic free radicals, mediated in part by the expression of inducible nitric-oxide synthase (iNOS). Here, we address the possibility that Abeta-stimulated iNOS expression might result from an initial induction of IL-1beta and TNFalpha. We find that in Abeta-stimulated astrocyte cultures, IL-1beta and TNFalpha production occur before iNOS production, new protein synthesis is required for increased iNOS mRNA levels, and the IL-1 receptor antagonist IL-1ra can inhibit nitrite accumulation. Likewise, dominant-negative mutants of tumor necrosis factor-alpha receptor-associated factor (TRAF) 6, TRAF2, and NFkappaB-inducing kinase (NIK), intracellular proteins involved in IL-1 and TNFalpha receptor signaling cascades, inhibit Abeta-stimulated iNOS promoter activity. Our data suggest that Abeta stimulation of astrocyte iNOS is mediated in part by IL-1beta and TNFalpha, and involves a TRAF6-, TRAF2-, and NIK-dependent signaling mechanism.