Interferon-β activates multiple signaling cascades in primary human microglia

Microglia, the resident brain macrophages, are the principal cells involved in the regulation of inflammatory and antimicrobial responses in the CNS. Interferon-β (IFNβ) is an antiviral cytokine induced by viral infection or following non-specific inflammatory challenges of the CNS. Because of the well-known anti-inflammatory properties of IFNβ, it is also used to treat multiple sclerosis, an inflammatory CNS disease. Despite the importance of IFNβ signaling in CNS cells, little has been studied, particularly in microglia. In this report, we investigated the molecular mechanisms underlying IFNβ-induced β-chemokine expression in primary human fetal microglia. Multiple signaling cascades are activated in microglia by IFNβ, including nuclear factor-κB (NF-κB), activator protein-1 (AP-1) and Jak/Stat. IFNβ induced IκBα degradation and NF-κB (p65:p50) DNA binding. Inhibition of NF-κB by either adenoviral transduction of a super repressor IκBα, or an antioxidant inhibitor of NF-κB reduced expression of the β-chemokines, regulated upon activation, normal T-cell expressed and secreted (RANTES) and macrophage inflammatory protein (MIP)-1β. IFNβ also induced phosphorylation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase, and the MAP kinase kinase 1 (MEK1) inhibitor PD98059 dose-dependently inhibited β-chemokine mRNA and protein expression. PD98059 did not inhibit NF-βB binding, demonstrating that ERK was not responsible for NF-κB activation. Two downstream targets of ERK were identified in microglia: AP-1 and Stat1. IFNβ induced AP-1 nuclear binding activity in microglia and this was suppressed by PD98059. Additionally, IFNβ induced Stat1 phosphorylation at both tyrosine 701 (Y701) and serine 727 (S727) residues. S727 phosphorylation of Stat1, which is known to be required for maximal transcriptional activation, was inhibited by PD98059. Our results demonstrating multiple signaling cascades initiated by IFNβ in primary human microglia are novel and have implications for inflammatory and infectious diseases of the CNS.