Amyloid peptide-induced cytokine and chemokine expression in THP-1 monocytes is blocked by small inhibitory RNA duplexes for early growth response-1 messenger RNA

In Alzheimer's disease (AD) one finds increased deposition of Abeta and also an increased presence of monocytes/macrophages in the vessel wall and activated microglial cells in the brain. AD patients show increased levels of proinflammatory cytokines by activated microglia. Here we used a human monocytic THP-1 cell line as a model for microglia to delineate the cellular signaling mechanism involved in amyloid peptides (Abeta(1-40) and Abeta(1-42))-induced expression of inflammatory cytokines and chemokines. We observed that Abeta peptides at physiological concentrations (125 nM) increased mRNA expression of cytokines (TNF-alpha, and IL-1beta) and chemokines (monocyte chemoattractant protein-1 (MCP-1), IL-8, and macrophage inflammatory protein-1beta (MIP-1beta)). The cellular signaling involved activation of c-Raf, extracellular signal-regulated kinase-1 (ERK-1)/ERK-2, and c-Jun N-terminal kinase, but not p38 mitogen-activated protein kinase. This is further supported by the data showing that Abeta causes phosphorylation of ERK-1/ERK-2, which, in turn, activates Elk-1. Furthermore, Abeta mediated a time-dependent increase in DNA binding activity of early growth response-1 (Egr-1) and AP-1, but not of NF-kappaB and CREB. Moreover, Abeta-induced Egr-1 DNA binding activity was reduced >60% in THP-1 cells transfected with small interfering RNA duplexes for Egr-1 mRNA. We show that Abeta-induced expression of TNF-alpha, IL-1beta, MCP-1, IL-8, and MIP-1beta was abrogated in Egr-1 small inhibitory RNA-transfected cells. Our results indicate that Abeta-induced expression of cytokines (TNF-alpha and IL-1beta) and chemokines'(MCP-1, IL-8, and MIP-1beta) in THP-1 monocytes involves activation of ERK-1/ERK-2 and downstream activation of Egr-1. The inhibition of Egr-1 by Egr-1 small inhibitory RNA may represent a potential therapeutic target to ameliorate the inflammation and progression of AD.