Lipopolysaccharide activation of the MEK-ERK1/2 pathway in human monocytic cells mediates tissue factor and tumor necrosis factor α expression by inducing Elk-1 phosphorylation and Egr-1 expression

Abstract

Lipopolysaccharide (LPS) induces human monocytes to express many proinflammatory mediators, including the procoagulant molecule tissue factor (TF) and the cytokine tumor necrosis factor alpha (TNF-α). The TF and TNF-α genes are regulated by various transcription factors, including nuclear factor (NF)-κB/Rel proteins and Egr-1. In this study, the role of the MEK-ERK1/2 mitogen-activated protein kinase (MAPK) pathway in LPS induction of TF and TNF-α gene expression in human monocytic cells was investigated. The MAPK kinase (MEK)1 inhibitor PD98059 reduced LPS induction of TF and TNF-α expression in a dose-dependent manner. PD98059 did not affect LPS-induced nuclear translocation of NF-κB/Rel proteins and minimally affected LPS induction of κB-dependent transcription. In contrast, PD98059 and dominant-negative mutants of the Ras-Raf1-MEK-ERK (extacellular signal–regulated kinase) pathway strongly inhibited LPS induction of Egr-1 expression. In kinetic experiments LPS induction of Egr-1 expression preceded induction of TF expression. In addition, mutation of the Egr-1 sites in the TF and TNF-α promoters reduced expression of these proinflammatory genes. It was demonstrated that LPS induction of the Egr-1 promoter was mediated by 3 SRE sites, which bound an LPS-inducible complex containing serum response factor and Elk-1. LPS stimulation transiently induced phosphorylation of Elk-1 and increased the functional activity of a GAL4–Elk-1TA chimeric protein via the MEK-ERK1/2 pathway. The data indicate that LPS induction of Egr-1 gene expression is required for maximal induction of the TNF-α and TF genes in human monocytic cells.