Suppression of tumor necrosis factor (TNF-α) gene expression by prostaglandin E2.: Role of early growth response protein-1 (Egr-1)

We examined the mechanism by which PGE(2) suppresses the expression of TNF-alpha in human macrophages and synovial fibroblasts. Prostaglandin E-2 increased, in a time and dose-dependent (EC50 75+/-15 ng/ml, mean+/-SD) fashion, the expression and synthesis of Egr-1/Krox24 as judged by Northern blotting and electrophoretic mobility gel-shift analysis, respectively. In human macrophagic THP-1 cells, rhIL-17 increased promoter activity by 7.6+/-0.35-fold over controls, an effect that was abrogated in a dose-dependent fashion by coincubations with PGE(2) (IC50 25+/-4 ng/ml). An intact Egr-1/Krox-24 enhancer sequence in the TNF-alpha promoter region was essential for the latter PGE(2)-dependent inhibitory effect as double base substitutions (GC --> TT) in the sequence curtailed promoter response to PGE(2). Overexpression of two dominant negative Egr-1/Krox-24 constructs in THP-1 cells considerably diminished the inhibitory effects of PGE, on rhIL-17-induced TNF-alpha mRNA expression. We conclude that PGE(2) inhibits induced TNF-alpha. expression in target cells through an Egr-1/Krox-24 mediated signaling process.