Analysis of functional elements in the human Egr-1 gene promoter
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The early growth response (Egr)-1 gene encoding a zinc-finger transcription factor is transiently induced in many different cell types upon various differentiation signals. However, in synovial fibroblasts of rheumatoid arthritis patients, Egr-1 is constitutively expressed at high levels, and several genes with Egr-1 binding sites in their promoter regions have been associated with disease progression of RA. We analyzed the control of Egr-1 transcription by characterizing those regulatory elements in the Egr-1 promoter that induce Egr-1 expression in fibroblasts. Using reporter gene assays and deletion mutants of the Egr-1 promoter we could demonstrate that Egr-1 transcription is mainly activated by a single serum response element, whereas other transcription factor binding sites, including binding sites for AP-1 or Egr-1, were found to play a minor role. Furthermore, we identified a novel regulatory element in the human Egr-1 promoter similar to a NFκ-B binding site. Deletion of this element enhanced Egr-1 promoter activity in 3T3 but not in L929 fibroblasts. Stimulation by phorbolester induced only transient Egr-1 expression in 3T3 fibroblasts but a extended expression of Egr-1 in L929 cells. These data suggest that in fibroblasts the most proximal serum response element in the Egr-1 promoter represents the major activation site, whereas binding of the NFkB-like factor may serve as negative regulatory signal for Egr-1 transcription in fibroblasts.
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