Transforming growth factor-β (TGF-β) stimulates collagen synthesis and accumulation, and aberrant TGF-β signaling is implicated in pathological organ fibrosis. Regulation of type I procollagen gene (COL1A2) transcription by TGF-β involves the canonical Smad signaling pathway as well as additional protein and lipid kinases, coactivators, and DNA-binding transcription factors that constitute alternate non-Smad pathways. By using Affymetrix microarrays to detect cellular genes whose expression is regulated by Smad3, we identified early growth response factor-1 (EGR-1) as a novel Smad3-inducible gene. Previous studies implicated Egr-1 in cell growth, differentiation, and survival. We found that TGF-β induced rapid and transient accumulation of Egr-1 protein and mRNA in human skin fibroblasts. In transient transfection assays, TGF-β stimulated the activity of the Egr-1 gene promoter, as well as that of a minimal Egr-1-responsive reporter construct. Furthermore, TGF-β enhanced endogenous Egr-1 interaction with a consensus Egr-1-binding site element and with GC-rich DNA sequences of the human COL1A2 promoter in vitro and in vivo. Forced expression of Egr-1 by itself caused dose-dependent up-regulation of COL1A2 promoter activity and further enhanced the stimulation induced by TGF-β. In contrast, the TGF-β response was abrogated when the Egr-1-binding sites of the COL1A2 promoter were mutated or deleted. Furthermore, Egr-1-deficient embryonic mouse fibroblasts showed attenuated TGF-β responses despite intact Smad activation, and forced expression of ectopic EGR-1 in these cells could restore COL1A2 stimulation in a dose-dependent manner. Taken together, these findings identify Egr-1 as a novel intracellular TGF-β target that is necessary for maximal stimulation of collagen gene expression in fibroblasts. The results therefore implicate Egr-1 in the profibrotic responses elicited by TGF-β and suggest that Egr-1 may play a new and important role in the pathogenesis of fibrosis.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology