| Elastin, a principal component of elastic fibers, maintains the resilience and structural integrity of airways and blood vessels in the mature lung. Elastin is a remarkably durable polymer, assembled from cross-linked monomers of tropoelastin, its soluble precursor. Once deposited, elastin essentially does not turn over in healthy lung during the life span of the organism. Emphysema, however, is characterized by irreversible destruction of elastic fibers, which is the consequence of increased elastolysis and insufficient repair of elastin. The mechanism underlying the insufficient repair of elastin is still under investigation. We have previously reported that neutrophil elastase downregulates tropoelastin expression in rat lung fibroblasts through the transactivation of the epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase kinase (Mek)/extracellular signal-regulated kinases 1 and 2 (Erk) signaling pathway, which is dependent on the release of epidermal growth factor (EGF)-like growth factors. In the present study, we investigated the mechanism by which EGF downregulates tropoelastin expression. We found that tropoelastin expression in the rat fetal lung fibroblast line RFL-6 is primarily maintained by autocrine transforming growth factor-beta (TGF-beta). We demonstrated that EGF, via the EGFR/Mek/Erk pathway, downregulates tropoelastin expression through inhibition of TGF-beta signaling. We showed that EGF does not prevent the TGF-beta-induced nuclear accumulation of Smad2/3; rather, EGF stabilizes Smad corepressor TG-interacting factor (TGIF) through EGFR/Mek/Erk-mediated phosphorylation of TGIF. We confirmed that elevated TGIF levels are sufficient to inhibit TGF-beta-induced tropoelastin expression in RFL-6 cells. Moreover, elevated TGIF levels are essential for EGF to downregulate tropoelastin expression, as the EGF inhibition of tropoelastin expression is prevented in cells whose TGIF levels are knocked down by RNAi. These results establish that EGF inhibits TGF-beta signaling and therefore downregulates tropoelastin expression through the stabilization of Smad corepressor TGIF. We also demonstrated that neutrophil elastase causes stabilization of TGIF through transactivation of the EGFR/Mek/Erk signaling pathway, which is dependent on the release of EGFR ligands. |
