Mechanisms of transforming growth factor β1/Smad signalling mediated by mitogen-activated protein kinase pathways in keloid fibroblasts

Background Keloids are recognized as benign tumours characterized by fibroblastic proliferation and accumulation of extracellular matrix, especially collagen deposition. The transforming growth factor (TGF)-beta(1)/Smad pathway plays an important role in keloid pathogenesis; however the underlying mechanisms are not fully understood. Objectives To define further the mechanisms of TGF-beta(1)/Smad signal transduction mediated by mitogen-activated protein kinases (MAPKs), including the extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 pathways, in keloid fibroblasts. Methods In the absence or presence of three MAPK (ERK, JNK and p38)-specific inhibitors, keloid fibroblasts were stimulated with exogenous TGF-beta(1) to activate Smad signalling. Smad protein expression was measured by immunoprecipitation/immunoblotting and immunofluorescence; plasminogen activator inhibitor (PAI)-1 transcriptional activity was measured by real-time reverse transcriptase-polymerase chain reaction analysis. Results TGF-beta(1) induced Smad2/3 phosphorylation at both the C-terminal and the linker region, thus promoting formation of the Smad2/3/4 complex and nuclear translocation, and PAI-1 mRNA expression in keloid fibroblasts; in addition, TGF-beta(1) decreased inhibitory Smad7 expression. Meanwhile, the p38 inhibitor significantly inhibited Smad2/3 phosphorylation, especially at the linker region, and furthermore blocked Smad2/3/4 complex formation, and thus decreased PAI-1 mRNA expression; decreased Smad7 expression induced by TGF-beta(1) was also reversed by the p38 inhibitor. The ERK and JNK inhibitors interrupted Smad2/3/4 complex translocation into the nucleus and consequently decreased PAI-1 mRNA expression. Conclusions These results suggested that the ERK, JNK and p38 pathways mediate TGF-beta(1)/Smad signal transduction and might be considered as specific targets of drug therapy for keloids.