Background Keloids are recognized as benign tumours characterized by fibroblastic proliferation and accumulation of extracellular matrix, especially collagen deposition. The transforming growth factor (TGF)-β1/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-β1/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-β1 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-β1 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-β1 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-β1 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-β1/Smad signal transduction and might be considered as specific targets of drug therapy for keloids.