Members of the transforming growth factor β (TGFβ) cytokine superfamily play critical roles in both homeostasis and disease. In light of their profibrotic effects, these molecules are implicated in the pathogenesis of fibrosis. In fibroblasts, TGFβ signals through the activin receptor–like kinase 5 (ALK‐5) type I TGFβ and triggers Smad and MAP kinase signaling pathways. Because targeting of TGFβ signaling represents a potential approach to the treatment of systemic sclerosis (SSc) and other fibrotic disorders, we investigated the modulation of intracellular TGFβ signal transduction by SB431542, the first small‐molecule inhibitor of ALK‐5 to be described.

Ligand‐induced activation of the Smad signaling pathway in human dermal fibroblasts was examined by Western blot analysis and confocal immunocytochemistry. Modulation of profibrotic gene expression was investigated using Northern blot analysis, transient transfection assays, and confocal microscopy. Induction of TGFβ production was evaluated by enzyme‐linked immunosorbent assay.

SB431542 abrogated TGFβ‐induced phosphorylation and nuclear importation of endogenous Smad2/3 and Smad4, and inhibited Smad3‐ and Smad2‐dependent gene transcription. Treatment with SB431542 prevented TGFβ‐induced stimulation of collagen, fibronectin, plasminogen activator inhibitor 1, and connective tissue growth factor gene expression, TGFβ autoinduction, and myofibroblast transdifferentiation, and it could reverse stimulation even when added to the cultures after TGFβ. In contrast, STAT‐6–mediated stimulation of collagen gene expression induced by interleukin‐13 was not prevented by SB431542, indicating the specificity of blockade for ALK‐5–dependent signaling. Furthermore, in contrast to its effects on receptor‐activated Smad activation, SB431542 failed to prevent TGFβ‐induced activation of MAP kinases.

The results indicate that SB431542 is a potent inhibitor of intracellular TGFβ signaling in normal fibroblasts through selective interference with ALK‐5–mediated Smad activation and Smad‐dependent transcriptional responses. Therefore, SB431542 is useful as a novel experimental tool for gaining a detailed understanding of normal and aberrant TGFβ signaling in SSc. Furthermore, as an anti‐TGFβ agent, SB431542 may represent a potential new approach to the treatment of fibrosis.