Objective— Transforming growth factor-β1 (TGFβ1) and fibroblast growth factor (FGF) families play a pivotal role during vascular development and in the pathogenesis of vascular disease. However, the interaction of intracellular signaling evoked by each of these growth factors is not well understood. The present study was undertaken to examine the molecular mechanisms that mediate the effects of TGFβ1 and basic FGF (bFGF) on smooth muscle cell (SMC) gene expression.

Methods and Results— TGFβ1 induction of SMC gene expression, including smooth muscle protein 22-α (SM22α) and smooth muscle α-actin, was examined in the pluripotent 10T1/2 cells. Marked increase in these mRNA levels by TGFβ1 was inhibited by c-Src-tyrosine kinase inhibitors and protein synthesis inhibitor cycloheximide. Functional studies with deletion and site-directed mutation analysis of the SM22α promoter demonstrated that TGFβ1 activated the SM22α promoter through a CC(A/T-rich)₆GG (CArG) box, which serves as a serum response factor (SRF)–binding site. TGFβ1 increased SRF expression through an increase in transcription of the SRF gene. In the presence of bFGF, TGFβ1 induction of SMC marker gene expression was significantly attenuated. Transient transfection assays showed that bFGF significantly suppressed induction of the SM22α promoter–driven luciferase activity by TGFβ1, whereas bFGF had no effects on the TGFβ1-mediated increase in SRF expression and SRF:DNA binding activity. Mitogen-activated protein kinase kinase-1 (MEK1) inhibitor PD98059 abrogated the bFGF-mediated suppression of TGFβ1-induced SMC gene expression.

Conclusion— Our data suggest that bFGF-induced MEK/extracellular signal-regulated kinase signaling plays an antagonistic role in TGFβ1-induced SMC gene expression through suppression of the SRF function. These data indicate that opposing effects of bFGF and TGFβ1 on SMC gene expression control the phenotypic plasticity of SMCs.