Essential role for Smad3 in regulating MCP-1 expression and vascular inflammation

Transforming growth factor (TGF)-beta(1) is a pleiotropic growth factor with known inhibitory effects on immune cell activation. However, the specific mechanism( s) and in vivo significance of the effectors of TGF-beta(1) modulation in the context of vascular inflammation are not well characterized. The chemokine monocyte chemoattractant protein ( MCP)- 1 is critical for the recruitment of macrophages in inflammatory disease states. In this study, we provide definitive evidence that the ability of TGF-beta(1) to inhibit MCP- 1 expression is mediated via its effector Smad3. Adenoviral overexpression of Smad3 potently repressed inducible expression of endogenous MCP- 1. Conversely, TGF-beta(1) inhibition of cytokine- mediated induction of MCP- 1 expression was completely blocked in Smad3- deficient macrophages. Consistent with this impaired response, cardiac allografts in Smad3- deficient mice developed accelerated intimal hyperplasia with increased infiltration of adventitial macrophages expressing MCP- 1. Previous studies show that MCP- 1 inducibility is regulated by an AP- 1 complex composed of c- Jun/ c- Fos heterodimers. We demonstrate that the inhibitory effect of Smad3 occurs via a novel antagonistic effect of Smad3 on AP- 1 DNA- protein binding and activity. Thus, Smad3 plays an essential role in modulating vascular inflammation characteristic of transplant- associated arteriopathy, is important in regulating MCP- 1 expression, and plays a critical role in the ability of TGF-beta(1) to repress stimuli from a major inflammatory signaling pathway.