Using the rat balloon catheter denudation model, we examined the role of transforming growth factor-beta (TGF-beta) isoforms in vascular repair processes. By en face in situ hybridization, proliferating and quiescent smooth muscle cells in denuded vessels expressed high levels of mRNA for TGF-beta1, TGF-beta2, TGF-beta3, and lower levels of TGF-beta receptor II (TGF-betaRII) mRNA. Compared with normal endothelium, TGF-beta1 and TGF-beta2, as well as TGF-betaRII, mRNA were upregulated in endothelium at the wound edge. Injected recombinant soluble TGF-betaRII (TGF-betaR:Fc) localized preferentially to the adventitia and developing neointima in the injured carotid artery, causing a reduction in intimal lesion formation (up to 65%) and an increase in lumen area (up to 88%). The gain in lumen area was largely due to inhibition of negative remodeling, which coincided with reduced adventitial fibrosis and collagen deposition. Four days after injury, TGF-betaR:Fc treatment almost completely inhibited the induction of smooth muscle alpha-actin expression in adventitial cells. In the vessel wall, TGF-betaR:Fc caused a marked reduction in mRNA levels for collagens type I and III. TGF-betaR:Fc had no effect on endothelial proliferation as determined by reendothelialization of the denuded rat aorta. Together, these findings identify the TGF-beta isoforms as major factors mediating adventitial fibrosis and negative remodeling after vascular injury, a major cause of restenosis after angioplasty.

Vascular remodeling is a response of blood vessels to both physiological and pathological stimuli, leading to either vessel enlargement (positive remodeling) or reduction in vessel diameter (negative remodeling). Examples of remodeling have been observed in fetal development 1 and after graft placement 2-5 or angioplasty. 6 -8 In humans, vascular remodeling but not intimal lesion formation was shown to account for the majority of the restenosis in response to angioplasty procedures. 9,10 We have recently established and characterized a mouse model of arterial remodeling. 11 In this model, flow in the common carotid artery is interrupted by ligation of the vessel near the carotid bifurcation. Using FVB/NJ mice, this resulted in a dramatic reduction in vessel diameter and formation of an intimal lesion. Neointima formation and the influx of inflammatory cells in this model are reduced in P-selectin-deficient mice, while the reduction in vessel diameter is not affected by the lack of P-selectin. 12 Additional specific factors that mediate the remodeling response are beginning to emerge. Several studies have implicated nitric oxide (NO) as an inhibitor of remodeling events. [13][14][15][16][17] Our own studies demonstrated that alterations in blood flow also lead to changes in gene expression of platelet-derived growth factor Achain and B-chain, factors known to modulate proliferation and migration of smooth muscle cells (SMC). 18 Preliminary experiments in our laboratory indicated that there is wide qualitative and quantitative variation in the vascular remodeling response of different mouse strains. To provide the basis for a genetic analysis, we subjected 11 different strains of inbred mice to carotid artery ligation for analysis of the remodeling response. Large differences were found between strains with regards to negative as well as positive remodeling and intimal lesion formation. The magnitude of neointima formation correlated with increased loss of SMC occurring immediately after ligation of the carotid artery as well as enhanced growth properties of SMC in vitro.

Vascular endothelial growth factor (VEGF) has traditionally been considered an endothelial cell-specific factor inducing angiogenesis and vascular permeability in vivo. In the present study, expression of VEGF and its receptors, fetal liver kinase-1 (flk-1) and fms-like tyrosine kinase-1 (flt-1), was examined in rat carotid arteries after balloon injury. Although VEGF and flk-1 were not detectable, high levels of flt-1 mRNA and protein were expressed by smooth muscle cells (SMCs) in the neointima, as demonstrated by en face in situ hybridization and Western blotting. Intimal SMC proliferation in chronically denuded rat carotid arteries was unaffected by intraluminal infusion of VEGF, whereas fibroblast growth factor (FGF)-2 increased the number of replicating SMCs 4-fold. Pretreatment with VEGF doubled the mitogenic response to infused FGF-2 by increasing SMC replication in deeper layers of the intima. VEGF increased the permeability of chronically denuded vessels to plasma proteins but had no effect on the uptake of locally infused biotinylated FGF-2. These findings demonstrate that vascular SMCs express functional flt-1 receptors after arterial injury and that VEGF has synergistic effects with FGF-2 on SMC proliferation. These effects are likely to be mediated by a VEGF-mediated increase in permeability as well as a direct interaction between the VEGF and FGF signaling pathways.