Approximately 30 to 40 percent of atherosclerotic coronary arteries treated by angioplasty or by bypass surgery occlude as a result of restenosis. This restenosis is due principally to the accumulation of neointimal smooth muscle cells, which is also a prominent feature of the advanced lesions of atherosclerosis. The factors responsible for the accumulation of intimal smooth muscle cells have not been identified. Platelet-derived growth factor (PDGF) is a potent smooth muscle chemoattractant and mitogen. It is present in platelets and can be formed by endothelium, smooth muscle, and monocyte-derived macrophages. The development of an intimal lesion in the carotid artery of athymic nude rats induced by intraarterial balloon catheter deendothelialization was inhibited by a polyclonal antibody to PDGF. These data demonstrate that endogenous PDGF is involved in the accumulation of neointimal smooth muscle cells associated with balloon injury and may be involved in restenosis after angioplasty, and perhaps in atherogenesis as well.

We have characterized matrix metalloproteinase expression in the rat carotid artery after two forms of arterial injury, balloon catheter denudation and nylon filament denudation. Gelatinolytic enzymes with molecular masses of 70 and 62 kD were produced constitutively in the rat carotid. Production of an 88-kD gelatinase was induced after balloon catheter injury, and proteinase production continued during the period of migration of smooth muscle cells from the media to the intima, from 6 hours to 6 days after balloon catheter injury. In addition, a marked increase in 62-kD gelatinolytic activity was observed between 4 and 14 days after arterial injury. Gelatinase activities (88 and 62 kD) were also increased after nylon filament denudation but were markedly less after this injury than after balloon catheter injury. These results suggested a correlation between gelatinase activity and smooth muscle cell migration after arterial injury. Administration of a metalloproteinase inhibitor after balloon catheter injury resulted in a 97% reduction in the number of smooth muscle cells migrating into the intima. Therefore, we hypothesize that gelatinase expression directly facilitates smooth muscle cell migration within the media and into the intima. These results suggest that gelatinases are involved in the vascular smooth muscle cell activation and neointimal formation that characterize arterial tissue remodeling after injury.

Communicated by Earl P. Benditt, February 7, 1991 ABSTRACT Proliferation of smooth muscle cells (SMCs) represents an important event in vascular lesion formation. Despite the common belief that growth factors contribute to the development of the atherosclerotic plaque, until now there has been no direct evidence for a role of mitogens in the development of arterial lesions. Balloon catheter injury of the rat carotid artery is accompanied by death of medial SMCs and is typically followed by proliferation of SMCs with subsequent formation of an intimal lesion. Our hypothesis is that injury causes mitogens to be released from dead cells, which then stimulate cell proliferation. One such mitogen that may be important in this process is basic fibroblast growth factor (bFGF), which can be detected immunocytochemically in SMCs and endothelial cells of adult rat carotid arteries.

In the present study, we established an injury model of the mouse carotid artery. Complete removal of the endothelium was achieved with a flexible wire. A platelet monolayer covered the denuded surface, and damage to underlying medial smooth muscle cells (SMCs) was detected. Injection of [3H]thymidine was used to determine the replication index for medial SMCs, which was found to be 1.6% at 2 days after denudation and 9.8% at 5 days. SMCs were observed in the intima by day 8 (replication index, 66%), and by 2 weeks the intimal lesion had a similar cell content as the media. In most animals, repair of the endothelial lining was complete 3 weeks after injury. The present model will allow us to use transgenic animals to address questions relevant to vascular biology and atherosclerosis.

Repair of arterial injury produced by balloon angioplasty leads to the formation of a neointima and a narrowing of the vascular lumen. In this study, we examined the possibility that smooth muscle cells (SMC) in injured rat carotid arteries are stimulated to produce type-i transforming growth factor-,# 61) during neointima formation in vivo. Levels of TGF-#l1 transcripts (2.4 kb) were significantly increased within 6 h after carotid injury and reached a maximum (five to sevenfold) by 24 h. Regenerating left carotids had sustained increases in TGF-j@1 mRNA levels (about fivefold) over the next 2 wk, during which time a substantial neointimal thickening was formed. No changes in basal TGF-#61 mRNA levels were found in contralateral uninjured carotids at any of the times examined. Immunohistochemical studies showed that a large majority of neointimal SMC were stained for TGF-,81 protein in an intracellular pattern, consistent with active TGF-fi1 synthesis in this tissue.Neointima formation and TGF-,61 immunoreactivity were correlated with increases in fibronectin, collagen a2(I), and collagen al(III) gene expression. Infusion of purified, recombinant TGF-#,1 into rats with a preexisting neointima produced a significant stimulation of carotid neointimal SMC DNA synthesis.These results suggest that TGF-,61 plays an important role as an endogenous growth regulatory factor produced by neointimal SMC themselves during progressive neointimal thickening after balloon angioplasty. (J. Clin. Invest. 1991. 88:904-910.)