Vascular smooth muscle cell (VSMC) proliferation and migration are responses to arterial injury that are highly important to the processes of restenosis and atherosclerosis. In the arterial balloon injury model in the rat, platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF) are induced in the vessel wall and regulate these VSMC activities. Novel insulin sensitizing agents, thiazolidinediones, have been demonstrated to inhibit insulin and epidermal growth factor-induced growth of VSMCs. We hypothesized that these agents might also inhibit the effect of PDGF and bFGF on cultured

Pharmacological activation of PPARgamma expressed in VSMCs inhibits their proliferation and migration, potentially limiting restenosis and atherosclerosis. These receptors are upregulated during vascular injury.

Abstract-Peroxisome proliferator-activated receptor-␥ (PPAR␥) is a ligand-activated nuclear receptor expressed in all of the major cell types found in atherosclerotic lesions: monocytes/macrophages, endothelial cells, and smooth muscle cells. In vitro, PPAR␥ ligands inhibit cell proliferation and migration, 2 processes critical for vascular lesion formation. In contrast to these putative antiatherogenic activities, PPAR␥ has been shown in vitro to upregulate the CD36 scavenger receptor, which could promote foam cell formation. Thus, it is unclear what impact PPAR␥ activation will have on the development and progression of atherosclerosis. This issue is important because thiazolidinediones, which are ligands for PPAR␥, have recently been approved for the treatment of type 2 diabetes, a state of accelerated atherosclerosis. We report herein that the PPAR␥ ligand, troglitazone, inhibited lesion formation in male low density lipoprotein receptor-deficient mice fed either a high-fat diet, which also induces type 2 diabetes, or a high-fructose diet.Troglitazone decreased the accumulation of macrophages in intimal xanthomas, consistent with our in vitro observation that troglitazone and another thiazolidinedione, rosiglitazone, inhibited monocyte chemoattractant protein-1-directed transendothelial migration of monocytes. Although troglitazone had some beneficial effects on metabolic risk factors (in particular, a reduction of insulin levels in the diabetic model), none of the systemic cardiovascular risk factors was consistently improved in either model. These observations suggest that the inhibition of early atherosclerotic lesion formation by troglitazone may result, at least in part, from direct effects of PPAR␥ activation in the artery wall.

Angiotensin II (A II ) is a critical factor in cardiac remodeling which involves hypertrophy, fibroblast proliferation, and extracellular matrix production. However, little is known about the mechanism by which A II accelerates these responses. Osteopontin is an acidic phosphoprotein with RGD (arginine-glycine-aspartate) sequences that are involved in the vascular smooth muscle cell remodeling process. We identified the presence of osteopontin mRNA and protein in cultured rat cardiac fibroblasts and its prominent regulation by A II (10 Ϫ 11 M). Osteopontin message levels were increased fourfold ( P Ͻ 0.01) and protein fivefold ( P Ͻ 0.05) at 24 h after addition of A II (10 Ϫ 7 M). This response was inhibited by the AT 1 receptor blocker, losartan. Osteopontin mRNA levels were increased in hypertrophied ventricles from animals with renovascular hypertension (1.6-fold, P Ͻ 0.05) and aortic banding (2.9-fold, P Ͻ 0.05). To examine the function of osteopontin, we determined its effects on ( a ) the ability of cardiac fibroblasts to contract three-dimensional collagen gels and ( b ) cardiac fibroblast growth. A monoclonal antibody against osteopontin partially blocked A II -induced three-dimensional collagen gel contraction by cardiac fibroblasts (64 Ϯ 4 vs. 86 Ϯ 5% in the presence of antibody, P Ͻ 0.05), while osteopontin itself promoted contraction of the gels by fibroblasts (71 Ϯ 5%, P Ͻ 0.05 compared with control). Either a monoclonal antibody against ␤ 3 integrin which is a ligand for osteopontin or the RGD peptide blocked both A II and osteopontin-induced collagen gel contraction. Thus, the osteopontin RGD sequence binds to ␤ 3 integrins on the fibroblast to promote fibroblast binding to collagen. A II induced a threefold increase in DNA synthesis of cardiac fibroblasts, which was completely blocked by antibodies against osteopontin and ␤ 3 integrin, or by RGD peptide, but not by controls. Thus, A II -induced growth of cardiac fibroblasts also requires osteopontin engagement of the ␤ 3 integrin. Taken together, these results provide the first evidence that osteopontin is a potentially important mediator of A II regulation of cardiac fibroblast behavior in the cardiac remodeling process. ( J. Clin. Invest. 1996. 98:2218-2227.)

The present study provides the first evidence that cardiomyocytes are a prominent source of OP in vivo and suggests that induction of OP expression is strongly associated with ventricular hypertrophy.

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