An increased plasma homocysteine level is an independent risk factor for vascular disease. However, the pathological mechanisms by which homocysteine promotes atherosclerosis are not yet clearly defined. Arterial smooth muscle cells cultured in the presence of homocysteine grew to a higher density and produced and accumulated collagen at levels significantly above control values. Homocysteine concentrations as low as 50 mumol/L significantly increased both cell density and collagen production. Cell density increased by as much as 43% in homocysteine-treated cultures. Homocysteine increased collagen production in a dose-dependent manner. Smooth muscle cells treated with homocysteine at concentrations observed in patients with hyperhomocysteinemia had collagen synthesis rates as high as 214% of control values. Likewise, collagen accumulation in the cell layer was nearly doubled in homocysteine-treated cultures. Addition of aquacobalamin to homocysteine-treated cultures controlled the increase in smooth muscle cell proliferation and collagen production. These results indicate a cellular mechanism for the atherogenicity of homocysteine and provide insight into a potential preventive treatment.

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To determine the effects of an intact extracellular matrix on collagen synthesis, arterial smooth muscle cells (SMCs) were plated sparsely on a cell-free, SMC-derived matrix and examined the following day. Collagen synthesis during a 5-hour incubation by cells on the matrix was reduced to 67% of the control values obtained from cultures on plastic. Total protein synthesis was unaffected. Treatment of the matrix with heparitinase to remove basic fibroblast growth factor (bFGF) before seeding the SMCs abolished the inhibitory effect of the matrix on collagen synthesis. The inhibitory effect was also eliminated by treating the matrix with a neutralizing polyclonal antibody directed against bFGF. Collagen synthesis by SMC cultures grown in wells coated with purified bFGF was only 61% that of control cultures, whereas total protein synthesis remained unchanged. Slot-blot analysis revealed that the relative message level for orl(III) procollagen was reduced in cultures grown on the preexisting matrix or on plastic precoated with bFGF, whereas the al(I) procollagen message was unaffected. These results demonstrate the ability of the extracellular matrix to modulate the synthesis of collagen by arterial SMCs and indicate that bFGF in the matrix is responsible for these effects. (Arteriosclerosis and Thrombosis 1993; 13:680-686) KEY WORDS • collagen • smooth muscle cells • basic fibroblast growth factor • extracellular matrixC ells in vivo are in contact with an insoluble matrix composed of collagens, elastin, proteoglycans, noncollagenous glycoproteins, and a variety of adherent nonstructural components including growth factors. This extracellular matrix has the capacity to modulate the behavior and phenotype of cells both in vivo and in vitro. Several types of cultured cells have been shown to respond to the substratum onto which they are plated by altering their adhesion, growth, morphology, and protein synthesis patterns.Numerous studies have shown that preformed matrices, when used as a substratum for cultured cells, may have significant effects on cell proliferation.1 " 3 In addition, the substratum on which cells are plated can have specific effects on the synthesis of extracellular matrix components.4 " 6 Previous studies in our laboratory have demonstrated that preconfluent smooth muscle cells (SMCs) grown on fibronectin synthesize significantly less collagen and fibronectin than control SMCs, whereas noncollagenous protein synthesis was unaltered. 7 In contrast, both collagen and fibronectin syn-

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