Cultured rat aortic vascular smooth muscle cells digest naturally produced extracellular matrix: Involvement of plasminogen-dependent and plasminogen-independent pathways

Abstract

Vascular smooth muscle (VSM) cell migration and proliferation play a major role in the development of atherosclerotic lesions, graft occlusion, and restenosis after angioplasty. Cell migration implies the digestion of the surrounding extracellular matrix. Cell-associated proteolysis has been extensively studied in neoplastic and inflammatory cells, but very little is known about the proteolytic properties of VSM. We have evaluated the ability of rat cultured VSM cells to solubilize [3H]amino acid-labeled extracellular matrices produced by bovine VSM. When plated at a density of 30,000 cells per well in 24 multiwell plates, VSM cells were able to solubilize 63.3±7.0% of the extracellular matrix after 10 days in culture. Extracellur matrix digestion occurred also when the cells were cultured in plasminogen-depleted serum but was higher in the presence of 10 μg/ml purified plasminogen (net percent digestion after the subtraction of the appropriate control, 8.6±3.0% versus 21.2±3.5% after 3 days in culture, p<0.005, respectively). The involvement of other enzymes in addition to plasmin is confirmed by the ability of VSM cells to degrade extracellular matrices from which the plasmin-sensitive component was removed with plasmin pretreatment. Rat VSM cells were able to solubilize 52.3±2.0 of this residual extracellular matrix-associated radioactivity after 6 days in culture versus 26.1±1.5% in the control dishes (p<0.01, n=5). Cell contact was required for extracellular matrix degradation: cell-conditioned medium did not have any effect on extracellular matrix digestion. Similarly, no extracellular matrix digestion was observed when the cells were cultured on porous membranes suspended at 1 mm above the extracellular matrices. These experiments demonstrate that VSM cells can potently digest naturally produced extracellular matrices and that both plasminogen-dependent and plasminogen-independent mechanisms requiring cell contact are involved. A better knowledge of the fibrinolytic/proteolytic properties of VSM cells might contribute to the design of new strategies to prevent VSM cell migration from the intima to the media in the vascular wall.