Inhibition of proteoglycan synthesis alters extracellular matrix deposition, proliferation, and cytoskeletal organization of rat aortic smooth muscle cells in culture

Arterial proteoglycans have been implicated in several important physiological processes ranging from lipid metabolism to regulation of smooth muscle cell growth. Vascular smooth muscle (VSM) cells are the major producers of proteoglycans in the medial layer of blood vessels. To study functional consequences of alterations in VSM proteoglycan metabolism we used 4-methylumbelliferyl-beta-D-xyloside to inhibit proteoglycan synthesis in primary and early passage cultures of rat aortic smooth muscle cells. Biochemical analysis of cultures labeled with 35SO4 showed the drug inhibited synthesis of different classes of proteoglycans by 50 to 62%. Inhibition of proteoglycan synthesis resulted in reduced accumulation of extracellular matrix, as shown by immunofluorescent staining with antibodies to chondroitin sulfate, fibronectin, thrombospondin, and laminin. There was also an inhibition of postconfluent (multilayered) growth of the smooth muscle cells, and a change in the morphology of the cells, with no apparent effect on subconfluent growth. In addition, in drug-treated cells there was a reduction in the number of cytoskeletal filaments that contained alpha-actin, the actin subtype synthesized by differentiated VSM cells. This occurred even though the total content of alpha-actin in the cells was not reduced. The effects of the inhibitor on growth and morphology could be reversed by switching the cultures to normal medium and could be prevented by growing the cells on preformed VSM extracellular matrix. These observations suggest the vascular extracellular matrix may play a role in regulating the growth and differentiation of smooth muscle cells.