Endothelial heparan sulfate proteoglycan. I. Inhibitory effects on smooth muscle cell proliferation

Proliferation of smooth muscle cells is an important component of pulmonary arterial morphogenesis, both during normal development and pathologic remodeling. However, little is known of the factors that regulate smooth muscle proliferation in these vessels. To investigate the hypothesis that factors produced by endothelial cells may regulate smooth muscle cell growth, we studied the effects of culture medium conditioned by fetal bovine pulmonary arterial endothelium on proliferation of smooth muscle cells in culture. This conditioned medium contains an inhibitor of smooth muscle proliferation that is degraded by nitrous acid, heparinase, and heparitinase, but resists degradation by protease, boiling, and chondroitin ABC lyase, indicating that the inhibitor is structurally similar to heparin. Inhibitor release occurs in both growing and confluent endothelial cell cultures and in the presence and absence of serum. A growth-inhibiting proteoglycan purified to homogeneity from endothelial cell-conditioned medium has physicochemical characteristics similar to those of the prototypic basement membrane heparan sulfate proteoglycan of the Englebreth-Holm-Swarm tumor: an overall size of approximately 10(6) D, heparan sulfate chains of 60,000 D, and a buoyant density of 1.33 g/ml. Antibody raised against the tumor basement proteoglycan recognizes this endothelial heparan sulfate proteoglycan, and Western blotting after SDS-PAGE demonstrates that the core proteins of both proteoglycans migrate as a doublet at apparent molecular weights of 450,000 and 360,000 D. Heparan sulfate glycosaminoglycan prepared from purified medium proteoglycan is a potent inhibitor of smooth muscle cell growth, exhibiting activity approximately 1,000 times greater than that of heparin. These results indicate that endothelial cells cultured from fetal bovine pulmonary arteries produce a basement membrane heparan sulfate proteoglycan that is a potent inhibitor of smooth muscle proliferation. This proteoglycan may mediate endothelial regulation of smooth muscle growth during development or pathologic pulmonary arterial remodeling.