Release of cell surface‐associated basic fibroblast growth factor by glycosylphosphatidylinositol‐specific phospholipase C

Abstract

Heparan sulfate proteoglycans (HSPG) are ubiquitous constituents of mammalian cell surfaces and most extracellular matrices. A portion of the cell surface HSPG is anchored via a covalently linked glycosyl-phosphatidylinositol (PI) residue, which can be released by treatment with a glycosyl-PI specific phospholipase C (PI-PLC). We report that exposure of bovine aortic endothelial and smooth muscle cells to PI-PLC resulted in release of cell surface-associated, growth-promoting activity that was neutralized by antibasic fibroblast growth factor (bFGF) antibodies. Active bFGF was also released by treating the cells with bacterial heparitinase. Under the same conditions there was no release of mitogenic activity from cells (BHK-21, NIH/3T3, PF-HR9) that expressed little or no bFGF, as opposed to PI-PLC-mediated release of active bFGF from the same cells transfected with the bFGF gene. The released bFGF competed with recombinant bFGF in a radioreceptor assay. Addition of PI-PLC to sparsely seeded vascular endothelial cells resulted in a marked stimulation of cell proliferation, but there was no mitogenic effect of PI-PLC on 3T3 fibroblasts. Studies with exogenously added ¹²⁵I-bFGF revealed that about 6.5% and 20% of the cell surface-bound bFGF were released by treatment with PI-PLC and heparitinase, respectively. Both enzymes also released sulfate-labeled heparan sulfate from metabolically labeled 3T3 fibroblasts. PI-PLC failed to release ¹²⁵I-bFGF from the subendothelial extracellular matrix (ECM), as compared to release of 60% of the ECM-bound bFGF by heparitinase. Our results indicate that 3–8% of the total cellular content of bFGF is associated with glycosyl-PI anchored cell surface HSPG. This FGF may exert both autocrine and paracrine effects, provided that it is released by PI-PLC and adequately presented to high affinity bFGF cell surface receptor sites.