Heparin inhibits the binding of basic fibroblast growth factor to cultured human aortic smooth-muscle cells

Basic fibroblast growth factor (bFGF) and its specific receptors have diverse roles on a variety of cell types, such as the induction of vascular smooth-muscle cell proliferation which contributes to restenosis after coronary balloon angioplasty. bFGF is also known to interact with heparan sulphate proteoglycans present on the cell surface or in the extracellular matrix. In this study, the binding of ¹²⁵I-bFGF to human aortic smooth-muscle cells was investigated. ¹²⁵I-bFGF binding to these cells was reversible and saturable. Scatchard analysis revealed the presence of two distinct binding sites: a high-affinity receptor (K_d = 38±7 pM; 1480±220 sites/cell) and a low-affinity non-saturable binding site (K_d= 8.0±2.0 nM). Pretreatment of the cells with heparinase resulted in a large reduction of ¹²⁵I-bFGF binding to its low-affinity receptors, suggesting that they are heparin-like molecules. The specificity of the low- and high-affinity binding sites for bFGF was determined with acidic FGF, platelet-derived growth factor-BB and epidermal growth factor, which did not compete for ¹²⁵I-bFGF binding. Expression of FGF receptor isoforms analysed by reverse transcriptase-PCR revealed the presence of only the type-1 receptor. Binding to low-affinity binding sites was antagonized by heparin, suramin, protamine sulphate and platelet factor 4. Unexpectedly, these molecules also reduced the binding of ¹²⁵I-bFGF to its high-affinity sites. Consistent with these results, heparin, suramin, protamine sulphate and platelet factor 4 inhibited bFGF-induced proliferation of human aortic smooth-muscle cells. Heparin abrogated bFGF-induced release of tissue-type plasminogen activator by these cells. These observations suggest that the interaction of bFGF with human aortic smooth-muscle cells is different from that described for other cells such as endothelial cells, in which heparin acts as a potentiating factor of the mitogenic activity of bFGF.