Differential structural requirements of heparin and heparan sulfate proteoglycans that promote binding of basic fibroblast growth factor to its receptor.

Abstract

Heparan sulfate proteoglycans (HSPG) are obligatory for receptor binding and mitogenic activity of basic fibroblast growth factor (bFGF). The capacity of various species of heparin and heparan sulfate (HS) to promote bFGF receptor binding was investigated using both Chinese hamster ovary mutant cells deficient in cell surface HSPG and a soluble bFGF receptor-alkaline phosphatase fusion protein. Highly sulfated oligosaccharides were more effective than medium and low sulfate fractions of the same size oligosaccharide. O-Sulfation in heparin was found to be critical for its capacity to promote binding of bFGF to its receptors. The highest level of bFGF-receptor binding was achieved in the presence of over-sulfated heparin fragments (% sulfur > 14) regardless of whether the N-position was sulfated or acetylated. Unlike receptor binding of bFGF which requires oligosaccharides containing at least 8-10 sugar units, displacement of heparin- or HS-bound bFGF was obtained by oligosaccharides containing as little as four sugar units and by an N-sulfated, O-desulfated heparin fragment (% sulfur = 5.3). A preparation of total cell surface-derived HS induced bFGF receptor binding. A preliminary survey of several defined and affinity purified species of cell surface HSPG, including syndecan, fibroglycan, and glypican failed to identify natural HSPG that promote high affinity receptor binding of bFGF. A similar lack of activity was observed with species of HS isolated from bovine arterial tissue and characterized for their effect on vascular smooth muscle cell proliferation. Moreover, most of these species of HS inhibited in a dose-dependent manner the restoration of bFGF-receptor binding induced by heparin or by total HSPG. These results suggest the involvement of defined heparin-like oligosaccharide sequences and unique species of cell surface and extracellular matrix HS in the regulation of bFGF receptor binding and biological activity.