Basic fibroblast growth factor (bFGF) dissociates rapidly from heparan sulfates but slowly from receptors. Implications for mechanisms of bFGF release from pericellular matrix

The effect of heparin on the rate of binding of basic fibroblast growth factor (bFGF) to high affinity (receptor) and low affinity (heparan sulfate) binding sites on endothelial cells and CHO cells transfected with FGF receptor-1 or FGF receptor-2 was investigated. Radiolabeled bFGF bound rapidly to both high and low affinity sites on all three types of cells. Addition of 10 micrograms/ml heparin eliminated binding to low affinity sites and decreased the rate of binding to high affinity sites to about 30% of the rate observed in the absence of heparin. However, the same amount of 125I-bFGF bound to high affinity sites at equilibrium in the presence and absence of heparin. The effect of heparin on the initial rate of binding to high affinity sites was related to the log of the heparin concentration. Depletion of the cells of heparan sulfates by treatment with heparinase also decreased the initial rate of binding to high affinity receptors. These results suggest that cell-surface heparan sulfates facilitate the interaction of bFGF with its receptor by concentrating bFGF at the cell surface. Dissociation rates for receptor-bound and heparan sulfate-bound bFGF were also measured. Dissociation from low affinity sites was rapid, with a half-time of 6 min for endothelial cell heparan sulfates and 0.5 min for Chinese hamster ovary heparan sulfates. In contrast, dissociation from receptors was slow, with a half-time of 46 min for endothelial cell receptors, 2.5 h for FGF receptor-1, and 1.4 h for FGF receptor-2. These results suggest that degradative enzymes may not be needed to release bFGF from the heparan sulfates in instances where receptors and heparan sulfate-bound bFGF are in close proximity because dissociation from heparan sulfates occurs rapidly enough to allow bFGF to bind to unoccupied receptors by laws of mass action.