Heparan sulphate proteoglycans modulate fibroblast growth factor-2 binding through a lipid raft-mediated mechanism

We investigated how lipid raft association of HSPG (heparan sulphate proteoglycans) modulates FGF-2 (fibroblast growth factor-2/basic fibroblast growth factor) interactions with vascular smooth-muscle cells. When lipid rafts were disrupted with sterol-binding agents, methyl-alpha-cyclodextrin and filipin, FGF-2 binding to HSPG was reduced 2-5-fold, yet the amount and turnover of cell-surface HSPG were unaffected. Approx. 50-65% of bound FGF-2 was in lipid raft-associated fractions based on insolubility in unlabelled Triton V 100 and flotation in OptiPrep density gradients, and this level was increased with higher FGF-2 concentrations. Less FGF-2 (50-90%) was associated in raft fractions when cholesterol was depleted or HSPG were degraded with heparinase III. To investigate how lipid raft-HSPG interactions altered binding, we compared the rates of FGF-2 dissociation with native, MbetaCD (methyl-beta-cyclodextrin)- and filipin-treated cells. We found that FGF-2 dissociation rates were increased when lipid rafts were disrupted. These results suggest that localization of HSPG within lipid rafts creates high local concentrations of binding sites such that dissociation of FGF-2 is hindered. The localization of FGF-2 and HSPG to lipid rafts also correlated with the activation of protein kinase Calpha. Thus raft association of HSPG might create growth factor traps resulting in increased binding and signal transduction to enhance cell sensitivity.