Potentiation and inhibition of bFGF binding by heparin: A model for regulation of cellular response

Basic fibroblast growth factor (bFGF) binds to cell surface tyrosine kinase receptor proteins and to heparan sulfate proteoglycans. The interaction of bFGF with heparan sulfate on the cell surface has been demonstrated to imp act receptor binding and biological activity. bFGF receptor binding affinit y is reduced on cells that do not express heparan sulfate. The addition of soluble heparin or heparan sulfate has been demonstrated to rescue the bFGF receptor binding affinity on heparan sulfate deficient cells yet has also been shown to inhibit binding under some conditions. While the chemical req uirements of the heparin-bFGF-receptor interactions have been studied in de tail, the possibility that heparin enhances bFGF binding in part by physica lly associating with the cell surface has not been fully evaluated. In the study presented here, we have investigated the possibility that heparin bin ding to the cell surface might play a role in modulating bFGF receptor bind ing and activity. Balb/c3T3 cells were treated with various concentrations of sodium chlorate, so as to express a range of endogenous heparan sulfate sites, and [I-125]bFGF binding was assessed in the presence of a range of h eparin concentrations. Low concentrations of heparin (0.1-30 nM) enhanced b FGF receptor binding to an extent that was inversely proportional to the am ount of endogenous heparan sulfate sites present. At high concentrations (1 0 mu M), heparin inhibited bFGF receptor binding in cells under all conditi ons. The ability of heparin to stimulate and inhibit bFGF-receptor binding correlated with altered bFGF-stimulated tyrosine kinase activity and cell p roliferation. Under control and chlorate-treated conditions, [I-125]heparin was observed to bind with a high affinity to a large number of binding sit es on the cells (K-d = 57 and 50 nM with 3.5 x 10(6) and 3.6 x 10(6) sites/ cell for control and chlorate-treated cells, respectively). A mathematical model of this process revealed that the dual functions of heparin in bFGF b inding were accurately represented by heparin cell binding-mediated stimula tion and soluble heparin-mediated inhibition of bFGF receptor binding.