DIFFERENTIAL EFFECT OF CELL-ASSOCIATED HEPARAN SULFATES ON THE BINDING OF KERATINOCYTE GROWTH-FACTOR (KGF) AND ACIDIC FIBROBLAST GROWTH-FACTOR TO THE KGF RECEPTOR

The fibroblast growth factors (FGFs) act through high affinity tyrosin e kinase receptors and, in addition, interact with lower affinity rece ptors that represent cellor matrix-associated heparan sulfate proteogl ycans. These lower affinity receptors modulate the biological activiti es of FGFs, but the mechanism by which they exert these effects is rat her controversial. We have previously shown (Ron, D., Bottaro, D. P., Finch, P. W., Morris, D., Rubin, J. S., and Aaronson, S. A. (1993) J. Biol. Chem, 268, 2984-2988) that heparin potentiates the mitogenic act ivity of acidic FGF (aFGF) but inhibits that of the keratinocyte growt h factor (HGF) in cells that express the KGF receptor (KGFR). Both gro wth factors bind the KGFR with high affinity, To gain an insight into the mechanism by which heparin modulates the biological activity of aF GF and KGF, we studied the effect of heparin and cell-associated hepar an sulfates on the binding of these two growth factors to the KGFR, To work in a well defined system, we expressed functional KGFR in L6E9 m yoblasts that lack detectable high affinity binding sites for FGFs, Lo w concentrations of heparin inhibited the binding of KGF to the KGFR. By contrast, similar concentrations of heparin enhanced the binding of aFGF to this receptor, The effect of heparin was not unique to L6E9 c ells expressing the KGFR; it was also observed in Balb/MK cells that n aturally express KGFR. Treatment of cells with sodium chlorate, which blocks sulfation of proteoglycans, reduced the binding of aFGF to its low and high affinity binding sites by 95 and 80%, respectively. In co ntrast, the binding of KGF to its high affinity binding sites was enha nced about S-fold, Similar results were obtained after degradation of cell-associated heparan sulfates by heparinase and heparitinase. Hepar in restored the high affinity binding of aFGF to chlorate-treated cell s and completely abolished the high affinity binding of KGF, Binding c ompetition experiments suggest that aFGF and KGF bind to the same popu lation of cell-associated heparan sulfates, In addition, KGF is appare ntly interacting with an as yet unidentified type of low affinity bind ing site that is not affected by chlorate or heparan sulfate-degrading enzymes. An important property of the FGF high affinity receptors is their ability to bind more than one ligand with high affinity, Based o n the differential effect of cell-associated heparan sulfates on the b inding of KGF and aFGF to the KGFR, we propose a regulatory role for c ell-associated heparan sulfates as coordinators of the interaction of aFGF and KGF with the KGFR, Such a regulatory mechanism may be importa nt for the coordination of cellular responses to KGF and aFGF.