HEPARIN INCREASES THE AFFINITY OF BASIC FIBROBLAST GROWTH-FACTOR FOR ITS RECEPTOR BUT IS NOT REQUIRED FOR BINDING

The role of heparin or heparan sulfates in the interaction of basic fi broblast growth factor (bFGF) with its high affinity receptor were inv estigated using purified extracellular ligand-binding region of FGF re ceptor-1 (FGFR-1) and intact receptors expressed in a myeloid cell lin e (32D) that does not express detectable levels of heparan sulfate pro teoglycans or in Chinese hamster ovary (CHO) cell mutants defective in heparan sulfate synthesis. The purified extracellular domain of FGFR- 1 formed complexes with I-125-bFGF both in the presence or absence of heparin. Intact FGFR-1 expressed in 32D cells also bound the same amou nt of I-125-bFGF in the presence or absence of heparin when saturating concentrations of bFGF were used. Varying the concentration of I-125- bFGF showed that heparin increased the amount of I-125-bFGF bound at l ow bFGF concentrations and increased the affinity of bFGF for its rece ptor by about 3-fold. To eliminate the possibility of alteration of bF GF properties through the chemical modification reactions, bFGF was la beled biosynthetically. The binding of biosynthetically labeled bFGF t o FGFR-1 also did not require heparin. When FGFR-1 or FGFR-2 were expr essed in mutant CHO cells deficient in heparan sulfate synthesis, the cells also bound I-125-bFGF in the absence of heparin, and the additio n of heparin increased the affinity of bFGF for its receptors 2-3-fold . Thus, heparin or heparan sulfate is not required for the binding of bFGF to its receptors but increases the binding affinity to a moderate degree. Finally, the requirement for heparin in signal transduction t hrough the receptor was investigated. Expression of c-fos mRNA was ind uced by bFGF in 32D cells expressing FGFR-1 to the same extent in the presence or absence of heparin.