aFGF binding to low and high affinity receptors induces both aFGF and aFGF receptors dimerization

Acidic Fibroblast Growth Factor (aFGF) binds on two classes of fibroblast growth factor receptors, the high affinity receptors (HAR) a family of four known transmembrane tyrosine kinases and the low affinity receptors (LAR), related to cell surface heparan sulfate proteoglycan (HSPG). We analysed the relationship between the binding of aFGF on the HAR and on the LAR in bovine lens epithelial (BEL) cells in the presence of heparin or suramin. Through Northern blotting analysis we demonstrated that the three immunoglobulin-like transcript of FGF receptor type 1 (FGF-R1) is the major expressed high affinity receptor in BEL cells. On the contrary, HAR-aFGF complexes are present in two forms (150 kDa and 135 kDa) revealed by cross-linking experiments with 125I aFGF. Moreover 125I aFGF binding to BEL cell surface induces the spontaneous formation of a 125I aFGF dimer (31 kDa) which is then internalized and degraded in the cells as the 15.5 kDa aFGF native form is. It has been observed that heparin at 10 micrograms/ml (1) in cross-linking experiments, reduces by half the total number of HAR complexes by preventing the formation of the 150 kDa complex but does not affect the 135 kDa complex, (2) in binding experiments, suppress the spontaneous formation of the 125I aFGF dimer bound to LAR, and then its internalization and degradation in the cells. Moreover, we demonstrate that (1) only HAR contributes specifically and directly to the aFGF internalization process, (2) HAR internalization is ligand concentration and time saturable, (3) there is no desensitization of aFGF internalization induced by ligand binding to HAR, (4) a FGF dimerization process is highly dependent on the apparent affinity of FGF for heparin, since aFGF mutant with a reduced affinity for heparin does not promote the dimerization. These data strongly suggest that a heteroreceptor-aFGF complex (150 kDa) is formed by one molecule of HAR (FGF-R1) associated to one molecule of LAR through their respective interactions with a very stable aFGF homodimer. Such a three component receptor induced by FGF dimerization may be a process involved in the mechanism of action of FGFs which could explain the diversity of the biological response of FGF depending on the presence of the HSPG on the extra cellular matrix. In addition prebinding of unlabelled aFGF to the cells induces a 4 fold increase in the affinity of HAR to 125IaFGF concomitant with its down regulation by 80% and initiates the formation of the HAR homodimer.(ABSTRACT TRUNCATED AT 400 WORDS)