Membrane heparan sulfate proteoglycan-supported FGF2-FGFR1 signaling - Evidence in support of the "cooperative end structures" model

Fibroblast growth factor 2 (FGF2)-initiated FGF receptor (FGFR)-signaling r equires the assistance of heparin/heparan sulfate. Here, we evaluated the e ffects of different heparan sulfate proteoglycan (HSPG)-expressing cell lin es and HSPGs derived from these cells on FGF2-induced FGFR1-phosphorylation in heparan sulfate-negative BaF3 cells. HSPGs supplied in membrane-associa ted form, by presenting cells, were all effective promotors of FGF2-initiat ed FGFR1 phosphorylation, independently of their nature (syndecan/glypican) or cellular origin (human lung fibroblasts, transfected Namalwa cells, or transfected K562 cells). A treatment with heparitinase initially stimulated , but finally completely inhibited, the activity of these presenting cells. In comparison, equivalent amounts of soluble HSPGs, obtained by trypsiniza tion of these cells or by immunopurification from cell extracts, did not pr omote FGF2-induced FGFR1-phosphorylation, yet removal of the less anionic s pecies or a further treatment with heparitinase converted these soluble fra ctions into potent activators of FGF2/FGFR1 signaling. Extrapolating from c urrent structural models, we suggest that FGFR dimerization and autophospho rylation is supported by cooperative "heparin-like end structures," and tha t cell surface association and concentration compensate for the relative sc arcity of such end structures in native HSPGs. In this model, "proteolytic" shedding of heparan sulfate would act as a diluting, down-regulatory mecha nism, while "heparanolytic" shedding might act as an up-regulatory mechanis m, by increasing the concentration of these end structures.