STIMULATION OF FIBROBLAST GROWTH-FACTOR RECEPTOR-1 OCCUPANCY AND SIGNALING BY CELL SURFACE-ASSOCIATED SYNDECANS AND GLYPICAN

The formation of distinctive basic FGF-heparan sulfate complexes is es sential for the binding of bFGF to its cognate receptor, In previous e xperiments, cell-surface heparan sulfate proteoglycans extracted from human lung fibroblasts could not be shown to promote high affinity bin ding of bFGF when added to heparan sulfate-deficient cells that expres s FGF receptor-1 (FGFR1) (Aviezer, D., D. Hecht, M. Safran, M. Eisinge r, G. David, and A. Yayon. 1994. Cell. 79:1005-1013). In alternative t ests to establish whether cell-surface proteoglycans can support the f ormation of the required complexes, K562 cells were first transfected with the IIIc splice variant of FGFR1 and then transfected with constr ucts coding for either syndecan-1, syndecan-2, syndecan-4 or glypican, or with an antisense syndecan-4 construct. Cells cotransfected with r eceptor and proteoglycan showed a two- to three-fold increase in neutr al salt-resistant specific I-125-bFGF binding in comparison to cells t ransfected with only receptor or cells cotransfected with receptor and anti-syndecan-4. Exogenous heparin enhanced the specific binding and affinity cross-linking of I-125-bFGF to FGFR1 in receptor transfectant s that were not cotransfected with proteoglycan, but had no effect on this binding and decreased the yield of bFGFR cross-links in cells tha t were cotransfected with proteoglycan, Receptor-transfectant cells sh owed a decrease in glycophorin A expression when exposed to bFGF, This suppression was dose-dependent and obtained at significantly lower co ncentrations of bFGF in proteoglycan-cotransfected cells. Finally, com plementary cell-free binding assays indicated that the affinity of I-1 25-bFGF for an immobilized FGFR1 ectodomain was increased threefold wh en the syndecan-4 ectodomain was coimmobilized with receptor. Equimola r amounts of soluble syndecan-4 ectodomain, in contrast, had no effect on this binding, We conclude that, at least in K562 cells, syndecans and glypican can support bFGF-FGFR1 interactions and signaling, and th at cell-surface association may augment their effectiveness.