Is the sensitivity of cells for FGF-1 and FGF-2 regulated by cell surface heparan sulfate proteoglycans?

We have examined the importance of cell surface heparan sulfate proteoglycans (HSPG) in fibroblast growth factor (FGF) signaling. 3T3 cells grown under conventional conditions were much more sensitive to FGF-2 compared to FGF-1. However, cells were equally sensitive to FGF-1 and FGF-2 using conditions which reduced the effect of endogenous HSPG. Addition of heparin, or treatment with chlorate, an inhibitor of proteoglycan sulfation, resulted in enhanced or reduced growth factor response, respectively, and eliminated the differences between FGF-1 and FGF-2. HSPGs isolated from trypsin digests of 3T3 cells had a much higher affinity for FGF-2 compared to that for FGF-1 when analyzed by affinity chromatography. Glycosaminoglycan chains or core protein fragments derived from the HSPG failed to show the same high apparent affinity for FGF-2, suggesting that an intact proteoglycan structure was important for the high FGF-2 affinity. Addition of HSPG ectodomains, isolated from cultured 3T3 cells or produced as recombinant molecules, to chlorate-treated cultures of 3T3 cells inhibited the mitogenic activity of FGF-2 and eliminated the effect of heparin as a potentiator of either growth factor. These results support the idea that the cell-associated HSPG is an integral component of the FGF signaling system and in 3T3 cells contributes to the increased sensitivity of these cells to FGF-2 compared to FGF-1. Since isolated ectodomains of HSPG inhibited rather than stimulated the mitogenic response of the FGFs, the proper anchoring of the HSPG in the cell membrane appears to be important for a stimulatory effect.