Heparan sulfate primed on β‐D‐xylosides restores binding of basic fibroblast growth factor

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Abstract
Heparan sulfate proteoglycans (HSPG) are obligatory for receptor binding and mitogenic activity of basic fibroblast growth factor (bFGF). Mutant Chinese hamster ovary cells (pgsA‐745) deficient in xylosyltransferase are unable to initiate glycosaminoglycan synthesis and hence can not bind bFGF to low‐ and high‐affinity cell surface receptors. Exposure of pgsA‐745 cells to β‐D‐xylopyranosides containing hydrophobic aglycones resulted in restoration of bFGF binding in a manner similar to that induced by soluble heparin or by heparan sulfate (HS) normally associated with cell sulfate. Restoration of bFGF binding correlated with the ability of the β‐D‐xylosides to prime the synthesis of heparan sulfate. Thus, both heparan sulfate synthesis and bFGF receptor binding were induced by low concentrations (10–30 μM) of estradiol‐β‐D‐xyloside and naphthyl‐β‐D‐xyloside, but not by cis/trans‐decahydro‐2‐naphthyl‐β‐D‐xyloside, which at low concentration primes mainly chondroitin sulfate. The obligatory involvement of xyloside‐primed heparan sulfate in restoration of bFGF‐receptor binding was also demonstrated by its sensitivity to heparinase treatment and by the lack of restoration activity in CHO cell mutants that lack enzymatic activities required to form the repeating disaccharide unit characteristic of heparan sulfate. Xyloside‐primed heparan sulfate binds to the cell surface. Restoration of bFGF receptor binding was induced by both soluble and cell bound xyloside‐primed heparan sulfate and was abolished in cells that were exposed to 0.5–1.0 M NaCl prior to the bFGF binding reaction. These results indicate that heparan sulfate chains produced on xyloside primers behave like heparan sulfate chains attached to cellular core proteins in terms of affinity for bFGF and ability to function as low‐affinity sites in a dual receptor mechanism characteristic of bFGF and other heparin‐binding growth promoting factors.