Evidence that carboxyl-reduced heparin fails to potentiate acidic fibroblast growth factor activity due to an inability to interact with cell surface heparin receptors.

Recently we reported that carboxyl-reduced heparin (CR-heparin), despite binding acidic fibroblast growth factor (aFGF) as effectively as native heparin, was much less potent at augmenting aFGF-induced mitogenesis. This paper describes experiments which examined this phenomenon in more detail in the hope that it would shed light on the mechanism by which heparin potentiates aFGF activity. Initial studies confirmed that heparin, with 60% of its carboxyl groups reduced, although binding aFGF with the same affinity as native heparin (Kd 35 +/- 5 nM), was a poor potentiator of aFGF-induced mitogenic activity. Proteolysis protection experiments also revealed that CR-heparin was as effective as native heparin at protecting aFGF from proteolytic degradation. In contrast, CR-heparin was considerably less effective than native heparin at enhancing the binding of aFGF to the fibroblast growth factor receptor (FGFR) on 3T3 cells. Furthermore, CR-heparin only bound to a subset (approximately 1/3) of heparin receptors on 3T3 cells. Based on these data, it is proposed that CR-heparin is less efficient than heparin at facilitating the formation of a quaternary complex among aFGF, the FGFR, and cell surface heparin receptors.