IMPORTANCE OF SIZE AND SULFATION OF HEPARIN IN RELEASE OF BASIC FIBROBLAST GROWTH-FACTOR FROM THE VASCULAR ENDOTHELIUM AND EXTRACELLULAR-MATRIX

We have characterized the importance of size, sulfation, and anticoagulant activity of heparin in release of basic fibroblast growth factor (bFGF) from the subendothelial extracellular matrix (ECM) and the luminal surface of the vascular endothelium. For this purpose, I-125-bFGF was first incubated with ECM and confluent endothelial cell cultures, or administered as a bolus into the blood of rats. the immobilized I-125-bFGF was then subjected to release by various chemically modified species of heparin and size-homogeneous oligosaccharides derived from depolymerized heparin. Both totally desulfated and N-desulfated heparin failed to release the ECM-bound bFGF. Likewise, substitution of N-sulfate groups of heparin and low molecular weight heparin (fragmin) by acetyl or hexanoyl residues resulted in an almost complete inhibition of bFGF release by these polysaccharides. The presence of O-sulfate groups in heparin increased but was not critical for release of ECM-bound bFGF. Similar structural requirements were identified for release of I-125-bFGF bound to low-affinity sites on the surface of vascular endothelial cells. Oligosaccharides derived from depolymerized heparin and containing as little as 8-10 sugar units were, on a weight basis, equivalent to whole heparin in their ability to release bFGF from ECM. Low-sulfate oligosaccharides were less effective releasers of bFGF as compared to medium- and high-sulfate fractions of the same size oligosaccharides. Heparin fractions with high and low affinity to antithrombin III exhibited a similar high bFGF-releasing activity despite a 200-fold difference in their anticoagulant activities. Studies on the fate of intravenously administered I-125-bFGF demonstrated that bFGF sequestered by the luminal surface of the vascular endothelium is accessible to release by heparin species in a manner similar to that observed for release of ECM-bound bFGF. Analysis of the distribution of I-125-bFGF in various organs revealed the presence of intact I-125-bFGF primarily in the rat liver and kidneys. Structural requirements for release of ECM-bound and cell-surface-bound bFGF by heparin and heparin-like molecules were different from those identified for inhibition of heparanase activity and lung colonization of B16 melanoma cells by these heparin molecules. These results indicate that various non-anticoagulant species of heparin having different size, sulfation, and substituted groups can be designed to elicit specific effects such as release of bFGF and inhibition of heparanase and hence for induction of neovascularization and inhibition of tumor metastasis, respectively.