Heparan sulfate (HS) is a glycosaminoglycan (GAG) widely distributed as a proteoglycan on the cell surface and in the extracellular matrix of animal tissues. Among other important physiological functions, its polysaccharide chains mediate cell proliferation by binding growth factors [fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF)], which are released in active form through the action of the enzyme heparanase overexpressed by tumor cells. HS is constituted of alternating disaccharide sequences of variously sulfated uronic acid (d-glucuronic, GlcA, or l-iduronic, IdoA) and glucosamine (N-acetylated, GlcNAc, or N-sulfated, GlcNSO3). HS mimics can be obtained by chemical modification of heparin, a more highly sulfated GAG clinically used as an anticoagulant and antithrombotic drug. With the aim of obtaining antagonists of FGFs as potential inhibitors of tumor neo-vascularization (angiogenesis), arrays of short FGF-binding sequences have been obtained by generating sulfation gaps within the prevalent (IdoA2SO3–GlcNSO36SO3)n sequences of heparin, by controlled base-catalyzed removal of 2-O-sulfate groups of IdoA2SO3 residues.The C(2)–C(3) bond of all nonsulfated uronic acid residues have then been split with periodate, to generate flexible joints along the polysaccharide chain. The novel heparin derivative (poly-PST.sU), chiefly made up of the repeating tetrasaccharide units –GlcNSO36SO3– IdoA2SO3– GlcNSO36SO3– sU– (where sU is a glycol-split and reduced uronic acid residue) binds FGF2 as strongly as heparin. However, it is a poor inducer of formation of FGF2 dimers and of complexes with FGF receptors, required for triggering mitogenic signals. NMR and molecular modeling studies indicate that formation of these higher-order complexes is prevented by the unfavorable conformation induced by glycol-split residues. In a parallel study, partially N-acetylated heparins have been obtained that efficiently inhibit heparanase upon glycol-splitting. It is noteworthy that glycol-splitting involves inactivation of the active site for antithrombin, with consequent loss of anticoagulant activity. In contrast, poly-PST.sU and some of its analogs show potent antiangiogenic activity in in vivo models in which heparin is either proangiogenic or inactive.
The antiproliferative activity of arterial heparan sulfate resides in domains enriched with 2-O-sulfated uronic acid residues.