Heparan sulfate proteoglycans

Studies of the involvement of ECM molecules in cell attachment, growth, and differentiation have revealed a central role of heparan sulfate (HS) proteoglycans (HSPGs) in early embryogenesis, morphogenesis, angiogenesis, and epithelial-mesenchymal interactions (1–3). HS chains bind a multitude of proteins and ensure that a wide variety of bioactive molecules (e.g., heparin-binding growth factors, chemokines, lipoproteins, and enzymes) cling to the cell surface and ECM. HSPGs can thus influence a variety of normal and pathologic processes, among which are tissue repair, neurite outgrowth, inflammation and autoimmunity, tumor growth and metastasis, vasculogenesis and angiogenesis (1–4). Binding to HS can modulate a tethered molecule’s biological activity or protect it from proteolytic cleavage and inactivation. Transmembrane and phospholipid-anchored HSPGs (syndecans and glypicans, respectively) mediate cell interactions with components of the microenvironment that control cell shape, adhesion, proliferation, survival, and differentiation (2, 3). These species of HSPGs can also serve as coreceptors along with the other cell surface molecules to form functional receptor complexes that transduce signals from various ligands (2, 3). Because of the important and multifaceted roles of HSPGs in cell physiology, their cleavage is likely to alter the integrity and functional state of tissues and to provide a mechanism by which cells can respond rapidly to changes in the extracellular environment. Enzymatic degradation of HS is, therefore, likely to be involved in fundamental biological phenomena, ranging from pregnancy, morphogenesis, and development to inflammation, angiogenesis, and cancer metastasis. Contrary to some early claims, there is no good evidence for more than one endogenous mammalian HS-degrading endoglycosidases. The recent cloning of a single gene by several groups (5–10), together with biochemical studies (11), suggests that mammalian cells express primarily a single dominant heparanase enzyme (12). This Perspective focuses on the molecular properties, expression pattern, biological functions, and clinical significance of this heparanase in normal and pathological processes, with emphasis on tumor metastasis and angiogenesis. Molecular and biochemical properties of heparanase HS degradation by mammalian endoglycosidic enzymes was first described in human placenta and rat liver hepatocytes. Since then, heparanase activity has been identified in a variety of normal and malignant cells and tissues, among which are cytotrophoblasts, endothelial cells (ECs), platelets, mast cells, neutrophils, macrophages, T and B lymphocytes, and lymphoma, melanoma, and carcinoma cells (5, 6, 12–16). Heparanase cleaves the glycosidic bond with a hydrolase mechanism and is thus distinct from bacterial heparinases, which depolymerize heparin and HS by eliminative cleavage. HS glycosaminoglycan chains are cleaved by heparanase at only a few sites, yielding HS fragments of appreciable size (10–20 sugar units), suggesting that the enzyme recognizes a particular and relatively rare HS structure (17). Heparanase purified from human platelets and hepatoma requires the presence of O-sulfation, with no essential requirement for N-sulfation or IdoA residues (17). The heparin-derived octasaccharide, which binds antithrombin III is cleaved at a single site (Figure 1, top, arrow), indicating that a 2-O-sulfate group on a hexuronic acid residue located two monosaccharides away from the cleavage site is essential for substrate recognition by heparanase (17) (Figure 1). In other studies, however, the presence of either Nor O-sulfates did not appear to be an absolute requirement for substrate cleavage (12, 18). The cloning of a single human heparanase cDNA sequence and its expression in mammalian cells was independently reported by several groups (5–10). The heparanase cDNA contains an open reading frame of 1629 bp encoding a 61.2-kDa polypeptide of 543 amino acids. The mature active 50 kDa enzyme, isolated from cells and tissues, has its N-terminus 157 amino acids downstream from the initiation codon (5–12), suggesting post-translational processing of the heparanase polypeptide at an unusual cleavage site (Gln157-Lys158). Processing and activation occur during incubation of the full-length 65-kDa recombinant enzyme with several normal and transformed cells and, Molecular properties and involvement of heparanase in cancer metastasis and angiogenesis