Activation, processing and trafficking of extracellular heparanase

Endoglycosidic heparan-sulfate-degrading heparanase has important roles in a variety of biological processes, including angiogenesis, inflammation, wound healing and metastatsis (Dempsey et al., 2000a; Irimura et al., 1986; Parish et al., 2001; Vlodavsky et al., 1990; Vlodavsky et al., 1994). A predominant cDNA encoding heparanase was independently cloned by several groups, and its gene localized to chromosome 4q.21.3 (Dempsey et al., 2000b; Hulett et al., 1999; Toyoshima et al., 1999; Vlodavsky et al., 1999a). Expression of the full-length heparanase cDNA yields a 65 kDa latent protein that is proteolytically processed at the N-terminus to yield a highly active 50 kDa form of the enzyme (Fairbanks et al., 1999; Hulett et al., 1999; Kussie et al., 1999; Toyoshima et al., 1999; Vlodavsky et al., 1999a). The major heparanase substrate, heparan sulfate proteoglycans (HSPGs), resides within the extracellular milieu of a variety of tissues. HSPGs are important constituents of the extracellular microenvironment, particularly basement membranes (BM), where they bind and assemble extracellular matrix (ECM) constituents (e.g. laminin, collagen) and hence contribute to the assembly and stability of these supramolecular structures (Bernfield et al., 1999; Iozzo, 1998; Kjellen and Lindahl, 1991). HSPGs are also utilized to sequester and store growth promoting factors, cytokines and enzymes in the microenvironment of cells (Bernfield et al., 1999; Iozzo, 1998; Vlodavsky et al., 1993; Vlodavsky et al., 1996; Wight et al., 1992). Heparanase is utilized by hematopoietic cells and by blood-borne metastatic tumor cells to disintegrate the structure of the subendothelial BM, thereby facilitating their trafficking through blood vessel walls (Dempsey et al., 2000a; Irimura et al., 1986; Parish et al., 2001; Vlodavsky et al., 1990; Vlodavsky et al., 1992; Vlodavsky et al., 1994). Highly metastatic cells and human tumors express high levels of heparanase compared with the respective non-invasive cells and normal tissues (Friedmann et al., 2000; Hulett et al., 1999; McKenzie et al., 2000; Vlodavsky et al., 1999a). Low metastatic murine T-lymphoma and melanoma cells become highly metastatic following transfection of the heparanase cDNA and expression of the protein, demonstrating a cause and effect relationship between heparanase expression and metastasis (Vlodavsky et al., 1999a). The extracellular location of HSPGs requires heparanase to be accessible to the extracellular milieu. Indeed, sequence analysis of the heparanase protein reveals a hydrophobic amino acid stretch at the N-terminus, which might function as a signal peptide for its secretion (Hulett et al., 1999; Vlodavsky et al., 1999a). Secretion of heparanase was found in activated platelets, granulocytes and lymphocytes and in highly metastatic tumor cells (Parish et al., 2001; Vlodavsky et al., 1992; Vlodavsky et al., 1994). Accumulation of 2179