Human Heparanase Is Localized within Lysosomes in a Stable Form

Heparanase is an endo--D-glucuronidase involved in degradation of heparan sulfate (HS) and extracellu- lar matrix (ECM) of a wide range of cells of vertebrate and invertebrate tissues. The enzymatic activity of heparanase is characterized by specific intrachain cleavage of glycosidic bonds with a hydrolase mecha- nism. This enzyme facilitates cell invasion and hence plays a role in tumor metastasis, angiogenesis, inflam- mation, and autoimmunity. Although the expression pattern and molecular properties of heparanase have been characterized, its subcellular localization has not been unequivocally determined. We have previ- ously suggested that heparanase subcellular localiza- tion is a major determinant in regulating the enzyme's biological functions. In the present study we examined heparanase localization in three different cell types, utilizing immunofluorescent staining and electron mi- croscopy. Our results indicate that heparanase is lo- calized primarily within lysosomes and the Golgi ap- paratus. A construct composed of heparanase cDNA fused to green fluorescent protein, utilized in order to visualize the enzyme within living cells, confirmed its localization in acidic vesicles. We suggest that follow- ing synthesis, heparanase is transported into the Golgi apparatus and subsequently accumulates in a stable form within the lysosomes, where it functions in HS turnover. The lysosomal compartment may also serve as a site for heparanase confinement within the cells, limiting its secretion and uncontrolled extracellular activities associated with tumor metastasis and angio- genesis. © 2002 Elsevier Science (USA)