The integrity of the extracellular matrix (ECM) is essential for tissue and organs organization. Heparan sulfate proteoglycans (HSPG) display important cellular functions, being present at the cell surface and extracellular matrix (ECM). Heparanase is an endo - b -glucucuronidase that degrades HSPG, and therefore, can alter biological roles performed by HSPG through the disassembly of the ECM and release bioactive heparan sulfate fragments that can interact with angiogenic and growth factors activating their activities. It is known that heparanase can be overexpressed in tumors and in some inflammatory processes. In order to better investigate heparanase effects at cellular level, we analyzed heparanase expression at different breast cell lines by RT-PCR (non neoplasic and metastatic lines) and we observed that there was no direct correlation between heparanase expression and more neoplasic cell line. We also cloned and transfected heparanase cDNA into MCF-7 cells to evaluate the effect of heparanase overexpression regarding cell shape, cell adhesion and cell proliferation, as well as the synthesis of sulfated glycosaminoglycans (GAG). The data obtained for GAG synthesis had shown that MCF-7 cells that overexpressed heparanase presented a significant reduction in the HS present in cell extract and also secreted to the medium, when compared to MCF-7 wild type cells. Adhesion assays demonstrated that heparanase transfected MCF-7 cells adhere much more to fibronectin and laminin when compared to the wild type cell. In addition, it was observed a decrease in the proliferation rate of heparanase transfected cells compared with non transfected MCF-7 cells. It was also demonstrated by confocal microscopy that the green fluorescent heparanase that this enzyme was present inside the cells in acidic vesicles like endosomes and lisosomes. These studies demonstrate that overexpression of heparanase can alter cellular characteristics of breast cell lines in culture like proliferation, adhesion and GAG profile.