Human heparanase: A molecular determinant of brain metastasis

Mechanisms responsible for the progression of malignant melanoma to highly aggressive brain-metastatic disease remain largely unknown. Brain neurotrophins (NT) can modulate brain invasion of melanoma cells and the activity of an ECM degradative enzyme, heparanase. This enzyme is an endo-β-D-glucuronidase that degrades the heparan sulfate chains of HSPG, essential and ubiquitous macromolecules associated with the cell surface and ECM of a wide range of cells and tissues. Human heparanase has been recently cloned as a single gene family, and it has been found that it is a potential target for antimetastasis drugs because of its critical role in angiogenic and invasive processes. Few studies have focused on the contributions of astrocytes in the brain-metastatic specificity of melanoma cells. To test the hypotheses that astrocytes may play an important role in the development of brain metastases and that heparanase is a critical mediator of brain-metastatic processes, we employed purified astrocyte cultures as in vitro models to stimulate heparanase and invasion. We found that astrocytes express both heparanase transcripts and functional enzyme that was upregulated by the prototypic NT, nerve growth factor (NGF). Coincubation of astrocytes (or their conditioned medium) with brain-metastatic cells resulted in both a super-additive effect on heparanase activity and an up to an eight-fold increase of in vitro chemoinvasion using purified HSPG as a substrate. Transfection of the heparanase gene into nonmetastatic melanoma cells resulted not only in augmented functional enzymatic activity but also significantly increased (7 to almost 14-fold) in vitro invasion by transfected cells. Additionally, heparanase mRNA was preferentially expressed in invasive and metastatic human melanoma tissue specimens. These observations indicate a role of heparanase in general invasion and metastatic mechanisms and demonstrate that astrocytic interactions with melanoma cells may contribute to brain invasion and colonization of melanoma cells. They support the concept that melanoma brain invasion results from establishing reciprocal circuits between the tumor cells and the normal glial cells present in the brain microenvironment.