Tumor invasiveness is an intrinsic feature of most glial tumors that accounts for their malignant and locally destructive nature. We evaluated the subcutaneous (sc) tumorigenicity and in vivo invasiveness of 9 astrocytoma cell lines together with their respective metalloprotease activity in order to establish their biologic behavior and malignant potential. Invasiveness was assessed with an in vivo invasion assay using tracheal xenotransplants subcutaneously implanted into Scid mice. This assay permitted us to evaluate the penetration of tumor cells into the transplanted deepithelialized tracheas previously inoculated with either normal primary glial cells or with astrocytoma-derived cell lines. Although only 2 cell lines were tumorigenic after sc inoculation, 5 out of 9 tumor cell lines were tumorigenic in the tracheal graft system. The astrocytoma cell lines showed varying levels of penetration into the tracheal wall. The tumor lines GOS3, M059K, CCFSTTG1 and A172, as well as primary normal astrocytes, were nontumorigenic and noninvasive in this experimental model. LN405, SW1088 and SW1783 cells that were not tumorigenic as sc xenotransplants, on the other hand, grew well in the tracheal graft system showing low levels of in vivo invasiveness. U87MG and U118MG cells were tumorigenic as sc xenotransplants and showed high levels of invasiveness. In parallel to these in vivo studies, the constitutive levels of secreted gelatinases and stromelysins (MMP-3 and MMP-11) were investigated using conditioned media submitted to gelatin or casein-substrate zymography and Western blot analysis. Neither the gelatinases (MMP-2 and MMP-9) nor MMP-11 showed a direct correlation with the levels of in vivo tumor cell invasiveness. Conversely, secretion of MMP-3 correlated closely with tumorigenicity and invasiveness. In vitro tumor cell invasiveness was significantly reduced after incubation with the metalloproteinase inhibitor GM6001. This positive correlation between MMP-3 and the depth of tracheal wall penetration led us to conclude that the invasive properties of brain tumor cells may be due to the direct or indirect proteolytic effects of MMP-3 on extracellular matrix (ECM) macromolecules and that this enzyme might be a potential target for future therapies.
Copyright 2003 Wiley-Liss, Inc.