Role of Glypican-6 and NG2 as metastasis promoting factors

Abstract

Cell surface proteoglycans (PGs) are key molecules in the regulation of tumour progression and metastasis formation. Eleven primary surface PGs, including syndecans-1-4, glypicans-1-6 and NG2, are currently known to act as mediators of the cancer cell’s interaction with the host microenvironment, with the potential to function synergically or antagonistically in the promotion of tumour growth and spreading. Using soft-tissue sarcomas as a model system we have observed that a given tumour cell may constitutively and coincidently express on average 3-5 of the 11 surface PGs, suggesting that diverse combinations of surface PGs may dictate the behaviour of cancer cells in different manners. To start to investigate the PG surface profiles as pro- and anti-tumorigenic we delineated strategies to modify the PG expression of 143B osteosarcoma cells by stable gene transduction and by examining how these modifications affected the cells adhesive and migratory capabilities in response to selected ECM substrates, and endothelial monolayers. To date there are no notice about GPC6 implications either in cell-ECM interactions or in the behaviour motility of cells. For these reasons 143B cells were stable transfected for overexpressing GPC6 showed a modulation of relative expression of all surface PGs respect to vector control cells. GPC6 overexpression induces morphological modification in 143B cells which are seen as changes in the organization of actin filaments containing protrusions similar to fillopodia/lamellipodia and an increase of motility cells in monodimensional assay which could be in relationship with cytoskelatal reorganization. Moreover these cells showed a spreading ability and the lower invasion ability on different ECM molecules that could be correlated by surface PG profile modulation. Since NG2 as a cell surface ligand for collagen type VI has been postulated to be involved in tumor progression, we have also examinated the interaction of the NG2+ and NG2- sarcoma cells with collagen type VI and other ECM molecules. Sarcoma cells were NG2 abrogated by RNAi or cells immunosorted for NG2 expression were found to exhibit a rather elective, impaired ability to adhere and migrate on purified collagen type VI. These findings confirmed that the NG2 was capable of mediating tumour cell adhesion and migration through interaction with this specific collagen. The outcome of these investigations provide a first evidence that NG2 may represent a unique, malignancy promoting factor in several types of soft-tissue sarcomas and that defined surface PGs pattern differentially control tumour progression, with some profiles being specifically associated with an aggressive behaviour, whereas others with a more benign phenotype.