Melanoma chondroitin sulfate proteoglycan (MCSP) is a cell surface proteoglycan expressed at high levels on the vast majority of human melanomas and is implicated in promoting tumor adhesion, migration and invasion. To further characterize MCSP in tumor progression we have cloned and stably expressed the MCSP core protein in two MCSP-negative melanoma cell lines. MCSP expression enhanced growth and tumor formation of melanoma cells in xenograft models, and monoclonal antibodies against the extracellular domain of MCSP inhibited tumor formation in vivo. Expression of MCSP stimulates integrin-mediated signal transduction as evidenced by MCSP-induced increases in cell spreading and focal adhesion kinase phosphorylation. Furthermore, expression of MCSP stimulates enhanced tyrosine phosphorylation and the phosphorylation of ERK, which is required for anchorage-independent growth, while a cytoplasmic tail- deleted MCSP failed to support anchorage-independent growth or enhance ERK activation. Members of the ERK/MAPK pathway, including BRAF (which is mutated to the V600E active form in these cells), pMEK and pERK all precipitated with a GST-MCSP cytoplasmic tail fusion protein, while a truncated fusion protein lacking the c-terminal end of the MCSP tail failed to bind these molecules. These results indicate that MCSP acts as a docking site for ERK/MAPK pathway members. We hypothesize that MCSP functions as a novel transmembrane scaffold protein that helps assemble and efficiently activate key signaling pathways to promote melanoma growth, survival and invasion. Aim #1 will study MCSP in tumor growth and maintanence using siRNA to inhibit MCSP expression in human melanoma cells. Aim #2 will focus on how members of the ERK/MAPK pathway link to the cytoplasmic domain of MCSP. Aim #3 will define structural features of the extracellular domain of MCSP required for activation of key signalling pathways important for tumor growth. Melanoma is a devastating disesase that is almost completely nonresponsive to current therapies. Since the vast majority of melanomas express MCSP in both primary tumors and in metastatic lesions, this suggests that melanoma cells may use this molecule to attain a competitive advantage over MCSP-negative cells at multiple stages of malignant progression. The long term goal of these studies is to determine the potential to exploit inhibitors of MCSP expression/function as novel therapies in the treatment of malignant melanoma.