[Truncated abstract] Melanoma cell adhesion molecule (MCAM) is highly expressed in more than 70% of metastatic melanoma and is correlated with invasive potential. However, the specific contribution MCAM makes to invasion and metastasis in melanoma is not clear. In this study, I have demonstrated that transfection of MCAM into MCAM-negative melanoma and CHO cells leads to changes in cell shape, and the modulation of cell-to-cell and cell-matrix interactions. MCAM positive cells were slower to spread on collagen type I, collagen type IV and laminin 1 than MCAM negative cells, although these differences were not apparent on vitronectin, fibronectin and laminin 10. In contrast, MCAM expression had little effect on cell adhesion to any of the matrices tested. MCAM positive (compared to negative) cells also showed morphological changes and a rearrangement of the actin cytoskeleton when plated on a matrix containing laminin 5. Taken together, these data suggest that MCAM expression modulates β1-integrinmediated spreading on matrix, but has little effect on αvβ3-mediated cell-matrix interactions. As this study provided little evidence to suggest that MCAM transfection altered β1 integrin expression levels on melanoma cells, it is proposed that a competitive interaction between the cytoplasmic domains of MCAM and β1 integrin may affect mature focal adhesion assembly. MCAM expression in melanoma cells was also associated with decreased cell movement over matrix into a scratch-wound site and an increased tendency to form cell cords on Matrigel. These two assays gauge the propensity of a cell to engage in cell-cell versus cell-matrix interactions, and suggest that MCAM positive cells favour cell-cell adhesion. Interestingly, MCAM transfection was also associated with an increased ability of melanoma cells to migrate through a basement membrane towards a chemoattractant. ... Analysis of the intracellular domain of MCAM revealed the presence of tyrosine and dileucine endocytosis signals. Interestingly, disruption of these two motifs did not seem to impair the internalization of MCAM from the cell surface. The di-leucine motif, however, was necessary for the recycling of MCAM back to the surface following endocytosis. Lastly, MCAM was found to exists as dimers within the cell membrane in the absence of ligand, although the exact location of the dimerization motif is not yet clearly defined. Collectively, findings from my study suggest: MCAM expression in melanoma cells facilitates cell-cell interactions, whilst concomitantly modulating cell-matrix interactions. MCAM transfection also leads to enhanced migration of melanoma cells through a basement membrane. Thus, MCAM expression may increase the ability of melanoma cells to migrate as a collective, a feature of highly invasive cancer. The intracellular domain of MCAM interacts with ApxL2, a novel member of the Shroom family of actin-binding proteins. It is likely that ApxL2 links a proportion of MCAM within the cell to the actin cytoskeleton, contributing to cell shape determination and other processes, such as migration. MCAM exists as dimers on the cell surface and is internalized at least partially by a clathrin-mediated mechanism.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2007|