Lipid raft adhesion receptors recruitment mediated by major histocompatibility complex (MHC) class II signalling in melanoma cells

Background: The lipid rafts are enriched in freely diffusing, stable assemblies of sphingolipids and cholesterol that are implicated in the selective protein-protein interactions as well as in the assembly of transient signalling platforms. The lipid raft proteins play important roles in cell-cell interaction and are docking site for the proteins involved in focal adhesion and cancer metastasis such as the adhesion receptors. Indeed, the alteration of adhesion receptors expression is related to metastatic progression of melanoma through the deregulation of adhesive functions, the subsequent detachment of tumour cells from the primary tumour site and the tissue borders. Interestingly, in different tumour cell lines has been reported that the signalling activated by the MHC class II molecules is associated to the lipid raft localization of these molecules but only reduced knowledge are available in this field for melanoma. In the aim to understand the molecular mechanisms used by melanomas for the progression to an invasive-metastatic state, we studied in MHC class II constitutive expressing melanoma cell lines, the membrane localization of adhesion receptors as well as the activated signalling. Material and methods: The class II constitutive expressing melanoma cells (A375 and HT144 cell lines) were stimulated with a specific anti-HLA-DR mAb (L243) that mimics the TCR interaction with the class II molecules for 24h and 48h or left unstimulated. The lipid rafts of stimulated and unstimulated melanoma cells were isolated and analysed by western blot. Exosomes secreted by stimulated and unstimulated melanoma cells were purified and analysed by western blot. Results: In the lipid rafts domains of stimulated A375 and HT144 cells we observed an increased localisation of HLA-DR?, Integrin ?1, MCAM, ICAM as well as FAK kinase. Therefore, we compared the expression of HLA-DR?, MCAM and ICAM receptors in exosomes secreted by stimulated and unstimulated A375 and HT144 melanoma cells. Conclusions: Therefore, our results underline the role played in melanoma cells by the MHC class II dependent signalling on motility and exosomes functionality. The signalling activated by class II molecules assembling class II, Integrins and CAMs receptors as well as the FAK kinase in the lipid rafts fraction, could elucidate the mechanisms of melanoma cells metastatic dissemination as well as the role of exosomes on the microenvironment of tumour sites.