Deacetylated GM3 promotes uPAR-associated membrane molecular complex to activate p38 MAPK in metastatic melanoma

GM3, the simplest ganglioside, regulates cell proliferation, migration, and invasion by influencing cell signaling at the membrane level. Although the classic N-acetylated form of GM3 (NeuAcLacCer) is commonly expressed and has been well studied, deacetylated GM3 (NeuNH2LacCer, d-GM3) has been poorly investigated, despite its presence in metastatic tumors but not in noninvasive melanomas or benign nevi. We have recently found that d-GM3 stimulates cell migration and invasion by activating urokinase plasminogen activator receptor (uPAR) signaling to augment matrix metalloproteinase-2 (MMP-2) function. However, the mechanisms by which d-GM3/uPAR increase MMP-2 expression and activation are not clear. By modifying the expression of d-GM3 genetically and biochemically, we found that decreasing d-GM3 expression inhibits, whereas overexpressing d-GM3 stimulates, p38 mitogen-activated protein kinase (MAPK) activity to influence MMP-2 expression and activation. p38 MAPK (p38) activation requires the formation of a membrane complex that contains uPAR, caveolin-1, and integrin α5β1 in membrane lipid rafts. In addition, knocking down or inhibiting focal adhesion kinase (FAK), phosphoinositide 3-kinase (PI3K), or Src kinase significantly reduces d-GM3-induced p38 phosphorylation and activation. Taken together, these results suggest that d-GM3 enhances the metastatic phenotype by activating p38 signaling through uPAR/integrin signaling with FAK, PI3K, and Src kinase as intermediates. Elucidation of the mechanisms by which d-GM3, a newly discovered, potential biomarker of metastatic melanomas, promotes cell metastasis will help us to understand the function of d-GM3 in metastatic melanomas and may lead to novel GM3-based cancer therapies.