Glia maturation factor-gamma modulation of signaling pathways in macrophages

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Monocyte migration requires the dynamic redistribution of integrins through a regulated endo-exocytosis cycle, but the complex molecular mechanisms underlying this process have not been fully elucidated. Glia maturation factor- (GMFG), a novel regulator of the Arp2/3 complex, has been shown to regulate directional migration of neutrophils and T-lymphocytes. In this study, we explored the important role of GMFG in monocyte chemotaxis, adhesion, and 1-integrin turnover. We found that knockdown of GMFG in monocytes resulted in impaired chemotactic migration toward formyl-Met-Leu-Phe (fMLP) and stromal cell-derived factor 1 (SDF-1) as well as decreased 51-integrin-mediated chemoattractant-stimulated adhesion. These GMFG knockdown impaired effects could be reversed by cotransfection of GFP-tagged full-length GMFG. GMFG knockdown cells reduced the cell surface and total protein levels of 51-integrin and increased its degradation. Importantly, we demonstrate that GMFG mediates the ubiquitination of 1-integrin through knockdown or overexpression of GMFG. Moreover, GMFG knockdown retarded the efficient recycling of 1-integrin back to the plasma membrane following normal endocytosis of 51-integrin, suggesting that the involvement of GMFG in maintaining 51-integrin stability may occur in part by preventing ubiquitin-mediated degradation and promoting 1-integrin recycling. Furthermore, we observed that GMFG interacted with syntaxin 4 (STX4) and syntaxin-binding protein 4 (STXBP4); however, only knockdown of STXBP4, but not STX4, reduced monocyte migration and decreased 1-integrin cell surface expression. Knockdown of STXBP4 also substantially inhibited 1-integrin recycling in human monocytes. These results indicate that the effects of GMFG on monocyte migration and adhesion probably occur through preventing ubiquitin-mediated proteasome degradation of 51-integrin and facilitating effective 1-integrin recycling back to the plasma membrane.