Galectin Binding to Mgat5-Modified N-Glycans Regulates Fibronectin Matrix Remodeling in Tumor Cells

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Abstract
Oncogenic signaling stimulates the dynamic remodeling of actin microfilaments and substrate adhesions, essential for cell spreading and motility. Transformation is associated with increased expression of β1,6GlcNAc-branched N-glycans, products of Golgi β1,6-acetylglucosaminyltransferase V (Mgat5) and the favored ligand for galectins. Herein we report that fibronectin fibrillogenesis and fibronectin-dependent cell spreading are deficient in Mgat5−/− mammary epithelial tumor cells and inhibited in Mgat5+/+ cells by blocking Golgi N-glycan processing with swainsonine or by competitive inhibition of galectin binding. At an optimum dosage, exogenous galectin-3 added to Mgat5+/+ cells activates focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K), recruits conformationally active α5β1-integrin to fibrillar adhesions, and increases F-actin turnover. RGD peptide inhibits PI3K-dependent fibronectin matrix remodeling and fibronectin-dependent cell motility, while galectin-3 stimulates and overrides the inhibitory effects of RGD. Antibodies to the galectin-3 N-terminal oligomerization domain stimulate α5β1 activation and recruitment to fibrillar adhesions in Mgat5+/+ cells, an effect that is blocked by disrupting galectin-glycan binding. Our results demonstrate that fibronectin polymerization and tumor cell motility are regulated by galectin-3 binding to branched N-glycan ligands that stimulate focal adhesion remodeling, FAK and PI3K activation, local F-actin instability, and α5β1 translocation to fibrillar adhesions.