A specific alpha5beta1-integrin conformation promotes directional integrin translocation and fibronectin matrix formation

The matrix reorganized: extracellular matrix remodeling and integrin signaling

Via integrins, cells can sense dimensionality and other physical and biochemical properties of the extracellular matrix (ECM). Cells respond differently to two-dimensional substrates and three-dimensional environments, activating distinct signaling pathways for each. Direct integrin signaling and indirect integrin modulation of growth factor and other intracellular signaling pathways regulate ECM remodeling and control subsequent cell behavior and tissue organization. ECM remodeling is critical for many developmental processes, and remodeled ECM contributes to tumorigenesis. These recent advances in the field provide new insights and raise new questions about the mechanisms of ECM synthesis and proteolytic degradation, as well as the roles of integrins and tension in ECM remodeling.

Galectin binding to Mgat5-modified N-glycans regulates fibronectin matrix remodeling in tumor cells

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 beta1,6GlcNAc-branched N-glycans, products of Golgi beta1,6-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 alpha5beta1-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 alpha5beta1 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 alpha5beta1 translocation to fibrillar adhesions.

The N-terminal 70-kDa fragment of fibronectin binds to cell surface fibronectin assembly sites in the absence of intact fibronectin

Binding of the N-terminal 70-kDa (70K) fragment of fibronectin to fibroblasts blocks assembly of intact fibronectin and is an accurate indicator of the ability of various agents to enhance or inhibit fibronectin assembly. Such binding is widely thought to be to already assembled fibronectin. We evaluated this hypothesis with fibronectin-null mouse fibroblasts plated on laminin-1 in the absence of intact fibronectin. As a proteolytic fragment or recombinant protein, 70K bound fibronectin-null cells specifically in linear arrays that extended outwards from the periphery of spread cells. At early time points, these arrays were similar to those formed by intact fibronectin. 70K arrays formed within 5 min following ligand addition at concentrations as low as 5 nM, indicating rapid and high affinity binding. Bound 70K was extractable with Triton X-100 or deoxycholate but became insoluble when cross-linked with a membrane-impermeable agent into large SDS-stable complexes. Intact fibronectin, in contrast, became progressively non-extractable in the absence of cross-linking. The detergent-resistant arrays of cross-linked 70K localized to tips of cellular extensions and partially overlapped with alpha6 and beta1 integrin subunits at the base of the extensions. alpha5 did not localize with 70K arrays, but became progressively co-localized with assemblies of intact fibronectin over time. These results support a model in which the 70-kDa region of fibronectin binds to linearly arrayed cell surface molecules of adherent cells to initiate assembly, display of the arrays is controlled by the integrin that mediates adhesion, and fibronectin-binding integrins promote fibronectin-fibronectin interactions during progression of assembly.