RGD-independent Cell Adhesion via a Tissue Transglutaminase-Fibronectin Matrix Promotes Fibronectin Fibril Deposition and Requires Syndecan-4/2 and α5β1 Integrin Co-signaling

Abstract

Fibronectin (FN) deposition mediated by fibroblasts is an important process in matrix remodeling and wound healing. By monitoring the deposition of soluble biotinylated FN, weshow that the stress-induced TG-FN matrix, a matrix complex of tissue transglutaminase (TG2) with its high affinity binding partner FN, can increase both exogenous and cellular FN deposition and also restore it when cell adhesion is interrupted via the presence of RGD-containing peptides. This mechanism does not require the transamidase activity of TG2 but is activated through an RGD-independent adhesion process requiring a heterocomplex of TG2 and FN and is mediated by a syndecan-4 and ␤1 integrin co-signaling pathway. By using ␣5 null cells, ␤1 integrin functional blocking antibody, and a ␣5␤1 integrin targeting peptide A5-1, we demonstrate that the ␣5 and ␤1 integrins are essential for TG-FN to compensate RGD-induced loss of cell adhesion and FN deposition. The importance of syndecan-2 in this process was shown using targeting siRNAs, which abolished the compensation effect of TG-FN on the RGD-induced loss of cell adhesion, resulting in disruption of actin skeleton formation and FN deposition. Unlike syndecan-4, syndecan-2 does not interact directly with TG2 but acts as a downstream effector in regulating actin cytoskeleton organization through the ROCK pathway. We demonstrate that PKC␣ is likely to be the important link between syndecan-4 and syndecan-2 signaling and that TG2 is the functional component of the TG-FN heterocomplex in mediating cell adhesion via its direct interaction with heparan sulfate chains. Fibronectin (FN)3 mediates the cell adhesion process by interacting with cell surface receptors through its different cell binding sites. It is essential to embryogenesis and found associated with the extracellular matrix in large quantities during wound healing and angiogenesis. The amino acid sequence Arg-Gly-Asp (RGD) found within FN and many other matrix proteins is the most widely occurring cell-adhesive motif (1). It is the most important recognition site for about half of all known integrins, such as ␣5␤1, ␣V␤1, ␣V␤3, and ␣V␤5 (2). However, this cell adhesion process can easily be inhibited by the presence of competitive peptides containing the RGD sequence, often leading to apoptosis (anoikis) (3, 4). Such peptides are commonly released during matrix remodeling, a process that is essential to both wound healing and angiogenesis (5-7). Of the integrins, ␣5␤1 integrin is probably the major cell surface integrin that interacts with the RGDcell binding site on FN, initiating the cell adhesion process (8), whereas the binding of ␣v␤3 integrin to the RGD domain can also support cell adhesion on FN (9). In addition to the RGDcell binding domains, the heparin binding sites within FN have also been reported to be involved in cell adhesion because, although cell binding to the RGD-cell binding domains of FN can initiate the cell adhesion process, it is not sufficient to fully support actin cytoskeleton formation, an observation tha...

Metastasis-associated S100A4 is a specific amine donor and an activity-independent binding partner of transglutaminase-2

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“…Another β1 integrin that has been linked to cardiac fibrotic disease is α5β1 integrin, the classic fibronectin receptor. Expression of α5β1 is increased in fibrotic myocardium and signaling downstream of α5β1 promotes the expression of additional fibronectin in an example of positive feedback (Sarrazy et al, 2014;Wang et al, 2010). Secretion of fibronectin contributes to further MyoFB differentiation by initiating signaling through FAK that facilitates the formation of new integrin adhesions and increases matrix stiffness (Fig.…”

Angiopoietin-2 differentially regulates angiogenesis through TIE2 and integrin signaling

“…Yet RGD-independent binding sites have been described (44). We therefore examined the ability of ANG-2 to interfere with fibronectin or vitronectin binding ( Figure 7F).…”