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Heterotropic association of tissue transglutaminase (TG2) with extracellular matrix-associated fibronectin (FN) can restore the adhesion of fibroblasts when the integrin-mediated direct binding to FN is impaired using RGD-containing peptide. We demonstrate that the compensatory effect of the TG-FN complex in the presence of RGD-containing peptides is mediated by TG2 binding to the heparan sulfate chains of the syndecan-4 cell surface receptor. This binding mediates activation of protein kinase C␣ (PKC␣) and its subsequent interaction with ␤ 1 integrin since disruption of PKC␣ binding to ␤ 1 integrins with a cell-permeant competitive peptide inhibits cell adhesion and the associated actin stress fiber formation. Cell signaling by this process leads to the activation of focal adhesion kinase and ERK1/2 mitogen-activated protein kinases. Fibroblasts deficient in Raf-1 do not respond fully to the TG-FN complex unless either the full-length kinase competent Raf-1 or the kinase-inactive domain of Raf-1 is reintroduced, indicating the involvement of the Raf-1 protein in the signaling mechanism. We propose a model for a novel RGD-independent cell adhesion process that could be important during tissue injury and/or remodeling whereby TG-FN binding to syndecan-4 activates PKC␣ leading to its association with ␤ 1 integrin, reinforcement of actin-stress fiber organization, and MAPK pathway activation. Tissue transglutaminase (TG2)2 belongs to a family of enzymes that in the presence of Ca 2ϩ catalyze the post-translational modification of proteins either by covalent cross-linking or through the incorporation of primary amines. TG2 has a GTP binding/hydrolysis site that negatively modulates the Ca 2ϩ -dependent transamidating activity of the enzyme by obstructing access to the active site (1). As a consequence, the enzyme is likely to be inactive under normal Ca 2ϩ homeostasis. TG2 localizes mainly in the cytoplasm, yet recent reports also suggest its presence in the nucleus, mitochondria, at the cell surface, and in the extracellular matrix (ECM) (1-3). TG2 is translocated to the plasma membrane and was subsequently deposited into the ECM via a non-classical secretory mechanism reportedly dependent on active site conformation and on an intact N-terminal ␤-sandwich domain (4, 5) as well as on its possible association with integrins (6). Deposition of the enzyme into the ECM after cell damage and stress is important in the remodeling and/or stabilization of the several ECM proteins, such as FN (7,8). FN is particularly interesting since TG2 binds to this ECM protein with high affinity promoting wideranging effects on cell-matrix interactions, including the regulation of cell adhesion and migration, matrix assembly, and adhesion-dependent signaling (6,7,9).Cell adhesion to FN involves a series of coordinated signaling events orchestrated by numerous transmembrane receptors including the integrins and the superfamily of cell-surface proteoglycans (10, 11). The identity of the receptor-ligand pairing defines the composition of f...

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...

Repair of tissue after injury depends on a series of concerted but overlapping events including, inflammation, re-epithelialization, neovascularization and synthesis and stabilization of a fibrous extracellular matrix (ECM) that is remodeled to emulate normal tissue over time. Particular members of the transglutaminase (TG) family are upregulated during wound healing and act as a novel class of wound-healing mediators during the repair process. This group of enzymes which crosslink proteins via epsilon(gamma-glutamyl) lysine bridges are involved in wound healing through their ability to stabilize proteins and also by regulating the behavior of a wide variety of cell types that are recruited to the damaged area in order to carry out tissue repair. In this article we discuss the function of the most widely expressed member of the TG family "tissue transglutaminase" (TG2) in wound repair. Using both early and recent evidence from the literature we demonstrate how the multifunctional TG2 affects the stability of the ECM, cell-ECM interactions and as a consequence cell behavior within the different phases of wound healing, and highlight how TG2 itself might be exploited for therapeutic use.

In fibrotic conditions increases in TG2 activity has been linked to an increase in the deposition of extracellular matrix proteins. Using TG2 transfected Swiss 3T3 fibroblasts expressing TG2 under the control of the tetracycline-regulated inducible promoter, we demonstrate that induction of TG2 not only stimulates an increase in collagen and fibronectin deposition but also an increase in the expression of these proteins. Increased TG2 expression in these fibroblasts led to NF-B activation, resulting in the increased expression of transforming growth factor (TGF) ␤ 1 . In addition, cells overexpressing TG2 demonstrated an increase in biologically active TGF␤ 1 in the extracellular environment. A specific site-directed inhibitor of TG abolished the NF-B and TGF␤1 activation and the subsequent elevation in the synthesis and deposition of extracellular matrix proteins, confirming that this process depends on the induction of transglutaminase activity. Treatment of TG2-induced fibroblasts with nontoxic doses of nitric oxide donor S-nitroso-N-acetylpenicillamine resulted in decreased TG2 activity and apprehension of the inactive enzyme on the cell surface. This was paralleled by a reduction in activation of NF-B and TGF␤ 1 production with a subsequent decrease in collagen expression and deposition. These findings support a role for NO in the regulation of TG2 function in the extracellular environment.

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