Thrombin upregulates tissue transglutaminase in endothelial cells: a potential role for tissue transglutaminase in stability of atherosclerotic plaque

Transglutaminase 2 Facilitates Murine Wound Healing in a Strain-Dependent Manner

Transglutaminase 2 (TG2) plays a role in cellular processes that are relevant to wound healing, but to date no studies of wound healing in TG2 knockout mice have been reported. Here, using 129T2/SvEmsJ (129)- or C57BL/6 (B6)-backcrossed TG2 knockout mice, we show that TG2 facilitates murine wound healing in a strain-dependent manner. Early healing of in vivo cutaneous wounds and closure of in vitro scratch wounds in murine embryonic fibroblast (MEF) monolayers were delayed in 129, but not B6, TG2 knockouts, relative to their wild-type counterparts, with wound closure in 129 being faster than in B6 wild-types. A single dose of exogenous recombinant wild-type TG2 to 129 TG2-/- mice or MEFs immediately post-wounding accelerated wound closure. Neutrophil and monocyte recruitment to 129 cutaneous wounds was not affected by Tgm2 deletion up to 5 days post-wounding. Tgm2 mRNA and TG2 protein abundance were higher in 129 than in B6 wild-types and increased in abundance following cutaneous and scratch wounding. Tgm1 and factor XIIA (F13A) mRNA abundance increased post-wounding, but there was no compensation by TG family members in TG2-/- relative to TG2+/+ mice in either strain before or after wounding. 129 TG2+/+ MEF adhesion was greater and spreading was faster than that of B6 TG2+/+ MEFs, and was dependent on syndecan binding in the presence, but not absence, of RGD inhibition of integrin binding. Adhesion and spreading of 129, but not B6, TG2-/- MEFs was impaired relative to their wild-type counterparts and was accelerated by exogenous addition or transfection of TG2 protein or cDNA, respectively, and was independent of the transamidase or GTP-binding activity of TG2. Rho-family GTPase activation, central to cytoskeletal organization, was altered in 129 TG2-/- MEFs, with delayed RhoA and earlier Rac1 activation than in TG2+/+ MEFs. These findings indicate that the rate of wound healing is different between 129 and B6 mouse strains, correlating with TG2 abundance, and although not essential for wound healing, TG2 facilitates integrin- and syndecan-mediated RhoA- and Rac1-activation in fibroblasts to promote efficient wound contraction.

Cross-linking by tissue transglutaminase-2 alters fibrinogen-directed macrophage proinflammatory activity

BACKGROUND

The blood coagulation factor fibrin(ogen) can modulate inflammation by altering leukocyte activity. Analyses of fibrin(ogen)-mediated proinflammatory activity have largely focused on leukocyte integrin binding activity revealed by conversion of fibrinogen to a stabilized fibrin polymer by blood coagulation enzymes. In addition to coagulation enzymes, fibrinogen is a substrate for tissue transglutaminase-2 (TG2), a widely expressed enzyme that produces unique fibrinogen Aα-γ chain cross-linked products.

OBJECTIVES

We tested the hypothesis that TG2 dependent cross-linking alters the proinflammatory activity of surface-adhered fibrinogen.

METHODS

Mouse bone marrow-derived macrophages (BMDMs) were cultured on tissue culture plates coated with fibrinogen or TG2-cross-linked fibrinogen (10 µg/ml) and then stimulated with lipopolysaccharide (LPS, 1 ng/ml) or vehicle for various times.

RESULTS

In the absence of LPS stimulation, TG2-cross-linked fibrin(ogen) enhanced inflammatory gene induction (e.g., Tnfα) compared with unmodified fibrinogen. LPS stimulation induced mitogen-activated protein kinase phosphorylation, IκBα degradation, and expression of proinflammatory cytokines (e.g., tumor necrosis factor α) within 60 min. This initial cellular activation was unaffected by unmodified or TG2-cross-linked fibrinogen. In contrast, LPS induction of interleukin-10 mRNA and protein and STAT3 phosphorylation was selectively attenuated by TG2-cross-linked fibrinogen, which was associated with enhanced proinflammatory cytokine secretion by LPS-stimulated BMDMs at later time points (6 and 24 h).

CONCLUSIONS

The results indicate that atypical cross-linking by TG2 imparts unique proinflammatory activity to surface-adhered fibrinogen. The results suggest a novel coagulation-independent mechanism controlling fibrinogen-directed macrophage activation.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Transglutaminase 2 moderates the expansion of mouse abdominal aortic aneurysms

Objective

Previously published work has indicated that transcripts encoding transglutaminase 2 (TG2) increase markedly in a rat model of abdominal aortic aneurysm. This study determines whether TG2 and the related TG, factor XIII-A (FXIII-A), protect against aortic aneurysm development in mice.

Methods

C57BL/6J wild-type, Tgm2 ^-/- knockout, F13a1 ^-/- knockout, and Tgm2 ^-/- /F13a1 ^-/- double knockout mice were subjected to laparotomy and periaortic application of CaCl₂.

Results

Tgm2 ^-/- mice showed slightly greater aortic dilatation at 6 weeks after treatment when compared with wild type. However, vessels from Tgm2 ^-/- mice, but not wild-type mice, continued to dilate up to 6 months after injury and by 24 weeks, a greater number of Tgm2 ^-/- mice had developed aneurysms (16/17 vs 10/19; P = .008). Laparotomy resulted in a high death rate in F13a1 ^-/- knockout mice, more frequently from cardiac complications than from hemorrhage, but among F13a1 ^-/- mice that survived for 6 weeks after CaCl₂ treatment, abdominal aortic aneurysm diameter was unaltered relative to wild-type mice. Laparotomy resulted in a higher death rate among Tgm2 ^-/- /F13a1 ^-/- double knockout mice, owing to an increased frequency of delayed bleeding. Surprisingly, Tgm2 ^-/- /F13a1 ^-/- double knockout mice showed a trend toward decreased dilatation of the aorta 6 weeks after injury, and this finding was replicated in Tgm2 ^-/- /F13a1 ^-/- mice subjected to carotid artery injury. Levels of transcripts encoding TG2 were not increased in the aortas of injured wild-type or F13a1 ^-/- knockout mice relative to uninjured mice, although changes in the levels of other transcripts accorded with previous descriptions of the CaCl₂ aneurysm model in mice.

Conclusions

Knockout of Tgm2, but not F13a1 exacerbates aortic dilatation, suggesting that TG2 confers protection. However, levels of TG2 messenger RNA are not acutely elevated after injury. FXIII-A plays a role in preventing postoperative damage after laparotomy, confirming previous reports that it prevents distal organ damage after trauma. TG2 promotes wound healing after surgery and, in its absence, the bleeding diathesis associated with FXIII-A deficiency is further exposed.

Spotlight on the transglutaminase 2 gene: a focus on genomic and transcriptional aspects

The type 2 isoenzyme is the most widely expressed transglutaminase in mammals displaying several intra- and extracellular activities depending on its location (protein modification, modulation of gene expression, membrane signalling and stabilization of cellular interactions with the extracellular matrix) in relation to cell death, survival and differentiation. In contrast with the appreciable knowledge about the regulation of the enzymatic activities, much less is known concerning its inducible expression, which is altered in inflammatory and neoplastic diseases. In this context, we first summarize the gene's basic features including single-nucleotide polymorphism characterization, epigenetic DNA methylation and identification of regulatory regions and of transcription factor-binding sites at the gene promoter, which could concur to direct gene expression. Further aspects related to alternative splicing events and to ncRNAs (microRNAs and lncRNAs) are involved in the modulation of its expression. Notably, this important gene displays transcriptional variants relevant for the protein's function with the occurrence of at least seven transcripts which support the synthesis of five isoforms with modified catalytic activities. The different expression of the TG2 (type 2 transglutaminase) variants might be useful for dictating the multiple biological features of the protein and their alterations in pathology, as well as from a therapeutic perspective.

Inhibition of transglutaminase 2 reduces efferocytosis in human macrophages: Role of CD14 and SR-AI receptors

BACKGROUND AND AIMS

Transglutaminase 2 (TGM2), a member of the transglutaminase family of enzymes, is a multifunctional protein involved in numerous events spanning from cell differentiation, to signal transduction, apoptosis, and wound healing. It is expressed in a variety of cells, macrophages included. Macrophage TGM2 promotes the clearance of apoptotic cells (efferocytosis) and emerging evidence suggests that defective efferocytosis contributes to the consequences of inflammation-associated diseases, including atherosclerotic lesion progression and its sequelae. Of interest, active TGM2 identified in human atherosclerotic lesions plays critical roles in plaque stability through effects on matrix cross-linking and TGFβ activity. This study explores the mechanisms by which TGM2 controls efferocytosis in human macrophages.

METHODS AND RESULTS

Herein we show that TGM2 increases progressively during monocyte differentiation towards macrophages and controls their efferocytic potential as well as morphology and viability. Two experimental approaches that took advantage of the inhibition of TGM2 activity and protein silencing give proof that TGM2 reduction significantly impairs macrophage efferocytosis. Among the mechanisms involved we highlighted a role of the receptors CD14 and SR-AI whose levels were markedly reduced by TGM2 inhibition. Conversely, CD36 receptor and αvβ3 integrin levels were not influenced. Of note, lipid accumulation and IL-10 secretion were reduced in macrophages displaying defective efferocytosis.

CONCLUSION

Overall, our data define a crucial role of TGM2 activity during macrophage differentiation via mechanisms involving CD14 and SR-AI receptors and show that TGM2 inhibition triggers a pro-inflammatory phenotype.

Acute traumatic spinal cord injury induces glial activation in the cynomolgus macaque (Macaca fascicularis)

BACKGROUND

Traumatic spinal cord injury leads to direct myelin and axonal damage and leads to the recruitment of inflammatory cells to site of injury. Although rodent models have provided the greatest insight into the genesis of traumatic spinal cord injury (TSCI), recent studies have attempted to develop an appropriate non-human primate model.

METHODS

We explored TSCI in a cynomolgus macaque model using a balloon catheter to mimic external trauma to further evaluate the underlying mechanisms of acute TSCI.

RESULTS

Following 1hour of spinal cord trauma, there were focal areas of hemorrhage and necrosis at the site of trauma. Additionally, there was a marked increased expression of macrophage-related protein 8, MMP9, IBA-1, and inducible nitric oxide synthase in macrophages and microglia at the site of injury.

CONCLUSIONS

This data indicate that acute TSCI in the cynomolgus macaque is an appropriate model and that the earliest immunohistochemical changes noted are within macrophage and microglia populations.

A crucial sequence for transglutaminase type 2 extracellular trafficking in renal tubular epithelial cells lies in its N-terminal beta-sandwich domain

Transglutaminase type 2 (TG2) catalyzes the formation of an ε-(γ-glutamyl)-lysine isopeptide bond between adjacent peptides or proteins including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlie tissue scarring and fibrosis. The extracellular trafficking of TG2 is crucial to its role in ECM homeostasis; however, the mechanism by which TG2 escapes the cell is unknown as it has no signal leader peptide and therefore cannot be transported classically. Understanding TG2 transport may highlight novel mechanisms to interfere with the extracellular function of TG2 as isoform-specific TG2 inhibitors remain elusive. Mammalian expression vectors were constructed containing domain deletions of TG2. These were transfected into three kidney tubular epithelial cell lines, and TG2 export was assessed to identify critical domains. Point mutation was then used to highlight specific sequences within the domain required for TG2 export. The removal of β-sandwich domain prevented all TG2 export. Mutations of Asp(94) and Asp(97) within the N-terminal β-sandwich domain were identified as crucial for TG2 externalization. These form part of a previously identified fibronectin binding domain ((88)WTATVVDQQDCTLSLQLTT(106)). However, siRNA knockdown of fibronectin failed to affect TG2 export. The sequence (88)WTATVVDQQDCTLSLQLTT(106) within the β-sandwich domain of TG2 is critical to its export in tubular epithelial cell lines. The extracellular trafficking of TG2 is independent of fibronectin.

Model thrombi formed under flow reveal the role of factor XIII-mediated cross-linking in resistance to fibrinolysis

BACKGROUND

Activated factor XIII (FXIIIa), a transglutaminase, introduces fibrin-fibrin and fibrin-inhibitor cross-links, resulting in more mechanically stable clots. The impact of cross-linking on resistance to fibrinolysis has proved challenging to evaluate quantitatively.

METHODS

We used a whole blood model thrombus system to characterize the role of cross-linking in resistance to fibrinolytic degradation. Model thrombi, which mimic arterial thrombi formed in vivo, were prepared with incorporated fluorescently labeled fibrinogen, in order to allow quantification of fibrinolysis as released fluorescence units per minute.

RESULTS

A site-specific inhibitor of transglutaminases, added to blood from normal donors, yielded model thrombi that lysed more easily, either spontaneously or by plasminogen activators. This was observed both in the cell/platelet-rich head and fibrin-rich tail. Model thrombi from an FXIII-deficient patient lysed more quickly than normal thrombi; replacement therapy with FXIII concentrate normalized lysis. In vitro addition of purified FXIII to the patient's preprophylaxis blood, but not to normal control blood, resulted in more stable thrombi, indicating no further efficacy of supraphysiologic FXIII. However, addition of tissue transglutaminase, which is synthesized by endothelial cells, generated thrombi that were more resistant to fibrinolysis; this may stabilize mural thrombi in vivo.

CONCLUSIONS

Model thrombi formed under flow, even those prepared as plasma 'thrombi', reveal the effect of FXIII on fibrinolysis. Although very low levels of FXIII are known to produce mechanical clot stability, and to achieve γ-dimerization, they appear to be suboptimal in conferring full resistance to fibrinolysis.

Role of transglutaminases in cuff-induced atherosclerotic lesion formation in femoral arteries of ApoE3 Leiden mice

Transglutaminases play an important role in vascular remodeling, calcification, cell adhesion and endothelial barrier function. In this study we investigate the influence of combined inhibition of both tissue-type transglutaminase (TG2) and the plasma transglutaminase FXIIIA on early lesion development.

METHODS

A cuff was placed around the femoral arteries of ApoE3 Leiden mice while fed a Western type diet to induce atherosclerotic lesion development. An osmotic minipump was placed in the intraperitoneal cavity containing an irreversible inhibitor of TG2 and FXIIIA activity ((1,3,4,5-tetramethyl-2-[(2-oxopropyl)thio]imidazolium chloride, Zedira). Atherosclerotic lesion composition was analyzed using immunohistochemistry and RT-PCR.

RESULTS

Inhibition of transglutaminases did not influence lesion size or geometric remodeling of the vessels. However, systemic transglutaminase inhibition resulted in 41% less macrophage infiltrate in the media of the vessels. Additional in vitro experiments on HL60 cells confirmed a decreased migratory response during transglutaminase inhibition.

CONCLUSION

Inhibition of TG2 and FXIIIA during early development of lesions reduced the macrophage content in the media of atherosclerotic vessels, while not affecting lesion size or geometric remodeling.

Effect of transglutaminase 2 (TG2) deficiency on atherosclerotic plaque stability in the apolipoprotein E deficient mouse

Background

Transglutaminase 2 (TG2), a cross-linking enzyme that confers supra-molecular structures with extra rigidity and resistance against proteolytic degradation, is expressed in the shoulder regions of human atherosclerotic plaques. It has been proposed that TG2 prevents tearing and promotes plaque repair at these potential weak points, and also promotes ectopic calcification of arteries. TG2 is also expressed within plaques that develop within the brachiocephalic arteries of apolipoprotein E (apoE) deficient mice.

Objectives

To determine the role that TG2 plays in plaque development and calcification, mice were bred that were doubly deficient in apoE and TG2, and were maintained on a high-fat diet for 6 months.

Results

Lesion size and composition were not significantly altered in the apoE/TG2 double-knockout mice, with the exception of a 9.7% decrease in the proportion of the plaque occupied by lipid (p = 0.032). The frequency of buried fibrous caps within brachiocephalic plaques was significantly higher in male than in female mice, but TG2 deficiency had no effect on either gender. The extent of lesion calcification varied markedly between individual mice, but it was not decreased in the apoE/TG2 double-knockout mice.

Conclusion

These data indicate that, in the apoE knockout mouse model of atherosclerosis, TG2 does not influence plaque composition or calcification. The data further suggest that TG2 does not influence plaque stability or repair in these mice.

Calcification locates to transglutaminases in advanced human atherosclerotic lesions

Transglutaminases play an important role in vascular smooth muscle cell-induced calcification in vitro. In this study, we determined whether these enzymes are also involved in human atherosclerotic calcification using nine carotid artery specimens obtained at endarterectomy. Sections of the carotid artery specimens were registered to micro-computed tomography images and stained for tissue-type transglutaminase, plasma transglutaminase factor XIIIA (FXIIIA), the N(epsilon)(gamma-glutamyl)lysine cross-link, and the macrophage marker CD68. Ex vivo micro-computed tomography revealed extensive calcification, which significantly correlated with the cross-link. FXIIIA was found to be the dominant transglutaminase, rather than tissue-type transglutaminase, although staining of both transglutaminases correlated with the cross-link. Staining for FXIIIA colocalized with CD68 at both the cellular and tissue level. In conclusion, areas of calcification locate to the presence and activity of transglutaminases in human atherosclerotic arteries. FXIIIA seems to be the dominant transglutaminase and may be derived from local macrophages. These results are consistent with the hypothesis that transglutaminases participate in the calcification process of human atherosclerotic arteries.

Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

Increased tissue transglutaminase expression in human atherosclerotic coronary arteries

BACKGROUND AND OBJECTIVE

Transglutaminase 2 (TGase 2) is a calcium-dependent cross-linking enzyme that catalyzes a covalent iso-peptide bond between two proteins. Interestingly, this catalysis can activate the nuclear factor-kappaB (NF-kappaB) through the polymerization of the inhibitory protein of NF-kappaB (I-kappaB). The objective of the present study was to investigate the expression of TGase 2 in the human atherosclerotic human coronary artery, and the possible roles of TGase 2 in NF-kappaB activation.

METHODS AND RESULTS

We explored whether expressions of TGase 2 and NF-kappaB are associated in atherosclerosis. Using human samples, we found that TGase 2 was markedly higher than normal in the neointimal tissue of atherosclerotic coronary arteries with atherosclerosis progression. TGase 2 activity was also increased approximately two-fold in the atherosclerotic vascular wall. In immunofluorescence analysis, NF-kappaB, COX-2, and TNF-alpha were co-localized at TGase 2-positive neointimal smooth muscle cells. A promoter assay test showed that NF-kappaB activity increased in both the human monocyte and human breast carcinoma cell by TGase 2, and that TGase 2-mediated NF-kappaB activation was reversed by TGase 2 siRNA.

CONCLUSION

According to these results, we suggest that TGase 2 may function as an activator in the NF-kappaB pathway; this effect may occur in the atherosclerotic vessel wall.

Extracellular transglutaminase 2 activates beta-catenin signaling in calcifying vascular smooth muscle cells

Accumulation of transglutaminase 2 (TG2) is often associated with mineral deposits in vasculature. Here, we demonstrate that purified TG2 stimulated a 3-fold increase in matrix mineralization and up-regulation of osteoblastic markers in cultured primary vascular smooth muscle cells (VSMCs). Extracellular TG2 interacts with the low density lipoprotein related-protein 5 receptor and activates beta-catenin signaling in VSMCs. These results suggest that TG2 may promote vascular calcification by activating the beta-catenin signaling pathway.

Transglutaminase 2 is central to induction of the arterial calcification program by smooth muscle cells

Arterial calcification is a phenotype of vascular repair in atherosclerosis, diabetes, hyperphosphatemic renal failure, and aging. Arterial calcification is modulated by transition of arterial smooth muscle cells (SMCs) from contractile to chondro-osseous differentiation programmed in response to increases in P(i), bone morphogenetic protein-2, and certain other stimuli. Transglutaminase (TG)2 release modulates tissue repair, partly by transamidation-catalyzed covalent crosslinking of extracellular matrix substrates. TG2 regulates cultured SMC differentiation, resistance artery remodeling to vasoconstriction, and atherosclerotic lesion size. Here, TG2 expression was required for the majority of TG activity in mouse and human aortic SMCs. TG2(-/-) SMCs lost the capacity for P(i) donor-induced formation of multicellular bone-like nodules and for increased expression of the type III sodium-dependent P(i) cotransporter Pit-1 and certain osteoblast and chondrocyte genes (tissue-nonspecific alkaline phosphatase, the osteoblast master transcription factor runx2, and chondrocyte-restricted aggrecan), and for P(i) donor- and bone morphogenetic protein-2-induced calcification. Uniquely in TG2(-/-) SMCs, P(i) donor treatment increased expression of the physiological SMC chondro-osseous differentiation and calcification inhibitors osteoprotegerin, matrix Gla protein, and osteopontin. Conversely, TG2(-/-) SMCs, unlike wild-type SMCs, failed to maintain contractile differentiation on laminin. Exogenous catalytically active TG2 augmented calcification by TG2(-/-) SMC in response to P(i) donor treatment. TG2 expression also drove P(i)-stimulated calcification of mouse aortic ring organ cultures, which was suppressed by the TG2 catalytic site-specific inhibitor Boc-DON-Gln-Ile-Val-OMe (10 micromol/L). Our results suggest that TG2 release in injured arteries is critical for programming chondro-osseous SMC differentiation and calcification in response to increased P(i) and bone morphogenetic protein-2.

Tissue transglutaminase in tumour progression: friend or foe?

Basic biological processes in which tissue transglutaminase (TG2, tTG) is thought to be important including apoptosis, cell adhesion and migration, ECM homeostasis and angiogenesis are key stages in the multistage tumour progression cascade. Studies undertaken with primary tumours and experimental models suggest that TG2 expression and activity in the tumour body and surrounding matrix generally decreases with tumour progression, favouring matrix destabilisation, but supporting angiogenesis and tumour invasion. In contrast, in the secondary metastatic tumour TG2 is often highly expressed whereby its potential roles in cell survival both at the intra- and extracellular level become important. In the following review the underlying molecular basis for the selection of these different phenotypes in tumour types and the anomaly for the requirement of TG2 is discussed in relation to the complex events of tumour progression.

Complex formation between tissue transglutaminase II (tTG) and vascular endothelial growth factor receptor 2 (VEGFR-2): Proposed mechanism for modulation of endothelial cell response to VEGF

We have recently demonstrated that thrombin-activated FXIII (FXIIIA-subunit), a plasma transglutaminase, activates VEGFR-2 by crosslinking it with the alpha(v)beta(3) integrin on the surface of endothelial cells (EC), thereby stimulating angiogenesis. Tissue transglutaminase (tTG), which is functionally and structurally related to FXIIIA, is expressed by numerous cell types, among them EC. However, its role in EC function has not been fully characterized. In the present study, we investigated the potential involvement of tTG in angiogenesis. Using co-immunoprecipitation and immunofluorescent staining experiments, we observed that tTG forms a complex with VEGFR-2 on the cell surface and within the cytoplasm of EC. Stimulation of EC with VEGF resulted in translocation of the tTG-VEGFR-2 complex from the cytoplasm to the nucleus. In VEGF-treated cells, tTG-VEGFR-2 interaction resulted in incorporation of VEGFR-2 into high molecular weight crosslinked complex (es), as revealed by an antibody against gamma-glutamyl-epsilon-lysine isopeptide bond. tTG -VEGFR-2 association was inhibited by a specific VEGFR-2 protein tyrosine kinase inhibitor (PTKI ), as well as by cystamine, inhibitor of the transglutaminase activity of tTG, but not by bacitracin which inhibits the protein-disulfide isomerase (PDI) activity of tTG. Furthermore, cystamine completely abolished the VEGF-induced nuclear translocation of the tTG-VEGFR-2 complex. Blockade of the crosslinking activity of tTG by cystamine enhanced VEGF-induced migration of EC in Boyden chamber by 31% (P < 0.02), and prolonged VEGF-induced signaling response, as demonstrated by sustained activation of the MAP kinase ERK. Taken together, our findings suggest that endothelial cell tTG might be involved in modulation of the cellular response to VEGF by forming an intracellular complex with VEGFR-2, and mediating its translocation into the nucleus upon VEGF stimulation.

Development of magnetic resonance imaging contrast material for in vivo mapping of tissue transglutaminase activity

Transglutaminases are a family of enzymes that play an important role in tissue remodeling by catalyzing covalent cross-links between proteins of the extracellular matrix. Elevated activity of transglutaminase was shown at the boundaries of invading tumors, in association with angiogenesis, in stabilization of atherosclerotic plaques, and in generation of blood clots. The aim of this work was to develop a low molecular weight substrate of transglutaminase that could serve for noninvasive magnetic resonance and optical mapping of transglutaminase-mediated cross-linking activity. A 2 kDa contrast material was generated which showed cross-linking by either tissue transglutaminase or factor XIII in the context of multicellular tumor spheroids or fibrin clots, respectively. Successful detection by nuclear magnetic resonance microscopy of transglutaminase-mediated cross-linking of the contrast material to MCF7 multicellular spheroids provides hope that this approach could potentially be developed for clinical demarcation of sites of transglutaminase activity.

Atorvastatin induces tissue transglutaminase in human endothelial cells

Tissue transglutaminase (tTgase) contributes to the organisation of the basement membrane and is therefore thought to be important for the integrity and stability of the vessel wall. In the present study, we hypothesised that the HMG-CoA reductase inhibitor atorvastatin may up-regulate the tTgase expression in endothelial cells and thereby exert beneficial effects on endothelial function. Treatment of human umbilical vein endothelial cells (HUVEC) with atorvastatin (1-10 microM) caused a clear increased expression of tTgase in both permeabilised and non-permeabilised HUVEC. In contrast, stimulation of HUVEC with TNFalpha had no substantial effect on tTgase expression or localisation but inhibited the atorvastatin-induced up-regulation and externalisation of tTgase. Propidium iodide staining revealed that statin-induced apoptosis is not responsible for the enhanced expression. By inducing the expression of tTgase, statins may promote tTgase-mediated stabilisation of the basement membrane. This effect of atorvastatin may contribute to the beneficial role of statins on endothelial function.

Identification of tissue transglutaminase as a novel molecule involved in human CD8+ T cell transendothelial migration

During inflammation, T lymphocytes migrate out of the blood across the vascular endothelium in a multistep process. The receptors mediating T cell adhesion to endothelium are well characterized; however, the molecules involved in T cell transendothelial migration (TEM) subsequent to lymphocyte adhesion to the endothelium are less clear. To identify receptors mediating TEM, mAbs were produced against human blood T cells adhering to IFN-gamma-activated HUVEC in mice and tested for inhibition of lymphocyte TEM across cytokine-activated HUVEC. Most of the mAbs were against beta(1) and beta(2) integrins, but one mAb, 6B9, significantly inhibited T cell TEM across IFN-gamma, TNF-alpha, and IFN-gamma plus TNF-alpha-stimulated HUVEC, and did not react with an integrin. 6B9 mAb did not inhibit T cell adhesion to HUVEC, suggesting that 6B9 blocked a novel pathway in T cell TEM. The 6B9 Ag was 80 kDa on SDS-PAGE, and was expressed by both blood leukocytes and HUVEC. Immunoaffinity purification and mass spectrometry identified this Ag as tissue transglutaminase (tTG), a molecule not known to mediate T cell TEM. Treatment of HUVEC with 6B9 was more effective than treatment of T cells. 6B9 blockade selectively inhibited CD4(-), but not CD4(+), T cell TEM, suggesting a role for tTG in recruitment of CD8(+) T lymphocytes. Thus, 6B9 is a new blocking mAb to human tTG, which demonstrates that tTG may have a novel role in mediating CD8(+) T cell migration across cytokine-activated endothelium and infiltration of tissues during inflammation.

Thyroxine stimulates transglutaminase activity in articular chondrocytes

OBJECTIVES

Thyroid hormones induce features of the hypertrophic phenotype in mature articular chondrocytes as well as in growth plate chondrocytes. Hypertrophic chondrocytes are responsible for extracellular matrix mineralization, with formation of bone mineral in growth plate cartilage and pathologic calcium crystals in aging articular cartilage. Elevated activity levels of the two transglutaminase (Tgase) enzymes (type II Tgase and Factor XIIIA (FXIIIA)) have recently been described as additional features of hypertrophic growth plate chondrocytes. Because Tgases may participate in pathologic mineralization in aging cartilage, we explored the effects of thyroid hormones on Tgase activity in articular chondrocytes.

METHODS

Adult porcine articular chondrocytes were incubated with or without 250-750nM L-thyroxine (T4) or 10-100 nM 3,3',5-tri-iodothyronine (T3). Tgase activity was measured with a standard radiometric assay. The effects of thyroid hormones on protein and mRNA levels of type II Tgase and FXIIIA were determined. As Tgase activity can be stimulated by proteases, endoproteinase levels were also measured. The mechanisms of these effects were explored.

RESULTS

T4 (750 nM) or T3 (100 nM) stimulated Tgase activity by twofold in articular chondrocytes at 4h and increased the percentage of Tgase activity in the extracellular matrix. Chondrocytes rapidly converted T4 to T3, but the time course suggests similar mechanisms for T4 and T3. T4-induced Tgase activity was suppressed with cycloheximide and protein kinase C inhibitors. The effects of T4 on type II Tgase and FXIIIA levels were modest, but T4 strongly induced endoproteinase activity in chondrocytes.

CONCLUSIONS

We report in this study that thyroid hormones increase Tgase activity in articular chondrocytes via a non-genomic mechanism, which may involve increased endoproteinase secretion.

Genes potentially involved in plaque rupture

PURPOSE OF REVIEW

Rupture of an atherosclerotic plaque is the predominant underlying event in the pathogenesis of acute coronary syndromes and stroke. While ruptured plaques are morphologically well described, the precise molecular mechanisms involved in plaque rupture are still incompletely understood. Over the last few years, techniques like microarray, suppression subtractive hybridization and differential display enabled us to study complex gene expression profiles that occur during the process of atherogenesis. In this review we focus on recent large-scale gene expression profiles performed on whole mount vascular specimens.

RECENT FINDINGS

The gene expression profiles on whole mount vascular tissue confirmed that at least three mechanisms are involved in plaque rupture: (1) a disturbed balance in extracellular matrix turnover, (2) disturbed regulation of cell turnover and (3) processes involved in lipid metabolism. Animal models exhibiting features of plaque rupture reflect the involvement of these three mechanisms. The most dramatic mouse phenotypes were observed after interventions in at least two of these mechanisms.

SUMMARY

The observation of plaque rupture in recent mice models is indicative of the multifactorial process of plaque rupture. This multifactorial character of plaque rupture suggests that interventions may be most effective when they influence more than one mechanisms at a time.