Transglutaminase 2 Inhibitors Research Articles

Modeling of celiac disease immune response and the therapeutic effect of potential drugs

BackgroundCeliac disease (CD) is an autoimmune disorder that occurs in genetically predisposed people and is caused by a reaction to the gluten protein found in wheat, which leads to intestinal villous atrophy. Currently there is no drug for treatment of CD. The only known treatment is lifelong gluten-free diet. The main aim of this work is to develop a mathematical model of the immune response in CD patients and to predict the efficacy of a transglutaminase-2 (TG-2) inhibitor as a potential drug for treatment of CD.ResultsA thorough analysis of the developed model provided the following results:1. TG-2 inhibitor treatment leads to insignificant decrease in antibody levels, and hence remains higher than in healthy individuals.2. TG-2 inhibitor treatment does not lead to any significant increase in villous area.3. The model predicts that the most effective treatment of CD would be the use of gluten peptide analogs that antagonize the binding of immunogenic gluten peptides to APC. The model predicts that the treatment of CD by such gluten peptide analogs can lead to a decrease in antibody levels to those of normal healthy people, and to a significant increase in villous area.ConclusionsThe developed mathematical model of immune response in CD allows prediction of the efficacy of TG-2 inhibitors and other possible drugs for the treatment of CD: their influence on the intestinal villous area and on the antibody levels. The model also allows to understand what processes in the immune response have the strongest influence on the efficacy of different drugs. This model could be applied in the pharmaceutical R&D arena for the design of drugs against autoimmune small intestine disorders and on the design of their corresponding clinical trials.

Gαh/transglutaminase-2 activity is required for maximal activation of adenylylcyclase 8 in human and rat glioma cells

Gαh (or transglutaminase-2 (TG2)) is an atypical guanine nucleotide binding-protein that associates with G protein-coupled receptors. TG2 also exerts transglutaminase activity that catalyzes posttranslational protein cross-linking with the formation of ε-(γ-glutamyl) lysine or (γ-glutamyl) polyamine bonds. Here, the role of Gαh/TG2 in signal transduction in glial cells was examined in detail. In 1321N1 human astrocytoma cells that lack Gαh/TG2, overexpression of Gαh/TG2 caused an enhancement of cAMP accumulation stimulated with the β-adrenergic receptor agonist, isoproterenol, or the adenylylcyclase activator, forskolin. This cAMP-enhancement was reversed by the TG2 inhibitor, ERW1069. In rat C6 glioma cells that express endogenous Gαh/TG2, cAMP accumulation induced by isoproterenol or forskolin was significantly inhibited by overexpression of Gαh/TG2-C277V, a dominant-negative mutant that lacks transglutaminase activity, but was not inhibited by the Gαh/TG2-S171E mutant that cannot bind GTP/GDP. These results suggest Gαh/TG2 potentiates adenylylcyclase activity by its transglutaminase activity and not by its G-protein activity. Gαh/TG2 also increased the activities of the cAMP response element and interleukin-6 promoter, accompanied by an of cAMP in both glioma cells. Since adenylylcyclase 8 plays a major role in cAMP production, we focused on post-translational modification of adenylylcyclase 8 by Gαh/TG2. Adenylylcyclase 8 is expressed in both 1321N1 and C6 cells; however, Gαh/TG2 affected neither adenylylcyclase 8 expression levels, glycosylation, nor dimerization status. In contrast, pentylamine, a substrate of Gαh/TG2, was incorporated into adenylylcyclase 8 in a transglutaminase activity-dependent manner. Taking these results together, Gαh/TG2 promotes cAMP production accompanied by a modification of adenylylcyclase 8 in glioma cells.

Targeting autophagy as a novel strategy for facilitating the therapeutic action of potentiators on ΔF508 cystic fibrosis transmembrane conductance regulator

Channel activators (potentiators) of cystic fibrosis (CF) transmembrane conductance regulator (CFTR), can be used for the treatment of the small subset of CF patients that carry plasma membrane-resident CFTR mutants. However, approximately 90% of CF patients carry the misfolded ΔF508-CFTR and are poorly responsive to potentiators, because ΔF508-CFTR is intrinsically unstable at the plasma membrane (PM) even if rescued by pharmacological correctors. We have demonstrated that human and mouse CF airways are autophagy deficient due to functional sequestration of BECN1 and that the tissue transglutaminase-2 inhibitor, cystamine, or antioxidants restore BECN1-dependent autophagy and reduce SQSTM1/p62 levels, thus favoring ΔF508-CFTR trafficking to the epithelial surface. Here, we investigated whether these treatments could facilitate the beneficial action of potentiators on ΔF508-CFTR homozygous airways. Cystamine or the superoxide dismutase (SOD)/catalase-mimetic EUK-134 stabilized ΔF508-CFTR at the plasma membrane of airway epithelial cells and sustained the expression of CFTR at the epithelial surface well beyond drug withdrawal, overexpressing BECN1 and depleting SQSTM1. This facilitates the beneficial action of potentiators in controlling inflammation in ex vivo ΔF508-CFTR homozygous human nasal biopsies and in vivo in mouse ΔF508-CFTR lungs. Direct depletion of Sqstm1 by shRNAs in vivo in ΔF508-CFTR mice synergized with potentiators in sustaining surface CFTR expression and suppressing inflammation. Cystamine pre-treatment restored ΔF508-CFTR response to the CFTR potentiators genistein, Vrx-532 or Vrx-770 in freshly isolated brushed nasal epithelial cells from ΔF508-CFTR homozygous patients. These findings delineate a novel therapeutic strategy for the treatment of CF patients with the ΔF508-CFTR mutation in which patients are first treated with cystamine and subsequently pulsed with CFTR potentiators.

Transglutaminase 2 exacerbates experimental autoimmune encephalomyelitis through positive regulation of encephalitogenic T cell differentiation and inflammation

The increased activity of transglutaminase 2 (TG2) in various inflammatory and fibrotic conditions results in the development of numerous disease processes. Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, is an inflammatory and demyelinating disease of the central nervous system and is mediated by many inflammatory cytokines and mediators. We examined the role of TG2 in encephalitogenic CD4(+) T cell responses and EAE development using mice lacking TG2 (TG2(-/-)). TG2(-/-) mice showed decreased disease severity as compared with wild-type (WT) mice. Treatment with cysteamine, a TG2 inhibitor, ameliorated disease severity in WT mice. Exacerbated disability in WT mice resulted from the increased infiltration of cytokine-producing CD4(+) T cells and sustained expression of inflammatory cytokines and mediators in the lesion. The increased number of IL-17- and IFN-γ-producing cells in the spinal cord resulted from peripheral expansion of these cells after immunization with myelin-derived antigen. In vitro differentiation of WT and TG2(-/-) splenocytes revealed that proliferation and activation-induced cell death did not differ, but differentiation into IL-17- or IFN-γ-producing cells was increased in WT mice. Adoptive transfer experiments revealed that pathogenic CD4(+) T cell differentiation and disease progression were caused by both the T cell-intrinsic and -extrinsic effects of TG2. This study is the first to report a pathogenic role for TG2 in the EAE progress and suggests that therapeutic targeting of TG2 may be effective against multiple sclerosis.

Are Transglutaminase 2 Inhibitors Able to Reduce Gliadin-Induced Toxicity Related to Celiac Disease? A Proof-of-Concept Study

Celiac disease is an autoimmune-mediated enteropathy characterized by adaptive and innate immune responses to dietary gluten in wheat, rye and barley in genetically susceptible individuals. Gluten-derived gliadin peptides are deamidated by transglutaminase 2 (TG2), leading to an immune response in the small-intestinal mucosa. TG2 inhibitors have therefore been suggested as putative drugs for celiac disease. In this proof-of-concept study we investigated whether two TG2 inhibitors, cell-impermeable R281 and cell-permeable R283, can prevent the toxic effects of gliadin in vitro and ex vivo. Intestinal epithelial Caco-2 cells were treated with peptic-tryptic-digested gliadin (PT-gliadin) with or without TG2 inhibitors and thereafter direct toxic effects (transepithelial resistance, cytoskeletal rearrangement, junction protein expression and phoshorylation of extracellular-signal-regulated kinase 1/2) were determined. In an organ culture of celiac-patient-derived small-intestinal biopsies we measured secretion of TG2-autoantibodies into the culture medium and the densities of CD25- and interleukin (IL) 15-positive cells, forkhead box P3 (FOXP3)-positive regulatory T cells (Tregs) and Ki-67-positive proliferating crypt cells. Both TG2 inhibitors evinced protective effects against gliadin-induced detrimental effects in Caco-2 cells but the cell-impermeable R281 seemed slightly more potent. In addition, TG2 inhibitor R281 modified the gluten-induced increase in CD25- and IL15-positive cells, Tregs and crypt cell proliferation, but had no effect on antibody secretion in celiac-patient-derived biopsies. Our results suggest that TG2 inhibitors are able to reduce certain gliadin-induced effects related to responses in vitro and ex vivo.

Cysteamine Prevents the Development of Lens Opacity in a Rat Model of Selenite-Induced Cataract

The activation of transglutaminase 2 (TG2) by oxidative stress through TGFβ has been reported to play a crucial role in cataract formation. The authors investigated whether TG2 is involved in selenite-induced cataract formation in rats and whether cysteamine, a chemical inhibitor of TG2, can prevent cataract formation in this model. Intracellular TG2 activity was monitored in a human lens epithelial cell (HLE-B3) line and cultured rat lenses after treatment with selenite. Rat pups (13 days old) were injected subcutaneously with sodium selenite (Na(2)SeO(3); 20 μmol/kg) and intraperitoneally with cysteamine (30, 40, and 60 mg/kg) for 14 days. Lenses were evaluated photographically at days 7 and 14. The concentrations of malondialdehyde and glutathione in the lenses were determined. In HLE-B3 cells or rat lenses, selenite induced intracellular TG activity, which was inhibited by cysteamine. In selenite-treated rats, the rate of cataract formation was significantly reduced by cysteamine (P < 0.001). The mean cataract area in the lenses of cysteamine-treated rats was smaller than that of control rats (P < 0.01). The levels of total and reduced glutathione in the lenses of cysteamine-treated rats extracted at day 14 were higher than those of control rats. Cysteamine suppresses cataract formation induced by selenite in rats, suggesting that cysteamine can be used as a pharmaceutical intervention to prevent or delay cataract formation.

Activation and Inhibition of Transglutaminase 2 in Mice

Transglutaminase 2 (TG2) is an allosterically regulated enzyme with transamidating, deamidating and cell signaling activities. It is thought to catalyze sequence-specific deamidation of dietary gluten peptides in the small intestines of celiac disease patients. Because this modification has profound consequences for disease pathogenesis, there is considerable interest in the design of small molecule TG2 inhibitors. Although many classes of TG2 inhibitors have been reported, thus far an animal model for screening them to identify promising celiac drug candidates has remained elusive. Using intraperitoneal administration of the toll-like receptor 3 (TLR3) ligand, polyinosinic-polycytidylic acid (poly(I∶C)), we induced rapid TG2 activation in the mouse small intestine. Dose dependence was observed in the activation of TG2 as well as the associated villous atrophy, gross clinical response, and rise in serum concentration of the IL-15/IL-15R complex. TG2 activity was most pronounced in the upper small intestine. No evidence of TG2 activation was observed in the lung mucosa, nor were TLR7/8 ligands able to elicit an analogous response. Introduction of ERW1041E, a small molecule TG2 inhibitor, in this mouse model resulted in TG2 inhibition in the small intestine. TG2 inhibition had no effect on villous atrophy, suggesting that activation of this enzyme is a consequence, rather than a cause, of poly(I∶C) induced enteropathy. Consistent with this finding, administration of poly(I∶C) to TG2 knockout mice also induced villous atrophy. Our findings pave the way for pharmacological evaluation of small molecule TG2 inhibitors as drug candidates for celiac disease.

Bcr is a substrate for Transglutaminase 2 cross-linking activity

BackgroundBreakpoint cluster region (Bcr) is a multi-domain protein that contains a C-terminal GTPase activating protein (GAP) domain for Rac. Transglutaminase 2 (TG2) regulates Bcr by direct binding to its GAP domain. Since TG2 has transglutaminase activity that has been implicated in the response to extreme stress, we investigated if Bcr can also act as a substrate for TG2.ResultsWe here report that activation of TG2 by calcium caused the formation of covalently cross-linked Bcr. Abr, a protein related to Bcr but lacking its N-terminal oligomerization domain, was not cross-linked by TG2 even though it forms a complex with it. A Bcr mutant missing the first 62 amino acid residues remained monomeric in the presence of activated TG2, showing that this specific domain is necessary for the cross-linking reaction. Calcium influx induced by a calcium ionophore in primary human endothelial cells caused cross-linking of endogenous Bcr, which was inhibited by the TG2 inhibitor cystamine. Treatment of cells with cobalt chloride, a hypoxia-mimetic that causes cellular stress, also generated high molecular weight Bcr complexes. Cross-linked Bcr protein appeared in the TritonX-100-insoluble cell fraction and further accumulated in cells treated with a proteasome inhibitor.ConclusionsBcr thus represents both an interacting partner under non-stressed conditions and a target of transglutaminase activity for TG2 during extreme stress.

Cytosolic Guanine Nucledotide Binding Deficient Form of Transglutaminase 2 (R580a) Potentiates Cell Death in Oxygen Glucose Deprivation

Transglutaminase 2 (TG2) is a hypoxia-responsive protein that is a calcium-activated transamidating enzyme, a GTPase and a scaffolding/linker protein. Upon activation TG2 undergoes a large conformational change, which likely affects not only its enzymatic activities but its non-catalytic functions as well. The focus of this study was on the role of transamidating activity, conformation and localization of TG2 in ischemic cell death. Cells expressing a GTP binding deficient form of TG2 (TG2-R580A) with high basal transamidation activity and a more extended conformation showed significantly increased cell death in response to oxygen-glucose deprivation; however, targeting TG2-R580A to the nucleus abrogated its detrimental role in oxygen-glucose deprivation. Treatment of cells expressing wild type TG2, TG2-C277S (a transamidating inactive mutant) and TG2-R580A with Cp4d, a reversible TG2 inhibitor, did not affect cell death in response to oxygen-glucose deprivation. These findings indicate that the pro-cell death effects of TG2 are dependent on its localization to the cytosol and independent of its transamidation activity. Further, the conformational state of TG2 is likely an important determinant in cell survival and the prominent function of TG2 in ischemic cell death is as a scaffold to modulate cellular processes.

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Programmed cell death, apoptosis, is a highly regulated process that removes damaged or unwanted cells in vivo and has significant immunological implications. Defective clearance of apoptotic cells by macrophages (professional phagocytes) is known to result in chronic inflammatory and autoimmune disease. Transglutaminase-2 (TG2) is a Ca2+-dependent protein crosslinking enzyme known to play an important role in apoptotic cell clearance by macrophages through an ill-defined mechanism. Several studies have implicated TG2 in the apoptosis programme e.g. raised TG2 levels in cells undergoing apoptosis; increased cell death with down-regulation of TG2; up-regulation of TG2 in monocytes upon differentiation into macrophages. Defective clearance of apoptotic cells by TG2 null mice has been described though in this context the role of TG2 is yet to be elucidated. Here we aim to characterise the role of TG2 in macrophage function with a focus on apoptotic cell clearance. THP-1 monocytes were induced to differentiate to macrophage-like cells by three different stimulants and were analysed for the presence of TG2. Macrophage-apoptotic cell interaction studies in the presence and absence of irreversible TG2 inhibitors resulted in significant inhibition of interaction indicating a possible role for TG2 in apoptotic cell clearance. TG2 was expressed at the macrophage cell surface and its association with s3 integrin expression suggests the possible link between TG2 and s3 integrins. Our current findings suggest that TG2 had got a crucial but yet to be defined role in apoptotic cell clearance.

Inhibition of transglutaminase 2 mitigates transcriptional dysregulation in models of Huntington disease

Caused by a polyglutamine expansion in the huntingtin protein, Huntington's disease leads to striatal degeneration via the transcriptional dysregulation of a number of genes, including those involved in mitochondrial biogenesis. Here we show that transglutaminase 2, which is upregulated in HD, exacerbates transcriptional dysregulation by acting as a selective corepressor of nuclear genes; transglutaminase 2 interacts directly with histone H3 in the nucleus. In a cellular model of HD, transglutaminase inhibition de-repressed two established regulators of mitochondrial function, PGC-1α and cytochrome c and reversed susceptibility of human HD cells to the mitochondrial toxin, 3-nitroproprionic acid; however, protection mediated by transglutaminase inhibition was not associated with improved mitochondrial bioenergetics. A gene microarray analysis indicated that transglutaminase inhibition normalized expression of not only mitochondrial genes but also 40% of genes that are dysregulated in HD striatal neurons, including chaperone and histone genes. Moreover, transglutaminase inhibition attenuated degeneration in a Drosophila model of HD and protected mouse HD striatal neurons from excitotoxicity. Altogether these findings demonstrate that selective TG inhibition broadly corrects transcriptional dysregulation in HD and defines a novel HDAC-independent epigenetic strategy for treating neurodegeneration.

A Profiling Platform for the Characterization of Transglutaminase 2 (TG2) Inhibitors

Huntington's disease (HD) is associated with increased expression levels and activity of tissue transglutaminase (TG2), an enzyme primarily known for its cross-linking of proteins. To validate TG2 as a therapeutic target for HD in transgenic models and for eventual clinical development, a selective and brain-permeable inhibitor is required. Here, a comprehensive profiling platform of biochemical and cellular assays is presented which has been established to evaluate the potency, cellular efficacy, subtype selectivity and the mechanism-of-action of known and novel TG2 inhibitors. Several classes of inhibitors have been characterized including: the commonly used pseudo-substrate inhibitors, cystamine and putrescine (which are generally nonspecific for TG2 and therefore not practical for drug development), the various peptidic inhibitors that target the active site cysteine residue (which display excellent selectivity but in general have poor cellular activity), and the allosteric reversible small-molecule hydrazides (which show poor selectivity and a lack of cellular activity and could not be improved despite considerable medicinal chemistry efforts). In addition, a set of inhibitors identified from a collection of pharmacologically active compounds was found to be unselective for TG2. Moreover, inhibition at the guanosine triphosphate binding site has been examined, but apart from guanine nucleotides, no such inhibitors have been identified. In addition, the promising pharmacological profile of a TG2 inhibitor is presented which is currently in lead optimization to be developed as a tool compound.

ANGIOTENSIN II-INDUCED TRANSGLUTAMINASE 2 REGULATES P-38 MAP-KINASE LONG TERM EXPRESSION IN SHR VASCULAR SMOOTH MUSCLE CELLS: 2C.02

Objective: Transglutaminase 2 (TG2) is ubiquitously expressed and induces transamidation and cross-linking of proteins. This process is important for cell-matrix interactions and vascular remodeling, and is modulated by angiotensin II (Ang II). We hypothesized that Ang II through the angiotensin type 1 receptor (AT1R) modulates TG2 expression, which differentially regulates MAP kinase expression in rat vascular smooth muscle cells (VSMCs). Methods: Mesenteric artery-derived VSMCs from Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) were studied. Cells were stimulated with Ang II (100 nM) ± the AT1R blocker valsartan (Val 10 μM) or ± the TG2 inhibitor cystamine (CYS 10 μM). Expression of TG2, p38 MAPK, AT1R and AT2R was evaluated by immunoblotting. Results: Basal TG2 expression in SHR VSMCs was significantly higher compared to basal TG2 expression in WKY VSMCs (+28 ± 12%, p < 0.05). Ang II significantly increased TG2 expression only in SHR VSMCs compared to vehicle (+57 ± 19%, p < 0.05). Val significantly reduced Ang II-induced TG2 expression in SHR VSMCs. Basal AT1R expression in cells from WKY was lower compared to SHR (1.15 ± 0.2 AU vs 1.8 ± 0.1 AU, p < 0.05). Basal AT2R expression was 2-fold higher in SHR cells than in WKY (p < 0.05). Ang II had no effect on AT1R expression in either WKY or SHR. In presence of Ang II, AT2R expression was significantly increased in WKY and reduced in SHR (224.6 ± 65.13% vs 47.87 ± 11.95%, p < 0.05). After one-hour stimulation, Ang II increased p38 MAPK expression only in SHR (3-fold increase vs vehicle, p < 0.05). Ang II-induced p38 MAPK expression was significantly reduced by CYS. Conclusions: Ang II positively regulates TG2 expression in rat VSMCs via AT1R. TG2 plays a role in Ang II-induced p38 MAPK long-term production. These effects are particularly evident in SHR VSMCs which presented high basal levels of Ang II receptors and TG2.

Inhibition of transglutaminase 2 enzymatic activity ameliorates the anti-angiogenic effects of coeliac disease autoantibodies

Objective. Earlier work has demonstrated that serum autoantibodies from coeliac patients targeted against transglutaminase 2 (TG2) inhibit in vitro angiogenesis. The aim of this study was to establish whether coeliac patient-derived monoclonal TG2-targeted antibodies produced by recombination technology exert similar anti-angiogenic effects to serum-derived coeliac autoantibodies. In addition, we studied whether the monoclonal patient autoantibodies modulate endothelial cell TG2 activity and whether such modulation is related to the anti-angiogenic effects. Material and methods. The influence of coeliac patient-derived monoclonal TG2-targeted antibodies on endothelial cell tubule formation was studied using a three-dimensional angiogenic cell culture model. Endothelial cell TG2 enzymatic activity was determined by means of a live-cell enzyme-linked immunosorbent assay. Results. Coeliac patient-derived monoclonal TG2-targeted antibodies produced by recombination technology inhibited endothelial tubule formation and enhanced the crosslinking activity of TG2. When this enzymatic activity was inhibited using site-directed irreversible TG2 inhibitors in the presence of autoantibodies, in vitro angiogenesis reverted to the control level. Conclusions. Since we found a significant negative correlation between endothelial cell angiogenesis and TG2 activity, we suggest that the anti-angiogenic effects of coeliac patient-derived TG2-targeted autoantibodies are exerted by enhanced enzymatic activity of TG2.

The Role of Transglutaminase-2 in Fibroproliferation after Lipopolysaccharide-induced Acute Lung Injury

Background: Transglutaminase-2 (TG-2) has been reported to play an important role in the process of fibrosis. However, TG-2 studies on fibroproliferation of acute lung injury (ALI) are absent. The purpose of this study was to investigate the role of TG-2 in the fibroproliferation of lipopolysaccharide (LPS)-induced ALI. Methods: The male C57BL/6 mice of 5 weeks age were divided into 3 groups; control group (n=30) in which 50 μL of saline was given intratracheally (IT), LPS group (n=30) in which LPS 0.5 mg/kg/50 μL of saline was given IT, and LPS＋Cyst group treated with intraperitoneal 200 mg/kg of cystamine, competitive inhibitor of TG-2, after induction of ALI by LPS. TG-2 activity and nuclear factor (NF)-κB were measured in lung tissue homogenate. Tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, myeloperoxidase (MPO), and transforming growth factor (TGF)-β1 were measured using bronchoalveolar lavage fluids. Histopathologic ALI score and Mallory`s phosphotunistic acid hematoxylin (PTAH) for collagen and fibronectin deposition were performed. Results: The TG-2 activities in the LPS group were significantly higher than the control and LPS＋Cyst groups (p＜0.05). The TNF-α and IL-1β concentrations and NF-κB activity were lower in the LPS＋Cyst group than the LPS group (p＜0.05). The LPS＋Cyst group showed lower MPO, ALI score, TGF-β1 concentration, and Mallory`s PTAH stain than the LPS group, but the differences were not significant (p＞0.05). Conclusion: Inhibition of TG-2 activity in the LPS-induced ALI prevented early inflammatory parameters, but had limited effects on late ALI and fibroproliferative parameters.

Transglutaminase 2 suppresses apoptosis by modulating caspase 3 and NF-κB activity in hypoxic tumor cells

The expression of hypoxia-inducible factor-1 (HIF-1) correlates with poor clinical outcomes and confers resistance to the apoptosis of the tumor cells that are exposed to hypoxia. Presently, the mechanism underlying this phenomenon is poorly understood. In this study we provide evidence that transglutaminase 2 (TG2), an enzyme that catalyses protein crosslinking reactions, is a transcriptional target of HIF-1 to enhance the survival of hypoxic cells. We found that hypoxia induces TG2 expression through an HIF-1 dependent pathway and concurrently activates intracellular TG2. The hypoxic cells overexpressing TG2 showed resistance to apoptosis. Conversely, the hypoxic cells treated with either TG2 inhibitor or small interfering RNA (siRNA) became sensitive to apoptosis. Activation of TG2 in response to hypoxic stress inhibited caspase-3 activity by forming crosslinked multimer, resulting in insoluble aggregates. TG2 also activates nuclear factor (NF)-kappaB pathway after hypoxic stress, and thereby induces the expression of cellular inhibitor of apoptosis 2. The anti-apoptotic role of TG2 was further confirmed in vivo using xenografts in athymic mice. Our results indicate that TG2 is an anti-apoptotic mediator of HIF-1 through modulating both apoptosis and survival pathways and may confer a selective growth advantage to tumor cells. These findings suggest that the inhibition of TG2 may offer a novel strategy for anticancer therapy.

Transglutaminase down‐regulates the dimerization of epidermal growth factor receptor in rat perivenous and periportal hepatocytes

Recently, we found that transglutaminase 2 (TG2) might be involved in the difference in proliferative capacities between periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) through down-regulation of high-affinity epidermal growth factor receptor (EGFR). However, it is uncertain whether this high-affinity EGFR contributes to the hepatocyte growth signalling pathway. Here, we have investigated the influence of TG2 on EGF-induced EGFR dimerization and its phosphorylation, which are important steps in the hepatocyte proliferative/growth signalling pathway, in PPH and PVH. PPH and PVH were isolated using the digitonin/collagenase perfusion technique. Amounts of TG2, EGFR dimerization and its phosphorylation were determined by Western blot analysis. Pretreatment with monodansylcadaverine, an inhibitor of TG2, greatly increased EGF-induced EGFR dimerization and its phosphorylation in PVH compared with PPH. Conversely, treatment with retinoic acid, an inducer of TG2, significantly decreased EGF-induced EGFR dimerization and its phosphorylation with a significant increase in TG2 expression and its catalysed products, isopeptide bonds, in both subpopulations. It was found that EGFR served as a substrate for TG2. The present data showed good correlation with our previous data on EGF-induced DNA synthesis and EGFR-binding affinity to EGF. These results suggest that zonal difference in cell growth between PPH and PVH may be caused by down-regulation of EGFR dimerization and subsequent autophosphorylation through TG2-mediated cross-linking of EGFR.

Role of Transglutaminase 2 in Liver Injury via Cross-linking and Silencing of Transcription Factor Sp1

Despite high morbidity and mortality of alcoholic liver disease worldwide, the molecular mechanisms underlying alcohol-induced liver cell death are not fully understood. Transglutaminase 2 (TG2) is a cross-linking enzyme implicated in apoptosis. TG2 levels and activity are increased in association with various types of liver injury. However, how TG2 induces hepatic apoptosis is not known. Human hepatic cells or primary hepatocytes from rats or TG2+/+ and TG2-/- mice were treated with ethanol. Mice were administered anti-Fas antibody or alcohol. Liver sections were prepared from patients with alcoholic steatohepatitis. Changes in TG2 levels, Sp1 cross-linking and its activities, expression of hepatocyte growth factor receptor, c-Met, and hepatic apoptosis were measured. Ethanol induced apoptosis in hepatic cells, enhanced activity and nuclear accumulation of TG2 as well as accumulation of cross-linked and inactivated Sp1, and reduced expression of the Sp1-responsive gene, c-Met. These effects were rescued by TG2 knockdown, restoration of functional Sp1, or addition of hepatocyte growth factor, whereas apoptosis was reproduced by Sp1 knockdown or TG2 overexpression. Compared with TG2+/+ mice, TG2-/- mice showed markedly reduced hepatocyte apoptosis and Sp1 cross-linking following ethanol or anti-Fas treatment. Treatment of TG2+/+ mice with the TG2 inhibitors putrescine or cystamine blocked anti-Fas-induced hepatic apoptosis and Sp1 silencing. Moreover, enhanced expression of cross-linked Sp1 and TG2 was evident in hepatocyte nuclei of patients with alcoholic steatohepatitis. TG2 induces hepatocyte apoptosis via Sp1 cross-linking and inactivation, with resultant inhibition of the expression of c-Met required for hepatic cell viability.

1002 Role of Intestinal Barrier Modulation By Non-Steroidal Anti-Inflammatory Drugs in Gluten-Induced Epithelial Damage in HLA-Dq8/Hcd4 Transgenic Mice

Background: Transglutaminase 2 (TG2) is a multifunctional protein with transamidase as well as signaling activities. Its aberrant catalytic activity has been implicated in the pathogenesis of celiac sprue. However, the physiological role of TG2 and the mechanisms for its In Vivo activation in the small intestine are largely unknown. Aims: To study the TG2 catalytic activity under basal conditions and to identify signals that induce TG2 activity. Methods: We used selective TG2 inhibitors and tagged nucleophilic substrates to monitor the In Vivo TG2 activity during experimental conditions such as fibroblast scratch assay or methotrexate and polyinosinic-polycytidylic acid (poly(I:C)) administration in mice. Results: The majority of TG2 is inactive under normal physiological conditions in cell culture and In Vivo. However, abundant TG2 activity can be detected around the wound in a standard cultured fibroblast scratch assay. To induce tissue injury model in mice, we used methotrexate and poly(I:C), since they trigger small intestinal injury characterized by villous atrophy similar to celiac sprue. The acute poly(I:C) injury model, but not the methotrexate model, resulted in activation of the small intestinal TG2 and in cellular protein crosslinking. Conclusions: Extracellular TG2 is predominantly inactive under physiological conditions. It is activated upon tissue injury or in response to innate immune signals such us poly (I:C). Our results illustrate a new tool to investigate the biology of TG2 in the small intestine. They also provide convenient assays for evaluating small molecule inhibitors of extracellular TG2 activity In Vitro and In Vivo.

Extracellular Transglutaminase 2 Is Catalytically Inactive, but Is Transiently Activated upon Tissue Injury

Transglutaminase 2 (TG2) is a multifunctional mammalian protein with transamidase and signaling properties. Using selective TG2 inhibitors and tagged nucleophilic amine substrates, we show that the majority of extracellular TG2 is inactive under normal physiological conditions in cell culture and in vivo. However, abundant TG2 activity was detected around the wound in a standard cultured fibroblast scratch assay. To demonstrate wounding-induced activation of TG2 in vivo, the toll-like receptor 3 ligand, polyinosinic-polycytidylic acid (poly(I:C)), was injected in mice to trigger small intestinal injury. Although no TG2 activity was detected in vehicle-treated mice, acute poly(I:C) injury resulted in rapid TG2 activation in the small intestinal mucosa. Our findings provide a new basis for understanding the role of TG2 in physiology and disease.