Gh: a GTP-binding protein with transglutaminase activity and receptor signaling function

The diamond anniversary of tissue transglutaminase: a protein of many talents

Tissue transglutaminase (tTG) is capable of binding and hydrolyzing GTP, as well as catalyzing an enzymatic transamidation reaction that crosslinks primary amines to glutamine residues. tTG adopts two vastly different conformations, depending on whether it is functioning as a GTP-binding protein or a crosslinking enzyme. It has been shown to have important roles in several different aspects of cancer progression, making it an attractive target for therapeutic intervention. Here, we highlight many of the major findings involving tTG since its discovery 60 years ago, and describe recent drug discovery efforts that target specific activities or conformations of this unique protein.

Induction of GD3/α1-adrenergic receptor/transglutaminase 2-mediated erythroid differentiation in chronic myelogenous leukemic K562 cells

The disialic acid-containing glycosphingolipid GD3 recruited membrane transglutaminase 2 (TG2) as a signaling molecule for erythroid differentiation in human chronic myelogenous leukemia (CML) K562 cells. The α1-adrenergic receptor (α1-AR)/TG2-mediated signaling pathway regulated GD3 functions, including gene expression and production, to differentiate CML K562 cells into erythroid lineage cells. Epinephrine, an AR agonist, increased membrane recruitment as well as GTP-photoaffinity of TG2, inducing GD3 synthase gene expression. Epinephrine activated PI3K/Akt signaling and GTPase downstream of TG2 activated Akt. The coupling of TG2 and GD3 production was specifically suppressed by prazosin (α1-AR antagonist), but not by propranolol (β-AR antagonist) or rauwolscine (α2-AR antagonist), indicating α1-AR specificity. Small interfering RNA (siRNA) experiment results indicated that the α1-AR/TG2-mediated signaling pathway activated PKCs α and δ to induce GD3 synthase gene expression. Transcription factors CREB, AP-1, and NF-κB regulated GD3 synthase gene expression during α1-AR-induced differentiation in CML K562 cells. In addition, GD3 synthase gene expression was upregulated in TG2-transfected cells via α1-AR with expression of erythroid lineage markers and benzidine-positive staining. α1-AR/TG2 signaling pathway-directed GD3 production is a crucial step in erythroid differentiation of K562 cells and GD3 interacts with α1-AR/TG2, inducing GD3/α1-AR/TG2-mediated erythroid differentiation. These results suggest that GD3, which acts as a membrane mediator of erythroid differentiation in CML cells, provides a therapeutic avenue for leukemia treatment.

Transglutaminase 2 regulates osteoclast differentiation via a Blimp1-dependent pathway

Transglutaminase 2 (TG2) performs multiple reactions, including transamidation, and also plays a role in signal transduction as a GTP-binding protein. In this study, we reveal that TG2 controls osteoclast differentiation and bone homeostasis in mice. Osteoclasts specifically expressed the TG2 isoform among eight TG family members. Suppression in TG2 expression with siRNA led to increased osteoclast formation from primary mouse precursor cells in response to receptor activator of nuclear factor kappaB ligand (RANKL). This osteoclastogenic effect of TG2 knockdown was associated with enhanced induction of c-Fos and NFATc1 by RANKL. Moreover, TG2 knockdown up-regulated B lymphocyte-induced maturation protein 1 (Blimp1), which represses anti-osteoclastogenic genes, in a manner dependent on the NF-κB signaling pathway. To the contrary, TG2 overexpression inhibited osteoclast formation and the expression of osteoclastogenic genes. Consistent with these in vitro results, TG2 knockout mice exhibited lower trabecular bone mass and increased number of osteoclasts compared with wild-type mice. Taken together, our results provide strong evidence that TG2 plays an important role in bone metabolism by suppressing excessive osteoclastogenesis via the regulation of the NF-κB-Blimp1 signaling pathway.

Browning deficiency and low mobilization of fatty acids in gonadal white adipose tissue leads to decreased cold-tolerance of transglutaminase 2 knock-out mice

During cold-exposure 'beige' adipocytes with increased mitochondrial content are activated in white adipose tissue (WAT). These cells, similarly to brown adipose tissue (BAT), dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). We investigated the effect of tissue transglutaminase (TG2) ablation on the function of ATs in mice. Although TG2^+/+ and TG2^-/- mice had the same amount of WAT and BAT, we found that TG2^+/+ animals could tolerate acute cold exposure for 4h, whereas TG2^-/- mice only for 3h. Both TG2^-/- and TG2^+/+ animals used up half of the triacylglycerol content of subcutaneous WAT (SCAT) after 3h treatment; however, TG2^-/- mice still possessed markedly whiter and higher amount of gonadal WAT (GONAT) as reflected in the larger size of adipocytes and lower free fatty acid levels in serum. Furthermore, lower expression of 'beige' marker genes such as UCP1, TBX1 and TNFRFS9 was observed after cold exposure in GONAT of TG2^-/- mice, paralleled with a lower level of UCP1 protein and a decreased mitochondrial content. The detected changes in gene expression of Resistin and Adiponectin did not provoke glucose intolerance in the investigated TG2^-/- mice, and TG2 deletion did not influence adrenaline, noradrenaline, glucagon and insulin production. Our data suggest that TG2 has a tissue-specific role in GONAT function and browning, which becomes apparent under acute cold exposure.

Atorvastatin has a protective effect in a mouse model of bronchial asthma through regulating tissue transglutaminase and triggering receptor expressed on myeloid cells-1 expression

Airway remodeling in asthma contributes to airway hyperreactivity, loss of lung function and persistent symptoms. Current therapies do not adequately treat the structural airway changes associated with asthma. Statin drugs have improved respiratory health and their therapeutic potential in asthma has been tested in clinical trials. However, the mechanism of action of statins in this context has remained elusive. The present study hypothesized that atorvastatin treatment of ovalbumin-exposed mice attenuates early features of airway remodeling via a mevalonate-dependent mechanism. BALB/c mice were sensitized with ovalbumin and atorvastatin was delivered via oral gavage prior to each ovalbumin exposure. Reverse transcription-semi-quantitative polymerase chain reaction (RT-semi-qPCR), ELISA and western blot analysis were used to assess the expression of a number of relevant genes, including tissue transglutaminase (tTG), triggering receptor expressed on myeloid cells (TREM)-1, nuclear factor erythroid 2-related factor (Nrf) 2, hypoxia-inducible factor (HIF)-1α, transforming growth factor (TGF)-β1, matrix metalloproteinase (MMP)-9 and tissue inhibitors of metalloproteinases (TIMP)-1 in lung tissue. α-Smooth muscle actin (α-SMA) activity was measured by immunohistochemistry. Airway hyperresponsiveness, lung collagen deposition, airway wall area, airway smooth muscle thickness and lung pathology were also assessed. Atorvastatin treatment led to downregulation of tTG and TREM-1 expression in lung tissue after ovalbumin sensitization, blocked the activity of MMP-9, vascular endothelial growth factor, nuclear factor-κB p65, α-SMA, HIF-α and TGF-β1 and up-regulated Nrf2 expression. Furthermore, the number of lymphocytes and eosinophils in the atorvastatin group was significantly lower than that in the control group. In addition, airway hyperresponsiveness, lung collagen deposition, airway wall area, airway smooth muscle thickness and pathological changes in the lung were significantly decreased in the atorvastatin group, and tumor necrosis factor-α, interleukin (IL)-8, IL-13 and IL-17 in serum were significantly decreased. Histological results demonstrated the attenuating effect of atorvastatin on ovalbumin-induced airway remodeling in asthma. In conclusion, the present study indicated that atorvastatin significantly alleviated ovalbumin-induced airway remodeling in asthma by downregulating tTG and TREM-1 expression. The marked protective effects of atorvastatin suggest its therapeutic potential in ovalbumin-induced airway remodeling in asthma treatment.

Transamidase site-targeted agents alter the conformation of the transglutaminase cancer stem cell survival protein to reduce GTP binding activity and cancer stem cell survival

Type 2 transglutaminase (TG2) is an important cancer stem cell survival protein that exists in open and closed conformations. The major intracellular form is the closed conformation that functions as a GTP-binding GTPase and is required for cancer stem cell survival. However, at a finite rate, TG2 transitions to an open conformation that exposes the transamidase catalytic site involved in protein-protein crosslinking. The activities are mutually exclusive, as the closed conformation has GTP binding/GTPase activity, and the open conformation transamidase activity. We recently showed that GTP binding, but not transamidase activity, is required for TG2-dependent cancer stem cell invasion, migration and tumour formation. However, we were surprised that transamidase site-specific inhibitors reduce cancer stem cell survival. We now show that compounds NC9, VA4 and VA5, which react exclusively at the TG2 transamidase site, inhibit both transamidase and GTP-binding activities. Transamidase activity is inhibited by direct inhibitor binding at the transamidase site, and GTP binding is blocked because inhibitor interaction at the transamidase site locks the protein in the extended/open conformation to disorganize/inactivate the GTP binding/GTPase site. These findings suggest that transamidase site-specific inhibitors can inhibit GTP binding/signalling by driving a conformation change that disorganizes the TG2 GTP binding to reduce TG2-dependent signalling, and that drugs designed to target this site may be potent anti-cancer agents.

Tissue transglutaminase-2 promotes gastric cancer progression via the ERK1/2 pathway

Gastric cancer (GC) is one of the most common tumors worldwide and involves extensive local tumor invasion, metastasis, and poor prognosis. Understanding mechanisms regulating progression of GC is necessary for developing effective therapeutic strategies. Tissue transglutaminase-2 (TG2), a multifunctional member of the transglutaminase family, has been shown to be critical for tumor initiation and progression. However, how TG2 promotes the progression of GC is unknown. We report that TG2 was highly expressed in GC tissues and positively associated with depth of tumor invasion and late TNM stage. With gain- and loss-of-function approaches, we observed that TG2 promoted GC cell proliferation, migration, invasion, as well as tumorigenesis and peritoneal metastasis in vivo. These events were associated with the ERK1/2 pathway activation and an ERK1/2 inhibitor (U0126) inhibited cell proliferation, migration, and invasion induced by overexpression of TG2. In summary, TG2 contributes to tumorigenesis and progression of GC by activating the ERK1/2 signaling pathway and is a potential therapeutic target of metastatic gastric cancer.

Effect of Some Growth Factors on Tissue Transglutaminase Overexpression Induced by β-Amyloid in Olfactory Ensheathing Cells

Herein, we assessed in a particular glial cell type, called olfactory ensheathing cells (OECs), the effect of some growth factors (GFs) on tissue transglutaminase (TG2) overexpression induced by amyloid-beta (Aβ) with native full-length peptide 1-42 or by fragments, 25-35 or 35-25, as control. Previously, we demonstrated that TG2 overexpression induced by some stressors was down-regulated by GFs exposure in OECs. To monitor cell viability, an MTT test was used, while TG2 expression was examined using immunocytochemical and Western blot analysis. We also considered the involvement of the TG2-mediated apoptotic pathway. Vimentin expression was evaluated as well. Reactive oxygen species and reduced glutathione levels were utilized to test the oxidative intracellular status. Lactate dehydrogenase released into the medium, as a marker of necrotic cell death, was evaluated. We found that in OECs exposed to Aβ(1-42) or Aβ(25-35) for 24 h, TG2 expression increased, and we observed that the protein appeared prevalently localized in the cytosol. The pre-treatment with GFs, basic fibroblast growth factor (bFGF) or glial-derived neurotrophic factor (GDNF), down-regulated the TG2 level, which was prevalently limited to the nuclear compartment. Vimentin expression and caspase cleavage showed a significant enhancement in Aβ(1-42) and Aβ(25-35) exposed cells. The pre-treatment with bFGF or GDNF was able to restore the levels of the proteins to control values, and the intracellular oxidative status modified by the exposure to Aβ(1-42) or Aβ(25-35). Our data suggest that both bFGF or GDNF could be an innovative mechanism to contrast TG2 expression, which plays a key role in Alzheimer's disease.

Is monocyte- and macrophage-derived tissue transglutaminase involved in inflammatory processes?

Monocytes and macrophages are key players in inflammatory processes following an infection or tissue damage. Monocytes adhere and extravasate into the inflamed tissue, differentiate into macrophages, and produce inflammatory mediators to combat the pathogens. In addition, they take up dead cells and debris and, therefore, take part in the resolution of inflammation. The multifunctional enzyme tissue Transglutaminase (TG2, tTG) is known to participate in most of those monocyte- and macrophage-mediated processes. Moreover, TG2 expression and activity can be regulated by inflammatory mediators. In the present review, we selectively elaborate on the expression, regulation, and contribution of TG2 derived from monocytes and macrophages to inflammatory processes mediated by those cells. In addition, we discuss the role of TG2 in certain pathological conditions, in which inflammation and monocytes and/or macrophages are prominently present, including atherosclerosis, sepsis, and multiple sclerosis. Based on the studies and considerations reported in this review, we conclude that monocyte- and macrophage-derived TG2 is clearly involved in various processes contributing to inflammation. However, TG2’s potential as a therapeutic target to counteract the possible detrimental effects or stimulate the potential beneficial effects on monocyte and macrophage responses during inflammation should be carefully considered. Alternatively, as TG2-related parameters can be used as a marker of disease, e.g., in celiac disease, or of disease-stage, e.g., in cancer, we put forward that this could be subject of research for monocyte- or macrophage-derived TG2 in inflammatory diseases.

Expression of Transglutaminase in Foreskin of Children with Balanitis Xerotica Obliterans

Balanitis xerotica obliterans (BXO) is a chronic inflammatory skin disorder of unclear etiology. The etiology and the exact molecular mechanisms underlying the disease are still unknown. The human transglutaminase (TG) family consists of several proteins with catalytic activity essential for biological processes. In the present research we investigated the transcript levels of three TGs in patients operated on for congenital phimosis without or with histologically confirmed BXO; Thirty children with acquired phimosis were enrolled. The removed foreskins were sent both for histological diagnosis and for quantitative real-time PCR to evaluate the transcript levels of keratinocyte (TG1), tissue (TG2), and epidermal (TG3) transglutaminase; We observed a decrease in TG1 and TG3 transcripts by about 70% (p < 0.001) in foreskins from patients with BXO (n = 15) in comparison with patients without BXO (n = 15) and an increase in TG2 mRNA levels by 2.9 folds (p < 0.001); Reduced expression of both TG1 and TG3 was associated with the altered structure of the foreskin in BXO and can be a consequence of damage to keratinocytes. Increased expression of TG2 can be the result of chronic inflammation. TG2 overexpression can play a pivotal role in triggering and maintaining the inflammatory response in BXO patients.

Transglutaminase-2: evolution from pedestrian protein to a promising therapeutic target

The ability of cancer cells to metastasize represents the most devastating feature of cancer. Currently, there are no specific biomarkers or therapeutic targets that can be used to predict the risk or to treat metastatic cancer. Many recent reports have demonstrated elevated expression of transglutaminase 2 (TG2) in multiple drug-resistant and metastatic cancer cells. TG2 is a multifunctional protein mostly known for catalyzing Ca²⁺-dependent -acyl transferase reaction to form protein crosslinks. Besides this transamidase activity, many Ca²⁺-independent and non-enzymatic activities of TG2 have been identified. Both, the enzymatic and non-enzymatic activities of TG2 have been implicated in diverse pathophysiological processes such as wound healing, cell growth, cell survival, extracellular matrix modification, apoptosis, and autophagy. Tumors have been frequently referred to as 'wounds that never heal'. Based on the observation that TG2 plays an important role in wound healing and inflammation is known to facilitate cancer growth and progression, we discuss the evidence that TG2 can reprogram inflammatory signaling networks that play fundamental roles in cancer progression. TG2-regulated signaling bestows on cancer cells the ability to proliferate, to resist cell death, to invade, to reprogram glucose metabolism and to metastasize, the attributes that are considered important hallmarks of cancer. Therefore, inhibiting TG2 may offer a novel therapeutic approach for managing and treatment of metastatic cancer. Strategies to inhibit TG2-regulated pathways will also be discussed.

Type II transglutaminase stimulates epidermal cancer stem cell epithelial-mesenchymal transition

Type II transglutaminase (TG2) is a multifunctional protein that has recently been implicated as having a role in ECS cell survival. In the present study we investigate the role of TG2 in regulating epithelial mesenchymal transition (EMT) in ECS cells. Our studies show that TG2 knockdown or treatment with TG2 inhibitor, results in a reduced EMT marker expression, and reduced cell migration and invasion. TG2 has several activities, but the most prominent are its transamidase and GTP binding activity. Analysis of a series of TG2 mutants reveals that TG2 GTP binding activity, but not the transamidase activity, is required for expression of EMT markers (Twist, Snail, Slug, vimentin, fibronectin, N-cadherin and HIF-1α), and increased ECS cell invasion and migration. This coupled with reduced expression of E-cadherin. Additional studies indicate that NF&#ξ03BA;B signaling, which has been implicated as mediating TG2 impact on EMT in breast cancer cells, is not involved in TG2 regulation of EMT in skin cancer. These studies suggest that TG2 is required for maintenance of ECS cell EMT, invasion and migration, and suggests that inhibiting TG2 GTP binding/G-protein related activity may reduce skin cancer tumor survival.

The Different Conformational States of Tissue Transglutaminase Have Opposing Affects on Cell Viability

Tissue transglutaminase (tTG) is an acyltransferase/GTP-binding protein that contributes to the development of various diseases. In human cancer cells, tTG activates signaling pathways that promote cell growth and survival, whereas in other disorders (i.e. neurodegeneration), overexpression of tTG enhances cell death. Therefore, it is important to understand how tTG is differentially regulated and functioning to promote diametrically distinct cellular outcomes. Previous structural studies revealed that tTG adopts either a nucleotide-bound closed conformation or a transamidation-competent open conformation. Here we provide evidence showing that these different conformational states determine whether tTG promotes, or is detrimental to, cell survival, with the open conformation of the protein being responsible for inducing cell death. First, we demonstrate that a nucleotide binding-defective form of tTG, which has previously been shown to induce cell death, assumes an open conformation in solution as assessed by an enhanced sensitivity to trypsin digestion and by small angle x-ray scattering (SAXS) analysis. We next identify two pairs of intramolecular hydrogen bonds that, based on existing x-ray structures, are predicted to form between the most C-terminal β-barrel domain and the catalytic core domain of tTG. By disrupting these hydrogen bonds, we are able to generate forms of tTG that constitutively assume an open conformation and induce apoptosis. These findings provide important insights into how tTG participates in the pathogenesis of neurodegenerative diseases, particularly with regard to the actions of a C-terminal truncated form of tTG (TG-Short) that has been linked to such disorders and induces apoptosis by assuming an open-like conformation.

Identification of transglutaminase 2 kinase substrates using a novel on-chip activity assay

Transglutaminase 2 (TG2) is an enzyme that plays a critical role in a wide variety of cellular processes through its multifunctional activities. TG2 kinase has emerged as an important regulator of apoptosis, as well as of chromatin structure and function. However, systematic investigation of TG2 kinase substrates is limited due to a lack of a suitable TG2 kinase activity assays. Thus, we developed a novel on-chip TG2 kinase activity assay for quantitative determination of TG2 kinase activity and for screening TG2 kinase substrate proteins in a high-throughput manner. Quantitative TG2 kinase activity was determined by selective detection of substrate protein phosphorylation on the surface of well-type amine arrays. The limit of detection (LOD) of this assay was 4.34μg/ml. We successfully applied this new activity assay to the kinetic analysis of 27 TG2-related proteins for TG2 kinase activity in a high-throughput manner and determined Michaelis-Menten constants (Km) of these proteins. We used the Km values and cellular locations of the TG2-related proteins to construct a substrate affinity map for TG2 kinase. Therefore, this on-chip TG2 kinase activity assay has a strong potential for the systematic investigation of substrate proteins and will be helpful for studying new physiological functions.

The transglutaminase type 2 and pyruvate kinase isoenzyme M2 interplay in autophagy regulation

Autophagy is a self-degradative physiological process by which the cell removes worn-out or damaged components. Constant at basal level it may become highly active in response to cellular stress. The type 2 transglutaminase (TG2), which accumulates under stressful cell conditions, plays an important role in the regulation of autophagy and cells lacking this enzyme display impaired autophagy/mitophagy and a consequent shift their metabolism to glycolysis. To further define the molecular partners of TG2 involved in these cellular processes, we analysed the TG2 interactome under normal and starved conditions discovering that TG2 interacts with various proteins belonging to different functional categories. Herein we show that TG2 interacts with pyruvate kinase M2 (PKM2), a rate limiting enzyme of glycolysis which is responsible for maintaining a glycolytic phenotype in malignant cells and displays non metabolic functions, including transcriptional co-activation and protein kinase activity. Interestingly, the ablation of PKM2 led to the decrease of intracellular TG2's transamidating activity paralleled by an increase of its tyrosine phosphorylation. Along with this, a significant decrease of ULK1 and Beclin1 was also recorded, thus suggesting a block in the upstream regulation of autophagosome formation. These data suggest that the PKM2/TG2 interplay plays an important role in the regulation of autophagy in particular under cellular stressful conditions such as those displayed by cancer cells.

Molecular mechanism by which acyclic retinoid induces nuclear localization of transglutaminase 2 in human hepatocellular carcinoma cells

Nuclear accumulation of transglutaminase 2 (TG2) is an important step in TG2-dependent cell death. However, the underlying molecular mechanisms for nuclear translocation of TG2 are still poorly understood. In this study, we demonstrated that acyclic retinoid (ACR) induced nuclear accumulation of TG2 in JHH-7 cells, a hepatocellular carcinoma (HCC) leading to their apoptosis. We further demonstrated molecular mechanism in nuclear-cytoplasmic trafficking of TG2 and an effect of ACR on it. We identified a novel 14-amino acid nuclear localization signal (NLS) (466)AEKEETGMAMRIRV(479) in the 'C' domain and a leucine-rich nuclear export signal (NES) (657)LHMGLHKL(664) in the 'D' domain that allowed TG2 to shuttle between the nuclear and cytosolic milieu. Increased nuclear import of GAPDH myc-HIS fused with the identified NLS was observed, confirming its nuclear import ability. Leptomycin B, an inhibitor of exportin-1 as well as point mutation of all leucine residues to glutamine residues in the NES of TG2 demolished its nuclear export. TG2 formed a trimeric complex with importin-α and importin-β independently from transamidase activity which strongly suggested the involvement of a NLS-based translocation of TG2 to the nucleus. ACR accelerated the formation of the trimeric complex and that may be at least in part responsible for enhanced nuclear localization of TG2 in HCC cells treated with ACR.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

Regulation of allergic lung inflammation by endothelial cell transglutaminase 2

Tissue transglutaminase 2 (TG2) is an enzyme with multiple functions, including catalysis of serotonin conjugation to proteins (serotonylation). Previous research indicates that TG2 expression is upregulated in human asthma and in the lung endothelium of ovalbumin (OVA)-challenged mice. It is not known whether endothelial cell TG2 is required for allergic inflammation. Therefore, to determine whether endothelial cell TG2 regulates allergic inflammation, mice with an endothelial cell-specific deletion of TG2 were generated, and these mice were sensitized and challenged in the airways with OVA. Deletion of TG2 in endothelial cells blocked OVA-induced serotonylation in lung endothelial cells, but not lung epithelial cells. Interestingly, deletion of endothelial TG2 reduced allergen-induced increases in respiratory system resistance, number of eosinophils in the bronchoalveolar lavage, and number of eosinophils in the lung tissue. Endothelial cell deletion of TG2 did not alter expression of adhesion molecules, cytokines, or chemokines that regulate leukocyte recruitment, consistent with other studies, demonstrating that deletion of endothelial cell signals does not alter lung cytokines and chemokines during allergic inflammation. Taken together, the data indicate that endothelial cell TG2 is required for allergic inflammation by regulating the recruitment of eosinophils into OVA-challenged lungs. In summary, TG2 functions as a critical signal for allergic lung responses. These data identify potential novel targets for intervention in allergy/asthma.

Transglutaminase Is Required for Epidermal Squamous Cell Carcinoma Stem Cell Survival

Cancer stem cells are thought to be responsible for rapid tumor growth, metastasis, and enhanced tumor survival following drug treatment. For this reason, there is a major emphasis on identifying proteins that can be targeted to kill cancer stem cells or control their growth, and transglutaminase type II (TGM2/TG2) is such a target in epidermal squamous cell carcinoma. TG2 was originally described as a transamidase in the extracellular matrix that crosslinks proteins by catalyzing ε-(γ-glutamyl)lysine bonds. However, subsequent studies have shown that TG2 is a GTP-binding protein that plays an important role in cell signaling and survival. In the present study, TG2 shows promise as a target for anticancer stem cell therapy in human squamous cell carcinoma. TG2 was determined to be highly elevated in epidermal cancer stem cells (ECS cells), and TG2 knockdown or suppression of TG2 function with inhibitors reduced ECS cell survival, spheroid formation, Matrigel invasion, and migration. The reduction in survival is associated with activation of apoptosis. Mechanistic studies, using TG2 mutants, revealed that the GTP-binding activity is required for maintenance of ECS cell growth and survival, and that the action of TG2 in ECS cells is not mediated by NF-κB signaling.

IMPLICATIONS

This study suggests that TG2 has an important role in maintaining cancer stem cell survival, invasive, and metastatic behavior and is an important therapeutic target to reduce survival of cancer stem cells in epidermal squamous cell carcinoma.

Monocytic Tissue Transglutaminase in a Rat Model for Reversible Acute Rejection and Chronic Renal Allograft Injury

Acute rejection is a major risk factor for chronic allograft injury (CAI). Blood leukocytes interacting with allograft endothelial cells during acute rejection were suggested to contribute to the still enigmatic pathogenesis of CAI. We hypothesize that tissue transglutaminase (Tgm2), a multifunctional protein and established marker of M2 macrophages, is involved in acute and chronic graft rejection. We focus on leukocytes accumulating in blood vessels of rat renal allografts (Fischer-344 to Lewis), an established model for reversible acute rejection and CAI. Monocytes in graft blood vessels overexpress Tgm2 when acute rejection peaks on day 9 after transplantation. Concomitantly, caspase-3 is activated, suggesting that Tgm2 expression is linked to apoptosis. After resolution of acute rejection on day 42, leukocytic Tgm2 levels are lower and activated caspase-3 does not differ among isografts and allografts. Cystamine was applied for 4 weeks after transplantation to inhibit extracellular transglutaminase activity, which did, however, not reduce CAI in the long run. In conclusion, this is the first report on Tgm2 expression by monocytes in vivo. Tgm2 may be involved in leukocytic apoptosis and thus in reversion of acute rejection. However, our data do not support a role of extracellular transglutaminase activity as a factor triggering CAI during self-limiting acute rejection.

Modulation of transglutaminase 2 activity in H9c2 cells by PKC and PKA signalling: a role for transglutaminase 2 in cytoprotection

BACKGROUND AND PURPOSE

Tissue transglutaminase (TG2) has been shown to mediate cell survival in many cell types. In this study, we investigated whether the role of TG2 in cytoprotection was mediated by the activation of PKA and PKC in cardiomyocyte-like H9c2 cells.

EXPERIMENTAL APPROACH

H9c2 cells were extracted following stimulation with phorbol-12-myristate-13-acetate (PMA) and forskolin. Transglutaminase activity was determined using an amine incorporating and a protein crosslinking assay. The presence of TG isoforms (TG1, 2, 3) was determined using Western blot analysis. The role of TG2 in PMA- and forskolin-induced cytoprotection was investigated by monitoring H₂O₂-induced oxidative stress in H9c2 cells.

KEY RESULTS

Western blotting showed TG2 >> TG1 protein expression but no detectable TG3. The amine incorporating activity of TG2 in H9c2 cells increased in a time and concentration-dependent manner following stimulation with PMA and forskolin. PMA and forskolin-induced TG2 activity was blocked by PKC (Ro 31-8220) and PKA (KT 5720 and Rp-8-Cl-cAMPS) inhibitors respectively. The PMA- and forskolin-induced increases in TG2 activity were attenuated by the TG2 inhibitors Z-DON and R283. Immunocytochemistry revealed TG2-mediated biotin-X-cadaverine incorporation into proteins and proteomic analysis identified known (β-tubulin) and novel (α-actinin) protein substrates for TG2. Pretreatment with PMA and forskolin reversed H2 O2 -induced decrease in MTT reduction and release of LDH. TG2 inhibitors R283 and Z-DON blocked PMA- and forskolin-induced cytoprotection.

CONCLUSIONS AND IMPLICATIONS

TG2 activity was stimulated via PKA- and PKC-dependent signalling pathways in H9c2 cells These results suggest a role for TG2 in cytoprotection induced by these kinases.

Lipopolysaccharide-Stimulated Transglutaminase 2 Expression Enhances Endocytosis Activity in the Mouse Microglial Cell Line BV-2

OBJECTIVES

In peripheral macrophages, tissue-type transglutaminase (TG2) is reported to be involved in phagocytosis of apoptotic cells. However, the contribution of TG2 to microglial phagocytosis has not been investigated. In this study, using a microglial cell line, BV-2, we examined the changes in TG2 expression, phagocytosis and pinocytosis in cells stimulated by lipopolysaccharide (LPS).

METHODS

Cells of the mouse microglial cell line BV-2 were stimulated by LPS with or without cystamine, an inhibitor of TG enzyme activity, for 24 h. TG2 expression was measured by real-time RT-PCR and Western blotting. TG activity was evaluated using biotinylated pentylamine as a substrate. Pinocytosis was determined by uptake of 1-µm fluorescent microbeads. Phagocytosis was assessed by uptake of dead cells, human neuroblastoma SH-SY5Y cells, which were pretreated with H2O2 for 24 h.

RESULTS

Phagocytosis of dead cells and pinocytosis of fluorescent microbeads were up-regulated by LPS stimulation together with TG2 expression. Blockade of TG enzyme activity by cystamine suppressed TG2 expression, phagocytosis and pinocytosis.

CONCLUSIONS

These results suggested that LPS-induced TG2 was involved in the mechanism of pinocytosis and phagocytosis in microglia.

Inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) catalyzes the cross-linking of proteins by the formation of isopeptide bonds between glutamine (Gln) and lysine (Lys) side chains. Although TG2 is essential for the stabilization of the extracellular matrix, its unregulated activity has been implicated in celiac disease, fibrosis, and cancer metastasis, among other disorders. Given the importance and range of TG2-related pathologies, recent work has focused on the development of potent and selective inhibitors against TG2. In this review, we present the latest and most noteworthy irreversible and reversible inhibitors of TG2, and offer perspectives for the design of future inhibitors, in the hope that lead compounds with therapeutic potential may soon be discovered.

Transglutaminase as a therapeutic target for celiac disease

INTRODUCTION

The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development.

AREAS COVERED

The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach.

EXPERT OPINION

Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

Activity-regulating structural changes and autoantibody epitopes in transglutaminase 2 assessed by hydrogen/deuterium exchange

The multifunctional enzyme transglutaminase 2 (TG2) is the target of autoantibodies in the gluten-sensitive enteropathy celiac disease. In addition, the enzyme is responsible for deamidation of gluten peptides, which are subsequently targeted by T cells. To understand the regulation of TG2 activity and the enzyme's role as an autoantigen in celiac disease, we have addressed structural properties of TG2 in solution by using hydrogen/deuterium exchange monitored by mass spectrometry. We demonstrate that Ca(2+) binding, which is necessary for TG2 activity, induces structural changes in the catalytic core domain of the enzyme. Cysteine oxidation was found to abolish these changes, suggesting a mechanism whereby disulfide bond formation inactivates the enzyme. Further, by using TG2-specific human monoclonal antibodies generated from intestinal plasma cells of celiac disease patients, we observed that binding of TG2 by autoantibodies can induce structural changes that could be relevant for the pathogenesis. Detailed mapping of two of the main epitopes targeted by celiac disease autoantibodies revealed that they are located adjacent to each other in the N-terminal part of the TG2 molecule.

Discovery of potent and specific dihydroisoxazole inhibitors of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes the posttranslational modification of glutamine residues on protein or peptide substrates. A growing body of literature has implicated aberrantly regulated activity of TG2 in the pathogenesis of various human inflammatory, fibrotic, and other diseases. Taken together with the fact that TG2 knockout mice are developmentally and reproductively normal, there is growing interest in the potential use of TG2 inhibitors in the treatment of these conditions. Targeted-covalent inhibitors based on the weakly electrophilic 3-bromo-4,5-dihydroisoxazole (DHI) scaffold have been widely used to study TG2 biology and are well tolerated in vivo, but these compounds have only modest potency, and their selectivity toward other transglutaminase homologues is largely unknown. In the present work, we first profiled the selectivity of existing inhibitors against the most pertinent TG isoforms (TG1, TG3, and FXIIIa). Significant cross-reactivity of these small molecules with TG1 was observed. Structure–activity and −selectivity analyses led to the identification of modifications that improved potency and isoform selectivity. Preliminary pharmacokinetic analysis of the most promising analogues was also undertaken. Our new data provides a clear basis for the rational selection of dihydroisoxazole inhibitors as tools for in vivo biological investigation.

Transglutaminase 2 ablation leads to mitophagy impairment associated with a metabolic shift towards aerobic glycolysis

Macroautophagy selectively degrades dysfunctional mitochondria by a process known as mitophagy. Here we demonstrate the involvement of transglutaminase 2 (TG2) in the turnover and degradation of damaged mitochondria. In TG2-ablated cells we observed the presence of a large number of fragmented mitochondria that display decreased membrane potential, downregulation of IF1 along with increased Drp1 and PINK1 levels, two key proteins regulating the mitochondrial fission. Of note, we demonstrate that in healthy mitochondria, TG2 interacts with the dynamic proteins Drp1 and Fis1; interestingly, their interaction is largely reduced upon induction of the fission process by carbonyl cyanide m-chlorophenyl hydrazine (CCCP). In keeping with these findings, mitochondria lacking TG2 are more susceptible to CCCP treatment. As a consequence of accumulation of damaged mitochondria, cells lacking TG2 increased their aerobic glycolysis and became sensitive to the glycolytic inhibitor 2-deoxy-D-glucose (2-DG). In contrast, TG2-proficient cells are more resistant to 2-DG-induced apoptosis as the caspase 3 is inactivated through the enzyme's crosslinking activity. The data presented in this study show that TG2 plays a key role in cellular dynamics and consequently influences the energetic metabolism.

Tissue transglutaminase: an emerging target for therapy and imaging

Tissue transglutaminase (transglutaminase 2) is a multifunctional enzyme with many interesting properties resulting in versatile roles in both physiology and pathophysiology. Herein, the particular involvement of the enzyme in human diseases will be outlined with special emphasis on its role in cancer and in tissue interactions with biomaterials. Despite recent progress in unraveling the different cellular functions of transglutaminase 2, several questions remain. Transglutaminase 2 features in both confirmed and some still ambiguous roles within pathological conditions, raising interest in developing inhibitors and imaging probes which target this enzyme. One important prerequisite for identifying and characterizing such molecular tools are reliable assay methods to measure the enzymatic activity. This digest Letter will provide clarification about the various assay methods described to date, accompanied by a discussion of recent progress in the development of inhibitors and imaging probes targeting transglutaminase 2.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

Conformational changes and translocation of tissue-transglutaminase to the plasma membranes: role in cancer cell migration

BACKGROUND

Tissue-transglutaminase (TG2), a dual function G-protein, plays key roles in cell differentiation and migration. In our previous studies we reported the mechanism of TG2-induced cell differentiation. In present study, we explored the mechanism of how TG2 may be involved in cell migration.

METHODS

To study the mechanism of TG2-mediated cell migration, we used neuroblastoma cells (SH-SY5Y) which do not express TG2, neuroblastoma cells expressing exogenous TG2 (SHYTG2), and pancreatic cancer cells which express high levels of endogenous TG2. Resveratrol, a natural compound previously shown to inhibit neuroblastoma and pancreatic cancer in the animal models, was utilized to investigate the role of TG2 in cancer cell migration. Immunofluorescence assays were employed to detect expression and intracellular localization of TG2, and calcium levels in the migrating cells. Native gel electrophoresis was performed to analyze resveratrol-induced cellular distribution and conformational states of TG2 in migrating cells. Data are presented as the mean and standard deviation of at least 3 independent experiments. Comparisons were made among groups using one-way ANOVA followed by Tukey-Kramer ad hoc test.

RESULTS

TG2 containing cells (SHYTG2 and pancreatic cancer cells) exhibit increased cell migration and invasion in collagen-coated and matrigel-coated transwell plate assays, respectively. Resveratrol (1 μM-10 μM) prevented migration of TG2-expressing cells. During the course of migration, resveratrol increased the immunoreactivity of TG2 without affecting the total TG2 protein level in migrating cells. In these cells, resveratrol increased calcium levels, and depletion of intracellular calcium by a calcium chelator, BAPTA, attenuated resveratrol-enhanced TG2 immunoreactivity. In native-polyacrylamide gels, we detected an additional TG2 protein band with slower migration in total cell lysates of resveratrol treated cells. This TG2 form is non-phosphorylated, exclusively present in plasma membrane fractions and sensitive to intracellular Ca(2+) concentration suggesting a calcium requirement in TG2-regulated cell migration.

CONCLUSIONS

Taken together, we conclude that resveratrol induces conformational changes in TG2, and that Ca(2+)-mediated TG2 association with the plasma membrane is responsible for the inhibitory effects of resveratrol on cell migration.

Transglutaminase 2 on the surface of dendritic cells is proposed to be involved in dendritic cell-T cell interaction

Transglutaminase 2 (TG2) is a ubiquitous enzyme involved in diverse biological processes. Recently, its function in adaptive immune responses has begun to emerge. Its presence and functions in B cells and T cells, for example, have been reported. However, those in dendritic cells (DCs), the principal antigen-presenting cells, are as yet unexplored in murine system. In this study, we first investigated the expression of TG2 in murine bone marrow-derived DCs, and then compared the functioning of these cells in the presence or absence of this enzyme using wild-type (WT) and TG2(-/-) mice. We found that the WT DCs expressed TG2 both in the cytoplasm and on the cell surface, both of which were elevated after LPS stimulation. Unexpectedly, between WT and TG2(-/-) DCs, there were no remarkable differences in cytokine secretion, IL-10 and IL-12, and neither in the expression of surface molecules CD80, CD86, and MHC II, excepting a moderate decrease of CD40 expression on the TG2(-/-) DCs. However, when T cells were stimulated with TG2(-/-) DCs, they showed decreased levels of proliferation, CD69 and CD25 expression, and IFN-γ secretion. The addition of anti-TG2 antibody to the WT DC-T cell co-culture resulted in decreased T cell activation. By immunofluorescence staining, TG2 was observed at DC-T cell interface (contact point). Taken together, we propose that TG2 on the surface of DCs modulates the DC-T cell interaction.

Biological functionalities of transglutaminase 2 and the possibility of its compensation by other members of the transglutaminase family

Transglutaminase 2 (TG2) is the most widely distributed and most abundantly expressed member of the transglutaminase family of enzymes, a group of intracellular and extracellular proteins that catalyze the Ca²⁺-dependent posttranslational modification of proteins. It is a unique member of the transglutaminase family owing to its specialized biochemical, structural and functional elements, ubiquitous tissue distribution and subcellular localization, and substrate specificity. The broad substrate specificity of TG2 and its flexible interaction with numerous other gene products may account for its multiple biological functions. In addition to the classic Ca²⁺-dependent transamidation of proteins, which is a hallmark of transglutaminase enzymes, additional Ca²⁺-independent enzymatic and nonenzymatic activities of TG2 have been identified. Many such activities have been directly or indirectly implicated in diverse cellular physiological events, including cell growth and differentiation, cell adhesion and morphology, extracellular matrix stabilization, wound healing, cellular development, receptor-mediated endocytosis, apoptosis, and disease pathology. Given the wide range of activities of the transglutaminase gene family it has been suggested that, in the absence of active versions of TG2, its function could be compensated for by other members of the transglutaminase family. It is in the light of this assertion that we review, herein, TG2 activities and the possibilities and premises for compensation for its absence.

A novel mechanism by which tissue transglutaminase activates signaling events that promote cell survival

Tissue transglutaminase (tTG) functions as a GTPase and an acyl transferase that catalyzes the formation of protein cross-links. tTG expression is frequently up-regulated in human cancer, where it has been implicated in various aspects of cancer progression, including cell survival and chemo-resistance. However, the extent to which tTG cooperates with other proteins within the context of a cancer cell, versus its intrinsic ability to confer transformed characteristics to cells, is poorly understood. To address this question, we asked what effect the ectopic expression of tTG in a non-transformed cellular background would have on the behavior of the cells. Using NIH3T3 fibroblasts stably expressing a Myc-tagged form of tTG, we found that tTG strongly protected these cells from serum starvation-induced apoptosis and triggered the activation of the PI3-kinase/mTOR Complex 1 (mTORC1)/p70 S6-kinase pathway. We determined that tTG forms a complex with the non-receptor tyrosine kinase c-Src and PI3-kinase, and that treating cells with inhibitors to block tTG function (monodansylcadaverine; MDC) or c-Src kinase activity (PP2) disrupted the formation of this complex, and prevented tTG from activating the PI3-kinase pathway. Moreover, treatment of fibroblasts over-expressing tTG with PP2, or with inhibitors that inactivate components of the PI3-kinase pathway, including PI3-kinase (LY294002) and mTORC1 (rapamycin), ablated the tTG-promoted survival of the cells. These findings demonstrate that tTG has an intrinsic capability to stimulate cell survival through a novel mechanism that activates PI3-kinase signaling events, thus highlighting tTG as a potential target for the treatment of human cancer.

Induction and translocation of tissue transglutaminase isoforms increased phosphorylation in retinoic acid treated erythroleukemia cells

Tissue transglutaminase (TGC, TG2, 80 kDa) is inactive in cross-linking reactions and is converted in vitro and in vivo to the TG (55 kDa) active isoform (Fraij in J Cell Biochem 112:2469-2489, 2011). Two isoforms of human TGC were cloned from human erythroleukemia (HEL) cells induced with retinoic acid (RA) and termed TGH, 63 kDa (Fraij et al. in J Biol Chem 267:22616-22673, 1992) and TGH2, 37 kDa. The purified TGC isoforms exhibited GTPase activity and TGH and TGH2 showed higher activities than the native TGC protein. In all normal cells examined, TGC was found in membrane fractions several fold higher than the supernatant fractions; however, in the natural tumor cell line HEL the TGC cellular distribution was reversed. Although TGC is the major enzyme in normal human erythrocytes, its expression level was significantly decreased in HEL cells. RA treatment induced a sevenfold increase in the level of TGC protein in HEL cells and was accompanied by its translocation to cell membranes. When isolated membrane and supernatant fractions from normal human foreskin (CF3), normal human embryonic lung (WI-38), and HEL cells treated with or without RA were incubated with [(32)P]-ATP at 37 °C for 1 h, more radio-labeled proteins were detected in the membrane fractions than the cytosolic fractions. More labeled protein bands were detected in RA treated HEL cells in comparison to control HEL cell extracts. Radio labeled proteins coimmunoprecipitated with the TGC isoforms in RA treated HEL membrane fractions thereby confirming that the radio-labeled material consists of endogenous proteins associated with TGC isoforms. Protein phosphorylation is related to the induction and translocation of the isoforms in RA treated cells. These results show that the TGC isoforms complexes with proteins in vivo and that the phosphorylation of these proteins is catalyzed directly by TGC kinase activity or indirectly by the TGC phosphorylation of other protein kinases.

Identification of a specific one amino acid change in recombinant human transglutaminase 2 that regulates its activity and calcium sensitivity

TG2 (transglutaminase 2) is a calcium-dependent protein cross-linking enzyme which is involved in a variety of cellular processes. The threshold level of calcium needed for endogenous and recombinant TG2 activity has been controversial, the former being more sensitive to calcium than the latter. In the present study we address this question by identifying a single amino acid change from conserved valine to glycine at position 224 in recombinant TG2 compared with the endogenous sequence present in the available genomic databases. Substituting a valine residue for Gly224 in the recombinant TG2 increased its calcium-binding affinity and transamidation activity 10-fold and isopeptidase activity severalfold, explaining the inactivity of widely used recombinant TG2 at physiological calcium concentrations. ITC (isothermal titration calorimetry) measurements showed 7-fold higher calcium-binding affinities for TG2 valine residues which could be activated inside cells. The two forms had comparable substrate- and GTP-binding affinities and also bound fibronectin similarly, but coeliac antibodies had a higher affinity for TG2 valine residues. Structural analysis indicated a higher stability for TG2 valine residues and a decrease in flexibility of the calcium-binding loop resulting in improved metal-binding affinity. The results of the present study suggest that Val224 increases TG2 activity by modulating its calcium-binding affinity enabling transamidation reactions inside cells.

Methylglyoxal in cells elicits a negative feedback loop entailing transglutaminase 2 and glyoxalase 1

Glyoxalase 1 (GlxI) is the key enzyme that converts the highly reactive α-oxo-aldehydes into the corresponding α-hydroxy acids using l-glutathione as a cofactor. In our preliminary data, GlxI was identified as a substrate of transglutaminase 2 (TG2), a ubiquitous enzyme with multiple functions. According to the catalytic properties of TG2, protein cross-linking, polyamine conjugation, and/or deamidation are potential post-translational modifications. In this article, we have demonstrated that TG2 catalyzes either polyamine conjugation or deamidation to GlxI depending on the presence of polyamines or not. Deamidation leads to activation of GlxI while polyamine conjugation results in activation of GlxI as well as stabilization of GlxI against denaturation treatment. In cultured HeLa cells, methylglyoxal challenge causes increase in intracellular levels of reactive oxygen species (ROS) and calcium leading to TG2 activation and subsequent transamidation and activation of GlxI. The inhibition of TG2 significantly weakens the cell resistance to the methylglyoxal challenge. Thus, GlxI is a novel substrate of TG2 and is activated by TG2 in vitro and in cellulo. Exposure to methylglyoxal elicits a negative feedback loop entailing ROS, calcium, TG2 and GlxI, thus leading to attenuation of the increase in the methylglyoxal level. The results imply that cancer cells highly express TG2 or GlxI can endure the oxidative stress derived from higher glycolytic flux and may gain extra growth advantage from the aerobic glycolysis.

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We have demonstrated novel modifications of glyoxalase I by transglutaminase 2.

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The modifications mediated by transglutaminse 2 modulate the glyoxalase I activities.

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Methylglyoxal treatment in cells induces increases in the levels of endogenous reactive oxygen species and activation transglutaminase 2 and glyoxalase I.

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Cells dispose the accumulated intracellular methylglyoxal by a negative feedback loop consisting of reactive oxygen species, calcium, transglutaminase 2 and glyoxalase I.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

A possible role of transglutaminase 2 in the nucleus of INS-1E and of cells of human pancreatic islets

Transglutaminase 2 (TG2) is a multifunctional protein with Ca^2 +-dependent transamidating and G protein activity. Previously we reported that the role of TG2 in insulin secretion may involve cytoplasmic actin remodeling and a regulative action on other proteins during granule movement. The aim of this study was to gain a better insight into the role of TG2 transamidating activity in mitochondria and in the nucleus of INS-1E rat insulinoma cell line (INS-1E) during insulin secretion. To this end we labeled INS-1E with an artificial donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins of the nuclear/mitochondrial-enriched fraction were analyzed using two-dimensional electrophoresis and mass spectrometry. Many mitochondrial proteins involved in Ca^2 + homeostasis (e.g. voltage-dependent anion-selective channel protein, prohibitin and different ATP synthase subunits) and many nuclear proteins involved in gene regulation (e.g. histone H3, barrier to autointegration factor and various heterogeneous nuclear ribonucleoprotein) were identified among a number of transamidating substrates of TG2 in INS-1E. The combined results provide evidence that a temporal link exists between glucose-stimulation, first phase insulin secretion and the action of TG on histone H3 both in INS-1E and human pancreatic islets.

Biological significance

Research into the role of transglutaminase 2 during insulin secretion in INS-1E rat insulinoma cellular model is depicting a complex role for this enzyme. Transglutaminase 2 acts in the different INS-1E compartments in the same way: catalyzing a post-translational modification event of its substrates. In this work we identify some mitochondrial and nuclear substrates of INS-1E during first phase insulin secretion. The finding that TG2 interacts with nuclear proteins that include BAF and histone H3 immediately after (2–5 min) glucose stimulus of INS-1E suggests that TG2 may be involved not only in insulin secretion, as suggested by our previous studies in cytoplasmic INS-1E fraction, but also in the regulation of glucose-induced gene transcription.

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Transglutaminase 2 localizes in the nucleus and in the mitochondrion of INS-1E.

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TG2 acts as a modifying enzyme in both compartments during FPIS.

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TG2 may contribute to Ca^2 + sensing in mitochondrion through its substrates.

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TG2 may contribute to chromatin condensation in nucleus through its substrates.

Tissue transglutaminase contributes to the pathogenesis of preeclampsia and stabilizes placental angiotensin receptor type 1 by ubiquitination-preventing isopeptide modification

Preeclampsia is a life-threatening pregnancy disorder that is widely thought to be triggered by impaired placental development. However, the placenta-related pathogenic factors are not fully identified, and their underlying mechanisms in disease development remain unclear. Here, we report that the protein level and enzyme activity of tissue transglutaminase (TG2 or tTG), the most ubiquitous member of a family of enzymes that conducts post-translational modification of proteins by forming ε-(γ-glutamyl)-lysine isopeptide bonds, are significantly elevated in placentas of preeclamptic women. TG2 is localized in the placental syncytiotrophoblasts of patients with preeclampsia where it catalyzes the isopeptide modification of the angiotensin receptor type 1 (AT1). To determine the role of elevated TG2 in preeclampsia, we used a mouse model of preeclampsia based on injection of AT1-agonistic autoantibody. A pathogenic role for TG2 in preeclampsia is suggested by in vivo experiments in which cystamine, a potent transglutaminase inhibitor, or small interfering RNA-mediated TG2 knockdown significantly attenuated autoantibody-induced hypertension and proteinuria in pregnant mice. Cystamine treatment also prevented isopeptide modification of placental AT1 receptors in preeclamptic mice. Mechanistically, we revealed that AT1-agonistic autoantibody stimulation enhances the interaction between AT1 receptor and TG2 and results in increased AT1 receptor stabilization via transglutaminase-mediated isopeptide modification in trophoblasts. Mutagenesis studies further demonstrated that TG2-mediated isopeptide modification of AT1 receptors prevents ubiquitination-dependent receptor degradation. Taken together, our studies not only identify a novel pathogenic involvement of TG2 in preeclampsia but also suggest a previously unrecognized role of TG2 in the regulation of G protein-coupled receptor stabilization by inhibiting ubiquitination-dependent degradation.

Anti-transglutaminase immunoreactivity and histological lesions of the duodenum in coeliac patients

Coeliac disease (CD) is characterized by several markers, including anti-transglutaminase auto-antibodies (tTGAb) directed against multiple epitopes of the gliadin protein. We aimed to investigate the correlation among CD duodenal lesions, tTGAb titres and the immunoreactivity against tTG constructs. A total of 345 CD patients (209 females, 136 males, overall median age: 7.3 years) were tested for full-length (fl) tTGAb with a fluid-phase radioimmunoassay. Out of the total, 231 patients were also tested for immunoreactivity against tTG fragments (F1: a.a. 227-687 and F2: a.a. 473-687). Patients were classified according to diffuse (D), patchy (P) or bulb (B) histological lesions. All sera were found fltTGAb positive. Patients with D, P and B lesions had a mean Ab index of 0.84±0.39, 0.57±0.39 and 0.45±0.24, respectively. Mean tTGAb titre varied between D and localized (P+B) patients (0.84±0.39 versus 0.52±0.34, P < 0.0001). Overall, 86.1% of patients were F1 auto-antibody (F1Ab) positive (D: 89%, P: 75%, B: 40%; D versus P+B: P = 0.004) and 49% of patients were F2 auto-antibody (F2Ab) positive (D: 53%, P: 19%, B: 10%; D versus P+B: P = 0.0006). Of the D patients 50.7% showed combined F1Ab-F2Ab (D versus P+B: P = 0.001), whereas 60% of B patients were negative for both F1Ab and F2Ab (B versus D: P < 0.0001). Coeliac-specific tTGAb immunoreactivity correlates with the grading and extension of histological duodenal lesions in CD patients at diagnosis. The immunoreactivity against single and combined tTG fragments is significantly higher in patients with D lesions. This is the first evidence of a distinct coeliac-specific immunoreactivity in patients with different duodenal involvement.

Glucose homeostasis in mice is transglutaminase 2 independent

Transglutaminase type 2 (TG2) has been reported to be a candidate gene for maturity onset diabetes of the young (MODY) because three different mutations that impair TG2 transamidase activity have been found in 3 families with MODY. TG2 null (TG2^−/−) mice have been reported to be glucose intolerant and have impaired glucose-stimulated insulin secretion (GSIS). Here we rigorously evaluated the role of TG2 in glucose metabolism using independently generated murine models of genetic TG2 disruption, which show no compensatory enhanced expression of other TGs in pancreatic islets or other tissues. First, we subjected chow- or fat-fed congenic SV129 or C57BL/6 wild type (WT) and TG2^−/− littermates, to oral glucose gavage. Blood glucose and serum insulin levels were similar for both genotypes. Pancreatic islets isolated from these animals and analysed in vitro for GSIS and cholinergic potentiation of GSIS, showed no significant difference between genotypes. Results from intraperitoneal glucose tolerance tests (GTTs) and insulin tolerance tests (ITTs) were similar for both genotypes. Second, we directly investigated the role of TG2 transamidase activity in insulin secretion using a coisogenic model that expresses a mutant form of TG2 (TG2^R579A), which is constitutively active for transamidase activity. Intraperitoneal GTTs and ITTs revealed no significant differences between WT and TG2^R579A/R579A mice. Given that neither deletion nor constitutive activation of TG2 transamidase activity altered basal responses, or responses to a glucose or insulin challenge, our data indicate that glucose homeostasis in mice is TG2 independent, and question a link between TG2 and diabetes.

Transglutaminase 2-specific autoantibodies in celiac disease target clustered, N-terminal epitopes not displayed on the surface of cells

The gluten-sensitive enteropathy celiac disease is tightly associated with the production of autoantibodies specific for the enzyme transglutaminase 2 (TG2). The mechanisms underlying the activation of autoreactive B cells, however, are not well defined. To gain more insight into this autoimmune response we have characterized the binding of TG2 by a panel of human mAbs generated by expression cloning of Ig genes from single plasma cells of the celiac disease lesion. The Abs were highly specific to TG2 and bound preferentially to the open, Ca(2+)-activated enzyme conformation. Epitope mapping revealed that they recognize few distinct conformational epitopes that cluster in the N-terminal half of the enzyme. Two of the epitopes were overlapping with the fibronectin binding site in TG2, and none of the epitopes was accessible when TG2 was in a cell surface-bound form. Based on our findings, we propose that the autoantibodies are generated against the soluble, catalytically active enzyme, whereas Abs reactive with cell surface-associated TG2 are absent from the response due to negative selection of B cells recognizing membrane-bound self-Ag. The findings give insight into the mechanisms controlling the formation of anti-TG2 autoantibodies in celiac disease.

Regulation of Pollen Tube Growth by Transglutaminase

In pollen tubes, cytoskeleton proteins are involved in many aspects of pollen germination and growth, from the transport of sperm cells to the asymmetrical distribution of organelles to the deposition of cell wall material. These activities are based on the dynamics of the cytoskeleton. Changes to both actin filaments and microtubules are triggered by specific proteins, resulting in different organization levels suitable for the different functions of the cytoskeleton. Transglutaminases are enzymes ubiquitous in all plant organs and cell compartments. They catalyze the post-translational conjugation of polyamines to different protein targets, such as the cytoskeleton. Transglutaminases are suggested to have a general role in the interaction between pollen tubes and the extracellular matrix during fertilization and a specific role during the self-incompatibility response. In such processes, the activity of transglutaminases is enhanced, leading to the formation of cross-linked products (including aggregates of tubulin and actin). Consequently, transglutaminases are suggested to act as regulators of cytoskeleton dynamics. The distribution of transglutaminases in pollen tubes is affected by both membrane dynamics and the cytoskeleton. Transglutaminases are also secreted in the extracellular matrix, where they may take part in the assembly and/or strengthening of the pollen tube cell wall.