Transglutaminase 2 kinase activity facilitates protein kinase A-induced phosphorylation of retinoblastoma protein

Effect of Ferulic Acid Loaded in Nanoparticle on Tissue Transglutaminase Expression Levels in Human Glioblastoma Cell Line

Glioblastoma (GBM) is one of the most aggressive cancers, characterized by a decrease in antioxidant levels. Evidence has demonstrated that ferulic acid (FA), a natural antioxidant particularly abundant in vegetables and fruits, could be a promising candidate for GBM treatment. Since FA shows a high instability that compromises its therapeutic application, it has been encapsulated into Nanostructured Lipid Carriers (NLCs) to improve its bioavailability in the brain. It has been demonstrated that tissue transglutaminase (TG2) is a multi-functional protein implicated in many physiological and pathological processes, including cancer. TG2 is also involved in GBM correlated with metastasis formation and drug resistance. Therefore, the evaluation of TG2 expression levels and its cellular localization are important to assess the anti-cancer effect of FA against GBM cancer. Our results have demonstrated that treatment with free FA and FA-NLCs in the U87-MG cancer cell line differently modified TG2 localization and expression levels. In the cells treated with free FA, TG2 appeared expressed both in the cytosol and in the nucleus, while the treatment with FA-NLCs showed that the protein is exclusively localized in the cytosol, exerting its pro-apoptotic effect. Therefore, our data suggest that FA loaded in NLCs could represent a promising natural agent for supplementing the current anti-cancer drugs used for the treatment of GBM.

Study of tissue transglutaminase spliced variants expressed in THP-1 derived macrophages exhibiting distinct functional phenotypes

Tissue transglutaminase (TG2) expressed in monocytes and macrophage is known to participate in processes during either early and resolution stages of inflammation. The alternative splicing of tissue transglutaminase gene is a mechanism that increases its functional diversity. Four spliced variants are known with truncated C-terminal domains (TGM2_v2, TGM2_v3, TGM2_v4a, TGM2_v4b) but scarce information is available about its expression in human monocyte and macrophages. We studied the expression of canonical TG2 (TGM2_v1) and its short spliced variants by RT-PCR during differentiation of TPH-1 derived macrophages (dTHP-1) using two protocols (condition I and II) that differ in Phorbol-12-myristate-13-acetate dose and time schedule. The production of TNF-α and IL-1β in supernatant of dTHP-1, measured by ELISA in supernatants showed higher proinflammatory milieu in condition I. We found that the expression of all mRNA TG2 spliced variants were up-regulated during macrophage differentiation and after IFN-γ treatment of dTHP-1 cells in both conditions. Nevertheless, the relative fold increase or TGM2_v3 in relation with TGM2_v1 was higher only with the condition I. M1/M2-like THP-1 macrophages obtained with IFN-γ/IL-4 treatments showed that the up-regulation of TGM2_v1 induced by IL-4 was higher in relation with any short spliced variants. The qualitative profile of relative contribution of spliced variants in M1/M2-like THP-1 cells showed a trend to higher expression of TGM2_v3 in the inflammatory functional phenotype. Our results contribute to the knowledge about TG2 spliced variants in the biology of monocyte/macrophage cells and show how the differentiation conditions can alter their expression and cell function.

TGM2 accelerates migration and differentiation of BMSCs by activating Wnt/β-catenin signaling

BACKGROUND

Transglutaminase 2 (TGM2) is a gene previously reported to be associated with the differentiation of bone marrow mesenchymal stem cells (BMSCs). The study was developed to reveal the impact of TGM2 on the migration and differentiation of BMSCs.

METHODS

Cells were isolated from bone marrow of mice and then the surface antigens were identified by flow cytometry. Wound healing assays were conducted to assess the migratory ability of BMSCs. The mRNA levels of TGM2 and osteoblast-associated genes (ALP, OCN, and RUNX2) were subjected to RT-qPCR analysis, and protein levels of these genes as well as β-catenin were quantitated by western blotting. Alizarin red staining was conducted for detection of osteogenic ability. The activation of Wnt signaling was assessed by TOP/FOP flash assays.

RESULTS

Surface antigens were positively identified in MSCs, indicating good multidirectional differentiation ability of cells. TGM2 silencing suppressed BMSC migration while weakening mRNA and protein levels of osteoblast-associated genes. While TGM2 overexpression exerts the opposite impact on cell migration and expression levels of osteoblast-associated genes. Additionally, overexpressed TGM2 promotes the mineralization of BMSCs according to results of Alizarin red staining. Moreover, TGM2 activated the Wnt/β-catenin signaling, and DKK1 (an inhibitor of Wnt signaling) reversed the promoting influence of TGM2 on cell migration and differentiation.

CONCLUSION

TGM2 promotes the migration and differentiation of BMSCs via activation of the Wnt/β-catenin signaling.

Transglutaminase 2 regulates terminal erythroid differentiation via cross-linking activity

Transglutaminase 2 (TGM2) is a versatile enzyme that modulates cell survival and differentiation. However, its role in terminal erythroid differentiation is poorly understood. In this study, we investigated the function of TGM2 in primary fetal liver erythroid differentiation. We predicted TGM2 as an upstream regulator via ingenuity pathway analysis (IPA), and found that its expression was increased at both RNA and protein level during terminal erythroid differentiation. TGM2 cross-linking activity inhibitors GK921 and Z-DON suppressed erythroid maturation and enucleation, while its GTPase inhibitor LDN27219 had no such effect. Z-DON treatment arrested differentiation at basophilic erythroblast stage, and interfered with cell cycle progression. RT-PCR demonstrated decreased GATA-1 and KLF1, and disarranged cyclin, CDKI and E2F family genes expression after Z-DON treatment. In conclusion, TGM2 regulates terminal erythroid differentiation through its cross-linking enzyme activity.

Type 2 transglutaminase in the nucleus: the new epigenetic face of a cytoplasmic enzyme

One of the major mysteries in science is how it is possible to pack the cellular chromatin with a total length of over 1 m, into a small sphere with a diameter of 5 mm "the nucleus", and even more difficult to envisage how to make it functional. Although we know that compaction is achieved through the histones, however, the DNA needs to be accessible to the transcription machinery and this is allowed thanks to a variety of very complex epigenetic mechanisms. Either DNA (methylation) or post-translational modifications of histone proteins (acetylation, methylation, ubiquitination and sumoylation) play a crucial role in chromatin remodelling and consequently on gene expression. Recently the serotonylation and dopaminylation of the histone 3, catalyzed by the Transglutaminase type 2 (TG2), has been reported. These novel post-translational modifications catalyzed by a predominantly cytoplasmic enzyme opens a new avenue for future investigations on the enzyme function itself and for the possibility that other biological amines, substrate of TG2, can influence the genome regulation under peculiar cellular conditions. In this review we analyzed the nuclear TG2's biology by discussing both its post-translational modification of various transcription factors and the implications of its epigenetic new face. Finally, we will focus on the potential impact of these events in human diseases.

Effect of Unloaded and Curcumin-Loaded Solid Lipid Nanoparticles on Tissue Transglutaminase Isoforms Expression Levels in an Experimental Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is a neurodegenerative disease representing the most prevalent cause of dementia. It is also related to the aberrant amyloid-beta (Aβ) protein deposition in the brain. Since oxidative stress is involved in AD, there is a possible role of antioxidants present in the effected person’s diet. Thus, we assessed the effect of the systemic administration of solid lipid nanoparticles (SLNs) to facilitate curcumin (CUR) delivery on TG2 isoform expression levels in Wild Type (WT) and in TgCRND8 (Tg) mice. An experimental model of AD, which expresses two mutated human amyloid precursor protein (APP) genes, was used. Behavioral studies were also performed to evaluate the improvement of cognitive performance and memory function induced by all treatments. The expression levels of Bcl-2, Cyclin-D₁, and caspase-3 cleavage were evaluated as well. In this research, for the first time, we demonstrated that the systemic administration of SLNs-CUR, both in WT and in Tg mice, allows one to differently modulate TG2 isoforms, which act either on apoptotic pathway activation or on the ability of the protein to repair cellular damage in the brains of Tg mice. In this study, we also suggest that SLNs-CUR could be an innovative tool for the treatment of AD.

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

Role of Transglutaminase 2 in Cell Death, Survival, and Fibrosis

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme catalyzing the crosslinking between Gln and Lys residues and involved in various pathophysiological events. Besides this crosslinking activity, TG2 functions as a deamidase, GTPase, isopeptidase, adapter/scaffold, protein disulfide isomerase, and kinase. It also plays a role in the regulation of hypusination and serotonylation. Through these activities, TG2 is involved in cell growth, differentiation, cell death, inflammation, tissue repair, and fibrosis. Depending on the cell type and stimulus, TG2 changes its subcellular localization and biological activity, leading to cell death or survival. In normal unstressed cells, intracellular TG2 exhibits a GTP-bound closed conformation, exerting prosurvival functions. However, upon cell stimulation with Ca²⁺ or other factors, TG2 adopts a Ca²⁺-bound open conformation, demonstrating a transamidase activity involved in cell death or survival. These functional discrepancies of TG2 open form might be caused by its multifunctional nature, the existence of splicing variants, the cell type and stimulus, and the genetic backgrounds and variations of the mouse models used. TG2 is also involved in the phagocytosis of dead cells by macrophages and in fibrosis during tissue repair. Here, we summarize and discuss the multifunctional and controversial roles of TG2, focusing on cell death/survival and fibrosis.

Amyloid-Beta Induces Different Expression Pattern of Tissue Transglutaminase and Its Isoforms on Olfactory Ensheathing Cells: Modulatory Effect of Indicaxanthin

Herein, we assessed the effect of full native peptide of amyloid-beta (Aβ) (1-42) and its fragments (25-35 and 35-25) on tissue transglutaminase (TG2) and its isoforms (TG2-Long and TG2-Short) expression levels on olfactory ensheathing cells (OECs). Vimentin and glial fibrillary acid protein (GFAP) were also studied. The effect of the pre-treatment with indicaxanthin from Opuntia ficus-indica fruit on TG2 expression levels and its isoforms, cell viability, total reactive oxygen species (ROS), superoxide anion (O₂⁻), and apoptotic pathway activation was assessed. The levels of Nestin and cyclin D1 were also evaluated. Our findings highlight that OECs exposure to Aβ(1-42) and its fragments induced an increase in TG2 expression levels and a different expression pattern of its isoforms. Indicaxanthin pre-treatment reduced TG2 overexpression, modulating the expression of TG2 isoforms. It reduced total ROS and O₂⁻ production, GFAP and Vimentin levels, inhibiting apoptotic pathway activation. It also induced an increase in the Nestin and cyclin D1 expression levels. Our data demonstrated that indicaxanthin pre-treatment stimulated OECs self-renewal through the reparative activity played by TG2. They also suggest that Aβ might modify TG2 conformation in OECs and that indicaxanthin pre-treatment might modulate TG2 conformation, stimulating neural regeneration in Alzheimer’s disease.

Transglutaminase Type 2 is Involved in the Hematopoietic Stem Cells Homeostasis

Type 2 transglutaminase (TG2) is a multifunctional protein involved in various biological processes playing a key regulatory role in cell homeostasis such as cell death and autophagy. New evidence is emerging that support an important role of autophagy in regulating normal hematopoiesis. Prompted by these findings, in this study we investigated in vivo involvement of TG2 in mouse hematopoiesis under normal or nutrient deprivation conditions. We found that the number and rate of differentiation of bone marrow hematopoietic stem cell was decreased in the TG2 knockout mice. We present evidence showing that these effects on hematopoietic system are very likely due to the TG2-dependent impairment of autophagy. In fact, stimulation of autophagy by starvation is able to rescue the block of the differentiation of stem cells progenitors in the TG2 KO mice. It was also shown that the RhoA/ERK½ pathway, known to be essential for regulation of the bone marrow progenitor cells homeostasis, was significantly impaired in the absence of TG2. Hence, this study expanded our knowledge about TG2 discovering a role of this enzyme in regulation of hematopoiesis.

Calcium relieves fluoride-induced bone damage through the PI3K/AKT pathway

Bone is the main target of fluorosis, and it has been perfectly elaborated that a moderate dosage of calcium (Ca) can alleviate bone fluorosis. However, whether Ca can alleviate fluorosis through the phosphatidylinositol 3 kinase (PI3K)/protein kinase B (AKT) signaling pathway has not yet been reported. Hence, we evaluated the histopathological structure, the imbalance of the biochemical index of bone metabolism, and the expression levels of PI3K/AKT apoptosis signaling pathway-related genes in rats treated with sodium fluoride (NaF, F) and/or calcium carbonate (CaCO3) for 120 days. Our results suggest that 100 mg L-1 NaF induced histopathological injury as alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (StrACP) activity increased, with a decrease in the serum Ca levels (p < 0.05). Moreover, the results of qRT-PCR and western blotting showed that F increased the expression levels of transglutaminase 2 (TGM2), focal adhesion kinase (FAK), PI3K, AKT, forkhead box O1 (Foxo1), Bcl-2 interacting mediator of cell death (BIM), Bcl2-associated x protein (Bax) and Caspase 3 (p < 0.05, p < 0.01). It also decreased the expression of AnnexinA5 (Anxa5), 3'-phosphoinositide-dependent kinase 1 (PDK1) and B-cell lymphoma-2 (Bcl-2) (p < 0.05, p < 0.01), which finally activated the PI3K/AKT pathway. On the other hand, CaCO3 supplementation reversed the histopathological injury along with the levels of ALP, StrACP and serum Ca, alleviating the gene expression levels of PI3K/AKT pathway-related markers. Altogether, we can conclude that CaCO3 supplementation mitigated F-induced bone damage via the PI3K/AKT signaling pathway.

Structures of Human Transglutaminase 2: Finding Clues for Interference in Cross-linking Mediated Activity

Human transglutaminase 2 (TGase2) has various functions, including roles in various cellular processes such as apoptosis, development, differentiation, wound healing, and angiogenesis, and is linked to many diseases such as cancer. Although TGase2 has been considered an optimized drug target for the treatment of cancer, fibrosis, and neurodegenerative disorders, it has been difficult to generate TGase2-targeted drugs for clinical use because of the relatively flat and broad active site on TGase2. To design more specific and powerful inhibitors, detailed structural information about TGase2 complexed with various effector and inhibitor molecules is required. In this review, we summarized the current structural studies on TGase2, which will aid in designing drugs that can overcome the aforementioned limitations.

Transglutaminase 2: The Maestro of the Oncogenic Mediators in Renal Cell Carcinoma

Transglutaminase 2 (TG2) is a multifunctional crosslinking enzyme that displays transamidation, protein disulfide isomerase, protein kinase, as well as GTPase and ATPase activities. TG2 can also act as an adhesion molecule involved in the syndecan and integrin receptor signaling. In recent years, TG2 was implicated in cancer progression, survival, invasion, migration, and stemness of many cancer types, including renal cell carcinoma (RCC). Von Hippel-Lindau mutations leading to the subsequent activation of Hypoxia Inducible Factor (HIF)-1-mediated signaling pathways, survival signaling via the PI3K/Akt pathway resulting in Epithelial Mesenchymal Transition (EMT) metastasis and angiogenesis are the main factors in RCC progression. A number of studies have shown that TG2 was important in HIF-1- and PI3K-mediated signaling, VHL and p53 stabilization, glycolytic metabolism and migratory phenotype in RCC. This review focuses on the role of TG2 in the regulation of molecular pathways nurturing not only the development and propagation of RCC, but also drug-resistance and metastatic potential.

Spotlight on the Transglutaminase 2-Heparan Sulfate Interaction

Heparan sulfate proteoglycans (HSPGs), syndecan-4 (Sdc4) especially, have been suggested as potential partners of transglutaminase-2 (TG2) in kidney and cardiac fibrosis, metastatic cancer, neurodegeneration and coeliac disease. The proposed role for HSPGs in the trafficking of TG2 at the cell surface and in the extracellular matrix (ECM) has been linked to the fibrogenic action of TG2 in experimental models of kidney fibrosis. As the TG2-HSPG interaction is largely mediated by the heparan sulfate (HS) chains of proteoglycans, in the past few years a number of studies have investigated the affinity of TG2 for HS, and the TG2 heparin binding site has been mapped with alternative outlooks. In this review, we aim to provide a compendium of the main literature available on the interaction of TG2 with HS, with reference to the pathological processes in which extracellular TG2 plays a role.

Transglutaminase in Receptor and Neurotransmitter-Regulated Functions

Transglutaminases (TGs) and especially TG2 play important roles in neurotransmitter and receptor signaling pathways. Three different mechanisms by which TG2 interacts with neurotransmitter and receptor signaling systems will be discussed in this review. The first way in which TG2 interacts with receptor signaling is via its function as a guanine nucleotide binding protein (G-protein) coupling to G-protein coupled receptors (GPCRs) to activate down-stream signaling pathways. TG2 can exist in a least two conformations, a closed GTP-bound conformation and an open calcium-bound conformation. In the closed GTP-bound conformation, TG2 is capable of functioning as a G-protein for GPCRs. In the open calcium-bound conformation, TG2 catalyzes a transamidation reaction cross-linking proteins or catalyzing the covalent binding of a mono- or polyamine to a protein. The second mechanism is regulation of the transamidation reaction catalyzed by TG2 via receptor stimulation which can increase local calcium concentrations and thereby increase transamidation reactions. The third way in which TG2 plays a role in neurotransmitter and receptor signaling systems is via its use of monoamine neurotransmitters as a substrate. Monoamine neurotransmitters including serotonin can be substrates for transamidation to a protein often a small G-protein (also known as a small GTPase) resulting in activation of the small G-protein. The transamidation of a monoamine neurotransmitter or serotonin has been designated as monoaminylation or more specifically serotonylation, respectively. Other proteins are also targets for monoaminylation such as fibronectin and cytoskeletal proteins. These receptor and neurotransmitter-regulated reactions by TG2 play roles in physiological and key pathophysiological processes.

Anti-type 2 transglutaminase antibodies as modulators of type 2 transglutaminase functions: a possible pathological role in celiac disease

Auto-antibodies to the ubiquitous enzyme type-2 transglutaminase (TG2) are a specific hallmark of celiac disease (CD), a widely diffused, multi-factorial disease, affecting genetically predisposed subjects. In CD an inflammatory response, at the intestinal level, is triggered by diet consumption of gluten-containing cereals. Intestinal mucosa displays various degrees of atrophy and hyperplasia, with consequent global intestinal dysfunction and other relevant extra-intestinal symptoms. Through deamidation of specific glutamines of gluten-derived gliadin peptides, TG2 strongly enhances gliadin immunogenicity. In addition, TG2 cross-linking activity may generate complexes between TG2 itself and gliadin peptides, and these complexes seem to cause the auto-immune response by means of an apten-carrier-like mechanism of antigen presentation. Anti-TG2 antibodies can be early detected in the intestinal mucosa of celiac patients and are also abundantly present into the serum, thus potentially reaching other organs and tissues by blood circulation. Recently, the possible pathogenetic role of auto-antibodies to TG2 in CD has been investigated. Here, we report an overview about the genesis of these antibodies, their specificity, their modulating ability toward TG2 enzymatic or non-enzymatic activities and their biological effects exerted by interacting with extracellular TG2 or with cell-surface TG2. We also discuss the auto-immune response occurring in CD against other TG members (i.e. type 3 and type 6) and analyze the occurrence of anti-TG2 antibodies in other auto-immune CD-related diseases. Data now available let us to suppose that, even if antibodies to TG2 do not represent the triggering molecules in CD, they could be important players in disease progression and manifestations.

Biocatalysis by Transglutaminases: A Review of Biotechnological Applications

The biocatalytic activity of transglutaminases (TGs) leads to the synthesis of new covalent isopeptide bonds (crosslinks) between peptide-bound glutamine and lysine residues, but also the transamidation of primary amines to glutamine residues, which ultimately can result into protein polymerisation. Operating with a cysteine/histidine/aspartic acid (Cys/His/Asp) catalytic triad, TGs induce the post-translational modification of proteins at both physiological and pathological conditions (e.g., accumulation of matrices in tissue fibrosis). Because of the disparate biotechnological applications, this large family of protein-remodelling enzymes have stimulated an escalation of interest. In the past 50 years, both mammalian and microbial TGs polymerising activity has been exploited in the food industry for the improvement of aliments' quality, texture, and nutritive value, other than to enhance the food appearance and increased marketability. At the same time, the ability of TGs to crosslink extracellular matrix proteins, like collagen, as well as synthetic biopolymers, has led to multiple applications in biomedicine, such as the production of biocompatible scaffolds and hydrogels for tissue engineering and drug delivery, or DNA-protein bio-conjugation and antibody functionalisation. Here, we summarise the most recent advances in the field, focusing on the utilisation of TGs-mediated protein multimerisation in biotechnological and bioengineering applications.

Mapping the minimum domain of the fibronectin binding site on transglutaminase 2 (TG2) and its importance in mediating signaling, adhesion, and migration in TG2-expressing cells

The interaction between the enzyme transglutaminase 2 (TG2) and fibronectin (FN) is involved in the cell-matrix interactions that regulate cell signaling, adhesion, and migration and play central roles in pathologic conditions, particularly fibrosis and cancer. A precise definition of the exact interaction domains on both proteins could provide a tool to design novel molecules with potential therapeutic applications. Although specific residues involved in the interaction within TG2 have been analyzed, little is known regarding the TG2 binding site on FN. This site has been mapped to a large internal 45-kDa protein fragment coincident with the gelatin binding domain (GBD). With the goal of defining the minimal FN interacting domain for TG2, we produced several expression constructs encoding different portions or modules of the GBD and tested their binding and functional properties. The results demonstrate that the I₈ module is necessary and sufficient for TG2-binding in vitro, but does not have functional effects on TG2-expressing cells. Modules I₇ and I₉ increase the strength of the binding and are required for cell adhesion. A 15-kDa fragment encompassing modules I_7-9 behaves as the whole 45-kDa GBD and mediates signaling, adhesion, spreading, and migration of TG2⁺ cells. This study provides new insights into the mechanism for TG2 binding to FN.-Soluri, M. F., Boccafoschi, F., Cotella, D., Moro, L., Forestieri, G., Autiero, I., Cavallo, L., Oliva, R., Griffin, M., Wang, Z., Santoro, C., Sblattero, D. Mapping the minimum domain of the fibronectin binding site on transglutaminase 2 (TG2) and its importance in mediating signaling, adhesion, and migration in TG2-expressing cells.

Structural aspects of transglutaminase 2: functional, structural, and regulatory diversity

Transglutaminase 2 (TG2) is a multi-functional protein that has both protein cross-linking and guanosine 5'-triphosphate (GTP) hydrolysis activities. The activities of this protein are controlled by many cellular factors, including calcium (Ca²⁺) and GTP, and have been implicated in several physiological activities, including apoptosis, angiogenesis, wound healing, cellular differentiation, neuronal regeneration, and bone development. TG2 is linked to many human diseases such as inflammatory disease, celiac disease, neurodegenerative disease, diabetes, tissue fibrosis, and various cancers and is one of the most dynamic enzymes in terms of its functions, structures, and regulatory mechanisms. The aim of this review was to summarize the functional, structural, and regulatory diversity of TG2, with a particular focus on the structure of TG2.

High-throughput investigation of transglutaminase 2 kinase regulation using a novel cysteine-modified peptide array

Transglutaminase 2 (TGase2) kinase has emerged as an important regulator of apoptosis as well as chromatin structure and function; however, details about the pathophysiological functions of TGase2 kinase have been limited because of the lack of a suitable activity assay for systematic investigation of TGase2 kinase regulation in a high-throughput manner. Thus, we developed a novel on-chip TGase2 kinase activity assay using a cysteine-modified insulin-like growth factor-binding protein-3-derived peptide (CMI peptide) on an array platform. This peptide array-based activity assay was reproducible, with a detection limit of 2.127 μg/ml. We successfully applied this assay to investigate the effects of thiol-reactive compounds and divalent cations on TGase2 kinase by determining the half maximal inhibitory concentrations (IC₅₀). Thiol-reactive compounds inhibited TGase2 kinase activity in a concentration-dependent manner, with IC₅₀ values ranging from 0.125 to 5.550 mM. Divalent metal cations also showed a concentration-dependent inhibition, with IC₅₀ values ranging from 0.005 to 1.937 mM; however, Ca²⁺ had no effect on TGase2 kinase activity. Thus, this novel kinase activity assay using the CMI peptide array described here is suitable for systematic investigation of TGase2 kinase regulation and may be useful for investigating the roles of TGase2 kinase in pathogenesis of kinase-mediated diseases.

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

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

Activation of transglutaminase 2 by nerve growth factor in differentiating neuroblastoma cells: A role in cell survival and neurite outgrowth

NGF (nerve growth factor) and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 transamidase activity by NGF in retinoic acid-induced differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. The role of TG2 in NGF-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with NGF in N2a and SH-SY5Y cells. NGF mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON (Z-ZON-Val-Pro-Leu-OMe; Benzyloxycarbonyl-(6-Diazo-5-oxonorleucinyl)-l-valinyl-l-prolinyl-l-leucinmethylester) and R283 (1,3,dimethyl-2[2-oxo-propyl]thio)imidazole chloride) and by pharmacological inhibition of extracellular signal-regulated kinases 1 and 2 (ERK1/2), protein kinase B (PKB) and protein kinase C (PKC), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated NGF induced in situ TG2 activity. TG2 inhibition blocked NGF-induced attenuation of hypoxia-induced cell death and neurite outgrowth in both cell lines. Together, these results demonstrate that NGF stimulates TG2 transamidase activity via a ERK1/2, PKB and PKC-dependent pathway in differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. Furthermore, NGF-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results suggest a novel and important role of TG2 in the cellular functions of NGF.

Mineralization-inhibiting effects of transglutaminase-crosslinked polymeric osteopontin

Osteopontin (OPN) belongs to the SIBLING family (Small, Integrin-Binding LIgand N-linked Glycoproteins) of mineral-binding matrix proteins found in bones and teeth. OPN is a well-known inhibitor of matrix mineralization, and enzymatic modification of OPN can affect this inhibitory function. In bone, OPN exists both as a monomer and as a high-molecular-weight polymer - the latter is formed by transglutaminase-mediated crosslinking of glutamine and lysine residues in OPN to create homotypic protein assemblies. OPN can be covalently crosslinked by transglutaminase 2 (TG2) and Factor XIII-A. Polymeric OPN has increased binding to collagen and promotes osteoblast adhesion, but despite these initial observations, its role in mineralization is not clear. In this study, we investigated the effect of polymerized OPN on mineralization using a hydroxyapatite crystal growth assay and mineralizing MC3T3-E1 osteoblast cultures. In the cultures, endogenous polymeric OPN was detected after mineralization occurred. In cell-free conditions, TG2 was used to crosslink bovine OPN into its polymeric form, and atomic force microscopy and dynamic light scattering revealed variably-sized, large branched aggregates ranging across hundreds of nanometers. These OPN polymers inhibited the growth of hydroxyapatite crystals in solution at concentrations similar to monomeric OPN, although the crosslinking slightly reduced its inhibitory potency. When added to MC3T3-E1 osteoblast cultures, this exogenous polymeric OPN essentially did not inhibit mineralization when given during the later mineralization stages of culture; however, cultures treated early and then continuously with polymeric OPN throughout both the matrix assembly and mineral deposition stages showed reduced mineralization. Immunoblotting of protein extracts from these continuously treated cultures revealed exogenous OPN polymers incorporated into mature matrix that had not yet mineralized. These results suggest that in bone, the increased size and branched structure of crosslinked inhibitory polymeric OPN near the mineralization front could hinder it from accessing focal mineralization sites in the dense collagen-rich matrix, suggesting that OPN-crosslinking into polymers may represent a way to fine-tune the inhibitory potency of OPN on bone mineralization.

Role of transglutaminase 2 in PAC1 receptor mediated protection against hypoxia-induced cell death and neurite outgrowth in differentiating N2a neuroblastoma cells

The PAC₁ receptor and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 activity by the PAC₁ receptor in retinoic acid-induced differentiating N2a neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. TG2 phosphorylation was monitored via immunoprecipitation and Western blotting. The role of TG2 in PAC₁ receptor-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27). PACAP-27 mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON and R283 and by pharmacological inhibition of protein kinase A (KT 5720 and Rp-cAMPs), protein kinase C (Ro 31-8220), MEK1/2 (PD 98059), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated PACAP-27 induced in situ TG2 activity. TG2 inhibition blocked PACAP-27 induced attenuation of hypoxia-induced cell death and outgrowth of axon-like processes. TG2 activation and cytoprotection were also observed in human SH-SY5Y cells. Together, these results demonstrate that TG2 activity was stimulated downstream of the PAC₁ receptor via a multi protein kinase dependent pathway. Furthermore, PAC₁ receptor-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results highlight the importance of TG2 in the cellular functions of the PAC₁ receptor.

Transglutaminase 2 has opposing roles in the regulation of cellular functions as well as cell growth and death

Transglutaminase 2 (TG2) is primarily known as the most ubiquitously expressed member of the transglutaminase family with Ca²⁺-dependent protein crosslinking activity; however, this enzyme exhibits multiple additional functions through GTPase, cell adhesion, protein disulfide isomerase, kinase, and scaffold activities and is associated with cell growth, differentiation, and apoptosis. TG2 is found in the extracellular matrix, plasma membrane, cytosol, mitochondria, recycling endosomes, and nucleus, and its subcellular localization is an important determinant of its function. Depending upon the cell type and stimuli, TG2 changes its subcellular localization and biological activities, playing both anti- and pro-apoptotic roles. Increasing evidence indicates that the GTP-bound form of the enzyme (in its closed form) protects cells from apoptosis but that the transamidation activity of TG2 (in its open form) participates in both facilitating and inhibiting apoptosis. A difficulty in the study and understanding of this enigmatic protein is that opposing effects have been reported regarding its roles in the same physiological and/or pathological systems. These include neuroprotective or neurodegenerative effects, hepatic cell growth-promoting or hepatic cell death-inducing effects, exacerbating or having no effect on liver fibrosis, and anti- and pro-apoptotic effects on cancer cells. The reasons for these discrepancies have been ascribed to TG2's multifunctional activities, genetic variants, conformational changes induced by the immediate environment, and differences in the genetic background of the mice used in each of the experiments. In this article, we first report that TG2 has opposing roles like the protagonist in the novel Dr. Jekyll and Mr. Hyde, followed by a summary of the controversies reported, and finally discuss the possible reasons for these discrepancies.

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.

O-GlcNAc glycosylation of p27(kip1) promotes astrocyte migration and functional recovery after spinal cord contusion

Glial scar formation derived from astrocyte proliferation and migration influences the functional recovery after spinal cord injury. Cyclin-dependent kinase inhibitor p27(kip1), whose activity is closely related to its phosphorylation state, reportedly regulates astrocyte proliferation and migration. In this study, we reported that p27(Kip1) undergoes O-GlcNAc modification at Ser 2, Ser 110 and Thr 197. Inhibiting O-GlcNAcylation on Ser 2 by gene mutation (S2A) attenuated the phosphorylation of Ser 10, and vice versa. Interestingly, compared with wild type p27(Kip1), S2A p27(Kip1) displayed a decreased interaction with CRM1 and reduced nuclear export following serum starvation and release. In addition, the interaction between stathmin and S2A p27(Kip1) was also decreased. Cytoskeletal proteins microtubules appeared high density in astrocytes transfected with S2A p27(Kip1) especially at the leading edge of the scratch wound. Accordingly, scratch-wound assay revealed that the motility of astrocytes transfected with S2A p27(Kip1) was faster than that of control. Finally, we injected lentiviral vectors immediately after spinal cord contusion, and found the lesion volume of the rat injected with S2A p27(Kip1) was smaller than that of rat injected with wild type p27(Kip1). Besides, the BBB and CBS behavioral tests showed greater functional recovery in S2A p27(Kip1) treated rats. Taken together, our findings revealed a novel function of O-GlcNAc modification of p27(Kip1) in mediating astrocytes migration and functional recovery after spinal cord contusion.

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.

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.

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.

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.

Celiac anti-type 2 transglutaminase antibodies induce phosphoproteome modification in intestinal epithelial Caco-2 cells

BACKGROUND

Celiac disease is an inflammatory condition of the small intestine that affects genetically predisposed individuals after dietary wheat gliadin ingestion. Type 2-transglutaminase (TG2) activity seems to be responsible for a strong autoimmune response in celiac disease, TG2 being the main autoantigen. Several studies support the concept that celiac anti-TG2 antibodies may contribute to disease pathogenesis. Our recent findings on the ability of anti-TG2 antibodies to induce a rapid intracellular mobilization of calcium ions, as well as extracellular signal-regulated kinase phosphorylation, suggest that they potentially act as signaling molecules. In line with this concept, we have investigated whether anti-TG2 antibodies can induce phosphoproteome modification in an intestinal epithelial cell line.

METHODS AND PRINCIPAL FINDINGS

We studied phosphoproteome modification in Caco-2 cells treated with recombinant celiac anti-TG2 antibodies. We performed a two-dimensional electrophoresis followed by specific staining of phosphoproteins and mass spectrometry analysis of differentially phosphorylated proteins. Of 14 identified proteins (excluding two uncharacterized proteins), three were hypophosphorylated and nine were hyperphosphorylated. Bioinformatics analyses confirmed the presence of phosphorylation sites in all the identified proteins and highlighted their involvement in several fundamental biological processes, such as cell cycle progression, cell stress response, cytoskeletal organization and apoptosis.

CONCLUSIONS

Identification of differentially phosphorylated proteins downstream of TG2-antibody stimulation suggests that in Caco-2 cells these antibodies perturb cell homeostasis by behaving as signaling molecules. We hypothesize that anti-TG2 autoantibodies may destabilize the integrity of the intestinal mucosa in celiac individuals, thus contributing to celiac disease establishment and progression. Since several proteins here identified in this study were already known as TG2 substrates, we can also suppose that transamidating activity and differential phosphorylation of the same targets may represent a novel regulatory mechanism whose relevance in celiac disease pathogenesis is still unexplored.

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.

Gα(h)/transglutaminase-2 activity is required for maximal activation of adenylylcyclase 8 in human and rat glioma cells

Gα(h) (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 Gα(h)/TG2 in signal transduction in glial cells was examined in detail. In 1321N1 human astrocytoma cells that lack Gα(h)/TG2, overexpression of Gα(h)/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 Gα(h)/TG2, cAMP accumulation induced by isoproterenol or forskolin was significantly inhibited by overexpression of Gα(h)/TG2-C277V, a dominant-negative mutant that lacks transglutaminase activity, but was not inhibited by the Gα(h)/TG2-S171E mutant that cannot bind GTP/GDP. These results suggest Gα(h)/TG2 potentiates adenylylcyclase activity by its transglutaminase activity and not by its G-protein activity. Gα(h)/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 Gα(h)/TG2. Adenylylcyclase 8 is expressed in both 1321N1 and C6 cells; however, Gα(h)/TG2 affected neither adenylylcyclase 8 expression levels, glycosylation, nor dimerization status. In contrast, pentylamine, a substrate of Gα(h)/TG2, was incorporated into adenylylcyclase 8 in a transglutaminase activity-dependent manner. Taking these results together, Gα(h)/TG2 promotes cAMP production accompanied by a modification of adenylylcyclase 8 in glioma cells.

Transglutaminase 2 facilitates or ameliorates HIF signaling and ischemic cell death depending on its conformation and localization

Transglutaminase 2 (TG2) is a widely expressed and multifunctional protein that modulates cell death/survival processes. We have previously shown that TG2 binds to hypoxia inducible factor 1β (HIF1β) and decreases the upregulation of HIF responsive genes; however, the relationship between these observations was not investigated. In this study, we investigated whether endogenous TG2 is sufficient to suppress HIF activity and whether the interaction between TG2 and HIF1β is required for this suppression. shRNA-mediated silencing of TG2 significantly enhanced HIF activation in response to hypoxia. In addition, nuclear localization of TG2 is required for its suppressive effect on HIF activity, with TG2 being recruited to HIF responsive promoters in hypoxic conditions. These observations suggest that TG2 directly regulates hypoxic transcriptional machinery; however, its interaction with HIF1β was not required for this regulation. We also examined whether TG2's effect on cell death/survival processes in ischemia is due to its effects on HIF signaling. Our results indicate that TG2 mediated HIF suppression can be separated from TG2's effect on cell survival in hypoxic/hypoglycemic conditions. Lastly, here we show that nuclear TG2 in the closed conformation and non-nuclear TG2 in the open conformation have opposing effects on hypoxic/hypoglycemic cell death, which could explain previous controversial results. Overall, our results further clarify the role of TG2 in mediating the cellular response to ischemia and suggest that manipulating the conformation of TG2 might be of pharmacological interest as a therapeutic strategy for the treatment of ischemia-related pathologies.

Phosphorylation of transglutaminase 2 (TG2) at serine-216 has a role in TG2 mediated activation of nuclear factor-kappa B and in the downregulation of PTEN

Background

Transglutaminase 2 (TG2) and its phosphorylation have been consistently found to be upregulated in a number of cancer cell types. At the molecular level, TG2 has been associated with the activation of nuclear factor-kappa B (NF-κB), protein kinase B (PKB/Akt) and in the downregulation of phosphatase and tensin homologue deleted on chromosome 10 (PTEN). However, the underlying mechanism involved is not known. We have reported that protein kinase A (PKA) induced phosphorylation of TG2 at serine-216 (Ser²¹⁶) regulates TG2 function and facilitates protein-protein interaction. However, the role of TG2 phosphorylation in the modulation of NF-κB, Akt and PTEN is not explored.

Methods

In this study we have investigated the effect of TG2 phosphorylation on NF-κB, Akt and PTEN using embryonic fibroblasts derived from TG2 null mice (MEF^tg2-/-) overexpressing native TG2 or mutant-TG2 (m-TG2) lacking Ser²¹⁶ phosphorylation site with and without dibutyryl cyclic-AMP (db-cAMP) stimulation. Functional consequences on cell cycle and cell motility were determined by fluorescence activated cell sorting (FACS) analysis and cell migration assay respectively.

Results

PKA activation in TG2 overexpressing MEF^tg2-/- cells resulted in an increased activation of NF-κB and Akt phosphorylation in comparison to empty vector transfected control cells as determined by the reporter-gene assay and immunoblot analysis respectively. These effects were not observed in MEF^tg2-/- cells overexpressing m-TG2. Similarly, a significant downregulation of PTEN at both, the mRNA and protein levels were found in cells overexpressing TG2 in comparison to empty vector control and m-TG2 transfected cells. Furthermore, Akt activation correlated with the simultaneous activation of NF-κB and a decrease in PTEN suggesting that the facilitatory effect of TG2 on Akt activation occurs in a PTEN-dependent manner. Similar results were found with MCF-7 and T-47D breast cancer cells overexpressing TG2 and m-TG2 further supporting the role of TG2 phosphorylation in NF-κB activation and in the downregulation of PTEN.

Conclusions

Collectively, these data suggest that phosphorylation of TG2 at Ser²¹⁶ plays a role in TG2 mediated activation of NF-κB, Akt and in the downregulation of PTEN. Blocking TG2 phosphorylation may provide a novel strategy to attenuate NF-κB activation and downregulation of PTEN in TG2 overexpressing cancers.

Functional characterization of naturally occurring transglutaminase 2 mutants implicated in early-onset type 2 diabetes

Transglutaminase 2 (TG2) is an enzyme with diverse biological functions. TG2 catalyzes transamidation reactions, has intrinsic kinase activity, and acts as a G-protein in intracellular signaling. TG2 (Tgm2)-null mice are glucose intolerant and have impaired glucose-stimulated insulin secretion (GSIS). Furthermore, three naturally occurring missense mutations in the human TGM2 gene, corresponding to amino acid substitutions of Met330Arg, Ile331Asn, and Asn333Ser in the TG2 protein, have been reported and found to be associated with early-onset type 2 diabetes. However, their effect on TG2 function is not fully understood. To determine this, we have reproduced naturally occurring mutations in TG2 using site-directed mutagenesis. Overexpression of Myc-TG2 mutants in INS-1E cells resulted in a reduction of GSIS in comparison with cells overexpressing wild-type Myc-TG2 (WT-TG2). The maximum reduction was found in cells overexpressing Ile331Asn-TG2 (32%) followed by Met330Arg-TG2 (20%), and the least in Asn333Ser-TG2 (7%). Enzymatic analysis revealed that TG2 mutants have impaired transamidation and kinase activities in comparison with WT-TG2. GTP-binding assays showed that TG2 mutants also have altered GTP-binding ability, which is found to be modulated in response to glucose stimulation. Collectively, these data suggest that naturally occurring mutations in TG2 affect transamidation, kinase, and GTP-binding functions of TG2. While reduced insulin secretion, as a result of naturally occurring mutations in TG2, is due to the impairment of more than one biological function of TG2, it is the transamidation function that appears to be impaired during the first phase, whereas the GTP-binding function affects the second phase of insulin secretion.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Expression of tissue transglutaminase on primary olfactory ensheathing cells cultures exposed to stress conditions

Tissue transglutaminase (TG2), a multifunctional enzyme implicated in cellular proliferation and differentiation processes, plays a modulatory role in the cell response to stressors. Herein, we used olfactory ensheathing cells (OECs), representing an unusual population of glial cells to promote axonal regeneration and to provide trophic support, as well as to assess whether the effect of some Growth Factors (GFs), NGF, bFGF or GDNF, on TG2 overexpression induced by stress conditions, such as glutamate or lipopolysaccaride (LPS). Glial Fibrillary Acidic Protein (GFAP) and vimentin were used as markers of astroglial differentiation and cytoskeleton component, respectively. Glutamate or LPS treatment induced a particular increase of TG2 expression. A pre-treatment of the cells with the GFs restored the levels of the protein to that of untreated ones. Our results demonstrate that the treatment of OECs with the GFs was able to restore the OECs oxidative status as modified by stress, also counteracting TG2 overexpression. It suggests that, in OECs, TG2 modulation or inhibition induced by GFs might represent a therapeutic target to control the excitotoxicity and/or inflammation, which are involved in several acute and chronic brain diseases.