Interaction of tissue transglutaminase with nuclear transport protein importin-alpha3

Pathogenetic Contributions and Therapeutic Implications of Transglutaminase 2 in Neurodegenerative Diseases

Neurodegenerative diseases encompass a heterogeneous group of disorders that afflict millions of people worldwide. Characteristic protein aggregates are histopathological hallmark features of these disorders, including Amyloid β (Aβ)-containing plaques and tau-containing neurofibrillary tangles in Alzheimer's disease, α-Synuclein (α-Syn)-containing Lewy bodies and Lewy neurites in Parkinson's disease and dementia with Lewy bodies, and mutant huntingtin (mHTT) in nuclear inclusions in Huntington's disease. These various aggregates are found in specific brain regions that are impacted by neurodegeneration and associated with clinical manifestations. Transglutaminase (TG2) (also known as tissue transglutaminase) is the most ubiquitously expressed member of the transglutaminase family with protein crosslinking activity. To date, Aβ, tau, α-Syn, and mHTT have been determined to be substrates of TG2, leading to their aggregation and implicating the involvement of TG2 in several pathophysiological events in neurodegenerative disorders. In this review, we summarize the biochemistry and physiologic functions of TG2 and describe recent advances in the pathogenetic role of TG2 in these diseases. We also review TG2 inhibitors tested in clinical trials and discuss recent TG2-targeting approaches, which offer new perspectives for the design of future highly potent and selective drugs with improved brain delivery as a disease-modifying treatment for neurodegenerative disorders.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

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.

Transglutaminase type 2-dependent crosslinking of IRF3 in dying melanoma cells

cGAS/STING axis is the major executor of cytosolic dsDNA sensing that leads to the production of type I interferon (IFNI) not only upon bacterial infection, but also in cancer cells, upon DNA damage. In fact, DNA damage caused by ionizing radiations and/or topoisomerase inhibitors leads to a release of free DNA into the cytosol, which activates the cGAS/STING pathway and the induction of IFNI expression. Doxorubicin-induced apoptotic cancer cells release damage-associated molecular patterns (DAMPs), including IFNI, which are able to stimulate the immune system. Our results indicate that Transglutaminase type 2 (TG2) is directly involved in the formation of a covalent cross-linked IRF3 (Interferon regulatory factor 3) dimers, thereby limiting the production of IFNI. Indeed, we demonstrated that upon doxorubicin treatment TG2 translocates into the nucleus of apoptotic melanoma cells interacting with IRF3 dimers. Interestingly, we show that both the knockdown of the enzyme as well as the inhibition of its transamidating activity lead to a decrease in the dimerization of IRF3 correlated with an increase in the IFNI mRNA levels. Taken together, these data demonstrate that TG2 negatively regulates the IRF3 pathway in human melanoma cells suggesting a so far unknown TG2-dependent mechanism by which cancer cells reduce the IFNI production after DNA damage to limit the immune system response.

The Multifaceted Role of HSF1 in Pathophysiology: Focus on Its Interplay with TG2

The cellular environment needs to be strongly regulated and the maintenance of protein homeostasis is crucial for cell function and survival. HSF1 is the main regulator of the heat shock response (HSR), the master pathway required to maintain proteostasis, as involved in the expression of the heat shock proteins (HSPs). HSF1 plays numerous physiological functions; however, the main role concerns the modulation of HSPs synthesis in response to stress. Alterations in HSF1 function impact protein homeostasis and are strongly linked to diseases, such as neurodegenerative disorders, metabolic diseases, and different types of cancers. In this context, type 2 Transglutaminase (TG2), a ubiquitous enzyme activated during stress condition has been shown to promote HSF1 activation. HSF1-TG2 axis regulates the HSR and its function is evolutionary conserved and implicated in pathological conditions. In this review, we discuss the role of HSF1 in the maintenance of proteostasis with regard to the HSF1-TG2 axis and we dissect the stress response pathways implicated in physiological and pathological conditions.

Transglutaminase Type 2 regulates the Wnt/β-catenin pathway in vertebrates

TG2 is a multifunctional enzyme involved in several cellular processes and has emerging as a potential regulator of gene expression. In this regard, we have recently shown that TG2 is able to activate HSF1, the master transcriptional regulator of the stress-responsive genes; however, its effect on the overall gene expression remains unclear. To address this point, we analyzed, by RNA-seq, the effect of TG2 on the overall transcriptome as well as we characterized the TG2 interactome in the nucleus. The data obtained from these omics approaches reveal that TG2 markedly influences the overall cellular transcriptome profile and specifically the Wnt and HSF1 pathways. In particular, its ablation leads to a drastic downregulation of many key members of these pathways. Interestingly, we found that key components of the Wnt/β-catenin pathway are also downregulated in cells lacking HSF1, thus confirming that TG2 regulates the HSF1 and this axis controls the Wnt signaling. Mechanistic studies revealed that TG2 can regulate the Wnt pathway by physically interacts with β-catenin and its nuclear interactome includes several proteins known to be involved in the regulation of the Wnt signaling. In order to verify whether this effect is playing a role in vivo, we ablated TG2 in Danio rerio. Our data show that the zebrafish lacking TG2 cannot complete the development and their death is associated with an evident downregulation of the Wnt pathway and a defective heat-shock response. Our findings show for the first time that TG2 is essential for the correct embryonal development of lower vertebrates, and its action is mediated by the Wnt/HSF1 axis.

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.

Transglutaminase-catalyzed incorporation of polyamines masks the DNA-binding region of the transcription factor Relish

In Drosophila, the final immune deficiency (IMD) pathway-dependent signal is transmitted through proteolytic conversion of the nuclear factor-κB (NF-κB)-like transcription factor Relish to the active N-terminal fragment Relish-N. Relish-N is then translocated from the cytosol into the nucleus for the expression of IMD-controlled genes. We previously demonstrated that transglutaminase (TG) suppresses the IMD pathway by polymerizing Relish-N to inhibit its nuclear translocation. Conversely, we also demonstrated that orally ingested synthetic amines, such as monodansylcadaverine (DCA) and biotin-labeled pentylamine, are TG-dependently incorporated into Relish-N, causing the nuclear translocation of modified Relish-N in gut epithelial cells. It remains unclear, however, whether polyamine-containing Relish-N retains transcriptional activity. Here, we used mass spectrometry analysis of a recombinant Relish-N modified with DCA by TG activity after proteolytic digestion and show that the DCA-modified Gln residues are located in the DNA-binding region of Relish-N. TG-catalyzed DCA incorporation inhibited binding of Relish-N to the Rel-responsive element in the NF-κB-binding DNA sequence. Subcellular fractionation of TG-expressing Drosophila S2 cells indicated that TG was localized in both the cytosol and nucleus. Of note, natural polyamines, including spermidine and spermine, competitively inhibited TG-dependent DCA incorporation into Relish-N. Moreover, in vivo experiments demonstrated that Relish-N was modified by spermine and that this modification reduced transcription of IMD pathway-controlled cecropin A1 and diptericin genes. These findings suggest that intracellular TG regulates Relish-N-mediated transcriptional activity by incorporating polyamines into Relish-N and via protein-protein cross-linking.

Role of tissue transglutaminase-2 (TG2)-mediated aminylation in biological processes

Post-translational modification (PTM) is an important mechanism in modulating a protein's structure and can lead to substantial diversity in biological function. Compared to other forms of PTMs such as phosphorylation, acetylation and glycosylation, the physiological significance of aminylation is limited. Aminylation refers to the covalent incorporation of biogenic/polyamines into target protein by calcium-dependent transglutaminases (TGs). The development of novel and more sensitive techniques has led to more proteins identified as tissue transglutaminase (TG2) substrates and potential targets for aminylation. Many of these substrate proteins play a role in cell signaling, cytoskeleton organization, muscle contraction, and inflammation. TG2 is well studied and widely expressed in a variety of tissues and will be the primary focus of this review on recent advance in transglutaminase-mediated aminylation.

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.

Moonlighting proteins in cancer

Since the 1980s, growing evidence suggested that the cellular localization of proteins determined their activity and biological functions. In a classical view, a protein is characterized by the single cellular compartment where it primarily resides and functions. It is now believed that when proteins appear in different subcellular locations, the cells surpass the expected activity of proteins given the same genomic information to fulfill complex biological behavior. Many proteins are recognized for having the potential to exist in multiple locations in cells. Dysregulation of translocation may cause cancer or contribute to poorer cancer prognosis. Thus, quantitative and comprehensive assessment of dynamic proteins and associated protein movements could be a promising indicator in determining cancer prognosis and efficiency of cancer treatment and therapy. This review will summarize these so-called moonlighting proteins, in terms of a coupled intracellular cancer signaling pathway. Determination of the detailed biological intracellular and extracellular transit and regulatory activity of moonlighting proteins permits a better understanding of cancer and identification of potential means of molecular intervention.

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.

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.

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.

Transglutaminase is a therapeutic target for oxidative stress, excitotoxicity and stroke: a new epigenetic kid on the CNS block

Transglutaminases (TGs) are multifunctional, calcium-dependent enzymes that have been recently implicated in stroke pathophysiology. Classically, these enzymes are thought to participate in cell injury and death in chronic neurodegenerative conditions via their ability to catalyze covalent, nondegradable crosslinks between proteins or to incorporate polyamines into protein substrates. Accumulating lines of inquiry indicate that specific TG isoforms can shuttle into the nucleus when they sense pathologic changes in calcium or oxidative stress, bind to chromatin and thereby transduce these changes into transcriptional repression of genes involved in metabolic or oxidant adaptation. Here, we review the evidence that supports principally a role for one isoform of this family, TG2, in cell injury and death associated with hemorrhagic or ischemic stroke. We also outline an evolving model in which TG2 is a critical mediator between pathologic signaling and epigenetic modifications that lead to gene repression. Accordingly, the salutary effects of TG inhibitors in stroke may derive from their ability to restore homeostasis by removing inappropriate deactivation of adaptive genetic programs by oxidative stress or extrasynaptic glutamate receptor signaling.

Role of transglutaminase 2 in celiac disease pathogenesis

A number of lines of evidence suggest that transglutaminase 2 (TG2) may be one of the earliest disease-relevant proteins to encounter immunotoxic gluten in the celiac gut. These and other investigations also suggest that the reaction catalyzed by TG2 on dietary gluten peptides is essential for the pathogenesis of celiac disease. If so, several questions are of critical significance. How is TG2 activated in the celiac gut? What are the disease-specific and general consequences of activating TG2? Can local inhibition of TG2 in the celiac intestine suppress gluten induced pathogenesis in a dose-responsive manner? And what are the long-term consequences of suppressing TG2 activity in the small intestinal mucosa? Answers to these questions will depend upon the development of judicious models and chemical tools. They also have the potential of yielding powerful next-generation drug candidates for treating this widespread but overlooked chronic disease.

Regulation of transglutaminase-mediated hepatic cell death in alcoholic steatohepatitis and non-alcoholic steatohepatitis

BACKGROUND AND AIM

Transglutaminase 2 (TG2), catalyzing crosslinking between lysine and glutamine residues, is involved in many liver diseases. We previously reported that TG2, induced in the nucleus of ethanol- or free fatty acids (FFAs)-treated hepatic cells, crosslinks and inactivates a transcription factor Sp1, leading to reduced expression of c-Met and thereby caspase independent hepatic apoptosis in culture systems, animal models, and both alcoholic steatohepatitis (ASH) and non-alcoholic steatohepatitis (NASH) patients. FFAs increase endoplasmic reticulum (ER) stress, NFkB activation and nuclear TG2 (nTG2) through pancreatic ER kinase (PERK)-dependent pathway, whereas ethanol induces nTG2 via retinoid signaling. However, the molecular mechanism by which ethanol/FFAs induce nuclear localization of TG2 has been unclear.

METHOD

A similar nTG2-mediated cell death is induced in acyclic retinoid (ACR)-treated hepatocellular carcinoma. Using cultured cells, we investigated how to control this novel apoptotic pathway by regulating nuclear localization of TG2.

RESULTS

TG2 is composed of N-terminal b-sandwich, catalytic core, b-barrel 1, and C-terminal b-barrel 2 domains. In a previous work, we identified a 14 amino acid nuclear localization signal (NLS) within the b-barrel 1 domain and a putative leucine-rich nuclear export signal (NES) at position 657 to 664 (LHMGLHKL) near the C-terminus in the b-barrel 2 domain, and found that ACR downregulated exportin-1 levels, thereby accumulation of TG2 in the nucleus. Here, we found that both ethanol and FFAs provoked generation of truncated short form of TG2 (TG2-S) defects in the putative NES at least in part through alternative splicing, thereby causing accumulation of TG2-S in the nucleus.

CONCLUSION

The generation of TG2-S in ethanol or FFAs-treated hepatic cells is a novel therapeutic target for prevention of hepatic cell death associated with ASH/NASH.

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.

Transglutaminase 2: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

Protein transamidation by transglutaminase 2 in cells: a disputed Ca2+-dependent action of a multifunctional protein

Transglutaminase 2 (TG2) is the first described cellular member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. During the last two decades its additional enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, protein kinase) and non-enzymatic (multiple interactions in protein scaffolds) activities, which do not require Ca(2+) , have been recognized. It became a prevailing view that TG2 is silent as a transamidase, except in extreme stress conditions, in the intracellular environment characterized by low Ca(2+) and high GTP concentrations. To counter this presumption a critical review of the experimental evidence supporting the role of this enzymatic activity in cellular processes is provided. It includes the structural basis of TG2 regulation through non-canonical Ca(2+) binding sites, mechanisms making it sensitive to low Ca(2+) concentrations, techniques developed for the detection of protein transamidation in cells and examples of basic cellular phenomena as well as pathological conditions influenced by this irreversible post-translational protein modification.

New insights into the functions and localization of nuclear transglutaminase 2

Transglutaminase 2 (TG2; EC 2.3.2.13) is the most abundantly expressed member of the transglutaminase family and exerts opposing effects on cell growth, differentiation and apoptosis via multiple activities, including transamidase, GTPase, cell adhesion, protein disulfide isomerase, kinase and scaffold activities. It is distributed in and around various parts of a cell, including the extracellular matrix, plasma membrane, cytosol, mitochondria and nucleus. Generally, nuclear TG2 represents only 5-7% of the total TG2 in a cell, and various stimuli will increase nuclear TG2 via cellular stress and/or an increased intracellular Ca(2+) concentration. There is increasing evidence indicating the importance of nuclear TG2 in regulating gene expression via post-translational modification of (or interaction with) transcriptional factors and related proteins. These include E2F1, hypoxia inducible factor 1, Sp1 and histones. Through this mechanism, TG2 controls cell growth or survival, differentiation and apoptosis, and is involved in the pathogenesis and/or treatment of neurodegenerative diseases, liver diseases and cancers. The balance between import from the cytoplasm to the nucleus, and export from the nucleus to the cytoplasm, determines the level of TG2 in the nucleus. Selective regulation of the expression, activity or localization of nuclear TG2 will be important for basic research, as well as clinical applications, suggesting a new era for this long-studied enzyme.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

TG2, a novel extracellular protein with multiple functions

TG2 is multifunctional enzyme which can be secreted to the cell surface by an unknown mechanism where its Ca(2+)-dependent transamidase activity is implicated in a number of events important to cell behaviour. However, this activity may only be transient due to the oxidation of the enzyme in the extracellular environment including its reaction with NO probably accounting for its many other roles, which are transamidation independent. In this review, we discuss the novel roles of TG2 at the cell surface and in the ECM acting either as a transamidating enzyme or as an extracellular scaffold protein involved in cell adhesion. Such roles include its ability to act as an FN co-receptor for β integrins or in a heterocomplex with FN interacting with the cell surface heparan sulphate proteoglycan syndecan-4 leading to activation of PKCα. These different properties of TG2 involve this protein in various physiological processes, which if not regulated appropriately can also lead to its involvement in a number of diseases. These include metastatic cancer, tissue fibrosis and coeliac disease, thus increasing its attractiveness as both a therapeutic target and diagnostic marker.

Role and predictive strength of transglutaminase type 2 expression in premalignant lesions of the cervix

The demonstration that type 2 transglutaminase (TG2) can incorporate polyamine into the E7 oncoprotein of human papillomavirus (HPV) type 18 has led to the hypothesis that TG2 can have a role in the host cellular response to HPV infection. The aim of this study was to investigate whether HPV-related pathology, in infected human cervical epithelium, was associated with modulation of TG2 expression. Normal controls and HPV-infected cervical biopsies were analyzed for the expression of TG2, and the findings were compared with lesion grade. The correlation between TG2 expression and p16, a marker for HPV-induced dysplasia, and the retinoblastoma protein (Rb), a target of the E7 protein of HPV, was also investigated. Results obtained showed that TG2 was absent in normal squamous mucosa, whereas it was present in 100% CIN I lesions. Low-grade lesions showed significantly higher TG2 expression than high-grade lesions (P<0.0001). In 94% of CIN I more than 50% of the cells were positive for TG2, with a strong staining intensity (+3), whereas a decreased staining intensity and a low number of positive cells were found in CIN II/III. In CIN I cases, both nuclear and cytoplasmic staining were found in cells exhibiting classical morphological features of HPV infection. In addition, during progression from low-grade squamous intraepithelial lesions to severe dysplasia, TG2 expression was inversely correlated with p16 (Pearson: -0.930), whereas a positive correlation was observed between the expression of TG2 and pRb (Pearson: 0.997). TG2 is expressed in HPV infection as an early phenomenon, not restricted to high-risk genotypes. TG2 upregulation is probably part of host cell reaction against HPV-induced tissue modification. It may act as a cellular antioxidant defense factor, playing an important role in counteracting oxidative damage in neoplastic disease.

Genome-wide expression analysis of roxarsone-stimulated growth of broiler chickens (Gallus gallus)

Roxarsone is a commonly used additive in chicken (Gallus gallus) industry. However, little is known on the intrinsic molecular mechanism via which the growth performance of birds improves. This study was therefore performed to investigate the expression profiles of genes induced by roxarsone. Fifty-six broiler chickens were divided into two groups, namely treated and untreated with roxarsone. The treated group was provided a diet of 45.4mg/kg roxarsone medication and the other group acted as control. Data analysis showed that roxarsone consistently and significantly (P<0.05) increased chicken growth performance. In addition to this a significant (P<0.05) increase of arsenic residue in liver has been seen. Microarray expression analysis of 8935 genes in liver showed that 22 genes (10 up- and 12 down-regulated) had altered expression throughout the experimental periods. Two novel genes (GenBank accession no. GU724343 and GU724344) were cloned through rapid amplification of cDNA ends (RACE). Gene GU724343 was predicted to encode an unidentified protein and the second gene GU724344 was presumed to encode a new member of immunoglobulin-like receptor (CHIR) family. Our results suggested for the first time that the role of roxarsone could be mainly to modify the expression levels of cell growth, immunity/defense and energy metabolism associated genes, as a result promoting animal growth. Further research on these genes should help to increase the knowledge of improving animal productivity safely and effectively.

Effect of therapeutic mild hypothermia on the genomics of the hippocampus after moderate traumatic brain injury in rats

BACKGROUND

Traumatic brain injury (TBI), a major cause of morbidity and mortality, is a serious public health concern.

OBJECTIVE

To evaluate the effect of mild hypothermia on gene expression in the hippocampus and to try to elucidate molecular mechanisms of hypothermic neuroprotection after TBI.

METHODS

Rats were subjected to mild hypothermia (group 1: n = 3, 33 degrees C, 3H) or normothermia (group 2: n = 3; 37 degrees C, 3H) after TBI. Six genome arrays were applied to detect the gene expression profiles of ipsilateral hippocampus. Functional clustering and gene ontology analysis were then carried out. Another 20 rats were randomly assigned to 4 groups (n = 5 per group): group 3, sham-normothermia; group 4, sham-hypothermia; group 5, TBI-normothermia; and group 6, TBI-hypothermia. Real-time fluorescent quantitative reverse-transcription polymerase chain reaction was used to detect specific selected genes.

RESULTS

We found that 133 transcripts in the hypothermia group were statistically different from those in the normothermia group, including 57 transcripts that were upregulated and 76 that were downregulated after TBI (P < .01). Most of these genes were involved in various pathophysiological processes, and some were critical to cell survival. Analysis showed that 9 gene ontology categories were significantly affected by hypothermia, including the most affected categories: synapse organization and biogenesis (upregulated) and regulation of inflammatory response (downregulated). The mRNA expression of Ank3, Cmbp, Nrxn3, Tgm2, and Fcgr3 was regulated by hypothermia, TBI, or a combination of TBI and hypothermia compared with the sham-normothermia group. Their mRNA expression was significantly regulated by hypothermia in TBI groups.

CONCLUSION

Posttraumatic mild hypothermia has a significant effect on the gene expression profiles of the hippocampus, especially those genes belonging to the 9 gene ontology categories. Differential expression of those genes may be involved in the most fundamental molecular mechanisms of cerebral protection by mild hypothermia after TBI.

Cytosolic guanine nucledotide binding deficient form of transglutaminase 2 (R580a) potentiates cell death in oxygen glucose deprivation

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

Hyperosmolarity-mediated mitochondrial dysfunction requires Transglutaminase-2 in human corneal epithelial cells

Hyperosmolar-induced ocular surface cell death is a key mitochondria-mediated event in inflammatory eye diseases. Transglutaminase (TGM)-2, a cross-linking enzyme, is purported to mediate cell death, but its link to mitochondria is unclear. In the cornea, the integrity of the epithelial cells is important for maintaining transparency of the cornea and therefore functional vision. We evaluated the role of TGM-2 and its involvement in hyperosmolarity-stimulated mitochondrial cell death in human corneal epithelial (HCE-T) cells. HCE-T cell lines stably expressing either shRNA targeting TGM-2 (shTG) or scrambled shRNA (shRNA) were constructed. Hyperosmolar conditions reduced viability and increased mitochondrial depolarization in shRNA cells. However, hyperosmolarity failed to induce mitochondrial depolarization to the same extent in shTG cells. Transient overexpression of TGM-2 resulted in very high levels of TGM-2 expression in shTG and shRNA cells. In the case of shTG cells after overexpression of TGM-2, hyperosmolarity induced the same extent of mitochondrial depolarization as similarly treated shRNA cells. Overexpression of TGM-2 also elevated transamidase activity and reduced viability. It also induced mitochondrial depolarization, increased caspase-3/7 and -9 activity, and these increases were partially suppressed by pan-caspase inhibitor Z-VAD-FMK. Corneal epithelial apoptosis via mitochondrial dysfunction after hyperosmolar stimulation is partially dependent on TGM-2. This TGM-2-dependent mechanism occurs in part via caspase-3/7 and -9. Protection against mitochondrial stress in the ocular surface targeting TGM-2 may have important implications in the survival of cells in hyperosmolar stress.

Dual induction of caspase 3- and transglutaminase-dependent apoptosis by acyclic retinoid in hepatocellular carcinoma cells

Background

Hepatocellular carcinoma has a high mortality rate due to its rate of recurrence. Acyclic retinoid prevents recurrence of hepatocellular carcinoma in patients after surgical removal of their primary tumors by inducing apoptosis in hepatocellular carcinoma cells, although the molecular mechanisms of action are not understood.

Methods

Human hepatocellular carcinoma cells in culture, as well as nude mice transplanted with hepatocellular carcinoma cells and rats given with N-diethylnitrosamine were treated with acyclic retinoid. Changes in activated caspase 3 and transglutaminase 2 (TG2) levels, Sp1 cross-linking and its activities, expression of epidermal growth factor receptor, and apoptotic levels were measured.

Results

Acyclic retinoid simultaneously stimulated the activation of caspase 3, and the expression, nuclear localization and crosslinking activity of TG2, resulting in crosslinking and inactivation of the transcription factor, Sp1, thereby reducing expression of epidermal growth factor receptor and cell death in three hepatocellular carcinoma cell lines. These effects were partially restored by a caspase inhibitor, transfection of antisense TG2, restoration of functional Sp1, or an excess of epidermal growth factor. Nuclear expression of TG2 and crosslinked Sp1, as also activated caspase 3 were found in both hepatocellular carcinoma cells transplanted into nude mice and cancerous regions within the liver in N-diethylnitrosamine-induced hepatocarcinogenesis model in rats, following treatment of animals with acyclic retinoid.

Conclusions

Treatment with acyclic retinoid produces a dual activation of caspase 3 and TG2 induced apoptosis of hepatocellular carcinoma cells via modification and inactivation of Sp1, resulting in reduced expression of epidermal growth factor receptor.

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

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

Type 2 transglutaminase in Huntington's disease: a double-edged sword with clinical potential

Huntington's disease (HD) is a dominant genetic neurodegenerative disorder. The pathology affects principally neurons in the basal ganglia circuits and terminates invariably in death. There is compelling necessity for safe and effective therapeutic strategies to arrest, or even retard the progression of the pathogenesis. Recent findings indicate the autophagy-lysosome systems as appealing targets for pharmacological intervention. Autophagy exerts a critical role in controlling neuronal protein homeostasis, which is perturbed in HD, and is compromised in the pathogenesis of several neurodegenerative diseases. Type 2 transglutaminase (TG2) plays an important role both in apoptosis and autophagy regulation, and accumulates at high levels in cells under stressful conditions. TG2 inhibition, achieved either via drug treatments or genetic approaches, has been shown to be beneficial for the treatment of HD in animal models. In this review we will discuss the relevance of TG2 to the pathogenesis of HD, in an effort to define novel therapeutic avenues.

Transglutaminase 2 protects against ischemic stroke

Transglutaminase 2 (TG2) is a multifunctional protein that modulates cell survival and death pathways. It is upregulated in numerous ischemic models, and protects primary neurons from oxygen and glucose deprivation. TG2 binds to the hypoxia inducible factor (HIF) 1beta and decreases the upregulation of hypoxic-induced proapoptotic genes. To investigate the role of TG2 in ischemic stroke in vivo, we used the murine, permanent middle cerebral artery (MCA) ligation model. TG2 mRNA levels are increased after MCA ligations, and transgenic mice that express human TG2 in neurons had significantly smaller infarct volumes than wild type littermates. Further, TG2 translocates into the nucleus within 2h post ligation. Nuclear-localized TG2 is also apparent in human stroke cases. TG2 suppressed the upregulation of the HIF-induced, proapoptotic gene, Noxa. The findings of this study indicate that TG2 plays a role in attenuating ischemic-induced cell death possibly by modulating hypoxic-induced transcriptional processes.

Transglutaminase 2: a multi-functional protein in multiple subcellular compartments

Transglutaminase 2 (TG2) is a multifunctional protein that can function as a transglutaminase, G protein, kinase, protein disulfide isomerase, and as an adaptor protein. These multiple biochemical activities of TG2 account for, at least in part, its involvement in a wide variety of cellular processes encompassing differentiation, cell death, inflammation, cell migration, and wound healing. The individual biochemical activities of TG2 are regulated by several cellular factors, including calcium, nucleotides, and redox potential, which vary depending on its subcellular location. Thus, the microenvironments of the subcellular compartments to which TG2 localizes, such as the cytosol, plasma membrane, nucleus, mitochondria, or extracellular space, are important determinants to switch on or off various TG2 biochemical activities. Furthermore, TG2 interacts with a distinct subset of proteins and/or substrates depending on its subcellular location. In this review, the biological functions and molecular interactions of TG2 will be discussed in the context of the unique environments of the subcellular compartments to which TG2 localizes.

Tissue transglutaminase 2 as a biomarker of cervical intraepithelial neoplasia (CIN) and its relationship to p16INK4A and nuclear factor kappaB expression

Tissue transglutaminase 2 (TG2) is a recently identified molecule with multifunctional physiological roles. This is the first report of the expression of TG2 in cervical intraepithelial neoplasia (CIN) and invasive squamous cell carcinoma (SCC). For comparison, the expression of p16, a known surrogate biomarker of HPV infection, was evaluated. The expression of nuclear factor kappa B (NF-kappaB), a molecule crucial to inflammation and neoplasia, was also determined to explore its possible linkage with TG2 expression. Twenty cases each with normal cervical histology, CIN1, CIN2, CIN3, and invasive SCC were analyzed for TG2, p16, and NF-kappaB expression by immunohistochemistry. Intergroup differences were analyzed by Friedman ANOVA. Cytoplasmic as well as nuclear TG2 expression was observed in the epithelial cells. As compared to normal controls, CIN1 showed markedly increased cytoplasmic TG2 expression (p = 0.006). In CIN2/3, additional nuclear TG2 expression was seen (p = 0.009 and 0.031, respectively). Marked extracellular stromal upregulation of TG2 was noted in CIN3/SCC versus normal controls (p = 0.054; p = 0.003). There was no relationship of TG2 with either p16 of NF-kappaB expression. Combining TG2 immunoreactivity with p16 increased the immunolabeling of dysplasia from 35% to 100% in CIN1, 45% to 60% in CIN2, and 60% to 85% in CIN3. TG2 serves as an additional biomarker for all grades of cervical dysplasia, especially for low-grade dysplasia.

Transglutaminases in inflammation and fibrosis of the gastrointestinal tract and the liver

Transglutaminases are a family of eight currently known calcium-dependent enzymes that catalyze the cross-linking or deamidation of proteins. They are involved in important biological processes such as wound healing, tissue repair, fibrogenesis, apoptosis, inflammation and cell-cycle control. Therefore, they play important roles in the pathomechanisms of autoimmune, inflammatory and degenerative diseases, many of which affect the gastrointestinal system. Transglutaminase 2 is prominent, since it is central to the pathogenesis of celiac disease, and modulates inflammation and fibrosis in inflammatory bowel and chronic liver diseases. This review highlights our present understanding of transglutaminase function in gastrointestinal and liver diseases and therapeutic strategies that target transglutaminase activities.

Role of transglutaminase 2 in liver injury via cross-linking and silencing of transcription factor Sp1

BACKGROUND & AIMS

Despite high morbidity and mortality of alcoholic liver disease worldwide, the molecular mechanisms underlying alcohol-induced liver cell death are not fully understood. Transglutaminase 2 (TG2) is a cross-linking enzyme implicated in apoptosis. TG2 levels and activity are increased in association with various types of liver injury. However, how TG2 induces hepatic apoptosis is not known.

METHODS

Human hepatic cells or primary hepatocytes from rats or TG2+/+ and TG2-/- mice were treated with ethanol. Mice were administered anti-Fas antibody or alcohol. Liver sections were prepared from patients with alcoholic steatohepatitis. Changes in TG2 levels, Sp1 cross-linking and its activities, expression of hepatocyte growth factor receptor, c-Met, and hepatic apoptosis were measured.

RESULTS

Ethanol induced apoptosis in hepatic cells, enhanced activity and nuclear accumulation of TG2 as well as accumulation of cross-linked and inactivated Sp1, and reduced expression of the Sp1-responsive gene, c-Met. These effects were rescued by TG2 knockdown, restoration of functional Sp1, or addition of hepatocyte growth factor, whereas apoptosis was reproduced by Sp1 knockdown or TG2 overexpression. Compared with TG2+/+ mice, TG2-/- mice showed markedly reduced hepatocyte apoptosis and Sp1 cross-linking following ethanol or anti-Fas treatment. Treatment of TG2+/+ mice with the TG2 inhibitors putrescine or cystamine blocked anti-Fas-induced hepatic apoptosis and Sp1 silencing. Moreover, enhanced expression of cross-linked Sp1 and TG2 was evident in hepatocyte nuclei of patients with alcoholic steatohepatitis.

CONCLUSIONS

TG2 induces hepatocyte apoptosis via Sp1 cross-linking and inactivation, with resultant inhibition of the expression of c-Met required for hepatic cell viability.

Differential activation of extracellular signal-regulated kinase 1 and a related complex in neuronal nuclei

The extracellular signal-regulated kinases 1 and 2 (ERKs 1/2) are known to participate in regulating transcription in response to moderate depolarization, such as synaptic stimulation, but how the same active enzyme can differentially regulate distinct transcriptional programs induced with abnormal depolarization (high potassium) is unknown. We hypothesized that ERK1 or 2 accomplishes this differential nuclear response through close association with other proteins in stable complexes. In support of this hypothesis, we have found that immunoreactivity for an apparent high molecular weight complex containing phospho-ERK1 increased in response to synaptic stimulation, but decreased in response to high potassium; p-ERK immunoreactivity at 44/42 kDa increased in both cases. Evidence supporting the conclusion that the band of interest contained ERK1 in a complex, as opposed to it being an unrelated protein crossreacting with antibodies against p-ERK, is that ERK1 (p44 MAPK) and 14-3-3 protein were electroeluted from the 160-kDa band cut from a gel. We also found the nuclear complexes to be exceptionally durable, suggesting a role for the crosslinking enzyme, transglutaminase, in its stabilization. In addition, we found other components of the ERK pathway, including MEK, ERK2, p90RSK, and Elk-1, migrating at higher-than-expected weights in brain nuclei. These results describe a novel stable complex of ERK1 in neuronal nuclei that responds differentially to synaptic and depolarizing stimulation, and thus may be capable of mediating gene transcription in a way distinct from the monomeric protein.

Identification of two GTP-independent alternatively spliced forms of tissue transglutaminase in human leukocytes, vascular smooth muscle, and endothelial cells

Tissue transglutaminase (tTG) is a multifunctional enzyme with transglutaminase crosslinking (TGase), GTP binding, and hydrolysis activities that play a role in many different disorders. We identified, characterized, and investigated the function and stability of two alternatively spliced forms of tTG using biochemical, cellular, and molecular biological approaches. Using a human aortic vascular smooth muscle cells (VSMC) cDNA library, we identified two cDNAs encoding C-terminal truncated forms, tTG(V1) and tTG(V2). tTG(V1,2) mRNAs were synthesized by a rare splicing event using alternate splice sites within exons 12 and 13 of the tTG gene, respectively. Quantitative PCR and immunoblotting demonstrated that there was unique expression and localization of tTG(V1,2) compared with tTG in human umbilical vein endothelial cells (HUVECs), VSMC, and leukocytes. The loss of C-terminal 52 amino acid residues (AAs) in tTG(V1,2) altered GTP binding, enhanced GTP hydrolysis, rendered the variants insensitive to GTP inhibition, and resulted in <10% residual Ca(+2)-dependent TGase activity. Transfection in HEK293 demonstrated a 28- and 5-fold reduction in the expression of tTG(V1) and tTG(V2), respectively, demonstrating that the C-terminal GTP-binding domain is important in stabilizing and promoting the half-life of tTG. The altered affinity for GTP allowed tTG(V1,2) to exhibit enhanced TGase activity when there is a transient increase in Ca(+2) levels. The abundance of tTG(V1,2) and its distinct intracellular expression patterns in human vascular cells and leukocytes indicate these isoforms likely have unique physiological functions.

Arterial adaptations to chronic changes in haemodynamic function: coupling vasomotor tone to structural remodelling

Healthy mature arteries are usually extremely quiescent tissues with cell proliferation rates much below 1%/day and with extracellular matrix constituents exhibiting half-lives of years to decades. However, chronic physiological or pathological changes in haemodynamic function elicit arterial remodelling processes that may involve substantial tissue synthesis, degradation or turnover. Although these remodelling processes accommodate changing demands placed upon the cardiovascular system by physiological adaptations, they can compromise further perfusion in the context of arterial occlusive disease and they entrench hypertension and may exacerbate its progression. Recent findings indicate that some of the most important such remodelling responses involve the integrated effects of persistently altered vascular tone that feed into restructuring responses, with common signalling pathways frequently interacting in the control of both phases of the response. Current efforts to define these signals and their targets may provide new directions for therapeutic interventions to treat important vascular disorders.

Implications of tissue transglutaminase expression in malignant melanoma

Human malignant melanoma is a highly aggressive form of cancer; the 5-year survival rate in patients with stage III or IV disease is <5%. In patients with metastatic melanoma, systemic therapy becomes ineffective because of the high resistance of melanoma cells to various anticancer therapies. We have found previously that development of the drug resistance and metastatic phenotypes in breast cancer cells is associated with increased tissue transglutaminase (TG2) expression. In the study reported here, we investigated TG2 expression and its implications in metastatic melanoma. We found that metastatic melanoma cell lines expressed levels of TG2 up to 24-fold higher than levels in radial growth phase of primary melanoma cell lines. Activation of endogenous TG2 by the calcium ionophore A23187 induced a rapid and strong apoptotic response in A375 cells and A23187-induced apoptosis could be blocked by TG2-specific inhibitors. These findings indicated that activation of endogenous TG2 could serve as a strategy for inducing apoptosis in malignant melanomas. Importantly, tumor samples from patients with malignant melanomas showed strong expression of TG2, suggesting that TG2 expression is selectively up-regulated during advanced developmental stages of melanoma. We observed that 20% to 30% of TG2 protein was present on cell membranes in association with beta1 and beta5 integrins. This association of TG2 with cell surface integrins promoted strong attachment of A375 cells to fibronectin-coated surfaces, resulting in increased cell survival in serum-free medium. Inhibition of TG2 by small interfering RNA inhibited fibronectin-mediated cell attachment and cell survival functions in A375 cells. Overall, our results suggest that TG2 expression contributes to the development of chemoresistance in malignant melanoma cells by exploiting integrin-mediated cell survival signaling pathways.

Tissue-transglutaminase contributes to neutrophil granulocyte differentiation and functions

Promyelocytic NB4 leukemia cells undergo differentiation to granulocytes following retinoic acid treatment. Here we report that tissue transglutaminase (TG2), a protein cross-linking enzyme, was induced, then partially translocated into the nucleus, and became strongly associated with the chromatin during the differentiation process. The transglutaminase-catalyzed cross-link content of both the cytosolic and the nuclear protein fractions increased while NB4 cells underwent cellular maturation. Inhibition of cross-linking activity of TG2 by monodansylcadaverin in these cells led to diminished nitroblue tetrazolium (NBT) positivity, production of less superoxide anion, and decreased expression of GP91PHOX, the membrane-associated subunit of NADPH oxidase. Neutrophils isolated from TG2–/– mice showed diminished NBT reduction capacity, reduced superoxide anion formation, and down-regulation of the gp91phox subunit of NADPH oxidase, compared with wild-type cells. It was also observed that TG2–/– mice exhibited increased neutrophil phagocytic activity, but had attenuated neutrophil chemotaxis and impaired neutrophil extravasation with higher neutrophil counts in their circulation during yeast extract–induced peritonitis. These results clearly suggest that TG2 may modulate the expression of genes related to neutrophil functions and is involved in several intracellular and extracellular functions of extravasating neutrophil.

Phosphorylation of histones by tissue transglutaminase

Tissue transglutaminase 2 (TG2) has recently been shown to have intrinsic serine/threonine kinase activity. Since histones are known to be cross-linked by TG2, we investigated whether histones are also substrates for TG2 kinase activity. TG2 was able to phosphorylate H1, H2A, H2B, H3, and H4 histones in vitro. Using peptide substrates and phosphospecific antibodies we demonstrated that TG2 phosphorylated Ser(10) in H3 and that this phosphorylation was reduced by acetylation, whereas phosphorylation of Ser(10) by TG2 enhanced acetylation. Furthermore we demonstrated that exogenous TG2 phosphorylated H1 and H3 in nucleosome preparations. We examined the abundance of TG2 in DNA-associated proteins from MCF-7 cells treated with phorbol ester (TPA) and 17beta-estradiol (E2). TG2 abundance was significantly reduced in E2-treated cells and enhanced in TPA-treated cells. In summary we have demonstrated that TG2 is able to phosphorylate purified histone proteins, and H3 and H1 in chromatin preparations, and it is associated with chromatin in breast cancer cells. These studies suggest a novel role for TG2 in the regulation of chromatin structure and function.

Cell type-specific activation of intracellular transglutaminase 2 by oxidative stress or ultraviolet irradiation: implications of transglutaminase 2 in age-related cataractogenesis

Transglutaminase (TGase) 2 is a ubiquitously expressed enzyme that modifies proteins by cross-linking or polyamination. An aberrant activity of TGase 2 has implicated its possible roles in a variety of diseases including age-related cataracts. However, the molecular mechanism by which TGase 2 is activated has not been elucidated. In this report, we showed that oxidative stress or UV irradiation elevates in situ TGase 2 activity. Neither the expression level nor the in vitro activity of TGase 2 appeared to correlate with the observed elevation of in situ TGase 2 activity. Screening a number of cell lines revealed that the level of TGase 2 activation depends on the cell type and also the environmental stress, suggesting that unrecognized cellular factor(s) may specifically regulate in situ TGase 2 activity. Concomitantly, we observed that human lens epithelial cells (HLE-B3) exhibited about 3-fold increase in in situ TGase 2 activity in response to the stresses. The activated TGase 2 catalyzed the formation of water-insoluble dimers or polymers of alphaB-crystallin, betaB(2)-crystallin, and vimentin in HLE-B3 cells, providing evidence that TGase 2 may play a role in cataractogenesis. Thus, our findings indicate that in situ TGase 2 activity must be evaluated instead of in vitro activity to study the regulation mechanism and function of TGase 2 in biological and pathological processes.

Intracellular localization and activity state of tissue transglutaminase differentially impacts cell death

Tissue transglutaminase (tTG) is a unique member of the transglutaminase family as it is both a transamidating enzyme and a GTPase. In the cell tTG is mostly cytosolic, however it is also found in the nucleus and associated with the plasma membrane. tTG can be proapoptotic, however anti-apoptotic activities of the enzyme have also been reported. To determine how the intracellular localization and transamidating activity of tTG modulates its effects on apoptosis, HEK293 cells were transiently transfected with tTG or [C277S]tTG (which lacks transamidating activity) constructs that were targeted to different intracellular compartments. Apoptosis was induced by thapsigargin treatment, which results in increased intracellular calcium concentrations. Cytosolic tTG was pro-apoptotic, while nuclear localization of [C277S]tTG attenuated apoptosis. Membrane-targeted tTG had neither pro- nor anti-apoptotic functions. This finding indicates for the first time that intracellular localization is an important determinant of the effect of tTG on apoptosis. Previous studies have suggested that tTG may modulate retinoblastoma (Rb) protein, an important suppressor of apoptosis. tTG interacted with Rb and after induction of apoptosis, the interaction of nuclear-targeted [C277S]tTG with Rb was increased significantly concomitant with an attenuation of apoptosis. In contrast, the interaction of nuclear-targeted tTG with Rb was significantly decreased and apoptosis was not attenuated. These data suggest that tTG protects cells against apoptosis in response to stimuli that do not result in increased transamidating activity by translocating to the nucleus, and that complexing with Rb may be an important aspect of the protective effects of tTG.

Transglutaminase 2 inhibits Rb binding of human papillomavirus E7 by incorporating polyamine

Transglutaminase 2 (TGase 2) is one of a family of enzymes that catalyze protein modification through the incorporation of polyamines into substrates or the formation of protein crosslinks. However, the physiological roles of TGase 2 are largely unknown. To elucidate the functions of TGase 2, we have searched for its interacting proteins. Here we show that TGase 2 interacts with E7 oncoprotein of human papillomavirus type 18 (HPV18) in vitro and in vivo. TGase 2 incorporates polyamines into a conserved glutamine residue in the zinc-binding domain of HPV18 E7 protein. This modification mediates the inhibition of E7's Rb binding ability. In contrast, TGase 2 does not affect HPV16 E7, due to absence of a glutamine residue at this polyamination site. Using E7 mutants, we demonstrate that TGase 2-dependent inhibition of HPV E7 function correlates with the presence of the polyamination site. Our results indicate that TGase 2 is an important cellular interfering factor and define a novel host-virus interaction, suggesting that the inability of TGase 2 to inactivate HPV16 E7 could explain the high prevalence of HPV16 in cervical cancer.

Effects of polyamines on histone polymerization

It is generally accepted that the nucleosome structure is not static, and that alternative conformations are adopted in response to several stimuli associated with the different functions. Histones are substrates for transglutaminase (TGase), and polymerized histone and polyamine binding histone have been suggested to play important roles in nucleus. We examined whether histone polymerization catalyzed by TGase was influenced by polyamines such as putrescine (PUT), spermidine (SPD), and spermine (SPM). PUT inhibited histone polymerization, and SPD slightly prevented it. However, SPM slightly enhanced histone polymerization. These results indicate that the nuclear accumulation of the polyamines may play an important role in nuclear remodeling by histone modification. We speculate that histone cross-linking by TGase may be involved in the chromatin structure. Also, we propose that histone cross-linking by TGase may be responsible for the changes in DNA function such as transcription and replication and that TGase may be involved in cell growth and differentiation.