Inhibition of "tissue" transglutaminase increases cell survival by preventing apoptosis

Transglutaminases are oncogenic biomarkers in human cancers and therapeutic targeting of TGM2 blocks chemoresistance and macrophage infiltration in pancreatic cancer

PURPOSE

Transglutaminases (TGs) are multifunctional enzymes exhibiting transglutaminase crosslinking, as well as atypical GTPase/ATPase and kinase activities. Here, we used an integrated comprehensive analysis to assess the genomic, transcriptomic and immunological landscapes of TGs across cancers.

METHODS

Gene expression and immune cell infiltration patterns across cancers were obtained from The Cancer Genome Atlas (TCGA) database and Gene Set Enrichment Analysis (GSEA) datasets. Western blotting, immunofluorescence staining, enzyme-linked immunosorbent assays, and orthotopic xenograft models were used to validate our database-derived results.

RESULTS

We found that the overall expression of TGs (designated as the TG score) is significantly upregulated in multiple cancers and related to a worse patient survival. The expression of TG family members can be regulated through multiple mechanisms at the genetic, epigenetic and transcriptional levels. The expression of transcription factors crucial for epithelial-to-mesenchymal transition (EMT) is commonly correlated with the TG score in many cancer types. Importantly, TGM2 expression displays a close connection with chemoresistance to a wide range of chemotherapeutic drugs. We found that TGM2 expression, F13A1 expression and the overall TG score were positively correlated with the infiltration of immune cells in all cancer types tested. Functional and clinical verification revealed that a higher TGM2 expression is linked with a worse patient survival, an increased IC50 value of gemcitabine, and a higher abundance of tumor-infiltrating macrophages in pancreatic cancer. Mechanistically, we found that increased C-C motif chemokine ligand 2 (CCL2) release mediated by TGM2 contributes to macrophage infiltration into the tumor microenvironment.

CONCLUSIONS

Our results reveal the relevance and molecular networks of TG genes in human cancers and highlight the importance of TGM2 in pancreatic cancer, which may provide promising directions for immunotherapy and for addressing chemoresistance.

Involvement and possible role of transglutaminases 1 and 2 in mediating fibrotic signalling, collagen cross-linking and cell proliferation in neonatal rat ventricular fibroblasts

Transglutaminase (TG) isoforms control diverse normal and pathophysiologic processes through their capacity to cross-link extracellular matrix (ECM) proteins. Their functional and signalling roles in cardiac fibrosis remain poorly understood, despite some evidence of TG2 involvement in abnormal ECM remodelling in heart diseases. In this study, we investigated the role of TG1 and TG2 in mediating fibrotic signalling, collagen cross-linking, and cell proliferation in healthy fibroblasts by siRNA-mediated knockdown. siRNA for TG1, TG2 or negative control was transfected into cultured neonatal rat ventricular fibroblasts and cardiomyocytes. mRNA expression of TGs and profibrotic, proliferation and apoptotic markers was assessed by qPCR. Cell proliferation and soluble and insoluble collagen were determined by ELISA and LC-MS/MS, respectively. TG1 and TG2 were both expressed in neonatal rat cardiomyocytes and fibroblasts before transfection. Other TGs were not detected before and after transfection. TG2 was predominantly expressed and more effectively silenced than TG1. Knocking down TG1 or TG2 significantly modified profibrotic markers mRNA expression in fibroblasts, decreasing connective tissue growth factor (CTGF) and increasing transforming growth factor-β1 compared to the negative siRNA control. Reduced expression of collagen 3A1 was found upon TG1 knockdown, while TG2 knockdown raised α-smooth muscle actin expression. TG2 knockdown further increased fibroblast proliferation and the expression of proliferation marker cyclin D1. Lower insoluble collagen content and collagen cross-linking were evidenced upon silencing TG1 or TG2. Transcript levels of collagen 1A1, fibronectin 1, matrix metalloproteinase-2, cyclin E2, and BCL-2-associated X protein/B-cell lymphoma 2 ratio were strongly correlated with TG1 mRNA expression, whereas TG2 expression correlated strongly with CTGF mRNA abundance. These findings support a functional and signalling role for TG1 and TG2 from fibroblasts in regulating key processes underlying myocardial ECM homeostasis and dysregulation, suggesting that these isoforms could be potential and promising targets for the development of cardiac fibrosis therapies.

Implications of enigmatic transglutaminase 2 (TG2) in cardiac diseases and therapeutic developments

Cardiac diseases are the leading cause of mortality and morbidity worldwide. Mounting evidence suggests that transglutaminases (TGs), tissue TG (TG2) in particular, are involved in numerous molecular responses underlying the pathogenesis of cardiac diseases. The TG family has several intra- and extracellular functions in the human body, including collagen cross-linking, angiogenesis, cell growth, differentiation, migration, adhesion as well as survival. TGs are thiol- and calcium-dependent acyl transferases that catalyze the formation of a covalent bond between the γ-carboxamide group of a glutamine residue and an amine group, thus increasing the stability, rigidity, and stiffness of the myocardial extracellular matrix (ECM). Excessive accumulation of cross-linked collagen leads to increase myocardial stiffness and fibrosis. Beyond TG2 extracellular protein cross-linking action, mounting evidence suggests that this pleiotropic TG isozyme may also promote fibrotic diseases through cell survival and profibrotic pathway activation at the signaling, transcriptional and translational levels. Due to its multiple functions and localizations, TG2 fulfils critical yet incompletely understood roles in myocardial fibrosis and associated heart diseases, such as cardiac hypertrophy, heart failure, and age-related myocardial stiffness under several conditions. This review summarizes current knowledge and existing gaps regarding the ECM-dependent and ECM-independent roles of TG2 and highlights the therapeutic prospects of targeting TG2 to treat cardiac diseases.

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.

The Biological and Biomechanical Role of Transglutaminase-2 in the Tumour Microenvironment

Transglutaminase-2 (TG2) is the most highly and ubiquitously expressed member of the transglutaminase enzyme family and is primarily involved in protein cross-linking. TG2 has been implicated in the development and progression of numerous cancers, with a direct role in multiple cellular processes and pathways linked to apoptosis, chemoresistance, epithelial-mesenchymal transition, and stem cell phenotype. The tumour microenvironment (TME) is critical in the formation, progression, and eventual metastasis of cancer, and increasing evidence points to a role for TG2 in matrix remodelling, modulation of biomechanical properties, cell adhesion, motility, and invasion. There is growing interest in targeting the TME therapeutically in response to advances in the understanding of its critical role in disease progression, and a number of approaches targeting biophysical properties and biomechanical signalling are beginning to show clinical promise. In this review we aim to highlight the wide array of processes in which TG2 influences the TME, focussing on its potential role in the dynamic tissue remodelling and biomechanical events increasingly linked to invasive and aggressive behaviour. Drug development efforts have yielded a range of TG2 inhibitors, and ongoing clinical trials may inform strategies for targeting the biomolecular and biomechanical function of TG2 in the TME.

Transglutaminase 2 promotes tumorigenicity of colon cancer cells by inactivation of the tumor suppressor p53

Despite a high clinical need for the treatment of colorectal carcinoma (CRC) as the second leading cause of cancer-related deaths, targeted therapies are still limited. The multifunctional enzyme Transglutaminase 2 (TGM2), which harbors transamidation and GTPase activity, has been implicated in the development and progression of different types of human cancers. However, the mechanism and role of TGM2 in colorectal cancer are poorly understood. Here, we present TGM2 as a promising drug target.In primary patient material of CRC patients, we detected an increased expression and enzymatic activity of TGM2 in colon cancer tissue in comparison to matched normal colon mucosa cells. The genetic ablation of TGM2 in CRC cell lines using shRNAs or CRISPR/Cas9 inhibited cell expansion and tumorsphere formation. In vivo, tumor initiation and growth were reduced upon genetic knockdown of TGM2 in xenotransplantations. TGM2 ablation led to the induction of Caspase-3-driven apoptosis in CRC cells. Functional rescue experiments with TGM2 variants revealed that the transamidation activity is critical for the pro-survival function of TGM2. Transcriptomic and protein-protein interaction analyses applying various methods including super-resolution and time-lapse microscopy showed that TGM2 directly binds to the tumor suppressor p53, leading to its inactivation and escape of apoptosis induction.We demonstrate here that TGM2 is an essential survival factor in CRC, highlighting the therapeutic potential of TGM2 inhibitors in CRC patients with high TGM2 expression. The inactivation of p53 by TGM2 binding indicates a general anti-apoptotic function, which may be relevant in cancers beyond CRC.

'Omics driven discoveries of gene targets for apoptosis attenuation in CHO cells

Chinese hamster ovary (CHO) cells are widely used in biopharmaceutical production. Improvements to cell lines and bioprocesses are constantly being explored. One of the major limitations of CHO cell culture is that the cells undergo apoptosis, leading to rapid cell death, which impedes reaching high recombinant protein titres. While several genetic engineering strategies have been successfully employed to reduce apoptosis, there is still room to further enhance CHO cell lines performance. 'Omics analysis is a powerful tool to better understand different phenotypes and for the identification of gene targets for engineering. Here, we present a comprehensive review of previous CHO 'omics studies that revealed changes in the expression of apoptosis-related genes. We highlight targets for genetic engineering that have reduced, or have the potential to reduce, apoptosis or to increase cell proliferation in CHO cells, with the final aim of increasing productivity.

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

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

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.

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.

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.

Effects of cystamine on antioxidant activities and regulatory T cells in lupus-prone mice

Attenuated antioxidant activities, irregular cytokines expressions and reduced regulatory T cells, are strongly associated with the pathogenesis of systemic lupus erythematosus (SLE). Despite the well-established beneficial effects of cystamine on lupus-prone mice, the extent to which cystamine contributes to antioxidant activity and the reduction of regulatory T cells has seldom been investigated. Therefore, this study elucidates how cystamine affects anti-oxidant activities in NZB/W F1 mice by performing assays of Glutathione (GSH), 1,1-diphenyl-2- picryl-hydrazyl (DPPH) and malondialdehyde thiobarbituric acid (MDA). In addition, investigations of the effects of cystamine on CD4⁺/CD25⁺ regulatory T cells and interleukin-6 (IL6)/STAT-3 signalling were performed with flow cytometry and immunoblots. Experimental results reveal more significantly reduced MDA and increased GSH and DPPH in NZB/W F1 mice receiving cystamine than in those mice receiving PBS. Meanwhile, CD4⁺/CD25⁺ regulatory T cells more significantly increase in NZB/W F1 mice receiving cystamine than in those mice receiving PBS, accompanied by significantly reduced IL-6/phosphorylated STAT-3 expression. The above findings suggest the beneficial effects of cystamine in terms of increasing antioxidant activities and CD4⁺/CD25⁺ regulatory T cells in lupus-prone mice by suppressing IL-6/STAT3 signalling.

Spinocerebellar ataxia type 35 (SCA35)-associated transglutaminase 6 mutants sensitize cells to apoptosis

Spinocerebellar ataxia type 35 (SCA35) is an autosomal dominant neurodegenerative disorder. In our previous study, using exome sequencing and linkage analysis, two missense mutations of the transglutaminase 6 (TGM6) gene were identified as causative for SCA35. TGM6 encodes transglutaminase 6 (TG6), a member of the transglutaminase family of enzymes that catalyze the formation of a covalent bond between a free amine group and the γ-carboxamide group of protein- or peptide-bound glutamine. However, the precise role of TG6 in contributing to SCA35 remains unclear. In this study, we analyzed the subcellular distribution, expression and in vitro activity of two missense mutations of TG6 (D327G, L517W) and found that both mutants exhibited decreased transglutaminase activity and stability. Furthermore, overexpressing the TG6 mutants sensitized cells to staurosporine-induced apoptosis by increasing the activity of caspases. We propose that the pro-apoptotic role of these mutants might underlie the pathogenesis of SCA35.

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.

Cysteamine treatment ameliorates alterations in GAD67 expression and spatial memory in heterozygous reeler mice

Brain-derived neurotrophic factor (BDNF) signalling through its receptor, TrkB is known to regulate GABAergic function and glutamic acid decarboxylase (GAD) 67 expression in neurons. Alterations in BDNF signalling have been implicated in the pathophysiology of schizophrenia and as a result, they are a potential therapeutic target. Interestingly, heterozygous reeler mice (HRM) have decreased GAD67 expression in the frontal cortex and hippocampus and they exhibit many behavioural and neurochemical abnormalities similar to schizophrenia. In this study, we evaluated the potential of cysteamine, a neuroprotective compound to improve the deficits in GAD67 expression and cognitive function in HRM. We found that cysteamine administration (150 mg/kg.d, through drinking water) for 30 d significantly ameliorated the decreases in GAD67, mature BDNF and full-length TrkB protein levels found in frontal cortex and hippocampus of HRM. A significant attenuation of the increased levels of truncated BDNF in frontal cortex and hippocampus, as well as truncated TrkB in frontal cortex of HRM was also observed following cysteamine treatment. In behavioural studies, HRM were impaired in a Y-maze spatial recognition memory task, but not in a spontaneous alternation task or a sensorimotor, prepulse inhibition (PPI) procedure. Cysteamine improved Y-maze spatial recognition in HRM to the level of wide-type controls and it improved PPI in both wild-type and HRM. Finally, mice deficient in TrkB, showed a reduced response to cysteamine in GAD67 expression suggesting that TrkB signalling plays an important role in GAD67 regulation by cysteamine.

Screening of therapeutic strategies for Huntington's disease in YAC128 transgenic mice

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by an unstable expansion of cytosine-adenine-guanine (CAG) repeats in the HD gene. The symptoms include cognitive dysfunction and severe motor impairment with loss of voluntary movement coordination that is later replaced by bradykinesia and rigidity. The neuropathology is characterized by neuronal loss mainly in the striatum and cortex, and the appearance of neuronal intranuclear inclusions of mutant huntingtin. The mechanisms responsible for neurodegeneration are still not fully understood although excitotoxicity and a consequent increase in intracellular calcium concentration as well as the activation of caspases and calapins are known to play a key role. There is currently no satisfactory treatment or cure for this disease. The YAC128 transgenic mice express the full-length human HD gene with 128 CAG repeats and constitute a unique model for the study of HD as they replicate the slow and biphasic progression of behavioral deficits characteristic of the human condition and show striatal neuronal loss. As such, these transgenic mice have been an invaluable model not only for the elucidation of the neurodegenerative pathways in HD, but also for the screening and development of new therapeutic approaches. Here, I will review the unique characteristics of this transgenic HD model and will provide a summary of the therapies that have been tested in these mice, namely: potentiation of the protective roles of wild-type huntingtin and mutant huntingtin aggregation, transglutaminase inhibition, inhibition of glutamate- and dopamine-induced toxicity, apoptosis inhibition, use of essential fatty acids, and the novel approach of intrabody gene therapy. The insights obtained from these and future studies will help identify potential candidates for clinical trials and will ultimately contribute to the discovery of a successful treatment for this devastating neurodegenerative disorder.

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.

Aloe-emodin as antiproliferative and differentiating agent on human U937 monoblastic leukemia cells

AIMS

Aloe-emodin (AE), a plant derived anthraquinone, has been shown to have anticancer activity in various human cancer cell lines. We have recently reported that AE possesses a differentiative potential on melanoma cells. The purpose of this study was to investigate the possible modulation of defined markers of monocytic differentiation of AE on human U937 cell line.

MAIN METHODS

U937 cells differentiation has been confirmed unequivocally by Griess and nitroblue tetrazolium reduction assays, protoporphyrin IX accumulation, expression of CD14 and CD11b surface antigens, phagocytic activity, migration and attachment ability. The effect on polyamine metabolism, apoptosis and cytokine production was also investigated.

KEY FINDINGS

We showed that AE-treated U937 cells exhibit a noticeably rise in transglutaminase activity. This enhanced enzyme activity correlates with AE-induced growth arrest and differentiation to functionally mature monocytes.

SIGNIFICANCE

Taken together, the results reported here show that AE can promote the macrophage differentiation of U937 cells, suggesting that this anthraquinone could be a potential candidate as a differentiation-inducing selective agent for therapeutic treatment of leukemia.

Transglutaminase 2: a molecular Swiss army knife

Transglutaminase 2 (TG2) is the most widely distributed member of the transglutaminase family with almost all cell types in the body expressing TG2 to varying extents. In addition to being widely expressed, TG2 is an extremely versatile protein exhibiting transamidating, protein disulphide isomerase and guanine and adenine nucleotide binding and hydrolyzing activities. TG2 can also act as a protein scaffold or linker. This unique protein also undergoes extreme conformational changes and exhibits localization diversity. Being mainly a cytosolic protein; it is also found in the nucleus, associated with the cell membrane (inner and outer side) and with the mitochondria, and also in the extracellular matrix. These different activities, conformations and localization need to be carefully considered while assessing the role of TG2 in physiological and pathological processes. For example, it is becoming evident that the role of TG2 in cell death processes is dependent upon the cell type, stimuli, subcellular localization and conformational state of the protein. In this review we discuss in depth the conformational and functional diversity of TG2 in the context of its role in numerous cellular processes. In particular, we have highlighted how differential localization, conformation and activities of TG2 may distinctly mediate cell death processes.

Potential roles of transglutaminase and thioredoxin in the release of gonad-stimulating factor in Penaeus monodon: implication from differential expression in the brain during ovarian maturation cycle

The synthesis of vitellogenin during ovarian maturation in crustacean is induced by gonad-stimulating factor(s) that are synthesized in the brain and thoracic ganglia. This process is negatively regulated by a gonad-inhibiting hormone (GIH) from the eyestalk. This study utilized differential-display RT-PCR technique to identify putative genes in brain and thoracic ganglia that may be involved in ovarian maturation of the black tiger shrimp, Penaeus monodon under the condition in which the expression of GIH was suppressed by GIH-specific dsRNA. After excluding redundant clones and subsequent verification by RT-PCR, 10 and 5 transcripts exhibited up-regulated and down-regulated expressions, respectively, in the GIH-dsRNA injected shrimp when compared with the Tris/NaCl injected shrimp. Among the up-regulated genes, a full sequence of thioredoxin cDNA was cloned, and nucleotide sequence analysis showed that it was highly similar to other crustacean thioredoxin. The thioredoxin gene as well as the other four genes including transglutaminase and three unknowns; U10-11, U10-15 and U13-11 that were up-regulated upon GIH-knockdown exhibited similar expression profile in the brain during ovarian maturation cycle. The highest expression level was detected in the brain of early-vitellogenic female shrimp suggesting that they are required for an initial stage of vitellogenesis. Our results posted for the first time a possible function of transglutaminase and thioredoxin in regulating the gonad-stimulating pathway in the brain of the shrimp.

TG2 transamidating activity acts as a reostat controlling the interplay between apoptosis and autophagy

Tissue transglutaminase (TG2) activity has been implicated in inflammatory disease processes such as Celiac disease, infectious diseases, cancer, and neurodegenerative diseases, such as Huntington's disease. Furthermore, four distinct biochemical activities have been described for TG2 including protein crosslinking via transamidation, GTPase, kinase and protein disulfide isomerase activities. Although the enzyme plays a complex role in the regulation of cell death and autophagy, the molecular mechanisms and the putative biochemical activity involved in each is unclear. Therefore, the goal of the present study was to determine how TG2 modulates autophagy and/or apoptosis and which of its biochemical activities is involved in those processes. To address this question, immortalized embryonic fibroblasts obtained from TG2 knock-out mice were reconstituted with either wild-type TG2 or TG2 lacking its transamidating activity and these were subjected to different treatments to induce autophagy or apoptosis. We found that knock out of the endogenous TG2 resulted in a significant exacerbation of caspase 3 activity and PARP cleavage in MEF cells subjected to apoptotic stimuli. Interestingly, the same cells showed the accumulation of LC3 II isoform following autophagy induction. These findings strongly suggest that TG2 transamidating activity plays a protective role in the response of MEF cells to death stimuli, because the expression of the wild-type TG2, but not its transamidation inactive C277S mutant, resulted in a suppression of caspase 3 as well as PARP cleavage upon apoptosis induction. Additionally, the same mutant was unable to catalyze the final steps in autophagosome formation during autophagy. Our findings clearly indicate that the TG2 transamidating activity is the primary biochemical function involved in the physiological regulation of both apoptosis and autophagy. These data also indicate that TG2 is a key regulator of cross-talk between autophagy and apoptosis.

Transglutaminase 2 inhibits apoptosis induced by calcium- overload through down-regulation of Bax

An abrupt increase of intracellular Ca(2+) is observed in cells under hypoxic or oxidatively stressed conditions. The dysregulated increase of cytosolic Ca(2+) triggers apoptotic cell death through mitochondrial swelling and activation of Ca(2+)-dependent enzymes. Transglutaminase 2 (TG2) is a Ca(2+)-dependent enzyme that catalyzes transamidation reaction producing cross-linked and polyaminated proteins. TG2 activity is known to be involved in the apoptotic process. However, the pro-apoptotic role of TG2 is still controversial. In this study, we investigate the role of TG2 in apoptosis induced by Ca(2+)-overload. Overexpression of TG2 inhibited the A23187-induced apoptosis through suppression of caspase-3 and -9 activities, cytochrome c release into cytosol, and mitochondria membrane depolarization. Conversely, down-regulation of TG2 caused the increases of cell death, caspase-3 activity and cytochrome c in cytosol in response to Ca(2+)-overload. Western blot analysis of Bcl-2 family proteins showed that TG2 reduced the expression level of Bax protein. Moreover, overexpression of Bax abrogated the anti-apoptotic effect of TG2, indicating that TG2-mediated suppression of Bax is responsible for inhibiting cell death under Ca(2+)-overloaded conditions. Our findings revealed a novel anti-apoptotic pathway involving TG2, and suggested the induction of TG2 as a novel strategy for promoting cell survival in diseases such as ischemia and neurodegeneration.

Tissue transglutaminase 2 promotes apoptosis of rat neonatal cardiomyocytes under oxidative stress

The role of tissue transglutaminase 2 (TG2) in cardiac myocyte apoptosis under oxidative stress induced by ischemic injury remains unclear. Here, we investigated the effects of TG2 on apoptosis of cardiomyocytes under oxidative stress. Ectopic expression of TG2 increased caspase-3 activity and calcium overload in cardiomyocytes. Expression levels of TG2 were significantly increased in H(2)O(2)-treated cardiomyocytes. Caspase-3 activity assay demonstrated its considerable correlation with TG2 expression, which supported that caspase-3 inhibitor inhibited the apoptosis induced by the ectopic overexpression of TG2. In addition, the other apoptotic signals, such as caspase-8, cytochrome c, and Bax, were increased dependent with TG2 expression in H(2)O(2)-treated cardiomyocytes. These results indicated that apoptotic signals had a positive correlation with TG2 expression. The decreased expression of phospholipase C (PLC)-δ1 and phospho-PKC in H(2)O(2)-treated cardiomyocytes were rescued by TG2 silencing. Together, our data strongly suggest that oxidative stress up-regulates TG2 expression in cardiomyocytes, leading to apoptosis.

Appearance of tissue transglutaminase in astrocytes in multiple sclerosis lesions: a role in cell adhesion and migration?

Multiple Sclerosis (MS) is a neuroinflammatory disease mainly affecting young adults. A major pathological hallmark of MS is the presence of demyelinated lesions in the central nervous system. In the active phase of the disease, astrocytes become activated, migrate and contribute to local tissue remodeling that ultimately can result in an astroglial scar. This process is facilitated by extracellular matrix proteins, including fibronectin. Tissue Transglutaminase (TG2) is a multifunctional enzyme with a ubiquitous tissue distribution and it has been shown that inflammatory cytokines can induce TG2 activity. In addition, TG2 is known to mediate cell adhesion and migration. We therefore hypothesized that TG2 is present in MS lesions and plays a role in cell adhesion and/or migration. Our studies showed that TG2 immunoreactivity appeared in astrocytes in active and chronic active MS lesions. These TG2 positive astrocytes partly co-localized with fibronectin. Additional in vitro studies showed that TG2 mediated astrocytoma adhesion to and migration on the extracellular matrix protein fibronectin. We therefore speculate that TG2 mediates the enhanced interaction of astrocytes with fibronectin in the extracellular matrix of MS lesions, thereby contributing to astrocyte adhesion and migration, and thus in tissue remodeling and possibly glial scarring.

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

OBJECTIVE

Earlier work has demonstrated that serum autoantibodies from coeliac patients targeted against transglutaminase 2 (TG2) inhibit in vitro angiogenesis. The aim of this study was to establish whether coeliac patient-derived monoclonal TG2-targeted antibodies produced by recombination technology exert similar anti-angiogenic effects to serum-derived coeliac autoantibodies. In addition, we studied whether the monoclonal patient autoantibodies modulate endothelial cell TG2 activity and whether such modulation is related to the anti-angiogenic effects.

MATERIAL AND METHODS

The influence of coeliac patient-derived monoclonal TG2-targeted antibodies on endothelial cell tubule formation was studied using a three-dimensional angiogenic cell culture model. Endothelial cell TG2 enzymatic activity was determined by means of a live-cell enzyme-linked immunosorbent assay.

RESULTS

Coeliac patient-derived monoclonal TG2-targeted antibodies produced by recombination technology inhibited endothelial tubule formation and enhanced the crosslinking activity of TG2. When this enzymatic activity was inhibited using site-directed irreversible TG2 inhibitors in the presence of autoantibodies, in vitro angiogenesis reverted to the control level.

CONCLUSIONS

Since we found a significant negative correlation between endothelial cell angiogenesis and TG2 activity, we suggest that the anti-angiogenic effects of coeliac patient-derived TG2-targeted autoantibodies are exerted by enhanced enzymatic activity of TG2.

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.

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.

Disialoganglioside GD3 synthase expression recruits membrane transglutaminase 2 during erythroid differentiation of the human chronic myelogenous leukemia K562 cells

By employing proteomics analysis tool, we examined the effects of GD3 synthase expression on the differentiation properties of chronic myelogenous leukemia (CML)-derived leukemia cells K562. Forced expression of GD3 synthase induced erythroid differentiation as determined by an increase in glycophorin A expression and synthesis of hemoglobins. The proteomic analysis revealed that 15 proteins were increased by GD3 synthase. In contrast, we observed three protein gel spots decreased in contents in the cell membranes of GD3 synthase-transfected K562 cells. Among the increased proteins, membrane transglutaminase 2 (TG2) was specifically increased in the cell membrane of GD3 synthase-transfected K562 cells. Then, we generated the GD3 synthase-transfected cells in the K562 cells. Interestingly, the TG2 level was increased in GD3 synthase-transfected cells compared with vector- and plasma membrane-associated ganglioside sialidase (Neu3)-transfected cells. In addition, its ability to be photoaffinity-labeled with [alpha-(32)P]GTP was also increased in the GD3 synthase- and TG2-transfected cells. Moreover, small interfering RNA (siRNA) analysis for the GD3 synthase showed the decrease or abolishment of the membrane TG2. Finally, GD3 synthase-transfected cells accelerated the erythroid differentiation. Therefore, we propose that the recruitment of TG2 into membranes by GD3 might play an important role in the erythroid differentiation in K562 cells.

Cystamine prevents haloperidol-induced decrease of BDNF/TrkB signaling in mouse frontal cortex

The role of brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology as well as treatment outcome of schizophrenia. Rodent studies indicate that several antipsychotic drugs have time-dependent (and differential) effects on BDNF levels in the brain. Earlier studies from our laboratory have indicated that long-term treatment with haloperidol (HAL) decreases BDNF, reduced GSH and anti-apoptotic marker, Bcl-xl protein levels and increases the expression of pro-apoptotic proteins in rat frontal cortex. Furthermore, findings from human as well as rodent studies suggest that treatment of schizophrenia must involve the neuroprotective strategies to improve the neuropathology and thereby clinical outcome. In the present study, we investigated the potential of cystamine (CYS), an anti-oxidant and anti-apoptotic compound, to prevent HAL-induced reduction in BDNF, GSH, and Bcl-xl protein levels in mice and the signaling mechanism(s) involved in the beneficial effects of CYS. The results indicated that CYS as well as cysteamine (the FDA-approved precursor of CYS) increased BDNF protein levels in mouse frontal cortex 7 days after treatment. CYS co-treatment prevented chronic HAL treatment-induced reduction in BDNF, GSH, and Bcl-xl protein levels. CYS treatment enhanced TrkB-tyrosine phosphorylation and activated Akt and extracellular signal-regulated kinase (ERK)1/2, downstream molecules of TrkB signaling. In addition, in vitro experiments with mouse cortical neurons showed that CYS prevented the HAL-induced reduction in neuronal cell viability and BDNF protein levels, and increase in apoptosis. BDNF-neutralizing antibody as well as K252a, a selective inhibitor of neurotrophin signaling blocked the CYS-mediated neuroprotection. Moreover, CYS-mediated neuroprotection is also blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor or PD98059, a mitogen-activated protein kinase kinase (MEK) inhibitor. Thus, CYS protects cortical neurons through a mechanism involving TrkB receptor activation, and a signaling pathway involving phosphatidylinositol 3-kinase and MAPK. The findings from the present study may be helpful for the development of novel neuroprotective strategies to improve the treatment outcome of schizophrenia.

Transglutaminase 2 protects against ischemic insult, interacts with HIF1beta, and attenuates HIF1 signaling

Transglutaminase 2 (TG2) is a multifunctional enzyme that has been implicated in the pathogenesis of neurodegenerative diseases, ischemia, and stroke. The mechanism by which TG2 modulates disease progression have not been elucidated. In this study we investigate the role of TG2 in the cellular response to ischemia and hypoxia. TG2 is up-regulated in neurons exposed to oxygen and glucose deprivation (OGD), and increased TG2 expression protects neurons against OGD-induced cell death independent of its transamidating activity. We identified hypoxia inducible factor 1beta (HIF1beta) as a TG2 binding partner. HIF1beta and HIF1alpha together form the heterodimeric transcription factor hypoxia inducible factor 1 (HIF1). TG2 and the transaminase-inactive mutant C277S-TG2 inhibited a HIF-dependent transcription reporter assay under hypoxic conditions without affecting nuclear protein levels for HIF1alpha or HIF1beta, their ability to form the HIF1 heterodimeric transcription factor, or HIF1 binding to its DNA response element. Interestingly, TG2 attenuates the up-regulation of the HIF-dependent proapoptotic gene Bnip3 in response to OGD but had no effect on the expression of VEGF, which has been linked to prosurvival processes. This study demonstrates for the first time that TG2 protects against OGD, interacts with HIF1beta, and attenuates the HIF1 hypoxic response pathway. These results indicate that TG2 may play an important role in protecting against the delayed neuronal cell death in ischemia and stroke.

GTP-binding-defective forms of tissue transglutaminase trigger cell death

Tissue transglutaminase (TGase-2), which binds GTP and catalyzes the cross-linking of proteins (transamidation), has been implicated both in the promotion of cell death and in the protection of cells against apoptotic insults. However, a novel transcript originally identified from the brains of Alzheimer's patients, encoding a truncated form of TGase-2 (called TGase-S), shows strong apoptotic activity. TGase-S exhibits no detectable GTP-binding capability, suggesting that its ability to induce cell death might be due to its inability to bind GTP. Thus, we have examined whether eliminating the GTP-binding capability of full-length human TGase-2 would prevent it from conferring protection against apoptotic challenges and instead convert it into a protein that causes cell death. A number of point mutants of human TGase-2 defective for binding GTP, as well as a mutant that shows impaired GTP-hydrolytic activity, were generated. Similar to what we had found for TGase-S, there was a time-dependent decrease in the expression of the GTP-binding-defective TGase-2 mutants in different cell lines, whereas the expression of wild-type TGase-2 and the GTP hydrolysis-defective mutant was sustained. Moreover, the GTP-binding-defective TGase-2 mutants induced cell death. The cell death responses triggered by these mutants were not due to their transdamidation activity, because double-mutants that were both GTP-binding- and transamidation-defective also stimulated cell death. Therefore, these results point to the inability to bind GTP as being sufficient for the apoptotic activity exhibited by the TGase-S protein. They also highlight a novel example of how the loss of GTP-binding activity can convert a protein that provides protection against apoptotic stimuli into a cell death-promoting factor.

Transglutaminase 2 inhibitors and their therapeutic role in disease states

Transglutaminase 2 (TG2) is a multi-domain, multi-functional enzyme that post-translationally modifies proteins by catalyzing the formation of intermolecular isopeptide bonds between glutamine and lysine side-chains. It plays a role in diverse biological functions, including extracellular matrix formation, integrin-mediated signaling, and signal transduction involving 7-transmembrane receptors. While some of the roles of TG2 under normal physiological conditions remain obscure, the protein is believed to participate in the pathogenesis of several unrelated diseases, including celiac sprue, neurodegenerative diseases, and certain types of cancer. A variety of small molecule and peptidomimetic inhibitors of the TG2 active site have been identified. Here, we summarize the biochemistry, biology, pharmacology and medicinal chemistry of human TG2.

Tissue transglutaminase-mediated chemoresistance in cancer cells

Drug resistance and metastasis are major impediments for the successful treatment of cancer. A common feature among drug resistant and metastatic tumor cells is that they exhibit profound resistance to apoptosis. This property enables cancer cells not only to grow and survive in stressful environments (metastasis) but also to display resistance against many anticancer agents. Therefore, perturbation of the intrinsic apoptotic pathways of cancer cells will affect their ability to respond to chemotherapy and to metastasize and survive in distant sites. Recent studies have demonstrated that cancer cells and cancer cell lines selected for resistance against chemotherapeutic drugs or isolated from metastatic sites, express elevated levels of the multifunctional protein, tissue transglutaminase (TG2). TG2 is the most diverse and ubiquitous member of the transglutaminase family of proteins that is implicated to play a role in apoptosis, wound healing, cell migration, cell attachment, cell growth, angiogenesis, and matrix assembly. TG2 can associate with certain beta members of the integrin family of proteins (beta1, beta3, beta4, and beta5) and promote stable interaction between cells and the extracellular matrix (ECM), resulting in increased cell survival, cell migration, and invasion. Additionally, TG2 forms a ternary complex with IkappaB/p65:p50 and results in constitutive activation of the nuclear transcription factor-kappaB (NF-kappaB). Moreover, TG2 expression in cancer cells leads to constitutive activation of the focal adhesion kinase (FAK) and its downstream PI3K/Akt survival pathway. Importantly, the inhibition of endogenous TG2 by small interfering RNA (siRNA) resulted in the reversal of drug resistance and the invasive phenotype. Conversely, ectopic expression of TG2 promoted cell survival, cell motility and invasive functions of cancer cells. This review discusses the current thinking and implications of increased TG2 expression in development of drug resistance and metastasis by cancer cells.

Tissue transglutaminase inhibits autophagy in pancreatic cancer cells

Elevated expression of tissue transglutaminase (TG2) in cancer cells has been implicated in the development of drug resistance and metastatic phenotypes. However, the role and the mechanisms that regulate TG2 expression remain elusive. Here, we provide evidence that protein kinase Cdelta (PKCdelta) regulates TG2 expression, which in turn inhibits autophagy, a type II programmed cell death, in pancreatic cancer cells that are frequently insensitive to standard chemotherapeutic agents. Rottlerin, a PKCdelta-specific inhibitor, and PKCdelta small interfering RNA (siRNA) down-regulated the expression of TG2 mRNA and protein and induced growth inhibition without inducing apoptosis in pancreatic cancer cells. Inhibition of PKCdelta by rottlerin or knockdown of TG2 protein by a TG2-specific siRNA resulted in a marked increase in autophagy shown by presence of autophagic vacuoles in the cytoplasm, formation of the acidic vesicular organelles, membrane association of microtubule-associated protein 1 light chain 3 (LC3) with autophagosomes, and a marked induction of LC3-II protein, important hallmarks of autophagy, and by electron microscopy. Furthermore, inhibition of TG2 by rottlerin or by the siRNA led to accumulation of green fluorescent protein (GFP)-LC3-II in autophagosomes in pancreatic cancer cells transfected with GFP-LC3 (GFP-ATG8) expression vector. Knockdown of Beclin-1, a specific autophagy-promoting protein and the product of Becn1 (ATG6), inhibited rottlerin-induced and TG2 siRNA-induced autophagy, indicating that Beclin-1 is required for this process. These results revealed that PKCdelta plays a critical role in the expression of TG2, which in turn regulates autophagy. In conclusion, these results suggest a novel mechanism of regulation of TG2 and TG2-mediated autophagy in pancreatic cancer cells.

Type 2 transglutaminase differentially modulates striatal cell death in the presence of wild type or mutant huntingtin

Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt), is characterized by extensive loss of striatal neurons. The dysregulation of type 2 transglutaminase (TG2) has been proposed to contribute to the pathogenesis in HD as TG2 is up-regulated in HD brain and knocking out TG2 in mouse models of HD ameliorates the disease process. To understand the role of TG2 in the pathogenesis of HD, immortalized striatal cells established from mice in which mutant htt with a polyglutamine stretch of 111 Gln had been knocked-in and wild type (WT) littermates, were stably transfected with human TG2 in a tetracycline inducible vector. Overexpression of TG2 in the WT striatal cells resulted in significantly greater cell death under basal conditions as well as in response to thapsigargin treatment, which causes increased intracellular calcium concentrations. Furthermore, in WT striatal cells TG2 overexpression potentiated mitochondrial membrane depolarization, intracellular reactive oxygen species production, and apoptotic cell death in response to thapsigargin. In contrast, in mutant striatal cells, TG2 overexpression did not increase cell death, nor did it potentiate thapsigargin-induced mitochondrial membrane depolarization or intracellular reactive oxygen species production. Instead, TG2 overexpression in mutant striatal cells attenuated the thapsigargin-activated apoptosis. When in situ transglutaminase activity was quantitatively analyzed in these cell lines, we found that in response to thapsigargin treatment TG2 was activated in WT, but not mutant striatal cells. These data suggest that mutant htt alters the activation of TG2 in response to certain stimuli and therefore differentially modulates how TG2 contributes to cell death processes.

Two isoforms of tissue transglutaminase mediate opposing cellular fates

Opposing cellular responses are typically regulated by distinct sets of genes. However, tissue transglutaminase (TGase) provides an interesting example of a single gene product that has been implicated both in affording protection against cellular insults as well as in promoting cell death. Here, we shed some light on how these conflicting activities might be manifested by demonstrating that alternative transcripts of TGase differentially affect cell viability. We show that although the full-length TGase protein affords strong protection against cell death signals, a shorter version of TGase that is truncated at the 3' end, and thus called TGase-short (TGase-S), is cytotoxic. The apoptotic activity of TGase-S is not dependent on its transamidation activity because the mutation of a cysteine residue that is essential for catalyzing this reaction does not compromise the ability of TGase-S to induce cell death. Intriguingly, TGase-S undergoes inappropriate oligomer formation in cells before cell death, suggesting a novel mechanism for the apoptotic effects of this protein.

Importance of Ca(2+)-dependent transamidation activity in the protection afforded by tissue transglutaminase against doxorubicin-induced apoptosis

Tissue transglutaminase II (TGase-II), which is capable of both GTP binding and transamidation activities, has been implicated in a variety of biological disorders ranging from cancer to neurodegenerative diseases. Recent studies have suggested that the transamidation activity of TGase-II is necessary for the survival of cancer cells confronted with different stresses and cellular insults. When assayed in vitro, the transamidation activity of TGase-II is Ca(2+)-dependent. However, at present, little is known with regard to how the regulation by Ca(2+) is manifested or if in fact it is important for the cellular functions of TGase-II. Here, we have set out to further examine the Ca(2+)-mediated regulation of TGase-II's transamidation activity, with our goals being to identify the Ca(2+)-regulatory sites on the protein and determine whether they are essential for TGase-II to confer survival to human breast cancer cells. On the basis of comparisons between the X-ray crystal structures of TGase-II and TGase-III, we identified three putative Ca(2+)-regulatory sites on TGase-II. Site-directed mutagenesis was performed to individually alter key residues at each of the sites. These substitutions did not affect the ability of TGase-II to bind guanine nucleotides, nor did they cause any obvious changes in its cellular localization. While substitutions at the different Ca(2+)-regulatory sites could either slightly enhance or markedly reduce the GTP hydrolytic activity of TGase-II, mutations at each of the three sites inhibited the Ca(2+)-responsive transamidation activity. We further showed that the same substitutions inhibited the ability of TGase-II to protect human breast cancer cells against the apoptotic activity of doxorubicin. Overall, these findings demonstrate that the Ca(2+)-mediated regulation of transamidation activity is essential for the ability of TGase-II to confer cell survival.

"Tissue" transglutaminase contributes to the formation of disulphide bridges in proteins of mitochondrial respiratory complexes

In this study we provide the first in vivo evidences showing that, under physiological conditions, "tissue" transglutaminase (TG2) might acts as a protein disulphide isomerase (PDI) and through this activity contributes to the correct assembly of the respiratory chain complexes. Mice lacking TG2 exhibit mitochondrial energy production impairment, evidenced by decreased ATP levels after physical challenge. This defect is phenotypically reflected in a dramatic decrease of motor behaviour of the animals. We propose that the molecular mechanism, underlying such a phenotype, resides in a defective disulphide bonds formation in ATP synthase (complex V), NADH-ubiquinone oxidoreductase (complex I), succinate-ubiquinone oxidoreductase (complex II) and cytochrome c oxidase (complex IV). In addition, TG2-PDI might control the respiratory chain by modulating the formation of the prohibitin complexes. These data elucidate a new pathway that directly links the TG2-PDI enzymatic activity with the regulation of mitochondrial respiratory chain function.

Neuroprotective effects of cystamine in aged parkinsonian mice

Accumulating evidence suggests an important role of oxidation in pathologies such as Parkinson's disease. Here, we investigated the effects of cystamine, which has shown neuroprotection in animal models of Huntington's disease, in a parkinsonian mouse generated by the toxin MPTP. Aged mice (16 months of age) were assigned to either a 10 or 50 mg/kg/day cystamine treatment administered (1) 2 days prior, during and 14 days after MPTP lesioning or (2) beginning on the day of the MPTP lesion and for the subsequent 14 days. Pre-treatment with lower doses of cystamine (10 mg/kg) revealed increased levels of tyrosine hydroxylase (TH) positive striatal fiber (p<0.01), increased density of TH-immunoreactive cells (p<0.01), increased substantia nigra Nurr1 mRNA levels (p<0.001), and increased density of substantia nigra cells expressing the dopamine transporter (p<0.001) as compared to MPTP treated mice. These results provide strong evidence for neuroprotective properties of cystamine in this animal model of Parkinson's disease.

Tissue transglutaminase serves as an inhibitor of apoptosis by cross-linking caspase 3 in thapsigargin-treated cells

Thapsigargin (THG) is an inhibitor of the endoplasmic reticulum Ca2+-ATPase that induces caspase 3 activation and apoptosis in HCT116 cells through a Bax-dependent pathway. In Bax-deficient HCT116 cells, however, THG specifically generates two additional species of caspase 3, termed p40 and p64, with molecular masses of approximately 40 and 64 kDa, respectively, through unknown mechanisms. Here, we report that the Ca2+-dependent protein cross-linking enzyme tissue transglutaminase (tTGase) is involved in THG-induced p40 and p64 formation by catalyzing caspase 3 cross-linking reactions, thereby inactivating caspase 3 and apoptosis in Bax-deficient cells. Overexpression of tTGase increases p40 and p64 in THG-treated cells, and purified tTGase catalyzes procaspase 3 cross-linking in vitro. Inhibition of tTGase activity by either the tTGase inhibitor monodansylcadaverine or short-hairpin RNA reduces the cross-linked species p40 and p64 and restores caspase 3 activation in response to THG treatment. Moreover, prolonged exposure to THG results in a decrease in protein levels of XIAP and cIAP-1, which is subsequently followed by an increase in tTGase protein expression and activity. Expression of cytosolic Smac sensitizes Bax-deficient cells to THG-induced apoptosis; however, this effect is diminished by coexpression of tTGase. Taken together, these results suggest a novel role for tTGase as a new type of caspase 3 inhibitor in THG-mediated apoptosis.

Cystamine treatment is neuroprotective in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult onset neurodegenerative disorder characterized by selective atrophy and cell loss within the striatum. There is currently no treatment that can prevent the striatal neuropathology. Transglutaminase (TG) activity is increased in HD patients, is associated with cell death, and has been suggested to contribute to striatal neuronal loss in HD. This work assesses the therapeutic potential of cystamine, an inhibitor of TG activity with additional potentially beneficial effects. Specifically, we examine the effect of cystamine on striatal neuronal loss in the YAC128 mouse model of HD. We demonstrate here for the first time that YAC128 mice show a forebrain-specific increase in TG activity compared with wild-type (WT) littermates which is decreased by oral delivery of cystamine. Treatment of symptomatic YAC128 mice with cystamine starting at 7 months prevented striatal neuronal loss. Cystamine treatment also ameliorated the striatal volume loss and striatal neuronal atrophy observed in these animals, but was unable to prevent motor dysfunction or the down-regulation of dopamine and cyclic adenosine monophsophate-regulated phosphoprotein (DARPP-32) expression in the striatum. While the exact mechanism responsible for the beneficial effects of cystamine in YAC128 mice is uncertain, our findings suggest that cystamine is neuroprotective and may be beneficial in the treatment of HD.

Invasion-associated MMP-2 and MMP-9 are Up-regulated Intracellularly in Concert with Apoptosis Linked to Melanoma Cell Detachment

Matrix metalloproteinases, like MMP-2 and MMP-9 gelatinases, show multiple functions as extracellular/cell-surface enzymes, and are broadly recognised for their matrix-degrading ability and involvement in cell motility. Given that adherent cells have reduced attachment during migration and also detach from their substratum during apoptosis, we now investigated whether extracellular matrix-bound gelatinases and intracellular MMP-2 and MMP-9 are modified with progression of death-inducing stimuli. This report shows that melanoma cells undergoing death in response to 2-acetyl furanonaphtoquinone (FNQ) as evidenced by greater Annexin V binding, increased cytosolic expression of pro-MMP-2 and intracellular activation of particulate MMP-9. These changes were associated with early activation of a substrate-attached 40 kDa gelatinase reciprocal with changes in extracellular matrix-bound activated MMP-2. A subsequent activation of secreted MMP-9 and induction of apoptosis-associated fragmentation of poly ADP-Ribose polymerase (PARP) correlated with cell detachment. Our data suggests that intracellularly activated gelatinases may cleave survival-associated substrates other than gelatin that share the Gly-Leu/Iso-Pro like collagen-binding acetylcholinesterase, thereby linking them to apoptosis associated with cell detachment.