Tissue transglutaminase and apoptosis: sense and antisense transfection studies with human neuroblastoma cells

Biological Implications and Functional Significance of Transglutaminase Type 2 in Nervous System Tumors

Transglutaminase type 2 (TG2) is the most ubiquitously expressed member of the transglutaminase family. TG2 catalyzes the transamidation reaction leading to several protein post-translational modifications and it is also implicated in signal transduction thanks to its GTP binding/hydrolyzing activity. In the nervous system, TG2 regulates multiple physiological processes, such as development, neuronal cell death and differentiation, and synaptic plasticity. Given its different enzymatic activities, aberrant expression or activity of TG2 can contribute to tumorigenesis, including in peripheral and central nervous system tumors. Indeed, TG2 dysregulation has been reported in meningiomas, medulloblastomas, neuroblastomas, glioblastomas, and other adult-type diffuse gliomas. The aim of this review is to provide an overview of the biological and functional relevance of TG2 in the pathogenesis of nervous system tumors, highlighting its involvement in survival, tumor inflammation, differentiation, and in the resistance to standard therapies.

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

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

Identification of Betamethasone-Regulated Target Genes and Cell Pathways in Fetal Rat Lung Mesenchymal Fibroblasts

Preterm birth is characterized by severe lung immaturity that is frequently treated antenatally or postnatally with the synthetic steroid betamethasone. The underlying cellular targets and pathways stimulated by betamethasone in the fetal lung are poorly defined. In this study, betamethasone was compared with corticosterone in steroid-treated primary cultures of fetal rat lung fibroblasts stimulated for 6 hours and analyzed by whole-cell transcriptome sequencing and glucocorticoid (GC) receptor (GR) chromatin immunoprecipitation sequencing (ChIP-Seq) analysis. Strikingly, betamethasone stimulated a much stronger transcriptional response compared with corticosterone for both induced and repressed genes. A total of 483 genes were significantly stimulated by betamethasone or corticosterone, with 476 stimulated by both steroids, indicating a strong overlap in regulation. Changes in mRNA levels were confirmed by quantitative PCR for eight induced and repressed target genes. Pathway analysis identified cell proliferation and cytoskeletal/cell matrix remodeling pathways as key processes regulated by both steroids. One target, transglutaminase 2 (Tgm2), was localized to fetal lung mesenchymal cells. Tgm2 mRNA and protein levels were strongly increased in fibroblasts by both steroids. Whole-genome GR ChIP-Seq analysis with betamethasone identified GC response element-binding sites close to the previously characterized GR target genes Per1, Dusp1, Fkbp5, and Sgk1 and near the genes identified by transcriptome sequencing encoding Crispld2, Tgm2, Hif3α, and Kdr, defining direct genomic induction of expression in fetal lung fibroblasts via the GR. These results demonstrate that betamethasone stimulates specific genes and cellular pathways controlling cell proliferation and extracellular matrix remodeling in lung mesenchymal fibroblasts, providing a basis for betamethasone's treatment efficacy in preterm birth.

A composite of urinary biomarkers for differentiating between tubulointerstitial inflammation and interstitial fibrosis/tubular atrophy in kidney allografts

Backgrounds/Aims

Compared with a single urinary biomarker, a composite of multiple urinary biomarkers may be more helpful for differentiating tubulointerstitial inflammation from interstitial fibrosis/tubular atrophy (IFTA) in kidney allografts.

Methods

In this cross-sectional cohort study, we collected urine samples from 115 patients with for-cause biopsy, 53 patients with stable allografts, and 50 living kidney donors. We measured the urinary levels of transglutaminase 2 (TG2), syndecan-4 (SDC4), alpha 1 microglobulin (A1M), interferon-inducible protein 10 (IP-10), interleukin 6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1).

Results

The for-cause biopsy group showed significantly higher levels of log_eTG2/Cr, log_eA1M/Cr, log_eIL-6/Cr, and log_eMCP-1/Cr compared with other groups. In the for-cause biopsy group, log_eTG2/Cr level was positively correlated with the severity of IFTA. After adjusting for age, sex, body mass index, diabetes, hypertension, cardiovascular disease, and the interval between kidney transplant and biopsy, TG2 and the interval between transplant and biopsy were significantly correlated variables for the severity of IFTA. Regarding tubulointerstitial inflammation, Body mass index, TG2, SDC4, and IP-10 were positively-correlated variables, and MCP-1 and the interval between transplant and biopsy were negatively-correlated variables.

Conclusions

Our results show that post-transplant urinary levels of TG2, SDC4, MCP-1 and IP-10 may be a useful biomarker for tubulointerstitial inflammation and IFTA.

Development of new scaffolds as reversible tissue transglutaminase inhibitors, with improved potency or resistance to glutathione addition

Previous studies within our group have yielded a class of cinnamoyl-based competitive reversible inhibitors for tissue transglutaminase (TG2), with K_i values as low as 1.0 μM (compound CP4d). However, due to the electrophilic nature of their alkene moiety, this class of inhibitors is susceptible to nucleophilic attack by glutathione, a key element in cellular metabolism and toxicity response. To address this issue, we made several modifications to the inhibitor scaffold, ultimately showing that a bis(triazole) scaffold increased resistance to nucleophilic attack, with compound 27d being the most potent (K_i = 10 μM). In the process of reducing reactivity, we also prepared a new class of inhibitors, replacing the alkene of CP4d with an alkyne, leading to a significant increase in potency for compound 22b (K_i = 420 nM).

Cyclin A1 expression and paclitaxel resistance in human ovarian cancer cells

BACKGROUND

The development of intrinsic and acquired resistance to antineoplastic agents is a major obstacle to successful chemotherapy in ovarian cancers. Identification and characterisation of chemoresponse-associated biomarkers are of paramount importance for novel therapeutic development.

METHODS

Global RNA expression profiles were obtained by high-throughput microarray analysis. Cell cycle, proliferation rate, and paclitaxel sensitivity of ovarian cancer cells harbouring cyclin A1-inducible expression construct were compared with and without tetracycline induction, as well as when the cyclin A1 expression was suppressed by short inhibiting RNA (siRNA). Cellular senescence was evaluated by β-galactosidase activity staining.

RESULTS

Global RNA expression profiling and subsequent correlation studies of gene expression level and drug response has identified that elevated expression of cyclin A1 (CCNA1) was significantly associated with cellular resistance to paclitaxel, doxorubicin and 5-fluorouracil. The role of cyclin A1 in paclitaxel resistance was confirmed in ovarian cancer cells that harbour an inducible cyclin A1 expression construct, which showed reduced paclitaxel-mediated growth inhibition and apoptosis when cyclin A1 expression was induced, whereas downregulation of cyclin A1 expression in the same cell lines using cyclin A1-specific siRNAs sensitised the cells to paclitaxel toxicity. However, ovarian cancer cells with ectopic expression of cyclin A1 demonstrated slowdown of proliferation and senescence-associated β-galactosidase activity.

CONCLUSIONS

Our profiling and correlation studies have identified cyclin A1 as one chemoresistance-associated biomarker in ovarian cancer. The results of the characterisation studies suggest that cyclin A1 functions as an oncogene that controls proliferative and survival activities in tumourigenesis and chemoresistance of ovarian cancer.

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.

Curcumin (Diferulolylmethane) Reduces Transglutaminase 2 Overexpression Induced by Retinoic Acid in Human Nervous Cell Lines

OBJECTIVES

Curcumin, a naturally occurring compound derived from turmeric (Curcuma longa) has long been suggested to have strong therapeutic or preventive potential against human diseases because of its antioxidative, anticancerous, and anti-inflammatory effects. Curcumin is known to exert anti-inflammatory effects by interrupting NF-κB signaling at multiple levels. Many observations indicate that curcumin shows its valuable potential by inhibiting the activity of I-κB kinase. Transglutaminase 2 (TG2) expression is increased in inflammatory diseases. Data in the literature suggest that this enzyme activates the proinflammatory transcriptional factor NF-κB by inducing the polymerization of its inhibitory subunit I-κBα, which in turn results in the dissociation of NF-κB and its translocation to the nucleus, where it is capable of upregulating host inflammatory genes. Interestingly, NF-κB regulatory response elements are also present in the TG2 promoter, suggesting a possible role for this pathway in the mechanism responsible for chronic inflammation. On the basis of these literature data, our objective was to analyze the effects of curcumin on TG2 expression in human nervous cell lines.

METHODS

Human nervous cell lines were treated with curcumin alone or in association with retinoic acid in order to induce TG2 overexpression. TG2 levels were analyzed by Western blot and real-time PCR analyses.

RESULTS

Curcumin was able to downregulate the expression of TG2 in human nervous cell lines, which was also the case after treatment with retinoic acid.

CONCLUSIONS

These results suggest a possible use of curcumin in reducing TG2 overexpression in human nervous cells.

Inhibitors of tissue transglutaminase

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

Acyl transfer mechanisms of tissue transglutaminase

Tissue transglutaminase (TG2) is a calcium-dependent enzyme that catalyses several acyl transfer reactions. The most biologically relevant of these involve protein-bound Gln residues as an acyl-donor substrate, and either water or a primary amine as an acyl-acceptor substrate. The former leads to deamidation of Gln to Glu, whereas the latter leads to transamidation, typically resulting in protein cross-linking when the amine substrate is a protein-bound Lys residue. In this review, we present an overview of over fifty years of mechanistic studies that have led to our current understanding of TG2-mediated hydrolysis and transamidation.

Transglutaminase 2 contributes to apoptosis induction in Jurkat T cells by modulating Ca2+ homeostasis via cross-linking RAP1GDS1

Background

Transglutaminase 2 (TG2) is a protein cross-linking enzyme known to be associated with the in vivo apoptosis program of T cells. However, its role in the T cell apoptosis program was not investigated yet.

Results

Here we report that timed overexpression of both the wild type (wt) and the cross-linking mutant of TG2 induced apoptosis in Jurkat T cells, the wt being more effective. Part of TG2 colocalised with mitochondria. WtTG2-induced apoptosis was characterized by enhanced mitochondrial Ca²⁺ uptake. Ca²⁺-activated wtTG2 cross-linked RAP1, GTP-GDP dissociation stimulator 1, an unusual guanine exchange factor acting on various small GTPases, to induce a yet uncharacterized signaling pathway that was able to promote the Ca²⁺ release from the endoplasmic reticulum via both Ins₃P and ryanodine sensitive receptors leading to a consequently enhanced mitochondrial Ca²⁺uptake.

Conclusions

Our data indicate that TG2 might act as a Ca²⁺ sensor to amplify endoplasmic reticulum-derived Ca²⁺ signals to enhance mitochondria Ca²⁺ uptake. Since enhanced mitochondrial Ca²⁺ levels were previously shown to sensitize mitochondria for various apoptotic signals, our data demonstrate a novel mechanism through which TG2 can contribute to the induction of apoptosis in certain cell types. Since, as compared to knock out cells, physiological levels of TG2 affected Ca²⁺ signals in mouse embryonic fibroblasts similar to Jurkat cells, our data might indicate a more general role of TG2 in the regulation of mitochondrial Ca²⁺ homeostasis.

ACTL6a enforces the epidermal progenitor state by suppressing SWI/SNF-dependent induction of KLF4

Somatic progenitors suppress differentiation to maintain tissue self-renewal. The mammalian SWI/SNF chromatin-remodeling complex regulates nucleosome packaging to control differentiation in embryonic and adult stem cells. Catalytic Brg1 and Brm subunits are required for these processes; however, the roles of SWI/SNF regulatory subunits are not fully understood. Here, we show that ACTL6a/BAF53A modulates the SWI/SNF complex to suppress differentiation in epidermis. Conditional loss of ACTL6a resulted in terminal differentiation, cell-cycle exit, and hypoplasia, whereas ectopic expression of ACTL6a promoted the progenitor state. A significant portion of genes regulated by ACTL6a were found to also be targets of KLF4, a known activator of epidermal differentiation. Mechanistically, we show that ACTL6a prevents SWI/SNF complex binding to promoters of KLF4 and other differentiation genes and that SWI/SNF catalytic subunits are required for full induction of KLF4 targets. Thus, ACTL6a controls the epidermal progenitor state by sequestering SWI/SNF to prevent activation of differentiation programs.

Transglutaminase type 2 in human abdominal aortic aneurysm is a potential factor in the stabilization of extracellular matrix

OBJECTIVE

The aim of this study was to evaluate transglutaminase type 2 (TG2) expression in human abdominal aortic aneurysm (AAA) tissue and to elucidate a potential role of TG2 in AAA formation. TG2, which is a Ca(2+)-dependent cross-linking enzyme, has been proven important for stabilizing the extracellular matrix. However, there is no evidence of the effect of TG2 on AAA formation in a human model.

METHODS

Aortic wall tissues were obtained during surgery in AAA patients (n = 38) and in patients with aortoiliac occlusive disease (Control; n = 4) in the Asan Medical Center from March 2011 to February 2012. In each AAA patient, the aortic neck (Neck) and maximally dilated portion (Max) of the aneurysm were sampled for analysis. TG2 expression was evaluated using immunohistochemistry and Western blotting. In addition, ex vivo experiments of isolated AAA tissue culture with the TG2 inhibitor cystamine and recombinant human TG2 were performed.

RESULTS

Among 38 AAA patients, 11 had ruptured (contained or free) AAAs. The mean maximal diameter of AAAs was 6.09 ± 1.46 cm. TG2 expressions of Max were significantly increased compared with those of Control (1.7-fold increase of Control; P = .00). Compared with Control, the intensities of tissue necrosis factor-α, matrix metalloproteinase (MMP)-2, MMP-9, and tissue inhibitors of metalloproteinase-2 were significantly upregulated in Max (1.7-fold, 1.5-fold, 1.3-fold, and 1.6-fold increases of Control; P = .00, P = .004, P = .046, and P = .007, respectively). Furthermore, double immunofluorescent staining showed that colocalization of TG2/transforming growth factor-β or TG2/fibronectin was prominent in Max compared with those of Neck or Control. In addition, MMP-2 intensity was upregulated in ruptured AAAs compared with unruptured AAAs, with marginal significance (P = .078). Ex vivo experiments showed that protein expressions of tissue necrosis factor-α, MMP-2, and MMP-9 in cultured AAA tissue were decreased by recombinant human TG2 but were increased by exogenous cystamine.

CONCLUSIONS

The TG2 expression in the maximally dilated portion of AAAs was enhanced compared with that of nondilated aorta. It is suggested that TG2 has a potential effect in stabilization of extracellular matrix by inhibition of proinflammatory cytokines and MMPs or by interaction with fibronectin and transforming growth factor-β.

Calcium signalling and the regulation of apoptosis

Apoptotic cell death is characterized by cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies and phagocytosis (Kerr et al., 1972). At the molecular level, activation of a family of cysteine proteases, caspases, related to interleukin-1beta-converting enzyme is believed to be a crucial event in apoptosis. This is associated with the proteolysis of nuclear and cytoskeletal proteins, cell shrinkage, glutathione efflux, exposure of phosphatidylserine on the cell surface, membrane blebbing, etc. In CD95- or TNF-mediated apoptosis, the proteolytic cascade is believed to be triggered directly by caspase binding to the activated plasma membrane receptor complex. In other forms of apoptosis, the mechanisms of activation of the proteolytic cascade are less well established but may involve imported proteases, such as granzyme B, or factors released from the mitochondria and, possibly, other organelles. Recently, the possibility that cytochrome c released from the mitochondria may serve to activate dormant caspases in the cytosol, and thereby to propagate the apoptotic process, has attracted considerable attention. A perturbation of intracellular Ca(2+) homeostasis has been found to trigger apoptosis in many experimental systems, and the apoptotic process has been related to either a sustained increase in cytosolic free Ca(2+) level or a depletion of intracellular Ca(2+) stores. Although many of the biochemical events involved in the apoptotic process are Ca(2+) dependent, the exact mechanism by which Ca(2+) triggers apoptosis remains unknown. The bcl-2 gene family, which includes both inhibitors and inducers of apoptosis, appears to regulate intracellular Ca(2+) compartmentalization. The induction of apoptosis by Ca(2+)-mobilizing agents results in caspase activation, which is similar to what is seen with other inducers of apoptosis. In addition, Ca(2+)-dependent proteases, such as calpain and a Ca(2+)-dependent nuclear scaffold-associated serine protease, are also activated by Ca(2+) signalling in some cell types where they appear to be involved in alpha-fodrin and lamin beta cleavage, respectively. Thus, a spectrum of proteases are activated during apoptosis depending on both cell type and inducer. This proteolytic cascade can involve both caspases and Ca(2+)-dependent proteases, which seem to interact during the apoptotic process.

Effect of aging on norepinephrine-related proliferative response in primary cultured periportal and perivenous hepatocytes

Norepinephrine (NE) amplifies the mitogenic effect of EGF in a rat liver through the adrenergic receptor coupled with G protein, Ghα. Ghα is also known as a transglutaminase 2 (TG2), whose cross-linking activity is implicated in hepatocyte growth. Recently, we found that NE-induced amplification of EGF-induced DNA synthesis in hepatocytes obtained from perivenous regions of liver is caused by inhibiting the downregulation of EGF receptor (EGFR) by TG2. In the present study, we investigated the effect of aging on NE-related proliferative response. Hepatocytes were obtained from the liver of 7- and 90-wk-old rats. To examine this in detail, periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) were isolated using the digitonin/collagenase perfusion technique. EGF or NE receptor binding was analyzed by Scatchard analysis. Changes in NE-induced DNA synthesis, G protein activity, and TG2 activity were measured. NE slightly potentiated [125I]EGF binding to EGFR, and EGF-induced DNA synthesis in PVH but not in PPH. [3H]NE binding studies indicated that PVH have a greater number of receptors than PPH, and that the number of receptors in both subpopulations increased with aging. NE-induced changes in G protein activity and TG2 activity in 90-wk-old rats were slight compared with 7-wk-old rats. These results suggest that NE results in a slight recovery effect on the age-related decline in EGF-induced DNA synthesis because of incomplete switching of the function from TG2 to Ghα.

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.

Presence of tissue transglutaminase in granular endoplasmic reticulum is characteristic of melanized neurons in Parkinson's disease brain

Parkinson's disease (PD) is characterized by the accumulation of α-synuclein aggregates and degeneration of melanized neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD brain, tTG-induced cross-links have been identified in α-synuclein monomers, oligomers and α-synuclein aggregates. However, whether tTG and α-synuclein occur together in PD affected neurons remains to be established. Interestingly, using immunohistochemistry, we observed a granular distribution pattern of tTG, characteristic of melanized neurons in PD brain. Apart from tTG, these granules were also positive for typical endoplasmic reticulum (ER)-resident chaperones, that is, protein disulphide isomerase, ERp57 and calreticulin, suggesting a direct link to the ER. Additionally, we observed the presence of phosphorylated pancreatic ER kinase (pPERK), a classical ER stress marker, in tTG granule positive neurons in PD brain, although no subcellular colocalization of tTG and pPERK was found. Our data therefore suggest that tTG localization to granular ER compartments is specific for stressed melanized neurons in PD brain. Moreover, as also α-synuclein aggregates were observed in tTG granule positive neurons, these results provide a clue to the cellular site of interaction between α-synuclein and tTG.

Extracellular transglutaminase 2 has a role in cell adhesion, whereas intracellular transglutaminase 2 is involved in regulation of endothelial cell proliferation and apoptosis

OBJECTIVE

Transglutaminase 2 (TG2) is a multifunctional protein with an important role in vascular biology, where it is involved in cell-matrix interaction, cell attachment and cell population expansion. In efforts to elucidate the role of TG2 in endothelial cell biology, in this study, we measured several endothelial cell characteristics in cells where TG2 was specifically knocked down by RNAi.

MATERIALS AND METHODS

The effect of small interfering RNA (siRNA)-TG2 on human umbilical vein endothelial cells was studied. Adhesion and cell viability were assessed by chemical reduction of MTT, and cell proliferation was analysed by flow cytometry. Apoptosis was evaluated by annexin V/PI dual staining and protein expression level was assayed by western blotting.

RESULTS

We found that siRNA-TG2 reduced endothelial cell number, lead to cell adhesion deficiency, cell cycle arrest in G₁ phase and induction of apoptosis. Our results show that exogenously added TG2 could reverse loss of adhesion but did not overcome the defect in cell proliferation, nor could it inhibit siRNA-TG2-induced apoptosis.

CONCLUSION

We conclude that TG2 loss in endothelial cells causes reduction in cell number as a result of cell cycle arrest, flaws in adhesion and induction of apoptosis. Our results imply that reduction in cell number and increased apoptosis in response to TG2 silencing is independent of the cell adhesion process. Altogether, our findings underline the significance of TG2 in endothelial cell cycle progression and cell survival, in vitro.

Plasmodium falciparum liver stage antigen-1 is cross-linked by tissue transglutaminase

Background

Plasmodium falciparum sporozoites injected by mosquitoes into the blood rapidly enter liver hepatocytes and undergo pre-erythrocytic developmental schizogony forming tens of thousands of merozoites per hepatocyte. Shortly after hepatocyte invasion, the parasite starts to produce Liver Stage Antigen-1 (LSA-1), which accumulates within the parasitophorous vacuole surrounding the mass of developing merozoites. The LSA-1 protein has been described as a flocculent mass, but its role in parasite development has not been determined.

Methods

Recombinant N-terminal, C-terminal or a construct containing both the N- and C- terminal regions flanking two 17 amino acid residue central repeat sequences (LSA-NRC) were subjected to in vitro modification by tissue transglutaminase-2 (TG2) to determine if cross-linking occurred. In addition, tissue sections of P. falciparum-infected human hepatocytes were probed with monoclonal antibodies to the isopeptide ε-(γ-glutamyl)lysine cross-bridge formed by TG2 enzymatic activity to determine if these antibodies co-localized with antibodies to LSA-1 in the growing liver schizonts.

Results

This study identified a substrate motif for (TG2) and a putative casein kinase 2 phosphorylation site within the central repeat region of LSA-1. The function of TG2 is the post-translational modification of proteins by the formation of a unique isopeptide ε-(γ-glutamyl)lysine cross-bridge between glutamine and lysine residues. When recombinant LSA-1 protein was crosslinked in vitro by purified TG2 in a calcium dependent reaction, a flocculent mass of protein was formed that was highly resistant to degradation. The cross-linking was not detectably affected by phosphorylation with plasmodial CK2 in vitro. Monoclonal antibodies specific to the very unique TG2 catalyzed ε- lysine cross-bridge co-localized with antibodies to LSA-1 in infected human hepatocytes providing visual evidence that LSA-1 was cross-linked in vivo.

Conclusions

While the role of LSA-1 is still unknown these results suggest that it becomes highly cross-linked which may aid in the protection of the parasite as it develops.

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.

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.

Protein crosslinking by transglutaminase controls cuticle morphogenesis in Drosophila

Transglutaminase (TG) plays important and diverse roles in mammals, such as blood coagulation and formation of the skin barrier, by catalyzing protein crosslinking. In invertebrates, TG is known to be involved in immobilization of invading pathogens at sites of injury. Here we demonstrate that Drosophila TG is an important enzyme for cuticle morphogenesis. Although TG activity was undetectable before the second instar larval stage, it dramatically increased in the third instar larval stage. RNA interference (RNAi) of the TG gene caused a pupal semi-lethal phenotype and abnormal morphology. Furthermore, TG-RNAi flies showed a significantly shorter life span than their counterparts, and approximately 90% of flies died within 30 days after eclosion. Stage-specific TG-RNAi before the third instar larval stage resulted in cuticle abnormality, but the TG-RNAi after the late pupal stage did not, indicating that TG plays a key role at or before the early pupal stage. Immediately following eclosion, acid-extractable protein from wild-type wings was nearly all converted to non-extractable protein due to wing maturation, whereas several proteins remained acid-extractable in the mature wings of TG-RNAi flies. We identified four proteins—two cuticular chitin-binding proteins, larval serum protein 2, and a putative C-type lectin—as TG substrates. RNAi of their corresponding genes caused a lethal phenotype or cuticle abnormality. Our results indicate that TG-dependent protein crosslinking in Drosophila plays a key role in cuticle morphogenesis and sclerotization.

Analysis of early changes in the articular cartilage transcriptisome in the rat meniscal tear model of osteoarthritis: pathway comparisons with the rat anterior cruciate transection model and with human osteoarthritic cartilage

OBJECTIVE

The purpose of this study was to use microarray technology to: (1) understand the early molecular events underlying the damage of articular cartilage initiated by this surgical procedure, and (2) determine whether these changes mimic those that are occurring in human osteoarthritic (OA) cartilage.

DESIGN

Cartilage was harvested from both medial and lateral sides of the tibial plateaus and femoral condyles of both meniscal tear (MT) and sham surgery groups on days 3, 7 and 21 post-surgery. mRNA prepared from these rat cartilage samples was used for microarray analysis.

RESULTS

Statistical analysis identified 475 genes that were differentially expressed between the sham and MT groups, at one or more of the time points that were analyzed. By integrating these genes with OA-related genes reported previously in a rat OA model and in human OA array studies, we identified 20 commonly changed genes. Six out of these 20 genes (Col5A1, Col6A2, INHBA, LTBP2, NBL1 and SERPINA1) were differentially expressed in two animal models and in human OA. Pathway analysis identified some key features of OA pathology, namely cartilage extracellular matrix remodeling, angiogenesis, and chondrocyte cell death that were recapitulated in the animal models. The rat models suggested increased inflammation and cholesterol metabolic pathways may play important role in early cartilage degeneration.

CONCLUSION

We identified a large number of differentially expressed genes in the articular cartilage of the MT model. While there was lack of overall identity in cartilage gene expression between the rat models and human OA, several key biological processes were recapitulated in the rat MT OA model.

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.

TG2 protects neuroblastoma cells against DNA-damage-induced stress, suppresses p53 activation

Tissue transglutaminase (TG2) is a multifunctional member of the transglutaminase (TGase) family (E.C.2.3.2.13), which catalyzes in a calcium-dependent reaction the formation of covalent bonds between the gamma-carboxamide groups of peptide-bound glutamine residues and various primary amines. Here, we investigated the role of TG2 in a response of the neuroblastoma SH-SY5Y cells to topoisomerase II inhibitor etoposide, known to trigger DNA-damage cell response. We found an early and transient (approximately 2 h) increase of the TG2 protein in SH-SY5Y cells treated with etoposide, along with the increase of phosphorylated and total levels of the p53 protein. Next, we showed that SH-SY5Y cells, which overexpress wild-type TG2 were significantly protected against etoposide-induced cell death. The TG2 protective effect was associated only with the transamidation active form of TG2, because overexpression the wild-type TG2, but not its transamidation inactive C277S form, resulted in a pronounced suppression of caspase-3 activity as well as p53 phosphorylation during the etoposide-induced stress. In addition, exacerbation of cell death with a significant increase in caspase-3 and p53 activation was observed in SH/anti-TG2 cells, in which expression of the endogenous TG2 protein has been greatly reduced by the antisense cDNA construct. Though the cell signaling and molecular mechanisms of the TG2-driven suppression of the cell death machinery remain to be investigated, our findings strongly suggest that TG2 plays an active role in the response of neuroblastoma cells to DNA-damage-induced stress by exerting a strong protective effect, likely by the suppression of p53 activation and p53-driven cell signaling events.

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

OBJECTIVE

Recently, we found that transglutaminase 2 (TG2) might be involved in the difference in proliferative capacities between periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) through down-regulation of high-affinity epidermal growth factor receptor (EGFR). However, it is uncertain whether this high-affinity EGFR contributes to the hepatocyte growth signalling pathway. Here, we have investigated the influence of TG2 on EGF-induced EGFR dimerization and its phosphorylation, which are important steps in the hepatocyte proliferative/growth signalling pathway, in PPH and PVH.

MATERIALS AND METHODS

PPH and PVH were isolated using the digitonin/collagenase perfusion technique. Amounts of TG2, EGFR dimerization and its phosphorylation were determined by Western blot analysis.

RESULTS

Pretreatment with monodansylcadaverine, an inhibitor of TG2, greatly increased EGF-induced EGFR dimerization and its phosphorylation in PVH compared with PPH. Conversely, treatment with retinoic acid, an inducer of TG2, significantly decreased EGF-induced EGFR dimerization and its phosphorylation with a significant increase in TG2 expression and its catalysed products, isopeptide bonds, in both subpopulations. It was found that EGFR served as a substrate for TG2.

CONCLUSION

The present data showed good correlation with our previous data on EGF-induced DNA synthesis and EGFR-binding affinity to EGF. These results suggest that zonal difference in cell growth between PPH and PVH may be caused by down-regulation of EGFR dimerization and subsequent autophosphorylation through TG2-mediated cross-linking of EGFR.

Apoptosis in human reproductive tissues: emerging concepts

In summary, apoptosis is an important concept in understanding many facets of human reproduction. Recent advances in the understanding of molecular mechanisms of apoptosis will allow us to understand this physiologically important process. How can the modulation of this process be applied to human reproduction? Studies to further understand the abnormalities of apoptosis, either too much or too little, may lead to a better understanding of the clinical problems in human reproduction.

We summarize future directions towards further understanding the roles of apoptotic processes in human reproduction in Table 3. The diseases listed in Table 3 are problems which could be approached from the apoptosis point of view. With further study using this concept as the lens, new diagnostic tools or therapies may be developed for these problems.

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.

The Differential Effects of R580A Mutation on Transamidation and GTP Binding Activity of Rat and Human Type 2 Transglutaminase

Type 2 transglutaminase (TG2) is an acyltransferase, which also undergoes a GTP-binding/GTPase cycle, with guanine nucleotide and calcium binding reciprocally regulating its transamidation (TG) activity. TG2 is expressed ubiquitously throughout the human body and is the predominant neuronal transglutaminase. Given a postulated role for TG2 in a number of physiological and pathological processes including neurodegenerative diseases, it is of critical importance to understand how TG2 and its enzymatic activities are regulated in the cells. The various aspects of TG2 regulation are addressed by using rat and human TG2 proteins, however, despite their homologous structure, regulation of their enzymatic activities may differ, especially in the cellular context. Here, we evaluate the role of Arg580 in human TG2 and Arg579 in rat TG2 in modulating GTP binding and TG activities in vitro and in situ. We confirm the importance of Arg580 and Arg579 in TG2 for GTP binding as their mutation to Ala completely abolished GTP binding activity in both human (R580A) and rat TG2 (R579A). Next, we showed that in transfected human embryonic kidney (HEK) 293 cells, basal in situ TG activity of human R580A TG2 and rat R579A TG2 was significantly greater than their wild-type (WT) counterparts. However, TG activity of the mutants and WT TG2 became equivalent when the intracellular calcium concentration was maximally increased with maitotoxin. Also, in vitro TG activity assay revealed an intriguing difference between rat and human TG2; at a calcium concentration when their activities were maximum, the protein level of human R580A TG2 was lower than its WT counterpart, whereas rat R579A and WT TG2 protein levels were similar. Taken together, our study underscores an essential role of Arg580 in human TG2 and Arg579 in rat TG2 for their GTP binding ability and also describes for the first time that these amino acid residues differentially influence the TG activity of human or rat TG2 by calcium in vitro and in situ.

Effect of retinoic acid on transglutaminase and ornithine decarboxylase activities during liver regeneration

Liver regeneration is regulated by several factors, including growth factors, cytokines, and post-translational modifications of several proteins. It is suggested that transglutaminase 2 (TG2) and ornithine decarboxylase (ODC) are involved in liver regeneration. To investigate the role of TG2 and ODC activities in regenerating liver, we used retinoic acid (RA), an inducer of TG2 and a suppressor of ODC. Regenerating rat liver was prepared by 70% partial hepatectomy (PH). Rats were sacrificed at 1, 2, 3, 4, and 6 days after surgery. RA was intraperitoneally injected immediately after PH. TG2 and ODC activities and products (epsilon-(gamma-glutamyl) lysine isopeptide (Gln-Lys) and polyamines, respectively) were examined at the indicated times. In RA-treated rat, DNA synthesis and ODC activity declined and the peak shifted to 2 days after PH, whereas TG2 activity increased at 1 day after PH. At that time, protein-polyamine, especially the protein-spermidine (SPD) bond, transiently decreased, whereas the formation of the Gln-Lys bond increased after PH. These results suggested that in regenerating liver, enhanced the formation of Gln-Lys bonds catalyzed by TG2 led to reduced DNA synthesis, whereas when ODC produced newly synthesized SPD, the inhibition of Gln-Lys bond production by the preferential formation of protein-SPD bonds led to an increase in DNA synthesis.

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.

Tissue transglutaminase, protein cross-linking and Alzheimer's disease: review and views

Extensive protein cross-linking and aggregation are some of the most common molecular events in the pathogenesis of Alzheimer's disease (AD). Both beta-amyloid (Abeta) plaques and neurofibrillary tangles, which are extracellular and intracellular proteinaceous aggregates, respectively, contribute to neuronal death and progressive cognitive decline. Although protein cross-linking has been recognized and extensively studied for many years, the underlying mechanisms are largely unknown. Recent data indicates that tissue transglutaminase (tTG), which catalyzes the cross-linking of a wide spectrum of proteins including Abeta, tau, alpha-synuclein and neurofilament proteins, may be involved in protein aggregation in AD. Many AD risk factors, such as trauma, inflammation, ischemia and stress, up-regulate tTG protein and activity levels. In this review, we summarize the evidence that tTG plays a role in AD, especially in cross-linking of Abeta, tau, alpha-synuclein and neurofilament proteins. An experimentally testable hypothesis is that tTG may play a central role in AD pathogenesis and that it provides a conceptual link between sporadic and familial AD through a shared pathogenic pathway.

Overexpression of tissue transglutaminase leads to constitutive activation of nuclear factor-kappaB in cancer cells: delineation of a novel pathway

The transcription factor nuclear factor-kappaB (NF-kappaB) plays an important role in regulating cell growth, apoptosis, and metastatic functions. Constitutive activation of NF-kappaB has been observed in various cancers; however, molecular mechanisms resulting in such activation remain elusive. Based on our previous results showing that drug-resistant and metastatic cancer cells have high levels of tissue transglutaminase (TG2) expression and that this expression can confer chemoresistance to certain types of cancer cells, we hypothesized that TG2 contributes to constitutive activation of NF-kappaB. Numerous lines of evidence showed that overexpression of TG2 is linked with constitutive activation of NF-kappaB. Tumor cells with overexpression of TG2 exhibited increased levels of constitutively active NF-kappaB. Activation of TG2 led to activation of NF-kappaB; conversely, inhibition of TG2 activity inhibited activation of NF-kappaB. Similarly, ectopic expression of TG2 caused activation of NF-kappaB, and inhibition of expression of TG2 by small interfering RNA abolished the activation of NF-kappaB. Our results further indicated that constitutive NF-kappaB reporter activity in pancreatic cancer cells is not affected by dominant-negative I kappaB alpha. Additionally, coimmunoprecipitation and confocal microscopy showed that I kappaB alpha is physically associated with TG2. Lastly, immunohistochemical analysis of pancreatic ductal carcinoma samples obtained from 61 patients further supported a strong correlation between TG2 expression and NF-kappaB activation/overexpression (P = 0.0098, Fisher's exact test). We conclude that TG2 induces constitutive activation of NF-kappaB in tumor cells via a novel pathway that is most likely independent of I kappaB alpha kinase. Therefore, TG2 may be an attractive alternate target for inhibiting constitutive NF-kappaB activation and rendering cancer cells sensitive to anticancer therapies.

Tissue transglutaminase in tumour progression: friend or foe?

Basic biological processes in which tissue transglutaminase (TG2, tTG) is thought to be important including apoptosis, cell adhesion and migration, ECM homeostasis and angiogenesis are key stages in the multistage tumour progression cascade. Studies undertaken with primary tumours and experimental models suggest that TG2 expression and activity in the tumour body and surrounding matrix generally decreases with tumour progression, favouring matrix destabilisation, but supporting angiogenesis and tumour invasion. In contrast, in the secondary metastatic tumour TG2 is often highly expressed whereby its potential roles in cell survival both at the intra- and extracellular level become important. In the following review the underlying molecular basis for the selection of these different phenotypes in tumour types and the anomaly for the requirement of TG2 is discussed in relation to the complex events of tumour progression.

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.

Involvement of retinoic acid-induced transglutaminase activity in zonal differences of hepatocyte proliferation after partial hepatectomy

BACKGROUND

The authors have recently demonstrated that there is inverse correlation between transglutaminase (TGase) activity and DNA synthesis in periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) at 1 day after partial hepatectomy in rats. In order to ensure the involvement of TGase in the differential growth capacities between periportal and perivenous regions of regenerating liver, the aim of this study was to investigate the effect of retinoic acid, an inducer of TGase expression, on zonal differences of hepatocyte proliferation between PPH and PVH isolated from regenerating rat liver.

METHODS

Regenerating liver was prepared by 70% partial hepatectomy. PPH and PVH subpopulations were isolated by the digitonin/collagenase perfusion technique. Cell cycle was evaluated for incorporation of BrdU into hepatocytes and detected by flow cytometric analysis. TGase activity was determined by incorporation of 14C-putrescine into dimethylcasein.

RESULTS

When retinoic acid was injected immediately after partial hepatectomy, TGase activity greatly increased in both PPH and PVH at 1 day after partial hepatectomy, and activity was higher in PPH than in PVH. DNA synthesis in both subpopulations did not increase 1 day after partial hepatectomy, with peaks of DNA synthesis shifting to 2 days, and synthesis was higher in PVH than in PPH.

CONCLUSION

These results suggest that TGase might be involved in differential growth capacities between periportal and perivenous regions of regenerating rat liver after partial hepatectomy.

Complex formation between tissue transglutaminase II (tTG) and vascular endothelial growth factor receptor 2 (VEGFR-2): Proposed mechanism for modulation of endothelial cell response to VEGF

We have recently demonstrated that thrombin-activated FXIII (FXIIIA-subunit), a plasma transglutaminase, activates VEGFR-2 by crosslinking it with the alpha(v)beta(3) integrin on the surface of endothelial cells (EC), thereby stimulating angiogenesis. Tissue transglutaminase (tTG), which is functionally and structurally related to FXIIIA, is expressed by numerous cell types, among them EC. However, its role in EC function has not been fully characterized. In the present study, we investigated the potential involvement of tTG in angiogenesis. Using co-immunoprecipitation and immunofluorescent staining experiments, we observed that tTG forms a complex with VEGFR-2 on the cell surface and within the cytoplasm of EC. Stimulation of EC with VEGF resulted in translocation of the tTG-VEGFR-2 complex from the cytoplasm to the nucleus. In VEGF-treated cells, tTG-VEGFR-2 interaction resulted in incorporation of VEGFR-2 into high molecular weight crosslinked complex (es), as revealed by an antibody against gamma-glutamyl-epsilon-lysine isopeptide bond. tTG -VEGFR-2 association was inhibited by a specific VEGFR-2 protein tyrosine kinase inhibitor (PTKI ), as well as by cystamine, inhibitor of the transglutaminase activity of tTG, but not by bacitracin which inhibits the protein-disulfide isomerase (PDI) activity of tTG. Furthermore, cystamine completely abolished the VEGF-induced nuclear translocation of the tTG-VEGFR-2 complex. Blockade of the crosslinking activity of tTG by cystamine enhanced VEGF-induced migration of EC in Boyden chamber by 31% (P < 0.02), and prolonged VEGF-induced signaling response, as demonstrated by sustained activation of the MAP kinase ERK. Taken together, our findings suggest that endothelial cell tTG might be involved in modulation of the cellular response to VEGF by forming an intracellular complex with VEGFR-2, and mediating its translocation into the nucleus upon VEGF stimulation.

An autocrine loop involving ret and glial cell-derived neurotrophic factor mediates retinoic acid-induced neuroblastoma cell differentiation

In several neuroblastoma cell lines, retinoic acid (RA)-induced differentiation is coupled to increased expression of functional neurotrophic factor receptors, including Trk family receptors and the glial cell-derived neurotrophic factor receptor, Ret. In several cases, increased expression is dependent on signaling through TrkB. Unlike TrkA and TrkB, Ret has never been implicated as a prognostic marker for neuroblastomas. SK-N-BE(2) cells do not express any of Trk family receptors; therefore, they are a choice system to study the specific role of Ret in RA-induced differentiation. Using a 2'-fluoro-RNA aptamer and a truncated Ret protein as specific inhibitors of Ret, we show that RA-induced differentiation is mediated by a positive autocrine loop that sustains Ret downstream signaling and depends on glial cell-derived neurotrophic factor expression and release. This report shows that in SK-N-BE(2) cells, stimulation of Ret is a major upstream mechanism needed to mediate RA-induced differentiation. These results provide important insights on the molecular mechanism of RA action, which might be relevant for the development of biologically based therapeutic strategies.

Protective effect against 17beta-estradiol on neuronal apoptosis in hippocampus tissue following transient ischemia/recirculation in mongolian gerbils via down-regulation of tissue transglutaminase activity

We analyzed the protective effect of 17beta-estradiol (17beta-ED) injection against delayed neuronal death in the hippocampus tissue of the brain in Mongolian gerbils after transient ischemia/recirculation treatment, especially in relation with bcl-2 gene expression and enzymatic activity changes of caspase-3 and tissue transglutaminase (tTGase). Daily intraperitoneal injection of 17beta-ED to the animal after the ischemia stimulated the expression of an apoptosis suppressor gene, bcl-2, in the hippocampal tissue for a week. The gradually increasing apoptotic enzyme activity of caspase-3 and increased number of TUNEL positive fragmented neuronal nuclei caused by ischemic attack in the gerbil brain were clearly suppressed by 17beta-ED administration. The reduced activity and enzyme protein of tTGase, a neurodegenerative marker of apoptosis in the hippocampus after ischemia, were also restored to nearly normal levels by 17beta-ED injection. These results suggest that daily 17beta-ED administration to the gerbil after transient ischemic insult with progressing neuronal deteriorative changes in hippocampus tissue can effectively prevent apoptotic changes through a molecular cascade involving gene expression regulation.

Coupling mitochondrial respiratory chain to cell death: an essential role of mitochondrial complex I in the interferon-β and retinoic acid-induced cancer cell death

Combination of retinoic acids (RAs) and interferons (IFNs) has synergistic apoptotic effects and is used in cancer treatment. However, the underlying mechanisms remain unknown. Here, we demonstrate that mitochondrial respiratory chain (MRC) plays an essential role in the IFN-beta/RA-induced cancer cell death. We found that IFN-beta/RA upregulates the expression of MRC complex subunits. Mitochondrial-nuclear translocation of these subunits was not observed, but overproduction of reactive oxygen species (ROS), which causes loss of mitochondrial function, was detected upon IFN-beta/RA treatment. Knockdown of GRIM-19 (gene associated with retinoid-interferon-induced mortality-19) and NDUFS3 (NADH dehydrogenase (ubiquinone) Fe-S protein 3), two subunits of MRC complex I, by siRNA in two cancer cell lines conferred resistance to IFN-beta/RA-induced apoptosis and reduced ROS production. In parallel, expression of late genes induced by IFN-beta/RA that are directly involved in growth inhibition and cell death was also repressed in the knockdown cells. Our data suggest that the MRC regulates IFN-beta/RA-induced cell death by modulating ROS production and late gene expression.