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

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

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

TSPO acts as an immune resistance gene involved in the T cell mediated immune control of glioblastoma

Glioblastoma (GB) IDH-wildtype is the most malignant primary brain tumor. It is particularly resistant to current immunotherapies. Translocator protein 18 kDa (TSPO) is upregulated in GB and correlates with malignancy and poor prognosis, but also with increased immune infiltration. Here, we studied the role of TSPO in the regulation of immune resistance of human GB cells. The role of TSPO in tumor immune resistance was experimentally determined in primary brain tumor initiating cells (BTICs) and cell lines through genetic manipulation of TSPO expression and subsequent cocultures with antigen specific cytotoxic T cells and autologous tumor-infiltrating T cells. Death inducing intrinsic and extrinsic apoptotic pathways affected by TSPO were investigated. TSPO-regulated genes mediating apoptosis resistance in BTICs were identified through gene expression analysis and subsequent functional analyses. TSPO transcription in primary GB cells correlated with CD8⁺ T cell infiltration, cytotoxic activity of T cell infiltrate, expression of TNFR and IFNGR and with the activity of their downstream signalling pathways, as well as with the expression of TRAIL receptors. Coculture of BTICs with tumor reactive cytotoxic T cells or with T cell-derived factors induced TSPO up-regulation through T cell derived TNFα and IFNγ. Silencing of TSPO sensitized BTICs against T cell-mediated cytotoxicity. TSPO selectively protected BTICs against TRAIL-induced apoptosis by regulating apoptosis pathways. TSPO also regulated the expression of multiple genes associated with resistance against apoptosis. We conclude that TSPO expression in GB is induced through T cell-derived cytokines TNFα and IFNγ and that TSPO expression protects GB cells against cytotoxic T cell attack through TRAIL. Our data thereby provide an indication that therapeutic targeting of TSPO may be a suitable approach to sensitize GB to immune cell-mediated cytotoxicity by circumventing tumor intrinsic TRAIL resistance.

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.

Toward Intracellular Bioconjugation Using Transition-Metal-Free Techniques

Bioconjugation is the chemical strategy of covalent modification of biomolecules, using either an external reagent or other biomolecules. Since its inception in the twentieth century, the technique has grown by leaps and bounds, and has a variety of applications in chemical biology. However, it is yet to reach its full potential in the study of biochemical processes in live cells, mainly because the bioconjugation strategies conflict with cellular processes. This has mostly been overcome by using transition metal catalysts, but the presence of metal centers limit them to in vitro use, or to the cell surface. These hurdles can potentially be circumvented by using metal-free strategies. However, the very modifications that are necessary to make such metal-free reactions proceed effectively may impact their biocompatibility. This is because biological processes are easily perturbed and greatly depend on the prevailing inter- and intracellular environment. With this taken into consideration, this review analyzes the applicability of the transition-metal-free strategies reported in this decade to the study of biochemical processes in vivo.

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.

Prostate transglutaminase (TGase-4, TGaseP) enhances the adhesion of prostate cancer cells to extracellular matrix, the potential role of TGase-core domain

Background

Transglutaminase-4 (TGase-4), also known as the Prostate Transglutaminase, is an enzyme found to be expressed predominately in the prostate gland. The protein has been recently reported to influence the migration and invasiveness of prostate cancer cells. The present study aimed to investigate the influence of TGase-4 on cell-matrix adhesion and search for the candidate active domain[s] within the protein.

Methods

Human prostate cancer cell lines and prostate tissues were used. Plasmids that encoded different domains and full length of TGase-4 were constructed and used to generate sublines that expressed different domains. The impact of TGase-4 on in vitro cell-matrix adhesion, cell migration, growth and in vivo growth were investigated. Interactions between TGase-4 and focal adhesion complex proteins were investigated using immunoprecipitation, immunofluorescence and phosphospecific antibodies.

Results

TGase-4 markedly increased cell-matrix adhesion and cellular migration, and resulted in a rapid growth of prostate tumours in vivo. This effect resided in the Core-domain of the TGase-4 protein. TGase-4 was found to co-precipitate and co-localise with focal adhesion kinase (FAK) and paxillin, in cells, human prostate tissues and tumour xenografts. FAK small inhibitor was able to block the action mediated by TGase-4 and TGase-4 core domain.

Conclusion

TGase-4 is an important regulator of cell-matrix adhesion of prostate cancer cells. This effect is predominately mediated by its core domain and requires the participation of focal adhesion complex proteins.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Implications of increased tissue transglutaminase (TG2) expression in drug-resistant breast cancer (MCF-7) cells

The development of resistance to chemotherapeutic drugs is a major obstacle to the successful treatment of breast cancer. Ways to block or overcome this resistance are objects of intense research. We have previously shown that cancer cells selected for resistance against chemotherapeutic drugs or isolated from metastatic tumor sites have high levels of a calcium-dependent protein crosslinking enzyme, tissue transglutaminase (TG2) but no direct link between TG2 and resistance was established. As TG2 can associate with the beta members of the integrin family of proteins, we hypothesized that TG2 promotes cell survival signaling pathways by activating integrins on the surface of these cells. To test this hypothesis, we studied the expression of TG2 and its interaction with various integrins in drug-resistant MCF-7 breast cancer cells. TG2 closely associated with beta1 and beta5 integrins on the surface of drug-resistant MCF-7 (MCF-7/Dox and MCF-7/RT) cells. The incubation of TG2-expressing drug-resistant MCF-7 cells on fibronectin (Fn)-coated surfaces strongly activated focal adhesion kinase, an event that leads to the activation of several downstream signaling pathways and, in turn, can confer apoptosis-resistant phenotype to cancer cells. The role of TG2 in Fn-mediated cell attachment, cell growth, and cell survival functions was further analysed by small interfering RNA (siRNA) approach. Inhibition of TG2 by siRNA-inhibited Fn-mediated cell attachment and cell survival functions in drug-resistant MCF-7 cells. We conclude that the expression of TG2 in breast cancer cells contributes to the development of the drug-resistance phenotype by promoting interaction between integrins and Fn.

Transglutaminase differentially regulates growth signalling in rat perivenous and periportal hepatocytes

The influence of transglutaminase 2 (TG2) activity on the proliferative effect of epidermal growth factor (EGF) and on EGF receptor affinity in periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) has been investigated using a primary culture system. PPH and PVH subpopulations have been isolated using the digitonin/collagenase perfusion technique. DNA synthesis was assessed by [3H] thymidine incorporation into hepatocytes. The assay for binding of [125I] EGF to cultured hepatocytes was analysed by Scatchard plot analysis. Pretreatment with the TG2 inhibitor monodansylcadaverine (MDC) greatly increased EGF-induced DNA synthesis in both PPH and PVH. Furthermore, [125I] EGF binding studies in PVH treated with MDC indicated that high-affinity EGF receptor expression was markedly up-regulated, whereas in PPH, there was no significant effect. Treatment with retinoic acid (RA), an inducer of TG2 expression, significantly decreased EGF-induced DNA synthesis in both PPH and PVH. Binding studies in the presence of RA revealed that the high-affinity EGF receptor was down-regulated and completely absent in both PPH and PVH. These results suggest that TG2 was involved in the differential growth capacities of PPH and PVH through down-regulation of high-affinity EGF receptors.

Transglutaminase 5 is regulated by guanine-adenine nucleotides

Transglutaminases (TGases) are Ca2+-dependent enzymes capable of catalysing transamidation of glutamine residues to form intermolecular isopeptide bonds. Nine distinct TGases have been described in mammals, and two of them (types 2 and 3) are regulated by GTP/ATP. TGase2 hydrolyses GTP and is therefore a bifunctional enzyme. In the present study, we report that TGase5 is also regulated by nucleotides. We have identified the putative TGase5 GTP-binding pocket by comparative amino acid sequence alignment and homology-derived three-dimensional modelling. GTP and ATP inhibit TGase5 cross-linking activity in vitro, and Ca2+ is capable of completely reversing this inhibition. In addition, TGase5 mRNA is not restricted to epidermal tissue, but is also present in different adult and foetal tissues, suggesting a role for TGase5 outside the epidermis. These results reveal the reciprocal actions of Ca2+ and nucleotides with respect to TGase5 activity. Taken together, these results indicate that TGases are a complex family of enzymes regulated by calcium, with at least three of them, namely TGase2, TGase3 and TGase5, also being regulated by ATP and GTP.

A novel function of tissue-type transglutaminase: protein disulphide isomerase

We have found that tissue-type transglutaminase (tTG), also called TGc, TGase2 and Galpha(h), has the activity of protein disulphide isomerase (PDI). We have shown that tTG converts completely reduced/denatured inactive RNase A molecule to the native active enzyme. The PDI activity of tTG was strongly inhibited by bacitracin, which is a frequently used inhibitor of conventional PDI activity. It was substantially inhibited by the simultaneous presence of other potential substrate proteins such as completely reduced BSA, but not by native BSA. This activity was especially high in the presence of GSSG, but not GSH. The addition of GSH to the reaction mixture in the presence of GSSG at a fixed concentration up to at least 200-fold excess did not very substantially inhibit the PDI activity. It is possible that tTG can exert PDI activity in a fairly reducing environment like cytosol, where most of tTG is found. It is quite obvious from the following observations that PDI activity of tTG is catalysed by a domain different from that used for the transglutaminase reaction. Although the alkylation of Cys residues in tTG completely abolished the transglutaminase activity, as was expected, it did not affect the PDI activity at all. This PDI activity did not require the presence of Ca(2+). It was not inhibited by nucleotides including GTP at all, unlike the other activity of tTG.

Transglutaminase 2-/- mice reveal a phagocytosis-associated crosstalk between macrophages and apoptotic cells

Tissue transglutaminase (TGase2) is a protein-crosslinking enzyme known to be associated with the in vivo apoptosis program. Here we report that apoptosis could be induced in TGase2-/- mice; however, the clearance of apoptotic cells was defective during the involution of thymus elicited by dexamethasone, anti-CD3 antibody, or gamma-irradiation, and in the liver after induced hyperplasia. The lack of TGase2 prevented the production of active transforming growth factor-beta1 in macrophages exposed to apoptotic cells, which is required for the up-regulation of TGase2 in the thymus in vivo, for accelerating deletion of CD4+CD8+ cells and for efficient phagocytosis of apoptotic bodies. The deficiency is associated with the development of splenomegaly, autoantibodies, and immune complex glomerulonephritis in TGase2-/- mice. These findings have broad implications not only for diseases linked to inflammation and autoimmunity but also for understanding the interrelationship between the apoptosis and phagocytosis process.

Transglutaminases: nature's biological glues

Transglutaminases (Tgases) are a widely distributed group of enzymes that catalyse the post-translational modification of proteins by the formation of isopeptide bonds. This occurs either through protein cross-linking via epsilon-(gamma-glutamyl)lysine bonds or through incorporation of primary amines at selected peptide-bound glutamine residues. The cross-linked products, often of high molecular mass, are highly resistant to mechanical challenge and proteolytic degradation, and their accumulation is found in a number of tissues and processes where such properties are important, including skin, hair, blood clotting and wound healing. However, deregulation of enzyme activity generally associated with major disruptions in cellular homoeostatic mechanisms has resulted in these enzymes contributing to a number of human diseases, including chronic neurodegeneration, neoplastic diseases, autoimmune diseases, diseases involving progressive tissue fibrosis and diseases related to the epidermis of the skin. In the present review we detail the structural and regulatory features important in mammalian Tgases, with particular focus on the ubiquitous type 2 tissue enzyme. Physiological roles and substrates are discussed with a view to increasing and understanding the pathogenesis of the diseases associated with transglutaminases. Moreover the ability of these enzymes to modify proteins and act as biological glues has not gone unnoticed by the commercial sector. As a consequence, we have included some of the present and future biotechnological applications of this increasingly important group of enzymes.

Ascorbate up-regulates MLH1 (Mut L homologue-1) and p73: implications for the cellular response to DNA damage

We have found previously that ascorbic acid (vitamin C), as well as acting as a radical scavenger, may modulate the expression of several genes [i.e. fra-1, glutathione S-transferase Pi (GSTpi) and Mut L homologue-1 (MLH1)] in human keratinocytes. In the present paper, we demonstrate that MLH1, as well as its downstream target p73, can be positively modulated by this antioxidant vitamin, indeed, up-regulation of the two mRNAs was observed after just 2 h, and increased further up to 16 h of treatment. Modulation of MLH1 and p73 gene expression improves cellular susceptibility to apoptosis triggered by the DNA-damaging agent cisplatin. Indeed, in ascorbate-supplemented cells, increased cisplatin-induced apoptosis was seen, involving activation of the MLH1/c-Abl/p73 signalling cascade. Our results were further confirmed by studies performed on genetically defined mutants, i.e. mouse embryo fibroblasts derived from knock-out animals for c-Abl or p53, as well as human colon carcinoma cell lines deficient in MLH1. The increased sensitivity to cisplatin observed in ascorbate-loaded cells appeared to be dependent exclusively on MLH1 and c-Abl expression, and independent of p53. These data suggest a potential mechanism accounting for the anti-carcinogenic and anti-cancer activities of vitamin C.

Gene disruption of tissue transglutaminase

Transglutaminase 2 (TGase 2), or tissue transglutaminase, catalyzes either epsilon-(gamma-glutamyl)lysine or N(1), N(8)-(gamma-glutamyl)spermidine isopeptide bonds. TGase 2 expression has been associated with apoptosis, and it has been proposed that its activation should lead to the irreversible assembly of a cross-linked protein scaffold in dead cells. Thus, TGase 2-catalyzed protein polymerization contributes to the ultrastructural changes typical of dying apoptotic cells; it stabilizes the integrity of the apoptotic cells, preventing the release of harmful intracellular components into the extracellular space and, consequently, inflammation and scar formation. In order to perform a targeted disruption of the enzyme, we prepared a construct deleting part of exons 5 and 6, containing the active site, and intron 5. Complete absence of TGase 2 was demonstrated by reverse transcription-PCR and Western blot analysis. TGase activity measured on liver and thymus extracts showed, however, a minimal residual activity in TGase 2(-/-) mice. PCR analysis of mRNA extracted from the same tissues demonstrated that at least TGase 1 (normally present in the skin) is also expressed in these tissues and contributes to this residual activity. TGase 2(-/-) mice showed no major developmental abnormalities, and histological examination of the major organs appeared normal. Induction of apoptosis ex vivo in TGase 2(-/-) thymocytes (by CD95, dexamethasone, etoposide, and H(2)O(2)) and in vitro on TGase 2(-/-) mouse embryonal fibroblasts (by retinoids, UV, and H(2)O(2)) showed no significant differences. A reduction in cross-linked apoptotic bodies with a modestly increased release of lactate dehydrogenase has been detected in some cases. Together our results show that TGase 2 is not a crucial component of the main pathway of the apoptotic program. It is possible that the residual enzymatic activity, due to TGase 1 or redundancy of other still-unidentified TGases, can compensate for the lack of TGase 2.

Bcl-2, tissue transglutaminase and p53 protein expression in the apoptotic cascade in ribs of premature infants

Apoptotic cells of the human growth plate have not previously been demonstrated in situ. We have investigated the distribution of apoptotic cells in costosternal growth plates and bone of premature infants aged 4-11 d with a gestational age of approximately 26 wk. In addition, we investigated the immunolocalisation of apoptosis-related proteins within the growth plates and associated bone. A proportion of late hypertrophic chondrocytes and osteocytes within newly formed primary spongiosa showed evidence of highly fragmented DNA. The incidence of osteocyte apoptosis decreased as the distance from the chondroosseous junction increased. Tissue transglutaminase (tTG) expression was associated with apoptosis of osteocytes and hypertrophic chondrocytes. In contrast the presence of tTG was demonstrated in osteoblasts and bone lining cells but it did not colocalise with evidence of apoptosis. The anti-apoptotic gene product Bcl-2 was absent from the growth plate but was present in osteocytes. Visual assessment indicated a greater occurrence of the protein in cells occupying regions of low apoptosis. P53 was not demonstrated in the growth plate or bone. These findings would indicate that human growth plate chondrocytes appear to show little provision for ensuring cell longevity. In contrast osteocyte apoptosis appears negatively correlated with the skeletal distribution of Bcl-2 protein in the human infant, implying a potential selective vulnerability in individual cells. Lack of Bcl-2 and the high incidence of osteocyte apoptosis in the more rapidly remodelling bone of the human infant suggest a potential role of osteocyte apoptosis in the remodelling process.

Apoptosis and tumorigenesis in human cholangiocarcinoma cells. Involvement of Fas/APO-1 (CD95) and calmodulin

We have previously demonstrated that tamoxifen inhibits the growth of human cholangiocarcinoma cells in culture and inhibits tumor growth when cells are injected into nude mice. However, the mechanism of action of tamoxifen remains unknown. Here we demonstrate that tamoxifen and trifluoperazine, both potent calmodulin antagonists, induce apoptosis in vitro, probably acting via the Fas system, in human cholangiocarcinoma cells. Human cholangiocarcinoma cell lines heterogeneously express Fas antigen on their surface. Fas-negative and Fas-positive surface-expressing cells were isolated, cloned, and cultured. Fas antibody, tamoxifen, and trifluoperazine induced dose-dependent apoptosis only in Fas-positive cells; Fas-negative cells were unaffected. Furthermore, apoptosis induced by tamoxifen in Fas-positive cells was blocked by an inhibitory Fas antibody. Tamoxifen was not acting through an anti-estrogenic mechanism, because neither Fas-negative nor Fas-positive cells expressed estrogen receptors and the pure anti-estrogen compound, ICI 182780, did not induce apoptosis in either cell line. Fas-negative cells, but not Fas-positive cells, were able to produce tumors when subcutaneously injected into nude mice. These findings suggest Fas may be a candidate oncogene involved in the pathogenesis of cholangiocarcinoma. Furthermore, the similarity between the pro-apoptotic effects of tamoxifen and trifluoperazine support an underlying molecular mechanism for Fas-mediated apoptosis that involves calmodulin.

Transglutaminase-catalyzed protein cross-linking in the molecular program of apoptosis and its relationship to neuronal processes

1. One type of transglutaminase is usually accumulated in various forms of naturally occurring cell death and apoptosis. The accumulated enzyme is activated during the death process, leading to the formation of cross-linked protein structures. Degradation of the cross-linked apoptotic bodies results in the elevation of the epsilon (gamma-glutamyl)lysine isodipeptide concentration in body fluids, which may provide a diagnostic tool to monitor the apoptosis rate in various tissues under normal and pathologic conditions. 2. Extensive protein cross-linking may be directly related to the act of killing in some cells. In others, the effect of protein cross-linking is palliative, preventing leakage of macromolecules and enhancing phagocytosis of the dead cells. 3. Tissue transglutaminase has been implicated in some physiologic functions of the nervous system. 4. The molecular machinery of apoptosis is present and easily evoked in neuronal cells. 5. Effector elements of the apoptosis process have been associated with the pathogenesis of neurologic disorders. Tissue transglutaminase, representing one of the effector elements of apoptosis, may be induced and activated in cells following ischemia. It may also participate in the formation of abnormal cell inclusions and A beta deposits in amyloid plaques.

Tissue transglutaminase-dependent posttranslational modification of the retinoblastoma gene product in promonocytic cells undergoing apoptosis

The retinoblastoma gene product (pRB) plays an important role in controlling both cell release from the G1 phase and apoptosis. We show here that in the early phases of apoptosis, pRB is posttranslationally modified by a tissue transglutaminase (tTG)-catalyzed reaction. In fact, by employing a novel haptenized lysis synthetic substrate which allows the isolation of glutaminyl-tTG substrates in vivo, we identified pRB as a potential tTG substrate in U937 cells undergoing apoptosis. In keeping with this finding, we showed that apoptosis of U937 cells is characterized by the rapid disappearance of the 105,000- to 110,000-molecular-weight pRB forms concomitantly with the appearance of a smear of immunoreactive products with a molecular weight of greater than 250,000. The shift in pRB molecular weight was reproduced by adding exogenous purified tTG to extracts obtained from viable U937 cells and was prevented by dansylcadaverine, a potent enzyme inhibitor. The effect of the pRB posttranslational modification during apoptosis was investigated by determining the E2F-1 levels and by isolating and characterizing pRB-null clones from U937 cells. Notably, the lack of pRB in these U937-derived clones renders these p53-null cells highly resistant to apoptosis induced by serum withdrawal, calphostin C, and ceramide. Taken together, these data suggest that tTG, acting on the pRB protein, might play an important role in the cell progression through the death program.

Regulation of the expression of the tissue transglutaminase gene by DNA methylation

We have investigated the role of DNA methylation in the regulation of the expression of the human tissue transglutaminase gene. Studies on the methylation of the transglutaminase promoter in normal and neoplastic human cells demonstrated that the promoter is methylated in vivo and hypomethylation of the promoter is correlated with constitutive gene expression. Demethylation of the promoter in vivo by treatment of the cells with 5-azacytidine increased transglutaminase expression and hypermethylation of the promoter in vitro suppressed its activity. These studies suggest that alternations in DNA methylation may be one of the mechanisms regulating the tissue-specific expression of the tissue transglutaminase gene.

Expression and retinoid modulation of N-arginine dibasic convertase and an aminopeptidase-B in human neuroblastoma cell lines

Under retinoic acid exposure, the three SK-N-BE(2)-derived human neuroblastoma cell lines, BE(2)-NA, BE(2)-SA and BE(2)-M17 undergo mainly differentiation, apoptosis or continue to proliferate, respectively. We have used this model system to study the modulation of the transcriptional expression of putative processing enzymes, two novel metallopeptidases; i.e. N-arginine dibasic convertase (NRD convertase; EC 3.4,24,61) and an aminopeptidase-B after exposure of the cells either to retinoic acid or to synthetic retinoid analogs. The data indicate that the two respective enzymes are differently modulated in the various cell lines. Whereas aminopeptidase-B expression is enhanced in most cases, NRD convertase appears to undergo opposite regulation in proliferating versus differentiating neuroblastoma cells. It is concluded that both genes might contain retinoic acid regulatory elements (RARE) in their promoters.

Retinoids and the control of growth/death decisions in human neuroblastoma cell lines

Cell proliferation, the balance between mitosis and apoptosis is the result of the continuous integration of a number of different signal transduction pathways stimulated in a cell at any given point in its life. Neuroblastoma cells regulate the switch between mitosis and death, according both to intrinsic factors and extrinsic factors, such as growth factor withdrawal and action of the vitamin A derivative, retinoic acid. In this review, we describe the balance of some factors regulating growth and death of human neuroblastoma cells in vitro. These dynamic studies are necessarily-performed on cell lines, which offer controlled conditions enabling the disection of the complex stimuli mediating survival and growth (IGF, trk, BDNF) and death (transglutaminase, free radicals, Bcl-2). Although the conclusions drawn may therefore not be directly applicable to tumour cells in vivo, the results herein discussed are of sufficient significance to warrant in vivo relevance.

Transglutaminase induction by various cell death and apoptosis pathways

Clarification of the molecular details of forms of natural cell death, including apoptosis, has become one of the most challenging issues of contemporary biomedical sciences. One of the effector elements of various cell death pathways is the covalent cross-linking of cellular proteins by transglutaminases. This review will discuss the accumulating data related to the induction and regulation of these enzymes, particularly of tissue type transglutaminase, in the molecular program of cell death. A wide range of signalling pathways can lead to the parallel induction of apoptosis and transglutaminase, providing a handle for better understanding the exact molecular interactions responsible for the mechanism of regulated cell death.

Induction of "tissue" transglutaminase in HIV pathogenesis: evidence for high rate of apoptosis of CD4+ T lymphocytes and accessory cells in lymphoid tissues

Peripheral blood mononuclear cells and lymphoid tissues from HIV-infected individuals display high levels of "tissue" transglutaminase (tTG) with respect to seronegative persons. In asymptomatic individuals, > 80% of the circulating CD4+ T cells synthesize tTG protein and the number of these cells matches the level of apoptosis detected in the peripheral blood mononuclear cells from the same patients. In HIV-infected lymph nodes tTG protein is localized in large number of cells (macrophages, follicular dendritic cells, and endothelial cells), showing distinctive morphological and biochemical features of apoptosis as well as in lymphocytes and syncytia. These findings demonstrate that during the course of HIV infection, high levels of apoptosis also occur in the accessory cells of lymphoid organs. The increased concentration of epsilon(gamma-glutamyl)lysine isodipeptide, the degradation product of tTG cross-linked proteins, observed in the blood of HIV-infected individuals demonstrates that the enzyme accumulated in the dying cells actively cross-links intracellular proteins. The enhanced levels of epsilon(gamma-glutamyl)lysine in the blood parallels the progression of HIV disease, suggesting that the isodipeptide determination might be a useful method to monitor the in vivo rate of apoptosis.

Cytoplasmic sequestration of wild-type p53 protein impairs the G1 checkpoint after DNA damage

Wild-type p53 protein is abnormally sequestered in the cytoplasm of a subset of primary human tumors including neuroblastomas (NB) (U. M. Moll, M. LaQuaglia, J. Benard, and G. Riou, Proc. Natl. Acad. Sci. USA 92:4407-4411, 1995; U. M. Moll, G. Riou, and A. J. Levine, Proc. Natl. Acad. Sci.USA 89:7262-7266, 1992). This may represent a nonmutational mechanism for abrogating p53 tumor suppressor function. To test this hypothesis, we established the first available in vitro model that accurately reflects the wild-type p53 sequestration found in NB tumors. We characterized a series of human NB cell lines that overexpress wild-type p53 and show that p53 is preferentially localized to discrete cytoplasmic structures, with no detectable nuclear p53. These cell lines, when challenged with a variety of DNA strand-breaking agents, all exhibit impaired p53-mediated G1 arrest. Induction analysis of p53 and p53-responsive genes show that this impairment is due to suppression of nuclear p53 accumulation. Thus, this naturally occurring translocation defect compromises the suppressor function of p53 and likely plays a role in the tumorigenesis of these tumors previously thought to be unaffected by p53 alterations.

Apoptosis induced in Jurkat cells by several agents is preceded by intracellular acidification

We have previously shown that in neutrophils deprived of granulocyte colony-stimulating factor, apoptosis is preceded by acidification and that the protection against apoptosis conferred on neutrophils by granulocyte colony-stimulating factor is dependent upon delay of this acidification. To test the hypothesis that acidification could be a general feature of apoptosis, we examined intracellular pH changes in another cell line. Jurkat cells, a T-lymphoblastoid line, were induced to undergo apoptosis with anti-Fas IgM, cycloheximide, or exposure to short-wavelength UV light. We found that acidification occurred in response to treatment with these agents and that acidification preceded DNA fragmentation. Jurkat cells were also found to possess an acid endonuclease that is active below pH 6.8, compatible with a possible role for this enzyme in chromatin digestion during apoptosis. Incubation of the cells with the bases imidazole or chloroquine during treatment with anti-Fas antibody or cycloheximide or after UV exposure decreased apoptosis as assessed by nuclear morphology and DNA content. The alkalinizing effect of imidazole and chloroquine was shown by the demonstration that the percentage of cells with an intracellular pH below 6.8 after treatment with anti-Fas antibody, cycloheximide, or UV was diminished in the presence of base as compared with similarly treated cells incubated in the absence of base. We conclude that acidification is an early event in programmed cell death and may be essential for genome destruction.

Transglutaminase induced by epidermal growth factor negatively regulates the growth signal in primary cultured hepatocytes

Transglutaminase (TGase) activity increased 2.5-fold at 6 h after treatment of rat hepatocytes with 17 nM epidermal growth factor (EGF). In the same manner, putrescine incorporation into the proteins of cells occurred in EGF-treated cells, but not in those pretreated with monodansylcadaverine (MDC), a TGase inhibitor, even in the presence of EGF. These results suggest that EGF-induced TGase was active and catalysed some cross-linkage reaction. Cycloheximide completely blocked the increase in TGase activity induced by EGF, suggesting that EGF stimulated de novo synthesis of TGase within 6 h. Furthermore, Northern blotting analysis indicated that EGF increased the expression of TGase mRNA. Pretreatment of cells with MDC additionally increased EGF-induced DNA-synthesis and the ratio of cells in S-phase. TGase antisense oligonucleotide inhibited de novo synthesis of TGase, resulting in increases in the ratios of S- and G2/M-phase cells in the presence of EGF. This effect was the result of inhibition of EGF-induced down-regulation of high-affinity receptor expression. These results suggest that the EGF-induced increase in TGase activity is a negative regulator of a growth signal in rat hepatocytes.

Association of a transglutaminase-related antigen with intermediate filaments

A mouse monoclonal antibody, G92.1.2, raised against guinea pig liver transglutaminase (TGase) recognizes an antigen present in primary mouse dermal fibroblasts. A filamentous pattern, bearing remarkable similarity to the vimentin intermediate filament (IF) network, is seen when these cells are fixed and processed for indirect immunofluorescence with the antibody. Double-label immunofluorescence reveals that the antigen reacting with the antibody colocalizes precisely with vimentin IF and that this colocalization is retained after the treatment of fibroblasts with colchicine, which induces a redistribution of the majority of IFs into perinuclear aggregates. These morphological observations are further supported by the finding that the protein reacting with G92.1.2 is retained in IF-enriched cytoskeletal preparations made by using nonionic detergent-containing high ionic strength solutions. Western blots of the IF fraction show that G92.1.2 recognizes a major band of approximately 280 kDa and does not cross react with vimentin. Furthermore, when the antibody is microinjected into live dermal fibroblasts, it causes a collapse of the vimentin IF network in the majority of injected cells. The results suggest that a form of TGase, or a TGase-related antigen, is closely associated with the vimentin IF network of primary cultures of mouse dermal fibroblasts.