Novel chemo-sensitizing agent, ERW1227B, impairs cellular motility and enhances cell death in glioblastomas

Inhibiting Transglutaminase 2 Mediates Kidney Fibrosis via Anti-Apoptosis

Transglutaminase 2 (TG2) is a calcium-dependent transamidating acyltransferase enzyme of the protein-glutamine γ-glutamyltransferase family implicated in kidney injury. In this study, we identified associations between TG2 and chronic kidney disease (CKD) identified by visualizing TG2 in kidney biopsy samples derived from CKD patients using immunohistochemistry and measuring the plasma TG2 concentrations. Our study revealed a connection between TG2 and the pathological markers of kidney disease. We showed high plasma TG2 levels in samples from patients with advanced CKD. In addition, we observed an increase in TG2 expression in tissues concomitant with advanced CKD in human samples. Moreover, we investigated the effect of TG2 inhibition on kidney injury using cystamine, a well-known competitive inhibitor of TG2. TG2 inhibition reduced apoptosis and accumulation of extracellular molecules (ECM) such as fibronectin and pro-inflammatory cytokine IL-8. Collectively, the increased expression of TG2 that was observed in advanced CKD, hence inhibiting TG2 activity, could protect kidney cells from ECM molecule accumulation, apoptosis, and inflammatory responses, thereby preventing kidney fibrosis.

Transglutaminase 2 takes center stage as a cancer cell survival factor and therapy target

Transglutaminase 2 (TG2) has emerged as a key cancer cell survival factor that drives epithelial to mesenchymal transition, angiogenesis, metastasis, inflammation, drug resistance, cancer stem cell survival and stemness, and invasion and migration. TG2 can exist in a GTP-bound signaling-active conformation or in a transamidase-active conformation. The GTP bound conformation of TG2 contributes to cell survival and the transamidase conformation can contribute to cell survival or death. We present evidence suggesting that TG2 has a role in human cancer, summarize what is known about the TG2 mechanism of action in a range of cancer types, and discuss TG2 as a cancer therapy target.

Transglutaminase is a tumor cell and cancer stem cell survival factor

Recent studies indicate that cancer cells express elevated levels of type II transglutaminase (TG2), and that expression is further highly enriched in cancer stem cells derived from these cancers. Moreover, elevated TG2 expression is associated with enhanced cancer stem cell marker expression, survival signaling, proliferation, migration, invasion, integrin-mediated adhesion, epithelial-mesenchymal transition, and drug resistance. TG2 expression is also associated with formation of aggressive and metastatic tumors that are resistant to conventional therapeutic intervention. This review summarizes the role of TG2 as a cancer cell survival factor in a range of tumor types, and as a target for preventive and therapeutic intervention. The literature supports the idea that TG2, in the closed/GTP-binding/signaling conformation, drives cancer cell and cancer stem cell survival, and that TG2, in the open/crosslinking conformation, is associated with cell death.

Transglutaminase 2 expression is increased as a function of malignancy grade and negatively regulates cell growth in meningioma

Most meningiomas are benign, but some clinical-aggressive tumors exhibit brain invasion and cannot be resected without significant complications. To identify molecular markers for these clinically-aggressive meningiomas, we performed microarray analyses on 24 primary cultures from 21 meningiomas and 3 arachnoid membranes. Using this approach, increased transglutaminase 2 (TGM2) expression was observed, which was subsequently validated in an independent set of 82 meningiomas by immunohistochemistry. Importantly, the TGM2 expression level was associated with increasing WHO malignancy grade as well as meningioma recurrence. Inhibition of TGM2 function by siRNA or cystamine induced meningioma cell death, which was associated with reduced AKT phosphorylation and caspase-3 activation. Collectively, these findings suggest that TGM2 expression increases as a function of malignancy grade and tumor recurrence and that inhibition of TGM2 reduces meningioma cell growth.

Mitochondria: gatekeepers of response to chemotherapy

Mitochondria are cellular organelles that regulate commitment to and execution of apoptosis. The intrinsic apoptotic pathway culminates in the permeabilization of the mitochondrial outer membrane and dismantling of the cell. Apoptosis of cancer cells is a favorable outcome when administering chemotherapeutic treatment, yet the basis for why some cancers are sensitive to chemotherapy whereas others are not has historically been poorly understood. In this review, we present recent work that has demonstrated the importance of mitochondrial apoptotic priming, or how close a cell is to the threshold of apoptosis, in determining whether a cell will undergo apoptosis after chemotherapy treatment. Differential levels of apoptotic priming in tumors create bona fide opportunities and challenges for effective use of targeted and cytotoxic chemotherapies.

Tissue transglutaminase as a central mediator in inflammation-induced progression of breast cancer

TGM2 is a stress-responsive gene that encodes a multifunctional and structurally complex protein called tissue transglutaminase (abbreviated as TG2 or tTG). TGM2 expression is frequently upregulated during inflammation and wounding. Emerging evidence indicates that TGM2 expression is aberrantly upregulated in multiple cancer cell types, particularly those selected for resistance to chemotherapy and radiation therapy and those isolated from metastatic sites. It is becoming increasingly evident that chronic expression of TG2 in epithelial cancer cells initiates a complex series of signaling networks which contributes to the development of drug resistance and an invasive phenotype. For example, forced or basal high expression of TG2 in mammary epithelial cells is associated with activation of nuclear transcription factor-kappa B (NF-κB), Akt, focal adhesion kinase, and hypoxia-inducible factor. All of these changes are considered hallmarks of aggressive tumors. TG2 expression is able to induce the developmentally regulated program of epithelial-to-mesenchymal transition (EMT) and to confer cancer stem cell (CSC) traits in mammary epithelial cells; both EMT and CSCs have been implicated in cancer metastasis and resistance to standard therapies. Importantly, TG2 expression in tumor samples is associated with poor disease outcome, increased drug resistance, and increased incidence of metastasis. These observations imply that TG2 plays a crucial role in promoting an aggressive phenotype in mammary epithelial cells. In this review, we discuss recent evidence that TG2-regulated pathways contribute to the aggressive phenotype in breast cancer.

Tissue transglutaminase constitutively activates HIF-1α promoter and nuclear factor-κB via a non-canonical pathway

Constitutive activation of nuclear factor kappa B (NF-κB) has been linked with carcinogenesis and cancer progression, including metastasis, chemoresistance, and radiation resistance. However, the molecular mechanisms that result in constitutive activation of NF-κB are poorly understood. Here we show that chronic expression of the pro-inflammatory protein tissue transglutaminase (TG2) reprograms the transcription regulatory network in epithelial cells via constitutive activation of NF-κB. TG2-induced NF-κB binds the functional NF-κB binding site in hypoxia-inducible factor-1 (HIF-1α) promoter and results in its increased expression at transcription and protein levels even under normoxic conditions. TG2/NF-κB-induced HIF-1 was deemed essential for increased expression of some transcription repressors, like Zeb1, Zeb2, Snail, and Twist. Unlike tumor necrosis factor-alpha (TNFα), TG2 did not require IκB kinase (IKK) for NF-κB activation. Our data suggest that TG2 binds with IκBα and results in its rapid degradation via a non-proteasomal pathway. Importantly, the catalytically inactive (C277S) mutant form of TG2 was as effective as was wild-type TG2 in activating NF-κB and inducing HIF-1 expression. We also found that TG2 interacted with p65/RelA protein, both in the cytosolic and the nuclear compartment. The TG2/p65(NF-κB) complex binds to the HIF-1 promoter and induced its transcriptional regulation. Inhibition of TG2 or p65/RelA also inhibited the HIF-1α expression and attenuated Zeb1, Zeb2, and Twist expression. To our knowledge, these findings show for the first time a direct link between TG2, NF-κB, and HIF-1α, demonstrating TG2's important role in cancer progression.

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.

Tissue transglutaminase: a new target to reverse cancer drug resistance

Cancer resistance mechanisms, which result from intrinsic genetic alterations of tumor cells or acquired genetic and epigenetic changes, limit the long-lasting benefits of anti-cancer treatments. Tissue transglutaminase (TG2) has emerged as a putative gene involved in tumor cell drug resistance and evasion of apoptosis. Although some reports have indicated that TG2 can suppress tumor growth and enhance the growth inhibitory effects of anti-tumor agents, several studies have presented both pro-survival and anti-apoptotic roles for TG2 in malignant cells. Increased TG2 expression has been found in several tumors, where it was considered a potential negative prognostic marker, and it is often associated with advanced stages of disease, metastatic spread and drug resistance. TG2 mediates drug resistance through the activation of survival pathways and the inhibition of apoptosis, but also by regulating extracellular matrix (ECM) formation, the epithelial-to-mesenchymal transition (EMT) or autophagy. Because TG2 knockdown or inhibition of TG2 enzymatic activity may reverse drug resistance and sensitize cancer cells to drug-induced apoptosis, many small molecules capable of blocking TG2 have recently been developed. Additional insight into the multifunctional nature of TG2 as well as translational studies concerning the correlation between TG2 expression, function or location and cancer behavior will aid in translating these findings into new therapeutic approaches for cancer patients.

Transglutaminase 2: a molecular Swiss army knife

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

Fibronectin matrix assembly suppresses dispersal of glioblastoma cells

Glioblastoma (GBM), the most aggressive and most common form of primary brain tumor, has a median survival of 12–15 months. Surgical excision, radiation and chemotherapy are rarely curative since tumor cells broadly disperse within the brain. Preventing dispersal could be of therapeutic benefit. Previous studies have reported that increased cell-cell cohesion can markedly reduce invasion by discouraging cell detachment from the tumor mass. We have previously reported that α5β1 integrin-fibronectin interaction is a powerful mediator of indirect cell-cell cohesion and that the process of fibronectin matrix assembly (FNMA) is crucial to establishing strong bonds between cells in 3D tumor-like spheroids. Here, we explore a potential role for FNMA in preventing dispersal of GBM cells from a tumor-like mass. Using a series of GBM-derived cell lines we developed an in vitro assay to measure the dispersal velocity of aggregates on a solid substrate. Despite their similar pathologic grade, aggregates from these lines spread at markedly different rates. Spreading velocity is inversely proportional to capacity for FNMA and restoring FNMA in GBM cells markedly reduces spreading velocity by keeping cells more connected. Blocking FNMA using the 70 KDa fibronectin fragment in FNMA-restored cells rescues spreading velocity, establishing a functional role for FNMA in mediating dispersal. Collectively, the data support a functional causation between restoration of FNMA and decreased dispersal velocity. This is a first demonstration that FNMA can play a suppressive role in GBM dispersal.