A Peptide with anti-transglutaminase activity decreases lipopolysaccharide-induced lung inflammation in mice

The Outside-In Journey of Tissue Transglutaminase in Cancer

Tissue transglutaminase (TG2) is a member of the transglutaminase family that catalyzes Ca²⁺-dependent protein crosslinks and hydrolyzes guanosine 5′-triphosphate (GTP). The conformation and functions of TG2 are regulated by Ca²⁺ and GTP levels; the TG2 enzymatically active open conformation is modulated by high Ca²⁺ concentrations, while high intracellular GTP promotes the closed conformation, with inhibition of the TG-ase activity. TG2’s unique characteristics and its ubiquitous distribution in the intracellular compartment, coupled with its secretion in the extracellular matrix, contribute to modulate the functions of the protein. Its aberrant expression has been observed in several cancer types where it was linked to metastatic progression, resistance to chemotherapy, stemness, and worse clinical outcomes. The N-terminal domain of TG2 binds to the 42 kDa gelatin-binding domain of fibronectin with high affinity, facilitating the formation of a complex with β-integrins, essential for cellular adhesion to the matrix. This mechanism allows TG2 to interact with key matrix proteins and to regulate epithelial to mesenchymal transition and stemness. Here, we highlight the current knowledge on TG2 involvement in cancer, focusing on its roles translating extracellular cues into activation of oncogenic programs. Improved understanding of these mechanisms could lead to new therapeutic strategies targeting this multi-functional protein.

Transglutaminase 2 has opposing roles in the regulation of cellular functions as well as cell growth and death

Transglutaminase 2 (TG2) is primarily known as the most ubiquitously expressed member of the transglutaminase family with Ca²⁺-dependent protein crosslinking activity; however, this enzyme exhibits multiple additional functions through GTPase, cell adhesion, protein disulfide isomerase, kinase, and scaffold activities and is associated with cell growth, differentiation, and apoptosis. TG2 is found in the extracellular matrix, plasma membrane, cytosol, mitochondria, recycling endosomes, and nucleus, and its subcellular localization is an important determinant of its function. Depending upon the cell type and stimuli, TG2 changes its subcellular localization and biological activities, playing both anti- and pro-apoptotic roles. Increasing evidence indicates that the GTP-bound form of the enzyme (in its closed form) protects cells from apoptosis but that the transamidation activity of TG2 (in its open form) participates in both facilitating and inhibiting apoptosis. A difficulty in the study and understanding of this enigmatic protein is that opposing effects have been reported regarding its roles in the same physiological and/or pathological systems. These include neuroprotective or neurodegenerative effects, hepatic cell growth-promoting or hepatic cell death-inducing effects, exacerbating or having no effect on liver fibrosis, and anti- and pro-apoptotic effects on cancer cells. The reasons for these discrepancies have been ascribed to TG2's multifunctional activities, genetic variants, conformational changes induced by the immediate environment, and differences in the genetic background of the mice used in each of the experiments. In this article, we first report that TG2 has opposing roles like the protagonist in the novel Dr. Jekyll and Mr. Hyde, followed by a summary of the controversies reported, and finally discuss the possible reasons for these discrepancies.

Transglutaminase II/microRNA-218/-181a loop regulates positive feedback relationship between allergic inflammation and tumor metastasis

The molecular mechanism of transglutaminase II (TGaseII)-mediated allergic inflammation remains largely unknown. TGaseII, induced by antigen stimulation, showed an interaction and co-localization with FcϵRI. TGaseII was necessary for in vivo allergic inflammation, such as triphasic cutaneous reaction, passive cutaneous anaphylaxis, and passive systemic anaphylaxis. TGaseII was necessary for the enhanced metastatic potential of B16F1 melanoma cells by passive systemic anaphylaxis. TGaseII was shown to be a secreted protein. Recombinant TGaseII protein increased the histamine release and β-hexosaminidase activity, and enhanced the metastatic potential of B16F1 mouse melanoma cells. Recombinant TGaseII protein induced the activation of EGF receptor and an interaction between EGF receptor and FcϵRI. Recombinant TGaseII protein displayed angiogenic potential accompanied by allergic inflammation. R2 peptide, an inhibitor of TGaseII, exerted negative effects on in vitro and in vivo allergic inflammation by regulating the expression of TGaseII and FcϵRI signaling. MicroRNA (miR)-218 and miR-181a, decreased during allergic inflammation, were predicted as negative regulators of TGaseII by microRNA array and TargetScan analysis. miR-218 and miR-181a formed a negative feedback loop with TGaseII and regulated the in vitro and in vivo allergic inflammation. TGaseII was necessary for the interaction between mast cells and macrophages during allergic inflammation. Mast cells and macrophages, activated during allergic inflammation, were responsible for the enhanced metastatic potential of tumor cells that are accompanied by allergic inflammation. In conclusion, the TGaseII/miR-218/-181a feedback loop can be employed for the development of anti-allergy therapeutics.

Effects of transglutaminase 2 inhibition on ventilator-induced lung injury

This study was performed to examine the role of transglutaminase 2 (TG2) in ventilator-induced lung injury (VILI). C57BL/6 mice were divided into six experimental groups: 1) control group; 2) lipopolysaccharide (LPS) group; 3) lung protective ventilation (LPV) group; 4) VILI group; 5) VILI with cystamine, a TG2 inhibitor, pretreatment (Cyst+VILI) group; and 6) LPV with cystamine pretreatment (Cyst+LPV) group. Acute lung injury (ALI) score, TG2 activity and gene expression, inflammatory cytokines, and nuclear factor-κB (NF-κB) activity were measured. TG2 activity and gene expression were significantly increased in the VILI group (P < 0.05). Cystamine pretreatment significantly decreased TG2 activity and gene expression in the Cyst+VILI group (P < 0.05). Inflammatory cytokines were higher in the VILI group than in the LPS and LPV groups (P < 0.05), and significantly lower in the Cyst+VILI group than the VILI group (P < 0.05). NF-κB activity was increased in the VILI group compared with the LPS and LPV groups (P < 0.05), and significantly decreased in the Cyst+VILI group compared to the VILI group (P = 0.029). The ALI score of the Cyst+VILI group was lower than the VILI group, but the difference was not statistically significant (P = 0.105). These results suggest potential roles of TG2 in the pathogenesis of VILI.

Transglutaminase 2 inhibitor abrogates renal cell carcinoma in xenograft models

PURPOSE

To test whether transglutaminase 2 (TGase 2) inhibitor GK921 alone reverses renal cell carcinoma (RCC) tumor growth. RCC is resistant to both radiation and chemotherapy, and the prognosis remains poor. Despite the recent therapeutic success of vascular endothelial growth factor inhibition in RCC, approximately one-third of RCC patients develop metastatic disease. The expression of TGase 2 is markedly increased in most RCC cell lines, as well as in clinical samples.

METHODS

Previously, we introduced the quinoxaline derivative GK13 as a lead compound for TGase 2 inhibitor. The inhibitory effect of GK13 on TGase 2 was improved in GK921 (3-(phenylethynyl)-2-(2-(pyridin-2-yl)ethoxy)pyrido[3,2-b]pyrazine). GK921 efficacy was tested using sulforhodamine in vitro as well as a xenograft tumor models using ACHN and CAKI-1 RCC cells.

RESULTS

GK921 showed cytotoxicity to RCC (average GI50 in eight RCC cell lines: 0.905 μM). A single treatment with GK921 almost completely reduced tumor growth by stabilizing p53 in the ACHN and CAKI-1 preclinical xenograft tumor models.

CONCLUSION

TGase 2 inhibitor GK921 abrogates RCC growth in xenograft tumor models, suggesting the possibility of a new therapeutic approach to RCC.

Transglutaminase 2-mediated serotonylation in pulmonary hypertension

The monoamine serotonin (5-HT) has been previously implicated in pulmonary arterial remodeling and is considered a potential therapeutic target for the disease pulmonary arterial hypertension (PAH). More recently, it has been recognized that the enzyme tissue transglutaminase (TG2) mediates cross-linking of proteins with 5-HT, a posttranslational process of monoaminylation known as "serotonylation." TG2 activity and serotonylation of protein participate in both smooth muscle proliferation and contraction produced by 5-HT. Indeed, markedly increased TG2 activity has now been identified in lung tissue of an experimental rodent model of pulmonary hypertension, and elevated serotonylation of fibronectin and the signaling molecule Rho, downstream products of transglutamidation, have been found in blood of patients with PAH. The basic mechanism by which TG2 is activated and the potential role(s) of serotonylated proteins in pulmonary hypertension remain a mystery. In the present review we have tried to address the current understanding of 5-HT metabolism in pulmonary hypertension and relate it to what is currently known about the evolving cellular process of serotonylation.

Anti-inflammatory effects of the R2 peptide, an inhibitor of transglutaminase 2, in a mouse model of allergic asthma, induced by ovalbumin

BACKGROUND AND PURPOSE

Transglutaminase 2 (TGase 2) expression is increased in inflammatory diseases, and TGase 2 inhibitors block these increases. We examined whether the R2 peptide inhibited the expression of TGase 2 in a mouse model of inflammatory allergic asthma.

EXPERIMENTAL APPROACH

C57BL/6 mice were sensitized and challenged by ovalbumin (OVA) to induce asthma. OVA-specific serum IgE and leukotrienes (LTs) levels were measured by enzyme-linked immunosorbent assay. Recruitment of inflammatory cells into bronchoalveolar lavage (BAL) fluid or lung tissues and goblet cell hyperplasia were assessed histologically. Airway hyperresponsiveness was determined in a barometric plethysmographic chamber. Expression of TGase 2, eosinophil major basic protein (EMBP), the adhesion molecule vascular cell adhesion molecule-1, Muc5ac and phospholipase A(2) (PLA(2) ) protein were determined by Western blot. Expression of mRNAs of Muc5ac, cytokines, matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs) were measured by reverse transcriptase-polymerase chain reaction and nuclear factor-κB (NF-κB) by electrophoretic mobility shift assay.

KEY RESULTS

R2 peptide reduced OVA-specific IgE levels; the number of total inflammatory cells, macrophages, neutrophils, lymphocytes and eosinophils in BAL fluid and the number of goblet cells. Airway hyperresponsiveness, TGase 2 and EMBP levels, mRNA levels of interleukin (IL)-4, IL-5, IL-6, IL-8, IL-13, RANTES, tumour necrosis factor-α, and MMP2/9, Muc5ac, NF-κB activity, PLA(2) activity and expressions, and LT levels in BAL cells and lung tissues were all reduced by R2 peptide. R2 peptide also restored expression of TIMP1/2.

CONCLUSION AND IMPLICATIONS

R2 peptide reduced allergic responses by regulating NF-κB/TGase 2 activity in a mouse model of allergic asthma. This peptide may be useful in the treatment of allergic asthma.

Tissue transglutaminase contributes to the all-trans-retinoic acid–induced differentiation syndrome phenotype in the NB4 model of acute promyelocytic leukemia

Treatment of acute promyelocytic leukemia (APL) with all-trans-retinoic acid (ATRA) results in terminal differentiation of leukemic cells toward neutrophil granulocytes. Administration of ATRA leads to massive changes in gene expression, including down-regulation of cell proliferation–related genes and induction of genes involved in immune function. One of the most induced genes in APL NB4 cells is transglutaminase 2 (TG2). RNA interference–mediated stable silencing of TG2 in NB4 cells (TG2-KD NB4) coupled with whole genome microarray analysis revealed that TG2 is involved in the expression of a large number of ATRA-regulated genes. The affected genes participate in granulocyte functions, and their silencing lead to reduced adhesive, migratory, and phagocytic capacity of neutrophils and less superoxide production. The expression of genes related to cell-cycle control also changed, suggesting that TG2 regulates myeloid cell differentiation. CC chemokines CCL2, CCL3, CCL22, CCL24, and cytokines IL1B and IL8 involved in the development of differentiation syndrome are expressed at significantly lower level in TG2-KD NB4 than in wild-type NB4 cells upon ATRA treatment. Based on our results, we propose that reduced expression of TG2 in differentiating APL cells may suppress effector functions of neutrophil granulocytes and attenuate the ATRA-induced inflammatory phenotype of differentiation syndrome.

A novel therapeutic target in inflammatory uveitis: transglutaminase 2 inhibitor

Purpose

Our goal was to investigate the effects of inhibition of transglutaminase 2 (TGase 2) on endotoxin-induced uveitis (EIU)

Methods

EIU was induced in female Lewis rats by single footpad injections of 200 µg of lipopolysaccharide (LPS). TGase 2 inhibitors were administered intraperitoneally 30 minutes before and at the time of LPS administration. Rats were sacrificed 24 hours after injection, and the effects of the TGase 2 inhibitors were evaluated by the number of intraocular inflammatory cells present on histologic sections and by measuring the TGase 2 activity and TGase products in the aqueous humor (AqH). TGase 2 substrates were also assayed in AqH from uveitis patients.

Results

Clinical indications of EIU, the number of cells present on histologic sections, and TGase 2 activity in AqH increased in a time-dependent manner, peaking 24 hours after LPS injection. Inflammation in EIU was significantly reversed by treatment with TGase inhibitors. A 23-kDa cross-linked TGase substrate was identified in the AqH from EIU rats and uveitis patients. MALDI-TOF analysis showed that this substrate in uveitis patients was human Ig kappa chain C region.

Conclusions

TGase 2 activity and its catalytic product were increased in the AqH of EIU rats. TGase 2 inhibition attenuated the degree of inflammation in EIU. Safe and stable TGase inhibitors may have great potential for the treatment of inflammatory uveitis.

A new regulatory mechanism of NF-kappaB activation by I-kappaBbeta in cancer cells

Transglutaminase 2 (TGase 2) catalyzes covalent isopeptide bond formation between glutamine and lysine residues. Recently, we reported that TGase 2 activates nuclear factor-kappa B (NF-kappaB) by depleting inhibitor of NF-kappaBalpha (I-kappaBalpha) levels via polymer formation. Furthermore, TGase 2 expression synergistically increases NF-kappaB activity with canonical pathway. The major I-kappaB proteins such as I-kappaBalpha and I-kappaBbeta resemble each other in both primary sequence and tertiary structure. However, I-kappaBbeta does not degrade fully, while I-kappaBalpha degrades immediately in response to most stimuli. We found that I-kappaBbeta does not contain any of the previously identified TGase 2 target sites. In this study, both an in vitro cross-linking assay and a TGase 2 transfection assay revealed that I-kappaBbeta is independent from TGase 2-mediated polymerization. Furthermore, increased I-kappaBbeta expression reversed NF-kappaB activation in cancer cells, compensating for the loss of I-kappaBalpha via TGase 2 polymerization.

Roles of transglutaminases in cardiac and vascular diseases

All transglutaminases share the common enzymatic activity of transamidation, or the cross-linking of glutamine and lysine residues to form N epsilon (gamma-glutamyl) lysyl isopeptide bonds. The plasma proenzyme factor XIII is responsible for stabilizing the fibrin clot against physical and fibrinolytic disruption. Another member of the transglutaminase family, tissue transglutaminase or TG2 is abundantly expressed in cardiomyocytes, vascular cells and macrophages. The transglutaminases have a variety of functions independent of their transamidating activity. For example, TG2 binds and hydrolyzes GTP, thereby fostering signal transduction by several G protein coupled receptors. Accumulating evidence points to novel roles for factor XIII and TG2 in cardiovascular biology including: (a) modulating platelet activity, (b) regulating glucose control, (c) contributing to the development of hypertension, (d) influencing the progression of atherosclerosis, (e) regulating vascular permeability and angiogenesis (f) and contributing to myocardial signaling, contractile activity and ischemia/reperfusion injury. In this review, we summarize the cardiovascular biology of two members of the family of transglutaminases, Factor XIII and TG2.

Transglutaminase 2 Mediates Polymer Formation of I-κBα through C-terminal Glutamine Cluster

Recently we reported that transglutaminase 2 (TGase 2) activates nuclear factor-kappaB (NF-kappaB) independently of I-kappaB kinase (IKK) activation, by inducing cross-linking and protein polymer formation of inhibitor of nuclear factor-kappaBalpha (I-kappaBalpha). TGase 2 catalyzes covalent isopeptide bond formation between the peptide bound-glutamine and the lysine residues. Using matrix-assisted laser desorption ionization time-of-flight mass spectra analysis of I-kappaBalpha polymers cross-linked by TGase 2, as well as synthetic peptides in an in vitro competition assay, we identified a glutamine cluster at the C terminus of I-kappaBalpha (amino acids 266-268) that appeared to play a key role in the formation of I-kappaBalpha polymers. Although there appeared to be no requirement for specific lysine residues, we found a considerably higher preference for the use of lysine residues at positions 21, 22, and 177 in TGase 2-mediated cross-linking of I-kappaBalpha. We demonstrated that synthetic peptides encompassing the glutamine cluster at amino acid positions 266-268 reversed I-kappaBalpha polymerization in vitro. Furthermore, the depletion of free I-kappaBalpha in EcR/TG cells was completely rescued in vivo by transfection of mutant I-kappaBalphas in glutamine sites (Q266G, Q267G, and Q313G) as well as in a lysine site (K177G). These findings provide additional clues into the mechanism by which TGase 2 contributes to the inflammatory process via activation of NF-kappaB.