Alpha1B-adrenoceptor signaling and cell motility: GTPase function of Gh/transglutaminase 2 inhibits cell migration through interaction with cytoplasmic tail of integrin alpha subunits

Metabolic characterisation of transglutaminase 2 inhibitor effects in breast cancer cell lines

Transglutaminase 2 (TG2), which mediates post-translational modifications of multiple intracellular enzymes, is involved in the pathogenesis and progression of cancer. We used 1 H-NMR metabolomics to study the effects of AA9, a novel TG2 inhibitor, on two breast cancer cell lines with distinct phenotypes, MCF-7 and MDA-MB-231. AA9 can promote apoptosis in both cell lines, but it is particularly effective in MD-MB-231, inhibiting transamidation reactions and decreasing cell migration and invasiveness. This metabolomics study provides evidence of a major effect of AA9 on MDA-MB-231 cells, impacting glutamate and aspartate metabolism, rather than on MCF-7 cells, characterised by choline and O-phosphocholine decrease. Interestingly, AA9 treatment induces myo-inositol alteration in both cell lines, indicating action on phosphatidylinositol metabolism, likely modulated by the G protein activity of TG2 on phospholipase C. Considering the metabolic deregulations that characterise various breast cancer subtypes, the existence of a metabolic pathway affected by AA9 further points to TG2 as a promising hot spot. The metabolomics approach provides a powerful tool to monitor the effectiveness of inhibitors and better understand the role of TG2 in cancer.

Current Developments on the Role of α1-Adrenergic Receptors in Cognition, Cardioprotection, and Metabolism

The α₁-adrenergic receptors (ARs) are G-protein coupled receptors that bind the endogenous catecholamines, norepinephrine, and epinephrine. They play a key role in the regulation of the sympathetic nervous system along with β and α₂-AR family members. While all of the adrenergic receptors bind with similar affinity to the catecholamines, they can regulate different physiologies and pathophysiologies in the body because they couple to different G-proteins and signal transduction pathways, commonly in opposition to one another. While α₁-AR subtypes (α_1A, α_1B, α_1C) have long been known to be primary regulators of vascular smooth muscle contraction, blood pressure, and cardiac hypertrophy, their role in neurotransmission, improving cognition, protecting the heart during ischemia and failure, and regulating whole body and organ metabolism are not well known and are more recent developments. These advancements have been made possible through the development of transgenic and knockout mouse models and more selective ligands to advance their research. Here, we will review the recent literature to provide new insights into these physiological functions and possible use as a therapeutic target.

Transglutaminase 2 as a novel target in chronic kidney disease - Methods, mechanisms and pharmacological inhibition

Chronic kidney disease (CKD) is a global health problem with a prevalence of 10-15%. Progressive fibrosis of the renal tissue is a main feature of CKD, but current treatment strategies are relatively unspecific and delay, but do not prevent, CKD. Exploration of novel pharmacological targets to inhibit fibrosis development are therefore important. Transglutaminase 2 (TG2) is known to be central for extracellular collagenous matrix formation, but TG2 is a multifunctional enzyme and novel research has broadened our view on its extra- and intracellular actions. TG2 exists in two conformational states with different catalytic properties as determined by substrate availability and local calcium concentrations. The open conformation of TG2 depends on calcium and has transamidase activity, central for protein modification and cross-linking of extracellular protein components, while the closed conformation is a GTPase involved in transmembrane signaling processes. We first describe different methodologies to assess TG2 activity in renal tissue and cell cultures such as biotin cadaverine incorporation. Then we systematically review animal CKD models and preliminary studies in humans (with diabetic, IgA- and chronic allograft nephropathy) to reveal the role of TG2 in renal fibrosis. Mechanisms behind TG2 activation, TG2 externalization dependent on Syndecan-4 and interactions between TG and profibrotic molecules including transforming growth factor β and the angiotensin II receptor are discussed. Pharmacological TG2 inhibition shows antifibrotic effects in CKD. However, the translation of TG2 inhibition to treat CKD in patients is a challenge as clinical information is limited, and further studies on pharmacokinetics and efficacy of the individual compounds are required.

Oláh A, Fésüs L, Mádi A. Transglutaminase 2 Has Metabolic and Vascular Regulatory Functions Revealed by In Vivo Activation of Alpha1-Adrenergic Receptor

The multifunctional tissue transglutaminase has been demonstrated to act as α1-adrenergic receptor-coupled G protein with GTPase activity in several cell types. To explore further the pathophysiological significance of this function we investigated the in vivo effects of the α1-adrenergic receptor agonist phenylephrine comparing responses in wild type and TG2-/- mice. Injection of phenylephrine, but not a beta3-adrenergic agonist (CL-316,243), resulted in the long-term decline of the respiratory exchange ratio and lower lactate concentration in TG2-/- mice indicating they preferred to utilize fatty acids instead of glucose as fuels. Measurement of tail blood pressure revealed that the vasoconstrictive effect of phenylephrine was milder in TG2-/- mice leading to lower levels of lactate dehydrogenase (LDH) isoenzymes in blood. LDH isoenzyme patterns indicated more damage in lung, liver, kidney, skeletal, and cardiac muscle of wild type mice; the latter was confirmed by a higher level of heart-specific CK-MB. Our data suggest that TG2 as an α1-adrenergic receptor-coupled G protein has important regulatory functions in alpha1-adrenergic receptor-mediated metabolic processes and vascular functions.

15(V/K) kinetic isotope effect and steady-state kinetic analysis for the transglutaminase 2 catalyzed deamidation and transamidation reactions

The Ca²⁺-dependent deamidation and transamidation activities of transglutaminase 2 (TG2) are important to numerous physiological and pathological processes. Herein, we have examined the steady-state kinetics and ¹⁵(V/K) kinetic isotope effects (KIEs) for the TG2-catalyzed deamidation and transamidation of N-Benzyloxycarbonyl-l-Glutaminylglycine (Z-Gln-Gly) using putrescine as the acyl acceptor substrate. Kinetic parameters determined from initial velocity plots are consistent with previously proposed mechanisms. Significant differences in the ¹⁵(V/K) KIEs on NH₃ release determined for the deamidation (0.2%) and the transamidation (2.3%) of Z-Gln-Gly suggest the rate-limiting steps of TG2 active site acylation are dependent on the presence of the acyl acceptor. We propose a plausible mechanistic explanation where substrate-induced conformational changes may play a role in promoting catalysis.

Induction of GD3/α1-adrenergic receptor/transglutaminase 2-mediated erythroid differentiation in chronic myelogenous leukemic K562 cells

The disialic acid-containing glycosphingolipid GD3 recruited membrane transglutaminase 2 (TG2) as a signaling molecule for erythroid differentiation in human chronic myelogenous leukemia (CML) K562 cells. The α1-adrenergic receptor (α1-AR)/TG2-mediated signaling pathway regulated GD3 functions, including gene expression and production, to differentiate CML K562 cells into erythroid lineage cells. Epinephrine, an AR agonist, increased membrane recruitment as well as GTP-photoaffinity of TG2, inducing GD3 synthase gene expression. Epinephrine activated PI3K/Akt signaling and GTPase downstream of TG2 activated Akt. The coupling of TG2 and GD3 production was specifically suppressed by prazosin (α1-AR antagonist), but not by propranolol (β-AR antagonist) or rauwolscine (α2-AR antagonist), indicating α1-AR specificity. Small interfering RNA (siRNA) experiment results indicated that the α1-AR/TG2-mediated signaling pathway activated PKCs α and δ to induce GD3 synthase gene expression. Transcription factors CREB, AP-1, and NF-κB regulated GD3 synthase gene expression during α1-AR-induced differentiation in CML K562 cells. In addition, GD3 synthase gene expression was upregulated in TG2-transfected cells via α1-AR with expression of erythroid lineage markers and benzidine-positive staining. α1-AR/TG2 signaling pathway-directed GD3 production is a crucial step in erythroid differentiation of K562 cells and GD3 interacts with α1-AR/TG2, inducing GD3/α1-AR/TG2-mediated erythroid differentiation. These results suggest that GD3, which acts as a membrane mediator of erythroid differentiation in CML cells, provides a therapeutic avenue for leukemia treatment.

Tissue Transglutaminase Modulates Vascular Stiffness and Function Through Crosslinking-Dependent and Crosslinking-Independent Functions

Background

The structural elements of the vascular wall, namely, extracellular matrix and smooth muscle cells (SMCs), contribute to the overall stiffness of the vessel. In this study, we examined the crosslinking‐dependent and crosslinking‐independent roles of tissue transglutaminase (TG2) in vascular function and stiffness.

Methods and Results

SMCs were isolated from the aortae of TG2−/− and wild‐type (WT) mice. Cell adhesion was examined by using electrical cell–substrate impedance sensing and PicoGreen assay. Cell motility was examined using a Boyden chamber assay. Cell proliferation was examined by electrical cell–substrate impedance sensing and EdU incorporation assays. Cell micromechanics were studied using magnetic torsion cytometry and spontaneous nanobead tracer motions. Aortic mechanics were examined by tensile testing. Vasoreactivity was studied by wire myography. SMCs from TG2−/− mice had delayed adhesion, reduced motility, and accelerated de‐adhesion and proliferation rates compared with those from WT. TG2−/− SMCs were stiffer and displayed fewer cytoskeletal remodeling events than WT. Collagen assembly was delayed in TG2−/− SMCs and recovered with adenoviral transduction of TG2. Aortic rings from TG2−/− mice were less stiff than those from WT; stiffness was partly recovered by incubation with guinea pig liver TG2 independent of crosslinking function. TG2−/− rings showed augmented response to phenylephrine‐mediated vasoconstriction when compared with WT. In human coronary arteries, vascular media and plaque, high abundance of fibronectin expression, and colocalization with TG2 were observed.

Conclusions

TG2 modulates vascular function/tone by altering SMC contractility independent of its crosslinking function and contributes to vascular stiffness by regulating SMC proliferation and matrix remodeling.

Inhibition or ablation of transglutaminase 2 impairs astrocyte migration

Astrocytes play numerous complex roles that support and facilitate the function of neurons. Further, when there is an injury to the central nervous system (CNS) they can both facilitate or ameliorate functional recovery depending on the location and severity of the injury. When a CNS injury is relatively severe a glial scar is formed, which is primarily composed of astrocytes. The glial scar can be both beneficial, by limiting inflammation, and detrimental, by preventing neuronal projections, to functional recovery. Thus, understanding the processes and proteins that regulate astrocyte migration in response to injury is still of fundamental importance. One protein that is likely involved in astrocyte migration is transglutaminase 2 (TG2); a multifunctional protein expressed ubiquitously throughout the brain. Its functions include transamidation and GTPase activity, among others, and previous studies have implicated TG2 as a regulator of migration. Therefore, we examined the role of TG2 in primary astrocyte migration subsequent to injury. Using wild type or TG2^-/- astrocytes, we manipulated the different functions and conformation of TG2 with novel irreversible inhibitors or mutant versions of the protein. Results showed that both inhibition and ablation of TG2 in primary astrocytes significantly inhibit migration. Additionally, we show that the deficiency in migration caused by deletion of TG2 can only be rescued with the native protein and not with mutants. Finally, the addition of TGFβ rescued the migration deficiency independent of TG2. Taken together, our study shows that transamidation and GTP/GDP-binding are necessary for inhibiting astrocyte migration and it is TGFβ independent.

Novel inhibitory effects of cardamonin on thromboxane A2-induced scratching response: Blocking of Gh/transglutaminase-2 binding to thromboxane A2 receptor

Alpinia katsumadai is known to suppress thromboxane A2 (TXA2) receptor agonist-induced scratching in mice. The specific components of A. katsumadai responsible for these biological effects, however, are not known. In the present study, we investigated whether cardamonin (CDN), one of major principles of A. katsumadai, has suppressive effects on TXA2-induced scratching in mice. Scratching induced by U46619 (the TXA2 receptor agonist) at a dose of 10nmol/site was shown to be suppressed by CDN (0.1nmol-0.5nmol/site). Suppression of the U46619-induced scratching response by CDN was found to be unrelated to competition with the ligand at the TXA2 receptor, since CDN did not suppress [(3)H] SQ29548 (the TXA2 receptor antagonist) binding to TXA2 receptor. TXA2 receptor expression in A549, HaCaT, and SH-SY5Y cell lines was examined and determined to be significant in the A549 and SH-SY5Y cell lines. Further, binding of high molecular G protein Gh/transglutaminase-2 (Gh/Tgase-2) to TXA2 receptor was confirmed in the A549 and SH-SY5Y cells by co-immunoprecipitation. CDN suppressed the binding of TXA2 receptor with Gh/Tgase-2, which also acts as a G protein involved in TXA2 signaling. These results suggested that CDN suppresses TXA2 receptor agonist-induced scratching by suppressing TXA2 signaling, specifically via blocking of the binding of Gh/Tgase-2 to TXA2 receptor.

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.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

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.

Transglutaminase 2 overexpression in tumor stroma identifies invasive ductal carcinomas of breast at high risk of recurrence

Introduction

Molecular markers for predicting breast cancer patients at high risk of recurrence are urgently needed for more effective disease management. The impact of alterations in extracellular matrix components on tumor aggressiveness is under intense investigation. Overexpression of Transglutaminase 2 (TG2), a multifunctional enzyme, in cancer cells impacts epithelial mesenchymal transition, growth, invasion and interactions with tumor microenvironment. The objective of our study is to determine the clinical relevance of stromal TG2 overexpression and explore its potential to identify breast cancers at high risk of recurrence.

Methods

This retrospective study is based on immunohistochemical analysis of TG2 expression in normal breast tissues (n = 40) and breast cancers (n = 253) with clinical, pathological and follow-up data available for up to 12 years. TG2 expression was correlated with clinical and pathological parameters as well as disease free survival (DFS) of breast cancer patients.

Results

Stromal TG2 overexpression was observed in 114/253 (45.0%) breast cancer tissues as compared to breast normal tissues. Among invasive ductal carcinomas (IDC) of the breast, 97/168 (57.7%) showed strong TG2 expression in tumor stroma. Importantly, IDC patients showing stromal TG2 accumulation had significantly reduced DFS (mean DFS = 110 months) in comparison with patients showing low expression (mean DFS = 130 months) in Kaplan-Meier survival analysis (p<0.001). In Cox multivariate regression analysis, stromal TG2 accumulation was an independent risk factor for recurrence (p = 0.006, Hazard’s ratio, H.R. = 3.79). Notably, these breast cancer patients also showed immunostaining of N-epsilon gamma-glutamyl lysine amino residues in tumor stroma demonstrating the transamidating activity of TG2.

Conclusions

Accumulation of TG2 in tumor stroma is an independent risk factor for identifying breast cancer patients at high risk of recurrence. TG2 overexpression in tumor stroma may serve as a predictor of poor prognosis for IDC of the breast.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

TG2, a novel extracellular protein with multiple functions

TG2 is multifunctional enzyme which can be secreted to the cell surface by an unknown mechanism where its Ca(2+)-dependent transamidase activity is implicated in a number of events important to cell behaviour. However, this activity may only be transient due to the oxidation of the enzyme in the extracellular environment including its reaction with NO probably accounting for its many other roles, which are transamidation independent. In this review, we discuss the novel roles of TG2 at the cell surface and in the ECM acting either as a transamidating enzyme or as an extracellular scaffold protein involved in cell adhesion. Such roles include its ability to act as an FN co-receptor for β integrins or in a heterocomplex with FN interacting with the cell surface heparan sulphate proteoglycan syndecan-4 leading to activation of PKCα. These different properties of TG2 involve this protein in various physiological processes, which if not regulated appropriately can also lead to its involvement in a number of diseases. These include metastatic cancer, tissue fibrosis and coeliac disease, thus increasing its attractiveness as both a therapeutic target and diagnostic marker.

Targeting ovarian tumor cell adhesion mediated by tissue transglutaminase

Tissue transglutaminase (TG2) is a transpeptidase involved in protein cross-linking through generation of ε-(γ-glutamyl)lysine isopeptide bonds. It also promotes cell adhesion through interaction with fibronectin and facilitates formation of fibronectin-integrin complexes. This interaction is involved in tumor cell adhesion to the matrix and in the process of tumor dissemination. Its inhibition by small molecules may therefore be useful in blocking metastasis. To that end, we screened more than 800,000 compounds following an in silico docking approach targeting two distinct cavities in the vicinity of the fibronectin-binding site on TG2. A total of 120 compounds were acquired and tested in cell culture-based assays for inhibition of ovarian tumor cell adhesion and proliferation. Seven compounds showed more than 50% inhibition of cell adhesion at a concentration of 25 μmol/L. A follow-up fluorescence polarization study revealed that one compound in particular (ITP-79) inhibited binding of a TG2 peptide to a 42-kDa fragment of fibronectin in a dose-dependent manner. This inhibition was confirmed in cancer cells by coimmunoprecipitation. A competition assay with surface plasmon resonance showed that ITP-79 modulated binding of TG2 to fibronectin. Direct binding of compounds that inhibited adhesion to TG2 were examined with differential scanning fluorimetry, which measures the effect of the compound on the melting temperature of the target. Two compounds, including ITP-79, reduced TG2 stabilization, mimicking the effects of GTP, a known negative allosteric regulator of TG2 enzymatic function. This suggests a potential allosteric mechanism for the compound in light of its distal target site.

Nonmuscle myosin IIA (myosin heavy polypeptide 9): a novel class of signal transducer mediating the activation of G alpha h/phospholipase C-delta 1 pathway

The dimeric Gh protein is comprised of alpha (tissue transglutaminase) and beta (Calreticulin) subunits and known to be associated with FSH-, oxytocin-, or epinephrine-receptors/functions in their respective target cells. After establishing the FSH-induced activation of G alpha h/phospholipase C (PLC)-delta 1 pathway in rat Sertoli cells (SCs), we have attempted to identify a possible G alpha h-coupled novel FSH receptor (FSH-R). Remarkably, a protein with approximately 240-kDa molecular mass was coimmunoprecipitated with G alpha h in the fractionated membrane proteins of rat SCs. The protein was identified as myosin heavy polypeptide 9 (MyH9) by mass spectrometric analysis and immunoblotting. In addition, immunoprecipitation analysis reveals that MyH9 is constitutively associated with classical Gs-coupled FSH-R and inactive GDP-bound G alpha h at resting state of rat SCs, but did not interact with FSH directly as judged by Far-Western analysis. Upon the stimulation of higher levels of extracellular FSH (>1000 IU/liter), classical FSH-R induces the phosphorylation of MyH9, the dissociation of active GTP-bound G alpha h from FSH-R:MyH9 complexes, and the elicitation of G alpha h/PLC-delta 1 pathway-dependent Ca(2+)-influx in rat SCs. Furthermore, the specific inhibition of MyH9 ATPase activity with Blebbistatin dose-dependently suppressed FSH-induced G alpha h/PLC-delta 1 signaling and Ca(2+)-influx, but not intracellular cAMP accumulation in rat SCs, implying that MyH9 mediates FSH-induced activation of G alpha h/PLC-delta 1/IP(3)/Ca(2+)-influx pathway in rat SCs. This is the first to demonstrate that the filament protein MyH9 constitutively forms a ternary complex with FSH-R and inactive GDP-bound G alpha h. At higher FSH levels, this ternary complex executes an alternative signaling of classical Gs-coupled FSH-R through activating a Gs/cAMP-independent, G alpha h/PLC-delta 1 pathway in rat SCs.

Transglutaminase 2: a multi-functional protein in multiple subcellular compartments

Transglutaminase 2 (TG2) is a multifunctional protein that can function as a transglutaminase, G protein, kinase, protein disulfide isomerase, and as an adaptor protein. These multiple biochemical activities of TG2 account for, at least in part, its involvement in a wide variety of cellular processes encompassing differentiation, cell death, inflammation, cell migration, and wound healing. The individual biochemical activities of TG2 are regulated by several cellular factors, including calcium, nucleotides, and redox potential, which vary depending on its subcellular location. Thus, the microenvironments of the subcellular compartments to which TG2 localizes, such as the cytosol, plasma membrane, nucleus, mitochondria, or extracellular space, are important determinants to switch on or off various TG2 biochemical activities. Furthermore, TG2 interacts with a distinct subset of proteins and/or substrates depending on its subcellular location. In this review, the biological functions and molecular interactions of TG2 will be discussed in the context of the unique environments of the subcellular compartments to which TG2 localizes.

Adhesion-GPCRs in the CNS

There are a total of 33 members of adhesion G protein-coupled receptors (GPCRs) in humans and 30 members in mice and rats. More than half of these receptors are expressed in the central nervous system (CNS), indicating their possible roles in the development and function of the CNS. Indeed, it has been shown that adhesion-GPCRs are involved in the regulation of neurulation, cortical development and neurite growth. Among the few adhesion-GPCRs being studied, GPR56 is so far the only member associated with a human brain malformation called bilateral frontoparietal polymicrogyria (BFPP). The histopathology of BFPP is a cobblestone-like brain malformation characterized by neuronal overmigration through a breached pial basement membrane (BM). Further studies in the Gpr56 knockout mouse model revealed that GPR56 is expressed in radial glial cells and regulates the integrity of the pial BM by binding a putative ligand in the extracellular matrix of the developing brain.

Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

Serotonylation of vascular proteins important to contraction

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) was named for its source (sero-) and ability to modify smooth muscle tone (tonin). The biological effects of 5-HT are believed to be carried out by stimulation of serotonin receptors at the plasma membrane. Serotonin has recently been shown to be synthesized in vascular smooth muscle and taken up from external sources, placing 5-HT inside the cell. The enzyme transglutaminase uses primary amines such as 5-HT to covalently modify proteins on glutamine residues. We tested the hypothesis that 5-HT is a substrate for transglutaminase in arterial vascular smooth muscle, with protein serotonylation having physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

The model was the rat aorta and cultured aortic smooth muscle cells. Western analysis demonstrated that transglutaminase II was present in vascular tissue, and transglutaminase activity was observed as a cystamine-inhibitable incorporation of the free amine pentylamine-biotin into arterial proteins. Serotonin-biotin was incorporated into alpha-actin, beta-actin, gamma-actin, myosin heavy chain and filamin A as shown through tandem mass spectrometry. Using antibodies directed against biotin or 5-HT, immunoprecipitation and immunocytochemistry confirmed serotonylation of smooth muscle alpha-actin. Importantly, the alpha-actin-dependent process of arterial isometric contraction to 5-HT was reduced by cystamine.

CONCLUSIONS

5-HT covalently modifies proteins integral to contractility and the cytoskeleton. These findings suggest new mechanisms of action for 5-HT in vascular smooth muscle and consideration for intracellular effects of primary amines.

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

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

Factor XIIIA mobilizes transglutaminase 2 to induce chondrocyte hypertrophic differentiation

Two transglutaminases (TGs), factor XIIIA (FXIIIA) and TG2, undergo physiologic upregulation in growth plate hypertrophic chondrocytes, and pathological upregulation in osteoarthritic cartilage. Externalization of guanine-nucleotide-bound TG2 drives chondrocyte maturation to hypertrophy, a state linked to matrix remodeling and calcification. Here, we tested the hypothesis that FXIIIA also promotes hypertrophic differentiation. Using human articular chondrocytes, we determined that extracellular FXIIIA induced chondrocyte hypertrophy associated with rapid movement of TG2 to the cell surface. Site-directed mutagenesis revealed that FXIIIA Pro37 bordering the thrombin endoproteolytic Arg38-Gly39 site, but not intrinsic TG catalytic activity, were necessary for FXIIIA to induce chondrocyte hypertrophy. TGs have been demonstrated to interact with certain integrins and, during osteoarthritis (OA), alpha1beta1 integrin is upregulated and associated with hypertrophic chondrocytes. FXIIIA engaged alpha1beta1 integrin in chondrocytes. Antibody crosslinking of alpha1beta1 integrin mobilized TG2. Conversely, an alpha1beta1-integrin-specific blocking antibody inhibited the capacity of FXIIIA to induce TG2 mobilization to the cell surface, phosphorylation of p38 MAP kinase, and chondrocyte hypertrophy. Our results identify a unique functional network between two cartilage TG isoenzymes that accelerates chondrocyte maturation without requirement for TG-catalyzed transamidation by either TG.

Transglutaminase 2 is central to induction of the arterial calcification program by smooth muscle cells

Arterial calcification is a phenotype of vascular repair in atherosclerosis, diabetes, hyperphosphatemic renal failure, and aging. Arterial calcification is modulated by transition of arterial smooth muscle cells (SMCs) from contractile to chondro-osseous differentiation programmed in response to increases in P(i), bone morphogenetic protein-2, and certain other stimuli. Transglutaminase (TG)2 release modulates tissue repair, partly by transamidation-catalyzed covalent crosslinking of extracellular matrix substrates. TG2 regulates cultured SMC differentiation, resistance artery remodeling to vasoconstriction, and atherosclerotic lesion size. Here, TG2 expression was required for the majority of TG activity in mouse and human aortic SMCs. TG2(-/-) SMCs lost the capacity for P(i) donor-induced formation of multicellular bone-like nodules and for increased expression of the type III sodium-dependent P(i) cotransporter Pit-1 and certain osteoblast and chondrocyte genes (tissue-nonspecific alkaline phosphatase, the osteoblast master transcription factor runx2, and chondrocyte-restricted aggrecan), and for P(i) donor- and bone morphogenetic protein-2-induced calcification. Uniquely in TG2(-/-) SMCs, P(i) donor treatment increased expression of the physiological SMC chondro-osseous differentiation and calcification inhibitors osteoprotegerin, matrix Gla protein, and osteopontin. Conversely, TG2(-/-) SMCs, unlike wild-type SMCs, failed to maintain contractile differentiation on laminin. Exogenous catalytically active TG2 augmented calcification by TG2(-/-) SMC in response to P(i) donor treatment. TG2 expression also drove P(i)-stimulated calcification of mouse aortic ring organ cultures, which was suppressed by the TG2 catalytic site-specific inhibitor Boc-DON-Gln-Ile-Val-OMe (10 micromol/L). Our results suggest that TG2 release in injured arteries is critical for programming chondro-osseous SMC differentiation and calcification in response to increased P(i) and bone morphogenetic protein-2.

Differentially expressed gene networks in cultured smooth muscle cells from normal and neuropathic bladder

Neuropathic bladder dysfunction results from abnormal development of the spine, spinal cord injuries, or diseases such as diabetics. Patients with neuropathic bladders often require surgical intervention such as bladder reconstruction to improve incontinence and prevent renal damage. Tissue engineering with ex-vivo cultured bladder cells has been suggested as one means for improving bladder function. However, we previously demonstrated that cultured bladder smooth muscle cells (SMCs) derived from neuropathic bladder exhibit and maintain altered pathologic phenotypes in culture. To identify genes that are responsible for the abnormal neuropathic phenotypes specifically elevated cell proliferation, the expression levels of 1,185 genes were compared between cultured SMCs derived from normal and neuropathic bladders using a cDNA array consisting of well-annotated genes. The expression data were analyzed using several methods to identify differentially expressed genes. The resulting sets of differentially expressed genes were examined by pathway analysis to identify the networks that remain abnormal in the culture-stable phenotype of neuropathic SMCs. A total of 18 genes that are differentially expressed between cultured normal and neuropathic bladder SMCs were identified. Of these 17 were up-regulated greater than 2-fold in neuropathic bladder SMCs, six of them along with one gene that was not up-regulated greater than 2-fold in cultured neuropathic bladder SMCs were confirmed and identified by more stringent analysis methods including significance analysis of microarrays, class comparison, and class prediction analyses. The major dysregulated pathways include fibroblast growth factor signaling, PTEN signaling, and integrin signaling. Our results further suggest that altered neuropathic bladder SMC phenotypes is stable in the culture environments and that SMCs derived from diseased bladders may not be appropriate for tissue engineering purpose without modification of pathologically altered genes expression.

The integrin alpha5beta1 regulates chondrocyte hypertrophic differentiation induced by GTP-bound transglutaminase 2

Soluble GTP-bound transglutaminase 2 (TG2) induces hypertrophic differentiation in chondrocyte cultures in a beta1 integrin-dependent fashion. beta1 integrin subfamily consists of 12 heterodimers with 12 different alpha subunits and a beta1 subunit. To identify the specific integrin heterodimer(s) responsible for this process, we specifically blocked individual beta1 integrins on the CH-8 immortalized human chondrocytes during hypertrophic differentiation. Blockade of alpha5beta1 inhibited matrix metalloproteinase 13 (MMP-13), type X collagen expression, alkaline phosphatase activity and matrix calcification by 30-50% associated with weak effects of anti-alpha3beta1 and -alpha4beta1. Anti-alpha1beta1, -alpha2beta1 and -alpha6beta1 had no effect. To examine whether the dominant effect of integrin alpha5beta1 was due to a direct interaction with TG2, we incubated the chondrocytic cells on plates coated with GTP-bound TG2. The immobilized GTP-bound TG2 induced hypertrophic differentiation to the same extent as the soluble GTP-bound TG2, which was also inhibited by anti-alpha5beta1. CH-8 cells grown on plates coated with GTP-bound TG2 demonstrated adherence associated with focal adhesion kinase phosphorylation. These properties were inhibited by anti-alpha5beta1. Furthermore, engagement of alpha5beta1 on CH-8 cells via anti-alpha5beta1 antibody did, in fact, induce differentiation. Although CH-8 cells adhered to GTP-free TG2 via integrin alpha5beta1, the cells failed to undergo hypertrophic differentiation. Thus, integrin alpha5beta1 is critical for the chondrocyte hypertrophic differentiation induced by GTP-bound TG2, and this induction is ligand dependent.

Mechanisms of vascular smooth muscle cell migration

Smooth muscle cell migration occurs during vascular development, in response to vascular injury, and during atherogenesis. Many proximal signals and signal transduction pathways activated during migration have been identified, as well as components of the cellular machinery that affect cell movement. In this review, a summary of promigratory and antimigratory molecules belonging to diverse chemical and functional families is presented, along with a summary of key signaling events mediating migration. Extracellular molecules that modulate migration include small biogenic amines, peptide growth factors, cytokines, extracellular matrix components, and drugs used in cardiovascular medicine. Promigratory stimuli activate signal transduction cascades that trigger remodeling of the cytoskeleton, change the adhesiveness of the cell to the matrix, and activate motor proteins. This review focuses on the signaling pathways and effector proteins regulated by promigratory and antimigratory molecules. Prominent pathways include phosphatidylinositol 3-kinases, calcium-dependent protein kinases, Rho-activated protein kinase, p21-activated protein kinases, LIM kinase, and mitogen-activated protein kinases. Important downstream targets include myosin II motors, actin capping and severing proteins, formins, profilin, cofilin, and the actin-related protein-2/3 complex. Actin filament remodeling, focal contact remodeling, and molecular motors are coordinated to cause cells to migrate along gradients of chemical cues, matrix adhesiveness, or matrix stiffness. The result is recruitment of cells to areas where the vessel wall is being remodeled. Vessel wall remodeling can be antagonized by common cardiovascular drugs that act in part by inhibiting vascular smooth muscle cell migration. Several therapeutically important drugs act by inhibiting cell cycle progression, which may reduce the population of migrating cells.

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 limits murine peritoneal acute gout-like inflammation by regulating macrophage clearance of apoptotic neutrophils

OBJECTIVE

Monosodium urate monohydrate (MSU) crystals have remarkable inflammatory potential. However, gouty inflammation is spontaneously self-limited, an occurrence recognized since antiquity. Gouty synovitis is driven and sustained by neutrophil influx. Importantly, macrophage phagocytosis of apoptotic (but not necrotic) neutrophils is antiinflammatory. Therefore, we tested the hypothesis that efficient clearance of apoptotic neutrophils by macrophages is one of the factors that restrains the progression of gouty inflammation. Macrophage expression of transglutaminase 2 (TG2), a multifunctional protein with reciprocally regulated transamidation and purine nucleotide-binding activities, promotes apoptotic leukocyte uptake. In this study, we tested the specific role of macrophage TG2 expression in MSU crystal-induced inflammation.

METHODS

We studied MSU crystal-induced peritonitis in TG2-/- and congenic TG2+/+ mice. We also studied the effects of TG2 on apoptotic cell uptake by cultured macrophages.

RESULTS

TG2-/- mice demonstrated more progressive neutrophilic accumulation than did TG2+/+ mice, which was associated with delayed clearance of apoptotic neutrophils during MSU crystal-induced peritonitis. We observed defective phagocytosis of apoptotic leukocytes by TG2-/- peritoneal macrophages, which was corrected by soluble extracellular TG2. Transamidation catalytic activity of TG2 was not required to mediate macrophage uptake of apoptotic leukocytes. In contrast, the TG2 nucleotide binding site residue K173 was critical for this TG2 function. TG2 bound to GDP, ADP, or ATP (but not to GTP) rescued defective apoptotic leukocyte uptake by TG2-/- macrophages.

CONCLUSION

Enhancement of apoptotic neutrophil uptake by macrophage-derived TG2 restrains gout-like neutrophilic peritoneal inflammation. Differential binding of TG2 by purine nucleotides may contribute to clinical variability in the extent and duration of gouty inflammation.

Tissue-transglutaminase contributes to neutrophil granulocyte differentiation and functions

Promyelocytic NB4 leukemia cells undergo differentiation to granulocytes following retinoic acid treatment. Here we report that tissue transglutaminase (TG2), a protein cross-linking enzyme, was induced, then partially translocated into the nucleus, and became strongly associated with the chromatin during the differentiation process. The transglutaminase-catalyzed cross-link content of both the cytosolic and the nuclear protein fractions increased while NB4 cells underwent cellular maturation. Inhibition of cross-linking activity of TG2 by monodansylcadaverin in these cells led to diminished nitroblue tetrazolium (NBT) positivity, production of less superoxide anion, and decreased expression of GP91PHOX, the membrane-associated subunit of NADPH oxidase. Neutrophils isolated from TG2–/– mice showed diminished NBT reduction capacity, reduced superoxide anion formation, and down-regulation of the gp91phox subunit of NADPH oxidase, compared with wild-type cells. It was also observed that TG2–/– mice exhibited increased neutrophil phagocytic activity, but had attenuated neutrophil chemotaxis and impaired neutrophil extravasation with higher neutrophil counts in their circulation during yeast extract–induced peritonitis. These results clearly suggest that TG2 may modulate the expression of genes related to neutrophil functions and is involved in several intracellular and extracellular functions of extravasating neutrophil.

Novel alpha1-adrenergic receptor signaling pathways: secreted factors and interactions with the extracellular matrix

alpha1-Adrenergic receptor (alpha1-ARs) subtypes (alpha1A, alpha1B, and alpha1D) regulate multiple signal pathways, such as phospholipase C, protein kinase C (PKC), and mitogen-activated protein kinases. We employed oligonucleotide microarray technology to explore the effects of both short- (1 h) and long-term (18 h) activation of the alpha1A-AR to enable RNA changes to occur downstream of earlier well characterized signaling pathways, promoting novel couplings. Polymerase chain reaction (PCR) studies confirmed that PKC was a critical regulator of alpha1A-AR-mediated gene expression, and secreted interleukin (IL)-6 also contributed to gene expression alterations. We next focused on two novel signaling pathways that might be mediated through alpha1A-AR stimulation because of the clustering of gene expression changes for cell adhesion/motility (syndecan-4 and tenascin-C) and hyaluronan (HA) signaling. We confirmed that alpha1-ARs induced adhesion in three cell types to vitronectin, an interaction that was also integrin-, FGF7-, and PKC-dependent. alpha1-AR activation also inhibited cell migration, which was integrin- and PKC-independent but still required secretion of FGF7. alpha1-AR activation also increased the expression and deposition of HA, a glycosaminoglycan, which displayed two distinct structures: pericellular coats and long cable structures, as well as increasing expression of the HA receptor, CD44. Long cable structures of HA can bind leukocytes, which this suggests that alpha1-ARs may be involved in proinflammatory responses. Our results indicate alpha1-ARs induce the secretion of factors that interact with the extracellular matrix to regulate cell adhesion, motility and proinflammatory responses through novel signaling pathways.

Retinoic acid receptors and tissue-transglutaminase mediate short-term effect of retinoic acid on migration and invasion of neuroblastoma SH-SY5Y cells

Long-term treatment with all trans-retinoic acid (RA) induces neuronal differentiation and apoptosis. However, the effect of short-term RA treatment on cell proliferation, migration and invasion of neuroblastoma cell lines (SH-SY5Y and IMR-32) remains unclear. RA induces expression of tissue-transglutaminase (TGase) and promotes migration and invasion after 24 h of treatment in SH-SY5Y cells, but not in IMR-32 cells. RA receptor (RAR) agonist (4-(E-2-[5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl]-1-propenyl) benzoic acid) and RAR/retinoid X receptor (RXR) agonist (9-cis-RA) promote expression of TGase, migration and invasion of SH-SY5Y cells, while RXR agonist has no significant effect. RAR antagonist blocks RA effect on migration and invasion, indicating that RAR receptors are required. Retinoid receptors are expressed and activated by RA in both cell lines. However, only transient activation of RAR is observed in IMR-32 cells. These findings suggest that different responses observed in SH-SY5Y and IMR-32 cells could be due to differential activation of retinoid receptors. Overexpression of TGase has no effect on migration or invasion, while overexpression of antisense TGase blocks RA-induced migration and invasion, indicating that other molecules along with TGase mediate RA effects. In addition to the long-term effects of RA that are coupled with cell differentiation, short-term effects involve migration and invasion of neuroblastoma SH-SY5Y cells.

External GTP-bound transglutaminase 2 is a molecular switch for chondrocyte hypertrophic differentiation and calcification

Chondrocyte maturation to hypertrophy, associated with up-regulated transglutaminase 2 (TG2) expression, mediates not only physiologic growth plate mineralization but also pathologic matrix calcification and dys-regulated matrix repair in osteoarthritic articular cartilage. TG2-/- mouse chondrocytes demonstrate markedly inhibited progression to hypertrophic differentiation in response to both retinoic acid and the chemokine CXCL1. Here, our objectives were to test if up-regulated TG2 alone is sufficient to promote chondrocyte hypertrophic differentiation and to identify TG2 molecular determinants and potential downstream signals involved. TG2 activities, regulated by nucleotides and calcium, include cross-linking of cartilage matrix proteins, binding of fibronectin, and hydrolysis of GTP and ATP. Following transfection of TG2 site-directed mutants into chondrocytic cells, we observed that wild type TG2, and TG catalytic site and fibronectin-binding mutants promoted type X collagen expression and matrix calcification consistent with chondrocyte hypertrophic differentiation. In contrast, transfected mutants of TG2 GTP binding (K173L) and externalization (Y274A) sites did not stimulate chondrocyte hypertrophy. Recombinant TG2 treatment of bovine cartilage explants demonstrated that extracellular TG2 induced hypertrophy more robustly in the GTP-bound state, confirming an essential role of TG2 GTP binding. Finally, TG2 treatment induced type X collagen in a beta1 integrin-mediated manner, associated with rapid phosphorylation of both Rac1 and p38 kinases that were inhibited by mutation of the TG2 GTP binding site. In conclusion, externalized GTP-bound TG2 serves as a molecular switch for differentiation of chondrocytes to a hypertrophic, calcifying phenotype in a manner that does not require either TG2 transamidation activity or fibronectin binding.

Overexpression of transglutaminase 2 accelerates the erythroid differentiation of human chronic myelogenous leukemia K562 cell line through PI3K/Akt signaling pathway

Transglutaminase 2 (TG2) is a GTP-binding protein with transglutaminase activity. Despite advances in the characterization of TG2 functions and their impact on cellular processes, the role of TG2 in Human chronic myelogenous leukemia K562 cell line is still poorly understood. To understand the biological significance of TG2 during the differentiation of K562 cells, we established and characterized K562 cells that specifically express TG2. Non-transfected K562 cells showed the increase of membrane-bound-TG2 level after 3 days in the response to Hemin and all trans-retinoic acid (tRA), indicating that membrane recruitment of TG2 is occurred during the erythroid differentiation. However, membrane recruitment of TG2 in TG2-transfected cells revealed within earlier time period, compared with that in vector-transfected cells. The ability of membrane-bound-TG2 to be photoaffinity-labeled with [alpha-32P]GTP was also increased in TG2-transfected cells. TG2-transfected cells activated Akt phosphorylation and inactivated ERK1/2 phosphorylation, compared with vector-transfected cells. Furthermore, phosphorylation of CREB, one of the Akt substrates, was increased in TG2-transfected cells and this phenomenon was confirmed by RT-PCR analysis of several marker genes related with erythroid lineage in the absence of PI3K specific inhibitor, Wortmannin, indicating that PI3K/Akt signaling pathway also involved in the differentiation of the cell. Finally, as results of benzidine positive staining as well as hemoglobinization analysis, overexpression of TG2 revealed acceleration of the erythroid differentiation of K562 cells. Taken together, there was no increased TG2 expression level in the response of Hemin/tRA and delayed differentiation in vector transfected cells than in TG2-transfected cells, suggesting that suppression of TG2 expression may retard the erythroid differentiation of K562 cells. Therefore, our study may give a new insight for another aspect of the development of this disease.