Cell surface transglutaminase promotes RhoA activation via integrin clustering and suppression of the Src-p190RhoGAP signaling pathway

Dithiothreitol prevents the spontaneous release of cortical granules in in vitro aged mouse oocytes by protecting regulatory proteins of cortical granules exocytosis and thickening the cortical actin cytoskeleton

In assisted fertility protocols, in vitro culture conditions mimic physiological conditions to preserve gametes in the best conditions. After collection, oocytes are maintained in a culture medium inside the incubator until in vitro fertilization (IVF) is performed. This time outside natural and physiological conditions exposes oocytes to an oxidative stress that renders in vitro aging. It has been described that in vitro aging produces a spontaneous cortical granule (CG) release decreasing the fertilization rate of oocytes. Nevertheless, this undesirable phenomenon has not been investigated, let alone prevented. In this work, we characterized the spontaneous CG secretion in in vitro aged oocytes. Using immunofluorescence indirect, quantification, and functional assays, we showed that the expression of regulatory proteins of CG exocytosis was affected. Our results demonstrated that in vitro oocyte aging by 4 and 8 h altered the expression and localization of alpha-SNAP and reduced the expression of NSF and Complexin. These alterations were prevented by supplementing culture medium with dithiothreitol (DTT), which in addition to having a protective effect on those proteins, also had an unexpected effect on the actin cytoskeleton. Indeed, DTT addition thickened the cortical layer of fibrillar actin. Both DTT effects, together, prevented the spontaneous secretion of CG and recovered the IVF rate in in vitro aged oocytes. We propose the use of DTT in culture media to avoid the spontaneous CG secretion and to improve the success rate of IVF protocols in in vitro aged oocytes.

Shaping Oncogenic Microenvironments: Contribution of Fibronectin

The extracellular matrix (ECM) is a complex network of proteins and glycans, dynamically remodeled and specifically tailored to the structure/function of each organ. The malignant transformation of cancer cells is determined by both cell intrinsic properties, such as mutations, and extrinsic variables, such as the mixture of surrounding cells in the tumor microenvironment and the biophysics of the ECM. During cancer progression, the ECM undergoes extensive remodeling, characterized by disruption of the basal lamina, vascular endothelial cell invasion, and development of fibrosis in and around the tumor cells resulting in increased tissue stiffness. This enhanced rigidity leads to aberrant mechanotransduction and further malignant transformation potentiating the de-differentiation, proliferation and invasion of tumor cells. Interestingly, this fibrotic microenvironment is primarily secreted and assembled by non-cancerous cells. Among them, the cancer-associated fibroblasts (CAFs) play a central role. CAFs massively produce fibronectin together with type I collagen. This review delves into the primary interactions and signaling pathways through which fibronectin can support tumorigenesis and metastasis, aiming to provide critical molecular insights for better therapy response prediction.

Mesenchymal Transglutaminase 2 Activates Epithelial ADAM17: Link to G-Protein-Coupled Receptor 56 (ADGRG1) Signalling

A wound healing model was developed to elucidate the role of mesenchymal-matrix-associated transglutaminase 2 (TG2) in keratinocyte re-epithelialisation. TG2 drives keratinocyte migratory responses by activation of disintegrin and metalloproteinase 17 (ADAM17). We demonstrate that epidermal growth factor (EGF) receptor ligand shedding leads to EGFR-transactivation and subsequent rapid keratinocyte migration on TG2-positive ECM. In contrast, keratinocyte migration was impaired in TG2 null conditions. We show that keratinocytes express the adhesion G-protein-coupled receptor, ADGRG1 (GPR56), which has been proposed as a TG2 receptor. Using ADAM17 activation as a readout and luciferase reporter assays, we demonstrate that TG2 activates GPR56. GPR56 activation by TG2 reached the same level as observed with an agonistic N-GPR56 antibody. The N-terminal GPR56 domain is required for TG2-regulated signalling response, as the constitutively active C-GPR56 receptor was not activated by TG2. Signalling required the C-terminal TG2 β-barrel domains and involved RhoA-associated protein kinase (ROCK) and ADAM17 activation, which was blocked by specific inhibitors. Cell surface binding of TG2 to the N-terminal GPR56 domain is rapid and is associated with TG2 and GPR56 endocytosis. TG2 and GPR56 represent a ligand receptor pair causing RhoA and EGFR transactivation. Furthermore, we determined a binding constant for the interaction of human TG2 with N-GPR56 and show for the first time that only the calcium-enabled "open" TG2 conformation associates with N-GPR56.

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.

Transglutaminase 2 has higher affinity for relaxed than for stretched fibronectin fibers

Transglutaminase 2 (TG2) plays a vital role in stabilizing extracellular matrix (ECM) proteins through enzymatic crosslinking during tissue growth, repair, and inflammation. TG2 also binds non-covalently to fibronectin (FN), an essential component of the ECM, facilitating cell adhesion, migration, proliferation, and survival. However, the interaction between TG2 and fibrillar FN remains poorly understood, as most studies have focused on soluble or surface-adsorbed FN or FN fragments, which differ in their conformations from insoluble FN fibers. Using a well-established in vitro FN fiber stretch assay, we discovered that the binding of a crosslinking enzyme to ECM fibers is mechano-regulated. TG2 binding to FN is tuned by the mechanical tension of FN fibers, whereby TG2 predominantly co-localizes to low-tension FN fibers, while fiber stretching reduces their affinity for TG2. This mechano-regulated binding relies on the proximity between the N-terminal β-sandwich and C-terminal β-barrels of TG2. Crosslinking mass spectrometry (XL-MS) revealed a novel TG2-FN synergy site within TG2's C-terminal β-barrels that interacts with FN regions located outside of the canonical gelatin binding domain, specifically FNI2 and FNIII14-15. Combining XL-MS distance restraints with molecular docking revealed the mechano-regulated binding mechanism between TG2 and modules FNI7-9 by which mechanical forces regulate TG2-FN interactions. This highlights a previously unrecognized role of TG2 as a tension sensor for FN fibers. This novel interaction mechanism has significant implications in physiology and mechanobiology, including how forces regulate cell adhesion, spreading, migration, phenotype modulation, depending on the tensional state of ECM fibers. Data are available via ProteomeXchange with identifier PXD043976.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Role of Moonlighting Proteins in Disease: Analyzing the Contribution of Canonical and Moonlighting Functions in Disease Progression

The term moonlighting proteins refers to those proteins that present alternative functions performed by a single polypeptide chain acquired throughout evolution (called canonical and moonlighting, respectively). Over 78% of moonlighting proteins are involved in human diseases, 48% are targeted by current drugs, and over 25% of them are involved in the virulence of pathogenic microorganisms. These facts encouraged us to study the link between the functions of moonlighting proteins and disease. We found a large number of moonlighting functions activated by pathological conditions that are highly involved in disease development and progression. The factors that activate some moonlighting functions take place only in pathological conditions, such as specific cellular translocations or changes in protein structure. Some moonlighting functions are involved in disease promotion while others are involved in curbing it. The disease-impairing moonlighting functions attempt to restore the homeostasis, or to reduce the damage linked to the imbalance caused by the disease. The disease-promoting moonlighting functions primarily involve the immune system, mesenchyme cross-talk, or excessive tissue proliferation. We often find moonlighting functions linked to the canonical function in a pathological context. Moonlighting functions are especially coordinated in inflammation and cancer. Wound healing and epithelial to mesenchymal transition are very representative. They involve multiple moonlighting proteins with a different role in each phase of the process, contributing to the current-phase phenotype or promoting a phase switch, mitigating the damage or intensifying the remodeling. All of this implies a new level of complexity in the study of pathology genesis, progression, and treatment. The specific protein function involved in a patient's progress or that is affected by a drug must be elucidated for the correct treatment of diseases.

Tissue transglutaminase activates integrin-linked kinase and β-catenin in ovarian cancer

Ovarian cancer (OC) is the most lethal gynecological cancer. OC cells have high proliferative capacity, are invasive, resist apoptosis, and tumors often display rearrangement of extracellular matrix (ECM) components, contributing to accelerated tumor progression. The multifunctional protein tissue transglutaminase (TG2) is known to be secreted in the tumor microenvironment (TME), where it interacts with fibronectin (FN) and the cell surface receptor β1 integrin. However, the mechanistic role of TG2 in cancer cell proliferation is unknown. Here, we demonstrate TG2 directly interacts with and facilitates the phosphorylation and activation of the integrin effector protein integrin-linked kinase (ILK) at Ser246. We show TG2 and p-Ser246-ILK form a complex that is detectable in patient-derived OC primary cells grown on FN-coated slides. In addition, we show co-expression of TGM2 and ILK correlates with poor clinical outcome. Mechanistically, we demonstrate TG2-mediated ILK activation causes phosphorylation of glycogen synthase kinase-3α/β (GSK-3α/β), allowing β-catenin nuclear translocation and transcriptional activity. Furthermore, inhibition of TG2 and ILK using small molecules, neutralizing antibodies, or shRNA-mediated knockdown block cell adhesion to the FN matrix, as well as the Wnt receptor response to the Wnt-3A ligand, and ultimately, cell adhesion, growth, and migration. In conclusion, we demonstrate TG2 directly interacts with and activates ILK in OC cells and tumors, and define a new mechanism which links ECM cues with β-catenin signaling in OC. These results suggest a central role of TG2/FN/integrin clusters in ECM rearrangement and indicate downstream effector ILK may represent a potential new therapeutic target in OC.

Hypoxia and inflammation conditions differentially affect the expression of tissue transglutaminase spliced variants and functional properties of extravillous trophoblast cells

PROBLEM

Persistent hypoxia and inflammation beyond early pregnancy are involved in a bad outcome because of defective trophoblast invasiveness. Tissue transglutaminase (TG2) coregulates several cell functions. An aberrant expression and/or transamidation activity could contribute to placental dysfunction.

METHOD OF STUDY

The first-trimester trophoblast cell line (Swan-71) was used to study TG2 expression and cell functions in the absence or presence of inflammatory cytokines (TNF-α, IL-1β) or chemical hypoxia (CoCl₂ ). We analyzed The concentration of cytokines in the supernatant by ELISA; Cell migration by scratch assay; NF-κB activation by detection of nuclear p65 by immunofluorescence or flow cytometry using a Swan-71 NF-κB-hrGFP reporter cell line. Tissue transglutaminase expression was analyzed by immunoblot and confocal microscopy. Expression of spliced mRNA variants of tissue transglutaminase was analyzed by RT-PCR. Transamidation activity was assessed by flow cytometry using 5-(biotinamido)-pentylamine substrate.

RESULTS

Chemical hypoxia and TGase inhibition, but not inflammatory stimuli, decreased Swan-71 migration. IL-6 production was also decreased by chemical hypoxia, but increased by inflammation. Intracellular TGase activity was increased by all stimuli, but NF-κB activation was observed only in the presence of proinflammatory cytokines. TG2 expression was decreased by CoCl₂ and TNF-α. Translocation of TG2 and p65 to nuclei was observed only with TNF-α, without colocalization. Differential relative expression of spliced variants of mRNA was observed between CoCl₂ and inflammatory stimuli.

CONCLUSION

The observed decrease in total TG2 expression and relative increase in short variants under hypoxia conditions could contribute to impaired trophoblast invasion and impact on pregnancy outcome.

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.

Hyperglycemia-induced transcriptional regulation of ROCK1 and TGM2 expression is involved in small artery remodeling in obese diabetic Göttingen Minipigs

Obesity and diabetes in humans are associated with hypertrophic remodeling and increased media:lumen ratio of small resistance arteries, which is an independent predictor of cardiovascular events. In order to minimize increases in media:lumen ratio, hypertrophic remodeling should be accompanied by outward remodeling. We aimed to investigate the mechanisms of structural remodeling in small pial arteries (PAs) and terminal mesenteric arteries (TMAs) from obese Göttingen Minipigs with or without diabetes. Göttingen Minipigs received either control diet (lean control (LC)), high fat/high fructose/high cholesterol diet (FFC), or FFC diet with streptozotocin (STZ)-induced diabetes (FFC/STZ) for 13 months. At the end of the study (20 months), we assessed body weight, fasting plasma biochemistry, passive vessel dimensions, mRNA expression (matrix metallopeptidases 2/9 (MMP2, MMP9), tissue inhibitor of metallopeptidase 1 (TIMP1), transglutaminase 2 (TGM2), Rho-kinase 1 (ROCK1), TGFβ-receptor 2 (TGFBR2), and IGF1-receptor (IGFR1) genes), and immunofluorescence in PAs and TMAs. We performed multiple linear correlation analyses using plasma values, structural data, and gene expression data. We detected outward hypertrophic remodeling in TMAs and hypertrophic remodeling in PAs from FFC/STZ animals. ROCK1 and TGM2 genes were up-regulated in PAs and TMAs from the FFC/STZ group. Passive lumen diameter (PLD) of TMAs was correlated with plasma values of glucose (GLU), fructosamine (FRA), total cholesterol (TC), and triglycerides (TGs). ROCK1 and TGM2 expressions in TMAs were correlated with PLD, plasma GLU, fructosamine, and TC. ROCK1 and TGM2 proteins were immunolocalized in the media of PAs and TMAs, and their fluorescence levels were increased in the FFC/STZ group. Hyperglycemia/hyperlipidemia is involved in regulation of ROCK1 and TGM2 expression leading to outward remodeling of small resistance arteries in obese diabetic Göttingen Minipigs.

Loss of transglutaminase 2 sensitizes for diet-induced obesity-related inflammation and insulin resistance due to enhanced macrophage c-Src signaling

Transglutaminase 2 (TG2) is a multifunctional protein that promotes clearance of apoptotic cells (efferocytosis) acting as integrin β₃ coreceptor. Accumulating evidence indicates that defective efferocytosis contributes to the development of chronic inflammatory diseases. Obesity is characterized by the accumulation of dead adipocytes and inflammatory macrophages in the adipose tissue leading to obesity-related metabolic syndrome. Here, we report that loss of TG2 from bone marrow-derived cells sensitizes for high fat diet (HFD)-induced pathologies. We find that metabolically activated TG2 null macrophages express more phospho-Src and integrin β₃, unexpectedly clear dying adipocytes more efficiently via lysosomal exocytosis, but produce more pro-inflammatory cytokines than the wild type ones. Anti-inflammatory treatment with an LXR agonist reverts the HFD-induced phenotype in mice lacking TG2 in bone marrow-derived cells with less hepatic steatosis than in wild type mice proving enhanced lipid clearance. Thus it is interesting to speculate whether LXR agonist treatment together with enhancing lysosomal exocytosis could be a beneficial therapeutic strategy in obesity.

Small Molecules Target the Interaction between Tissue Transglutaminase and Fibronectin

Tissue transglutaminase (TG2) is a multifunctional protein with enzymatic, GTP-ase, and scaffold properties. TG2 interacts with fibronectin (FN) through its N-terminus domain, stabilizing integrin complexes, which regulate cell adhesion to the matrix. Through this mechanism, TG2 participates in key steps involved in metastasis in ovarian and other cancers. High-throughput screening identified several small molecule inhibitors (SMI) for the TG2/FN complex. Rational medicinal chemistry optimization of the hit compound (TG53) led to second-generation analogues (MT1-6). ELISA demonstrated that these analogues blocked TG2/FN interaction, and bio-layer interferometry (BLI) showed that the SMIs bound to TG2. The compounds also potently inhibited cancer cell adhesion to FN and decreased outside-in signaling mediated through the focal adhesion kinase. Blockade of TG2/FN interaction by the small molecules caused membrane ruffling, delaying the formation of stable focal contacts and mature adhesions points and disrupted organization of the actin cytoskeleton. In an in vivo model measuring intraperitoneal dissemination, MT4 and MT6 inhibited the adhesion of ovarian cancer cells to the peritoneum. Pretreatment with MT4 also sensitized ovarian cancer cells to paclitaxel. The data support continued optimization of the new class of SMIs that block the TG2/FN complex at the interface between cancer cells and the tumor niche.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Transglutaminase 2: The Maestro of the Oncogenic Mediators in Renal Cell Carcinoma

Transglutaminase 2 (TG2) is a multifunctional crosslinking enzyme that displays transamidation, protein disulfide isomerase, protein kinase, as well as GTPase and ATPase activities. TG2 can also act as an adhesion molecule involved in the syndecan and integrin receptor signaling. In recent years, TG2 was implicated in cancer progression, survival, invasion, migration, and stemness of many cancer types, including renal cell carcinoma (RCC). Von Hippel-Lindau mutations leading to the subsequent activation of Hypoxia Inducible Factor (HIF)-1-mediated signaling pathways, survival signaling via the PI3K/Akt pathway resulting in Epithelial Mesenchymal Transition (EMT) metastasis and angiogenesis are the main factors in RCC progression. A number of studies have shown that TG2 was important in HIF-1- and PI3K-mediated signaling, VHL and p53 stabilization, glycolytic metabolism and migratory phenotype in RCC. This review focuses on the role of TG2 in the regulation of molecular pathways nurturing not only the development and propagation of RCC, but also drug-resistance and metastatic potential.

The Role of Tissue Transglutaminase in Cancer Cell Initiation, Survival and Progression

Tissue transglutaminase (transglutaminase type 2; TG2) is the most ubiquitously expressed member of the transglutaminase family (EC 2.3.2.13) that catalyzes specific post-translational modifications of proteins through a calcium-dependent acyl-transfer reaction (transamidation). In addition, this enzyme displays multiple additional enzymatic activities, such as guanine nucleotide binding and hydrolysis, protein kinase, disulfide isomerase activities, and is involved in cell adhesion. Transglutaminase 2 has been reported as one of key enzymes that is involved in all stages of carcinogenesis; the molecular mechanisms of action and physiopathological effects depend on its expression or activities, cellular localization, and specific cancer model. Since it has been reported as both a potential tumor suppressor and a tumor-promoting factor, the role of this enzyme in cancer is still controversial. Indeed, TG2 overexpression has been frequently associated with cancer stem cells’ survival, inflammation, metastatic spread, and drug resistance. On the other hand, the use of inducers of TG2 transamidating activity seems to inhibit tumor cell plasticity and invasion. This review covers the extensive and rapidly growing field of the role of TG2 in cancer stem cells survival and epithelial–mesenchymal transition, apoptosis and differentiation, and formation of aggressive metastatic phenotypes.

Mechanistic role of transglutaminase-2 in focal adhesions

Transglutaminase (TG)-2 interacts with matrix proteins and integrins, forming focal adhesions (FA) to initiate cell migration, thus playing a vital role in wound healing. Previously we showed that TG-2 influenced phosphorylation of paxillin and other FA proteins. Here, we aimed to investigate the molecular mechanism of TG-2 regulation of paxillin. Human corneal epithelial cells expressing shRNA against TG-2 (shTG) and scrambled sequence control (shRNA) were cultured. TG-2 was pulled down by anti-paxillin antibody, but not MAP3K12. Cell-free interaction assay with immobilized paxillin shows that TG-2 bind to paxillin directly. JNK was the strongest kinase for paxillin phosphorylation in the in-vitro kinase screen, but TG-2 could not phosphorylate paxillin directly. Increasing TG-2 concentrations did not increase the amount of JNK in the TG-2/paxillin complex. Immunofluoresent staining shows that TG-2 colocalises with vinculin and paxillin in FA of migrating cells. TG-2 binds to paxillin and JNK-containing FA but does not recruit JNK directly. Taken together with previous findings, TG-2 binds paxillin non-covalently, and JNK can phosphorylate paxillin, these processes critically regulate corneal epithelial adhesion and migration.

Targeting the Microenvironment in High Grade Serous Ovarian Cancer

Cancer⁻stroma interactions play a key role in cancer progression and response to standard chemotherapy. Here, we provide a summary of the mechanisms by which the major cellular components of the ovarian cancer (OC) tumor microenvironment (TME) including cancer-associated fibroblasts (CAFs), myeloid, immune, endothelial, and mesothelial cells potentiate cancer progression. High-grade serous ovarian cancer (HGSOC) is characterized by a pro-inflammatory and angiogenic signature. This profile is correlated with clinical outcomes and can be a target for therapy. Accumulation of malignant ascites in the peritoneal cavity allows for secreted factors to fuel paracrine and autocrine circuits that augment cancer cell proliferation and invasiveness. Adhesion of cancer cells to the mesothelial matrix promotes peritoneal tumor dissemination and represents another attractive target to prevent metastasis. The immunosuppressed tumor milieu of HGSOC is permissive for tumor growth and can be modulated therapeutically. Results of emerging preclinical and clinical trials testing TME-modulating therapeutics for the treatment of OC are highlighted.

Transglutaminase activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics

Osteoclasts, bone resorbing cells, derive from monocyte/macrophage cell lineage. Increased osteoclast activity is responsible for bone destruction in diseases such as osteoporosis, periodontitis and rheumatoid arthritis. Transglutaminases (TGs), protein crosslinking enzymes, were recently found involved in osteoclastogenesis in vivo, however their mechanisms of action have remained unknown. In this study, we have investigated the role of TG activity in osteoclastogenesis in vitro using four TG inhibitors, NC9, Z006, T101, and monodansyl cadaverine. Our results showed that all TG inhibitors were capable of blocking the entire osteoclastogenesis process. The most potent of the inhibitors, NC9 when added to cultures at different phases of osteoclastogenesis, inhibited differentiation, migration, and fusion of pre-osteoclasts as well as resorption activity of mature osteoclasts. Further investigation into the mechanisms revealed that NC9 increased RhoA levels and blocked podosome belt formation suggesting that TG activity regulates actin dynamics in pre-osteoclasts. The inhibitory effect of NC9 on osteoclastogenesis as well as podosome belt formation was completely reversed with a Rho-family inhibitor Exoenzyme C3. Microtubule architecture, acetylation, and detyrosination of α-tubulin were not affected. Finally, we demonstrated that macrophages and osteoclasts expressed mRNA of three TGs:TG1, TG2, and Factor XIII-A which were all differentially regulated in these cells during differentiation. Immunofluoresence microscopic analysis showed that all three enzymes co-localized to podosomes in osteoclasts. Taken together, our data suggests that TG activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics and that this may involve contribution from all three TG enzymes.

Transglutaminase 2 in human diseases

Transglutaminase 2 (TG2) is an inducible transamidating acyltransferase that catalyzes Ca(2+)-dependent protein modifications. In addition to being an enzyme, TG2 also serves as a G protein for several seven transmembrane receptors and acts as a co-receptor for integrin β1 and β3 integrins distinguishing it from other members of the transglutaminase family. TG2 is ubiquitously expressed in almost all cell types and all cell compartments, and is also present on the cell surface and gets secreted to the extracellular matrix via non-classical mechanisms. TG2 has been associated with various human diseases including inflammation, cancer, fibrosis, cardiovascular disease, neurodegenerative diseases, celiac disease in which it plays either a protective role, or contributes to the pathogenesis. Thus modulating the biological activities of TG2 in these diseases will have a therapeutic value.

Tissue transglutaminase expression is necessary for adhesion, metastatic potential and cancer stemness of renal cell carcinoma

Tissue transglutaminase (TG2) is the ubiquitously expressed member of transglutaminase family and shown to play a critical role in the development and progression of drug resistance malignancies. We have previously showed the association of TG2 upregulation with progression and metastasis of renal cell carcinoma (RCC) and low disease-free survival. In the present study we further investigate the role of TG2 in cell adhesion, migration and invasion of RCC by silencing TG2 expression in Caki-2 and A-498 primary site and Caki-1 and ACHN metastatic site RCC cell lines. Downregulation of TG2 expression led up to a 60% decrease in actin stress fiber formation and adhesion to β 1 integrin (ITGB1) substrates fibronectin, collagen type I and laminin in both primary and metastatic site RCC cell lines. In addition, treatment with siRNAs against TG2 impaired the migration capacity and cellular invasiveness of ITGB1 substrates in all 4 RCC cell lines. Lastly, the knockdown of TG2 in metastatic Caki-1 cells diminished the expression of CD44, CD73-and CD105 cancer stem cell-like markers. We conclude, for the first time, that TG2 expression is critical for cancer cell adhesion, migration, invasiveness and cancer cell-stemness during RCC progression and dissemination. Therefore, combined targeting of TG2 with drugs widely used in the treatment of RCC may be a promising therapeutic strategy for RCC.

Lipopolysaccharide and Curcumin Co-Stimulation Potentiates Olfactory Ensheathing Cell Phagocytosis Via Enhancing Their Activation

The gradual deterioration following central nervous system (CNS) injuries or neurodegenerative disorders is usually accompanied by infiltration of degenerated and apoptotic neural tissue debris. A rapid and efficient clearance of these deteriorated cell products is of pivotal importance in creating a permissive environment for regeneration of those damaged neurons. Our recent report revealed that the phagocytic activity of olfactory ensheathing cells (OECs) can make a substantial contribution to neuronal growth in such a hostile environment. However, little is known about how to further increase the ability of OECs in phagocytosing deleterious products. Here, we used an in vitro model of primary cells to investigate the effects of lipopolysaccharide (LPS) and curcumin (CCM) co-stimulation on phagocytic activity of OECs and the possible underlying mechanisms. Our results showed that co-stimulation using LPS and CCM can significantly enhance the activation of OECs, displaying a remarkable up-regulation in chemokine (C-X-C motif) ligand 1, chemokine (C-X-C motif) ligand 2, tumor necrosis factor-α, and Toll-like receptor 4, increased OEC proliferative activity, and improved phagocytic capacity compared with normal and LPS- or CCM-treated OECs. More importantly, this potentiated phagocytosis activity greatly facilitated neuronal growth under hostile culture conditions. Moreover, the up-regulation of transglutaminase-2 and phosphatidylserine receptor in OECs activated by LPS and CCM co-stimulation are likely responsible for mechanisms underlying the observed cellular events, because cystamine (a specific inhibitor of transglutaminase-2) and neutrophil elastase (a cleavage enzyme of phosphatidylserine receptor) can effectively abrogate all the positive effects of OECs, including phagocytic capacity and promotive effects on neuronal growth. This study provides an alternative strategy for the repair of traumatic nerve injury and neurologic diseases with the application of OECs in combination with LPS and CCM.

Tissue transglutaminase-2 promotes gastric cancer progression via the ERK1/2 pathway

Gastric cancer (GC) is one of the most common tumors worldwide and involves extensive local tumor invasion, metastasis, and poor prognosis. Understanding mechanisms regulating progression of GC is necessary for developing effective therapeutic strategies. Tissue transglutaminase-2 (TG2), a multifunctional member of the transglutaminase family, has been shown to be critical for tumor initiation and progression. However, how TG2 promotes the progression of GC is unknown. We report that TG2 was highly expressed in GC tissues and positively associated with depth of tumor invasion and late TNM stage. With gain- and loss-of-function approaches, we observed that TG2 promoted GC cell proliferation, migration, invasion, as well as tumorigenesis and peritoneal metastasis in vivo. These events were associated with the ERK1/2 pathway activation and an ERK1/2 inhibitor (U0126) inhibited cell proliferation, migration, and invasion induced by overexpression of TG2. In summary, TG2 contributes to tumorigenesis and progression of GC by activating the ERK1/2 signaling pathway and is a potential therapeutic target of metastatic gastric cancer.

Molecular Cues Guiding Matrix Stiffness in Liver Fibrosis

Tissue and matrix stiffness affect cell properties during morphogenesis, cell growth, differentiation, and migration and are altered in the tissue remodeling following injury and the pathological progression. However, detailed molecular mechanisms underlying alterations of stiffness in vivo are still poorly understood. Recent engineering technologies have developed powerful techniques to characterize the mechanical properties of cell and matrix at nanoscale levels. Extracellular matrix (ECM) influences mechanical tension and activation of pathogenic signaling during the development of chronic fibrotic diseases. In this short review, we will focus on the present knowledge of the mechanisms of how ECM stiffness is regulated during the development of liver fibrosis and the molecules involved in ECM stiffness as a potential therapeutic target for liver fibrosis.

Inhibition of transglutaminase 2 reduces efferocytosis in human macrophages: Role of CD14 and SR-AI receptors

BACKGROUND AND AIMS

Transglutaminase 2 (TGM2), a member of the transglutaminase family of enzymes, is a multifunctional protein involved in numerous events spanning from cell differentiation, to signal transduction, apoptosis, and wound healing. It is expressed in a variety of cells, macrophages included. Macrophage TGM2 promotes the clearance of apoptotic cells (efferocytosis) and emerging evidence suggests that defective efferocytosis contributes to the consequences of inflammation-associated diseases, including atherosclerotic lesion progression and its sequelae. Of interest, active TGM2 identified in human atherosclerotic lesions plays critical roles in plaque stability through effects on matrix cross-linking and TGFβ activity. This study explores the mechanisms by which TGM2 controls efferocytosis in human macrophages.

METHODS AND RESULTS

Herein we show that TGM2 increases progressively during monocyte differentiation towards macrophages and controls their efferocytic potential as well as morphology and viability. Two experimental approaches that took advantage of the inhibition of TGM2 activity and protein silencing give proof that TGM2 reduction significantly impairs macrophage efferocytosis. Among the mechanisms involved we highlighted a role of the receptors CD14 and SR-AI whose levels were markedly reduced by TGM2 inhibition. Conversely, CD36 receptor and αvβ3 integrin levels were not influenced. Of note, lipid accumulation and IL-10 secretion were reduced in macrophages displaying defective efferocytosis.

CONCLUSION

Overall, our data define a crucial role of TGM2 activity during macrophage differentiation via mechanisms involving CD14 and SR-AI receptors and show that TGM2 inhibition triggers a pro-inflammatory phenotype.

Is monocyte- and macrophage-derived tissue transglutaminase involved in inflammatory processes?

Monocytes and macrophages are key players in inflammatory processes following an infection or tissue damage. Monocytes adhere and extravasate into the inflamed tissue, differentiate into macrophages, and produce inflammatory mediators to combat the pathogens. In addition, they take up dead cells and debris and, therefore, take part in the resolution of inflammation. The multifunctional enzyme tissue Transglutaminase (TG2, tTG) is known to participate in most of those monocyte- and macrophage-mediated processes. Moreover, TG2 expression and activity can be regulated by inflammatory mediators. In the present review, we selectively elaborate on the expression, regulation, and contribution of TG2 derived from monocytes and macrophages to inflammatory processes mediated by those cells. In addition, we discuss the role of TG2 in certain pathological conditions, in which inflammation and monocytes and/or macrophages are prominently present, including atherosclerosis, sepsis, and multiple sclerosis. Based on the studies and considerations reported in this review, we conclude that monocyte- and macrophage-derived TG2 is clearly involved in various processes contributing to inflammation. However, TG2’s potential as a therapeutic target to counteract the possible detrimental effects or stimulate the potential beneficial effects on monocyte and macrophage responses during inflammation should be carefully considered. Alternatively, as TG2-related parameters can be used as a marker of disease, e.g., in celiac disease, or of disease-stage, e.g., in cancer, we put forward that this could be subject of research for monocyte- or macrophage-derived TG2 in inflammatory diseases.

Transglutaminase 2--a novel inhibitor of adipogenesis

Differentiation of preadipocytes to lipid storing adipocytes involves extracellular signaling pathways, matrix remodeling and cytoskeletal changes. A number of factors have been implicated in maintaining the preadipocyte state and preventing their differentiation to adipocytes. We have previously reported that a multifunctional and protein crosslinking enzyme, transglutaminase 2 (TG2) is present in white adipose tissue. In this study, we have investigated TG2 function during adipocyte differentiation. We show that TG2 deficient mouse embryonic fibroblasts (Tgm2-/- MEFs) display increased and accelerated lipid accumulation due to increased expression of major adipogenic transcription factors, PPARγ and C/EBPα. Examination of Pref-1/Dlk1, an early negative regulator of adipogenesis, showed that the Pref-1/Dlk1 protein was completely absent in Tgm2-/- MEFs during early differentiation. Similarly, Tgm2-/- MEFs displayed defective canonical Wnt/β-catenin signaling with reduced β-catenin nuclear translocation. TG2 deficiency also resulted in reduced ROCK kinase activity, actin stress fiber formation and increased Akt phosphorylation in MEFs, but did not alter fibronectin matrix levels or solubility. TG2 protein levels were unaltered during adipogenic differentiation, and was found predominantly in the extracellular compartment of MEFs and mouse WAT. Addition of exogenous TG2 to Tgm2+/+ and Tgm2-/- MEFs significantly inhibited lipid accumulation, reduced expression of PPARγ and C/EBPα, promoted the nuclear accumulation of β-catenin, and recovered Pref-1/Dlk1 protein levels. Our study identifies TG2 as a novel negative regulator of adipogenesis.

Cellular Factor XIIIA Transglutaminase Localizes in Caveolae and Regulates Caveolin-1 Phosphorylation, Homo-oligomerization and c-Src Signaling in Osteoblasts

Transglutaminases (TGs) are a family of widely distributed enzymes that catalyze protein crosslinking by forming a covalent isopeptide bond between the substrate proteins. We have shown that MC3T3-E1 osteoblasts express Factor XIII-A (FXIII-A), and that the extracellular crosslinking activity of FXIII-A is involved in regulating matrix secretion and deposition. In this study, we have investigated the localization and potential role of intracellular FXIII-A. Conventional immunofluorescence microscopy and TIRF microscopy analyses showed that FXIII-A co-localizes with caveolin-1 in specialized membrane structures, caveolae, in differentiating osteoblasts. The caveolae-disrupting agent methyl-β-cyclodextrin abolished FXIII-A staining and co-localization with caveolin-1 from the osteoblast plasma membrane. The presence of FXIII-A in caveolae was confirmed by preparing caveolae-enriched cellular fractions using sucrose density gradient ultracentrifugation followed by western blotting. Despite this association of FXIII-A with caveolae, there was no detectable transglutaminase activity in caveolae, as measured by monodansylcadaverine incorporation. TG inhibitor NC9--which can alter TG enzyme conformation--localized to caveolae and displaced FXIII-A from these structures when added to the osteoblast cultures. The decreased FXIII-A levels in caveolae after NC9 treatment increased c-Src activation, which resulted in caveolin-1 phosphorylation, homo-oligomerization and Akt phosphorylation, suggesting cellular FXIII-A has a role in regulating c-Src signaling in osteoblasts.

Transglutaminase as a therapeutic target for celiac disease

INTRODUCTION

The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development.

AREAS COVERED

The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach.

EXPERT OPINION

Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

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.

Celiac disease patient IgA antibodies induce endothelial adhesion and cell polarization defects via extracellular transglutaminase 2

We have recently found that celiac disease patient serum-derived autoantibodies targeted against transglutaminase 2 interfere with several steps of angiogenesis, including endothelial sprouting and migration, though the mechanism involved remained to be fully characterized. This study now investigated the processes underlying the antiangiogenic effects exerted by celiac disease patient antibodies on endothelial cells, with particular regard to the adhesion, migration, and polarization signaling pathway. We observed that celiac IgA reduced endothelial cell numbers by affecting adhesion without increasing apoptosis. Endothelial cells in the presence of celiac IgA showed weak attachment, a high susceptibility to detach from fibronectin, and a disorganized extracellular matrix due to a reduction of protein cross-links. Furthermore, celiac patient IgA led to secretion of active transglutaminase 2 from endothelial cells into the culture supernatants. Additionally, cell surface transglutaminase 2 mediated integrin clustering in the presence of celiac IgA was coupled to augmented expression of β1-integrin. We also observed that celiac patient IgA-treated endothelial cells had migratory defects and a less polarized phenotype when compared to control groups, and this was associated with the RhoA signaling pathway. These biological effects mediated by celiac IgA on endothelial cells were partially influenced but not completely abolished by R281, an irreversible extracellular transglutaminase 2 enzymatic activity inhibitor. Taken together, our results imply that celiac patient IgA antibodies disturb the extracellular protein cross-linking function of transglutaminase 2, thus altering cell-extracellular matrix interactions and thereby affecting endothelial cell adhesion, polarization, and motility.

Transglutaminase 2 on the surface of dendritic cells is proposed to be involved in dendritic cell-T cell interaction

Transglutaminase 2 (TG2) is a ubiquitous enzyme involved in diverse biological processes. Recently, its function in adaptive immune responses has begun to emerge. Its presence and functions in B cells and T cells, for example, have been reported. However, those in dendritic cells (DCs), the principal antigen-presenting cells, are as yet unexplored in murine system. In this study, we first investigated the expression of TG2 in murine bone marrow-derived DCs, and then compared the functioning of these cells in the presence or absence of this enzyme using wild-type (WT) and TG2(-/-) mice. We found that the WT DCs expressed TG2 both in the cytoplasm and on the cell surface, both of which were elevated after LPS stimulation. Unexpectedly, between WT and TG2(-/-) DCs, there were no remarkable differences in cytokine secretion, IL-10 and IL-12, and neither in the expression of surface molecules CD80, CD86, and MHC II, excepting a moderate decrease of CD40 expression on the TG2(-/-) DCs. However, when T cells were stimulated with TG2(-/-) DCs, they showed decreased levels of proliferation, CD69 and CD25 expression, and IFN-γ secretion. The addition of anti-TG2 antibody to the WT DC-T cell co-culture resulted in decreased T cell activation. By immunofluorescence staining, TG2 was observed at DC-T cell interface (contact point). Taken together, we propose that TG2 on the surface of DCs modulates the DC-T cell interaction.

Biological functionalities of transglutaminase 2 and the possibility of its compensation by other members of the transglutaminase family

Transglutaminase 2 (TG2) is the most widely distributed and most abundantly expressed member of the transglutaminase family of enzymes, a group of intracellular and extracellular proteins that catalyze the Ca²⁺-dependent posttranslational modification of proteins. It is a unique member of the transglutaminase family owing to its specialized biochemical, structural and functional elements, ubiquitous tissue distribution and subcellular localization, and substrate specificity. The broad substrate specificity of TG2 and its flexible interaction with numerous other gene products may account for its multiple biological functions. In addition to the classic Ca²⁺-dependent transamidation of proteins, which is a hallmark of transglutaminase enzymes, additional Ca²⁺-independent enzymatic and nonenzymatic activities of TG2 have been identified. Many such activities have been directly or indirectly implicated in diverse cellular physiological events, including cell growth and differentiation, cell adhesion and morphology, extracellular matrix stabilization, wound healing, cellular development, receptor-mediated endocytosis, apoptosis, and disease pathology. Given the wide range of activities of the transglutaminase gene family it has been suggested that, in the absence of active versions of TG2, its function could be compensated for by other members of the transglutaminase family. It is in the light of this assertion that we review, herein, TG2 activities and the possibilities and premises for compensation for its absence.

Celiac anti-type 2 transglutaminase antibodies induce phosphoproteome modification in intestinal epithelial Caco-2 cells

BACKGROUND

Celiac disease is an inflammatory condition of the small intestine that affects genetically predisposed individuals after dietary wheat gliadin ingestion. Type 2-transglutaminase (TG2) activity seems to be responsible for a strong autoimmune response in celiac disease, TG2 being the main autoantigen. Several studies support the concept that celiac anti-TG2 antibodies may contribute to disease pathogenesis. Our recent findings on the ability of anti-TG2 antibodies to induce a rapid intracellular mobilization of calcium ions, as well as extracellular signal-regulated kinase phosphorylation, suggest that they potentially act as signaling molecules. In line with this concept, we have investigated whether anti-TG2 antibodies can induce phosphoproteome modification in an intestinal epithelial cell line.

METHODS AND PRINCIPAL FINDINGS

We studied phosphoproteome modification in Caco-2 cells treated with recombinant celiac anti-TG2 antibodies. We performed a two-dimensional electrophoresis followed by specific staining of phosphoproteins and mass spectrometry analysis of differentially phosphorylated proteins. Of 14 identified proteins (excluding two uncharacterized proteins), three were hypophosphorylated and nine were hyperphosphorylated. Bioinformatics analyses confirmed the presence of phosphorylation sites in all the identified proteins and highlighted their involvement in several fundamental biological processes, such as cell cycle progression, cell stress response, cytoskeletal organization and apoptosis.

CONCLUSIONS

Identification of differentially phosphorylated proteins downstream of TG2-antibody stimulation suggests that in Caco-2 cells these antibodies perturb cell homeostasis by behaving as signaling molecules. We hypothesize that anti-TG2 autoantibodies may destabilize the integrity of the intestinal mucosa in celiac individuals, thus contributing to celiac disease establishment and progression. Since several proteins here identified in this study were already known as TG2 substrates, we can also suppose that transamidating activity and differential phosphorylation of the same targets may represent a novel regulatory mechanism whose relevance in celiac disease pathogenesis is still unexplored.

The obligatory role of the actin cytoskeleton on inward remodeling induced by dithiothreitol activation of endogenous transglutaminase in isolated arterioles

Inward remodeling is the most prevalent structural change found in the resistance arteries and arterioles of hypertensive individuals. Separate studies have shown that the inward remodeling process requires transglutaminase activation and the polymerization of actin. Therefore, we hypothesize that inward remodeling induced via endogenous transglutaminase activation requires and depends on actin cytoskeletal structures. To test this hypothesis, isolated and cannulated rat cremaster arterioles were exposed to dithiothreitol (DTT) to activate endogenous transglutaminases. DTT induced concentration-dependent vasoconstriction that was suppressed by coincubation with cystamine or cytochalasin-D to inhibit tranglutaminase activity or actin polymerization, respectively. Prolonged (4 h) exposure to DTT caused arteriolar inward remodeling that was also blocked by the presence of cystamine or cytochalasin-D. DTT inwardly remodeled arterioles had reduced passive diameters, augmented wall thickness-to-lumen ratios and altered elastic characteristics that were reverted upon disruption of the actin cytoskeleton with mycalolide-B. In freshly isolated arterioles, exposure to mycalolide-B caused no changes in their passive diameters or their elastic characteristics. These results suggest that, in arterioles, the early stages of the inward remodeling process induced by prolonged endogenous transglutaminase activation require actin dynamics and depend on changes in actin cytoskeletal structures.

Transglutaminase-2 in cell adhesion: all roads lead to paxillin?

Cell-matrix adhesion is a fundamental biological process that governs survival, migration, and proliferation of living eukaryotic cells. Paxillin is an important central player in a network of adhesome proteins that form focal adhesion complexes. Phosphorylation of tyrosine and serine residues in paxillin is critical for the coordinated sequential recruitment of other adaptor and kinase proteins to adhesion complexes. Recently, the phosphorylation of serine178 in paxillin has been shown to be vital for epithelial cell adhesion and migration. In vivo and in vitro evidence have shown that transglutaminase (TG)-2 positively regulates this phosphorylation. Here, we propose three possible mechanisms that may explain these observations. First, TG-2 itself may be an adhesome member directly interacting with paxillin in a non-covalent way. Second, TG-2 may cross link a mitogen-activated protein kinase kinase kinase (MAP3K), which eventually activates c-Jun N-terminal kinase (JNK), and the latter phosphorylates paxillin. Lastly, TG-2 may have intrinsic kinase activity that phosphorylates paxillin. Future studies investigating these hypotheses on TG-2-paxillin relationships are necessary in order to address this fundamental process in cell matrix adhesion signaling.

Molecular mechanism of transglutaminase-2 in corneal epithelial migration and adhesion

Migration of cells in the ocular surface underpins physiological wound healing as well as many human diseases. Transglutaminase (TG)-2 is a multifunctional cross-linking enzyme involved in the migration of skin fibroblasts and wound healing, however, its functional role in epithelial migration has not been evaluated. This study investigated the importance of TG-2 in a murine corneal wound healing model as well as the mechanistic role of TG-2 in the regulation of related biological processes such as cell adhesion and migration of cultured human corneal epithelial (HCE-T) cells. Corneal wound closure was delayed in homozygous TG-2 deleted mice compared to wild type mice. HCE-T cells that were knocked-down for TG-2 expression through stable expression of a short-hairpin (sh) RNA targeting TG-2, were delayed in closure of scratch wounds (48 compared to 12h in control cells expressing scrambled shRNA). TG-2 knockdown did not influence epithelial cell cycle progression or proliferation, rather, it led to reduced epithelial cell adhesion, spreading and velocity of migration. At the molecular level, TG-2 knockdown reduced phosphorylation of β-3 integrin at Tyr747, paxillin at Ser178, vinculin at Tyr822 and focal adhesion kinase at Tyr925 simultaneous with reduced activation of Rac and CDC42. Phosphorylation of paxillin at Ser178A has been shown to be indispensable for the migration of corneal epithelial cells (Kimura et al., 2008) [18]. TG-2 dependent β-3 integrin activation, serine-phosphorylation of paxillin, and Rac and CDC42 activation may thus play a key functional role in enhancing corneal epithelial cell adhesion and migration during wound healing.

Transglutaminase 2 accelerates vascular calcification in chronic kidney disease

BACKGROUND

Transglutaminase 2 (TGM2) is a calcium-dependent enzyme that can cross-link nearly all extracellular matrix (ECM) proteins and can facilitate cell-ECM interaction through integrins. Given the importance of the ECM in vascular calcification we tested the hypothesis that increased TGM2 activity may accelerate vascular calcification in chronic kidney disease (CKD).

METHODS

We utilized thoracic aortas and vascular smooth muscle cells (VSMC) from the Cy/+ rat, a model of progressive CKD that develops arterial calcification on a normal phosphorus diet, compared to normal rats.

RESULTS

VSMC isolated from CKD rats had increased expression and activity of TGM2 compared to cells from normal rats. The increased calcification and expression of alkaline phosphatase activity observed in VSMC from CKD rats compared to normal was inhibited in a dose-dependent manner with the TGM inhibitors cystamine and Z006. Matrix vesicles (MV) from CKD rat VSMC also had increased TGM2 expression and the calcification of MV on type I collagen could be inhibited with cystamine and accelerated by exogenous cross-linking of fibronectin or type I collagen with TGM2. Finally, the calcification of aorta rings from CKD rats in ex vivo cultures was inhibited with TGM2 inhibitor.

CONCLUSION

These data demonstrate a role of TGM2 in the pathogenesis of vascular calcification in CKD through enhancement of MV-ECM calcification.

Activation of Extracellular Transglutaminase 2 by Mechanical Force in the Arterial Wall

Inward remodeling of small arteries occurs after prolonged vasoconstriction, low blood flow, and in several models of hypertension. The cross-linking enzyme, transglutaminases 2 (TG2), is able to induce inward remodeling and stiffening of arteries. The activity of TG2 is dependent on its conformation, which can be open or closed, and on its redox state. Several factors have been shown to be involved in modulating TG2 activity, including Ca²⁺ and GTP/GDP concentrations, as well as the redox state of the environment. This review introduces the hypothesis that mechanical force could be involved in regulating the activity of TG2 during inward remodeling by promoting its open and reduced active state. Several aspects of TG2, such as its structure and localization, are assessed in order to provide arguments that support the hypothesis. We conclude that a direct activation of TG2 by mechanical force exerted by smooth muscle cells may explain the link between smooth muscle activation and inward remodeling, as observed in several physiological and pathological conditions.

Rotavirus infection of cells in culture induces activation of RhoA and changes in the actin and tubulin cytoskeleton

Rotavirus infection induces an increase in [Ca²⁺]_cyto, which in turn may affect the distribution of the cytoskeleton proteins in the infected cell. Changes in microfilaments, including the formation of stress fibers, were observed starting at 0.5 h.p.i. using fluorescent phalloidin. Western blot analysis indicated that RhoA is activated between 0.5 and 1 h.p.i. Neither the phosphorylation of RhoA nor the formation of stress fibers were observed in cells infected with virions pre-treated with an anti-VP5* non-neutralizing mAb, suggesting that RhoA activation is stimulated by the interaction of the virus with integrins forming the cell receptor complex. In addition, the structure of the tubulin cytoskeleton was also studied. Alterations of the microtubules were evident starting at 3 h.p.i. and by 7 h.p.i. when microtubules were markedly displaced toward the periphery of the cell cytoplasm. Loading of rotavirus-infected cells with either a Ca²⁺ chelator (BAPTA) or transfection with siRNAs to silence NSP4, reversed the changes observed in both the microfilaments and microtubules distribution, but not the appearance of stress fibers. These results indicate that alterations in the distribution of actin microfilaments are initiated early during infection by the activation of RhoA, and that latter changes in the Ca²⁺ homeostasis promoted by NSP4 during infection may be responsible for other alterations in the actin and tubulin cytoskeleton.

RhoB is associated with the anti-angiogenic effects of celiac patient transglutaminase 2-targeted autoantibodies

Celiac patient-derived anti-transglutaminase 2 (TG2) antibodies disturb several steps in angiogenesis, but the detailed molecular basis is not known. Therefore, we here analyzed by microarray technology the expression of a set of genes related to angiogenesis and endothelial cell biology in order to identify factors that could explain our previous data related to vascular biology in the context of celiac disease. To this end, in vitro models using human umbilical vein endothelial cells (HUVECs) or in vivo models of angiogenesis were used. A total of 116 genes were analyzed after treatment with celiac patient autoantibodies against TG2. Compared to treatment with control IgA celiac patient, total IgA induced a consistent expression change of 10 genes, the up-regulation of four and down-regulation of six. Of these genes the up-regulated RhoB was selected for further studies. RhoB expression was found to be up-regulated at both messenger RNA and protein level in response to celiac patient total IgA as well as anti-TG2-specific antibody derived from a celiac patient. Interestingly, down-regulation of RhoB by specific small interfering RNA treatment in endothelial cells could rescue the deranged endothelial length and tubule formation caused by celiac disease autoantibodies. RhoB function is controlled by its post-translational modification by farnesylation. This modification of RhoB required for its correct function can be prevented by the cholesterol lowering drug simvastatin, which was also able to abolish the anti-angiogenic effects of celiac anti-TG2 autoantibodies. Taken together, our results would suggest that RhoB plays a key role in the response of endothelial cells to celiac disease-specific anti-TG2 autoantibodies.

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.

Transglutaminase 2 as a biomarker of osteoarthritis: an update

Osteoarthritis is a progressive joint disease characterized by cartilage degradation and bone remodelling. Under physiologic conditions, articular cartilage displays a stable chondrocyte phenotype, whereas in osteoarthritis a chondrocyte hypertrophy develops near the sites of cartilage surface damage and associates to the pathologic expression of type X collagen. Transglutaminases (TGs) include a family of Ca(2+)-dependent enzymes that catalyze the formation of γ-glutamyl cross-links. Their substrates include a variety of intracellular and extracellular macromolecular components. TGs are ubiquitously and abundantly expressed and implicated in a variety of physiopathological processes. TGs activity is modulated by inflammatory cytokines. TG2 (also known as tissue transglutaminase) mediates the hypertrophic differentiation of joint chondrocytes and interleukin-1-induced calcification. Histomorphometrical and biomolecular investigations document increased TG2 expression in human and experimental osteoarthritis. Consequently, the level of TG2 expression may represent an adjuvant additional marker to monitor tissue remodelling occurring in osteoarthritic joint tissue. Experimental induction of osteoarthritis in TG2 knockout mice is followed from reduced cartilage destruction and increased osteophyte formation compared to wild-type mice, suggesting a different influence on joint bone and cartilage remodelling. The capacity of transamidation by TG2 to regulate activation of latent TGF-β seems to have a potential impact on the regulation of inflammatory response in osteoarthritic tissues. Additional studies are needed to define TG2-regulated pathways that are differently modulated in osteoblasts and chondrocytes during osteoarthritis.