Extracellular transglutaminase 2 activates beta-catenin signaling in calcifying vascular smooth muscle cells

TGM2 accelerates migration and differentiation of BMSCs by activating Wnt/β-catenin signaling

BACKGROUND

Transglutaminase 2 (TGM2) is a gene previously reported to be associated with the differentiation of bone marrow mesenchymal stem cells (BMSCs). The study was developed to reveal the impact of TGM2 on the migration and differentiation of BMSCs.

METHODS

Cells were isolated from bone marrow of mice and then the surface antigens were identified by flow cytometry. Wound healing assays were conducted to assess the migratory ability of BMSCs. The mRNA levels of TGM2 and osteoblast-associated genes (ALP, OCN, and RUNX2) were subjected to RT-qPCR analysis, and protein levels of these genes as well as β-catenin were quantitated by western blotting. Alizarin red staining was conducted for detection of osteogenic ability. The activation of Wnt signaling was assessed by TOP/FOP flash assays.

RESULTS

Surface antigens were positively identified in MSCs, indicating good multidirectional differentiation ability of cells. TGM2 silencing suppressed BMSC migration while weakening mRNA and protein levels of osteoblast-associated genes. While TGM2 overexpression exerts the opposite impact on cell migration and expression levels of osteoblast-associated genes. Additionally, overexpressed TGM2 promotes the mineralization of BMSCs according to results of Alizarin red staining. Moreover, TGM2 activated the Wnt/β-catenin signaling, and DKK1 (an inhibitor of Wnt signaling) reversed the promoting influence of TGM2 on cell migration and differentiation.

CONCLUSION

TGM2 promotes the migration and differentiation of BMSCs via activation of the Wnt/β-catenin signaling.

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.

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.

Dextromethorphan Reduces Oxidative Stress and Inhibits Uremic Artery Calcification

Medial vascular calcification has emerged as a key factor contributing to cardiovascular mortality in patients with chronic kidney disease (CKD). Vascular smooth muscle cells (VSMCs) with osteogenic transdifferentiation play a role in vascular calcification. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors reduce reactive oxygen species (ROS) production and calcified-medium-induced calcification of VSMCs. This study investigates the effects of dextromethorphan (DXM), an NADPH oxidase inhibitor, on vascular calcification. We used in vitro and in vivo studies to evaluate the effect of DXM on artery changes in the presence of hyperphosphatemia. The anti-vascular calcification effect of DXM was tested in adenine-fed Wistar rats. High-phosphate medium induced ROS production and calcification of VSMCs. DXM significantly attenuated the increase in ROS production, the decrease in ATP, and mitochondria membrane potential during the calcified-medium-induced VSMC calcification process (p < 0.05). The protective effect of DXM in calcified-medium-induced VSMC calcification was not further increased by NADPH oxidase inhibitors, indicating that NADPH oxidase mediates the effect of DXM. Furthermore, DXM decreased aortic calcification in Wistar rats with CKD. Our results suggest that treatment with DXM can attenuate vascular oxidative stress and ameliorate vascular calcification.

Transglutaminase-2 regulates Wnt and FoxO3a signaling to determine the severity of osteoarthritis

Transglutaminase 2 (TG2), also known as tissue transglutaminase, is a calcium-dependent enzyme that has a variety of intracellular and extracellular substrates. TG2 not only increases in osteoarthritis (OA) tissue but also affects the progression of OA. However, it is still unclear how TG2 affects cartilage degradation in OA at the molecular level. Surgically induced OA lead to an increase of TG2 in the articular cartilage and growth plate, and it was dependent on TGFβ1 in primary chondrocytes. The inhibition of TG2 enzymatic activity with intra-articular injection of ZDON, the peptide-based specific TG2 inhibitor, ameliorated the severity of surgically induced OA as well as the expression of MMP-3 and MMP-13. ZDON attenuated MMP-3 and MMP-13 expression in TGFβ- and calcium ionophore-treated chondrocytes in a Runx2-independent manner. TG2 inhibition with ZDON suppressed canonical Wnt signaling through a reduction of β-catenin, which was mediated by ubiquitination-dependent proteasomal degradation. In addition, TG2 activation by a calcium ionophore enhanced the phosphorylation of AMPK and FoxO3a and the nuclear translocation of FoxO3a, which was responsible for the increase in MMP-13. In conclusion, TG2 plays an important role in the pathogenesis of OA as a major catabolic mediator that affects the stability of β-catenin and FoxO3a-mediated MMP-13 production.

The Role of Sclerostin in Bone and Ectopic Calcification

Sclerostin, a 22-kDa glycoprotein that is mainly secreted by the osteocytes, is a soluble inhibitor of canonical Wnt signaling. Therefore, when present at increased concentrations, it leads to an increased bone resorption and decreased bone formation. Serum sclerostin levels are known to be increased in the elderly and in patients with chronic kidney disease. In these patient populations, there is a high incidence of ectopic cardiovascular calcification. These calcifications are strongly associated with cardiovascular morbidity and mortality. Although data are still controversial, it is likely that there is a link between ectopic calcification and serum sclerostin levels. The main question, however, remains whether sclerostin exerts either a protective or deleterious role in the ectopic calcification process.

Anti-type 2 transglutaminase antibodies as modulators of type 2 transglutaminase functions: a possible pathological role in celiac disease

Auto-antibodies to the ubiquitous enzyme type-2 transglutaminase (TG2) are a specific hallmark of celiac disease (CD), a widely diffused, multi-factorial disease, affecting genetically predisposed subjects. In CD an inflammatory response, at the intestinal level, is triggered by diet consumption of gluten-containing cereals. Intestinal mucosa displays various degrees of atrophy and hyperplasia, with consequent global intestinal dysfunction and other relevant extra-intestinal symptoms. Through deamidation of specific glutamines of gluten-derived gliadin peptides, TG2 strongly enhances gliadin immunogenicity. In addition, TG2 cross-linking activity may generate complexes between TG2 itself and gliadin peptides, and these complexes seem to cause the auto-immune response by means of an apten-carrier-like mechanism of antigen presentation. Anti-TG2 antibodies can be early detected in the intestinal mucosa of celiac patients and are also abundantly present into the serum, thus potentially reaching other organs and tissues by blood circulation. Recently, the possible pathogenetic role of auto-antibodies to TG2 in CD has been investigated. Here, we report an overview about the genesis of these antibodies, their specificity, their modulating ability toward TG2 enzymatic or non-enzymatic activities and their biological effects exerted by interacting with extracellular TG2 or with cell-surface TG2. We also discuss the auto-immune response occurring in CD against other TG members (i.e. type 3 and type 6) and analyze the occurrence of anti-TG2 antibodies in other auto-immune CD-related diseases. Data now available let us to suppose that, even if antibodies to TG2 do not represent the triggering molecules in CD, they could be important players in disease progression and manifestations.

Cortistatin inhibits arterial calcification in rats via GSK3β/β-catenin and protein kinase C signalling but not c-Jun N-terminal kinase signalling

AIM

Cortistatin (CST) is a newly discovered endogenous active peptide that exerts protective effects on the cardiovascular system. However, the relationship between CST and aortic calcification and the underlying mechanism remain obscure. Therefore, we investigated effects of CST on aortic calcification and its signalling pathways.

METHODS

Calcium content and alkaline phosphatase (ALP) activity were measured using the o-cresolphthalein colorimetric method and ALP assay kit respectively. Protein expression of smooth muscle (SM)-ɑ-actin, osteocalcin (OCN), β-catenin, glycogen synthase kinase 3β (GSK3β), p-GSK3β, protein kinase C (PKC), p-PKC, c-Jun N-terminal kinase (JNK) and p-JNK was determined using Western blotting.

RESULTS

In aorta from a rat vitamin D3 calcification model, CST abrogated calcium deposition and pathological damage, decreased the protein expression of OCN and β-catenin and increased SM-ɑ-actin expression. In a rat cultured vascular smooth muscular cell (VSMC) calcification model induced by β-glycerophosphate (β-GP), CST inhibited the increase in ALP activity, calcium content and OCN protein and the decrease in SM-α-actin expression. CST also inhibited the β-GP-induced increase in p-GSK3β and β-catenin protein (both P < .05). The inhibitory effects of CST on ALP activity, calcium deposition and β-catenin protein were abolished by pretreatment with lithium chloride, a GSK3β inhibitor. CST promoted the protein expression of p-PKC by 68.5% (P < .01), but not p-JNK. The ability of CST to attenuate β-GP-induced increase in ALP activity, calcium content and OCN expression in the VSMC model was abolished by pretreatment with the PKC inhibitor Go6976.

CONCLUSION

These results indicate that CST inhibits aortic calcification and osteogenic differentiation of VSMCs likely via the GSK3β/β-catenin and PKC signalling pathways, but not JNK signalling pathway.

Tissue Tranglutaminase Regulates Interactions between Ovarian Cancer Stem Cells and the Tumor Niche

Cancer progression and recurrence are linked to a rare population of cancer stem cells (CSC). Here, we hypothesized that interactions with the extracellular matrix drive CSC proliferation and tumor-initiating capacity and investigated the functions of scaffold protein tissue transglutaminase (TG2) in ovarian CSC. Complexes formed by TG2, fibronectin (FN), and integrin β1 were enriched in ovarian CSC and detectable in tumors. A function-inhibiting antibody against the TG2 FN-binding domain suppressed complex formation, CSC proliferation as spheroids, tumor-initiating capacity, and stemness-associated Wnt/β-catenin signaling. Disruption of the interaction between TG2 and FN also blocked spheroid formation and the response to Wnt ligands. TG2 and the Wnt receptor Frizzled 7 (Fzd7) form a complex in cancer cells and tumors, leading to Wnt pathway activation. Protein docking and peptide inhibition demonstrate that the interaction between TG2 and Fzd7 overlaps with the FN-binding domain of TG2. These results support a new function of TG2 in ovarian CSC, linked to spheroid proliferation and tumor-initiating capacity and mediated through direct interactions with Fzd7. We propose this complex as a new stem cell target.Significance: These findings reveal a new mechanism by which ovarian CSCs interact with the tumor microenvironment, promoting cell proliferation and tumor initiation. Cancer Res; 78(11); 2990-3001. ©2018 AACR.

Tissue transglutaminase induces Epithelial-Mesenchymal-Transition and the acquisition of stem cell like characteristics in colorectal cancer cells

Human colon cancer cell lines (CRCs) RKO, SW480 and SW620 were investigated for TG2 involvement in tumour advancement and aggression. TG2 expression correlated with tumour advancement and expression of markers of epithelial-mesenchymal transition (EMT). The metastatic cell line SW620 showed high TG2 expression compared to the primary tumour cell lines SW480 and RKO and could form tumour spheroids under non- adherent conditions. TG2 manipulation in the CRCs by shRNA or TG2 transduction confirmed the relationship between TG2 and EMT. TGFβ1 expression in CRC cells, and its level in the cell medium and extracellular matrix was increased in primary tumour CRCs overexpressing TG2 and could regulate TG2 expression and EMT by both canonical (RKO) and non-canonical (RKO and SW480) signalling. TGFβ1 regulation was not observed in the metastatic SW620 cell line, but TG2 knockdown or inhibition in SW620 reversed EMT. In SW620, TG2 expression and EMT was associated with increased presence of nuclear β-catenin which could be mediated by association of TG2 with the Wnt signalling co-receptor LRP5. TG2 inhibition/knockdown increased interaction between β-catenin and ubiquitin shown by co-immunoprecipitation, suggesting that TG2 could be important in β-catenin regulation. β-Catenin and TG2 was also upregulated in SW620 spheroid cells enriched with cancer stem cell marker CD44 and TG2 inhibition/knockdown reduced the spheroid forming potential of SW620 cells. Our data suggests that TG2 could hold both prognostic and therapeutic significance in colon cancer.

Characterization of dermal myofibroblast differentiation in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a rare hereditary disorder which is caused by ABCC6 (ATP-binding cassette subfamily C member 6) gene mutations. Characteristic hallmarks of PXE are progressive calcification and degradation of the elastic fibers in skin, cardiovascular system and ocular fundus. Since the underlying pathomechanisms of PXE remain unidentified, the aim of this study was to get new insights into PXE pathophysiology by characterizing dermal myofibroblast differentiation. Fibroblasts are the key cells of extracellular matrix (ECM) remodeling and, therefore, participate not only in physiological processes, such as calcification or wound healing, but also in pathologic events, such as fibrotization. We revealed that human dermal PXE fibroblasts possess exaggerated migration capability in wound healing and attenuated myofibroblast contractility in comparison to controls. Subsequent analyses reinforced these observations and indicated a diminished induction of the myofibroblast differentiation markers α-smooth muscle actin and xylosyltransferase-I as well as poor transforming growth factor-β1 responsiveness in PXE fibroblasts. In summary, we describe pathological deviations of dermal myofibroblast differentiation in PXE which might be mediated by aberrant supramolecular ECM organization. These results not only improve our insights into cellular PXE pathophysiology, but might also qualify us to interfere with ECM remodeling in the future.

Differential regulation of renal Klotho and FGFR1 in normal and uremic rats

In renal failure, hyperphosphatemia occurs despite a marked elevation in serum fibroblast growth factor (FGF)-23. Abnormal regulation of the FGFR1-Klotho receptor complex may cause a resistance to the phosphaturic action of FGF23. The purpose of the present study was to investigate the regulation of renal Klotho and FGF receptor (FEFR)-1 in healthy and uremic rats induced by 5/6 nephrectomy. In normal rats, the infusion of rat recombinant FGF23 enhanced phosphaturia and increased renal FGFR1 expression; however, Klotho expression was reduced. Uremic rats on a high-phosphate (HP) diet presented hyperphosphatemia with marked elevation of FGF23 and an increased fractional excretion of phosphate (P) that was associated with a marked reduction of Klotho expression and an increase in FGFR1. After neutralization of FGF23 by anti-FGF23 administration, phosphaturia was still abundant, Klotho expression remained low, and the FGFR1 level was reduced. These results suggest that the expression of renal Klotho is modulated by phosphaturia, whereas the FGFR1 expression is regulated by FGF23. Calcitriol (CTR) administration prevented a decrease in renal Klotho expression. In HEK293 cells HP produced nuclear translocation of β-catenin, together with a reduction in Klotho. Wnt/β-catenin inhibition with Dkk-1 prevented the P-induced down-regulation of Klotho. The addition of CTR to HP medium was able to recover Klotho expression. In summary, high FGF23 levels increase FGFR1, whereas phosphaturia decreases Klotho expression through the activation of Wnt/β-catenin pathway.-Muñoz-Castañeda, J. R., Herencia, C., Pendón-Ruiz de Mier, M. V., Rodriguez-Ortiz, M. E., Diaz-Tocados, J. M., Vergara, N., Martínez-Moreno, J. M., Salmerón, M. D., Richards, W. G., Felsenfeld, A., Kuro-O, M., Almadén, Y., Rodríguez, M. Differential regulation of renal Klotho and FGFR1 in normal and uremic rats.

A single gene connects stiffness in glaucoma and the vascular system

Arterial calcification results in arterial stiffness and higher systolic blood pressure. Arterial calcification is prevented by the high expression of the Matrix-Gla gene (MGP) in the vascular smooth muscle cells (VSMC) of the arteries' tunica media. Originally, MGP, a gene highly expressed in cartilage and VSMC, was found to be one of the top expressed genes in the trabecular meshwork. The creation of an Mgp-lacZ Knock-In mouse and the use of mouse genetics revealed that in the eye, Mgp's abundant expression is localized and restricted to glaucoma-associated tissues from the anterior and posterior segments. In particular, it is specifically expressed in the regions of the trabecular meshwork and of the peripapillary sclera that surrounds the optic nerve. Because stiffness in these tissues would significantly alter outflow facility and biomechanical scleral stress in the optic nerve head (ONH), we propose MGP as a strong candidate for the regulation of stiffness in glaucoma. MGP further illustrates the presence of a common function affecting key glaucomatous parameters in the front and back of the eye, and thus offers the possibility for a sole therapeutic target for the disease.

Arterial Calcification in Diabetes Mellitus: Preclinical Models and Translational Implications

Diabetes mellitus increasingly afflicts our aging and dysmetabolic population. Type 2 diabetes mellitus and the antecedent metabolic syndrome represent the vast majority of the disease burden-increasingly prevalent in children and older adults. However, type 1 diabetes mellitus is also advancing in preadolescent children. As such, a crushing wave of cardiometabolic disease burden now faces our society. Arteriosclerotic calcification is increased in metabolic syndrome, type 2 diabetes mellitus, and type 1 diabetes mellitus-impairing conduit vessel compliance and function, thereby increasing the risk for dementia, stroke, heart attack, limb ischemia, renal insufficiency, and lower extremity amputation. Preclinical models of these dysmetabolic settings have provided insights into the pathobiology of arterial calcification. Osteochondrogenic morphogens in the BMP-Wnt signaling relay and transcriptional regulatory programs driven by Msx and Runx gene families are entrained to innate immune responses-responses activated by the dysmetabolic state-to direct arterial matrix deposition and mineralization. Recent studies implicate the endothelial-mesenchymal transition in contributing to the phenotypic drift of mineralizing vascular progenitors. In this brief overview, we discuss preclinical disease models that provide mechanistic insights-and point to challenges and opportunities to translate these insights into new therapeutic strategies for our patients afflicted with diabetes mellitus and its arteriosclerotic complications.

WNT/β-catenin signaling promotes VSMCs to osteogenic transdifferentiation and calcification through directly modulating Runx2 gene expression

Arterial medial calcification (AMC) is prevalent in patients with chronic kidney disease (CKD) and contributes to elevated risk of cardiovascular events and mortality. Vascular smooth muscle cells (VSMCs) to osteogenic transdifferentiation (VOT) in a high-phosphate environment is involved in the pathogenesis of AMC in CKD. WNT/β-catenin signaling is indicated to play a crucial role in osteogenesis via promoting Runx2 expression in osteoprogenitor cells, however, its role in Runx2 regulation and VOT remains incompletely clarified. In this study, Runx2 was induced and β-catenin was activated by high-phosphate in VSMCs. Two forms of active β-catenin, dephosphorylated on Ser37/Thr41 and phosphorylated on Ser675 sites, were upregulated by high-phosphate. Activation of β-catenin, through ectopic expression of stabilized β-catenin, inhibition of GSK-3β, or WNT-3A protein, induced Runx2 expression, whereas blockade of WNT/β-catenin signaling with Porcupine (PORCN) inhibitor or Dickkopf-1 (DKK1) protein inhibited Runx2 induction by high-phosphate. WNT-3A promoted osteocalcin expression and calcium deposition in VSMCs, whereas DKK1 ameliorated calcification of VSMCs induced by high-phosphate. Two functional T cell factor (TCF)/lymphoid enhancer-binding factor binding sites were identified in the promoter region of Runx2 gene in VSMCs, which interacted with TCF upon β-catenin activation. Site-directed mutation of each of them attenuated Runx2 response to β-catenin, and deletion or destruction of both of them completely abolished this responsiveness. In the aortic tunica media of rats with chronic renal failure, followed by AMC, Runx2 and β-catenin was induced, and the Runx2 mRNA level was positively associated with the abundance of phosphorylated β-catenin (Ser675). Collectively, our study suggested that high-phosphate may activate WNT/β-catenin signaling through different pathways, and the activated WNT/β-catenin signaling, through direct downstream target Runx2, could play an important role in promoting VOT and AMC.

Angiotensin II promotes an osteoblast-like phenotype in porcine aortic valve myofibroblasts

OBJECTIVES

The mechanisms for pathogenesis of cardiac valve calcification were explored by studying the regulation of the Wnt signaling pathway during the transformation from cardiac valvular myofibroblasts to osteoblast-like phenotype.

METHODS

Studies were carried on primary cultured porcine aortic valvular myofibroblasts. The cells were randomly divided into four groups and treated with angiotensin II (Ang II) according to the following: Ang II (10(-6) mol/l), Valsartan (Val) (10(-5) mol/l), Ang II plus Val (Ang II 10(-6) mol/l + Val 10(-5) mol/l) or mock treated as the control. Protein expression of Bone morphogenetic protein 2 (BMP2), Alkaline phosphatase (ALP), and Wnt pathway components, Wnt3a and β-catenin, was investigated to assess the activation of the Wnt signaling pathway and determine whether cells undergo the transformation to osteoblast-like phenotype.

RESULT

Ang II treatment of myofibroblasts led to significant up-regulation of α-SMA expression and activation of the cells. Neither the BMP2 or ALP proteins, nor the mRNA was detectable in the control group or the Val-treated group; however, there was a significant increase in Ang II-treated group (P < 0.01). The Wnt/β-catenin signaling ligand, Wnt3a, was not expressed in the control or Val-treated groups, whereas in Ang II-treated cells, both Wnt3a and β-catenin gene expression were enhanced (P < 0.01).The effect of Ang II can be inhibited by the addition of Val (P < 0.05).

CONCLUSION

Ang II might act on the Ang II receptor on valvular interstitial cells (VICs) and lead to activation of the Wnt/β-catenin pathway and hence cause the activation, differentiation and proliferation of myofibroblasts, and finally, osteoblast-like phenotype transformation, leading to calcification of heart valves.

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.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

Effects of oxidized low density lipoprotein on transformation of valvular myofibroblasts to osteoblast-like phenotype

In order to investigate the roles of Wnt signal pathway in transformation of cardiac valvular myofibroblasts to the osteoblast-like phenotype, the primary cultured porcine aortic valve myofibroblasts were incubated with oxidized low density lipoprotein (ox-LDL, 50 mg/L), and divided into four groups according to the ox-LDL treatment time: control group, ox-LDL 24-h group, ox-LDL 48-h group, and ox-LDL 72-h group. Wnt signal pathway blocker Dickkopf-1 (DDK-1, 100 μg/L) was added in ox-LDL 72-h group. The expression of a-smooth muscle actin (α-SMA), bone morphogenetic protein 2 (BMP2), alkaline phosphatase (ALP), and osteogenic transcription factor Cbfa-1 was detected by Western blotting, and that of β-catenin, a key mediator of Wnt signal pathway by immunocytochemical staining method. The Wnt/β-catenin was observed and the transformation of myofibroblasts to the osteoblast-like phenotype was examined. The expression of α-SMA, BMP2, ALP and Cbfa-1 proteins in the control group was weaker than in the ox-LDL-treated groups. In ox-LDL-treated groups, the protein expression of a-SMA, BMP2, ALP, and Cbfa-1 was significantly increased in a time-dependent manner as compared with the control group, and there was significant difference among the three ox-LDL-treated groups (P<0.05 for all); β-catenin protein was also up-regulated in the ox-LDL-treated groups in a time-dependent manner as compared with the control group (P<0.05), and its transfer from cytoplasm to nucleus and accumulation in the nucleus were increased in the same fashion (P<0.05). After addition of DKK-1, the expression of α-SMA, bone-related proteins and β-catenin protein was significantly reduced as compared with ox-LDL 72-h group (P<0.05). The Wnt/ β-catenin signaling pathway may play an important role in transformation of valvular myofibroblasts to the osteoblast-like phenotype.

Transglutaminase 2 promotes PDGF-mediated activation of PDGFR/Akt1 and β-catenin signaling in vascular smooth muscle cells and supports neointima formation

BACKGROUND

Phenotypic switch of vascular smooth muscle cells (VSMCs) accompanies neointima formation and associates with vascular diseases. Platelet-derived growth factor (PDGF)-induced activation of PDGFR/Akt1 and β-catenin signaling pathways in VSMCs has been implicated in vessel occlusion. Transglutaminase 2 (TG2) regulates these pathways and its levels are increased in the neointima.

OBJECTIVE

The aim of this study was to evaluate the role of TG2 in PDGF/β-catenin signaling cross-talk and assess its contribution to neointima.

METHODS

Aortic VSMCs from wild-type and TG2 knockout mice were tested in vitro for levels of VSMC markers, proliferation, migration and PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways. Neointima in these mice was studied ex vivo in coronary vessels using a heart slice model and in vivo using a carotid artery ligation model.

RESULTS

Genetic deletion of TG2 attenuated the PDGF-induced phenotypic switch of aortic VSMCs, reduced their proliferation and migration rates, and inhibited PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways in both ex vivo and in vivo neointima models. Importantly, genetic deletion of TG2 also markedly attenuated vessel occlusion.

CONCLUSIONS

TG2 promotes neointima formation by mediating the PDGF-induced activation of the PDGFR/Akt1 and β-catenin pathways in VSMCs. This study identifies TG2 as a potential therapeutic target for blocking neointima in blood vessels.

MicroRNAs Regulate Vascular Medial Calcification

Vascular calcification is highly prevalent in patients with coronary artery disease and, when present, is associated with major adverse cardiovascular events, including an increased risk of cardiovascular mortality. The pathogenesis of vascular calcification is complex and is now recognized to recapitulate skeletal bone formation. Vascular smooth muscle cells (SMC) play an integral role in this process by undergoing transdifferentiation to osteoblast-like cells, elaborating calcifying matrix vesicles and secreting factors that diminish the activity of osteoclast-like cells with mineral resorbing capacity. Recent advances have identified microRNAs (miRs) as key regulators of this process by directing the complex genetic reprogramming of SMCs and the functional responses of other relevant cell types relevant for vascular calcification. This review will detail SMC and bone biology as it relates to vascular calcification and relate what is known to date regarding the regulatory role of miRs in SMC-mediated vascular calcification.

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.

The long non-coding HOTAIR is modulated by cyclic stretch and WNT/β-CATENIN in human aortic valve cells and is a novel repressor of calcification genes

Aortic valve calcification is a significant and serious clinical problem for which there are no effective medical treatments. Individuals born with bicuspid aortic valves, 1–2% of the population, are at the highest risk of developing aortic valve calcification. Aortic valve calcification involves increased expression of calcification and inflammatory genes. Bicuspid aortic valve leaflets experience increased biomechanical strain as compared to normal tricuspid aortic valves. The molecular pathogenesis involved in the calcification of BAVs are not well understood, especially the molecular response to mechanical stretch. HOTAIR is a long non-coding RNA (lncRNA) that has been implicated with cancer but has not been studied in cardiac disease. We have found that HOTAIR levels are decreased in BAVs and in human aortic interstitial cells (AVICs) exposed to cyclic stretch. Reducing HOTAIR levels via siRNA in AVICs results in increased expression of calcification genes. Our data suggest that β-CATENIN is a stretch responsive signaling pathway that represses HOTAIR. This is the first report demonstrating that HOTAIR is mechanoresponsive and repressed by WNT β-CATENIN signaling. These findings provide novel evidence that HOTAIR is involved in aortic valve calcification.

Magnesium inhibits Wnt/β-catenin activity and reverses the osteogenic transformation of vascular smooth muscle cells

Magnesium reduces vascular smooth muscle cell (VSMC) calcification in vitro but the mechanism has not been revealed so far. This work used only slightly increased magnesium levels and aimed at determining: a) whether inhibition of magnesium transport into the cell influences VSMC calcification, b) whether Wnt/β-catenin signaling, a key mediator of osteogenic differentiation, is modified by magnesium and c) whether magnesium can influence already established vascular calcification. Human VSMC incubated with high phosphate (3.3 mM) and moderately elevated magnesium (1.4 mM) significantly reduced VSMC calcification and expression of the osteogenic transcription factors Cbfa-1 and osterix, and up-regulated expression of the natural calcification inhibitors matrix Gla protein (MGP) and osteoprotegerin (OPG). The protective effects of magnesium on calcification and expression of osteogenic markers were no longer observed in VSMC cultured with an inhibitor of cellular magnesium transport (2-aminoethoxy-diphenylborate [2-APB]). High phosphate induced activation of Wnt/β-catenin pathway as demonstrated by the translocation of β-catenin into the nucleus, increased expression of the frizzled-3 gene, and downregulation of Dkk-1 gene, a specific antagonist of the Wnt/β-catenin signaling pathway. The addition of magnesium however inhibited phosphate-induced activation of Wnt/β-catenin signaling pathway. Furthermore, TRPM7 silencing using siRNA resulted in activation of Wnt/β-catenin signaling pathway. Additional experiments were performed to test the ability of magnesium to halt the progression of already established VSMC calcification in vitro. The delayed addition of magnesium decreased calcium content, down-regulated Cbfa-1 and osterix and up-regulated MGP and OPG, when compared with a control group. This effect was not observed when 2-APB was added. In conclusion, magnesium transport through the cell membrane is important to inhibit VSMC calcification in vitro. Inhibition of Wnt/β-catenin by magnesium is one potential intracellular mechanism by which this anti-calcifying effect is achieved.

The effects of myocilin expression on functionally relevant trabecular meshwork genes: a mini-review

Myocilin is a secreted glaucoma-associated protein, specifically induced by dexamethasone in human trabecular meshwork cells, where it was discovered. Myocilin is expressed in several tissues of the body, but it causes disease only in the eye. The protein contains two domains: an N-terminal region with significant homologies to nonmuscle myosin, and a C-terminal region, which is similar to the olfactomedin proteins. Forty percent of myocilin undergoes an intracellular endoproteolytic cleavage by calpain II, a calcium-dependent cysteine protease, which releases the 2 domains. The protein is known to interact with intracellular and extracellular matrix proteins, and some is released into the extracellular space associated with exosomes. Myocilin mutations are linked to glaucoma and induce elevated intraocular pressure. Most of the glaucoma-causative mutations map to the olfactomedin domain, which appears to be a critical domain for the function of the protein. Myocilin mutants are misfolded, aggregate in the endoplasmic reticulum, and are not secreted. Overexpression of myocilin and of its mutants in primary human trabecular meshwork cells triggers changes in the expression of numerous genes, many of which have been known to be involved in mechanisms important for the physiology and pathology of the tissue. Here we review recent studies from our laboratory and those of others that deal with trabecular meshwork genes, which are altered by the overexpression of wild-type and glaucoma-causative mutant myocilin genes.

Two sides of MGP null arterial disease: chondrogenic lesions dependent on transglutaminase 2 and elastin fragmentation associated with induction of adipsin

Mutations in matrix Gla protein (MGP) have been correlated with vascular calcification. In the mouse model, MGP null vascular disease presents as calcifying cartilaginous lesions and mineral deposition along elastin lamellae (elastocalcinosis). Here we examined the mechanisms underlying both of these manifestations. Genetic ablation of enzyme transglutaminase 2 (TG2) in Mgp(-/-) mice dramatically reduced the size of cartilaginous lesions in the aortic media, attenuated calcium accrual more than 2-fold, and doubled longevity as compared with control Mgp(-/-) animals. Nonetheless, the Mgp(-/-);Tgm2(-/-) mice still died prematurely as compared with wild-type and retained the elastocalcinosis phenotype. This pathology in Mgp(-/-) animals was developmentally preceded by extensive fragmentation of elastic lamellae and associated with elevated serine elastase activity in aortic tissue and vascular smooth muscle cells. Systematic gene expression analysis followed by an immunoprecipitation study identified adipsin as the major elastase that is induced in the Mgp(-/-) vascular smooth muscle even in the TG2 null background. These results reveal a central role for TG2 in chondrogenic transformation of vascular smooth muscle and implicate adipsin in elastin fragmentation and ensuing elastocalcinosis. The importance of elastin calcification in MGP null vascular disease is highlighted by significant residual vascular calcification and mortality in Mgp(-/-);Tgm2(-/-) mice with reduced cartilaginous lesions. Our studies identify two potential therapeutic targets in vascular calcification associated with MGP dysfunction and emphasize the need for a comprehensive approach to this multifaceted disorder.

Attenuation of chondrogenic transformation in vascular smooth muscle by dietary quercetin in the MGP-deficient mouse model

RATIONALE

Cartilaginous metaplasia of vascular smooth muscle (VSM) is characteristic for arterial calcification in diabetes and uremia and in the background of genetic alterations in matrix Gla protein (MGP). A better understanding of the molecular details of this process is critical for the development of novel therapeutic approaches to VSM transformation and arterial calcification.

OBJECTIVE

This study aimed to identify the effects of bioflavonoid quercetin on chondrogenic transformation and calcification of VSM in the MGP-null mouse model and upon TGF-β3 stimulation in vitro, and to characterize the associated alterations in cell signaling.

METHODS AND RESULTS

Molecular analysis revealed activation of β-catenin signaling in cartilaginous metaplasia in Mgp-/- aortae in vivo and during chondrogenic transformation of VSMCs in vitro. Quercetin intercepted chondrogenic transformation of VSM and blocked activation of β-catenin both in vivo and in vitro. Although dietary quercetin drastically attenuated calcifying cartilaginous metaplasia in Mgp-/- animals, approximately one-half of total vascular calcium mineral remained as depositions along elastic lamellae.

CONCLUSION

Quercetin is potent in preventing VSM chondrogenic transformation caused by diverse stimuli. Combined with the demonstrated efficiency of dietary quercetin in preventing ectopic chondrogenesis in the MGP-null vasculature, these findings indicate a potentially broad therapeutic applicability of this safe for human consumption bioflavonoid in the therapy of cardiovascular conditions linked to cartilaginous metaplasia of VSM. Elastocalcinosis is a major component of MGP-null vascular disease and is controlled by a mechanism different from chondrogenic transformation of VSM and not sensitive to quercetin.

Transglutaminase 2 as a novel activator of LRP6/β-catenin signaling

The β-catenin signaling axis is critical for normal embryonic development and tissue homeostasis in adults. We have previously shown that extracellular enzyme transglutaminase 2 (TG2) activates β-catenin signaling in vascular smooth muscle cells (VSMCs). In this study, we provide several lines of evidence that TG2 functions as an activating ligand of the LRP5/6 receptors. Specifically, we show that TG2 synergizes with LRP6 in the activation of β-catenin-dependent gene expression in Cos-7 cells. Interfering with the LRP5/6 receptors attenuates TG2-induced activation of β-catenin in Cos-7 cells. Further, we show that TG2 binds directly to the extracellular domain of LRP6, which is also able to act as a substrate for TG2-mediated protein cross-linking. Furthermore, inhibitors of TG2 protein cross-linking quench the observed TG2-induced β-catenin activation, implicating protein cross-linking as a novel regulatory mechanism for this pathway. Together, our findings identify and characterize a new activating ligand of the LRP5/6 receptors and uncover a novel activity of TG2 as an agonist of β-catenin signaling, contributing to the understanding of diverse developmental events and pathological conditions in which transglutaminase and β-catenin signaling are implicated.

Tissue transglutaminase regulates β-catenin signaling through a c-Src-dependent mechanism

Tissue transglutaminase (TG2) is a multifunctional enzyme involved in protein cross-linking and cell adhesion to fibronectin (FN). In cancer, TG2 induces an epithelial to mesenchymal transition, contributing to metastasis. Because cadherins bind β-catenin at cell-cell junctions, disruption of adherens junctions destabilizes cadherin-catenin complexes. The goal of the present study was to analyze whether and how TG2 interacts with and regulates β-catenin signaling in ovarian cancer (OC) cells. We observed a significant correlation between TG2 and β-catenin expression levels in OC cells and tumors. TG2 augmented Wnt/β-catenin signaling, as evidenced by enhanced β-catenin transcriptional activity, inducing transcription of target genes cyclin D1 and c-Myc. By promoting integrin-mediated cell adhesion to FN, TG2 physically associates with and recruits c-Src, which in turn phosphorylates β-catenin at Tyr(654), releasing it from E-cadherin and rendering it available for transcriptional regulation. By interacting with FN and enhancing β-catenin signaling, complexed TG2 stimulates OC cell proliferation. In summary, our data demonstrate that TG2 regulates β-catenin expression and function in OC cells and define the c-Src-dependent mechanism through which this occurs.

MicroRNA in cardiovascular calcification: focus on targets and extracellular vesicle delivery mechanisms

Cardiovascular calcification is a prominent feature of chronic inflammatory disorders-such as chronic kidney disease, type 2 diabetes mellitus, and atherosclerosis-that associate with significant morbidity and mortality. The concept that similar pathways control both bone remodeling and vascular calcification is widely accepted, but the precise mechanisms of calcification remain largely unknown. The central role of microRNAs (miRNA) as fine-tune regulators in the cardiovascular system and bone biology has gained acceptance and has raised the possibility for novel therapeutic targets. Additionally, circulating miRNAs have been proposed as biomarkers for a wide range of cardiovascular diseases, but knowledge of miRNA biology in cardiovascular calcification is very limited. This review focuses on the role of miRNAs in cardiovascular disease, with emphasis on osteogenic processes. Herein, we discuss the current understanding of miRNAs in cardiovascular calcification. Furthermore, we identify a set of miRNAs common to diseases associated with cardiovascular calcification (chronic kidney disease, type 2 diabetes mellitus, and atherosclerosis), and we hypothesize that these miRNAs may provide a molecular signature for calcification. Finally, we discuss this novel hypothesis with emphasis on known biological and pathological osteogenic processes (eg, osteogenic differentiation, release of calcifying matrix vesicles). The aim of this review is to provide an organized discussion of the known links between miRNA and calcification that provide emerging concepts for future studies on miRNA biology in cardiovascular calcification, which will be critical for developing new therapeutic strategies.

Quercetin attenuates warfarin-induced vascular calcification in vitro independently from matrix Gla protein

Warfarin can stimulate vascular calcification in vitro via activation of β-catenin signaling and/or inhibition of matrix Gla protein (MGP) carboxylation. Calcification was induced in vascular smooth muscle cells (VSMCs) with therapeutic levels of warfarin in normal calcium and clinically acceptable phosphate levels. Although TGF/BMP and PKA pathways are activated in calcifying VSMCs, pharmacologic analysis reveals that their activation is not contributory. However, β-catenin activity is important because inhibition of β-catenin with shRNA or bioflavonoid quercetin prevents calcification in primary human VSMCs, rodent aortic rings, and rat A10 VSMC line. In the presence of quercetin, reactivation of β-catenin using the glycogen synthase kinase-3β (GSK-3β) inhibitor LiCl restores calcium accumulation, confirming that quercetin mechanism of action hinges on inhibition of the β-catenin pathway. Calcification in VSMCs induced by 10 μm warfarin does not associate with reduced levels of carboxylated MGP, and inhibitory effects of quercetin do not involve induction of MGP carboxylation. Further, down-regulation of MGP by shRNA does not alter the effect of quercetin. These results suggest a new β-catenin-targeting strategy to prevent vascular calcification induced by warfarin and identify quercetin as a potential therapeutic in this pathology.

Transglutaminase inhibitors attenuate vascular calcification in a preclinical model

OBJECTIVE

In vitro, transglutaminase-2 (TG2)-mediated activation of the β-catenin signaling pathway is central in warfarin-induced calcification, warranting inquiry into the importance of this signaling axis as a target for preventive therapy of vascular calcification in vivo.

METHODS AND RESULTS

The adverse effects of warfarin-induced elastocalcinosis in a rat model include calcification of the aortic media, loss of the cellular component in the vessel wall, and isolated systolic hypertension, associated with accumulation and activation of TG2 and activation of β-catenin signaling. These effects of warfarin can be completely reversed by intraperitoneal administration of the TG2-specific inhibitor KCC-009 or dietary supplementation with the bioflavonoid quercetin, known to inhibit β-catenin signaling. Our study also uncovers a previously uncharacterized ability of quercetin to inhibit TG2. Quercetin reversed the warfarin-induced increase in systolic pressure, underlying the functional consequence of this treatment. Molecular analysis shows that quercetin diet stabilizes the phenotype of smooth muscle and prevents its transformation into osteoblastic cells.

CONCLUSIONS

Inhibition of the TG2/β-catenin signaling axis seems to prevent warfarin-induced elastocalcinosis and to control isolated systolic hypertension.

Arterial calcification and bone physiology: role of the bone-vascular axis

Bone never forms without vascular interactions. This simple statement of fact does not adequately reflect the physiological and pharmacological implications of the relationship. The vasculature is the conduit for nutrient exchange between bone and the rest of the body. The vasculature provides the sustentacular niche for development of osteoblast progenitors and is the conduit for egress of bone marrow cell products arising, in turn, from the osteoblast-dependent haematopoietic niche. Importantly, the second most calcified structure in humans after the skeleton is the vasculature. Once considered a passive process of dead and dying cells, vascular calcification has emerged as an actively regulated form of tissue biomineralization. Skeletal morphogens and osteochondrogenic transcription factors are expressed by cells within the vessel wall, which regulates the deposition of vascular calcium. Osteotropic hormones, including parathyroid hormone, regulate both vascular and skeletal mineralization. Cellular, endocrine and metabolic signals that flow bidirectionally between the vasculature and bone are necessary for both bone health and vascular health. Dysmetabolic states including diabetes mellitus, uraemia and hyperlipidaemia perturb the bone-vascular axis, giving rise to devastating vascular and skeletal disease. A detailed understanding of bone-vascular interactions is necessary to address the unmet clinical needs of an increasingly aged and dysmetabolic population.

In vascular smooth muscle cells paricalcitol prevents phosphate-induced Wnt/β-catenin activation

The present study investigates the differential effect of two vitamin D receptor agonists, calcitriol and paricalcitol, on human aortic smooth muscle cells calcification in vitro. Human vascular smooth muscle cells were incubated in a high phosphate (HP) medium alone or supplemented with either calcitriol 10(-8)M (HP + CTR) or paricalcitol 3·10(-8) M (HP + PC). HP medium induced calcification, which was associated with the upregulation of mRNA expression of osteogenic factors such as bone morphogenetic protein 2 (BMP2), Runx2/Cbfa1, Msx2, and osteocalcin. In these cells, activation of Wnt/β-catenin signaling was evidenced by the translocation of β-catenin into the nucleus and the increase in the expression of direct target genes as cyclin D1, axin 2, and VCAN/versican. Addition of calcitriol to HP medium (HP + CTR) further increased calcification and also enhanced the expression of osteogenic factors together with a significant elevation of nuclear β-catenin levels and the expression of cyclin D1, axin 2, and VCAN. By contrast, the addition of paricalcitol (HP + PC) not only reduced calcification but also downregulated the expression of BMP2 and other osteoblastic phenotype markers as well as the levels of nuclear β-catenin and the expression of its target genes. The role of Wnt/β-catenin on phosphate- and calcitriol-induced calcification was further demonstrated by the inhibition of calcification after addition of Dickkopf-related protein 1 (DKK-1), a specific natural antagonist of the Wnt/β-catenin signaling pathway. In conclusion, the differential effect of calcitriol and paricalcitol on vascular calcification appears to be mediated by a distinct regulation of the BMP and Wnt/β-catenin signaling pathways.

Implication for transglutaminase 2-mediated activation of β-catenin signaling in neointimal vascular smooth muscle cells in chronic cardiac allograft rejection

BACKGROUND

Cardiac allograft vasculopathy (CAV) remains the main cause of long-term transplant rejection. CAV is characterized by hyperproliferation of vascular smooth muscle cells (VSMCs). Canonical β-catenin signaling is a critical regulator of VSMC proliferation in development; however, the role of this pathway and its regulation in CAV progression are obscure. We investigated the activity of β-catenin signaling and the role for a putative activating ligand, transglutaminase 2 (TG2), in chronic cardiac rejection.

METHODS

Hearts from Bm12 mice were transplanted into C57BL/6 mice (class II mismatch), and allografts were harvested 8 weeks after transplantation. Accumulation and sub-cellular distribution of β-catenin protein and expression of several components of β-catenin signaling were analyzed as hallmarks of pathway activation. In vitro, platelet-derived growth factor treatment was used to mimic the inflammatory milieu in VSMC and organotypic heart slice cultures.

RESULTS

Activation of β-catenin in allografts compared with isografts or naïve hearts was evidenced by the augmented expression of β-catenin target genes, as well as the accumulation and nuclear localization of the β-catenin protein in VSMCs of the occluded allograft vessels. Expression of TG2, an activator of β-catenin signaling in VSMCs, was dramatically increased in allografts. Further, our ex vivo data demonstrate that TG2 is required for VSMC proliferation and for β-catenin activation by platelet-derived growth factor in cardiac tissue.

CONCLUSIONS

β-Catenin signaling is activated in occluded vessels in murine cardiac allografts. TG2 is implicated as an endogenous activator of this signaling pathway and may therefore have a role in the pathogenesis of CAV during chronic allograft rejection.

Tissue transglutaminase promotes PDGF/PDGFR-mediated signaling and responses in vascular smooth muscle cells

Although the pivotal role of platelet derived growth factor (PDGF)-mediated signaling in vascular diseases was demonstrated, the pathophysiological mechanisms driving its over-activation remain incompletely understood. Tissue transglutaminase (tTG) is a multifunctional protein expressed in the vasculature, including smooth muscle cells (SMCs), and implicated in several vascular pathologies. The goal of this study is to define the regulation of PDGF-BB/PDGFRβ-induced signaling pathways and cell responses by tTG in vascular SMCs. We find that in human aortic SMCs, shRNA-mediated depletion and over-expression of tTG reveals its ability to down-regulate PDGFRβ levels and induce receptor clustering. In these cells, tTG specifically amplifies the activation of PDGFRβ and its multiple downstream signaling targets in response to PDGF-BB. Furthermore, tTG promotes dedifferentiation and increases survival, proliferation, and migration of human aortic SMCs mediated by this growth factor. Finally, PDGF-BB stimulates tTG expression in human aortic SMCs in culture and in the blood vessels in response to injury. Together, our results show that tTG in vascular SMCs acts as a principal enhancer within the PDGF-BB/PDGFRβ signaling axis involved in phenotypic modulation of these cells, thereby suggesting a novel role for this protein in the progression of vascular diseases.

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: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

Extracellular TG2: emerging functions and regulation

Tissue transglutaminase (TG2) is a ubiquitously expressed member of the transglutaminase family of Ca(2+)-dependent crosslinking enzymes. Unlike other family members, TG2 is a multifunctional protein, which has several other well documented enzymatic and non-enzymatic functions. A significant body of evidence accumulated over the last decade reveals multiple and complex activities of this protein on the cell surface and in the extracellular matrix (ECM), including its role in the regulation of cell-ECM interactions and outside-in signaling by several types of transmembrane receptors. Moreover, recent findings indicate a dynamic regulation of the levels and functions of extracellular TG2 by several complementary mechanisms. This review summarizes and assesses recent research into the emerging functions and regulation of extracellular TG2.

Transglutaminase 2-mediated activation of β-catenin signaling has a critical role in warfarin-induced vascular calcification

OBJECTIVE

Accumulating experimental evidence implicates β-catenin signaling and enzyme transglutaminase 2 (TG2) in the progression of vascular calcification, and our previous studies have shown that TG2 can activate β-catenin signaling in vascular smooth muscle cells (VSMCs). Here we investigated the role of the TG2/β-catenin signaling axis in vascular calcification induced by warfarin.

METHODS AND RESULTS

Warfarin-induced calcification in rat A10 VSMCs is associated with the activation of β-catenin signaling and is independent of oxidative stress. The canonical β-catenin inhibitor Dkk1, but not the Wnt antagonist Wif-1, prevents warfarin-induced activation of β-catenin, calcification, and osteogenic transdifferentiation in VSMCs. TG2 expression and activity are increased in warfarin-treated cells, in contrast to canonical Wnt ligands. Vascular cells with genetically or pharmacologically reduced TG2 activity fail to activate β-catenin in response to warfarin. Moreover, warfarin-induced calcification is significantly reduced on the background of attenuated TG2 both in vitro and in vivo.

CONCLUSIONS

TG2 is a critical mediator of warfarin-induced vascular calcification that acts through the activation of β-catenin signaling in VSMCs. Inhibition of canonical β-catenin pathway or TG2 activity prevents warfarin-regulated calcification, identifying the TG2/β-catenin axis as a novel therapeutic target in vascular calcification.

The regulation of valvular and vascular sclerosis by osteogenic morphogens

Vascular calcification increasingly afflicts our aging, dysmetabolic population. Once considered only a passive process of dead and dying cells, vascular calcification has now emerged as a highly regulated form of biomineralization organized by collagenous and elastin extracellular matrices. During skeletal bone formation, paracrine epithelial-mesenchymal and endothelial-mesenchymal interactions control osteochondrocytic differentiation of multipotent mesenchymal progenitor cells. These paracrine osteogenic signals, mediated by potent morphogens of the bone morphogenetic protein and wingless-type MMTV integration site family member (Wnt) superfamilies, are also active in the programming of arterial osteoprogenitor cells during vascular and valve calcification. Inflammatory cytokines, reactive oxygen species, and oxylipids-increased in the clinical settings of atherosclerosis, diabetes, and uremia that promote arteriosclerotic calcification-elicit the ectopic vascular activation of osteogenic morphogens. Specific extracellular and intracellular inhibitors of bone morphogenetic protein-Wnt signaling have been identified as contributing to the regulation of osteogenic mineralization during development and disease. These inhibitory pathways and their regulators afford the development of novel therapeutic strategies to prevent and treat valve and vascular sclerosis.

The redox state of transglutaminase 2 controls arterial remodeling

While inward remodeling of small arteries in response to low blood flow, hypertension, and chronic vasoconstriction depends on type 2 transglutaminase (TG2), the mechanisms of action have remained unresolved. We studied the regulation of TG2 activity, its (sub) cellular localization, substrates, and its specific mode of action during small artery inward remodeling. We found that inward remodeling of isolated mouse mesenteric arteries by exogenous TG2 required the presence of a reducing agent. The effect of TG2 depended on its cross-linking activity, as indicated by the lack of effect of mutant TG2. The cell-permeable reducing agent DTT, but not the cell-impermeable reducing agent TCEP, induced translocation of endogenous TG2 and high membrane-bound transglutaminase activity. This coincided with inward remodeling, characterized by a stiffening of the artery. The remodeling could be inhibited by a TG2 inhibitor and by the nitric oxide donor, SNAP. Using a pull-down assay and mass spectrometry, 21 proteins were identified as TG2 cross-linking substrates, including fibronectin, collagen and nidogen. Inward remodeling induced by low blood flow was associated with the upregulation of several anti-oxidant proteins, notably glutathione-S-transferase, and selenoprotein P. In conclusion, these results show that a reduced state induces smooth muscle membrane-bound TG2 activity. Inward remodeling results from the cross-linking of vicinal matrix proteins, causing a stiffening of the arterial wall.

Tissue transglutaminase regulates chondrogenesis in mesenchymal stem cells on collagen type XI matrices

Tissue transglutaminase (tTG) is a multifunctional enzyme with a plethora of potential applications in regenerative medicine and tissue bioengineering. In this study, we examined the role of tTG as a regulator of chondrogenesis in human mesenchymal stem cells (MSC) using nanofibrous scaffolds coated with collagen type XI. Transient treatment of collagen type XI films and 3D scaffolds with tTG results in enhanced attachment of MSC and supports rounded cell morphology compared to the untreated matrices or those incubated in the continuous presence of tTG. Accordingly, enhanced cell aggregation and augmented chondrogenic differentiation have been observed on the collagen type XI-coated poly-(L-lactide) nanofibrous scaffolds treated with tTG prior to cell seeding. These changes implicate that MSC chondrogenesis is enhanced by the tTG-mediated modifications of the collagen matrix. For example, exogenous tTG increases resistance to collagenolysis in collagen type XI matrices by catalyzing intermolecular cross-linking, detected by a shift in the denaturation temperature. In addition, tTG auto-crosslinks to collagen type XI as detected by western blot and immunofluorescent analysis. This study identifies tTG as a novel regulator of MSC chondrogenesis further contributing to the expanding use of these cells in cartilage bioengineering.

Characterization of the transglutaminase gene family in zebrafish and in vivo analysis of transglutaminase-dependent bone mineralization

We have characterized the protein cross-linking enzyme transglutaminase (TGs) genes in zebrafish, Danio rerio, based on the analysis of their genomic organization and phylogenetics. Thirteen zebrafish TG genes (zTGs) have been identified, of which 11 show high homology to only 3 mammalian enzymes: TG1, TG2 and FXIIIa. No zebrafish homologues were identified for mammalian TGs 3-7. Real-time PCR analysis demonstrated distinct temporal expression profiles for zTGs in larvae and adult fish. Analysis by in situ hybridization revealed restricted expression of zTG2b and zFXIIIa in skeletal elements, resembling expression of their mammalian homologues in osteo-chondrogenic cells. Mammalian TG2 and FXIIIa have been implicated in promoting osteoblast differentiation and bone mineralization in vitro, however, mouse models lacking either gene have no skeletal phenotype likely due to a compensation effect. We show in this study that mineralization of the newly formed vertebrae is significantly reduced in fish grown for 5 days in the presence of TG inhibitor KCC-009 added at 3-5 days post fertilization. This treatment reduces average vertebrae mineralization by 30%, with complete inhibition in some fish, and no effect on the overall growth and vertebrae number. This is the first in vivo demonstration of the crucial requirement for the TG-catalyzed cross-linking activity in bone mineralization.

Transglutaminase 2 regulates early chondrogenesis and glycosaminoglycan synthesis

The expression pattern for tissue transglutaminase (TG2) suggests that it regulates cartilage formation. We analyzed the role of TG2 in early stages of chondrogenesis using differentiating high-density cultures of mesenchymal cells from chicken limb bud as a model. We demonstrate that TG2 promotes cell differentiation towards a pre-hypertrophic stage without inducing precocious hypertrophic maturation. This finding, combined with distinctive up-regulation of extracellular TG2 in the pre-hypertrophic cartilage of the growth plate, indicates that TG2 is an autocrine regulator of chondrocyte differentiation. We also show that TG2 regulates synthesis of the cartilaginous glycosaminoglycan (GAG)-rich extracellular matrix. Elevated levels of TG2 down-regulate xylosyltransferase activity which mediates the key steps in chondroitin sulfate synthesis. On the contrary, inhibition of endogenous transglutaminase activity in differentiating chondrogenic micromasses results in increased GAG deposition and enhancement of early chondrogenic markers. Regulation of GAG synthesis by TG2 appears independent of TGF-β activity, which is a downstream mediator of the TG2 functions in some biological systems. Instead, our data suggest a major role for cAMP/PKA signaling in transmitting TG2 signals in early chondrogenic differentiation. In summary, we demonstrate that matrix synthesis and early stages of chondrogenic differentiation are regulated through a novel mechanism involving TG2-dependent inhibition of PKA. These findings further advance understanding of cartilage formation and disease, and contribute to cartilage bioengineering.

Fluorescent probes of tissue transglutaminase reveal its association with arterial stiffening

Tissue transglutaminase (TG2) catalyzes the crosslinking of proteins. TG2 has been implicated in fibrosis and vascular calcification, both of which lead to a common feature of aging known as arterial stiffness. In order to probe the role of TG2 in arterial rigidification, we have prepared a fluorescent irreversible inhibitor as a probe for TG2 activity (RhodB-PGG-K(Acr)-LPF-OH). This probe was synthesized on solid support, characterized kinetically (k(inact) = 0.68 min⁻¹, K(I) = 79 μM), and then used to stain the aorta from rats used as a model of isolated systolic hypertension (ISH). Interestingly, TG2 activity was thus shown to increase over 4 weeks of the hypertension model, corresponding with the previously observed increase in arterial stiffness. These results clearly suggest an association between TG2 and the phenomenon of arterial rigidification.

Plasma membrane factor XIIIA transglutaminase activity regulates osteoblast matrix secretion and deposition by affecting microtubule dynamics

Transglutaminase activity, arising potentially from transglutaminase 2 (TG2) and Factor XIIIA (FXIIIA), has been linked to osteoblast differentiation where it is required for type I collagen and fibronectin matrix deposition. In this study we have used an irreversible TG-inhibitor to ‘block –and-track’ enzyme(s) targeted during osteoblast differentiation. We show that the irreversible TG-inhibitor is highly potent in inhibiting osteoblast differentiation and mineralization and reduces secretion of both fibronectin and type I collagen and their release from the cell surface. Tracking of the dansyl probe by Western blotting and immunofluorescence microscopy demonstrated that the inhibitor targets plasma membrane-associated FXIIIA. TG2 appears not to contribute to crosslinking activity on the osteoblast surface. Inhibition of FXIIIA with NC9 resulted in defective secretory vesicle delivery to the plasma membrane which was attributable to a disorganized microtubule network and decreased microtubule association with the plasma membrane. NC9 inhibition of FXIIIA resulted in destabilization of microtubules as assessed by cellular Glu-tubulin levels. Furthermore, NC9 blocked modification of Glu-tubulin into 150 kDa high-molecular weight Glu-tubulin form which was specifically localized to the plasma membrane. FXIIIA enzyme and its crosslinking activity were colocalized with plasma membrane-associated tubulin, and thus, it appears that FXIIIA crosslinking activity is directed towards stabilizing the interaction of microtubules with the plasma membrane. Our work provides the first mechanistic cues as to how transglutaminase activity could affect protein secretion and matrix deposition in osteoblasts and suggests a novel function for plasma membrane FXIIIA in microtubule dynamics.