Characterization of tissue transglutaminase in human osteoblast-like cells

Preclinical evaluation of an 18F-labeled Nε-acryloyllysine piperazide for covalent targeting of transglutaminase 2

BACKGROUND

Transglutaminase 2 (TGase 2) is a multifunctional protein and has a prominent role in various (patho)physiological processes. In particular, its transamidase activity, which is rather latent under physiological conditions, gains importance in malignant cells. Thus, there is a great need of theranostic probes for targeting tumor-associated TGase 2, and targeted covalent inhibitors appear to be particularly attractive as vector molecules. Such an inhibitor, equipped with a radionuclide suitable for noninvasive imaging, would be supportive for answering the general question on the possibility for functional characterization of tumor-associated TGase 2. For this purpose, the recently developed 18F-labeled Nε-acryloyllysine piperazide [18F]7b, which is a potent and selective irreversible inhibitor of TGase 2, was subject to a detailed radiopharmacological characterization herein.

RESULTS

An alternative radiosynthesis of [18F]7b is presented, which demands less than 300 µg of the respective trimethylammonio precursor per synthesis and provides [18F]7b in good radiochemical yields (17 ± 7%) and high (radio)chemical purities (≥ 99%). Ex vivo biodistribution studies in healthy mice at 5 and 60 min p.i. revealed no permanent enrichment of 18F-activity in tissues with the exception of the bone tissue. In vivo pretreatment with ketoconazole and in vitro murine liver microsome studies complemented by mass spectrometric analysis demonstrated that bone uptake originates from metabolically released [18F]fluoride. Further metabolic transformations of [18F]7b include mono-hydroxylation and glucuronidation. Based on blood sampling data and liver microsome experiments, pharmacokinetic parameters such as plasma and intrinsic clearance were derived, which substantiated the apparently rapid distribution of [18F]7b in and elimination from the organisms. A TGase 2-mediated uptake of [18F]7b in different tumor cell lines could not be proven. Moreover, evaluation of [18F]7b in melanoma tumor xenograft models based on A375-hS100A4 (TGase 2 +) and MeWo (TGase 2 -) cells by ex vivo biodistribution and PET imaging studies were not indicative for a specific targeting.

CONCLUSION

[18F]7b is a valuable radiometric tool to study TGase 2 in vitro under various conditions. However, its suitability for targeting tumor-associated TGase 2 is strongly limited due its unfavorable pharmacokinetic properties as demonstrated in rodents. Consequently, from a radiochemical perspective [18F]7b requires appropriate structural modifications to overcome these limitations.

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.

Transglutaminases and Obesity in Humans: Association of F13A1 to Adipocyte Hypertrophy and Adipose Tissue Immune Response

Transglutaminases TG2 and FXIII-A have recently been linked to adipose tissue biology and obesity, however, human studies for TG family members in adipocytes have not been conducted. In this study, we investigated the association of TGM family members to acquired weight gain in a rare set of monozygotic (MZ) twins discordant for body weight, i.e., heavy-lean twin pairs. We report that F13A1 is the only TGM family member showing significantly altered, higher expression in adipose tissue of the heavier twin. Our previous work linked adipocyte F13A1 to increased weight, body fat mass, adipocyte size, and pro-inflammatory pathways. Here, we explored further the link of F13A1 to adipocyte size in the MZ twins via a previously conducted TWA study that was further mined for genes that specifically associate to hypertrophic adipocytes. We report that differential expression of F13A1 (ΔHeavy-Lean) associated with 47 genes which were linked via gene enrichment analysis to immune response, leucocyte and neutrophil activation, as well as cytokine response and signaling. Our work brings further support to the role of F13A1 in the human adipose tissue pathology, suggesting a role in the cascade that links hypertrophic adipocytes with inflammation.

Assessment of expression and specific activities of transglutaminases TG1, TG2, and FXIII-A during osteoclastogenesis

Osteoclasts are large multinucleated bone-resorbing cells derived from monocyte/macrophage lineage. Macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL) drive the multi-stage osteoclastogenesis. Transglutaminases (TGs) are Ca²⁺- and thiol-dependent acyl transferases and protein crosslinking enzymes. TG enzyme family contains eight catalytically active enzymes TG1-7 and Factor XIII-A (FXIII-A). Recent studies have shown that TG1, TG2, and FXIII-A are present in osteoclasts and that TG2 and FXIII-A regulate osteoclastogenesis. In this study, we examined gene and protein expression and specific activities of TG1, TG2, and FXIII-A during osteoclastogenesis using "Hitomi peptides" in a day-by-day manner. We report that TG activities are highest in the differentiation and early fusion phases and then decrease dramatically. TG activities were upregulated by M-CSF and downregulated by addition of RANKL. FXIII-A was dramatically downregulated by RANKL, suggesting its involvement in M-CSF-mediated precursor commitment phase. TG1 and TG2 proteins were present throughout osteoclastogenesis, suggesting that they may have functions in both differentiation and fusion. In summary, the three TGs likely exert distinct functions at different stages of osteoclastogenesis. Our work also demonstrates that the "Hitomi peptides" are highly specific tools for detection of distinct TGs in a system where multiple TGs are present.

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.

Mineralization-inhibiting effects of transglutaminase-crosslinked polymeric osteopontin

Osteopontin (OPN) belongs to the SIBLING family (Small, Integrin-Binding LIgand N-linked Glycoproteins) of mineral-binding matrix proteins found in bones and teeth. OPN is a well-known inhibitor of matrix mineralization, and enzymatic modification of OPN can affect this inhibitory function. In bone, OPN exists both as a monomer and as a high-molecular-weight polymer - the latter is formed by transglutaminase-mediated crosslinking of glutamine and lysine residues in OPN to create homotypic protein assemblies. OPN can be covalently crosslinked by transglutaminase 2 (TG2) and Factor XIII-A. Polymeric OPN has increased binding to collagen and promotes osteoblast adhesion, but despite these initial observations, its role in mineralization is not clear. In this study, we investigated the effect of polymerized OPN on mineralization using a hydroxyapatite crystal growth assay and mineralizing MC3T3-E1 osteoblast cultures. In the cultures, endogenous polymeric OPN was detected after mineralization occurred. In cell-free conditions, TG2 was used to crosslink bovine OPN into its polymeric form, and atomic force microscopy and dynamic light scattering revealed variably-sized, large branched aggregates ranging across hundreds of nanometers. These OPN polymers inhibited the growth of hydroxyapatite crystals in solution at concentrations similar to monomeric OPN, although the crosslinking slightly reduced its inhibitory potency. When added to MC3T3-E1 osteoblast cultures, this exogenous polymeric OPN essentially did not inhibit mineralization when given during the later mineralization stages of culture; however, cultures treated early and then continuously with polymeric OPN throughout both the matrix assembly and mineral deposition stages showed reduced mineralization. Immunoblotting of protein extracts from these continuously treated cultures revealed exogenous OPN polymers incorporated into mature matrix that had not yet mineralized. These results suggest that in bone, the increased size and branched structure of crosslinked inhibitory polymeric OPN near the mineralization front could hinder it from accessing focal mineralization sites in the dense collagen-rich matrix, suggesting that OPN-crosslinking into polymers may represent a way to fine-tune the inhibitory potency of OPN on bone mineralization.

Cross-linked matrix rigidity and soluble retinoids synergize in nuclear lamina regulation of stem cell differentiation

A nanofilm of cross-linked collagen-I is equivalent to a relatively stiff matrix, which stiffens the nucleus, correlating broadly with lamin-A (including mutant progerin), retinoic acid transcription factor level and activity, and osteoinduction. In vitro results are supported by studies of ectopic bone formation in vivo.

Synergistic cues from extracellular matrix and soluble factors are often obscure in differentiation. Here the rigidity of cross-linked collagen synergizes with retinoids in the osteogenesis of human marrow mesenchymal stem cells (MSCs). Collagen nanofilms serve as a model matrix that MSCs can easily deform unless the film is enzymatically cross-linked, which promotes the spreading of cells and the stiffening of nuclei as both actomyosin assembly and nucleoskeletal lamin-A increase. Expression of lamin-A is known to be controlled by retinoic acid receptor (RAR) transcription factors, but soft matrix prevents any response to any retinoids. Rigid matrix is needed to induce rapid nuclear accumulation of the RARG isoform and for RARG-specific antagonist to increase or maintain expression of lamin-A as well as for RARG-agonist to repress expression. A progerin allele of lamin-A is regulated in the same manner in iPSC-derived MSCs. Rigid matrices are further required for eventual expression of osteogenic markers, and RARG-antagonist strongly drives lamin-A–dependent osteogenesis on rigid substrates, with pretreated xenografts calcifying in vivo to a similar extent as native bone. Proteomics-detected targets of mechanosensitive lamin-A and retinoids underscore the convergent synergy of insoluble and soluble cues in differentiation.

Transglutaminases factor XIII-A and TG2 regulate resorption, adipogenesis and plasma fibronectin homeostasis in bone and bone marrow

Appropriate bone mass is maintained by bone-forming osteoblast and bone-resorbing osteoclasts. Mesenchymal stem cell (MSC) lineage cells control osteoclastogenesis via expression of RANKL and OPG (receptor activator of nuclear factor κB ligand and osteoprotegerin), which promote and inhibit bone resorption, respectively. Protein crosslinking enzymes transglutaminase 2 (TG2) and Factor XIII-A (FXIII-A) have been linked to activity of myeloid and MSC lineage cells; however, in vivo evidence has been lacking to support their function. In this study, we show in mice that TG2 and FXIII-A control monocyte-macrophage cell differentiation into osteoclasts as well as RANKL production in MSCs and in adipocytes. Long bones of mice lacking TG2 and FXIII-A transglutaminases, show compromised biomechanical properties and trabecular bone loss in axial and appendicular skeleton. This was caused by increased osteoclastogenesis, a cellular phenotype that persists in vitro. The increased potential of TG2 and FXIII-A deficient monocytes to form osteoclasts was reversed by chemical inhibition of TG activity, which revealed the presence of TG1 in osteoclasts and assigned different roles for the TGs as regulators of osteoclastogenesis. TG2- and FXIII-A-deficient mice had normal osteoblast activity, but increased bone marrow adipogenesis, MSCs lacking TG2 and FXIII-A showed high adipogenic potential and significantly increased RANKL expression as well as upregulated TG1 expression. Chemical inhibition of TG activity in the null cells further increased adipogenic potential and RANKL production. Altered differentiation of TG2 and FXIII-A null MSCs was associated with plasma fibronectin (FN) assembly defect in cultures and FN retention in serum and marrow in vivo instead of assembly into bone. Our findings provide new functions for TG2, FXIII-A and TG1 in bone cells and identify them as novel regulators of bone mass, plasma FN homeostasis, RANKL production and myeloid and MSC cell differentiation.

Transglutaminase Crosslinking of SIBLING Proteins in Teeth

Transglutaminase 2 (TG2), a protein-crosslinking enzyme, participates in extracellular matrix maturation and cell adhesion in cartilage and bone. We hypothesized that TG2 has similar roles in teeth. A TG activity assay and immunoblotting of rat tooth extracts showed TG activity and the presence of high-molecular-weight forms of the SIBLING (Small Integrin-Binding LIgand N-linked Glycoprotein) proteins: dentin matrix protein 1 (DMP1), dentin phosphoprotein (DPP), and bone sialoprotein (BSP). DMP1 and BSP, each containing both glutamine and lysine residues critical for crosslink formation, readily formed polymers in vitro when incubated with TG2. The ability of glutamine-lacking DPP to form polymers in vitro and in vivo demonstrates that it could act as a lysine donor for crosslinking, potentially having protein crosslinking partner(s) in teeth. Consistent with a role in cell adhesion, the TG2 isoform was co-localized by immunohistochemistry with its substrates at cell-matrix adhesion sites, including along odontoblast tubules (DMP1 and DPP), in the pericellular matrix of cementocytes (DMP1), and in predentin (BSP).

Structural Stability of Intelectin-1

We study the structural stability of helical and nonhelical regions in chain A of human intelectin-1. Using a geometrical model introduced previously, we carried out a computational analysis based on the recently reported crystal structure of this protein by Kiessling et al. to quantify the resiliency of the native state to steric perturbations. Response to these perturbations is characterized by calculating, relative to the native state, the lateral, radial, and angular displacements of n-residue segments of the polypeptide chain centered on each residue. By quantifying the stability of the protein through six stages of unfolding, we are able to identify regions in chain A of intelectin-1 that are markedly affected by structural perturbations versus those that are relatively unaffected, the latter suggesting that the native-state geometry of these regions is essentially conserved. Importantly, residues in the vicinity of calcium ions comprise a conserved region, suggesting that Ca ions play a role not only in the coordination of carbohydrate hydroxyl groups but also in preserving the integrity of the structure.

Comparison of tissue transglutaminase 2 and bone biological markers osteocalcin, osteopontin and sclerostin expression in human osteoporosis and osteoarthritis

Osteoporosis (OP) and osteoarthritis (OA) are the most common joint diseases, with a high incidence in the elderly population. OP is characterized by trabecular bone remodeling and reabsorption, whereas articular cartilage and subchondral bone remodeling are major features of OA. Although classically considered as independent or even conflicting processes, clinical coexistence of OP and OA was recently described. Transglutaminase 2 (TG2) expression is considered a biomarker of OA, but its role in osteoporotic bone remodeling is still uncertain. We investigated TG2 and bone biological markers (Osteocalcin, Osteopontin, and Sclerostin) in osteoporotic and osteoarthritic osteocartilagineous tissue (n = 54) and human chondrocyte cultures in vitro by immunohistochemistry, immunofluorescence and RT-PCR. Histomorphometric evaluation of bone trabecular remodeling was also performed. In cartilage, TG2 expression was faint in control and OP and significantly less than in OA and OP + OA chondrocytes; the opposite was found for Osteocalcin, whereas Osteopontin and Sclerostin expression was similar. In the subchondral trabecular bone, osteocytes/osteoblasts TG2 expression was slight and similar comparing control, OP, OA, and OP + OA group, whereas Osteocalcin and Osteopontin expression was lower in OP compared to control, OA and OP + OA. Increased TG2 and reduced Osteocalcin expression were maintained in human osteoarthritic chondrocytes in vitro. Histomorphometric analysis confirmed reduced trabecular bone mass in OP and OP + OA compared with OA patients. TG2 represented a suitable biomarker of osteoarthritic chondrocyte activation, whereas osteocalcin and osteopontin characterized osteoporotic osteocyte/osteoblast changes; differences were lost in OP + OA patients, suggesting careful consideration when coexistence of the two diseases occurs.

Effect of retinoic acid and vitamin D3 on osteoblast differentiation and activity in aging

Several studies have evidenced that in aging, osteoblast functional activity is impaired: osteoblast proliferation is slower and matrix deposition is less efficient. Because peroxisome-proliferator-activated receptor γ2 (PPARγ2) and fatty acids are important inhibitory signals in osteoblast development, we have investigated in human primary osteoblasts obtained from patients of different ages, the influence of retinoic acid and calcitriol on enzymes involved in differentiative (PPARγ2, β-catenin, and insulin-like growth factor 1) and metabolic (carnitine palmitoyltransferase 1) intracellular pathways, and on transglutaminase 2, as enzyme fundamental for stabilizing the newly deposited extracellular matrix in bone. Retinoic acid and calcitriol influenced, respectively, proliferation and differentiation of osteoblasts, and an increase in PPARγ2 expression was observed following retinoic acid administration, whereas a decrease was observed following calcitriol administration. Aging widely influenced all parameters analyzed (the proliferation, differentiation, and new matrix deposition are significantly reduced in aged osteoblasts), with the exception of PPARγ2, which we found to be constitutively overexpressed and not modulated by retinoic acid or calcitriol administration. Our findings show the impaired ability of aged osteoblasts to perform adequate functional response and draw attention to the therapeutic approaches for bone healing in elderly patients.

Tissue transglutaminase: an emerging target for therapy and imaging

Tissue transglutaminase (transglutaminase 2) is a multifunctional enzyme with many interesting properties resulting in versatile roles in both physiology and pathophysiology. Herein, the particular involvement of the enzyme in human diseases will be outlined with special emphasis on its role in cancer and in tissue interactions with biomaterials. Despite recent progress in unraveling the different cellular functions of transglutaminase 2, several questions remain. Transglutaminase 2 features in both confirmed and some still ambiguous roles within pathological conditions, raising interest in developing inhibitors and imaging probes which target this enzyme. One important prerequisite for identifying and characterizing such molecular tools are reliable assay methods to measure the enzymatic activity. This digest Letter will provide clarification about the various assay methods described to date, accompanied by a discussion of recent progress in the development of inhibitors and imaging probes targeting transglutaminase 2.

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.

Polyamine delivery as a tool to modulate stem cell differentiation in skeletal tissue engineering

The first step in skeleton development is the condensation of mesenchymal precursors followed by any of two different types of ossification, depending on the type of bone segment: in intramembranous ossification, the bone is deposed directly in the mesenchymal anlagen, whereas in endochondral ossification, the bone is deposed onto a template of cartilage that is subsequently substituted by bone. Polyamines and polyamine-related enzymes have been implicated in bone development as global regulators of the transcriptional and translational activity of stem cells and pivotal transcription factors. Therefore, it is tempting to investigate their use as a tool to improve regenerative medicine strategies in orthopedics. Growing evidence in vitro suggests a role for polyamines in enhancing differentiation in both adult stem cells and differentiated chondrocytes. Adipose-derived stem cells have recently proved to be a convenient alternative to bone marrow stromal cells, due to their easy accessibility and the high frequency of stem cell precursors per volume unit. State-of-the-art "prolotherapy" approaches for skeleton regeneration include the use of adipose-derived stem cells and platelet concentrates, such as platelet-rich plasma (PRP). Besides several growth factors, PRP also contains polyamines in the micromolar range, which may also exert an anti-apoptotic effect, thus helping to explain the efficacy of PRP in enhancing osteogenesis in vitro and in vivo. On the other hand, spermidine and spermine are both able to enhance hypertrophy and terminal differentiation of chondrocytes and therefore appear to be inducers of endochondral ossification. Finally, the peculiar activity of spermidine as an inducer of autophagy suggests the possibility of exploiting its use to enhance this cytoprotective mechanism to counteract the degenerative changes underlying either the aging or degenerative diseases that affect bone or cartilage.

Transglutaminase-2 Is Involved in Expression of Osteoprotegerin in MG-63 Osteosarcoma Cells

Osteoprotegerin (OPG) is a secreted glycoprotein and a member of the tumor necrosis factor receptor superfamily. It usually functions in bone remodeling, by inhibiting osteoclastogenesis through interaction with a receptor activator of the nuclear factor κB (RANKL). Transglutaminases-2 (Tgase-2) is a group of multifunctional enzymes that plays a role in cancer cell metastasis and bone formation. However, relationship between OPG and Tgase-2 is not studied. Therefore, we investigated the involvement of 12-O-Tetradecanoylphorbol 13-acetate in the expression of OPG in MG-63 osteosarcoma cells. Interleukin-1β time-dependently induced OPG and Tgase-2 expression in cell lysates and media of the MG-63 cells by a Western blot. Additional 110 kda band was found in the media of MG-63 cells. 12-O-Tetradecanoylphorbol 13-acetate also induced OPG and Tgase-2 expression. However, an 110 kda band was not found in TPA-treated media of MG-63 cells. Cystamine, a Tgase-2 inhibitor, dose-dependently suppressed the expression of OPG in MG-63 cells. Gene silencing of Tgase-2 also signifi cantly suppressed the expression of OPG in MG-63 cells. Next, we examined whether a band of 110 kda of OPG contains an isopeptide bond, an indication of Tgase-2 action, by monoclonal antibody specifi c for the isopeptide bond. However, we could not fi nd the isopeptide bond at 110 kda but 77 kda, which is believed to be the band position of Tgase-2. This suggested that 110 kda is not the direct product of Tgase-2’s action. All together, OPG and Tgase-2 is induced by IL-1β or TPA in MG-63 cells and Tgase-2 is involved in OPG expression in MG-63 cells.

Tissue transglutaminase (TG2) activity regulates osteoblast differentiation and mineralization in the SAOS-2 cell line

Tissue transglutaminase (type II, TG2) has long been postulated to directly promote skeletal matrix calcification and play an important role in ossification. However, limited information is available on the expression, function and modulating mechanism of TG2 during osteoblast differentiation and mineralization. To address these issues, we cultured the well-established human osteosarcoma cell line SAOS-2 with osteo-inductive conditioned medium and set up three time points (culture days 4, 7, and 14) to represent different stages of SAOS-2 differentiation. Osteoblast markers, mineralization, as well as TG2 expression and activity, were then assayed in each stage. Furthermore, we inhibited TG activity with cystamine and then checked SAOS-2 differentiation and mineralization in each stage. The results showed that during the progression of osteoblast differentiation SAOS-2 cells presented significantly high levels of osteocalcin (OC) mRNA, bone morphogenetic protein-2 (BMP-2) and collagen I, significantly high alkaline phosphatase (ALP) activity, and the increased formation of calcified matrix. With the same tendency, TG2 expression and activity were up-regulated. Furthermore, inhibition of TG activity resulted in a significant decrease of OC, collagen I, and BMP-2 mRNA and of ALP activity and mineralization. This study demonstrated that TG2 is involved in osteoblast differentiation and may play a role in the initiation and regulation of the mineralization processes. Moreover, the modulating effects of TG2 on osteoblasts may be related to BMP-2.

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.

Astrocyte-derived tissue transglutaminase interacts with fibronectin: a role in astrocyte adhesion and migration?

An important neuropathological feature of neuroinflammatory processes that occur during e.g. Multiple Sclerosis (MS) is the formation of an astroglial scar. Astroglial scar formation is facilitated by the interaction between astrocytes and extracellular matrix proteins (ECM) such as fibronectin. Since there is evidence indicating that glial scars strongly inhibit both axon growth and (re)myelination in brain lesions, it is important to understand the factors that contribute to the interaction between astrocytes and ECM proteins. Tissue Transglutaminase (TG2) is a multifunctional enzyme with an ubiquitous tissue distribution, being clearly present within the brain. It has been shown that inflammatory cytokines can enhance TG2 activity. In addition, TG2 can mediate cell adhesion and migration and it binds fibronectin with high affinity. We therefore hypothesized that TG2 is involved in astrocyte-fibronectin interactions. Our studies using primary rat astrocytes show that intracellular and cell surface expression and activity of TG2 is increased after treatment with pro-inflammatory cytokines. Astrocyte-derived TG2 interacts with fibronectin and is involved in astrocyte adhesion onto and migration across fibronectin. TG2 is involved in stimulating focal adhesion formation which is necessary for the interaction of astrocytes with ECM proteins. We conclude that astrocyte-derived TG2 contributes to the interaction between astrocytes and fibronectin. It might thereby regulate ECM remodeling and possibly glial scarring.

Transglutaminase-mediated oligomerization promotes osteoblast adhesive properties of osteopontin and bone sialoprotein

Tissue transglutaminase (TG2) is a widely distributed, protein-crosslinking enzyme having a prominent role in cell adhesion as a β1 integrin co-receptor for fibronectin. In bone and teeth, its substrates include the matricellular proteins osteopontin (OPN) and bone sialoprotein (BSP). The aim of this study was to examine effects of TG2-mediated crosslinking and oligomerization of OPN and BSP on osteoblast cell adhesion. We show that surfaces coated with oligomerized OPN and BSP promote MC3T3-E1/C4 osteoblastic cell adhesion significantly better than surfaces coated with the monomeric form of the proteins. Both OPN and BSP oligomer-adherent cells showed more cytoplasmic extensions than those cells grown on the monomer-coated surfaces indicative of increased cell connectivity. Our study suggests a role for TG2 in promoting the cell adhesion function of two matricellular substrate proteins prominent in bone, tooth cementum and certain tumors.

Importance of syndecan-4 and syndecan -2 in osteoblast cell adhesion and survival mediated by a tissue transglutaminase-fibronectin complex

Tissue transglutaminase (TG2) has been identified as an important extracellular crosslinking enzyme involved in matrix turnover and in bone differentiation. Here we report a novel cell adhesion/survival mechanism in human osteoblasts (HOB) which requires association of FN bound TG2 with the cell surface heparan sulphates in a transamidase independent manner. This novel pathway not only enhances cell adhesion on FN but also mediates cell adhesion and survival in the presence of integrin competing RGD peptides. We investigate the involvement of cell surface receptors and their intracellular signalling molecules to further explore the pathway mediated by this novel TG-FN heterocomplex. We demonstrate by siRNA silencing the crucial importance of the cell surface heparan sulphate proteoglycans syndecan-2 and syndecan-4 in regulating the compensatory effect of TG-FN on osteoblast cell adhesion and actin cytoskeletal formation in the presence of RGD peptides. By use of immunoprecipitation and inhibitory peptides we show that syndecan-4 interacts with TG2 and demonstrate that syndecan-2 and the α5β1 integrins, but not α4β1 function as downstream modulators in this pathway. Using function blocking antibodies, we show activation of α5β1 occurs by an inside out signalling mechanism involving activation and binding of protein kinase PKCα and phosphorylation of focal adhesion kinase (FAK) at Tyr(861) and activation of ERK1/2.

Tissue transglutaminase expression and activity in human ligamentum flavum cells derived from thoracic ossification of ligamentum flavum

STUDY DESIGN

The study was undertaken to compare the expression and activity of tissue transglutaminase (TG2) in human ligamentum flavum cells derived from ossification of the ligamentum flavum (OLF) and non-OLF patients.

OBJECTIVE

To determine whether TG2 is involved in the pathologic process of OLF.

SUMMARY OF BACKGROUND DATA

OLF is a disease characterized by heterotopic formation of new bone in the flavum ligament. Recently, TG2 is proved to directly promote skeletal matrix mineralization and play an important role in the ossification. TG2 activity is vital to the differentiation of osteoblasts and the formation of mineralization. But whether TG2 is involved in the pathologic process of OLF is unknown. We investigated the relations between TG2 expression and OLF.

METHODS

OLF and non-OLF cells were cultured and osteocalcin, bone morphogenetic protein-2(BMP-2) and TG2 mRNA expressions were assayed by reverse transcription polymerase chain reaction. Meanwhile, alkaline phosphatase activity and calcified nodules were compared between OLF and non-OLF cells. To detect TG2 expression, Western blot and immunohistochemical analysis were carried out, and TG2 activity was compared between OLF and non-OLF cells.

RESULTS

Our experiments demonstrated that OLF cells showed osteoblast-like activity and increased mRNA expression of BMP-2. More interesting, compared with non-OLF cells, OLF cells showed elevated expression levels of TG2 mRNA and protein, as well as enzyme activity.

CONCLUSION

TG2 expression and enzyme activity are upregulated in the OLF cells and TG2 may be involved in the pathologic process of OLF.

Serotonin--more than a neurotransmitter: transglutaminase-mediated serotonylation of C6 glioma cells and fibronectin

In the central nervous system serotonin plays important roles as a neurotransmitter as well as during neuronal development and in synaptogenesis. Outside the central nervous system, serotonin is covalently transamidated to procoagulant proteins involved in blood clotting. This process is mediated by transglutaminases and named "serotonylation". Serotonylated proteins then tightly bind to specific serotonin binding sites on fibrinogen and thrombospondin to form stable extracellular multivalent complexes needed for thrombus formation. Here, we have investigated whether transglutaminases can also covalently incorporate extracellular serotonin to neural proteins and whether this might affect extracellular protein expression. Our data reveal that recombinant transglutaminase specifically transamidates [(3)H]-serotonin to cell-surface proteins from C6 glioma cells and the extracellular matrix protein fibronectin. Serotonylation of [(3)H]-serotonin was inhibited by the transglutaminase inhibitor cystamine and unlabelled serotonin. Transglutaminase-mediated transamidation of unlabelled serotonin to C6 cells induced an aggregation of extracellular protein matrices adjacent to and between single cells. Transglutaminase also transamidated the autofluorescent serotonin analogue 5,7-dihydroxytryptamine and monodansylcadaverine (MDC) into living C6 glioma cells. Electrophoretic separation of MDC-labelled C6 cells identified several distinct fluorescent proteins one of which was fibronectin.

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

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

Heparan sulfate proteoglycans are receptors for the cell-surface trafficking and biological activity of transglutaminase-2

Transglutaminase type 2 (TG2) is both a protein cross-linking enzyme and a cell adhesion molecule with an elusive unconventional secretion pathway. In normal conditions, TG2-mediated modification of the extracellular matrix modulates cell motility, proliferation and tissue repair, but under continuous cell insult, higher expression and elevated extracellular trafficking of TG2 contribute to the pathogenesis of tissue scarring. In search of TG2 ligands that could contribute to its regulation, we characterized the affinity of TG2 for heparan sulfate (HS) and heparin, an analogue of the chains of HS proteoglycans (HSPGs). By using heparin/HS solid-binding assays and surface plasmon resonance we showed that purified TG2 has high affinity for heparin/HS, comparable to that for fibronectin, and that cell-surface TG2 interacts with heparin/HS. We demonstrated that cell-surface TG2 directly associates with the HS chains of syndecan-4 without the mediation of fibronectin, which has affinity for both syndecan-4 and TG2. Functional inhibition of the cell-surface HS chains of wild-type and syndecan-4-null fibroblasts revealed that the extracellular cross-linking activity of TG2 depends on the HS of HSPG and that syndecan-4 plays a major but not exclusive role. We found that heparin binding did not alter TG2 activity per se. Conversely, fibroblasts deprived of syndecan-4 were unable to effectively externalize TG2, resulting in its cytosolic accumulation. We propose that the membrane trafficking of TG2, and hence its extracellular activity, is linked to TG2 binding to cell-surface HSPG.

Use of tissue transglutaminase and fibronectin to influence osteoblast responses to tricalcium phosphate scaffolds

To explore the possibility of controlling cell interaction with biomaterials, tricalcium phosphate scaffolds were modified using the enzyme tissue transglutaminase (tTgase) in conjunction with fibronectin. Previous reports in the literature have highlighted a number of favourable responses that this protein-enzyme complex can stimulate, including enhancing both cell adhesion, and mineralisation. Fibronectin and tTgase alone were used as controls, and a series of different concentrations of tTgase and fibronectin in combination were assessed. Cell metabolic activity, alkaline phosphatase production, and collagen content were all measured in cultures up to 28 days. Using tetracycline labelling, calcium containing multilayered regions were imaged and quantified. Addition of 6 microg fibronectin resulted in increased alkaline phosphatase activity in all combinations, while increased transglutaminase resulted in more collagen in the cell lysates. Samples treated with fibronectin produced many small mineralised areas, those with 6 microg fibronectin and transglutaminase produced numerous large mineralised areas. The mixture of fibronectin and transglutaminase may prove to be a useful treatment for producing increased osteoblast differentiation on scaffolds.

Enhanced osteoblast adhesion on transglutaminase 2-crosslinked fibronectin

Fibronectin (FN) is a cell adhesion protein that binds integrins in a process also involving the protein-crosslinking enzyme transglutaminase 2 (TG2) as a co-receptor. The cell-adhesive property of TG2 has been linked to a complex formation with FN and to its ability to crosslink and polymerize FN on the cell surface. We tested here the effects of extracellular FN, before and after in vitro crosslinking and polymerization by TG2, on MC3T3-E1 osteoblast adhesion. We show that TG2-mediated crosslinking creates large, compacted chain-like protein clusters that include both TG2 and FN molecules as analyzed by Western blotting and atomic force microscopy. Crosslinking of FN significantly promotes osteoblast adhesion as measured by crystal violet staining, and enhances beta(1)-integrin clustering on the cell surface as visualized by immunofluorescence microscopy. We hypothesize that TG2-mediated crosslinking enhances the cell-adhesive properties of FN by increasing the molecular rigidity of FN in the extracellular matrix.

Factor XIIIA mobilizes transglutaminase 2 to induce chondrocyte hypertrophic differentiation

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

Transglutaminase activity regulates osteoblast differentiation and matrix mineralization in MC3T3-E1 osteoblast cultures

Transglutaminase (TG) enzymes and protein crosslinking have long been implicated in the formation of mineralized tissues. The aim of this study was to analyze the expression, activity and function of TGs in differentiating osteoblasts to gain further insight into the role of extracellular matrix protein crosslinking in bone formation. MC3T3-E1 (subclone 14) pre-osteoblast cultures were treated with ascorbic acid and beta-glycerophosphate to induce cell differentiation and matrix mineralization. Expression of TG isoforms was analyzed by RT-PCR. TG activity was assessed during osteoblast differentiation by in vitro biochemical assays and by in situ labeling of live cell cultures. We demonstrate that MC3T3-E1/C14 osteoblasts express two TG isoforms--TG2 and FXIIIA. Abundant TG activity was observed during cell differentiation which increased significantly after thrombin treatment, a result confirming the presence of FXIIIA in the cultures. Ascorbic acid treatment, which stimulated collagen secretion and assembly, also stimulated externalization of TG activity, likely from FXIIIA which was externalized upon this treatment as analyzed by immunofluoresence microscopy. Inhibition of TG activity in the cultures by cystamine resulted in complete abrogation of mineralization, attributable to decreased matrix accumulation and an arrested state of osteoblast differentiation as measured by decreased levels of bone sialoprotein, osteocalcin and alkaline phosphatase. Additional functional studies and substrate characterization showed that TG activity was required for the formation of a fibronectin-collagen network during the early stages of matrix formation and assembly. This network, in turn, appeared to be essential for further matrix production and progression of the osteoblast differentiation program, and ultimately for mineralization.

Effect of retinoic acid-induced transglutaminase on cell growth in regenerating liver

Transglutaminase 2 (TG2) is implicated in the inhibitory regulation of the hepatocyte growth in vitro. In vivo, however, the role of TG2 in liver regeneration after partial hepatectomy (PH) is almost unknown. A dramatic increase of TG2 expression and activation is induced by retinoic acid (RA). Here we show the effect of the RA-induced overexpression of TG2 on liver regeneration after PH. Regenerating rat liver was prepared by 70% PH. RA was intraperitoneally injected immediately after PH. TG2 activity was determined by incorporation of 14C-putrescine into dimethylcasein. Cell cycle was evaluated for incorporation of BrdU into hepatocytes and detected by a flow cytometric analysis. The treatment of RA greatly increased TG2 activity at 1 day after PH. At that time, DNA synthesis was significantly reduced by the treatment of RA. The recovery of liver weight after PH was significantly delayed by the treatment of RA. These results suggested that TG2 was involved in growth capacity in regenerating rat liver after PH.

Relationship between Liver Injury and Transglutaminase Activities in Guinea Pigs and Rats

We investigated the effect of transglutaminase (TGase) on in guinea pigs and rats. Serum alanine aminotransferase (ALT) level increased 1 d after CCl(4) treatment of both in guinea pigs and rats since TGaese activity was greatly higher in guinea pigs than rats. However, serum ALT level in guinea pigs was very much lower than that in rats. Liver TGase activities decreased after CCl(4) treatment in both guinea pigs and rats. However, TGase activities in the liver from guinea pigs were higher than that from rats. Decreased TGase activities by CCl(4) in the liver from guinea pigs and rats were significantly recovered by retinoic acid treatment that was reported to increase TGase. Degree of recovery of serum ALT level by retinoic acid in rats was larger than in guinea pigs. These results suggested that the distinction of the effect of retinoic acid on serum ALT level in CCl(4)-treated animals was due to the different TGase activity that increased membrane stability.

A novel RGD-independent cel adhesion pathway mediated by fibronectin-bound tissue transglutaminase rescues cells from anoikis

Specific association of tissue transglutaminase (tTG) with matrix fibronectin (FN) results in the formation of an extracellular complex (tTG-FN) with distinct adhesive and pro-survival characteristics. tTG-FN supports RGD-independent cell adhesion of different cell types and the formation of distinctive RhoA-dependent focal adhesions following inhibition of integrin function by competitive RGD peptides and function blocking anti-integrin antibodies alpha5beta1. Association of tTG with its binding site on the 70-kDa amino-terminal FN fragment does not support this cell adhesion process, which seems to involve the entire FN molecule. RGD-independent cell adhesion to tTG-FN does not require transamidating activity, is mediated by the binding of tTG to cell-surface heparan sulfate chains, is dependent on the function of protein kinase Calpha, and leads to activation of the cell survival focal adhesion kinase. The tTG-FN complex can maintain cell viability of tTG-null mouse dermal fibroblasts when apoptosis is induced by inhibition of RGD-dependent adhesion (anoikis), suggesting an extracellular survival role for tTG. We propose a novel RGD-independent cell adhesion mechanism that promotes cell survival when the anti-apoptotic role mediated by RGD-dependent integrin function is reduced as in tissue injury, which is consistent with the externalization and binding of tTG to fibronectin following cell damage/stress.

Establishment of osteoblast culture from human ethmoidal sinus

OBJECTIVE

Chronic sinusitis is characterized by persistent chronic inflammation of the sinus system and local expression and release of various cytokines, such as IL-1 beta, IL-4, IL-6, IL-8, TGF-beta and TNF-alpha are deeply involved in the pathogenesis of the disease. We hypothesized that not only the sinus mucosa-containing cells, such as epithelial cells and fibroblasts, but also osteoblasts, of which sinus bone structure consists, may contribute to this inflammatory network. This study evaluates the development and establishment of an osteoblasts culture system derived from human ethmoidal sinus as an initial step toward verifying this hypothesis.

METHODS

Ethmoidal sinus bone was obtained from patients at the time of sinus surgery for chronic sinusitis and outgrowth cell sheets were obtained according to the explant-outgrowth cell culture technique. In order to examine the specific characteristics of osteoblasts in the obtained cells, four major features of osteoblasts (collagen type I, osteocalcin synthesis, alkaline phosphatase activity and extracellular matrix mineralization ability) were investigated at the third passage of the culture and productions of TGF-beta 1, which modulate osteoblast proliferation and maturation in an autocrine fashion, in the cultured medium was also investigated in time of culture up to 20 days.

RESULTS

The cells obtained in our study clearly show collagen type I synthesis, osteocalcin synthesis, alkaline phosphatase activity and production of visible extracellular matrix mineralization. Production of TGF-beta 1 in the medium did not significantly different in time of culture up to 20 days.

CONCLUSION

Our results, the first of their kind, indicate that osteoblast-like cells can be cultured from adult human ethmoidal sinus bones. It suggested that proliferation and maturation of the osteoblast were continuously modulated in an autocrine fashion by producing TGF-beta 1.

Tissue transglutaminase and its substrates in bone

Tissue transglutaminase (tTG) is an intra- and extracellular, protein-cross-linking enzyme that has been implicated in apoptosis, matrix stabilization, and cell attachment in a variety of tissues. This study provides in vivo evidence in bone of TG activity, its tissue localization, and identification of its substrates. In microplate- and blotting-based activity assays using biotinylated primary amine as a probe, we show TG activity in protein extracts from the mineralized compartment of intramembranous rat bone. Avidin affinity purification of bone extract labeled with biotinylated primary amine in the presence of tTG, in conjunction with Western blotting, permitted identification of three major noncollagenous TG substrates in bone: osteopontin (OPN), bone sialoprotein (BSP), and alpha2 HS-glycoprotein (AHSG), of which the latter two are novel substrates. Cross-linking and labeling of purified proteins confirmed their ability to serve as TG substrates, because they readily incorporated biotinylated primary amine and formed large protein aggregates in the presence of tTG. All three proteins were also identified in the high molecular weight complexes extractable from the mineralized compartment of bone. Two-dimensional (2D) gel electrophoretic analysis combined with Western blotting indicated that the proteins are not cross-linked to each other, but form distinct homotypic polymers. In the extracellular matrix of bone, tTG and isopeptide bonds were localized by immunohistochemistry in the osteoid and in the pericellular matrix surrounding osteocytes. At the cellular level, osteoblasts and osteocytes were immunostained for tTG. Collectively, these data suggest a role for tTG and its covalently cross-linked substrates in cell adhesion and possibly also in bone matrix maturation and calcification.

Involvement of tissue transglutaminase in the stabilisation of biomaterial/tissue interfaces important in medical devices

Tissue transglutaminase (tTG) has recently been established as a novel cell surface adhesion protein that binds with high affinity to fibronectin in the pericellular matrix. In this study, we have made use of this property to enhance the biocompatibility of poly(epsilon-caprolactone) (PCL), a biomaterial currently used in bone repair. Poly(epsilon-caprolactone) discs were first coated with fibronectin and then tTG. The surface localisation of the two proteins was confirmed using ELISA and the tTG shown to be active on the surface by incorporation of biotin cadaverine into the fibronectin coating. When human osteoblasts (HOBs) were seeded onto the coated polymer surfaces in serum free medium, the surface coated with fibronectin and then tTG showed an increase in the spreading of the cells as compared to the surface coated with fibronectin alone, when analysed using environmental scanning electron microscopy. The presence of tTG had no effect on HOB cell differentiation when analysed by determining alkaline phosphatase activity. The use of tTG as a novel adhesion protein in this way may therefore have considerable potential in forming a stable tissue/biomaterial interface for application in medical devices.