Association of a transglutaminase-related antigen with intermediate filaments

The Motility and Mesenchymal Features of Breast Cancer Cells Correlate with the Levels and Intracellular Localization of Transglutaminase Type 2

We have investigated motility in breast cancer cell lines in association with the expression of Transglutaminase type 2 (TG2) as well as upon the administration of Doxorubicin (Dox), an active cytotoxic agent that is employed in chemotherapy. The exposure of MCF-7 cells to the drug increased TG2 levels, triggering epithelial–mesenchymal transition (EMT), thereby supporting cell motility. The effects of Dox on the movement of MCF-7 cells were counteracted by treatment with NC9, a TG2 inhibitor, which induced morphological changes and also reduced the migration of MDA-MB-231 cells exhibiting high levels of TG2. The physical association of TG2 with the cytoskeletal component vimentin appeared pivotal both in drug-treated MCF-7 and in MDA-MB-231 cells and seemed to be independent of the catalytic activity of TG2. NC9 altered the subcellular distribution of TG2 and, consequently, the co-localization of TG2 with vimentin. Furthermore, NC9 induced a nuclear accumulation of TG2 as a prelude to TG2-dependent gene expression modifications. Since enzyme activity can affect both motility and nuclear functions, targeting of this protein could represent a method to improve therapeutic interventions in breast tumors, particularly those to control progression and to limit drug resistance.

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.

Polymorphism of transglutaminase 2: unusually low frequency of genomic variants with deficient functions

Transglutaminase 2 (TG2) is a multifunctional member of an enzyme family: it modifies glutamine residues by cross-linking proteins and incorporating primary amines into them, has protein disulphide isomerase and protein kinase activities, mediates trans-membrane signal transduction and interactions between cell surface proteins and the extracellular matrix. These unusual multiple roles encoded into one polypeptide chain suggest that genomic variations in the TGM2 gene should be limited. Indeed, the available information in databases shows that unlike in the case of most other transglutaminases there are no common single nucleotide polymorphisms in exons of human TGM2. We collected data on and produced some of the rare genetic variants of TGM2 by site directed mutagenesis and found that some were less stable than the most abundant (wild type) enzyme variant and the majority had deficient transamidating activity. Further studies are required to clarify the pathologic significance of these rare TGM2 alleles in the human population.

Deamidated gliadin peptides form epitopes that transglutaminase antibodies recognize

OBJECTIVE

Deamidated gliadin peptides are efficient antigens in diagnostic tests for celiac disease, and results correlate better with transglutaminase 2-based assays than those with native gliadin. We investigated whether deamidated gliadin antigens are structurally similar to transglutaminase 2 or could mimic transglutaminase epitopes.

PATIENTS AND METHODS

Serum samples from 74 celiac and 65 control patients, and 13 different transglutaminase 2-specific monoclonal mouse antibodies were investigated for their binding to commercially available deamidated gliadin peptides using enzyme-linked immunosorbent assay, competition studies, and molecular modelling.

RESULTS

The enzyme-linked immunosorbent assay with deamidated gliadin peptides had 100% sensitivity and 98.5% specificity in patients. Deamidated gliadin epitopes also were recognized by 3 transglutaminase-specific monoclonal antibodies, and antibodies affinity-purified with deamidated gliadin peptides from celiac patient sera reacted with transglutaminase but did not show endomysial binding. The binding of the monoclonal antibodies to deamidated gliadin was inhibited dose dependently by full-length recombinant human transglutaminase, its fragments containing the binding sites of these monoclonal antibodies, or by celiac patient antibodies. Deamidated gliadin peptides decreased the binding of transglutaminase-specific monoclonal antibodies to transglutaminase. Three different cross-reacting transglutaminase epitopes were found, of which 2 are located in the C-terminal domain and 1 is conformational. The binding of celiac serum samples to deamidated gliadin peptides could not be abolished by transglutaminase or by any of the transglutaminase-specific monoclonals, indicating that celiac sera also contain additional antibodies to gliadin epitopes different from transglutaminase.

CONCLUSIONS

Certain deamidated gliadin-derived peptides and transglutaminase 2 epitopes have similar 3-dimensional appearance. This homology may contribute to the induction of transglutaminase autoantibodies by molecular mimicry.

Arterial vimentin is a transglutaminase substrate: a link between vasomotor activity and remodeling?

BACKGROUND/AIMS

The transglutaminases (TG2 and factor XIIIa) may contribute to the stability of arteries by cross-linking a variety of substrates, including extracellular matrix proteins and protease inhibitors. The preferred substrates have never been determined, however.

METHODS

We used an amine donor, 5-biotinamidopentylamine, that is covalently linked to acceptor glutamine residues, to determine transglutaminase substrates in carotid endarterectomy tissue.

RESULTS

The incorporation of 5-biotinamidopentylamine was calcium dependent, resulting in the labeling of several proteins that were detected by streptavidin-peroxidase and purified over a monomeric avidin affinity column. A major band of 42 kDa that was eluted from the column was sequenced along with 2 additional bands (80 and 65 kDa), revealing an internal fragment of vimentin, transferrin and albumin, respectively. Vimentin dimers were detected in 5 out of 5 carotid plaque homogenates.

CONCLUSIONS

Vimentin is a major arterial substrate for transglutaminases. It has previously been shown that TG2 activity and vimentin contribute to vasomotor activity of arteries. Furthermore, transglutaminases (both TG2 and factor XIIIa), as well as vimentin, regulate structural alterations (inward remodeling) that occur in response to low flow states. Transglutaminase-mediated vimentin dimerization produces a novel unifying pathway by which vasodilatory and remodeling responses may be regulated.

Endomysial antibody-negative coeliac disease: clinical characteristics and intestinal autoantibody deposits

BACKGROUND

Some patients with untreated coeliac disease are negative for serum endomysial autoantibodies (EmA) targeted against transglutaminase 2 (TG2).

AIMS

To evaluate the clinical and histological features of EmA-negative coeliac disease, and to examine whether EmA-equivalent autoantibodies against TG2 can be seen in the small-bowel mucosa when absent in serum.

PATIENTS

Serum EmA was studied in 177 biopsy-proved specimens from adult patients with coeliac disease. 20 patients with intestinal diseases served as non-coeliac controls; three had autoimmune enteropathy with villous atrophy.

METHODS

Clinical manifestations, small-bowel mucosal morphology, intraepithelial inflammation and TG2-specific extracellular immunoglobulin A (IgA) deposits were investigated in both serum EmA-negative and EmA-positive patients.

RESULTS

22 patients with IgA-competent coeliac disease were negative for serum EmA. Three of these had small-bowel lymphoma. Patients with EmA-negative coeliac disease were older, had abdominal symptoms more often, and the density of gammadelta+ intraepithelial lymphocytes in their intestinal mucosa was lower than in EmA-positive patients; otherwise the histology was similar. All serum EmA-negative patients with coeliac disease, but none of the disease controls, had gluten-dependent mucosal IgA deposits alongside TG2 in the small-bowel mucosal specimens. In vivo deposited IgA was shown to be TG2-specific by its ability to bind recombinant TG2.

CONCLUSIONS

Negative serum EmA might be associated with advanced coeliac disease. TG2-targeted autoantibodies were deposited in the small-bowel mucosa even when absent in serum. This finding can be used in the diagnosis of seronegative coeliac disease when the histology is equivocal. It may also be helpful in the differential diagnosis between autoimmune enteropathy and coeliac disease.

Mitochondrial injury in steatohepatitis

Rich diet and lack of exercise are causing a surge in obesity, insulin resistance and steatosis, which can evolve into steatohepatitis. Patients with non-alcoholic steatohepatitis have increased lipid peroxidation, increased tumour necrosis factor-alpha (TNF-alpha) and increased mitochondrial beta-oxidation rates. Their in-vivo ability to re-synthesize ATP after a fructose challenge is decreased, and their hepatic mitochondria exhibit ultrastructural lesions, depletion of mitochondrial DNA and decreased activity of respiratory chain complexes. Although the mechanisms for these effects is unknown, the basal cellular formation of reactive oxygen species (ROS) may oxidize fat deposits to cause lipid peroxidation, which damages mitochondrial DNA, proteins and cardiolipin to partially hamper the flow of electrons within the respiratory chain. This flow may be further decreased by TNF-alpha, which can release cytochrome c from mitochondria. Concomitantly, the increased mitochondrial fatty acid beta-oxidation rate augments the delivery of electrons to the respiratory chain. Due to the imbalance between a high electron input and a restricted outflow, electrons may accumulate within complexes I and III, and react with oxygen to form the superoxide anion radical. Increased mitochondrial ROS formation could in turn directly oxidize mitochondrial DNA, proteins and lipids, enhance lipid peroxidation-related mitochondrial damage, trigger hepatic TNF-alpha formation and deplete antioxidants, thus further blocking electron flow and further increasing mitochondrial ROS formation. Mitochondrial dysfunction plays an important role in liver lesions, through the ROS-induced release of both biologically active lipid peroxidation products and cytokines. In particular, the up-regulation of both TNF-alpha and Fas triggers mitochondrial membrane permeability and apoptosis. The ingestion of apoptotic bodies by stellate cells stimulates fibrogenesis, which is further activated by lipid peroxidation products and high leptin levels. Chronic apoptosis is compensated by increased cell proliferation, which, together with oxidative DNA damage, may cause gene mutations and cancer.

Activation of transglutaminase in mu-calpain null erythrocytes

Intracellular transglutaminases (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) are calcium-dependent thiol enzymes that catalyze the covalent cross-linking of proteins, including those in the erythrocyte membrane. Several studies suggest that the activation of some transglutaminases is positively regulated by the calcium-dependent cysteine protease, mu-calpain. Using mu-calpain null (Capn1(-/-)) mouse erythrocytes, we demonstrate that the activation of soluble as well as membrane-bound forms of transglutaminase (TG2) in mouse erythrocytes was independent of mu-calpain. Also, the absence of mu-calpain or any detectable cysteine protease did not affect the transglutaminase activity in the erythrocyte lysate. Our studies also identify physiological substrates of mu-calpain in the erythrocyte membrane and show that their cleavage has no discernible effect on the transglutaminase mediated cross-linking of membrane proteins. Taken together, these data suggest the existence of a calpain-independent mechanism for the activation of transglutaminase 2 by calcium ions in the mouse erythrocytes and presumably also in non-erythroid cells.

Role of tissue transglutaminase in GTP depletion-induced apoptosis of insulin-secreting (HIT-T15) cells

The role of tissue transglutaminase (tTG), a calcium-dependent and GTP-modulated enzyme, in apoptotic death induced by GTP depletion in islet beta-cells was investigated. GTP depletion and apoptosis were induced by mycophenolic acid (MPA) in insulin-secreting HIT-T15 cells. MPA treatment increased in situ tTG activity (but not protein levels) in a dose- and time-dependent manner in parallel with the induction of apoptosis. MPA-induced increases of both tTG activity and apoptosis were entirely blocked by co-provision of guanosine but not adenosine. MPA-enhanced tTG activity could be substantially reduced by co-exposure to monodansylcadaverine or putrescine (tTG inhibitors), and largely blocked by lowering free Ca(2+) concentrations in the culture medium. However, MPA-induced cell death was either not changed or was only slightly reduced under these conditions. By contrast, a pan-caspase inhibitor (Z-VAD-FMK) entirely prevented apoptosis induced by MPA, but did not block the enhanced tTG activity, indicating that GTP depletion can induce apoptosis and activate tTG either independently or as part of a cascade of events involving caspases. Importantly, the morphological changes accompanying apoptosis could be markedly prevented by tTG inhibitors. These findings suggest that the effect of the marked increase in tTG activity in GTP depletion-induced apoptosis of insulin-secreting cells may be restricted to some terminal morphological changes.

Role of transglutaminase 2 in glucose tolerance: knockout mice studies and a putative mutation in a MODY patient

Transglutaminase 2 (TGase 2) is a Ca+2-dependent enzyme that catalyzes both intracellular and extracellular cross-linking reactions by transamidation of specific glutamine residues. TGase 2 is known to be involved in the membrane-mediated events required for glucose-stimulated insulin release from the pancreatic beta cells. Here we show that targeted disruption of TGase 2 impairs glucose-stimulated insulin secretion. TGase 2-/- mice show glucose intolerance after intraperitoneal glucose loading. TGase 2-/- mice manifest a tendency to develop hypoglycemia after administration of exogenous insulin as a consequence of enhanced insulin receptor substrate 2 (IRS-2) phosphorylation. We suggest that the increased peripheral sensitivity to insulin partially compensates for the defective secretion in this animal model. TGase 2-/- mouse phenotype resembles that of the maturity-onset diabetes of young (MODY) patients. In the course of screening for human TGase 2 gene in Italian subjects with the clinical features of MODY, we detected a missense mutation (N333S) in the active site of the enzyme. Collectively, these results identify TGase 2 as a potential candidate gene in type 2 diabetes.

Osmotic resistance of high-density erythrocytes in transglutaminase 2-deficient mice

Transglutaminase 2 (TGase 2) is a Ca(2+)-dependent enzyme responsible for the posttransttranslational modification of proteins by transamidation of specific polypeptide-bound glutamine residues. Elevating the intracellular concentration of Ca(2+)-ions in human erythrocytes leads to the formation of cytoskeletal and cytoplasmic protein polymers. The Ca(2+)-dependent TGase 2-dependent cross-linking activity has been proposed for its involvement in erythrocyte aging, by inducing irreversible modification of their cell shape and deformability. Accordingly, we found that high-density ("old") TGase 2(minus sign/minus sign) red blood cells (RBCs) were more resistant to osmotic stress-induced hemolysis than those from wild type mice. In addition, elevating the intracellular concentration of Ca(2+) by treatment of total RBCs with ionophore A23187 resulted in enhanced resistance of TGase 2-deficient erythrocytes compared to their normal counterpart. These findings indicate that TGase 2 may have a role in regulating structural flexibility of RBCs, possibly affecting their life span in physiopathological conditions, such as erythrocyte senescence, which are accompanied by increases in intracellular Ca(2+) concentration.

Transglutaminase 5 cross-links loricrin, involucrin, and small proline-rich proteins in vitro

Transglutaminases (TGases) are seven enzymes, cross-linking proteins by gamma-glutamil-epsilon-lysine bonds, four of which are expressed in the skin. A new member of the TGase family, TGase 5, has been identified recently, and in the present study we evaluated its role in keratinocyte differentiation in vitro. In addition to the previously described isoforms, full-length TGase 5 and Delta3 (deletion of exon 3), we identified two new splicing variants, Delta11 and Delta3Delta11 (deletion of exons 11 or 3, 11). We expressed full-length TGase 5, Delta3, Delta11, and Delta3Delta11 isoforms in the keratinocyte and baculovirus systems. The results indicate that both full-length TGase 5 and Delta11 are active, whereas Delta3 and Delta3Delta11 have very low activity. Expression studies show that full-length TGase 5 is induced during the early stages of keratinocyte differentiation and is differently regulated in comparison with the other epidermal TGases. Kinetic and in vitro cross-linking experiments indicate that full-length TGase 5 is very efficient in using specific epidermal substrates (loricrin, involucrin, and SPR3). In keratinocyte expression system, TGase 5 isoforms are retained in an intermediate filament-enriched fraction, suggesting its association with insoluble proteins. Indeed, TGase 5 co-localize with vimentin and it is able to cross-link vimentin in vitro.

A novel interaction of the Golgi complex with the vimentin intermediate filament cytoskeleton

The integration of the vimentin intermediate filament (IF) cytoskeleton and cellular organelles in vivo is an incompletely understood process, and the identities of proteins participating in such events are largely unknown. Here, we show that the Golgi complex interacts with the vimentin IF cytoskeleton, and that the Golgi protein formiminotransferase cyclodeaminase (FTCD) participates in this interaction. We show that the peripherally associated Golgi protein FTCD binds directly to vimentin subunits and to polymerized vimentin filaments in vivo and in vitro. Expression of FTCD in cultured cells results in the formation of extensive FTCD-containing fibers originating from the Golgi region, and is paralleled by a dramatic rearrangements of the vimentin IF cytoskeleton in a coordinate process in which vimentin filaments and FTCD integrate into chimeric fibers. Formation of the FTCD fibers is obligatorily coupled to vimentin assembly and does not occur in vim(-/-) cells. The FTCD-mediated regulation of vimentin IF is not a secondary effect of changes in the microtubule or the actin cytoskeletons, since those cytoskeletal systems appear unaffected by FTCD expression. The assembly of the FTCD/vimentin fibers causes a coordinate change in the structure of the Golgi complex and results in Golgi fragmentation into individual elements that are tethered to the FTCD/vimentin fibers. The observed interaction of Golgi elements with vimentin filaments and the ability of FTCD to specifically interacts with both Golgi membrane and vimentin filaments and promote their association suggest that FTCD might be a candidate protein integrating the Golgi compartment with the IF cytoskeleton.

Protein crosslinking in assembly and remodelling of extracellular matrices: the role of transglutaminases

Transglutaminases form a family of proteins that have evolved for specialized functions such as protein crosslinking in haemostasis, semen coagulation, or keratinocyte cornified envelope formation. In contrast to the other members of this protein family, tissue transglutaminase is a multifunctional enzyme apparently involved in very disparate biological processes. By virtue of its reciprocal Ca2+-dependent crosslinking activity or GTP-dependent signal transducing activity, tissue transglutaminase exhibits true multifunctionality at the molecular level. The crosslinking activity can subserve disparate biological phenomena depending on the location of the target proteins. Intracellular activation of tissue transglutaminase can give rise to crosslinked protein envelopes in apoptotic cells, whereas extracellular activation contributes to stabilization of the extracellular matrix and promotes cell-substrate interaction. While tissue transglutaminase synthesis and activation is normally part of a protective cellular response contributing to tissue homeostasis, the enzyme has also been implicated in a number of pathological conditions including fibrosis, atherosclerosis, neurodegenerative diseases, celiac disease, and cancer metastasis. This review discusses the role of transglutaminases in extracellular matrix crosslinking with a focus on the multifunctional enzyme tissue transglutaminase.

Identification of a cellular protein that functionally interacts with the C2 domain of cytosolic phospholipase A(2)alpha

Cytosolic phospholipase A(2) (cPLA(2)) alpha plays critical roles in lipid mediator synthesis. We performed far-Western analysis and identified a 60-kDa protein (P60) that interacted with cPLA(2)alpha in a Ca(2+)-dependent manner. Peptide microsequencing revealed that purified P60 was identical to vimentin, a major component of the intermediate filament. The interaction occurred between the C2 domain of cPLA(2)alpha and the head domain of vimentin. Immunofluorescence microscopic analysis demonstrated that cPLA(2)alpha and vimentin colocalized around the perinuclear area in cPLA(2)alpha-overexpressing human embryonic kidney 293 cells following A23187 stimulation. Forcible expression of vimentin in vimentin-deficient SW13 cells augmented A23187-induced arachidonate release. Moreover, overexpression of the vimentin head domain in rat fibroblastic 3Y1 cells exerted a dominant inhibitory effect on arachidonate metabolism, significantly reducing A23187-induced arachidonate release and attendant prostanoid generation. These results suggest that vimentin is an adaptor for cPLA(2)alpha to function properly during the eicosanoid-biosynthetic process.

Interactions of G(h)/transglutaminase with phospholipase Cdelta1 and with GTP

The inositol phosphate hydrolyzing activity of human phospholipase Cdelta1 (PLCdelta1) is markedly inhibited when the enzyme is coexpressed with the human heart G(h)/transglutaminase (TG) in human embryonic kidney cells. Because the cotransfection does not affect the amount of PLCdelta1 in the cells, the depression of phospholipase activity probably is a result of a direct interaction between the two proteins. An ELISA procedure was employed to document the associations of purified TG preparations from a variety of tissues (human red cells, rabbit lens, guinea pig liver) with PLCdelta1. Nucleotides (GTP > GDP > ATP > GMP = ADP, in order of decreasing efficiency) interfered with the formation of the PLCdelta1:TG complex. A conformational change in the TG partner, occurring with nucleotide binding, is thought to be responsible for dissociating the two proteins. The structural rearrangement produces a remarkable shift in the anodic mobility of TG in electrophoresis: TG(slow) + GTP -->/<-- [TG:GTP](fast). Altogether, our findings indicate that GTP controls PLCdelta1 activity by releasing this protein from an inhibitory association with G(h)/transglutaminase.

Properties of purified lens transglutaminase and regulation of its transamidase/crosslinking activity by GTP

On account of its protein crosslinking activity, the Ca2+-dependent transglutaminase of the lens is likely to be involved in the formation of cataracts. We have now purified the rabbit lens enzyme to near homogeneity as judged by SDS-PAGE (Mr approximately 78 kDa), and a key feature of the procedure was the use of a highly selective affinity chromatographic step with a fibronectin fragment as ligand. The catalytic activity of the lens transglutaminase, measured by the incorporation of dansylcadaverine into dimethylcasein, was compared with those of two similar enzymes isolated from human red cells and from guinea pig liver, respectively. All three enzymes were inhibited by GTP, but the lens enzyme was most sensitive to inhibition by the nucleotide. Moreover, GTP was also shown to inhibit the formation of the approximately 55 kDa betacrystallin dimers in the Ca2+-treated rabbit lens homogenate, proving that the nucleotide is a negative regulator for the crosslinking activity of transglutaminase in this tissue.

Decreased synthesis of glycosphingolipids in cells lacking vimentin intermediate filaments

We are studying defects in glycosphingolipid synthesis in cells lacking vimentin intermediate filaments (vimentin-). Sugars can be incorporated into glycolipids whose ceramide is synthesized either de novo (pathway 1) or from sphingoid bases salvaged from hydrolysis of sphingolipids (pathway 2) and into glycolipids recycling from the endosomal pathway through the Golgi (pathway 3). Vimentin- embryonic fibroblasts, obtained from vimentin-knockout mice, incorporate less sugar into glycolipids than vimentin+ fibroblasts. Using two inhibitors of ceramide synthesis, beta-chloroalanine and fumonisin B1, we found the major defect in synthesis to be in pathway 2 and not in de novo synthesis. We used two additional approaches to analyze the functions of pathways 2 and 3. First, we used exogenous glucosylthioceramide ([14C]C8-Glc-S-Cer), a synthetic, nonhydrolyzable glycosphingolipid, as a precursor for synthesis of larger glycolipids. Vimentin- SW13 cells and embryonic fibroblasts glycosylated [14C]C8-Glc-S-Cer less extensively than their vimentin+ counterparts. Second, we used chloroquine to inhibit the hydrolysis of sphingolipids in endosomes and lysosomes. Chloroquine markedly decreased the incorporation of sugars into glycolipids larger than glucosylceramide. The defect in glycolipid synthesis in vimentin- cells probably results from impaired intracellular transport of glycolipids and sphingoid bases between the endosomal/lysosomal pathway and the Golgi apparatus and endoplasmic reticulum. Intermediate filaments may accomplish this function by contributing to the organization of subcellular organelles and/or by binding proteins that participate in transport processes.

The intermediate filament protein, vimentin, in the lens is a target for cross-linking by transglutaminase

Mere addition of Ca2+ to a lens cortical homogenate (bovine) generates a series of products composed of a variety of high molecular weight vimentin species. The Ca2+-induced cross-linking of this cytoskeletal element seems to be mediated by the intrinsic transglutaminase of lens, because the reaction could be blocked at the monomeric state of vimentin by the inclusion of small synthetic substrates of the enzyme dansylcadaverine or dansyl-epsilon-aminocaproyl-Gln-Gln-Ile-Val. These compounds are known to compete against the Gln or Lys functionalities of proteins that would participate in forming the Nepsilon(gamma-glutamyl)lysine protein-to-protein cross-links. The cytosolic transglutaminase-catalyzed reactions could be reproduced with purified bovine lens vimentin and also with recombinant human vimentin preparations. Employing the latter system, we have titrated the transglutaminase-reactive sites of vimentin and, by sequencing the dansyl-tracer-labeled segments of the protein, we have shown that residues Gln453 and Gln460 served as acceptor functionalities and Lys97, Lys104, Lys294, and Lys439 as electron donor functionalities in vimentin. The transglutaminase-dependent reaction of this intermediate filament protein might influence the shape and plasticity of the fiber cells, and the enzyme-catalyzed cross-linking of vimentin, in conjunction with other lens constituents, may contribute to the process of cataract formation.

Differences in transglutaminase mRNA after polyamine depletion in two cell lines

Polyamines serve as natural substrates for the transglutaminase that catalyzes covalent cross-linking of proteins and is involved in cellular adhesion and proliferation. This study tests the hypothesis that intracellular polyamines play a role in the regulation of transglutaminase expression in rat small intestinal crypt cells (IEC-6 cell line) and human colon carcinoma cells (Caco-2 cell line). Treatment with alpha-difluoromethylornithine (DFMO; a specific inhibitor of polyamine synthesis) significantly depleted the cellular polyamines putrescine, spermidine, and spermine in both cell lines. In IEC-6 cells, polyamine depletion was associated with a decrease in the levels of transglutaminase mRNA. In Caco-2 cells, however, polyamine depletion significantly increased the levels of transglutaminase mRNA and enzyme activity. In both cell lines, ornithine decarboxylase mRNA levels increased and protooncogene c-myc mRNA decreased in the presence of DFMO. Addition of polyamines to cells treated with DFMO reversed the effect of DFMO on the levels of mRNA for these genes in both lines. There was no significant change in the stability of transglutaminase mRNA between control and DFMO-treated IEC-6 cells. In contrast, the half-life of mRNA for transglutaminase in Caco-2 cells was dramatically increased after polyamine depletion. Spermidine, when given together with DFMO, completely prevented increased half-life of transglutaminase mRNA in Caco-2 cells. These results indicate that 1) expression of transglutaminase requires polyamines in IEC-6 cells but is inhibited by these agents in Caco-2 cells, 2) polyamines modulate transglutaminase expression at the level of mRNA through different pathways in these two cell lines, and 3) posttranscriptional regulation plays a major role in the induction of transglutaminase mRNA in polyamine-deficient Caco-2 cells.

A transglutaminase-related antigen associates with keratin filaments in some mouse epidermal cells

A mouse monoclonal IgG, G82, directed against guinea pig liver transglutaminase recognizes a transglutaminase-related antigen that is associated with the keratin intermediate filament network in some primary mouse keratinocytes. The association can be seen at the resolution of individual keratin tonofibrils following fixation and staining for double-label indirect immunofluorescence. Western blots indicate that G82 reacts with two proteins of 95 kDa and 280 kDa, respectively, in extracts of these cells. The 95-kDa band is also recognized by a polyclonal antibody against purified guinea pig liver transglutaminase, and the 280-kDa protein seems to correspond to a similar protein that was shown to be recognized by G92.1.2 in the intermediate filament fraction of primary mouse fibroblasts. The transglutaminase-related antigen was shown by confocal microscopy to co-localize only with nonbasal cell specific keratin intermediate filaments.

A new transglutaminase-like from the ascidian Ciona intestinalis

A cDNA clone encoding a transglutaminase (TGase) was isolated from a cDNA library prepared from the larval stage of Ciona intestinalis. The cDNA sequence has an open reading frame encoding a protein of 696 amino acids and is about 36% identical to 11 other TGase sequences. In addition, the critical residues thought to form the catalytic center are conserved. The Ciona TGase (CiTGase) has an extension of 39 amino acids in the NH2-terminal region similar to that reported for keratinocyte TGases. A phylogenetic analysis among other types of TGases demonstrated that CiTGase represents a new type of the enzyme.

Colocalization of tissue transglutaminase and stress fibers in human vascular smooth muscle cells and human umbilical vein endothelial cells

The subcellular distribution of tissue transglutaminase in human umbilical vein endothelial cells and human arterial and venous smooth muscle cells was examined. Double-immunofluorescence staining of smooth muscle cells and endothelial cells with anti-transglutaminase antisera and rhodamine-tagged phalloidin revealed codistribution of transglutaminase with the stress fibers, with endothelial cells also containing a cytoplasmic pool. This pattern of distribution was confirmed by confocal microscopy. Immunoprecipitation experiments demonstrated that transglutaminase co-immunoprecipitated with myosin in high-molecular-weight complexes, but not with actin, suggesting that the association of transglutaminase with the stress fibers was due to its cross-linking to myosin. About 97% of endothelial cell transglutaminase activity was present in the cytosolic fraction and 3% in the particulate fraction. The detergent-insoluble fraction was practically devoid of activity as measured by the putrescine assay, but was active as evidenced by the covalent cross-linking of 125I-fibronectin. Western blotting with a polyclonal rabbit antiserum raised against human erythrocyte transglutaminase detected high levels of enzyme in endothelial cell cytosol and both detergent-soluble and detergent-insoluble membrane fractions. In contrast, smooth muscle cells contained much less cytosolic transglutaminase, as determined either functionally or antigenically. Furthermore, within the particulate fraction of the smooth muscle cells, most of the enzyme was located in the detergent-insoluble fraction, as assessed by Western blot analysis. Retinoic acid increased the levels of enzyme in the cytosol of all cell types and the increases were correlated with increases in mRNA. Thus, tissue transglutaminase is present in various particulate fractions of vascular smooth muscle cells and endothelial cells and may be present in this cellular fraction by virtue of autocross-linking of the enzyme itself to stress fiber-associated myosin.