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Transglutaminases: new target molecules for inflammatory bowel disease?
  1. B Siegmund,
  2. M Zeitz
  1. Charité Universitätsmedizin Berlin, Campus Benjamin Franklin, Medizinische Klinik I, Berlin, Germany
  1. Correspondence to:
    Dr B Siegmund
    Charité Universitätsmedizin Berlin, Campus Benjamin Franklin, Medizinische Klinik I, Hindenburgdamm 30, 12200 Berlin, Germany; britta.siegmundcharite.de

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Transglutaminases may be involved in intestinal inflammation and may even represent therapeutic targets for ulcerative colitis

The family of transglutaminases (TG) includes the plasma form factor XIIIa as well as the tissue transglutaminase (tTG) and keratinocyte transglutaminase (TGk). In particular, tTG reminds every gastroenterologist primarily of coeliac disease where tTG represents the key autoantigen.1 Disease induction is confined to tTG, a ubiquitous enzyme which is released from fibroblasts, endothelial, and inflammatory cells during mechanical irritation or inflammation. At acidic pH, which occurs with inflammation, tTG can, apart from its physiological function described in more detail below, also simply deamidate some of the glutamine residues of the gluten peptides. In coeliac disease, deamidation introduces a negative charge into the gluten peptides which can increase the binding affinity to HLA-DQ2 or HLA-DQ8, the primary HLA association in coeliac disease. Binding of gluten peptide to either HLA-DQ2 or HLA-DQ8 results in an increase in their capacity to stimulate T cells, thus inducing intestinal inflammation.2,3

As indicated in this issue of Gut by D’Argenio and colleagues,4 TG may be involved in intestinal inflammation in a completely different way and may even represent a therapeutic target (see page 496). But how can the physiological function of TG be defined? TG are enzymes catalysing the covalent cross linking between proteins by forming ε(γ-glutamyl)-lysine isopeptide bonds. These bonds are formed between glutaminase and lysine residues thus stabilising intra- and extracellular proteins, a process which is required for a variety of essential physiological purposes such as barrier function in epithelia, apoptosis, and extracellular matrix formation.5 Substrates include intracellular (actin cytoskeleton, histones) as well as extracellular (collagen, vitronectin, fibronectin) proteins many of which are found in the matrix of the colonic mucosa. The balance of cross linking alters the susceptibility of the matrix to proteolysis and thus affects the rate of its formation and remodelling during mucosal damage and repair.

In the work presented in this issue of Gut, D’Argenio and colleagues4 hypothesise that due to impaired healing in ulcerative colitis, an imbalance of TG may participate in the inflammatory process. To approach this question, peripheral blood as well as colonic biopsies from patients with active and inactive ulcerative colitis as well as non-inflamed control samples were examined for the three members of the TG family, tTG, factor XIIIa, and TGk. Endoscopic and histological disease indices were applied to determine disease activity. Factor XIIIa activity was evaluated in plasma. In addition, tTG, TGk, and factor XIIIa protein as well as RNA content in colonic tissue was analysed by western blot and reverse transcription-polymerase chain reaction, respectively. An elegant tool in this study proved to be the immunohistochemical studies visualising colonic localisation of TG and their reaction products, the ε(γ-glutamyl)-lysine isopeptide bonds. With the results obtained, a scheme can be suggested as to how TG act in active ulcerative colitis. Primarily, factor XIIIa activity in plasma is significantly reduced in active disease compared with inactive ulcerative colitis or healthy controls. Immunohistochemical analysis indicated that tTG and factor XIIIa colocalise in damaged areas with the isopeptide bonds, which may explain the decrease in factor XIIIa in plasma. The TGk protein as well as RNA expression is significantly reduced in active disease in colonic tissue. tTG RNA is upregulated while tTG protein expression remains unaltered. Interestingly, in active disease, tTG appeared with two bands in the western blot analysis representing degradation probably due to endogenous proteolysis by calpain, an enzyme that inactivates tTG, known to be increased in active ulcerative colitis.6 Importantly, TGk was detected for the first time in colonic tissue where it was localised at the upper part of the crypts and it was shown that TGk was significantly downregulated in active disease. Based on these results the authors conclude that the abnormal pattern of TG contributes to the course of ulcerative colitis.

Whether or not these changes are specific for ulcerative colitis or whether they also occur in other intestinal inflammations, for example, infectious colitis, is unknown. Similar data have been obtained for Crohn’s disease.7,8 Here, in active disease, factor XIIIa was equally decreased in serum, and immunhistochemical staining also showed colocalisation of factor XIIIa and tTG to the extracellular matrix. Interestingly, the western blot from colonic tissue from patients with active Crohn’s disease showed an identical double band in active disease, an observation that was not discussed further by the authors. These data indicate that independent of the type of chronic inflammatory bowel disease, a similar pattern of TG changes can be seen. These data as well as data from the literature indicate a protective role of TG in Crohn’s disease referred to above. This protective function is already evident given the functional properties of TG: TGk is abundantly expressed in epithelial cells cross linking a series of defined structural proteins, thus exerting barrier function. Consequently, the decrease in TGk is associated with a barrier defect, a well described phenomenon in intestinal inflammation. tTG serves as G protein in most mammalian cells; when Ca2+ levels rise this protein becomes active in cross linking TG reactions. The potential protective effect is supported by the observation that inhibition of calpain, which can inactivate tTG, reduced colonic injury in experimental colitis.2 Thus wound healing or tissue remodelling is an active process consisting of a destructive part, including extracellular proteinases such as metalloproteinases or calpain, and a constructive part involving processing enzymes such as TG. During acute inflammation the presence of extracellular matrix proteins increases, thus inactivating at least part of the TG, as indicated in the present study as well as in the study referred to above in Crohn’s disease patients.9 Additional experimental data support this theory. Na-butyrate enemas have been demonstrated to ameliorate experimental colitis. Na-butyrate is known to upregulate tTG expression.10 Furthermore, a case report including three patients demonstrated a beneficial effect for recombinant factor XIII concentrate in patients with inflammatory bowel disease.11 These results could not be confirmed in a prospective, double blind, placebo controlled study in steroid refractory patients with ulcerative colitis. A total of 28 patients were included in this study and were treated with either intravenous application of factor XIII concentrates or placebo. No beneficial effect of additional factor XIII treatment was demonstrated.12

In contrast, a proinflammatory role for TG was demonstrated in an animal model of allergic conjunctivitis.13 More precisely, dual inhibition of phospholipase A2 (PLA2) and TG resulted in dramatic anti-inflammatory activity. The PLA2 enzyme family comprises cell bound as well as secretory isoforms which play a key role in arachidonic acid release during acute inflammation.14 TG catalysed post-translational modifications activate secretory PLA2, thus potentially increasing eicosanoid production during acute inflammation.14 Are these data contrary to the results obtained in the present study or are there other explanations? With regard to the data obtained in Crohn’s disease patients, one could argue that this is a T helper cell type 1 (Th1) disease while the allergic model represents a classic T helper cell type 2 (Th2) dependent model.15 However, this explanation has to be disregarded as ulcerative colitis is considered a Th2 dependent disease, with interleukin 13 representing the key inflammatory mediator.16 Another anti-inflammatory pharmacological group, the non-steroidal anti-inflammatory agents, also significantly inhibit the release of arachidonic acid. Simultaneously, non-steroidal anti-inflammatory agents have been associated with a deteriorating effect in inflammatory bowel disease.17 One might hypothesise that in the intestinal mucosa TG are mandatory for the constructive mucosal healing process, thus the anti-inflammatory properties are superior, and eventually activation of PLA2 may even be partially anti-inflammatory in this setting. In addition, bactericidal activity in the intestine has been described for PLA2 which contributes to epithelial barrier function.18

In summary, D’Argenio and colleagues4 have provided important information for an old concept. In light of the new data, one has to consider whether the highly dynamic process modulated by TG can be used to our advantage for therapeutic interventions.

Acknowledgments

The authors wish to thank the Deutsche Forschungsgemeinschaft and the Broad Medical Research Program of the Eli and Edythe L Broad Foundation for their continued support of our research.

Transglutaminases may be involved in intestinal inflammation and may even represent therapeutic targets for ulcerative colitis

REFERENCES

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Footnotes

  • Conflict of interest: None declared.

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