Transglutaminase 2 cross-linking activity is linked to invadopodia formation and cartilage breakdown in arthritis

α9β1 integrin & its ligands as new potential biomarkers in FMF

Background & objectives Familial Mediterranean Fever (FMF) manifests as a hereditary condition characterized by repeated bouts of fever, abdominal, chest, and joint discomfort, and swelling. Colchicine is the most common form of treatment, but it does not eliminate the disease. The underlying causes of the inflammatory mechanism are still not fully known. Methods A total of 20 healthy controls, 16 individuals with FMF in the attack period, and 14 in the remission period participated in the study. ITGA9, ITGB1, OPN, TNC, VEGF, VCAM-1, TGM2, TSP-1, Emilin-1, and vWF levels were measured by ELISA by obtaining serum from blood samples of individuals. In addition, gene expressions of α9β1 (ITGA9, ITGB1) and its best known ligands (TNC, SPP1) were analyzed by quantitative real-time PCR (qPCR). Results The findings of this study showed that serum levels of α9β1 and its ligands were higher in individuals with FMF in the attack period than in the healthy controls and the FMF group in the remission period (P<0.05). The marker levels of the healthy group were also higher than those in the remission period (p<0.05). In addition, when the gene expressions were compared between the healthy controls and FMF group, no significant difference was found for ITGA9, ITGB1, TNC, and SPP1 genes. Interpretation & conclusions The function of α9β1 and its ligands in FMF disease was investigated for the first time in this study as per our knowledge. Serum levels of these biomarkers may help identify potential new targets for FMF disease diagnosis and treatment approaches.

Assessment of α9β1 ıntegrın as a new dıagnostıc and therapeutıc target ın Behcet's dısease

This study aimed to investigate the roles of α9β1 integrin and its ligands in Behçet's disease (BD) by examining serum levels and gene expressions. 15 healthy controls and 30 BD patients (14 active and 16 inactive) were included in the study. Serum levels of ITGA9, ITGB1, TNC, OPN, VCAM-1, VEGF, TSP1, TGM2, Emilin-1, and vWF, were measured by ELISA. Gene expressions of α9β1 (ITGA9 and ITGB1) and its ligands (TNC and SPP1) were evaluated by RT-PCR. Laboratory findings (CRP, ESR, HGB, WBC, RBC, neutrophil, lymphocyte, PLT, RDW, MPV, PCT, and HLA-B51) were obtained from the electronic database. Active BD patients had higher serum levels of α9β1 integrin and its ligands than inactive patients and healthy controls. No significant difference was observed between healthy controls and inactive patients. Gene expressions of ITGB1 and SPP1 were increased in both patient groups compared to healthy controls. ITGA9 and TNC gene expression levels were lower in the active group than in the inactive group. No noticeable differences were found in ITGB1 and SPP1 gene expressions between the patient groups. BD patients exhibited elevated CRP, ESR, WBC, neutrophil, PLT, and PCT levels, while HGB, RBC, and RDW values were lower than healthy controls. Active patients had higher CRP, ESR, WBC, neutrophil, and PLT levels. Significant positive correlations were found between CRP, ESR, WBC, neutrophil, PLT, PCT and serum levels of α9β1 integrin and its ligands. Increased release of α9β1 integrin and its ligands is associated with BD, suggesting their potential as markers for disease severity.

Cross-reactivity and sequence similarity between microbial transglutaminase and human tissue antigens

Microbial transglutaminase (mTG) is a bacterial survival factor, frequently used as a food additive to glue processed nutrients. As a result, new immunogenic epitopes are generated that might drive autoimmunity. Presently, its contribution to autoimmunity through epitope similarity and cross-reactivity was investigated. Emboss Matcher was used to perform sequence alignment between mTG and various antigens implicated in many autoimmune diseases. Monoclonal and polyclonal antibodies made specifically against mTG were applied to 77 different human tissue antigens using ELISA. Six antigens were detected to share significant homology with mTG immunogenic sequences, representing major targets of common autoimmune conditions. Polyclonal antibody to mTG reacted significantly with 17 out of 77 tissue antigens. This reaction was most pronounced with mitochondrial M2, ANA, and extractable nuclear antigens. The results indicate that sequence similarity and cross-reactivity between mTG and various tissue antigens are possible, supporting the relationship between mTG and the development of autoimmune disorders 150W.

The P2X7 Receptor in Autoimmunity

The P2X7 receptor (P2X7R) is an ATP-gated nonselective cationic channel that, upon intense stimulation, can progress to the opening of a pore permeable to molecules up to 900 Da. Apart from its broad expression in cells of the innate and adaptive immune systems, it is expressed in multiple cell types in different tissues. The dual gating property of P2X7R is instrumental in determining cellular responses, which depend on the expression level of the receptor, timing of stimulation, and microenvironmental cues, thus often complicating the interpretation of experimental data in comprehensive settings. Here we review the existing literature on P2X7R activity in autoimmunity, pinpointing the different functions in cells involved in the immunopathological processes that can make it difficult to model as a druggable target.

Peripheral blood cellular dynamics of rheumatoid arthritis treatment informs about efficacy of response to disease modifying drugs

Rheumatoid arthritis (RA) is an autoimmune disease characterized by systemic inflammation and is mediated by multiple immune cell types. In this work, we aimed to determine the relevance of changes in cell proportions in peripheral blood mononuclear cells (PBMCs) during the development of disease and following treatment. Samples from healthy blood donors, newly diagnosed RA patients, and established RA patients that had an inadequate response to MTX and were about to start tumor necrosis factor inhibitors (TNFi) treatment were collected before and after 3 months of treatment. We used in parallel a computational deconvolution approach based on RNA expression and flow cytometry to determine the relative cell-type frequencies. Cell-type frequencies from deconvolution of gene expression indicate that monocytes (both classical and non-classical) and CD4⁺ cells (T_h1 and T_h2) were increased in RA patients compared to controls, while NK cells and B cells (naïve and mature) were significantly decreased in RA patients. Treatment with MTX caused a decrease in B cells (memory and plasma cell), and a decrease in CD4 T_h cells (T_h1 and T_h17), while treatment with TNFi resulted in a significant increase in the population of B cells. Characterization of the RNA expression patterns found that most of the differentially expressed genes in RA subjects after treatment can be explained by changes in cell frequencies (98% and 74% respectively for MTX and TNFi).

Osthole regulates N6-methyladenosine-modified TGM2 to inhibit the progression of rheumatoid arthritis and associated interstitial lung disease

Rheumatoid arthritis (RA) is an inflammatory autoimmune disease, and RA interstitial lung disease (ILD) is a severe complication of RA. This investigation aims to determine the effect and underlying mechanism of osthole (OS), which could be extracted from Cnidium, Angelica, and Citrus plants and evaluate the role of transglutaminase 2 (TGM2) in RA and RA-ILD. In this work, OS downregulated TGM2 to exert its additive effect with methotrexate and suppress the proliferation, migration, and invasion of RA-fibroblast-like synoviocytes (FLS) by attenuating NF-κB signaling, resulting in the suppression of RA progression. Interestingly, WTAP-mediated N6-methyladenosine modification of TGM2 and Myc-mediated WTAP transcription cooperatively contributed to the formation of a TGM2/Myc/WTAP-positive feedback loop through upregulating NF-κB signaling. Moreover, OS could downregulate the activation of the TGM2/Myc/WTAP-positive feedback circuit. Furthermore, OS restrained the proliferation and polarization of M2 macrophages to inhibit the aggregation of lung interstitial CD11b+ macrophages, and the effectiveness and non-toxicity of OS in suppressing RA and RA-ILD progression were verified in vivo. Finally, bioinformatics analyses validated the importance and the clinical significance of the OS-regulated molecular network. Taken together, our work emphasized OS as an effective drug candidate and TGM2 as a promising target for RA and RA-ILD treatment.

Transglutaminase 2 inhibitors attenuate osteoarthritic degeneration of TMJ-osteoarthritis by suppressing NF-κB activation

BACKGROUND

The temporomandibular joint osteoarthritis (TMJ-OA) is characterized by progressive cartilage degradation, subchondral bone erosion, and chronic pain, leading to articular damage and chewing dysfunction. Studies have shown that interleukin-1β (IL-1β) plays a critical role in the development of TMJ-OA. Transglutaminase 2 (TG2) has been identified as a marker of chondrocyte hypertrophy and IL-1β was able to increase TG2 expression in chondrocytes. Therefore, the aim of this study was to explore the ability of TG2 inhibitors to suppress TMJ-OA progression.

METHODS

Firstly, toluidine blue staining, cell counting kit-8 assay, immunocytofluorescent staining and western blot were used to investigate the anti-inflammatory effects of TG2 inhibitors in IL-1β-stimulated murine chondrocytes and the underlying mechanisms. Afterwards, micro-CT analysis, histological staining, immunohistochemical and immunohistofluorescent staining were used to evaluate the therapeutic efficacy of TG2 inhibitors in monosodium iodoacetate (MIA)-induced TMJ-OA in rats.

RESULTS

TG2 inhibitors suppressed the IL-1β-induced upregulation of COX-2, iNOS, MMP-13, and MMP-3 and reversed the IL-1β-induced proteoglycan loss in chondrocytes through inhibiting NF-κB activation. Consistently, the MIA-induced upregulation of MMP-13 and MMP-3, and loss of structural integrity of the articular cartilage and subchondral bone were markedly reversed by TG2 inhibitors via inhibiting NF-κB activation.

CONCLUSIONS

TG2 inhibitors demonstrated a potent therapeutic efficacy on cartilage and subchondral bone structures of TMJ-OA by reducing inflammation and cartilage degradation through suppressing NF-κB activation.

Cytosolic TGM2 promotes malignant progression in gastric cancer by suppressing the TRIM21-mediated ubiquitination/degradation of STAT1 in a GTP binding-dependent modality

BACKGROUND

Previous studies have revealed the critical role of transglutaminase 2 (TGM2) as a potential therapeutic target in cancers, but the oncogenic roles and underlying mechanisms of TGM2 in gastric cancer (GC) are not fully understood. In this study, we examined the role and potential mechanism of TGM2 in GC.

METHODS

Western blotting, immunohistochemistry, CCK8, colony formation and transwell assays were used to measure TGM2 expression in the GC cells and tissues and to examine the in vitro role of TGM2 in GC. Xenograft and in vivo metastasis experiments were performed to examine the in vivo role of TGM2 in GC. Gene set enrichment analysis, quantitative PCR and western blotting were conducted to screen for potential TGM2 targets involved in GC. Gain/loss-of-function and rescue experiments were conducted to detect the biological roles of STAT1 in GC cells in the context of TGM2. Co-immunoprecipitation, mass spectrometry, quantitative PCR and western blotting were conducted to identify STAT1-interacting proteins and elucidate their regulatory mechanisms. Mutations in TGM2 and two molecules (ZM39923 and A23187) were used to identify the enzymatic activity of TGM2 involved in the malignant progression of GC and elucidate the underlying mechanism.

RESULTS

In this study, we demonstrated elevated TGM2 expression in the GC tissues, which closely related to pathological grade, and predicted poor survival in patients with GC. TGM2 overexpression or knockdown promoted (and inhibited) cell proliferation, migration, and invasion, which were reversed by STAT1 knockdown or overexpression. Further studies showed that TGM2 promoted GC progression by inhibiting STAT1 ubiquitination/degradation. Then, tripartite motif-containing protein 21 (TRIM21) was identified as a ubiquitin E3 ligase of STAT1 in GC. TGM2 maintained STAT1 stability by facilitating the dissociation of TRIM21 and STAT1 with GTP-binding enzymatic activity. A23187 abolished the role of TGM2 in STAT1 and reversed the pro-tumor role of TGM2 in vitro and in vivo.

CONCLUSIONS

This study revealed a critical role and regulatory mechanism of TGM2 on STAT1 in GC and highlighted the potential of TGM2 as a therapeutic target, which elucidates the development of medicine or strategies by regulating the GTP-binding activity of TGM2 in GC.

N6-methyladenosine modification of TGM2 mRNA contributes to the inhibitory activity of sarsasapogenin in rheumatoid arthritis fibroblast-like synoviocytes

BACKGROUND

Developing alternative targets and drugs for rheumatoid arthritis (RA) treatment is currently an urgent issue. The relationship between TGM2 and the abnormal immune microenvironment in synovium tissues, as well as the specific role of TGM2 in RA are yet to be elucidated. Sarsasapogenin (Sar) is a sapogenin extracted from the Chinese medical herb Anemarrhena asphodeloides Bunge. and served as a representative anti-inflammatory drug capable of ameliorating inflammatory responses in several human diseases. However, the therapeutic effect of Sar on RA remains unknown.

PURPOSE

This investigation aims to elucidate the role of TGM2 in RA and investigate whether Sar is a candidate drug to target TGM2 of fibroblast-like synoviocytes (FLS).

METHODS

Bioinformatics analyses were applied for elucidating the role of N(6)-methyladenine (m6A) RNA methylation in RA and identifying the specific target regulated by m6A methylation in RA-FLS. Methylated RNA immunoprecipitation, CCK8 assay, Edu assay, flow cytometry, RT-qPCR and Western blot were utilized to investigate the function of Sar and TGM2 in RA-FLS.

RESULTS

Bioinformatics analyses emphasized the importance of m6A RNA methylation in RA and identified an m6A methylation-mediated gene TGM2. Interestingly, both m6A RNA methylation and TGM2 expression in RA synovium tissues correlated with activated immuno-inflammatory phenotype and associated with clinical characteristics and therapy response of RA patients. TGM2 served as a promoter of RA-FLS proliferation by inducing DNA replication and cell cycle transition and inhibiting apoptosis through activating NF-κB signaling. Intriguingly, Sar could impair m6A methylation of TGM2 mRNA and downregulate TGM2 expression. Downregulated TGM2 contributed to the suppressive role of Sar in DNA replication and the stimulatory role of Sar in cell cycle arrest and apoptosis of RA-FLS. Mechanically, Sar inhibited the expression of key regulators in DNA replication, cell cycle, and apoptosis by impairing NF-κB signaling, thus abolishing FLS proliferation to ameliorate RA progression.

CONCLUSIONS

This cross-validated work based on three independent datasets is detailedly delineated using cell lines and clinical samples, recognizing that TGM2 can be an attractive target and Sar might be a novel anti-RA drug.

14-3-3η Promotes Invadosome Formation via the FOXO3-Snail Axis in Rheumatoid Arthritis Fibroblast-like Synoviocytes

Erosive destruction of joint structures is a critical event in the progression of rheumatoid arthritis (RA), in which fibroblast-like synoviocytes (FLS) are the primary effectors. We previously reported that the ability of RA FLS to degrade extracellular matrix (ECM) components depends on the formation of actin-rich membrane protrusions, called invadosomes, through processes that remain elusive. 14-3-3η belongs to a family of scaffolding proteins involved in a wide range of cellular functions, and its expression is closely related to joint damage and disease activity in RA patients. In this study, we sought to assess the role of 14-3-3η in joint damage by examining its contribution to the invadosome formation phenotype of FLS. Using human primary FLS, we show that 14-3-3η expression is closely associated with their ability to form invadosomes. Furthermore, knockdown of 14-3-3η using shRNAs decreases the level of invadosome formation in RA FLS, whereas addition of the recombinant protein to FLS from healthy individuals promotes their formation. Mechanistic studies suggest that 14-3-3η regulates invadosome formation by increasing Snail expression, a mechanism that involves nuclear exclusion of the transcription repressor FOXO3. Our results implicate the 14-3-3η–FOXO3–Snail axis in promoting the aggressive ECM-degrading phenotype of RA FLS, and suggest a role for this scaffolding protein in cartilage degradation.

Targeting MyD88 Downregulates Inflammatory Mediators and Pathogenic Processes in PBMC From DMARDs-Naïve Rheumatoid Arthritis Patients

MyD88-dependent intracellular signalling cascades and subsequently NF-kappaB-mediated transcription lead to the dynamic inflammatory processes underlying the pathogenesis of rheumatoid arthritis (RA) and related autoimmune diseases. This study aimed to identify the effect of the MyD88 dimerization inhibitor, ST2825, as a modulator of pathogenic gene expression signatures and systemic inflammation in disease-modifying antirheumatic drugs (DMARDs)-naïve RA patients. We analyzed bulk RNA-seq from peripheral blood mononuclear cells (PBMC) in DMARDs-naïve RA patients after stimulation with LPS and IL-1β. The transcriptional profiles of ST2825-treated PBMC were analyzed to identify its therapeutic potential. Ingenuity Pathway Analysis was implemented to identify downregulated pathogenic processes. Our analysis revealed 631 differentially expressed genes between DMARDs-naïve RA patients before and after ST2825 treatment. ST2825-treated RA PBMC exhibited a gene expression signature similar to that of healthy controls PBMC by downregulating the expression of proinflammatory cytokines, chemokines and matrix metalloproteases. In addition, B cell receptor, IL-17 and IL-15 signalling were critically downregulated pathways by ST2825. Furthermore, we identified eight genes (MMP9, CXCL9, MZB1, FUT7, TGM2, IGLV1-51, LINC01010, and CDK1) involved in pathogenic processes that ST2825 can potentially inhibit in distinct cell types within the RA synovium. Overall, our findings indicate that targeting MyD88 effectively downregulates systemic inflammatory mediators and modulates the pathogenic processes in PBMC from DMARDs-naïve RA patients. ST2825 could also potentially inhibit upregulated genes in the RA synovium, preventing synovitis and joint degeneration.

ECM Cross-Linking Regulates Invadopodia Dynamics

Invadopodia are membrane protrusions dynamically assembled by invasive cancer cells in contact with the extracellular matrix (ECM). Invadopodia are enriched by the structural proteins actin and cortactin as well as metalloproteases such as MT1-MMP, whose function is to degrade the surrounding ECM. During metastasis, invadopodia are necessary for cancer cell intravasation and extravasation. Although signaling pathways involved in the assembly and function of invadopodia are well studied, few studies address invadopodia dynamics and how the cell-ECM interactions contribute to cell invasion. Using iterative analysis based on time-lapse microscopy and mathematical modeling of invasive cancer cells, we found that cells oscillate between invadopodia presence and cell stasis-termed the "invadopodia state"-and invadopodia absence during cell translocation-termed the "migration state." Our data suggest that β1-integrin-ECM binding and ECM cross-linking control the duration of each of the two states. By changing the concentration of cross-linkers in two-dimensional and three-dimensional cultures, we generate an ECM in which 0-0.92 of total lysine residues are cross-linked. Using an ECM with a range of cross-linking degrees, we demonstrate that the dynamics of invadopodia-related functions have a biphasic relationship to ECM cross-linking. At intermediate levels of ECM cross-linking (0.39), cells exhibit rapid invadopodia protrusion-retraction cycles and rapid calcium spikes, which lead to more frequent MT1-MMP delivery, causing maximal invadopodia-mediated ECM degradation. In contrast, both extremely high or low levels of cross-linking lead to slower invadopodia-related dynamics and lower ECM degradation. Additionally, β1-integrin inhibition modifies the dynamics of invadopodia-related functions as well as the length of time cells spend in either of the states. Collectively, these data suggest that β1-integrin-ECM binding nonlinearly translates small physical differences in the extracellular environment to differences in the dynamics of cancer cell behaviors. Understanding the conditions under which invadopodia can be reduced by subtle environment-targeting treatments may lead to combination therapies for preventing metastatic spread.

Towards frailty biomarkers: Candidates from genes and pathways regulated in aging and age-related diseases

OBJECTIVE

Use of the frailty index to measure an accumulation of deficits has been proven a valuable method for identifying elderly people at risk for increased vulnerability, disease, injury, and mortality. However, complementary molecular frailty biomarkers or ideally biomarker panels have not yet been identified. We conducted a systematic search to identify biomarker candidates for a frailty biomarker panel.

METHODS

Gene expression databases were searched (http://genomics.senescence.info/genes including GenAge, AnAge, LongevityMap, CellAge, DrugAge, Digital Aging Atlas) to identify genes regulated in aging, longevity, and age-related diseases with a focus on secreted factors or molecules detectable in body fluids as potential frailty biomarkers. Factors broadly expressed, related to several "hallmark of aging" pathways as well as used or predicted as biomarkers in other disease settings, particularly age-related pathologies, were identified. This set of biomarkers was further expanded according to the expertise and experience of the authors. In the next step, biomarkers were assigned to six "hallmark of aging" pathways, namely (1) inflammation, (2) mitochondria and apoptosis, (3) calcium homeostasis, (4) fibrosis, (5) NMJ (neuromuscular junction) and neurons, (6) cytoskeleton and hormones, or (7) other principles and an extensive literature search was performed for each candidate to explore their potential and priority as frailty biomarkers.

RESULTS

A total of 44 markers were evaluated in the seven categories listed above, and 19 were awarded a high priority score, 22 identified as medium priority and three were low priority. In each category high and medium priority markers were identified.

CONCLUSION

Biomarker panels for frailty would be of high value and better than single markers. Based on our search we would propose a core panel of frailty biomarkers consisting of (1) CXCL10 (C-X-C motif chemokine ligand 10), IL-6 (interleukin 6), CX3CL1 (C-X3-C motif chemokine ligand 1), (2) GDF15 (growth differentiation factor 15), FNDC5 (fibronectin type III domain containing 5), vimentin (VIM), (3) regucalcin (RGN/SMP30), calreticulin, (4) PLAU (plasminogen activator, urokinase), AGT (angiotensinogen), (5) BDNF (brain derived neurotrophic factor), progranulin (PGRN), (6) α-klotho (KL), FGF23 (fibroblast growth factor 23), FGF21, leptin (LEP), (7) miRNA (micro Ribonucleic acid) panel (to be further defined), AHCY (adenosylhomocysteinase) and KRT18 (keratin 18). An expanded panel would also include (1) pentraxin (PTX3), sVCAM/ICAM (soluble vascular cell adhesion molecule 1/Intercellular adhesion molecule 1), defensin α, (2) APP (amyloid beta precursor protein), LDH (lactate dehydrogenase), (3) S100B (S100 calcium binding protein B), (4) TGFβ (transforming growth factor beta), PAI-1 (plasminogen activator inhibitor 1), TGM2 (transglutaminase 2), (5) sRAGE (soluble receptor for advanced glycosylation end products), HMGB1 (high mobility group box 1), C3/C1Q (complement factor 3/1Q), ST2 (Interleukin 1 receptor like 1), agrin (AGRN), (6) IGF-1 (insulin-like growth factor 1), resistin (RETN), adiponectin (ADIPOQ), ghrelin (GHRL), growth hormone (GH), (7) microparticle panel (to be further defined), GpnmB (glycoprotein nonmetastatic melanoma protein B) and lactoferrin (LTF). We believe that these predicted panels need to be experimentally explored in animal models and frail cohorts in order to ascertain their diagnostic, prognostic and therapeutic potential.

TGM2 knockdown reverses cisplatin chemoresistance in osteosarcoma

In the past decades, chemotherapy has resulted in improved outcomes for patients with osteosarcoma. However, resistance to chemotherapy often leads to poor prognoses. Cisplatin is a standard drug for osteosarcoma therapy, and chemoresistance to cisplatin in osteosarcoma limits the effectiveness of chemotherapy drugs. Transglutaminase 2 (TGM2) is a member of the transglutaminase family, and it is reported to be associated with chemoresistance in various types of cancer. The present study aimed to investigate the function of TGM2 in regulating chemosensitivity of osteosarcoma cells to cisplatin. For in vitro experiments, a cisplatin-resistant osteosarcoma cell line (Saos2-CIS-R) was established, and TGM2 was demonstrated to be upregulated in the resistant Saos2-CIS-R cells compared with the normal Saos2 cells. The present study also revealed that TGM2 was associated with chemoresistance to cisplatin in osteosarcoma cells, and knockdown of TGM2 enhanced their chemosensitivity. In addition, TGM2 was demonstrated to affect the chemosensitivity of osteosarcoma cells via regulation of the activation of mitogen-activated protein kinase and AKT serine/threonine kinase pathways. Expression of BCL2 apoptosis regulator, BCL2 associated X and caspase-3 was also involved in chemoresistance development in osteosarcoma. For in vivo experiments, a mouse model was used to detect that the cisplatin sensitivity of Saos2-CIS-R cells was reversed following TGM2 knockdown. Taken together, the present data suggested a potentially important role for TGM2 in the regulation of osteosarcoma chemosensitivity. TGM2 might therefore serve as a therapeutic target for osteosarcoma.

The role of α9β1 integrin and its ligands in the development of autoimmune diseases

Adhesion of cells to extracellular matrix proteins through integrins expressed on the cell surface is important for cell adhesion/motility, survival, and differentiation. Recently, α9β1 integrin was reported to be important for the development of autoimmune diseases including rheumatoid arthritis, multiple sclerosis, and their murine models. In addition, ligands for α9β1 integrin, such as osteopontin and tenascin-C, are well established as key regulators of autoimmune diseases. Therefore, this review focused on the role of interactions between α9β1 integrin and its ligands in the development of autoimmune diseases.

Transglutaminase 2 in human diseases

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

Transglutaminase 2 in cartilage homoeostasis: novel links with inflammatory osteoarthritis

Transglutaminase 2 (TG2) is highly expressed during chondrocyte maturation and contributes to the formation of a mineralised scaffold by introducing crosslinks between extracellular matrix (ECM) proteins. In healthy cartilage, TG2 stabilises integrity of ECM and likely influences cartilage stiffness and mechanistic properties. At the same time, the abnormal accumulation of TG2 in the ECM promotes chondrocyte hypertrophy and cartilage calcification, which might be an important aspect of osteoarthritis (OA) initiation. Although excessive joint loading and injuries are one of the main causes leading to OA development, it is now being recognised that the presence of inflammatory mediators accelerates OA progression. Inflammatory signalling is known to stimulate the extracellular TG2 activity in cartilage and promote TG2-catalysed crosslinking of molecules that promote chondrocyte osteoarthritic differentiation. It is, however, unclear whether TG2 activity aims to resolve or aggravate damages within the arthritic joint. Better understanding of the complex signalling pathways linking inflammation with TG2 activities is needed to identify the role of TG2 in OA and to define possible avenues for therapeutic interventions.

Fibroblast-like synoviocytes-dependent effector molecules as a critical mediator for rheumatoid arthritis: Current status and future directions

Rheumatoid arthritis (RA) is a systemic-autoimmune-mediated disease characterized by synovial hyperplasia and progressive destruction of joint. Currently available biological agents and inhibitor therapy that specifically target tumor necrosis factor-α, interleukin 1β (IL-1β), IL-6, T cells, B cells, and subcellular molecules (p38 mitogen-activated protein kinase and janus kinase) cannot facilitate complete remission in all patients and are unable to cure the disease. Therefore, further potent therapeutic targets need to be identified for effective treatment and successful clinical outcomes in patients with RA. Scientific breakthroughs have brought new insights regarding fibroblast-like synoviocytes (FLS), a major constituent of the synovial hyperplasia. These play a pivotal role in RA invading cartilage and bone tissue. Currently there are no effective therapies available that specifically target these aggressive cells. Recent evidences indicate that FLS-dependent effector molecules (toll-like receptors, nodal effector molecules, hypoxia-inducible factor, and IL-17) have emerged as important mediators of RA. In this review, we discuss the pathological features and recent advances in understanding the role of FLS-dependent effector molecules in the disease onset of RA. Pharmacological inhibition of FLS-dependent effector molecules might be a promising option for FLS-targeted therapy in RA.

Transglutaminase-2 is Involved in Cell Apoptosis of Osteosarcoma Cell Line U2OS Under Hypoxia Condition

Osteosarcoma is the most common type of solid bone cancer, which is the second leading cause of cancer-related death. Hypoxia is an ordinary phenomenon in solid tumor tissues and can induce cell apoptosis but the specific molecular mechanism remains unclear. In this study, we explored the effect and the molecular mechanism of Transglutaminase 2 (TG2) on cell apoptosis in osteosarcoma U2OS cells under hypoxia. We found the enzymatic activity of TG2 is significantly increased and the expression of TG2 is remarkably up-regulated under hypoxia condition. Cell apoptotic rate is markedly increased upon knockdown of TG2 by siRNA under hypoxia. We further investigated the mechanism of cell apoptosis and found Bax protein is significantly increased after depletion of TG2 under hypoxia. Moreover, our data also show that cytochrome C (Cyt C) is significantly increased in cytoplasm and markedly decreased in mitochondria of U2OS cells after depletion of TG2 under hypoxia. Our results suggest that TG2 can inhibit tumor cell apoptosis through down-regulation of Bax and prevention of release Cyt C from mitochondria into cytoplasm.

P2X7 receptor-mediated TG2 externalization: a link to inflammatory arthritis?

Transglutaminases have important roles in stabilizing extracellular protein assemblies in tissue repair processes but some reaction products can stimulate immune activation, leading to chronic inflammatory conditions or autoimmunity. Exacerbated disease in models of inflammatory arthritis has been ascribed to sustained extracellular enzyme activity alongside formation of select protein modifications. Here, we review the evidence, with a focus on the link between P2X7R signaling and TG2 export, a pathway that we have recently discovered which ties extracellular protein modifications into the danger signal-mediated innate immune response. These recent insights offer new opportunities for therapeutic intervention.

Snail Is a Critical Mediator of Invadosome Formation and Joint Degradation in Arthritis

Progressive cartilage destruction, mediated by invasive fibroblast-like synoviocytes, is a central feature in the pathogenesis of rheumatoid arthritis (RA). Members of the Snail family of transcription factors are required for cell migration and invasion, but their role in joint destruction remains unknown. Herein, we demonstrate that Snail is essential for the formation of extracellular matrix-degrading invadosomal structures by synovial cells from collagen-induced arthritis (CIA) rats and RA patients. Mechanistically, Snail induces extracellular matrix degradation in synovial cells by repressing PTEN, resulting in increased phosphorylation of platelet-derived growth factor receptor and activation of the phosphatidylinositol 3-kinase/AKT pathway. Of significance, Snail is overexpressed in synovial cells and tissues of CIA rats and RA patients, whereas knockdown of Snail in CIA joints prevents cartilage invasion and joint damage. Furthermore, Snail expression is associated with an epithelial-mesenchymal transition gene signature characteristic of transglutaminase 2/transforming growth factor-β activation. Transforming growth factor-β and transglutaminase 2 stimulate Snail-dependent invadosome formation in rat and human synoviocytes. Our results identify the Snail-PTEN platelet-derived growth factor receptor/phosphatidylinositol 3-kinase axis as a novel regulator of the prodestructive invadosome-forming phenotype of synovial cells. New therapies for RA target inflammation, and are only partly effective in preventing joint damage. Blocking Snail and/or its associated gene expression program may provide an additional tool to improve the efficacy of treatments to prevent joint destruction.

Rheumatoid arthritis-celiac disease relationship: joints get that gut feeling

Rheumatoid arthritis (RA) and celiac disease (CD) belong to the autoimmune disease family. Despite being separate entities they share multiple aspects. Epidemiologically they share comparable incidence environmental influences, associated antibodies and a recent incidental surge. They differ in their HLA pre-dispositions and specific predictive and diagnostic biomarkers. At the clinical level, celiac disease exhibits extra-intestinal rheumatic manifestations and RA gastrointestinal ones. Small bowel pathology exists in rheumatic patients. A trend towards responsiveness to a gluten free diet has been observed, ameliorating celiac rheumatic manifestations, whereas dietary interventions for rheumatoid arthritis remain controversial. Pathophysiologically, both diseases are mediated by endogenous enzymes in the target organs. The infectious, dysbiotic and increased intestinal permeability theories, as drivers of the autoimmune cascade, apply to both diseases. Contrary to their specific HLA pre-disposition, the diseases share multiple non-HLA loci. Those genes are crucial for activation and regulation of adaptive and innate immunity. Recently, light was shed on the interaction between host genetics and microbiota composition in relation to CD and RA susceptibility, connecting bugs and us and autoimmunity. A better understanding of the above mentioned similarities in the gut-joint inter-relationship, may elucidate additional facets in the mosaic of autoimmunity, relating CD to RA.

Role of tissue transglutaminase in the pathogenesis of diabetic cardiomyopathy and the intervention effect of rutin

The aim of this study was to investigate the role of tissue transglutaminase (tTG) in the pathogenesis of diabetic cardiomyopathy (DCM) and the intervention effect of rutin. DCM was induced in rats by the injection of streptozotocin (STZ; 25 mg/kg). After a preliminary examination, the rats were randomly divided into four groups: Control (n=8), STZ-induced DCM (n=8), STZ + positive drug (captopril; n=6) and STZ + rutin (n=8) groups. The DCM model was evaluated using blood sugar values, serum enzyme levels, hematoxylin and eosin staining and Masson’s staining, ex vivo. The protein and mRNA expression of tTG was assessed with immunohistochemistry, western blotting and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The rat model of DCM was successfully established by STZ administration, and the expression levels of tTG were significantly increased in the DCM model. Following the injection of captopril or rutin, the blood sugar values, collagen content and expression levels of tTG were gradually reduced and serum enzyme levels were increased, as compared with those in the STZ-induced DCM group. In conclusion, tTG plays an important role in STZ-induced DCM. In addition, rutin may inhibit the expression of tTG and regulate myocardial injury in STZ-induced DCM.

Zinc is the modulator of the calcium-dependent activation of post-translationally acting thiol-enzymes in autoimmune diseases

Post-translational modifications of proteins can generate antigenic conformations that may cause autoimmune diseases in persons with specific HLA-haplotypes. Monocytes and macrophages, attracted to an inflamed site, can release post-translationally acting enzymes, such as transglutaminases and peptidylarginine deiminases. In vivo, the activation of these enzymes is crucial for the further course of event. Our hypothesis is that zinc modulates the activation of these calcium-dependent thiol-enzymes. Persons with celiac disease carry antibodies against deamidated dietary gluten and against transglutaminase type 2. Similarly, antibodies against citrulline-containing peptides and against peptidylarginine deiminase are detected in patients with rheumatoid arthritis. Thus, in two major autoimmune diseases, antibodies are detected against post-translationally modified proteins and against the thiol-enzymes responsible for catalyzing the modifications. In vitro, physiological concentrations of zinc reversibly inhibit the calcium-dependent activation of transglutaminases. Zinc attenuates the calcium-induced increase in affinity between transglutaminase 2 and serum from patients with celiac disease. Peptidylarginine deiminases are also inhibited by zinc. Moreover, zinc is rapidly redistributed in animals when an infection is induced. This pathway starting with an unspecific inflammation and ending up with an immune reaction against a specific tissue constitutes a theme with variations in other autoimmune diseases, such as dermatitis herpetiformis, multiple sclerosis, and type 1 diabetes. Inhibitors against transglutaminases and peptidylarginine deiminases have a great pharmacological potential. Interestingly, a large portion of the population may have been exposed to such an inhibitor. The primary metabolite of ethanol, acetaldehyde, can probably function as an irreversible inhibitor of these enzymes by forming a hemithioacetal with the thiol group of the active site. Not surprisingly, epidemiological studies have shown that alcohol is beneficial in rheumatoid arthritis. We predict that a similar situation will be observed in multiple sclerosis. The affinity of chelators such as EDTA and EGTA for Zn(2+) is three orders of magnitude greater than that for Ca(2+). This frequently overlooked complication imposes problems in biomedical research since a restoration of the zinc level can never be achieved in a blood sample which has been anti-coagulated by calcium chelators. The new synthetic direct thrombin inhibitors may offer a better way of preventing coagulation in vitro.

CONCLUSIONS

Post-translational modifications are of potential interest in autoimmune diseases. The in vivo activation of calcium-dependent thiol-enzymes catalyzing these alterations, such as the transglutaminases and the peptidylarginine deiminases, is crucial for this pathway. According to our hypothesis, zinc is the modulator of this key function.