Increased levels of citrullinated antithrombin in plasma of patients with rheumatoid arthritis and colorectal adenocarcinoma determined by a newly developed ELISA using a specific monoclonal antibody

Atherothrombotic Risk Factors in Patients With Rheumatoid Arthritis

BACKGROUND

The onset of cardiovascular complications has increased the mortality rate in rheumatoid arthritis (RA) patients. Presently, there is a need to diagnose cardiovascular co-morbidity in rheumatic disease. While biomarkers such as P-selectin glycoprotein ligand-1 (PSGL-1), fibrinogen, anti-thrombin III (AT-III), hsCRP, lipoprotein (a) (lp(a)), leptin, adiponectin, and asymmetric dimethyl arginine (ADMA) are already established as independent risk factors for the development of atherosclerosis, the association of these biomarkers with disease activity in RA patients is unclear.

METHODS

The case-control study comprised 40 cases along with age- and gender-matched controls recruited from a tertiary care hospital in southern India. Platelet activation in plasma was analyzed by flow cytometry using CD41 per CPCY 5.5 (platelet marker) and human CD62P FITC monoclonal antibody (P-selectin marker). Other parameters were quantified through nephelometry and ELISA. The association between the risk factors and RA disease severity, as per the disease activity score (DAS/DAS28), was analyzed. Furthermore, an ROC analysis was done to assess the utility of these biomarkers in the diagnosis of RA.

RESULTS

With the exception of leptin, adiponectin, and ADMA, there was a significant increase in the levels of PSGL-1, fibrinogen, AT-III, hsCRP, and lp(a) when compared to healthy controls. Conventional risk factors contributing to dyslipidemia were also assessed, in which the low-density lipoprotein (LDL)/high-density lipoprotein (HDL) ratio was found to be significantly higher in RA patients compared to controls. Moreover, a significant positive correlation was identified between DAS score and activated platelets, fibrinogen, and hsCRP. ROC analysis identified that fibrinogen could predict the RA disease status with 95% accuracy, followed by activated platelets and hsCRP.

CONCLUSION

Several of the studied atherothrombotic risk factors were significantly altered in patients with RA. Activated platelets, fibrinogen, and hsCRP were associated with disease activity and also served as good diagnostic predictors for RA. Based on our findings, further studies could explore the potential of introducing anti-thrombotic agents in the treatment regimen of patients with RA.

Role and intervention of PAD4 in NETs in acute respiratory distress syndrome

BACKGROUND

Sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection. Acute respiratory distress syndrome (ARDS) is a common sepsis-associated injury that can increase postoperative mortality but the mechanism is still unclear.

MAIN TEXT

The role of neutrophils in the pathophysiology of sepsis was deeply challenged after the discovery of NETosis, a process resulting in neutrophil extracellular traps (NETs) release. NETs can support thrombin generation and the concept of immunothrombosis has emerged as a new innate response to infection. Immunothrombosis leads to thrombosis in microvessels and supports immune cells together with specific thrombus-related molecules. ARDS is a common sepsis-associated organ injury. Immunothrombosis participates in thrombosis in pulmonary capillaries. Intervention regarding immunothrombosis in ARDS is a key scientific problem. PAD4 is the key enzyme regulating the NET skeleton protein histone H3 to citrulline histone to form NETs in immune thrombosis. This review summarizes NETosis and immunohaemostasis, ARDS and therapeutic opportunities targeting PAD4 via PAD4 inhibitors and lncRNAs potentially, providing future therapies.

CONCLUSIONS

We identified and summarized the fundamental definition of ARDS and the concept of immune thrombosis and its composition. NETs activation has become particularly relevant in the formation of immune thrombosis. The taskforce highlighted the intervention targets of PAD4, including noncoding RNAs, potentially providing future therapeutic targets to confront the high postoperative mortality of ARDS.

Role of non-canonical post-translational modifications in gastrointestinal tumors

Post-translational modifications (PTMs) of proteins contribute to the occurrence and development of tumors. Previous studies have suggested that canonical PTMs such as ubiquitination, glycosylation, and phosphorylation are closely implicated in different aspects of gastrointestinal tumors. Recently, emerging evidence showed that non-canonical PTMs play an essential role in the carcinogenesis, metastasis and treatment of gastrointestinal tumors. Therefore, we summarized recent advances in sumoylation, neddylation, isoprenylation, succinylation and other non-canonical PTMs in gastrointestinal tumors, which comprehensively describe the mechanisms and functions of non-classical PTMs in gastrointestinal tumors. It is anticipated that targeting specific PTMs could benefit the treatment as well as improve the prognosis of gastrointestinal tumors.

The role of SERPIN citrullination in thrombosis

Aberrant protein citrullination is associated with many pathologies; however, the specific effects of this modification remain unknown. We have previously demonstrated that serine protease inhibitors (SERPINs) are highly citrullinated in rheumatoid arthritis (RA) patients. These citrullinated SERPINs include antithrombin, antiplasmin, and t-PAI, which regulate the coagulation and fibrinolysis cascades. Notably, citrullination eliminates their inhibitory activity. Here, we demonstrate that citrullination of antithrombin and t-PAI impairs their binding to their cognate proteases. By contrast, citrullination converts antiplasmin into a substrate. We recapitulate the effects of SERPIN citrullination using in vitro plasma clotting and fibrinolysis assays. Moreover, we show that citrullinated antithrombin and antiplasmin are increased and decreased in a deep vein thrombosis (DVT) model, accounting for how SERPIN citrullination shifts the equilibrium toward thrombus formation. These data provide a direct link between increased citrullination and the risk of thrombosis in autoimmunity and indicate that aberrant SERPIN citrullination promotes pathological thrombus formation.

PAD4 takes charge during neutrophil activation: Impact of PAD4 mediated NET formation on immune-mediated disease

BACKGROUND

Peptidyl arginine deiminase 4 (PAD4) is an enzyme that converts arginine into citrulline. PAD4 is expressed in neutrophils that, when activated, can drive the formation of neutrophil extracellular traps (NETs). Uncontrolled activation of PAD4 and subsequent citrullination of proteins is increasingly recognized as a driver of (auto)immune diseases. Currently, our understanding of PAD4 structure-function relationships and activity control in vivo is incomplete.

AIMS

To provide the current state-of-the-art on PAD4 structure-activity relationships and involvement of PAD4 in autoimmune disorders as well as in thrombo-inflammatory disease.

MATERIALS & METHODS

Literature review and molecular modelling Results: In this review, we used molecular modelling to generate a three-dimensional structure of the complete PAD4 molecule. Using our model, we discuss the catalytic conversion of the arginine substrate to citrulline. Besides mechanistic insight into PAD4 function, we give an overview of biological functions of PAD4 and mechanisms that influence its activation. In addition, we discuss the crucial role of PAD4-mediated citrullination of histones during the formation of NETs. Subsequently, we focus on the role of PAD4-mediated NET formation and its role in pathogenesis of rheumatoid arthritis, sepsis and (immune-)thrombosis. Finally, we summarize current efforts to design different classes of PAD4 inhibitors that are being developed for improved treatment of autoimmune disorders as well as thrombo-inflammatory disease.

DISCUSSION

Advances in PAD4 structure-function are still necessary to gain a complete insight in mechanisms that control PAD4 activity in vivo. The involvement of PAD4 in several diseases signifies the need for a PAD4 inhibitor. Although progress has been made to produce an isotype specific and potent PAD4 inhibitor, currently no PAD4 inhibitor is ready for clinical use.

CONCLUSION

More research into PAD4 structure and function and into the regulation of its activity is required for the development of PAD4 specific inhibitors that may prove vital to combat and prevent autoimmune disorders and (thrombo)inflammatory disease.

A Novel Citrullinated Modification of Histone 3 and Its Regulatory Mechanisms Related to IPO-38 Antibody-Labeled Protein

IPO-38 is a potential biomarker for early diagnosis of gastric cancer that we recently identified. Although we characterized its chemical nature as a nucleosome histone, we suspected the existence of histone modification for the IPO-38 antibody-labeled protein. Here, we used a commercially available modified histone peptide array to identify the type and site of histone modification labeled by the IPO-38 monoclonal antibody. In protein array analysis, the citrulline modification of histone 3 on arginine 26 (H3R26Cit) yielded the strongest signal. Although peptidyl arginine deiminase-2 and -4 (PADI2 and PADI4, respectively) can catalyze the conversion of arginine to citrulline, we observed that only PADI4 expression correlated with the citrulline histone modification of H3R26Cit. Overexpression of PADI4, via transfection of a eukaryotic expression vector, and knockdown of PADI4 gene expression, by a PADI4 CRISPR/Cas9 vector, confirmed the crucial function of PADI4 on the increased level of H3R26Cit in gastric cancer cell lines. By immunoprecipitation and immunoblotting, we found an interaction between H3R26Cit and H3K27me3. Our study established the first link between the IPO-38 antigen and citrullinated histone 3, and clarified the upstream regulatory enzyme PADI4. The new findings suggest an important role for the citrullination modification of histone in gastric cancer biology, and should help us optimize the development of a sensitive and specific diagnostic reagent.

Progression on Citrullination of Proteins in Gastrointestinal Cancers

The citrullination modification (Cit) of proteins has received increasing attention in recent years. This kind of protein modification was first discovered in autoimmune diseases such as rheumatoid arthritis. The citrullination modification process is catalyzed by the peptidyl arginine deiminases (PADIs) family. A well-known citrullination of histone involves the key mechanism of neutrophil extracellular traps (NETs) of inflammation in the peripheral blood. Further studies revealed that citrullination modification of proteins also involves in carcinogenesis in human being. Citrullinated proteins disturbed the stability of proteins and caused DNA damages. There is increasing evidence that citrullinated proteins can be used as potential targets for cancer diagnosis or treatment. This review introduces the concept of citrullination modification of proteins, substrate proteins, examining methods and biological significances.

Peptidylarginine deiminase 4 overexpression resensitizes MCF-7/ADR breast cancer cells to adriamycin via GSK3β/p53 activation

Background

Adriamycin (ADR) is widely used in the clinical chemotherapy against breast cancer. But its efficacy is strongly limited due to the acquisition of multidrug resistance (MDR). Therefore, acquisition of the resistance to ADR is still a major cause of chemotherapy failure in breast cancer patients. Peptidylarginine deiminase IV (PAD4) is reported to target non-histone proteins for citrullination, regulate their substrate activities, and thereby play critical roles in maintaining cell phenotype in breast cancer cells. However, whether PAD4 is involved in the development of MDR in breast cancer is poorly understood.

Materials and methods

We examined the expression of PAD family members, including PAD4 in ADR-resistant MCF-7 cells compared with the parental control cells by real-time PCR and Western blotting analyses. Rescue of PAD4 expression in MCF-7/ADR cells was performed to assess whether PAD4 could restore the sensitivity of MCF-7/ADR cells to ADR treatment with cell counting kit-8, flow cytometry, TUNEL, nuclear and cytoplasmic extract preparations, and immunofluorescence staining analyses.

Results

Both PAD2 and PAD4 were significantly decreased in ADR-resistant cells. However, only PAD4 overexpression can increase the sensitivity of MCF-7/ADR cells to ADR treatment and decrease MDR1 gene expression. Overexpression of PAD4 in MCF-7/ADR cells inhibited cell proliferation by inducing cell apoptosis. Under ADR treatment, overexpression of PAD4 promoted nuclear accumulation of glycogen synthase kinase-3β and p53, which further activated proapoptotic gene expression and downregulated MDR1 expression. Moreover, PAD4 activity was required for activating proapoptotic gene transcripts.

Conclusion

We demonstrate the previously unappreciated role of PAD4 in reversing ADR resistance in MCF-7/ADR cells and help establish PAD4 as a candidate biomarker of prognosis and chemotherapy target for MDR in breast cancers.

Pollution Level and Health Risk Assessment of PM2.5-Bound Metals in Baoding City Before and After the Heating Period

In order to assess the pollution levels and health risks of PM2.5-bound metals in Baoding City before and after the heating period, samples were collected in 2016 at Hebei University from September 25th to November 14th during the non-heating period, and November 15th to December 26th during the heating period, respectively. ICP-MS was applied to analyze seven heavy metals (Cr, Zn, Cu, Pb, Ni, Cd and Fe). The statistical analysis, enrichment factor (EF), pollution load index method, and Risk Assessment Method proposed by U.S. EPA were used to evaluate the non-carcinogenic risks of six of these heavy metals (Cr, Zn, Cu, Pb, Ni and Cd) and carcinogenic risks of three of these heavy metals (Cr, Ni and Cd). The results showed three main results. First, the average daily PM2.5 concentrations of the national air monitoring stations was 155.66 μg·m-3 which was 2.08 times as high as that of the second level criterion in China (75 μg·m-3) during the observation period. Compared with the non-heating period, all heavy metals concentrations increased during heating period. The growth rates of Pb and Ni were the highest and the lowest, which were 88.03 and 5.11 percent, respectively. Second, the results of enrichment factor indicated that the EF values of all heavy metals were higher during the heating period in comparison with during the non-heating period, but the degree of enrichment of all heavy metals remained unchanged. Not only those, Cr and Ni were minimally enriched and were affected by both human and natural factors, Pb, Cu and Zn were significantly enriched and were mainly affected by human factors, the enrichment of Cd was much higher than that of the other heavy metals, exhibiting extremely high enrichment, mainly due to human factors during the whole sampling period. The results of the pollution load index indicated that the proportions of the number of highly and very highly polluted PM2.5-bound metals were the highest during the heating period, while the proportion of moderately polluted PM2.5-bound metals was the highest during the non-heating period. The combined pollution degree of heavy metals was more serious during the heating period. Third, according to the health risk assessment model, we concluded that the non-carcinogenic and carcinogenic risks caused by inhalation exposure were the highest and by dermal exposure were the lowest for all kinds of people. The overall non-carcinogenic risk of heavy metals via inhalation and subsequent ingestion exposure caused significant harm to children during the non-heating and the heating periods, and the risk values were 2.64, 4.47, 1.20 and 1.47, respectively. Pb and Cr exhibited the biggest contributions to the non-carcinogenic risk. All the above non-carcinogenic risks exceeded the standard limits suggested by EPA (HI or HQ < 1). The carcinogenic risk via inhalation exposure to children, adult men and women were 2.10 × 10-4, 1.80 × 10-4, and 1.03 × 10-4 during the non-heating period, respectively, and 2.52 × 10-4, 2.16 × 10-4 and 1.23 × 10-4 during the heating period, respectively. All the above carcinogenic risks exceeded the threshold ranges (10-6~10-4), and Cr posed a carcinogenic risk to all people.

Biochemical and cellular consequences of the antithrombin p.Met1? mutation identified in a severe thrombophilic family

Nature is always the best inspiration for basic research. A family with severe thrombosis and antithrombin deficiency, the strongest anticoagulant, carried a new mutation affecting the translation-start codon of SERPINC1, the gene encoding antithrombin. Expression of this variant in a eukaryotic cell system produced three different antithrombins. Two downstream methionines were used as alternative initiation codons, generating highly expressed small aglycosylated antithrombins with cytoplasmic localization. Wild-type antithrombin was generated by the use of the mutated AUU as initiation codon. Actually, any codon except for the three stop codons might be used to initiate translation in this strong Kozak context. We show unexpected consequences of natural mutations affecting translation-start codons. Downstream alternative initiation AUG codons may be used when the start codon is mutated, generating smaller molecules with potential different cell localization, biochemical features and unexplored consequences. Additionally, our data further support the use of other codons apart from AUG for initiation of translation in eukaryotes.

PADI4 has genetic susceptibility to gastric carcinoma and upregulates CXCR2, KRT14 and TNF-α expression levels

PADI4 (peptidyl deiminase isoform 4) is overexpressed in many tumor tissues and converts arginine residues to citrulline residues. This study used an Illumina SNP microarray and a TaqMan assay to determine the possible association of the PADI4 gene with various tumor risks. Both genotyping methods demonstrated significant associations between the tag SNPs rs1635566 and rs882537 in the PADI4 locus with gastric carcinoma in two independent cohorts. Based on this genotyping result, we used the Cancer Pathway Finder, p53 Signaling, Signal Transduction and Tumor Metastasis PCR arrays to investigate the tumorigenic pathway of PADI4 in MNK-45 cells derived from gastric carcinoma. We detected significantly decreased expression levels of CXCR2, KRT14 and TNF-α in MNK-45 cells that were treated with anti-PADI4 siRNA. We also detected increased expression of these three genes in MNK-45 cells transfected with a pcDNA3.1 plasmid overexpressing PADI4. A highly similar result was also obtained for SGC 7901 cells, which also originate from gastric carcinoma. Our result indicates that the PADI4 gene has genetic susceptibility in gastric carcinoma. PADI4 contributes to gastric tumorigenesis by upregulating CXCR2, KRT14 and TNF-α expression, which are well known to activate angiogenesis, cell proliferation, cell migration and the immune microenvironment in tumors.

PAD1 promotes epithelial-mesenchymal transition and metastasis in triple-negative breast cancer cells by regulating MEK1-ERK1/2-MMP2 signaling

Peptidylargininedeiminase 1 (PAD1) catalyzes protein for citrullination, and this activity has been linked to the epidermal cornification. However, a role for PAD1 in tumorigenesis, including breast cancers has not been previously explored. Here we first showed that PAD1 is overexpressed in human triple negative breast cancer (TNBC). In cultured cells and xenograft mouse models, PAD1 depletion or inhibition reduced cell proliferation, suppressed epithelial-mesenchymal transition, and prevented metastasis of MDA-MB-231 cells. These changes were correlated with a dramatic decrease in MMP2/9 expression. Furthermore, ERK1/2 and P38 MAPK signaling pathways are activated upon PAD1 silencing. Treatment with MEK1/2 inhibitor in PAD1 knockdown cells significantly recovered MMP2 expression, while inhibiting P38 activation only slightly elevated MMP9 levels. We then showed that PAD1 interacts with and citrullinates MEK1 thereby disrupting MEK1-catalyzed ERK1/2 phosphorylation, thus leading to the MMP2 overexpression. Collectively, our data indicate that PAD1 appears to promote tumorigenesis by regulating MEK1-ERK1/2-MMP2 signaling in TNBC. These results also raise the possibility that PAD1 may function as an important new biomarker for TNBC tumors and suggest that PAD1-specific inhibitors could potentially be utilized to treat metastatic breast cancer.

Hypoxia-induced production of peptidylarginine deiminases and citrullinated proteins in malignant glioma cells

BACKGROUND

Recently, it has been reported that hypoxia highly enhances expression of peptidylarginine deiminase (PAD) 4 and production of citrullinated proteins in some tumor cells. However, little is known about malignant gliomas on this issue. Therefore, we here investigated whether expression of PADs was induced by hypoxia and whether PADs citrullinated intracellular proteins if induced using U-251 MG cells of a human malignant glioma cell line.

METHODS

Expression of PADs in U-251 MG cells, cultured under hypoxia or normoxia for 24 h, was investigated by quantitative polymerase chain reaction (qPCR). Citrullination of proteins in the cells and the cell lysates incubated for 48 h with or without Ca²⁺ was detected by western blotting. Citrullinated proteins were identified by mass spectrometry.

RESULTS

The mRNA levels of PAD1, 2, 3, and 4 were up-regulated by hypoxia in a hypoxia-inducible factor-1-dependent manner in U-251 MG cells. In spite of the increased expression, intracellular proteins were not citrullinated. However, the induced PADs citrullinated U-251 MG cell-derived proteins when the cells were lysed. Multiple proteins citrullinated by hypoxia-induced PADs were identified. In addition, the extracellular domain of vascular endothelial growth factor receptor 2 was citrullinated by human PAD2 in vitro.

CONCLUSION

Our data may contribute to understanding of pathophysiology of malignant gliomas from the aspects of protein citrullination.

Role of peptidylarginine deiminase type 4 in gastric cancer

Peptidylarginine deiminase type 4 (PADI4) post-translationally converts peptidylarginine to citrulline, appearing to be overexpressed in numerous carcinomas. The current study aimed to investigate the expression of PADI4 in gastric cancer tissues and its effect on the biological activities of SGC-7901 and AGS tumor cell lines. The expression of PADI4 was determined in gastric cancer and normal gastric mucosa tissues using western blot analysis and reverse transcription-quantitative polymerase chain reaction. Gastric cancer cell lines were divided into the following groups: Mock group (subjected to transfection reagent); negative group [subjected to small interfering RNA (siRNA) transfection]; PADI4 siRNA group (subjected to PADI4 siRNA transfection); 5-fluorouracil (5-Fu) group (subjected to 5-Fu); and 5-Fu + siRNA transfection group (subjected to 5-Fu and PADI4 siRNA transfection). The effects of silencing PADI4 with the above measures on the proliferation and invasion of SGC-7901 and AGS cells were determined by MTT and Transwell chamber assays. In addition, propidium iodide staining was performed to detect the effects of PADI4 on the cell cycle. A significant increase in the expression of PADI4 mRNA in gastric cancer tissue compared with normal mucosa tissue was identified (P<0.05). The proliferation and invasion of SGC-7901 and AGS cells were significantly decreased in the PADI4 siRNA group. Furthermore, flow cytometry DNA analysis revealed that silencing PADI4 resulted in significant S phase arrest and marked decrease of cells in the G2/M phase. PADI4 siRNA coupled with 5-Fu significantly enhanced its inhibitory effect on the proliferation of gastric cancer cells. In conclusion, PADI4 demonstrated high expression in gastric cancer and served an important role in the biological activities of gastric cancer cells involving cell proliferation, invasion and cell cycle. As a result, PADI4 may be a valid cancer susceptibility gene and potential target for cancer therapy.

Importance of post-translational modifications on the function of key haemostatic proteins

Post-translational modifications (PTMs) such as glycosylation and phosphorylation play an important role on the function of haemostatic proteins and are critical in the setting of disease. Such secondary level changes to haemostatic proteins have wide ranging effects on their ability to interact with other proteins. This review aimed to summarize the knowledge of the common PTMs associated with haemostatic proteins and the implications of such modifications on protein function. Haemostatic proteins that represent the main focus for studies specific to PTMs are von Willebrand factor, tissue factor, factor VIII, antithrombin and fibrinogen. These proteins are susceptible to PTMs by glycosylation, phosphorylation, sulphation, citrullination and nitration, respectively, with a significant impact on their function. During synthesis, vWF must undergo extensive PTMs, with N-linked glycosylation being the most common. Increased phosphorylation of tissue factor results in increased affinity for platelets to the vessel endothelium. Citrullination of antithrombin leads to an increased anticoagulant function of this protein and therefore an anticoagulant state that inhibits clot formation. On the contrary, nitration of fibrinogen has been shown to result in a prothrombotic state, whilst sulphation is required for the normal function of Factor VIII. From this review, it is evident that PTMs of haemostatic proteins as a change in protein structure at a secondary level greatly influences the behaviour of the protein at a tertiary level.

Detection and identification of protein citrullination in complex biological systems

Protein citrullination is a post-translational modification of arginine that is catalyzed by the Protein Arginine Deiminase (PAD) family of enzymes. Aberrantly increased citrullination is associated with a host of inflammatory diseases and cancer and PAD inhibitors have shown remarkable efficacy in a range of diseases including rheumatoid arthritis, lupus, atherosclerosis, and ulcerative colitis. In rheumatoid arthritis, citrullinated proteins serve as key antigens for rheumatoid arthritis-associated autoantibodies. These data suggest that citrullinated proteins may serve more generally as biomarkers of specific disease states, however, the identification of citrullinated residues remains challenging due to the small 1Da mass change that occurs upon citrullination. Herein, we highlight the available techniques to identify citrullinated proteins/residues focusing on advanced MS techniques as well as chemical derivatization strategies that are currently being employed to identify citrullinated proteins as well as the specific residues modified within those proteins.

Investigating citrullinated proteins in tumour cell lines

BACKGROUND

The conversion of arginine into citrulline, termed citrullination, has important consequences for the structure and function of proteins. Studies have found PADI4, an enzyme performing citrullination, to be highly expressed in a variety of malignant tumours and have shown that PADI4 participates in the process of tumorigenesis. However, as citrullinated proteins have not been systematically investigated in tumours, the present study aimed to identify novel citrullinated proteins in tumours by 2-D western blotting (2-D WB).

METHODS

Two identical two-dimensional electrophoresis (2-DE) gels were prepared using extracts from ECA, H292, HeLa, HEPG2, Lovo, MCF-7, PANC-1, SGC, and SKOV3 tumour cell lines. The expression profiles on a 2-DE gel were trans-blotted to PVDF membranes, and the blots were then probed with an anti-citrulline antibody. By comparing the 2-DE profile with the parallel 2-D WB profile at a global level, protein spots with immuno-signals were collected from the second 2-DE gel and identified using mass spectrometry. Immunoprecipitation was used to verify the expression and citrullination of the targeted proteins in tumour cell lines.

RESULTS

2-D WB and mass spectrometry identified citrullinated α-enolase (ENO1), heat shock protein 60 (HSP60), keratin 8 (KRT8), tubulin beta (TUBB), T cell receptor chain and vimentin in these cell lines. Immunoprecipitation analyses verified the expression and citrullination of ENO1, HSP60, KRT8, and TUBB in the total protein lysates of the tumour cell lines.

CONCLUSIONS

The citrullination of these proteins suggests a new mechanism in the tumorigenic process.

Screening citrullinated proteins in synovial tissues of rheumatoid arthritis using 2-dimensional western blotting

OBJECTIVE

Citrullination, a reaction converting arginine residue into citrulline residue, is essential for autoimmunity of rheumatoid arthritis (RA). We conducted 2-dimensional Western blot analyses (2-D WB) to screen for novel citrullinated proteins in synovial tissues from patients with RA.

METHODS

Total proteins were extracted from the synovial membranes of patients with RA (n = 10) and pooled. Four identical 2-D electrophoresis (2-DE) gels were prepared, and 2 gels were transblotted to polyvinylidene fluoride membranes that were separately probed with sera from patients with RA (n = 10) or an anticitrulline antibody. The protein profiles of the 2-DE gels were compared with the hybridization results on a global level. The immunoreactive protein spots were collected from the 2-DE gels and identified using mass spectrometry. Proteins that were detected by both RA sera and anticitrulline antibody were considered citrullinated proteins. The result was confirmed through routine WB, immunoprecipitation, and ELISA. Autoantibodies against these potential antigens were also examined in the blood of patients with RA by ELISA.

RESULTS

RA sera and the anticitrulline antibody on 2-D WB detected α-1-antitrypsin (A1AT), dynein heavy-chain 3, fibrinogen β chain, keratin type II cuticular Hb4 (KRT84), lumican, tubulin β-chain (TUBB), and vimentin. A1AT, KRT84, and TUBB had high expression in the synovial membranes (n = 5) of patients with RA and A1AT and KRT84 had high expression in RA synovial fluids (n = 40). A1AT, KRT84, and TUBB immunoprecipitated from synovial tissues showed citrullination. A high level of autoantibodies against KRT84 was detected in the blood of patients with RA (n = 92) compared to that of healthy controls (n = 92).

CONCLUSION

Our study identified some new citrullinated proteins in RA synovial tissues using 2-D WB.

Potential role of peptidylarginine deiminase enzymes and protein citrullination in cancer pathogenesis

The peptidylarginine deiminases (PADs) are a family of posttranslational modification enzymes that catalyze the conversion of positively charged protein-bound arginine and methylarginine residues to the uncharged, nonstandard amino acid citrulline. This enzymatic activity is referred to as citrullination or, alternatively, deimination. Citrullination can significantly affect biochemical pathways by altering the structure and function of target proteins. Five mammalian PAD family members (PADs 1–4 and 6) have been described and show tissue-specific distribution. Recent reviews on PADs have focused on their role in autoimmune diseases. Here, we will discuss the potential role of PADs in tumor progression and tumor-associated inflammation. In the context of cancer, increasing clinical evidence suggests that PAD4 (and possibly PAD2) has important roles in tumor progression. The link between PADs and cancer is strengthened by recent findings showing that treatment of cell lines and mice with PAD inhibitors significantly suppresses tumor growth and, interestingly, inflammatory symptoms. At the molecular level, transcription factors, coregulators, and histones are functional targets for citrullination by PADs, and citrullination of these targets can affect gene expression in multiple tumor cell lines. Next generation isozyme-specific PAD inhibitors may have therapeutic potential to regulate both the inflammatory tumor microenvironment and tumor cell growth.

Detection and quantification of citrullinated chemokines

Background

Posttranslational deimination or citrullination by peptidylarginine deiminases (PAD) regulates the biological function of proteins and may be involved in the development of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. This posttranslational modification of arginine was recently discovered on inflammatory chemokines including CXCL8 and CXCL10, and significantly reduced their biological activity. To evaluate the importance of these modified chemokines in patients, methods for the detection and quantification of citrullinated chemokines are needed. Since citrullination only results in an increase of the protein mass with one mass unit and the loss of one positive charge, selective biochemical detection is difficult. Therefore, we developed an antibody-based method to specifically detect and quantify citrullination on a protein of interest.

Methodology/Principal Findings

First, the citrullinated proteins were chemically modified with antipyrine and 2,3-butanedione at low pH. Such selectively modified citrullines were subsequently detected and quantified by specific antibodies raised against a modified citrulline-containing peptide. The specificity of this two-step procedure was validated for citrullinated CXCL8 ([Cit⁵]CXCL8). Specific detection of [Cit⁵]CXCL8 concentrations between 1 and 50 ng/ml was possible, also in complex samples containing an excess of contaminating proteins. This novel detection method was used to evaluate the effect of lipopolysaccharide (LPS) on the citrullination of inflammatory chemokines induced in peripheral blood mononuclear cells (PBMCs) and granulocytes. LPS had no significant effect on the induction of CXCL8 citrullination in human PBMCs and granulocytes. However, granulocytes, known to contain PAD, were essential for the production of significant amounts of [Cit⁵]CXCL8.

Conclusion/Significance

The newly developed antibody-based method to specifically detect and quantify chemically modified citrullinated proteins is proven to be effective. This study furthermore demonstrates that granulocytes were essential to obtain significant levels of [Cit⁵]CXCL8. For human PBMCs and granulocytes stimulation with LPS did not affect the citrullination of CXCL8.