The Peptidylarginine Deiminase Inhibitor Cl-Amidine Suppresses Inducible Nitric Oxide Synthase Expression in Dendritic Cells

The conversion of peptidylarginine into peptidylcitrulline by calcium-dependent peptidylarginine deiminases (PADs) has been implicated in the pathogenesis of a number of diseases, identifying PADs as therapeutic targets for various diseases. The PAD inhibitor Cl-amidine ameliorates the disease course, severity, and clinical manifestation in multiple disease models, and it also modulates dendritic cell (DC) functions such as cytokine production, antigen presentation, and T cell proliferation. The beneficial effects of Cl-amidine make it an attractive compound for PAD-targeting therapeutic strategies in inflammatory diseases. Here, we found that Cl-amidine inhibited nitric oxide (NO) generation in a time- and dose-dependent manner in maturing DCs activated by lipopolysaccharide (LPS). This suppression of NO generation was independent of changes in NO synthase (NOS) enzyme activity levels but was instead dependent on changes in inducible NO synthase (iNOS) transcription and expression levels. Several upstream signaling pathways for iNOS expression, including the mitogen-activated protein kinase, nuclear factor-κB p65 (NF-κB p65), and hypoxia-inducible factor 1 pathways, were not affected by Cl-amidine. By contrast, the LPS-induced signal transducer and the activator of transcription (STAT) phosphorylation and activator protein-1 (AP-1) transcriptional activities (c-Fos, JunD, and phosphorylated c-Jun) were decreased in Cl-amidine-treated DCs. Inhibition of Janus kinase/STAT signaling dramatically suppressed iNOS expression and NO production, whereas AP-1 inhibition had no effect. These results indicate that Cl-amidine-inhibited STAT activation may suppress iNOS expression. Additionally, we found mildly reduced cyclooxygenase-2 expression and prostaglandin E2 production in Cl-amidine-treated DCs. Our findings indicate that Cl-amidine acts as a novel suppressor of iNOS expression, suggesting that Cl-amidine has the potential to ameliorate the effects of excessive iNOS/NO-linked immune responses.


Introduction
Peptidylarginine deiminase (PAD) enzymes mediate the citrullination process-which involves the conversion of arginine to citrulline on a protein or peptide but not to free arginine-through

Cl-Amidine Suppresses NO Generation in Lipopolysaccharide-Stimulated Dendritic Cells
During LPS-induced DC maturation, we determined NO levels in the absence or presence of Cl-amidine. In DCs treated with Cl-amidine (50, 100, or 200 µM), LPS-induced NO production was significantly reduced by up to 30%, 60%, and 85%, respectively ( Figure 1A). The pan-NOS inhibitor N G -methyl-L-arginine (L-NMMA) was used as a positive control and completely inhibited NO production. We also found that the decreased NO production induced by Cl-amidine occurs at an early time point (6 h) and in a time-dependent manner ( Figure 1B). These results suggest that Cl-amidine acts as a suppressor of NO production. significantly reduced by up to 30%, 60%, and 85%, respectively ( Figure 1A). The pan-NOS inhibitor N G -methyl-L-arginine (L-NMMA) was used as a positive control and completely inhibited NO production. We also found that the decreased NO production induced by Cl-amidine occurs at an early time point (6 h) and in a time-dependent manner ( Figure 1B). These results suggest that Clamidine acts as a suppressor of NO production.

NOS Activity Is not Inhibited by Cl-Amidine
Next, we determined whether the decreased NO production we observed was caused by Clamidine-induced inhibition of NOS activity. Recombinant NOS enzymes (iNOS, eNOS, and nNOS) were pretreated with Cl-amidine, and we then measured NOS activity. As shown in Figure 2, Clamidine had no effect on the enzymatic activity of any NOS enzymes we tested. These results indicate that the inhibition of NO production by Cl-amidine likely occurs through the misregulation of an upstream signaling pathway for NO production.

NOS Activity Is not Inhibited by Cl-Amidine
Next, we determined whether the decreased NO production we observed was caused by Cl-amidine-induced inhibition of NOS activity. Recombinant NOS enzymes (iNOS, eNOS, and nNOS) were pretreated with Cl-amidine, and we then measured NOS activity. As shown in Figure 2, Cl-amidine had no effect on the enzymatic activity of any NOS enzymes we tested. These results indicate that the inhibition of NO production by Cl-amidine likely occurs through the misregulation of an upstream signaling pathway for NO production. significantly reduced by up to 30%, 60%, and 85%, respectively ( Figure 1A). The pan-NOS inhibitor N G -methyl-L-arginine (L-NMMA) was used as a positive control and completely inhibited NO production. We also found that the decreased NO production induced by Cl-amidine occurs at an early time point (6 h) and in a time-dependent manner ( Figure 1B). These results suggest that Clamidine acts as a suppressor of NO production.

NOS Activity Is not Inhibited by Cl-Amidine
Next, we determined whether the decreased NO production we observed was caused by Clamidine-induced inhibition of NOS activity. Recombinant NOS enzymes (iNOS, eNOS, and nNOS) were pretreated with Cl-amidine, and we then measured NOS activity. As shown in Figure 2, Clamidine had no effect on the enzymatic activity of any NOS enzymes we tested. These results indicate that the inhibition of NO production by Cl-amidine likely occurs through the misregulation of an upstream signaling pathway for NO production.  ; and 420 ng nNOS) was preincubated with or without Cl-amidine (200 µM) for 1 h, and then activity was measured for 1 h. Quantitative levels are represented by bars (mean ± SD; n = 3).

Cl-Amidine Suppresses iNOS Expression
In DCs, iNOS is responsible for NO production. Therefore, we measured iNOS expression during DC maturation induced by LPS in the absence or presence of Cl-amidine. In the DCs that were treated with 50 µM, 100 µM, and 200 µM of Cl-amidine, LPS-induced iNOS expression was markedly suppressed by~30%, 60%, and 80%, respectively ( Figure 3A), in a dose-dependent manner. To confirm whether decreased iNOS expression was due to the cell death, we analyzed cell viability using Cell Counting Kit-8 (CCK-8) staining. No significant decrease in viability was observed in our experimental conditions ( Figure 3B). Similar to our NO production results, iNOS expression was suppressed at an early time point (6 h) by Cl-amidine ( Figure 3C). These results indicate that the down-regulation of NO production by Cl-amidine treatment is caused by suppression of iNOS expression.
Next, we used RT-PCR to investigate whether Cl-amidine affects the production of iNOS mRNA transcripts. As shown in Figure 3D, Cl-amidine affected transcription of the iNOS gene at an early time point (3 h). Taken together, these findings suggest that the pan-PAD inhibitor Cl-amidine is a candidate for suppression of iNOS.

Cl-Amidine Suppresses iNOS Expression
In DCs, iNOS is responsible for NO production. Therefore, we measured iNOS expression during DC maturation induced by LPS in the absence or presence of Cl-amidine. In the DCs that were treated with 50 μM, 100 μM, and 200 μM of Cl-amidine, LPS-induced iNOS expression was markedly suppressed by ~30%, 60%, and 80%, respectively ( Figure 3A), in a dose-dependent manner. To confirm whether decreased iNOS expression was due to the cell death, we analyzed cell viability using Cell Counting Kit-8 (CCK-8) staining. No significant decrease in viability was observed in our experimental conditions ( Figure 3B). Similar to our NO production results, iNOS expression was suppressed at an early time point (6 h) by Cl-amidine ( Figure 3C). These results indicate that the down-regulation of NO production by Cl-amidine treatment is caused by suppression of iNOS expression.
Next, we used RT-PCR to investigate whether Cl-amidine affects the production of iNOS mRNA transcripts. As shown in Figure 3D, Cl-amidine affected transcription of the iNOS gene at an early time point (3 h). Taken together, these findings suggest that the pan-PAD inhibitor Cl-amidine is a candidate for suppression of iNOS.

Cl-Amidine Impairs AP-1 Family Member Activity
Next, we examined whether Cl-amidine can affect the activity of the AP-1 family, a candidate regulatory factor for iNOS expression. When AP-1 dimers are activated, they bind to a specific double-stranded DNA sequence containing the TPA-responsive element (5′-TGAGTCA-3′), and this AP-1 family activity also regulates iNOS expression [24]. Therefore, we examined whether Clamidine regulates LPS-induced activation of AP-1 using nuclear extract. As shown in Figure 5, the expression levels of the LPS-induced AP-1 family members (c-Fos, JunD, p-c-Jun, and JunB) were significantly decreased by Cl-amidine treatment.
In addition, we determined whether the inhibition of AP-1 activity can decrease NO production and iNOS expression. As shown in Figure 5B, treatment with the AP-1 inhibitor T-5224 (20 μM) slightly inhibited NO production (~11%) but not iNOS expression. Moreover, the other AP-1 inhibitor, SR113201, had no effect on NO production. These results indicate that AP-1 activity may not be involved in the regulation of iNOS expression in LPS-activated DCs.

Cl-Amidine Impairs AP-1 Family Member Activity
Next, we examined whether Cl-amidine can affect the activity of the AP-1 family, a candidate regulatory factor for iNOS expression. When AP-1 dimers are activated, they bind to a specific double-stranded DNA sequence containing the TPA-responsive element (5 -TGAGTCA-3 ), and this AP-1 family activity also regulates iNOS expression [24]. Therefore, we examined whether Cl-amidine regulates LPS-induced activation of AP-1 using nuclear extract. As shown in Figure 5, the expression levels of the LPS-induced AP-1 family members (c-Fos, JunD, p-c-Jun, and JunB) were significantly decreased by Cl-amidine treatment.
In addition, we determined whether the inhibition of AP-1 activity can decrease NO production and iNOS expression. As shown in Figure 5B, treatment with the AP-1 inhibitor T-5224 (20 µM) slightly inhibited NO production (~11%) but not iNOS expression. Moreover, the other AP-1 inhibitor, SR113201, had no effect on NO production. These results indicate that AP-1 activity may not be involved in the regulation of iNOS expression in LPS-activated DCs.

Ineffective Regulation of Cyclooxygenase 2 (COX-2) Expression and Prostaglandin E2 (PGE2) Production due to Cl-Amidine Treatment
PGE2 is a major inflammatory mediator along with NO and is derived from cyclooxygenase 2 (COX-2). Inflammatory stimuli elicit the synthesis of iNOS and COX-2 proteins over similar time courses, and these two proteins also interact [26][27][28]. To investigate whether Cl-amidine affects COX-2 expression and PGE2 production, we measured COX-2 expression by Western blotting with an anti-COX-2 antibody and PGE2 levels using ELISA in LPS-stimulated DCs in the absence or presence of Cl-amidine. Expression of the COX-2 protein was down-regulated by ~20% in the presence of Clamidine treatment compared with the LPS alone treatment ( Figure 6A,B), concomitant with a similar reduction in PGE2 formation ( Figure 6C). This result indicates that Cl-amidine also exerts a mild effect on COX-2 induction and PGE2 production.

Ineffective Regulation of Cyclooxygenase 2 (COX-2) Expression and Prostaglandin E2 (PGE2) Production due to Cl-Amidine Treatment
PGE 2 is a major inflammatory mediator along with NO and is derived from cyclooxygenase 2 (COX-2). Inflammatory stimuli elicit the synthesis of iNOS and COX-2 proteins over similar time courses, and these two proteins also interact [26][27][28]. To investigate whether Cl-amidine affects COX-2 expression and PGE 2 production, we measured COX-2 expression by Western blotting with an anti-COX-2 antibody and PGE 2 levels using ELISA in LPS-stimulated DCs in the absence or presence of Cl-amidine. Expression of the COX-2 protein was down-regulated by~20% in the presence of Cl-amidine treatment compared with the LPS alone treatment ( Figure 6A,B), concomitant with a similar reduction in PGE 2 formation ( Figure 6C). This result indicates that Cl-amidine also exerts a mild effect on COX-2 induction and PGE 2 production.

Ineffective Regulation of Cyclooxygenase 2 (COX-2) Expression and Prostaglandin E2 (PGE2) Production due to Cl-Amidine Treatment
PGE2 is a major inflammatory mediator along with NO and is derived from cyclooxygenase 2 (COX-2). Inflammatory stimuli elicit the synthesis of iNOS and COX-2 proteins over similar time courses, and these two proteins also interact [26][27][28]. To investigate whether Cl-amidine affects COX-2 expression and PGE2 production, we measured COX-2 expression by Western blotting with an anti-COX-2 antibody and PGE2 levels using ELISA in LPS-stimulated DCs in the absence or presence of Cl-amidine. Expression of the COX-2 protein was down-regulated by ~20% in the presence of Clamidine treatment compared with the LPS alone treatment ( Figure 6A,B), concomitant with a similar reduction in PGE2 formation ( Figure 6C). This result indicates that Cl-amidine also exerts a mild effect on COX-2 induction and PGE2 production.

Discussion
Recently, PAD enzymes and protein citrullination have been reported to possess a variety of physiological activities, such as tumorigenesis, bactericidal effects, and immunomodulatory effects [5,18,29], suggesting that the inhibition of PAD is a potential therapeutic strategy for the treatment of various disease models. Indeed, the PAD inhibitor Cl-amidine abrogates citrullinated, histone-linked NET formation in vitro and in vivo [3,13], enhances p53-targeted gene expression [5], and regulates the function of immune cells such as T cells and DCs [18]. In the present study, we found that Cl-amidine acts as a potential inhibitor for NO production and iNOS expression (Figures 1 and 3). NO is involved in NET activity, autoimmune diseases, and inflammatory diseases [23,[30][31][32][33]. Therefore, it is also possible that the suppression of NO production by Cl-amidine, as described here, might attenuate NO/iNOS-involved disease phenotypes.
NETs have roles in many infectious and noninfectious diseases, including lupus, RA, and cancer [34,35]. NETs are highly decondensed chromatin structures containing histones, protease, and extracellular DNA, which can result in suicidal NETosis or vital NETosis [29,34,35]. The generation of NETs primarily protects the host from infection, but reducing NETs protects against tissue injury [34,36]. PAD4 deficiency and Cl-amidine treatment block NET formation and its release [29,37]. Moreover, exogenous NO promotes NET release, which is blocked by the reactive oxygen species scavenger N-acetyl cysteine [38]. In addition, NO is produced during NET release, and the iNOS inhibitor L-N G -nitroarginine methyl ester (L-NAME) reduces NET formation [39,40]. Therefore, we hypothesized that reduced NET formation by Cl-amidine treatment not only blocked histone citrullination but also inhibited NO generation by suppressing iNOS expression. In other words, reduced disease activity by Cl-amidine treatment may be attributed to PAD inactivation with reduced NO generation in a number of diseases, such as lupus, MS, and RA [41].
The expression of iNOS is regulated by multiple upstream signaling pathways in mouse macrophages and DCs, including MAPK, NF-κB, STAT, PI3K, HIF-1α, and the AP1 family [23][24][25]. The inhibition of the MAPK, NF-κB p65, and PI3K signaling pathways inhibited NO production ( Figure 4A). The p38 (SB203580) and NF-κB p65 (BAY 11-7085) inhibitors suppressed~50% of NO production compared with controls. The JNK and PI3K inhibitors were less effective, reducing NO production by~22% and~18%, respectively. Both a PI3K deficiency and a PI3K inhibitor impaired NO generation in peritoneal macrophages [42]. By contrast, it was found that two PI3K inhibitors, wortmannin and LY294002, actually increased iNOS and NO production in RAW264.7 macrophages [43]. This discrepancy may have been due to the different types of cells used in each set of experiments. Altogether, our observations indicate that multiple signaling pathways are involved in NO generation in DCs. Interestingly, the ERK inhibitor U0126 enhanced NO production ( Figure 4A). U126 was previously shown to enhance NO production in RAW264.7 macrophages [44]. Therefore, ERK may negatively regulate NO production in LPS-stimulated DCs. Because Cl-amidine had no effect on MAPK and NF-κB p65 signaling pathways ( Figure 4B) and the DNA-binding activity of NF-κB p65 ( Figure 4C), these signaling pathways are not likely associated with Cl-amidine-mediated iNOS suppression, although the MAPK and NF-κB p65 signaling pathways are involved in the regulation of iNOS expression.
JAK/STAT were implicated in NO generation in a variety of cells [47][48][49]. As JAK undergoes transphosphorylation and, subsequently, phosphorylate STATs, activated STAT proteins dimerize and translocate into the nucleus, where they activate or repress target gene promoters [50]. In this study, Cl-amidine down-regulated STAT phosphorylation ( Figure 4D). Involvement of STAT signaling in iNOS expression was also confirmed by treatment with the JAK inhibitor tofacitinib and the STAT inhibitor niclosamide (Figure 4E,F). Although AP-1 activity, a potential regulator for iNOS expression, was significantly decreased by Cl-amidine, both iNOS expression and NO production in DCs were not affected by AP-1 inhibition (T-5224) ( Figure 5). Therefore, Cl-amidine-mediated suppression of iNOS expression may be due to the suppression of STAT phosphorylation but not the MAPK, NF-κB, HIF-1α, and AP-1 signaling pathways.
Inflammatory stimuli elicit both NO and PGE 2 production through the induction of iNOS and COX-2 over similar time courses. More specifically, NO enhances PGE 2 production through S-nitrosylation of COX-2 [28]. Our data showed that LPS-induced COX-2 expression and PGE 2 production still maintained~80% of their normal expression levels by Cl-amidine ( Figure 6). Therefore, our results indicate that Cl-amidine selectively inhibits iNOS induction rather than COX-2 induction signaling. It is possible that a combination of Cl-amidine with a COX-2 inhibitor may have synergistic therapeutic benefits in inflammatory diseases.
Physiologically, NO attacks the thiol groups of cysteines on proteins, forms S-nitrosothiols, and reacts with superoxide anions radical (O 2 − ) to generate peroxynitrite (ONOO − ), which can modify proteins (e.g., tyrosine nitration) [23]. These modifications are involved in a number of disease models and cellular processes, such as signal transduction and apoptosis [51,52]. Furthermore, ONOO − secreted from DCs can kill tumor cells [53]. Therefore, further studies are needed to determine whether Cl-amidine can reduce S-nitrosothiol levels and nitration formation and modulate associated signaling pathways.
In this study, we determined that Cl-amidine inhibits NO generation by suppressing iNOS transcription in DCs. It is also possible that Cl-amidine may have effects on iNOS expression in other cell types, including macrophages. Indeed, Witalison et al. showed that Cl-amidine suppressed iNOS induction in mouse macrophages [54]. NO also plays an important role in epithelial cells and endothelial cells, and further studies will be needed to determine whether Cl-amidine has an effect on these cell types.
In summary, iNOS expression and NO production are reduced in DCs by the PAD inhibitor Cl-amidine through the repression of STAT activities. We hypothesized that Cl-amidine may downregulate NO production via modulation of iNOS expression in the other cell types, including macrophages. Taken together, these findings indicate that Cl-amidine could improve excessive NO-mediated inflammatory responses and damage in certain disease states. Therefore, PAD inhibitors should be further examined to determine whether they have potential therapeutic effect on the disease(s) associated with iNOS activity and/or NO production.

NO Detection and NOS Activity
The culture medium was obtained from LPS (0.1 µg/mL)or vehicle-stimulated DCs (5 × 10 5 or 1 × 10 6 cells) in the absence or presence of Cl-amidine (50-200 µM). The levels of NO in the culture medium were measured by ELISA according to the manufacturer's instructions (Intron Biotechnology, Seongnam, Korea). To assay NOS activity, recombinant NOS (180 ng iNOS; 210 ng eNOS; 420 ng nNOS; Enzo Life Sciences, New York, NY, USA) was incubated with or without Cl-amidine (200 µM) for 1 h, and then activity was measured for 1 h by ELISA, according to the manufacturer's instructions (Biovision, Milpitas, CA, USA). The primary antibodies were detected with appropriate horseradish peroxidase-conjugated goat anti-mouse, rabbit anti-goat, or goat anti-rabbit secondary antibodies (1:5000; Cell Signaling or Merck-Millipore). Chemiluminescent signals were detected on X-ray film (Agfa HealthCare, Mortsel, Belgium) using ECL Western blotting detection reagents (ATTO, Tokyo, Japan).

Semi-Quantitative Reverse Transcriptase Polymerase Chain Reaction (RT-PCR)
DCs were stimulated with LPS (0.1 µg/mL) with or without Cl-amidine (200 µM) for the indicated times, and then total RNA was extracted using the Ribospin RNA extraction kit following the manufacturer's instructions (GeneAll, Seoul, Korea). The cDNA synthesis was performed with AMV reverse transcriptase (Promega, Madison, WI, USA) according to the instructions of the manufacturer. RT-PCR was performed using primers specific to the iNOS (600 bp; forward, 5 -GGTATGCTGTGTTTGGCCTT-3 and reverse, 5 -GCAGCCTCTTGTCTTTGACC-3 ) and GAPDH (200 bp; forward, 5 -TGGTATCGTGGAAGGACTCATGAC-3 and reverse, and 5 -ATGC CAGTGAGCTTCCCGTTCAGC-3 ) genes with GoTaq DNA polymerase, according to the manufacturer's instructions (Promega). The resulting RT-PCR products were separated by gel electrophoresis on a 1.0% agarose gel with the SafeView nucleic acid gel stain (ABM, Richmond, BC, Canada) and then visualized using UV light.

Activator Protein-1 (AP-1) Transcription Activity Assay
DCs (1 × 10 7 cells) were stimulated with LPS (0.1 µg/mL), with or without Cl-amidine (200 µM), for 1 h, and then nuclear proteins were isolated using the Nuclear Extraction Kit, following the manufacturer's instructions (Abcam). To determine AP-1 family member activity, ten micrograms of total nuclear protein used with the AP-1 Transcription Factor Assay Kit, according to the manufacturer's instructions (Abcam).

Statistical Analysis
All data are presented as mean ± standard deviation (SD) values. The probability of statistically significant differences between experimental groups was assessed by a two-sample t test or by one-way analysis of variance.

COX-2
Cyclooxygenase 2 PGE 2 Prostaglandin E 2 JAK Janus kinase STAT Signal transducer and activator of transcription