Cell-specific inhibition of inducible nitric oxide synthase activation by leflunomide

Direct Inhibition of IRF-Dependent Transcriptional Regulatory Mechanisms Associated With Disease

Interferon regulatory factors (IRFs) are a family of homologous proteins that regulate the transcription of interferons (IFNs) and IFN-induced gene expression. As such they are important modulating proteins in the Toll-like receptor (TLR) and IFN signaling pathways, which are vital elements of the innate immune system. IRFs have a multi-domain structure, with the N-terminal part acting as a DNA binding domain (DBD) that recognizes a DNA-binding motif similar to the IFN-stimulated response element (ISRE). The C-terminal part contains the IRF-association domain (IAD), with which they can self-associate, bind to IRF family members or interact with other transcription factors. This complex formation is crucial for DNA binding and the commencing of target-gene expression. IRFs bind DNA and exert their activating potential as homo or heterodimers with other IRFs. Moreover, they can form complexes (e.g., with Signal transducers and activators of transcription, STATs) and collaborate with other co-acting transcription factors such as Nuclear factor-κB (NF-κB) and PU.1. In time, more of these IRF co-activating mechanisms have been discovered, which may play a key role in the pathogenesis of many diseases, such as acute and chronic inflammation, autoimmune diseases, and cancer. Detailed knowledge of IRFs structure and activating mechanisms predisposes IRFs as potential targets for inhibition in therapeutic strategies connected to numerous immune system-originated diseases. Until now only indirect IRF modulation has been studied in terms of antiviral response regulation and cancer treatment, using mainly antisense oligonucleotides and siRNA knockdown strategies. However, none of these approaches so far entered clinical trials. Moreover, no direct IRF-inhibitory strategies have been reported. In this review, we summarize current knowledge of the different IRF-mediated transcriptional regulatory mechanisms and how they reflect the diverse functions of IRFs in homeostasis and in TLR and IFN signaling. Moreover, we present IRFs as promising inhibitory targets and propose a novel direct IRF-modulating strategy employing a pipeline approach that combines comparative in silico docking to the IRF-DBD with in vitro validation of IRF inhibition. We hypothesize that our methodology will enable the efficient identification of IRF-specific and pan-IRF inhibitors that can be used for the treatment of IRF-dependent disorders and malignancies.

Toxicity of teriflunomide in aryl hydrocarbon receptor deficient mice

The intracellular transcription factor aryl hydrocarbon receptor (AHR) is bound and activated by xenobiotics, thereby promoting their catabolism by inducing expression of cytochrome P450 oxidase (CYP) genes through binding xenobiotic response elements (XRE) in their promoter region. In addition, it is involved in several cellular pathways like cell proliferation, differentiation, regeneration, tumor invasiveness and immune responses. Several pharmaceutical compounds like benzimidazoles activate the AHR and induce their own metabolic degradation. Using newly generated XRE-reporter mice, which allow in vivo bioluminescence imaging of AHR activation, we show here that the AHR is activated in vivo by teriflunomide (TER), which has recently been approved for the treatment of multiple sclerosis. While we did not find any evidence that the AHR mediates the immunomodulatory effects of TER, AHR activation led to metabolism and detoxification of teriflunomide, most likely via CYP. Mice deficient for the AHR show higher blood levels of teriflunomide, suffer from enhanced thrombo- and leukopenia and elevated liver enzymes as well as from severe gastrointestinal ulcers and bleeding which are lethal after 8-11 days of treatment. Leukopenia, acute liver damage and diarrhea have also been described as common side effects in human trials with TER. These data suggest that the AHR is relevant for detoxification not only of environmental toxins but also of drugs in clinical use, with potential implications for the application of AHR-modifying therapies in conjunction to TER in humans. The XRE-reporter mouse is a useful novel tool for monitoring AHR activation using in vivo imaging.

Protective effect of leflunomide against oxidative intestinal injury in a rodent model of sepsis

BACKGROUND

Sepsis is defined as an uncontrolled inflammatory response in a host. The process may lead to severe sepsis, multisystem organ failure and even death. Leflunomide has important immunomodulatory and anti-inflammatory effects, which may mitigate host response to bacterial translocation. The goal of our study was to measure the effects leflunomide administration had on a variety of biochemical markers upregulated in systemic inflammatory response syndrome, sepsis, and multiple organ failure syndrome.

MATERIALS AND METHODS

Wistar albino type rats were randomly divided into five groups: control, sham, leflunomide, sepsis, and sepsis + leflunomide. Sepsis was achieved by means of the cecal ligation and puncture method. Leflunomide 2 × 10 mg/kg/d was administered before the experiment. At the end of 24 h, the tissue levels of superoxide dismutase, catalase activity, malondialdehyde, nitric oxide, and protein carbonyl were measured.

RESULTS

The level of the bowel superoxide dismutase and catalase levels of the sepsis group is significantly lower than those of the control, sham, and leflunomide groups (P < 0.05). Malondialdehyde, nitric oxide, and protein carbonyl levels are significantly higher in sepsis compared with other groups (P < 0.05).

CONCLUSIONS

Leflunomide's prevention of protein and lipid peroxidation was observed in septic bowel tissue. Use of leflunomide could have protective effects against both the onset and the progressive stages of sepsis.

An update of teriflunomide for treatment of multiple sclerosis

There are a number of oral agents emerging as potential disease-modifying agents in multiple sclerosis (MS). Among these investigational agents, teriflunomide has shown promise in large, multicenter, phase III clinical trials with respect to safety and efficacy in relapsing MS patients, and is the latest disease-modifying agent approved for use in MS patients in the United States. This review will summarize teriflunomide’s historical development, clinical pharmacology, studies in animals, clinical trials, and safety data, and will end with a discussion of the role of teriflunomide in MS in the context of existing treatment options.

Role of oral teriflunomide in the management of multiple sclerosis

The landscape of the treatment of relapsing-remitting multiple sclerosis is changing fast. Several oral treatments have shown benefit and generate much interest because of the convenience of their administration. Two oral compounds, fingolimod and teriflunomide, have been approved in relapsing-remitting multiple sclerosis, while others have completed Phase III trials and are awaiting review for registration. Teriflunomide is a pyrimidine synthesis inhibitor with selective immunomodulatory and immunosuppressive properties that have shown consistent efficacy in clinical trials, and a good safety profile. This paper provides an overview of the mechanisms of action and efficacy and safety results from clinical trials with this drug. The role of teriflunomide in the treatment of relapsing-remitting multiple sclerosis is discussed.

Emerging oral immunomodulating agents - focus on teriflunomide for the treatment of multiple sclerosis

Treatment for multiple sclerosis (MS), a chronic disease of the central nervous system, has historically relied exclusively on the use of injectable therapies. As the disease requires lifelong therapy, the development of oral therapies that are safe and effective would provide a more convenient dosage form that may improve patient compliance. One oral medication (fingolimod) was recently approved for treatment of MS. Teriflunomide, an immunomodulator, is one of four oral therapies currently undergoing Phase III trials. Teriflunomide exerts its clinical effects via selective inhibition of de novo pyrimidine synthesis, primarily targeting proliferating T and B lymphocytes in the periphery. Teriflunomide was effective as monotherapy in reducing magnetic resonance imaging lesions and annual relapse rates in Phase II and Phase III trials. When teriflunomide was added to interferon or glatiramer acetate therapy in Phase II trials, teriflunomide reduced magnetic resonance imaging lesions significantly more than either interferon or glatiramer acetate alone. Treatment-emergent adverse events occurred at similar rates among all groups in teriflunomide studies, with a trend towards a higher treatment emergent adverse events rate in the higher dosage group of teriflunomide (14 mg daily). Treatment discontinuations in teriflunomide trials were relatively low, suggesting that teriflunomide monotherapy is well tolerated. This article reviews the mode of action of teriflunomide, its pharmacokinetic, clinical efficacy, and safety profiles.

Emerging strategies for the treatment of multiple sclerosis

Despite extraordinary advances in the field of neuroimmunology, ideal treatment for patients with multiple sclerosis remains an unmet need. Existing treatments are only partially effective in preventing multiple sclerosis relapses, have a limited impact on the accrual of disability, have not been effective in progressive forms of the disease, and treatment remains preventive rather than restorative. This review provides an overview of emerging therapies and targets, and incorporates strategies for two different approaches to multiple sclerosis: prevention, through immune modulation; and repair, through neuroprotection and remyelination. Agents at all stages of development, from late-stage clinical trials of BG-12, teriflunomide, alemtuzumab, daclizumab and anti-CD20 agents, to novel approaches in preclinical testing, are discussed.

Hepatic damage in biliary-obstructed rats is ameliorated by leflunomide treatment

Cholestasis, or impaired bile flow, occurs in a wide variety of liver diseases and causes hepatic damage by retention and accumulation of toxic hydrophobic bile salts inducing persistent inflammation and oxidative stress. In the present research, we studied the effect of leflunomide, a novel immunosuppressive and anti-inflammatory agent against autoimmune disease, on hepatic damage produced by double ligature of the extrahepatic biliary duct in Wistar Albino rats. Cholestasis was done by double ligature and section of the extrahepatic biliary duct (BDL). Leflunomide was given i.g. 10 mg/kg/day. The severity of cholestasis and hepatic injury was determined by changes in the plasma enzyme activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and levels of direct bilirubin. Malondialdehyde (MDA), protein carbonyl (PC), nitric oxide (NO), catalase (CAT) and superoxide dismutase (SOD) were determined to the oxidative status in the liver tissue. Myeloperoxidase (MPO) activity and levels of tissue hydroxyproline (HPR) were determined to neutrophil activation and collagen accumulation, respectively. Further, histological changes were studied. Treatment with leflunomide markedly reduced serum transaminase activities as compared to BDL rats. At the same time leflunomide significantly inhibited increases in liver MDA, PC and NO levels and also attenuated the depletion of CAT and SOD in the liver after bile duct ligation. Similarly, increase in tissue MPO activity and HPR due to BDL was also attenuated by leflunomide treatment. These findings were supported by histopathological findings. These findings suggested that leflunomide can attenuate hepatic damage in extrahepatic cholestasis by prevention of oxidative stress and inflammatory process.

Acidosis affects tumor cell survival through modulation of nitric oxide release

The influence of environmental pH on the production of tumoricidal free radical nitric oxide (NO) was investigated in mouse fibrosarcoma L929 and rat glioma C6 cell lines. A combination of IFN-gamma and IL-1 induced a significant NO release and subsequent reduction of cell viability in tumor cell lines. Acidification of cell culture medium reduced tumor cell NO production in a pH-dependent manner. While the inhibitory effect of acidosis on NO production in C6 cells was associated with a further decrease in cell viability, it completely rescued L929 cells from NO-dependent apoptotic and necrotic death. Acidic pH diminished IFN-gamma+ IL-1-induced expression of inducible NO synthase (iNOS) mRNA and protein, and abolished the activation of iNOS transcription factor IRF-1 in L929 cells. Moreover, extracellular acidosis significantly impaired cytokine-induced phosphorylation of MAP kinase p44/42 (ERK1/2) and subsequent expression of transcription factor c-Fos in L929 cells. Finally, mild acidosis (pH 6.8) augmented, while severe acidosis (pH 6.0) reduced, IFN-gamma-induced iNOS activation/NO release and NO-dependent anticancer activity of rat and mouse macrophages. Taken together, our findings indicate that modulation of macrophage and tumor cell iNOS by an acidic microenvironment might influence the progression of NO-sensitive solid tumors.

[Pt(HPxSC)Cl3], a novel platinum(IV) compound with anticancer properties

There has been a continuing effort for the discovery of novel platinum(IV)-based antitumor compounds with better therapeutic performances than cisplatin. In the present work, the anticancer action of recently synthesized Pt(IV)-based complex [Pt(HPxSC)Cl(3)] was investigated using rat and human astrocytoma cell lines C6 and U251. [Pt(HPxSC)Cl(3)] markedly reduced the number of cultured astrocytoma cells (IC(50), 80 microM), as determined by crystal violet assay. The Pt(IV) complex induced apoptotic death of tumor cells, as flow cytometry analysis of the propidium iodide-stained cellular DNA revealed approx. 30% of hypodiploid cells in [Pt(HPxSC)Cl(3)]-treated astrocytoma cell cultures. On the other hand, [Pt(HPxSC)Cl(3)] at 200 microM did not affect the viability of rat primary astrocytes, unlike the established anticancer drug cisplatin, which displayed high toxicity toward both astrocytoma cells (IC(50), 15 microM) and primary astrocytes (IC(50), 20 microM). Moreover, [Pt(HPxSC)Cl(3)] at 100 microM did not interfere with the ability of rat peritoneal macrophages to produce important antitumor molecules nitric oxide and tumor necrosis factor-alpha. Finally, we assessed the ability of [Pt(HPxSC)Cl(3)] to restrain growth of some bacterial and yeast strains, but it showed rather limited antimicrobial activity.

Leflunomide reduces nitric oxide production in patients with active rheumatoid arthritis

OBJECTIVES

Leflunomide is an immunomodulatory agent that was recently approved for the treatment of rheumatoid arthritis (RA). The mechanism of action is not fully understood. Nitric oxide (NO) plays an important role in the pathogenesis of RA. Leflunomide has been shown to cause cell specific inhibition of inducible nitric oxide synthase (iNOS) activation in animal models. We carried out this study to determine if there was alteration in NO production in patients with RA.

METHODS

An 8-week open label study was carried out on patients with adult onset active RA. We measured levels of nitrite and citrulline spectrophotometrically as surrogate markers of NO production. Within-patient serum levels of nitrite and citrulline were compared with leflunomide therapy at three points of time (at 0, 4 and 8 weeks of therapy).

RESULTS

Thirty-three patients with active RA were enrolled for this study. These patients were a subset of 63 individuals who are studied for clinical efficacy of leflunomide. Three patients were lost to follow-up. Median nitrite levels were 817.2 (nmol/ml) at the start of therapy and this declined to 440.9 nmol/ml and 301.1 nmol/ml at 4 and 8 weeks of therapy. Median citrulline levels were 649.3 nmol/ml at the start of the study, which declined to 549.2 nmol/ml and 485.4 nmol/ml at 4 and 8 weeks, respectively. Statistically significant decrease in median values for serum nitrite and citrulline levels was documented after 4 weeks of leflunomide therapy (p<0.01), which was sustained at 8 weeks (p<0.01), although there was no further fall between 4 and 8 weeks (p>0.1).

CONCLUSIONS

Leflunomide inhibits nitric oxide production in patients with active RA. Inhibition of NO synthesis may be one of the mechanisms responsible for the immunomodulatory activity of leflunomide.

Iron down-regulates macrophage anti-tumour activity by blocking nitric oxide production

Although the inhibitory effect of iron on macrophage production of tumoricidal free radical nitric oxide (NO) has been reported, its possible influence on macrophage anti-tumour activity has not been established. In the present study, FeSO4 markedly reduced IFN-gamma + LPS-induced NO synthesis in mouse and rat macrophages. The effect of iron coincided with the loss of macrophage cytotoxic activity against NO-sensitive C6 rat astrocytoma and L929 mouse fibrosarcoma cell lines, as measured by MTT assay for cellular respiration and the crystal violet test for cell viability. Tumour cell survival did not improve further in the presence of FeSO4 if macrophage NO release and cytotoxicity were already blocked by aminoguanidine. In accordance with the results obtained with exogenous iron, cell membrane permeable iron chelator o-phenanthroline enhanced both macrophage NO release and anti-tumour activity. Iron also down-regulated NO production and increased the viability of L929 fibrosarcoma cells stimulated with IFN-gamma + LPS in the absence of macrophages. However, neither NO release nor cell viability was affected by iron addition to cultures of the C6 astrocytoma cell line. Iron was unable to prevent L929 and C6 cell death induced by the NO releasing chemicals SNP and SIN-1, indicating that iron-mediated inhibition of NO synthesis, rather than interference with its cytotoxic action, was responsible for the protection of tumour cells. Collectively, these results indicate that iron might protect tumour cells by reducing both macrophage and tumour cell-derived NO release.

Differential effects of leflunomide and methotrexate on cytokine production in rheumatoid arthritis

BACKGROUND

T cells have a pivotal role in RA. Leflunomide inhibits pyrimidine biosynthesis, to which T cells are especially susceptible, and therefore may have a different cytokine profile than methotrexate.

MATERIALS AND METHODS

Serum samples of 100 patients with RA, treated with leflunomide (n = 50) or methotrexate (n = 50), were collected at baseline, after 16 weeks and after 1 year's treatment. Serum levels of interleukin 6 (IL6), and interferon (IFN) gamma were determined by ELISA. Additionally, peripheral blood mononuclear cells (PBMC) of five healthy volunteers and three patients with RA were isolated and the effects of the active metabolite of leflunomide (A77-1726, 0-200 mmol/l) on cell proliferation and on IL6 and IFNgamma production were determined by ELISA. In peripheral blood lymphocytes (PBL) and monocytes (PBM) from two healthy volunteers the effects of A77-1726 on IL6 production were measured by ELISA and PCR.

RESULTS

Mean (SEM) serum levels of IFNgamma were significantly reduced after leflunomide treatment (baseline 43 (10) pg/ml; 1 year 29 (7) (p = 0.015), but there was no change in IL6 levels (baseline 158 (41), 1 year 151 (48)). Both IFNgamma and IL6 levels were significantly reduced after methotrexate treatment. This observation was supported by in vitro experiments. The production of IFNgamma by PBL was inhibited by A77-1726, but IL6 production by PBM was not inhibited.

CONCLUSION

The differential effect on IFNgamma and IL6 production supports the hypothesis that activated T cells are preferentially inhibited by leflunomide. An explanation may be either inhibition of uridine synthesis or effects on signal transduction pathways.

5-Aza-2′-deoxycytidine and paclitaxel inhibit inducible nitric oxide synthase activation in fibrosarcoma cells

Given the important role of gaseous free radical nitric oxide (NO) in tumor cell biology, we investigated the ability of the anti-cancer drugs 5-Aza-2'-deoxycytidine (ADC) and paclitaxel to modulate NO production in mouse L929 fibrosarcoma cells. Both drugs reduced IFN-gamma-stimulated NO release in cultures of L929 and primary fibroblasts, but not in mouse peritoneal macrophages. The inhibitory effect was due to the reduced expression of inducible NO synthase (iNOS), the enzyme responsible for cytokine-induced intracellular NO synthesis, as both agents markedly suppressed the interferon-gamma (IFN-gamma)-triggered increase in iNOS concentration in L929 cells. In addition, ADC and paclitaxel prevented the IFN-gamma-triggered activation of p44/p42 mitogen-activated protein (MAP) kinase in L929 fibroblasts, suggesting a possible mechanism for the observed inhibition of iNOS expression. These results might have important implications for the therapeutic effect of ADC and paclitaxel, since their inhibitory action on NO release partly neutralized the NO-dependent toxicity of IFN-gamma on L929 fibrosarcoma cells.

Active leflunomide metabolite inhibits interleukin 1beta, tumour necrosis factor alpha, nitric oxide, and metalloproteinase-3 production in activated human synovial tissue cultures

BACKGROUND

Leflunomide is now an approved agent for the management of adult rheumatoid arthritis (RA). Its active metabolite A771726 inhibits de novo pyrimidine biosynthesis. Although considered to be an immunosuppressive agent, its mechanism of action remains obscure.

OBJECTIVES

Evaluation of the leflunomide active metabolite A771726 (LEF) effect on interleukin 1beta (IL1beta), tumour necrosis factor (TNFalpha), nitric oxide (NO), and stromelysin (metalloproteinase-3 (MMP-3)) production by activated human synovial tissue in culture.

METHODS

Synovial tissue was obtained during surgery from patients undergoing total knee replacement owing to RA or osteoarthritis (OA), cut into small pieces, and cultured in Petri dishes with test materials as previously described. IL1beta, TNFalpha, NO, and MMP-3 were measured in the culture media after 48 hours incubation with different doses of LEF by methods previously described.

RESULTS

LEF (0.3, 3, and 9 micro g/ml) inhibited IL1beta production in the presence of lipopolysaccharide (LPS; 3 micro g/ml) in a dose dependent manner (p<0.01) at LEF 0.3 micro g/ml. TNFalpha production in the presence of IL1beta (1 ng/ml) was also inhibited in a dose dependent manner (p<0.05 at LEF 0.3 micro g/ml). NO and MMP-3 production in the presence of LPS (3 micro g/ml) was inhibited as well (p<0.01 at LEF 1 micro g/ml and at LEF 0.3 micro g/ml, respectively). Synovial cell viability evaluated by the tetrazolium salt XTT was unaffected by the LEF concentration used. There was no qualitative difference in the response of OA and RA synovial tissue.

CONCLUSION

Leflunomide may modulate the rheumatoid articular process by inhibition of local production of IL1beta, TNFalpha, NO, and MMP-3.

Mycophenolic acid inhibits activation of inducible nitric oxide synthase in rodent fibroblasts

Mycophenolate mofetil (MMF) is an immunosuppressive drug that acts as a selective inhibitor of inosine monophosphate dehydrogenase (IMPDH). MMF has recently been shown to inhibit the enzymatic activity of inducible NO synthase (iNOS) and subsequent production of the cytotoxic free radical nitric oxide (NO) in endothelial cells. We here investigated the effect of bioactive MMF compound mycophenolic acid (MPA) on iNOS-mediated NO synthesis in fibroblasts, which are important source of NO in rheumatoid arthritis and during rejection of solid organ transplants. MPA exerted dose-dependent inhibition of NO synthesis, measured as nitrite accumulation, in IFN-gamma + LPS-stimulated L929 mouse fibroblast cell line and rat primary fibroblasts. The effect of MPA was not mediated through interference with IMPDH-dependent synthesis of iNOS co-factor BH4 and subsequent suppression of iNOS enzymatic activity, as direct BH4 precursor sepiapterin failed to block the action of the drug. MPA suppressed the IFN-gamma + LPS-induced expression of fibroblast iNOS protein, as well as mRNA for iNOS and its transcription factor IRF-1, as assessed by cell-based ELISA and semiquantitative RT-PCR, respectively. MPA suppression of fibroblast NO release, iNOS, and IRF-1 activation, was efficiently prevented by exogenous guanosine, indicating that the drug acted through reduction of IMPDH-dependent synthesis of guanosine nucleotides. These results suggest that MPA inhibits NO production in fibroblasts by blocking guanosine nucleotide-dependent expression of iNOS gene, through mechanisms that might involve the interference with the induction of iNOS transcription factor IRF-1.

Immunosuppressants leflunomide and mycophenolic acid inhibit fibroblast IL-6 production by distinct mechanisms

Mycophenolic acid (MPA) and A77 1726, the active components of the immunosuppressants mycophenolate mophetil and leflunomide, respectively, in a dose-dependent manner inhibited interferon (IFN)-gamma/LPS-induced interleukin (IL)-6 release in confluent cultures of mouse L929 fibrosarcoma cells. In addition, both drugs markedly reduced the production of the free radical gas nitric oxide (NO), without affecting the viability of L929 cells. The inhibitors of NO synthase, aminoguanidine and L-NMMA, but not L-NMMA inactive counterpart D-NMMA, mimicked the effects of A77 1726 and MPA on IL-6 generation in L929 fibroblasts. Furthermore, NO-releasing substance SNP completely reverted IL-6 accumulation in L929 cultures treated with A77 1726, while only partial recovery of IL-6 production was observed in the presence of MPA. MPA, but not A77 1726, significantly suppressed NO-independent IL-6 release triggered by cAMP-elevating agent rolipram. Thus, while A77 1726 effect on IL-6 production was mediated through concomitant reduction of NO synthesis, MPA action was mainly independent of the interference with NO generation. Finally, both agents inhibited IFN-gamma/LPS-triggered IL-6 production in mouse primary fibroblasts, but not in mouse peritoneal macrophages, indicating cell-specificity of this novel anti-inflammatory action of A77 1726 and MPA.

Effect of leflunomide on immunological liver injury in mice

AIM: To study the effect of leflunomide on immunological liver injury (ILI) in mice.

METHODS: ILI was induced by tail vein injection of 2.5 mg Bacillus Calmette-Guerin (BCG), and 10 d later with 10 mg lipopolysaccharide (LPS) in 0.2 mL saline (BCG + LPS). The alanine aminotransferase (ALT), aspartate aminotransferase (AST), nitric oxide (NO) level in plasma and molondiadehyde (MDA), glutathione peroxidase (GSHpx) in liver homogenate were assayed by spectroscopy. The serum content of tumor necrosis factors-α (TNF-α) was determined by ELISA. Interleukin-1 (IL-1), interleukin-2 (IL-2) and Concanavalin A (ConA)-induced splenocyte proliferation response were determined by methods of ³H-infiltrated cell proliferation.

RESULTS: Leflunomide (4, 12, 36 mg·kg^-1) was found to significantly decrease the serum transaminase (ALT, AST) activity and MDA content in liver homogenate, and improve reduced GSHpx level of liver homogenate. Leflunomide (4, 12, 36 mg·kg^-1) significantly lowered TNF-α and NO level in serum, and IL-1 produced by intraperitoneal macrophages (PMF). Moreover, the decreased IL-2 production and ConA-induced splenocyte proliferation response were further inhibited.

CONCLUSION: These findings suggested that leflunomide had significant protective action on ILI in mice.

Mycophenolic acid downregulates inducible nitric oxide synthase induction in astrocytes

Free radical nitric oxide (NO), generated by inducible nitric oxide synthase (iNOS) in astrocytes and macrophages, has been implicated in CNS inflammatory disorders such as multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). Mycophenolic acid (MPA), a selective inhibitor of inosine monophosphate dehydrogenase (IMPDH), inhibited interferon-gamma (IFN-gamma) + lipopolysaccharide (LPS)-induced NO production dose-dependently in astrocytes, but not in macrophages. The effect of MPA was not mediated through interference with IMPDH-dependent synthesis of iNOS cofactor BH4 and subsequent suppression of iNOS enzymatic activity, as direct BH4 precursor sepiapterin failed to block the action of the drug. However, MPA markedly inhibited IFN-gamma + LPS-triggered astrocyte expression of mRNA for iNOS and its transcription factor IRF-1, while the expression of tumor necrosis factor-alpha (TNF-alpha) gene was not altered. The observed MPA suppression of NO release and iNOS and IRF-1 induction in astrocytes were efficiently prevented by exogenous guanosine, indicating that the drug acted through reduction of IMPDH-dependent synthesis of guanosine nucleotides. This IRF-1-dependent inhibition of iNOS activation might be partly responsible for the protective effect of MPA in EAE, prompting investigation of its potential use in multiple sclerosis.

Necrotic tumor cells oppositely affect nitric oxide production in tumor cell lines and macrophages

Nitric oxide (NO) is a highly reactive free radical with profound tumoricidal activity, produced by both macrophages and tumor cells. While it has been postulated that necrotic tumor cells can augment macrophage anti-tumor action, we investigated the effect of tumor cell necrosis on NO synthesis and viability of L929 fibrosarcoma and C6 astrocytoma cell lines. The presence of necrotic tumor cells dose-dependently reduced NO production in IFN-gamma stimulated L929 cells, and rescued them from NO-dependent autotoxicity. This effect was mediated through soluble products, since it was completely preserved after blocking the contact between the necrotic and live cells. On the other hand, apoptotic tumor cells were unable to suppress IFN-gamma-triggered NO release and subsequent decrease of cell respiration in L929 cultures. Similar results were obtained with C6 astrocytoma cell line. This down-regulation of NO synthesis in response to necrotic cell products was not specific for tumor cell lines, since necrotic tumor cells markedly suppressed NO production in cytokine-stimulated primary fibroblasts and astrocytes. In contrast, both murine and rat peritoneal macrophages readily increased their basal or IFN-gamma-induced NO production when incubated with necrotic tumor cells. Taken together, these results suggest that tumor cell necrosis might promote or restrict tumor growth through suppression or enhancement of NO synthesis in tumor cells and macrophages, respectively, with net effect presumably depending on the extent of macrophage infiltration.

STAT1 is required for iNOS activation, but not IL-6 production in murine fibroblasts

The role of transcription factor STAT1 in production of pro-inflammatory mediators nitric oxide (NO) and IL-6 was examined in murine embryonic fibroblasts. While cells from wild-type animals released large amounts of NO after stimulation with IFN-gamma in combination with LPS, TNF-alpha or IL-1, their STAT1-deficient counterparts failed to synthesise detectable levels of this free radical gas. Inability of STAT1-/- fibroblasts to produce NO was accompanied by complete absence of mRNA for iNOS and its transcription factor IRF-1, both readily upregulated in wild-type cells. However, treatment with cytokines (IFN-gamma, TNF-alpha, IL-1, IL-17) significantly increased IL-6 generation in STAT1-deficient fibroblasts. These results indicate that STAT1 activation and subsequent IRF-1 transcription are required for induction of iNOS, but not IL-6 in murine fibroblasts.

Leflunomide inhibits activation of inducible nitric oxide synthase in rat astrocytes

Highly reactive gaseous free radical nitric oxide (NO), generated by astrocytes and infiltrating macrophages is implicated in inflammatory destruction of brain tissue, including that occurring in multiple sclerosis. Therefore, the influence of immunosuppressive drug leflunomide on inducible nitric oxide synthase (iNOS)-dependent NO production in rat astrocytes and macrophages was investigated. Under the same cultivating conditions, leflunomide's active metabolite A77 1726 caused a dose-dependent decrease of NO production in IFN-gamma+LPS-stimulated primary astrocytes, but not in macrophages. While A77 1726 did not alter iNOS enzymatic activity, it markedly suppressed IFN-gamma+LPS-triggered expression of iNOS mRNA in astrocytes. In the presence of transcription inhibitor actinomycin D, A77 1726 failed to inhibit astrocyte NO production, suggesting transcriptional regulation of iNOS by leflunomide. This assumption was further supported by the ability of A77 1726 to inhibit IFN-gamma+LPS-induced expression of mRNA for an important iNOS transcription factor IRF-1. PD98059, a specific inhibitor of mitogen-activated protein kinase kinase (MAPKK/MEK), but not genistein, an unselective protein tyrosine kinase inhibitor, completely mimicked cell type-specific inhibition of NO synthesis by A77 1726. Therefore, previously described inhibition of MEK/MAP pathway by leflunomide could present a possible mechanism for A77 1726-mediated suppression of iNOS activation in astrocytes. Accordingly to results obtained with primary astrocytes, both A77 1726 and PD98059 significantly reduced IFN-gamma+LPS-induced NO synthesis in the cultures of rat astrocytoma cell line C6. The ability to suppress iNOS induction in astrocytes supports potential use of leflunomide in the treatment of multiple sclerosis and other NO-dependent inflammatory brain disorders.