Inhibition of p38 mitogen-activated protein kinase enhances c-Jun N-terminal kinase activity: Implication in inducible nitric oxide synthase expression
© Lahti et al; licensee BioMed Central Ltd. 2006
Received: 18 January 2006
Accepted: 21 February 2006
Published: 21 February 2006
Nitric oxide (NO) is an inflammatory mediator, which acts as a cytotoxic agent and modulates immune responses and inflammation. p38 mitogen-activated protein kinase (MAPK) signal transduction pathway is activated by chemical and physical stress and regulates immune responses. Previous studies have shown that p38 MAPK pathway regulates NO production induced by inflammatory stimuli. The aim of the present study was to investigate the mechanisms involved in the regulation of inducible NO synthesis by p38 MAPK pathway.
p38 MAPK inhibitors SB203580 and SB220025 stimulated lipopolysaccharide (LPS)-induced inducible nitric oxide synthase (iNOS) expression and NO production in J774.2 murine macrophages. Increased iNOS mRNA expression was associated with reduced degradation of iNOS mRNA. Treatment with SB220025 increased also LPS-induced c-Jun N-terminal kinase (JNK) activity. Interestingly, JNK inhibitor SP600125 reversed the effect of SB220025 on LPS-induced iNOS mRNA expression and NO production.
The results suggest that inhibition of p38 MAPK by SB220025 results in increased JNK activity, which leads to stabilisation of iNOS mRNA, to enhanced iNOS expression and to increased NO production.
Nitric oxide (NO) is a highly reactive signaling molecule and inflammatory mediator, which acts as a cytotoxic agent and modulates immune responses and inflammation [1, 2]. High amounts of NO are produced for prolonged times by inducible nitric oxide synthase (iNOS) in response to proinflammatory cytokines and bacterial products [3, 4]. iNOS expression is regulated both at transcriptional and posttranscriptional level. Several transcription factors which regulate iNOS promoter activity have been characterized, but the mechanisms and factors regulating iNOS mRNA stability are largely unknown [2, 5].
Mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that are part of the signal transduction pathways, which connect inflammatory and various other extracellular signals to intracellular responses e.g. gene expression . p38 MAPK and c-Jun N-terminal kinase (JNK) are members of the MAPK family, and they are activated by chemical and physical stress. p38 and JNK regulate immune responses and expression of various cytokines e.g. tumor necrosis factor-α, interleukin-1 and interleukin-6 .
JNK and p38 MAPK are also involved in regulation of iNOS expression. Previous studies have shown that JNK pathway belongs to the factors that mediate the up-regulation of iNOS expression [8–10]. Depending on the cell-type and stimulation used, p38 MAPK has been reported to have either up-regulatory role [11–13], down-regulatory role [14–16] or no role [17, 18] in iNOS expression. We have previously reported that p38 MAPK inhibitors enhance iNOS expression and NO production in LPS-stimulated J774 macrophages . The detailed mechanism behind those stimulatory effects is not known.
The aim of the present study was to investigate the mechanism by which p38 inhibition leads to increase in NO production. The results suggest that inhibition of p38 MAPK increases LPS-induced JNK activity, which leads to stabilisation of iNOS mRNA and increased production of NO in activated macrophages.
p38 MAPK inhibitor SB220025 increases LPS-induced NO production and iNOS expression
SB220025 stabilises iNOS mRNA
Because SB220025 had no effect on iNOS mRNA levels when measured 4 h after LPS, but significantly increased the mRNA levels when measured 10 h after LPS, we hypothesized that SB220025 might stabilize iNOS mRNA.
Actinomycin D (an inhibitor of transcription) was added to cells 6 h after LPS in an attempt to test whether the slowed disappearance of iNOS mRNA in cells treated with LPS+SB220025 was due to increased rate of transcription of iNOS gene or reduced degradation of mRNA. Interestingly, the level of mRNA was reducing at the same or slower rate in cells treated with LPS+actinomycin D compared with cells treated with LPS only, suggesting that no significant transcription of iNOS gene occurs in cells 6 – 12 h after LPS stimulation and that actinomycin D itself inhibits the degradation of iNOS mRNA. Thus, the slowed disappearance of iNOS mRNA in cells treated with SB220025 was most likely due to reduced degradation of mRNA.
p38α and p38β expression in J774 macrophages
There are four known isoforms of p38 MAPK (α, β, γ and δ) , and SB203580 has been shown to inhibit p38α and p38β but not p38γ and p38δ isoforms . p38α and p38β have been recently reported to differently regulate iNOS expression . Therefore we wanted to investigate whether J774.2 macrophages express p38α and p38β isoenzymes.
SB220025 increases LPS-induced JNK activity
Opposite roles for p38 MAPK and JNK have recently been reported on thrombin induced iNOS expression in RAW264.7 macrophages . JNK and p38 MAPK have common target proteins and there is crosstalk between these signaling cascades . Furthermore, we have previously reported that JNK inhibition destabilizes iNOS mRNA . Therefore we hypothesized that the roles of JNK and p38 MAPK pathways on LPS-induced iNOS expression may be coupled. We continued by investigating whether inhibition of p38 MAPK modulates the activity of JNK.
To rule out the possibility, that increased c-Jun phosphorylation was a result of reduced dephosphorylation, we tested whether the effect of SB220025 could be reversed with JNK inhibitor SP600125. Treatment with LPS and SB220025 induced a 6 fold increase in c-Jun Ser63 phosphorylation compared with cells treated with LPS only (Fig. 8B). In contrast, the negative control compound SB202474 had no effect on c-Jun phosphorylation. The SB220025-stimulated increase in c-Jun phosphorylation was almost completely reversed by SP600125, suggesting that the increase in c-Jun phosphorylation was due to increased JNK activity and not due to reduced dephosphorylation.
The stimulatory effect of SB220025 on LPS-induced NO production and iNOS mRNA expression can be reversed by SP600125
To continue, we hypothesized that the stimulatory effect of SB220025 on LPS-induced NO production was due to increased JNK activity and therefore we tested the effect of JNK inhibitor SP600125 on SB220025-stimulated NO production.
The same result was observed at the level of iNOS mRNA expression. SB220025 increased the amounts of iNOS mRNA to almost two fold compared with cells treated with LPS only, whereas the negative control compound SB202474 had no effect (Fig. 9B). SP600125 alone reduced the LPS-stimulated iNOS mRNA levels slightly. In addition, in the presence of the JNK inhibitor SP600125, SB220025 had no stimulatory effect on iNOS mRNA levels.
Cycloheximide increases JNK activity and iNOS mRNA expression
In the present study we have shown that inhibition of p38 MAPK by SB220025 increases LPS-induced JNK activity, which leads to stabilization of iNOS mRNA and increased iNOS expression and NO production in J774.2 macrophages.
Inhibitors of p38 MAPK have been shown to up-regulate iNOS expression in IL-1β-stimulated rat mesangial cells [14, 24], in LPS+IFN-γ-stimulated RAW264.7γ macrophages , in interferon-γ (IFN-γ)+mannose-capped lipoarabinomannan-stimulated RAW264.7γ macrophages  and in LPS-stimulated J774.A1 macrophages . In this study, a novel p38 MAPK inhibitor SB220025 increased LPS-induced NO production with an EC50 of ~100 nM, which is close to its IC50 value of p38 MAPK inhibition (~60 nM) . Furthermore, a structurally related inactive control compound SB202474 had no effect. These results together suggest that the observed increase in NO production and iNOS expression was due to inhibition of p38 MAPK. SB203580 inhibits the p38α and p38β isoforms at equipotent efficiency, but does not inhibit p38γ or p38δ . To our knowledge there is no published data about the isoform specificity of SB220025. In the present study both SB203580 and SB220025 had similar effect on LPS-induced NO production, thus it is likely that the observed effects are mediated by p38α and/or p38β. J774 macrophages were found to express p38α mRNA and p38α protein at relatively high levels whereas only low amounts of p38β mRNA were detected. Similar pattern of p38α and p38β expression was reported by Lui et al. (2004) in rat renal mesangial cells, in which p38 MAPK inhibition was also found to increase iNOS expression. In their further transfection experiments, Lui et al. (2004) found that p38α mutant and p38β wild-type isoforms inhibited IL-1-induced iNOS expression suggesting that the two isoforms have reciprocal effects on iNOS expression in renal mesangial cells. Our results show that inhibition of p38 enhances iNOS expression and NO production in macrophages activated by LPS but further studies are required to clarify the roles of different p38 MAPK isoforms in that process.
The mechanisms how p38 MAPK inhibitors enhance iNOS expression and NO production have been unclear. The present data suggest that inhibition of p38 enhances JNK activity that results in stabilization of iNOS mRNA and enhanced iNOS protein expression. Our results are in line with those of Avdi et al. (2002) in which inhibition of p38 MAPK by SB203580 led to increased activity of JNK in human neutrophils . The inhibition of p38 MAPK was found to reduce the activity of protein phosphatase-2A which resulted in reduced dephosphorylation and increased activity of JNK. Various protein phosphatases are able to dephosphorylate MAPKs and are thus important regulators of MAPK activity . It is possible that p38 MAPK regulates the activity of protein phosphatase-2A or some other protein phosphatase and inhibition of p38 MAPK by SB220025 reduces protein phosphatase activity, which leads to increased JNK activity observed in the present study. Interestingly, we found that JNK inhibitor SP600125 reversed the SB220025 stimulated increase in JNK activity, NO production and iNOS expression, suggesting that increased iNOS expression by SB220025 results from increased JNK activity. In addition, cycloheximide, a known JNK activator, also increased LPS-induced iNOS mRNA expression in a similar manner as SB220025. The stimulatory effect of cycloheximide on iNOS mRNA expression was reversed by SP600125, suggesting that the effect of cycloheximide is at least partially mediated through increased JNK activity. Up-regulatory role for JNK in iNOS expression has been previously shown in IL-1+IFN-γ-stimulated human fetal astrocytes , in LPS+IFN-γ-stimulated RAW264.7γ macrophages , IL-1β-stimulated rat primary mesangial cells and LPS-stimulated J774.A1 macrophages .
Regulation of iNOS mRNA stability seems to be an important mean to regulate iNOS expression. However, the mechanisms regulating iNOS mRNA stability are poorly known. HuR is a mRNA stabilizing factor, which has been shown to bind an AU-rich sequence element in the 3' untranslated region of human iNOS mRNA and to stabilise iNOS mRNA . Tristetraprolin seems to have a role as a mRNA stabilizing factor for human iNOS  while the KH-type splicing regulatory protein (KSRP) has been identified as a destabilizing factor . Heterogeneous nuclear ribonucleoproteins I and L have been reported to interact with murine iNOS mRNA . In addition, dexamethasone , protein kinase Cδ  and β-adrenergic stimulation  have been shown to regulate iNOS mRNA stability. Recently, we have shown that JNK inhibitor SP600125 reduces iNOS mRNA stability . In the present study, treatment with SB220025 had no effect on iNOS mRNA levels when measured 4 h after LPS stimulation, whereas a two fold increase in mRNA levels was observed 10 h after LPS. Furthermore, mRNA levels decreased slower in SB220025 treated cells than in cells treated with LPS alone. These results together suggest that SB220025 increases iNOS mRNA expression by stabilising mRNA. Also actinomycin D seems to have a stabilising effect on iNOS mRNA. Actinomycin D has previously been reported to stabilise mRNAs of transferrin receptor  and cyclooxygenase-2  but the mechanisms are not known in detail.
The present results show that inhibition of p38 MAPK enhances JNK activity, which leads to stabilisation of iNOS mRNA, and to increased iNOS expression and NO production. p38 MAPK regulates activity of JNK pathway and it is therefore important to consider whether results obtained by inhibiting p38 MAPK might result from increased JNK activity rather than from reduced p38 MAPK activity directly. Finally, based on our results, it seems that JNK is an important post-transcriptional regulator of LPS-induced iNOS expression and NO production.
Reagents were obtained as follows: anthra(1,9-cd)pyrazol-6(2H)-one (SP600125), 4-ethyl-2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-5-(4pyridyl)-imidazole (SB202474), 5-(2-amino-4-pyrimidinyl)-4-(4-fluorophenyl)-1-(4-piperidinyl)-imidazole(SB220025) and 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-imidazole (SB203580) (Calbiochem), rabbit polyclonal mouse iNOS, c-Jun, JNK1 and actin antibodies, goat polyclonal p38β antibody, donkey anti-goat antibody and goat anti-rabbit polyclonal antibody (Santa Cruz Biotechnology Inc.), rabbit polyclonal p38 MAPK, phospho-p38 MAPK (Thr180/Tyr182), p38α, phospho-SAPK/JNK (Thr183/Tyr185) and phospho-c-Jun (Ser63) II antibodies (Cell Signaling technology). All other reagents were from Sigma.
J774.2 macrophages (The European Collection of Cell Cultures) were cultured at 37°C, 5% CO2 atmosphere, in Dulbecco's Modified Eagle's Medium with glutamax-I (Cambrex Bioproducts) containing 10% of heat inactivated foetal bovine serum, 100 U/ml penicillin, 100 μg/ml streptomycin and 250 ng/ml amphotericin B (all from Invitrogen). Cells were seeded on 24 well plates for nitrite measurements and in 6 well plates for Western blot and RT-PCR and grown for 48 h prior to experiments.
At indicated time points the culture medium was collected for nitrite measurement, which was used as a measure of NO production. Culture medium (100 μl) was incubated with 100 μl of Griess reagent (0.1% napthalethylenediamine dihydrochloride, 1% sulphanilamine, 2.5% H3PO4) and the absorbance was measured at 540 nm.
Preparation of cell lysates
At indicated time points cells were rapidly washed with ice cold PBS and solubilised in cold lysis buffer containing 10 mM Tris-base, 5 mM EDTA, 50 mM NaCl, 1% Triton-X-100, 0.5 mM phenylmethylsulfonyl fluoride, 2 mM sodiumorthovanadate, 10 μg/ml leupeptin, 25 μg/ml aprotinin, 1.25 mM NaF, 1 mM sodiumpyrophosphate and 10 mM n-octyl-β-D-glucopyranoside. After incubation for 20 min on ice, lysates were centrifuged (14500 g, 15 min) and supernatants were mixed 1:4 with SDS loading buffer (62.5 mM Tris-HCl, pH 6.8, 10% glycerol, 2% SDS, 0.025% bromophenol blue, 5% β-mercaptoethanol) and boiled for 5 min. Protein concentrations in the samples were measured by the Coomassie blue method .
Protein (30 μg) was loaded on 10% SDS-polyacrylamide electrophoresis gel and electrophoresed for 4 h at 100 V in buffer containing 95 mM Tris-HCl, 960 mM glycine and 0.5% SDS. After electrophoresis the proteins were transferred to Hybond ECL™ nitrocellulose membrane (Amersham) with semi-dry blotter at 2.5 mA/cm2 for 60 min. After transfer the membrane was blocked in TBS/T (20 mM Tris-base pH 7.6, 150 mM NaCl, 0.1% Tween-20) containing 5% bovine serum albumin for 1 h at room temperature and incubated with primary antibody in the blocking solution at 4°C overnight. Thereafter the membrane was washed 4× with TBS/T for 5 min, incubated with secondary antibody in the blocking solution for 0.5 h at room temperature and washed 4× with TBS/T for 5 min. Bound antibody was detected using SuperSignal® West Pico chemiluminescent substrate (Pierce) and FluorChem™ 8800 imaging system (Alpha Innotech). The quantitation of chemiluminescent signal was carried out with FluorChem™ software v. 3.1.
RNA extraction and real-time RT-PCR
At indicated time points cell monolayers were rapidly washed with ice cold PBS and cells were homogenised using QIAshredder™ (QIAGEN Inc.). RNA extraction was carried out with RNeasy® kit for isolation of total RNA (QIAGEN inc.). Total RNA (25 ng) was reverse transcribed to cDNA using TaqMan Reverse Transcription reagents and random hexamers (Applied Biosystems). Reverse transcriptase (RT) reaction parameters were as follows: incubation at 25°C for 10 min, RT at 48°C for 30 min and RT inactivation at 95°C for 5 min. cDNA obtained from the RT reaction (amount corresponding approximately 1 ng of total RNA) was subjected to PCR using TaqMan® Universal PCR Master Mix and ABI PRISM® 7000 Sequence detection system (Applied Biosystems). GAPDH and iNOS primer and probe sequences and concentrations were optimised according to manufacturers guidelines in TaqMan® Universal PCR Master Mix Protocol Part Number 4304449 Rev. C and were as follows: 5'-CCTGGTACGGGCATTGCT-3', 5'-GCTCATGCGGCCTCCTT-3' (forward and reverse mouse iNOS primer respectively, both 300 nM), 5'-CAGCAGCGGCTCCATGACTCCC-3'(mouse iNOS probe 150 nM, containing 6-FAM as 5'-reporter dye) and 5'-GCATGGCCTTCCGTGTTC-3', 5'-GATGTCATCATACTTGGCAGGTTT-3' (forward and reverse mouse glyceraldehyde-3-phosphate dehydrogenase (GAPDH) primer respectively, both 300 nM), 5'-TCGTGGATCTGACGTGCCGCC-3'(mouse GAPDH probe 150 nM, containing 6-FAM as 5'-reporter dye) (Metabion). Primers and probes for p38α (product Mm00442497_m1) and p38β (product Mm00440955_m1) (Applied Biosystems) were used as recommended by the manufacturer. PCR reaction parameters were as follows: incubation at 50°C for 2 min, incubation at 95°C for 10 min and thereafter 40 cycles of denaturation at 95°C for 15 sec and annealing and extension at 60°C for 1 min. Each sample was determined in duplicate.
A standard curve method was used to determine the relative iNOS and GAPDH mRNA levels as described in Applied Biosystems User Bulletin #2. In short, a standard curve for each gene was created using mRNA isolated from LPS-stimulated J774.2 macrophages. Isolated RNA was reverse transcribed as described. Dilution series were made from obtained cDNA ranging from 10 ng to 1 pg and were subjected to real time PCR as described. Obtained threshold cycle values were plotted against dilution factor to create a standard curve. Relative mRNA levels in test samples were then calculated from the standard curve.
Results are expressed as mean ± standard error of mean (S.E.M.). When indicated, statistical significance was calculated by analysis of variances supported by Bonferroni multiple comparisons test. Differences were considered significant at P < 0.05.
We thank Mrs. Niina Ikonen and Ms. Jaana Laine for the skillful technical assistance and Mrs. Heli Määttä for secretarial help. This work was supported by The Academy of Finland, The Medical Research Fund of Tampere University Hospital, Tampere Tuberculosis Foundation, The Pirkanmaa Regional Fund and National Technology Agency (Tekes).
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