Glutamine Protects Mice from Lethal Endotoxic Shock via a Rapid Induction of MAPK Phosphatase-1

Hyun-Mi Ko, Sin-Hye Oh, Hwa-Suk Bang, Nam-In Kang, Baik-Hwan Cho, Suhn-Young Im and Hern-Ku Lee
J Immunol June 15, 2009, 182 (12) 7957-7962; DOI: https://doi.org/10.4049/jimmunol.0900043

Abstract

The nonessential amino acid l-glutamine (Gln) is the most abundant amino acid in plasma. Clinical trials have demonstrated that Gln therapy is safe and improves clinical outcomes in critically ill patients. We have previously shown that Gln protect animals from endotoxic shock through the inhibition of cytosolic phospholipase A2 activity. In this study, we investigated how Gln regulates MAPK activation, as the molecular mechanism underlying Gln-induced cytosolic phospholipase A2 inactivation. Gln rapidly (within 10 min) inactivated p38 and JNK, but not ERK, by dephosphorylating them only when these MAPKs were phosphorylated in response to LPS in vivo as well as in vitro. Western blot analysis revealed that Gln administration resulted in rapid (∼5 min) phosphorylation and protein induction of MAP kinase phosphatase-1 (MKP-1). MKP-1 siRNA abrogated the Gln-mediated 1) inactivation of p38 and JNK, 2) induction of MKP-1, and 3) protection against endotoxic shock. The ERK inhibitor U0126 blocked Gln-induced MKP-1 phosphorylation and protein induction, as well as Gln’s protective activity against endotoxic shock. These data suggest that Gln exerts a beneficial effect on endotoxic shock by inactivating p38 and JNK via a rapid induction of MKP-1 protein in an ERK-dependent way.

The nonessential amino acid l-glutamine (Gln)4 is the most abundant amino acid in plasma (1), and Gln is an energy substrate for most cells (2, 3). Consequently, Gln plays an important role in nitrogen and carbon-skeleton exchange among different tissues, where this amino acid fulfills many different physiological functions (4). Following tissue injury or severe infections, critically ill patients experience metabolic alterations leading to muscle proteolysis activation, enhanced liver gluconeogenesis, and tissue insulin resistance (5). In critically ill patients, glutamine depletion is proportional to severity of illness (6). Clinical trials in human subjects have also demonstrated that Gln treatment decreases infectious complications, shortens hospital stay, and decreases hospital costs in a number of patient populations (7).

Phospholipase A2 (PLA2) comprises a diverse family of enzymes that cleave the sn-2 fatty acyl ester bond of glycerophospholipids to yield a free fatty acid and a lysophospholipid (8, 9). Among PLA2s, a 85-kDa cytoplasmic PLA2 (cPLA2) is known to play a critical role in the pathogenesis and progression of inflammation, including endotoxic shock. The protective effects of PLA2 inhibitors in endotoxic shock (10, 11), or reduced septic symptoms in PLA2-deficient mice (12, 13), indicate that PLA2 is an essential component in the pathogenesis of endotoxic shock. We have previously shown that Gln protects animals from endotoxic shock through its inhibition of cPLA2 activity (14), and this activity of Gln was also associated with suppression of asthma in mice (15).

In this study, we investigated the molecular mechanism underlying Gln-induced cPLA2 inactivation. Given that phosphorylation of cPLA2 is a key step in the activation of this enzyme, and the MAPK family, including p38 kinases, ERK1/2 (p42/p44); and c-Jun NH2-terminal kinase (JNK) is a major downstream pathway for cPLA2 phosphorylation (16), we looked at how Gln regulated MAPK activation. We found that Gln inactivated LPS-induced p38 and JNK via a rapid induction of MAPK phosphatase-1 (MKP-1) protein in an ERK-dependent way, and that this activity of Gln was associated with its protective activity against endotoxic shock.

Materials and Methods

Animals

Specific pathogen-free female BALB/c mice were purchased from Samtako Bio Korea, and kept in our animal facility for at least 1 wk before use. All mice were used at 7 to 8 wk of age at the start of each experiment. All experimental animals used in this study were under the protocol approved by the Institutional Animal Care and Use Committee of the Chonbuk National University Medical School.

Reagents

LPS derived from Escherichia coli (O127:B8, L3024) and l-Gln (Biotechnology performance certified, G-8540) were purchased from Sigma-Aldrich. LPS was injected via lateral tail vein. Gln was dissolved in Hartmann’s solution (30 mg/ml), and 750 mg/kg of Gln was i.p. administered as described previously (14). Quercetin from Sigma-Aldrich (400 mg/kg) was administered i.p. 6 h before LPS injection, as described elsewhere (17). U0126, a specific inhibitor of MEK1/2, was obtained from Calbiochem. U0126 (12.5 mg/kg) was injected i.p. 48 h before LPS treatment. Triptolide (Calbiochem), a MKP-1 inhibitor, was added to the culture 30 min before LPS stimulation. U0126 and triptolide were dissolved in DMSO. The solution containing the same concentration of DMSO was used as a control vehicle.

Cell culture

Murine alveolar macrophage cells, MH-S (ATCC CRL-2019), were maintained in RPMI 1640 containing 2 mM Gln (Life Technologies) supplemented with 10% FBS (Invitrogen) and 1% antibiotics at 37°C in a 5% CO2 atmosphere.

RNA interference

Small interfering RNA (siRNA) strands for mouse MKP-1 and controls were obtained from Santa Cruz Biotechnology. In vivo delivery of siRNA was performed using in vivo-jet polyethylene imine (PEI) (Polyplus-transfection), according to the instructions of manufacturer. In brief, MKP-1 siRNA and PEI dissolved in 5% glucose were mixed in a volume of 50 or 200 μl for intratracheal or i.v. injection, respectively, at room temperature for 20 min and the mixture was administered 24 h before LPS treatment. The mixture containing control siRNA and PEI dissolved in 5% glucose without siRNA were used as controls. To confirm that the MKP-1 siRNA used really block the synthesis of its target, an immunoblotting analysis was performed.

Immunoblotting analysis

Mice were sacrificed by cervical dislocation and the lungs were collected immediately thereafter and briefly washed with cold PBS and then dried with blotting paper. The isolated lung tissues were frozen in liquid nitrogen and were stored in −70°C until analysis. Small lung specimens were homogenized in the PhosphoSafe Extraction Reagent (Novagen). Equal amounts of cell lysates were separated on a 10% SDS polyacrylamide gel under reducing conditions, and were then transferred onto Protran nitrocellulose membranes (Schleicher and Schuell). Membranes were blocked by incubation for 1 h at room temperature in 5% skimmed milk in TBS, followed by a further 2 h of incubation with primary Abs against mouse phospho-p38, phospho-JNK, phospho-ERK phospho-MKP-1 (Cell Signaling Technology), MKP-1 (Santa Cruz Biotechnology), and β-actin (Sigma-Aldrich). Blots were washed for 15 min with three changes of TBS-0.05% Tween 20 solution, followed by incubation for 1 h at room temperature with the HRP-conjugated anti-rabbit IgG Ab (Santa Cruz Biotechnology). Finally, they were developed in LumiGLO reagent (Cell Signaling Technology). p38 was used as a loading control.

Statistical analysis

Statistical differences in survival were assessed by Fisher’s Exact test. p < 0.05 was considered statistically significant.

Results

Gln inactivates LPS-induced p38 and JNK

To investigate the mechanism of down-regulation of activity of cPLA2 by Gln, we first assessed the kinetics of MAPK activation in the lungs. MAPKs were activated by LPS injection, and the activation process was detected by Western blotting using phospho-specific Abs in the lungs (Fig. 1⇓A). p38 and JNK were rapidly activated by LPS, reaching their maximal activities within 10 min, which declined thereafter and returned to a basal level within 60 min. ERK was also rapidly activated, but in contrast to p38 and JNK, its activity was sustained at relatively high levels for up to 60 min. Gln was administered 10 min or 5 min before LPS injection, or 2 min, 5 min or 10 min after LPS injection, and MAPK activation was examined at 20 min post-LPS (Fig. 1⇓B). When Gln was administered before or 2 min after LPS injection, it did not affect LPS-induced MAPK activation at all. However, Gln inactivated p38 and JNK in the 5 min post-LPS, and such activity was complete at 10 min. In contrast to p38 and JNK, Gln had no effect on the activation of ERK. To further determine the interval between Gln administration and p38 and JNK inactivation, Gln was administered 10 min after LPS, and the lungs of different groups of mice were removed at 2 min intervals thereafter. Gln almost totally inactivated p38 within 6 min, but it took 10 min for JNK (Fig. 1⇓C). These data indicate that Gln inactivates p38 and JNK by dephosphorylating them only when these MAPKs were phosphorylated in response to LPS. In addition to the lungs, Gln-induced dephosphorylation of p38 and JNK, were also observed in the spleen and kidneys, but not observed in the liver (data not shown).

FIGURE 1.
FIGURE 1.

Gln inactivates LPS-induced p38 and JNK MAPKs. A, Lungs were removed at the indicated times after i.v. injection of LPS (2.5 mg/kg). B, Gln (750 mg/kg) was given i.p. at the indicated times before (−) or after (+) LPS injection, and the lungs were removed at 20 min post-LPS. C, Gln was given i.p.10 min after LPS injection and the lungs were removed at 2 min intervals thereafter. A representative of three to five independent experiments with three mice/time point/experiment is shown. D, Gln (20 mM) was added to the MS-H cells (2 × 106) 10 min after LPS (100 ng/ml) stimulation. A representative of two or three independent experiments is shown.

We examined the effect of Gln in the murine alveolar macrophage cell line MH-S. Treatment of the cells with LPS resulted in phosphorylation of the three MAPKs, and addition of Gln 10 min after LPS stimulation dephosphorylated the phosphorylated p38 and JNK at 20 min. However, Gln did not affect ERK phosphorylation (Fig. 1⇑D), as seen in in vivo study.

Gln-induced dephosphorylation of p38 and JNK was associated with the rapid induction of MKP-1

The observations detailed above prompted us to directly examine the role of the MKPs in this process. MKPs are a family of protein phosphatases that inactivate MAPKs through dephosphorylation of threonine and/or tyrosine residues (18). We examined the possible involvement of MKP-1, as this has been shown to preferentially dephosphorylate p38 and JNK, as compared with ERK (19, 20, 21, 22). Western blot analysis revealed that MKP-1 protein increased dramatically from 30 min and continued to rise over the 2 h of the experiment in response to LPS in the lungs (Fig. 2⇓A). The appearance of MKP-1 protein also correlated with the dephosphorylation of p38 and JNK seen in Fig. 1⇑A. To examine how Gln regulated MKP-1 protein induction, Gln was administered 10 min after LPS and the lungs were removed at 2 min intervals thereafter. Gln injection resulted in an induction of MKP-1 within 4–6 min. (Fig. 2⇓B). This Gln-induced induction of MKP-1 also coincided with a Gln-induced dephosphorylation of p38 and JNK seen in Fig. 1⇑C.

FIGURE 2.
FIGURE 2.

Gln induces the early induction of MKP-1 in response to LPS. A, Lungs were removed at the indicated times after i.v. injection of LPS (2.5 mg/kg). B, Gln (750 mg/kg) was given i.p. 10 min after LPS injection and the lungs were removed at the indicated times after LPS. C, Mice were i.v. injected with 200 μl of the PEI mixture containing 0.15 or 0.6 nmol. siRNA, 24 h before LPS injection. D, Mice were intratracheally injected with 50 μl of the PEI mixture containing 0.06 or 0.15 nmol siRNA 24 h before LPS injection. Gln was i.p. administered 10 min after LPS and the lungs were removed at 20 min post-LPS. A representative of three independent experiments with three mice/time point/experiment is shown. E and F, Gln (20 mM) was added to the MS-H cells (2 × 106) 10 min after LPS (100 ng/ml) stimulation. Triptolide (1 μM) was added 30 min before LPS. A representative of two or three independent experiments is shown. G, Quercetin (400 mg/kg) was given i.p. 6 h before LPS injection and the lungs were removed at 20 min after LPS.

To clarify the role of MKP-1 in Gln-induced MAPK inactivation, we tried to block MKP-1 induction using MKP-1 siRNA. Mice were i.v. administered with 0.15 or 0.6 nmol of MKP-1 siRNA 24 h before LPS injection. Gln was administered 10 min after LPS injection, and MKP-1 induction was examined at 20 min post-LPS. MKP-1 siRNA, but not control siRNA, abrogated the Gln-mediated induction of MKP-1 (Fig. 2⇑C). Under these conditions, Gln no longer dephosphorylated the preactivated p38 and JNK. These data clearly indicate that Gln dephosphorylates these MAPKs through the early induction of MKP-1. Intratracheal administration of MKP-1 siRNA (either 0.06 or 0.15 nmol.) displayed a similar inhibitory activity on Gln-induced MKP-1 induction (Fig. 2⇑D).

We also observed the effect of Gln on MKP-1 induction in MH-S cells. Gln addition resulted in the early induction of MKP-1 in LPS-treated cells (Fig. 2⇑E), which was abrogated by pretreatment with the MKP-1 inhibitor, triptolide. Triptolide also inhibited Gln-induced dephosphorylation of p38 and JNK (Fig. 2⇑F).

Heat shock proteins (HSP) have been reported to be associated with the beneficial role of Gln in endotoxic shock, (23, 24). Furthermore, HSP 70 has been shown to phosphorylate MKP-1 (25). This suggests a possibility that HSP may be involved in Gln-mediated MKP-1 induction. Therefore, we investigated whether the HSP inhibitor, quercetin, could affect Gln-mediated MKP-1 induction. Quercetin (400 mg/kg) was administered i.p. 6 h before LPS injection. However, quercetin had no effect on Gln-mediated MKP-1 induction (Fig. 2⇑G).

MKP-1 siRNA abrogates Gln-induced protection against endotoxic shock

Using MKP-1 siRNA, we next examined whether the ability of Gln to induce MKP-1 protein was associated with Gln’s beneficial effect on septic shock. Mice were injected with LPS (8.5 mg/kg = LD90). Administration of Gln 10 min after LPS significantly reduced LPS-induced mortality. Pretreatment of animals with MKP-1 siRNA, but not control siRNA, abrogated Gln’s protective activity in a dose dependent manner (Fig. 3⇓).

FIGURE 3.
FIGURE 3.

MKP-1 siRNA abrogates Gln’s protective activity against endotoxic shock. Mice were i.v. injected with 200 μl of the PEI mixture containing 0.15 or 0.6 nmol. siRNA 24 h before i.v. administration of LPS (8.5 mg/kg = LD90). Gln (750 mg/kg) was i.p. administered 10 min after LPS injection. Each group includes 20 mice from two independent experiments. *, p < 0.05; **, p < 0.01.

ERK-dependent induction of MKP-1

MKP-1 has been reported to be a labile protein that is normally degraded via the ubiquitin/proteasome pathway and its phosphorylation reduces its ubiquitination and degradation (26, 27). Based on this information, we speculated that Gln might induce the early induction of MKP-1 protein via the prevention of its degradation by inducing MKP-1 phosphorylation. Gln injection 10 min after LPS resulted in MKP-1 phosphorylation, which coincided with MKP-1 protein induction (Fig. 4⇓A), suggesting that Gln-induced early induction of MKP-1 was probably associated with its phosphorylation.

FIGURE 4.
FIGURE 4.

ERK inhibitor abrogates Gln-induced MKP-1 induction. A, Lungs were removed at the indicated times after i.v. injection of LPS (2.5 mg/kg). B, U0126 (12.5 mg/kg) was injected i.p. 48 h before LPS injection. C, Gln (750 mg/kg) was i.p. administered 10 min after LPS. A representative of three independent experiments with three mice/time points/experiments is shown. D and E, Gln (20 mM) was added to the MS-H cells (2 × 106) 10 min after LPS (100 ng/ml) stimulation. U0126 was added to the culture 1 h before LPS. A representative of two or three independent experiments is shown.

We next assessed the possible effect of ERK on Gln-induced MKP-1 phosphorylation, as the involvement of ERK here has been mentioned previously (26). This was performed by examining how the ERK inhibitor, U0126, affected Gln-induced MKP-1 phosphorylation and protein induction. We first verified whether the concentration of U0126 used in this in vivo study would effectively block ERK phosphorylation. U0126 inhibited the phosphorylation of its target molecule (Fig. 4⇑B). U0126 blocked not only Gln-induced MKP-1 phosphorylation and protein induction, but also Gln-induced p38 and JNK inactivation in the lungs (Fig. 4⇑C). Similar findings were observed in LPS-treated MH-S cells. Gln addition resulted in an early phosphorylation of MKP-1 (Fig. 4⇑D), and pretreatment of the cells with U0126 abrogated Gln-induced phosphorylation and protein induction of MKP-1, as well as dephosphorylation of p38 and JNK (Fig. 4⇑E).

We finally examined how the ERK inhibitor affected Gln’s protective activity against endotoxic shock. Gln’s protective activity was completely abrogated by pretreatment of the mice with U0126 (Fig. 5⇓). Taken together, these data indicate that Gln induces MKP-1 phosphorylation and protein induction in an ERK-dependent way.

FIGURE 5.
FIGURE 5.

ERK inhibitor abrogates Gln’s protective activity against endotoxic shock. U0126 (12.5 mg/kg) was injected i.p. 48 h before i.v. administration of LPS (8.5 mg/kg = LD90). Gln (750 mg/kg) was i.p. administered 10 min after LPS injection. Each group includes 20 mice from two independent experiments. *, p < 0.01.

Discussion

The present study has demonstrated for the first time that Gln functions as a potent MAPK inhibitor in vivo, as well as in vitro. Gln inactivated p38 and JNK, but not ERK, and this activity of Gln was observed only when p38 and JNK were phosphorylated in response to LPS. Hence, Gln inactivated those MAPKs by dephosphorylating them. These findings might explain our previous observations of the protective effect of Gln against endotoxic shock when it was administered after (+10 min), but not before (−10 min), LPS injection (14).

We found that Gln-induced dephosphorylation of p38 and JNK was attributed to the early induction of the dual phosphatase MKP-1 protein. This conclusion came from the observations that 1) Gln injection resulted in a rapid induction of MKP-1 within 4–6 min, which coincided with Gln-induced dephosphorylation of p38 and JNK, 2) MKP-1 siRNA abrogated the ability of Gln not only to induce MKP-1 induction and dephosphorylation of p38 and JNK, but also to protect animals from septic shock. MKP-1 was originally identified as an ERK-specific phosphatase (28, 29), but has since been also recognized as being able to dephosphorylate and inactivate both p38 and JNK MAPKs, its substrate specificity being dependent on cell type and context (19, 20, 21, 22). Among MKPs, in addition to MKP-1, MKP-5 (30) and MKP-7 (31) have also been reported to have specificity for p38 and JNK. Although abrogation of the majority of activities of Gln by MKP-1 siRNA suggests that MKP-1 appears to be the critical MKP in this process, the possible involvement of MKP-5, and −7 cannot be excluded. Further studies are required to address this issue.

MKP-1 can be rapidly induced in mammalian cells in response to an array of stress stimuli, including oxidative stress and heat shock (32), UV light (33), and DNA-damaging anti-cancer drugs (34, 35) through transcriptional (36, 37) and posttranscriptional mechanisms (26, 38). Under our experimental conditions, MKP-1 induction occurred within 5–6 min following Gln injection. This rapid induction suggested that Gln-mediated MKP-1 induction occurs through posttranscriptional mechanisms rather than transcriptional. In this context, Brondello et al. (26) reported that MKP-1 is a target in vivo and in vitro for p42MAPK or p44MAPK, which phosphorylates MKP-1 on two carboxyl-terminal serine residues: serine 359 and serine 364. This phosphorylation does not modify MKP-1’s intrinsic ability, but does lead to stabilization of the protein. In agreement with this report, we have demonstrated that the ERK inhibitor, U0126, blocked not only Gln-induced MKP-1 phosphorylation and protein induction, but also Gln’s protective activity against endotoxic shock. This further supports a role for ERK in Gln-induced MKP-1 induction. This also further supports the reason why Gln exerts its beneficial role for endotoxic shock only when given after LPS administration. In fact, Gln was significantly protective when given 1–2 h after LPS injection. Furthermore, it still significantly delayed the mortality even when given 6 h later (data not shown). We are currently investigating whether MKP-1 is also importantly involved in this situation.

HSP are a family of highly conserved proteins that act as molecular chaperones during periods of cell stress (39). Septic shock leads to the down-regulation of HSP-70 expression in the lungs (40). HSP has been reported to be associated with the beneficial role of Gln in endotoxic shock, i.e., Gln enhances cell survival in vitro against a variety of stressful stimuli (41), and protects animals from endotoxic shock through the induction of HSP (23, 24). Furthermore, HSP 70 has been shown to phosphorylate MKP-1 (25). Therefore, we investigated whether the HSP inhibitor, quercetin, could affect Gln-mediated MKP-1 induction. However, quercetin had no effect on Gln-mediated MKP-1 induction, suggesting that HSP is not required in this rapid induction of MKP-1 protein in response to Gln.

p38 has a central role in the production of inflammatory molecules through both transcription-dependent mechanisms and posttranscriptional regulation (42, 43, 44, 45, 46). As a result, the p38 pathway is considered to be a central regulator of inflammation. Owing to its vital role in inflammation, p38 is an obvious therapeutic target for potential drugs to treat inflammatory diseases (43). JNK is a multifactorial kinase involved in several physiological and pathological processes. Recent studies have demonstrated that the JNK pathway is considered to be a potentially relevant target for therapy regarding a variety of diseases, including neurodegenerative disease (47), metabolic disorders (48), inflammation (49), and cancer (50).

We have shown that Gln-induced inactivation of p38 and JNK through the early induction of MKP-1. In this regard, given that Gln has been safely administered to a wide spectrum of critically ill patients without any significant toxicity, it might be of great therapeutic value in the control of inflammatory, and other many diseases, that have constitutively active p38 and JNK MAPKs.

Disclosures

The authors have no financial conflict of interest.

Footnotes

  • The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

  • ↵1 This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-313-C00723).

  • ↵2 S.-Y.I. and H.-K.L. contributed equally to this work.

  • ↵3 Address correspondence and reprint requests to Dr. Hern-Ku Lee, Department of Immunology, Chonbuk National University Medical School, Chonju, Chonbuk, Republic of Korea. E-mail address: leeh-k{at}chonbuk.ac.kr

  • ↵4 Abbreviations used in this paper: Gln, l-glutamine; PLA2, phospholipase A2; c PLA2, cytoplasmic PLA2; MKP-1, MAPK phosphatase-1; PEI, polyethylene imine; siRNA, small interfering RNA; HSP, heat shock protein.

  • Received January 7, 2009.
  • Accepted April 15, 2009.

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