- Research article
- Open Access
Glucocorticoids synergize with IL-1β to induce TLR2 expression via MAP Kinase Phosphatase-1-dependent dual Inhibition of MAPK JNK and p38 in epithelial cells
© Sakai et al; licensee BioMed Central Ltd. 2004
Received: 17 December 2003
Accepted: 04 May 2004
Published: 04 May 2004
Despite the importance of glucocorticoids in suppressing immune and inflammatory responses, their role in enhancing host immune and defense response against invading bacteria is poorly understood. Toll-like receptor 2 (TLR2) has recently gained importance as one of the major host defense receptors. The increased expression of TLR2 in response to bacteria-induced cytokines has been thought to be crucial for the accelerated immune response and resensitization of epithelial cells to invading pathogens.
We show that IL-1β, a key proinflammatory cytokine, greatly up-regulates TLR2 expression in human epithelial cells via a positive IKKβ-IκBα-dependent NF-κB pathway and negative MEKK1-MKK4/7-JNK1/2 and MKK3/6-p38 α/β pathways. Glucocorticoids synergistically enhance IL-1β-induced TLR2 expression via specific up-regulation of the MAP kinase phosphatase-1 that, in turn, leads to dephosphorylation and inactivation of both MAPK JNK and p38, the negative regulators for TLR2 induction.
These results indicate that glucocorticoids not only suppress immune and inflammatory response, but also enhance the expression of the host defense receptor, TLR2. Thus, our studies may bring new insights into the novel role of glucocorticoids in orchestrating and optimizing host immune and defense responses during bacterial infections and enhance our understanding of the signaling mechanisms underlying the glucocorticoid-mediated attenuation of MAPK.
Epithelial cells are often situated at host-environment boundaries, and thus act as the first line of defense against bacterial pathogens . The epithelial cells are not merely a passive barrier but can detect foreign pathogens and generate a range of mediators that play important roles in the activation of innate and adaptive immunity. For effective host defense, the epithelial cells recognize highly conserved structural motifs only expressed by microbial pathogens, called pathogen-associated molecular patterns (PAMPs). Toll-like receptors (TLRs) play a critical role in early innate immunity to invading microorganisms by sensing PAMPs . Stimulation of TLRs by PAMPs initiates a signaling cascade that induces the production and secretion of proinflammatory cytokines . Moreover, stimulation of TLRs also induces the production of effector cytokines that leads to activation of adaptive immunity.
Mammalian TLRs were originally found as homologues of the Drosophila Toll . TLRs are type I transmembrane receptors, that possess extracellular leucine-rich repeats domains flanked by cytoplasmic domains [4, 5]. Although at least 10 TLRs have been identified, only two TLRs, TLR2 and TLR4, have been well-studied. While TLR4 is mainly involved in Gram-negative bacteria lipopolysaccharide (LPS) signaling, TLR2 can respond to a variety of Gram-positive bacterial products, including peptidoglycan, lipoprotein, lipoteichoic acid and lipoarabinomannan. In addition to Gram-positive bacterial PAMPs, TLR2 also recognizes factors released by Gram-negative bacteria including nontypeable Haemophilus influenzae (NTHi) [6, 7] and coccobacilli, Neisseria meningitidis  as well as the Mycoplasma lipopeptides [9, 10]. The importance of TLR2 in host defense was further demonstrated by studies of knockout mice showing decreased survival of TLR2-deficient mice after infection with Gram-positive Staphylococcus aureus . Thus, it is clear that TLR2 plays a crucial role in host defense against a variety of microbial pathogens.
In contrast to its relatively high level of expression in lymphoid tissues, TLR2 is expressed at low levels in epithelial cells. A key issue that has thus been raised is whether the low amount of TLR2 expressed in epithelial cells is sufficient for mediating the host defense response against invading microbial pathogens. Our recent studies revealed that TLR2, although expressed at very low level in unstimulated human epithelial cells, is greatly up-regulated by NTHi via a positive IKKβ-IκBα-dependent NF-κB pathway and a negative MKK3/6-p38α/β pathway . Moreover, glucocorticoids synergistically enhance NTHi-induced expression of TLR2 via a negative cross-talk with p38 MAP kinase pathway, supporting the notion that glucocorticoids plays an important role in orchestrating and optimizing immune functions, including host defense, during bacterial infections [13, 14]. However, still unclear is whether up-regulation of TLR2 expression in epithelial cells can also be generalized to other key mediators such as proinflammatory cytokines, e.g. interleukin 1-β (IL-1β) and if so, whether the cytokine-mediated up-regulation of TLR2 can also be further enhanced by glucocorticoids.
IL-1β, a proinflammatory cytokine, is produced by various cell types including epithelial cells and can be strongly induced during bacterial infections . It has been recognized as one of the key mediators of the host response to microbial invasion, inflammation, immunological reactions and tissue injury . Although it has been shown to induce the expression of a variety of non-structural, function-associated genes that are expressed during inflammation, particularly other cytokines, whether IL-1β also regulates the expression of host defense receptor TLR2 in human epithelial cells is still unknown. In addition, it is still unclear whether the IL-1β-mediated TLR2 expression can also be enhanced by glucocorticoids.
Here we show that IL-1β up-regulates TLR2 via a positive IKKβ-IκBα-dependent NF-κB signaling pathway and negative MKK3/6-p38α/β and MEKK1-MKK4/7-JNK1/2 pathways in human epithelial cells. Surprisingly, glucocorticoids synergistically enhance the IL-1β-induced TLR2 expression via a MKP-1-dependent dual inhibition of MAPK p38 and JNK signaling pathways. These studies may provide novel insights into the role of IL-1β and glucocorticoids in orchestrating and optimizing host immune and defense responses and lead to new therapeutic strategies for modulating innate immune and inflammatory responses for bacterial infections.
IL-1β selectively up-regulates TLR2, but not TLR4, in human epithelial cells
Activation of NF-κB via IKKβ-IκBα signaling pathway is required for IL-1β-induced TLR2 up-regulation
Activation of MKK3/6-p38 α/β MAPK pathway is negatively involved in IL-1β-induced TLR2 expression
Activation of MEKK1-MKK4/7-JNK1/2 pathway is also negatively involved in IL-1β-induced TLR2 expression
On the basis that JNK activation is mediated by two upstream MAPK kinases (MKKs), MKK4 and MKK7, and that both MKKs are activated by the further upstream MAPK/Erk kinase kinase 1 (MEKK1), we next sought to investigate whether the MEKK1-MKK4/7 signaling pathway is also negatively involved in TLR2 induction. We first confirmed whether MKK4 is activated by IL-1β in HeLa cells. Figure 4d showed a strong phosphorylation of MKK4 induced by IL-1β. To further determine the involvement of MEKK1-MKK4/7 pathway, HeLa cells were transfected with a dominant-negative mutant of either MKK4 or MKK7 or MEKK1. As shown in Figure 4e, overexpression of these dominant-negative mutants enhanced IL-1β-induced TLR2 expression (Figure 4e). Similar results were also observed in primary human bronchial epithelial NHBE cells (Figure 4f). Together, these results indicate that activation of MEKK1-MKK4/7-JNK1/2 signaling pathway is also negatively involved in IL-1β-induced TLR2 expression in human epithelial cells.
Glucocorticoids synergistically enhance IL-1β-induced TLR2 expression likely via negative cross-talk with MAPK p38 and JNK pathways
Because glucocorticoids have been shown to inhibit a number of MAPK family members, we next sought to determine whether DEX inhibits the IL-1β-induced activation of p38 or JNK by performing Western blot analysis. As shown in Figure 5d and 5e, DEX inhibited IL-1β-induced phosphorylation of both p38 and JNK, but not MKK3/6 or MKK4 and the inhibitory effects of DEX were blocked by GR antagonist RU486. Therefore, it is likely that DEX may synergistically enhance IL-1β-induced TLR2 up-regulation via negative cross-talk with p38 and JNK, but not their upstream kinases MKK3/6 or MKK4.
Glucocorticoids may enhance IL-1β-induced TLR2 up-regulation via up-regulation of MAP kinase phosphatase 1 (MKP-1)
Inhibition of MKP-1 expression attenuates glucocorticoids-mediated inhibition of IL-1β-induced p38 and JNK phosphorylation and enhancement of TLR2 expression
To further confirm whether the up-regulation of MKP-1 by glucocorticoids is indeed required for the enhancement of IL-1β-induced TLR2 expression, we investigated the effect of overexpressing an antisense full-length MKP-1 construct on the glucocorticoid-mediated enhancement of TLR2 induction . As shown in Figure 7e, we first confirmed that overexpression of the antisense MKP-1 construct specifically inhibited MKP-1 protein expression, but not MKP-2 or MKP-3 expression. In addition, overexpression of the antisense MKP-1 construct antagonized the inhibitory effect of DEX on IL-1β-induced phosphorylation of p38 or JNK (Figure 7f). Moreover, overexpression of the same antisense MKP-1 also inhibited the DEX-mediated enhancement of IL-1β-induced TLR2 expression in both HeLa cells and primary human bronchial epithelial NHBE cells (Figure 7g and 7h). Taken together, our data demonstrate that glucocorticoids synergistically enhance IL-1β-induced TLR2 expression via up-regulation of MKP-1 that, in turn, leads to dephosphorylation and inactivation of p38 and JNK, the negative regulators for TLR2 expression in human epithelial cells.
Despite the importance of glucocorticoids in suppressing immune and inflammatory responses, their role in enhancing host immune and defense response against invading bacterial pathogens is poorly understood [13, 14, 28, 29]. Although there are reports that glucocorticoids may have a beneficial role in certain bacterial infections, the molecular basis for their beneficial effects still remains unclear. In the present study, we provided evidence that glucocorticoids synergistically enhance the TLR2 expression, a key receptor that has been shown to play a crucial role in host defense response, as evidenced by the studies of TLR2 knockout mice showing decreased survival after infection with Gram-positive S. aureus . Thus, it appears that glucocorticoids may not only suppress but also likely enhance host immune and defense response. In addition, our study may provide the molecular basis for the explanation of the beneficial role of glucocorticoids in certain bacterial infections. Given the fact that TLR2 expression is low in unstimulated epithelial cells, the synergistic enhancement of IL-1β-induced TLR2 expression by glucocorticoids will probably contribute to the accelerated immune response by epithelial cells as well as resensitization of epithelial cells to invading pathogens. It should be noted that glucocorticoids enhance the TLR2 expression induced not only by IL-1β, but also by a variety of other pathogenic inducers including the Gram-negative bacterium nontypeable Haemophilus influenzae, the Gram-positive bacterium Streptococcus pneumoniae, and the proinflammatory cytokines TNF-α and IL-1α (data not shown). Therefore, up-regulation of the host defense receptor, TLR2, may be one of the important positive immune-regulatory mechanisms widely involved in the glucocorticoid-mediated host defense against many microbial pathogens. Our studies may bring new insights into the novel role of glucocorticoids in orchestrating and optimizing immune functions, including host defense, during bacterial infections.
Another interesting finding in this study is that glucocorticoids synergistically enhance the IL-1β-induced TLR2 expression via a MKP-1-mediated inactivation of both MAPK p38 and JNK, the negative regulators for TLR2 induction. Although several MAPKs have been identified as potential targets for negative regulation by glucocorticoids [25, 30–36], the signaling mechanisms underlying the inhibition of MAPKs by glucocorticoids still remain largely unknown. Our finding that glucocorticoids inhibit the IL-1β-induced phosphorylation and activation of MAPKs p38 and JNK via up-regulation of MKP-1 may enhance our understanding of the signaling mechanisms underlying the glucocorticoid-mediated attenuation of MAPKs. In agreement with recent reports that glucocorticoids inhibit p38 via up-regulation of MKP-1 [24, 26], our data showed that glucocorticoids enhanced IL-1β-induced TLR2 expression via up-regulating MKP-1 expression, that, in turn, leads to the dephosphorylation and inactivation of p38 in HeLa cells and primary NHBE cells. However, our finding that glucocorticoids inhibit the IL-1β-induced phosphorylation of JNK via an up-regulation of MKP-1 is rather unexpected, because it is in sharp contrast with the previous report that glucocorticoid-mediated inhibition of JNK does not require any de novo protein synthesis and independent of the transactivation function of GR . Similar to the glucocorticoid-mediated inactivation of p38, glucocorticoids require ongoing MKP-1 synthesis to inhibit phosphorylation of ERK1/2 [25, 37]. Hence, multiple signaling mechanisms are likely to underlie the regulation of MAPKs by glucocorticoids.
The molecular mechanisms underlying p38 and JNK-mediated inhibition of TLR2 induction by IL-1β still remain unclear. The human TLR2 promoter contains multiple transcription factor binding elements including Sp1 and Ets-1 . Sp1 and Ets-1 have been shown to be regulated by p38 signaling pathway [39, 40]. Activation of Sp1 and Ets family through JNK has been also reported [41, 42]. Thus, Sp1 and Ets-1 could be involved in p38 and JNK-mediated inhibition of TLR2 expression. TLR2 promoter region also contains some uncharacterized regulatory elements . These elements may also play a role in transcriptional inhibition of TLR2 mediated by p38 and JNK signaling. In addition to these transcription factors, the possible involvement of the regulation of mRNA stability by p38 and JNK can not be ruled out because both have been shown to be involved in regulating the mRNA stability of certain genes [43–46].
Although we have shown the involvement of multiple signaling pathways including the positive IKKβ-IκBα-dependent NF-κB pathway and the negative MKK3/6-p38 α/β and MEKK1-MKK4/7-JNK1/2 pathways, our data do not preclude the involvement of other signaling mechanisms such a direct transcriptional activation of TLR2 by glucocorticoids, or direct cross-talk between GR and other transcription factors that can be activated by the above signaling pathways. Future studies will focus on elucidating the signaling mechanisms underlying the glucocorticoid-mediated synergistic enhancement of IL-1β-induced TLR2 at the transcriptional level by taking advantage of the recent success in cloning the regulatory region of human TLR2. In addition, the role of MKP-1 in mediating the enhancement of TLR2 induction by glucocorticoids will also be confirmed in vivo by using an MKP-1 knockout mouse strain. Moreover, it is also interesting to determine whether glucocorticoids also enhance antibacterial defense responses.
Human IL-1β was purchased from PIERCE ENDOGEN (Woburn, MA). MG-132, SB203580 and Ro-31-8220 were purchased from Calbiochem (La Jolla, CA). Dexamethasone and RU486 were purchased from Sigma (St. Louis, MO).
The human cervix epithelial cell line, HeLa, was maintained as described . Primary human bronchial epithelial cells (NHBE) (Clonetics, Walkersville, MD) were maintained in Bronchial Epithelial Basal Medium (BEBM) (Clonetics) according to the manufacturer's instructions.
Real-time quantitative PCR analysis of TLR2 and TLR4
Total RNA was isolated from the cells using a TRIzol Reagent (Invitrogen Corporation, Carlsbad, CA) following manufacturer's instruction. For the RT reaction, TaqMan Reverse Transcription Regents (Applied Biosystems, Foster City, CA) were used. The real-time quantitative PCR was performed using an ABI 7700 sequence detector (Applied Biosystems). The primers and probes used for the PCR amplification of TLR2 were; p r i m e r s (5'-GGCCAGCAAATTACCTGTGTG-3' a n d 5'-AGGCGGACATCCTGAACCT-3') and a probe (5'-TCCATCCCATGTGCGTGGCC-3'). The primers and probes used for the PCR amplification of TLR4 were; primers (5'-CCAGTGAGGATGATGCCAGGAT-3' and 5'-GCCATGGCTGGGATCAGAGT-3') and a probe (5'-TGTCTGCCTCGCGCCTGGC-3'). All primers and probes were synthesized by Applied Biosystems Custom Oligo Synthesis Service. Relative quantity of TLR2 and TLR4 mRNA were obtained using Comparative CT Method, and was normalized using Pre-Developed TaqMan Assay Reagent Human Cyclophilin (Applied Biosystems) as an endogenous control as previously described .
Plasmids and transfections
The expression plasmids IκBα(S32/36A), IKKβ (K49A), fp38α (AF) and wild-type (WT), fp38β2 (AF) and WT, MKK3b (A), MKK6b (A), JNK1 (AF), JNK2 (AF), MKK4 DN, MKK7 DN and MEKK1 DN were previously described [6, 47]. The expression plasmid of the wild-type and mutant MKP-1 were kindly provided by Dr. N. Tonks (Cold Spring Harbor Laboratory, NY) . The antisense full-length MKP-1 construct was kindly provided by Dr. Desbois-Mouthon (INSERM U-402, Faculté de Médecine Saint-Antoine, 75571 Paris, France) and was described previously . All transient transfections were carried out in duplicate for RT-PCR analysis using TransIT-LT1 reagent (Mirus, Madison, WI) following manufacturer's instruction, unless otherwise indicated. In all transfections with expression plasmids of signaling molecules, an empty vector was used as a control.
Western Blot analysis
Phosphorylation of IκBα, JNK, p38, MKK4 and MKK3/6 was assessed using antibodies against phospho-IκBα (Ser32), IκBα, phospho-JNK (Thr183/Tyr185), JNK, phospho-p38 (Thr180/182), p38, phospho-MKK4 (Thr261), phospho-MKK3/MKK6 (Ser189/207) and MKK3 (New England Biolabs, Beverly, MA). Anti-MKK4 (K-18) was purchased from Santa Cruz Biotechnology, Inc. Santa Cruz, CA). Phosphorylation of of IκBα, JNK, p38, MKK4 and MKK3/6 was detected as described . Anti-MKP-1 (V-15), anti-MKP-2 (H-67) and anti-MKP-3 (C-20) antibodies were purchased from Santa Cruz, and used to assess protein expression. TLR2 expression was assessed with an anti-TLR2 antibody (Imgenex Corporation, San Diego, CA). Monoclonal anti-β-Actin antibody (Sigma) was used as a protein loading control for Western blot analyses of MKP-1, MKP-2, MKP-3 and TLR2.
Cells were cultured on 4-chamber microscopeslides. After IL-1β treatment, the cells were fixed in paraformaldehyde solution (4%), incubated with mouse anti-p65 NF-κB monoclonal antibodies for 1 h (Santa Cruz Biotechnology, Inc.). Primary antibody was detected with fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse IgG (Santa Cruz Biotechnology, Inc.). Samples were viewed and photographed using a Zeiss Axiophot microscope.
Data were analyzed using Student's t test or ANOVA and Tukey's post hoc test. A value of p < 0.05 was considered significant.
We are grateful to Dr. N. Tonks and Dr. C. Desbois-Mouthon for various reagents. We thank Dr. A. Andalibi for critically reviewing this manuscript. This work is supported by grants from NIH HL070293, DC004562, and DC005843 (to J.D. Li).
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