TLR2 and AP-1/NF-kappaB are involved in the regulation of MMP-9 elicited by heat killed Listeria monocytogenes in human monocytic THP-1 cells
© Shihab et al.; licensee BioMed Central. 2015
Received: 27 May 2014
Accepted: 9 March 2015
Published: 18 April 2015
MMP-9 is crucial for a normal immune response, but excessive release of this enzyme leads to severe tissue damage. Listeria monocytogenes (LM) is an opportunistic food-borne pathogen causing listerosis, meningitis and sepsis. Heat killed Listeria monocytogenes (HKLM) activates immune system and leads production of cytokines and chemokines. However, nothing is known about the involvement of HKLM in MMP-9 regulation. Therefore we investigated the role of HKLM in the regulation of MMP-9 gene expression in THP-1 cells.
Commercially available heat killed Listeria monocytogenes was used in this study. HKLM-induced MMP-9 expression was assessed with quantitative real-time qPCR and ELISA. Action of HKLM in different signaling pathways were studied by using THP-1-XBlue™ cells (THP-1-cells with NF-κB/AP-1 reporter construct), THP-1-XBlue™-defMyD cells (MyD88−/− THP-1 cells), anti-TLR2 mAb and pharmacological inhibitors. Phospho and total proteins were determined by Western blotting.
Increased MMP-9 production (mRNA: 395-Fold; Protein: 8141 pg/ml; P < 0.05) was observed in HKLM stimulated THP-1 cells as compared to the un-stimulated THP-1 cells. This production of MMP-9 was completely abrogated by anti-TLR2 blocking mAb (P = 0.0024). Furthermore, THP-1-XBlue™-defMyD cells were unable to produce MMP-9 in response to HKLM. HKLM- induced activation of NF-kappaB/AP-1 was also observed in THP-1-XBlue™ Cells. In addition, inhibitors of JNK (SP600125), MEK/ERK (U0126; PD98056), p38 MAPK (SB203580) and NF-kappaB (BAY 11–7085, Triptolide and Resveratrol) significantly suppressed (P < 0.05) HKLM-stimulated MMP-9 expression.
Our results indicate that HKLM activates TLR2 and NF-κB/AP-1 signaling pathways, leading to up-regulation of MMP-9 production in THP-1 cells. Thus, MMP-9 could be an appropriate therapeutic target to stop severe tissue damage caused by infection or chronic inflammation.
Listeria monocytogenes is a Gram-positive foodborne pathogen that is widely distributed in nature, occurring in soil, water, various food products, animals, and humans . Infection by Listeria monocytogenes occurs almost exclusively after ingestion of contaminated food . Immunocompromised individuals, neonates, pregnant woman, elderly persons, and patients suffering from transplantation events are most susceptible to infections. Listeriosis causes invasive disease including septicemia and meningitis . Although the listeriosis incidence is low, the high mortality rates (about 24%) due to septicemia and meningitis make L. monocytogenes one of the most deadly human food-borne pathogens . Immediate immune responses are triggered during LM infection. Innate immunity to LM is mediated via toll like receptors or nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) . Toll-like receptors (TLRs) have been shown to play an important role in the host’s innate immune responses to microbial infections through the induction of proinflammatory cytokines, chemokines, and type I interferons by macrophages and dendritic cells [6,7]. Member of the TLR family, namely TLR2 has been shown to be critical in the initiation of innate immune responses to LM infection in the mouse model [8,9]. Recognition of a microbial invasion through the TLRs triggers the activation of signaling pathways, resulting in the recruitment of several adaptor proteins to the TIR domain. However, myeloid differentiation factor 88 (MyD88) is a key adaptor protein which is common to almost all TLRs except TLR3 . MyD88 activates in turn IL-1 receptor–associated kinases (IRAK) family members and tumor necrosis factor-alpha receptor–associated factor 6 (TRAF6) [11,12]. These adaptor proteins have essential role in the activation of NF-κB and mitogen-activated protein kinase (MAPK) pathways [13-18]. NF-kappaB and AP-1 transcription pathways are involved in the regulation of inflammatory mediators that trigger the migration of the inflammatory cells into the tissue. Inflammatory cells migration into tissues is dependent on several events including adherence to endothelial cells and penetration through the vessel wall into the extracellular matrix [19-21].
Matrix metalloproteinases (MMPs) form a family of zinc-containing proteases that degrade all extracellular matrix components and have an important role in tissue remodeling and immunomodulatory functions [22,23]. As gelatin is a major component of extra- cellular matrix (ECM) and in view of their collagen type IV-specific degradation capacity, MMP-9 plays a key role in ECM breakdown. MMP-9 is predominantly secreted by monocytes which are central cells in developing immune response to infection. The production of MMP-9 by monocytes is of interest in the context of facilitating leukocyte infiltration into infected sites through degrading type IV collagen in vascular basement membranes . MMP-9 production is tightly controlled at the level of gene transcription and its unrestricted release/activity may contribute to host tissue damage during infection. Elevated levels of MMP-9 were found in different inflammatory and infectious diseases [25-28]. MMP-9 gene expression in monocytic cells is regulated by different cytokines, including TNF-alpha IL-1beta, IL-18 and microbial components [29-31].
Previous work has shown that Heat killed listeria monocytogenes (HKLM) activates immune system by regulating the expression of cytokines (IL-1β, IL-6, IL-8, IL-12 and TNFα and chemokines [32-34]. However, nothing is known about the regulation of MMP-9 by HKLM in monocytic cells. In this study we therefore looked at the influence of HKLM on monocyte production of MMP-9. We show that HKLM induces MMP-9 in the monocytic cell line THP-1 via activation of MAPK and NF-kappaB. MMP-9 secretion was blocked by neutralizing TLR2. MyD88−/− cells abrogate the HKLM stimulated MMP-9 secretion.
Materials and methods
Cell culture and stimulation
Human monocytic leukemia cell line THP-1 was purchased from American Type Culture Collection (ATCC) and grown in RPMI-1640 culture medium (Gibco, Life Technologies, Grand Island, USA) supplemented with 10% fetal bovine serum (Gibco, Life Technologies, Grand Island, NY, USA), 2 mM glutamine (Gibco, Invitrogen, Grand Island, NY, USA), 1 mM sodium pyruvate, 10 mM HEPES, 100ug/ml Normocin 50 U/ml penicillin and 50 μg/ml streptomycin (P/S; (Gibco, Invitrogen, Grand Island, NY, USA), and incubation at 37°C (with humidity) in 5% CO2. THP-1-XBlue cells stably expressing a secreted embryonic alkaline phosphatase (SEAP) reporter inducible by NF-κB and AP-1 were purchased from InvivoGen (InvivoGen, San Diego, CA, USA). THP-1-XBlue cells show similar response to HKLM purchased from InvivoGen, (San Diego, CA, USA) as THP-1 cells. All the experiments were performed with THP-1-XBlue cells at a cell density of 1 × 106 /ml in 12-well plates. THP-1-XBlue™-defMyD cells (cells deficient in MyD88 activity; MyD88−/− THP-1 cells) were also purchased from InvivoGen (InvivoGen, San Diego, CA, USA). THP-1-XBlue cells were cultured in complete RPMI medium with the addition of zeocin (200 μg/ml) (InvivoGen, San Diego, CA, USA) to select for cells expressing the SEAP -NF-κB/AP-1 reporter. THP-1-XBlue™-defMyD cells were cultured in complete RPMI medium with the addition of Zeocin (200ug/ml) and HygroGold (100ug/ml) (InvivoGen, San Diego, CA, USA).
Prior to stimulation, THP-1 cells were transferred into normal medium and plated in 12-well plates (Costar, Corning Incorporated, Corning, NY, USA) at 1 × 106 cells/well cell density unless indicated otherwise. In dose–response experiment, the following HKLM concentrations were used to stimulate THP-1 cells: 1-9×107 particles/ml. In subsequent experiments, the optimal (non-cytotoxic) concentration of HKLM (3×107 particles/ml) or TNF-alpha (25 ng/ml) were used to stimulate cells for 24 hr at 37°C. Cells were harvested for RNA isolation and conditioned media were collected for measuring MMP-9 secretion levels and SEAP activity. Conditioned media were collected and stored at −80°C.
Quantification of NF-κB/AP-1 activity
THP-1 XBlue cells (InvivoGen, San Diego, CA) are THP-1 cells stably transfected with a reporter construct, expressing a secreted embryonic alkaline phosphatase (SEAP) gene under the control of a promoter inducible by the transcription factors NF-κB and AP-1. Upon stimulation, NF-κB and AP-1 are activated and subsequently the secretion of SEAP is promoted. Levels of SEAP were detected in the conditioned media after 4 hr incubation of supernatants with Quanti-Blue medium (InvivoGen, San Diego, CA, USA) at 650 nm wave length by ELISA reader.
Real time quantitative RT-PCR
Total RNA was extracted using RNeasy Mini Kit (Qiagen, Valencia. CA, USA). The cDNA was synthesized using 1 μg of total RNA using high capacity cDNA reverse transcription kit (Applied Biosystems, Foster city, CA, USA). 50 ng cDNA was used in each real-time PCR reaction. For real-time polymerase chain reaction (PCR), complementary DNA was amplified with Inventoried TaqMan Gene Expression Assay products (MMP-9: Hs00234579_m1; GAPDH: Hs03929097_g1) containing two gene-specific primers and one TaqMan MGB probe (6-FAM dye-labeled) using a TaqMan® Gene Expression Master Mix (Applied Biosystems, Foster city, CA, USA) in a 7500 Fast Real-Time PCR System (Applied Biosystems, Foster City, CA, USA). The mRNA levels were normalized against GAPDH mRNA and the amounts of MMP-9 mRNA relative to control were calculated with ΔΔCt-method . Relative mRNA expression was expressed as fold expression over average of control gene expression. The expression level in control treatment was assumed to be 1. Values are presented as mean ± SEM. Results were analyzed statistically; P < 0.05 was considered significant.
ELISA for secreted MMP-9 and TIMP-1 in cell culture supernatants
Concentrations of MMP-9 and TIMP-1 in cell culture supernatants were measured using sandwich ELISA according to the manufacturer’s instructions (R&D systems, Minneapolis, USA).
Cellular lysates were prepared as described previously [7,36]. Briefly THP-1 cells were incubated for 30 min with lysis buffer (Tris 62.5 mM (pH 7.5), 1% Triton X-100, 10% glycerol). The lysates were then centrifuged at 14000 rpm for 10 min and the supernatants were collected. Protein concentration in the lysates was measured by Quickstart Bradford Dye Reagent, 1x Protein Assay kit (BioRad Laboratories, Inc, CA). Protein (20 μg) samples were mixed with sample loading buffer, heated for 5 min at 95°C and were resolved on SDS-12% SDS-PAGE and transferred to immobilon polyvinyldifluoride (PVDF) membranes (Bio-Rad Laboratories, USA) by electro blotting. The blots were blocked with 5% non-fat milk in PBS at room temperature and then probed with rabbit anti-human antibodies against p-MEK1/2, pERK1/2, p-JNK, p-p38, p-c-jun, p-IKKα/β, p-IKB, p-NF-kappaB and Beta Actin in 1:1000 dilution at 4°C overnight. All the primary antibodies were purchased from Cell Signalling (Cell Signalling Technology, Inc). The blots were then washed three times with TBS and incubated for 2 h with HRP-conjugated secondary antibody (Promega, Madison, WI, USA). Immunoreactive bands were developed using an Amersham ECL Plus Western Blotting Detection System (GE Health Care, Buckinghamshire, UK) and visualized by Molecular Imager ® VersaDoc™ MP Imaging Systems (Bio-Rad Laboratories, Hercules, CA, USA).
Statsitical analysis was performed using GraphPad Prism software (La Jolla, CA, USA). Data are shown as mean ± standard deviation values, unless otherwise indicated. Unpaired Student t-test was used to compare means between groups. In all cases, P value < 0.05 was considered significant.
HKLM induces MMP-9 production in THP-1 cells
Studies have revealed an essential role of NF-kappaB and AP-1 activation in MMP-9 secretion in different cell types by several external stimuli [38,39]. To investigate the involvement of NF-kappaB and AP-1 in HKLM induced MMP-9 gene expression, the THP-1 X-blue cells, expressing a reporter gene SEAP driven by NF-kappaB and AP-1 response elements, were treated with HKLM or TNF-alpha. Elevated SEAP activity (NF-kappaB/AP-1 activation; P = 0.0018) was determined in the condition media obtained from THP-1 cells treated with HKLM as compared to unstimulated cells (Figure 1F).
Involvement of TLR2 in HKLM induced MMP-9 production
Role of MyD88 in HKLM induced MMP-9 production
MAPK and NF-kappaB signaling pathways are involved in HKLM-induced MMP-9 upregulation
MMP-9 is essential for normal physiological conditions but its increase in production could be involved in the pathogenesis of various diseases such as chronic inflammation, tumor cell metastasis, arthritis, obesity and in the progression of various infectious diseases ([25-28]. Also, MMP involvement in the pathogenesis of central nervous system diseases have been reported. A study reported the presence of Listeria monocytogenes antigens and MMP-7/9 in sheep brains . Previously, we studied the MMP-9 production induced by FSL-1, a synthetic lipoprotein derived from Mycoplasma salivarium ; whereas in this study, we have demonstrated that MMP-9 was up-regulated in monocytic cells upon exposure to Heat killed Listeria monocytogenes (HKLM). This MMP-9 up-regulation requires TLR2/MyD88-dependent activation of NF-κB/AP-1. Our study shows that HKLM mediated induction of MMP-9 in monocytic cells is due to HKLM interaction with surface receptor on the target cells. The interaction of different bacteria with TLRs has been shown to be decisive in the outcome of immune responses and bacterial pathogenesis. For instance, TLR2 serves as the signaling receptor for molecules such as LP, LTA, and PGN. PGN, a cell wall component of gram positive bacteria, induces IL-6 and MMP-9 gene expression in microglia and neurtophils, respectively 9 [47,48]. Other studies have also shown that macrophages from TLR2-deficient mice lost the ability to secrete the inflammatory cytokines TNF-alpha and IL-6 in response to bacterial component from gram-positive bacteria , which suggests that interaction of the TLR on the cells with the bacterial component is responsible, in part, to induce immune response in the host. In the present study, to demonstrate that TLR2 participates in the induction of MMP-9 by HKLM, we achieved no cellular responses to the HKLM after neutralization of TLR2 on THP-1 cells. Previous studies have shown that the cytokine production induced by gram positive bacteria in cells of the monocytic linage depends on TLR2 stimulation [50,51].
Recognition of a microbial invasion through the TLRs triggers the recruitment and activation of several adaptor proteins to the TIR domain. MyD88 is a key adaptor protein and is common to almost all TLRs except TLR3 . The involvement of MyD88 is well known in the induction of various inflammatory mediators. Thus, we also suggested that inducing effect of HKLM on MMP-9 was blocked in MyD88 deficient cells. It is noteworthy in this regard that the role of MyD88 has provided a further evidence for the involvement of the TLR2 in this induction of MMP-9. The interaction of MyD88 with the TLR receptor promotes the recruitment of other adaptor proteins that in turn activates downstream kinases including NF-κB–inducing kinase (NIK), IKKα/β/γ, and mitogen-activated protein kinases (MAPKs) [13-18]. In the classical pathway, activated IKKβ which is part of an IKKα/β/γ complex, phosphorylates IKBα or IKBβ leading to their proteosomal degradation. As a result, NF-κB gets activated. In case of TLRs, che classical NF-κB pathway gets activated. Whereas in case of lymphoid development, the alternate pathway is involved in which NIK activates IKKα which phosphorylates NF-κB2 . These signaling cascades thus activate multiple transcription factors such as NF-kappaB and activator protein 1 (AP-1) . Indeed, previous studies indicate that AP-1 and NF-kappaB have binding sites in the MMP-9 promoter region and play important role in MMP-9 gene regulation . Therefore, we show the involvement of NF-κB/AP-1 activation in HKLM-induced MMP-9 gene expression in THP-1 cells. This is also confirmed by our observations that neutralization of TLR2 and deficiency of MyD88 blocked NF-κB/AP-1 activation along with the inhibition of MMP-9 expression. As NF-kappaB and MAPK pathways have been extensively studied downstream of various inflammatory stimuli. It is well established that stimulation of monocytes/macrophages by microbial components induces phosphorylation of p38, ERK1/2, and c-Jun NH2-terminal kinase (JNK). MAPKs are activated largely by bacterial products through TLRs and participated in the inflammatory response induced by TLR2 activation in monocytes/macrophages [53,55]. Our results showed that HKLM induced phosphorylation of p38, ERK1/2, and c-Jun NH2-terminal kinase (JNK) are involved in the regulation of MMP-9 gene expression. Moreover, this is confirmed by our findings that HKLM induced MMP-9 was reduced by inhibition of MEK/ERK, JNK and p38. The involvement of MEK/ERK as well as that of other kinases (JNK and p38) in MMP-9 expression has been reported in other cell systems [54,56]. Since our data shows that NF-kappaB signaling pathways are also very effective in the regulation MMP-9 along with MAPK signaling pathways. Thus, we established that HKLM induced phosphorylation of the IKKα/β, IKB and NF-kappaB. Furthermore, we found that effect of HKLM on MMP-9 regulation was suppressed in the cells treated with inhibitors of NF-kappaB signaling pathways. Our results suggested that signaling pathways induced by HKLM that regulate MMP-9 expression in monocytic cells is also dependent on NF-kappaB signalling pathways. Many studies have revealed an essential role of NF-kappaB and AP-1 activation in MMP-9 secretion has been revealed in different cell types by several external stimuli [38,39]. A few studies support that NF-kappaB and AP-1 transcription factors are regulated by the similar intracellular signal transduction cascades [57-60]. For example, in the activation of JNK by inflammatory or stress stimuli and the nuclear traslocation of NF-kappaB, the simultaneous activation of NF-kappaB and AP-1 suggest that these transcription factors work cooperatively . Another indication of the interaction between AP-1 and NF-kappaB activation pathways comes from the studies showing that the MAPK pathway activation leads to activation of JNK and IkB kinase complexes . This cooperative interaction between AP-1 and NF-kappaB is further supported by the presence of a scaffold protein which was shown to be involved in the activation of JNK pathways and NF-kappaB nuclear translocation . From these studies, taken together, there is possibility that NF-kappaB and AP-1 can modulate the activity of each other.
In summary, we have shown that TLR2 regulates the expression of MMP-9 in THP-1 cells in response to HKLM by multiple cooperative mechanisms. In particular, we have identified HKLM-mediated activation of MAPK, AP-1 and NF-κB signaling pathways as critical steps for transcriptional up-regulation of MMP-9.
This study was financially supported by Kuwait Foundation for the Advancement of Sciences (KFAS). We thankfully acknowledge the support by Mr. Azadali K. Moorji and the staff of the Tissue Bank Core Facility.
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