Transforming growth factor-β1 induces matrix metalloproteinase-9 and cell migration in astrocytes: roles of ROS-dependent ERK- and JNK-NF-κB pathways
© Hsieh et al; licensee BioMed Central Ltd. 2010
Received: 6 August 2010
Accepted: 6 December 2010
Published: 6 December 2010
Transforming growth factor-β (TGF-β) and matrix metalloproteinases (MMPs) are the multifunctional factors during diverse physiological and pathological processes including development, wound healing, proliferation, and cancer metastasis. Both TGF-β and MMPs have been shown to play crucial roles in brain pathological changes. Thus, we investigated the molecular mechanisms underlying TGF-β1-induced MMP-9 expression in brain astrocytes.
Rat brain astrocytes (RBA-1) were used. MMP-9 expression was analyzed by gelatin zymography and RT-PCR. The involvement of signaling molecules including MAPKs and NF-κB in the responses was investigated using pharmacological inhibitors and dominant negative mutants, determined by western blot and gene promoter assay. The functional activity of MMP-9 was evaluated by cell migration assay.
Here we report that TGF-β1 induces MMP-9 expression and enzymatic activity via a TGF-β receptor-activated reactive oxygen species (ROS)-dependent signaling pathway. ROS production leads to activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun-N-terminal kinase (JNK) and then activation of the NF-κB transcription factor. Activated NF-κB turns on transcription of the MMP-9 gene. The rat MMP-9 promoter, containing a NF-κB cis-binding site, was identified as a crucial domain linking to TGF-β1 action.
Collectively, in RBA-1 cells, activation of ERK1/2- and JNK-NF-κB cascades by a ROS-dependent manner is essential for MMP-9 up-regulation/activation and cell migration induced by TGF-β1. These findings indicate a new regulatory pathway of TGF-β1 in regulating expression of MMP-9 in brain astrocytes, which is involved in physiological and pathological tissue remodeling of central nervous system.
Matrix metalloproteinases (MMPs) are a large family of zinc-dependent endopeptidases that play an important role in the turnover of extracellular matrix (ECM) and function in physiological and pathological processes . In the central nervous system (CNS), MMPs, and MMP-9 especially, are implicated in development, morphogenesis, wounding healing, neurite outgrowth, and immune cell migration . In addition, they also participate in the pathogenesis of several CNS diseases such as stroke, Alzheimer's disease, neuroinflammation, and malignant glioma . Among members of the MMP family, MMP-9 has been shown to be elevated in various brain disorders [4–6]. Moreover, several pro-inflammatory mediators such as interleukin-1β (IL-1β), lipopolysaccharide, bradykinin (BK), and oxidized low-density lipoprotein (oxLDL) can induce MMP-9 expression and activity in cultured rat astrocytes [7–10], indicating that the expression and activation of MMP-9 may be regulated during brain injuries and inflammation.
Transforming growth factor-β (TGF-β) is a multifunctional cytokine that regulates a broad diversity of physiological and pathological processes, including tissue wound healing, inflammation, cell proliferation, differentiation, migration, and extracellualr matrix (ECM) synthesis [11–13]. Accordingly, TGF-β family members play an important role in early embryogenesis and in the homeostasis of adult tissues. However, several lines of evidence show that lack of coordination of TGF-β-dependent signaling often leads to a number of human diseases, including fibrosis [14, 15], cancer [16, 17], and autoimmune diseases . Moreover, TGF-β is a key immune system modulator, TGF-β1 especially, that may have both pro- and anti-inflammatory effects in immune system depending on the cell type (11-13). Within the CNS, all three isoforms of TGF-βs family, i.e. TGF-β1, -β2, and -β3, are produced by both glial and neural cells . Previous reports have suggested a relationship between increased TGF-β1 levels and cerebral ischemic injury [20, 21]. Following CNS injury, elevated TGF-β levels in astrocytes has been proven to be associated with astrocytic scar formation . Emerging evidence has also demonstrated that TGF-β1 is a crucial mediator in the pathogenesis of several CNS disorders, such as in organization of glial scars in response to injury and in several neurodegenerative disorders [11, 15, 23].
TGF-βs binds to two serine/threonine kinase receptors which consist of TGF-βRI and TGF-βRII. When a ligand binds, TGF-βRII phosphorylates TGF-βRI and activates Smad-dependent intracellular signaling pathways and thus leads to expression of several genes [24–26]. In addition to activation of Smad-dependent pathways, TGF-β can affect several signal transduction pathways in a Smad-independent manner, such as mitogen-activated protein kinases (MAPKs), including extracellular-signal-related protein kinase (ERK1/2), p38 MAPK, and c-Jun N-terminal kinase (JNK) [12, 25, 27]. In human gingival and skin fibroblasts, both p38 MAPK and Smad3 cooperate in regulating TGF-β-induced MMP-13 expression, whereas ERK1/2 cooperates with Smad3 in regulating connective tissue growth factor expression [25, 28, 29]. Recently, increasing evidence has attributed the cellular damage in neurodegenerative disorders to oxidative stress that leads to generation of reactive oxygen species (ROS) that are responsible for brain inflammatory disorders and that have deleterious effects during CNS pathogenic processes [30–32]. TGF-β can stimulate ROS production, which participates in the expression of diverse genes, such as those for MMPs, in the processes of several human diseases like lung fibrosis [33, 34]. However, very little information is available concerning the intracellular pathways involved in the effects of TGF-β1 in brain cells.
Recently, several studies have shown that TGF-β1 can up-regulate MMP-9 expression and activity in several cell types such as human skin  and corneal epithelial cells , implying a crucial role of TGF-β1 in the regulation of MMP-9 in tissue remodeling and wound healing during physiological and pathological processes. The MMP-9 expression is regulated by various mechanisms such as transcriptional and translational regulation in MMP-9 synthesis. The promoter of MMP-9 has been characterized to possess a series of functional enhancer element-binding sites, such as nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), but not in MMP-2 promoter [37, 38]. In RBA-1 cells, our previous studies have demonstrated that IL-1β and BK can up-regulate MMP-9 expression via activation of NF-κB [39, 40]. However, the possibility of MAPKs and NF-κB activation and their roles in MMP-9 up-regulation and cellular function (migration) induced by TGF-β1 in astrocytes (RBA-1 cells) are poorly defined.
In this study, we investigated the molecular mechanisms and the functional responses underlying TGF-β1-induced MMP-9 expression in RBA-1 cells. These findings indicate that TGF-β1-induced MMP-9 expression via TGF-β receptors is mediated through a ROS-dependent activation of ERK1/2, JNK1/2, and NF-κB pathway, finally leading to cell migration in RBA-1 cells. These results suggest that TGF-β1-induced astrocytic MMP-9 up-regulation might play a key role in physiological and pathological brain tissue remodeling such as wound healing and scar formation.
DMEM/F-12 medium, fetal bovine serum (FBS), and TRIzol were from Invitrogen (Carlsbad, CA, USA). Hybond C membrane and enhanced chemiluminescence (ECL) western blotting detection system were from GE Healthcare Biosciences (Buckinghamshire, UK). Phospho-(Thr202/Tyr204)-ERK1/2, phospho-(Ser176/180)-JNK1/2, and phospho-(Ser536)-p65 antibody kits were from Cell Signaling (Danver, MA, USA). GAPDH antibody was from Biogenesis (Boumemouth, UK). All primary antibodies were diluted at 1:1000 in phosphate-buffered saline (PBS) with 1% BSA (Calbiochem). Actinomycin D, cycloheximide, SB431542, U0126, SB202190, SP600125, helenalin, and Bay11-7082 were from Biomol (Plymouth Meeting, PA, USA). Bicinchoninic acid (BCA) protein assay reagent was from Pierce (Rockford, IL, USA). TGF-β1 was from R&D Systems (Minneapolis, MN, USA). N-acetyl cysteine (NAC), enzymes, XTT assay kit, and other chemicals were from Sigma (St. Louis, MO, USA).
Rat brain astrocyte culture
RBA-1 cells were used throughout this study. This cell line originated from a primary astrocyte culture of neonatal rat cerebrum and naturally developed through successive cell passages . Staining of RBA-1 with the astrocyte-specific marker, glial fibrillary acid protein (GFAP), showed nearly 95% positive staining. In this study, the RBA-1 cells within 40 passages were used that showed normal cellular morphological characteristics and had steady growth and proliferation in the monolayer system. Cells were cultured and treated as previously described . Primary astrocyte cultures were prepared from the cortex of 6-day-old Sprague-Dawley rat pups as previously described . The purity of primary astrocyte cultures was assessed with the astrocyte-specific marker, GFAP, showing over 95% GFAP-positive astrocytes. The cells were plated on 12-well plates and 10-cm culture dishes for MMP gelatin zymography and RT-PCR, respectively. The culture medium was changed every 3 days.
MMP gelatin zymography
After TGF-β1 treatment, the culture medium was collected, mixed with equal amounts of non-reduced sample buffer, and electrophoresed on 10% SDS-polyacrylamide gels containing 1 mg/ml gelatin as a protease substrate. Following electrophoresis, gelatinolytic activity was determined as previously described . Mixed human MMP-2 and MMP-9 standards (Chemicon, Temecula, CA, USA) were used as positive controls. Because cleaved MMPs were not reliably detectable, only proform zymogens were quantified. When inhibitors were used, they were added 1 h prior to the application of TGF-β1. Treatment of RBA-1 cells with the pharmacological inhibitors alone had no significant effect on cell viability determined by an XTT assay (data not shown).
Total RNA extraction and RT-PCR analysis
For RT-PCR analysis of MMP-9 mRNA expression, total RNA was extracted from RBA-1 cells stimulated by TGF-β1 as previously described . The cDNA obtained from 1 μg total RNA was used as a template for PCR amplification. Oligonucleotide primers were designed based on Genbank entries for rat MMP-9 and β-actin. The following primers were used for amplification reaction: for MMP-9: 5'-AGTTTGGTGTCGCGGAGCAC-3' (sense), 5'-TACATGAGCGCTTCCGGCAC-3' (anti-sense); for β-actin: 5'-GAACCCTAAGGCCAACCGTG-3' (sense), 5'-TGGCATAGAGGTCTTTACGG-3' (anti-sense). The PCR amplification was performed in 30 cycles at 55°C, 30 s; 72°C, 1 min; 94°C, 30 s. PCR fragments were analyzed on 2% agarose 1X TAE gel containing ethidium bromide and their size was compared to a molecular weight markers. Amplification of β-actin, a relatively invariant internal reference RNA, was performed in parallel, and cDNA amounts were standardized to equivalent β-actin mRNA levels. These primer sets specifically recognize only the genes of interest as indicated by amplification of a single band of the expected size (754 bp for MMP-9 and 514 bp for β-actin) and direct sequence analysis of the PCR product.
Cell migration (wound healing) assay
RBA-1 cells were grown to confluence in 6 well plates and starved with serum-free DMEM/F-12 medium for 24 h. The monolayer cells were manually scratched with a pipette tip to create extended and definite scratches in the center of the dishes with a bright and clear field. The detached cells were removed by washing the cells once with PBS. Serum-free DMEM/F-12 medium with or without TGF-β1 was added to each dish as indicated after pretreatment with the inhibitors for 1 h. Images of migratory cells from the scratched boundary were observed and acquired at 48 h with a digital camera and a light microscope (Olympus, Japan). The images shown represent one of three individual experiments.
Preparation of cell extracts and western blot analysis
Growth-arrested RBA-1 cells were incubated with TGF-β1 at 37°C for the indicated time intervals. The cells were washed with ice-cold PBS, scraped, and collected by centrifugation at 45,000 × g for 1 h at 4°C to yield the whole cell extract, as previously described . Samples were denatured, subjected to SDS-PAGE using a 10% (w/v) running gel, and transferred to nitrocellulose membrane. Membranes were incubated overnight using an anti-phospho-ERK1/2, phospho-JNK1/2, phospho-p65, or GAPDH antibody. Membranes were washed with TTBS four times for 5 min each, incubated with a 1:2000 dilution of anti-rabbit horseradish peroxidase antibody for 1 h. The immunoreactive bands were detected by ECL reagents.
Measurement of intracellular ROS generation
The peroxide-sensitive fluorescent probe 2',7'-dichlorofluorescein diacetate (DCF-DA) was used to assess the generation of intracellular ROS  with minor modifications. RBA-1 cells in monolayers were incubated with RPMI-1640 supplemented with 5 μM DCF-DA for 45 min at 37°C. The supernatant was removed and replaced with fresh RPMI-1640 media before stimulation with TGF-β1. Relative fluorescence intensity was recorded over time (3 to 60 minutes) by using a fluorescent plate reader (Thermo, Appliskan) at an excitation wavelength of 485 nm and emission was measured at a wavelength of 530 nm.
Plasmid construction, transient transfection, and promoter activity assays
The dominant negative plasmids encoding ERK1 (ΔERK1), ERK2 (ΔERK2), p38 (Δp38), and JNK (ΔJNK) were kindly provided by Dr. K.L. Guan (Department of Biological Chemistry, University of Michigan, MI), Dr. J. Han (The Scripps Research Institute, La Jolla, CA, USA), and C.C. Chen (Department of Pharmacology, National Taiwan University, Taipei, Taiwan), respectively. The rat MMP-9 promoter was constructed as previously described  with some modifications. The upstream region (-1280 to +108) of the rat MMP-9 promoter was cloned into the pGL3-basic vector containing the luciferase reporter system. Introduction of a double-point mutation into the NF-κB-binding site (κB domain; GGAATTCC to GGAATTGG) to generate pGL-MMP-9-DκB (mt-κB-MMP-9) was performed using the following (forward) primer: 5'-GGGTTGCCCCGTGGAATTGGCCCAAATCCTGC-3' (corresponding to a region from -572 to -541). The underlined nucleotides indicate the positions of substituted bases. All plasmids were prepared by using QIAGEN plasmid DNA preparation kits. The MMP-9 promoter reporter constructs were transfected into RBA-1 cells using the Lipofetamine™RNAiMAX reagent according to the instructions of manufacture (Invitrogen, Carlsbad, CA). The transfection efficiency (~60%) was determined by transfection with enhanced EGFP. To assess promoter activity, cells were collected and disrupted by sonication in lysis buffer (25 mM Tris-phosphate, pH 7.8, 2 mM EDTA, 1% Triton X-100, and 10% glycerol). After centrifugation, aliquots of the supernatants were tested for luciferase activity using a luciferase assay system. Firefly luciferase activities were standardized to b-galactosidase activity.
Analysis of data
All data were estimated using GraphPad Prism Program (GraphPad, San Diego, CA, USA). Quantitative data were analyzed by one-way ANOVA followed by Tukey's honestly significant difference tests between individual groups. Data were expressed as mean ± SEM. A value of P < 0.05 was considered significant.
TGF-β1 induces de novo synthesis of MMP-9 and cell migration in RBA-1 cells
TGF-β1 induces MMP-9 expression and cell migration via a TGF-β type I receptor
TGF-β1-induced MMP-9 expression is mediated through ERK1/2
JNK1/2, but not p38 MAPK, is involved in TGF-β1-induced MMP-9 expression
Involvement of ROS-dependent ERK1/2 and JNK1/2 pathways in TGF-β1-induced MMP-9 expression
NF-κB is required for TGF-β1-induced MMP-9 expression and cell migration in RBA-1 cells
Involvement of NF-κB binding site in regulation of the rat MMP-9 promoter by TGF-β1
MMPs contribute to a wide range of biological activities in several CNS diseases, such as stroke, Alzheimer's disease, and malignant glioma . Among MMPs, MMP-9 expression and activation have been shown to be predominantly elevated by various brain injuries [4, 6], suggesting that MMP-9 may be a critical molecule in the degradation of ECM and in the pathophysiology of many brain diseases. Another gelatinase, gelatinase A (MMP-2, 72 kDa), is constitutively expressed and its expression is usually not inducible in several cell types including brain cells. Moreover, TGF-β and related peptides are simultaneously produced and released following injury to the human CNS [48, 57]. Despite an obviously important role of TGF-β in brain trauma and diseases, the processes by which TGF-β is implicated in astrocytic functions are not completely understood. A well-established rat astroglial cell line (RBA-1) is derived from dissociated cultures of normal neonatal rat brain tissues . According to various analyses in previous studies, the properties of RBA-1 cells are similar to those of normal astrocytes . Thus, we used a culture model of RBA-1 cells to investigate the mechanisms underlying TGF-β1-induced MMP-9 expression and cellular functional responses. These results suggest that in RBA-1 cells, activation of ROS-dependent ERK1/2 and JNK1/2 linking to NF-κB, mediated through a TGF-β receptor, is essential for TGF-β1-induced MMP-9 gene expression and cell migration. However, previous studies have demonstrated that MMP-2 can be up-regulated by some stimuli such as TGF-β, but usually participates in development of cancer including growth, invasion, and metastasis [25, 58].
Abnormal regulation of MAPKs might be implicated in several CNS disorders . Moreover, TGF-β1 has been reported to act as a multifunctional factor through activation of MAPK cascades in different cell types [19, 25, 34]. In the present study, we found that ERK1/2 and JNK1/2 are required for MMP-9 expression, since RBA-1 cells transfected with dominant negative ERK1 (ΔERK1), ERK2 (ΔERK2) or JNK (ΔJNK) plasmid led to down-regulation of MMP-9 (Figures 3E and 4E). These results are consistent with the MMP-9 expression and secretion through ERK1/2 in rat cortical astrocytes [8, 40, 59] and the induction of MMP-9 by oxidized low-density lipoprotein via ERK1/2 and JNK1/2 pathways in RBA-1 cells . Our results are consistent with MMP-9 expression through ERK1/2 in transformed keratinocytes . Previously, many reports have indicated that long-term activation of MAPKs may participate in regulating some cellular functions such as gene expression and cell survival [61, 62]. Consistent with these reports, our data show that TGF-β1 stimulated JNK1/2 phosphorylation with a maximal response observed within 4 h (Figure 4D), suggesting that long-term phosphorylation of JNK1/2 by TGF-β1 may play a sustained role in up-regulation of MMP-9 in RBA-1 cells. Moreover, we have also demonstrated that either p38 MAPK inhibitor SB202190 or dominant negative mutant (Δp38) have no effect on TGF-β1-induced MMP-9 expression (Figures 4A, C, and 4E). However, recent reports have also indicated that TGF-β-induced MMP-9 expression is mediated through activation of p38 MAPK, but not ERK1/2, in MCF10A human breast epithelial cells  and in human glioblastoma cells . The different results may be due to diverse cell types and experimental conditions.
ROS have been shown to exert a key role in the physiological functions and pathological processes [65–67]. In the brain, ROS also extend to the control of vascular tone which is tightly modulated by metabolic activity within neurons . Moreover, increasing oxidative stress (i.e. ROS production) by diverse stimuli can regulate the expression of inflammatory genes linked to pathogenesis of human CNS disorders [65–69]. Recently, increasing evidence attributes the cellular damage in neurodegenerative disorders such as AD to oxidative stress that is due to generation of free radicals implicated in brain inflammatory disorders [30, 32]. The effects of TGF-β on ROS generation have been reported to be involved in pathogenesis of tumor progression, connective tissue degradation, and lung disease [33, 34, 70]. In this study, we found that TGF-β1-induced MMP-9 expression is mediated through ROS generation, since pretreatment with ROS scavenger NAC significantly attenuated TGF-β1-induced responses (Figures 5A-C). The role of ROS in TGF-β1-induced ERK1/2 and JNK1/2 phosphorylation was further confirmed by pretreatment with NAC (Figure 5D), suggesting that ROS-dependent activation of ERK1/2 and JNK1/2 is involved in TGF-β1-induced MMP-9 expression in RBA-1 cells. Consistently, many reports have also shown that MAPKs are the down-stream signaling molecules regulated by ROS [34, 70]. In addition, we demonstrated that ROS participates in up-regulation of MMP-9 by direct exposure of RBA-1 cells to H2O2 (Figure 5F). Herein we are the first to establish that intracellular ROS generation contributes to up-regulation of MMP-9 induced by TGF-β1 in RBA-1 cells.
NF-κB is a well-known redox-regulated transcription factor for expression of genes induced by diverse stress signals, including mutagenic, oxidative, and hypoxic stresses associated with physiological and pathological events. Our results reveal that TGF-β1-induced MMP-9 expression via NF-κB phosphorylation, is mediated through ROS-dependent ERK1/2 and JNK1/2 cascades in RBA-1 cells (Figure 6). The requirement of NF-κB signaling for MMP-9 induction has been confirmed by in vitro and in vivo studies [40, 53], which demonstrate a relationship between MMP-9 expression and enhancing cell motility [9, 10] and tumor invasion . In RBA-1 cells and human U87 astrocytoma cells, ERK1/2 has been suggested to be necessary for NF-κB activation [40, 71]. In addition, accumulating evidence also indicates that TGF-β1-triggered urokinase up-regulation and promotion of invasion is mediated through an ERK1/2-dependent, but not p38 MAPK, activation of NF-κB in human ovarian cancer cells . Our previous study of RBA-1 cells has indicated that up-regulation of MMP-9 by BK is mediated through an ERK1/2-dependent NF-κB pathway . Recently, the JNK/NF-κB cascade has also been shown to participate in TGF-β1-induced MMP-9 expression in corneal epithelial cells . These data imply that different MAPK members are differentially involved in NF-κB activation in various cell types. These studies are consistent with our presented results in RBA-1 cells challenged with TGF-β1.
Cell migration is essential for the organization and maintenance of tissue integrity and plays a role in embryonic development, wound healing, inflammation, and invasiveness through ECM [73, 74]. It has been reported that ROS, MAPKs, and NF-κB are involved in MMP-9 up-regulation, which is crucial for regulating cell motility in different cell types [9, 56, 75, 76]. In this study, we demonstrated that TGF-β1-enhanced cell migration is mediated through up-regulation of MMP-9 protein and activity (Figure 1E) via TGF-β receptor and ROS-dependent NF-κB cascade (Figures 2C, 5E, and 6E). Moreover, to rule out the possibility of cell proliferation in TGF-β1-induced cell migration, hydroxyurea, an inhibitor of DNA synthesis , was used to prevent proliferation of astrocytes during the period of observation in the migration (wound healing) assay. Therefore, these results suggest that up-regulation of MMP-9 by TGF-β1 is essential for enhancing migration of RBA-1 cells.
In the study, we have demonstrated that TGF-β1 directly induces MMP-9 expression via TGF-β receptor, ROS-dependent activation of ERK1/2 and JNK1/2, and transcription factor NF-κB pathway, which results in the promotion of cell migration in RBA-1 cells. Based on observations from the literature and on our findings, Figure 8C depicts a model for the molecular mechanisms underlying TGF-β1-induced MMP-9 expression and migration of RBA-1 cells. These findings imply that TGF-β1 might play a critical role in the processes of wound healing and scar formation after brain injuries and diseases. Pharmacological approaches suggest that targeting MMP-9 and their upstream signaling components may yield useful therapeutic targets for the treatment of brain injury, tumors, and inflammatory diseases.
The authors appreciated Drs. K.L. Guan (Department of Biological Chemistry, University of Michigan, MI), J. Han (The Scripps Research Institute, La Jolla, CA, USA), and C.C. Chen (Department of Pharmacology, National Taiwan University, Taipei, Taiwan), for providing dominant negative mutants of ERK1 (ΔERK1), ERK2 (ΔERK2), p38 MAPK(Δp38), and JNK (ΔJNK), respectively. This work was supported by National Science Council, Taiwan; Grant numbers: NSC97-2321-B-182-007 and NSC98-2321-B-182-004 (CMY); NSC96-2320-B-182-009 and NSC98-2320-B-255-001-MY3 (HLH) and Chang Gung Medical Research Foundation, Grant number: CMRPD150313, CMRPD140253, CMRPD170492, CMRPD180371, CMRPD150253 (CMY) and CMRPF170023 (HLH).
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