Background
Pulmonary arterial hypertension (PAH) is a life-threatening illness characterized by increased pulmonary vascular resistance (PVR) following right heart dysfunction [
1]. Several changes in the diagnosis and management of this disease have been implemented by the National Institute of Health (NIH) registry since the 1980s [
2], but the outcome of this fatal disease although improved still remains poor [
3,
4]. Recent research revealed that the median survival of PAH was between 3 and 5 years [
5,
6].
The pathogenesis of PAH still remains elusive and there is general agreement that the endothelial dysfunction and pulmonary vascular remodeling appear to be the key prerequisite reasons for the initiation of the disease. Any stimuli leading to vascular endothelial injury, vasoconstriction, cell proliferation, proinflammatory, thrombogenic functions and vascular remodeling are likely to contribute to PAH [
7,
8]. The increase in PVR is progressive and finally leads to right heart failure and death. Many factors may be involved in this progressive process and an understanding of molecular mechanisms of PAH has given rise to numerous lines of research and important discoveries in the last decade. The presence of inflammatory cytokines and increased expression of growth and transcriptional factors are thought to contribute directly to further recruitment of inflammatory cells and proliferation of smooth muscle and endothelial cells resulting in increased PVR [
9]. ET-1, prostacyclin, TGF-β family and nitric oxide (NO) are closely related to pulmonary arterial smooth muscle cell (PASMC) proliferation [
10].
ET-1 is a 21-AA peptide which regulates vasoconstriction and proliferative responses in numerous cell types and recent findings have re-established the role of ET-1 in the pulmonary vascular remodeling process. ET-1 plasma levels are prominently increased in PAH patients and correlate with PVR and PAH [
10‐
13]. The concentration of ET-1 in pulmonary circulation correlated with the increased levels of PVR, as well as the severity of the structural abnormalities found in distal pulmonary arteries as measured by intravascular ultrasound [
14]. Factors that can affect the expression of ET-1 will also affect pulmonary vascular remodeling. TGF-β1 is one of the multifunctional peptides that regulate proliferation, differentiation and other functions in several cell types. Increased expression of TGF-β1 has been observed in PAH vessel and contribute to PASMC growth and collagen deposition [
15]. The effects of ET-1 leading to pulmonary vascular remodeling are enhanced by the presence of TGF-β1 in human PASMC [
16]. The pathophysiology of pulmonary hypertension differs according to the presence or absence of lung disease. Idiopathic pulmonary fibrosis (IPF) is associated with a high incidence of pulmonary hypertension [
17,
18]. Epithelial to mesenchymal transformation (EMT) of alveolar epithelial cells has been recognized as a potential contributor to IPF and TGF-β1 has a close relationship with EMT in A549 cells [
19,
20]. Previous studies suggested that TGF-β1-induced A549 cells undergo EMT via phosphorylation of Smad2 [
21,
22] and peroxisome proliferator-activated receptor gamma (PPAR-γ) ligands inhibited profibrotic changes in TGF-β1-stimulated cells [
23,
24]. PPAR-γ is a ligand-activated nuclear receptor which regulates the transcription of genes involved in adipogenesis, insulin sensitization, inflammation, as well as vascular remodeling [
25,
26]. Early research suggested that PPAR-γ activators inhibited oxidized low-density lipoprotein-induced induced ET-1 production in endothelial cells [
27]. The expression of PPAR-γ was reduced in the pulmonary tissue of rat models of this disease [
28] and pharmacological activation of PPAR-γ could effectively attenuate the upregulation of ET-1 signaling in mice or human pulmonary artery endothelial cells [
29].
The way in which TGF-β1 and PPAR-γ regulate the expression of ET-1 and what signaling pathways participate in this process remain unclear. We hypothesize that TGF-β1 can stimulate A549 cells to produce ET-1 and that while PPAR-γ may has some effects on this progress. We measured the effects of TGF-β1 and PPAR-γ on ET-1 expression and production in A549 cells by using RT-PCR, ELISA, western blot and confocal laser scanning microscopy (CLSM).
Methods
Cell culture
Human type II alveolar epithelial cell line A549 was purchased from the American Type Culture Collection (VR-15™). The cells were propagated in Roswell Park Memorial Institute 1640 (RPMI 1640) media (Gibco, USA) supplemented with 10 % fetal bovine serum (FBS; Hyclone,USA), 1 % penicillin/streptomycin (Solarbio, China) and maintained at 37 °C in a humidified atmosphere of 95 % air: 5 % CO2. Cells were subcultured every 3–4 days when 70–80 % confluence was reached. For all experiments, A549 cells were seeded (5,000 cells/cm2) into six-well plates for total RNA isolation, into ninety-six-well plates for ELISA experiments, or into 60-mm dishes for protein extraction. Experiments were repeated at three independent times and performed in triplicate each time. Experiments were performed when a monolayer of A549 cells achieved 70–80 % confluence and cells were serum-deprived for 24 h before the drug treatments.
Cell treatment and sample collection
The drug treatments consisted of 4 parts as follow: A. A549 cells were treated with TGF-β1 (Peprotech, USA) at 10 ng/mL, BMP-2 at 100 ng/mL, BMP-4 at 100 ng/mL and BMP-7 at 100 ng/mL respectively, bovine serum albumin (BSA) was used as a control. B. A549 cells were treated with SB203580 (Cell Signaling Technology, USA) and/or TGF-β1. The cells were pretreated with 10 μM SB203580 for 60 min before 10 ng/mL TGF-β1 stimulation while 0.1 % DMSO used was as vehicle control. C. A549 cells were treated with S2871 (Selleckchem, USA, USA) and/or TGF-β1. The cells were pretreated with 10 μM S2871 for 60 min before 10 ng/mL TGF-β1 stimulation while 0.1 % DMSO was used as vehicle control. D. A549 cells were treated with S2505 (Selleckchem, USA) and/or TGF-β1. The cells were pretreated with 10 μM S2505 for 60 min before 10 ng/mL TGF-β1 stimulation while 0.1 % DMSO was used as vehicle control.
The culture supernatants were harvested 12 h after drug treatment for ELISA measurements of ET-1. After incubated with agonists or antagonists for 2 h, total RNA was isolated from the cells by TRIzol Reagent (Invitrogen, USA) according to the manufacturer’s instructions. RNA integrity was checked electrophoretically and purity was quantified by using spectrophotometry. Cell lysate were collected with cell lysis buffer (P0013, Beyotime Biotechnology, China) with 1 μM phenylmethanesulfonyl fluoride (PMSF) at 0, 5, 10, 15 and 30 min after drug treatment respectively according to the experimental conditions for western blotting. Total protein concentration was measured via a spectrophotometer using the bicinchoninic acid (BCA) protein assay kit (P0010S, Beyotime Biotechnology, China) with BSA utilised as the protein standard. For immunofluorescence analysis by CLSM, the cells were fixed in 4 % paraformaldehyde for 30 min after TGF-β1 treatment for 15, 30 and 60 min respectively.
Real-time PCR
After incubations with agonists or antagonists for 2 h, total RNA was isolated from the cells by TRIzol Reagent and reverse transcription was performed on 1 μg RNA with oligo (dT) primers in 20 μL reactions by using the PrimeScriptTM RT reagent kit with gDNA Eraser (Perfect Real Time) (TAKARA BIO INC, RR047A, Japan) according to the manufacturer’s instructions. Gene expression of ET-1, PPAR-γ and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was evaluated using real-time PCR and SYBR Green I dye (Bio-Rad, Hercules, CA). The primers of ET-1, PPAR-γ and GAPDH were obtained from Invitrogen (Life Technologies, Shanghai). The sequence of primers for the amplification of ET-1, PPAR-γ and GAPDH were as follows: ET-1 (human) forward: 5-CCAATCTTGGAACAGTCTTTTCCT-3, reverse: 5-GGACATCATTTGGGTCAACACTCC-3; PPAR-γ (human) forward: 5-ACTCCCTCATGGCAATTGAATGTC-3, reverse: 5-ATACTCTGTGATCTCCTGCACAGCC-3; GAPDH (human) forward: 5-GAAGGTGAAGGTCGGAGT-3, reverse: 5-GAAGATGGTGATGGGATTC-3. After 15 min of initial activation at 95 °C, PCR was carried out for 40 cycles at 94 °C for 15 s and 56.5 °C (ET-1) or 58.0 °C (PPAR-γ) for 30s. GAPDH was performed simultaneously and used as the housekeeping gene. The threshold cycle (Ct) value was measured, and the comparative gene expression was calculated by 2
−△△Ct method as described previously [
30].
ET-1 ELISA
Confluent cells were incubated with serum-free RM1640 for 24 h prior to drug treatment. TGF-β1, SB203580, S2871 and S2505 were added into the culture supernatant for 12 h and the supernatant was harvested and stored at − 80 °C. For the SB203580 + TGF-β1, S2871 + TGF-β1 and S2505 + TGF-β1 groups, SB203580, S2871 and S2505 were added into the supernatant respectively for 60 min then incubated with TGF-β1 for 12 h. All assays were performed in triplicates and the ET-1 protein concentration in the supernatants was measured using human endothelin-1QuantiGlo ELISA kit (R&D Systems, USA) according to the manufacturer’s instructions.
Western Blotting
20 μg of total protein from each sample was separated by 10 % SDS polyacrylamide gels (SDS-PAGE). After electrophoresis, separated proteins were transferred onto the polyvinylidene fluoride (PVDF) membranes (Millipore, Billerica, MA, USA) and the membranes were blocked for 2 h at room temperature with 5 % BSA in TBST. Membranes were then probed with primary antibodies (1:10,000 for β-actin and 1:1000 for other antibodies) at 4 °C overnight. The antibodies used were phospho-p38 MAPK (Thr180/Tyr182) (D3F9) XP® Rabbit mAb (4511, Cell Signaling Technology), p38 MAPK (D13E1) XP® Rabbit mAb (8690, Cell Signaling Technology), phospho-Smad2 (Ser465/467) (138D4) rabbit mAb (3108, Cell Signaling Technology), Smad2 (D43B4) XP® Rabbit mAb (5339, Cell Signaling Technology), phospho-SAPK/JNK (Thr183/Tyr185) (81E11) rabbit mAb (4668, Cell Signaling Technology), phospho-NF-kB p65 (Ser536) (93H1) rabbit mAb (3033, Cell Signaling Technology), NF-kB p65 (C22B4) rabbit mAb (4764, Cell Signaling Technology) and beta-actin rabbit polyclonal antibody (#4967, Cell Signaling Technology). After washing with TBST, membranes were probed with secondary antibodies (anti-rabbit IgG, HRP-linked antibody, #7074, Cell Signaling Technology) for 2 h at room temperature. Immunoblots were visualised by enhanced chemiluminescence and the image of western blots was scanned by Quantity One software. Band densities were quantified with freeware image analysis software, NIH Image (National Institute of Health, Bethesda MD, USA). The results of phosphorylated protein were normalized against the intensity of the total protein in each sample. In some western blots beta-actin was used as a protein loading control.
Plasmid transfection
A549 cells were allowed to grow till about 80 % confluency and then transfected with PPAR-γ positive or negative plasmid. The glycerol bacterial samples containing plasmids of human PPAR gamma sequence or small interfering RNA (siRNA) were synthesized by Invitrogen (Life Technologies, Shanghai). The glycerol bacterial samples were amplified in Luria-Bertani (LB) medium with 0.5 % amikacin at 37 °C overnight and the plasmid DNA extraction was performed using endofree maxi plasmid kit (TIANGEN BIOTECH, Beijing). The plasmid DNA was quantified by using spectrophotometry. Transfection was performed using LipofectamineTM 2000 reagent (Invitrogen) following the manufacturer’s instruction. Cells were incubated at 37 °C for 24 h in a humidified incubator containing 95 % air: 5 % CO2 and then treated with 10 ng/mL TGF-β1. Detailed methods are provided in the online supplementary data.
Immunofluorescence analysis by confocal laser scanning microscopy (CLSM)
A549 cells were serum-deprived for 24 h before treatment after reaching 70–80 % confluence. TGF-β1 was added into the cultures at 10 ng/mL for 15, 30 and 60 min respectively. After treatment the culture supernatant was removed and the cells were washed with PBS 3 times and then fixed in 4 % paraformaldehyde for 30 min at room temperature, permeabilized by 0.1 % Triton X-100 in PBS for 20 min, blocked with 5 % BSA for 30 min. The cells were incubated with primary antibody of phospho-Smad2 rabbit mAb (3108, Cell Signaling Technology) at 4 °C overnight. After PBS washing for 3 times, secondary antibody (Alexa Fluor 488-labeled goat anti-rabbit IgG, Beyotime Inc.) was added to the cells and incubated in the dark at room temperature for 1 h. The cells were washed with PBS for 3 times and mounted with propidium iodide (PI)-containing mounting media (ZSGB-BIO, Beijing, China) for 5 min. After washing with PBS 3 times, the cells were observed and photographed by a confocal laser scanning microscope.
Statistical analysis
Data are presented as means ± standard deviation of multiple determinations. Statistical analysis was performed using SPSS for windows (version 16.0, Chicago, USA). Differences between multiple groups were compared using One-way repeated measures ANOVA. The difference between a control and a treatment group or between two treatment groups was compared by two-sample Student’s t-test where stated in the figure legends. The difference was considered statistically significant at P < 0.05.
Discussion
The pathogenesis of PAH still remains elusive, partly because many diseases can lead to PAH and multiple signaling pathways are implicated in this process. There is increasing evidence that the increased expression of ET-1 and the activation of MAPKs are linked to PAH [
10,
31]. Interaction of MAPK with TGF-β1 activated signaling cascades were associated with PAH [
15,
32]. The serum levels of TGF-β1 were significantly higher in patients with schistosomiasis-associated PAH compared with patients with schistosomiasis but without PAH [
33] and TGF-β induced ET-1 expression [
34‐
36]. PPAR-γ is a ligand-activated nuclear receptor and early research suggested that PPAR-γ activators suppressed the ET-1 production both in vitro and in vivo [
27,
29,
37]. The way in which TGF-β1 and PPAR-γ regulate the expression of ET-1 and what signaling pathways participate in this process remains unclear. The aim of our study was to investigate the interactions between PPAR-γ, TGF-β1 and ET-1 and the underlying biochemical mechanisms whereby this occurs in A549 cells.
We demonstrated that TGF-β1 could significantly promote the expression of ET-1 mRNA and protein in A549 cells, increase the phosphorylation status of P38 MAPK and Smad2. SB203580 pre-treated A549 cells, resulted in the inhibition of p38 MAPK transduction and nuclear transfer of Smad2 respectively and a suppressive effect on the TGF-β1-induced production of ET-1. These results suggest that both p38 MAPK and Smad2 are involved in TGF-β1 mediated release of ET-1 by A549 cells. These results are consistent with previous studies suggesting that TGF-β1 strongly stimulated the synthesis and secretion of ET-1 in endothelial cells [
36,
38], pancreatic stellate cells [
35] and lung fibroblasts [
34]. TGF-β signaling should be a complex phenomenon and several studies with cell and animal models have reported that TGF-β induced ET-1 expression is mediated through the ALK5/Smad3 and p38 MAPK pathways [
36,
39]. TGF-β increased Smad2 and Smad3 phosphorylation and promoted Smad hetero-complex formation and nuclear accumulation in myofibroblasts [
40]. Our results demonstrate that TGF-β1, 100 ng/mL BMP-2 and 100 ng/mL BMP-7 increased the relative ET-1 mRNA levels respectively in A549 cells but only the TGF-β1 treatment significantly increased the levels of ET-1 protein expression. There may be several reasons as to why BMP-2 and BMP-7 could not significantly increase the expression of ET-1 protein as well as ET-1 mRNA. The process from ET-1 mRNA to protein expression included translation, post-translational processing and secretion to certain tissues, and multiple factors participate in this process. Any alterations in the various steps in this process would lead to the inconsistencies between mRNA and protein levels while the result of protein expression is probably more reliable index. We investigated the possible pathways by western blot analysis and the results showed that TGF-β1 allowed increases in protein phosphorylation of NF-kB p65, JNK/SAPK but the inhibitors of these pathways could not suppress the ET-1 protein expression when the results of ELISA were considered. It was also found that TGF-β1 allowed increases in protein phosphorylation of p38 MAPK and Smad2 and that SB203580 can effectively inhibit the expression of phspho-p38 MAPK and the nuclear transfer Smad2. Therefore our results strongly suggest that both p38 MAPK and Smad2 were involved in the release of ET-1 mediated by TGF-β1.
Moreover, we have demonstrated that transfecting cells with PPAR-γ siRNA plasmid or pre-treating cells with S2871 increased the expression of phospho-p38 MAPK and phospho-Smad2 and enhanced the expression of ET-1 mRNA and protein mediated by TGF-β1. However, transfecting cells with PPAR-γ plasmid or pre-treating cells with S2505 decreased the expression of phospho-P38 MAPK and phospho-Smad2 and suppressed the expression of ET-1 mRNA and protein mediated by TGF-β1. These results suggest the TGF-induced ET-1 release was dependent on PPAR-γ. Others have shown that PPAR-γ agonist rosiglitazone attenuated ET-1-induced pulmonary vasoconstriction in Sprague-Dawley rats [
37,
41‐
43] and inhibited the activation of p38 MAPK in murine macrophages [
44]. Rosiglitazone increased expression of PPAR-γ mRNA and also suppressed the expression of ET-1 mRNA in animal models [
28,
45]. When human pulmonary artery endothelial cells were transfected with PPAR-γ siRNA and this was shown to reduce PPAR-γ protein and increased ET-1 protein [
46].
In this study, we showed that TGF-β1 promoted the synthesis and secretion of ET-1 in A549 cells, increased the expression of phospho-p38 MAPK and phospho-Smad2 and induced an increase in nuclear transfer of Smad2. SB203580 suppressed the activation of MAPK P38 signal pathway, nuclear transfer of Smad2 and the expression of ET-1. Inhibition of PPAR-γ or PPAR-γ gene silencing suppressed the expression of PPAR-γ mRNA and increased the expression of ET-1 mRNA and protein mediated by TGF-β1 and also increased the expression of phospho-P38 MAPK and phospho-Smad2. However, activation of PPAR-γ or PPAR-γ gene over-expression suppressed the expression of ET-1 mRNA and protein mediated by TGF-β1 and increased the expression of phospho-p38 and phospho-Smad2. These results suggest that the release of TGF-induced ET-1 depended on PPAR-γ through the p38 MAPK and Smad2 pathways. PPAR-γ could, therefore, be considered as a potential therapeutic target for PAH. However, in the present study, we investigated only the interaction between PPAR-γ, TGF-β1 and ET-1 in vitro. Therefore, there is a need to further study the relationship between PPAR-γ, TGF-β1 and ET-1 in vivo and to investigate the anti- vasoconstriction effect of PPAR-γ and its potential role in PAH treatment.
Conclusions
In conclusion, this study shows that TGF-β1-induced production of ET-1, increased phosphorylation of p38 MAPK and Smad2 nuclear transfer could be prevented by using SB20308 and regulated by PPAR-γ, suggested that p38 MAPK and Smad2 signaling transduction is involved in TGF-β1 induced ET-1 release and that this process was PPAR-γ dependent in A549 cells.
Abbreviations
CLSM, confocal laser scanning microscopy; ECM, extracellular matrix; ELISA, enzyme-linked immunosorbent assay; ET-1, endothelin-1; mAb, monoclonal antibody; PAH, pulmonary arterial hypertension; PASMC, pulmonary arterial smooth muscle cell; PPAR-γ, peroxisome proliferator-activated receptor gamma; PVR, pulmonary vascular resistance; TGF-β1, transforming growth factor beta 1
Acknowledgements
This study was supported by grants from the National Natural Science Foundation of China, Nos. 81160013, 81060007 and 81560321, Science and Technique Research Projects of Guangxi, No.1140003B-93 and the Key Programs of Natural Science Foundation of Guangxi, No.2011GXNSFD018039.