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Published ahead of print on January 26, 2006, doi:10.1165/rcmb.2005-0309OC
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American Journal of Respiratory Cell and Molecular Biology. Vol. 34, pp. 746-753, 2006
© 2006 American Thoracic Society
DOI: 10.1165/rcmb.2005-0309OC

Fibroblast Growth Factor 2 and Transforming Growth Factor beta1 Synergism in Human Bronchial Smooth Muscle Cell Proliferation

Ynuk Bossé, Charles Thompson, Jana Stankova and Marek Rola-Pleszczynski

Immunology Division, Department of Pediatrics, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, Québec, Canada

Correspondence and requests for reprints should be addressed to Dr. Marek Rola-Pleszczynski, Department of Pediatrics, Immunology Division, Faculty of Medicine, Université de Sherbrooke, 3001 North 12th Avenue, Sherbrooke, PQ, J1H 5N4 Canada. E-mail: marek.rola-pleszczynski{at}usherbrooke.ca


   Abstract
Top
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Bronchial smooth muscle cell (BSMC) hyperplasia is a typical feature of airway remodeling and contributes to airway obstruction and hyperresponsiveness in asthma. Fibroblast growth factor 2 (FGF-2) and transforming growth factor beta1 (TGF-beta1) are sequentially upregulated in asthmatic airways after allergic challenge. Whereas FGF-2 induces BSMC proliferation, the mitogenic effect of TGF-beta1 remains controversial, and the effect of sequential FGF-2 and TGF-beta1 co-stimulation on BSMC proliferation is unknown. This study aimed to assess the individual and sequential cooperative effects of FGF-2 and TGF-beta1 on human BSMC proliferation and define the underlying mechanisms. Mitogenic response was measured using crystal violet staining and [3H]-thymidine incorporation. Steady-state mRNA and protein levels were measured by semiquantitative RT-PCR, Western blot, and ELISA, respectively. TGF-beta1 (0.1–20 ng/ml) alone had no effect on BSMC proliferation, but increased the proliferative effect of FGF-2 (2 ng/ml) in a concentration-dependent manner (up to 6-fold). Two distinct platelet-derived growth factor receptor (PDGFR) inhibitors, AG1296 and Inhibitor III, as well as a neutralizing Ab against PDGFR{alpha}, partially blocked the synergism between these two growth factors. In this regard, TGF-beta1 increased PDGF-A and PDGF-C mRNA expression as well as PDGF-AA protein expression. Moreover, FGF-2 pretreatment increased the mRNA and protein expression of PDGFR{alpha} and the proliferative effect of exogenous PDGF-AA (140%). Our data suggest that FGF-2 and TGF-beta1 synergize in BSMC proliferation and that this synergism is partially mediated by a PDGF loop, where FGF-2 and TGF-beta1 upregulate the receptor (PDGFR{alpha}) and the ligands (PDGF-AA and PDGF-CC), respectively. This powerful synergistic effect may thus contribute to the hyperplastic phenotype of BSMC in remodeled asthmatic airways.

Key Words: airway remodeling • asthma • hyperplasia • PDGF • smooth muscle


   Introduction
Top
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Bronchial smooth muscle cell (BSMC) hyperplasia had been recognized almost a century ago as a feature of airway remodeling in patients with asthma (1). Since then, several groups of investigators have suggested that BSMC hyperplasia is the major histologic alteration contributing to bronchial hyperresponsiveness (2, 3). BSMC are also active participant in the inflammatory response during asthma exacerbation by their ability to synthesize and secrete a diverse array of mediators involved in recruitment, activation, and enhanced survival of inflammatory cells (4). Thus, elucidating the factors involved in BSMC hyperplasia is of major significance to the understanding of airway wall thickening and hyperresponsiveness, but is also likely relevant to the elaboration of improved therapeutics for the treatment of airway inflammation that arises in patients with asthma.

Numerous individual mediators have been shown to induce BSMC proliferation in vitro, including growth factors, proteases, spasmogenes, reactive oxygen species (ROS), and cytokines (5). Most of these mitogens are upregulated during asthma exacerbation, and the order in which the expression of certain growth factors increase after phlogogenic challenge have been documented. In this regard, Redington and coworkers (6) have reported a rapid (10 min) FGF-2 increase in bronchoalveolar lavage fluid (BALF) after a segmental allergen challenge, whereas TGF-beta1 upregulation was observed only 24 h after challenge (7).

Despite the lack of secretory signal peptide (8), the low-molecular-weight (LMW; 18 kD) cytoplasmic isoform of FGF-2 was found in the extracellular milieu (8, 9) and has been shown to support BSMC proliferation in vitro (10, 11). The airway epithelium is the main source of this LMW isoform in the lung (9), but inflammatory cells such as mast cells (12), macrophages (13), and eosinophils (14) can also secrete FGF-2. Its elevated expression in asthmatic airways has been confirmed by immunohistochemistry in humans (9, 14) and in a nonhuman primate model (15).

Increased TGF-beta1 expression in humans with asthma (7, 16–23) and in animal models of airway inflammation (24–26) has been extensively documented as well. Various cells in the airway could contribute to TGF-beta1 upregulation. However, its role in BSMC proliferation is still questionable. Some in vitro data reported antimitogenic activity (27–29), whereas other studies supported the opposite, reporting a mitogenic activity for this cytokine, acting by itself (29–33) or in synergy with other mitogens (31, 33).

The aim of the current study was to clarify the individual mitogenic effect of TGF-beta1, and to measure the combined effects of sequentially added FGF-2 and TGF-beta1 on the proliferation of human primary BSMC. Our results showed that TGF-beta1 had no significant mitogenic effect, but synergized with FGF-2– induced proliferation in a concentration-dependent manner. Moreover, the results suggested that part of this synergism involved an autocrine platelet-derived growth factor (PDGF) loop, where FGF-2 increased PDGF receptor {alpha} chain (PDGFR{alpha}) expression and TGF-beta1 increased production of PDGFR ligands (PDGF-AA and PDGF-C).


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Cell Culture
Human primary bronchial smooth muscle cells (BSMC; BioWhittaker, Inc., Walkersville, MD) were used for all the experiments. BSMC were derived from a 5-wk-old black male and 1-yr-old and 21-yr-old white females. All donors, had negative history of smoking and were free of pre-existing lung disease. Upon reception, cryopreserved cells were cultured in T-75 flasks in Smooth muscle Growth Medium (SmGM) (SmGM-2 Bulletkit; BioWhittaker) consisting of Smooth muscle Basal Medium (SmBM), 5% fetal bovine serum (FBS), and a mixture of growth factors, including fibroblast growth factor 2 (FGF-2; 2 ng/ml), epidermal growth factor (EGF; 0.5 ng/ml), and insulin (5 µg/ml), as well as a mixture of antibiotics, including Gentamicin (100 ng/ml) and Amphotericin B (0.1 ng/ml). Thawing, subculturing, and harvesting procedures were performed according to manufacturer's instructions. Experiments were performed with cells at the fourth passage.

Cell Proliferation: Crystal Violet Staining
Cells were subcultured into 96-well plates at 3,000 cells/well in a starvation medium, consisting of SmBM + 1% FBS, with or without FGF-2 (2 ng/ml). Cells were maintained in these conditions for 24 h before TGFbeta1 (PeproTech Canada, Inc., Ottawa, ON, Canada) or PDGF-AA (10 ng/ml, unless otherwise specified; PeproTech, Inc., Rocky Hill, NJ) stimulations. BSMC proliferation was measured 96 h after stimulation using the DNA staining property of crystal violet (Sigma, Oakville, ON, Canada). Briefly, cells were washed once with HBSS solution containing 2 mM CaCl2 and 10 mM Hepes, fixed 20 min with ethanol (70%) at –20°C, incubated 15 min in crystal violet dilution (1% wt/vol) at room temperature, washed six times under tap water, and dissolved in acetic acid (33%). Optical density (OD) was determined at 550 nm with an ELISA reader (Bio-Rad Laboratory, Inc., Hercules, CA).

To confirm the validity of crystal violet staining as a surrogate of cell proliferation in the BSMC cell line, preliminary tests were performed where different numbers of cells were cultured in a 96-well plate (from 1 to 11 thousands). Simple regression model demonstrated significant value (P < 0.001; data not shown) with a very high coefficient of determination (r2 = 0.997), indicating that the variance in one variable (OD) would predict almost all the variance in the other variable (number of cells). Based on the slope of the linear regression model, 0.01 unit of OD corresponds to 1,370 cells. Thus, the crystal violet staining method is a valid approach to quantify BSMC proliferation.

To investigate signaling pathways involved in BSMC proliferation, pharmacologic inhibitors were added 1 h before TGF-beta1 treatment at the following concentrations: phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 (10 µM), and Wortmannin (1 µM) (Biomol, Plymouth Meeting, PA); Src kinase inhibitors, PP1 and PP2 (10 µM) (Biomol); PDGF receptor tyrosine kinase inhibitor, tyrphostin AG1296 (10 µM, unless otherwise specified; Biomol) and PDGF receptor inhibitor III (Calbiochem, San Diego, CA); and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor, PD98059 (10 µM; Biomol). DMSO (0.1% unless otherwise specified) was used as the control vehicle. To investigate the contribution of an autocrine PDGF loop in the observed FGF-2– TGF-beta1 synergism, neutralizing mAb against human PDGFR{alpha} (R&D Systems, Minneapolis, MN) was also evaluated. Anti-PDGFR{alpha} was administered 1 h before TGF-beta1 treatment at concentrations of 0.2, 2, or 20 µg/ml.

DNA Synthesis: [3H]-Thymidine Incorporation
Thymidine incorporation assays were used to measure DNA synthesis. Cells were seeded in 96-well plates at a density of 3,000 cells/well in SmBM 1% FBS and stimulated thereafter at different intervals during a 5-d time-course. FGF-2 (2 ng/ml) and TGF-beta1 (10 ng/ml) treatment were administered alone or sequentially with a 24-h interval in their respective order. BSMC were pulsed with 2.5 µCi/ml of [methyl-3H]-thymidine (91 Ci/mmol; Amersham Biosciences, Piscataway, NJ) 4 h before being placed at –20°C until further processed. DNA of individual wells was then transferred onto a Whatman membrane using a cell harvester (Titertek Cell Harvester, Rockville, MD) and placed in scintillation vials for radioactivity quantification using a 1,215 Rackbeta II liquid scintillation counter (LKB Wallac, Turku, Finland).

RT-PCR
Semiquantitative RT-PCR was used to measure mRNA expression. After reaching confluence in 6-well plates, cells were starved for 24 h in SmBM 1% FBS before TGF-beta1 (10 ng/ml) stimulations in fresh medium. Cells were then harvested, centrifuged, and resuspended in TriPure solution (Roche Diagnostics Canada, Laval, PQ, Canada) to perform mRNA extraction as described by the manufacturer. To avoid DNA contamination and mRNA degradation, mRNA extracts were treated for 15 min with dexoxyRNase (DNase 1 from Amersham Biosciences) and RNase inhibitor (RNasin from Promega, Madison, WI) at 37°C. RT reactions were performed using 1 µg of mRNA extract and M-MLV reverse transcriptase kit (BioCan Scientific, Inc., Mississauga, ON, Canada) and PCR was performed with a Taq polymerase kit (New England BioLabs, Ltd., Pickering, ON, Canada). For all experiments, GAPDH mRNA was used as an internal housekeeping gene control. The PCR primers used were: PDGF-A forward, 5'-gaccaggacggtcatttacg-3'; PDGF-A reverse, 3'-cctcacatccgtgtcctctt-5'; PDGF-C forward, 5'-ttcagcaa caaggaacagaac-3'; PDGF-C reverse, 3'-ctgaagggggtagctctgaa-3'; PDGFR{alpha} forward, 5'-gaagctgtcaacctgcatga-3'; PDGFR{alpha} reverse, 3'-atcgaccaagtcca gaatgg-5'; tissue plasminogen activator (tPA) forward, 5'-cccagatcgagact caaagc-3'; tPA reverse, 3'-tggggttctgtgctgtgtaa-5'; GAPDH forward, 5'-gatgacatcaagaaggtggtgaa-3'; GAPDH reverse, 3'-gtcttactccttggaggcc atgt-5'.

Protein Determination: ELISA
Cells were allowed to reach confluence in 6-well plates in SmGM before being washed once in PBS and starved for 24 h in SmBM 1% FBS. TGF-beta1 (10 ng/ml) was then added in fresh starvation medium, and the conditioned medium (CM) was recuperated 24 h later. Protein quantification in the CM was determined by the human/mouse PDGF-AA Quantikine ELISA kit (R&D Systems).

Western Analysis
Cells reached confluence in 6-well plates in SmGM before being starved for at least 24 h in SmBM 1% FBS. FGF-2 stimulation was then initiated for 24 h by adding fresh medium in all wells. Cells were harvested as described above and lysed in RIPA solution (NaCl, 0.15M; Tris-HCl, 0.05M; Igepal 1%, vol:vol; Na-Deoxycholate 0.5%, vol:vol, SDS 0.1%, wt:vol; EDTA, 5 mM) containing the following inhibitors: aprotinin (2 µg/ml), leupeptin, (1µM), soybean trypsin inhibitor (10 µg/ml), AEBSF (0.1 mg/ml), NaF (10 mM), and Na3VO4 (1 mM). Lysates were subsequently exposed to reducing condition (2.86 M mercaptoethanol in the 4x loading buffer) and boiled for 3 min before being submitted to electrophoresis in 7% SDS polyacrylamyde gel and transferred overnight onto nitrocellulose membranes. After blocking (milk), membranes were immunoblotted with a mouse anti-human PDGFR{alpha} mAb (1 µg/ml; R&D Systems) and the secondary goat anti-mouse mAb conjugated with HRP (1:2,500; Amersham Biosciences, Baie D'Urfé, PQ, Canada) with appropriated washing after each step (TBS: 0.1% Tween 20, vol:vol). Thereafter, HRP substrate (ECL Western Blotting Detection Reagents; Amersham Biosciences) was added and protein amount revealed by exposure on ECL detecting film (Hyperfilm ECL; Amersham Biosciences). To control for equal loading, membranes were stripped (SDS 2%, wt:vol; mercaptoethanol, 100 mM; Tris, 50 mM, pH 6.8) for 20 min at 50°C and reblotted after similar procedures with a mouse anti-human vinculin mAb (1:1,000; mouse monoclonal anti-vinculin clone VIN-11–5 from Sigma-Aldrich, Oakville, ON, Canada) as the primary antibody.

Data Analysis
Results illustrated in the figures, as well as their statistical analysis, are compiled data obtained using BSMC derived from the 5-wk-old donor. Unless otherwise specified, raw data were used for statistical analysis. ANOVA was first performed to determine overall significance of differences among conditions tested and was followed by Fisher's PLSD test to specify which conditions significantly differed from each other. In RT-PCR analysis, ratios over GAPDH were standardized in Z score within each experiment before the data obtained for every experiment were compiled and analyzed as described above. In ELISA analysis, unpaired Student's t test was used to compare the protein expression levels between TGF-beta1–treated versus untreated cells. P <= 0.05 was arbitrarily considered sufficient to reject the null hypothesis.


   RESULTS
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
TGF-beta1 Synergizes in a Concentration-Dependent Manner with FGF-2 to Induce BSMC Proliferation
The proliferative effect of TGF-beta1, alone or 24 h after FGF-2 stimulation, in human primary BSMC was examined. Results illustrated in the figures are calculated data obtained using BSMC from the 5-wk-old donor. As shown in Figure 1A, increasing concentrations of TGF-beta1 on its own had no effect on BSMC proliferation. However, when FGF-2 (2 ng/ml) was added to the culture medium 24 h before TGF-beta1 stimulation, the latter induced a concentration-dependent increase in BSMC proliferation. At 10 ng/ml, TGF-beta1 induced a 5-fold increase in BSMC proliferation relative to FGF-2 (2 ng/ml) alone. Conversely, a fixed concentration of TGF-beta1 (10 ng/ml) also increased the mitogenic effect of increasing concentrations of FGF-2 (Figure 1B). Dose–response curves for FGF-2–TGF-beta1 proliferative synergism with the two other donors gave similar results, although the overall effect was smaller. At the concentrations used above (2 ng/ml for FGF-2 and 10 ng/ml for TGF-beta1), TGF-beta1 increased FGF-2–induced BSMC proliferation by 89 and 129% in cells from the 1-yr-old and the 21-yr-old white female donors, respectively (data not shown). Unless specified otherwise, the remaining experiments were performed with BSMC obtained from the 5-wk-old black male donor.


Figure 1
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  		Figure 1. TGF-beta1 and FGF-2 synergized in a concentration-dependent manner to induce BSMC proliferation. BSMC were seeded with a fixed (A) or increasing (B) concentration of FGF-2 (2 ng/ml) and stimulated 24 h later with increasing (A) or fixed (B) concentrations of TGF-beta1. Cell proliferation was measured 4 d after TGF-beta1 stimulation as described in MATERIALS AND METHODS. Points are means ± SEM of quadruplicate measurements of at least three independent experiments. In A, all concentrations over 0.25 ng/ml of TGF-beta1 are significantly higher compared with FGF-2 alone (P < 0.01). In B, the effect of TGF-beta1 is significantly higher compared with respective concentration of FGF-2 alone, starting at 0.25 ng/ml (P <= 0.01).

 
TGF-beta1 Increases and Prolongs DNA Synthesis Induced by FGF-2
[3H]-thymidine incorporation assays, as a surrogate of DNA synthesis and cell cycle activation, were also performed after FGF-2 and TGF-beta1 co-stimulation. As shown in Figure 2, the mitogenic effect of FGF-2 (2 ng/ml) peaked at 48 h and returned to baseline levels at 96 h after stimulation. As expected, TGF-beta1 (10 ng/ml) alone had no effect on DNA synthesis from 24–96 h after stimulation. However, TGF-beta1 increased (at 96 h), but most importantly prolonged (to at least 120 h), the mitogenic effect of FGF-2. Time-course experiments were also performed using crystal violet staining during five consecutive days after TGF-beta1 administration. As depicted in Figure 2B, FGF-2 alone increased BSMC proliferation, starting at Day 1 and peaking at Day 4. On the other hand, TGF-beta1 alone did not affect BSMC proliferation over baseline conditions (FBS 1%) at any particular day. However, when added after a 24-h pretreatment with FGF-2, TGF-beta1 strikingly increased FGF-2–induced proliferation, starting at Day 2 and continuing for at least 5 d after stimulation. Based on a standard curve and a calculated simple linear regression model obtained by plotting OD values with known quantity of BSMC seeded (see MATERIALS AND METHODS), the number of cells per well at Day 5 in this particular experiment was estimated at 5,192 for control, 5,155 for TGF-beta1, 15,467 for FGF-2, and 57,526 for the sequential FGF-2 and TGF-beta1 co-stimulation.


Figure 2
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  		Figure 2. TGF-beta1 prolonged mitogenicity when administered 24 h after FGF-2 treatment. (A) Cells were stimulated with FGF-2 (2 ng/ml) or TGF-beta1 (10 ng/ml) individually or sequentially at 24-h intervals in their respective order during a 5-d time-course experiment. DNA synthesis was evaluated with a 4-h pulse of [3H]-thymidine at the end of four consecutive days following TGF-beta1 administration. Bars are means ± SEM of quadruplicate measurements of at least three independent experiments. *P < 0.05 compared with baseline. (B) Five-day time-course experiment performed with crystal violet staining to assess BSMC proliferation. Bars are means of quadruplicate measurements of a single experiment.

 
Tyrosine Kinase Activity of PDGFR Is Involved in the FGF-2–TGF-beta1 Synergism
BSMC were treated with different pharmacologic inhibitors or the vehicle control DMSO (0.1%) for 1 h before TGF-beta1 administration. As shown in Figure 3A, the Src inhibitor PP1 abrogated the proliferative effect of FGF-2 and of FGF-2–TGF-beta1 co-stimulation. On the other hand, tyrphostin AG1296 (10 µM), a PDGFR tyrosine kinase inhibitor, partially blocked FGF-2– TGF-beta1 synergism without any effect on FGF-2–induced proliferation. To confirm this partial inhibition of FGF-2–TGF-beta1 synergy, increasing concentrations of the tyrphostin AG1296 and another potent and selective ATP-competitive inhibitor of PDGFR tyrosine kinase activity, namely PDGF Receptor Tyrosine Kinase Inhibitor III (refered herein as Inhibitor III), were tested (Figure 3B). Whereas 20 µM of AG1296 as well as 1 µM of Inhibitor III did not affect the FGF-2–induced proliferation (data not shown), the synergism between FGF-2 and TGF-beta1 was greatly reduced (66 and 52%, respectively). At higher concentrations (50 µM and 10 µM of AG1296 and Inhibitor III, respectively), both PDGFR inhibitors abrogated the synergism between FGF-2 and TGF-beta1 (Figure 3B). However, these concentrations also partially reduced FGF-2–induced proliferation (data not shown), suggesting a nonspecific effect of these inhibitors on BSMC proliferation when used at these higher concentrations. Proliferative synergism between FGF-2 and TGF-beta1 observed in cells derived from the other two donors was also reduced by AG1296 (10 µM) (23 and 37% for the 1-yr-old and the 21-yr-old white females, respectively; data not shown). Additional experiments were performed using a neutralizing mAb against PDGFR{alpha}. At 0.2, 2, and 20 µg/ml, the synergism between FGF-2 and TGF-beta1 was reduced by 5, 22, and 33%, respectively (data not shown).


Figure 3
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  		Figure 3. Tyrosine kinase inhibitors of PDGFR reduced the proliferative synergism obtained by sequential FGF-2 and TGF-beta1 co-stimulation. (A) Effects of different pharmacologic inhibitors on FGF-2 (2 ng/ml)– and FGF-2 (2 ng/ml) + TGF-beta1 (10 ng/ml)–induced proliferation. Pharmalogic inhibitors were administered 1 h before TGF-beta1 treatment and, hence, 23 h after FGF-2 treatment. Results are expressed as percentage of baseline OD (i.e., without any stimulus, inhibitor, or vehicle), where each bar represents means ± SEM of quadruplicate measurements of at least four independent experiments. *P <= 0.05 compared with their respective vehicle (DMSO)-treated control. (B) The effect of increasing concentration of AG1296 (shaded bars), Inhibitor III (solid bars), or DMSO (open bars) on FGF-2–TGF-beta1 synergy. Results are expressed as percentage of the synergism (defined as the difference between FGF-2- and FGF-2 + TGF-beta1–induced proliferation) for every concentration of AG1296, Inhibitor III, or equivalent AG1296-concentration of DMSO tested. Each bar represents means ± SD of quadruplicate measurements of at least three experiments. *P <= 0.05 compared with control FGF-2–TGF-beta1 synergism. Inhibitor III was used in one experiment in quadruplicate.

 
TGF-beta1 Increases PDGFR Ligand Expression
The expression of PDGF-A, -B, and -C mRNA was measured by semiquantitative RT-PCR during a time-course stimulation with TGF-beta1 (10 ng/ml). Relative PDGF-A mRNA increased significantly after 1 h stimulation with TGF-beta1 and persisted for 12 h after stimulation (Figure 4A). At the protein level, conditioned medium (CM) from BSMC treated for 24 h with TGF-beta1 (10 ng/ml) showed an ~ 5-fold increase in PDGF-AA expression compared with CM from nontreated cells (from 5.60 ± 1.71 to 29.29 ± 4.93 pg/ml, P = 0.001) (Figure 4B). PDGF-C was also upregulated after TGF-beta1 stimulation (Figure 4C). However, in contrast to the transient induction of PDGF-A, the increase in PDGF-C expression was maintained for at least 2 d. PDGF-B, on the other hand, was not detected in BSMC (data not shown).


Figure 4
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  		Figure 4. TGF-beta1 increased PDGF-AA and PDGF-C expression. Representative kinetics (upper panels) and densitometric analysis (lower panels) of PDGF-A (A) and PDGF-C (C) mRNA expression in response to TGF-beta1 (10 ng/ml) stimulation. Each point represents a compilation of at least three independent experiments. *P <= 0.05 compared with time 0. (B) PDGF-AA expression level in CM from TGF-beta1 (10 ng/ml)-treated or nontreated BSMC. Bars represent means ± SEM of duplicate measurements of six independent experiments. *P <= 0.05 compared with nontreated cells.

 
FGF-2 Increases PDGFR{alpha} as well as the Proliferation Induced by Exogenous PDGF-AA
Semiquantitative RT-PCR revealed that FGF-2 (2 ng/ml) time-dependently increased the mRNA expression level of PDGFR{alpha}, reaching a peak at 1 h after stimulation (Figure 5A). To ensure that PDGFR{alpha} protein was upregulated at the time of TGF-beta1 stimulation, expression of PDGFR{alpha} was measured by Western blot analysis in whole cell lysates 24 h after FGF-2 administration. As demonstrated in Figure 5B, levels of PDGFR{alpha} were elevated 24 h after FGF-2 stimulation relative to levels found in unstimulated cells. In contrast, expression of the structural protein vinculin did not differ between baseline and stimulated conditions, indicating that equal amounts of protein had been loaded in each well.


Figure 5
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  		Figure 5. FGF-2 increases PDGFR{alpha} expression as well as the proliferation induced by increasing concentrations of exogenous PDGF-AA. Representative kinetics (upper panels) and densitometric analysis (lower panels) of PDGFR{alpha} expression (A) in response to FGF-2 (2 ng/ml) stimulation. Each point represents a compilation of at least three independent experiments. *P <= 0.05 compared with time 0. (B) Western blot analysis of PDGFR{alpha} (~ 180 kD) and vinculin (~ 113 kD) expression at baseline and after 24 h stimulation with FGF-2 (2 ng/ml). Molecular markers are indicated on the left. (C) BSMC were seeded with or without a fixed concentration of FGF-2 (2 ng/ml) and stimulated 24 h later with increasing concentrations of PDGF-AA. Points are means ± SEM of quadruplicate measurements of at least three independent experiments. *P <= 0.05 compared with FGF-2 alone.

 
The effect of exogenous PDGF-AA with or without a 24 h pretreatment with FGF-2 was then evaluated to confirm the functionality of the induced receptor. Recombinant PDGF-AA alone (1–50 ng/ml) tended to increase BSMC proliferation in a concentration-dependent manner (not statistically significant at any concentration tested) (Figure 5C). However, with a prior FGF-2 stimulation (2 ng/ml), the proliferative effect of increasing concentrations of PDGF-AA was enhanced, reaching a statistically significant difference at 10 ng/ml and demonstrating a 140% increase at 50 ng/ml compared with FGF-2 alone.

FGF-2 Increases tPA mRNA Expression
To investigate whether FGF-2 stimulation would induce activation of latent PDGF-CC after its secretion, kinetic studies of mRNA expression of the PDGF-CC–activating protease tPA were performed. As demonstrated in Figure 6A, FGF-2 increased in a time-dependent fashion the mRNA expression of tPA, reaching its peak levels between 12 and 24 h of stimulation. In additional experiments, leupeptin (50 µM), a serine and cysteine inhibitor that inhibits tPA activity, administered 1 h before TGF-beta1 stimulation, reduced significantly (36%) the proliferative synergism induced by sequential FGF-2 and TGF-beta1 co-stimulation.


Figure 6
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  		Figure 6. FGF-2 increases tPA expression and protease activity is required for optimal proliferative synergism between FGF-2 and TGF-beta1. Representative kinetics (upper panels) and densitometric analysis (lower panels) of tPA mRNA expression (A) in response to FGF-2 (2 ng/ml) stimulation. Each point represents a compilation of at least three independent experiments. *P <= 0.05 compared with time 0. (B) FGF-2– TGF-beta1 synergism was investigated in the presence of leupeptin (50 µM; administered 1 h before TGFbeta1 stimulation). Points are means ± SEM of quadruplicate measurements of three independent experiments. *P <= 0.05 compared with FGF-2–TGF-beta1 without inhibitor.

 

   DISCUSSION
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
This is the first demonstration of FGF-2 and TGF-beta1 synergism in human BSMC proliferation. The mechanisms responsible for this synergy are partially elucidated here and indicate that members of the well-known PDGF family of mitogens are involved. In support of this hypothesis, our data demonstrated that: (1) inhibitors of PDGFR tyrosine kinase activity (AG1296 and Inhibitor III) reduced significantly the synergism between FGF-2 and TGF-beta1; (2) TGF-beta1 induced PDGF-AA mRNA and protein as well as PDGF-C mRNA expression; (3) FGF-2 increased PDGFR{alpha} mRNA and protein expression and potentiated the proliferative effect of exogenous PDGF-AA; (4) FGF-2 also increased the mRNA expression of the PDGF-CC–activating protease tPA, and the tPA protease inhibitor leupeptin partially blocked the FGF-2–TGF-beta1 synergy; and (5) neutralizing Ab against the receptor engaged by both PDGF-AA and PDGF-CC, namely PDGFR{alpha}, also reduced the proliferative synergism triggered by sequential FGF-2 and TGF-beta1 co-stimulation (Figure 7). The mechanisms involved in the remaining part of the synergism, which is not blocked by inhibitors of PDGFR signaling, are still unresolved and are the subject of current investigations in our laboratory.


Figure 7
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  		Figure 7. Proliferative synergism between FGF-2 and TGF-beta1 involves a PDGF loop. Whereas FGF-2 increases PDGF receptor {alpha}-chain (PDGFR{alpha}) and PDGF-CC activating protease (tPA) expression in human bronchial smooth muscle cells, TGF-beta1 increases the expression of PDGFR{alpha} ligands, namely PDGF-AA and PDGF-CC. When FGF-2 and TGF-beta1 are administered sequentially at a 24-h interval in their respective order, as demonstrated in the schematic representation, a proliferative synergism occurs.

 
For the last two decades, the effect of TGF-beta1 on airway smooth muscle proliferation in vitro has been a matter of debate (27–33). However, many insights have recently been gained concerning its potential contribution to BSMC hyperplasia in vivo: (1) TGF-beta1 was consistently reported to be upregulated in animal (24–26) or human (7, 16–21, 23) airways with asthma; (2) its expression further increased after allergic challenge in humans with asthma (7); (3) active TGF-beta1 signaling (measured by nuclear phospho-Smad2 immunostaining) has been observed in airways of animals (34, 35) and humans (22, 36) with asthma before and after allergic challenge; (4) targeted overexpression of TGF-beta1 in the adult lung of mice induced BSMC hyperplasia (37); and (5) therapeutically administered anti–TGF-beta1 antibodies prevented BSMC hyperplasia in a prolonged allergen sensitization/challenge mouse model (35). However, the mechanisms that govern TGF-beta1–induced BSMC hyperplasia in vivo remain unknown, and its controversial effect observed in in vitro condition necessitates elucidation.

In this regard, the temporal increase in TGF-beta1 expression after phlogogenic challenge relative to other growth factors could potentially bring information regarding the etiology of BSMC hyperplasia. Contrary to other rapidly released mediators such as histamine, leukotrienes or FGF-2, the upregulation of TGF-beta1 during an asthma attack is delayed (7). In this work, we studied the effect of TGF-beta1, alone or with a prior stimulation with FGF-2, on human BSMC proliferation.

In our proliferation assay, TGF-beta1 alone had no detectable effect on BSMC proliferation. However, when administered 24 h after FGF-2 stimulation, TGF-beta1 markedly increased BSMC proliferation. The mechanisms involved were tested by screening the effect of different pharmacologic inhibitors. Src inhibitors PP1 and PP2 abrogated the proliferation induced by FGF-2 alone. Considering that both signals from FGF-2 and TGF-beta1 are required for the synergy to occur, the blocking effect of Src inhibitors on the synergy was expected. More surprisingly, tyrphostin AG1296 (20 µM) and Inhibitor III (1 µM), which inhibit tyrosine kinase activity of the {alpha} and beta chains of PDGF receptors, significantly blocked the synergism between FGF-2 and TGF-beta1 without affecting FGF-2–induced proliferation.

Based on time-course proliferation assays with [3H]-thymidine and crystal violet staining, TGF-beta1 did not increase FGF-2 mitogenicity, but rather prolonged DNA synthesis induced by FGF-2. The delayed mitogenic effect of TGF-beta1, together with the involvement of PDGFR tyrosine kinase activity in the FGF-2– TGF-beta1 synergy, were suggestive of an induced expression of PDGFR ligands by TGF-beta1. Our findings indicate that this may indeed be the case, since expression of both PDGF-AA and PDGF-C was induced by TGF-beta1. Moreover, neutralizing antibody against PDGFR{alpha} reduced in a concentration-dependent manner FGF-2–TGF-beta1 synergy. Collectively, these results suggest that PDGF-AA and PDGF-C are upregulated by TGF-beta1 and could likely act as autocrine growth factors to increase BSMC proliferation.

TGF-beta1 alone was sufficient to increase PDGF-AA and PDGF-C expression, but was unable to induce BSMC proliferation. Bonner and coworkers (11) have already demonstrated that FGF-2 could potentiate the mitogenic response of BSMC to exogenous PDGF-AA by its ability to increase PDGFR{alpha} expression. This mechanism seems operational in our study as well, since we found FGF-2 to increase PDGFR{alpha} mRNA and protein expression. In addition, based on the enhanced proliferative response of BSMC to exogenous PDGF-AA after a 24-h pretreatment with FGF-2, these newly synthesized receptors were shown to be functional. The magnitude of this enhanced proliferation may, however, be underestimated compared with the role of PDGF in the FGF-2–TGF-beta1 synergy, since stimulations were performed with PDGF-AA only, rather than with both PDGF-AA and PDGF-CC. In any case, the lack of mitogenic effect of TGF-beta1 on its own supports the notion that TGF-beta1–induced PDGF-AA and PDGF-CC were insufficient to stimulate mitogenesis, but would likely sustain proliferation after PDGFR{alpha} upregulation by FGF-2.

In contrast to PDGF-AA, PDGF-CC is secreted in a latent form. To be of any significance in the proliferative synergism between FGF-2 and TGF-beta1, latent PDGF-CC would need to be activated by proteolytic cleavage. The only protease known to activate latent PDGF-CC is tPA (38). Accordingly, we demonstrated that FGF-2 induced tPA mRNA expression in BSMC. Moreover, the serine and cysteine protease inhibitor leupeptin partially blocked the synergism between FGF-2 and TGF-beta1. These results highlight the possibility that latent PDGF-CC induced by TGF-beta1 would be cleaved by FGF-2–induced tPA, which will subsequently permit binding to and dimerization-induced activation of its receptors. This also corroborates the lack of proliferative effect of TGF-beta1 on its own.

In conclusion, we speculate that the proliferative synergism obtained in in vitro conditions with human BSMC after such sequential FGF-2 and TGF-beta1 co-stimulation represents potential etiologic events leading to BSMC hyperplasia in individuals with asthma. Since this observed synergism has been reproduced in BSMC derived from two other donors with different age, sex, and ethnicity, we propose that this proliferative response is a generalized mechanism leading to BSMC hyperplasia, regardless of the genetic heterogeneity among the population. Our results also suggest that the well-known mitogenic receptor PDGFR{alpha} contributes significantly to this response via an autocrine agonist–dependent activation, but other mechanisms are likely operational as well.


   Footnotes
 
This work was supported by grants (J.S. and M.R.P.) and studentships (Y.B. and C.T.) from the Canadian Institutes of Health Research.

Originally Published in Press as DOI: 10.1165/rcmb.2005-0309OC on January 26, 2006

Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Received in original form August 11, 2005

Accepted in final form December 22, 2005


   References
Top
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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