Targeting of Discoidin Domain Receptor 2 (DDR2) Prevents Myofibroblast Activation and Neovessel Formation During Pulmonary Fibrosis

Abstract

Idiopathic pulmonary fibrosis (IPF) is a lethal human disease with short survival time and few treatment options. Herein, we demonstrated that discoidin domain receptor 2 (DDR2), a receptor tyrosine kinase that predominantly transduces signals from fibrillar collagens, plays a critical role in the induction of fibrosis and angiogenesis in the lung. In vitro cell studies showed that DDR2 can synergize the actions of both transforming growth factor (TGF)-β and fibrillar collagen to stimulate lung fibroblasts to undergo myofibroblastic changes and vascular endothelial growth factor (VEGF) expression. In addition, we confirmed that late treatment of the injured mice with specific siRNA against DDR2 or its kinase inhibitor exhibited therapeutic efficacy against lung fibrosis. Thus, this study not only elucidated novel mechanisms by which DDR2 controls the development of pulmonary fibrosis, but also provided candidate target for the intervention of this stubborn disease.

Figure 1

Discoidin domain receptor 2 (DDR2) facilitates experimental lung fibrosis. (a–d) Ddr2 deficient mice were resistant to bleomycin-induced fibrosis in the lung. Wild-type or slie mutant mice were administered bleomycin and the lung tissues were harvested 4 weeks later. Representative images of H&E a and Masson's trichrome staining b are shown. Scale bars, 100 µm. The whole lung homogenates from three mice were used for hydroxyproline content assay and immunoblot analysis, respectively c. Immunofluorescent staining of myofibroblast marker α-smooth muscle actin (α-SMA) (red) was demonstrated in d. Right histogram shows the percentage of α-SMA staining area with regard to the total picture area (10 fields/group). **P < 0.01. (e–g) DDR2 knockdown prevents pulmonary fibrosis. One day before bleomycin challenge, C57BL/6 mice were instilled with small interfering RNA (siRNA) solutions and this treatment was repeated once a week until the lung tissues were collected on day 28 (e). The protein extracts from three mice were pooled and subjected to immunoblot (f). The images in g show H&E and Masson's trichrome staining, respectively. Scale bars, 100 µm. H&E, Hematoxylin and Eosin.

Figure 2

Discoidin domain receptor 2 (DDR2) controls lung myofibroblast activation. (a) DDR2 is highly expressed in human idiopathic pulmonary fibrosis (IPF) patients. Represent images of immunohistochemistry. (b) DDR2 expression is increased during the differentiation of lung myofibroblasts. Primary mouse lung fibroblasts were treated transforming growth factor (TGF)-β1 for various durations. (c–d) The absent expression of DDR2 impedes the TGF-β1-induced differentiation of lung fibroblasts. Primary lung fibroblasts from control or slie mice were stimulated with TGF-β1 and then subjected to quantitative polymerase chain reaction (qPCR) c and immunoblot analysis d, respectively. ***P < 0.001. (e–f) DDR2 enhances both TGF-β1 and collagen I-driven myofibroblast activation. Lung fibroblasts from slie mice were infected with either EGFP- or DDR2-expressing adenovirus for 24 hours and then stimulated with TGF-β1 for 48 hours e. Lung fibroblasts from wild-type mice were treated with recombinant adenovirus for 24 hours and then cultured on collagen I-coated surface for another 24 hours. For detection of phosphorylated DDR2, the cell lysates were immunoprecipitated with anti-DDR2 and then subjected to immunoblot with antiphosphotyrosine f.

Figure 3

The protein kinases involved in mediating discoidin domain receptor 2 (DDR2) control of lung myofibroblast activation. (a) DDR2 deficiency in lung fibroblasts impairs the transforming growth factor (TGF)-β1-induced activation of p38 and Akt. Wild-type or slie lung fibroblasts were treated with TGF-β1 for 30 minutes and the cell lysates were then subjected to immunoblot analysis of the indicated proteins. (b,c) The influence of DDR2 overexpression and collagen treatment on TGF-β1 signaling. The indicated lung fibroblasts were infected with recombinant adenovirus for 24 hours and then were treated with either TGF-β1 or the combination of TGF-β1 and collagen I. For the combination treatment, the cells were firstly incubated with or without 10 μg/ml collagen solutions and then stimulated with TGF-β1 for another 30 minutes. (d) DDR2 affects TGF-β1 pathway independent of its activation status. Slie lung fibroblasts were infected lentivirus expressing either wild-type DDR2 or a kinase dead mutant form of DDR2 (K608EDDR2). Fortyeight hours after virus infection, the cells were treated with collagen I for 2 hours and then stimulated with TGF-β1 for another 30 minutes. (e) DDR2 strengthens the collagen I-triggered activation of ERK1/2. Mouse lung fibroblasts expressing EGFP or DDR2 were treated with collagen I (10 μg/ml) for various durations. (f) ERK1/2 cascade is responsible for the collagen I-induced lung myofibroblast activation. Twenty four hours after adenovirus infection, mouse lung fibroblasts were cultured on collagen-coated surface for another 24 hours in the presence of dimethyl sulfoxide or 20 μmol/l PD98059. (g) The kinase activities in bleomycin-injured mouse lung. ERK, extracellular signal-regulated kinase; EGFP, green fluorescent protein.

Figure 4

Discoidin domain receptor 2 (DDR2) promotes lung fibrosis-related angiogenesis in vivo and myofibroblast expression of vascular endothelial growth factor (VEGF) in vitro. (a) C57BL/6 mice were challenged with bleomycin for various durations and the lung tissues were subjected to immunofluorescent staining of endothelial marker CD31. Scale bars, 100 µm. (b,c) DDR2 deficiency inhibits the bleomycin-induced vascular remodeling in the lung. Wild-type or slie mutant mice were treated with bleomycin for 21 days and the lung tissues were immunofluorescent stained with antibody recognizing mouse CD31. Right histogram shows the percentage of CD31 staining area with regard to the total picture area (10 fields/group). **P < 0.01 (b). The lung tissues were subjected to quantitative polymerase chain reaction (qPCR) analysis of the mRNA expression of the indicated proangiogenic cytokines. **P < 0.001 (c). (d,e) DDR2 is required for transforming growth factor (TGF) -β1 induction of VEGF-A in lung fibroblasts. Primary lung fibroblasts from control or slie mice were stimulated with TGF-β and the relative expression level of VEGF-A was determined by qPCR (d) and western blot (e), respectively. *P < 0.05. (f) DDR2 up-regulates the collagen I-induced VEGF-A expression. The green fluorescent protein (EGFP)- or DDR2-overexpressing mouse lung fibroblasts were treated with collagen I for 24 hours. The protein levels of VEGF-A and DDR2 were analyzed by immunoblot.

Figure 5

Discoidin domain receptor 2 (DDR2) -targeting strategies have therapeutic efficacy against lung fibrosis. (a,b) The expression and activation pattern of DDR2 during bleomycin-induced lung fibrosis. Lung tissues from bleomycin-treated C57BL/6 mice were collected at various time points for analysis of the mRNA (a) and protein (b) expression of the indicated genes. The histograms in b depict results quantified using densitometry scanning. (c,d) Late treatment with DDR2 small interfering RNA (siRNA) still potently inhibits the development of lung fibrosis. 1 day after bleomycin instillation, C57BL/6 mice were treated with siRNA immediately or from 14 days later. At day 28, the mouse lungs were subjected to Hematoxylin and Eosin (H&E) staining c and immunoblot analysis d. (e,f) DDR2 kinase inhibitor dasatinib attenuates pulmonary fibrosis. Fourteen days after bleomycin treatment, the C57BL/6 mice received daily oral gavage of dasatinib. Two weeks later, the mouse lung tissues were collected for analysis of the lung structure e and vascular density f. The histogram in f represents the percentage of CD31 staining. **P < 0.001.

Figure 6

Discoidin domain receptor 2 (DDR2) knockdown limits the progression of established lung fibrosis. (a) Schematic of late treatment with DDR2 small interfering RNA (siRNA). (b, c) 21 days after bleomycin challenge, the mice were either sacrificed or continuously received treatment with siRNA. On day 28, the mouse lung tissues were subjected to histological analysis b and hydroxyproline content assay c, respectively. Data are representative of three independent experiments. **P < 0.01.

Figure 7

Schematic depicting the role of discoidin domain receptor 2 (DDR2) in mediating the activation of myofibroblasts during lung fibrosis. In the early phase of fibrotic reaction, transforming growth factor (TGF)-β stimulation of lung fibroblasts induces up-regulated expression of DDR2, which may enable DDR2 to cooperate with TGF-β receptor to activate p38 and Akt, consequently leading to the cell phenotypic transition toward myofibroblasts. This action of DDR2 is independent of the presence of its ligands, fibrillar collagens (left). When the fibrotic process is already established, the abundant fibrillar collagens within fibrotic foci may gradually augment the activation of myofibroblasts via the activation of DDR2/ extracellular signal-regulated kinase (ERK) axis in a positive feedback manner (right). In both phases, DDR2 promotes the oversynthesis of extracellular matrix (ECM) components as well as the secretion of vascular endothelial growth factor (VEGF), which eventually causes massive fibrosis and angiogenesis.