Chop Deficiency Protects Mice Against Bleomycin-induced Pulmonary Fibrosis by Attenuating M2 Macrophage Production

Abstract

C/EBP homologous protein (Chop) has been shown to have altered expression in patients with idiopathic pulmonary fibrosis (IPF), but its exact role in IPF pathoaetiology has not been fully addressed. Studies conducted in patients with IPF and Chop(-/-) mice have dissected the role of Chop and endoplasmic reticulum (ER) stress in pulmonary fibrosis pathogenesis. The effect of Chop deficiency on macrophage polarization and related signalling pathways were investigated to identify the underlying mechanisms. Patients with IPF and mice with bleomycin (BLM)-induced pulmonary fibrosis were affected by the altered Chop expression and ER stress. In particular, Chop deficiency protected mice against BLM-induced lung injury and fibrosis. Loss of Chop significantly attenuated transforming growth factor β (TGF-β) production and reduced M2 macrophage infiltration in the lung following BLM induction. Mechanistic studies showed that Chop deficiency repressed the M2 program in macrophages, which then attenuated TGF-β secretion. Specifically, loss of Chop promoted the expression of suppressors of cytokine signaling 1 and suppressors of cytokine signaling 3, and through which Chop deficiency repressed signal transducer and activator of transcription 6/peroxisome proliferator-activated receptor gamma signaling, the essential pathway for the M2 program in macrophages. Together, our data support the idea that Chop and ER stress are implicated in IPF pathoaetiology, involving at least the induction and differentiation of M2 macrophages.

Figures

Figure 1

Analysis of Chop expression and ER stress in patients with IPF and in BLM-induced mice. (a) Representative results for coimmunostaining of Chop and CD206, an M2 macrophage marker in the lung sections from patients with IPF and healthy subjects. The nuclei were stained blue by DAPI, and the images were taken under original magnification ×400. A total of eight patients with IPF and six control subjects were analyzed. (b) Western blot analysis of Chop expression in the lung of mice 21 days after BLM induction. (c–e) Analysis of additional ER stress markers: p-eIF2α (c), Atf4 (d) and Bip (e) in the lung of mice 21 days after BLM induction. Twelve mice were analyzed in each group. Bar graphs indicate the differences (mean ± SEM) between BLM- and saline-treated mice. **P < 0.01; ***P < 0.001. Atf4, activating transcriptional factor 4; BLM, bleomycin; ER, endoplasmic reticulum; IPF, idiopathic pulmonary fibrosis; p-eIF2α, phosphorylated eukaryotic initiation factor 2 α.

Figure 2

Comparison of the severity of lung fibrosis between Chop−/−and WT mice 21 days after BLM induction. (a) Histological analysis for the severity of lung fibrosis in mice after BLM induction. Left panel: representative results for H&E (Top), Sirius red (center), and Masson staining (bottom). The inset shows higher magnification for a particular location. Right panel: a bar graph showing the semiquantitative Ashcroft scores for the severity of fibrosis. Images were taken under original magnification ×200. b, c Western blot analysis of fibrotic markers collagen I (b) and fibronectin (c). Left panel: a representative western blot result. Right panel: a bar graph showing the mean data of all mice analyzed in each group. (d) Body weight change during the course of BLM-induced fibrosis. Each bar represents mean ± SEM of 12 mice analyzed. *P < 0.05; **P < 0.01; ***P < 0.001. BLM, bleomycin; Coll, collagen I; Fib, fibronectin; WT, wild type.

Figure 3

Loss of Chop attenuates TGF-β signalling after BLM induction. (a) Western blot analysis of TGF-β1 expression in the lung homogenates. Left panel: representative western blot results. Right panel: a bar graph showing the data of all mice studied. (b) ELISA results for TGF-β1 levels in the BALF. (c) Results for western blot analysis of TGF-β downstream Smad2 and 3 activities. Left panel: a representative western blot result for Smad2/3, p-smad2, and p-smad3 in the lung homogenates. Right panel: a bar graph showing the results of all mice examined. (d) Coimmunostaining of TGF-β1 and F4/80 in the lung sections. The images were taken under original magnification ×400. Each bar represents the mean ± SEM of 12 mice studied. **P < 0.01; ***P < 0.001. BALF, bronchoalveolar lavage fluids; BLM, bleomycin; TGF-β, transforming growth factor β.

Figure 4

Chop is predominantly overexpressed by the infiltrated M2 macrophages in the lung following BLM induction. (a,b) Results for coimmunostaining of Chop and F4/80 (a), and arginase-1 and F4/80 (b) in BLM-induced lung sections. Chop was significantly overexpressed in infiltrated macrophages, and type II alveolar epithelial cells also resulted in increased Chop expression in BLM-induced mice, while arginase-1 was predominantly localized within the infiltrated macrophages. (c) Results for arginase-1 expression in the lung homogenates. Left panel: a representative western blot result. Right panel: a bar graph showing the expression levels of arginase-1 of all mice examined for each group. (d) Real-time PCR results for analysis of YM1 expression in the lung. (e) Real-time PCR analysis of FIZZ1 expression in the lung. Each bar represents the mean ± SEM of 12 mice examined. *P < 0.05; **P < 0.01. Arg-1, arginase-1; BLM, bleomycin; FIZZ1, Found in Inflammatory Zone-1.

Figure 5

Loss of Chop inhibits the macrophage M2 program. (a) Flow cytometry analysis of CD206 expression in BMDMs following IL-4 stimulation. Left panel: representative results for flow cytometry analysis. Right panel: a bar graph showing the data with three replications. (b) Real-time PCR analysis of YM1 expression in IL-4-stimulated BMDMs. (c) Real-time PCR results for FIZZ1 in BMDMs after IL-4 stimulation. (d) ELISA analysis of TGF-β1 production in BMDMs following IL-4 stimulation. (e) Temporal Chop expression changes in BMDMs during the course of IL-4 stimulation. (f) Flow cytometry analysis of BMDMs following lipopolysaccharides stimulation. Each bar represents the mean ± SEM of at least six mice. *P < 0.05; **P < 0.01; ***P < 0.001. BMDMs, bone marrow-derived macrophages; FIZZ1, Found in Inflammatory Zone-1.

Figure 6

Loss of Chop protects mice from BLM-induced pulmonary fibrosis dependent on its effect on the macrophage M2 program. (a) Clodronate liposome efficiently depleted the macrophages in the lung; macrophages were almost undetectable in the BALF of clodronate liposome-treated mice along with a significant reduction for the number of total cells. (b) Depletion of macrophages restored Chop−/− mice with manifestations similar as WT mice after BLM induction, as evidenced by the comparable histological changes and Ashcroft scores. Left panel: representative results for H&E, Sirius red, and Masson staining and an inset picture was employed to show the indicated area at higher magnification. Right panel: a bar graph showing the semiquantitative Ashcroft scores for the severity of fibrosis. (c) Macrophage depleted WT and Chop−/− mice manifested comparable levels of collagen I and fibronectin expression in the lung after BLM induction. (d) Adoptive transfer of WT macrophages into Chop−/− mice restored their susceptibility to BLM-induced pulmonary fibrosis. Similar as above, the left panel presents representative results for H&E, Sirius red, and Masson staining, and the right panel displays the semiquantitative Ashcroft scores relevant to the severity of fibrosis. (e) western blot results for analysis of collagen I and fibronectin expression in the lung homogenates of above WT and Chop−/− mice with adoptive transfer studies. All images were taken under original magnification ×200, and eight mice were included in each study group. *P < 0.05; **P < 0.01; and ***P < 0.001. BALF, bronchoalveolar lavage fluids; BLM, bleomycin; Coll I, collagen I; Fib, fibronectin; WT, wild type.

Figure 7

The impact of Chop deficiency on IL-4-stimulated STAT6/PPAR-γ signalling and SOCS1 and 3 expressions in macrophages. (a) Loss of Chop attenuated IL-4-induced STAT6/PPAR-γ signalling. Left panel: a representative western blotting results for STAT6, p-STAT6, and PPAR-γ at different time points of IL-4 stimulation. Right panel: figures showing the data with three replications. (b) Chop deficiency enhanced SOCS1 and 3 expression during the course of IL-4 stimulation. Left panel: representative western blotting results for SOCS1 and SOCS3 at indicated time points of IL-4 simulation. Right panel: figures showing the data with three replications. (c) Chop deficiency did not affect MAPK (p38, ERK1/2, and JNK) and PI3K signalling. *P < 0.05; **P < 0.01; ***P < 0.001. PPAR-γ, peroxisome proliferator-activated receptor gamma; p-STAT6, phosphorylated STAT6; SOCS1 and SOCS3, suppressors of cytokine signaling 1 and suppressors of cytokine signaling 3; STAT6, signal transducer and activator of transcription 6.