Monocyte-derived alveolar macrophage apolipoprotein E participates in pulmonary fibrosis resolution
Huachun Cui, Dingyuan Jiang, Sami Banerjee, Na Xie, Tejaswini Kulkarni, Rui-Ming Liu, Steven R. Duncan, Gang Liu
Huachun Cui, Dingyuan Jiang, Sami Banerjee, Na Xie, Tejaswini Kulkarni, Rui-Ming Liu, Steven R. Duncan, Gang Liu
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Research Article Pulmonology

Monocyte-derived alveolar macrophage apolipoprotein E participates in pulmonary fibrosis resolution

Abstract

Recent studies have presented compelling evidence that it is not tissue-resident, but rather monocyte-derived alveolar macrophages (TR-AMs and Mo-AMs, respectively) that are essential to development of experimental lung fibrosis. However, whether apolipoprotein E (ApoE), which is produced abundantly by Mo-AMs in the lung, plays a role in the pathogenesis is unclear. In this study, we found that pulmonary ApoE was almost exclusively produced by Mo-AMs in mice with bleomycin-induced lung fibrosis. We showed that, although ApoE was not necessary for developing maximal fibrosis in bleomycin-injured lung, it was required for the resolution of this pathology. We found that ApoE directly bound to Collagen I and mediated Collagen I phagocytosis in vitro and in vivo, and this process was dependent on low-density lipoprotein receptor–related protein 1 (LPR1). Furthermore, interference of ApoE/LRP1 interaction impaired the resolution of lung fibrosis in bleomycin-treated WT mice. In contrast, supplementation of ApoE promoted this process in ApoE–/– animals. In conclusion, Mo-AM–derived ApoE is beneficial to the resolution of lung fibrosis, supporting the notion that Mo-AMs may have distinct functions in different phases of lung fibrogenesis. The findings also suggest a potentially novel therapeutic target for treating lung fibrosis, to which effective remedies remain scarce.

Authors

Huachun Cui, Dingyuan Jiang, Sami Banerjee, Na Xie, Tejaswini Kulkarni, Rui-Ming Liu, Steven R. Duncan, Gang Liu

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Figure 1

ApoE is one of the most differentially expressed signature markers in Mo-AMs versus TR-AMs.

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ApoE is one of the most differentially expressed signature markers in Mo...
(A) Eight-week-old C57BL/6 mice were i.t. instilled with saline or bleomycin (BLM, 1.5 U/kg in 50 μL saline). Three weeks after treatment, mice were sacrificed and BALF harvested. BAL cells then underwent fluorescence activated cell sorting for isolation of TR-AMs and Mo-AMs, as described in detail in Methods. Total RNAs of AMs were purified and real-time PCR performed to assess the expression of the indicated genes. n = 3 mice for each group; mean ± SEM; ***P < 0.001 by 1-way ANOVA with Bonferroni’s post hoc test. (B) Experiments were done as in A. ApoE levels in BALF were determined by ELISA. n = 3 mice for each group; mean ± SEM; **P < 0.01 by 2-tailed Student’s t test. (C) Experiments were done as in A. BALF protein was precipitated and resolved by SDS-PAGE. Levels of the indicated proteins in BALF were determined by Western blotting. (D) Mice were treated as in A. Lungs were harvested and single cell suspensions prepared. AMs, alveolar epithelial cells (AECs), and fibroblasts (Fbs) were isolated as described in detail in Methods. Total RNAs were purified and real-time PCR performed to assess ApoE expression. n = 3 mice for each group; mean ± SEM; *P < 0.05 by 1-way ANOVA with Bonferroni’s post hoc test. (E) Mice were treated as in A. Lungs were harvested and single cell suspension prepared. Mo-AMs were purified by fluorescence activated cell sorting, and all of the remaining lung cells were treated as non–Mo-AMs. Total RNAs from the Mo-AM and non–Mo-AM population were isolated, and the relative level of ApoE in the Mo-AMs and non–Mo-AMs was determined. The ratio of the absolute amount of ApoE produced by the Mo-AMs to that by the non–Mo-AMs was derived by factoring the respective percentage of the Mo-AMs and non–Mo-AMs into this relative level. n = 3; mean ± SEM; *P < 0.05 by 2-tailed Student’s t test. (F) Slices of normal control and IPF lungs were prepared. Immunofluorescence staining and fluorescence microscopy were performed to determine the expression of ApoE. Nuclei were counterstained with DAPI. Original magnification, ×200. Scale bars: 100 μm.