SUMMARYMembers of the 18 glycosyl hydrolase (GH 18) gene family have been conserved over species and time and are dysregulated in inflammatory, infectious, remodeling, and neoplastic disorders. This is particularly striking for the prototypic chitinase-like protein chitinase 3-like 1 (Chi3l1), which plays a critical role in antipathogen responses where it augments bacterial killing while stimulating disease tolerance by controlling cell death, inflammation, and remodeling. However, receptors that mediate the effects of GH 18 moieties have not been defined. Here, we demonstrate that Chi3l1 binds to interleukin-13 receptor α2 (IL-13Rα2) and that Chi3l1, IL-13Rα2, and IL-13 are in a multimeric complex. We also demonstrate that Chi3l1 activates macrophage mitogen-activated protein kinase, protein kinase B/AKT, and Wnt/β-catenin signaling and regulates oxidant injury, apoptosis, pyroptosis, inflammasome activation, antibacterial responses, melanoma metastasis, and TGF-β1 production via IL-13Rα2-dependent mechanisms. Thus, IL-13Rα2 is a GH 18 receptor that plays a critical role in Chi3l1 effector responses.

Idiopathic Pulmonary Fibrosis (IPF) is a lethal lung disease with progressive fibrosis and death within 2–3 years of diagnosis. IPF incidence and prevalence rates are increasing annually with few effective treatments available. Inhibition of interleukin 6 (IL-6) results in the attenuation of pulmonary fibrosis in mice. It is unclear whether this is due to blockade of classical signaling, mediated by membrane-bound IL-6 receptor alpha (mIL-6Rα), or trans signaling, mediated by soluble IL-6Rα (sIL-6Rα). Our study assessed the role of sL-6Rα in IPF. We demonstrated elevations of sIL-6Rα in IPF patients and in mice during the onset and progression of fibrosis. We demonstrated that protease-mediated cleavage from lung macrophages was important in production of sL-6Rα. In vivo neutralization of sIL-6Rα attenuated pulmonary fibrosis in mice as seen by reductions in myofibroblasts, fibronectin and collagen in the lung. In vitro activation of IL-6 trans signaling enhanced fibroblast proliferation and extracellular matrix protein production, effects relevant in the progression of pulmonary fibrosis. Together these findings demonstrate that the production of sL-6Rα from macrophages in the diseased lung contributes to IL-6 trans signaling that in turn influences events crucial in pulmonary fibrosis.

Epithelial injury, alternative macrophage accumulation, and fibroproliferation coexist in the lungs of patients with idiopathic pulmonary fibrosis (IPF). Chitinase 3–like 1 (CHI3L1) is a prototypic chitinase-like protein that has been retained over species and evolutionary time. However, the regulation of CHI3L1 in IPF and its ability to regulate injury and/or fibroproliferative repair have not been fully defined. We demonstrated that CHI3L1 levels were elevated in patients with IPF. High levels of CHI3L1 are associated with progression—as defined by lung transplantation or death—and with scavenger receptor–expressing circulating monocytes in an ambulatory IPF population. In preterminal acute exacerbations of IPF, CHI3L1 levels were reduced and associated with increased levels of apoptosis. We also demonstrated that in bleomycin-treated mice, CHI3L1 expression was acutely and transiently decreased during the injury phase and returned toward and eventually exceeded baseline levels during the fibrotic phase. In this model, CHI3L1 played a protective role in injury by ameliorating inflammation and cell death, and a profibrotic role in the repair phase by augmenting alternative macrophage activation, fibroblast proliferation, and matrix deposition. Using three-dimensional culture system of a human fibroblast cell line, we found that CHI3L1 is sufficient to induce low grade myofibroblast transformation. In combination, these studies demonstrate that CHI3L1 is stimulated in IPF, where it represents an attempt to diminish injury and induce repair. They also demonstrate that high levels of CHI3L1 are associated with disease progression in ambulatory patients and that a failure of the CHI3L1 antiapoptotic response might contribute to preterminal disease exacerbations.

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BackgroundAdenosine is generated in response to cellular stress and damage and is elevated in the lungs of patients with chronic lung disease. Adenosine signaling through its cell surface receptors serves as an amplifier of chronic lung disorders, suggesting adenosine-based therapeutics may be beneficial in the treatment of lung diseases such as chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). Previous studies in mouse models of chronic lung disease demonstrate that the key components of adenosine metabolism and signaling are altered. Changes include an up-regulation of CD73, the major enzyme of adenosine production and down-regulation of adenosine deaminase (ADA), the major enzyme for adenosine metabolism. In addition, adenosine receptors are elevated.Methodology/Principal FindingsThe focus of this study was to utilize tissues from patients with COPD or IPF to examine whether changes in purinergic metabolism and signaling occur in human disease. Results demonstrate that the levels of CD73 and A2BR are elevated in surgical lung biopsies from severe COPD and IPF patients. Immunolocalization assays revealed abundant expression of CD73 and the A2BR in alternatively activated macrophages in both COPD and IPF samples. In addition, mediators that are regulated by the A2BR, such as IL-6, IL-8 and osteopontin were elevated in these samples and activation of the A2BR on cells isolated from the airways of COPD and IPF patients was shown to directly induce the production of these mediators.Conclusions/SignificanceThese findings suggest that components of adenosine metabolism and signaling are altered in a manner that promotes adenosine production and signaling in the lungs of patients with COPD and IPF, and provide proof of concept information that these disorders may benefit from adenosine-based therapeutics. Furthermore, this study provides the first evidence that A2BR signaling can promote the production of inflammatory and fibrotic mediators in patients with these disorders.

Adenosine is an extracellular signaling molecule that is generated in response to cell injury where it orchestrates tissue protection and repair. Whereas adenosine is best known for promoting anti-inflammatory activities during acute injury responses, prolonged elevations can enhance destructive tissue remodeling processes associated with chronic disease states. The generation of adenosine and the subsequent activation of the adenosine 2B receptor (A2BR) is an important processes in the regulation of both acute and chronic lung disease. The goal of this study was to examine the contribution of the A2BR in models of bleomycin-induced lung injury that exhibit varying degrees of acute and chronic injury. Intratracheal bleomycin exposure results in substantial acute lung injury followed by progressive fibrosis. In this model, genetic removal of the A2BR resulted in enhanced loss of barrier function and increased pulmonary inflammation, with few differences in indexes of pulmonary fibrosis. These results support an anti-inflammatory role for this receptor in this model of acute lung injury. In contrast, systemic exposure of mice to bleomycin resulted in modest acute lung injury together with progressive pulmonary fibrosis. In this model, the effects of A2BR removal on acute lung injury were negligible; however, there were substantial reductions in pulmonary fibrosis, supporting a profibrotic role for this receptor. A2BR-dependent regulation of IL-6 production was identified as a potential mechanism involved in the diminished pulmonary fibrosis seen in A2BR knockout mice exposed to i.p. bleomycin. These studies highlight the distinct roles of A2BR signaling during acute and chronic stages of lung injury.

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease and interstitial lung disease are characterized by inflammation and tissue remodeling processes that compromise pulmonary function. Adenosine is produced in the inflamed and damaged lung where it plays numerous roles in the regulation of inflammation and tissue remodeling. Extracellular adenosine serves as an autocrine and paracrine signaling molecule by engaging cell surface adenosine receptors. Preclinical and cellular studies suggest that adenosine plays an anti-inflammatory role in processes associated with acute lung disease, where activation of the A2AR and A2BR have promising implications for the treatment of these disorders. In contrast, there is growing evidence that adenosine signaling through the A1R, A2BR and A3R may serve pro-inflammatory and tissue remodeling functions in chronic lung diseases. This review discusses the current progress of research efforts and clinical trials aimed at understanding the complexities of this signaling pathway as they pertain to the development of treatment strategies for chronic lung diseases.

Interstitial lung disease (ILD) with pulmonary fibrosis is an important manifestation in systemic sclerosis (SSc, scleroderma) where it portends a poor prognosis. However, biomarkers that predict the development and or severity of SSc-ILD have not been validated, and the pathogenetic mechanisms that engender this pulmonary response are poorly understood. In this study, we demonstrate in two different patient cohorts that the levels of chitotriosidase (Chit1) bioactivity and protein are significantly increased in the circulation and lungs of SSc patients compared with demographically matched controls. We also demonstrate that, compared with patients without lung involvement, patients with ILD show high levels of circulating Chit1 activity that correlate with disease severity. Murine modeling shows that in comparison with wild-type mice, bleomycin-induced pulmonary fibrosis was significantly reduced in Chit1−/− mice and significantly enhanced in lungs from Chit1 overexpressing transgenic animals. In vitro studies also demonstrated that Chit1 interacts with TGF-β1 to augment fibroblast TGF-β receptors 1 and 2 expression and TGF-β–induced Smad and MAPK/ERK activation. These studies indicate that Chit1 is potential biomarker for ILD in SSc and a therapeutic target in SSc-associated lung fibrosis and demonstrate that Chit1 augments TGF-β1 effects by increasing receptor expression and canonical and noncanonical TGF-β1 signaling.