Extensive phenotyping of individuals at risk for familial interstitial pneumonia reveals clues to the pathogenesis of interstitial lung disease.

RATIONALE: Asymptomatic relatives of patients with familial interstitial pneumonia (FIP), the inherited form of idiopathic interstitial pneumonia, carry increased risk for developing interstitial lung disease. OBJECTIVES: Studying these at-risk individuals provides a unique opportunity to investigate early stages of FIP pathogenesis and develop predictive models of disease onset. METHODS: Seventy-five asymptomatic first-degree relatives of FIP patients (mean age, 50.8 yr) underwent blood sampling and high-resolution chest computed tomography (HRCT) scanning in an ongoing cohort study; 72 consented to bronchoscopy with bronchoalveolar lavage (BAL) and transbronchial biopsies. Twenty-seven healthy individuals were used as control subjects. MEASUREMENTS AND MAIN RESULTS: Eleven of 75 at-risk subjects (14%) had evidence of interstitial changes by HRCT, whereas 35.2% had abnormalities on transbronchial biopsies. No differences were noted in inflammatory cells in BAL between at-risk individuals and control subjects. At-risk subjects had increased herpesvirus DNA in cell-free BAL and evidence of herpesvirus antigen expression in alveolar epithelial cells (AECs), which correlated with expression of endoplasmic reticulum stress markers in AECs. Peripheral blood mononuclear cell and AEC telomere length were shorter in at-risk individuals than healthy control subjects. The minor allele frequency of the Muc5B rs35705950 promoter polymorphism was increased in at-risk subjects. Levels of several plasma biomarkers differed between at-risk subjects and control subjects, and correlated with abnormal HRCT scans. CONCLUSIONS: Evidence of lung parenchymal remodeling and epithelial dysfunction was identified in asymptomatic individuals at risk for FIP. Together, these findings offer new insights into the early pathogenesis of idiopathic interstitial pneumonia and provide an ongoing opportunity to characterize presymptomatic abnormalities that predict progression to clinical disease.

ß-catenin in the alveolar epithelium protects from lung fibrosis after intratracheal bleomycin.

RATIONALE: Alveolar epithelial cells (AECs) play central roles in the response to lung injury and the pathogenesis of pulmonary fibrosis. OBJECTIVES: We aimed to determine the role of ß-catenin in alveolar epithelium during bleomycin-induced lung fibrosis. METHODS: Genetically modified mice were developed to selectively delete ß-catenin in AECs and were crossed to cell fate reporter mice that express ß-galactosidase (ßgal) in cells of AEC lineage. Mice were given intratracheal bleomycin (0.04 units) and assessed for AEC death, inflammation, lung injury, and fibrotic remodeling. Mouse lung epithelial cells (MLE12) with small interfering RNA knockdown of ß-catenin underwent evaluation for wound closure, proliferation, and bleomycin-induced cytotoxicity. MEASUREMENTS AND MAIN RESULTS: Increased ß-catenin expression was noted in lung parenchyma after bleomycin. Mice with selective deletion of ß-catenin in AECs had greater AEC death at 1 week after bleomycin, followed by increased numbers of fibroblasts and enhanced lung fibrosis as determined by semiquantitative histological scoring and total collagen content. However, no differences in lung inflammation or protein levels in bronchoalveolar lavage were noted. In vitro, ß-catenin-deficient AECs showed increased bleomycin-induced cytotoxicity as well as reduced proliferation and impaired wound closure. Consistent with these findings, mice with AEC ß-catenin deficiency showed delayed recovery after bleomycin. CONCLUSIONS: ß-Catenin in the alveolar epithelium protects against bleomycin-induced fibrosis. Our studies suggest that AEC survival and wound healing are enhanced through ß-catenin-dependent mechanisms. Activation of the developmentally important ß-catenin pathway in AECs appears to contribute to epithelial repair after epithelial injury.

Endoplasmic reticulum stress enhances fibrotic remodeling in the lungs.

Evidence of endoplasmic reticulum (ER) stress has been found in lungs of patients with familial and sporadic idiopathic pulmonary fibrosis. We tested whether ER stress causes or exacerbates lung fibrosis by (i) conditional expression of a mutant form of surfactant protein C (L188Q SFTPC) found in familial interstitial pneumonia and (ii) intratracheal treatment with the protein misfolding agent tunicamycin. We developed transgenic mice expressing L188Q SFTPC exclusively in type II alveolar epithelium by using the Tet-On system. Expression of L188Q SFTPC induced ER stress, as determined by increased expression of heavy-chain Ig binding protein (BiP) and splicing of X-box binding protein 1 (XBP1) mRNA, but no lung fibrosis was identified in the absence of a second profibrotic stimulus. After intratracheal bleomycin, L188Q SFTPC-expressing mice developed exaggerated lung fibrosis and reduced static lung compliance compared with controls. Bleomycin-treated L188Q SFTPC mice also demonstrated increased apoptosis of alveolar epithelial cells and greater numbers of fibroblasts in the lungs. With a complementary model, intratracheal tunicamycin treatment failed to induce lung remodeling yet resulted in augmentation of bleomycin-induced fibrosis. These data support the concept that ER stress produces a dysfunctional epithelial cell phenotype that facilitates fibrotic remodeling. ER stress pathways may serve as important therapeutic targets in idiopathic pulmonary fibrosis.

A critical role for macrophages in promotion of urethane-induced lung carcinogenesis.

Macrophages have established roles in tumor growth and metastasis, but information about their role in lung tumor promotion is limited. To assess the role of macrophages in lung tumorigenesis, we developed a method of minimally invasive, long-term macrophage depletion by repetitive intratracheal instillation of liposomal clodronate. Compared with controls treated with repetitive doses of PBS-containing liposomes, long-term macrophage depletion resulted in a marked reduction in tumor number and size at 4 mo after a single i.p. injection of the carcinogen urethane. After urethane treatment, lung macrophages developed increased M1 macrophage marker expression during the first 2-3 wk, followed by increased M2 marker expression by week 6. Using a strategy to reduce alveolar macrophages during tumor initiation and early promotion stages (weeks 1-2) or during late promotion and progression stages (weeks 4-16), we found significantly fewer and smaller lung tumors in both groups compared with controls. Late-stage macrophage depletion reduced VEGF expression and impaired vascular growth in tumors. In contrast, early-stage depletion of alveolar macrophages impaired urethane-induced NF-κB activation in the lungs and reduced the development of premalignant atypical adenomatous hyperplasia lesions at 6 wk after urethane injection. Together, these studies elucidate an important role for macrophages in lung tumor promotion and indicate that these cells have distinct roles during different stages of lung carcinogenesis.

Alveolar epithelial cells undergo epithelial-to-mesenchymal transition in response to endoplasmic reticulum stress.

Expression of mutant surfactant protein C (SFTPC) results in endoplasmic reticulum (ER) stress in type II alveolar epithelial cells (AECs). AECs have been implicated as a source of lung fibroblasts via epithelial-to-mesenchymal transition (EMT); therefore, we investigated whether ER stress contributes to EMT as a possible mechanism for fibrotic remodeling. ER stress was induced by tunicamyin administration or stable expression of mutant (L188Q) SFTPC in type II AEC lines. Both tunicamycin treatment and mutant SFTPC expression induced ER stress and the unfolded protein response. With tunicamycin or mutant SFTPC expression, phase contrast imaging revealed a change to a fibroblast-like appearance. During ER stress, expression of epithelial markers E-cadherin and Zonula occludens-1 decreased while expression of mesenchymal markers S100A4 and α-smooth muscle actin increased. Following induction of ER stress, we found activation of a number of pathways, including MAPK, Smad, ß-catenin, and Src kinase. Using specific inhibitors, the combination of a Smad2/3 inhibitor (SB431542) and a Src kinase inhibitor (PP2) blocked EMT with maintenance of epithelial appearance and epithelial marker expression. Similar results were noted with siRNA targeting Smad2 and Src kinase. Together, these studies reveal that induction of ER stress leads to EMT in lung epithelial cells, suggesting possible cross-talk between Smad and Src kinase pathways. Dissecting pathways involved in ER stress-induced EMT may lead to new treatment strategies to limit fibrosis.

Intratracheal bleomycin causes airway remodeling and airflow obstruction in mice.

In addition to parenchymal fibrosis, fibrotic remodeling of the distal airways has been reported in interstitial lung diseases. Mechanisms of airway wall remodeling, which occurs in a variety of chronic lung diseases, are not well defined and current animal models are limited. The authors quantified airway remodeling in lung sections from subjects with idiopathic pulmonary fibrosis (IPF) and controls. To investigate intratracheal bleomycin as a potential animal model for fibrotic airway remodeling, the authors evaluated lungs from C57BL/6 mice after bleomycin treatment by histologic scoring for fibrosis and peribronchial inflammation, morphometric evaluation of subepithelial connective tissue volume density, TUNEL (terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling) assay, and immunohistochemistry for transforming growth factor ß1 (TGFß1), TGFß2, and the fibroblast marker S100A4. Lung mechanics were determined at 3 weeks post bleomycin. IPF lungs had small airway remodeling with increased bronchial wall thickness compared to controls. Similarly, bleomycin-treated mice developed dose-dependent airway wall inflammation and fibrosis and greater airflow resistance after high-dose bleomycin. Increased TUNEL(+) bronchial epithelial cells and peribronchial inflammation were noted by 1 week, and expression of TGFß1 and TGFß2 and accumulation of S100A4(+) fibroblasts correlated with airway remodeling in a bleomycin dose-dependent fashion. IPF is characterized by small airway remodeling in addition to parenchymal fibrosis, a pattern also seen with intratracheal bleomycin. Bronchial remodeling from intratracheal bleomycin follows a cascade of events including epithelial cell injury, airway inflammation, profibrotic cytokine expression, fibroblast accumulation, and peribronchial fibrosis. Thus, this model can be utilized to investigate mechanisms of airway remodeling.

Telomerase deficiency does not alter bleomycin-induced fibrosis in mice.

Idiopathic pulmonary fibrosis (IPF) is characterized by interstitial lung infiltrates, dyspnea, and progressive respiratory failure. Reports linking telomerase mutations to familial interstitial pneumonia (FIP) suggest that telomerase activity and telomere length maintenance are important in disease pathogenesis. To investigate the role of telomerase in lung fibrotic remodeling, intratracheal bleomycin was administered to mice deficient in telomerase reverse transcriptase (TERT) or telomerase RNA component (TERC) and to wild-type controls. TERT-deficient and TERC-deficient mice were interbred to the F6 and F4 generation, respectively, when they developed skin manifestations and infertility. Fibrosis was scored using a semiquantitative scale and total lung collagen was measured using a hydroxyprolinemicroplate assay. Telomere lengths were measured in peripheral blood leukocytes and isolated type II alveolar epithelial cells (AECs). Telomerase activity in type II AECs was measured using a real-time polymerase chain reaction (PCR)-based system. Following bleomycin, TERT-deficient and TERC-deficient mice developed an equivalent inflammatory response and similar lung fibrosis (by scoring of lung sections and total lung collagen content) compared to controls, a pattern seen in both early (F1) and later (F6 TERT and F4 TERC) generations. Telomere lengths were reduced in peripheral blood leukocytes and isolated type II AECs from F6 TERT-deficient and F4 TERC-deficient mice compared to controls. Telomerase deficiency in a murine model leads to telomere shortening, but does not predispose to enhanced bleomycin-induced lung fibrosis. Additional genetic or environmental factors may be necessary for development of fibrosis in the presence of telomerase deficiency.

TGFß signaling in lung epithelium regulates bleomycin-induced alveolar injury and fibroblast recruitment.

The response of alveolar epithelial cells (AECs) to lung injury plays a central role in the pathogenesis of pulmonary fibrosis, but the mechanisms by which AECs regulate fibrotic processes are not well defined. We aimed to elucidate how transforming growth factor-ß (TGFß) signaling in lung epithelium impacts lung fibrosis in the intratracheal bleomycin model. Mice with selective deficiency of TGFß receptor 2 (TGFßR2) in lung epithelium were generated and crossed to cell fate reporter mice that express ß-galactosidase (ß-gal) in cells of lung epithelial lineage. Mice were given intratracheal bleomycin (0.08 U), and the following parameters were assessed: AEC death by terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling assay, inflammation by total and differential cell counts from bronchoalveolar lavage, fibrosis by scoring of trichrome-stained lung sections, and total lung collagen content. Mice with lung epithelial deficiency of TGFßR2 had improved AEC survival, despite greater lung inflammation, after bleomycin administration. At 3 wk after bleomycin administration, mice with epithelial TGFßR2 deficiency showed a significantly attenuated fibrotic response in the lungs, as determined by semiquantitatve scoring and total collagen content. The reduction in lung fibrosis in these mice was associated with a marked decrease in the lung fibroblast population, both total lung fibroblasts and epithelial-to-mesenchymal transition-derived (S100A4(+)/ß-gal(+)) fibroblasts. Attenuation of TGFß signaling in lung epithelium provides protection from bleomycin-induced fibrosis, indicating a critical role for the epithelium in transducing the profibrotic effects of this cytokine.

Repetitive intratracheal bleomycin models several features of idiopathic pulmonary fibrosis.

Single-dose intratracheal bleomycin has been instrumental for understanding fibrotic lung remodeling, but fails to recapitulate several features of idiopathic pulmonary fibrosis (IPF). Since IPF is thought to result from recurrent alveolar injury, we aimed to develop a repetitive bleomycin model that results in lung fibrosis with key characteristics of human disease, including alveolar epithelial cell (AEC) hyperplasia. Wild-type and cell fate reporter mice expressing ß-galactosidase in cells of lung epithelial lineage were given intratracheal bleomycin after intubation, and lungs were harvested 2 wk after a single or eighth biweekly dose. Lungs were evaluated for fibrosis and collagen content. Bronchoalveolar lavage (BAL) was performed for cell counts. TUNEL staining and immunohistochemistry were performed for pro-surfactant protein C (pro-SP-C), Clara cell 10 (CC-10), ß-galactosidase, S100A4, and α-smooth muscle actin. Lungs from repetitive bleomycin mice had marked fibrosis with prominent AEC hyperplasia, similar to usual interstitial pneumonia (UIP). Compared with single dosing, repetitive bleomycin mice had greater fibrosis by scoring, morphometry, and collagen content; increased TUNEL+ AECs; and reduced inflammatory cells in BAL. Sixty-four percent of pro-SP-C+ cells in areas of fibrosis expressed CC-10 in the repetitive model, suggesting expansion of a bronchoalveolar stem cell-like population. In reporter mice, 50% of S100A4+ lung fibroblasts were derived from epithelial mesenchymal transition compared with 33% in the single-dose model. With repetitive bleomycin, fibrotic remodeling persisted 10 wk after the eighth dose. Repetitive intratracheal bleomycin results in marked lung fibrosis with prominent AEC hyperplasia, features reminiscent of UIP.

Contribution of epithelial-derived fibroblasts to bleomycin-induced lung fibrosis.

RATIONALE: Lung fibroblasts are key mediators of fibrosis resulting in accumulation of excessive interstitial collagen and extracellular matrix, but their origins are not well defined. OBJECTIVES: We aimed to elucidate the contribution of lung epithelium-derived fibroblasts via epithelial-mesenchymal transition (EMT) in the intratracheal bleomycin model. METHODS: Primary type II alveolar epithelial cells were cultured from Immortomice and exposed to transforming growth factor-beta(1) and epidermal growth factor. Cell fate reporter mice that permanently mark cells of lung epithelial lineage with beta-galactosidase were developed to study EMT, and bone marrow chimeras expressing green fluorescent protein under the control of the fibroblast-associated S100A4 promoter were generated to examine bone marrow-derived fibroblasts. Mice were given intratracheal bleomycin (0.08 unit). Immunostaining was performed for S100A4, beta-galactosidase, green fluorescent protein, and alpha-smooth muscle actin. MEASUREMENTS AND MAIN RESULTS: In vitro, primary type II alveolar epithelial cells undergo phenotypic changes of EMT when exposed to transforming growth factor-beta(1) and epidermal growth factor with loss of prosurfactant protein C and E-cadherin and gain of S100A4 and type I procollagen. In vivo, using cell fate reporter mice, approximately one-third of S100A4-positive fibroblasts were derived from lung epithelium 2 weeks after bleomycin administration. From bone marrow chimera studies, one-fifth of S100A4-positive fibroblasts were derived from bone marrow at this same time point. Myofibroblasts rarely derived from EMT or bone marrow progenitors. CONCLUSIONS: Both EMT and bone marrow progenitors contribute to S100A4-positive fibroblasts in bleomycin-induced lung fibrosis. However, neither origin is a principal contributor to lung myofibroblasts.

Progress toward improving animal models for idiopathic pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) remains a disease with an unknown cause and a poor prognosis. Among attempts to define disease pathogenesis, animal models of experimental lung fibrosis have a prominent role. Commonly used models include exposure to bleomycin, silica, fluorescein isothiocyanate; irradiation; or expression of specific genes through a viral vector or transgenic system. These all have been instrumental in the study of lung fibrosis, but all have limitations and fall short of recapitulating a pattern of usual interstitial pneumonia, the pathologic correlate to IPF. A model of repetitive bleomycin lung injury has recently been reported that results in marked lung fibrosis, prominent alveolar epithelial cell hyperplasia, a pattern of temporal heterogeneity and persistence of aberrant remodeling well after stimulus removal, representing a significant addition to the collection of animal lung fibrosis models. Taken together, animal models remain a key component in research strategies to better define IPF pathogenesis.

Genetics in pulmonary fibrosis--familial cases provide clues to the pathogenesis of idiopathic pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is the most common form of the idiopathic interstitial pneumonias and remains a disease with a poor prognosis. Familial interstitial pneumonia (FIP) occurs when 2 or more individuals from a given family have an idiopathic interstitial pneumonia. FIP cases have been linked to mutations in surfactant protein C, surfactant protein A2, telomerase reverse transcriptase and telomerase RNA component. Together, mutations in these 4 genes likely explain only 15% to 20% of FIP cases and are even less frequent in sporadic IPF. However, dysfunctional aspects of the pathways that are involved with these genes are present in sporadic forms of IPF even in the absence of mutations, suggesting common underlying disease mechanisms. By serving as a resource for identifying the current and future genetic links to disease, FIP families hold great promise in defining IPF pathogenesis, potentially suggesting targets for the development of future therapies.