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Cystic fibrosis (CF) produces variable lung disease phenotypes that are, in part, independent of the CF transmembrane conductance regulator ( CFTR) genotype. Transforming growth factor-β (TGFβ) is the best described genetic modifier of the CF phenotype, but its mechanism of action is unknown. We hypothesized that TGFβ is sufficient to drive pathognomonic features of CF lung disease in vivo and that CFTR deficiency enhances susceptibility to pathological TGFβ effects. A CF mouse model and littermate controls were exposed intratracheally to an adenoviral vector containing the TGFβ1 cDNA (Ad-TGFβ), empty vector, or PBS only. Studies were performed 1 wk after treatment, including lung mechanics, collection of bronchoalveolar lavage fluid, and analysis of lung histology, RNA, and protein. CF and non-CF mice showed similar weight loss, inflammation, goblet cell hyperplasia, and Smad pathway activation after Ad-TGFβ treatment. Ad-TGFβ produced greater abnormalities in lung mechanics in CF versus control mice, which was uniquely associated with induction of phosphoinositide 3-kinase and mitogen-activated protein kinase signaling. CFTR transcripts were reduced, and epithelial sodium channel transcripts were increased in CF and non-CF mice, whereas the goblet cell transcription factors, forkhead ortholog A3 and SAM-pointed domain-containing ETS-like factor, were increased in non-CF but not CF mice following Ad-TGFβ treatment. Pulmonary TGFβ1 expression was sufficient to produce pulmonary remodeling and abnormalities in lung mechanics that were associated with both shared and unique cell signaling pathway activation in CF and non-CF mice. These results highlight the multifunctional impact of TGFβ on pulmonary pathology in vivo and identify cellular-response differences that may impact CF lung pathology.

Pulmonary fibrosis contributes to morbidity and mortality in a range of diseases, and there are no approved therapies for reversing its progression. To understand the mechanisms underlying pulmonary fibrosis and assess potential therapies, mouse models are central to basic and translational research. Unfortunately, metrics commonly used to assess murine pulmonary fibrosis require animals to be grouped and euthanized, increasing experimental difficulty and cost. We examined the ability of magnetic resonance imaging (MRI) to noninvasively assess lung fibrosis progression and resolution in a doxycycline (Dox) regulatable, transgenic mouse model that overexpresses transforming growth factor-α (TGF-α) under control of a lung-epithelial-specific promoter. During 7 wk of Dox treatment, fibrotic lesions were readily observed as high-signal tissue. Mean weighted signal and percent signal volume were found to be the most robust MRI-derived measures of fibrosis, and these metrics correlated significantly with pleural thickness, histology scores, and hydroxyproline content ( R  = 0.75-0.89). When applied longitudinally, percent high signal volume increased by 1.5% wk -1 ( P < 0.001) and mean weighted signal increased at a rate of 0.0065 wk -1 ( P = 0.0062). Following Dox treatment, lesions partially resolved, with percent high signal volume decreasing by -3.2% wk -1 ( P = 0.0034) and weighted mean signal decreasing at -0.015 wk -1 ( P = 0.0028). Additionally, longitudinal MRI revealed dynamic remodeling in a subset of lesions, a previously unobserved behavior in this model. These results demonstrate MRI can noninvasively assess experimental lung fibrosis progression and resolution and provide unique insights into its pathobiology., (Copyright © 2017 the American Physiological Society.)

The p70 ribosomal S6 kinase (S6K) is a downstream substrate that is phosphorylated and activated by the mammalian target of rapamycin complex and regulates multiple cellular processes associated with fibrogenesis. Recent studies demonstrate that aberrant mTORC1-S6K signaling contributes to various pathological conditions, but a direct role in pulmonary fibroproliferation has not been established. Increased phosphorylation of the S6K pathway is detected immediately following transforming growth factor-α (TGF-α) expression in a transgenic model of progressive lung fibrosis. To test the hypothesis that the S6K directly regulates pulmonary fibroproliferative disease we determined the cellular sites of S6K phosphorylation during the induction of fibrosis in the TGF-α model and tested the efficacy of specific pharmacological inhibition of the S6K pathway to prevent and reverse fibrotic disease. Following TGF-α expression increased phosphorylation of the S6K was detected in the airway and alveolar epithelium and the mesenchyme of advanced subpleural fibrotic regions. Specific inhibition of the S6K with the small molecule inhibitor LY-2584702 decreased TGF-α and platelet-derived growth factor-β-induced proliferation of lung fibroblasts in vitro. Administration of S6K inhibitors to TGF-α mice prevented the development of extensive subpleural fibrosis and alterations in lung mechanics, and attenuated the increase in total lung hydroxyproline. S6K inhibition after fibrosis was established attenuated the progression of subpleural fibrosis. Together these studies demonstrate targeting the S6K pathway selectively modifies the progression of pulmonary fibrosis in the subpleural compartment of the lung., (Copyright © 2016 the American Physiological Society.)

A number of growth factors and signaling pathways regulate matrix deposition and fibroblast proliferation in the lung. The epidermal growth factor receptor (EGFR) family of receptors and the transforming growth factor-β (TGF-β) family are active in diverse biological processes and are central mediators in the initiation and maintenance of fibrosis in many diseases. Transforming growth factor-α (TGF-α) is a ligand for the EGFR, and doxycycline (Dox)-inducible transgenic mice conditionally expressing TGF-α specifically in the lung epithelium develop progressive fibrosis accompanied with cachexia, changes in lung mechanics, and marked pleural thickening. Although recent studies demonstrate that EGFR activation modulates the fibroproliferative effects involved in the pathogenesis of TGF-β induced pulmonary fibrosis, in converse, the direct role of EGFR induction of the TGF-β pathway in the lung is unknown. The αvβ6 integrin is an important in vivo activator of TGF-β activation in the lung. Immunohistochemical analysis of αvβ6 protein expression and bronchoalveolar analysis of TGF-β pathway signaling indicates activation of the αvβ6/TGF-β pathway only at later time points after lung fibrosis was already established in the TGF-α model. To determine the contribution of the αvβ6/TGF-β pathway on the progression of established fibrotic disease, TGF-α transgenic mice were administered Dox for 4 wk, which leads to extensive fibrosis; these mice were then treated with a function-blocking anti-αvβ6 antibody with continued administration of Dox for an additional 4 wk. Compared with TGF-α transgenic mice treated with control antibody, αvβ6 inhibition significantly attenuated pleural thickening and altered the decline in lung mechanics. To test the effects of genetic loss of the β6 integrin, TGF-α transgenic mice were mated with β6-null mice and the degree of fibrosis was compared in adult mice following 8 wk of Dox administration. Genetic ablation of the β6 integrin attenuated histological and physiological changes in the lungs of TGF-α transgenic mice although a significant degree of fibrosis still developed. In summary, inhibition of the β6 integrin led to a modest, albeit significant, effect on pleural thickening and lung function decline observed with TGF-α-induced pulmonary fibrosis. These data support activation of the αvβ6/TGF-β pathway as a secondary effect contributing to TGF-α-induced pleural fibrosis and suggest a complex contribution of multiple mediators to the maintenance of progressive fibrosis in the lung.

Increases in the epidermal growth factor receptor (EGFR) have been associated with the severity of airway thickening in chronic asthmatic subjects, and EGFR signaling is induced by asthma-related cytokines and inflammation. The goal of this study was to determine the role of EGFR signaling in a chronic allergic model of asthma and specifically in epithelial cells, which are increasingly recognized as playing an important role in asthma. EGFR activation was assessed in mice treated with intranasal house dust mite (HDM) for 3 wk. EGFR signaling was inhibited in mice treated with HDM for 6 wk, by using either the drug erlotinib or a genetic approach that utilizes transgenic mice expressing a mutant dominant negative epidermal growth factor receptor in the lung epithelium (EGFR-M mice). Airway hyperreactivity (AHR) was assessed by use of a flexiVent system after increasing doses of nebulized methacholine. Airway smooth muscle (ASM) thickening was measured by morphometric analysis. Sensitization to HDM (IgG and IgE), inflammatory cells, and goblet cell changes were also assessed. Increased EGFR activation was detected in HDM-treated mice, including in bronchiolar epithelial cells. In mice exposed to HDM for 6 wk, AHR and ASM thickening were reduced after erlotinib treatment and in EGFR-M mice. Sensitization to HDM and inflammatory cell counts were similar in all groups, except neutrophil counts, which were lower in the EGFR-M mice. Goblet cell metaplasia with HDM treatment was reduced by erlotinib, but not in EGFR-M transgenic mice. This study demonstrates that EGFR signaling, especially in the airway epithelium, plays an important role in mediating AHR and remodeling in a chronic allergic asthma model.

Transforming growth factor-alpha (TGF-alpha) is a ligand for the EGF receptor (EGFR). EGFR activation is associated with fibroproliferative processes in human lung disease and animal models of pulmonary fibrosis. We determined the effects of EGFR tyrosine kinase inhibitors gefitinib (Iressa) and erlotinib (Tarceva) on the development and progression of TGF-alpha-induced pulmonary fibrosis. Using a doxycycline-regulatable transgenic mouse model of lung-specific TGF-alpha expression, we determined effects of treatment with gefitinib and erlotinib on changes in lung histology, total lung collagen, pulmonary mechanics, pulmonary hypertension, and expression of genes associated with synthesis of ECM and vascular remodeling. Induction in the lung of TGF-alpha caused progressive pulmonary fibrosis over an 8-wk period. Daily administration of gefitinib or erlotinib prevented development of fibrosis, reduced accumulation of total lung collagen, prevented weight loss, and prevented changes in pulmonary mechanics. Treatment of mice with gefitinib 4 wk after the induction of TGF-alpha prevented further increases in and partially reversed total collagen levels and changes in pulmonary mechanics and pulmonary hypertension. Increases in expression of genes associated with synthesis of ECM as well as decreases of genes associated with vascular remodeling were also prevented or partially reversed. Administration of gefitinib or erlotinib did not cause interstitial fibrosis or increases in lavage cell counts. Administration of small molecule EGFR tyrosine kinase inhibitors prevented further increases in and partially reversed pulmonary fibrosis induced directly by EGFR activation without inducing inflammatory cell influx or additional lung injury.

Transforming growth factor-alpha (TGF-alpha) and its receptor, the epithelial growth factor receptor (EGFR), have been associated with lung remodeling in premature infants with bronchopulmonary dysplasia (BPD). The goal of this study was to target TGF-alpha overexpression to the saccular phase of lung morphogenesis and determine early alterations in gene expression. Conditional lung-specific TGF-alpha bitransgenic mice and single-transgene control mice were generated. TGF-alpha overexpression was induced by doxycycline (Dox) treatment from embryonic day 16.5 (E16.5) to E18.5. After birth, all bitransgenic pups died by postnatal day 7 (P7). Lung histology at E18.5 and P1 showed abnormal lung morphogenesis in bitransgenic mice, characterized by mesenchymal thickening, vascular remodeling, and poor apposition of capillaries to distal air spaces. Surfactant levels (saturated phosphatidylcholine) were not reduced in bitransgenic mice. Microarray analysis was performed after 1 or 2 days of Dox treatment during the saccular (E17.5, E18.5) and alveolar phases (P4, P5) to identify genes induced by EGFR signaling that were shared or unique to each phase. We found 196 genes to be altered (>1.5-fold change; P < 0.01 for at least 2 time points), with only 32% similarly altered in both saccular and alveolar phases. Western blot analysis and immunostaining showed that five genes selected from the microarrays (egr-1, SP-B, SP-D, S100A4, and pleiotrophin) were also increased at the protein level. Pathological changes in TGF-alpha-overexpressing mice bore similarities to premature infants born in the saccular phase who develop BPD, including remodeling of the distal lung septae and arteries.

To determine potential relationships between transforming growth factor (TGF)-alpha and surfactant homeostasis, the metabolism, function, and composition of surfactant phospholipid and proteins were assessed in transgenic mice in which TGF-alpha was expressed in respiratory epithelial cells. Secretion of saturated phosphatidylcholine was decreased 40-60% by expression of TGF-alpha. Although SP-A, SP-B, and SP-C mRNA levels were unchanged by expression of TGF-alpha, SP-A and SP-B content in bronchoalveolar lavage fluid was decreased. The minimum surface tension of surfactant isolated from the transgenic mice was significantly increased. Incubation of cultured normal mice type II cells with TGF-alpha in vitro did not change secretion of surfactant phosphatidylcholine and SP-B, indicating that TGF-alpha does not directly influence surfactant secretion. Expression of a dominant negative (mutant) EGF receptor in the respiratory epithelium blocked the TGF-alpha-induced changes in lung morphology and surfactant secretion, indicating that EGF receptor signaling in distal epithelial cells was required for TGF-alpha effects on surfactant homeostasis. Because many epithelial cells were embedded in fibrotic lesions caused by TGF-alpha, changes in surfactant homeostasis may at least in part be influenced by tissue remodeling that results in decreased surfactant secretion. The number of nonembedded type II cells was decreased 30% when TGF-alpha was expressed during development and was increased threefold by TGF-alpha expression in adulthood, suggesting possible alteration of type II cells on surfactant metabolism in the adult lung. Abnormalities in surfactant function and decreased surfactant level in the airways may contribute to the pathophysiology induced by TGF-alpha in both the developing and adult lung.

To determine whether overexpression of transforming growth factor (TGF)-alpha in the adult lung causes remodeling independently of developmental influences, we generated conditional transgenic mice expressing TGF-alpha in the epithelium under control of the doxycycline (Dox)-regulatable Clara cell secretory protein promoter. Two transgenic lines were generated, and following 4 days of Dox-induction TGF-alpha levels in whole lung homogenate were increased 13- to 18-fold above nontransgenic levels. After TGF-alpha induction, transgenic mice developed progressive pulmonary fibrosis and body weight loss, with mice losing 15% of their weight after 6 wk of TGF-alpha induction. Fibrosis was detected within 4 days of TGF-alpha induction and developed initially in the perivascular, peribronchial, and pleural regions but later extended into the interstitium. Fibrotic regions were composed of increased collagen and cellular proliferation and were adjacent to airway and alveolar epithelial sites of TGF-alpha expression. Fibrosis progressed in the absence of inflammatory cell infiltrates as determined by histology, without changes in bronchiolar alveolar lavage total or differential cell counts and without changes in proinflammatory cytokines TNF-alpha or IL-6. Active TGF-beta in whole lung homogenate was not altered 1 and 4 days after TGF-alpha induction, and immunostaining was not increased in the peribronchial/perivascular areas at all time points. Chronic epithelial expression of TGF-alpha in adult mice caused progressive pulmonary fibrosis associated with increased collagen and extracellular matrix deposition and increased cellular proliferation. Induction of pulmonary fibrosis by TGF-alpha was independent of inflammation or early activation of TGF-beta.

Pulmonary vascular disease plays a major role in morbidity and mortality in infant and adult lung diseases in which increased levels of transforming growth factor (TGF)-alpha and its receptor EGFR have been associated. The aim of this study was to determine whether overexpression of TGF-alpha disrupts pulmonary vascular development and causes pulmonary hypertension. Lung-specific expression of TGF-alpha in transgenic mice was driven with the human surfactant protein (SP)-C promoter. Pulmonary arteriograms and arterial counts show that pulmonary vascular development was severely disrupted in TGF-alpha mice. TGF-alpha mice developed severe pulmonary hypertension and vascular remodeling characterized by abnormally extensive muscularization of small pulmonary arteries. Pulmonary vascular development was significantly improved and pulmonary hypertension and vascular remodeling were prevented in bi-transgenic mice expressing both TGF-alpha and a dominant-negative mutant EGF receptor under the control of the SP-C promoter. Vascular endothelial growth factor (VEGF-A), an important angiogenic factor produced by the distal epithelium, was decreased in the lungs of TGF-alpha adults and in the lungs of infant TGF-alpha mice before detectable abnormalities in pulmonary vascular development. Hence, overexpression of TGF-alpha caused severe pulmonary vascular disease, which was mediated through EGFR signaling in distal epithelial cells. Reductions in VEGF may contribute to the pathogenesis of pulmonary vascular disease in TGF-alpha mice.