Predisposition for disrepair in the aged lung

An Official American Thoracic Society Workshop Report: Use of Animal Models for the Preclinical Assessment of Potential Therapies for Pulmonary Fibrosis

Numerous compounds have shown efficacy in limiting development of pulmonary fibrosis using animal models, yet few of these compounds have replicated these beneficial effects in clinical trials. Given the challenges associated with performing clinical trials in patients with idiopathic pulmonary fibrosis (IPF), it is imperative that preclinical data packages be robust in their analyses and interpretations to have the best chance of selecting promising drug candidates to advance to clinical trials. The American Thoracic Society has convened a group of experts in lung fibrosis to discuss and formalize recommendations for preclinical assessment of antifibrotic compounds. The panel considered three major themes (choice of animal, practical considerations of fibrosis modeling, and fibrotic endpoints for evaluation). Recognizing the need for practical considerations, we have taken a pragmatic approach. The consensus view is that use of the murine intratracheal bleomycin model in animals of both genders, using hydroxyproline measurements for collagen accumulation along with histologic assessments, is the best-characterized animal model available for preclinical testing. Testing of antifibrotic compounds in this model is recommended to occur after the acute inflammatory phase has subsided (generally after Day 7). Robust analyses may also include confirmatory studies in human IPF specimens and validation of results in a second system using in vivo or in vitro approaches. The panel also strongly encourages the publication of negative results to inform the lung fibrosis community. These recommendations are for preclinical therapeutic evaluation only and are not intended to dissuade development of emerging technologies to better understand IPF pathogenesis.

Exploring Animal Models That Resemble Idiopathic Pulmonary Fibrosis

Large multicenter clinical trials have led to two recently approved drugs for patients with idiopathic pulmonary fibrosis (IPF); yet, both of these therapies only slow disease progression and do not provide a definitive cure. Traditionally, preclinical trials have utilized mouse models of bleomycin (BLM)-induced pulmonary fibrosis—though several limitations prevent direct translation to human IPF. Spontaneous pulmonary fibrosis occurs in other animal species, including dogs, horses, donkeys, and cats. While the fibrotic lungs of these animals share many characteristics with lungs of patients with IPF, current veterinary classifications of fibrotic lung disease are not entirely equivalent. Additional studies that profile these examples of spontaneous fibroses in animals for similarities to human IPF should prove useful for both human and animal investigators. In the meantime, studies of BLM-induced fibrosis in aged male mice remain the most clinically relevant model for preclinical study for human IPF. Addressing issues such as time course of treatment, animal size and characteristics, clinically irrelevant treatment endpoints, and reproducibility of therapeutic outcomes will improve the current status of preclinical studies. Elucidating the mechanisms responsible for the development of fibrosis and disrepair associated with aging through a collaborative approach between researchers will promote the development of models that more accurately represent the realm of interstitial lung diseases in humans.

Nicotine stimulates collagen type I expression in lung via α7 nicotinic acetylcholine receptors

Background

Tobacco-related chronic lung diseases are characterized by alterations in lung architecture leading to decreased lung function. Knowledge of the exact mechanisms involved in tobacco-induced tissue remodeling and inflammation remains incomplete. We hypothesize that nicotine stimulates the expression of extracellular matrix proteins, leading to relative changes in lung matrix composition, which may affect immune cells entering the lung after injury.

Methods

Pulmonary fibroblasts from wildtype and α7 nicotinic acetylcholine receptor knockout (α7KO) mice were exposed to nicotine and examined for collagen type 1 mRNA and protein expression. Testing the potential role on immune cell function, pulmonary fibroblasts were retained in culture for 120 h. The fibroblasts were eliminated by osmotic lysis and the remaining matrix-coated dishes were washed thoroughly. U937 cells were incubated on the matrix-coated dishes for 24 h followed by evaluation of IL-1β gene expression. Wildtype or α7KO C57BL/6 mice (female, 8–12 weeks) were fed normal diet and exposed to nicotine in their drinking water (100 μg/ml) for 8-12weeks. Lungs were processed for mRNA, protein, and histology. Statistical significance was determined at p ≤ .05 by two-tailed test or 2-way ANOVA with Bonferroni posttest.

Results

We found that nicotine stimulated collagen type I mRNA and protein expression in a dose-dependent manner and up to 72 h in primary lung fibroblasts. The stimulatory effect of nicotine was inhibited in α7KO primary lung fibroblasts. Testing the potential role of these events on immune cell function, U937 monocytic cells were cultured atop matrices derived from nicotine-treated lung fibroblasts. These cells expressed more IL-1β than those cultured atop matrices derived from untreated fibroblasts, and antibodies against the α2β1 collagen integrin receptor inhibited the effect. Nicotine also stimulated fibroblast proliferation via MEK-1/ERK, unveiling a potentially amplifying pathway. In vivo, nicotine increased collagen type I expression was detected in wildtype, but not in α7KO mice. Wildtype mice showed increased collagen staining in lung, primarily around the airways.

Conclusions

These observations suggest that nicotine stimulates fibroblast proliferation and their expression of collagen type I through α7 nAChRs, thereby altering the relative composition of the lung matrix without impacting the overall lung architecture; this may influence inflammatory responses after injury.

Sirtuin 3 Deregulation Promotes Pulmonary Fibrosis

Oxidative stress leads to alveolar epithelial cell injury and fibroblast-myofibroblast differentiation (FMD), key events in the pathobiology of pulmonary fibrosis (PF). Sirtuin 3 (SIRT3) is a mitochondrial protein deacetylase regulator of antioxidant response and mitochondrial homeostasis. Here, we demonstrate reduced SIRT3 expression in the lungs of old mice compared to young mice, as well as in two murine models of PF. The analysis of the pattern of SIRT3 expression in the lungs of patients with PF revealed low SIRT3 staining within the fibrotic regions. We also demonstrated, using murine models of PF and human lung fibroblasts, that reduced SIRT3 expression in response to transforming growth factor beta 1 (TGFβ1) promotes acetylation (inactivation) of major oxidative stress response regulators, such as SOD2 and isocitrate dehydrogenase 2. Reduction of SIRT3 in human lung fibroblasts promoted FMD. By contrast, overexpression of SIRT3 attenuated TGFβ1-mediated FMD and significantly reduced the levels of SMAD family member 3 (SMAD3). Resveratrol induced SIRT3 expression and ameliorated acetylation changes induced by TGFβ1. We demonstrated that SIRT3-deficient mice are more susceptible to PF compared to control mice, and concomitantly exhibit enhanced SMAD3 expression. Collectively, these data define a SIRT3/TGFβ1 interaction during aging that may play a significant role in the pathobiology of PF.

Mitochondria in the spotlight of aging and idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic age-related lung disease with high mortality that is characterized by abnormal scarring of the lung parenchyma. There has been a recent attempt to define the age-associated changes predisposing individuals to develop IPF. Age-related perturbations that are increasingly found in epithelial cells and fibroblasts from IPF lungs compared with age-matched cells from normal lungs include defective autophagy, telomere attrition, altered proteostasis, and cell senescence. These divergent processes seem to converge in mitochondrial dysfunction and metabolic distress, which potentiate maladaptation to stress and susceptibility to age-related diseases such as IPF. Therapeutic approaches that target aging processes may be beneficial for halting the progression of disease and improving quality of life in IPF patients.

Aging and Lung Disease. Clinical Impact and Cellular and Molecular Pathways

With the expected rapid growth of the aging population worldwide, there is a clear need to understand the complex process of aging to develop interventions that might extend the health span in this group of patients. Aging is associated with increased susceptibility to a variety of chronic diseases, and lung pathologies are no exception. The prevalence of lung diseases such as idiopathic pulmonary fibrosis and chronic obstructive pulmonary disease has been found to increase considerably with age. In October 2014, the Division of Pulmonary, Allergy, and Critical Care of the University of Pittsburgh cohosted the Pittsburgh-Munich Lung Conference focused in aging and lung disease with the Comprehensive Pneumology Center, Institute of Lung Biology and Disease, Ludwig-Maximilians University and Helmholtz Zentrum Munich Germany. The purpose of the conference was to disseminate novel concepts in aging mechanisms that have an impact in lung physiology and pathogenesis of pulmonary diseases that commonly occur in older populations. The conference included 28 presentations on diverse topics, which are summarized in this report. The participants identified priorities for future basic and translational investigations that will assist in the identification of molecular insights involved in the pathogenesis of age-related pulmonary diseases and the design of therapeutic interventions for these lung conditions.

Differential Regulation of the Extracellular Cysteine/Cystine Redox State (EhCySS) by Lung Fibroblasts from Young and Old Mice

Aging is associated with progressive oxidation of plasma cysteine (Cys)/cystine (CySS) redox state, expressed as E_hCySS. Cultured cells condition their media to reproduce physiological E_hCySS, but it is unknown whether aged cells produce a more oxidized extracellular environment reflective of that seen in vivo. In the current study, we isolated primary lung fibroblasts from young and old female mice and measured the media E_hCySS before and after challenge with Cys or CySS. We also measured expression of genes related to redox regulation and fibroblast function. These studies revealed that old fibroblasts produced a more oxidizing extracellular E_hCySS than young fibroblasts and that old fibroblasts had a decreased capacity to recover from an oxidative challenge due to a slower rate of reduction of CySS to Cys. These defects were associated with 10-fold lower expression of the Slc7a11 subunit of the xCT cystine-glutamate transporter. Extracellular superoxide dismutase (Sod3) was the only antioxidant or thiol-disulfide regulating enzyme among 36 examined that was downregulated in old fibroblasts by more than 2-fold, but there were numerous changes in extracellular matrix components. Thus, aging fibroblasts not only contribute to remodeling of the extracellular matrix but also have a profound effect on the extracellular redox environment.

What Should Be Chronic: The Animal, the Model, or Both?

The systematic review of mesenchymal stromal cell (MSC) therapy in pulmonary fibrosis by Srour and Thébaud published in a recent issue of Stem Cells Translational Medicine is critically reviewed; Rubio et al. suggest that bleomycin-induced lung injury in aged mice studied over time is a more clinically applicable model for human idiopathic pulmonary fibrosis.

"Scar-cinoma": viewing the fibrotic lung mesenchymal cell in the context of cancer biology

Lung cancer and pulmonary fibrosis are common, yet distinct, pathological processes that represent urgent unmet medical needs. Striking clinical and mechanistic parallels exist between these distinct disease entities. The goal of this article is to examine lung fibrosis from the perspective of cancer-associated phenotypic hallmarks, to discuss areas of mechanistic overlap and distinction, and to highlight profibrotic mechanisms that contribute to carcinogenesis. Ultimately, we speculate that such comparisons might identify opportunities to leverage our current understanding of the pathobiology of each disease process in order to advance novel therapeutic approaches for both. We anticipate that such "outside the box" concepts could be translated to a more precise and individualised approach to fibrotic diseases of the lung.

Mechanisms of Lung Fibrosis Resolution

Fibrogenesis involves a dynamic interplay between factors that promote the biosynthesis and deposition of extracellular matrix along with pathways that degrade the extracellular matrix and eliminate the primary effector cells. Opposing the often held perception that fibrotic tissue is permanent, animal studies and clinical data now demonstrate the highly plastic nature of organ fibrosis that can, under certain circumstances, regress. This review describes the current understanding of the mechanisms whereby the lung is known to resolve fibrosis focusing on degradation of the extracellular matrix, removal of myofibroblasts, and the role of inflammatory cells. Although there are significant gaps in understanding lung fibrosis resolution, accelerated improvements in biotechnology and bioinformatics are expected to improve the understanding of these mechanisms and have high potential to lead to novel and effective restorative therapies in the treatment not only of pulmonary fibrosis, but also of a wide-ranging spectrum of chronic disorders.

Decreased Laminin Expression by Human Lung Epithelial Cells and Fibroblasts Cultured in Acellular Lung Scaffolds from Aged Mice

The lung changes functionally and structurally with aging. However, age-related effects on the extracellular matrix (ECM) and corresponding effects on lung cell behavior are not well understood. We hypothesized that ECM from aged animals would induce aging-related phenotypic changes in healthy inoculated cells. Decellularized whole organ scaffolds provide a powerful model for examining how ECM cues affect cell phenotype. The effects of age on ECM composition in both native and decellularized mouse lungs were assessed as was the effect of young vs old acellular ECM on human bronchial epithelial cells (hBECs) and lung fibroblasts (hLFs). Native aged (1 year) lungs demonstrated decreased expression of laminins α3 and α4, elastin and fibronectin, and elevated collagen, compared to young (3 week) lungs. Proteomic analyses of decellularized ECM demonstrated similar findings, and decellularized aged lung ECM contained less diversity in structural proteins compared to young ECM. When seeded in old ECM, hBECs and hLFs demonstrated lower gene expression of laminins α3 and α4, respectively, as compared to young ECM, paralleling the laminin deficiency of aged ECM. ECM changes appear to be important factors in potentiating aging-related phenotypes and may provide clues to mechanisms that allow for aging-related lung diseases.

Conformational coupling of integrin and Thy-1 regulates Fyn priming and fibroblast mechanotransduction

Lateral associations between inactive α_v integrin and Thy-1 glycoprotein control integrin avidity to extracellular matrix ligand, the localization and kinetics of downstream signal activity, and mechanosensitive remodeling of the cytoskeleton.

Progressive fibrosis is characterized by excessive deposition of extracellular matrix (ECM), resulting in gross alterations in tissue mechanics. Changes in tissue mechanics can further augment scar deposition through fibroblast mechanotransduction. In idiopathic pulmonary fibrosis, a fatal form of progressive lung fibrosis, previous work has shown that loss of Thy-1 (CD90) expression in fibroblasts correlates with regions of active fibrogenesis, thus representing a pathologically relevant fibroblast subpopulation. We now show that Thy-1 is a regulator of fibroblast rigidity sensing. Thy-1 physically couples to inactive α_vβ₃ integrins via its RGD-like motif, altering baseline integrin avidity to ECM ligands and also facilitating preadhesion clustering of integrin and membrane rafts via Thy-1’s glycophosphatidylinositol tether. Disruption of Thy-1–α_vβ₃ coupling altered recruitment of Src family kinases to adhesion complexes and impaired mechanosensitive, force-induced Rho signaling, and rigidity sensing. Loss of Thy-1 was sufficient to induce myofibroblast differentiation in soft ECMs and may represent a physiological mechanism important in wound healing and fibrosis.

Therapeutic benefits of young, but not old, adipose-derived mesenchymal stem cells in a chronic mouse model of bleomycin-induced pulmonary fibrosis

The observation that pulmonary inflammatory lesions and bleomycin (BLM)-induced pulmonary fibrosis spontaneously resolve in young mice, whereas remaining irreversible in aged mice suggests that impairment of pulmonary regeneration and repair is associated with aging. Because mesenchymal stem cells (MSCs) may promote repair after injury, we postulated that differences in MSCs from aged mice may underlie postinjury fibrosis in aging. The potential for young-donor MSCs to inhibit BLM-induced pulmonary fibrosis in aged male mice (>22 months) has not been studied. Adipose-derived MSCs (ASCs) from young (4 months) and old (22 months) male mice were infused 1 day after intratracheal BLM administration. At 21-day sacrifice, aged BLM mice demonstrated lung fibrosis by Ashcroft score, collagen content, and α(v)-integrin messenger RNA (mRNA) expression. Lung tissue from aged BLM mice receiving young ASCs exhibited decreased fibrosis, matrix metalloproteinase (MMP)-2 activity, oxidative stress, and markers of apoptosis vs BLM controls. Lung mRNA expression of tumor necrosis factor-alpha was also decreased in aged BLM mice receiving young-donor ASCs vs BLM controls. In contrast, old-donor ASC treatment in aged BLM mice did not reduce fibrosis and related markers. On examination of the cells, young-donor ASCs had decreased mRNA expression of MMP-2, insulin-like growth factor (IGF) receptor, and protein kinase B (AKT) activation compared with old-donor ASCs. These results show that the BLM-induced pulmonary fibrosis in aged mice could be blocked by young-donor ASCs and that the mechanisms involve changes in collagen turnover and markers of inflammation.

Matrix metalloproteinases as therapeutic targets for idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a restrictive lung disease that is associated with high morbidity and mortality. Current medical therapies are not fully effective at limiting mortality in patients with IPF, and new therapies are urgently needed. Matrix metalloproteinases (MMPs) are proteinases that, together, can degrade all components of the extracellular matrix and numerous nonmatrix proteins. MMPs and their inhibitors, tissue inhibitors of MMPs (TIMPs), have been implicated in the pathogenesis of IPF based upon the results of clinical studies reporting elevated levels of MMPs (including MMP-1, MMP-7, MMP-8, and MMP-9) in IPF blood and/or lung samples. Surprisingly, studies of gene-targeted mice in murine models of pulmonary fibrosis (PF) have demonstrated that most MMPs promote (rather than inhibit) the development of PF and have identified diverse mechanisms involved. These mechanisms include MMPs: (1) promoting epithelial-to-mesenchymal transition (MMP-3 and MMP-7); (2) increasing lung levels or activity of profibrotic mediators or reducing lung levels of antifibrotic mediators (MMP-3, MMP-7, and MMP-8); (3) promoting abnormal epithelial cell migration and other aberrant repair processes (MMP-3 and MMP-9); (4) inducing the switching of lung macrophage phenotypes from M1 to M2 types (MMP-10 and MMP-28); and (5) promoting fibrocyte migration (MMP-8). Two MMPs, MMP-13 and MMP-19, have antifibrotic activities in murine models of PF, and two MMPs, MMP-1 and MMP-10, have the potential to limit fibrotic responses to injury. Herein, we review what is known about the contributions of MMPs and TIMPs to the pathogenesis of IPF and discuss their potential as therapeutic targets for IPF.

Alteration of Aging-Dependent MicroRNAs in Idiopathic Pulmonary Fibrosis

Preclinical Research Idiopathic Pulmonary Fibrosis (IPF) is the most severe fibrotic lung disease and characterized by the accumulation of (myo)fibroblasts and collagen within the alveolar wall resulting in obliteration of the gas-exchange surface. Although the detailed pathogenesis is not understood, recent studies have found that several microRNAs (miRNAs) are associated with the progression of lung diseases including IPF. IPF is a fibrotic disease and, most frequently found in an aged population. In this review, the functional roles of miRNAs that are deregulated in IPF progression are discussed together with how aging affects the miRNA signature, altering the fibroblast phenotype and promoting lung fibrosis. Finally, the possibility of targeting miRNAs as a therapeutic approach for the treatment of IPF is discussed.

Deregulation of selective autophagy during aging and pulmonary fibrosis: the role of TGFβ1

Aging constitutes a significant risk factor for fibrosis, and idiopathic pulmonary fibrosis (IPF) is characteristically associated with advancing age. We propose that age-dependent defects in the quality of protein and cellular organelle catabolism may be causally related to pulmonary fibrosis. Our research found that autophagy diminished with corresponding elevated levels of oxidized proteins and lipofuscin in response to lung injury in old mice and middle-aged mice compared to younger animals. More importantly, older mice expose to lung injury are characterized by deficient autophagic response and reduced selective targeting of mitochondria for autophagy (mitophagy). Fibroblast to myofibroblast differentiation (FMD) is an important feature of pulmonary fibrosis in which the profibrotic cytokine TGFβ1 plays a pivotal role. Promotion of autophagy is necessary and sufficient to maintain normal lung fibroblasts’ fate. On the contrary, FMD mediated by TGFβ1 is characterized by reduced autophagy flux, altered mitophagy, and defects in mitochondrial function. In accord with these findings, PINK1 expression appeared to be reduced in fibrotic lung tissue from bleomycin and a TGFβ1-adenoviral model of lung fibrosis. PINK1 expression is also reduced in the aging murine lung and biopsies from IPF patients compared to controls. Furthermore, deficient PINK1 promotes a profibrotic environment. Collectively, this study indicates that an age-related decline in autophagy and mitophagy responses to lung injury may contribute to the promotion and/or perpetuation of pulmonary fibrosis. We propose that promotion of autophagy and mitochondrial quality control may offer an intervention against age-related fibrotic diseases.

The effect of environmental chemicals on the tumor microenvironment

Potentially carcinogenic compounds may cause cancer through direct DNA damage or through indirect cellular or physiological effects. To study possible carcinogens, the fields of endocrinology, genetics, epigenetics, medicine, environmental health, toxicology, pharmacology and oncology must be considered. Disruptive chemicals may also contribute to multiple stages of tumor development through effects on the tumor microenvironment. In turn, the tumor microenvironment consists of a complex interaction among blood vessels that feed the tumor, the extracellular matrix that provides structural and biochemical support, signaling molecules that send messages and soluble factors such as cytokines. The tumor microenvironment also consists of many host cellular effectors including multipotent stromal cells/mesenchymal stem cells, fibroblasts, endothelial cell precursors, antigen-presenting cells, lymphocytes and innate immune cells. Carcinogens can influence the tumor microenvironment through effects on epithelial cells, the most common origin of cancer, as well as on stromal cells, extracellular matrix components and immune cells. Here, we review how environmental exposures can perturb the tumor microenvironment. We suggest a role for disrupting chemicals such as nickel chloride, Bisphenol A, butyltins, methylmercury and paraquat as well as more traditional carcinogens, such as radiation, and pharmaceuticals, such as diabetes medications, in the disruption of the tumor microenvironment. Further studies interrogating the role of chemicals and their mixtures in dose-dependent effects on the tumor microenvironment could have important general mechanistic implications for the etiology and prevention of tumorigenesis.

TGF-β1 epigenetically modifies Thy-1 expression in primary lung fibroblasts

Idiopathic pulmonary fibrosis is a progressive lung disease that increases in incidence with age. We identified a profibrotic lung phenotype in aging mice characterized by an increase in the number of fibroblasts lacking the expression of thymocyte differentiation antigen 1 (Thy-1) and an increase in transforming growth factor (TGF)-β1 expression. It has been shown that Thy-1 expression can be epigenetically modified. Lung fibroblasts (PLFs) were treated with TGF-β1 ± DNA methyltransferase (DNMT) inhibitor 5-aza-2'-deoxycytidine (5-AZA) and analyzed for Thy-1 gene and protein expression, DNMT protein expression, and activity. α-Smooth muscle actin (α-SMA) and collagen type 1 (Col1A1) gene and protein expression was assessed. PLFs were transfected with DNMT1 silencing RNA ± TGF-β1. TGF-β1 inhibited Thy-1 gene and protein expression in PLFs, and cotreatment with 5-AZA ameliorated this effect and appeared to inhibit DNMT1 activation. TGF-β1 induced Thy-1 promoter methylation as assessed by quantitative methyl PCR. Treatment with 5-AZA attenuated TGF-β1-induced Col1A1 gene and protein expression and α-SMA gene expression (but not α-SMA protein expression). Inhibiting DNMT1 with silencing RNA attenuated TGF-β1-induced DNMT activity and its downstream suppression of Thy-1 mRNA and protein expression as well as inhibited α-SMA mRNA and Col1A1 mRNA and protein expression, and showed a decreased trend in Thy-1 promoter methylation. Immunofluorescence for α-SMA suggested that 5-AZA inhibited stress fiber formation. These findings suggest that TGF-β1 epigenetically regulates lung fibroblast phenotype through methylation of the Thy-1 promoter. Targeted inhibition of DNMT in the right clinical context might prevent fibroblast to myofibroblast transdifferentiation and collagen deposition, which in turn could prevent fibrogenesis in the lung and other organs.

Association between Maturation and Aging and Pulmonary Responses in Animal Models of Lung Injury

Background:

Advanced age is associated with an increased susceptibility and mortality of the acute respiratory distress syndrome. This may be due to the progressive changes in innate immune responses and intrinsic properties of the lung that occur during the process of aging. Therefore, this study assesses the association between maturation and aging and pulmonary responses to injury in animal models of lung injury.

Methods:

A systematic search was conducted in PubMed, EMBASE (up to June 2014) and in the references of relevant articles to identify the studies using in vivo models of lung injury caused by an acute pulmonary insult, in which at least two age groups were compared. Because methodological diversity precluded combining these studies in a quantitative meta-analysis, data are presented based on the qualitative comparison with the adult group.

Results:

Of the 2,840 identified studies, 51 were included in this review. Most studies showed that, in response to a pulmonary insult, increasing age is associated with more pulmonary inflammation, edema, alveolar damage, and higher mortality. In addition, results indicate the existence of age-dependent changes in key components of the intracellular signaling pathways involved in the inflammatory response.

Conclusions:

Increasing age seems to be correlated with exaggerated pulmonary responses to injury, ultimately leading to more severe lung injury. Pulmonary inflammation seems relatively suppressed in infants/juveniles, whereas in the middle aged/elderly, the inflammatory response seems delayed but aggravated. This implies that investigators and clinicians need to use caution about extrapolating results from adolescent or youngadult animals to pediatric or elderly patients in clinical practice.

TGFβ1 mediates alcohol-induced Nrf2 suppression in lung fibroblasts

Background

Chronic alcohol ingestion induces the expression of transforming growth factor beta-1(TGFβ1), inhibits nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-mediated activation of the antioxidant response element (ARE), depletes alveolar glutathione pools, and potentiates acute lung injury. In this study, we examined the mechanistic relationship between TGFβ1 and Nrf2-ARE signaling in the experimental alcoholic lung.

Methods

Wild-type mice were treated ± alcohol in drinking water for 8 weeks and their lungs were assessed for Nrf2 expression. In parallel, mouse lung fibroblasts were cultured ± alcohol and treated ± sulforaphane (SFP; an activator of Nrf2), ±TGFβ1, ±TGFβ1 neutralizing antibody, and/or ±activin receptor-like kinase 5 inhibitors (to block TGβ1 receptor signaling) and then analyzed for the expression of Nrf2, Kelch-like ECH-associated protein 1 (Keap1) and TGFβ1, Nrf2-ARE activity, and the expression of the Nrf2-ARE-dependent antioxidants glutathione s-transferase theta 2 (GSTT2) and glutamate-cysteine ligase catalytic subunit (GCLC). Finally, silencing RNA (siRNA) of Nrf2 was then performed prior to alcohol exposure and subsequent analysis of TGFβ1 expression.

Results

Alcohol treatment in vivo or in vitro decreased Nrf2 expression in murine whole lung and lung fibroblasts, respectively. In parallel, alcohol exposure in vitro decreased Keap1 gene and protein expression in lung fibroblasts. Furthermore, alcohol exposure increased TGFβ1 expression but decreased Nrf2-ARE activity and expression of the ARE-dependent genes for GSTT2 and GCLC. These effects of alcohol were prevented by treatment with SFP; in contrast, Nrf2 SiRNA expression exacerbated alcohol-induced TGFβ1 expression. Finally, TGFβ1 treatment directly suppressed Nrf2-ARE activity whereas blocking TGFβ1 signaling attenuated alcohol-induced suppression of Nrf2-ARE activity.

Conclusions

Alcohol-induced oxidative stress is mediated by TGFβ1, which suppresses Nrf2-ARE-dependent expression of antioxidant defenses and creates a vicious cycle that feeds back to further increase TGFβ1 expression. These effects of alcohol can be mitigated by activation of Nrf2, suggesting a potential therapy in individuals at risk for lung injury due to alcohol abuse.

Translational models of lung disease

The 2nd Cross Company Respiratory Symposium (CCRS), held in Horsham, U.K. in 2012, brought together representatives from across the pharmaceutical industry with expert academics, in the common interest of improving the design and translational predictiveness of in vivo models of respiratory disease. Organized by the respiratory representatives of the European Federation of Pharmaceutical Industries and Federations (EFPIA) group of companies involved in the EU-funded project (U-BIOPRED), the aim of the symposium was to identify state-of-the-art improvements in the utility and design of models of respiratory disease, with a view to improving their translational potential and reducing wasteful animal usage. The respiratory research and development community is responding to the challenge of improving translation in several ways: greater collaboration and open sharing of data, careful selection of the species, complexity and chronicity of the models, improved practices in preclinical research, continued refinement in models of respiratory diseases and their sub-types, greater understanding of the biology underlying human respiratory diseases and their sub-types, and finally greater use of human (and especially disease-relevant) cells, tissues and explants. The present review highlights these initiatives, combining lessons from the symposium and papers published in Clinical Science arising from the symposium, with critiques of the models currently used in the settings of asthma, idiopathic pulmonary fibrosis and COPD. The ultimate hope is that this will contribute to a more rational, efficient and sustainable development of a range of new treatments for respiratory diseases that continue to cause substantial morbidity and mortality across the world.

Blue journal conference. Aging and susceptibility to lung disease

The aging of the population in the United States and throughout the developed world has increased morbidity and mortality attributable to lung disease, while the morbidity and mortality from other prevalent diseases has declined or remained stable. Recognizing the importance of aging in the development of lung disease, the American Thoracic Society (ATS) highlighted this topic as a core theme for the 2014 annual meeting. The relationship between aging and lung disease was discussed in several oral symposiums and poster sessions at the annual ATS meeting. In this article, we used the input gathered at the conference to develop a broad framework and perspective to stimulate basic, clinical, and translational research to understand how the aging process contributes to the onset and/or progression of lung diseases. A consistent theme that emerged from the conference was the need to apply novel, systems-based approaches to integrate a growing body of genomic, epigenomic, transcriptomic, and proteomic data and elucidate the relationship between biologic hallmarks of aging, altered lung function, and increased susceptibility to lung diseases in the older population. The challenge remains to causally link the molecular and cellular changes of aging with age-related changes in lung physiology and disease susceptibility. The purpose of this review is to stimulate further research to identify new strategies to prevent or treat age-related lung disease.

Senescent Cells Contribute to the Physiological Remodeling of Aged Lungs

Age-associated decline in organ function governs life span. We determined the effect of aging on lung function and cellular/molecular changes of 8- to 32-month old mice. Proteomic analysis of lung matrix indicated significant compositional changes with advanced age consistent with a profibrotic environment that leads to a significant increase in dynamic compliance and airway resistance. The excess of matrix proteins deposition was associated modestly with the activation of myofibroblasts and transforming growth factor-beta signaling pathway. More importantly, detection of senescent cells in the lungs increased with age and these cells contributed toward the excess extracellular matrix deposition observed in our aged mouse model and in elderly human samples. Mechanistic target of rapamycin (mTOR)/AKT activity was enhanced in aged mouse lungs compared with those from younger mice associated with the increased expression of the histone variant protein, MH2A, a marker for aging and potentially for senescence. Introduction in the mouse diet of rapamycin, significantly blocked the mTOR activity and limited the activation of myofibroblasts but did not result in a reduction in lung collagen deposition unless it was associated with prevention of cellular senescence. Together these data indicate that cellular senescence significantly contributes to the extracellular matrix changes associated with aging in a mTOR 1-dependent mechanism.

Plasminogen activator inhibitor 1, fibroblast apoptosis resistance, and aging-related susceptibility to lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal lung disorder with unknown cause and no effective treatment. The incidence of and mortality from IPF increase with age, suggesting that advanced age is a major risk factor for IPF. The mechanism underlying the increased susceptibility of the elderly to IPF, however, is unknown. In this study, we show for the first time that the protein level of plasminogen activator inhibitor 1 (PAI-1), a protease inhibitor which plays an essential role in the control of fibrinolysis, was significantly increased with age in mouse lung homogenate and lung fibroblasts. Upon bleomycin challenge, old mice experienced augmented PAI-1 induction and lung fibrosis as compared to young mice. Most interestingly, we show that fewer (myo)fibroblasts underwent apoptosis and more (myo)fibroblasts with increased level of PAI-1 accumulated in the lung of old than in young mice after bleomycin challenge. In vitro studies further demonstrate that fibroblasts isolated from lungs of old mice were resistant to H2O2 and tumor necrosis factor alpha-induced apoptosis and had augmented fibrotic responses to TGF-β1, compared to fibroblasts isolated from young mice. Inhibition of PAI-1 activity with a PAI-1 inhibitor, on the other hand, eliminated the aging-related apoptosis resistance and TGF-β1 sensitivity in isolated fibroblasts. Moreover, we show that knocking down PAI-1 in human lung fibroblasts with PAI-1 siRNA significantly increased their sensitivity to apoptosis and inhibited their responses to TGF-β1. Together, the results suggest that increased PAI-1 expression may underlie the aging-related sensitivity to lung fibrosis in part by protecting fibroblasts from apoptosis.

Aging mesenchymal stem cells fail to protect because of impaired migration and antiinflammatory response

RATIONALE

Aging is characterized by functional impairment and reduced capacity to respond appropriately to environmental stimuli and injury. With age, there is an increase in the incidence and severity of chronic and acute lung diseases. However, the relationship between age and the lung's reduced ability to repair is far from established and necessitates further research in the field.

OBJECTIVES

Little is currently known about age-related phenomena in mesenchymal stem cells (MSCs). On account of their ability to protect the endothelium and the alveolar epithelium through multiple paracrine mechanisms, we looked for adverse effects that aging might cause in MSC biology. Such age-related changes might partly account for the increased susceptibility of the aging lung to injury.

MEASUREMENTS AND MAIN RESULTS

We demonstrated that old mice have more inflammation in response to acute lung injury. To investigate the causes, we compared the global gene expression of aged and young bone marrow-derived MSCs (B-MSCs). Our results revealed that the expression levels of inflammatory response genes depended on the age of the B-MSCs. We demonstrated that the age-dependent decrease in expression of several cytokine and chemokine receptors is important for the migration and activation of B-MSCs. Finally, we showed by adoptive transfer of aged B-MSCs to young endotoxemic mice that aged cells lacked the antiinflammatory protective effect of their young counterparts.

CONCLUSIONS

Taken together, the decreased expression of cytokine and chemokine receptors in aged B-MSCs compromises their protective role by perturbing the potential of B-MSCs to become activated and mobilize to the site of injury.

Reversal of persistent fibrosis in aging by targeting Nox4-Nrf2 redox imbalance

The incidence and prevalence of pathological fibrosis increase with advancing age, although mechanisms for this association are unclear. We assessed the capacity for repair of lung injury in young (2 months) and aged (18 months) mice. Whereas the severity of fibrosis was not different between these groups, aged mice demonstrated an impaired capacity for fibrosis resolution. Persistent fibrosis in lungs of aged mice was characterized by the accumulation of senescent and apoptosis-resistant myofibroblasts. These cellular phenotypes were sustained by alterations in cellular redox homeostasis resulting from elevated expression of the reactive oxygen species-generating enzyme Nox4 [NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase-4] and an impaired capacity to induce the Nrf2 (NFE2-related factor 2) antioxidant response. Lung tissues from human subjects with idiopathic pulmonary fibrosis (IPF), a progressive and fatal lung disease, also demonstrated this Nox4-Nrf2 imbalance. Nox4 mediated senescence and apoptosis resistance in IPF fibroblasts. Genetic and pharmacological targeting of Nox4 in aged mice with established fibrosis attenuated the senescent, antiapoptotic myofibroblast phenotype and led to a reversal of persistent fibrosis. These studies suggest that loss of cellular redox homeostasis promotes profibrotic myofibroblast phenotypes that result in persistent fibrosis associated with aging. Our studies suggest that restoration of Nox4-Nrf2 redox balance in myofibroblasts may be a therapeutic strategy in age-associated fibrotic disorders, potentially able to resolve persistent fibrosis or even reverse its progression.

Aging promotes pro-fibrotic matrix production and increases fibrocyte recruitment during acute lung injury

Fibrotic lung diseases increase with age. Previously we determined that senescence increases tissue expression of fibronectin EDA (Fn-EDA) and decreases fibroblast expression of Thy-1, and that fibrocytes contribute to fibrosis following bleomycin-induced lung injury in mice. In this study we hypothesized that fibroblasts lacking Thy-1 expression produce an extracellular matrix that promotes fibrocyte retention and myofibroblast transdifferentiation, thereby promoting fibrogenesis. Young and old mice were treated with bleomycin intratracheally; fibrocytes in the bone marrow, blood, and lungs were quantified, and lung fibroblast Thy-1 expression assessed. Bone marrow-derived fibrocytes were cultured on matrices derived from Thy-1(+) or Thy-1(-) fibroblasts ± the pro-fibrotic cytokine TGFβ1. Older mice had more fibrocytes in their bone marrows at baseline and more fibrocytes in their lungs following bleomycin treatment. In parallel, lung fibroblasts in older mice had lower expression of Thy-1 at baseline that increased transiently 7 days after bleomycin treatment but then rapidly waned such that 14 days after bleomycin treatment Thy-1 expression was again markedly lower. Fibrocytes cultured on matrices derived from Thy-1(-) fibroblasts + TGFβ1 had increased gene expression for collagen type 1, fibronectin, Fn-EDA, and α-smooth muscle actin. In parallel, whereas the matrices derived from Thy-1(-) fibroblasts stimulated phosphorylation of Akt in cultured fibrocytes, the matrices derived from Thy-1(+) fibroblasts induced apoptosis. These findings suggest that senescence increases fibrocyte recruitment to the lung following injury and that loss of Thy-1 expression by lung fibroblasts promotes fibrocyte retention and myofibroblast trans-differentiation that renders the "aging lung" susceptible to fibrosis.

Novel therapeutic strategies for lung disorders associated with airway remodelling and fibrosis

Inflammatory cell infiltration, cytokine release, epithelial damage, airway/lung remodelling and fibrosis are central features of inflammatory lung disorders, which include asthma, chronic obstructive pulmonary disease, acute respiratory distress syndrome and idiopathic pulmonary fibrosis. Although the lung has some ability to repair itself from acute injury, in the presence of ongoing pathological stimuli and/or insults that lead to chronic disease, it no longer retains the capacity to heal, resulting in fibrosis, the final common pathway that causes an irreversible loss of lung function. Despite inflammation, genetic predisposition/factors, epithelial-mesenchymal transition and mechanotransduction being able to independently contribute to airway remodelling and fibrosis, current therapies for inflammatory lung diseases are limited by their ability to only target the inflammatory component of the disease without having any marked effects on remodelling (epithelial damage and fibrosis) that can cause lung dysfunction independently of inflammation. Furthermore, as subsets of patients suffering from these diseases are resistant to currently available therapies (such as corticosteroids), novel therapeutic approaches are required to combat all aspects of disease pathology. This review discusses emerging therapeutic approaches, such as trefoil factors, relaxin, histone deacetylase inhibitors and stem cells, amongst others that have been able to target airway inflammation and airway remodelling while improving related lung dysfunction. A better understanding of the mode of action of these therapies and their possible combined effects may lead to the identification of their clinical potential in the setting of lung disease, either as adjunct or alternative therapies to currently available treatments.

Type V collagen induced tolerance suppresses collagen deposition, TGF-β and associated transcripts in pulmonary fibrosis

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a fatal interstitial lung disease characterized by progressive scarring and matrix deposition. Recent reports highlight an autoimmune component in IPF pathogenesis. We have reported anti-col(V) immunity in IPF patients. The objective of our study was to determine the specificity of col(V) expression profile and anti-col(V) immunity relative to col(I) in clinical IPF and the efficacy of nebulized col(V) in pre-clinical IPF models.

METHODS

Col(V) and col(I) expression profile was analyzed in normal human and IPF tissues. C57-BL6 mice were intratracheally instilled with bleomycin (0.025 U) followed by col(V) nebulization at pre-/post-fibrotic stage and analyzed for systemic and local responses.

RESULTS

Compared to normal lungs, IPF lungs had higher protein and transcript expression of the alpha 1 chain of col(V) and col(I). Systemic anti-col(V) antibody concentrations, but not of anti-col(I), were higher in IPF patients. Nebulized col(V), but not col(I), prevented bleomycin-induced fibrosis, collagen deposition, and myofibroblast differentiation. Col(V) treatment suppressed systemic levels of anti-col(V) antibodies, IL-6 and TNF-α; and local Il-17a transcripts. Compared to controls, nebulized col(V)-induced tolerance abrogated antigen-specific proliferation in mediastinal lymphocytes and production of IL-17A, IL-6, TNF-α and IFN-γ. In a clinically relevant established fibrosis model, nebulized col(V) decreased collagen deposition. mRNA array revealed downregulation of genes specific to fibrosis (Tgf-β, Il-1β, Pdgfb), matrix (Acta2, Col1a2, Col3a1, Lox, Itgb1/6, Itga2/3) and members of the TGF-β superfamily (Tgfbr1/2, Smad2/3, Ltbp1, Serpine1, Nfkb/Sp1/Cebpb).

CONCLUSIONS

Anti-col(V) immunity is pathogenic in IPF, and col(V)-induced tolerance abrogates bleomycin-induced fibrogenesis and down regulates TGF- β-related signaling pathways.

Animal models of fibrotic lung disease

Interstitial lung fibrosis can develop as a consequence of occupational or medical exposure, as a result of genetic defects, and after trauma or acute lung injury leading to fibroproliferative acute respiratory distress syndrome, or it can develop in an idiopathic manner. The pathogenesis of each form of lung fibrosis remains poorly understood. They each result in a progressive loss of lung function with increasing dyspnea, and most forms ultimately result in mortality. To better understand the pathogenesis of lung fibrotic disorders, multiple animal models have been developed. This review summarizes the common and emerging models of lung fibrosis to highlight their usefulness in understanding the cell-cell and soluble mediator interactions that drive fibrotic responses. Recent advances have allowed for the development of models to study targeted injuries of Type II alveolar epithelial cells, fibroblastic autonomous effects, and targeted genetic defects. Repetitive dosing in some models has more closely mimicked the pathology of human fibrotic lung disease. We also have a much better understanding of the fact that the aged lung has increased susceptibility to fibrosis. Each of the models reviewed in this report offers a powerful tool for studying some aspect of fibrotic lung disease.

Pathogenesis of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fibrosing interstitial lung disease associated with aging that is characterized by the histopathological pattern of usual interstitial pneumonia. Although an understanding of the pathogenesis of IPF is incomplete, recent advances delineating specific clinical and pathologic features of IPF have led to better definition of the molecular pathways that are pathologically activated in the disease. In this review we highlight several of these advances, with a focus on genetic predisposition to IPF and how genetic changes, which occur primarily in epithelial cells, lead to activation of profibrotic pathways in epithelial cells. We then discuss the pathologic changes within IPF fibroblasts and the extracellular matrix, and we conclude with a summary of how these profibrotic pathways may be interrelated.

The potential of cell-based therapy in lung diseases

INTRODUCTION

Many lung diseases have high morbidity and mortality rates and there are no cures or treatments apart from mechanical ventilation or transplantation. Cell-based therapies are currently an area of intense research, and many groups are working to translate successful in vitro results into treatments that are safe for patients.

AREAS COVERED

This review discusses several types of stem and progenitor cells that have been proven likely candidates for cell therapies, as well as their applications so far in specific acute and chronic lung diseases, focusing on their mechanisms of action and how best they can be directed toward clinical aims.

EXPERT OPINION

The research on cell therapies for the lung, particularly regarding mesenchymal stem cells (MSCs), is promising, but there is still much uncertainty surrounding the mechanisms of MSC action and the factors relevant to clinical applications such as the optimal timing of dosage. Future studies will focus on the microenvironment of the stem cells, including the role of microRNAs and extracellular vesicles.

Detecting splicing variants in idiopathic pulmonary fibrosis from non-differentially expressed genes

Idiopathic pulmonary fibrosis (IPF) is an interstitial lung disease of unknown cause that lacks a proven therapy for altering its high mortality rate. Microarrays have been employed to investigate the pathogenesis of IPF, but are presented mostly at the gene-expression level due to technologic limitations. In as much as, alternative RNA splicing isoforms are increasingly identified as potential regulators of human diseases, including IPF, we propose a new approach with the capacity to detect splicing variants using RNA-seq data. We conducted a joint analysis of differential expression and differential splicing on annotated human genes and isoforms, and identified 122 non-differentially expressed genes with a high degree of "switch" between major and minor isoforms. Three cases with variant mechanisms for alternative splicing were validated using qRT-PCR, among the group of genes in which expression was not significantly changed at the gene level. We also identified 35 novel transcripts that were unique to the fibrotic lungs using exon-exon junction evidence, and selected a representative for qRT-PCR validation. The results of our study are likely to provide new insight into the pathogenesis of pulmonary fibrosis and may eventuate in new treatment targets.

Influence of age on wound healing and fibrosis

The incidence and severity of fibrotic lung diseases increase with age, but very little is known about how age-related changes affect the mechanisms that underlie disease emergence and progression. Normal ageing includes accumulation of DNA mutations, oxidative and cell stresses, mitochondria dysfunction, increased susceptibility to apoptosis, telomere length dysfunction and differential gene expression as a consequence of epigenetic changes and miR regulation. These inevitable ageing-related phenomena may cause dysfunction and impaired repair capacity of lung epithelial cells, fibroblasts and MSCs. As a consequence, the composition of the extracellular matrix changes and the dynamic interaction between cells and their environment is damaged, resulting ultimately in predisposition for several diseases. This review summarizes what is known about age-related molecular changes that are implicated in the pathobiology of lung fibrosis in lung tissue.