Mechanosensing by the α6-integrin confers an invasive fibroblast phenotype and mediates lung fibrosis

Matrix stiffening is a prominent feature of pulmonary fibrosis. In this study, we demonstrate that matrix stiffness regulates the ability of fibrotic lung myofibroblasts to invade the basement membrane (BM). We identify α₆-integrin as a mechanosensing integrin subunit that mediates matrix stiffness-regulated myofibroblast invasion. Increasing α₆-expression, specifically the B isoform (α₆B), couples β₁-integrin to mediate MMP-2-dependent pericellular proteolysis of BM collagen IV, leading to myofibroblast invasion. Human idiopathic pulmonary fibrosis lung myofibroblasts express high levels of α₆-integrin in vitro and in vivo. Genetic ablation of α₆ in collagen-expressing mesenchymal cells or pharmacological blockade of matrix stiffness-regulated α₆-expression protects mice against bleomycin injury-induced experimental lung fibrosis. These findings suggest that α₆-integrin is a matrix stiffness-regulated mechanosensitive molecule which confers an invasive fibroblast phenotype and mediates experimental lung fibrosis. Targeting this mechanosensing α₆(β₁)-integrin offers a novel anti-fibrotic strategy against lung fibrosis.

Matrix stiffening is a feature of pulmonary fibrosis, and is amplified by lung myofibroblasts. Here the authors find that a6 integrin expression is upregulated on lung myofibroblasts in response to matrix stiffness, and this integrin is required for myofibroblast invasion, and fibrosis in an experimental disease model.

Redox mechanisms in age-related lung fibrosis

Redox signaling and oxidative stress are associated with tissue fibrosis and aging. Aging is recognized as a major risk factor for fibrotic diseases involving multiple organ systems, including that of the lung. A number of oxidant generating enzymes are upregulated while antioxidant defenses are deficient with aging and cellular senescence, leading to redox imbalance and oxidative stress. However, the precise mechanisms by which redox signaling and oxidative stress contribute to the pathogenesis of lung fibrosis are not well understood. Tissue repair is a highly regulated process that involves the interactions of several cell types, including epithelial cells, fibroblasts and inflammatory cells. Fibrosis may develop when these interactions are dysregulated with the acquisition of pro-fibrotic cellular phenotypes. In this review, we explore the roles of redox mechanisms that promote and perpetuate fibrosis in the context of cellular senescence and aging.

Alveolar epithelial disintegrity in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease characterized by progressive decline in lung function, resulting in significant morbidity and mortality. Current concepts of the pathogenesis of IPF primarily center on dysregulated epithelial cell repair and altered epithelial-mesenchymal communication and extracellular matrix deposition following chronic exposure to cigarette smoke or environmental toxins. In recent years, increasing attention has been directed toward the role of the intercellular junctional complex in determining the specific properties of epithelia in pulmonary diseases. Additionally, recent genomewide association studies suggest that specific genetic variants predictive of epithelial cell dysfunction may confer susceptibility to the development of sporadic idiopathic pulmonary fibrosis. A number of genetic disorders linked to pulmonary fibrosis and familial interstitial pneumonias are associated with loss of epithelial integrity. However, the potential links between extrapulmonary clinical syndromes associated with defects in epithelial cells and the development of pulmonary fibrosis are not well understood. Here, we report a case of hereditary mucoepithelial dysplasia that presented with pulmonary fibrosis and emphysema on high-resolution computed tomography. This case illustrates a more generalizable concept of epithelial disintegrity in the development of fibrotic lung diseases, which is explored in greater detail in this review article.

Targeted Therapy for Idiopathic Pulmonary Fibrosis: Where To Now?

Idiopathic pulmonary fibrosis (IPF) is an aging-associated, recalcitrant lung disease with historically limited therapeutic options. The recent approval of two drugs, pirfenidone and nintedanib, by the US Food and Drug Administration in 2014 has heralded a new era in its management. Both drugs have demonstrated efficacy in phase III clinical trials by retarding the rate of progression of IPF; neither drug appears to be able to completely arrest disease progression. Advances in the understanding of IPF pathobiology have led to an unprecedented expansion in the number of potential therapeutic targets. Drugs targeting several of these are under investigation in various stages of clinical development. Here, we provide a brief overview of the drugs that are currently approved and others in phase II clinical trials. Future therapeutic opportunities that target novel pathways, including some that are associated with the biology of aging, are examined. A multi-targeted approach, potentially with combination therapies, and identification of individual patients (or subsets of patients) who may respond more favourably to specific agents are likely to be more effective.

Macrophage Akt1 Kinase-Mediated Mitophagy Modulates Apoptosis Resistance and Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating lung disorder with increasing incidence. Mitochondrial oxidative stress in alveolar macrophages is directly linked to pulmonary fibrosis. Mitophagy, the selective engulfment of dysfunctional mitochondria by autophagasomes, is important for cellular homeostasis and can be induced by mitochondrial oxidative stress. Here, we show Akt1 induced macrophage mitochondrial reactive oxygen species (ROS) and mitophagy. Mice harboring a conditional deletion of Akt1 in macrophages (Akt1(-/-)Lyz2-cre) and Park2(-/-) mice had impaired mitophagy and reduced active transforming growth factor-β1 (TGF-β1). Although Akt1 increased TGF-β1 expression, mitophagy inhibition in Akt1-overexpressing macrophages abrogated TGF-β1 expression and fibroblast differentiation. Importantly, conditional Akt1(-/-)Lyz2-cre mice and Park2(-/-) mice had increased macrophage apoptosis and were protected from pulmonary fibrosis. Moreover, IPF alveolar macrophages had evidence of increased mitophagy and displayed apoptosis resistance. These observations suggest that Akt1-mediated mitophagy contributes to alveolar macrophage apoptosis resistance and is required for pulmonary fibrosis development.

The matricellular protein CCN1 enhances TGF-β1/SMAD3-dependent profibrotic signaling in fibroblasts and contributes to fibrogenic responses to lung injury

Matricellular proteins mediate pleiotropic effects during tissue injury and repair. CCN1 is a matricellular protein that has been implicated in angiogenesis, inflammation, and wound repair. In this study, we identified CCN1 as a gene that is differentially up-regulated in alveolar mesenchymal cells of human subjects with rapidly progressive idiopathic pulmonary fibrosis (IPF). Elevated levels of CCN1 mRNA were confirmed in lung tissues of IPF subjects undergoing lung transplantation, and CCN1 protein was predominantly localized to fibroblastic foci. CCN1 expression in ex vivo IPF lung fibroblasts correlated with gene expression of the extracellular matrix proteins, collagen (Col)1a1, Col1a2, and fibronectin as well as the myofibroblast marker, α-smooth muscle actin. RNA interference (RNAi)-mediated knockdown of CCN1 down-regulated the constitutive expression of these profibrotic genes in IPF fibroblasts. TGF-β1, a known mediator of tissue fibrogenesis, induces gene and protein expression of CCN1 via a mothers against decapentaplegic homolog 3 (SMAD3)-dependent mechanism. Importantly, endogenous CCN1 potentiates TGF-β1-induced SMAD3 activation and induction of profibrotic genes, supporting a positive feedback loop leading to myofibroblast activation. In vivo RNAi-mediated silencing of CCN1 attenuates fibrogenic responses to bleomycin-induced lung injury. These studies support previously unrecognized, cooperative interaction between the CCN1 matricellular protein and canonical TGF-β1/SMAD3 signaling that promotes lung fibrosis.-Kurundkar, A. R., Kurundkar, D., Rangarajan, S., Locy, M. L., Zhou, Y., Liu, R.-M., Zmijewski, J., Thannickal, V. J. The matricellular protein CCN1 enhances TGF-β1/SMAD3-dependent profibrotic signaling in fibroblasts and contributes to fibrogenic responses to lung injury.

Novel Mechanisms for the Antifibrotic Action of Nintedanib

Idiopathic pulmonary fibrosis (IPF) is a disease with relentless course and limited therapeutic options. Nintedanib (BIBF-1120) is a multiple tyrosine kinase inhibitor recently approved by the U.S. Food and Drug Administration for the treatment of IPF. The precise antifibrotic mechanism(s) of action of nintedanib, however, is not known. Therefore, we studied the effects of nintedanib on fibroblasts isolated from the lungs of patients with IPF. Protein and gene expression of profibrotic markers were assessed by Western immunoblotting and real-time PCR. Autophagy markers and signaling events were monitored by biochemical assays, Western immunoblotting, microscopy, and immunofluorescence staining. Silencing of autophagy effector proteins was achieved with small interfering RNAs. Nintedanib down-regulated protein and mRNA expression of extracellular matrix (ECM) proteins, fibronectin, and collagen 1a1 while inhibiting transforming growth factor (TGF)-β1-induced myofibroblast differentiation. Nintedanib also induced beclin-1-dependent, ATG7-independent autophagy. Nintedanib's ECM-suppressive actions were not mediated by canonical autophagy. Nintedanib inhibited early events in TGF-β signaling, specifically tyrosine phosphorylation of the type II TGF-β receptor, activation of SMAD3, and p38 mitogen-activated protein kinase. Nintedanib down-regulates ECM production and induces noncanonical autophagy in IPF fibroblasts while inhibiting TGF-β signaling. These mechanisms appear to be uncoupled and function independently to mediate its putative antifibrotic effects.

Therapeutic potential of an orally effective small molecule inhibitor of plasminogen activator inhibitor for asthma

Asthma is one of the most common respiratory diseases. Although progress has been made in our understanding of airway pathology and many drugs are available to relieve asthma symptoms, there is no cure for chronic asthma. Plasminogen activator inhibitor 1 (PAI-1), a primary inhibitor of tissue-type and urokinase-type plasminogen activators, has pleiotropic functions besides suppression of fibrinolysis. In this study, we show that administration of TM5275, an orally effective small-molecule PAI-1 inhibitor, 25 days after ovalbumin (OVA) sensitization-challenge, significantly ameliorated airway hyperresponsiveness in an OVA-induced chronic asthma model. Furthermore, we show that TM5275 administration significantly attenuated OVA-induced infiltration of inflammatory cells (neutrophils, eosinophils, and monocytes), the increase in the levels of OVA-specific IgE and Th2 cytokines (IL-4 and IL-5), the production of mucin in the airways, and airway subepithelial fibrosis. Together, the results suggest that the PAI-1 inhibitor TM5275 may have therapeutic potential for asthma through suppressing eosinophilic allergic response and ameliorating airway remodeling.

Glycolytic Reprogramming in Myofibroblast Differentiation and Lung Fibrosis

RATIONALE

Dysregulation of cellular metabolism has been shown to participate in several pathologic processes. However, the role of metabolic reprogramming is not well appreciated in the pathogenesis of organ fibrosis.

OBJECTIVES

To determine if glycolytic reprogramming participates in the pathogenesis of lung fibrosis and assess the therapeutic potential of glycolytic inhibition in treating lung fibrosis.

METHODS

A cell metabolism assay was performed to determine glycolytic flux and mitochondrial respiration. Lactate levels were measured to assess glycolysis in fibroblasts and lungs. Glycolytic inhibition by genetic and pharmacologic approaches was used to demonstrate the critical role of glycolysis in lung fibrosis.

MEASUREMENTS AND MAIN RESULTS

Augmentation of glycolysis is an early and sustained event during myofibroblast differentiation, which is dependent on the increased expression of critical glycolytic enzymes, in particular, 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3). Augmented glycolysis contributes to the stabilization of hypoxia-inducible factor 1-α, a master regulator of glycolytic enzymes implicated in organ fibrosis, by increasing cellular levels of tricarboxylic acid cycle intermediate succinate in lung myofibroblasts. Inhibition of glycolysis by the PFKFB3 inhibitor 3PO or genomic disruption of the PFKFB3 gene blunted the differentiation of lung fibroblasts into myofibroblasts, and attenuated profibrotic phenotypes in myofibroblasts isolated from the lungs of patients with idiopathic pulmonary fibrosis. Inhibition of glycolysis by 3PO demonstrates therapeutic benefit in bleomycin-induced and transforming growth factor-β1-induced lung fibrosis in mice.

CONCLUSIONS

Our data support the novel concept of glycolytic reprogramming in the pathogenesis of lung fibrosis and provide proof-of-concept that targeting this pathway may be efficacious in treating fibrotic disorders, such as idiopathic pulmonary fibrosis.

Noninvasive imaging of experimental lung fibrosis

Small animal models of lung fibrosis are essential for unraveling the molecular mechanisms underlying human fibrotic lung diseases; additionally, they are useful for preclinical testing of candidate antifibrotic agents. The current end-point measures of experimental lung fibrosis involve labor-intensive histological and biochemical analyses. These measures fail to account for dynamic changes in the disease process in individual animals and are limited by the need for large numbers of animals for longitudinal studies. The emergence of noninvasive imaging technologies provides exciting opportunities to image lung fibrosis in live animals as often as needed and to longitudinally track the efficacy of novel antifibrotic compounds. Data obtained by noninvasive imaging provide complementary information to histological and biochemical measurements. In addition, the use of noninvasive imaging in animal studies reduces animal usage, thus satisfying animal welfare concerns. In this article, we review these new imaging modalities with the potential for evaluation of lung fibrosis in small animal models. Such techniques include micro-computed tomography (micro-CT), magnetic resonance imaging, positron emission tomography (PET), single photon emission computed tomography (SPECT), and multimodal imaging systems including PET/CT and SPECT/CT. It is anticipated that noninvasive imaging will be increasingly used in animal models of fibrosis to gain insights into disease pathogenesis and as preclinical tools to assess drug efficacy.

Metabolic Reprogramming Is Required for Myofibroblast Contractility and Differentiation

Contraction is crucial in maintaining the differentiated phenotype of myofibroblasts. Contraction is an energy-dependent mechanism that relies on the production of ATP by mitochondria and/or glycolysis. Although the role of mitochondrial biogenesis in the adaptive responses of skeletal muscle to exercise is well appreciated, mechanisms governing energetic adaptation of myofibroblasts are not well understood. Our study demonstrates induction of mitochondrial biogenesis and aerobic glycolysis in response to the differentiation-inducing factor transforming growth factor β1 (TGF-β1). This metabolic reprogramming is linked to the activation of the p38 mitogen-activated protein kinase (MAPK) pathway. Inhibition of p38 MAPK decreased accumulation of active peroxisome proliferator-activated receptor γ coactivator 1α in the nucleus and altered the translocation of mitochondrial transcription factor A to the mitochondria. Genetic or pharmacologic approaches that block mitochondrial biogenesis or glycolysis resulted in decreased contraction and reduced expression of TGF-β1-induced α-smooth muscle actin and collagen α-2(I) but not of fibronectin or collagen α-1(I). These data indicate a critical role for TGF-β1-induced metabolic reprogramming in regulating myofibroblast-specific contractile signaling and support the concept of integrating bioenergetics with cellular differentiation.

Heme oxygenase-1-mediated autophagy protects against pulmonary endothelial cell death and development of emphysema in cadmium-treated mice

Pulmonary exposure to cadmium, a major component of cigarette smoke, has a dramatic impact on lung function and the development of emphysema. Cigarette smoke exposure induces heme oxygenase-1 (HO-1), a cytoprotective enzyme. In this study, we employed a truncated mouse model of emphysema by intratracheal instillation of cadmium (CdCl2) solution (0.025% per 1 mg/kg body wt) in HO-1(+/+), HO-1(-/-), and overexpressing humanized HO-1 bacterial artificial chromosome (hHO-1BAC) mice. We evaluated the role of HO-1 in cadmium-induced emphysema in mice by analyzing histopathology, micro-computed tomography scans, and lung function tests. CdCl2-exposed HO-1(-/-) mice exhibited more severe emphysema compared with HO-1(+/+) or hHO-1BAC mice. Loss of pulmonary endothelial cells (PECs) from the alveolar capillary membrane is recognized to be a target in emphysema. PECs from HO-1(+/+), HO-1(-/-), and hHO-1BAC were employed to define the underlying molecular mechanism for the protection from emphysema by HO-1. Electron microscopy, expression of autophagic markers (microtubule-associated protein 1B-light chain 3 II, autophagy protein 5, and Beclin1) and apoptotic marker (cleaved caspase 3) suggested induction of autophagy and apoptosis in PECs after CdCl2 treatment. CdCl2-treated HO-1(-/-) PECs exhibited downregulation of autophagic markers and significantly increased cleaved caspase 3 expression and activity (∼4-fold higher). Moreover, hHO-1BAC PECs demonstrated upregulated autophagy and absence of cleaved caspase 3 expression or activity. Pretreatment of HO-1(+/+) PECs with rapamycin induced autophagy and resulted in reduced cell death upon cadmium treatment. Induction of autophagy following CdCl2 treatment was found to be protective from apoptotic cell death. HO-1 induced protective autophagy in PECs and mitigated cadmium-induced emphysema.

The code of non-coding RNAs in lung fibrosis

The pathogenesis of pulmonary fibrosis is a complicated and complex process that involves phenotypic abnormalities of a variety of cell types and dysregulations of multiple signaling pathways. There are numerous genetic, epigenetic and post-transcriptional mechanisms that have been identified to participate in the pathogenesis of this disease. However, efficacious therapeutics developed from these studies have been disappointingly limited. In the past several years, a group of new molecules, i.e., non-coding RNAs (ncRNAs), has been increasingly appreciated to have critical roles in the pathological progression of lung fibrosis. In this review, we summarize the recent findings on the roles of ncRNAs in the pathogenesis of this disorder. We analyze the translational potential of this group of molecules in treating lung fibrosis. We also discuss challenges and future opportunities of studying and utilizing ncRNAs in lung fibrosis.

Pyruvate dehydrogenase kinase 1 participates in macrophage polarization via regulating glucose metabolism

The M1 and M2 polarized phenotypes dictate distinctive roles for macrophages as they participate in inflammatory disorders. There has been growing interest in the role of cellular metabolism in macrophage polarization. However, it is currently unclear whether different aspects of a specific metabolic program coordinately regulate this cellular process. In this study, we found that pyruvate dehydrogenase kinase 1 (PDK1), a key regulatory enzyme in glucose metabolism, plays an important role in the differential activation of macrophages. Knockdown of PDK1 diminished M1, whereas it enhanced M2 activation of macrophages. Mechanistically, PDK1 knockdown led to diminished aerobic glycolysis in M1 macrophages, which likely accounts for the attenuated inflammatory response in these cells. Furthermore, we found that mitochondrial respiration is enhanced during and required by the early activation of M2 macrophages. Suppression of glucose oxidation, but not that of fatty acids, inhibits this process. Consistent with its inhibitory role in early M2 activation, knockdown of PDK1 enhanced mitochondrial respiration in macrophages. Our data suggest that two arms of the glucose metabolism synergistically regulate the differential activation of macrophages. Our findings also highlight the central role of PDK1 in this event via controlling glycolysis and glucose oxidation.

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.

Heme oxygenase-1 protects corexit 9500A-induced respiratory epithelial injury across species

The effects of Corexit 9500A (CE) on respiratory epithelial surfaces of terrestrial mammals and marine animals are largely unknown. This study investigated the role of CE-induced heme oxygenase-1 (HO-1), a cytoprotective enzyme with anti-apoptotic and antioxidant activity, in human bronchial airway epithelium and the gills of exposed aquatic animals. We evaluated CE-mediated alterations in human airway epithelial cells, mice lungs and gills from zebrafish and blue crabs. Our results demonstrated that CE induced an increase in gill epithelial edema and human epithelial monolayer permeability, suggesting an acute injury caused by CE exposure. CE induced the expression of HO-1 as well as C-reactive protein (CRP) and NADPH oxidase 4 (NOX4), which are associated with ROS production. Importantly, CE induced caspase-3 activation and subsequent apoptosis of epithelial cells. The expression of the intercellular junctional proteins, such as tight junction proteins occludin, zonula occludens (ZO-1), ZO-2 and adherens junctional proteins E-cadherin and Focal Adhesion Kinase (FAK), were remarkably inhibited by CE, suggesting that these proteins are involved in CE-induced increased permeability and subsequent apoptosis. The cytoskeletal protein F-actin was also disrupted by CE. Treatment with carbon monoxide releasing molecule-2 (CORM-2) significantly inhibited CE-induced ROS production, while the addition of HO-1 inhibitor, significantly increased CE-induced ROS production and apoptosis, suggesting a protective role of HO-1 or its reaction product, CO, in CE-induced apoptosis. Using HO-1 knockout mice, we further demonstrated that HO-1 protected against CE-induced inflammation and cellular apoptosis and corrected CE-mediated inhibition of E-cadherin and FAK. These observations suggest that CE activates CRP and NOX4-mediated ROS production, alters permeability by inhibition of junctional proteins, and leads to caspase-3 dependent apoptosis of epithelial cells, while HO-1 and its reaction products protect against oxidative stress and apoptosis.

SMAD-independent down-regulation of caveolin-1 by TGF-β: effects on proliferation and survival of myofibroblasts

Transforming growth factor-β (TGF-β) mediates growth-inhibitory effects on most target cells via activation of the canonical SMAD signaling pathway. This growth-inhibitory activity may be coupled with cellular differentiation. Our studies demonstrate that TGF-β1 inhibits proliferation of primary, non-transformed human lung fibroblasts in association with the induction of myofibroblast differentiation. Differentiated myofibroblasts maintain the capacity to proliferate in response to exogenous mitogenic stimuli and are resistant to serum deprivation-induced apoptosis. These proliferative and anti-apoptotic properties of myofibroblasts are related, in part, to the down-regulation of caveolin-1 (Cav-1) by TGF-β1. Cav-1 down-regulation is mediated by early activation of p38 MAPK and does not require SMAD signaling. In contrast, myofibroblast differentiation is dependent on activation of the SMAD pathway, but not on p38 MAPK. Thus, combinatorial signaling by TGF-β1 of myofibroblast differentiation and down-regulation of Cav-1 by SMAD and p38 MAPK pathways, respectively, confer proliferative and apoptosis-resistant properties to myofibroblasts. Selective targeting of this SMAD-independent, p38-MAPK/Cav-1-dependent pathway is likely to be effective in the treatment of pathological conditions characterized by TGF-β signaling and myofibroblast activation.

Epigenetic mechanisms regulate NADPH oxidase-4 expression in cellular senescence

Aging is a well-known risk factor for a large number of chronic diseases, including those of the lung. Cellular senescence is one of the hallmarks of aging, and contributes to the pathogenesis of age-related diseases. Recent studies implicate the reactive oxygen species (ROS)-generating enzyme, NADPH oxidase 4 (Nox4) in cellular senescence. In this study, we investigated potential mechanisms for epigenetic regulation of Nox4. We observed constitutively high levels of Nox4 gene/protein and activity in a model of replication-induced cellular senescence of lung fibroblasts. In replicative senescent fibroblasts, the Nox4 gene is enriched with the activation histone mark, H4K16Ac, and inversely associated with the repressive histone mark, H4K20Me3, supporting an active transcriptional chromatin conformation. Silencing of the histone acetyltransferase Mof, which specifically acetylates H4K16, down-regulates Nox4 gene/protein expression. The Nox4 gene promoter is rich in CpG sites; mixed copies of methylated and unmethylated Nox4 DNA were detected in both nonsenescent and senescent cells. Interestingly, the Nox4 gene is variably associated with specific DNA methyltransferases and methyl binding proteins in these two cell populations. These results indicate a critical role for histone modifications involving H4K16Ac in epigenetic activation of the Nox4 gene, while the role of DNA methylation may be contextual. Defining mechanisms for the epigenetic regulation of Nox4 will aid in the development of novel therapeutic strategies for age-related diseases in which this gene is overexpressed, in particular idiopathic pulmonary fibrosis and cancer.

Exposure to cigarette smoke impacts myeloid-derived regulatory cell function and exacerbates airway hyper-responsiveness

Cigarette smoking enhances oxidative stress and airway inflammation in asthma, the mechanisms of which are largely unknown. Myeloid-derived regulatory cells (MDRC) are free radical producing immature myeloid cells with immunoregulatory properties that have recently been demonstrated as critical regulators of allergic airway inflammation. NO (nitric oxide)-producing immunosuppressive MDRC suppress T-cell proliferation and airway-hyper responsiveness (AHR), while the O2(•-) (superoxide)-producing MDRC are proinflammatory. We hypothesized that cigarette smoke (CS) exposure may impact MDRC function and contribute to exacerbations in asthma. Exposure of bone marrow (BM)-derived NO-producing MDRC to CS reduced the production of NO and its metabolites and inhibited their potential to suppress T-cell proliferation. Production of immunoregulatory cytokine IL-10 was significantly inhibited, while proinflammatory cytokines IL-6, IL-1β, TNF-α and IL-33 were enhanced in CS-exposed BM-MDRC. Additionally, CS exposure increased NF-κB activation and induced BM-MDRC-mediated production of O2(•-), via NF-κB-dependent pathway. Intratracheal transfer of smoke-exposed MDRC-producing proinflammatory cytokines increased NF-κB activation, reactive oxygen species and mucin production in vivo and exacerbated AHR in C57BL/6 mice, mice deficient in Type I IFNR and MyD88, both with reduced numbers of endogenous MDRC. Thus CS exposure modulates MDRC function and contributes to asthma exacerbation and identifies MDRC as potential targets for asthma therapy.

The monocarboxylate transporter 4 is required for glycolytic reprogramming and inflammatory response in macrophages

There has been fast growing evidence showing that glycolysis plays a critical role in the activation of immune cells. Enhanced glycolysis leads to increased formation of intracellular lactate that is exported to the extracellular environment by monocarboxylate transporter 4 (MCT4). Although the biological activities of extracellular lactate have been well studied, it is less understood how the lactate export is regulated or whether lactate export affects glycolysis during inflammatory activation. In this study, we found that MCT4 is up-regulated by TLR2 and TLR4, but not TLR3 agonists in a variety of macrophages. The increased expression of MCT4 was mediated by MYD88 in a NF-κB-dependent manner. Furthermore, we found that MCT4 is required for macrophage activation upon TLR2 and TLR4 stimulations, as evidenced by attenuated expression of proinflammatory mediators in macrophages with MCT4 knockdown. Mechanistically, we found that MCT4 knockdown leads to enhanced intracellular accumulation of lactate and decreased glycolysis in LPS-treated macrophages. We found that LPS-induced expression of key glycolytic enzymes hexokinase 2 and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 is diminished in macrophages with MCT4 knockdown. Our data suggest that MCT4 up-regulation represents a positive feedback mechanism in macrophages to maintain a high glycolytic rate that is essential to a fully activated inflammatory response.

Fibrosis: ultimate and proximate causes

Fibrotic disorders account for an increasing burden of disease-associated morbidity and mortality worldwide. Although numerous risk factors have been recognized, the etiologies of many of these clinical syndromes have not been identified, and they are often termed idiopathic or cryptogenic. Here, we provide an evolutionary perspective on fibrosis aimed at elucidating its etiopathogenesis. By asking the ultimate question of "why" this process evolved in multicellular organisms, we hope to uncover proximate explanations for "how" it causes disease in humans. We posit that physiological fibrosis-like reactions evolved as an essential process in host defense against pathogens and in normal wound healing. Based on this premise, we reason that pathological fibrosis is related to one or more of the following: unidentified infectious or noninfectious antigens, autoimmunity, impaired regenerative responses, and the antagonistically pleiotropic action of genes involved in wound healing or development. The importance of genetic susceptibility, epigenetics, aging, and the modern-day environment are highlighted. Consideration of both ultimate and proximate causation goes beyond philosophical cogitations, as it will better inform pathobiological mechanisms of disease and aid in the prevention and treatment of fibrotic diseases.

Therapeutic targeting of SRC kinase in myofibroblast differentiation and pulmonary fibrosis

Myofibroblasts are effector cells in fibrotic disorders that synthesize and remodel the extracellular matrix (ECM). This study investigated the role of the Src kinase pathway in myofibroblast activation in vitro and fibrogenesis in vivo. The profibrotic cytokine, transforming growth factor β1 (TGF-β1), induced rapid activation of Src kinase, which led to myofibroblast differentiation of human lung fibroblasts. The Src kinase inhibitor AZD0530 (saracatinib) blocked TGF-β1-induced Src kinase activation in a dose-dependent manner. Inhibition of Src kinase significantly reduced α-smooth muscle actin (α-SMA) expression, a marker of myofibroblast differentiation, in TGF-β1-treated lung fibroblasts. In addition, the induced expression of collagen and fibronectin and three-dimensional collagen gel contraction were also significantly inhibited in AZD0530-treated fibroblasts. The therapeutic efficiency of Src kinase inhibition in vivo was tested in the bleomycin murine lung fibrosis model. Src kinase activation and collagen accumulation were significantly reduced in the lungs of AZD0530-treated mice when compared with controls. Furthermore, the total fibrotic area and expression of α-SMA and ECM proteins were significantly decreased in lungs of AZD0530-treated mice. These results indicate that Src kinase promotes myofibroblast differentiation and activation of lung fibroblasts. Additionally, these studies provide proof-of-concept for targeting the noncanonical TGF-β signaling pathway involving Src kinase as an effective therapeutic strategy for lung fibrosis.

NADPH oxidases in lung health and disease

SIGNIFICANCE

The evolution of the lungs and circulatory systems in vertebrates ensured the availability of molecular oxygen (O2; dioxygen) for aerobic cellular metabolism of internal organs in large animals. O2 serves as the physiologic terminal acceptor of mitochondrial electron transfer and of the NADPH oxidase (Nox) family of oxidoreductases to generate primarily water and reactive oxygen species (ROS), respectively.

RECENT ADVANCES

The purposeful generation of ROS by Nox family enzymes suggests important roles in normal physiology and adaptation, most notably in host defense against invading pathogens and in cellular signaling.

CRITICAL ISSUES

However, there is emerging evidence that, in the context of chronic stress and/or aging, Nox enzymes contribute to the pathogenesis of a number of lung diseases.

FUTURE DIRECTIONS

Here, we review evolving functions of Nox enzymes in normal lung physiology and emerging pathophysiologic roles in lung disease.

Development and maintenance of a biospecimen repository for clinical samples derived from pulmonary patients

The Pulmonary Biospecimen Repository (PBR) at the University of Alabama at Birmingham (UAB) was launched in 2009. The purpose of the UAB PBR is to provide investigators within the pulmonary community at UAB and elsewhere with clinical samples derived from multiple lung diseases, including transplant recipients, idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, cystic fibrosis, and asthma. Cell and fluid samples isolated from bronchoalveolar lavage (BAL), plasma, and serum are collected and stored; samples are assessed routinely for viability. Each sample is linked directly with the respective patient information via the Pulmonary Translational Research and Clinical Database, a Health Insurance Portability and Accountability Act compliant database that includes detailed information allowing for the study of specific patient cohorts. To access samples, investigators must complete a request form, which is reviewed by the UAB PBR Steering Committee. To date, more than 800 patients have provided approximately 7,000 BAL, serum and plasma fluid, and cell samples. Over the past 4 years, nearly 800 of these samples have been distributed to investigators at UAB and elsewhere. Future plans for the UAB PBR include expanding sample collection to additional pulmonary diseases, such as mycobacterial infections, increasing the number of sample users and obtaining external funding to ensure its continued sustainability.

miR-27a regulates inflammatory response of macrophages by targeting IL-10

Although microRNAs were shown to participate in innate immune responses, it is not completely understood how they regulate negative immunomodulatory events. IL-10 is an important anti-inflammatory mediator that prevents excessive inflammation and associated immunological pathologies. Although the regulation of IL-10 expression has been well studied at both the transcriptional and translational levels, it is less clear how microRNAs control IL-10 expression during inflammation. In this study, we found that miR-27a is downregulated in macrophages following stimulation through TLR2 and TLR4, but not TLR3. Upregulation of miR-27a enhanced the expression of proinflammatory cytokines in TLR2/4-activated macrophages. Conversely, knockdown of miR-27a diminished cytokine expression. Mechanistically, we found that miR-27a negatively regulates IL-10 expression; upregulation of miR-27a decreases, whereas downregulation of miR-27a increases, IL-10 expression in activated macrophages. Likely due to the decreased expression of IL-10, upregulation of miR-27a diminished IL-10-dependent STAT3 phosphorylation in TLR4-activated macrophages. Consistent with IL-10 being a potential mediator for the role of miR-27a in the immune response, blocking IL-10 abolished the enhancing effect of miR-27a on TLR4-activated inflammation. In conclusion, our study identified miR-27a downregulation as a negative-regulatory mechanism that prevents overly exuberant TLR2- and TLR4-driven inflammatory responses.

Negative regulation of NADPH oxidase 4 by hydrogen peroxide-inducible clone 5 (Hic-5) protein

Hydrogen peroxide-inducible clone 5 (Hic-5) is a focal adhesion adaptor protein induced by the profibrotic cytokine TGF-β1. We have demonstrated previously that TGF-β1 induces myofibroblast differentiation and lung fibrosis by activation of the reactive oxygen species-generating enzyme NADPH oxidase 4 (Nox4). Here we investigated a potential role for Hic-5 in regulating Nox4, myofibroblast differentiation, and senescence. In normal human diploid fibroblasts, TGF-β1 induces Hic-5 expression in a delayed manner relative to the induction of Nox4 and myofibroblast differentiation. Hic-5 silencing induced constitutive Nox4 expression and enhanced TGF-β1-inducible Nox4 levels. The induction of constitutive Nox4 protein in Hic-5-silenced cells was independent of transcription and translation and controlled by the ubiquitin-proteasomal system. Hic-5 associates with the ubiquitin ligase Cbl-c and the ubiquitin-binding protein heat shock protein 27 (HSP27). The interaction of these proteins is required for the ubiquitination of Nox4 and for maintaining low basal levels of this reactive oxygen species-generating enzyme. Our model suggests that TGF-β1-induced Hic-5 functions as a negative feedback mechanism to limit myofibroblast differentiation and senescence by promoting the ubiquitin-proteasomal system-mediated degradation of Nox4. Together, these studies indicate that endogenous Hic-5 suppresses senescence and profibrotic activities of myofibroblasts by down-regulating Nox4 protein expression. Additionally, these are the first studies, to our knowledge, to demonstrate posttranslational regulation of Nox4.

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.

Future directions in idiopathic pulmonary fibrosis research. An NHLBI workshop report

The median survival of patients with idiopathic pulmonary fibrosis (IPF) continues to be approximately 3 years from the time of diagnosis, underscoring the lack of effective medical therapies for this disease. In the United States alone, approximately 40,000 patients die of this disease annually. In November 2012, the NHLBI held a workshop aimed at coordinating research efforts and accelerating the development of IPF therapies. Basic, translational, and clinical researchers gathered with representatives from the NHLBI, patient advocacy groups, pharmaceutical companies, and the U.S. Food and Drug Administration to review the current state of IPF research and identify priority areas, opportunities for collaborations, and directions for future research. The workshop was organized into groups that were tasked with assessing and making recommendations to promote progress in one of the following six critical areas of research: (1) biology of alveolar epithelial injury and aberrant repair; (2) role of extracellular matrix; (3) preclinical modeling; (4) role of inflammation and immunity; (5) genetic, epigenetic, and environmental determinants; (6) translation of discoveries into diagnostics and therapeutics. The workshop recommendations provide a basis for directing future research and strategic planning by scientific, professional, and patient communities and the NHLBI.

A far-upstream AP-1/Smad binding box regulates human NOX4 promoter activation by transforming growth factor-β

NADPH oxidase 4 (NOX4) is a member of the NADPH oxidase gene family that regulates cellular differentiation, innate immunity and tissue fibrosis. Transforming growth factor-β (TGF-β1) is known to induce expression of NOX4 mRNA in mesenchymal cells. However, the mechanisms of transcriptional regulation of NOX4 are not well understood. In this study, we examined the transcriptional regulation of NOX4 in human lung fibroblasts by TGF-β1. Five promoter-reporter constructs containing DNA fragments of 0.74kb, 1.35kb, 1.84kb, 3.97kb and 4.76kb upstream from the transcriptional start site (TSS) of the human NOX4 gene were generated and their relative responsiveness to TGF-β1 analyzed. TGF-β1-induced NOX4 gene promoter activation requires a region between -3.97kb and -4.76kb. Bioinformatics analysis revealed a 15bp AP-1/Smad binding element in this region. Mutation or deletion of either the AP-1 or the Smad element attenuated TGF-β1 responsiveness of the -4.76kb NOX4 promoter. Furthermore, insertion of this AP-1/Smad box conferred TGF-β1 inducibility to the non-responsive -3.97kb NOX4 promoter construct. Chromatin immunoprecipitation analysis indicated that phospho-Smad3 and cJun associate with this element in a TGF-β1-inducible manner. These results demonstrate that the AP-1/Smad box located between 3.97kb and 4.76kb upstream of the TSS site of the NOX4 promoter is essential for NOX4 gene transcription induced by TGF-β1 in human lung fibroblasts. Our study provides insights into the molecular mechanisms of NOX4 gene expression, informing novel therapeutic approaches to interfere with upregulation of NOX4 in diseases characterized by activation of the TGF-β1/NOX4 pathway.

Wilms' tumor 1 (Wt1) regulates pleural mesothelial cell plasticity and transition into myofibroblasts in idiopathic pulmonary fibrosis

Pleural mesothelial cells (PMCs), which are derived from the mesoderm, exhibit an extraordinary capacity to undergo phenotypic changes during development and disease. PMC transformation and trafficking has a newly defined role in idiopathic pulmonary fibrosis (IPF); however, the contribution of Wilms' tumor 1 (Wt1)-positive PMCs to the generation of pathognomonic myofibroblasts remains unclear. PMCs were obtained from IPF lung explants and healthy donor lungs that were not used for transplantation. Short hairpin Wt1-knockdown PMCs (sh Wt1) were generated with Wt1 shRNA, and morphologic and functional assays were performed in vitro. Loss of Wt1 abrogated the PMC phenotype and showed evidence of mesothelial-to-mesenchymal transition (MMT), with a reduced expression of E-cadherin and an increase in the profibrotic markers α-smooth muscle actin (α-SMA) and fibronectin, along with increased migration and contractility, compared with that of the control. Migration of PMCs in response to active transforming growth factor (TGF)-β1 was assessed by live-cell imaging with 2-photon microscopy and 3D imaging, of Wt1-EGFP transgenic mice. Lineage-tracing experiments to map the fate of Wt1(+) PMCs in mouse lung in response to TGF-β1 were also performed by using a Cre-loxP system. Our results, for the first time, demonstrate that Wt1 is necessary for the morphologic integrity of pleural membrane and that loss of Wt1 contributes to IPF via MMT of PMCs into a myofibroblast phenotype.

Enhancement of antitumor immunity in lung cancer by targeting myeloid-derived suppressor cell pathways

Chemoresistance due to heterogeneity of the tumor microenvironment (TME) hampers the long-term efficacy of first-line therapies for lung cancer. Current combination therapies for lung cancer provide only modest improvement in survival, implicating necessity for novel approaches that suppress malignant growth and stimulate long-term antitumor immunity. Oxidative stress in the TME promotes immunosuppression by tumor-infiltrating myeloid-derived suppressor cells (MDSC), which inhibit host protective antitumor immunity. Using a murine model of lung cancer, we demonstrate that a combination treatment with gemcitabine and a superoxide dismutase mimetic targets immunosuppressive MDSC in the TME and enhances the quantity and quality of both effector and memory CD8(+) T-cell responses. At the effector cell function level, the unique combination therapy targeting MDSC and redox signaling greatly enhanced cytolytic CD8(+) T-cell response and further decreased regulatory T cell infiltration. For long-term antitumor effects, this therapy altered the metabolism of memory cells with self-renewing phenotype and provided a preferential advantage for survival of memory subsets with long-term efficacy and persistence. Adoptive transfer of memory cells from this combination therapy prolonged survival of tumor-bearing recipients. Furthermore, the adoptively transferred memory cells responded to tumor rechallenge exerting long-term persistence. This approach offers a new paradigm to inhibit immunosuppression by direct targeting of MDSC function, to generate effector and persistent memory cells for tumor eradication, and to prevent lung cancer relapse.

Heritability of pulmonary function estimated from pedigree and whole-genome markers

Asthma and chronic obstructive pulmonary disease (COPD) are major worldwide health problems. Pulmonary function testing is a useful diagnostic tool for these diseases, and is known to be influenced by genetic and environmental factors. Previous studies have demonstrated that a substantial proportion of the variation in pulmonary function phenotypes can be explained by familial relationships. The availability of whole-genome single nucleotide polymorphism (SNP) data enables us to further evaluate the extent to which genetic factors account for variation in pulmonary function and to compare pedigree- to SNP-based estimates of heritability. Here, we employ methods developed in the animal breeding field to estimate the heritability of forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and the ratio of these two measures (FEV1/FVC) among subjects in the Framingham Heart Study dataset. We compare heritability estimates based on pedigree-based relationships to those based on genome-wide SNPs. We find that, in a family-based study, estimates of heritability using SNP data are nearly identical to estimates based on pedigree information, and range from 0.50 for FEV1 to 0.66 for FEV1/FVC. Therefore, we conclude that genetic factors account for a sizable proportion of inter-individual differences in pulmonary function, and that estimates of heritability based on SNP data are nearly identical to estimates based on pedigree data. Finally, our findings suggest a higher heritability for FEV1/FVC compared to either FEV1 or FVC.

Mechanistic links between aging and lung fibrosis

Our understanding of the biology of aging has advanced significantly in recent years. This has resulted in the recent formulation of the "hallmarks of aging" that include genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal lung disease that results from the accumulation of scar tissue in the lungs of affected individuals. IPF is a disease of aging that most commonly affects human subjects older than 60 years of age. While progress has been made in elucidating key pathological processes in IPF, the relationship of these processes to those that occur during aging are not well defined. In this review, we explore existing and emerging paradigms in the pathogenesis of IPF in light of the recently defined hallmarks of aging.

miR-145 regulates myofibroblast differentiation and lung fibrosis

The expression of smooth muscle actin-α (SMA-α) by fibroblasts defines phenotypic transition to myofibroblasts and is a primary contributor to contractile force generation by these differentiated cells. Although the regulation of SMA-α expression has been the focus of many studies, there is presently only limited information concerning miRNA regulation of lung myofibroblast differentiation and the involvement of these miRNAs in pulmonary fibrosis. To determine the role of miR-145 in regulating lung myofibroblast differentiation and pulmonary fibrosis. Wild-type and miR-145(-/-) mice were studied. Lung fibrosis models and cell culture systems were employed. miR-145 mimics or inhibitors were transfected into pulmonary fibroblasts. Fibrogenic and contractile activities of lung fibroblasts were determined. We found that miR-145 expression is upregulated in TGF-β1-treated lung fibroblasts. miR-145 expression is also increased in the lungs of patients with idiopathic pulmonary fibrosis as compared to in normal human lungs. Overexpression of miR-145 in lung fibroblasts increased SMA-α expression, enhanced contractility, and promoted formation of focal and fibrillar adhesions. In contrast, miR-145 deficiency diminished TGF-β1 induced SMA-α expression. miR-145 did not affect the activity of TGF-β1, but promoted the activation of latent TGF-β1. miR-145 targets KLF4, a known negative regulator of SMA-α expression. Finally, we found that miR-145(-/-) mice are protected from bleomycin-induced pulmonary fibrosis. miR-145 plays an important role in the differentiation of lung myofibroblasts. miR-145 deficiency is protective against bleomycin-induced lung fibrosis, suggesting that miR-145 may be a potential target in the development of novel therapies to treat pathological fibrotic disorders.

Inhibition of mechanosensitive signaling in myofibroblasts ameliorates experimental pulmonary fibrosis

Matrix stiffening and myofibroblast resistance to apoptosis are cardinal features of chronic fibrotic diseases involving diverse organ systems. The interactions between altered tissue biomechanics and cellular signaling that sustain progressive fibrosis are not well defined. In this study, we used ex vivo and in vivo approaches to define a mechanotransduction pathway involving Rho/Rho kinase (Rho/ROCK), actin cytoskeletal remodeling, and a mechanosensitive transcription factor, megakaryoblastic leukemia 1 (MKL1), that coordinately regulate myofibroblast differentiation and survival. Both in an experimental mouse model of lung fibrosis and in human subjects with idiopathic pulmonary fibrosis (IPF), we observed activation of the Rho/ROCK pathway, enhanced actin cytoskeletal polymerization, and MKL1 cytoplasmic-nuclear shuttling. Pharmacologic disruption of this mechanotransduction pathway with the ROCK inhibitor fasudil induced myofibroblast apoptosis through a mechanism involving downregulation of BCL-2 and activation of the intrinsic mitochondrial apoptotic pathway. Treatment with fasudil during the postinflammatory fibrotic phase of lung injury or genetic ablation of Mkl1 protected mice from experimental lung fibrosis. These studies indicate that targeting mechanosensitive signaling in myofibroblasts to trigger the intrinsic apoptosis pathway may be an effective approach for treatment of fibrotic disorders.

Pleural mesothelial cell differentiation and invasion in fibrogenic lung injury

The origin of the myofibroblast in fibrotic lung disease is uncertain, and no effective medical therapy for fibrosis exists. We have previously demonstrated that transforming growth factor-β1 (TGF-β1) induces pleural mesothelial cell (PMC) transformation into myofibroblasts and haptotactic migration in vitro. Whether PMC differentiation and migration occurs in vivo, and whether this response can be modulated for therapeutic benefit, is unknown. Here, using mice recombinant for green fluorescent protein (GFP) driven by the Wilms tumor-1 (WT-1) promoter, we demonstrate PMC trafficking into the lung and differentiation into myofibroblasts. Carbon monoxide or the induction of heme oxygenase-1 (HO-1) inhibited the expression of myofibroblast markers, contractility, and haptotaxis in PMCs treated with TGF-β1. Intrapleural HO-1 induction inhibited PMC migration after intratracheal fibrogenic injury. PMCs from patients with idiopathic pulmonary fibrosis (IPF) exhibited increased expression of myofibroblast markers and enhanced contractility and haptotaxis, compared with normal PMCs. Carbon monoxide reversed this IPF PMC profibrotic phenotype. WT-1-expressing cells were present within fibrotic regions of the lungs in IPF subjects, supporting a role for PMC differentiation and trafficking as contributors to the myofibroblast population in lung fibrosis. Our findings also support a potential role for pleural-based therapies to modulate pleural mesothelial activation and parenchymal fibrosis progression.

Histone modifications in senescence-associated resistance to apoptosis by oxidative stress

Aging and age-related diseases are associated with cellular senescence that results in variable apoptosis susceptibility to oxidative stress. Although fibroblast senescence has been associated with apoptosis resistance, mechanisms for this have not been well defined. In this report, we studied epigenetic mechanisms involving histone modifications that confer apoptosis resistance to senescent human diploid fibroblasts (HDFs). HDFs that undergo replicative senescence display typical morphological features, express senescence-associated β-galactosidase, and increased levels of the tumor suppressor genes, p16, p21, and caveolin-1. Senescent HDFs are more resistant to oxidative stress (exogenous H₂O₂)-induced apoptosis in comparison to non-senescent (control) HDFs; this is associated with constitutively high levels of the anti-apoptotic gene, Bcl-2, and low expression of the pro-apoptotic gene, Bax. Cellular senescence is characterized by global increases in H4K20 trimethylation and decreases in H4K16 acetylation in association with increased activity of Suv420h2 histone methyltransferase (which targets H4K20), decreased activity of the histone acetyltransferase, Mof (which targets H4K16), as well as decreased total histone acetyltransferase activity. In contrast to Bax gene, chromatin immunoprecipitation studies demonstrate marked enrichment of the Bcl-2 gene with H4K16Ac, and depletion with H4K20Me3, predicting active transcription of this gene in senescent HDFs. These data indicate that both global and locus-specific histone modifications of chromatin regulate altered Bcl-2:Bax gene expression in senescent fibroblasts, contributing to its apoptosis-resistant phenotype.

Vitronectin inhibits efferocytosis through interactions with apoptotic cells as well as with macrophages

Effective removal of apoptotic cells, particularly apoptotic neutrophils, is essential for the successful resolution of acute inflammatory conditions. In these experiments, we found that whereas interaction between vitronectin and integrins diminished the ability of macrophages to ingest apoptotic cells, interaction between vitronectin with urokinase-type plasminogen activator receptor (uPAR) on the surface of apoptotic cells also had equally important inhibitory effects on efferocytosis. Preincubation of vitronectin with plasminogen activator inhibitor-1 eliminated its ability to inhibit phagocytosis of apoptotic cells. Similarly, incubation of apoptotic cells with soluble uPAR or Abs to uPAR significantly diminished efferocytosis. In the setting of LPS-induced ALI, enhanced efferocytosis and decreased numbers of neutrophils were found in bronchoalveolar lavage obtained from vitronectin-deficient (vtn(-/-)) mice compared with wild type (vtn(+/+)) mice. Furthermore, there was increased clearance of apoptotic vtn(-/-) as compared with vtn(+/+) neutrophils after introduction into the lungs of vtn(-/-) mice. Incubation of apoptotic vtn(-/-) neutrophils with purified vitronectin before intratracheal instillation decreased efferocytosis in vivo. These findings demonstrate that the inhibitory effects of vitronectin on efferocytosis involve interactions with both the engulfing phagocyte and the apoptotic target cell.

HMGB1 promotes neutrophil extracellular trap formation through interactions with Toll-like receptor 4

Although neutrophil extracellular traps (NETs) form to prevent dissemination of pathogenic microorganisms, excessive release of DNA and DNA-associated proteins can also perpetuate sterile inflammation. In this study, we found that the danger-associated molecular pattern protein high-mobility group box 1 (HMGB1) can induce NET formation. NET formation was found after exposure of wild-type and receptor for advanced glycation end products-deficient neutrophil to HMGB1, whereas deficiency of Toll-like receptor (TLR)4 diminished the ability of neutrophils to produce NETs. Incubation of neutrophils with HMGB1 significantly increased the amount of DNA and histone 3 released as well as intracellular histone 3 citrullination, a signaling event that precedes chromatin decondensation. In vivo, neutrophils isolated from bronchoalveolar lavages of mice exposed to LPS and HMGB1 showed consistently greater ability to produce NETs compared with pulmonary neutrophils from mice that received LPS alone. In contrast, mice treated with LPS and neutralizing antibody to HMGB1 had decreased amounts of the inflammatory cytokines TNF-α and macrophage inflammatory protein 2, as well as of free DNA and histone 3 in bronchoalveolar lavage fluids. Airway neutrophils from LPS-exposed mice that had been treated with anti-HMGB1 antibodies showed decreased citrullination of histone 3. These results demonstrate that interactions between HMGB1 and TLR4 enhance the formation of NETs and provide a novel mechanism through which HMGB1 may contribute to the severity of neutrophil-associated inflammatory conditions.

Vascular peroxidase 1 catalyzes the formation of hypohalous acids: characterization of its substrate specificity and enzymatic properties

The heme-containing peroxidase family comprises eight members in humans. The physiological and pathophysiological roles of heme-containing peroxidases are not well understood. Phagocyte-derived myeloperoxidase (MPO) utilizes chloride and bromide, in the presence of hydrogen peroxide (H(2)O(2)), to generate hypochlorous acid and hypobromous acid, potent oxidizing species that are known to kill invading pathogens. Vascular peroxidase 1 (VPO1) is a new member of the heme-containing peroxidase family; VPO1 is highly expressed in the cardiovascular system, lung, liver, pancreas, and spleen. However, functional roles of VPO1 have not been defined. In this report, we demonstrate the capacity for VPO1 to catalyze the formation of hypohalous acids, and characterize its enzymatic properties. VPO1, like MPO but unlike lactoperoxidase, is able to generate hypochlorous acid, hypobromous acid, and hypothiocyanous acid in the presence of H(2)O(2). Under physiological pH and concentrations of halides (100μM KBr, 100μM KSCN, and 100mM NaCl), VPO1 utilizes approximately 45% of H(2)O(2) for the generation of hypobromous acid, 35% for hypothiocyanous acid, and 18% for hypochlorous acid. The specific activity of VPO1 is ∼10- to 70-fold lower than that of MPO, depending on the specific substrate. These studies demonstrate that the enzymatic properties and substrate specificity of VPO1 are similar to MPO; however, significantly lower catalytic rate constants of VPO1 relative to MPO suggest the possibility of other physiologic roles for this novel heme-containing peroxidase.

Host responses in tissue repair and fibrosis

Myofibroblasts accumulate in the spaces between organ structures and produce extracellular matrix (ECM) proteins, including collagen I. They are the primary "effector" cells in tissue remodeling and fibrosis. Previously, leukocyte progenitors termed fibrocytes and myofibroblasts generated from epithelial cells through epithelial-to-mesenchymal transition (EMT) were considered the primary sources of ECM-producing myofibroblasts in injured tissues. However, genetic fate mapping experiments suggest that mesenchyme-derived cells, known as resident fibroblasts, and pericytes are the primary precursors of scar-forming myofibroblasts, whereas epithelial cells, endothelial cells, and myeloid leukocytes contribute to fibrogenesis predominantly by producing key fibrogenic cytokines and by promoting cell-to-cell communication. Numerous cytokines derived from T cells, macrophages, and other myeloid cell populations are important drivers of myofibroblast differentiation. Monocyte-derived cell populations are key regulators of the fibrotic process: They act as a brake on the processes driving fibrogenesis, and they dismantle and degrade established fibrosis. We discuss the origins, modes of activation, and fate of myofibroblasts in various important fibrotic diseases and describe how manipulation of macrophage activation could help ameliorate fibrosis.

Neuronal Wiskott-Aldrich syndrome protein (N-WASP) is critical for formation of α-smooth muscle actin filaments during myofibroblast differentiation

Myofibroblasts are implicated in pathological stromal responses associated with lung fibrosis. One prominent phenotypic marker of fully differentiated myofibroblasts is the polymerized, thick cytoplasmic filaments containing newly synthesized α-smooth muscle actin (α-SMA). These α-SMA-containing cytoplasmic filaments are important for myofibroblast contractility during tissue remodeling. However, the molecular mechanisms regulating the formation and maturation of α-SMA-containing filaments have not been defined. This study demonstrates a critical role for neuronal Wiskott-Aldrich syndrome protein (N-WASP) in regulating the formation of α-SMA-containing cytoplasmic filaments during myofibroblast differentiation and in myofibroblast contractility. Focal adhesion kinase (FAK) is activated by transforming growth factor-β1 (TGF-β1) and is required for phosphorylation of tyrosine residue 256 (Y256) of N-WASP. Phosphorylation of Y256 of N-WASP is essential for TGF-β1-induced formation of α-SMA-containing cytoplasmic filaments in primary human lung fibroblasts. In addition, we demonstrate that actin-related protein (Arp) 2/3 complex is downstream of N-WASP and mediates the maturation of α-SMA-containing cytoplasmic filaments. Together, this study supports a critical role of N-WASP in integrating FAK and Arp2/3 signaling to mediate formation of α-SMA-containing cytoplasmic filaments during myofibroblast differentiation and maturation.

Altered DNA methylation profile in idiopathic pulmonary fibrosis

RATIONALE

DNA methylation is an important epigenetic mechanism, which often occurs in response to environmental stimuli and is crucial in regulating gene expression. It is likely that epigenetic alterations contribute to pathogenesis in idiopathic pulmonary fibrosis (IPF).

OBJECTIVES

To determine the DNA methylation changes in IPF and their effects on gene expression.

METHODS

Total DNA methylation and DNA methyltransferase expression were compared in IPF and normal control lung tissues. IPF and normal tissues were subjected to comparative analysis of genome-wide DNA methylation and RNA expression using DNA hybridization to the Illumina HumanMethylation27 BeadChip and RNA hybridization to Illumina HumanHT-12 BeadChip. Functional analyses of differentially expressed and differentially methylated genes were done. Selected genes were validated at DNA, RNA, and protein levels.

MEASUREMENTS AND MAIN RESULTS

DNA methylation status was altered in IPF. IPF samples demonstrated higher DNA methyltransferase expression without observed alterations in global DNA methylation. Genome-wide differences in DNA methylation status and RNA expression were demonstrated by array hybridization. Among the genes whose DNA methylation status and RNA expression were both significantly altered, 16 genes were hypermethylated in DNA associated with decreased mRNA expression or vice versa. We validated CLDN5, ZNF467, TP53INP1, and DDAH1 genes at the level of DNA methylation status, RNA, and protein-level expression.

CONCLUSIONS

Changes in DNA methylation correspond to altered mRNA expression of a number of genes, some with known and others with previously uncharacterized roles in IPF, suggesting that DNA methylation is important in the pathogenesis of IPF.

Mechanisms of pulmonary fibrosis: role of activated myofibroblasts and NADPH oxidase

A common feature of pathological fibrosis involving the lung and other organs is the persistent activation of myofibroblasts in injured tissues. Recent evidence supports the role of a member of the NADPH oxidase (NOX) gene family, NOX4, in myofibroblast differentiation, matrix synthesis and contractility. Additionally, NOX4 may contribute directly or indirectly to alveolar epithelial cell death, while myofibroblasts themselves acquire an apoptosis-resistant phenotype. Thus, NOX4 may be responsible for the cardinal features of progressive fibrosis - myofibroblast activation and epithelial cell dysrepair. Therapeutic targeting of NOX4 is likely to be effective in progressive cases of fibrosis involving multiple organs.

miR-31 is a negative regulator of fibrogenesis and pulmonary fibrosis

Aberrant expression of miRNAs is closely associated with initiation and progression of pathological processes, including diabetes, cancer, and cardiovascular disease. However, the role of miRNAs in lung fibrosis is not well characterized. We sought to determine the role of miR-31 in regulating the fibrogenic, contractile, and migratory activities of lung fibroblasts and modulating of pulmonary fibrosis in vivo. In vivo lung fibrosis models and ex vivo cell culture systems were employed. Real-time PCR and Western blot analysis were used to determine gene expression levels. miR-31 mimics or inhibitors were transfected into pulmonary fibroblasts. Fibrogenic, contractile, and migratory activities of lung fibroblasts were determined. We found that miR-31 expression is reduced in the lungs of mice with experimental pulmonary fibrosis and in IPF fibroblasts. miR-31 inhibits the profibrotic activity of TGF-β1 in normal lung fibroblasts and diminishes the fibrogenic, contractile, and migratory activities of IPF fibroblasts. In these experiments, miR-31 was shown to directly target integrin α(5) and RhoA, two proteins that have been shown to regulate activation of fibroblasts. We found that levels of integrin α(5) and RhoA are up-regulated in fibrotic mouse lungs. Knockdown of integrin α(5) and RhoA attenuated fibrogenic, contractile, and migratory activities of IPF fibroblasts, in a manner similar to that observed with miR-31. We also found that introduction of miR-31 ameliorated experimental lung fibrosis in mice. Our data suggest that miR-31 is an important regulator of the pathological activities of lung fibroblasts and may be a potential target in the development of novel therapies to treat pathological fibrotic disorders, including pulmonary fibrosis.