Cellular mechanisms of tissue fibrosis. 7. New insights into the cellular mechanisms of pulmonary fibrosis

Whole-Lung Irradiation Results in Pulmonary Macrophage Alterations that are Subpopulation and Strain Specific

Exposure of the lung to radiation produces injury and inflammatory responses that result in microenvironmental alterations, which can promote the development of pneumonitis and/or pulmonary fibrosis. It has been shown that after other toxic insults, macrophages become phenotypically polarized in response to microenvironmental signals, orchestrating the downstream inflammatory responses. However, their contribution to the development of the late consequences of pulmonary radiation exposure remains unclear. To address this issue, fibrosis-prone C57BL/6J mice or pneumonitis-prone C3H/HeJ mice were whole-lung irradiated with 0 or 12.5 Gy and lung digests were collected between 3 and 26 weeks after radiation exposure. CD45(+) leukocytes were isolated and characterized by flow cytometry, and alveolar, interstitial and infiltrating macrophages were also detected. Ly6C, expressed by pro-inflammatory monocytes and macrophages, and mannose receptor (CD206), a marker of alternative activation, were assessed in each subpopulation. While the total number of pulmonary macrophages was depleted at 3 weeks after lung irradiation relative to age-matched controls in both C57 and C3H mice, identification of discrete subpopulations showed that this loss in cell number occurred in the alveolar, but not the interstitial or infiltrating, subsets. In the alveolar macrophages of both C57 and C3H mice, this correlated with a loss in the proportion of cells that expressed CD206 and F4/80. In contrast, in interstitial and infiltrating macrophages, the proportion of cells expressing these markers was increased at several time points after irradiation, with this response generally more pronounced in C3H mice. Radiation exposure was also associated with elevations in the proportion of alveolar and interstitial macrophage subpopulations expressing Ly6C and F4/80, with this response occurring at earlier time points in C57 mice. Although the radiation dose used in this study was not isoeffective for the inflammatory response in the two strains, the differences observed in the responses of these discrete macrophage populations between the fibrosis-prone versus pneumonitis-prone mice nonetheless suggest a possible role for these cells in the development of long-term consequences of pulmonary radiation exposure.

Antifibrotic properties of receptor for advanced glycation end products in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive chronic interstitial lung disease with poor survival. Previous reports suggested the contributory effect of receptor for advanced glycation end products (RAGE) to the pathogenesis of IPF. But the findings are controversial. The present in vivo study with RAGE null mice, we further confirmed the evidence that lack of RAGE evolves worse bleomycin-induced pulmonary fibrosis compared with control mice. Moreover, RAGE null mice spontaneously developed similar pathogenesis of lung fibrosis via immunohistochemical staining. In addition, we investigated the negative roles of RAGE on epithelial-mesenchymal transition (EMT) indicated by elevated α-smooth muscle actin (α-SMA) and collagen-I (Col-I) deposition in A549 cell treated with transforming growth factor-β (TGF-β), all of which were blocked by sRAGE, a decoy receptor. Furthermore, interacting with the specific ligand as AGE, RAGE blocked TGF-β-induced activation of Smad2, ERK and JNK signals in A549 cells, which were also challenged by sRAGE administration. This present study confirmed an important role of RAGE in vivo and vitro models of pulmonary fibrosis and suggested the therapeutic possibility for pulmonary fibrosis via RAGE regulation.

Fibrocytes Regulate Wilms Tumor 1-Positive Cell Accumulation in Severe Fibrotic Lung Disease

Collagen-producing myofibroblast transdifferentiation is considered a crucial determinant in the formation of scar tissue in the lungs of patients with idiopathic pulmonary fibrosis. Multiple resident pulmonary cell types and bone marrow-derived fibrocytes have been implicated as contributors to fibrotic lesions because of the transdifferentiation potential of these cells into myofibroblasts. In this study, we assessed the expression of Wilms tumor 1 (WT1), a known marker of mesothelial cells, in various cell types in normal and fibrotic lungs. We demonstrate that WT1 is expressed by both mesothelial and mesenchymal cells in idiopathic pulmonary fibrosis lungs but has limited or no expression in normal human lungs. We also demonstrate that WT1(+) cells accumulate in fibrotic lung lesions, using two different mouse models of pulmonary fibrosis and WT1 promoter-driven fluorescent reporter mice. Reconstitution of bone marrow cells into a TGF-α transgenic mouse model demonstrated that fibrocytes do not transform into WT1(+) mesenchymal cells, but they do augment accumulation of WT1(+) cells in severe fibrotic lung disease. Importantly, the number of WT1(+) cells in fibrotic lesions was correlated with severity of lung disease as assessed by changes in lung function, histology, and hydroxyproline levels in mice. Finally, inhibition of WT1 expression was sufficient to attenuate collagen and other extracellular matrix gene production by mesenchymal cells from both murine and human fibrotic lungs. Thus, the results of this study demonstrate a novel association between fibrocyte-driven WT1(+) cell accumulation and severe fibrotic lung disease.

Vocal fold myofibroblast profile of scarring

OBJECTIVES/HYPOTHESIS

Vocal fold fibroblasts (VFF) are responsible for extracellular matrix synthesis supporting lamina propria in normal and diseased conditions. When tissue is injured, VFF become activated and differentiate into myofibroblasts to facilitate wound healing response. We investigated if vocal fold myofibroblasts can be utilized as surrogate cells for scarred VFF.

STUDY DESIGN

In vitro.

METHODS

Normal VFF cell lines from a 21-year-old male (N21), 59-year-old female (N59), and a scar VFF cell line from a 56-year-old female (S56) were used in this study. 10 ng/mL of transforming growth factor (TGFβ1) was applied for 5 days to normal VFF. Myofibroblast differentiation was determined with immunocytochemistry and western blot, measuring alpha smooth muscle actin (α-SMA). Cell growth, proliferation, contractile properties, and gene expression profiles were evaluated.

RESULTS

N21, N59, and S56 VFF presented elongated configuration. N21+ and N21- VFF demonstrated significantly greater proliferation compared to N59+, N59-, and S56 VFF at 6 days. α-SMA was expressed in all cells. Fibronectin, alpha smooth actin, connective tissue growth factor, and metallopeptidase inhibitor were the highest genes expression in VFF treated with transforming growth factor β1 (TGFβ1). At 24 hours, S56 VFF showed lower contraction compared to N21+ and N59+ VFF, but at 60 hours S56 VFF had lower collagen contraction compared to all cell groups. Highest collagen contraction matrices were measured with VFF treated with TGFβ1 at 24 hours and N59- VFF at 60 hours.

CONCLUSION

VFF treated with TGFβ1 (myofibroblasts) appear to have similar phenotypic characteristics but different genotypic behavior compared to scar VFF.

LEVEL OF EVIDENCE

N/A. Laryngoscope, 126:E110-E117, 2016.

miR-9-5p suppresses pro-fibrogenic transformation of fibroblasts and prevents organ fibrosis by targeting NOX4 and TGFBR2

Uncontrolled extracellular matrix (ECM) production by fibroblasts in response to injury contributes to fibrotic diseases, including idiopathic pulmonary fibrosis (IPF). Reactive oxygen species (ROS) generation is involved in the pathogenesis of IPF. Transforming growth factor-β1 (TGF-β1) stimulates the production of NADPH oxidase 4 (NOX4)-dependent ROS, promoting lung fibrosis (LF). Dysregulation of microRNAs (miRNAs) has been shown to contribute to LF. To identify miRNAs involved in redox regulation relevant for IPF, we performed arrays in human lung fibroblasts exposed to ROS. miR-9-5p was selected as the best candidate and we demonstrate its inhibitory effect on TGF-β receptor type II (TGFBR2) and NOX4 expression. Increased expression of miR-9-5p abrogates TGF-β1-dependent myofibroblast phenotypic transformation. In the mouse model of bleomycin-induced LF, miR-9-5p dramatically reduces fibrogenesis and inhibition of miR-9-5p and prevents its anti-fibrotic effect both in vitro and in vivo. In lung specimens from patients with IPF, high levels of miR-9-5p are found. In omentum-derived mesothelial cells (MCs) from patients subjected to peritoneal dialysis (PD), miR-9-5p also inhibits mesothelial to myofibroblast transformation. We propose that TGF-β1 induces miR-9-5p expression as a self-limiting homeostatic response.

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.

Regenerative medicine in the treatment of idiopathic pulmonary fibrosis: current position

Idiopathic pulmonary fibrosis (IPF) is a progressive, irreversible disease of the lung that has no lasting option for therapy other than transplantation. It is characterized by replacement of the normal lung tissue by fibrotic scarring, honeycombing, and increased levels of myofibroblasts. The underlying causes of IPF are still largely unknown. The focus of the current review is the possible use of stem cell therapy, specifically mesenchymal stem cells (MSCs), a multipotent stromal cell population, which have demonstrated promising data in multiple animal models of pulmonary fibrosis (PF). The most studied source of MSCs is the bone marrow, although they can be found also in the adipose tissue and umbilical cord, as well as in the placenta. MSCs have immunomodulatory and tissue-protective properties that allow them to manipulate the local environment of the injured tissue, ameliorating the inflammation and promoting repair. Because IPF primarily affects older patients, the issue of aging is intrinsically linked to many aspects of the disease, including the age of the stem cells. Animal models have shown the success of MSC therapy in mitigating the fibrotic effects of bleomycin-induced PF. However, bleomycin, the most commonly used model for PF, is imperfect in mimicking IPF as it presents in humans, as the duration of the illness is not parallel or reversible, and honeycombing is not produced. Furthermore, the time of MSC dosage has proven to be critical in determining whether the cells will ultimately have a positive or negative effect on disease progression, since it has been demonstrated that the maximal beneficial effect of MSCs occurs during the early inflammatory phase of the disease and that there is no or negative effect during the late fibrotic phase. Therefore, all the current clinical trials of MSCs and IPF, though promising, should proceed with caution as we move toward true stem cell therapy for this disease.

Tissue is an issue in the search for biomarkers in idiopathic pulmonary fibrosis

Biological markers, i.e., biomarkers, in lung tissue may make it possible to connect cell biological phenomena to the pathogenetic mechanisms in idiopathic pulmonary fibrosis (IPF). This review focuses on the lung tissue biomarkers, which have been compared with relevant clinical endpoints or with the most common differential diagnostic lung diseases. In addition, studies conducted on lung tissue samples and investigated by transcriptomic or proteomic methodologies have been included. Several studies have observed changes in alveolar epithelium and extracellular matrix supporting the current hypotheses of the pathogenesis of IPF. In many studies, however, alterations in inflammatory cells have been revealed, a phenomenon not currently incorporated into pathogenetic theories. Combining lung tissue material with other non-solid organs with clinically meaningful endpoints may prove to be the most beneficial approach in the search for non-invasive biomarkers.

Epigenetic targets for novel therapies of lung diseases

In spite of substantial advances in defining the immunobiology and function of structural cells in lung diseases there is still insufficient knowledge to develop fundamentally new classes of drugs to treat many lung diseases. For example, there is a compelling need for new therapeutic approaches to address severe persistent asthma that is insensitive to inhaled corticosteroids. Although the prevalence of steroid-resistant asthma is 5-10%, severe asthmatics require a disproportionate level of health care spending and constitute a majority of fatal asthma episodes. None of the established drug therapies including long-acting beta agonists or inhaled corticosteroids reverse established airway remodeling. Obstructive airways remodeling in patients with chronic obstructive pulmonary disease (COPD), restrictive remodeling in idiopathic pulmonary fibrosis (IPF) and occlusive vascular remodeling in pulmonary hypertension are similarly unresponsive to current drug therapy. Therefore, drugs are needed to achieve long-acting suppression and reversal of pathological airway and vascular remodeling. Novel drug classes are emerging from advances in epigenetics. Novel mechanisms are emerging by which cells adapt to environmental cues, which include changes in DNA methylation, histone modifications and regulation of transcription and translation by noncoding RNAs. In this review we will summarize current epigenetic approaches being applied to preclinical drug development addressing important therapeutic challenges in lung diseases. These challenges are being addressed by advances in lung delivery of oligonucleotides and small molecules that modify the histone code, DNA methylation patterns and miRNA function.

TRPV4 mediates myofibroblast differentiation and pulmonary fibrosis in mice

Idiopathic pulmonary fibrosis (IPF) is a fatal fibrotic lung disorder with no effective medical treatments available. The generation of myofibroblasts, which are critical for fibrogenesis, requires both a mechanical signal and activated TGF-β; however, it is not clear how fibroblasts sense and transmit the mechanical signal(s) that promote differentiation into myofibroblasts. As transient receptor potential vanilloid 4 (TRPV4) channels are activated in response to changes in plasma membrane stretch/matrix stiffness, we investigated whether TRPV4 contributes to generation of myofibroblasts and/or experimental lung fibrosis. We determined that TRPV4 activity is upregulated in lung fibroblasts derived from patients with IPF. Moreover, TRPV4-deficient mice were protected from fibrosis. Furthermore, genetic ablation or pharmacological inhibition of TRPV4 function abrogated myofibroblast differentiation, which was restored by TRPV4 reintroduction. TRPV4 channel activity was elevated when cells were plated on matrices of increasing stiffness or on fibrotic lung tissue, and matrix stiffness-dependent myofibroblast differentiation was reduced in response to TRVP4 inhibition. TRPV4 activity modulated TGF-β1-dependent actions in a SMAD-independent manner, enhanced actomyosin remodeling, and increased nuclear translocation of the α-SMA transcription coactivator (MRTF-A). Together, these data indicate that TRPV4 activity mediates pulmonary fibrogenesis and suggest that manipulation of TRPV4 channel activity has potential as a therapeutic approach for fibrotic diseases.

Ca2+ influx and ATP release mediated by mechanical stretch in human lung fibroblasts

One cause of progressive pulmonary fibrosis is dysregulated wound healing after lung inflammation or damage in patients with idiopathic pulmonary fibrosis and severe acute respiratory distress syndrome. The mechanical forces are considered to regulate pulmonary fibrosis via activation of lung fibroblasts. In this study, the effects of mechanical stretch on the intracellular Ca(2+) concentration ([Ca(2+)]i) and ATP release were investigated in primary human lung fibroblasts. Uniaxial stretch (10-30% in strain) was applied to fibroblasts cultured in a silicone chamber coated with type I collagen using a stretching apparatus. Following stretching and subsequent unloading, [Ca(2+)]i transiently increased in a strain-dependent manner. Hypotonic stress, which causes plasma membrane stretching, also transiently increased the [Ca(2+)]i. The stretch-induced [Ca(2+)]i elevation was attenuated in Ca(2+)-free solution. In contrast, the increase of [Ca(2+)]i by a 20% stretch was not inhibited by the inhibitor of stretch-activated channels GsMTx-4, Gd(3+), ruthenium red, or cytochalasin D. Cyclic stretching induced significant ATP releases from fibroblasts. However, the stretch-induced [Ca(2+)]i elevation was not inhibited by ATP diphosphohydrolase apyrase or a purinergic receptor antagonist suramin. Taken together, mechanical stretch induces Ca(2+) influx independently of conventional stretch-sensitive ion channels, the actin cytoskeleton, and released ATP.

Cellular mechanisms of tissue fibrosis. 6. Purinergic signaling and response in fibroblasts and tissue fibrosis

Tissue fibrosis occurs as a result of the dysregulation of extracellular matrix (ECM) synthesis. Tissue fibroblasts, resident cells responsible for the synthesis and turnover of ECM, are regulated via numerous hormonal and mechanical signals. The release of intracellular nucleotides and their resultant autocrine/paracrine signaling have been shown to play key roles in the homeostatic maintenance of tissue remodeling and in fibrotic response post-injury. Extracellular nucleotides signal through P2 nucleotide and P1 adenosine receptors to activate signaling networks that regulate the proliferation and activity of fibroblasts, which, in turn, influence tissue structure and pathologic remodeling. An important component in the signaling and functional responses of fibroblasts to extracellular ATP and adenosine is the expression and activity of ectonucleotideases that attenuate nucleotide-mediated signaling, and thereby integrate P2 receptor- and subsequent adenosine receptor-initiated responses. Results of studies of the mechanisms of cellular nucleotide release and the effects of this autocrine/paracrine signaling axis on fibroblast-to-myofibroblast conversion and the fibrotic phenotype have advanced understanding of tissue remodeling and fibrosis. This review summarizes recent findings related to purinergic signaling in the regulation of fibroblasts and the development of tissue fibrosis in the heart, lungs, liver, and kidney.