Role of Sonic Hedgehog in idiopathic pulmonary fibrosis

Sonic hedgehog signaling in the lung. From development to disease

Over the past two decades, the secreted protein sonic hedgehog (SHH) has emerged as a critical morphogen during embryonic lung development, regulating the interaction between epithelial and mesenchymal cell populations in the airway and alveolar compartments. There is increasing evidence that the SHH pathway is active in adult lung diseases such as pulmonary fibrosis, asthma, chronic obstructive pulmonary disease, and lung cancer, which raises two questions: (1) What role does SHH signaling play in these diseases? and (2) Is it a primary driver of the disease or a response (perhaps beneficial) to the primary disturbance? In this review we aim to fill the gap between the well-studied period of embryonic lung development and the adult diseased lung by reviewing the hedgehog (HH) pathway during the postnatal period and in adult uninjured and injured lungs. We elucidate the similarities and differences in the epithelial-mesenchymal interplay during the fibrosis response to injury in lung compared with other organs and present a critical appraisal of tools and agents available to evaluate HH signaling.

An overview of hedgehog signaling in fibrosis

The Hedgehog (Hh) signaling pathway plays a key role during embryogenesis and tissue regeneration. Recently, studies revealed that overactivated Hh signaling leads to fibrogenesis in many types of tissues. The activation of Hh signaling is involved in the epithelial-mesenchymal transition and excessive extracellular matrix deposition. Blockade of Hh signaling abolishes the induction of the epithelial-mesenchymal transition and ameliorates tissue fibrosis. Therefore, new therapeutic targets to alleviate fibrosis based on the Hh signaling have attracted a great deal of attention. This is a new strategy for treating fibrosis and other related diseases. In this review, we discuss the crucial role of Hh signaling in fibrogenesis to provide a better understanding of their relationship and to encourage the study of novel targeted therapies.

Epigenetics of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a complex lung disease of unknown etiology. Development of IPF is influenced by both genetic and environmental factors. Recent work by our and other groups has identified strong genetic predisposition factors for the development of pulmonary fibrosis, and cigarette smoke remains the most strongly associated environmental exposure risk factor. Gene expression profiling studies of IPF lung have taught us quite a bit about the biology of this fatal disease, and those of peripheral blood have provided important biomarkers. However, epigenetic marks may be the missing link that connects the environmental exposure in genetically predisposed individuals to transcriptional changes associated with disease development. Moreover, epigenetic marks represent a promising therapeutic target for IPF. In this review, the disease is introduced, genetic and gene expression studies in IPF are summarized, exposures relevant to IPF and known epigenetic changes associated with cigarette smoke exposure are discussed, and epigenetic studies conducted so far in IPF are summarized. Limitations, challenges, and future opportunities in this field are also discussed.

New therapeutic avenues for treatment of fibrosis: can we learn from other diseases?

Crohn's disease (CD) is characterized by the frequent occurrence of complications, such as fibrotic strictures and subsequently the need for CD-related surgery. Chronic or recurrent inflammation is generally regarded to be a necessary precondition for the initiation of intestinal fibrosis. In this view, fibrosis is a pathologically augmented healing response to inflammation-induced mucosal tissue destruction and injury. At present, there are no approved or effective medical therapies aimed specifically at fibrosis or stricture in IBD. Indirect benefits may occur from anti-inflammatory therapies, although there is no consensus on this. Therapy for fibrosis is complicated by the fact that a wound-healing response is essential in CD and ulcerative colitis. Several pharmaceutical companies are now working on the therapy of fibrosis in other diseases. Strategies interfering with TGF-β expression and activation are promising. Pirfenidone has been studied in several clinical trials. Further therapeutic options are second-generation and wide-spectrum tyrosine kinase inhibitors. These inhibit growth factor receptor signaling, thus reducing fibrosis in animal models and some patients with tumor-associated fibrosis. At present, the development of antifibrotic therapies takes place in other diseases such as lung and liver fibrosis. This is partially due to a lack of experimental models for gut fibrosis and the fact that reliable readouts (MRI, serum markers) in patients are lacking. It will be important to test the above-mentioned newly available treatment strategies in IBD to profit from progress in other fibrotic diseases.

Promising new treatment targets in patients with fibrosing lung disorders

The processes of lung fibrogenesis and fibrotic healing are common to a number of conditions with different etiologies. The lungs are the only affected organ in some cases, whereas in others, several organ systems are involved. Therapeutic options can be discussed from various perspectives. In this review, we address the localization of therapeutic targets with regard to cell compartments, including secreted ligands, cell surface, plasma membrane-cytosol interplay, cytosol and nucleus. Complex approach using stem cell therapy is also discussed. As the prognosis of patients with these disorders remains grim, treatment combinations targeting different molecules within the cell should sometimes be considered. It is reasonable to assume that blocking specific pathways will more likely lead to disease stabilization, while stem cell-based treatments could potentially restore lung architecture. Gene therapy could be a candidate for preventive care in families with proven specific gene polymorphisms and documented familial lung fibrosis. Chronobiology, that takes into account effect of circadian rhythm on cell biology, has demonstrated that timed drug administration can improve treatment outcomes. However, the specific recommendations for optimal approaches are still under debate. A multifaceted approach to interstitial lung disorders, including cooperation between those doing basic research and clinical doctors as well as tailoring research and treatment strategies toward (until now) unmet medical needs, could improve our understanding of the diseases and, above all, provide benefits for our patients.

Targeting the hedgehog-glioma-associated oncogene homolog pathway inhibits bleomycin-induced lung fibrosis in mice

Idiopathic pulmonary fibrosis has been associated with the reactivation of developmental pathways, notably the Hedgehog-Glioma-associated oncogene homolog (GLI) pathway. In this study, we determined whether the Hedgehog pathway was activated in bleomycin-induced lung injury in mice, and whether targeting the Hedgehog-Gli pathway could decrease bleomycin-induced lung fibrosis. After intratracheal injection of bleomycin on Day 0, C57Bl6 mice received GDC-0449 (an inhibitor of Smoothened, the transducer of the pathway), or 2,2'-[[Dihydro-2-(4-pyridinyl)-1,3(2H,4H)-pyrimidinediyl]bis(methylene)]bis[N,N dimethylbenzenamine (GANT61; an inhibitor of GLI transcription factors in the nucleus), from Day 7 to Day 13. At Day 14, whole-lung homogenates were obtained for morphological analysis, assessment of cell apoptosis and proliferation, collagen quantification, and evaluation of profibrotic (transforming growth factor-β, connective tissue growth factor, plasminogen activator inhibitor 1, vascular endothelial growth factor-A) and proinflammatory mediators (IL-1β) expression. We showed that the Hedgehog pathway was activated in bleomycin-induced lung fibrosis on Day 14 after injury, with an increased lung expression of the ligand, Sonic Hedgehog, and with increased messenger RNA expression and nuclear localization of GLI1 and GLI2. Inhibition of Smoothened with GDC-0449 did not influence the development of bleomycin-induced lung fibrosis. By contrast, the inhibition of GLI activity with GANT61 decreased lung fibrosis and lung collagen accumulation, and promoted an antifibrotic and anti-inflammatory environment. Our results identify the hedgehog-Gli pathway as a profibrotic pathway in experimental fibrosis. Inhibition of the Hedgehog-Gli pathway at the level of GLI transcriptional activity could be a therapeutic option in fibrotic lung diseases.

In search of the elusive lipofibroblast in human lungs

Although the pulmonary interstitial lipofibroblast (LF) has been widely recognized in rat and mouse lungs, their presence in human lungs remains controversial. In a recent issue of the Journal, Tahedl and associates (Tahedl D, Wirkes A, Tschanz SA, Ochs M, Mühlfeld C. Am J Physiol Lung Cell Mol Physiol 307: L386-L394, 2014) address this controversy and provide the most detailed stereological analysis of LFs in mammals other than rodents. Strikingly, their observations demonstrate that LFs were only observed in rodents, which contrasts with earlier reports. This editorial reviews the anatomical, physiological, and biochemical characteristics of the LF to better understand the significance of LFs for lung development and disease. Although lipid droplets are a signature of the LF cell type, it remains unclear whether lipid storage is the defining characteristic of LFs, or whether other less overt properties determine the importance of LFs. Are lipid droplets an adaptation to the neonatal environment, or are LFs a surrogate for other properties that promote alveolar development, and do lipid droplets modify physiology or disease in adults?

Revealing the pathogenic and aging-related mechanisms of the enigmatic idiopathic pulmonary fibrosis. an integral model

A growing body of evidence indicates that aberrant activation of alveolar epithelial cells and fibroblasts in an aging lung plays a critical role in the pathogenesis of idiopathic pulmonary fibrosis (IPF). However, the biopathological processes linking aging with IPF and the mechanisms responsible for the abnormal activation of epithelial cells and fibroblasts have not been elucidated. Many of the hallmarks of aging (e.g., genomic instability, telomere attrition, epigenetic alterations, mitochondrial dysfunction, and cellular senescence) have been proposed as essential mechanisms for the development of IPF; however, these disturbances are not restricted to IPF and also occur in other aging-related lung disorders, primarily chronic obstructive pulmonary disease (COPD). Therefore, an unanswered question is why a current/former smoker of about 60 years of age with shorter telomeres, alveolar epithelial senescence, excessive oxidative stress, and mitochondrial dysfunction develops IPF and not COPD; in other words, what makes old lungs specifically susceptible to develop IPF? In this Perspective, we propose an integral model in which the combination of some gene variants and/or gene expression in the aging lung results in the loss of epithelial integrity and consequently in the failure of the alveoli to correctly respond to injury and to face the stress associated with mechanical stretch. Afterward, a distinctive epigenetic "reprogramming" that affects both epithelial cells and fibroblasts provokes, among others, the recapitulation of developmental pathways and the aberrant activation and miscommunication between both cell types, resulting in the exaggerated production and accumulation of extracellular matrix and the subsequent destruction of the lung architecture.

Interleukin-1β attenuates myofibroblast formation and extracellular matrix production in dermal and lung fibroblasts exposed to transforming growth factor-β1

One of the most potent pro-fibrotic cytokines is transforming growth factor (TGFβ). TGFβ is involved in the activation of fibroblasts into myofibroblasts, resulting in the hallmark of fibrosis: the pathological accumulation of collagen. Interleukin-1β (IL1β) can influence the severity of fibrosis, however much less is known about the direct effects on fibroblasts. Using lung and dermal fibroblasts, we have investigated the effects of IL1β, TGFβ1, and IL1β in combination with TGFβ1 on myofibroblast formation, collagen synthesis and collagen modification (including prolyl hydroxylase, lysyl hydroxylase and lysyl oxidase), and matrix metalloproteinases (MMPs). We found that IL1β alone has no obvious pro-fibrotic effect on fibroblasts. However, IL1β is able to inhibit the TGFβ1-induced myofibroblast formation as well as collagen synthesis. Glioma-associated oncogene homolog 1 (GLI1), the Hedgehog transcription factor that is involved in the transformation of fibroblasts into myofibroblasts is upregulated by TGFβ1. The addition of IL1β reduced the expression of GLI1 and thereby also indirectly inhibits myofibroblast formation. Other potentially anti-fibrotic effects of IL1β that were observed are the increased levels of MMP1, −2, −9 and −14 produced by fibroblasts exposed to TGFβ1/IL1β in comparison with fibroblasts exposed to TGFβ1 alone. In addition, IL1β decreased the TGFβ1-induced upregulation of lysyl oxidase, an enzyme involved in collagen cross-linking. Furthermore, we found that lung and dermal fibroblasts do not always behave identically towards IL1β. Suppression of COL1A1 by IL1β in the presence of TGFβ1 is more pronounced in lung fibroblasts compared to dermal fibroblasts, whereas a higher upregulation of MMP1 is seen in dermal fibroblasts. The role of IL1β in fibrosis should be reconsidered, and the differences in phenotypical properties of fibroblasts derived from different organs should be taken into account in future anti-fibrotic treatment regimes.

Interleukin-1β attenuates myofibroblast formation and extracellular matrix production in dermal and lung fibroblasts exposed to transforming growth factor-β1

One of the most potent pro-fibrotic cytokines is transforming growth factor (TGFβ). TGFβ is involved in the activation of fibroblasts into myofibroblasts, resulting in the hallmark of fibrosis: the pathological accumulation of collagen. Interleukin-1β (IL1β) can influence the severity of fibrosis, however much less is known about the direct effects on fibroblasts. Using lung and dermal fibroblasts, we have investigated the effects of IL1β, TGFβ1, and IL1β in combination with TGFβ1 on myofibroblast formation, collagen synthesis and collagen modification (including prolyl hydroxylase, lysyl hydroxylase and lysyl oxidase), and matrix metalloproteinases (MMPs). We found that IL1β alone has no obvious pro-fibrotic effect on fibroblasts. However, IL1β is able to inhibit the TGFβ1-induced myofibroblast formation as well as collagen synthesis. Glioma-associated oncogene homolog 1 (GLI1), the Hedgehog transcription factor that is involved in the transformation of fibroblasts into myofibroblasts is upregulated by TGFβ1. The addition of IL1β reduced the expression of GLI1 and thereby also indirectly inhibits myofibroblast formation. Other potentially anti-fibrotic effects of IL1β that were observed are the increased levels of MMP1, -2, -9 and -14 produced by fibroblasts exposed to TGFβ1/IL1β in comparison with fibroblasts exposed to TGFβ1 alone. In addition, IL1β decreased the TGFβ1-induced upregulation of lysyl oxidase, an enzyme involved in collagen cross-linking. Furthermore, we found that lung and dermal fibroblasts do not always behave identically towards IL1β. Suppression of COL1A1 by IL1β in the presence of TGFβ1 is more pronounced in lung fibroblasts compared to dermal fibroblasts, whereas a higher upregulation of MMP1 is seen in dermal fibroblasts. The role of IL1β in fibrosis should be reconsidered, and the differences in phenotypical properties of fibroblasts derived from different organs should be taken into account in future anti-fibrotic treatment regimes.

Repair-associated inflammation in nonalcoholic fatty liver disease

The mechanisms that drive non-alcoholic fatty liver disease (NAFLD) progression from simple steatosis to non-alcoholic steatohepatitis (NASH) and NASH-fibrosis and/or cirrhosis are complex. Recent studies suggest that the liver progenitor cell (ie liver stem cell) population expands during chronic liver injury, and is an essential component of the repair process. Hedgehog (Hh) is a developmental morphogen that has an important role in the adult tissue repair (and progenitor) response. Accumulating data in mice and human show that resurrection of the Hh pathway occurs during progressive NAFLD, and that activity of this pathway correlates with NASH-fibrosis stage. Importantly, Hh ligands secreted by dying (or stressed) hepatocytes, hepatic stellate cells (i.e. myofibroblasts), cholangiocytes and recruited immune cells can act on neighbouring cells to perpetuate the fibrogenic response. Intriguingly, Hh ligands can also stimulate cholangiocytes to secrete chemokines that recruit immune cell subsets (such as natural killer T cells), which could explain why fibrosis generally occurs in the context of chronic inflammation (i.e. fibrosis-associated inflammatory response). Finally, the administration of Hh inhibitors led to reduced fibrosis in a model of NASH. Future studies are needed to evaluate the utility of these inhibitors in other models of chronic liver disease. If successful, this could pave the way for the development of new therapy for patients with NASH, because Hh pathway inhibitors have now been licensed for use in patients with advanced basal cell carcinoma.

Reconstructing dynamic microRNA-regulated interaction networks

The regulation of gene expression in cells, including by microRNAs (miRNAs), is a dynamic process. Current methods for identifying miRNA targets by combining sequence and miRNA and mRNA expression data do not adequately use the temporal information and thus miss important miRNAs and their targets. We developed the MIRna Dynamic Regulatory Events Miner (mirDREM), a probabilistic modeling method that uses input-output hidden Markov models to reconstruct dynamic regulatory networks that explain how temporal gene expression is jointly regulated by miRNAs and transcription factors. We measured miRNA and mRNA expression for postnatal lung development in mice and used mirDREM to study the regulation of this process. The reconstructed dynamic network correctly identified known miRNAs and transcription factors. The method has also provided predictions about additional miRNAs regulating this process and the specific developmental phases they regulate, several of which were experimentally validated. Our analysis uncovered links between miRNAs involved in lung development and differentially expressed miRNAs in idiopathic pulmonary fibrosis patients, some of which we have experimentally validated using proliferation assays. These results indicate that some disease progression pathways in idiopathic pulmonary fibrosis may represent partial reversal of lung differentiation.

Hedgehog signaling in neonatal and adult lung

Sonic Hedgehog (Shh) signaling is essential during embryonic lung development, but its role in postnatal lung development and adult lung are not known. Using Gli1(nlacZ) reporter mice to identify cells with active Hh signaling, we found that Gli1(nlacZ)-positive mesenchymal cells are densely and diffusely present up to 2 weeks after birth and decline in number thereafter. In adult mice, Gli1(nlacZ)-positive cells are present around large airways and vessels and are sparse in alveolar septa. Hh-stimulated cells are mostly fibroblasts; only 10% of Gli1(nlacZ)-positive cells are smooth muscle cells, and most smooth muscle cells do not have activation of Hh signaling. To assess its functional relevance, we influenced Hh signaling in the developing postnatal lung and adult injured lung. Inhibition of Hh signaling during early postnatal lung development causes airspace enlargement without diminished alveolar septation. After bleomycin injury in the adult lung, there are abundant Gli1(nlacZ)-positive mesenchymal cells in fibrotic lesions and increased numbers of Gli1(nlacZ)-positive cells in preserved alveolar septa. Inhibition of Hh signaling with an antibody against all Hedgehog isoforms does not reduce bleomycin-induced fibrosis, but adenovirus-mediated overexpression of Shh increases collagen production in this model. Our data provide strong evidence that Hh signaling can regulate lung stromal cell function in two critical scenarios: normal development in postnatal lung and lung fibrosis in adult lung.

Abrogation of Eya1/Six1 disrupts the saccular phase of lung morphogenesis and causes remodeling

The Eya1 gene encodes a transcriptional co-activator that acts with Six1 to control the development of different organs. However, Six1-Eya1 interactions and functional roles in mesenchymal cell proliferation and differentiation as well as alveolarization during the saccular stage of lung development are still unknown. Herein, we provide the first evidence that Six1 and Eya1 act together to regulate mesenchymal development as well as alveolarization during the saccular phase of lung morphogenesis. Deletion of either or both Six1 and Eya1 genes results in a severe saccular phenotype, including defects of mesenchymal cell development and remodeling of the distal lung septae and arteries. Mutant lung histology at the saccular phase shows mesenchymal and saccular wall thickening, and abnormal proliferation of α-smooth muscle actin-positive cells, as well as increased mesenchymal/fibroblast cell differentiation, which become more sever when deleting both genes. Our study indicates that SHH but not TGF-β signaling pathway is a central mediator for the histologic alterations described in the saccular phenotype of Eya1(-/-) or Six1(-/-) lungs. Indeed, genetic reduction of SHH activity in vivo or inhibition of its activity in vitro substantially rescues lung mesenchymal and alveolar phenotype of mutant mice at the saccular phase. These findings uncover novel functions for Six1-Eya1-SHH pathway during the saccular phase of lung morphogenesis, providing a conceptual framework for future mechanistic and translational studies in this area.

Fibrosis of two: Epithelial cell-fibroblast interactions in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by the progressive and ultimately fatal accumulation of fibroblasts and extracellular matrix in the lung that distorts its architecture and compromises its function. IPF is now thought to result from wound-healing processes that, although initiated to protect the host from injurious environmental stimuli, lead to pathological fibrosis due to these processes becoming aberrant or over-exuberant. Although the environmental stimuli that trigger IPF remain to be identified, recent evidence suggests that they initially injure the alveolar epithelium. Repetitive cycles of epithelial injury and resultant alveolar epithelial cell death provoke the migration, proliferation, activation and myofibroblast differentiation of fibroblasts, causing the accumulation of these cells and the extracellular matrix that they synthesize. In turn, these activated fibroblasts induce further alveolar epithelial cell injury and death, thereby creating a vicious cycle of pro-fibrotic epithelial cell-fibroblast interactions. Though other cell types certainly make important contributions, we focus here on the "pas de deux" (steps of two), or perhaps more appropriate to IPF pathogenesis, the "folie à deux" (madness of two) of epithelial cells and fibroblasts that drives the progression of pulmonary fibrosis. We describe the signaling molecules that mediate the interactions of these cell types in their "fibrosis of two", including transforming growth factor-β, connective tissue growth factor, sonic hedgehog, prostaglandin E2, angiotensin II and reactive oxygen species. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

A simple dot-blot-Sirius red-based assay for collagen quantification

The assessment of collagen content in tissues is important in biomedical research, since this protein is altered in numerous diseases. Hydroxyproline and Sirius red based assays are the most common methods for collagen quantification. However, these procedures have some pitfalls, such as the requirement of oxygen-free medium or expensive equipment and large sample size or being unsuitable for hydrolyzed collagen, respectively. Our objective was to develop a specific, versatile, and user-friendly quantitative method applicable to small tissue samples and extracts obtained from elastin purification, therefore, suitable for simultaneous quantification of elastin. This method is based on the binding of Sirius red to collagen present in a sample immobilized on a PVDF membrane, as in the dot-blot technique, and quantified by a scanner and image analysis software. Sample loading, Sirius red concentration, temperature and incubation time, type of standard substance, albumin interference, and quantification time are optimized. The method enabled the quantification of (1) intact collagen in several rat tissue homogenates, including small resistance-sized arteries, (2) partially hydrolyzed collagen obtained from NaOH extracts, compatible with elastin purification, and (3) for the detection of differences in collagen content between hypertensive and normotensive rats. We conclude that the developed technique can be widely used since it is versatile (quantifies intact and hydrolyzed collagen), requires small sample volumes, is user-friendly (low-cost, easy to use, minimum toxic materials, and reduced time of test), and is specific (minimal interference with serum albumin).

Gli1 mediates lung cancer cell proliferation and Sonic Hedgehog-dependent mesenchymal cell activation

Non-Small-Cell-Lung-Cancer (NSCLC) represents approximately 85% of all lung cancers and remains poorly understood. While signaling pathways operative during organ development, including Sonic Hedgehog (Shh) and associated Gli transcription factors (Gli1-3), have recently been found to be reactivated in NSCLC, their functional role remains unclear. Here, we hypothesized that Shh/Gli1-3 could mediate NSCLC autonomous proliferation and epithelial/stromal signaling in the tumoral tissue. In this context, we have investigated the activity of Shh/Gli1-3 signaling in NSCLC in both, cancer and stromal cells. We report here that inhibition of Shh signaling induces a significant decrease in the proliferation of NSCLC cells. This effect is mediated by Gli1 and Gli2, but not Gli3, through regulation of cyclin D1 and cyclin D2 expression. While exogenous Shh was unable to induce signaling in either A549 lung adenocarcinoma or H520 lung squamous carcinoma cells, both cells were found to secrete Shh ligand, which induced fibroblast proliferation, survival, migration, invasion, and collagen synthesis. Furthermore, Shh secreted by NSCLC mediates the production of proangiogenic and metastatic factors in lung fibroblasts. Our results thus provide evidence that Shh plays an important role in mediating epithelial/mesenchymal crosstalk in NSCLC. While autonomous Gli activity controls NSCLC proliferation, increased Shh expression by NSCLC is associated with fibroblast activation in tumor-associated stroma. Our study highlights the relevance of studying stromal-associated cells in the context of NSCLC regarding new prognosis and therapeutic options.