Targeting Inhibitor of Apoptosis Proteins Protects from Bleomycin-Induced Lung Fibrosis

Discoidin domain receptor 2 signaling through PIK3C2α in fibroblasts promotes lung fibrosis

Pulmonary fibrosis, especially idiopathic pulmonary fibrosis (IPF), portends significant morbidity and mortality, and current therapeutic options are suboptimal. We have previously shown that type I collagen signaling through discoidin domain receptor 2 (DDR2), a receptor tyrosine kinase expressed by fibroblasts, is critical for the regulation of fibroblast apoptosis and progressive fibrosis. However, the downstream signaling pathways for DDR2 remain poorly defined and could also be attractive potential targets for therapy. A recent phosphoproteomic approach indicated that PIK3C2α, a poorly studied member of the PI3 kinase family, could be a downstream mediator of DDR2 signaling. We hypothesized that collagen I/DDR2 signaling through PIK3C2α regulates fibroblast activity during progressive fibrosis. To test this hypothesis, we found that primary murine fibroblasts and IPF-derived fibroblasts stimulated with endogenous or exogenous type I collagen led to the formation of a DDR2/PIK3C2α complex, resulting in phosphorylation of PIK3C2α. Fibroblasts treated with an inhibitor of PIK3C2α or with deletion of PIK3C2α had fewer markers of activation after stimulation with TGFβ and more apoptosis after stimulation with a Fas-activating antibody. Finally, mice with fibroblast-specific deletion of PIK3C2α had less fibrosis after bleomycin treatment than did littermate control mice with intact expression of PIK3Cα. Collectively, these data support the notion that collagen/DDR2/PIK3C2α signaling is critical for fibroblast function during progressive fibrosis, making this pathway a potential target for antifibrotic therapy. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

The alveolus: Our current knowledge of how the gas exchange unit of the lung is constructed and repaired

The mammalian lung completes its last step of development, alveologenesis, to generate sufficient surface area for gas exchange. In this process, multiple cell types that include alveolar epithelial cells, endothelial cells, and fibroblasts undergo coordinated cell proliferation, cell migration and/or contraction, cell shape changes, and cell-cell and cell-matrix interactions to produce the gas exchange unit: the alveolus. Full functioning of alveoli also involves immune cells and the lymphatic and autonomic nervous system. With the advent of lineage tracing, conditional gene inactivation, transcriptome analysis, live imaging, and lung organoids, our molecular understanding of alveologenesis has advanced significantly. In this review, we summarize the current knowledge of the constituents of the alveolus and the molecular pathways that control alveolar formation. We also discuss how insight into alveolar formation may inform us of alveolar repair/regeneration mechanisms following lung injury and the pathogenic processes that lead to loss of alveoli or tissue fibrosis.

Comprehensive analysis of an endoplasmic reticulum stress-related gene prediction model and immune infiltration in idiopathic pulmonary fibrosis

Background

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive interstitial lung disease. This study aimed to investigate the involvement of endoplasmic reticulum stress (ERS) in IPF and explore its correlation with immune infiltration.

Methods

ERS-related differentially expressed genes (ERSRDEGs) were identified by intersecting differentially expressed genes (DEGs) from three Gene Expression Omnibus datasets with ERS-related gene sets. Gene Set Variation Analysis and Gene Ontology were used to explore the potential biological mechanisms underlying ERS. A nomogram was developed using the risk signature derived from the ERSRDEGs to perform risk assessment. The diagnostic value of the risk signature was evaluated using receiver operating characteristics, calibration, and decision curve analyses. The ERS score of patients with IPF was measured using a single-sample Gene Set Enrichment Analysis (ssGSEA) algorithm. Subsequently, a prognostic model based on the ERS scores was established. The proportion of immune cell infiltration was assessed using the ssGSEA and CIBERSORT algorithms. Finally, the expression of ERSRDEGs was validated in vivo and in vitro via RT-qPCR.

Results

This study developed an 8-ERSRDEGs signature. Based on the expression of these genes, we constructed a diagnostic nomogram model in which agouti-related neuropeptide had a significantly greater impact on the model. The area under the curve values for the predictive value of the ERSRDEGs signature were 0.975 and 1.000 for GSE70866 and GSE110147, respectively. We developed a prognostic model based on the ERS scores of patients with IPF. Furthermore, we classified patients with IPF into two subtypes based on their signatures. The RT-qPCR validation results supported the reliability of most of our conclusions.

Conclusion

We developed and verified a risk model using eight ERSRDEGs. These eight genes can potentially affect the progression of IPF by regulating ERS and immune responses.

Inhibition of discoidin domain receptor 2 reveals kinase-dependent and kinase-independent functions in regulating fibroblast activity

Progressive pulmonary fibrosis is a devastating condition and current treatment is suboptimal. There has been considerable interest in the role of tyrosine kinase signaling as mediators of pro- and antifibrotic processes. Nintedanib is a nonspecific tyrosine kinase that has been shown to have therapeutic benefit in lung fibrosis. However, the precise mechanism of action remains unclear because nintedanib inhibits several tyrosine kinases, which are often expressed on multiple cell types with different activities during fibrosis. Discoidin domain receptor 2 (DDR2) has been suggested as a potential target of nintedanib. DDR2 is a receptor tyrosine kinase that is activated by fibrillar collagens such as type I collagen. DDR2 is primarily expressed by fibroblasts. The effectiveness of specifically targeting DDR2 signaling during fibrosis remains undefined. In the present study, we show that nintedanib acts as a direct and indirect inhibitor of DDR2. We then utilize a novel allosteric inhibitor of DDR2, WRG-28, which blocks ligand binding and activation of DDR2. We find that WRG-28 augments fibroblast apoptosis and attenuates fibrosis. Finally, we show that fibroblast type I collagen autocrine signaling is regulated by DDR2 through both kinase-dependent and kinase-independent functions of DDR2. These findings highlight the importance of type I collagen autocrine signaling by fibroblasts during fibrosis and demonstrate that DDR2 has a central role in this pathway making it a potential therapeutic target.NEW & NOTEWORTHY Type I collagen is a major component of fibrosis and can signal through cell surface receptors such as discoidin domain receptor 2 (DDR2). DDR2 activation can lead to further collagen deposition by fibroblasts setting up a profibrotic positive feedback loop. In this report, we find that inhibition of DDR2 with nintedanib or a specific DDR2 inhibitor, WRG-28, can disrupt this cycle and prevent fibrosis through augmented fibroblast apoptosis and inhibited activation.

p53 and Myofibroblast Apoptosis in Organ Fibrosis

Organ fibrosis represents a dysregulated, maladaptive wound repair response that results in progressive disruption of normal tissue architecture leading to detrimental deterioration in physiological function, and significant morbidity/mortality. Fibrosis is thought to contribute to nearly 50% of all deaths in the Western world with current treatment modalities effective in slowing disease progression but not effective in restoring organ function or reversing fibrotic changes. When physiological wound repair is complete, myofibroblasts are programmed to undergo cell death and self-clearance, however, in fibrosis there is a characteristic absence of myofibroblast apoptosis. It has been shown that in fibrosis, myofibroblasts adopt an apoptotic-resistant, highly proliferative phenotype leading to persistent myofibroblast activation and perpetuation of the fibrotic disease process. Recently, this pathological adaptation has been linked to dysregulated expression of tumour suppressor gene p53. In this review, we discuss p53 dysregulation and apoptotic failure in myofibroblasts and demonstrate its consistent link to fibrotic disease development in all types of organ fibrosis. An enhanced understanding of the role of p53 dysregulation and myofibroblast apoptosis may aid in future novel therapeutic and/or diagnostic strategies in organ fibrosis.

A four-part guide to lung immunology: Invasion, inflammation, immunity, and intervention

Lungs are important respiratory organs primarily involved in gas exchange. Lungs interact directly with the environment and their primary function is affected by several inflammatory responses caused by allergens, inflammatory mediators, and pathogens, eventually leading to disease. The immune architecture of the lung consists of an extensive network of innate immune cells, which induce adaptive immune responses based on the nature of the pathogen(s). The balance of immune responses is critical for maintaining immune homeostasis in the lung. Infection by pathogens and physical or genetic dysregulation of immune homeostasis result in inflammatory diseases. These responses culminate in the production of a plethora of cytokines such as TSLP, IL-9, IL-25, and IL-33, which have been implicated in the pathogenesis of several inflammatory and autoimmune diseases. Shifting the balance of Th1, Th2, Th9, and Th17 responses have been the targets of therapeutic interventions in the treatment of these diseases. Here, we have briefly reviewed the innate and adaptive i3mmune responses in the lung. Genetic and environmental factors, and infection are the major causes of dysregulation of various functions of the lung. We have elaborated on the impact of inflammatory and infectious diseases, advances in therapies, and drug delivery devices on this critical organ. Finally, we have provided a comprehensive compilation of different inflammatory and infectious diseases of the lungs and commented on the pros and cons of different inhalation devices for the management of lung diseases. The review is intended to provide a summary of the immunology of the lung, with an emphasis on drug and device development.

Regulation of Mesenchymal Cell Fate by Transfer of Active Gasdermin-D via Monocyte-Derived Extracellular Vesicles

Fibrosis is characterized by inappropriately persistent myofibroblast accumulation and excessive extracellular matrix deposition with the disruption of tissue architecture and organ dysfunction. Regulated death of reparative mesenchymal cells is critical for normal wound repair, but profibrotic signaling promotes myofibroblast resistance to apoptotic stimuli. A complex interplay between immune cells and structural cells underlies lung fibrogenesis. However, there is a paucity of knowledge on how these cell populations interact to orchestrate physiologic and pathologic repair of the injured lung. In this context, gasdermin-D (GsdmD) is a cytoplasmic protein that is activated following cleavage by inflammatory caspases and induces regulated cell death by forming pores in cell membranes. This study was undertaken to evaluate the impact of human (Thp-1) monocyte-derived extracellular vesicles and GsdmD on human lung fibroblast death. Our data show that active GsdmD delivered by monocyte-derived extracellular vesicles induces caspase-independent fibroblast and myofibroblast death. This cell death was partly mediated by GsdmD-independent induction of cellular inhibitor of apoptosis 2 (cIAP-2) in the recipient fibroblast population. Our findings, to our knowledge, define a novel paradigm by which inflammatory monocytes may orchestrate the death of mesenchymal cells in physiologic wound healing, illustrating the potential to leverage this mechanism to eliminate mesenchymal cells and facilitate the resolution of fibrotic repair.

Inhibition of antiapoptotic BCL-2 proteins with ABT-263 induces fibroblast apoptosis, reversing persistent pulmonary fibrosis

Patients with progressive fibrosing interstitial lung diseases (PF-ILDs) carry a poor prognosis and have limited therapeutic options. A hallmark feature is fibroblast resistance to apoptosis, leading to their persistence, accumulation, and excessive deposition of extracellular matrix. A complex balance of the B cell lymphoma 2 (BCL-2) protein family controlling the intrinsic pathway of apoptosis and fibroblast reliance on antiapoptotic proteins has been hypothesized to contribute to this resistant phenotype. Examination of lung tissue from patients with PF-ILD (idiopathic pulmonary fibrosis and silicosis) and mice with PF-ILD (repetitive bleomycin and silicosis) showed increased expression of antiapoptotic BCL-2 family members in α-smooth muscle actin-positive fibroblasts, suggesting that fibroblasts from fibrotic lungs may exhibit increased susceptibility to inhibition of antiapoptotic BCL-2 family members BCL-2, BCL-XL, and BCL-W with the BH3 mimetic ABT-263. We used 2 murine models of PF-ILD to test the efficacy of ABT-263 in reversing established persistent pulmonary fibrosis. Treatment with ABT-263 induced fibroblast apoptosis, decreased fibroblast numbers, and reduced lung collagen levels, radiographic disease, and histologically evident fibrosis. Our studies provide insight into how fibroblasts gain resistance to apoptosis and become sensitive to the therapeutic inhibition of antiapoptotic proteins. By targeting profibrotic fibroblasts, ABT-263 offers a promising therapeutic option for PF-ILDs.

Blockade of caspase cascade overcomes malaria-associated acute respiratory distress syndrome in mice

Malaria is an enormous burden on global health that caused 409,000 deaths in 2019. Severe malaria can manifest in the lungs, an illness known as acute respiratory distress syndrome (ARDS). Not much is known about the development of malaria-associated ARDS (MA-ARDS), especially regarding cell death in the lungs. We had previously established a murine model that mimics various human ARDS aspects, such as pulmonary edema, hemorrhages, pleural effusion, and hypoxemia, using DBA/2 mice infected with Plasmodium berghei ANKA. Here, we explored the mechanisms and the involvement of apoptosis in this syndrome. We found that apoptosis contributes to the pathogenesis of MA-ARDS, primarily as facilitators of the alveolar-capillary barrier breakdown. The protection of pulmonary endothelium by inhibiting caspase activation could be a promising therapeutic strategy to prevent the pathogenicity of MA-ARDS. Therefore, intervention in the programmed death cell mechanism could help patients not to develop severe malaria.

Targeting cIAPs attenuates CCl4-induced liver fibrosis by increasing MMP9 expression derived from neutrophils

AIMS

Liver fibrosis is a growing public health concern without effective medical treatment. Recent reports have indicated that inhibitors of apoptosis proteins (IAPs) were potential targets for idiopathic pulmonary fibrosis therapy. However, their roles have not been well identified in liver fibrosis.

METHODS

The expression of IAPs were examined in human liver tissue and experimental mouse models. Liver fibrosis in CCl₄-induced mouse models were investigated by Sirius red staining, RT-PCR, Western blotting after hepatocytes-specific cIAP2 knockout or IAPs inhibitor APG-1387 treatment. The underlying molecular mechanism of APG-1387 action was explored by apoptosis analysis, matrix metalloprotein 9 (MMP9) inhibition, neutrophils depletion, and CC Motif Chemokine Ligand 5 (CCL5) gene knockout in vitro and in vivo.

FINDINGS

Our study showed that increased expression of cIAP2 was associated with liver fibrosis severity in liver tissues. Deletion of cIAP2 from hepatocytes or degrading cIAPs by APG-1387 ameliorated liver fibrosis induced by CCl₄. APG-1387 treatment exhibited increased expression of MMP9 and resulted in higher ratio of MMP9 to tissue inhibitor of metalloproteinase-1. MMP9 was mainly derived from CCL5 chemotactic neutrophils. Further, MMP9 inhibition by CTT peptide, neutrophil depletion by Ly6G antibody or CCL5 deficiency blocked the anti-fibrotic effects of APG-1387 in vivo.

SIGNIFICANCE

These results suggested that cIAPs, especially cIAP2, might play a novel role in the pathogenesis of liver fibrosis, and targeting cIAPs represented a promising therapeutic strategy for liver fibrosis by increasing MMP9 expression induced by CCL5 chemotactic neutrophils.

Antibody-drug conjugates targeting CD248+ myofibroblasts effectively alleviate renal fibrosis in mice

Myofibroblasts, or activated fibroblasts, play a critical role in the process of renal fibrosis. Targeting myofibroblasts to inhibit their activation or induce specific cell death has been considered to be an effective strategy to attenuate renal fibrosis. However, specific biomarkers for myofibroblasts are needed to ensure the efficacy of these strategies. Here, we verified that CD248 was mainly expressed in myofibroblasts in patients with chronic kidney disease, which was inversely correlated with renal function. The same result was also confirmed in renal fibrotic mice induced by unilateral ureteral obstruction and aristolochic acid nephropathy. By using an antibody-drug conjugate (ADC) named IgG78-DM1, in which maytansinoid (DM1) was linked to a fully human antibody IgG78 through an uncleavable SMCC linker, we demonstrated that it could effectively bind with and kill CD248⁺ fibroblasts in vitro and alleviate renal fibrosis in mice models. Besides, we confirmed that IgG78-DM1 had qualified biosafety in vivo. Our results confirmed that CD248 can be used as a specific marker for myofibroblasts, and specific killing of CD248⁺ myofibroblasts by IgG78-DM1 has excellent anti-fibrotic effect in renal fibrotic mice. Our study expanded the application of ADC and provided a novel strategy for the treatment of renal fibrosis.

XIAP Knockdown in Alcohol-Associated Liver Disease Models Exhibits Divergent in vitro and in vivo Phenotypes Owing to a Potential Zonal Inhibitory Role of SMAC

Alcohol-associated liver disease (ALD) has been recognized as the most common cause of advanced liver disease worldwide, though mechanisms of pathogenesis remain incompletely understood. The X-linked inhibitor of apoptosis (XIAP) protein was originally described as an anti-apoptotic protein that directly binds and inhibits caspases-3, 7, and 9. Here, we investigated the function of XIAP in hepatocytes in vitro using gain and loss-of-function approaches. We noted an XIAP-dependent increase in caspase activation as well as increased inflammatory markers and pro-inflammatory EV release from hepatocytes in vitro. Primary hepatocytes (PMH) from Xiap Alb.Cre and Xiap loxP mice exhibited higher cell death but surprisingly, lower expression of inflammation markers. Conditioned media from these isolated Xiap deleted PMH further decrease inflammation in bone marrow-derived macrophages. Also, interestingly, when administered an ethanol plus Fas-agonist-Jo2 model and an ethanol plus CCl4 model, these animals failed to develop an exacerbated disease phenotype in vivo. Of note, neither Xiap Alb . Cre nor Xiap AAV8.Cre mice presented with aggravated liver injury, hepatocyte apoptosis, liver steatosis, or fibrosis. Since therapeutics targeting XIAP are currently in clinical trials and caspase-induced death is very important for development of ALD, we sought to explore the potential basis of this unexpected lack of effect. We utilized scRNA-seq and spatially reconstructed hepatocyte transcriptome data from human liver tissue and observed that XIAP was significantly zonated, along with its endogenous inhibitor second mitochondria-derived activator of caspases (SMAC) in periportal region. This contrasted with pericentral zonation of other IAPs including cIAP1 and Apollon as well as caspases 3, 7, and 9. Thus providing a potential explanation for compensation of the effect of Xiap deletion by other IAPs. In conclusion, our findings implicate a potential zonallydependent role for SMAC that prevented development of a phenotype in XIAP knockout mice in ALD models. Targeting SMAC may also be important in addition to current efforts of targeting XIAP in treatment of ALD.

Role of various imbalances centered on alveolar epithelial cell/fibroblast apoptosis imbalance in the pathogenesis of idiopathic pulmonary fibrosis

There have been recent extensive studies and rapid advancement on the pathogenesis underlying idiopathic pulmonary fibrosis (IPF), and intricate pathogenesis of IPF has been suggested. The purpose of this study was to clarify the logical relationship between these mechanisms. An extensive search was undertaken of the PubMed using the following keywords: “etiology,” “pathogenesis,” “alveolar epithelial cell (AEC),” “fibroblast,” “lymphocyte,” “macrophage,” “epigenomics,” “histone,” acetylation,” “methylation,” “endoplasmic reticulum stress,” “mitochondrial dysfunction,” “telomerase,” “proteases,” “plasminogen,” “epithelial-mesenchymal transition,” “oxidative stress,” “inflammation,” “apoptosis,” and “idiopathic pulmonary fibrosis.” This search covered relevant research articles published up to April 30, 2020. Original articles, reviews, and other articles were searched and reviewed for content; 240 highly relevant studies were obtained after screening. IPF is likely the result of complex interactions between environmental, genetic, and epigenetic factors: environmental exposures affect epigenetic marks; epigenetic processes translate environmental exposures into the regulation of chromatin; epigenetic processes shape gene expression profiles; in turn, an individual's genetic background determines epigenetic marks; finally, these genetic and epigenetic factors act in concert to dysregulate gene expression in IPF lung tissue. The pathogenesis of IPF involves various imbalances including endoplasmic reticulum, telomere length homeostasis, mitochondrial dysfunction, oxidant/antioxidant imbalance, Th1/Th2 imbalance, M1–M2 polarization of macrophages, protease/antiprotease imbalance, and plasminogen activation/inhibition imbalance. These affect each other, promote each other, and ultimately promote AEC/fibroblast apoptosis imbalance directly or indirectly. Excessive AEC apoptosis and impaired apoptosis of fibroblasts contribute to fibrosis. IPF is likely the result of complex interactions between environmental, genetic, and epigenetic factors. The pathogenesis of IPF involves various imbalances centered on AEC/fibroblast apoptosis imbalance.

Type I Collagen Signaling Regulates Opposing Fibrotic Pathways through α2β1 Integrin

Fibrosis is characterized by fibroblast activation, leading to matrix remodeling culminating in a stiff, type I collagen-rich fibrotic matrix. Alveolar epithelial cell (AEC) apoptosis is also a major feature of fibrogenesis, and AEC apoptosis is sufficient to initiate a robust lung fibrotic response. TGF-β (transforming growth factor-β) is a major driver of fibrosis and can induce both AEC apoptosis and fibroblast activation. We and others have previously shown that changes in extracellular matrix stiffness and composition can regulate the cellular response to TGF-β. In the present study, we find that type I collagen signaling promotes TGF-β-mediated fibroblast activation and inhibits TGF-β-induced AEC death. Fibroblasts cultured on type I collagen or fibrotic decellularized lung matrix had augmented activation in response to TGF-β, whereas AECs on cultured on type I collagen or fibrotic lung matrix were more resistant to TGF-β-induced apoptosis. Both of these responses were mediated by integrin α₂β₁, a major collagen receptor. AECs treated with an α₂ integrin inhibitor or with deletion of α₂ integrin had loss of collagen-mediated protection from apoptosis. We found that mice with fibroblast-specific deletion of α₂ integrin were protected from fibrosis whereas mice with AEC-specific deletion of α₂ integrin had more lung injury and a greater fibrotic response to bleomycin. Intrapulmonary delivery of an α₂ integrin-activating collagen peptide inhibited AEC apoptosis in vitro and in vivo and attenuated the fibrotic response. These studies underscore the need for a thorough understanding of the divergent response to matrix signaling.

You Say You Want a Resolution (of Fibrosis)

In pathological fibrosis, aberrant tissue remodeling with excess extracellular matrix leads to organ dysfunction and eventual morbidity. Diseases of fibrosis create significant global health and economic burdens and are often deadly. Although fibrosis has traditionally been thought of as an irreversible process, a growing body of evidence demonstrates that organ fibrosis can reverse in certain circumstances, especially if an underlying cause of injury can be removed. This body of evidence has uncovered more and more contributors to persistent and nonresolving tissue fibrosis. Here, we review the present knowledge on resolution of organ fibrosis and restoration of near-normal tissue architecture. We emphasize three critical areas of tissue homeostasis that are necessary for fibrosis resolution, namely, the elimination of matrix-producing cells, the clearance of excess matrix, and the regeneration of normal tissue constituents. In so doing, we also highlight how profibrotic pathways interact with one another and where there may be therapeutic opportunities to intervene and remediate pathological persistent fibrosis.

Retinoic acid signalling in fibro/adipogenic progenitors robustly enhances muscle regeneration

BACKGROUND

During muscle regeneration, excessive formation of adipogenic and fibrogenic tissues, from their respective fibro/adipogenic progenitors (FAPs), impairs functional recovery. Intrinsic mechanisms controlling the proliferation and differentiation of FAPs remain largely unexplored.

METHODS

Here, we investigated the role of retinoic acid (RA) signalling in regulating FAPs and the subsequent effects on muscle restoration from a cardiotoxin-induced injury. Blockage of retinoic acid receptor (RAR) signalling was achieved through dominant negative retinoic acid receptor α (RARα403) expression specific in PDGFRα+ FAPs in vivo and by BMS493 treatment in vitro. Effects of RAR-signalling on FAP cellularity and muscle regeneration were also investigated in a high-fat diet-induced obese mice model.

FINDINGS

Supplementation of RA increased the proliferation of FAPs during the early stages of regeneration while suppressing FAP differentiation and promoting apoptosis during the remodelling stage. Loss of RAR-signalling caused ectopic adipogenic differentiation of FAPs and impaired muscle regeneration. Furthermore, obesity disrupted the cellular transition of FAPs and attenuated muscle regeneration. Supplementation of RA to obese mice not only rescued impaired muscle fibre regeneration, but also inhibited infiltration of fat and fibrotic tissues during muscle repair. These beneficial effects were abolished after blocking RAR-signalling in FAPs of obese mice.

INTERPRETATION

These data suggest that RAR-signalling in FAPs is a critical therapeutic target for suppressing differentiation of FAPs and facilitating the regeneration of muscle and other tissues.

FUNDING

This study was supported by grants from the National Institutes of Health (R01-HD067449 and R21-AG049976) to M.D.

Diverse Injury Pathways Induce Alveolar Epithelial Cell CCL2/12, Which Promotes Lung Fibrosis

Accumulating evidence suggests that fibrosis is a multicellular process with contributions from alveolar epithelial cells (AECs), recruited monocytes/macrophages, and fibroblasts. We have previously shown that AEC injury is sufficient to induce fibrosis, but the precise mechanism remains unclear. Several cell types, including AECs, can produce CCL2 and CCL12, which can promote fibrosis through CCR2 activation. CCR2 signaling is critical for the initiation and progression of pulmonary fibrosis, in part through recruitment of profibrotic bone marrow-derived monocytes. Attempts at inhibiting CCL2 in patients with fibrosis demonstrated a marked upregulation of CCL2 production and no therapeutic response. To better understand the mechanisms involved in CCL2/CCR2 signaling, we generated mice with conditional deletion of CCL12, a murine homolog of human CCL2. Surprisingly, we found that mice with complete deletion of CCL12 had markedly increased concentrations of other CCR2 ligands and were not protected from fibrosis after bleomycin injury. In contrast, mice with lung epithelial cell-specific deletion of CCL12 were protected from bleomycin-induced fibrosis and had expression of CCL2 and CCL7 similar to that of control mice treated with bleomycin. Deletion of CCL12 within AECs led to decreased recruitment of exudate macrophages. Finally, injury to murine and human primary AECs resulted in increased production of CCL2 and CCL12, in part through activation of the mTOR pathway. In conclusion, these data suggest that targeting CCL2 may be a viable antifibrotic strategy once the pathways involved in the production and function of CCL2 and other CCR2 ligands are better defined.

Transcriptomic profiling of microglia and astrocytes throughout aging

Background

Activation of microglia and astrocytes, a prominent hallmark of both aging and Alzheimer’s disease (AD), has been suggested to contribute to aging and AD progression, but the underlying cellular and molecular mechanisms are largely unknown.

Methods

We performed RNA-seq analyses on microglia and astrocytes freshly isolated from wild-type and APP-PS1 (AD) mouse brains at five time points to elucidate their age-related gene-expression profiles.

Results

Our results showed that from 4 months onward, a set of age-related genes in microglia and astrocytes exhibited consistent upregulation or downregulation (termed “age-up”/“age-down” genes) relative to their expression at the young-adult stage (2 months). And most age-up genes were more highly expressed in AD mice at the same time points. Bioinformatic analyses revealed that the age-up genes in microglia were associated with the inflammatory response, whereas these genes in astrocytes included widely recognized AD risk genes, genes associated with synaptic transmission or elimination, and peptidase-inhibitor genes.

Conclusions

Overall, our RNA-seq data provide a valuable resource for future investigations into the roles of microglia and astrocytes in aging- and amyloid-β-induced AD pathologies.

Evasion of apoptosis by myofibroblasts: a hallmark of fibrotic diseases

Organ fibrosis is a lethal outcome of autoimmune rheumatic diseases such as systemic sclerosis. Myofibroblasts are scar-forming cells that are ultimately responsible for the excessive synthesis, deposition and remodelling of extracellular matrix proteins in fibrosis. Advances have been made in our understanding of the mechanisms that keep myofibroblasts in an activated state and control myofibroblast functions. However, the mechanisms that help myofibroblasts to persist in fibrotic tissues remain poorly understood. Myofibroblasts evade apoptosis by activating molecular mechanisms in response to pro-survival biomechanical and growth factor signals from the fibrotic microenvironment, which can ultimately lead to the acquisition of a senescent phenotype. Growing evidence suggests that myofibroblasts and senescent myofibroblasts, rather than being resistant to apoptosis, are actually primed for apoptosis owing to concomitant activation of cell death signalling pathways; these cells are poised to apoptose when survival pathways are inhibited. This knowledge of apoptotic priming has paved the way for new therapies that trigger apoptosis in myofibroblasts by blocking pro-survival mechanisms, target senescent myofibroblast for apoptosis or promote the reprogramming of myofibroblasts into scar-resolving cells. These novel strategies are not only poised to prevent progressive tissue scarring, but also have the potential to reverse established fibrosis and to regenerate chronically injured tissues.

Glutaminolysis Epigenetically Regulates Antiapoptotic Gene Expression in Idiopathic Pulmonary Fibrosis Fibroblasts

Fibrotic responses involve multiple cellular processes, including epigenetic changes. Epigenetic changes are sensitive to alterations in the tissue microenvironment such as the flux of tricarboxylic acid (TCA) cycle metabolites. TCA metabolites directly regulate epigenetic states, in part by regulating histone modification-related enzymes. Glutaminolysis is a critical metabolic process by which glutamine is converted to glutamate by glutaminase and then to α-ketoglutarate (α-KG), a TCA cycle metabolite. Idiopathic pulmonary fibrosis (IPF) is a disease characterized by aberrant metabolism, including enhanced glutaminolysis. IPF fibroblasts are apoptosis resistant. In this study, we explored the relationship between glutaminolysis and the resistance to apoptosis of IPF fibroblasts. Inhibition of glutaminolysis decreased expression of XIAP and survivin, members of the inhibitor of apoptosis protein (IAP) family. α-KG is a cofactor for JMJD3 histone demethylase, which targets H3K27me3. In the absence of glutamine, JMJD3 activity in fibroblasts is significantly decreased, whereas H3K27me3 levels are increased. Chromatin immunoprecipitation assays confirmed that JMJD3 directly interacts with XIAP and survivin promoter regions in a glutamine-dependent manner. Exogenous α-KG partially restores JMJD3 function and its interaction with the XIAP and survivin promoter regions under glutamine-deficient conditions. Interestingly, α-KG upregulates XIAP, but not survivin, suggesting differential α-KG-dependent and -independent mechanisms by which glutamine regulates these IAPs. Our data demonstrate a novel mechanism of metabolic regulation in which glutaminolysis promotes apoptosis resistance of IPF fibroblasts through epigenetic regulation of XIAP and survivin.

Urokinase Plasminogen Activator Overexpression Reverses Established Lung Fibrosis

INTRODUCTION

 Impaired plasminogen activation (PA) is causally related to the development of lung fibrosis. Prior studies demonstrate that enhanced PA in the lung limits the severity of scarring following injury and in vitro studies indicate that PA promotes matrix degradation and fibroblast apoptosis. These findings led us to hypothesize that increased PA in an in vivo model would enhance the resolution of established lung fibrosis in conjunction with increased myofibroblast apoptosis.

METHODS

 Transgenic C57BL/6 mice with doxycycline inducible lung-specific urokinase plasminogen activator (uPA) expression or littermate controls were treated (day 0) with bleomycin or saline. Doxycycline was initiated on days 1, 9, 14, or 21. Lung fibrosis, stiffness, apoptosis, epithelial barrier integrity, and inflammation were assessed.

RESULTS

 Protection from fibrosis with uPA upregulation from day 1 through day 28 was associated with reduced parenchymal stiffness as determined by atomic force microscopy. Initiation of uPA expression beginning in the late inflammatory or the early fibrotic phase reduced stiffness and fibrosis at day 28. Induction of uPA activity in mice with established fibrosis decreased lung collagen and lung stiffness while increasing myofibroblast apoptosis. Upregulation of uPA did not alter lung inflammation but was associated with improved epithelial cell homeostasis.

CONCLUSION

 Restoring intrapulmonary PA activity diminishes lung fibrogenesis and enhances the resolution of established lung fibrosis. This PA-mediated resolution is associated with increased myofibroblast apoptosis and improved epithelial cell homeostasis. These studies support the potential capacity of the lung to resolve existing scar in murine models.

Covalent Inhibitors of Protein-Protein Interactions Targeting Lysine, Tyrosine, or Histidine Residues

We have recently reported a series of Lys-covalent agents targeting the BIR3 domain of the X-linked inhibitor of apoptosis protein (XIAP) using a benzamide-sulfonyl fluoride warhead. Using XIAP as a model system, we further investigated a variety of additional warheads that can be easily incorporated into binding peptides and analyzed their ability to form covalent adducts with lysine and other amino acids, including tyrosine, histidine, serine, and threonine, using biochemical and biophysical assays. Moreover, we tested aqueous, plasma stability, cell permeability, and cellular efficacy of the most effective agents. These studies identified aryl-fluoro sulfates as likely the most suitable electrophiles to effectively form covalent adducts with Lys, Tyr, and His residues, given that these agents were cell permeable and stable in aqueous buffer and in plasma. Our studies contain a number of general findings that open new possible avenues for the design of potent covalent protein-protein interaction antagonists.

Discoidin Domain Receptor 2 Signaling Regulates Fibroblast Apoptosis through PDK1/Akt

Progressive fibrosis is a complication of many chronic diseases, and collectively, organ fibrosis is the leading cause of death in the United States. Fibrosis is characterized by accumulation of activated fibroblasts and excessive deposition of extracellular matrix proteins, especially type I collagen. Extensive research has supported a role for matrix signaling in propagating fibrosis, but type I collagen itself is often considered an end product of fibrosis rather than an important regulator of continued collagen deposition. Type I collagen can activate several cell surface receptors, including α₂β₁ integrin and discoidin domain receptor 2 (DDR2). We have previously shown that mice deficient in type I collagen have reduced activation of DDR2 and reduced accumulation of activated myofibroblasts. In the present study, we found that DDR2-null mice are protected from fibrosis. Surprisingly, DDR2-null fibroblasts have a normal and possibly exaggerated activation response to transforming growth factor-β and do not have diminished proliferation compared with wild-type fibroblasts. DDR2-null fibroblasts are significantly more prone to apoptosis, in vitro and in vivo, than wild-type fibroblasts, supporting a paradigm in which fibroblast resistance to apoptosis is critical for progression of fibrosis. We have identified a novel molecular mechanism by which DDR2 can promote the activation of a PDK1 (3-phosphoinositide dependent protein kinase-1)/Akt survival pathway, and we have found that inhibition of PDK1 can augment fibroblast apoptosis. Furthermore, our studies demonstrate that DDR2 expression is heavily skewed to mesenchymal cells compared with epithelial cells and that idiopathic pulmonary fibrosis cells and tissue demonstrate increased activation of DDR2 and PDK1. Collectively, these findings identify a promising target for fibrosis therapy.

Mechanisms for the Resolution of Organ Fibrosis

Fibrosis is a dynamic process with the potential for reversibility and restoration of near-normal tissue architecture and organ function. Herein, we review mechanisms for resolution of organ fibrosis, in particular that involving the lung, with an emphasis on the critical roles of myofibroblast apoptosis and clearance of deposited matrix.

Cell Death in the Lung: The Apoptosis-Necroptosis Axis

Regulated cell death is a major mechanism to eliminate damaged, infected, or superfluous cells. Previously, apoptosis was thought to be the only regulated cell death mechanism; however, new modalities of caspase-independent regulated cell death have been identified, including necroptosis, pyroptosis, and autophagic cell death. As an understanding of the cellular mechanisms that mediate regulated cell death continues to grow, there is increasing evidence that these pathways are implicated in the pathogenesis of many pulmonary disorders. This review summarizes our understanding of regulated cell death as it pertains to the pathogenesis of chronic obstructive pulmonary disease, asthma, idiopathic pulmonary fibrosis, acute respiratory distress syndrome, and pulmonary arterial hypertension.

Evaluation of 99mTc-rhAnnexin V-128 SPECT/CT as a diagnostic tool for early stages of interstitial lung disease associated with systemic sclerosis

Background

Given the need for early detection of organ involvement in systemic sclerosis, we evaluated ^99mTc-rhAnnexin V-128 for the detection of early stages of interstitial lung disease (ILD) in respective animal models using single photon emission computed tomography (SPECT/CT).

Methods

In bleomycin (BLM)-challenged mice, fos-related antigen 2 (Fra-2) transgenic (tg) mice and respective controls, lung injury was evaluated by analysis of hematoxylin and eosin (HE) and Sirius red staining, with semi-quantification of fibrosis by the Ashcroft score. Apoptotic cells were identified by TUNEL assay, cleaved caspase 3 staining and double staining with specific cell markers. To detect early stages of lung remodeling by visualization of apoptosis, mice were injected intravenously with ^99mTc-rhAnnexin V-128 and imaged by small animal SPECT/CT. For confirmation, biodistribution and ex vivo autoradiography studies were performed.

Results

In BLM-induced lung fibrosis, inflammatory infiltrates occurred as early as day 3 with peak at day 7, whereas pulmonary fibrosis developed from day 7 and was most pronounced at day 21. In accordance, the number of apoptotic cells was highest at day 3 compared with saline controls and then decreased over time. Epithelial cells (E-cadherin+) and inflammatory cells (CD45+) were the primary cells undergoing apoptosis in the earliest remodeling stages of experimental ILD. This was also true in the pathophysiologically different Fra-2 tg mice, where apoptosis of CD45+ cells occurred in the inflammatory stage. In accordance with the findings on tissue level, at day 3 in the BLM and at week 16 in the Fra-2 tg model, biodistribution and/or ex vivo autoradiography showed increased pulmonary uptake of ^99mTc-rhAnnexin V-128 compared with controls. However, accumulation of the radiotracer and thus the signal intensity in lungs was too low to allow the differentiation of healthy and injured lungs in vivo.

Conclusion

At the tissue level, ^99mTc-rhAnnexin V-128 successfully demonstrated early stages of ILD in two animal models by detection of apoptotic epithelial and/or inflammatory cells. In vivo, however, we did not detect early lung injury. It remains to be investigated whether the same applies to human ILD.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1681-1) contains supplementary material, which is available to authorized users.

Metformin reverses established lung fibrosis in a bleomycin model

Fibrosis is a pathological result of a dysfunctional repair response to tissue injury and occurs in a number of organs, including the lungs1. Cellular metabolism regulates tissue repair and remodelling responses to injury2-4. AMPK is a critical sensor of cellular bioenergetics and controls the switch from anabolic to catabolic metabolism5. However, the role of AMPK in fibrosis is not well understood. Here, we demonstrate that in humans with idiopathic pulmonary fibrosis (IPF) and in an experimental mouse model of lung fibrosis, AMPK activity is lower in fibrotic regions associated with metabolically active and apoptosis-resistant myofibroblasts. Pharmacological activation of AMPK in myofibroblasts from lungs of humans with IPF display lower fibrotic activity, along with enhanced mitochondrial biogenesis and normalization of sensitivity to apoptosis. In a bleomycin model of lung fibrosis in mice, metformin therapeutically accelerates the resolution of well-established fibrosis in an AMPK-dependent manner. These studies implicate deficient AMPK activation in non-resolving, pathologic fibrotic processes, and support a role for metformin (or other AMPK activators) to reverse established fibrosis by facilitating deactivation and apoptosis of myofibroblasts.

Design of Potent pan-IAP and Lys-Covalent XIAP Selective Inhibitors Using a Thermodynamics Driven Approach

Recently we reported that rapid determination of enthalpy of binding can be achieved for a large number of congeneric agents or in combinatorial libraries fairly efficiently. We show that using a thermodynamic Craig plot can be very useful in dissecting the enthalpy and entropy contribution of different substituents on a common scaffold, in order to design potent, selective, or pan-active compounds. In our implementation, the approach identified a critical Lys residue in the BIR3 domain of XIAP. We report for the first time that it is possible to target such residue covalently to attain potent and selective agents. Preliminary cellular studies in various models of leukemia, multiple myeloma, and pancreatic cancers suggest that the derived agents possess a potentially intriguing pattern of activity, especially for cell lines that are resistant to the pan-IAP antagonist and clinical candidate LCL161.

Regulation of fibroblast Fas expression by soluble and mechanical pro-fibrotic stimuli

Background

Fibroblast apoptosis is a critical component of normal repair and the acquisition of an apoptosis-resistant phenotype contributes to the pathogenesis of fibrotic repair. Fibroblasts from fibrotic lungs of humans and mice demonstrate resistance to apoptosis induced by Fas-ligand and prior studies have shown that susceptibility to apoptosis is enhanced when Fas (CD95) expression is increased in these cells. Moreover, prior work shows that Fas expression in fibrotic lung fibroblasts is reduced by epigenetic silencing of the Fas promoter. However, the mechanisms by which microenvironmental stimuli such as TGF-β1 and substrate stiffness affect fibroblast Fas expression are not well understood.

Methods

Primary normal human lung fibroblasts (IMR-90) were cultured on tissue culture plastic or on polyacrylamide hydrogels with Young’s moduli to recapitulate the compliance of normal (400 Pa) or fibrotic (6400 Pa) lung tissue and treated with or without TGF-β1 (10 ng/mL) in the presence or absence of protein kinase inhibitors and/or inflammatory cytokines. Expression of Fas was assessed by quantitative real time RT-PCR, ELISA and Western blotting. Soluble Fas (sFas) was measured in conditioned media by ELISA. Apoptosis was assessed using the Cell Death Detection Kit and by Western blotting for cleaved PARP.

Results

Fas expression and susceptibility to apoptosis was diminished in fibroblasts cultured on 6400 Pa substrates compared to 400 Pa substrates. TGF-β1 reduced Fas mRNA and protein in a time- and dose-dependent manner dependent on focal adhesion kinase (FAK). Surprisingly, TGF-β1 did not significantly alter cell-surface Fas expression, but did stimulate secretion of sFas. Finally, enhanced Fas expression and increased susceptibility to apoptosis was induced by combined treatment with TNF-α/IFN-γ and was not inhibited by TGF-β1.

Conclusions

Soluble and matrix-mediated pro-fibrotic stimuli promote fibroblast resistance to apoptosis by decreasing Fas transcription while stimulating soluble Fas secretion. These findings suggest that distinct mechanisms regulating Fas expression in fibroblasts may serve different functions in the complex temporal and spatial evolution of normal and fibrotic wound-repair responses.

Electronic supplementary material

The online version of this article (10.1186/s12931-018-0801-4) contains supplementary material, which is available to authorized users.

Transplantation of adipose-derived mesenchymal stem cells attenuates pulmonary fibrosis of silicosis via anti-inflammatory and anti-apoptosis effects in rats

Background

Silicosis has been topping the list of high-incidence occupational diseases in developing countries and cannot be completely cured. Recent advances in stem cell research have made possible the treatment of various diseases including lung fibrosis. The application of stem cell therapy in occupational diseases, in particular the use of adipose-derived mesenchymal stem cells (AD-MSCs) in treatment of silicosis, has not yet been reported. The aim of the study is to explore the intervening effect of silica-induced lung fibrosis in rats.

Methods

In this study, we investigated the anti-pulmonary fibrosis effects of the transplantation of AD-MSCs in rats in which lung fibrosis was induced by oral tracheal intubation with silica suspension. Twenty rats were divided into four groups: control group (n = 5), exposure group (n = 5), vehicle group (n = 5) and treatment group (n = 5). AD-MSCs were given to rats after exposure to silica for 24 h. Twenty-eight days after AD-MSC transplantation, we examined the organ coefficient, inflammatory cytokines, apoptosis, pathological and fibrotic changes in lung tissue.

Results

Results showed that exposure to silica for 28 days induced an increase of the lung coefficient with significant pulmonary fibrosis. Treatment with AD-MSC transplantation led to a remissive effect on pulmonary fibrosis. We found that after AD-MSC transplantation the inflammatory response decreased and Caspase-3 protein expression significantly decreased with a significant increase of the Bcl-2/Bax ratio.

Conclusions

Anti-inflammatory and anti-apoptosis of AD-MSCs may play important roles in their anti-pulmonary fibrosis effect. Our data suggest that transplantation of AD-MSCs holds promise for potential interference in the formation of silicosis through regulating inflammatory and apoptotic processes.

Lung carcinogenesis and fibrosis taken together: just coincidence?

PURPOSE OF REVIEW

The pathogenesis of lung cancer and pulmonary fibrotic disorders partially overlaps. This review focuses on the common features of the two disease categories, aimed at advancing our translational understanding of their pathobiology and at fostering the development of new therapies.

RECENT FINDINGS

Both malignant and collagen-producing lung cells display enhanced cellular proliferation, increased resistance to apoptosis, a propensity for invading and distorting the lung parenchyma, as well as stemness potential. These characteristics are reinforced by the tissue microenvironment and inflammation seems to play an important adjuvant role in both types of disorders.

SUMMARY

Unraveling the thread of the common and distinct characteristics of lung fibrosis and cancer might contribute to a more comprehensive approach of the pathobiology of both diseases and to a pathfinder for novel and personalized therapeutic strategies.

Resolution of organ fibrosis

Fibrosis is the excessive accumulation of extracellular matrix that often occurs as a wound healing response to repeated or chronic tissue injury, and may lead to the disruption of organ architecture and loss of function. Although fibrosis was previously thought to be irreversible, recent evidence indicates that certain circumstances permit the resolution of fibrosis when the underlying causes of injury are eradicated. The mechanism of fibrosis resolution encompasses degradation of the fibrotic extracellular matrix as well as elimination of fibrogenic myofibroblasts through their adaptation of various cell fates, including apoptosis, senescence, dedifferentiation, and reprogramming. In this Review, we discuss the present knowledge and gaps in our understanding of how matrix degradation is regulated and how myofibroblast cell fates can be manipulated, areas that may identify potential therapeutic approaches for fibrosis.

Lung epithelial cell focal adhesion kinase signaling inhibits lung injury and fibrosis

Progressive pulmonary fibrosis is a devastating consequence of many acute and chronic insults to the lung. Lung injury leads to alveolar epithelial cell (AEC) death, destruction of the basement membrane, and activation of transforming growth factor-β (TGF-β). There is subsequent resolution of the injury and a coordinated and concurrent initiation of fibrosis. Both of these processes may involve activation of similar intracellular signaling pathways regulated in part by dynamic changes to the extracellular matrix. Matrix signaling can augment the profibrotic fibroblast response to TGF-β. However, similar matrix/integrin signaling pathways may also be involved in the inhibition of ongoing TGF-β-induced AEC apoptosis. Focal adhesion kinase (FAK) is an integrin-associated signaling molecule expressed by many cell types. We used mice with AEC-specific FAK deletion to isolate the epithelial aspect of integrin signaling in the bleomycin model of lung injury and fibrosis. Mice with AEC-specific deletion of FAK did not exhibit spontaneous lung injury but did have significantly greater terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling-positive cells (18.6 vs. 7.1) per ×200 field, greater bronchoalveolar lavage protein (3.2 vs. 1.8 mg/ml), and significantly greater death (77 vs. 19%) after bleomycin injury compared with littermate control mice. Within primary AECs, activated FAK directly associates with caspase-8 and inhibits activation of the caspase cascade resulting in less apoptosis in response to TGF-β. Our studies support a model in which dynamic changes to the extracellular matrix after injury promote fibroblast activation and inhibition of epithelial cell apoptosis in response to TGF-β through FAK activation potentially complicating attempts to nonspecifically target this pathway for antifibrotic therapy.

Periostin regulates fibrocyte function to promote myofibroblast differentiation and lung fibrosis

Fibrocytes are circulating mesenchymal precursors (CD45+, col 1+) recruited to fibrotic areas. Fibrocytes secrete profibrotic mediators including periostin; a matricellular protein that regulates cellular interactions with extracellular matrix (ECM) components. In bleomycin-induced fibrosis, periostin deficiency in structural or hematopoietic cells limits development of pulmonary fibrosis. To determine if hematopoietic-derived fibrocytes might secrete soluble factors to activate structural myofibroblast differentiation, wild-type (WT) fibroblasts were treated with conditioned medium from fibrocytes isolated from bleomycin-treated WT or periostin^-/- mice. After 24 h we saw less α-smooth muscle actin expression in cells treated with conditioned medium from periostin^-/- fibrocytes. Adoptive transfer of WT fibrocytes augmented lung fibrosis to a greater extent than transfer of fibrocytes from periostin^-/- mice. In vitro analysis of fibrocytes and fibroblasts isolated from WT and periostin^-/- mice treated with TGFβ1 or periostin demonstrated co-regulation of mesenchymal activation and beta 1 integrin as a potential receptor for periostin on fibrocytes. Additionally, connective tissue growth factor (CTGF) mRNA expression was increased in fibrocytes treated with periostin whereas CTGF and lysl oxidase (LOX) mRNA expression was low in bleomycin-treated periostin^-/- fibrocytes. These data suggest fibrocytes may augment bleomycin-induced fibrosis via secretion of periostin and other soluble factors that promote myofibroblast differentiation.

The vitronectin RGD motif regulates TGF-β-induced alveolar epithelial cell apoptosis

Transforming growth factor-β (TGF-β) is a critical driver of acute lung injury and fibrosis. Injury leads to activation of TGF-β, which regulates changes in the cellular and matrix makeup of the lung during the repair and fibrosis phase. TGF-β can also initiate alveolar epithelial cell (AEC) apoptosis. Injury leads to destruction of the laminin-rich basement membrane, which is replaced by a provisional matrix composed of arginine-glycine-aspartate (RGD) motif-containing plasma matrix proteins, including vitronectin and fibronectin. To determine the role of specific matrix proteins on TGF-β-induced apoptosis, we studied primary AECs cultured on different matrix conditions and utilized mice with deletion of vitronectin (Vtn(-/-)) or mice in which the vitronectin RGD motif is mutated to nonintegrin-binding arginine-glycine-glutamate (RGE) (Vtn(RGE/RGE)). We found that AECs cultured on fibronectin and vitronectin or in wild-type mouse serum are resistant to TGF-β-induced apoptosis. In contrast, AECs cultured on laminin or in serum from Vtn(-/-) or Vtn(RGE/RGE) mice undergo robust TGF-β-induced apoptosis. Plasminogen activator inhibitor-1 (PAI-1) sensitizes AECs to greater apoptosis by disrupting AEC engagement to vitronectin. Inhibition of integrin-associated signaling proteins augments AEC apoptosis. Mice with transgenic deletion of PAI-1 have less apoptosis after bleomycin, but deletion of vitronectin or disruption of the vitronectin RGD motif reverses this protection, suggesting that the proapoptotic function of PAI-1 is mediated through vitronectin inhibition. Collectively, these data suggest that integrin-matrix signaling is an important regulator of TGF-β-mediated AEC apoptosis and that PAI-1 functions as a natural regulator of this interaction.

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

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

Mechanisms of Lung Fibrosis Resolution

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