Regenerative activity of the lung after epithelial injury

p53 governs an AT1 differentiation programme in lung cancer suppression

Lung cancer is the leading cause of cancer deaths worldwide1. Mutations in the tumour suppressor gene TP53 occur in 50% of lung adenocarcinomas (LUADs) and are linked to poor prognosis1-4, but how p53 suppresses LUAD development remains enigmatic. We show here that p53 suppresses LUAD by governing cell state, specifically by promoting alveolar type 1 (AT1) differentiation. Using mice that express oncogenic Kras and null, wild-type or hypermorphic Trp53 alleles in alveolar type 2 (AT2) cells, we observed graded effects of p53 on LUAD initiation and progression. RNA sequencing and ATAC sequencing of LUAD cells uncovered a p53-induced AT1 differentiation programme during tumour suppression in vivo through direct DNA binding, chromatin remodelling and induction of genes characteristic of AT1 cells. Single-cell transcriptomics analyses revealed that during LUAD evolution, p53 promotes AT1 differentiation through action in a transitional cell state analogous to a transient intermediary seen during AT2-to-AT1 cell differentiation in alveolar injury repair. Notably, p53 inactivation results in the inappropriate persistence of these transitional cancer cells accompanied by upregulated growth signalling and divergence from lung lineage identity, characteristics associated with LUAD progression. Analysis of Trp53 wild-type and Trp53-null mice showed that p53 also directs alveolar regeneration after injury by regulating AT2 cell self-renewal and promoting transitional cell differentiation into AT1 cells. Collectively, these findings illuminate mechanisms of p53-mediated LUAD suppression, in which p53 governs alveolar differentiation, and suggest that tumour suppression reflects a fundamental role of p53 in orchestrating tissue repair after injury.

ΔNp63 drives dysplastic alveolar remodeling and restricts epithelial plasticity upon severe lung injury

The lung exhibits a robust, multifaceted regenerative response to severe injuries such as influenza infection, during which quiescent lung-resident epithelial progenitors participate in two distinct reparative pathways: functionally beneficial regeneration via alveolar type 2 (AT2) cell proliferation and differentiation, and dysplastic tissue remodeling via intrapulmonary airway-resident basal p63+ progenitors. Here we show that the basal cell transcription factor ΔNp63 is required for intrapulmonary basal progenitors to participate in dysplastic alveolar remodeling following injury. We find that ΔNp63 restricts the plasticity of intrapulmonary basal progenitors by maintaining either active or repressive histone modifications at key differentiation gene loci. Following loss of ΔNp63, intrapulmonary basal progenitors are capable of either airway or alveolar differentiation depending on their surrounding environment both in vitro and in vivo. Uncovering these regulatory mechanisms of dysplastic repair and lung basal cell fate choice highlight potential therapeutic targets to promote functional alveolar regeneration following severe lung injuries.

Developmental switch from morphological replication to compensatory growth for salamander lung regeneration

Salamanders possess a pair of lungs for active air breathing, but the lung respiration is fully operational only during the late stage of development, particularly after metamorphosis. Larval salamanders mainly exchange air through the gills and skin, thus sparing the developing lungs. Salamanders can repair their lungs after injury, but a comparative analysis of regenerative responses between the lungs of young and adult animals is lacking. In this study, lung resections were performed in both larval and adult newts (Pleurodeles waltl). The cellular dynamics, tissue morphology and organ function during lung regeneration were examined and the Yap mutants were produced with CRISPR tools. We found that salamander switches the regenerative strategies from morphological replication through the blastema formation to compensatory growth via resident epithelial cells proliferation upon pulmonary resection injury as it transitions beyond metamorphosis. The larval animals achieve lung regeneration by forming a transient blastema-like structure and regrowing full-sized developing lungs, albeit unventilated. The adults repair injured lungs via massive proliferating epithelial cells and by expanding the existing alveolar epithelium without neo-alveolarization. Yap signalling promotes epithelial cell proliferation and prevents epithelial-to-mesenchymal transition to restore functional respiration. The salamanders have evolved distinct regenerative strategies for lung repair during different phases of life. Our results demonstrate a novel strategy for functional lung recovery by inducing epithelial cell proliferation to strengthen the remaining alveoli without rebuilding new alveoli.

RNA Sequencing of Idiopathic Subglottic Stenosis Tissues Uncovers Putative Profibrotic Mechanisms and Identifies a Prognostic Biomarker

Idiopathic subglottic stenosis (iSGS) is a localized airway disease that almost exclusively affects females. Understanding the molecular mechanisms involved may provide insights leading to therapeutic interventions. Next-generation sequencing was performed on tissue sections from patients with iSGS (n = 22), antineutrophil cytoplasmic antibody-associated vasculitis (AAV; n = 5), and matched controls (n = 9) to explore candidate genes and mechanisms of disease. Gene expression changes were validated, and selected markers were identified by immunofluorescence staining. Epithelial-mesenchymal transition (EMT) and leukocyte extravasation pathways were the biological mechanisms most relevant to iSGS pathogenesis. Alternatively activated macrophages (M2) were abundant in the subepithelium and perisubmucosal glands of the airway in iSGS and AAV. Increased expression of the mesenchymal marker S100A4 and decreased expression of the epithelial marker epithelial cell adhesion molecule (EPCAM) further supported a role for EMT, but to different extents, in iSGS and antineutrophil cytoplasmic antibody-associated subglottic stenosis. In patients with iSGS, high expression of prostate transmembrane protein, androgen induced 1 (PMEPA1), an EMT regulator, was associated with a shorter recurrence interval (25 versus 116 months: hazard ratio = 4.16; P = 0.041; 95% CI, 1.056-15.60). Thus, EMT is a key pathogenetic mechanism of subglottic stenosis in iSGS and AAV. M2 macrophages contribute to the pathogenesis of both diseases, suggesting a shared profibrotic mechanism, and PMEPA1 may be a biomarker for predicting disease recurrence in iSGS.

Modeling explains prolonged SARS-CoV-2 nasal shedding relative to lung shedding in remdesivir treated rhesus macaques

In clinical trials, remdesivir decreased recovery time in hospitalized patients with SARS- CoV-2 and prevented hospitalization when given early during infection, despite not reducing nasal viral loads. In rhesus macaques, early remdesivir prevented pneumonia and lowered lung viral loads, but viral loads increased in nasal passages after five days. We developed mathematical models to explain these results. Our model raises the hypotheses that: 1) in contrast to nasal passages viral load monotonically decreases in lungs during therapy because of infection-dependent generation of refractory cells, 2) slight reduction in lung viral loads with an imperfect agent may result in a substantial decrease in lung damage, and 3) increases in nasal viral load may occur due to a blunting of peak viral load which decreases the intensity of the innate immune response. We demonstrate that a higher potency drug could lower viral loads in nasal passages and lung.

Transmural pressure signals through retinoic acid to regulate lung branching

During development, the mammalian lung undergoes several rounds of branching, the rate of which is tuned by the relative pressure of the fluid within the lumen of the lung. We carried out bioinformatics analysis of RNA-sequencing of embryonic mouse lungs cultured under physiologic or sub-physiologic transmural pressure and identified transcription factor-binding motifs near genes whose expression changes in response to pressure. Surprisingly, we found retinoic acid (RA) receptor binding sites significantly overrepresented in the promoters and enhancers of pressure-responsive genes. Consistently, increasing transmural pressure activates RA signaling, and pharmacologically inhibiting RA signaling decreases airway epithelial branching and smooth muscle wrapping. We found that pressure activates RA signaling through the mechanosensor Yap. A computational model predicts that mechanical signaling through Yap and RA affects lung branching by altering the balance between epithelial proliferation and smooth muscle wrapping, which we test experimentally. Our results reveal that transmural pressure signals through RA to balance the relative rates of epithelial growth and smooth muscle differentiation in the developing mouse lung and identify RA as a previously unreported component in the mechanotransduction machinery of embryonic tissues.

YAP regulates alveolar epithelial cell differentiation and AGER via NFIB/KLF5/NKX2-1

Ventilation is dependent upon pulmonary alveoli lined by two major epithelial cell types, alveolar type-1 (AT1) and 2 (AT2) cells. AT1 cells mediate gas exchange while AT2 cells synthesize and secrete pulmonary surfactants and serve as progenitor cells which repair the alveoli. We developed transgenic mice in which YAP was activated or deleted to determine its roles in alveolar epithelial cell differentiation. Postnatal YAP activation increased epithelial cell proliferation, increased AT1 cell numbers, and caused indeterminate differentiation of subsets of alveolar cells expressing atypical genes normally restricted to airway epithelial cells. YAP deletion increased expression of genes associated with mature AT2 cells. YAP activation enhanced DNA accessibility in promoters of transcription factors and motif enrichment analysis predicted target genes associated with alveolar cell differentiation. YAP participated with KLF5, NFIB, and NKX2-1 to regulate AGER. YAP plays a central role in a transcriptional network that regulates alveolar epithelial differentiation.

Evaluation on epithelial-mesenchymal state and microRNAs focusing on isolated alveolar epithelial cells from bleomycin injured rat lung

Several studies using bleomycin (BLM)-induced lung injury rat model revealed that epithelial-mesenchymal transition (EMT) contributes to pulmonary fibrosis. Conversely, microRNAs (miRNAs) are considered as useful markers of various diseases. In the present study, we aimed to characterize the EMT state through focusing on alveolar epithelial cells and identify the miRNAs that can be used as markers to predict pulmonary fibrosis using a BLM-induced lung injury rat model. Intratracheal administration of BLM increased hydroxyproline, a component of collagen, in lung tissues at day 14, but not at day 7. However, BLM induced EMT at day 7, which was accompanied with increased mRNA expression of α-smooth muscle actin, a representative EMT marker, in alveolar epithelium, thereby suggesting that EMT occurs prior to pulmonary fibrosis in alveolar epithelial cells. Using this rat model, the expression levels of several EMT-associated miRNAs were examined, and miR-222 was found to be upregulated in alveolar epithelial cells as well as bronchoalveolar lavage fluid from day 3. Our findings indicate that EMT in alveolar epithelial cells may occur before pulmonary fibrosis, and miR-222 may be used as a potential marker for early prediction of pulmonary fibrosis.

Pulmonary mesenchymal stem cells are engaged in distinct steps of host response to respiratory syncytial virus infection

Lung-resident (LR) mesenchymal stem and stromal cells (MSCs) are key elements of the alveolar niche and fundamental regulators of homeostasis and regeneration. We interrogated their function during virus-induced lung injury using the highly prevalent respiratory syncytial virus (RSV) which causes severe outcomes in infants. We applied complementary approaches with primary pediatric LR-MSCs and a state-of-the-art model of human RSV infection in lamb. Remarkably, RSV-infection of pediatric LR-MSCs led to a robust activation, characterized by a strong antiviral and pro-inflammatory phenotype combined with mediators related to T cell function. In line with this, following in vivo infection, RSV invades and activates LR-MSCs, resulting in the expansion of the pulmonary MSC pool. Moreover, the global transcriptional response of LR-MSCs appears to follow RSV disease, switching from an early antiviral signature to repair mechanisms including differentiation, tissue remodeling, and angiogenesis. These findings demonstrate the involvement of LR-MSCs during virus-mediated acute lung injury and may have therapeutic implications.

Linking EMT programmes to normal and neoplastic epithelial stem cells

Epithelial stem cells serve critical physiological functions in the generation, maintenance and repair of diverse tissues through their ability to self-renew and spawn more specialized, differentiated cell types. In an analogous fashion, cancer stem cells have been proposed to fuel the growth, progression and recurrence of many carcinomas. Activation of an epithelial-mesenchymal transition (EMT), a latent cell-biological programme involved in development and wound healing, has been linked to the formation of both normal and neoplastic stem cells, but the mechanistic basis underlying this connection remains unclear. In this Perspective, we outline the instances where aspects of an EMT have been implicated in normal and neoplastic epithelial stem cells and consider the involvement of this programme during tissue regeneration and repair. We also discuss emerging concepts and evidence related to the heterogeneous and plastic cell states generated by EMT programmes and how these bear on our understanding of cancer stem cell biology and cancer metastasis. A more comprehensive accounting of the still-elusive links between EMT programmes and the stem cell state will surely advance our understanding of both normal stem cell biology and cancer pathogenesis.

Aging Reprograms the Hematopoietic-Vascular Niche to Impede Regeneration and Promote Fibrosis

Regenerative capacity is frequently impaired in aged organs. Stress to aged organs often causes scar formation (fibrosis) at the expense of regeneration. It remains to be defined how hematopoietic and vascular cells contribute to aging-induced regeneration to fibrotic transition. Here, we find that aging aberrantly reprograms the crosstalk between hematopoietic and vascular cells to impede the regenerative capacity and enhance fibrosis. In aged lung, liver, and kidney, induction of Neuropilin-1/hypoxia-inducible-factor 2α (HIF2α) suppresses anti-thrombotic and anti-inflammatory endothelial protein C receptor (EPCR) pathway, leading to formation of pro-fibrotic platelet-macrophage rosette. Activated platelets via supplying interleukin 1α synergize with endothelial-produced angiocrine chemokine to recruit fibrogenic TIMP1^high macrophages. In mouse models, genetic targeting of endothelial Neuropilin-1-HIF2α, platelet interleukin 1α, or macrophage TIMP1 normalized the pro-fibrotic hematopoietic-vascular niche and restored the regenerative capacity of old organs. Targeting of aberrant endothelial node molecules might help propel "regeneration without scarring" in the repair of multiple organs.

Pantoea agglomerans chronic exposure induces epithelial-mesenchymal transition in human lung epithelial cells and mice lungs

Pantoea agglomerans is gram-negative bacteria widely distributed in nature. It predominates in inhalable dust from grain, herbs, and flax, and was identified as the most important cause of hypersensitivity pneumonitis (HP) in eastern Poland. To better understand the molecular mechanism of HP development studies focused on the interactions between P. agglomerans and alveolar epithelial cells as well as lung tissue with particular emphasis on the epithelial-mesenchymal transition (EMT). The studies were conducted on human normal lung epithelial NL20 cells and mice strain C57BL/6J. Cells and mice underwent chronic exposure to saline extract of P. agglomerans (SE-PA). Morphological changes were evaluated under light microscopy, the concentration of fibrosis markers was examined by the ELISA method, while the expression of genes involved in EMT was evaluated by RealTime PCR. During incubation with SE-PA epithelial cells underwent conversion and assumed fibroblast phenotype characterized by a decrease in epithelial cells markers (CDH1, CLDN1, JUP) and increase in mesenchymal cells markers (FN1, VIM, CDH2). Mice lungs collected after 14 days of SE-PA treatment revealed inflammation with marked lymphocytes infiltration. The intensified inflammatory process accompanied by increased proliferation of fibrous connective tissue was noted in mice lungs after 28 days of SE-PA exposure. Histological changes correlated with an increase of fibrosis markers (hydroxyproline, collagens), downregulation of epithelial markers (Cdh1, Cldn1, Jup, Ocln) and upregulation of myofibroblasts markers (Acta2, Cdh2, Fn1, Vim). Obtained results revealed SE-PA ability to induce EMT in human lung epithelial cells and mice lung tissue, with the scale of changes proportional to the time of treatment.

Concentration- and Time-Dependent Effects of Benzalkonium Chloride in Human Lung Epithelial Cells: Necrosis, Apoptosis, or Epithelial Mesenchymal Transition

Benzalkonium chloride (BAC), an antimicrobial agent in inhalable medications and household sprays, has been reported to be toxic to pulmonary organs. Although cell membrane damage has been considered as the main cytotoxic mechanism of BAC, its concentration- and time-dependent cellular effects on lung epithelium have not been fully understood. In the present study, human lung epithelial (H358) cells were exposed to 0.2–40 μg/mL of BAC for 30 min or 21 days. Cell membranes were rapidly disrupted by 30 min exposure, but 24 h incubation of BAC (4–40 μg/mL) predominantly caused apoptosis rather than necrosis. BAC (2–4 μg/mL) induced mitochondrial depolarization, which may be associated with increased expression of pro-apoptotic proteins (caspase-3, PARP, Bax, p53, and p21), and decreased levels of the anti-apoptotic protein Bcl-2. The protein expression levels of IRE1α, BiP, CHOP, and p-JNK were also elevated by BAC (2–4 μg/mL) suggesting the possible involvement of endoplasmic reticulum stress in inducing apoptosis. Long-term (7–21 days) incubation with BAC (0.2–0.6 μg/mL) did not affect cell viability but led to epithelial-mesenchymal transition (EMT) as shown by the decrease of E-cadherin and the increase of N-cadherin, fibronectin, and vimentin, caused by the upregulation of EMT transcription factors, such as Snail, Slug, Twist1, Zeb1, and Zeb2. Therefore, we conclude that apoptosis could be an important mechanism of acute BAC cytotoxicity in lung epithelial cells, and chronic exposure to BAC even at sub-lethal doses can promote pulmonary EMT.

Glycyrrhizin suppresses epithelial-mesenchymal transition by inhibiting high-mobility group box1 via the TGF-β1/Smad2/3 pathway in lung epithelial cells

Background

Epithelial-mesenchymal transition (EMT) plays an important role in fibrosis, chronic inflammation, tumor metastasis, etc. Glycyrrhizin, an active component extracted from licorice plant, has been reported to treat a variety of inflammatory reactions through inhibiting high-mobility group box1 (HMGB1), which has been suggested to be a significant mediator in EMT process. However, whether glycyrrhizin affects the EMT process or not remains unclear.

Methods

Human alveolar epithelial cell line A549 and normal human bronchial epithelial cell line BEAS-2B were treated with extrinsic TGF-β1 to induce EMT. Elisa was used to detect HMGB1 concentrations in cell supernatant. RNA interference and lentivirus infection experiments were performed to investigate the involvement of HMGB1 in EMT process. Cell Counting Kit-8 (CCK-8) was used to detect the viability of A549 and BEAS-2B cells treated with glycyrrhizin. Finally, the effects of glycyrrhizin on EMT changes, as well as the underlying mechanisms, were evaluated via Western blot, immunofluorescence and transwell assays.

Results

Our results showed that HMGB1 expression was increased by TGF-β1, and knockdown of HMGB1 expression reversed TGF-β1-induced EMT in A549 and BEAS-2B cells. Ectopic HMGB1 expression or TGF-β1 treatment caused a significant increase in HMGB1 release. Notably, we found that glycyrrhizin treatment effectively suppressed TGF-β1-induced EMT process by inhibiting HMGB1. Also, glycyrrhizin significantly inhibited the migration of both A549 and BEAS-2B cells promoted by TGF-β1. Mechanistically, HMGB1 overexpression could activate Smad2/3 signaling in A549 and BEAS-2B cells. Glycyrrhizin significantly blocked the phosphorylation of Smad2/3 stimulated either by TGF-β1 or by ectopic HMGB1 in A549 and BEAS-2B cells.

Conclusions

HMGB1 is a vital mediator of EMT changes induced by TGF-β1 in lung epithelial cells. Importantly, glycyrrhizin can effectively block Smad2/3 signaling pathway through inhibiting HMGB1, thereby suppressing the EMT progress.

Distinct Chronic Post-Viral Lung Diseases upon Infection with Influenza or Parainfluenza Viruses Differentially Impact Superinfection Outcome

Chronic obstructive pulmonary disease (COPD) and asthma remain prevalent human lung diseases. Variability in epithelial and inflammatory components that results in pathologic heterogeneity complicates the development of treatments for these diseases. Early childhood infection with parainfluenza virus or respiratory syncytial virus is strongly associated with the development of asthma and COPD later in life, and exacerbations of these diseases correlate with the presence of viral RNA in the lung. Well-characterized animal models of postviral chronic lung diseases are necessary to study the underlying mechanisms of viral-related COPD and asthma and to develop appropriate therapies. In this study, we cross-analyzed chronic lung disease caused by infection with Sendai virus (SeV) or influenza A virus in mice. Differences were observed in lesion composition and inflammatory profiles between SeV- and influenza A virus-induced long-term lung disease. In addition, a primary SeV infection led to worsened pathologic findings on secondary heterologous viral challenge, whereas the reversed infection scheme protected against disease in response to a secondary viral challenge >1 month after the primary infection. These data demonstrate the differential effect of primary viral infections in the susceptibility to disease exacerbation in response to a different secondary viral infection and highlight the usefulness of these viral models as tools to understand the underlying mechanisms that mediate distinct chronic postviral lung diseases.

Akt and Notch pathways mediate polyhexamethylene guanidine phosphate-induced epithelial-mesenchymal transition via ZEB2

Polyhexamethylene guanidine phosphate (PHMG-p), an antimicrobial additive, was used as a humidifier disinfectant in Korea and caused severe lung injuries, including lung fibrosis, in hundreds of victims. As PHMG-p-induced lung fibrosis is different from that induced by known fibrogenic agents such as bleomycin, it is important to understand the molecular mechanisms underlying this effect. A recent study showed that epithelial-mesenchymal transition (EMT) could play key roles in PHMG-p-induced pulmonary fibrosis. Therefore, we aimed to characterize the molecular mechanisms associated with PHMG-p-induced EMT. We observed EMT, macrophage infiltration, and fibrosis in mouse lung tissues after intratracheal instillation of PHMG-p. Furthermore, PHMG-p-induced EMT was observed in A549 cells by the evaluation of cell morphology and quantitation of mRNA and protein expression. The use of EMT inhibitors revealed that PHMG-p induced EMT through the activation of Akt and Notch signaling. Moreover, the transcription factor ZEB2 was observed in PHMG-p-treated A549 cells and mouse lungs. The results indicated that upstream regulators, including Akt and Notch 1, acted as intracellular effectors that triggered ZEB2 expression after exposure to PHMG-p. Attenuation of PHMG-p-induced EMT following inhibition or silencing of Akt and Notch signaling or ZEB2 implied that PHMG-p-induced EMT was a result of Akt, Notch, and ZEB2 activation. Our findings showed that PHMG-p induced EMT through Akt/Notch signaling pathways and that ZEB2 played an important role in PHMG-p-induced lung toxicity. This study will help to understand the mechanisms of action of PHMG-p associated with lung fibrogenesis.

Cigarette Smoke Induced Lung Barrier Dysfunction, EMT, and Tissue Remodeling: A Possible Link between COPD and Lung Cancer

Chronic obstructive pulmonary disease (COPD) and lung cancer, closely related to smoking, are major lung diseases affecting millions of individuals worldwide. The generated gas mixture of smoking is proved to contain about 4,500 components such as carbon monoxide, nicotine, oxidants, fine particulate matter, and aldehydes. These components were considered to be the principle factor driving the pathogenesis and progression of pulmonary disease. A large proportion of lung cancer patients showed a history of COPD, which demonstrated that there might be a close relationship between COPD and lung cancer. In the early stages of smoking, lung barrier provoked protective response and DNA repair are likely to suppress these changes to a certain extent. In the presence of long-term smoking exposure, these mechanisms seem to be malfunctioned and lead to disease progression. The infiltration of inflammatory cells to mucosa, submucosa, and glandular tissue caused by inhaled cigarette smoke is responsible for the destruction of matrix, blood supply shortage, and epithelial cell death. Conversely, cancer cells have the capacity to modulate the proliferation of epithelial cells and produce of new vascular networks. Comprehension understanding of mechanisms responsible for both pathologies is necessary for the prevention and treatment of COPD and lung cancer. In this review, we will summarize related articles and give a glance of possible mechanism between cigarette smoking induced COPD and lung cancer.

Macrophages promote epithelial proliferation following infectious and non-infectious lung injury through a Trefoil factor 2-dependent mechanism

Coordinated efforts between macrophages and epithelia are considered essential for wound healing, but the macrophage-derived molecules responsible for repair are poorly defined. This work demonstrates that lung macrophages rely upon Trefoil factor 2 to promote epithelial proliferation following damage caused by sterile wounding, Nippostrongylus brasiliensis or Bleomycin sulfate. Unexpectedly, the presence of T, B, or ILC populations was not essential for macrophage-driven repair. Instead, conditional deletion of TFF2 in myeloid-restricted CD11c^Cre TFF2 ^flox mice exacerbated lung pathology and reduced the proliferative expansion of CD45^- EpCAM⁺ pro-SPC⁺ alveolar type 2 cells. TFF2 deficient macrophages had reduced expression of the Wnt genes Wnt4 and Wnt16 and reconstitution of hookworm-infected CD11c^Cre TFF2^flox mice with rWnt4 and rWnt16 restored the proliferative defect in lung epithelia post-injury. These data reveal a previously unrecognized mechanism wherein lung myeloid phagocytes utilize a TFF2/Wnt axis as a mechanism that drives epithelial proliferation following lung injury.

Selective activation and proliferation of a quiescent stem cell population in the neuroepithelial body microenvironment

BACKGROUND

The microenvironment (ME) of neuroepithelial bodies (NEBs) harbors densely innervated groups of pulmonary neuroendocrine cells that are covered by Clara-like cells (CLCs) and is believed to be important during development and for adult airway epithelial repair after severe injury. Yet, little is known about its potential stem cell characteristics in healthy postnatal lungs.

METHODS

Transient mild lung inflammation was induced in mice via a single low-dose intratracheal instillation of lipopolysaccharide (LPS). Bronchoalveolar lavage fluid (BALF), collected 16 h after LPS instillation, was used to challenge the NEB ME in ex vivo lung slices of control mice. Proliferating cells in the NEB ME were identified and quantified following simultaneous LPS instillation and BrdU injection.

RESULTS

The applied LPS protocol induced very mild and transient lung injury. Challenge of lung slices with BALF of LPS-treated mice resulted in selective Ca²⁺-mediated activation of CLCs in the NEB ME of control mice. Forty-eight hours after LPS challenge, a remarkably selective and significant increase in the number of divided (BrdU-labeled) cells surrounding NEBs was observed in lung sections of LPS-challenged mice. Proliferating cells were identified as CLCs.

CONCLUSIONS

A highly reproducible and minimally invasive lung inflammation model was validated for inducing selective activation of a quiescent stem cell population in the NEB ME. The model creates new opportunities for unraveling the cellular mechanisms/pathways regulating silencing, activation, proliferation and differentiation of this unique postnatal airway epithelial stem cell population.

MEG3 is increased in idiopathic pulmonary fibrosis and regulates epithelial cell differentiation

Idiopathic pulmonary fibrosis (IPF) is a chronic interstitial lung disease causing fibrotic remodeling of the peripheral lung, leading to respiratory failure. Peripheral pulmonary epithelial cells lose normal alveolar epithelial gene expression patterns and variably express genes associated with diverse conducting airway epithelial cells, including basal cells. Single-cell RNA sequencing of pulmonary epithelial cells isolated from IPF lung tissue demonstrated altered expression of LncRNAs, including increased MEG3. MEG3 RNA was highly expressed in subsets of the atypical IPF epithelial cells and correlated with conducting airway epithelial gene expression patterns. Expression of MEG3 in human pulmonary epithelial cell lines increased basal cell-associated RNAs, including TP63, KRT14, STAT3, and YAP1, and enhanced cell migration, consistent with a role for MEG3 in regulating basal cell identity. MEG3 reduced expression of TP73, SOX2, and Notch-associated RNAs HES1 and HEY1, in primary human bronchial epithelial cells, demonstrating a role for MEG3 in the inhibition of genes influencing basal cell differentiation into club, ciliated, or goblet cells. MEG3 induced basal cell genes and suppressed genes associated with terminal differentiation of airway cells, supporting a role for MEG3 in regulation of basal progenitor cell functions, which may contribute to tissue remodeling in IPF.

Active epithelial Hippo signaling in idiopathic pulmonary fibrosis

Hippo/YAP signaling plays pleiotropic roles in the regulation of cell proliferation and differentiation during organogenesis and tissue repair. Herein we demonstrate increased YAP activity in respiratory epithelial cells in lungs of patients with idiopathic pulmonary fibrosis (IPF), a common, lethal form of interstitial lung disease (ILD). Immunofluorescence staining in IPF epithelial cells demonstrated increased nuclear YAP and loss of MST1/2. Bioinformatic analyses of epithelial cell RNA profiles predicted increased activity of YAP and increased canonical mTOR/PI3K/AKT signaling in IPF. Phospho-S6 (p-S6) and p-PTEN were increased in IPF epithelial cells, consistent with activation of mTOR signaling. Expression of YAP (S127A), a constitutively active form of YAP, in human bronchial epithelial cells (HBEC3s) increased p-S6 and p-PI3K, cell proliferation and migration, processes that were inhibited by the YAP-TEAD inhibitor verteporfin. Activation of p-S6 was required for enhancing and stabilizing YAP, and the p-S6 inhibitor temsirolimus blocked nuclear YAP localization and suppressed expression of YAP target genes CTGF, AXL, and AJUBA (JUB). YAP and mTOR/p-S6 signaling pathways interact to induce cell proliferation and migration, and inhibit epithelial cell differentiation that may contribute to the pathogenesis of IPF.

EMT/MET at the Crossroad of Stemness, Regeneration and Oncogenesis: The Ying-Yang Equilibrium Recapitulated in Cell Spheroids

The epithelial-to-mesenchymal transition (EMT) is an essential trans-differentiation process, which plays a critical role in embryonic development, wound healing, tissue regeneration, organ fibrosis, and cancer progression. It is the fundamental mechanism by which epithelial cells lose many of their characteristics while acquiring features typical of mesenchymal cells, such as migratory capacity and invasiveness. Depending on the contest, EMT is complemented and balanced by the reverse process, the mesenchymal-to-epithelial transition (MET). In the saving economy of the living organisms, the same (Ying-Yang) tool is integrated as a physiological strategy in embryonic development, as well as in the course of reparative or disease processes, prominently fibrosis, tumor invasion and metastasis. These mechanisms and their related signaling (e.g., TGF-β and BMPs) have been effectively studied in vitro by tissue-derived cell spheroids models. These three-dimensional (3D) cell culture systems, whose phenotype has been shown to be strongly dependent on TGF-β-regulated EMT/MET processes, present the advantage of recapitulating in vitro the hypoxic in vivo micro-environment of tissue stem cell niches and their formation. These spheroids, therefore, nicely reproduce the finely regulated Ying-Yang equilibrium, which, together with other mechanisms, can be determinant in cell fate decisions in many pathophysiological scenarios, such as differentiation, fibrosis, regeneration, and oncogenesis. In this review, current progress in the knowledge of signaling pathways affecting EMT/MET and stemness regulation will be outlined by comparing data obtained from cellular spheroids systems, as ex vivo niches of stem cells derived from normal and tumoral tissues. The mechanistic correspondence in vivo and the possible pharmacological perspective will be also explored, focusing especially on the TGF-β-related networks, as well as others, such as SNAI1, PTEN, and EGR1. This latter, in particular, for its ability to convey multiple types of stimuli into relevant changes of the cell transcriptional program, can be regarded as a heterogeneous "stress-sensor" for EMT-related inducers (growth factor, hypoxia, mechano-stress), and thus as a therapeutic target.

Protein Kinase Cζ Inhibitor Promotes Resolution of Bleomycin-Induced Acute Lung Injury

Protein kinase Cζ (PKCζ) is highly expressed in the lung, where it plays several regulating roles in the pathogenesis of acute lung injury (ALI). Proliferation and differentiation of integrin β4⁺ distal lung epithelial progenitor cells seem to play a key role in proper lung regeneration. We investigated the effects of a myristoylated PKCζ inhibitor (PKCζi) in a murine model of bleomycin-induced ALI. After intratracheal injury, we treated mice three times a week with PKCζi or its vehicle, DMSO. We found that mice injured with bleomycin and then treated with PKCζi for one week showed decreased activation of PKCζ, improved lung compliance, and decreased lung protein permeability compared to injured mice treated with DMSO. Mice treated continuously with PKCζi for 6 weeks showed reduced evidence of lung fibrosis by computed tomographic images, decreased lung collagen deposition, and decreased active transforming growth factor-β in the bronchoalveolar lavage fluid. In addition, we found an increased number of lung β4⁺ cells compared to DMSO at Week 6. Therefore, we grew isolated integrin β4⁺ lung progenitor cells in the presence of PKCζi or DMSO and found that β4⁺ cells treated with PKCζi proliferated more in vitro compared to DMSO. We conclude that the use of a PKCζi promotes resolution of lung fibrosis in a bleomycin ALI model and increases the number of β4⁺ progenitor cells with regenerative potential in the lung.

Selective gene expression analysis of the neuroepithelial body microenvironment in postnatal lungs with special interest for potential stem cell characteristics

BACKGROUND

The pulmonary neuroepithelial body (NEB) microenvironment (ME) consists of innervated cell clusters that occur sparsely distributed in the airway epithelium, an organization that has so far hampered reliable selective gene expression analysis. Although the NEB ME has been suggested to be important for airway epithelial repair after ablation, little is known about their potential stem cell characteristics in healthy postnatal lungs. Here we report on a large-scale selective gene expression analysis of the NEB ME.

METHODS

A GAD67-GFP mouse model was used that harbors GFP-fluorescent NEBs, allowing quick selection and pooling by laser microdissection (LMD) without further treatment. A panel of stem cell-related PCR arrays was used to selectively compare mRNA expression in the NEB ME to control airway epithelium (CAE). For genes that showed a higher expression in the NEB ME, a ranking was made based on the relative expression level. Single qPCR and immunohistochemistry were used to validate and quantify the PCR array data.

RESULTS

Careful optimization of all protocols appeared to be essential to finally obtain high-quality RNA from pooled LMD samples of NEB ME. About 30% of the more than 600 analyzed genes showed an at least two-fold higher expression compared to CAE. The gene that showed the highest relative expression in the NEB ME, Delta-like ligand 3 (Dll3), was investigated in more detail. Selective Dll3 gene expression in the NEB ME could be quantified via single qPCR experiments, and Dll3 protein expression could be localized specifically to NEB cell surface membranes.

CONCLUSIONS

This study emphasized the importance of good protocols and RNA quality controls because of the, often neglected, fast RNA degradation in postnatal lung samples. It was shown that sufficient amounts of high-quality RNA for reliable complex gene expression analysis can be obtained from pooled LMD-collected NEB ME samples of postnatal lungs. Dll3 expression, which has also been reported to be important in high-grade pulmonary tumor-initiating cells, was used as a proof-of-concept to confirm that the described methodology represents a promising tool for further unraveling the molecular basis of NEB ME physiology in general, and its postnatal stem cell capacities in particular.

Plasma membrane wounding and repair in pulmonary diseases

Various pathophysiological conditions such as surfactant dysfunction, mechanical ventilation, inflammation, pathogen products, environmental exposures, and gastric acid aspiration stress lung cells, and the compromise of plasma membranes occurs as a result. The mechanisms necessary for cells to repair plasma membrane defects have been extensively investigated in the last two decades, and some of these key repair mechanisms are also shown to occur following lung cell injury. Because it was theorized that lung wounding and repair are involved in the pathogenesis of acute respiratory distress syndrome (ARDS) and idiopathic pulmonary fibrosis (IPF), in this review, we summarized the experimental evidence of lung cell injury in these two devastating syndromes and discuss relevant genetic, physical, and biological injury mechanisms, as well as mechanisms used by lung cells for cell survival and membrane repair. Finally, we discuss relevant signaling pathways that may be activated by chronic or repeated lung cell injury as an extension of our cell injury and repair focus in this review. We hope that a holistic view of injurious stimuli relevant for ARDS and IPF could lead to updated experimental models. In addition, parallel discussion of membrane repair mechanisms in lung cells and injury-activated signaling pathways would encourage research to bridge gaps in current knowledge. Indeed, deep understanding of lung cell wounding and repair, and discovery of relevant repair moieties for lung cells, should inspire the development of new therapies that are likely preventive and broadly effective for targeting injurious pulmonary diseases.

EMT: 2016

The significant parallels between cell plasticity during embryonic development and carcinoma progression have helped us understand the importance of the epithelial-mesenchymal transition (EMT) in human disease. Our expanding knowledge of EMT has led to a clarification of the EMT program as a set of multiple and dynamic transitional states between the epithelial and mesenchymal phenotypes, as opposed to a process involving a single binary decision. EMT and its intermediate states have recently been identified as crucial drivers of organ fibrosis and tumor progression, although there is some need for caution when interpreting its contribution to metastatic colonization. Here, we discuss the current state-of-the-art and latest findings regarding the concept of cellular plasticity and heterogeneity in EMT. We raise some of the questions pending and identify the challenges faced in this fast-moving field.

The biological role of epithelial-mesenchymal transition in lung cancer (Review)

Epithelial-mesenchymal transition (EMT) is a process whereby epithelial cells gradually transform into mesenchymal-like cells losing their epithelial functionality and characteristics. EMT is thought to be involved in the pathogenesis of numerous lung diseases ranging from developmental disorders and fibrotic tissue remodeling to lung cancer. Lung cancer is the most lethal form of cancer worldwide, and despite significant therapeutic improvements, the patient survival rate still remains low. Activation of EMT endows invasive and metastatic properties upon cancer cells that favor successful colonization of distal target organs. The present review provides a brief insight into the mechanism and biological assessment methods of EMT in lung cancer and summarizes the recent literature highlighting the controversial experimental data and conclusions.

Neonatal Type II Alveolar Epithelial Cell Transplant Facilitates Lung Reparation in Piglets With Acute Lung Injury and Extracorporeal Life Support

OBJECTIVES

Type II alveolar epithelial cells have potential for lung growth and reparation. Extracorporeal membrane oxygenation is used as life support for lung impairment resulting from acute respiratory distress syndrome. We hypothesized that intratracheal transplantation of isogeneic primary type II alveolar epithelial cells in combination with extracorporeal membrane oxygenation may facilitate lung reparation for acute lung injury (ALI).

DESIGN

A randomized, controlled experiment.

SETTING

An animal laboratory in a university pediatric center.

SUBJECTS

Twenty-eight 4- to 6-week young piglets, weighing 7-8 kg.

INTERVENTIONS

Type II alveolar epithelial cells from neonatal male piglet lungs were isolated, purified, cultured, and labeled with chemical stain PKH26. After 3-6 hours of induction of ALI by IV endotoxin and mechanical ventilation (MV), young female piglets were allocated to five groups (n = 5): ALI-MV, ALI treated with MV; ALI-EC, ALI treated with both MV and venovenous extracorporeal membrane oxygenation; ALI-EC-T, ALI-EC protocol plus intratracheal type II alveolar epithelial cell transplant; CON-MV, healthy animals treated with MV; and CON-EC-T, healthy animals treated with venovenous extracorporeal membrane oxygenation. After 24 hours, animals were weaned from treatment for recovery in the ensuing 14 days, with their lungs assessed for injury and reparation.

MEASUREMENTS AND MAIN RESULTS

Lung injury for animals in ALI-MV was moderate to severe, whereas much milder injuries in ALI-EC-T and ALI-EC were found. More PKH26-labeled type II alveolar epithelial cells were detected by fluorescence in the lungs of ALI-EC-T than in CON-EC-T as further verified by the expression of messenger RNA of sex-determining region of Y chromosome. Electromicroscopically intact type II alveolar epithelial cells and prominent lattice-like tubular myelin were also found in ALI-EC-T and CON-MV but not in ALI-EC. The hydroxyproline level in lung tissue was significantly lower in ALI-EC-T than in ALI-EC and ALI-MV, with most of the lung histopathologic and pathobiologic manifestations in favor of ALI-EC-T.

CONCLUSIONS

The preliminary data suggested that type II alveolar epithelial cell transplant facilitated lung reparation for ALI in this model.

An official American Thoracic Society workshop report: stem cells and cell therapies in lung biology and diseases

The University of Vermont College of Medicine and the Vermont Lung Center, in collaboration with the NHLBI, Alpha-1 Foundation, American Thoracic Society, European Respiratory Society, International Society for Cell Therapy, and the Pulmonary Fibrosis Foundation, convened a workshop, "Stem Cells and Cell Therapies in Lung Biology and Lung Diseases," held July 29 to August 1, 2013 at the University of Vermont. The conference objectives were to review the current understanding of the role of stem and progenitor cells in lung repair after injury and to review the current status of cell therapy and ex vivo bioengineering approaches for lung diseases. These are all rapidly expanding areas of study that both provide further insight into and challenge traditional views of mechanisms of lung repair after injury and pathogenesis of several lung diseases. The goals of the conference were to summarize the current state of the field, discuss and debate current controversies, and identify future research directions and opportunities for both basic and translational research in cell-based therapies for lung diseases. This conference was a follow-up to four previous biennial conferences held at the University of Vermont in 2005, 2007, 2009, and 2011. Each of those conferences, also sponsored by the National Institutes of Health, American Thoracic Society, and Respiratory Disease Foundations, has been important in helping guide research and funding priorities. The major conference recommendations are summarized at the end of the report and highlight both the significant progress and major challenges in these rapidly progressing fields.

Hypoxia-Induced Epithelial-Mesenchymal Transition Is Involved in Bleomycin-Induced Lung Fibrosis

Pulmonary fibrosis is a severe disease that contributes to the morbidity and mortality of a number of lung diseases. However, the molecular and cellular mechanisms leading to lung fibrosis are poorly understood. This study investigated the roles of epithelial-mesenchymal transition (EMT) and the associated molecular mechanisms in bleomycin-induced lung fibrosis. The bleomycin-induced fibrosis animal model was established by intratracheal injection of a single dose of bleomycin. Protein expression was measured by Western blot, immunohistochemistry, and immunofluorescence. Typical lesions of lung fibrosis were observed 1 week after bleomycin injection. A progressive increase in MMP-2, S100A4, α-SMA, HIF-1α, ZEB1, CD44, phospho-p44/42 (p-p44/42), and phospho-p38 MAPK (p-p38) protein levels as well as activation of EMT was observed in the lung tissues of bleomycin mice. Hypoxia increased HIF-1α and ZEB1 expression and activated EMT in H358 cells. Also, continuous incubation of cells under mild hypoxic conditions increased CD44, p-p44/42, and p-p38 protein levels in H358 cells, which correlated with the increase in S100A4 expression. In conclusion, bleomycin induces progressive lung fibrosis, which may be associated with activation of EMT. The fibrosis-induced hypoxia may further activate EMT in distal alveoli through a hypoxia-HIF-1α-ZEB1 pathway and promote the differentiation of lung epithelial cells into fibroblasts through phosphorylation of p38 MAPK and Erk1/2 proteins.

Activation of type II cells into regenerative stem cell antigen-1(+) cells during alveolar repair

The alveolar epithelium is composed of two cell types: type I cells comprise 95% of the gas exchange surface area, whereas type II cells secrete surfactant, while retaining the ability to convert into type I cells to induce alveolar repair. Using lineage-tracing analyses in the mouse model of Pseudomonas aeruginosa-induced lung injury, we identified a population of stem cell antigen (Sca)-1-expressing type II cells with progenitor cell properties that mediate alveolar repair. These cells were shown to be distinct from previously reported Sca-1-expressing bronchioalveolar stem cells. Microarray and Wnt reporter studies showed that surfactant protein (Sp)-C(+)Sca-1(+) cells expressed Wnt signaling pathway genes, and inhibiting Wnt/β-catenin signaling prevented the regenerative function of Sp-C(+)Sca-1(+) cells in vitro. Thus, P. aeruginosa-mediated lung injury induces the generation of a Sca-1(+) subset of type II cells. The progenitor phenotype of the Sp-C(+)Sca-1(+) cells that mediates alveolar epithelial repair might involve Wnt signaling.

Inhibition of epithelial-to-mesenchymal transition and pulmonary fibrosis by methacycline

A high-throughput small-molecule screen was conducted to identify inhibitors of epithelial-mesenchymal transition (EMT) that could be used as tool compounds to test the importance of EMT signaling in vivo during fibrogenesis. Transforming growth factor (TGF)-β1-induced fibronectin expression and E-cadherin repression in A549 cells were used as 48-hour endpoints in a cell-based imaging screen. Compounds that directly blocked Smad2/3 phosphorylation were excluded. From 2,100 bioactive compounds, methacycline was identified as an inhibitor of A549 EMT with the half maximal inhibitory concentration (IC50) of roughly 5 μM. In vitro, methacycline inhibited TGF-β1-induced α-smooth muscle actin, Snail1, and collagen I of primary alveolar epithelial cells . Methacycline inhibited TGF-β1-induced non-Smad pathways, including c-Jun N-terminal kinase, p38, and Akt activation, but not Smad or β-catenin transcriptional activity. Methacycline had no effect on baseline c-Jun N-terminal kinase, p38, or Akt activities or lung fibroblast responses to TGF-β1. In vivo, 100 mg/kg intraperitoneal methacycline delivered daily beginning 10 days after intratracheal bleomycin improved survival at Day 17 (P < 0.01). Bleomycin-induced canonical EMT markers, Snail1, Twist1, collagen I, as well as fibronectin protein and mRNA, were attenuated by methacycline (Day 17). Methacycline did not attenuate inflammatory cell accumulation or alter TGF-β1-responsive genes in alveolar macrophages. These studies identify a novel inhibitor of EMT as a potent suppressor of fibrogenesis, further supporting the concept that EMT signaling is important to lung fibrosis. The findings also provide support for testing the impact of methacycline or doxycycline, an active analog, on progression of human pulmonary fibrosis.

Progenitor cells of the distal lung and their potential role in neonatal lung disease

Bronchopulmonary dysplasia (BPD) is the most common adverse outcome in extreme preterm neonates (born before 28 weeks gestation). BPD is characterized by interrupted lung growth and may predispose to early-onset emphysema and poor lung function in later life. At present, there is no treatment for BPD. Recent advances in stem/progenitor cell biology have enabled the exploration of endogenous lung progenitor populations in health and disease. In parallel, exogenous stem/progenitor cell administration has shown promise in protecting the lung from injury in the experimental setting. This review will provide an outline of the progenitor populations that have currently been identified in all tissue compartments of the distal lung and how they may be affected in BPD. A thorough understanding of the lung's endogenous progenitor populations during normal development, injury and repair may one day allow us to harness their regenerative capacity.

Hyperoxia induces alveolar epithelial-to-mesenchymal cell transition

Myofibroblast accumulation is a pathological feature of lung diseases requiring oxygen therapy. One possible source for myofibroblasts is through the epithelial-to-mesenchymal transition (EMT) of alveolar epithelial cells (AEC). To study the effects of oxygen on alveolar EMT, we used RLE-6TN and ex vivo lung slices and found that hyperoxia (85% O2, H85) decreased epithelial proteins, presurfactant protein B (pre-SpB), pro-SpC, and lamellar protein by 50% and increased myofibroblast proteins, α-smooth muscle actin (α-SMA), and vimentin by over 200% (P < 0.05). In AEC freshly isolated from H85-treated rats, mRNA for pre-SpB and pro-SpC was diminished by ∼50% and α-SMA was increased by 100% (P < 0.05). Additionally, H85 increased H2O2 content, and H2O2 (25-50 μM) activated endogenous transforming growth factor-β1 (TGF-β1), as evident by H2DCFDA immunofluorescence and ELISA (P < 0.05). Both hyperoxia and H2O2 increased SMAD3 phosphorylation (260% of control, P < 0.05). Treating cultured cells with TGF-β1 inhibitors did not prevent H85-induced H2O2 production but did prevent H85-mediated α-SMA increases and E-cadherin downregulation. Finally, to determine the role of TGF-β1 in hyperoxia-induced EMT in vivo, we evaluated AEC from H85-treated rats and found that vimentin increased ∼10-fold (P < 0.05) and that this effect was prevented by intraperitoneal TGF-β1 inhibitor SB-431542. Additionally, SB-431542 treatment attenuated changes in alveolar histology caused by hyperoxia. Our studies indicate that hyperoxia promotes alveolar EMT through a mechanism that is dependent on activation of TGF-β1 signaling.

Epithelial-mesenchymal transition in lung development and disease: does it exist and is it important?

Epithelial-mesenchymal transition (EMT) is a process when epithelial cells gradually transform into mesenchymal-like cells losing their epithelial functionality and characteristics. EMT is thought to be involved in the pathogenesis of numerous lung diseases ranging from developmental disorders, fibrotic tissue remodelling to lung cancer. The most important question--namely what is the importance and contribution of EMT in the pathogenesis of several chronic lung conditions (asthma, COPD, bronchiolitis obliterans syndrome and lung fibrosis)--is currently intensely debated. This review gives a brief insight into the mechanism and assessment methods of EMT in various pulmonary diseases and summarises the recent literature highlighting the controversial experimental data and conclusions.