Fibrillar collagen clamps lung mesenchymal cells in a nonproliferative and noncontractile phenotype

ACE2 Expression in Organotypic Human Airway Epithelial Cultures and Airway Biopsies

Coronavirus disease 2019 (COVID-19) caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an acute respiratory disease with systemic complications. Therapeutic strategies for COVID-19, including repurposing (partially) developed drugs are urgently needed, regardless of the increasingly successful vaccination outcomes. We characterized two-dimensional (2D) and three-dimensional models (3D) to establish a physiologically relevant airway epithelial model with potential for investigating SARS-CoV-2 therapeutics. Human airway basal epithelial cells maintained in submerged 2D culture were used at low passage to retain the capacity to differentiate into ciliated, club, and goblet cells in both air-liquid interface culture (ALI) and airway organoid cultures, which were then analyzed for cell phenotype makers. Airway biopsies from non-asthmatic and asthmatic donors enabled comparative evaluation of the level and distribution of immunoreactive angiotensin-converting enzyme 2 (ACE2). ACE2 and transmembrane serine proteinase 2 (TMPRSS2) mRNA were expressed in ALI and airway organoids at levels similar to those of native (i.e., non-cultured) human bronchial epithelial cells, whereas furin expression was more faithfully represented in ALI. ACE2 was mainly localized to ciliated and basal epithelial cells in human airway biopsies, ALI, and airway organoids. Cystic fibrosis appeared to have no influence on ACE2 gene expression. Neither asthma nor smoking status had consistent marked influence on the expression or distribution of ACE2 in airway biopsies. SARS-CoV-2 infection of ALI cultures did not increase the levels of selected cytokines. Organotypic, and particularly ALI airway cultures are useful and practical tools for investigation of SARS-CoV-2 infection and evaluating the clinical potential of therapeutics for COVID-19.

Resident interstitial lung fibroblasts and their role in alveolar stem cell niche development, homeostasis, injury, and regeneration

Developing, regenerating, and repairing a lung all require interstitial resident fibroblasts (iReFs) to direct the behavior of the epithelial stem cell niche. During lung development, distal lung fibroblasts, in the form of matrix-, myo-, and lipofibroblasts, form the extra cellular matrix (ECM), create tensile strength, and support distal epithelial differentiation, respectively. During de novo septation in a murine pneumonectomy lung regeneration model, developmental processes are reactivated within the iReFs, indicating progenitor function well into adulthood. In contrast to the regenerative activation of fibroblasts upon acute injury, chronic injury results in fibrotic activation. In murine lung fibrosis models, fibroblasts can pathologically differentiate into lineages beyond their normal commitment during homeostasis. In lung injury, recently defined alveolar niche cells support the expansion of alveolar epithelial progenitors to regenerate the epithelium. In human fibrotic lung diseases like bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and chronic obstructive pulmonary disease (COPD), dynamic changes in matrix-, myo-, lipofibroblasts, and alveolar niche cells suggest differential requirements for injury pathogenesis and repair. In this review, we summarize the role of alveolar fibroblasts and their activation stage in alveolar septation and regeneration and incorporate them into the context of human lung disease, discussing fibroblast activation stages and how they contribute to BPD, IPF, and COPD.

Mitofusion 2 Overexpression Decreased Proliferation of Human Embryonic Lung Fibroblasts in Acute Respiratory Distress Syndrome through Inhibiting RAS-RAF-1-ERK1/2 Pathway

Acute respiratory distress syndrome (ARDS) is one of the most fatal diseases worldwide. Pulmonary fibrosis occurs early in ARDS, and its severity plays a crucial role in ARDS mortality rate. Some studies suggested that fibroproliferation is an essential mechanism in ARDS. Mitofusion2 (Mfn2) overexpression plays a role in inhibiting cell proliferation. However, the role and potential mechanism of Mfn2 on the proliferation of fibroblasts is still unknown. In this study, we aimed at exploring the effect of Mfn2 on the human embryonic lung fibroblasts (HELF) and discussed its related mechanism. The HELF were treated with the Mfn2 overexpressing lentivirus (adv-Mfn2). The cell cycle was detected by flow cytometry. MTT, PCR and Western blotting were used to investigate the effect of Mfn2 on the proliferation of the HELF, collagen expression, the RAS-RAF-1-ERK1/2 pathway and the expression of cycle-related proteins (p21, p27, Rb, Raf-1, p-Raf-1, Erk1/2 and p-Erk1/2). The co-immunoprecipitation assay was used to explore the interaction between Mfn2 and Ras. The results showed that the overexpression of Mfn2 inhibited the proliferation of the HELF and induced the cell cycle arrest at the G₀/G₁ phase. Meanwhile, Mfn2 also inhibited the expression of collagen I, p-Erk and p-Raf-1. In addition, an interaction between Mfn2 and Ras existed in the HELF. This study suggests that the overexpression of Mfn2 can decrease the proliferation of HELF in ARDS, which was associated with the inhibition of the RAS-RAF-1-ERK1/2 pathway. The results may offer a potential therapeutic intervention for patients with ARDS.

Cellular Microenvironment Stiffness Regulates Eicosanoid Production and Signaling Pathways

Pathological changes in the biomechanical environment are implicated in the progression of idiopathic pulmonary fibrosis (IPF). Stiffened matrix augments fibroblast proliferation and differentiation and activates TGF-β1 (transforming growth factor-β1). Stiffened matrix impairs the synthesis of the antifibrogenic lipid mediator prostaglandin E₂ (PGE₂) and reduces the expression of the rate-limiting prostanoid biosynthetic enzyme cyclooxygenase-2 (COX-2). We now show that prostaglandin E synthase (PTGES), the final enzyme in the PGE₂ biosynthetic pathway, is expressed at lower levels in the lungs of patients with IPF. We also show substantial induction of COX-2, PTGES, prostaglandin E receptor 4 (EP4), and cytosolic phospholipase A₂ (cPLA₂) expression in human lung fibroblasts cultured in soft collagen hydrogels or in spheroids compared with conventional culture on stiff plastic culture plates. Induction of COX-2, cPLA₂, and PTGES expression in spheroid cultures was moderately inhibited by the p38 mitogen-activated protein kinase inhibitor SB203580. The induction of prostanoid biosynthetic enzyme expression was accompanied by an increase in PGE₂ levels only in non-IPF-derived fibroblast spheroids. Our study reveals an extensive dysregulation of prostanoid biosynthesis and signaling pathways in IPF-derived fibroblasts, which are only partially abrogated by culture in soft microenvironments.

Self DNA perpetuates IPF lung fibroblast senescence in a cGAS-dependent manner

Senescence and mitochondrial stress are mutually reinforcing age-related processes that contribute to idiopathic pulmonary fibrosis (IPF); a lethal disease that manifests primarily in the elderly. Whilst evidence is accumulating that GMP-AMP synthase (cGAS) is crucial in perpetuating senescence by binding damaged DNA released into the cytosol, its role in IPF is not known. The present study examines the contributions of cGAS and self DNA to the senescence of lung fibroblasts from IPF patients (IPF-LFs) and age-matched controls (Ctrl-LFs). cGAS immunoreactivity was observed in regions of fibrosis associated with fibroblasts in lung tissue of IPF patients. Pharmacological inhibition of cGAS or its knockdown by silencing RNA (siRNA) diminished the escalation of IPF-LF senescence in culture over 7 days as measured by decreased p21 and p16 expression, histone 2AXγ phosphorylation and/or IL-6 production (P < 0.05, n = 5-8). The targeting of cGAS also attenuated etoposide-induced senescence in Ctrl-LFs (P < 0.05, n = 5-8). Levels of mitochondrial DNA (mDNA) detected by qPCR in the cytosol and medium of IPF-LFs or senescence-induced Ctrl-LFs were higher than Ctrl-LFs at baseline (P < 0.05, n = 5-7). The addition of DNAse I (100 U/ml) deaccelerated IPF-LF senescence (P < 0.05, n = 5), whereas ectopic mDNA or the induction of endogenous mDNA release augmented Ctrl-LF senescence in a cGAS-dependent manner (P < 0.05, n = 5). In conclusion, we provide evidence that cGAS reinforces lung fibroblast senescence involving damaged self DNA. The targeting of cGAS to supress senescent-like responses may have potential important therapeutic implications in the treatment of IPF.

Rhythm on a chip: circadian entrainment in vitro is the next frontier in body-on-a chip technology

Organoids, bioprinted mini-tissues and body-on-a-chip technologies are poised to transform the practice of preclinical pharmacology, with a view to achieving better predictive value. We review the need for further refinement in static and dynamic biomechanical aspects of such microenvironments. Further consideration of the developments required in perfusion systems to enable delivery of an appropriate soluble microenvironment are argued. We place particular emphasis on a major deficiency in these systems, being the absence or aberrant circadian behaviour of cells used in such settings, and consider the technical challenges that are needing to be met in order to achieve rhythm-on-a-chip.

Mitochondrial dysfunction contributes to the senescent phenotype of IPF lung fibroblasts

Increasing evidence highlights that senescence plays an important role in idiopathic pulmonary fibrosis (IPF). This study delineates the specific contribution of mitochondria and the superoxide they form to the senescent phenotype of lung fibroblasts from IPF patients (IPF‐LFs). Primary cultures of IPF‐LFs exhibited an intensified DNA damage response (DDR) and were more senescent than age‐matched fibroblasts from control donors (Ctrl‐LFs). Furthermore, IPF‐LFs exhibited mitochondrial dysfunction, exemplified by increases in mitochondrial superoxide, DNA, stress and activation of mTORC1. The DNA damaging agent etoposide elicited a DDR and augmented senescence in Ctrl‐LFs, which were accompanied by disturbances in mitochondrial homoeostasis including heightened superoxide production. However, etoposide had no effect on IPF‐LFs. Mitochondrial perturbation by rotenone involving sharp increases in superoxide production also evoked a DDR and senescence in Ctrl‐LFs, but not IPF‐LFs. Inhibition of mTORC1, antioxidant treatment and a mitochondrial targeting antioxidant decelerated IPF‐LF senescence and/or attenuated pharmacologically induced Ctrl‐LF senescence. In conclusion, increased superoxide production by dysfunctional mitochondria reinforces lung fibroblast senescence via prolongation of the DDR. As part of an auto‐amplifying loop, mTORC1 is activated, altering mitochondrial homoeostasis and increasing superoxide production. Deeper understanding the mechanisms by which mitochondria contribute to fibroblast senescence in IPF has potentially important therapeutic implications.

Casein Kinase 1δ/ε Inhibitor, PF670462 Attenuates the Fibrogenic Effects of Transforming Growth Factor-β in Pulmonary Fibrosis

Transforming growth factor-beta (TGF-β) is a major mediator of fibrotic diseases, including idiopathic pulmonary fibrosis (IPF). However, therapeutic global inhibition of TGF-β is limited by unwanted immunosuppression and mitral valve defects. We performed an extensive literature search to uncover a little-known connection between TGF-β signaling and casein kinase (CK) activity. We have examined the abundance of CK1 delta and epsilon (CK1δ/ε) in lung tissue from IPF patients and non-diseased controls, and investigated whether inhibition of CK1δ/ε with PF670462 inhibits pulmonary fibrosis. CK1δ/ε levels in lung tissue from IPF patients and non-diseased controls were assessed by immunohistochemistry. Anti-fibrotic effects of the CK1δ/ε inhibitor PF670462 were assessed in pre-clinical models, including acute and chronic bleomycin mouse models and in vitro experiments on spheroids made from primary human lung fibroblast cells from IPF and control donors, and human A549 alveolar-like adenocarcinoma-derived epithelial cells. Increased expression of CK1δ and ε in IPF lungs compared to non-diseased controls was accompanied by increased levels of the product, phospho-period 2. In vitro, PF670462 prevented TGF-β-induced epithelial-mesenchymal transition. The stiffness of IPF-derived spheroids was reduced by PF670462 and TGF-β-induced fibrogenic gene expression was inhibited. The CK1δ/ε inhibitor PF670462 administered systemically or locally by inhalation prevented both acute and chronic bleomycin-induced pulmonary fibrosis in mice. PF670462 administered in a 'therapeutic' regimen (day 7 onward) prevented bleomycin-induced lung collagen accumulation. Elevated expression and activity of CK1 δ and ε in IPF and anti-fibrogenic effects of the dual CK1δ/ε inhibitor, PF670462, support CK1δ/ε as novel therapeutic targets for IPF.

The Processes and Mechanisms of Cardiac and Pulmonary Fibrosis

Fibrosis is the formation of fibrous connective tissue in response to injury. It is characterized by the accumulation of extracellular matrix components, particularly collagen, at the site of injury. Fibrosis is an adaptive response that is a vital component of wound healing and tissue repair. However, its continued activation is highly detrimental and a common final pathway of numerous disease states including cardiovascular and respiratory disease. Worldwide, fibrotic diseases cause over 800,000 deaths per year, accounting for ~45% of total deaths. With an aging population, the incidence of fibrotic disease and subsequently the number of fibrosis-related deaths will rise further. Although, fibrosis is a well-recognized cause of morbidity and mortality in a range of disease states, there are currently no viable therapies to reverse the effects of chronic fibrosis. Numerous predisposing factors contribute to the development of fibrosis. Biological aging in particular, interferes with repair of damaged tissue, accelerating the transition to pathological remodeling, rather than a process of resolution and regeneration. When fibrosis progresses in an uncontrolled manner, it results in the irreversible stiffening of the affected tissue, which can lead to organ malfunction and death. Further investigation into the mechanisms of fibrosis is necessary to elucidate novel, much needed, therapeutic targets. Fibrosis of the heart and lung make up a significant proportion of fibrosis-related deaths. It has long been established that the heart and lung are functionally and geographically linked when it comes to health and disease, and thus exploring the processes and mechanisms that contribute to fibrosis of each organ, the focus of this review, may help to highlight potential avenues of therapeutic investigation.

Annexin A2 contributes to lung injury and fibrosis by augmenting factor Xa fibrogenic activity

In lung injury and disease, including idiopathic pulmonary fibrosis (IPF), extravascular factor X is converted into factor Xa (FXa), a coagulant protease with fibrogenic actions. Extracellular annexin A2 binds to FXa, augmenting activation of the protease-activated receptor-1 (PAR-1). In this study, the contribution of annexin A2 in lung injury and fibrosis was investigated. Annexin A2 immunoreactivity was observed in regions of fibrosis, including those associated with fibroblasts in lung tissue of IPF patients. Furthermore, annexin A2 was detected in the conditioned media and an EGTA membrane wash of human lung fibroblast (LF) cultures. Incubation with human plasma (5% vol/vol) or purified FXa (15–50 nM) evoked fibrogenic responses in LF cultures, with FXa increasing interleukin-6 (IL-6) production and cell number by 270 and 46%, respectively ( P < 0.05, n = 5–8). The fibrogenic actions of plasma or FXa were attenuated by the selective FXa inhibitor apixaban (10 μM, or antibodies raised against annexin A2 or PAR-1 (2 μg/ml). FXa-stimulated LFs from IPF patients ( n = 6) produced twice as much IL-6 as controls ( n = 10) ( P < 0.05), corresponding with increased levels of extracellular annexin A2. Annexin A2 gene deletion in mice reduced bleomycin-induced increases in bronchoalveolar lavage fluid (BALF) IL-6 levels and cell number (* P < 0.05; n = 4–12). Lung fibrogenic gene expression and dry weight were reduced by annexin A2 gene deletion, but lung levels of collagen were not. Our data suggest that annexin A2 contributes to lung injury and fibrotic disease by mediating the fibrogenic actions of FXa. Extracellular annexin A2 is a potential target for the treatment of IPF.

The fibrogenic actions of lung fibroblast-derived urokinase: a potential drug target in IPF

The role of urokinase plasminogen activator (uPA) in idiopathic pulmonary fibrosis (IPF) remains unclear. uPA-generated plasmin has potent fibrogenic actions involving protease activated receptor-1 (PAR-1) and interleukin-6 (IL-6). Here we characterize uPA distribution or levels in lung tissue and sera from IPF patients to establish the mechanism of its fibrogenic actions on lung fibroblasts (LFs). uPA immunoreactivity was detected in regions of fibrosis including fibroblasts of lung tissue from IPF patients (n = 7). Serum uPA levels and activity were also higher in IPF patients (n = 18) than controls (n = 18) (P < 0.05), being negatively correlated with lung function as measured by forced vital capacity (FVC) %predicted (P < 0.05). The culture supernatants of LFs from IPF patients, as compared to controls, showed an increase in plasmin activity after plasminogen incubation (5–15 μg/mL), corresponding with increased levels of uPA and IL-6 (n = 5–6, P < 0.05). Plasminogen-induced increases in plasmin activity and IL-6 levels were attenuated by reducing uPA and/or PAR-1 expression by RNAi. Plasmin(ogen)-induced mitogenesis was also attenuated by targeting uPA, PAR-1 or IL-6. Our data shows uPA is formed in active regions of fibrosis in IPF lung and contributes to LF plasmin generation, IL-6 production and proliferation. Urokinase is a potential target for the treatment of lung fibrosis.

Diversity of Interstitial Lung Fibroblasts Is Regulated by Platelet-Derived Growth Factor Receptor α Kinase Activity

Epithelial-mesenchymal cell interactions and factors that control normal lung development are key players in lung injury, repair, and fibrosis. A number of studies have investigated the roles and sources of epithelial progenitors during lung regeneration; such information, however, is limited in lung fibroblasts. Thus, understanding the origin, phenotype, and roles of fibroblast progenitors in lung development, repair, and regeneration helps address these limitations. Using a combination of platelet-derived growth factor receptor α-green fluorescent protein (PDGFRα-GFP) reporter mice, microarray, real-time polymerase chain reaction, flow cytometry, and immunofluorescence, we characterized two distinct interstitial resident fibroblasts, myo- and matrix fibroblasts, and identified a role for PDGFRα kinase activity in regulating their activation during lung regeneration. Transcriptional profiling of the two populations revealed a myo- and matrix fibroblast gene signature. Differences in proliferation, smooth muscle actin induction, and lipid content in the two subpopulations of PDGFRα-expressing fibroblasts during alveolar regeneration were observed. Although CD140α(+)CD29(+) cells behaved as myofibroblasts, CD140α(+)CD34(+) appeared as matrix and/or lipofibroblasts. Gain or loss of PDGFRα kinase activity using the inhibitor nilotinib and a dominant-active PDGFRα-D842V mutation revealed that PDGFRα was important for matrix fibroblast differentiation. We demonstrated that PDGFRα signaling promotes alveolar septation by regulating fibroblast activation and matrix fibroblast differentiation, whereas myofibroblast differentiation was largely PDGFRα independent. These studies provide evidence for the phenotypic and functional diversity as well as the extent of specificity of interstitial resident fibroblasts differentiation during regeneration after partial pneumonectomy.

The Coagulant Factor Xa Induces Protease-Activated Receptor-1 and Annexin A2-Dependent Airway Smooth Muscle Cytokine Production and Cell Proliferation

During asthma exacerbation, plasma circulating coagulant factor X (FX) enters the inflamed airways and is activated (FXa). FXa may have an important role in asthma, being involved in thrombin activation and an agonist of protease-activated receptor-1 (PAR-1). Extracellular annexin A2 and integrins are also implicated in PAR-1 signaling. In this study, the potential role of PAR-1 in mediating the effects of FXa on human airway smooth muscle (ASM) cell cytokine production and proliferation was investigated. FXa (5-50 nM), but not FX, stimulated increases in ASM IL-6 production and cell number after 24- and 48-hour incubation, respectively (P < 0.05; n = 5). FXa (15 nM) also stimulated increases in the levels of mRNA for cytokines (IL-6), cell cycle-related protein (cyclin D1), and proremodeling proteins (FGF-2, PDGF-B, CTGF, SM22, and PAI-1) after 3-hour incubation (P < 0.05; n = 4). The actions of FXa were insensitive to inhibition by hirudin (1 U/ml), a selective thrombin inhibitor, but were attenuated by SCH79797 (100 nM), a PAR-1 antagonist, or Cpd 22 (1 μM), an inhibitor of integrin-linked kinase. The selective targeting of PAR-1, annexin A2, or β1-integrin by small interfering RNA and/or by functional blocking antibodies also attenuated FXa-evoked responses. In contrast, the targeting of annexin A2 did not inhibit thrombin-stimulated ASM function. In airway biopsies of patients with asthma, FXa and annexin A2 were detected in the ASM bundle by immunohistochemistry. These findings establish FXa as a potentially important asthma mediator, stimulating ASM function through actions requiring PAR-1 and annexin A2 and involving integrin coactivation.

Plasminogen-stimulated airway smooth muscle cell proliferation is mediated by urokinase and annexin A2, involving plasmin-activated cell signalling

BACKGROUND AND PURPOSE

The conversion of plasminogen into plasmin by interstitial urokinase plasminogen activator (uPA) is potentially important in asthma pathophysiology. In this study, the effect of uPA-mediated plasminogen activation on airway smooth muscle (ASM) cell proliferation was investigated.

EXPERIMENTAL APPROACH

Human ASM cells were incubated with plasminogen (0.5-50 μg·mL(-1) ) or plasmin (0.5-50 mU·mL(-1) ) in the presence of pharmacological inhibitors, including UK122, an inhibitor of uPA. Proliferation was assessed by increases in cell number or MTT reduction after 48 h incubation with plasmin(ogen), and by earlier increases in [(3) H]-thymidine incorporation and cyclin D1 expression.

KEY RESULTS

Plasminogen (5 μg·mL(-1) )-stimulated increases in cell proliferation were attenuated by UK122 (10 μM) or by transfection with uPA gene-specific siRNA. Exogenous plasmin (5 mU·mL(-1) ) also stimulated increases in cell proliferation. Inhibition of plasmin-stimulated ERK1/2 or PI3K/Akt signalling attenuated plasmin-stimulated increases in ASM proliferation. Furthermore, pharmacological inhibition of cell signalling mediated by the EGF receptor, a receptor trans-activated by plasmin, also reduced plasmin(ogen)-stimulated cell proliferation. Knock down of annexin A2, which has dual roles in both plasminogen activation and plasmin-signal transduction, also attenuated ASM cell proliferation following incubation with either plasminogen or plasmin.

CONCLUSIONS AND IMPLICATIONS

Plasminogen stimulates ASM cell proliferation in a manner mediated by uPA and involving multiple signalling pathways downstream of plasmin. Targeting mediators of plasminogen-evoked ASM responses, such as uPA or annexin A2, may be useful in the treatment of asthma.

Extra-cellular matrix proteins induce matrix metalloproteinase-1 (MMP-1) activity and increase airway smooth muscle contraction in asthma

Airway remodelling describes the histopathological changes leading to fixed airway obstruction in patients with asthma and includes extra-cellular matrix (ECM) deposition. Matrix metalloproteinase-1 (MMP-1) is present in remodelled airways but its relationship with ECM proteins and the resulting functional consequences are unknown. We used airway smooth muscle cells (ASM) and bronchial biopsies from control donors and patients with asthma to examine the regulation of MMP-1 by ECM in ASM cells and the effect of MMP-1 on ASM contraction. Collagen-I and tenascin-C induced MMP-1 protein expression, which for tenascin-C, was greater in asthma derived ASM cells. Tenascin-C induced MMP-1 expression was dependent on ERK1/2, JNK and p38 MAPK activation and attenuated by function blocking antibodies against the β1 and β3 integrin subunits. Tenascin-C and MMP-1 were not expressed in normal airways but co-localised in the ASM bundles and reticular basement membrane of patients with asthma. Further, ECM from asthma derived ASM cells stimulated MMP-1 expression to a greater degree than ECM from normal ASM. Bradykinin induced contraction of ASM cells seeded in 3D collagen gels was reduced by the MMP inhibitor ilomastat and by siRNA knockdown of MMP-1. In summary, the induction of MMP-1 in ASM cells by tenascin-C occurs in part via integrin mediated MAPK signalling. MMP-1 and tenascin-C are co-localised in the smooth muscle bundles of patients with asthma where this interaction may contribute to enhanced airway contraction. Our findings suggest that ECM changes in airway remodelling via MMP-1 could contribute to an environment promoting greater airway narrowing in response to broncho-constrictor stimuli and worsening asthma symptoms.

Plasminogen-stimulated inflammatory cytokine production by airway smooth muscle cells is regulated by annexin A2

Plasminogen has a role in airway inflammation. Airway smooth muscle (ASM) cells cleave plasminogen into plasmin, a protease with proinflammatory activity. In this study, the effect of plasminogen on cytokine production by human ASM cells was investigated in vitro. Levels of IL-6 and IL-8 in the medium of ASM cells were increased by incubation with plasminogen (5-50 μg/ml) for 24 hours (P < 0.05; n = 6-9), corresponding to changes in the levels of cytokine mRNA at 4 hours. The effects of plasminogen were attenuated by α2-antiplasmin (1 μg/ml), a plasmin inhibitor (P < 0.05; n = 6-12). Exogenous plasmin (5-15 mU/ml) also stimulated cytokine production (P < 0.05; n = 6-8) in a manner sensitive to serine-protease inhibition by aprotinin (10 KIU/ml). Plasminogen-stimulated cytokine production was increased in cells pretreated with basic fibroblast growth factor (300 pM) in a manner associated with increases in urokinase plasminogen activator expression and plasmin formation. The knockdown of annexin A2, a component of the putative plasminogen receptor comprised of annexin A2 and S100A10, attenuated plasminogen conversion into plasmin and plasmin-stimulated cytokine production by ASM cells. Moreover, a role for annexin A2 in airway inflammation was demonstrated in annexin A2-/- mice in which antigen-induced increases in inflammatory cell number and IL-6 levels in the bronchoalveolar lavage fluid were reduced (P < 0.01; n = 10-14). In conclusion, plasminogen stimulates ASM cytokine production in a manner regulated by annexin A2. Our study shows for the first time that targeting annexin A2-mediated signaling may provide a novel therapeutic approach to the treatment of airway inflammation in diseases such as chronic asthma.

Transforming growth factor-β-induced differentiation of airway smooth muscle cells is inhibited by fibroblast growth factor-2

In asthma, basic fibroblast growth factor (FGF-2) plays an important (patho)physiological role. This study examines the effects of FGF-2 on the transforming growth factor-β (TGF-β)-stimulated differentiation of airway smooth muscle (ASM) cells in vitro. The differentiation of human ASM cells after incubation with TGF-β (100 pM) and/or FGF-2 (300 pM) for 48 hours was assessed by increases in contractile protein expression, actin-cytoskeleton reorganization, enhancements in cell stiffness, and collagen remodeling. FGF-2 inhibited TGF-β-stimulated increases in transgelin (SM22) and calponin gene expression (n = 15, P < 0.01) in an extracellular signal-regulated kinase 1/2 (ERK1/2) signal transduction-dependent manner. The abundance of ordered α-smooth muscle actin (α-SMA) filaments formed in the presence of TGF-β were also reduced by FGF-2, as was the ratio of F-actin to G-actin (n = 8, P < 0.01). Furthermore, FGF-2 attenuated TGF-β-stimulated increases in ASM cell stiffness and the ASM-mediated contraction of lattices, composed of collagen fibrils (n = 5, P < 0.01). However, the TGF-β-stimulated production of IL-6 was not influenced by FGF-2 (n = 4, P > 0.05), suggesting that FGF-2 antagonism is selective for the regulation of ASM cell contractile protein expression, organization, and function. Another mitogen, thrombin (0.3 U ml(-1)), exerted no effect on TGF-β-regulated contractile protein expression (n = 8, P > 0.05), α-SMA organization, or the ratio of F-actin to G-actin (n = 4, P > 0.05), suggesting that the inhibitory effect of FGF-2 is dissociated from its mitogenic actions. The addition of FGF-2, 24 hours after TGF-β treatment, still reduced contractile protein expression, even when the TGF-β-receptor kinase inhibitor, SB431542 (10 μM), was added 1 hour before FGF-2. We conclude that the ASM cell differentiation promoted by TGF-β is antagonized by FGF-2. A better understanding of the mechanism of action for FGF-2 is necessary to develop a strategy for therapeutic exploitation in the treatment of asthma.

Novel non-canonical TGF-β signaling networks: emerging roles in airway smooth muscle phenotype and function

The airway smooth muscle (ASM) plays an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease (COPD). ASM cells express a wide range of receptors involved in contraction, growth, matrix protein production and the secretion of cytokines and chemokines. Transforming growth factor beta (TGF-β) is one of the major players in determining the structural and functional abnormalities of the ASM in asthma and COPD. It is increasingly evident that TGF-β functions as a master switch, controlling a network of intracellular and autocrine signaling loops that effect ASM phenotype and function. In this review, the various elements that participate in non-canonical TGF-β signaling, including MAPK, PI3K, WNT/β-catenin, and Ca(2+), are discussed, focusing on their effect on ASM phenotype and function. In addition, new aspects of ASM biology and their possible association with non-canonical TGF-β signaling will be discussed.

Ligand-independent transforming growth factor-β type I receptor signalling mediates type I collagen-induced epithelial-mesenchymal transition

Evidence suggests epithelial-mesenchymal transition (EMT) as one potential source of fibroblasts in idiopathic pulmonary fibrosis. To assess the contribution of alveolar epithelial cell (AEC) EMT to fibroblast accumulation in vivo following lung injury and the influence of extracellular matrix on AEC phenotype in vitro, Nkx2.1-Cre;mT/mG mice were generated in which AECs permanently express green fluorescent protein (GFP). On days 17-21 following intratracheal bleomycin administration, ~4% of GFP-positive epithelial-derived cells expressed vimentin or α-smooth muscle actin (α-SMA). Primary AECs from Nkx2.1-Cre;mT/mG mice cultured on laminin-5 or fibronectin maintained an epithelial phenotype. In contrast, on type I collagen, cells of epithelial origin displayed nuclear localization of Smad3, acquired spindle-shaped morphology, expressed α-SMA and phospho-Smad3, consistent with activation of the transforming growth factor-β (TGFβ) signalling pathway and EMT. α-SMA induction and Smad3 nuclear localization were blocked by the TGFβ type I receptor (TβRI, otherwise known as Alk5) inhibitor SB431542, while AEC derived from Nkx2.1-Cre;Alk5(flox/KO) mice did not undergo EMT on collagen, consistent with a requirement for signalling via Alk5 in collagen-induced EMT. Inability of a pan-specific TGFβ neutralizing antibody to inhibit effects of collagen together with absence of active TGFβ in culture supernatants is consistent with TGFβ ligand-independent activation of Smad signalling. These results support the notion that AECs can acquire a mesenchymal phenotype following injury in vivo and implicate type I collagen as a key regulator of EMT in AECs through signalling via Alk5, likely in a TGFβ ligand-independent manner.

Modulation of human mesenchymal stem cell function in a three-dimensional matrix promotes attenuation of adverse remodelling after myocardial infarction

The application of tissue engineering (TE) practices for cell delivery offers a unique approach to cellular cardiomyoplasty. We hypothesized that human mesenchymal stem cells (hMSCs) applied to the heart in a collagen matrix would outperform the same cells grown in a monolayer and directly injected for cardiac cell replacement after myocardial infarction in a rat model. When hMSC patches were transplanted to infarcted hearts, several measures for left ventricle (LV) remodelling and function were improved, including fractional area change, wall thickness, -dP/dt and LV end-diastolic pressure. Neovessel formation throughout the LV infarct wall after hMSC patch treatment increased by 37% when compared to direct injection of hMSCs. This observation was correlated with increased secretion of angiogenic factors, with accompanying evidence that these factors enhanced vessel formation (30% increase) and endothelial cell growth (48% increase) in vitro. These observations may explain the in vivo observations of increased vessel formation and improved cardiac function with patch-mediated cell delivery. Although culture of hMSC in collagen patches enhanced angiogenic responses, there was no effect on cell potency or viability. Therefore, hMSCs delivered as a cardiac patch showed benefits above those derived from monolayers and directly injected. hMSCs cultured and delivered within TE constructs may represent a good option to maximize the effects of cellular cardiomyoplasty.

Plasminogen activation by airway smooth muscle is regulated by type I collagen

Plasmin, the activated protease product of plasminogen, is involved in collagen remodeling, and is strongly implicated in asthma pathophysiology by recent genome-wide association studies. This study examines plasminogen "activation" by airway smooth muscle cells, and its regulation in a fibrotic environment created by culture on type I collagen and incubation with transforming growth factor (TGF)-β. Urokinase plasminogen activator (uPA) activity was detected in the supernatants of human airway smooth muscle cell cultures maintained in serum-free conditions. Incubation with plasminogen (1.5-50.0 μg/ml, 24 h) increased plasmin activity in a concentration-dependent manner (P < 0.001). uPA activity was higher in cultures maintained on fibrillar type I collagen substrata than in those on plastic, as was plasmin activity after incubation with plasminogen (20 μg/ml). Pretreatment with TGF-β (100 pM) for 18 hours inhibited plasminogen activation by airway smooth muscle cells maintained on plastic, but not on collagen. TGF-β stimulated an increase in the level of uPA mRNA in airway smooth muscle cells grown on collagen, but not on plastic. Reducing the levels of β1-integrin collagen receptor, using interference RNA, attenuated plasmin formation by airway smooth muscle cells grown on collagen, and restored the inhibitory effect of TGF-β. This study shows that airway smooth muscle activation of plasminogen by uPA is accelerated in a collagen-rich environment in which the inhibitory effect of TGF-β is attenuated in association with greater uPA expression induced via β1-integrin signaling. These findings suggest that the plasminogen-activation system involving uPA has the potential to contribute to airway wall remodeling in asthma.

Functional expression of IgG-Fc receptors in human airway smooth muscle cells

IgE-Fc receptors and IgG-Fc receptors are expressed on hematopoietic cells, but some evidence suggests that these receptors are also found on nonhematopoietic cells, including human airway smooth muscle (hASM) cells. Our study characterizes the expression of IgE-Fc receptors (FcεRI/CD23) and IgG-Fc receptors (FcγRs-I, -II, and -III) in cultured hASM cells by flow cytometry and Western blotting, and the functional activity of receptors was determined through quantification of cell proliferation and released cytokines. Expression of Fc receptor-linked intracellular signaling proteins and phosphorylation of the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1/2 and p38(MAPK) in hASM cells was examined by Western blotting. Expression of FcεRI and CD23 was not detectable in hASM cells. However, FcγRI and FcγRII were shown to be expressed on these cells. Specific antibodies, validated using transfected cell lines, revealed that the inhibitory IgG receptor, FcγRIIb, was the most abundant Fc receptor subtype expressed. Although cross-linking FcγR with heat-aggregated γ globulin (HAGG) did not induce detectable cell stimulation, pretreating hASM cells with HAGG significantly inhibited IL-1α-induced increases in cytokine levels and basic fibroblast growth factor-induced cell proliferation. This inhibitory effect of HAGG was abrogated by preincubation of cells with an anti-FcγRIIb antigen-binding fragment (Fab). Expression of proteins involved in the canonical FcγRIIb inhibitory signaling pathway was established in hASM cells. Pretreatment of hASM cells with HAGG significantly inhibited IL-1α- and basic fibroblast growth factor-induced extracellular signal-regulated kinase 1/2 and p38(MAPK) phosphorylation. This study identifies functional expression of FcγRIIb in hASM cells, with the potential to suppress their remodeling and immunomodulatory roles.

Airway smooth muscle remodels pericellular collagen fibrils: implications for proliferation

Airway wall remodeling comprises a broad range of structural changes including increases in the volume of airway smooth muscle (ASM) and fibrillar collagen. The impact of fibrillar collagen remodeling on ASM proliferation was investigated. Human ASM cultured on type I fibrillar collagen remodeled the collagen substrate by both degradation (collagenolysis) and formation of networks comprised of thicker reticular collagen fibrils (fibrillogenesis). In cultures maintained on fibrillar collagen, the levels of matrix metalloproteases (MMPs) -1 and -14 mRNA and active MMP-2 were higher than in cultures maintained on nonfibrillar type I collagen (gelatin) or plastic. Although there was no apparent increase in cytotoxicity or apoptosis, the number of ASM was lower on fibrillar collagen than on gelatin or plastic for control conditions. Furthermore, maintenance on fibrillar collagen attenuated basic fibroblast growth factor-stimulated increases in cell number and the percentage of cells entering S-phase. In cultures maintained on fibrillar collagen, the MMP inhibitor ilomastat (2.5 μM) 1) attenuated collagenolysis, 2) enhanced fibrillogenesis, and 3) inhibited proliferation. In contrast, knockdown of the β1-integrin gene in ASM maintained on fibrillar collagen led to an increase in proliferation and reduced MMP-1 and -14 expression. Thus, ASM remodel the pericellular environment by degrading collagen fibrils and spinning them into larger collagen assemblies. Moreover, the collagen fibrils limit proliferation and activate autocrine MMPs in a β-integrin-dependent manner, suggesting a potential negative feedback on modeling executed through fibrillar collagen activation of β1-integrins.

Human lung parenchyma but not proximal bronchi produces fibroblasts with enhanced TGF-beta signaling and alpha-SMA expression

Given the contribution various fibroblast subsets make to wound healing and tissue remodeling, the concept of lung fibroblast heterogeneity is of great interest. However, the mechanisms contributing to this heterogeneity are unknown. To this aim, we compared molecular and biophysical characteristics of fibroblasts concurrently isolated from normal human proximal bronchi (B-FBR) and distal lung parenchyma (P-FBR). Using quantitative RT-PCR, spontaneous expression of more than 30 genes related to repair and remodeling was analyzed. All P-FBR lines demonstrated significantly increased basal α-smooth muscle actin (α-SMA) mRNA and protein expression levels when compared with donor-matched B-FBR. These differences were not associated with sex, age, or disease history of lung tissue donors. In contrast to B-FBR, P-FBR displayed enhanced transforming growth factor (TGF)-β/Smad signaling at baseline, and inhibition of either ALK-5 or neutralization of endogenously produced and activated TGF-β substantially decreased basal α-SMA protein in P-FBR. Both B-FBR and P-FBR up-regulated α-SMA after stimulation with TGF-β1, and basal expression levels of TGF-β1, TGF-βRI, and TGF-βRII were not significantly different between fibroblast pairs. Blockade of metalloproteinase-dependent activation of endogenous TGF-β did not significantly modify α-SMA expression in P-FBR. However, resistance to mechanical tension of these cells was significantly higher in comparison with B-FBR, and added TGF-β1 significantly increased stiffness of both cell monolayers. Our data suggest that in contrast with human normal bronchial tissue explants, lung parenchyma produces mesenchymal cells with a myofibroblastic phenotype by intrinsic mechanisms of TGF-β activation in feed-forward manner. These results also offer a new insight into mechanisms of human fibroblast heterogeneity and their function in the airway and lung tissue repair and remodeling.