Fibrotic Repair Research Articles

Enhancing Clinical Diagnosis for Patients With Persistent Pulmonary Abnormalities After COVID-19 Infection: The Potential Benefit of 68 Ga-FAPI PET/CT.

Six post COVID-19 patients suspected for pulmonary fibrosis were scheduled for dual-tracer PET/CT with 18 F-FDG and 68 Ga-fibroblast activation protein inhibitor (FAPI)-46. The uptake of 68 Ga-FAPI-46 in the involved lung was compared with a control group of 9 non-COVID-19 patients. Clinical data and PET/CT imaging were collected and analyzed. PET/CT revealed in all 6 pulmonary impaired patients the reduced glucose avidity on 18 F-FDG and clear positivity on 68 Ga-FAPI-46 PET/CT in comparison to the control group. Enhancing fibrotic repair mechanisms, 68 Ga-FAPI PET/CT may improve noninvasive clinical diagnostic performance in patients with long-term CT abnormalities after severe COVID-19. Although this study shows promising results, additional studies in larger populations are required to establish a general diagnostic guideline.

Single-cell analysis highlights differences in druggable pathways underlying adaptive or fibrotic kidney regeneration

The kidney has tremendous capacity to repair after acute injury, however, pathways guiding adaptive and fibrotic repair are poorly understood. We developed a model of adaptive and fibrotic kidney regeneration by titrating ischemic injury dose. We performed detailed biochemical and histological analysis and profiled transcriptomic changes at bulk and single-cell level (> 110,000 cells) over time. Our analysis highlights kidney proximal tubule cells as key susceptible cells to injury. Adaptive proximal tubule repair correlated with fatty acid oxidation and oxidative phosphorylation. We identify a specific maladaptive/profibrotic proximal tubule cluster after long ischemia, which expresses proinflammatory and profibrotic cytokines and myeloid cell chemotactic factors. Druggability analysis highlights pyroptosis/ferroptosis as vulnerable pathways in these profibrotic cells. Pharmacological targeting of pyroptosis/ferroptosis in vivo pushed cells towards adaptive repair and ameliorates fibrosis. In summary, our single-cell analysis defines key differences in adaptive and fibrotic repair and identifies druggable pathways for pharmacological intervention to prevent kidney fibrosis.

NETs promote pathogenic cardiac fibrosis and participate in ventricular aneurysm formation after ischemia injury through the facilitation of perivascular fibrosis

Fibrosis has been widely investigated in acute phase of myocardial infarction (MI). However, the mechanism of sustained fibrosis after MI hasn't been elucidated, which eventually gives rise to ventricular aneurysm (VA) formation chronic while lethal. Neutrophil as vital cell facilitating the fibrotic repair after acute MI may not project its effect to chronic phase unless neutrophil extracellular traps (NETs) were secreted and accumulating. The aim of this study was to investigate whether NETs contribute to the sustained fibrosis and VA formation after MI. We identified NETs in ventricular aneurysm of patients. Accordingly, NETs increased in peripheral blood of VA patients. Moreover, in rat VA NETs were also identified. Stimulated by NETs, the migration of fibroblast was enhanced and the differentiation of cardiac myofibroblast was initiated. Smad, MAPK and RhoA signaling pathways were activated by NETs incubation. And additional deposition with DNase I to disrupt NETs and abrogated NETs induced fibrosis both in vivo and vitro. These results collectively demonstrate a novel profibrotic role for NETs in chronic cardiac fibrosis and VA formation.

Alveolar Regeneration in COVID-19 Patients: A Network Perspective.

A viral infection involves entry and replication of viral nucleic acid in a host organism, subsequently leading to biochemical and structural alterations in the host cell. In the case of SARS-CoV-2 viral infection, over-activation of the host immune system may lead to lung damage. Albeit the regeneration and fibrotic repair processes being the two protective host responses, prolonged injury may lead to excessive fibrosis, a pathological state that can result in lung collapse. In this review, we discuss regeneration and fibrosis processes in response to SARS-CoV-2 and provide our viewpoint on the triggering of alveolar regeneration in coronavirus disease 2019 (COVID-19) patients.

Lymphatic Clearance of Immune Cells in Cardiovascular Disease.

Recent advances in our understanding of the lymphatic system, its function, development, and role in pathophysiology have changed our views on its importance. Historically thought to be solely involved in the transport of tissue fluid, lipids, and immune cells, the lymphatic system displays great heterogeneity and plasticity and is actively involved in immune cell regulation. Interference in any of these processes can be deleterious, both at the developmental and adult level. Preclinical studies into the cardiac lymphatic system have shown that invoking lymphangiogenesis and enhancing immune cell trafficking in ischaemic hearts can reduce myocardial oedema, reduce inflammation, and improve cardiac outcome. Understanding how immune cells and the lymphatic endothelium interact is also vital to understanding how the lymphatic vascular network can be manipulated to improve immune cell clearance. In this Review, we examine the different types of immune cells involved in fibrotic repair following myocardial infarction. We also discuss the development and function of the cardiac lymphatic vasculature and how some immune cells interact with the lymphatic endothelium in the heart. Finally, we establish how promoting lymphangiogenesis is now a prime therapeutic target for reducing immune cell persistence, inflammation, and oedema to restore heart function in ischaemic heart disease.

Comparison of Cellular Responses to TGF-β1 and BMP-2 Between Healthy and Torn Tendons

Background: Tendons heal by fibrotic repair, increasing the likelihood of reinjury. Animal tendon injury and overuse models have identified transforming growth factor beta (TGF-β) and bone morphogenetic proteins (BMPs) as growth factors actively involved in the development of fibrosis, by mediating extracellular matrix synthesis and cell differentiation. Purpose: To understand how TGF-β and BMPs contribute to fibrotic processes using tendon-derived cells isolated from healthy and diseased human tendons. Study Design: Controlled laboratory study. Methods: Tendon-derived cells were isolated from patients with a chronic rotator cuff tendon tear (large to massive, diseased) and healthy hamstring tendons of patients undergoing anterior cruciate ligament repair. Isolated cells were incubated with TGF-β1 (10 ng/mL) or BMP-2 (100 ng/mL) for 3 days. Gene expression was measured by real-time quantitative polymerase chain reaction. Cell signaling pathway activation was determined by Western blotting. Results: TGF-β1 treatment induced ACAN mRNA expression in both cell types but less in the diseased compared with healthy cells (P < .05). BMP-2 treatment induced BGN mRNA expression in healthy but not diseased cells (P < .01). In the diseased cells, TGF-β1 treatment induced increased ACTA2 mRNA expression (P < .01) and increased small mothers against decapentaplegic (SMAD) signaling (P < .05) compared with those of healthy cells. Moreover, BMP-2 treatment induced ACTA2 mRNA expression in the diseased cells only (P < .05). Conclusion: Diseased tendon–derived cells show reduced expression of the proteoglycans aggrecan and biglycan in response to TGF-β1 and BMP-2 treatments. These same treatments induced enhanced fibrotic differentiation and canonical SMAD cell signaling in diseased compared with healthy cells. Clinical Relevance: Findings from this study suggest that diseased tendon–derived cells respond differently than healthy cells in the presence of TGF-β1 and BMP-2. The altered responses of diseased cells may influence fibrotic repair processes during tendon healing.

Post-translational regulation of PGC-1α modulates fibrotic repair.

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease associated with mitochondrial oxidative stress. Mitochondrial reactive oxygen species (mtROS) are important for cell homeostasis by regulating mitochondrial dynamics. Here, we show that IPF BAL cells exhibited increased mitochondrial biogenesis that is, in part, due to increased nuclear expression of peroxisome proliferator‐activated receptor‐ɣ (PPARɣ) coactivator (PGC)‐1α. Increased PPARGC1A mRNA expression directly correlated with reduced pulmonary function in IPF subjects. Oxidant‐mediated activation of the p38 MAPK via Akt1 regulated PGC‐1α activation to increase mitochondrial biogenesis in monocyte‐derived macrophages. Demonstrating the importance of PGC‐1α in fibrotic repair, mice harboring a conditional deletion of Ppargc1a in monocyte‐derived macrophages or mice administered a chemical inhibitor of mitochondrial division had reduced biogenesis and increased apoptosis, and the mice were protected from pulmonary fibrosis. These observations suggest that Akt1‐mediated regulation of PGC‐1α maintains mitochondrial homeostasis in monocyte‐derived macrophages to induce apoptosis resistance, which contributes to the pathogenesis of pulmonary fibrosis.

Fibroblast-Specific Proteotranscriptomes Reveal Distinct Fibrotic Signatures of Human Sinoatrial Node in Nonfailing and Failing Hearts.

Up to 50% of the adult human sinoatrial node (SAN) is composed of dense connective tissue. Cardiac diseases including heart failure (HF) may increase fibrosis within the SAN pacemaker complex, leading to impaired automaticity and conduction of electric activity to the atria. Unlike the role of cardiac fibroblasts in pathologic fibrotic remodeling and tissue repair, nothing is known about fibroblasts that maintain the inherently fibrotic SAN environment. Intact SAN pacemaker complex was dissected from cardioplegically arrested explanted nonfailing hearts (non-HF; n=22; 48.7±3.1 years of age) and human failing hearts (n=16; 54.9±2.6 years of age). Connective tissue content was quantified from Masson trichrome-stained head-center and center-tail SAN sections. Expression of extracellular matrix proteins, including collagens 1 and 3A1, CILP1 (cartilage intermediate layer protein 1), and POSTN (periostin), and fibroblast and myofibroblast numbers were quantified by in situ and in vitro immunolabeling. Fibroblasts from the central intramural SAN pacemaker compartment (≈10×5×2 mm3) and right atria were isolated, cultured, passaged once, and treated ± transforming growth factor β1 and subjected to comprehensive high-throughput next-generation sequencing of whole transcriptome, microRNA, and proteomic analyses. Intranodal fibrotic content was significantly higher in SAN pacemaker complex from HF versus non-HF hearts (57.7±2.6% versus 44.0±1.2%; P<0.0001). Proliferating phosphorylated histone 3+/vimentin+/CD31- (cluster of differentiation 31) fibroblasts were higher in HF SAN. Vimentin+/α-smooth muscle actin+/CD31- myofibroblasts along with increased interstitial POSTN expression were found only in HF SAN. RNA sequencing and proteomic analyses identified unique differences in mRNA, long noncoding RNA, microRNA, and proteomic profiles between non-HF and HF SAN and right atria fibroblasts and transforming growth factor β1-induced myofibroblasts. Specifically, proteins and signaling pathways associated with extracellular matrix flexibility, stiffness, focal adhesion, and metabolism were altered in HF SAN fibroblasts compared with non-HF SAN. This study revealed increased SAN-specific fibrosis with presence of myofibroblasts, CILP1, and POSTN-positive interstitial fibrosis only in HF versus non-HF human hearts. Comprehensive proteotranscriptomic profiles of SAN fibroblasts identified upregulation of genes and proteins promoting stiffer SAN extracellular matrix in HF hearts. Fibroblast-specific profiles generated by our proteotranscriptomic analyses of the human SAN provide a comprehensive framework for future studies to investigate the role of SAN-specific fibrosis in cardiac rhythm regulation and arrhythmias.

Immune Inhibitory Properties and Therapeutic Prospects of Transforming Growth Factor-Beta and Interleukin 10 in Autoimmune Hepatitis.

Transforming growth factor-beta and interleukin 10 have diverse immune inhibitory properties that have restored homeostatic defense mechanisms in experimental models of autoimmune disease. The goals of this review are to describe the actions of each cytokine, review their investigational use in animal models and patients, and indicate their prospects as interventions in autoimmune hepatitis. English abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed. Transforming growth factor-beta expands the natural and inducible populations of regulatory T cells, limits the proliferation of natural killer cells, suppresses the activation of naïve CD8+ T cells, decreases the production of interferon-gamma, and stimulates fibrotic repair. Interleukin 10 selectively inhibits the CD28 co-stimulatory signal for antigen recognition and impairs antigen-specific activation of uncommitted CD4+ and CD8+ T cells. It also inhibits maturation of dendritic cells, suppresses Th17 cells, supports regulatory T cells, and limits production of diverse pro-inflammatory cytokines. Contradictory immune stimulatory effects have been associated with each cytokine and may relate to the dose and accompanying cytokine milieu. Experimental findings have not translated into successful early clinical trials. The recombinant preparation of each agent in low dosage has been safe in human studies. In conclusion, transforming growth factor-beta and interleukin 10 have powerful immune inhibitory actions of potential therapeutic value in autoimmune hepatitis. The keys to their therapeutic application will be to match their predominant non-redundant function with the pivotal pathogenic mechanism or cytokine deficiency and to avoid contradictory immune stimulatory actions.

The evolving cardiac lymphatic vasculature in development, repair and regeneration.

The lymphatic vasculature has an essential role in maintaining normal fluid balance in tissues and modulating the inflammatory response to injury or pathogens. Disruption of normal development or function of lymphatic vessels can have severe consequences. In the heart, reduced lymphatic function can lead to myocardial oedema and persistent inflammation. Macrophages, which are phagocytic cells of the innate immune system, contribute to cardiac development and to fibrotic repair and regeneration of cardiac tissue after myocardial infarction. In this Review, we discuss the cardiac lymphatic vasculature with a focus on developments over the past 5 years arising from the study of mammalian and zebrafish model organisms. In addition, we examine the interplay between the cardiac lymphatics and macrophages during fibrotic repair and regeneration after myocardial infarction. Finally, we discuss the therapeutic potential of targeting the cardiac lymphatic network to regulate immune cell content and alleviate inflammation in patients with ischaemic heart disease.

Metabolic regulation of innate immune cell phenotypes during wound repair and regeneration.

Metabolism regulates an array of cellular processes from embryonic development through adulthood. These include proliferation, differentiation and the effector functions of adult cells to maintain homeostasis and repair. It is becoming clear that bioenergetic shifts can control how cells respond to environmental disruptions during tissue injury to initiate a healing response. Specifically, innate immune cells shift their phenotypes to initiate and resolve inflammation, and there is intense interest to understand how these responses might regulate healing outcomes. Here, we review recent literature describing how cellular metabolism and metabolic byproducts regulate phenotype conversions among innate immune cells. Although most studies of this kind do not focus on tissue damage, we discuss how metabolic regulation of these phenotypes promotes tissue repair. In particular, we provide a framework for considering the extent to which altering the innate immune response might shift fibrotic repair towards regenerative healing.

Gli1+ mesenchymal stromal cells form a pathological niche to promote airway progenitor metaplasia in the fibrotic lung.

Aberrant epithelial reprogramming can induce metaplastic differentiation at sites of tissue injury, culminating in transformed barriers composed of scar and metaplastic epithelium. While the plasticity of epithelial stem cells is well-characterized, the identity and role of the niche has not been delineated in metaplasia. Here we show that Gli1+ mesenchymal stromal cells (MSCs), previously shown to contribute to myofibroblasts during scarring, promote metaplastic differentiation of airway progenitors into KRT5+ basal cells. During fibrotic repair, Gli1+ MSCs integrate hedgehog activation to upregulate BMP antagonism in the progenitor niche that promotes metaplasia. Restoring the balance towards BMP activation attenuated metaplastic KRT5+ differentiation while promoting adaptive alveolar differentiation into SFTPC+ epithelium. Finally, fibrotic human lungs demonstrate altered BMP activation in the metaplastic epithelium. These findings show that Gli1+ MSCs integrate hedgehog signaling as a rheostat to control BMP activation in the progenitor niche to determine regenerative outcome in fibrosis.

Advances in the research of effects of epithelial-mesenchymal interaction on wound healing and scar formation

Wound healing is a dynamic process which involves interaction of various types of cells, cytokines, and extracellular matrix. Among them, epithelial cells and mesenchymal cells are the key components which involve in wound healing and scar formation. Related scholars had done a great number of studies about the functions of epithelial cells and fibroblasts(Fbs) in wound healing and scar formation. The results showed that under the stimulation of complex microenvironment, epithelial cells would lose their epithelial characteristics and acquire the typical characteristics and migration ability of mesenchymal cells. At the same time, with the complex changes of cell structure and cell behavior, they would participate in the process of tissue wound repair, including normal or fibrotic repair, by covering the wound with migration. Fbs are the key cells for the wound fibrotic repair, and play important roles in the process of wound healing, including excessive wound healing or delayed wound healing. In the recent years, the researchers realized that the cross-talk between epithelial cells and Fbs in wound healing, which is referred to as epithelial-mesenchymal interaction, significantly changes the biological behaviors of these two cell types, which affects the dermal remodeling and re-epithelialization quality of wound. Epithelial-mesenchymal interaction plays an important role in skin morphogenesis during embryonic development and maintaining the structural integrity of adult skin. In the process of re-epithelialization, Fbs could promote the proliferation and migration of keratinocytes, meanwhile keratinocytes would receive the signals from Fbs to reconstruct functional epithelium, which has become a hot topic in the field of wound healing at present. In this paper, a comprehensive analysis of the literature on the role of epithelial-mesenchymal interaction in wound healing and scar formation at home and abroad in recent years is presented for the reference of relevant scholars.

Alveolar macrophage - derived exosomes modulate severity and outcome of acute lung injury

Severe acute lung injury (ALI) can cause death, and the survivals may develop acute respiratory distress syndrome (ARDS) due to fibrotic repair of the lung. Alveolar macrophages play a demonstrative role during the pathogenesis of ALI, and the timing and degree of differentially polarization of macrophages determine the severity of disease and outcome. Exosomes are important mediators of cellular communication and play critical roles during macrophage differentiation, proliferation and function. Nevertheless, the exact effects of alveolar macrophage - derived exosomes on ALI remain unknow. Here, we used lipopolysaccharide (LPS) to induce ALI in mice and analyzed the exosome population in bronchoalveolar lavage fluid (BALF) from macrophages, neutrophils and epithelial cells at different time points after treatment. Our data showed that macrophages were the major secretors for early secreted pro-inflammatory cytokines in the BALF-exosomes, which likely activated neutrophils to produce a variety of pro-inflammatory cytokines and IL-10. IL-10 by neutrophils in BALF-exosomes likely in turn polarized macrophages to M2c, which may be responsible for post-ALI fibrosis. Our study thus reveals a previous non-acknowledged role of BALF-exosomes as a mediator of inflammatory response and cell crosstalk during ALI.

SUN-316 Binding of β-catenin to TCF1 and FoxO1 controls TGF-beta fibrogenic signalling pathways and predicts adverse outcomes in transplanted kidneys

Chronic injury and maladptive fibrotic repair in kidney transplants remains the major cause of allograft loss. Transforming growth factor β (TGF-β) is a key cytokine causing fibrosis. Our recent work has demonstrated that β-catenin binding to Foxo or TCF determines the signaling direction of TGF-β, oscillating between profibrotic and anti-inflammatory pathways in murine models of kidney fibrosis. To explore the relevance of our findings to human disease, we examined β-catenin/FoxO1 and β-catenin/TCF1 in relation to the severity of kidney inflammation, fibrosis and dysfunction in biopsies of kidney transplant patients. Our aim was to examine control of fibrogenic pathways, and determine their prognostic value in clincially relevant outcomes in kidney transplantation. We simultaneously quantified binding of FoxO1 and TCF1 to β-catenin within tubular cells using a proximity ligation assay in 164 transplanted kidneys. eGFR and serum creatinine were measured on the day of biopsy and all biopsies were evaluated for fibrosis by renal pathologists according to the Banff classification. The ratio of β-catenin/TCF1 to β-catenin/FoxO1 binding (TF ratio) was calculated, and multivariable linear and logistic regression models were used for predictive analysis. Cox regression analysis was used to explore the relationship between TF ratio and chronic fibrosis. ROC curve was used for specificity and sensitivity analysis. The one month TF ratio correlated with reduced eGFR (r=-0.556, P<0.001) and increased serum creatinine (r=0.507, P<0.001), albuminuria (r=0.276, P=0.036) Banff ci (r=0.273, P=0.002) and ct scores (r=0.316, P<0.001) at 12 months in kidney transplant patients. Using multivariable linear regression, one year eGFR was only predictable by eGFR (β=0.548, p<0.001) and TF ratio (β=-5.905, p=0.002) at one month. Progression to 1-year moderate fibrosis (Banff ci≥2), was predicted by one month TF ratio (OR=3.221, 95%CI 1.610-6.444, P<0.001) adjusted for baseline pathology using logistic regression. Multivariable models confirmed TF ratio was a consistent and independent risk factor for progressive fibrosis, withstanding adjustment for multiple confounding clinical risk factors. The adjusted hazard ratio of one month TF was 1.773 (95%CI 1.416-2.221, P<0.001). The one month TF ratio predicted Banff ci≥1 fibrosis (AUC=0.717, 95%CI 0.522-0.712, P<0.001) or ci≥2 fibrosis (AUC=0.769, 95%CI 0.660-0.857, P<0.001) at 12-months. The predictive ability of one month TF ratio for 12 month functional impairment was impressive (AUC 0.817, 95%CI 0.746-0.887, P<0.001). The full model including TF ratio was optimal and highly predictive of late renal impairment (AUC 0.914, 95%CI 0.863-0.964, P<0.001). This study demonstrated that differences in binding of TCF1 or FoxO1 to β-catenin affects the direction of fibrogenic signalling and renal transplant outcomes, including intra-renal fibrogenesis and inflammation. The TF ratio at one month was associated with adverse renal fibrosis and clinical outcomes at one year. It may serve as a novel and powerful prognostic biomarker of progressive fibrosis, proteinuria and graft dysfunction. Altering the balance of β-catenin-associated transcriptional activity may be an interventional target for preventative anti-fibrotic therapy.

Pericyte-Mediated Tissue Repair through PDGFRβ Promotes Peri-Infarct Astrogliosis, Oligodendrogenesis, and Functional Recovery after Acute Ischemic Stroke.

Post-stroke functional recovery can occur spontaneously during the subacute phase; however, how post-stroke fibrotic repair affects functional recovery is highly debated. Platelet-derived growth factor receptor β (PDGFRβ)-expressing pericytes are responsible for post-stroke fibrotic repair within infarct areas; therefore, we examined peri-infarct neural reorganization and functional recovery after permanent middle cerebral artery occlusion (pMCAO) using pericyte-deficient Pdgfrb+/- mice. Time-dependent reduction of infarct area sizes, i.e., repair, was significantly impaired in Pdgfrb+/- mice with recovery of cerebral blood flow (CBF) in ischemic areas attenuated by defective leptomeningeal arteriogenesis and intrainfarct angiogenesis. Peri-infarct astrogliosis, accompanied by increased STAT3 phosphorylation, was attenuated in Pdgfrb+/- mice. Pericyte-conditioned medium (PCM), particularly when treated with platelet-derived growth factor subunit B (PDGFB) homodimer (PDGF-BB; PCM/PDGF-BB), activated STAT3 and enhanced the proliferation and activity of cultured astrocytes. Although peri-infarct proliferation of oligodendrocyte (OL) precursor cells (OPCs) was induced promptly after pMCAO regardless of intrainfarct repair, OPC differentiation and remyelination were significantly attenuated in Pdgfrb+/- mice. Consistently, astrocyte-CM (ACM) promoted OPC differentiation and myelination, which were enhanced remarkably by adding PCM/PDGF-BB to the medium. Post-stroke functional recovery correlated well with the extent and process of intrainfarct repair and peri-infarct oligodendrogenesis. Overall, pericyte-mediated intrainfarct fibrotic repair through PDGFRβ may promote functional recovery through enhancement of peri-infarct oligodendrogenesis as well as astrogliosis after acute ischemic stroke.

How Do Innate Immune Cells Contribute to Airway Remodeling in COPD Progression?

Recently, the therapeutic potential of immune-modulation during the progression of chronic obstructive pulmonary disease (COPD) has been attracting increasing interest. However, chronic inflammatory response has been over-simplified in descriptions of the mechanism of COPD progression. As a form of first-line airway defense, epithelial cells exhibit phenotypic alteration, and participate in epithelial layer disorganization, mucus hypersecretion, and extracellular matrix deposition. Dendritic cells (DCs) exhibit attenuated antigen-presenting capacity in patients with advanced COPD. Immature DCs migrate into small airways, where they promote a pro-inflammatory microenvironment and bacterial colonization. In response to damage-associated molecular patterns (DAMPs) in lung tissue affected by COPD, neutrophils are excessively recruited and activated, where they promote a proteolytic microenvironment and fibrotic repair in small airways. Macrophages exhibit decreased phagocytosis in the large airways, while they demonstrate high pro-inflammatory potential in the small airways, and mediate alveolar destruction and chronic airway inflammation. Natural killer T (NKT) cells, eosinophils, and mast cells also play supplementary roles in COPD progression; however, their cellular activities are not yet entirely clear. Overall, during COPD progression, “exhausted” innate immune responses can be observed in the large airways. On the other hand, the innate immune response is enhanced in the small airways. Approaches that inhibit the inflammatory cascade, chemotaxis, or the activation of inflammatory cells could possibly delay the progression of airway remodeling in COPD, and may thus have potential clinical significance.

Increased flux through the mevalonate pathway mediates fibrotic repair without injury.

Macrophages are important in mounting an innate immune response to injury as well as in repair of injury. Gene expression of Rho proteins is known to be increased in fibrotic models; however, the role of these proteins in idiopathic pulmonary fibrosis (IPF) is not known. Here, we show that BAL cells from patients with IPF have a profibrotic phenotype secondary to increased activation of the small GTPase Rac1. Rac1 activation requires a posttranslational modification, geranylgeranylation, of the C-terminal cysteine residue. We found that by supplying more substrate for geranylgeranylation, Rac1 activation was substantially increased, resulting in profibrotic polarization by increasing flux through the mevalonate pathway. The increased flux was secondary to greater levels of acetyl-CoA from metabolic reprogramming to β oxidation. The polarization mediated fibrotic repair in the absence of injury by enhancing macrophage/fibroblast signaling. These observations suggest that targeting the mevalonate pathway may abrogate the role of macrophages in dysregulated fibrotic repair.

Mitochondrial calcium uniporter regulates PGC-1α expression to mediate metabolic reprogramming in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive disease with an increased mortality. Metabolic reprogramming has a critical role in multiple chronic diseases. Lung macrophages expressing the mitochondrial calcium uniporter (MCU) have a critical role in fibrotic repair, but the contribution of MCU in macrophage metabolism is not known. Here, we show that MCU regulates peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and metabolic reprogramming to fatty acid oxidation (FAO) in macrophages. MCU regulated PGC-1α expression by increasing the phosphorylation of ATF-2 by the p38 MAPK in a redox-dependent manner. The expression and activation of PGC-1α via the p38 MAPK was regulated by MCU-mediated mitochondrial calcium uptake, which is linked to increased mitochondrial ROS (mtROS) production. Mice harboring a conditional expression of dominant-negative MCU in macrophages had a marked reduction in mtROS and FAO and were protected from pulmonary fibrosis. Moreover, IPF lung macrophages had evidence of increased MCU and mitochondrial calcium, increased phosphorylation of ATF2 and p38, as well as increased expression of PGC-1α. These observations suggest that macrophage MCU-mediated metabolic reprogramming contributes to fibrotic repair after lung injury.

NOX4 modulates macrophage phenotype and mitochondrial biogenesis in asbestosis.

Macrophage activation is implicated in the development of pulmonary fibrosis by generation of profibrotic molecules. Although NADPH oxidase 4 (NOX4) is known to contribute to pulmonary fibrosis, its effects on macrophage activation and mitochondrial redox signaling are unclear. Here, we show that NOX4 is crucial for lung macrophage profibrotic polarization and fibrotic repair after asbestos exposure. NOX4 was elevated in lung macrophages from subjects with asbestosis, and mice harboring a deletion of NOX4 in lung macrophages were protected from asbestos-induced fibrosis. NOX4 promoted lung macrophage profibrotic polarization and increased production of profibrotic molecules that induce collagen deposition. Mechanistically, NOX4 further augmented mitochondrial ROS production and induced mitochondrial biogenesis. Targeting redox signaling and mitochondrial biogenesis prevented the profibrotic polarization of lung macrophages by reducing the production of profibrotic molecules. These observations provide evidence that macrophage NOX4 is a potentially novel therapeutic target to halt the development of asbestos-induced pulmonary fibrosis.