Involvement of leukotrienes in the pathogenesis of silica-induced pulmonary fibrosis in mice

Montelukast Influence on Lung in Experimental Diabetes

Background and Objectives: The influence of montelukast (MK), an antagonist of cysLT1 leukotriene receptors, on lung lesions caused by experimental diabetes was studied. Materials and Methods: The study was conducted on four groups of six adult male Wistar rats. Diabetes was produced by administration of streptozotocin 65 mg/kg ip. in a single dose. Before the administration of streptozotocin, after 72 h, and after 8 weeks, the serum values of glucose, SOD, MDA, and total antioxidant capacity (TAS) were determined. After 8 weeks, the animals were anesthetized and sacrificed, and the lungs were harvested and examined by optical microscopy. Pulmonary fibrosis, the extent of lung lesions, and the lung wet-weight/dry-weight ratio were evaluated. Results: The obtained results showed that MK significantly reduced pulmonary fibrosis (3.34 ± 0.41 in the STZ group vs. 1.73 ± 0.24 in the STZ+MK group p < 0.01) and lung lesion scores and also decreased the lung wet-weight/dry-weight (W/D) ratio. SOD and TAS values increased significantly when MK was administered to animals with diabetes (77.2 ± 11 U/mL in the STZ group vs. 95.7 ± 13.3 U/mL in the STZ+MK group, p < 0.05, and 25.52 ± 2.09 Trolox units in the STZ group vs. 33.29 ± 1.64 Trolox units in the STZ+MK group, respectively, p < 0.01), and MDA values decreased. MK administered alone did not significantly alter any of these parameters in normal animals. Conclusions: The obtained data showed that by blocking the action of peptide leukotrienes on cysLT1 receptors, montelukast significantly reduced the lung lesions caused by diabetes. The involvement of these leukotrienes in the pathogenesis of fibrosis and other lung diabetic lesions was also demonstrated.

TRIB3 promotes pulmonary fibrosis through inhibiting SLUG degradation by physically interacting with MDM2

Pulmonary fibrosis (PF) is the pathological structure of incurable fibroproliferative lung diseases that are attributed to the repeated lung injury-caused failure of lung alveolar regeneration (LAR). Here, we report that repetitive lung damage results in a progressive accumulation of the transcriptional repressor SLUG in alveolar epithelial type II cells (AEC2s). The abnormal increased SLUG inhibits AEC2s from self-renewal and differentiation into alveolar epithelial type I cells (AEC1s). We found that the elevated SLUG represses the expression of the phosphate transporter SLC34A2 in AEC2s, which reduces intracellular phosphate and represses the phosphorylation of JNK and P38 MAPK, two critical kinases supporting LAR, leading to LAR failure. TRIB3, a stress sensor, interacts with the E3 ligase MDM2 to suppress SLUG degradation in AEC2s by impeding MDM2-catalyzed SLUG ubiquitination. Targeting SLUG degradation by disturbing the TRIB3/MDM2 interaction using a new synthetic staple peptide restores LAR capacity and exhibits potent therapeutic efficacy against experimental PF. Our study reveals a mechanism of the TRIB3-MDM2-SLUG-SLC34A2 axis causing the LAR failure in PF, which confers a potential strategy for treating patients with fibroproliferative lung diseases.

Baicalin Ameliorates Radiation-Induced Lung Injury by Inhibiting the CysLTs/CysLT1 Signaling Pathway

Objective

Radiation-induced lung injury (RILI) is a common complication of radiotherapy for thoracic tumors. This study investigated the alleviating effect of baicalin (BA) on RILI and its possible mechanism.

Methods

RILI model was established by chest irradiation (IR) of C57BL/6 mice for 16 weeks. Different concentrations of BA were administered, and dexamethasone (DXM) was used as a positive control. Then, the lung pathological changes were observed by HE and Masson staining. The levels of TGF-β, TNF-α, IL-1β, IL-6, CysLT, LTC4, and LTE4 were measured by ELISA. The CysLT1 expression was detected by qPCR, immunohistochemistry, and western blot. Type II AEC cells were pretreated with LTD-4 to establish the RILI cell model and intervened with different concentrations of BA. Then, the collagen I protein level was measured by ELISA. The CysLT1 and α-SMA expression were detected by qPCR, immunofluorescence, and western blot.

Results

BA could effectively improve lung histopathological changes and pulmonary fibrosis. In vivo, BA could inhibit the levels of TGF-β, TNF-α, IL-1β, and IL-6 and reduce the levels of CysLT, LTC4, and LTE4. In vitro, different concentrations of LTD4 could reduce the viability of type II AEC cells, which could be reversed by the administration of different concentrations of BA. In addition, BA could reduce CysLT1 mRNA, as well as CysLT1 and α-SMA protein levels in vitro and in vivo.

Conclusion

BA attenuated lung inflammation and pulmonary fibrosis by inhibiting the CysLTs/CysLT1 pathway, thereby protecting against RILI.

Longitudinal micro-computed tomography-derived biomarkers quantify non-resolving lung fibrosis in a silicosis mouse model

In spite of many compounds identified as antifibrotic in preclinical studies, pulmonary fibrosis remains a life-threatening condition for which highly effective treatment is still lacking. Towards improving the success-rate of bench-to-bedside translation, we investigated in vivo µCT-derived biomarkers to repeatedly quantify experimental silica-induced pulmonary fibrosis and assessed clinically relevant readouts up to several months after silicosis induction. Mice were oropharyngeally instilled with crystalline silica or saline and longitudinally monitored with respiratory-gated-high-resolution µCT to evaluate disease onset and progress using scan-derived biomarkers. At weeks 1, 5, 9 and 15, we assessed lung function, inflammation and fibrosis in subsets of mice in a cross-sectional manner. Silica-instillation increased the non-aerated lung volume, corresponding to onset and progression of inflammatory and fibrotic processes not resolving with time. Moreover, total lung volume progressively increased with silicosis. The volume of healthy, aerated lung first dropped then increased, corresponding to an acute inflammatory response followed by recovery into lower elevated aerated lung volume. Imaging results were confirmed by a significantly decreased Tiffeneau index, increased neutrophilic inflammation, increased IL-13, MCP-1, MIP-2 and TNF-α concentration in bronchoalveolar lavage fluid, increased collagen content and fibrotic nodules. µCT-derived biomarkers enable longitudinal evaluation of early onset inflammation and non-resolving pulmonary fibrosis as well as lung volumes in a sensitive and non-invasive manner. This approach and model of non-resolving lung fibrosis provides quantitative assessment of disease progression and stabilization over weeks and months, essential towards evaluation of fibrotic disease burden and antifibrotic therapy evaluation in preclinical studies.

Lipid Mediators Regulate Pulmonary Fibrosis: Potential Mechanisms and Signaling Pathways

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease of unknown etiology characterized by distorted distal lung architecture, inflammation, and fibrosis. The molecular mechanisms involved in the pathophysiology of IPF are incompletely defined. Several lung cell types including alveolar epithelial cells, fibroblasts, monocyte-derived macrophages, and endothelial cells have been implicated in the development and progression of fibrosis. Regardless of the cell types involved, changes in gene expression, disrupted glycolysis, and mitochondrial oxidation, dysregulated protein folding, and altered phospholipid and sphingolipid metabolism result in activation of myofibroblast, deposition of extracellular matrix proteins, remodeling of lung architecture and fibrosis. Lipid mediators derived from phospholipids, sphingolipids, and polyunsaturated fatty acids play an important role in the pathogenesis of pulmonary fibrosis and have been described to exhibit pro- and anti-fibrotic effects in IPF and in preclinical animal models of lung fibrosis. This review describes the current understanding of the role and signaling pathways of prostanoids, lysophospholipids, and sphingolipids and their metabolizing enzymes in the development of lung fibrosis. Further, several of the lipid mediators and enzymes involved in their metabolism are therapeutic targets for drug development to treat IPF.

The mercapturomic profile of health and non-communicable diseases

The mercapturate pathway is a unique metabolic circuitry that detoxifies electrophiles upon adducts formation with glutathione. Since its discovery over a century ago, most of the knowledge on the mercapturate pathway has been provided from biomonitoring studies on environmental exposure to toxicants. However, the mercapturate pathway-related metabolites that is formed in humans—the mercapturomic profile—in health and disease is yet to be established. In this paper, we put forward the hypothesis that these metabolites are key pathophysiologic factors behind the onset and development of non-communicable chronic inflammatory diseases. This review goes from the evidence in the formation of endogenous metabolites undergoing the mercapturate pathway to the methodologies for their assessment and their association with cancer and respiratory, neurologic and cardiometabolic diseases.

Inflammasome-Independent Leukotriene B4 Production Drives Crystalline Silica-Induced Sterile Inflammation

Silicosis is a lung inflammatory disease caused by chronic exposure to crystalline silica (CS). Leukotriene B₄ (LTB₄) plays an important role in neutrophilic inflammation, which drives silicosis and promotes lung cancer. In this study, we examined the mechanisms involved in CS-induced inflammatory pathways. Phagocytosis of CS particles is essential for the production of LTB₄ and IL-1β in mouse macrophages, mast cells, and neutrophils. Phagosomes enclosing CS particles trigger the assembly of lipidosome in the cytoplasm, which is likely the primary source of CS-induced LTB₄ production. Activation of the JNK pathway is essential for both CS-induced LTB₄ and IL-1β production. Studies with bafilomycin-A1- and NLRP3-deficient mice revealed that LTB₄ synthesis in the lipidosome is independent of inflammasome activation. Small interfering RNA knockdown and confocal microscopy studies showed that GTPases Rab5c, Rab40c along with JNK1 are essential for lipidosome formation and LTB₄ production. BI-78D3, a JNK inhibitor, abrogated CS-induced neutrophilic inflammation in vivo in an air pouch model. These results highlight an inflammasome-independent and JNK activation-dependent lipidosome pathway as a regulator of LTB₄ synthesis and CS-induced sterile inflammation.

Elastase modifies bleomycin-induced pulmonary fibrosis in mice

Pulmonary fibrosis (PF) is characterized by excessive accumulation of collagen in the lungs. Emphysema is characterized by loss of the extracellular matrix (ECM) and alveolar enlargement. We studied the co-participation of elastase-induced mild emphysema in bleomycin-induced PF in mice by analyzing oxidative stress, inflammation and lung histology. C57BL/6 mice were divided into four groups: control; bleomycin (0.1U/mouse); elastase (using porcine pancreatic elastase (PPE)+bleomycin (3U/mouse 14 days before 0.1U/mouse of bleomycin; PPE+B); elastase (3U/mouse). Mice were humanely sacrificed 7, 14 and 21 days after treatment with bleomycin or vehicle. PF was observed 14 days and 21 days after bleomycin treatment but was observed after 14 days only in the PPE+B group. In the PPE+B group at 21 days, we observed many alveoli and alveolar septa with few PF areas. We also observed marked and progressive increases of collagens 7, 14 and 21 days after bleomycin treatment whereas, in the PPE+B group, collagen deposition was observed only at 14 days. There was a reduction in activities of the antioxidant enzymes superoxide dismutase (p<0.05), catalase (p<0.01) and glutathione peroxidase (p<0.01) parallel with an increase in nitrite (p<0.01) 21 days after bleomycin treatment compared with the control group. These endpoints were also reduced (p<0.05, p<0.05 and p<0.01, respectively) and increased (p<0.01) in the PPE+B group at 21 days compared with the control group. Interleukin (IL)-1β expression was upregulated (p<0.01) whereas IL-6 was downregulated (p<0.05) in the PPE+B group at 21 days compared with the control group. PF and emphysema did not coexist in our model of lung disease and despite increased levels of oxidative stress and inflammatory markers after combined stimulus (elastase and bleomycin) overall histology was improved to that of the nearest control group.

Reduced supply of monocyte-derived macrophages leads to a transition from nodular to diffuse lesions and tissue cell activation in silica-induced pulmonary fibrosis in mice

Pulmonary fibrosis (PF) is an intractable disorder with a poor prognosis. Lung macrophages have been reported to regulate both progression and remission of bleomycin-induced diffuse PF. However, it remains unclear how macrophages contribute to silica-induced progressive nodular PF and the associated tissue cell responses in vivo. We found that lack of monocyte-derived macrophages results in the formation of diffuse PF after silica instillation. We found that the proportion and the number of monocyte-derived macrophages were persistently higher in silica-induced progressive PF compared with bleomycin-induced PF. Surprisingly, in Ccr2(-/-) mice, in which monocyte-derived macrophage infiltration is impaired, silica administration induced diffuse PF with loose nodule formation and greater activation of tissue cells. In the diffuse lesions, the distribution of epithelial cells, distribution of myofibroblasts, and architecture of the basement membrane were disrupted. Consistent with the development of diffuse lesions, genes that were differentially expressed in CD45(-) tissue cells from the lung of wild-type and Ccr2(-/-) mice were highly enriched in human diffuse, progressive PF. In gene ontology network analyses, many of these genes were associated with tissue remodeling and included genes not previously associated with PF, such as Mmp14, Thbs2, and Fgfr4. Overall, these results indicate that monocyte-derived macrophages prevent transition from nodular to diffuse silica-induced PF, potentially by regulating tissue cell responses.

Qualitative rather than quantitative changes are hallmarks of fibroblasts in bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is characterized by accumulation of activated fibroblasts that produce excessive amounts of extracellular matrix components such as collagen type I. However, the dynamics and activation signatures of fibroblasts during fibrogenesis remain poorly understood, especially in vivo. We examined changes in lung tissue cell populations and in the phenotype of activated fibroblasts after acute injury in a model of bleomycin-induced pulmonary fibrosis. Despite clustering of collagen type I-producing fibroblasts in fibrotic regions, flow cytometry-based quantitative analysis of whole lungs revealed that the number of fibroblasts in the lungs remained constant. At the peak of inflammation, fibroblast proliferation and apoptosis were both increased, suggesting that the clustering was not merely a result of proliferation, but also of fibroblast migration from nearby alveolar walls. Parabiosis experiments demonstrated that fibroblasts were not supplied from the circulation. Comprehensive gene expression analysis of freshly isolated fibroblasts revealed a detailed activation signature associated with fibrogenesis, including changes in genes responsible for migration and extracellular matrix construction. The Spp1 gene, which encodes osteopontin, was highly up-regulated and was an identifying characteristic of activated fibroblasts present at the sites of remodeling. Osteopontin may serve as a useful marker of profibrotic fibroblasts. These results provide insights into the cellular and molecular mechanisms underlying pulmonary fibrosis and provide a foundation for development of specific antifibrotic therapies.

Bioreactivity of the crystalline silica polymorphs, quartz and cristobalite, and implications for occupational exposure limits (OELs)

Silica or silicon dioxides (SiO₂) are naturally occurring substances that comprise the vast majority of the earth's crust. Because of their prevalence and commercial applications, they have been widely studied for their potential to induce pulmonary fibrosis and other disorders. Historically, the focus in the workplace has been on the development of inflammation and fibrotic lung disease, the basis for promulgating workplace standards to protect workers. Crystalline silica (CS) polymorphs, predominantly quartz and cristobalite, are used in industry but are different in their mineralogy, chemistry, surface features, size dimensions and association with other elements naturally and during industrial applications. Epidemiologic, clinical and experimental studies in the literature historically have predominantly focused on quartz polymorphs. Thus, in this review, we summarize past scientific evaluations and recent peer-reviewed literature with an emphasis on cristobalite, in an attempt to determine whether quartz and cristobalite polymorphs differ in their health effects, toxicity and other properties that may dictate the need for various standards of protection in the workplace. In addition to current epidemiological and clinical reports, we review in vivo studies in rodents as well as cell culture studies that shed light on mechanisms intrinsic to the toxicity, altered cell responses and protective or defense mechanisms in response to these minerals. The medical and scientific literature indicates that the mechanisms of injury and potential causation of inflammation and fibrotic lung disease are similar for quartz and cristobalite. Our analysis of these data suggests similar occupational exposure limits (OELs) for these minerals in the workplace.

Pranlukast, a cysteinyl leukotriene type 1 receptor antagonist, attenuates the progression but not the onset of silica-induced pulmonary fibrosis in mice

BACKGROUND

Although cysteinyl leukotrienes (CysLTs) have been implicated in the etiology of acute inflammatory diseases, recent studies have suggested that they also directly stimulate fibroblasts. However, their precise role in the pathogenesis of pulmonary fibrosis is unclear.

METHODS

In this study, we evaluated the effect of both short- and long-term treatment with pranlukast, a CysLT type 1 (CysLT(1)) receptor antagonist, on silica-induced pulmonary fibrosis in mice, which is characterized by persistent progression of fibrosis in the chronic phase. Pranlukast (30 mg/kg/day) was administered orally to mice for 2 or 10 weeks after intratracheal silica instillation.

RESULTS

Pranlukast treatment for 10 weeks significantly attenuated the progression of pulmonary fibrosis, and decreased the content of CysLTs and LTB(4), which were markedly increased in the bronchoalveolar lavage fluid (BALF) and lung tissues of silica-instilled mice in the chronic phase. However, pranlukast treatment for 2 weeks neither affected the acute inflammatory response induced by silica instillation nor inhibited the onset of fibrosis. The expression of TGF-β1 and TNF-α was not affected by pranlukast treatment for either 2 or 10 weeks.

CONCLUSIONS

Pranlukast attenuates the progression of pulmonary fibrosis in the chronic phase but has no effect on the acute inflammatory response or on the onset of pulmonary fibrosis. The antifibrotic effect of pranlukast may be exhibited by antagonizing the direct profibrotic effect of CysLTs, without affecting the expression of other profibrotic cytokines such as TGF-β1 and TNF-α, and also by decreasing the production of CysLTs and LTB(4).

Macrophages and tissue injury: agents of defense or destruction?

The past several years have seen the accumulation of evidence demonstrating that tissue injury induced by diverse toxicants is due not only to their direct effects on target tissues but also indirectly to the actions of resident and infiltrating macrophages. These cells release an array of mediators with cytotoxic, pro- and anti-inflammatory, angiogenic, fibrogenic, and mitogenic activity, which function to fight infections, limit tissue injury, and promote wound healing. However, following exposure to toxicants, macrophages can become hyperresponsive, resulting in uncontrolled or dysregulated release of mediators that exacerbate acute tissue injury and/or promote the development of chronic diseases such as fibrosis and cancer. Evidence suggests that the diverse activity of macrophages is mediated by distinct subpopulations that develop in response to signals within their microenvironment. Understanding the precise roles of these different macrophage populations in the pathogenic response to toxicants is key to designing effective treatments for minimizing tissue damage and chronic disease and for facilitating wound repair.