The effect of a liver-X-receptor ligand on bleomycin induced pulmonary fibrosis in mice

Polyhexamethylene guanidine accelerates the macrophage foamy formation mediated pulmonary fibrosis

Polyhexamethylene guanidine (PHMG) is manufactured and applied extensively due to its superior disinfectant capabilities. However, the inhalatory exposure to PHMG aerosols is increasingly recognized as a potential instigator of pulmonary fibrosis, prompting an urgent call for elucidation of the underlying pathophysiological mechanisms. Within this context, alveolar macrophages play a pivotal role in the primary immune defense in the respiratory tract. Dysregulated lipid metabolism within alveolar macrophages leads to the accumulation of foam cells, a process that is intimately linked with the pathogenesis of pulmonary fibrosis. Therefore, this study examines PHMG's effects on alveolar macrophage foaminess and its underlying mechanisms. We conducted a 3-week inhalation exposure followed by a 3-week recovery period in C57BL/6 J mice using a whole-body exposure system equipped with a disinfection aerosol generator (WESDAG). The presence of lipid-laden alveolar macrophages and downregulation of pulmonary tissue lipid transport proteins ABCA1 and ABCG1 were observed in mice. In cell culture models involving lipid-loaded macrophages, we demonstrated that PHMG promotes foam cell formation by inhibiting lipid efflux in mouse alveolar macrophages. Furthermore, PHMG-induced foam cells were found to promote an increase in the release of TGF-β1, fibronectin deposition, and collagen remodeling. In vivo interventions were subsequently implemented on mice exposed to PHMG aerosols, aiming to restore macrophage lipid efflux function. Remarkably, this intervention demonstrated the potential to retard the progression of pulmonary fibrosis. In conclusion, this study underscores the pivotal role of macrophage foaming in the pathogenesis of PHMG disinfectants-induced pulmonary fibrosis. Moreover, it provides compelling evidence to suggest that the regulation of macrophage efflux function holds promise for mitigating the progression of pulmonary fibrosis, thereby offering novel insights into the mechanisms underlying inhaled PHMG disinfectants-induced pulmonary fibrosis.

Assessing the Effects of Nicotinamide Mononucleotide Supplementation on Pulmonary Inflammation in Male Mice Subchronically Exposed to Ambient Particulate Matter

BACKGROUND

Chronic lung injury and dysregulated cellular homeostasis in response to particulate matter (PM) exposure are closely associated with adverse health effects. However, an effective intervention for preventing the adverse health effects has not been developed.

OBJECTIVES

This study aimed to evaluate the protective effects of nicotinamide mononucleotide (NMN) supplementation on lung injury and elucidate the mechanism by which NMN improved immune function following subchronic PM exposure.

METHODS

Six-week-old male C57BL/6J mice were placed in a real-ambient PM exposure system or filtered air-equipped chambers (control) for 16 wk with or without NMN supplementation in drinking water (regarded as Con-H2O, Exp-H2O, Con-NMN and Exp-NMN groups, respectively) in Shijiazhuang City, China (n=20/group). The effects of NMN supplementation (500mg/kg) on PM-induced chronic pulmonary inflammation were assessed, and its mechanism was characterized using single-cell transcriptomic sequencing (scRNA-seq) analysis of whole lung cells.

RESULTS

The NMN-treated mice exhibited higher NAD+ levels in multiple tissues. Following 16-wk PM exposure, slightly less pulmonary inflammation and less collagen deposition were noted in mice with NMN supplementation in response to real-ambient PM exposure (Exp-NMN group) compared with the Exp-H2O group (all p<0.05). Mouse lung tissue isolated from the Exp-NMN group was characterized by fewer neutrophils, monocyte-derived cells, fibroblasts, and myeloid-derived suppressor cells induced by subchronic PM exposure as detected by scRNA-seq transcriptomic analysis. The improved immune functions were further characterized by interleukin-17 signaling pathway inhibition and lower secretion of profibrotic cytokines in the Exp-NMN group compared with the Exp-H2O group. In addition, reduced proportions of differentiated myofibroblasts and profibrotic interstitial macrophages were identified in the NMN-supplemented mice in response to PM exposure. Furthermore, less immune function suppression and altered differentiation of pathological cell phenotypes NMN was related to intracellular lipid metabolism activation.

DISCUSSION

Our novel findings suggest that NMN supplementation mitigated PM-induced lung injury by regulating immune functions and improving lipid metabolism in male mice, providing a putative intervention method for prevention of human health effects associated with PM exposure. https://doi.org/10.1289/EHP12259.

PPARγ/LXRα axis mediated phenotypic plasticity of lung fibroblasts in silica-induced experimental silicosis

Silicosis is a disease mainly caused by pulmonary interstitial fibrosis caused by long-term inhalation of dust with excessively high content of free SiO₂. Transdifferentiation of lung fibroblasts into myofibroblasts is an important cellular basis for silicosis, but the key transcription factors (TFs) involved in this process are still unclear. In order to explore the biological regulation of transcription factor PPARγ/LXRα in silica-induced pulmonary fibrosis, this study explored the molecular mechanism of PPARγ/LXRα involved in regulating transcription factors related to SiO₂-induced lung injury at the cellular level and in animal models. ChIP-qPCR detected that PPARγ directly regulated the transcriptional activity of the LXRα gene promoter, while the PPARγ agonist RSG increased the expression of LXRα. In addition, we demonstrated in the cell model that upregulation of LXRα can inhibit silica-mediated fibroblast transdifferentiation, accompanied by an increase in the expression of SREBF1, PLTP and ABCA1. The results of LXRα silencing experiment matched those of overexpression experiment. These studies explored the role of LXRα in plasticity and phenotypic transformation between lung fibroblasts and myofibroblasts. Therefore, inhibiting or reversing the transdifferentiation of lung fibroblasts to myofibroblasts by intervening PPARγ/LXRα may provide a new therapeutic target for the treatment of silicosis.

Reduction of fibrosis and immune suppressive cells in ErbB2-dependent tumorigenesis by an LXR agonist

One of the central challenges for cancer therapy is the identification of factors in the tumor microenvironment that increase tumor progression and prevent immune surveillance. One such element associated with breast cancer is stromal fibrosis, a histopathologic criterion for invasive cancer and poor survival. Fibrosis is caused by inflammatory factors and remodeling of the extracellular matrix that elicit an immune tolerant microenvironment. To address the role of fibrosis in tumorigenesis, we developed NeuT/ATTAC transgenic mice expressing a constitutively active NeuT/erbB2 transgene, and an inducible, fat-directed caspase-8 fusion protein, which upon activation results in selective and partial ablation of mammary fat and its replacement with fibrotic tissue. Induction of fibrosis in NeuT/ATTAC mice led to more rapid tumor development and an inflammatory and fibrotic stromal environment. In an effort to explore therapeutic options that could reduce fibrosis and immune tolerance, mice were treated with the oxysterol liver X receptor (LXR) pan agonist, N,N-dimethyl-3-β-hydroxy-cholenamide (DMHCA), an agent known to reduce fibrosis in non-malignant diseases. DMHCA reduced tumor progression, tumor multiplicity and fibrosis, and improved immune surveillance by reducing infiltrating myeloid-derived suppressor cells and increasing CD4 and CD8 effector T cells. These effects were associated with downregulation of an LXR-dependent gene network related to reduced breast cancer survival that included Spp1, S100a9, Anxa1, Mfge8 and Cd14. These findings suggest that the use of DMHCA may be a potentially effective approach to reduce desmoplasia and immune tolerance and increase the efficacy of cancer therapy.

Preterm Lung Exhibits Distinct Spatiotemporal Proteome Expression at Initiation of Lung Injury

The development of regional lung injury in the preterm lung is not well understood. This study aimed to characterize time-dependent and regionally specific injury patterns associated with early ventilation of the preterm lung using a mass spectrometry-based proteomic approach. Preterm lambs delivered at 124-127 days gestation received 15 or 90 minutes of mechanical ventilation (positive end-expiratory pressure = 8 cm H₂O, Vt = 6-8 ml/kg) and were compared with unventilated control lambs. At study completion, lung tissue was taken from standardized nondependent and dependent regions, and assessed for lung injury via histology, quantitative PCR, and proteomic analysis using Orbitrap-mass spectrometry. Ingenuity pathway analysis software was used to identify temporal and region-specific enrichments in pathways and functions. Apoptotic cell numbers were ninefold higher in nondependent lung at 15 and 90 minutes compared with controls, whereas proliferative cells were increased fourfold in the dependent lung at 90 minutes. The relative gene expression of lung injury markers was increased at 90 minutes in nondependent lung and unchanged in gravity-dependent lung. Within the proteome, the number of differentially expressed proteins was fourfold higher in the nondependent lung than the dependent lung. The number of differential proteins increased over time in both lung regions. A total of 95% of enriched canonical pathways and 94% of enriched cellular and molecular functions were identified only in nondependent lung tissue from the 90-minute ventilation group. In conclusion, complex injury pathways are initiated within the preterm lung after 15 minutes of ventilation and amplified by continuing ventilation. Injury development is region specific, with greater alterations within the proteome of nondependent lung.

Therapeutic molecular targets of SSc-ILD

Systemic sclerosis is a fibrosing chronic connective tissue disease of unknown etiology. A major hallmark of systemic sclerosis is the uncontrolled and persistent activation of fibroblasts, which release excessive amounts of extracellular matrix, lead to organ dysfunction, and cause high mobility and motility of patients. Systemic sclerosis–associated interstitial lung disease is one of the most common fibrotic organ manifestations in systemic sclerosis and a major cause of death. Treatment options for systemic sclerosis–associated interstitial lung disease and other fibrotic manifestations, however, remain very limited. Thus, there is a huge medical need for effective therapies that target tissue fibrosis, vascular alterations, inflammation, and autoimmune disease in systemic sclerosis–associated interstitial lung disease. In this review, we discuss data suggesting therapeutic ways to target different genes in distinct tissues/organs that contribute to the development of SSc.

Transcriptome network analysis identifies protective role of the LXR/SREBP-1c axis in murine pulmonary fibrosis

Pulmonary fibrosis (PF) is an intractable disorder with a poor prognosis. Although lung fibroblasts play a central role in PF, the key regulatory molecules involved in this process remain unknown. To address this issue, we performed a time-course transcriptome analysis on lung fibroblasts of bleomycin- and silica-treated murine lungs. We found gene modules whose expression kinetics were associated with the progression of PF and human idiopathic PF (IPF). Upstream analysis of a transcriptome network helped in identifying 55 hub transcription factors that were highly connected with PF-associated gene modules. Of these hubs, the expression of Srebf1 decreased in line with progression of PF and human IPF, suggesting its suppressive role in fibroblast activation. Consistently, adoptive transfer and genetic modification studies revealed that the hub transcription factor SREBP-1c suppressed PF-associated gene expression changes in lung fibroblasts and PF pathology in vivo. Moreover, therapeutic pharmacological activation of LXR, an SREBP-1c activator, suppressed the Srebf1-dependent activation of fibroblasts and progression of PF. Thus, SREBP-1c acts as a protective hub of lung fibroblast activation in PF. Collectively, the findings of the current study may prove to be valuable in the development of effective therapeutic strategies for PF.

Therapeutic pro-fibrogenic signaling pathways in fibroblasts

Myofibroblasts (MFs) play a critical role in the progression of chronic inflammatory and fibroproliferative diseases in different tissues/organs, whatever the etiology. Fibrosis is preceded and sustained by persistent injury and inflammatory response in a profibrogenic scenario involving mutual interactions, operated by several mediators and pathways, of MFs and related precursor cells with innate immunity cells and virtually any cell type in a defined tissue. These interactions, mediators and related signaling pathways are critical in initiating and perpetuating the differentiation of precursor cells into MFs that in different tissues share peculiar traits and phenotypic responses, including the ability to proliferate, produce ECM components, migrate and contribute to the modulation of inflammatory response and tissue angiogenesis. Literature studies related to liver, lung and kidney fibrosis have outlined a number of MF-related core regulatory fibrogenic signaling pathways conserved across these different organs and potentially targetable in order to develop effective antifibrotic therapeutic strategies.

The challenges and promise of targeting the Liver X Receptors for treatment of inflammatory disease

The Liver X Receptors (LXRs) are oxysterol-activated transcription factors that upregulate a suite of genes that together promote coordinated mobilization of excess cholesterol from cells and from the body. The LXRs, like other nuclear receptors, are anti-inflammatory, inhibiting signal-dependent induction of pro-inflammatory genes by nuclear factor-κB, activating protein-1, and other transcription factors. Synthetic LXR agonists have been shown to ameliorate atherosclerosis and a wide range of inflammatory disorders in preclinical animal models. Although this has suggested potential for application to human disease, systemic LXR activation is complicated by hepatic steatosis and hypertriglyceridemia, consequences of lipogenic gene induction in the liver by LXRα. The past several years have seen the development of multiple advanced LXR therapeutics aiming to avoid hepatic lipogenesis, including LXRβ-selective agonists, tissue-selective agonists, and transrepression-selective agonists. Although several synthetic LXR agonists have made it to phase I clinical trials, none have progressed due to unforeseen adverse reactions or undisclosed reasons. Nonetheless, several sophisticated pharmacologic strategies, including structure-guided drug design, cell-specific drug targeting, as well as non-systemic drug routes have been initiated and remain to be comprehensively explored. In addition, recent studies have identified potential utility for targeting the LXRs during therapy with other agents, such as glucocorticoids and rexinoids. Despite the pitfalls encountered to date in translation of LXR agonists to human disease, it appears likely that this accelerating field will ultimately yield effective and safe applications for LXR targeting in humans.

Bone marrow mesenchymal stem cells inhibited bleomycin-induced lung fibrosis

The present study was performed to evaluate the protective effect of bone marrow mesenchymal stem cells (BMSCs), TLR2-silencing BMSCs (BMSC^TLR2−/−), on bleomycin (BLM)-induced lung fibrosis and elucidate the critical role of TLR2 during the process.