Alveolar Macrophage ABCG1 Deficiency Promotes Pulmonary Granulomatous Inflammation

Biological Functions and Clinical Significance of the ABCG1 Transporter

ATP-binding cassette (ABC) transporters are a large family of proteins that transport various substances across cell membranes using energy from ATP hydrolysis. ATP-binding cassette sub-family G member 1 (ABCG1) is a member of the ABCG subfamily of transporters and performs many important functions, such as the export of cholesterol and some other lipids across the membranes of various cells. Cholesterol transport is the mechanism that links metabolism and the innate immune system. Due to its lipid transport function, ABCG1 may contribute to the prevention of atherosclerosis and is involved in the functioning of the lung, pancreas, and other organs and systems. However, the full clinical significance of ABCG1 is still unknown and is a promising area for future research.

Development of a Mouse Cardiac Sarcoidosis Model Using Carbon Nanotubes

Sarcoidosis, a granulomatous disorder of unknown etiology affecting multiple organs. It is often a benign disease but can have significant morbidity and mortality when the heart is involved (often presenting with clinical manifestations such as conduction irregularities and heart failure). This study addresses a critical gap in cardiac sarcoidosis (CS) research by developing a robust animal model. The absence of a reliable animal model for cardiac sarcoidosis is a significant obstacle in advancing understanding and treatment of this condition. The proposed model utilizes carbon nanotube injection and transverse aortic constriction as stressors. Intramyocardial injection of carbon nanotubes induces histiocytes typical of sarcoid granulomas in the heart but shows limited effects on fibrosis or cardiac function. Priming the immune system with transverse aortic constriction prior to intramyocardial injection of carbon nanotubes enhances cardiac fibrosis, diminishes cardiac function, and impairs cardiac conduction. This novel, easily executable model may serve as a valuable tool for disease profiling, biomarker identification, and therapeutic exploration.

Pulmonary Sarcoidosis: Experimental Models and Perspectives of Molecular Diagnostics Using Quantum Dots

Sarcoidosis is a complex inflammatory multisystem disease of unknown etiology that is characterised by epithelioid cell granulomatous lesions affecting various organs, mainly the lungs. In general, sarcoidosis is asymptomatic, but some cases result in severe complications and organ failure. So far, no accurate and validated modelling for clinical and pathohistological manifestations of sarcoidosis is suggested. Moreover, knowledge about disease-specific diagnostic markers for sarcoidosis is scarce. For instance, pulmonary granulomatosis is associated with the upregulated production of proinflammatory molecules: TNF-α, IL-6, CXCL1, CCL2, CCL18, CD163, serum angiotensin-converting enzyme (sACE), lysozyme, neopterin, and serum amyloid A (SAA). Quantum dots (QDs) are widely applied for molecular diagnostics of various diseases. QDs are semiconductor nanoparticles of a few nanometres in size, made from ZnS, CdS, ZnSe, etc., with unique physical and chemical properties that are useful for the labelling and detection in biological experiments. QDs can conjugate with various antibodies or oligonucleotides, allowing for high-sensitivity detection of various targets in organs and cells. Our review describes existing experimental models for sarcoidosis (in vitro, in vivo, and in silico), their advantages and restrictions, as well as the physical properties of quantum dots and their potential applications in the molecular diagnostics of sarcoidosis. The most promising experimental models include mice with TSC2 deletion and an in silico multiscale computational model of sarcoidosis (SarcoidSim), developed using transcriptomics and flow cytometry of human sarcoid biopsies. Both models are most efficient to test different candidate drugs for sarcoidosis.

First Characterization of the Transcriptome of Lung Fibroblasts of SSc Patients and Healthy Donors of African Ancestry

Systemic sclerosis (SSc) is a connective tissue disorder that results in fibrosis of the skin and visceral organs. SSc-associated pulmonary fibrosis (SSc-PF) is the leading cause of death amongst SSc patients. Racial disparity is noted in SSc as African Americans (AA) have a higher frequency and severity of disease than European Americans (EA). Using RNAseq, we determined differentially expressed genes (DEGs; q < 0.1, log2FC > |0.6|) in primary pulmonary fibroblasts from SSc lungs (SScL) and normal lungs (NL) of AA and EA patients to characterize the unique transcriptomic signatures of AA-NL and AA-SScL fibroblasts using systems-level analysis. We identified 69 DEGs in "AA-NL vs. EA-NL" and 384 DEGs in "AA-SScL vs. EA-SScL" analyses, and a comparison of disease mechanisms revealed that only 7.5% of DEGs were commonly deregulated in AA and EA patients. Surprisingly, we also identified an SSc-like signature in AA-NL fibroblasts. Our data highlight differences in disease mechanisms between AA and EA SScL fibroblasts and suggest that AA-NL fibroblasts are in a "pre-fibrosis" state, poised to respond to potential fibrotic triggers. The DEGs and pathways identified in our study provide a wealth of novel targets to better understand disease mechanisms leading to racial disparity in SSc-PF and develop more effective and personalized therapies.

Activation of Kruppel-like factor 6 by multi-walled carbon nanotubes in a diameter-dependent manner in THP-1 macrophages in vitro and bronchoalveolar lavage cells in vivo

MWCNTs activated KLF6-signaling pathways in THP-1 macrophages and bronchoalveolar lavage cells.

The Importance of the Plasma Membrane in Atherogenesis

Atherosclerotic cardiovascular diseases are an important medical problem due to their high prevalence, impact on quality of life and prognosis. The pathogenesis of atherosclerosis is an urgent medical and social problem, the solution of which may improve the quality of diagnosis and treatment of patients. Atherosclerosis is a complex chain of events, which proceeds over many years and in which many cells in the bloodstream and the vascular wall are involved. A growing body of evidence suggests that there are complex, closely linked molecular mechanisms that occur in the plasma membranes of cells involved in atherogenesis. Lipid transport, innate immune system receptor function, and hemodynamic regulation are linked to plasma membranes and their biophysical properties. A better understanding of these interrelationships will improve diagnostic quality and treatment efficacy.

HDL metabolism and functions impacting on cell cholesterol homeostasis are specifically altered in patients with abdominal aortic aneurysm

Background

The etiopathogenesis of abdominal aortic aneurysm (AAA) is still unclarified, but vascular inflammation and matrix metalloproteases activation have a recognized role in AAA development and progression. Circulating lipoproteins are involved in tissue inflammation and repair, particularly through the regulation of intracellular cholesterol, whose excess is associated to cell damage and proinflammatory activation. We analyzed lipoprotein metabolism and function in AAA and in control vasculopathic patients, to highlight possible non-atherosclerosis-related, specific abnormalities.

Methods

We measured fluorometrically serum esterified/total cholesterol ratio, as an index of lecithin-cholesterol acyltransferase (LCAT) activity, and cholesteryl ester transfer protein (CETP) activity in patients referred to vascular surgery either for AAA (n=30) or stenotic aortic/peripheral atherosclerosis (n=21) having similar burden of cardiovascular risk factors and disease. We measured high-density lipoprotein (HDL)-cholesterol efflux capacity (CEC), through the ATP-binding cassette G1 (ABCG1) and A1 (ABCA1) pathways and serum cell cholesterol loading capacity (CLC), by radioisotopic and fluorimetric methods, respectively.

Results

We found higher LCAT (+23%; p &lt; 0.0001) and CETP (+49%; p &lt; 0.0001) activity in AAA sera. HDL ABCG1-CEC was lower (−16%; p &lt; 0.001) and ABCA1-CEC was higher (+31.7%; p &lt; 0.0001) in AAA. Stratification suggests that smoking may partly contribute to these modifications. CEC and CETP activity correlated with CLC only in AAA.

Conclusions

We demonstrated that compared to patients with stenotic atherosclerosis, patients with AAA had altered HDL metabolism and functions involved in their anti-inflammatory and tissue repair activity, particularly through the ABCG1-related intracellular signaling. Clarifying the relevance of this mechanism for AAA evolution might help in developing new diagnostic parameters and therapeutic targets for the early management of this condition.

Myeloid ABCG1 Deficiency Enhances Apoptosis and Initiates Efferocytosis in Bronchoalveolar Lavage Cells of Murine Multi-Walled Carbon Nanotube-Induced Granuloma Model

The use of carbon nanotubes has increased in the past few decades. Carbon nanotubes are implicated in the pathogenesis of pulmonary sarcoidosis, a chronic granulomatous inflammatory condition. We developed a murine model of chronic granulomatous inflammation using multiwall carbon nanotubes (MWCNT) to investigate mechanisms of granuloma formation. Using this model, we demonstrated that myeloid deficiency of ATP-binding cassette (ABC) cholesterol transporter (ABCG1) promotes granuloma formation and fibrosis with MWCNT instillation; however, the mechanism remains unclear. Our previous studies showed that MWCNT induced apoptosis in bronchoalveolar lavage (BAL) cells of wild-type (C57BL/6) mice. Given that continual apoptosis causes persistent severe lung inflammation, we hypothesized that ABCG1 deficiency would increase MWCNT-induced apoptosis thereby promoting granulomatous inflammation and fibrosis. To test our hypothesis, we utilized myeloid-specific ABCG1 knockout (ABCG1 KO) mice. Our results demonstrate that MWCNT instillation enhances pulmonary fibrosis in ABCG1 KO mice compared to wild-type controls. Enhanced fibrosis is indicated by increased trichrome staining and transforming growth factor-beta (TGF-β) expression in lungs, together with an increased expression of TGF-β related signaling molecules, interleukin-13 (IL-13) and Smad-3. MWCNT induced more apoptosis in BAL cells of ABCG1 KO mice. Initiation of apoptosis is most likely mediated by the extrinsic pathway since caspase 8 activity and Fas expression are significantly higher in MWCNT instilled ABCG1 KO mice compared to the wild type. In addition, TUNEL staining shows that ABCG1 KO mice instilled with MWCNT have a higher percentage of TUNEL positive BAL cells and more efferocytosis than the WT control. Furthermore, BAL cells of ABCG1 KO mice instilled with MWCNT exhibit an increase in efferocytosis markers, milk fat globule-EGF factor 8 (MFG-E8) and integrin β3. Therefore, our observations suggest that ABCG1 deficiency promotes pulmonary fibrosis by MWCNT, and this effect may be due to an increase in apoptosis and efferocytosis in BAL cells.

Pulmonary delivery of the broad-spectrum matrix metalloproteinase inhibitor marimastat diminishes multiwalled carbon nanotube-induced circulating bioactivity without reducing pulmonary inflammation

BACKGROUND

Multiwalled carbon nanotubes (MWCNT) are an increasingly utilized engineered nanomaterial that pose the potential for significant risk of exposure-related health outcomes. The mechanism(s) underlying MWCNT-induced toxicity to extrapulmonary sites are still being defined. MWCNT-induced serum-borne bioactivity appears to dysregulate systemic endothelial cell function. The serum compositional changes after MWCNT exposure have been identified as a surge of fragmented endogenous peptides, likely derived from matrix metalloproteinase (MMP) activity. In the present study, we utilize a broad-spectrum MMP inhibitor, Marimastat, along with a previously described oropharyngeal aspiration model of MWCNT administration to investigate the role of MMPs in MWCNT-derived serum peptide generation and endothelial bioactivity.

RESULTS

C57BL/6 mice were treated with Marimastat or vehicle by oropharyngeal aspiration 1 h prior to MWCNT treatment. Pulmonary neutrophil infiltration and total bronchoalveolar lavage fluid protein increased independent of MMP blockade. The lung cytokine profile similarly increased following MWCNT exposure for major inflammatory markers (IL-1β, IL-6, and TNF-α), with minimal impact from MMP inhibition. However, serum peptidomic analysis revealed differential peptide compositional profiles, with MMP blockade abrogating MWCNT-derived serum peptide fragments. The serum, in turn, exhibited differential potency in terms of inflammatory bioactivity when incubated with primary murine cerebrovascular endothelial cells. Serum from MWCNT-treated mice led to inflammatory responses in endothelial cells that were significantly blunted with serum from Marimastat-treated mice.

CONCLUSIONS

Thus, MWCNT exposure induced pulmonary inflammation that was largely independent of MMP activity but generated circulating bioactive peptides through predominantly MMP-dependent pathways. This MWCNT-induced lung-derived bioactivity caused pathological consequences of endothelial inflammation and barrier disruption.

Sex Differences in Pulmonary Eicosanoids and Specialized Pro-Resolving Mediators in Response to Ozone Exposure

Ozone (O3) is a criteria air pollutant known to increase the morbidity and mortality of cardiopulmonary diseases. This occurs through a pulmonary inflammatory response characterized by increased recruitment of immune cells into the airspace, pro-inflammatory cytokines, and pro-inflammatory lipid mediators. Recent evidence has demonstrated sex-dependent differences in the O3-induced pulmonary inflammatory response. However, it is unknown if this dimorphic response is evident in pulmonary lipid mediator metabolism. We hypothesized that there are sex-dependent differences in lipid mediator production following acute O3 exposure. Male and female C57BL/6J mice were exposed to 1 part per million O3 for 3 hours and were necropsied at 6 or 24 hours following exposure. Lung lavage was collected for cell differential and total protein analysis, and lung tissue was collected for mRNA analysis, metabololipidomics, and immunohistochemistry. Compared to males, O3-exposed female mice had increases in airspace neutrophilia, neutrophil chemokine mRNA, pro-inflammatory eicosanoids such as prostaglandin E2, and specialized pro-resolving mediators (SPMs) such as resolvin D5 in lung tissue. Likewise, precursor fatty acids (arachidonic and docosahexaenoic acid; DHA) were increased in female lung tissue following O3 exposure compared to males. Experiments with ovariectomized females revealed that loss of ovarian hormones exacerbates pulmonary inflammation and injury. However, eicosanoid and SPM production were not altered by ovariectomy despite depleted pulmonary DHA concentrations. Taken together, these data indicate that O3 drives an increased pulmonary inflammatory and bioactive lipid mediator response in females. Furthermore, ovariectomy increases susceptibility to O3-induced pulmonary inflammation and injury, as well as decreases pulmonary DHA concentrations.

Experimental models of sarcoidosis: where are we now?

PURPOSE OF REVIEW

Sarcoidosis remains a mysterious disease that presents many challenges both in pathogenetic understanding and in the management of patients. This review presents experimental models for sarcoidosis developed since 2016 and discusses their strengths and weaknesses and how they have contributed to the understanding and therapeutic approaches in this disease. In addition, future directions are proposed to overcome the limitations of current models.

RECENT FINDINGS

New cellular models using infectious antigen as trigger, and transgenic models in mice have recently been developed to study signaling pathways potentially implicated in the pathogenesis of sarcoidosis.

SUMMARY

No model fully reproduces sarcoidosis, but most of them generate data supporting key concepts and some open up therapeutic perspectives, like mTOR inhibition or IL-1β blocking. However, there are still many limitations to these models, largely related to the complexity of sarcoidosis, which might be overcome with new technologies, such as mathematical modeling.

Studies in a Murine Granuloma Model of Instilled Carbon Nanotubes: Relevance to Sarcoidosis

Poorly soluble environmental antigens, including carbon pollutants, are thought to play a role in the incidence of human sarcoidosis, a chronic inflammatory granulomatous disease of unknown causation. Currently, engineered carbon products such as multiwall carbon nanotubes (MWCNT) are manufactured commercially and have been shown to elicit acute and chronic inflammatory responses in experimental animals, including the production of granulomas or fibrosis. Several years ago, we hypothesized that constructing an experimental model of chronic granulomatosis resembling that associated with sarcoidosis might be achieved by oropharyngeal instillation of MWCNT into mice. This review summarizes the results of our efforts to define mechanisms of granuloma formation and identify potential therapeutic targets for sarcoidosis. Evidence is presented linking findings from the murine MWCNT granuloma model to sarcoidosis pathophysiology. As our goal was to determine what pulmonary inflammatory pathways might be involved, we utilized mice of knock-out (KO) backgrounds which corresponded to deficiencies noted in sarcoidosis patients. A primary example of this approach was to study mice with a myeloid-specific knock-out of the lipid-regulated transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ) which is strikingly depressed in sarcoidosis. Among the major findings associated with PPARγ KO mice compared to wild-type were: (1) exacerbation of granulomatous and fibrotic histopathology in response to MWCNT; (2) elevation of inflammatory mediators; and (3) pulmonary retention of a potentially antigenic ESAT-6 peptide co-instilled with MWCNT. In line with these data, we also observed that activation of PPARγ in wild-type mice by the PPARγ-specific ligand, rosiglitazone, significantly reduced both pulmonary granuloma and inflammatory mediator production. Similarly, recognition of a deficiency of ATP-binding cassette (ABC) lipid transporter ABCG1 in sarcoidosis led us to study MWCNT instillation in myeloid-specific ABCG1 KO mice. As anticipated, ABCG1 deficiency was associated with larger granulomas and increased levels of inflammatory mediators. Finally, a transcriptional survey of alveolar macrophages from MWCNT-instilled wild-type mice and human sarcoidosis patients revealed several common themes. One of the most prominent mediators identified in both human and mouse transcriptomic analyses was MMP12. Studies with MMP12 KO mice revealed similar acute reactions to those in wild-type but at chronic time points where wild-type maintained granulomatous disease, resolution occurred with MMP12 KO mice suggesting MMP12 is necessary for granuloma progression. In conclusion, these studies suggest that the MWCNT granuloma model has relevance to human sarcoidosis study, particularly with respect to immune-specific pathways.

ABCG1 Attenuates Oxidative Stress Induced by H2O2 through the Inhibition of NADPH Oxidase and the Upregulation of Nrf2-Mediated Antioxidant Defense in Endothelial Cells

Summary. Oxidative stress is an important factor that is related to endothelial dysfunction. ATP-binding cassette transporter G1 (ABCG1), a regulator of intracellular cholesterol efflux, has been found to prevent endothelial activation in vessel walls. To explore the role of ABCG1 in oxidative stress production in endothelial cells, HUAECs were exposed to H₂O₂ and transfected with the specific ABCG1 siRNA or ABCG1 overexpression plasmid. The results showed that overexpression of ABCG1 by ABCG1 plasmid or liver X receptor (LXR) agonist T0901317 treatment inhibited ROS production and MDA content induced by H₂O₂ in HUAECs. Furthermore, ABCG1 upregulation blunted the activity of prooxidant NADPH oxidase and the expression of Nox4, one of the NADPH oxidase subunits. Moreover, the increased migration of Nrf2 from the cytoplasm to the nucleus and antioxidant HO-1 expression were detected in HUAECs with upregulation of ABCG1. Conversely, ABCG1 downregulation by ABCG1 siRNA increased NADPH oxidase activity and Nox4 expression and abrogated the increase at Nrf2 nuclear protein levels. In addition, intracellular cholesterol load interfered with the balance between NADPH oxidase activity and HO-1 expression. It was suggested that ABCG1 attenuated oxidative stress induced by H₂O₂ in endothelial cells, which might be involved in the balance between decreased NADPH oxidase activity and increased Nrf2/OH-1 antioxidant defense signaling via its regulation for intracellular cholesterol accumulation.

Mammalian ABCG-transporters, sterols and lipids: To bind perchance to transport?

Members of the ATP binding cassette (ABC) transporter family perform a critical function in maintaining lipid homeostasis in cells as well as the transport of drugs. In this review, we provide an update on the ABCG-transporter subfamily member proteins, which include the homodimers ABCG1, ABCG2 and ABCG4 as well as the heterodimeric complex formed between ABCG5 and ABCG8. This review focusses on progress made in this field of research with respect to their function in health and disease and the recognised transporter substrates. We also provide an update on post-translational regulation, including by transporter substrates, and well as the involvement of microRNA as regulators of transporter expression and activity. In addition, we describe progress made in identifying structural elements that have been recognised as important for transport activity. We furthermore discuss the role of lipids such as cholesterol on the transport function of ABCG2, traditionally thought of as a drug transporter, and provide a model of potential cholesterol binding sites for ABCG2.

Matrix Metalloproteinase-12 Is Required for Granuloma Progression

Background

Sarcoidosis is a chronic inflammatory disease of unknown cause characterized by granuloma formation. Mechanisms for chronic persistence of granulomas are unknown. Matrix Metalloproteinase-12 (MMP12) degrades extracellular matrix elastin and enables infiltration of immune cells responsible for inflammation and granuloma formation. Previous studies report increased MMP12 in sarcoidosis patients and association between MMP12 expression and disease severity. We also observed elevated MMP12 in our multiwall carbon nanotube (MWCNT) murine model of granulomatous inflammation. Here we hypothesized that MMP12 is important to acute and late phases of granuloma pathogenesis. To test this hypothesis, we analyzed granulomatous and inflammatory responses of Mmp12 knock-out (KO) mice at 10 (acute) and 60 days (late) after MWCNT instillation.

Methods

C57BL/6 (wildtype) and Mmp12 KO mice underwent oropharyngeal instillation of MWCNT. Lungs were harvested at 3, 10, 20, and 60 days post instillation for evaluation of MMP12 expression and granulomatous changes. Bronchoalveolar lavage (BAL) cells were analyzed 60 days after MWCNT instillation for expression of mediators thought to play a role in sarcoid granulomatosis: peroxisome proliferator-activated receptor-gamma (PPARγ), interferon-gamma (IFN-γ), and CCL2 (MCP-1).

Results

Pulmonary granuloma appearance at 10 days after MWCNT instillation showed no differences between wildtype and Mmp12 KO mice. In contrast, by 60 days after MWCNT instillation, Mmp12 KO mice revealed markedly attenuated granuloma formation together with elevated PPARγ and reduced IFNγ expression in BAL cells compared to wildtype. Unexpectedly, Mmp12 KO mice further demonstrated increased alveolar macrophages with increased CCL2 at 60 days.

Conclusions

The striking reduction of granuloma formation at day 60 in Mmp12 KO mice suggests that MMP12 is required to maintain chronic granuloma pathophysiology. The increased PPARγ and decreased IFNγ findings suggest that these mediators also may be involved since previous studies have shown that PPARγ suppresses IFNγ and PPARγ deficiency amplifies granuloma formation. Interestingly, a role of MMP12 in granuloma resolution is also suggested by increases in both macrophage influx and CCL2. Overall, our results strongly implicate MMP12 as a key factor in granuloma persistence and as a possible therapeutic target in chronic pulmonary sarcoidosis.