TREM2 Insufficiency Protects against Pulmonary Fibrosis by Inhibiting M2 Macrophage Polarization

TREM2 as a regulator of obesity-induced cardiac remodeling: mechanisms and therapeutic insights

Obesity and type 2 diabetes mellitus (T2DM) are global health challenges that significantly increase the risk of cardiovascular diseases (CVD). Advances in immunometabolism have identified triggering receptor expressed on myeloid cells 2 (TREM2) as a key regulator of macrophage function, lipid metabolism, and inflammation resolution. Although extensively studied in neurodegenerative diseases, TREM2's role in metabolic disorders and cardiovascular health is an emerging area of research. This review explores TREM2's molecular structure and functions, emphasizing its contributions to immunometabolic regulation in obesity and T2DM. Evidence from preclinical models demonstrates that TREM2 modulates macrophage-driven inflammatory responses, lipid clearance, plaque stability, fibrosis, and myocardial remodeling. Translational findings suggest that TREM2 expression correlates with cardiometabolic outcomes, underscoring its potential as a therapeutic target. Key knowledge gaps include TREM2's temporal dynamics during disease progression, sex-specific effects, and interactions with recruited or resident macrophage activation in obesity and T2DM. Integrating mechanistic and translational insights is critical to harness TREM2's immunoregulatory potential for improving CVD outcomes in metabolic disorders.

CH25H/25-HC promotes pulmonary fibrosis via AMPK/STAT6 pathway-dependent M2 macrophage polarization in COPD

OBJECTIVE

Chronic obstructive pulmonary disease (COPD) is intricately linked to pulmonary fibrosis, yet the underlying mechanisms remain unclear. This study investigates whether CH25H/25-hydroxycholesterol (25-HC) promotes pulmonary fibrosis in COPD by modulating AMPK/STAT6-dependent M2 macrophage polarization.

METHODS

Using GEO datasets and a cigarette smoke-induced COPD mouse model, we analyzed CH25H expression and fibrotic pathology. CH25H was silenced via adeno-associated virus (AAV)-delivered shRNA. Histopathology, flow cytometry, qPCR, and Western blotting assessed fibrosis, macrophage polarization (M1/M2), and AMPK/STAT6 pathway activity. Bone marrow-derived macrophages (BMDMs) were employed to validate polarization mechanisms. The role of the AMPK/STAT6 pathway was investigated using the specific activator.

RESULTS

Analysis of the GEO database and experimental verification showed significantly increased CH25H expression in both lung tissues and macrophages from COPD mice. CH25H knockdown alleviated alveolar damage, airway remodeling, and pulmonary fibrosis in COPD mice, evidenced by reduced expression of fibrosis-related proteins, improved lung function, and attenuated inflammatory responses. Moreover, CH25H knockdown inhibited M2 macrophage polarization and decreased the proportion of M2-type macrophages. Importantly, decreased levels of 25-HC following CH25H knockdown were asso ciated with suppressed activation of the AMPK/STAT6 pathway. Rescue experiments demonstrated that the protective effects of CH25H knockdown could be reversed by adding the AMPKα activator GSK621.

CONCLUSION

Our findings demonstrate that CH25H/25-HC exacerbates COPD-associated pulmonary fibrosis by promoting AMPK/STAT6-dependent M2 macrophage polarization. Targeting CH25H may represent a novel therapeutic strategy for mitigating fibrosis in COPD.

m6A reader IGF2BP1 facilitates macrophage glycolytic metabolism and fibrotic phenotype by stabilizing THBS1 mRNA to promote pulmonary fibrosis

N6-methyladenosine (m6A) modification, a dynamically reversible epigenetic mechanism, is implicated in pulmonary fibrosis (PF) progression. The function and molecular mechanisms of m6A reader, insulin-like growth factor 2 mRNA binding protein 1 (IGF2BP1) in PF remain elusive. This study investigates the mechanistic contributions of IGF2BP1 to PF development. We found IGF2BP1 was overexpressed in macrophages of PF mice. IGF2BP1 knockdown markedly attenuated bleomycin (BLM)-induced lung pathology, as evidenced by reduced inflammatory cell infiltration, fibroblast accumulation, Ashcroft fibrosis scores, and hydroxyproline deposition. Furthermore, IGF2BP1 knockdown downregulated PF-associated markers in lung tissues and embryonic lung fibroblasts (ELFs), including TGF-β1, α-SMA, Collagen-I/III, Arg1, CCL18, Ym1, CD163, IL-6, IL-1β, and TIMP1, while decreasing the CD68+/CD163+ macrophage proportion. Mechanistic studies revealed that IGF2BP1 bound to and stabilized thrombospondin-1 (THBS1) in an m6A-dependent manner. THBS1 overexpression rescued the suppression of macrophage M2 polarization caused by IGF2BP1 knockdown. Additionally, THBS1 overexpression counteracted IGF2BP1 knockdown-mediated inhibition of glycolysis, restoring HK2, LDHA, and PKM2 expression, lactate/glucose metabolism, and ATP production. Intriguingly, THBS1 physically interacted with toll-like receptor 4 (TLR4), and TLR4 overexpression reversed the inhibitory effect of THBS1 knockdown on macrophage M2 polarization and glycolytic reprogramming. Collectively, our findings demonstrate that IGF2BP1 drives PF progression by stabilizing THBS1 mRNA via m6A modification, thereby promoting TLR4-mediated macrophage M2 polarization and glycolytic activation. This study unveils a novel IGF2BP1/THBS1/TLR4 regulatory axis in PF pathogenesis, offering potential therapeutic targets.

TREM2+ macrophages: a key role in disease development

Triggering receptors expressed on myeloid cells 2 (TREM2), an immune receptor expressed on myeloid cells, has garnered considerable attention in recent years due to its role in unique signaling pathways and diverse biological functions, including phagocytosis, lipid metabolism, cell survival, and inflammatory responses. Although TREM2 is expressed in various cell types, such as macrophages, dendritic cells (DCs), osteoclasts, and others, where it exhibits context-dependent functional characteristics, it is mainly expressed in macrophages. Notably, TREM2 is implicated in the development and progression of multiple diseases, playing dual and often opposing roles in noncancerous diseases and cancers. This review aims to highlight the pivotal role of TREM2 in macrophages and immune-related diseases, elucidate its underlying mechanisms of action, explore its potential as a clinical diagnostic and prognostic marker, and propose therapeutic strategies targeting TREM2 based on current clinical trial data, providing comprehensive guidance and references for clinical practice.

Soluble epoxide hydrolase inhibition impairs triggering receptor expressed on myeloid cells-1 in periodontal tissue

AIMS

Periodontitis is a prevalent inflammatory disorder affecting the oral cavity, driven by dysbiotic oral biofilm and host immune response interactions. While the major clinical focus of periodontitis treatment is currently controlling oral biofilm, understanding the immune response is crucial to prevent disease progression. Soluble epoxide hydrolase (sEH) inhibition has shown promise in preventing alveolar bone resorption. Triggering receptors expressed on myeloid cells (TREMs) play pivotal roles in regulating inflammation and bone homeostasis, and dysregulation of TREM signaling is implicated in periodontitis. Here, we investigated the impact of sEH inhibition on TREM 1 and 2 expression, associated with inflammatory cytokines, and histologically assessed the inflammatory infiltrate in periodontal tissue.

METHODS

The experimental periodontitis model was induced by placing a ligature around the upper second molar. For 14 days, animals were treated daily with a sEH inhibitor (TPPU) or vehicle. The alveolar bone loss was examined using a methylene blue stain. Gingival tissues were used to measure the mRNA expression of TREM-1, TREM-2, IKKβ, NF-κB, IL-1β, IL-6, IL-8, and TNF-α by RT-qPCR. Another set of experiments was performed to determine the histological inflammatory scores.

RESULTS

In a ligature-induced periodontitis model, sEH inhibition prevented alveolar bone loss and reduced TREM1 expression, albeit with a slight elevation compared to the disease-free group. In contrast, TREM2 expression remained elevated, suggesting sustained immunomodulation favoring resolution. The inhibition of sEH reduced the expression of NF-κB, IL-1β, and TNF-α, while no differences were found in the expression of IL-6, IL-8, and IKKβ. In histological analysis, sEH inhibition reduced the inflammatory leukocyte infiltrate in periodontal tissues close to the ligature.

CONCLUSION

These findings underscore the potential of sEH inhibition to modulate periodontal inflammation by regulating TREM-1 alongside decreased IL-1β and TNF-α expression, highlighting a promising therapeutic approach for periodontitis management.

ITIH5-mediated fibroblast/macrophage crosstalk exacerbates cardiac remodelling after myocardial infarction

BACKGROUND

Myocardial infarction (MI) and subsequent ischaemic cardiomyopathy (ICM) are the primary causes of heart failure. Inter-α trypsin inhibitor heavy chain 5 (ITIH5) is an extracellular matrix (ECM) protein and has been identified as a myocardial marker of ICM. However, its diagnostic value in patients with ICM and its function and molecular mechanism in regulating cardiac repair and remodelling after MI remain unknown.

METHODS

Three microarray datasets including 117 ICM and 152 non-failing (NF) myocardial tissue samples were merged and analysed. Peripheral blood and clinical information were collected from 53 patients with ICM and 40 NF controls. The effects of ITIH5 on cellular interactions and cardiac remodelling was studied using ITIH5 RNAi adeno-associated virus and mouse MI model in vivo and in fibroblast-macrophage co-culture model in vitro.

RESULTS

ITIH5 was upregulated in the myocardial tissue and peripheral blood of patients with ICM and could be an independent risk factor for ICM. Experiments in mice suggested that ITIH5 promotes cardiac fibrotic remodelling at all phases after MI. Downregulation of ITIH5 increased the risk of death within 7 d after MI but inhibited ventricular remodelling and improved cardiac function on the long-term. ITIH5 promotes the primary cardiac fibroblasts (CFs) proliferation, migration, and improves survival rather than activiation. Morover, ITIH5 directly promotes macrophage tissue infiltration, maturation, and profibrotic phenotype transformation, thereby promoting fibrotic remodelling. By using fibroblast-macrophage co-culture model, we demonstrated ITIH5 enhanced the fibroblast/macrophage crosstalk manifest as macrophage profibrotic phenotype transformation and CFs activation, mainly by enhancing the hyaluronan stability, the ability of ITIH5 to bind macrophage CD44 receptors and the downstream activation of the signal transduction and activator of transcription 3 pathway in macrophages.

CONCLUSIONS

ITIH5 could be used as a diagnostic marker for ICM. Moreover, ITIH5 expression was upregulated after MI, which accelerated ECM-fibroblast-macrophage interaction, thereby promoting macrophage profibrotic phenotype transformation, CFs activation, and cardiac fibrotic remodelling.

PFKFB3 decreases α-ketoglutarate production while partial PFKFB3 knockdown in macrophages ameliorates arthritis in tumor necrosis factor-transgenic mice

OBJECTIVE

Aberrant 6-phosphofructo-2kinase/fructose-2,6-bisphoshatase 3 (PFKFB3) expression is tightly correlated with multiple steps of tumorigenesis; however, the pathological significance of PFKFB3 in macrophages in patients with rheumatoid arthritis (RA) remains obscure. In this study, we examined whether PFKFB3 modulates macrophage activation and promotes RA development.

METHOD

Peripheral blood mononuclear cells (PBMCs) from patients with RA, THP-1 cells, and bone marrow-derived macrophages from conditional PFKFB3-knockout mice were used to investigate the mechanism underlying PFKFB3-induced macrophage regulation of RA.

RESULT

We demonstrated that patients with RA have higher PFKFB3 levels than healthy volunteers. PFKFB3 silencing suppressed M1 macrophage polarization and downregulated IL-1β, CD80, IFIT1, CCL8, and CXCL10 in macrophages of patients with RA. PFKFB3 overexpression markedly upregulated IRF5, HIF1α, IL-1β, CD80, IFI27, IFI44, IFIT1, IFIT3, CCL2, CCL8, CXCL10, CXCL11, and MMP13 in phorbol 12-myristate 13-acetate-induced THP-1 cells, although these changes were partially reversed by PFK15, an inhibitor of PFKFB3 enzyme activity. Co-immunoprecipitation assays revealed that PFKFB3 interacted with GLUD1 and decreased glutamate dehydrogenase (GDH) activity and α-ketoglutarate production. PFKFB3, TNFα, IL-6, IFNγ, CXCL9, CXCL10, CXCL11, MMP13, and MMP19 were downregulated in bone marrow-derived macrophages of conditional PFKFB3-knockout mice relative to those of wild-type mice. Partial PFKFB3 knockdown in macrophages ameliorated the clinical signs of arthritis and bone destruction, inhibited proinflammatory factor expression, and promoted GDH activity and α-ketoglutarate production in tumor necrosis factor-transgenic mice. Single-cell sequencing revealed that macrophages were the most abundant cells in the ankles of arthritic mice, and partial PFKFB3 knockdown promoted M2-like polarization and was correlated with TREM2, SPP1, APOE, and C1Q expression.

CONCLUSION

PFKFB3 is upregulated in macrophages in patients with RA. PFKFB3 aggravates arthritis by modulating macrophage activity, which may be related to decreased α-ketoglutarate production.

The role of natural products targeting macrophage polarization in sepsis-induced lung injury

Sepsis-induced acute lung injury (SALI) is characterized by a dysregulated inflammatory and immune response. As a key component of the innate immune system, macrophages play a vital role in SALI, in which a macrophage phenotype imbalance caused by an increase in M1 macrophages or a decrease in M2 macrophages is common. Despite significant advances in SALI research, effective drug therapies are still lacking. Therefore, the development of new treatments for SALI is urgently needed. An increasing number of studies suggest that natural products (NPs) can alleviate SALI by modulating macrophage polarization through various targets and pathways. This review examines the regulatory mechanisms of macrophage polarization and their involvement in the progression of SALI. It highlights how NPs mitigate macrophage imbalances to alleviate SALI, focusing on key signaling pathways such as PI3K/AKT, TLR4/NF-κB, JAK/STAT, IRF, HIF, NRF2, HMGB1, TREM2, PKM2, and exosome-mediated signaling. NPs influencing macrophage polarization are classified into five groups: terpenoids, polyphenols, alkaloids, flavonoids, and others. This work provides valuable insights into the therapeutic potential of NPs in targeting macrophage polarization to treat SALI.

Examining the Impact of Microglia on Ischemic Stroke With an Emphasis on the Metabolism of Immune Cells

BACKGROUND

Ischemic stroke, a major cause of disability and the second leading cause of death, poses a significant public health challenge. Post-stroke inflammation can harm the blood-brain barrier and worsen neurological deficits, which are key factors in neuronal damage in patients with ischemic stroke. Microglia are crucial in the central nervous system, involved in inflammation, neuronal damage, and repair after cerebral ischemia. While cellular immune metabolism has been widely studied, its role in ischamic stroke remains unclear.

AIM

This review aims to examine how inflammation affects the phenotypic characteristics of immune cells after ischemic stroke and to explore the effects of the immune metabolic microenvironment on the phenotypic profiles and functions of microglia in ischemic stroke.

METHOD

The review refers to the available literature in PubMed, searching for critical terms related to Ischemic stroke, neuroinflammation, microglia, and immunometabolism.

RESULT

In this review, we found that during stroke progression, microglia can dynamically switch between pro-inflammatory and anti-inflammatory phenotypes. Microglial glycometabolism includes oxidative phosphorylation and glycolysis, and lipid metabolism involves lipid synthesis and breakdown. Modulating the production of inflammatory mediator precursors can induce an anti-inflammatory phenotype in microglia.

CONCLUSION

Thus, studying microglial metabolic pathways and their products may offer new insights for ischemic stroke treatment.

Genetic variation reveals the therapeutic potential of BRSK2 in idiopathic pulmonary fibrosis

BACKGROUND

Current research underscores the need to better understand the pathogenic mechanisms and treatment strategies for idiopathic pulmonary fibrosis (IPF). This study aimed to identify key targets involved in the progression of IPF.

METHODS

We employed Mendelian randomization (MR) with three genome-wide association studies and four quantitative trait loci datasets to identify key driver genes for IPF. Prioritized targets were evaluated for respiratory insufficiency and transplant-free survival. The therapeutic efficacy of the core gene was validated in cellular and animal models. Additionally, we conducted a comprehensive evaluation of therapeutic value, pathogenic mechanisms, and safety through phenome-wide association study (PheWAS), mediation analysis, transcriptomic analyses, shared causal variant exploration, DNA methylation MR, and protein interactions.

RESULTS

Multiple MR results revealed that BRSK2 has a significant pathogenic impact on IPF at both transcriptional and translational levels, with a lung tissue-specific association (OR = 1.596; CI, 1.300-1.961; Pval = 8.290 × 10 - 6). BRSK2 was associated with IPF progression driven by high-risk factors, with mediation effects ranging from 34.452 to 69.665%. Elevated BRSK2 expression in peripheral blood mononuclear cells correlated with reduced pulmonary function, while increased circulating BRSK2 levels suggested respiratory failure and shorter transplant-free survival in IPF patients. BRSK2 silencing attenuated lung fibrosis progression in cellular and animal models. Transcriptomic integration identified PSMB1, CTSD, and CTSH as significant downstream effectors of BRSK2, with PSMB1 showing robust shared causal variant support (PPH4 = 0.800). Colocalization analysis and phenotype scan deepened the pathogenic association of BRSK2 with IPF, while methylation MR analysis highlighted the critical role of epigenetic regulation in BRSK2-driven IPF pathogenesis. PheWAS revealed no significant drug-related toxicities for BRSK2, and its therapeutic potential was further underscored by protein interaction analyses.

CONCLUSIONS

BRSK2 is identified as a critical pathogenic factor in IPF, with strong potential as a therapeutic target. Future studies should focus on its translational implications and the development of targeted therapies to improve patient outcomes.

Ultrasonic Microfluidic Method Used for siHSP47 Loaded in Human Embryonic Kidney Cell-Derived Exosomes for Inhibiting TGF-β1 Induced Fibroblast Differentiation and Migration

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and devastating lung disorder. In response to transforming growth factor-β (TGF-β), normal lung cells proliferate and differentiate into myofibroblasts, which are instrumental in promoting disease progression. Small interfering RNA (siRNA) targeting heat shock protein 47 (HSP47) has been demonstrated to alleviate IPF by blocking collagen synthesis and secretion. Exosomes (EXOs) have been investigated for drug delivery due to their superior carrier properties. However, their loading efficiency has been a limiting factor in widely application as drug carriers. In this study, an ultrasonic microfluidic method was employed to enhance the loading efficiency of siHSP47 into EXOs, achieving 31.1% efficiency rate. EXOs were isolated from human embryonic kidney cells (293F) and loaded with siHSP47 (EXO-siHSP47). The findings indicated that EXO-siHSP47 penetrated the collagen barrier and effectively silenced HSP47 expression in activated fibroblasts in vitro. Western blotting and immunofluorescence analyses confirmed that EXO-siHSP47 significantly reduced the secretion and deposition of extracellular matrix (ECM) proteins. Wound healing and Transwell migration assays demonstrated that EXO-siHSP47 inhibited fibroblast differentiation and migration. In conclusion, 293F-derived EXOs loaded with siHSP47 present a promising therapeutic strategy for IPF.

Blockade of TREM2 ameliorates pulmonary inflammation and fibrosis by modulating sphingolipid metabolism

Pulmonary fibrosis is a chronic interstitial lung disease involving systemic inflammation and abnormal collagen deposition. Dysregulations in lipid metabolism, such as macrophage-dependent lipid catabolism, have been recognized as critical factors for the development of pulmonary fibrosis. However, little is known about the signaling pathways involved and the key regulators. Here we found that triggering receptor expressed on myeloid cells 2 (TREM2) plays a pivotal role in regulating the lipid handling capacities of pulmonary macrophages and triggering fibrosis. By integrating analysis of single-cell and bulk RNA sequencing data from patients and mice with pulmonary fibrosis, we revealed that pulmonary macrophages consist of heterogeneous populations with distinct pro-fibrotic properties, and found that both sphingolipid metabolism and the expression of chemotaxis-related genes are elevated in fibrotic lungs. TREM2, a sensor recognizing multiple lipid species, is specifically upregulated in a subset of monocyte-derived macrophages. Blockade of TREM2 by conventional/conditional knock-out or soluble TREM2 administration can attenuate bleomycin-induced pulmonary fibrosis. By utilizing scRNA Seq and lipidomics, we found that Trem2 deficiency downregulates the synthesis of various sphingomyelins, and inhibits the expression of chemokines such as Ccl2. Together, our findings not only reveal the alterations in lipidomic profiles and the atlas of pulmonary macrophages during pulmonary fibrosis, but also suggest that targeting TREM2, the crucial regulator affecting both pulmonary sphingolipid metabolism and the chemokines secretion, can benefit pulmonary fibrosis patients in the future.

The role of macrophage polarization in rheumatoid arthritis and osteoarthritis: Pathogenesis and therapeutic strategies

Rheumatoid arthritis (RA) and osteoarthritis (OA) are common and debilitating joint disorders affecting millions of individuals worldwide. Despite their distinct pathological features, both conditions share a crucial role of macrophages in disease progression. Macrophages exhibit remarkable plasticity, polarizing into pro-inflammatory M1 or anti-inflammatory M2 phenotypes in response to environmental cues. An imbalance in macrophage polarization, particularly a shift towards the M1 phenotype, contributes to chronic inflammation and joint damage in RA and OA. This review explores the complex interplay between macrophages and various cell types, including T cells, B cells, synovial fibroblasts, osteoclasts, chondrocytes, and adipocytes, in the pathogenesis of these diseases. We discuss the current understanding of macrophage polarization in RA and OA, highlighting the molecular mechanisms involved. Furthermore, we provide an overview of potential therapeutic strategies targeting macrophage polarization, such as disease-modifying anti-rheumatic drugs, traditional Chinese medicine, nanomedicines, proteins, chemical compounds, and physical therapies. By elucidating the precise mechanisms governing macrophage polarization and its interactions with other cells in the joint microenvironment, researchers can identify novel therapeutic targets and develop targeted interventions to alleviate disease progression and improve patient outcomes in RA and OA.

Compromised C3b-VSIG4 axis between decidual NK cells and macrophages contributes to recurrent spontaneous abortion

NK cells and macrophages constitute the predominant immune cell subsets in the decidua during the first trimester of pregnancy, with macrophages typically adopting an anti-inflammatory phenotype. Conversely, in the third trimester, macrophages undergo a shift towards a pro-inflammatory phenotype concurrent with a reduction in NK cell numbers. The direct regulatory impact of NK cells on macrophage phenotype remains poorly explored. In our investigation, we observed that ICAM1+ macrophages stimulate the expression of intracellular C3 in LFA1+ decidual NK cells. Notably, Cathepsin W within NK cells exhibit the potential to generate active C3b fragments, effectively inhibit the proinflammatory phenotype of macrophages by binding to VSIG4. Our study unveils a direct regulatory mechanism orchestrated by decidual NK cells over macrophages, providing a potential pathogenic explanation for recurrent spontaneous abortion.

Zerumbone alleviated bleomycin-induced pulmonary fibrosis in mice via SIRT1/Nrf2 pathway

Pulmonary fibrosis (PF) is a persistent interstitial lung condition for which effective treatment options are currently lacking. Zerumbone (zerum), a humulane sesquiterpenoid extracted from Zingiber zerumbet Smith, has been documented in previous studies to possess various pharmacological benefits. The aim of this study was to observe and investigate the therapeutic effects and mechanisms of zerum on pulmonary fibrosis. We utilized a transforming growth factor (TGF)-β1-induced human lung fibroblast (MRC-5) activation model and a bleomycin-induced pulmonary fibrosis mouse model. Cell counting kit 8 (CCK8) and cell migration assays were performed to assess the effects of zerum on MRC-5 cells. Masson's trichrome, Hematoxylin and Eosin (HE), and Sirius Red staining were conducted for pathological evaluation of lung tissue. Western blot experiments were conducted to measure the protein expression levels of Collagen I, α-SMA, Nrf2, and SIRT1. Immunofluorescence and immunohistochemistry assays were used to detect the expression of reactive oxygen species (ROS), Nrf2, and α-SMA. ELISA was employed to measure the levels of MDA, SOD, and GSH-Px. Our findings from in vitro and in vivo studies demonstrated that zerum significantly inhibited the migration ability of TGF-β1-induced MRC-5 cells, reduced ROS production in TGF-β1-induced MRC-5 cells and pulmonary fibrosis mice, and decreased the expression of Collagen I and α-SMA proteins. Additionally, zerum activated the SIRT1/Nrf2 signaling pathway in TGF-β1-induced MRC-5 cells and pulmonary fibrosis mice. Knockdown of SIRT1 abolished the anti-fibrotic effects of zerum. These results suggest that zerum inhibits TGF-β1 and BLM-induced cell and mouse pulmonary fibrosis by activating the SIRT1/Nrf2 pathway.

Unveiling the impact of TREM-2+ Macrophages in metabolic disorders

The Triggering Receptor Expressed on Myeloid cells 2 (TREM-2) has been widely known by its anti-inflammatory activity. It can be activated in response to microbes and tissue damage, leading to phagocytosis, autophagy, cell polarization and migration, counter inflammation, and tissue repair. So far, the receptor has been largely explored in neurodegenerative disorders, however, a growing number of studies have been investigating its contribution in different pathological conditions, including metabolic diseases, in which (resident) macrophages play a crucial role. In this regard, TREM-2 + macrophages have been implicated in the onset and development of obesity, atherosclerosis, and fibrotic liver disease. These macrophages can be detected in the brain, white adipose tissue, liver, and vascular endothelium. In this review we discuss how different murine models have been demonstrating the ability of such cells to contribute to tissue and body homeostasis by phagocytosing cellular debris and lipid structures, besides contributing to lipid homeostasis in metabolic diseases. Therefore, understanding the role of TREM-2 in metabolic disorders is crucial to expand our current knowledge concerning their immunopathology as well as to foster the development of more targeted therapies to treat such conditions.

TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway in mice

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that affects cellular phenotypes by modulating phagocytosis and metabolism, promoting cell survival, and counteracting inflammation. Its role in renal injury, in particular, unilateral ureteral obstruction (UUO) or ischemia-reperfusion injury (IRI)-induced renal injury remains unclear. In our study, WT and Trem2-/- mice were employed to evaluate the role of TREM2 in renal macrophage infiltration and tissue injury after UUO. Bone marrow-derived macrophages (BMDM) from both mouse genotypes were cultured and polarized for in vitro experiments. Next, the effects of TREM2 on renal injury and macrophage polarization in IRI mice were also explored. We found that TREM2 expression was upregulated in the obstructed kidneys. TREM2 deficiency exacerbated renal inflammation and fibrosis 3 and 7 days after UUO, in association with reduced macrophage infiltration. Trem2-/- BMDM exhibited increased apoptosis and poorer survival compared with WT BMDM. Meanwhile, TREM2 deficiency augmented M1 and M2 polarization after UUO. Consistent with the in vivo observations, TREM2 deficiency led to increased polarization of BMDM towards the M1 proinflammatory phenotype. Mechanistically, TREM2 deficiency promoted M1 and M2 polarization via the JAK-STAT pathway in the presence of TGF-β1, thereby affecting cell survival by regulating mTOR signaling. Furthermore, cyclocreatine supplementation alleviated cell death caused by TREM2 deficiency. Additionally, we found that TREM2 deficiency promoted renal injury, fibrosis, and macrophage polarization in IRI mice. The current data suggest that TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway. These findings have implications for the role of TREM2 in the regulation of renal injury that justify further evaluation.

TREM2 regulates microglial lipid droplet formation and represses post-ischemic brain injury

Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor protein predominantly expressed in microglia within the central nervous system (CNS). TREM2 regulates multiple microglial functions, including lipid metabolism, immune reaction, inflammation, and microglial phagocytosis. Recent studies have found that TREM2 is highly expressed in activated microglia after ischemic stroke. However, the role of TREM2 in the pathologic response after stroke remains unclear. Herein, TREM2-deficient microglia exhibit an impaired phagocytosis rate and cholesteryl ester (CE) accumulation, leading to lipid droplet formation and upregulation of Perilipin-2 (PLIN2) expression after hypoxia. Knockdown of TREM2 results in increased lipid synthesis (PLIN2, SOAT1) and decreased cholesterol clearance and lipid hydrolysis (LIPA, ApoE, ABCA1, NECH1, and NPC2), further impacting microglial phenotypes. In these lipid droplet-rich microglia, the TGF-β1/Smad2/3 signaling pathway is downregulated, driving microglia towards a pro-inflammatory phenotype. Meanwhile, in a neuron-microglia co-culture system under hypoxic conditions, we found that microglia lost their protective effect against neuronal injury and apoptosis when TREM2 was knocked down. Under in vivo conditions, TREM2 knockdown mice express lower TGF-β1 expression levels and a lower number of anti-inflammatory M2 phenotype microglia, resulting in increased cerebral infarct size, exacerbated neuronal apoptosis, and aggravated neuronal impairment. Our work suggests that TREM2 attenuates stroke-induced neuroinflammation by modulating the TGF-β1/Smad2/3 signaling pathway. TREM2 may play a direct role in the regulation of inflammation and also exert an influence on the post-ischemic inflammation and the stroke pathology progression via regulation of lipid metabolism processes. Thus, underscoring the therapeutic potential of TREM2 agonists in ischemic stroke and making TREM2 an attractive new clinical target for the treatment of ischemic stroke and other inflammation-related diseases.

Tumor-related exosomal circ_0001715 promotes lung adenocarcinoma cell proliferation and metastasis via enhancing M2 macrophage polarization by regulating triggering receptor expressed on myeloid cells-2

BACKGROUND

Circular RNAs (circRNAs) have been shown to mediate tumor-associated macrophages (TAMs) to regulate the development of many cancers, including lung adenocarcinoma (LUAD). However, whether circ_0001715 regulates LUAD progression by mediating TAMs polarization remains uncertain.

METHODS

Monocytes (THP-1) were treated with PMA to induce M0 macrophages. M0 macrophages were incubated with LUAD cells-derived exosomes and then cocultured with LUAD cells. The levels of circ_0001715, M2 macrophage markers, microRNA (miR)-205-5p, and triggering receptor expressed on myeloid cells-2 (TREM2) were examined using quantitative real-time PCR. Flow cytometry was performed to assess M2 macrophage surface marker CD206. Cell proliferation, migration and invasion were determined using cell counting kit 8, EdU, colony formation and transwell assays. Dual-luciferase reporter assay was used to investigate the interactions between miR-205-5p and circ_0001715 or TREM2.

RESULTS

Circ_0001715 knockdown inhibited M2 macrophage polarization and its overexpression had an opposite effect. After M0 macrophages transfected with si-circ_0001715 were cocultured with LUAD cells, the proliferation and metastasis of LUAD cells were markedly reduced. Exosomes transferred circ_0001715 between M0 macrophages and LUAD cells. Exosomal circ_0001715 promoted M2 macrophage polarization to increase LUAD cell proliferation and metastasis. In terms of mechanism, circ_0001715 sponged miR-205-5p to positively regulate TREM2. TREM2 upregulation also could promote LUAD cell proliferation and metastasis via increasing M2 macrophage polarization. In addition, TREM2 knockdown reversed the effect of exosomal circ_0001715 on M2 macrophage polarization and LUAD cell progression.

CONCLUSION

Exosomal circ_0001715 led to LUAD cell proliferation and metastasis by promoting M2 macrophage polarization via the miR-205-5p/TREM2 axis.

Macrophage polarization in tissue fibrosis

Fibrosis can occur in all major organs with relentless progress, ultimately leading to organ failure and potentially death. Unfortunately, current clinical treatments cannot prevent or reverse tissue fibrosis. Thus, new and effective antifibrotic therapeutics are urgently needed. In recent years, a growing body of research shows that macrophages are involved in fibrosis. Macrophages are highly heterogeneous, polarizing into different phenotypes. Some studies have found that regulating macrophage polarization can inhibit the development of inflammation and cancer. However, the exact mechanism of macrophage polarization in different tissue fibrosis has not been fully elucidated. This review will discuss the major signaling pathways relevant to macrophage-driven fibrosis and profibrotic macrophage polarization, the role of macrophage polarization in fibrosis of lung, kidney, liver, skin, and heart, potential therapeutics targets, and investigational drugs currently in development, and hopefully, provide a useful review for the future treatment of fibrosis.

Betulinic acid regulates tumor-associated macrophage M2 polarization and plays a role in inhibiting the liver cancer progression

OBJECTIVE

To investigate the regulatory role and mechanism of betulinic acid (BET) in tumor-associated M2 macrophage polarization.

METHODS

For in vitro experiments, RAW246.7 and J774A.1 cells were used, and differentiation of M2 macrophages was induced using recombinant interleukin-4/13. The levels of M2 cell marker cytokines were measured, and the proportion of F4/80+CD206+ cells was evaluated using flow cytometry. Furthermore, STAT6 signaling was detected, and H22 and RAW246.7 cells were cocultured to assess the effect of BET on M2 macrophage polarization. Changes in the malignant behavior of H22 cells after coculturing were observed and a tumor-bearing mouse model was constructed to determine CD206 cell infiltration after BET intervention.

RESULTS

In vitro experiments showed that BET inhibited M2 macrophage polarization and phospho-STAT6 signal modification. Moreover, the ability to promote the malignant behavior of H22 cells was reduced in BET-treated M2 macrophages. Furthermore, in vivo experiments indicated that BET decreased M2 macrophage polarization and infiltration in the microenvironment of liver cancer. BET was noted to predominantly bind to the STAT6 site to inhibit STAT6 phosphorylation.

CONCLUSION

BET bound chiefly to STAT6 to inhibit STAT6 phosphorylation and decrease M2 polarization in the microenvironment of liver cancer. These findings suggest that BET exerts an antitumor effect by modulating M2 macrophage function.

Macrophage Implication in IPF: Updates on Immune, Epigenetic, and Metabolic Pathways

Idiopathic pulmonary fibrosis (IPF) is a lethal interstitial lung disease of unknown etiology with a poor prognosis. It is a chronic and progressive disease that has a distinct radiological and pathological pattern from common interstitial pneumonia. The use of immunosuppressive medication was shown to be completely ineffective in clinical trials, resulting in years of neglect of the immune component. However, recent developments in fundamental and translational science demonstrate that immune cells play a significant regulatory role in IPF, and macrophages appear to be among the most crucial. These highly plastic cells generate multiple growth factors and mediators that highly affect the initiation and progression of IPF. In this review, we will provide an update on the role of macrophages in IPF through a systemic discussion of various regulatory mechanisms involving immune receptors, cytokines, metabolism, and epigenetics.