TREM2 Insufficiency Protects against Pulmonary Fibrosis by Inhibiting M2 Macrophage Polarization

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.

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.

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.

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.

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.