Activated Kupffer cells inhibit insulin sensitivity in obese mice

Macrophage: Hidden Criminal in Therapy Resistance

BACKGROUND

Although substantial efforts have been made by researchers to develop drugs, a disappointing reality is that the emergence of drug resistance is an unavoidable reality for the majority of patients. In recent years, emerging evidence suggests a connection between drug resistance and immune dysregulation.

SUMMARY

As a ubiquitously distributed, versatile innate immune cell, macrophages play essential roles in maintaining tissue homeostasis in a steady state. Nevertheless, it is becoming aware that macrophages undermine the action of therapeutic drugs across various disease types. Reprogramming macrophage function has been proven to be effective in restoring patient responsiveness to treatment. Herein, we comprehensively reviewed how macrophages respond to drugs and the mechanisms by which they contribute to treatment unresponsiveness in cancer, inflammatory diseases, and metabolic diseases. In addition, future prospects in macrophage-based combination therapy were discussed.

KEY MESSAGES

Targeting macrophages is a promising strategy for overcoming drug resistance in immune disorders.

Hepatocyte CHRNA4 mediates the MASH-promotive effects of immune cell-produced acetylcholine and smoking exposure in mice and humans

Metabolic dysfunction-associated steatohepatitis (MASH) is a leading risk factor for liver cirrhosis and hepatocellular carcinoma. Here, we report that CHRNA4, a subunit of nicotinic acetylcholine receptors (nAChRs), is an accelerator of MASH progression. CHRNA4 also mediates the MASH-promotive effects induced by smoking. Chrna4 was expressed specifically in hepatocytes and exhibited increased levels in mice and patients with MASH. Elevated CHRNA4 levels were positively correlated with MASH severity. We further revealed that during MASH development, acetylcholine released from immune cells or nicotine derived from smoking functioned as an agonist to activate hepatocyte-intrinsic CHRNA4, inducing calcium influx and activation of inflammatory signaling. The communication between immune cells and hepatocytes via the acetylcholine-CHRNA4 axis led to the production of a variety of cytokines, eliciting inflammation in liver and promoting the pathogenesis of MASH. Genetic and pharmacological inhibition of CHRNA4 protected mice from diet-induced MASH. Targeting CHRNA4 might be a promising strategy for MASH therapeutics.

Macrophage-derived insulin antagonist ImpL2 induces lipoprotein mobilization upon bacterial infection

The immune response is an energy-demanding process that must be coordinated with systemic metabolic changes redirecting nutrients from stores to the immune system. Although this interplay is fundamental for the function of the immune system, the underlying mechanisms remain elusive. Our data show that the pro-inflammatory polarization of Drosophila macrophages is coupled to the production of the insulin antagonist ImpL2 through the activity of the transcription factor HIF1α. ImpL2 production, reflecting nutritional demands of activated macrophages, subsequently impairs insulin signaling in the fat body, thereby triggering FOXO-driven mobilization of lipoproteins. This metabolic adaptation is fundamental for the function of the immune system and an individual's resistance to infection. We demonstrated that analogically to Drosophila, mammalian immune-activated macrophages produce ImpL2 homolog IGFBP7 in a HIF1α-dependent manner and that enhanced IGFBP7 production by these cells induces mobilization of lipoproteins from hepatocytes. Hence, the production of ImpL2/IGFBP7 by macrophages represents an evolutionarily conserved mechanism by which macrophages alleviate insulin signaling in the central metabolic organ to secure nutrients necessary for their function upon bacterial infection.

Chronic inflammation in high-fat diet-fed mice: Unveiling the early pathogenic connection between liver and adipose tissue

Obesity-induced chronic inflammation has been linked to several autoimmune diseases, including rheumatoid arthritis, type 1 diabetes, and multiple sclerosis. The underlying mechanisms are not yet fully understood, but it is believed that chronic inflammation in adipose tissue can lead to the production of pro-inflammatory cytokines and chemokines, which can trigger immune responses and contribute to the development of autoimmune diseases. However, the underlying mechanisms that lead to the infiltration of immune cells into adipose tissue are not fully understood. In this study, we observed a time-dependent response to a high-fat diet in the liver and epididymal white adipose tissue using gene set enrichment analysis. Our findings revealed a correlation between early abnormal innate immune responses in the liver and late inflammatory response in the adipose tissue, that eventually leads to systemic inflammation. Specifically, our data suggest that the dysregulated NADH homeostasis in the mitochondrial matrix, interacting with the mitochondrial translation process, could serve as a sign marking the transition from liver inflammation to adipose tissue inflammation. Taken together, our study provides valuable insights into the molecular mechanisms underlying the development of chronic inflammation and associated autoimmune diseases in obesity.

The miR-23-27-24 clusters drive lipid-associated macrophage proliferation in obese adipose tissue

Identifying molecular circuits that control adipose tissue macrophage (ATM) function is necessary to understand how ATMs contribute to tissue homeostasis and obesity-induced insulin resistance. In this study, we find that mice with a myeloid-specific knockout of the miR-23-27-24 clusters of microRNAs (miRNAs) gain less weight on a high-fat diet but exhibit worsened glucose and insulin tolerance. Analysis of ATMs from these mice shows selectively reduced numbers and proliferation of a recently reported subset of lipid-associated CD9+Trem2+ ATMs (lipid-associated macrophages [LAMs]). Leveraging the role of miRNAs to control networks of genes, we use RNA sequencing (RNA-seq), functional screens, and biochemical assays to identify candidate target transcripts that regulate proliferation-associated signaling. We determine that miR-23 directly targets the mRNA of Eif4ebp2, a gene that restricts protein synthesis and proliferation in macrophages. Altogether, our study demonstrates that control of proliferation of a protective subset of LAMs by noncoding RNAs contributes to protection against diet-induced obesity metabolic dysfunction.

Modeling and therapeutic targeting of inflammation-induced hepatic insulin resistance using human iPSC-derived hepatocytes and macrophages

Hepatic insulin resistance is recognized as a driver of type 2 diabetes and fatty liver disease but specific therapies are lacking. Here we explore the potential of human induced pluripotent stem cells (iPSCs) for modeling hepatic insulin resistance in vitro, with a focus on resolving the controversy about the impact of inflammation in the absence of steatosis. For this, we establish the complex insulin signaling cascade and the multiple inter-dependent functions constituting hepatic glucose metabolism in iPSC-derived hepatocytes (iPSC-Heps). Co-culture of these insulin-sensitive iPSC-Heps with isogenic iPSC-derived pro-inflammatory macrophages induces glucose output by preventing insulin from inhibiting gluconeogenesis and glycogenolysis and activating glycolysis. Screening identifies TNFα and IL1β as the mediators of insulin resistance in iPSC-Heps. Neutralizing these cytokines together restores insulin sensitivity in iPSC-Heps more effectively than individual inhibition, reflecting specific effects on insulin signaling and glucose metabolism mediated by NF-κB or JNK. These results show that inflammation is sufficient to induce hepatic insulin resistance and establish a human iPSC-based in vitro model to mechanistically dissect and therapeutically target this metabolic disease driver.

The hepato-ovarian axis: genetic evidence for a causal association between non-alcoholic fatty liver disease and polycystic ovary syndrome

BACKGROUND

Recent studies found associations between non-alcoholic fatty liver disease (NAFLD) and polycystic ovary syndrome (PCOS), but the causal nature of this association is still uncertain.

METHODS

We performed a bidirectional two-sample Mendelian randomization (MR) analysis to test for the causal association between NAFLD and PCOS using data from a large-scale biopsy-confirmed NAFLD genome-wide association study (GWAS) (1483 cases and 17,781 controls) and PCOS GWAS (10,074 cases and 103,164 controls) in European ancestries. Data from glycemic-related traits GWAS (in up to 200,622 individuals) and sex hormones GWAS (in 189,473 women) in the UK Biobank (UKB) were used in the MR mediation analysis to assess potential mediating roles of these molecules in the causal pathway between NAFLD and PCOS. Replication analysis was conducted using two independent datasets from NAFLD and PCOS GWASs in the UKB and a meta-analysis of data from FinnGen and the Estonian Biobank, respectively. A linkage disequilibrium score regression was conducted to assess genetic correlations between NAFLD, PCOS, glycemic-related traits, and sex hormones using full summary statistics.

RESULTS

Individuals with higher genetic liability to NAFLD were more likely to develop PCOS (OR per one-unit log odds increase in NAFLD: 1.10, 95% CI: 1.02-1.18; P = 0.013). Indirect causal effects of NAFLD on PCOS via fasting insulin only (OR: 1.02, 95% CI: 1.01-1.03; P = 0.004) and further a suggestive indirect causal effect via fasting insulin in concert with androgen levels were revealed in MR mediation analyses. However, the conditional F statistics of NAFLD and fasting insulin were less than 10, suggesting likely weak instrument bias in the MVMR and MR mediation analyses.

CONCLUSIONS

Our study suggests that genetically predicted NAFLD was associated with a higher risk of developing PCOS but less evidence for vice versa. Fasting insulin and sex hormones might mediate the link between NAFLD and PCOS.

Essential Minerals and Metabolic Adaptation of Immune Cells

Modern lifestyles deviated considerably from the ancestral routines towards major shifts in diets and increased sedentarism. The trace elements status of the human body is no longer adequately supported by micronutrient-inferior farmed meats and crop commodities produced by the existing agricultural food systems. This is particular evident in the increased obesogenic adipogenesis and low-grade inflammation that fails to resolve with time. The metabolically restrictive environment of the inflamed tissues drives activation and proliferation of transient and resident populations of immune cells in favor of pro-inflammatory phenotypes, as well as a part of the enhanced autoimmune response. As different stages of the immune activation and resolution depend on the availability of specific minerals to maintain the structural integrity of skin and mucus membranes, activation and migration of immune cells, activation of the complement system, and the release of pro-inflammatory cytokines and chemokines, this review discusses recent advances in our understanding of the contribution of select minerals in optimizing the responses of innate and adaptive immune outcomes. An abbreviated view on the absorption, transport, and delivery of minerals to the body tissues as related to metabolic adaptation is considered.

The Tumor Microenvironment of Hepatocellular Carcinoma: Untying an Intricate Immunological Network

HCC, the most prevalent form of primary liver cancer, is prototypically an inflammation-driven cancer developing after years of inflammatory insults. Consequently, the hepatic microenvironment is a site of complex immunological activities. Moreover, the tolerogenic nature of the liver can act as a barrier to anti-tumor immunity, fostering cancer progression and resistance to immunotherapies based on immune checkpoint inhibitors (ICB). In addition to being a site of primary carcinogenesis, many cancer types have high tropism for the liver, and patients diagnosed with liver metastasis have a dismal prognosis. Therefore, understanding the immunological networks characterizing the tumor microenvironment (TME) of HCC will deepen our understanding of liver immunity, and it will underpin the dominant mechanisms controlling both spontaneous and therapy-induced anti-tumor immune responses. Herein, we discuss the contributions of the cellular and molecular components of the liver immune contexture during HCC onset and progression by underscoring how the balance between antagonistic immune responses can recast the properties of the TME and the response to ICB.

Spotlight on liver macrophages for halting injury and progression in non-alcoholic fatty liver disease

INTRODUCTION

Non-alcoholic fatty liver disease (NAFLD) is considered the hepatic manifestation of the metabolic syndrome and is rapidly emerging as the leading cause of liver-related morbidity and mortality. Macrophages play an essential role in the development and progression of NAFLD.

AREAS COVERED

In this review, we provide an update on recent studies of drugs, which directly or indirectly affect macrophages in NAFLD, and discuss the implication of macrophage biomarkers to monitor the disease stage and progression/regression.

EXPERT OPINION

There is an unmet need for better understanding of disease pathogenesis from hepatic fat accumulation to disease progression with inflammation and fibrosis. We expect that future research will uncover additional objects/pathways as treatment targets. We speculate that this will involve better characterization of the gut microbiome, damage-associated molecular patterns (DAMPS) or molecules and pathways involved in development of DAMPS, and advanced molecular biology studies including single-cell sequencing of macrophage subpopulations. In addition, we speculate that studies focusing on pharmaceuticals that improve insulin resistance, diminish the metabolic syndrome and reduce fibrosis will prevail.

Macrophage functional diversity in NAFLD - more than inflammation

Macrophages have diverse phenotypes and functions due to differences in their origin, location and pathophysiological context. Although their main role in the liver has been described as immunoregulatory and detoxifying, changes in macrophage phenotypes, diversity, dynamics and function have been reported during obesity-related complications such as non-alcoholic fatty liver disease (NAFLD). NAFLD encompasses multiple disease states from hepatic steatosis to non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis and hepatocarcinoma. Obesity and insulin resistance are prominent risk factors for NASH, a disease with a high worldwide prevalence and no approved treatment. In this Review, we discuss the turnover and function of liver-resident macrophages (Kupffer cells) and monocyte-derived hepatic macrophages. We examine these populations in both steady state and during NAFLD, with an emphasis on NASH. The explosion in high-throughput gene expression analysis using single-cell RNA sequencing (scRNA-seq) within the last 5 years has revolutionized the study of macrophage heterogeneity, substantially increasing our understanding of the composition and diversity of tissue macrophages, including in the liver. Here, we highlight scRNA-seq findings from the last 5 years on the diversity of liver macrophages in homeostasis and metabolic disease, and reveal hepatic macrophage function beyond their classically described inflammatory role in the progression of NAFLD and NASH pathogenesis.

Macrophages, Low-Grade Inflammation, Insulin Resistance and Hyperinsulinemia: A Mutual Ambiguous Relationship in the Development of Metabolic Diseases

Metabolic derangement with poor glycemic control accompanying overweight and obesity is associated with chronic low-grade inflammation and hyperinsulinemia. Macrophages, which present a very heterogeneous population of cells, play a key role in the maintenance of normal tissue homeostasis, but functional alterations in the resident macrophage pool as well as newly recruited monocyte-derived macrophages are important drivers in the development of low-grade inflammation. While metabolic dysfunction, insulin resistance and tissue damage may trigger or advance pro-inflammatory responses in macrophages, the inflammation itself contributes to the development of insulin resistance and the resulting hyperinsulinemia. Macrophages express insulin receptors whose downstream signaling networks share a number of knots with the signaling pathways of pattern recognition and cytokine receptors, which shape macrophage polarity. The shared knots allow insulin to enhance or attenuate both pro-inflammatory and anti-inflammatory macrophage responses. This supposedly physiological function may be impaired by hyperinsulinemia or insulin resistance in macrophages. This review discusses the mutual ambiguous relationship of low-grade inflammation, insulin resistance, hyperinsulinemia and the insulin-dependent modulation of macrophage activity with a focus on adipose tissue and liver.

Yeast as carrier for drug delivery and vaccine construction

Yeast has been employed as an effective derived drug carrier as a unicellular microorganism. Many research works have been devoted to the encapsulation of nucleic acid compounds, insoluble small molecule drugs, small molecules, liposomes, polymers, and various nanoparticles in yeast for the treatment of disease. Recombinant yeast-based vaccine carriers (WYV) have played a major role in the development of vaccines. Herein, the latest reports on the application of yeast carriers and the development of related research are summarized, a conceptual description of gastrointestinal absorption of yeast carriers, as well as the various package forms of different drug molecules and nanoparticles in yeast carriers are introduced. In addition, the advantages and development of recombinant yeast vaccine carriers for the disease, veterinary and aquaculture applications are discussed. Moreover, the current challenges and future directions of yeast carriers are proposed.

Macrophage activation marker sCD163 is associated with liver injury and hepatic insulin resistance in obese patients before and after Roux-en-Y gastric bypass

BACKGROUND

Macrophages are associated with metabolic complications to obesity including fatty liver disease and impaired hepatic and muscle insulin sensitivity (IS). Bariatric surgery induces weight loss and improves IS. We investigated associations between the macrophage activation marker soluble (s)CD163, alanine-aminotransferase (ALT), and IS before and after Roux-en-Y Gastric Bypass (RYGB).

METHODS

We analyzed sCD163 from 10 type 2 diabetes (T2D) and 10 obese patients with normal glucose tolerance (NGT) undergoing RYGB for associations with hepatic, adipose tissue, and muscle IS and ALT after 1-week, 3, and 12 months postoperatively. IS was evaluated by hyperinsulinemic-euglycemic clamp in combination with glucose tracer technique.

RESULTS

Preoperative sCD163 correlated with ALT (r = 0.58, p = 0.007) and tended to associate inversely with hepatic (r = -0.39, p = 0.1) and adipose tissue (r = -0.39, p = 0.09), but not muscle IS. Following RYGB, sCD163 decreased significantly in all patients. The decrease in sCD163 during the first 3 months correlated inversely with the improvement of hepatic IS (r = -0.65, p = 0.01) and tended to be associated with changes in muscle IS (r = -0.45, p = 0.09). After 3 months sCD163 remained associated with ALT (r = 0.75, p < 0.001) and inversely with hepatic IS (r = -0.39, p = 0.1), but not muscle or adipose tissue IS. One year after RYGB, sCD163 correlated with ALT (r = 0.61, p = 0.007), but not with hepatic, adipose tissue, or muscle IS.

CONCLUSION

Macrophage activation is associated with liver injury and hepatic IS in obese patients. Improvements in these measures correlate during the first 3 months following RYGB, supporting a link between macrophages and hepatic IS in severe obesity and diabetes.

A subset of Kupffer cells regulates metabolism through the expression of CD36

Tissue macrophages are immune cells whose phenotypes and functions are dictated by origin and niches. However, tissues are complex environments, and macrophage heterogeneity within the same organ has been overlooked so far. Here, we used high-dimensional approaches to characterize macrophage populations in the murine liver. We identified two distinct populations among embryonically derived Kupffer cells (KCs) sharing a core signature while differentially expressing numerous genes and proteins: a major CD206^loESAM^- population (KC1) and a minor CD206^hiESAM⁺ population (KC2). KC2 expressed genes involved in metabolic processes, including fatty acid metabolism both in steady-state and in diet-induced obesity and hepatic steatosis. Functional characterization by depletion of KC2 or targeted silencing of the fatty acid transporter Cd36 highlighted a crucial contribution of KC2 in the liver oxidative stress associated with obesity. In summary, our study reveals that KCs are more heterogeneous than anticipated, notably describing a subpopulation wired with metabolic functions.

Macrophage tumor necrosis factor-alpha deletion does not protect against obesity-associated metabolic dysfunction

The pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-α), has been suggested to be a key factor in the induction of obesity-associated metabolic dysfunction. However, the role that macrophage-derived TNF-α has on regulating metabolic perturbations in obesity is not completely understood. Therefore, we utilized the TNF-αFlox/Flox(F/F) , LyzMcre± mouse model to determine the impact that macrophage TNF-α deletion has on the development of high-fat diet (HFD)-induced obesity. At 10 weeks of age, male littermates were randomly assigned to 1 of 4 groups: TNF-αF/F low-fat diet (TNF-αF/F LFD), TNF-αF/F,LyzMCre LFD, TNF-αF/F HFD, or TNF-αF/F,LyzMCre HFD (n = 16-28/group) and were fed their respective diets for 18 weeks. Body weight was assessed throughout the course of the experiment. Body composition, hepatic lipid accumulation, and metabolic outcomes were also examined. A microarray gene expression experiment was performed from RNA isolated from epididymal adipose tissue of the HFD-fed groups (n = 10/group) and results were verified via qRT-PCR for all groups. Macrophage-derived TNF-α deletion significantly reduced adipose tissue TNF-α gene expression and circulating TNF-α and downregulated genes linked to the toll-like receptor (TLR) and NFκB signaling pathways. However, macrophage TNF-α deletion had no effect on hindering the development of obesity, hepatic lipid accumulation, or improving glucose metabolism or insulin sensitivity. In conclusion, macrophage-derived TNF-α is not a causative factor for the induction of obesity-associated metabolic dysfunction.

Myeloid Endoplasmic Reticulum Resident Chaperone GP96 Facilitates Inflammation and Steatosis in Alcohol-Associated Liver Disease

Cellular stress-mediated chaperones are linked to liver macrophage activation and inflammation in alcohol-associated liver disease (ALD). In this study, we investigate the role of endoplasmic reticulum (ER) resident stress chaperone GP96/HSP90B1/GRP94, paralog of the HSP90 family, in ALD pathogenesis. We hypothesize that ER resident chaperone, heat shock protein GP96, plays a crucial role in alcohol-associated liver inflammation and contributes to liver injury. We show high expression of GP96/HSP90B1 and GRP78/HSPA5 in human alcohol-associated hepatitis livers as well as in mouse ALD livers with induction of GP96 prominent in alcohol-exposed macrophages. Myeloid-specific GP96 deficient (M-GP96KO) mice failed to induce alcohol-associated liver injury. Alcohol-fed M-GP96KO mice exhibit significant reduction in steatosis, serum endotoxin, and pro-inflammatory cytokines compared with wild-type mice. Anti-inflammatory cytokines interleukin-10 and transforming growth factor β, as well as activating transcription factor 3 and triggering receptor expressed on myeloid cells 2, markers of restorative macrophages, were higher in alcohol-fed M-GP96KO livers. M-GP96KO mice exhibit protection in a model of endotoxin-mediated liver injury in vivo, which is in agreement with reduced inflammatory responses during ex vivo lipopolysaccharide/endotoxin- stimulated bone marrow-derived macrophages from M-GP96KO mice. Furthermore, we show that liver macrophages from alcohol-fed M-GP96KO mice show compensatory induction of GRP78 messenger RNA, likely due to increased splicing of X-box binding protein-1. Finally, we show that inhibition of GP96 using a specific pharmacological agent, PU-WS13 or small interfering RNA, alleviates inflammatory responses in primary macrophages. Conclusion: Myeloid ER resident GP96 promotes alcohol-induced liver damage through activation of liver macrophage inflammatory responses, alteration in lipid homeostasis, and ER stress. These findings highlight a critical role for liver macrophage ER resident chaperone GP96/HSP90B1 in ALD, and its targeted inhibition represents a promising therapeutic approach in ALD.

Liver macrophages inhibit the endogenous antioxidant response in obesity-associated insulin resistance

Obesity and insulin resistance are risk factors for nonalcoholic fatty liver disease (NAFLD), the most common chronic liver disease worldwide. Because no approved medication nor an accurate and noninvasive diagnosis is currently available for NAFLD, there is a clear need to better understand the link between obesity and NAFLD. Lipid accumulation during obesity is known to be associated with oxidative stress and inflammatory activation of liver macrophages (LMs). However, we show that although LMs do not become proinflammatory during obesity, they display signs of oxidative stress. In livers of both humans and mice, antioxidant nuclear factor erythroid 2-related factor 2 (NRF2) was down-regulated with obesity and insulin resistance, yielding an impaired response to lipid accumulation. At the molecular level, a microRNA-targeting NRF2 protein, miR-144, was elevated in the livers of obese insulin-resistant humans and mice, and specific silencing of miR-144 in murine and human LMs was sufficient to restore NRF2 protein expression and the antioxidant response. These results highlight the pathological role of LMs and their therapeutic potential to restore the impaired endogenous antioxidant response in obesity-associated NAFLD.

Tissue-resident macrophages: guardians of organ homeostasis

Tissue-resident macrophages (M_TR) have recently emerged as a key rheostat capable of regulating the balance between organ health and disease. In most organs, ontogenetically and functionally distinct macrophage subsets fulfill a plethora of functions specific to their tissue environment. In this review, we summarize recent findings regarding the ontogeny and functions of macrophage populations in different mammalian tissues, describing how these cells regulate tissue homeostasis and how they can contribute to inflammation. Furthermore, we highlight new developments concerning certain general principles of tissue macrophage biology, including the importance of metabolism for understanding macrophage activation states and the influence of intrinsic and extrinsic factors on macrophage metabolic control. We also shed light on certain open questions in the field and how answering these might pave the way for tissue-specific therapeutic approaches.

Hepatic macrophage accumulation with aging: cause for concern?

Aging is associated with chronic, low-grade inflammation that adversely affects physiological function. The liver regulates systemic inflammation; it is a source of cytokine production and also scavenges bacteria from the portal circulation to prevent infection of other organs. The cells with primary roles in these functions, hepatic macrophages, become more numerous in the liver with "normal" aging (i.e., in the absence of disease). Here, we demonstrate evidence and potential mechanisms for this phenomenon, which include augmented tumor necrosis factor-α (TNF-α) and intercellular adhesion molecule-1 (ICAM-1) expression in the liver. Also, we discuss how an age-related impairment in autophagy within macrophages leads to a pro-oxidative state and ensuing production of proinflammatory cytokines, particularly interleukin 6 (IL-6). Given that the liver is a rich source of macrophages, we posit that it represents a major source of the elevated systemic IL-6 observed with aging, which is associated with physiological dysfunction. Testing a causal role for liver macrophage production of IL-6 during aging remains a challenge, yet interventions that have targeted macrophages and/or IL-6 have demonstrated promise in treating age-related diseases. These studies have demonstrated an age-related, deleterious reprogramming of macrophage function, which worsens pathology. Therefore, hepatic macrophage accrual is indeed a cause for concern, and therapies that attenuate the aged phenotype of macrophages will likely prove useful in promoting healthy aging.

Immunotherapy for Infarcts: In Vivo Postinfarction Macrophage Modulation Using Intramyocardial Microparticle Delivery of Map4k4 Small Interfering RNA

The myeloid cells infiltrating the heart early after acute myocardial infarction elaborate a secretome that largely orchestrates subsequent ventricular wall repair. Regulating this innate immune response could be a means to improve infarct healing. To pilot this concept, we utilized (β1,3-d-) glucan-encapsulated small interfering RNA (siRNA)-containing particles (GeRPs), targeting mononuclear phagocytes, delivered to mice as a one-time intramyocardial injection immediately after acute infarction. Findings demonstrated that cardiac macrophages phagocytosed GeRPs in vivo and had little systemic dissemination, thus providing a means to deliver local therapeutics. Acute infarcts were then injected in vivo with phosphate-buffered saline (PBS; vehicle) or GeRPs loaded with siRNA to Map4k4, and excised hearts were examined at 3 and 7 days by quantitative polymerase chain reaction, flow cytometry, and histology. Compared with infarcted PBS-treated hearts, hearts with intrainfarct injections of siRNA-loaded GeRPs exhibited 69–89% reductions in transcripts for Map4k4 (mitogen-activated protein kinase kinase kinase kinase 4), interleukin (IL)-1β, and tumor necrosis factor α at 3 days. Expression of other factors relevant to matrix remodeling—monocyte chemoattractant protein-1 (MCP-1), matrix metalloproteinases, hyaluronan synthases, matricellular proteins, and profibrotic factors transforming growth factor beta (TGF-β), and connective tissue growth factor (CTGF)—were also decreased. Most effects peaked at 3 days, but, in some instances (Map4k4, IL-1β, TGF-β, CTGF, versican, and periostin), suppression persisted to 7 days. Thus, direct intramyocardial GeRP injection could serve as a novel and clinically translatable platform for in vivo RNA delivery to intracardiac macrophages for local and selective immunomodulation of the infarct microenvironment.

Kupffer Cell Protein Isolation and Detection by Western Blot

Kupffer cells are the majority of tissue macrophages present in the body and are actively involved in the regulation of liver homeostasis as well as in the progression of liver diseases. Here we describe a methodology to successfully isolate and detect proteins from Kupffer cells and avoid the contamination of other liver cell populations.

High Glucose-Aggravated Hepatic Insulin Resistance: Role of the NLRP3 Inflammasome in Kupffer Cells

OBJECTIVE

This study aimed to investigate whether the NLRP3 inflammasome in Kupffer cells (KCs) can be activated in response to high glucose (HG) and to evaluate its influence on hepatic insulin sensitivity.

METHODS

Primary KCs and hepatocytes were isolated from mice, and lipid accumulation, glucose output, and insulin sensitivity of hepatocytes were investigated after culturing either alone or with KCs exposed to HG. The influence of HG-induced NLRP3 inflammasome activation in KCs on insulin sensitivity of hepatocytes was examined. Treatment with gadolinium trichloride caused KC depletion, and, subsequently, a streptozotocin-induced hyperglycemic mouse model was used to confirm the influence of KCs on hepatic insulin sensitivity.

RESULTS

Hepatocytes cocultured with KCs showed enhanced lipid accumulation, glucose output, and impaired insulin sensitivity when exposed to HG. Enhanced NLRP3 inflammasome activation was also evident in both hepatocytes and KCs. Moreover, KCs that were pretreated with caspase-1 inhibitor, NLRP3 inhibitor, and NLRP3 small interfering RNA corrected coculture-induced aberrances in insulin action and NLRP3 inflammasome activation in hepatocytes. KC coculture also increased interleukin-1β (IL-1β)-mediated nuclear factor-κB (NF-κB) activation in hepatocytes. In hyperglycemic mice, KC depletion inhibited NLRP3 inflammasome activation and improved hepatic insulin sensitivity.

CONCLUSIONS

NLRP3 inflammasome activation impaired insulin sensitivity through KC-derived IL-1β-mediated NF-κB activation in hepatocytes exposed to HG.

Vitamin D Receptor Activation in Liver Macrophages Ameliorates Hepatic Inflammation, Steatosis, and Insulin Resistance in Mice

BACKGROUND AND AIMS

Obesity-induced chronic inflammation is a key component in the pathogenesis of nonalcoholic fatty liver disease (NAFLD) and insulin resistance. Increased secretion of proinflammatory cytokines by macrophages in metabolic tissues promotes disease progression. In the diet-induced obesity (DIO) mouse model, activation of liver resident macrophages, or Kupffer cells (KCs), drives inflammatory responses, which recruits circulating macrophages and promotes fatty liver development, and ultimately contributes to impaired hepatic insulin sensitivity. Hepatic macrophages express the highest level of vitamin D receptors (VDRs) among nonparenchymal cells, whereas VDR expression is very low in hepatocytes. VDR activation exerts anti-inflammatory effects in immune cells.

APPROACH AND RESULTS

Here we found that VDR activation exhibits strong anti-inflammatory effects in mouse hepatic macrophages, including those isolated from DIO livers, and mice with genetic loss of Vdr developed spontaneous hepatic inflammation at 6 months of age. Under the chronic inflammation conditions of the DIO model, VDR activation by the vitamin D analog calcipotriol reduced liver inflammation and hepatic steatosis, significantly improving insulin sensitivity. The hyperinsulinemic euglycemic clamp revealed that VDR activation greatly increased the glucose infusion rate, while hepatic glucose production was remarkably decreased. Glucose uptake in muscle and adipose did not show similar effects, suggesting that improved hepatic insulin sensitivity is the primary contributor to the beneficial effects of VDR activation. Finally, specifically ablating liver macrophages by treatment with clodronate liposomes largely abolished the beneficial metabolic effects of calcipotriol, confirming that VDR activation in liver macrophages is required for the antidiabetic effect.

CONCLUSIONS

Activation of liver macrophage VDRs by vitamin D ligands ameliorates liver inflammation, steatosis and insulin resistance. Our results suggest therapeutic paradigms for treatment of NAFLD and type 2 diabetes mellitus.

Macrophages in metabolic associated fatty liver disease

Metabolic associated fatty liver disease (MAFLD), formerly named non-alcoholic fatty liver disease is the most common liver disorder in many countries. The inflammatory subtype termed steatohepatitis is a driver of disease progression to cirrhosis, hepatocellular carcinoma, liver transplantation, and death, but also to extrahepatic complications including cardiovascular disease, diabetes and chronic kidney disease. The plasticity of macrophages in response to various environmental cues and the fact that they can orchestrate cross talk between different cellular players during disease development and progression render them an ideal target for drug development. This report reviews recent advances in our understanding of macrophage biology during the entire spectrum of MAFLD including steatosis, inflammation, fibrosis, and hepatocellular carcinoma, as well as for the extra-hepatic manifestations of MAFLD. We discuss the underlying molecular mechanisms of macrophage activation and polarization as well as cross talk with other cell types such as hepatocytes, hepatic stellate cells, and adipose tissue. We conclude with a discussion on the potential translational implications and challenges for macrophage based therapeutics for MAFLD.

Activation of α7nAChR via vagus nerve prevents obesity-induced insulin resistance via suppressing endoplasmic reticulum stress-induced inflammation in Kupffer cells

Obesity is a major risk factor for type 2 diabetes mellitus and insulin resistance (IR). In the state of obesity, excess fat accumulates in the liver, a key organ in systemic metabolism, altering the inflammatory and metabolic signals contributing substantially to the development of hepatic IR. Current therapies for these metabolic disorders have not been able to reverse their rapidly rising prevalence. One of the reasons is that the effects of existing drugs are predominantly non-lasting [1,2]. The vagus nerve (VN) is known to play an essential role in maintaining metabolic homeostasis while decreased VN activity has been suggested to contribute to obesity associated metabolic syndrome [3,4]. Several studies have reported that activation of α7 nicotinic acetylcholine receptor (α7nAChR) cholinergic signaling with or without VN intervention has protective effects against obesity-related inflammation and other metabolic complications [5]. However, the molecular mechanisms are still not elucidated. Exaggerated endoplasmic reticulum (ER) stress and consequent dysregulated inflammation has been implicated in the development of lipid accumulation and IR [6]. Whether targeting α7nAChR can regulate IR through these pathways is rarely reported. Accordingly, the present proposal posits that activation of the α7nAChR by VNS attenuates ER stress induced inflammation, thus ameliorating hepatic IR in Kupffer cell. We will focus on the specific interaction between vagal cholinergic activity and the modulation of ER stress induced inflammation via the α7nAChR associated pathway during IR development. Recently, the Endocrine Society has emphasized the absence of specific evidence from basic science, clinical, and epidemiological literature to assess current knowledge regarding underlying mechanisms of obesity [7]. In this proposal, we assign a significant role to α7nAChR in obesity-induced hepatic IR, and suggest a possible therapeutic strategy with VNS intervention.

Immunometabolism in type 2 diabetes mellitus: tissue-specific interactions

The immune system is frequently described in the context of its protective function against infections and its role in the development of autoimmunity. For more than a decade, the interactions between the immune system and metabolic processes have been reported, in effect creating a new research field, termed immunometabolism. Accumulating evidence supports the hypothesis that the development of metabolic diseases may be linked to inflammation, and reflects, in some cases, the activation of immune responses. As such, immunometabolism is defined by 1) inflammation as a driver of disease development and/or 2) metabolic processes stimulating cellular differentiation of the immune components. In this review, the main factors capable of altering the immuno-metabolic communication leading to the development and establishment of obesity and diabetes are comprehensively presented. Tissue-specific immune responses suggested to impair metabolic processes are described, with an emphasis on the adipose tissue, gut, muscle, liver, and pancreas.

Chronic Adipose Tissue Inflammation Linking Obesity to Insulin Resistance and Type 2 Diabetes

Obesity is one of the major health burdens of the 21st century as it contributes to the growing prevalence of its related comorbidities, including insulin resistance and type 2 diabetes. Growing evidence suggests a critical role for overnutrition in the development of low-grade inflammation. Specifically, chronic inflammation in adipose tissue is considered a crucial risk factor for the development of insulin resistance and type 2 diabetes in obese individuals. The triggers for adipose tissue inflammation are still poorly defined. However, obesity-induced adipose tissue expansion provides a plethora of intrinsic signals (e.g., adipocyte death, hypoxia, and mechanical stress) capable of initiating the inflammatory response. Immune dysregulation in adipose tissue of obese subjects results in a chronic low-grade inflammation characterized by increased infiltration and activation of innate and adaptive immune cells. Macrophages are the most abundant innate immune cells infiltrating and accumulating into adipose tissue of obese individuals; they constitute up to 40% of all adipose tissue cells in obesity. In obesity, adipose tissue macrophages are polarized into pro-inflammatory M1 macrophages and secrete many pro-inflammatory cytokines capable of impairing insulin signaling, therefore promoting the progression of insulin resistance. Besides macrophages, many other immune cells (e.g., dendritic cells, mast cells, neutrophils, B cells, and T cells) reside in adipose tissue during obesity, playing a key role in the development of adipose tissue inflammation and insulin resistance. The association of obesity, adipose tissue inflammation, and metabolic diseases makes inflammatory pathways an appealing target for the treatment of obesity-related metabolic complications. In this review, we summarize the molecular mechanisms responsible for the obesity-induced adipose tissue inflammation and progression toward obesity-associated comorbidities and highlight the current therapeutic strategies.

Kupffer Cell and Monocyte-Derived Macrophage Identification by Immunofluorescence on Formalin-Fixed, Paraffin-Embedded (FFPE) Mouse Liver Sections

Kupffer cells are the liver-resident macrophages and represent the first line of defense between the pathogens circulating from the intestines through the portal vein and systemic circulation. Recent works have highlighted the complex heterogeneity of macrophage functions and origins, thus raising awareness on the need for a better characterization of macrophage populations. The immunohistochemistry method here described, allows for a rapid distinction between Kupffer cells and monocyte-derived macrophages present on formalin-fixed, paraffin-embedded mouse liver samples. This protocol has been optimized for its reproducibility, reliability, and simplicity.

NK cells induce hepatic ER stress to promote insulin resistance in obesity through osteopontin production

High-fat diet (HFD) induced hepatic endoplasmic reticulum (ER) stress drives insulin resistance (IR) and steatosis. NK cells in adipose tissue play an important role in the pathogenesis of IR in obesity. Whether NK cells in the liver can induce hepatic ER stress and thus promote IR in obesity is still unknown. We demonstrate that HFD-fed mice display elevated production of proinflammatory cytokine osteopontin (OPN) in hepatic NK cells, especially in CD49a⁺ DX5^- tissue-resident NK (trNK) cells. Obesity-induced ER stress, IR, and steatosis in the liver are ameliorated by ablating NK cells with neutralizing antibody in HFD-fed mice. OPN treatment enhances the expression of ER stress markers, including p-PERK, p-eIF2, ATF4, and CHOP in both murine liver tissues and HL-7702, a human liver cell line. Pretreatment of HL-7702 cells with OPN promotes hyperactivation of JNK and subsequent decrease of tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), resulting in impaired insulin signaling, which can be reversed by inhibiting ER stress. Collectively, we demonstrate that hepatic NK cells induce obesity-induced hepatic ER stress, and IR through OPN production.

Dysregulated liver lipid metabolism and innate immunity associated with hepatic steatosis in neonatal BBdp rats and NOD mice

In a previous study we reported that prediabetic rats have a unique gene signature that was apparent even in neonates. Several of the changes we observed, including enhanced expression of pro-inflammatory genes and dysregulated UPR and metabolism genes were first observed in the liver followed by the pancreas. In the present study we investigated further early changes in hepatic innate immunity and metabolism in two models of type 1 diabetes (T1D), the BBdp rat and NOD mouse. There was a striking increase in lipid deposits in liver, particularly in neonatal BBdp rats, with a less striking but significant increase in neonatal NOD mice in association with dysregulated expression of lipid metabolism genes. This was associated with a decreased number of extramedullary hematopoietic clusters as well as CD68⁺ macrophages in the liver of both models. In addition, PPARɣ and phosphorylated AMPKα protein were decreased in neonatal BBdp rats. BBdp rats displayed decreased expression of antimicrobial genes in neonates and decreased M2 genes at 30 days. This suggests hepatic steatosis could be a common early feature in development of T1D that impacts metabolic homeostasis and tolerogenic phenotype in the prediabetic liver.

Immune Regulation of Metabolic Homeostasis by Helminths and Their Molecules

Since time immemorial, humans have coevolved with a wide variety of parasitic helminths that have contributed to shape their immune system. The recent eradication of helminth infections in modern societies has coincided with a spectacular rise in inflammatory metabolic diseases, such as obesity, nonalcoholic steatohepatitis, and type 2 diabetes. Landmark studies in the emerging field of immunometabolism have highlighted the central role of the immune system in regulating metabolic functions, notably in adipose tissue, liver, and the gut. In this review we discuss how helminths, which are among the strongest natural inducers of type 2 immunity, and some of their unique immunomodulatory molecules, may contribute to the maintenance of tissue-specific and whole-body metabolic homeostasis and protection against obesity-associated meta-inflammation.

Dietary Methionine Restriction Reduces Inflammation Independent of FGF21 Action

OBJECTIVE

Methionine restriction (MR) decreases inflammation and improves markers of metabolic disease in rodents. MR also increases hepatic and circulating concentrations of fibroblast growth factor 21 (FGF21). Emerging evidence has suggested that FGF21 exerts anti-inflammatory effects. The purpose of this study was to determine the role of FGF21 in mediating the MR-induced reduction in inflammation.

METHODS

Wild-type and Fgf21^-/- mice were fed a high-fat (HF) control or HF-MR diet for 8 weeks. In a separate experiment, mice were fed a HF diet (HFD) for 10 weeks. Vehicle or recombinant FGF21 (13.6 µg/d) was administered via osmotic minipump for an additional 2 weeks. Inflammation and metabolic parameters were measured.

RESULTS

Fgf21^-/- mice were more susceptible to HFD-induced inflammation, and MR reduced inflammation in white adipose tissue (WAT) and liver of Fgf21^-/- mice. MR downregulated activity of signal transducer and activator of transcription 3 in WAT of both genotypes. FGF21 administration reduced hepatic lipids and blood glucose concentrations. However, there was little effect of FGF21 on inflammatory gene expression in liver or adipose tissue or circulating cytokines.

CONCLUSIONS

MR reduces inflammation independent of FGF21 action. Endogenous FGF21 is important to protect against the development of HFD-induced inflammation in liver and WAT, yet administration of low-dose FGF21 has little effect on markers of inflammation.

How Does Inflammation-Induced Hyperglycemia Cause Mitochondrial Dysfunction in Immune Cells?

Inflammatory mediators have an established role in inducing insulin resistance and promoting hyperglycemia. In turn, hyperglycemia has been argued to drive immune cell dysfunction as a result of mitochondrial dysfunction. Here, the authors review the evidence challenging this view. First, it is pointed out that inflammatory mediators are known to induce altered mitochondrial function. In this regard, critical care patients suffer both an elevated inflammatory tone as well as hyperglycemia, rendering it difficult to distinguish between the effects of inflammation and hyperglycemia. Second, emerging evidence indicates that a decrease in mitochondrial respiration and an increase in reactive oxygen species (ROS) production are not necessarily manifestations of pathology, but adaptations taking shape as the mitochondria is abdicating its adenosine triphosphate (ATP)-producing function (which is taken over by glycolysis) and instead becomes "retooled" for an immunological role. Collectively, these observations challenge the commonly held belief that acute hyperglycemia induces mitochondrial damage leading to immune cell dysfunction.

Liver macrophages regulate systemic metabolism through non-inflammatory factors

Liver macrophages (LMs) have been proposed to contribute to metabolic disease through secretion of inflammatory cytokines. However, anti-inflammatory drugs lead to only modest improvements in systemic metabolism. Here we show that LMs do not undergo a proinflammatory phenotypic switch in obesity-induced insulin resistance in flies, mice and humans. Instead, we find that LMs produce non-inflammatory factors, such as insulin-like growth factor-binding protein 7 (IGFBP7), that directly regulate liver metabolism. IGFBP7 binds to the insulin receptor and induces lipogenesis and gluconeogenesis via activation of extracellular-signal-regulated kinase (ERK) signalling. We further show that IGFBP7 is subject to RNA editing at a higher frequency in insulin-resistant than in insulin-sensitive obese patients (90% versus 30%, respectively), resulting in an IGFBP7 isoform with potentially higher capacity to bind to the insulin receptor. Our study demonstrates that LMs can contribute to insulin resistance independently of their inflammatory status and indicates that non-inflammatory factors produced by macrophages might represent new drug targets for the treatment of metabolic diseases.

The role of macrophages in nonalcoholic fatty liver disease and nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) and its inflammatory and often progressive subtype nonalcoholic steatohepatitis (NASH) are becoming the leading cause of liver-related morbidity and mortality worldwide, and a primary indication for liver transplantation. The pathophysiology of NASH is multifactorial and not yet completely understood; however, innate immunity is a major contributing factor in which liver-resident macrophages (Kupffer cells) and recruited macrophages play a central part in disease progression. In this Review, we assess the evidence for macrophage involvement in the development of steatosis, inflammation and fibrosis in NASH. In this process, not only the polarization of liver macrophages towards a pro-inflammatory phenotype is important, but adipose tissue macrophages, especially in the visceral compartment, also contribute to disease severity and insulin resistance. Macrophage activation is mediated by factors such as endotoxins and translocated bacteria owing to increased intestinal permeability, factors released from damaged or lipoapoptotic hepatocytes, as well as alterations in gut microbiota and defined nutritional components, including certain free fatty acids, cholesterol and their metabolites. Reflecting the important role of macrophages in NASH, we also review studies investigating drugs that target macrophage recruitment to the liver, macrophage polarization and their inflammatory effects as potential treatment options for patients with NASH.

Yeast glucan particles enable intracellular protein delivery in Drosophila without compromising the immune system

Glucan particles spread through the whole organism quickly, accumulate in sites of macrophage occurrence and can deliver cargo into the macrophages with a negligible effect on immune response activation.

Glucan-Encapsulated siRNA Particles (GeRPs) for Specific Gene Silencing in Adipose Tissue Macrophages

Macrophages are cells of the immune system that have been suggested as important regulators of whole-body metabolism in mammals. In obesity, adipose tissue macrophages (ATMs) are thought to play both a detrimental and a beneficial role in the regulation of insulin sensitivity. Here, we describe a protocol to prepare and administer glucan-encapsulated RNAi particles (GeRPs), for specific delivery of siRNA and subsequent gene silencing in ATMs in obese mice. Using the GeRP technology to silence genes provides a unique method to study the function of factors expressed by ATMs in the regulation of metabolism.

Myeloid-specific deletion of Zfp36 protects against insulin resistance and fatty liver in diet-induced obese mice

Obesity is associated with adipose tissue inflammation that contributes to insulin resistance. Zinc finger protein 36 (Zfp36) is an mRNA-binding protein that reduces inflammation by binding to cytokine transcripts and promoting their degradation. We hypothesized that myeloid-specific deficiency of Zfp36 would lead to increased adipose tissue inflammation and reduced insulin sensitivity in diet-induced obese mice. As expected, wild-type (Control) mice became obese and diabetic on a high-fat diet, and obese mice with myeloid-specific loss of Zfp36 [knockout (KO)] demonstrated increased adipose tissue and liver cytokine mRNA expression compared with Control mice. Unexpectedly, in glucose tolerance testing and hyperinsulinemic-euglycemic clamp studies, myeloid Zfp36 KO mice demonstrated improved insulin sensitivity compared with Control mice. Obese KO and Control mice had similar macrophage infiltration of the adipose depots and similar peripheral cytokine levels, but lean and obese KO mice demonstrated increased Kupffer cell (KC; the hepatic macrophage)-expressed Mac2 compared with lean Control mice. Insulin resistance in obese Control mice was associated with enhanced Zfp36 expression in KCs. Compared with Control mice, KO mice demonstrated increased hepatic mRNA expression of a multitude of classical (M1) inflammatory cytokines/chemokines, and this M1-inflammatory hepatic milieu was associated with enhanced nuclear localization of IKKβ and the p65 subunit of NF-κB. Our data confirm the important role of innate immune cells in regulating hepatic insulin sensitivity and lipid metabolism, challenge-prevailing models in which M1 inflammatory responses predict insulin resistance, and indicate that myeloid-expressed Zfp36 modulates the response to insulin in mice.

Decreasing CB1 receptor signaling in Kupffer cells improves insulin sensitivity in obese mice

Objective

Obesity-induced accumulation of ectopic fat in the liver is thought to contribute to the development of insulin resistance, and increased activity of hepatic CB₁R has been shown to promote both processes. However, lipid accumulation in liver can be experimentally dissociated from insulin resistance under certain conditions, suggesting the involvement of additional mechanisms. Obesity is also associated with pro-inflammatory changes which, in turn, can promote insulin resistance. Kupffer cells (KCs), the liver's resident macrophages, are the major source of pro-inflammatory cytokines in the liver, such as TNF-α, which has been shown to inhibit insulin signaling in multiple cell types, including hepatocytes. Here, we sought to identify the role of CB₁R in KCs in obesity-induced hepatic insulin resistance.

Methods

We used intravenously administered β-D-glucan-encapsulated siRNA to knock-down CB₁R gene expression selectively in KCs.

Results

We demonstrate that a robust knock-down of the expression of Cnr1, the gene encoding CB₁R, results in improved glucose tolerance and insulin sensitivity in diet-induced obese mice, without affecting hepatic lipid content or body weight. Moreover, Cnr1 knock-down in KCs was associated with a shift from pro-inflammatory M1 to anti-inflammatory M2 cytokine profile and improved insulin signaling as reflected by increased insulin-induced Akt phosphorylation.

Conclusion

These findings suggest that CB₁R expressed in KCs plays a critical role in obesity-related hepatic insulin resistance via a pro-inflammatory mechanism.

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CB₁R signaling promotes hepatic insulin resistance by promoting hepatic steatosis and hepatic inflammation.

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CB₁R knock-down in liver macrophages (Kupffer cells, KCs) improves global insulin resistance and glucose homeostasis.

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CB₁R expressed in KCs play a critical role in hepatic insulin resistance independent of ectopic fat in the liver or adipose tissue inflammation.

Macrophage heterogeneity and energy metabolism

Macrophages are versatile and multifunctional cell types present in most vertebrate tissues. They are the first line of defense against pathogens through phagocytosis of microbial infections, particles and dead cells. Macrophages harbor additional functions besides immune protection by participating in essential homeostatic and tissue development functions. The immune response requires a concomitant and coordinated regulation of the energetic metabolism. In this review, we will discuss how macrophages influence metabolic tissues and in turn how metabolic pathways, particularly glucose and lipid metabolism, affect macrophage phenotypes.

Inflammation and the Metabolic Syndrome: The Tissue-Specific Functions of NF-κB

Obesity is becoming a major health concern in Western society, and medical conditions associated with obesity are grouped in the metabolic syndrome. Overnutrition activates several proinflammatory signaling pathways, leading to a condition of chronic low-grade inflammation in several metabolic tissues affecting their proper function. Nuclear factor kappa B (NF-κB) signaling is a crucial pathway in this process and has been studied extensively in the context of obesity and the metabolic syndrome. Here we give an overview of the molecular mechanisms behind the inflammatory function of NF-κB in response to overnutrition and the effect this has on several metabolic tissues.

SREBP1 Contributes to Resolution of Pro-inflammatory TLR4 Signaling by Reprogramming Fatty Acid Metabolism

Macrophages play pivotal roles in both the induction and resolution phases of inflammatory processes. Macrophages have been shown to synthesize anti-inflammatory fatty acids in an LXR-dependent manner, but whether the production of these species contributes to the resolution phase of inflammatory responses has not been established. Here, we identify a biphasic program of gene expression that drives production of anti-inflammatory fatty acids 12-24 hr following TLR4 activation and contributes to downregulation of mRNAs encoding pro-inflammatory mediators. Unexpectedly, rather than requiring LXRs, this late program of anti-inflammatory fatty acid biosynthesis is dependent on SREBP1 and results in the uncoupling of NFκB binding from gene activation. In contrast to previously identified roles of SREBP1 in promoting production of IL1β during the induction phase of inflammation, these studies provide evidence that SREBP1 also contributes to the resolution phase of TLR4-induced gene activation by reprogramming macrophage lipid metabolism.

Isolation of Kupffer Cells and Hepatocytes from a Single Mouse Liver

Liver perfusion is a common technique used to isolate parenchymal and non-parenchymal liver cells for in vitro experiments. This method allows hepatic cells to be separated based on their size and weight, by centrifugation using a density gradient. To date, other methods allow the isolation of only one viable hepatic cellular fraction from a single mouse; either parenchymal (hepatocytes) or non-parenchymal cells (i.e., Kupffer cells or hepatic stellate cells). Here, we describe a method to isolate both hepatocytes and Kupffer cells from a single mouse liver, thereby providing the unique advantage of studying different liver cell types that have been isolated from the same organism.

Liver innate immune cells and insulin resistance: the multiple facets of Kupffer cells

Obesity, which affects 600 million adults worldwide, is a major risk factor for type 2 diabetes (T2D) and insulin resistance. Current therapies for these metabolic disorders include weight management by lifestyle intervention or bariatric surgery and pharmacological treatment with the aim of regulating blood glucose. Probably because of their short-term effectiveness, these therapies have not been able to stop the rapidly rising prevalence of T2D over the past decades, highlighting an urgent need to develop new therapeutic strategies. The role of immune cells, such as macrophages, in insulin resistance has been extensively studied. Major advances have been made to elucidate the role of adipose tissue macrophages in these pathogeneses. Recently, anti-inflammatory drugs have been suggested as an alternative treatment for T2D, and clinical trials of these agents are currently ongoing. In addition, results of previous clinical trials using antibodies against inflammatory cytokines, which showed modest effects, are now being rigorously re-evaluated. However, it is still unclear how liver macrophages [termed Kupffer cells (KCs)], which constitute the major source of macrophages in the body, contribute to the development of insulin resistance. In this review, we will discuss the present understanding of the role of liver immune cells in the development of insulin resistance. We will particularly focus on KCs, which could represent an attractive target for the treatment of metabolic diseases.

Macrophages in age-related chronic inflammatory diseases

Chronic inflammation is the common pathological basis for such age-associated diseases as cardiovascular disease, diabetes, cancer and Alzheimer’s disease. A multitude of bodily changes occur with aging that contribute to the initiation and development of inflammation. In particular, the immune system of elderly individuals often exhibits diminished efficiency and fidelity, termed immunosenescence. But, although immune responses to new pathogens and vaccines are impaired, immunosenescence is also characterized by a basal systemic inflammatory state. This alteration in immune system function likely promotes chronic inflammation. Changes in the tissue microenvironment, such as the accumulation of cell debris, and systemic changes in metabolic and hormonal signals, also likely contribute to the development of chronic inflammation. Monocyte/macrophage lineage cells are crucial to these age-associated changes, which culminate in the development of chronic inflammatory diseases. In this review, we will summarize the diverse physiological and pathological roles of macrophages in the chronic inflammation underlying age-associated diseases.

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Peptide- and Amine-Modified Glucan Particles for the Delivery of Therapeutic siRNA

Translation of siRNA technology into the clinic is limited by the need for improved delivery systems that target specific cell types. Macrophages are particularly attractive targets for RNAi therapy because they promote pathogenic inflammatory responses in a number of important human diseases. We previously demonstrated that a multicomponent formulation of β-1,3-d-glucan-encapsulated siRNA particles (GeRPs) can specifically and potently silence genes in mouse macrophages. A major advance would be to simplify the GeRP system by reducing the number of delivery components, thus enabling more facile manufacturing and future commercialization. Here we report the synthesis and evaluation of a simplified glucan-based particle (GP) capable of delivering siRNA in vivo to selectively silence macrophage genes. Covalent attachment of small-molecule amines and short peptides containing weak bases to GPs facilitated electrostatic interaction of the particles with siRNA and aided in the endosomal release of siRNA by the proton-sponge effect. Modified GPs were nontoxic and were efficiently internalized by macrophages in vitro. When injected intraperitoneally (i.p.), several of the new peptide-modified GPs were found to efficiently deliver siRNA to peritoneal macrophages in lean, healthy mice. In an animal model of obesity-induced inflammation, i.p. administration of one of the peptide-modified GPs (GP-EP14) bound to siRNA selectively reduced the expression of target inflammatory cytokines in the visceral adipose tissue macrophages. Decreasing adipose tissue inflammation resulted in an improvement of glucose metabolism in these metabolically challenged animals. Thus, modified GPs represent a promising new simplified system for the efficient delivery of therapeutic siRNAs specifically to phagocytic cells in vivo for modulation of inflammation responses.

Regulation of metabolism by the innate immune system

Low-grade tissue inflammation induced by obesity can result in insulin resistance, which in turn is a key cause of type 2 diabetes mellitus. Cells of the innate immune system produce cytokines and other factors that impair insulin signalling, which contributes to the connection between obesity and the onset of type 2 diabetes mellitus. Here, we review the innate immune cells involved in secreting inflammatory factors in the obese state. In the adipose tissue, these cells include proinflammatory adipose tissue macrophages and natural killer cells. We also discuss the role of innate immune cells, such as anti-inflammatory adipose tissue macrophages, eosinophils, group 2 innate lymphoid cells and invariant natural killer T cells, in maintaining an anti-inflammatory and insulin-sensitive environment in the lean state. In the liver, both Kupffer cells and recruited hepatic macrophages can contribute to decreased hepatic insulin sensitivity. Proinflammatory macrophages might also adversely affect insulin sensitivity in the skeletal muscle and pancreatic β-cell function. Finally, this Review provides an overview of the mechanisms for regulating proinflammatory immune responses that could lead to future therapeutic opportunities to improve insulin sensitivity.