The Pivotal Role of Macrophages in Metabolic Distress

*Joseph Roberts, Padraic G. Fallon and Emily Hams*

## **Abstract**

Obesity is a prevalent condition with several associated co-morbidities including the development of metabolic diseases. In obesity there is immune cell infiltration into the white adipose tissue and this is associated with the generation of inflammation and insulin resistance (IR). A large majority of the infiltrating leukocytes in obese adipose tissue are pro-inflammatory macrophages, which upon activation induce a switch in metabolism from oxidative phosphorylation, as is utilised by macrophages in lean adipose tissue, towards aerobic glycolysis. The signalling pathways evoked in the recruited macrophages induce the release of pro-inflammatory cytokines, in signalling pathways which directly interfere with insulin signalling and thus induce a state of IR. As macrophages appear to play such a pivotal role in the generation of IR and are the largest leukocyte population in the adipose tissue, they provide a promising therapeutic target. Indeed, there are several strategies currently being studied to induce a 'switch' in macrophages associated with obese adipose tissue, towards the phenotype of those associated with lean adipose tissue, with arguably the most promising being those strategies designed to target the metabolic pathways within the macrophages. This chapter will discuss the polarisation and activation of macrophages within lean and obese adipose tissue and how these cells can be targeted therapeutically.

**Keywords:** macrophage, obesity, metabolism, inflammation

### **1. Introduction**

Obesity is defined as abnormal or excessive fat accumulation and is linked with increased risk of development of multiple co-morbidities, including cardiovascular disease, type 2 diabetes, musculoskeletal disorders and certain cancers. Obesity and its associated co-morbidities are a significant health concern facing the global population. Worldwide obesity has tripled since 1975, with 39% of adults considered overweight and 13% considered obese [1]. This situation is prominent in childhood, with 41 million of the global under five population overweight or obese [1].

Obesity induces a state of low-grade systemic inflammation, characterized by increased serum levels of pro-inflammatory mediators, including C Reactive Protein (CRP), Tumour Necrosis Factor (TNF)-α, Interleukin (IL)-1β and IL-6, which contributes to metabolic dysfunction and insulin resistance (IR) [2]. Although the mechanisms underlying this inflammatory response are not fully understood, activation of adipose tissue macrophages (ATM) contributes to this inflammatory state, and therefore to the development of insulin resistance (IR) [3, 4]. Conversely, in lean individuals the immune repertoire constitutes a more anti-inflammatory phenotype, with ATM alongside regulatory T cells (Tregs) releasing cytokines such as IL-10 and transforming growth factor (TGF)-β, which increase insulin sensitivity [5]. Therefore, the role of ATM in metabolic function is clearly an area of interest, indeed transcriptional profiling has identified how quickly macrophages can respond and adapt to alterations in their microenvironment [6]. This chapter will focus on the role macrophages play in the pathogenesis of metabolic disorders and explore if reeducation of these cells provides a target for therapeutic intervention in obesity and its related co-morbidities.

### **2. The microenvironment of the adipose tissue in lean and obese individuals**

Obesity historically was believed to be due to a combination of genetic predisposition and environmental factors, however, more recently it has been recognised that immunological factors can also contribute to the pathogenesis of obesity. Indeed, while over 30 gene loci combinations have been associated with the development of obesity and metabolic disease, these loci are only associated with 2–3% of the incidence of these conditions [7]. Further the energy-dense modern Western diet combined with a sedentary lifestyle undoubtedly adds to the obesity epidemic. Recent work has identified the links between dysbiosis in the intestinal microbiome and immune cell activation, linked to the ingestion of high-fat, low-fibre diets, and the development of obesity [8].

#### **2.1 The role of the microbiome**

The intestinal microbiome is essential for processing dietary polysaccharides and has been identified as a key regulator of systemic inflammation in obesity [9]. Mouse studies are routinely used to study the mechanisms underlying obesity and metabolic disease. Due to the nature of obesity being largely related to diet, diet-induced models are often favoured over genetic models (for example, leptin deficient ob/ob mice). Indeed, studies using a high-fat diet (HFD; equivalent to 60% animal-derived fats in the diet) have been used to study the potential implications of alteration in the microbiome related to diet as well as other obesity-related pathogenesis. It has been shown that the microbiome in obese mice has an increased capacity to harvest energy from the diet compared to the microbiome from lean mice [10]. Microbiome transfer studies, in which intestinal microbiota from mice raised in conventional housing was transferred into germ-free mice, induced a 60% increase in body fat and IR within 14 days, despite a reduction in food consumption [11]. The transfer of microbiota-derived products such as lipopolysaccharides and peptidoglycans, have shown to promote metabolic endotoxemia, which induces proinflammation in adipose tissue [12]. In contrast, the microbiota of the gut bacterial fermentation of dietary fibre was shown to have anti-inflammatory effects [12]. Indeed, it was shown that the transfer of intestinal microbiota from lean donors increased the insulin sensitivity in individuals with metabolic syndrome [13].

#### **2.2 Adipose tissue**

Adipose tissue (AT) is an important metabolic organ, which helps orchestrates metabolic and endocrine functions as well as immune responses [12]. AT functions to store excess nutrients as triacylglycerides and releases fatty acids in the fasted

*The Pivotal Role of Macrophages in Metabolic Distress DOI: http://dx.doi.org/10.5772/intechopen.86474*

state, provide cold insulation and protection of vital organs. In the AT of obese individuals, there is significant adipocyte hyperplasia and adipose tissue hypertrophy [14]. AT consists of mature adipocytes, pre-adipocytes, fibroblasts, endothelial cells, histocytes and populations of immune cells including monocytes, macrophages, natural killer (NK) cells, innate lymphoid cells (ILCs) and lymphocytes. AT is classified into three categories, namely white (WAT), beige or 'brite' (beige/brite) and brown (BAT). WAT accounts for approximately 50% of body mass and can release free fatty acids (FFA) into circulation when glucose levels are low. Whilst BAT plays an important role in thermogenesis and the production of heat [15].

The AT of obese individuals is in a state of chronic low-grade inflammation with marked infiltration various pro-inflammatory immune cells such as CD8 cells, NK cells, ILC1, Th1 cells, neutrophils and pro-inflammatory macrophages [16]. Conversely, the immune repertoire of AT from lean individuals comprises anti-inflammatory cell populations, including eosinophils, ILC2, Tregs, Th2 cells and anti-inflammatory macrophages [16] (**Figure 1**). In lean mice, ATM constitutes approximately 5% of cells, conversely in obese mice ATM can account for up to 50% of the cells [3]. Whilst in lean human AT, ATM comprises 4% of cells compared to 12% in excess adiposity [17]. In addition to macrophages, lymphocytes and ILCs also play roles in the regulation of AT inflammation, the roles of which seem to largely involve supporting the polarisation state of the ATM populations. For example, eosinophils provide a source of IL-4 promoting an M2 phenotype. In obese mice adipose eosinophils are decreased, whilst depletion of eosinophils results in increased M1 ATM, weight gain and systemic IR [18]. Furthermore, ILC2

#### **Figure 1.**

*Immune cell composition of adipose tissue in a lean and obese state. In the lean state, eosinophils and type 2 innate lymphoid cells (ILC2s) produce Th2 cytokines (IL-4, IL-5 and IL-13), which promotes eosinophil recruitment and anti-inflammatory polarisation of macrophages towards an M2 phenotype, which is supported in the normoxic state of lean adipose tissue. In turn, M2 macrophages secrete anti-inflammatory cytokines such as IL-10. In the obese state, adipocyte hypertrophy, hyperplasia and hypoxia cause necrotic adipocytes, resulting in pro-inflammatory state and macrophage recruitment, forming crown like structures (CLS) surrounding the adipocytes. These macrophages are polarised towards an M1 phenotype and secrete the pro-inflammatory cytokines IL-6, IL-1β and TNF-α.*

have been identified as a source of IL-5, a key cytokine in eosinophil recruitment. Thus, accumulation of eosinophils and maintenance of M2 ATM relies on ILC2 [19].

#### **2.3 The role of the adaptive immune system in obesity**

The adaptive immune system also plays an important role in obesity and metabolic disease. B cells have been shown to be involved in obesity induced inflammation and IR [20]. In obese mice, there was an identified increased in IgG<sup>+</sup> B cells and IgG production, associated with activation of M1 ATM, increased Th1 cells and conversely, attrition of Treg cells [21]. In addition, transferring B cells into B cell deficient mice induced IR [20]. Furthermore, in obese mice, there is increased CD8<sup>+</sup> effector T cell recruitment in epididymal AT. Interestingly, it is reported that CD8+ T cells precede macrophage infiltration and deletion of CD8+ T cells resulted in reduced macrophage infiltration and AT inflammation whilst improving IR [22]. Conversely, both Treg cells and iNKT cells are negatively associated with obesityinduced inflammation and are enriched in lean AT. Indeed, both these immune cells are known to secrete IL-10 which promotes M2 macrophage polarisation [23, 24].

#### **2.4 Adipose tissue macrophages**

ATMs appear to play a major role in the regulation of obesity-related inflammation, with different macrophage phenotypes associated with divergent roles in the AT. In lean animals, ATM function to maintain the homeostatic micro-environment in AT by taking up excess lipids and phagocytosing dead adipocytes. Broadly speaking macrophages present in lean AT are of an M2 phenotype, which have been shown to suppress inflammation in AT [25]. Furthermore, M2 macrophages in lean AT have been associated with brown fat activation and 'beiging' of WAT in mouse models of obesity, via expression of tyrosine hydroxylase, which induces thermogenesis [26, 27]. However, this process has recently been queried, with IL-4-stimulated macrophages failing to generate sufficient levels of catecholamines to contribute to adipose tissue adaptive thermogenesis [28]. Conversely, excess lipid uptake in obese AT, induces M1 polarisation and along with excess lipid droplets, immune cells and necrotic adipocytes this forms a component called 'crown-like' structures (CLS) [29, 30]. Indeed, it has been shown that more than 90% of all macrophages in WAT of obese mice and humans are localized to dead adipocytes [31]. This metabolic activation of M1 macrophages in obese AT is associated with increased pro-inflammatory cytokines in the AT and recruitment and activation of M1 macrophages in the AT [32].

ATM in lean AT is considered a resident macrophage population, which originates from yolk-sac progenitors and self-renews via proliferation under homeostatic conditions. Over time into adulthood resident ATMs are replaced with circulating monocytes derived from bone marrow [33]. Using mouse bone marrow chimera experiments, following transplanting donor CD45.1+ bone marrow into recipient CD45.2+ mice, and maintenance on obesity-inducing HFD, 85% of the ATM were donor-derived compared to 15% that were recipient-derived [3]. Interestingly, the polarization of macrophages in obesity from an M2 to an M1 phenotype has been mainly attributed to the recruitment of monocytes to AT, rather than the conversion of tissue resident M2 macrophages [34]. Murine monocytes can be classified through the expression of Ly6C, with Ly6Chi monocytes considered inflammatory. In the steady state Ly6Chi monocytes differentiate into Ly6Clo monocytes in the circulation, which are believed to differentiate into M2 macrophages in the tissue. However, in obese AT in response to inflammatory stimuli such as the monocyte chemoattractant CCL2, Ly6Chi macrophages are recruited to the AT where they

differentiate to M1-like ATM [25]. Indeed, in absence of Ccl2 expression macrophages expressed an M2 gene profile [35].

ATM represents the largest population of leukocytes within the AT and plays many vital homeostatic roles including tissue remodelling and insulin sensitivity. However, with progressive obesity ATM are the key mediators of inflammation, IR and the impairment of adipocyte function.
