**2.2. Adipokines**

Adipose tissue was considered just an energy (triglycerides) storage site until obesity arises as a health problem worldwide. Adipose tissue came to the fore as an active secretory organ involved in various physiological and pathophysiological processes. Adipokines is a term used to identify molecules released from adipose tissue; some of them are secreted by others tissues (i.e. TNF-α, IL-1A, -1β, -5, -6, -8, 10, -15, -18) and certain are mainly or exclusively synthetized by adipocytes (i.e. leptin, adiponectin, resistin), these adipokines deserve that proposed term adipokinome [40, 41].

#### *2.2.1. Leptin*

Leptin is an adipokine secreted principally by white adipose tissue that regulates food intake and energy expenditure; furthermore, it also plays an important role in glucose homeostasis, immunity and fertility among others [42, 43]. Leptin exerts its action through leptin receptors, which are transmembrane proteins, members of the class I cytokine receptor superfamily, their pathway involves JAK/STAT, PI3K, MAPK/ERK systems, and PKC [42, 43]. There are six (a–f) isoforms of the leptin receptor generated by alternative splicing, b-isoform is the longest one, and have all the signalling motifs; moreover, it is the most expressed in diverse cell lineages (i.e. adipocytes, myocytes, immune cells, neurons) permitting to leptin act in autocrine, paracrine and endocrine ways [44, 45].

Leptin production is proportional to the amount of adipose tissue; so, in subjects within a normal range weight, increasing leptin levels suppresses the need to eat by inhibiting the release of orexigenic neuropeptides (e.g. neuropeptide Y and Agouti-related protein) in the arcuate nucleus of hypothalamus, while obese individuals do not have this physiological response, a state called 'leptin resistance' [46].

It has been proposed that the establishment of this condition is a consequence of the combination of three main mechanisms: diminished intracellular leptin-receptor signalling, abnormal transport of leptin across the blood-brain barrier and development programming disorders; however, the molecular mechanisms by which lesser sensitivity to leptin is present in obesity have not yet been defined [46, 47].

#### *2.2.2. Adiponectin*

diabetes mellitus type 2 or obesity, there are reports in fed mice with high fat diet, linking these

In recent years, studies have been published to identify innate lymphoid cells (ILCs), all members of this new family are characterized by a similar lymphocyte morphology, however, lack markers on its surface that identifies them as another immune cell type, because this is

pathways: (1) the first called cytotoxic ILCs, integrated by classic NK; (2) on the other hand, we have non-cytotoxical ILCs. The last group is subdivided into three types: ILC1s, ILC2s and ILC3s; they express T-bet, GATA-3 and ROR-γT, respectively, which is the main difference between them. ILCs can directly communicate with several varieties of cells and regulate

ILC2s plays an important role in the regulation of glucose metabolism, lipid storage and redox balance in lean individuals. It accomplishes these by communicating with other immune cells associated with the type 2 immune axis (i.e. M2, eosinophils and invariant natural killer T) and participates in cross-talk with adipocytes [8, 39]. These cells produce cytokines associated with lymphocyte T-helper 2, cytokines that are required for immunity against helminths, allergic

In contrast, it was found that cytotoxic ILCs (individuals and mice) and non-cytotoxic ILC1s (mice) are increased in visceral adipose tissue when obesity is present, accompanied by an increase in the production of interferon gamma, the latter contributes to change of the phenotype of macrophages to M1, thus, favours an inflammatory environment and increased

Adipose tissue was considered just an energy (triglycerides) storage site until obesity arises as a health problem worldwide. Adipose tissue came to the fore as an active secretory organ involved in various physiological and pathophysiological processes. Adipokines is a term used to identify molecules released from adipose tissue; some of them are secreted by others tissues (i.e. TNF-α, IL-1A, -1β, -5, -6, -8, 10, -15, -18) and certain are mainly or exclusively synthetized by adipocytes (i.e. leptin, adiponectin, resistin), these adipokines deserve that proposed term

Leptin is an adipokine secreted principally by white adipose tissue that regulates food intake and energy expenditure; furthermore, it also plays an important role in glucose homeostasis, immunity and fertility among others [42, 43]. Leptin exerts its action through leptin receptors, which are transmembrane proteins, members of the class I cytokine receptor superfamily, their pathway involves JAK/STAT, PI3K, MAPK/ERK systems, and PKC [42, 43]. There are six (a–f) isoforms of the leptin receptor generated by alternative splicing, b-isoform is the longest one,

) [39]. The ILCs come from two development

cells with an increasing adiposity and insulin resistance [8, 10].

immunity, inflammation and homeostasis in different tissues [8, 39].

defined as lacking cells lineage markers (Lin−

inflammation and tissue repair [8].

**2.2. Adipokines**

adipokinome [40, 41].

*2.2.1. Leptin*

recruitment of immune cells type 1 axis [8, 10, 12].

*2.1.4. Innate lymphoid cells*

162 Adiposity - Omics and Molecular Understanding

Adiponectin is a multifunctional and multi-named adipokine (adipocyte complement-related protein of 30 kDa, Acrp30; gelatin binding protein of 28 kDa, GBP-28; adipose most abundant gene transcript 1, apM1), coded by *ADIPOQ* gene, is a major adipocyte-secreted protein and is down-regulated in obesity and its co-morbidities. Adiponectin regulates metabolic homeostasis by acting on organs such as the brain, kidney, liver, pancreas and skeletal muscle by exerting potent insulin-sensitizing, anti-atherogenic and anti-inflammatory activities [8, 48].

Adiponectin is synthesized as a monomer, however, suffers extensive post-translational modifications to form trimers, hexamers and high molecular weight species (HMW, 12–18 monomers) before being secreted by adipocytes. Recent evidence suggests that depending on the degree of multimerization, different biological effects have been obtained [49, 50].

Biological activity of adiponectin is mainly mediated by binding to one of its two adiponectin receptors: AdipoR1 and AdipoR2. These receptors are differentially expressed, and adiponectin shows distinct affinity to them according to its multimerization degree [51]. AdipoR1 is most commonly found in skeletal muscle and binds preferably to low molecular weight species (trimers and hexamers), whereas AdipoR2 is abundant in liver and binds easily to HMW adiponectin [49, 51]. Liver and skeletal muscle have a crucial role in the IR process, therapeutic effect of thiazolidinediones is in part due to the enhanced expression of adiponectin and its receptors through PPAR-γ activation [52].

#### *2.2.3. Resistin*

Resistin was described in mice as the responsible molecule of IR; however in humans, results were not conclusive, in part because its specific receptor has not been identified yet. It is an adipokine that stimulates the synthesis of pro-inflammatory cytokines among which are: TNFα, IL-1, IL-6 and IL-12; in various types of cells through pathway-dependent signalling nuclear factor (NF)-κB [17, 53]. It also induces increased expression of adhesion molecules (i.e. VCAM-1, ICAM-1) and chemokines (i.e. CX3CL1, CX3CR1) in human endothelial cells [17, 53]. Various studies report positive correlations of serum resistin levels with the amount of body fat, however, other studies have found no correlation [53–55]. The most important association of circulating resistin levels reported is with C-reactive protein, which could be a marker of systemic inflammation [53].

#### *2.2.4. Chemerin*

It is secreted by adipocytes; it is closely associated with amount and distribution of adipose tissue. As a chemoattractant protein, chemerin acts as a ligand for the coupled G-receptor protein (ChemR23) and participates in both adaptive and innate immunity [56]. In humans, chemerin gene (*RRARES2*) is highly expressed in WAT and to a lesser extent in liver and lungs. On immune cells, chemerin is known to stimulate chemotaxis of dendritic cells, macrophages and natural killer (NK) cells. Meanwhile, its receptor, ChemR23 gene (*CMKLR1*), is expressed in dendritic cells, monocyte/macrophages and endothelial cells [18, 56, 57]. ChemR23 is involved in the differentiation of adipocytes and increased intracellular glucose or lipids promote its expression [18].

The interaction of chemerin/ChemR23 has been shown to reduce cytokines, chemokines and phagocytosis, proving to be important in the inflammatory process associated with obesity [18, 57]. In this context, chemerin/ChemR23 axis has been shown to impact IR development, which influences the clinical course and severity of obesity-related diseases.

As has been exposed, dysregulation of adipokinome due to accumulation of adipose tissue in obesity establishes and perpetuates a vicious circle from which emerges the chronic low-grade inflammation state.

#### **2.3. Low-grade inflammation state in obesity**

Inflammation is a physiological response to a stimulus (i.e. injury or infection) described by Celsus and Galen and is characterized by five classical signs: pain, heat, redness, swelling and loss of function [58, 59]. The inflammation resolution is an active process influenced, in part, by the time and especially regulated by the formation of a group of lipid mediators, which are identified as LXs, protectins and resolvins [60].

The low-grade inflammation state is a term used to define the activation of the vascular endothelium and presence of inflammatory cells in the absence of the five classical signs (subclinical) [61]. This state is due, at least initially, to adipose tissue hypertrophy present in obese individuals because different pathological processes occurs (i.e. fatty acids in excess, hypoxia, cell infiltration and activation of the inflammasome), this pro-inflammatory state is chronic in obesity and now is considered the etiologic agent of its co-morbidities [58, 62]. Effects of this 'unresolved' inflammation state can be appreciated in other context not explored here, but in which we cannot ignore the nervous system [63, 64]; these three: metabolism, immunity and nervous system are so interdependent that now they are considered as branches of a higher hierarchical level, the neuroendocrine-immune system.

#### *2.3.1. Recruitment of immune cells to adipose tissue*

Various studies report positive correlations of serum resistin levels with the amount of body fat, however, other studies have found no correlation [53–55]. The most important association of circulating resistin levels reported is with C-reactive protein, which could be a marker of

It is secreted by adipocytes; it is closely associated with amount and distribution of adipose tissue. As a chemoattractant protein, chemerin acts as a ligand for the coupled G-receptor protein (ChemR23) and participates in both adaptive and innate immunity [56]. In humans, chemerin gene (*RRARES2*) is highly expressed in WAT and to a lesser extent in liver and lungs. On immune cells, chemerin is known to stimulate chemotaxis of dendritic cells, macrophages and natural killer (NK) cells. Meanwhile, its receptor, ChemR23 gene (*CMKLR1*), is expressed in dendritic cells, monocyte/macrophages and endothelial cells [18, 56, 57]. ChemR23 is involved in the differentiation of adipocytes and increased intracellular glucose or lipids

The interaction of chemerin/ChemR23 has been shown to reduce cytokines, chemokines and phagocytosis, proving to be important in the inflammatory process associated with obesity [18, 57]. In this context, chemerin/ChemR23 axis has been shown to impact IR development,

As has been exposed, dysregulation of adipokinome due to accumulation of adipose tissue in obesity establishes and perpetuates a vicious circle from which emerges the chronic low-grade

Inflammation is a physiological response to a stimulus (i.e. injury or infection) described by Celsus and Galen and is characterized by five classical signs: pain, heat, redness, swelling and loss of function [58, 59]. The inflammation resolution is an active process influenced, in part, by the time and especially regulated by the formation of a group of lipid mediators, which are

The low-grade inflammation state is a term used to define the activation of the vascular endothelium and presence of inflammatory cells in the absence of the five classical signs (subclinical) [61]. This state is due, at least initially, to adipose tissue hypertrophy present in obese individuals because different pathological processes occurs (i.e. fatty acids in excess, hypoxia, cell infiltration and activation of the inflammasome), this pro-inflammatory state is chronic in obesity and now is considered the etiologic agent of its co-morbidities [58, 62]. Effects of this 'unresolved' inflammation state can be appreciated in other context not explored here, but in which we cannot ignore the nervous system [63, 64]; these three: metabolism, immunity and nervous system are so interdependent that now they are considered as branches of a higher

which influences the clinical course and severity of obesity-related diseases.

systemic inflammation [53].

164 Adiposity - Omics and Molecular Understanding

promote its expression [18].

inflammation state.

**2.3. Low-grade inflammation state in obesity**

identified as LXs, protectins and resolvins [60].

hierarchical level, the neuroendocrine-immune system.

*2.2.4. Chemerin*

The corresponding number of immune cells in adipose tissue is increased in obesity, mainly due to circulating cells' recruitment, when compared to lean individuals. [10, 65]. The main infiltrating cells are monocytes, however, other cell types such as NK, LB and LT may also migrate principally [10, 65, 66]. The infiltrated cells promote a positive feedback loop for a chronic low-grade inflammation state.

A key molecule for this recruitment of macrophages is CCL2 chemokine (formerly MCP-1), its expression displays positive correlation with the amount of adipose tissue [10, 19, 67], which is produced by macrophages and other cell types after stimulation. *In vitro* studies have shown that free fatty acids and TNF-α can stimulate production in chemotactic molecules of adipocytes [68].

CCL2 is the most important chemoattractant in the recruitment of monocytes, but possibly not the only one, mice fed with high-fat diets also showed an increase in expression of leukotriene B4 (LTB4) in muscle, liver and adipose tissue [10].
