ChemR23 knockout mice display mild obesity but no deficit in adipocyte differentiation

Progress in the contrary effects of glucagon-like peptide-1 and chemerin on obesity development

Glucagon-like peptide-1 (GLP-1), secreted by intestinal L-cells, plays a pivotal role in the modulation of β-cell insulin secretion in a glucose-dependent manner, concurrently promoting β-cell survival and β-cell mass. Notably, GLP-1 has emerged as an effective second-line treatment for type 2 diabetes mellitus, gaining further prominence for its pronounced impact on body weight reduction, positioning it as a potent antiobesity agent. However, the mechanism by which GLP-1 improves obesity remains unclear. Some reports suggest that this mechanism may be associated with the regulation of adipokine synthesis within adipose tissue. Chemerin, a multifunctional adipokine and chemokine, has been identified as a pivotal player in adipocyte differentiation and the propagation of systemic inflammation, a hallmark of obesity. This review provides a comprehensive overview of the mechanisms by which GLP-1 and chemerin play crucial roles in obesity and obesity-related diseases. It discusses well-established aspects, such as their effects on food intake and glycolipid metabolism, as well as recent insights, including their influence on macrophage polarization and adipose tissue thermogenesis. GLP-1 has been shown to increase the population of anti-inflammatory M2 macrophages, promote brown adipose tissue thermogenesis, and induce the browning of white adipose tissue. In contrast, chemerin exhibits opposite effects in these processes. In addition, recent research findings have demonstrated the promising potential of GLP-1-based therapies in directly or indirectly regulating chemerin expression. In an intriguing reciprocal relationship, chemerin has also been newly identified as a negative regulator of GLP-1 in vivo. This review delineates the intricate interplay between GLP-1 and chemerin, unraveling their mutual inhibitory interactions. To the best of our knowledge, no previous reviews have focused on this specific topic, making this review particularly valuable in expanding our understanding of the endocrine mechanisms of obesity and providing potential strategies for the treatment of obesity and related diseases.

The Role of Chemerin in Metabolic and Cardiovascular Disease: A Literature Review of Its Physiology and Pathology from a Nutritional Perspective

Chemerin is a novel adipokine that plays a major role in adipogenesis and lipid metabolism. It also induces inflammation and affects insulin signaling, steroidogenesis and thermogenesis. Consequently, it likely contributes to a variety of metabolic and cardiovascular diseases, including atherosclerosis, diabetes, hypertension and pre-eclampsia. This review describes its origin and receptors, as well as its role in various diseases, and subsequently summarizes how nutrition affects its levels. It concludes that vitamin A, fat, glucose and alcohol generally upregulate chemerin, while omega-3, salt and vitamin D suppress it. Dietary measures rather than drugs acting as chemerin receptor antagonists might become a novel tool to suppress chemerin effects, thereby potentially improving the aforementioned diseases. However, more detailed studies are required to fully understand chemerin regulation.

Intrauterine androgen exposure impairs gonadal adipose tissue functions of adult female rats

Prenatal androgen exposure induces fetal programming leading to alterations in offspring health and phenotypes that resemble those seen in women with Polycystic Ovary Syndrome. It has been described that prenatal androgenization affects the reproductive axis and leads to metabolic and endocrine disorders. Adipose tissue plays a crucial role in all these functions and is susceptible to programming effects. Particularly, gonadal adipose tissue is involved in reproductive functions, so dysfunctions in this tissue could be related to fertility alterations. We aimed to investigate the extent to which prenatal hyperandrogenization is able to alter the functionality of gonadal adipose tissue in female adult rats, including lipid metabolism, adipokines expression, and de novo synthesis of steroids. Pregnant rats were treated with 1 mg of testosterone from day 16 to day 19 of pregnancy, and female offspring were followed until 90 days of age, when they were euthanized. The prenatally hyperandrogenized (PH) female offspring displayed two phenotypes: irregular ovulatory (PHiov) and anovulatory (PHanov). Regarding lipid metabolism, both PH groups displayed disruptions in the main lipid pathways with altered levels of triglyceride and increased lipid peroxidation levels. In addition, we found that Peroxisome Proliferator-Activated Receptors (PPARs) alpha protein expression was decreased in both PH phenotypes (p < 0.05), but no changes were found in PPARγ protein levels. Furthermore, regarding adipokines, no changes were found in Leptin and Adiponectin protein levels, but Chemerin protein levels were decreased in the PHiov group (p < 0.05). Regarding de novo synthesis of steroids, the PHanov group showed increased protein levels of Cyp17a1 and Cyp19, while the PHiov group only showed decreased protein levels of Cyp19 (p < 0.05). These results suggest that prenatal androgen exposure affects females' gonadal adipose tissue in adulthood, disturbing different lipid pathways, Chemerin expression, and de novo synthesis of steroids.

GPCR in Adipose Tissue Function-Focus on Lipolysis

Adipose tissue can be divided anatomically, histologically, and functionally into two major entities white and brown adipose tissues (WAT and BAT, respectively). WAT is the primary energy depot, storing most of the bioavailable triacylglycerol molecules of the body, whereas BAT is designed for dissipating energy in the form of heat, a process also known as non-shivering thermogenesis as a defense against a cold environment. Importantly, BAT-dependent energy dissipation directly correlates with cardiometabolic health and has been postulated as an intriguing target for anti-obesity therapies. In general, adipose tissue (AT) lipid content is defined by lipid uptake and lipogenesis on one side, and, on the other side, it is defined by the breakdown of lipids and the release of fatty acids by lipolysis. The equilibrium between lipogenesis and lipolysis is important for adipocyte and general metabolic homeostasis. Overloading adipocytes with lipids causes cell stress, leading to the recruitment of immune cells and adipose tissue inflammation, which can affect the whole organism (metaflammation). The most important consequence of energy and lipid overload is obesity and associated pathophysiologies, including insulin resistance, type 2 diabetes, and cardiovascular disease. The fate of lipolysis products (fatty acids and glycerol) largely differs between AT: WAT releases fatty acids into the blood to deliver energy to other tissues (e.g., muscle). Activation of BAT, instead, liberates fatty acids that are used within brown adipocyte mitochondria for thermogenesis. The enzymes involved in lipolysis are tightly regulated by the second messenger cyclic adenosine monophosphate (cAMP), which is activated or inhibited by G protein-coupled receptors (GPCRs) that interact with heterotrimeric G proteins (G proteins). Thus, GPCRs are the upstream regulators of the equilibrium between lipogenesis and lipolysis. Moreover, GPCRs are of special pharmacological interest because about one third of the approved drugs target GPCRs. Here, we will discuss the effects of some of most studied as well as "novel" GPCRs and their ligands. We will review different facets of in vitro, ex vivo, and in vivo studies, obtained with both pharmacological and genetic approaches. Finally, we will report some possible therapeutic strategies to treat obesity employing GPCRs as primary target.

Formylpeptide receptor 2: Nomenclature, structure, signalling and translational perspectives: IUPHAR review 35

We discuss the fascinating pharmacology of formylpeptide receptor 2 (FPR2; often referred to as FPR2/ALX since it binds lipoxin A₄ ). Initially identified as a low-affinity 'relative' of FPR1, FPR2 presents complex and diverse biology. For instance, it is activated by several classes of agonists (from peptides to proteins and lipid mediators) and displays diverse expression patterns on myeloid cells as well as epithelial cells and endothelial cells, to name a few. Over the last decade, the pharmacology of FPR2 has progressed from being considered a weak chemotactic receptor to a master-regulator of the resolution of inflammation, the second phase of the acute inflammatory response. We propose that exploitation of the biology of FPR2 offers innovative ways to rectify chronic inflammatory states and represents a viable avenue to develop novel therapies. Recent elucidation of FPR2 structure will facilitate development of the anti-inflammatory and pro-resolving drugs of next decade.

The Chemerin-CMKLR1 Axis is Functionally important for Central Regulation of Energy Homeostasis

Chemerin is an adipokine involved in inflammation, adipogenesis, angiogenesis and energy metabolism, and has been hypothesized as a link between obesity and type II diabetes. In humans affected by obesity, chemerin gene expression in peripheral tissues and circulating levels are elevated. In mice, plasma levels of chemerin are upregulated by high-fat feeding and gain and loss of function studies show an association of chemerin with body weight, food intake and glucose homeostasis. Therefore, chemerin is an important blood-borne mediator that, amongst its other functions, controls appetite and body weight. Almost all studies of chemerin to date have focused on its release from adipose tissue and its effects on peripheral tissues with the central effects largely overlooked. To demonstrate a central role of chemerin, we manipulated chemerin signaling in the hypothalamus, a brain region associated with appetite regulation, using pharmacological and genetic manipulation approaches. Firstly, the selective chemerin receptor CMKLR1 antagonist α-NETA was administered i.c.v. to rats to test for an acute physiological effect. Secondly, we designed a short-hairpin-RNA (shRNA) lentivirus construct targeting expression of CMKLR1. This shRNA construct, or a control construct was injected bilaterally into the arcuate nucleus of male Sprague Dawley rats on high-fat diet (45%). After surgery, rats were maintained on high-fat diet for 2 weeks and then switched to chow diet for a further 2 weeks. We found a significant weight loss acutely and inhibition of weight gain chronically. This difference became apparent after diet switch in arcuate nucleus-CMKLR1 knockdown rats. This was not accompanied by a difference in blood glucose levels. Interestingly, appetite-regulating neuropeptides remained unaltered, however, we found a significant reduction of the inflammatory marker TNF-α suggesting reduced expression of CMKLR1 protects from high-fat diet induced neuroinflammation. In white and brown adipose tissue, mRNA expression of chemerin, its receptors and markers of adipogenesis, lipogenesis and brown adipocyte activation remained unchanged confirming that the effects are driven by the brain. Our behavioral analyses suggest that knockdown of CMKLR1 had an impact on object recognition. Our data demonstrate that CMKLR1 is functionally important for the central effects of chemerin on body weight regulation and neuroinflammation.

Thromboinflammatory Processes at the Nexus of Metabolic Dysfunction and Prostate Cancer: The Emerging Role of Periprostatic Adipose Tissue

Simple Summary

As overweight and obesity increase among the population worldwide, a parallel increase in the number of individuals diagnosed with prostate cancer was observed. There appears to be a relationship between both diseases where the increase in the mass of fat tissue can lead to inflammation. Such a state of inflammation could produce many factors that increase the aggressiveness of prostate cancer, especially if this inflammation occurred in the fat stores adjacent to the prostate. Another important observation that links obesity, fat tissue inflammation, and prostate cancer is the increased production of blood clotting factors. In this article, we attempt to explain the role of these latter factors in the effect of increased body weight on the progression of prostate cancer and propose new ways of treatment that act by affecting how these clotting factors work.

Abstract

The increased global prevalence of metabolic disorders including obesity, insulin resistance, metabolic syndrome and diabetes is mirrored by an increased incidence of prostate cancer (PCa). Ample evidence suggests that these metabolic disorders, being characterized by adipose tissue (AT) expansion and inflammation, not only present as risk factors for the development of PCa, but also drive its increased aggressiveness, enhanced progression, and metastasis. Despite the emerging molecular mechanisms linking AT dysfunction to the various hallmarks of PCa, thromboinflammatory processes implicated in the crosstalk between these diseases have not been thoroughly investigated. This is of particular importance as both diseases present states of hypercoagulability. Accumulating evidence implicates tissue factor, thrombin, and active factor X as well as other players of the coagulation cascade in the pathophysiological processes driving cancer development and progression. In this regard, it becomes pivotal to elucidate the thromboinflammatory processes occurring in the periprostatic adipose tissue (PPAT), a fundamental microenvironmental niche of the prostate. Here, we highlight key findings linking thromboinflammation and the pleiotropic effects of coagulation factors and their inhibitors in metabolic diseases, PCa, and their crosstalk. We also propose several novel therapeutic targets and therapeutic interventions possibly modulating the interaction between these pathological states.

Role of chemerin in the control of glucose homeostasis

Chemerin is an adipokine produced by the white adipose tissue and other tissues, which plays various roles in the pathogenesis of inflammatory and metabolic diseases in multiple organs. The present review aims at gathering scientific evidence reported in the last ten years, concerning the relationship of chemerin with alterations of glycaemic control, such as insulin resistance, type 2 diabetes and gestational diabetes in humans. Although the vast majority of the studies have shown a positive correlation between the chemerin level and a bad glycaemic control, a general consensus has not been reached. The reported results come from case-control and observational longitudinal studies, thereby limiting their interpretation. In fact, it cannot be stated whether insulin resistance and diabetes lead to an increase in chemerin levels or, on the contrary, if high levels of chemerin contribute to an impaired glycaemic control. Elevated levels of circulating chemerin are also associated with gestational diabetes mellitus. Chemerin gene polymorphisms could be proposed as mediators of glucose-related diseases. Nevertheless, to date very little is known about their implication in glucose metabolism. With regard to the mechanisms of action, chemerin impairs insulin cascade signaling by acting on several proteins of this cascade and by inducing inflammation.

Chemerin - exploring a versatile adipokine

Chemerin is a small chemotactic protein and a key player in initiating the early immune response. As an adipokine, chemerin is also involved in energy homeostasis and the regulation of reproductive functions. Secreted as inactive prochemerin, it relies on proteolytic activation by serine proteases to exert biological activity. Chemerin binds to three distinct G protein-coupled receptors (GPCR), namely chemokine-like receptor 1 (CMKLR1, recently named chemerin₁), G protein-coupled receptor 1 (GPR1, recently named chemerin₂), and CC-motif chemokine receptor-like 2 (CCRL2). Only CMKLR1 displays conventional G protein signaling, while GPR1 only recruits arrestin in response to ligand stimulation, and no CCRL2-mediated signaling events have been described to date. However, GPR1 undergoes constitutive endocytosis, making this receptor perfectly adapted as decoy receptor. Here, we discuss expression pattern, activation, and receptor binding of chemerin. Moreover, we review the current literature regarding the involvement of chemerin in cancer and several obesity-related diseases, as well as recent developments in therapeutic targeting of the chemerin system.

Resolvin E1-ChemR23 Axis Regulates the Hepatic Metabolic and Inflammatory Transcriptional Landscape in Obesity at the Whole Genome and Exon Level

Resolvin E1 (RvE1) is an immunoresolvent that is synthesized from eicosapentaenoic acid and can bind the receptor ERV1/ChemR23. We previously showed activation of the RvE1-ChemR23 axis improves hyperglycemia and hyperinsulinemia of obese mice; however, it remains unclear how RvE1 controls glucose homeostasis. Here we investigated hepatic metabolic and inflammatory transcriptional targets of the RvE1-ChemR23 axis using lean and obese wild type (WT) and ChemR23 knockout (KO) mice. We conducted an in-depth transcriptional study by preforming whole gene-level and exon-level analyses, which provide insight into alternative splicing variants and miRNA regulation. Compared to controls, WT and KO obese mice in the absence of RvE1 displayed similar gene-level profiles, which entailed dysregulated pathways related to glucose homeostasis. Notably, obese WT mice relative to lean controls showed a robust decrease in pathways related to the biosynthesis of unsaturated fatty acids. At the exon-level, obese ChemR23 KOs compared to obese WT mice displayed changes in pathways related to hepatic lipid transport, cholesterol metabolism, and immunological functions such as complement cascades and platelet activation. Importantly, upon RvE1 administration to WT obese mice, we discovered upregulated genes in pathways relating to insulin sensitivity and downregulated genes related to regulators of TGF-β signaling. This transcriptional profile was generally not recapitulated with obese ChemR23 KO mice administered RvE1. Collectively, gene and exon-level analyses suggest RvE1 controls the hepatic transcriptional profile related to glucose homeostasis, insulin sensitivity, and inflammation in a manner that is largely dependent on ChemR23. These studies will drive future mechanistic experiments on the RvE1-ChemR23 axis.

Chemerin regulates normal angiogenesis and hypoxia-driven neovascularization

Chemerin is a multifunctional protein initially characterized in our laboratory as a chemoattractant factor for leukocyte populations. Its main functional receptor is CMKLR1. We identified previously chemerin as an anti-tumoral factor inhibiting the vascularization of tumor grafts. We show here that overexpression of bioactive chemerin in mice results in a reduction of the density of the retinal vascular network during its development and in adults. Chemerin did not affect vascular sprouting during the post-natal development of the network, but rather promoted endothelial cell apoptosis and vessel pruning. This phenotype was reversed to normal in CMKLR1-deficient mice, demonstrating the role of this receptor. Chemerin inhibited also neoangiogenesis in a model of pathological proliferative retinopathy, and in response to hind-limb ischemia. Mechanistically, PTEN and FOXO1 antagonists could almost completely restore the density of the retinal vasculature, suggesting the involvement of the PI3-kinase/AKT pathway in the chemerin-induced vessel regression process.

The Role of Kallikrein 7 in Tumorigenesis

Kallikrein 7 (KLK7) is a secreted serine protease with chymotrypsic protease activity. Abnormally high expression of KLK7 is closely related to the occurrence and development of various types of cancer. Therefore, KLK7 has been identified as a potential target for cancer drug development design in recent years. KLK7 mediates various biological and pathological processes in tumorigenesis, including cell proliferation, migration, invasion, angiogenesis, and cell metabolism, by hydrolyzing a series of substrates such as membrane proteins, extracellular matrix proteins, and cytokines. This review mainly introduces the downstream cell signaling pathways involved in the activation of KLK7 and its substrate-related proteins. This review will not only help us to better understand the mechanisms of KLK7 in regulating biological and pathological processes of cancer cells, but also lay a solid foundation for the design of inhibitors targeting KLK7.

The antitumoral effects of chemerin are independent from leukocyte recruitment and mediated by inhibition of neoangiogenesis

Chemerin, a multifunctional protein acting through the receptor ChemR23/CMKLR1, is downregulated in various human tumors and was shown to display antitumoral properties in mouse models of cancer. In the present study, we report that bioactive chemerin expression by tumor cells delays the growth of B16 melanoma and Lewis lung carcinoma in vivo. A similar delay is observed when chemerin is not expressed by tumor cells but by keratinocytes of the host mice. The protective effect of chemerin is mediated by CMKLR1 and appears unrelated to the recruitment of leukocyte populations. Rather, tumors grown in the presence of chemerin display a much smaller number of blood vessels, hypoxic regions early in their development, and larger necrotic areas. These observations likely explain the slower growth of the tumors. The anti-angiogenic effects of chemerin were confirmed in a bead sprouting assay using human umbilical vein endothelial cells. These results suggest that CMKLR1 agonists might constitute therapeutic molecules inhibiting the neoangiogenesis process in solid tumors.

The chemerin-CMKLR1 axis limits thermogenesis by controlling a beige adipocyte/IL-33/type 2 innate immunity circuit

IL-33-associated type 2 innate immunity has been shown to support beige fat formation and thermogenesis in subcutaneous inguinal white adipose tissue (iWAT), but little is known about how it is regulated in iWAT. Chemerin, as a newly identified adipokine, is clinically associated with obesity and metabolic disorders. We here show that cold exposure specifically reduces chemerin and its receptor chemerin chemokine-like receptor 1 (CMKLR1) expression in iWAT. Lack of chemerin or adipocytic CMKLR1 enhances cold-induced thermogenic beige fat via potentiating type 2 innate immune responses. Mechanistically, we identify adipocytes, particularly beige adipocytes, as the main source for cold-induced IL-33, which is restricted by the chemerin-CMKLR1 axis via dampening cAMP-PKA signaling, thereby interrupting a feed-forward circuit between beige adipocytes and type 2 innate immunity that is required for cold-induced beige fat and thermogenesis. Moreover, specific deletion of adipocytic IL-33 inhibits cold-induced beige fat and type 2 innate immune responses. Last, genetic blockade of adipocytic CMKLR1 protects against diet-induced obesity and enhances the metabolic benefits of cold stimulation in preestablished obese mice. Thus, our study identifies the chemerin-CMKLR1 axis as a physiological negative regulator of thermogenic beige fat via interrupting adipose-immune communication and suggests targeting adipose CMKLR1 as a potential therapeutic strategy for obesity-related metabolic disorders.

Changes in brown adipose tissue lipid mediator signatures with aging, obesity, and DHA supplementation in female mice

Brown adipose tissue (BAT) dysfunction in aging and obesity has been related to chronic unresolved inflammation, which could be mediated by an impaired production of specialized proresolving lipid mediators (SPMs), such as Lipoxins-LXs, Resolvins-Rvs, Protectins-PDs, and Maresins-MaRs. Our aim was to characterize the changes in BAT SPMs signatures and their association with BAT dysfunction during aging, especially under obesogenic conditions, and their modulation by a docosahexaenoic acid (DHA)-rich diet. Lipidomic, functional, and molecular studies were performed in BAT of 2- and 18-month-old lean (CT) female mice and in 18-month-old diet-induced obese (DIO) mice fed with a high-fat diet (HFD), or a DHA-enriched HFD. Aging downregulated Prdm16 and UCP1 levels, especially in DIO mice, while DHA partially restored them. Arachidonic acid (AA)-derived LXs and DHA-derived MaRs and PDs were the most abundant SPMs in BAT of young CT mice. Interestingly, the sum of LXs and of PDs were significantly lower in aged DIO mice compared to young CT mice. Some of the SPMs most significantly reduced in obese-aged mice included LXB₄ , MaR2, 4S,14S-diHDHA, 10S,17S-diHDHA (a.k.a. PDX), and RvD6. In contrast, DHA increased DHA-derived SPMs, without modifying LXs. However, MicroPET studies showed that DHA was not able to counteract the impaired cold exposure response in BAT of obese-aged mice. Our data suggest that a defective SPMs production could underlie the decrease of BAT activity observed in obese-aged mice, and highlight the relevance to further characterize the physiological role and therapeutic potential of specific SPMs on BAT development and function.

The complex role of adipokines in obesity, inflammation, and autoimmunity

The global obesity epidemic is a major contributor to chronic disease and disability in the world today. Since the discovery of leptin in 1994, a multitude of studies have characterized the pathological changes that occur within adipose tissue in the obese state. One significant change is the dysregulation of adipokine production. Adipokines are an indispensable link between metabolism and optimal immune system function; however, their dysregulation in obesity contributes to chronic low-grade inflammation and disease pathology. Herein, I will highlight current knowledge on adipokine structure and physiological function, and focus on the known roles of these factors in the modulation of the immune response. I will also discuss adipokines in rheumatic and autoimmune diseases.

Measurement of Serum Chemerin, Oxidized LDL, and Vitamin D Levels in Prader–Willi Syndrome: A Cross-Sectional Study in Pediatric Egyptian Patients

Prader–Willi syndrome (PWS) is the commonest genetic cause of obesity. Oxidative stress and chronic low-grade inflammation play a crucial role in the pathogenesis of obesity. Alterations of vitamin D (25-OHD) levels are commonly encountered with obesity. The aim of this study was to analyze serum chemerin, oxidized low-density lipoprotein (ox-LDL), and 25-OHD values in pediatric PWS patients in comparison with obese healthy children and nonobese control groups, highlighting possible correlations with body mass index (BMI) and obesity. Twenty-six PWS Egyptian patients and 26 obese healthy individuals referred to the outpatient clinic of the Clinical Genetics Department, National Research Centre, Cairo, Egypt, and 20 control patients with matching age and sex were enrolled in the study. Patients were clinically diagnosed and confirmed by routine cytogenetic and fluorescence in-situ hybridization analysis. Anthropometric measurements were performed, and BMI was calculated by weight/height2 (kg/m2), and BMI z score was also determined. Serum chemerin, ox-LDL, and vitamin D were determined by enzyme-linked immunosorbent assay. Chemerin levels, which reflected chronic inflammation, were significantly elevated as compared with obese and nonobese controls (p ≤ 0.0001). Concerning oxidative damage, children with PWS showed higher Ox-LDL levels compared with obese and nonobese controls (p < 0.0001). Vitamin D levels were significantly lower in PWS patients compared with obese and nonobese controls (p ≤ 0.0001). Our data showed that obesity in PWS is associated with oxidative stress and chronic low-grade inflammation. Ox-LDL is a good indicator of oxidative stress, and chemerin could be used as a biomarker for the chronic inflammatory state. Furthermore, vitamin D supplementation is recommended in PWS patients

Chemerin: A Potential Regulator of Inflammation and Metabolism for Chronic Obstructive Pulmonary Disease and Pulmonary Rehabilitation

Chronic obstructive pulmonary disease (COPD) features chronic inflammatory reactions of both intra- and extrapulmonary nature. Moreover, COPD is associated with abnormal glucose and lipid metabolism in patients, which influences the prognosis and chronicity of this disease. Abnormal glucose and lipid metabolism are also closely related to inflammation processes. Further insights into the interactions of inflammation and glucose and lipid metabolism might therefore inspire novel therapeutic interventions to promote lung rehabilitation. Chemerin, as a recently discovered adipokine, has been shown to play a role in inflammatory response and glucose and lipid metabolism in many diseases (including COPD). Chemerin recruits inflammatory cells to sites of inflammation during the early stages of COPD, leading to endothelial barrier dysfunction, early vascular remodeling, and angiogenesis. Moreover, it supports the recruitment of antigen-presenting cells that guide immune cells as part of the body's inflammatory responses. Chemerin also regulates metabolism via activation of its cognate receptors. Glucose homeostasis is affected via effects on insulin secretion and sensitivity, and lipid metabolism is changed by increased transformation of preadipocytes to mature adipocytes through chemerin-binding receptors. Controlling chemerin signaling may be a promising approach to improve various aspects of COPD-related dysfunction. Importantly, several studies indicate that chemerin expression in vivo is influenced by exercise. Although available evidence is still limited, therapeutic alterations of chemerin activity may be a promising target of therapeutic approaches aimed at the rehabilitation of COPD patients based on exercises. In conclusion, chemerin plays an essential role in COPD, especially in the inflammatory responses and metabolism, and has a potential to become a target for, and a biomarker of, curative mechanisms underlying exercise-mediated lung rehabilitation.

CMKLR1 deficiency attenuates androgen-induced lipid accumulation in mice

Excess androgen-induced obesity has become a public health problem, and its prevalence has increased substantially in recent years. Chemokine-like receptor 1 (CMKLR1), a receptor of chemerin secreted by adipose tissue, is linked to adipocyte differentiation, adipose tissue development, and obesity. However, the effect of CMKLR1 signaling on androgen-mediated adiposity in vivo remains unclear. Using CMKLR1-knockout mice, we constructed an androgen-excess female mouse model through 5α-dihydrotestosterone (DHT) treatment and an androgen-deficient male mouse model by orchidectomy (ORX). For mechanism investigation, we used 2-(α-Naphthoyl) ethyltrimethylammonium iodide (α-NETA), an antagonist of CMKLR1, to suppress CMKLR1 in vivo and wortmannin, a PI3K signaling antagonist, to treat brown adipose tissue (BAT) explant cultures in vitro. Furthermore, we used histological examination and quantitative PCR, as well as Western blot analysis, glucose tolerance tests, and biochemical analysis of serum, to describe the phenotypes and the changes in gene expression. We demonstrated that excess androgen in the female mice resulted in larger cells in the white adipose tissue (WAT) and the BAT, whereas androgen deprivation in the male mice induced a reduction in cell size. Both of these adipocyte size effects could be attenuated in the CMKLR1-knockout mice. CMKLR1 deficiency influenced the effect of androgen treatment on adipose tissue by regulating the mRNA expression of the androgen receptor (AR) and adipocyte markers (such as Fabp4 and Cidea). Moreover, suppression of CMKLR1 by α-NETA could also reduce the extent of the adipocyte cell enlargement caused by DHT. Furthermore, we found that DHT could reduce the levels of phosphorylated ERK (pERK) in the BAT, while CMKLR1 inactivation inhibited this effect, which had been induced by DHT, through the PI3K signaling pathway. These findings reveal an antiobesity role of CMKLR1 deficiency in regulating lipid accumulation, highlighting the scientific importance for the further development of small-molecule CMKLR1 antagonists as fundamental research tools and/or as potential drugs for use in the treatment of adiposity.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.

Chemokine-like receptor 1 deficiency leads to lower bone mass in male mice

The adipokine Chemerin and its receptor, chemokine-like receptor 1 (CMKLR1), are associated with osteoblastogenic differentiation of mesenchymal stem cells (MSCs) and osteoclastogenic differentiation of osteoclast precursors in vitro, suggesting that CMKLR1 would affect the bone mineral density (BMD). However, the role of CMKLR1 on BMD in vivo remains unknown. Here, using CMKLR1 knockout mouse model, we unveiled that CMKLR1 effected the amount of Leydig cells in testis and regulated androgen-dependent bone maintenance in male mice, which exhibited lower serum testosterone levels, thereby reducing the trabecular bone mass. Correspondingly, the mRNA expression of testosterone synthesis enzymes in testis decreased. The bone tissue also showed decreased mRNAs expression of osteogenic markers and increased mRNA levels for osteoclast markers. Furthermore, by in vitro differentiation models, we found CMKLR1-deficiency could break the balance between osteoblastogenesis and osteoclastogenesis that caused a shift from osteogenic to adipogenic differentiation in MSCs and enhanced osteoclast formation. In addition, bone mass increase in CMKLR1 KO male mice can be promoted by treatment with 5α-dihydrotestosterone (DHT), and the inactivation of CMKLR1 in male wild-type (WT) mice with antagonist treatment can lead to low bone mass. Taken together, these data indicate that CMKLR1 positively regulates bone metabolism through mediating testosterone production and the balance between osteoblast and osteoclast formation.

Chemerin receptor blockade improves vascular function in diabetic obese mice via redox-sensitive and Akt-dependent pathways

Chemerin and its G protein-coupled receptor [chemerin receptor 23 (ChemR23)] have been associated with endothelial dysfunction, inflammation, and insulin resistance. However, the role of chemerin on insulin signaling in the vasculature is still unknown. We aimed to determine whether chemerin reduces vascular insulin signaling and whether there is interplay between chemerin/ChemR23, insulin resistance, and vascular complications associated with type 2 diabetes (T2D). Molecular and vascular mechanisms were probed in mesenteric arteries and cultured vascular smooth muscle cells (VSMCs) from C57BL/6J, nondiabetic lean db/m, and diabetic obese db/db mice as well as in human microvascular endothelial cells (HMECs). Chemerin decreased insulin-induced vasodilatation in C57BL/6J mice, an effect prevented by CCX832 (ChemR23 antagonist) treatment. In VSMCs, chemerin, via oxidative stress- and ChemR23-dependent mechanisms, decreased insulin-induced Akt phosphorylation, glucose transporter 4 translocation to the membrane, and glucose uptake. In HMECs, chemerin decreased insulin-activated nitric oxide signaling. AMP-activated protein kinase phosphorylation was reduced by chemerin in both HMECs and VSMCs. CCX832 treatment of db/db mice decreased body weight, insulin, and glucose levels as well as vascular oxidative stress. CCX832 also partially restored vascular insulin responses in db/db and high-fat diet-fed mice. Our novel in vivo findings highlight chemerin/ChemR23 as a promising therapeutic target to limit insulin resistance and vascular complications associated with obesity-related diabetes. NEW & NOTEWORTHY Our novel findings show that the chemerin/chemerin receptor 23 axis plays a critical role in diabetes-associated vascular oxidative stress and altered insulin signaling. Targeting chemerin/chemerin receptor 23 may be an attractive strategy to improve insulin signaling and vascular function in obesity-associated diabetes.

The impact of chemerin or chemokine-like receptor 1 loss on the mouse gut microbiome

Chemerin is an adipocyte derived signalling molecule (adipokine) that serves as a ligand activator of Chemokine-like receptor 1(CMKLR1). Chemerin/CMKLR1 signalling is well established to regulate fundamental processes in metabolism and inflammation. The composition and function of gut microbiota has also been shown to impact the development of metabolic and inflammatory diseases such as obesity, diabetes and inflammatory bowel disease. In this study, we assessed the microbiome composition of fecal samples isolated from wildtype, chemerin, or CMKLR1 knockout mice using Illumina-based sequencing. Moreover, the knockout mice and respective wildtype mice used in this study were housed at different universities allowing us to compare facility-dependent effects on microbiome composition. While there was no difference in alpha diversity within samples when compared by either facility or genotype, we observed a dramatic difference in the presence and abundance of numerous taxa between facilities. There were minor differences in bacterial abundance between wildtype and chemerin knockout mice, but significantly more differences in taxa abundance between wildtype and CMKLR1 knockout mice. Specifically, CMKLR1 knockout mice exhibited decreased abundance of Akkermansia and Prevotella, which correlated with body weight in CMKLR1 knockout, but not wildtype mice. This is the first study to investigate a linkage between chemerin/CMKLR1 signaling and microbiome composition. The results of our study suggest that chemerin/CMKLR1 signaling influences metabolic processes through effects on the gut microbiome. Furthermore, the dramatic difference in microbiome composition between facilities might contribute to discrepancies in the metabolic phenotype of CMKLR1 knockout mice reported by independent groups. Considered altogether, these findings establish a foundation for future studies to investigate the relationship between chemerin signaling and the gut microbiome on the development and progression of metabolic and inflammatory disease.

Chemerin: a multifaceted adipokine involved in metabolic disorders

Metabolic syndrome is a global public health problem and predisposes individuals to obesity, diabetes and cardiovascular disease. Although the underlying mechanisms remain to be elucidated, accumulating evidence has uncovered a critical role of adipokines. Chemerin, encoded by the gene Rarres2, is a newly discovered adipokine involved in inflammation, adipogenesis, angiogenesis and energy metabolism. In humans, local and circulating levels of chemerin are positively correlated with BMI and obesity-related biomarkers. In this review, we discuss both peripheral and central roles of chemerin in regulating body metabolism. In general, chemerin is upregulated in obese and diabetic animals. Previous studies by gain or loss of function show an association of chemerin with adipogenesis, glucose homeostasis, food intake and body weight. In the brain, the hypothalamus integrates peripheral afferent signals including adipokines to regulate appetite and energy homeostasis. Chemerin increases food intake in seasonal animals by acting on hypothalamic stem cells, the tanycytes. In peripheral tissues, chemerin increases cell expansion, inflammation and angiogenesis in adipose tissue, collectively resulting in adiposity. While chemerin signalling enhances insulin secretion from pancreatic islets, contradictory results have been reported on how chemerin links to obesity and insulin resistance. Given the association of chemerin with obesity comorbidities in humans, advances in translational research targeting chemerin are expected to mitigate metabolic disorders. Together, the exciting findings gathered in the last decade clearly indicate a crucial multifaceted role for chemerin in the regulation of energy balance, making it a promising candidate for urgently needed pharmacological treatment strategies for obesity.

International Union of Basic and Clinical Pharmacology CIII: Chemerin Receptors CMKLR1 (Chemerin1) and GPR1 (Chemerin2) Nomenclature, Pharmacology, and Function

Chemerin, a chemoattractant protein and adipokine, has been identified as the endogenous ligand for a G protein–coupled receptor encoded by the gene CMKLR1 (also known as ChemR23), and as a consequence the receptor protein was renamed the chemerin receptor in 2013. Since then, chemerin has been identified as the endogenous ligand for a second G protein–coupled receptor, encoded by the gene GPR1. Therefore, the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification recommends that the official name of the receptor protein for chemokine-like receptor 1 (CMKLR1) is chemerin receptor 1, and G protein–coupled receptor 1 is chemerin receptor 2 to follow the convention of naming the receptor protein after the endogenous ligand. Chemerin receptor 1 and chemerin receptor 2 can be abbreviated to Chemerin₁ and Chemerin₂, respectively. Chemerin requires C-terminal processing for activity, and human chemerin21–157 is reported to be the most active form, with peptide fragments derived from the C terminus biologically active at both receptors. Small-molecule antagonist, CCX832, selectively blocks CMKLR1, and resolvin E1 activation of CMKLR1 is discussed. Activation of both receptors by chemerin is via coupling to G_i/o, causing inhibition of adenylyl cyclase and increased Ca²⁺ flux. Receptors and ligand are widely expressed in humans, rats, and mice, and both receptors share ∼80% identity across these species. CMKLR1 knockout mice highlight the role of this receptor in inflammation and obesity, and similarly, GPR1 knockout mice exhibit glucose intolerance. In addition, the chemerin receptors have been implicated in cardiovascular disease, cancer, steroidogenesis, human immunodeficiency virus replication, and neurogenerative disease.

Absence of the Non-Signalling Chemerin Receptor CCRL2 Exacerbates Acute Inflammatory Responses In Vivo

Chemerin is a chemotactic protein that induces migration of several immune cells including macrophages, immature dendritic cells, and NK cells. Chemerin binds to three G protein-coupled receptors (GPCRs), including CCRL2. The exact function of CCRL2 remains unclear. CCRL2 expression is rapidly upregulated during inflammation, but it lacks the intracellular DRYLAIV motif required for classical GPCR downstream signalling pathways, and it has not been reported to internalise chemerin upon binding. The aim of this study was to investigate what role if any CCRL2 plays during acute inflammation. Using the zymosan- and thioglycollate-induced murine models of acute inflammation, we report that mice deficient in the Ccrl2 gene display exaggerated local and systemic inflammatory responses, characterised by increased myeloid cell recruitment. This amplified myeloid cell recruitment was associated with increased chemerin and CXCL1 levels. Furthermore, we report that the inflammatory phenotype observed in these mice is dependent upon elevated levels of endogenous chemerin. Antibody neutralisation of chemerin activity in Ccrl2-/- mice abrogated the amplified inflammatory responses. Importantly, chemerin did not directly recruit myeloid cells but rather increased the production of other chemotactic proteins such as CXCL1. Administration of recombinant chemerin to wild-type mice before inflammatory challenge recapitulated the increased myeloid cell recruitment and inflammatory mediator production observed in Ccrl2-/- mice. We have demonstrated that the absence of CCRL2 results in increased levels of local and systemic chemerin levels and exacerbated inflammatory responses during acute inflammatory challenge. These results further highlight the importance of chemerin as a therapeutic target in inflammatory diseases.

Association of a variant in the gene encoding for ERV1/ChemR23 with reduced inflammation in visceral adipose tissue from morbidly obese individuals

Obesity comorbidities are closely associated with chronic low-grade adipose tissue inflammation. A number of SNPs associated with inflammation has been identified, underscoring the impact of genetic determinants on this process. Here, we screened SNPs in genes with pro-inflammatory (IL-1β, IL-6, STAT3 and JAK2), anti-inflammatory (IL-10 and SOCS3) and pro-resolving (ERV1/ChemR23) properties in 101 obese and 99 non-obese individuals. Among the SNPs analyzed, we identified that individuals carrying a C allele in the rs1878022 polymorphism of the ERV1/ChemR23 gene, which encodes for the receptor of the pro-resolving mediator RvE1, had increased ERV1/ChemR23 protein expression and reduced levels of the inflammatory cytokine IL-6 in adipose tissue. Moreover, patients carrying the C allele in homozygosity had lower plasma levels of IL-6, IFN-α2, IL-15, IL-1ra, IL-10, GM-CSF, G-CSF and VEGF and enhanced leukocyte responsiveness to RvE1. C-carriers also exhibited decreased TAG to HDL ratio, a surrogate marker of insulin resistance and a predictor of incident fatty liver. Finally, we confirmed in vivo that the ERV1/ChemR23 receptor regulates systemic and tissue inflammation since mice lacking ERV1/ChemR23 expression showed increased IL-6 levels in adipose tissue and peritoneal macrophages. Together, our study identified an ERV1/ChemR23 variant that protects patients with obesity from excessive inflammatory burden.

Chemokine-Like Receptor 1 mRNA Weakly Correlates with Non-Alcoholic Steatohepatitis Score in Male but Not Female Individuals

The chemokine-like receptor 1 (CMKLR1) ligands resolvin E1 and chemerin are known to modulate inflammatory response. The progression of non-alcoholic fatty liver disease (NAFLD) to non-alcoholic steatohepatitis (NASH) is associated with inflammation. Here it was analyzed whether hepatic CMKLR1 expression is related to histological features of NASH. Therefore, CMKLR1 mRNA was quantified in liver tissue of 33 patients without NAFLD, 47 patients with borderline NASH and 38 patients with NASH. Hepatic CMKLR1 mRNA was not associated with gender and body mass index (BMI) in the controls and the whole study group. CMKLR1 expression was similar in controls and in patients with borderline NASH and NASH. In male patients weak positive correlations with inflammation, fibrosis and NASH score were identified. In females CMKLR1 was not associated with features of NAFLD. Liver CMKLR1 mRNA tended to be higher in type 2 diabetes patients of both genders and in hypercholesterolemic women. In summary, this study shows that hepatic CMKLR1 mRNA is weakly associated with features of NASH in male patients only.

CMKLR1 deficiency influences glucose tolerance and thermogenesis in mice on high fat diet

Obesity has become a global epidemic disease, contributing to increases in the prevalence of type 2 diabetes. CMKLR1, one of the receptors for chemerin, has a wide range of functions in physiological and pathological activity, including innate and adaptive immunity, inflammation, metabolism and reproduction. In our study, CMKLR1 deficiency did not influence the gain of body weight but did exacerbate glucose intolerance, increase serum insulin level, and promote insulin resistance in mice on high fat diets. The expression of thermogenesis related genes was examined and indicated to decrease in CMKLR1 knockout (KO) mice in both normal and cold environments, which indicated CMKLR1 influence the thermogenesis process. Cold exposure induced significant body mass decrease and improved glucose tolerance and insulin resistance in wild type HFD mice but had no obvious effect on CMKLR1 KO HFD mice. In vitro, loss of CMKLR1 did not significantly influence the differentiation of stromal vascular fibroblasts (SVFs) derived from adipose tissue, but did suppress the expression of thermogenesis related genes. Collectively, these data demonstrate that CMKLR1 deficiency induces inbalance of glucose metabolism and impairs the cold induced-thermogenesis process in high diet models.

Evidence from studies in rodents and in isolated adipocytes that agonists of the chemerin receptor CMKLR1 may be beneficial in the treatment of type 2 diabetes

The literature is unclear on whether the adipokine chemerin has pro- or anti-inflammatory properties or plays any role in the aetiology of type 2 diabetes or obesity. To address these questions, and in particular the potential of agonists or antagonists of the chemerin receptor CMKLR1 in the treatment of type 2 diabetes and obesity, we studied the metabolic phenotypes of both male and female, CMKLR1 knockout and heterozygote mice. We also investigated changes in plasma chemerin levels and chemerin gene mRNA content in adipose tissue in models of obesity and diabetes, and in response to fasting or administration of the insulin sensitizing drug rosiglitazone, which also has anti-inflammatory properties. The effects of murine chemerin and specific C-terminal peptides on glucose uptake in wild-type and CMKLR1 knockout adipocytes were investigated as a possible mechanism by which chemerin affects the blood glucose concentration. Both male and female CMKLR1 knockout and heterozygote mice displayed a mild tendency to obesity and impaired glucose homeostasis, but only when they were fed on a high-fat died, rather than a standard low-fat diet. Obesity and impaired glucose homeostasis did not occur concurrently, suggesting that obesity was not the sole cause of impaired glucose homeostasis. Picomolar concentrations of chemerin and its C15- and C19-terminal peptides stimulated glucose uptake in the presence of insulin by rat and mouse wild-type epididymal adipocytes, but not by murine CMKLR1 knockout adipocytes. The insulin concentration-response curve was shifted to the left in the presence of 40 pM chemerin or its C-15 terminal peptide. The plasma chemerin level was raised in diet-induced obesity and ob/ob but not db/db mice, and was reduced by fasting and, in ob/ob mice, by treatment with rosiglitazone. These findings suggest that an agonist of CMKLR1 is more likely than an antagonist to be of value in the treatment of type 2 diabetes and to have associated anti-obesity and anti-inflammatory activities. One mechanism by which an agonist of CMKLR1 might improve glucose homeostasis is by increasing insulin-stimulated glucose uptake by adipocytes.

Gpr1 is an active chemerin receptor influencing glucose homeostasis in obese mice

Chemerin is an adipose-derived signaling protein (adipokine) that regulates adipocyte differentiation and function, immune function, metabolism, and glucose homeostasis through activation of chemokine-like receptor 1 (CMKLR1). A second chemerin receptor, G protein-coupled receptor 1 (GPR1) in mammals, binds chemerin with an affinity similar to CMKLR1; however, the function of GPR1 in mammals is essentially unknown. Herein, we report that expression of murine Gpr1 mRNA is high in brown adipose tissue and white adipose tissue (WAT) and skeletal muscle. In contrast to chemerin (Rarres2) and Cmklr1, Gpr1 expression predominates in the non-adipocyte stromal vascular fraction of WAT. Heterozygous and homozygous Gpr1-knockout mice fed on a high-fat diet developed more severe glucose intolerance than WT mice despite having no difference in body weight, adiposity, or energy expenditure. Moreover, mice lacking Gpr1 exhibited reduced glucose-stimulated insulin levels and elevated glucose levels in a pyruvate tolerance test. This study is the first, to our knowledge, to report the effects of Gpr1 deficiency on adiposity, energy balance, and glucose homeostasis in vivo. Moreover, these novel results demonstrate that GPR1 is an active chemerin receptor that contributes to the regulation of glucose homeostasis during obesity.

Chemokine-like receptor 1 deficiency does not affect the development of insulin resistance and nonalcoholic fatty liver disease in mice

The adipokine chemerin and its receptor, chemokine-like receptor 1 (Cmklr1), are associated with insulin resistance and nonalcoholic fatty liver disease (NAFLD), which covers a broad spectrum of liver diseases, ranging from simple steatosis to nonalcoholic steatohepatitis (NASH). It is possible that chemerin and/or Cmklr1 exert their effects on these disorders through inflammation, but so far the data have been controversial. To gain further insight into this matter, we studied the effect of whole-body Cmklr1 deficiency on insulin resistance and NAFLD. In view of the primary role of macrophages in hepatic inflammation, we also transplanted bone marrow from Cmklr1 knock-out (Cmklr1-/-) mice and wild type (WT) mice into low-density lipoprotein receptor knock-out (Ldlr-/-) mice, a mouse model for NASH. All mice were fed a high fat, high cholesterol diet containing 21% fat from milk butter and 0.2% cholesterol for 12 weeks. Insulin resistance was assessed by an oral glucose tolerance test, an insulin tolerance test, and by measurement of plasma glucose and insulin levels. Liver pathology was determined by measuring hepatic inflammation, fibrosis, lipid accumulation and the NAFLD activity score (NAS). Whole-body Cmklr1 deficiency did not affect body weight gain or food intake. In addition, we observed no differences between WT and Cmklr1-/- mice for hepatic inflammatory and fibrotic gene expression, immune cell infiltration, lipid accumulation or NAS. In line with this, we detected no differences in insulin resistance. In concordance with whole-body Cmklr1 deficiency, the absence of Cmklr1 in bone marrow-derived cells in Ldlr-/- mice did not affect their insulin resistance or liver pathology. Our results indicate that Cmklr1 is not involved in the pathogenesis of insulin resistance or NAFLD. Thus, we recommend that the associations reported between Cmklr1 and insulin resistance or NAFLD should be interpreted with caution.

Association of chemerin with oxidative stress, inflammation and classical adipokines in non-diabetic obese patients

The prevalence of obesity has been increasing worldwide. Chemerin is a recently discovered adipokine secreted by the enlarged adipose tissue with diverse biological effects that are not well detailed yet. This study aimed to elucidate the potential role of chemerin in oxidative stress and inflammation that are characteristics for excess weight and may eventually lead to insulin resistance and atherosclerotic complications. We also analysed the associations between chemerin and classical adipokines, namely leptin and adiponectin. Therefore, we investigated non-diabetic obese patients without manifest cardiovascular disease and compared their data to healthy lean individuals. Chemerin correlated positively with markers of oxidative stress and inflammation, while it showed a negative correlation with the measure of antioxidant status, characterized by the HDL-linked paraoxonase-1 enzyme. Chemerin also correlated positively with leptin and negatively with adiponectin respectively. In our study population, oxidized low-density lipoprotein and high-sensitivity C-reactive protein were found to be the strongest predictors of chemerin level. We conclude that chemerin may contribute to chronic inflammation and increased oxidative stress in obese individuals, even in the absence of manifest insulin resistance.

Processing, signaling, and physiological function of chemerin

Chemerin is an immunomodulating factor secreted predominantly by adipose tissue and skin. Processed by a variety of proteases linked to inflammation, it activates the G-protein coupled receptor chemokine-like receptor 1 (CMKLR1) and induces chemotaxis in natural killer cells, macrophages, and immature dendritic cells. Recent developments revealed the role of the nonsignaling chemerin receptor C-C chemokine receptor-like 2 (CCRL2) in inflammation. Besides further research establishing its link to inflammatory skin conditions such as psoriasis, functions in healthy skin have also been reported. Here, the current understanding of chemerin processing, signaling and physiological function has been summarized, focusing on the regulation of its activity, its different receptors and its controversially discussed role in diseases.