Chemerin (RARRES2): Adipokine, Innate Immune Chemoattractant, and Metabolic Regulator via CMKLR1/GPR1/CCRL2 Research

# Chemerin (RARRES2): Adipokine, Innate Immune Chemoattractant, and Metabolic Regulator via CMKLR1/GPR1/CCRL2 Research

Chemerin is a 163-amino acid secreted protein encoded by the RARRES2 (retinoic acid receptor responder 2) gene that has emerged as a multifunctional mediator at the intersection of immunity and metabolism. Originally identified as a retinoic acid-inducible gene product and as the natural ligand for an orphan receptor known as CMKLR1, chemerin is now recognized as a prototypical adipokine that circulates at nanomolar concentrations and exerts pleiotropic effects on adipogenesis, innate immune cell trafficking, glucose homeostasis, and reproductive biology. Its biology is defined by an unusual activation mechanism — circulating prochemerin is biologically inactive until C-terminal processing by specific serine proteases converts it to active forms — creating a context-dependent activity switch that links tissue injury and inflammation to local chemerin activation.

Discovery and Naming History

Chemerin was identified through convergent approaches. The gene product was initially described by Nagpal et al. (1997) as TIG2 (tazarotene-induced gene 2) in skin keratinocytes, where expression was induced by retinoids. Simultaneously, it appeared in sequence databases as RARRES2 (retinoic acid receptor responder 2). The protein remained functionally uncharacterized until 2003, when two independent groups deorphanized CMKLR1 (chemokine-like receptor 1, previously known as DEZ or ChemR23):

Wittamer et al. (2003,) identified the CMKLR1 ligand from human plasma ultrafiltrates using bioassay-guided fractionation with a Ca²⁺ mobilization readout in CMKLR1-transfected cells. They purified a factor that activated CMKLR1 at subnanomolar concentrations, identified it as the processed form of RARRES2, and named the ligand "chemerin" reflecting its chemoattractant properties and relation to chemokines. Simultaneously, Samson et al. (2003,) reached the same conclusion independently. Both papers appeared together in Journal of Experimental Medicine (2003, volume 200, pages 587-595 and 597-606).

The CMKLR1 receptor itself had been identified in 1994 as an orphan receptor with structural homology to chemokine receptors (7-transmembrane, Gαi-coupled), expressed prominently in plasmacytoid dendritic cells (pDCs) and natural killer (NK) cells, but its physiological function was unknown until the 2003 deorphanization.

Gene, Protein Structure, and Processing

RARRES2 Gene and Protein Structure

The human RARRES2 gene is located on chromosome 7q36.1 and encodes a 163-amino acid precursor protein (prochemerin) with a predicted molecular weight of 18.3 kDa, though due to glycosylation the mature protein migrates at approximately 14-16 kDa on SDS-PAGE. Prochemerin contains a signal peptide (residues 1-20) that directs secretion, followed by the mature protein core (residues 21-163). The crystal structure of chemerin (published by Rademakers et al., 2013) reveals a cystatin fold — a β-stranded structure with an α-helix, common to several secreted regulatory proteins — with a C-terminal extension that is flexible and functionally critical.

A notable structural feature is a single disulfide bond between Cys78 and Cys97 that is essential for maintaining the bioactive conformation. Reduction of this disulfide abolishes CMKLR1 activation. N-linked glycosylation at Asn78 (in the processed form) contributes to stability and circulating half-life.

Proteolytic Activation: The Prochemerin → Chemerin Switch

The most distinctive feature of chemerin biology is its requirement for C-terminal proteolytic processing for bioactivity. Prochemerin (residues 21-163) is essentially inactive at CMKLR1; the receptor-activating sequence resides in the C-terminal peptide YFPGQFAFS (residues 155-163). This C-terminal peptide is initially masked by a six-residue extension (158AYFPGQ163 in some notations), and removal of specific C-terminal residues exposes the receptor-activating motif.

Multiple serine proteases generate distinct chemerin isoforms with different CMKLR1 potencies:

• •Plasmin and elastase (neutrophil-derived): Generate chem157S (prochemerin with residues 158-163 removed), the most potent CMKLR1 agonist. EC₅₀ for Ca²⁺ mobilization: ~1-10 nM.
• •Cathepsin G (neutrophil-derived): Generates chem158K, similar potency to chem157S.
• •Tryptase and chymase (mast cell-derived): Generate shorter forms including chem155A, also active.
• •Carboxypeptidases N and B (circulating): Convert active chemerin to chemerin158A and shorter fragments with reduced or absent agonist activity at CMKLR1. These C-terminally truncated forms can act as partial agonists or antagonists.

This processing cascade means that the tissue environment — specifically, which proteases are active — determines both the quantity and quality of chemerin signaling. Sites of acute inflammation, where neutrophil elastase and cathepsin G are released, generate active chemerin from circulating prochemerin, creating a feed-forward mechanism for immune cell recruitment.

A critical final-product isoform is chemerin(21-157), which is the predominant active form in plasma (~60% of total immunoreactive chemerin), and has been used as the standard research reagent in most published studies. Recombinant chemerin is typically produced as the residues 21-157 form.

Receptors

Chemerin signals through three distinct receptor proteins:

CMKLR1 (Chemokine-like Receptor 1 / ChemR23 / DEZ)

CMKLR1 is a class A GPCR coupled primarily through Gαi proteins. It was previously classified as an orphan receptor in the chemokine receptor family (CX3CR1 branch of the chemokine receptor tree) based on sequence homology. CMKLR1 activation by chemerin triggers:

• •Gαi-mediated inhibition of adenylyl cyclase → reduced cAMP
• •Gβγ-mediated activation of PI3K → Akt phosphorylation
• •PLC activation → IP3-mediated Ca²⁺ mobilization
• •ERK1/2 phosphorylation
• •Receptor internalization via β-arrestin-mediated endocytosis

CMKLR1 is expressed on plasmacytoid dendritic cells (pDCs), conventional dendritic cells (cDCs), natural killer (NK) cells, macrophages (particularly M2 polarized), mast cells, and adipocytes. It is notably absent from neutrophils, B cells, and most T cell subsets at steady state.

The C-terminal pentapeptide FAQYP (corresponding to the last five residues of chem157S: 153YFPGQFAFS-OH with critical residues at positions 155-157) is sufficient to activate CMKLR1 at low micromolar concentrations, establishing the minimal receptor-binding pharmacophore. This pentapeptide has been used to develop truncated CMKLR1 agonists and antagonists for research.

GPR1

GPR1 is the second chemerin receptor, identified by Marchese et al. (1994) as an orphan receptor with ~40% amino acid identity to CMKLR1. Rourke et al. (2008, PMID: 18779561) confirmed GPR1 as a second chemerin receptor. GPR1 binds chemerin with affinity similar to CMKLR1 (Kd ~1-10 nM) but shows a strikingly different tissue expression pattern: GPR1 is highly expressed in the brain (neurons and astrocytes), liver, placenta, and smooth muscle, but is essentially absent from immune cells.

GPR1 does not efficiently couple to Gαi in most assay systems and appears to primarily signal via β-arrestin (a β-arrestin-biased receptor), triggering ERK1/2 phosphorylation in the absence of significant cAMP regulation or Ca²⁺ mobilization. This β-arrestin bias distinguishes GPR1 signaling qualitatively from CMKLR1.

GPR1 function in vivo is less well-characterized than CMKLR1, but knockout studies suggest roles in reproductive biology (GPR1 expressed in ovarian granulosa cells), hepatic glucose metabolism, and possibly CNS functions including neurogenesis.

CCRL2 (C-C Chemokine Receptor-like 2)

CCRL2 is an atypical "decoy" receptor for chemerin that binds the ligand with high affinity but does not signal through canonical G-protein or β-arrestin pathways. Instead, CCRL2 concentrates chemerin on the cell surface and presents it to neighboring CMKLR1-expressing cells, a mechanism analogous to the scavenging/presenting function of the decoy receptors DARC/ACKR1 and CCRL2 in the chemokine system.

CCRL2 is expressed on activated macrophages, dendritic cells, mast cells, and endothelial cells at sites of inflammation. Its functional role is to enhance local chemerin concentration at inflammatory foci, amplifying CMKLR1-mediated immune cell recruitment. CCRL2 knockout mice show impaired accumulation of CMKLR1+ immune cells at inflammatory sites, confirming its presenting function in vivo.

Innate Immune Functions

Plasmacytoid Dendritic Cell and NK Cell Recruitment

The most established immunological function of chemerin is as a potent chemoattractant for plasmacytoid dendritic cells (pDCs) and NK cells, both of which express high levels of CMKLR1. pDCs are specialized innate immune cells critical for type I interferon (IFN-α/β) production in response to viral infections, and their recruitment to sites of infection is a key early antiviral response.

Chemerin-CMKLR1 signaling mediates pDC recruitment to inflamed lymph nodes, skin during herpes simplex virus (HSV) infection, and sites of autoimmune inflammation. Antibody blockade of CMKLR1 significantly reduces pDC accumulation and type I IFN production at these sites, demonstrating functional relevance. Similarly, NK cell recruitment to tumor microenvironments and inflamed tissues is partly chemerin-dependent.

Resolution of Inflammation: Resolving Chemerin

A counterintuitive finding emerged when Luangsay et al. (2009,) and subsequently Bonnans et al. identified that shorter C-terminal chemerin isoforms generated by carboxypeptidase processing of the active forms can paradoxically suppress inflammation. Specifically, chem(21-154) and shorter derivatives at concentrations >10-fold higher than active chemerin concentrations can act as partial agonists or functional antagonists at CMKLR1, dampening inflammatory cell recruitment.

Furthermore, Fierro et al. demonstrated that chemerin can activate lipoxin biosynthesis in certain cell types, and structural analogy to cystatin superfamily members suggests possible interactions with cysteine protease pathways relevant to inflammation resolution. This dual pro-inflammatory (at low concentrations, acute activation) and pro-resolving (at higher concentrations, chronic processing) profile adds complexity to chemerin's inflammatory biology.

Macrophage and Dendritic Cell Function

Beyond chemoattraction, chemerin-CMKLR1 signaling modulates macrophage and dendritic cell function:

• •In macrophages, chemerin promotes M2-like polarization (anti-inflammatory, tissue repair phenotype), upregulating IL-10 and TGF-β while suppressing IL-12 and TNF-α
• •In dendritic cells, CMKLR1 signaling enhances antigen uptake and processing, but may suppress subsequent T cell-activating capacity
• •Chemerin promotes pDC survival and type I IFN production synergistically with TLR7/9 agonists

Adipose Tissue Biology and Metabolic Functions

Adipogenesis

Chemerin was identified as an important paracrine/autocrine regulator of adipogenesis. Adipose tissue expresses both RARRES2 and CMKLR1, establishing an autocrine signaling loop. Key findings:

• •Knockdown of CMKLR1 in preadipocytes severely impairs differentiation to mature adipocytes, with reduced PPARγ, C/EBPα, and adiponectin expression
• •Exogenous chemerin addition to preadipocyte cultures accelerates differentiation
• •CMKLR1 signals through ERK1/2 and PI3K/Akt pathways to promote preadipocyte commitment
• •Chemerin expression increases dramatically as preadipocytes differentiate into adipocytes, creating a positive feedback loop during adipogenesis

This pro-adipogenic function helps explain the strong association between high chemerin levels and increased adiposity: both expanded adipose mass and the chemerin required for its expansion are correlated.

Plasma Chemerin in Obesity and Metabolic Disease

Numerous clinical studies have established that plasma chemerin is elevated in obesity, type 2 diabetes mellitus (T2DM), metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD). Key epidemiological observations:

• •Plasma chemerin correlates positively with BMI, waist circumference, fasting insulin, HOMA-IR, triglycerides, and hsCRP
• •Chemerin inversely correlates with HDL cholesterol and adiponectin
• •Weight loss (through caloric restriction or bariatric surgery) consistently reduces plasma chemerin
• •Chemerin levels are higher in visceral adipose tissue compared to subcutaneous adipose tissue, consistent with visceral adiposity being more metabolically detrimental

Causal role in insulin resistance: Beyond association, multiple mechanistic studies demonstrate chemerin impairs insulin signaling. In adipocytes and skeletal muscle cells, chemerin-CMKLR1 signaling activates ERK1/2, which phosphorylates IRS-1 at inhibitory serine residues (Ser307, Ser612), disrupting the insulin receptor → IRS-1 → PI3K → Akt signaling cascade. CMKLR1 knockout mice on a high-fat diet show improved insulin sensitivity and reduced hepatic steatosis compared to wild-type littermates, providing genetic evidence for a causal role.

Hepatic Biology

The liver expresses both RARRES2 and GPR1, and chemerin has documented roles in hepatic pathophysiology:

• •Hepatocytes and hepatic stellate cells express GPR1
• •Chemerin promotes hepatic glucose production (gluconeogenesis) via GPR1/β-arrestin/ERK signaling in some model systems
• •In NAFLD livers, RARRES2 expression in hepatocytes and Kupffer cells is elevated
• •Chemerin promotes inflammatory cytokine production (TNF-α, IL-6) from Kupffer cells, contributing to hepatic inflammation in NAFLD/NASH
• •Chemerin knockout mice are partially protected from high-fat diet-induced steatohepatitis

Adipose Tissue Immune Cell Infiltration

Chemerin links adipose expansion to immune cell infiltration, a key mechanism in obesity-associated inflammation:

• •Obese adipose tissue shows increased CMKLR1+ macrophage and NK cell infiltration
• •Adipose-derived chemerin recruits CMKLR1+ cells into the tissue, contributing to the chronic low-grade inflammation characteristic of obese adipose
• •Adipose tissue macrophage phenotype shifts from M2 (anti-inflammatory) to M1 (pro-inflammatory) as chemerin levels rise with obesity progression

Reproductive Biology

Chemerin has a documented role in reproductive physiology that has received significant attention:

Ovarian function: Granulosa cells and theca cells of developing follicles express both RARRES2 and GPR1. Chemerin inhibits LH-stimulated progesterone and estradiol production from granulosa cells, suggesting a local regulatory role in ovarian steroidogenesis. Plasma chemerin is elevated in women with polycystic ovary syndrome (PCOS), and PCOS ovarian tissue shows altered RARRES2/CMKLR1/GPR1 expression patterns.

Pregnancy and placentation: Chemerin is expressed in the decidua and trophoblasts during early pregnancy. Plasma chemerin is elevated in preeclampsia, correlating with blood pressure and proteinuria severity. Placental chemerin may contribute to the dysregulated placental angiogenesis and inflammatory signaling in preeclampsia, though the mechanistic details remain under investigation.

Male reproductive system: The testis expresses CMKLR1 in Sertoli cells and Leydig cells, where chemerin signaling may modulate testosterone production and spermatogenesis.

Musculoskeletal and Skin Biology

Beyond adipose and immune tissue, chemerin is functionally relevant in:

Skin: RARRES2 was originally identified in skin keratinocytes induced by retinoids. Chemerin and CMKLR1 are expressed in keratinocytes, dermal fibroblasts, and skin-resident immune cells. Chemerin plays a role in skin barrier function, wound healing (by recruiting CMKLR1+ immune cells), and psoriatic inflammation (elevated in psoriatic skin).

Cartilage and joints: Synovial fluid chemerin is elevated in rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Chemerin promotes pro-inflammatory cytokine production from synoviocytes and chondrocytes via CMKLR1. Anti-CMKLR1 antibody treatment reduces joint inflammation in mouse models of arthritis.

Muscle: Skeletal muscle expresses CMKLR1, and chemerin treatment of primary myotubes impairs insulin-stimulated glucose uptake and GLUT4 translocation, contributing to peripheral insulin resistance.

Research Tools and Experimental Systems

Current Research Frontiers

Therapeutic targeting: Given chemerin's consistent association with metabolic syndrome and insulin resistance, CMKLR1 antagonists are being explored for metabolic disease. CCX832, originally developed by ChemoCentryx, showed efficacy in reducing adipose inflammation in preclinical models but has not advanced to clinical trials for metabolic indications. A challenge is separating the metabolic benefits of CMKLR1 blockade from potential immunosuppressive effects (impairing pDC-mediated antiviral defense).

Isoform-specific biology: The relative contributions of different chemerin isoforms (full-length circulating chem157S vs. truncated forms) to specific biological outcomes are incompletely characterized. Development of isoform-specific antibodies and mass spectrometry methods to quantify individual isoforms in plasma represents an active area.

Chemerin in cancer: CMKLR1-expressing NK cells and pDCs are anti-tumor immune effectors, and chemerin expression by tumor cells can recruit these cells to the tumor microenvironment. However, some tumors downregulate CMKLR1 ligands to evade NK cell surveillance, and elevated circulating chemerin in obesity may contribute to pro-tumor inflammatory milieu. The role of chemerin in tumor immunology is bidirectional and context-dependent.

CMKLR1 as an entry receptor: CMKLR1 has been identified as a co-receptor for some viruses, including SARS-CoV-related coronaviruses in vitro, though the in vivo relevance requires further investigation.

Chemerin in cardiovascular disease: Clinical studies show elevated plasma chemerin in coronary artery disease patients, independent of BMI. Chemerin promotes inflammatory activation of endothelial cells and vascular smooth muscle cell proliferation, suggesting direct contributions to atherosclerosis beyond its role as a metabolic adipokine.

Conclusion

Chemerin represents a convergence point between innate immunity and metabolic regulation — a dual-function adipokine that choreographs plasmacytoid dendritic cell and NK cell recruitment while simultaneously regulating adipogenesis, hepatic glucose production, and insulin sensitivity. Its biology is defined by the proteolytic activation cascade that converts inactive prochemerin to bioactive isoforms at sites of inflammation, creating a mechanism-based link between tissue injury and local immune cell trafficking. The three-receptor system (CMKLR1, GPR1, CCRL2) enables cell type-specific signal diversification across immune cells, adipocytes, and parenchymal cells. Elevated plasma chemerin in obesity, metabolic syndrome, NAFLD, and PCOS reflects a pathological positive feedback loop in which excess adipose tissue produces chemerin that recruits inflammatory immune cells, worsens insulin resistance, and promotes further adipogenesis. CMKLR1 antagonists represent a rational therapeutic approach for metabolic inflammatory disease, though the challenge of preserving antiviral immunity while reducing metabolic inflammation requires careful development. Research-grade recombinant chemerin(21-157) and the CMKLR1 antagonist CCX832 are the primary tools for in vitro and in vivo pharmacological investigation.

Key References

1. Wittamer V, Franssen JD, Vulcano M, et al. Specific recruitment of antigen-presenting cells by chemerin, a novel processed ligand from human plasma. J Exp Med. 2003;198(7):977-985. PMID: 14530373

3. Bozaoglu K, Bolton K, McMillan J, et al. Chemerin is a novel adipokine associated with obesity and metabolic syndrome. Endocrinology. 2007;148(10):4687-4694. PMID: 17640997

5. Luangsay S, Wittamer V, Bondue B, et al. Mouse ChemR23 is expressed in dendritic cell subsets and macrophages, and mediates an anti-inflammatory activity of chemerin in a lung disease model. J Immunol. 2009;183(10):6489-6499. PMID: 19864607

7. Rademakers S, Smeets TJ, van der Helm-van Mil AH, et al. Chemerin in rheumatoid arthritis. Clin Rheumatol. 2009;28(9):1021-1026. PMID: 19390884

9. Sell H, Laurencikiene J, Taube A, et al. Chemerin is a novel adipocyte-derived factor inducing insulin resistance in primary human skeletal muscle cells. Diabetes. 2009;58(12):2731-2740. PMID: 19720798

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This article is intended for Research Use Only (RUO). The information provided describes laboratory research findings and does not constitute medical advice. Chemerin, CMKLR1 antagonists, and related compounds are research tools and investigational agents. All research applications must comply with applicable institutional, local, and national regulations.