RARRES2

RARRES2 encodes chemerin, a secreted protein originally identified as a retinoid-responsive transcript whose induction requires the 3D tissue architecture of keratinocytes and fibroblasts and is driven specifically by RAR-selective retinoids (PMID:9204961). The prepropeptide undergoes C-terminal proteolytic processing to generate bioactive forms — circulating species corresponding to residues ~21–154/155/156 of the 163-residue precursor — that serve as the endogenous ligand for the orphan GPCR ChemR23/CMKLR1 (PMID:14675762, PMID:15522723). Through CMKLR1, chemerin acts as a negative regulator of ovarian steroidogenesis, suppressing progesterone and estradiol output, lowering STAR, CYP19A1 and HMGCR levels, inhibiting MAPK3/MAPK1 phosphorylation, and arresting oocyte maturation, with all effects blocked by CMKLR1 antagonism (PMID:24671882). Beyond classical ligand–receptor signaling, RARRES2 also exerts cell-autonomous, CMKLR1-independent functions in cancer: it promotes β-catenin phosphorylation/degradation and inhibits p38 MAPK to suppress adrenocortical carcinoma growth (PMID:28114280), and its loss reprograms lipid metabolism through the PTEN-mTOR-SREBP1 axis to favor breast cancer brain metastasis (PMID:37491281). In fibroblasts, RARRES2 drives a pro-fibrotic program by phosphorylating STAT3, which binds the HSPG2 promoter to induce HSPG2 expression (PMID:42170108). RARRES2 has additionally been characterized as a host factor supporting respiratory syncytial virus replication (PMID:26277777).

1. 1997 Medium

   Established RARRES2 (TIG2) as a retinoid-inducible gene and defined an unusual requirement for 3D tissue context, distinguishing it from simple monolayer-inducible retinoid targets.

   Evidence Subtraction hybridization and Northern blot in skin raft cultures with RAR- vs RXR-selective retinoid controls

   PMID:9204961

   Open questions at the time

   • Did not identify the protein product's function or whether it is secreted
   • Mechanism of the 3D-dependence of induction not resolved
   • No receptor or downstream pathway identified
2. 2003 High

   Resolved the function of the RARRES2 product by identifying chemerin as the endogenous ligand for the orphan GPCR ChemR23/CMKLR1 and defining the bioactive circulating species.

   Evidence Reverse pharmacology screening of a human hemofiltrate peptide library with receptor binding/activation

   PMID:14675762

   Open questions at the time

   • Did not define the protease(s) generating the bioactive form
   • Downstream signaling consequences of CMKLR1 activation not characterized here
3. 2004 Medium

   Confirmed that C-terminal proteolytic processing of the precursor generates the bioactive chemerin forms, defining the precise residue boundaries.

   Evidence Heparin-affinity chromatography, RP-HPLC, Edman sequencing and MALDI-TOF MS of the hamster ortholog

   PMID:15522723

   Open questions at the time

   • Performed in hamster ortholog rather than human
   • Identity of the activating proteases not established
4. 2014 High

   Demonstrated a receptor-dependent physiological role: chemerin signals through CMKLR1 to suppress ovarian steroidogenesis and MAPK1/3 activation.

   Evidence Recombinant chemerin on bovine granulosa cells and cumulus-oocyte complexes with anti-CMKLR1 blockade, steroid assays and Western blot

   PMID:24671882

   Open questions at the time

   • G-protein coupling and proximal signaling steps downstream of CMKLR1 not dissected
   • Validated in bovine cells; human in vivo relevance not addressed
5. 2015 Medium

   Identified an unexpected role for RARRES2 as a host factor supporting RSV replication, separate from its endocrine ligand function.

   Evidence cDNA gain-of-function in MDCK cells and siRNA knockdown in HEp-2/A549 with viral replication readout

   PMID:26277777

   Open questions at the time

   • Molecular mechanism linking RARRES2 to viral replication unknown
   • Whether secreted or intracellular RARRES2 mediates the effect not determined
6. 2017 Medium

   Revealed a cell-autonomous, CMKLR1-independent tumor-suppressive function of RARRES2 acting on Wnt/β-catenin and p38 MAPK.

   Evidence RARRES2 overexpression in adrenocortical carcinoma lines with Western blot, in vitro proliferation/invasion assays and xenografts

   PMID:28114280

   Open questions at the time

   • How intracellular RARRES2 engages β-catenin and p38 pathways mechanistically is unresolved
   • Generalizability beyond adrenocortical carcinoma not tested
7. 2023 Medium

   Linked RARRES2 loss to lipid metabolic reprogramming via PTEN-mTOR-SREBP1, promoting breast cancer brain metastasis.

   Evidence RARRES2 manipulation with multi-omics and lipidomics in TNBC models in vitro and in vivo

   PMID:37491281

   Open questions at the time

   • Direct biochemical link between RARRES2 and PTEN-mTOR-SREBP1 not established
   • Single lab, single tumor type
8. 2024 Low

   Placed RARRES2 in an intercellular IGF-1 axis maintaining osteosarcoma stem cell stemness.

   Evidence Single-cell transcriptomics and cell communication analysis with IGF-1 treatment and RARRES2 manipulation

   PMID:38280909

   Open questions at the time

   • Primarily computational with limited in vitro validation
   • Mechanism by which RARRES2 maintains stemness undefined
9. 2026 Medium

   Defined a RARRES2-STAT3-HSPG2 transcriptional axis driving fibroblast pro-scarring activity.

   Evidence Mendelian randomization, multiomics, p-STAT3 promoter binding on HSPG2, and RARRES2 manipulation in primary fibroblasts and animal models

   PMID:42170108

   Open questions at the time

   • How RARRES2 triggers STAT3 phosphorylation is not resolved
   • Not yet independently replicated
10. 2025 Low

    Proposed a paracrine role for hepatocyte-secreted Rarres2 in modulating hepatic stellate cells during fibrosis regression.

    Evidence Single-cell RNA profiling of TAA-induced mouse liver cirrhosis with NicheNet inference (preprint)

    Open questions at the time

    • Computational inference with limited direct functional validation
    • Receptor and signaling pathway on stellate cells not defined

• It remains unresolved how RARRES2 switches between extracellular CMKLR1-dependent ligand signaling and cell-autonomous intracellular functions, and what proteases and cell-context cues govern this dichotomy.
• No unified mechanism reconciling secreted-ligand and cell-autonomous roles
• Activating proteases not mapped across tissues
• Structural basis of intracellular signaling functions unknown