RARRES2

RARRES2 encodes chemerin, a secreted chemoattractant protein that functions as the natural ligand for the GPCR ChemR23/CMKLR1, regulating immune cell chemotaxis, steroidogenesis, and tumor-suppressive signaling. Originally identified as a retinoid-responsive gene induced by RAR-selective (but not RXR-selective) retinoids in a skin raft culture system (PMID:9204961), RARRES2 is synthesized as a 163-amino-acid prepropeptide that undergoes C-terminal proteolytic processing to yield bioactive forms (residues 21–154 or 20–155/156) (PMID:14675762, PMID:15522723). Through CMKLR1, chemerin suppresses steroidogenesis by reducing STAR, CYP19A1, and MAPK3/1 phosphorylation in granulosa cells (PMID:24671882), and acts as a cell-intrinsic tumor suppressor by promoting β-catenin degradation and inhibiting p38 MAPK in adrenocortical carcinoma (PMID:28114280), while its loss in triple-negative breast cancer activates the PTEN–mTOR–SREBP1 axis to reprogram lipid metabolism and promote brain metastasis (PMID:37491281).

1. 1997 High

   The identification of TIG2/RARRES2 as a retinoid-responsive gene established that RAR-specific signaling transcriptionally controls a previously unknown secreted factor, linking retinoid biology to a novel downstream effector.

   Evidence Subtraction hybridization and Northern blot in skin raft cultures treated with receptor-selective retinoids

   PMID:9204961

   Open questions at the time

   • Protein product function was unknown
   • Receptor for the gene product had not been identified
   • Mechanism of RAR-dependent transcriptional regulation not defined
2. 2003 High

   Identification of chemerin (RARRES2 product) as the natural ligand of the orphan GPCR ChemR23/CMKLR1 resolved the receptor-ligand pairing and revealed that the bioactive form (residues 21–154) is generated by proteolytic processing of a 163-aa propeptide.

   Evidence Reverse pharmacology screening of hemofiltrate-derived peptides; biochemical characterization of processed forms

   PMID:14675762

   Open questions at the time

   • The specific proteases responsible for C-terminal processing were not identified
   • Whether additional processed forms exist was unknown
   • Downstream intracellular signaling cascades activated by ChemR23 were uncharacterized
3. 2004 High

   Biochemical isolation of two distinct C-terminally processed bioactive chemerin forms (ending at F156 and A155) demonstrated that multiple cleavage events generate active species, establishing that regulated proteolysis tunes chemerin bioactivity.

   Evidence Heparin-affinity and reversed-phase chromatography purification from CHO supernatant; Edman sequencing and MALDI-TOF-MS

   PMID:15522723

   Open questions at the time

   • Identity of the processing proteases remained unknown
   • Relative potencies of different processed forms on ChemR23 were not compared
   • In vivo relevance of each form was not determined
4. 2014 High

   Demonstrating that chemerin suppresses progesterone/estradiol production and MAPK3/1 phosphorylation in granulosa cells via CMKLR1 established a CMKLR1-dependent mechanism for chemerin's regulation of reproductive steroidogenesis and oocyte maturation.

   Evidence In vitro granulosa cell and cumulus-oocyte complex cultures with recombinant chemerin; anti-CMKLR1 blocking antibody; immunoblotting

   PMID:24671882

   Open questions at the time

   • In vivo reproductive phenotype of chemerin deficiency was not shown
   • Whether additional chemerin receptors (GPR1, CCRL2) contribute in ovarian context was not tested
   • Precise mechanism linking CMKLR1 to STAR/CYP19A1 downregulation was not fully delineated
5. 2015 Medium

   Gain- and loss-of-function experiments showing RARRES2 promotes RSV replication identified chemerin as a host cell factor exploited by respiratory syncytial virus, expanding its functional repertoire beyond chemotaxis and metabolic regulation.

   Evidence cDNA overexpression in MDCK cells and siRNA knockdown in HEp-2/A549 cells with viral replication assays

   PMID:26277777

   Open questions at the time

   • Mechanism by which RARRES2 promotes viral replication was not elucidated
   • Whether the effect requires CMKLR1 signaling or is receptor-independent was not tested
   • Not independently replicated by other groups
6. 2017 High

   Showing that RARRES2 overexpression promotes β-catenin phosphorylation/degradation and inhibits p38 MAPK in adrenocortical carcinoma cells in a cell-intrinsic, immune-independent manner established chemerin as a tumor suppressor acting through Wnt/β-catenin and MAPK pathways.

   Evidence RARRES2 overexpression in ACC cell lines; proliferation/invasion assays; immunodeficient mouse xenograft; immunoblotting

   PMID:28114280

   Open questions at the time

   • Whether the tumor-suppressive activity requires CMKLR1 or another receptor was not determined
   • Direct molecular target linking chemerin to β-catenin phosphorylation machinery was not identified
   • Relevance beyond adrenocortical carcinoma was not demonstrated
7. 2023 Medium

   Multi-omics analysis revealing that RARRES2 loss activates PTEN–mTOR–SREBP1 signaling and reprograms lipid metabolism to promote brain metastasis in TNBC established chemerin as a regulator of lipid metabolic adaptation in cancer dissemination.

   Evidence Transcriptomics and lipidomics with RARRES2 knockdown/overexpression in TNBC cells; in vivo brain metastasis models; PTEN/mTOR/SREBP1 immunoblotting

   PMID:37491281

   Open questions at the time

   • Direct biochemical link between chemerin and PTEN regulation was not defined
   • Single laboratory finding without independent replication
   • Whether the PTEN–mTOR axis operates through CMKLR1 in this context was not addressed

• Key unresolved questions include the identity of the specific proteases that generate bioactive chemerin forms in vivo, the relative contributions of CMKLR1, GPR1, and CCRL2 to chemerin's diverse tissue-specific functions, and the direct molecular mechanism by which chemerin controls β-catenin degradation and PTEN–mTOR signaling.
• In vivo processing proteases not identified
• Receptor specificity for tumor-suppressive and metabolic functions not resolved
• No structural model of the chemerin–CMKLR1 signaling complex