IL20RA

IL-20RA (IL-20R1) is a shared signaling subunit of heterodimeric type II cytokine receptor complexes that transduce signals from IL-19, IL-20, and IL-24 to regulate inflammation, tissue repair, and immune cell trafficking. IL-20RA pairs with IL-20R2 to bind IL-20 and IL-19, and IL-24 signals through both IL-20RA/IL-20R2 and IL-22R1/IL-20R2 heterodimers, activating STAT transcription factors, JAK1-STAT3, and ERK/Nrf2 pathways depending on cellular context (PMID:11706020, PMID:33532035, PMID:33456560). Transcriptional control of IL-20RA itself is mediated by an NF-κB-bound enhancer element at the 6q23 locus, which contacts the IL-20RA promoter through chromatin looping (PMID:29483615). In disease settings, IL-20RA signaling through JAK1-STAT3 drives liver fibrosis via TGF-β1 upregulation, promotes cancer stemness and immune evasion through SOX2 and PD-L1 induction, and regulates leukocyte transmigration across lymphatic endothelium in Crohn's disease (PMID:24763901, PMID:33456560, PMID:31426584).

1. 2001 High

   Establishing that IL-20RA functions as a ligand-binding subunit in two heterodimeric receptor complexes for IL-24 resolved how a single cytokine engages distinct receptor pairs to activate STAT signaling.

   Evidence Ligand-receptor binding and STAT activation assays in COS cells, keratinocytes, and BHK cells transfected with IL-20R1/IL-20R2 or IL-22R1/IL-20R2

   PMID:11706020

   Open questions at the time

   • Downstream STAT family member specificity not resolved
   • Binding affinities for IL-19 and IL-20 through these complexes not measured
   • No structural basis for receptor assembly
2. 2011 Medium

   Crystallization of the IL-20–IL-20RA–IL-20R2 ternary complex provided the first structural evidence that IL-20RA directly contacts both ligand and co-receptor in a defined stoichiometric assembly.

   Evidence Protein purification and X-ray crystallography at 3 Å resolution

   PMID:22232181

   Open questions at the time

   • Full atomic structure and interface residue analysis not yet reported
   • No mutagenesis validation of predicted contact sites
   • Structure of IL-19 or IL-24 ternary complexes not determined
3. 2014 High

   Demonstrating that IL-20RA-deficient mice and anti-IL-20RA antibody blockade prevented liver fibrosis established IL-20RA as a required upstream activator of TGF-β1 and pro-fibrotic gene expression in hepatic stellate cells.

   Evidence IL-20R1 knockout mice and anti-IL-20R1 monoclonal antibody (51D) in CCl4-induced liver fibrosis model

   PMID:24763901

   Open questions at the time

   • Precise signaling intermediates between IL-20RA and TGF-β1 induction not mapped
   • Whether IL-19 or IL-24 also drives fibrosis through this receptor not tested
   • Human disease relevance not validated
4. 2018 High

   Identification of an NF-κB-bound intergenic enhancer at 6q23 that controls IL-20RA expression via chromatin looping revealed how genetic variation at this locus can tune receptor abundance.

   Evidence CRISPR/Cas9 enhancer deletion, EMSA, chromatin conformation capture (3C), and TALE-based transcriptional analysis

   PMID:29483615

   Open questions at the time

   • Cell-type specificity of enhancer activity not fully characterized
   • Quantitative relationship between enhancer variant genotype and surface IL-20RA protein levels not established
   • Whether this enhancer co-regulates TNFAIP3 independently of IL-20RA not resolved
5. 2019 High

   Structural and mutagenesis studies on IL-24 identified T198 as a critical residue governing promiscuous binding to IL-20RA and IL-22R1, explaining how flexible ligand regions enable receptor sharing.

   Evidence Protein engineering, binding affinity measurements, signaling assays, and crystal structure of IL-24B variant (PDB 6GG1)

   PMID:31152679

   Open questions at the time

   • Reciprocal mutations on IL-20RA that affect binding not identified
   • Whether IL-19 uses analogous structural determinants unknown
   • No kinetic (SPR) data for mutant binding
6. 2019 Medium

   Finding that mTOR signaling drives IL-20RA expression in Crohn's disease lymphatic endothelial cells and that IL-20RA mediates leukocyte transmigration placed the receptor in a pathway controlling immune cell trafficking during intestinal inflammation.

   Evidence Transcriptomic profiling of isolated human intestinal lymphatic endothelial cells and transwell transmigration assay

   PMID:31426584

   Open questions at the time

   • Direct ligand (IL-19/IL-20/IL-24) responsible for transmigration effect not identified
   • Downstream signaling from IL-20RA in lymphatic endothelium not mapped
   • In vivo confirmation in animal models lacking
7. 2021 Medium

   Convergent studies established that IL-20RA activates the JAK1-STAT3 axis in multiple contexts: in breast cancer it drives SOX2/PD-L1-mediated immune evasion, and miR-452 negatively regulates IL-20RA to suppress this pathway.

   Evidence Gain/loss-of-function experiments with in vivo tumor models (breast cancer); luciferase reporter assay validating miR-452 targeting of IL-20RA mRNA; CRISPR knockdown in rat GMH model confirming ERK/Nrf2 branch

   PMID:33456560 PMID:33532035 PMID:34283251

   Open questions at the time

   • Whether JAK1-STAT3 and ERK/Nrf2 are activated simultaneously or context-dependently not resolved
   • Direct physical interaction between IL-20RA and JAK1 not demonstrated biochemically
   • Relative contribution of each ligand (IL-19/IL-20/IL-24) to STAT3 activation in tumors unknown

• Key unresolved questions include the full atomic structure of IL-20RA-containing ternary complexes, the mechanism of JAK1 recruitment to IL-20RA's intracellular domain, and whether IL-20RA signals through additional non-canonical pathways beyond JAK-STAT and ERK-Nrf2.
• No high-resolution ternary complex structure with full interface analysis published
• JAK1 recruitment mechanism to IL-20RA intracellular domain unknown
• Ligand-specific versus shared downstream signaling programs not systematically dissected