PROK1

PROK1 (Prokineticin-1/EG-VEGF) is a secreted cysteine-rich ligand that signals through two G protein-coupled receptors, PROKR1 and PROKR2, to regulate angiogenesis, vascular permeability, trophoblast invasion, and enteric neural crest cell development. PROKR1 mediates proliferation, migration, and survival of endothelial cells and activates downstream Gq/11–PLC–ERK, cSrc–EGFR–MAPK, and calcineurin–NFAT cascades to induce target genes including IL-11, CTGF, and MMPs, while PROKR2 mediates vascular permeability and inhibition of trophoblast invasion with associated downregulation of MMP-2/MMP-9 (PMID:20587779, PMID:19801577, PMID:21098624, PMID:19602057). PROKR1 localizes to primary cilia in trophoblasts, where ciliogenesis is required for EG-VEGF-induced ERK1/2 activation and MMP-dependent invasion (PMID:27736039). PROK1 expression is transcriptionally regulated by hCG via cAMP/CRE elements and by PPARγ, as demonstrated by PPARγ-null mouse placentas showing decreased EG-VEGF levels (PMID:22138749, PMID:26081281).

1. 2005 Low

   Early work established that EG-VEGF/PK-1 is a mitogen and survival factor for endothelial cells acting through MAPK, and suggested receptor-subtype-specific roles based on differential expression under stress conditions.

   Evidence Proliferation, MAPK activation, and apoptosis assays in luteal endothelial and steroidogenic cells with RT-PCR-based receptor profiling

   PMID:16320832

   Open questions at the time

   • Receptor-subtype assignment was based on expression patterns rather than direct knockdown or antagonism
   • Bovine system findings not validated in human cells
   • VEGF upregulation downstream of PK-R1 not confirmed by loss-of-function
2. 2007 Medium

   PROK1 was shown to function beyond the vasculature, cooperating with GDNF/RET signaling to drive enteric neural crest cell proliferation and differentiation, establishing it as a neurodevelopmental factor.

   Evidence Microarray, enteric NCC cultures, and c-Ret-deficient mouse epistasis analysis

   PMID:18006159

   Open questions at the time

   • Single-lab finding without independent replication
   • Downstream signaling pathway in NCCs not fully dissected
   • In vivo enteric phenotype of Prok1 loss not characterized
3. 2009 High

   Two studies resolved receptor-specific functions: PROKR1 was identified as the conduit for Gq/11–ERK–calcineurin–NFAT signaling to IL-11 in endometrium, while PROKR2 was shown to mediate inhibition of trophoblast invasion and MMP-2/9 suppression.

   Evidence siRNA knockdown of PROKR1/PROKR2, pathway inhibitors, and gain/loss-of-function in Ishikawa cells, HTR-8/SVneo cells, and villous explants

   PMID:19602057 PMID:19801577

   Open questions at the time

   • NFAT-target gene repertoire beyond IL-11 not defined
   • Whether PROKR1 and PROKR2 signal independently or form heterodimers in trophoblasts is unknown
4. 2010 High

   Receptor dichotomy was extended to placental endothelial cells: PROKR1 drives angiogenic responses (proliferation, migration, survival) while PROKR2 controls permeability, and a PROKR1-specific Gq–PLC–cSrc–EGFR–ERK cascade inducing CTGF was mapped in decidua.

   Evidence siRNA and neutralizing antibody approaches in HPEC/HUVEC; pharmacological inhibitor cascade dissection with functional trophoblast adhesion and tube-formation readouts

   PMID:20587779 PMID:21098624

   Open questions at the time

   • EGFR transactivation mechanism (ligand shedding vs. intracellular) not resolved
   • In vivo confirmation of receptor-specific angiogenic vs. permeability functions lacking
5. 2011 Medium

   hCG was identified as a major upstream regulator of PROK1 transcription via cAMP and CRE elements in the EG-VEGF promoter, linking gonadotropin signaling to prokineticin expression in early pregnancy.

   Evidence Placental explants and primary trophoblast cultures with dose–time responses, cAMP pathway analysis, and promoter element mapping

   PMID:22138749

   Open questions at the time

   • CRE element identity not confirmed by mutagenesis
   • Post-transcriptional regulatory mechanism not molecularly defined
6. 2015 Medium

   PPARγ was established as a second transcriptional activator of EG-VEGF, with PPARγ-null mouse placentas showing reduced EG-VEGF levels and PPARγ-mediated effects on trophoblast vascularization and invasion routed through PROKR1 and PROKR2 respectively.

   Evidence PPARγ agonist treatment in human trophoblasts, receptor-specific antagonists, and PPARγ-knockout mouse placental analysis

   PMID:26081281

   Open questions at the time

   • Direct PPARγ binding to PROK1 promoter not demonstrated (e.g., ChIP)
   • Relative contributions of hCG vs. PPARγ to PROK1 expression in vivo not determined
7. 2016 Medium

   PROKR1 was found to localize to primary cilia in trophoblasts, and ciliary integrity (via IFT88) was shown to be required for EG-VEGF-induced ERK1/2 activation and MMP-dependent invasion, revealing a novel subcellular compartment for prokineticin signaling.

   Evidence PROKR1 immunolocalization to cilia in HTR-8/SVneo and first-trimester placenta; IFT88 siRNA abrogated ciliary growth, ERK activation, MMP upregulation, and invasion

   PMID:27736039

   Open questions at the time

   • Mechanism of ciliary enrichment of PROKR1 not defined
   • Whether ciliary PROKR1 signaling differs qualitatively from non-ciliary signaling is unclear
   • In vivo ciliary requirement not tested
8. 2017 Medium

   The PROK1–PROKR1/R2 axis was shown to drive choriocarcinoma cell proliferation, migration, and invasion, and receptor antagonism reduced tumor growth and metastasis in vivo, establishing PROK1 as a potential therapeutic target in gestational trophoblastic neoplasia.

   Evidence JEG3 2D/3D assays and luciferase-labeled placental xenograft SCID mouse model with receptor antagonists

   PMID:28899975

   Open questions at the time

   • Antagonist specificity and off-target effects not fully characterized
   • Human clinical relevance beyond xenograft model not established

• Key unresolved questions include the structural basis of PROKR1 vs. PROKR2 signaling selectivity, the identity of the full downstream transcriptional program of PROK1 in different tissues, and the in vivo requirement for PROK1 in human placental development and enteric nervous system formation.
• No structural model of PROK1–receptor complexes exists
• Prok1-knockout phenotype in mouse placenta and ENS not reported in the literature
• Comprehensive transcriptomic profiling of PROK1 target genes across tissues is lacking