Prokineticin-1

Overview

Prokineticin-1 (PK1), also known by its alternative name endocrine gland-derived vascular endothelial growth factor (EG-VEGF), is one of two mammalian prokineticins — the other being prokineticin-2 (PK2, or Bv8-related). PK1 was originally identified in 2001 by Ferrara and colleagues as a factor selectively produced by steroidogenic endocrine glands (ovary, testis, placenta, adrenal) that promoted angiogenesis in these tissues while sparing vascular endothelium elsewhere.

The name "prokineticin" derives from the peptide family's original description by Chen and colleagues as factors that stimulated gastrointestinal smooth muscle contractility. The two mammalian prokineticins share structural homology with the non-mammalian bioactive peptides Bv8 (from frog skin secretions) and mamba intestinal toxin (MIT1), suggesting an ancient evolutionary origin.

PK1 and PK2 signal through two GPCRs, PKR1 and PKR2, which they bind with different relative affinities. Beyond angiogenesis and GI motility, the prokineticin system is involved in circadian rhythm regulation, olfactory bulb neurogenesis, reproduction, pain, and neuroinflammation. PK1 has been studied particularly intensively in reproductive biology, where it is essential for placental development and implantation.

Structure/Sequence

Human Prokineticin-1 mature protein: 86 amino acids after signal peptide cleavage

• Precursor length: 105 amino acids (with 19-aa signal peptide)
• Mature length: 86 amino acids
• Molecular weight: ~10 kDa
• Gene: PROK1 (chromosome 1p13.3)
• Conserved motifs:
  • N-terminal AVITGA hexapeptide: Absolutely required for receptor activation — mutation of any residue abolishes bioactivity
  • Colipase-fold C-terminal domain: Stabilized by five intramolecular disulfide bonds
• Disulfide architecture: 10 conserved cysteines forming five disulfide bridges (shared with PK2 and Bv8)

The AVITGA motif at the N-terminus is absolutely conserved across all prokineticin family members from invertebrates to mammals. N-terminal processing or modification obliterates activity, making this motif a strict pharmacophore.

Mechanism of Action

Receptor Binding

PK1 signals through two GPCRs:

Prokineticin Receptor 1 (PKR1 / PROKR1):

• Expressed in peripheral tissues: endocrine glands, GI tract, lymphoid organs
• Couples to Gq, Gi, and Gs depending on context
• PK1 has higher affinity for PKR1 than for PKR2

Prokineticin Receptor 2 (PKR2 / PROKR2):

• Enriched in CNS: suprachiasmatic nucleus, olfactory bulb, hypothalamus
• PK2 binds PKR2 with higher affinity
• Mediates central effects on circadian rhythm and olfaction

Angiogenesis in Endocrine Tissues

PK1 promotes proliferation, migration, and survival of endothelial cells within steroidogenic tissues — the ovarian corpus luteum, placental trophoblast, adrenal cortex, and testis. Unlike VEGF, PK1 does not act broadly on all vascular endothelium but is restricted by tissue-specific PKR1 expression.

Placental Development

PK1 is highly expressed in first-trimester placental trophoblast and supports:

• Trophoblast invasion
• Maternal spiral artery remodeling
• Placental angiogenesis

Dysregulated PK1 expression has been implicated in preeclampsia and intrauterine growth restriction research.

Gastrointestinal Motility

PK1 and PK2 stimulate smooth muscle contraction in the gastrointestinal tract through PKR1 on enteric neurons and smooth muscle cells — the "prokinetic" activity for which the family is named.

Circadian and CNS Effects (primarily PK2)

While PK2 dominates CNS actions, PK1 contributes to peripheral clock output signaling and neuroendocrine regulation.

Immune Cell Effects

PKR1 is expressed on monocytes and macrophages; prokineticin signaling modulates cytokine production and chemotaxis.

Research Summary

Common Discussion Topics

1. Tissue-selective angiogenesis — PK1's restriction to steroidogenic endocrine glands contrasts with the pan-endothelial activity of VEGF. This selectivity is a function of where PKR1 is expressed rather than any intrinsic property of the ligand, illustrating how receptor tissue distribution shapes pharmacological profiles.

2. Placental biology — PK1 has emerged as one of the more interesting trophoblast-expressed factors in placental research. Its role in first-trimester trophoblast invasion and maternal vessel remodeling places it at the interface of reproductive and vascular biology.

3. AVITGA motif as strict pharmacophore — Few peptide families are as strictly dependent on a single N-terminal motif as the prokineticins. This has major implications for analog design: any N-terminal modification inactivates the molecule, constraining medicinal chemistry approaches.

4. PK1 vs PK2 partitioning — While PK1 dominates endocrine and peripheral biology, PK2 dominates CNS signaling through PKR2. This ligand-receptor pairing reflects co-evolution of each ligand-receptor pair with particular physiological niches.

5. Bv8 relationship — The frog-skin peptide Bv8 is a prokineticin orthologue that informed early pharmacology of the family. Amphibian skin peptides frequently provided initial leads for mammalian peptide receptor discovery (cf. dermorphin, bombesin).

Related Compounds

• Prokineticin — broader entry covering the prokineticin family including PK2
• Endothelin-1 — related vascular peptide with distinct structure and receptors