VIP: Mechanism of Action

1. Primary Receptor Targets — VPAC1, VPAC2, and PAC1

VIP exerts its biological effects primarily by binding to two specific G-protein-coupled receptors (GPCRs) belonging to the Class B (secretin-like) family: [3]

• VPAC1 (VIPR1): Constitutively expressed in the lung (alveolar type II cells), T-lymphocytes, liver, and brain cortex.
• VPAC2 (VIPR2): Predominantly expressed in smooth muscle, the suprachiasmatic nucleus (SCN), pancreatic β-cells, and inducible in immune cells upon stimulation.
• PAC1: VIP also binds to the PACAP receptor PAC1, but with significantly lower affinity (>500 nM). [8]

VIP-receptor interaction follows a “two-site” binding model: the N-terminal ectodomain (structured as a “Sushi” domain) captures VIP’s central/C-terminal regions (residues 6–28), then the N-terminus of VIP (His1) activates transmembrane domain 1 (TM1). [9]

2. Canonical Signaling — Gs/cAMP/PKA/CREB Pathway

In most cell types, VIP binding triggers the exchange of GDP for GTP on the Gαs subunit, activating adenylyl cyclase (AC) and increasing intracellular cyclic AMP (cAMP). Elevated cAMP activates Protein Kinase A (PKA), which phosphorylates cAMP response element-binding protein (CREB). This pathway drives surfactant production in lungs and insulin secretion in the pancreas. [10]

3. Alternative Signaling Pathways

• NF-κB Inhibition (PKA-Independent): In macrophages and monocytes, VIP inhibits nuclear translocation of NF-κB through a PKA-independent mechanism that prevents phosphorylation of IκB and inhibits IκB kinase (IKK), suppressing pro-inflammatory cytokine production. [11]
• Dual Gs/AC + Gq/PLC Pathway (Neurons): In GnRH neurons, VIP excitation requires both Gs/AC/PKA and Gq/Phospholipase C (PLC) activation, leading to PIP2 depletion and inhibition of KCa3.1 channels. [12]
• Epac Pathway (β-Cells): In pancreatic β-cells, VIP signaling involves both PKA (closing ATP-dependent K⁺ channels, causing depolarization and Ca²⁺ influx) and the Epac pathway (mobilizing intracellular Ca²⁺ stores to drive insulin secretion). [13]
• EGFR/HER2 Transactivation (Cancer): In certain cancer cells (lung, breast), VIP/PACAP signaling can transactivate EGFR and HER2, promoting cell growth and VEGF secretion. [14]

4. Tissue-Level Effects

Immunomodulation: VIP inhibits production of pro-inflammatory cytokines (TNF-α, IL-6, IL-12) and chemokines in macrophages and microglia. It shifts T-cell differentiation from Th1 toward Th2 and Treg phenotypes, and downregulates TLR2 and TLR4 expression on macrophages and dendritic cells. [11]

Pulmonary System: VIP upregulates choline phosphate cytidylyltransferase and C-Fos protein in alveolar type II (ATII) cells, increasing surfactant production. It acts as a potent bronchodilator — 100-fold more potent than isoproterenol. [15]

Central Nervous System: In the suprachiasmatic nucleus (SCN), VIP synchronizes neuronal firing via VPAC2, producing long-lasting increases in electrical activity (2–4 hours) dependent on the clock gene Per1 and Kv3 channels. [16]

Metabolic System: VIP stimulates insulin secretion in a glucose-dependent manner via VPAC2 receptors on pancreatic β-cells — negligible at low glucose (protecting against hypoglycemia) but potent during hyperglycemia. [13]

5. Pharmacokinetics — Ultra-Short Half-Life

VIP has a serum half-life of approximately 1–2 minutes, with rapid degradation by DPP-4 and other peptidases in the liver, kidneys, and lung. [5] Following IV administration, approximately 45% of the dose distributes to the lungs within 30 minutes. Apparent volume of distribution is ~14 mL/kg with a metabolic clearance rate of ~9 mL/kg/min. [17]

6. Dose-Response Relationships

• CNS Firing Rate (SCN): 1 µM and 10 µM VIP produced significant increases in SCN neuronal firing; 0.1 µM had no effect (threshold response). [16]
• Circadian Phase Shifting: Threshold ~100 nM, EC₅₀ ~500 nM, saturation at ~10 µM. [18]
• Neuroprotection (VIPR2 agonist LBT-3627): Bell-shaped dose-response — 2.0 mg/kg provided optimal neuroprotection and Treg rescue in rat Parkinson’s models. [19]
• Antiviral Activity: 10 nM VIP provided maximal anti-SARS-CoV-2 effects in cell models; effects seen at 1 nM. [4]