Vasoactive Intestinal Peptide (VIP)

Overview

Vasoactive intestinal peptide (VIP) is a 28-amino-acid neuropeptide first isolated from porcine small intestine by Sami Said and Viktor Mutt in 1970. Its initial identification was based on its potent vasodilatory activity — the property that gave the peptide its name — but subsequent decades of research have revealed VIP to be one of the most functionally versatile neuropeptides in mammalian biology, with significant roles in the gastrointestinal, cardiovascular, immune, respiratory, reproductive, and central nervous systems.

VIP belongs to the secretin-glucagon superfamily of peptides, which includes pituitary adenylate cyclase-activating polypeptide (PACAP), secretin, glucagon, growth hormone-releasing hormone (GHRH), and glucose-dependent insulinotropic polypeptide (GIP). These peptides share structural homology and act through related G-protein-coupled receptors of the class B (secretin) receptor family. VIP and PACAP are the closest relatives within this superfamily, sharing 68% sequence identity and overlapping receptor pharmacology.

VIP is widely expressed as both a neurotransmitter and a neuromodulator. It is found in parasympathetic and sensory nerve fibers throughout the body, in specific populations of interneurons in the cerebral cortex and hippocampus, and in the master circadian pacemaker of the suprachiasmatic nucleus (SCN). VIP's role in the SCN as an essential synchronizing signal for circadian rhythms has attracted particular interest, linking this gut-derived neuropeptide to the fundamental biology of the body clock.

In the immune system, VIP functions as a potent anti-inflammatory and immunomodulatory peptide, inhibiting pro-inflammatory cytokine production and promoting regulatory T cell differentiation. This immunoregulatory profile has driven research into VIP-based therapies for autoimmune and inflammatory disorders.

Structure

VIP is a linear, non-cyclized 28-amino-acid peptide:

Sequence: His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn-NH₂

• Molecular formula: C₁₄₇H₂₃₈N₄₄O₄₃S₁
• Molecular weight: 3,326.82 Da
• CAS Number: 37221-79-7
• C-terminal modification: Amidated (Asn28-NH₂)
• Gene: VIP gene on chromosome 6q25

VIP is synthesized from a 170-amino-acid prepro-VIP precursor that also encodes peptide histidine methionine (PHM-27, in humans) or peptide histidine isoleucine (PHI-27, in other mammals). Both VIP and PHM/PHI are co-released and activate similar receptor targets, though VIP is the more potent ligand.

The N-terminal region (residues 1-7) is critical for receptor activation, while the C-terminal alpha-helical domain (residues 15-28) is important for receptor binding affinity. The C-terminal amidation protects against carboxypeptidase degradation and contributes to receptor interaction.

VIP shares significant structural homology with PACAP-27 (68% identity), and both peptides activate the same VPAC1 and VPAC2 receptors, though PACAP additionally activates the PAC1 receptor. This receptor overlap creates functional redundancy in some physiological systems while allowing distinct signaling in tissues where receptor expression differs.

Mechanism of Action

Receptor Pharmacology

VIP signals primarily through two class B G-protein-coupled receptors:

VPAC1 (formerly VIP/PACAP receptor 1):

• Widely expressed: lung, liver, intestinal epithelium, T lymphocytes, brain cortex
• Binds VIP and PACAP with approximately equal affinity
• Coupled primarily to Gs — activates adenylyl cyclase, increasing cAMP

VPAC2 (formerly VIP/PACAP receptor 2):

• Expression: suprachiasmatic nucleus, pancreatic beta cells, smooth muscle, immune cells
• Binds VIP and PACAP with approximately equal affinity
• Coupled to Gs — cAMP-mediated signaling
• Critical role in circadian rhythm maintenance in the SCN

Both receptors activate downstream signaling through:

1. cAMP/PKA pathway — the primary signaling cascade driving most VIP effects
2. EPAC (Exchange Protein Activated by cAMP) — cAMP-dependent, PKA-independent signaling
3. Calcium mobilization — some VPAC signaling involves phospholipase C and intracellular calcium
4. PI3K/Akt — activated in some cell types, contributing to anti-apoptotic effects

Vasodilation and Smooth Muscle Relaxation

VIP is one of the most potent endogenous vasodilators. It relaxes vascular smooth muscle through cAMP-dependent mechanisms, including:

• Activation of large-conductance calcium-activated potassium (BK) channels
• Direct inhibition of myosin light chain phosphorylation
• Stimulation of endothelial nitric oxide synthase (eNOS) and nitric oxide release

This vasodilatory activity is particularly prominent in the cerebral, coronary, and pulmonary circulations. VIP also relaxes non-vascular smooth muscle in the airways (bronchodilation), gastrointestinal tract, and urogenital system.

Immunomodulation

VIP's immunoregulatory effects are mediated through both VPAC1 and VPAC2 receptors on immune cells:

• Anti-inflammatory cytokine shift — suppresses TNF-alpha, IL-6, IL-12, and iNOS expression in activated macrophages; promotes IL-10 production
• Dendritic cell modulation — generates tolerogenic DCs that promote regulatory T cell (Treg) differentiation over Th1/Th17 responses
• T cell regulation — inhibits T cell proliferation and promotes Th2/Treg polarization over Th1
• NF-kB suppression — inhibits NF-kB and AP-1 transcriptional activity in immune cells
• Chemotaxis inhibition — reduces neutrophil and macrophage recruitment to inflammatory sites

Circadian Rhythm Regulation

In the suprachiasmatic nucleus, VIP-expressing neurons constitute a critical subpopulation responsible for synchronizing individual cellular oscillators into a coherent circadian output. VIP signaling through VPAC2 receptors in the SCN:

• Maintains intercellular coupling between clock neurons
• Mediates light-induced phase shifts of the circadian clock
• Sustains rhythmic gene expression (Per1, Per2, Bmal1)

VIP knockout mice display fragmented circadian rhythms, loss of coordinated SCN neuronal firing, and disrupted sleep-wake cycles, demonstrating VIP's indispensable role in clock function.

Research Summary

Area of Study	Key Finding	Notable Reference
Discovery and isolation	First isolation from porcine intestinal extracts based on vasodilatory activity	Said and Mutt, Science, 1970
Receptor cloning	VPAC1 and VPAC2 receptors cloned and characterized	Ishihara et al., Neuron, 1992; Lutz et al., FEBS Letters, 1993
Circadian rhythm	VIP essential for SCN neuronal synchronization and circadian output	Aton et al., Nature Neuroscience, 2005
Rheumatoid arthritis	VIP reduced joint inflammation and disease severity in collagen-induced arthritis models	Delgado et al., Nature Medicine, 2001
Sepsis	VIP improved survival and reduced inflammatory markers in experimental sepsis	Delgado et al., Journal of Leukocyte Biology, 2008
Treg induction	VIP-conditioned dendritic cells generated antigen-specific Tregs in autoimmune models	Gonzalez-Rey et al., Blood, 2006
Pulmonary hypertension	Inhaled VIP reduced pulmonary vascular resistance and improved exercise capacity	Petkov et al., Journal of Clinical Investigation, 2003
Neuroprotection	VIP protected against glutamate excitotoxicity and ischemic neuronal death	Brenneman et al., Peptides, 1985; Dejda et al., Neuroscience, 2005
VIPoma syndrome	Characterization of VIP-secreting tumors causing watery diarrhea, hypokalemia, achlorhydria (WDHA/Verner-Morrison syndrome)	Bloom et al., Gut, 1973
Inflammatory bowel disease	VIP reduced intestinal inflammation in experimental colitis models	Abad et al., Gastroenterology, 2003

Applications

Clinical Relevance

VIPoma (Verner-Morrison Syndrome): VIP-secreting tumors of the pancreatic islets produce a characteristic syndrome of profuse watery diarrhea, hypokalemia, and achlorhydria (WDHA syndrome). Diagnosis relies on elevated plasma VIP levels (normally <75 pg/mL; VIPoma: typically >200 pg/mL). Treatment involves surgical resection when possible, with somatostatin analogs (octreotide, lanreotide) providing effective medical management of secretory diarrhea.

Pulmonary Hypertension: Inhaled VIP has been investigated for primary pulmonary arterial hypertension based on its potent pulmonary vasodilatory effects. Early clinical results showed improvements in hemodynamics and exercise tolerance, though the peptide's short half-life (approximately 1-2 minutes in plasma) presents formulation challenges.

Autoimmune Disease Research: VIP's immunomodulatory profile has driven preclinical and early clinical research in:

• Rheumatoid arthritis
• Multiple sclerosis
• Type 1 diabetes
• Inflammatory bowel disease
• Transplant rejection

Pharmacological Challenges

Therapeutic development of VIP-based drugs faces significant obstacles:

• Short half-life — rapid degradation by dipeptidyl peptidase IV (DPP-IV) and neutral endopeptidases; plasma half-life of 1-2 minutes
• Poor oral bioavailability — as a 28-amino-acid peptide, VIP requires parenteral or inhaled delivery
• Hypotension — systemic VIP administration produces significant blood pressure reduction, limiting tolerable doses
• Receptor non-selectivity — VIP activates both VPAC1 and VPAC2; selective agonists may be needed for specific therapeutic applications

Strategies under development include PEGylated VIP analogs, nanoparticle encapsulation, selective VPAC1 or VPAC2 agonists, and gene therapy approaches delivering VIP locally to target tissues.

Biomarker Applications

Plasma VIP measurement is clinically used for:

• Diagnosis and monitoring of VIPoma
• Assessment of gut neuroendocrine function
• Research biomarker in circadian rhythm studies

Dosing Protocols

As an endogenous neuropeptide, VIP is not typically administered exogenously in routine clinical practice. It is primarily studied as a neuromodulator of gastrointestinal, pulmonary, and immune function, or through receptor-targeted interventions.

Investigational uses of VIP include:

• Pulmonary arterial hypertension: Inhaled VIP (100-200 mcg via nebulizer, 3-4 times daily) has been explored in small clinical studies with modest results
• Sarcoidosis: Inhaled VIP investigated for anti-inflammatory effects in pulmonary sarcoidosis
• Erectile dysfunction: VIP in combination with phentolamine (Invicorp) has been used as an intracavernosal injection in some countries

VIP analogs with improved stability are under development for various therapeutic applications. The rapid enzymatic degradation of native VIP (half-life approximately 1-2 minutes) limits its direct therapeutic utility.

Related Compounds

• Somatostatin — an inhibitory peptide that suppresses VIP release from gastrointestinal neurons; octreotide is the primary medical treatment for VIPoma
• PACAP (Pituitary Adenylate Cyclase-Activating Polypeptide) — the closest structural relative of VIP (68% identity), sharing VPAC1/VPAC2 receptor activation with additional PAC1 signaling
• Alpha-MSH — another neuropeptide with anti-inflammatory properties acting through a distinct receptor system (melanocortin receptors)
• ACTH — a pituitary hormone from a parallel neuroendocrine axis; VIP stimulates ACTH release in some contexts
• Secretin — a related peptide of the same superfamily with overlapping but distinct gastrointestinal functions
• GHRH — a member of the same peptide superfamily involved in growth hormone regulation