Vasoactive intestinal peptide receptors in pancreas and liver. Structure-function relationship

VIP as a trophic factor in the CNS and cancer cells

The effects of vasoactive intestinal peptide (VIP) on the proliferation of central nervous system (CNS) and cancer cells were investigated. VIP has important actions during CNS development. During neurogenesis, VIP stimulates the proliferation and differentiation of brain neurons. Addition of VIP to embryonic mouse spinal cord cultures increases neuronal survival and activity dependent neurotrophic factor (ADNF) secretion from astroglial cells. VIP is an integrative regulator of brain growth and development during neurogenesis and embryogenesis. Also, VIP causes increased proliferation of human breast and lung cancer cells in vitro. VIP binds with high affinity to cancer cells, elevates the cAMP and increases gene expression of c-fos, c-jun, c-myc and vascular endothelial cell growth factor. The effects of VIP on cancer cells are reversed by VIPhybrid, a synthetic VPAC(1) receptor antagonist. VIPhyb inhibits the basal growth of lung cancer cells in vitro and tumors in vivo and potentiates the ability of chemotherapeutic drugs to kill cancer cells. Due to the high density of VPAC(1) receptors in cancer cells, VIP has been radiolabeled with 123I, 18F and 99mTc to image tumors. It remains to be determined if radiolabeled VIP analogs will be useful agents for early detection of cancer in patients.

Structural motifs of pituitary adenylate cyclase-activating polypeptide (PACAP) defining PAC1-receptor selectivity

Pituitary adenylate cyclase-activating polypeptide (PACAP) interacts with three types of PACAP/VIP-receptors. The PAC1-receptor accepts PACAP as a high affinity ligand but not vasoactive intestinal peptide (VIP) similarly binding to VPAC1- and VPAC2-receptors. To identify those amino acids not present in VIP defining PAC1-receptor selectivity of PACAP, radio receptor binding assays on AR4-2J cells were performed. It could be shown that PACAP(1-27) exhibited a distinct and much higher susceptibility to VIP-amino acid substitutions, compared to PACAP(1-38). Positions 4 and 5 seem to be most important for receptor binding of PACAP(1-27), whereas position 13 was identified to be crucial for maximal affinity of PACAP(1-38). PACAP(29-38) extension analogues of VIP revealed a stabilizing effect of the C-terminus of PACAP(1-38) on the optimal peptide conformation. The substitution analogues were also checked for their capacity to stimulate IP3 and cAMP formation in AR4-2J cells. Compared to PACAP(1-27) and PACAP(1-38), most analogues revealed potencies reduced congruously to their lower binding affinities. However, one of the analogues, PACAP(1-27) substituted in position 5, may represent a weak antagonist since this peptide was less potent in inducing second messengers than in label displacement. Our findings indicate that PACAP(1-27) and PACAP(1-38) differ in terms of their requirement of the amino acids in positions 4, 5, 9, 11 and 13 for maximal interaction with the PAC1-receptor.

(Stearyl, Norleucine17)VIP hybrid antagonizes VIP receptors on non-small cell lung cancer cells

The effects of VIP receptor antagonists were investigated using non-small cell lung cancer (NSCLC) cells. By Northern blot and RT-PCR, VIP1 receptors were detected on NSCLC cell line NCI-H1299. VIPhybrid,(N-Stearyl-Norleucine17) VIPhybrid ((SN) VIPhybrid) and PTC4495 inhibited 125I-VIP binding to NCI-H1299 cells with IC50 values of 500, 30 and 5000 nM respectively. (SN) VIPhybrid (1 microM) had no effect on basal cAMP but strongly inhibited the increase in cAMP caused by 10 nM VIP. The order of peptide potency to inhibit cAMP was (SN) VIPhybrid > VIPhybrid > PTC4495. (SN) VIPhybrid was more potent than VIPhybrid at inhibiting NCI-H1299 colony formation. Also, (SN) VIPhybrid was more potent than VIPhybrid at inhibiting NCI-H1299 xenograft formation in nude mice. These data suggest that (SN) VIPhybrid antagonizes VIP1 receptors on NSCLC cells.

Growth hormone-releasing factor stimulates meiotic maturation in follicle- and cumulus-enclosed rat oocyte

This study was designed in order to assess the possible role of growth hormone-releasing factor (GRF) on oocyte maturation. This effect was analyzed in follicle-enclosed, cumulus-enclosed and denuded oocytes obtained from immature pregnant mare's serum gonadotropin-treated rats. The addition of GRF to the cultures significantly accelerated maturation in follicle- and cumulus-enclosed oocytes while no effect was seen on denuded oocytes. Also, the neuropeptide was able to induce maturation in follicle-enclosed oocytes obtained from immature untreated rats. The GRF action was probably not mediated by the vasoactive intestinal peptide (VIP) receptors since the two hormones had different effects on oocyte maturation and on cAMP production by granulosa cells. In addition the disappearance of the GRF effect observed in the presence of antibodies anti-GH suggested that GRF required the intermediacy of GH to accomplish its effect on oocyte maturation. Finally, GRF did not affect meiotic maturation when dbcAMP was added to the cultures. Our results demonstrate the ability of GRF to accelerate maturation in oocytes from both primed and unprimed rats. Since the presence and the involvement of GRF at the ovarian levels is now well established, the present data strongly suggest an important potential role of GRF in the ovarian physiology.

A chimeric VIP-PACAP analogue but not VIP pseudopeptides function as VIP receptor antagonists

The ability to assess the importance of VIP in different physiological processes is limited by the lack of specific potent antagonists. In the present study, we have adopted two different approaches used successfully with other peptides in an attempt to identify new VIP receptor antagonists. One involves the formation of pseudopeptides by insertion of reduced peptide bonds in the NH2-terminus from position 2 to 8 of VIP. The other methodology involves the formation of a COOH-terminal chimeric analogue by combining VIP(6-28) and PACAP(28-38). The ability of each of these peptides to function as an antagonist was compared with reported VIP antagonists. All of the peptides inhibited [125I]VIP binding to VIP receptors on guinea pig pancreatic acini. For the pseudopeptides the affinities were: [psi 3-4]VIP (0.2 microM) = 4 x [psi 4-5]VIP = 8 x [psi 8-9]VIP = 14 x [psi 6-7]VIP, [psi 2-3]VIP = 25 x [psi 5-6]VIP. Each nonpseudopeptide analogue also inhibited VIP binding with relative potencies of VIP(6-28)-PACAP(28-38) (1 microM) = 2.5 x [4-Cl-D-Phe6,Leu17]VIP, VIP(10-28), neurotensin(6-11)-VIP(7-28) = 6 x [Ac-Tyr1,D-Phe2]GRF. All pseudopeptides were agonists with relative potencies: [psi 3-4]VIP > [psi 6-7], [psi 4-5]VIP > [psi 5-6] > [psi 8- 9]VIP > [psi 2-3]VIP. The reported VIP receptor antagonist, neurotensin(6-11)-VIP(7-28), was also an agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

Effects of peptide histidine isoleucine on pancreatic exocrine secretion in anaesthetized dogs

1. The effects of peptide histidine isoleucine (PHI) on pancreatic exocrine secretion were investigated in preparations of the isolated and blood-perfused dog pancreas as compared with those of vasoactive intestinal peptide (VIP), secretin and glucagon. 2. Each peptide tested was injected intra-arterially (i.a.) as a single bolus. Graded doses of PHI (3-300 nmol/kg), VIP (1-100 nmol/kg) and secretin (0.01-0.3 nmol/kg) caused dose-dependent increases in the secretion of pancreatic juice and bicarbonate outputs, but had little effect on the protein outputs. Glucagon (0.1-10 mumol/kg) produced a bell-shaped dose-response curve for the secretory rate, bicarbonate and protein outputs. 3. The secretory activity of 30 nmol/kg of PHI corresponded roughly to that of 80 pmol/kg of secretin, 9 nmol/kg of VIP and 0.6 mumol/kg of glucagon, respectively. Thus, based on administered dose, PHI was about 375 x less potent than secretin, 3 x less potent than VIP and 20 x more potent than glucagon. 4. The PHI- and VIP-stimulated secretions were inhibited by a VIP antagonist, but not by a glucagon antagonist, SCH23390 (a dopamine D-1 antagonist), L-364718 (a cholecystokinin antagonist) or atropine. 5. Each peptide increased cyclic AMP concentration, but not cyclic GMP concentration, concomitant with the increase in pancreatic secretion. 6. From these results, it is concluded that PHI produces an increase in pancreatic secretion by acting on VIP-preferring receptors on the exocrine pancreatic gland of the dogs. This may be mediated at least in part through the increase of intracellular cyclic AMP concentrations.

Molecular properties of the vasoactive intestinal peptide receptor in aorta and other tissues

The molecular weight of the vasoactive intestinal peptide (VIP) receptor was assessed in bovine aorta, and rat liver, lung, and brain by covalent cross-linking and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The receptor in all four tissues was found to be a single polypeptide of approximate M(r) 54,000, contradicting previous claims for substantial heterogeneity in the molecular weight of this receptor. Guanine nucleotides inhibit cross-linking of 125I-VIP to its receptor, and cross-linking with ethylene glycolbis(succinimidylsuccinate) provides further evidence for complex formation between VIP, its receptor and a guanine nucleotide-binding regulatory protein (G-protein). The precise mechanism of receptor-G-protein coupling may differ between the aorta and other tissues.

The preparation of biotinyl-epsilon-aminocaproylated forms of the vasoactive intestinal polypeptide (VIP) as probes for the VIP receptor

Vasoactive intestinal polypeptide (VIP) was biotinyl-epsilon-aminocaproylated using sulfosuccinimidyl-6-(biotinamido) hexanoate thereby producing a series of products that were separated by high performance liquid chromatography (HPLC). Seven VIP-derivatives were isolated and the number and location of biotinyl-epsilon-aminocaproylation was determined by a combination of enzymatic degradation and plasma desorption mass spectrometry (PDMS). Receptor binding experiments with the VIP biotinyl-epsilon-aminocaproylated derivatives revealed IC50 values for the monobiotinyl-epsilon-aminocaproylated peptides that were 1.3-3.2 times higher than for natural VIP. All isolated biotinyl-epsilon-aminocaproylated derivatives possess VIP-like bioactivity as shown by an assay measuring pancreatic juice secretion in cat, VIP biotinyl-epsilon-aminocaproylated in position lysine being almost equipotent with natural VIP.

Localisation and characterisation of functional vasoactive intestinal peptide receptors in feline kidney

Specific 125I-labelled vasoactive intestinal peptide (VIP) binding was determined in feline renal cortical and medullary plasma membranes. For the cortex, Scatchard analysis of the data resulted in a curvilinear plot with a high-affinity site K0.5 of 8.4 +/- 2.6 nmol l-1 (SE, n = 6) and a second low-affinity site K0.5 204 +/- 16 nmol l-1 with binding site concentrations (Bmax) of 385 +/- 44.5 and 2710 +/- 181.3 fmol mg protein-1 respectively. Conversely a similar analysis of the results obtained for outer medullary membranes gave a single site with a K0.5 of 1.2 +/- 0.2 nmol l-1 (SE, n = 4) and Bmax of 157.8 +/- 24.7 fmol mg-1. Inner medullary membrane binding data. Gave a single site of lower affinity (K0.5 = 62.5 +/- 21.6 nmol l-1; n = 3). Structurally related peptides, glucagon and secretin, were ineffective (up to 1 mumol l-1) in displacing VIP from specific sites in both cortex and medulla. Porcine PHI 1-27 (a peptide having N-terminal histidine and C-terminal isoleucine) and a VIP antagonist [4-Cl-D-Phe6Leu17]VIP both displaced 125I-VIP from cortical and medullary membrane binding sites with IC50 values of 43.0 nmol l-1 and 1.3 mumol l-1 (cortex) and 132.0 nmol l-1 and 1.5 mumol l-1 (medulla) respectively. The localisation of specific VIP binding sites in feline kidney was investigated further by in vitro autoradiography.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization of active and stable receptors for vasoactive intestinal peptide from rat liver

Vasoactive intestinal peptide (VIP) receptors were solubilized from rat liver using the zwitterionic detergent CHAPS. Optimal conditions of solubilization were obtained with 5 mM CHAPS and 2.5 mg protein/ml. The binding of 125I-VIP to CHAPS extracts was time- and pH-dependent, saturable and reversible. The following order of potency of unlabeled VIP-related peptides for inhibiting 125I-VIP binding was observed: VIP greater than helodermin greater than peptide histidine isoleucine amide (PHI) greater than rat growth hormone releasing factor (rGRF) greater than secretin. This peptide specificity is identical to that of rat liver membrane-bound receptors. VIP binding activity in the CHAPS extract was destroyed by trypsin or dithiothreitol in accordance with the known sensitivity of membrane-bound receptors to these agents. VIP receptors in CHAPS extracts were stable for at least 5 days at 4 degrees C. Scatchard analysis of equilibrium binding data indicated the presence in CHAPS extracts of high (H) and low (L) affinity binding sites with the following characteristics: KdH = 0.27 nM and BmH = 34 fmol/mg protein; KdL = 51 nM and BmL = 1078 fmol/mg protein. The guanine nucleotide GTP inhibited 125I-VIP binding to soluble receptors and enhanced the dissociation of soluble VIP-receptor complexes, suggesting that GTP-binding proteins were functionally associated with VIP receptors in solution. Gel filtration of solubilized VIP receptors on Sephacryl S-300 revealed a single binding component with a Stokes radius of 6.1 nm. It is concluded that active VIP receptors can be extracted from liver membranes by CHAPS. The availability of this CHAPS-soluble, stable and functional receptor from a tissue which can be obtained in large amounts represents a major step toward the purification of VIP receptors.