Characterization of a vasoactive intestinal peptide-sensitive adenylate cyclase in rat intestinal epithelial cell membranes

Transcriptional regulation of the glucose-6-phosphatase gene by cAMP/vasoactive intestinal peptide in the intestine. Role of HNF4alpha, CREM, HNF1alpha, and C/EBPalpha

Gluconeogenesis is induced in both the liver and intestine by increased cAMP levels. However, hepatic and intestinal glucose production can have opposite effects on glucose homeostasis. Glucose release into the portal vein by the intestine increases glucose uptake and reduces food intake. In contrast, glucose production by the liver contributes to hyperglycemia in type II diabetes. Glucose-6-phosphatase (Glc6Pase) is the key enzyme of gluconeogenesis in both the liver and intestine. Here we specify the cAMP/protein kinase A regulation of the Glc6Pase gene in the intestine compared with the liver. Similarly to the liver, the molecular mechanism of cAMP/protein kinase A regulation involves cAMP-response element-binding protein, HNF4alpha, CAAT/enhancer-binding protein, and HNF1. In contrast to the situation in the liver, we find that different isoforms of CAAT/enhancer-binding protein and HNF1 contribute to the specific regulation of the Glc6Pase gene in the intestine. Moreover, we show that cAMP-response element binding modulator specifically contributes to the regulation of the Glc6Pase gene in the intestine but not in the liver. These results allow us to identify intestine-specific regulators of the Glc6Pase gene and to improve the understanding of the differences in the regulation of gluconeogenesis in the intestine compared with the liver.

A lymphocyte-generated fragment of vasoactive intestinal peptide with VPAC1 agonist activity and VPAC2 antagonist effects

Vasoactive intestinal peptide receptors 1 (VPAC1) and 2 (VPAC2) have been identified in humans. Cell lines expressing only VPAC1 (HT-29) or VPAC2 (Molt-4b) were identified using real-time reverse transcriptase polymerase chain reaction. Vasoactive intestinal peptide (VIP) and related peptides, VIP-6-28, VIP4-28, and VIP10-28, previously isolated from cultures of human leukocytes, were evaluated for their ability to bind to VPAC1 and VPAC2 and to increase the levels of cAMP in HT-29 and Molt-4b cells. VIP bound to membranes of HT-29 colon carcinoma cells and Molt-4b lymphoblasts with high affinity (KD = 1.6 +/- 0.2 and 1.7 +/- 0.9 nM, respectively). VIP4-28 also demonstrated high-affinity binding (KD = 1.7 +/- 0.2 and 1.7 +/- 0.7 nM in HT-29 and Molt-4b, respectively). VIP and VIP4-28 are potent VPAC1 agonists, inducing maximal 200- and 400-fold increases in cAMP, respectively. VIP demonstrated weak VPAC2 agonist activity, inducing a maximal 14-fold increase in cAMP. VIP4-28 had no VPAC2 agonist activity but demonstrated potent VPAC2 antagonist activity. VIP4-28 inhibited VPAC2-mediated increases in cAMP in Molt-4b cells up to 95%, but had no antagonistic effect on VPAC1. Lymphoblasts did not hydrolyze VIP4-28 to a form with VPAC1 antagonist activity. VIP4-28 thus is a lymphocyte-generated VIP fragment with potent agonist activity for VPAC1 and potent antagonist activity for VPAC2.

Tryptophan 67 in the human VPAC(1) receptor: crucial role for VIP binding

The human receptor subtype for VIP and PACAP, referred to as VPAC(1) receptor, has a large N-terminal extracellular domain which is critical for VIP binding. We further investigated this domain by mutating 12 amino acid residues which could participate in the formation of a tight bend (W67) or a coiled coil motif. They were changed to alanine (A) and the cDNAs were transiently transfected into Cos cells. All mutants but W67A exhibited K(d) values similar to that of the wild-type receptor. For the W67A mutant, no specific (125)I-VIP binding could be observed. Mutants at the W67 site were further characterized after stable transfection of epitope-tagged VPAC(1) receptor-GFP fusion proteins into CHO cells. W67A, W67E, W67H, and W67K mutants neither bound VIP nor mediated adenylyl cyclase activation by VIP. The W67F mutant mediated stimulation of adenylyl cyclase only at high VIP concentrations. Microscopic analysis and antibody binding experiments showed that all mutants were similarly expressed at the cell surface of CHO cells. Therefore tryptophan 67 in the human VPAC(1) receptor plays a crucial role in VIP binding due, in part, to its aromatic moiety.

Intraduodenal neuropeptide levels, but not plasma levels, vary in a cyclic fashion with the migrating motor complex

The neurohormonal control of the migrating motor complex (MMC) is not fully understood. The hypothesis of the present study was that neuropeptide levels might vary with the different phases of the MMC and that a similar variation might be found in the secretions of the gastrointestinal tract. Thus, plasma and intraduodenal concentrations of vasoactive intestinal peptide (VIP), somatostatin (SOM), substance P (SP) and neurokinin A (NKA) were determined by radioimmunoassay every 10 min during two complete MMC cycles in eight male subjects. For comparison, plasma motilin (MOT) concentrations were measured. Plasma concentrations of MOT (mean peak value +/- SEM; 39 +/- 6 pmol L-1), but none of the neuropeptides studied, showed a cyclic variation in plasma with the different phases of the MMC. Peak intraduodenal concentrations of VIP (79 +/- 23 pmol L-1), SOM (2437 +/- 432 pmol L-1) and SP (718 +/- 326 pmol L-1) occurred at or at the time point before the onset of phase III of the MMC. No such correlation was observed for NKA. These results demonstrate that intraduodenal but not plasma concentrations of the neuropeptides VIP, SOM and SP show an association with phase III of the MMC. The biological relevance of this finding is yet unclear, but the results raise the possibility that gut neuropeptides may regulate fasting motility through a luminal release.

Inhibition of prostaglandin-induced intestinal secretion by igmesine in healthy volunteers

BACKGROUND & AIMS

Igmesine, a final sigma ligand, has been shown to inhibit intestinal secretion and diarrhea in animal models. The purpose of this study was to measure the inhibitory effect of igmesine on basal and prostaglandin E2 (PGE2)-induced jejunal secretion in normal volunteers.

METHODS

Jejunal absorption of water and electrolytes was measured with a three-lumen open-segment perfusion method in 16 volunteers. A double-blind crossover study was performed involving intraluminal infusion of PGE2 after oral administration of placebo or igmesine at two doses.

RESULTS

PGE2 induced net secretion of water and electrolytes (P < 0.01 vs. basal conditions). The effect of PGE2 on water and electrolytes was not changed by 25 mg of igmesine but was suppressed by 200 mg of igmesine. This effect lasted at least 3 hours after a single oral dose. Igmesine at a dose of 200 mg also produced a significant decrease in basal rates of water and electrolyte absorption.

CONCLUSIONS

Igmesine, a final sigma ligand, inhibits PGE2-induced intestinal secretion in normal humans. Evaluating the drug in chronic diarrheas may be of interest.

Cyclic nucleotides and vasoactive intestinal peptide production in a rabbit model of Escherichia coli septicemia

Nitric oxide and vasoactive intestinal peptide (VIP) are potent vasodilators and postulated as inducers of hypotension. These mediators activate guanylate cyclase and adenylate cyclase, respectively, with subsequent biosynthesis of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) producing vascular smooth muscle relaxation and vasodilatation. Cyclic nucleotides and VIP were evaluated during Escherichia coli septicemia in two groups of rabbits; 1) sepsis alone and 2) sepsis and a competitive inhibitor of nitric oxide synthase, NG-monomethyl-L-arginine. Arterial blood was obtained for determination of bacteremia, lactic acidemia, nucleotides, nitrites, and VIP levels. Significant bacteremia, endotoxemia, tachycardia, lactic acidosis, and hypotension occurred in all animals (P < 0.005). Circulating blood levels of cGMP, nitrites, cAMP, and VIP (P < 0.005) increased with development of shock. The NG-monomethyl-L-arginine treated animals had less cGMP, nitrites, cAMP, and VIP produced (P < 0.01). Plasma cGMP levels remained stable, suggesting that stimulated phagocytes in whole blood were responsible for increased cGMP levels. Infusion of VIP produced profound hypotension and lactic acidemia. Results of these experiments provide definitive evidence that nitric oxide and VIP are mediators during septic shock and their messengers are cGMP and cAMP, respectively. In addition, phagocytic stimulation with increased production of cGMP may initiate shock, with these mediators acting synergistically to prolong hypotension.

Immunoprecipitation and characterization of a binding protein specific for the peptide, intestinal trefoil factor

Recombinant rat intestinal trefoil factor (rITF) and human spasmolytic polypeptide (hSP) were irreversibly cross-linked to specific binding sites in solubilized rat intestinal epithelial membranes and human adenocarcinoma cells. Analysis of the immunoprecipitates by immunoblotting identified a cross-linked protein complex of approximately 45 kDa, which under reducing conditions appeared as a approximately 28-kDa band and the latter displayed ligand-stimulated phosphorylation of a tyrosine, but not a threonine or serine, residue in the binding complex. [125I]rITF was used to localize binding sites by autoradiography of frozen sections from rat gastrointestinal tissues. A high density of specific [125I]rITF binding sites was present within gastric, colonic, and jejunal mucosal glands. Unlabeled hSP partially inhibited [125I]rITF binding at a concentration of 1 microM when compared with the same concentration of unlabeled rITF. These studies support earlier observations for the existence of trefoil binding sites in the gastrointestinal tract and further suggest that hSP has affinity for the mucosal rITF binding site.

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.

Binding and internalization of VIP in rat intestinal epithelial cells

We have prepared villous cells from the jejunum of the rat small intestine and studied the effects of divalent cations and bacitracin on the binding and internalization of VIP. Villous epithelial cells (4 x 10(6) cells/ml) were suspended in a Hepes-NaCl buffer with 1.0% BSA, (pH 7.4) and the cells were incubated for varying periods of time with 125I-VIP at 24 degrees C. Specific binding of radiolabeled VIP was maximal within 10 min (10%) and slowly declined to 9.0 percent after 30 min. In the presence of 1.0 mg/ml bacitracin, however, maximal specific binding of VIP was only 2.7 percent (P less than or equal to 0.001). The addition of CA2+ or Mg2+ to the buffer significantly decreased binding of VIP in a concentration dependent manner. At 8.0, 4.0, 2.0 and 1.0 mM Ca2+, binding of 125I-VIP decreased by 70, 60, 40 and 25 percent, whereas in the presence of the same concentrations of Mg2+ binding was decreased to 50, 38, 25 and 10 percent (P less than or equal to 0.01). To determine if epithelial cells internalize VIP, we bound 125I-VIP to villous cells and then differentiated surface-bound and internalized radioactivity by treating with trypsin (150 micrograms/ml). Surface bound radioligand was the same at both 24 and 4 degrees C (5.3%), while internalized 125I-VIP was 4.0% at 24 degrees C compared to only 1.0% at 4 degrees C (P less than or equal to 0.001). At 24 and 4 degrees C, both Ca2+ (4.0 mM) and Mg2+ (8.0 mM) decreased surface bound radioligand by 60 percent (P less than or equal to 0.01) and lowered internalized radioactivity. These data demonstrate that (1) bacitracin decreases the binding of VIP to small intestinal epithelial cells, (2) both Ca2+ and Mg2+ affect the binding of VIP to its surface receptor and (3) VIP is internalized into epithelial cells.

A clonal rat pancreatic delta cell line (Rin14B) expresses a high number of galanin receptors negatively coupled to a pertussis-toxin-sensitive cAMP-production pathway

Galanin, an ubiquitous neuropeptide, was recently shown to inhibit somatostatin release by the rat islet tumor cell line, Rin-m. By using the clonal pancreatic delta cell line Rin14B, originating from Rin-m cells, we were able to identify the presence of one type of specific galanin-binding site of high affinity (Kd = 1.6 nM; maximal binding capacity = 270 fmol/mg protein) and high specificity for the peptide. Binding of 125I-galanin to these receptors was time-dependent and highly sensitive to guanine nucleotides. Using the cross-linker disuccinimidyl tartrate, covalent linking of the galanin receptor to 125I-galanin in membranes from Rin14B cells, followed by SDS/PAGE analysis of membrane proteins, indicated that the galanin receptor is a protein of 54 kDa. 0.1-100 nM galanin also exerted a marked inhibitory effect on the cAMP-production system under basal conditions, as well as in the presence of the pancreatic peptide glucagon. At a maximal dose, galanin induces a 90-100% decrease of basal and glucagon-stimulated cAMP production levels, with a median inhibition concentration (IC50) of 3 nM galanin. The direct inhibitory effect of galanin on the adenylate cyclase activity in Rin14B cell membranes was also demonstrated (IC50 = 3 nM galanin). The inhibitory effect of galanin on the basal and glucagon-stimulated cAMP production in Rin14B cells was reversed by pertussis toxin. The toxin was also shown to specifically ADP-ribosylate a protein of 41 kDa in membranes from Rin14B cells. Taken together, these data show that the pancreatic delta cell line Rin14B expresses high affinity galanin receptors negatively coupled to a pertussis-toxin-sensitive cAMP-production system.

Characterization of vasoactive intestinal peptide receptors in retina

Vasoactive intestinal peptide (VIP) is a neuropeptide having a wide range of effects on a large number of tissues. To gain insight into the role VIP plays in retinal function, VIP receptors in bovine retinal membranes were analyzed in competition binding assays and by affinity labeling studies and compared to VIP receptors in rat liver membranes. In both membrane preparations, high affinity VIP binding sites (KD approximately 1 nM) were detected. Secretin and glucagon, each having close structural homology to VIP, were found to have negligible effects on [125I]VIP binding in retina. In contrast, secretin (KD = 70 nM) was modestly effective in inhibiting [125I]VIP binding to rat liver membranes. Affinity labeling analysis revealed a VIP binding site of 59 kDa in both bovine retinal and rat liver membranes. Digestion of affinity-labeled receptor proteins with endoglycosidase F generated final cleavage products of approx. 45 kDa for both receptors. These results indicate that the retina expresses a high affinity, highly selective VIP receptor thereby supporting a specific function for VIP in this tissue.

Secretory actions of vasoactive intestinal polypeptide, peptide histidine isoleucine and helodermin in rat small intestine: the effects of putative VIP antagonists upon VIP-induced ion secretion

Vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), and helodermin stimulate electrogenic anion secretion in preparations of rat jejunum stripped of muscularis propria. Concentration-response curves to exogenously applied peptides yielded EC50 values of 12 nM, 12 nM and 100 nM for VIP, PHI and helodermin respectively. These secretory responses were most probably mediated via the same receptor population given that cross-desensitisation was observed between all 3 analogues. Four putative VIP antagonists, namely, two growth hormone releasing factors (GRF); [AcTyr1, D-Phe2]GRF-(1-29)-NH2 and [AcTyr1]hGRF-(1-40)-OH as well as [4Cl-D-Phe6,Leu17]VIP and VIP-(10-28) were tested for their ability to inhibit VIP induced electrogenic ion secretion. None of the above exhibited any intrinsic agonist activity nor were they competitive antagonists, although some inhibition was observed with [AcTyr1]hGRF-(1-40)-OH and VIP-(10-28). Their use as selective VIP antagonists is therefore limited in rat jejunal mucosa.

Solubilization and hydrodynamic characterization of guanine nucleotide sensitive vasoactive intestinal peptide-receptor complexes from rat intestine

The purpose of this work was to solubilize vasoactive intestinal peptide (VIP) receptors from rat small intestinal plasma membranes and to analyze the nature and function of its molecular form(s) in a nondenaturing environment. Membranes were incubated with 3 nM 125I-VIP, washed, and treated with 1% Triton X-100. Chromatography on Sephadex G-50 showed that 60% of the extractable radioactivity was eluted with macromolecular components in the void volume. This radioactive material was dramatically reduced when 1 microM unlabeled VIP was present in the incubation medium or when membranes were pretreated with trypsin or dithiothreitol. Macromolecular components that had bound 125I-VIP were further chromatographed on Sephacryl S-300. Two peaks were observed: a major one (80%) and a minor one (20%) with Stokes radii of 5.2 and 3.1 nm, respectively. The labeling of both components was inhibited by unlabeled VIP or peptide with NH2-terminal histidine and COOH-terminal isoleucine amide (a VIP agonist). The presence of GTP (0.1 mM) in the incubation medium of membranes completely abolished the labeling of the 5.2-nm component but did not affect that of the 3.1-nm one. Moreover, GTP induced dissociation of 125I-VIP from the 5.2-nm component isolated by Sephacryl S-300 chromatography. This effect was time dependent and nucleotide specific. In contrast, GTP did not affect the stability of the 3.1-nm component. After cholera toxin catalyzed [32P]ADP-ribosylation of membranes, chromatography of solubilized material on Sephacryl S-300 showed that a peak of 32P radioactivity was coeluted with the 5.2-nm component.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y antagonises secretagogue evoked chloride transport in rat jejunal epithelium

Neuropeptide Y (NPY) inhibits electrogenic Cl secretion in rat jejunal epithelium under voltage clamp conditions. This effect is dependent upon endogenous eicosanoid formation since it is blocked by the cyclooxygenase inhibitor, piroxicam, which itself has an inhibitory action upon chloride secretion. A number of chloride secretagogues have been examined for their ability to restore the antisecretory effects of NPY. Data presented here shows that NPY responsiveness is restored, in piroxicam pretreated tissues, by vasoactive intestinal polypeptide (VIP), forskolin, prostaglandin E2 (PGE2) isobutyl-1-methyl-xanthine (IBMX) and dibutyryl cAMP added prior to the neuropeptide. While all these agents cause chloride secretion by elevating intracellular cAMP, NPY is also effective in inhibiting the secretory effects of carbachol (CCh) and substance P (SP), agents believed to act by raising intracellular calcium (Cai). Although there is evidence that NPY can inhibit adenylate cyclase, its ability to attenuate chloride secretion brought about by secretagogues acting through both adenylate cyclase and calcium mechanisms, implies that NPY has either a more general fundamental mechanism or has multiple interactions with different second messenger systems.

Mechanism of galanin-inhibited insulin release. Occurrence of a pertussis-toxin-sensitive inhibition of adenylate cyclase

In the insulin-secreting beta cell line Rin m 5F, galanin, a newly discovered ubiquitous neuropeptide, inhibited, by 50%, the stimulation of insulin release induced by gastric inhibitory polypeptide (GIP) or forskolin, i.e. two cAMP-generating effectors. In contrast, it failed to decrease the stimulation of insulin release elicited by either the Ca2+-mobilizing agent, carbamoylcholine, or by dibutyryl-cAMP. Concomitantly, galanin inhibited the GIP- and forskolin-stimulated cAMP production. Furthermore, adenylate cyclase in membranes from Rin m 5F cells was highly sensitive to galanin, which exerted a marked inhibitory effect on the forskolin-stimulated enzyme activity. All these galanin effects were observed at low physiological doses, in the nanomolar range. Overnight treatment of the Rin m 5F cells with pertussis toxin completely abolished the inhibitory effect of galanin on insulin release, cAMP production and adenylate cyclase activity. Moreover, pertussis toxin specifically ADP-ribosylated a 39-kDa protein present in membranes from those cells. Taken together, these data show that the galanin inhibition of insulin release most likely occurs through the inhibition of adenylate cyclase, involving a petussis-toxin-sensitive inhibitory GTP-binding regulatory protein.

Permeability properties of isolated enterocytes from rat small intestine

Metabolic and permeability properties of enterocytes isolated by treatment of rat small intestine with hyaluronidase or EDTA were compared. No significant difference was observed in the ability of the two types of cell to produce lactate from glucose. However, while cells obtained with hyaluronidase accumulate alpha-methylglucoside, cells obtained with EDTA were unable to accumulate the sugar above the medium concentrations. When resuspended in a medium designed to resemble the intracellular medium, potentiometric measurements showed that cells obtained with hyaluronidase released Ca2+ to the medium while cells obtained with EDTA accumulated it. Using 45Ca transport assays, this was shown to be an ATP-dependent process, the accumulated 45Ca being totally released by the addition of the ionophore A23187. When cells obtained with EDTA were resuspended in a medium containing concentrations of free Ca2+ higher that 10 microM, the uptake was partially inhibited by sodium orthovanadate and also by oligomycin and antimycin. At free Ca2+ concentrations lower than 1 microM, the accumulation was inhibited up to 87% by sodium orthovanadate while mitochondrial inhibitors inhibited only 5%. Thus, it appears that during their preparation cells obtained with hyaluronidase retain their integrity while cells obtained with EDTA become permeable to Ca2+ and other ions. The usefulness of both types of preparation in metabolic and transport studies is discussed.

Pathogenesis and pharmacology of diarrhea

The etiological factors involved in diarrhea are multiple. Also the mechanisms and mediators involved are multiple: intracellular mediators (Ca, cAMP, cGMP, calmodulin, phospholipids), extracellular mediators (hormones, neurotransmitters, prostaglandins, enterotoxins...), intramural blood flow and oxygen, intestinal motility (local- and peristaltic motility). Till now, antidiarrheals are not so versatile that they provide a solution to all types of diarrhea. The mechanisms of action of fluid replacement therapy, loperamide, alpha 2 agonist and some nonsteroidal anti-inflammatory substances are reviewed.

cAMP immunocytochemistry provides evidence for functional VIP receptors in trachea

Vasoactive intestinal peptide (VIP), first isolated from porcine intestine (S. I. Said and V. Mutt, Science Wash. DC 169: 1217-1218, 1970), has been identified in postganglionic autonomic axons of many tissues. VIP has potent regulatory effects on the function of various cell types within these tissues, ranging from relaxation of smooth muscle to ion transport. Recently, VIP has been implicated in the regulation of mucus secretion in the respiratory tract, a process involving release of macromolecules from exocrine cells and transport of ions and water across the airway mucosa. However, because airway glands and mucosa both consist of mixed cell populations, it was unclear which specific cells contained VIP receptors and contributed to VIP-evoked responses. We identified these specific cells by using immunocytochemical techniques to monitor concentration changes in adenosine 3',5'-cyclic monophosphate (cAMP), the intracellular compound known to mediate VIP responses. Serous and mucous cells of ferret tracheal submucosal glands and ciliated and basal cells of dog tracheal mucosa all increased cAMP in response to VIP stimulation. We conclude that these cell types possess VIP receptors and thus participate in VIP-stimulated responses. In contrast, ferret tracheal epithelium and dog epithelial goblet cells showed little or no reactivity after VIP, and thus we believe that these cells lack VIP receptors.

Role of phospholipids in the binding activity of vasoactive intestinal peptide receptors

Phospholipase digestion of rat intestinal epithelial cell membranes was performed in order to study the influence of membrane phospholipids on the binding activity of VIP receptors. Phospholipases A2 and C strongly (ED50 congruent to 4 X 10(-2) and 4 X 10(-1) micrograms/ml, respectively) and rapidly reduced 125I-VIP binding to membranes whereas phospholipase D was ineffective. This suggests an important role of both hydrophobic and hydrophilic groups of phospholipids on VIP receptor binding activity.

Interaction of PHM, PHI and 24-glutamine PHI with human VIP receptors from colonic epithelium: comparison with rat intestinal receptors

PHM, the human counterpart of porcine Peptide Histidine Isoleucine amide (PHI), is shown to be a VIP agonist with low potency on human VIP receptors located in colonic epithelial cell membranes. Its potency is identical to that of PHI but by 3 orders of magnitude lower than that of VIP itself in inhibiting 125I-VIP binding and in stimulating adenylate cyclase activity. This contrasts markedly with the behaviour of PHI on rat VIP receptors located in intestinal epithelial cell membranes where PHI is a potent agonist with a potency that is 1/5 that of VIP. In another connection, we show that 24-glutamine PHI has the same affinity as 24-glutamic acid PHI (the natural peptide) for rat or human VIP receptors. These results indicate that while PHI may exert some physiological function through its interaction with VIP receptors in rodents, its human counterpart PHM is a very poor agonist of VIP in human. Furthermore, they show that the drastic change in position 24 of PHI (neutral versus acid residue) does not affect the activity of PHI, at least on VIP receptors.

The rat liver vasoactive intestinal peptide binding site. Molecular characterization by covalent cross-linking and evidence for differences from the intestinal receptor

To identify the molecular components of the vasoactive intestinal peptide (VIP) binding sites in the liver, 125I-labelled VIP was covalently linked to liver membranes by using the cleavable cross-linker dithiobis(succinimidylpropionate). Purified rat liver plasma membranes were incubated with 125I-VIP, washed and treated with 1 mM-cross-linker. Polyacrylamide-gel electrophoresis of membrane proteins followed by autoradiography revealed a major 125I-VIP-protein complex of Mr 51 000. A minor Mr 89 000 complex was also observed. An identical pattern of protein labelling was obtained using crude membranes from rat liver. Labelling of the Mr 51 000 and 89 000 species was specific in that it could be abolished by native VIP, but was unaffected by 1 microM-glucagon and cholecystokinin octapeptide. Densitometric scanning of autoradiographs indicated that the labelling of the two species was abolished by similar low VIP concentrations (0.1-100 nM). It was also reduced by two VIP agonists, peptide histidine isoleucine amide and secretin, with a potency that is 1/7 and 1/200 that of native VIP, respectively. The guanine nucleotide GTP in the concentration range between 10(-7) and 10(-3) M reduces the labelling of the major Mr 51 000 protein and that of the minor Mr 89 000 protein, but with a slightly higher potency. Assuming one molecule of 125I-VIP was bound per molecule of protein, a major Mr 48 000 protein and a minor Mr 86 000 protein were identified as components of the high-affinity VIP binding sites in liver. This contrasts markedly with the pattern of labelling of rat intestinal epithelial membranes, where a Mr 73 000 protein was identified as a high-affinity VIP receptor and a Mr 33 000 protein as a low-affinity VIP binding site [Laburthe, Bréant & Rouyer-Fessard (1984) Eur. J. Biochem. 139, 181-187], suggesting structural differences between VIP binding sites in rat liver and intestinal epithelium.

Forskolin stimulates adenylate cyclase in human colonic crypts: interaction with VIP

Forskolin in the 10(-8)-10(-4) M concentration range (ED50 2 microM) strongly stimulated the cyclic AMP production of epithelial crypts isolated from the human colon. At a maximal dose, production increased up to 500 and 700 times the basal cyclic AMP levels at 15 and 37 degrees C, respectively. Forskolin was thus much more efficient than VIP, which is the physiological regulator of this system. Forskolin (ED50 7 microM) also stimulated colonic membrane adenylate cyclase. The stimulation was immediate, did not require guanyl nucleotides and was inhibited by calcium (10(-5)-10(-3) M). In the concentration range between 10(-9) and 10(-5) M (ED50 0.04 microM), forskolin strongly potentiated the stimulation of adenylate cyclase by VIP. We conclude that: (1) forskolin exerts a double dose-dependent action on cyclic AMP production in human colonic crypts, i.e. direct activation of basal adenylate cyclase activity and potentiation of VIP effect; (2) forskolin may be a unique pharmacological tool to investigate the cyclic AMP-dependent processes in human intestine.

The GIP receptor on pancreatic beta cell tumor: molecular identification by covalent cross-linking

125I-GIP binds reversibly to a high affinity binding site in crude plasma membranes prepared from a hamster pancreatic beta cell tumor. The treatment of labeled membranes with the cross-linker dithiobis (succinimidylpropionate) prevents, to a greater extent, the rapid dissociation of 125I-GIP-membrane complexes which is observed when 10(-6) M native GIP is added. Polyacrylamide gel electrophoresis of membrane proteins reveals a major 125I-GIP-protein complex of Mr 64,000. This labeling decreases when increasing concentrations (10(-9) -10(-6)M) of native GIP are added but is not altered by other peptide hormones (tested at 10(-6)M) including glucagon, VIP and insulin. The Mr 64,000 complex is not observed in tissues which have no specific binding sites for GIP such as intestinal epithelium. Assuming one molecule of 125I-GIP is bound per molecule of protein, one protein with Mr 59,000 is identified as the specific GIP binding site.

Structural requirements for VIP interaction with specific receptors in human and rat intestinal membranes: effect of nine partial sequences

Nine VIP sequences have been tested for their ability to inhibit the specific binding of 125I-VIP and to stimulate adenylate cyclase activity in intestinal epithelial membranes from rat and man. They are VIP 2-28; VIP 1-14; VIP 2-14; VIP 14-28; VIP 15-28; VIP 20-28; VIP 21-28 and two sequences where the N-terminal VIP 1-6 or VIP 1-9 have been joined covalently with the C-terminal VIP 20-28 or VIP 21-28. It appears that only VIP 2-28, VIP 14-28 and VIP 15-18 are able to inhibit competitively the binding of 125I-VIP to human and rat membranes. These analogues are respectively 88, 8,300 and 25,000 times less potent than VIP 1-28 in rat; they are respectively 70, 7,900 and 13,000 times less potent than VIP 1-28 in man. With respect to adenylate cyclase activation, VIP 14-28 and VIP 15-28 are very weak stimulators in the membranes from both species. VIP 2-28 behaves as a full VIP agonist in man whereas it is a partial VIP agonist in rat. These results indicate the structural importance of the whole VIP sequence for interacting with human and rat VIP receptors and further argue for a different structural requirement of rat and human receptors.

Adrenergic versus VIPergic control of cyclic AMP in human colonic crypts

The actions of catecholamines on VIP-induced cyclic AMP is studied in human colon. We show that: (1) Epinephrine in the 10(-7)-10(-3) M concentration range (ED50 = 11.10(-6) M) inhibits VIP-induced cyclic AMP rise in isolated colonic epithelial cells; the maximal inhibition reaches 30% of VIP effect; epinephrine alters the efficacy of the peptide and does not modify its potency; epinephrine also reduces the basal cyclic AMP level. (2) The inhibition is found with other alpha adrenergic agonists with the order of potencies epinephrine greater than norepinephrine greater than phenylephrine. Clonidine has a poor intrinsic activity but antagonizes the action of epinephrine. (3) The inhibition of VIP action by epinephrine is reversed by the alpha antagonists dihydroergotamine, phentolamine and the alpha 2 antagonist yohimbine, while unaffected by the beta antagonist propranolol and the alpha 1 antagonist prazosin, (4) Epinephrine inhibits VIP-stimulated adenylate cyclase activity in preparations of colonic plasma membranes. Thus catecholamines exert through an alpha 2 adrenoreceptor a negative control on basal and VIP-stimulated cyclic AMP formation in human colon. We suggest that colonic cyclic AMP metabolism undergoes a dual control: VIPergic, activator and adrenergic, inhibitor.

Molecular identification of receptors for vasoactive intestinal peptide in rat intestinal epithelium by covalent cross-linking. Evidence for two classes of binding sites with different structural and functional properties

The cleavable cross-linking reagent dithiobis (succinimidyl propionate) or DTSP was shown to link 125I-labeled vasoactive intestinal peptide (125I-VIP) covalently to its receptors in rat intestinal epithelial membranes. DTSP treatment of 125I-VIP-labeled membranes inhibited the dissociation of VIP-receptor complexes in a way which was dependent on both time and concentration (ED50 = 200 microM). Polyacrylamide gel electrophoresis of membrane proteins revealed three 125I-VIP-protein complexes of Mr 76 000, 36 000 and 17 000. The labeling of those compounds was not observed when: (a) treatment of membranes by DTSP was omitted; (b) the reagent quench, ammonium acetate, was added together with DTSP; (c) DTSP-treated membranes were incubated with 2-mercaptoethanol which reduces the disulfide bond present within DTSP. Labeling of Mr-76 000 and Mr-36 000 complexes was specific in that it could be abolished by native VIP, while the labeling of the Mr-17 000 was not. Densitometric scanning of autoradiographs indicated that: (a) labeling of the Mr-76 000 complex was abolished by low VIP concentrations (0.03--10 nM), by VIP agonists with the relative potency VIP greater than a peptide having N-terminal histidine and C-terminal isoleucine amide greater than secretin, and by GTP (10(-5)--1 mM) but was unaffected by various other peptide hormones; (b) labeling of the Mr-36 000 complex was inhibited by high VIP concentrations (1--300 nM), by VIP agonists at high concentrations but was not affected by GTP and various peptide hormones. Assuming one molecule of 125I-VIP was bound per molecule of protein, two proteins with Mr-73 000 and 33 000 were identified as VIP binding sites. The Mr-73 000 protein displays many characteristics (affinity, specificity, discriminating power toward agonists, sensitivity to GTP regulation) of the high-affinity VIP receptors mediating adenylate cyclase activation. The Mr-33 000 protein displays the characteristics (affinity, specificity) of a low-affinity VIP binding site. This study thus shows the molecular characteristics of the VIP receptor and further argues for the molecular heterogeneity of VIP binding sites.

Interaction of Gila monster venom with VIP receptors in intestinal epithelium of human. A comparison with rat

Gila monster venom (1-300 micrograms/ml) is shown to inhibit completely the binding of [125I]VIP to human and rat intestinal epithelial cell membranes. In both models, the venom inhibits [125I]VIP binding and stimulates adenylate cyclase with a maximal efficiency that is similar to that of VIP and a potency that is 10000-50000 times lower than that of the peptide, on a weight basis. At maximal doses, VIP and Gila monster venom do not exert an additive effect on adenylate cyclase, suggesting that the activation of the enzyme by the venom occurs through VIP receptors. As is the case for VIP, adenylate cyclase activation by Gila monster venom requires the presence of GTP in the incubation medium. Finally, no VIP-like immunoreactivity was detected in the venom using an antiserum raised against mammalian VIP. All these data suggest the presence in the venom of the Gila monster, of a new substance which behaves as a VIP agonist in human as well as rat intestine.

Interaction of GRF with VIP receptors and stimulation of adenylate cyclase in rat and human intestinal epithelial membranes. Comparison with PHI and secretin

GRF (10(-8) - 10(-5) M) is shown to inhibit competitively the binding of [125I]VIP to human and rat intestinal epithelial membranes. The affinity of GRF for VIP receptor is 700-800-times lower than that of VIP in both species. The order of affinity of different peptides is VIP greater than PHI greater than secretin greater than GRF in rat, and VIP greater than GRF greater than PHI greater than secretin in man. The important species specificity of VIP receptors in recognizing PHI and secretin does not occur in the case of GRF. GRF stimulates adenylate cyclase through its interaction with VIP receptors in rat and human membranes. However, while GRF behaves as a VIP agonist in human tissue, it is a partial agonist/antagonist of VIP in the rat.

Intestinal VIP receptors: differential effect of trypsin on the high and low affinity binding sites

The effects of trypsin treatment on VIP binding to rat intestinal epithelial cell membranes were examined. The decrease in specific binding of [125I]VIP is dependent on the amount of trypsin used and digestion time. Specific binding decreases by 50% after 8 min with 20 micrograms/ml trypsin. Trypsin is active in the 1-100 micrograms/ml concentration range (ED50 approximately equal to 5 micrograms/ml). Non-specific binding is unaltered by the enzyme. The effect of trypsin is abolished by trypsin inhibitor. Scatchard analysis of VIP binding reveals two types of binding sites: sites I characterized by a high affinity, a low capacity and a high sensitivity to low trypsin levels (1-5 micrograms/ml); sites II characterized by a low affinity, a high capacity, resistant to low trypsin levels (1-5 micrograms/ml) but sensitive to a high trypsin level (20 micrograms/ml). Trypsin decreases the binding capacity by lowering the site number without altering their affinity. Sites not destroyed by trypsin retain their functional characteristics: KD, sensitivity to GTP and coupling with adenylate cyclase. It is concluded that sites I and II are proteins with different structures and/or differently localized in the membrane.

Involvement of calmodulin in the regulation of adenylate cyclase activity in guinea-pig enterocytes

The involvement of calmodulin as an activator of adenylate cyclase activity was examined in isolated guinea-pig enterocytes and in a membrane preparation. In enterocytes, which responded to prostaglandin E1, vasoactive intestinal peptide and cholera toxin with a significant increase in the rate of cAMP formation trifluoperazine, a calmodulin antagonist, completely inhibited cAMP formation. In a membrane preparation adenylate cyclase activity was stimulated 10-20-fold by the GTP analog, guanosine 5'-[beta-imido]5'-triphosphate (Gpp[NH]p). Prostaglandin E1 and vasoactive intestinal peptide enhanced cAMP formation in this system by 2-3- and 1.2-1.6-fold. respectively. Addition of 200 nM calmodulin to membranes, in which endogenous calmodulin was decreased from 1.4 microgram/mg protein to 0.5 microgram/mg protein by washing with buffer containing EGTA and EDTA, resulted in a 3-4-fold increase of adenylate cyclase activity. The absolute increment in adenylate cyclase activity caused by calmodulin (10-15 pmol cAMP/min per mg protein) was approximately the same in the absence or presence of Gpp[NH]p. The apparent Ka for Gpp[NH]p (6 . 10-7 M) was not significantly changed by the addition of calmodulin. Although endogenous calcium (approx. 10 microM) in the enzyme assay was adequate to affect stimulation by calmodulin, a maximal effect was observed at a calcium concentration of 100 microM. These findings indicate that a calmodulin-sensitive form of adenylate cyclase is present in guinea-pig enterocytes, and that stimulation of cAMP formation in the intestinal mucosa may involve a calmodulin-mediated mechanism.

The effects of vasoactive intestinal polypeptide on the electrical activity of guinea-pig intestinal smooth muscle

The effect of vasoactive intestinal polypeptide (VIP) on the electrical activity of the smooth muscle of the guinea-pig taenia coli has been examined using the single sucrose gap apparatus. VIP usually caused a slowly developing, long-lasting membrane hyperpolarisation, although sometimes it reduced the frequency of spontaneous spike discharge without hyperpolarising the membrane. In contrast, non-adrenergic, non-cholinergic nerve stimulation initiated rapid membrane hyperpolarisation followed by post inhibitory excitation. In the presence of apamin, the VIP response was little affected, whereas the responses to non-adrenergic, non-cholinergic nerve stimulation were either markedly antagonised or reversed to membrane depolarisation. Consideration is given to the possible role of either VIP or ATP as the neurotransmitter responsible for the inhibitory junction potential elicited in the guinea-pig taenia coli in response to non-adrenergic inhibitory nerve stimulation.

A new neuroregulator: the vasoactive intestinal peptide or VIP

VIP is a neuroregulator occurring in the central and peripheral nervous system which exhibits the function of neurotransmitter in the brain, neuroendocrine substance at the pituitary level, and neuroparacrine substance in peripheral organs. The structure and the specificity of the molecule as studied by antibody and receptor, and its location in brain and peripheral organs are summarized as well as its numerous biological effects. The method used to demonstrate the involvement of VIP in a physiological regulation is described and illustrated by two examples: the effect of VIP on gut epithelium and the neuroendocrine action of VIP in inducing prolactin release from pituitary cells. The consequence of this recent progress in the knowledge of VIP release and action in human physiology and disease is indicated.

VIP and PGE1 activate adenylate cyclase in rat intestinal epithelial cell membranes via different mechanisms

The effect of vasoactive intestinal polypeptide (VIP) and of prostaglandin E1 (PGE1) alone and in combination was tested in an adenylate cyclase (AC) preparation from rat intestinal epithelial cell membranes. AC activity increased linearly with time up to 15 min in the absence or presence of VIP or PGE1, respectively. NaF, VIP and PGE1 concentration-dependently stimulated AC activity. The dose-response curve of VIP in the presence of PGE1 was shifted to the left and the maximal effect of VIP was significantly enhanced. The dose-response curve obtained experimentally was significantly different from theoretical additive dose-response curve, indicating that PGE1 potentiates the effect of VIP on AC activity. The possible mechanisms of potentiation are discussed. The results suggest that VIP and PGE1 activate AC via two different but not entirely independent mechanisms.

Importance of the vasoactive intestinal peptide receptor in the stimulation of cyclic adenosine 3',5'-monophosphate in gallbladder epithelial cells of man. Comparison with the guinea pig

An EDTA procedure was used to prepare isolated epithelial cells of human gallbladder devoid of endogenous vasoactive intestinal peptide (VIP) as measured by radioimmunoassay. Specific binding sites for VIP were characterized in these cells. At 37 degrees C, the binding of (125)I-labeled VIP reached a peak within 20 min and then declined rapidly. At 15 degrees C, binding was stable between 90 and 180 min of incubation. Binding of the labeled peptide was inhibited by concentrations of native VIP of 30 pM-0.1 muM. Half-maximal inhibition was observed at 2 nM. Scatchard analysis indicated two functionally independent classes of receptor sites: 62,000 high affinity sites/cell with a dissociation constant (K(d)) of 1.3 nM, and 510,000 low affinity sites/cell with a K(d) of 16.2 nM. Secretin inhibited tracer binding but with a 1,000 times lower potency than native VIP. VIP strongly stimulated adenosine 3':5' monophosphate (cyclic AMP) production in human gallbladder epithelial cells. At 37 degrees C, 0.1 nM and 10 nM VIP raised cyclic AMP levels 44 and 100 times above the basal level, respectively. Maximal values remained constant between 60 and 90 min at 15 degrees C. The importance of the VIP-induced cyclic AMP rise was related, at least in part, to a low phosphodiesterase activity in human gallbladder epithelial cells. At equilibrium, during a 60-min incubation at 15 degrees C, cyclic AMP production was noted at concentrations of VIP as low as 3 pM. Maximal and half-maximal stimulations were observed at 10 nM and 0.2 nM VIP, respectively. Secretin also stimulated cyclic AMP production but with a 10,000 lower potency than VIP. In the guinea pig, VIP and secretin were equipotent stimulators of cyclic AMP in gallbladder epithelial cells. This particular feature was shown to be due to receptors specific for each peptide that were present in these cells.

Influence of vasoactive intestinal polypeptide on net water flux and cyclic adenosine 3',5'-monophosphate formation in the rat jejunum

1. The effects were studied of vasoactive intestinal polypeptide (VIP), theophylline, and morphine on net water flux and cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels in the jejunum of anaesthetized rats in vivo and of VIP and morphine on adenylate cyclase activity in rat epithelial cell membranes in vitro. 2. Infusion of VIP (0.1-2 x 10(-9) mol/min/kg) dose dependently caused a reversal from net water absorption to net secretion; 2 x 10(-9) mol/min/kg enhanced the mucosal cyclic AMP content by 67%. 3. Theophylline (5 mg/ml, intraluminally) enhanced the effect of intra-arterial infusion of VIP (2 x 10(-9) mol/min/kg) as to net water secretion and increase in mucosal cyclic AMP content. 4. Pretreatment with morphine (5 mg/kg, s.c.) did not influence the effects of VIP on net water flux and on mucosal cyclic AMP content. 5. Atropine (2 mg/kg, s.c.) also failed to reduced the effect of VIP (0.4 x 10(-9) mol/min/kg) on net water flux. 6. Stimulation of adenylate cyclase activity was a function of VIP concentration over a range of 1 x 10(-10)-1 x 10(-7) M. Morphine (up to 1 x 10(-3) M) failed to influence stimulation of adenylate cyclase by VIP. 7. The finding that low doses of VIP, which already have an effect on net water flux, fail to increase cyclic AMP levels makes it likely that other mediators besides cyclic AMP are involved in the effect of VIP on net water flux. Some of the present results, however, support the assumption that VIP stimulates intestinal fluid secretion by increasing mucosal cyclic AMP levels.

Characterization of specific binding sites for vasoactive intestinal peptide in rat intestinal epithelial cell membranes

Specific binding sites for vasoactive intestinal peptide were characterized in plasma membranes from rat intestinal epithelial cells. At 30 degrees C, the interaction of 125I-labelled peptide with intestinal membranes was rapid, reversible, specific and saturable. At equilibrium, the binding of 125I-labelled peptide was competitively inhibted by native peptide in the 3 . 10(-11)--3 . 10-(7) M range concentration. Scatchard analysis of binding data suggested the presence of two distinct classes of vasoactive intestinal peptide binding sites: a class with a high affinity (Kd = 0.28 nM) and a low capacity (0.8 pmol peptide/mg membrane protein) and a class with a low affininty (Kd = 152 nM) and a high capacity (161 pmol peptide/mg membrane protein). Secretin competitively inhibited binding of 125I-labelled peptide but its potency was 1/1000 that of native peptide. Glucagon and the gastric inhibitory peptide were ineffective. The guanine nucleotides, GTP and Gpp(NH)p inhibited markedly the interaction of 125I-labelled peptide with its binding sites, by increasing the rate of dissociation of peptide bound to membranes. The other nucleotides triphosphate tested (ATP, ITP, UTP, CTP) were also effective in inhibiting binding of 125I-labelled peptide to membranes but their potencies were 1/100--1/1000 that of guanine nucleotides. The specificity and affinity of the vasoactive intestinal peptide-binding sites in plasma membranes prepared from rat intestinal epithelial cells, which is in agreement with an adenylate cyclase highly sensitive to the peptide recently characterized in these membranes (Amiranoff, B., Laburthe, M., Dupont, C. and Rosselin, G. (1978) Biochim. Biophys. Acta 544, 474--481) further argue for a physiological role of the peptide in the regulation of intestinal epithelial function.