Regulation of the vasoactive intestinal peptide receptor

Comparison of inotropic and chronotropic effects of vasoactive intestinal peptide in isolated dog atria

The positive chronotropic and inotropic effects of vasoactive intestinal peptide, VIP, were studied in an isolated canine right atrial preparation. Atria were removed, maintained in a bath, and perfused with Tyrode's solution. Contractile force and atrial depolarization were measured. VIP (18.8-600 pmol) was injected into a cannulated sinoatrial nodal artery and dose response curves were obtained. The mean EC50 was similar for the inotropic and the chronotropic responses (136 and 144 pmol, respectively). Time courses of the onset and of recovery from the responses were measured. Times for onset of VIP effects were similar but, once the effect was initiated, rate of development of the response and recovery time from the responses were dose dependent. The increases in atrial rate lasted two to four times longer than did the increases in contractile force. Recovery from the chronotropic and inotropic responses to VIP differ, suggesting that the intracellular responses are coupled differently to the receptors. The responses to VIP were compared to those of 100 pmol isoproterenol, another positive chronotropic and inotropic agent. Isoproterenol was a slightly more potent chronotropic and inotropic agent than VIP. Desensitization of the responses was determined. Repeated exposures to VIP decreased the chronotropic response but not the inotropic response to VIP. There was no significant decrease in responsiveness to isoproterenol.

Effect of castration on the VIPergic innervation and 125I-labelled vasoactive intestinal peptide (VIP) binding sites in the hamster seminal vesicle. A quantitative immunohistochemical and receptor autoradiographic study

In the present work we have investigated the effects of medium- (15 days) and long-term (2 months) castration on vasoactive intestinal peptide (VIP)-immunoreactive nerve fibres and 125I-labelled VIP binding sites in the adult hamster seminal vesicle. The density of VIP- and synaptophysin (general neuronal marker)-containing nerve fibres was determined in immunofluorescently stained cryostat sections using a computerised image analysis system. The morphological analysis of 125I-VIP binding sites in seminal vesicle cryostat sections was performed by quantitative receptor autoradiography. Our results show that the densities of the overall (synaptophysin immunoreactive) and VIPergic innervation increase in both medium and long-term castrated animals. In absolute terms, the quantity of VIP- and synaptophysin- containing nerves is not altered in medium-term castrates, but decreases for synaptophysin in long-term castrates. Medium-term castration does not affect the density of 125I-VIP binding sites in the gland muscular coat, but a significant decrease is observed after long-term castration. In conclusion, our results indicate that whereas VIP nerves are apparently unaffected by castration, 125I-VIP binding sites in the muscular coat of hamster seminal vesicle are sensitive to androgen levels.

Regulation of VIP gene expression in general. Human lung cancer cells in particular

Vasoactive intestinal peptide (VIP) is a neuropeptide of multiple functions affecting development and aging. In cancer, for example, VIP was found to function as an autocrine growth factor in nonsmall cell lung cancer (NSCLC) promotion. Furthermore, a VIP hybrid antagonist (neurotensin(6-11)-VIP(7-28)) was found to inhibit NSCLC growth. In the present study, the expression of VIP mRNA was studied using human lung cancer cells. RNA prepared from 19 cell lines was fractionated by 1% agarose gel electrophoresis followed by blotting onto nitrocellulose membranes and hybridization to a VIP-specific RNA probe. VIP mRNA was detected in about 50% of the cell lines tested with a greater abundance in NSCLC. Cultures of the NSCLC NCI-H727 cell line were treated with forskolin, an activator of cyclic AMP (cAMP), and separately with the tumor promoter phorbol 12-myristate 13-acetate (PMA). Northern blot hybridization analysis showed an increase in VIP mRNA levels after 4 h treatment with 50 microM forskolin. Incubation with PMA also showed a significant increase in the levels of VIP transcripts. Cultures were then incubated with PMA in the presence of actinomycin D, a transcription blocker. Results indicated that PMA treatment may induce both VIP mRNA synthesis as well as VIP mRNA stabilization, and suggested a 4-5 h half-life for the VIP mRNA in the absence of PMA. Thus, lung cancer tumor proliferation may be regulated, in part, at the level of VIP gene expression.

Stable expression of the recombinant human VIP1 receptor in clonal Chinese hamster ovary cells: pharmacological, functional and molecular properties

We stably transfected Chinese hamster ovary (CHO) cells with the recombinant human vasoactive intestinal peptide (VIP)1 receptor. A clone referred to as Clone 15 was isolated and studied for receptor properties. The following data were obtained: (1) one class of binding site was identified by Scatchard analysis of [125I]VIP binding to cell membranes with a Kd of 0.41 nM and a Bmax of 1.62 pmol/mg protein; (2) the constant Ki for the inhibition of [125I]VIP binding by VIP and related peptides was: VIP (0.9 nM) = pituitary adenylate cyclase-activating peptide (PACAP)-27 (1.3 nM) < PACAP-38 (6.8 nM) < helodermin (46.0 nM) < human growth hormone-releasing factor (GRF) (0.6 microM) < peptide histidine methionineamide (2.0 microM) < secretin (> 10 microM); (3) cross-linking experiments using [125I]VIP identified a single M(r) 67000 recombinant receptor; (4) VIP stimulated cAMP production in Clone 15 cells with an EC50 of 0.20 nM; (5) some previously described VIP receptor antagonists including [4-Cl-D-Phe6, Leu17]VIP, [Ac-Tyr1,D-Phe2]GRF-(1-29) amide and VIP-(10-28) inhibited [125I]VIP binding with a Ki of 0.7, 1.6 and 2.5 microM, respectively. They failed to stimulate cAMP production in Clone 15 cells and inhibited, at least partially, the VIP (0.3 nM)-evoked cAMP production; (6) exposure of Clone 15 cells to 10 nM VIP for 24 h resulted in a sharp decrease in Bmax in Clone 15 cells (0.43 vs. 1.62 pmol/mg protein in control cells) and in the potency and efficacy of VIP in stimulating cAMP. Moreover, immunofluorescence studies using confocal microscopy indicated that the receptor was internalized and sequestered in vesicular structures within the cells. It is concluded that Clone 15 cells provide a valuable tool to further characterize various functional and pharmacological aspects of the human VIP1 receptor.

Inhibition of human neuroblastoma growth by a specific VIP antagonist

The 28-amino-acid neuropeptide, vasoactive intestinal peptide (VIP), is a potent mitogen during embryonic development and plays a vital role in brain growth. VIP is also mitogenic for tumor cells, including the human neuroblastoma (NMB). Northern blot analysis has revealed VIP mRNA transcripts in NMB. We now report VIP-like immunoreactivity within these neuroblastoma cells that increased during logarithmic growth and decreased after attaining confluency. About 10(6) seeded cells secreted 5-40 pg of VIP-like immunoreactivity into the medium. These results suggest an autocrine role for VIP in the regulation of neuroblastoma growth. A VIP hybrid antagonist (neurotensin6-11 VIP7-28) that has been shown to inhibit lung cancer proliferation was now tested for inhibition of neuroblastoma growth. Receptor binding studies indicated that the hybrid antagonist displaced [125I]-VIP binding in the neuroblastoma cells (EC50 = 5 x 10(-6)M). Furthermore, as measured by thymidine incorporation and by cell counts, the potent VIP hybrid antagonist inhibited neuroblastoma multiplication in a dose-dependent manner. In conclusion, VIP may be an important regulator of growth of nerve cell progenitors and of tumors derived from neuronal origin and intervening with VIP function may lead to improved treatment of cancer.

Learning and sexual deficiencies in transgenic mice carrying a chimeric vasoactive intestinal peptide gene

The molecular mechanisms responsible for behavior are largely unknown. A state of the art model, paving the path from genes to behavior, is offered by transgenic animals. Candidate molecules are classic neuropeptides, such as vasoactive intestinal peptide (VIP). Transgenic mice harboring a chimeric VIP gene driven by the polyoma promoter were produced. Behavioral studies revealed learning impairment and prolonged retardation in memory acquisition in the genetically altered animals. Furthermore, reduced performance was observed when the male transgenic mice were tested for sexual activity in the presence of receptive females. Surprisingly, radioimmunoassays showed an approx 20% decrease in the VIP content of the transgenic mice brains. To directly assess genetically reduced VIP content as a cause for learning impairment, transgenic mice carrying diphtheria toxin-encoding sequences driven by the rat VIP promoter were created. These animals had reduced brain VIP and exhibited deficiencies in learning abilities, strongly supporting an important neurobiological function for VIP in vivo.

VIP modulates intracellular calcium oscillations in human lymphoblasts

Vasoactive intestinal polypeptide (VIP) has been shown to stimulate adenylate cyclase in a human lymphoblast cell line (MOLT 4). In the present study, we monitored fluorescence in cell suspensions and in single fura-2 loaded MOLT 4 lymphoblasts to determine if VIP modulates intracellular calcium concentrations ([Ca2+]i), and if this modulation is mediated by adenylate cyclase. The distribution of [Ca2+]i in resting and stimulated cells was non-homogeneous, with gradients of high [Ca2+]i present in the subplasmalemmal space. In a subset of cells (10-30% of all cells studied), [Ca2+]i showed La(3+)-sensitive, temporal changes in the form of [Ca2+]i oscillations with a baseline [Ca2+]i value of 115 +/- 10 nM, an oscillation amplitude of 150 +/- 18 nM and a mean period of 9.2 +/- 2 s. The remaining non-oscillating cells showed a constant [Ca2+]i level of 75 +/- 5 nM (n = 65 cells from 4 experiments). In the subset of cells with spontaneous [Ca2+]i oscillations, VIP dose-dependently (10(-12) to 10(-8) M) increased the amplitude of oscillations but did not stimulate their frequency. The stimulatory effect of VIP was correlated with baseline [Ca2+]i in these cells, was attenuated in the presence of La3+ (25 microM), but was unaffected by cell depolarization (126 mM KCl). Dibutyryl cyclic AMP (10(-4) to 10(-3) M) and forskolin (10(-4) M) had no effect on [Ca2+]i oscillations, or on [Ca2+]i in cells without oscillations. In cell suspensions, baseline [Ca2+]i was found to be 55.1 +/- 11.2 nM (mean +/- S.E.M., n = 11); VIP, cyclic AMP analogues or forskolin had no significant effect on [Ca2+]i. These findings suggest that: a) VIP modulates the amplitude of [Ca2+]i oscillations generated by a cytosolic [Ca2+] oscillator in a subset of cells at a concentration of 10(-12) M, a thousand-fold below the KD for the VIP receptor; b) baseline [Ca2+] values may be related to both the ability of cells to generate spontaneous [Ca2+] oscillations and of oscillating cells to respond to VIP; c) due to the small number of responding cells, VIP-induced [Ca2+]i changes are not detectable when studied in cell suspensions.

Cholesterol modulation of membrane fluidity and VIP receptor/effector system in rat prostatic epithelial cells

Treatment of rat prostatic epithelial cells with cholesteryl hemisuccinate (ChH) resulted in a time- and dose-dependent inhibition of the stimulatory effect of the neuropeptide vasoactive intestinal peptide (VIP) on cyclic AMP accumulation, with a 40% decrease in the response to a maximally effective VIP concentration. Cell treatment with ChH led also to a similar blocking of isoproterenol (a beta-adrenergic agonist) action but did not modify forskolin (which is assumed to act directly on the catalytic unit of adenylate cyclase) activity upon cyclic AMP levels. The levels of the transduction protein Gs were similar in membranes from both control and ChH-treated cells as suggested by experiments on cholera toxin-catalyzed ADP-ribosylation. The inhibitory effect of ChH was accompanied by an increase of membrane microviscosity as estimated by measurements of fluorescence polarization. Experiments on VIP binding indicated that increasing cholesterol concentration in the plasma membrane led to a higher VIP binding capacity without changes in the affinity of VIP receptors. These data suggest that membrane cholesterol incorporation diminishes the coupling efficiency between adenylate cyclase and the VIP-receptor complex or other receptor systems (i.e., desensitization) due to an increase of plasma membrane rigidity.

VIP: molecular biology and neurobiological function

In the mammalian brain, a major regulatory peptide is vasoactive intestinal peptide (VIP). This 28 amino acid peptide, originally isolated from the porcine duodenum, was later found in the central and peripheral nervous systems and in endocrine cells, where it exhibits neurotransmitter and hormonal roles. Increasing evidence points to VIP's importance as a mediator or a modulator of several basic functions. Thus, VIP is a major factor in brain activity, neuroendocrine functions, cardiac activity, respiration, digestion, and sexual potency. In view of this peptide's importance, the mechanisms controlling its production and the pathways regulating its functions have been reviewed. VIP is a member of a peptide family, including peptides such as glucagon, secretin, and growth hormone releasing hormone. These peptides may have evolved by exon duplication coupled with gene duplication. The human VIP gene contains seven exons, each encoding a distinct functional domain on the protein precursor or the mRNA. VIP gene transcripts are mainly found in neurons or neuron-related cells. VIP gene expression is regulated by neuronal and endocrine signals that contribute to its developmental control. VIP exerts its function via receptor-mediated systems, activating signal transduction pathways, including cAMP. It can act as a neurotransmitter, neuromodulator, and a secretagog. As a growth and developmental regulator, VIP may have a crucial effect as a neuronal survival factor. We shall proceed from the gene to its multiple functions.