Human vasoactive intestinal peptide1 receptors expressed by stable transfectants couple to two distinct signaling pathways

Targeting VIP and PACAP Receptor Signaling: New Insights into Designing Drugs for the PACAP Subfamily of Receptors

Pituitary Adenylate Cyclase-Activating Peptide (PACAP) and Vasoactive Intestinal Peptide (VIP) are neuropeptides involved in a diverse array of physiological and pathological processes through activating the PACAP subfamily of class B1 G protein-coupled receptors (GPCRs): VIP receptor 1 (VPAC1R), VIP receptor 2 (VPAC2R), and PACAP type I receptor (PAC1R). VIP and PACAP share nearly 70% amino acid sequence identity, while their receptors PAC1R, VPAC1R, and VPAC2R share 60% homology in the transmembrane regions of the receptor. PACAP binds with high affinity to all three receptors, while VIP binds with high affinity to VPAC1R and VPAC2R, and has a thousand-fold lower affinity for PAC1R compared to PACAP. Due to the wide distribution of VIP and PACAP receptors in the body, potential therapeutic applications of drugs targeting these receptors, as well as expected undesired side effects, are numerous. Designing selective therapeutics targeting these receptors remains challenging due to their structural similarities. This review discusses recent discoveries on the molecular mechanisms involved in the selectivity and signaling of the PACAP subfamily of receptors, and future considerations for therapeutic targeting.

Pathophysiological Role and Drug Modulation of Calcium Transport in Ocular Surface Cells

The ocular surface structure and extraocular accessory organs constitute the ocular surface system, which includes the cornea, conjunctiva, eyelids, lacrimal organs, and lacrimal passages. This system is composed of, and stabilized by, the corneal epithelium, conjunctival cells, conjunctival goblet cells, lacrimal acinar cells and Tenon's fibroblasts, all of which maintain the healthy eyeball surface system. Ocular surface diseases are commonly referred to corneal and conjunctival disease and external ocular disease, resulting from damage to the ocular surface structure. A growing body of evidence has indicated that abnormal activation of the KCa3.1 channel and Ca2+/ calmodulin-dependent kinase initiates ocular injury. Signaling pathways downstream of the irregular Ca2+ influx induce cell progression and migration, and impair tight junctions, epithelial transport and secretory function. In this overview, we summarize the current knowledge regarding ocular surface disease in terms of physical and pathological alteration of the ocular system. We dissect in-depth, the mechanisms underlying disease progression, and we describe the current calcium transport therapeutics and the obstacles that remain to be solved. Finally, we summarize how to integrate the research results into clinical practice in the future.

Pituitary adenylate cyclase-activating polypeptide receptors in the trigeminovascular system: implications for migraine

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been implicated in a wide range of functions including vasodilatation, neuroprotection, nociception and neurogenic inflammation. PACAP activates three distinct receptors, the PAC₁ receptor, which responds to PACAP, and the VPAC₁ and VPAC₂ receptors, which respond to both PACAP and vasoactive intestinal polypeptide. The trigeminovascular system plays a key role in migraine and contains the trigeminal nerve, which is the major conduit of craniofacial pain. PACAP is expressed throughout the trigeminovascular system and in higher brain regions involved in processing pain. Evidence from human clinical studies suggests that PACAP may act outside the blood-brain barrier in the pathogenesis of migraine. However, the precise mechanisms involved remain unclear. PACAP potentially induces migraine attacks by activating different receptors in different cell types and tissues. This complexity prompted this review of PACAP receptor pharmacology, expression and function in the trigeminovascular system. Current evidence suggests that the PAC₁ receptor is the likely pathophysiological target of PACAP in migraine. However, multiple PACAP receptors are expressed in key parts of the trigeminovascular system and further work is required to determine their contribution to PACAP physiology and the pathology of migraine.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Pituitary adenylate cyclase-activating polypeptide induces postsynaptically expressed potentiation in the intra-amygdala circuit

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide expressed in the brain, where it may act as a neuromodulator or neurotransmitter contributing to different behavioral processes and stress responses. PACAP is highly expressed in the amygdala, a subcortical brain area involved in both innate and learned fear, suggesting a role for PACAP-mediated signaling in fear-related behaviors. It remains unknown, however, whether and how PACAP affects neuronal and synaptic functions in the amygdala. In this study, we focused on neurons in the lateral division of the central nucleus (CeL), where PACAP-positive presynaptic terminals were predominantly found within the amygdala. In our experiments on rat brain slices, exogenous application of PACAP did not affect either resting membrane potential or membrane excitability of CeL neurons. PACAP enhanced, however, excitatory synaptic transmission in projections from the basolateral nucleus (BLA) to the CeL, while inhibitory transmission in the same pathway was unaffected. PACAP-induced potentiation of glutamatergic synaptic responses persisted after the washout of PACAP and was blocked by the VPAC1 receptor antagonist, suggesting that VPAC1 receptors might mediate synaptic effects of PACAP in the CeL. Moreover, potentiation of synaptic transmission by PACAP was dependent on postsynaptic activation of protein kinase A and calcium/calmodulin-dependent protein kinase II, as well as synaptic targeting of GluR1 subunit-containing AMPA receptors. Thus, PACAP may upregulate excitatory neurotransmission in the BLA-CeL pathway postsynaptically, consistent with the known roles of PACAP in control of fear-related behaviors.

Mechanisms involved in VPAC receptors activation and regulation: lessons from pharmacological and mutagenesis studies

Vasoactive intestinal peptide (VIP) plays diverse and important role in human physiology and physiopathology and their receptors constitute potential targets for the treatment of several diseases such as neurodegenerative disorder, asthma, diabetes, and inflammatory diseases. This article reviews the current knowledge regarding the two VIP receptors, VPAC(1) and VPAC(2), with respect to mechanisms involved in receptor activation, G protein coupling, signaling, regulation, and oligomerization.

Neurotransmitter influence on human meibomian gland epithelial cells

PURPOSE

A striking characteristic of the human meibomian gland is its rich sensory, sympathetic, and parasympathetic innervation, yet the functional relevance of these nerve fibers remains unknown. Acting on the hypothesis that neurotransmitters are released in the vicinity of the gland, act on glandular receptors, and influence the production, secretion, and/or delivery of meibomian gland secretions to the ocular surface, the goal in this study was to begin to determine whether neurotransmitters influence the meibomian gland.

METHODS

Immortalized human meibomian gland epithelial (SLHMG) cells were examined for the presence of vasoactive intestinal peptide (VIP) and muscarinic acetylcholine (mACh) receptor transcripts and proteins. Cells were also exposed to VIP, carbachol, forskolin, and/or 3-isobutyl-1-methylxanthine (IBMX) to determine whether these agents, alone or in combination, modulate the adenylyl cyclase pathway, the accumulation of intracellular free calcium ([Ca2+]i), or cell proliferation.

RESULTS

Results demonstrate that SLHMG cells transcribe and translate VIP and mACh receptors; VIP, with either IBMX or forskolin, activates the adenylyl cyclase pathway, and the effect of VIP and forskolin together is synergistic; both VIP and carbachol increase intracellular [Ca2+] in SLHMG cells; and VIP with forskolin stimulates SLHMG cell proliferation.

CONCLUSIONS

This study shows that parasympathetic neurotransmitters and their agonists influence the function of human meibomian gland epithelial cells. It remains to be determined whether this action alters the production, secretion, and/or delivery of meibum to the ocular surface.

VIPoma with expression of both VIP and VPAC1 receptors in a patient with WDHA syndrome

We report a case of VIPoma in a 72-year-old female patient who presented with excessive diarrhea, severe hypokalemia, and acidemia. She had been referred to our hospital three times because of severe diarrhea. No primary tumor site was found by conventional techniques, including contrast-enhanced CT and MRI, angiography, endoscopy, and positron emission tomography (PET), but a tumor was subsequently found in the head of the pancreas by octreotide scanning. Her diarrhea diminished dramatically after octreotide treatment, while her diarrhea has ceased without the therapy of octreotide at the first admission in the course of 2 years of her disease. Immunohistochemial analysis of the excised tumor tissue revealed the expression of both vasoactive intestinal peptide (VIP) and VIP and pituitary adenylate cyclase-activating peptide 1 (VPAC1) receptors. This is the first case report of a VIPoma that immunostains for VIP and VPAC1 receptors and indicates that abundant VIP produced by VIPoma might inhibit its growth and reduce VIP secretion via the VPAC1 receptor in vivo.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.

A systematic comparison of intracellular cyclic AMP and calcium signalling highlights complexities in human VPAC/PAC receptor pharmacology

VPAC/PAC receptor activation classically results in cyclic-AMP production, with limited reports evaluating calcium signalling. These studies systematically characterise intracellular cyclic-AMP ([cAMP](i)) and calcium ([Ca(2+)](i)) responses in CHO-cells expressing recombinant human (h) VPAC/PAC receptors (hVPAC(1)R, hVPAC(2)R, hPAC(1)R), using two simple, non-radioactive, HT-amenable assays. The rank order of potency (ROP) of the agonists VIP, PACAP-27 and PACAP-38 was similar in both assays for each individual receptor subtype, although potencies (EC(50)) in the [Ca(2+)](i) assay were approximately 100-fold lower. Importantly, this shift was also evident in SHSY-5Y cells endogenously expressing hPAC(1)R. Furthermore, [Ala(11,22,28)]VIP and maxadilan were selective hVPAC(1)R and hPAC(1)R agonists, respectively, and although R3P65 had no demonstrable hVPAC(2)R selectivity, these compounds exhibited comparable reductions in [Ca(2+)](i) EC(50) values. In contrast, PG97-269 and PG99-465, putatively selective hVPAC(1)R and hVPAC(2)R antagonists, respectively, were marginally less potent in [cAMP](i) studies, whereas M65 was equipotent at hPAC(1)R. Moreover, PG99-465 alone increased [cAMP](i) at all three hVPAC/PAC receptor subtypes, with full hVPAC(1)R and hPAC(1)R agonism. With equivalent agonist ROPs generated in both assays, [Ca(2+)](i) signalling provides an alternative approach to examine hVPAC/PAC receptor pharmacology. However, these studies underscore the paucity of receptor selective compounds, complexities in comparing drug potencies across assays, and the pleiotropic nature of VPAC/PAC-receptor signalling.

A natural variant type II G protein-coupled receptor for vasoactive intestinal peptide with altered function

The vasoactive intestinal peptide (VIP) and its G protein-coupled receptors VPAC1 and VPAC2 prominently mediate diverse physiological functions in the neural, endocrine, and immune systems. A deletion variant of mouse VPAC2 has been identified in immune cells that lacks amino acids 367-380 at the carboxyl-terminal end of the seventh transmembrane domain. When expressed at equivalent levels in a human Jurkat T cell line, which has very low endogenous expression of human VPAC1 and VPAC2, wild-type and deletion-variant VPAC2 bound the same amount of 125I-VIP with similar affinity. Unlike wild-type VPAC2, however, deletion-variant VPAC2 did not transduce VIP-elicited increases in intracellular concentration of cyclic AMP, chemotaxis, or suppression of generation of interleukin-2. Natural deletion of part of the last transmembrane domain of VPAC2 thus abrogates signaling functions without apparent alterations of expression or ligand binding.

Enhancement of aquaporin-3 by vasoactive intestinal polypeptide in a human colonic epithelial cell line

BACKGROUND

Vasoactive intestinal polypeptide (VIP) plays an important role in water transport in the intestine. Several specialized channels termed aquaporins (AQP) facilitate water transport in the gastrointestinal tract. Aquaporin-3 localizes to epithelial cells in the human small intestine and colon. However, the regulatory mechanisms underlying the functions of AQP3 remain unclear. To characterize the regulation of AQP3 expression by VIP, we studied messenger (m)RNA expression, protein expression and DNA binding activity in a human colonic epithelial cell line, HT-29.

METHOD

Human colonic epithelial cells, HT-29, were incubated with VIP (10-12-10-7 M). The cells were treated with protein kinase-A (PK-A) inhibitors (H-89, H-9) or chloride channel-blockers (diphenylamine-2-carboxylate (DPC), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPD)). The expression of AQP3 mRNA and protein was determined by Northern blot and Western blot, respectively. The DNA-binding activities of cyclic adenosine monophosphate (cAMP) response elements/activating transcription factor (CRE/ATF)) in the nuclear extract were determined by electrophoretic mobility shift assay.

RESULTS

Aquaporin-3 mRNA was up-regulated at a concentration of 10-10 M VIP. The expression of AQP3 protein was enhanced at 3 h after addition of VIP. The PK-A inhibitors (H-89, H-9) inhibited the expression of AQP3 mRNA enhanced by VIP and cAMP. The gel shift assay of CRE/ATF in HT-29 cells revealed a single band.

CONCLUSION

These results indicate that VIP upregulated the expression of AQP3 mRNA and protein, and that a cAMP-dependent pathway mediated this effect in a human colonic epithelial cell line, HT-29.

Lysine 195 and aspartate 196 in the first extracellular loop of the VPAC1 receptor are essential for high affinity binding of agonists but not of antagonists

The role in ligand recognition and receptor activation of two adjacent charged residues (lysine 195 and aspartate 196) in the first extracellular loop of the human VPAC(1) receptor was investigated in stably transfected CHO cells expressing the wild type or point mutated receptors.Replacement of lysine 195 by glutamine or of aspartate 196 by asparagine reduced the agonists' ability to stimulate adenylate cyclase activity; VIP behaved like a partial agonist and a partial agonist behaved as an antagonist. The receptor's capacity to recognize agonists was reduced but antagonists' affinity was unaffected. Both results suggesting that the two charged residues are essential for VPAC(1) receptor activation. On the other hand, the double mutant was less severely affected than single mutants suggesting that hydrogen bonds may partially compensate the loss of charged residues. But the inversion of the residues affected receptor recognition and activation more markedly suggesting that the two charged residues do not interact directly.

VPAC(1) receptors have different agonist efficacy profiles on membrane and intact cells

The vasoactive intestinal peptide receptor VPAC(1) is preferentially coupled to G(alpha s) protein but also increases [Ca(2+)](i) through interaction with G(alpha i)/G(alpha q) protein. We evaluated a panel of full, partial and null agonists for their capability to stimulate adenylate cyclase activity in both intact cells and membrane and [Ca(2+)](i) in intact cells transfected with the reporter gene aequorin. In intact cells, the agonists efficacy for cAMP and calcium increase were well, but not linearly correlated: VPAC(1) receptors activated G(alpha s) protein more efficiently but with the same pharmacological profile as the other G proteins. In contrast, there was a difference between cAMP increase in intact and broken cell membranes: EC(50) values were generally lower in intact cells whereas the efficacy was higher. There was, however, no correlation between the shift in the EC(50) value and the intrinsic activity. Of interest, the (4-28) fragment, a reported antagonist on cell membrane, was a full agonist in intact cells. We concluded that the active states of the VPAC(1) receptor resulting from the coupling to different effector are undistinguishable by the VIP analogs tested but that receptor properties are different when evaluated in intact cells or cell membranes.

A small sequence in the third intracellular loop of the VPAC(1) receptor is responsible for its efficient coupling to the calcium effector

The stimulatory effect of VIP on intracellular calcium concentration ([Ca(2+)](i)) has been investigated in Chinese hamster ovary cells stably transfected with the reporter gene aequorin, and expressing human VPAC(1), VPAC(2), chimeric VPAC(1)/VPAC(2), or mutated receptors. The VIP-induced [Ca(2+)](i) increase was linearly correlated with receptor density and was higher in cells expressing VPAC(1) receptors than in cells expressing a similar VPAC(2) receptor density. The study was performed to establish the receptor sequence responsible for that difference. VPAC(1)/VPAC(2) chimeric receptors were first used for a broad positioning: those having the third intracellular loop (IC(3)) of the VPAC(1) or of the VPAC(2) receptor behaved, in that respect, phenotypically like VPAC(1) and VPAC(2) receptor, respectively. Replacement in the VPAC(2) receptor of the sequence 315-318 (VGGN) within the IC(3) by its VPAC(1) receptor counterpart 328-331 (IRKS) and the introduction of VGGN in state of IRKS in VPAC(1) was sufficient to mimic the VPAC(1) and VPAC(2) receptor characteristics, respectively. Thus, a small sequence in the IC(3) of the VPAC(1) receptor, probably through interaction with G(alphai) and G(alphaq) proteins, is responsible for the efficient agonist-stimulated [Ca(2+)](i) increase.

Identification of G-proteins coupling to the vasoactive intestinal peptide receptor VPAC(1) using immunoaffinity chromatography: evidence for precoupling

VPAC(1) receptor subtype-specific G-protein interactions were identified using a strategy that exploits an essential initial signaling event, namely the functional and physical association of the receptor with G-protein. An immunoaffinity purification column was constructed using a previously characterized antibody that had been raised against the first extracellular loop of the VPAC(1) receptor. VPAC(1)/G-protein complexes were solubilized from membranes and copurified. Receptor and Galpha-proteins were detected in eluates using (125)I-VIP labeling and immunoblotting, respectively. Human VPAC(1) transfected in HEK293 cells couples to Gs but not Gi3, Gi1/2, or Gq. Rat VPAC(1) in brain membranes is coupled to Gs and Gi3. Rat VPAC(1) in lung membranes couples to Gs, Gi3, and Gq. Pretreatment of membranes with VIP increased the level of all G-proteins copurifying with VPAC(1). Immunoaffinity chromatography also revealed VPAC(1) receptor precoupling to G-protein in the absence of VIP pretreatment. This was confirmed using a cross-linking procedure to capture VIP receptor/G-protein complexes in the native membrane milieu prior to solubilization. Precoupling suggests that there is a significant basal level of VPAC(1) receptor activity especially in cells, such as some human malignant tumor cells, that express high levels of receptor.

Enhanced delayed-type hypersensitivity and diminished immediate-type hypersensitivity in mice lacking the inducible VPAC(2) receptor for vasoactive intestinal peptide

Vasoactive intestinal peptide (VIP) and its G protein-coupled receptors, VPAC(1)R and VPAC(2)R, are prominent in the immune system and regulate many aspects of T cell-dependent immunity. In mouse T cells, VPAC(1)R is expressed constitutively, whereas VPAC(2)R is induced by immune stimuli. VPAC(2)R-null (VPAC(2)R(-/-)) mice on a C57BL/6 background are shown here to have normal basic immune characteristics, including serum Ig concentrations, blood levels of all leukocytes, and spleen number of total T cells (CD3(+)) and T cells bearing CD4, CD8, and CD28. Hapten-evoked cutaneous delayed-type hypersensitivity (DTH) was significantly enhanced in VPAC(2)R-null mice compared with age- and sex-matched wild-type mice. In contrast, generation of IgE anti-hapten antibodies and active cutaneous anaphylaxis were > or =70% lower in VPAC(2)R-null mice than in wild-type controls. Cytokine production by splenic CD4(+) T cells, stimulated with adherent anti-CD3 plus anti-CD28 antibodies, revealed higher levels of IL-2 (mean = 3-fold) and IFN-gamma (mean = 3-fold), and lower levels of IL-4 (mean = one-fifth) in VPAC(2)R-null mice than wild-type controls. Loss of VIP-VPAC(2)R maintenance of the normal ratio of Th2/Th1 cytokines thus leads to a state of enhanced DTH and depressed immediate-type hypersensitivity, which may alter both host defense and susceptibility to immune-mediated diseases.

Allergic diathesis in transgenic mice with constitutive T cell expression of inducible vasoactive intestinal peptide receptor

Vasoactive intestinal peptide (VIP) and its G-protein-coupled receptors (VPAC1 and VPAC2 Rs) are prominent in the immune system. In T cells, VPAC1 R is expressed constitutively whereas VPAC2 R is induced only after stimulation of the T cell receptor (TCR) or exposure to some cytokines. VPAC1 R and VPAC2 R also transduce different effects of VIP on T cells. Constitutive expression of VPAC2 R selectively in CD4+ T cells (helper-inducer Th cells) of transgenic (TG) C57BL/6 mice directed by the lck tyrosine kinase promoter is now shown to evoke production of more Th2-type interleukins 4 and 5, and less Th1-type interferon gamma after TCR activation. VPAC2 R TG mice consequently have significant elevations of blood IgE, IgG1, and eosinophils. VPAC2 R TG mice also show increased IgE antibody responses, which mediate heightened cutaneous allergic reactions, and have depressed delayed-type hypersensitivity. VIP enhancement of the ratio of Th2 cell to Th1 cell cytokines thus evokes an allergic state in normally nonallergic mice, which suggests the possibility of neuropeptide contributions to immune phenotypic alterations in human hypersensitivity diseases.

Vasoactive intestinal peptide (VIP) stimulates [Ca2+]i and cyclic AMPin CHO cells expressing Galpha16

The stimulatory effect of vasoactive intestinal peptide (VIP) and analogues on [Ca2+]i has been investigated in chinese hamster ovary (CHO) cells stably transfected with the reporter gene aequorin, and expressing either the human VPAC1or VPAC2 receptor in absence or in presence of the Galpha16. In cells that were not transfected with Galpha16 and expressed a similar density of receptors, the VIP induced [Ca2+]i ncrease was higher in VPAC1 than in VPAC2 receptor expressing cells. In aequorin/Galpha16 cotransfected cells, the VIP-induced response was higher, reaching 70 to 80% of the maximal calcium response, obtained after digitonin treatment, in response to both VPAC1 and VPAC2 receptor stimulation. The results suggest that in hematopoietic cells, which express both VIP receptors and Galpha16, the signalling pathway of VIP could be mediated through both cyclic AMP and [Ca2+]i increase.

Regulation of cholangiocyte bicarbonate secretion

The objective of this review article is to discuss the role of secretin and its receptor in the regulation of the secretory activity of intrahepatic bile duct epithelial cells (i.e., cholangiocytes). After a brief overview of cholangiocyte functions, we provide an historical background for the role of secretin and its receptor in the regulation of ductal secretion. We review the newly developed experimental in vivo and in vitro tools, which lead to understanding of the mechanisms of secretin regulation of cholangiocyte functions. After a description of the intracellular mechanisms by which secretin stimulates ductal secretion, we discuss the heterogeneous responses of different-sized intrahepatic bile ducts to gastrointestinal hormones. Furthermore, we outline the role of a number of cooperative factors (e.g., nerves, alkaline phosphatase, gastrointestinal hormones, neuropeptides, and bile acids) in the regulation of secretin-stimulated ductal secretion. Finally, we discuss other factors that may also play an important role in the regulation of secretin-stimulated ductal secretion.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Pituitary adelylate cyclase-activating peptide is an activator of vasoactive intestinal polypeptide gene transcription in human neuroblastoma cells

In many ganglia, the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) innervates nerve cell bodies containing the homologous neuropeptide vasoactive intestinal polypeptide (VIP). We therefore investigated whether PACAP affected the VIP gene expression and elucidated the molecular mechanisms using the human neuroblastoma cell line NB-1. A concentration dependent induction of the VIP mRNA level was found upon PACAP stimulation. Five nM PACAP mediated transient elevation of the VIP mRNA being evident after 2 h, the maximal 65-fold induction was reached after 6-8 h and hereafter the level decreased rapidly. In cell extracts, the concentration of immunoreactive VIP was elevated four-fold upon PACAP stimulation for 8 h, and it remained elevated during the next 40 h. In conditioned medium, a stable 20-fold VIP increase was seen after 8-24 h. Experiments with the translational inhibitor cycloheximide showed a direct effect of PACAP on the VIP mRNA level, and nuclear run-on assays revealed a three- to four-fold enhancement of the VIP gene transcription rate after PACAP stimulation. The VIP mRNA induction was abolished by transcriptional inhibition with the actinomycin D, and PACAP did not seem to mediate any changes in the VIP mRNA half-life. However, the VIP mRNA level seemed very stable during the transcriptional cessation. Reporter gene constructs were used to evaluate involvement of the VIP CRE site in the PACAP mediated induction of the VIP gene transcription. Mutation of the CRE site did not abolish the induction suggesting it to be of minor if any importance for the induction. In conclusion, the PACAP mediated induction of the VIP gene expression suggests that PACAP released from nerve terminals could influence the function of VIP'ergic neurons in target tissues.

VIP activates G(s) and G(i3) in rat alveolar macrophages and G(s) in HEK293 cells transfected with the human VPAC(1) receptor

We have characterized vasoactive intestinal peptide (VIP) receptor/G-protein coupling in rat alveolar macrophage (AM) membranes and find that pertussis toxin treatment and antisera against G(alphai3) and G(alphas) reduce high-affinity (125)I-VIP binding, indicating that both G(alphas) and G(alphai3) couple to the VIP-receptor. The predominant VIP-receptor subtype in AM is VPAC(1) and we examined the G-protein interactions of the human VPAC(1) that had been transfected into HEK293 cells. VPAC(1) has a molecular mass of 56 kDa; GTP analogs reduced (125)I-VIP binding to this protein demonstrating that high-affinity binding of VIP to the receptor requires coupling to G-protein. Functional VIP/VPAC(1)/G-protein complexes were captured by covalent cross-linking and analyzed by Western blotting. The transfected human VPAC(1) receptor in HEK293 was found to be coupled to G(alphas) but not G(alphai) or G(alphaq). Furthermore, pertussis toxin treatment had no effect on VPAC(1)/G-protein coupling in these cells. These observations suggest that the G-proteins activated by VPAC(1) may be dependent upon species and cell type.

The G protein alpha subunit has a key role in determining the specificity of coupling to, but not the activation of, G protein-gated inwardly rectifying K(+) channels

In neuronal and atrial tissue, G protein-gated inwardly rectifying K(+) channels (Kir3.x family) are responsible for mediating inhibitory postsynaptic potentials and slowing the heart rate. They are activated by Gbetagamma dimers released in response to the stimulation of receptors coupled to inhibitory G proteins of the G(i/o) family but not receptors coupled to the stimulatory G protein G(s). We have used biochemical, electrophysiological, and molecular biology techniques to examine this specificity of channel activation. In this study we have succeeded in reconstituting such specificity in an heterologous expression system stably expressing a cloned counterpart of the neuronal channel (Kir3.1 and Kir3.2A heteromultimers). The use of pertussis toxin-resistant G protein alpha subunits and chimeras between G(i1) and G(s) indicate a central role for the G protein alpha subunits in determining receptor specificity of coupling to, but not activation of, G protein-gated inwardly rectifying K(+) channels.

The rat growth hormone-releasing hormone receptor gene: structure, regulation, and generation of receptor isoforms with different signaling properties

The interaction of GHRH with membrane-bound receptors on somatotroph cells of the anterior pituitary is an important step in the regulation of GH synthesis and secretion. The identification of a G protein-coupled receptor for GHRH has made it possible to investigate the pathway by which GHRH regulates pituitary somatotroph cell function. To initiate an analysis of the mechanisms regulating expression and function of the GHRH receptor, the structure of the gene and its promoter region were analyzed. The coding sequence of the rat GHRH receptor gene is contained within 14 exons spanning approximately 15 kb of genomic DNA. Four transcription start sites are located within 286 bp upstream of the initiation codon. The 5' flanking region of the GHRH receptor gene acts as a functional promoter in rat pituitary tumor GH3 cells, and basal promoter activity is enhanced in GH3 and COS7 cells by cotransfection of an expression construct encoding the pituitary-specific transcription factor Pit-1. The rat GHRH receptor gene is subject to at least 1 alternative RNA processing event that generates 2 receptor isoforms differing by 41 amino acids within the third intracellular loop (IL) of the protein. The short isoform of the GHRH receptor is predominant in pituitary cells. The MtT/S pituitary tumor cell line was found to express the GHRH receptor, and different populations of these cells produce predominantly the long or short isoforms of the receptor messenger RNA, suggesting that the alternative splicing can be regulated. Functional analysis of the two GHRH receptor isoforms demonstrates that both bind GHRH, but only the short isoform signals through a cAMP-mediated pathway. Neither receptor isoform is able to stimulate calcium mobilization from internal stores after GHRH treatment. Our findings indicate that the pituitary-specific transcription factor Pit-1 is involved in the somatotroph-specific expression of the GHRH receptor gene and that functionally distinct receptor proteins are generated by an alternative RNA processing mechanism.

Pituitary adenylate cyclase-activating polypeptide receptors during development: expression in the rat embryo at primitive streak stage

The distribution and localization of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor the PAC1 receptor (previously called the type 1 PACAP receptor or PVR1), which binds PACAP, but not vasoactive intestinal peptide, with high affinity] were first investigated in rats with in situ hybridization for its messenger RNA, and with immunohistochemical methods during prenatal and postnatal development. The expression of PACAP receptor messenger RNA was first detected in the rat embryo at the primitive streak stage as early as embryonic day 9, and it was intensely expressed in the neural plate. PACAP receptor messenger RNA was also intensely expressed in the neuroepithelia of the mesencephalon and rhombencephalon at embryonic day 11, and expressed in the basal telencephalon, hippocampal formation neuroepithelium, cortical neuroepithelium and cerebellar neuroepithelium after embryonic day 13. It was also expressed in the olfactory bulb neuroepithelium after embryonic day 16, and in mature regions of the older embryos. In postnatal developing brains, PACAP receptor messenger RNA was intensely expressed in the olfactory bulb, hippocampal formation, cerebellum and other scattered regions. The localization of PACAP receptor-like immunoreactivity coincided well with that of the gene transcripts. We also used reverse transcription-polymerase chain reaction methods to determine the expression of the splice variants of the PACAP receptor gene. At each ontogenetic stage of the rat from embryonic day 9 to postnatal day 60, two major products were detected with reverse transcription-polymerase chain reaction, a thick band (303 base pairs) corresponding to the short splice variant of the receptor that lacks both the "hip" and "hop" cassettes, and a thin band (387 base pairs) corresponding to the splice variant that contains one cassette of "hop" or "hip". There was no evidence for the other larger splice variants. Some of the amplified products were sequenced and found to have the exact sequences of "PACAP receptor" and "PACAP receptor-hopl", which are coupled to different signal transduction pathways. These results indicate that the PACAP receptor is actively expressed in different neuroepithelia from early developmental stages and expressed in various brain regions during prenatal and postnatal development, and that the major splice variants are "PACAP receptor" and "PACAP receptor-hopl". The initial mapping of ontogenetic localization of the PACAP receptor provides the basis for a better understanding of the functions of PACAP and its receptors during the development of the brain.

Functional and molecular diversity of PACAP/VIP receptors in cortical neurons and type I astrocytes

In the present study we determined the mRNA-expression of pituitary adenylate cyclase activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) receptors in primary cultures of rat cortical neurons and type I astrocytes, and investigated the effects of PACAP38 on adenylyl cyclase, inositol phospholipid hydrolysis and intracellular calcium homeostasis. PACAP38 elicited a concentration-dependent (1 nM-100 nM) increase in inositol phosphate levels and [Ca2+]i in neurons but not in type I astrocytes. The PACAP-induced increase of intracellular calcium concentration, [Ca2+]i, was characterized by a spike, compatible with inositol trisphosphate (IP3) -induced calcium mobilization from intracellular stores, and a plateau phase, sustained by activation of capacitative calcium entry triggered by depletion of IP3-sensitive calcium stores. In the absence of extracellular calcium, only the spike phase was present while the plateau phase was clearly reduced. In addition, thapsigargin pretreatment abolished the PACAP38-induced [Ca2+]i rise. Treatment with 1 microM VIP did not affect [Ca2+]i in either neurons or type I astrocytes, clearly indicating the coupling of PAC1-HOP subtype to phospholipase-C in neurons. In addition, as previously reported, PACAP38 stimulated cAMP formation in both neurons and type I astrocytes. Using the reverse transcription polymerase chain reaction, we found mRNA-expression of PAC1 (PACAP - HOP variant) and VPAC2 in neurons, PAC1 (PACAP - R variant), VPAC1 and VPAC2 in astrocytes. These data indicate both a functional and molecular diversity of PACAP and VIP receptors in these cell types and support the view that the PAC1-HOP variant may be responsible for phospholipase-C activation and [Ca2+]i elevation in cortical neurons.

Effects of pituitary adenylate cyclase activating polypeptide-27 (PACAP) and vasoactive intestinal polypeptide (VIP) on chloride in HT29 cells studied by X-ray microanalysis

The colon cancer cell line HT29 is a useful model to study intestinal chloride secretion. These cells have both cAMP-activated and calcium-activated chloride channels. Changes in elemental content of the cells after stimulation with agonists were determined by X-ray microanalysis in the scanning or scanning transmission electron microscope. Exposure of HT29 cells to pituitary adenylate cyclase activating polypeptide-27 (PACAP) caused a transient decrease in the cellular Cl and K concentrations, indicating (net) efflux of chloride. The effect of PACAP is inhibited by somatostatin, which is known to inhibit cAMP-activated as well as calcium-activated chloride secretion and by U-73122, an inhibitor of phospholipase C. Alloxan, an inhibitor of adenylate cyclase, did not significantly affect the PACAP-induced loss of chloride. The calcium-chelating agent EGTA inhibited the PACAP-induced loss of chloride, indicating the need for extracellular calcium ions. Also vasointestinal polypeptide (VIP) caused a decrease of the cellular chloride concentration in HT29 cells. VIP-induced loss of chloride could be inhibited by pre-treating the cells with somatostatin or UK14,304, an alpha-2 adrenergic agonist that has been shown previously to inhibit purinergically activated chloride efflux. Our results indicate that there is cross-talk between the cAMP- and the calcium-activated pathways for chloride secretion in HT29 cells.

Perspectives on pituitary adenylate cyclase activating polypeptide (PACAP) in the neuroendocrine, endocrine, and nervous systems

PACAP is a pleiotropic neuropeptide that belongs to the secretin/glucagon/VIP family. PACAP functions as a hypothalamic hormone, neurotransmitter, neuromodulator, vasodilator, and neurotrophic factor. Its structure has been remarkably conserved during evolution. The PACAP receptor is G protein-coupled with seven transmembrane domains and also belongs to the VIP receptor family. PACAP, but not VIP, binds to PAC1-R, whereas PACAP and VIP bind to VPAC1-R and VPAC2-R with a similar affinity. Despite the sizable homology of the structures of PACAP and VIP and their receptors, the distribution of these peptides and receptors is quite different. At least eight subtypes of PACAP specific, or PAC1-R, result from alternate splicing. Each subtype is coupled with specific signaling pathways, and its expression is tissue or cell specific. Although PACAP fulfills most requirements for a physiological hypothalamic hypophysiotropic hormone, it does not consistently stimulate secretion of the adenohypophysial hormones, except for stimulation of IL-6 release from the FS cells of the pituitary. The major regulatory role of PACAP in pituitary cells appears to be the regulation of gene expression of pituitary hormones and/or regulatory proteins that control growth and differentiation of the pituitary glandular cells. These effects appear to be exhibited directly and indirectly through a paracrine or autocrine action. Although PACAP stimulates the release of AVP, the physiological role of neurohypophysial PACAP remains unknown. One important action of PACAP in the endocrine system is its role as a potent secretagogue for adrenaline from the adrenal medulla through activation of TH. PACAP also stimulates the release of insulin and increases [Ca2+]i from pancreatic beta-cells at an extremely small concentration. The stage-specific expression of PACAP in testicular germ cells during spermatogenesis suggests its regulatory role in the maturation of germ cells. In the ovary, PACAP is transiently expressed in the granulosa cells of the preovulatory follicles and appears to be involved in the LH-induced cellular events in the ovary, including prevention of follicular apoptosis. In the central nervous system, PACAP acts as a neurotransmitter or neuromodulator, which has been supported by IHC and electrophysiological methods. More important, PACAP is a neurotrophic factor that may play an important role during the development of the brain. In the adult brain, PACAP appears to function as a neuroprotective factor that attenuates the neuronal damage resulting from various insults.

Functional and molecular expression of PACAP/VIP receptors in the rat retina

Receptor binding sites for pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP), positively coupled to adenylate cyclase, have been previously described in the retina of different mammalian species. In the present study, we determined the mRNA expression of PACAP/VIP receptor variants in the rat retina and investigated their coupling to phospholipase C in addition to adenylate cyclase. The two forms of PACAP, PACAP27 and PACAP38, induced a dose-dependent (1-100 nM) increase of cAMP and [3H]inositol monophosphate levels, whereas VIP stimulated, with lower potency and efficacy, cAMP formation only. Reverse transcription-PCR analysis in the rat retina detected both type-I (PACAP-R and PACAP-HOP splice variants) and type-II (VIP-I and -2) receptor-mRNAs. These data indicate that PACAP and VIP may interact with multiple receptor subtypes and activate one (VIP) or two (PACAP) signal transduction mechanisms in the rat retina.

Human vascular endothelium heterogeneity. A comparative study of cerebral and peripheral cultured vascular endothelial cells

BACKGROUND AND PURPOSE

Hormones, neurotransmitters, and autacoids play a key role in the regulation of vascular tone as a result of their interaction with the endothelium. The aim of this study was to compare selected properties of three human endothelial cell lines isolated from cerebral pial arteries (PEC) and two peripheral vessels, the superficial temporal (SEC) and omental (OEC) arteries.

METHODS

Intracellular free calcium concentration ([Ca2+]i) and receptor protein expression were measured in characterized primary cultures of human endothelial cells.

RESULTS

All cell lines labeled positively for factor VIII/von Willebrand factor. Growth rate and constitutive release of endothelin-1, expressed as a function of protein, were both significantly lower in cerebral cells (PEC) than in endothelial cells derived from peripheral vessels. Basal [Ca2+]i measured with the fluorescent calcium indicator fura 2-AM (2 mumol/L) did not differ in either of the three cell lines. Although PEC responded to endothelin-1 (0.1 mumol/L) and vasoactive intestinal peptide (1 mumol/L) by a twofold to threefold increase in [Ca2+]i, OEC were unresponsive to these peptides. Moreover, the calcium response to alpha-thrombin (10 nmol/L) was greater in cerebral (PEC) than in peripheral (SEC, OEC) endothelial cells, while bradykinin (100 nmol/L) increased [Ca2+]i to a similar level in all three cell types.

CONCLUSIONS

This study demonstrates that endothelial cells from different sites of the vasculature exhibit different growth rates and vary in their response to agonists.

alpha-Thrombin upregulates G alpha i3 in human vascular endothelial cells

BACKGROUND AND PURPOSE

During thrombosis, alpha-thrombin becomes sequestered by fibrin and the subendothelial basement membrane, and it is available to interact with the vasculature over prolonged periods. In this study, we investigated the long-term effect of alpha-thrombin on G alpha i3 and G alpha s levels in human vascular endothelial cells (EC). Because obesity is associated with changes in receptor signaling in many animal models, we also explored the influence of this clinical risk factor.

METHODS

Primary cultures of human EC were exposed to alpha-thrombin for 16 hours, and immunologically detectable G alpha i3 and G alpha s levels were measured.

RESULTS

alpha-Thrombin (100 nmol/L) increased G alpha i3 levels in EC derived from the cerebral microvasculature and superficial temporal artery (4.2 +/- 1.2-fold and 2.8 +/- 0.32-fold, respectively) but had no significant effect on EC derived from omental artery (P > .6) or from the superficial temporal artery of obese (body mass index > or = 28 kg/m2) patients (P > .4). The expression of G alpha s was unchanged in all cell types (P > or = .1). Two other circulating peptides, vasoactive intestinal peptide and endothelin-1, failed to alter the expression of either G protein in EC from the cerebral microvasculature, further demonstrating the specificity of the alpha-thrombin effect. However, thrombin receptor activating protein-14 mimicked the alpha-thrombin response and increased G alpha i3 in EC derived from the cerebral microvasculature and superficial temporal artery.

CONCLUSIONS

We conclude that alpha-thrombin increases G alpha i3 expression in some EC through activation of its tethered liganded receptor. Obesity appears to suppress this action of alpha-thrombin.

The functional investigation of a human adenocarcinoma cell line, stably transfected with the neuropeptide Y Y1 receptor

1. The human adenocarcinoma cell line, HT-29, has been stably transfected with the cDNA sequence for the rat neuropeptide Y (NPY) Y1 receptor, and three Y1 clones (Y1-4, Y1-7 and Y1-16) have been isolated which express high levels of specific [125I]-PYY binding. We have studied the functional responses or lack of responses to peptide YY (PYY) and its analogues in the three transfected clones and HT-29 wild type (wt) cells. 2. Vasoactive intestinal polypeptide (VIP) produced long-lasting increases in short-circuit current (SCC) in both HT-29 wt cells and the Y1 clones. VIP EC50 values were 8.4-11.7 nM in all four cases. The elevation in SCC after a maximal concentration of VIP (30 nM) was significantly greater in Y1-7 cells than in either HT-29 wt epithelia or the other Y1 cell lines. 3. PYY (100 nM) and human pancreatic polypeptide (hPP; 1 microM) were ineffective in HT-29 wt cells under either basal or stimulated conditions. In contrast, basolateral additions of PYY reduced both basal and VIP-stimulated SCC in all three Y1 clones. After VIP, the PYY EC50 values (in nM) were 18.6 in Y1-4, 8.0 in Y1-7 and 52.5 in Y1-16 hPP (1 microM) produced only small and transient responses in each transfected cell type. 4. The Y1 receptor agonist, [Leu31, Pro34] NPY (1 microM) was also effective in the three Y1 cell lines. In the Y1-7 clone the EC50 value for the effect of this peptide was 149 nM, 18.6 fold less potent than PYY. 5. PYY and the Y1-selective non-peptide antagonist, BIBP 3226 displaced [125I]-PYY binding from Y1-7 cell membranes with Ki values of 2.0 and 3.1 nM respectively. In the Y1-7 clone, BIBP 3226 fully inhibited the reductions in VIP-stimulated SCC induced by 30 nM PYY, with an IC50 of 27.2 nM and 30 nM BIBP 3226 caused a parallel rightward shift on the PYY concentration-response curve, with an approximate pKB of 8.0. 6. HT-29 clones stably expressing the Y1 receptor therefore show responses to PYY and its analogues that are characteristic of that subtype, and the Y1-7 clone in particular will be useful in the assessment of novel Y1-specific drugs. This approach will also allow the functional study of NPY Yi receptors with selected mutations.

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.

Predominant expression of type II vasoactive intestinal peptide receptors by human T lymphoblastoma cells: transduction of both Ca2+ and cyclic AMP signals

An immunoregulatory role for vasoactive intestinal peptide (VIP) is suggested by the high concentrations in subsets of neurons supplying lymphoid organs and by the capacity of VIP to affect T lymphocyte functions. The Tsup-1 line of human T lymphoblastoma cells expresses both type I and type II G protein-coupled VIP receptors (Rs), as shown by detection of the encoding mRNAs with reverse transcription-polymerase chain reaction analyses. Northern blot quantification of the relative amounts of mRNA encoding the two VIPRs in Tsup-1 cells indicated that type II predominates over type I, as it does in human blood CD4+ T cells. Tsup-1 cells bound 125I-VIP to 8.95 x 10(4) high-affinity sites/cell (Kd = 6.0 nM) and 7.45 x 10(5) low-affinity sites/cell (Kd = 210 nM). VIP increased [cAMP]i in Tsup-1 cells (EC50 = 14.4 nM) and stimulated a rapid and transient increase in [Ca2+]i (EC50 = 30 nM). Functional coupling of G proteins to type II VIPRs was suggested by the change in binding of 125I-VIP to Tsup-1 cell membranes from two sites with Kd values of 3.8 and 109 nM to one site of Kd 30 nM by GTP-gamma-S and the suppression by pertussis toxin of increases in [Ca2+]i evoked by VIP. The VIP antagonists, VIP4-28 and (4-Cl-D-Phe6-Leu17) VIP, inhibited 125I-VIP binding by type II VIPRs, as well as VIP-elicited increases in [Ca2+]i and [cAMP]i. Type II VIPRs thus are the major transducers of VIP signals to a subset of human T cells.

High levels of vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor mRNA expression in primary and tumor lymphoid cells

Neuropeptides exert a variety of putative immunomodulatory actions. Despite the molecular cloning of multiple forms of receptors for several neuropeptides with putative immunomodulatory effects, including vasoactive intestinal peptide (VIP), the related peptide pituitary adenylate cyclase-activating peptide (PACAP), the opiate peptides, tachykinins, somatostatin and corticotropin-releasing factor, it has not been reported that any of the receptor genes are expressed at significant levels in cells of the immune system. The low level of expression of these receptors and lack of knowledge concerning receptor subtype has impeded progress in understanding how neuropeptides regulate immune function. For example, it is not understood why VIP produces immunomodulatory effects at concentrations far below its receptor-binding affinity. Receptors for VIP and PACAP have recently been cloned. We show here by Northern blot analysis that the VIP/PACAP1 receptor mRNA is present in total RNA prepared from mouse spleen B- and T-lymphocytes. The VIP/PACAP1 receptor mRNA was also present in human peripheral blood lymphocytes, and in a B-lymphocyte and a myelocytic cell line. The mRNA for a second form of the receptor, the VIP/PACAP2 receptor, was not expressed at detectable levels in normal cells, but was detected in several human T-cell lines and a murine mast cell line. The results indicate that VIP/PACAP1 and perhaps VIP/PACAP2 receptors mediate the diverse effects of VIP and PACAP on immune cells.

Differential expression of VIP/PACAP receptor genes in breast, intestinal, and pancreatic cell lines

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) are structurally-related neuropeptides that function as trophic factors in addition to their more classical roles as neurotransmitters. Binding and molecular cloning studies have shown that their actions are mediated by receptors encoded by at least three different genes. VIP binding has been demonstrated on many tumor types, and radiolabeled VIP has recently been used as a novel method to localize intestinal tumors in humans and their sites of metastasis. To determine the receptor subtype and level of gene expression, we screened breast, intestinal, and pancreatic, cell lines by Northern blot analysis. Breast lines expressed VIP/PACAP1 receptor mRNA levels comparable to intestinal lines, in agreement with the studies showing particularly high VIP binding in these tumors and their derived cell lines. Pancreatic cell lines expressed mRNA for several receptor types. This extends the potential utility of VIP and PACAP in the localization of tumors, and because VIP and PACAP may regulate the growth rate of some tumors by autocrine or other mechanisms, the identification of receptor subtypes on these lines sets the stage for studies in which the activity of these individual receptors in growth and other processes can be investigated.

Structure, expression, and chromosomal localization of the type I human vasoactive intestinal peptide receptor gene

Vasoactive intestinal peptide (VIP) and other members of the pituitary adenylyl cyclase-activating peptide (PACAP) and secretin neuroendocrine peptide family are recognized with specificity by related G protein-coupled receptors. We report here the cloning, characterization, and chromosomal location of the gene encoding the human type I VIP receptor (HVR1), also termed the type II PACAP receptor. The gene spans approximately 22 kb and is composed of 13 exons ranging from 42 to 1400 bp and 12 introns ranging from 0.3 to 6.1 kb. Primer extension analysis with poly(A)+ RNA from human HT29 colonic adenocarcinoma cells indicated that the transcription initiation site is located at position -110 upstream of the first nucleotide (+1) of the translation start codon, and 75 nt downstream of a consensus CCAAT-box motif. The G+C-rich 5' flanking region contains potential binding sites for several nuclear factors, including Sp1, AP2, ATF, interferon regulatory factor 1, NF-IL6, acute-phase response factor, and NF-kappa B. The HVR1 gene is expressed selectively in human tissues with a relative prevalence of lung > prostate > peripheral blood leukocytes, liver, brain, small intestine > colon, heart, spleen > placenta, kidney, thymus, testis. Fluorescence in situ hybridization localized the HVR1 gene to the short arm of human chromosome 3 (3p22), in a region associated with small-cell lung cancer.