Gene Symbol: ADCYAP1

1. General Information

PACAP Neuropeptide

Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) is a gastrointestinal neuropeptide, which belongs to the family of regulatory peptides that also includes Vasoactive Intestinal Polypeptide (VIP), secretin, glucagon and GLP-1 hormones (38). It was identified in 1989 by Arimura and colleagues from ovine hypothalamus (24). This peptide occurs naturally as both a 27-amino acid or 38-amino acid biologically active forms that have equal biological activities and are thus, named PACAP-27 and PACAP-38, respectively (Figure 1). Both peptides are identical in sequence and conformation at N-terminal region whereas the C-terminal segment of PACAP-38 exhibits a short helix attached by a flexible hinge to the 1–27 region.  Most of the endogenous PACAP exists as PACAP-38. The primary sequence of PACAP is 68% identical to its closest hormone relative, VIP (38). In humans, the PACAP gene is located in the P11 region of chromosome 18. It is composed of five exons; the sequence of PACAP being encoded by exon 5. In humans, the cDNA encoding the PACAP precursor encodes a 176-amino acid prepro-protein.  The sequence of PACAP-38 is located in the C-terminal domain of the precursor. Since its discovery, PACAP has been identified in both the peripheral and central nervous system and in the gastrointestinal tract where it has been shown to have potent physiological activity.

Receptors for PACAP

The high affinity type I PACAP receptor, PAC1, was cloned and pharmacologically characterized by Pisegna and Wank (29). The PAC1 receptor was demonstrated to be a heptahelical, G protein coupled receptor of the Type 2 family and related molecularly and pharmacologically to receptors in the VIP, secretin, glucagon and GLP-1 superfamily. PACAP has binding affinity for the type 1 PACAP receptor (PAC1) and the two VIP receptors VPAC1 and VPAC2. PAC1 exhibits a 1000-fold greater affinity for PACAP compared to VIP. The gene for the human PAC1 is localized on chromosome 7 (38). The pharmacology and functions of the VIP-PACAP receptor family have been well characterized (12).

Signal transduction of PAC1

Unlike other members of its superfamily, PAC1 receptors are coupled to dual signal transduction pathways acting through cAMP and Ca²⁺. PAC1 receptor is coupled to G_s protein which activates adenylyl cyclase to form cAMP that in turn activates protein kinase A.  PAC1 receptor signaling also couples to Gq and thereby activates phospholipase C which produces inositol phosphate which increases cytosolic calcium release from intra-cellular calcium stores. Another PACAP signaling pathway is the elevation of intra-cellular sodium levels via activation of nonselective cation channels (18, 38).

Localization of PACAP and its receptor PAC1

PACAP and PAC1 are expressed in a wide range of organs including central and peripheral nervous systems, and various endocrine glands including thyroid, pituitary, gonads, adrenals, and pancreas. In the gastro-intestinal tract, PACAP and PAC1 immune-reactivity show distribution in the myenteric ganglia and nerve fibers localized in the muscle layers of the esophagus, stomach, duodenum, small and large intestine (17, 22. In the pancreas, PACAP containing neurons innervate both the endocrine islet cells and the exocrine pancreatic acinar cells. In localization studies, PACAP immune-reactivity is found in the pancreatic nerves with accumulation in intra-pancreatic ganglia in both mice and rats. Furthermore, in situ hybridization, using oligo-deoxyribonucleotide probes recognizing mRNA for PACAP receptors, demonstrated that mouse and rat pancreas, and the insulinoma cell lines HIT-T15 and RINm5F, express PAC1 and the VPAC2 receptors (8, 37). PACAP mRNA expression is seen in pancreatic beta-cells (30), and PACAP peptide has been localized in the secretory granules of alpha and beta cells of  human and rodent pancreas (39).

2. Effects on Pancreas

Effects of PACAP on endocrine pancreas

PACAP has been shown to have a potent stinulatory effect on the endogenous release of insulin in both humans and rodents (1, 6-8, 26, 28). The effect of PACAP on insulin secretion is mainly mediated through the cAMP pathway and its effect on the K_ATP channels to stimulate exocytosis of insulin containing granules in the beta cells (1, 26). Glucose induced insulin response was blunted in mice pre-treated with PACAP antagonist (PACAP 6-38) (10). In PAC1 receptor null mice, both PACAP induced insulin release and glucose induced insulin release was significantly reduced (15). PACAP exerts its stimulatory effect on insulin secretion through activation of PAC1-R and VPAC2-R as shown by in situ hybridization, RT-PCR, and functional analyses studies (3, 13, 40). PACAP may also regulate beta cell mass. Transgenic mice overexpressing PACAP in beta cells were found to have significantly large islets (41). In-vitro studies have indicated that PACAP may also exert anti-apoptotic effect on the pancreatic beta-cells, thus protecting them from toxicity of oxidative stress, inflammation, and hyperglycemia (27). The observation that PACAP has potent trophic effects to control both proliferation and cell viability of beta-cells suggest that it may have a role in the treatment of diabetes (33). In addition to its role to facilitate insulin release, PACAP exerts long-term effects on beta-cells, such as transcriptional regulation of the insulin gene and genes of the glucose-sensing system such as GLUT1 and hexokinase 1 (3).

Interestingly, PACAP is also a potent stimulator of glucagon secretion. PACAP injection has been shown to enhance glucagon release in mice and perfused rat pancreas (8). In humans, post glucose injection, glucose levels were found to be higher during PACAP infusion than during saline infusion. This study demonstrated that in concordance with animal studies, PACAP stimulates both insulin and glucagon secretion in humans (7). The stimulatory effect of PACAP on both insulin and glucagon release remains under the inhibitory control of somatostatin (42).

Effects of PACAP on exocrine pancreas

PACAP acts as a secretagogue for the exocrine pancreas as well. The first description of a role for PACAP on exocrine pancreas demonstrated that PACAP-38 stimulated cAMP production and amylase release from dispersed rat pancreatic acini and appeared to have a synergistic effect with bombesin, CCK-8 or carbachol (31).  In rat AR4-2 J pancreatic carcinoma cell line and in isolated rat pancreatic acini, PACAP was demonstrated to be a ligand for both PAC1 and VPAC receptors. PACAP and VIP equipotently stimulated acinar lipase release and cyclic AMP production in pancreatic acini. (35). PACAP induced vasodilation and increases pancreatic blood flow in dogs and rats (4, 14). In-vitro experiments conducted in isolated rat pancreatic acini showed release of amylase and lipase after treatment with PACAP (25). In-vivo injections with PACAP agonist were demonstrated to trigger the release of amylase, bicarbonate, and pancreatic fluid (2). The stimulatory effect of PACAP on flow of pancreatic secretions could be inhibited by the PACAP antagonist (PACAP 6-38) in pigs (37).

The role of PACAP in the development of pancreatitis is currently being investigated in rodent models. The over-expression of PACAP in pancreas was shown to enhance the cerulein-induced inflammatory response of acinar cells, leading to aggravated acute pancreatitis (11). In chronic pancreatitis, an imbalance between the pro and anti-inflammatory cytokines is thought to play a role in the development of inflammation and pain. Because PACAP is contained within neurons and activated lymphoid cells, it is a useful target to investigate its role in the development of chronic pancreatitis. Animal studies in mice deficient in either PACAP or PAC1 have suggested that PACAP is involved in pain responses (16, 21). Expression of PACAP and its receptors is increased in human chronic pancreatitis (23). In the peripheral blood mononuclear cells there appeared to be a correlation with the up-regulation of PACAP expression in macrophages encountering apoptotic pancreatic acini. The authors in this study also demonstrated that the pain response of patients with chronic pancreatitis was related to the pancreatic PACAP levels (21, 23).

The expression of PACAP and PAC1 on pancreatic tumors

PACAP receptors have been demonstrated on pancreatic tumors suggesting a molecular basis for the development of in vivo scintigraphy and radiotherapy or the use of PACAP analogues for the treatment of malignant tumors (32). The human PACAP response gene 1 (p22/PRG1) was shown to be expressed in the human pancreatic carcinoma cell lines and is a growth-associated early response gene (34). The receptors for PACAP have also been demonstrated on pancreatic neuroendocrine tumors. Studies conducted on the neuroendocrine tumor cell line, BON, demonstrate the expression of PAC1. PACAP stimulation of these receptors results in an increase in intracellular cAMP, the secretion of biogenic amines such as serotonin and result in cell proliferation (9, 19). The insulin-secreting pancreatic insulinoma cells have also been shown to express the mRNA encoding PAC1 (3).

3. Tools to Study PACAP

a. Agonists and antagonist

PACAP peptide (PACAP-27 and PACAP-38) and competitive partial antagonist (PACAP 6-38) are available commercially. More recently, several longer acting analogues of PACAP have been developed.  We obtain these peptides from American Peptides Corporation. 

b. Transgenic mice

Mice with genetic deletion of PACAP or PAC1 receptor are available. PACAP gene knockout mice developed by Sherwood et al were temperature and diet type sensitive (36). However, similar phenotype was not observed in the PACAP null mice developed by the group led by Waschek (5). Mice lacking PAC1 receptor developed by Brabet et al are best suited to the study the specific physiologic role of PACAP and its receptor (16, 20).

References

1. Ahren B. Role of pituitary adenylate cyclase-activating polypeptide in the pancreatic endocrine system. Ann New York Acad of Sci 1144: 28-35, 2008. PMID: 19076360
2. Alonso RM, Rodriguez AM, Garcia LJ, Lopez MA, and Calvo JJ. Comparison between the effects of VIP and the novel peptide PACAP on the exocrine pancreatic secretion of the rat. Pancreas 9: 123-128, 1994. PMID: 7509062
3. Borboni P, Porzio O, Pierucci D, Cicconi S, Magnaterra R, Federici M, Sesti G, Lauro D, D'Agata V, Cavallaro S, and Marlier LN. Molecular and functional characterization of pituitary adenylate cyclase-activating polypeptide (PACAP-38)/vasoactive intestinal polypeptide receptors in pancreatic beta-cells and effects of PACAP-38 on components of the insulin secretory system. Endocrinology 140: 5530-5537, 1999. PMID: 10579316
4. Carlsson PO, Ostenson CG, Efendic S, Langel U, and Jansson L. Pituitary adenylate cyclase activating polypeptide (PACAP) redistributes the blood within the pancreas of anesthetized rats. Regulatory peptides 63: 123-128, 1996. PMID: 8837220
5. Colwell CS, Michel S, Itri J, Rodriguez W, Tam J, Lelievre V, Hu Z, and Waschek JA. Selective deficits in the circadian light response in mice lacking PACAP. Am J Physiol Regulatory, Integrative and Comparative Physiology 287: R1194-1201, 2004. PMID: 15217792
6. Filipsson K, Sundler F, Hannibal J, and Ahren B. PACAP and PACAP receptors in insulin producing tissues: localization and effects. Regulatory Peptides 74: 167-175, 1998. PMID: 9712178
7. Filipsson K, Tornoe K, Holst J, and Ahren B. Pituitary adenylate cyclase-activating polypeptide stimulates insulin and glucagon secretion in humans. J Clin Endocrinolo Metab 82: 3093-3098, 1997. PMID: 9284750
8. Fridolf T, Sundler F, and Ahren B. Pituitary adenylate cyclase-activating polypeptide (PACAP): occurrence in rodent pancreas and effects on insulin and glucagon secretion in the mouse. Cell Tiss Rese 269: 275-279, 1992. PMID: 1423494
9. Germano PM, Lieu SN, Xue J, Cooke HJ, Christofi FL, Lu Y, and Pisegna JR. PACAP induces signaling and stimulation of 5-hydroxytryptamine release and growth in neuroendocrine tumor cells. J Mol Neurosci : MN 39: 391-401, 2009. PMID: 19701709
10. Green BD, Irwin N, Cassidy RS, Gault VA, and Flatt PR. Long-term administration of PACAP receptor antagonist, PACAP(6-27), impairs glucose tolerance and insulin sensitivity in obese diabetic ob/ob mice. Peptides 27: 2343-2349, 2006. PMID: 16730098
11. Hamagami K, Sakurai Y, Shintani N, Higuchi N, Ikeda K, Hashimoto H, Suzuki A, Kiyama H, and Baba A. Over-expression of pancreatic pituitary adenylate cyclase-activating polypeptide (PACAP) aggravates cerulein-induced acute pancreatitis in mice. J Pharmacol Sci  110: 451-458, 2009. PMID: 19672038
12. Harmar AJ, Fahrenkrug J, Gozes I, Laburthe M, May V, Pisegna JR, Vaudry D, Vaudry H, Waschek JA, and Said SI. Pharmacology and functions of receptors for vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide: IUPHAR review 1. Brit J of Pharmacol 166: 4-17, 2012. PMID: 22289055
13. Inagaki N, Yoshida H, Mizuta M, Mizuno N, Fujii Y, Gonoi T, Miyazaki J, and Seino S. Cloning and functional characterization of a third pituitary adenylate cyclase-activating polypeptide receptor subtype expressed in insulin-secreting cells. Proc Natl Acad Sci USA 91: 2679-2683, 1994. PMID: 8146174
14. Ito O, Naruse S, Kitagawa M, Ishiguro H, Ko S, Nakajima M, and Hayakawa T. The effect of VIP/PACAP family of peptides on pancreatic blood flow and secretion in conscious dogs. Regulatory Peptides 78: 105-112, 1998. PMID: 9879753
15. Jamen F, Persson K, Bertrand G, Rodriguez-Henche N, Puech R, Bockaert J, Ahren B, and Brabet P. PAC1 receptor-deficient mice display impaired insulinotropic response to glucose and reduced glucose tolerance. J Clin Invest 105: 1307-1315, 2000. PMID: 10792006
16. Jongsma H, Pettersson LM, Zhang Y, Reimer MK, Kanje M, Waldenstrom A, Sundler F, and Danielsen N. Markedly reduced chronic nociceptive response in mice lacking the PAC1 receptor. Neuroreport 12: 2215-2219, 2001. PMID: 11447337
17. Koves K, Arimura A, Vigh S, Somogyvari-Vigh A, and Miller J. Immunohistochemical localization of PACAP in the ovine digestive system. Peptides 14: 449-455, 1993. PMID: 8332545
18. Laburthe M, Couvineau A, and Tan V. Class II G protein-coupled receptors for VIP and PACAP: structure, models of activation and pharmacology. Peptides 28: 1631-1639, 2007. PMID: 17574305
19. Lieu SN, Oh DS, Pisegna JR, and Germano PM. Neuroendocrine tumors express PAC1 receptors. Ann New York Acad of Sci 1070: 399-404, 2006. PMID: 16888199
20. Lu Y, Germano P, Ohning GV, Vu JP, and Pisegna JR. PAC1 deficiency in a murine model induces gastric mucosa hypertrophy and higher basal gastric acid output. J Mol Neurosci 43: 76-84, 2011. PMID: 20821075
21. Mabuchi T, Shintani N, Matsumura S, Okuda-Ashitaka E, Hashimoto H, Muratani T, Minami T, Baba A, and Ito S. Pituitary adenylate cyclase-activating polypeptide is required for the development of spinal sensitization and induction of neuropathic pain. J Neurosci 24: 7283-7291, 2004. PMID: 15317855
22. Miampamba M, Germano PM, Arli S, Wong HH, Scott D, Tache Y, and Pisegna JR. Expression of pituitary adenylate cyclase-activating polypeptide and PACAP type 1 receptor in the rat gastric and colonic myenteric neurons. Regulatory Peptides 105: 145-154, 2002. PMID: 11959368
23. Michalski CW, Selvaggi F, Bartel M, Mitkus T, Gorbachevski A, Giese T, Sebastiano PD, Giese NA, and Friess H. Altered anti-inflammatory response of mononuclear cells to neuropeptide PACAP is associated with deregulation of NF-{kappa}B in chronic pancreatitis. Am J Physiol gastrointest Liver Physiol 294: G50-57, 2008. PMID: 17962362
24. Miyata A, Arimura A, Dahl RR, Minamino N, Uehara A, Jiang L, Culler MD, and Coy DH. Isolation of a novel 38 residue-hypothalamic polypeptide which stimulates adenylate cyclase in pituitary cells. Biochem Biophys Res Commun 164: 567-574, 1989. PMID: 2803320
25. Mungan Z, Ertan A, Hammer RA, and Arimura A. Effect of pituitary adenylate cyclase activating polypeptide on rat pancreatic exocrine secretion. Peptides 12: 559-562, 1991. PMID: 1717954
26. Nakata M and Yada T. PACAP in the glucose and energy homeostasis: physiological role and therapeutic potential. Current Pharmaceuti Design 13: 1105-1112, 2007. PMID: 17430174
27. Onoue S, Hanato J, and Yamada S. Pituitary adenylate cyclase-activating polypeptide attenuates streptozotocin-induced apoptotic death of RIN-m5F cells through regulation of Bcl-2 family protein mRNA expression.  FEBS Journal 275: 5542-5551, 2008. PMID: 18959742
28. Persson-Sjogren S, Forsgren S, and Lindstrom P. Vasoactive intestinal polypeptide and pituitary adenylate cyclase activating polypeptide: effects on insulin release in isolated mouse islets in relation to metabolic status and age. Neuropeptides 40: 283-290, 2006. PMID: 16797701
29. Pisegna JR and Wank SA. Cloning and characterization of the signal transduction of four splice variants of the human pituitary adenylate cyclase activating polypeptide receptor. Evidence for dual coupling to adenylate cyclase and phospholipase C. Biolo Chem 271: 17267-17274, 1996. PMID: 8663363
30. Portela-Gomes GM, Lukinius A, Ljungberg O, Efendic S, Ahren B, and Abdel-Halim SM. PACAP is expressed in secretory granules of insulin and glucagon cells in human and rodent pancreas. Evidence for generation of cAMP compartments uncoupled from hormone release in diabetic islets. Regulatory Peptides 113: 31-39, 2003. PMID: 12686458
31. Raufman JP, Malhotra R, and Singh L. PACAP-38, a novel peptide from ovine hypothalamus, is a potent modulator of amylase release from dispersed acini from rat pancreas. Regulatory Peptides 36: 121-129, 1991. PMID: 1724566
32. Reubi JC. In vitro evaluation of VIP/PACAP receptors in healthy and diseased human tissues. Clinical implications. Ann New York Acad of Sci  921: 1-25, 2000. PMID: 11193811
33. Sakurai Y, Shintani N, Hayata A, Hashimoto H, and Baba A. Trophic effects of PACAP on pancreatic islets: a mini-review. J Mol Neurosci : 3-7, 2011. PMID: 20645023
34. Schafer H, Lettau P, Trauzold A, Banasch M, and Schmidt WE. Human PACAP response gene 1 (p22/PRG1): proliferation-associated expression in pancreatic carcinoma cells. Pancreas 18: 378-384, 1999. PMID: 10231843
35. Schmidt WE, Seebeck J, Hocker M, Schwarzhoff R, Schafer H, Fornefeld H, Morys-Wortmann C, Folsch UR, and Creutzfeldt W. PACAP and VIP stimulate enzyme secretion in rat pancreatic acini via interaction with VIP/PACAP-2 receptors: additive augmentation of CCK/carbachol-induced enzyme release. Pancreas 8: 476-487, 1993. PMID: 8103217
36. Sherwood NM, Adams BA, Isaac ER, Wu S, and Fradinger EA. Knocked down and out: PACAP in development, reproduction and feeding. Peptides 28: 1680-1687, 2007. PMID: 17467121
37. Tornoe K, Hannibal J, Fahrenkrug J, and Holst JJ. PACAP-(1-38) as neurotransmitter in pig pancreas: receptor activation revealed by the antagonist PACAP-(6-38). Am J Physiol gastrointest Liver Physiol 273: G436-446, 1997. PMID: 9277423
38. Vaudry D, Falluel-Morel A, Bourgault S, Basille M, Burel D, Wurtz O, Fournier A, Chow BK, Hashimoto H, Galas L, and Vaudry H. Pituitary adenylate cyclase-activating polypeptide and its receptors: 20 years after the discovery. Pharmacol Rev 61: 283-357, 2009. PMID: 19805477
39. Yada T, Sakurada M, Ishihara H, Nakata M, Shioda S, Yaekura K, Hamakawa N, Yanagida K, Kikuchi M, and Oka Y. Pituitary adenylate cyclase-activating polypeptide (PACAP) is an islet substance serving as an intra-islet amplifier of glucose-induced insulin secretion in rats. J Physiol 505: 319-328, 1997. PMID: 9423175
40. Yamada H, Watanabe M, and Yada T. Cytosolic Ca2+ responses to sub-picomolar and nanomolar PACAP in pancreatic beta-cells are mediated by VPAC2 and PAC1 receptors. Regulatory Peptides 123: 147-153, 2004. PMID: 15518905
41. Yamamoto K, Hashimoto H, Tomimoto S, Shintani N, Miyazaki J, Tashiro F, Aihara H, Nammo T, Li M, Yamagata K, Miyagawa J, Matsuzawa Y, Kawabata Y, Fukuyama Y, Koga K, Mori W, Tanaka K, Matsuda T, and Baba A. Overexpression of PACAP in transgenic mouse pancreatic beta-cells enhances insulin secretion and ameliorates streptozotocin-induced diabetes. Diabetes 52: 1155-1162, 2003. PMID: 12716746
42. Yokota C, Kawai K, Ohashi S, Watanabe Y, Suzuki S, and Yamashita K. Stimulatory effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on insulin and glucagon release from the isolated perfused rat pancreas. Acta Endocrinol 129: 473-479, 1993. PMID: 7904113