Interaction of peptides related to VIP and secretin with guinea pig pancreatic acini

Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na+,K+-ATPase in pancreatic acinar cells

Rat pancreatic acinar cells possess only the p21-activated kinase (PAKs), PAK4 of the group II PAK, and it is activated by gastrointestinal hormones/neurotransmitters stimulating PLC and by a number of growth factors. However, little is known generally of cAMP agents causing PAK4 activation, and there are no studies with gastrointestinal hormones/neurotransmitters activating cAMP cascades. In the present study, we examined the ability of VIP and secretin, which stimulate cAMP generation in pancreatic acini, to stimulate PAK4 activation, the signaling cascades involved, and their possible role in activating sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase). PAK4 activation was compared with activation of the well-established cAMP target, cyclic AMP response element binding protein (CREB). Secretin-stimulated PAK4 activation was inhibited by KT-5720 and PKA Type II inhibitor (PKI), protein kinase A (PKA) inhibitors, whereas VIP activation was inhibited by ESI-09 and HJC0197, exchange protein directly activated by cAMP (EPAC) inhibitors. In contrast, both VIP/secretin-stimulated phosphorylation of CREB (pCREB) via EPAC activation; however, it was inhibited by the p44/42 inhibitor PD98059 and the p38 inhibitor SB202190. The specific EPAC agonist 8-CPT-2- O-Me-cAMP as well 8-Br-cAMP and forskolin stimulated PAK4 activation. Secretin/VIP activation of Na⁺,K⁺-ATPase, was inhibited by PAK4 inhibitors (PF-3758309, LCH-7749944). These results demonstrate PAK4 is activated in pancreatic acini by stimulation of both VIP-/secretin-preferring receptors, as is CREB. However, they differ in their signaling cascades. Furthermore, PAK4 activation is needed for Na⁺,K⁺ATPase activation, which mediates pancreatic fluid secretion. These results, coupled with recent studies reporting PAKs are involved in both pancreatitis/pancreatic cancer growth/enzyme secretion, show that PAK4, similar to PAK2, likely plays an important role in both pancreatic physiological/pathological responses. NEW & NOTEWORTHY Pancreatic acini possess only the group II p21-activated kinase, PAK4, which is activated by PLC-stimulating agents/growth factors and is important in enzyme-secretion/growth/pancreatitis. Little information exists on cAMP-activating agents stimulating group II PAKs. We studied ability/effect of cyclic AMP-stimulating agents [vasoactive intestinal polypeptide (VIP), secretin] on PAK4 activity in rat pancreatic-acini. Both VIP/secretin activated PAK4/CREB, but the cAMP signaling cascades differed for EPAC, MAPK, and PKA pathways. Both hormones require PAK4 activation to stimulate sodium-potassium adenosine triphosphatase activity. This study shows PAK4 plays an important role in VIP-/secretin-stimulated pancreatic fluid secretion and suggests it plays important roles in pancreatic acinar physiological/pathophysiological responses mediated by cAMP-activating agents.

PACAP and its role in primary headaches

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide implicated in a wide range of functions, such as nociception and in primary headaches. Regarding its localization, PACAP has been observed in the sensory trigeminal ganglion (TG), in the parasympathetic sphenopalatine (SPG) and otic ganglia (OTG), and in the brainstem trigeminocervical complex. Immunohistochemistry has shown PACAP-38 in numerous cell bodies of SPG/OTG, co-stored with vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS) and, to a minor degree, with choline acetyltransferase. PACAP has in addition been found in a subpopulation of calcitonin gene-related peptide (CGRP)-immunoreactive cells in the trigeminal system. The PACAP/VIP receptors (PAC₁, VPAC₁, and VPAC₂) are present in sensory neurons and in vascular smooth muscle related to the trigeminovascular system. It is postulated that PACAP is involved in nociception. In support, abolishment of PACAP synthesis or reception leads to diminished pain responses, whereas systemic PACAP-38 infusion triggers pain behavior in animals and delayed migraine-like attacks in migraine patients without marked vasodilatory effects. In addition, increased plasma levels have been documented in acute migraine attacks and in cluster headache, in accordance with findings in experimental models of trigeminal activation. This suggest that the activation of the trigeminal system may result in elevated venous levels of PACAP, a change that can be reduced when headache is treated. The data presented in this review indicate that PACAP and its receptors may be promising targets for migraine therapeutics.

A structure-function study of PACAP using conformationally restricted analogs: Identification of PAC1 receptor-selective PACAP agonists

Pituitary adenylate cyclase-activating polypeptide (PACAP) has widespread physiological/pathophysiological actions and there is increased interest for its use therapeutically, especially in the CNS (neuroprotection). Unfortunately, no selective PACAP-analogs exist for PACAP-preferring PAC1-receptors, primarily because of its high sequence identity to VIP and particularly, because of the inability of structure-function studies to separate the pharmacophore of PAC1-R from VPAC1-R, which has high affinity for PACAP and VIP. The present study attempted to develop PAC1-R-selective agonists primarily by making conformationally restricted PACAP-analogs in positions important for receptor-selectivity/affinity. Forty-six PACAP-related-analogs were synthesized with substitutions in positions 1-4, 14-17, 20-22, 28, 34, 38 and receptor-selectivity determined in PAC1-R,VPAC1-R,VPAC2-R-transfected or native cells from binding or cAMP-generation experiments. Fifteen PACAP-analogs had 6-78-fold higher affinities for PAC1-R than VPAC1-R and 13 were agonists. Although binding-affinities correlated significantly with agonist potency, the degree of receptor-spareness varied markedly for the different PACAP-analogs, resulting in selective potencies for activating the PAC1 receptor over the VPAC1 receptor from 0- to 103-fold. In addition, a number of PACAP-analogs were identified that had high selectivity for PAC1-R over VPAC2-R as well as PACAP-analogs that could prove more useful therapeutically because of substitutions known to extend their half-lives (substitutions at potential sites of proteolysis and attachment of long-chain fatty acids). This study provides for the first time a separation of the pharmacophores for PAC1-R and VPAC1-R, resulting in PACAP-related analogs that are PAC1-R-preferring. Some of these analogs, or their modifications, could prove useful as therapeutic agents for various diseases.

Rat and guinea pig pancreatic acini possess both VIP(1) and VIP(2) receptors, which mediate enzyme secretion

Pancreatic acini from most species possess vasoactive intestinal peptide (VIP) receptors. Recently, two subtypes of VIP receptors, VIP(1)-R and VIP(2)-R, were cloned. Which subtype exists on pancreatic acini or mediates secretion is unclear. To address this, we examined pancreatic acini from both rat and guinea pig. VIP(1)-R and VIP(2)-R mRNA were identified in dispersed acini from both species by Northern blot analysis and in rat by Southern blot analysis. With the use of the VIP(2)-R-selective ligand Ro-25-1553 in both species, inhibition of binding of (125)I-labeled VIP to acini showed a biphasic pattern with a high-affinity component (10%) and a second representing 90%. The VIP(1)-R-selective ligand, [Lys(15),Arg(16),Leu(27)]VIP-(1-7)-GRF-(8-27), gave a monophasic pattern. Binding of Ro-25-1553 was better fit by a two-site model. In both rat and guinea pig acini, the dose-response curve of Ro-25-1553 for stimulation of enzyme secretion was biphasic, with a high-affinity component of 10-15% of the maximal secretion and a low-affinity component accounting for 85-90%. At low concentrations (10 nM) of Ro-25-1553 and [Lys(15),Arg(16), Leu(27)]VIP-(1-7)-GRF(8-27), which only occupy VIP receptors, a 4-fold and a 56-fold increase in cAMP occurred, respectively. These results show that both VIP(1)-R and VIP(2)-R subtypes exist on pancreatic acini of rat and guinea pig, their activation stimulates enzyme secretion by a cAMP-mediated mechanism, and the effects of VIP are mediated 90% by activation of VIP(1)-R and 10% by VIP(2)-R. Because VIP has a high affinity for both VIP-R subtypes, its effect on pancreatic acini is mediated by two receptor subtypes, which will need to be considered in future studies of the action of VIP in the pancreas.

Postnatal development of circulating cholecystokinin and secretin, pancreatic growth, and exocrine function in guinea pigs

Concentrations of cholecystokinin (CCK) and secretin from neonatal guinea pig plasma were evaluated in relation to pancreatic growth, and secretion from dispersed pancreatic acini. Plasma CCK was low at birth (3.1 +/- 0.8 pmol/L) but rose markedly by day 15 (11.0 +/- 0.8 pmol/L, p < 0.001). Plasma secretin was also low at birth (3.8 +/- 0.8 pmol/L) but peaked on day 4 (17.0 +/- 1.4 pmol/L, p < 0.001) and remained elevated through d 15. Adult nonfasting plasma CCK and secretin levels were 12.3 +/- 0.8 and 8.9 +/- 1.5 pmol/L, respectively. Pancreatic weight more than doubled during the first week of life, with the greatest increase during the first 4 d (156 +/- 4 to 262 +/- 10 mg/100g body wt, p < 0.001). Body weight increased most dramatically during the second week (126 +/- 9 to 190 +/- 7 g p < 0.001). Amylase secretion from isolated acini stimulated by CCK-8, carbachol, phorbol ester, forskolin, and calcium ionophore (A23187) was present at birth. The percentage of total amylase content secreted in response to CCK, carbamylcholine, and secretin did not change during the first 2 wk of neonatal life, and was independent of surges in circulating CCK and secretin. These results indicate that functional maturity of the guinea pig pancreas for amylase secretion occurs early and is different from the rat, mouse, pig, and human. Since results can readily be compared between animals of different postnatal ages, the neonatal guinea pig is ideally suited for the study of hormonal effects on acinar cell function.

Activation of neuromedin B-preferring bombesin receptors on rat glioblastoma C-6 cells increases cellular Ca2+ and phosphoinositides

Recent cloning studies confirm the presence of two subtypes of bombesin (Bn) receptors. In contrast to the gastrin-releasing peptide (GRP)-preferring subtype, which has been widely studied, nothing is known about the cellular mechanisms of the neuromedin B (NMB)-preferring subtype, which occurs widely in the central nervous system and gastrointestinal tissues, partially because of the lack of a cell line with functional receptors. In the present study we have investigated Bn receptors on the rat glioblastoma cell line C-6, reported to contain mRNA of the NMB receptor subtype. Binding of 125I-[D-Tyr0]NMB to these cells was time- and temperature-dependent, saturable, reversible, and only inhibited by Bn receptor agonists or antagonists. For Bn receptor agonists the relative potencies were: NMB (1.7 nM) approximately equal to litorin (3 nM) greater than ranatensin (8 nM) greater than Bn (19 nM) greater than neuromedin C (NMC) (210 nM) greater than GRP (500 nM). These relative affinities were almost identical to those for the NMB receptor subtype on rat oesophageal tissue and for Balb 3T3 cells stably transfected with the NMB receptor subtype. These potencies differed from those for the GRP receptor subtype on rat pancreatic acini [Bn approximately equal to litorin (4 nM) greater than ranatensin, NMC, GRP (15-20 nM) much greater than NMB (351 nM)]. The relative potencies of four different classes of Bn receptor antagonists were compared. Results from C-6 tumour cells agreed closely with those for binding to the NMB receptor subtype on rat oesophageal tissue and in Balb 3T3 cells stably transfected with this receptor, and differed markedly from those for binding to the GRP receptor subtype on rat pancreatic acini. Four Bn receptor antagonists had a higher affinity for the GRP subtype ([D-Phe6]Bn-(6-13)ethyl ester (500 x), [D-Phe6][psi 13-14,Cpa14]Bn- (6-14) (70 x) (where psi 13-14 refers to the replacement of the -CONH- peptide bond between Leu13 and Met14 by -CH2NH2) [psi 13-14,Leu14]Bn, [D-Phe6]Bn-(6-13) propylamide (30 x)] and two had a higher affinity for the NMB subtype on C-6 cells and transfected cells ([D-Pro4,D-Trp7,9,10] substance P-(4-11) (9 x) and [Tyr4,D-Phe12]Bn (18 x)]. In C-6 tumour cells, Bn receptor agonists caused an increase in cytosolic Ca2+ and the generation of inositol phosphates. For both responses, NMB was more than 50-fold more potent than GRP. Neither NMB nor GRP increased cyclic AMP. These results demonstrate that the rat glioblastoma cell line C-6 possesses functional NMB-preferring Bn receptors, and agonist occupation activates phospholipase C, thus increasing cytosolic Ca2+ and inositol phosphate formation. Because the interaction of Bn-related peptides with C-6 cell receptors is identical with that reported in other tissues containing the mRNA for the NMB subtype, this cell line should prove useful in exploring further the cellular basis of action of the peptides that interact with this receptor in the central nervous system and various other tissues.

Distribution and co-localization of immunoreactive helospectin with vasoactive intestinal polypeptide and peptide histidine methionine in human nasal mucosa, soft palate and larynx

Regulatory peptide immunoreactivities reported in the upper respiratory system of man include vasoactive intestinal polypeptide (VIP) and peptide histidine methionine (PHM), which are co-localized in a network of fine varicose nerve fibers. The present study was undertaken to examine the possible occurrence and distribution of the recently described VIP-like peptide helospectin. Double immunofluorescence labelling showed that helospectin is co-localized with VIP and PHM. In addition to nerve fibers containing all three peptides, scattered nerve fibers were detected that were only immunoreactive to helospectin but not to VIP and/or PHM. The distribution of helospectin/VIP/PHM immunoreactive nerve fibers around blood vessels and in close relationship to seromucous glands indicates their possible involvement in the regulation of blood flow and secretion.