Different actions of secretin and Gly-extended secretin predict secretin receptor subtypes

Selective gene expression by rat gastric corpus epithelium

The gastrointestinal (GI) tract is divided into several segments that have distinct functional properties, largely absorptive. The gastric corpus is the only segment thought of as largely secretory. Microarray hybridization of the gastric corpus mucosal epithelial cells was used to compare gene expression with other segments of the columnar GI tract followed by statistical data subtraction to identify genes selectively expressed by the rat gastric corpus mucosa. This provides a means of identifying less obvious specific functions of the corpus in addition to its secretion-related genes. For example, important properties found by this GI tract comparative transcriptome reflect the energy demand of acid secretion, a role in lipid metabolism, the large variety of resident neuroendocrine cells, responses to damaging agents and transcription factors defining differentiation of its epithelium. In terms of overlap of gastric corpus genes with the rest of the GI tract, the distal small bowel appears to express many of the gastric corpus genes in contrast to proximal small and large bowel. This differential map of gene expression by the gastric corpus epithelium will allow a more detailed description of major properties of the gastric corpus and may lead to the discovery of gastric corpus cell differentiation genes and those mis-regulated in gastric carcinomas.

Secretin: Should we revisit its metabolic outcomes?

Metabolic pathologies such as Type 2 Diabetes have become a major health problem for worldwide populations. Unfortunately, efforts to cure and especially to prevent these significant global problems have so far been met with disappointment. Recently, the involvement of the gut-derived hormonal dysregulation in the development of obesity-related disturbances has been intensively studied. For instance, studies of gut-derived peptides such as peptide YY 3-36, glucagon-like peptide-1, oxyntomodulin and, more recently, ghrelin have significantly improved our understanding of mechanisms underlying weight and metabolic regulation. Even though early reports of the existence of secretin, the first peptide hormone to be described, date back as far as 1825, so much and yet so little is still known about its physiological role in mammals, including humans. However, recent years have provided a better understanding of how the release of secretin is regulated by enteral secretagogues. On the other hand, most basic questions about its role in the post-prandial regulation of metabolic functions in normal and pathophysiological conditions remain to be elucidated. The present work intends to review the physiology of secretin along with its central and peripheral outcomes on metabolic functions.

Peptide processing and biology in human disease

PURPOSE OF REVIEW

To describe recent advances in the processing of gastrointestinal hormones, and the consequences for human disease of mutations in the enzymes involved.

RECENT FINDINGS

Although gastrointestinal prohormones were long regarded as devoid of biological activity, recent data indicate that the prohormones for both gastrin and gastrin-releasing peptide are bioactive, through different receptors from the mature hormones. Mutations in the family of prohormone convertases responsible for the initial steps in the processing of gastrointestinal hormones are associated with several different pathophysiological conditions in humans.

SUMMARY

Human mutational studies, when taken together with the phenotypes observed in mice deficient in the prohormone convertases, emphasize the crucial importance of the processing enzymes in mammalian biology. Although the phenotypes may often be ascribed to defective production of a mature hormone or growth factor, the recognition that the precursors are independently bioactive suggests that the increased precursor concentrations may also contribute to the symptoms. The observation that the precursors often act through different receptors from the mature hormones may permit the development of precursor-selective antagonists for therapeutic use.

A new endogenous form of PYY isolated from canine ileum: Gly-extended PYY(1-36)

We purified and identified the peptide YY (PYY) forms present and determined their levels from a portion of the canine ileum directly adjacent to the cecum by a new extraction method designed to prevent and evaluate degradation of endogenous peptides. We used three reverse phase chromatography steps with radioimmunoassay of fractions for PYY-like-immunoreactivity (PYY-LI). The purified fractions underwent intact protein/peptide mass spectrometry identification and sequencing (i.e. "top-down" MS analysis). This analysis confirmed the identity of a new form of PYY, PYY(1-36)-Gly, which co-elutes with PYY(1-36)-NH(2) through all three of separation steps used. The PYY(1-36)-Gly form represents approximately 20% of the total PYY found in this region of the canine intestine. In addition, we also found that the PYY(3-36)-NH(2) form represents 6% of the total PYY in the canine ileo-cecal junction. The physiological implication of the Gly-extended form of PYY(1-36) warrants further investigation.

Water and enzyme secretion are tightly coupled in pancreatic secretion stimulated by food or CCK-58 but not by CCK-8

Pancreatic secretion of protein, water, chloride, and bicarbonate under basal conditions and in response to intravenous and intraduodenal stimuli were studied in awake rats fully recovered from surgery. During the basal phase of pancreatic secretion, protein output and water output were weakly correlated or uncorrelated, consistent with separate regulation and distinct cellular origin of enzyme (acinar cells) and water (duct cells), referred to as the two-component paradigm of pancreatic secretion. When pancreatic secretion was stimulated physiologically, water and protein output abruptly became strongly and significantly correlated, suggesting that protein secretion and water secretion are tightly coupled or that protein secretion is dependent on water secretion. The apparent function of this coupling is to resist or prevent increases in protein concentration as protein output increases. This pattern of secretion was reproduced by intravenous infusion of the CCK-58 form of cholecystokinin, which strongly stimulates pancreatic water and chloride secretion, but not by CCK-8, which only weakly stimulates water and chloride secretion in a non-dose-dependent manner. The remarkably tight association of water and protein secretion in food-stimulated and CCK-58-stimulated pancreatic secretion is consistent with a single cell type as the origin of both water and enzyme secretion, i.e., the acinar cell, and is not consistent with the two-component paradigm of pancreatic secretion. Because CCK-58 is the only detectable endocrine form of cholecystokinin in the rat and its bioactivity pattern is markedly and qualitatively different from CCK-8, actions previously recorded for CCK-8 should be reexamined.

Stimulation of proliferation and migration of a colorectal cancer cell line by amidated and glycine-extended gastrin-releasing peptide via the same receptor

Although amidated forms of gastrin-releasing peptide (GRP) have been identified as autocrine growth factors in small cell lung cancer, their role in the development and progression of colorectal carcinoma is less clear. In addition, the biological activity of non-amidated gastrin-releasing peptide has not been investigated in colorectal carcinoma cells. We therefore investigated the effect of bombesin (a homologue of gastrin-releasing peptide) on proliferation, migration and inositol phosphate production in the human colorectal carcinoma cell line DLD-1, and determined the ability of gastrin-releasing peptide receptor antagonists to inhibit these effects. We also compared the biological activities of amidated and non-amidated GRP in the same assays. Treatment with either bombesin, or amidated or non-amidated GRP resulted in significant increase in proliferation, and in migration in a wound-healing assay. Both the mitogenic and migratory effects of amidated and non-amidated forms were inhibited by the GRP receptor antagonist [D-Phe(6), Leu-NHet(13), des-Met(14)]-bombesin(6-13). The presence of GRP receptor mRNA and GRP binding sites in three colorectal carcinoma cell lines was demonstrated by RT-PCR and by binding of radiolabelled bombesin, respectively. Transfection of DLD-1 cells with a dominant negative phosphatidylinositol 3-kinase did not affect bombesin-stimulated cell proliferation, but inhibited bombesin-stimulated cell migration. Bombesin and GRPgly activated phospholipase C, mitogen-activated protein kinase and focal adhesion kinase. We conclude that both amidated and non-amidated forms of gastrin-releasing peptide accelerate proliferation and migration of DLD-1 human colorectal carcinoma cells via the gastrin-releasing peptide receptor, but that phosphatidylinositol 3-kinase is only involved in the cell migration signalling pathway. Our results suggest a potential role for gastrin-releasing peptide receptor antagonists in the management of colorectal carcinoma.

Receptor subtypes: species variations in secretin affect potency for pancreatic but not gastric secretion

INTRODUCTION

Receptor subtypes can be distinguished by different actions of agonists on physiologic responses. In this study, we compared effects of four species variants of secretin (rat, porcine, canine, and human) on pancreatic secretion and gastrin-induced acid secretion in urethane-anesthetized rats. These secretins differ by one to three residues in position 14, 15, or 16 and were used to probe for the presence of different secretin receptor subtypes in the rat.

METHODOLOGY

Pancreatic responses were measured in a two-point parallel line bioassay with porcine secretin (3 and 30 pmol/kg IV bolus) as standard. Inhibition of gastric acid secretion by each secretin (100 pmol/[kg x h]) was quantitated against a threshold dosage of gastrin-17 (200 pmol/[kg x h]), and percent inhibition of incremental acid responses was determined.

RESULTS

Rat secretin was significantly more potent than other secretins for pancreatic secretion, in the order of rat > porcine > canine > human. The four secretins significantly inhibited gastrin-induced acid secretion by 37% to 49%, with no statistically significant differences among the forms.

CONCLUSIONS

Stimulation of pancreatic secretion was influenced by species variations in secretin structure, but inhibition of gastric acid secretion was not. This finding suggests that secretin receptor subtypes with different recognition patterns mediate these responses.

Stimulation of amylase release by Orexin is mediated by Orexin 2 receptor in AR42J cells

INTRODUCTION AND AIMS

Orexins have been demonstrated to have mainly central physiological functions, including regulation of food and water intake, sleep, and arousal. However, little is known about their direct peripheral effects, if any. As a first step toward understanding the role of Orexin in non-neuronal tissues or cells, we initiated studies to examine expression of Orexin receptors (OXR) in an established pancreatic tumor cell line AR42J. Secondly, we wanted to determine whether Orexins, in various molecular forms, are active to stimulate any known pancreatic cell functions in AR42J cells.

METHODOLOGY

Reverse transcription-PCR analysis was performed to identify the presence of specific Orexin receptor subtypes. Intracellular calcium mobilization and cAMP levels were measured following stimulation by Orexin A and B peptides, their respective C-terminal decapeptide fragments, and hypocretin-2-gly (glycine-extended Orexin B). Release of alpha-amylase was measured in conditioned media after acute stimulation with the set of Orexin peptides for 30 minutes. Cell proliferation was determined by H-thymidine incorporation after 24 hours following treatment with Orexins under serum-free condition.

RESULTS

RT-PCR and sequencing results showed that Orexin receptor subtype 2 (OX2R) was the main form expressed in AR42J cells. Orexins stimulated dose-dependent increases in intracellular calcium mobilization with EC50 0.05 nM for Orexin A and 0.1 nM for Orexin B but were unable to stimulate any significant cAMP accumulation or DNA synthesis even at micromolar concentrations. Both Orexin-A and -B, but not hypocretin-2-gly, also stimulated dose-dependent increases in amylase release in the AR42J cells. Orexin-A and -B carboxyl-terminal decapeptides elicited significant but much lower calcium and amylase responses.

CONCLUSION

Our data demonstrate that OX2R mediates Ca -dependent amylase release in AR42J cells, suggesting that Orexins may have secretory functions in pancreatic tumor cells.

Secretin-mediated gene delivery, a specific targeting mechanism with potential for treatment of biliary and pancreatic disease in cystic fibrosis

Gene therapy directed to the gastroenterological manifestations of cystic fibrosis (CF) would ideally be administered systemically. Such delivery would require efficient targeting at the cellular level to achieve a safe and effective therapy. Here we describe gene delivery using the secretin receptor (SR) as a basolateral target specific to the biliary and pancreatic epithelia affected in CF patients. We describe here targeting of a polycation-based nonviral gene delivery vector and retargeting of an adenoviral vector to cells expressing the SR in vitro. We were able to transfect cells expressing the SR up to 10-fold more efficiently than those not expressing the SR with a targeted polycation, SecGGC-lPEI. This targeting effect was secretin-specific and substantially reduced by competing secretin. SR-retargeted adenovirus transduced SR-expressing cells at more than sixfold higher levels than adenovirus alone. The SR may be an effective target for targeting systemically applied viral and nonviral gene delivery constructs to disease-affected tissues in CF patients.