Functional GIP receptors in a hamster pancreatic beta cell line, In 111: specific binding and biological effects

Pharmacological Advances in Incretin-Based Polyagonism: What We Know and What We Don't

The prevalence of obesity continues to rise in both adolescents and adults, in parallel obesity is strongly associated with the increased incidence of type 2 diabetes, heart failure, certain types of cancer, and all-cause mortality. In relation to obesity, many pharmacological approaches of the past have tried and failed to combat the rising obesity epidemic, particularly due to insufficient efficacy or unacceptable side effects. However, while the history of antiobesity medication is plagued by failures and disappointments, we have witnessed over the last 10 years substantial progress, particularly in regard to biochemically optimized agonists at the receptor for glucagon-like peptide-1 (GLP-1R) and unimolecular coagonists at the receptors for GLP-1 and the glucose-dependent insulinotropic polypeptide (GIP). Although the GIP receptor:GLP-1R coagonists are being heralded as premier pharmacological tools for the treatment of obesity and diabetes, uncertainty remains as to why these drugs testify superiority over best-in-class GLP-1R monoagonists. Particularly with regard to GIP, there remains great uncertainty if and how GIP acts on systems metabolism and if the GIP system should be activated or inhibited to improve metabolic outcome in adjunct to GLP-1R agonism. In this review, we summarize recent advances in GLP-1- and GIP-based pharmacology and discuss recent findings and open questions related to how the GIP system affects systemic energy and glucose metabolism.

The Role of GIP Receptor in the CNS for the Pathogenesis of Obesity

Glucose-dependent insulinotropic polypeptide (GIP) (also known as gastric inhibitory polypeptide) is a hormone produced in the upper gut and secreted to the circulation in response to the ingestion of foods, especially fatty foods. Growing evidence supports the physiological and pharmacological relevance of GIP in obesity. In an obesity setting, inhibition of endogenous GIP or its receptor leads to decreased energy intake, increased energy expenditure, or both, eventually causing weight loss. Further, supraphysiological dosing of exogenous long-lasting GIP agonists alters energy balance and has a marked antiobesity effect. This remarkable yet paradoxical antiobesity effect is suggested to occur primarily via the brain. The brain is capable of regulating both energy intake and expenditure and plays a critical role in human obesity. In addition, the GIP receptor is widely distributed throughout the brain, including areas responsible for energy homeostasis. Recent studies have uncovered previously underappreciated roles of the GIP receptor in the brain in the context of obesity. This article highlights how the GIP receptor expressed by the brain impacts obesity-related pathogenesis.

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

Structural basis for ligand recognition of incretin receptors

The glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor are homologous G-protein-coupled receptors (GPCRs). Incretin receptor agonists stimulate the synthesis and secretion of insulin from pancreatic β-cells and are therefore promising agents for the treatment of type 2 diabetes. It is well established that the N-terminal extracellular domain (ECD) of incretin receptors is important for ligand binding and ligand specificity, whereas the transmembrane domain is involved in receptor activation. Structures of the ligand-bound ECD of incretin receptors have been solved recently by X-ray crystallography. The crystal structures reveal a similar fold of the ECD and a similar mechanism of ligand binding, where the ligand adopts an α-helical conformation. Residues in the C-terminal part of the ligand interact directly with the ECD and hydrophobic interactions appear to be the main driving force for ligand binding to the ECD of incretin receptors. Obviously, the-still missing-structures of full-length incretin receptors are required to construct a complete picture of receptor function at the molecular level. However, the progress made recently in structural analysis of the ECDs of incretin receptors and related GPCRs has shed new light on the process of ligand recognition and binding and provided a basis to disclose some of the mechanisms underlying receptor activation at high resolution.

Targeting beta-cell mass in type 2 diabetes: promise and limitations of new drugs based on incretins

Progressive insulin secretory defects, due to either functional abnormalities of the pancreatic beta-cells or a reduction in beta-cell mass, are the cornerstone of type 2 diabetes. Incretin-based drugs hold the potential to improve glucose tolerance by immediate favorable effect on beta-cell physiology as well as by expanding or at least maintaining beta-cell mass, which may delay the progression of the disease. Long-term studies in humans are needed to elaborate on these effects.

Characterisation and biological activity of Glu3 amino acid substituted GIP receptor antagonists

Glucose-dependent insulinotropic polypeptide (GIP) is an important gastrointestinal hormone, which regulates insulin release and glucose homeostasis, but is rapidly inactivated by enzymatic N-terminal truncation. Here we report the enzyme resistance and biological activity of several Glu(3)-substituted analogues of GIP namely; (Ala(3))GIP, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP. Only (Lys(3))GIP demonstrated moderately enhanced resistance to DPP-IV (p<0.05 to p<0.01) compared to native GIP. All analogues demonstrated a decreased potency in cAMP production (EC(50) 1.47 to 11.02 nM; p<0.01 to p<0.001) with (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated cAMP production (p<0.05). In BRIN-BD11 cells, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP did not stimulate insulin secretion with both (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated insulin secretion (p<0.05). Injection of each GIP analogue together with glucose in ob/ob mice significantly increased the glycaemic excursion compared to control (p<0.05 to p<0.001). This was associated with lack of significant insulin responses. (Ala(3))GIP, (Phe(3))GIP and (Tyr(3))GIP, when administered together with GIP, significantly reduced plasma insulin (p<0.05 to p<0.01) and impaired the glucose-lowering ability (p<0.05 to p<0.01) of the native peptide. The DPP-IV resistance and GIP antagonism observed were similar but less pronounced than (Pro(3))GIP. These data demonstrate that position 3 amino acid substitution of GIP with (Ala(3)), (Phe(3)), (Tyr(3)) or (Pro(3)) provides a new class of functional GIP receptor antagonists.

The bioactive conformation of glucose-dependent insulinotropic polypeptide by NMR and CD spectroscopy

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal incretin hormone, which modulates physiological insulin secretion. Because of its glucose-sensitive insulinotropic activity, there has been a considerable interest in utilizing the hormone as a potential treatment for type 2 diabetes. Structural parameters obtained from NMR spectroscopy combined with molecular modeling techniques play a vital role in the design of new therapeutic drugs. Therefore, to understand the structural requirements for the biological activity of GIP, the solution structure of GIP was investigated by circular dichroism (CD) followed by proton nuclear magnetic resonance (NMR) spectroscopy. CD studies showed an increase in the helical character of the peptide with increasing concentration of trifluoroethanol (TFE) up to 50%. Therefore, the solution structure of GIP in 50% TFE was determined. It was found that there was an alpha-helix between residues 6 and 29, which tends to extend further up to residue 36. The implications of the C-terminal extended helical segment in the inhibitory properties of GIP on gastric acid secretion are discussed. It is shown that the adoption by GIP of an alpha-helical secondary structure is a requirement for its biological activity. Knowledge of the solution structure of GIP will help in the understanding of how the peptide interacts with its receptor and aids in the design of new therapeutic agents useful for the treatment of diabetes.

Chemical ablation of gastric inhibitory polypeptide receptor action by daily (Pro3)GIP administration improves glucose tolerance and ameliorates insulin resistance and abnormalities of islet structure in obesity-related diabetes

Glucose-dependent insulinotropic polypeptide (gastric inhibitory polypeptide [GIP]) is an important incretin hormone secreted by endocrine K-cells in response to nutrient ingestion. In this study, we investigated the effects of chemical ablation of GIP receptor (GIP-R) action on aspects of obesity-related diabetes using a stable and specific GIP-R antagonist, (Pro3)GIP. Young adult ob/ob mice received once-daily intraperitoneal injections of saline vehicle or (Pro3)GIP over an 11-day period. Nonfasting plasma glucose levels and the overall glycemic excursion (area under the curve) to a glucose load were significantly reduced (1.6-fold; P < 0.05) in (Pro3)GIP-treated mice compared with controls. GIP-R ablation also significantly lowered overall plasma glucose (1.4-fold; P < 0.05) and insulin (1.5-fold; P < 0.05) responses to feeding. These changes were associated with significantly enhanced (1.6-fold; P < 0.05) insulin sensitivity in the (Pro3)GIP-treated group. Daily injection of (Pro3)GIP reduced pancreatic insulin content (1.3-fold; P < 0.05) and partially corrected the obesity-related islet hypertrophy and beta-cell hyperplasia of ob/ob mice. These comprehensive beneficial effects of (Pro3)GIP were reversed 9 days after cessation of treatment and were independent of food intake and body weight, which were unchanged. These studies highlight a role for GIP in obesity-related glucose intolerance and emphasize the potential of specific GIP-R antagonists as a new class of drugs for the alleviation of insulin resistance and treatment of type 2 diabetes.

Gut peptides in the treatment of diabetes mellitus

It has been known for at least one century that agents secreted from the intestine during meal absorption regulates glucose assimilation. Extensive research during the past three decades has identified two gut hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, also known as gastric inhibitory polypeptide) that are important in postprandial glucose metabolism. Both peptides are incretins; they are secreted during carbohydrate absorption and increase insulin secretion. Since they are potent insulin secretagogues, GIP and GLP-1 have received considerable attention as potential diabetes therapeutics. However, only GLP-1 exerts insulinotropic properties when administered to patients with Type 2 diabetes. Both GLP-1 and GIP are rapidly inactivated in the circulation by the enzyme dipeptidyl peptidase IV (DPP-IV). The application of GLP-1 into clinical practice has been delayed due to the need to develop compounds that overcome this rapid inactivation. Two approaches have been taken to utilise the insulinotropic and glucose-lowering actions of GLP-1 as an antidiabetic agent: the development of DPP-IV-resistant analogues and the inhibition of DPP-IV. This review focuses on the physiology of GLP-1 and GIP and the advances that have been made thus far in developing treatments based on these physiological incretins for Type 2 diabetes.

Glucose-dependent insulinotropic polypeptide promotes beta-(INS-1) cell survival via cyclic adenosine monophosphate-mediated caspase-3 inhibition and regulation of p38 mitogen-activated protein kinase

The incretin glucose-dependent insulinotropic polypeptide (GIP) is a major regulator of postprandial insulin secretion in mammals. Recent studies in our laboratory, and others have suggested that GIP is a potent stimulus for protein kinase activation, including the MAPK (ERK1/2) module. Based on these studies, we hypothesized that GIP could regulate cell fate and sought to examine the underlying mechanisms involved in GIP stimulation of cell survival. GIP potentiated glucose-induced beta-(INS-1)-cell growth to levels comparable with GH and GLP-1 while promoting cell survival in the face of serum and glucose-deprivation or treatment with wortmannin or streptozotocin. In the absence of GIP, 50% of cells died after 48 h of serum and glucose withdrawal, whereas 91 +/- 10% of cells remained viable in the presence of GIP [n = 3, P < 0.05; EC50 of 1.24 +/- 0.48 nm GIP (n = 4)]. Effects of GIP on cell survival and inhibition of caspase-3 were mimicked by forskolin, but pharmacological experiments excluded roles for MAPK kinase (Mek)1/2, phosphatidylinositol 3-kinase, protein kinase A, Epac, and Rap 1. Survival effects of GIP were ablated by the inhibitor SB202190, indicating a role for p38 MAPK. Furthermore, caspase-3 activity was also regulated by p38 MAPK, with a lesser role for Mek1/2, based on RNA interference studies. We propose that GIP is able to reverse caspase-3 activation via inhibition of long-term p38 MAPK phosphorylation in response to glucose deprivation (+/-wortmannin). Intriguingly, these findings contrasted with short-term phosphorylation of MKK3/6-->p38 MAPK-->ATF-2 by GIP. Thus, these data suggest that GIP is able to regulate INS-1 cell survival by dynamic control of p38 MAPK phosphorylation via cAMP signaling and lend further support to the notion that GIP regulation of MAPK signaling is critical for its regulation of cell fate.

Glucose-dependent insulinotropic polypeptide induced growth hormone secretion in acromegaly

Glucose-dependent insulinotropic polypeptide (GIP), a peptide released from the intestines after meals, is thought to stimulate insulin secretion. GIP receptor cDNA has recently been cloned and its mRNA has been recognized in several organs including the pituitary, but the physiological roles of GIP receptors of the pituitary have yet to be determined. We have demonstrated the possibility that GIP stimulates GH secretions from the pituitary adenoma cells of acromegalics. GIP-stimulated GH responses were studied in four acromegalics. In two acromegalics whose GH showed paradoxical secretion to oral glucose tolerance test (OGTT), GIP infusion (0.6 microg/kg/h) drove GH secretion (13.7 to 68.1, 22.5 to 76.2 ng/ml, respectively). However, in the other two acromegalics whose GH showed no paradoxical response to OGTT, GIP infusion did not induce GH secretion. One of the patients who was studied extensively had a GH that responded to OGTT. This patient's serum GH levels increased after meals while adenomectomy abolished both the paradoxical GH secretions by OGTT and GH responses to the GIP infusion. These data suggested that some somatotroph adenoma cells have an aberrant response to GIP which may go toward explaining paradoxical GH secretions to OGTT in acromegalics.

Glucose-dependent insulinotropic peptide stimulates thymidine incorporation in endothelial cells: role of endothelin-1

We have previously characterized the receptor for glucose-dependent insulinotropic polypeptide (GIPR) in vascular endothelial cells (EC). Different EC types were found to contain distinct GIPR splice variants. To determine whether activation of the GIPR splice variants resulted in different cellular responses, we examined GIP effects on human umbilical vein endothelial cells (HUVEC), which contain two GIPR splice variants, and compared them with a spontaneously transformed human umbilical vein EC line, ECV 304, which contains four GIPR splice variants. GIP dose-dependently stimulated HUVEC and ECV 304 proliferation as measured by [3H]thymidine incorporation. GIP increased endothelin-1 (ET-1) secretion from HUVEC but not from ECV 304. Use of the endothelin B receptor blocker BQ-788 resulted in an inhibition of [3H]thymidine incorporation in HUVEC but not in ECV 304. These findings suggest that, although GIP increases [3H]thymidine incorporation in both HUVEC and ECV 304, this proliferative response is mediated by ET-1 only in HUVEC. These differences in cellular response to GIP may be related to differences in activation of GIPR splice variants.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

Characterization of the carboxyl-terminal domain of the rat glucose-dependent insulinotropic polypeptide (GIP) receptor. A role for serines 426 and 427 in regulating the rate of internalization

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone involved in the regulation of insulin secretion. In non-insulin-dependent diabetes mellitus insulin responses to GIP are blunted, possibly due to altered signal transduction or reduced receptor number. Site-directed mutagenesis was used to construct truncated GIP receptors to study the importance of the carboxyl-terminal tail (CT) in binding, signaling, and receptor internalization. Receptors truncated at amino acids 425, 418, and 405, expressed in COS-7 or CHO-K1 cells, exhibited similar binding to wild type receptors. GIP-dependent cAMP production with the 405 mutant was decreased in COS-7 cells. Maximal cAMP production in CHO-K1 cells was reduced with all truncated forms. Binding was undetectable with a receptor truncated at amino acid 400; increasing tail length by adding 5 alanines restored binding and signaling. Mutants produced by alanine scanning of residues 394-401, adjacent to transmembrane domain 7, were all functional. CT truncation by 30 or more amino acids, mutation of serines 426/427, singly or combined, or complete CT serine knockout all reduced receptor internalization rate. The majority of the GIP receptor CT is therefore not required for signaling, a minimum chain length of approximately 405 amino acids is needed for receptor expression, and serines 426 and 427 are important for regulating rate of receptor internalization.

Food-dependent Cushing's syndrome: characterization and functional role of gastric inhibitory polypeptide receptor in the adrenals of three patients

In the present work, the presence of gastric inhibitory polypeptide (GIP) receptors and their functional role in the adrenal cells of three patients with food-dependent Cushing's syndrome were studied. RT-PCR and in situ hybridization studies demonstrated the presence of GIP receptor in the adrenals of the three patients. The presence of this receptor was also demonstrated in two human fetal adrenals, but not in two normal adult human adrenals or in the adrenals of one patient with nonfood-dependent Cushing's syndrome. Freshly isolated cells from patient adrenals responded in a dose-dependent manner to the steroidogenic action of both ACTH and GIP, whereas cells from normal adrenals responded only to ACTH. Treatment of cultured normal adrenal cells with ACTH, but not with GIP, increased the messenger ribonucleic acid (mRNA) levels of cholesterol side-chain cleavage cytochrome P-450, P450c17, and 3beta-hydroxysteroid dehydrogenase, whereas both hormones enhanced these mRNAs in patients' adrenal cells, although the effects of ACTH were greater than those of GIP. Moreover, pretreatment with ACTH enhanced the steroidogenic responsiveness of both normal and patient adrenal cells, whereas GIP caused homologous desensitization, and this was associated with a marked reduction of GIP receptor mRNA levels, as demonstrated by RT-PCR and in situ hybridization. Finally, both ACTH and GIP inhibited DNA synthesis in one patient's adrenal cells, whereas in normal adrenal cells only ACTH had this effect. In conclusion, the present data demonstrate that ectopic expression of functional GIP receptors is the main cause of food-dependent Cushing's syndrome.

Cloning and tissue distribution of a new rat olfactory receptor-like (OL2)

Polymerase chain reaction (PCR) was used to clone an intronless cDNA encoding a new member (named OL2) of the G protein-coupled receptor superfamily. The coding region of the rat OL2 receptor gene predicts a seven transmembrane domain receptor of 315 amino acids. OL2 has 46.4 percent amino acid identity with OL1, an olfactory receptor expressed in the developing rat heart, and slightly lower percent indentities with several other olfactory receptors. PCR analysis reveals that the transcript is present mainly in the rat spleen and in a mouse insulin-secreting cell line (MIN6). No correlation was found between the tissue distribution of OL2 and that of the olfaction-related GTP-binding protein Golf alpha subunit. These findings suggest a role for this new hypothetical G-protein coupled receptor and for its still unknown ligand in the spleen and in the insulin-secreting beta cells.

Gastric inhibitory polypeptide activates MAP kinase through the wortmannin-sensitive and -insensitive pathways

The signal transduction pathways of a cloned human gastric inhibitory polypeptide (GIP) receptor have been investigated in CHO cells stably expressing this receptor. Exposure of GIP receptor expressing cells to GIP significantly increased MAP kinase activity. Time course analysis showed that a rapid and marked increase in MAP kinase activation was detected and that this activation reached maximal levels 10 min after the addition of GIP. Dose-response analysis showed that GIP activated MAP kinase activity in a dose-dependent manner with an ED50 value of 5.9 x 10(-10) M of GIP. Wortmannin, a potent inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), partially inhibited GIP-induced MAP kinase activation, suggesting that GIP activates MAP kinase through two different, wortmannin-sensitive and -insensitive pathways. It has been demonstrated that in CHO cells cAMP attenuates MAP kinase activity by inhibiting Raf-1. Since GIP elevates intracellular cAMP, we examined the effects of cAMP on MAP kinase activation. Interestingly, forskolin, which increased intracellular cAMP levels, significantly inhibited MAP kinase activation by GIP, but did not affect MAP kinase activation by GIP in the presence of wortmannin, suggesting that the wortmannin-sensitive pathway activates an MAP kinase cascade at or above the level of Raf-1 and that the wortmannin-insensitive pathway activates an MAP kinase cascade below the level of Raf-1. These findings demonstrate that the GIP receptor is linked to the MAP kinase cascade via at least two different pathways.

GLP-1/GIP chimeric peptides define the structural requirements for specific ligand-receptor interaction of GLP-1

The gastrointestinal hormones glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) strongly stimulate insulin release. Despite their high N-terminal sequence similarity, GLP-1 does not bind to the GIP receptor and vice versa. To characterize the domains required for interaction of the peptide ligands with their specific receptors, we performed displacement studies with various synthetic GLP-1/GIP hybrid peptides on RINm5F insulinoma cells. Displacement of 125I-GIP and 125I-GLP-1 was measured using GLP-1/GIP chimeras which comprised GIP and GLP-1 sequences at different positions. The binding affinity to the GLP-1 receptor was found to be sensitive to GIP-like exchanges in the N-terminal 22 amino acids as well as in positions 13 and 15 (loss of affinity 280-fold to more than 1000-fold). C-terminal substitution of the GLP-1 sequence by GIP diminished the affinity towards the GLP-1 receptor only 20-fold. All hybrid peptides investigated showed minimal binding affinity for the GIP receptor, indicating that the entire GIP-sequence (1-31) is important for receptor recognition. These findings provide insight into the structural requirements for the specific interaction of two important insulinotropic peptides with their specific receptors.

Characterization of GIP(1-30) and GIP(1-42) as stimulators of proinsulin gene transcription

Originally characterized in terms of its gastric acid inhibitory properties, GIP (gastric inhibitory polypeptide) expressed in the upper small intestine, was subsequently shown to exert strong glucose-dependent insulin-releasing properties. This action is generally attributed to GIP(1-42) and, so far, no evidence for the contribution of other relevant GIP forms exists. In this study, we compared the effects of GIP(1-42) and C-terminally truncated GIP(1-30) on cAMP production and proinsulin gene transcription at clonal insulin-secreting cell lines (RIN 1046-38, beta TC-3). Both peptides were equally potent stimulators of cAMP generation in both cell lines. Insulin release from RIN 1046-38 cells stimulated by both GIP forms was identical. In both B-cell lines GIP(1-42) and GIP(1-30) stimulated proinsulin gene expression equipotently. GIP not only enhances insulin secretion but also insulin gene expression and, therefore, it is a true insulinotropic hormone.

Effects of vasoactive intestinal peptide and its homologues on the noradrenaline-mediated secretion of fluid and protein from the rat submandibular gland

1. Noradrenaline-mediated secretion of fluid and protein from rat submandibular glands was enhanced by vasoactive intestinal peptide (VIP) and secretin but not by peptide histidine isoleucine (PHI) or gastric inhibitory peptide (GIP). 2. The synergistic effect of the combination of VIP with noradrenaline (NA) was antagonized by pretreatment with prazosin or phentolamine but not by pretreatment with yohimbine or propranolol. 3. These results suggest that VIP and secretin but not PHI and GIP can significantly enhance the secretion of fluid and protein that is mediated by NA in rat submandibular gland and that the synergistic effect of VIP and NA involves both alpha 1-adrenergic receptors and receptors for VIP.

Effects of vasoactive intestinal peptide and its homologues on the substance P-mediated secretion of fluid and protein from the rat submandibular gland

1. Vasoactive intestinal peptide (VIP) and secretin elicited slight secretion of saliva but peptide histidine isoleucine (PHI) and gastric inhibitory peptide (GIP) failed to elicit secretion of saliva from rat submandibular glands. 2. Substance P (SP)-mediated secretion of fluid and protein were enhanced by VIP and secretin but not by PHI or GIP. 3. These results suggest that the effects of VIP, PHI, secretin and GIP on the secretion of fluid from rat submandibular glands and the synergistic effects of VIP and its homologues on the SP-mediated secretion of fluid and protein do not correspond to the extent of the structural homology of each analogue to VIP.

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

Gastric inhibitory polypeptide-dependent cortisol hypersecretion--a new cause of Cushing's syndrome

BACKGROUND

Corticotropin-independent nodular adrenal hyperplasia is a rare cause of Cushing's syndrome, and the factors responsible for the adrenal hyperplasia are not known.

METHODS

We studied a 48-year-old woman with Cushing's syndrome, nodular adrenal hyperplasia, and undetectable plasma corticotropin concentrations in whom food stimulated cortisol secretion.

RESULTS

Cortisol secretion had an inverse diurnal rhythm in this patient, with low-to-normal fasting plasma cortisol concentrations and elevated postprandial cortisol concentrations that could not be suppressed with dexamethasone. The cortisol concentrations increased in response to oral glucose (4-fold increase) and a lipid-rich meal (4.8-fold increase) or a protein-rich meal (2.6-fold increase), but not intravenous glucose. The infusion of somatostatin blunted the plasma cortisol response to oral glucose. Intravenous infusion of gastric inhibitory polypeptide (GIP) for one hour increased the plasma cortisol concentration in the patient but not in four normal subjects. Fasting plasma GIP concentrations in the patient were similar to those in the normal subjects; feeding the patient test meals induced increases in plasma GIP concentrations that paralleled those in plasma cortisol concentrations. Cell suspensions of adrenal tissue from the patient produced more cortisol when stimulated by GIP than when stimulated by corticotropin. In contrast, adrenal cells from normal adults and fetuses or patients with cortisol-producting or aldosterone-producing adenomas responded to corticotropin but not to GIP.

CONCLUSIONS

Nodular adrenal hyperplasia and Cushing's syndrome may be food-dependent as a result of abnormal responsiveness of adrenal cells to physiologic secretion of GIP. "Illicit" (ectopic) expression of GIP receptors on adrenal cells presumably underlies this disorder.

A clonal rat pancreatic delta cell line (Rin14B) expresses a high number of galanin receptors negatively coupled to a pertussis-toxin-sensitive cAMP-production pathway

Galanin, an ubiquitous neuropeptide, was recently shown to inhibit somatostatin release by the rat islet tumor cell line, Rin-m. By using the clonal pancreatic delta cell line Rin14B, originating from Rin-m cells, we were able to identify the presence of one type of specific galanin-binding site of high affinity (Kd = 1.6 nM; maximal binding capacity = 270 fmol/mg protein) and high specificity for the peptide. Binding of 125I-galanin to these receptors was time-dependent and highly sensitive to guanine nucleotides. Using the cross-linker disuccinimidyl tartrate, covalent linking of the galanin receptor to 125I-galanin in membranes from Rin14B cells, followed by SDS/PAGE analysis of membrane proteins, indicated that the galanin receptor is a protein of 54 kDa. 0.1-100 nM galanin also exerted a marked inhibitory effect on the cAMP-production system under basal conditions, as well as in the presence of the pancreatic peptide glucagon. At a maximal dose, galanin induces a 90-100% decrease of basal and glucagon-stimulated cAMP production levels, with a median inhibition concentration (IC50) of 3 nM galanin. The direct inhibitory effect of galanin on the adenylate cyclase activity in Rin14B cell membranes was also demonstrated (IC50 = 3 nM galanin). The inhibitory effect of galanin on the basal and glucagon-stimulated cAMP production in Rin14B cells was reversed by pertussis toxin. The toxin was also shown to specifically ADP-ribosylate a protein of 41 kDa in membranes from Rin14B cells. Taken together, these data show that the pancreatic delta cell line Rin14B expresses high affinity galanin receptors negatively coupled to a pertussis-toxin-sensitive cAMP-production system.

Mechanism of galanin-inhibited insulin release. Occurrence of a pertussis-toxin-sensitive inhibition of adenylate cyclase

In the insulin-secreting beta cell line Rin m 5F, galanin, a newly discovered ubiquitous neuropeptide, inhibited, by 50%, the stimulation of insulin release induced by gastric inhibitory polypeptide (GIP) or forskolin, i.e. two cAMP-generating effectors. In contrast, it failed to decrease the stimulation of insulin release elicited by either the Ca2+-mobilizing agent, carbamoylcholine, or by dibutyryl-cAMP. Concomitantly, galanin inhibited the GIP- and forskolin-stimulated cAMP production. Furthermore, adenylate cyclase in membranes from Rin m 5F cells was highly sensitive to galanin, which exerted a marked inhibitory effect on the forskolin-stimulated enzyme activity. All these galanin effects were observed at low physiological doses, in the nanomolar range. Overnight treatment of the Rin m 5F cells with pertussis toxin completely abolished the inhibitory effect of galanin on insulin release, cAMP production and adenylate cyclase activity. Moreover, pertussis toxin specifically ADP-ribosylated a 39-kDa protein present in membranes from those cells. Taken together, these data show that the galanin inhibition of insulin release most likely occurs through the inhibition of adenylate cyclase, involving a petussis-toxin-sensitive inhibitory GTP-binding regulatory protein.

Enteroinsular axis

A physiological role for GIP as an insulinotropic hormone involved in the enteroinsular axis has been established and ingestion of glucose, fatty acids and certain amino acids will produce an increase in circulating IR-GIP levels. The insulinotropic action of GIP is glucose concentration dependent in normal animals. A role for GIP in NIDDM is equivocal although several studies have demonstrated elevated serum levels. Animal models have indicated a disturbance of GIP receptor function associated with hyperinsulinaemia, i.e. lowering of the minimum glucose concentration at which GIP is insulinotropic.

Truncated glucagon-like peptide-1 (proglucagon 78-107 amide), an intestinal insulin-releasing peptide, has specific receptors on rat insulinoma cells (RIN 5AH)

We studied binding of 125I-labelled truncated-glucagon-like peptide-1 (proglucagon 78-107 amide) to a cloned rat insulin-producing cell line, RIN 5AH, in monolayer culture. Interaction of the peptide with pancreatic insulinoma cells was saturable and time dependent. Half-maximal binding was obtained when the cells were incubated in the presence of 3.3 x 10(-9) mol/l unlabelled truncated-glucagon-like peptide-1 (proglucagon 78-107 amide). Neither glucagon, full-length glucagon-like peptide-1 (proglucagon 72-107 amide) nor gastric inhibitory peptide competed for binding in concentrations up to 10(-6) mol/l.

The effects of glucose-dependent insulinotropic polypeptide infused at physiological concentrations in normal subjects and type 2 (non-insulin-dependent) diabetic patients on glucose tolerance and B-cell secretion

The effects of porcine glucose-dependent insulinotropic polypeptide given by continuous intravenous infusion in normal subjects (n = 6) and Type 2 (non-insulin-dependent) diabetic patients (n = 6) have been investigated. The subjects were studied on 2 separate days after overnight fasts. On each day 25 g of glucose was infused from 0-30 min plus an infusion of either porcine glucose-dependent insulinotropic polypeptide (0.75 pmol . kg-1 . min-1) or control solution. During the glucose-dependent insulinotropic polypeptide infusion plasma glucose values were reduced in normal subjects from 30-60 min (p less than 0.01) and in Type 2 diabetic patients at 45 and 60 min (p less than 0.05). In the normal subjects insulin concentrations were greater from 10-35 min (p less than 0.01) following glucose-dependent insulinotropic polypeptide infusion and peak values were increased by 123%. In the Type 2 diabetic patients following glucose-dependent insulinotropic polypeptide infusion insulin levels were increased from 4-40 min (p less than 0.01) but peak values were only increased by 27%. In the normal subjects C-peptide values were greater from 25-45 min (p less than 0.01) following glucose-dependent insulinotropic polypeptide infusion and peak C-peptide levels were increased by 82%. In the Type 2 diabetic patients following the glucose-dependent insulinotropic polypeptide infusion C-peptide levels were increased from 6-55 min (p less than 0.01) and peak values were increased by 20%. Plasma glucose-dependent insulinotropic polypeptide levels were within the physiological post prandial range during the glucose-dependent insulinotropic polypeptide infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastric inhibitory polypeptide receptor in hamster pancreatic beta cells. Direct cross-linking, solubilization and characterization as a glycoprotein

125I-labelled gastric inhibitory polypeptide (125I-GIP) is directly cross-linked to its specific receptor in hamster pancreatic beta cell membranes by using an ultraviolet irradiation procedure. This approach results in the identification of a GIP-protein complex of apparent Mr 64,000. The labelling of this protein species is specific since it is inhibited when incubating the membranes with increasing doses of native GIP (0.1 nM-1 microM) together with 125I-GIP, half-maximal inhibition being elicited by 5 nM peptide. Reduction of the GIP-protein complex by 100 mM dithiothreitol induces a decrease of the electrophoretic mobility of the complex. Alternatively pretreatment of membranes with dithiothreitol (up to 1 M) does not prevent the binding of 125I-GIP to its receptor. When prelabelled membranes are extracted by 0.5% Triton X-100 (v/v) and the extract is layered on a Sephadex G-50 column, a high peak of radioactivity is eluted with the void volume of the column. Treatment of this peak by 10 min ultraviolet irradiation followed by SDS-PAGE leads to identification of a major band of Mr 64,000. When the peak is further layered on Sephacryl S-200 it yields a single peak of radioactivity corresponding to a protein species with a Stokes radius of 3.2 nm and an apparent Mr of 65,000. The solubilized GIP-receptor complex is specifically adsorbed by Sepharose coupled to wheat germ agglutinin and concanavalin A and eluted from these lectins by their respective sugars. In conclusion the GIP receptor in pancreatic beta cells is a protein monomer of apparent Mr 59 000; its structure is maintained by intrachain disulfide bridges, these bonds being, however, not involved in the interaction of GIP with its receptor; the GIP receptor is a glycoprotein containing N-acetylglucosamine, mannose and probably sialic acid in its carbohydrate moiety.

Potentiation of glucose-induced insulin secretion in the perfused rat pancreas by porcine GIP (gastric inhibitory polypeptide), bovine GIP, and bovine GIP(1-39)

Porcine GIP (gastric inhibitory polypeptide) potentiates glucose-induced insulin secretion under a variety of experimental conditions. Recently GIP was isolated also from bovine intestine, and found to differ from porcine GIP by having isoleucine instead of lysine in position 37. We have compared the effects of porcine GIP to that of bovine GIP and bovine GIP(1-39) on glucose-induced insulin secretion from the perfused rat pancreas. We found that porcine GIP, bovine GIP, and bovine GIP(1-39) all strongly potentiated both first and second phases of glucose-induced insulin secretion (glucose concentration 6.7 mM; polypeptide concentration 1 nM). There was no significant difference between the polypeptides with regard to the potency to potentiate glucose-induced insulin secretion. We conclude that bovine GIP, as porcine GIP, potentiates glucose-induced insulin secretion, and that the insulinotropic activity of GIP is not confined to the last three amino acids at the C-terminal end.

The receptors of the VIP family peptides (VIP, secretin, GRF, PHI, PHM, GIP, glucagon and oxyntomodulin). Specificities and identity

A model is proposed for the receptors of the VIP family peptides including a ligand and a cellular domain. Specificities of the receptors are due to different ligand binding sites. Three subgroups of the family can be distinguished accordingly: glucagon and oxyntomodulin; GIP; VIP, secretin r and hGRF, PHI and PHM. In the same species, the expression of these different sites is cell-specific resulting in a stoichiometry of the ligand-receptor interaction which is compatible with physiological regulation of cell function. Specificities of the interaction as studied by native and synthetic analogs is supported both by restricted sequences of amino acids (such as that including the N-terminal histidine residue), and membrane-induced configuration of the ligand. Identity of the receptors is related to their interactions with subunits of the adenylate cyclase system. Arguments are put forward indicating that the alpha subunit of the guanyl regulatory protein is a reasonable candidate for directly transducing to the adenylyl cyclase the information contained in the activated ligand-binding site subunits. Evidence of functional and molecular heterogeneity of the recognizing site and of the alpha subunits leads to the supposition that some types of specific complementarity is retained at this level of interaction, further enhancing the possibility of species and cell differences. On the other hand, the identities found in other sequences of the alpha and ras oncogene products extend to the receptor of the VIP family peptides a pattern of organization which is similar to that recently described for the insulin family of receptors. The role of ligand specific receptor mediated regulation in homologous or heterologous desensitization is reviewed in brief for the peptides of the VIP family as well as the appearance of the specific receptor during the ontogenesis or the cell differentiation. The co-distribution of plasma membrane receptors from other families further adds to the cell specificity resulting for each differentiated cell in unique patterns of recognizing site. Some examples of receptor-receptor interaction are given, indicating that the integration of the different signals by cells might occur at an early step through the transmembranair domain of the receptor.

Functional receptors for VIP, GIP, glucagon-29 and -37 in the HGT-1 human gastric cancer cell line

Three separate sets of receptors sensitive to VIP, GIP and pancreatic/entero-glucagons, have been characterized in HGT-1 cells. The order of relative potencies of VIP receptor agonists was VIP greater than rh GRF-43, rh GRF-29 greater than PHI greater than hp GRF-40, secretin. G-37 was about 4 times less potent than G-29 in HGT-1 cells (G-29 greater than G-37), whereas it was about 20 times more potent than G-29 in rat fundic glands (G-37 greater than G-29). Adenylate cyclase in HGT-1 cells was stimulated by VIP, G-29, G-37 and GIP, over a concentration from 3.16 X 10(-9) to 3.16 X 10(-7) M GIP. The experimental data: (1) support the enterogastrone activity of GIP, via adenylate cyclase activation and somatostatin release by gastric D cells; (2) demonstrate that HGT-1 cells originating from a human fundic tumor are sensitive to the glucagon-like peptides G-29 and -37, as rat fundic glands; (3) indicate that the pharmacological properties of the VIP receptor in this human gastric cell line are similar to those characterized in normal human gastric glands.

Interaction of gastric inhibitory polypeptide (GIP) with the insulin-secreting pancreatic beta cell line, In lll: characteristics of GIP binding sites

The interaction of GIP with its receptors in the hamster pancreatic insulin-secreting beta cell line, In lll, has been analyzed. 125I-labelled GIP used as tracer showed the same affinity as native GIP for the GIP binding sites. Binding of the tracer was time, temperature and cell concentration dependent. It was saturable, reversible and highly specific. Under equilibrium conditions, i.e. 2 hours at 13 degrees C, 20% and 25% of the tracer and of GIP binding sites were inactivated, respectively. Native GIP inhibited binding of 125I-labelled GIP in a dose-dependent manner, saturation of the GIP binding sites being obtained at 3. 10(-7) M peptide. Two types of GIP binding sites were found by Scatchard analysis, a small population with a high affinity for GIP (KD = 7 nM) and a large population with a low affinity (KD = 800 nM). The biphasic dissociation process confirmed the GIP binding sites heterogeneity. Apart from GIP, no peptide tested influenced the binding of the 125I-labelled GIP. The present data represents the first analysis of functionally relevant GIP binding sites in a insulin-secreting cell.

The GIP receptor on pancreatic beta cell tumor: molecular identification by covalent cross-linking

125I-GIP binds reversibly to a high affinity binding site in crude plasma membranes prepared from a hamster pancreatic beta cell tumor. The treatment of labeled membranes with the cross-linker dithiobis (succinimidylpropionate) prevents, to a greater extent, the rapid dissociation of 125I-GIP-membrane complexes which is observed when 10(-6) M native GIP is added. Polyacrylamide gel electrophoresis of membrane proteins reveals a major 125I-GIP-protein complex of Mr 64,000. This labeling decreases when increasing concentrations (10(-9) -10(-6)M) of native GIP are added but is not altered by other peptide hormones (tested at 10(-6)M) including glucagon, VIP and insulin. The Mr 64,000 complex is not observed in tissues which have no specific binding sites for GIP such as intestinal epithelium. Assuming one molecule of 125I-GIP is bound per molecule of protein, one protein with Mr 59,000 is identified as the specific GIP binding site.