Differences between natural and recombinant G protein-coupled receptor systems with varying receptor/G protein stoichiometry

The increasing accessibility of genetically engineered receptor systems for the study of drug-receptor interaction has led to a corresponding increase in the testing of new drug entities in recombinant receptor systems. In this article Terry Kenakin illustrates some possible conditions within these recombinant systems where the relative stoichiometry of the receptors to other cellular components may differ from that found in natural systems and where this difference may lead to anomalies in drug testing.

Coexistence of novel amylin-binding sites with calcitonin receptors in human breast carcinoma MCF-7 cells

Amylin, calcitonin (CT) and calcitonin gene-related peptide (CGRP) share limited structural homology including amino-terminal ring structures linked by a disulfide bridge and amidated carboxy-termini. Here, we have compared [125I]Bolton-Hunter-[Lys1] rat amylin ([125I]amylin) binding and the stimulation of cyclic AMP accumulation by human (h) amylin, hCT and hCGRP-I in the human breast carcinoma cell lines MCF-7 and T47D, which predominantly express hCT1a and hCT1b receptor isoforms (hCTR1a, hCTR1b) at a similar total number of hCT-binding sites. In MCF-7 cells, half-maximal inhibition (IC50) of [125I]amylin binding by human amylin was observed at 3.6 +/- 0.8 nM (n = 6). hCT and hCGRP-I displaced [125I]amylin binding with 22 and 66 times higher IC50. [125I]hCT binding was inhibited by hCT with an IC50 of 8.1 +/- 1.9 nM (n = 5), and human amylin and hCGRP-I were over 100 times less potent. In T47D cells, on the other hand, specific binding of [125I]amylin was not observed, but hCT inhibited [125I]hCT binding with an IC50 of 3.2 +/- 0.4 nM (n = 3), and human amylin and hCGRP-I had over 200 times higher IC50. In MCF-7 cells, half-maximal stimulation (EC50) of cyclic AMP accumulation by human amylin, hCT and hCGRP-I occurred at 1.4 +/- 0.2, 1.7 +/- 0.4 and 6.3 +/- 1.3 nM respectively. In T47D cells, the EC50 of hCT was 0.32 +/- 0.02 nM (n = 3), and 30- and 1900-fold higher with human amylin and hCGRP-I. In conclusion, the expression of hCTR1a and hCTR1b and [125I]hCT binding were indistinguishable in MCF-7 and T47D cells. Yet, [125I]amylin binding was only recognized in MCF-7 cells, consistent with a distinct amylin receptor.

Characterization of amylin and calcitonin receptor binding in the mouse alpha-thyroid-stimulating hormone thyrotroph cell line

Recently, a high affinity amylin binding site was identified in the mouse alpha-TSH thyrotroph cell line. In this study, we have characterized binding sites for 125I-salmon calcitonin (125I-sCT), 125I-rat alpha-calcitonin gene-related peptide (125I-CGRP), and 125I-rat amylin in alpha-TSH cells. Using 125I-CGRP or 125I-rat amylin, equilibrium was rapidly reached, and binding was fully reversible. Competition binding revealed the relative potency of peptides was sCT>amylin, CGRP>>rCT, which is similar to the specificity profile of amylin receptors characterized in rat brain. Furthermore, specific binding of 125I-rat amylin and 125I-CGRP to membrane preparations was reduced by 52% and 39%, respectively, in the presence of 20 microM GTP-gamma-s, indicating a requirement of G protein coupling for high affinity binding. In contrast, 125I-sCT binding reached equilibrium more slowly, was essentially irreversible, and was unaltered by GTP-gamma-s. Competition binding studies using 125I-sCT as radioligand demonstrated only weak interaction by CGRP or amylin, consistent with other described CT receptors. Assessment of ligand-induced cAMP accumulation and intracellular calcium signaling revealed a relative specificity profile of sCT>rCT with little or no second messenger signaling stimulated by amylin or CGRP, consistent with a C1-CT receptor phenotype. RT-PCR amplification of messenger RNA indicated that the predominant isoform was the C1a CT receptor. In cross-linking studies, 125I-rat amylin and 125I-CGRP specifically labeled a major band of relative molecular mass (Mr) approximately 80K, being approximately 10 kDa higher than the major 125I-sCT binding protein. Full deglycosylation of N-linked carbohydrates with endoglycosidase F reduced the Mr of each of the labeled proteins to approximately 50K. Cross-linked amylin or CT receptors were immunoprecipitated with C-terminally directed antimouse or antirat CT receptor antibodies but were not immunoprecipitated with nonimmune sera or antihuman CT receptor antibodies. The current data demonstrate expression of two biochemically distinct receptor phenotypes in mouse alpha-TSH cells, a CT receptor phenotype and an amylin receptor phenotype that have highly similar protein backbones.

Evidence for a physiological role of central calcitonin gene-related peptide (CGRP) receptors in the control of food intake in rats

In the present study, we investigated the role of central calcitonin gene-related peptide (CGRP) and amylin receptors in mediating the anorectic effects of CGRP and amylin in rats chronically cannulated in the lateral brain ventricle. Intracerebroventricular (ICV) injection of the CGRP and amylin receptor antagonist CGRP(8-37) failed to influence the anorectic effects of peripherally injected CGRP and amylin. CGRP(8-37) alone, however, increased food intake in food deprived rats when administered 2 h before food presentation. Under the same experimental conditions, the more specific amylin receptor antagonists amylin(8-37) or AC 187 did not affect food intake. We therefore conclude, that CGRP is a physiological regulator of food intake within the central nervous system, acting at central CGRP receptors. Peripheral receptors, however, are likely to mediate the anorectic effects of peripherally administered amylin and CGRP.

The selectivity and structural determinants of peptide antagonists at the CGRP receptor of rat, L6 myocytes

1. Potency orders were determined for a series of agonists and antagonists on the calcitonin gene-related peptide (CGRP) receptor of rat L6 myocytes. The agents tested were all shown to have been active against CGRP, amylin or adrenomedullin receptors. 2. AC187 had a pIC50 of 6.8 +/- 0.10, making it 14 fold less potent as an antagonist than CGRP8-37 (pIC50, 7.95 +/- 0.14). Amyline8-37 was equipotent to AC187 (pIC50, 6.6 +/- 0.16) and CGRP19-32 was 3 fold less potent than either (pIC50, 6.1 +/- 0.24). 3. [Ala11]-CGRP8-37 was 6 fold less potent than CGRP8-37, (pIC50, 7.13 +/- 0.14), whereas [Ala18]-CGRP8-37 was approximately equipotent to CGRP8-37 (pIC50, 7.52 +/- 0.15). However, [Ala11,Ala18]-CGRP8-37 was over 300 fold less potent than CGRP8-37 (pIC50, 5.30 +/- 0.04). 4. [Tyr0]-CGRP28-37, amylin19-37 and adrenomedullin22-52 were inactive as antagonists at concentrations of up to 1 microM. 5. Biotinyl-human alpha-CGRP was 150 fold less potent than human alpha-CGRP itself (EC50 values of 48 +/- 17 nM and 0.31 +/- 0.13 nM, respectively). At 1 microM, [Cys(acetomethoxy)2,7]-CGRP was inactive as an agonist. 6. These results confirm a role for Arg11 in maintaining the high affinity binding of CGRP8-37. Arg18 is of less direct significance for high affinity binding, but it may be important in maintaining the amphipathic nature of CGRP and its analogues.

Role of amylin in nutrient intake - animal studies

The pancreatic hormone amylin is co-secreted with insulin by beta-cells in response to nutrient intake. Studies performed in experimental animals have provided evidence that amylin may have several effects associated with carbohydrate metabolism. Amylin is a potent inhibitor of gastric emptying. This effect appears to require an intact vagus nerve and it is over-ridden by hypoglycaemia. These observations, coupled with the identification of putative amylin receptors in the area postrema of the hindbrain (a region implicated in the regulation of gastric motility) suggest that the effects of amylin on gastric emptying are mediated, at least in part, by the central nervous system. There is also evidence that amylin acts to inhibit food intake, an action which is distinct from its effects on gastric emptying. In addition, amylin has been shown to inhibit amino acid-stimulated glucagon secretion, suggesting that it may reduce endogenous glucose production in the postprandial period.

Rat amylin mediates a pressor response in the anaesthetised rat: implications for the association between hypertension and diabetes mellitus

Amylin (or islet amyloid polypeptide) has been reported to have binding sites in the central nervous system and the kidney and has been shown to activate plasma renin. It has been postulated that this peptide may be an important mechanistic link between hypertension and diabetes in the insulin resistance syndrome. To explore this issue, the effects of rat amylin on mean arterial blood pressure were investigated in anaesthetised rats. Amylin elicited a pressor response of approximately 10 mmHg (maximal at 100 pmol.kg-1) which was apparent within 30-60 s and persisted over 15 min. At higher concentrations amylin elicited a hypotensive response (negative log IC50 8.52 mol.kg-1). The novel amylin receptor antagonist AC413 (12 nmol.kg-1.min-1) reduced the pressor response but not the hypotensive effects of amylin. The peptide antagonist calcitonin gene-related peptide (CGRP)8-37 (12 nmol.kg-1.min-1) reduced the pressor response elicited by amylin and also antagonized the hypotensive effect of amylin. Pre-treatment of animals with the ganglion blocker mecamylamine (3 mg.kg-1 s.c.) reduced the pressor effect of amylin. Following the administration of the angiotensin converting enzyme inhibitor ramiprilat (300 nmol.kg-1 i.v.) the pressor response to amylin was reduced. Salmon calcitonin also elevated blood pressure in the anaesthetised rat; doses of amylin and salmon calcitonin associated with a pressor effect were associated with increases in plasma renin activity. We conclude that amylin may act centrally to elevate blood pressure in the anaesthetised rat, possibly through activation of the renin angiotensin system.

Multiple receptors for calcitonin gene-related peptide and amylin on guinea-pig ileum and vas deferens

1. The responses of the electrically stimulated guinea-pig ileum and vas deferens to human and rat calcitonin gene-related peptide (CGRP) and amylin were investigated. 2. The inhibition of contraction of the ileum produced by human alpha CGRP was antagonized by human alpha CGRP8-37 (apparent pA2 estimated at 7.15 +/- 0.23) > human alpha CGRP19-37 (apparent pA2 estimated as 6.67 +/- 0.33) > [Tyr0]-human alpha CGRP28-37. The amylin antagonist, AC187, was three fold less potent than CGRP8-37 in antagonizing human alpha CGRP. 3. Both human beta- and rat alpha CGRP inhibited contractions of the ileum, but this was less sensitive to inhibition by CGRP8-37 than the effect of human alpha CGRP. However, CGRP19-37 was twenty times more effective in inhibiting the response to rat alpha CGRP (apparent pA2 estimated as 8.0 +/- 0.1) compared to human alpha CGRP. 4. Rat amylin inhibited contractions in about 10% of ileal preparations; this effect was not antagonized by any CGRP fragment. Human amylin had no action on this preparation. 5. Both human and rat alpha CGRP inhibited electrically stimulated contractions of the vas deferens, which were not antagonized by 3 microM CGRP8-37 or 10 microM AC187. 6. Rat amylin inhibited the stimulated contractions of the vas deferens (EC50 = 77 +/- 9 nM); human amylin was less potent (EC50 = 213 +/- 22 nM). The response to rat amylin was antagonized by 10 microM CGRP8-37 (EC50 = 242 +/- 25 nM) and 10 microM AC187 (EC50 = 610 +/- 22 nM). 7. It is concluded that human alpha CGRP relaxes the guinea-pig ileum via CGRP1-like receptors, but that human beta CGRP and rat alpha CGRP may use additional receptors. These are distinct CGRP2-like and amylin receptors on guinea-pig vas deferens.

Amylin mobilizes [Ca2+]i and stimulates the release of pancreatic digestive enzymes from rat acinar AR42J cells: evidence for an exclusive receptor system of amylin

Amylin dose-dependently stimulated the secretion of amylase and cholesterol esterase from rat pancreatic acinar AR42J cells. The biochemical basis of this action was investigated using fura-2-loaded AR42J cells. Amylin increased intracellular free calcium. [Ca2+]i, in a dose-dependent manner. The Ca2+ signal persisted even in Ca(2+)-free medium, suggesting mobilization from intracellular stores rather than influx. Consistently, thapsigargin abolished amylin-induced responses, suggesting that Ca2+ is released from an inositol 1,4,5-triphosphate (IP3)-sensitive pool. This was confirmed by the finding that amylin elevated IP3 levels. AR42J cells pretreated with amylin did not respond to amylin, suggesting that the receptors mediating this response undergo homologous desensitization. However, pretreatment with related peptides, calcitonin gene-related peptide (CGRP) and salmon calcitonin, did not diminish [Ca2+]i mobilization by amylin. CGRP and calcitonin also failed to mobilize [Ca2+]i even at 10 microM. These results suggests that the stimulatory effects of amylin on pancreatic digestive enzyme secretion from AR42J cells are mediated by a G-protein-linked membrane receptor coupled to IP3-dependent calcium pools.

Attenuation of the anorectic effects of glucagon, cholecystokinin, and bombesin by the amylin receptor antagonist CGRP(8-37)

The anorectic effect of IP injection of amylin (1 microgram/kg) was abolished by simultaneous IP injection of the amylin receptor antagonist calcitonin gene-related peptide-(8-37) [CGRP(8-37), 10 micrograms/kg]. The IP injection of pancreatic glucagon (400 micrograms/kg) at dark onset also reduced food intake in 24-h food-deprived rats, and this effect was also totally blocked by coadministration of CGRP(8-37) (10 micrograms/kg). In another feeding paradigm with glucagon (540 micrograms/kg IP 3 h into the light phase in 3 h-prefed rats), however, the anorectic effect of glucagon was not significantly antagonized by CGRP(8-37). The anorectic effect of cholecystokinin (CCK) (0.25 microgram/kg) and bombesin (BBS) (2 micrograms/kg) was partly neutralized by CGRP(8-37). In contrast, the anorectic effect of vasopressin (VP) (2.5 micrograms/kg) was not influenced by CGRP(8-37). As glucagon has been shown previously to increase the secretion of amylin, we conclude that the anorectic effect of peripherally administered glucagon is mediated by the release of amylin, at least under certain conditions. This may also be true for CCK and BBS, as these peptides are insulinotropic and may therefore be presumed to increase amylin release.

Pharmacology of receptors for calcitonin gene-related peptide and amylin

Calcitonin gene-related peptide (CGRP), a widespread neuropeptide with multiple actions, has substantial homology with amylin, a peptide implicated in insulin-resistant diabetes, and adrenomedullin, a recently discovered potent vasodilator. There is controversy over the existence of CGRP receptor subtypes, and whether independent receptors exist for amylin and adrenomedullin. In this article, the current status of CGRP receptor classification is reviewed by David Poyner, taking particular account of species differences, and evidence is presented supporting the existence of multiple receptors for CGRP, as well as independent binding sites for amylin.

Differential calcitonin gene-related peptide (CGRP) and amylin binding sites in nucleus accumbens and lung: potential models for studying CGRP/amylin receptor subtypes

Calcitonin gene-related peptide (CGRP), a 37-amino-acid peptide, is a member of a small family of peptides including amylin or islet amyloid polypeptide and salmon calcitonin. These related peptides have been shown to display similar effects on in vitro and in vivo carbohydrate metabolism. The present study was initiated to identify and characterize the binding sites for these peptides in lung and nucleus accumbens membranes prepared from pig and guinea pig. Both tissues in either species displayed high-affinity (2-[125I]iodohistidyl10)humanCGRP alpha ([125I]hCGRP alpha) binding (IC50 = 0.4-7.7 nM), which was displaced by hCGRP8-37 alpha with equally high affinity (IC50 = 0.4-7.3 nM). High-affinity binding for [125I]Bolton-Hunter human amylin ([125I]BH-h-amylin) was also observed in these tissues (IC50 = 0.2-6.0 nM). In membranes from the nucleus accumbens of both species, salmon calcitonin competed for amylin binding sites with high affinity (IC50 = 0.1 nM) but was poor in competing for amylin binding in lung membranes. Rat amylin8-37 competed for [125I]hCGRP alpha binding with higher affinity (IC50 = 5.4 nM) compared with [125I]BH-h-amylin binding (IC50 = 200 nM) in porcine nucleus accumbens, whereas in guinea pig nucleus accumbens, the IC50 values for rat amylin8-37 were 117 and 12 nM against [125I]hCGRP alpha and [125I]BH-h-amylin, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative affinities of human adrenomedullin for 125I-labelled human alpha calcitonin gene related peptide ([125I]hCGRP alpha) and 125I-labelled Bolton-Hunter rat amylin ([125I]BHrAMY) specific binding sites in the rat brain

Adrenomedullin (ADM) is a recently identified peptide that shows some homology (approximately 25%) with calcitonin gene related peptide (CGRP) and is now considered to be a new member of this peptide family. Because it shares biological effects with CGRP, we evaluated the possible affinity of human adrenomedullin (hADM) for 125I-labelled human CGRP alpha ([125I]hCGRP alpha) binding sites in the rat brain. Moreover, we evaluated the potential existence of cross-reactivity for 125I-labelled Bolton-Hunter rat amylin ([125I]BHrAMY), another member of this peptide family. In all brain areas investigated, hADM only competed with relatively low affinities for both [125I]hCGRP alpha and [125I]BHrAMY binding sites, with IC50 values generally in the high nanomolar-low micromolar range, the lowest affinity being observed for [125I]BHrAMY binding sites. Interestingly, the lowest affinities of hADM against both radioligands were detected in the nucleus accumbens and ventral striatum. These areas are known to be enriched with atypical CGRP - salmon calcitonin - amylin sensitive sites. It thus appears that hADM is unlikely to bind to this atypical site. Moreover, hADM demonstrated limited affinity for either [125I]hCGRP alpha or [125I]BHrAMY binding sites in the rat brain. This suggests that the potential biological effects of ADM in the brain could be mediated through a different class of receptors with higher affinity for this newly isolated peptide.

Receptors for calcitonin, calcitonin gene related peptide, amylin, and adrenomedullin

Calcitonin, calcitonin gene related peptide, amylin, and adrenomedullin are structurally related polypeptides characterized by a six or seven amino acid ring structure linked by a disulfide bridge and an amidated C-terminus. They exhibit overlapping biological actions as a result of cross-reactivity between the different receptors. In this article, the respective receptors and G-protein-coupled postreceptor events are reviewed in relation to some of the biological actions of the peptides.

Comparative distribution of receptors for amylin and the related peptides calcitonin gene related peptide and calcitonin in rat and monkey brain

The distribution of amylin receptors (125I-labelled rat amylin) in brains of rat and monkey were mapped and compared with the distribution of receptors for calcitonin (CT) (125I-labelled salmon CT) and calcitonin gene related peptide (CGRP) (rat, 125I-labelled rat CGRP alpha; monkey, 125I-labelled human CGRP alpha. In rat, amylin receptors were discretely distributed with the highest receptor densities found in mid-caudal accumbens nucleus, parts of the bed nucleus of the stria terminalis, amygdala, and hypothalamus. Moderate to high densities of binding also occurred in the area postrema, subfornical organ, vascular organ of the lamina terminalis, locus ceruleus, dorsal raphe, and caudal solitary tract nucleus. In monkey, the distribution of amylin binding sites was similar, although the highest densities of receptors were in the hypothalamus, with relatively fewer sites present in the accumbens nucleus. In rat, the distribution of amylin receptors formed a subset of the receptor distributions for 125I-labelled salmon CT and 125I-labelled rat CGRP alpha. In contrast, in monkey, although the amylin receptors again formed a subset of the binding sites identified with 125I-labelled salmon CT, there was very little overlap with the pattern of CGRP receptor distribution. This suggests that the specificity profile of amylin receptors in primates differs from that of amylin receptors in the rat, with CGRP alpha having relatively lower affinity for the primate amylin receptors.

Differential antagonism of amylin's metabolic and vascular actions with amylin receptor antagonists

High affinity amylin binding sites are present in the rat nucleus accumbens. These sites bind [125I]amylin with an affinity of 27 pM and have high affinity for salmon calcitonin (sCT) and moderately high affinity for calcitonin gene related peptide (CGRP). N-terminally truncated peptides were tested for their ability to compete for [125I]amylin binding to these sites and to antagonize the metabolic and vascular actions of amylin. CGRP(8-37), sCT(8-32), and ac-[Asn30,Tyr32]sCT(8-32) (AC187) inhibited [125I]amylin binding to rat nucleus accumbens. Order of potency at inhibiting amylin binding (AC187 > sCT(8-32) > CGRP(8-37)) differed from the order of potency at inhibiting [125I]CGRP binding to SK-N-MC neuroblastoma cells (CGRP(8-37) > AC187 > sCT(8-32)) . AC187 was the most potent antagonist of amylin's effects on isolated rat soleus muscle glycogen metabolism, and it was more effective than either sCT(8-32) or CGRP(8-37) at reducing amylin-stimulated hyperlactemia in rats. In contrast, CGRP(8-37) was the most potent peptide at antagonizing amylin-induced hypotension in rats. Amylin's hypotensive actions appear to be mediated by a weak action at CGRP receptors, while its metabolic actions are mediated by receptors with a distinct antagonist profile. AC187 is a potent antagonist of amylin binding sites in nucleus accumbens and of amylin's metabolic actions.

Binding sites for islet amyloid polypeptide in mammalian lung: species variation and effects on adenylyl cyclase

Islet amyloid polypeptide (IAPP) and calcitonin gene related peptide (CGRP) share a 47% sequence homology. IAPP can interact with adenylyl cyclase coupled CGRP receptors. We have examined [125I]IAPP binding in mouse, pig, and guinea pig lung membranes in competition with IAPP, CGRP, and CGRP(8-37). Three types of site were shown by order of potency: (i) mouse, IAPP > CGRP(8-37) >> CGRP; (ii) pig, CGRP > IAPP > CGRP(8-37); and (iii) guinea pig, CGRP = IAPP = CGRP(8-37). Chemical cross-linking of [125I]IAPP and [125I]CGRP binding sites in lung demonstrated that both sites had similar molecular weights in any one species but differed across species, i.e., mouse M(r) = 70,000 and 98,000; pig M(r) = 68,000, 56,000, and 47,000; and guinea pig M(r) = 106,000 and 56,000. Adenylyl cyclase activity was stimulated by forskolin and AlCl3-NaF in rat, mouse, pig, and guinea pig membranes. Only in mouse and pig were CGRP and IAPP able to stimulate adenylyl cyclase activity. In mouse lung CGRP and IAPP stimulated adenylyl cyclase activity with EC50 values of 642 +/- 222 nM (n = 4) and 325 +/- 115 nM (n = 4), respectively. In pig lung membranes EC50 values were 5.7 +/- nM (n = 4) for CGRP and 1230 +/- 1130 nM (n = 4) for IAPP. Thus IAPP either did not stimulate adenylyl cyclase activity in these lung membranes or did so with a low potency.

Calcitonin, calcitonin gene-related peptide, adrenomedullin and amylin: homologous peptides, separate receptors and overlapping biological actions

Calcitonin, calcitonin gene-related peptide, adrenomedullin and amylin are structurally related peptides with N-terminal 6-7 amino acid ring structures linked by a disulfide bridge and with amidated C-termini. Among the related bioactive peptides, the structures of the calcitonin receptor and subtypes thereof have been identified so far through molecular cloning. Cross-reaction between receptors of calcitonin, calcitonin gene-related peptide, adrenomedullin and amylin, as well as overlapping biological actions, anticipate that the respective receptors belong to a family of G-protein-coupled receptors that include those of parathyroid hormone, secretin and vasointestinal peptide.

Islet amyloid polypeptide: does it play a pathophysiological role in the development of diabetes?

There is suggestive evidence that amylin acts physiologically in an autocrine manner within the islet to restrain insulin secretion, but conversely there is little indication that this action of amylin plays any role in the development of NIDDM. Deposition of amylin within pancreatic islets is a feature in patients with NIDDM but is of sufficient degree to disrupt beta-cell function in only a small minority of individuals. Current evidence suggests that amylin does not have any physiologically important extra-islet metabolic effects. The potential exists for the development of amylin antagonists as pharmacological agents to enhance insulin secretion in NIDDM but antagonism of systematic CGRP would need to be avoided. There is little, if any, indication that either replacement of amylin or treatment with amylin agonists are likely to have any beneficial role in patients with IDDM.

In vitro autoradiographic localization of amylin binding sites in rat brain

Amylin is a recently discovered 37 amino acid peptide which is co-secreted from the pancreas with insulin and acts to modulate carbohydrate metabolism. Recently, high-affinity binding sites for [125I]rat amylin have been identified in the rat central nervous system. These sites also have high affinity for the structurally related peptides calcitonin gene-related peptide and salmon calcitonin. In the present study we have used in vitro autoradiography to map the distribution of these [125I]rat amylin binding sites in rat brain. High to moderate levels of binding were present in mid-caudal accumbens nucleus, fundus striati and parts of the bed nucleus of the stria terminalis and substantia inominata. This binding extended caudally into parts of the amygdalostriatal transition zone and the central and medial amygdaloid nuclei. High to moderate levels of binding also occurred in much of the hypothalamus including the medial preoptic, dorsomedial hypothalamic and medial tuberal nuclei as well as the ventrolateral subnucleus of the ventromedial hypothalamic nucleus. Other regions of high level binding included the subfornical organ, the vascular organ of the lamina terminalis, area postrema, locus coeruleus, dorsal raphe and caudal parts of the nucleus of the solitary tract. The subfornical organ, vascular organ of the lamina terminalis and area postrema, which display some of the highest binding densities, lack a patent blood-brain barrier and thus could be responsive to blood-borne amylin. In conclusion we have mapped, in detail, the distribution of amylin binding sites in rat brain. The location of binding is consistent with potential roles for these sites in appetite, fluid and electrolyte homeostasis, autonomic function and regulation of mood.

Autoradiographic distribution and receptor binding profile of [125I]Bolton Hunter-rat amylin binding sites in the rat brain

Amylin is a recently isolated peptide from amyloid plaques in noninsulin-dependent diabetic patients and showed high sequence homology with calcitonin gene-related peptide. We investigated the distribution and the binding profile of [125I]Bolton Hunter-rat amylin ([125I]BH-rat amylin) binding sites in the rat brain, as well as the affinity of rat amylin for [125I]hCGRP alpha binding sites in the brain, atrium (CGRP1 receptor-enriched tissue) and vas deferens (CGRP2 receptor-enriched tissue). High amounts of high affinity [125I]BH-rat amylin binding sites were observed in the nucleus accumbens, various hypothalamic nuclei, amygdaloid body, dorsal raphe, tegmental and parabrachial nuclei and the locus ceruleus. Interestingly, both rat amylin and salmon calcitonin revealed low nanomolar affinities (2-19 nM) for [125I] BH-rat amylin binding sites in the various brain areas, whereas human calcitonin gene-related peptide-alpha (hCGRP alpha) showed lower affinities ranging between 13 to 150 nM. Moreover, the affinity of rat amylin was much lower than that of hCGRP alpha for [125I]hCGRP alpha binding in the brain, atrium and vas deferens, except for very few areas such as the nucleus accumbens and ventral striatum. Similarly, rat amylin was much weaker (100- to 400-fold) than hCGRP alpha to induce a biological effect in the atrium and vas deferens. These results thus suggest the existence of unique [125I]BH-rat amylin binding sites in the rat brain as well as limited cross-reactivity between rat amylin and [125I]hCGRP alpha receptors present in the brain, atrium and vas deferens.

Calcitonin gene-related peptide and islet amyloid polypeptide stimulate insulin secretion in RINm5F cells through a common receptor coupled to a generation of cAMP

The question as to whether the homologous peptides CGRP and IAPP can regulate insulin secretion in RINm5F cells was addressed. Chicken CGRP displayed a reproducible inhibitory effect on insulin secretion within 0.1 and 1 nM concentrations and a stimulatory effect at higher concentrations. The maximal stimulatory effects on insulin secretion were obtained with 1.0 microM of chicken CGRP (cCGRP), human alpha-CGRP (h alpha-CGRP) and human IAPP (hIAPP) which caused 246 +/- 22, 302 +/- 63 and 224 +/- 14 percent increases of control levels, respectively (p < 0.001). Similarly, maximal accumulations of cAMP were obtained with 1.0 microM of cCGRP, h alpha-CGRP and hIAPP with the respective percent increases of control levels of 587 +/- 24, 436 +/- 41 and 410 +/- 25 (p < 0.005). Thus the stimulatory effects on insulin secretion in RINm5F cells by cCGRP, h alpha-CGRP and hIAPP appear to be mediated by the cAMP pathway. Chicken CGRP, the most potent peptide tested, displayed a correlated dose response stimulation of intracellular cAMP and insulin release within the concentration range of 10-1000nM. The EC50 values of cCGRP for cAMP accumulation and insulin release were similar (20nM and 10nM respectively). The stimulatory effect of IAPP on cAMP was not additive with that of cCGRP suggesting that IAPP action was mediated by CGRP receptors. This hypothesis was further sustained by a preferential inhibition of 125I[His]h alpha-CGRP binding to RINm5F cells by cCGRP as compared to IAPP. We conclude that CGRP and IAPP, through a direct action on a chicken CGRP preferring receptor present in beta cells, stimulated insulin by a cAMP mediated pathway.

Solubilization of binding sites for IAPP and CGRP from rat lung

Functional binding sites for [125I]IAPP and [125I]CGRP were solubilized from rat lung membranes with CHAPSO (10 mM). Rat IAPP had a higher affinity (Ki = 22.9 nM) for [125I]IAPP binding and rat alpha CGRP (Ki = 0.904 nM) had a higher affinity for [125I]CGRP binding over related peptides. [125I]IAPP binding was unaffected by GTP gamma S, but [125I]CGRP binding was 50% inhibited, indicating solubilization of a G-protein-receptor complex for CGRP but not IAPP binding. Wheat germ agglutinin affinity columns gave a 25-fold purification of IAPP binding sites, but no CGRP binding sites were eluted from the column, indicating different patterns of glycosylation of the two sites.

Regulation of hepatocyte adenylate cyclase by amylin and CGRP: a single receptor displaying apparent negative cooperatively towards CGRP and simple saturation kinetics for amylin, a requirement for phosphodiesterase inhibition to observe elevated hepatocyte cyclic AMP levels and the phosphorylation of Gi-2

Challenge of intact hepatocytes with amylin only succeeded in elevating intracellular cyclic AMP levels and activating phosphorylase in the presence of the cAMP phosphodiesterase inhibitor IBMX. Both amylin and CGRP similarly activated adenylate cyclase, around 5-fold, although approximately 400-fold higher levels of amylin were required to elicit half maximal activation. Amylin activated adenylate cyclase though apparently simple Michaelien kinetics whereas CGRP elicited activation by kinetics indicative of apparent negative co-operativity. Use of the antagonist CGPP(8-37) showed that both CGRP and amylin activated hepatocyte adenylate cyclase through a common receptor by a mnemonical mechanism where it was proposed that the receptor co-existed in interconvertible high and low affinity states for CGRP. It is suggested that this model may serve as a paradigm for G-protein linked receptors in general. Amylin failed to both stimulate inositol phospholipid metabolism in hepatocytes and to elicit the desensitization of glucagon-stimulated adenylate cyclase. Amylin did, however, elicit the phosphorylation of the inhibitory guanine nucleotide regulatory protein Gi-2 in hepatocytes and prevented the action of insulin in reducing the level of phosphorylation of this G-protein.

High affinity amylin binding sites in rat brain

Amylin, a 37-amino acid peptide structurally related to calcitonin gene-related peptide, is synthesized in and released along with insulin from pancreatic beta-cells. Amylin is proposed to act as an endocrine partner to insulin, in part through actions upon skeletal muscle that promote cycling of gluconeogenic precursors to liver. We report here that binding sites with high affinity (Kd = 27 pm) for radioiodinated rat amylin are present in the nucleus accumbens region of rat brain. Competition experiments show that sites measured in nucleus accumbens membranes have high affinity for rat amylin, lower affinity for rat calcitonin gene-related peptides, and very low affinity for rat calcitonin. In contrast to rat calcitonin, salmon calcitonin has a high affinity for these sites, indicating that it shares critical binding determinants with amylin. We further tested whether salmon calcitonin shares with amylin the ability to regulate glycogen metabolism in rat skeletal muscle. Salmon calcitonin potently inhibits insulin-stimulated glucose incorporation into rat soleus muscle glycogen, suggesting that rat skeletal muscle may also contain receptor populations that have high affinity for both amylin and salmon calcitonin.

Islet amyloid polypeptide: a review of its biology and potential roles in the pathogenesis of diabetes mellitus

Islet amyloidosis (IA) is the principal lesion in the endocrine pancreas of human beings with non-insulin-dependent diabetes mellitus (NIDDM) and in the similar forms of diabetes mellitus in domestic cats and macaques. As such, the delineation of the pathogenesis of this form of amyloidosis may be crucial to the understanding of the development and progression of NIDDM. Islet amyloid polypeptide (IAPP) is a recently discovered polypeptide that is the principal constituent of IA in human beings, cats, and macaques. IAPP is produced by the pancreatic beta-cells and is co-packaged with insulin in the beta-cell secretory vesicles. Immunohistochemical and physiologic evidence supports the notion that the beta-cells are heterogenous with respect to their relative contents of insulin and IAPP. Therefore, although IAPP is co-secreted with insulin in response to a variety of well-known insulin secretagogues, the molar ratio of these two proteins that is released from the islets may vary, depending upon the glucose concentration and prevailing metabolic milieu. IAPP is highly conserved among mammalian species and has about 45% homology to another neuropeptide, calcitonin gene-related peptide. IAPP is encoded by a single-copy gene located, in the human being, on chromosome 12. IAPP is expressed as a 93 (murine)-89 (human)-amino acid prepropolypeptide that is processed enzymatically, resulting in the removal of amino- and carboxy-terminal propeptide segments. The 20-29 region of the IAPP molecule is most important in the ability of IAPP to form amyloid fibrils. The role of IAPP and IA in the pathogenesis of human NIDDM and similar forms of diabetes mellitus in cats and macaques may involve several possible mechanisms, including 1) direct physical/chemical damage to beta-cells, resulting in necrosis and loss of functional islet tissue, 2) biologic activities of IAPP that oppose those of insulin or abnormally suppress insulin secretion, and 3) interference by IA deposits of passage of insulin out of beta-cells and/or entrance of glucose and other secretogogues into the islet. The roles of each of these possible mechanisms have yet to be demonstrated. In addition, the physiological significance of the apparent IAPP deficiency in both insulin-dependent diabetes mellitus and NIDDM is currently unknown.

A mnemonical or negative-co-operativity model for the activation of adenylate cyclase by a common G-protein-coupled calcitonin-gene-related neuropeptide (CGRP)/amylin receptor

Both amylin and calcitonin-gene-related neuropeptide (CGRP) activated adenylate cyclase activity in hepatocyte membranes around 5-fold in a dose-dependent fashion, with EC50 values of 120 +/- 14 and 0.3 +/- 0.14 nM respectively. Whereas amylin exhibited normal activation kinetics (Hill coefficient, h approximately 1), CGRP showed kinetics indicative of either multiple sites/receptor species having different affinities for this ligand or a single receptor species exhibiting apparent negative co-operativity (h approximately 0.21). The CGRP antagonist CGRP-(8-37)-peptide inhibited adenylate cyclase stimulated by EC50 concentrations of either amylin or CGRP. Inhibition by CGRP-(8-37) was selective in that markedly lower concentrations were required to block the action of amylin (IC50 = 3 +/- 1 nM) compared with that of CGRP itself (IC50 = 120 +/- 11 nM). Dose-effect data for inhibition of CGRP action by CGRP-(8-37) showed normal saturation kinetics (h approximately 1), whereas CGRP-(8-37) inhibited amylin-stimulated adenylate cyclase activity in a fashion which was indicative of either multiple sites or apparent negative co-operativity (h approximately 0.24). Observed changes in the kinetics of inhibition by CGRP-(8-37) of CGRP, but not amylin-stimulated adenylate cyclase, at concentrations of agonists below their EC50 values militated against a model of two distinct populations of non-interacting receptors each able to bind both amylin and CGRP. A kinetic model is proposed whereby a single receptor, capable of being activated by both CGRP and amylin, obeys either a mnemonical kinetic mechanism or one of negative co-operativity with respect to CGRP but not to amylin. The relative merits of these two models are discussed together with a proposal suggesting that the activation of adenylate cyclase by various G-protein-linked receptors may be described by a mnemonical model mechanism.

Characterization of amylin binding sites in a human hepatoblastoma cell line

Amylin binding sites in a human hepatoblastoma cell line (HepG2) have been characterized in detail. 125I-Amylin (rat) bound to HepG2 cells with high affinity. Binding was reversible and selective, and dependent on time and temperature. Scatchard analysis revealed the presence of high (Kd = 0.11 +/- 0.04 nM) and low (Kd = 1.3 +/- 0.4 microM) affinity binding sites for 125I-amylin in HepG2 cells. The dissociation experiments also showed that 125I-amylin dissociated from high- and low-affinity sites. The association data, however, indicated the presence of only one binding site. Rat amylin was more potent than human amylin and rat calcitonin gene-related peptide (CGRP) in displacing 125I-amylin bound to HepG2 cells. Nonhomologous peptides did not displace 125I-amylin. Rat amylin was, however, less potent than rat CGRP in displacing 125I[Tyr0]CGRP from HepG2 cells. Pretreatment of HepG2 cells with rat amylin (10 nM) reduced the specific binding of 125I-amylin by 75%, whereas rat CGRP (10 nM) pretreatment had no effect on amylin binding. Calcitonin gene-related peptide, as well as rat and human amylin, stimulated the adenylate cyclase activity of HepG2 cell membrane preparation in a dose-dependent manner, with an order of potency of CGRP > rat amylin > human amylin. A CGRP antagonist, CGRP(8-37), significantly attenuated the stimulatory effect of both amylin and CGRP on adenylate cyclase activity. These investigations show that distinct receptors of amylin and CGRP are present in HepG2 cells, and that amylin stimulates adenylate cyclase activity through CGRP receptors. This system could now be exploited for studying amylin receptors and amylin-mediated signal transduction.

8-37hCGRP, an amylin receptor antagonist, enhances the insulin response and perturbs the glucose response to infused arginine in anesthetized rats

When 2 mmol L-arginine was infused into non-fasted, anesthetized rats at a rate slow enough to avoid hemodynamic disturbance, there was a rise in plasma glucose concentration followed by a decline to pre-infusion levels. In animals pre-infused with 8-37hCGRP, a fragment of calcitonin gene-related peptide that blocks amylin's hyperglycemic action, the normal initial rise in plasma glucose was accompanied by an enhanced rise in plasma insulin and was then followed by an enhanced fall in plasma glucose. These perturbations of the insulin and glucose response during amylin receptor blockade are difficult to explain without invoking a role for endogenous amylin; they further suggest an autocrine/paracrine role for amylin at the pancreatic islet.

Investigation and characterization of binding sites for islet amyloid polypeptide in rat membranes

Islet amyloid polypeptide (IAPP) is a 37-amino acid peptide shown to be cosecreted with insulin from the pancreatic beta-cells. We have investigated the existence and characteristics of IAPP binding sites in the rat. Specific binding sites for [125I]IAPP were found to be highest in the lung followed by the stomach fundus, spleen, brain stem, hypothalamus, and the liver, respectively. The interaction of [125I]IAPP with its binding site was rapid and temperature dependent, displaying optimum binding at 4 C. This may be explained by the rapid degradation of the label observed at 22 C and 37 C, as determined by fast protein liquid chromatography analysis, and also degradation of the receptor at 37 C. Binding of [125I]IAPP was rapidly dissociated by the addition of 200 nM unlabeled peptide. The presence of nonmetabolizable GTP-gamma-S (0.5 microM) reduced binding, thus suggesting the coupling of the binding site to a G protein. Rat IAPP displaced [125I]IAPP displaying an IC50 of 5.75 x 10(-9) M (mean, n = 4). Displacement was also seen with human IAPP (IC50 = 5.53 x 10(-8) M), human alpha-calcitonin gene-related peptide (CGRP) (IC50 = 3.8 x 10(-8) M), rat alpha-CGRP (IC50 = 9.0 x 10(-7) M), and rat beta-CGRP (IC50 = 5.53 x 10(-8) M); suggesting an IAPP-specific binding site. Scatchard plots for rat IAPP binding in the lung gave a dissociation constant of 10.4 +/- 2.63 nM (mean +/- SE, n = 4) and maximal binding of 3.1 +/- 0.97 pmol/mg (mean +/- SE, n = 4), displaying a single class of binding site. Chemical cross-linking analysis showed binding of IAPP to sites of Mr 67,000, 64,000, and 38,000. These findings suggest that specific IAPP binding sites exist which differ from the CGRP receptors in rat tissues. This indicates a possible novel autocrine/paracrine role for IAPP.