Physiology of GIP--a lesson from GIP receptor knockout mice.

A much greater insulin response is observed after oral glucose load than after intravenous injection of glucose. The hormonal factor(s) implicated as transmitters of signals from the gut to pancreatic beta-cells was referred to incretin; gastric inhibitory polypeptide or glucose-dependent insulinotropic polypeptide (GIP) is identified as one of the incretins. GIP exerts its effects by binding to its specific receptor, the GIP receptor, which is expressed in various tissues including pancreatic islets, adipose tissue, and brain. However, the physiological role of GIP has been generally thought to stimulate insulin secretion from pancreatic beta-cells, and the other actions of GIP have received little attention. We have bred and characterized mice with a targeted mutation of the GIP receptor gene. From these studies, we now know that GIP not only mediates early insulin secretion by acting on pancreatic beta-cells, but also links overnutrition to obesity by acting on adipocytes.     

Biphasic effects of insulin on islet amyloid polypeptide membrane disruption

Type II diabetes, in its late stages, is often associated with the formation of extracellular islet amyloid deposits composed of islet amyloid polypeptide (IAPP or amylin). IAPP is stored before secretion at millimolar concentrations within secretory granules inside the β-cells. Of interest, at these same concentrations in vitro, IAPP rapidly aggregates and forms fibrils, yet within secretory granules of healthy individuals, IAPP does not fibrillize. Insulin is also stored within the secretory granules before secretion, and has been shown in vitro to inhibit IAPP fibril formation. Because of insulin's inhibitory effect on IAPP fibrillization, it has been suggested that insulin may also inhibit IAPP-mediated permeabilization of the β-cell plasma membrane in vivo. We show that although insulin is effective at preventing fiber-dependent membrane disruption, it is not effective at stopping the initial phase of membrane disruption before fibrillogenesis, and does not prevent the formation of small IAPP oligomers on the membrane. These results suggest that insulin has a more complicated role in inhibiting IAPP fibrillogenesis, and that other factors, such as the low pH of the secretory granule, may also play a role.     

Assessing the presence of abnormal regulation of cortisol secretion by membrane hormone receptors: in vivo and in vitro studies in patients with functioning and non-functioning adrenal adenoma.

Regulation of cortisol secretion by aberrant hormone receptors may play a role in the pathogenesis of ACTH-independent Cushing's syndrome. In this study, the topic was evaluated by combining in vivo and in vitro approaches. Cortisol responses to various stimuli (standard meal, GnRH + TRH, cisapride, vasopressin, glucagon) were assessed in 6 patients with clinical or subclinical adrenal Cushing's syndrome, and non-functioning adrenal adenoma in two cases. Abnormal responses were observed in three patients with Cushing's syndrome; one patient showed a gastric inhibitory polypeptide (GIP)-dependent cortisol rise after meal, together with responses after GnRH and cisapride; the second patient showed an LH-dependent cortisol response to GnRH, and in the third cortisol rose after cisapride. The pattern of receptor expression performed by RT-PCR showed that while GIP-R was only expressed in tumor from the responsive patient, 5-hydroxytryptamine type 4 receptor and LH-R were also present in normal adrenal tissues and tissues from non-responsive patients. Interestingly, an activating mutation of Gsalpha gene was identified in one of these tumors. Therefore, cortisol responses to agents operating via Gs protein coupled receptors (in one case associated with Gsalpha mutation) were found in Cushing's patients, while these responses were absent in the others. The finding of receptor expression in normal and non-responsive tumors suggests that different mechanisms are probably involved in inducing in vivo cortisol responses.     

Lack of inhibitory effect of alpha-human atrial natriuretic polypeptide on cortisol secretion in cultured adrenocortical adenoma cells from the patients with Cushing's syndrome

The effects of synthetic alpha-human atrial natriuretic polypeptide (alpha-hANP) on cortisol secretion by adrenocortical adenoma cells from patients with Cushing's syndrome (CS cells) in primary monolayer cultures, compared to cultured normal adrenal cells, were studied. alpha-hANP significantly inhibited cortisol secretion by human normal adrenal cells in culture, but had no direct effect on cortisol secretion from CS cells, in the presence or absence of 10(-8) M ACTH. alpha-hANP enhanced the accumulation of intracellular cyclic GMP in normal adrenal cells in culture, but not in CS cells. Visualization of [125I] iodo-alpha-hANP-specific binding sites by an in vitro receptor autoradiographic technique showed that these sites were lacking in adrenocortical adenoma tissues. These results suggest that the loss of alpha-hANP inhibitory effect on cortisol secretion in CS cells may be due to the absence of alpha-hANP receptor sites.     

Prolactin, progesterone, and dexamethasone coordinately and adversely regulate glucokinase and cAMP/PDE cascades in MIN6 beta-cells

Islet cells undergo major changes in structure and function to meet the demand for increased insulin secretion during pregnancy, but the nature of the hormonal interactions and signaling events is incompletely understood. Here, we used the glucose-responsive MIN6 beta-cell line treated with prolactin (PRL), progesterone (PRG), and dexamethasone (DEX, a synthetic glucocorticoid), all elevated during late pregnancy, to study their effects on mechanisms of insulin secretion. DEX alone or combined with PRL and PRG inhibited insulin secretion in response to 16 mM glucose-stimulating concentrations. However, in the basal state (3 mM glucose), the insulin levels in response to DEX treatment were unchanged, and the three hormones together maintained higher insulin release. There were no changes of protein levels of GLUT2 or glucokinase (GK), but PRL or PRG treatment increased GK activity, whereas DEX had an inhibitory effect on GK activity. alpha-Ketoisocaproate (alpha-KIC)-stimulated insulin secretion was also reduced by DEX alone or combined with PRL and PRG, suggesting that DEX may inhibit distal steps in the insulin-exocytotic process. PRL treatment increased the concentration of intracellular cAMP in response to 16 mM glucose, suggesting a role for cAMP in potentiation of insulin secretion, whereas DEX alone or combined with PRL and PRG reduced cAMP levels by increasing phosphodiesterase (PDE) activity. These data provide evidence that PRL and to a lesser extent PRG, which increase in early pregnancy, enhance basal and glucose-stimulated insulin secretion in part by increasing GK activity and amplifying cAMP levels. Glucocorticoid, which increases throughout gestation, counteracts only glucose-stimulated insulin secretion under high glucose concentrations by dominantly inhibiting GK activity and increasing PDE activity to reduce cAMP levels. These adaptations in the beta-cell may play an important role in maintaining the basal hyperinsulinemia of pregnancy while limiting the capacity of PRL and PRG to promote glucose-stimulated insulin secretion during late gestation.     

Dissimilar effects of transglutaminase inhibitors on peptide hormone-indiced secretion from testis and pituitary

Inhibitors of transglutaminase (monodansylcadaverine and bacitracin) reduced luteinizing hormone stimulated androgen and adenosine - 3′: 5′ - monophosphate production by testicular tissue but had no inhibitory effect on gonadotropin releasing hormone-stimulated luteinizing hormone secretion by the pituitary. These results indicate that there are differences in the mechanisms by which these polypeptide hormones stimulate hormone secretion and suggest a role for protein cross-linking in the mechanism of luteinizing hormone action in the testis.     

Cellular glucose availability and glucagon-like peptide-1

Glucagon-like peptide (GLP)-1 and gastric inhibitory polypeptide (GIP, glucose-dependent insulinotropic polypeptide) are produced in enteroendocrine L-cells and K-cells, respectively. They are known as incretins because they potentiate postprandial insulin secretion. Although unresponsiveness of type 2 diabetes (T2D) patients to GIP has now been reconsidered, GLP-1 mimetics and inhibitors of the GLP-1 degradation enzyme dipeptidyl peptidase (DPP)-4 have now been launched as drugs against T2D. The major roles of GLP-1 in T2D are reduction of appetite, gastric motility, glucagon secretion, enhancement of insulin secretion and β-cell survival. For insulin secretion and peripheral insulin function, GLP-1 and its mimetics sensitise β-cells to glucose; accelerate blood glucose withdrawal, in-cell glucose utilisation and glycogen synthesis in insulin-sensitive tissues; and assist in the function and survival of neurons mainly using glucose as an energy source. Taken together, GLP-1 acts to potentiate glucose availability of various cells or tissues to assist with their essential functions and/or survival. Herein, we review the signalling pathways and clinical relevance of GLP-1 in enhancing cellular glucose availability. On the basis of our recent research results, we also describe a mechanism that regulates GLP-1 for glucokinase activity. Because diabetic tissues including β-cells resist glucose, GLP-1 may be useful for treating T2D.     

Effect of glucagon-like peptide-1 on insulin secretion

The insulinotropic actions of two forms of glucagon-like peptide 1 (GLP-1) containing 31 and 37 amino acid residues on perfused rat pancreas were compared with that of gastric inhibitory polypeptide (GIP), hitherto the most potent intestinal insulinotropic polypeptide known. The smaller form, C-terminally amidated GLP-1-(7-36), strongly enhanced insulin secretion stimulated by 11.1 mM D-glucose at a concentration as low as 0.1 nM. Comparable effects of GIP and GLP-1-(1-37) on insulin secretion were observed at concentrations of 1.0 nM and 10.0 nM, respectively. At the doses tested, neither GLP-1s nor GIP had any effect on insulin secretion induced by 3.3 mM D-glucose. At a concentration of 1.0 nM, GLP-1-(7-36 amide) also enhanced insulin secretion induced by 5 mM L-arginine whereas at concentrations of up to 10.0 nM, GLP-1-(1-37) did not. The results show that the smaller form of GLP-1 is more strongly insulinotropic than GIP. These findings suggest that the smaller GLP-1 may have a physiologically more important role as a modulator of insulin release.     

Potentiation of Insulin Action in Normal Subjects by a Pituitary Polypeptide

A pituitary polypeptide, acceleratory polypeptide-growth hormone (ACG), has been found to increase the sensitivity of fasting normal people to intravenous insulin. It is suggested that diminished activity of this polypeptide—or increased activity of another polypeptide, inhibitory polypeptide-growth hormone (ING)—may have a role in the genesis of diabetes mellitus.     

Regulation of gastric acid secretion.

The advent of the H2-histamine-receptor antagonists has given new life to the old hypothesis that histamine might be the final common chemical mediator of acid secretion. The available evidence, however, does not prove this hypothesis but does confer on histamine a role in the regulation of acid secretion in normal physiology. Evidence is mounting that, in addition to its stimulatory action, the vagus may play an inhibitory role in acid secretion and gastrin release. Our concepts of the gastric phase of acid secretion have been extended by the discovery of cross distension reflexes in the stomach: the pyloro-oxyntic reflex for acid secretion and the oxyntopyloric reflex for gastrin release. In addition, digested protein has been shown to stimulate directly the oxyntic gland mucosa, but the evidence is against a role for this mechanism in the intact stomach. The hormone(s) responsible for the intestinal phase have not been isolated but the physiological characteristics of entero-oxyntin (a nongastrin, enteric substance that acts on the oxyntic cell) have been defined. Gastric inhibitory polypeptide is an excellent candidate for the entero-gastrone released by fat, but whether it is the sole enterogastrone released is yet to be determined.     

Effect of isradipine on urinary excretion of catecholamines and their metabolites in patients with pheochromocytoma]

An effect of calcium antagonist-isradipine-on catecholamines and their metabolites excretion with the urine in 4 patients with pheochromocytoma has been analysed. It was found that the excretion of OFFnoradrenaline and vanilaminomandelic acid is reduced. It may indicate inhibitory effect of calcium antagonists on catecholamines secretion in patients with pheochromocytoma.     

Enkephalinergic control of somatostatin secretion from the perfused rat stomach

A role for the enkephalins in the regulation of gastric somatostatin (SLI) secretion has been investigated in an isolated perfused rat stomach model. Both methionine- and leucine-enkephalins caused a dose-dependent inhibition of gastric inhibitory polypeptide (GIP) stimulated SLI secretion. Leu-enkephalin was one order of magnitude less potent than met-enkephalin: 50% inhibition by met-enkephalin was at 4 X 10(-9) M and with leu-enkephalin 3.5 X 10(-8) M. Naloxone (100 nM) had no effect on basal secretion but blocked the inhibitory action of met-enkephalin (1 nM or 1 microM). Vagal stimulation (7 V, 10 Hz, 5 ms) inhibited GIP-stimulated SLI release. Administration of naloxone partially reversed this inhibition, suggesting that endogenous opioids were at least partially responsible for vagally induced inhibition. A number of possible pathways by which endogenous enkephalins may modulate SLI release have been proposed.     

Cot kinase plays a critical role in <Emphasis Type="Italic">Helicobacter pylori</Emphasis>-induced IL-8 expression

Helicobacter pylori is a major pathogen causing various gastric diseases including gastric cancer. Infection of H. pylori induces pro-inflammatory cytokine IL-8 expression in gastric epithelial cells in the initial inflammatory process. It has been known that H. pylori can modulate Ras-Raf-Mek-Erk signal pathway for IL-8 induction. Recently, it has been shown that another signal molecule, cancer Osaka thyroid oncogene/tumor progression locus 2 (Cot/Tpl2) kinase, activates Mek and Erk and plays a role in the Erk pathway, similar to MAP3K signal molecule Raf kinase. Therefore, the objective of this study was to determine whether Cot kinase might be involved in IL-8 induction caused by H. pylori infection. AGS gastric epithelial cells were infected by H. pylori strain G27 or its isogenic mutants lacking cagA or type IV secretion system followed by treatment with Cot kinase inhibitor (KI) or siRNA specific for Cot kinase. Activation of Erk was assessed by Western blot analysis and expression of IL-8 was measured by ELISA. Treatment with Cot KI reduced both transient and sustained Erk activation. It also reduced early and late IL-8 secretion in the gastric epithelial cell line. Furthermore, siRNA knockdown of Cot inhibited early and late IL-8 secretion induced by H. pylori infection. Taken together, these results suggest that Cot kinase might play a critical role in H. pylori type IV secretion apparatus-dependent early IL-8 secretion and CagA-dependent late IL-8 secretion as an alternative signaling molecule in the Erk pathway.     

Autophagy mediates the secretion of macrophage migration inhibitory factor from cardiomyocytes upon serum-starvation

Macrophage migration inhibitory factor(MIF) is an inflammatory cytokine. It is elevated early in the blood of acute myocardial infarction patients. However, it is unclear whether and how MIF is released. This study investigated the cellular source and mechanism of MIF release from hearts. An ischemia-mimic treatment induced the secretion of MIF from neonatal rat cardiomyocytes but not from fibroblasts. The treatment did not cause significant leakage of lactate dehydrogenase, suggesting that ischemia induced the MIF secretion without causing severe cell damage. Plasma samples from patients with acute chest pain at the emergency department were collected for the detection of MIF. MIF levels in patients with acute coronary syndrome(ACS)increased early, when cardiac injury markers were not yet elevated, suggesting that ischemia can induce MIF secretion before the occurrence of severe myocardial damage. Serum-starvation caused MIF secretion from rat cardiomyocytes and Langendorffperfused rat hearts. The secretion was suppressed by the inhibition of autophagy by inhibitors or by silencing of Atg5. In conclusion, serum-starvation induces the secretion of MIF from cardiomyocytes via autophagy dependent pathway. Clarifying the mechanism of MIF secretion will be helpful for its application in the early diagnosis and treatment of ACS.     

Sperm dumping in a haplogyne spider

The present study shows that females of Silhouettella loricatula (Arachnida: Araneae: Oonopidae) manage to process sperm in an unusual and previously unknown way. The male ejaculate consisting of spermatozoa and globular secretion is enclosed in a secretory sac. This may avoid the mixing of sperm from different males and at least severely limit sperm competition. The process of sperm enclosure occurs within the female's sperm storage site (receptaculum) as the ejaculate is not surrounded by a sac inside the male's sperm-transferring organs (palpal bulbs). The secretion forming the sac is produced by glands adjoining the receptaculum. The possibility that globular secretions in the male palpal bulbs partly contribute to the sac cannot be ruled out completely. It is suggested that in S. loricatula , the main function of sperm enclosure in a sac is enabling females to dump the ejaculate of a male. The present study represents the first report on sperm dumping in the family Oonopidae. During five first and three second copulations in the laboratory, the dumping of a sac was observed. One dumped sac was sectioned and contained spermatozoa. Two couples were flash-fixed with liquid nitrogen early during copulation, which revealed the mechanism of the sac dumping. By muscle contractions, the receptaculum is bent backwards and the sac moved into the genital opening. The actual sac dumping occurs most probably in cooperation with the male, which moves his pedipalps rhythmically during the entire copulation. Extensions and furrows on the emboli suggest that they may additionally be used as copulatory courtship devices. The enclosure of sperm from the current male in secretion takes place during or immediately following copulation as all mated females sacrificed after copulation had a new sac containing spermatozoa in the receptaculum. Dumping sperm of a previous male during the next copulation may allow females to bias sperm precedence.     

Reduced gastric acid inhibitory effect of a pGIP(1-30)NH2 fragment with potent pancreatic amylase inhibitory activity.

Gastric inhibitory polypeptide (GIP) strongly stimulates insulin secretion in the presence of glucose and also stimulates somatostatin release from gastric mucosa. It was reported recently that both stimulatory activities can be dissociated by removing the C-terminal 12 amino acid residues. Since insulin and somatostatin are involved in regulation of exocrine pancreatic and gastric secretion in rats, we compared the inhibitory effects of pGIP and the pGIP(1-30)NH2 fragment on pancreatic amylase and gastric acid secretion. pGIP(1-30)NH2 displayed full activity on inhibition of bombesin (BN)-stimulated amylase release relative to GIP itself, but was about 10-fold less potent in inhibiting gastric acid secretion. These results suggest that the receptors involved in these two events have quite different ligand binding requirements and that more specific analogues of GIP can be designed which should be of value in elucidating the physiological roles of this hormone.     

Role of incretin hormones in the regulation of insulin secretion in diabetic and nondiabetic humans

The available evidence suggests that about two-thirds of the insulin response to an oral glucose load is due to the potentiating effect of gut-derived incretin hormones. The strongest candidates for the incretin effect are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). In patients with type 2 diabetes, however, the incretin effect is lost or greatly impaired. It is hypothesized that this loss explains an important part of the impaired insulin secretion in patients. Further analysis of the incretin effects in patients has revealed that the secretion of GIP is near normal, whereas the secretion of GLP-1 is decreased. On the other hand, the insulintropic effect of GLP-1 is preserved, whereas the effect of GIP is greatly reduced, mainly because of a complete loss of the normal GIP-induced potentiation of second-phase insulin secretion. These two features, therefore, explain the incretin defect of type 2 diabetes. Strong support for the hypothesis that the defect plays an important role in the insulin deficiency of patients is provided by the finding that administration of excess GLP-1 to patients may completely restore the glucose-induced insulin secretion as well as the beta-cells' sensitivity to glucose. Because of this, analogs of GLP-1 or GLP-1 receptor activations are currently being developed for diabetes treatment, so far with very promising results.     

Inhibitory effect of rat amylin on the insulin responses to glucose and arginine in the perfused rat pancreas

Amylin, a 37-amino acid polypeptide, is the main component of amyloid deposits in the islets of Langerhans, and has been identified in the B-cell secretory granules. We have investigated the effect of rat amylin on the insulin and glucagon release by the isolated, perfused rat pancreas. Amylin infusion at 750 nM, markedly reduced unstimulated insulin release (ca. 50%, P less than 0.025), whereas it did not modify glucagon output. At the same concentration, amylin also blocked the insulin response to 9 mM glucose (ca. 80%, P less than 0.025) without affecting the suppressor effect of glucose on glucagon release. The inhibitory effect of amylin on glucose-induced insulin secretion was confirmed by lowering the amylin concentration (500 nM) and increasing the glucose stimulus (11 mM); again, no effect of amylin on glucagon release was observed. Finally, amylin, at 500 nM, reduced the insulin response to 3.5 mM arginine (ca. 40%, P less than 0.025) without modifying the secretion of glucagon elicited by this amino acid. It can be concluded that, in the rat pancreas, the inhibitory effect of homologous amylin on unstimulated insulin secretion, as well as on the insulin responses to metabolic substrates (glucose and arginine), favours the concept of this novel peptide as a potential diabetogenic agent.     

Abnormality of circadian rhythm of serum melatonin and other biochemical parameters in fibromyalgia syndrome

Fibromyalgia syndrome (FMS) is a complex chronic condition causing widespread pain and variety of other symptoms. It produces pain in the soft tissues located around joints throughout the body. FMS has unknown etiology and its pathophysiology is not fully understood. However, abnormality in circadian rhythm of hormonal profiles and cytokines has been observed in this disorder. Moreover, there are reports of deficiency of serotonin, melatonin, cortisol and cytokines in FMS patients, which are fully regulated by circadian rhythm. Melatonin, the primary hormone of the pineal gland regulates the body's circadian rhythm and normally its levels begin to rise in the mid-to-late evening, remain high for most of the night, and then decrease in the early morning. FMS patients have lower melatonin secretion during the hours of darkness than the healthy subjects. This may contribute to impaired sleep at night, fatigue during the day and changed pain perception. Studies have shown blunting of normal diurnal cortisol rhythm, with elevated evening serum cortisol level in patients with FMS. Thus, due to perturbed level of cortisol secretion several symptoms of FMS may occur. Moreover, disturbed cytokine levels have also been reported in FMS patients. Therefore, circadian rhythm can be an important factor in the pathophysiology, diagnosis and treatment of FMS. This article explores the circadian pattern of abnormalities in FMS patients, as this may help in better understanding the role of variation in symptoms of FMS and its possible relationship with circadian variations of melatonin, cortisol, cytokines and serotonin levels.