Glucagon-like peptide 1 immunoreactivity in gastroentero-pancreatic endocrine tumors: a light- and electron-microscopic study

Myricetin: a potent approach for the treatment of type 2 diabetes as a natural class B GPCR agonist

The physiologic properties of glucagon-like peptide 1 (GLP-1) make it a potent candidate drug target in the treatment of type 2 diabetes mellitus (T2DM). GLP-1 is capable of regulating the blood glucose level by insulin secretion after administration of oral glucose. The advantages of GLP-1 for the avoidance of hypoglycemia and the control of body weight are attractive despite its poor stability. The clinical efficacies of long-acting GLP-1 derivatives strongly support discovery pursuits aimed at identifying and developing orally active, small-molecule GLP-1 receptor (GLP-1R) agonists. The purpose of this study was to identify and characterize a novel oral agonist of GLP-1R (i.e., myricetin). The insulinotropic characterization of myricetin was performed in isolated islets and in Wistar rats. Long-term oral administration of myricetin demonstrated glucoregulatory activity. The data in this study suggest that myricetin might be a potential drug candidate for the treatment of T2DM as a GLP-1R agonist. Further structural modifications on myricetin might improve its pharmacology and pharmacokinetics.-Li, Y., Zheng, X., Yi, X., Liu, C., Kong, D., Zhang, J., Gong, M. Myricetin: a potent approach for the treatment of type 2 diabetes as a natural class B GPCR agonist.

Novel application of hydrophobin in medical science: a drug carrier for improving serum stability

Multiple physiological properties of glucagon-like peptide-1 (GLP-1) ensure that it is a promising drug candidate for the treatment of type 2 diabetes. However, the in vivo half-life of GLP-1 is short because of rapid degradation by dipeptidyl peptidase-IV (DPP-IV) and renal clearance. The poor serum stability of GLP-1 has significantly limited its clinical utility, although many studies are focused on extending the serum stability of this molecule. Hydrophobin, a self-assembling protein, was first applied as drug carrier to stabilize GLP-1 against protease degradation by forming a cavity. The glucose tolerance test clarified that the complex retained blood glucose clearance activity for 72 hours suggesting that this complex might be utilized as a drug candidate administered every 2–3 days. Additionally, it was found that the mutagenesis of hydrophobin preferred a unique pH condition for self-assembly. These findings suggested that hydrophobin might be a powerful tool as a drug carrier or a pH sensitive drug-release compound. The novel pharmaceutical applications of hydrophobin might result in future widespread interest in hydrophobin.

Altered expression of aquaporin 4 and H+/K+-ATPase in the stomachs of peptide YY (PYY) transgenic mice

BACKGROUND INFORMATION

The hormone PYY (peptide YY), synthesized by endocrine cells in the pancreas, ileum, colon and stomach has widespread inhibitory effects on gastrointestinal and pancreatic fluid secretion. Transgenic mice expressing a viral oncoprotein under the control of the PYY gene 5'-flanking region develop well-differentiated colonic endocrine tumours producing mainly PYY and enteroglucagon. In the present study, we investigated the expression of AQP4 (aquaporin 4) water channel and H(+)/K(+)-ATPase in stomachs from both control and transgenic mice.

RESULTS

Semi-quantitative RT (reverse transcriptase)-PCR showed an increase in the AQP4 transcript compared with control mice. Quantitative Western-blot analysis of stomachs from control and transgenic mice confirmed a significant increase in the 30 kDa AQP4 protein in transgenic mice. In control mice, AQP4 is specifically expressed in the basolateral membrane of gastric parietal cells, located in the basal region of the fundic glands. This particular location suggests that parietal cells in the base region of gastric pits might have a major role in water transport when compared with the more superficial parietal cells. Interestingly, immunofluorescence studies on transgenic mice revealed that the quantitative increase of AQP4 expression was actually due to an increase in the number of AQP4-expressing epithelial cells rather than to a higher expression of AQP4 in parietal cells. In fact, immunofluorescence experiments using the specific antibody raised against the AE2 isoform of Cl(-)/HCO3- exchanger specifically expressed in parietal cells confirmed that the number of parietal cells was comparable in both PYY and control stomachs. Moreover, in transgenic mice, a parallel significant decrease in the expression of H(+)/K(+)-ATPase was observed, as revealed by RT-PCR, quantitative immunoblotting and immunofluorescence.

CONCLUSIONS

In the present study, we demonstrate that the sustained inhibition of gastric secretion due to tumours producing PYY/enteroglucagon in transgenic mice is associated with an increase in AQP4 expression and a down-regulation of H(+)/K(+)-ATPase in parietal cells that acquire the characteristics of basal parietal cells. The absence of H2 receptors-mediated signalling due to the inhibition of histamine release from ECL (enterochromaffin-like) cells by PYY may be in part responsible for the observed increase in the number of parietal cells expressing AQP4.

Glucagon Secreting Cells Responds to Insulin Secretion In vitro Using Immunocytochemistry

In the present study, pancreas of rats were dissected and transferred to HEPES buffer (25 mM, pH 7.4). The control tissue pieces were kept in culture medium for one hour and the treated tissues were kept in same medium for 30 minutes and incubated with Insulin (10 nm and 100 nm) for another half hour, then tissues were transferred to Bouin‘s fixative (overnight at 40 ° Cc), cryosectioned (15 µm at -16 0 c) and subjected to immunocytochemical labeling with antibodies against Glucagon.

Glial fibrillary acidic protein (GFAP)-like immunoreactivity in rat endocrine pancreas

The study of intermediate filament expression in the pancreatic epithelium has been previously focused almost exclusively on cytokeratins. Transient vimentin immunoreactivity has also been detected in duct cells of rat fetal pancreas. Here we report that, in rat pancreas, intense GFAP-like immunoreactivity is detectable in a subpopulation of endocrine cells located in the periphery of the islet of Langerhans. In addition, staining appeared to be preferentially localized to the apical pole of the cells. Two different polyclonal antibodies were employed in this study, with analogous results. Staining of consecutive sections with anti-GFAP, anti-glucagon, and anti-somatostatin antibodies demonstrates that GFAP-like immunoreactivity is present in glucagon-secreting cells. The relevance of this finding is discussed. (J Histochem Cytochem 48:259-265, 2000)

Ultrastructural pattern of glucagon producing-cells in the gastric mucosa of the developing opossum Didelphis albiventris (Marsupialia)

Gastric glucagon-producing cells of the pouch young, weaned and adult opossum Didelphis albiventris were investigated by immunocytochemical and ultrastructural methods. In sections of the oxyntic mucosa stained by immunoperoxidase it was observed that glucagon-immunoreactive cells increased in number during pouch development (from 7.9 +/- 4.7 to 35.4 +/- 5.9 cells/mm2), decreasing progressively to weaned (14.2 +/- 4.8 cells/mm2) and adult animals (10.4 +/- 3.0 cells/mm2). The glucagon-immunoreactive cells presented numerous electrondense, round cytoplasmic granules surrounded by a membrane. Frequently, the dense core of the granules was separated from the enveloping membrane by a delicate electronlucent halo or by a less electrondense peripheral halo. This pattern of the granular component was similar to the ultrastructural morphology observed in pancreatic islet glucagon-producing cells and A-like cells of the gastric mucosa of those eutherian mammals studied so far.

Glucagon-like peptide-1 (GLP-1): a gut hormone of potential interest in the treatment of diabetes

GLP-1 (glucagon-like peptide-1) is a gut hormone which is released into the blood stream after feeding. Its main action is to stimulate insulin secretion through potentiating the insulinotropic action of glucose. The peptide is encoded in the glucagon gene and expressed mainly in the gut L cells. It exerts its actions through activating specific receptors of the seven transmembraneous domain-G-protein-coupled type with 463 amino acids. Its main signalling mechanism is activation of adenylate cyclase and formation of cyclic AMP. The peptide also increases the cytoplasmic concentration of Ca2 which is thought to be executed both through a Na(+)-dependent uptake of extracellular Ca2+ and through release of Ca2+ from intracellular Ca2+ stores. GLP-1 also inhibits glucagon secretion and inhibits gastric emptying and gastric acid and pancreatic exocrine secretion. Its integrated action on carbohydrate metabolism results in reduction of circulating glucose, and GLP-1 has therefore been suggested as a therapeutic alternative in diabetes. Finally, GLP-1 is also expressed in neurons in the hypothalamus, and may be involved in the regulation of feeding behaviour, since it inhibits food intake.

Galanin is a potent inhibitor of glucagon-like peptide-1 secretion from rat ileum

The insulinotropic glucagon-like peptide 1 (GLP-1) originates from the lower intestines. Surprisingly, food ingestion induces a rapid increase of GLP-1 plasma levels. Therefore, a complex regulation for postprandial GLP-1 secretion must exist, which cannot be solely explained by direct contact of nutrients in the gut lumen with the GLP-1-releasing L cells. This was addressed in the present study utilizing an isolated vascularly perfused rat ileum preparation. Cholinergic (methacholine) as well as peptidergic stimulation by glucose-dependent insulin-releasing polypeptide (synonym: gastric inhibitory polypeptide) (GIP) strongly enhanced GLP-1 secretion from the rat ileum. The stimulation of GLP-1 secretion by methacholine was abolished by addition of atropine and partly reduced by galanin. Galanin dose-dependently antagonized the stimulatory effect of GIP on GLP-1 release. Atropine was without effect. Furthermore, employing double immunohistochemistry labeling techniques galanin-immunoreactive nerves were detected in the vicinity of GLP-1-immunostained cells. Our data indicate that stimulatory and inhibitory mediators regulate GLP-1 secretion and that galanin is a likely inhibitor.