The effect of glucose and incretins on islet hormone secretion and osteopontin production

Abstract

Abstract

Impaired glycemic control of circulating glucose through storage as glycogen or immediate utilization through glycolysis and eventually ATP synthesis through the tricarboxylic acid cycle can have dire systemic effects. Glucose metabolism capabilities are of critical importance to many patients in the prevention and management of diabetes mellitus. Therefore this study focused on the effects of high glucose and incretins on insulin and glucagon secretions as well as osteopontin secretion. Osteopontin (OPN) is a glycoprotein synthesized and secreted by different cell types from various tissues. Over expression of OPN has been associated with several physiological and pathological conditions in response to inflammation and other immunologic disorders. In diabetes, previous studies have revealed that OPN is upregulated in the serum of type 1 diabetic patients, in diabetic vascular walls and kidneys. Our study has been carried out in a diabetic mouse model SUR1-E1506K, as well as human pancreatic islets. We compared and demonstrated the species differences between mouse and human pancreatic islets in insulin and glucagon secretion when challenged by GIP and GLP-1. We also showed that OPN secretion is regulated by high glucose as well as incretins in both mouse and human islets.

Abstract

Popular science summary

The effect of glucose and incretin proteins on insulin secretion and OPN production

Insulin secretion is a sensitive cellular process with ramifications in diabetes mellitus development. Diabetes mellitus is characterized by hyperglycemia, a chronic condition defined by high levels of circulating glucose. Insulin hormone is the primary regulator of glucose absorption from blood into target tissues. Thus, impaired insulin production or insulin sensitivity in target tissues can lead to hyperglycemia, and is of critical importance to patients with diabetes mellitus.

We studied the effects of varying glucose concentrations and incretin hormones on cytokine osteopontin (OPN) secretion and on pancreatic islet hormone secretion: insulin and glucagon. The pancreatic islet consists of many cell types with those which produce insulin (β cells) and glucagon (α cells) in highest proportion. To study the secretion of these hormones we must first understand the process of secretion. When glucose is taken into the α- or β-cell, it is quickly converted to glucose-6-phosphate which immediately undergoes metabolic processing. The result of glucose metabolism increases the ATP/ADP ratio which triggers the closure of KATP channels which causes the membrane to depolarize. This depolarization opens voltage gated Ca2+ channels (additionally Na+ channels in α-cells), calcium ions influx, and insulin or glucagon is secreted.

In order to investigate the effect of hyperglycemia on the secretion of islet hormones we needed a good diabetic mouse model. We utilized the Sur1-E1506K mouse strain which has a functional mutation in KATP channels which closes them and induces chronic insulin secretion. This mouse becomes hyperglycemic after 8 weeks. Once the mice were in a diabetic state, we euthanized them and excised their pancreas to isolate the islets. We proceeded to test the effects of glucose and incretins on OPN, insulin and glucagon secretion through immunostaining, confocal microscopy, hormone secretion assay, and ELISA analyses in both mouse and human islets.

Results
High glucose stimulated insulin secretion while inhibiting glucagon secretion; this coincides with normal islet hormone secretion. In mouse, incretins showed decreased insulin secretion within high glucose conditions and no change in low glucose. Furthermore incretins stimulated the secretion of glucagon.

Islet hormone immunostaining revealed species differences between human and mouse islet architecture. High glucose and incretins increased OPN production in mice and yet decreased in human. To further distinguish a species difference more experiments are necessary due to limited number of human tissue samples.

Advisor: Marita Cohn
Master´s Degree Project in Molecular Genetics 45 credits VT 2013
Department of Biology, Lund University

• author: Acosta, Juan Ramon
• supervisor: Yang De Marinis; Marita Cohn
• organization: Degree Projects in Molecular Biology
• course: MOBN19 20122
• year: 2013
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 4075245
• date added to LUP: 2013-10-02 15:59:26
• date last changed: 2013-10-02 15:59:26