Hypoglycemia but not hyperglycemia induces rapid changes in pancreatic beta-cell gene transcription

Control of Insulin Secretion by Production of Reactive Oxygen Species: Study Performed in Pancreatic Islets from Fed and 48-Hour Fasted Wistar Rats

Mitochondria and NADPH oxidase are important sources of reactive oxygen species in particular the superoxide radical (ROS) in pancreatic islets. These molecules derived from molecular oxygen are involved in pancreatic β-cells signaling and control of insulin secretion. We examined the involvement of ROS produced through NADPH oxidase in the leucine- and/or glucose-induced insulin secretion by pancreatic islets from fed or 48-hour fasted rats. Glucose-stimulated insulin secretion (GSIS) in isolated islets was evaluated at low (2.8 mM) or high (16.7 mM) glucose concentrations in the presence or absence of leucine (20 mM) and/or NADPH oxidase inhibitors (VAS2870–20 μM or diphenylene iodonium—DPI—5 μM). ROS production was determined in islets treated with dihydroethidium (DHE) or MitoSOX Red reagent for 20 min and dispersed for fluorescence measurement by flow cytometry. NADPH content variation was examined in INS-1E cells (an insulin secreting cell line) after incubation in the presence of glucose (2.8 or 16.7 mM) and leucine (20 mM). At 2.8 mM glucose, VAS2870 and DPI reduced net ROS production (by 30%) and increased GSIS (by 70%) in a negative correlation manner (r = -0.93). At 16.7 mM glucose or 20 mM leucine, both NADPH oxidase inhibitors did not alter insulin secretion neither net ROS production. Pentose phosphate pathway inhibition by treatment with DHEA (75 μM) at low glucose led to an increase in net ROS production in pancreatic islets from fed rats (by 40%) and induced a marked increase (by 144%) in islets from 48-hour fasted rats. The NADPH/NADP⁺ ratio was increased when INS-1E cells were exposed to high glucose (by 4.3-fold) or leucine (by 3-fold). In conclusion, increased ROS production through NADPH oxidase prevents the occurrence of hypoglycemia in fasting conditions, however, in the presence of high glucose or high leucine levels, the increased production of NADPH and the consequent enhancement of the activity of the antioxidant defenses mitigate the excess of ROS production and allow the secretory process of insulin to take place.

Cellular and molecular characterization of a feline insulinoma

BACKGROUND

Insulinoma is an autonomous insulin-secreting islet cell neoplasm that is rarely diagnosed in cats. The clinical and pathological aspects of feline insulinoma have been described previously, but the molecular characteristics of these tumors have not been investigated.

OBJECTIVES

The study objectives were to characterize peptide hormone production and determine expression of selected genes involved in glucose metabolism and insulin secretion in a feline insulinoma.

METHODS

Immunohistochemistry and RT-PCR were used to examine hormone and gene expression, respectively, by insulinoma cells.

RESULTS

Immunohistochemistry examination indicated that the tumor cells expressed insulin, chromogranin A, and somatostatin but not glucagon or pancreatic polypeptide. The tumor expressed several genes characteristic of pancreatic beta cells (beta cells) including insulin (INS), glucose transporter 2 (GLUT2), and glucokinase (GCK). The tumor also expressed hexokinase 1 (HK1), a glycolytic enzyme not normally expressed in beta cells. GCK expression was higher in the insulinoma than in normal pancreas from the same cat. The GCK : HK1 ratio was >20-fold higher in insulinoma tissue than in normal pancreas.

CONCLUSIONS AND CLINICAL IMPORTANCE

The feline insulinoma produced several peptide hormones and expressed genes consistent with a beta-cell phenotype. The pattern of hexokinase gene expression in tumor cells differed from that of normal pancreas. These findings suggest insulinoma cells may have an increased sensitivity to glucose that could contribute to the abnormal insulin secretory response observed at low serum glucose concentrations.

Ablation of the glucagon receptor gene increases fetal lethality and produces alterations in islet development and maturation

Although glucagon (GLU) plays a pivotal role in glucose homeostasis, its role in the regulation of fetal growth and maturation is poorly understood. These issues were examined in a line of mice with a global deletion of the GLU receptor (Gcgr-/-), which are characterized by lower blood glucose levels and by alpha- and delta-cell hyperplasia in adults. Ablation of Gcgr was deleterious to fetal survival; it delayed beta-cell differentiation and perturbed the proportion of beta- to alpha-cells in embryonic islets. In adults, the mutation inhibited the progression of alpha-cells to maturity, affected the expression of several beta-cell-specific genes, and resulted in an augmentation of the alpha-, beta-, and delta-cell mass. This increase was due to an augmentation in both islet number and in the rate of proliferation of cells expressing GLU or insulin. These findings suggest that GLU participates in a feedback loop that regulates the proportion of the different endocrine cell types in islets, the number of islets per pancreas, and development of the mature alpha-cell phenotype.

Adaptation of beta-cell mass to substrate oversupply: enhanced function with normal gene expression

Although type 2 diabetes mellitus is associated with insulin resistance, many individuals compensate by increasing insulin secretion. Putative mechanisms underlying this compensation were assessed in the present study by use of 4-day glucose (GLC; 35% Glc, 2 ml/h) and lipid (LIH; 10% Intralipid + 20 U/ml heparin; 2 ml/h) infusions to rats. Within 2 days of beginning the infusion of either lipid or glucose, plasma glucose profiles were normalized (relative to saline-infused control rats; SAL; 0.45% 2 ml/h). During glucose infusion, plasma glucose was maintained in the normal range by an approximately twofold increase in plasma insulin and an approximately 80% increase in beta-cell mass. During LIH infusion, glucose profiles were also maintained in the normal range. Plasma insulin responses during feeding were doubled, and beta-cell mass increased 54%. For both groups, the increase in beta-cell mass was associated with increased beta-cell proliferation (98% increase during GLC and 125% increase during LIH). At the end of the 4-day infusions, no significant changes were observed in islet-specific gene transcription (i.e., the expression of islet hormone genes, glucose metabolism genes, and insulin transcription factors were unaffected). Two days after termination of the infusions, the glucose-stimulated plasma insulin response was increased approximately 67% in glucose-infused animals. No sustained effect on insulin secretory capacity was observed in the LIH animals. The increase in plasma insulin response after glucose infusion was achieved in the absence of any change in insulin clearance. We conclude that, in rats, an increase in insulin demand after an increase in glucose appearance or free fatty acid leads to an increase in beta-cell mass, mediated in part by an increase in beta-cell proliferation, and that these compensatory changes lead to increased insulin secretion, normal plasma glucose levels, and the maintenance of normal islet gene expression.

Short-term regulation of insulin gene transcription by glucose

Whereas short-term regulation of insulin biosynthesis at the level of translation is well accepted, glucose-dependent transcriptional control is still believed to be a long-term effect occurring after more than 2 hr of glucose stimulation. Because pancreatic beta cells are exposed to elevated glucose levels for minutes rather than hours after food uptake, we hypothesized the existence of a short-term transcriptional control. By studying the dynamics of newly synthesized (prepro)insulin RNA and by employing on-line monitoring of gene expression in single, insulin-producing cells, we were able to provide convincing evidence that insulin gene transcription indeed is affected by glucose within minutes. Exposure of insulinoma cells and isolated pancreatic islets to elevated glucose for only 15 min resulted in a 2- to 5-fold elevation in (prepro)insulin mRNA levels within 60-90 min. Similarly, insulin promoter-driven green fluorescent protein expression in single insulin-producing cells was significantly enhanced after transient glucose stimulation. Thus, short-term signaling, such as that involved in insulin secretion, also may regulate insulin gene transcription.