Glucose uptake, utilization, and signaling in GLUT2-null islets.

Abstract

We previously reported that pancreatic islet ␤-cells from GLUT2-null mice lost the first phase but preserved the second phase of glucose-stimulated insulin secretion (GSIS). Furthermore, we showed that the remaining secretory activity required glucose uptake and metabolism because it can be blocked by inhibition of oxidative phosphorylation. Here, we extend these previous studies by analyzing, in GLUT2-null islets, glucose transporter isoforms and glucokinase expression and by measuring glucose usage, GSIS, and glucosestimulated insulin mRNA biosynthesis. We show that in the absence of GLUT2, no compensatory expression of either GLUT1 or GLUT3 is observed and that glucokinase is expressed at normal levels. Glucose usage by isolated islets was increased between 1 and 6 mmol/l glucose but was not further increased between 6 and 20 mmol/l glucose. Parallel GSIS measurements showed that insulin secretion was not stimulated between 2.8 and 6 mmol/l glucose but was increased by >4-fold between 6 and 20 mmol/l glucose. Stimulation by glucose of total protein and insulin biosynthesis was also markedly impaired in the absence of GLUT2. Finally, we re-expressed GLUT2 in GLUT2-null ␤-cells using recombinant lentiviruses and demonstrated a restoration of normal GSIS. Together, these data show that in the absence of GLUT2, glucose can still be taken up by ␤-cells, albeit at a low rate, and that this transport activity is unlikely to be attributed to GLUT1 or GLUT3. This uptake activity, however, is limiting for normal glucose utilization and signaling to secretion and translation. These data further demonstrate the key role of GLUT2 in murine ␤-cells for glucose signaling to insulin secretion and biosynthesis. Diabetes

Evidence That Extrapancreatic GLUT2-Dependent Glucose Sensors Control Glucagon Secretion

“…In the absence of the transporter, a response can be stimulated only by extreme glycemic conditions. This would be similar to the secretion of insulin by GLUT2-null ␤-cells, which is mostly suppressed in the presence of moderate hyperglycemia but still detected with extreme glycemic levels (29,34). Second, there may be separate glucose sensors, activated at different hypoglycemic levels, that rely on different glucose transporter isoforms.…”

Identification of Insulin‐Producing Cells Derived from Embryonic Stem Cells by Zinc‐Chelating Dithizone

“…Furthermore, they expressed GLUT2 and IGRP. GLUT2 is an essential gene that plays an important role in pancreatic β-cell functions, including glucose-stimulated insulin secretion [34,35], while IGRP is a newly cloned islet-specific gene encoding the catalytic subunit of glucose-6-phosphatase, a multicomponent system located in the endoplasmic reticulum that comprises a catalytic subunit and transporters for glucose-6-phosphate, inorganic phosphate, and glucose [36,37]. Based on the variety of gene expression seen, including GLUT2 and IGRP, we believe that it was unlikely that the DTZ-stained clusters were of extraembryonic origin and unrelated to pancreatic islets.…”

“…Using this characteristic of DTZ, we identified insulin-producing cells in EB outgrowths derived from mouse ES cells as well as cellular clusters. We then analyzed the characteristic features of these DTZ-stained clusters by immunohistochemistry for insulin production and by reverse transcriptase-polymerase chain reaction (RT-PCR) for gene expression of the pancreatic β-cell markers, including proinsulin 1, proinsulin 2, pancreatic transcription factor pancreatic-duodenal homeobox 1 (PDX1) [32,33], glucose transporter-2 (GLUT2) [34,35], and islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) [36,37]. We also examined insulin secretion using an enzyme-linked immunoassay (ELISA).…”

Morphogenetic plasticity of adult human pancreatic islets of Langerhans

“…Taken together, these data strongly suggest that the DEC-to-ILS transformation process appears to be transcriptionally regulated by factors implicated in islet development that normally occurs in vivo. 29,53 Given that PDX-1 is required for proper physiologic islet function, 59 and that the presence of GLUT-2 and C-peptide suggests normal insulin processing and secretion, 60 we hypothesized that ILS process and secrete insulin in a physiologic manner. To test this hypothesis, we assessed insulin content and glucose-stimulated insulin secretion in freshly isolated adult human islets, DECs and ILS by ELISA.…”