Functions of pancreatic β cells and adipocytes in bombesin receptor subtype-3-deficient mice

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Abstract

We previously reported that mice lacking bombesin receptor subtype-3 (BRS-3) exhibit mild late-onset obesity and glucose intolerance [Nature 390 (1997) 160]. To examine the mechanism by which glucose intolerance is developed in these mice, we studied insulin release and proinsulin biosynthesis in isolated pancreatic islets and glucose uptake and facilitative glucose transporter (GLUT)-4 translocation in adipose tissues. Although islet insulin contents and the size and number of islets of Langerhans in BRS-3-deficient mice decreased, there was no difference in glucose-stimulated insulin release and proinsulin biosynthesis between BRS-3-deficient and wild-type control mice. In contrast, adipose tissues exhibited a marked difference: the uptake of [14C]2-deoxy-d-glucose by adipocytes isolated from BRS-3-deficient mice was not stimulated by 10−7 M insulin addition, and membrane fractionation analysis showed that GLUT4 was barely detected in the fraction of plasma membrane in BRS-3-deficient mice in the presence of 10−7 M insulin. Quantitative reverse transcription-PCR (RT-PCR) showed that mRNA levels of GLUT4, insulin receptor, insulin receptor substrate (IRS)-1 and IRS-2, syntaxin 4, SNAP23, and VAMP-2 in adipose tissues of BRS-3-deficient mice were unchanged compared with those in wild-type control mice. We concluded that impaired glucose metabolism observed in BRS-3-deficient mice was mainly caused by impaired GLUT4 translocation in adipocytes.

Section snippets

Materials and methods

Animals. Male BRS-3-deficient mice [13] and male wild-type control littermates were used at the age of 12 weeks. The mice were given free access to food and water until the start of the experiments.

Islet isolation, insulin release, and proinsulin biosynthesis. Pancreatic islets were isolated by collagenase digestion as described previously [14]. For insulin-release experiments, islets were preincubated for 1 h in Krebs–Ringer buffer (KRB) containing 2.2 mM glucose and then challenged by 22 mM

Studies using pancreatic β cell

Because mice lacking functional BRS-3 develop the impairments of glucose tolerance and insulin tolerance as described [13], we intended to characterize the functions of both pancreatic β cells and adipose cells in this study. First, we studied both insulin secretion and proinsulin biosynthesis in pancreatic β cells. Fig. 1A shows the glucose responsiveness of insulin release from islets of BRS-3-deficient and wild-type control mice. There was no statistical difference among levels of insulin

Conclusion deduced from the two studies

The growth of islets in the pancreas was disturbed by the disruption of BRS-3, but insulin release and biosynthesis were preserved. On the other hand, the GLUT4 translocation system in the adipocytes was impaired by the disruption of BRS-3. Thus, the impairment of glucose metabolism observed in BRS-3-deficient mice may be the result of the impaired GLUT4 translocation in the adipocytes and the stress on β cells.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (C) 14570130 (to M.O.-I.) and (B) 15390108 (to S.N.) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.

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    Abbreviations: ELISA, enzyme-linked immunosorbent assay; KRB, Krebs–Ringer buffer; PCR, polymerase chain reaction; SDS, sodium dodecyl sulfate; PAGE, polyacrylamide gel electrophoresis; VAMP-2, vesicle associated membrane protein-2; GLUT, facilitative glucose transporter; GRP-R, gastrin-releasing peptide receptor; NMB-R, neuromedin B receptor.

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