title: Functional differences between aggregated and dispersed insulin-producing cells.
abstract: OBJECTIVE
  Beta cells situated in the islet of Langerhans respond more vigorously to glucose than do dissociated beta cells. Mechanisms for this discrepancy were studied by comparing insulin-producing MIN6 cells aggregated into pseudoislets with MIN6 monolayer cells and mouse and human islets.
  
  
  METHODS
  MIN6 monolayers, pseudoislets and mouse and human islets were exposed to glucose, α-ketoisocaproic acid (KIC), pyruvate, KIC plus glutamine and the phosphatidylinositol 3-kinase (PI3K) inhibitors LY294002 or wortmannin. Insulin secretion (ELISA), cytoplasmic Ca(2+) concentration ([Ca(2+)]c; microfluorometry), glucose oxidation (radiolabelling), the expression of genes involved in mitochondrial metabolism (PCR) and the phosphorylation of insulin receptor signalling proteins (western blotting) were measured.
  
  
  RESULTS
  Insulin secretory responses to glucose, pyruvate, KIC and glutamine were higher in pseudoislets than monolayers and comparable to those of human islets. Glucose oxidation and genes for mitochondrial metabolism were upregulated in pseudoislets compared with single cells and monolayers, respectively. Phosphorylation at the inhibitory S636/639 site of IRS-1 was significantly higher in monolayers and dispersed human and mouse cells than pseudoislets and intact human and mouse islets. PI3K inhibition only slightly attenuated glucose-stimulated insulin secretion from monolayers, but substantially reduced that from pseudoislets and human and mouse islets without suppressing the glucose-induced [Ca(2+)]c response.
  
  
  CONCLUSIONS
  We propose that islet architecture is critical for proper beta cell mitochondrial metabolism and IRS-1 signalling, and that PI3K regulates insulin secretion at a step distal to the elevation of [Ca(2+)]c.
authors.affiliation: Department of Medical Cell Biology, Uppsala University, Box 571, 75123, Uppsala, Sweden. azazul.chowdhury@mcb.uu.se
authors.forename: A
authors.initials: A
authors.lastname: Chowdhury
authors.forename: O
authors.initials: O
authors.lastname: Dyachok
authors.forename: A
authors.initials: A
authors.lastname: Tengholm
authors.forename: S
authors.initials: S
authors.lastname: Sandler
authors.forename: P
authors.initials: P
authors.lastname: Bergsten
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000378:metabolism; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction
chemical_list: D007328:Insulin; D019869:Phosphatidylinositol 3-Kinases; D002118:Calcium