Evidence that insulin secretion influences SNAP-25 through proteasomal activation.
Regulation of SNARE proteins by glucose in pancreatic islets is complex and insufficiently clarified. We aimed to study effects of glucose per se separate from enhancing effects on exocytosis. A 24h culture of rat islets at elevated glucose (27 mmol/L) increased t-SNARES (SNAP-25, syntaxin) (Western blotting). Co-culture with diazoxide, which inhibits glucose-induced insulin secretion, reversed these effects. Effects on SNAP-25 were similar in human and rat islets. Effects of diazoxide were mimicked by blocking secretion with somatostatin (rat islets). Blocking secretion by cooling abolished both glucose and diazoxide effects on SNAP-25. Total SNAP-25 mRNA as well as isoforms alpha and beta were increased by 24-h elevated glucose. Diazoxide failed to reverse the glucose effects on mRNA. However, effects of diazoxide on SNAP-25 protein were nullified by proteasome inhibitors (ALLN, MG-132, and epoxomicin) but not by lysosomal inhibition (NH(4)Cl). Exocytosis per se modifies SNAREs by a process linked to proteasomal activation.
Ma
Zuheng
Z
Department of Molecular Medicine, Endocrine and Diabetes Unit, Rolf Luft Center of Diabetes Research, Karolinska Institutet, Karolinska Hospital, S-171 76 Stockholm, Sweden.
Portwood
Neil
N
Foss
Aksel
A
Grill
Valdemar
V
Björklund
Anneli
A
D000818:Animals; D002478:Cells, Cultured; D003981:Diazoxide / Q000494:pharmacology*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005947:Glucose / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D050825:Synaptosomal-Associated Protein 25
D007328:Insulin; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; C494208:SNAP25 protein, human; C494210:Snap25 protein, rat; D050825:Synaptosomal-Associated Protein 25; D046988:Proteasome Endopeptidase Complex; D005947:Glucose; D003981:Diazoxide