Proteasomal activation mediates down-regulation of inositol 1,4,5-trisphosphate receptor and calcium mobilization in rat pancreatic islets.
Inositol 1,4,5-trisphosphate receptor (IP3R) protein levels in isolated rat pancreatic islets were investigated in response to carbachol (CCh) and sulfated cholecystokinin 26-33 amide stimulation. Within 2 h, CCh reduced IP3R-I protein levels by 22% and IP3R-II and -III levels to 65% or more below basal. Sulfated cholecystokinin 26-33 amide decreased the levels of IP3R-I, -II, and -III by 34%, 60%, and 66% below basal, respectively. The effect of CCh was concentration- and time-dependent, with a persistent decline in IP3R levels for up to 6 h after the onset of stimulation. CCh-pretreated islets also showed an inhibition of glucose-stimulated insulin secretion. Proteasome inhibition completely blocked the down-regulatory effects of CCh on IP3Rs and significantly increased the insulin secretory response to glucose stimulation in the presence of CCH: Islet stimulation by glucose, alpha-ketoisocaproic acid, and tolbutamide completely protected IP3Rs against the down-regulatory effects of CCH: 2-deoxyglucose and 3-O-methyl glucose failed to affect CCh-induced IP3R down-regulation. The protective effects of glucose on IP3R down-regulation were completely inhibited by the Ca(2+) channel-blocking agent nimodipine. Intracellular Ca(2+) ([Ca(2+)](i)) levels in Fura-2 (fluorescent Ca(2+) indicator)-loaded islets, in the absence of extracellular Ca(2+), increased in response to glucose stimulation; but in islets pretreated with CCh, glucose did not increase [Ca(2+)](i) above basal levels. However, in islets pretreated with CCh and the proteasomal inhibitor MG-132 (carbobenzoxyl-leucinyl-leucinyl-leucinyl-H), the glucose-stimulated increase in [Ca(2+)](i) was significantly higher than the change observed for glucose-stimulated [Ca(2+)](i) in the absence of MG-132. The results suggest that muscarinic receptor stimulation modulates IP3R protein levels in islets through a proteasomal activation pathway, and that down-regulation of IP3Rs has a profound effect on Ca(2+) mobilization in islets that may relate to insulin secretory responsiveness.
Lee
B
B
Department of Pharmacology and Toxicology, School of Medicine and Biomedical Sciences, State University of New York at Buffalo, Buffalo, New York 14214, USA.
Gai
W
W
Laychock
S G
SG
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000032:analysis* / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D002766:Cholecystokinin / Q000494:pharmacology; D003546:Cysteine Endopeptidases / Q000502:physiology*; D015536:Down-Regulation; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism*; D008297:Male; D009097:Multienzyme Complexes / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear / Q000032:analysis*
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007328:Insulin; D009097:Multienzyme Complexes; D018160:Receptors, Cytoplasmic and Nuclear; D002217:Carbachol; D002766:Cholecystokinin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D002118:Calcium