Proteasomal activation mediates down-regulation of inositol 1,4,5-trisphosphate receptor and calcium mobilization in rat pancreatic islets

Abstract

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, α-ketoisocaproic acid, and tolbutamide completely protected IP3Rs against the down-regulatory effects ofCCh. 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 Ca2+ channel-blocking agent nimodipine. Intracellular Ca2+ ([Ca2+]i) levels in Fura-2 (fluorescent Ca2+ indicator)-loaded islets, in the absence of extracellular Ca2+, increased in response to glucose stimulation; but in islets pretreated with CCh, glucose did not increase [Ca2+]i above basal levels. However, in islets pretreated with CCh and the proteasomal inhibitor MG-132 (carbo-benzoxyl-leucinyl-leucinyl-leucinyl-H), the glucose-stimulated increase in [Ca2+]i was significantly higher than the change observed for glucose-stimulated [Ca2+]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 Ca2+ mobilization in islets that may relate to insulin secretory responsiveness.