Secretagogues cause ubiquitination and down-regulation of inositol 1,4,5-trisphosphate receptors in rat pancreatic acinar cells

Abstract

Background & Aims: The action of several exocrine pancreas secretagogues depends on the second messenger inositol 1,4,5-trisphosphate (IP 3), which, via endoplasmic reticulum–located IP 3 receptors, mobilizes intracellular Ca 2+ stores. Signaling pathways like this one are regulated at multiple loci. To determine whether IP 3 receptors are one of these loci, we measured IP 3 receptor concentration, distribution, and modification in secretagogue-stimulated rat pancreatic acinar cells. Methods: Isolated rat pancreatic acinar cells were exposed to cholecystokinin and other secretagogues, or rats were injected intraperitoneally with cerulein. Then samples of cells or pancreata were probed for IP 3 receptor content and distribution as well as for ubiquitin association with IP 3 receptors. Results: Secretagogues rapidly down-regulated acinar cell IP 3 receptors both in vitro and in vivo. They also elicited receptor redistribution and caused receptors to become ubiquitinated, indicating that the ubiquitin/proteasome proteolytic pathway contributes to the down-regulation. Surprisingly, however, proteasome inhibitors did not block IP 3 receptor down-regulation, and phospholipase Cβ1 and protein kinase Cϵ also were down-regulated. Thus, secretagogues simultaneously activate an additional proteolytic pathway. Conclusions: Secretagogues rapidly down-regulate IP 3 receptors and other proteins involved in intracellular signaling by a mechanism that involves, but is not limited to, the ubiquitin/proteasome pathway. Loss of these proteins may account for the disruption of Ca 2+ mobilization that occurs in models of acute pancreatitis, and may contribute to cell adaptation under physiological conditions. GASTROENTEROLOGY 1999;116:1194-1201