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

Background & Aims: The action of several exocrine pancreas secretagogues de pends on the second messenger inositol 1,4,5-trisphosphate (IP3), which, vi a endoplasmic reticulum-located IP3 receptors, mobilizes intracellular Ca2 stores. Signaling pathways like this one are regulated at multiple loci. T o determine whether IP3 receptors are one of these loci, we measured IP3 re ceptor concentration, distribution, and modification in secretagogue-stimul ated rat pancreatic acinar cells. Methods: Isolated rat pancreatic acinar c ells were exposed to cholecystokinin and other secretagogues, or rats were injected intraperitoneally with cerulein, Then samples of cells or pancreat a were probed for IP3 receptor content and distribution as well as for ubiq uitin association with IP3 receptors. Results: Secretagogues rapidly down-r egulated acinar cell IP3 receptors both in vitro and in vivo. They also eli cited receptor redistribution and caused receptors to become ubiquitinated, indicating that the ubiquitin/proteasome proteolytic pathway contributes t o the down-regulation. Surprisingly, however, proteasome inhibitors did not block IP(3)receptor down-regulation, and phospholipase C beta 1 and protei n kinase C epsilon also were down-regulated. Thus, secretagogues simultaneo usly activate an additional proteolytic pathway. Conclusions: Secretagogues rapidly down-regulate IP3 receptors and other proteins involved in intrace llular signaling by a mechanism that involves, but is not limited to, the u biquitin/proteasome pathway. Loss of these proteins may account for the dis ruption of Ca2+ mobilization that occurs in models of acute pancreatitis, a nd may contribute to cell adaptation under physiological conditions.