CONTROL OF CHOLECYSTOKININ RECEPTOR DEPHOSPHORYLATION IN PANCREATIC ACINAR-CELLS

Phosphorylation of cell surface receptors regulates the physiological response to many hormones and neurotransmitters. We have demonstrated that the receptor for cholecystokinin (CCK), the major secretagogue fo r the exocrine pancreas, is phosphorylated on serines in response to C CK (Klueppelberg, U. G., Gates, L. K., Gorelick, F. S., and Miller, L. J. (1991) J. Biol. Chem. 266, 2403-2408). Receptor phosphorylation wa s transient, even in the continued presence of agonist, and suggested a role for pancreatic protein phosphatases (PP) in regulating receptor phosphorylation in the intact cell. Treatment of acinar cells with ok adaic acid increased the extent and duration of receptor phosphorylati on induced by CCK. Receptor phosphorylated in the intact cell in respo nse to CCK was used as substrate to analyze protein phosphatases in pa ncreatic acinar cell extracts. The predominant CCK receptor phosphatas e activity was found in the cytosol and was potently inhibited by okad aic acid (IC50 = 0.2 nM). This phosphatase activity was unaffected by inhibitor-2, Ca2+, or Me+, suggesting that the major receptor phosphat ase was PP-2A. Stimulation of cells with CCK resulted in a 3-fold incr ease in protein phosphatase activity toward the CCK receptor, at the s ame time that phosphorylase a phosphatase activity was unchanged. This increase in receptor phosphatase activity was reflected only in the c ytosol, with the particulate activity unchanged. Consistent with this subcellular localization, the hormone-regulated receptor phosphatase a ctivity was PP-2A, with chromatographic separation demonstrating activ ity in both PP-2A1 and PP-2A2 forms. These data suggest that CCK coord inates the activity of both protein kinases and phosphatases acting on the CCK receptor to control the extent and duration of receptor phosp horylation in the pancreatic acinar cell.