Specific protein phosphorylation during stimulation of amylase secretion by beta-agonists or dibutyryl adenosine 3',5'-monophosphate in the rat parotid gland

Cyclic AMP in the regulation of exocytosis in the rat parotid gland. Evidence obtained with cholera toxin

The effects of cholera toxin on rat parotid gland function were determined in order to further characterize the relationship between cyclic AMP and exocytosis in this tissue. Cholera toxin induced the release of alpha-amylase from rat parotid minces in vitro. This release was accompanied by an activation of adenylate cyclase, elevated cyclic AMP levels, an elevated protein kinase activity ratio, and changes in the degree of phosphorylation of three endogenous phosphoproteins. Two of the phosphoproteins became more phosphorylated upon cholera toxin stimulation while the phosphorylation of the other decreased. The effects of cholera toxin on endogenous phosphoprotein labelling appeared to mimic those of the beta-adrenergic agonist isoproterenol but were of a smaller magnitude. These results are consistent with cyclic AMP functioning as a major mediator of exocytosis in this gland exerting its effects, at least in part, via activation of cyclic AMP dependent protein kinase. The mechanism by which an increased cyclic AMP level results in the decreased phosphorylation of an endogenous phosphoprotein is not known.

Phosphopeptide patterns of the ribosomal protein S6 following stimulation of guinea pig parotid glands by secretagogues involving either cAMP or calcium as second messenger

Stimulation of secretion in exocrine cells is associated with the incorporation of up to 3 to 4 phosphates into the ribosomal protein S6. This occurs with secretagogues involving either cAMP or free calcium as second messenger. An analysis of the phosphorylation pattern of S6 from stimulated guinea pig parotid glands reveals 3 phosphopeptides (termed A,B,C). The phosphopeptide pattern was identical for cAMP- or calcium-mediated stimulation, whereas phosphorylation of the S6 protein in vitro with catalytic subunit of cAMP-dependent protein kinase resulted only in the formation of phosphopeptides A and C. Therefore, secretagogue-mediated phosphorylation is not or not exclusively catalyzed by cAMP-dependent protein kinase even when cAMP is the second messenger.

Role of cyclic AMP-dependent protein kinase activation in regulating rat submandibular mucin secretion

The extent of activation of rat submandibular gland cyclic AMP-dependent protein kinase (EC 2.7.1.37) was determined in vitro using dispersed cells to assess the involvement of this enzyme in submandibular mucin secretion. cAMP-dependent protein kinase activation, as determined by activity ratio method, was markedly increased following beta-adrenergic receptor activation. 0.5 M NaCl was required in the homogenization buffer for stabilization of the hormonally activated cAMP-dependent protein kinase. A role for cAMP-dependent protein kinase activation in regulating mucin secretion was strongly suggested by the following: (1) the kinase activity ratio increased rapidly after beta-adrenergic receptor stimulation; (2) dose-response relationship of the kinase activation following beta-adrenergic receptor activation correlated with isoproterenol induced mucin release; (3) termination of beta-adrenergic mediated mucin secretion caused a rapid decrease in the kinase activity ratio; (4) dibutyryl cyclic AMP stimulation caused an increase in the kinase ratio; whereas (5) pure cholinergic and pure alpha-adrenergic receptor stimulation had no effect on endogenous kinase activity. Although cAMP-dependent protein kinase activation may not be the only regulator of mucin secretion, these data suggest an important regulatory role for this kinase activation during rat submandibular mucin release.

The involvement of protein phosphorylation in stimulus-secretion coupling in the mouse exocrine pancreas

Secretagogue-induced protein phosphorylation was studied in the mouse pancreas in vitro, by using polyacrylamide-gel electrophoresis to separate the labelled proteins. Muscarinic cholinergic agonists increased the phosphorylation of a single band, which corresponded to Mr 32000, when the tissue was incubated with Ca2+ present in the extracellular medium, but not in Ca2+-free Krebs solution. In the presence of Ca2+, ionophore A23187 stimulated phosphorylation of the same band. The dose-response curve for carbachol-induced phosphorylation was biphasic, with maximum response at 1.0 microM-carbachol, and lesser responses when greater concentrations were used. This resembles the dose-response curve for carbachol-induced amylase secretion. The data suggest that the muscarinic-agonist-induced protein phosphorylation is stimulated secondarily to elevation of cytosol [Ca2+] and do not support the idea that diacylglycerol formed from hydrolysis of phosphatidylinositol is the activator of the protein kinase. Derivatives of cyclic AMP stimulated phosphorylation of bands corresponding to Mr 95500, 32000 and 20000. The effects of dibutyryl cyclic AMP and bethanechol on the protein of Mr 32000 were not additive, suggesting that the two agents produced phosphorylation of the same site(s) on this protein. Since derivatives of cyclic AMP, which are not very effective secretagogues in the exocrine pancreas, stimulate phosphorylation of the protein of Mr 32000, it is difficult to argue that phosphorylation of this particular protein leads to protein secretion.

Dispersed rat parotid cells: role of calcium and cAMP in the regulation of amylase secretion

The inter-relationship between cellular calcium and cAMP in the regulation of amylase secretion in dispersed rat parotid cells was evaluated. α- adrenergic stimulation augmented β-adrenergic receptor-mediated amylase release. Cellular calcium depletion was unable to completely block amylase secretion.

Hormone-induced protein phosphorylation. I. Relationship between secretagogue action and endogenous protein phosphorylation in intact cells from the exocrine pancreas and parotid

We undertook studies to determine whether secretagogue action on the exocrine pancreas and parotid is accompanied by phosphorylation of proteins in intact cells. For this purpose, rat pancreatic, and parotid lobules were preincubated with 32Pi for 45 min at 37 degrees C, washed, and then incubated at 37 degrees C in the presence or absence of secretagogues that effect discharge through different second messengers. Among a variety of polypeptides exhibiting enhanced phosphorylation in pancreatic lobules upon a 30-s incubation in the presence of the secretagogues carbamylcholine, cholecystokinin octapeptide, or secretin, one species with an Mr of 29,000 was especially notable for three reasons: (a) its enhanced level of phosphorylation was dependent on the dose of secretagogue used and was still apparent after incubation for 30 min at 37 degrees C; (b) an analogous phosphorylated polypeptide was observed in isoproterenol-stimulated parotid lobules; and (c) in both tissues its selective dephosphorylation was observed upon termination of stimulation by administration of atropine to carbamylcholine-stimulated pancreatic lobules and propranolol to isoproterenol-stimulated parotid lobules. These results suggest that the phosphorylation of one protein with an Mr of 29,000 is closely correlated both temporally and in a dose-dependent fashion with secretagogue action in both the exocrine pancreas and parotid.

Phosphoproteins of the adrenal chromaffin granule membrane

A fraction of chromaffin granule membranes contained a number of substrates for endogenous protein kinase activity as well as endogenous phosphatase activity. The major 32P-labelled polypeptide of molecular weight 43,000 appeared to be the alpha-subunit of pyruvate dehydrogenase of residual mitochondria. Several polypeptides showed cyclic AMP stimulation of phosphorylation of which the major polypeptide of molecular weight 59,000 shows half-maximal phosphorylation with 0.49 microM cyclic AMP. The phosphorylation of several other polypeptides is inhibited at high cyclic AMP concentrations. From studies with immunoprecipitation and two-dimensional electrophoresis it was found that alpha- and beta-tubulin and actin were absent from the granule membranes. However 32P labelling of a proportion of the copies of dopamine-beta-hydroxylase was demonstrated. The majority of the substrates for endogenous protein kinase activity are probably on the cytoplasmic side of the granule membrane.

Effects of Ca2+, calmodulin and cyclic AMP on the phosphorylation of endogenous proteins by homogenates of rt islets of langerhans

Activation of Ca2+ -calmodulin- and cyclic AMP-dependent protein kinases has been suggested to be involved in stimulus-secretion coupling in the pancreatic beta-cell. To study the properties of suc kinases and their endogenous protein substrates homogenates of rat islets of Langerhans were incubated with [gamma-32P]ATP. Phosphorylated proteins were separated by sodium dodecyl sulphate polyacrylamide gel electrophoresis and detected by autoradiography. The phosphorylation of certain proteins could be enhanced by Ca2+ plus calmodulin or by cyclic AMP. The major effect of Ca2+ and calmodulin was to stimulate the phosphorylation of a protein (P53) of molecular weight 53,100 +/- 500 (n = 15). Maximum phosphorylation of protein P53 occurred within 2 min with 2 micrometers free Ca2+ and 0.7 micrometers calmodulin. Incorporation of label into protein P53 was inhibited by trifluoperazine or W7 but not by cyclic AMP-dependent protein kinase inhibitor. Phosphorylation of a proteins of similar molecular weight could be enhanced to a lesser extent in the absence of Ca2+ but in the presence of cyclic AMP and 3-isobutylmethylxanthine: this phosphorylation was blocked by cyclic AMP-dependent protein kinase inhibitor. Cyclic AMP also stimulated incorporation of label into polypeptides of molecular weights 55,000 and 70-80,000. The results are consistent with the hypothesis that protein phosphorylation mechanisms may play a role in the regulation of insulin secretion.

Phosphorylation of the same specific protein during amylase release evoked by beta-adrenergic or cholinergic agonists in rat and mouse parotid glands

Stimulation of amylase secretion from the rat parotid gland by beta-adrenergic agonists is associated with a specific phosphorylation of three membrane-bound proteins designated as proteins I, II, and III [Jahn, R., Unger, C. & Söling, H. D. (1980) Eur. J. Biochem. 112, 345-352]. In contrast, stimuliation by carbachol induced significant phosphorylation of only protein I. This phosphorylation was low compared to isoproterenol-induced phosphorylation but corresponded to the smaller enhancement of amylase secretion. The mouse organ, however, is almost equally sensitive to beta-adrenergic and to cholinergic agonists. Incubation of mouse parotid gland slices with either 20 microM isoproterenol or 10 microM carbachol resulted in strong and comparable releases of amylase, which were accompanied by comparable phosphorylations of protein I. Proteins II and III were phosphorylated only in the presence of isoproterenol. Removal of external calcium by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetate abolished the carbachol-induced release of amylase but not the phosphorylation of protein I. Isoproterenol-induced secretion of amylase and phosphorylation of proteins I, II, and III were not inhibited under these conditions. Amylase release stimulated by the ionophore A-23187 was accompanied by the phosphorylation of protein I. Two-dimensional electrophoresis revealed that the radioactive spot corresponding to protein I was located at the same position after cholinergic and after beta-adrenergic stimulation, indicating that both stimuli led to the phosphorylation of the same membrane-associated protein. These findings strongly support the view that the phosphorylation of protein I is an important step in the sequence of events leading from receptor activation to exocytosis.