Regulation of protein kinases in exocrine secretory cells during agonist-induced exocytosis

Sialagogue-stimulated protein phosphorylation related to ornithine decarboxylase induction in cultured rat parotid explants

Both beta-adrenergic (isoproterenol) and cholinergic (carbachol) sialagogues increase amylase secretion, ornithine decarboxylase activity and DNA synthesis in murine parotid gland in vivo and in vitro. These agonists enhanced the incorporation of labelled inorganic orthophosphate into parotid proteins in rat parotid explants cultured on siliconized lens paper floating on serum-free 199 medium. Analysis of the labelled proteins by SDS-PAGE and autoradiography revealed that isoproterenol enhanced the phosphorylation of four proteins with apparent molecular weights of 17, 20, 31 and 32 kDa and carbachol stimulated the phosphorylation of 31 and 32 K proteins. Isoproterenol-dependent ornithine decarboxylase induction and phosphorylation of the proteins were selectively suppressed by monensin but not by polymyxin B, whereas carbachol-dependent ornithine decarboxylase induction and protein phosphorylation were inhibited by polymyxin B but not by monensin. Neither monensin nor polymyxin B suppressed isoproterenol- or carbachol-stimulated amylase secretion. Time course experiments showed that sialagogue-stimulated protein phosphorylation preceded the increase of ornithine decarboxylase activity and had almost disappeared when it was maximal. Propranolol and atropine, antagonists of isoproterenol and carbachol, respectively, completely inhibited not only amylase secretion and ornithine decarboxylase induction but also protein phosphorylation stimulated by the corresponding agonists. These findings suggest that increased phosphorylation of specific proteins is associated with sialagogue-stimulated ornithine decarboxylase induction but not amylase secretion.

Co-ordinated changes in the cyclic AMP signalling system and the phosphorylation of two nuclear proteins of Mr 130,000 and 110,000 during proliferative stimulation of the rat parotid gland by isoprenaline. Possible identity of the two proteins with pp135 and nucleolin

Parotid glands were stimulated to growth by repeated injection of the beta-agonist isoprenaline into rats. Incubation of intact parotid-gland lobules with [32P]Pi and subsequent analysis of nuclear proteins revealed in the stimulated glands an increased 32P incorporation into two acid-soluble non-histone proteins with apparent Mr values of 110,000 and 130,000 (p110 and p130). After a single injection of isoprenaline, leading to a biphasic increase in DNA synthesis (maximum at 24 h), the same two proteins showed a transiently increased 32P incorporation at 17 h after injection. At this time point at the onset of DNA synthesis the total activity of soluble cyclic AMP-dependent protein kinase decreased. No change in p110/p130 phosphorylation was observed at 0.3 h after stimulation, a time of maximal stimulation of secretion. Administration of the beta-antagonist propranolol 8 h after the injection of isoprenaline suppressed the increase in DNA synthesis, the preceding changes in the concentration of cyclic AMP and in the activity of cyclic AMP-dependent protein kinase, as well as the increased phosphorylation of p110 and p130. Cross-reactivity of p110 and p130 with specific antisera against two nucleolar phosphoproteins of similar molecular mass (nucleolin and pp135), as well as their localization in a nucleolar cell fraction, indicated a possible identity of p110 and p130 with these two proteins. Our results suggest that nucleolin and pp135 are nuclear target proteins of cyclic AMP in the cyclic AMP-influenced regulation of the transition of cells from the G1 to the S phase.

Ca2+/calmodulin-dependent protein phosphatase in bovine parotid gland: purification and characterization

Calmodulin-dependent protein phosphatase (CaM-PPase) was isolated from bovine parotid gland by sequential application of DEAE-52, Affi-gel blue and calmodulin-affinity chromatography followed by gel filtration and high performance liquid chromatography. The enzyme was activated in the simultaneous presence of Ni2+ or Mn2+ and Ca2+ plus calmodulin. Ca2+/calmodulin-dependent activation of CaM-PPase was antagonized by inhibitors of calmodulin action, such as W-7 and trifluoperazine. Tryptophan fluorescence was quenched in the presence of Ni2+. CaM-PPase was a heterodimer. The molecular weights of large subunits which bound calmodulin (CaM) were 68 kD and 58 kD - the 68 kD subunit was predominant. Polyclonal antibodies against bovine calcineurin cross-reacted with both types of larger subunits. Using polyclonal antibodies against bovine calcineurin or the monoclonal antibody against subunit B of bovine calcineurin, the smaller molecular weight subunit (19 kD) was found to be immunologically identical to subunit B of bovine calcineurin. In bovine parotid gland, CaM-PPase was found both in acinar and duct cells.

Signal transduction and control of lacrimal gland protein secretion: a review

Proteins in lacrimal gland fluid are secreted primarily by the acinar cells. Secretory proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, stored in secretory granules, and released upon a change in the cellular level of second messenger. The second messenger level is controlled by a process termed signal transduction. Agonists, primarily neurotransmitters in the lacrimal gland, bind to receptors in the basolateral membrane of secretory cells. This interaction activates enzymes in the membrane that cause production of second messengers. It has been hypothesized that second messengers stimulate secretion by activating specific protein kinases to phosphorylate proteins important for secretion. In the lacrimal gland, cholinergic agonists stimulate protein secretion. They act by activating phospholipase C to break down phosphatidylinositol bisphosphate into 1,4,5-inositol trisphosphate (1,4,5-IP3) and diacylglycerol (DAG). 1,4,5-IP3 causes release of Ca2+ from intracellular stores. This Ca2+, perhaps in conjunction with calmodulin, activates specific protein kinases that may be involved in secretion. DAG activates protein kinase C which stimulates protein secretion. alpha 1-Adrenergic agonists also stimulate lacrimal gland protein secretion. These agonists use a pathway that is separate from that utilized by cholinergic agonists and vasoactive intestinal peptide (VIP). The specific pathway has not been identified but may be DAG and protein kinase C. VIP, beta-adrenergic agonists, alpha-melanocyte stimulating hormone, and adrenocorticotropic hormone are lacrimal gland secretagogues. They activate adenylate cyclase to produce cAMP. cAMP stimulates protein kinase A, which perhaps causes protein secretion. Thus, three separate cellular pathways stimulate lacrimal gland protein secretion. Cholinergic agonists and VIP also stimulate lacrimal gland fluid secretion, and the same signal transduction pathways utilized by these agonists to stimulate protein secretion are most likely used for electrolyte and water secretion.

Protein phosphatases of the guinea-pig parotid gland

The nature of protein phosphatases of the guinea-pig parotid gland was investigated. The protein phosphatases were characterized by (a) the use of five different 32P-labelled substrate proteins (phosphorylase a, histone H2B, casein, and the alpha and beta subunits of phosphorylase kinase), (b) their behaviour during ion-exchange chromatography, (c) their relative molecular mass distribution during gel filtration, (d) their sensitivity towards inhibition by inhibitor 2, (e) their ability to be stimulated by protamine and (f) by their behaviour during freezing and thawing in the presence of 2-mercaptoethanol. The following results were obtained. 1. The 'cytosol' (100,000 X g supernatant) contains protein phosphatases of the types 1, 2A and 2B. 2. On the basis of inhibition with inhibitor 2 (1.2 micrograms/ml) the 'cytosolic' phosphorylase phosphatase activity consists to about 40% of protein phosphatase 1 and to about 60% of protein phosphatase 2A. 3. In the cytosol about 80-90% of the protein phosphatases 1 and 2A exist in an inactive state. 4. A 5-10-fold activation can be achieved by ethanol precipitation, which results in the generation of a mixture of forms of low apparent molecular mass of about 30 kDa. 5. Microsome-associated phosphorylase phosphatase activities can be extracted in a highly active state by detergent (1% Triton X-100) or by 0.8 M NaCl. 6. Activity measurements in the presence of inhibitor 2 (1.2 micrograms/ml) indicate that the microsomal activities consist to about 75% of protein phosphatase 1 and to about 25% of protein phosphatase 2A. Activities corresponding to protein phosphatases 2B and 2C could not be detected. 7. The 'microsomal' protein phosphatase activities exhibit lower apparent molecular masses (70 kDa and 30 kDa) than the 'cytosolic' protein phosphatases (about 260 kDa). 8. After ethanol treatment of the microsomal protein phosphatases only activities with apparent molecular masses of about 30 kDa can be detected. These share several similarities with the ethanol-treated cytosolic protein phosphatases. 9. Both cytosolic and microsomal protein phosphatases display activity towards histone H2B and casein.

Effects of stimulation of muscarinic and of beta-catecholamine receptors on the intracellular distribution of protein kinase C in guinea pig exocrine glands

Stimulation of exocrine cells via muscarinic receptors is associated with an activation of protein kinase C [Padel & Söling (1985) Eur. J. Biochem. 151, 1-10]. We show here that stimulation of isolated parotid gland lobules with 8 X 10(-6) M-carbamoylcholine leads to a translocation of protein kinase C from the cytosolic to the particulate compartment within 30 s (25% and 45% of total activity recovered in the particulate fraction of controls and stimulated samples respectively). The specific enzyme activity in the particulate fraction increased to 169% of the corresponding control value. After 10 min the changes started to reverse and, after 30 min, cytosolic protein kinase C was higher in stimulated than in unstimulated lobules. Isoproterenol (2 X 10(-5) M) stimulated the release of amylase more than did carbamoylcholine, but did not significantly affect intracellular distribution of protein kinase C during the observation time of 30 min. In isolated pancreatic lobules a significant carbamoylcholine-mediated translocation of protein kinase C into the particulate fraction could be observed after 5 and 20 min, but not after 1 min. After 5 min the specific enzyme activity in the particulate fraction had increased to 153% of the corresponding controls. The corresponding decrease (-38%) in the specific enzymic activity of cytosolic protein kinase C stayed constant up to 30 min. In isolated parotid gland lobules alpha-amylase secretion proceeded at a linear rate already during the first 1 min of stimulation, whereas in pancreatic lobules a measurable rate of alpha-amylase secretion did not occur before 5 min. These differences in time course paralleled the differences in the onset of translocation of protein kinase C. The results support a direct involvement of protein kinase C in carbamoylcholine-mediated but not in isoproterenol-mediated stimulation of exocytosis in exocrine cells.

Phosphorylation of the ribosomal protein S6 during agonist-induced exocytosis in exocrine glands is catalyzed by calcium-phospholipid-dependent protein kinase (protein kinase C). Experiments with guinea pig parotid glands

The ribosomal protein S6 in exocrine cells is phosphorylated during stimulation of exocytosis by cAMP-dependent or calcium-dependent agonists. Under both conditions the same tryptic S6 phosphopeptides (termed A, B, and C) were found [Padel, Kruppa, Jahn & Söling (1983) FEBS Lett. 159, 112-118]. Studies have now been made of the phosphorylation pattern of protein S6 from purified guinea pig parotid ribosomes following in vitro phosphorylation with calmodulin-dependent, phospholipid-dependent, and cAMP-dependent protein kinases. Only the phospholipid-dependent enzyme led to the phosphorylation of peptides A, B, and C, while the cAMP-dependent enzyme phosphorylated only peptides A and C, and the calmodulin-dependent enzyme did not phosphorylate any of the phosphopeptides found in S6 from unstimulated or stimulated intact cells. Guinea pig parotid microsomes contain substantial phospholipid-dependent protein kinase activity. Stimulation of intact parotid glands with tetradecanoylphorbol acetate led to a significant phosphorylation of S6 and a similar tryptic S6 phosphopeptide pattern as seen with carbamoylcholine. It is concluded that activation of phospholipid-dependent protein kinase is responsible for the phosphorylation of protein S6 during stimulation with calcium-dependent and cAMP-dependent secretagogues.