Protein phosphorylation during secretion in the rat lacrimal gland. A general role of EC-protein stimulus-secretion coupling in exocrine organs?

Protein phosphorylation in Golgi, endosomal, and endoplasmic reticulum membrane fractions of lacrimal gland

Ca2+/calmodulin- and cAMP-dependent protein kinase activities were characterized in two subcellular membrane samples. Membranes from rat lacrimal gland were isolated by differential and density gradient centrifugation into six density windows. The present study focused on membranes from density windows III and V which contain mixtures of apical, Golgi, endosomal, and endoplasmic reticulum membranes in different proportions. Phosphorylation of membrane proteins was measured by incubating the samples in [g-32P]ATP and separating the proteins by discontinuous SDS-PAGE followed by autoradiography. The amount of phosphate incorporated into specific peptide bands was quantified by densitometry. Ca2+/calmodulin-dependent protein kinase phosphorylated a 52,000 MW peptide in membranes from both density windows with a maximal increase from 0.3 to 66 microM free Ca2+. Trifluoperazine and promethazine, two inhibitors of Ca2+/calmodulin-dependent protein kinases, inhibited this phosphorylation. cAMP-dependent protein kinase phosphorylated a 22,000 MW peptide and a 91,000 MW peptide which were present in membranes from density window III only. We conclude that a Ca2+/calmodulin-dependent protein kinase activity is present in membranes from both density window III and V whereas a cAMP-dependent protein kinase activity is present only in membranes from density window III.

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.

Sympathomimetic protein secretion by young and aged lacrimal gland

The diminished basal tear flow in aged individuals is associated with lymphocytic infiltrations and atrophy of the lacrimal ducts and acini. We have investigated the age-related physiological changes to sympathomimetic stimulation of lacrimal tissue from F344 rats to determine if the responses are uniformly diminished as would be expected by glandular atrophy. The quantitative and temporal pattern of protein and peroxidase secretion by lacrimal gland fragments from young (4 month) and aged (24 month) F344 male rats was examined in a perifusion system. Upon stimulation of tissue from young animals with 0.01 mM phenylephrine for 40 min, secretion above baseline levels of protein was 570.8 micrograms/g tissue and of peroxidase was 45.2 delta A X min-1/g tissue. The response of the aged tissue to phenylephrine was not significantly different from that of the young tissue. beta-adrenergic stimulation by isoproterenol (0.01 mM) evoked only a modest secretion of protein and no consistently measurable peroxidase from young tissue. IBMX alone and in combination with isoproterenol (0.1 mM and 0.01 mM respectively) evoked a large secretion of protein, 1345.7 micrograms/g tissue, and a modest secretion of peroxidase, 9.5 delta A X min-1/g tissue by young tissue. The aged tissue, upon stimulation with the combination of IBMX and isoproterenol, secreted significantly less protein and peroxidase than the young tissue. In separate experiments, the production of cAMP was measured. In young tissue, isoproterenol did not cause a measurable increase of intracellular cAMP. IBMX caused a 2-3 fold increase in cellular cAMP which was not increased further by addition of isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Stimulation of exocytosis in exocrine glands is associated with an increased phosphorylation of several particulate proteins. Irrespective of the type of secretagogue (cAMP-dependent agonists, calcium-dependent agonists, calcium ionophores, phorbol esters) exocytosis is always accompanied by an enhanced phosphorylation of the ribosomal protein S6. It is shown by an analysis of the phosphopeptide pattern of the in vivo and the in vitro phosphorylated S6 protein that the protein kinase responsible for phosphorylation of the S6 protein during enhanced exocytosis is protein kinase C. This is so irrespective of whether the agonist uses cAMP or calcium as second messenger. Experiments with isolated guinea pig parotid gland lobules reveal that not only the acetylcholine analog carbamoylcholine, but also the beta-agonist isoproterenol lead within seconds to an increased formation of diacylglycerol. As diacylglycerol increases the affinity of protein kinase C for calcium this finding would explain why the phosphorylation pattern of the S6 protein reflects activation of protein kinase C also under conditions where (as in the case of stimulation with beta-agonists) cAMP is the primary second messenger. It would further explain why the changes of the phosphorylation of individual histones observed during agonist-induced exocytosis in the parotid gland are quite similar for isoproterenol on one hand and carbamoylcholine on the other. A 22 K protein which becomes phosphorylated only when cAMP serves as second messenger is located in the membrane of the endoplasmic reticulum. A possible relationship of this protein with the calcium transport ATPase of the endoplasmic reticulum is under investigation.

Protein secretion induced by isoproterenol or pentoxifylline in lacrimal gland: Ca2+ effects

In exorbital lacrimal glands, pentoxifylline (a methylxanthine) induces labeled protein secretion in a dose-related manner: the half-maximal and maximal stimulations are at 4 and 10 mM, respectively. In the presence of papaverine (10(-5) M), a phosphodiesterase inhibitor, labeled protein discharge is strongly stimulated by isoproterenol, via beta-adrenergic receptors: the maximal response is at 10(-6) M. l-Propranolol specifically inhibits the secretory stimulation to isoproterenol in a dose-related manner: for 5 X 10(-6) M isoproterenol in the presence of 10(-5) M papaverine, the half-maximal and maximal inhibitions are at 3 X 10(-7) and 10(-5) M, respectively. The beta-adrenergic response is mimicked by the adenosine 3',5'-cyclic monophosphate (cAMP) analogue dibutyryl cAMP (DBcAMP) at a 10(-3) M concentration. The time course of labeled protein secretion induced by pentoxifylline, DBcAMP, and isoproterenol shows a latency. In the presence or absence of extracellular calcium, pentoxifylline and isoproterenol immediately increase the cAMP intracellular level. Extracellular calcium omission increases the observed latency and also affects the maximal rate of protein secretion. As opposed to the cholinergic agonist, pentoxifylline has only a slight but sustained effect on 45Ca efflux, whereas isoproterenol has none. These data suggest that labeled protein secretion, such as that of peroxidase, can also be stimulated in rat exorbital lacrimal gland, through beta-adrenergic receptors; in the stimulation evoked by a beta-adrenergic agonist, DBcAMP, or methylxanthine, calcium could play a key role.

Muscarinic stimulation of phospholipid turnover in dissociated avian salt gland cells

Addition of carbamylcholine to 32P-prelabeled dissociated avian salt gland cells resulted in increased turnover of phosphatidic acid, phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which could be prevented by the inclusion of atropine. Carbamylcholine had no discernable effect on protein phosphorylation, measured either in the total preparation or in subcellular fractions. It is concluded that for the avian salt gland, no obligatory link is indicated between protein phosphorylation and either phospholipid turnover or salt secretion.

Forskolin as a tool to study the beta-adrenergic receptor-elicited, labeled protein secretion in rat lacrimal gland

In rat lacrimal glands, Forskolin induces a dose-dependent [3H]protein release. This effect can be potentiated by papaverine. As for the other inducers whose effects on protein secretion are assumed to be cAMP-mediated, Forskolin secretion time course shows a latency. Isoproterenol decreases the Forskolin EC50 at least 60-times. On the other hand, Forskolin enhances the efficacy of isoproterenol without affecting its potency. As a whole, the data collected show that isoproterenol-induced [3H]protein secretion in rat lacrimal glands involved adenylate cyclase activation by coupling with beta-adrenergic receptors.

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.