Enhancement of cyclic AMP modulated salivary amylase secretion by protein kinase C activators

Functional effects of proinflammatory factors present in Sjögren's syndrome salivary microenvironment in an in vitro model of human salivary gland

Primary Sjögren’s syndrome (pSS) is an autoimmune exocrinopathy in which the role that the immune response plays in reducing exocrine gland function, including the glandular microenvironment of cytokines, has not been fully understood. Epithelial cells from biopsies of human parotid gland (HPG) were used to establish a model of human salivary gland in vitro. In this model, the functional consequences of several proinflammatory soluble factors present in the pSS glandular microenvironment were assessed. Stimulation with isoproterenol and calcium produced a significant increase in the basal activity of amylase in the HPG cell supernatants. Under these conditions, the presence of TNF-α and CXCL12 increased amylase mRNA cellular abundance, but reduced the amylase activity in the cell-free supernatant in a dose-dependent manner. IL-1β and IFN-γ, but not TGF-β, also diminished amylase secretion by HPG cells. These results suggest that the glandular microenvironment of cytokine, by acting post-transcriptionally, may be responsible, at least in part, for the reduced exocrine function observed in pSS patients. These data may help to a better understanding of the pathogenesis of SS, which in turn would facilitate the identification of new therapeutic targets for this disorder.

Role of protein kinase C-delta in isoproterenol-induced amylase release in rat parotid acinar cells

In parotid acinar cells, beta-adrenergic receptor activation results in accumulation of intracellular cAMP. Subsequently, cAMP-dependent protein kinase (PKA) is activated and consequently amylase release is provoked. In this paper, we investigated involvement of protein kinase C-delta (PKC delta), a novel isoform of PKC, in amylase release induced by beta-adrenergic receptor stimulation. Amylase release stimulated with the beta-agonist isoproterenol (IPR) was inhibited by rottlerin, an inhibitor of PKC delta. IPR activated PKC delta and the effect of IPR were inhibited by a PKA inhibitor, H89. Myristoylated alanine-rich C kinase substrate (MARCKS), a major cellular substrate for PKC, was detected in rat parotid acinar cells, and a MARCKS inhibitor, MARCKS-related peptide, inhibited the IPR-induced amylase release. IPR stimulated MARCKS phosphorylation, which was found to be inhibited by H89 and rottlerin. These observations suggest that PKC delta activation is a downstream pathway of PKA activation and is involved in amylase release via MARCKS phosphorylation in rat parotid acinar cells stimulated with beta-adrenergic agonist.

cAMP and cGMP in human parotid saliva: relationships to taste and smell dysfunction, gender, and age

Among the chemical moieties present in human parotid saliva, some, such as gustin or carbonic anhydrase VI, have been useful to distinguish patients with taste and smell dysfunction from normal subjects. To continue these studies we compared levels of salivary cAMP and cGMP in patients with taste and smell dysfunction with those in normal subjects. We were also interested in exploring physiological characteristics of salivary cAMP and cGMP including changes with gender and age because previous studies had not clearly defined these issues. To perform these studies parotid saliva was collected from 61 normal volunteers and 253 patients with taste and smell dysfunction. cAMP and cGMP were measured by a spectrophotometric 96 plate ELISA technique; parotid salivary protein and flow rate were also measured. Both cAMP and cGMP were found in saliva of normal subjects and patients in the detection range of the assay used. In patients mean concentrations of both cAMP and cGMP were lower than in normal subjects; for cAMP levels were lower among both men and women patients. cAMP was 7 to 10 times higher than cGMP in both normal subjects and patients. Concentrations of cAMP were consistently higher in normal women than in normal men. cAMP levels were generally lower and cGMP levels were generally higher than in previously reported studies. There was a complex pattern of change for both cAMP and cGMP with age with concentrations increasing to about age 50, then decreasing, then increasing again at age >70 years.

Cyclic AMP potentiates substance P-induced amylase secretion by augmenting the effect of calcium in the rat parotid acinar cells

Potentiation of amylase secretion by the combination of isoproterenol and substance P was examined in perfused rat parotid acinar cells. Combined additions of substance P and isoproterenol evoked biphasic changes in amylase secretion, an initial large peak and the following sustained plateau: the magnitudes of the both responses were higher than the sum of the responses induced by each agonist alone. Isoproterenol also increased the maximum response and the apparent affinity (EC50) for substance P to evoke the initial peak response; the EC50 values were about 20 and 0.8 nM, respectively, in the absence and the presence of isoproterenol. On the other hand, 1 nM substance P was sufficient for evoking the maximum potentiation of the sustained plateau response. Substance P did not change the EC50 for isoproterenol. The effect of isoproterenol was mimicked with dibutyryl cyclic AMP and agonists that increase parotid cyclic AMP. Omission of Ca2+ or addition of 5 mM nickel chloride almost completely abolished the potentiation of the sustained plateau, but little decreased that of the initial peak. Depletion of Ca2+ in InsP3-sensitive intracellular stores with thapsigargin, on the other hand, decreased the initial peak response, but not the sustained plateau, to substance P. The potentiation was also observed between isoproterenol and Ca2+ ionophores. Switching to the solutions containing higher concentrations of Ca2+ during the continuous stimulation with isoproterenol or IBMX evoked a large, but transient, response of amylase secretion. Time course of changes in amylase secretion induced by isoproterenol and substance P in combination was very similar to that of substance P, but not of isoproterenol. Isoproterenol did not enhance the effect of substance P on [Ca2+]i. These results show that the potentiation is mainly, if not totally, caused by cyclic AMP-induced enhancement of the potency and the efficacy in the pathway regulated by Ca2+.

Calcium-calmodulin-stimulated phosphorylation of rat parotid secretion granule proteins

In studies designed to determine the mechanism by which Ca++ and calmodulin stimulate the fusion of parotid secretion granules with plasma membrane vesicles, the hypothesis tested was that Ca++ and calmodulin act by stimulating protein phosphorylation. It was earlier found that Ca++ and calmodulin, but neither alone, stimulated the phosphorylation of four secretion granule proteins with molecular masses of 64, 58, 55 and 31 kDa, and decreased the degree of phosphorylation of a 36-kDa protein. Further studies have shown that in the presence of an optimal concentration of calmodulin (2.4 microM), half-maximal activation of phosphorylation of the four proteins occurred at approx. 8 microM Ca++, and at a maximally effective Ca++ concentration (10(-4) M), half-maximal stimulation occurred at calmodulin concentrations between 0.13 and 1.1 microM for the different proteins. The studies now described also demonstrate that the need for calmodulin for stimulating the phosphorylation, but not the dephosphorylation, is specific; two other Ca(++)-binding proteins, parvalbumin and troponin, could not replace calmodulin in stimulating phosphorylation of the four secretion granule proteins, but either one could substitute for calmodulin in stimulating dephosphorylation of the 36-kDa protein. Additionally, the phosphorylated proteins appear to be located on the granule surface. When secretion granules were subjected to mild treatment with a concentration of trypsin that did not lyse the granules, the 31-, 36-, 55-, 58- and 64-kDa proteins were no longer observed. In the presence of optimal concentrations of Ca++ and calmodulin, a dose-dependent inhibition of the phosphorylation of the various proteins by two calmodulin antagonists, trifluoperazine and calmidazolium, was observed; 50% inhibition of phosphorylation of the different proteins was obtained at approx. 20-40 microM trifluoperazine and at about 2.5-3.0 microM calmidazolium. Inhibition of the dephosphorylation of the 36-kDa protein required greater concentrations of trifluoperazine and calmidazolium; 128 microM and 50 microM, respectively. These results are consistent with the hypothesis that the phosphorylation of one or more of the 31-, 55-, 58- and 64-kDa proteins, but not the dephosphorylation of the 36-kDa protein, may be involved in the action of Ca++ and calmodulin in secretion granule-plasma membrane fusion.

Chelerythrine, a protein kinase C inhibitor, interacts with cyclic nucleotide phosphodiesterases

Chelerythrine, a potent inhibitor of protein kinase C, was evaluated for its effect on cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta. Chelerythrine activated basal PDE2 and inhibited activated PDE2, PDE4 and PDE5. The effect of chelerythrine (10 microM) was also investigated on vasorelaxation induced by a beta-adrenoceptor agonist or a PDE3 inhibitor. Chelerythrine attenuated the isoprenaline-mediated relaxation whereas it potentiated the relaxation induced by SK & F 94120 (5-(4)acetamidophenyl)pyrazin-2(1 H)-one, a PDE3 inhibitor). The present study demonstrates that chelerythrine, at a concentration generally reported in the literature to inhibit protein kinase C, interacts with cyclic nucleotide phosphodiesterases and consequently modulates vasorelaxation. These results cast some doubt on the use of chelerythrine as a specific inhibitor of protein kinase C.

The role of protein kinase C in carbachol-induced and of cAMP-dependent protein kinase in isoproterenol-induced secretion in primary cultured guinea pig parotid acinar cells

Stimulation of secretion by muscarinic agonists in guinea pig parotid or pancreatic acini is accompanied by a translocation of protein kinase C (PKC) from the cytosol to the particulate fraction [Machado-De Domenech and Söling (1987) Biochem. J. 242, 749-754] and by a PKC-mediated phosphorylation of the ribosomal protein S6 [Padel and Söling (1985) Eur. J. Biochem. 151, 1-10]. In order to decide whether PKC is directly involved in the secretory process, the effect of down regulation of PKC by phorbol 12-myristate 13-acetate (PMA) was studied in primary cultured guinea pig parotid acinar cells. These cells secrete in response to carbachol and isoproterenol. Only the carbachol response is associated with an increase in cytosolic calcium. Carbachol plus isoproterenol lead to an over-additive stimulation of secretion, an effect which depends completely on the presence of external calcium. Down regulation of PKC by about 90% did not significantly affect carbachol-induced exocytosis, whereas isoproterenol-stimulated secretion was almost doubled. The secretory response to permeable cAMP analogues was also enhanced in PKC-down-regulated acini, indicating a post-receptor effect. The increased response to isoproterenol was also observed in the absence of external calcium. The isoproterenol effect was significantly inhibited by the relatively specific cAMP-dependent protein kinase inhibitor H-89, which had only a minor effect on carbachol-induced exocytosis. Although down regulation of total PKC by up to 90% did not significantly affect the secretory response to carbachol, RO 31-8220, a relatively specific inhibitor of PKC, abolished carbachol-induced secretion in normal as well as in PMA-down-regulated cells. This indicates that a PKC isoform resistant to down regulation by PMA is involved in carbachol- but not in cAMP-mediated secretion.

Amylase secretion by cultured porcine parotid cells

The similarity of porcine and human physiology and the availability of slaughterhouse tissues suggests the use of porcine parotid cells as a model for amylase secretion. A procedure is described for the isolation of porcine parotid cells by collagenase-P/dispase digestion of the tissue. The preparation consisted of individual cells and small aggregates that were maintained in primary culture, during which the cells formed aggregates that firmly attached to the plastic substrate. The amylase content of the cultured cells remained adequate for assay of secretory activity during culture for one week after isolation. Depending upon variations in experimental treatments, the cultured cells secreted approx. 35-65% of cellular amylase in response to a carbachol challenge. The cells were slightly responsive to long exposures to isoproterenol, and were unresponsive to nicotine, elevated extracellular K+ or substance P. Secretion induced by carbachol required extracellular Ca2+, was inhibited by atropine and occurred with a nearly linear response over a 30-min period. The Ca2+ ionophore A23187 was also a potent secretagogue for amylase secretion, producing levels of secretion equal to that induced by carbachol. The ease of preparation and the retention of amylase during primary culture suggests that the preparation will be useful in studies on muscarinic receptor-mediated control of amylase secretion.

Translocation of the alpha-isozyme of protein kinase C during stimulation of rat parotid acinar cells by phorbol ester and carbachol

Protein kinase C activity was detected in the cytosolic fraction of quiescent parotid acinar cells; the particulate fraction contained a much smaller proportion of the enzyme. Protein kinase C activity was increased in the membrane fraction and decreased in the cytosol after exposure of intact cells to phorbol 12-myristate 13-acetate (PMA) or the muscarinic-receptor agonist carbachol. The effect of PMA was potentiated by a subthreshold concentration of ionomycin. Immunoblot analysis with anti-protein kinase C antibodies revealed that the protein kinase C-alpha isoform is expressed in rat parotid cells. Other Ca(2+)-dependent isoforms were not detected. Further, agonist stimulation caused the redistribution of protein kinase C-alpha from cytosol to a membrane fraction. Agonists may promote parotid acinar cell activity, including amylase secretion, by increasing the affinity of protein kinase C-alpha for the membrane fraction, presumably via a rise in Ca2+ and diacylglycerol derived from polyphosphoinositide hydrolysis.

Inhibitory interactions between stimulus-secretion pathways in the exocrine rat pancreas

In many tissues the cellular responses mediated through different intracellular messenger systems are mutually interactive. In the exocrine pancreas the secretagogues acting via adenosine cyclic monophosphate (cAMP) and those acting via calcium-phosphoinositides can potentiate one another. On the other hand, protein kinase C (PK-C) modulates receptor-induced responses in exocrine pancreatic cells and other cell types. Recording total protein output, monitored on-line at 280 nm, from superfused rat pancreatic segments, we demonstrate that secretin (a cAMP-acting hormone) reduces the efficacy of the calcium-mediated secretagogue cholecystokinin-octapeptide (CCK-8). Likewise, the PK-C activator 12,O,tetradecanoyl phorbol 13 acetate (TPA) reduces both the efficacy of secretin and the potency of cholecystokinin. Thus, the hypothesis of potentiation between different stimulus-secretion coupling mechanisms must be revised, and receptor-activated responses in the exocrine pancreas must be considered a complex model with multiple inhibitory and stimulatory interactions.

Phorbol ester has different effects on forskolin and beta-adrenergic-stimulated cAMP accumulation in mouse parotid acini

Phorbol 12-myristate 13-acetate (TPA) augmented the effects of forskolin, and inhibited the effects of isoproterenol on cAMP accumulation in mouse parotid acini. Treatment of intact cells with the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (MIX), blocked TPA inhibition of isoproterenol but not forskolin-stimulated cAMP accumulation. TPA also caused the translocation of protein kinase C (PKC) from cytosol to membrane. Pre-treatment of parotid acini with TPA for 30 min enhanced the forskolin and isoproterenol-stimulated adenylate cyclase activity in isolated parotid membranes. Addition of purified PKC (pPKC) to parotid membranes mimicked the effects of TPA in increasing cAMP synthesis; the effects were blocked in the absence of calcium and phospholipid, and in the presence of the synthetic peptide (PKC 19-36). Purified PKC also mimicked the effects of TPA in augmenting forskolin and isoproterenol-stimulated adenylate cyclase activities in the cell-free system. Data suggest that the differential regulation of forskolin and isoproterenol-stimulated cAMP accumulation by TPA results from modification of enzymes that synthesize and degrade cAMP.

Influence of aging on the beta- and glucagon-receptor-mediated glycogenolysis in rat hepatocytes

The influence of aging on beta-receptor and glucagon-receptor control of glycogenolysis was investigated in rat hepatocytes. The beta-receptor-induced glucose output was detectable only in senescent rats, was partly dependent on extracellular Ca2+, and was inhibited by 4 beta-phorbol 12-myristate 13-acetate (PMA), insulin, and the Ca(2+)-antagonists, verapamil and nifedipine. Chelation of extracellular Ca2+ potentiated the effect of nifedipine only. In contrast, glucagon-stimulated glycogenolysis, similar in mature and senescent rats, was independent on extracellular Ca2+ and was unaffected by PMA. Verapamil, in senescent rats only, and nifedipine, in mature and senescent rats, inhibited glucagon-stimulated glucose output only in the presence of Ca2+. Insulin inhibited glucagon-induced glucose output, irrespective of the age of the rat and the presence of Ca2+. We conclude that the beta-receptor component in the adrenergic regulation of glycogenolysis in senescent rats consists of a major Ca(2+)-independent and a minor Ca(2+)-dependent part, displaying different sensitivity towards protein kinase C (PKC), Ca(2+)-antagonists, and insulin. Aging does not change the capacity of glucagon to induce a full glycogenolytic response in the absence of extracellular Ca2+; Ca(2+)-influx, however, seems to be involved when extracellular Ca2+ is present, and this sensitivity is increased on aging.

Relationship between cytosolic Ca2+ concentration and amylase release in rat parotid acinar cells following muscarinic stimulation

Carbachol (CCh), a muscarinic-cholinergic agonist, increased both cytosolic free calcium concentration ([Ca2+]i) and amylase release in rat parotid acinar cells or acini in a dose-dependent manner. Treatment of acinar cells with the intracellular Ca2+ antagonist, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), or the intracellular Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid (BAPTA), strongly attenuated the increases in [Ca2+]i evoked by CCh, but amylase release from acini was not significantly suppressed by the treatment with TMB-8 or BAPTA. Low concentrations (0.02-0.5 microM) of ionomycin, a Ca2+ ionophore, caused increases in [Ca2+]i comparable to those induced by CCh, but the same concentrations had only a little effect on amylase release. The protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), stimulated amylase release in quantities similar to those induced by CCh, although TPA alone did not cause any change in [Ca2+]i. Combined addition of TPA and ionomycin potentiated only modestly amylase release stimulated by TPA alone. Staurosporine, a protein kinase C-inhibitor, similarly inhibited both the CCh- and TPA-induced amylase release. These results suggest that an increase in [Ca2+]i elicited by CCh does not play an essential role for inducing amylase release in rat parotid acini. Amylase release by muscarinic stimulation may be mediated mainly by activation of protein kinase C.

Interaction between the calcium and cyclic AMP messenger systems in perifused rat parotid acinar cells

Potentiation of amylase secretion induced by a combination of isoproterenol and carbamylcholine was examined in perifused rat parotid acinar cells. The time course of changes in the augmented amylase secretion induced by isoproterenol plus carbamylcholine was similar to that induced by carbamylcholine alone, but not to that caused by isoproterenol. Concentration-response analysis showed that isoproterenol increased the apparent affinity for carbamylcholine to stimulate amylase secretion with the maximum effect attained by isoproterenol plus carbamylcholine being higher than that attained by isoproterenol or carbamylcholine. 8-Bromo cyclic AMP, forskolin and 3-isobutyl-1-methylxanthine mimicked the effect of isoproterenol. Calcium ionophores (A23187 and ionomycin), but not phorbol 12,13-dibutyrate, mimicked the effect of carbamylcholine. Chelation of intracellular free calcium with 1,2-bis-[2-aminophenoxyl]-ethane-N,N,N',N'-tetraacetic acid, but not that of extracellular calcium with [ethylenebis(oxyethylenenitrile)]-tetraacetic acid (EGTA), abolished the potentiation. Calmodulin antagonists inhibited amylase secretion induced by isoproterenol plus carbamylcholine or carbamylcholine alone, but not that induced by isoproterenol alone. These results suggest that the potentiation is mainly, if not completely, caused by a coordinated interaction between the cyclic AMP system and the Ca2+ system at a step distal to second messenger generation, probably via a cyclic AMP-induced increase in the sensitivity of the Ca2+ response element to calcium.

Microplate reader-based kinetic determination of alpha-amylase activity: application to quantitation of secretion from rat parotid acini

A coupled enzyme assay for measuring alpha-amylase activity was adapted for analysis with a microplate reader. Activity was quantified by monitoring the cleavage of p-nitrophenol from a chemically defined substrate at 405 nm. Features of this assay method include: low sample volume (10 microliters); economical use of reagent (200 microliters); increased precision due to kinetic nature of assay; linearity with amylase content to 2600 U/liter; capability of processing up to 96 samples within 10 min; facilitated data analysis using readily available software. The ability to rapidly measure amylase content of a large number of samples permitted sophisticated analysis of alpha-amylase secretion patterns from dispersed rat parotid acinar cells. Examination of the dose-dependent increase in the rate of amylase secretion stimulated by carbachol revealed a biphasic response at higher concentrations, where a rapid and transient increase in amylase release was consistently observed during the first 10 min. This initial phase of amylase release was additive with the slower, sustained secretion which was stimulated by carbachol in a dose-dependent manner (Kd = 2.5 microM). Such a biphasic release pattern was not seen with other potent secretagogues (isoproterenol, dibutyryl cAMP) that are not thought to act via a Ca-dependent mechanism. These results suggest that parotid secretion patterns should be studied at a number of time points, which is feasible using the method reported herein.

Evidence for the convergence of beta-adrenergic and muscarinic signalling systems at a post-receptor site

The beta-adrenergic agonist isoproterenol stimulates inositol trisphosphate (IP3) formation and cytosolic Ca2+ [( Ca2+]i) mobilization in rat parotid acini via a cAMP-dependent process. Atropine, a muscarinic antagonist, inhibited these isoproterenol responses without affecting isoproterenol-induced amylase secretion or peak [Ca2+]i and IP3 responses elicited by alpha 1-adrenergic stimulation with epinephrine. Atropine had no effect on isoproterenol-induced [Ca2+]i responses in a cell line which lacked muscarinic receptors and did not alter beta-adrenoreceptor ligand binding. These results suggest that the inhibition by atropine results from a post-receptor effect on cAMP-mediated stimulation of phosphatidylinositol 4,5 bisphosphate (PIP2) hydrolysis.

Convergence of cAMP and phosphoinositide pathways during rat parotid secretion

Rat parotid acinar cells were employed to investigate the mechanism by which receptor agonists that activate the phosphoinositide pathway enhance the stimulatory effects of adenosine 3',5'-cyclic monophosphate (cAMP) on amylase secretion. Norepinephrine (NE), which activates both alpha- and beta-adrenoceptors, evoked a secretory response that was greater than the sum of the responses obtained when NE was employed as a beta-agonist (in the presence of prazosin) and as an alpha-agonist (in the presence of propranolol). The enhancement of amylase secretion induced by NE was accompanied by an augmented rise in Ca2+ influx, as determined by fura-2 analysis. NE-induced cAMP production was comparable to that evoked by NE as a beta-agonist, and the accumulation of [3H]inositol 1,4,5-trisphosphate (IP3) evoked by NE was comparable to that elicited by NE as an alpha-agonist. The beta-adrenoceptor agonist isoproterenol potentiated the rise in cytosolic Ca2+ elicited by the muscarinic agonist carbachol, while possessing no stimulatory effect of its own. Isoproterenol had no effect on carbachol-induced stimulation of [3H]IP3 or 1,3,4,5-[3H]inositol tetrakisphosphate accumulation. Ionomycin and dibutyryl cAMP in combination produced a similar enhancing effect on the Ca2+ signal and amylase release as adrenergic and muscarinic receptor agonists. These results suggest that the synergism between the phosphoinositide and cAMP-signaling systems in parotid cells resides in enhanced Ca2+ availability.