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 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.

Purification and characterization of multiple S6 phosphatases from the rat parotid gland

S6 phosphatase activities, which dephosphorylate the phosphorylated S6 synthetic peptide, RRLSSLRASTSKSESSQK, were purified to near homogeneity from the membrane and cytosolic fractions of the rat parotid gland. Multiple S6 phosphatases were fractionated on Mono Q and gel filtration columns. In the cytosolic fraction, at least three forms of S6 phosphatase, termed peaks I, II, and III, were differentially resolved. The three forms had different sizes and protein compositions. The peak I enzyme, which had an approximately Mr of 68 kDa on gel filtration, appears to represent a dimeric form of the 39 kDa protein. This S6 phosphatase showed the high activity in the presence of EGTA and was completely inhibited by nanomolar concentrations of either okadaic acid or inhibitor 2. The peak II S6 phosphatase enzyme, with an Mr of 35 kDa, was activated by Mn2+. This form could be a proteolytic product of the catalytic subunit of type 1 phosphatase, due to its sensitivities to okadaic acid and inhibitor 2. The peak III enzyme, with an Mr of 55 kDa, is a Mn(2+)-dependent S6 phosphatase. This S6 phosphatase can be classified as a type 1 phosphatase, due to its sensitivity to okadaic acid, since the IC50 of okadaic acid is 4 nM. However, the molecular mass of this S6 phosphatase differs from that of the type 1 catalytic subunit (37 kDa) and showed less sensitivity to inhibitor 2. On the other hand, the membrane fraction contained one form of the S6 phosphatases, termed peak V (Mr 34 and 28 kDa), which could be classified as a type 1 phosphatase. This S6 phosphatase activity was greatly stimulated by Mn2+.

Carbachol, substance P, and phorbol ester promote the tyrosine phosphorylation of protein kinase C delta in salivary gland epithelial cells

The initiation of saliva formation by parotid acinar cells, which comprise the majority of cells in this salivary gland, is initiated by the release of neurotransmitters (acetylcholine, substance P) from parasympathetic nerves. In response to substance P and the muscarinic agonist carbachol, two ligands that activate phospholipase C-linked receptors, which stimulate fluid secretion, PKC delta was phosphorylated on tyrosine residues. The maximal agonist-dependent tyrosine phosphorylation occurred within seconds of the addition of either agonist and then returned rapidly to a smaller increased level. Phorbol ester also caused a rapid increase in tyrosine phosphorylation, which reached a maximal level 5 min after the addition of phorbol 12-myristate 13-acetate. The increase in tyrosine phosphorylation of PKC delta was blocked by tyrosine kinase inhibitors genistein and staurosporine. Ionophore-mediated elevation of [Ca2+]i or activation of the beta-adrenergic receptor, epidermal growth factor receptor, or insulin receptor did not promote the tyrosine phosphorylation of PKC delta. These results indicate that tyrosine phosphorylation plays a role in early signal transduction events promoted by the activation of muscarinic and substance P receptors and suggests that the tyrosine phosphorylation of PKC delta has a role in the activation of fluid secretion by neurotransmitters binding to phospholipase C-linked receptors.

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.

Purification and partial characterization of analogous 26-kDa rat submandibular and parotid gland integral membrane phosphoproteins that may have a role in exocytosis

Rat submandibular and parotid gland exocytosis is primarily controlled by beta-adrenergic receptor stimulation. Although its precise role in the regulation of salivary gland exocytosis is not fully understood, protein phosphorylation, mediated by the activation of cAMP-dependent protein kinase, may be directly involved. Previous studies suggest that analogous 26-kDa integral membrane phosphoproteins may play a direct role in regulating exocytosis. Studies were here undertaken to purify and partially characterize both phosphoproteins. After endogenous phosphorylation with 32P, subcellular fraction and solubilization of the microsomal fraction in n-octyl beta-glucopyranoside, the 26-kDa integral membrane phosphoproteins were purified by high performance liquid chromatography (HPLC), followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and electroelution of the proteins. Amino acid analysis indicated a significant number of serine amino acids: N-terminal sequence data demonstrated a high level of homology; and trypsin digestion followed by reversed-phase HPLC indicated the possibility of multiple phosphorylation sites.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Mitogen-responsive S6 kinase

Many cell lines respond to mitogenic stimuli (serum, growth factors) with rapid phosphorylation of the ribosomal protein S6 at several serine sites. We have tried to identify the protein kinase(s) mediating this effect of growth stimuli. Examining post-DEAE chromatography fractions of S49 kin- cell extracts, we could detect a highly active effector-independent S6 kinase with specificity for serine residues. The study was extended to the presumably homologous human enzyme, using HeLa S3 cells as model system. Activity yields increased up to sevenfold when exhausted HeLa cells were supplied with fresh medium plus serum. The enzyme uses ATP, not GTP, as cosubstrate, 40-S or 80-S (reassociated from subunits) ribosomal particles being substrate. The optimal K+ concentration, measured at 3 mM Mg2+, is 35 mM. Under optimized assay conditions S6 phosphorylation proceeded faster in vitro than it appeared to do in vivo. The apparent Mr of the enzyme, as estimated by gel filtration on Sephadex G-100, is 56,000 (determination in the presence of 200 mM KCl in 25 mM phosphate buffer). Tighter binding to DEAE-Sephacel and higher specificity for S6 distinguishes this enzyme from the following S6-phosphorylating protein kinases: protein kinase C, protease-activated kinase II, histone-4 phosphotransferase and an enzyme with the properties of casein kinase I. In published summaries of observations shown here and in a follow-up study with chick embryo fibroblasts, the enzyme(s) has been referred to as mitogen-responsive S6 kinase(s) [Martini, O. H. W. and Lawen, A. (1985) in Hormones and cell regulation (Dumont, J. E., Hamprecht, B. and Nunez, J., eds) vol. 9, pp. 411-412, Elsevier Company, North-Holland, Amsterdam; Lawen, A. and Martini, O. H. W. (1985) FEBS Lett. 185, 272-276].

Insulin-induced S6 kinase activation in HeLa cells and its reversal by hyperthermic stress

Insulin treatment of HeLa S3 cells activates an S6-phosphorylating protein kinase. Although this enzyme has chromatographic properties resembling those of described proteolytic fragments of other protein kinases, namely protein kinase C, protease-activated kinase II and histone-4 protein kinase, and although insulin has been proposed by others to cause S6 phosphorylation via proteolytic protein kinase activation, the insulin-induced increase in S6-kinase activity described here is probably not due to proteolysis. Rather, the activity indicates the existence, in HeLa cells, of an interconvertible S6 kinase, since the insulin-induced activity increase was rapidly reversed under hyperthermic stress, and since this effect of hyperthermia was itself reversible. The S6-kinase activities from serum- and from insulin-stimulated HeLa cells resemble each other closely and are likely to represent the same enzyme. The enzyme may therefore mediate both signals delivered by mitogens and the insulin signal. Analysed at an in vitro transfer of 1 mol phosphate/mol S6, this S6 kinase activity does not phosphorylate the (principal) S6 site recognized by the cAMP-dependent protein kinase.

Comparative studies on phosphorylation of synthetic peptide analogue of ribosomal protein S6 and 40-S ribosomal subunits between Ca2+/phospholipid-dependent protein kinase and its protease-activated form

Ca2+/phospholipid-dependent protein kinase (protein kinase C) and trypsin-activated protein kinase C (protein kinase M) phosphorylated the synthetic peptide R1-A13 (Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala-Ser-Thr-Ser-Lys-Ala) which contains both cAMP- and insulin-regulated phosphorylation sites in rat liver ribosomal protein S6 [Wettenhall, R. E. H. & Morgan, F. J. (1984) J. Biol. Chem. 259, 2084-2091]. Both enzymes showed essentially the same kinetic properties; V and apparent Km were determined to be 0.16 mumol min-1 mg-1 and 30 microM, respectively. At first, tryptic phosphopeptides were prepared at the early stage of phosphorylation and purified by high-performance liquid chromatography (HPLC). Through these analyses, four radioactive peptides were isolated. When protein kinase C was employed, phosphorylation was observed on all four peptides in a Ca2+/phospholipid-dependent manner. Irrespective of the protein kinase employed, phosphate incorporation into these peptides increased linearly with time; the peptide concentration did not affect the ratio of phosphate distribution into these four peptides. Analysis of amino acid composition and phosphoamino acid of radioactive peptides obtained after extensive phosphorylation showed that phosphates were incorporated into Ser-4, Ser-5, Ser-9 and Ser-11. The latter three serine residues were major phosphorylated sites. When rat liver 40-S ribosomal subunits were employed as substrate for protein kinases C and M, a radioactive protein with Mr,app = 31,000, which corresponded to S6 protein, was detected on an autoradiogram of a sodium dodecyl sulfate/polyacrylamide slab gel. The rate of phosphorylation with protein kinase M was twice as fast as that with protein kinase C. The elution profile of radioactive tryptic peptides in HPLC suggest that phosphorylation occurred on the sites in S6 protein corresponding to Ser-5, Ser-9 and Ser-11 as major sites and Ser-4 as the minor one. These results indicate that protein kinase C has an ability to recognize at least four sites derived from hormone-dependent phosphorylation sites in ribosomal protein S6 irrespective of the mode of activation of this enzyme.

Purification and properties of a multifunctional calcium/calmodulin-dependent protein kinase from rat pancreas

A calcium/calmodulin-dependent protein kinase (Ca/calmodulin protein kinase) was purified from rat pancreas using hydrophobic chromatography followed by gel filtration and affinity chromatography. Ca/calmodulin protein kinase from pancreas resembled previously described multifunctional Ca/calmodulin protein kinases from other tissues with respect to substrate specificity, autophosphorylation on serine and threonine residues, and catalytic and hydrodynamic properties. While Ca/calmodulin protein kinase from other tissues contains subunits of 53-60 kDa with variable proportions of a smaller 50-52 kDa subunit, pancreatic Ca/calmodulin protein kinase was found to contain a single component of 51 kDa. Experiments mixing brain Ca/calmodulin protein kinase with pancreatic homogenate suggest that the absence of a larger subunit in the pancreatic Ca/calmodulin protein kinase is not due to proteolytic degradation during enzyme preparation. Ca/calmodulin protein kinase binding to 125I-labeled calmodulin in solution was demonstrated using the photoaffinity cross-linker, N-hydroxysuccinimidyl-4-azidobenzoate. 125I-labeled calmodulin binding to Ca/calmodulin protein kinase was also demonstrated using filters containing Ca/calmodulin protein kinase transferred from polyacrylamide gels after two-dimensional gel electrophoresis. Finally, the ribosomal substrate for Ca/calmodulin protein kinase was identified as the ribosomal protein, S6. The purification procedure presented in this study promises to be useful in characterizing Ca/calmodulin protein kinase in other tissues and in clarifying the role of these enzymes in cellular function.

Characteristics of thyroid protein kinase C. Different Ca2 requirement for the phosphorylation of endogenous proteins and of H1 histone

Thyroid protein kinase C (PKc) from cytosols of porcine and rat thyroid glands has been characterized using histone H1 or endogenous proteins as substrates. As in many other tissues histone H1 is by far the preferred exogenous substrate of thyroid PKc. Kinetic studies with H1 showed that, compared to rat thyroids, porcine glands are particularly rich in PKc, the predominant kinase activity in this tissue. The cAMP-dependent protein kinase (PKa) level, on the contrary, is very similar in both rat and porcine thyroids. Consequently, for the same type of tissue, there may be great species differences in the PKc level and the ratios between PKc and PKa kinase activities. Chromatographic properties of thyroid PKc are similar to those described in other tissues (one major peak followed by a small shoulder) except that elution of the main peak can vary depending on the nature of the salt gradient (approximately 55 mM for NaCl and 15 mM for sodium phosphate). In the first case PKc is completely separated from the PKa activity, in the second it is coeluted with the peak of PKa type I. The one-dimensional PAGE pattern of proteins phosphorylated by porcine PKc is very similar to the pattern obtained by rat enzyme. Protein bands of 18 kDa, 22-25 kDa and 32-36 kDa are specific substrates of the thyroid PKc, after in vitro phosphorylation of cytosol proteins. A great difference in Ca2+ requirement for PKc activation was noted, depending whether histone H1 or endogenous proteins were substrates. As in other tissues, calcium was absolutely necessary for phosphorylation of histone H1 by PKc. The addition of calcium was not absolutely necessary when endogenous proteins were the substrates, either for the activation of the enzyme or for phosphorylation of the PKc-specific substrates. Almost the same rate of phosphorylation was obtained with or without calcium in the incubation medium. However the one-dimensional PAGE pattern of phosphorylated proteins was different in the presence or absence of calcium. While addition of calcium was not absolutely necessary for the phosphorylation of a great number of proteins by the PKc, its presence was indispensable for the phosphorylation of certain endogenous substrates. However, calcium alone, in the absence of phospholipids had no effect on the phosphorylation of these proteins. Endogenous proteins, phosphorylated by the PKc only when calcium was present, were resolved by the two-dimensional PAGE into several distinct spots with molecular masses of 32-35 kDa and pI range of 5-7.5.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction, partial purification and characterization of a hamster fibroblast protein kinase activity that phosphorylates ribosomal protein S6

When BHK cells were grown to confluence and the growth medium replenished, there was a large and rapid increase in the phosphorylation of ribosomal protein S6. In postribosomal extracts of these cells, prepared in the presence of glycerol 2-phosphate and EGTA, a ribosomal protein S6 kinase was detected. The increase in activity of this protein kinase broadly reflected the increase in phosphorylation of ribosomal protein S6 observed in vivo. This ribosomal protein S6 kinase activity was substantially purified by a combination of phosphocellulose, DEAE-cellulose, Mono Q and heparin-Sepharose chromatography, and some of its characteristics were examined. When the products of phosphorylation of 40S ribosomal subunits by purified enzyme in vitro were analysed using two-dimensional gel electrophoresis, monophosphorylated and diphosphorylated forms of ribosomal protein S6 were observed to be the predominant radioactively labelled species.

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.

Endogenous substrates of protein kinase C in experimentally induced and regressed rat thyroid goitres

The presence of endogenous substrates of the protein kinase C (PKc) in rat thyroid glands has been demonstrated in in vitro phosphorylated cytosolic proteins by polyacrylamide gel electrophoresis (PAGE). Rat thyroid PKc specifically catalyzes the phosphorylation of the 35 kDa and 18 kDa proteins. These proteins were not labelled in the presence of Ca2+ alone, but they were phosphorylated when phospholipids alone were added. In hyperplastic glands the total phosphorylation of endogenous proteins is stimulated, due to the increased labelling of the 35 kDa and 18 kDa proteins. No extra phosphorylated bands were revealed by PAGE analysis. After suppression of growth activity the labelling of the two PKc-specific substrates was strongly inhibited.

Altered protein kinase activities of lymphoid cells transformed by Abelson and Moloney leukemia viruses

Five different types of protein kinase activities have been evaluated in cell lines from murine lymphomas induced by Abelson leukemia virus (A-MuLV), whose oncogene codes for a tyrosine protein kinase. Such activities were compared with those of normal cells and of cells transformed by Moloney leukemia virus (M-MuLV), lacking oncogene sequences in its genome. While cAMP-dependent protein kinase and casein kinase-1 do not undergo significant changes, casein kinase-2 rises in both A-MuLV and M-MuLV infected lymphocytes, becoming largely associated with the particulate fraction of transformed cells. Protein kinase-C on the other hand is unchanged in M-MuLV transformed cells but it undergoes a 2-3-fold increment in both soluble and particulate fractions of A-MuLV transformed lymphocytes, which also display high tyrosine protein kinase activity.

Studies and perspectives of protein kinase C

Protein kinase C, an enzyme that is activated by the receptor-mediated hydrolysis of inositol phospholipids, relays information in the form of a variety of extracellular signals across the membrane to regulate many Ca2+-dependent processes. At an early phase of cellular responses, the enzyme appears to have a dual effect, providing positive forward as well as negative feedback controls over various steps of its own and other signaling pathways, such as the receptors that are coupled to inositol phospholipid hydrolysis and those of some growth factors. In biological systems, a positive signal is frequently followed by immediate negative feedback regulation. Such a novel role of this protein kinase system seems to give a logical basis for clarifying the biochemical mechanism of signal transduction, and to add a new dimension essential to our understanding of cell-to-cell communication.

Regulation of protein phosphorylation in pancreatic acini. Distinct effects of Ca2+ ionophore A23187 and 12-O-tetradecanoylphorbol 13-acetate

Regulation of protein phosphorylation in isolated pancreatic acini by the intracellular messengers Ca2+ and diacylglycerol was studied by using the Ca2+ ionophore A23187 and the tumour-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate. As assessed by two-dimensional polyacrylamide-gel electrophoresis, the phorbol ester (1 microM) and Ca2+ ionophore (2 microM) altered the phosphorylation of distinct sets of proteins between Mr 83,000 and 23,000 in mouse and guinea-pig acini. The phorbol ester increased the phosphorylation of four proteins, whereas the ionophore increased the phosphorylation of two proteins and, in mouse acini, decreased the phosphorylation of one other protein. In addition, the phorbol ester and ionophore each caused the dephosphorylation of two proteins, of Mr 20,000 and 20,500. Administered together, these agents reproduced the changes in phosphorylation induced by the cholinergic agonist carbamoylcholine. The effects of the phorbol ester and ionophore on acinar amylase release were also studied. In mouse pancreatic acini, a maximally effective concentration of phorbol ester (1 microM) produced a secretory response that was only 28% of that produced by a maximally effective concentration of carbamoylcholine, whereas the ionophore (0.3 microM) stimulated amylase release to two-thirds of the maximal response to carbamoylcholine. In contrast, in guinea-pig acini, the phorbol ester and carbamoylcholine evoked similar maximal secretory responses, whereas the maximal secretory response to the ionophore was only 35% of that to carbamoylcholine. Combination of phorbol ester and ionophore resulted in a modest synergistic effect on amylase release in both species. It is concluded that cholinergic agonists act via both diacylglycerol and Ca2+ to regulate pancreatic protein phosphorylation, but that synergism between these intracellular messengers is of limited importance in stimulating enzyme secretion.