Activation of protein kinase C is not required for exocytosis from bovine adrenal chromaffin cells. The effects of protein kinase C(19-31), Ca/CaM kinase II(291-317), and staurosporine

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Regulation of catecholamine release in human adrenal chromaffin cells by β-adrenoceptors

The adrenal gland plays a fundamental role in the response to a variety of stress situations. After a stress condition, adrenal medullary chromaffin cells release, by exocytosis, high quantities of catecholamine (epinephrine, EP; norepinephrine, NE), especially EP. Once in the blood stream, catecholamines reach different target organs, and induce their biological actions through the activation of different adrenoceptors. Adrenal gland cells may also be activated by catecholamines, through hormonal, paracrine and/or autocrine system. The presence of functional adrenoceptors on human adrenal medulla and their involvement on catecholamines secretion was not previously evaluated. In the present study we investigated the role of β(1)-, β(2)- and β(3)-adrenoceptors on catecholamine release from human adrenal chromaffin cells in culture. We observed that the β-adrenoceptor agonist (isoproterenol) and β(2)-adrenoceptor agonist (salbutamol) stimulated catecholamine (NE and EP) release from human adrenal chromaffin cells. Furthermore, the β(2)-adrenoceptor antagonist (ICI 118,551; 100 nM) and β(3)-adrenoceptor antagonist (SR 59230A; 100 nM) inhibited the catecholamine release stimulated by isoproterenol and nicotine in chromaffin cells. The β(1)-adrenoceptor antagonist (atenolol; 100 nM) did not change the isoproterenol- neither the nicotine-evoked catecholamine release from human adrenal chromaffin cells. Moreover, our results show that the protein kinase A (PKA), protein kinase C (PKC), mitogen-activated protein kinase (MAPK) and phospholipase C (PLC) are intracellular mechanisms involved in the catecholamine release evoked by salbutamol. In conclusion, our data suggest that the activation of β(2)- and β(3)-adrenoceptors modulate the basal and evoked catecholamine release, NE and EP, via an autocrine positive feedback loop in human adrenal chromaffin cells.

Regulation of catecholamine release and tyrosine hydroxylase in human adrenal chromaffin cells by interleukin-1beta: role of neuropeptide Y and nitric oxide

Adrenal chromaffin cells synthesize and secrete catecholamines and neuropeptides that may regulate hormonal and paracrine signaling in stress and also during inflammation. The aim of our work was to study the role of the cytokine interleukin-1beta (IL-1beta) on catecholamine release and synthesis from primary cell cultures of human adrenal chromaffin cells. The effect of IL-1beta on neuropeptide Y (NPY) release and the intracellular pathways involved in catecholamine release evoked by IL-1beta and NPY were also investigated. We observed that IL-1beta increases the release of NPY, norepinephrine (NE), and epinephrine (EP) from human chromaffin cells. Moreover, the immunoneutralization of released NPY inhibits catecholamine release evoked by IL-1beta. Moreover, IL-1beta regulates catecholamine synthesis as the inhibition of tyrosine hydroxylase decreases IL-1beta-evoked catecholamine release and the cytokine induces tyrosine hydroxylase Ser40 phosphorylation. Moreover, IL-1beta induces catecholamine release by a mitogen-activated protein kinase (MAPK)-dependent mechanism, and by nitric oxide synthase activation. Furthermore, MAPK, protein kinase C (PKC), protein kinase A (PKA), and nitric oxide (NO) production are involved in catecholamine release evoked by NPY. Using human chromaffin cells, our data suggest that IL-1beta, NPY, and nitric oxide (NO) may contribute to a regulatory loop between the immune and the adrenal systems, and this is relevant in pathological conditions such as infection, trauma, stress, or in hypertension.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Regulation of exocytosis by protein kinase C

PKC (protein kinase C) has been known for many years to modulate regulated exocytosis in a wide variety of cell types. In neurons and neuroendocrine cells, PKC regulates several different stages of the exocytotic process, suggesting that these multiple actions of PKC are mediated by phosphorylation of distinct protein targets. In recent years, a variety of exocytotic proteins have been identified as PKC substrates, the best characterized of which are SNAP-25 (25 kDa synaptosome-associated protein) and Munc18. In the present study, we review recent evidence suggesting that site-specific phosphorylation of SNAP-25 and Munc18 by PKC regulates distinct stages of exocytosis.

Staurosporine restores GTPgammaS induced block of rapid endocytosis in chromaffin cells

Fast capacitance measurements demonstrated that chromaffin cells retrieve membrane by several kinetically different pathways. Here, we show that rapid endocytosis is blocked and slow endocytosis reduced by intracellular application of GTPgammaS, an activator of G-proteins, but not by the competitive blocker GDPbetaS. The inhibition of rapid endocytosis by GTPgammaS can be restored with GDPbetaS or staurosporine completely. But only staurosporine partially abolishes the reduction of slow endocytosis by GTPgammaS. Besides triggering exocytosis, GTPgammaS elicits large exo- and endocytotic vesicles that contributed significantly to the total membrane traffic, indicating a third pathway of membrane shuttle.

Phosphorylation of Munc18 by protein kinase C regulates the kinetics of exocytosis

Protein phosphorylation by protein kinase C (PKC) has been implicated in the control of neurotransmitter release and various forms of synaptic plasticity. The PKC substrates responsible for phosphorylation-dependent changes in regulated exocytosis in vivo have not been identified. Munc18a is essential for neurotransmitter release by exocytosis and can be phosphorylated by PKC in vitro on Ser-306 and Ser-313. We demonstrate that it is phosphorylated on Ser-313 in response to phorbol ester treatment in adrenal chromaffin cells. Mutation of both phosphorylation sites to glutamate reduces its affinity for syntaxin and so acts as a phosphomimetic mutation. Unlike phorbol ester treatment, expression of Munc18 with this phosphomimetic mutation in PKC phosphorylation sites did not affect the number of exocytotic events. The mutant did, however, produce changes in single vesicle release kinetics, assayed by amperometry, which were identical to those caused by phorbol ester treatment. Furthermore, the effects of phorbol ester treatment on release kinetics were occluded in cells expressing phosphomimetic Munc18. These results suggest that the dynamics of vesicle release events during exocytosis are controlled by PKC directly through phosphorylation of Munc18 on Ser-313. Phosphorylation of Munc18 by PKC may provide a mechanism for the control of exocytosis and thereby synaptic plasticity.

Chromaffin cell catecholamine secretion: bisindolylmaleimide compounds exhibit novel and potent antagonist effects at the nicotinic cholinergic receptor in pheochromocytoma cells

Activation of protein kinase C (PKC) stimulates nicotine-induced catecholamine secretion. PKC down-regulation by prolonged pretreatment with phorbol 12-myristate 13-acetate diminished nicotine-induced catecholamine secretion only slightly (approximately 16%), suggesting substantial PKC independence of nicotinic receptor activation. However, we found that bisindolylmaleimide compounds (which are also putative PKC chemical inhibitors) dramatically inhibited nicotine-induced catecholamine secretion (IC(50) values of approximately 24-37 nM). This inhibition was specific for the nicotinic cholinergic receptor. Catecholamine secretion induced by other nicotinic agonists (such as epibatidine, anatoxin, or cytisine) was also powerfully antagonized by bisindolylmaleimide II (IC(50) values of approximately 60-90 nM). Even high-dose nicotinic agonists failed to overcome the inhibition by bisindolylmaleimide II, suggesting noncompetitive nicotinic antagonism by this class of compounds. Nicotinic inhibition by bisindolylmaleimide seemed not to be readily reversible. Structure-activity studies of bisindolylmaleimide compounds revealed that bisindolylmaleimides I through III are the most potent nicotinic antagonists at the nicotinic cholinergic receptor in PC-12 cells (IC(50) < or =37 nM), whereas bisindolylmaleimide IV and V have far less nicotinic antagonist activity (IC(50) >1 microM); the active compounds I through III have cationic tails at an indole nitrogen, whereas the least potent compounds IV and V do not. By contrast, a free NH within the maleimide ring is crucial for PKC inhibition by this class of compounds. We conclude that bisindolylmaleimides I through III are some of the most potent noncompetitive neuronal nicotinic antagonists, indeed the most potent such antagonists we have observed in PC-12 cells. Nicotinic antagonism of these compounds seems to be independent of PKC inhibition.

Activation of annexin 7 by protein kinase C in vitro and in vivo

Annexin 7, a Ca(2+)/GTP-activated membrane fusion protein, is preferentially phosphorylated in intact chromaffin cells, and the levels of annexin 7 phosphorylation increase quantitatively in proportion to the extent of catecholamine secretion. Consistently, various protein kinase C inhibitors proportionately reduce both secretion and phosphorylation of annexin 7 in these cells. In vitro, annexin 7 is quantitatively phosphorylated by protein kinase C to a mole ratio of 2.0, and phosphorylation is extraordinarily sensitive to variables such as pH, calcium, phospholipid, phorbol ester, and annexin 7 concentration. Phosphorylation of annexin 7 by protein kinase C significantly potentiates the ability of the protein to fuse phospholipid vesicles and lowers the half-maximal concentration of calcium needed for this fusion process. Furthermore, other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and protein-tyrosine kinase pp60(c-)(src), also label annexin 7 with high efficiency but do not have this effect on membrane fusion. In the case of pp60(c-)(src), we note that this kinase, if anything, modestly suppresses the membrane fusion activity of annexin 7. These results thus lead us to hypothesize that annexin 7 may be a positive mediator for protein kinase C action in the exocytotic membrane fusion reaction in chromaffin cells.

Protein kinase C-dependent supply of secretory granules to the plasma membrane

To elucidate the mechanism for supplying secretory granules to the cell membrane, chromaffin cells isolated from the bovine adrenal medulla were observed by the evanescent wave microscopy after staining their granules with acridine orange. The secretory granules showed only a very small fluctuation, indicating their docking to the plasma membrane. The rate and range of movement increased greatly by application of botulinum toxin A or C. The number of secretory granules docked to the plasma membrane significantly decreased by botulinum toxin C. Conversely, the number increased greatly by activation of protein kinase C with phorbol 12,13-dibutyrate (PDBu). In the presence of an anti-actin reagent cytochalasin D, no increasing effect of PDBu on the number of docked granules was observed. While in the presence of an anti-mitotic reagent, colchicine, a clear increasing effect of PDBu was observed. The final step for supplying granules to the plasma membrane in endocrine cells is concluded to be mediated by a phosphorylation-dependent and actin-based transport system.

Latrotoxin stimulates secretion in permeabilized cells by regulating an intracellular Ca2+ - and ATP-dependent event: a role for protein kinase C

alpha-Latrotoxin, a component of black widow spider venom, stimulates transmitter release from nerve terminals and intact chromaffin cells and enhances secretion from permeabilized chromaffin cells already maximally stimulated by Ca(2+). In this study we demonstrate that chromaffin cells contain a protein antigenically similar to the cloned Ca(2+)-independent receptor for alpha-latrotoxin. Although this receptor has homology to the secretin family of G-protein-linked receptors, pertussis toxin has no effect on the ability of alpha-latrotoxin to enhance secretion, suggesting that neither G(i) nor G(o) is involved in the response. Furthermore, in the absence of Ca(2+), alpha-latrotoxin does not stimulate polyphosphoinositide-specific phospholipase C. alpha-Latrotoxin specifically enhances ATP-dependent secretion in permeabilized cells. An in situ assay for protein kinase C reveals that alpha-latrotoxin augments the activation of protein kinase C by Ca(2+), and use of protein kinase inhibitors demonstrates that this activation is important for the toxin's enhancing effect. This enhancement of secretion requires Ca(2+) concentrations above 3 microm and is not supported by Ba(2+) or nonhydrolyzable guanine nucleotides, which do not stimulate protein kinase C. We conclude that alpha-latrotoxin stimulates secretion in permeabilized cells by regulating a Ca(2+)- and ATP-dependent event involving protein kinase C.

Carbachol-induced [Ca2+]i increase, but not activation of protein kinase C, stimulates exocytosis in rat parotid acini

1. A column perfusion system was applied to rat parotid acinar cells to clarify the roles of Ca2+ and protein kinase C (PKC) in the mechanisms of carbachol (CCh)-induced amylase secretion. 2. CCh evoked a biphasic response of amylase secretion with an initial rapid and large peak that reached maximum at about 10 s followed by a sustained plateau. The time profile and the dose-response relationship paralleled with those of cytosolic free Ca2+ concentration ([Ca2+]i). 3. The CCh-induced sustained response of amylase secretion maintained by Ca2+ influx into cells was ATP dependent, while the initial peak response regulated by Ca2+ released from InsP3-sensitive stores was relatively ATP independent. 4. Restoration of extracellular Ca2+ during continuous stimulation with CCh in Ca(2+)-free medium evoked a second rapid and large peak of amylase secretion. 5. Phorbol 12,13-dibutyrate (PDBu) potentiated the CCh-induced amylase secretion in both the initial peak and the sustained plateau without enhancing CCh-induced [Ca2+]i changes. 6. PKC inhibitors such as Ro 31-8220 inhibited the potentiating effect of PDBu but only slightly reduced amylase secretion induced by CCh alone. 7. These results suggest that a CCh-induced rise in [Ca2+]i triggers the final fusion and/or exocytosis of amylase secretion. CCh also has some ability to promote ATP-dependent priming of secretory granules that, together with Ca2+ influxed into cells, contributes to the CCh-induced sustained plateau of amylase secretion. PDBu-induced activation of PKC promotes the priming of secretory granules, thereby enhancing the efficacy for Ca2+ to trigger fusion/exocytosis.

Pseudosubstrate peptide inhibitors of beta-cell protein kinases: altered selectivity after myristoylation

Inhibitors of protein kinases are widely used to study stimulus-response pathways in pancreatic beta-cells. Synthetic peptides modelled on the pseudosubstrate sites of protein kinases, or of their endogenous inhibitor proteins, offer potentially specific inhibitors of individual protein kinases or kinase isoforms. However, the use of these inhibitors in studies of beta-cell physiology has been limited, since such peptide sequences are usually poorly membrane permeant. Myristoylation of these peptides enhances their ability to cross intact plasma membranes and thus inhibit intracellular protein kinases, and this approach is becoming increasingly common in identifying the cellular role(s) of particular protein kinases. In this study, using insulin-secreting beta-cells, we demonstrate that myristoylation alters the specificity of pseudosubstrate peptides such that all myristoylated peptides tested, even those lacking pseudosubstrate domains, acted as protein kinase C (PKC) inhibitors. This effect of myristoylation was limited to the inhibition of PKC, since the specificity of peptide inhibitors towards beta-cell protein kinase A activity was not affected by myristoylation. These results demonstrate that myristoylated pseudosubstrate peptides have value as protein kinase inhibitors in intact beta-cells, but emphasise that studies using them to ascribe role(s) for protein kinases in beta-cells must be interpreted with caution.

1,25-dihydroxyvitamin D3 but not TPA activates PLD in Caco-2 cells via pp60(c-src) and RhoA

In the accompanying paper [Khare et al., Am. J. Physiol. 276 (Gastrointest. Liver Physiol. 39): G993-G1004, 1999], activation of protein kinase C-alpha (PKC-alpha) was shown to be involved in the stimulation of phospholipase D (PLD) by 1,25-dihydroxyvitamin D3 [1, 25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) in Caco-2 cells. Monomeric or heterotrimeric G proteins, as well as pp60(c-src) have been implicated in PLD activation. We therefore determined whether these signal transduction elements were involved in PLD stimulation by 1,25(OH)2D3 or TPA. Treatment with C3 transferase, which inhibits members of the Rho family of monomeric G proteins, markedly diminished the ability of 1,25(OH)2D3, but not TPA, to stimulate PLD. Brefeldin A, an inhibitor of ADP-ribosylation factor proteins, did not, however, significantly reduce the stimulation of PLD by either of these agents. Moreover, 1,25(OH)2D3, but not TPA, activated pp60(c-src) and treatment with PP1, a specific inhibitor of the pp60(c-src) family, blocked the ability of 1,25(OH)2D3 to activate PLD. Pretreatment of cells with pertussis toxin (PTx) markedly reduced the stimulation of PLD by either agonist. PTx, moreover, inhibited the stimulation of pp60(c-src) and PKC-alpha by 1,25(OH)2D3. PTx did not, however, block the membrane translocation of RhoA induced by 1,25(OH)2D3 or inhibit the stimulation of PKC-alpha by TPA. These findings, taken together with those of the accompanying paper, indicate that although 1,25(OH)2D3 and TPA each activate PLD in Caco-2 cells in part via PKC-alpha, their stimulation of PLD differs in a number of important aspects, including the requirement for pp60(c-src) and RhoA in the activation of PLD by 1,25(OH)2D3, but not TPA. Moreover, the requirement for different signal transduction elements by 1,25(OH)2D3 and TPA to induce the stimulation of PLD may potentially underlie differences in the physiological effects of these agents in Caco-2 cells.

Regulation of Ca2+ influx by a protein kinase C activator in chromaffin cells: differential role of P/Q- and L-type Ca2+ channels

Phorbol esters reduce depolarization-evoked Ca2+ influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca2+ channels (VSCCs) are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that L- and P/Q-type Ca2+ channel subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term (10 min) incubations with phorbol 12-myristate 13-acetate (PMA) inhibited early high K+-evoked rises in cytosolic free Ca2+ concentration ([Ca2+]i) and the early component of the depolarization-evoked Mn2+ quenching of fura-2 fluorescence in a dose-dependent manner (IC50: 18 and 7 nM; maximal inhibitions: 45 and 48%, respectively). The protein kinase C inhibitor staurosporine (100 nM) reverted the inhibitory action of PMA. PMA (0.1-1 microM) inhibited the early and late phases of the ionomycin (2 microM)-evoked [Ca2+]i transients by 14-23%. Omega-agatoxin IVA, a blocker of P/Q-type Ca2+ channels, inhibited high K+-evoked [Ca2+]i rises in a dose-dependent fashion (IC50 = 50 nM). In contrast, 0.1 microM omega-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable (< 45%) component of early Ca2+ influx persisted in the combined presence of omega-agatoxin IVA (100 nM) and nitrendipine (1 microM). Simultaneous exposure to omega-agatoxin IVA and PMA inhibited both the early [Ca2+]i transients and Mn2+ quenching to a much greater extent than each drug separately. Inhibition of the [Ca2+]i transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over P/Q type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-P/Q type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells.

Effects of R(-)-1-(benzo[b]thiophen-5-yl)-2-[2-N,N-diethylamino)ethoxy]ethan ol hydrochloride (T-588), a novel cognitive enhancer, on noradrenaline release in rat cerebral cortical slices

We investigated the effects of R(-)-1-(benzo[b]thiophen-5-yl)-2-[2-(N,N-diethylamino)ethoxy]ethan ol hydrochloride (T-588), a novel cognitive enhancer, on noradrenaline (NA) release from rat cerebral cortical slices in vitro. Addition of T-588 in an assay mixture stimulated [3H]NA release from prelabeled slices in the presence or absence of extracellular CaCl2, and in the presence of the Ca2+/calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide and trifluoperazine. T-588 stimulated NA release with a time lag of about 1 min, and the high level of release was maintained for at least 10 min, whereas maximal KCl-evoked NA release was observed within 1 min after the addition of KCl, and the effect declined subsequently. The effect of T-588 was reversible (pretreatment with T-588 showed no effect on NA release after two washes by centrifugation). We also compared the effects of T-588 and N-ethylmaleimide (NEM), a sulfhydryl alkylating agent known to stimulate neurotransmitter release in several types of cells. The addition of NEM stimulated NA release irreversibly from the slices in a Ca2+-independent manner, and the effect of NEM, but not that of T-588, was inhibited by the simultaneous addition of dithiothreitol, a sulfhydryl group reducing agent. The addition of T-588, which stimulated NA release by itself, inhibited the NA release by 0.6 mM NEM, although the effect of T-588 was additive in the presence of 0.2 mM NEM. These findings suggest that T-588 stimulates NA release from rat cerebral cortical slices in a Ca2+- and calmodulin-independent manner, possibly via an NEM-sensitive factor(s), although the mechanism of the effects of T-588 seems to be different from that of NEM.

Pseudosubstrate inhibition of cyclic AMP-dependent protein kinase in intact pancreatic islets: effects on cyclic AMP-dependent and glucose-dependent insulin secretion

Synthetic peptides derived from the endogenous protein kinase A inhibitor (PKI) offer a specific means of inhibiting cyclic AMP-dependent protein kinase A (PKA), but their use in whole cells is restricted by the plasma membrane. We have now modified PKI sequences by N-terminal myristoylation to enhance their membrane permeability, and have used the myristoylated (myr) peptides to investigate the role of PKA activation in glucose-induced insulin secretion from intact pancreatic beta-cells. The myristoylated PKI peptides, myr PKI14-22 and myrPKI6-22, were effective inhibitors in vitro of PKA activity extracted from rat islets of Langerhans. In experiments using intact islets, myr PKI14-22 caused a concentration-dependent inhibition of insulin secretion in response to the PKA activators dibutyryl cyclic AMP and forskolin, suggesting that it gained access to the cytosolic compartment of intact beta-cells and inhibited PKA in situ. However, these concentrations of myr PKI14-22 did not inhibit insulin secretion in response to glucose suggesting that the activation of PKA is not required for the initiation of glucose-induced insulin secretion.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

A myristoylated pseudosubstrate peptide inhibitor of protein kinase C: effects on glucose- and carbachol-induced insulin secretion

We have used synthetic pseudosubstrate peptide inhibitors of protein kinase C (PKC) to re-examine the role of conventional isoforms of PKC in the insulin secretory response of intact rat islets of Langerhans to glucose and to the cholinergic agonist carbachol (CCh). One peptide was modified by N-terminal myristoylation (PKC-myr20-28) to allow its use in intact beta-cells. Maximal inhibition of PKC activity in vitro required 10-fold less of this peptide (PKC-myr20-28) than of its non-myristoylated analogue. The maximum inhibitory concentration of PKC-myr20-28 had little effect on islet protein kinase A or Ca2+/calmodulin kinase activities. PKC-myr20-28 (25-100 microM) caused a dose-dependent inhibition of phorbol myristate acetate (PMA)-induced insulin secretion from intact rat islets but non-myristoylated peptides had little effect on the secretory response to PMA. A concentration of PKC-myr20-28 (100 microM) which maximally inhibited PMA-induced insulin secretion, also inhibited the secretory response to CCh, but did not affect glucose-stimulated insulin secretion from intact islets. These results indicate that myristoylation of pseudosubstrate peptides increases their potency as inhibitors and that PKC-myr20-28 is a selective and cell-permeant inhibitor of PMA-sensitive isoforms of PKC. They also suggest that the activation of PMA-sensitive PKC isoforms mediates the stimulatory effects of CCh, but is not obligatory for glucose-induced insulin secretion from pancreatic beta-cells.

Protein kinase C enhances exocytosis from chromaffin cells by increasing the size of the readily releasable pool of secretory granules

We have used membrane capacitance measurements to assay Ca2+-triggered exocytosis in single bovine adrenal chromatin cells. Brief application of phorbol ester (PMA) enhances depolarization-evoked exocytosis severalfold while actually decreasing the Ca2+ current. Ca2+ metabolism is unchanged. Three different protocols were used to show that PMA increases the size of the readily releasable pool of secretory granules. PMA treatment leads to a large increase in amplitude, but little change in the time course of the exocytic burst that results from rapid elevation of [Ca2+]i upon photolysis of DMI-Nitrophen. Thus, PKC appears to affect a late step in secretion but not the Ca2+ sensitivity of the final step.

Annexin II tetramer: structure and function

The annexins are a family of proteins that bind acidic phospholipids in the presence of Ca2+. The interaction of these proteins with biological membranes has led to the suggestion that these proteins may play a role in membrane trafficking events such as exocytosis, endocytosis and cell-cell adhesion. One member of the annexin family, annexin II, has been shown to exist as a monomer, heterodimer or heterotetramer. The ability of annexin II tetramer to bridge secretory granules to plasma membrane has suggested that this protein may play a role in Ca(2+)-dependent exocytosis. Annexin II tetramer has also been demonstrated on the extracellular face of some metastatic cells where it mediates the binding of certain metastatic cells to normal cells. Annexin II tetramer is a major cellular substrate of protein kinase C and pp60src. Phosphorylation of annexin II tetramer is a negative modulator of protein function.

Inhibition of regulated catecholamine secretion from PC12 cells by the Ca2+/calmodulin kinase II inhibitor KN-62

When stimulated by the cholinergic agonist carbachol, PC12 cells rapidly secrete a large fraction of the intracellular catecholamines by exocytotic release from the large dense-core secretory vesicles in a Ca(2+)-dependent manner. To investigate whether Ca2+/calmodulin kinase II plays a role in the regulated secretion of catecholamines, we examined the effect of the specific Ca2+/calmodulin kinase II inhibitor KN-62 on the carbachol-induced release of norepinephrine from PC12 cells. Approximately 50% of the regulated release of norepinephrine, stimulated either by carbachol or direct depolarization, was inhibited by pretreatment with KN-62, while the remaining 50% was resistant to KN-62 and therefore independent of Ca2+/calmodulin kinase II. In contrast, H7, an inhibitor of protein kinase C, had no effect on any of the stimulated release. FURA 2 imaging experiments demonstrated that KN-62 does not act by blocking the stimulation-induced increase in intracellular [Ca2+]. The most likely model consistent with these data is that all the dense-core vesicles fuse with the plasma membrane in a Ca(2+)-dependent process, but that approximately 50% of the vesicles require an additional step that is dependent on the action of Ca2+/calmodulin kinase II. This step occurs between the influx of Ca2+ and the fusion of vesicle membranes with the plasma membrane, and may be analogous to the Ca2+/calmodulin kinase II phosphorylation of synapsin which mobilizes small, clear synaptic vesicles for exocytosis at the synapse.

Protein phosphorylation in the regulation of insulin secretion: the use of site-directed inhibitory peptides in electrically permeabilised islets of Langerhans

We have used electrically permeabilised rat islets of Langerhans to investigate the role of protein phosphorylation in the regulation of insulin secretion using pseudosubstrate inhibitory peptides for cyclic AMP-dependent protein kinase (PKA) and for protein kinase C (PKC). The protein kinase inhibitor (PKI) peptide, PKI(6-22), completely inhibited the effects of cyclic AMP on islet PKA activity in vitro, on endogenous protein phosphorylation and on insulin secretion. This peptide had no significant effect on islet PKC activity in vitro, on Ca(2+)-induced protein phosphorylation and on secretory responses to Ca2+ or to the PKC activator, 4 beta-phorbol myristate acetate (PMA). The PKC pseudosubstrate inhibitory peptide, PKC(19-36), caused a marked inhibition of islet PKC activity in vitro and inhibite PMA-induced insulin secretion without affecting secretory responses to cyclic AMP and Ca2+. These results demonstrate that PKA- and PKC-induced protein phosphorylation is obligatory for cyclic AMP- and PMA-stimulated insulin secretion, respectively, and suggest that there is little "crosstalk" between the response elements of the secretory pathways to the different second messengers, at least after the generation of the messengers within the beta-cells.

Effect of cAMP-dependent protein kinase on catecholamine secretion from bovine adrenal chromaffin cells

We examined the role of cAMP-dependent protein kinase in Ca(2+)-elicited catecholamine secretion from bovine adrenal chromaffin cells. When the digitonin-treated cells were incubated with the catalytic subunit of cAMP-dependent protein kinase, the secretion of catecholamines from the cells occurred in the absence of Ca2+. The effect of the catalytic subunit was dependent on its activity (50-100 units/ml) and the presence of ATP-Mg2+ in the incubation medium. However, incubation of the cells with the regulatory subunit of cAMP-dependent protein kinase did not affect the secretion. Ca2+ (43 nM-10 microM) also increased the secretion, which was ATP-Mg(2+)-dependent. The catalytic subunit (25-200 units/ml) enhanced the Ca(2+)-evoked secretion at the suboptimal but not optimal Ca2+ concentration, which induced maximal secretion. A potent synthetic peptide inhibitor of cAMP-dependent protein kinase abolished the catalytic subunit-induced secretion, but not the Ca(2+)-evoked secretion. On the other hand, K-252a, a potent inhibitor of protein kinases, inhibited both the catalytic subunit-induced and the Ca(2+)-evoked secretion, but not KT5823, a much less potent inhibitor of protein kinases. These results strongly suggest that the catalytic subunit of cAMP-dependent protein kinase produces the secretion of catecholamines via protein phosphorylation. The results further suggest that the cAMP-dependent protein kinase does not participate in an intrinsic process of Ca(2+)-elicited secretion but it may act as a modulator.

Botulinum A and the light chain of tetanus toxins inhibit distinct stages of Mg.ATP-dependent catecholamine exocytosis from permeabilised chromaffin cells

Susceptibilities of Mg.ATP-independent and Mg.ATP-requiring components of catecholamine secretion from digitonin-permeabilised chromaffin cells to inhibition by Clostridial botulinum type A and tetanus toxins were investigated. These toxins are Zn(2+)-dependent proteases which specifically cleave the 25-kDa synaptosomal-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) II, respectively. When applied to permeabilised chromaffin cells they rapidly inhibited secretion in the presence of Mg.ATP but the catecholamine released in the absence of Mg.ATP, thought to represent fusion of primed granules, was not perturbed. The toxins can exert their effects per se in the absence of the nucleotide complex; therefore, Mg.ATP-requiring steps of secretion are implicated as roles for their targets. Primed release was lost rapidly after permeabilisation of the cells but could be maintained by including Mg.ATP during the incubation before stimulating release with Ca2+. This ability of Mg.ATP to maintain primed release was only partially inhibited by botulinum neurotoxin A whereas it was abolished by tetanus toxin, consistent with the distinct substrates for these toxins. This study reveals a component of release within which these proteins are either resistant to cleavage by these toxins or in such a position that degradation can no longer prevent granule fusion. Differences in the steps of release at which these toxins can affect inhibition are also revealed.

Tetraethylammonium selectively stimulates secretion from noradrenergic bovine chromaffin cells

1. The effects of tetraethylammonium chloride (TEA) on catecholamine secretion from primary cultures of noradrenaline-rich (noradrenergic) and adrenaline-rich (adrenergic) bovine chromaffin cells were studied. TEA stimulated catecholamine secretion from both cell types but was a much more effective secretory stimulus for noradrenergic cells. 2. TEA-induced catecholamine secretion was dependent on extracellular Ca2+, was partially inhibited by nifedipine and by tetrodotoxin, and was potentiated by ouabain. Other K+ channel blocking agents including 4-aminopyridine, glibenclamide, and tolbutamide did not stimulate catecholamine secretion. 3. TEA had no effect on Ca(2+)-induced secretion from digitonin-permeabilized chromaffin cells. 4. TEA presumably evokes secretion by inhibiting K+ channels, depolarizing chromaffin cells, and activating voltage-gated Ca2+ channels in the cells. Noradrenergic cells appear to be more sensitive to K+ channel inhibition than are adrenergic cells. 5. The secretory response of the chromaffin cells to TEA increased with time in culture. 6. In addition to being a more effective secretagogue in noradrenergic cells, TEA was also more effective in stimulating catecholamine synthesis in these cells.

Evidence that the ability to respond to a calcium stimulus in exocytosis is determined by the secretory granule membrane: comparison of exocytosis of injected bovine chromaffin granule membranes and endogenous cortical granules in Xenopus laevis oocytes

1. To understand better the mechanisms which govern the sensitivity of secretory vesicles to a calcium stimulus, we compared the abilities of injected chromaffin granule membranes and of endogenous cortical granules to undergo exocytosis in Xenopus laevis oocytes and eggs in response to cytosolic Ca2+. Exocytosis of chromaffin granule membranes was detected by the appearance of dopamine-beta-hydroxylase of the chromaffin granule membrane in the oocyte or egg plasma membrane. Cortical granule exocytosis was detected by release of cortical granule lectin, a soluble constituent of cortical granules, from individual cells. 2. Injected chromaffin granule membranes undergo exocytosis equally well in frog oocytes and eggs in response to a rise in cytosolic Ca2+ induced by incubation with ionomycin. 3. Elevated Ca2+ triggered cortical granule exocytosis in eggs but not in oocytes. 4. Injected chromaffin granule membranes do not contribute factors to the oocyte that allow calcium-dependent exocytosis of the endogenous cortical granules. 5. Protein kinase C activation by phorbol esters stimulates cortical granule exocytosis in both Xenopus laevis oocytes and X. laevis eggs (Bement, W. M., and Capco, D. G., J. Cell Biol. 108, 885-892, 1989). Activation of protein kinase C by phorbol ester also stimulated chromaffin granule membrane exocytosis in oocytes, indicating that although cortical granules and chromaffin granule membranes differ in calcium responsiveness, PKC activation is an effective secretory stimulus for both. 6. These results suggest that structural or biochemical characteristics of the chromaffin granule membrane result in its ability to respond to a Ca2+ stimulus. In the oocytes, cortical granule components necessary for Ca(2+)-dependent exocytosis may be missing, nonfunctional, or unable to couple to the Ca2+ stimulus and downstream events.

Insulin-like growth factor-I-enhanced secretion is abolished in protein kinase C-deficient chromaffin cells

Previous studies have demonstrated that bovine chromaffin cells cultured in medium with 10 nM insulin-like growth factor-I (IGF-I) secrete about twofold more catecholamine when exposed to secretory stimuli than do cells cultured without IGF-I. The purpose of this study was to determine whether protein kinase C (PKC) is involved in the effect of IGF-I on secretion from these cells. PKC was down-regulated in the cells by 16-18 h of treatment with beta-phorbol didecanoate (beta-PDD; 100 nM). Such treatment had no effect on high-K(+)-stimulated secretion from cells cultured without IGF-I; however, secretion from cells cultured with IGF-I was reduced to a level comparable to that in cells cultured without the peptide. The inactive isomer, alpha-PDD (100 nM), had no effect on secretion from untreated or IGF-I-treated chromaffin cells. The effect of beta-PDD was time and concentration dependent, with 100 nM beta-PDD producing a maximal effect in 8-10 h. In situ PKC activity measured in permeabilized cells treated with PMA (300 nM) was decreased by approximately 40% by 10 h and was reduced to almost basal levels by 18 h. Immunoblotting experiments demonstrated that both alpha- and epsilon-PKC were lost from the cells with time courses similar to that seen in the in situ PKC assay. Overnight treatment with the PKC inhibitor H7 (100 microM) prevented the enhanced secretion normally seen in IGF-I-treated cells, whereas HA1004 had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Staurosporine-induced reduction of secretory function in cultured bovine adrenal chromaffin cells

Staurosporine, a potent inhibitor of protein kinases, has been used to investigate the involvement of protein kinases in cellular processes such as secretory function and differentiation. We have been examining the effects of staurosporine on secretory function under the same conditions it induces dramatic changes in cell morphology in cultured bovine adrenal chromaffin cells. Our results show that treatment with 100 nM staurosporine reduces catecholamine release stimulated by 56 mM KCl, 10 microM nicotine, and 2 mM BaCl2 in a time-dependent manner (t1/2s, 42, 32, and 31 min, respectively). However, we demonstrate that the time-dependent effects on secretory function are not the direct result of staurosporine-induced changes in cell morphology. The effects of staurosporine on secretion stimulated by KCl, nicotine, and BaCl2 are concentration-dependent (IC50s, 6.3, 29.3, and 34.9 nM, respectively). Staurosporine pretreatment does not inhibit activated 45Ca2+ influx, but does reduce catecholamine release stimulated directly by Ca2+ from permeabilized cells. Furthermore, staurosporine also inhibits basal release with time- and concentration-dependencies (IC50, 9.3 nM and t1/2, 21 min) similar to those found for stimulated release. These results suggest that prolonged staurosporine pretreatment may result in the depletion/alteration of a component essential for the more terminal steps of the secretory process.

Intracellular mechanism of Pb(2+)-induced norepinephrine release from bovine chromaffin cells

The intracellular mechanism of Pb(2+)-induced release of norepinephrine (NE) was investigated in comparison with Ca2+ in bovine chromaffin cells permeabilized with staphylococcal alpha-toxin. Pb2+ activated NE release at considerably lower concentrations [concentration of free metal giving half maximal metal-dependent release (K0.5) 4.6 nM] than Ca2+ (K0.5 2.4 microM). The release of NE was associated with the release of dopamine-beta-hydroxylase but not lactate dehydrogenase. The maximal secretory responses produced by Pb2+ and Ca2+ were similar and nonadditive. Pb(2+)- and Ca(2+)-dependent releases showed a similar requirement for MgATP and were equally enhanced by protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA) but not by kinase A activator 8-bromoadenosine 3',5'-cyclic monophosphate free base. The protein kinase C inhibitor staurosporine blocked the TPA-stimulated component of secretion but had no effect on the NE release in the absence of TPA. Calmidazolium, an inhibitor of calmodulin, inhibited the secretion evoked by both metals to similar extent. Agents interacting with microtubules (colchicine and vinblastine) or microfilaments (cytochalasin B and phalloidin) had no effect on secretion induced by either metal cation. These observations indicate that both Pb2+ and Ca2+ act at a common site and activate the exocytotic release of NE by an analogous mechanism.

Protein kinase C and clostridial neurotoxins affect discrete and related steps in the secretory pathway

1. The effects on catecholamine secretion of activation of protein kinase C and clostridial neurotoxins were examined in digitonin-permeabilized bovine adrenal chromaffin cells. 2. The enhancement by phorbol esters increased only the initial rate of secretion; later rates were unaffected. This enhancement was present over a wide range of Ca2+ concentrations and was elicited at 18 as well as at 27 degrees C. 3. Tetanus toxin inhibited both ATP-dependent and ATP-independent secretion, indicating that the tetanus toxin target is important during the final steps in the pathway. 4. Prior activation of protein kinase C by the phorbol ester 12-O-tetradecanoyl phorbol acetate rendered the primed state more sensitive to inhibition by tetanus toxin. The data indicate that a phosphorylated protein kinase C substrate is either identical to or closely associated with the tetanus toxin target protein at the final steps in the pathway. 5. The interaction between the effect of protein kinase activation and that of tetanus toxin suggests that protein kinase C activation does not stimulate a separate pathway of secretion but, rather, modulates the activity of the ongoing pathway. 6. The enhancement of secretion by protein kinase C is caused, at least in part, by a qualitative change in the characteristics of the primed state. This is indicated by the increased sensitivity of primed secretion to inhibition by tetanus toxin and a threefold increase in sensitivity of primed secretion to Ca2+. 7. Because activation of protein kinase C does not increase the later rates of secretion that are limited by ATP-dependent priming reactions, it is unlikely that enhancement of the maximal rate of secretion by TPA is due to an increased amount of the primed state. Instead, protein kinase C activation may increase the efficacy with which Ca2+ stimulates secretion at all Ca2+ concentrations.

Synaptosomal amino acid release: effect of inhibiting protein phosphatases with okadaic acid

The protein phosphatase inhibitor okadaic acid was used to investigate the role of protein phosphatases in regulating the release of amino acids from synaptosomes. Okadaic acid increased the basal release of the amino acids glutamate, aspartate and GABA. The effect was specific in that taurine was not released by either KCl or okadaic acid and there was no synaptosomal lysis or change in ATP/ADP ratios in the presence of okadaic acid. The okadaic acid-stimulated release of amino acids was, however, only a small proportion of that produced by KCl depolarisation. Since okadaic acid raised synaptosomal protein phosphorylation levels to those equivalent to that produced by KCl depolarisation, it is unlikely therefore that there is a direct causal relationship between protein phosphorylation and the release of amino acids. Nevertheless, that release of amino acids from synaptosomes can be elevated under basal conditions by okadaic acid treatment does suggest that okadaic acid-sensitive protein phosphatases have a modulatory role in this process.

Role of omega-conotoxin GVIA-sensitive Ca2+ entry in angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding in bovine adrenal medullary cells

The relative contributions of Ca2+ influx and intracellular Ca2+ mobilization were examined for angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding, which reflects the level of activated protein kinase C in bovine chromaffin cells. Angiotensin II receptors activate phospholipase C in chromaffin cells, leading to a short-lived mobilization of intracellular Ca2+. Angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding was largely blocked in Ca(2+)-free buffer and by pretreatment with the Ca(2+)-channel blocker omega-conotoxin GVIA. The [3H]phorbol 12,13-dibutyrate binding response to [Sar1]angiotensin II also appeared to be voltage sensitive, as no additivity was observed with the response to the depolarizing agent 4-aminopyridine (3 mM). Threshold sensitivities of the extra- and intracellular Ca(2+)-mobilizing pathways to angiotensin II were similar, and all examined effects of angiotensin II in these cells were apparently mediated by losartan-sensitive (AT1-like) receptors. The dependence of angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding on extracellular Ca2+ entry, in contrast to stimulation by other phospholipase C-linked receptor agonists (bradykinin and methacholine), suggests that angiotensin II preferentially stimulates protein kinase C translocation to the plasma membrane, rather than to internal membranes, in bovine adrenal medullary cells.

Difference in the effectiveness of Ca2+ to evoke catecholamine secretion between adrenaline- and noradrenaline-containing cells of bovine adrenal medulla

Differential adrenaline (Ad) and noradrenaline (NA) secretions evoked by secretagogues were investigated using digitonin-permeabilized adrenal chromaffin cells, cultured adrenal chromaffin cells, and perfused adrenal glands of the ox. In digitonin-permeabilized cells, Ca2+ (0.8-160 microM) caused a concentration-dependent increase in catecholamine secretion, which was characterized by a predominance of NA over Ad secretion. Acetylcholine (10-1,000 microM), high K+ (14-56 mM), and bradykinin (0.1-1,000 nM) all were confirmed to induce the release of more NA than Ad at all concentrations used. There was no apparent difference in the ratios of NA/Ad between Ca(2+)-induced catecholamine secretion from digitonin-permeabilized cells and those induced by secretagogues from cultured cells. Qualitatively the same result was obtained in the secretory responses to acetylcholine and high K+ in perfused adrenal glands. These results indicate that the effectiveness of Ca2+ for catecholamine secretion is higher in the secretory apparatus of NA cells than in that of Ad cells of the bovine adrenal medulla. This may be one of the reasons why the secretagogues cause a predominance of NA secretion over Ad secretion in the bovine adrenal medulla.

Transduction of a protein kinase C-generated signal into the long-lasting facilitation of glutamate release

The present study investigated the delayed and persistent effects of 4 beta-phorbol 12,13-dibutyrate (PDBu) on the K(+)-evoked release of endogenous glutamate and dynorphin B-like immunoreactivity from a subcellular fraction (P3) that is enriched in hippocampal mossy fiber synaptosomes. It is demonstrated that the alpha, beta, gamma, epsilon, and zeta isoforms of protein kinase C (PKC) are present in the P3 fraction obtained using the guinea pig hippocampus as starting tissue. The K(+)-evoked release of glutamate was found to be selectively enhanced when mossy fiber-enriched synaptosomes were preincubated with PDBu for 15 minutes and extensively washed with a PDBu-free medium. The persistent enhancement of glutamate release observed under this condition was not reversed by the protein kinase inhibitor staurosporine and was desensitized to the potentiating effects of an acute reexposure to PDBu. The overall content and activity of PKC was not substantially altered during the initial 15 minutes of treatment with PDBu (10 microM). More prolonged pretreatments with PDBu altered the substrate specificity of PKC and decreased the content of all PKC isoforms, but did not reverse the facilitation of glutamate release that followed preincubation in the presence of PDBu. It is concluded that the persistent activation of PKC enhances K(+)-evoked glutamate release from hippocampal mossy fiber-enriched synaptosomes and that, once established, this presynaptic facilitation is sustained by a process that is no longer directly dependent on continued PKC phosphotransferase activity.

Studies on the role of B-50 (GAP-43) in the mechanism of Ca(2+)-induced noradrenaline release: lack of involvement of protein kinase C after the Ca2+ trigger

The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca(2+)-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (> 60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10(-8) to 10(-5) M Ca2+. The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca(2+)-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca(2+)-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca(2+)-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10(-5) M), failed to inhibit Ca(2+)-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca(2+)-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca(2+)- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca(2+)-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA.

Protein kinase C activation by phorbol esters induces chromaffin cell cortical filamentous actin disassembly and increases the initial rate of exocytosis in response to nicotinic receptor stimulation

Nicotinic stimulation and high K+ depolarization of bovine chromaffin cells cause disassembly of cortical filamentous actin networks. Previous work from our laboratory has demonstrated that disassembly of actin filaments is Ca(2+)-dependent, precedes exocytosis and occurs in cortical areas of low cytoplasmic viscosity which are the sites of exocytosis. It has also been suggested that protein kinase C is involved in catecholamine secretion from chromaffin cells. Therefore, the possibility that protein kinase C activation might be implicated in cortical filamentous actin disassembly was investigated. Here we report that phorbol myristate acetate, a protein kinase C activator, causes cortical filamentous actin disassembly. Short-term phorbol ester treatment does not alter the morphology of chromaffin cells; however, 1 h after phorbol ester exposure an increase in cell flattening and membrane ruffling is observed. Phorbol ester-induced cortical filamentous actin disassembly is inhibited by protein kinase C activity inhibitors, is independent of extracellular Ca2+ and has a slower time course than that induced by either nicotinic receptor stimulation or K(+)-depolarization. Phorbol ester effects are likely to be mediated by activation of protein kinase C and not by any changes in intracellular Ca2+ levels, as indicated by measurements of Ca2+ transients. Pretreatment of chromaffin cells with phorbol myristate acetate increases the initial rate of nicotine-evoked catecholamine release. Nicotine-induced cortical actin filament disassembly and catecholamine secretion are partially (29-40%) inhibited by pretreatment of cells with either calphostin C, staurosporine or sphingosine. The results suggest that protein kinase C may be involved in the reorganization of the cortical actin filament network priming the cells for release by removing a barrier to secretory granule mobility. However, its role in exocytosis is modulatory but not essential.

Time-dependent inhibition of stimulated adrenal catecholamine release by staurosporine

Staurosporine, a potent inhibitor of protein kinases, is used to study the involvement of protein kinases in cellular processes. In the present studies, the effect of prolonged staurosporine treatment on catecholamine secretion in cultured bovine adrenal chromaffin cells was examined. Staurosporine inhibits catecholamine release stimulated by 10 microM nicotine, depolarizing concentrations of potassium (56 mM KCl), and 2 mM BaCl2. The effects of staurosporine on KCl-stimulated release are time dependent, with a half-time of approximately 50 min and a maximal inhibition at 2 h. Our results indicate that activation of a staurosporine-sensitive protein kinase is not directly involved in the stimulus-secretion coupling process. This does not rule out the possibility that Ca2+/phospholipid-dependent protein kinase or other protein kinases may acutely modulate release. However, these results suggest that a protein(s), which is phosphorylated by a staurosporine-sensitive protein kinase(s), is required to maintain the integrity of the stimulus-secretion coupling process.

Serotonergic signalling between thyroid cells: protein kinase C and 5-HT2 receptors in the secretion and action of serotonin

Parafollicular (PF) cells of the thyroid gland are neural crest derivatives, which costore the neurotransmitter, 5-hydroxytryptamine (5-HT) with calcitonin. PF cells are located adjacent to follicular (F) cells within the basement membrane of thyroid follicles. It has been proposed that 5-HT serves an intercellular signalling function in the thyroid and that F cells are its target. This proposal was tested by using cell lines derived from PF (medullary thyroid carcinoma [MTC]) and F (FRTL-5) cells to study the mechanisms that mediate the secretion and action of 5-HT. Secretion of 5-HT by MTC cells was evoked by thyroid stimulating hormone, thyrotropin (TSH), elevated extracellular calcium (increases [Ca2+]e), or by agents that increase intracellular cAMP (increases [cAMP]i). When protein kinase C (PKC) was down-regulated by prolonged treatment of MTC cells with phorbol 12-myristate 13-acetate (PMA), or PKC was inhibited by staurosporin, the TSH- or PMA-evoked secretion of 5-HT was blocked; however, interference with PKC function did not affect 5-HT secretion evoked by increases [Ca2+]e or increases [cAMP]i. In the putative targets, FRTL-5 cells, 5-HT increased the turnover of phosphoinositides (PI), cytosolic calcium (increases [Ca2+]i), increases [cAMP]i, and biphasically modified the effect of TSH on cAMP. All of these 5-HT effects were inhibited by 5-HT2 receptor antagonists (spiperone and ketanserin) and by pertussis toxin (PTx), suggesting that the actions of 5-HT are mediated by 5-HT2 receptors, which are coupled to a G protein. This suggestion was supported by the following additional observations: FRTL-5 membranes bound the 5-HT2 agonist, [125I]2,5-dimethoxy-4-iodophenylisopropylamine ([125I]-DOI), and anti-idiotypic antibodies, which recognize 5-HT2 receptors. [125I]-DOI binding was inhibited by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) and the antibodies were displaced by spiperone. Data are consistent with the hypothesis that 5-HT serves as a PF to F cell messenger.

Synthetic peptides of the effector-binding domain of rab enhance secretion from digitonin-permeabilized chromaffin cells

There is evidence that the rab class of low molecular weight GTP-binding proteins is involved in vesicular transfer from endoplasmic reticulum to Golgi and between Golgi cisternae. To determine whether similar proteins play a role in regulated exocytosis, the effects of synthetic peptides derived from low molecular weight GTP-binding proteins on catecholamine secretion from digitonin-permeabilized chromaffin cells were investigated. The synthetic peptides represent the putative effector-binding domains of the rab, ras and ral classes of low molecular weight GTP-binding proteins and correspond to ras(33-48). Two rab peptides but neither a ras nor a ral peptide enhanced Ca(2+)-dependent secretion by approximately 30%. Maximal secretion in response to Ca2+ was increased. The enhancement was not blocked by the pseudosubstrate inhibitor of protein kinase C, PKC(19-31), thus indicating that activation of protein kinase C was not responsible for the enhancement of secretion. Similarly a rab peptide but neither a ras nor a ral peptide enhanced GppNHp-induced secretion 30-70%. The peptides had little or no effect in the absence of Ca2+ or GppNHp. The data are consistent with a protein of the rab class playing a role in regulated exocytosis.

Inhibition of peptide release from invertebrate neurons by the protein kinase inhibitor H-7

The protein kinase inhibitor H-7 has been shown to prevent the potentiation of action potentials that normally accompanies an afterdischarge in the bag cell neurons of Aplysia. We have now shown that H-7 attenuates the release of ELH from these neurons during an afterdischarge without influencing the firing frequency or length of the afterdischarge.