Activation of protein kinase C by a tumor-promoting phorbol ester in pancreatic islets

Single-molecule studies reveal a hidden key step in the activation mechanism of membrane-bound protein kinase C-α

Protein kinase C-α (PKCα) is a member of the conventional family of protein kinase C isoforms (cPKCs) that regulate diverse cellular signaling pathways, share a common activation mechanism, and are linked to multiple pathologies. The cPKC domain structure is modular, consisting of an N-terminal pseudosubstrate peptide, two inhibitory domains (C1A and C1B), a targeting domain (C2), and a kinase domain. Mature, cytoplasmic cPKCs are inactive until they are switched on by a multistep activation reaction that occurs largely on the plasma membrane surface. Often, this activation begins with a cytoplasmic Ca²⁺ signal that triggers C2 domain targeting to the plasma membrane where it binds phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP₂). Subsequently, the appearance of the signaling lipid diacylglycerol (DAG) activates the membrane-bound enzyme by recruiting the inhibitory pseudosubstrate and one or both C1 domains away from the kinase domain. To further investigate this mechanism, this study has utilized single-molecule total internal reflection fluorescence microscopy (TIRFM) to quantitate the binding and lateral diffusion of full-length PKCα and fragments missing specific domain(s) on supported lipid bilayers. Lipid binding events, and events during which additional protein is inserted into the bilayer, were detected by their effects on the equilibrium bound particle density and the two-dimensional diffusion rate. In addition to the previously proposed activation steps, the findings reveal a major, undescribed, kinase-inactive intermediate. On bilayers containing PS or PS and PIP₂, full-length PKCα first docks to the membrane via its C2 domain, and then its C1A domain embeds itself in the bilayer even before DAG appears. The resulting pre-DAG intermediate with membrane-bound C1A and C2 domains is the predominant state of PKCα while it awaits the DAG signal. The newly detected, membrane-embedded C1A domain of this pre-DAG intermediate confers multiple useful features, including enhanced membrane affinity and longer bound state lifetime. The findings also identify the key molecular step in kinase activation: because C1A is already membrane-embedded in the kinase off state, recruitment of C1B to the bilayer by DAG or phorbol ester is the key regulatory event that stabilizes the kinase on state. More broadly, this study illustrates the power of single-molecule methods in elucidating the activation mechanisms and hidden regulatory states of membrane-bound signaling proteins.

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

Identification and analysis of the promoter region of the human PLC-δ4 gene

The delta4 isoform of phospholipase C (PLC-delta4) is thought to be associated with various cellular functions and disease status. However, little is known about how its function is controlled in cells, particularly in terms of the regulation of its expression. To understand the regulation mechanisms of the PLC-delta4 gene transcription, the 5'-flanking region (-2046 approximately +5) (the nucleotide sequence data reported in this paper have been submitted to the EMBL/GenBank/DDBJ data bank under accession numbers DQ302751) of the human PLC-delta4 gene was isolated from human genomic DNA. It was a TATA-less promoter with very GC-rich sequences near the transcription start site. The activity of the PLC-delta4 promoter was shown in various human and mouse cell lines by luciferase reporter assay. Serial deletion analysis identified the core promoter region as being between -402 and -67, in which an E-box and an AP-1 binding site played important roles in the promoter activity. In addition, we also showed that 12-O-tetradecanoylphorbol-1,3-acetate (TPA), a PKC activator and tumor promoter, induced the activity of the PLC-delta4 promoter via the AP-1 binding site. In summary, this study identified a core promoter region of the hPLC-delta4 gene and the factor binding sites responsible for the promoter activity. These results will provide important new information to further understand the regulatory mechanism of the PLC-delta4 function.

Signal transduction by protein kinase C in mammalian cells

1. The past two decades have witnessed great advances in our understanding of the role of protein kinase C (PKC) in signal transduction. The Ca(2+)-activated, phospholipid-dependent protein kinase discovered by Nishizuka's group in 1977 is now a family of at least 11 isoforms. Protein kinase C isoforms exist in different proportions in a host of mammalian cells and each isoform has a characteristic subcellular distribution in each cell type. 2. Stimulation of a specific PKC isoform often causes redistribution of the isoform from one subcellular compartment to another compartments where it complexes with and phosphorylates a specific protein substrate. 3. The interaction of a specific PKC isoform with its protein substrate may directly activate a specific function of the cell or may trigger a cascade of protein kinases that ultimately stimulates a specific response in differentiated cells or regulates growth and proliferation in undifferentiated cells.

Involvement of protein kinase A in the induction of arginase in murine bone marrow-derived macrophages

Arginase is induced in bone marrow-derived macrophages by agents that increase the intracellular concentrations of cAMP (Br-cAMP, prostaglandin E2) and, in their presence, the LPS induced NO synthesis is down regulated. Moreover, interleukin 10 which induces arginase in macrophages is able to increase the cAMP-dependent protein kinase A activity. In contrast, suppressors of NOS synthesis like protein kinase C inhibitors and calmodulin antagonists (W7), or NO activators (A23187) have no effect on the induction of arginase by LPS. These results strongly suggest that PKA is involved in the induction of arginase and supports the hypothesis that there is a reciprocal regulation of these two enzymes that drives the macrophages towards opposite functional states.

A role for reactive oxygen species in zymosan and beta-glucan induced protein tyrosine phosphorylation and phospholipase A2 activation in murine macrophages

Previously we have shown that reactive oxygen species (ROS) formation induced by phorbol ester in association with vanadate is essential for protein tyrosine phosphorylation and phospholipase A2 (PLA2) activation. Here we show that the interaction of beta-glucan particles (glucanp) or zymosan with complement receptor type 3 (CR3) leads, when associated with vanadate, to a cascade of reactions culminating in PLA2 activation. Vanadate + zymosan (or glucanp) markedly enhance protein tyrosine phosphorylation in bone marrow derived macrophages (BMMs), whereas neither of the agents alone has any effect. The enhancement was due to both sustained activation of protein tyrosine kinase (PTK) and inactivation of protein tyrosine phosphatase (PTP) as assessed in lysates of treated cells. Zymosan elevates membranal PKC, an effect that is potentiated by vanadate. Activation of both PTK and PKC leads to the activation of NADPH oxidase and to ROS formation. The formed ROS together with vanadate are potent inactivators of PTP leading to amplification of tyrosine phosphorylation and myelin basic protein kinase (MBP-K) activation. The activation of the cascade of protein kinases eventually leads to activation of PLA2. All the activation steps, i.e., activation of PTK, NADPH oxidase, MBP-K,PLA2 and the inactivation of PTP are sensitive to the NADPH oxidase inhibitor diphenyleneiodonium (DPI), to antioxidants and to PKC inhibitors. Thus, ROS formation (in the presence of vanadate) is critical for protein phosphorylation processes constituting the regulatory pathway of PLA2 activation by ligand-CR3 interaction.

Reactive oxygen species mediate phorbol ester-regulated tyrosine phosphorylation and phospholipase A2 activation: potentiation by vanadate

We have previously shown that vanadate potentiates the activating effect of phorbol ester (TPA) on cellular phospholipase A2 (PLA2) in a pathway dependent on the formation of reactive oxygen species (ROS). Here we evaluate the chain of enzymes (protein kinases and phosphatases) that participate in this process. Treatment of macrophages with vanadate plus TPA led to activation of protein kinase C (PKC) and NADPH oxidase (O2- generation in intact cells), massive cellular protein tyrosine phosphorylation, suppression of protein tyrosine phosphatase (PTP) activity and a sustained activation of protein tyrosine kinase (PTK) and myelin basic protein kinase activity (the latter three enzyme activities were assessed in cell lysates). Inhibition of ROS formation by diphenyleneiodonium (DPI) prevented PTP inhibition, PTK activation and protein tyrosine phosphorylation by vanadate plus TPA. Vanadate plus H2O2 mimicked the effect of vanadate plus TPA on PKC activation, cellular protein tyrosine phosphorylation, PTP and PTK, but their effects were resistant to DPI. Suppression of PKC activity (down-regulation; selective inhibitors) prevented the above-mentioned effects of vanadate plus TPA, but not of vanadate plus H2O2. Collectively, the results show that ROS formation induced by TPA in association with vanadate is essential in the modulation of protein tyrosine phosphorylation and PLA2 activity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters pancreatic membrane tyrosine phosphorylation following acute treatment

To understand the basic mechanisms of TCDD's action to cause hypoinsulinemia in several experimental animals, we have studied TCDD-induced changes in various protein kinase activities in membrane preparations of guinea pig pancreas. For this purpose, young male guinea pigs were treated through a single intraperitoneal injection with 1 or 3 micrograms/kg of TCDD in vivo, and, after given time periods, pancreas samples were obtained and membranes were isolated through homogenization and centrifugation procedures. Several sets of incubation conditions were selected for protein kinase activity assay, each favoring a specific type of protein kinase. It was found that overall protein phosphorylation activities were higher in the preparation from TCDD-treated animals as compared to those found in pair-fed controls and that this trend was more pronounced when the assay medium contained Mn2+ in place of Mg2+ and EGTA. These are the conditions that are known to favor protein tyrosine kinases. Other types of protein kinases from the treated animals did not show any significant differences from the pair-fed control animals, though that of protein kinase C in the treated preparation showed a modest increase. To establish that the type of protein kinases stimulated by TCDD are protein tyrosine kinases, we have carried out phosphoamino acid analyses, KOH digestion, and western blot analyses using an antibody to phosphotyrosine. All the results were consistent in supporting the idea that TCDD causes a rise in protein-tyrosine kinases in pancreas at early stages of poisoning.

Activation of PKC inhibits NaF-induced inositol phospholipid turnover in rat insulinoma cells

An investigation was done to elucidate the regulatory role of protein kinase C (PKC) in insulin release and also the effects of PKC activation on NaF-induced inositol phospholipid (PI) turnover in and insulin release from rat insulinoma cells (RINr). NaF stimulated insulin secretion in association with an increase in [3H]inositol phosphate formation in RINr cells. Furthermore, NaF induced a rapid decrease in 32P-labeling of phosphatidylinositol-4,5-diphosphate (PIP2) with a concomitant increase of [32P]phosphatidic acid in prelabeled cells. In contrast, NaF had no effect on cyclic AMP production. Although phorbol 12,13-dibutyrate (PDBu) also stimulated insulin release, on concomitant administration of NaF and PDBu, insulin secretion was clearly less than that expected on the basis of an additive action. Moreover, PDBu significantly inhibited NaF-enhanced PI turnover. However, this inhibition was abolished after downregulating PKC by pretreating RINr cells with PDBu. Thus NaF-induced insulin release from RINr cells appears to involve enhancement of PI turnover. Moreover, because NaF is known to activate guanine nucleotide binding proteins (G proteins) directly, PKC activation appears to induce a mechanism that inhibits stimulus-secretion coupling at a level between G protein and phospholipase C-induced PIP2 hydrolysis.

The effect of Escherichia coli heat-stable enterotoxin on protein kinase activity

Isolated rat enterocytes were incubated with E. coli heat-stable enterotoxin or buffer alone and the protein kinase activity and cyclic GMP level determined on the particulate fraction or cytosol, respectively. In the control cells, particulate protein kinase activity and cyclic GMP concentration were at a maximum after 20 sec and 1 min of incubation, respectively. In heat-stable enterotoxin-treated cells the particulate protein kinase activity was significantly increased (P less than 0.05) after 20 sec of incubation, but decreased (P less than 0.05) after 30 sec, 1 min and 2 min, when compared to the control reaction. During this time period the concentration of intracellular cyclic GMP increased 10-fold. The effect of heat-stable enterotoxin on particulate protein kinase activity and cyclic GMP concentration was dose-dependent. Analysis of radioactive membrane phosphorylation products indicate a role for phosphoproteins with a mol. wt of 25,000 and 120,000. These results suggest that the action of heat-stable enterotoxin may involve an effect on protein kinase.

The role of protein kinase C in cholinergic stimulation of insulin secretion from rat islets of Langerhans

The role of the Ca2+/phospholipid-dependent protein kinase C (PKC) in cholinergic potentiation of insulin release was investigated by measuring islet PKC activity and insulin secretion in response to carbachol (CCh), a cholinergic agonist. CCh caused a dose-dependent increase in insulin secretion from cultured rat islets at stimulatory glucose concentrations (greater than or equal to 7 mM), with maximal effects observed at 100 microM. Short-term exposure (5 min) of islets to 500 microM-CCh at 2 mM- or 20 mM-glucose resulted in redistribution of islet PKC activity from a predominantly cytosolic location to a membrane-associated form. Prolonged exposure (greater than 20 h) of islets to 200 nM-phorbol myristate acetate caused a virtual depletion of PKC activity associated with the islet cytosolic fraction. Under these conditions of PKC down-regulation, the potentiation of glucose-stimulated insulin secretion by CCh (500 microM) was significantly decreased, but not abolished. CCh stimulated the hydrolysis of inositol phospholipids in both normal and PKC-depleted islets, as assessed by the generation of radiolabelled inositol phosphates. These results suggest that the potentiation of glucose-induced insulin secretion by cholinergic agonists is partly mediated by activation of PKC as a consequence of phospholipid hydrolysis.

Comparison of effects of phorbol esters and glucose on protein kinase C activation and insulin secretion in pancreatic islets

The tumour-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) induces insulin secretion from isolated pancreatic islets, and this suggests a potential role for protein kinase C in the regulation of stimulus-secretion coupling in islets. In the present study, the hypothesis that the insulinotropic effect of TPA is mediated by activation of protein kinase C in pancreatic islets has been examined. TPA induced a gradual translocation of protein kinase C from the cytosol to a membrane-associated state which correlated with the gradual onset of insulin secretion. The pharmacologically inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not mimic this effect. TPA also induced a rapid time-dependent decline of total protein kinase C activity in islets and the appearance of a Ca2+- and phospholipid-independent protein kinase activity. Insulin secretion induced by TPA was completely suppressed (IC50 approximately 10 nM) by staurosporine, a potent protein kinase C inhibitor. Staurosporine also inhibited islet cytosolic protein kinase C activity at similar concentrations (IC50 approximately 2 nM). In addition, staurosporine partially (approximately 60%) inhibited glucose-induced insulin secretion at concentrations (IC50 approximately 10 nM) similar to those required to inhibit TPA-induced insulin secretion, suggesting that staurosporine may act at a step common to both mechanisms, possibly the activation of protein kinase C. However, stimulatory concentrations of glucose did not induce down-regulation of translocation of protein kinase C, and the inhibition of glucose-induced insulin release by staurosporine was incomplete. Significant questions therefore remain unresolved as to the possible involvement of protein kinase C in glucose-induced insulin secretion.

Effects of low-concentration polymyxin B on insulin secretion induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA), glucose, or tolbutamide from the isolated perfused rat pancreas

To evaluate the role of protein kinase C on insulin secretion, we investigated the effects of low-concentration polymyxin B (100 mumol/L) on insulin secretion from the isolated perfused rat pancreas induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA, 10 nmol/L), a protein kinase C activator, glucose (10 mmol/L), or tolbutamide (738 mumol/L). Polymyxin B, a potent and relatively selective protein kinase C inhibitor at this low concentration, did significantly inhibit the gradual rise of insulin secretion induced by TPA (P less than .05). As for glucose or tolbutamide stimulation, polymyxin B significantly inhibited not only the second phase but also the first phase of insulin secretion (P less than .05) without changing the secretion patterns. Although the possibility of nonspecific effects of polymyxin B other than protein kinase C inhibition could not be excluded, the data suggest that protein kinase C might be involved in insulin secretion as a potentiating modulator.

Translocation of protein kinase C in rat islets of Langerhans. Effects of a phorbol ester, carbachol and glucose

In unstimulated rat islets (2 mM glucose), most of the ion-exchange purified protein kinase C (PKC) activity was associated with the cytosolic fraction. Both carbachol and phorbol myristate acetate caused a significant translocation of PKC activity from cytosolic to membrane fractions, but under the same conditions, glucose (20 mM) did not cause such a redistribution of PKC activity. PMA-induced translocation of PKC to the membrane fraction was also observed in electrically permeabilised islets, in which recovery of the enzyme activity was enhanced by buffering the intracellular Ca2+ concentration to 50 nM and supplying the permeabilised islets with protease inhibitors.

Hydrogen peroxide generation in whole rat pancreatic islets; synergistic regulation by cytoplasmic free calcium and protein kinase-C

This is the first report to show that pancreatic islet cells generate H2O2 and this H2O2 generation is regulated synergistically by cytoplasmic free calcium ([Ca2+]i) and protein kinase-C. Effects of calcium ionophore A23187 and 12-O-tetradecanoylphorbol 13-acetate (TPA), a tumor promoter, on H2O2 generation were studied in whole pancreatic islets obtained from male Wistar rats. We employed A23187 to elevate cytoplasmic free calcium, and TPA to activate protein kinase-C and monitored continuously their effects on H2O2 generation, measured using homovanillic acid and horseradish peroxidase. A23187 stimulates H2O2 generation. TPA, which activates protein kinase-C, augments this A23187-stimulated H2O2 generation. H2O2 generation is stimulated by an increase in [Ca2+]i and regulated synergistically by [Ca2+]i and protein kinase-C.

Translocation of phospholipase A2 from cytosol to membranes induced by 1-oleoyl-2-acetyl-glycerol in serum-free cultured macrophages

Macrophages exhibit high activities of a phospholipase A2 which preferentially cleaves arachidonic acid (I. Flesch, B. Schmidt, and E. Ferber, Z. Naturforsch. 40c, 356-363, 1985). In unstimulated cells more than 90% of the total activity of this enzyme is localized in the cytosol. Treatment of these cells with 100 microM 1-oleoyl-2-acetyl-glycerol (OAG) for 30 min induced a translocation of phospholipase A2 to cellular membranes. The amount of translocated phospholipase A2 was about 30% of the total activity and correlated with a similar translocation of protein kinase C to membranes. These data suggest that the translocation of phospholipase A2 to membranes is related to the activation process of this enzyme.

A re-assessment of the role of protein kinase C in glucose-stimulated insulin secretion

Isolated rat islets of Langerhans which had been pretreated with 200 nM-phorbol 12-myristate 13-acetate (PMA) for 20-24 h, a treatment reported in other cell types to deplete cells of protein kinase C activity, were found not to contain detectable Ca2+/phospholipid-dependent protein kinase activity. These islets did not secrete insulin in response to a subsequent exposure to PMA (0.1 or 1 microM) during a 30 min incubation, although insulin secretion could be stimulated by 20 mM-glucose, a response which was enhanced by 20 microM-forskolin. PMA-pretreated islets that had been permeabilized by high-voltage discharge showed unimpaired secretory responses to an increase in Ca2+ concentration, cyclic AMP and forskolin. These results suggest that (i) pretreatment of islets with tumour-promoting phorbol esters may be a useful means of investigating the role of protein kinase C in stimulus-secretion coupling in the pancreatic beta-cell and (ii) protein kinase C may not play an essential role in glucose-induced insulin secretion.

Glucose-induced phospholipid hydrolysis in isolated pancreatic islets: quantitative effects on the phospholipid content of arachidonate and other fatty acids

Our recent findings indicate that glucose-induced insulin secretion from isolated pancreatic islets is temporally associated with accumulation of substantial amounts of free arachidonic acid and that arachidonate may serve as a second messenger for intracellular calcium mobilization in islets. In an effort to determine the source of this released arachidonate, the endogenous fatty acid composition of phospholipids from islets has been determined by thin-layer chromatographic separation of the phospholipids, methanolysis to the fatty acid methyl esters, and quantitative gas chromatographic analyses. The relative abundance of phospholipids in islets as judged by their fatty acid content was phosphatidylcholine (PC), 0.63; phosphatidylethanolamine (PE), 0.23; phosphatidylinositol (PI), 0.067; phosphatidylserine (PS), 0.049. Arachidonate constituted 17% of the total islet fatty acid content, and PC contained 43% of total islet arachidonate. Islets incubated with [3H]arachidonate in the presence of 28 mM D-glucose incorporated radiolabel into PC with a considerably higher specific activity than that of PE, PS or PI. The total fatty acid content of PC from islets incubated with 28 mM glucose for 30 min was significantly lower than that of islets incubated with 3 mM glucose, and smaller effects were observed with PE, PS and PI. The molar decrement in PC arachidonate was 3.2 pmol/islet under these conditions, which is sufficient to account for the previously observed accumulation of free arachidonate (2 pmol/islet). A sensitive method involving negative ion-chemical ionization-mass spectrometric analyses of the pentafluorobenzyl esters of fatty acids derived from trace amounts of lysophosphatidylcholine (lyso-PC) was developed, and glucose-stimulation was found to reduce islet lyso-PC content by about 10-fold. These findings indicate that the insulin secretagogue D-glucose induces phospholipid hydrolysis in islets and suggest that PC may be the major source of free arachidonate which accumulates in glucose-stimulated islets.

The coupling of metabolic to secretory events in pancreatic islets: inhibition by 2-cyclohexene-1-one of the secretory response to cyclic AMP and cytochalasin B

In rat pancreatic islets perifused in the presence of 2-cyclohexene-1-one (CHX; 1.0 mM), the secretory response to either D-glucose or 2-ketoisocaproate, but not that evoked by the association of L-leucine and L-glutamine, was severely decreased. This coincided with a decreased stimulation of [45Ca] efflux from prelabelled islets, whereas the inhibitory action of D-glucose or 2-ketoisocaproate upon both [86Rb] and [45Ca] efflux appeared little or not affected. In the presence of D-glucose, the islets exposed to CHX were virtually unresponsive to either forskolin, theophylline or cytochalasin B. A severe decrease in the secretory response to forskolin was also observed in CHX-treated islets exposed to L-leucine and L-glutamine. Except for a somewhat lower sensitivity to NaF, no major change in adenylate cyclase activity or cyclic AMP production was observed in CHX-treated islets. The activity of protein kinase A was decreased in such islets but its responsiveness to cyclic AMP appeared unaltered. Transglutaminase activity was severely decreased in homogenates derived from CHX-treated islets. These findings suggest that CHX, possibly by lowering the GSH content of islet cells, impairs the functional capacity of the effector system for insulin release, in addition to and independently of any effect that it may exert upon nutrient catabolism and cationic fluxes in the islet cells.

Evaluation of granule exocytosis in toad urinary bladder

Addition of phorbol myristate acetate (PMA) to the mucosal bathing solution induces exocytosis of the granules found in the apical cytoplasm of toad urinary bladder epithelial cells. Because mucosal application of PMA also increases epithelial water permeability in the complete absence of vasopressin, these observations have suggested that granules might have a role in mediating the permeability response to vasopressin. From electron microscopic immunolocalization studies we have identified a 70-kDa protein as a component of the apical surface glycocalyx that is stored in the granules. Using antibodies to this protein in a competitive immunoassay, we found that addition of PMA to the mucosal side results in more than a fourfold increase in antigen release into the mucosal bath. The amount of antigen detectable on the mucosal surface of the cells was doubled by mucosal PMA exposure. Serosal application of PMA or addition of the inactive analogue phorbol didecanoate to either surface failed to produce significant antigen release. These results are consistent with electron microscopic studies showing exocytosis of granules only with mucosal exposure to PMA. However, immunoassay of granule antigen after vasopressin stimulation of water permeability failed to show a detectable change in granule exocytosis. We conclude that granule exocytosis does not play an important role in mediating vasopressin-induced changes in permeability.

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.

The effect of phorbol esters on the chloride secreting epithelium of the rabbit cornea

Tumor-promoting phorbol esters have been shown to modulate a number of physiological events in various cell types, and these events are associated with the activation of protein kinase C. The purpose of this study was to examine the effects of phorbol ester stimulation on protein kinase C activity and epithelial ion transport in the Cl- secreting rabbit corneal epithelium. We report here that nanomolar concentrations of tumor-promoting phorbol ester, 12-0-tetradecanoyl-phorbol-13 acetate (TPA), stimulate active ion transport via a putative receptor located in the corneal epithelium. This stimulation is a Cl- -dependent process, but is independent of beta-adrenergic receptor activation and prostaglandin formation. Biochemical data support the idea that TPA activates Ca++/phospholipid-dependent protein kinase C in the corneal epithelium. Therefore, we suggest that protein kinase C may have a role in the regulation of membrane Cl- transport in the mammalian corneal epithelium.

Na+-H+ exchange in the process of glucose-induced insulin release from the pancreatic B-cell. Effects of amiloride on 86Rb, 45Ca fluxes and insulin release

The effect of amiloride, an inhibitor of Na+-H+ exchange, on intracellular pH (pHi), 86Rb outflow, 45Ca outflow and insulin release from pancreatic rat islets was examined. In the 0.1-1 mM range, amiloride transiently reduced pHi of glucose-deprived islets and allowed glucose to induce a sustained decrease in pHi of the islet cells. Amiloride reproduced the effect of glucose to decrease 86Rb and 45Ca outflow. In the presence of glucose (5.6 mM or more), amiloride (100 microM) acted synergistically with the sugar to reduce K+ outflow, and to stimulate 40Ca inflow and insulin release from perifused islets. These results add strong support to the view that the generation of protons through the metabolism of glucose represents an important step in the process of glucose-induced release. The stimulation by glucose of Na+-H+ exchange apparently masks and even overcomes the glucose-induced decrease in pHi otherwise expected from the increase in catabolic fluxes.

The effects of polymyxin B, a protein kinase C inhibitor, on insulin secretion from intact and permeabilized islets of Langerhans

Polymyxin B (0.01-1 mM), a polyamine antibiotic, inhibited both phorbol ester- and glucose-stimulated insulin secretion from isolated rat islets of Langerhans. This inhibition was rapidly reversible. Assay of the cytosolic protein kinase C by measurement of incorporation of labelled phosphate into a histone substrate demonstrated the presence of activity in islet extracts which could be stimulated by 12-O-tetradecanoylphorbol-13-acetate and inhibited by polymyxin B. These results suggest that protein kinase C plays a role in glucose-induced insulin secretion.

Cholinergic stimulation of ion fluxes in pancreatic islets

Cholinergic agents are known to stimulate the hydrolysis of polyphosphoinositides in pancreatic islets. The effect of carbamylcholine upon ion fluxes in the islet cells was investigated. Carbamylcholine provoked a rapid but poorly sustained increase in 45Ca and 86Rb outflow from perifused islets. Such a cationic response was observed at different glucose concentrations (zero to 16.7 mM), at three concentrations of carbamylcholine (10 microM, 100 microM and 1.0 mM), and in the absence or presence of extracellular Ca2+. It coincided with a biphasic stimulation of insulin release, both the cationic and secretory responses being abolished in the presence of atropine (10 microM). At variance with nutrient secretagogues, carbamylcholine failed to affect the net production of cyclic AMP and caused a transient decrease in 32P outflow from islets prelabelled with [32P]phosphate. It is proposed that cholinergic agents mobilize Ca2+ from intracellular sites, possibly through generation of inositol, 1,4,5-triphosphate from phosphatidylinositol 4,5-bisphosphate. The intracellular redistribution of Ca2+ does not appear sufficient, however, to account fully for the secretory response, which may also involve activation of protein kinase C by diacylglycerol.

Stimulation by glucose and carbamylcholine of phospholipase C in pancreatic islets

Phosphoinositide hydrolysis in intact pancreatic islet cells was investigated in an indirect but dynamic manner by monitoring the efflux of radioactivity from islets prelabelled with [3H]inositol. A rise in glucose concentration provoked a rapid, modest but sustained increase in effluent radioactivity, this phenomenon being abolished in the absence of extracellular Ca2+ or presence of verapamil. The release of [3H]inositol was also stimulated at high extracellular K+ concentration, but not by gliclazide. Whether in the presence or absence of glucose, carbamylcholine provoked a marked increase in effluent radioactivity. The response to the cholinergic agent was decreased in the presence of verapamil or absence of extracellular Ca2+ and abolished in the presence of atropine or LiCl. These results suggest that an increase in cytosolic Ca activity, as caused by glucose or membrane depolarization, may cause activation of phospholipase C. In response to cholinergic agents, however, the enzymic activation, although modulated by Ca2+ availability, may result directly from the occupation of muscarinic receptors.

Studies on the mechanism by which melittin stimulates insulin secretion from isolated rat islets of Langerhans

Incubation of isolated rat islets of Langerhans with melittin resulted in a dose-dependent stimulation of insulin secretion with half the maximal response occurring at 4 micrograms/ml melittin. The effect of melittin on insulin secretion was dependent on extracellular calcium, was inhibited by the phospholipase A2 inhibitor quinacrine and by the lipoxygenase inhibitor nordihydroguaiaretic acid. Stimulation of insulin secretion by melittin was associated with a calcium-dependent loss of [3H]arachidonic acid from phospholipids in islet cells prelabelled with [3H]arachidonic acid. Analysis of the islet phospholipids involved in this response revealed that the [3H]arachidonic acid was released predominantly from phosphatidylcholine. These results suggest that melittin may stimulate insulin secretion by activating phospholipase A2 in islet cells, causing the release of arachidonic acid from membrane phospholipid. The results are consistent with suggestions that the subsequent metabolism of arachidonic acid via the lipoxygenase pathway may be involved in regulating the insulin secretory response.

Stimulation of protein kinase C and insulin release by 1-oleoyl-2-acetyl-glycerol

The membrane-accessible diacylglycerol 1-oleoyl-2-acetyl-sn-glycerol (OAG, 5-500 microM) caused a dose-related activation of protein kinase C in rat islet homogenates. In islet cell membranes exposed to [gamma-32P]ATP, OAG (100 microM) stimulated the net production of labelled phosphatidate and inhibited that of labelled phosphatidylinositol 4-phosphate. In intact islets exposed to 5.6 mM D-glucose, OAG (100 microM) decreased the outflow of 86Rb, increased that of 45Ca and caused a rapid stimulation of insulin release. The secretory response to OAG was dose-related in the 50-500 microM range, being most marked, in relative terms, at a glucose concentration close to the threshold value for stimulation of insulin release by this hexose. It was decreased but not abolished in the absence of CaCl2 and presence of EGTA. At variance with tumor-promoting phorbol esters, OAG failed to potentiate insulin release stimulated by a hypoglycaemic sulphonylurea. Although these findings support the view that activation of protein kinase C by diacylglycerol represents an efficient modality for stimulation of insulin release, they suggest that the effect of OAG upon islet function may not be solely attributable to such an activation.

Effect of glucose on polyphosphoinositide metabolism in isolated rat islets of Langerhans

The metabolism of inositol-containing phospholipids during insulin secretion was studied in rat islets of Langerhans preincubated with [3H]inositol to label their phospholipids. Glucose (20 mM) caused a rapid breakdown of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and an accumulation of inositol trisphosphate and inositol bisphosphate. This effect was maximal at 60s, did not require the presence of extracellular Ca2+, and was abolished by mannoheptulose (15 mM), but not by noradrenaline (1 microM). Mannose (20 mM) and DL-glyceraldehyde (10 mM) produced similar effects to those of glucose, but galactose (20 mM) and KCl (30 mM) were without effect. These results are compatible with the hypothesis that an early event in the stimulus-secretion coupling mechanism in the pancreatic B-cell is the rapid breakdown of polyphosphoinositides catalysed by phospholipase C. Moreover, they suggest that the breakdown of polyphosphoinositides is linked to sugar metabolism in the B-cell. This observation is important, since it demonstrates that events in a cell other than plasma-membrane receptor occupancy can promote polyphosphoinositide hydrolysis.

Tumour-promoting phorbol esters inhibit agonist-induced phosphatidate formation and Ca2+ flux in human platelets

Tumour-promoting phorbol esters (phorbol-12-myristate-13-acetate, PMA; phorbol-12,13-dibutyrate, PDBu) but not 4 beta-phorbol, activate protein kinase C. Using human platelets pre-labelled with quin2 or 32PO4 we examined the effects of these compounds on human platelet cytosolic free Ca2+ ([Ca2+]i) and on [32P]phosphatidic acid ([32P]PtdOH). PMA and PDBu, but not 4 beta-phorbol inhibited thrombin-, PAF- and vasopressin-induced elevation of [Ca2+]i and [32P]PtdOH formation. It is suggested that protein kinase C may act to terminate the transduction processes that link receptor occupancy to cellular activation.