Effects of a phorbol ester and clomiphene on protein phosphorylation and insulin secretion in rat pancreatic islets

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.

Intracellular Ca(2+) modulation of ATP-sensitive K(+) channel activity in acetylcholine-induced activation of rat pancreatic beta-cells

We investigated the mechanism by which acetylcholine (ACh) regulates insulin secretion from rat pancreatic beta-cells. In an extracellular solution with 5.5 mM glucose, ACh increased the rate of insulin secretion from rat islets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, ACh still increased insulin secretion, but the increment was lower than that without BIM. In the presence of nifedipine, an L-type Ca(2+) channel blocker, on the other hand, ACh did not increase insulin secretion. In isolated rat pancreatic beta-cells, ACh caused depolarization followed by action potentials. This ACh effect was observed even in cells treated with BIM. In the presence of nifedipine, ACh caused only depolarization. These ACh effects were prevented by atropine. In the perforated whole-cell configuration, ramp pulses from -90 to -50 mV induced membrane currents mostly through ATP-sensitive K(+) channels (K(ATP)). These currents were reduced in size by ACh in cells either treated or untreated with BIM; whereas the loading of cells with U-73122 (a phospholipase C inhibitor) or BAPTA/AM (a Ca(2+) chelator) abolished the ACh effect. In the standard whole-cell configuration, ACh reduced the currents through K(ATP) with 0.5 mM EGTA, but not with 10 mM EGTA, in the pipette solution. Intracellular application of GDPbetaS or heparin also inhibited the ACh effect. In the inside-out single-channel recordings, elevation of the Ca(2+) concentration inside the membrane from 10 nM-10 microM decreased K(ATP) activity only in the presence of ATP. The affinity of ATP to K(ATP) became 4.5 times higher with the higher concentration of Ca(2+). These results suggest that Ca(2+) from ACh receptor signaling modulates the sensitivity of K(ATP) to ATP. A positive-feedback mechanism of intracellular Ca(2+)-dependent Ca(2+) influx was also demonstrated.

Phorbol ester impairs electrical excitation of rat pancreatic beta-cells through PKC-independent activation of KATP channels

BACKGROUND

Phorbol 12-myristate 13-acetate (PMA) is often used as an activating phorbol ester of protein kinase C (PKC) to investigate the roles of the kinase in cellular functions. Accumulating lines of evidence indicate that in addition to activating PKC, PMA also produces some regulatory effects in a PKC-independent manner. In this study, we investigated the non-PKC effects of PMA on electrical excitability of rat pancreatic beta-cells by using patch-clamp techniques.

RESULTS

In current-clamp recording, PMA (80 nM) reversibly inhibited 15 mM glucose-induced action potential spikes superimposed on a slow membrane depolarization and this inhibition can not be prevented by pre-treatment of the cell with a specific PKC inhibitor, bisindolylmaleimide (BIM, 1 microM). In the presence of a subthreshold concentration (5.5 mM) of glucose, PMA hyperpolarized beta-cells in a concentration-dependent manner (0.8-240 nM), even in the presence of BIM. Based on cell-attached single channel recordings, PMA increased ATP-sensitive K+ channel (KATP) activity. Based on inside-out patch-clamp recordings, PMA had little effect on KATP activity if no ATP was in the bath, while PMA restored KATP activity that was suppressed by 10 microM ATP in the bath. In voltage-clamp recording, PMA enhanced tolbutamide-sensitive membrane currents elicited by repetitive ramp pulses from -90 to -50 mV in a concentration-dependent manner, and this potentiation could not be prevented by pre-treatment of cell with BIM. 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), a non-PKC-activating phorbol ester, mimicked the effect of PMA on both current-clamp and voltage-clamp recording configurations. With either 5.5 or 16.6 mM glucose in the extracellular solution, PMA (80 nM) increased insulin secretion from rat islets. However, in islets pretreated with BIM (1 microM), PMA did not increase, but rather reduced insulin secretion.

CONCLUSION

In rat pancreatic beta-cells, PMA modulates insulin secretion through a mixed mechanism: increases insulin secretion by activation of PKC, and meanwhile decrease insulin secretion by impairing beta-cell excitability in a PKC-independent manner. The enhancement of KATP activity by reducing sensitivity of KATP to ATP seems to underlie the PMA-induced impairment of beta-cells electrical excitation in response to glucose stimulation.

Protein kinase C-dependent and -independent inhibition of Ca(2+) influx by phorbol ester in rat pancreatic beta-cells

Phorbol esters were used to investigate the action of protein kinase C (PKC) on insulin secretion from pancreatic beta-cells. Application of 80 nM phorbol 12-myristate 13-acetate (PMA), a PKC-activating phorbol ester, had little effect on glucose (15 mM)-induced insulin secretion from intact rat islets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, PMA significantly reduced the glucose-induced insulin secretion. PMA decreased the level of intracellular Ca(2+) concentration ([Ca(2+)](i)) elevated by the glucose stimulation when tested in isolated rat beta-cells. This inhibitory effect of PMA was not prevented by BIM. PMA inhibited glucose-induced action potentials, and this effect was not prevented by BIM. Further, 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), a non-PKC-activating phorbol ester, produced an effect similar to PMA. In the presence of nifedipine, the glucose stimulation produced only depolarization, and PMA applied on top of glucose repolarized the cell. When applied at the resting state, PMA hyperpolarized beta-cells with an increase in the membrane conductance. Recorded under the voltage-clamp condition, PMA reduced the magnitude of Ca(2+) currents through L-type Ca(2+) channels. BIM prevented the PMA inhibition of the Ca(2+) currents. These results suggest that activation of PKC maintains glucose-stimulated insulin secretion in pancreatic beta-cells, defeating its own inhibition of the Ca(2+) influx through L-type Ca(2+) channels. PKC-independent inhibition of electrical excitability by phorbol esters was also demonstrated.

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.

Protein kinase C in beta-cells: expression of multiple isoforms and involvement in cholinergic stimulation of insulin secretion

The mammalian protein kinase C (PKC) family consists of at least 11 distinct isotypes with marked differences in tissue distribution, localization, cofactor dependence and substrate specificity. Evidence exists for the expression of some of the PKC isoforms in pancreatic beta-cells but no comprehensive analysis of all the known PKC types has been accomplished. To assess the functional relevance of phosphorylation by PKC in the mechanism of insulin secretion we firstly investigated the expression of PKC isoforms in pancreatic beta-cells. The combination of reverse transcription-polymerase chain reaction (RT-PCR), Northern analysis and immunoblotting demonstrated the expression of PKC-alpha, beta II, epsilon, zeta, lambda and mu in MIN6 beta-cells. PKC-mu has not previously been detected in beta-cells. Expression of PKC-delta was also observed at the mRNA level; however, the protein could not be detected by Western blotting in MIN6 cells but was readily observed in RINm5F beta-cells. In short-term incubations, insulin release from MIN6 cells was augmented by 12-0-tetradecanoyl-phorbol-13-acetate (TPA), by carbachol, and by 40 mM K+. Culture of MIN6 cells overnight with TPA resulted in down-regulation of PKC-alpha (totally) and epsilon (partially), without significant change in the other isoforms. In such TPA-treated cells, the secretory response to TPA and to carbachol was abolished but not that elicited by high K+. It is suggested that PKC-alpha and/or epsilon may play a role in cholinergic potentiation of insulin secretion.

Increase in insulin release from rat pancreatic islets by quinolone antibiotics

1. The present study was undertaken to elucidate the mechanism(s) of hypoglycaemia caused by quinolone antibiotics. We investigated the effects of various quinolone antibiotics on insulin release in rat pancreatic islets. 2. At a non-stimulatory concentration of 3 mM glucose, lomefloxacin (LFLX) or sparfloxacin at 1 mM and pipemidic acid (0.1-1 mM) induced slight insulin release but tosufloxacin or enoxacin up to 100 microM did not. 3. At the stimulatory concentration of 10 mM glucose, all quinolones augmented insulin release in a dose-dependent manner. LFLX (100 microM) shifted the dose-response curve of glucose-induced insulin release to the left without altering the maximal response. 4. At 10 mM glucose, LFLX (100 microM) increased insulin release augmented by forskolin (5 microM) or 12-O-tetradecanoyl phorbol-13-acetate (100 nM) but not by raising the K+ concentration from 6 to 25 mM. 5. Verapamil (50 microM) or diazoxide (50-400 microM) antagonized the insulinotropic effect of LFLX. 6. These data suggest that quinolone antibiotics may cause hypoglycaemia by increasing insulin release via blockade of ATP-sensitive K+ channels.

Protein phosphorylation and beta-cell function

The central role of reversible protein phosphorylation in regulation of beta-cell function is reviewed and the properties of the protein kinases so far defined in beta cells are summarised. The key effect of Ca2+ to initiate insulin secretion involves activation of a Ca2+/calmodulin-dependent protein kinase. Potentiation of secretion by agents activating protein kinase A or C appears to involve an increase in the sensitivity of the secretory system to intracellular Ca2+. The effects of MgATP on the binding of [3H]-glibenclamide to the beta-cell sulphonylurea receptor suggest that the properties of this receptor, which controls the activity of ATP-sensitive K-channels, are modulated by phosphorylation. The identity of the kinases and phosphatases responsible is not known but the presence in beta-cell membranes of various kinases not dependent on Ca2+ or cyclic AMP, and including tyrosine kinase, is documented, together with the presence of both Ca(2+)-dependent and Ca(2+)-independent protein phosphatases. Protein phosphorylation is also involved in regulation of beta-cell Ca2+ fluxes and evidence is presented that protein kinase C activation inhibits Ca2+ signalling by reducing influx of Ca2+ into the beta cell. The identity of the Ca2+/calmodulin-dependent protein kinase activity in beta cells is discussed. Comparison of its properties towards substrates and inhibitors with those of brain Ca2+/calmodulin-dependent protein kinase II suggests that the beta-cell enzyme may be similar or identical to the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of protein-kinase-C--dependent cell proliferation of human lung cancer cell lines by the dihydropyridine dexniguldipine

The dihydropyridine, dexniguldipine hydrochloride (B859-35), has shown therapeutic activity in experimentally induced neuroendocrine hamster lung tumors and demonstrated antiproliferative effects in a mammary cancer cell line via inhibition of Ca2+ calmodulin. Studies in NIH 3T3 fibroblasts have provided evidence that dexniguldipine may also inhibit protein kinase C (PKC). In this study, we have tested the hypothesis that dexniguldipine may inhibit the proliferation of lung cancer cells in response to autocrine or exogenous activation of PKC. Using a panel of human lung cancer cell lines, we show that dexniguldipine is a potent inhibitor of mitogenic signal transduction pathways dependent on PKC activation in several small-cell and non-small-cell lung cancer cell lines while it failed to inhibit cyclic-AMP-dependent cell proliferation.

The role of reduced glucose transporter content and glucose metabolism in the immature secretory responses of fetal rat pancreatic islets

Isolated fetal islets show an immature or poor secretory response to nutrient secretagogues which may result from impaired mitochondrial oxidative processes. Insulin secretion, glucose metabolism and detection of metabolic enzymes by radiolabelling and immunoprecipitation were compared in islets isolated from neonatal (aged 5 days) and fetal rats (at 20 days gestation). The insulin secretory dynamics of fetal islets were abnormal in response to stimulation by glucose (10 mmol/l); a rapid release of insulin reaching a maximum 6 min after stimulation was observed with no rising second phase release. However, when the data were expressed as percentage of islet insulin content released, fetal islets released significantly more insulin than neonatal islets in response to glucose (4.86 +/- 0.45% vs 1.81 +/- 0.62%, p < 0.01) or 100 nmol/l glibenclamide (2.49 +/- 0.17% vs 0.25 +/- 0.06%, p < 0.001). Fetal islets however, failed to release insulin in response to stimulation by glyceraldehyde (10 mmol/l) unlike neonatal islets. Both glucose utilisation (as measured by the formation of [3H] H2O from 5-[3H] glucose) and glucose oxidation (as measured by the formation of [14C] CO2 from U-[14C] glucose) did not increase significantly in response to increasing the medium glucose concentration to 10 mmol/l whereas in neonatal islets, glucose utilisation and glucose oxidation were significantly increased 2.5- and 2.7-fold, respectively. When islets were incubated with both radiolabelled glucoses simultaneously, the rate of glucose oxidation was shown to be directly proportional to the rate of glucose utilisation. The relationship between glucose utilisation and glucose oxidation was similar in fetal and neonatal islets.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus-secretion coupling in pancreatic beta cells

Insulin secretion is triggered by a rise in the intracellular Ca2+ concentration that results from the activation of voltage-gated Ca2+ channels in the beta-cell plasma membrane. Multiple types of beta-cell Ca2+ channel have been identified in both electrophysiological and molecular biological studies, but it appears that the L-type Ca2+ channel plays a dominant role in regulating Ca2+ influx. Activity of this channel is potentiated by protein kinases A and C and is inhibited by GTP-binding proteins, which may mediate the effects of potentiators and inhibitors of insulin secretion on Ca2+ influx, respectively. The mechanisms by which elevation of intracellular Ca2+ leads to the release of insulin granules is not fully understood but appears to involve activation of Ca2+/calmodulin-dependent protein kinase. Phosphorylation by either protein kinase A or C, probably at different substrates, potentiates insulin secretion by acting at some late stage in the secretory process. There is also evidence that small GTP-binding proteins are involved in regulating exocytosis in beta cells. The identification and characterisation of the proteins involved in exocytosis in beta cells and clarification of the mechanism(s) of action of Ca2+ is clearly an important goal for the future.

Staurosporine inhibits protein kinases activated by Ca2+ and cyclic AMP in addition to inhibiting protein kinase C in rat islets of Langerhans

Staurosporine has been used in several studies to investigate the role of protein kinase C (PKC) in secretory responses of islets of Langerhans to insulin secretagogues. We have assessed the effect of staurosporine on: [i] islet PKC activity in vitro; [ii] the stimulation of insulin secretion by nutrient secretagogues and [iii] the stimulation of protein phosphorylation and insulin secretion in electrically permeabilised islets. All experiments were carried out on rat isolated islets of Langerhans, either intact or permeabilised by high voltage discharge (3.4 kV/cm). The activity of PKC partially purified from rat islets was inhibited by staurosporine (1.6-400 nM) in a concentration-dependent manner. Staurosporine also inhibited insulin secretion stimulated by both glucose and glyceraldehyde, with maximal effects at 50 nM. After prolonged exposure of islets to the tumour-promoting phorbol ester, 4 beta phorbol myristate acetate (4 beta PMA), a procedure which depletes islet PKC activity, staurosporine still inhibited both glucose- and glyceraldehyde-stimulated insulin release. In electrically permeabilised islets, staurosporine inhibited both Ca(2+)- and cyclic AMP-stimulated protein phosphorylation and insulin secretion. These results suggest that staurosporine should not be used as a selective inhibitor of PKC in rat islets.

Effects of cyclosporine A on cyclic AMP generation and GTP-binding proteins in isolated islets

The role of cyclosporine A (CsA) in cAMP generation and its relationship with guanine nucleotide-binding proteins (G-proteins) was investigated in isolated islets. cAMP accumulation in response to glucose, 3-isobutyl-1-methyl-xanthine (a phosphodiesterase inhibitor) and the calcium ionophore A23187 increased significantly (P less than 0.05) in the presence of 0.5 microgram/mL CsA. CsA (0.5 microgram/mL) was unable to affect the 2.1-fold increase in cAMP formation induced by 30 microM forskolin (an adenylate cyclase complex activator). The pertussis toxin-induced cAMP generation in the presence of 20 mM glucose was suppressed by CsA by 34%. On the other hand, CsA enhanced cAMP levels in cholera toxin-treated islets. CsA caused a non-competitive inhibition of phosphodiesterase activity with half-maximal inhibition at 5 micrograms/mL CsA. CsA blocked the pertussis toxin ADP-ribosylation of a 41-kDa and a 21-kDa islet protein, but not the cholera toxin ADP-ribosylation of a 45-kDa and a 21-kDa islet protein. These data indicate that CsA increases cAMP content by a non-competitive inhibition of phosphodiesterase activity and by acting through G-proteins involved in the modulation of adenylate cyclase activity. An inhibitory effect of CsA on a 21-kDa pertussis toxin-sensitive G-protein was also observed.

Insulin secretion and protein phosphorylation in PKC-depleted islets of Langerhans

Protein kinase C (PKC)-dependent phosphorylation of endogenous substrates was measured in electrically permeabilised rat islets of Langerhans. The PKC-activating phorbol ester, 4 beta-phorbol myristate acetate (PMA), caused a slow but prolonged increase in insulin secretion from permeabilised islets, which was accompanied by increased 32P incorporation into several islet proteins of apparent M.W. 30-50 kDa. Depletion of islet PKC by prolonged exposure to PMA abolished subsequent secretory and phosphorylating responses to the phorbol ester. However, PKC-depleted islets did not show diminished responses to glucose, suggesting that PKC-mediated phosphorylation of these proteins is not essential for nutrient-induced insulin secretion.

The role of cytosolic free Ca2+ and protein kinase C in acetylcholine-induced insulin release in the clonal beta-cell line, HIT-T15

We examined the contribution of signal-transduction pathways to acetylcholine-induced insulin release in the clonal beta-cell line HIT-T15. To assess the importance of changes in cytosolic free Ca2+ [( Ca2+]i), we studied time courses of the effects of glucose and acetylcholine on [Ca2+]i and insulin release in quin 2-loaded HIT cells. Incubation in the presence of glucose (2 mM) resulted in a sustained increase in [Ca2+]i in HIT cells from 98 +/- 7 nM to 195 +/- 12 nM measured after 9 min, whereas subsequent addition of acetylcholine (50 microM) produced a transient increase in [Ca2+]i which reached a peak after 30 s (at 274 +/- 10 nM), returning to pre-stimulus levels after 3 min. In contrast, incubation of HIT cells with acetylcholine in the presence of glucose produced a sustained increase in insulin release over and above that stimulated by glucose alone; after 10 min acetylcholine had potentiated glucose-stimulated insulin release by an additional increment of 135%. The transient increase in [Ca2+]i induced by acetylcholine was dose-dependent, and was prevented by omission of glucose or extracellular Ca2+ from the incubation medium. It was also inhibited by inclusion of 50 microM-verapamil in the incubation medium (by 87 +/- 3%) or by decreasing the Na+ concentration in the medium (by 73 +/- 6%). To evaluate the role of the protein kinase C pathway, we have pretreated HIT cells with the phorbol ester 12-O-tetradecanoylphorbol acetate (TPA), to deplete the protein kinase C activity, and have compared their secretory activity with that of control cells. Protein kinase C activity was decreased by 73% in HIT cells cultured in the presence of 200 nM-TPA for 22-24 h. TPA pre-treatment also significantly decreased the insulin content of HIT cells, but had no effect on cell number or the increases in [Ca2+]i induced by glucose or acetylcholine. TPA-pre-treated cells responded comparatively less well to secretagogues than did control cells: glucose-stimulated insulin release was decreased by 40%, whereas potentiation by TPA was significantly decreased by 50% in comparison with control cells (P less than 0.05, n = 24). Acetylcholine (50 microM) potentiated glucose-stimulated insulin release by 61% in control cells. This effect was abolished in HIT cells pre-treated with TPA, whereas these cells still retained their normal secretory response to stimulation by forskolin. These data suggest that an early increase in [Ca2+]i may be important for the initial increase in insulin release induced by acetylcholine in HIT cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Phorbol-ester-induced down-regulation of protein kinase C in mouse pancreatic islets. Potentiation of phase 1 and inhibition of phase 2 of glucose-induced insulin secretion

The influence of down-regulation of protein kinase C on glucose-induced insulin secretion was studied. A 22-24 h exposure of mouse pancreatic islets to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 0.16 microM) in RPMI 1640 culture medium (8.3 mM-glucose, 0.43 mM-Ca2+) abolished TPA (0.16 microM)-induced insulin secretion and led to a potentiation of phase 1 and a decrease in phase 2 of glucose-induced insulin secretion. Thus, although the total insulin release during 40 min of perfusion with glucose (16.7 mM) (45-85 min) was unaffected, the percentage released during phase 1 (45-55 min) was increased from 12.9 +/- 1.5 (4)% in controls to 35.8 +/- 3.9 (4)% in TPA-treated islets (P less than 0.01), and the percentage released during phase 2 (65-85 min) was decreased from 63.2 +/- 3.9 (4)% to 35.3 +/- 1.4 (4)% (P less than 0.005). In contrast, TPA exposure in TCM 199 medium (5.5 mM-glucose, 1.26 mM-Ca2+) caused a total abolition of both phases 1 and 2 of glucose-induced secretion. However, inclusion of the alpha 2-adrenergic agonists adrenaline (10 microM) or clonidine (10 microM), or lowering of the Ca2+ concentration in TCM 199 during down-regulation, preserved and potentiated phase 1 of glucose-induced secretion. Furthermore, perifusion of islets in the presence of staurosporine (1 microM), an inhibitor of protein kinase C, potentiated phase 1 and inhibited phase 2 of glucose-induced secretion. In addition, down-regulation of protein kinase C potentiated phase 1 and inhibited phase 2 of carbamoylcholine (100 microM)-induced insulin secretion at 3.3 mM-glucose, and abolished the potentiating effect of carbamoylcholine (100 microM) at 16.7 mM-glucose. These results substantiate a role for protein kinase C in insulin secretion, and suggest that protein kinase C inhibits phase 1 and stimulates phase 2 of both glucose-induced and carbamoylcholine-induced insulin secretion.

Protein kinase A and/or C inhibitors potentiate isoproterenol-induced cyclic AMP accumulation in intact human lymphocytes

The objective of the present study was to investigate the roles of protein kinase A and/or C in agonist-induced beta adrenoceptor activation in intact human lymphocytes. LYmphocytes from healthy subjects were incubated with isoproterenol and phosphodiesterase inhibitor (IBMX, 1.0 mM) after 20 minutes of preincubation with (or without) various compounds possessing protein kinase A and/or C inhibitory activities. These compounds included the relatively selective protein kinase C (PK-C) inhibitors (W-7, calmidazolium, polymyxin B, neomycin, tamoxifen and clomiphene), purified protein inhibitors of protein kinase A (PK-A) (obtained synthetically, or purified from bovine hearts and porcine hearts) and the two compounds (H-7, H-9), which have been found to inhibit both PK-A and PK-C. The results showed that all PK-C inhibitors alone decreased cellular basal cAMP levels while inhibitors of PK-A as well as both H-7 and H-9 increased basal cAMP levels in a dose dependent manner at certain concentrations. All inhibitors studied potentiated isoproterenol-induced cAMP accumulation. The protein kinase A and C inhibitor, H-7, also potentiated PGE1 (but not forskolin)-induced cAMP accumulation. In contrast, the protein kinase C activator, PMA, inhibited isoproterenol- and PGE1- (but not forskolin) induced cAMP accumulation. These data suggest that the potentiating effects of PK-A and/or C inhibitors may be related to the inhibition of PK-A and/or PK-C, both of which have been shown to be involved in beta 2 adrenoceptor desensitization and phosphorylation.

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 protein kinase C activation on the regulation of the stimulus-secretion coupling in pancreatic beta-cells

Effects of protein kinase C (PKC) activation on the insulin-secretory process were investigated, by using beta-cell-rich suspensions obtained from pancreatic islets of obese-hyperglycaemic mice. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), which is known to activate PKC directly, the muscarinic-receptor agonist carbamoylcholine and high glucose concentration enhanced the phosphorylation of a specific 80 kDa PKC substrate in the beta-cells. At a non-stimulatory glucose concentration, 10 nM-TPA increased insulin release, although there were no changes in either the cytoplasmic free Ca2+ concentration ([Ca2+]i) or membrane potential, as measured with the fluorescent indicators quin-2 and bisoxonol respectively. At a stimulatory glucose concentration TPA caused a lowering in [Ca2+]i, whereas membrane potential was unaffected. Despite the decrease in [Ca2+]i, there was a large stimulation of insulin release. Addition of TPA lowered [Ca2+]i also in beta-cells stimulated by tolbutamide or high K+, although to a lesser extent than in those stimulated by glucose. There was no effect of TPA on either Ca2+ buffering or the ability of Ins(1,4,5)P3 to release Ca2+ in permeabilized beta-cells. However, the phorbol ester inhibited the rise in [Ca2+]i in response to carbamoylcholine, which stimulates the formation of InsP3, in intact beta-cells. Down-regulation of PKC influenced neither glucose-induced insulin release nor the increase in [Ca2+]i. Hence, although PKC activation is of no major importance in glucose-stimulated insulin release, this enzyme can serve as a modulator of the glucose-induced insulin-secretory response. Such a modulation involves mechanisms promoting both amplification of the secretory response and lowering of [Ca2+]i.

Effect of secretagogues on cytosolic free Ca2+ and insulin release at different extracellular Ca2+ concentrations in the hamster clonal beta-cell line HIT-T15

We have examined the relationship between extracellular Ca2+, cytosolic free Ca2+ and insulin release in the clonal beta-cell line HIT-T15. Glucose-stimulated insulin release was dependent on the extracellular Ca2+ concentration in a dose-related manner; the threshold medium Ca2+ concentration for glucose-stimulated insulin release was 0.5 mM. Both forskolin and 12-O-tetradecanoylphorbol 13-acetate (TPA) increased insulin release in the presence of glucose at all extracellular Ca2+ concentration tested (0.1-2.5 mM) but not in the absence of Ca2+. Thus, the threshold medium Ca2+ concentration for glucose-stimulated insulin release was reduced to 0.1 mM by forskolin or TPA. Step-wise increases in the medium Ca2+ concentration in the presence of an initiator of insulin release resulted in a dose-related increase in cytosolic free Ca2+. In the presence of 10 mM glucose, cytosolic free Ca2+ in HIT cells was increased from 60 +/- 5 nM in Ca2+-free medium to 290 +/- 46 nM in medium containing 2.5 mM Ca2+. The effects of increasing extracellular Ca2+ in the presence of 40 mM K+ were similar but considerably more pronounced. Inclusion of either TPA or forskolin in the incubation medium had no significant effect on the steady-state cytosolic free Ca2+ levels in the absence of glucose but in the presence of 10 mM glucose forskolin caused modest (11-18%) increases in steady-state cytosolic free Ca2+ levels at extracellular Ca2+ concentrations of 0.25 mM or above. In contrast, in the presence of glucose TPA significantly reduced the steady-state levels of cytosolic free Ca2+ by 17-21% at extracellular Ca2+ concentrations of 0.25 mM or above. These data provide further evidence that insulin release mediated by activation of beta-cell protein kinases involves primarily an increase in sensitivity of the secretory system to intracellular Ca2+.

The effect of monooleoylglycerol on insulin secretion from isolated perifused rat islets

The effect of monooleoylglycerol on cholecystokinin- and tolbutamide-induced insulin secretion was examined in isolated perifused rat islets. In the presence of 5.5 mmol/l glucose, addition of 10 nmol/l cholecystokinin or 50 mumol/l tolbutamide had practically no effect on insulin secretion. Combined tolbutamide and cholecystokinin led to a biphasic insulin secretory response which was significantly enhanced by addition of 50 mumol/l monooleoylglycerol, an inhibitor of diacylglycerol kinase. Monooleoylglycerol (50 mumol/l) alone had a minimal stimulatory effect on insulin release in the presence of 5.5 mmol/l glucose. Perifusion of islets with 1 mumol/l forskolin had no significant effect on basal insulin secretion in the presence of 5.5 mmol/l glucose, but markedly enhanced the responses to both cholecystokinin plus tolbutamide, and to the combination of cholecystokinin, tolbutamide and monooleoylglycerol. Lowering the glucose level to 2.75 mmol/l abolished the profound stimulatory effect to these agonist combinations on insulin release. Finally, monooleoylglycerol also enhanced the first and second phase insulin secretory responses induced by 20 mmol/l glucose. These results are discussed in relationship to the possible role of protein kinase C in mediating insulin secretion.

Protein phosphorylation in electrically permeabilized islets of Langerhans. Effects of Ca2+, cyclic AMP, a phorbol ester and noradrenaline

The incorporation of 32P from [gamma-32P]ATP into intracellular proteins was studied in electrically permeabilized rat islets of Langerhans. Ca2+ (10 microM), cyclic AMP (100 microM) and a protein kinase C-activating phorbol ester, phorbol 13-myristate 12-acetate (PMA; 100 nM) produced marked changes in the phosphorylation state of a number of proteins in permeabilized islets after incubation for 1 min at 37 degrees C. Ca2+ modified the effects of cyclic AMP and PMA on protein phosphorylation. Noradrenaline (10 microM) had no detectable effects on Ca2+-dependent protein phosphorylation, but significantly inhibited Ca2+-induced insulin secretion from electrically permeabilized islets. These results suggest that electrically permeabilized islets offer a useful model in which to study rapid events in protein phosphorylation as a mechanism of stimulus-secretion coupling. If the rapid Ca2+-induced effects on protein phosphorylation are involved in the control of insulin secretion, the results of this study also imply that part of the catecholamine inhibition of insulin secretion occurs at a stage in the secretory pathway beyond the activation of the regulated protein kinases.