Potentiation of stimulus-induced insulin secretion in protein kinase C-deficient RINm5F cells

The diverse roles of protein kinase C in pancreatic beta-cell function

Members of the serine/threonine PKC (protein kinase C) family perform diverse functions in multiple cell types. All members of the family are activated in signalling cascades triggered by occupation of cell surface receptors, but the cPKC (conventional PKC) and nPKC (novel PKC) isoforms are also responsive to fatty acid metabolites. PKC isoforms are involved in various aspects of pancreatic beta-cell function, including cell proliferation, differentiation and death, as well as regulation of secretion in response to glucose and muscarinic receptor agonists. Recently, the nPKC isoform, PKCepsilon, has also been implicated in the loss of insulin secretory responsiveness that underpins the development of Type 2 diabetes.

Potentiation of insulin secretion by phorbol esters is mediated by PKC-alpha and nPKC isoforms

Culturing clonal beta-cells (HIT-T15) overnight in the presence of phorbol ester [phorbol myristate acetate (PMA)] enhanced insulin secretion while causing downregulation of some protein kinase C (PKC) isoforms and most PKC activity. We show here that this enhanced secretion required the retention of PMA in the cell. Hence, it could not be because of long-lived phosphorylation of cellular substrates by the isoforms that were downregulated, namely PKC-alpha, -betaII, and -epsilon, but could be because of the continued activation of the two remaining diacylglycerol-sensitive isoforms delta and mu. The enhanced secretion did not involve changes in glucose metabolism, cell membrane potential, or intracellular Ca2+ handling, suggesting a distal effect. PMA washout caused the loss of the enhanced response, but secretion was then stimulated by acute readdition of PMA or bombesin. The magnitude of this restimulation appeared dependent on the mass of PKC-alpha, which was rapidly resynthesized during PMA washout. Therefore, stimulation of insulin secretion by PMA, and presumably by endogenous diacylglycerol, involves the activation of PKC isoforms delta and/or mu, and also PKC-alpha.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Fatty acids rapidly induce the carnitine palmitoyltransferase I gene in the pancreatic beta-cell line INS-1

Fatty acids are important metabolic substrates for the pancreatic beta-cell, and long term exposure of pancreatic islets to elevated concentrations of fatty acids results in an alteration of glucose-induced insulin secretion. Previous work suggested that exaggerated fatty acid oxidation may be implicated in this process by a mechanism requiring changes in metabolic enzyme expression. We have therefore studied the regulation of carnitine palmitoyltransferase I (CPT I) gene expression by fatty acids in the pancreatic beta-cell line INS-1 since this enzyme catalyzes the limiting step of fatty acid oxidation in various tissues. Palmitate, oleate, and linoleate (0.35 mM) elicited a 4-6-fold increase in CPT I mRNA. The effect was dose-dependent and was similar for saturated and unsaturated fatty acids. It was detectable after 1 h and reached a maximum after 3 h. The induction of CPT I mRNA by fatty acids did not require their oxidation, and 2-bromopalmitate, a nonoxidizable fatty acid, increased CPT I mRNA to the same extent as palmitate. The induction was not prevented by cycloheximide treatment of cells indicating that it was mediated by pre-existing transcription factors. Neither glucose nor pyruvate and various secretagogues had a significant effect except glutamine (7 mM) which slightly induced CPT I mRNA. The half-life of the CPT I transcript was unchanged by fatty acids, and nuclear run-on analysis showed a rapid (less than 45 min) and pronounced transcriptional activation of the CPT I gene by fatty acids. The increase in CPT I mRNA was followed by a 2-3-fold increase in CPT I enzymatic activity measured in isolated mitochondria. The increase in activity was time-dependent, detectable after 4 h, and close to maximal after 24 h. Fatty acid oxidation by INS-1 cells, measured at low glucose, was also 2-3-fold higher in cells cultured with fatty acid in comparison with control cells. Long term exposure of INS-1 cells to fatty acid was associated with elevated secretion of insulin at a low (5 mM) concentration of glucose and a decreased effect of higher glucose concentrations. It also resulted in a decreased oxidation of glucose. The results indicate that the CPT I gene is an early response gene induced by fatty acids at the transcriptional level in beta- (INS-1) cells. It is suggested that exaggerated fatty acid oxidation caused by CPT-1 induction is implicated in the process whereby fatty acids alter glucose-induced insulin secretion.

Up-regulation of L- and non-L, non-N-type Ca2+ channels by basal and stimulated protein kinase C activation in insulin-secreting RINm5F cells

We studied the effect of protein kinase C (PKC) inhibition and activation on voltage-dependent Ca2+ channels in rat insulinoma RINm5F cells. PKC down-regulation by chronic (24 h) treatment with the PKC activator phorbol 12-myristate 13-acetate (PMA) reduced by about 60% the Ba2+ currents through L- and non-L, non-N-type high-voltage-activated Ca2+ channels, indicating that PKC tonically up-regulates the two main Ca2+ channel subtypes of RINm5F cells under basal conditions. Consistently, PKC activation by acute PMA application caused only a modest increase (average 23%) of Ba2+ currents in a minority of cells (24%). L- and non-L, non-N-type channels were differentially up-regulated by either basal or stimulated PKC activation. Acute up-regulation was predominant on L-type channels and caused an I/V shift of the Ba2+ currents in the hyperpolarizing direction. Non-L, non-N-type channels were less affected by acute PMA application, possibly reflecting a more effective tonic PKC up-regulatory action. Unexpectedly, the increase of Ba2+ currents during acute PMA application was followed by a progressive current decrease, which was also observed in isolation in another 24% of the cells and could be ascribed to PKC-induced ATP depletion, rather than to a direct effect of PKC on Ca2+ channels. We also provide evidence that PKC-mediated phosphorylation is not involved in the G-protein-mediated noradrenergic modulation of Ca2+ channels in RINm5F cells.

Inhibition of glucose-stimulated insulin secretion by Ro 31-8220, a protein kinase C inhibitor

The involvement of the family of protein kinase C (PKC) isoenzymes in the secretory response of rat islets of Langerhans to glucose, the major insulin secretagogue, was investigated using the PKC inhibitor Ro 31-8220, a derivative of staurosporine. Ro 31-8220 was a more selective PKC inhibitor than staurosporine in islets, having minimal effects on protein kinases activated by cyclic AMP or by Ca(2+) and calmodulin. The secretory response to 4βPMA, an activator of phorbol ester-sensitive isoforms of PKC, was abolished by Ro 31-8220. Basal insulin secretion (2MM: glucose) was not affected by Ro 31-8220, but 20MM: glucose-induced insulin release was inhibited in a dose-dependent manner, maximally by ∼50% at 10 µM: Ro 31-8220. Higher concentrations of Ro 31-8220 (507gmM: ) did not further inhibit the secretory response to glucose and also caused ∼50% inhibition of insulin secretion stimulated by 10MM: glyceraldehyde. Ca(2+)-stimulated insulin secretion from electrically permeabilised islets was not inhibited by Ro 31-8220. Calphostin C, which inhibits some isoforms of PKC by interacting with the diacylglycerol binding site, unexpectedly caused a large (∼10-fold) increase in secretion at 2MM: glucose, so could not be used in islets to further investigate the involvement of phorbol ester-sensitive PKC isoforms in the insulin secretory process. One possible explanation for our results using Ro 31-8220 is that phorbol ester-insensitive isoforms of PKC (ζ and/orι) are involved in glucose-stimulated insulin secretion from rat islets.

Vasopressin induces frequency-modulated repetitive calcium transients in single insulin-secreting hit cells

Ca2+ is central to the stimulation of insulin secretion from pancreatic beta-cells. Arginine-vasopressin (AVP) may participate in the modulation of insulin release. In the present study, the AVP-induced changes in cytosolic free Ca2+ ([Ca2+]i) were investigated in single fura-2 loaded insulin-secreting HIT cells. Stimulation with AVP (0.1-5 nM) caused repetitive Ca2+ transients. The frequency but not the amplitude of the Ca2+ transients was modulated by the concentration of AVP. High concentrations of AVP (10-100 nM) triggered a biphasic rise in [Ca2+]i. In Ca(2+)-free medium AVP caused only one or two Ca2+ transients. Withdrawal of extracellular Ca2+ rapidly abolished the AVP-induced Ca2+ transients in all cells tested. The Ca2+ channel blocker, verapamil (50 microM), reduced amplitude and frequency of the Ca2+ transients by about 25% and 60%, respectively, and terminated the Ca2+ transients in 2 of 6 cells. When HIT cells were incubated in Ca(2+)-free medium, and extracellular Ca2+ was restored, there was a small increase in [Ca2+]i. If, however, the agonist-sensitive Ca2+ pool was functionally depleted by repetitive stimulation with high concentrations of AVP or thapsigargin in Ca(2+)-free medium before extracellular Ca2+ was restored, an agonist-independent increase in [Ca2+]i was observed, which was transiently larger than in the control cells, and was mainly preserved in the presence of verapamil. Thus, depletion of the agonist-sensitive Ca2+ pool enhances the influx of extracellular Ca2+ through a Ca2+ entry mechanism independent from verapamil-sensitive voltage-dependent Ca2+ channels (VDCC).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of disruption of actin filaments by Clostridium botulinum C2 toxin on insulin secretion in HIT-T15 cells and pancreatic islets

To examine their role in insulin secretion, actin filaments (AFs) were disrupted by Clostridium botulinum C2 toxin that ADP-ribosylates G-actin. Ribosylation also prevents polymerization of G-actin to F-actin and inhibits AF assembly by capping the fast-growing end of F-actin. Pretreatment of HIT-T15 cells with the toxin inhibited stimulated insulin secretion in a time- and dose-dependent manner. The toxin did not affect cellular insulin content or nonstimulated secretion. In static incubation, toxin treatment caused 45-50% inhibition of secretion induced by nutrients alone (10 mM glucose + 5 mM glutamine + 5 mM leucine) or combined with bombesin (phospholipase C-activator) and 20% reduction of that potentiated by forskolin (stimulator of adenylyl cyclase). In perifusion, the stimulated secretion during the first phase was marginally diminished, whereas the second phase was inhibited by approximately 80%. Pretreatment of HIT cells with wartmannin, a myosin light chain kinase inhibitor, caused a similar pattern of inhibition of the biphasic insulin release as C2 toxin. Nutrient metabolism and bombesin-evoked rise in cytosolic free Ca2+ were not affected by C2 toxin, indicating that nutrient recognition and the coupling between receptor activation and second messenger generation was not changed. In the toxin-treated cells, the AF web beneath the plasma membrane and the diffuse cytoplasmic F-actin fibers disappeared, as shown both by staining with an antibody against G- and F-actin and by staining F-actin with fluorescent phallacidin. C2 toxin dose-dependently reduced cellular F-actin content. Stimulation of insulin secretion was not associated with changes in F-actin content and organization. Treatment of cells with cytochalasin E and B, which shorten AFs, inhibited the stimulated insulin release by 30-50% although differing in their effects on F-actin content. In contrast to HIT-T15 cells, insulin secretion was potentiated in isolated rat islets after disruption of microfilaments with C2 toxin, most notably during the first phase. This effect was, however, diminished, and the second phase became slightly inhibited when the islets were degranulated. These results indicate an important role for AFs in insulin secretion. In the poorly granulated HIT-T15 cells actin-myosin interactions may participate in the recruitment of secretory granules to the releasable pool. In native islet beta-cells the predominant function of AFs appears to be the limitation of the access of granules to the plasma membrane.

Phorbol myristate acetate and ATP-sensitive potassium channels in insulin-secreting cells

The actions of 4 beta-phorbol 12-myristate 13-acetate (PMA; 10-100 nM) on the ATP-sensitive K+ channel have been studied in the RINm5F insulin-secreting cell line. These experiments were carried out using the inside-out patch and open-cell recording configurations of the patch-clamp technique. In the presence of intracellular ATP and ADP, PMA was found to have complex effects. Over short periods of time, i.e., less than 5 min, PMA promoted channel inhibition. However, in the sustained presence of the phorbol ester, this inhibition was only transient and was followed by a period of recovery and then stimulation of the channels. There were no effects of PMA on ATP-sensitive K+ (K+ATP) channels in the absence of intracellular ATP/ADP and in patches of membrane excised from cells that had been pretreated overnight with 1 microM PMA to downregulate endogenous C-kinase. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate (10 nM-1 microM) was also found to have no actions on ATP-sensitive K+ channels. Overall, these data may suggest that through the modulation of protein kinase(s) associated with K+ATP channels, PMA is capable of causing both activation and inhibition of K+ channels in RINm5F cells.

Polymyxin B has multiple blocking actions on the ATP-sensitive potassium channel in insulin-secreting cells

The action of polymyxin B (0.1 microM) on ATP-sensitive K+ (K+ATP) channels in RINm5F insulin-secreting cells was investigated by patch-clamp techniques. Using inside-out patches, open-cells and outside-out patches, polymyxin B was found to block K+ATP channels by, on average, approximately 90-95% of the initial control level of channel activity. The effects were rapid in onset, sustained and readily reversible. Similar effects were found in patches excised from cells pretreated overnight with 1 microM of the phorbol ester phorbol myristate acetate (PMA). External block of channels was associated with a marked decrease in single-channel current amplitude, whereas these effects were not seen when polymyxin B was added to the inside face of the membrane. In patches bathed with internally applied ATP (0.5 mM) and ADP (0.5 mM), polymyxin B inhibited channels but its actions were not reversible upon removal of the compound. However, when the same protocol was undertaken upon cells pre-treated with PMA, the effects of polymyxin B were readily reversed. Our data suggests that polymyxin B is a novel modulator of K+ATP channels, exhibiting multiple blocking actions that may possibly involve a direct effect upon the channel and indirect effects mediated through the inhibition of endogenous protein kinase(s).

Cross-talk between muscarinic- and adenosine-receptor signalling in the regulation of cytosolic free Ca2+ and insulin secretion

The effects of A1-adenosine-receptor occupation on Ca2+ handling in the insulin-secreting RINm5F cell line were investigated. The selective A1-agonist N6-cyclopentyladenosine (CPA) had no effect itself on the cytosolic free Ca2+ concentration in cells loaded with Fura 2. However, CPA (1) attenuated the rise due to activation of voltage-gated Ca2+ channels with Bay K 8644, and (2) caused a secondary increase (EC50 approx. 300 nM) if added after the primary Ca(2+)-mobilizing agonists vasopressin or carbamoylcholine (carbachol). Prior addition of CPA (10 microM) also potentiated (by approx. 20%) the subsequent Ca2+ peak due to maximal (100 microM) carbachol, but did not alter the EC50 of the carbachol response. Detailed analysis of the secondary rise in Ca2+ revealed further features. First, it was due to mobilization from intracellular stores, since it persisted in the absence of extracellular Ca2+. Second, it was associated with a rapid (5-15 s) increase in phospholipase C (PLC) activity, as measured by h.p.l.c. analysis of Ins(1,4,5)P3. This increase was only apparent after prior stimulation with carbachol. Third, and unlike the response to carbachol, it was mediated by a pertussis-toxin-sensitive G-protein. Fourth, it was not secondary to a decrease in cyclic AMP. Fifth, it was absolutely dependent on continued occupation of the primary receptor, since it was abolished if carbachol was displaced with the antagonist atropine. This implies a dynamic cross-talk between the two receptor coupling systems, rather than covalent modification as a result of the prior activation of PLC. Sixth, it was not associated with any desensitization of the ability of CPA to inhibit forskolin-stimulated adenylate cyclase activity. Glyceraldehyde (10 mM)-induced insulin secretion was also potently inhibited by CPA > 10 nM, but the secretory response to 100 microM carbachol was unaffected up to 10 microM. The results suggest that, in vivo, adenosine would inhibit secretion due to carbohydrate nutrients much more effectively than that due to stimuli which activate PLC.

Intracellular pH and the stimulus-secretion coupling in insulin-producing RINm5F cells

The regulation of intracellular pH (pHi) and its role in the insulin-secretory process were evaluated, by using the clonal insulin-secreting cell line RINm5F. Glyceraldehyde, lactate and dihydroxyacetone decreased pHi, but only the first two released insulin. In the presence of extracellular Na+ the cells counteracted the acidification. Blocking the Na+/H+ exchange in acidic cells resulted in a drastic further lowering of pHi, an effect not obtained under basal conditions. Whereas glyceraldehyde depolarized the cells, lactate was without effect. Dihydroxyacetone hyperpolarized the cells in the presence of extracellular Na+, but this effect disappeared when Na+ was excluded from the medium. Stimulation with glyceraldehyde resulted in increased free cytoplasmic Ca2+ concentration ([Ca2+]i). Dihydroxyacetone and lactate had no effect on [Ca2+]i in the presence of Na+, but lactate induced a decrease in [Ca2+]i in Na(+)-deficient medium. In RINm5F cells the activity of the Na+/H+ antiport could not be augmented by activation of protein kinase C (PKC). Hence, in insulin-secreting cells a PKC-insensitive Na+/H+ antiport is the major mechanism restoring a decrease in pHi. Acidification itself does not affect membrane potential, but may directly interact with the mechanisms regulating exocytosis.

Protein kinase C activity affects glucose-induced oscillations in cytoplasmic free Ca2+ in the pancreatic B-cell

Acute stimulation of protein kinase C (PKC) inhibited glucose-induced slow oscillations in cytoplasmic free Ca(2+)-concentration, [Ca2+]i, in mouse pancreatic B-cells. In PKC-depleted cells glucose induced rapid transients in [Ca2+]i, lasting for approximately 10 s, superimposed on the slow oscillations in [Ca2+]i. It was demonstrated that the transients did not occur in the absence of extracellular Ca2+. Each transient typically was preceded by a slow increase in [Ca2+]i, representing the rising phase of an ordinary glucose-induced slow oscillation, and the [Ca2+]i, immediately after a transient was lower than just before the spike. These data further emphasize the interplay between voltage-dependent Ca(2+)-channels and the phospholipase C system in the regulation of B-cell [Ca2+]i-oscillations.

Ca(2+)- and ATP-dependent reversible inactivation of pancreatic islet phosphoinositide-specific phospholipase C activity

Phosphoinositide-specific phospholipase C (PI-PLC) activity in whole homogenates of mouse pancreatic islets decreased 60-85% when the homogenates were incubated at 37 degrees C for 1 h in the presence of down to micromolar concentrations of Ca2+. Ca(2+)-induced inactivation was augmented by calmodulin, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate in the presence of ATP-Mg, and by Mg2+. Inactivation was inhibited when ATP was removed and completely abolished by trifluoperazine and EGTA. Inactivation was not affected by the non-phosphorylating ATP analogue, AMP-PCP, GMP-PNP, glucose, Zn2+ or a series of protease inhibitors. These observations suggest that PI-PLC in broken cell preparations of pancreatic islets may be inactivated via phosphorylation by Ca(2+)-calmodulin-stimulated protein kinase and/or protein kinase C. Inactivation of PI-PLC was reversible. Reactivation started after approx. 2 h incubation, when the concentration of ATP in the homogenate was below 0.15 x 10(-6) M. PI-PLC activity returned to values approx. 25% higher than the initial values. PI-PLC inactivation via phosphorylation by the mentioned protein kinases may constitute a feedback control on the phosphoinositide response, attenuating subsequent diacylglycerol formation and/or Ca2+ mobilization by inositol trisphosphate.

Activation of protein kinase C is not required for glyceraldehyde-stimulated insulin secretion from rat islets

Glyceraldehyde-induced insulin release from rat islets of Langerhans was not affected following down-regulation of protein kinase C (PKC) by prolonged exposure to the tumour-promoting phorbol ester, 4 beta-phorbol myristate acetate (PMA). Glyceraldehyde did not cause translocation of islet PKC under conditions in which PMA stimulated redistribution of enzyme activity. These results indicate that activation of PKC is not required for glyceraldehyde stimulation of insulin secretion from normal rat islets.

Activation of voltage-sensitive Ca2+ currents by vasopressin in an insulin-secreting cell line

The effect of vasopressin on voltage-sensitive Ca2+ currents in the rat insulinoma cell line RINm5F has been investigated in patch-clamp whole-cell and single-channel current recording experiments. In the whole-cell recording configuration the dominant inward current in the presence of tetrodotoxin was noninactivating and had a high voltage threshold. This current was much enhanced when external Ca2+ was replaced by Ba2+ and was blocked by 1 microM nifedipine. It can therefore be classified as an L-current. Vasopressin enhanced the L-current without changing the voltage threshold of activation or the voltage at which the peak current was observed. Vasopressin effects were seen at concentrations as low as 0.01 nM, and the maximal effect was observed at about 1 nM. In higher concentrations the vasopressin effects were weaker, with effects at 50 nM of about the same magnitude as at 0.01 nM. In single-channel current recording experiments carried out with the cell-attached configuration there were no effects on single L-channel currents when vasopressin was added to the bath solution, but in experiments in which vasopressin (5 nM) was infused into the patch pipette a marked increase in the apparent channel open state probability was observed. We conclude that vasopressin, a peptide that is known to markedly enhance glucose-evoked insulin secretion, stimulates opening of the voltage-sensitive Ca2+ channels in insulin-secreting cells.