Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells. Modulation in vivo by calcium, GTP, and pertussis toxin.

The gamma subunits of trimeric G-proteins (gamma1, gamma2, gamma5, and gamma7 isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTPgammaS significantly stimulated the carboxyl methylation selectively of gamma2 and gamma5 isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists--a stimulatory concentration of glucose or a depolarizing concentration of K+--resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either. The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alphabetagamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+. Glucose-induced carboxyl methylation of gamma2 and gamma5 isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE2, which activates Gi and Go in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma2 and gamma5 isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose- and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits via nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.

Evidence of a role for GTP in the potentiation of Ca(2+)-induced insulin secretion by glucose in intact rat islets.

Glucose initiates insulin secretion by closing K(+)-ATP channels, leading to Ca2+ influx (E1); it also potentiates Ca(2+)-induced secretion (E2) when the K(+)-ATP channel is kept open using diazoxide and depolarizing concentrations of K+ are provided. To examine the roles of purine nucleotides in E2, we compared the effects of glucose to those of the mitochondrial fuel monomethylsuccinate. Either agonist could induce E2 accompanied by significant increases in ATP, ATP/ADP ratio, and GTP/GDP ratio; GTP increased significantly only with glucose. Mycophenolic acid (MPA), an inhibitor of cytosolic GTP synthesis, markedly inhibited glucose-induced E2 (either in perifusions or in static incubations) and decreased GTP and the GTP/GDP ratio, but did not alter the ATP/ADP ratio. Provision of guanine (but not adenine) reversed these changes pari passu. In contrast, MPA had no effect on succinate-induced E2, despite generally similar changes in nucleotides. A similar lack of effect of MPA on E2 was seen with a second mitochondrial fuel, alpha-ketoisocaproic acid (KIC). However, in the absence of diazoxide and K+, MPA blunted the secretory effects of either glucose, succinate, or KIC. These studies suggest that GTP plays a role in both glucose and succinate or KIC-induced insulin secretion at a step dependent on mitochondrial metabolism and the K(+)-ATP channel. In addition to mitochondrial effects, glucose appears to have extramitochondrial effects important to its potentiation of Ca(2+)-induced insulin secretion that are also dependent on GTP.

Small elevations of glucose concentration redirect and amplify the synthesis of guanosine 5'-triphosphate in rat islets.

Recent studies suggest a permissive requirement for guanosine 5'-triphosphate (GTP) in insulin release, based on the use of GTP synthesis inhibitors (such as myocophenolic acid) acting at inosine monophosphate (IMP) dehydrogenase; herein, we examine the glucose dependency of GTP synthesis. Mycophenolic acid inhibited insulin secretion equally well after islet culture at 7.8 or 11.1 mM glucose (51% inhibition) but its effect was dramatically attenuated when provided at < or = 6.4 mM glucose (13% inhibition; P < 0.001). These observations were explicable by a stimulation of islet GTP synthesis derived from IMP since, at high glucose: (a) total GTP content was augmented; (b) a greater decrement in GTP (1.75 vs. 1.05 pmol/islet) was induced by mycophenolic acid; and (c) a smaller "pool" of residual GTP persisted after drug treatment. Glucose also accelerated GTP synthesis from exogenous guanine ("salvage" pathway) and increased content of a pyrimidine, uridine 5'-triphosphate (UTP), suggesting that glucose augments production of a common regulatory intermediate (probably 5-phosphoribosyl-1-pyrophosphate). Pathway-specific radiolabeling studies confirmed that glucose tripled both salvage and de novo synthesis of nucleotides. We conclude that steep changes in the biosynthesis of cytosolic pools of GTP occur at modest changes in glucose concentrations, a finding which may have relevance to the adaptive (patho) physiologic responses of islets to changes in ambient glucose levels.

Glucose- and GTP-dependent stimulation of the carboxyl methylation of CDC42 in rodent and human pancreatic islets and pure beta cells. Evidence for an essential role of GTP-binding proteins in nutrient-induced insulin secretion.

Several GTP-binding proteins (G-proteins) undergo post-translational modifications (isoprenylation and carboxyl methylation) in pancreatic beta cells. Herein, two of these were identified as CDC42 and rap 1, using Western blotting and immunoprecipitation. Confocal microscopic data indicated that CDC42 is localized only in islet endocrine cells but not in acinar cells of the pancreas. CDC42 undergoes a guanine nucleotide-specific membrane association and carboxyl methylation in normal rat islets, human islets, and pure beta (HIT or INS-1) cells. GTPgammaS-dependent carboxyl methylation of a 23-kD protein was also demonstrable in secretory granule fractions from normal islets or beta cells. AFC (a specific inhibitor of prenyl-cysteine carboxyl methyl transferases) blocked the carboxyl methylation of CDC42 in five types of insulin-secreting cells, without blocking GTPgammaS-induced translocation, implying that methylation is a consequence (not a cause) of transfer to membrane sites. High glucose (but not a depolarizing concentration of K+) induced the carboxyl methylation of CDC42 in intact cells, as assessed after specific immunoprecipitation. This effect was abrogated by GTP depletion using mycophenolic acid and was restored upon GTP repletion by coprovision of guanosine. In contrast, although rap 1 was also carboxyl methylated, it was not translocated to the particulate fraction by GTPgammaS; furthermore, its methylation was also stimulated by 40 mM K+ (suggesting a role which is not specific to nutrient stimulation). AFC also impeded nutrient-induced (but not K+-induced) insulin secretion from islets and beta cells under static or perifusion conditions, whereas an inactive structural analogue of AFC failed to inhibit insulin release. These effects were reproduced not only by S-adenosylhomocysteine (another methylation inhibitor), but also by GTP depletion. Thus, the glucose- and GTP-dependent carboxyl methylation of G-proteins such as CDC42 is an obligate step in the stimulus-secretion coupling of nutrient-induced insulin secretion, but not in the exocytotic event itself. Furthermore, AFC blocked glucose-activated phosphoinositide turnover, which may provide a partial biochemical explanation for its effect on secretion, and implies that certain G-proteins must be carboxyl methylated for their interaction with signaling effector molecules, a step which can be regulated by intracellular availability of GTP.

Mobilization of cellular Ca2+ by lysophospholipids in rat islets of Langerhans.

To determine whether lysophospholipids mobilize cellular Ca2+, intact rat islets were prelabelled with 45Ca2+ and subjected to three maneuvers designed to simulate the physiologic accumulation of lysophospholipids: (1) exogenous provision; (2) addition of porcine pancreatic phospholipase A2; and (3) provision of p-hydroxymercuribenzoic acid, which impedes both the reacylation and hydrolysis of endogenous lysophospholipids, leading to their accumulation in islets. Each maneuver provoked 45Ca2+ efflux at concentrations nearly identical to those previously reported to induce insulin release in the absence of toxic effects on the islets. Lysophosphatidylcholine (lysoPC) and lysophosphatidylinositol were active, whereas the ethanolamine and serine derivatives, and lysophosphatidic acid, were much less effective. The effects of lysoPC were reversible; they also were reduced by lanthanum or gentamicin (which are probes of superficial, plasma membrane-bound stores of Ca2+) or by prior depletion of membrane-bound cellular Ca2+ stores using ionomycin, but not by removal of extracellular Ca2+ or Na+. The effects of lysoPC, phospholipase A2 and p-hydroxymercuribenzoic acid were largely independent of any hydrolysis to, or accumulation of, free fatty acids as assessed by resistance to dantrolene or trifluoperazine (which selectively reduce arachidonic acid-induced 45Ca2+ efflux and insulin release). Thus, lysophospholipids are a newly recognized class of lipid mediators which may promote insulin release at least in part via mobilization of a pool(s) of Ca2+ ('trigger Ca2+') bound in the plasma membrane and possibly in other cellular membranes.

Regulation of guanine-nucleotide binding proteins in islet subcellular fractions by phospholipase-derived lipid mediators of insulin secretion.

In the accompanying article (Kowluru, A., Rabaglia, M.E., Mose, K.E. and Metz, S.A. (1994) Biochim. Biophys. Acta 1222, 348-359) we identified three specific GTPase activities in islet subcellular fractions; most notably, two of these were enriched in the secretory granules. In the present study, we describe the regulation of GTPase activity in subcellular fractions of normal rat and human islets by insulinotropic lipids with a similar rank order as their insulin-releasing capacity. Arachidonic acid (AA), lysophosphatidylcholine (LPC), or phosphatidic acid (PA) inhibited the GTPase activities significantly (by 60-80%) in islet homogenates; each also selectively inhibited certain GTPases in specific individual fractions. Less insulinotropic fatty acids, such as linoleic acid and oleic acid, inhibited GTPase to a lesser degree, whereas lysophosphatidic acid (LPA), phosphatidylcholine (PC) or palmitic acid, which do not acutely promote secretion, were ineffective. Similar inhibitory effects of these lipids were also demonstrable in fractions of human islets as well as those of transformed beta-cells (HIT cells). The effects of lipids were not attributable to their detergent properties (since several detergents failed to mimic lipid effects) or to inhibition of GTP binding (since they actually increased GTP gamma S binding modestly, and moreover, in reconstituted fractions, they potentiated GDP/GTP exchange activity up to 2-fold). These data indicate that the insulinotropic nature of the lipids might be due, in part, to their ability to maintain G-proteins in their GTP-bound (active) configuration by increasing GTP binding and decreasing its hydrolysis. These studies comprise the first evidence for the regulation by biologically active lipids of endocrine cell G-proteins at a locus distal to plasma membrane events (i.e., on endocrine secretory granules), and provide thereby a possible novel mechanism whereby the activation of islet endogenous phospholipases might culminate in insulin exocytosis.

Subcellular localization and kinetic characterization of guanine nucleotide binding proteins in normal rat and human pancreatic islets and transformed beta cells.

The subcellular localization and the kinetics of the GTPase activities of monomeric and heterotrimeric GTP-binding proteins were investigated in normal rat and human pancreatic islets and were compared to those obtained using a transformed hamster beta cell line (HIT cells). The [alpha-32P]GTP overlay technique revealed the presence of at least four low-molecular-mass proteins (approx. 20-27 kDa) in normal rat islets, which were enriched in the secretory granule fraction compared to the membrane fraction (with little abundance of these proteins in the cytosolic fraction). In contrast, in HIT cells, these proteins (at least six) were predominantly cytosolic. Three of these proteins were immunologically identified as rab3A, rac2, and CDC42Hs in islets as well as in HIT cells. In addition, pertussis toxin augmented the ribosylation of at least one heterotrimeric G-protein of about 39 kDa (probably G(i) and/or G(o)) in the membrane and secretory granule fractions of normal rat and human islets, whereas at least three such Ptx substrates (36-39 kDa) were found in HIT cell membranes. Kinetic activities revealed the presence of at least three such activities (Km for GTP of 372 nM, 2.2 microM, and 724 microM) in islet homogenates which were differentially distributed in various subcellular fractions; similar activities were also demonstrable in HIT cell homogenates. Thus, these studies demonstrate the presence of both monomeric G-proteins intrinsic to the secretory granules of normal rat islets which can be ascribed to beta cells; since these G-proteins are regulated by insulinotropic lipids (as described in the accompanying article), such proteins may couple the activation of phospholipases (endogenous to islets) to the exocytotic secretion of insulin. These findings also suggest that caution is necessary in extrapolating data concerning G-proteins from cultured, transformed beta cell lines to the physiology of normal islets, in view of both qualitative and quantitative differences between the two preparations.

p53-independent induction of p21(waf1/cip1) contributes to the activation of caspases in GTP-depletion-induced apoptosis of insulin-secreting cells.

We investigated the role of some key regulators of cell cycle in the activation of caspases during apoptosis of insulin-secreting cells after sustained depletion of GTP by a specific inosine 5'-monophosphate dehydrogenase inhibitor, mycophenolic acid (MPA). p21(Waf1/Cip1) was significantly increased following MPA treatment, an event closely correlated with the time course of caspase activation under the same conditions. MPA-induced p21(Waf1/Cip1) was not mediated by p53, since p53 mass was gradually reduced over time of MPA treatment. The increment of p21(Waf1/Cip1) by MPA was further enhanced in the presence of a pan-caspase inhibitor, indicating that the increased p21(Waf1/Cip1) may occur prior to caspase activation. This notion of association of p21(Waf1/Cip1) accumulation with caspase activation and apoptosis was substantiated by using mimosine, a selective p21(Waf1/Cip1) inducer independent of p53. Mimosine, like MPA, also increased p21(Waf1/Cip1), promoted apoptosis and simultaneously increased the activity of caspases. Furthermore, knocking down of p21(Waf1/Cip1) transfection of siRNA duplex inhibited caspase activation and apoptosis due to GTP depletion. In contrast to p21(Waf1/Cip1), a reduction in p27(Kip1) occurred in MPA-treated cells. These results indicate that p21(Waf1/Cip1) may act as an upstream signal to block mitogenesis and activate caspases which in turn contribute to induction of apoptosis.

Epinephrine impairs insulin release by a mechanism distal to calcium mobilization. Similarity to lipoxygenase inhibitors.

The mechanisms that enable epinephrine (EPI) and lipoxygenase inhibitors to impede insulin secretion are unknown. We examined the possibility that EPI inhibits Ca2+ fluxes as its major mechanism by studying 45Ca efflux from prelabeled, intact rat islets. EPI (2.5 x 10(-7) to 1 x 10(-5) M) inhibited insulin release induced by the influx of extracellular Ca2+ (46 mM K+) or the mobilization of intracellular Ca2+ stores (2 mM Ba2+), but it did not reduce the 45Ca efflux stimulated by either agonist. EPI also nullified insulin release induced by isobutylmethylxanthine or dibutyryl cAMP, with minimal or no effects on 45Ca efflux, and blocked the insulinotropic effects of 12-O-tetradecanoylphorbol-13-acetate (a direct activator of protein kinase C), which is believed primarily to sensitize the exocytotic apparatus to Ca2+ without mobilizing additional Ca2+. Previously we reported that similar effects were induced by inhibitors of pancreatic islet lipoxygenase. In this study, however, pretreatment with either the alpha 2-adrenergic antagonist yohimbine or pertussis toxin did not block the effects of lipoxygenase inhibitors, although either agent did block the effects of EPI. Thus, EPI, via an alpha 2-receptor mechanism, is able to reduce exocytosis largely distal to, or independent of, changes in Ca2+ flux, cAMP formation or its Ca2+-mobilizing action, or generation of protein kinase C activators. Therefore, EPI may reduce the sensitivity of the exocytotic apparatus to Ca2+. Inhibition of islet lipoxygenase may have a similar effect; however, in this case, the effect would have to be unrelated, or distal, to stimulation of alpha 2-receptors.

Exogenous arachidonic acid promotes insulin release from intact or permeabilized rat islets by dual mechanisms. Putative activation of Ca2+ mobilization and protein kinase C.

A number of indirect studies suggest a role for endogenous arachidonic acid (AA) in pancreatic islet function. To probe the effects of this fatty acid, AA and other polyunsaturated fatty acids were exogenously provided in Ca2+-free medium to avoid the formation of insoluble or impermeant Ca2+-arachidonate complexes. Concentrations of AA of greater than or equal to 3 microM induced potent and sustained but reversible 45Ca efflux from prelabeled intact (or digitonin-permeabilized) islets; AA also induced insulin release at somewhat higher concentrations. Other unsaturated fatty acids (erucic, oleic, linoleic, linolenic, dihomo-gamma-linolenic, eicosapentaenoic, docosahexaenoic acids) were generally less active than AA itself, indicating a structure-function relationship. The effects of AA were saturable, were inhibitable by cooling, and were not accompanied in parallel by 51Cr release or trypan blue retention, suggesting a nontoxic mechanism. At low concentrations (3.3-16 microM), at which AA does not stimulate insulin release, AA-induced 45Ca efflux was not reduced by pretreatment with ionomycin (to deplete membrane-bound Ca2+ stores), suggesting stimulation of Ca2+ extrusion through the plasma membrane. At higher concentrations (greater than or equal to 25 microM), at which AA promotes insulin release, further Ca2+ efflux was stimulated, which was blunted by pretreatment with ionomycin (as well as by trifluoperazine). Conversely, pretreatment with AA obliterated the effects of ionomycin (3 microM) on cellular Ca2+ mobilization. Thus, AA also mobilizes Ca2+ from intracellular organelles, leading to a rise in free cytosolic Ca2+ (as previously reported). AA-induced 45Ca efflux and insulin release were independent of the presence of extracellular Na+ and did not require the oxygenation of AA. Dose-response curves comparing 45Ca efflux and insulin secretion suggested that AA also stimulates hormone release by at least one other mechanism in addition to Ca2+ mobilization. This second stimulatory effect of AA could be seen in digitonin-permeabilized islets, where changes in cytosolic free Ca2+ concentration were vitiated by EGTA-containing buffers. Such secretion was also saturable and was inhibited by cooling or by spermine (which inhibits protein kinase C in the islet). Furthermore, AA-induced secretion from either intact or permeabilized islets was blunted by prolonged pretreatment of islets with a phorbol ester to deplete them of protein kinase C. Thus, exogenous arachidonic acid seems to be a complete secretagogue, having stimulatory effects both on Ca2+ mobilization and Ca2+-related secretory processes, putatively the activation of protein kinase C.

Perspectives in diabetes. Is protein kinase C required for physiologic insulin release?

Extant data suggest that a Ca2+- and phospholipid-dependent protein kinase C (PKC) exists (as a single enzyme or possibly a family of related enzymes) in rodent beta-cells. PKC activators probably induce secretion primarily through phosphorylation of key proteins, thereby sensitizing the exocytotic apparatus to Ca2+. PKC can be activated by several pharmacologic probes and by endogenous diacylglycerol (and possibly arachidonic acid) released by nutrient-activated phospholipases. Several nonspecific pharmacologic agents inhibit both PKC and physiologic insulin release. However, when a more specific inhibitor of PKC, H7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], was studied, it did not reduce glucose-induced insulin secretion. Moreover, prolonged preexposure of islets to a phorbol ester (believed to induce selective depletion of PKC) also failed to substantially reduce the subsequent secretory response to glucose. Thus, indisputable evidence for an obligatory physiological role of PKC in the islet is still missing, and the enzyme's status as a critical coupling signal should be viewed as putative only.

The pancreatic islet as Rubik's Cube. Is phospholipid hydrolysis a piece of the puzzle?

At least three types of phospholipase exist in the beta-cells of the pancreatic islet. Data regarding their physiological activation are incomplete but suggest that glucose (or its metabolite glyceraldehyde) either activates or potentiates the activation of several phospholipases. At least seven phospholipid hydrolysis by-products (diacylglycerol, myo-inositol 1,4,5-trisphosphate, lysophospholipids, arachidonic acid and its cyclooxygenase- and lipoxygenase-derived metabolites, phosphatidate) have been demonstrated to have effects compatible with their postulated roles as mediators or modulators of islet function. Presumptive mechanisms of action have been tentatively identified for these metabolites. However, key studies in the puzzle are missing, and current methodologies have important limitations. Shortcomings include the paucity of measurements of the mass of metabolites; the frequent use of static incubations rather than perfusions; a lack of complete time- and agonist concentration-dependence curves; the equation of metabolite accumulation with rates of metabolite generation (which ignores metabolite removal as a key variable); the use of nonspecific, insensitive, or ambiguous phospholipase assays; and the need for more studies directly correlating lipid metabolism and insulin secretion in physiologically functioning preparations. Like Rubik's Cube, the pancreatic islet is a dynamic puzzle comprised of many interrelated components requiring proper alignment and integration. Phospholipid turnover is one "panel" in the islet; however, an obligate role for phospholipase activation in glucose-induced insulin secretion is not yet rigorously established, despite tantalizing, inferential evidence. It may be that glucose serves principally to potentiate the phospholipase and secretory responses to other signals that act by initiating phospholipid hydrolysis.

Ether-linked lysophospholipids initiate insulin secretion. Lysophospholipids may mediate effects of phospholipase A2 activation on hormone release.

Phospholipase A2 activation may be a pivotal step in glucose-induced insulin secretion; however, recent studies have focused on only one by-product (arachidonic acid). To examine the possible role of the other by-product (lysophospholipids), the lysoderivatives of alkylacyl- (ether linked) or diacylphospholipids were applied to rat islets in static incubations. 1-O-alkyl-2-lyso-sn-glyceryl-3-phosphorylcholine [lyso-PAF, the precursor of platelet-activating factor (PAF)] or lysophosphatidylcholine initiated insulin release at 1.7 mM glucose. Two preparations of PAF itself (0.005-5000 ng/ml) were without effect at 1.7 or 16.7 mM glucose, but PAF was nearly equipotent to lyso-PAF at greater than or equal to 20 micrograms/ml. A precursor-product relationship was suggested because the precursors (alkylacyl- or diacylglyceryl-phosphorylcholine) of all three active metabolites were inactive. The stimulatory effect of lyso-PAF is largely independent of any toxic or lytic effect, being biphasic, reversible, unassociated with impairment of the subsequent physiologic functioning of treated islets, and inhibitable (by Ni2+, La3+, or nordihydroguaiaretic acid but not by other lipoxygenase inhibitors). It also occurred at threshold concentrations at which islet morphology and 51Cr retention were preserved. Furthermore, lyso-PAF-induced insulin secretion was markedly impaired by reduced ambient temperature (16 degrees C) or by the impermeant anion isethionate, further implying initiation of true exocytotic granule release and fission. Lyso-PAF (but not arachidonic acid) also circumvented the inhibition of glucose-induced insulin release caused by phospholipase inhibitors. Generation of endogenous lysophospholipids through exogenous application of phospholipase A2 also initiated insulin release, an effect responding to a panel of potential inhibitors identically to that induced by exogenously provided lysophospholipids. We propose that glucose activates phospholipase A2 in the pancreatic islet, leading to the generation of lysophospholipids; the latter may couple energy production to insulin release, at least in part via the promotion of Ca2+ translocation.

Phasic glucose-stimulated insulin secretion by neonatal rat pancreatic islet cells. Enhancement by sodium salicylate.

UNLABELLED: Monolayer cultures of neonatal rat pancreatic islets were superfused to examine phasic insulin secretion. When stimulated with a constant glucose concentration of 300 mg/dl, insulin secretion promptly rose to a peak more than eightfold higher than the basal levels observed with glucose 30 mg/dl. This first-phase peak was followed by a quick decline in insulin to a level about four- to fivefold higher than basal, representing second-phase insulin secretion. Addition of sodium salicylate 20 mg/dl enhanced glucose-stimulated insulin secretion. Addition of salicylate concurrently with glucose greatly enhanced second-phase insulin secretion, but did not affect the first-phase peak, thereby converting the biphasic pattern to one that appeared to be monophasic. However, when cultures were superfused with salicylate both before and concurrent with stimulation by glucose, both the first-phase peak and the second phase of insulin secretion were increased, resulting in not only preservation but enhancement of the biphasic pattern. Salicylate had no effect on basal insulin secretion at glucose 30 mg/dl. During the transition from glucose 300 mg/dl to 30 mg/dl, immediately as glucose concentration began to fall in the superfusate, insulin secretion showed a transient increase ("off response"). CONCLUSIONS: (1) Cultures of neonatal rat pancreatic islet cells respond with a biphasic pattern of insulin secretion when exposed to a continuous and constant glucose stimulus. (2) When endogenous prostaglandin synthesis is inhibited by sodium salicylate, the biphasic pattern not only remains but is enhanced, indicating that endogenous prostaglandin synthesis exerts a tonic restraint throughout the entire period of glucose-stimulated phasic insulin release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects on glucose-induced insulin secretion of lipoxygenase-derived metabolites of arachidonic acid.

Our previous data suggested that lipoxygenation of endogenously released arachidonic acid (AA) is a critical step in stimulus-secretion coupling in the pancreatic beta cell. In the current study using monolayer cultures of neonatal rat islet cells, exogenous arachidonic acid (AA) (5 micrograms/ml) potently stimulated insulin release in the presence of a substimulatory glucose concentration, and potentiated release induced by glucose. Since the latter stimulatory effect of AA is prevented by inhibitors of the lipoxygenase pathway, we examined the effects of various lipoxygenase pathway products on glucose-induced insulin secretion. The mediator was not one of the stable end-products of either limb of the lipoxygenase pathway: 12- or 5-hydroxyeicosatetraenoic acid (HETE) (0.5-2000 ng/ml) did not alter insulin release, whereas 11-HETE, 15-HETE, leukotriene (LT)B4 and the delta 6 trans isomers of LTB4, LTC4 and 11-trans LTC4 all inhibited insulin release. Furthermore, diethylcarbamazine, a selective leukotriene synthesis inhibitor, did not prevent AA- or glucose-induced insulin release, arguing against a role for LTs as the mediator of AA's stimulatory effect. However, the unstable intermediate 12-hydroperoxyeicosatetraenoic acid (12-HPETE), and positional isomers of 12-HPETE, potentiated glucose-induced insulin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual functional effects of interleukin-1beta on purine nucleotides and insulin secretion in rat islets and INS-1 cells.

Interleukin-1beta (IL-1beta) has been shown to inhibit glucose-induced insulin secretion from rat islets and purified beta-cells, primarily through the generation of nitric oxide (NO). However, the mechanisms by which NO exerts its effects remain unclear. To examine the role of purine nucleotides, we cultured intact rat islets or INS-1 (glucose-responsive transformed rat) beta-cells for 18 h in the presence or absence of IL-1beta. In islets, the exposure to IL-1beta (100 pmol/l) inhibited subsequent glucose-induced insulin secretion by 91% with no significant effect on insulin content or basal insulin release. IL-1beta also diminished insulin secretion induced by pure mitochondrial fuels, 40 mmol/l K+, or a phorbol ester. Concomitantly, IL-1beta significantly decreased islet ATP (-45%), GTP (-33%), ATP/ADP (-54%), and GTP/GDP (-46%). These effects were totally reversed by provision of N(omega)-nitro-L-arginine methyl ester (NAME) in arginine-free media that inhibited NO production. In contrast, in INS-1 cells, IL-1beta (10 or 100 pmol/l) reduced both basal and glucose-induced insulin secretion by 50%, but insulin content was also reduced by 35%. Therefore, the INS-1 cells were still able to respond to glucose stimulation with a 1.8-2.0-fold increase in insulin release in either the presence or absence of IL-1beta. Concomitantly, in INS-1 cells, IL-1beta had no effect on ATP/ADP or GTP/GDP ratios, although it modestly decreased ATP (-25%) and GTP (-22%). As in islets, all effects of IL-1beta in INS-1 cells were prevented by NAME. Thus, in rat islets, IL-1beta (via the generation of NO) abolishes insulin exocytosis in association with large decreases in the ATP/ADP (and GTP/GDP) ratio, implying the impairment of mitochondrial function. Furthermore, IL-1beta inhibits cytosolic synthesis of new purine nucleotides (via the salvage pathway), as assessed by a decrease in their specific activity after labeling with [3H]hypoxanthine. In contrast, in INS-1 cells, IL-1beta appears to impair cytosolic synthesis of purine nucleotides and insulin biosynthesis selectively (both possibly reflecting decreased glycolysis) with little direct effect on insulin exocytosis itself.

A defect late in stimulus-secretion coupling impairs insulin secretion in Goto-Kakizaki diabetic rats.

A widely accepted genetically determined rodent model for human type 2 diabetes is the Goto-Kakizaki (GK) rat; however, the lesion(s) in the pancreatic islets of these rats has not been identified. Herein, intact islets from GK rats (aged 8-14 weeks) were studied, both immediately after isolation and after 18 h in tissue culture. Despite intact contents of insulin and protein, GK islets had markedly deficient insulin release in response to glucose, as well as to pure mitochondrial fuels or a non-nutrient membrane-depolarizing stimulus (40 mmol/l K+). In contrast, mastoparan (which activates GTP-binding proteins [GBPs]) completely circumvented any secretory defect. Basal and stimulated levels of adenine and guanine nucleotides, the activation of phospholipase C by Ca2+ or glucose, the secretory response to pertussis toxin, and the activation of selected low-molecular weight GBPs were not impaired. Defects were found, however, in the autophosphorylation and catalytic activity of cytosolic nucleoside diphosphokinase (NDPK), which may provide compartmentalized GTP pools to activate G-proteins; a deficient content of phosphoinositides was also detected. These studies identify novel, heretofore unappreciated, defects late in signal transduction in the islets of our colony of GK rats, possibly occurring at the site of activation by NDPK of a mastoparan-sensitive G-protein-dependent step in exocytosis.

Inhibition of prostaglandin E synthesis augments glucose-induced insulin secretion is cultured pancreas.

Monolayer cultures of neonatal rat pancreatic cells were examined to ascertain whether they synthesize prostaglandin E (PGE) and to determine the effects on insulin secretion caused by PGE and drugs that inhibit its synthesis. PGE release into the medium was observed. Sodium salicylate and ibuprofen (at drug concentrations similar to those achieved therapeutically in humans in vivo) inhibited PGE synthesis in a dose-responsive fashion to a maximum of 70-80% inhibition. Inhibition of PGE synthesis was accompanied by augmented insulin secretion. Both PGE synthesis inhibitors shifted the glucose dose-insulin response curves to the left at low glucose concentrations and augmented maximal insulin release at high glucose concentrations. Increments in glucose-induced insulin secretion induced by sodium salicylate correlated well (r = 0.89) with inhibition of PGE synthesis and addition of exogenous PGE1 to the cultures reversed the augmenting effects of the drug on insulin secretion. It is concluded that cultures of pancreatic cells synthesize PGE and that a function of PGE in these cultures appears to be a tonic negative modulation of glucose-induced insulin secretion.

Genetic obesity unmasks nonlinear interactions between murine type 2 diabetes susceptibility loci.

Nonlinear interactions between obesity and genetic risk factors are thought to determine susceptibility to type 2 diabetes. We used genetic obesity as a tool to uncover latent differences in diabetes susceptibility between two mouse strains, C57BL/6J (B6) and BTBR. Although both BTBR and B6 lean mice are euglycemic and glucose tolerant, lean BTBR x B6 F1 male mice are profoundly insulin resistant. We hypothesized that the genetic determinants of the insulin resistance syndrome might also predispose genetically obese mice to severe diabetes. Introgressing the ob allele into BTBR revealed large differences in diabetes susceptibility between the strain backgrounds. In a population of F2-ob/ob mice segregating for BTBR and B6 alleles, we observed large variation in pancreatic compensation for the underlying insulin resistance. We also detected two loci that substantially modify diabetes severity, and a third locus that strongly links to fasting plasma insulin levels. Amplification of the genetic signal from these latent diabetes susceptibility alleles in F2-ob/ob mice permitted discovery of an interaction between the two loci that substantially increased the risk of severe type 2 diabetes.

Prostaglandin E2 metabolite levels during diabetic ketoacidosis.

Insulin therapy was withdrawn from 15 well-controlled type I diabetic subjects for no longer than 18 h to examine the sequence with which 13,14-dihydro-15-keto-PGE2 (PGE-m), glucagon, norepinephrine, and epinephrine increased in circulating blood in diabetic subjects becoming ketoacidotic. Fourteen of 15 patients had increments in PGE-m; 12/12, 12/15, and 13/15 had increments in glucagon, norepinephrine, and epinephrine, respectively. Six of the 15 patients developed mild diabetic ketoacidosis (DKA) by 12-18 h; all had nonmeasurable C-peptide levels. This DKA group had significantly greater increments of PGE-m (835 +/- 130 versus 276 +/- 111 pg/ml, mean +/- SEM, P less than 0.01) but not glucagon, norepinephrine, or epinephrine compared with the 9 non-DKA patients. In the DKA group, there were significant PGE-m and glucagon increments in the circulation by 3 h, significant norepinephrine increments by 9 h, and epinephrine increments in 5/6 patients by 12 h (not statistically significant) of insulin withdrawal. These studies document that (1) PGE-m accumulates in the circulation during DKA, (2) PGE-m and glucagon increase before catecholamines, and (3) PGE-m, glucagon, and catecholamine levels promptly return to normal levels when insulin therapy is reinstituted. It is suggested that elevated PGE-m levels early in the onset of DKA may represent a host-defense mechanism.