A local glucagon-like peptide 1 (GLP-1) system in human pancreatic islets.

AIMS/HYPOTHESIS: Glucagon-like peptide 1 (GLP-1) is a major incretin, mainly produced by the intestinal L cells, with beneficial actions on pancreatic beta cells. However, while in vivo only very small amounts of GLP-1 reach the pancreas in bioactive form, some observations indicate that GLP-1 may also be produced in the islets. We performed comprehensive morphological, functional and molecular studies to evaluate the presence and various features of a local GLP-1 system in human pancreatic islet cells, including those from type 2 diabetic patients. METHODS: The presence of insulin, glucagon, GLP-1, proconvertase (PC) 1/3 and PC2 was determined in human pancreas by immunohistochemistry with confocal microscopy. Islets were isolated from non-diabetic and type 2 diabetic donors. GLP-1 protein abundance was evaluated by immunoblotting and matrix-assisted laser desorption-ionisation-time of flight (MALDI-TOF) mass spectrometry. Single alpha and beta cell suspensions were obtained by enzymatic dissociation and FACS sorting. Glucagon and GLP-1 release were measured in response to nutrients. RESULTS: Confocal microscopy showed the presence of GLP-1-like and PC1/3 immunoreactivity in subsets of alpha cells, whereas GLP-1 was not observed in beta cells. The presence of GLP-1 in isolated islets was confirmed by immunoblotting, followed by mass spectrometry. Isolated islets and alpha (but not beta) cell fractions released GLP-1, which was regulated by glucose and arginine. PC1/3 (also known as PCSK1) gene expression was shown in alpha cells. GLP-1 release was significantly higher from type 2 diabetic than from non-diabetic isolated islets. CONCLUSIONS/INTERPRETATION: We have shown the presence of a functionally competent GLP-1 system in human pancreatic islets, which resides in alpha cells and might be modulated by type 2 diabetes.

Insulin promoter DNA methylation correlates negatively with insulin gene expression and positively with HbA(1c) levels in human pancreatic islets.

AIMS/HYPOTHESIS: Although recent studies propose that epigenetic factors influence insulin expression, the regulation of the insulin gene in type 2 diabetic islets is still not fully understood. Here, we examined DNA methylation of the insulin gene promoter in pancreatic islets from patients with type 2 diabetes and non-diabetic human donors and related it to insulin expression, HbA(1c) levels, BMI and age. METHODS: DNA methylation was analysed in 25 CpG sites of the insulin promoter and insulin mRNA expression was analysed using quantitative RT-PCR in pancreatic islets from nine donors with type 2 diabetes and 48 non-diabetic donors. RESULTS: Insulin mRNA expression (p = 0.002), insulin content (p = 0.004) and glucose-stimulated insulin secretion (p = 0.04) were reduced in pancreatic islets from patients with type 2 diabetes compared with non-diabetic donors. Moreover, four CpG sites located 234 bp, 180 and 102 bp upstream and 63 bp downstream of the transcription start site (CpG -234, -180, -102 and +63, respectively), showed increased DNA methylation in type 2 diabetic compared with non-diabetic islets (7.8%, p = 0.03; 7.1%, p = 0.02; 4.4%, p = 0.03 and 9.3%, p = 0.03, respectively). While insulin mRNA expression correlated negatively (p < 1 × 10(-6)), the level of HbA(1c) correlated positively (p ≤ 0.01) with the degree of DNA methylation for CpG -234, -180 and +63. Furthermore, DNA methylation for nine additional CpG sites correlated negatively with insulin mRNA expression (p ≤ 0.01). Also, exposure to hyperglycaemia for 72 h increased insulin promoter DNA methylation in clonal rat beta cells (p = 0.005). CONCLUSIONS/INTERPRETATIONS: This study demonstrates that DNA methylation of the insulin promoter is increased in patients with type 2 diabetes and correlates negatively with insulin gene expression in human pancreatic islets.

Taspoglutide, a novel human once-weekly GLP-1 analogue, protects pancreatic ß-cells in vitro and preserves islet structure and function in the Zucker diabetic fatty rat in vivo.

AIM: Glucagon-like peptide-1 (GLP-1) has protective effects on pancreatic ß-cells. We evaluated the effects of a novel, long-acting human GLP-1 analogue, taspoglutide, on ß-cells in vitro and in vivo. METHODS: Proliferation of murine pancreatic ß (MIN6B1) cells and rat islets in culture was assessed by imaging of 5-ethynyl-2'-deoxyuridine-positive cells after culture with taspoglutide. Apoptosis was evaluated with the transferase-mediated 2'-deoxyuridine 5'-triphosphate nick-end labelling assay in rat insulinoma (INS-1E) cells and isolated human islets exposed to cytokines (recombinant interleukin-1ß, interferon-γ, tumour necrosis factor-α) or lipotoxicity (palmitate) in the presence or absence of taspoglutide. Islet morphology and survival and glucose-stimulated insulin secretion in perfused pancreata were assessed 3-4 weeks after a single application of taspoglutide to prediabetic 6-week-old male Zucker diabetic fatty (ZDF) rats. RESULTS: Proliferation was increased in a concentration-dependent manner up to fourfold by taspoglutide in MIN6B1 cells and was significantly stimulated in isolated rat islets. Taspoglutide almost completely prevented cytokine- or lipotoxicity-induced apoptosis in INS-1E cells (control 0.5%, cytokines alone 2.2%, taspoglutide + cytokines 0.6%, p < 0.001; palmitate alone 8.1%, taspoglutide + palmitate 0.5%, p < 0.001) and reduced apoptosis in isolated human islets. Treatment of ZDF rats with taspoglutide significantly prevented ß-cell apoptosis and preserved healthy islet architecture and insulin staining intensity as shown in pancreatic islet cross sections. Basal and glucose-stimulated insulin secretion of in situ perfused ZDF rat pancreata was normalized after taspoglutide treatment. CONCLUSIONS: Taspoglutide promoted ß-cell proliferation, prevented apoptosis in vitro and exerted multiple ß-cell protective effects on islet architecture and function in vivo in ZDF rats.

Thyrotropin-releasing hormone increases cytosolic free Ca2+ in clonal pituitary cells (GH3 cells): direct evidence for the mobilization of cellular calcium.

Changes in the cytosolic free Ca2+ concentration following cell surface receptor activation have been proposed to mediate a wide variety of cellular responses. Using the specific Ca2+ chelator quin2 as a fluorescent intracellular probe, we measured the Ca2+ levels in the cytosol of clonal rat pituitary cells, GH3 cells. We demonstrate that thyrotropin-releasing hormone (TRH) at nanomolar concentrations leads to a rapid and transient increase in cytosolic Ca2+. This increase was found to occur in Ca2+-free media in the presence of EGTA, thus at extracellular Ca2+ levels that are below the cytosolic concentrations, and was not prevented by verapamil, a Ca2+ channel blocker. Depolarization of GH3 cells with K+, which can mimic the action of TRH on prolactin release, increased cytosolic Ca2+ levels only in the presence of free extracellular Ca2+, and this increase could be blocked by verapamil. These data show that the mobilization of intracellular Ca2+ due to TRH action that has been proposed by previous studies actually leads to an increase in cytosolic free Ca2+. The kinetic features of this response emphasize the key role of cytosolic free Ca2+ in stimulus-secretion coupling.

Modulation of cytosolic-free calcium transients by changes in intracellular calcium-buffering capacity: correlation with exocytosis and O2-production in human neutrophils.

The intracellularly trapped fluorescent calcium indicator, quin 2, was used not only to monitor changes in cytosolic-free calcium, [Ca2+]i, but also to assess the role of [Ca2+]i in neutrophil function. To increase cytosolic calcium buffering, human neutrophils were loaded with various quin 2 concentrations, and [Ca2+]i transients, granule content release as well as superoxide [O2-] production were measured in response to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP) and the calcium ionophore ionomycin. Receptor-mediated cell activation induced by fMLP caused a rapid rise in [Ca2+]i. The extent of [Ca2+]i rise and granule release were inversely correlated with the intracellular concentration of quin 2, [quin 2]i. These effects of [quin 2]i were more pronounced in the absence of extracellular Ca2+. The initial rate and extent of fMLP-induced O2- production were also inhibited by [quin 2]i. The rates of increase of [Ca2+]i and granule release elicited by ionomycin were also inversely correlated with [quin 2]i in Ca2+-containing medium. As the effects of ionomycin, in contrast to those of fMLP, are sustained, the final increase in [Ca2+]i and granule release were not affected by [quin 2]i. A further reduction of fMLP effects was seen when intracellular calcium stores were depleted by incubating the cells in Ca2+-free medium with ionomycin. The specificity of quin 2 effects on cellular calcium were confirmed by loading the cells with Anis/AM, a structural analog of quin 2 with low affinity for calcium which did not inhibit granule release. In addition, functional responses to phorbol myristate acetate (PMA), which stimulates neutrophils without raising [Ca2+]i, were not affected by [quin 2]i. The findings indicate that rises in [Ca2+]i control the rate and extent of granule exocytosis and O2-generation in human neutrophils exposed to the chemotactic peptide fMLP.

SNAP-25 is expressed in islets of Langerhans and is involved in insulin release.

SNAP-25 is known as a neuron specific molecule involved in the fusion of small synaptic vesicles with the presynaptic plasma membrane. By immunolocalization and Western blot analysis, it is now shown that SNAP-25 is also expressed in pancreatic endocrine cells. Botulinum neurotoxins (BoNT) A and E were used to study the role of SNAP-25 in insulin secretion. These neurotoxins inhibit transmitter release by cleaving SNAP-25 in neurons. Cells from a pancreatic B cell line (HIT) and primary rat islet cells were permeabilized with streptolysin-O to allow toxin entry. SNAP-25 was cleaved by BoNT/A and BoNT/E, resulting in a molecular mass shift of approximately 1 and 3 kD, respectively. Cleavage was accompanied by an inhibition of Ca(++)-stimulated insulin release in both cell types. In HIT cells, a concentration of 30-40 nM BoNT/E gave maximal inhibition of stimulated insulin secretion of approximately 60%, coinciding with essentially complete cleavage of SNAP-25. Half maximal effects in terms of cleavage and inhibition of insulin release were obtained at a concentration of 5-10 nM. The A type toxin showed maximal and half-maximal effects at concentrations of 4 and 2 nM, respectively. In conclusion, the results suggest a role for SNAP-25 in fusion of dense core secretory granules with the plasma membrane in an endocrine cell type- the pancreatic B cell.

Is cytosolic ionized calcium regulating neutrophil activation?

The concentration of cytosolic ionized calcium, [Ca2+]i, was measured in intact neutrophils by use of a fluorescent indicator trapped in the icytoplasm. A given rise of [Ca2+]i elicited by the chemotactic peptide formylmethionylleucylphenylalanine (FMLP) was associated with a much greater degree of superoxide generation and myeloperoxidase secretion than was the same or larger [Ca2+]i produced by a specific calcium ionophore, ionomycin, which bypasses cell surface receptors. Thus, FMLP appears to generate some important excitatory signal in addition to a rise in [Ca2+]i and exocytosis and superoxide generation in neutrophils may not be simply dependent on [Ca2+]i as is widely supposed.

Increased uptake of oxLDL does not exert lipotoxic effects in insulin-secreting cells.

Modified lipoproteins can negatively affect beta cell function and survival. However, the mechanisms behind interactions of modified lipoproteins with beta cells - and in particular, relationships to increased uptake - are only partly clarified. By over-expressing the scavenger receptor CD36 (Tet-on), we increased the uptake of fluorescent low-density modified lipoprotein (oxLDL) into insulin-secreting INS-1 cells. The magnitude of uptake followed the degree of CD36 over-expression. CD36 over-expression increased concomitant efflux of 3H-cholesterol in proportion to the cellular contents of 3H-cholesterol. Exposure to concentrations of oxLDL from 20 to 100 µg/mL dose-dependently increased toxicity (evaluated by MTT) as well as apoptosis. However, the increased uptake of oxLDL due to CD36 over-expression did not exert additive effects on oxLDL toxicity - neither on viability, nor on glucose-induced insulin release and cellular content. Reciprocally, blocking CD36 receptors by Sulfo-N-Succinimidyl Oleate decreased the uptake of oxLDL but did not diminish the toxicity. Pancreatic islets of CD36-/- mice displayed reduced uptake of 3H-cholesterol-labeled oxLDL vs wild type but similar toxicity to oxLDL. OxLDL was found to increase the expression of CD36 in islets and INS-1 cells. In summary, given the experimental conditions, our results indicate that (1) increased uptake of oxLDL is not responsible for toxicity of oxLDL, (2) increased efflux of the cholesterol moiety of oxLDL counterbalances, at least in part, increased uptake and (3) oxLDL participates in the regulation of CD36 in pancreatic islets and in INS-1 cells.

The transcription factor PAX4 acts as a survival gene in INS-1E insulinoma cells.

The paired/homeodomain transcription factor Pax4 is essential for islet beta-cell generation during pancreas development and their survival in adulthood. High Pax4 expression was reported in human insulinomas indicating that deregulation of the gene may be associated with tumorigenesis. We report that rat insulinoma INS-1E cells express 25-fold higher Pax4 mRNA levels than rat islets. In contrast to primary beta-cells, activin A but not betacellulin or glucose induced Pax4 mRNA levels indicating dissociation of Pax4 expression from insulinoma cell proliferation. Short hairpin RNA adenoviral constructs targeted to the paired domain or homeodomain (viPax4PD and viPax4HD) were generated. Pax4 mRNA levels were lowered by 73 and 50% in cells expressing either viPax4PD or viPax4HD. Transcript levels of the Pax4 target gene bcl-xl were reduced by 53 and 47%, whereas Pax6 and Pdx1 mRNA levels were unchanged. viPax4PD-infected cells displayed a twofold increase in spontaneous apoptosis and were more susceptible to cytokine-induced cell death. In contrast, proliferation was unaltered. RNA interference-mediated repression of insulin had no adverse effects on either Pax4 or Pdx1 expression as well as on cell replication or apoptosis. These results indicate that Pax4 is redundant for proliferation of insulinoma cells, whereas it is essential for survival through upregulation of the antiapoptotic gene bcl-xl.

Reduced insulin secretion and content in VEGF-a deficient mouse pancreatic islets.

Mice, deficient for vascular endothelial growth factor VEGF-A in pancreatic islets, have reduced insulin gene expression levels and an impaired glucose tolerance. Here, we investigated whether VEGF-A was required for physiological glucose-stimulated insulin secretion and insulin content. We performed in situ pancreas perfusions and islet perifusions on mice lacking VEGF-A in the pancreatic epithelium in order to study their ability to secrete insulin in response to glucose. We identified insulin secretion defects in the pancreata of VEGF-A deficient mice, including a delayed and blunted response to glucose. Islet perifusion experiments revealed a missing first phase and weaker second phase of insulin secretion, in two of three VEGF-A deficient mice. On average, insulin content in VEGF-A deficient islets was significantly reduced when compared with control islets. We conclude that VEGF-A is required in pancreatic islets for normal glucose-stimulated insulin secretion and physiological insulin content. Thus, VEGF-A is a key factor for pancreatic islet function.

Evidence for the involvement of Na/Ca exchange in glucose-induced insulin release from rat pancreatic islets.

Glucose-induced inhibition of Ca(++) extrusion from the beta-cell may contribute to the rise in cytosol Ca(++) that leads to insulin release. To study whether interference with Na/Ca exchange is involved in this inhibition the effects of glucose were compared to those of ouabain. This substance inhibits Na/K ATPase, decreases the transmembrane Na(+) gradient in islets, and thus interferes with Na/Ca exchange. Collagenase isolated rat islets were maintained for 2 d in tissue culture with a trace amount of (45)Ca(++). Insulin release and (45)Ca(++) efflux were then measured during perifusion. In Ca(++)-deprived medium (to avoid changes in tissue specific radioactivity) 16.7 mM glucose inhibited (45)Ca(++) efflux. Initially 1 mM ouabain inhibited (45)Ca(++) efflux in a similar fashion, the onset being even faster than that of glucose. The effects of 16.7 mM glucose and ouabain were not additive, indicating that both substances may interfere with Na/Ca exchange. In the presence of Ca(++), 16.7 mM glucose induced biphasic insulin release. Ouabain alone caused a gradual increase of insulin release. Again, the effects of ouabain and 16.7 mM glucose were not additive. In contrast, at a submaximal glucose concentration (7 mM) ouabain enhanced both phases of release. An important role for Na/Ca exchange is suggested from experiments in which Ca(++) was removed at the time of glucose-stimulation (16.7 mM). The resulting marked inhibition of insulin release was completely overcome during first phase by ouabain added at the time of Ca(++) removal; second phase was restored to 60%. This could be due to the rapid inhibitory action of ouabain on Ca(++) efflux thereby preventing loss of cellular calcium critical for glucose to induce insulin release. It appears, therefore, that interference with Na/Ca exchange is an important event in the stimulation of insulin release by glucose.

Possible role for calmodulin in insulin release. Studies with trifluoperazine in rat pancreatic islets.

The role of calmodulin in insulin secretion from rat pancreatic islets has been examined by the use of trifluoperazine, an inhibitor of calmodulin-Ca++-directed functions. It was found that 30 microM trifluoperazine caused 50% inhibition, and 100 microM, up to 73% inhibition of 16.7 mM glucose-stimulated insulin release. 100 microM trifluoperazine caused a similar inhibition of 10 mM glyceraldehyde-stimulated release. Therefore, the site of action of trifluoperazine in glucose stimulus-secretion coupling appears to be after the trioses. As trifluoperazine had no effect upon insulin release stimulated by 1 mM 3-isobutyl-1-methylxanthine, the inhibitory effect of trifluoperazine appears to be rather specific. Further, the process of exocytosis per se is not affected. It was also found that although trifluoperazine inhibited the effect of glucose to stimulate insulin release, it did not affect the synergism between glucose and 3-isobutyl-1-methylxanthine to potentiate insulin release. It may be concluded that trifluoperazine selectively inhibits one part of the mechanism by which glucose stimulates insulin release. Calmodulin plays a role in the stimulation of insulin release by glucose at a site between metabolism of trioses and elevation of cytosol Ca++, but is not involved in the final process of exocytosis.

Glucose and carbachol generate 1,2-diacylglycerols by different mechanisms in pancreatic islets.

Diacylglycerols (DAG) modulate secretory responses by the activation of protein kinase C. Early changes in DAG formation induced by the muscarinic receptor agonist carbachol were compared to those caused by the nutrient secretagogue glucose in pancreatic islets. Turnover rates of DAG were investigated in radiolabeling experiments, whereas changes in total mass and fatty acid composition of DAG were assessed by gas-liquid chromatography. When islet lipids were labeled to steady state in tissue culture with [3H]glycerol, carbachol induced a rapid (10 s) and sustained increase of [3H]DAG generation. In contrast, glucose stimulation failed to increase [3H]glycerol containing DAG, and this was probably due to the isotopic dilution of the label secondary to enhanced glycolysis. This was substantiated by following the transfer of 14C from glucose into DAG. Within 1 min of acute exposure of islets to D-[U-14C]-glucose at stimulatory concentrations, DAG labeling increased fivefold representing up to 2% of total glucose usage. Similar stimulation of 14C incorporation into other neutral lipids and inositol phospholipids was observed, suggesting the enhanced de novo synthesis of phosphatidic acid, the common precursor for DAG, and inositol phospholipids from glycolytic intermediates. Transfer of 14C from glucose was not stimulated by agents such as carbachol and exogenous phospholipase C that act primarily on inositol phospholipid breakdown. The total mass of islet DAG was increased by 60% after both carbachol and glucose stimulation. However, analysis of the fatty acid composition of carbachol-generated DAG revealed at the early time point (10 s) a prevalent stearoyl-arachidonoyl configuration similar to that reported for inositol phospholipids. This pattern shifted to a DAG enriched in palmitic acid at a later time point. Glucose-stimulated islets displayed a predominance of palmitic acid containing DAG, indicating increased de novo synthesis of the putative second messenger rather than its formation by inositol phospholipid hydrolysis. Indeed, steady-state labeling of these phospholipids with [3H]inositol confirmed this idea since only carbachol caused detectable inositol phospholipid hydrolysis. Thus, although protein kinase C may be activated by both carbachol and glucose, the two secretagogues generate diacylglycerols through different mechanisms.

Somatostatin- and epinephrine-induced modifications of 45Ca++ fluxes and insulin release in rat pancreatic islets maintained in tissue culture.

The effects of somatostatin and epinephrine have been studied with regard to glucose-induced insulin release and (45)Ca(++) uptake by rat pancreatic islets after 2 days in tissue culture and with regard to (45)Ca(++) efflux from islets loaded with the radio-isotope during the 2 days of culture. (45)Ca(++) uptake, measured simultaneously with insulin release, was linear with time for 5 min. (45)Ca(++) efflux and insulin release were also measured simultaneously from perifused islets. Glucose (16.7 mM) markedly stimulated insulin release and (45)Ca(++) uptake. Somatostatin inhibited the stimulation of insulin release by glucose in a concentration-related manner (1-1,000 ng/ml) but was without effect on the glucose-induced stimulation of (45)Ca(++) uptake. Similarly, under perifusion conditions, both phases of insulin release were inhibited by somatostatin while no effect was observed on the pattern of (45)Ca(++) efflux after glucose.Epinephrine, in contrast to somatostatin, caused a concentration-dependent inhibition of the stimulation of both insulin release and (45)Ca(++) uptake by glucose. Both phases of insulin release were inhibited by epinephrine and marked inhibition could be observed with no change in the characteristic glucose-evoked pattern of (45)Ca(++) efflux (e.g., with 10 nM epinephrine). The inhibitory effect of epinephrine on (45)Ca(++) uptake and insulin release appeared to be mediated via an alpha-adrenergic mechanism, since is was abolished in the presence of phentolamine. Somatostatin inhibits insulin release without any detectable effect upon the handling of calcium by the islets. In contrast, inhibition of insulin release by epinephrine is accompanied by a partial inhibition of glucose-induced Ca(++) uptake.

The roles of intracellular and extracellular Ca++ in glucose-stimulated biphasic insulin release by rat islets.

Verapamil, an agent known rapidly to block calcium uptake into islets of Langerhans, has been used to study the roles of intra- and extracellular calcium in the two phases of glucose-induced insulin release. Rates of calcium uptake and insulin release during the first phase were measured simultaneously over 5 min in rat islets after maintenance in tissue culture for 2 days. Rates of (45)Ca(++) efflux and insulin release during the first and second phases were also measured simultaneously under perifusion conditions. For this, islets were loaded with (45)Ca(++) during the entire maintenance period to complete isotopic equilibrium. Under static incubation conditions 5 muM Verapamil had no effect upon Ca(++) uptake or insulin release in the presence of 2.8 mM glucose. By contrast, glucose-stimulated calcium influx was totally abolished without there being any significant effect upon first phase insulin release. Thus first phase insulin release is independent of increased uptake of extracellular calcium. The lack of effect of 5 muM Verapamil blockade on first phase insulin release was confirmed, under perifusion conditions, and was in marked contrast to the observed 55% inhibition of second phase release. (45)Ca(++) efflux was inhibited during both phases of the insulin release response. The results show that increased calcium uptake in response to glucose is not involved in the mechanism of first phase insulin release but is required for the full development and maintenance of the second phase release. It seems possible that intracellular calcium is the major regulatory control for first phase insulin release and that intracellular calcium and increased uptake of extracellular calcium contribute almost equally to the second phase of glucose-induced release.

Functional differences between rat islets of ventral and dorsal pancreatic origin.

Do functional linkages between islet endocrine cells exist? The effect of differences in frequency and distribution of islet endocrine cells on B cell function was examined in islets from the ventral (ventral islets) and dorsal (dorsal islets) areas of the rat pancreas. Dorsal islets contained 10 times as much glucagon as ventral islets, whereas insulin and total protein contents were similar. Basal rates of insulin secretion and proinsulin biosynthesis were similar in the two types of islet, but, under conditions of glucose stimulation, both insulin secretion and proinsulin biosynthesis were significantly greater in the glucagon-rich dorsal islets. Similarly, glucose utilization rates an ATP levels were greater in dorsal islets. In contrast, the rates of processing of newly synthesized proinsulin were similar in ventral and dorsal islets. That the islet glucagon content may have affected B cell function is inferred from two independent findings. Firstly, basal and glucose-stimulated cyclic AMP contents of glucagon-rich dorsal islets were greater than those of ventral islets. Secondly, in the presence of excess exogenous glucagon (1 microgram/ml), the differences in glucose-induced insulin secretion and proinsulin biosynthesis rates between the two types of islets were eliminated. These results strongly suggest that changes in the relative proportions of the different islet endocrine cells exert marked effects on islet function. In particular, a greater A cell and glucagon content is associated with higher rates of glucose-induced insulin secretion and biosynthesis.

Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in beta cells.

Previous investigations revealed low activities of lactate dehydrogenase (LDH) and plasma membrane monocarboxylate transporters (MCT) in the pancreatic beta cell. In this study the significance of these characteristics was explored by overexpressing type A LDH (LDH-A) and/or type 1 MCT (MCT-1) in the clonal INS-1 beta cells and isolated rat islets. Inducible overexpression of LDH-A resulted in an 87-fold increase in LDH activity in INS-1 cells. Adenovirus-mediated overexpression of MCT-1 increased lactate transport activity 3.7-fold in INS-1 cells. Although overexpression of LDH-A, and/or MCT-1 did not affect glucose-stimulated insulin secretion, LDH-A overexpression resulted in stimulation of insulin secretion even at a low lactate concentration with a concomitant increase in its oxidation in INS-1 cells regardless of MCT-1 co-overexpression. Adenovirus-mediated overexpression of MCT-1 caused an increase in pyruvate oxidation and conferred pyruvate-stimulated insulin release to isolated rat islets. Although lactate did not stimulate insulin secretion from control or MCT-1-overexpressing islets, co-overexpression of LDH-A and MCT-1 evoked lactate-stimulated insulin secretion with a concomitant increase in lactate oxidation in rat islets. These results suggest that low expression of MCT and LDH is requisite to the specificity of glucose in insulin secretion, protecting the organism from undesired hypoglycemic actions of pyruvate and lactate during exercise and other catabolic states.

Dependency of cyclic AMP-induced insulin release on intra- and extracellular calcium in rat islets of Langerhans.

Calcium and cyclic AMP are important in the stimulation of insulin release. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) raises islet cAMP levels and causes insulin release at nonstimulatory glucose concentrations. In isolated rat pancreatic islets maintained for 2 d in tissue culture, the effects of IBMX on insulin release and 45Ca++ fluxes were compared with those of glucose. During perifusion at 1 mM Ca++, 16.7 mM glucose elicited a biphasic insulin release, whereas 1 mM IBMX in the presence of 2.8 mM glucose caused a monophasic release. Decreasing extracellular Ca++ a monophasic release. Decreasing extracellular Ca++ to 0.1 mM during stimulation reduced the glucose effect by 80% but did not alter IBMX-induced release. Both glucose and IBMX stimulated 45Ca++ uptake (5 min). 45Ca++ efflux from islets loaded to isotopic equilibrium (46 h) was increased by both substances. IBMX stimulation of insulin release, of 45Ca++ uptake, and of efflux were not inhibited by blockade of Ca++ uptake with verapamil, whereas glucose-induced changes are known to be inhibited. Because IBMX-induced insulin release remained unaltered at 0.1 mM calcium, it appears that cAMP-stimulated insulin release is controlled by intracellular calcium. This is supported by perifusion experiments at 0 Ca++ when IBMX stimulated net Ca++ efflux. In addition, glucose-stimulated insulin release was potentiated by IBMX. These results suggest that cAMP induced insulin release is mediated by increases in cytosolic Ca++ and that cAMP causes dislocation of Ca++ from intracellular stores.

Chemotactic peptide activation of human neutrophils and HL-60 cells. Pertussis toxin reveals correlation between inositol trisphosphate generation, calcium ion transients, and cellular activation.

The mechanism of neutrophil activation by the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP) has been studied by pretreatment of human neutrophils with pertussis toxin. Upon stimulation with FMLP, the cytosolic-free calcium concentration, [Ca2+]i, is increased both by stimulation of calcium influx and mobilization of cellular calcium. We have measured [Ca2+]i as well as the generation of the phospholipid breakdown product inositol trisphosphate (IP3), which is thought to mediate Ca2+ mobilization. As the phosphoinositide pool in human neutrophils is difficult to prelabel with [3H]myoinositol, experiments were also carried out in the cultured human promyelocytic leukemia cell line HL-60 after differentiation with dimethylsulfoxide. Pertussis toxin pretreatment of both cell types inhibited FMLP stimulated membrane depolarization, exocytosis, and superoxide production in a dose-dependent manner. This toxin effect was selective for the receptor agonist, since stimulation of these parameters by two substances bypassing the transduction mechanism, the calcium ionophore ionomycin and the phorbolester phorbol myristate acetate, were unaffected. Rises in [Ca2+]i, as well as generation of IP3 in response to FMLP, were inhibited in parallel; for the inhibition of functional responses, slightly lower toxin concentrations were required. The attentuation of the [Ca2+]i rise was more marked in the absence of extracellular calcium, i.e., when the rise is due only to calcium mobilization. The results provide evidence that phospholipase C stimulation by FMLP resulting in IP3 generation is involved in the signal transduction mechanism. Coupling of FMLP receptor occupancy to phospholipase C activation is sensitive to pertussis toxin, suggesting the involvement of a GTP binding protein (N protein), which has been shown to be a pertussis toxin substrate. The parallel changes in [Ca2+]i and IP3 further support the hypothesis that IP3 is the calcium-mobilizing mediator in FMLP-activated cells.

Glucagon release induced by pancreatic nerve stimulation in the dog.

A direct neural role in the regulation of immunoreactive glucagon (IRG) secretion has been investigated during stimulation of mixed autonomic nerves to the pancreas in anesthetized dogs. The responses were evaluated by measurement of blood flow and hormone concentration in the venous effluent from the stimulated region of pancreas. Electrical stimulation of the distal end of the discrete bundles of nerve fibers isolated along the superior pancreaticoduodenal artery was invariably followed by an increase in IRG output. With 10-min periods of nerve stimulation, the integrated response showed that the higher the control glucagon output, the greater was the increment. Atropinization did not influence the response to stimulation. That the preparation behaved in physiologic fashion was confirmed by a fall in IRG output, and a rise in immunoreactive insulin (IRI) output, during hyperglycemia induced by intravenous glucose (0.1 g/kg). The kinetics of this glucose effect on IRG showed characteristics opposite to those of nerve stimulation: the lower the control output, the less the decrement. Furthermore, during the control steady state, blood glucose concentration was tightly correlated with the IRI/IRG molar output ratio, the function relating the two parameters being markedly nonlinear. Injection or primed infusion of glucose diminished the IRG response to simultaneous nerve stimulation. Measurement of IRG was inferred to reflect response of pancreatic glucagon secretion on the basis of the site of sample collection (the superior pancreaticoduodenal vein), the absence of changes in arterial IRG, and similar responses being obtained using an antibody specific for pancreatic glucagon. THESE STUDIES SUPPORT A ROLE FOR THE AUTONOMIC NERVOUS SYSTEM IN THE CONTROL OF GLUCAGON SECRETION: direct nerve stimulation induces glucagon release. Such sympathetic activation may be interpreted as capable of shifting the sensitivity of the A cell to glucose in the direction of higher glycemia for a given glucagon output. The experimental model employed is valid for further studies of regulatory mechanisms of endocrine pancreatic function in vivo.