Exercise at anaerobic threshold intensity and insulin secretion by isolated pancreatic islets of rats

To evaluate the effect of acute exercise and exercise training at the anaerobic threshold (AT) intensity on aerobic conditioning and insulin secretion by pancreatic islets, adult male Wistar rats were submitted to the lactate minimum test (LMT) for AT determination. Half of the animals were submitted to swimming exercise training (trained), 1 h/day, 5 days/week during 8 weeks, with an overload equivalent to the AT. The other half was kept sedentary. At the end of the experimental period, the rats were submitted to an oral glucose tolerance test and to another LMT. Then, the animals were sacrificed at rest or immediately after 20 minutes of swimming exercise at the AT intensity for pancreatic islets isolation. At the end of the experiment mean workload (% bw) at AT was higher and blood lactate concentration (mmol/L) was lower in the trained than in the control group. Rats trained at the AT intensity showed no alteration in the areas under blood glucose and insulin during OGTT test. Islet insulin content of trained rats was higher than in the sedentary rats while islet glucose uptake did not differ among the groups. The static insulin secretion in response to the high glucose concentration (16.7 mM) of the sedentary group at rest was lower than the sedentary group submitted to the acute exercise and the inverse was observed in relation to the trained groups. physical training at the AT intensity improved the aerobic condition and altered insulin secretory pattern by pancreatic islets. © 2010 Landes Bioscience.

Glucocorticoids in vivo induce both insulin hypersecretion and enhanced glucose sensitivity of stimulus-secretion coupling in isolated rat islets

Although glucocorticoids are widely used as antiinflammatory agents in clinical therapies, they may cause serious side effects that include insulin resistance and hyperinsulinemia. To study the potential functional adaptations of the islet of Langerhans to in vivo glucocorticoid treatment, adult Wistar rats received dexamethasone (DEX) for 5 consecutive days, whereas controls (CTL) received only saline. The analysis of insulin release in freshly isolated islets showed an enhanced secretion in response to glucose in DEX-treated rats. The study of Ca2 2+ signals by fluorescence microscopy also demonstrated a higher response to glucose in islets from DEX-treated animals. However, no differences in Ca2 2+signals were found between both groups with tolbutamide or KCl, indicating that the alterations were probably related to metabolism. Thus, mitochondrial function was explored by monitoring oxidation of nicotinamide dinucleotide phosphate autofluorescence and mitochondrial membrane potential. Both parameters revealed a higher response to glucose in islets from DEX-treated rats. The mRNA and protein content of glucose transporter-2, glucokinase, and pyruvate kinase was similar in both groups, indicating that changes in these proteins were probably not involved in the increased mitochondrial function. Additionally,weexplored the status of Ca2 2+-dependent signaling kinases. Unlike calmodulin kinase II, we found an augmented phosphorylation level of protein kinase Cα as well as an increased response of the phospholipase C/inositol 1,4,5-triphosphate pathway in DEX-treated rats. Finally, an increased number of docked secretory granules were observed in the β-cells of DEX animals using transmission electron microscopy. Thus, these results demonstrate that islets from glucocorticoid-treated rats develop several adaptations that lead to an enhanced stimulus-secretion coupling and secretory capacity. Copyright © 2010 by The Endocrine Society.

Agelaia MP-I: A peptide isolated from the venom of the social wasp, Agelaia pallipes pallipes, enhances insulin secretion in mice pancreatic islets

Peptides isolated from animal venoms have shown the ability to regulate pancreatic beta cell function. Characterization of wasp venoms is important, since some components of these venoms present large molecular variability, and potential interactions with different signal transduction pathways. For example, the well studied mastoparan peptides interact with a diversity of cell types and cellular components and stimulate insulin secretion via the inhibition of ATP dependent K + (K ATP) channels, increasing intracellular Ca 2+ concentration. In this study, the insulin secretion of isolated pancreatic islets from adult Swiss mice was evaluated in the presence of synthetic Agelaia MP-I (AMP-I) peptide, and some mechanisms of action of this peptide on endocrine pancreatic function were characterized. AMP-I was manually synthesized using the Fmoc strategy, purified by RP-HPLC and analyzed using ESI-IT-TOF mass spectrometry. Isolated islets were incubated at increasing glucose concentrations (2.8, 11.1 and 22.2 mM) without (Control group: CTL) or with 10 μM AMP-I (AMP-I group). AMP-I increased insulin release at all tested glucose concentrations, when compared with CTL (P < 0.05). Since molecular analysis showed a potential role of the peptide interaction with ionic channels, insulin secretion was also analyzed in the presence of 250 μM diazoxide, a K ATP channel opener and 10 μM nifedipine, a Ca 2+ channel blocker. These drugs abolished insulin secretion in the CTL group in the presence of 2.8 and 11.1 mM glucose, whereas AMP-I also enhanced insulin secretory capacity, under these glucose conditions, when incubated with diazoxide and nifedipine. In conclusion, AMP-I increased beta cell secretion without interfering in K ATP and L-type Ca 2+ channel function, suggesting a different mechanism for this peptide, possibly by G protein interaction, due to the structural similarity of this peptide with Mastoparan-X, as obtained by modeling. © 2012 Elsevier Ltd.

Rôle de l’enzyme PAS kinase dans la régulation du facteur de transcription PDX-1 dans la cellule bêta pancréatique.

Le diabète de type 2 (DT2) se caractérise par une production insuffisante d'insuline par le pancréas ainsi qu'une résistance des tissus périphériques à l'action de l'insuline. Dans les cellules bêta pancréatiques, le glucose stimule la production de l'insuline en induisant la transcription de son gène et la traduction ainsi que la sécrétion de sa protéine. Paradoxalement, une exposition prolongée et simultanée de ces cellules à de hautes concentrations de glucose en présence d'acides gras conduit à la détérioration de la fonction bêta pancréatique et au développement du DT2. Toutefois, les mécanismes moléculaires responsables de ces effets du glucose ne sont que partiellement connus. L'objectif du travail décrit dans cette thèse est d'identifier les mécanismes responsables de la régulation de la transcription du gène de l'insuline. PDX-1 (de l’anglais pour pancreatic and duodenal homeobox 1) est un facteur de transcription majeur et essentiel tant pour le développement du pancréas que pour le maintien de sa fonction à l'état adulte. En réponse au glucose, PDX-1 se lie au promoteur du gène de l'insuline et induit sa transcription. Ceci est inhibé par l'acide gras palmitate. Dans la première partie des travaux effectués dans le cadre de cette thèse, nous avons identifié deux mécanismes de régulation de la transcription du gène de l'insuline: le premier via ERK1/2 (de l'anglais pour extracellular-signal-regulated protein kinases 1 and 2) et le second par l’enzyme PASK (pour per-arnt-sim kinase). Nous avons également mis en évidence l'existence d'un troisième mécanisme impliquant l'inhibition de l'expression du facteur de transcription MafA par le palmitate. Nos travaux indiquent que la contribution de la signalisation via PASK est majeure. L'expression de PASK est augmentée par le glucose et inhibée par le palmitate. Sa surexpression dans les cellules MIN6 et les îlots isolés de rats, mime les effets du glucose sur l'expression du gène de l'insuline ainsi que sur l'expression de PDX-1 et prévient les effets délétères du palmitate. Dans la deuxième partie de la thèse, nous avons identifié un nouveau mécanisme par lequel PASK augmente la stabilité protéique de PDX-1, soit via la phosphorylation et l'inactivation de la protéine kinase GSK3 bêta (de l'anglais pour glycogen synthase kinase 3 beta). Le glucose induit la translocation de PDX-1 du cytoplasme vers le noyau, ce qui est essentiel à sa liaison au promoteur de ses gènes cibles. L'exclusion nucléaire de PDX-1 a été observée dans plusieurs modèles ex vivo et in vivo de dysfonction de la cellule bêta pancréatique. Dans le dernier volet de cette thèse, nous avons démontré l'importance de l'utilisation de cellules primaires (îlots isolés et dispersés) pour étudier la translocation nucléaire de PDX-1 endogène étant donné que ce mode de régulation est absent dans les lignées insulino-sécrétrices MIN6 et HIT-T15. Ces études nous ont permis d'identifier et de mieux comprendre les mécanismes régulant la transcription du gène de l'insuline via le facteur de transcription PDX-1. Les cibles moléculaires ainsi identifiées pourraient contribuer au développement de nouvelles approches thérapeutiques pour le traitement du diabète de type 2. Mots-clés : Diabète, îlots de Langerhans, cellule bêta pancréatique, gène de l'insuline, PDX-1, PASK, GSK3 bêta, ERK1/2, PKB, glucose, palmitate.

Glucose-induced insulin secretion is impaired and insulin-induced phosphorylation of the insulin receptor and insulin receptor substrate-1 are increased in protein-deficient rats

Malnutrition is related to diabetes in tropical countries. In experimental animals, protein deficiency may affect insulin secretion. However, the effect of malnutrition on insulin receptor phosphorylation and further intracellular signaling events is not known. Therefore, we decided to evaluate the rate of insulin secretion and the early molecular steps of insulin action in insulin-sensitive tissues of an animal model of protein deficiency. Pancreatic islets isolated from rats fed a standard (17%) or a low (6%) protein diet were studied for their secretory response to increasing concentrations of glucose in the culture medium. Basal as well as maximal rates of insulin secretion were significantly lower in the islets isolated from rats fed a low protein diet. Moreover, the dose-response curve to glucose was significantly shifted to the right in the islets from malnourished rats compared with islets from control rats. During an oral glucose tolerance test, there were significantly lower circulating concentrations of insulin in the serum of rats fed a low protein diet in spite of no difference in serum glucose concentration between the groups, suggesting an increased peripheral insulin sensitivity. Immunoblotting and immunoprecipitation were used to study the phosphorylation of the insulin receptor and the insulin receptor substrate-1 as well as the insulin receptor substrate-1-p85 subunit of phosphatidylinositol 3-kinase association in response to insulin. Values were greater in hind-limb muscle from rats fed a low protein diet compared with controls. No differences were detected in the total amount of protein corresponding to the insulin receptor or insulin receptor substrate-1 between muscle from rats fed the two diets. Therefore, we conclude that a decreased glucose-induced insulin secretion in pancreatic islets from protein-malnourished rats is responsible, at least in part, for an increased phosphorylation of the insulin receptor, insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase. These might represent some of the factors influencing the equilibrium in glucose concentrations observed in animal models of malnutrition and undernourished subjects.

Protein deficiency and nutritional recovery modulate insulin secretion and the early steps of insulin action in rats

Maternal malnutrition was shown to affect early growth and leads to permanent alterations in insulin secretion and sensitivity of offspring. In addition, epidemiological studies showed an association between low birth weight and glucose intolerance in adult life. To understand these interactions better, we investigated the insulin secretion by isolated islets and the early events related to insulin action in the hind-limb muscle of adult rats fed a diet of 17% protein (control) or 6% protein [low (LP) protein] during fetal life, suckling and after weaning, and in rats receiving 6% protein during fetal life and suckling followed by a 17% protein diet after weaning (recovered). The basal and maximal insulin secretion by islets from rats fed LP diet and the basal release by islets from recovered rats were significantly lower than that of control rats. The dose-response curves to glucose of islets from LP and recovered groups were shifted to the right compared to control islets, with the half-maximal response (EC 50) occurring at 16.9 ± 1.3, 12.4 ± 0.5 and 8.4 ± 0.1 mmol/L, respectively. The levels of insulin receptor, as well as insulin receptor substrate-1 and phosphorylation and the association between insulin receptor substrate-1 and phosphatidylinositol 3-kinase were greater in rats fed a LP diet than in control rats. In recovered rats, these variables were not significantly different from those of the other two groups. These results suggest that glucose homeostasis is maintained in LP and recovered rats by an increased sensitivity to insulin as a result of alterations in the early steps of the insulin signal transduction pathway.

Insulin secretion in monosodium glutamate (MSG) obese rats submitted to aerobic exercise training

The present study was designed to evaluate the effects of aerobic exercise training on glucose tolerance and insulin secretion of obese male Wistar rats (monosodium glutamate [MSG] administration, 4mg/g-body weight, each other day, from birth to the 14th day). Fourteen weeks after the drug administration, the rats were separated into two groups: MSG-S (sedentary) and MSG-T (T = swimming, 1 h/day, 5 days/week, with an overload of 5% body weight for 10 weeks). Rats of the same age and strain injected with saline were used as control (C) and subdivided into two groups: C-S and C-T. Insulin and glucose responses during an oral glucose tolerance test (GTT) were evaluated by the estimation of the total areas under serum insulin (AI) and glucose (AG) curves. Glucose-induced insulin secretion by isolated pancreatic islets was also evaluated. MSG-S rats showed higher AI than C-rats while MSG-T rats presented lower AI than MSG-S rats. No differences in AG were observed among the 4 groups. Pancreatic islets from MSG-rats showed higher insulin secretion in response to low (2.8) and moderate (8.3 mM) concentrations of glucose than those from their control counterparts and no differences were observed between MSG-S and MSG-T rats. These results provide evidences that the hyperinsulinemia at low or moderate glucose concentrations observed in MSG-obese rats is, at least in part, a consequence of direct hypersecretion of the B cells and that chronic aerobic exercise is able to partially counteract the hyperinsulinemic state of these animals without disrupting glucose homeostasis.

Glucose homeostasis in pregnant rats submitted to dietary protein restriction

In the present work, we examined the effects of feeding a low protein diet during pregnancy on glucose-induced insulin secretion and glucose homeostasis in rats. Young (60 days), pregnant (P) or non-pregnant (NP) rats were fed during pregnancy or for 21 days (the NP) a normal (17%) or a low (6%) protein diet. Serum glucose and insulin levels and pancreas insulin content in the fed state; total area under serum glucose curve (AG) after a glucose load and serum glucose disappearance rate (Kitt) after insulin administration; as well as 86Rb outflow, 45Ca uptake and insulin secretion by isolated pancreatic islets in response to glucose were evaluated. Serum glucose was lower in 17%-P (12%) and 6%-P (27%) than in corresponding NP-rats. Serum insulin was higher in 17%- P (153%) and 6%-P (77%) compared to the corresponding NP-rats. Pancreatic insulin was higher in 6%-rats (55%) than in 17%-rats. No differences were found in AG among the groups whereas Kitt was lower in 6%-NP and higher in 6%-P than in the equivalent 17% rats. Increasing glucose concentration from 2.8 to 16.7 mmol/l, reduced 86Rb outflow from isolated islets from all groups. Increasing glucose concentration from 2.8 to 16.7 mmol/l elevated 45Ca uptake by 17%-NP (47%), 17%-P (40%) and 6%-P (214%) islets but not by 6%-NP ones. The increase in 45Ca uptake was followed by an increase in insulin release by the 17%-NP (2767%), 17%-P (2850%) and 6%-P (1200%) islets. In conclusion, 6%-P rats show impaired glucose induced insulin secretion related to reduced calcium uptake by pancreatic islets. However, the poor insulin secretion did not fully compensate the high peripheral sensitivity to the hormone, resulting in hypoglycemia.

Modulation of insulin secretion by physical training during recovery from protein malnutrition in rats

Objective: The purpose of the present study was to examine insulin secretion in rats submitted to protein restriction and nutritional recovery associated or not to physical training. Methods: The experiment was designed in two sets of five weeks each. In the first set the rats were fed a nonnal-protein diet(17%-control group) or a low-protein diet (6%-malnourished group) for five weeks. After this, all animals were fed the 17% protein diet and separated into four groups: sedentary control(SC); trained eontrol(TC); sedentary recovered(SR) and trained recovered(TR). TC and TR rats performed swimming exercise. Results: The results indicated efficiency of the 6% protein diet in producing signs of malnutrition, as reduction in body weight gain and serum albumin levels, as well as liver fat. Serum insulin in the fed state and insulin secretion by isolated pancreatic islets in response to glucose were Keduced,but peripheral sensitivity to insulin was increased and glucose tolerance was not changed in the protein deficient rats, indicating adaptation to malnutrition. Diet protocol for nutritional recovery was efficient in repairing body weight gain, serum albumin and liver fat levels of the previously malnourished rats. Glucose induced insulin release by pancreatic islets remained low after nutritional recovery. Insulin secretion by the islets isolated from rats submitted to exercise training during nutritional recovery was improved when compared with the sedentary animals. Conclusion: This indicates that exercise training may be useful in the treatment of protein calorie malnutrition, concerning to glucose induced insulip secretion.

Targeting histone deacetylases for the treatment of immune, endocrine and metabolic disorders

The 'histone code' is a well-established hypothesis describing the idea that specific patterns of post-translational modifications to histones act like a molecular "code" recognised and used by non-histone proteins to regulate specific chromatin functions. One modification which has received significant attention is that of histone acetylation. The enzymes which regulate this modification are described as histone acetyltransferases or HATs, and histone deacetylases or HDACs. Due to their conserved catalytic domain HDACs have been actively targeted as a therapeutic target. The proinflammatory environment is increasingly being recognised as a critical element for both degenerative diseases and cancer. The present review will discuss the current knowledge surrounding the clinical potential & current development of histone deacetylases for the treatment of diseases for which a proinflammatory environment plays important roles, and the molecular mechanisms by which such inhibitors may play important functions in modulating the proinflammatory environment. © 2009 Bentham Science Publishers Ltd.

Ebselen and cytokine-induced nitric oxide synthase expression in insulin-producing cells

Interleukin-1 (IL-1) may be a mediator of β-cell damage in insulin-dependent diabetes mellitus (IDDM). The IL-1 mechanism of action on insulin-producing cells probably includes activation of the transcription nuclear factor κB (NF-κB), increased transcription of the inducible form of nitric oxide synthase (iNOS) and the subsequent production of nitric oxide (NO). Reactive oxygen intermediates, particularly H2O2, have been proposed as second messengers for NF-κB activation. In the present study, we tested whether ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one), a glutathione peroxidase mimicking compound, could counteract the effects of IL-1β, H2O2 and alloxan in rat pancreatic islets and in the rat insulinoma cell line RINm5F (RIN cells). Some of these experiments were also reproduced in human pancreatic islets. Ebselen (20 μM) prevented the increase in nitrite production by rat islets exposed to IL-1β for 6 hr and induced significant protection against the acute inhibitory effects of alloxan or H2O2 exposure, as judged by the preserved glucose oxidation rates. However, ebselen failed to prevent the increase in nitrite production and the decrease in glucose oxidation and insulin release by rat islets exposed to IL-1β for 24 hr. Ebselen prevented the increase in nitrite production by human islets exposed for 14 hr to a combination of cytokines (IL-1β, tumor necrosis factor-α and interferon-γ). In RIN cells, ebselen counteracted both the expression of iNOS mRNA and the increase in nitrite production induced by 6 hr exposure to IL-β but failed to block IL-1β-induced iNOS expression following 24 hr exposure to the cytokine. Moreover, ebselen did not prevent IL-1β-induced NF-κB activation. As a whole, these data indicate that ebselen partially counteracts cytokine-induced NOS activation in pancreatic β-cells, an effect not associated with inhibition of NF-κB activation.

Synergistic effect of glucose and prolactin on GLUT2 expression in cultured neonatal rat islets

We studied the synergistic effect of glucose and prolactin (PRL) on insulin secretion and GLUT2 expression in cultured neonatal rat islets. After 7 days in culture, basal insulin secretion (2.8 mM glucose) was similar in control and PRL-treated islets (1.84 ± 0.06% and 2.08 ± 0.07% of the islet insulin content, respectively). At 5.6 and 22 mM glucose, insulin secretion was significantly higher in PRL-treated than in control islets, achieving 1.38 ± 0.15% and 3.09 ± 0.21 % of the islet insulin content in control and 2.43 ± 0.16% and 4.31 ± 0.24% of the islet insulin content in PRL-treated islets, respectively. The expression of the glucose transporter GLUT2 in B-cell membranes was dose-dependently increased by exposure of the islet to increasing glucose concentrations. This effect was potentiated in islets cultured for 7 days in the presence of 2 μg/ml PRL. At 5.6 and 10 mM glucose, the increase in GLUT2 expression in PRL-treated islets was 75% and 150% higher than that registered in the respective control. The data presented here indicate that insulin secretion, induced by different concentrations of glucose, correlates well with the expression of the B-cell-specific glucose transporter GLUT2 in pancreatic islets.

Long-term effect of prolactin treatment on glucose-induced insulin secretion in cultured neonatal rat islets

Insulin secretion and 45SCa2+ uptake and efflux were studied in neonatal rat islets maintained in culture for 7 or 19 days in the absence or presence of prolactin (PRL). Insulin secretion in response to glucose (G), leucine (Leu), arginine (Arg) and carbachol (Cch) was augmented after 7 and 19 days in culture, compared to basal secretion (G 2.8 mM), in both PRL- treated and control islets. However, the increase in insulin secretion induced by the above secretagogues was higher in islets cultured in the presence of PRL for 19 days. In PRL-treated islets, the 45Ca2+ content after a 5 min incubation in the presence of G, Leu, Arg and Cch was significantly higher than the control only in islets cultured for 19 days. Except with Arg, the 45Ca2+ uptake in PRL-treated islets after a 90 min incubation was also significantly higher than the control only in islets cultured for 19 days. Finally, Leu-induced alterations in the 45Ca2+ efflux were higher in PRL-treated than in control islets cultured for 7 or 19 days. In the absence of external Ca2+, the reduction in 45Ca2+ efflux induced by glucose was also significantly higher in PRL-treated than in control islets. This effect was slightly potentiated after 19 days in culture. These data further support the hypothesis that PRL treatment enhances maturation of the secretory mechanism in neonatal islets. This effect can be potentiated even more if the treatment is prolonged.

Pancreatic β-cell defects in women at risk of type 2 diabetes

We evaluated insulin release and insulin sensitivity in women with basal and/or postprandial hyperglycemia but normal oral glucose tolerance test (OGTT) in previous pregnancy (GHG). These women were individually matched with females without previous hyperglycemia (NGT). Both groups consisted of normal glucose-tolerant women at the time of this study. They underwent OGTT (75g; n= 32 pairs) and hyperglycemic clamp experiments (10mmoll-1; n=27 pairs) with plasma glucose, insulin, and C-peptide measurements and calculation of insulinogenic index, first- and second-phase insulin release, and insulin sensitivity index (ISI). The GHG group showed higher glycosylated hemoglobin levels (6.2±0.6% versus 5.8±0.8%; P<0.05); lower insulinogenic index at 30min (134.03±62.69pmolmmol-1 versus 181.59±70.26pmolmmoll-1; P<0.05) and diminished C-peptide response in relation to glucose (4.05±0.36nmolmmol-1 versus 4.23±0.36nmolmmol-1; P<0.05) at OGTT. Both groups did not show difference in insulin secretion and ISI by hyperglycemic clamp technique. We concluded that in up to 12 years from index pregnancy, women with previous GHG, presenting normal glucose tolerance and well-matched with their controls, showed β-cell dysfunction without change in ISI. As women with previous GHG are at risk of type 2 diabetes, β-cell dysfunction may be its primary defect. © 2003 Elsevier B.V. All rights reserved.

Localization of myosin-Va in subpopulations of cells in rat endocrine organs

Myosin-Va is a Ca 2+/calmodulin-regulated unconventional myosin involved in the transport of vesicles, membranous organelles, and macromolecular complexes composed of proteins and mRNA. The cellular localization of myosin-Va has been described in great detail in several vertebrate cell types, including neurons, melanocytes, lymphocytes, auditory tissues, and a number of cultured cells. Here, we provide an immunohistochemical view of the tissue distribution of myosin-Va in the major endocrine organs. Myosin-Va is highly expressed in the pineal and pituitary glands and in specific cell populations of other endocrine glands, especially the parafollicular cells of the thyroid, the principal cells of the parathyroid, the islets of Langerhans of the pancreas, the chromaffin cells of the adrenal medulla, and a subpopulation of interstitial testicular cells. Weak to moderate staining has been detected in steroidogenic cells of the adrenal cortex, ovary, and Leydig cells. Myosin-Va has also been localized to non-endocrine cells, such as the germ cells of the seminiferous epithelium and maturing oocytes and in the intercalated ducts of the exocrine pancreas. These data provide the first systematic description of myosin-Va localization in the major endocrine organs of rat. © 2008 Springer-Verlag.