Glucocorticoids in Vivo Induce Both Insulin Hypersecretion and Enhanced Glucose Sensitivity of Stimulus-Secretion Coupling in Isolated Rat Islets

Instituto de Bioingeniería, and Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas (CIBERDEM), Universidad Miguel Hernández de Elche, Elche 03202, Spain.
Endocrinology (Impact Factor: 4.5). 10/2009; 151(1):85-95. DOI: 10.1210/en.2009-0704
Source: PubMed


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 Ca(2+) signals by fluorescence microscopy also demonstrated a higher response to glucose in islets from DEX-treated animals. However, no differences in Ca(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, we explored the status of Ca(2+)-dependent signaling kinases. Unlike calmodulin kinase II, we found an augmented phosphorylation level of protein kinase C alpha 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 beta-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.

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Available from: Alex Rafacho, May 15, 2015
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    • "Additionally, ob/ob islets displayed higher glucose-induced ψm hyperpolarization, further supporting an enhanced β-cell mitochondrial performance in obese mice. All these findings are in agreement with previous reports showing similar metabolic responses in a rat model of insulin resistance (Rafacho et al., 2010). Since enhanced NAD(P)H production and Ψm hyperpolarization should be coupled to increased ATP synthesis (Quesada et al., 2006), the mitochondrial responses in ob/ob islets and their higher glucose sensitivity may explain the electrical activity at lower glucose levels compared with controls (Fig. 3). "
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    ABSTRACT: Obesity is associated with insulin resistance and is known to be a risk factor for type-2 diabetes. In obese individuals, pancreatic beta-cells try to compensate for the increased insulin demand in order to maintain euglycemia. Most studies have reported that this adaptation is due to morphological changes. However, the involvement of beta-cell functional adaptations in this process needs to be clarified. For this purpose, we evaluated different key steps in the glucose-stimulated insulin secretion (GSIS) in intact islets from female ob/ob obese mice and lean controls. Obese mice showed increased body weight, insulin resistance, hyperinsulinemia, glucose intolerance and fed hyperglycemia. Islets from ob/ob mice exhibited increased glucose-induced mitochondrial activity, reflected by enhanced NAD(P)H production and mitochondrial membrane potential hyperpolarization. Perforated patch-clamp examination of beta-cells within intact islets revealed several alterations in the electrical activity such as increased firing frequency and higher sensitivity to low glucose concentrations. A higher intracellular Ca(2+) mobilization in response to glucose was also found in ob/ob islets. Additionally, they displayed a change in the oscillatory pattern and Ca(2+) signals at low glucose levels. Capacitance experiments in intact islets revealed increased exocytosis in individual ob/ob beta-cells. All these up-regulated processes led to increased GSIS. In contrast, we found a lack of beta-cell Ca(2+) signal coupling, which could be a manifestation of early defects that lead to beta-cell malfunction in the progression to diabetes. These findings indicate that beta-cells functional adaptations are an important process in the compensatory response to obesity. Copyright © 2015. Published by Elsevier Ireland Ltd.
    Molecular and Cellular Endocrinology 01/2015; 404(C). DOI:10.1016/j.mce.2015.01.033 · 4.41 Impact Factor
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    • "Thus, when í µí»½-cells can no longer compensate, a glucolipotoxicity process progressively develops that induces í µí»½cell death accompanied by hypoinsulinemia, hyperglycemia, and hyperlipidemia [15]. The í µí»½-cell compensations [17] can be rapidly obtained experimentally by 5-day treatment with DEX (5 days) [18] [19] that induces peripheral IR [20] [21], which is associated with increased hepatic gluconeogenesis and lipolysis [8]. "
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    ABSTRACT: Glucocorticoid (GC) therapies may adversely cause insulin resistance (IR) that lead to a compensatory hyperinsulinemia due to insulin hypersecretion. The increased íµí»½-cell function is associated with increased insulin signaling that has the protein kinase B (AKT) substrate with 160 kDa (AS160) as an important downstream AKT effector. In muscle, both insulin and AMP-activated protein kinase (AMPK) signaling phosphorylate and inactivate AS160, which favors the glucose transporter (GLUT)-4 translocation to plasma membrane. Whether AS160 phosphorylation is modulated in islets from GC-treated subjects is unknown. For this, two animal models, Swiss mice and Wistar rats, were treated with dexamethasone (DEX) (1 mg/kg body weight) for 5 consecutive days. DEX treatment induced IR, hyperinsulinemia, and dyslipidemia in both species, but glucose intolerance and hyperglycemia only in rats. DEX treatment caused increased insulin secretion in response to glucose and augmented íµí»½-cell mass in both species that were associated with increased islet content and increased phosphorylation of the AS160 protein. Protein AKT phosphorylation, but not AMPK phosphorylation, was found significantly enhanced in islets from DEX-treated animals. We conclude that the augmented íµí»½-cell function developed in response to the GC-induced IR involves inhibition of the islet AS160 protein activity.
    International Journal of Endocrinology 09/2014; 2014(1). DOI:10.1155/2014/983453 · 1.95 Impact Factor
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    • "Assessment of HPA-axis function revealed that both peak and stress induced corticosterone levels were elevated in NRic-KO mice. The metabolic derangements observed in NRic-KO mice are consistent with the systemic effects of glucocorticoid action to increase hepatic TG [71] and glucose production [72], and promote insulin secretion [73]. Reduced energy expenditure and lower RER, together with decreased body temperature, observed in NRic-KO mice are also consistent with the reported actions of corticosterone to promote preferential oxidation of fat over glucose in muscle [74], while simultaneously decreasing the rate of fat oxidized through non-shivering thermogenesis in brown adipose tissue [75]. "
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    ABSTRACT: Insulin signaling in the central nervous system (CNS) regulates energy balance and peripheral glucose homeostasis. Rictor is a key regulatory/structural subunit of the mTORC2 complex and is required for hydrophobic motif site phosphorylation of Akt at serine 473. To examine the contribution of neuronal Rictor/mTORC2 signaling to CNS regulation of energy and glucose homeostasis, we utilized Cre-LoxP technology to generate mice lacking Rictor in all neurons, or in either POMC or AgRP expressing neurons. Rictor deletion in all neurons led to increased fat mass and adiposity, glucose intolerance and behavioral leptin resistance. Disrupting Rictor in POMC neurons also caused obesity and hyperphagia, fasting hyperglycemia and pronounced glucose intolerance. AgRP neuron specific deletion did not impact energy balance but led to mild glucose intolerance. Collectively, we show that Rictor/mTORC2 signaling, especially in POMC-expressing neurons, is important for central regulation of energy and glucose homeostasis.
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