ABSTRACT
A regimen of low-protein diet induces a reduction of pancreatic islet function that is associated with development of metabolic disorders including diabetes and obesity afterward. In the present study, the influence of leucine supplementation on metabolic parameters, insulin secretion to glucose and to amino acids, as well as the levels of proteins that participate in the phosphatidylinositol 3-phosphate kinase (PI3K) pathway was investigated in malnourished rats. Four groups were fed with different diets for 12 weeks: a normal protein diet (17%) without (NP) or with leucine supplementation (NPL) or a low (6%)-protein diet without (LP) or with leucine supplementation (LPL). Leucine was given in the drinking water during the last 4 weeks. As indicated by the intraperitoneal glucose tolerance test, LPL rats exhibited increased glucose tolerance as compared with NPL group. Both NPL and LPL rats had higher circulating insulin levels than controls. The LPL rats also showed increased insulin secretion by pancreatic islets in response to glucose or arginine compared with those observed in islets from LP animals. Glucose oxidation was significantly reduced in NPL, LP, and LPL isolated islets as compared with NP; but no alteration was observed for leucine and glutamate oxidation among the 4 groups. Western blotting analysis demonstrated increased PI3K and mammalian target protein of rapamycin protein contents in LPL compared with LP islets. A significant increase in insulin-induced insulin receptor substrate 1-associated PI3K activation was also observed in LPL compared with LP islets. These findings indicate that leucine supplementation can augment islet function in malnourished rats and that activation of the PI3K/mammalian target protein of rapamycin pathway may play a role in this process.
Subject(s)
Diabetes Mellitus, Type 2/metabolism , Insulin-Secreting Cells/metabolism , Insulin/metabolism , Leucine/administration & dosage , Malnutrition/metabolism , Animals , Blood Proteins/metabolism , Body Weight , Diabetes Mellitus, Type 2/drug therapy , Dietary Supplements , Glucose Tolerance Test , Insulin Secretion , Insulin-Secreting Cells/drug effects , Intracellular Signaling Peptides and Proteins/genetics , Intracellular Signaling Peptides and Proteins/metabolism , Male , Malnutrition/drug therapy , Phosphatidylinositol Phosphates/genetics , Phosphatidylinositol Phosphates/metabolism , Protein Serine-Threonine Kinases/genetics , Protein Serine-Threonine Kinases/metabolism , RNA/chemistry , RNA/genetics , Rats , Rats, Wistar , Reverse Transcriptase Polymerase Chain Reaction , Ribosomal Protein S6 Kinases/genetics , Ribosomal Protein S6 Kinases/metabolism , TOR Serine-Threonine KinasesABSTRACT
Low-protein diet impairs insulin secretion in response to nutrients and may induce several metabolic disorders including diabetes, obesity, and cardiovascular disease. In the present study, the influence of leucine supplementation on glutamate dehydrogenase (GDH) expression and glucose-induced insulin secretion (GIIS) was investigated in malnourished rats. Four groups were fed with different diets for 12 weeks: a normal-protein diet (17%) without or with leucine supplementation or a low (6%)-protein diet without (LP) or with leucine supplementation (LPL). Leucine (1.5%) was supplied in the drinking water. Western blotting analysis revealed reduced GDH expression in LP, whereas LPL displayed improved GDH expression, similar to control. The GIIS and leucine-induced insulin release were also enhanced in LPL compared with LP and similar to those observed in rats fed a normal-protein diet without leucine supplementation. In addition, GDH allosteric activators produced an increased insulin secretion in LPL. These findings indicate that leucine supplementation was able to increase GDH expression leading to GIIS restoration, probably by improved leucine metabolic pathways.
Subject(s)
Glucose/pharmacology , Glutamate Dehydrogenase/biosynthesis , Insulin/metabolism , Leucine/therapeutic use , Protein-Energy Malnutrition/drug therapy , Protein-Energy Malnutrition/metabolism , Animals , Blotting, Western , Glutamate Dehydrogenase/genetics , In Vitro Techniques , Islets of Langerhans/drug effects , Islets of Langerhans/metabolism , Male , Protein-Energy Malnutrition/enzymology , RatsABSTRACT
Glucagon secreted from pancreatic alpha-cells plays a critical role in glycemia, mainly by hepatic glucose mobilization. In diabetic patients, an impaired control of glucagon release can worsen glucose homeostasis. Despite its importance, the mechanisms that regulate its secretion are still poorly understood. Since alpha-cells are particularly sensitive to neural and paracrine factors, in this report we studied the role of purinergic receptors and extracellular ATP, which can be released from nerve terminals and beta-cell secretory granules. Using immunocytochemistry, we identified in alpha-cells the P2 receptor subtype P2Y1, as well as the P1 receptors A1 and A2A. In contrast, only P2Y1 and A1 receptors were localized in beta-cells. To analyze the role of purinergic receptors in alpha-cell function, we studied their participation in Ca2+ signaling. At low glucose concentrations, mouse alpha-cells exhibited the characteristic oscillatory Ca2+ signals that lead to secretion. Application of ATP (1-10 microM) abolished these oscillations or reduced their frequency in alpha-cells within intact islets and isolated in culture. ATPgammaS, a nonhydrolyzable ATP derivative, indicated that the ATP effect was mainly direct rather than through ATP-hydrolytic products. Additionally, adenosine (1-10 microM) was also found to reduce Ca2+ signals. ATP-mediated inhibition of Ca2+ signaling was accompanied by a decrease in glucagon release from intact islets in contrast to the adenosine effect. Using pharmacological agonists, we found that only P2Y1 and A2A were likely involved in the inhibitory effect on Ca2+ signaling. All these findings indicate that extracellular ATP and purinergic stimulation are effective regulators of the alpha-cell function.
Subject(s)
Adenosine Triphosphate/pharmacology , Calcium Signaling/drug effects , Glucagon-Secreting Cells/drug effects , Glucagon-Secreting Cells/metabolism , Glucagon/metabolism , Receptors, Purinergic/physiology , Adenosine/pharmacology , Adenosine Triphosphate/analogs & derivatives , Animals , Extracellular Space/drug effects , Extracellular Space/physiology , Glucose/pharmacology , Immunohistochemistry , In Vitro Techniques , Mice , Microscopy, Confocal , Paracrine Communication/drug effects , Paracrine Communication/physiology , Receptor, Adenosine A1/drug effects , Receptor, Adenosine A2A/drug effects , Receptors, Purinergic/drug effects , Receptors, Purinergic P2/drug effects , Receptors, Purinergic P2Y1ABSTRACT
Low protein diet has been shown to affect the levels and activities of several enzymes from pancreatic islets. To further extend the knowledge on how malnutrition affects insulin secretion pathway, we investigated in this work the insulin release induced by glucose or leucine, an insulin secretagogue, and the expression of insulin receptor (IR), insulin receptor substrate 1 (IRS1), phosphatidylinositol 3-kinase (PI3K), and p70S6K1 (S6K-1) proteins from pancreatic islets of rats fed a normal (17%; NP) or a low (6%; LP) protein diet for 8 weeks. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 16.7 mmol/L of glucose, or 2.8 mmol/L of glucose in the presence or absence of 20 mmol/L of leucine. Glucose- and leucine-induced insulin secretions were higher in NP than in LP islets. Western blotting analysis showed an increase in the expression of IR and PI3K protein levels whereas IRS1 and S6K-1 protein expression were lower in LP compared to NP islets. In addition, S6K-1 mRNA expression was also reduced in islets from LP rats. Our data indicate that a low protein diet modulates the levels of several proteins involved in the insulin secretion pathway. Particularly, the decrease in S6K-1 expression might be an important factor affecting either glucose- or leucine-induced insulin secretion.
Subject(s)
Diet, Protein-Restricted , Insulin/metabolism , Islets of Langerhans/metabolism , Ribosomal Protein S6 Kinases/metabolism , Animals , Animals, Newborn , Blotting, Western , Disease Models, Animal , Gene Expression Regulation, Enzymologic/drug effects , Glucose/pharmacology , In Vitro Techniques , Insulin Secretion , Leucine/pharmacology , Malnutrition/genetics , Malnutrition/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , Rats , Rats, Wistar , Reverse Transcriptase Polymerase Chain Reaction , Ribosomal Protein S6 Kinases/genetics , Signal Transduction/drug effectsABSTRACT
B cell destruction during the onset of diabetes mellitus is associated with oxidative stress. In this work, we attempted to further trace the fate of H2O2 inside the pancreatic islets and determine whether it is mediated by enzymatic (peroxidase) activity or by chemical reaction with thiols from any protein chain. Our results suggest that the islet cells have a very similar peroxidase activity at the hydrophilic (cytoplasm) and hydrophobic compartments (organelles and nucleus), independent of the catalase content of the samples. This activity is composed of sacrificial thiols and by proteins with Fe3+/Mn3+ ions at non-heme catalytic sites. The capacity of the hydrophobic fraction to scavenge O2- was increased in the presence of high concentrations of NADP* and RS* and was highly dependent on RSH. On the contrary, the hydrophilic fraction exhibited a low RSH-dependent activity where the O2- scavenging is related to metal Cu2+/Fe3+/Mn3+ ions attached to the protein molecules.
Subject(s)
Hydrogen Peroxide/pharmacology , Islets of Langerhans/metabolism , Animals , Animals, Newborn , Catalase/metabolism , Catalytic Domain , Cell Nucleus/metabolism , Cells, Cultured , Copper/chemistry , Cytoplasm/metabolism , Dose-Response Relationship, Drug , Heme/chemistry , Ions , Iron/metabolism , Kinetics , Magnesium/metabolism , Models, Chemical , NADP/chemistry , NADP/metabolism , NADP/physiology , Oxidation-Reduction , Oxidative Stress , Oxygen/metabolism , Peroxidase/metabolism , Rats , Sulfhydryl Compounds/chemistry , Superoxides/metabolism , Time FactorsABSTRACT
Undernutrition has been shown to affect the autonomic nervous system, leading to permanent alterations in insulin secretion. To understand these interactions better, we investigated the effects of carbamylcholine (CCh) and phorbol 12-myristate 13-acetate (PMA) on insulin secretion in pancreatic islets from rats fed a normal (17%; NP) or low (6%; LP) protein diet for 8 wk. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 8.3 mmol glucose/L, with or without PMA (400 nmol/L) and CCh. Increasing concentrations of CCh (0.1-1000 micro mol/L) dose dependently increased insulin secretion by islets from both groups of rats. However, insulin secretion by islets from rats fed the NP diet was significantly higher than that of rats fed the LP diet, and the dose-response curve to CCh was shifted to the right in islets from rats fed LP with a 50% effective concentration (EC(50)) of 2.15 +/- 0.7 and 4.64 +/- 0.1 micro mol CCh/L in islets of rats fed NP and LP diets, respectively (P < 0.05). PMA-induced insulin secretion was higher in islets of rats fed NP compared with those fed LP. Western blotting revealed that the protein kinase (PK)Calpha and phospholipase (PL)Cbeta(1) contents of islets of rats fed LP were 30% lower than those of islets of rats fed NP (P < 0.05). In addition, PKCalpha mRNA expression was reduced by 50% in islets from rats fed LP. In conclusion, a reduced expression of PKCalpha and PLCbeta(1) may be involved in the decreased insulin secretion by islets from LP rats after stimulation with CCh and PMA.
