Diabetes & Metabolism Journal

Original Articles

Impairment of Insulin Secretion by Fat Overload in Rat Pancreatic Islets and Effects of Antioxidants.: Chul Hee Kim, Chan Hee Kim, Hyeong Kyu Park, Kyo Il Suh, Ki Up Lee; Korean Diabetes J. 2002;26(5):347-356. Published online October 1, 2002

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It has recently been suggested that fat overload on pancreatic beta cells is responsible for the abnormal pattern of insulin secretion in type 2 diabetes mellitus. Antioxidant treatment was reported to preserve beta cell function in animal models of diabetes. This study was undertaken to examine the effects of various free fatty acids and triglyceride on insulin secretion in isolated rat pancreatic islets. In addition, we examined the effects of antioxidants. METHODS: Pancreatic islets of normal Sprague-Dawley rats were isolated by intraductal injection of collagenase and Ficoll-gradient centrifugation. The islets were treated with palmitat0e (C16:0), oleate (C18:1), linoleate (C18:2), and triglyceride emulsions (intralipid) for 72hours. Basal and glucose-stimulated insulin secretions were measured. The effects of the antioxidants, vitamin E, alpha-lipoic acid, and N-acetyl cysteine, were examined on the fat-induced change of insulin secretion. RESULTS: All of the free fatty acids and the triglyceride increased the basal insulin secretion. In contrast, insulin secretion stimulated by 27 mM glucose was significantly decreased after the treatment with free fatty acids or triglycerides. The antioxidant could not prevent the fat-induced inhibition of insulin secretion. CONCLUSION: These results show that various free fatty acids and triglyceride commonly cause defects in insulin secretion. However, we could not confirm the the hypothesis that increased oxidative stress may be involved in the pathogenesis of insulin secretory defect associated with fat overload.

Effects of Hydrogen Peroxide on Insulin Secretion in Rat Pancreatic Islets.: Chul Hee Kim, Chan Hee Kim, Hyeong Kyu Park, Kyo Il Suh, Ki Up Lee; Korean Diabetes J. 2002;26(4):265-273. Published online August 1, 2002

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It has been hypothesized that reactive oxygen species (ROS) are involved in the progression of beta cell dysfunction in both type 1 and type 2 diabetes mellitus. On the other hand, recent evidence suggests that ROS might be an integral component of intracellular signaling. This study was undertaken to examine effects of hydrogen peroxide (H2O2) on insulin secretion by various secretagogues in isolated rat pancreatic islets. METHODS: Pancreatic islets from normal Sprague-Dawley rats were isolated by intraductal injection of collagenase and Ficoll-gradient centrifugation. Isolated islets were treated with H2O2 directly added to the culture media or continuously generated by glucose-glucose oxidase system for 24 hours. Insulin secretion stimulated by glucose, arginine, and KCl was measured by radioimmunoassay. RESULTS: Basal insulin secretion was increased after treatment with H2O2. Treatment with low concentration of H2O2 stimulated insulin secretion in response to 27 mM glucose. In contrast, insulin secretion stimulated by 27 mM glucose was significantly decreased after treatment with high concentrations of H2O2. Arginine- stimulated insulin secretion was increased by both low- and high concentrations of H2O2. Insulin secretion stimulated by KCl was not affected by treatment with H2O2. CONCLUSION: These results suggest that the effect of H2O2 is diverse according to its concentration and different insulin secretagogues. In particular, H2O2 has a dual action on glucose-induced insulin secretion. At low concentration, H2O2 can stimulate insulin secretion probably by acting on signaling pathway of stimulus- secretion coupling. In contrast, high concentrations of H2O2 impairs glucose- induced insulin secretion, probably by acting as an oxidative stress.

Effects of Antioxidants on Ethidium Bromide-induced Inhibition of Insulin Secretion in Rat Pancreatic Islets.: Chul Hee Kim, Chan Hee Kim, Hyeong Kyu Park, Kyo Il Suh, Ki Up Lee; Korean Diabetes J. 2002;26(3):179-187. Published online June 1, 2002

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It was recently shown that mitochondrial function in pancreatic beta-cells is essential in nutrient-stimulated insulin secretion. The inhibition of mitochondrial DNA (mtDNA) transcription by ethidium bromide (EtBr) has been reported to suppress glucose-induced insulin secretion in beta-cell lines. This study was undertaken to examine the effects of EtBr on insulin secretion in isolated normal rat pancreatic islets, and to see whether antioxidants could protect the beta-cell function against the EtBr-induced impairment. METHODS: Pancreatic islets of normal Sprague-Dawley rats were isolated by intraductal injection of collagenase followed by Ficoll-gradient centrifugation. Isolated islets were treated with 0.2 +/- 2.0 microgram/mL of EtBr for 2 to 6 days, and the glucose-stimulated insulin secretion measured. The effects of the antioxidant, vitamin E and alpha-lipoic acid, on the EtBr-induced inhibition of insulin secretion were also examined. RESULTS: EtBr inhibited the basal and glucose-stimulated insulin secretion in normal rat pancreatic islets in a dose- and time-dependent manner. Vitamin E and alpha-lipoic acid prevented the EtBr-induced inhibition of insulin secretion. CONCLUSION: Our results show that antioxidant can protect normal rat pancreatic islets from the EtBr-induced inhibition of insulin secretion. This suggests that oxidative stress is involved in the pathogenesis of the insulin secretory defect associated with mitochondrial dysfunction.

Regulation of Guanine Nucleotide Binding Protein Activity in Normal Rat Pancreatic Islet Secretory Granule by Arachidonic Acid and its Metabolites.: Yeon Ah Sung; Korean Diabetes J. 1999;23(2):120-130. Published online January 1, 2001

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Abstract PDF: BACKGROUND
Arachidonic acid (AA) generated by the activation of phospholipase A2. and 12hydroxyeicasateraenoic acid (12-HEIE), a lipoxygenase metabolite of AA stimulates glucose induced insulin secretion in pancreatic islets, whereas prostaglandin E. (PGE2), a cyclooxygenase metabolite of AA, inhibits it. This effect of PGE2 is largely reversible by pertussis toxin, suggesting a possible involvement of Gi or Go-like proteins. The purpose of this study was to investigate the regulation of guanine nucleotide binding protein (G-protein) activity by AA and its metabolites (12-HETE and PGE2) in the secretory granule of normal rat pancreatic islets. METHODS: After isolation and subcellular fractionation of pancreatic islets from male Sprague Dawley rats, measurements of GTPase activity using [y-P32]GTP hydrolysis and GTP binding by [y-S35] GTPvS in secretory granules were performed. RESULTS: l. AA inhibited a high affinity low Km and a low affinity high Km GTPase activity in the normal rat islet secretory granule in a concentration dependent manner. Half maximal inhibition was demonstrable at 90 pM AA concentration known to stimulate both phases of glucose-induced insulin secretion. 2. PGE2 stimulated a high affinity low Km and a low affinity high Km GTPase activity in the normal rat islet secretory granule in a concentration dependent manner and this effect was more prominent in the high affinity low Km GTPase. Half maximal stimulation was demonstrable at 0.8uM PGE2, a concentration known to inhibit both phases of glucose-induced insulin secretion from pure beta cell lines and pancreatic islets. 3. PGE2 as well as other inhibitors of insulin secretion (such as epinephrine or clonidine) also stimulated high and low Km GTPase activity and these effects on low Km GTPase were recovered by pertussis pretreatment. Of the five prostaglandins (PGF2, PGA2, PGD2, and PGB2), only PGE2. stiniulated GTPase activity significantly (p<0.05). 4. 12-HETE had no effecl on GTPase activity. 5. AA increased GTP binding significantly(p< 0.05), but 12-HETE and PGE, had no measurable effects on GTP binding. CONCLUSION: In secretory granules of normal rat pancreatic islet cells, AA might have their stimulatory effect on insulin secretion via inhibition of GTPase activity and increased GTP binding, whereas PGE might represent their action via stimulation of low Km GTPase activity.

Effects of Free Fatty Acid on Insulin Secretion in Cultured Rat Pancreatic Islets.: Hong Kyu Kim, Young Il Kim, Chul Hee Kim, Joong Yoel Park, Sung Kwan Hong, Jae Dam Lee, Ki Up Lee; Korean Diabetes J. 1997;21(4):381-387. Published online January 1, 2001

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It has been recently suggested that enhanced fat oxidation is responsible for the abnormal insulin secretory pattern in non-insulin-dependent diabetes mellitus. This study was undertaken to assess the effect of chronic exposure of pancreatic islets to free fatty acid on insulin secretion. METHODS: Rat pancreatic islets were cultured in various concentrations of glucose(5.5, 11, 27 mM) for 48 hrs with or without addition of free fatty acid(90 upM linoleic acid), and the basal and glucose-stimulated insulin secretion were measured. The effect of fatty acid oxidation inhibitor(2-bromopalmitate) was also tested. RESULTS: Islets cultured in high glucose concentrations showed a marked increase in basal insulin secretion. Free fatty acid stimulated the basal insulin secretion in islets cultured at 5.5 or 11 mM glucose, but no additional effect was seen in islets eultured at 27 mM glucose. In contrast, glucose-stimulated insulin secretion was decreased in islets cultured in high glucose media. Exposure to free fatty acid exerted an additive inhibitory effect on glucose-induced insulin secretion in islets cultured at 5.5 or 1 1 mM glucose, but not in islets cultured at 27M glucose, An inhibitor of fatty acid oxidation, 2-bromopalmitate, prevented the fatty acid-induced changes in both basal and glucosestimulated insulin secretion. CONCLUSION: These results showed that longterm exposure of pancreatic islets to free fatty acid altered the dynamics of insulin secretion, probably through a glucosefatty acid cycle.

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