triacsin-c and Diabetes-Mellitus

Like obese humans, Zucker diabetic fatty (ZDF) rats exhibit early beta cell compensation for insulin resistance (4-fold beta cell hyperplasia) followed by decompensation (>50% loss of beta cells). In prediabetic and diabetic ZDF islets, apoptosis measured by DNA laddering is increased 3- and >7-fold, respectively, compared with lean ZDF controls. Ceramide, a fatty acid-containing messenger in cytokine-induced apoptosis, was significantly increased (P < 0.01) in prediabetic and diabetic islets. Free fatty acids (FFAs) in plasma are high (>1 mM) in prediabetic and diabetic ZDF rats; therefore, we cultured prediabetic islets in 1 mM FFA. DNA laddering rose to 19.6% vs. 4.6% in lean control islets, preceded by an 82% increase in ceramide. C2-Ceramide without FFA induced DNA laddering, but fumonisin B1, a ceramide synthetase inhibitor, completely blocked FFA-induced DNA laddering in cultured ZDF islets. [3H]Palmitate incorporation in [3H]ceramide in ZDF islets was twice that of controls, but [3H]palmitate oxidation was 77% less. Triacsin C, an inhibitor of fatty acyl-CoA synthetase, and troglitazone, an enhancer of FFA oxidation in ZDF islets, both blocked DNA laddering. These agents also reduced inducible nitric oxide (NO) synthase mRNA and NO production, which are involved in FFA-induced apoptosis. In ZDF obesity, beta cell apoptosis is induced by increased FFA via de novo ceramide formation and increased NO production.

Topics: Animals; Apoptosis; Cells, Cultured; Ceramides; Chromans; Coenzyme A Ligases; Diabetes Mellitus; Diabetes Mellitus, Type 2; DNA Fragmentation; Enzyme Induction; Enzyme Inhibitors; Fatty Acids, Nonesterified; Guanidines; Homozygote; Humans; Hypoglycemic Agents; Islets of Langerhans; Male; Nitric Oxide Synthase; Nitric Oxide Synthase Type II; Obesity; Palmitic Acid; Prediabetic State; Rats; Rats, Zucker; Repressor Proteins; RNA, Messenger; Saccharomyces cerevisiae Proteins; Thiazoles; Thiazolidinediones; Transcription, Genetic; Triazenes; Troglitazone

Diabetic states are characterized by a raised serum/islet level of long chain fatty acids and a lowered ED50 for glucose-induced insulin secretion. Prolonged culture (> 6 h) of islets with long chain fatty acids replicates the basal insulin hypersecretion. We examined this effect in rat islets cultured for 24 h with 0.25 mM oleate. Insulin secretion at 2.8 mM glucose was doubled in combination with a 60% lowered islet content of glucose-6-phosphate (G6P). Investigation of the lowered G6P showed: (a) increased glucose usage from 0.5 to 100 mM glucose with identical values measured by [2-3H]glucose and [5-3H]glucose, (c) indicating little glucose- 6-phosphatase activity, (b) unchanged low pentose phosphate shunt activity, (c) 50% increased phosphofructokinase (PFK) Vmax, (d) a normal ATP/ADP ratio, and (e) unchanged fructose 2,6 bisphosphate content. Triacsin C, an inhibitor of fatty acyl-CoA synthetase, prevented the increase in PFK activity and the lowered G6P content. These results suggest that long chain acyl-CoA mediates the rise in PFK activity, which in turn lowers the G6P level. We speculate that the inhibition of hexokinase by G6P is thus attenuated, thereby causing the basal insulin hypersecretion.

Topics: Allosteric Regulation; Animals; Coenzyme A Ligases; Diabetes Mellitus; Drug Interactions; Fructosediphosphates; Glucose; Glucose-6-Phosphate; Insulin; Insulin Secretion; Islets of Langerhans; Oleic Acid; Oxidation-Reduction; Pentose Phosphate Pathway; Phosphofructokinase-1; Rats; Rats, Sprague-Dawley; Repressor Proteins; Saccharomyces cerevisiae Proteins; Triazenes