Decreased levels of metabolic enzymes in pancreatic islets of patients with type 2 diabetes

Abstract

Aims/hypothesis Glucose-stimulated insulin secretion is defective in patients with type 2 diabetes. We sought to acquire new information about enzymes of glucose metabolism, with an emphasis on mitochondrial enzymes, by comparing pancreatic islets of type 2 diabetes patients with those of non-diabetic controls. Methods Expression of genes encoding 13 metabolic enzymes was estimated with microarrays and activities of up to nine metabolic enzymes were measured. Results The activities of the mitochondrial enzymes, glycerol phosphate dehydrogenase, pyruvate carboxylase (PC) and succinyl-CoA:3-ketoacid-CoA transferase (SCOT) were decreased by 73%, 65% and 92%, respectively, in the diabetic compared with the non-diabetic islets. ATP citrate lyase, a cytosolic enzyme of the mitochondrial citrate pyruvate shuttle, was decreased 57%. Activities of propionyl-CoA carboxylase, NADP-isocitrate dehydrogenase, cytosolic malic enzyme, aspartate aminotransferase and malate dehydrogenase were not significantly different from those of the control. The low activities of PC and SCOT were confirmed with western blots, which showed that their protein levels were low. The correlation of relative mRNA signals with enzyme activities was good in four instances, moderate in four instances and poor in one instance. In diabetic islets, the mRNA signal of the islet cell-enriched transcription factor musculoaponeurotic fibrosarcoma oncogene homologue A, which regulates expression of islet genes, including the PC gene, was decreased to 54% of the control level. PC activity and protein levels in the non-diabetic islets were significantly lower than in islets from non-diabetic rodents. Conclusions/interpretation Low levels of certain islet metabolic enzymes, especially mitochondrial enzymes, are associated with human type 2 diabetes.

ATP-Citrate Lyase Reduction Mediates Palmitate-induced Apoptosis in Pancreatic Beta Cells

“…After a 4-week high fat diet, islet ACLY protein levels were significantly reduced (Fig. 2), to a similar degree as we observed in vitro and as has been reported for type 2 diabetic human islets (15). Together, these data validate our in vitro culture system and demonstrate the physiological relevance of our observations.…”

“…ACLY is also expressed and active in pancreatic beta cells (14). ACLY levels and activity are significantly reduced in pancreatic islets from patients with type 2 diabetes (15). Several groups have investigated the role of ACLY in glucose-stimulated insulin release and have obtained conflicting results (16,17).…”

Peroxisome Proliferator-activated Receptor γ (PPARγ) and Its Target Genes Are Downstream Effectors of FoxO1 Protein in Islet β-Cells

“…We have shown that islet PPAR␥ and its target genes are hyperexpressed in nondiabetic insulin-resistant rats (present study and Refs. 6,8) and rats during the ␤-cell adaptation after a partial pancreatectomy (4, 6), plus we and others have shown impaired expression of these same genes in diabetic animals (6,29,44) and humans with type 2 diabetes (30,45,46). Collectively, these findings support, but do not prove, a FoxO1/PPAR␥-mediated transcriptional network functioning as a core element of how ␤-cells adapt to metabolic stress, with failure of this response causing or exacerbating diabetes.…”

“…This finding is not unique to the Px ZF model, as nuclear FoxO1 and reduced overall islet FoxO1 expression were found in insulin-resistant diabetic mice created by deletion of insulin receptors in GLUT4-expressing tissues (43). Also, below normal islet expression of Pdx1, GIP receptor, and PC is well known to occur in animals (6,28,29,44) and humans (30,45,46) with type 2 diabetes. Based on this collective evidence, we propose that nuclear retention of FoxO1 impairing PPAR␥-mediated defenses against metabolic stresses is an unrecognized feature of failed ␤-cell adaptation.…”

Supplement of TCA cycle intermediates protects against high glucose/palmitate-induced INS-1 beta cell death

“…Recently, a report that incomplete fatty acid oxidation through depletion of TCA cycle intermediates contributed to FFAinduced insulin resistance suggest that mitochondrial dysfunction, rather than activation of lipid-mediated signals, may be a direct mediator for FFA-induced toxicity to beta cells [31]. Furthermore, decrease of various mitochondrial enzymes such as pyruvate carboxylase, glutamate dehydrogenase, isocitrate dehydrogenase, and ATP synthase in beta cells under hyperglycemic and hyperlipidemic conditions supports that mitochondrial dysfunction through metabolic failure may be a critical contributor to high glucose/FFAinduced cytotoxicity to beta cells [23,32,33].…”