Role of uncoupling protein-2 up-regulation and triglyceride accumulation in impaired glucose-stimulated insulin secretion in a beta-cell lipotoxicity model overexpressing sterol regulatory element-binding protein-1c.

Triglyceride (TG) accumulation in pancreatic beta-cells is thought to be associated with impaired insulin secretory response to glucose (lipotoxicity). To better understand the mechanism of the impaired insulin secretory response to glucose in beta-cell lipotoxicity, we overexpressed a constitutively active form of the sterol regulatory element-binding protein- 1c (SREBP-1c), a master transcriptional factor of lipogenesis, in INS-1 cells with an adenoviral vector. This treatment was associated with strong activation of transcription of the genes involved in fatty acid biosynthesis, increased cellular TG content, severely blunted glucose-stimulated insulin secretion, and enhanced expression of the uncoupling protein-2 (UCP-2), which supposedly dissipates the mitochondrial electrochemical potential. To decrease the up-regulated UCP-2 expression, small interfering RNA for UCP-2 was used. Introduction of the small interfering RNA increased the ATP/ADP ratio and partially rescued the glucose-stimulated insulin secretion in the cells overexpressing SREBP-1c, but did not affect the cellular TG content. Next, the effect of the AMP-activated protein kinase (AMPK) agonist, 5-amino-4-imidazolecarboxamide riboside, was examined in the lipotoxicity model. Exposure of the cells with lipotoxicity to 5-amino-4-imidazolecarboxamide riboside increased free fatty acid oxidation, partially reversed the TG accumulation, phosphorylated AMPK and acetyl-coenzyme A carboxylase, and improved the impaired glucose-stimulated insulin secretion. These results suggest that UCP-2 down-regulation and AMPK activation could be candidate targets for releasing beta-cells from lipotoxicity.