t0901317 and Insulinoma

Increased basal and loss of glucose-stimulated insulin secretion (GSIS) are hallmarks of beta-cell dysfunction associated with type 2 diabetes. It has been proposed that elevated glucose promotes insulin secretory defects by activating sterol regulatory element binding protein (SREBP)-1c, lipogenic gene expression, and neutral lipid storage. Activation of liver X receptors (LXRs) also activates SREBP-1c and increases lipogenic gene expression and neutral lipid storage but increases basal and GSIS. This study was designed to characterize the changes in de novo fatty acid and triacylglyceride (TAG) synthesis in LXR-activated beta-cells and determine how these changes contribute to elevated basal and GSIS. Treatment of INS-1 beta-cells with LXR agonist T0901317 and elevated glucose led to markedly increased nuclear localization of SREBP-1, lipogenic gene expression, de novo synthesis of monounsaturated fatty acids and TAG, and basal and GSIS. LXR-activated cells had increased fatty acid oxidation and expression of genes involved in mitochondrial beta-oxidation, particularly carnitine palmitoyltransferase-1. Increased basal insulin release from LXR-activated cells coincided with rapid turnover of newly synthesized TAG and required acyl-coenzyme A synthesis and mitochondrial beta-oxidation. GSIS from LXR-activated INS-1 cells required influx of extracellular calcium and lipolysis, suggesting production of lipid-signaling molecules from TAG. Inhibition of diacylglyceride (DAG)-binding proteins, but not classic isoforms of protein kinase C, attenuated GSIS from LXR-activated INS-1 cells. In conclusion, LXR activation in beta-cells exposed to elevated glucose concentrations increases de novo TAG synthesis; subsequent lipolysis produces free fatty acids and DAG, which are oxidized to increase basal insulin release and activate DAG-binding proteins to enhance GSIS, respectively.

Topics: Animals; Carnitine O-Palmitoyltransferase; Cell Line, Tumor; Cells, Cultured; Disease Models, Animal; DNA-Binding Proteins; Fatty Acids; Glucose; Humans; Hydrocarbons, Fluorinated; Insulin; Insulin Secretion; Insulin-Secreting Cells; Insulinoma; Lipid Metabolism; Liver X Receptors; Orphan Nuclear Receptors; Pancreatic Neoplasms; Rats; Receptors, Cytoplasmic and Nuclear; Sterol Regulatory Element Binding Protein 1; Sulfonamides; Triglycerides

Liver X receptors (LXRalpha and LXRbeta) regulate glucose and lipid metabolism. Pancreatic beta-cells and INS-1E insulinoma cells express only the LXRbeta isoform. Activation of LXRbeta with the synthetic agonist T0901317 increased glucose-induced insulin secretion and insulin content, whereas deletion of the receptor in LXRbeta knockout mice severely blunted insulin secretion. Analysis of gene expression in LXR agonist-treated INS-1E cells and islets from LXRbeta-deficient mice revealed that LXRbeta positively regulated expression of ATP-binding cassette transporter A1 (ABCA1), sterol regulatory element-binding protein 1 (SREBP-1), insulin, PDX-1, glucokinase, and glucose transporter 2 (Glut2). Down-regulation of SREBP-1 expression with the specific small interfering RNA blocked basal and LXRbeta-induced expression of pancreatic duodenal homeobox 1 (PDX-1), insulin, and Glut2 genes. SREBP-1 small interfering RNA also prevented an increase in insulin secretion and insulin content induced by T0901317. Moreover, 5-(tetradecyloxy)-2-furoic acid, an inhibitor of the SREBP-1 target gene acetyl-coenzyme A carboxylase, blocked T0901317-induced stimulation of insulin secretion. In conclusion, activation of LXRbeta in pancreatic beta-cells increases insulin secretion and insulin mRNA expression via SREBP-1-regulated pathway. These data support the role of LXRbeta, SREBP-1, and cataplerosis/anaplerosis pathways in the control of insulin secretion in pancreatic beta-cells.

Topics: Alternative Splicing; Animals; Cell Line, Tumor; DNA-Binding Proteins; Gene Expression Regulation; Glucose Intolerance; Homeodomain Proteins; Hydrocarbons, Fluorinated; Insulin; Insulin Secretion; Insulinoma; Islets of Langerhans; Liver X Receptors; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Orphan Nuclear Receptors; Pancreatic Neoplasms; Receptors, Cytoplasmic and Nuclear; RNA, Messenger; RNA, Small Interfering; Sterol Regulatory Element Binding Protein 1; Sulfonamides; Trans-Activators