Sterol regulatory element-binding protein 1 mediates liver X receptor-β-induced increases in insulin secretion and insulin messenger ribonucleic acid levels

Liver X receptors (LXR alpha and LXR beta) regulate glucose and lipid metabolism. Pancreatic beta-cells and INS-1E insulinoma cells express only the LXR beta isoform. Activation of LXR beta with the synthetic agonist T0901317 increased glucose-induced insulin secretion and insulin content, whereas deletion of the receptor in LXR beta knockout mice severely blunted insulin secretion. Analysis of gene expression in LXR agonist-treated INS-1E cells and islets from LXR beta-deficient mice revealed that LXR beta 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 LXR beta-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 LXR beta in pancreatic beta-cells increases insulin secretion and insulin mRNA expression via SREBP-1-regulated pathway. These data support the role of LXR beta, SREBP-1, and cataplerosis/ anaplerosis pathways in the control of insulin secretion in pancreatic beta-cells.