Three isoforms of a novel member of the steroid hormone nuclear receptor superfamily related to the retinoic acid receptors have been identified. The three isoforms, referred to as RORal, R0Ra2, and R0Ra3, share common DNA-and putative ligand-binding domains but are characterized by distinct amino-terminal domains generated by alternative RNA processing. An exon encoding a functionally important subregion of the amino-terminal domain of the RORa2 isoform resides on the opposite strand of a cytochrome c-processed pseudogene. Binding site selection using in vitro-synthesized proteins reveals that the RORal and R0Ra2 isoforms bind DNA as monomers to hormone response elements composed of a 6-bp AT-rich sequence preceding a half-site core motif PuGGTCA (RORE). However, RORal and RORa2 display different binding specificities: RORal binds to and constitutively activates transcription from a large subset of ROREs, whereas R0Ra2 recognizes ROREs with strict specificity and displays weaker transcriptional activity. The differential DNA-binding activity of each isoform maps to their respective amino-terminal domains. Whereas truncation of the amino-terminal domain diminishes the ability of RORal to bind DNA, a similar deletion relaxes RORa2-binding specificity to that displayed by RORal. Remarkably, transfer of the entire amino-terminal region of RORal or amino-terminal deletion of RORa2 confers RORE-binding specificities to het^erologous receptors. These results demonstrate that the amino-terminal domain and the zinc finger region work in concert to confer high affinity and specific DNA-binding properties to the ROR isoforms and suggest a novel strategy to control DNA-binding activity of nuclear receptors.

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) control glucose homeostasis through well-defined actions on the islet β cell via stimulation of insulin secretion and preservation and expansion of β cell mass. We examined the importance of endogenous incretin receptors for control of glucose homeostasis through analysis of Glp1r -/-, Gipr -/-, and double incretin receptor knockout (DIRKO) mice fed a high-fat (HF) diet. DIRKO mice failed to upregulate levels of plasma insulin, pancreatic insulin mRNA transcripts, and insulin content following several months of HF feeding. Both single incretin receptor knockout and DIRKO mice exhibited resistance to diet-induced obesity, preservation of insulin sensitivity, and increased energy expenditure associated with increased locomotor activity. Moreover, plasma levels of plasminogen activator inhibitor-1 and resistin failed to increase significantly in DIRKO mice after HF feeding, and the GIP receptor agonist [D-Ala 2 ]GIP, but not the GLP-1 receptor agonist exendin-4, increased the levels of plasma resistin in studies of both acute and chronic administration. These findings extend our understanding of how endogenous incretin circuits regulate glucose homeostasis independent of the β cell via control of adipokine secretion and energy expenditure.

OBJECTIVE-Dipeptidyl peptidase-4 (DPP4) inhibitors lower blood glucose in diabetic subjects; however, the mechanism of action through which these agents improve glucose homeostasis remains incompletely understood. Although glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) represent important targets for DPP4 activity, whether additional substrates are important for the glucose-lowering actions of DPP4 inhibitors remains uncertain. RESEARCH DESIGN AND METHODS-We examined the efficacy of continuous vildagliptin administration in wild-type (WT) and dual incretin receptor knockout (DIRKO) mice after 8 weeks of a high-fat diet.RESULTS-Vildagliptin had no significant effect on food intake, energy expenditure, body composition, body weight gain, or insulin sensitivity in WT or DIRKO mice. However, glycemic excursion after oral glucose challenge was significantly reduced in WT but not in DIRKO mice after vildagliptin treatment. Moreover, vildagliptin increased levels of glucose-stimulated plasma insulin and reduced levels of cholesterol and triglycerides in WT but not in DIRKO mice. Vildagliptin treatment reduced the hepatic expression of genes important for cholesterol synthesis and fatty acid oxidation, including phospho-mevalonate kinase (Mvk), acyl-coenzyme dehydrogenase medium chain (Acadm), mevalonate (diphospho)decarboxylase (Mvd), and Acyl-CoA synthetase (Acsl1), in WT but not in DIRKO mice. However, vildagliptin also reduced levels of hepatic mRNA transcripts for farnesyl di-phosphate transferase (Fdft1), acetyl coenzyme A acyltransferase 1 (Acaa1), and carnitine palmitoyl transferase 1 (Cpt 1) in DIRKO mice. No direct effect of GLP-1 receptor agonists was detected on cholesterol or triglyceride synthesis and secretion in WT hepatocytes.CONCLUSIONS-These findings illustrate that although GLP-1 and GIP receptors represent the dominant molecular mechanisms for transducing the glucoregulatory actions of DPP4 inhibitors, prolonged DPP4 inhibition modulates the expression of genes important for lipid metabolism independent of incretin receptor action in vivo. Diabetes 56:3006-3013, 2007