Molecular basis for negative regulation of the glucagon receptor.

Members of the class B family of G protein-coupled receptors (GPCRs) bind peptide hormones and have causal roles in many diseases, ranging from diabetes and osteoporosis to anxiety. Although peptide, small-molecule, and antibody inhibitors of these GPCRs have been identified, structure-based descriptions of receptor antagonism are scarce. Here we report the mechanisms of glucagon receptor inhibition by blocking antibodies targeting the receptor's extracellular domain (ECD).

Exenatide blocks JAK1-STAT1 in pancreatic beta cells.

Exenatide (Ex-4) is an antidiabetic drug that acts through the glucagon-like peptide 1 receptor and has recently been approved for the treatment of type 2 diabetes mellitus. Ex-4 also has been shown to affect beta cell gene expression and increase beta cell mass in rodent models of type 1 diabetes mellitus, but the mechanisms are not fully understood. We therefore analyzed the pathways affected by Ex-4 in human islets by using oligonucleotide microarrays and the PathwayStudio software (Ariadne Genomics, Rockville, MD).

Metabolism-independent sugar effects on gene transcription: the role of 3-O-methylglucose.

Glucose effects on cellular functions such as gene expression require, in general, glucose metabolism at least to glucose-6-phosphate (G-6-P). However, the example of thioredoxin-interacting protein (TXNIP), a glucose-regulated gene involved in the cellular redox state and pancreatic beta cell apoptosis, demonstrates that this rule may not always apply. We found that aside form glucose, the nonmetabolizable sugars 2-deoxyglucose, which is still converted to G-6-P as well as 3-O-methylglucose (3-MG), which cannot be phosphorylated by glucokinase, stimulate TXNIP expression.

Exenatide inhibits beta-cell apoptosis by decreasing thioredoxin-interacting protein.

Exenatide (Ex-4) is a novel anti-diabetic drug that stimulates insulin secretion and enhances beta-cell mass, but the mechanisms involved are not fully understood. We found that Ex-4 protects INS-1 beta-cells against oxidative stress-induced apoptosis (TUNEL) and also reduces expression (mRNA and protein) of thioredoxin-interacting protein (TXNIP), a pro-apoptotic factor involved in beta-cell glucose toxicity and oxidative stress. This reduction was observed in INS-1 cells, mouse, and human islets as well as in wild-type mice receiving Ex-4 and was accompanied by decreased expression of the apoptotic factors caspase-3 and Bax.

Gene expression profiling in INS-1 cells overexpressing thioredoxin-interacting protein.

Thioredoxin-interacting protein (TXNIP) is overexpressed in diabetes and has deleterious effects on pancreatic beta-cells and the cardiovascular system. TXNIP is a regulator of the cellular redox state, but has also been suggested to act as a transcriptional repressor. However, the genes and pathways regulated by TXNIP remain unknown.

Thioredoxin-interacting protein is stimulated by glucose through a carbohydrate response element and induces beta-cell apoptosis.

Recently, we identified thioredoxin-interacting protein (TXNIP) as the most dramatically glucose-induced gene in our human islet microarray study. TXNIP is a regulator of the cellular redox state, but its role in pancreatic beta-cells and the mechanism of its regulation by glucose remain unknown. We therefore generated a stable transfected beta-cell line (INS-1) overexpressing human TXNIP and found that TXNIP overexpression induced apoptosis as assessed by Bax, Bcl2, caspase-3, and cleaved caspase-9 as well as Hoechst staining.

Increased insulin translation from an insulin splice-variant overexpressed in diabetes, obesity, and insulin resistance.

Type 2 diabetes occurs when pancreatic beta-cells become unable to compensate for the underlying insulin resistance. Insulin secretion requires beta-cell insulin stores to be replenished by insulin biosynthesis, which is mainly regulated at the translational level. Such translational regulation often involves the 5'-untranslated region.

Insulinomas and expression of an insulin splice variant.

Background: Insulinomas are beta-cell tumours characterised by uncontrolled insulin secretion even in the presence of hypoglycaemia. However, the mechanisms allowing such excessive insulin secretion are not known. Insulin secretion can occur only when the beta-cell insulin stores have been replenished by insulin biosynthesis, which is mainly controlled by translation.

Resistin is expressed in pancreatic islets.

Resistin, a recently described adipocyte factor, is regulated by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists. While resistin has been proposed to mediate insulin resistance in rodents, little is known about human resistin and its expression in pancreatic islets has not been tested. The goal of the present study was therefore to analyze whether resistin, like PPARgamma, is expressed in islets.