Carrie Millward

Mice with a Deletion in the Gene for CCAAT/Enhancer-binding Protein β Have an Attenuated Response to cAMP and Impaired Carbohydrate Metabolism

Fifty percent of the mice homozygous for a deletion in the gene for CCAAT/enhancer-binding protein β (C/EBPβ−/− mice; B phenotype) die within 1 to 2 h after birth of hypoglycemia. They do not mobilize their hepatic glycogen or induce the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK). Administration of cAMP resulted in mobilization of glycogen, induction of PEPCK mRNA, and a normal blood glucose; these mice survived beyond 2 h postpartum. Adult C/EBPβ−/− mice (A phenotype) also had difficulty in maintaining blood glucose levels during starvation. Fasting these mice for 16 or 30 h resulted in lower levels of hepatic PEPCK mRNA, blood glucose, β-hydroxybutyrate, blood urea nitrogen, and gluconeogenesis when compared with control mice. The concentration of hepatic cAMP in these mice was 50% of controls, but injection of theophylline, together with glucagon, resulted in a normal cAMP levels. Agonists (glucagon, epinephrine, and isoproterenol) and other effectors of activation of adenylyl cyclase were the same in liver membranes isolated from C/EBPβ−/− mice and littermates. The hepatic activity of cAMP-dependent protein kinase was 80% of wild type mice. There was a 79% increase in the concentration of RIα and 27% increase in RIIα in the particulate fraction of the livers of C/EBPβ−/− mice relative to wild type mice, with no change in the catalytic subunit (Cα). Thus, a 45% increase in hepatic cAMP (relative to the wild type) would be required in C/EBPβ−/− mice to activate protein kinase A by 50%. In addition, the total activity of phosphodiesterase in the livers of C/EBPβ−/− mice, as well as the concentration of mRNA for phosphodiesterase 3A (PDE3A) and PDE3B was approximately 25% higher than in control animals, suggesting accelerated degradation of cAMP. C/EBPβ influences the regulation of carbohydrate metabolism by altering the level of hepatic cAMP and the activity of protein kinase A.

Mice With a Deletion in the Gene for CCAAT/Enhancer-Binding Protein β Are Protected Against Diet-Induced Obesity

The CCAAT/enhancer-binding protein β (C/EBPβ) is required for adipocyte differentiation and maturation. We have studied the role of the transcription factor, C/EBPβ, in the development of diet-induced obesity. Mice with a deletion in the gene for C/EBPβ (C/EBPβ−/−) and wild-type mice were fed a high-fat diet (60% fat) for 12 weeks. The C/EBPβ−/− mice lost body fat, whereas the wild-type mice increased their total body fat on a high-fat diet. The C/EBPβ−/− mice had lower levels of blood triglycerides, free fatty acids, cholesterol, and hepatic triglyceride accumulation compared with the wild-type mice, thus protecting them from diet-induced obesity and fatty liver on a high-fat diet. Deletion of C/EBPβ gene resulted in greatly reducing hepatic lipogenic genes, acetyl CoA carboxylase, and fatty acid synthase and increasing the expression of β-oxidation genes in the brown adipose tissue. CO2 production was significantly higher in the C/EBPβ−/− mice as was the level of uncoupling protein (UCP)-1 and UCP-3 in the muscle. In conclusion, the transcription factor C/EBPβ is an important regulator in controlling lipid metabolism and in the development of diet-induced obesity.

Phosphoenolpyruvate carboxykinase (Pck1) helps regulate the triglyceride/fatty acid cycle and development of insulin resistance in mice

The aim of this study was to investigate the role of the cytosolic form of phosphoenolpyruvate carboxykinase (Pck1) in the development of insulin resistance. Previous studies have shown that the roles of Pck1 in white adipose tissue (WAT) in glyceroneogenesis and reesterification of free fatty acids (FFA) to generate triglyceride are vital for the prevention of diabetes. We hypothesized that insulin resistance develops when dysregulation of Pck1 occurs in the triglyceride/fatty acid cycle, which regulates lipid synthesis and transport between adipose tissue and the liver. We examined this by analyzing mice with a deletion of the PPARγ binding site in the promoter of Pck1 (PPARE−/−). This mutation reduced the fasting Pck1 mRNA expression in WAT in brown adipose tissue (BAT). To analyze insulin resistance, we performed hyperinsulinemic-euglycemic glucose clamp analyses. PPARE−/− mice were profoundly insulin resistant and had more FFA and glycerol released during the hyperinsulinemic-euglycemic clamp compared with wild-type mice (WT). Finally, we analyzed insulin secretion in isolated islets. We found a 2-fold increase in insulin secretion in the PPARE−/− mice at 16.7 mM glucose. Thus, the PPARE site in the Pck1 promoter is essential for maintenance of lipid metabolism and glucose homeostasis and disease prevention.