Knocking down liver CCAAT/enhancer-binding protein α by adenovirus-transduced silent interfering ribonucleic acid improves hepatic gluconeogenesis and lipid homeostasis in db/db ice

CCAAT/ enhancer- binding protein-alpha (C/EBP alpha) is a member of the basic leucine zipper transcription factor family and regulates expression of several enzymes in the liver that control glucose and lipid metabolism. Using adenovirus-transduced silent interfering (si) RNA against C/EBP alpha, endogenous liver C/EBP alpha protein was knocked down by 70-80% in 8-wk-old wild-type (WT) and db/db mice. In WT mice, fasting blood glucose concentrations were reduced approximately 24% without changes in plasma free fatty acid and triglycerides, when compared with LacZ adenovirus-treated control mice. Ad-C/EBP alpha siRNA treatment nearly normalized fasting glucose and significantly reduced plasma insulin and free fatty acid content, even though there was no elevation of C/EBP alpha protein in the livers of db/db mice. In parallel with the changes in glucose levels, hepatic glucose production was significantly reduced in C/EBP alpha siRNA-treated WT and db/db mice. mRNA levels of phyosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and liver glycogen synthase were decreased in the C/EBP alpha siRNA-treated WT and db/db mice. Interestingly, the magnitude of reduction in these enzymes was more profound in db/db mice. C/EBP alpha siRNA also decreased mRNA levels of proliferator activator protein-gamma coactivator-1 alpha in both the WT and db/db mice but reduced cAMP response element-binding protein only in WT and did not alter hepatic nuclear factor-4 alpha and CBP/p300 expression. Expression of genes involved in lipogenesis, such as fatty acid synthase, acetyl-CoA carboxylase, and sterol regulatory element-binding protein-1c was robustly suppressed in the C/EBP alpha siRNA-treated db/db mice. Taken together, these results indicate that C/EBP alpha plays an important role in maintaining glucose and lipid homeostasis in the liver.