Activation of mammalian target of rapamycin complex 1 and insulin resistance induced by palmitate in hepatocytes.

Excessive supply of fatty acids to the liver might be a contributing factor to hepatic insulin resistance associated with obesity and type 2 diabetes mellitus. The aim of this study was to investigate direct effects of palmitate on insulin signaling in hepatocytes. The ability of metformin to reverse changes induced by palmitate was also studied.

Immunocytochemical localization of glucose 6-phosphatase and cytosolic phosphoenolpyruvate carboxykinase in gluconeogenic tissues reveals unsuspected metabolic zonation.

Immunohistochemical analysis was used to define the precise cell-specific localization of Glucose-6-phosphatase (Glc6Pase) and cytosolic form of the phosphoenolpyruvate carboxykinase (PEPCK-C) in the digestive system (liver, small intestine and pancreas) and the kidney. Co-expression of Glc6Pase and PEPCK-C was shown to take place in hepatocytes, in proximal tubules of the cortex kidney and at the top of the villi of the small intestine suggesting that these tissues are all able to perform complete gluconeogenesis. On the other hand, intrahepatic bile ducts, collecting tubes of the nephron and the urinary epithelium in the calices of the kidney, as well as the crypts of the small intestine, express Glc6Pase without significant levels of PEPCK-C.

Insulin induction of glucokinase and fatty acid synthase in hepatocytes: analysis of the roles of sterol-regulatory-element-binding protein-1c and liver X receptor.

The transcription activator SREBP-1c (sterol-regulatory-element-binding protein-1c) is induced by insulin in the liver and is considered a master regulator of lipogenic genes such as FASN (fatty acid synthase). The question of whether SREBP-1c is also a mediator of insulin action on the regulatory enzyme of glucose metabolism GCK (glucokinase) is controversial. In the present paper, we induced SREBP-1c to various levels with insulin or the liver X receptor ligand T0901317 in primary hepatocytes and asked if these levels correlated with those of GCK or FASN mRNA expression, using the latter as positive control.

Lack of evidence for a role of TRB3/NIPK as an inhibitor of PKB-mediated insulin signalling in primary hepatocytes.

The protein TRB3 (tribbles 3), also called NIPK (neuronal cell death-inducible protein kinase), was recently identified as a protein-protein interaction partner and an inhibitor of PKB (protein kinase B). To explore the hypothesis that TRB3/NIPK might act as a negative regulator of insulin signalling in the liver, this protein was overexpressed by adenoviral transduction of primary cultures of rat hepatocytes, and various aspects of insulin action were investigated. The insulin-induced phosphorylation of Ser-473 and Thr-308 of PKB was found to be undiminished in transduced hepatocytes with a molar excess of TRB3/NIPK over PKB of more than 25-fold.

Analysis of the role of protein kinase B (cAKT) in insulin-dependent induction of glucokinase and sterol regulatory element-binding protein 1 (SREBP1) mRNAs in hepatocytes.

Previous work showed that acute stimulation of a conditionally active protein kinase B (PKB or cAKT) was sufficient to elicit insulin-like induction of GCK (glucokinase) and SREBP1 (sterol regulatory element-binding protein 1) in hepatocytes [Iynedjian, Roth, Fleischmann and Gjinovci (2000) Biochem. J. 351, 621-627; Fleischmann and Iynedjian (2000) Biochem.

Discrimination between signaling pathways in regulation of specific gene expression by insulin and growth hormone in hepatocytes.

Insulin and GH can activate common signaling elements in many tissues and cell lines. We investigated the possibility of overlap in signaling pathways activated by insulin and GH in a key target cell, the hepatocyte. In primary cultures of rat hepatocytes, GH caused a dose- and time-dependent increase in tyrosine phosphorylation of signal transducer and activator of transcription 5.