Metabolic dysregulation in the SHROB rat reflects abnormal expression of transcription factors and enzymes that regulate carbohydrate metabolism.

The Koletsky (SHROB) strain of rats is spontaneously hypertensive and displays insulin resistance, hyperglucagonemia and hypertriglyceridemia but is normoglycemic under fasting conditions. The aim of this study was to unravel the pattern of expression of genes encoding key regulatory enzymes involved in carbohydrate metabolism in the liver and kidney that may be impacted in this strain. We found that SHROB animals have decreased beta-adrenergic receptor density and, consequently, blunted increases in cAMP levels in response to beta-adrenergic agonists. They also have lower levels of hepatic as well as renal phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) mRNA and protein than their lean littermates. Expression of the genes for glycogen phosphorylase and glycogen synthase was also decreased. Hepatocytes from the SHROB animals exhibited glycogen depletion of only 50% compared to 86% by hepatocytes from lean littermates when challenged with either glucagon or forskolin to stimulate adenylyl cyclase. The expression of C/EBPalpha and C/EBPbeta, two key transcription factors that are essential for the coordinated expression of genes involved in glucose homeostasis, was depressed in livers of the SHROB rats, as were levels of HNF-4alpha, PPARalpha and PGC-1alpha. We conclude that overproduction of glucose is prevented in the SHROB rats by decreased expression of the genes for glycogen phosphorylase and the gluconeogenic enzymes PEPCK and G6Pase, which may prevent progression to diabetes in this model.

Several transcription factors are recruited to the glucose-6-phosphatase gene promoter in response to palmitate in rat hepatocytes and H4IIE cells.

Fatty acids and glucose are strong modulators of the expression of glucose-6-phosphatase (Glc-6-Pase), an enzyme that plays a key role in glucose homeostasis. PUFA inhibit, whereas SFA and monounsaturated fatty acids induce the expression of the Glc-6-Pase gene. Palmitate and oleate are the most abundant fatty acid species in circulation during food deprivation in mammals. Although dietary fats have been shown to modulate the expression of genes involved in both lipid and carbohydrate metabolism in liver, little is known regarding the molecular mechanism of transcriptional response of the Glc-6-Pase gene to long-chain fatty acids. Using H4IIE hepatoma cells and hepatocytes from adult rats, we investigated the mechanism of the induction of this gene by palmitate and oleate. Both of these fatty acids stimulated Glc-6-Pase gene transcription but did not affect the stability of its mRNA. In transient transfection assays, transcription from the Glc-6-Pase gene promoter was markedly enhanced by both palmitate and oleate but not by arachidonate. Chromatin immunoprecipitation analysis was used to show that palmitate induced the recruitment of an array of transcription factors viz hepatic nuclear factor(NF)-4alpha, CAAT/enhancer binding proteinbeta, PPARalpha, chicken ovalbumin upstream promoter transcription factor (COUP-TF), cAMP regulatory element binding protein, and NF-kappaB to this gene promoter. Although it is presently unclear how these various transcription factors interact at this promoter, the data are consistent with the view that multiple regulatory elements in the Glc-6-Pase gene promoter are responsible for the modulation of gene transcription by fatty acids.