Gluconeogenesis in the chicken: regulation of phosphoenolpyruvate carboxykinase gene expression

In chickens, the liver functions in gluconeogenesis to recycle lactate carbon (Cori cycle) and the kidney is the major organ for net gluconeogenesis from substrates such as pyruvate and amino acids. This is markedly different from mammalian systems where the liver is the primary gluconeogenic organ. The limited ability of chicken hepatocytes to synthesize glucose is explained, at least in part, by the observation that phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) in these cells is located exclusively in the mitochondria. The kidney possesses a cytosolic form of this enzyme that adapts to dietary and acid-base stimuli. The relative abundance of mRNA coding for the cytosolic enzyme has been detected by using a specific cDNA probe. Starvation increases the level of this mRNA in chicken kidney and also results in the appearance of the message in chicken liver. Isolated hepatocytes have been used to determine which hormones regulate expression of the hepatic gene. Incubations with glucagon, epinephrine, norepinephrine, dexamethasone, or dibutyryl cyclic AMP increase the relative abundance of the message in liver cells isolated from fed chickens. Despite considerable levels of this mRNA in the liver of starved chickens, functional cytosolic enzyme activity is not detected. This indicates some form of posttranscriptional regulation. The studies summarized illustrate the usefulness of isolated hepatocytes and recombinant DNA probes in the study of hormonal regulation of hepatic gene expression.