Stimulation of glucose-6-phosphatase gene expression by glucose and fructose-2,6-bisphosphate

Abstract

Glucose-6-phosphatase, a key enzyme in the homeostatic regulation of blood glucose concentration, catalyzes the terminal step in gluconeogenesis and glycogenolysis. Glucose, the product of the glucose-6-phosphatase reaction, dramatically increases the level of glucose-6-phosphatase mRNA transcripts in primary hepatocytes (20-fold), and the maximum response is obtained at a glucose concentration as low as 11 mM. Glucose specifically increases glucose-6-phosphatase mRNA and L-type pyruvate kinase mRNA. In the rat hepatoma-derived cell line, Fao, glucose increases the glucose-6-phosphatase mRNA only modestly (3-fold). In the presence of high glucose concentrations, overexpression of glucokinase in Fao cells via recombinant adenovirus vectors increases lactate production to the level found in primary hepatocytes and increases glucose-6-phosphatase gene expression by 21-fold. Similar overexpression of hexokinase I in Fao cells with high levels of glucose does not increase lactate production nor does it change the response of glucose-6-phosphatase mRNA to glucose. Glucokinase overexpression in Fao cells blunts the previously reported inhibitory effect of insulin on glucose-6-phosphatase gene expression in these cells. Raising the cellular concentration of fructose-2,6-bisphosphate, a potent effector of the direction of carbon flux through the gluconeogenic and glycolytic pathways, also stimulated glucose-6-phosphatase gene expression in Fao cells. Increasing the fructose-2,6-bisphosphate concentration over a 15-fold range (12 +/- 1 to 187 +/- 17 pmol/plate) via an adenoviral vector overexpression system, led to a 6-fold increase (0.32 +/- 0. 03 to 2.2 +/- 0.33 arbitrary units of mRNA) in glucose-6-phosphatase gene expression with a concomitant increase in glycolysis and a decrease in gluconeogenesis. Also, the effects of fructose-2, 6-bisphosphate concentrations on fructose-1,6-bisphosphatase gene expression were stimulatory, leading to a 5-6-fold increase in mRNA level over a 15-fold range in fructose-2,6-bisphosphate level. Liver pyruvate kinase and phosphoenolpyruvate carboxykinase mRNA were unchanged by the manipulation of fructose-2,6-bisphosphate level.