STIMULATION OF GLUCOSE-6-PHOSPHATASE GENE-EXPRESSION BY GLUCOSE AND FRUCTOSE-2,6-BISPHOSPHATE

Glucose-6-phosphatase, a key enzyme in the homeostatic regulation of b lood glucose concentration, catalyzes the terminal step in gluconeogen esis and glycogenolysis. Glucose, the product of the glucose-6-phospha tase reaction, dramatically increases the level of glu cose-6-phosphat ase mRNA transcripts in primary hepatocytes (20-fold), and the maximum response is obtained at a glucose concentration as low as 11 mM. Gluc ose specifically increases glucose-6-phosphatase mRNA and L-type pyruv ate kinase mRNA. In the rat hepatoma-derived cell line, Fao, glucose i ncreases the glucose-6-phosphatase mRNA only modestly (3-fold). In the presence of high glucose concentrations, overexpression of glucokinas e in Fao cells via recombinant adenovirus vectors increases lactate pr oduction to the level found in primary hepatocytes and increases gluco se g-phosphatase gene expression by al-fold, Similar overexpression of hexokinase I in Fao cells with high levels of glucose does not increa se lactate production nor does it change the response of glucose-g-pho sphatase mRNA to glucose, Glucokinase overexpression in Fao cells blun ts the previously reported inhibitory effect of insulin on glucose 6-p hosphatase gene expression in these cells. Raising the cellular concen tration of fructose-2,6-bisphosphate, a potent effector of the directi on of carbon flux through the gluconeogenic and glycolytic pathways, a lso stimulated glucose-6-phosphatase gene expression in Fao cells. Inc reasing the fructose-2,6-bisphosphate concentration over a 15-fold ran ge (12+/-1 to 187+/-17 pmol/plate) via an adenoviral vector overexpres sion system, led to a 6-fold increase (0.32+/-0.03 to 2.2+/-0.33 arbit rary units of mRNA) in glucose-6-phosphatase gene expression with a co ncomitant increase in glycolysis and a decrease in gluconeogenesis, Al so, the effects of fructose-2,6-bisphosphate concentrations on fructos e-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-bis phosphate level. Liver pyruvate kinase and phosphoenolpyruvate carboxy kinase mRNA were unchanged by the manipulation of fructose-2,6-bisphos phate level.