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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.

The mRNA level of the catalytic subunit of rat liver glucose-6-phosphatase (Glu-6-Pase) was regulated by hormones commensurate with activity changes in vivo. Insulin exerts a dominant negative effect on the mRNA levels of Glu-6-Pase. Both mRNA levels and activities of the enzyme are low in the fed and refed state where insulin levels are elevated. Insulin administration to diabetic rats also decreases levels of mRNA and Glu-6-Pase activity. Insulin at a concentration of 1 nmol/l completely overcomes the stimulatory effect of glucocorticoids on Glu-6-Pase message levels in FAO hepatoma cells. The stimulatory response to glucocorticoid in FAO cells is biphasic, with maxima seen at 3 and 18 h after hormone addition (respectively 1.6- and 3.3-fold). 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) causes a fourfold increase in Glu-6-Pase mRNA at 3 h in FAO cells. The gene of rat liver Glu-6-Pase is 13 kilobases in length and comprised of 5 exons. The exon-intron structure is completely conserved when compared with the mouse and human genes. A 0.5-kb 3'-untranslated region, which is present in rat and mouse liver Glu-6-Pase cDNA, is absent in the Glu-6-Pase gene reported here, indicating the possible duplication of either the terminal fifth exon or the entire gene. The promoter region contains a consensus core CCAAT element at position -207 and a TATAAA at position -31. Several possible response elements have been identified in the 5'-flanking region (from a HindIII site at position -1641). A consensus glucocorticoid response element is located at base pair -1552, a 9/10 match of the insulin response sequence is located at position -1449, and a 7/8 match of the cAMP response element is located at position -164.

cDNA clones coding for the catalytic subunit of rat liver glucose-6-phosphatase (EC 3.1.3.9) were isolated from a rat liver cDNA library in lambda gt11 phage. The sequence of the cDNA and the amino acid sequence derived from it were greater than 90% identical to the corresponding sequences for the mouse and human forms of liver glucose-6-phosphatase. Northern blot analysis of RNA from FAO hepatoma cells revealed that dexamethasone induced the glucose-6-phosphatase mRNA while insulin suppressed its expression. When both hormones were added together insulin completely suppressed the effect of glucocorticoid. cAMP addition alone decreased the abundance of glucose-6-phosphatase mRNA. The results demonstrate multihormonal regulation of gene expression of hepatic glucose-6-phosphatase and support a dominant role for insulin.