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J Biol Chem. 1995 Oct 13;270(41):24229-36.

Adenovirus-mediated overexpression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in gluconeogenic rat hepatoma cells. Paradoxical effect on Fru-2,6-P2 levels.

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Department of Physiology and Biophysics, SUNY at Stony Brook 11794, USA.


6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase has been postulated to be a metabolic signaling enzyme, which acts as a switch between glycolysis and gluconeogenesis in mammalian liver by regulating the level of fructose 2,6-bisphosphate. The effect of overexpressing the bifunctional enzyme was studied in FAO cells transduced with recombinant adenoviral constructs of either the wild-type enzyme or a double mutant that has no bisphosphatase activity or protein kinase phosphorylation site. With both constructs, the mRNA and protein were overexpressed by 150- and 40-fold, respectively. Addition of cAMP to cells overexpressing the wild-type enzyme increased the S0.5 for fructose 6-phosphate of the kinase by 1.5-fold but had no effect on the overexpressed double mutant. When the wild-type enzyme was overexpressed, there was a decrease in fructose 2,6-bisphosphate levels, even though 6-phosphofructo-2-kinase maximal activity increased more than 22-fold and was in excess of fructose-2,6-bisphosphatase maximal activity. The kinase:bisphosphatase maximal activity ratio was decreased, indicating that the overexpressed enzyme was phosphorylated by cAMP-dependent protein kinase. Overexpression of the double mutant resulted in a 28-fold increase in kinase maximal activity and a 3-4-fold increase in fructose 2,6-bisphosphate levels. Overexpression of this form inhibited the rate of glucose production from dihydroxyacetone by 90% and stimulated the rate of lactate plus pyruvate production by 200%. In contrast, overexpression of the wild-type enzyme enhanced glucose production and inhibited lactate plus pyruvate production. These results provide direct support for fructose 2,6-bisphosphate as a regulator of gluconeogenic/glycolytic pathway flux and suggest that regulation of bifunctional enzyme activities by covalent modification is more important than the amount of the protein.

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