Phosphoenolpyruvate carboxykinase (PEPCK) is a rate-controlling
enzyme in hepatic gluconeogenesis, and it therefore plays a central role in
glucose homeostasis. The rate of transcription of the PEPCK gene is increased by
glucagon (via cAMP) and
glucocorticoids and is inhibited by
insulin. Under certain circumstances
glucose also decreases PEPCK gene expression, but the mechanism of this effect is poorly understood. The
glucose-mediated stimulation of a number of glycolytic and lipogenic genes requires the expression of
glucokinase (GK) and increased
glucose metabolism. HL1C rat
hepatoma cells are a stably transfected line of H4IIE rat
hepatoma cells that express a PEPCK promoter-
chloramphenicol acetyltransferase fusion gene that is regulated in the same manner as the endogenous PEPCK gene. These cells do not express GK and do not normally exhibit a response of either the endogenous PEPCK gene, or of the trans-gene, to
glucose. A recombinant adenovirus that directs the expression of
glucokinase (AdCMV-GK) was used to increase
glucose metabolism in HL1C cells to test whether increased
glucose flux is also required for the repression of PEPCK gene expression. In AdCMV-GK-treated cells
glucose strongly inhibits
hormone-activated transcription of the endogenous PEPCK gene and of the expressed fusion gene. The
glucose effect on PEPCK gene promoter activity is blocked by 5 mM
mannoheptulose, a specific inhibitor of GK activity. The
glucose analog,
2-deoxyglucose mimics the
glucose response, but this effect does not require GK expression.
3-O-methylglucose is ineffective.
Glucose exerts its effect on the PEPCK gene within 4 h, at physiologic concentrations, and with an EC50 of 6.5 mM, which approximates the Km of
glucokinase. The effects of
glucose and
insulin on PEPCK gene expression are additive, but only at suboptimal concentrations of both agents. The results of these studies demonstrate that, by inhibiting PEPCK gene transcription,
glucose participates in a feedback control loop that governs its production from gluconeogenesis.