To understand how glucose regulates the expression of the glucose-6-phosphatase gene, the effect of glucose was studied in primary cultures of rat hepatocytes. Glucose-6-phosphatase mRNA levels increased about 10-fold when hepatocytes were incubated with 20 mM glucose. The rate of transcription of the glucose-6-phosphatase gene increased about 3-fold in hepatocytes incubated with glucose. The half-life of glucose-6-phosphatase mRNA was estimated to be 90 min in the absence of glucose and 3 h in its presence. Inhibition of the oxidative and the nonoxidative branches of the pentose phosphate pathway blocked the stimulation of glucose-6-phosphatase expression by glucose but not by xylitol or carbohydrates that enter the glycolytic/gluconeogenic pathways at the level of the triose phosphates. These results indicate that (i) the glucose induction of the mRNA for the catalytic unit of glucose-6-phosphatase occurs by transcriptional and post-transcriptional mechanisms and that (ii) xylitol and glucose increase the expression of this gene through different signaling pathways.Glc-6-Pase 1 (EC 3.1.3.9) is a multicomponent protein complex comprising catalytic and transporting entities (1-5). The complex is tightly associated with the endoplasmic reticulum, and the enzymatic component catalyzes the hydrolysis of glucose 6-phosphate to glucose, a final common step to both the pathways of glycogenolysis and gluconeogenesis. Hepatic Glc-6-Pase activity is effectively regulated by hormonal and nutritional status. For example, fasting and hormones that increase cAMP concentration stimulate its gene expression while refeeding and insulin decrease it (1, 2, 6 -12).The expression of the genes for several other proteins is regulated by glucose; these include genes for L-type pyruvate kinase (13, 14), fatty-acid synthase (15, 16), PEPCK (17, 18), and the type 2 glucose transporter (GLUT-2) (19,20). The mechanism by which glucose regulates the expression of these genes remains largely unknown. Recently, Kahn and colleagues (21, 22) have proposed a signaling pathway model to explain the molecular mechanism by which glucose regulates the expression of the L-type pyruvate kinase. This model consists of the following details: (i) the presence of glucose is sensed in the cell; (ii) this information is transduced by intracellular messengers; (iii) a second messenger, presumably xylulose 5-phosphate, rises and then modulates the activity of protein kinase and protein phosphatase involved in a cascade of phosphorylation/dephosphorylation. This cascade then leads to a modification of the phosphorylation state of the glucoseresponsive complex, followed by an increase in the transcriptional rate of the target gene. In this model, the presence of an active glucokinase that phosphorylates glucose to glucose 6-phosphate (Glc-6-P) is paramount.The molecular mechanism by which glucose regulates the expression of the Glc-6-Pase gene is currently unknown. Here we show that glucose regulation of the expression of this gene involves metabolism of glucose...