Concurrent with the well-known inhibitory effect of insulin, glucose inhibits transcription of PEPCK in the liver (Kahn et al, 1987; Meyer et al., 1991). This effect is independent of the presence of insulin. As described above for lipogenic enzymes, the effect of glucose requires phosphorylation of glucose to produce glucose-6-phosphate. The inhibitory effect of glucose is observed in hepato-cytes from adult rats, but not in hepatocytes from suckling rats, which do not express glucokinase (Cournarie et al., 1999). Moreover, the effect of glucose on PEPCK expression is absent in H4IIE hepatoma cells, which express hexokinase I but not glucokinase (Scott et al., 1998). The overexpression of glucokinase by adenovirus is sufficient to recover the glucose effect (Scott et al., 1998). At the present time, the localization of the glucose-sensitive region in the PEPCK promoter and the identity of transcription factors involved remain unknown.
Glucose-6-phosphatase is another key gluconeogenic enzyme. As stated previously, its expression is increased in response to glucose shortage in the diet and is repressed by a high-carbohydrate diet and by insulin. Repression by insulin seems to involve a transcription factor of the forkhead family (Nakae et al, 2001; Foufelle and Ferré, 2002). Surprisingly, it has been shown that high glucose concentrations in culture media induce glucose-6-phosphatase expression in both
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