Volume 88, Issue 6, December (2002), pp. 607-614 © The Author 2002
doi:10.1079/BJN2002701

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Nutritional regulation of glucose-6-phosphatase gene expression in liver of the gilthead sea bream (Sparus aurata)

A. Caseras¹, I. Metón¹, C. Vives¹, M. Egea¹, F. Fernández², I. V. Baanante^1
1Department de Bioquímica i Biologia Molecular, Facultat de Farmàcia, Universitat de Barcelona, Barcelona, Spain
²Department d’Ecologia, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain

 (Received 19 November 2001–Revised 31 May 2002–Accepted 2 July 2002)

To examine the role of glucose-6-phosphatase (G6Pase) in glucose homeostasis in the diabetes-like experimental model of carnivorous fish, we analysed postprandial variations and the effect of starvation, ration size and diet composition on the regulation of G6Pase expression at the enzyme activity and mRNA level in the liver of gilthead sea bream (Sparus aurata). G6Pase expression increased in long-term starved or energy-restricted fish. In contrast to data reported for other fish species, short-term regulation of G6Pase expression was found in regularly fed S. aurata. G6Pase mRNA levels were lowest between 4 and 15 h after food intake, whereas minimal enzyme activity was observed 10–15 h postprandially. Alterations of plasma glucose levels affect G6Pase in mammals. However, the carbohydrate content of the diet did not affect hepatic expression of G6Pase in S. aurata, suggesting that a different molecular mechanism is involved in the control of G6Pase expression in fish. Although G6Pase was unaffected, high-carbohydrate low-protein diets increased glucokinase (GK) expression and thus allowed a metabolic adaptation favouring glycolysis over gluconeogenesis. Interestingly, only the nutritional conditions that promoted variations in the blood glucose levels resulted in changes in the hepatic expression of G6Pase. These findings indicate a concerted regulation of G6Pase and GK expression and suggest that the direction and rate of the glucose–glucose-6-phosphate substrate cycle flux is finely regulated in the liver of S. aurata, challenging the role attributed to deficient regulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolize glucose.

Keywords:
Sparus aurata: Glucose-6-phosphatase: Glucokinase: Gene expression