Glucose administration induces the premature expression of liver glucokinase gene in newborn rats

Abstract

Glucokinase first appears in the liver of the rat 2 weeks after birth and its activity rapidly increases after weaning on to a high-carbohydrate diet. The appearance of glucokinase is principally due to the increase of plasma insulin and to the decrease of plasma glucagon concentrations. Oral glucose administration to 1- or 10-day-old suckling rats induced an increase in plasma insulin and a fall in plasma glucagon and allowed a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription. When unrestrained late pregnant rats were infused with glucose during 36 h to induce an increase in fetal plasma insulin and a decrease in fetal plasma glucagon concentrations, glucokinase mRNA was detectable in fetal liver but the level was 100-fold lower than that observed in 1- or 10-day-old suckling rats. It is suggested that the hormonal environment did not allow glucokinase gene expression to be induced in fetal liver and that the absence of expression of glucokinase in suckling rat liver is due to the presence of low plasma insulin and high plasma glucagon levels. The chromatin structure of the glucokinase gene was examined during development by identification of DNase-I-hypersensitive sites from the region comprised between -8 kb upstream and +4 kb downstream of the cap site. Five hypersensitive sites were found: four liver-specific sites upstream of the cap site and one non-specific site in the first intron. These sites are already present in term fetus but the intensity of the two proximal sites located upstream of the cap site increase markedly after birth. This suggests that these sites could be implicated in the regulation of glucokinase gene expression by insulin and glucagon. Full DNase-I-hypersensitivity of these two proximal sites seems necessary for the mature response of glucokinase gene in response to changes in pancreatic hormones concentrations.