Effect of thyroid hormones on glucokinase gene transcription in rat liver

Thyroid hormones contribute to the regulation of blood sugar by accelerating the turnover of glucose. The mechanism by which thyroid hormones stimulate the rate of glucose utilization in the liver was determined by investigating the effect of different thyroid states on the expression of the glucokinase gene, a key enzyme of glycolysis. In euthyroid rats the mass of glucokinase mRNA increased 8-fold during the first 4 h of refeeding a high carbohydrate diet to 48-h starved rats. In hypothyroid rats under the same conditions only a 2-fold induction was observed. In euthyroid rats a 5-fold increase was obtained 1 h after refeeding, while hypothyroid rats displayed no significant response in glucokinase mRNA within this time. Basal levels of glucokinase mRNA in starved rats were the same observed in eu- and hypothyroid rats. Injection of 3,3',5-triiodothyronine (T3) into hypothyroid rats restored the mRNA levels of refed hypothyroid rats to euthyroid levels within 24 h. However, a 3-fold increase over untreated animals was already observed 3 h after T3 administration. Even subphysiological doses of T3 (0.1 microgram/100 g body weight) led to an significant increase in glucokinase mRNA levels (1.5-fold, p less than 0.05) in hypothyroid rats, whereas higher doses (100 micrograms/100 g body weight) restored the mRNA levels to those of euthyroid controls. Parallel increases in the cytosolic mRNA levels and rate of glucokinase gene transcription were detected when hypo- and euthyroid fasted rats were compared after 4 h of refeeding. It is concluded that thyroid hormones are permissive for the induction of glucokinase during refeeding but have no effect during starvation. They rapidly enhance the rate of gene transcription within the range of their physiologically circulating concentrations. The results suggest a major importance of thyroid hormones in regulating glucose utilization.