Differential effects of glucose and fructose on liver L-type pyruvate kinase gene expression in vivo.

Dietary carbohydrates are known to stimulate L-type pyruvate kinase gene expression at the transcriptional level in the liver. However, the short-term effects, the time course, and the mechanism of the gene activation by elemental hexoses in normal fasted rats remain unknown. In the present study, both glucose and fructose were found to stimulate the gene expression at the transcriptional level in liver. However, the kinetics and the extent of the mRNA induction differed according to the carbohydrate given. Fructose stimulated early (2-4 h) and transiently the gene transcription, the RNA precursor, and the mRNA accumulation in 48-h-fasted rats while maximum stimulation of the RNA synthesis by glucose was delayed until the 12th h of refeeding, despite an early rise of plasma insulin. In contrast, insulin release was not required for fructose to trigger the gene transcription, nor did the high cyclic AMP levels in fasted rat liver prevent RNA synthesis by fructose. The agent(s) operating early in fructose-fed animals might be powerful enough to not require insulin for gene activation and to balance the inhibitory action of glucagon in the liver.