Rapid regulation of L-type pyruvate kinase mRNA by fructose in diabetic rat liver.

The effect of fructose on the induction of L-type pyruvate kinase mRNA in diabetic rat liver was studied by using a cloned cDNA probe. Fructose feeding resulted in a 5- to 6-fold increase in the L-type enzyme mRNA level after 1 to 3 days. These changes were approximately proportional to the changes in the level of translatable mRNA of this enzyme. A significant increase in total cellular L-type enzyme mRNA level was observed within 2 h after fructose feeding and the level reached a maximum after 8 h. Dietary glycerol also markedly increased the L-type mRNA level. These alterations were essentially due to the changes in the cytosolic mRNA. Northern blot analysis of total cellular RNA revealed that two L-type enzyme mRNA species with molecular sizes of 2.1 and 3.6 kilobases were proportionally increased during the fructose induction. The two mRNA forms were found in immunopurified L-type enzyme mRNA and directed synthesis of the L-type subunit in vitro; they are therefore functional mature forms. In contrast, analysis of nuclear RNA showed five putative precursor RNA species for the enzyme, up to 9.4 kilobases in length, in the liver of fructose-fed rats, while no band of the RNA species was found in the nuclei of control liver. The changes in the number of bands of these RNA species and their intensities after fructose feeding preceded the changes in the level of total cellular L-type enzyme mRNA sequences. These results indicate that dietary fructose causes a rapid increase in the level of L-type pyruvate kinase mRNA sequences by acting at the nuclear level.