Changes in the rates of synthesis and messenger RNA levels of hepatic glucose-6-phosphate and 6-phosphogluconate dehydrogenases following induction by diet or thyroid hormone

Abstract

The lipogenic capacity of rat liver is increased in animals fed a high carbohydrate, fat-free diet or by the administration of 2,2',5'-triiodo-L-thyronine. Underlying this change is a generalized induction of the enzymes involved in lipogenesis, including glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme, which together serve to generate the additional NADPH required for increased fatty acid synthesis. This report presents evidence indicating that induction of the hexose-shunt dehydrogenases involves increased enzyme synthesis secondary to elevated enzyme specific mRNA levels, as has previously been shown for malic enzyme. Activities of specific mRNAs, estimated by cell-free translation of hepatic poly(A)-containing RNA in the mRNA dependent rabbit reticulocyte lysate, were compared with enzyme specific activities and relative rates of specific enzyme synthesis. The 2-fold increase in glucose-6-phosphate dehydrogenase specific activity in hyperthyroid rats and the 13-fold increase in rats fed a high carbohydrate, fat-free diet, relative to euthyroid, chow-fed controls were paralleled by comparable increases in the synthetic rates and mRNA levels of this enzyme. Similarly, consonant changes in the rate of enzyme synthesis and concentration of 6-phosphogluconate dehydrogenase mRNA accompanied the 2.5- and 3-fold increases in specific activity of this enzyme observed in response to hormonal and dietary induction, respectively. Thus, both thyroid hormone and carbohydrate feeding appear to induce glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase primarily by increasing the effective cellular concentrations of their respective mRNAs and, consequently, their rates of synthesis.