Kinetics for changes in enzyme synthesis and mRNA content and hormones required for induction of 6-phosphogluconate dehydrogenase in hepatocytes

Abstract Rats fasted for 2 days were refed a 60% glucose diet for varying periods of time in order to follow the kinetics for changes in 6-phosphogluconate dehydrogenase synthesis and mRNA content. Hepatocytes isolated from control or induced rats were incubated with actinomycin D and the rate of decline in 6-phosphogluconate dehydrogenase mRNA was determined by translating RNA in a nuclease-treated reticulocyte lysate. The half-life for 6-phosphogluconate dehydrogenase mRNA under both of these conditions was about 2 h. Thus, increases in transcription or the processing of nuclear RNA may increase 6-phosphogluconate dehydrogenase mRNA during the dietary induction of this enzyme. Hepatocytes prepared from fasted rats were cultured with 5% serum and various hormones and energy sources. If hepatocytes were isolated from thyroidectomized rats and cultured in serum from a thyroidectomized calf, the 4-fold induction of 6-phosphogluconate dehydrogenase was primarily dependent upon added insulin. In the presence of optimal insulin concentrations (10 −7 M) triiodothyronine slightly stimulated 6-phosphogluconate dehydrogenase induction. The gut hormones somatostatin and secretin had no effect on 6-phosphogluconate dehydrogenase induction in cultured hepatocytes. Hepatocytes cultured in carbohydrate-free medium and 5% serum required added insulin for maximal induction. 8-Br-cGMP did not significantly affect 6-phosphogluconate dehydrogenase induction in hepatocytes either in the presence or absence of added insulin. Dibutyryl cAMP did not alter the time course or extent of 6-phosphogluconate dehydrogenase induction in cultured hepatocytes. We have concluded that under these conditions insulin is a potent signal regulating the levels of 6-phosphogluconate dehydrogenase mRNA and that this induction is not mediated by cyclic nucleotides.