Regulation of glucose-6-p dehydrogenase synthesis in rat epididymal fat pads

Nutritional regulation of mRNA processing

Understanding how a cell adapts to dietary energy in the form of carbohydrate versus energy in the form of triacylglycerol requires knowledge of how the activity of the enzymes involved in lipogenesis is regulated. Changes in the activity of these enzymes are largely caused by changes in the rate at which their proteins are synthesized. Nutrients within the diet can signal these changes either via altering hormone concentrations or via their own unique signal transduction pathways. Most of the lipogenic genes are regulated by changes in the rate of their transcription. Glucose-6-phosphate dehydrogenase (G6PD) is unique in this group of enzymes in that nutritional regulation of its synthesis involves steps exclusively at a posttranscriptional level. G6PD activity is enhanced by the consumption of diets high in carbohydrate and is inhibited by the consumption of polyunsaturated fat. In this review, evidence is presented that changes in the rate of synthesis of the mature G6PD mRNA involves regulation of the efficiency of splicing of the nascent G6PD transcript. Furthermore, this regulation involves the activity of a cis-acting sequence in the G6PD primary transcript. This sequence in exon 12 is essential for the inhibition of G6PD mRNA splicing by PUFA. Understanding the mechanisms by which nutrients alter nuclear posttranscriptional events will provide new information on the breadth of mechanisms involved in gene regulation.

Dietary regulation of expression of glucose-6-phosphate dehydrogenase

The family of enzymes involved in lipogenesis is a model system for understanding how a cell adapts to dietary energy in the form of carbohydrate versus energy in the form of triacylglycerol. Glucose-6-phosphate dehydrogenase (G6PD) is unique in this group of enzymes in that it participates in multiple metabolic pathways: reductive biosynthesis, including lipogenesis; protection from oxidative stress; and cellular growth. G6PD activity is enhanced by dietary carbohydrates and is inhibited by dietary polyunsaturated fats. These changes in G6PD activity are a consequence of changes in the expression of the G6PD gene. Nutrients can regulate the expression of genes at both transcriptional and posttranscriptional steps. Most lipogenic enzymes undergo large changes in the rate of gene transcription in response to dietary changes; however, G6PD is regulated at a step subsequent to transcription. This step is involved in the rate of synthesis of the mature mRNA in the nucleus, specifically regulation of the efficiency of splicing of the nascent G6PD transcript. Understanding the mechanisms by which nutrients alter nuclear posttranscriptional events will help uncover new information on the breadth of mechanisms involved in gene regulation.

Regulation of liver glucose-6-P dehydrogenase levels in female rats

1. Gender differences in the dietary regulation of rat liver glucose-6-P dehydrogenase activity, synthesis and mRNA levels were examined. 2. As expected, in normal rats fed a standard chow diet, females have higher G6PD activity than males because they have more G6PD mRNA and therefore a higher rate of G6PD synthesis. 3. In contrast, the decreased dietary induction in female rats is due to a more rapid rate of G6PD degradation rather than a decrease in G6PD mRNA or synthesis.

Solution hybridization quantitation of G6PD mRNA in rat epididymal fat pads

A solution hybridization assay is systematically characterized and used to quantitate glucose-6-phosphate dehydrogenase (G6PD) mRNA from epididymal fat pads in fasted and glucose-induced rats. G6PD mRNA and specific activity increase 9-fold and 2-fold, respectively. The 9-fold increase in G6PD synthesis reported previously (Wolfe et al. (1979) Biochem. Biophys. Res. Commun. 89, 108-115) can, therefore, be accounted for by the increase in G6PD mRNA. This solution hybridization assay is sensitive enough to quantitative levels of G6PD mRNA in total liver RNA from a fasted rat, one of the least abundant sources of this mRNA. It can, therefore, be used to answer several questions about the regulation of G6PD synthesis in rat tissues. Preliminary results suggest that the dietary regulation of G6PD mRNA in rat liver is much larger than previously reported.

Increased cerebral glucose-6-phosphate dehydrogenase activity in Alzheimer's disease may reflect oxidative stress

The activities of the hexose monophosphate pathway enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were measured at autopsy in control and Alzheimer's disease brains. Enzyme activities did not vary between different areas of brain and were unaltered by age. In Alzheimer's disease, the activities of both enzymes were increased, the glucose-6-phosphate dehydrogenase activity being almost double the activity of normal controls. We propose that this increased enzyme activity is a response to elevated brain peroxide metabolism.