Quantitation of messenger RNA levels for rat liver 6-phosphogluconate dehydrogenase

Glucose sensing by ChREBP/MondoA-Mlx transcription factors

The paralogous transcription factors ChREBP and MondoA, together with their common binding partner Mlx, have emerged as key mediators of intracellular glucose sensing. By regulating target genes involved in glycolysis and lipogenesis, they mediate metabolic adaptation to changing glucose levels. As disturbed glucose homeostasis plays a central role in human metabolic diseases and as cancer cells often display altered glucose metabolism, better understanding of cellular glucose sensing will likely uncover new therapeutic opportunities. Here we review the regulation, function and evolutionary conservation of the ChREBP/MondoA-Mlx glucose sensing system and discuss possible directions for future research.

Gender- and age-related changes in 6-phosphogluconate dehydrogenase gene expression in white adipose tissue of rats (Rattus norvegicus) are not related to serum testosterone concentration

Previously, we have shown that the age-related changes in 6-phosphogluconate dehydrogenase (6PGDH) activity depend on sex, and that oestradiol is playing a crucial role in the regulation of 6PGDH gene expression in rat liver, but not in other tissues [Pankiewicz, A., Sledzinski, T., Nogalska, A., Swierczynski, J., 2003. Tissue specific, sex and age-related differences in the 6-phosphogluconate dehydrogenase gene expression. Int. J. Biochem. Cell Biol. 35, 235-245.]. To complete the knowledge on the influence of sex hormones on 6PGDH activity, experiments have been performed on the effect of testosterone on 6PGDH gene expression in rat white adipose tissue and liver. The results presented here disclosed that in young male rats high serum testosterone concentration was associated with high white adipose tissue 6PGDH activity. After orchidectomy, a decrease in serum testosterone concentration (both in young and old rats) was observed. In contrast, no changes in white adipose tissue and liver 6PGDH activity were found. In female rats, both young and old, serum testosterone concentration was below the limit of detection, whereas 6PGDH activity was much higher in young than in old animals. Moreover, the testosterone administration to 9-month old male rats (which displayed much lower serum testosterone concentration that young animals) resulted in no effect on 6PGDH activity either in WAT or in the liver. In conclusion, the results presented in this paper indicate that testosterone does not play any role in the age- and gender-related differences in 6PGDH gene expression in white adipose tissue.

Tissue specific, sex and age--related differences in the 6-phosphogluconate dehydrogenase gene expression

Data presented in thid paper indicate that: (1) the age-related changes in 6-phosphogluconate dehydrogenase (6PGDH) activity depend on sex and tissue. No differences in the liver 6PGDH activity between young (1-month-old) males and females were found. In adult males, the activity was the same as in young animals but, in adult females, it reached the value twice as high as in the young. In adipose tissue (both white and brown) and kidney cortex, the enzyme activity decreased with age both in males and females. There were no differences between males and females 6PGDH activity in brain, heart and skeletal muscle. (2) The sex and age-related changes in the liver 6PGDH activity occur predominantly at the level of mRNA cellular concentration. (3) In the liver of ovariectomized rats decrease of 6PGDH activity and mRNA level was observed. Oestradiol administration to ovariectomized rats restored liver 6PGDH activity and liver 6PGDH mRNA levels to that observed in controls. No changes in 6PGDH activity and mRNA levels were found in white adipose tissue (WAT) of ovariectomized adult rats and in ovariectomized rats treated with oestradiol. (4) Oestradiol administration to males caused an increase of liver 6PGDH activity and mRNA levels to values observed in females, but was without an effect on WAT 6PGDH activity and mRNA level. (5) These results suggest that 6PGDH activity in different tissues is not regulated in coordinate fashion and that oestradiol plays an important role in the liver enzyme activity regulation.

Kinetic properties of hexose-monophosphate dehydrogenases. II. Isolation and partial purification of 6-phosphogluconate dehydrogenase from rat liver and kidney cortex

6-Phosphogluconate dehydrogenase (6PGDH) from rat-liver and kidney-cortex cytosol has been partially purified and almost completely isolated (more than 95%) from glucose-6-phosphate dehydrogenase activity. The purification and isolation procedures included high-speed centrifugation, 60-75% ammonium-sulphate fractionation, by which both hexose-monophosphate dehydrogenases activities were separated, and finally the protein fraction was applied to a chromatographic column of Sephadex G-25 equilibrated with 10 mM Tris-EDTA-NADP buffer, pH 7.6, to eliminate any contaminating metabolites. The kinetic properties of the isolated partially purified liver and renal 6PGDH were examined. The saturation curves of this enzyme in both rat tissues showed a typical Michaelis-Menten kinetic, with no evidence of co-operativity. The optimum pH for both liver and kidney-cortex 6PGDH was 8.0. The Km values of liver 6PGDH for 6-phosphogluconate (6PG) and for NADP were 157 microM and 258 microM respectively, while the specific activity measured at optimum conditions (pH 8.0 and 37 degrees C) was 424.2 mU/mg of protein. NADPH caused a competitive inhibition against NADP with an inhibition constant (Ki) of 21 microM. The Km values for 6PG and NADP from kidney-cortex 6PGDH were 49 microM and 56 microM respectively. The specific activity at pH 8.0 and 37 degrees C was 120.7 mU/mg of protein. NADPH also competitively inhibited 6PGDH activity, with a Ki of 41 microM. This paper describes a quick, easy and reliable method for the separation of the two dehydrogenases present in the oxidative segment of the pentose-phosphate pathway in animal tissues, eliminating interference in the measurements of their activities.

Stimulation of malic enzyme formation in hepatocyte culture by metabolites: evidence favoring a nonglycolytic metabolite as the proximate induction signal

Recent studies have shown that the addition of increasing concentrations of glucose to the medium of primary adult rat hepatocyte cultures results in the progressive induction of malic enzyme. We have undertaken experiments to determine (1) whether metabolism of glucose was an essential prerequisite for such induction, and (2) whether a specific glycolytic intermediate could be shown to constitute the proximate carbohydrate signal triggering such induction. In line with these objectives we investigated the ability of various sugars and glycolytic metabolites to induce malic enzyme in this system and assessed the influence of insulin, glucagon, and thyroid hormone (triiodothyronine, T3) on this process. Our results show that only those sugars capable of entering the cell and being metabolized induce malic enzyme (glucose, fructose, and galactose). The nonmetabolizable sugars 3-O-methylglucose and 2-deoxyglucose are ineffective. Incubation with 20 mmol/L lactate, pyruvate, dihydroxyacetone, or glycerol resulted in malic enzyme induction, whereas incubation with acetate, citrate, and alpha-ketoisocaproate was without effect. The induction by all sugars and metabolites required presence of insulin. As previously reported for glucose, addition of T3, under all metabolic conditions, resulted in a constant 3.6-fold increase in the rate of malic enzyme induction and further supports the proposal T3 acts to multiply the effect of a common carbohydrate-generated signal. Glucagon administration led to a dose-dependent inhibition of the carbohydrate effect with a half-maximal effect and maximal effect at 2 and 100 nmol/L, respectively. None of the glycolytic metabolites tested could reverse the glucagon inhibition completely.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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.

NADP-dependent dehydrogenases in rat liver parenchyma. III. The description of a liponeogenic area on the basis of histochemically demonstrated enzyme activities and the neutral fat content during fasting and refeeding

The activities of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase(6PGDH), malic enzyme (ME) and isocitrate dehydrogenase (ICDh) were investigated with optimized histochemical methods (Rieder it al 1978), and the activity of 3-hydroxybutyrate dehydrogenase (3HBDH) and neutral fat content with conventional techniques in the liver of male rats under the following experimental dietary conditions: (A) Fasting for 0, 12 and 84h; (B) 84-h fasting followed by refeeding with a low-fat, high-carbohydrate diet for 6 h and for 2, 3, 5, 7, 11 and 14 nights; (C) refeeding with standard diet for 5 nights; (D) low-fat high-carbohydrate diet for 7 an 14 nights. The activities of G6PDH, 6PGDH and ME decreased slightly during fasting primarily in zone 1 and increased dramatically on refeeding with a low-fat, high-carbohydrate diet. This activity increase was confined mainly to zone 3 during the first 3 days and was accompanied by a deposition of neutral fats that began in zone 3 and progressed to zone 1. Neutral for accumulation was maximal after 3 nights, with a uniform accumulation of large droplets in all the hepatocytes; this was followed by a release that started in zone 3 and proceeded in a periportal direction. On the other hand, G6PDH, 6PGDH and ME attained their maximum activities after 5 amd 7 nights of low-fat diet, the activities being nearly homogeneously distributed over the liver acinus in a few cases. Subsequently the activities fill mainly in zone 1, causing the activity patterns and levels to approach those of the animals in group (D). In contrast to this, the activity of ICDH increased during fasting principally in zone 1, so that the otherwise steep activity gradient in favor of zone 3 lessened. Refeeding led at first to a fall of activity below the initial value, but later the normal distribution pattern was restored. The activity of 3HBDH showed a behavior similar to that of ICDH. The findings are discussed with reference to the functional heterogeneity of the liver parenchyma, and the existence of a liponeogenic area in zone 3 is proposed.

Changes in the hepatic levels of messenger ribonucleic acid for malic enzyme during induction by thyroid hormone or diet

Levels of hepatic messenger ribonucleic acid (mRNA) for malic enzyme [L-malate:NADP oxidoreductase (decarboxylating), EC 1.1.1.40] were quantitated in different dietary and hormonal states of the rat. Polysomal or total cellular poly(A)-containing RNA was translated in the rabbit reticulocyte lysate system, which had been treated to reduce endogenous mRNA activity. The relative level of incorporation of radiolabeled amino acid into malic enzyme was determined by immunoprecipitation with antibody to malic enzyme and formaldehyde-fixed Staphylococcus aureus (Cowens I strain) as an immunoadsorbent. The immunoprecipitated product comigrated with purified malic enzyme on sodium dodecyl sulfate--polyacrylamide gel electrophoresis. No malic enzyme was detected when nonspecific antisera or an excess of unlabeled malic enzyme was added during immunoprecipitation. The level of malic enzyme mRNA was found to markedly increase relative to euthyroid, chow-fed rats when the animal was either fed a high carbohydrate, fat-free diet or made hyperthyroid. Animals receiving both treatments had a further increase in mRNA activity to a level which was approximately 0.2% of the total incorporation of [3H]leucine. Levels of malic enzyme activity and the relative rate of synthesis were found to increase roughly in proportion to mRNA levels in these three states. Thus, the induction of malic enzyme by thyroid hormone or high carbohydrate, fat-free diet is due largely to an increase in the mRNA coding for this enzyme.