Levels of rat glucose-6-phosphate dehydrogenase messenger RNA

The hypocholesterolemic effect of sulfur-substituted fatty acid analogues in rats fed a high carbohydrate diet

Sulfur-substituted fatty acid analogues have been administered to rats fed a high carbohydrate diet, and the effect on plasma and hepatic lipid metabolism was investigated. Two of the analogues studied, 3-thiadicarboxylic acid and tetradecylthioacetic acid, reduced the plasma cholesterol level significantly, whereas the effect on plasma triacylglycerol level was only marginal. 3-Thiadicarboxylic acid was the most potent, decreasing the cholesterol level faster and at a lower dose than tetradecylthioacetic acid. The relative effects on plasma cholesterol and triacylglycerol levels were different from what have been observed in rats fed a conventional pellet diet. Tetradecylthiopropionic acid had no hypocholesterolemic effect. The activities of three lipogenic enzymes: ATP-citrate lyase, acetyl-CoA carboxylase and fatty acid synthase was measured. The two hypocholesterolemic analogues reduced the activities of these enzymes in a coordinated manner. The enzyme activities was found to correlate with the the plasma cholesterol level, indicating a coordinated regulation of these enzymes and cholesterol synthesis or secretion. The effect on two enzymes involved in cholesterol metabolism was also studied. The activity of acyl-CoA:cholesterol acyltransferase (ACAT) was reduced by the two hypocholesterolemic analogues, in contrast to the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase, which tended to increase. The cholesterol lowering effect of 3-thiadicarboxylic acid and tetradecylthioacetic acid can probably be ascribed to diminished cholesterol synthesis due to a reduced availability of acetyl-CoA. A reduction in the esterification of hepatic cholesterol may be a contributing factor.

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.

Regulation of glucose-6-phosphate dehydrogenase by diet and thyroid hormone

The regulation of hepatic glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) RNA by thyroid hormone and high carbohydrate (sucrose) diet was studied. Previous studies from several laboratories have demonstrated that thyroid hormone modulates G6PDH activity. However, the point at which thyroid hormone exerts this regulation has not been adequately addressed. In order to assess the role of thyroid hormone in this regulation, levels of G6PDH mRNA were determined in hypothyroid rats maintained on normal or high carbohydrate diets with or without thyroid hormone (triiodothyronine; T3) supplementation. A dot-blot hybridization procedure with nick-translated cDNA probes was used to directly assess the relative concentrations of G6PDH mRNA. Enzyme activity increased when the animals were treated with T3 and/or placed on a high carbohydrate diet. However, there was no effect of T3 and diet, alone or in combination, on G6PDH mRNA levels in hypothyroid rats. The data suggests that thyroid hormone and high carbohydrate diet are acting at a translational level to increase G6PDH enzyme activity in these animals.

Sex-linked changes in immunoreactive glucose-6-phosphate dehydrogenase in rat liver

The level of hepatic immunoreactive glucose-6-phosphate dehydrogenase protein was found to correlate well with the enzyme activity in adult rats fed the stock laboratory diet in a variety of hormonal conditions. The amount of immunoreactive protein and enzyme activity was 2-fold greater in sexually mature female rats compared with aged matched male animals. However, this difference was absent in diabetic animals, and furthermore although triiodothyronine administration to the diabetic male rat could restore the level of enzyme activity to that of the normoglycaemic animal, it was much less effective in the female animal. In contrast, administration of insulin to the normoglycaemic animal increased the level of glucose-6-phosphate dehydrogenase in the female, but was without effect in the male. These results are discussed in relation to the possible role of thyroid status and steroid sex hormones in the regulation of hepatic glucose-6-phosphate dehydrogenase.

Effects of dietary nutrients on lipogenic enzyme and mRNA activities in rat liver during induction

By feeding a carbohydrate diet (without protein) to fasted rats, malic enzyme mRNA activity in the liver was increased to the level in rats fed a carbohydrate and protein diet, whereas the enzyme activity itself was increased to 60% of that level. It appears that malic enzyme mRNA activity was increased by dietary carbohydrate, while dietary protein contributed to an increase in the translation of mRNA. In the animals fed carbohydrate without protein, glucose-6-phosphate dehydrogenase mRNA activity increased to 50% of the level in rats fed the carbohydrate and protein diet, whereas the enzyme activity increased to only 25%. By feeding a protein diet (without carbohydrate), glucose-6-phosphate dehydrogenase activity increased to 65% of the level in rats fed both carbohydrate and protein. This enzyme induction appears to be more dependent on protein than carbohydrate. With the carbohydrate diet, acetyl-CoA carboxylase was induced up to the level in the carbohydrate and protein diet group, whereas fatty acid synthetase was induced to only 33%. Acetyl-CoA carboxylase induction appears to be carbohydrate dependent. On the other hand, isotopic leucine incorporation studies showed that the magnitudes of the enzyme inductions caused by the dietary nutrients should be ascribed to the enzyme synthesis rates rather than the degradation. By fat feeding, the mRNA activities of malic enzyme and glucose-6-phosphate dehydrogenase were markedly decreased along with the enzyme induction. Fat appears to reduce these enzyme inductions before the translation of mRNA.

Regulation of glucose-6-phosphate dehydrogenase mRNA by insulin and the glucocorticoids in primary cultures of rat hepatocytes

The levels of functional mRNA encoding glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) were examined in hepatocytes from fasted and fasted/carbohydrate-refed rats and in hepatocytes inoculated into primary culture. Functional G6PDH mRNA was assessed in a cell-free protein synthesis system in vitro. We observed that hepatocytes from fasted/carbohydrate-refed rats had a 12-fold higher level of mRNA than did hepatocytes from fasted rats. The possibility that the adrenal glucocorticoids and insulin were responsible for the increase in G6PDH mRNA in refed rats was examined by studying the effect of insulin and the synthetic glucocorticoid, dexamethasone, on the level of functional G6PDH mRNA in primary cultures of rat hepatocytes maintained in a chemically defined medium. Hepatocytes from fasted rats were inoculated into primary culture and maintained for 48 h either in the absence of hormones or in the presence of insulin alone, dexamethasone alone or both hormones together. We observed that dexamethasone alone caused a fourfold increase in G6PDH mRNA while insulin caused about a twofold increase. Both hormones together elicited an increase that was additive. A comparison of functional G6PDH mRNA levels with the effect of the hormones on G6PDH activity and relative rate of enzyme synthesis suggests that the glucocorticoid elevates the level of G6PDH mRNA within the cell without causing a concommitant increase in the rate of synthesis of the enzyme or the level of G6PDH activity. The results obtained with the primary cultures of hepatocytes indicate that insulin and the glucocorticoids are probably involved with the regulation of hepatic G6PDH mRNA. However, involvement of other hormones, such as thyroid hormone, seems likely since the induced levels of G6PDH mRNA in hepatocytes in culture was one-third of that observed in refed rats.

Macromolecular weight specificity in covalent binding of bromobenzene

Bromobenzene is a hepatotoxicant that causes centrilobular necrosis. Pretreatment of animals with 3-methylcholanthrene decreases and phenobarbital pretreatment enhances the hepatotoxic action of this compound. We have investigated the macromolecular weight specificity of the covalent interactions of bromobenzene with liver macromolecules following incubation of [14C]bromobenzene in isolated hepatocytes. Hepatocytes were prepared from Fischer-344 rats treated for 3 days with 3-methylcholanthrene, phenobarbital, or normal saline. After a 1-hr incubation, total covalent binding, as measured by sodium dodecyl sulfate-equilibrium dialysis, was twofold less in hepatocytes from 3-methylcholanthrene-treated rats and sixfold greater in hepatocytes from phenobarbital-treated rats, as compared to hepatocytes from control animals. Analysis of the arylated macromolecules by electrophoresis on 15% sodium dodecyl sulfate-polyacrylamide disc gels indicated that in the first 1 to 3 min of incubation substantial amounts of covalently bound radiolabel were associated with macromolecules of between 20,000 and 40,000. The amount of radioactivity associated with these macromolecules rapidly diminished in hepatocytes from control and 3-methylcholanthrene-treated animals. In hepatocytes from phenobarbital-treated animals, the amount of radioactivity associated with macromolecules, 20,000, increased throughout the incubation. The amount of radiolabel associated with macromolecules, 20,000, increased in all incubations. When nontoxic doses of phenylmethylsulfonyl fluoride, a specific inhibitor of serine proteases, were added to control hepatocytes incubated with [14C]-bromobenzene, the decrease in radioactivity associated with larger (greater than 20,000) macromolecules was inhibited and a corresponding lack of increase in radioactivity associated with smaller macromolecules was observed. In hepatocytes from phenobarbital-treated rats, either the rate of adduct formation with higher molecular weight macromolecules greatly exceeded the rate of their breakdown or the phenobarbital treatment compromised the degradation process. The toxicity induced by bromobenzene may result from the covalently bound material altering the biological function of macromolecules. The result of this study suggest that cellular degradation of the arylated macromolecules may be one mechanism of detoxification. Persistence of the arylated macromolecules within the cell may be associated with the toxic action of bromobenzene.

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)

Dietary induction of glucose-6-phosphate dehydrogenase synthesis

The effect of dietary carbohydrate on rat liver glucose-6-phosphate dehydrogenase synthesis has been determined by using a method which can accurately quantitate relative rates of synthesis as low as 0.001 percent of total protein synthesis. Hepatocytes were incubated with (3H) leucine for 60 min and G6PD was separated from all other proteins by immunoprecipitation and electrophoresis on two-dimensional O'Farrell gels. The relative rate of synthesis of G6PD increased 70-fold (from 0.0015 to 0.11% of total protein synthesis) in hepatocytes from fasted rats refed a high carbohydrate diet. We have concluded that the 20-30 fold dietary induction of G6PD is due to a 70 and 3 fold increase in synthesis and degradation, respectively.

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.

Dietary carbohydrate induces lipogenesis and very-low-density lipoprotein synthesis

Hepatocytes obtained from sucrose-fed rats secreted triacylglycerol at a rate that was about twice that of cells from control rats. The increased rate of triacylglycerol secretion by cells from sucrose-fed rats was accompanied by a twofold increase in its rate of synthesis as determined by 3H2O incorporation. In addition, cells from sucrose-fed rats had a two- to threefold increase in apolipoprotein synthesis. Differences between the two groups became even more marked when cells were challenged in vitro with glucose. Double-reciprocal analysis showed that compared with controls, cells from sucrose-fed rats had a fourfold increase in the Vmax that described the glucose stimulation of [3H]triacylglycerol secretion. In contrast to in vivo carbohydrate (sucrose) induction of apolipoprotein synthesis, glucose added in vitro did not affect apolipoprotein synthesis. These data suggest that in vivo induction by dietary carbohydrate requires factors in addition to increased hexose that are not contained within the isolated hepatocyte system. The coinduction by dietary carbohydrate of both lipogenesis and apolipoprotein synthesis is likely to play a role in the increased capacity of cells from sucrose-fed rats to both assemble and secrete triacylglycerol-rich lipoproteins.

Hormonal regulation of translatable mRNA of glucose-6-phosphate dehydrogenase in primary cultures of adult rat hepatocytes

The quantity of translatable mRNA of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) in primary cultures of adult rat hepatocytes subjected to different hormonal conditions was determined with a reticulocyte-lysate, cell-free system. The level of glucose-6-phosphate dehydrogenase mRNA was about 5-fold higher in the presence of insulin than in its absence. This increase of glucose-6-phosphate dehydrogenase mRNA reached a maximum 12 h after the addition of insulin. The maximum level of induction of glucose-6-phosphate dehydrogenase mRNA required 10(-8) M insulin. Glucagon and triiodothyronine had no effect on the glucose-6-phosphate dehydrogenase mRNA level. The increase of glucose-6-phosphate dehydrogenase activity correlated with the increase in level of mRNA of this enzyme. This suggests that the changes in glucose-6-phosphate dehydrogenase activity in response to the above hormonal changes are primarily due to changes in the amount of mRNA coding for this enzyme.

A new method for measuring covalent binding of chemicals to cellular macromolecules

A new method has been developed for measuring the total covalent binding of metabolically activated compounds to cellular macromolecules. This method employs equilibrium dialysis, in the presence of the detergent sodium dodecyl sulfate (SDS), to remove unbound radiolabeled compound and its metabolites from cellular macromolecules. [14C] Bromobenzene (80 microM), [14C]aflatoxin B1 (5 microM) or 3-[14C]methylcholanthrene (100 microM) was incubated (37 degrees C) with primary hepatocytes or liver microsomes isolated from Fischer-344 rats. The covalent binding of 14C-radiolabel to hepatic or microsomal macromolecules was measured by SDS-equilibrium dialysis and compared with that measured by exhaustive extraction. After 1 h of incubation with hepatocytes or microsomes, 2--7 times more covalent binding was detected by SDS-equilibrium dialysis, than by exhaustive extraction. The radioactivity associated with these hepatic or microsomal macromolecules migrated to discrete positions on SDS-polyacrylamide disc gels. The non-dialysable radioactivity from incubations with [14C] bromobenzene could not be extracted with diethyl ether even after treatment of the dialysin with beta-glucuronidase-sulfatase or dilute acid. This was taken to indicate that the radioactivity in the dialysin did not include free bromobenzene or its metabolites, a conclusion supported by thin-layer chromatography analysis of the dialysin. The lower amount of covalent binding detected by exhaustive extraction may be related to the inability of trichloroacetic acid to quantitatively precipitate small molecular weight macromolecules. SDS-equilibrium dialysis is an easy, rapid and non-destructive technique for measuring covalent binding. The macromolecular integrity of the sample is maintained and allows further studies concerning the specificity of the covalent interactions.

Regulation of rat-liver tyrosine-aminotransferase mRNA by hydrocortisone and by N6,O2'-dibutyryladenosine 3',5'-phosphate

The intraperitoneal injection of either hydrocortisone of N6,O2'-dibutyryladenosine 3',5'-phosphate (Bt2cAMP) results in a specific increase in functional tyrosine aminotransferase mRNA (mRNATAT) activity in rat liver that is proportional to the degree of enzyme induction. Both require continuous RNA synthesis. There are several differences in the response to these inducers: (a) the magnitude of the increase is greater following hydrocortisone injection than after Bt2cAMP; (b) the peak response is seen within 1 h following the injection of Bt2cAMP as compared to the 5 h required for the maximal response following hydrocortisone injection; (c) finally, although both responses are rapid, the lag period which precedes the accumulation of functional tyrosine aminotransferase mRNA activity following the injection of hydrocortisone is at least 20 min whereas following Bt2cAMP it is 5-10 min. The administration of actinomycin D to rats 5 h after they were treated with hydrocortisone causes an additional twofold increase in tyrosine aminotransferase enzymatic activity, a phenomenon known as superinduction, but does not prevent the normal decrease in its mRNA seen at this time. This dissociation of enzyme and mRNA activities indicates that superinduction of tyrosine aminotransferase is not due to a selective stabilization of the mRNA which codes for this protein.

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.