Effect of glycerol and dihydroxyacetone on hepatic lipogenesis

Cell energy metabolism: An update

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and maintenance processes. During growth, both ATP/ADP and NADH/NAD⁺ molecules play a key role. Cell energy metabolism hence refers to metabolic pathways involved in ATP synthesis linked to NADH turnover. Two main pathways are thus involved in cell energy metabolism: glycolysis/fermentation and oxidative phosphorylation. Glycolysis and mitochondrial oxidative phosphorylation are intertwined through thermodynamic and kinetic constraints that are reviewed herein. Further, our current knowledge of short-term and long term regulation of cell energy metabolism will be reviewed using examples such as the Crabtree and the Warburg effect.

A Novel Acetaminophen Soft-Chew Formulation Produced via Hot-Melt Extrusion with In-line Near-Infrared Monitoring as a Process Analytical Technology Tool

Patients who suffer from dysphagia have difficulty in swallowing hard tablets and capsules; hence, gelatin-based soft-chew dosages are used as an alternative and novel drug delivery approach to overcome this problem. However, the conventional method of producing gelatin-based soft-chew dosages has many potential issues. The objective of this study was to use glycerol and the hot-melt extrusion technique to address potential issues and optimize the formulation. Gelatin, acetaminophen, saccharin, xylitol, and sodium chloride and six different ratios of water and glycerol were used in the seven formulations. Extrusion process temperature of formulations 1-6 and formulation 7 were 90°C and 140°C, respectively. Near-infrared spectra were collected during extrusion to monitor quality consistency. Scanning electron microscopic images of the cross-section of the soft-chew dosages were recorded. Differential scanning calorimetry (DSC) was used to characterize the crystal states of each formulation. Texture profile analysis was used to evaluate the physical properties of the tablets. In vitro drug release characteristics were studied. A 45-day stability study was carried out to evaluate the stability of each formulation. Near-infrared spectra showed that formulations 1-6 were uniform while formulation 7 was not. From the DSC results, formulations 1 and 2 showed crystallinity of acetaminophen. Formulation 5 displayed the desired physical and chemical stability in texture profile analysis and in the in vitro drug release studies. By using glycerol and hot-melt extrusion, the potential issues of conventional methods were successfully addressed.

The effect of glutamine and dihydroxyacetone supplementation on food intake, weight gain, and postprandial glycogen synthesis in female Zucker rats

OBJECTIVE

We sought to test the hypothesis that increasing postprandial hepatic glycogen synthesis rate would decrease food intake and growth rate in obese Zucker rats.

DESIGN

Supplements of glutamine, with and without dihydroxyacetone (DHA), which have previously been shown to stimulate hepatic glycogen synthesis, were administered in the diet of obese Zucker rats for periods of 1 and 3 wk.

MEASUREMENTS

Food intake and body weight were monitored throughout the experiments. At the end of the feeding period the rats were fed a test meal and injected with (3)H(2)O to measure in vivo rates of glycogen and lipid synthesis. Final plasma glucose and triacylglycerol and hepatic glycogen content were also determined. Carcass fat and water contents were also measured in the 3-wk study.

RESULTS

Dietary glutamine had no effect on food intake, weight gain, or body composition. Addition of DHA caused a reduction in food intake and weight gain and a stimulation of in vivo hepatic glycogen synthesis after 1 wk, but these changes were abolished by the end of 3 wk. Hepatic lipogenesis in vivo was increased by DHA treatment for 1 and 3 wk.

CONCLUSIONS

Stimulation of hepatic glycogen synthesis by DHA treatment was associated with a reduction in food intake. However, the effect of DHA on glycogen synthesis and food intake disappeared after 3 wk of supplementation.

Effect of diet supplementation with glutamine, dihydroxyacetone, and leucine on food intake, weight gain, and postprandial glycogen metabolism of rats

OBJECTIVE

We tested the hypothesis that increasing the rate of postprandial hepatic glycogen synthesis would decrease food intake and growth rate in normal rats.

METHODS

Diets supplemented with glutamine, glutamine plus dihydroxyacetone, and glutamine plus dihydroxyacetone plus leucine were administered to male Sprague-Dawley rats for 1 wk. These are combinations that have been shown to stimulate hepatic glycogen synthesis in vitro. Food intake and body weight were monitored throughout the experiment. At the end of the feeding period, rats were fed a test meal and injected with 3H2O to measure in vivo rates of glycogen and lipid synthesis. Positional analysis of the 3H incorporated into glycogen was used to determine the proportion of glycogen synthesized via pyruvate. Final levels of plasma glucose and triacylglycerol and hepatic glycogen were also measured.

RESULTS

Dietary glutamine increased hepatic glycogen synthesis. Addition of dihydroxyacetone, with or without additional leucine, caused an additional increase in hepatic glycogen synthesis and increased the proportion of glycogen synthesized via pyruvate. Lipogenesis was not altered in the liver or adipose tissue. None of the dietary treatments had any effect on food intake, but the diets that contained dihydroxyacetone decreased the rate of weight gain.

CONCLUSIONS

Increasing glycogen synthesis had no effect on food intake. Increasing the proportion of glycogen synthesized by the indirect pathway through pyruvate was associated with a decrease in weight gain.

Allosteric activation of pyruvate kinase via NAD+ in rat liver cells

In isolated rat hepatocytes, it has previously been reported that a rise in the ATP content induces a proportional increase in cytosolic NAD+ concentration [Devin, A., Guérin, B. & Rigoulet, M. (1997) FEBS Lett. 410, 329-332]. This occurs under physiological conditions such as various substrates or different energetic states. To investigate the effect of a physiological rise in cytosolic [NAD+] per se on glycolysis and gluconeogenesis, an increase in [NAD+] induced by exogenous nicotinamide addition was obtained without a change in redox potential, ATP/ADP ratio and ATP concentration. Using dihydroxyacetone as substrate, we found that an increase in cytosolic [NAD+] decreases gluconeogenesis and enhances glycolysis without significant alteration of dihydroxyacetone consumption rate. These modifications are the consequence of an allosteric activation of pyruvate kinase via cytosolic NAD+ content. Thus, in addition to the well-known thermodynamic control of glycolysis by pyridine-nucleotide redox status, our study points to a new mechanism of glycolytic flux regulation by NAD+ concentration at the level of pyruvate kinase activity.

Cytosolic NAD+ content strictly depends on ATP concentration in isolated liver cells

By focusing on the question of the thermodynamic relationships involved in the regulation of biological energy conversion, bioenergetic studies usually consider the free pyridine and adenine nucleotide rather than their total pools, in either cytosol or mitochondria. In this study, we report a new observation that, at steady state, nicotinamide nucleotide content is increased by a rise in the ATP content of the whole cell under physiological conditions. It is a straight line relationship when only NAD+ and ATP are considered. When regarding the compartmentation of this phenomenon, it appears that the linear relationship between [NAD+] and [ATP] occurs only in the cytosol. Such a dependence could be a supplementary mechanism of regulation between various metabolic pathways in the liver cell.

Evidence for a glycerol pathway through aquaporin 1 (CHIP28) channels

Permeabilities to glycerol and small non-electrolytes of three Aquaporin 1 CHIP (AQP1) water channels were measured in AQP1 cRNA-injected Xenopus laevis oocytes and in human AQP1 channels reconstituted in proteoliposomes. By an "osmotic" swelling assay, significant increases of ethylene glycol, glycerol and 1,3-propanediol apparent permeability coefficients (P'solutes) were found in oocytes expressing human, rat and frog AQP1. p-Chloromercuribenzene sulphonate (pCMBS) and CuSO4 inhibited, by 95% and 58% respectively, apparent glycerol permeability (P'gly) in oocytes expressing human AQP1. pCMBS inhibition was reversed by beta-mercaptoethanol and CuSO4 inhibition was partly reversed by the Cu(2+)-binding peptide Gly-Gly-His. Tritiated glycerol uptakes confirmed the augmented P'gly value of AQP1 cRNA-injected oocytes. In contrast, no increases of urea, meso-erythritol, D-or L-threitol, xylitol and mannitol uptakes were detected. Stopped-flow light scattering experiments performed with human AQP1 proteoliposomes also revealed a much greater increase of P'gly than did those with protein-free liposomes; the initial rate of proteoliposomes also swelling was inhibited by 96.2% with HgCl2 and by 72.5% with CuSO4. In AQP1 cRNA-injected oocytes and in proteoliposomes, the value of the glycerol reflection coefficient was 0.74-0.80, indicating that water and glycerol share the same pathway. All these results provide strong evidence that water and certain small solutes permeate the AQP1 channels expressed at the surface of X. laevis oocytes or reconstituted in proteoliposomes. The urea exclusion suggests that the selectivity of the AQP1 channels not only depends on the size of the solutes but probably also on their flexibility and their ability to form H-bonds.

Interleukin-6, but not tumour necrosis factor-alpha, increases lipogenesis in rat hepatocyte primary cultures

The Kupffer-cell products interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha) have been shown to stimulate hepatic lipogenesis in vivo. Studies were performed to define the direct effects of these cytokines on lipogenesis in primary-culture rat hepatocytes. Hepatocytes were cultured in the presence of IL-6 or TNF-alpha for periods of 24-72 h. IL-6 increased hepatocyte protein content per microgram of DNA. IL-6 also caused a dose- and time-dependent induction of hepatocyte capacity for incorporation of [2-14C]pyruvate into fatty acids (56% increase by 12.5 ng/ml IL-6 after 72 h of cytokine exposure). This increase in cellular lipogenic capacity was confirmed by using 3H2O incorporation into fatty acids as tracer. TNF-alpha did not increase hepatocyte lipogenesis. In contrast with studies in vivo, neither IL-6 nor TNF-alpha had any acute (2 h of exposure) effects on rates of lipogenesis. Both IL-6 and TNF-alpha are known to increase macrophage prostaglandin synthesis acutely. The prostaglandin E agonist misoprostol free acid (0.1 microM) acutely increased hepatocyte lipogenic rates by 14%. Thus, IL-6 can directly induce hepatocyte lipogenic capacity, and E-series prostaglandins can antagonize the acute inhibition of lipogenesis by glucagon. The observations provide further evidence for the role of Kupffer-cell products in the regulation of hepatocyte metabolism.

Adaptive changes in total pyruvate dehydrogenase activity in lipogenic tissues of rats fed high-sucrose or high-fat diets

1. Pyruvate dehydrogenase complex (PDC) activity was measured in several tissues of rats fed for 7 or 15 days on control, or high-sucrose or high-fat diets. 2. Total activity in adipose tissue increased in the three groups 3-4 fold as compared with chow-fed animals in the first week. Total activity was 60% lower in rats fed the diet containing 22% corn oil for 2 weeks. 3. Hepatic total and PDCa activities were 50-80% higher in rats fed the sucrose diet for 7 or 15 days and decreased 30-40% in those fed on the high-fat diet for 2 weeks.

Aromatic aldehydes and aromatic ketones open ATP-sensitive K+ channels in guinea-pig ventricular myocytes

Patch-clamp techniques were used to study the effects of three carbonyl compounds, 3,4-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxyacetophenone, on the adenosine-5'-triphosphate(ATP)-sensitive K+ channel current (IK.ATP) in guinea-pig ventricular myocytes. 3,4-Dihydroxybenzaldehyde (0.5-1 mM) shortened the action potential duration, and this effect was inhibited by application of a specific blocker of IK.ATP, glibenclamide. The shortening of the action potential duration was shown to be caused by a time-independent outward current. In the cell-attached patch configuration, all three compounds activated a kind of single-channel current, which showed an inward rectification at positive potentials and which had a linear current/voltage relation at negative potentials, having a conductance of 90 pS. The current reversed at about 0 mV in symmetrical K+ concentrations on both sides of the membrane. In excised patches this current was blocked by internal application of ATP. Thus we identified this channel as IK.ATP. The activation effects of two aromatic aldehydes were stronger than that of the aromatic ketone. The effect of these compounds on IK.ATP was not reduced by addition of cysteine (10 mM). In inside-out patches, 3,4-dihydroxybenzaldehyde increased the activity of IK.ATP, which had been blocked by 0.5 mM MgATP in the presence of 0.5 mM ADP, but the activation effect was variable and much weaker than that in the cell-attached configuration, and was completely eliminated in the absence of ADP.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes

Glycogen synthesis in hepatocyte cultures is dependent on: (1) the nutritional state of the donor rat, (2) the acinar origin of the hepatocytes, (3) the concentrations of glucose and gluconeogenic precursors, and (4) insulin. High concentrations of glucose (15-25 mM) and gluconeogenic precursors (10 mM-lactate and 1 mM-pyruvate) had a synergistic effect on glycogen deposition in both periportal and perivenous hepatocytes. When hepatocytes were challenged with glucose, lactate and pyruvate in the absence of insulin, glycogen was deposited at a linear rate for 2 h and then reached a plateau. However, in the presence of insulin, the initial rate of glycogen deposition was increased (20-40%) and glycogen deposition continued for more than 4 h. Consequently, insulin had a more marked effect on the glycogen accumulated in the cell after 4 h (100-200% increase) than on the initial rate of glycogen deposition. Glycogen accumulation in hepatocyte cultures prepared from rats that were fasted for 24 h and then re-fed for 3 h before liver perfusion was 2-fold higher than in hepatocytes from rats fed ad libitum and 4-fold higher than in hepatocytes from fasted rats. The incorporation of [14C]lactate into glycogen was 2-4-fold higher in periportal than in perivenous hepatocytes in both the absence and the presence of insulin, whereas the incorporation of [14C]glucose into glycogen was similar in periportal and perivenous hepatocytes in the absence of insulin, but higher in perivenous hepatocytes in the presence of insulin. Rates of glycogen deposition in the combined presence of glucose and gluconeogenic precursors were similar in periportal and perivenous hepatocytes, whereas in the presence of glucose alone, rates of glycogen deposition paralleled the incorporation of [14C]glucose into glycogen and were higher in perivenous hepatocytes in the presence of insulin. It is concluded that periportal and perivenous hepatocytes utilize different substrates for glycogen synthesis, but differences between the two cell populations in the relative utilization of glucose and gluconeogenic precursors are dependent on the presence of insulin and on the nutritional state of the rat.