Contributions of dietary carbohydrate and ethanol to alterations in liver glycogen levels and glycolytic activity

The environmental pollutant, polychlorinated biphenyl 126, alters liver function in a rodent model of alcohol-associated liver disease

BACKGROUND

The prevalence of alcohol-associated liver disease (ALD), a subtype of fatty liver disease (FLD), continues to rise. ALD is a major cause of preventable death. Polychlorinated biphenyl (PCB) 126 is an environmentally relevant, dioxin-like pollutant whose negative metabolic effects have been well documented. In human and animal studies, PCB has been associated with the severity of nonalcoholic fatty liver disease (NAFLD). However, few studies have investigated whether exposures to environmental toxicants can worsen ALD. Thus, the objective of the current study was to develop an alcohol-plus-toxicant model to study how an environmental pollutant, PCB 126, impacts rodent ALD pathology.

METHODS

Briefly, male C57BL/6J mice were exposed to 0.2 mg/kg PCB 126 or corn oil vehicle four days prior to ethanol feeding using the chronic-binge (10-plus-one) model.

RESULTS

Concentrations of macromolecules, including hepatic lipids, carbohydrates, and protein (albumin) were impacted. Exposure to PCB 126 exacerbated hepatic steatosis and hepatomegaly in mice exposed to the chemical and fed an ethanol diet. Gene expression and the analysis of blood chemistry showed a potential net increase and retention of hepatic lipids and reductions in lipid oxidation and clearance capabilities. Depletion of glycogen and glucose was evident, which contributes to disease progression by generating systemic malnutrition. Granulocytic immune infiltrates were present but driven solely by ethanol feeding. Hepatic albumin gene expression and plasma levels were decreased by ~50% indicating a potential compromise of liver function. Finally, gene expression analyses indicated that the aryl hydrocarbon receptor and constitutive androstane receptor were activated by PCB 126 and ethanol, respectively.

CONCLUSIONS

Various environmental toxicants are known to modify or enhance FLD in high-fat diet models. Findings from the present study suggest that they interact with other lifestyle factors such as alcohol consumption to reprogram intermediary metabolism resulting in exacerbated ethanol-associated systemic malnutrition in ALD.

Cellular Bioenergetics: Experimental Evidence for Alcohol-induced Adaptations

At-risk alcohol use is associated with multisystemic effects and end-organ injury, and significantly contributes to global health burden. Several alcohol-mediated mechanisms have been identified, with bioenergetic maladaptation gaining credence as an underlying pathophysiological mechanism contributing to cellular injury. This evidence-based review focuses on the current knowledge of alcohol-induced bioenergetic adaptations in metabolically active tissues: liver, cardiac and skeletal muscle, pancreas, and brain. Alcohol metabolism itself significantly interferes with bioenergetic pathways in tissues, particularly the liver. Alcohol decreases states of respiration in the electron transport chain, and activity and expression of respiratory complexes, with a net effect to decrease ATP content. In addition, alcohol dysregulates major metabolic pathways, including glycolysis, the tricarboxylic acid cycle, and fatty acid oxidation. These bioenergetic alterations are influenced by alcohol-mediated changes in mitochondrial morphology, biogenesis, and dynamics. The review highlights similarities and differences in bioenergetic adaptations according to tissue type, pattern of (acute vs. chronic) alcohol use, and energy substrate availability. The compromised bioenergetics synergizes with other critical pathophysiological mechanisms, including increased oxidative stress and accelerates cellular dysfunction, promoting senescence, programmed cell death, and end-organ injury.

Genetic deletion of the circadian clock transcription factor BMAL1 and chronic alcohol consumption differentially alter hepatic glycogen in mice

Multiple metabolic pathways exhibit time-of-day-dependent rhythms that are controlled by the molecular circadian clock. We have shown that chronic alcohol is capable of altering the molecular clock and diurnal oscillations in several elements of hepatic glycogen metabolism ( 19 , 44 ). Herein, we sought to determine whether genetic disruption of the hepatocyte clock differentially impacts hepatic glycogen content in chronic alcohol-fed mice. Male hepatocyte-specific BMAL1 knockout (HBK) and littermate controls were fed control or alcohol-containing diets for 5 wk to alter hepatic glycogen content. Glycogen displayed a significant diurnal rhythm in livers of control genotype mice fed the control diet. While rhythmic, alcohol significantly altered the diurnal oscillation of glycogen in livers of control genotype mice. The glycogen rhythm was mildly altered in livers of control-fed HBK mice. Importantly, glycogen content was arrhythmic in livers of alcohol-fed HBK mice. Consistent with these changes in hepatic glycogen content, we observed that some glycogen and glucose metabolism genes were differentially altered by chronic alcohol consumption in livers of HBK and littermate control mice. Diurnal rhythms in glycogen synthase (mRNA and protein) were significantly altered by alcohol feeding and clock disruption. Alcohol consumption significantly altered Gck, Glut2, and Ppp1r3g rhythms in livers of control genotype mice, with diurnal rhythms of Pklr, Glut2, Ppp1r3c, and Ppp1r3g further disrupted (dampened or arrhythmic) in livers of HBK mice. Taken together, these findings show that chronic alcohol consumption and hepatocyte clock disruption differentially influence the diurnal rhythm of glycogen and various key glycogen metabolism-related genes in the liver. NEW & NOTEWORTHY We report that circadian clock disruption exacerbates alcohol-mediated alterations in hepatic glycogen. We observed differential responsiveness in diurnal rhythms of glycogen and glycogen metabolism genes and proteins in livers of hepatocyte-specific BMAL1 knockout and littermate control mice fed alcohol. Our findings provide new insights into potential mechanisms by which alcohol alters glycogen, an important energy source for liver and other organs.

The Molecular Circadian Clock and Alcohol-Induced Liver Injury

Emerging evidence from both experimental animal studies and clinical human investigations demonstrates strong connections among circadian processes, alcohol use, and alcohol-induced tissue injury. Components of the circadian clock have been shown to influence the pathophysiological effects of alcohol. Conversely, alcohol may alter the expression of circadian clock genes and the rhythmic behavioral and metabolic processes they regulate. Therefore, we propose that alcohol-mediated disruption in circadian rhythms likely underpins many adverse health effects of alcohol that cut across multiple organ systems. In this review, we provide an overview of the circadian clock mechanism and showcase results from new studies in the alcohol field implicating the circadian clock as a key target of alcohol action and toxicity in the liver. We discuss various molecular events through which alcohol may work to negatively impact circadian clock-mediated processes in the liver, and contribute to tissue pathology. Illuminating the mechanistic connections between the circadian clock and alcohol will be critical to the development of new preventative and pharmacological treatments for alcohol use disorders and alcohol-mediated organ diseases.

Time-dependent effect of ethanol force-feeding on glycogen repletion: NMR evidence of a link with ATP turnover in rat liver

The purpose was to study the hepatic effects of low-dose ethanol on the links between ATP and glycogen production. Fasted male Wistar rats received a single force-feeding of glucose plus ethanol or isocaloric glucose. At different times after force-feeding (0-10 h), glycogen repletion and ATP characteristics (content, apparent catalytic time constant, mitochondrial turnover) were monitored by (13)C- or (31)P-nuclear magnetic resonance (NMR) in perfused and isolated liver. In vivo glycogen repletion after force-feeding was slower after glucose plus ethanol vs. glucose (12.04 ± 0.68 and 8.50 ± 0.86 μmol/h/g liver wet weight [ww], respectively), reaching a maximum at the 6th hour. From the 3rd to the 8th hour, glycogen content was lower after glucose plus ethanol vs. glucose. After glucose plus ethanol, the correlation between glycogen and ATP contents presented two linear steps: before and after the 3rd hour (30 and 102 μmol glycogen/g ww per μmol ATP/g ww, respectively, the latter being near the single step measured in glucose). After glucose plus ethanol, ATP turnover remained stable for 2 h, was 3-fold higher from the 3rd hour to the 8th hour, and was higher than after glucose (2.59 ± 0.45 and 1.39 ± 0.19 μmol/min/g ww, respectively). In the 1st hour, glucose plus ethanol induced a transient acidosis and an increase in the phosphomonoesters signal. In conclusion, after ethanol consumption, a large part of the ATP production was diverted to redox re-equilibrium during the first 2 h, thereby reducing the glycogen synthesis. Thereafter, the maintenance of a large oxidative phosphorylation allowed the stimulation of glycogen synthesis requiring ATP.

Chronic ethanol consumption disrupts diurnal rhythms of hepatic glycogen metabolism in mice

Chronic ethanol consumption has been shown to significantly decrease hepatic glycogen content; however, the mechanisms responsible for this adverse metabolic effect are unknown. In this study, we examined the impact chronic ethanol consumption has on time-of-day-dependent oscillations (rhythms) in glycogen metabolism processes in the liver. For this, male C57BL/6J mice were fed either a control or ethanol-containing liquid diet for 5 wk, and livers were collected every 4 h for 24 h and analyzed for changes in various genes and proteins involved in hepatic glycogen metabolism. Glycogen displayed a robust diurnal rhythm in the livers of mice fed the control diet, with the peak occurring during the active (dark) period of the day. The diurnal glycogen rhythm was significantly altered in livers of ethanol-fed mice, with the glycogen peak shifted into the inactive (light) period and the overall content of glycogen decreased compared with controls. Chronic ethanol consumption further disrupted diurnal rhythms in gene expression (glycogen synthase 1 and 2, glycogenin, glucokinase, protein targeting to glycogen, and pyruvate kinase), total and phosphorylated glycogen synthase protein, and enzyme activities of glycogen synthase and glycogen phosphorylase, the rate-limiting enzymes of glycogen metabolism. In summary, these results show for the first time that chronic ethanol consumption disrupts diurnal rhythms in hepatic glycogen metabolism at the gene and protein level. Chronic ethanol-induced disruption in these daily rhythms likely contributes to glycogen depletion and disruption of hepatic energy homeostasis, a recognized risk factor in the etiology of alcoholic liver disease.

Effect of chronic ethanol consumption in female rats subjected to experimental sepsis

The objective of this research was to evaluate the interference of ethanol consumption by female rats with cytokines involved in the sepsis process and its correlation with mortality, the main outcome of sepsis. Female Wistar rats in estrus phase were evaluated in three experiments. Experiment 1 (n=40) was performed to determine survival rates. Experiment 2 (n=69) was designed for biochemical analysis, measurement of cytokine and estrogen levels before and after sepsis, and experiment 3 (n=10) was performed to evaluate bacterial growth by colony counts of peritoneal fluid. In all experiments, treated animals were exposed to a 10% ethanol/water solution (v/v) as the single drinking source, while untreated animals were given tap water. After 4 weeks, sepsis was induced in the rats by ip injection of feces. In experiment 1, mortality in ethanol-exposed animals was delayed compared with those that drank water (48 h; P=0.0001). Experiment 2 showed increased tumor necrosis factor alpha (TNF-α) and decreased interleukin-6 (IL-6) and macrophage migration inhibitory factor in septic animals exposed to ethanol compared to septic animals not exposed. Sepsis also increased TNF-α and IL-6 levels in both ethanol- and water-exposed groups. Biochemical analysis showed higher creatinine, alanine aminotransferase and aspartate aminotransferase and decreased glucose levels in septic animals that were exposed to ethanol. In experiment 3, septic animals exposed to ethanol showed decreased numbers of colony-forming units than septic animals exposed to water. These results suggest that ethanol consumption delays the mortality of female rats in estrus phase after sepsis induction. Female characteristics, most probably sex hormones, may be involved in cytokine expression.

Chronic ethanol consumption decreases mitochondrial and glycolytic production of ATP in liver

AIMS

The synthesis of ATP in the liver of the chronic ethanol consumer is suppressed, particularly if the tissue becomes hypoxic. Moreover, the perivenous region of the liver lobule becomes even more oxygen deficient as a result of ethanol consumption. Synthesis of ATP in the perivenous region of the lobule may be depressed in the chronic ethanol consumer due to decreases in both mitochondrial and glycolytic activities. In this study the effects of hypoxia on hepatic ATP levels derived from synthesis by both oxidative phosphorylation and the glycolytic mechanisms were investigated.

METHODS

Rats were pair-fed liquid diets containing 36% of calories as ethanol or an isocaloric control diet. The contributions of glycolysis and mitochondria to ATP production were assessed employing oligomycin, an inhibitor of oxidative phosphorylation. In order to localize the ethanol-elicited lesion in the glycolytic pathway, the metabolism of [3-(3)H] D-glucose was followed in hepatocytes from ethanol-fed and control animals.

RESULTS

Under both hypoxic and normoxic conditions ATP losses were due to decreases in both glycolytic and mitochondrial ATP production. The rate of production of tritiated water from [3-(3)H] D-glucose was significantly decreased in hepatocytes from ethanol-fed animals, which indicates there is an ethanol-elicited lesion in glycolysis between glucose and glyceraldehyde-3-phosphate.

Ethanol Stimulates Glycogenolysis and Inhibits both Glycogenesis via Gluconeogenesis and from Exogenous Glucose in Perfused Rat Liver

AIMS

Although it is commonly recognized that ethanol suppresses gluconeogenesis, the influence of alcohol intake on blood glucose levels remains controversial. Ethanol may act on both glucose production and glucose consumption in the liver. Thus, we studied each effect of ethanol on glucose oxidation, gluconeogenesis, glycogenesis and glycogenolysis in the liver.

METHODS

The rat liver was isolated and cyclically perfused with a medium containing 50 mmol/l ethanol.

RESULTS

Ethanol enhanced 14C-glucose oxidation in the liver from 1.09 +/- 0.11 to 1.41 +/- 0.14 micromol for 20 min (p < 0.05). Gluconeogenesis from 14C-lactate was markedly reduced by ethanol from 8.0 +/- 1.3 to 1.5 +/- 0.6 micromol for 12 min (p < 0.01). Ethanol increased glycogenolysis (net hepatic glucose output, 0.47 +/- 0.10 vs. 0.22 +/- 0.04 mmol/30 min, p < 0.01), and then decreased hepatic glycogen content (179 +/- 38 vs. 273 +/- 39 mg in the presence of 1 mU/ml insulin after 30 min of perfusion, p < 0.05). Ethanol decreased the direct glycogenesis from 14C-glucose from 0.55 +/- 0.08 to 0.33 +/- 0.05 micromol per 100 mg glycogen for 30 min (p < 0.01). Ethanol inhibited the indirect glycogenesis from 14C-lactate from 0.21 +/- 0.04 to 0.09 +/- 0.01 micromol per 100 mg glycogen for 30 min (p < 0.01).

DISCUSSION

The influence of ethanol on the blood glucose regulation by the liver seems to be different between fasted and fed states. Namely, ethanol has both the hypoglycemic effects through decreased gluconeogenesis and increased glucose oxidation and the hyperglycemic effects through decreased glycogenesis and increased glycogenolysis.

Some processes of energy saving and expenditure occurring during ethanol perfusion in the isolated liver of fed rats; a Nuclear Magnetic Resonance study

Background

In the isolated liver of fed rats, a 10 mM ethanol perfusion rapidly induced a rapid 25% decrease in the total ATP content, the new steady state resulting from both synthesis and consumption. The in situ rate of mitochondrial ATP synthesis without activation of the respiration was increased by 27%, implying an increased energy demand. An attempt to identify the ethanol-induced ATP-consuming pathways was performed using ³¹P and ¹³C Nuclear Magnetic Resonance.

Results

Ethanol (i) transiently increased sn-glycerol-3-phosphate formation whereas glycogenolysis was continuously maintained; (ii) decreased the glycolytic ATP supply and (iii) diminished the intracellular pH in a dose-dependent manner in a slight extend. Although the cytosolic oxidation of ethanol largely generated H⁺ (and NADH), intracellular pHi was maintained by (i) the large and passive excretion of cellular acetic acid arising from ethanol oxidation (evidenced by exogenous acetate administration), without energetic cost or (ii) proton extrusion via the Na⁺-HCO₃^- symport (implying the indirect activation of the Na⁺-K⁺-ATPase pump and thus an energy use), demonstrated during the addition of their specific inhibitors SITS and ouabaïn, respectively.

Conclusion

Various cellular mechanisms diminish the cytosolic concentration of H⁺ and NADH produced by ethanol oxidation, such as (i) the large but transient contribution of the dihydroxyacetone phosphate / sn-glycerol-3-phosphate shuttle between cytosol and mitochondria, mainly implicated in the redox state and (ii) the major participation of acetic acid in passive proton extrusion out of the cell. These processes are not ATP-consuming and the latter is a cellular way to save some energy. Their starting in conjunction with the increase in mitochondrial ATP synthesis in ethanol-perfused whole liver was however insufficient to alleviate either the inhibition of glycolytic ATP synthesis and/or the implication of Na⁺-HCO₃^- symport and Na⁺-K⁺-ATPase in the pHi homeostasis, energy-consuming carriers.

Chronic ethanol consumption and liver glycogen synthesis

Chronic ethanol consumption results in a dramatic decrease in liver glycogen concentrations, which could be related to either a depressed rate of synthesis or an increased rate of breakdown. Earlier studies suggested that there is not an increase in the rate of glycogenolysis as glycogen phosphorylase activities are not elevated. In the present study it was observed that the incorporation of radiolabeled glucose into glycogen was significantly depressed in hepatocytes from ethanol-fed rats under both anaerobic and aerobic conditions. Chronic ethanol consumption decreased the total glycogen synthase (a + b) activity, which correlated closely with a loss in glycogen synthase protein. However, glycogen synthase messenger RNA levels were not depressed, which indicated posttranscriptional modifications affecting both activity and protein levels. The concentration of glucose transporter 1 was also decreased due to ethanol consumption, but glucose transporter 2 levels were not altered. This latter result suggests that glucose transport in the perivenous region of the liver lobule may be decreased in chronic ethanol consumers. The alterations in glucose transport protein and glycogen synthesis observed in this study may contribute to lowered glycogen synthesis, but do not appear to account for the magnitude of the decreases in glycogen levels and rate of synthesis. Indeed, ethanol effects on glycogen metabolism are likely to be exerted at several levels, including posttranslational modulation of enzyme activities.