Regulation of hepatocyte glutathione by amino acid precursors and cAMP in protein-energy malnourished rats

Rumen-protected methionine compared with rumen-protected choline improves immunometabolic status in dairy cows during the peripartal period

The immunometabolic status of peripartal cows is altered due to changes in liver function, inflammation, and oxidative stress. Nutritional management during this physiological state can affect the biological components of immunometabolism. The objectives of this study were to measure concentrations of biomarkers in plasma, liver tissue, and milk, and also polymorphonuclear leukocyte function to assess the immunometabolic status of cows supplemented with rumen-protected methionine (Met) or choline (CHOL). Forty-eight multiparous Holstein cows were used in a randomized complete block design with 2×2 factorial arrangement of Met (Smartamine M, Adisseo NA, Alpharetta, GA) and CHOL (ReaShure, Balchem Inc., New Hampton, NY) level (with or without). Treatments (12 cows each) were control (CON), no Met or CHOL; CON and Met (SMA); CON and CHOL (REA); and CON and Met and CHOL (MIX). From -50 to -21d before expected calving, all cows received the same diet [1.40Mcal of net energy for lactation (NE_L)/kg of DM] with no Met or CHOL. From -21d to calving, cows received the same close-up diet (1.52Mcal of NE_L/kg of DM) and were assigned randomly to each treatment. From calving to 30d, cows were on the same postpartal diet (1.71Mcal of NE_L/kg of DM) and continued to receive the same treatments until 30d. The Met supplementation was adjusted daily at 0.08% DM of diet, and CHOL was supplemented at 60g/cow per day. Liver (-10, 7, 21, and 30d) and blood (-10, 4, 8, 20, and 30d) samples were harvested for biomarker analyses. Neutrophil and monocyte phagocytosis and oxidative burst were assessed at d 1, 4, 14, and 28d. The Met-supplemented cows tended to have greater plasma paraoxonase. Greater plasma albumin and IL-6 as well as a tendency for lower haptoglobin were detected in Met- but not CHOL-supplemented cows. Similarly, cows fed Met compared with CHOL had greater concentrations of total and reduced glutathione (a potent intracellular antioxidant) in liver tissue. Upon a pathogen challenge in vitro, blood polymorphonuclear leukocyte phagocytosis capacity and oxidative burst activity were greater in Met-supplemented cows. Overall, liver and blood biomarker analyses revealed favorable changes in liver function, inflammation status, and immune response in Met-supplemented cows.

Acute glucagon induces postprandial peripheral insulin resistance

Glucagon levels are often moderately elevated in diabetes. It is known that glucagon leads to a decrease in hepatic glutathione (GSH) synthesis that in turn is associated with decreased postprandial insulin sensitivity. Given that cAMP pathway controls GSH levels we tested whether insulin sensitivity decreases after intraportal (ipv) administration of a cAMP analog (DBcAMP), and investigated whether glucagon promotes insulin resistance through decreasing hepatic GSH levels.Insulin sensitivity was determined in fed male Sprague-Dawley rats using a modified euglycemic hyperinsulinemic clamp in the postprandial state upon ipv administration of DBcAMP as well as glucagon infusion. Glucagon effects on insulin sensitivity was assessed in the presence or absence of postprandial insulin sensitivity inhibition by administration of L-NMMA. Hepatic GSH and NO content and plasma levels of NO were measured after acute ipv glucagon infusion. Insulin sensitivity was assessed in the fed state and after ipv glucagon infusion in the presence of GSH-E. We founf that DBcAMP and glucagon produce a decrease of insulin sensitivity, in a dose-dependent manner. Glucagon-induced decrease of postprandial insulin sensitivity correlated with decreased hepatic GSH content and was restored by administration of GSH-E. Furthermore, inhibition of postprandial decrease of insulin sensitivity L-NMMA was not overcome by glucagon, but glucagon did not affect hepatic and plasma levels of NO. These results show that glucagon decreases postprandial insulin sensitivity through reducing hepatic GSH levels, an effect that is mimicked by increasing cAMP hepatic levels and requires physiological NO levels. These observations support the hypothesis that glucagon acts via adenylate cyclase to decrease hepatic GSH levels and induce insulin resistance. We suggest that the glucagon-cAMP-GSH axis is a potential therapeutic target to address insulin resistance in pathological conditions.

Recombinant protein glutathione S-transferases P1 attenuates inflammation in mice

We have reported that intracellular glutathione S-transferases P1 (GSTP1) suppresses LPS (lipopolysaccharide)-induced excessive production of pro-inflammatory factors by inhibiting LPS-stimulated MAPKs (mitogen-activated protein kinases) as well as NF-kappaB activation. But under pathogenic circumstances, physiologic levels of GSTP1 are insufficient to stem pro-inflammatory signaling. Here we show that LPS-induced up-regulation of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in RAW246.7 cells is significantly reduced by incubating cells with recombinant GSTP1 protein. In vivo study demonstrates that treatment of mice (i.p.) with recombinant GSTP1 protein effectively suppresses the devastating effects of acute inflammation, which includes reduction of mortality rate of endotoxic shock, alleviation of LPS-induced acute lung injury and abrogation of thioglycolate (TG)-induced peritoneal deposition of leukocytes and polymorphonuclear cells (PMNs). Meanwhile, GSTP1 prevented LPS-induced TNF-alpha, IL-1beta, MCP-1 and NO production. Further investigation by using confocal microscopy and flow cytometry shows that recombinant GSTP1 protein can be delivered into RAW246.7 cells, mouse peritoneal macrophages and HEK 293 cells suggesting that extracellular GSTP1 protein could be transported across plasma membrane and act as a cytosolic protein. In conclusion our research demonstrates a new finding that increasing cellular GSTP1 level by supplement of recombinant GSTP1 effectively suppresses the devastating effects of acute inflammation.

Production of intracellular 35S-glutathione by rat and human hepatocytes for the quantification of xenobiotic reactive intermediates

The quantification and identification of xenobiotic reactive intermediates is difficult in the absence of highly radiolabeled drug. We have developed a method for identifying these intermediates by measuring the formation of adducts to intracellularly generated radiolabeled glutathione (GSH). Freshly isolated adherent rat and human hepatocytes were incubated overnight in methionine and cystine-free ('thio-free') medium. They were then exposed to 100 microM methionine and 10 microCi 35S-labeled methionine in otherwise thio-free medium to replete cellular GSH pools with intracellularly generated 35S-labeled GSH. After 3 h, acetaminophen was added as a test compound and the cells were incubated for an additional 24 h. Intracellular GSH and its specific activity were quantified after reaction with monobromobimane followed by HPLC analysis with fluorescence and radiochemical detection. Radiolabeled GSH was detectable at 3 h and maintained high specific activity and physiological concentrations for up to 24 h. Incubation medium from acetaminophen treated and nontreated hepatocytes were analyzed for radiolabeled peaks by HPLC using radiochemical detection. Radiolabeled peaks not present in nontreated hepatocytes were identified as acetaminophen GSH adducts by LC-MS. Formation of acetaminophen 35S-GSH adducts by rat hepatocytes containing endogenously synthesized 35S-GSH was increased with acetaminophen concentrations ranging from 500 to 2 mM.

Long-term high protein intake does not increase oxidative stress in rats

The maximum dietary protein intake that does not cause adverse effects in a healthy population is uncertain. We tested whether a high protein intake enhances oxidative stress. Adult rats were adapted to different casein-based diets containing either an adequate (13.8%; AP), medium (25.7%; MP), or high (51.3%; HP) level of crude protein; a fourth group received a HP diet but no RRR-alpha-tocopherol acetate (HP-toc). After 15 wk of feeding, plasma protein carbonyl concentration, liver lipid peroxide levels [thiobarbituric acid-reacting substances (TBARS)], reduced glutathione (GSH) status and leucine kinetics ([1-(13)C]leucine) were measured. Higher concentrations of protein carbonyls and TBARS were found in rats fed the AP and the HP-toc diets compared with those fed the MP and HP diets (P: < 0.05). GSH concentrations in plasma did not differ but total blood GSH concentrations were significantly (P: < 0.05) lower in rats fed the HP-toc diet compared with those fed the AP, MP and HP diets. Liver GSH concentrations were significantly (P: < 0.01) lower in rats fed the AP diet compared with the other groups. Rates of postabsorptive leucine oxidation (LeuOX) and flux (Q(Leu)) were positively correlated with the dietary protein level (for AP, MP, and HP, respectively: LeuOX, 74.9 +/- 28.5, 109 +/- 35.2, 142.3 +/- 38.4 micromol/(kg. h); Q(Leu), 425 +/- 102, 483 +/- 82, 505 +/- 80 micromol/(kg. h). Only HP-toc resulted in a significantly greater protein breakdown (PB(Leu)) and Q(Leu). No difference was seen in nonoxidative leucine disposal. Long-term intake of high protein diets did not increase variables of oxidative stress, in contrast to our initial hypothesis. An unexpected finding was that adequate protein feeding (AP) may in fact induce oxidative stress.

Optimization of rat hepatocyte culture in citrated human plasma

BACKGROUND

Maintenance of liver-specific functions in hepatocyte cultures during plasma exposure is critical for the clinical application of bioartificial liver assist systems. Sodium citrate is a common anticoagulant but has been shown to be cytotoxic to hepatocytes. We have tested the effect of various supplements on the viability and function of adult primary rat hepatocytes exposed to citrated plasma.

MATERIALS AND METHODS

Freshly isolated rat hepatocytes were cultured in the collagen gel sandwich configuration in culture medium for 6 days followed by exposure to citrated human plasma with various supplements for 1 week. Controls were left in culture medium throughout. Viability and synthetic functions were evaluated.

RESULTS

Hepatocytes exposed to unsupplemented citrated plasma lost significant viability and function within the first 2 days. Cells cultured in plasma supplemented with a fivefold concentrate of standard hepatocyte culture medium maintained urea (1. 2-2.1 micromol/day/10(6) cells) and albumin (51-62 microg/day/10(6) cells) synthesis rates equal to or higher than those of controls. Among the various components of the concentrated medium supplement, calcium chloride (1.8 mM), magnesium sulfate (0.8 mM), amino acids (fourfold Basal Medium Eagle amino acids including 4 mM glutamine), and glucagon (14 ng/ml) were found to be essential in maintaining urea synthesis. Maintenance of a high albumin synthesis rate also required the addition of hydrocortisone (7.5 microg/ml) and insulin (0.5 U/ml).

CONCLUSIONS

Appropriate metabolic and hormonal supplementation of citrated human plasma prevents its cytotoxic effects and may be used in conjunction with in vivo use of bioartificial liver assist systems.

Augmented resistance to oxidative stress in fatty rat livers induced by a short-term sucrose-rich diet

Hepatic steatosis and the accompanying oxidative stress have been associated with a variety of liver diseases. It is not known if fat accumulation per se plays a direct role in the oxidative stress of the organ. This study tested if steatosis induced by a short-term carbohydrate-rich diet results in an increased hepatic sensitivity to oxidative stress. Antioxidant status was determined in a liver perfusion system and in isolated parenchymal, endothelial and Kupffer cells from rats kept on sucrose-rich diet or on regular diet for 48 h. t-Butyl hydroperoxide addition (2 mM) to the perfusion fluid resulted in a release of alanine aminotransferase (ALT) in livers from controls, whereas no ALT release was observed in fatty livers. After t-butyl hydroperoxide addition, oxidized glutathione release was 40% less in fatty than in control livers, whereas reduced glutathione (GSH) release was not different. Sinusoidal oxidant stress was mimicked by the addition of lipopolysaccharide (LPS) from Escherichia coli (10 microg/ml) followed by the addition of opsonized zymosan (8 mg/ml) to the perfusion medium. LPS plus zymosan treatments resulted in the release of ALT in control but not in fatty livers. At the end of perfusion, liver glutathione content was 3-fold elevated, and the tissue content of lipid peroxidation products was approx. 40% less in fatty livers compared to controls. GSH content was doubled and glucose-6-phosphate dehydrogenase (G6PD) expression was elevated by 3- and 10-fold in sinusoidal endothelial and parenchymal cells form fatty livers compared to cells from control animals. Following H(2)O(2) administration in vitro (0.2-1 mM), GSH remained elevated in endothelial and parenchymal cells from fatty livers compared to cells from controls. In contrast, G6PD activity and GSH content were similar in Kupffer cells isolated from fatty or control livers. The study shows that hepatic fat accumulation caused by a short-term sucrose diet is not accompanied by elevated hepatic lipid peroxidation, and an elevated hepatic antioxidant activity can be manifested in the presence of prominent steatosis. The diet-induced increase in G6PD expression and, thus, the efficient maintenance of reduced glutathione in endothelial and parenchymal cells are a supportive mechanism in the observed hepatic resistance against intracellular or sinusoidal oxidative stress.

Efeito da restrição energética na atividade hepática da gama-glutamiltranspeptidase e nos níveis de glutationa

Avaliou-se o efeito da restrição energética aguda (crescimento pós-desmame) e crônica (lactantes desnutridas desde o período pré-natal e seus filhotes) na atividade da gama-glutamiltranspeptidase e nos níveis de glutationa. O consumo da dieta foi ad libitum (controle) ou com restrição de 30% (desnutrido), tendo-se conduzido dois ensaios, um de desnutrição aguda e outro de desnutrição crônica. A restrição aguda provocou redução significativa no ganho ponderal dos ratos em crescimento, mas as mães cronicamente desnutridas ganharam significativamente mais peso/filhote, pois apresentaram menor número de filhotes. Não foram observadas alterações significativas nos níveis de glutationa, na atividade da gama-glutamiltranspeptidase e mesmo na concentração de proteína hepática sob o efeito da restrição energética, aguda ou crônica. Menor desequilíbrio no perfil de aminoácidos plasmáticos e na síntese protéica, conseqüente à desnutrição energética e não à protéica, pode justificar estes resultados.

Biologic and pharmacologic regulation of mammalian glutathione synthesis

Glutathione (L-gamma-glutamyl-L-cysteinylglycine, GSH) is synthesized from its constituent amino acids by the sequential action of gamma-glutamylcysteine synthetase (gamma-GCS) and GSH synthetase. The intracellular GSH concentration, typically 1-8 mM, reflects a dynamic balance between the rate of GSH synthesis and the combined rate of GSH consumption within the cell and loss through efflux. The gamma-GCS reaction is rate limiting for GSH synthesis, and regulation of gamma-GCS expression and activity is critical for GSH homeostasis. Transcription of the gamma-GCS subunit genes is controlled by a variety of factors through mechanisms that are not yet fully elucidated. Glutathione synthesis is also modulated by the availability of gamma-GCS substrates, primarily L-cysteine, by feedback inhibition of gamma-GCS by GSH, and by covalent inhibition of gamma-GCS by phosphorylation or nitrosation. Because GSH plays a critical role in cellular defenses against electrophiles, oxidative stress and nitrosating species, pharmacologic manipulation of GSH synthesis has received much attention. Administration of L-cysteine precursors and other strategies allow GSH levels to be maintained under conditions that would otherwise result in GSH depletion and cytotoxicity. Conversely, inhibitors of gamma-GCS have been used to deplete GSH as a strategy for increasing the sensitivity of tumors and parasites to certain therapeutic interventions.

DL-methionine supplementation of rice-and-bean diets affects gamma-glutamyltranspeptidase activity and glutathione content in livers of growing rats

Gamma-glutamyltranspeptidase (GGT-EC 2.3.2.2) activity and glutathione (GSH) content were measured in livers of female weanling Wistar rats (N = 5-18), submitted to rice-and-bean diets (13 and 6% w/w protein), both supplemented or not with DL-methionine (0.5 and 0.23 g/100 g dry diet, respectively). After 28 days, the rats on the rice-and-bean diets showed significantly higher levels (four times higher) of liver GGT activity and a concomitant 50% lower concentration of liver GSH in comparison with control groups feeding on casein. The addition of DL-methionine to rice-and-bean diets significantly increased the liver GSH content, which reached levels 50% higher than those found in animals on casein diets. The increase in GSH was accompanied by a decrease in liver GGT activity, which did not reach levels as low as those observed in the control groups. No significant correlation could be established between GGT and GSH changes under the present experimental conditions. Linear correlation analysis only revealed that in animals submitted to unsupplemented rice-and-bean diets GSH concentration was positively associated (P < 0.05) with weight gain, food intake and food efficiency. GGT, however, was negatively correlated (P < 0.05) with food intake only, and exclusively for supplemented rice-and-bean diets. The high levels of GGT activity observed in the present study for rats receiving a rice-and-bean mixture could be a result of the poor quality of these diets associated with their deficiency in sulfur amino acids. The results also suggest that diet supplementation with methionine could be important in the reduction of the deleterious effects of GSH depletion by restoring the intracellular concentration of this tripeptide.

Moderate dietary protein and energy restriction modulate cAMP-dependent protein kinase activity in rat liver

Very low protein diets result in a desensitization of hepatic cAMP signaling in rats, which is characterized by a loss of cAMP-dependent protein kinase (PKA) activity and type I regulatory subunit (RI). Here we have tested whether more moderate protein restriction (Trial 1) or energy restriction (Trial 2) also modulates hepatic PKA quantity and activity. In trial 1, weanling rats were allowed free access to diets containing normal protein (15%, AL-NP), moderately restricted protein (12.5%, AL-MP) and low protein (7.5%, AL-LP); in trial 2, rats were allowed free access to diet containing 15% (AL-NP) or 0.5% protein (very low protein, AL-VLP) or were energy restricted by pair-feeding a diet isonitrogenous to AL-NP but at 65% of the energy intake (ER-IN) for 14 d. Body weights were lower (P < 0.05) by d 14 in all restricted groups compared with the AL-NP group. The quantity of cytosolic RI was lower (P < 0.05) in AL-LP and AL-VLP, but not in AL-MP or ER-IN, compared with AL-NP. In contrast, there was no effect of diet on RI in the particulate fraction. RII was not changed by moderate and low protein diets in either the cytosol or particulate fraction. However, type II regulatory subunit (RII) was greater in the cytosol of ER-IN and in the particulate fraction of AL-VLP (P < 0.05) compared with AL-NP. Specific activity of PKA was lower in the cytosol and particulate fraction (P < 0.05) in the AL-VLP and ER-IN groups compared with the AL-NP group. In contrast, specific activity of PKA was maintained in cytosol from AL-LP, but lower in the particulate fraction (P < 0.05) compared with AL-NP. In summary, protein restricted-diets lower RI subunit in the cytosol; however, only in rats fed very low protein diets is this loss of RI associated with lower cytosolic PKA activity. In contrast, energy restriction lowers PKA activity in the cytosol and particulate fractions, independent of signficant reduction in RI or RII subunits. Taken together, these data indicate that moderate protein and energy restrictions have differential effects on activity and quantity of PKA.

Methionine and cysteine affect glutathione level, glutathione-related enzyme activities and the expression of glutathione S-transferase isozymes in rat hepatocytes

Methionine and cysteine are constituents of glutathione. To understand the effects of these two sulfur amino acids on the glutathione (GSH)-dependent detoxification defense system, intracellular GSH and GSH-related enzyme activities, including GSH peroxidase, GSH reductase, GSH S-transferase (GST) and gamma-glutamylcysteine synthetase, were determined. In addition, the expression of three GST isozymes and carbonic anhydrase III (CA III) was examined. Hepatocytes isolated from male Sprague-Dawley rats were cultured with 0.1, 0.3, 0.5 or 1.0 mmol/L each of L-methionine and L-cysteine, for up to 7 d. Cells incubated with 0.5 or 1.0 mmol/L methionine and cysteine had increased intracellular GSH. A twofold increase was observed on d 6 compared with freshly isolated hepatocytes (P < 0.05). However, intracellular GSH was lower in cells treated with 0.3 or 0.1 mmol/L each of methionine and cysteine than in cells tested with 0.5 or 1.0 mmol/L. Although the GSH level differed significantly between cells cultured with 0.3 or 1.0 mmol/L of methionine and cysteine, GSH-related enzymes did not differ at these two concentrations. The activity generally remained constant for the first 24 h, then increased up to d 4. Immunodetection analysis revealed no difference in the level of CA III and GST isoforms, Ya, Yb and Yp, with amino acids each at a concentration of at least 0.3 mmol/L. Yp expression steadily increased up to d 7. Most proteins decreased rapidly after 48 h when cultured with 0.1 mmol/L of methionine and cysteine; however, the Yp level increased up to d 6. In conclusion, results indicate that a twofold increase of intracellular GSH is reached by adding methionine and cysteine at a concentration >0.5 mmol/L to the culture medium. The concentrations of methionine and cysteine for maintaining hepatic GSH are higher than for GSH-related enzyme activity and for GST isoform expression.

Effect of essential fatty acid deficiency on G-proteins, cAMP-dependent protein kinase activity and mucin secretion in the rat submandibular salivary glands

Studies were conducted to determine whether beta-adrenergic cell signalling is altered in submandibular salivary glands (SMSG) is essential fatty acid (EFA) deficiency. Three groups of rats were fed diets which were deficient in EFA (EFAD), marginally deficient in EFA (MEFAD) or contained sufficient amount of EFA (Control). Rats were killed after 20 wk on diets, SMSG were dissected out and cyclic AMP-dependent protein kinase (PKA) activity was measured. The specific enzyme activities were higher in the homogenates and supernatant fractions of the gland from EFAD and MEFAD rats compared with the controls. The relative levels of guanine nucleotide-binding regulatory proteins (Gs and Gi) were also measured in the SMSG membranes of rats fed the 3 diets. The levels of Gs were significantly higher in the EFAD and MEFAD groups than in the controls. No significant differences were observed in the secretion of trichloroacetic acid-phosphotungstic acid (TCA-PTA) precipitable glycoproteins from the SMSG slices among the 3 dietary groups.