Independent and combined actions of interleukin-1 beta, tumor necrosis factor alpha, and glucagon on amino acid metabolism in the isolated perfused rat liver

Proteins and amino acids are fundamental to optimal nutrition support in critically ill patients

Proteins and amino acids are widely considered to be subcomponents in nutritional support. However, proteins and amino acids are fundamental to recovery and survival, not only for their ability to preserve active tissue (protein) mass but also for a variety of other functions. Understanding the optimal amount of protein intake during nutritional support is therefore fundamental to appropriate clinical care. Although the body adapts in some ways to starvation, metabolic stress in patients causes increased protein turnover and loss of lean body mass. In this review, we present the growing scientific evidence showing the importance of protein and amino acid provision in nutritional support and their impact on preservation of muscle mass and patient outcomes. Studies identifying optimal dosing for proteins and amino acids are not currently available. We discuss the challenges physicians face in administering the optimal amount of protein and amino acids. We present protein-related nutrition concepts, including adaptation to starvation and stress, anabolic resistance, and potential adverse effects of amino acid provision. We describe the methods for assessment of protein status, and outcomes related to protein nutritional support for critically ill patients. The identification of a protein target for individual critically ill patients is crucial for outcomes, particularly for specific subpopulations, such as obese and older patients. Additional research is urgently needed to address these issues.

Consequences of head injury and static cold storage on hepatic function: ex vivo experiments using a model of isolated perfused rat liver

The purpose of the study was to evaluate the effect of head injury (HI) on the metabolic and energy functions of the liver and its consequences after cold storage. In male SD rats with HI, livers were isolated 4 days after injury and perfused either immediately (HI) or after 24 hours of cold preservation. Livers isolated from healthy rats were treated identically. The hepatic functions were explored with an isolated perfused liver model. Head injury induced a liver atrophy without significant difference in the intrahepatic energy level versus healthy rats. After cold storage, hepatic adenosine triphosphate and glycogen contents in HI rats were similar to those of healthy rats. The livers of the HI group that underwent cold preservation had a lower protein catabolism. The portal flow rate at the time of reperfusion was significantly increased in the HI group. In conclusion, static cold storage of livers harvested from HI rats revealed a net protein catabolism reduction and a modification of hepatic microcirculation.

Low Doses of Tumour Necrosis Factor ? and Interleukin 1? Diminish Hepatic Gluconeogenesis from Alanine in vivo

Previous reports have attributed a stimulating action on hepatic gluconeogenesis to tumour necrosis factor alpha (TNFalpha) administered to rats at high doses (250 mug/kg). However, in adjuvant-induced arthritic rats, which present TNFalpha and other interleukins in the circulation, hepatic gluconeogenesis is diminished. The same occurs in some types of experimental cancer models as, for example, rats bearing the Walker-256 tumour. The present work represents an attempt of reproducing in rats gluconeogenesis inhibition by interleukins using low instead of high doses of both TNFalpha and interleukin 1beta (IL1beta). TNFalpha and IL1beta at doses of up to 10 mug/kg were given endovenously to rats and, after six hours, gluconeogenesis from alanine and several related parameters were evaluated in the isolated haemoglobin-free perfused rat liver. Livers from rats injected with TNFalpha and IL1beta, either alone or in combination, presented diminished gluconeogenesis. The degrees of inhibition caused by TNFalpha+IL1beta, TNFalpha and IL1beta were, respectively, 48.5, 38.8 and 30.4%. TNFalpha also diminished oxygen uptake. No action on urea and ammonia production was found. Possibly, both TNFalpha and IL1beta contribute to the decreased rates of hepatic gluconeogenesis that were found in rats with arthritis, sepsis and some kinds of cancer, but not to the decreased rates of ureagenesis.

Down-regulated expression of PPARalpha target genes, reduced fatty acid oxidation and altered fatty acid composition in the liver of mice transgenic for hTNFalpha

The present study investigated the hepatic regulation of fatty acid metabolism in hTNFalpha transgenic mice. Reduced hepatic mRNA levels and activities of carnitine palmitoyltransferase-II (CPT-II) and mitochondrial HMG-CoA synthase were observed, accompanied by decreased fatty acid oxidation, fatty acyl-CoA oxidase and fatty acid synthase (FAS) activities and down-regulated gene expression of mitochondrial acetyl-CoA carboxylase 2 (ACC2). The mRNA levels of peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARdelta were reduced. The hepatic fatty acid composition was altered, with increased amounts of saturated and polyunsaturated fatty acids. The relative amounts of Delta(9) desaturated fatty acids were decreased, as was Delta(9)desaturase mRNA. The CPT-I mRNA level remained unchanged. The PPARalpha targeted genes CPT-II and HMG-CoA synthase are potential regulators of mitochondrial fatty acid oxidation and ketogenesis in hTNFalpha transgenic mice, and the increased propionyl-CoA level found is a possible inhibitor of these processes. Reduced mitochondrial and peroxisomal fatty acid oxidation may explain the increased hepatic triglyceride level induced by TNFalpha. This is not due to de novo fatty acid synthesis as both FAS activity and gene expression of ACC2 were reduced.

Interorgan amino acid transport and its regulation

Interorgan amino acid transport is a highly active and regulated process that provides amino acids to all tissues of the body, both for protein synthesis and to enable amino acids to be used for specific metabolic functions. It is also an important component of plasma amino acid homeostasis. Net movement of amino acids depends on the physiological and nutritional state. For example, in the fed state the dominant flux is from the intestine to the other tissues. In starvation the dominant flux is from muscle to the liver and kidney. A number of general principles underlie many amino acid fluxes: i) The body does not have a store for amino acids. This means that dietary amino acids, in excess of those required for protein synthesis, are rapidly catabolized; ii) Amino acid catabolism must occur in a manner that does not elevate blood ammonia. Thus, extrasplanchnic amino acid metabolism often involves an innocuous means of transporting nitrogen to the liver; iii) Because most amino acids are glucogenic, there will be a considerable flux of amino acids to the gluconeogenic organs when there is a need to produce glucose. In addition to these bulk flows, fluxes of many specific amino acids underlie specific organ function. These include intestinal oxidation of enteral amino acids, the intestinal/renal axis for arginine production, the brain uptake of neurotransmitter precursors and renal glutamine metabolism. There is no single means of regulating amino acid fluxes; rather, such varied mechanisms as substrate supply, enzyme activity, transporter activity and competitive inhibition of transport are all found.

Plasma amino acid levels with a note on membrane transport: characteristics, regulation, and metabolic significance

The plasma concentration of an amino acid (AA) is the result of its rates of appearance (Ra) in and disappearance (Rd) from plasma. As for most nutrients, AA Ra and Rd are tightly regulated and at the postabsorptive state Ra equals Rd. Factors controlling Ra are protein intake and tissue release; those controlling Rd are tissue uptake and body losses (urine, sweat, etc.). Regulation of plasma AA concentrations involves hormones, in particular insulin and glucagon, both of which induce hypoaminoacidemia (but for quite different reasons), and cortisol, which induces hyperaminoacidemia. In addition, in pathologic states, catecholamines, thyroid hormones, and cytokines modulate plasma AA levels. Peripheral availability of AAs after protein ingestion is controlled by the liver, with an activation of ureagenesis in hyperprotein feeding and repression during a hypoprotein diet. The arginine-to-citrulline pathway in the intestine plays a key role in this adaptative process. In some circumstances tissue uptake of AAs and further metabolism depend on plasma AA concentrations. Plasma glutamine level may be the driving force controlling the flux of this AA at the muscle level. Also, channeling of the arginine cellular pathways means that plasma arginine is a major controlling component of nitric oxide synthesis in endothelial and immune cells. All these features explain the excessive increase in glutamine and arginine demands, in particular for energy expenditure, leading to morbidity (e.g., gut atrophy, muscle wasting, and immune dysfunction) in stressed patients. Normoaminoacidemia is not synonymous with health because this state is observed in level 2 starvation (Ra and Rd decrease) or after minor injury (Ra and Rd increase). Hyperaminoacidemia may be the consequence of organ failure (Rd decreases) or excessive AA intake during parenteral nutrition (Ra increases). Hypoaminoacidemia is observed after organ removal (Ra decreases, e.g., decrease in citrulline concentration in short bowel syndrome) or in stress situations (Rd increases). Mere determinations of plasma AA concentrations at the basal state (i.e., postabsorptive) provide rather limited information. Their usefulness can be improved by measuring arteriovenous differences or performing time course measurements, but techniques based on stable isotopes are necessary to obtain more precise information on the behavior of a particular AA or group of AAs.

Pulmonary-specific expression of tumor necrosis factor-alpha alters surfactant lipid metabolism

Tumor necrosis factor (TNF)-alpha is a major cytokine implicated in inducing acute and chronic lung injury, conditions associated with surfactant phosphatidylcholine (PtdCho) deficiency. Acutely, TNF-alpha decreases PtdCho synthesis but stimulates surfactant secretion. To investigate chronic effects of TNF-alpha, we investigated PtdCho metabolism in a murine transgenic model exhibiting lung-specific TNF-alpha overexpression. Compared with controls, TNF-alpha transgenic mice exhibited a discordant pattern of PtdCho metabolism, with a decrease in PtdCho and disaturated PtdCho (DSPtdCho) content in the lung, but increased levels in alveolar lavage. Transgenics had lower activities and increased immunoreactive levels of cytidylyltransferase (CCT), a key PtdCho biosynthetic enzyme. Ceramide, a CCT inhibitor, was elevated, and linoleic acid, a CCT activator, was decreased in transgenics. Radiolabeling studies revealed that alveolar reuptake of DSPtdCho was significantly decreased in transgenic mice. These observations suggest that chronic expression of TNF-alpha results in a complex pattern of PtdCho metabolism where elevated lavage PtdCho may originate from alveolar inflammatory cells, decreased surfactant reuptake, or altered surfactant secretion. Reduced parenchymal PtdCho synthesis appears to be attributed to CCT enzyme that is physiologically inactivated by ceramide or by diminished availability of activating lipids.

Intrahepatic nuclear factor-kappa B activity and alpha 1-acid glycoprotein transcription do not predict outcome after cecal ligation and puncture in the rat

OBJECTIVE

Sepsis is the leading cause of death in critically ill surgical patients. Septic hepatic dysfunction, an important determinant of outcome, is poorly understood but includes inappropriate transcriptional down-regulation. This may be modulated by proinflammatory cytokines. We hypothesized that intrahepatic changes in tumor necrosis factor (TNF)/interleukin (IL)-1-linked processes, such as the activation of the p50 homodimeric and the p65/p50 heterodimeric isoforms of the transcription factor nuclear factor (NF)-kappaB or transcription of the acute phase reactant alpha1-acid glycoprotein (AGP), would correlate with recovery from sepsis.

DESIGN

Prospective experimental comparison of sham operation and nonlethal and lethal sepsis in male Sprague-Dawley rats.

INTERVENTIONS

Nonlethal sepsis was induced by using single-puncture cecal ligation and puncture (CLP). Lethal sepsis was induced via double-puncture CLP. NF-kappaB DNA binding activity was determined by using electrophoretic mobility shift analysis with differentiation of p50/p50 and p50/p65 isoforms by using appropriate antibodies. AGP transcription was assessed with transcription elongation analysis, intrahepatic IL-1beta, and TNF-alpha abundance by using immunohistochemistry, and serum IL-1beta was assessed by using ELISA.

MAIN RESULTS

Overall NF-kappaB activity increased equivalently over time after both single- and double-puncture CLP, with a peak occurring 3 hrs after intervention. In single-puncture CLP, there was an increase in the binding of the p50 homodimer form over time. After double-puncture CLP, no such change was observed. AGP transcription was increased equivalently in both models. Intrahepatic IL-1beta was detected 16 and 24 hrs after single-puncture CLP and 6 hrs after double-puncture CLP. After double-puncture CLP, intrahepatic TNF-alpha was detected at 6, 16, and 24 hrs. Serum IL-1beta was undetectable after both single- and double-puncture CLP.

CONCLUSIONS

Although AGP transcription was similar in mild and fulminant sepsis, double-puncture CLP increased the binding activity of the p50 homodimer relative to binding of the p50/p65 NF-kappaB heterodimer. These results imply that transcriptional activity not linked to acute phase responses is an important determinant of outcome in sepsis.

Effects of intratracheal instillation of TNF-alpha on surfactant metabolism

Tumor necrosis factor-alpha (TNF-alpha) has been shown to play an integral role in the pathogenesis of the acute respiratory distress syndrome. This disorder is characterized by a deficiency of alveolar surfactant, a surface-active material that is composed of key hydrophobic proteins and the major lipid disaturated phosphatidylcholine (DSPC). We investigated how TNF-alpha might alter DSPC content in rat lungs by instilling the cytokine (2.5 microg) intratracheally for 10 min and then assaying parameters of DSPC synthesis and degradation in alveolar type II epithelial cells, which produce surfactant. Cells isolated from rats given TNF-alpha had 26% lower levels of phosphatidylcholine compared with control. TNF-alpha treatment also decreased the ability of these cells to incorporate [(3)H]choline into DSPC by 45% compared with control isolates. There were no significant differences in the levels of choline substrate or choline transport between the groups. However, TNF-alpha produced a 64% decrease in the activity of cytidylyltransferase, the rate-regulatory enzyme required for DSPC synthesis. TNF-alpha administration in vivo also tended to stimulate phospholipase A(2) activity, but it did not alter other parameters for DSPC degradation such as activities for phosphatidylcholine-specific phospholipase C or phospholipase D. These observations indicate that TNF-alpha decreases the levels of surfactant lipid by decreasing the activity of a key enzyme involved in surfactant lipid synthesis. The results do not exclude stimulatory effects of the cytokine on phosphatidylcholine breakdown.

Effects of amino acids on bile acid-dependent and independent bile flow in the isolated perfused rat liver

BACKGROUND/AIMS

Conflicting data on the effects of amino acids on biliary function led us to investigate their interaction with taurocholate in the perfused rat liver model.

METHODS

To investigate the influence of amino acids on the bile acid-independent component of bile flow, 12 livers were perfused with (n = 6) and without (n = 6) amino acid addition from t30 min. For the study of bile acid-dependent bile flow, 24 livers were perfused under 8 experimental conditions according to the perfusate taurocholate concentration (12.5, 25, 37.5 or 50 microM) and whether amino acids were or were not added from t30 min.

RESULTS

In the absence of taurocholate, amino acids induced a 40% (p<0.01) decrease in bile flow together with an increase in hepatic water content (17.8%, p< 0.05). Thus, amino acids exert an inhibitory effect on bile acid-independent bile flow despite the postulated cell swelling-dependent increase in bile flow. When livers were perfused at various taurocholate concentrations, amino acids induced, in addition to their inhibitory effect on bile acid-independent bile flow, a significant increase in taurocholate apparent choleretic activity (13.2 microl/micromol vs. 10.6 microl/micromol; p = 0.05), while taurocholate intrinsic clearance was significantly decreased (4.5+/-1.2 ml x min(-1) x g(-1) vs. 6.1+/-1.3 ml x min(-1) x g(-1); p<0.01).

CONCLUSIONS

These data suggest that at physiological bile acid concentrations amino acids exert an inhibitory effect on both bile acid-dependent and- independent bile flow, whereas at higher taurocholate concentrations this inhibitory effect disappears, probably because of cell swelling-dependent mechanisms.

Abnormalities in branched-chain amino acid metabolism in cirrhosis: influence of hormonal and nutritional factors and directions for future research

Plasma branched-chain amino acid (BCAA) levels are decreased in patients with liver cirrhosis, owing to an increase in BCAA tissue uptake and/or catabolism and a decrease in BCAA production from proteins. Non-specific factors such as malnutrition worsen this picture. Studies of BCAA fluxes and protein turnover in cirrhotic patients have given conflicting results due to patient heterogeneity, differences in method and bias in the expression of results. In well compensated cirrhosis, muscle wasting is moderate and probably due more to decreased protein synthesis than to increased protein catabolism. Hyperinsulinemia has been suggested as the main cause of decreased BCAA levels, by increasing BCAA uptake in muscle and additionally in adipose tissue. However, as depletion of fat stores is frequent in cirrhosis, this effect is certainly quantitatively weak. Also, there is no correlation between state of hyperinsulinemia and decrease in BCAA levels. An effect of cytokines (IL1 and TNF) on muscle BCAA catabolism is a possibility. Until recently, the contribution of the liver to abnormal BCAA metabolism has been underestimated. In cirrhotic liver an increase in liver transamination of branched-chain keto acids (BCKAs) has been suggested and may result from inhibition of liver BCKA dehydrogenase. A modification of protein turnover in cirrhotic liver must be also considered. Lastly, the contribution of non-hepatocyte liver cells, which are activated in cirrhosis, remains to be assessed.

No evidence for a tumor necrosis factor alpha stimulated 2-methylaminoisobutyric acid uptake in hepatocyte monolayer

This study investigates the short-term effects of glucagon and human recombinant tumor necrosis factor alpha (TNF alpha) singly and in association on 2-methylaminoisobutyric acid (MeAIB) transport in hepatocyte monolayers. As expected, glucagon induced a time-dependent stimulation of MeAIB transport. In our experimental conditions, TNF alpha did not induce cytolysis. A 2 hour exposure to TNF alpha (0.05-500 ng/l) with or without glucagon (10(-9) to 10(-6) M) did not modify the basal or glucagon-stimulated MeAIB transport. Varying the duration of exposure to TNF alpha 5 ng/l up to 6 h was equally ineffective. The presence of hydrocortisone potentiated the glucagon-stimulated transport, but TNF alpha remained ineffective. Finally, the association of interferon (IFN gamma) with TNF alpha and/or glucagon was unable to modify the transport activity. These data demonstrate that TNF alpha does not exert a direct effect on MeAIB transport in hepatocytes, at least on a short-term basis.