Quantitative effects of thermal injury and insulin on the metabolism of the skeletal muscle using the perfused rat hindquarter preparation

Modulation of Burn Hypermetabolism in Preclinical Models

Severe burns elicit a state of physiological stress and increased metabolism to help the body compensate for the changes associated with the traumatic injury. However, this hypermetabolic state is associated with increased insulin resistance, cardiovascular dysfunction, skeletal muscle catabolism, impaired wound healing, and delayed recovery. Several interventions were attempted to modulate burn hypermetabolism, including nutritional support, early excision and grafting, and growth hormone application. However, burn hypermetabolism still imposes significant morbidity and mortality in burn patients. Due to the limitations of in vitro models, animal models are indispensable in burn research. Animal models provide researchers with invaluable tools to test the safety and efficacy of novel treatments or advance our knowledge of previously utilized agents. Several animal studies evaluated novel therapies to modulate burn hypermetabolism in the last few years, including recombinant human growth hormone, erythropoietin, acipimox, apelin, anti-interleukin-6 monoclonal antibody, and ghrelin therapies. Results from these studies are promising and may be effectively translated into human studies. In addition, other studies revisited drugs previously used in clinical practice, such as insulin and metformin, to further investigate their underlying mechanisms as modulators of burn hypermetabolism. This review aims to update burn experts with the novel therapies under investigation in burn hypermetabolism with a focus on applicability and translation. Furthermore, we aim to guide researchers in selecting the correct animal model for their experiments by providing a summary of the methodology and the rationale of the latest studies.

Development of Metabolic Indicators of Burn Injury: Very Low Density Lipoprotein (VLDL) and Acetoacetate Are Highly Correlated to Severity of Burn Injury in Rats

Hypermetabolism is a significant sequela to severe trauma such as burns, as well as critical illnesses such as cancer. It persists in parallel to, or beyond, the original pathology for many months as an often-fatal comorbidity. Currently, diagnosis is based solely on clinical observations of increased energy expenditure, severe muscle wasting and progressive organ dysfunction. In order to identify the minimum number of necessary variables, and to develop a rat model of burn injury-induced hypermetabolism, we utilized data mining approaches to identify the metabolic variables that strongly correlate to the severity of injury. A clustering-based algorithm was introduced into a regression model of the extent of burn injury. As a result, a neural network model which employs VLDL and acetoacetate levels was demonstrated to predict the extent of burn injury with 88% accuracy in the rat model. The physiological importance of the identified variables in the context of hypermetabolism, and necessary steps in extension of this preliminary model to a clinically utilizable index of severity of burn injury are outlined.

Advanced stoichiometric analysis of metabolic networks of mammalian systems

Metabolic engineering tools have been widely applied to living organisms to gain a comprehensive understanding about cellular networks and to improve cellular properties. Metabolic flux analysis (MFA), flux balance analysis (FBA), and metabolic pathway analysis (MPA) are among the most popular tools in stoichiometric network analysis. Although application of these tools into well-known microbial systems is extensive in the literature, various barriers prevent them from being utilized in mammalian cells. Limited experimental data, complex regulatory mechanisms, and the requirement of more complex nutrient media are some major obstacles in mammalian cell systems. However, mammalian cells have been used to produce therapeutic proteins, to characterize disease states or related abnormal metabolic conditions, and to analyze the toxicological effects of some medicinally important drugs. Therefore, there is a growing need for extending metabolic engineering principles to mammalian cells in order to understand their underlying metabolic functions. In this review article, advanced metabolic engineering tools developed for stoichiometric analysis including MFA, FBA, and MPA are described. Applications of these tools in mammalian cells are discussed in detail, and the challenges and opportunities are highlighted.

Soft constraints-based multiobjective framework for flux balance analysis

The current state of the art for linear optimization in Flux Balance Analysis has been limited to single objective functions. Since mammalian systems perform various functions, a multiobjective approach is needed when seeking optimal flux distributions in these systems. In most of the available multiobjective optimization methods, there is a lack of understanding of when to use a particular objective, and how to combine and/or prioritize mutually competing objectives to achieve a truly optimal solution. To address these limitations we developed a soft constraints based linear physical programming-based flux balance analysis (LPPFBA) framework to obtain a multiobjective optimal solutions. The developed framework was first applied to compute a set of multiobjective optimal solutions for various pairs of objectives relevant to hepatocyte function (urea secretion, albumin, NADPH, and glutathione syntheses) in bioartificial liver systems. Next, simultaneous analysis of the optimal solutions for three objectives was carried out. Further, this framework was utilized to obtain true optimal conditions to improve the hepatic functions in a simulated bioartificial liver system. The combined quantitative and visualization framework of LPPFBA is applicable to any large-scale metabolic network system, including those derived by genomic analyses.

Application of metabolic flux analysis to identify the mechanisms of free fatty acid toxicity to human hepatoma cell line

Chronic exposure to elevated levels of free fatty acids (FFAs) has been shown to cause cell death (lipotoxicity), but the underlying mechanisms of lipotoxicity in hepatocytes remain unclear. We have previously shown that the saturated FFAs cause much greater toxicity to human hepatoma cells (HepG2) than the unsaturated ones (Srivastava and Chan, 2007). In this study, metabolic flux analysis (MFA) was applied to identify the metabolic changes associated with the cytotoxicity of saturated FFA. Measurements of the fluxes revealed that the saturated FFA, palmitate, was oxidized to a greater extent than the non-toxic oleate and had comparatively less triglyceride synthesis and reduced cystine uptake. Although fatty acid oxidation had a high positive correlation to the cytotoxicity, inhibitor experiments indicated that the cytotoxicity was not due to the higher fatty acid oxidation. Application of MFA revealed that cells exposed to palmitate also had a consistently reduced flux of glutathione (GSH) synthesis but greater de novo ceramide synthesis. These predictions were experimentally confirmed. In silico sensitivity analyses identified that the GSH synthesis was limited by the uptake of cysteine. Western blot analyses revealed that the levels of the cystine transporter xCT, but not that of the GSH-synthesis enzyme glutamyl-cysteine synthase (GCS), were reduced in the palmitate cultures, suggesting the limitation of cysteine import as the cause of the reduced GSH synthesis. Finally, supplementing with N-acetyl L-cysteine (NAC), a cysteine-provider whose uptake does not depend on xCT levels, reduced the FFA-toxicity significantly. Thus, the metabolic alterations that contributed to the toxicity and suggested treatments to reduce the toxicity were identified, which were experimentally validated.

Contribution of gene expression to metabolic fluxes in hypermetabolic livers induced through burn injury and cecal ligation and puncture in rats

Severe injury activates many stress-related and inflammatory pathways that can lead to a systemic hypermetabolic state. Prior studies using perfused hypermetabolic rat livers have identified intrinsic metabolic flux changes that were not dependent upon the continual presence of elevated stress hormones and substrate loads. We investigated the hypothesis that such changes may be due to persistent alterations in gene expression. A systemic hypermetabolic response was induced in rats by applying a moderate burn injury followed 2 days later by cecum ligation and puncture (CLP) to produce sepsis. Control animals received a sham-burn followed by CLP, or a sham-burn followed by sham-CLP. Two days after CLP, livers were analyzed for gene expression changes using DNA microarrays and for metabolism alterations by ex vivo perfusion coupled with Metabolic Flux Analysis. Burn injury prior to CLP increased fluxes while decreases in gene expression levels were observed. Conversely, CLP alone significantly increased metabolic gene expression, but decreased many of the corresponding metabolic fluxes. Burn injury combined with CLP led to the most dramatic changes, where concurrent changes in fluxes and gene expression levels occurred in about 1/3 of the reactions. The data are consistent with the notion that in this model, burn injury prior to CLP increased fluxes through post-translational mechanisms with little contribution of gene expression, while CLP treatment up-regulated the metabolic machinery by transcriptional mechanisms. Overall, these data show that mRNA changes measured at a single time point by DNA microarray analysis do not reliably predict metabolic flux changes in perfused livers.

Integrated energy and flux balance based multiobjective framework for large-scale metabolic networks

Flux balance analysis (FBA) provides a framework for the estimation of intracellular fluxes and energy balance analysis (EBA) ensures the thermodynamic feasibility of the computed optimal fluxes. Previously, these techniques have been used to obtain optimal fluxes that maximize a single objective. Because mammalian systems perform various functions, a multi-objective approach is needed when seeking optimal flux distributions in such systems. For example, hepatocytes perform several metabolic functions at various levels depending on environmental conditions; furthermore, there is a potential benefit to enhance some of these functions for applications such as bioartificial liver (BAL) support devices. Herein we developed a multi-objective optimization approach that couples the normalized Normal Constraint (NC) with both FBA and EBA to obtain multi-objective Pareto-optimal solutions. We investigated the Pareto frontiers in gluconeogenic and glycolytic hepatocytes for various combinations of liver-specific objectives (albumin synthesis, glutathione synthesis, NADPH synthesis, ATP generation, and urea secretion). Next, we evaluated the impact of experimental flux measurements on the Pareto frontiers. We found that measurements induce dramatic changes in Pareto frontiers and further constrain the network fluxes. This multi-objective optimality analysis may help explain certain features of the metabolic control of hepatocytes, which is relevant to the response to hepatocytes and liver to various physiological stimuli and disease states.

Burn-induced increase in atrogin-1 and MuRF-1 in skeletal muscle is glucocorticoid independent but downregulated by IGF-I

The present study determined whether thermal injury increases the expression of the ubiquitin (Ub) E3 ligases referred to as muscle ring finger (MuRF)-1 and muscle atrophy F-box (MAFbx; aka atrogin-1), which are muscle specific and responsible for the increased protein breakdown observed in other catabolic conditions. After 48 h of burn injury (40% total body surface area full-thickness scald burn) gastrocnemius weight was reduced, and this change was associated with an increased mRNA abundance for atrogin-1 and MuRF-1 (3.1- to 8-fold, respectively). Similarly, burn increased polyUb mRNA content in the gastrocnemius twofold. In contrast, there was no burn-induced atrophy of the soleus and no significant change in atrogin-1, MuRF-1, or polyUb mRNA. Burns also did not alter E3 ligase expression in heart. Four hours after administration of the anabolic agent insulin-like growth factor (IGF)-I to burned rats, the mRNA content of atrogin-1 and polyUb in gastrocnemius had returned to control values and the elevation in MuRF-1 was reduced 50%. In contrast, leucine did not alter E3 ligase expression. In a separate study, in vivo administration of the proteasome inhibitor Velcade prevented burn-induced loss of muscle mass determined at 48 h. Finally, administration of the glucocorticoid receptor antagonist RU-486 did not prevent burn-induced atrophy of the gastrocnemius or the associated elevation in atrogin-1, MuRF-1, or polyUb. In summary, the acute muscle wasting accompanying thermal injury is associated with a glucocorticoid-independent increase in the expression of several Ub E3 ligases that can be downregulated by IGF-I.

Effects of dehydroepiandrosterone administration on rat hepatic metabolism following thermal injury

BACKGROUND

Severe burns cause dramatic alterations in liver and whole-body metabolism. Recently, there has been interest in using dehydroepiandrosterone (DHEA) as a treatment for trauma patients, and enhanced survival and immune function have been reported using DHEA in animal trauma models. The specific effects of DHEA on hepatic metabolism following burn injury have not been explored.

MATERIALS AND METHODS

Male rats received either (1) a burn covering approximately 20% of the total body surface area or a sham burn or (2) burn injury followed by two intraperitoneal injections of DHEA or vehicle. After 4 days, the livers were isolated and perfused in vitro, and 28 metabolite fluxes were measured. Metabolic flux analysis was used to obtain the intracellular metabolic flux distribution and provide an overview of the metabolic state of the livers in each experimental group.

RESULTS

Burn injury decreased the uptake of lactate and the production of beta-hydroxybutyrate and increased the deamination of glutamine to glutamate and asparagine to aspartate. DHEA, compared to vehicle treatment, decreased pentose phosphate pathway (PPP) fluxes and the uptake of several amino acids in burned rats. Furthermore, DHEA treatment restored liver metabolism in burned rats to a state that was very similar to that of the sham control group.

CONCLUSIONS

DHEA administration appears to normalize hepatocellular metabolism in burned rats but also decreases the PPP flux, which may impair the liver's ability to recycle endogenous antioxidants. DHEA treatment combined with exogenous antioxidants should receive further consideration in the management of burn and trauma patients.