Bench-to-bedside review: Glucose and stress conditions in the intensive care unit

The physiological response to blood glucose elevation is the pancreatic release of insulin, which blocks hepatic glucose production and release, and stimulates glucose uptake and storage in insulin-dependent tissues. When this first regulatory level is overwhelmed (that is, by exogenous glucose supplementation), persistent hyperglycaemia occurs with intricate consequences related to the glucose acting as a metabolic substrate and as an intracellular mediator. It is thus very important to unravel the glucose metabolic pathways that come into play during stress as well as the consequences of these on cellular functions. During acute injuries, activation of serial hormonal and humoral responses inducing hyperglycaemia is called the 'stress response'. Central activation of the nervous system and of the neuroendocrine axes is involved, releasing hormones that in most cases act to worsen the hyperglycaemia. These hormones in turn induce profound modifications of the inflammatory response, such as cytokine and mediator profiles. The hallmarks of stress-induced hyperglycaemia include 'insulin resistance' associated with an increase in hepatic glucose output and insufficient release of insulin with regard to glycaemia. Although both acute and chronic hyperglycaemia may induce deleterious effects on cells and organs, the initial acute endogenous hyperglycaemia appears to be adaptive. This acute hyperglycaemia participates in the maintenance of an adequate inflammatory response and consequently should not be treated aggressively. Hyperglycaemia induced by an exogenous glucose supply may, in turn, amplify the inflammatory response such that it becomes a disproportionate response. Since chronic exposure to glucose metabolites, as encountered in diabetes, induces adverse effects, the proper roles of these metabolites during acute conditions need further elucidation.


Introduction
Acute life-threatening situations cause an intense stress response. Th ese situations promote immuno-infl ammatory and metabolic responses that are entangled in an intricate way, as the cells involved in these key events onto genetically originate from a unique primordial organ combining both immune and metabolic functions, namely the 'fat body' [1]. Acute stress-induced hyperglycaemia [2] is observed in many conditions, such as myocardial infarct [3], and shock states, especially septic [4], but also traumatic [5], as well as stroke [6]. Th e observed concordance between elevated blood glucose and mortality raised the question of a causative relationship between hyperglycaemia and prognosis [7].
A landmark monocenter study published in 2001 suggested that hyperglycaemia has a deleterious impact on prognosis in mostly surgical ICU patients, since tight glucose control by intravenous insulin dramatically improved mortality [8]. Th e large debate following this publication questioned the population studied (mainly cardiovascular surgical patients), the respective roles of glycaemia control versus additional insulin, and the impact of the amount of exogenous carbohydrate [9]. In 2006, the same group published another study performed on medical ICU patients testing the same protocol used in the fi rst study [10]. In this new study, global mortality did not improve with tight control of glycaemia and a worsening of the death rate in a subgroup of patients staying less than 3 days in the ICU was observed. Th e group treated with tight control of glycaemia for more than 3 days had a reduction in severity and number of organ failures, which surprisingly did not translate to outcome benefi t. Subsequent ICU trials published recently [11][12][13][14][15] have failed to confi rm a benefi t of tight control of glycaemia on prognosis in critically ill patients while emphasizing the potential role of hypoglycaemia in explaining the divergent results.
Th e recently published meta-analysis by Marik and Preiser [9] showed that, overall, tight glycaemic control did not reduce 28-day mortality (odds ratio (OR) 0.95; 95% confi dence interval (CI), 0.87 to 1.05), the incidence of blood stream infections (OR 1.04; 95% CI, 0.93 to 1.17), or the requirement for renal replacement therapy (OR 1.01; 95% CI, 0.89 to 1.13). Th e incidence of hypoglycaemia was signifi cantly higher in patients randomized to tight glycaemic control (OR 7.7; 95% CI, 6.0 to 9.9; P < 0.001). Metaregression demonstrated a signifi cant relationship between the 28-day mortality and the proportion of calories provided parenterally (P = 0.005), suggesting that the diff erence in outcome between the two Leuven Intensive Insulin Th erapy Trials and the subsequent trials could be related to the use of parenteral nutrition. More importantly, when the two Leuven Intensive Insulin Th erapy Trials were excluded from the meta-analysis, mortality was lower in the control patients (OR 0.90; 95% CI, 0.81 to 0.99; P = 0.04; I(2) = 0%).
Th e focus of this review is an integrative description of the main pathways and mechanisms involved in the acute stress conditions responsible for hyperglycaemia, and the description of complex situations involving both the stimulation of systemic infl ammation and changes in metabolic requirements [16] in an attempt to clarify apparent contradictory results.

Metabolic pathways using glucose during acute critical conditions
Th e normal response to a stress situation associates the activation of central nervous system and neuroendocrine axes with increased release of hormones such as cortisol, macrophage inhibiting factor (MIF) [17,18], epinephrine and norepinephrine, growth hormone, and glucagon. Th ese hormones profoundly modify the infl ammatory response, especially cytokine release. Stress hormones generate globally a systemic pro-infl ammatory profi le while anti-infl am mation is predominant at the tissue level (for a review, see [19]). Th ese hormones, except for MIF, also stimu late, among other mechanisms, gluco neogenesis and hepatic glucose production, thus aggravating hypergly caemia [20].
Th e pancreatic insulin release in response to blood glucose elevation leads to the blocking of hepatic glucose production and the stimulation of glucose uptake and storage by the liver, muscle and adipose tissue. If this fi rst line of regulation fails to control glucose levels, the micro environment of cells will contain high levels of glucose. To enter the cell, glucose uses transporters that allow facilitated diff usion (via concentration gradients) through the cytoplasmic membrane. Th ese transporters are part of the superfamily of glucose transporters encoded by the GLUT genes; there are several isoforms, such as GLUT4, and their expression on the cell surface is amplifi ed by insulin [21].
After entering the cell, glucose may go through diff erent metabolic pathways in addition to glycolysis, as summarized in Figure 1. During the early hours of stress, the metabolic stimulation of the cell corresponds to increased mitochondrial energy production (ATP) with increased O 2 and glucose consumption [22]. Similarly, during cell proliferation, glucose availability is necessary for the induction of glycolytic enzymes, such as hexokinase, pyruvate kinase or lactate dehydrogenase. Th is glycolysis favours lactate production despite O 2 availability [23], and regeneration of NAD + , which is required for additional cycles of glycolysis [24].

Recognition and cellular mechanisms of acute conditions
Acute critical conditions cause cellular injuries that are known to initiate repair or cell death pathways ( Figure 2). Th ese integrative mechanisms tend to either contain the response at the local level or, on the contrary, spread it by recruiting circulating cells and factors for repair.
Damaged cells communicate with innate immune cells by releasing intracellular factors named damage-associated molecular pattern molecules (DAMPs), such as calgranulines [25] and alarmines [26,27] (Figure 2). Together with pathogen-associated molecular pattern molecules (PAMPs), they activate the cellular expression of Toll-like receptors (TLRs) [28]. Accumulation of abnormal proteins, which are processed by the proteasome S26 system in the endoplasmic reticulum [29], as well as fl uctuations of nutrients or energy availability, hypoxia, viruses and toxins activate a complex transcriptional response called the endoplasmic reticulum stress response ( Figure 2), or the unfolded protein response [30].
Receptors for recognition of infl ammation appear on both target cells and infl ammatory cells. Th e alteration of the extracellular milieu is transmitted into the cell, modifying its functions. In peripheral blood mononuclear cells, for instance [31], an increased energy demand associated with a simultaneous metabolic failure can occur [32,33]. Th e increased permeability of the injured mitochondria leads to energy loss and cell death, which by itself fuels the infl ammatory process through the release of the cell contents.
While glycolysis is activated by hypoxia, phosphofructokinase-1 and lactate dehydrogenase activity is stimu lated by increased lactate production [44] associated with decreased mitochondrial oxygen consump tion. Th is mechanism, described since 1910 in tumour cells as the 'Warburg eff ect' [45], seems to be adaptive to the lack of oxygen while maintaining cell redox status. A suffi cient amount of energy is then produced but without an increase in reactive oxygen species (ROS) production by the mitochondria [46].

Adenosine
Adenosine production mainly results from ATP degradation during stress when it is released into the extracellular space. Adenosine regulates innate and adaptive immune functions by interacting with almost every immune cell Glucose is known to be oxidized through cytoplasmic glycolysis to produce pyruvate. Pyruvate may be reduced into lactate by lactate dehydrogenase or it may enter the mitochondria to participate in the citric acid cycle and the production of ATP by the mitochondrial respiratory chain and ATPase. However, glucose can be involved in other pathways. Glycogen synthesis is a major way to store glucose in muscle and liver. In the polyol pathway, aldose reductase reduces toxic aldehyde to inactive alcohol and glucose to sorbitol and fructose. In reducing NADPH to NADP + , this enzyme may be deleterious by consuming the essential cofactor needed to regenerate reduced glutathione, an essential antioxidant factor in cells. The hexosamine pathway originates from glycolysis at the fructose-6-phosphate level. In this pathway, glutamine fructose-6-phosphate amidotransferase is involved in the synthesis of glucosamine-6-phosphate, which is ultimately converted to uridine diphosphate (UDP)-N-acetyl-glucosamine. This glucosamine is able to activate transcription factors such as Sp-1 and to induce the production of pro-infl ammatory cytokines. Diacylglycerol, which activates isoforms of protein kinase C (PKC), may be produced from dihydroxyacetone phosphate. The PKC activation can induce several pro-infl ammatory patterns, such as activation of the transcription factor NF-κB, and the production of NADPH oxidase or pro-infl ammatory cytokines. The pentose phosphate pathway may use glucose-6-phosphate to produce pentoses for nucleic acid production. This pathway is also able to produce NADPH for use in lipid, nitric oxide and reduced glutathione production, and also the synthesis of reactive oxygen species by NADPH oxidase. Advanced glycation end product (AGE) synthesis is linked to high intracellular glucose concentrations. AGEs can induce cell dysfunction by modifying cell proteins, and extracellular matrix proteins, which changes signalling between the matrix and the cell, or by activating receptors for advanced glycation end products (RAGEs), which induce the production of the transcription factors NF-κB and TNF-α or other pro-infl ammatory molecules. GLUT, glucose transporter. [47]. It inhibits antigen presentation, pro-infl ammatory cytokine production and immune cell proliferation, and participates in tissue repair and remodelling. Adenosine induces increased intracellular cAMP, which stimulates protein kinase (PK)A, which in turn activates the transcription factor CREB (cAMP response element-binding), thus linking the infl ammatory response to alterations of glucose metabolism [48].

Oxidative stress
Oxidative stress produces ROS, which alter normal cell function. ROS are permanently released at a low rate at the cytoplasmic membrane (NADPH oxidase, myeloperoxidase, cyclooxygenase) and in the cytoplasm (heme oxygenase, xanthine oxidase), and also within the mitochondria. When activated, phagocytic cells display a specifi c response called the 'respiratory burst' , which is an acute overproduction of ROS by the activation of the NADPH oxidase Nox 2. Oxidative stress may indirectly modify glucose metabolism since it induces DNA alterations that activate the nuclear enzyme poly-ADP-ribose polymerase 1 (PARP-1). Th is activation consumes NAD + and depletes its intracellular stores, which in turn hampers glycolysis and ATP production, in parallel with altered cell functions [49]. A transient low level of oxidative stress with redox alterations stimulates glucose uptake via insulin-independent GLUT transporters mediated by the AMP kinase pathway [50,51]. . During cell death, these molecules, such as calgranulines from the protein S100 A superfamily or alarmines such as the nuclear protein high-mobility group box 1 (HMGB1), are released into the extracellular space to activate the immune system. These molecules associate with pathogen-associated molecular pattern molecules (PAMPs) from destroyed pathogens to activate cellular expression of Toll-like receptors (TLRs) of the pattern recognition receptor (PRR) superfamily. Some of these receptors, specifi cally TLR2, 4 and 9, recognize multiple DAMPS released during stress and cell death. Proteins with abnormal conformation are processed by the proteasome S26 system in the endoplasmic reticulum, where protein kinase R-like endoplasmic reticulum kinase (PERK)-type kinases are activated; these pathways depend on Ire1 (which requires inositol) and nuclear factors, such as NF-κB and Nrf2 (NF-E2 related factor). Nrf2 controls the expression of genes encoding enzymes that remove reactive oxygen species (ROS), including heme oxygenase 1 (HO-1) and glutathione S-transferase (GST). PERK-dependent phosphorylation of Nrf2 thus coordinates a transcriptional program connecting oxidative stress and endoplasmic reticulum stress. Activation of the transcription factor CREB-H can be achieved through this endoplasmic reticulum stress; CREB-H is responsible for the acute infl ammatory response in the liver with acute phase protein synthesis. Adapted from [1]. GLUT, glucose transporter; HIF, hypoxia-inducible factor; HO, heme oxygenase; IKK, IκB kinase; JNK, c-Jun N-terminal kinase; LPS, lipopolysaccharide; NOS, nitric oxide synthase; PKC, protein kinase C; RAGE, receptors for advanced glycation end products; ssRNA, single-stranded RNA.

Sepsis, an integrative condition
Sepsis corresponds to a systemic infl ammation related to the abnormal presence of bacterial antigens and involves diff erent mechanisms such as hypoxia and oxidative stress. At the early phase, inhibition of glycogen synthesis results in increased global glucose availability and increased cellular uptake [52][53][54]. Glucose uptake appears to be most increased in organs containing a vast population of phagocytic cells (liver, spleen, gut, lung) [55][56][57]. In rats injected with endotoxin or TNF-α, insulin-independent glucose uptake is increased in liver non-parenchymal cells (Küpff er cells, endothelial cells) [58], as observed in circulating immune cells, including polymorphonuclear leukocytes [59,60], lymphocytes, monocytes and macrophages [61][62][63]. Skeletal muscle displays only a limited increase in glucose uptake, probably because of the development of insulin resistance.
Sepsis also modifi es cytoplasmic glycolysis at the transcriptional level. In healthy volunteers receiving intravenous endotoxin, there was an early under-expression of genes encoding metabolic enzymes [64]. In particular, the key enzymes of glycolysis and those of the , and numerous molecules involved in vascular reactivity (such as nitric oxide synthase (NOS)), recruitment of endothelial progenitors, and cytoprotection through angiogenic growth factors such as vascular endothelial growth factor (VEGF). During hypoxia, glycogenolysis is stimulated, increasing glucose availability. An increase in glucose transporter (GLUT) expression enables augmented glucose uptake. This overexpression of GLUTs is mediated by the activation of AMP kinase (AMPK) and p38 mitogen-activated kinase. Stimulation of AMPK results from a decreased cytoplasmic ATP/AMP ratio together with altered cellular redox status. Phosphofructokinase-1 and lactate dehydrogenase activity is stimulated by increased lactate production. HIF decreases mitochondrial oxygen consumption and induces the expression of pyruvate dehydrogenase kinase, the main inhibitor of pyruvate dehydrogenase and of the entry of acetylCoA into mitochondria. Adapted from [36]. OXPHOS, oxidative phosphorylation; TGF, transforming growth factor. mito chon drial respiratory chain (MRC) were transiently under-expressed. In the diaphragm of septic rats, transcription, synthesis and activity of the constituents of the MRC, as well as of phosphofructokinase-1, a keyenzyme of glycolysis, are reduced [65]. In muscle of septic rats, the activity of pyruvate dehydrogenase is reduced, with a simultaneous increase in the activity of its inhibitor, pyruvate dehydrogenase kinase. Th e net result of these modifi cations is a reduction in pyruvate entering the mitochondria while the conversion of pyruvate to lactate is promoted [66].
In septic shock patients, increased use of glucose and increased lactate production was observed under aerobic conditions [67]. A microdialysis study of quadriceps muscles showed lactate overproduction during septic shock resulting form exaggerated aerobic glycolysis through Na/K-ATPase stimulation. To maintain cell func tions, stimulation of glycolysis was shown to adaptively compensate for the metabolic rate increase [68]. Elevated circulating epinephrine stimulates Na/K-ATPase, which promotes lactate hyperproduction without any oxygen debt [69].
Mitochondrial dysfunction during sepsis [70] involves alterations in the structure [71] and function of the MRC, including impairment of key enzymes of electron transport and ATP synthesis [72,73] and mitochondrial biogenesis [74]. Th ese results were also found with monocytes [75] and skeletal muscle [76] harvested from septic shock patients. ATP levels in skeletal muscle cells were main tained despite mitochondrial ultrastructural alterations [76]. Th is mitochondrial dysfunction results from plasmatic factors that promote uncoupled MRC oxygen consumption [77] that correlates with sepsisinduced modifi cations of the immune phenotype and is associated with increased mitochondrial permeability [78].
In summary, glucose metabolism alterations in acute critical conditions can be viewed as a 'redistribution of glucose consumption away from mitochondrial oxidative phosphorylation' towards other metabolic pathways, such as lactate production. Th is re-channelling does not seem to aff ect energy supply to the cells. Th is may result from decreased ATP consumption by the cells, which in turn lose some of their characteristics, indicating metabolic failure [79].

Why does glycaemia fi nally increase during acute injury?
Stress-induced hyperglycaemia results from the combined eff ects of increased counter-regulatory hormones that stimulate glucose production and reduced uptake associated with insulin resistance, that is, decreased insulin activity. Th ere is also inadequate pancreatic insulin release with regard to glycaemia (or adaptive 'pancreas tolerance'). Insulin release during stress is decreased mainly through the stimulation of α-adrenergic pancreatic receptors [20]. Pro-infl ammatory cytokines may directly inhibit insulin release by β pancreatic cells [80]. A new glucose balance results, allowing a higher blood 'glucose pressure' , which aff ects tissues diff erently depending on whether they are insulin-dependent or not.
Glucose availability also relies on delivery to cells, analogous to oxygen diff usion. For glucose to arrive at a cell with reduced blood fl ow (ischemia, sepsis), it must move from the blood stream across the interstitial space. Glucose movement is dependent entirely on a concentration gradient, and for adequate delivery to occur across an increased distance, the concentration at the origin (blood) must be greater. Th erefore, in the face of reduced or redistributed blood fl ow, hyperglycaemia is adaptive.

Development of insulin resistance
Insulin resistance (IR) is a reduction in the direct eff ect of insulin on its signalling process leading to metabolic consequences [81], very similar to type 2 diabetes, and is commonly observed during sepsis [82].
Insulin acts mainly on the liver, muscle and fat (metabolic eff ects), but it also targets many cellular subtypes to stimulate essentially protein and DNA synthesis as well as apoptosis (mitogenic eff ects). Hepatic IR involves increased hepatic glucose production (gluconeogenesis) together with decreased glycogen synthesis. During sepsis, however, gluconeogenesis can be limited by inhibi tion of important enzymes [83,84]. Muscle IR corresponds to decreased glycogen deposition and glucose uptake linked to decreased expression of GLUT4, while a transient defect in insulin signalling has also been described [85]. IR in adipocytes leads to inhibition of lipogenesis and activation of lipolysis.
MIF is not only produced by various immune cells [93] and the anterior pituitary gland, but also by islet β cells, where it positively regulates insulin secretion [94]. During infl ammation in skeletal muscle, locally produced MIF stimulates glucose use and lactate production [95]. In endotoxemic mice genetically defi cient in MIF, glucose metabolism is almost normalized when compared to wild-type mice [96]. Increased circulating cortisol partici pates in the maintenance of blood glucose not only by increasing its production or decreasing its utilization, but also by directly inhibiting insulin secretion by ß cells [97].
Endogenous catecholamines are also involved in the alteration of glucose metabolism during endotoxaemia [98], especially in the liver [99]. Exogenous epinephrine metabolic eff ects on glucose turnover were, however, attenuated in endotoxic rats when compared to controls [100].

Role of exogenous glucose supply and induced hyperglycaemia
Glucose acts not only as an energetic substrate but also as a signalling molecule of the cellular environment, as shown in diabetes with chronic hyperglycaemia. Stressinduced acute hyperglycaemia has been less studied up to now as it has been considered an adaptive response. Some concepts from chronic hyperglycaemia may, however, be used in acute conditions. Intravenous administration of exogenous glucose yielded similar glycaemia in control and septic animals despite higher insulin levels in the septic group [101]. Hepatic glycogen deposition was observed only when glucose was infused via the portal vein [31,102].

Glucose-controlled genomic modifi cations
In fasted animals, increased circulating glucagon induces a gluconeogenic program by activating the nuclear transription factor CREB through a molecule named Crtc2 (CREB regulated transcription coactivator 2) or TORC 2 (Transducer of regulated CREB activity 2) [103,104]. Th e expression rate of gluconeogenic enzymes is thus increased, especially for glucose-6-phosphatase.
Re-feeding in turn increases insulin levels, which inhibits hepatic glucose production partly by ubiquitindependent destruction of Crtc2 [105]. During sustained hyperglycaemia, the hexosamine pathway can be activated [106]. In hepatocytes, Crtc2 is then O-glycosylated on a serine residue instead of being phosphorylated. It can thus migrate into the nucleus to activate CREB and the gluconeogenic program, contributing to maintain hyperglycaemia [107]. Th is has been described as the 'sweet conundrum' [105]. Regulation of this pathway during acute injury remains to be proven.

Hyperglycaemia and the infl ammatory response
In diabetics, glucose channelling through alternative glycolytic pathways seems to depend on MRC activity [106,108]. Th e accumulation of energy substrates induced by isolated hyperglycaemia without a concomitant increase in energy demand may enhance the fl ux of carbon hydrates to the mitochondria with increased activity of the MRC and proton driving force. Once the activity of ATPase is saturated, intermediate radicals from the MRC will accumulate and may react with the surrounding available O 2 to produce ROS [109], as shown in bovine endothelial cells. When inhibiting this radical production, the activity of alternative glycolytic pathways is decreased as well as the expression of transcription factor NF-κB [110]. Inhibition of glyceraldehyde-3-phosphate dehydrogenase, an enzyme involved in cytoplasmic glycolysis, has also been observed. Metabolites accumulate upstream of this enzyme and are funnelled towards alternative pathways (Figure 1). Polymers of ADP-ribose, produced by nuclear PARP to repair DNA altered by mitochondrial ROS, may be involved in this inhibition. PARP, by migrating into the cytosol, may be a key to glucose toxicity [111]. Th ere is still a lack of evidence to fully extrapolate these theories to explain mitochondrial dysfunction and organ failure observed during stressinduced hyperglycaemia.
Glucose also acts as a pro-infl ammatory molecule [81,112]. Glucose ingestion in healthy volunteers rapidly increases the activity of NF-κB [113] and the production of mRNA for TNF-α [114]. Under the same conditions, acute hyperglycaemia increased the activity of the transcription factors AP-1 (Activator protein-1) and EGR-1 (Early growth response-1), which in turn activate the production of matrix metalloproteinase-2 (MMP-2) by monocytes, an enzyme that facilitates the diff usion of infl am mation by hydrolysing extracellular matrix. Produc tion of tissue factor, a prothrombotic and proaggregant molecule [115], is increased, as is production of cellular adhesion molecules [116]. Acute hyperglycaemia induced in healthy volunteers by octreotid, an inhibitor of insulin release, leads to a rapid and transient secretion of pro infl ammatory cytokines (IL-6, TNFα, IL-8). Th is eff ect is amplifi ed in insulin-resistant subjects and blunted with antiradical treatment [117].

Glucose-cytokine interactions
In vitro, an increased release of IL-1ß has been measured in the culture medium of human monocytes exposed to hyperglycaemic conditions after endotoxin stimulation [118]. In our model of endotoxaemia [102], glucose supply interfered with haemodynamic, metabolic and infl am matory responses, with a dramatic increase in circulating TNF-α when intraportal glucose was administered. Fasting on the other hand seemed to attenuate the response to endotoxin.
In liver transplant patients, glucose feeding during the early postoperative period induced major haemodynamic modifi cations within the graft, where the immunoinfl ammatory insult occurs [119], including almost halted arterial hepatic infl ow. Th is vasoconstriction was specifi cally related to glucose since fructose, amino acids and fatty acids did not provoke this eff ect. One tempting hypothesis for this eff ect involves increased production of ROS, which are well known to vasoconstrict arteries.

Glucose and ROS production
Nutrients, and especially glucose, are able to stimulate oxidative stress and infl ammatory responses [106]. Th e body thus needs to regulate nutrient excesses in order to maintain metabolic homeostasis. STAMP2 (Six-transmembrane protein of prostate 2) has recently been detected in adipose tissue, a key organ in the management of nutrient excesses, and is also expressed in the heart, liver, lung and platelets [120]. In STAMP2 genetically defi cient mice, the eff ects of insulin on liver, muscle and adipose tissue are altered, all three being essential organs for glucose homeostasis. STAMP2 is a metalloreductase involved in iron handling, which may infl uence ROS production [121]. Ingestion of glucose in healthy volunteers led to increased production of ROS in circulating monocytes and polymorphonuclear leukocytes. Th is was associated with the rapidly increased synthesis of NADPH oxidase subunits [122].
Th ese data suggest that glucose induces profound modi fi cations of the monocyte pro-infl ammatory response. In peripheral blood mononuclear cells harvested from healthy volunteers and septic patients [77,123], increasing extracellular glucose increased glucose uptake. Subsequent stimulation of these cells by various agonists increased ROS production via NADPH oxidase in both the healthy volunteers and the septic patients. Th e link between ROS and intracellular glucose levels could be the increased production of NADPH via the pentose phosphate cycle [123] (Figure 4). More studies are needed to confi rm these multifaceted eff ects and to confi rm that such a coordinated regulation between nutrient availability and the intensity of the infl ammatory response is also at play during acute insults. To cite Leverve, 'it appears that glucose obviously plays a very subtle role in oxidant cellular signaling. It can either increase or decrease ROS production and can either increase or decrease the antioxidant defense […]. Th erefore it is not surprising that any change in blood glucose must be considered as a complex event, and taking care of glycemia and redox homeostasis will be probably central in the management of ICU patients in the next years' [124].
Recent in vitro data suggest that giving glucose boluses after hypoglycaemia may trigger neuronal death due to ROS overproduction [125]. In healthy volunteers, hyperglycaemic spikes induced increased pro-infl ammatory cytokine levels that were blunted by antioxidant pretreatment [117]. Th is introduces the concept of glucose variability, which by itself seems to be deleterious with regard to outcome in critically ill patients [126,127].

Future prospects
Many questions regarding glycaemia remain to be solved for daily critical care practice. How should we achieve gly caemic control: should we take into account nutritional support, especially parenteral nutrition [9]? Should we control the physiological response to an induced hyper glycaemia or should we control the endogenous stress-induced hyperglycaemia that may be adaptive in the absence of exogenous glucose intake? Is there a place for new therapeutics such as incretins [128]? Does endogenous hyperglycaemia have a similar impact as hyperglycaemia induced by nutritional support? Th ese questions in turn prompt investigation of the role of glucose deprivation induced by fasting with regard to normoglycemia achieved by insulin therapy. Similarly, the consequences of spontaneous versus insulin-induced hypoglycaemia remain to be investigated. Answers to these questions will probably help to solve the confl ict between supporters and opponents of tight glycaemia control in the ICU. Th is discussion is in accordance with the concerns raised by several authors about early initiation of parenteral nutrition in acute critical patients [129,130], as supported by the results of two large multicentre studies, Nice-Sugar [14] and Glucontrol [15].

Conclusion
Glucose metabolism is profoundly altered during acute conditions, from its uptake to the induction of complex programs of gene expression [14]. Th e increased glucose availability in cells is not necessarily used to produce ATP by mitochondria. Glucose seems able to activate proinfl ammatory metabolic pathways. While chronic exposure to these end products seems deleterious (diabetes), their actual roles during acute conditions need to be further elucidated. Early stress-induced hypergly caemia has been described as an adaptive response that could in turn sustain an adaptive infl ammatory response (host . Glucose and reactive oxygen species production during sepsis: hypothesis. In peripheral blood mononuclear cells harvested from healthy volunteers and septic patients, increasing extracellular glucose increased glucose uptake. Subsequent stimulation of these cells by various agonists, such as PMA (phorbol 12-myristate 13-acetate), a protein kinase C (PKC) activator, increased reactive oxygen species (ROS) production via NADPH oxidase in both the healthy volunteers and septic patients. The link between ROS and intracellular glucose levels could be increased production of NADPH via the pentose phosphate pathway.
defence, wound healing, and so on). Glucose intakeinduced hyper glycaemia may, however, lead to maladjusted and disproportionate infl ammation that should be avoided. How then should this subsequent hyper glycaemia be prevented: should we limit glucose supply at the early phase of infl ammation?