8
Molecular mechanisms behind clinical benefits of intensive
insulin therapy during critical illness: Glucose versus insulin
Ilse Vanhorebeek, PhD, Associate Professor
*, Lies Langouche, PhD,
Post-doctoral Fellow
1Department of Intensive Care Medicine, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium
Keywords: critical illness
intensive insulin therapy glucose toxicity insulin resistance
High blood glucose levels have been associated with morbidity and poor outcome in critically ill patients, irrespective of underlying pathology. In a large, randomised, controlled study the use of insulin therapy to maintain normoglycaemia for at least a few days improved survival and reduced morbidity of patients who are in a surgical intensive care unit (ICU). Since the publication of this landmark study, several other investigators have provided support for, whereas others have questioned, the beneficial effects of intensive insulin therapy.
In this review, we discuss the investigated potential molecular mechanisms behind the clinical benefits of intensive insulin therapy. We first describe the molecular origin of hyperglycaemia and the impact of the therapy on insulin sensitivity. Next, the molecular basis of glucose toxicity in critical illness and the impact of intensive insulin therapy hereon are described, as well as other non-glucose-toxicity-related metabolic effects of intensive insulin therapy.
Ó 2009 Elsevier Ltd. All rights reserved.
Clinical benefits of intensive insulin therapy
Critical illness is hallmarked by numerous endocrine and metabolic disturbances, including the development of hyperglycaemia consequent to insulin resistance and increased hepatic glucose production, which is referred to as ‘stress diabetes’ or ‘diabetes of injury’.1High blood glucose levels
*Corresponding author. Tel.: þ32 16 330242; Fax þ32 16 330932.
E-mail addresses:[email protected](I. Vanhorebeek),[email protected](L. Langouche).
1 Tel.: þ32 16 330242; Fax: þ32 16 330932.
Contents lists available atScienceDirect
Best Practice & Research Clinical
Anaesthesiology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b e a n
1521-6896/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.bpa.2009.08.008
have been associated with morbidity and poor outcome in critically ill patients, irrespective of underlying pathology.2 This association may merely reflect that the degree of hyperglycaemia is a marker of the severity of illness. Alternatively, it may indicate that hyperglycaemia itself contributes to the disease and that there is a causal relationship between hyperglycaemia and clinical complications.
For a long time, it was believed that a mild degree of hyperglycaemia would be beneficial for organs that largely rely on glucose for energy provision, but do not depend on insulin for glucose uptake. With the publication of the first, landmark Leuven randomised clinical study on intensive insulin therapy in adult surgical critically ill patients this concept was challenged.3At the time of the study, standard care consisted of treating only excessive hyperglycaemia above the renal threshold (>220 mg dl1, known
to induce osmotic diuresis and infectious complications) with infusion of exogenous insulin, which was discontinued when levels fell below 180 mg dl1. This approach was compared with strict glycaemic control to normal fasting blood glucose levels (80–110 mg dl1) with insulin infusion, labelled ‘intensive insulin therapy’. Intensive insulin therapy strikingly lowered mortality ICU and in-hospital, most pronounced for long-stay patients, and improved long-term outcome.3,4Furthermore, it reduced the incidence of several common critical illness-associated clinical complications. These included the prevention of bloodstream infections, acute renal failure, critical illness polyneuropathy and hyperbilirubinaemia and reduced need for red blood cell transfusions and prolonged mechanical ventilation, all culminating in a reduced need for prolonged intensive care.3The therapy also protected the central and peripheral nervous system from secondary insults and improved long-term rehabili-tation of patients with isolated brain injury.5Subsequently, two large randomised clinical studies were performed by the same investigators in a strictly medical population of critically ill adults6and in critically ill children7, again comparing standard care versus fasting glucose levels targeted to age-adjusted normal levels. These studies largely reproduced the previously observed clinical benefits of intensive insulin therapy.3,5–9Since the publication of the first Leuven study, several other investigators have provided support for the beneficial effects of intensive insulin therapy10–15, whereas in other studies no clinical benefits were seen.16,17Due to these apparently conflicting results, the optimal level and modality of glucose control remain an area of heavy debate.2,18–20In general, however, studies that were unable to detect clinical benefit already started with lower glucose levels in the standard care group than did the Leuven studies.21,22This may suggest that prevention of excessive hyperglycaemia is what evokes the benefit, although the combination with inadequate glucose monitoring and poor achievement of the glycaemic targets may have played a role.18–20
Hence, whether strict normoglycaemia should be maintained, as in the Leuven studies, or whether an intermediate glucose range should be targeted needs to be studied further using appropriate glucose monitoring tools. An indication for the answer to this question has been provided by multi-variate analyses on the two adult randomised controlled trials performed in Leuven.8,23Compared with the intermediate blood glucose levels of 110–150 mg dl1, mortality was higher with blood glucose
>150 mg dl1and lower with glucose <110 mg dl1. The largest benefit was gained by prevention of the excessive hyperglycaemia. However, the reduction of blood glucose levels below 110 mg dl1
seemed to be crucial for the prevention of events that cause morbidity such as bacteraemia, anaemia and acute renal failure.
Mechanism of stress hyperglycaemia and blood glucose control Insulin resistance and glucose uptake
The stress imposed by any type of acute illness or injury results in insulin resistance, glucose intol-erance and hyperglycaemia. Despite high blood glucose levels and abundantly released insulin, hepatic glucose production is up-regulated in the acute phase of critical illness. Hepatic insulin resistance is further characterised by elevated circulating levels of IGF-binding protein-1 (IGFBP-1).24–26Elevated levels of cytokines, growth hormone, glucagon and cortisol might play a role in this increased gluco-neogenesis. Several effects of these hormones oppose the normal action of insulin, resulting in an increased lipolysis and proteolysis and providing substrates for gluconeogenesis. Catecholamines, which are released in response to acute injury, also enhance hepatic glycogenolysis and inhibit
glycogenesis.27The most severe cases of stress-induced hyperglycaemia1and highest levels of circu-lating IGFBP-124–26are observed in patients with the highest risk of death. Glucose uptake mechanisms are also affected during critical illness and contribute to the development of hyperglycaemia. As a result of the immobilisation of critically ill patients, exercise-stimulated glucose uptake in skeletal muscle disappears. Furthermore, because of impaired insulin-stimulated glucose uptake and impaired glycogen synthase activity, glucose uptake in heart, skeletal muscle and adipose tissue is compromised.28–31 In tissues not depending on insulin for glucose uptake, such as the nervous system, hepatocytes, gastrointestinal mucosa, pancreatic
b
-cells, renal tubular cells and endothelial and immune cells, hyperglycaemia may increase glucose uptake.1The higher levels of insulin, impaired peripheral glucose uptake and elevated hepatic glucose production reflect the development of insulin resistance during critical illness.The mechanism by which intensive insulin therapy lowers blood glucose levels in critically ill patients is not completely clear. Insulin actions in metabolism, cell growth and differentiation are mediated by intracellular signalling pathways. Insulin binding to its receptor can activate two sig-nalling pathways: the ‘mitogenic’ pathway, which proceeds through Shc/Grb2 activation leading to activation of different mitogen-activated protein kinase (MAPK) isoforms, and the ‘metabolic’ pathway, which proceeds through the insulin receptor substrates (IRSs) and depends on the activation of PI3K.32 In critically ill patients, it was demonstrated that tight glycaemic control with intensive insulin therapy activated the metabolic insulin signalling pathway in muscle33and increased the levels of mRNA encoding GLUT4 (transporter responsible for insulin-stimulated glucose uptake) and of hexokinase II (rate-limiting enzyme in intracellular insulin-stimulated glucose metabolism).34By contrast, hepatic insulin signalling remained impaired under intensive insulin therapy.33In addition, hepatic expression of phospoenolpyruvate carboxykinase (rate-limiting enzyme in gluconeogenesis) and of glucokinase (rate-limiting enzyme for insulin-mediated glucose uptake and glycogen synthesis) was unaffected by insulin therapy.34,35Moreover, circulating levels of insulin-like growth factor binding protein 1, which is normally under inhibitory control of insulin, was also refractory to the therapy.35Intensive insulin therapy also increased the insulin-sensitising hormone adiponectin.33 These studies suggest that intensive insulin therapy does not affect hepatic insulin resistance and lowers blood glucose mainly through stimulation of skeletal muscle glucose uptake. Whether also other peripheral tissues are involved, remains to be investigated.
Blood glucose control or insulin?
Hyperglycaemia has been identified as an independent risk factor for adverse outcome in numerous different clinical settings. Even a modest degree of hyperglycaemia occurring after admission to an intensive care unit (ICU) is associated with a substantial increase in mortality while in the hospital.36 Multivariate logistic regression analyses on the two adult Leuven studies indicated that blood glucose control, rather than the insulin dose administered, statistically explains most of the beneficial effects of insulin therapy on the outcomes of critical illness.8,23However, a higher mean daily insulin dose for any given blood glucose level was associated with higher hospital mortality in the intention-to-treat population.8This statistical association between a high insulin dose for any given blood glucose level and risk of death can point to the known association between severity of illness and degree of insulin resistance, or it may suggest that hyperinsulinaemia is deleterious. However, although patients under tight glycaemic control regimen received several-fold higher insulin doses to maintain normoglycae-mia than patients under standard care or ‘conventional insulin therapy’, it was demonstrated that serum insulin levels were only transiently higher with intensive insulin therapy, despite the much lower blood glucose levels.33,37Patients under conventional insulin therapy demonstrated elevated serum C-peptide levels, suggesting increased endogenous insulin release, which appeared to be counteracted by intensive insulin therapy.33,37
A recent animal study by Ellger et al., in which plasma insulin and blood glucose levels were manipulated independently over 7 days in burn-injured, parenterally fed rabbits, addressed the separate effects of glucose and insulin in detail.38The study showed that reduction of glucose levels, independent from the circulating insulin levels was responsible for improved morbidity and mortality, although both factors contributed to improved leucocyte and myocardial functions.
Molecular pathways contributing to the clinical benefits of intensive insulin therapy Protection against hyperglycaemia-induced bioenergetic failure and oxidative stress
Two possible pathways to organ failure in the critically ill are currently suggested. The first is a disturbed microcirculation due to endothelial dysfunction39, leading to inadequate oxygen supply and cellular hypoxia. This compromises cellular energy metabolism and function and causes organ failure and death. The other pathway involves a disturbed mitochondrial oxygen utilisation labelled ‘cytopathic hypoxia’ as a cause of bioenergetic failure, rather than inadequate oxygen delivery.40Such mitochondrial dysfunction has been related to lethal outcome in patients as well as in a resuscitated long-term rat model of sepsis.41,42Intensive insulin therapy targeting strict normoglycaemia has been shown to protect the vascular endothelium and the mitochondrial compartment.43,44
(i) Endothelial dysfunction and hypoxia
Reduced endothelial activation, as reflected by lower levels of adhesion molecules, related to prevention of organ failure and death with intensive insulin therapy in critically ill patients.43This effect was likely mediated in part via inhibition of excessive nitric oxide release by inducible nitric oxide synthase (iNOS). Levels of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) were also reduced45, which was associated with a better outcome. Here, attenuation of the inhibition of the constitutively expressed endothelial nitric oxide synthase (eNOS), contributing to preservation of organ blood flow, may have played a role. In a rabbit model of critical illness, where blood glucose and insulin were independently manipulated to normal or high levels, it was demonstrated that it was the glycaemic control and not glycaemia-independent effects of insulin that mediated the observed endothelial protection. This included improved endothelium-mediated relaxation of aortic rings38, prevention of excessive systemic NO release46and maintenance of physiological ADMA levels in plasma and tissues by preservation of physiological activity of the ADMA metabolising enzyme dimethylarginine dimethyla-minohydrolase (DDAH).47However, no effect was seen on tissue perfusion and oxygen delivery.48 (ii) Cytopathic hypoxia
Whereas liver biopsies taken from patients who received the conventional standard care glucose control regimen tolerating hyperglycaemia up to 215 mg dl1 showed an abundant presence of severely abnormal mitochondria that were swollen, had an increased number of abnormal and irregular cristae and were up to 10-fold enlarged, patients who received intensive insulin therapy were virtually protected from such severe morphological damage to mitochondria.44At the functional level, this correlated with higher activities of the mitochondrial respiratory chain enzyme complexes I and IV. No morphological or functional differences were observed in skeletal muscle. Like for the endothelial protection, glucose control appeared crucial to preserve mitochondrial integrity, as shown in critically ill rabbits.48,49Severe morphological and/or functional damage to mitochondria was only observed in the presence of hyperglycaemia whereas maintenance of normoglycaemia, but not elevating insulin levels, attenuated or prevented such abnormalities. The role of glucose toxicity was further under-scored by the strong correlations observed between tissue levels of glucose, strongly elevated in hyperglycaemic animals and mitochondrial enzyme activities. Importantly, mitochondrial enzyme activities correlated with biomarkers of hepatic, myocardial and renal damage, independent of oxygen delivery.48,49Mitochondrial respiratory chain enzyme activities remained normal in skeletal muscle of these burn-injured critically ill rabbits, regardless of the glucose and insulin levels48, which is in agreement with the absence of an effect of intensive insulin therapy in skeletal muscle of critically ill patients.44The tissue-specific effects may be explained by the different mechanisms of glucose uptake, allowing cellular glucose overload in tissues with insulin-independent glucose transporters, whereas tissues that rely predominantly on insulin-dependent glucose uptake may be relatively well protected in view of the development of insulin resistance.
(iii) Proximal pathways of glucose toxicity
Multiple mechanisms may contribute to glucose toxicity, as revealed by the diabetes literature. Several of them have been linked to a central event whereby the glycolytic enzyme glyceraldehyde-3-phosphate
dehydrogenase (GAPDH) is inactivated, causing a block in glycolysis and a shift of glucose into proximal toxic pathways.50These include an increased flux through the polyol and hexosamine pathways, activation of protein kinase C and increased formation of advanced glycation end products (AGEs). GAPDH appeared to be not or hardly compromised by hyperglycaemia in critical illness, unlike in diabetes, as shown in burn-injured rabbits and critically ill patients.44,48,49Furthermore, glycolysis was suggested to be increased rather than inhibited in critical illness, supported by animal data48and increased pyruvate formation as a result of increased glucose turnover in critically ill patients.51,52The described toxic pathways may however be fed by such overloading of the glycolytic pathway, as shown for the formation of methylglyoxal (the most important dicarbonyl precursor of AGEs), being proportionate to the glycolytic flux.53The dicarbonyl metabolism was disturbed in hyperglycaemic critically ill rabbits, with elevated dicarbonyl levels (glyoxal, methylglyoxal and 3-deoxyglucosone) and impaired detoxification as compared with normoglycaemic sick rabbits.48 Dicarbonyl levels strongly correlated inversely with mitochondrial enzyme activities, which may support involvement of this pathway in critical illness. Administration of dicarbonyls has been shown to reproduce the complications of diabetes.54Furthermore, mitochondrial toxicity of dicarbonyls has been demonstrated and has included severe ultra-structural alterations, post-translational protein modification and inhibition of the citric acid cycle and respiratory chain.55–57 (iv) Oxidative stress
Multiple organ failure in critical illness has been linked to increased oxidative stress that develops secondary to an increased production of reactive oxygen species and/or a decrease in its scaven-gers.58,59 Substantial evidence links diabetes and hyperglycaemia to the development of increased oxidative stress, and several mechanisms contribute to the elevated production of reactive oxygen species in the presence of high glucose concentrations. One of them is an increased mitochondrial superoxide production, which actually is central in the unifying hypothesis on hyperglycaemic damage pathways described above.50Indeed, consequent DNA damage has been described to activate poly (ADP-ribose) polymerase (PARP) that in its turn inactivates GAPDH.60Other sources of ROS production include transition metal-catalysed glucose oxidation, activation of NADPH oxidase and also AGEs themselves are able to produce oxygen free radicals due to their strong reducing properties.61–63 Furthermore, antioxidant defence systems may be reduced.64In critical illness, cytokines enhance the synthesis of nitric oxide, whereas hypoxia–reperfusion aggravates the production of superoxide, thus promoting the formation of peroxynitrite. This reactive nitrogen species may interfere with the normal function of several proteins65, including the mitochondrial enzyme complex I of which the activity is suppressed by tyrosine nitration.66Avoiding hyperglycaemia in critically ill rabbits prevented the rise of malondialdehyde, a marker of oxidative stress-induced lipid peroxidation, and the depletion of the oxidant scavenger reduced glutathione, which correlated with improved mitochondrial function.48 Innate immunity, inflammation and coagulation
High glucose levels compromise all major components of innate immunity67, including poly-morphonuclear neutrophil function and intracellular bactericidal and opsonic activity.68–70 Hyper-glycaemia can lead to glycosylation and thus inactivation of immunoglobulins, hence contributing to the risk of infection.71This may play a role in the increased risk of infections observed for patients who are exposed to such high glucose levels.72,73Tight glycaemic control with intensive insulin therapy has been shown to prevent nosocomial infections and lethal sepsis in critically ill patients.3Independent of its effect on prevention of infection, intensive insulin therapy also lowered excessive inflammation, as illustrated by the lower levels of C-reactive protein (CRP) and mannose-binding lectin.74In critically ill children, tight glycaemic control with intensive insulin therapy attenuated the inflammatory response as evidenced by the time course of CRP.7 Secondary infections were reduced, most importantly, pulmonary and bloodstream infections.7Similar observations were seen in an animal model of pro-longed critical illness induced by a third-degree burn injury75, where glucose control with insulin beneficially affected innate immunity by preservation of phagocytosis and the oxidative burst function of monocytes and attenuated the CRP response.76 When glucose and insulin were manipulated independently in this rabbit model of critical illness, a partial rescue by insulin and not tight glycaemic control could be demonstrated on leucocyte dysfunction.38These data indicate that both prevention of
hyperglycaemia and direct insulin effects appear to contribute to the anti-inflammatory effects of intensive insulin therapy.
In view of the link between inflammation, endothelial dysfunction and coagulation abnormalities, and as both insulin therapy and glucose control may affect coagulation and fibrinolysis77–80, endo-thelial function43,81,82and inflammation74,83–85, one could expect that intensive insulin therapy may prevent severe coagulation abnormalities, thereby contributing to organ protection and improved survival. However, in a post hoc analysis on a subset of the second Leuven trial on medical ICU patients, no effect of insulin therapy was found on the tested fibrinolytic and coagulation parameters.37 By contrast, in a prospective randomised clinical trial in patients with sepsis, a small but significant improvement of fibrinolysis was observed in patients receiving tight glycaemic control.86
Anabolic actions of insulin
Muscle protein synthesis is stimulated, whereas protein breakdown is inhibited by insulin87,88, illustrating its anabolic properties. Hence, insulin administration may counteract the hypercatabolic state and feeding-resistant wasting syndrome of prolonged critical illness.89From clinical observation, such an anabolic effect was not obvious, but intensive insulin therapy increased the total protein content in the skeletal muscle of critically ill patients.44Improvement of insulin sensitivity may have played a role.33Whereas insulin therapy in diabetes mellitus was able to restore the levels of the anabolic insulin-like growth factor-1 (IGF-1), altered regulation at the level of the growth hormone axis appeared not to be involved in the beneficial effects of insulin during critical illness, as IGF-1 levels surprisingly were further suppressed.35Beneficial effects of insulin were also observed in animal models. For instance, it prevented weight loss in a rabbit model of prolonged critical illness90and suppressed muscle proteolysis in burn-injured rats, with involvement of the ubiquitin–proteasome pathway and lysosomal proteases.91However, when considering potential anabolic effects of intensive insulin therapy, a contribution of glycaemic control may not be excluded as it has been shown that hyperglycaemia induces catabolic responses.92,93 Maintaining normoglycaemia may therefore be required to obtain anabolic effects with insulin administration.
Myocardial protection by insulin
Infusion of insulin together with glucose and potassium, labelled GIK therapy, emerged several decades ago as a metabolic cocktail to reduce early mortality and morbidity of patients with myocardial infarction.94It has also been shown to protect and improve myocardial function during open heart surgery, endotoxic shock and other critical conditions.95Initially promising results96were, however, followed by the disappointing results of two recent large randomised clinical studies in patients with myocardial infarction where GIK failed to improve outcome.97,98
Direct anti-apoptotic properties of insulin, independent of glucose uptake and involving insulin sig-nalling, may play a role in myocardial protection.99However, avoiding hyperglycaemia is likely crucial for insulin to exert any beneficial effect on the myocardium.95Hyperinsulinaemia was able to increase myocardial contractility in critically ill rabbits, but only when blood glucose levels were maintained normal.38 Furthermore, simultaneous toleration of hyperglycaemia compromised contractility and mitochondrial function.38,48These data, suggesting that insulin administration in the absence of adequate blood glucose control may be deleterious, may offer an explanation for the controversy surrounding GIK and the disappointment of the recent clinical studies. Indeed, neither of the studies performed or suc-ceeded in tight blood glucose control.97,98Similarly, GIK infusion after acute stroke failed to reduce mortality in the absence of glycaemic control.100By contrast, intensive insulin therapy targeting and reaching age-adjusted normal glucose levels provided myocardial protection in critically ill children.7 Effect of insulin on the lipid profile
Lipid metabolism in critically ill patients is strongly deranged with an elevated triglyceride level (due to an increase in very-low-density lipoprotein (VLDL)) and very low levels of high-density lipo-protein (HDL) and low-density lipolipo-protein (LDL) cholesterol.101,102These disturbances were almost
completely (hypertriglyceridaemia) or partially reversed (HDL and LDL cholesterol) by insulin therapy.34Insulin treatment also decreased serum triglycerides and free fatty acids in burned chil-dren.103 The important role of lipoproteins in transportation of lipid components (i.e., cholesterol, triglycerides, phospholipids and lipid-soluble vitamins) and scavenging of pro-inflammatory bacterial endotoxin may have contributed to the improved outcome.104 Furthermore, multivariate logistic regression analysis revealed that improvement of the dyslipidaemia with insulin therapy explained a significant part of the reduced mortality and organ failure in critically ill patients.34
Cholestatic liver dysfunction and biliary sludge are common problems in critically ill patients. In animal studies, it was shown that hyperglycaemia induces cholestasis.105 In humans, gallbladder dysmotility is also associated with hyperglycaemia as well as with insulin resistance.106,107Intensive insulin therapy significantly reduced both the cholestatic liver dysfunction and biliary sludge, but not ischaemic hepatitis in prolonged critically ill patients.108
The mechanism behind the improved lipid profile with intensive insulin therapy remains unre-solved although the ameliorated liver and gallbladder function is likely involved. One could also expect an increased peripheral lipid uptake or a decreased lipid release.
Function of the adrenal axis
An adequate cortisol response to critical illness is vital, as both excessive and insufficient activation of the hypothalamic–pituitary–adrenal axis is associated with increased mortality. Insulin theoretically could have an ambiguous effect on the cortisol response to critical illness. A stimulatory effect of insulin on free cortisol levels could be inferred by its ability to suppress the levels of cortisol-binding glob-ulin.109 However, attenuation of the cortisol response may also be expected in view of the anti-inflammatory effects of insulin, which may reduce the levels of cytokines that directly drive cortisol secretion and metabolism. Circulating total and free cortisol levels were lowered by intensive insulin therapy, without altering cortisol-binding activity.110This effect statistically related to the improved outcome of the patients who received intensive insulin therapy. The mRNA expression of the gluco-corticoid receptor splice variants in postmortem liver and skeletal muscle biopsies of critically ill patients was not affected by insulin therapy.111
Summary
Hyperglycemia in critical illness has been associated with increased mortality. Simply maintaining normoglycaemia with insulin therapy improves survival and reduces morbidity in ICU patients, as shown by several randomised controlled studies. Multivariate logistic regression analyses on the two Leuven studies indicated that blood glucose control, rather than the insulin dose administered, statistically explains most of the beneficial effects of insulin therapy on the outcome of critical illness. Amelioration of insulin sensitivity thereby improving glucose uptake in insulin-sensitive tissues appears to play a major role in the mechanism by which blood glucose is lowered.
Several molecular pathways are beneficially affected by intensive insulin therapy. Prevention of hyperglycaemia-induced bioenergetic failure likely plays an important role in organ protection by this therapy. In addition, other metabolic effects of insulin therapy contributed to the clinical benefits, although most often with the requirement of simultaneously avoiding hyperglycaemia. Insulin therapy appeared to improve innate immunity and lowered excessive inflammation, pre-vented weight loss and ameliorated muscle mass, protected the myocardium and had an impact on the cortisol response.
Acknowledgements
This work was supported by the Fund for Scientific Research Flanders, Belgium (F.W.O, G.0533.06 and G.0585.09), by the Research Council of the Katholieke Universiteit Leuven (GOA2007/14) and by long-term structural funding – Methusalem funding by the Flemish Government. IV received a doctoral Fellowship from the Research Council of the Katholieke Universiteit Leuven and LL is a Post-doctoral Fellow for the F.W.O.
Conflict of interest
The authors report to have no conflict of interest. References
1. McCowen KC, Malhotra A & Bistrian BR. Stress-induced hyperglycemia. Critical Care Clinics 2001; 17: 107–124. 2. Vanhorebeek I, Langouche L & Van den Berghe G. Tight blood glucose control with insulin in the ICU: facts and
controversies. Chest 2007; 132: 268–278.
*3. Van den Berghe G, Wouters P, Weekers F et al. Intensive insulin therapy in critically ill patients. The New England Journal of Medicine 2001; 345: 1359–1367.
4. Ingels C, Debaveye Y, Milants I et al. Strict blood glucose control with insulin during intensive care after cardiac surgery: impact on 4-years survival, dependency on medical care, and quality-of-life. European Heart Journal 2006; 27: 2716–2724.
5. Van den Berghe G, Schoonheydt K, Becx P et al. Insulin therapy protects the central and peripheral nervous system of intensive care patients. Neurology 2005; 64: 1348–1353.
6. Van den Berghe G, Wilmer A, Hermans G et al. Intensive insulin therapy in the medical ICU. The New England Journal of Medicine 2006; 354: 449–461.
*7. Vlasselaers D, Milants I, Desmet L et al. Intensive insulin therapy for patients in paediatric intensive care: a prospective, randomised controlled study. Lancet 2009; 373: 547–556.
8. Van den Berghe G, Wilmer A, Milants I et al. Intensive insulin therapy in mixed medical/surgical intensive care units: benefit versus harm. Diabetes 2006; 55: 3151–3159.
9. Hermans G, Wilmer A, Meersseman W et al. Impact of intensive insulin therapy on neuromuscular complications and ventilator dependency in the medical intensive care unit. American Journal of Respiratory and Critical Care Medicine 2007; 175: 480–489.
10. Grey NJ & Perdrizet GA. Reduction of nosocomial infections in the surgical intensive-care unit by strict glycemic control. Endocrine Practice 2004; 10(Suppl. 2): 46–52.
11. Krinsley JS. Effect of an intensive glucose management protocol on the mortality of critically ill adult patients. Mayo Clinic Proceedings 2004; 79: 992–1000.
12. Furnary AP & Wu Y. Eliminating the diabetic disadvantage: the Portland Diabetic Project. Seminars in Thoracic and Cardiovascular Surgery 2006; 18: 302–308.
13. Reed CC, Stewart RM, Sherman M et al. Intensive insulin protocol improves glucose control and is associated with a reduction in intensive care unit mortality. Journal of American College of Surgeons 2007; 204: 1048–1054. 14. Bilotta F, Spinelli A, Giovannini F et al. The effect of intensive insulin therapy on infection rate, vasospasm, neurologic
outcome, and mortality in neurointensive care unit after intracranial aneurysm clipping in patients with acute subarachnoid hemorrhage: a randomized prospective pilot trial. Journal of Neurosurgical Anesthesiology 2007; 19: 156–160. 15. Lecomte P, Van Vlem B, Coddens J et al. Tight perioperative glucose control is associated with a reduction in renal impairment and renal failure in non-diabetic cardiac surgical patients. Critical Care (London, England) 2008; 12: R154. 16. Finfer S, Chittock DR, Su SY et al. Intensive versus conventional glucose control in critically ill patients. The New England
Journal of Medicine 2009; 360: 1283–1297.
17. Brunkhorst FM, Engel C, Bloos F et al. Intensive insulin therapy and pentastarch resuscitation in severe sepsis. The New England Journal of Medicine 2008; 358: 125–139.
*18. Van den Berghe G, Schetz M, Vlasselaers D et al. Intensive insulin therapy in critically ill patients: NICE-SUGAR or Leuven blood glucose target? The Journal of Clinical Endocrinology and Metabolism 2009; 94: 3163–3170.
19. Van den Berghe G, Mesotten D & Vanhorebeek I. Intensive insulin therapy in the intensive care unit. Canadian Medical Association Journal 2009; 180: 799–800.
20. Inzucchi SE & Siegel MD. Glucose control in the ICU–how tight is too tight? The New England Journal of Medicine 2009; 360: 1346–1349.
21. Griesdale DE, de Souza RJ, van Dam RM et al. Intensive insulin therapy and mortality among critically ill patients: a meta-analysis including NICE-SUGAR study data. Canadian Medical Association Journal 2009; 180: 821–827. 22. Pittas AG, Siegel RD & Lau J. Insulin therapy and in-hospital mortality in critically ill patients: systematic review and
meta-analysis of randomized controlled trials. JPEN Journal of Parenteral and Enteral Nutrition 2006; 30: 164–172. 23. Van den Berghe G, Wouters PJ, Bouillon R et al. Outcome benefit of intensive insulin therapy in the critically ill: insulin
dose versus glycemic control. Critical Care Medicine 2003; 31: 359–366.
24. Van den Berghe G, Wouters P, Weekers F et al. Reactivation of pituitary hormone release and metabolic improvement by infusion of growth hormone-releasing peptide and thyrotropin-releasing hormone in patients with protracted critical illness. Journal of Clinical Endocrinology and Metabolism 1999; 84: 1311–1323.
25. Van den Berghe G, Baxter RC, Weekers F et al. A paradoxical gender dissociation within the growth hormone/insulin-like growth factor I axis during protracted critical illness. Journal of Clinical Endocrinology and Metabolism 2000; 85: 183–192. 26. Mesotten D, Delhanty PJ, Vanderhoydonc F et al. Regulation of insulin-like growth factor binding protein-1 during
protracted critical illness. Journal of Clinical Endocrinology and Metabolism 2002; 87: 5516–5523.
27. Watt MJ, Howlett KF, Febbraio MA et al. Adrenaline increases skeletal muscle glycogenolysis, pyruvate dehydrogenase activation and carbohydrate oxidation during moderate exercise in humans. The Journal of Physiology 2001; 534: 269–278. 28. Wolfe RR, Durkot MJ, Allsop JR & Burke JF. Glucose metabolism in severely burned patients. Metabolism 1979; 28:
1031–1039.
29. Wolfe RR, Herndon DN, Jahoor F et al. Effect of severe burn injury on substrate cycling by glucose and fatty acids. The New England Journal of Medicine 1987; 317: 403–408.
30. Virkamaki A & Yki-Jarvinen H. Mechanisms of insulin resistance during acute endotoxemia. Endocrinology 1994; 134: 2072–2078.
31. Stephens JM, Bagby GJ, Pekala PH et al. Differential regulation of glucose transporter gene expression in adipose tissue or septic rats. Biochemical and Biophysical Research 1992; 183: 417–422.
32. Virkamaki A, Ueki K & Kahn CR. Protein-protein interaction in insulin signaling and the molecular mechanisms of insulin resistance. The Journal of Clinical Investigation 1999; 103: 931–943.
*33. Langouche L, Vander Perre S, Wouters PJ et al. Effect of intensive insulin therapy on insulin sensitivity in the critically ill. Journal of Clinical Endocrinology and Metabolism 2007; 92: 3890–3897.
*34. Mesotten D, Swinnen JV, Vanderhoydonc F et al. Contribution of circulating lipids to the improved outcome of critical illness by glycemic control with intensive insulin therapy. Journal of Clinical Endocrinology and Metabolism 2004; 89: 219–226.
35. Mesotten D, Wouters PJ, Peeters RP et al. Regulation of the somatotropic axis by intensive insulin therapy during protracted critical illness. Journal of Clinical Endocrinology and Metabolism 2004; 89: 3105–3113.
36. Krinsley JS. Association between hyperglycemia and increased hospital mortality in a heterogeneous population of critically ill patients. Mayo Clinic Proceedings 2003; 78: 1471–1478.
37. Langouche L, Meersseman W, Vander Perre S et al. Effect of insulin therapy on coagulation and fibrinolysis in medical intensive care patients. Critical Care Medicine 2008; 36: 1475–1480.
*38. Ellger B, Debaveye Y, Vanhorebeek I et al. Survival benefits of intensive insulin therapy in critical illness: impact of maintaining normoglycemia versus glycemia-independent actions of insulin. Diabetes 2006; 55: 1096–1105. 39. Aird WC. The role of the endothelium in severe sepsis and multiple organ dysfunction syndrome. Blood 2003; 101:
3765–3777.
40. Singer M & Brealey D. Mitochondrial dysfunction in sepsis. Biochemical Society Symposis 1999; 66: 149–166. 41. Brealey D, Brand M, Hargreaves I et al. Association between mitochondrial dysfunction and severity and outcome of
septic shock. Lancet 2002; 360: 219–223.
42. Brealey D, Karyampudi S, Jacques TS et al. Mitochondrial dysfunction in a long-term rodent model of sepsis and organ failure. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 2004; 286: R491–R497.
*43. Langouche L, Vanhorebeek I, Vlasselaers D et al. Intensive insulin therapy protects the endothelium of critically ill patients. The Journal of Clinical Investigation 2005; 115: 2277–2286.
*44. Vanhorebeek I, De Vos R, Mesotten D et al. Protection of hepatocyte mitochondrial ultrastructure and function by strict blood glucose control with insulin in critically ill patients. Lancet 2005; 365: 53–59.
45. Siroen MP, van Leeuwen PA, Nijveldt RJ et al. Modulation of asymmetric dimethylarginine in critically ill patients receiving intensive insulin treatment: a possible explanation of reduced morbidity and mortality? Critical Care Medicine 2005; 33: 504–510.
46. Ellger B, Langouche L, Richir M et al. Modulation of regional nitric oxide metabolism: blood glucose control or insulin? Intensive Care Medicine 2008; 34: 1525–1533.
47. Ellger B, Richir MC, van Leeuwen PA et al. Glycemic control modulates arginine and asymmetrical-dimethylarginine levels during critical illness by preserving dimethylarginine-dimethylaminohydrolase activity. Endocrinology 2008; 149: 3148–3157.
*48. Vanhorebeek I, Ellger B, De Vos R et al. Tissue-specific glucose toxicity induces mitochondrial damage in a burn injury model of critical illness. Critical Care Medicine 2009; 37: 1355–1364.
49. Vanhorebeek I, Gunst J, Ellger B et al. Hyperglycemic kidney damage in an animal model of prolonged critical illness. Kidney International 2009; 76: 512–520.
50. Brownlee M. Biochemistry and molecular cell biology of diabetic complications. Nature 2001; 414: 813–820. 51. Gore DC, Jahoor F, Hibbert JM & DeMaria EJ. Lactic acidosis during sepsis is related to increased pyruvate production,
not deficits in tissue oxygen availability. Annals of Surgery 1996; 224: 97–102.
52. Revelly JP, Tappy L, Martinez A et al. Lactate and glucose metabolism in severe sepsis and cardiogenic shock. Critical Care Medicine 2005; 33: 2235–2240.
53. Thornalley PJ. Modification of the glyoxalase system in human red blood cells by glucose in vitro. Biochemical Journal 1988; 254: 751–755.
54. Berlanga J, Cibrian D, Guillen I et al. Methylglyoxal administration induces diabetes-like microvascular changes and perturbs the healing process of cutaneous wounds. Clinical Science (London) 2005; 109: 83–95.
55. Rosca MG, Monnier VM, Szweda LI & Weiss MF. Alterations in renal mitochondrial respiration in response to the reactive oxoaldehyde methylglyoxal. American Journal of Physiology. Renal Physiology 2002; 283: F52–F59.
56. Shangari N, Bruce WR, Poon R & O’Brien PJ. Toxicity of glyoxals–role of oxidative stress, metabolic detoxification and thiamine deficiency. Biochemical Society Transactions 2003; 31: 1390–1393.
57. Rosca MG, Mustata TG, Kinter MT et al. Glycation of mitochondrial proteins from diabetic rat kidney is associated with excess superoxide formation. American Journal of Physiology. Renal Physiology 2005; 289: F420–F430.
58. Grune T & Berger MM. Markers of oxidative stress in ICU clinical settings: present and future. Current Opinion in Clinical Nutrition and Metabolic Care 2007; 10: 712–717.
59. Eaton S. The biochemical basis of antioxidant therapy in critical illness. Proceedings of the Nutrition Society 2006; 65: 242–249.
60. Pacher P & Szabo C. Role of poly(ADP-ribose) polymerase-1 activation in the pathogenesis of diabetic complications: endothelial dysfunction, as a common underlying theme. Antioxidants & Redox Signaling 2005; 7: 1568–1580. 61. Hunt JV, Dean RT & Wolff SP. Hydroxyl radical production and autoxidative glycosylation. Glucose autoxidation as the
cause of protein damage in the experimental glycation model of diabetes mellitus and ageing. Biochemical Journal 1988; 256: 205–212.
62. Bonnefont-Rousselot D. Glucose and reactive oxygen species. Current Opinion in Clinical Nutrition and Metabolic Care 2002; 5: 561–568.
63. Suh SW, Gum ET, Hamby AM et al. Hypoglycemic neuronal death is triggered by glucose reperfusion and activation of neuronal NADPH oxidase. The Journal of Clinical Investigation 2007; 117: 910–918.
65. Aulak KS, Koeck T, Crabb JW & Stuehr DJ. Dynamics of protein nitration in cells and mitochondria. American Journal of Physiology. Heart and Circulatory Physiology 2004; 286: H30–H38.
66. Frost MT, Wang Q, Moncada S & Singer M. Hypoxia accelerates nitric oxide-dependent inhibition of mitochondrial complex I in activated macrophages. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 2005; 288: R394–R400.
67. Turina M, Fry DE & Polk Jr. HC. Acute hyperglycemia and the innate immune system: clinical, cellular, and molecular aspects. Critical Care Medicine 2005; 33: 1624–1633.
68. Nielson CP & Hindson DA. Inhibition of polymorphonuclear leukocyte respiratory burst by elevated glucose concen-trations in vitro. Diabetes 1989; 38: 1031–1035.
69. Perner A, Nielsen SE & Rask-Madsen J. High glucose impairs superoxide production from isolated blood neutrophils. Intensive Care Medicine 2003; 29: 642–645.
70. Rassias AJ, Marrin CA, Arruda J et al. Insulin infusion improves neutrophil function in diabetic cardiac surgery patients. Anesthesia & Analgesia 1999; 88: 1011–1016.
71. Black CT, Hennessey PJ & Andrassy RJ. Short-term hyperglycemia depresses immunity through nonenzymatic glyco-sylation of circulating immunoglobulin. The Journal of Trauma 1990; 30: 830–832.
72. Furnary AP, Zerr KJ, Grunkemeier GL & Starr A. Continuous intravenous insulin infusion reduces the incidence of deep sternal wound infection in diabetic patients after cardiac surgical procedures. Annals of Thoracic Surgery 1999; 67: 352–360.
73. Pozzilli P & Leslie RD. Infections and diabetes: mechanisms and prospects for prevention. Diabetic Medicine 1994; 11: 935–941.
74. Hansen TK, Thiel S, Wouters PJ et al. Intensive insulin therapy exerts antiinflammatory effects in critically ill patients and counteracts the adverse effect of low mannose-binding lectin levels. Journal of Clinical Endocrinology and Metab-olism 2003; 88: 1082–1088.
75. Weekers F, Van Herck E, Coopmans W et al. A novel in vivo rabbit model of hypercatabolic critical illness reveals a biphasic neuroendocrine stress response. Endocrinology 2002; 143: 764–774.
76. Weekers F, Michalaki M, Coopmans W et al. Endocrine and metabolic effects of growth hormone (GH) compared with GH-releasing peptide, thyrotropin-releasing hormone, and insulin infusion in a rabbit model of prolonged critical illness. Endocrinology 2004; 145: 205–213.
77. Meigs JB, Mittleman MA, Nathan DM et al. Hyperinsulinemia, hyperglycemia, and impaired hemostasis: the Fra-mingham Offspring Study. Journal of American Medical Association 2000; 283: 221–228.
78. Nordt TK, Sawa H, Fujii S & Sobel BE. Induction of plasminogen activator inhibitor type-1 (PAI-1) by proinsulin and insulin in vivo. Circulation 1995; 91: 764–770.
79. Stegenga ME, van der Crabben SN, Levi M et al. Hyperglycemia stimulates coagulation, whereas hyperinsulinemia impairs fibrinolysis in healthy humans. Diabetes 2006; 55: 1807–1812.
80. Vaidyula VR, Rao AK, Mozzoli M et al. Effects of hyperglycemia and hyperinsulinemia on circulating tissue factor procoagulant activity and platelet CD40 ligand. Diabetes 2006; 55: 202–208.
81. Aljada A & Dandona P. Effect of insulin on human aortic endothelial nitric oxide synthase. Metabolism 2000; 49: 147–150. 82. Dandona P, Aljada A, Mohanty P et al. Insulin suppresses plasma concentration of vascular endothelial growth factor
and matrix metalloproteinase-9. Diabetes Care 2003; 26: 3310–3314.
83. Dandona P, Aljada A, Mohanty P et al. Insulin inhibits intranuclear nuclear factor kappaB and stimulates IkappaB in mononuclear cells in obese subjects: evidence for an anti-inflammatory effect? Journal of Clinical Endocrinology and Metabolism 2001; 86: 3257–3265.
84. Dandona P, Aljada A & Bandyopadhyay A. Inflammation: the link between insulin resistance, obesity and diabetes. Trends in Immunology 2004; 25: 4–7.
85. Aljada A, Ghanim H, Mohanty P et al. Insulin inhibits the pro-inflammatory transcription factor early growth response gene-1 (Egr)-1 expression in mononuclear cells (MNC) and reduces plasma tissue factor (TF) and plasminogen activator inhibitor-1 (PAI-1) concentrations. Journal of Clinical Endocrinology and Metabolism 2002; 87: 1419–1422.
86. Savioli M, Cugno M, Polli F et al. Tight glycemic control may favor fibrinolysis in patients with sepsis. Critical Care Medicine 2009; 37: 424–431.
87. Agus MS, Javid PJ, Ryan DP & Jaksic T. Intravenous insulin decreases protein breakdown in infants on extracorporeal membrane oxygenation. Journal of Pediatric Surgery 2004; 39: 839–844.
88. Gore DC, Wolf SE, Sanford AP et al. Extremity hyperinsulinemia stimulates muscle protein synthesis in severely injured patients. American Journal of Physiology. Endocrinology and Metabolism 2004; 286: E529–E534.
89. Vanhorebeek I & Van den Berghe G. Hormonal and metabolic strategies to attenuate catabolism in critically ill patients. Current Opinion in Pharmacology 2004; 4: 621–628.
*90. Weekers F, Giulietti AP, Michalaki M et al. Metabolic, endocrine, and immune effects of stress hyperglycemia in a rabbit model of prolonged critical illness. Endocrinology 2003; 144: 5329–5338.
91. Solomon V, Madihally S, Yarmush M & Toner M. Insulin suppresses the increased activities of lysosomal cathepsins and ubiquitin conjugation system in burn-injured rats. The Journal of Surgical Research 2000; 93: 120–126.
92. Flakoll PJ, Hill JO & Abumrad NN. Acute hyperglycemia enhances proteolysis in normal man. American Journal of Physiology 1993; 265: E715–E721.
93. Gore DC, Chinkes DL, Hart DW et al. Hyperglycemia exacerbates muscle protein catabolism in burn-injured patients. Critical Care Medicine 2002; 30: 2438–2442.
94. Sodi-Pallares D, Testelli MR, Fishleder BL et al. Effects of an intravenous infusion of a potassium-glucose-insulin solution on the electrocardiographic signs of myocardial infarction. A preliminary clinical report. American Journal of Cardiology 1962; 9: 166–181.
95. Das UN. Insulin: an endogenous cardioprotector. Current Opinion in Critical Care 2003; 9: 375–383.
96. Malmberg K, Ryden L, Efendic S et al. Randomized trial of insulin-glucose infusion followed by subcutaneous insulin treatment in diabetic patients with acute myocardial infarction (DIGAMI study): effects on mortality at 1 year. Journal of the American College of Cardiology 1995; 26: 57–65.
97. Malmberg K, Ryden L, Wedel H et al. Intense metabolic control by means of insulin in patients with diabetes mellitus and acute myocardial infarction (DIGAMI 2): effects on mortality and morbidity. European Heart Journal 2005; 26: 650–661. 98. The CREATE-ECLA Trial Group Investigators. Effect of glucose-insulin-potassium infusion on mortality in patients with acute ST-segment elevation myocardial infarction: the CREATE-ECLA randomized controlled trial. Journal of American Medical Association 2005; 293: 437–446.
99. Gao F, Gao E, Yue TL et al. Nitric oxide mediates the antiapoptotic effect of insulin in myocardial ischemia-reperfusion: the roles of PI3-kinase, Akt, and endothelial nitric oxide synthase phosphorylation. Circulation 2002; 105: 1497–1502. 100. Gray CS, Hildreth AJ, Sandercock PA et al. Glucose-potassium-insulin infusions in the management of post-stroke
hyperglycaemia: the UK Glucose Insulin in Stroke Trial (GIST-UK). Lancet Neurology 2007; 6: 397–406.
101. Carpentier YA & Scruel O. Changes in the concentration and composition of plasma lipoproteins during the acute phase response. Current Opinion in Clinical Nutrition and Metabolic Care 2002; 5: 153–158.
102. Lanza-Jacoby S, Wong SH, Tabares A et al. Disturbances in the composition of plasma lipoproteins during gram-negative sepsis in the rat. Biochimica et Biophysica Acta 1992; 1124: 233–240.
103. Jeschke MG, Klein D & Herndon DN. Insulin treatment improves the systemic inflammatory reaction to severe trauma. Annals of Surgery 2004; 239: 553–560.
104. Harris HW, Grunfeld C, Feingold KR & Rapp JH. Human very low density lipoproteins and chylomicrons can protect against endotoxin-induced death in mice. The Journal of Clinical Investigation 1990; 86: 696–702.
105. Garcia-Marin JJ, Villanueva GR & Esteller A. Diabetes-induced cholestasis in the rat: possible role of hyperglycemia and hypoinsulinemia. Hepatology 1988; 8: 332–340.
106. Gielkens HA, Lam WF, Coenraad M et al. Effect of insulin on basal and cholecystokinin-stimulated gallbladder motility in humans. Journal of Hepatology 1998; 28: 595–602.
107. Nakeeb A, Comuzzie AG, Al-Azzawi H et al. Insulin resistance causes human gallbladder dysmotility. Journal of Gastrointestinal Surgery 2006; 10: 940–948.
108. Mesotten D, Wauters J, Van den Berghe G et al. The effect of strict blood glucose control on biliary sludge and cholestasis in critically ill patients. Journal of Clinical Endocrinology and Metabolism 2009.
109. Fernandez-Real JM, Pugeat M, Emptoz-Bonneton A & Ricart W. Study of the effect of changing glucose, insulin, and insulin-like growth factor-I levels on serum corticosteroid binding globulin in lean, obese, and obese subjects with glucose intolerance. Metabolism 2001; 50: 1248–1252.
110. Vanhorebeek I, Peeters RP, Vander Perre S et al. Cortisol response to critical illness: effect of intensive insulin therapy. Journal of Clinical Endocrinology and Metabolism 2006; 91: 3803–3813.
111. Peeters RP, Hagendorf A, Vanhorebeek I et al. Tissue mRNA expression of the glucocorticoid receptor and its splice variants in fatal critical illness. Clinical Endocrinology (Oxford) 2009; 71: 145–153.
