Research: advances in cell biology relevant to critical illness

The role of impaired mitochondrial lipid oxidation in obesity

Obesity represents a disruption in balancing fuel intake with energy expenditure in favor of energy conservation. Adiposity is known to be carefully regulated and, over time, highly resistant to major changes, raising questions about how energy homeostasis can become dysregulated in favor of fat accumulation. In obesity, the excess lipid accumulation represents a surfeit of energy, but those who are obese often experience rapid fatigue and decreased physical endurance, reflecting an energy deficiency. To develop an explanation for this apparent contradiction in energy homeostasis and the chronic overeating relative to energy used in obesity, a review of the literature was conducted. The resulting model of obesity is based on a growing body of research demonstrating that altered mitochondrial energy production, particularly in skeletal muscles, is a major anomaly capable of setting off a chain of metabolic events leading to obesity. Alterations in skeletal muscle mitochondria distribution and their oxidative and glycolytic energy capacities in obesity are described. The metabolic responses of obese and normal individuals to exercise are contrasted, and the effects of weight loss on energy production are presented. The effect of altered fat oxidation is considered in relation to energy regulation by the central nervous system and the development of major obesity comorbidities, including systemic inflammation, insulin resistance and diabetes, and cardiovascular disease. Recommendations for clinical intervention and additional research are proposed based on the model presented of impaired mitochondrial function in obesity.

Dysregulation of mitochondrial dynamics and the muscle transcriptome in ICU patients suffering from sepsis induced multiple organ failure

Background

Septic patients treated in the intensive care unit (ICU) often develop multiple organ failure including persistent skeletal muscle dysfunction which results in the patient's protracted recovery process. We have demonstrated that muscle mitochondrial enzyme activities are impaired in septic ICU patients impairing cellular energy balance, which will interfere with muscle function and metabolism. Here we use detailed phenotyping and genomics to elucidate mechanisms leading to these impairments and the molecular consequences.

Methodology/Principal Findings

Utilising biopsy material from seventeen patients and ten age-matched controls we demonstrate that neither mitochondrial in vivo protein synthesis nor expression of mitochondrial genes are compromised. Indeed, there was partial activation of the mitochondrial biogenesis pathway involving NRF2α/GABP and its target genes TFAM, TFB1M and TFB2M yet clearly this failed to maintain mitochondrial function. We therefore utilised transcript profiling and pathway analysis of ICU patient skeletal muscle to generate insight into the molecular defects driving loss of muscle function and metabolic homeostasis. Gene ontology analysis of Affymetrix analysis demonstrated substantial loss of muscle specific genes, a global oxidative stress response related to most probably cytokine signalling, altered insulin related signalling and a substantial overlap between patients and muscle wasting/inflammatory animal models. MicroRNA 21 processing appeared defective suggesting that post-transcriptional protein synthesis regulation is altered by disruption of tissue microRNA expression. Finally, we were able to demonstrate that the phenotype of skeletal muscle in ICU patients is not merely one of inactivity, it appears to be an actively remodelling tissue, influenced by several mediators, all of which may be open to manipulation with the aim to improve clinical outcome.

Conclusions/Significance

This first combined protein and transcriptome based analysis of human skeletal muscle obtained from septic patients demonstrated that losses of mitochondria and muscle mass are accompanied by sustained protein synthesis (anabolic process) while dysregulation of transcription programmes appears to fail to compensate for increased damage and proteolysis. Our analysis identified both validated and novel clinically tractable targets to manipulate these failing processes and pursuit of these could lead to new potential treatments.

Understanding proteomics

The purpose of this article is to describe proteomics, to discuss the importance of proteomics, to review different methods for protein measurement, and to illustrate how knowledge of proteomics might improve patient care. Among common laboratory determinations are those involving enzymatic (protein) function. Although the presence or activity of proteins may be seen clinically as incidental, proteins represent the engines through which critical life processes ensue. A selected review of the literature is presented to define and explain proteomics and to review the various techniques to measure proteins. A case-study approach is used to illustrate how proteomics can be utilized for detecting and monitoring disease. The human genome has been completely sequenced and proteomics has emerged as a way to unravel the biochemical and physiological mechanisms of diseases at the functional level. This review includes the recent discoveries regarding proteomics and its importance in the detection and treatment of disease.

The role of apoptosis in respiratory diseases

PURPOSE

The purpose of this article is to define apoptosis and describe how this cellular pathway is relevant to the pathogenesis of different respiratory diseases. This will assist clinical nurse specialists in understanding how new drugs and therapies inhibit and stimulate apoptotic pathways.

BACKGROUND

Clinical nurse specialists need to expand their knowledge concerning the role of apoptosis so that they can better expand their spheres of influence. The 4 stages of apoptosis are discussed, as well as the various apoptotic pathways involved with asthma, emphysema, and acute respiratory distress syndrome that promote and inhibit apoptosis in patients.

CONCLUSION

It is crucial for clinical nurse specialists to know what apoptosis is and how it relates to different pathophysiologic states. The challenge facing clinical nurse specialists is how to be kept informed and current concerning molecular and cellular mechanisms that are important in the practice setting. Strategies needed to maintain expertise include acquiring new knowledge, developing new skills, and changing attitudes about molecular biology. Apoptosis must become a significant part of any health professionals' continuing educational program because it has been recognized as the pathway to most any disease. Clinical nurse specialists who understand apoptosis and its pathways can use this knowledge to aid in the prevention and treatment of respiratory diseases.

[Hyperglycemia and acute illness]

Hyperglycemia due to acute illness is frequently observed in non-diabetic patients. Considered as a physiological response to inflammation, it has now been shown to be an independent factor of morbid-mortality in critically ill patients. Hyperglycemia reduces the immune system response to aggression by decreasing the efficacy of some complement factors and polynuclear cells chemotactism and phagocytosis and by increasing the inflammatory response (cytokines, NF-kB and CRP). Glycemia near normalization using intensive insulin therapy significantly improves mortality and morbidity in several critical illnesses such as cardiac or infectious diseases. This improvement is probably due to the neutralization of deleterious effects caused by hyperglycemia and to the specific actions of insulin on the inflammatory response. Except for ICU patients, precise management protocols of hyperglycemia due to acute illness remain to be proposed and evaluated in clinical practice.

Voltage-dependent anion channel (VDAC) as mitochondrial governator--thinking outside the box

Despite a detailed understanding of their metabolism, mitochondria often behave anomalously. In particular, global suppression of mitochondrial metabolism and metabolite exchange occurs in apoptosis, ischemia and anoxia, cytopathic hypoxia of sepsis and multiple organ failure, alcoholic liver disease, aerobic glycolysis in cancer cells (Warburg effect) and unstimulated pancreatic beta cells. Here, we propose that closure of voltage-dependent anion channels (VDAC) in the mitochondrial outer membrane accounts for global mitochondrial suppression. In anoxia, cytopathic hypoxia and ethanol treatment, reactive oxygen and nitrogen species, cytokines, kinase cascades and increased NADH act to inhibit VDAC conductance and promote selective oxidation of membrane-permeable respiratory substrates like short chain fatty acids and acetaldehyde. In cancer cells, highly expressed hexokinase binds to and inhibits VDAC to suppress mitochondrial function while stimulating glycolysis, but an escape mechanism intervenes when glucose-6-phosphate accumulates and dissociates hexokinase from VDAC. Similarly, glucokinase binds mitochondria of insulin-secreting beta cells, possibly blocking VDAC and suppressing mitochondrial function. We propose that glucose metabolism leads to glucose-6-phosphate-dependent unbinding of glucokinase, relief of VDAC inhibition, release of ATP from mitochondria and ATP-dependent insulin release. In support of the overall proposal, ethanol treatment of isolated rat hepatocytes inhibited mitochondrial respiration and accessibility to adenylate kinase in the intermembrane space, effects that were overcome by digitonin permeabilization of the outer membrane. Overall, these considerations suggest that VDAC is a dynamic regulator, or governator, of global mitochondrial function both in health and disease.