BALANCING THE SCALES USING A NOVEL CELLULAR ENERGY SENSOR

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1 The West London Medical Journal 2010 Vol 2 No 4 pp BALANCING THE SCALES USING A NOVEL CELLULAR ENERGY SENSOR Sairah Akbar The topic of obesity is rarely out of the public eye with an increasingly bleak picture portrayed for our obesogenic nation if current trends continue, by 2010 half of all school children will be obese and in 25 yrs a phenomenal half of the population will follow suit; on an economic level, this will equate to almost half of the total healthcare budget being spent on treating the associated complications cardiovascular disease, type 2 diabetes, hypertension and dyslipidaemia (the so-called metabolic syndrome).(1) The scale of the problem has resulted in an ongoing debate regarding apportioning of responsibility. Fast food stores have borne the brunt of the criticism for their seemingly unscrupulous advertising, spending billions annually in promoting their products. But what about the individuals themselves, should they not be held responsible for their food intake? Even in our rights obsessed society, there remains a huge stigmatisation of overweight individuals, the general consensus being that they are fat and lazy. As the debate continues to be waged, a considerable amount of time and money has been invested into understanding the pathogenesis of obesity, particularly the cellular mechanisms involved in the control of energy expenditure. In the last decade, exciting developments have been made at the forefront is evidence that mutations in energy regulating pathways cause substantial weight gain, suggesting there may be a genetic basis for the development of obesity. STRUCTURE AND REGULATION OF AMPK ACTIVITY Figure 1(2) 29

2 THE WEST LONDON MEDICAL JOURNAL , 4 At the forefront of current research is a novel cellular energy sensor, AMP-kinase (AMPK). AMPK is the downstream product of a protein kinase cascade(3). Its pivotal role as a regulator of energy balance arises from the ability of AMPK to detect intracellular levels of ATP(4). Within a cell, the energy producing reaction is catalysed by adenylate kinase: 2 ADP ATP + AMP AMPK acts as an energy sensor by detecting the ratio of AMP:ATP. Under normal conditions, cells exist under high energy states with an ATP:ADP ratio usually of the order 10:1. When energy is used within the cell, ATP levels are depleted resulting in an increased ratio of AMP: ATP; crucially it is this increased ratio, indicating a reduced energy state that is responsible for activating AMPK(4). Once activated, AMPK phosphorylates downstream substrates and in doing so, switches on catabolic (i.e. energy generating) pathways and switches off anabolic (energy-using) pathways; in this sense it has been dubbed a cellular fuel gauge(2). Figure 2(4) The structure of AMPK is essential to its functioning. AMPK exists as a heterotrimer composed of α, β and γ subunits(4). It can be activated by AMP in 2 distinct ways: firstly, AMP has a higher affinity for the enzyme than ATP with the result that AMP binding (signifying ATP depletion) causes allosteric activation. AMP binding also indirectly activates AMPK by making it a 30

3 BALANCING THE SCALES USING A NOVEL CELLULAR ENERGY SENSOR better substrate for an upstream kinase, LKB1. Activation of LKB1 causes phosphorylation of AMPK at the α-subunit at a specific threonine residue (Thr172), thus an activation loop is established within which a kinase cascade is activated. High concentrations of ATP inhibit these methods of activation.(3) ROLES OF AMPK Table 1 Targets of AMPK and their biologic effects(3) Cell/Organ type Glucose homeostasis Heart Various Pancreatic Target site (1 or 2 ) Glycogen synthase 6-phosphofructo-2-kinase Transcription factor ChREBP Gluconeogenic genes GLUT 4 GLUT 1? Immediate outcome Enzyme activity DNA binding Expression Membrane translocation Transporter activity Insulin release End result Glycogen synthesis Glycolysis Pyruvate kinase expression Gluconeogenesis Glucose uptake Glucose uptake Plasma insulin Lipid metabolism Adipose tissue Acetyl-CoA carboxylase- 1α Acetyl-CoA carboxylase- 2β HMG-CoA reductase Hormone-sensitive lipase Activation by PKA Fatty acid synthesis Fatty acid oxidation Cholesterol synthesis Lipolysis Others Heart Endothelial NO synthase Transcription factor NRF-1 Enzyme activity DNA binding NO production Mitochondrial biogenesis As Table 1 indicates, the roles of AMPK are wide-ranging; amongst its catalogued functions, AMPK exerts an effect on all the key organs involved in energy homeostasis heart, skeletal muscle, adipose tissue and liver with activation resulting in inhibition of fatty acid synthesis, increased fatty acid oxidation, enhanced glycolysis and reduced gluconeogenesis. Many of these effects are mediated by inhibition of rate limiting steps to exert a powerful catabolic effect.(3) It is now recognised that AMPK is not only involved in energy homeostasis within individual cells, but also plays an integral part in whole body energy (and thus weight) balance. This is because AMPK activity is influenced by hormonal systems, most importantly adipose tissue(3). 31

4 THE WEST LONDON MEDICAL JOURNAL , 4 Figure 3(3) Leptin and adiponectin are two hormones secreted by adipocytes. Recently, leptin has been the epicentre of research as a potential anti-obesity agent, based on the lipolytic effect of endogenous administration in adipose tissue in rats. This effect, both in animals and humans appears to be effected by AMPK-mediated stimulation of fatty acid oxidation.(3) Leptin and adiponectin bind to their respective receptors to activate the protein kinase; once activated, AMPK phosphorylates acetyl CoA carboxylase (ACC), the rate limiting step in the conversion of acetyl-coa to malonyl-coa. When phosphorylated, the activity of ACC is inhibited with a consequent reduction in malonyl-coa levels. The resultant activation of carnitine palmitoyl transferase (CPT1) increases mitochondrial import of long chain fatty acids and enhances oxidation. The activation of this pathway is thought to be central to the depletion of fat stores in muscle to exert a powerful anti-obesity effect. AMPK AS MASTER REGULATOR OF FOOD INTAKE One of the most important advances in our understanding of the crucial involvement of AMPK in obesity is the discovery of its role as a regulator of food intake.(5) Within the hypothalamus, the arcuate and paraventricular nuclei are integrative in the regulation of appetite; thus, for the first time, a mechanism has been postulated for central regulation of energy intake and expenditure. 32

5 BALANCING THE SCALES USING A NOVEL CELLULAR ENERGY SENSOR Figure 4(3) The 2 hormones involved in this process are leptin and ghrelin, a hormone derived from the gastrointestinal tract. Leptin acts as a satiety hormone, acting on neurones to reduce the release of orexigenic neuropeptides and increase the release of anorexigenic neuropeptides. Ghrelin, on the other hand, is synthesised by the stomach and acts to increase food intake through the same anorexigenic pathways as leptin but acting in opposite directions.(6) In terms of AMPK activity, rat studies have found that intraperitoneal injection of leptin results in a delayed but sustained decrease in AMPK activity after 60 mins, activity was reduced by 25-30%; this effect persisted for up to 180 mins(5). Injecting ghrelin into mice was also associated with a similar trend of a delayed change in AMPK levels but this time activity levels were increased(5). When AMPK is activated in the hypothalamus by injecting AICA riboside into the paraventricular nucleus (it is converted to AICA ribotide which increases levels of AMP and activates AMPK) and food intake is monitored, studies have found activation is associated with a significant increase in appetite and consumption.(5) UNCOUPLING FUTURE FOR AMPK? Some of the most fascinating research into the role of AMPK has involved the respiratory chain. Essentially, respiration involves the transfer of electrons from reduced carriers across a series of complexes to oxygen; this process is concomitantly associated with the transfer of protons across the inner mitochondrial membrane to create an electro potential difference across the 33

6 THE WEST LONDON MEDICAL JOURNAL , 4 membrane; the energy from this gradient is then used to drive ATP synthesis.(7) Tissues such as brown fat are described as metabolically insufficient because they possess an uncoupling protein (Ucp) which results in disruption of the electrochemical gradient. Therefore, instead of the energy being used to derive ATP it is dissipated as heat(7). If this process could be replicated in fat stores of obese people, their excess adipose tissue would literally be burned off. Indeed, experiments in mice with low expression of Ucp (Ucp-L) found that skeletal muscle O 2 consumption was 98% higher than wild-type mice and were resistant to obesity induced by 2 types of high fat diet(8). Figure 5(3) So where does AMPK fit into this? With the loss of potential sources of energy, uncoupling leads to an intracellular depletion of ATP with a subsequent rise in the AMP:ATP ratio resulting in activation of AMPK. Once activated, AMPK through the pathways already discussed, phosphorylates and inactivates the enzymes involved in lipid metabolism with an increase in fatty acid oxidation, resulting in depletion of fat stores(3). In this way, the downstream effects of uncoupling seem to be in agreement with the changes associated with AMPK activation. CONCLUSION After researching the topical issue of obesity, the firmly held beliefs that the responsibility and ultimately the solution lies in altering our couchpotato lifestyles can be challenged. In particular, the influences of genetic factors have to be taken into consideration in prehistoric times when food availability was limited, it was our ability to utilise the calorie content of food 34

7 BALANCING THE SCALES USING A NOVEL CELLULAR ENERGY SENSOR that determined our survival as a species. Now, however, the situation is very different, and a food surplus has meant this key adaptive mechanism is maladaptive as excessive calories are associated with significant health morbidities. Therefore, going back to the original question, perhaps it is our genes that are to blame for obesity. AMPK is a protein kinase that acts both on cellular and whole body homeostasis to alter energy regulating pathways and there is undisputable evidence that dysfunctioning of this protein kinase is implicated in obesity. Therefore, It is entirely conceivable that AMPK or one of its downstream products may be functioning inappropriately to contribute to this complex illness; if true, this may be the future for tackling the obesity epidemic that is crippling the Western world. REFERENCES [1] National Obesity Forum [online]. Cited Feb Accessed from URL: [2] Hardie DG. The AMP-activated protein kinase pathway new players upstream and downstream. Journal of Cell Science; 117: [3] Kahn BB, Alquier T, Carling D, Hardie G. AMP-activated protein kinase: Ancient energy gauge provides clues to modern understanding of metabolism. Cell Met; 1: [4] Carling D. The AMP-activated protein kinase cascade a unifying system for energy control. Trends in Biochemical Sciences; 29(1): [5] Andersson U, Fillipsson K, Abbott CR, Woods A, Smith K, Bloom SR et al. AMP-activated Protein Kinase Plays a Role in the Control of Food Intake. Journal of Biological Chemistry; 279(13): [6] Spiegelman BM, Flier JS. Obesity and the regulation of energy balance. Cell Feb 23;104(4): [7] Li B, Nolte LA, Ju J, Han DH, Coleman T, Holloszy JO, Semenkovich CF. Skeletal muscle respiratory uncoupling prevents diet-induced obesity and insulin resistance in mice. Nature Medicine; 6: [8] Rossmeisl MR, Flachs P, Sponarova J, Matejkova O, Prazak T, Ruzickova J et al. Role of energy charge and AMP-activated protein kinase in adipocytes in the control of body fat stores. International Journal of Obesity; 28: