Targeting AMP-activated protein kinase as a novel therapeutic approach for the treatment of metabolic disorders

In the light of recent studies in humans and rodents, AMP-activated protein kinase (AMPK), a phylogenetically conserved serine/threonine protein kinase, has been described as an integrator of regulatory signals monitoring systemic and cellular energy status. AMP-activated protein kinase (AMPK) has been proposed to function as a 'fuel gauge' to monitor cellular energy status in response to nutritional environmental variations. Recently, it has been proposed that AMPK could provide a link in metabolic defects underlying progression to the metabolic syndrome. AMPK is a heterotrimeric enzyme complex consisting of a catalytic subunit and two regulatory subunits β and γ. AMPK is activated by rising AMP and falling ATP. AMP activates the system by binding to the γ subunit that triggers phosphorylation of the catalytic subunit by the upstream kinases LKB1 and CaMKKβ (calmodulin-dependent protein kinase kinase). AMPK system is a regulator of energy balance that, once activated by low energy status, switches on ATP-producing catabolic pathways (such as fatty acid oxidation and glycolysis), and switches off ATP-consuming anabolic pathways (such as lipogenesis), both by short-term effect on phosphorylation of regulatory proteins and by long-term effect on gene expression. As well as acting at the level of the individual cell, the system also regulates food intake and energy expenditure at the whole body level, in particular by mediating the effects of insulin sensitizing adipokines leptin and adiponectin. AMPK is robustly activated during skeletal muscle contraction and myocardial ischaemia playing a role in glucose transport and fatty acid oxidation. In liver, activation of AMPK results in enhanced fatty acid oxidation as well as decreased glucose production. Moreover, the AMPK system is one of the probable targets for the anti-diabetic drugs biguanides and thiazolidinediones. Thus, the relationship between AMPK activation and beneficial metabolic effects provide the rationale for the development of new therapeutic strategies in metabolic disorders.     

Metabolic adaptation to malnutrition

Resting and nonresting energy expenditure are reduced in restricted volunteers.This decrease is due to fat-free mass depletion and also to a reduction in energy expenditure per kilogram of fat-free mass. Protein deprivation leads to a reduction in protein turn-over or oxidation in the postabsorptive state and to a decrease in the amplitude of diurnal cycling of protein turn-over, synthesis and breakdown. Such a metabolic adaptation tends to maintain fat mass and to reduce the need for essential amino acids. In adults with naturally occurring states of malnutrition, the results vary according to the cause of malnutrition. One may observe or not a reduction in energy expenditure or protein metabolism. The lack of reduction of metabolic adaptation could be due to the long term energy and protein deprivation and to the preservation of visceral mass relative to muscle mass     

β-Subunit myristoylation is the gatekeeper for initiating metabolic stress sensing by AMP-activated protein kinase (AMPK)

The AMP-activated protein kinase (AMPK) is an αβγ heterotrimer that acts as a master metabolic regulator to maintain cellular energy balance following increased energy demand and increases in the AMP/ATP ratio. This regulation provides dynamic control of energy metabolism, matching energy supply with demand that is essential for the function and survival of organisms. AMPK is inactive unless phosphorylated on Thr172 in the α-catalytic subunit activation loop by upstream kinases (LKB1 or calcium-calmodulin-dependent protein kinase kinase β). How a rise in AMP levels triggers AMPK α-Thr172 phosphorylation and activation is incompletely understood. Here we demonstrate unequivocally that AMP directly stimulates α-Thr172 phosphorylation provided the AMPK β-subunit is myristoylated. Loss of the myristoyl group abolishes AMP activation and reduces the extent of α-Thr172 phosphorylation. Once AMPK is phosphorylated, AMP further activates allosterically but this activation does not require β-subunit myristoylation. AMP and glucose deprivation also promote membrane association of myristoylated AMPK, indicative of a myristoyl-switch mechanism. Our results show that AMP regulates AMPK activation at the initial phosphorylation step, and that β-subunit myristoylation is important for transducing the metabolic stress signal.     

Multiorgan failure is an adaptive, endocrine-mediated, metabolic response to overwhelming systemic inflammation

Sepsis and other critical illnesses produce a biphasic inflammatory, immune, hormonal, and metabolic response. The acute phase is marked by an abrupt rise in the secretion of so-called stress hormones with an associated increase in mitochondrial and metabolic activity. The combination of severe inflammation and secondary changes in endocrine profile diminish energy production, metabolic rate, and normal cellular processes, leading to multiple organ dysfunction. This perceived failure of organs might instead be a potentially protective mechanism, because reduced cellular metabolism could increase the chances of survival of cells, and thus organs, in the face of an overwhelming insult. We propose that, first, multiple organ failure induced by critical illness is primarily a functional, rather than structural, abnormality. Indeed, it may not be failure as such, but a potentially protective, reactive mechanism. Second, the decline in organ function is triggered by a decrease in mitochondrial activity and oxidative phosphorylation, leading to reduced cellular metabolism. Third, this effect on mitochondria might be the consequence of acute-phase changes in hormones and inflammatory mediators.     

Effects of active muscle mass size on cardiopulmonary responses to exercise in congestive heart failure

Previous studies from this laboratory demonstrated that in healthy young men, cardiac output is closely coupled to oxygen uptake during dynamic exercise, regardless of its mode or relative intensity, whereas other physiologic responses such as heart rate, blood pressure and ventilation are inversely related to the size of the active muscle mass when expressed as functions of oxygen uptake. The purpose of the current investigation was to determine whether congestive heart failure alters the pattern of physiologic responses to various modes of arm and leg exercise in proportion to the size of the active muscle mass. Cardiopulmonary responses to four modes of dynamic work (one arm curl, one arm cycle ergometry, one leg cycle ergometry and two leg cycle ergometry) were characterized in terms of absolute and relative intensities (oxygen uptake and mode-specific percent of peak oxygen uptake, respectively) in middle-aged men with congestive heart failure and control groups of healthy subjects and patients after myocardial infarction without heart failure. Peak oxygen uptake was reduced to the greatest extent in patients with heart failure for large muscle mass work (-13% for curl, -32% for one arm and one leg cycle ergometry and -37% for two leg cycle ergometry; p less than 0.05 versus the normal group for the three modes of ergometry). This finding was paralleled by a markedly blunted slope for the cardiac output-oxygen uptake relation for leg but not arm exercise that was only partially compensated for by a widened arteriovenous oxygen difference. Blood pressure expressed as a function of oxygen uptake remained inversely related to active muscle mass size in all groups of subjects despite attenuation of systolic pressure for heavy large muscle mass effort in the group with heart failure. Pulmonary ventilation at a given metabolic rate was not influenced by active muscle mass size. Thus, saturation of capacity for systemic oxygen transport occurs in conjunction with blunted cardiac output reserve in patients with heart failure during exercise involving a smaller muscle mass than in healthy subjects. The basic inverse relation between size of the active muscle mass and blood pressure at a given metabolic rate is not altered by aging or reduced cardiac reserve. The muscle mass effect on ventilation seen in young healthy subjects disappears with aging.     

Resistance training,visceral obesity and inflammatory response: a review of the evidence

Intra‐abdominal obesity is an important risk factor for low‐grade inflammation, which is associated with increased risk for diabetes mellitus and cardiovascular disease. For the most part, recommendations to treat or prevent overweight and obesity via physical activity have focused on aerobic endurance training as it is clear that aerobic training is associated with much greater energy expenditure during the exercise session than resistance training. However, due to the metabolic consequences of reduced muscle mass, it is understood that normal ageing and/or decreased physical activity may lead to a higher prevalence of metabolic disorders. Whether resistance training alters visceral fat and the levels of several pro‐inflammatory cytokines produced in adipose tissue has not been addressed in earlier reviews. Because evidence suggests that resistance training may promote a negative energy balance and may change body fat distribution, it is possible that an increase in muscle mass after resistance training may be a key mediator leading to a better metabolic control. Considering the benefits of resistance training on visceral fat and inflammatory response, an important question is: how much resistance training is needed to confer such benefits? Therefore, the purpose of this review was to address the importance of resistance training on abdominal obesity, visceral fat and inflammatory response.     

Sarcopenia and Osteoporotic Fractures

Low bone mass is strongly associated with increased fracture risk. However, the importance of low muscle mass and strength—known as sarcopenia—as a risk factor for osteoporotic fractures remains overlooked and sometimes controversial. Bone and muscle are closely interconnected not only anatomically, but also physically, chemically and metabolically. Indeed, a significant proportion of individuals with sarcopenia also suffer from osteopenia/osteoporosis suggesting a link between the two tissues. This subgroup of osteosarcopenic individuals are at higher risk of falls and fractures. Therefore, we suggest that lean mass and muscle strength/function assessments should be an integral part in any fracture prevention protocol. A combination of lean mass quantification by dual-energy X-ray absorptiometry scan and assessment of muscle function by gait velocity could not only confirm the diagnosis of sarcopenia but also optimize any fracture prevention interventions. In the absence of specific therapies for sarcopenia, simple interventions such as resistance (weight-bearing) training, protein supplements and appropriate levels of vitamin D have a dual effect on bone and muscle and could have a significant effect on reducing falls and fractures in this high-risk population.     

A novel therapeutic approach to treating obesity through modulation of TGFβ signaling

Obesity results from disproportionately high energy intake relative to energy expenditure. Many therapeutic strategies have focused on the intake side of the equation, including pharmaceutical targeting of appetite and digestion. An alternative approach is to increase energy expenditure through physical activity or adaptive thermogenesis. A pharmacological way to increase muscle mass and hence exercise capacity is through inhibition of the activin receptor type IIB (ActRIIB). Muscle mass and strength is regulated, at least in part, by growth factors that signal via ActRIIB. Administration of a soluble ActRIIB protein comprised of a form of the extracellular domain of ActRIIB fused to a human Fc (ActRIIB-Fc) results in a substantial muscle mass increase in normal mice. However, ActRIIB is also present on and mediates the action of growth factors in adipose tissue, although the function of this system is poorly understood. In the current study, we report the effect of ActRIIB-Fc to suppress diet-induced obesity and linked metabolic dysfunctions in mice fed a high-fat diet. ActRIIB-Fc induced a brown fat-like thermogenic gene program in epididymal white fat, as shown by robustly increased expression of the thermogenic genes uncoupling protein 1 and peroxisomal proliferator-activated receptor-γ coactivator 1α. Finally, we identified multiple ligands capable of reducing thermogenesis that represent likely target ligands for the ActRIIB-Fc effects on the white fat depots. These data demonstrate that novel therapeutic ActRIIB-Fc improves obesity and obesity-linked metabolic disease by both increasing skeletal muscle mass and by inducing a gene program of thermogenesis in the white adipose tissues.     

The microcirculation of skeletal muscle in aging

Microvascular structure and function are key aspects of tissue and organ health. At approximately 40% of total body mass, skeletal muscle contains more microvessels than any other organ system in the body. Moreover, skeletal muscle is the most dynamic tissue in the body with the capacity to increase blood flow and metabolic rate 30- to 50- fold. Aging is associated with decrements in microvascular function and exercise tolerance that are poorly understood. Here, experts in their respective fields of microvascular structure and function are brought together to review the current state of knowledge regarding microvascular adaptations to aging. Reviews are drawn from human and animal studies and focus on age-related changes in sympathetic nervous system control of microvessels, capillary hemodynamics and oxygen pressure, microvascular network structure and functional integration, microvascular reactivity, whole muscle perfusion, and cellular contacts and inflammation.     

Adenosine 5'-monophosphate-activated protein kinase regulation of fatty acid oxidation in skeletal muscle

AMP-activated protein kinase (AMPK) is master regulator of energy balance through suppression of ATP-consuming anabolic pathways and enhancement of ATP-producing catabolic pathways. AMPK is activated by external metabolic stresses and subsequently orchestrates a complex downstream signaling cascade that mobilizes the cell for efficient energy production. AMPK has emerged as a key kinase driving lipid oxidation in skeletal muscle, and this function has important implications for exercise adaptations as well as metabolic defects associated with obesity.     

Regulation of energy metabolism by creatine in cardiac and skeletal muscle cells in culture

Cardiac and skeletal muscle cells grown in culture were used in experiments designed to test the regulatory function of creatine. An increased concentration of intracellular phosphorylcreatine resulted from the addition of creatine to the growth media and the addition of metabolic inhibitors prevented the new high steady state concentration without depletion of ATP. With 1-fluoro-2, 4-dinitrobenzene the high concentration of phosphorylcreatine remained unchanged, while ATP was depleted. The use of the inhibitors provided additional evidence for creatine as the effector molecule in a feedback regulation of energy production, and for phosphorylcreatine in the regulation of energy charge at the contractile site. The effect of creatine on the fusion of myoblasts into myotubes supported its role in the control of cellular functions other than those involved in the energy metabolism. The correlation of the growth of muscle with increased muscular activity might be mediated through creatine, the end product of muscle activity. Creatine, synthesized in tissues other than muscle, might be involved in the coordination of muscle development throughout the body. Furthermore, it is suggested that the metabolic regulatory function of creatine is limited to the cells which do not synthesize creatine de novo, while in the tissues, such as the liver and kidney which have the mechanism for de novo synthesis of creatine, the absence of mitochondrial CPK precludes metabolic regulation of ～ P synthesis by creatine.     

The principles of metabolic therapy for heart disease

Metabolic therapy involves the administration of a substance normally found in the body to enhance a metabolic reaction within the cell. This may be achieved in two ways. First, for some systems, a substance can be given to achieve greater than normal levels in the body so as to drive an enzymic reaction in a preferred direction. Second, metabolic therapy may be used to correct an absolute or relative deficiency of a cellular component. Thus, metabolic therapy differs greatly from most standard cardiovascular pharmacologic therapy such as the use of ACE Inhibitors b-blockers, statins and calcium channel antagonists that are given to block rather than enhance cellular processes. In this review we highlight some metabolic substances that have potential benefit in treating heart disease or improving outcomes after cardiovascular interventions. Glucose-insulin-potassium therapy is protective against myocardial ischaemia by elevating myocardial glycogen levels. Coenzyme Q(10) is a lipid-soluble antioxidant that plays a crucial role in cellular ATP production. Magnesium orotate, a key intermediate in the biosynthetic pathway of glycogen, has been shown to improve the energy status of the cell and improve recovery from cardioplegic arrest. The amino acid aspartate plays an important role in providing energy substrates for oxidative phosphorylation in the myocyte. By improving cellular energy production, metabolic therapy has the potential to benefit cardiac function during the stress of cardiac surgery, myocardial infarction and cardiac failure.     

Importance of anti-angiogenic factors in the regulation of skeletal muscle angiogenesis

The microcirculation is essential for delivery of oxygen and nutrients to maintain skeletal muscle health and function. The network of microvessels surrounding skeletal myocytes has a remarkable plasticity that ensures a good match between muscle perfusion capacities and myofiber metabolic needs. Depending on physiologic conditions, this vascular plasticity can either involve growth (e.g., exercise-induced angiogenesis) or regression (e.g., physical deconditioning) of capillaries. This angio-adaptative response is thought to be controlled by a balance between pro- and anti-angiogenic factors and their receptors. While changes in the expression or activity for pro-angiogenic factors have been well studied in response to acute and chronic exercise during the past two decades, little attention thus far has been devoted to endogenous negative regulators that are also likely to be important in regulating capillary growth/regression. Indeed, the importance and contribution of anti-angiogenic factors in controlling skeletal muscle angiogenesis remains poorly understood. Here, we highlight the emerging research related to skeletal muscle expression of several negative angiogenic factors and discuss their potential importance in controlling skeletal muscle angio-adaptation, particularly in physiologic response to physical activity.     

Role of energy charge and AMP-activated protein kinase in adipocytes in the control of body fat stores

As indicated by in vitro studies, both lipogenesis and lipolysis in adipocytes depend on the cellular ATP levels. Ectopic expression of mitochondrial uncoupling protein 1 (UCP1) in the white adipose tissue of the aP2-Ucp1 transgenic mice reduced obesity induced by genetic or dietary manipulations. Furthermore, respiratory uncoupling lowered the cellular energy charge in adipocytes, while the synthesis of fatty acids (FA) was inhibited and their oxidation increased. Importantly, the complex metabolic changes triggered by ectopic UCP1 were associated with the activation of AMP-activated protein kinase (AMPK), a metabolic master switch, in adipocytes. Effects of several typical treatments that reduce adiposity, such as administration of leptin, beta-adrenoceptor agonists, bezafibrate, dietary n-3 polyunsaturated FA or fasting, can be compared with a phenotype of the aP2-Ucp1 mice. These situations generally lead to the upregulation of mitochondrial UCPs and suppression of the cellular energy charge and FA synthesis in adipocytes. On the other hand, FA oxidation is increased. Moreover, it has been shown that AMPK in adipocytes can be activated by adipocyte-derived hormones leptin and adiponectin, and also by insulin-sensitizes thiazolidinediones. Thus, it is evident that metabolism of adipose tissue itself is important for the control of body fat content and that the cellular energy charge and AMPK are involved in the control of lipid metabolism in adipocytes. The reciprocal link between synthesis and oxidation of FA in adipocytes represents a prospective target for the new treatment strategies aimed at reducing obesity.     

Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate

It has long been accepted that thyroid hormone is an important determinant of overall energy expenditure and the basal metabolic rate. Indeed, regulating thermogenesis is one of the major tasks of thyroid hormone in adult humans. A wealth of data have demonstrated the effects of thyroid hormone on cellular processes involved with energy expenditure, yet in spite of this body of work it remains unclear which 3,3'-triiodothyronine-responsive energetic processes are most relevant for the determination of the basal metabolic rate. Recently, a novel metabolic role for thyroid hormone has been recognized based on the observation that bile acids can activate local production of thyroid hormone via induction of the type 2 deiodinase. Nevertheless, more work must be done before it can be fully explained how thyroid hormone determines the metabolic rate.     

α-Lipoic acid increases energy expenditure by enhancing adenosine monophosphate-activated protein kinase-peroxisome proliferator-activated receptor-γ coactivator-1α signaling in the skeletal muscle of aged mice

Skeletal muscle mitochondrial dysfunction is associated with aging and diabetes, which decreases respiratory capacity and increases reactive oxygen species. Lipoic acid (LA) possesses antioxidative and antidiabetic properties. Metabolic action of LA is mediated by activation of adenosine monophosphate-activated protein kinase (AMPK), a cellular energy sensor that can regulate peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a master regulator of mitochondrial biogenesis. We hypothesized that LA improves energy metabolism and mitochondrial biogenesis by enhancing AMPK-PGC-1α signaling in the skeletal muscle of aged mice. C57BL/6 mice (24 months old, male) were supplemented with or without α-LA (0.75% in drinking water) for 1 month. In addition, metabolic action and cellular signaling of LA were studied in cultured mouse myoblastoma C2C12 cells. Lipoic acid supplementation improved body composition, glucose tolerance, and energy expenditure in the aged mice. Lipoic acid increased skeletal muscle mitochondrial biogenesis with increased phosphorylation of AMPK and messenger RNA expression of PGC-1α and glucose transporter-4. Besides body fat mass, LA decreased lean mass and attenuated phosphorylation of mammalian target of rapamycin (mTOR) signaling in the skeletal muscle. In cultured C2C12 cells, LA increased glucose uptake and palmitate β-oxidation, but decreased protein synthesis, which was associated with increased phosphorylation of AMPK and expression of PGC-1α and glucose transporter-4, and attenuated phosphorylation of mTOR and p70S6 kinase. We conclude that LA improves skeletal muscle energy metabolism in the aged mouse possibly through enhancing AMPK-PGC-1α-mediated mitochondrial biogenesis and function. Moreover, LA increases lean mass loss possibly by suppressing protein synthesis in the skeletal muscle by down-regulating the mTOR signaling pathway. Thus, LA may be a promising supplement for treatment of obesity and/or insulin resistance in older patients.