Metabolic signals regulate and integrate many vital functions throughout the human body, including energy homeostasis, blood pressure, heart performance, food intake, hormonal status and brain performance [30
]. Recent evidence suggests the general importance of adenylate kinase-mediated AMP signaling in appetite, wakefulness and sleep control and in hormonal, food and antidiabetic drug actions which are coupled to alterations of cellular AMP levels and associated signaling [4
]. AMP signaling also plays an important role in hypoxic response, immune function and taste reception [60
]. Adenylate kinase integrates and tunes AMP signals coming from different sources and delivers them to metabolic sensors to elicit a regulatory response (). Reduced or increased adenylate kinase activity would distort integration of AMP signals and, depending on tissue metabolic phenotype and intensity of AMP-generating reactions, could produce both positive and negative impact on the activity of metabolic sensors [42
AMPing up and down — integration cellular AMP signals by adenylate kinase.
Recent evidence suggests that ingestion of fructose, a major constituent of sugar and high-fructose corn syrup, causes increase in cellular AMP and AMP/ATP ratio leading to activation of AMPK [68
]. Contrary to glucose, which usually lowers cellular AMP levels and inhibits AMPK, fructose activates hypothalamic AMPK and increases food consumption () [68
]. Compared with glucose-sweetened beverages, consumption of fructose-sweetened beverages with meals elevates postprandial plasma triglycerides and lowers 24-h insulin and leptin profiles in normal weight women [154
]. Fructose metabolism bypasses the rate-limiting step of the glucose pathway, and is metabolized much more quickly than glucose. Excessive fructose intake depletes cellular ATP by trapping inorganic phosphate required for ATP resynthesis, consequently inducing nucleotide degradation and increasing plasma uric acid and allantoin levels [155
]. These metabolic alterations induced by fructose may play an important role in the epidemic of metabolic syndrome and may contribute the development of diabetes and cardiovascular disease [156
]. A diet high in fructose can give rise to hyperlipidemia, insulin resistance and hypertension [157
]. Similarly, ethanol consumption and subsequent acetate metabolism causes AMP accumulation in liver and other tissues resulting in increased AMP and adenosine signaling and blood flow [158
]. Excess ethanol consumption can also cause nucleotide degradation and depletion of cellular adenine nucleotide pool [160
]. However, short term and limited exposure to fructose and ethanol could be beneficial in stimulating energy metabolism and metabolic signaling, thus “AMPing” your body [153
]. In this regard, cardioprotection induced by both fructose and ethanol preconditioning stimuli could be related to augmented AMP signaling [162
Normal human blood AMP levels are in the 10–20 μM range of which only a fraction is free due to binding to serum proteins [164
]. Intracellular and blood AMP levels are increased during physical activity and are sensitive indicators of myocardial ischemia rising within minutes after insult [165
]. Adenylate kinase isoforms provide fine tuning of intracellular, interstitial and circulating AMP levels due to different kinetic properties and localization (). Deficiency of AK1 isoform reduces the AMP signal stress response [4
], however at the basal level it could result in higher AMP signaling due to compromised tune-up mechanisms [152
]. Circulating AMP is emerging as a molecular mediator of hibernation and constant darkness effect, switching mice from a glucoseburning, fat-storing state to a fat-burning, glucose-conserving lethargy [66
]. In hibernating mammals AMP originating, at least partly, from the brown fat also down-regulates the seasonally-dependent proliferation of the thymus [167
]. In addition, overworked brains release adenosine, usually originating from AMP, to slow cell activity and trigger sleep [168
]. These data strongly support the notion that AMP and adenosine play a key role as endogenous modulators of wakefulness and sleep which fits with our proposed role of phosphotransfer reactions in regulating brain activity and information processing [30
Regulation of intracellular AMP levels.
Available evidence suggests that growth factors, which alter cellular energy metabolism, and hormones, such as adipocyte-derived leptin and adiponectin, activate AMPK through local or temporal regulation of AMP levels [170
]. Leptin alters muscle adenine nucleotide homeostasis (decreases ATP) and increases AMP dynamics (inferred from elevated AMP and IMP levels) followed by activation of AMPK [173
]. This could be also due to increased expression of uncoupling proteins (UCP) - mitochondrial carriers that dissipate the electrochemical gradient across the mitochondrial inner membrane [176
]. Mild uncoupling of mitochondria shift cellular nucleotide balance and, due to the monitoring function of adenylate kinase, AMP levels would go up triggering metabolic signaling cascades [176
]. Interestingly, AMP by itself through interaction with ANT can produce uncoupling that could facilitate respiration and reduce ROS production [178
], which could be beneficial during ischemia reperfusion.
AMP signaling plays a significant role in cellular senescence. It’s been shown by proteome analysis that AK1 protein is markedly increased in senescent skeletal muscle fibers [179
] and that lifespan of worms can be extended by the addition of copies of the AMPK gene and by chronic activation of AMPK as is seen on calorie-restricted diets [180
]. Indeed, AMP/ATP ratios are several folds higher in senescent fibroblasts compared with young fibroblasts and this is accompanied by a marked elevation in AMPK activity [181
]. This could be viewed as a compensatory measure to cope with declining capacity of energy metabolism during aging. AMP and AMPK signaling is critical in cell differentiation, maintaining cell polarity and completing normal cell division as well as in induction of meiotic maturation in mammalian oocytes [183
]. AMP directly or through AMPK plays a physiological role in modulating activity of cystic fibrosis transmembrane conductance regulator (CFTR) in polarized epithelia cells [186
]. CFTP nucleotide binding folds possess an intrinsic adenylate kinase activity which could facilitate metabolic signal reception [61
]. Thus, AMP signaling plays a critical role in altered hormonal and energetic states including cell differentiation, maintenance of polarity and senescence.
AMP is important mediator of insulin and metabolic protein kinase Akt signaling. It has been proposed that protein kinase Akt mediates inhibitory effects of insulin on AMPK [189
]. Since Akt does not directly phosphorylate AMPK, changes in Akt activity induced by insulin can regulate AMPK through changes in cellular AMP/ATP ratio. Indeed, it was demonstrated that Akt activation reduces cellular AMP/ATP ratio causing decline in AMPK activity [190
]. Insulin and Akt-mediated inhibition of AMPK can be overcome by metformin, which is known to act on site I of mitochondrial respiratory chain causing increase in AMP levels [189
]. In this regard, free fatty acids, which activation generates AMP, prevents AMPK inhibition by insulin [192
]. Thus, insulin-Akt signaling axis can expand the range of metabolic effects through tuning up AMP signals and the activity of AMPK.
Most importantly, recent studies indicate that pharmacological actions of popular antidiabetic drugs metformin and thiazolidinediones and cholesterol lowering statins are related to their ability to alter cellular AMP levels and consequently AMPK activity [151
]. Time course studies revealed that troglitazone-induced increases in phosphorylated forms of AMPK and ACC are paralleled by an increase in the AMP-to-ATP ratio [193
]. A similar increase in AMP is seen following incubation of cells with rosiglitazone [196
]. Moreover, livers of rats treated with resveratrol, a constituent of red grapes and red wine, show a strong tendency for AMPK activation, as well as increase phosphorylation of two downstream indicators of its activity [197
]. Besides inhibiting HMG-CoA reductase, statins preserve CD39/ATPDase activity in thrombin-treated endothelial cells [198
]. Preservation of adenine nucleotide metabolism may directly contribute to the observed anti-thrombotic and anti-inflammatory actions of statins. In this regard, metformin, a biguanidine compound from French lilac and more recently extracts from bitter melon, a traditional Chinese medicine, activate AMPK and exerts its glucose lowering effect through mild interference with the efficiency of energy metabolism resulting in changes in intracellular nucleotide dynamics and AMP levels [151
]. Through activating AMP signaling metformin also improves cardiac function after ischemia in rats [200
]. Interestingly that metformin increases glucose utilization and lactate production in cells with a dominant-negative mutant form of AMPK (DN-AMPK) [202
], suggesting existence of AMPKindependent pathways of metabolic signaling including direct effects of AMP on other metabolic sensors and enzymes of energetic pathways.
Recent interest in clinical use of adenosine as “adenosine flush” and in reperfusion therapy is also associated with AMP signaling, activation of AMPK and replenishment of cellular ATP levels [203
]. Adenosine, besides signaling through adenosine receptors, can be taken up by cells and phosphorylated to AMP initiating metabolic signaling cascades [104
]. Subsequent conversion of AMP to ADP and ATP by reactions involving adenylate kinase and ATP synthesis in mitochondrial oxidative phosphorylation and glycolysis would replenish cellular ATP and total adenine nucleotide pool which is diminished during ischemic insult [104
]. Thus, adenosine and AMP have pleiotropic metabolic signaling and energetic effects which could be further explored in reperfusion therapy.
Last but not least, AMP through interaction with taste receptors has the bitterness-suppressing quality that allows taste buds to interpret food as seeming “sweeter” [207
]. This makes lower-calorie food products more palatable. Recently AMP has been approved by the FDA as a ‘Bitter Blocker’ additive to foodstuffs (Linguagen Corp.). AMP is found in a wide range of natural foods — including breast milk. Calcium compounds in breast milk are usually bitter and thus breast milk may be natural system using bitter-blockers. AMP also inhibits behavioral and electrophysiological responses of mice to bitter tastants [207
]. A number of AMP containing medications are used for nutritional supplementation and for treating dietary shortage or imbalance and disease conditions [208
]. Nucleotides such as AMP affect a number of immune functions, including the reversal of malnutrition and starvation-induced immunosuppression due to adenine nucleotide shortage, the enhancement of Tcell maturation and function and natural killer cell activity [208
]. However AMP by itself has immunosuppressive properties. In fact, mosquito and fly saliva contains AMP and adenosine as vasodilatative agents which also have immunosuppressant activity facilitating pathogen or parasite transmission [209
]. To this end, secreted and cell associated adenylate kinase has been identified as a virulence factor in a number of pathogens affecting nucleotide signaling and host immune defense [210
]. Due to immunomodulatory function, a promising therapeutic potential for the AMP analog AICAR/(ZMP) exist in the treatment of multiple sclerosis and other Th1 cell-mediated inflammatory diseases such as psoriasis and arthritis [211
]. Thus, AMP is emerging as pivotal metabolic signal conveying information about food consumption, hormonal and energy metabolism status, a regulator of brain activity associated with wakefulness and appetite control as well as a mediator of drug action and therapeutic agent.