Review articles
AMP-activated protein kinase: the energy charge hypothesis revisited D. Grahame Hardie* and Simon A. Hawley
Summary The AMP-activated protein kinase cascade is a sensor of cellular energy charge, and its existence provides strong support for the energy charge hypothesis first proposed by Daniel Atkinson in the 1960s. The system is activated in an ultrasensitive manner by cellular stresses that deplete ATP (and consequently elevate AMP), either by inhibiting ATP production (e.g., hypoxia), or by accelerating ATP consumption (e.g., exercise in muscle). Once activated, it switches on catabolic pathways, both acutely by phosphorylation of metabolic enzymes and chronically by effects on gene expression, and switches off many ATP-consuming processes. Recent work suggests that activation of AMPK is responsible for many of the effects of physical exercise, both the rapid metabolic effects and the adaptations that occur during training. Dominant mutations in regulatory subunit isoforms (g2 and g3) of AMPK, which appear to increase the basal activity in the absence of AMP, lead to hypertrophy of cardiac and skeletal muscle respectively. BioEssays 23:1112±1119, 2001. ß 2001 John Wiley & Sons, Inc. Introduction One of the most fundamental parameters that any healthy cell must maintain is a high ratio of ATP to ADP (of the order of 10:1). Almost all energy-requiring processes in the cell are driven, either directly or indirectly, by hydrolysis of one or other of the acid anhydride bonds in ATP, yielding ADP or AMP (reactions 1 and 2, Box 1). Healthy cells maintain the reactants and products of these two reactions many orders of magnitude away from their equilibrium ratios. This is why ATP hydrolysis is able to perform useful work when coupled to processes requiring an input of energy. A useful analogy can be drawn between the adenine nucleotides in a living cell and the chemicals in an electrical cell or battery (strictly, the latter term refers to a number of cells arranged in series). The ``battery'' of the cell is charged up by catabolism or photosynthesis, converting ADP and Pi to ATP (reaction 3, Box 1). Almost all other cellular processes are coupled to ATP breakdown and
Division of Molecular Physiology, Dundee University, DUNDEE, Scotland. *Correspondence to: D. Grahame Hardie, Wellcome Trust Biocentre, Division of Molecular Physiology, School of Life Sciences, Dundee University, DUNDEE DD1 5EH, Scotland, UK. E-mail:
[email protected]
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Box 1: Reactions interconverting ATP, ADP, and AMP ATPases: ATP!ADP Pi.....................(1) Ligases: ATP!AMP PPi...................(2) ATP synthases: ADP Pi!ATP.............................(3) Adenylate kinase: 2ADP$ATP AMP.................(4) If adenylate kinase is at equilibrium: ATPAMP ADP2
K
;ATPAMP K ADP2 Divide both sides of the equation by [ATP]2: AMP ADP 2 ; / ATP ATP i.e., the AMP:ATP ratio varies as the square of the ADP:ATP ratio.
therefore tend to discharge the ``battery''. Given the critical importance to the cell of the maintenance of appropriate ratios of ATP:ADP and ATP:AMP, it is not surprising that sophisticated mechanisms to regulate these ratios have evolved. Work over the last decade has highlighted the key role of the AMP-activated protein kinase cascade in this process.(1±3) That the nucleotides themselves should be the signals that mediate this regulation is, with hindsight, rather obvious, but it appears to have been first proposed by Daniel Atkinson in 1964.(4) By the early 1960s a small number of metabolic enzymes (e.g., muscle phosphorylase and phosphofructokinase) had been shown to be allosterically regulated by adenine nucleotides, with AMP and ATP tending to act in reciprocal directions. Atkinson extrapolated from these initial examples and proposed that adenine nucleotides would regulate all branch points between anabolism and catabolism. He called this the adenylate control hypothesis or (drawing on the analogy with an electrical cell described above) the energy charge hypothesis. The idea stimulated considerable interest at the time, but rather few additional enzymes regulated directly by adenine nucleotides were subsequently discov-
BioEssays 23:1112±1119, ß 2001 John Wiley & Sons, Inc. DOI 10.1002/bies.10009
Review articles ered. Thus the idea remained somewhat in abeyance until the arrival on the scene of the AMP-activated protein kinase. At this point, it is worth discussing why AMP, rather than ADP, should be the key regulatory molecule. Eukaryotic cells have a very active adenylate kinase that interconverts ATP, ADP and AMP (reaction 4, Box 1), and maintains this reaction close to equilibrium. If it is at equilibrium, the AMP:ATP ratio will vary as the square of the ADP:ATP ratio (see lower part of Box 1). Healthy cells under ideal conditions maintain an ADP:ATP ratio of the order of 1:10 due to the operation of ATP synthases (reaction 3). Under these conditions, adenylate kinase will operate from right to left, keeping AMP very low (AMP:ATP 1:100). If a cellular stress causes the rate of ATPases (reaction 1) to exceed that of the ATP synthases (reaction 3), however, the ADP:ATP ratio will rise, and the adenylate kinase reaction will operate from left to right, generating AMP. If the ADP:ATP ratio rises by 5-fold, the AMP:ATP ratio will rise 25-fold. Thus, as pointed out by Hans Krebs in 1963,(5) the cellular concentrations of AMP change much more dramatically than do those of ATP or ADP. It therefore makes sense for any system that monitors cellular energy status to respond to AMP (or the AMP:ATP ratio) rather than ADP (or the ADP:ATP ratio). Structure and regulation of AMP-activated protein kinase AMP-activated protein kinase was originally discovered by its ability to inactivate HMG-CoA reductase(6) and acetyl-CoA carboxylase.(7) In 1980 Kim's group reported that an acetylCoA carboxylase kinase that they were studying was stimulated by 50 -AMP, and suggested that it might inhibit fatty acid synthesis in response to falling energy charge.(8) Five years later Hegardt's group reported that an HMG-CoA reductase kinase was also stimulated by AMP.(9) Shortly after this our laboratory provided evidence that a single protein kinase could account for both of these observations.(10) When it became clear that the kinase had multiple physiological substrates we renamed it the AMP-activated protein kinase (AMPK) after its allosteric activator.(11) AMPK is now known to exist as heterotrimeric complexes comprising a, b and g subunits (Fig. 1). In mammals, each subunit is encoded by two or three genes (a1, a2, b1, b2, g1, g2, g3) and at least 12 heterotrimeric combinations are possible.(12±14) The a subunit (63 kDa) contains the kinase domain at the N terminus, plus a C-terminal regulatory domain containing an autoinhibitory region that inhibits the kinase in the absence of AMP.(15) The b subunits appear to be the scaffold on which a and g assemble, via binding to their conserved KIS and ASC domains, respectively.(13) The g subunits contain four tandem repeats of a structural module called a CBS domain, examples of which are also found in various other proteins.(16) The functions of CBS domains are not known, although the example in cystathione b-synthase (after which they are named) appears to be
involved in allosteric activation by S-adenosyl methionine.(17) It is therefore tempting to speculate that one or more of the CBS domains of the g subunit of AMP is involved in binding the adenosine moiety of AMP. In support of this, the photoaffinity analogue 8-azido-[32P]AMP labels the g subunit of rat liver AMPK, and this is specifically prevented by the presence of AMP.(14) A current model to explain AMP activation(14) proposes that AMP binds in the interface between the a and g subunits, preventing association of the autoinhibitory segment of the a subunit with the kinase domain on the same subunit (Fig. 1). However, the exact location of the AMPbinding site is currently not well defined. The degree of activation by AMP depends on the particular isoform of a and g present in the complex,(14) and can be up to 13-fold with recombinant a2b1g1 complex.(18) This allosteric activation is only a part, however, and indeed a small part, of the activation mechanism. AMPK is activated 50- to 100-fold by an upstream kinase (AMP-activated protein kinase kinase or AMPKK, whose molecular identity remains unclear)
Figure 1. Model for the changes in interdomain interactions in the AMPK complex, based in part on two-hybrid analysis in the yeast system.(65,66) In both the inactive and active conformations, the b subunit acts as a ``scaffold'' that binds a and g via the conserved KIS and ASC domains. In the inactive, T conformation (top) the kinase domain of a is inhibited by interactions with the autoinhibitory region on the same subunit. In the active, R conformation (bottom), this interaction is prevented because the autoinhibitory region on a now interacts with the CBS domains on g, instead of with the kinase domain. AMP promotes this conformation by stabilizing the a$g interaction, while ATP binding at the allosteric site would disrupt it. In the active conformation, the kinase domain is free to be phosphorylated and activated by the upstream kinase, and to phosphorylate downstream targets.
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Review articles which phosphorylates a threonine residue (Thr-172) within the ``activation loop'' of the kinase domain.(18,19) AMP promotes this phosphorylation both by binding to dephospho-AMPK and making it a better substrate, and by activating AMPKK itself.(20) In addition, AMP binding to phospho-AMPK almost completely prevents dephosphorylation of Thr-172.(21) This complex mechanism of AMP activation is summarized in Figure 2. The multiple effects of AMP, coupled with a very low Km of AMPKK for AMPK, make the AMPK cascade an ultrasensitive system in which, over a critical range of concentrations, there is a large response to a small increase in AMP. Our laboratory recently showed that there is a sigmoidal (i.e., ultrasensitive) response to the concentration of the activating nucleotide in intact cells.(22) Through the action of adenylate kinase (Box 1), AMP and ATP almost always vary in reciprocal directions in the cell. Although both AMPK and AMPKK require low concentrations of ATP to function as protein kinases, high (mM) concentrations of ATP inhibit activation of the system by antagonizing binding of AMP at the allosteric site(s) on AMPK. In a manner highly analogous to the effects of these nucleotides on muscle phosphorylase and phosphofructokinase, rising AMP and falling ATP activate the AMPK system, which therefore acts as an ``energy charge sensor''. AMPK is also allosterically inhibited by phosphocreatine at concentrations that lie within the physiological range.(23) Since phosphocreatine in muscle and some other cells acts as a phosphagen, i.e., a short-term reservoir of ATP, this fits in well with the energy sensor concept. Phosphocreatine appears to have no effect on phosphorylation of AMPK by AMPKK (DGH and SAH, unpublished).
Figure 2. Model for the regulation of AMPK by AMP and by phosphorylation, based on the classical Monod/Wyman/ Changeux model for allosteric enzymes.(67) AMPK is proposed to exist in two conformations, i.e., R and T, each of which can also exist in phosphorylated and dephosphorylated forms, making four states in all. AMP binding promotes the T!R transitions by stabilizing the R states (see Fig. 1). Only the R state is a substrate for AMPKK, while only the T state is a substrate for the protein phosphatase. The figures in square boxes indicate the approximate kinase activity of that form, relative to that of the phosphorylated T state. The activity of the phosphorylated R state (bottom right) relative to that of the phosphorylated T state (bottom left) varies according to the identity of the a and g subunit isoform. Redrawn from Ref. 2
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Physiological regulation of AMP-activated protein kinase A misconception sometimes encountered by the authors is that adenine nucleotides cannot be important cellular regulators because their concentrations never vary. On inspection of the pathways and processes by which ATP is produced and consumed, however, it is not at all obvious why they should remain so perfectly balanced. Observations that the levels of ATP, ADP and AMP are usually very constant in cells may merely reflect the success of the systems that preserve this balance. Because of this, the easiest way to observe AMPK in action is to look at situations of cellular stress where this normal balance has been disturbed. Thus, AMPK is activated by any stress treatment that interferes with ATP production. Such stresses include heat shock,(24) metabolic poisons,(24) glucose deprivation (hypoglycaemia),(25) oxygen deprivation (hypoxia),(26) and interruption of the blood supply (ischaemia, which can be regarded as a combination of hypoxia and hypoglycaemia).(27) These are all abnormal, pathological events, but a physiological stress that activates AMPK by increasing ATP consumption is exercise in skeletal muscle, as demonstrated in both animals(28) and humans.(29,30) Using transgenic mice in which AMPK activity was almost abolished in muscle by expression of a dominant negative mutant, Birnbaum's group have recently shown that the effect of contraction on muscle glucose uptake was partly eliminated, while the effect of hypoxia was completely eliminated.(31) The mice were also ``lazy'' in that they took less voluntary exercise, presumably because their muscles became fatigued more easily. This work elegantly confirms the important role of AMPK in the response of muscle to exercise and hypoxia. Much of the recent growth in interest in the AMPK system has stemmed from this aspect of its function, and its potential role in mediating the beneficial effects of exercise on humans with Type 2 diabetes(32) (see below). Homologues in other species Recent genome sequencing has revealed that there are homologues of the a, b, and g subunits of AMPK throughout the eukaryotic domain, including Arabidopsis, Dictyostelium, Drosophila and C. elegans. However the non-mammalian species in which an AMPK homologue is best characterized is the yeast Saccharomyces cerevisiae, where the homologues of the a and g subunits are termed Snf1p and Snf4p,(33) which form complexes with one of three alternate b subunits termed Sip1p, Sip2p and Gal83p. Deletion of either SNF1 or SNF4 (or all three b subunit genes) produces a similar phenotype characterized by failure to grow on carbon sources other than glucose. In the presence of glucose (the preferred carbon source) many genes are repressed, and functional SNF1 complexes are required for their derepression. When glucose is removed from cells in mid-log phase, there is a dramatic activation of the SNF1 kinase due to phosphorylation(34) that is
Review articles associated with very large increases in AMP and decreases in ATP. However, AMP does not appear to activate the SNF1 kinase in cell-free assays, so it remains unclear whether a rise in the AMP:ATP ratio is the signal that switches on the yeast kinase in vivo.(34) What is known is that derepression of many glucose-repressed genes involves a direct phosphorylation of the repressor protein Mig1 by the SNF1 kinase, causing the repressor to become inactive and to be exported from the nucleus.(35,36) This provides an interesting model to explain how AMPK might regulate gene expression in mammals (see below). Targets of AMP-activated protein kinase In his paper in 1964(4) Atkinson wrote: ``These indications that the level of AMP, or the AMP to ATP ratio, may regulate the metabolic direction (towards energy release or towards energy storage) ...... suggest that the level of AMP may function more generally as a mediator of energy metabolism'' (my italics). In the last few years there has been an explosive growth in our knowledge of the downstream targets of the AMPK system, and this has fully vindicated Atkinson's far-sighted proposal. Much of this progress came from the use of 5-aminoimidazole4-carboxamide (AICA) riboside. This nucleoside is taken up into cells and phosphorylated by adenosine kinase to the monophosphorylated nucleotide, usually referred to as ZMP. ZMP mimics all four effects of AMP on the AMPK cascade(37) and, although it is much less potent than AMP itself, in most cells it accumulates to sufficiently high concentrations when they are incubated with the riboside. This represents a method for activating AMPK without disturbing the cellular levels of ATP, ADP or AMP, thus avoiding the many possible sideeffects of the latter. Its absolute specificity remains uncertain and, like any pharmacological approach, the results should be interpreted with caution. A novel alternative approach has been developed recently, in which a constitutively active form of the AMPK kinase domain is expressed from an adenoviral vector.(38) This has its own drawbacks: one is overexpressing a form of the kinase lacking the accessory subunits, which may therefore not be correctly localized in the cell. However, a role for AMPK is strengthened when the same results are obtained using both AICA riboside and the constitutively active mutant, as has been done in the case of inhibition of transcription of lipogenic genes in liver.(38) The many proposed downstream responses to AMPK activation are summarized in Table 1. A full description of them is beyond the scope of this article, but I will make a few general comments and discuss a few specific examples. In most cases, the effects have been demonstrated using AICA riboside only, although in some of these the effects are nevertheless convincing because the target protein and the site of phosphorylation on the protein has been identified and found to correspond to the site phosphorylated by AMPK in vitro. Many of the effects would change the balance between
anabolism and catabolism, exactly as predicted in Atkinson's energy charge hypothesis. For examples involving anabolism, AMPK activation acutely inhibits fatty acid, triglyceride and sterol synthesis while, in the longer term, it inhibits the expression of enzymes involved in fatty acid synthesis and gluconeogenesis. For examples involving catabolism, AMPK activation acutely stimulates glucose uptake (via both GLUT1 and GLUT4), glycolysis (at least in the heart), and fatty acid oxidation while, in the longer term, it increases the expression of GLUT4, hexokinase, and mitochondrial enzymes involved in the TCA cycle and respiratory chain in muscle (see Table 1 for details). Intriguingly, the latter long-term effects are also seen in response to exercise training, raising the important possibility that training improves athletic performance in part by causing regular AMPK activation. Whether AMPK also increases expression of other muscle proteins (e.g., contractile proteins), thus explaining the hypertrophic response to training, remains to be tested. Atkinson does not appear to have anticipated that the systems that monitor cellular energy charge would regulate processes other than metabolism. In principle, any cellular process that consumes ATP, and is not essential for shortterm survival, is a potential target for AMPK. Early examples of non-metabolic processes that may be regulated by the system include autophagy and apoptosis (Table 1). The evidence for AMPK involvement in these cases is based entirely on the use of AICA riboside and the targets for phosphorylation have not been identified, although it has been proposed that the effect on apoptosis in astrocytes is mediated by inhibition of ceramide synthesis.(39) Involvement of AMPK in metabolic disorders Diabetes is characterized by abnormally high blood glucose. The type 2 form (which accounts for > 90% of all cases) is not caused by a primary deficiency in insulin (as in type 1) but by reduced insulin sensitivity of the major glucose-metabolizing tissues, especially muscle, and increased glucose production by the liver. Treatment of type 2 diabetes and its complications are currently estimated to account for 10±20% of all healthcare spending in developed countries, and the incidence is rising world-wide, with projections of 200 million people being affected by the end of this decade.(40) This increase is partly due to the ageing population, but is also thought to be due to changes in lifestyle that are associated with obesity, i.e., more frequent consumption of high calorie foods and lack of physical exercise. Many of the metabolic abnormalities associated with type 2 diabetes would be expected to be reversed by activation of AMPK.(32) Indeed, this has been confirmed by chronic administration of AICA riboside to animal models of diabetes/ insulin resistance such as the obese Zucker rat, which in one study improved glucose tolerance,(41) and in another lowered plasma triglycerides and fatty acids, and reduced endogenous glucose production, presumably by inhibiting
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Table 1. Proposed downstream physiological effects of AMPK activation Pathway affected A) Rapid, Acute effects: Inhibition of anabolism: # sterol/isoprenoid synthesis # fatty acid synthesis # triacylglycerol synthesis Stimulation of catabolism: " fatty acid oxidation " glucose uptake via GLUT4 translocation via GLUT1 activation " glycolysis (heart) Other effects: # lipolysis (adipocytes) " nitric oxide production # apoptosis # autophagy Gene affected
Direct AMPK target?
Approach
(24,37)
HMG-CoA reductase Acetyl-CoA carboxylase Glycerol-P acyl transferase?
AR,* others AR,* others AR*
Acetyl-CoA carboxylase
AR*
(52,53)
Endothelial NO synthase? ? 6-phosphofructo-2-kinase
AR* AR* Ischaemia, oligomycin
(54,55)
Hormone sensitive lipase Endothelial NO synthase ? (Ceramide synthesis(39)?) ?
AR* Ischaemia AR* AR*
(37,57)
Pathway affected?
Approach
B) Chronic effects via changes in gene expression: Inhibition of gene expression: # acetyl-CoA carboxylase # Fatty acid synthesis # fatty acid synthase # Fatty acid synthesis # S14 # Lipogenesis? # L-pyruvate kinase # Glycolysis, lipogenesis # PEP carboxykinase # Gluconeogenesis Stimulation of gene expression (all effects below in skeletal muscle): " GLUT4 " Glucose uptake " hexokinase " Glycolysis " mitochondrial enzymes " Oxidative phosphorylation " UCP3 " Protection against oxidative damage? *
Reference
(24,37) (51)
(56) (26)
(58) (39,59,60) (61)
Reference
CAM** CAM** CAM** CAM** arsenite
(38)
AR* AR* AR* AR,* hypoxia
(44)
AR,* AR,* AR,* AR,* AR,*
(38) (38) (38) (62)
(44) (63) (64)
AR AICA riboside.
gluconeogenesis.(42) Chronic AICA riboside treatment also increases the insulin-sensitivity of glucose uptake in muscles isolated from normal rats(43) (possibly because of increased expression of GLUT4, Ref. 44) and prevented the development of glucose-induced insulin resistance in vitro.(45) Since AMPK is activated by exercise,(28) it might explain the beneficial effects of physical exercise on patients with type 2 diabetes.(46) There is therefore much current interest in the development of AMPK-activating drugs as potential treatments for Type 2 diabetes. Equally there is a danger that such drugs would be abused by athletes! Intriguing links between the AMPK system and disease have also come from recent studies on naturally occurring mutations. Three severe forms of familial hypertrophic cardiomyopathy, a hereditary heart disorder characterized by thickened heart muscle that results in premature deaths, have been traced to mutations in the gene encoding the g2 subunit of AMPK (which is expressed at high levels in the heart, Ref. 14). Two of these (R302Q and H383R) involve mutations affecting basic residues in equivalent positions in the CBS1 and CBS2 domains, while the third inserts an extra
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residue (leucine) between a conserved Arg-Glu in the linker between CBS1 and CBS2.(47,48) Intriguingly, a mutation in g3, an isoform that is highly expressed in skeletal muscle, produces a strain of pigs with a high carcass meat content, i.e., another hypertrophic response.(49) This mutation, R200Q, is in CBS1 of g3 and aligns perfectly with the R302Q and H383R mutations in CBS1 and CBS2 of g2. Also, a mutation in cystathione b-synthase that affects activation by S-adenosyl methionine occurs in an equivalent position in its single CBS domain (see alignment in Fig. 3). Whether these mutations cause loss of function or gain in function in AMPK is an important question. This is not known for g2 or g3, but an engineered mutation in g1 (R70Q, equivalent to R302Q (g2) or R200Q (g3)) results in a ``constitutively active'' form of the a1b1g1 complex that is less dependent on AMP and is more highly phosphorylated, and therefore more active, under basal conditions.(50) According to the model shown in Fig. 1, this mutation may stabilize the activating a$g interaction even in the absence of AMP. Alternatively, it might prevent binding of the inhibitory nucleotide, ATP, that normally disrupts this interaction. It is therefore conceivable
Review articles
Figure 3. Alignment of selected CBS domains from cystathionine b-synthase (CBS) and g subunits of AMPK, showing the location of naturally occurring and engineered mutations. Residues that tend to be conserved across all CBS domains (bold type), and the likely locations of b strands (bbbb...) and a-helices (hhhh...), are as discussed by Bateman.(16)
that all of these mutations cause forms of AMPK that are more active under basal conditions in the absence of a rise in AMP. It is easier to explain the dominant nature of these mutations if they cause constitutive activation, rather than inhibition, of the kinase activity. If AMPK activation does indeed cause muscle hypertrophy as discussed earlier, these findings could also help to explain the hypertrophy in both cardiac (g2) and skeletal muscles (g3), as the muscle would respond as if it sensed a depletion of ATP even when none had happened. These are exciting times for those of us working in the AMPK field, with new papers appearing almost weekly. An idea that started as a theoretical concept with Daniel Atkinson has been provided with a firm experimental basis and is now turning out to have great relevance to important clinical conditions. Acknowledgments Work in the authors' laboratory is supported by the Wellcome Trust, Diabetes UK and the Medical Research Council. We thank Lee Witters and Bruce Kemp for permission to cite their work prior to its publication. References 1. Hardie DG, Carling D, Carlson M. The AMP-activated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell? Ann Rev Biochem 1998;67:821±855. 2. Hardie DG, Carling D. The AMP-activated protein kinase: fuel gauge of the mammalian cell? Eur J Biochem 1997;246:259±273. 3. Kemp BE, Mitchelhill KI, Stapleton D, Michell BJ, Chen ZP, Witters LA. Dealing with energy demand: the AMP activated protein kinase. Trends Biochem Sci 1999;24:22±25. 4. Ramaiah A, Hathaway JA, Atkinson DE. Adenylate as a metabolic regulator. Effect on yeast phosphofructokinase kinetics. J Biol Chem 1964;239:3619±3622. 5. Krebs H. The Croonian lecture ± gluconeogenesis. Proc Roy Soc Lond B 1963;159:545±564. 6. Beg ZH, Allmann DW, Gibson DM. Modulation of 3-hydroxy-3-methylglutaryl coenzyme: A reductase activity with cAMP and with protein fractions of rat liver cytosol. Biochem Biophys Res Comm 1973;54:1362± 1369.
7. Carlson CA, Kim KH. Regulation of hepatic acetyl coenzyme A carboxylase by phosphorylation and dephosphorylation. J Biol Chem 1973;8:378±380. 8. Yeh LA, Lee KH, Kim KH. Regulation of rat liver acetyl-CoA carboxylase. Regulation of phosphorylation and inactivation of acetyl-CoA carboxylase by the adenylate energy charge. J Biol Chem 1980;255:2308± 2314. 9. Ferrer A, Caelles C, Massot N, Hegardt FG. Activation of rat liver cytosolic 3-hydroxy-3-methylglutaryl Coenzyme A reductase kinase by adenosine 50 -monophosphate. Biochem Biophys Res Comm 1985;132:497±504. 10. Carling D, Zammit VA, Hardie DG. A common bicyclic protein kinase cascade inactivates the regulatory enzymes of fatty acid and cholesterol biosynthesis. FEBS Lett 1987;223:217±222. 11. Hardie DG, Carling D, Sim ATR. The AMP-activated protein kinaseÐa multisubstrate regulator of lipid metabolism. Trends Biochem Sci 1989;14:20±23. 12. Stapleton D, Woollatt E, Mitchelhill KI, Nicholl JK, Fernandez CS, Michell BJ, Witters LA, Power DA, Sutherland GR, Kemp BE. AMP-activated protein kinase isoenzyme family: subunit structure and chromosomal location. FEBS Lett 1997;409:452±456. 13. Thornton C, Snowden MA, Carling D. Identification of a novel AMPactivated protein kinase b subunit isoform which is highly expressed in skeletal muscle. J Biol Chem 1998;273:12443±12450. 14. Cheung PCF, Salt IP, Davies SP, Hardie DG, Carling D. Characterization of AMP-activated protein kinase g subunit isoforms and their role in AMP binding. Biochem J 2000;346:659±669. 15. Crute BE, Seefeld K, Gamble J, Kemp BE, Witters LA. Functional domains of the alpha1 catalytic subunit of the AMP-activated protein kinase. J Biol Chem 1998;273:35347±35354. 16. Bateman A. The structure of a domain common to archaebacteria and the homocystinuria disease protein. Trends Biochem Sci 1997;22:12±13. 17. Kluijtmans LA, Boers GH, Stevens EM, Renier WO, Kraus JP, Trijbels FJ, van den Heuvel LP, Blom HJ. Defective cystathionine b-synthase regulation by S-adenosylmethionine in a partially pyridoxine responsive homocystinuria patient. J Clin Invest 1996;98:285±289. 18. Stein SC, Woods A, Jones NA, Davison MD, Carling D. The regulation of AMP-activated protein kinase by phosphorylation. Biochem J 2000;345 Pt 3:437±443. 19. Hawley SA, Davison M, Woods A, Davies SP, Beri RK, Carling D, Hardie DG. Characterization of the AMP-activated protein kinase kinase from rat liver, and identification of threonine-172 as the major site at which it phosphorylates and activates AMP-activated protein kinase. J Biol Chem 1996;271:27879±27887. 20. Hawley SA, Selbert MA, Goldstein EG, Edelman AM, Carling D, Hardie DG. 50 -AMP activates the AMP-activated protein kinase cascade, and Ca2/calmodulin the calmodulin-dependent protein kinase I cascade, via three independent mechanisms. J Biol Chem 1995;270:27186± 27191.
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21. Davies SP, Helps NR, Cohen PTW, Hardie DG. 50 -AMP inhibits dephosphorylation, as well as promoting phosphorylation, of the AMPactivated protein kinase. Studies using bacterially expressed human protein phosphatase-2Ca and native bovine protein phosphatase-2AC. FEBS Lett 1995;377:421±425. 22. Hardie DG, Salt IP, Hawley SA, Davies SP. AMP-activated protein kinase: an ultrasensitive system for monitoring cellular energy charge. Biochem J 1999;338:717±722. 23. Ponticos M, Lu QL, Morgan JE, Hardie DG, Partridge TA, Carling D. Dual regulation of the AMP-activated protein kinase provides a novel mechanism for the control of creatine kinase in skeletal muscle. EMBO J 1998;17:1688±1699. 24. Corton JM, Gillespie JG, Hardie DG. Role of the AMP-activated protein kinase in the cellular stress response. Current Biol 1994;4:315±324. 25. Salt IP, Johnson G, Ashcroft SJH, Hardie DG. AMP-activated protein kinase is activated by low glucose in cell lines derived from pancreatic b cells, and may regulate insulin release. Biochem J 1998;335:533± 539. 26. Marsin AS, Bertrand L, Rider MH, Deprez J, Beauloye C, Vincent MF, Van den Berghe G, Carling D, Hue L. Phosphorylation and activation of heart PFK-2 by AMPK has a role in the stimulation of glycolysis during ischaemia. Curr Biol 2000;10:1247±1255. 27. Kudo N, Barr AJ, Barr RL, Desai S, Lopaschuk GD. High rates of fatty acid oxidation during reperfusion of ischemic hearts are associated with a decrease in malonyl-CoA levels due to an increase in 50 -AMP-activated protein kinase inhibition of acetyl-CoA carboxylase. J Biol Chem 1995;270:17513±17520. 28. Winder WW, Hardie DG. Inactivation of acetyl-CoA carboxylase and activation of AMP-activated protein kinase in muscle during exercise. Am J Physiol 1996;270:E299±E304. 29. Fujii N, Hayashi T, Hirshman MF, Smith JT, Habinowski SA, Kaijser L, Mu J, Ljungqvist O, Birnbaum MJ, Witters LA, Thorell A, Goodyear LJ. Exercise induces isoform-specific increase in 50 AMP-activated protein kinase activity in human skeletal muscle. Biochem Biophys Res Commun 2000;273:1150±1155. 30. Wojtaszewski JF, Nielsen P, Hansen BF, Richter EA, Kiens B. Isoformspecific and exercise intensity-dependent activation of 50 -AMP-activated protein kinase in human skeletal muscle. J Physiol 2000;528:221± 226. 31. Mu J, Brozinick JT, Valladares O, Bucan M, Birnbaum MJ. A role for AMP-activated protein kinase in contraction- and hypoxia-regulated glucose transport in skeletal muscle. Mol Cell 2001;7:1085±1094. 32. Winder WW, Hardie DG. The AMP-activated protein kinase, a metabolic master switch: possible roles in Type 2 diabetes. Am J Physiol 1999; 277:E1±E10. 33. Carlson M. Glucose repression in yeast. Curr Opin Microbiol 1999; 2:202±207. 34. Wilson WA, Hawley SA, Hardie DG. The mechanism of glucose repression/derepression in yeast: SNF1 protein kinase is activated by phosphorylation under derepressing conditions, and this correlates with a high AMP:ATP ratio. Curr Biol 1996;6:1426±1434. 35. Smith FC, Davies SP, Wilson WA, Carling D, Hardie DG. The SNF1 kinase complex from Saccharomyces cerevisiae phosphorylates the repressor protein Mig1p in vitro at four sites within or near Regulatory Domain 1. FEBS Lett 1999;453:219±223. 36. DeVit MJ, Johnston M. The nuclear exportin Msn5 is required for nuclear export of the Mig1 glucose repressor of Saccharomyces cerevisiae. Curr Biol 1999;9:1231±1241. 37. Corton JM, Gillespie JG, Hawley SA, Hardie DG. 5-Aminoimidazole-4carboxamide ribonucleoside: a specific method for activating AMPactivated protein kinase in intact cells? Eur J Biochem 1995;229:558± 565. 38. Woods A, Azzout-Marniche D, Foretz M, Stein SC, Lemarchand P, Ferre P, Foufelle F, Carling D. Characterization of the role of AMP-activated protein kinase in the regulation of glucose-activated gene expression using constitutively active and dominant negative forms of the kinase. Mol Cell Biol 2000;20:6704±6711. 39. Blazquez C, Geelen MJ, Velasco G, Guzman M. The AMP-activated protein kinase prevents ceramide synthesis de novo and apoptosis in astrocytes. FEBS Lett 2001;489:149±153.
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