AMP-activated protein kinase: greater AMP dependence, and preferential nuclear localization, of complexes containing the alpha2 isoform

AMPK activation increases uncoupling protein-3 expression and mitochondrial enzyme activities in rat muscle without fibre type transitions

The present study examined the effect of chronic activation of 5'-AMP-activated protein kinase (AMPK) on the metabolic profile, including uncoupling protein-3 (UCP-3) and myosin heavy chain (MHC)-based fibre phenotype of rodent fast-twitch tibialis anterior muscle. Sprague-Dawley rats were given daily injections of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), a known activator of AMPK, or vehicle (control) for 28 days. After AICAR treatment, UCP-3 expression at the mRNA level was elevated 1.6 +/- 0.1-fold (P < 0.006) and corresponded to a 3.3 +/- 0.2-fold increase in UCP-3 protein content (P < 0.0001). In addition, the activities of the mitochondrial reference enzymes citrate synthase (EC 4.1.3.7) and 3-hydroxyacyl-CoA-dehydrogenase (EC 1.1.1.35), which are known to increase in proportion to mitochondrial volume density, were elevated 1.6-fold (P < 0.006), while the activity of lactate dehydrogenase (EC 1.1.1.27) was reduced to 80 % of control (P < 0.02). No differences were detected after AICAR treatment in the activities of the glycolytic reference enzymes glyceraldehydephosphate dehydrogenase (EC 1.2.1.12) or phosphofructokinase (EC 2.7.1.11), nor were MHC-based fibre-type transitions observed, using immunohistochemical or electrophoretic analytical methods. These changes could not be attributed to variations in inter-organ signalling by metabolic substrates or insulin. We conclude that an AMPK-dependent pathway of signal transduction does mimic some of the metabolic changes associated with chronic exercise training, but does not affect expression of the MHC-based structural phenotype. Thus, the metabolic and MHC-based fibre types do not appear to be regulated in a co-ordinated way, but may be independently modified by different signalling pathways.

Mammalian AMP-activated protein kinase: functional, heterotrimeric complexes by co-expression of subunits in Escherichia coli

The 5'-AMP-activated protein kinase (AMPK) plays a critical role in the regulation of cellular energy homeostasis. AMPK is a heterotrimer composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma). To date, purified AMPK has only been obtained in small, microgram quantities from tissues. Here, we describe an expression and purification system for production of functional AMPK in Escherichia coli. A plasmid carrying all three subunits of AMPK (alpha1, beta1, and gamma1) for T7 RNA polymerase-driven transcription of a single tricistronic messenger was constructed, allowing spontaneous formation of the heterotrimeric complex in the bacterial cytosol. AMPK was purified from the bacterial lysates by single-step nickel-ion chromatography, utilizing a poly-histidine tag fused to the N-terminus of the alpha-subunit. The recombinant AMPK complex was monodisperse, as shown by gel filtration chromatography with elution of a single peak at a Stokes radius of 52A. Bacterially expressed AMPK was entirely inactive, yet it could be activated by upstream kinase in the presence of AMP. Sufficient quantities of purified functional AMPK should prove to be an invaluable tool to solve many of the pertinent questions about its molecular structure and function, in particular facilitating protein crystallization for X-ray structure analysis.

AMP-activated protein kinase regulates HNF4alpha transcriptional activity by inhibiting dimer formation and decreasing protein stability

AMP-activated protein kinase (AMPK) is the central component of a cellular signaling system that regulates multiple metabolic enzymes and pathways in response to reduced intracellular energy levels. The transcription factor hepatic nuclear factor 4alpha (HNF4alpha) is an orphan nuclear receptor that regulates the expression of genes involved in energy metabolism in the liver, intestine, and endocrine pancreas. Inheritance of a single null allele of HNF4alpha causes diabetes in humans. Here we demonstrate that AMPK directly phosphorylates HNF4alpha and represses its transcriptional activity. AMPK-mediated phosphorylation of HNF4alpha on serine 304 had a 2-fold effect, reducing the ability of the transcription factor to form homodimers and bind DNA and increasing its degradation rate in vivo. These results demonstrate that HNF4alpha is a downstream target of AMPK and raise the possibility that one of the effects of AMPK activation is reduced expression of HNF4alpha target genes.

AMP-activated protein kinase (AMPK) control of fatty acid and glucose metabolism in the ischemic heart

Myocardial ischemia is the leading cause of all cardiovascular deaths in North America. Myocardial ischemia is accompanied by profound changes in metabolism including alterations in glucose and fatty acid metabolism, increased uncoupling of glucose oxidation from glycolysis and accumulation of protons within the myocardium. These changes can contribute to a poor functional recovery of the heart. One key player in the ischemia-induced alteration in fatty acid and glucose metabolism is 5'AMP-activated protein kinase (AMPK). Accumulating evidence suggest that activation of AMPK during myocardial ischemia both increases glucose uptake and glycolysis while also increasing fatty acid oxidation during reperfusion. Gain-of-function mutations of AMPK in cardiac muscle may also be causally related to the development of hypertrophic cardiomyopathies. Therefore, a better understanding of role of AMPK in cardiac metabolism is necessary to appropriately modulate its activity as a potential therapeutic target in treating ischemia reperfusion injuries. This review attempts to update some of the recent findings that delineate various pathways through which AMPK regulates glucose and fatty acid metabolism in the ischemic myocardium.

Exercise increases nuclear AMPK alpha2 in human skeletal muscle

An acute bout of exercise increases skeletal muscle glucose uptake, improves glucose homeostasis and insulin sensitivity, and enhances muscle oxidative capacity. Recent studies have shown an association between these adaptations and the energy-sensing 5' AMP-activated protein kinase (AMPK), the activity of which is increased in response to exercise. Activation of AMPK has been associated with enhanced expression of key metabolic proteins such as GLUT-4, hexokinase II (HKII), and mitochondrial enzymes, similar to exercise. It has been hypothesized that AMPK might regulate gene and protein expression through direct interaction with the nucleus. The purpose of this study was to determine if nuclear AMPK alpha(2) content in human skeletal muscle was increased by exercise. Following 60 min of cycling at 72 +/- 1% of VO(2peak) in six male volunteers (20.6 +/- 2.1 years; 72.9 +/- 2.1 kg; VO(2peak) = 3.62 +/- 0.18 l/min), nuclear AMPK alpha(2) content was increased 1.9 +/- 0.4-fold (P = 0.024). There was no change in whole-cell AMPK alpha(2) content or AMPK alpha(2) mRNA abundance. These results suggest that nuclear translocation of AMPK might mediate the effects of exercise on skeletal muscle gene and protein expression.

5'-AMP-activated protein kinase activity and subunit expression in exercise-trained human skeletal muscle

5'-AMP-activated protein kinase (AMPK) has been proposed to be a pivotal factor in cellular responses to both acute exercise and exercise training. To investigate whether protein levels and gene expression of catalytic (alpha(1), alpha(2)) and regulatory (beta(1), beta(2), gamma(1), gamma(2), gamma(3)) AMPK subunits and exercise-induced AMPK activity are influenced by exercise training status, muscle biopsies were obtained from seven endurance exercise-trained and seven sedentary young healthy men. The alpha(1)- and alpha(2)-AMPK mRNA contents in trained subjects were both 117 +/- 2% of that in sedentary subjects (not significant), whereas mRNA for gamma(3) was 61 +/- 1% of that in sedentary subjects (not significant). The level of alpha(1)-AMPK protein in trained subjects was 185 +/- 34% of that in sedentary subjects (P < 0.05), whereas the levels of the remaining subunits (alpha(2), beta(1), beta(2), gamma(1), gamma(2), gamma(3)) were similar in trained and sedentary subjects. At the end of 20 min of cycle exercise at 80% of peak O(2) uptake, the increase in phosphorylation of alpha-AMPK (Thr(172)) was blunted in the trained group (138 +/- 38% above rest) compared with the sedentary group (353 +/- 63% above rest) (P < 0.05). Acetyl CoA-carboxylase beta-phosphorylation (Ser(221)), which is a marker for in vivo AMPK activity, was increased by exercise in both groups but to a lower level in trained subjects (32 +/- 5 arbitrary units) than in sedentary controls (45 +/- 1 arbitrary units) (P < 0.01). In conclusion, trained human skeletal muscle has increased alpha(1)-AMPK protein levels and blunted AMPK activation during exercise.

AMP- and stress-activated protein kinases: key regulators of glucose-dependent gene transcription in mammalian cells?

This article will discuss the role of two classes of serine/threonine protein kinases in the regulation of gene transcription in mammals. The first is AMP-activated protein kinase (AMPK), which is responsive to changes in the intracellular energy status. The second is the 'stress-activated" family of protein kinases, members of the mitogen-activated protein (MAP) kinase superfamily, whose regulation by a number of extracellular agents (including osmotic stresses, cytokines, and heat) is less well understood. Interest in these enzymes has grown in the past few years due to mounting evidence (both pharmacological and genetic) which has implicated them in the regulation of a number genes important in mammalian metabolism.

Metabolic and mitogenic signal transduction in human skeletal muscle after intense cycling exercise

We determined whether mitogen-activated protein kinase (MAPK) and 5'-AMP-activated protein kinase (AMPK) signalling cascades are activated in response to intense exercise in skeletal muscle from six highly trained cyclists (peak O(2) uptake (.V(O2,peak)) 5.14 +/- 0.1 l min(-1)) and four control subjects (Vdot;(O(2))(,peak) 3.8 +/- 0.1 l min(-1)) matched for age and body mass. Trained subjects completed eight 5 min bouts of cycling at approximately 85% of .V(O2,peak) with 60 s recovery between work bouts. Control subjects performed four 5 min work bouts commencing at the same relative, but a lower absolute intensity, with a comparable rest interval. Vastus lateralis muscle biopsies were taken at rest and immediately after exercise. Extracellular regulated kinase (ERK1/2), p38 MAPK, histone H3, AMPK and acetyl CoA-carboxylase (ACC) phosphorylation was determined by immunoblot analysis using phosphospecific antibodies. Activity of mitogen and stress-activated kinase 1 (MSK1; a substrate of ERK1/2 and p38 MAPK) and alpha(1) and alpha(2) subunits of AMPK were determined by immune complex assay. ERK1/2 and p38 MAPK phosphorylation and MSK1 activity increased (P < 0.05) after exercise 2.6-, 2.1- and 2.0-fold, respectively, in control subjects and 1.5-, 1.6- and 1.4-fold, respectively, in trained subjects. Phosphorylation of histone H3, a substrate of MSK1, increased (P < 0.05) approximately 1.8-fold in both control and trained subject. AMPKalpha(2) activity increased (P < 0.05) after exercise 4.2- and 2.3-fold in control and trained subjects, respectively, whereas AMPKalpha(1) activity was not altered. Exercise increased ACC phosphorylation (P < 0.05) 1.9- and 2.8-fold in control and trained subjects. In conclusion, intense cycling exercise in subjects with a prolonged history of endurance training increases MAPK signalling to the downstream targets MSK1 and histone H3 and isoform-specific AMPK signalling to ACC. Importantly, exercise-induced signalling responses were greater in untrained men, even at the same relative exercise intensity, suggesting muscle from previously well-trained individuals requires a greater stimulus to activate signal transduction via these pathways.

Functional analysis of mutations in the gamma 2 subunit of AMP-activated protein kinase associated with cardiac hypertrophy and Wolff-Parkinson-White syndrome

Mutations in the gene encoding the gamma(2) subunit of the AMP-activated protein kinase (AMPK) have recently been shown to cause cardiac hypertrophy and ventricular pre-excitation (Wolff-Parkinson-White syndrome). We have examined the effect of four of these mutations on AMPK activity. The mutant gamma(2) polypeptides are all able to form functional complexes following co-expression with either alpha(1)beta(1) or alpha(2)beta(1) in mammalian cells. None of the mutations caused any detectable change in the phosphorylation of threonine 172 within the alpha subunit of AMPK. Consequently, in the absence of an appropriate stimulus the mutant complexes, like the wild-type complex, exist in an inactive form demonstrating that the mutations do not lead to constitutive activation of the kinase. Three of the mutations we studied occur within the cystathionine beta-synthase (CBS) domains of gamma(2). Two of these mutations lead to a marked decrease in AMP dependence, whereas the third reduces AMP sensitivity. These findings suggest that the CBS domains play an important role in AMP-binding within the complex. In contrast, a fourth mutation, which lies between adjacent CBS domains, has no significant effect on AMPK activity in vitro. These results indicate that mutations in gamma(2) have different effects on AMPK function, suggesting that they may lead to abnormal development of the heart through distinct mechanisms.

AMP kinase is required for mitochondrial biogenesis in skeletal muscle in response to chronic energy deprivation

Mitochondrial biogenesis is a critical adaptation to chronic energy deprivation, yet the signaling mechanisms responsible for this response are poorly understood. To examine the role of AMP-activated protein kinase (AMPK), an evolutionarily conserved fuel sensor, in mitochondrial biogenesis we studied transgenic mice expressing a dominant-negative mutant of AMPK in muscle (DN-AMPK). Both DN-AMPK and WT mice were treated with beta-guanidinopropionic acid (GPA), a creatine analog, which led to similar reductions in the intramuscular ATPAMP ratio and phosphocreatine concentrations. In WT mice, GPA treatment resulted in activation of muscle AMPK and mitochondrial biogenesis. However, the same GPA treatment in DN-AMPK mice had no effect on AMPK activity or mitochondrial content. Furthermore, AMPK inactivation abrogated GPA-induced increases in the expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha and calciumcalmodulin-dependent protein kinase IV (both master regulators of mitochondrial biogenesis). These data demonstrate that by sensing the energy status of the muscle cell, AMPK is a critical regulator involved in initiating mitochondrial biogenesis.

AMP-activated protein kinase activates transcription of the UCP3 and HKII genes in rat skeletal muscle

AMP-activated protein kinase (AMPK) has recently emerged as a key signaling protein in skeletal muscle, coordinating the activation of both glucose and fatty acid metabolism in response to increased cellular energy demand. To determine whether AMPK signaling may also regulate gene transcription in muscle, rats were given a single subcutaneous injection (1 mg/g) of the AMP analog 5-aminoimidazole-4-carboxamide-1-beta-d-ribonucleoside (AICAR). AICAR injection activated (P < 0.05) AMPK-alpha 2 ( approximately 2.5-fold) and transcription of the uncoupling protein-3 (UCP3, approximately 4-fold) and hexokinase II (HKII, approximately 10-fold) genes in both red and white skeletal muscle. However, AICAR injection also elicited (P < 0.05) an acute drop (60%) in blood glucose and a sustained (2-h) increase in blood lactate, prompting concern regarding the specificity of AICAR on transcription. To maximize AMPK activation in muscle while minimizing potential systemic counterregulatory responses, a single-leg arterial infusion technique was employed in fully conscious rats. Relative to saline-infused controls, single-leg arterial infusion of AICAR (0.125, 0.5, and 2.5 micro g. g(-1). min(-1) for 60 min) induced a dose-dependent increase (2- to 4-fold, P < 0.05) in UCP3 and HKII transcription in both red and white skeletal muscle. Importantly, AICAR infusion activated transcription only in muscle from the infused leg and had no effect on blood glucose or lactate levels. These data provide evidence that AMPK signaling is linked to the transcriptional regulation of select metabolic genes in skeletal muscle.

A homologue of AMP-activated protein kinase in Drosophila melanogaster is sensitive to AMP and is activated by ATP depletion

We have identified single genes encoding homologues of the alpha, beta and gamma subunits of mammalian AMP-activated protein kinase (AMPK) in the genome of Drosophila melanogaster. Kinase activity could be detected in extracts of a Drosophila cell line using the SAMS peptide, which is a relatively specific substrate for the AMPK/SNF1 kinases in mammals and yeast. Expression of double stranded (ds) RNAs targeted at any of the putative alpha, beta or gamma subunits ablated this activity, and abolished expression of the alpha subunit. The Drosophila kinase (DmAMPK) was activated by AMP in cell-free assays (albeit to a smaller extent than mammalian AMPK), and by stresses that deplete ATP (oligomycin and hypoxia), as well as by carbohydrate deprivation, in intact cells. Using a phosphospecific antibody, we showed that activation was associated with phosphorylation of a threonine residue (Thr-184) within the 'activation loop' of the alpha subunit. We also identified a homologue of acetyl-CoA carboxylase (DmACC) in Drosophila and, using a phosphospecific antibody, showed that the site corresponding to the regulatory AMPK site on the mammalian enzyme became phosphorylated in response to oligomycin or hypoxia. By immunofluorescence microscopy of oligomycin-treated Dmel2 cells using the phosphospecific antibody, the phosphorylated DmAMPK alpha subunit was mainly detected in the nucleus. Our results show that the AMPK system is highly conserved between insects and mammals. Drosophila cells now represent an attractive system to study this pathway, because of the small, well-defined genome and the ability to ablate expression of specific gene products using interfering dsRNAs.

Invited review: Exercise training-induced changes in insulin signaling in skeletal muscle

This review will provide insight on the current understanding of the intracellular signaling mechanisms by which exercise training increases glucose metabolism and gene expression in skeletal muscle. Participation in regular exercise programs can have important clinical implications, leading to improved health in insulin-resistant persons. Evidence is emerging that insulin signal transduction at the level of insulin receptor substrates 1 and 2, as well as phosphatidylinositol 3-kinase, is enhanced in skeletal muscle after exercise training. This is clinically relevant because insulin signaling is impaired in skeletal muscle from insulin-resistant Type 2 diabetic and obese humans. The molecular mechanism for enhanced insulin-stimulated glucose uptake after exercise training may be partly related to increased expression and activity of key proteins known to regulate glucose metabolism in skeletal muscle. Exercise also leads to an insulin-independent increase in glucose transport, mediated in part by AMP-activated protein kinase. Changes in protein expression may be related to increased signal transduction through the mitogen-activated protein kinase signaling cascades, a pathway known to regulate transcriptional activity. Understanding the molecular mechanism for the activation of insulin signal transduction pathways after exercise training may provide novel entry points for new strategies to enhance glucose metabolism and for improved health in the general population.

Invited review: intracellular signaling in contracting skeletal muscle

Physical exercise is a significant stimulus for the regulation of multiple metabolic and transcriptional processes in skeletal muscle. For example, exercise increases skeletal muscle glucose uptake, and, after exercise, there are increases in the rates of both glucose uptake and glycogen synthesis. A single bout of exercise can also induce transient changes in skeletal muscle gene transcription and can alter rates of protein metabolism, both of which may be mechanisms for chronic adaptations to repeated bouts of exercise. A central issue in exercise biology is to elucidate the underlying molecular signaling mechanisms that regulate these important metabolic and transcriptional events in skeletal muscle. In this review, we summarize research from the past several years that has demonstrated that physical exercise can regulate multiple intracellular signaling cascades in skeletal muscle. It is now well established that physical exercise or muscle contractile activity can activate three of the mitogen-activated protein kinase signaling pathways, including the extracellular signal-regulated kinase 1 and 2, the c-Jun NH(2)-terminal kinase, and the p38. Exercise can also robustly increase activity of the AMP-activated protein kinase, as well as several additional molecules, including glycogen synthase kinase 3, Akt, and the p70 S6 kinase. A fundamental goal of signaling research is to determine the biological consequences of exercise-induced signaling through these molecules, and this review also provides an update of progress in this area.

Effect of fiber type and nutritional state on AICAR- and contraction-stimulated glucose transport in rat muscle

AMP-activated protein kinase (AMPK) may mediate the stimulatory effect of contraction and 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) on glucose transport in skeletal muscle. In muscles with different fiber type composition from fasted rats, AICAR increased 2-deoxyglucose transport and total AMPK activity approximately twofold in epitrochlearis (EPI), less in flexor digitorum brevis, and not at all in soleus muscles. Contraction increased both transport and AMPK activity more than AICAR did. In EPI muscles, the effects of AICAR and contractions on glucose transport were partially additive despite a lower AMPK activity with AICAR compared with contraction alone. In EPI from fed rats, glucose transport responses were smaller than what was seen in fasted rats, and AICAR did not increase transport despite an increase in AMPK activity. AICAR and contraction activated both alpha(1)- and alpha(2)-isoforms of AMPK. Expression of both isoforms varied with fiber types, and alpha(2) was highly expressed in nuclei. In conclusion, AICAR-stimulated glucose transport varies with muscle fiber type and nutritional state. AMPK is unlikely to be the sole mediator of contraction-stimulated glucose transport.

AMP-activated protein kinase kinase: detection with recombinant AMPK alpha1 subunit

The AMP-activated protein kinase (AMPK) is a heterotrimeric serine/threonine protein kinase important for the responses to metabolic stress. It consists of a catalytic alpha subunit and two non-catalytic subunits, beta and gamma, and is regulated both by the allosteric action of AMP and by phosphorylation of the alpha and beta subunits catalyzed by AMPKK(s) and autophosphorylation. The Thr172 site on the alpha subunit has been previously characterized as an activating phosphorylation site. Using bacterially expressed AMPK alpha1 subunit proteins, we have explored the role of Thr172-directed AMPKKs in alpha subunit regulation. Recombinant alpha1 subunit proteins, representing the N-terminus, have been expressed as maltose binding protein (MBP) 6x His fusion proteins and purified to homogeneity by Ni(2+) chromatography. Both wild-type alpha1(1-312) and alpha1(1-312)T172D are inactive when expressed in bacteria, but the former can be fully phosphorylated (1 mol/mol) on Thr172 and activated by a surrogate AMPKK, CaMKKbeta. The corresponding AMPKalpha1(1-392), an alpha construct containing its autoinhibitory sequence, can be similarly phosphorylated, but it remains inactive. In an insulinoma cell line, either low glucose or 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) treatment leads to activation and T172 phosphorylation of endogenous AMPK. Under the same conditions of cell incubation, we have identified an AMPKK activity that both phosphorylates and activates the recombinant alpha1(1-312), but this Thr172-directed AMPKK activity is unaltered by low glucose or AICAR, indicating that it is constitutively active.

A potential role for AMP-activated protein kinase in meiotic induction in mouse oocytes

Cyclic adenosine monophosphate (cAMP) has been implicated as an important regulator of meiotic maturation in mammalian oocytes. A decrease in cAMP, brought about by the action of cAMP phosphodiesterase (PDE), is thought to initiate germinal vesicle breakdown (GVB) by the inactivation of cAMP-dependent protein kinase. However, the product of PDE activity, 5'-AMP, is a potent activator of an important regulatory enzyme, AMP-activated protein kinase (AMPK). The aim of this study was to evaluate a possible role for AMPK in meiotic induction, using oocytes obtained from eCG-primed, immature mice. Alpha-1 and -2 isoforms of the catalytic subunit of AMPK were detected in both oocytes and cumulus cells. When 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside (AICA riboside), an activator of AMPK, was tested on denuded oocytes (DO) and cumulus cell-enclosed oocytes (CEO) maintained in meiotic arrest by dbcAMP or hypoxanthine, GVB was dose-dependently induced. Meiotic induction by AICA riboside in dbcAMP-supplemented medium was initiated within 3 h in DO and 4 h in CEO and was accompanied by increased AMPK activity in the oocyte. AICA riboside also triggered GVB when meiotic arrest was maintained with hypoxanthine, 8-AHA-cAMP, guanosine, or milrinone, but was ineffective in olomoucine- or roscovitine-arrested oocytes, indicating that it acts upstream of maturation-promoting factor. Adenosine monophosphate dose-dependently stimulated GVB in DO when meiotic arrest was maintained with dbcAMP or hypoxanthine. This effect was not mimicked by other monophosphate or adenosine nucleotides and was not affected by inhibitors of ectophosphatases. Combined treatment with adenosine and deoxycoformycin, an adenosine deaminase inhibitor, stimulated GVB in dbcAMP-arrested CEO, suggesting AMPK activation due to AMP accumulation. It is concluded that phosphodiesterase-generated AMP may serve as a transducer of the meiotic induction process through activation of AMPK.

Exercise-induced transcription of the muscle glucose transporter (GLUT 4) gene

We studied the effects of exercise on GLUT4 gene transcription in several lines of transgenic mice expressing the chloramphenicol acyltransferase (CAT) reporter gene, driven by various lengths of the human GLUT4 promoter (2400, 1600, 895, and 730 bp). In all transgenic lines examined, endogenous GLUT4 mRNA increased in response to exercise (19-90%, P < 0.05). Exercise increased CAT mRNA (51-83%, P < 0.05) in mice when the transgene was driven by at least 895 bp of the promoter but showed no effect in mice in which the transgene was driven by only 730 bp. These results suggest that the exercise-induced increase in the transcriptional activity of the human GLUT4 gene is mediated, at least in part, by element(s) within -895 bp of the promoter. These observations reveal a striking similarity to the time course and regional promoter requirements of AMPK-induced GLUT4 gene expression, providing further evidence that AMPK may be mediating the effects of exercise on GLUT4 expression.

Protein kinase substrate recognition studied using the recombinant catalytic domain of AMP-activated protein kinase and a model substrate

We have expressed a truncated form of the alpha1 kinase domain of AMP-activated protein kinase (AMPK) in Escherichia coli as a glutathione-S-transferase fusion (GST-KD). A T172D mutant version did not require prior phosphorylation and was utilized for most subsequent studies. We have also created a recombinant substrate (GST-ACC) by expressing 34 residues around the major phosphorylation site (Ser79) on rat acetyl-CoA carboxylase-1/alpha (ACC1) as a GST fusion. This was an excellent substrate that was phosphorylated with similar kinetic parameters to ACC1 by both native AMPK and the bacterially expressed kinase domain. We also constructed a structural model for the binding of the ACC1 sequence to the kinase domain, based on crystal structures for related protein kinases. The model was tested by making a total of 25 mutants of GST-ACC and seven mutants of GST-KD, and measuring kinetic parameters with different combinations. The results reveal that AMPK and ACC1 interact over a much wider region than previously realized (>20 residues). The features of the interaction can be summarised as follows: (i) an amphipathic helix from P-16 to P-5 on the substrate binds in a hydrophobic groove on the large lobe of the kinase; (ii) basic residues at P-6 and P-4 bind to two acidic patches (D215/D216/D217 and E103/D100/E143, respectively), on the large lobe; (iii) a histidine at P+3 interacts with D56 on the small lobe; (iv) the side-chain of P+4 leucine could not be precisely positioned, but a new finding was that asparagine or glutamine could replace a hydrophobic residue at this position. These interactions position the serine residue to be phosphorylated in close proximity to the gamma-phosphate group of ATP. Although based on modelling rather than a determined structure, this represents one of the most detailed studies of the interaction between a kinase and its substrate achieved to date.

Adaptations of skeletal muscle to prolonged, intense endurance training

1. Endurance exercise induces a variety of metabolic and morphological responses/adaptations in skeletal muscle that function to minimize cellular disturbances during subsequent training sessions. 2. Chronic adaptations in skeletal muscle are likely to be the result of the cumulative effect of repeated bouts of exercise, with the initial signalling responses leading to such adaptations occurring after each training session. 3. Recently, activation of the mitogen-activated protein kinase signalling cascade has been proposed as a possible mechanism involved in the regulation of many of the exercise-induced adaptations in skeletal muscle. 4. The protein targets of AMP-activated protein kinase also appear to be involved in both the regulation of acute metabolic responses and chronic adaptations to exercise. 5. Endurance training is associated with an increase in the activities of key enzymes of the mitochondrial electron transport chain and a concomitant increase in mitochondrial protein concentration. These morphological changes, along with increased capillary supply, result in a shift in trained muscle to a greater reliance on fat as a fuel with a concomitant reduction in glycolytic flux and tighter control of acid-base status. Taken collectively, these adaptations result in an enhanced performance capacity.

A role for peroxisome proliferator-activated receptor alpha (PPARalpha ) in the control of cardiac malonyl-CoA levels: reduced fatty acid oxidation rates and increased glucose oxidation rates in the hearts of mice lacking PPARalpha are associated with higher concentrations of malonyl-CoA and reduced expression of malonyl-CoA decarboxylase

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a nuclear receptor transcription factor that has an important role in controlling cardiac metabolic gene expression. We determined whether mice lacking PPARalpha (PPARalpha (-/-) mice) have alterations in cardiac energy metabolism. Rates of palmitate oxidation were significantly decreased in isolated working hearts from PPARalpha (-/-) hearts compared with hearts from age-matched wild type mice (PPARalpha (+/+) mice), (62 +/- 12 versus 154 +/- 65 nmol/g dry weight/min, respectively, p < 0.05). This was compensated for by significant increases in the rates of glucose oxidation and glycolysis. The decreased fatty acid oxidation in PPARalpha (-/-) hearts was associated with increased levels of cardiac malonyl-CoA compared with PPARalpha (+/+) hearts (15.15 +/- 1.63 versus 7.37 +/- 1.31 nmol/g, dry weight, respectively, p < 0.05). Since malonyl-CoA is an important regulator of cardiac fatty acid oxidation, we also determined if the enzymes that control malonyl-CoA levels in the heart are under transcriptional control of PPARalpha. Expression of both mRNA and protein as well as the activity of malonyl-CoA decarboxylase, which degrades malonyl-CoA, were significantly decreased in the PPARalpha (-/-) hearts. In contrast, the expression and activity of acetyl-CoA carboxylase, which synthesizes malonyl-CoA and 5'-AMP-activated protein kinase, which regulates acetyl-CoA carboxylase, were not altered. Glucose transporter expression (GLUT1 and GLUT4) was not different between PPARalpha (-/-) and PPARalpha (+/+) hearts, suggesting that the increase in glycolysis and glucose oxidation in the PPARalpha null mice was not due to direct effects on glucose uptake but rather was occurring secondary to the decrease in fatty acid oxidation. This study demonstrates that PPARalpha is an important regulator of fatty acid oxidation in the heart and that this regulation of fatty acid oxidation may in part occur due to the transcriptional control of malonyl-CoA decarboxylase.

Glycogen-dependent effects of 5-aminoimidazole-4-carboxamide (AICA)-riboside on AMP-activated protein kinase and glycogen synthase activities in rat skeletal muscle

5'-AMP-activated protein kinase (AMPK) functions as a metabolic switch in mammalian cells and can be artificially activated by 5-aminoimidazole-4-carboxamide (AICA)-riboside. AMPK activation during muscle contraction is dependent on muscle glycogen concentrations, but whether glycogen also modifies the activation of AMPK and its possible downstream effectors (glycogen synthase and glucose transport) by AICA-riboside in resting muscle is not known. Thus, we have altered muscle glycogen levels in rats by a combination of swimming exercise and diet and investigated the effects of AICA-riboside in the perfused rat hindlimb muscle. Two groups of rats, one with super-compensated muscle glycogen content (approximately 200-300% of normal; high glycogen [HG]) and one with moderately lowered muscle glycogen content (approximately 80% of normal; low glycogen [LG]), were generated. In both groups, the degree of activation of the alpha2 isoform of AMPK by AICA-riboside depended on muscle type (white gastrocnemius >> red gastrocnemius > soleus). Basal and AICA-riboside-induced alpha2-AMPK activity were markedly lowered in the HG group (approximately 50%) compared with the LG group. Muscle 2-deoxyglucose uptake was also increased and glycogen synthase activity decreased by AICA-riboside. Especially in white gastrocnemius, these effects, as well as the absolute activity levels of AMPK-alpha2, were markedly reduced in the HG group compared with the LG group. The inactivation of glycogen synthase by AICA-riboside was accompanied by decreased gel mobility and was eliminated by protein phosphatase treatment. We conclude that acute AICA-riboside treatment leads to phosphorylation and deactivation of glycogen synthase in skeletal muscle. Although the data do not exclude a role of other kinases/phosphatases, they suggest that glycogen synthase may be a target for AMPK in vivo. Both basal and AICA-riboside-induced AMPK-alpha2 and glycogen synthase activities, as well as glucose transport, are depressed when the glycogen stores are plentiful. Because the glycogen level did not affect adenine nucleotide concentrations, our data suggest that glycogen may directly affect the activation state of AMPK in skeletal muscle.

5-amino-4-imidazolecarboxamide riboside confers strong tolerance to glucose starvation in a 5'-AMP-activated protein kinase-dependent fashion

Acadesine, 5-amino-4-imidazolecarboxamide riboside (AICAR), has been claimed to protect the heart, lung, and small intestine against ischemic damage. The biochemical mechanisms of this effect of AICAR are not yet fully understood. To understand the mechanism, we examined the effect of AICAR on glucose starvation, since cellular responses to ischemia could be regarded as a protective response to an insufficient blood supply, cells might display adaptive reactions not only to oxygen deficiency but to nutrient deficiency. AICAR was found to confer strong tolerance to glucose starvation. By using antisense RNA expression vector for alpha subunit of 5'-AMP-activated protein kinase, the effect of AICAR was found to be dependent on 5'-AMP-activated protein kinase containing the alpha2 subunit. The AICAR effect was also dependent on the presence of amino acids, indicating an energy source switch from glucose to amino acids.

Chronic activation of AMP kinase results in NRF-1 activation and mitochondrial biogenesis

The underlying mechanism by which skeletal muscle adapts to exercise training or chronic energy deprivation is largely unknown. To examine this question, rats were fed for 9 wk either with or without beta-guanadinopropionic acid (beta-GPA; 1% enriched diet), a creatine analog that is known to induce muscle adaptations similar to those induced by exercise training. Muscle phosphocreatine, ATP, and ATP/AMP ratios were all markedly decreased and led to the activation of AMP-activated protein kinase (AMPK) in the beta-GPA-fed rats compared with control rats. Under these conditions, nuclear respiratory factor-1 (NRF-1) binding activity, measured using a cDNA probe containing a sequence encoding for the promoter of delta-aminolevulinate (ALA) synthase, was increased by about eightfold in the muscle of beta-GPA-fed rats compared with the control group. Concomitantly, muscle ALA synthase mRNA and cytochrome c content were also increased. Mitochondrial density in both extensor digitorum longus and epitrochlearis from beta-GPA-fed rats was also increased by more than twofold compared with the control group. In conclusion, chronic phosphocreatine depletion during beta-GPA supplementation led to the activation of muscle AMPK that was associated with increased NRF-1 binding activity, increased cytochrome c content, and increased muscle mitochondrial density. Our data suggest that AMPK may play an important role in muscle adaptations to chronic energy stress and that it promotes mitochondrial biogenesis and expression of respiratory proteins through activation of NRF-1.

AMP-activated protein kinase activation prevents denervation-induced decline in gastrocnemius GLUT-4

This study was designed to determine whether the reductions in GLUT-4 seen in 3-day-denervated muscles can be prevented through chemical activation of 5'-AMP-activated protein kinase (AMPK). Muscle AMPK can be chemically activated in rats using subcutaneous injections with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). In this study, the tibial nerve was sectioned on one side; the other was sham operated but without nerve section. Acute injections of AICAR resulted in significantly increased AMPK activity in denervated gastrocnemius but not soleus muscles. Acetyl-CoA carboxylase activity, a reporter of AMPK activation, declined in both gastrocnemius and soleus in both denervated and contralateral muscles. Three days after denervation, GLUT-4 levels were significantly decreased by approximately 40% in gastrocnemius muscles and by approximately 30% in soleus muscles. When rats were injected with AICAR (1 mg/g body wt) for 3 days, the decline in GLUT-4 levels was prevented in denervated gastrocnemius muscles but not in denervated soleus muscles. The extent of denervation-induced muscle atrophy was similar in AICAR-treated vs. saline-treated rats. These studies provide evidence that some effects of denervation may be prevented by chemical activation of the appropriate signaling pathways.

Regulation of transcription by AMP-activated protein kinase: phosphorylation of p300 blocks its interaction with nuclear receptors

AMP-activated protein kinase (AMP-kinase) modulates many metabolic processes in response to fluctuations in cellular energy status. Although most of its known targets are metabolic enzymes, it has been proposed that AMP-kinase might also regulate gene expression. Here we demonstrate that the transcriptional coactivator p300 is a substrate of AMP-kinase. Phosphorylation of p300 at serine 89 by AMP-kinase dramatically reduced its interaction, in vitro and in vivo, with the nuclear receptors peroxisome proliferator-activated receptor gamma, thyroid receptor, retinoic acid receptor, and retinoid X receptor, but did not affect its interaction with the non-nuclear receptor transcription factors E1a, p53, or GATA4. These findings indicate that the AMP-kinase signaling pathway selectively modulates a subset of p300 activities and represent the first example of a transcriptional component regulated by AMP-kinase. Our results suggest a direct link between cellular energy metabolism and gene expression.

Increased adenosine monophosphate-activated protein kinase activity in rat hearts with pressure-overload hypertrophy

BACKGROUND

Recent reports suggest that activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK), in response to acute changes in cellular energy status in cardiac and skeletal muscles, results in altered substrate utilization. We hypothesized that chronic alterations in myocardial energetics in hypertrophied hearts (left ventricular hypertrophy, LVH) will lead to elevated AMPK activity, which in turn regulates substrate utilization.

METHODS AND RESULTS

Using (31)P NMR spectroscopy and biochemical assays, we found that in LVH hearts, adenosine triphosphate (ATP) concentration decreased by 10%, phosphocreatine concentration decreased by 30%, and total creatine concentration was unchanged. Thus, the ratio of phosphocreatine/creatine decreased to one third of controls, and the ratio of AMP/ATP increased to 5 times above controls. These changes were associated with increased alpha(1) and alpha(2) AMPK activity (3.5- and 4.8-fold above controls, respectively). The increase in AMPK alpha(1) activity was accompanied by a 2-fold increase in alpha(1) expression, whereas alpha(2) expression was decreased by 30% in LVH. The basal rate of 2-deoxyglucose uptake increased by 3-fold in LVH, which was associated with an increased amount of glucose transporters present on the plasma membrane.

CONCLUSIONS

These results demonstrate for the first time that chronic changes in myocardial energetics in hypertrophied hearts are accompanied by significant elevations in AMPK activity and isoform-specific alterations in AMPK expression. It also raises the possibility that AMPK signaling plays an important role in regulating substrate utilization in hypertrophied hearts.

Insulin antagonizes AMP-activated protein kinase activation by ischemia or anoxia in rat hearts, without affecting total adenine nucleotides

AMP-activated protein kinase (AMPK) is known to be activated by phosphorylation on Thr172 in response to an increased AMP/ATP ratio. We report here that such an activation indeed occurred in anaerobic rat hearts and that it was antagonized (40-50%) when the hearts were pre-treated with 100 nM insulin. The effect of insulin (1) was blocked by wortmannin, an inhibitor of phosphatidylinositol-3-kinase; (2) only occurred when insulin was added before anoxia, suggesting a hierarchical control; (3) resulted in a decreased phosphorylation state of Thr172 in AMPK and (4) was unrelated to changes in the AMP/ATP ratio. This is the first demonstration that AMPK activity could be changed without a detectable change in the AMP/ATP ratio of the cardiac cell.

The regulation of AMP-activated protein kinase by H(2)O(2)

AMP-activated protein kinase (AMPK), a heterotrimeric serine/threonine kinase, is activated by conditions leading to an increase of the intracellular AMP:ATP ratio. However, how AMPK is regulated under the oxidative stress is completely unknown. In the present study, we examined effects of the oxidative agent H(2)O(2) on AMPK. AMPK was transiently and concentration-dependently activated by H(2)O(2) in NIH-3T3 cells. This activation was tightly associated with an increased AMP:ATP ratio, an electrophoretic mobility shift of AMPK alpha1 catalytic subunit, and an increased phosphorylation level of AMPK alpha1 threonine 172, which is a major in vitro phosphorylation site by the upstream AMPK kinase. All of these events were significantly blocked by the pretreatment of 0.5% dimethyl sulfoxide, a potent hydroxyl radical scavenger, indicating that AMPK cascades are highly sensitive to the oxidative stress. Interestingly, a specific tyrosine kinase inhibitor, genistein, further stimulated the H(2)O(2)-induced AMPK activity by 70% without altering the AMP:ATP. Taken together, our results suggest that AMP:ATP ratio is the major parameter to which AMPK responds under the oxidative stress, but AMPK may be regulated in part by a tyrosine kinase-dependent pathway, which is independent of the cellular adenosine nucleotides level.

Regulation of muscle GLUT-4 transcription by AMP-activated protein kinase

Skeletal muscle GLUT-4 transcription in response to treatment with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), a known activator of AMP-activated protein kinase (AMPK), was studied in rats and mice. The increase in GLUT-4 mRNA levels in response to a single subcutaneous injection of AICAR, peaked at 13 h in white and red quadriceps muscles but not in the soleus muscle. The mRNA level of chloramphenicol acyltransferase reporter gene which is driven by 1,154 or 895 bp of the human GLUT-4 proximal promoter was increased in AICAR-treated transgenic mice, demonstrating the transcriptional upregulation of the GLUT-4 gene by AICAR. However, this induction of transcription was not apparent with 730 bp of the promoter. In addition, nuclear extracts from AICAR-treated mice bound to the consensus sequence of myocyte enhancer factor-2 (from -473 to -464) to a greater extent than from saline-injected mice. Thus AMP-activated protein kinase activation by AICAR increases GLUT-4 transcription by a mechanism that requires response elements within 895 bp of human GLUT-4 proximal promoter and that may be cooperatively mediated by myocyte enhancer factor-2.

Energy-sensing and signaling by AMP-activated protein kinase in skeletal muscle

AMP-activated protein kinase (AMPK) is emerging as an important energy-sensing/signaling system in skeletal muscle. This kinase is activated allosterically by 5'-AMP and inhibited allosterically by creatine phosphate. Phosphorylation of AMPK by an upstream kinase, AMPK kinase (also activated allosterically by 5'-AMP), results in activation. It is activated in both rat and human muscle in response to muscle contraction, the extent of activation depending on work rate and muscle glycogen concentration. AMPK can also be activated chemically in resting muscle with 5-aminoimidazole-4-carboxamide-riboside, which enters the muscle and is phosphorylated to form ZMP, a nucleotide that mimics the effect of 5'-AMP. Once activated, AMPK is hypothesized to phosphorylate proteins involved in triggering fatty acid oxidation and glucose uptake. Evidence is also accumulating for a role of AMPK in inducing some of the adaptations to endurance training, including the increase in muscle GLUT-4, hexokinase, uncoupling protein 3, and some of the mitochondrial oxidative enzymes. It thus appears that AMPK has the capability of monitoring intramuscular energy charge and then acutely stimulating fat oxidation and glucose uptake to counteract the increased rates of ATP utilization during muscle contraction. In addition, this system may have the capability of enhancing capacity for ATP production when the muscle is exposed to endurance training.

AMP-activated protein kinase (AMPK) is activated in muscle of subjects with type 2 diabetes during exercise

Insulin-stimulated GLUT4 translocation is impaired in people with type 2 diabetes. In contrast, exercise results in a normal increase in GLUT4 translocation and glucose uptake in these patients. Several groups have recently hypothesized that exercise increases glucose uptake via an insulin-independent mechanism mediated by the activation of AMP-activated protein kinase (AMPK). If this hypothesis is correct, people with type 2 diabetes should have normal AMPK activation in response to exercise. Seven subjects with type 2 diabetes and eight matched control subjects exercised on a cycle ergometer for 45 min at 70% of maximum workload. Biopsies of vastus lateralis muscle were taken before exercise, after 20 and 45 min of exercise, and at 30 min postexercise. Blood glucose concentrations decreased from 7.6 to 4.77 mmol/l with 45 min of exercise in the diabetic group and did not change in the control group. Exercise significantly increased AMPK alpha2 activity 2.7-fold over basal at 20 min in both groups and remained elevated throughout the protocol, but there was no effect of exercise on AMPK alpha1 activity. Subjects with type 2 diabetes had similar protein expression of AMPK alpha1, alpha2, and beta1 in muscle compared with control subjects. AMPK alpha2 was shown to represent approximately two-thirds of the total alpha mRNA in the muscle from both groups. In conclusion, people with type 2 diabetes have normal exercise-induced AMPK alpha2 activity and normal expression of the alpha1, alpha2 and beta1 isoforms. Pharmacological activation of AMPK may be an attractive target for the treatment of type 2 diabetes.

AMP-activated protein kinase activity and glucose uptake in rat skeletal muscle

The AMP-activated protein kinase (AMPK) has been hypothesized to mediate contraction and 5-aminoimidazole-4-carboxamide 1-beta-D-ribonucleoside (AICAR)-induced increases in glucose uptake in skeletal muscle. The purpose of the current study was to determine whether treadmill exercise and isolated muscle contractions in rat skeletal muscle increase the activity of the AMPK alpha 1 and AMPK alpha 2 catalytic subunits in a dose-dependent manner and to evaluate the effects of the putative AMPK inhibitors adenine 9-beta-D-arabinofuranoside (ara-A), 8-bromo-AMP, and iodotubercidin on AMPK activity and 3-O-methyl-D-glucose (3-MG) uptake. There were dose-dependent increases in AMPK alpha 2 activity and 3-MG uptake in rat epitrochlearis muscles with treadmill running exercise but no effect of exercise on AMPK alpha1 activity. Tetanic contractions of isolated epitrochlearis muscles in vitro significantly increased the activity of both AMPK isoforms in a dose-dependent manner and at a similar rate compared with increases in 3-MG uptake. In isolated muscles, the putative AMPK inhibitors ara-A, 8-bromo-AMP, and iodotubercidin fully inhibited AICAR-stimulated AMPK alpha 2 activity and 3-MG uptake but had little effect on AMPK alpha 1 activity. In contrast, these compounds had absent or minimal effects on contraction-stimulated AMPK alpha 1 and -alpha 2 activity and 3-MG uptake. Although the AMPK alpha 1 and -alpha 2 isoforms are activated during tetanic muscle contractions in vitro, in fast-glycolytic fibers, the activation of AMPK alpha 2-containing complexes may be more important in regulating exercise-mediated skeletal muscle metabolism in vivo. Development of new compounds will be required to study contraction regulation of AMPK by pharmacological inhibition.

Subcellular localization of the Snf1 kinase is regulated by specific beta subunits and a novel glucose signaling mechanism

The Snf1/AMP-activated protein kinase family has broad roles in transcriptional, metabolic, and developmental regulation in response to stress. In Saccharomyces cerevisiae, Snf1 is required for the response to glucose limitation. Snf1 kinase complexes contain the alpha (catalytic) subunit Snf1, one of the three related beta subunits Gal83, Sip1, or Sip2, and the gamma subunit Snf4. We present evidence that the beta subunits regulate the subcellular localization of the Snf1 kinase. Green fluorescent protein fusions to Gal83, Sip1, and Sip2 show different patterns of localization to the nucleus, vacuole, and/or cytoplasm. We show that Gal83 directs Snf1 to the nucleus in a glucose-regulated manner. We further identify a novel signaling pathway that controls this nuclear localization in response to glucose phosphorylation. This pathway is distinct from the glucose signaling pathway that inhibits Snf1 kinase activity and responds not only to glucose but also to galactose and sucrose. Such independent regulation of the localization and the activity of the Snf1 kinase, combined with the distinct localization of kinases containing different beta subunits, affords versatility in regulating physiological responses.

Identification and characterization of a novel sucrose-non-fermenting protein kinase/AMP-activated protein kinase-related protein kinase, SNARK

Subtraction hybridization after the exposure of keratinocytes to ultraviolet radiation identified a differentially expressed cDNA that encodes a protein of 630 amino acid residues possessing significant similarity to the catalytic domain of the sucrose-non-fermenting protein kinase (SNF1)/AMP-activated protein kinase (AMPK) family of serine/threonine protein kinases. Northern blotting and reverse-transcriptase-mediated PCR demonstrated that mRNA transcripts for the SNF1/AMPK-related kinase (SNARK) were widely expressed in rodent tissues. The SNARK gene was localized to human chromosome 1q32 by fluorescent in situ hybridization. SNARK was translated in vitro to yield a single protein band of approx. 76 kDa; Western analysis of transfected baby hamster kidney (BHK) cells detected two SNARK-immunoreactive bands of approx. 76-80 kDa. SNARK was capable of autophosphorylation in vitro; immunoprecipitated SNARK exhibited phosphotransferase activity with the synthetic peptide substrate HMRSAMSGLHLVKRR (SAMS) as a kinase substrate. SNARK activity was significantly increased by AMP and 5-amino-4-imidazolecarboxamide riboside (AICAriboside) in rat keratinocyte cells, implying that SNARK might be activated by an AMPK kinase-dependent pathway. Furthermore, glucose deprivation increased SNARK activity 3-fold in BHK fibroblasts. These findings identify SNARK as a glucose- and AICAriboside-regulated member of the AMPK-related gene family that represents a new candidate mediator of the cellular response to metabolic stress.

Post-translational modifications of the beta-1 subunit of AMP-activated protein kinase affect enzyme activity and cellular localization

The AMP-activated protein kinase (AMPK) is a ubiquitous mammalian protein kinase important in the adaptation of cells to metabolic stress. The enzyme is a heterotrimer, consisting of a catalytic alpha subunit and regulatory beta and gamma subunits, each of which is a member of a larger isoform family. The enzyme is allosterically regulated by AMP and by phosphorylation of the alpha subunit. The beta subunit is post-translationally modified by myristoylation and multi-site phosphorylation. In the present study, we have examined the impact of post-translational modification of the beta-1 subunit on enzyme activity, heterotrimer assembly and subcellular localization, using site-directed mutagenesis and expression of subunits in mammalian cells. Removal of the myristoylation site (G2A mutant) results in a 4-fold activation of the enzyme and relocalization of the beta subunit from a particulate extranuclear distribution to a more homogenous cell distribution. Mutation of the serine-108 phosphorylation site to alanine is associated with enzyme inhibition, but no change in cell localization. In contrast, the phosphorylation site mutations, SS24, 25AA and S182A, while having no effects on enzyme activity, are associated with nuclear redistribution of the subunit. Taken together, these results indicate that both myristoylation and phosphorylation of the beta subunit of AMPK modulate enzyme activity and subunit cellular localization, increasing the complexity of AMPK regulation.

Molecular cloning, genomic organization, and mapping of PRKAG2, a heart abundant gamma2 subunit of 5'-AMP-activated protein kinase, to human chromosome 7q36

5'-AMP-activated protein kinase (AMPK) acts as a major regulator of cellular ATP levels and protects cells against stresses that cause ATP depletion. AMPK is a protein heterotrimer composed of a catalytic alpha subunit and two regulatory subunits, beta and gamma. In the present study, a homologue of the AMPK gamma1-subunit cDNA with an open reading frame encoding 328 amino acids was identified. The putative protein sequence is about 76% identical to the 331-amino-acid gamma1 subunit and also has four consecutive cystathionine-beta-synthase (CBS) domains, a characteristic structure of AMPK gamma subunits from various species. This cDNA (tentatively termed PRKAG2-b) is identical to a recently reported cDNA (tentatively termed PRKAG2-a) of human AMPK gamma subunits except in their 5'-end regions, suggesting that these two cDNAs are two different transcripts of the same gene. To determine the expression pattern of the gene, two probes, one from the 3'-UTR of PRKAG2-b and the other from the 5'- unique region of PRKAG2-a, were used to hybridize MTN membranes. Three transcripts (3.8, 3.0, and 2.4 kb) were observed when the first probe was used, whereas only 3.8- and 3.0-kb transcripts were seen when the second probe was used. Thus, the PRKAG2-b corresponded to the 2.4-kb transcript, which is ubiquitously expressed except in liver and thymus. The highest level was detected in heart, while abundant expression also existed in placenta and testis. The expression pattern of PRKAG2-b is completely different from those of PRKAG2-a and PRKAG1, whose expression patterns were also determined in the current study. The PRKAG2 gene was located to human chromosome 7q36 between markers D7S2439 and D7S2462 by radiation hybrid mapping. The genomic organization of PRKAG2-b was identified by comparing its cDNA sequence with two genomic sequences AC006358 and AC006966, which showed that PRKAG2-b spanned an approximately 80-kb region and was composed of 12 exons.

Isoform-specific and exercise intensity-dependent activation of 5'-AMP-activated protein kinase in human skeletal muscle

1. 5'-AMP-activated protein kinase (AMPK) has been suggested to play a key role in the regulation of metabolism in skeletal muscle. AMPK is activated in treadmill-exercised and electrically stimulated rodent muscles. Whether AMPK is activated during exercise in humans is unknown. 2. We investigated the degree of activation and deactivation of alpha-isoforms of AMPK during and after exercise. Healthy human subjects performed bicycle exercise on two separate occasions at either a low ( approximately 50% maximum rate of O2 uptake (VO2,max) for 90 min) or a high ( approximately 75% VO2,max for 60 min) intensity. Biopsies from the vastus lateralis muscle were obtained before and immediately after exercise, and after 3 h of recovery. 3. We observed a 3- to 4-fold activation of the alpha2-AMPK isoform immediately after high intensity exercise, whereas no activation was observed after low intensity exercise. The activation of alpha2-AMPK was totally reversed 3 h after exercise. In contrast, alpha1-AMPK was not activated during either of the two exercise trials. 4. The in vitro AMP dependency of alpha2-AMPK was significantly greater than that of alpha1-AMPK ( approximately 3- vs. approximately 2-fold). 5. We conclude that in humans activation of alpha2-AMPK during exercise is dependent upon exercise intensity. The stable activation of alpha2-AMPK, presumably due to the activation of an upstream AMPK kinase, is compatible with a role for this kinase complex in the regulation of skeletal muscle metabolism during exercise, whereas the lack of stable alpha1-AMPK activation makes this kinase complex a less likely candidate.

beta-subunits of Snf1 kinase are required for kinase function and substrate definition

The Snf1 kinase and its mammalian homolog, the AMP-activated protein kinase, are heterotrimeric enzymes composed of a catalytic alpha-subunit, a regulatory gamma-subunit and a beta-subunit that mediates heterotrimer formation. Saccharomyces cerevisiae encodes three beta-subunit genes, SIP1, SIP2 and GAL83. Earlier studies suggested that these subunits may not be required for Snf1 kinase function. We show here that complete and precise deletion of all three beta-subunit genes inactivates the Snf1 kinase. The sip1Delta sip2Delta gal83Delta strain is unable to derepress invertase, grows poorly on alternative carbon sources and fails to direct the phosphorylation of the Mig1 and Sip4 proteins in vivo. The SIP1 sip2Delta gal83Delta strain manifests a subset of Snf phenotypes (Raf(+), Gly(-)) observed in the snf1Delta 10 strain (Raf(-), Gly(-)), suggesting that individual beta-subunits direct the Snf1 kinase to a subset of its targets in vivo. Indeed, deletion of individual beta-subunit genes causes distinct differences in the induction and phosphorylation of Sip4, strongly suggesting that the beta-subunits play an important role in substrate definition.

UCP-3 expression in skeletal muscle: effects of exercise, hypoxia, and AMP-activated protein kinase

Uncoupling protein 3 (UCP-3), a member of the mitochondrial transporter superfamily, is expressed primarily in skeletal muscle where it may play a role in altering metabolic function under conditions of fuel depletion caused, for example, by fasting and exercise. Here, we show that treadmill running by rats rapidly (30 min) induces skeletal muscle UCP-3 mRNA expression (sevenfold after 200 min), as do hypoxia and swimming in a comparably rapid and substantial fashion. The expression of the mitochondrial transporters, carnitine palmitoyltransferase 1 and the tricarboxylate carrier, is unaffected under these conditions. Hypoxia and exercise-mediated induction of UCP-3 mRNA result in a corresponding four- to sixfold increase in rat UCP-3 protein. We treated extensor digitorum longus (EDL) muscle with 5'-amino-4-imidazolecarboxamide ribonucleoside (AICAR), a compound that activates AMP-activated protein kinase (AMPK), an enzyme known to be stimulated during exercise and hypoxia. Incubation of rat EDL muscle in vitro for 30 min with 2 mM AICAR causes a threefold increase in UCP-3 mRNA and a 1.5-fold increase of UCP-3 protein compared with untreated muscle. These data are consistent with the notion that activation of AMPK, presumably as a result of fuel depletion, rapidly regulates UCP-3 gene expression.

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

In the liver, glucose induces the expression of a number of genes involved in glucose and lipid metabolism, e.g., those encoding L-type pyruvate kinase and fatty acid synthase. Recent evidence has indicated a role for the AMP-activated protein kinase (AMPK) in the inhibition of glucose-activated gene expression in hepatocytes. It remains unclear, however, whether AMPK is involved in the glucose induction of these genes. In order to study further the role of AMPK in regulating gene expression, we have generated two mutant forms of AMPK. One of these (alpha1(312)) acts as a constitutively active kinase, while the other (alpha1DN) acts as a dominant negative inhibitor of endogenous AMPK. We have used adenovirus-mediated gene transfer to express these mutants in primary rat hepatocytes in culture in order to determine their effect on AMPK activity and the transcription of glucose-activated genes. Expression of alpha1(312) increased AMPK activity in hepatocytes and blocked completely the induction of a number of glucose-activated genes in response to 25 mM glucose. This effect is similar to that observed following activation of AMPK by 5-amino-imidazolecarboxamide riboside. Expression of alpha1DN markedly inhibited both basal and stimulated activity of endogenous AMPK but had no effect on the transcription of glucose-activated genes. Our results suggest that AMPK is involved in the inhibition of glucose-activated gene expression but not in the induction pathway. This study demonstrates that the two mutants we have described will provide valuable tools for studying the wider physiological role of AMPK.

SNRK, a member of the SNF1 family, is related to low K(+)-induced apoptosis of cultured rat cerebellar granule neurons

When cerebellar granule neurons obtained from 11-day-old rats were cultured first in high K(+) medium for 4 days, followed by culture in low K(+) medium, the neurons underwent apoptosis and died. This cell death was prevented by actinomycin D, an inhibitor of RNA synthesis. Commitment time of the protective effect of RNA synthesis inhibition on the cell death was examined by adding actinomycin D at various time points after the switch to the low K(+) medium. More than 50% of the cells died when actinomycin D was added 3 h after changing to the low K(+) medium. To identify what kinds of newly synthesized genes are involved in regulation of the low K(+)-induced death, we performed PCR-based differential subtraction analysis using RNA prepared from the cultured neurons 0 and 3 h after changing to low K(+) medium. We isolated a clone that showed an increase in its mRNA level after changing to the low K(+) medium. This clone encoded the 3' untranslated region of SNRK, a serine/threonine kinase. Tissue distribution analysis showed that the mRNA was expressed mainly in the brain and testis. Developmental analysis in the brain showed that the mRNA expression increased in an age-dependent manner until P28, and was slightly decreased in adults. In situ hybridization analysis showed that the mRNA was expressed throughout the brain. The mRNA was shown to be expressed in neurons by double staining with anti-MAP2 antibody. In addition, anti-N-terminal SNRK antibody stained the nuclei of cultured rat cerebellar granule neurons. These results suggested that SNRK may be involved in regulation of low K(+)-induced apoptosis of cultured cerebellar granule neurons.

Exercise induces isoform-specific increase in 5'AMP-activated protein kinase activity in human skeletal muscle

The 5'AMP-activated protein kinase (AMPK) is stimulated by contractile activity in rat skeletal muscle. AMPK has emerged as an important signaling intermediary in the regulation of cell metabolism being linked to exercise-induced changes in muscle glucose and fatty acid metabolism. In the present study, we determined the effects of exercise on isoform-specific AMPK activity (alpha1 and alpha2) in human skeletal muscle. Needle biopsies of vastus lateralis muscle were obtained from seven healthy subjects at rest, after 20 and 60 min of cycle ergometer exercise at 70% of VO(2)max, and 30 min following the 60 min exercise bout. In comparison to the resting state, AMPK alpha2 activity significantly increased at 20 and 60 min of exercise, and remained at a higher level with 30 min of recovery. AMPK alpha1 activity tended to slightly decrease with 20 min of exercise at 70%VO(2)max; however, the change was not statistically significant. AMPK alpha1 activities were at basal levels at 60 min of exercise and 30 min of recovery. On a separate day, the same subjects exercised for 20 min at 50% of VO(2)max. Exercise at this intensity did not change alpha2 activity, and similar to exercise at 70% of VO(2)max, there was no significant change in alpha1 activity. In conclusion, exercise at a higher intensity for only 20 min leads to increases in AMPK alpha2 activity but not alpha1 activity. These results suggest that the alpha2-containing AMPK complex, rather than alpha1, may be involved in the metabolic responses to exercise in human skeletal muscle.

Role of AMP-activated protein kinase in the regulation by glucose of islet beta cell gene expression

Elevated glucose concentrations stimulate the transcription of the pre-proinsulin (PPI), L-type pyruvate kinase (L-PK), and other genes in islet beta cells. In liver cells, pharmacological activation by 5-amino-4-imidazolecarboxamide riboside (AICAR) of AMP-activated protein kinase (AMPK), the mammalian homologue of the yeast SNF1 kinase complex, inhibits the effects of glucose, suggesting a key signaling role for this kinase. Here, we demonstrate that AMPK activity is inhibited by elevated glucose concentrations in MIN6 beta cells and that activation of the enzyme with AICAR prevents the activation of the L-PK gene by elevated glucose. Furthermore, microinjection of antibodies to the alpha2- (catalytic) or beta2-subunits of AMPK complex, but not to the alpha1-subunit or extracellular stimulus-regulated kinase, mimics the effects of elevated glucose on the L-PK and PPI promoter activities as assessed by single-cell imaging of promoter luciferase constructs. In each case, injection of antibodies into the nucleus and cytosol, but not the nucleus alone, was necessary, indicating the importance of either a cytosolic phosphorylation event or the subcellular localization of the alpha2-subunits. Incubation with AICAR diminished, but did not abolish, the effect of glucose on PPI transcription. These data suggest that glucose-induced changes in AMPK activity are necessary and sufficient for the regulation of the L-PK gene by the sugar and also play an important role in the regulation of the PPI promoter.

Characterization of AMP-activated protein kinase gamma-subunit isoforms and their role in AMP binding

The AMP-activated protein kinase (AMPK) cascade plays an important role in the regulation of energy homeostasis within the cell. AMPK is a heterotrimer composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma). We have isolated and characterized two isoforms of the gamma subunit, termed gamma2 and gamma3. Both gamma2 (569 amino acids) and gamma3 (492 amino acids) have a long N-terminal domain which is not present in the previously characterized isoform, gamma1. As with gamma1, mRNA encoding gamma2 is widely expressed in human tissues, whereas significant expression of gamma3 mRNA was only detected in skeletal muscle. Using isoform-specific antibodies, we determined the AMPK activity associated with the different gamma isoforms in a number of rat tissues. In most tissues examined more than 80% of total AMPK activity was associated with the gamma1 isoform, with the remaining activity being accounted for mainly by the gamma2 isoform. Exceptions to this were testis and, more notably, brain where all three isoforms contributed approximately equally to activity. There was no evidence for any selective association between the alpha1 and alpha2isoforms and the various gamma isoforms. However, the AMP-dependence of the kinase complex is markedly affected by the identity of the gamma isoform present, with gamma2-containing complexes having the greatest AMP-dependence, gamma3 the lowest, and gamma1 having an intermediate effect. Labelling studies, using the reactive AMP analogue 8-azido-[(32)P]AMP, indicate that the gamma subunit may participate directly in the binding of AMP within the complex.

The regulation of AMP-activated protein kinase by phosphorylation

The AMP-activated protein kinase (AMPK) cascade is activated by an increase in the AMP/ATP ratio within the cell. AMPK is regulated allosterically by AMP and by reversible phosphorylation. Threonine-172 within the catalytic subunit (alpha) of AMPK (Thr(172)) was identified as the major site phosphorylated by the AMP-activated protein kinase kinase (AMPKK) in vitro. We have used site-directed mutagenesis to study the role of phosphorylation of Thr(172) on AMPK activity. Mutation of Thr(172) to an aspartic acid residue (T172D) in either alpha1 or alpha2 resulted in a kinase complex with approx. 50% the activity of the corresponding wild-type complex. The activity of wild-type AMPK decreased by greater than 90% following treatment with protein phosphatases, whereas the activity of the T172D mutant complex fell by only 10-15%. Mutation of Thr(172) to an alanine residue (T172A) almost completely abolished kinase activity. These results indicate that phosphorylation of Thr(172) accounts for most of the activation by AMPKK, but that other sites are involved. In support of this we have shown that AMPKK phosphorylates at least two other sites on the alpha subunit and one site on the beta subunit. Furthermore, we provide evidence that phosphorylation of Thr(172) may be involved in the sensitivity of the AMPK complex to AMP.

Growth suppression by Lkb1 is mediated by a G(1) cell cycle arrest

Germ-line mutations of LKB1 (STK11) lead to Peutz-Jeghers syndrome characterized by gastrointestinal polyps and cancer of different organ systems. The mutations lead to loss or severe impairment of Lkb1 serine/threonine kinase activity. Therefore LKB1 has been implicated as a tumor suppressor gene, but only a few mutations in the coding exons of LKB1 have been detected in sporadic tumors. Here, we have identified tumor cell lines with severely reduced mRNA levels and impaired Lkb1 kinase activity. Reintroducing Lkb1 into these cells suppressed cell growth. The Lkb1-mediated growth inhibition was caused by a G(1) cell cycle block and was not detected with several naturally occurring Lkb1 mutants. These results indicate that LKB1 has functional and specific growth-suppressing activity.

AMP-activated protein kinase, a metabolic master switch: possible roles in type 2 diabetes

Adenosine 5'-monophosphate-activated protein kinase (AMPK) now appears to be a metabolic master switch, phosphorylating key target proteins that control flux through metabolic pathways of hepatic ketogenesis, cholesterol synthesis, lipogenesis, and triglyceride synthesis, adipocyte lipolysis, and skeletal muscle fatty acid oxidation. Recent evidence also implicates AMPK as being responsible for mediating the stimulation of glucose uptake induced by muscle contraction. In addition, the secretion of insulin by insulin secreting (INS-1) cells in culture is modulated by AMPK activation. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipolysis and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells. In skeletal muscle, AMPK is activated by contraction. Type 2 diabetes mellitus is likely to be a disease of numerous etiologies. However, defects or disuse (due to a sedentary lifestyle) of the AMPK signaling system would be predicted to result in many of the metabolic perturbations observed in Type 2 diabetes mellitus. Increased recruitment of the AMPK signaling system, either by exercise or pharmaceutical activators, may be effective in correcting insulin resistance in patients with forms of impaired glucose tolerance and Type 2 diabetes resulting from defects in the insulin signaling cascade.

Cellular distribution and developmental expression of AMP-activated protein kinase isoforms in mouse central nervous system

The mammalian AMP-activated protein kinase is a heterotrimeric serine/threonine protein kinase with multiple isoforms for each subunit (alpha, beta, and gamma) and is activated under conditions of metabolic stress. It is widely expressed in many tissues, including the brain, although its expression pattern throughout the CNS is unknown. We show that brain mRNA levels for the alpha2 and beta2 subunits were increased between embryonic days 10 and 14, whereas expression of alpha1, beta1, and gamma1 subunits was consistent at all ages examined. Immunostaining revealed a mainly neuronal distribution of all isoforms. The alpha2 catalytic subunit was highly expressed in neurons and activated astrocytes, whereas the alpha1 catalytic subunit showed low expression in neuropil. The gamma1 noncatalytic subunit was highly expressed by neurons, but not by astrocytes. Expression of the beta1 and beta2 noncatalytic subunits varied, but some neurons, such as granule cells of olfactory bulb, did not express detectable levels of either beta isoform. Preferential nuclear localization of the alpha2, beta1, and gamma1 subunits suggests new functions of the AMP-activated protein kinase, and the different expression patterns and cellular localization between the two catalytic subunits alpha1 and alpha2 point to different physiological roles.