AMPK-Mediated AS160 Phosphorylation in Skeletal Muscle Is Dependent on AMPK Catalytic and Regulatory Subunits
http://www.100md.com
糖尿病学杂志 2006年第7期
1 Department of Human Physiology, Institute of Exercise and Sport Sciences, Copenhagen Muscle Research Centre, University of Copenhagen, Copenhagen, Denmark
2 Department of Molecular Medicine and Surgery, Section Integrative Physiology, Karolinska Institute, Stockholm, Sweden
3 Renee Descartes University, Institute Cochin, Paris, France
4 Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala Biomedical Center, Uppsala, Sweden
5 Swiss Federal Institute of Technology, Zurich, Switzerland
ACC, acetyl-CoA carboxylase; AICAR, 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside; AMPK, AMP-activated protein kinase; CaMKK, calmodulin-dependent protein kinase kinase; EDL, extensor digitorum longus; GST, glutathione S-transferase; PAS, phospho-Akt substrate; TSC, tuberous sclerosis complex
ABSTRACT
AMP-activated protein kinase (AMPK) is a heterotrimeric protein that regulates glucose transport mediated by cellular stress or pharmacological agonists such as 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside (AICAR). AS160, a Rab GTPase-activating protein, provides a mechanism linking AMPK signaling to glucose uptake. We show that AICAR increases AMPK, acetyl-CoA carboxylase, and AS160 phosphorylation by insulin-independent mechanisms in isolated skeletal muscle. Recombinant AMPK heterotrimeric complexes (111 and 221) phosphorylate AS160 in a cell-free assay. In mice deficient in AMPK signaling (2 AMPK knockout [KO], 2 AMPK kinase dead [KD], and 3 AMPK KO), AICAR effects on AS160 phosphorylation were severely blunted, highlighting that complexes containing 2 and 3 are necessary for AICAR-stimulated AS160 phosphorylation in intact skeletal muscle. Contraction-mediated AS160 phosphorylation was also impaired in 2 AMPK KO and KD but not 3 AMPK KO mice. Our results implicate AS160 as a downstream target of AMPK.
The 5'AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that is ubiquitously expressed and functions as an intracellular fuel sensor activated by depletion of high-energy phospho-compounds (1). Activation of AMPK initiates a complex series of signaling events that trigger increases in the uptake and oxidation of substrates important for ATP synthesis and decreases in ATP-consuming biosynthetic processes such as protein, lipid, and glycogen synthesis (2). Activation of AMPK has been linked to the regulation of glucose transport (3), but the substrates linking these events are elusive.
AMPK regulates GLUT4-dependent glucose transport across the sarcolemma in skeletal muscle in response to diverse forms of cellular stress including exercise, hypoxia, and agents that disrupt the intracellular ATP-to-AMP ratio (4eC6). Evidence linking AMPK and glucose uptake has been provided by the use of different genetic approaches, including knockout (KO) and transgenic dominant-negative kinase mouse models (4,5,6). Expression of a dominant-negative 2 AMPK construct in skeletal muscle suppresses 2 and 1 isoform-specific AMPK activity and completely prevents 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside (AICAR)-induced glucose transport (6). This observation was reinforced by recent reports showing that knockout of either the catalytic 2 (but not 1 AMPK isoform) or the regulatory 3 AMPK subunit completely abolishes AICAR-induced glucose transport (4,5). Collectively, these data provide evidence to suggest that 2 and 3 containing AMPK heterotrimeric complexes are involved in AICAR-induced glucose transport. Although AICAR-induced glucose transport in resting skeletal muscle is mediated by GLUT4 translocation to the plasma membrane (7,8), the link between AMPK signaling and GLUT4 translocation is presently unknown.
AS160 is a substrate for the protein kinase Akt that links insulin signaling and GLUT4 trafficking (9eC11). AS160 contains a GTPase-activating protein homology domain that has been shown to regulate the GTPase activity of certain Rab proteins in vitro (12). Phosphorylation of AS160 by Akt is likely to inhibit its GTPase-activating protein activity, such that as a consequence, the GTP form of a Rab protein is elevated and this elevation in turn increases GLUT4 vesicle movement to, and/or fusion with, the plasma membrane (9eC11). In a cell-free assay, Akt phosphorylates AS160 (13). In 3T3-L1 adipocytes, insulin induces phosphorylation of at least six residues on AS160, of which five are placed in the Akt phosphorylation motif RXRXXS/T (10). Mutational analysis reveals that Ser-588 and Thr-642 on AS160 are of particular importance for the regulation of insulin-induced GLUT4 translocation (10). Insulin increases AS160 phosphorylation in a wortmannin-sensitive manner in 3T3-L1 adipocytes (13) and skeletal muscle (14). Thus, AS160 plays a pivotal role in insulin-mediated GLUT4 exocytosis at a step before the fusion of GLUT4-containing vesicles with the plasma membrane, such that when AS160 is dephosphorylated, GLUT4 vesicles remain in an intracellular compartment and upon phosphorylation by Akt, or other related kinases, GLUT4 is released for fusion with the plasma membrane (9,11).
AICAR treatment induces AS160 phosphorylation in isolated rat skeletal muscle (14). Insulin also increases AS160 phosphorylation in rat epitrochlearis (14) and human skeletal muscle (15eC17). In type 2 diabetic patients (16), and in healthy humans infused with the cytokine tumor necrosis factor- (17), AS160 phosphorylation and glucose transport are reduced in skeletal muscle. In contrast to effects of insulin, AICAR-induced AS160 phosphorylation occurs in the absence of Akt Ser-473 phosphorylation, a site necessary for full activation of the kinase, suggesting an Akt-independent regulation of AS160 (14). Since AICAR is a potent AMPK activator, we determined whether AS160 is an AMPK substrate in skeletal muscle. We hypothesized that AMPK mediates glucose transport in resting skeletal muscle via phosphorylation of AS160. This question is of clinical relevance since activation of AMPK bypasses insulin signaling defects, thereby increasing glucose transport and controlling glucose homeostasis (7). We also investigated whether contraction, a physiological stimulus for glucose uptake in skeletal muscle, increased AS160 phosphorylation by an AMPK-dependent mechanism. To this end, skeletal muscle from three genetically modified AMPK mouse models were studied: the whole-body 2 AMPK KO (18), 3 AMPK KO (4,19), and the skeletal muscleeCspecific 2 AMPK kinase dead (KD) (6).
RESEARCH DESIGN AND METHODS
All experiments were approved by the Danish Animal Experimental Inspectorate and the Regional Animal Ethical Committee (Stockholm, Sweden) and complied with the European Convention for the Protection of Vertebrate Animals Used for Experiments and Other Scientific Purposes (Council of Europe 123, Strasbourg, France). Male mice, 4eC5 months of age, from four different strains were used: 2 AMPK KO (18), transgenic mice expressing a KD-dominant inhibitory mutant of 2 AMPK (kindly provided by Morris J. Birnbaum, Howard Hughes Medical Institute and University of Pennsylvania School of Medicine, Philadelphia, PA) (6), 3 AMPK KO (4), and C57Bl/6 mice (Taconic, Ry, Denmark). Wild-type littermates were used as controls for each genetic model studied. Within each transgenic strain, mice of different genotypes were produced by intercross breeding (hetero-hetero for 2 and 3 and hetero-wild type for KD). Mice were maintained on a 10 h/14 h light/dark cycle and received standard rodent diet.
Muscle incubation procedures.
Extensor digitorum longus (EDL) muscles were quickly removed from anesthetized fed mice and incubated in Krebs-Henseleit prebuffer at 30°C oxygenated with a gas containing 95% O2 and 5% CO2. Mice were anesthetized with either Avertin (2,2,2-tribromo ethanol 99% and tertiary amyl alcohol, at 0.015eC0.017 ml/g mouse body wt i.p. for 3 AMPK KO) or pentobarbital (6 mg/100 g body wt i.p. for 2 AMPK KO and 2 AMPK KD mice). The content of the prebuffer has been described previously (4,18). Incubations were carried out using prebuffer, to which AICAR (2 mmol/l; Toronto Research Chemicals, Toronto, Canada), wortmannin (500 nmol/l; Sigma), insulin (60 nmol/l Actrapid; Novo Nordisk, Bagsvrd, Denmark), and/or DMSO (0.01%; Bie & Berntsen, Rdovre, Denmark) was added. To test the time-dependent effect of AICAR on AS160 phosphorylation, EDL muscles from wild-type C57Bl/6 mice were preincubated for 30 min in prebuffer, followed by incubation with or without 2 mmol/l AICAR for 10, 20, 40, or 60 min. To evaluate whether wortmannin inhibits AICAR- or insulin-induced AS160 phosphorylation, EDL muscles from wild-type C57Bl/6 mice were preincubated for 30 min in prebuffer with or without wortmannin, after which muscles were incubated for 40 min in the absence or presence of either AICAR or insulin. To test whether AICAR increased AS160 phosphorylation in an AMPK-dependent manner, EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice and corresponding wild-type littermates were preincubated for 10 min in prebuffer, followed by a 40-min exposure to AICAR. To assess the effects of contraction on AMPK signaling and AS160 phosphorylation, isolated EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice and corresponding wild-type littermates were incubated for 40 min in prebuffer. Thereafter, contraction was induced by electrical stimulation with a 10-s train (100 Hz, 0.2-ms impulse, 30 V) per minute for 10 min. Contralateral muscles were incubated for 10 min under resting condition. To assess the effect of insulin on AS160 phosphorylation in the 2 AMPK KO and wild-type animals, EDL muscles were preincubated for 10 min in prebuffer, followed by a 10-min exposure to insulin. After incubation, muscles were harvested, washed in ice-cold Krebs-Henseleit buffer, blotted on filter paper, quickly frozen with aluminum tongs precooled in liquid nitrogen, and stored at eC80°C.
Muscle lysate preparation.
Muscles were homogenized in ice-cold buffer (10% glycerol, 20 mmol/l sodium pyrophosphate, 150 mmol/l NaCl, 50 mmol/l HEPES [pH 7.5], 1% NP-40, 20 mmol/l -glycerophosphate, 10 mmol/l sodium fluoride, 1 mmol/l EDTA, 1 mmol/l EGTA, 2 mmol/l phenylmethylsulfonyl fluoride, 10 e蘥/ml aprotinin, 10 e蘥/ml leupeptin, 2 mmol/l sodium orthovanadate, 3 mmol/l benzamidine [pH 7.4]) for 20 s using a homogenizer (PT 3100; Brinkman Instruments). Homogenates were rotated end-over-end for 1 h at 4°C and then subjected to centrifugation (17,000g for 20 min) at 4°C. Supernatants were quickly frozen in liquid nitrogen and stored at eC80°C. Protein content in lysates was measured by the bicinchoninic acid method (Pierce, Rockford, IL).
Immunoblot analysis.
Phosphorylation of AMPK on Thr-172 was measured using a phospho-specific antibody (Cell Signaling Technology, Danvers, MA). Acetyl-CoA carboxylase (ACC) Ser-227 phosphorylation was determined using a phospho-specific antibody (Upstate Biotechnologies, Waltham, MA). Phosphorylation of AS160 was measured using a phospho (Ser/Thr)-Akt substrate (PAS) antibody (Cell Signaling Technology, Danvers, MA). Phosphorylation of Akt on Ser-473 was measured with a phospho-specific antibody (Cell Signaling Technology). Total AS160 protein level was determined using an affinity-purified rabbit antibody prepared against a glutathione S-transferase (GST) fusion protein with a portion of mouse AS160 corresponding to amino acids 584eC833 of human AS160 (kindly provided by Gustav E. Lienhard, Dartmouth Medical School, Hanover, NH). Muscle lysates were adjusted to equal protein concentration and boiled in Laemmli buffer, loaded on either 7.5% Tris-HCl Criterion gels (Bio-Rad, Copenhagen, Denmark) or self-cast 6eC12% or 6eC15% gradient gels, and transferred to polyvinylidene difluoride membrane (Immobilon Transfer Membrane; Millipore, Copenhagen, Denmark). Membranes were blocked in washing buffer (10 mmol/l Tris-base, 150 mmol/l NaCl, 0.25% Tween 20) containing 7.5% low-fat milk protein for 1 h at room temperature. Membranes were then incubated with primary antibodies overnight at 4°C, followed by incubation with appropriate horseradish peroxidaseeCconjugated secondary antibody (Dako, Glostrup, Denmark) for 2 h at room temperature. Bands were visualized using a Kodak Image Station 2000MM and enhanced chemiluminescence. Immunoreactive proteins were quantified (Kodak 1D 3.6 software), and results were expressed in relative units in comparison with basal samples loaded on each gel.
Immunoprecipitation.
EDL muscle lysate (350 e蘥 protein) was immunoprecipitated with 3.5 e蘥 COOH-terminal anti-AS160 antibody at 4°C with gentle rotation overnight. Samples were incubated with equal mixtures of protein A sepharose (Amersham, Uppsala, Sweden) and protein G agarose (Sigma-Aldrich, St. Louis, MO) for 3 h at 4°C and subsequently washed three times with homogenization buffer and four times with PBS. The immunocomplex was boiled in Laemmli buffer containing -mercaptoethanol and subjected to SDS-PAGE.
Cell-free AMPK kinase assay.
Recombinant functional AMPK heterotrimers (111 or 221) were expressed in Escherichia coli (20) and purified from bacterial lysates by a single-step nickel-ion chromatography, utilizing a polyhistidine tag fused to the NH2-terminus of the subunit, as previously described (20). A GST fusion protein of calmodulin-dependent protein kinase kinase (CaMKK) was expressed in E. coli, purified, and cleaved from GST as previously described (21). For activation of recombinant wild-type AMPK (111 or 221), 0.5 mg enzyme (20) was incubated for 60 min at 30°C with 0.5 mg recombinant CaMKK (21) in activation buffer (10 mmol/l HEPES [pH 7.5], 5 mmol/l MgCl2, 1 mmol/l AMP, 2 mmol/l ATP). EDL muscle lysates (100 mg) were incubated (30 min at 30°C) in assay buffer (10 mmol/l HEPES [pH 7.5], 5 mmol/l MgCl2, 1 mmol/l EGTA, 0.5 mmol/l Na3VO4, 40 e蘭ol/l P1,P5-di(adenosine-5') pentaphosphate, 100 e蘭ol/l AMP, 200 e蘭ol/l ATP) with or without recombinant-activated wild-type AMPK. Assays were performed under six conditions: 1) lysate and assay buffer; 2) lysate, assay buffer, and AMP; 3) lysate, assay buffer, AMP, and ATP; and 4-6) lysate, assay buffer, AMP, ATP, and recombinant CaMKK or AMPK (111 or 221). The reaction was stopped by addition of sample buffer (6x) and heating (96°C for 4 min). Samples were subjected to SDS-PAGE, followed by Western blot analysis with the PAS antibody.
Statistics.
Data are expressed as means ± SE. Levene’s test of equality of variance was used to test whether groups had equal variances, and data were log-transformed if Levene’s test was significant. In all cases, this was sufficient to obtain equal variances among groups. Statistical evaluation was performed by two-way ANOVA with repeated measures or one-way ANOVA, using Fisher’s least significant difference post hoc analysis to identify significant differences between groups. All statistical analyses were performed using SPSS 13.0. P < 0.05 was considered significant.
RESULTS
AICAR treatment in vitro leads to AS160 phosphorylation in skeletal muscle.
Immunoblot analysis with the PAS antibody revealed that AICAR and insulin independently increased the phosphorylation of multiple proteins in EDL muscle from C57BL/6 mice (Fig. 1A). Previous studies provide evidence that exposure of isolated rat epitrochlearis muscle to AICAR increases AS160 phosphorylation (14). A phospho-protein of 160 kDa, confirmed as AS160 in the immunoprecipitation experiments (Fig. 1B), was phosphorylated in response to insulin exposure. AICAR increased AS160 phosphorylation in a time-dependent manner (Fig. 1C) paralleled by increased phosphorylation of AMPK on Thr-172 and ACC, an AMPK substrate, on Ser-227 (Fig. 1D and E).
AICAR-induced AS160 phosphorylation is wortmannin independent.
Insulin action on AS160 phosphorylation occurred by a wortmannin-sensitive mechanism (Fig. 2A), suggesting that this effect was mediated by signal transduction over a phosphatidylinositol-3 kinaseeCand Akt-dependent pathway. In contrast, AICAR increased AS160 phosphorylation in the presence of wortmannin (Fig. 2A) and without effect on Akt (Ser-473) phosphorylation (Fig. 2B), implicating that this effect was mediated by a pathway independent of the canonical insulin signaling cascade. We hypothesized that the AICAR effect on AS160 occurs via the AMPK-signaling pathway. Consistent with this hypothesis, wortmannin did not alter AICAR-induced phosphorylation of AMPK (Thr-172) (Fig. 2C) or ACC (Ser-227) (Fig. 2D) in murine EDL muscle.
To test whether AMPK phosphorylates AS160, we performed a cell-free kinase assay adding either 111 or 221 recombinant AMPK heterotrimeric complexes to EDL muscle lysate (n = 3). Several phosphoproteins were identified by the PAS antibody in subsequent Western blot analysis (Fig. 2E). Phosphorylation of AS160 was enhanced in vitro by the addition of either 111 or 221 recombinant AMPK heterotrimeric complexes in all preparations tested, whereas the addition of recombinant CaMKK was without effect (Fig. 2F). This in vitro approach could indicate that AS160 is an AMPK substrate.
AS160 phosphorylation is regulated by AMPK in isolated EDL muscle.
To establish the role of AS160 as an AMPK target, EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice were isolated and incubated in the absence or presence of AICAR. In all experiments, genetically modified mice were compared with wild-type littermates to exclude any influence of genetic background on the measured phenotype. Differences in signaling responses were noted among wild-type mice of the three strains, further reinforcing the need to study each genetically modified animal against their own wild-type littermate. AICAR increased AMPK phosphorylation in wild-type mice from each genotype. This effect was blunted in 2 AMPK KO and 3 AMPK KO but not in 2 AMPK KD mice (Table 1). The AICAR response on AMPK observed in the 2 AMPK KD mice was expected, since the dominant-negative construct can be phosphorylated but not activated (6). AICAR increased ACC phosphorylation in wild-type mice, with effects severely blunted in 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice (Table 1). In contrast to wild-type mice, the effect of AICAR on AS160 phosphorylation was significantly (P = 0.02) blunted (2 AMPK KO) or ablated (2 AMPK KD and 3 AMPK KO) in EDL muscle from KO or transgenic mice (Fig. 3A). In EDL muscle from 2 AMPK KO and 2 AMPK KD mice, basal AS160 phosphorylation was also reduced compared with wild-type mice. This difference in basal AS160 phosphorylation may be related to a slight and nonsignificant reduction in AS160 protein expression in 2 AMPK KO and 2 AMPK KD mice (Fig. 3B). In contrast, basal AS160 phosphorylation and protein expression was similar between 3 AMPK KO and wild-type mice.
To address whether physiological activation of AMPK by muscle contraction is associated with AS160 phosphorylation, isolated EDL muscle was subjected to intermittent electrical stimulation and forced to contract. Here, we provide evidence that AMPK phosphorylation after electrically induced muscle contraction is markedly increased in wild-type mice from all strains and blunted in 2 AMPK KO mice (Table 2). In contrast, contraction-induced AMPK phosphorylation was unaltered in 2 AMPK KD and 3 AMPK KO mice. ACC phosphorylation, after electrically induced muscle contraction, was markedly increased in wild-type and 3 AMPK KO mice and reduced in 2 AMPK KO and 2 AMPK KD mice. Similarly, contraction markedly increased AS160 phosphorylation in wild-type mice, with contraction-induced responses completely suppressed in 2 AMPK KO and 2 AMPK KD but not in 3 AMPK KO mice (Fig. 3C).
Akt signaling is unaltered in 2 AMPK KO mice.
Since AS160 phosphorylation was reduced under basal and AICAR-stimulated conditions in 2 AMPK KO and 2 AMPK KD mice, additional experiments were performed to determine whether insulin-mediated responses were impaired in these mice. Phosphorylation of Akt (Ser-473) and AS160 phosphorylation was increased in response to 60 nmol/l insulin, irrespective of genotype (Fig. 4A and B). Although basal and insulin-stimulated Akt phosphorylation was similar between 2 AMPK KO and wild-type mice, the absolute level of AS160 phosphorylation under basal and insulin-stimulated conditions was reduced. Nevertheless, the increase in AS160 phosphorylation in response to insulin was maintained, indicating an intact response to insulin. AMPK and ACC phosphorylation in 2 AMPK KO and wild-type mice was unaffected by insulin treatment (data not shown).
DISCUSSION
Using three different animal models in which the predominant catalytic and regulatory AMPK isoforms expressed in skeletal muscle have been genetically manipulated, we provide evidence that AS160 is a downstream target of AMPK in skeletal muscle. Exposure of isolated skeletal muscle to AICAR, an AMPK activator, increased AS160 phosphorylation in wild-type but only slightly or not at all in 2 AMPK KO, 3 AMPK KO, or 2 AMPK KD mice. Taken together with previous observations that AICAR-induced glucose transport is totally abolished in these KO and transgenic mouse models (4eC6), our results indicate that AICAR could increase glucose transport by a pathway involving AMPK and likely AS160. Moreover, heterotrimeric complexes containing the AMPK 2/3 subunits are likely to play a major role in the regulation of AS160 phosphorylation upon pharmacological activation by AICAR, since the AICAR effects on AS160 were ablated in 2 and 3 AMPK KO mice. This is of physiological relevance, since the 3 isoform is mainly found in complexes containing 2 and 2 subunits in skeletal muscle (22,23).
We also provide evidence that AMPK is the major contributor to the regulation of AS160 in response to muscle contraction. Despite our evidence supporting a role for AS160 as an AMPK-dependent target important for GLUT4-mediated glucose transport in resting muscle, the connection between AS160 and glucose transport is unresolved in contracting skeletal muscle. Previous studies indicate that contraction-induced glucose uptake is unaltered (5) or only partly decreased (6) in the same AMPK-deficient mouse models in which contraction-induced AS160 phosphorylation was completely prevented. Thus, although contraction increases AS160 phosphorylation in skeletal muscle, AS160-independent mechanisms may elicit contraction-induced responses on glucose transport. In accordance, Ca2+-mediated mechanisms have been proposed in the regulation of contraction-induced skeletal muscle glucose transport (24). Thus, AS160 is unlikely to be the sole mechanism involved in the release of GLUT4-containing vesicles. This hypothesis is supported by recent evidence showing that silencing of AS160 by short-hairpin RNA in 3T3-L1 adipocytes only partially released basal GLUT4 retention, thereby implicating AS160-dependent and -independent mechanisms in insulin signaling to GLUT4 exocytosis (25).
AMPK may phosphorylate AS160 directly or indirectly through unidentified substrates. Using an in vitro AMPK activity assay, where addition of activated recombinant 111 or 221 complexes to skeletal muscle lysate increased AS160 phosphorylation, we provided evidence to suggest that AMPK, per se, phosphorylates AS160. In contrast, recombinant CaMKK was without effect. Our results do not eliminate a possible indirect effect of AMPK on AS160. For example, the recombinant AMPK may have phosphorylated another kinase, which in turn may have directly phosphorylated AS160. However, if AMPK is an indirect regulator of AS160, wortmannin-sensitive components of the canonical insulin signaling cascade, including Akt, are unlikely candidates.
AS160 is a negative regulator of basal GLUT4 exocytosis in 3T3-L1 adipocytes (9,25). In 2 AMPK KO and 2 AMPK KD mice, basal AS160 phosphorylation was impaired. Based on studies in 3T3-L1 adipocytes, in which AS160 expression is silenced (9,25), basal glucose uptake in 2 AMPK KO and 2 AMPK KD mice would be predicted to be decreased due to the reduction in AS160 phosphorylation. However, basal glucose transport is unaltered by modification of the AMPK 2 subunit (5,6). Thus, there may be a threshold effect, such that only a permissive level of AS160 phosphorylation may be necessary for basal glucose uptake. The reduced basal AS160 phosphorylation may be related to a slight but nonsignificant reduction in AS160 protein expression in 2 AMPK KO and 2 AMPK KD mice, although this alternative seems unlikely. In contrast, basal AS160 phosphorylation was preserved in the 3 AMPK KO mice, an observation that may suggest that heterotrimeric complexes containing this isoform are dispensable for basal AS160 phosphorylation. The reduced basal AS160 phosphorylation in the 2 AMPK KO mice was not associated with impairments in insulin signaling, as insulin-induced phosphorylation of Akt and AS160, as well as glucose transport, are unaltered (18). Likewise, insulin-stimulated glucose uptake and Akt signaling is unaltered in 2 AMPK KD mice (6). Nevertheless, there may be excess signaling, such that partial phosphorylation of AS160 may be sufficient for full activation of insulin-mediated glucose transport.
The PAS antibody used to detect AS160 phosphorylation recognizes proteins with phosphorylated serine or threonine residues that follow arginine at positions P-3 and P-5 (RXRXXS/T). The recognition sequences of Akt and AMPK differ (13,26,27); consequently, AMPK might target other unidentified sites on AS160, which could regulate AS160 activity or localization. Interestingly, tuberous sclerosis complex (TSC)2 in the TSC1-TSC2 complex, which functions as a GTPase-activating protein for the small GTPase Ras homologue enriched in brain (Rheb) in the mammalian target of rapamycin pathway, is phosphorylated by Akt and AMPK but on different sites, leading to either inhibition or activation, respectively, of TSC2 (28,29).
The regulation of glucose transport is complex and requires multiple signaling inputs that converge on GLUT4 vesicle transport (30,31). The identification of AS160 as an AMPK substrate provides a mechanism to partly account for insulin-independent glucose uptake in resting skeletal muscle, whereas a role in contraction-mediated glucose uptake is unclear. Our results reveal that AMPK complexes containing 2 and 3 are required for AS160 phosphorylation in response to the AMPK activator AICAR. Furthermore, AMPK increases AS160 phosphorylation in response to AICAR via a phosphatidylinositol-3 kinase/Akt-independent pathway. We also provide evidence that complexes containing the AMPK 2 subunit are required for AS160 phosphorylation in response to muscle contraction, whereas complexes containing the 3 subunit are dispensable. Thus, AS160 may be viewed as a point of convergence for insulin-dependent and -independent signaling to glucose transport in resting skeletal muscle.
ACKNOWLEDGMENTS
This work was supported by the Swedish Research Council, the Swedish Diabetes Association, the Foundation for Scientific Studies of Diabetology, the Strategic Research Foundation, the Swedish Centre for Sports Research, the Commission of the European Communities (contract nos. LSHM-CT-2004-005272 EXGENESIS and LSHM-CT-2004-512013 EUGENE2), the Danish Medical Research Council, the Novo Nordisk Foundation, the Danish Diabetes Association, and the Copenhagen Muscle Research Centre.
J.F.P.W. was supported by a Hallas Mller fellowship from the Novo Nordisk Foundation.
We are especially grateful to Hiroshi Tokumitsu (Kagawa University, Kagawa, Japan), Professor Gustav E. Lienhard (Dartmouth Medical School, Hanover, NH), and Professor Morris J. Birnbaum (Howard Hughes Medical Institute and University of Pennsylvania School of Medicine, Philadelphia, PA) for providing valuable reagents.
FOOTNOTES
J.T.T., S.G., and A.D. contributed equally to this work.
See related article by Kramer et al., p. 2067.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact
REFERENCES
Kahn BB, Alquier T, Carling D, Hardie DG: AMP-activated protein kinase: ancient energy gauge provides clues to modern understanding of metabolism. Cell Metab 1:15eC25, 2005
Hardie DG: The AMP-activated protein kinase pathway: new players upstream and downstream. J Cell Sci 117:5479eC5487, 2004
Merrill GF, Kurth EJ, Hardie DG, Winder WW: AICA riboside increases AMP-activated protein kinase, fatty acid oxidation, and glucose uptake in rat muscle. Am J Physiol 273:E1107eCE1112, 1997
Barnes BR, Marklund S, Steiler TL, Walter M, Hjalm G, Amarger V, Mahlapuu M, Leng Y, Johansson C, Galuska D, Lindgren K, Abrink M, Stapleton D, Zierath JR, Andersson L: The 5'-AMP-activated protein kinase gamma3 isoform has a key role in carbohydrate and lipid metabolism in glycolytic skeletal muscle. J Biol Chem 279:38441eC38447, 2004
Jrgensen SB, Viollet B, Andreelli F, Frsig C, Birk JB, Schjerling P, Vaulont S, Richter EA, Wojtaszewski JF: Knockout of the alpha2 but not alpha1 5'-AMP-activated protein kinase isoform abolishes 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranosidebut not contraction-induced glucose uptake in skeletal muscle. J Biol Chem 279:1070eC1079, 2004
Mu J, Brozinick JT Jr, 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 7:1085eC1094, 2001
Koistinen HA, Galuska D, Chibalin AV, Yang J, Zierath JR, Holman GD, Wallberg-Henriksson H: 5-amino-imidazole carboxamide riboside increases glucose transport and cell-surface GLUT4 content in skeletal muscle from subjects with type 2 diabetes. Diabetes 52:1066eC1072, 2003
Lemieux K, Konrad D, Klip A, Marette A: The AMP-activated protein kinase activator AICAR does not induce GLUT4 translocation to transverse tubules but stimulates glucose uptake and p38 mitogen-activated protein kinases alpha and beta in skeletal muscle. FASEB J 17:1658eC1665, 2003
Larance M, Ramm G, Stockli J, van Dam EM, Winata S, Wasinger V, Simpson F, Graham M, Junutula JR, Guilhaus M, James DE: Characterisation of the role of the RabGAP AS160 in insulin-regulated GLUT4 trafficking. J Biol Chem 280:37803eC37813, 2005
Sano H, Kane S, Sano E, Miinea CP, Asara JM, Lane WS, Garner CW, Lienhard GE: Insulin-stimulated phosphorylation of a Rab GTPase-activating protein regulates GLUT4 translocation. J Biol Chem 278:14599eC14602, 2003
Zeigerer A, McBrayer MK, McGraw TE: Insulin stimulation of GLUT4 exocytosis, but not its inhibition of endocytosis, is dependent on RabGAP AS160. Mol Biol Cell 15:4406eC4415, 2004
Miinea CP, Sano H, Kane S, Sano E, Fukuda M, Peranen J, Lane WS, Lienhard GE: AS160, the Akt substrate regulating GLUT4 translocation, has a functional Rab GTPase-activating protein domain. Biochem J 391:87eC93, 2005
Kane S, Sano H, Liu SC, Asara JM, Lane WS, Garner CC, Lienhard GE: A method to identify serine kinase substrates: Akt phosphorylates a novel adipocyte protein with a Rab GTPase-activating protein (GAP) domain. J Biol Chem 277:22115eC22118, 2002
Bruss MD, Arias EB, Lienhard GE, Cartee GD: Increased phosphorylation of Akt substrate of 160 kDa (AS160) in rat skeletal muscle in response to insulin or contractile activity. Diabetes 54:41eC50, 2005
Karlsson HK, Hllsten K, Bjrnholm M, Tsuchida H, Chibalin AV, Virtanen KA, Heinonen OJ, Lnnqvist F, Nuutila P, Zierath JR: Effects of metformin and rosiglitazone treatment on insulin signaling and glucose uptake in patients with newly diagnosed type 2 diabetes: a randomized controlled study. Diabetes 54:1459eC1467, 2005
Karlsson HK, Zierath JR, Kane S, Krook A, Lienhard GE, Wallberg-Henriksson H: Insulin-stimulated phosphorylation of the Akt substrate AS160 is impaired in skeletal muscle of type 2 diabetic subjects. Diabetes 54:1692eC1697, 2005
Plomgaard P, Bouzakri K, Krogh-Madsen R, Mittendorfer B, Zierath JR, Pedersen BK: Tumor necrosis factor- induces skeletal muscle insulin resistance in healthy human subjects via inhibition of Akt substrate 160 phosphorylation. Diabetes 54:2939eC2945, 2005
Viollet B, Andreelli F, Jorgensen SB, Perrin C, Geloen A, Flamez D, Mu J, Lenzner C, Baud O, Bennoun M, Gomas E, Nicolas G, Wojtaszewski JF, Kahn A, Carling D, Schuit FC, Birnbaum MJ, Richter EA, Burcelin R, Vaulont S: The AMP-activated protein kinase alpha2 catalytic subunit controls whole-body insulin sensitivity. J Clin Invest 111:91eC98, 2003
Barnes BR, Glund S, Long YC, Hjalm G, Andersson L, Zierath JR: 5'-AMP-activated protein kinase regulates skeletal muscle glycogen content and ergogenics. FASEB J 19:773eC779, 2005
Neumann D, Woods A, Carling D, Wallimann T, Schlattner U: Mammalian AMP-activated protein kinase: functional, heterotrimeric complexes by co-expression of subunits in Escherichia coli. Protein Expr Purif 30:230eC237, 2003
Tokumitsu H, Iwabu M, Ishikawa Y, Kobayashi R: Differential regulatory mechanism of Ca2+/calmodulin-dependent protein kinase kinase isoforms. Biochemistry 40:13925eC13932, 2001
Mahlapuu M, Johansson C, Lindgren K, Hjalm G, Barnes BR, Krook A, Zierath JR, Andersson L, Marklund S: Expression profiling of the gamma-subunit isoforms of AMP-activated protein kinase suggests a major role for gamma3 in white skeletal muscle. Am J Physiol Endocrinol Metab 286:E194eCE200, 2004
Wojtaszewski JF, Birk JB, Frosig C, Holten M, Pilegaard H, Dela F: 5'AMP activated protein kinase expression in human skeletal muscle: effects of strength training and type 2 diabetes. J Physiol 564:563eC573, 2005
Holloszy JO: A forty-year memoir of research on the regulation of glucose transport into muscle. Am J Physiol Endocrinol Metab 284:E453eCE467, 2003
Eguez L, Lee A, Chavez JA, Miinea CP, Kane S, Lienhard GE, McGraw TE: Full intracellular retention of GLUT4 requires AS160 Rab GTPase activating protein. Cell Metab 2:263eC272, 2005
Hardie DG, Carling D, Carlson M: The AMP-activated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell Annu Rev Biochem 67:821eC855, 1998
Weekes J, Ball KL, Caudwell FB, Hardie DG: Specificity determinants for the AMP-activated protein kinase and its plant homologue analysed using synthetic peptides. FEBS Lett 334:335eC339, 1993
Inoki K, Li Y, Zhu T, Wu J, Guan KL: TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat Cell Biol 4:648eC657, 2002
Inoki K, Zhu T, Guan KL: TSC2 mediates cellular energy response to control cell growth and survival. Cell 115:577eC590, 2003
Ramm G, James DE: GLUT4 trafficking in a test tube (Letter). Cell Metab 2:150eC152, 2005
Thong FS, Dugani CB, Klip A: Turning signals on and off: GLUT4 traffic in the insulin-signaling highway. Physiology (Bethesda) 20:271eC284, 2005(Jonas T. Treebak, Stephan)
2 Department of Molecular Medicine and Surgery, Section Integrative Physiology, Karolinska Institute, Stockholm, Sweden
3 Renee Descartes University, Institute Cochin, Paris, France
4 Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala Biomedical Center, Uppsala, Sweden
5 Swiss Federal Institute of Technology, Zurich, Switzerland
ACC, acetyl-CoA carboxylase; AICAR, 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside; AMPK, AMP-activated protein kinase; CaMKK, calmodulin-dependent protein kinase kinase; EDL, extensor digitorum longus; GST, glutathione S-transferase; PAS, phospho-Akt substrate; TSC, tuberous sclerosis complex
ABSTRACT
AMP-activated protein kinase (AMPK) is a heterotrimeric protein that regulates glucose transport mediated by cellular stress or pharmacological agonists such as 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside (AICAR). AS160, a Rab GTPase-activating protein, provides a mechanism linking AMPK signaling to glucose uptake. We show that AICAR increases AMPK, acetyl-CoA carboxylase, and AS160 phosphorylation by insulin-independent mechanisms in isolated skeletal muscle. Recombinant AMPK heterotrimeric complexes (111 and 221) phosphorylate AS160 in a cell-free assay. In mice deficient in AMPK signaling (2 AMPK knockout [KO], 2 AMPK kinase dead [KD], and 3 AMPK KO), AICAR effects on AS160 phosphorylation were severely blunted, highlighting that complexes containing 2 and 3 are necessary for AICAR-stimulated AS160 phosphorylation in intact skeletal muscle. Contraction-mediated AS160 phosphorylation was also impaired in 2 AMPK KO and KD but not 3 AMPK KO mice. Our results implicate AS160 as a downstream target of AMPK.
The 5'AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that is ubiquitously expressed and functions as an intracellular fuel sensor activated by depletion of high-energy phospho-compounds (1). Activation of AMPK initiates a complex series of signaling events that trigger increases in the uptake and oxidation of substrates important for ATP synthesis and decreases in ATP-consuming biosynthetic processes such as protein, lipid, and glycogen synthesis (2). Activation of AMPK has been linked to the regulation of glucose transport (3), but the substrates linking these events are elusive.
AMPK regulates GLUT4-dependent glucose transport across the sarcolemma in skeletal muscle in response to diverse forms of cellular stress including exercise, hypoxia, and agents that disrupt the intracellular ATP-to-AMP ratio (4eC6). Evidence linking AMPK and glucose uptake has been provided by the use of different genetic approaches, including knockout (KO) and transgenic dominant-negative kinase mouse models (4,5,6). Expression of a dominant-negative 2 AMPK construct in skeletal muscle suppresses 2 and 1 isoform-specific AMPK activity and completely prevents 5-aminoimidazole-4-carboxamide 1 -D-ribonucleoside (AICAR)-induced glucose transport (6). This observation was reinforced by recent reports showing that knockout of either the catalytic 2 (but not 1 AMPK isoform) or the regulatory 3 AMPK subunit completely abolishes AICAR-induced glucose transport (4,5). Collectively, these data provide evidence to suggest that 2 and 3 containing AMPK heterotrimeric complexes are involved in AICAR-induced glucose transport. Although AICAR-induced glucose transport in resting skeletal muscle is mediated by GLUT4 translocation to the plasma membrane (7,8), the link between AMPK signaling and GLUT4 translocation is presently unknown.
AS160 is a substrate for the protein kinase Akt that links insulin signaling and GLUT4 trafficking (9eC11). AS160 contains a GTPase-activating protein homology domain that has been shown to regulate the GTPase activity of certain Rab proteins in vitro (12). Phosphorylation of AS160 by Akt is likely to inhibit its GTPase-activating protein activity, such that as a consequence, the GTP form of a Rab protein is elevated and this elevation in turn increases GLUT4 vesicle movement to, and/or fusion with, the plasma membrane (9eC11). In a cell-free assay, Akt phosphorylates AS160 (13). In 3T3-L1 adipocytes, insulin induces phosphorylation of at least six residues on AS160, of which five are placed in the Akt phosphorylation motif RXRXXS/T (10). Mutational analysis reveals that Ser-588 and Thr-642 on AS160 are of particular importance for the regulation of insulin-induced GLUT4 translocation (10). Insulin increases AS160 phosphorylation in a wortmannin-sensitive manner in 3T3-L1 adipocytes (13) and skeletal muscle (14). Thus, AS160 plays a pivotal role in insulin-mediated GLUT4 exocytosis at a step before the fusion of GLUT4-containing vesicles with the plasma membrane, such that when AS160 is dephosphorylated, GLUT4 vesicles remain in an intracellular compartment and upon phosphorylation by Akt, or other related kinases, GLUT4 is released for fusion with the plasma membrane (9,11).
AICAR treatment induces AS160 phosphorylation in isolated rat skeletal muscle (14). Insulin also increases AS160 phosphorylation in rat epitrochlearis (14) and human skeletal muscle (15eC17). In type 2 diabetic patients (16), and in healthy humans infused with the cytokine tumor necrosis factor- (17), AS160 phosphorylation and glucose transport are reduced in skeletal muscle. In contrast to effects of insulin, AICAR-induced AS160 phosphorylation occurs in the absence of Akt Ser-473 phosphorylation, a site necessary for full activation of the kinase, suggesting an Akt-independent regulation of AS160 (14). Since AICAR is a potent AMPK activator, we determined whether AS160 is an AMPK substrate in skeletal muscle. We hypothesized that AMPK mediates glucose transport in resting skeletal muscle via phosphorylation of AS160. This question is of clinical relevance since activation of AMPK bypasses insulin signaling defects, thereby increasing glucose transport and controlling glucose homeostasis (7). We also investigated whether contraction, a physiological stimulus for glucose uptake in skeletal muscle, increased AS160 phosphorylation by an AMPK-dependent mechanism. To this end, skeletal muscle from three genetically modified AMPK mouse models were studied: the whole-body 2 AMPK KO (18), 3 AMPK KO (4,19), and the skeletal muscleeCspecific 2 AMPK kinase dead (KD) (6).
RESEARCH DESIGN AND METHODS
All experiments were approved by the Danish Animal Experimental Inspectorate and the Regional Animal Ethical Committee (Stockholm, Sweden) and complied with the European Convention for the Protection of Vertebrate Animals Used for Experiments and Other Scientific Purposes (Council of Europe 123, Strasbourg, France). Male mice, 4eC5 months of age, from four different strains were used: 2 AMPK KO (18), transgenic mice expressing a KD-dominant inhibitory mutant of 2 AMPK (kindly provided by Morris J. Birnbaum, Howard Hughes Medical Institute and University of Pennsylvania School of Medicine, Philadelphia, PA) (6), 3 AMPK KO (4), and C57Bl/6 mice (Taconic, Ry, Denmark). Wild-type littermates were used as controls for each genetic model studied. Within each transgenic strain, mice of different genotypes were produced by intercross breeding (hetero-hetero for 2 and 3 and hetero-wild type for KD). Mice were maintained on a 10 h/14 h light/dark cycle and received standard rodent diet.
Muscle incubation procedures.
Extensor digitorum longus (EDL) muscles were quickly removed from anesthetized fed mice and incubated in Krebs-Henseleit prebuffer at 30°C oxygenated with a gas containing 95% O2 and 5% CO2. Mice were anesthetized with either Avertin (2,2,2-tribromo ethanol 99% and tertiary amyl alcohol, at 0.015eC0.017 ml/g mouse body wt i.p. for 3 AMPK KO) or pentobarbital (6 mg/100 g body wt i.p. for 2 AMPK KO and 2 AMPK KD mice). The content of the prebuffer has been described previously (4,18). Incubations were carried out using prebuffer, to which AICAR (2 mmol/l; Toronto Research Chemicals, Toronto, Canada), wortmannin (500 nmol/l; Sigma), insulin (60 nmol/l Actrapid; Novo Nordisk, Bagsvrd, Denmark), and/or DMSO (0.01%; Bie & Berntsen, Rdovre, Denmark) was added. To test the time-dependent effect of AICAR on AS160 phosphorylation, EDL muscles from wild-type C57Bl/6 mice were preincubated for 30 min in prebuffer, followed by incubation with or without 2 mmol/l AICAR for 10, 20, 40, or 60 min. To evaluate whether wortmannin inhibits AICAR- or insulin-induced AS160 phosphorylation, EDL muscles from wild-type C57Bl/6 mice were preincubated for 30 min in prebuffer with or without wortmannin, after which muscles were incubated for 40 min in the absence or presence of either AICAR or insulin. To test whether AICAR increased AS160 phosphorylation in an AMPK-dependent manner, EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice and corresponding wild-type littermates were preincubated for 10 min in prebuffer, followed by a 40-min exposure to AICAR. To assess the effects of contraction on AMPK signaling and AS160 phosphorylation, isolated EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice and corresponding wild-type littermates were incubated for 40 min in prebuffer. Thereafter, contraction was induced by electrical stimulation with a 10-s train (100 Hz, 0.2-ms impulse, 30 V) per minute for 10 min. Contralateral muscles were incubated for 10 min under resting condition. To assess the effect of insulin on AS160 phosphorylation in the 2 AMPK KO and wild-type animals, EDL muscles were preincubated for 10 min in prebuffer, followed by a 10-min exposure to insulin. After incubation, muscles were harvested, washed in ice-cold Krebs-Henseleit buffer, blotted on filter paper, quickly frozen with aluminum tongs precooled in liquid nitrogen, and stored at eC80°C.
Muscle lysate preparation.
Muscles were homogenized in ice-cold buffer (10% glycerol, 20 mmol/l sodium pyrophosphate, 150 mmol/l NaCl, 50 mmol/l HEPES [pH 7.5], 1% NP-40, 20 mmol/l -glycerophosphate, 10 mmol/l sodium fluoride, 1 mmol/l EDTA, 1 mmol/l EGTA, 2 mmol/l phenylmethylsulfonyl fluoride, 10 e蘥/ml aprotinin, 10 e蘥/ml leupeptin, 2 mmol/l sodium orthovanadate, 3 mmol/l benzamidine [pH 7.4]) for 20 s using a homogenizer (PT 3100; Brinkman Instruments). Homogenates were rotated end-over-end for 1 h at 4°C and then subjected to centrifugation (17,000g for 20 min) at 4°C. Supernatants were quickly frozen in liquid nitrogen and stored at eC80°C. Protein content in lysates was measured by the bicinchoninic acid method (Pierce, Rockford, IL).
Immunoblot analysis.
Phosphorylation of AMPK on Thr-172 was measured using a phospho-specific antibody (Cell Signaling Technology, Danvers, MA). Acetyl-CoA carboxylase (ACC) Ser-227 phosphorylation was determined using a phospho-specific antibody (Upstate Biotechnologies, Waltham, MA). Phosphorylation of AS160 was measured using a phospho (Ser/Thr)-Akt substrate (PAS) antibody (Cell Signaling Technology, Danvers, MA). Phosphorylation of Akt on Ser-473 was measured with a phospho-specific antibody (Cell Signaling Technology). Total AS160 protein level was determined using an affinity-purified rabbit antibody prepared against a glutathione S-transferase (GST) fusion protein with a portion of mouse AS160 corresponding to amino acids 584eC833 of human AS160 (kindly provided by Gustav E. Lienhard, Dartmouth Medical School, Hanover, NH). Muscle lysates were adjusted to equal protein concentration and boiled in Laemmli buffer, loaded on either 7.5% Tris-HCl Criterion gels (Bio-Rad, Copenhagen, Denmark) or self-cast 6eC12% or 6eC15% gradient gels, and transferred to polyvinylidene difluoride membrane (Immobilon Transfer Membrane; Millipore, Copenhagen, Denmark). Membranes were blocked in washing buffer (10 mmol/l Tris-base, 150 mmol/l NaCl, 0.25% Tween 20) containing 7.5% low-fat milk protein for 1 h at room temperature. Membranes were then incubated with primary antibodies overnight at 4°C, followed by incubation with appropriate horseradish peroxidaseeCconjugated secondary antibody (Dako, Glostrup, Denmark) for 2 h at room temperature. Bands were visualized using a Kodak Image Station 2000MM and enhanced chemiluminescence. Immunoreactive proteins were quantified (Kodak 1D 3.6 software), and results were expressed in relative units in comparison with basal samples loaded on each gel.
Immunoprecipitation.
EDL muscle lysate (350 e蘥 protein) was immunoprecipitated with 3.5 e蘥 COOH-terminal anti-AS160 antibody at 4°C with gentle rotation overnight. Samples were incubated with equal mixtures of protein A sepharose (Amersham, Uppsala, Sweden) and protein G agarose (Sigma-Aldrich, St. Louis, MO) for 3 h at 4°C and subsequently washed three times with homogenization buffer and four times with PBS. The immunocomplex was boiled in Laemmli buffer containing -mercaptoethanol and subjected to SDS-PAGE.
Cell-free AMPK kinase assay.
Recombinant functional AMPK heterotrimers (111 or 221) were expressed in Escherichia coli (20) and purified from bacterial lysates by a single-step nickel-ion chromatography, utilizing a polyhistidine tag fused to the NH2-terminus of the subunit, as previously described (20). A GST fusion protein of calmodulin-dependent protein kinase kinase (CaMKK) was expressed in E. coli, purified, and cleaved from GST as previously described (21). For activation of recombinant wild-type AMPK (111 or 221), 0.5 mg enzyme (20) was incubated for 60 min at 30°C with 0.5 mg recombinant CaMKK (21) in activation buffer (10 mmol/l HEPES [pH 7.5], 5 mmol/l MgCl2, 1 mmol/l AMP, 2 mmol/l ATP). EDL muscle lysates (100 mg) were incubated (30 min at 30°C) in assay buffer (10 mmol/l HEPES [pH 7.5], 5 mmol/l MgCl2, 1 mmol/l EGTA, 0.5 mmol/l Na3VO4, 40 e蘭ol/l P1,P5-di(adenosine-5') pentaphosphate, 100 e蘭ol/l AMP, 200 e蘭ol/l ATP) with or without recombinant-activated wild-type AMPK. Assays were performed under six conditions: 1) lysate and assay buffer; 2) lysate, assay buffer, and AMP; 3) lysate, assay buffer, AMP, and ATP; and 4-6) lysate, assay buffer, AMP, ATP, and recombinant CaMKK or AMPK (111 or 221). The reaction was stopped by addition of sample buffer (6x) and heating (96°C for 4 min). Samples were subjected to SDS-PAGE, followed by Western blot analysis with the PAS antibody.
Statistics.
Data are expressed as means ± SE. Levene’s test of equality of variance was used to test whether groups had equal variances, and data were log-transformed if Levene’s test was significant. In all cases, this was sufficient to obtain equal variances among groups. Statistical evaluation was performed by two-way ANOVA with repeated measures or one-way ANOVA, using Fisher’s least significant difference post hoc analysis to identify significant differences between groups. All statistical analyses were performed using SPSS 13.0. P < 0.05 was considered significant.
RESULTS
AICAR treatment in vitro leads to AS160 phosphorylation in skeletal muscle.
Immunoblot analysis with the PAS antibody revealed that AICAR and insulin independently increased the phosphorylation of multiple proteins in EDL muscle from C57BL/6 mice (Fig. 1A). Previous studies provide evidence that exposure of isolated rat epitrochlearis muscle to AICAR increases AS160 phosphorylation (14). A phospho-protein of 160 kDa, confirmed as AS160 in the immunoprecipitation experiments (Fig. 1B), was phosphorylated in response to insulin exposure. AICAR increased AS160 phosphorylation in a time-dependent manner (Fig. 1C) paralleled by increased phosphorylation of AMPK on Thr-172 and ACC, an AMPK substrate, on Ser-227 (Fig. 1D and E).
AICAR-induced AS160 phosphorylation is wortmannin independent.
Insulin action on AS160 phosphorylation occurred by a wortmannin-sensitive mechanism (Fig. 2A), suggesting that this effect was mediated by signal transduction over a phosphatidylinositol-3 kinaseeCand Akt-dependent pathway. In contrast, AICAR increased AS160 phosphorylation in the presence of wortmannin (Fig. 2A) and without effect on Akt (Ser-473) phosphorylation (Fig. 2B), implicating that this effect was mediated by a pathway independent of the canonical insulin signaling cascade. We hypothesized that the AICAR effect on AS160 occurs via the AMPK-signaling pathway. Consistent with this hypothesis, wortmannin did not alter AICAR-induced phosphorylation of AMPK (Thr-172) (Fig. 2C) or ACC (Ser-227) (Fig. 2D) in murine EDL muscle.
To test whether AMPK phosphorylates AS160, we performed a cell-free kinase assay adding either 111 or 221 recombinant AMPK heterotrimeric complexes to EDL muscle lysate (n = 3). Several phosphoproteins were identified by the PAS antibody in subsequent Western blot analysis (Fig. 2E). Phosphorylation of AS160 was enhanced in vitro by the addition of either 111 or 221 recombinant AMPK heterotrimeric complexes in all preparations tested, whereas the addition of recombinant CaMKK was without effect (Fig. 2F). This in vitro approach could indicate that AS160 is an AMPK substrate.
AS160 phosphorylation is regulated by AMPK in isolated EDL muscle.
To establish the role of AS160 as an AMPK target, EDL muscles from 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice were isolated and incubated in the absence or presence of AICAR. In all experiments, genetically modified mice were compared with wild-type littermates to exclude any influence of genetic background on the measured phenotype. Differences in signaling responses were noted among wild-type mice of the three strains, further reinforcing the need to study each genetically modified animal against their own wild-type littermate. AICAR increased AMPK phosphorylation in wild-type mice from each genotype. This effect was blunted in 2 AMPK KO and 3 AMPK KO but not in 2 AMPK KD mice (Table 1). The AICAR response on AMPK observed in the 2 AMPK KD mice was expected, since the dominant-negative construct can be phosphorylated but not activated (6). AICAR increased ACC phosphorylation in wild-type mice, with effects severely blunted in 2 AMPK KO, 2 AMPK KD, and 3 AMPK KO mice (Table 1). In contrast to wild-type mice, the effect of AICAR on AS160 phosphorylation was significantly (P = 0.02) blunted (2 AMPK KO) or ablated (2 AMPK KD and 3 AMPK KO) in EDL muscle from KO or transgenic mice (Fig. 3A). In EDL muscle from 2 AMPK KO and 2 AMPK KD mice, basal AS160 phosphorylation was also reduced compared with wild-type mice. This difference in basal AS160 phosphorylation may be related to a slight and nonsignificant reduction in AS160 protein expression in 2 AMPK KO and 2 AMPK KD mice (Fig. 3B). In contrast, basal AS160 phosphorylation and protein expression was similar between 3 AMPK KO and wild-type mice.
To address whether physiological activation of AMPK by muscle contraction is associated with AS160 phosphorylation, isolated EDL muscle was subjected to intermittent electrical stimulation and forced to contract. Here, we provide evidence that AMPK phosphorylation after electrically induced muscle contraction is markedly increased in wild-type mice from all strains and blunted in 2 AMPK KO mice (Table 2). In contrast, contraction-induced AMPK phosphorylation was unaltered in 2 AMPK KD and 3 AMPK KO mice. ACC phosphorylation, after electrically induced muscle contraction, was markedly increased in wild-type and 3 AMPK KO mice and reduced in 2 AMPK KO and 2 AMPK KD mice. Similarly, contraction markedly increased AS160 phosphorylation in wild-type mice, with contraction-induced responses completely suppressed in 2 AMPK KO and 2 AMPK KD but not in 3 AMPK KO mice (Fig. 3C).
Akt signaling is unaltered in 2 AMPK KO mice.
Since AS160 phosphorylation was reduced under basal and AICAR-stimulated conditions in 2 AMPK KO and 2 AMPK KD mice, additional experiments were performed to determine whether insulin-mediated responses were impaired in these mice. Phosphorylation of Akt (Ser-473) and AS160 phosphorylation was increased in response to 60 nmol/l insulin, irrespective of genotype (Fig. 4A and B). Although basal and insulin-stimulated Akt phosphorylation was similar between 2 AMPK KO and wild-type mice, the absolute level of AS160 phosphorylation under basal and insulin-stimulated conditions was reduced. Nevertheless, the increase in AS160 phosphorylation in response to insulin was maintained, indicating an intact response to insulin. AMPK and ACC phosphorylation in 2 AMPK KO and wild-type mice was unaffected by insulin treatment (data not shown).
DISCUSSION
Using three different animal models in which the predominant catalytic and regulatory AMPK isoforms expressed in skeletal muscle have been genetically manipulated, we provide evidence that AS160 is a downstream target of AMPK in skeletal muscle. Exposure of isolated skeletal muscle to AICAR, an AMPK activator, increased AS160 phosphorylation in wild-type but only slightly or not at all in 2 AMPK KO, 3 AMPK KO, or 2 AMPK KD mice. Taken together with previous observations that AICAR-induced glucose transport is totally abolished in these KO and transgenic mouse models (4eC6), our results indicate that AICAR could increase glucose transport by a pathway involving AMPK and likely AS160. Moreover, heterotrimeric complexes containing the AMPK 2/3 subunits are likely to play a major role in the regulation of AS160 phosphorylation upon pharmacological activation by AICAR, since the AICAR effects on AS160 were ablated in 2 and 3 AMPK KO mice. This is of physiological relevance, since the 3 isoform is mainly found in complexes containing 2 and 2 subunits in skeletal muscle (22,23).
We also provide evidence that AMPK is the major contributor to the regulation of AS160 in response to muscle contraction. Despite our evidence supporting a role for AS160 as an AMPK-dependent target important for GLUT4-mediated glucose transport in resting muscle, the connection between AS160 and glucose transport is unresolved in contracting skeletal muscle. Previous studies indicate that contraction-induced glucose uptake is unaltered (5) or only partly decreased (6) in the same AMPK-deficient mouse models in which contraction-induced AS160 phosphorylation was completely prevented. Thus, although contraction increases AS160 phosphorylation in skeletal muscle, AS160-independent mechanisms may elicit contraction-induced responses on glucose transport. In accordance, Ca2+-mediated mechanisms have been proposed in the regulation of contraction-induced skeletal muscle glucose transport (24). Thus, AS160 is unlikely to be the sole mechanism involved in the release of GLUT4-containing vesicles. This hypothesis is supported by recent evidence showing that silencing of AS160 by short-hairpin RNA in 3T3-L1 adipocytes only partially released basal GLUT4 retention, thereby implicating AS160-dependent and -independent mechanisms in insulin signaling to GLUT4 exocytosis (25).
AMPK may phosphorylate AS160 directly or indirectly through unidentified substrates. Using an in vitro AMPK activity assay, where addition of activated recombinant 111 or 221 complexes to skeletal muscle lysate increased AS160 phosphorylation, we provided evidence to suggest that AMPK, per se, phosphorylates AS160. In contrast, recombinant CaMKK was without effect. Our results do not eliminate a possible indirect effect of AMPK on AS160. For example, the recombinant AMPK may have phosphorylated another kinase, which in turn may have directly phosphorylated AS160. However, if AMPK is an indirect regulator of AS160, wortmannin-sensitive components of the canonical insulin signaling cascade, including Akt, are unlikely candidates.
AS160 is a negative regulator of basal GLUT4 exocytosis in 3T3-L1 adipocytes (9,25). In 2 AMPK KO and 2 AMPK KD mice, basal AS160 phosphorylation was impaired. Based on studies in 3T3-L1 adipocytes, in which AS160 expression is silenced (9,25), basal glucose uptake in 2 AMPK KO and 2 AMPK KD mice would be predicted to be decreased due to the reduction in AS160 phosphorylation. However, basal glucose transport is unaltered by modification of the AMPK 2 subunit (5,6). Thus, there may be a threshold effect, such that only a permissive level of AS160 phosphorylation may be necessary for basal glucose uptake. The reduced basal AS160 phosphorylation may be related to a slight but nonsignificant reduction in AS160 protein expression in 2 AMPK KO and 2 AMPK KD mice, although this alternative seems unlikely. In contrast, basal AS160 phosphorylation was preserved in the 3 AMPK KO mice, an observation that may suggest that heterotrimeric complexes containing this isoform are dispensable for basal AS160 phosphorylation. The reduced basal AS160 phosphorylation in the 2 AMPK KO mice was not associated with impairments in insulin signaling, as insulin-induced phosphorylation of Akt and AS160, as well as glucose transport, are unaltered (18). Likewise, insulin-stimulated glucose uptake and Akt signaling is unaltered in 2 AMPK KD mice (6). Nevertheless, there may be excess signaling, such that partial phosphorylation of AS160 may be sufficient for full activation of insulin-mediated glucose transport.
The PAS antibody used to detect AS160 phosphorylation recognizes proteins with phosphorylated serine or threonine residues that follow arginine at positions P-3 and P-5 (RXRXXS/T). The recognition sequences of Akt and AMPK differ (13,26,27); consequently, AMPK might target other unidentified sites on AS160, which could regulate AS160 activity or localization. Interestingly, tuberous sclerosis complex (TSC)2 in the TSC1-TSC2 complex, which functions as a GTPase-activating protein for the small GTPase Ras homologue enriched in brain (Rheb) in the mammalian target of rapamycin pathway, is phosphorylated by Akt and AMPK but on different sites, leading to either inhibition or activation, respectively, of TSC2 (28,29).
The regulation of glucose transport is complex and requires multiple signaling inputs that converge on GLUT4 vesicle transport (30,31). The identification of AS160 as an AMPK substrate provides a mechanism to partly account for insulin-independent glucose uptake in resting skeletal muscle, whereas a role in contraction-mediated glucose uptake is unclear. Our results reveal that AMPK complexes containing 2 and 3 are required for AS160 phosphorylation in response to the AMPK activator AICAR. Furthermore, AMPK increases AS160 phosphorylation in response to AICAR via a phosphatidylinositol-3 kinase/Akt-independent pathway. We also provide evidence that complexes containing the AMPK 2 subunit are required for AS160 phosphorylation in response to muscle contraction, whereas complexes containing the 3 subunit are dispensable. Thus, AS160 may be viewed as a point of convergence for insulin-dependent and -independent signaling to glucose transport in resting skeletal muscle.
ACKNOWLEDGMENTS
This work was supported by the Swedish Research Council, the Swedish Diabetes Association, the Foundation for Scientific Studies of Diabetology, the Strategic Research Foundation, the Swedish Centre for Sports Research, the Commission of the European Communities (contract nos. LSHM-CT-2004-005272 EXGENESIS and LSHM-CT-2004-512013 EUGENE2), the Danish Medical Research Council, the Novo Nordisk Foundation, the Danish Diabetes Association, and the Copenhagen Muscle Research Centre.
J.F.P.W. was supported by a Hallas Mller fellowship from the Novo Nordisk Foundation.
We are especially grateful to Hiroshi Tokumitsu (Kagawa University, Kagawa, Japan), Professor Gustav E. Lienhard (Dartmouth Medical School, Hanover, NH), and Professor Morris J. Birnbaum (Howard Hughes Medical Institute and University of Pennsylvania School of Medicine, Philadelphia, PA) for providing valuable reagents.
FOOTNOTES
J.T.T., S.G., and A.D. contributed equally to this work.
See related article by Kramer et al., p. 2067.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact
REFERENCES
Kahn BB, Alquier T, Carling D, Hardie DG: AMP-activated protein kinase: ancient energy gauge provides clues to modern understanding of metabolism. Cell Metab 1:15eC25, 2005
Hardie DG: The AMP-activated protein kinase pathway: new players upstream and downstream. J Cell Sci 117:5479eC5487, 2004
Merrill GF, Kurth EJ, Hardie DG, Winder WW: AICA riboside increases AMP-activated protein kinase, fatty acid oxidation, and glucose uptake in rat muscle. Am J Physiol 273:E1107eCE1112, 1997
Barnes BR, Marklund S, Steiler TL, Walter M, Hjalm G, Amarger V, Mahlapuu M, Leng Y, Johansson C, Galuska D, Lindgren K, Abrink M, Stapleton D, Zierath JR, Andersson L: The 5'-AMP-activated protein kinase gamma3 isoform has a key role in carbohydrate and lipid metabolism in glycolytic skeletal muscle. J Biol Chem 279:38441eC38447, 2004
Jrgensen SB, Viollet B, Andreelli F, Frsig C, Birk JB, Schjerling P, Vaulont S, Richter EA, Wojtaszewski JF: Knockout of the alpha2 but not alpha1 5'-AMP-activated protein kinase isoform abolishes 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranosidebut not contraction-induced glucose uptake in skeletal muscle. J Biol Chem 279:1070eC1079, 2004
Mu J, Brozinick JT Jr, 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 7:1085eC1094, 2001
Koistinen HA, Galuska D, Chibalin AV, Yang J, Zierath JR, Holman GD, Wallberg-Henriksson H: 5-amino-imidazole carboxamide riboside increases glucose transport and cell-surface GLUT4 content in skeletal muscle from subjects with type 2 diabetes. Diabetes 52:1066eC1072, 2003
Lemieux K, Konrad D, Klip A, Marette A: The AMP-activated protein kinase activator AICAR does not induce GLUT4 translocation to transverse tubules but stimulates glucose uptake and p38 mitogen-activated protein kinases alpha and beta in skeletal muscle. FASEB J 17:1658eC1665, 2003
Larance M, Ramm G, Stockli J, van Dam EM, Winata S, Wasinger V, Simpson F, Graham M, Junutula JR, Guilhaus M, James DE: Characterisation of the role of the RabGAP AS160 in insulin-regulated GLUT4 trafficking. J Biol Chem 280:37803eC37813, 2005
Sano H, Kane S, Sano E, Miinea CP, Asara JM, Lane WS, Garner CW, Lienhard GE: Insulin-stimulated phosphorylation of a Rab GTPase-activating protein regulates GLUT4 translocation. J Biol Chem 278:14599eC14602, 2003
Zeigerer A, McBrayer MK, McGraw TE: Insulin stimulation of GLUT4 exocytosis, but not its inhibition of endocytosis, is dependent on RabGAP AS160. Mol Biol Cell 15:4406eC4415, 2004
Miinea CP, Sano H, Kane S, Sano E, Fukuda M, Peranen J, Lane WS, Lienhard GE: AS160, the Akt substrate regulating GLUT4 translocation, has a functional Rab GTPase-activating protein domain. Biochem J 391:87eC93, 2005
Kane S, Sano H, Liu SC, Asara JM, Lane WS, Garner CC, Lienhard GE: A method to identify serine kinase substrates: Akt phosphorylates a novel adipocyte protein with a Rab GTPase-activating protein (GAP) domain. J Biol Chem 277:22115eC22118, 2002
Bruss MD, Arias EB, Lienhard GE, Cartee GD: Increased phosphorylation of Akt substrate of 160 kDa (AS160) in rat skeletal muscle in response to insulin or contractile activity. Diabetes 54:41eC50, 2005
Karlsson HK, Hllsten K, Bjrnholm M, Tsuchida H, Chibalin AV, Virtanen KA, Heinonen OJ, Lnnqvist F, Nuutila P, Zierath JR: Effects of metformin and rosiglitazone treatment on insulin signaling and glucose uptake in patients with newly diagnosed type 2 diabetes: a randomized controlled study. Diabetes 54:1459eC1467, 2005
Karlsson HK, Zierath JR, Kane S, Krook A, Lienhard GE, Wallberg-Henriksson H: Insulin-stimulated phosphorylation of the Akt substrate AS160 is impaired in skeletal muscle of type 2 diabetic subjects. Diabetes 54:1692eC1697, 2005
Plomgaard P, Bouzakri K, Krogh-Madsen R, Mittendorfer B, Zierath JR, Pedersen BK: Tumor necrosis factor- induces skeletal muscle insulin resistance in healthy human subjects via inhibition of Akt substrate 160 phosphorylation. Diabetes 54:2939eC2945, 2005
Viollet B, Andreelli F, Jorgensen SB, Perrin C, Geloen A, Flamez D, Mu J, Lenzner C, Baud O, Bennoun M, Gomas E, Nicolas G, Wojtaszewski JF, Kahn A, Carling D, Schuit FC, Birnbaum MJ, Richter EA, Burcelin R, Vaulont S: The AMP-activated protein kinase alpha2 catalytic subunit controls whole-body insulin sensitivity. J Clin Invest 111:91eC98, 2003
Barnes BR, Glund S, Long YC, Hjalm G, Andersson L, Zierath JR: 5'-AMP-activated protein kinase regulates skeletal muscle glycogen content and ergogenics. FASEB J 19:773eC779, 2005
Neumann D, Woods A, Carling D, Wallimann T, Schlattner U: Mammalian AMP-activated protein kinase: functional, heterotrimeric complexes by co-expression of subunits in Escherichia coli. Protein Expr Purif 30:230eC237, 2003
Tokumitsu H, Iwabu M, Ishikawa Y, Kobayashi R: Differential regulatory mechanism of Ca2+/calmodulin-dependent protein kinase kinase isoforms. Biochemistry 40:13925eC13932, 2001
Mahlapuu M, Johansson C, Lindgren K, Hjalm G, Barnes BR, Krook A, Zierath JR, Andersson L, Marklund S: Expression profiling of the gamma-subunit isoforms of AMP-activated protein kinase suggests a major role for gamma3 in white skeletal muscle. Am J Physiol Endocrinol Metab 286:E194eCE200, 2004
Wojtaszewski JF, Birk JB, Frosig C, Holten M, Pilegaard H, Dela F: 5'AMP activated protein kinase expression in human skeletal muscle: effects of strength training and type 2 diabetes. J Physiol 564:563eC573, 2005
Holloszy JO: A forty-year memoir of research on the regulation of glucose transport into muscle. Am J Physiol Endocrinol Metab 284:E453eCE467, 2003
Eguez L, Lee A, Chavez JA, Miinea CP, Kane S, Lienhard GE, McGraw TE: Full intracellular retention of GLUT4 requires AS160 Rab GTPase activating protein. Cell Metab 2:263eC272, 2005
Hardie DG, Carling D, Carlson M: The AMP-activated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell Annu Rev Biochem 67:821eC855, 1998
Weekes J, Ball KL, Caudwell FB, Hardie DG: Specificity determinants for the AMP-activated protein kinase and its plant homologue analysed using synthetic peptides. FEBS Lett 334:335eC339, 1993
Inoki K, Li Y, Zhu T, Wu J, Guan KL: TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat Cell Biol 4:648eC657, 2002
Inoki K, Zhu T, Guan KL: TSC2 mediates cellular energy response to control cell growth and survival. Cell 115:577eC590, 2003
Ramm G, James DE: GLUT4 trafficking in a test tube (Letter). Cell Metab 2:150eC152, 2005
Thong FS, Dugani CB, Klip A: Turning signals on and off: GLUT4 traffic in the insulin-signaling highway. Physiology (Bethesda) 20:271eC284, 2005(Jonas T. Treebak, Stephan)