Department of Diabetes Biology, Novo Nordisk, 2880 Bagsvaerd, Denmark
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We examined whether acute
activation of 5'-AMP-activated protein kinase (AMPK) by
5'-aminoimidazole-4-carboxamide-1--D-ribonucleoside (AICAR) ameliorates insulin resistance in isolated rat skeletal muscle.
Insulin resistance was induced in extensor digitorum longus (EDL)
muscles by prolonged exposure to 1.6 mM palmitate, which inhibited
insulin-stimulated glycogen synthesis to 51% of control after 5 h
of incubation. Insulin-stimulated glucose transport was less affected
(22% of control). The decrease in glycogen synthesis was accompanied
by decreased glycogen synthase (GS) activity and increased GS
phosphorylation. When including 2 mM AICAR in the last hour of the 5-h
incubation with palmitate, the inhibitory effect of palmitate on
insulin-stimulated glycogen synthesis and glucose transport was
eliminated. This effect of AICAR was accompanied by activation of AMPK.
Importantly, AMPK inhibition was able to prevent this effect. Neither
treatment affected total glycogen content. However, glucose 6-phosphate
was increased after inclusion of AICAR, indicating increased influx of
glucose. No effect of AICAR on the inhibited insulin-stimulated GS
activity or increased GS phosphorylation by palmitate could be
detected. Thus the mechanism by which AMPK activation ameliorates the
lipid-induced insulin resistance probably involves induction of
compensatory mechanisms overriding the insulin resistance. Our results
emphasize AMPK as a promising molecular target for treatment of insulin resistance.
insulin sensitivity; 5'-aminoimidazole-4-carboxamide-1--D-ribonucleoside; glycogen synthesis; palmitate
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
SKELETAL MUSCLE IS the most important tissue responsible for glucose disposal under postprandial conditions, and muscle insulin resistance is a major characteristic of type 2 diabetes (10). Although the mechanism by which the muscle becomes insulin resistant is unclear, there is a strong correlation between increased levels of plasma free fatty acids and intramuscular fatty acid metabolites (long-chain acyl-CoA, diacylglycerol, and triglycerides) and insulin resistance (23). Thus excess levels of lipids appear to be of major importance for the development of insulin resistance in muscle (23).
The enzyme 5'-AMP-activated protein kinase (AMPK) has emerged as
a key regulator of carbohydrate and fat metabolism, working as a
"fuel sensor" in most tissues (30). AMPK activity is
regulated by variations in the intracellular levels of AMP, ATP, and
creatine phosphate through allosteric regulation and phosphorylation
induced by upstream kinases (AMPKK; see Ref. 30). In
skeletal muscle, AMPK is activated in response to various metabolic
stresses such as hypoxia, hyperosmolarity, and exercise
(15-17, 28, 29, 31). Chemically, AMPK can be
activated in muscle by treatment with 5'-aminoimidazole-4-carboxamide-1--D-ribonucleoside
(AICAR), which leads to AMP-like activation of the AMPK system
(24). In muscle, AMPK activation by AICAR increases
glucose transport in a wortmannin [inhibitor of phosphatidylinositol
(PI) 3-kinase]-insensitive manner (3, 17, 21) and
increases lipid oxidation through inhibition of acetyl-CoA carboxylase
(24, 29), with both effects being very similar to
exercise-induced regulation of muscle metabolism. Thus AMPK is believed
to play a role in the effects of exercise on carbohydrate and fat metabolism.
In relation to diabetes and insulin resistance, physical exercise improves insulin sensitivity in animal models of diabetes and in type 2 diabetic patients (9, 12). Because of the possible involvement of AMPK in exercise-induced metabolic regulation, AMPK has emerged as an attractive molecular target for the possible treatment of type 2 diabetes (2). However, only a few studies have examined the effects of activating AMPK on insulin resistance. In these studies, where AICAR was infused in animal models of diabetes, plasma glucose levels were decreased or glucose transport in muscle was increased despite the insulin resistance (2, 13). However, because of possible indirect effects driven by alterations in whole body metabolism combined with the nonspecificity of AICAR, it remains unclear whether AICAR treatment can ameliorate insulin resistance directly in skeletal muscle.
Therefore, the aim of this study was to examine if acute activation of AMPK by AICAR could ameliorate insulin resistance induced by exposure to free fatty acids (palmitate) in isolated rat extensor digitorum longus (EDL) muscle.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Source of tissue. All experiments were approved by the Danish Animal Experiments Inspectorate and complied with the "European Convention for the Protection of Vertebrate Animals Used for Experiments and Other Scientific Purposes" (Council of Europe no. 123, Strasbourg, France, 1985). Male Wistar rats (~50 g) purchased from Moellegaard (Lille Skensved, Denmark) were anesthetized by brief exposure to CO2 and killed by cervical dislocation, and the EDL was gently dissected free. EDL muscle was chosen because its reported fiber type composition (42-56-2%) is close to the fiber type composition of the rat hindquarter (1).
Chemicals. All chemicals, if not described differently, were purchased from Merck.
Muscle incubations. The intact EDL muscles were incubated free floating in pregassed (95% O2-5% CO2) Krebs-Henseleit (KRH) buffer (in mM: 118.5 NaCl, 4.7 KCl, 1.2 KH2PO4, 25 NaHCO3, 2.5 CaCl2, 1.2 MgSO4, and 10 HEPES) supplemented with 5 mM glucose, 10 pM insulin (human from Novo Nordisk), and 4% BSA with or without 1.6 mM palmitate (Sigma) for 5 h or as indicated. In some experiments, 2 mM AICAR (Sigma) was included for the last hour. Palmitate was conjugated to fatty acid-free BSA (Sigma), as described previously (27). When the insulin response was to be investigated, muscles were stimulated with insulin (6 nM or as indicated) for 20 min in palmitate-free KRH containing 5 mM glucose, 1% BSA, and 10 pM insulin in the basal stimulations. The AMPK inhibitor 6-[4-(2-piperidin-1-yl-ethoxy)-phenyl]-3-pyridin-4-yl-pyyrazolo[1,5-a]pyrimidine was purchased from Syncrome and was synthesized as described previously (33). All incubations were performed at 30°C, with gentle agitation (110/min), under continuous gassing with 95% O2-5% CO2 and with buffer change every 1-2 h. Thus the incubation conditions fulfilled the requirements for long-term incubations to retain muscle viability (6). After incubation, muscles were quickly freeze-clamped in liquid nitrogen.
Glycogen synthesis.
Glycogen synthesis rate was measured by adding
D-[U-14C]glucose (Amersham) together with
insulin for 20 min after the 5-h incubation. The muscles were frozen in
liquid nitrogen, weighed, and boiled in 1 N NaOH for 30 min. Glycogen
was precipitated overnight at 20°C in ethanol after addition of
0.35 µg/µl unlabeled glycogen (Sigma). After centrifugation (20 min, 2,800 g), the glycogen pellet was washed in ice-cold
ethanol, solubilized in water, and counted (Tri-Carp 1500; Packard).
Glucose transport. Glucose transport was measured by addition of 2-deoxy-D-[1-3H]glucose (2-DG; Amersham) after 10 min of insulin stimulation to the incubation buffer containing 1% BSA and 5 mM glucose for the last 10 min of incubation. Extracellular volume was measured by addition of [14C]mannitol (Amersham) together with 2-DG, and glucose transport was corrected for the presence of 2-DG in the extracellular volume. After incubation, the muscles were snap-frozen in liquid nitrogen, solubilized in Solvable (Packard), and counted (Tri-Carp 1500).
AMPK activity.
AMPK activity was measured in vitro by immunoprecipitation with
anti-AMPK-1 and anti-AMPK-
2 antibodies as
described (8) using the AMARAASAAAKARRR peptide.
Alternatively, AMPK activity was measured by Western blot analysis
using phospho- and dephosphospecific ACC antibodies. The antibodies
were raised against the peptides TMRPSMSpGLHLVK or TMRPSMSGLHLVK
(residues 221-233 of human ACC2, where Sp is phosphoserine) using
previously described methods (25).
Glycogen synthase activity and phosphorylation, total glycogen content, and glucose 6-phosphate. Glycogen synthase (GS) activity was measured as described (26) with minor modifications. Precipitation of labeled glycogen was performed in ethanol prewet Whatman Unifilter 350 polytronic plates (50 µl reaction and 200 µl ice-cold 66% ethanol for 1 h; Frisenette). The plate was then washed five times with 66% ethanol, and dried, after which 75 µl Microscint 20 (Packard) were added. After 2 h, the plate was counted in a Topcount-NXT (Packard). GS activity was determined in the absence or presence of 8 mM glucose 6-phosphate (G-6-P). Phosphorylation of site 3a3b on GS was measured by Western blot analysis using a phosphospecific GS antibody. The antibody was raised against the peptide RYPRPASpVPPSpPSLSR (residues 634-649 of human muscle GS) using previously described methods (25). Total glycogen concentration was determined by measuring free glycosyl units using a hexokinase-based assay kit (Sigma) after acid hydrolysis as described (22), and G-6-P concentrations were determined according to Lowry and Passonneau (22) in perchloric acid extracts of frozen muscle.
Statistical analysis.
Differences between groups were assessed by one-way ANOVA followed by
Bonferroni's post hoc comparison between selected groups (see Fig. 6),
in case of paired muscles by paired t-test, and in Fig. 3 by
two-way ANOVA [3 (treatments) × 4 (concentrations of
insulin) table]. Data are expressed as means + SE from
n = 6-12 muscles in each group, as indicated in
the legends for Figs. 1-6.
|
|
|
|
|
|
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Insulin resistance in rat EDL muscle induced by prolonged exposure to palmitate. Upon exposure to palmitate, the rate of glycogen synthesis in response to 6 nM insulin was decreased by up to 51% in the EDL muscle compared with muscles incubated without palmitate (Fig. 1A). This inhibition of the insulin response occurred in a time (P < 0.001)- and concentration (P < 0.05)-dependent manner, with maximal inhibition after 4 h of incubation using 1.6 mM palmitate (Fig. 1, A and B). No significant effect of palmitate incubation on basal (10 pM insulin) glycogen synthesis could be detected. Palmitate (1.6 mM) incubation for 5 h had no significant effect on basal (10 pM insulin) glucose transport (Fig. 2). However, insulin (6 nM)-stimulated glucose transport was significantly inhibited with palmitate by 22% compared with muscles incubated without palmitate (P < 0.05).
Amelioration of insulin resistance by acute treatment with AICAR. To determine whether acute activation of AMPK would ameliorate lipid-induced insulin resistance, 2 mM AICAR was included in the last hour of a 5-h incubation with palmitate. Focusing on glycogen synthesis as the major site of insulin resistance in this system, the insulin response was measured as [14C]glucose incorporation into glycogen. Palmitate exposure led to an inhibition of insulin-induced glycogen synthesis (P < 0.001 for treatments; Fig. 3). When AICAR was included for the last hour of the incubation, the inhibitory effect of prolonged exposure to palmitate on the insulin response was eliminated. The increased glycogen synthesis by AICAR inclusion was accompanied by a significantly (P < 0.05) increased insulin-stimulated glucose transport (Fig. 2).
Activation of AMPK.
To verify that AMPK was activated by the AICAR treatment, AMPK activity
was measured. Figure 4A shows
the AMPK activity in muscles, incubated as indicated, measured by an in
vitro kinase assay in AMPK-1/
2
imunnoprecipitates. No effect of palmitate treatment could be detected.
When AICAR was included for the last hour of incubation, the activity
was increased twofold (P < 0.01). The same increase in AMPK
activity was found in response to AICAR in muscles incubated without
palmitate (P < 0.001). In addition, AMPK activity was
measured using anti-phospho- and anti-dephosphoantibodies directed
against Ser221 in acetyl-CoA carboxylase, a well-known
substrate of AMPK (Fig. 4B). With this approach, any
allosteric regulation of AMPK and activation by phosphorylation is
detected. Thus normally this approach results in a more sensitive
measure for AMPK activity. The results using these antibodies confirm
the data obtained by the in vitro kinase assay.
Specific involvement of AMPK in the ameliorating effect of AICAR. Because AICAR is known to be somewhat unspecific, an AMPK inhibitor (33) was used to verify the involvement of AMPK in AICAR-induced amelioration of the insulin resistance. Figure 5 shows that AMPK inhibition prevented the effect of AICAR on palmitate-induced insulin resistance (P < 0.001 for 100 µM inhibitor).
Glycogen, G-6-P, and GS activity.
To examine the mechanism by which AICAR ameliorates palmitate-induced
insulin resistance, total glycogen content, G-6-P levels, and GS activity were measured. Neither palmitate exposure nor inclusion
of AICAR had any effect on the total glycogen content in the EDL muscle
(Table 1). However, the inclusion of
AICAR significantly (P < 0.01 vs. palmitate treated)
increased G-6-P levels (Table 1). Palmitate incubation
significantly inhibited insulin-stimulated in vitro GS activity
(P < 0.05), indicating an inhibition of
insulin-stimulated dephosphorylation of the GS (Fig.
6A). In agreement, by use of a
phosphospecific antibody, insulin-stimulated dephosphorylation of GS on
site 3a3b [phosphorylation site of e.g., glycogen synthase kinase 3 (GSK3)] was abolished by palmitate exposure (Fig.
6B). Inclusion of AICAR had no significant effect on
the inhibited in vitro GS activity or the phosphorylation state of site
3a3b.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the present study, we provide evidence that AICAR, directly in skeletal muscle, ameliorates lipid-induced insulin resistance accompanied by increased AMPK activity. Importantly, an AMPK inhibitor was able to prevent this effect, ensuring that the effect is specific for AMPK activation in the muscle. This finding is parallel to results previously seen with glucose-induced insulin resistance (18), indicating that activation of AMPK in general can ameliorate insulin resistance induced by an energy surplus.
An increasing body of evidence suggests a strong relationship between obesity, increased lipid availability, and the impaired insulin sensitivity observed in obese and type 2 diabetic patients (23). In the present study, we show that insulin resistance can be induced directly by prolonged incubation with free fatty acids (palmitate) in the EDL muscle, representing fast-twitch fibers. This is in agreement with results obtained in soleus (representing slow-twitch fibers) by Thompson et al. (27) performed under similar conditions. The inhibitory effect of palmitate on insulin action is most pronounced at the level of glycogen synthesis, in both fast- and slow-twitch fibers, whereas glucose transport seems to be affected less [Thompson et al. (27)] (Figs. 1 and 2). Thus prolonged exposure to excess lipids directly induces insulin resistance in both slow- and fast-twitch skeletal muscle fibers, in agreement with in vivo rat and human lipid infusion studies (4, 5, 7, 19).
Alterations in the insulin-signaling pathway have been implicated in the mechanism of lipid-induced insulin resistance in skeletal muscle (11, 27). In this study, we provide evidence that insulin-stimulated in vitro GS activity and dephosphorylation of site 3a3b on GS (phosphorylation sites of, e.g., GSK3) were affected, whereas G-6-P and total glycogen content were unaffected by prolonged palmitate exposure. These findings indicate that prolonged exposure to palmitate modulates the insulin-signaling pathway, in agreement with the findings in isolated soleus muscles, where insulin-induced protein kinase B (PKB) phosphorylation was reported decreased (27), and after in vivo infusions of fatty acid, where decreased insulin receptor substrate (IRS)-1- and IRS-2-associated PI 3-K and Akt1 activity in muscle were seen (11, 20). Interestingly, Kim et al. (20) reported that lipid infusion had no effect on insulin-stimulated inhibition of GSK3 activity, whereas a reduction in GS activity was seen. The latter finding agrees nicely with the present study, and, since we also can demonstrate decreased insulin-induced dephosphorylation of GS (site 3a3b), this could indicate activation of a yet to be identified kinase by exposure to excess levels of fatty acids, which could be responsible for the increased GS phosphorylation. Alternatively, it could be speculated that fatty acid excess inhibited insulin-stimulated protein phosphatase-1 (PP1) activity. This possibility was ruled out by Kim et al. (20), since they could not demonstrate an effect of lipid infusion on insulin-stimulated PP1 activity. In this respect, however, it should be remembered that the PP1 activity measured by Kim et al. is a crude whole cell estimate, and changes in PP1 activity toward GS might be overlooked, since this activity most likely is associated with the glycogen particles.
It is well known that the level of glycogen in the muscle affects the activity of the GS (32), and this might therefore be part of the molecular mechanism of palmitate-induced insulin resistance. However, neither we in EDL muscles nor Thompson et al. (27) in soleus muscles could demonstrate any difference in the glycogen level after induction of insulin resistance by palmitate.
No matter the mechanism behind fatty acid-induced insulin resistance, we find that AICAR very efficiently prevents the development of insulin resistance. In previous in vivo studies in Zucker obese rats and KKAy-CETP mice, AICAR was found to improve whole body glucose homeostasis (2, 13). In these studies, the main effect of AICAR appeared to be on glucose production from the liver rather than peripheral glucose disposal. However, our present results suggest that a major effect of AICAR should be observable in skeletal muscle. The explanation for this discrepancy is unclear, but changes in the plasma lipid profiles were observed in these in vivo experiments, which are well known to modulate glucose homeostasis. Furthermore, AICAR is not a specific AMPK activator, since it will affect other AMP-sensitive enzymes, which will complicate interpretation of whole body experiments. Consequently, it is not clear if the beneficial effects of AICAR on glucose metabolism observed in those in vivo studies were the result of an effect of AMPK activation on peripheral glucose utilization or because of, primarily, an effect on the liver. Nevertheless, using in vitro muscle incubations, we and others (3, 14, 17, 21, 24) find that AICAR indeed has a direct effect on skeletal muscle glucose metabolism. A more specific AMPK activator is needed to evaluate the effects of AMPK activation on whole body glucose metabolism.
The molecular mechanisms by which AICAR ameliorates the inhibitory effect of palmitate on insulin-stimulated glucose metabolism in muscle might occur by either relieving the lipid-induced defect or by inducing compensatory mechanisms overriding the insulin resistance. Because AICAR is known to activate glucose transport independent of the insulin-signaling pathway, overriding the insulin resistance by increasing glucose influx is a likely mechanism. Indeed we find an increased G-6-P concentration when including AICAR and increased insulin-stimulated glucose transport. Alternatively, AMPK activation could lead to a relief of the palmitate-induced insulin resistance by increasing fatty acid oxidation and thereby removing the inhibitory fatty acid metabolites, or by activating a protein with a direct effect on the insulin-signaling pathway. However, in this study, we find that the inhibited insulin-stimulated GS activity and dephosphorylation of GS by palmitate are unaffected by the AICAR treatment, making a relieving mechanism unlikely.
In conclusion, acute treatment with AICAR ameliorates insulin resistance induced by prolonged treatment with free fatty acids (palmitate) in isolated EDL muscle. The AICAR effect seems to be specific for AMPK activation, since the ameliorating effect of AICAR can be prevented by an AMPK inhibitor. The mechanism by which activation of AMPK ameliorates palmitate-induced insulin resistance most likely occurs by inducing increased glucose transport and thereby overriding the insulin resistance rather than relieving the insulin resistance. Our data provide further support for pursuing AMPK as a potential molecular target for the treatment of insulin resistance.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank Prof. D. G. Hardie for the AMP-activated protein
kinase-1/
2 activity protocol and
materials and Drs. J. G. McCormack and E. Nishimura for critical
review of the manuscript.
![]() |
FOOTNOTES |
---|
The phosphorylated glycogen synthase antibody was made in collaboration with Sir Philip Cohen and Dr. Chris Armstrong. G. S. Olsen was partly supported by European Commission Grant QLRT-2000-01488.
Address for reprint requests and other correspondence: G. S. Olsen, Dept. of Diabetes Biology, Novo Nordisk, Novo Allé 6A (1.060), 2880 Bagsvaerd, Denmark (E-mail: gso{at}novonordisk.com).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
July 9, 2002;10.1152/ajpendo.00118.2002
Received 25 February 2002; accepted in final form 4 July 2002.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Armstrong, RB,
and
Phelps RO.
Muscle fiber type composition of the rat hindlimb.
Am J Anat
171:
259-272,
1984[ISI][Medline].
2.
Bergeron, R,
Previs SF,
Cline GW,
Perret P,
Russell RR,
Young LH,
and
Shulman GI.
Effect of 5-aminoimidazole-4-carboxamide-1-beta-D- ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats.
Diabetes
50:
1076-1082,
2001
3.
Bergeron, R,
Russell RR,
Young LH,
Ren JM,
Marcucci M,
Lee A,
and
Shulman GI.
Effect of AMPK activation on muscle glucose metabolism in conscious rats.
Am J Physiol Endocrinol Metab
276:
E938-E944,
1999
4.
Boden, G,
Chen X,
Ruiz J,
White JV,
and
Rossetti L.
Mechanisms of fatty acid-induced inhibition of glucose uptake.
J Clin Invest
93:
2438-2446,
1994[ISI][Medline].
5.
Boden, G,
Jadali F,
White J,
Liang Y,
Mozzoli M,
Chen X,
Coleman E,
and
Smith C.
Effects of fat on insulin-stimulated carbohydrate metabolism in normal men.
J Clin Invest
88:
960-966,
1991[ISI][Medline].
6.
Bonen, A,
Clark MG,
and
Henriksen EJ.
Experimental approaches in muscle metabolism: hindlimb perfusion and isolated muscle incubations.
Am J Physiol Endocrinol Metab
266:
E1-E16,
1994
7.
Chalkley, SM,
Hettiarachchi M,
Chisholm DJ,
and
Kraegen EW.
Five-hour fatty acid elevation increases muscle lipids and impairs glycogen synthesis in the rat.
Metabolism
47:
1121-1126,
1998[ISI][Medline].
8.
Cheung, PCF,
Salt IP,
Davies SP,
Hardie DG,
and
Carling D.
Characterization of AMP-activated protein kinase gamma- subunit isoforms and their role in AMP binding.
Biochem J
346:
659-669,
2000[ISI][Medline].
9.
Cortez, MY,
Torgan CE,
Brozinick JT,
and
Ivy JL.
Insulin resistance of obese Zucker rats exercise trained at two different intensities.
Am J Physiol Endocrinol Metab
261:
E613-E619,
1991
10.
DeFronzo, RA,
Gunnarsson R,
Bjorkman O,
Olsson M,
and
Wahren J.
Effects of insulin on peripheral and splanchnic glucose metabolism in noninsulin-dependent (type II) diabetes mellitus.
J Clin Invest
76:
149-155,
1985[ISI][Medline].
11.
Dresner, A,
Laurent D,
Marcucci M,
Griffin ME,
Dufour S,
Cline GW,
Slezak LA,
Andersen DK,
Hundal RS,
Rothman DL,
Petersen KF,
and
Shulman GI.
Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity.
J Clin Invest
103:
253-259,
1999
12.
Eriksson, JG.
Exercise and the treatment of type 2 diabetes mellitus. An update.
Sports Med
27:
381-391,
1999[ISI][Medline].
13.
Fiedler, M,
Zierath JR,
Selen G,
Wallberg-Henriksson H,
Liang Y,
and
Sakariassen KS.
5-Aminoimidazole-4-carboxy-amide-1-beta-D-ribofuranoside treatment ameliorates hyperglycaemia and hyperinsulinaemia but not dyslipidaemia in KKA(y)-CETP mice.
Diabetologia
44:
2180-2186,
2001[ISI][Medline].
14.
Fisher, JS,
Gao JP,
Han DH,
Holloszy JO,
and
Nolte LA.
Activation of AMP kinase enhances sensitivity of muscle glucose transport to insulin.
Am J Physiol Endocrinol Metab
282:
E18-E23,
2002
15.
Fujii, N,
Hayashi T,
Hirshman MF,
Smith JT,
Habinowski SA,
Kaijser L,
Mu J,
Ljungqvist O,
Birnbaum MJ,
Witters LA,
Thorell A,
and
Goodyear LJ.
Exercise induces isoform-specific increase in 5 ' AMP- activated protein kinase activity in human skeletal muscle.
Biochem Biophys Res Commun
273:
1150-1155,
2000[ISI][Medline].
16.
Hayashi, T,
Hirshman MF,
Fujii N,
Habinowski SA,
Witters LA,
and
Goodyear LJ.
Metabolic stress and altered glucose transport: activation of AMP-activated protein kinase as a unifying coupling mechanism.
Diabetes
49:
527-531,
2000[Abstract].
17.
Hayashi, T,
Hirshman MF,
Kurth EJ,
Winder WW,
and
Goodyear LJ.
Evidence for 5'AMP-activated protein-kinase mediation of the effect of muscle contraction on glucose transport.
Diabetes
47:
1369-1373,
1998[Abstract].
18.
Kawanaka, K,
Han DH,
Gao JP,
Nolte LA,
and
Holloszy JO.
Development of glucose-induced insulin resistance in muscle requires protein synthesis.
J Biol Chem
276:
20101-20107,
2001
19.
Kelley, DE,
Mokan M,
Simoneau JA,
and
Mandarino LJ.
Interaction between glucose and free fatty acid metabolism in human skeletal muscle.
J Clin Invest
92:
91-98,
1993[ISI][Medline].
20.
Kim JB, Shulman GI, and Kahn BB. Fatty acid infusion selectively
impairs insulin action on Akt1 and PKC/ but not on glycogen synthase
kinase-3. J Biol Chem. In press.
21.
Kurth-Kraczek, EJ,
Hirshman MF,
Goodyear LJ,
and
Winder WW.
5'-AMP-activated protein kinase activation causes GLUT4 translocation in skeletal muscle.
Diabetes
48:
1667-1671,
1999[Abstract].
22.
Lowry, OH,
and
Passonneau JV.
A Flexible System of Enzymatic Analysis. London: Academic, 1972, p. 1-291.
23.
McGarry, J.
Dysregulation of fatty acid metabolism in the etiology of type 2 diabetes.
Diabetes
51:
7-18,
2002
24.
Merrill, GF,
Kurth EJ,
Hardie DG,
and
Winder WW.
AICA riboside increases AMP-activated protein kinase, fatty acid oxidation, and glucose uptake in rat muscle.
Am J Physiol Endocrinol Metab
273:
E1107-E1112,
1997
25.
Sugden, C,
Crawford RM,
Halford NG,
and
Hardie DG.
Regulation of spinach SNF1-related (SnRK1) kinases by protein kinases and phosphatases is associated with phosphorylation of the T loop and is regulated by 5'-AMP.
Plant J
19:
433-439,
1999[ISI][Medline].
26.
Thomas, JA,
Schlender KK,
and
Larner J.
A rapid filter paper assay for UDPglucose-glycogen glucosyltransferase, including an improved biosynthesis of UDP-14C-glucose.
Anal Biochem
25:
486-499,
1968[ISI][Medline].
27.
Thompson, A,
Lim-Fraser MYC,
Kraegen EW,
and
Cooney GJ.
Effects of individual fatty acids on glucose uptake and glycogen synthesis in soleus muscle in vitro.
Am J Physiol Endocrinol Metab
279:
E577-E584,
2000
28.
Vavvas, D,
Apazidis A,
Saha AK,
Gamble J,
Patel A,
Kemp BE,
Witters LA,
and
Ruderman NB.
Contraction-induced changes in acetyl-CoA carboxylase and 5'-AMP-activated kinase in skeletal-muscle.
J Biol Chem
272:
13255-13261,
1997
29.
Winder, WW,
and
Hardie DG.
Inactivation of acetyl-CoA carboxylase and activation of AMP-activated protein kinase in muscle during exercise.
Am J Physiol Endocrinol Metab
270:
E299-E304,
1996
30.
Winder, WW,
and
Hardie DG.
AMP-activated protein kinase, a metabolic master switch: possible roles in type 2 diabetes.
Am J Physiol Endocrinol Metab
277:
E1-E10,
1999
31.
Wojtaszewski, JFP,
Nielsen P,
Hansen BF,
Richter EA,
and
Kiens B.
Isoform-specific and exercise intensity-dependent activation of 5'-AMP-activated protein kinase in human skeletal muscle.
J Physiol (Lond)
528:
221-226,
2000
32.
Wojtaszewski, JFP,
Nielsen JN,
and
Richter EA.
Exercise effects on muscle insulin signaling and action: effect of acute exercise on insulin signaling and action in humans.
J Appl Physiol
93:
384-392,
2002
33.
Zhou, GC,
Myers R,
Li Y,
Chen YL,
Shen XL,
Fenyk-Melody J,
Wu M,
Ventre J,
Doebber T,
Fujii N,
Musi N,
Hirshman MF,
Goodyear LJ,
and
Moller DE.
Role of AMP-activated protein kinase in mechanism of metformin action.
J Clin Invest
108:
1167-1174,
2001