Downregulation of AMPK accompanies leucine- and glucose-induced increases in protein synthesis and insulin resistance in rat skeletal muscle.
OBJECTIVE
Branched-chain amino acids, such as leucine and glucose, stimulate protein synthesis and increase the phosphorylation and activity of the mammalian target of rapamycin (mTOR) and its downstream target p70S6 kinase (p70S6K). We examined in skeletal muscle whether the effects of leucine and glucose on these parameters and on insulin resistance are mediated by the fuel-sensing enzyme AMP-activated protein kinase (AMPK).


METHODS
Rat extensor digitorum longus (EDL) muscle was incubated with different concentrations of leucine and glucose with or without AMPK activators. Muscle obtained from glucose-infused rats was also used as a model.


RESULTS
In the EDL, incubation with 100 or 200 μmol/l leucine versus no added leucine suppressed the activity of the α2 isoform of AMPK by 50 and 70%, respectively, and caused concentration-dependent increases in protein synthesis and mTOR and p70S6K phosphorylation. Very similar changes were observed in EDL incubated with 5.5 or 25 mmol/l versus no added glucose and in muscle of rats infused with glucose in vivo. Incubation of the EDL with the higher concentrations of both leucine and glucose also caused insulin resistance, reflected by a decrease in insulin-stimulated Akt phosphorylation. Coincubation with the AMPK activators AICAR and α-lipoic acid substantially prevented all of those changes and increased the phosphorylation of specific sites of mTOR inhibitors raptor and tuberous sclerosis complex 2 (TSC2). In contrast, decreases in AMPK activity induced by leucine and glucose were not associated with a decrease in raptor or TSC2 phosphorylation.


CONCLUSIONS
The results indicate that both leucine and glucose modulate protein synthesis and mTOR/p70S6 and insulin signaling in skeletal muscle by a common mechanism. They also suggest that the effects of both molecules are associated with a decrease in AMPK activity and that AMPK activation prevents them.
Saha
Asish K
AK
Diabetes Research Unit, Division of Endocrinology, Department of Medicine, Boston University Medical Center, Boston, Massachusetts, USA. aksaha@bu.edu
Xu
X Julia
XJ
Lawson
Ebony
E
Deoliveira
Rosangela
R
Brandon
Amanda E
AE
Kraegen
Edward W
EW
Ruderman
Neil B
NB
D055372:AMP-Activated Protein Kinases / Q000378:metabolism*; D000263:Adenylate Kinase / Q000378:metabolism*; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects; D005947:Glucose / Q000494:pharmacology*; D007333:Insulin Resistance / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins / Q000187:drug effects / Q000378:metabolism; D007700:Kinetics; D007773:Lactates / Q000378:metabolism; D007930:Leucine / Q000494:pharmacology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000201:enzymology*; D010750:Phosphoproteins / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000187:drug effects / Q000378:metabolism; D011773:Pyruvates / Q000378:metabolism; D051381:Rats; D012265:Ribonucleotides / Q000378:metabolism; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000187:drug effects / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases
D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D047908:Intracellular Signaling Peptides and Proteins; D007773:Lactates; D010750:Phosphoproteins; D011773:Pyruvates; D012265:Ribonucleotides; D000620:Aminoimidazole Carboxamide; C546845:mTOR protein, rat; D017346:Protein Serine-Threonine Kinases; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; D000263:Adenylate Kinase; C031143:AICA ribonucleotide; D007930:Leucine; D005947:Glucose