title: Insulin down-regulates the expression of ubiquitin E3 ligases partially by inhibiting the activity and expression of AMP-activated protein kinase in L6 myotubes.
abstract: While insulin is an anabolic hormone, AMP-activated protein kinase (AMPK) is not only a key energy regulator, but it can also control substrate metabolism directly by inducing skeletal muscle protein degradation. The hypothesis of the present study was that insulin inhibits AMPK and thus down-regulates the expression of the ubiquitin E3 ligases, muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1) in skeletal muscle cells. Differentiated L6 myotubes were treated with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) and/or compound C to stimulate and/or block AMPK respectively. These treatments were also conducted in the presence or absence of insulin and the cells were analysed by western blot and quantitative real-time PCR. In addition, nucleotide levels were determined using HPLC. The activation of AMPK with AICAR enhanced the mRNA levels of MAFbx and MuRF1. Insulin reduced the phosphorylation and activity AMPK, which was accompanied by reduced MAFbx and MuRF1 mRNA levels. Using a protein kinase B (PKB/Akt) inhibitor, we found that insulin regulates AMPK through the activation of Akt. Furthermore, insulin down-regulated AMPK α2 mRNA. We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. In addition, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Hu-Ping
authors.initials: HP
authors.lastname: Deng
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China cjk304@163.com.
authors.forename: Jia-Ke
authors.initials: JK
authors.lastname: Chai
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Chuan-An
authors.initials: CA
authors.lastname: Shen
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Xi-Bo
authors.initials: XB
authors.lastname: Zhang
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Li
authors.initials: L
authors.lastname: Ma
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Tian-Jun
authors.initials: TJ
authors.lastname: Sun
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Qing-Gang
authors.initials: QG
authors.lastname: Hu
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Yun-Fei
authors.initials: YF
authors.lastname: Chi
authors.affiliation: Burns Institute, Department of Burn and Plastic Surgery, Burn and Plastic Surgery Hospital, First Affiliated Hospital of General Hospital of PLA, Beijing 100048, China.
authors.forename: Ning
authors.initials: N
authors.lastname: Dong
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000096:biosynthesis*; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives; D000818:Animals; D002460:Cell Line; D015536:Down-Regulation / Q000187:drug effects*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D007328:Insulin / Q000494:pharmacology*; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000201:enzymology*; D009124:Muscle Proteins / Q000378:metabolism; D051381:Rats; D012265:Ribonucleotides / Q000096:biosynthesis; D044843:SKP Cullin F-Box Protein Ligases / Q000378:metabolism; D000070584:Tripartite Motif Proteins; D044767:Ubiquitin-Protein Ligases / Q000096:biosynthesis* / Q000378:metabolism
chemical_list: D007328:Insulin; D009124:Muscle Proteins; D012265:Ribonucleotides; D000070584:Tripartite Motif Proteins; D000620:Aminoimidazole Carboxamide; C486692:Fbxo32 protein, rat; D044843:SKP Cullin F-Box Protein Ligases; C499086:Trim63 protein, rat; D044767:Ubiquitin-Protein Ligases; D055372:AMP-Activated Protein Kinases; C568305:Prkaa1 protein, rat; C031143:AICA ribonucleotide
keywords: AMP-activated protein kinase (AMPK); insulin; muscle RING finger 1 (MuRF1); muscle atrophy F-box (MAFbx); myotubes