Regulation of muscle protein degradation: coordinated control of apoptotic and ubiquitin-proteasome systems by phosphatidylinositol 3 kinase.
Muscle proteolysis from catabolic conditions, including chronic kidney disease, requires coordinated activation of both the apoptotic and ATP-ubiquitin-proteasome systems (Ub-P'some), including upregulation of components of the Ub-P'some system. Activation of the apoptotic system is required because caspase-3 initially cleaves myofibrils, yielding substrates for the Ub-P'some system plus a characteristic 14-kD actin fragment. The authors studied insulin deficiency, a model of accelerated muscle atrophy, to understand how regulation of the apoptotic and the Ub-P'some systems could be coordinated. As expected, phosphatidylinositol 3 kinase activity (PI3K) was suppressed in muscle; in addition to decreased insulin, the mechanism includes IRS-1 phosphorylation at serine-307. Caspase-3 activity was also increased, and the authors linked it to a low PI3K-induced activation of the apoptotic system that includes a conformational change in Bax and release of cytochrome C. Coordinated atrogin-1/MAFbx expression is required as a critical factor for Ub-P'some system-dependent muscle proteolysis in diabetes and other catabolic states. The mechanism that regulates atrogin-1/MAFbx expression is unknown. Atrogin-1/MAFbx expression increased when the authors suppressed PI3K activity in muscle cells. The forkhead transcriptional factor, a downstream substrate of PI3K, stimulated atrogin-1/MAFbx promoter transcriptional activity markedly. The authors found in diabetic muscle that mRNA of the forkhead transcriptional factor, its nuclear translocation, and binding to the atrogin-1/MAFbx promoter were increased. When PI3K activity is low, both apoptotic and Ub-P'some pathways are activated coordinately to cause muscle proteolysis. This mechanism could increase muscle atrophy in conditions with impaired insulin responsiveness.
Lee
Seoung Woo
SW
Nephrology Division, Department of Medicine, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555, USA.
Dai
Guoli
G
Hu
Zhaoyong
Z
Wang
Xiaonan
X
Du
Jie
J
Mitch
William E
WE
D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017209:Apoptosis; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D045304:Cytochromes c / Q000378:metabolism; D004789:Enzyme Activation; D007150:Immunohistochemistry; D007328:Insulin / Q000378:metabolism; D008297:Male; D009097:Multienzyme Complexes / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism* / Q000473:pathology; D009133:Muscular Atrophy; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D046988:Proteasome Endopeptidase Complex; D011487:Protein Conformation; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012694:Serine / Q000737:chemistry; D015398:Signal Transduction; D014158:Transcription, Genetic; D025801:Ubiquitin / Q000378:metabolism*; D015854:Up-Regulation
D000199:Actins; D007328:Insulin; D009097:Multienzyme Complexes; D012333:RNA, Messenger; D025801:Ubiquitin; D012694:Serine; D000255:Adenosine Triphosphate; D045304:Cytochromes c; D019869:Phosphatidylinositol 3-Kinases; C505410:Casp3 protein, rat; D053148:Caspase 3; D020169:Caspases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex