Abstract
Myotubularin is a 3-phosphoinositide phosphatase that is mutated in X-linked myotubular myopathy, a severe neonatal disorder in which skeletal muscle development and/or regeneration is impaired. In this report we provide evidence that siRNA-mediated silencing of myotubularin expression markedly inhibits growth factor-stimulated Akt phosphorylation, leading to activation of caspase-dependent pro-apoptotic signaling in HeLa cells and primary human skeletal muscle myotubes. Myotubularin silencing also inhibits Akt-dependent signaling through the mammalian target of rapamycin complex 1 as assessed by p70 S6-kinase and 4E-BP1 phosphorylation. Similarly, phosphorylation of FoxO transcription factors is also significantly reduced in myotubularin-deficient cells. Our data further suggest that inhibition of Akt activation and downstream survival signaling in myotubularin-deficient cells is caused by accumulation of the MTMR substrate lipid phosphatidylinositol 3-phosphate generated from the type II phosphatidylinositol 3-kinase PIK3C2B. Our findings are significant because they suggest that myotubularin regulates Akt activation via a cellular pool of phosphatidylinositol 3-phosphate that is distinct from that generated by the type III phosphatidylinositol 3-kinase hVps34. Because impaired Akt signaling has been tightly linked to skeletal muscle atrophy, we hypothesize that loss of Akt-dependent growth/survival cues due to impaired myotubularin function may be a critical factor underlying the severe skeletal muscle atrophy characteristic of muscle fibers in patients with X-linked myotubular myopathy.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
MeSH terms
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Adaptor Proteins, Signal Transducing / genetics
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Adaptor Proteins, Signal Transducing / metabolism
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Cell Cycle Proteins
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Cell Survival / genetics
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Class II Phosphatidylinositol 3-Kinases
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Class III Phosphatidylinositol 3-Kinases / genetics
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Class III Phosphatidylinositol 3-Kinases / metabolism
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Enzyme Activation / genetics
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HeLa Cells
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Humans
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Muscle Fibers, Skeletal / metabolism*
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Muscle Fibers, Skeletal / pathology
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Muscle Proteins / genetics
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Muscle Proteins / metabolism*
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Myopathies, Structural, Congenital / genetics
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Myopathies, Structural, Congenital / metabolism
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Myopathies, Structural, Congenital / pathology
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Phosphatidylinositol 3-Kinases / genetics
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Phosphatidylinositol 3-Kinases / metabolism
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Phosphatidylinositol Phosphates / genetics
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Phosphatidylinositol Phosphates / metabolism*
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Phosphoproteins / genetics
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Phosphoproteins / metabolism
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Phosphorylation / genetics
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Protein Tyrosine Phosphatases, Non-Receptor / genetics
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Protein Tyrosine Phosphatases, Non-Receptor / metabolism*
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Proto-Oncogene Proteins c-akt / genetics
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Proto-Oncogene Proteins c-akt / metabolism*
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Ribosomal Protein S6 Kinases, 70-kDa / genetics
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Ribosomal Protein S6 Kinases, 70-kDa / metabolism
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Signal Transduction*
Substances
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Adaptor Proteins, Signal Transducing
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Cell Cycle Proteins
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EIF4EBP1 protein, human
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Muscle Proteins
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Phosphatidylinositol Phosphates
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Phosphoproteins
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Class II Phosphatidylinositol 3-Kinases
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Class III Phosphatidylinositol 3-Kinases
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PIK3C2B protein, human
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Proto-Oncogene Proteins c-akt
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Ribosomal Protein S6 Kinases, 70-kDa
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Protein Tyrosine Phosphatases, Non-Receptor
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myotubularin