Acylated and unacylated ghrelin impair skeletal muscle atrophy in mice
Paolo E. Porporato, … , Stefano Geuna, Andrea Graziani
Paolo E. Porporato, … , Stefano Geuna, Andrea Graziani
Published January 2, 2013
Citation Information: J Clin Invest. 2013;123(2):611-622. https://doi.org/10.1172/JCI39920.
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Research Article Muscle biology

Acylated and unacylated ghrelin impair skeletal muscle atrophy in mice

Abstract

Cachexia is a wasting syndrome associated with cancer, AIDS, multiple sclerosis, and several other disease states. It is characterized by weight loss, fatigue, loss of appetite, and skeletal muscle atrophy and is associated with poor patient prognosis, making it an important treatment target. Ghrelin is a peptide hormone that stimulates growth hormone (GH) release and positive energy balance through binding to the receptor GHSR-1a. Only acylated ghrelin (AG), but not the unacylated form (UnAG), can bind GHSR-1a; however, UnAG and AG share several GHSR-1a–independent biological activities. Here we investigated whether UnAG and AG could protect against skeletal muscle atrophy in a GHSR-1a–independent manner. We found that both AG and UnAG inhibited dexamethasone-induced skeletal muscle atrophy and atrogene expression through PI3Kβ-, mTORC2-, and p38-mediated pathways in myotubes. Upregulation of circulating UnAG in mice impaired skeletal muscle atrophy induced by either fasting or denervation without stimulating muscle hypertrophy and GHSR-1a–mediated activation of the GH/IGF-1 axis. In Ghsr-deficient mice, both AG and UnAG induced phosphorylation of Akt in skeletal muscle and impaired fasting-induced atrophy. These results demonstrate that AG and UnAG act on a common, unidentified receptor to block skeletal muscle atrophy in a GH-independent manner.

Authors

Paolo E. Porporato, Nicoletta Filigheddu, Simone Reano, Michele Ferrara, Elia Angelino, Viola F. Gnocchi, Flavia Prodam, Giulia Ronchi, Sharmila Fagoonee, Michele Fornaro, Federica Chianale, Gianluca Baldanzi, Nicola Surico, Fabiola Sinigaglia, Isabelle Perroteau, Roy G. Smith, Yuxiang Sun, Stefano Geuna, Andrea Graziani

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Figure 1

AG and UnAG protect C2C12 myotubes from dexamethasone-induced atrophy without induction of protein synthesis or hypertrophy.

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AG and UnAG protect C2C12 myotubes from dexamethasone-induced atrophy wi...
(A) Myotube diameters were measured after 24-hour treatment in differentiation medium (DM) with 10 nM AG, 10 nM UnAG, and/or 1 μM dexamethasone (DEXA). In every experiment, 10 ng/ml IGF-1 was used as positive control for antiatrophic/hypertrophic activity. (B and C) Atrogin-1 and MuRF1 expression analysis upon dexamethasone treatment with or without AG and UnAG. (D) Treatment with 100 nM wortmannin (W) or 20 ng/ml rapamycin (R) reverted the antiatrophic activity of AG and UnAG on myotube diameter. Control myotubes in differentiation medium were treated with DMSO, a vehicle for both wortmannin and rapamycin. (E and F) Phosphorylation of AktS473 and FoxO3aT32, detected by Western blotting, upon treatment for 20 minutes with 1 μM AG or UnAG. Shown are representative blots and quantification of 3 independent experiments. (G–I) IGF-1, but not AG and UnAG, induced protein synthesis, as determined by phosphorylation of S6KT389 (G) or S6S235/236 (H) and by incorporation of [3H]-leucine (I). (J) Effect of raptor and rictor silencing on protein levels, detected by Western blotting. (K) Silencing of rictor, but not of raptor, reverted the antiatrophic activity of AG and UnAG on the diameter of myotubes treated as in A. #P < 0.05, §P < 0.01 vs. DM control; *P < 0.01 vs. DEXA treatment.