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Testosterone enables growth and hypertrophy in fusion impaired myoblasts that display myotube atrophy: deciphering the role of androgen and IGF-I receptors

Hughes, DH and Stewart, CE and Sculthorpe, N and Dugdale, HF and Yousefian, F and Lewis, MP and Sharples, AP (2015) Testosterone enables growth and hypertrophy in fusion impaired myoblasts that display myotube atrophy: deciphering the role of androgen and IGF-I receptors. Biogerontology, Epub A. pp. 1-21. ISSN 1573-6768

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Abstract

We have previously highlighted the ability of testosterone to improve differentiation and myotube hypertrophy in fusion impaired myoblasts that display reduced myotube hypertrophy at 72hrs (after transfer to low serum media) via multiple population doublings (PD) vs. their parental controls (CON); an observation which is abrogated via PI3K/Akt inhibition (Deane et al. 2013). However, whether the most predominant molecular mechanism responsible for T induced hypertrophy occurs directly via androgen receptor or indirectly via IGF-IR/PI3K/Akt pathway is currently debated. PD and CON C2C12 muscle cells were exposed to low serum conditions in the presence or absence of T (100 nM) ± inhibitors of AR (flutamide/F, 40 μm) and IGF-IR (Picropodophyllin/PPP, 150 nM) for 72 hrs and 7 days (early/late muscle differentiation respectively). T increased AR and Akt abundance, myogenin expression, and myotube hypertrophy, but not ERK1/2 activity in both CON and PD cell types. Akt activity was not increased significantly in either cell type with T. Testosterone was unable to promote early differentiation in the presence of IGF-IR inhibitor (PPP) yet still able to promote appropriate later increases in myotube hypertrophy and AR abundance despite IGF-IR inhibition. The addition of the AR inhibitor powerfully attenuated all T induced increases in differentiation and myotube hypertrophy with corresponding reductions in AR abundance, phosphorylated Akt, ERK1/2 and gene expression of IGF-I, myoD and myogenin with increases in myostatin mRNA both cell types. Interestingly, despite basally reduced differentiation and myotube hypertrophy, PD cells showed larger increased in AR abundance vs. CON cells, a response abrogated in the presence of AR but not IGF-IR inhibitors. Furthermore, T induced increases in Akt abundance were sustained despite the presence of IGF-IR inhibition in PD cells only. However, flutamide alone reduced IGF-IR mRNA in both cell types across time points, with an observed reduction in activity of ERK and Akt, perhaps suggesting that IGF-IR was transcriptionally regulated by AR. However, where testosterone increased AR protein content there was no increases observed in IGF-IR gene expression. Overall, this suggested that sufficient AR was important to enable normal gene expression of IGF-IR and downstream signalling, yet elevated levels of AR due to testosterone had no further effect on IGF-IR, despite testosterone increasing Akt abundance in the presence of IGF-IR inhibitor. In conclusion, testosterones ability to improve differentiation and myotube hypertrophy occurred predominately via increases in AR and Akt abundance in both CON and PD cells, with fusion impaired cells (PD) showing an increased responsiveness to T induced AR levels. Finally, T induced increases in myotube hypertrophy (but not early differentiation) occurred independently of upstream IGF-IR input, however it appears that normal AR function in basal conditions is required for adequate IGF-IR gene expression and downstream Akt abundance.

Item Type: Article
Additional Information: The final publication is available at Springer via http://dx.doi.org/10.1007/s10522-015-9621-9
Uncontrolled Keywords: 1103 Clinical Sciences
Subjects: Q Science > QM Human anatomy
R Medicine > RC Internal medicine > RC1200 Sports Medicine
Divisions: Sport & Exercise Sciences
Publisher: Springer Verlag (Germany)
Related URLs:
Date Deposited: 11 Nov 2015 07:54
Last Modified: 11 Nov 2015 07:54
URI: http://researchonline.ljmu.ac.uk/id/eprint/2289

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