Background

Long-term testosterone replacement therapy (TRT) increases muscle mass in elderly men with subnormal testosterone levels. However, the molecular mechanisms underlying this effect of TRT on protein balance in human skeletal muscle in vivo remain to be established.

Methods

Here, we examined skeletal muscle biopsies obtained before and 24-h after the last dose of treatment with either testosterone gel (n = 12) or placebo (n = 13) for 6 months in aging men with subnormal bioavailable testosterone levels. The placebo-controlled, testosterone-induced changes (β-coefficients) in mRNA levels, protein expression and phosphorylation were examined by quantitative real-time PCR and western blotting.

Results

Long-term TRT increased muscle mass by β = 1.6 kg (p = 0.01) but had no significant effect on mRNA levels of genes involved in myostatin/activin/SMAD or IGF1/FOXO3 signalling, muscle-specific E3-ubiquitin ligases, upstream transcription factors (MEF2C, PPARGC1A-4) or myogenic factors. However, TRT caused a sustained decrease in protein expression of SMAD2 (β = −36%, p = 0.004) and SMAD3 (β = −32%, p = 0.001), which was accompanied by reduced protein expression of the muscle-specific E3-ubiquitin ligases, MuRF1 (β = −26%, p = 0.004) and Atrogin-1/MAFbx (β = −20%, p = 0.04), but with no changes in FOXO3 signalling. Importantly, TRT did not affect muscle fibre type distribution between slow-oxidative (type 1), fast-oxidative (type 2a) and fast-glycolytic (type 2×) muscle fibres.

Conclusions

Our results indicate that long-term TRT of elderly men with subnormal testosterone levels increases muscle mass, at least in part, by decreasing protein breakdown through the ubiquitin proteasome pathway mediated by a sustained suppression of SMAD-signalling and muscle-specific E3-ubiquitin ligases.

中文翻译：

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长期睾酮治疗对睾酮低于正常的老年男性骨骼肌质量和纤维类型分布的分子调节剂的影响。

背景

长期睾酮替代疗法 (TRT) 可增加睾酮水平低于正常的老年男性的肌肉质量。然而，TRT 对体内人类骨骼肌蛋白质平衡的这种影响的分子机制仍有待确定。

方法

在这里，我们检查了在使用睾酮凝胶 ( n  = 12) 或安慰剂 ( n  = 13) 治疗 6 个月之前和之后 24 小时在生物可利用睾酮水平低于正常的老年男性中获得的骨骼肌活检。通过定量实时 PCR 和蛋白质印迹检查安慰剂控制的睾酮诱导的 mRNA 水平、蛋白质表达和磷酸化变化（β 系数）。

结果

长期 TRT 使肌肉质量增加 β = 1.6 kg ( p  = 0.01)，但对肌肉生长抑制素/激活素/SMAD 或 IGF1/FOXO3 信号、肌肉特异性 E3-泛素连接酶、上游转录相关基因的 mRNA 水平没有显着影响因素（MEF2C，PPARGC1A-4）或生肌因素。然而，TRT 导致 SMAD2 (β = -36%, p  = 0.004) 和 SMAD3 (β = -32%, p  = 0.001) 的蛋白质表达持续下降，伴随着肌肉特异性 E3 的蛋白质表达降低-泛素连接酶，MuRF1 (β = -26%, p  = 0.004) 和 Atrogin-1/MAFbx (β = -20%, p = 0.04)，但 FOXO3 信号没有变化。重要的是，TRT 不影响慢氧化（1 型）、快氧化（2a 型）和快速糖酵解（2x 型）肌纤维之间的肌纤维类型分布。

结论

我们的结果表明，睾酮水平低于正常的老年男性的长期 TRT 增加肌肉质量，至少部分是通过持续抑制 SMAD 信号传导和肌肉特异性 E3-泛素连接酶介导的泛素蛋白酶体途径减少蛋白质分解.