Department of Physiology, Ribeirão Preto Medical School/University of São Paulo, Ribeirão Preto, SP, Brazil.
Department of Biochemistry/Immunology, Ribeirão Preto Medical School/University of São Paulo, Ribeirão Preto, SP, Brazil.
J Cachexia Sarcopenia Muscle. 2019 Apr;10(2):455-475. doi: 10.1002/jcsm.12395. Epub 2019 Apr 1.
Stimulation of β -adrenoceptors can promote muscle hypertrophy and fibre type shift, and it can counteract atrophy and weakness. The underlying mechanisms remain elusive.
Fed wild type (WT), 2-day fasted WT, muscle-specific insulin (INS) receptor (IR) knockout (M-IR ), and MKR mice were studied with regard to acute effects of the β -agonist formoterol (FOR) on protein metabolism and signalling events. MKR mice express a dominant negative IGF1 receptor, which blocks both INS/IGF1 signalling. All received one injection of FOR (300 μg kg subcutaneously) or saline. Skeletal muscles and serum samples were analysed from 30 to 240 min. For the study of chronic effects of FOR on muscle plasticity and function as well as intracellular signalling pathways, fed WT and MKR mice were treated with formoterol (300 μg kg day ) for 30 days.
In fed and fasted mice, one injection of FOR inhibited autophagosome formation (LC3-II content, 65%, P ≤ 0.05) that was paralleled by an increase in serum INS levels (4-fold to 25-fold, P ≤ 0.05) and the phosphorylation of Akt (4.4-fold to 6.5-fold, P ≤ 0.05) and ERK1/2 (50% to two-fold, P ≤ 0.05). This led to the suppression (40-70%, P ≤ 0.05) of the master regulators of atrophy, FoxOs, and the mRNA levels of their target genes. FOR enhanced (41%, P ≤ 0.05) protein synthesis only in fed condition and stimulated (4.4-fold to 35-fold, P ≤ 0.05) the prosynthetic Akt/mTOR/p70S6K pathway in both fed and fasted states. FOR effects on Akt signalling during fasting were blunted in both M-IR and MKR mice. Inhibition of proteolysis markers by FOR was prevented only in MKR mice. Blockade of PI3K/Akt axis and mTORC1, but not ERK1/2, in fasted mice also suppressed the acute FOR effects on proteolysis and autophagy. Chronic stimulation of β -adrenoceptors in fed WT mice increased body (11%, P ≤ 0.05) and muscle (15%, P ≤ 0.05) growth and downregulated atrophy-related genes (30-40%, P ≤ 0.05), but these effects were abolished in MKR mice. Increases in muscle force caused by FOR (WT, 24%, P ≤ 0.05) were only partially impaired in MKR mice (12%, P ≤ 0.05), and FOR-induced slow-to-fast fibre type shift was not blocked at all in these animals. In MKR mice, FOR also restored the lower levels of muscle SDH activity to basal WT values and caused a marked reduction (57%, P ≤ 0.05) in the number of centrally nucleated fibers.
NS/IGF1 signalling is necessary for the anti-proteolytic and hypertrophic effects of in vivo β -adrenergic stimulation and appears to mediate FOR-induced enhancement of protein synthesis. INS/IGF1 signalling only partially contributes to gain in strength and does not mediate fibre type transition induced by FOR.
β-肾上腺素受体的刺激可以促进肌肉肥大和纤维类型转变，并可以对抗萎缩和虚弱。其潜在机制仍不清楚。
研究了急性给予β-激动剂福莫特罗（FOR）对蛋白质代谢和信号事件的影响，实验对象为 fed 野生型（WT）、禁食 2 天的 WT、肌肉特异性胰岛素（INS）受体（IR）敲除（M-IR）和 MKR 小鼠。MKR 小鼠表达一种显性负 IGF1 受体，该受体阻断 INS/IGF1 信号传导。所有小鼠均接受一次 FOR（300μg/kg 皮下注射）或生理盐水注射。从 30 到 240 分钟分析骨骼肌和血清样本。为了研究 FOR 对肌肉可塑性和功能以及细胞内信号通路的慢性影响，fed WT 和 MKR 小鼠接受 FOR（300μg/kg 天）治疗 30 天。
在 fed 和禁食的小鼠中，一次注射 FOR 抑制自噬体形成（LC3-II 含量，65%，P≤0.05），同时血清 INS 水平升高（4 到 25 倍，P≤0.05），Akt（4.4 到 6.5 倍，P≤0.05）和 ERK1/2（50%到两倍，P≤0.05）的磷酸化增加。这导致萎缩的主要调节因子 FoxOs 的抑制（40-70%，P≤0.05），及其靶基因的 mRNA 水平降低。FOR 仅在 fed 状态下（41%，P≤0.05）增强蛋白质合成，并在 fed 和禁食状态下均刺激 prosynthetic Akt/mTOR/p70S6K 通路（4.4 到 35 倍，P≤0.05）。在禁食状态下，FOR 对 Akt 信号的影响在 M-IR 和 MKR 小鼠中均减弱。仅在 MKR 小鼠中，FOR 对蛋白酶解标志物的抑制作用才被阻止。在禁食小鼠中阻断 PI3K/Akt 轴和 mTORC1，而不是 ERK1/2，也抑制了 FOR 对蛋白水解和自噬的急性作用。在 fed WT 小鼠中，慢性刺激β-肾上腺素受体增加了体重（11%，P≤0.05）和肌肉（15%，P≤0.05）的生长，并下调了与萎缩相关的基因（30-40%，P≤0.05），但这些作用在 MKR 小鼠中被消除。FOR 引起的肌肉力量增加（WT，24%，P≤0.05）在 MKR 小鼠中仅部分受损（12%，P≤0.05），并且 FOR 诱导的慢肌向快肌纤维类型转变在这些动物中并未完全阻断。在 MKR 小鼠中，FOR 还将较低的肌肉 SDH 活性水平恢复到基础 WT 值，并导致中央核纤维数量显著减少（57%，P≤0.05）。
INS/IGF1 信号传导对于体内β-肾上腺素刺激的抗蛋白水解和肥大作用是必要的，并且似乎介导了 FOR 诱导的蛋白质合成增强。INS/IGF1 信号传导仅部分有助于力量的增加，并且不介导 FOR 诱导的纤维类型转变。
