Insulin/IGF1 signalling mediates the effects of β₂‐adrenergic agonist on muscle proteostasis and growth

Abstract

Background Stimulation of β 2 ‐adrenoceptors can promote muscle hypertrophy and fibre type shift, and it can counteract atrophy and weakness. The underlying mechanisms remain elusive. Methods 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 2 ‐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 −1 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 −1 day −1 ) for 30 days. Results 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 2 ‐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 nucleat...

α−Calcitonin gene-related peptide inhibits autophagy and calpain systems and maintains the stability of neuromuscular junction in denervated muscles

“…Because CGRP was found to control AChR abundance at the plasma membrane of differentiated myotubes independent of protein synthesis [16], our current and previous data [38] suggest that the ability of CGRP to prevent NMJ degeneration in the setting of DEN is due to the activation of mTORC1, leading to reduced autophagy and AChR degradation. The fact that CGRP activates mTORC1 but does not stimulate protein synthesis is consistent with recent studies, which demonstrated that mTORC1 can inhibit autophagy independent of the control of protein synthesis [68], [69]. Mechanistically, the full activity of mTORC1 depends on its lysosomal localization mediated by the Ragulator-Rag complex [70], [71] followed by its interaction with Rheb [71], a key activator of mTORC1 stimulated by insulin [69].…”

Molecular Mechanisms of Skeletal Muscle Hypertrophy

“…Binding of ␤2 agonists to the receptor activates adenylate cyclase with generation of cyclic AMP and activation of protein kinase A (PKA). Chronic treatment with ␤2 agonists as clenbuterol leads to muscle hypertrophy through still poorly defined pathways, which appear to involve the IGF1-PI3K-Akt-mTOR cascade [18,19]. The role of PKA-dependent phosphorylation of the transcription factor CREB (cAMP response element binding protein) and associated coactivators in mediating muscle hypertrophy is not known, although the pro-hypertrophic factor MEF2 (see below) could be involved, as a dominant-negative CREB in postnatal mouse muscles caused muscle wasting that was associated with reduced expression of MEF2 target genes [see 20].…”

Signaling Pathways That Control Muscle Mass

“…This effect is facilitated by Akt/mTOR signaling as treatment with rapamycin abolishes this effect [ 39 ]. Recent studies have demonstrated that β2-adrenergic signaling impinges on insulin/IGF-1 receptor signaling [ 40 ], where genetic or pharmacological inhibition of the insulin or IGF-1 receptor abolishes the anti-proteolytic effect of β2-adrenergic agonists. Interestingly, formoterol also induces the expression of JunB, a transcription factor that promotes muscle hypertrophy independently of mTOR [ 41 ].…”