Article

Insulin like growth factor-1-induced phosphorylation and altered distribution of tuberous sclerosis complex (TSC)1/TSC2 in C2C12 myotubes

Center for Muscle Biology, Department of Physiology, University of Kentucky, Lexington, USA.
FEBS Journal (Impact Factor: 4). 05/2010; 277(9):2180-91. DOI: 10.1111/j.1742-4658.2010.07635.x
Source: PubMed

ABSTRACT

Insulin like growth factor-1 (IGF-1) is established as an anabolic factor that can induce skeletal muscle growth by activating the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway. Although this signaling pathway has been the subject of much study, the molecular mechanisms linking IGF-1 binding to mTOR activation remain poorly defined in muscle. The present study aimed to test the hypothesis that IGF-1 activation of mTOR in C2C12 myotubes requires a phosphorylation-dependent, altered distribution of the tuberous sclerosis complex (TSC)1/TSC2 complex from the membrane to the cytosol. We found that IGF-1 treatment does not affect complex formation between TSC1 and TSC2, but rather IGF-1 induces an altered distribution of the TSC1/TSC2 complex in C2C12 myotubes. In response to IGF-1 treatment, there was a relative redistribution of the TSC1/TSC2 complex, composed of TSC1 and phosphorylated TSC2, from the membrane to the cytosol. IGF-1-stimulated TSC1/TSC2 phosphorylation and redistribution were completely prevented by the phosphoinositide 3-kinase inhibitor wortmannin, but were not with the downstream mTOR inhibitor, rapamycin. When a nonphosphorylatable form of TSC2 (S939A) was overexpressed, phosphorylation-dependent binding of the scaffold protein 14-3-3 to TSC2 was diminished and no redistribution of the TSC1/TSC2 complex was observed after IGF-1 stimulation. These results indicate that TSC2 phosphorylation in response to IGF-1 treatment is necessary for the altered distribution of the TSC1/TSC2 complex to the cytosol. We suggest that this translocation is likely critical for mTOR activation by dissociating the interaction between the GTPase activating protein activity of the TSC1/TSC2 complex and its downstream target, Ras homolog enriched in brain.
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    • "It has also been demonstrated that growth factor stimulation causes TSC2 to translocate from a crude membrane to cytosolic fraction. Furthermore, this translocation event requires the phosphorylation of TSC2 on residues that lie within the RxRxxS*/T* consensus motifs that are commonly utilized by members of the AGC family of kinases such as PKB (Cai et al. 2006; Miyazaki et al. 2010). Based on these points, we reasoned that growth factor-induced changes in the phosphorylation of TSC2 might cause it to translocate away from the LEL, and therefore, spatially inhibit the ability of TSC2 to effectively function as a GAP for Rheb at the LEL (Jacobs et al. 2013). "
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    • "Growth factor (e.g. IGF-1 and/or insulin) stimulation of muscle leads to activation of phosphoinositide 3-kinase (PI3K) and its downstream effector Akt (also referred to as protein kinase B), triggering multiple downstream signalling events including mTORC1 activation (Frost & Lang, 2007; Miyazaki et al. 2010). The ability of IGF-1 to act as an anabolic agent in an autocrine/paracrine fashion in skeletal muscle has been clearly demonstrated in both the in vitro cell culture model and the in vivo animal model. "
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    • "Growth factor (e.g. IGF-1 and/or insulin) stimulation of muscle leads to activation of phosphoinositide 3-kinase (PI3K) and its downstream effector Akt (also referred to as protein kinase B), triggering multiple downstream signalling events including mTORC1 activation (Frost & Lang, 2007; Miyazaki et al. 2010). The ability of IGF-1 to act as an anabolic agent in an autocrine/paracrine fashion in skeletal muscle has been clearly demonstrated in both the in vitro cell culture model and the in vivo animal model. "

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