Regulation of insulin receptor substrate-1 by mTORC2 (mammalian target of rapamycin complex 2).

mTOR (mammalian target of rapamycin) responds to the presence of nutrients, energy and growth factors to link cellular metabolism, growth and proliferation. The rapamycin-sensitive mTORC (mTOR complex) 1 activates the translational regulator S6K (S6 kinase), leading to increased protein synthesis in the presence of nutrients. On the other hand, the rapamycin-insensitive mTORC2 responds to the presence of growth factors such as insulin by phosphorylating Akt to promote its maturation and allosteric activation. We recently found that mTORC2 can also regulate insulin signalling at the level of IRS-1 (insulin receptor substrate-1). Whereas mTORC1 promotes IRS-1 serine phosphorylation that is linked to IRS-1 down-regulation, we uncovered that mTORC2 mediates its degradation. In mTORC2-disrupted cells, inactive IRS-1 accumulated despite undergoing phosphorylation at the mTORC1-mediated serine sites. Defective IRS-1 degradation was due to attenuated expression of the CUL7 (Cullin 7) ubiquitin ligase substrate-targeting sub-unit Fbw8. mTORC2 and Fbw8 co-localize at the membrane where mTORC2 phosphorylates Ser86 to stabilize Fbw8 and promotes its cytosolic localization upon insulin stimulation. Under conditions of chronic insulin exposure, inactive serine-phosphorylated IRS-1 and Fbw8 co-localize to the cytosol where the former becomes ubiquitylated via CUL7/Fbw8. Thus mTORC2 negatively feeds back to IRS-1 via control of Fbw8 stability and localization. Our findings reveal that, in addition to persistent mTORC1 signalling, increased mTORC2 signals can promote insulin resistance due to mTORC2-mediated degradation of IRS-1.

mTOR complex 2 regulates proper turnover of insulin receptor substrate-1 via the ubiquitin ligase subunit Fbw8.

The mammalian target of rapamycin (mTOR) integrates signals from nutrients and insulin via two distinct complexes, mTORC1 and mTORC2. Disruption of mTORC2 impairs the insulin-induced activation of Akt, an mTORC2 substrate. Here, we found that mTORC2 can also regulate insulin signaling at the level of insulin receptor substrate-1 (IRS-1). Despite phosphorylation at the mTORC1-mediated serine sites, which supposedly triggers IRS-1 downregulation, inactive IRS-1 accumulated in mTORC2-disrupted cells. Defective IRS-1 degradation was due to attenuated expression and phosphorylation of the ubiquitin ligase substrate-targeting subunit, Fbw8. mTORC2 stabilizes Fbw8 by phosphorylation at Ser86, allowing the insulin-induced translocation of Fbw8 to the cytosol where it mediates IRS-1 degradation. Thus, mTORC2 negatively feeds back to IRS-1 via control of Fbw8 stability and localization. Our findings reveal that in addition to persistent mTORC1 signaling, heightened mTORC2 signals can promote insulin resistance due to mTORC2-mediated degradation of IRS-1.