Novel Functions of mTOR in the Regulation of Insulin Signaling and Brain Metabolism
Michael A. DeStefano
M.S., Rutgers University - 2010
Thesis Advisor: Estela Jacinto, Ph.D.
Graduate Program in Physiology and Integrative Biology
SPH, Room 258
Tuesday, November 19, 2013
The mammalian target of rapamycin (mTOR) integrates signals from nutrients and insulin via two distinct protein 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. In this thesis we also examined mTOR control of brain metabolism in a model of autism. Tuberous sclerosis (TSC) is associated with autism spectrum disorders (ASD) and both diseases have been linked to metabolic dysfunction and unrestrained signaling of mTOR. Inhibition of mTOR by rapamycin can mitigate some of the phenotypic abnormalities associated with ASD but whether this is due to the mTOR-related functions on energy metabolism remains to be elucidated. In young Eker rats, an animal model of TSC and autism, which harbors a germline heterozygous Tsc2 mutation, we previously reported that cerebral oxygen consumption was pronouncedly elevated. Here, we found that despite TSC2 haploinsufficiency in the Eker brain, mTORC1 signaling is augmented. Enhanced S6K1 activity triggered a negative feedback loop that decreased IRS-1/Akt signals. However, restoring basal oxygen consumption by rapamycin treatment is specifically linked to rescue of cytosolic phosphorylation of mTOR and Akt but not S6K1 or 4E-BP1. In the cytosol, levels of lactate dehydrogenase A were elevated in the Eker that can be restored to normal levels by rapamycin. Dampening mTORC1 signals also rescued defective phosphorylation of AMPK and LKB1. Our findings reveal that enhanced oxygen consumption in the Eker brain is coupled to increased glycolytic metabolism. Drugs that can restore defective mTOR, LDHA and AMPK signals could alleviate aberrant brain energy metabolism and have therapeutic potential in tuberous sclerosis and autism linked to TSC.
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