Sin1 phosphorylation impairs mTORC2 complex integrity and inhibits downstream Akt signalling to suppress tumorigenesis

Abstract

The mechanistic target of rapamycin (mTOR) functions as a critical regulator of cellular growth and metabolism by forming multi-component, yet functionally distinct complexes mTORC1 and mTORC2. Although mTORC2 has been implicated in mTORC1 activation, little is known about how mTORC2 is regulated. Here we report that phosphorylation of Sin1 at T86 and T398 suppresses mTORC2 kinase activity by dissociating Sin1 from mTORC2. Importantly, Sin1 phosphorylation, triggered by S6K or Akt, in a cellular context-dependent manner, inhibits not only insulin/IGF-1-mediated, but also PDGF or EGF-induced Akt phosphorylation by mTORC2, demonstrating a negative regulation of mTORC2 independent of IRS-1 and Grb10. Lastly, a cancer patient-derived Sin1-R81T mutation impairs Sin1 phosphorylation, leading to hyper-mTORC2 activation via bypassing this negative regulation. Together, our work reveals a Sin1 phosphorylation-dependent mTORC2 regulation, providing a potential molecular mechanism by which mutations in the mTORC1/S6K/Sin1 signaling axis might cause aberrant hyper-activation of mTORC2/Akt that facilitates tumorigenesis.

Pharmacological inhibition of S6K1 increases glucose metabolism and Akt signalling in vitro and in diet-induced obese mice

“…In contrast, we found that PF-4708671 increased Akt S473 phosphorylation, which was associated with blunted inhibitory S1101 phosphorylation on IRS1 in vitro and in vivo. However, S6K1 is also involved in a negative feedback loop towards mTORC2/Akt signalling, which has been recently linked to inhibitory phosphorylation of the mTORC2 subunit Sin1 on Thr86 and Thr398 by S6K1 [11,34,36]. Indeed, Sin1 phosphorylation by S6K1 inhibits the ability of mTORC2 to phosphorylate Akt at S473.…”

“…We have further reported that Ser 1101 is an i m p o r t a n t t a rg e t o f S 6 K 1 a n d t h a t t h i s s i t e i s hyperphosphorylated in animal models of obesity and upon nutrient excess in humans [10]. Recently, Liu et al [11] reported that S6K1 can also disrupt mTORC2 by phosphorylating Sin1 on both T86 and T398 residues. All these negative feedback control mechanisms reveal the importance of S6K1 in the regulation of insulin's pleiotropic actions.…”

mTORC2 promotes type I insulin-like growth factor receptor and insulin receptor activation through the tyrosine kinase activity of mTOR

“…Previous studies have revealed that S6K1 phosphorylates rictor and SIN1 thereby negatively regulating mTORC2 activity [11,12]. Therefore, treatment with rapamycin or downregulation of raptor and S6K1 may paradoxically activate mTORC2.…”

“…In addition, mTORC2 was shown to regulate protein kinase Cα and serum/ glucocorticoid-induced protein kinase 1 [8,9]. Recently, it has been found that mTORC1 and its downstream effector S6K1 negatively regulate mTORC2 via the phosphorylation of rictor and SIN1 [10][11][12], suggesting a regulatory link between these two complexes. In addition, prolonged treatment with rapamycin, an antibiotic product that predominantly inhibits mTORC1, leads to ERK1/2 phosphorylation [13].…”

mTORC1-independent autophagy regulates receptor tyrosine kinase phosphorylation in colorectal cancer cells via an mTORC2-mediated mechanism

“…To this end, we employed the mTORC1 inhibitor Rapamycin, which is known to upregulate mTORC2 activity through inhibition of a mTORC1-mediated negative feedback loop. [75][76][77] Autophagy-proficient and-deficient HCT-116 cells were treated with Rapamycin and analysed at different time points. Rapamycin downregulated mTORC1 activity and led to increased mTORC2-mediated phosphorylation of AKT at S473.…”