mTORC1-Activated S6K1 Phosphorylates Rictor on Threonine 1135 and Regulates mTORC2 Signaling

Abstract

The mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that forms two functionally distinct complexes important for nutrient and growth factor signaling. While mTOR complex 1 (mTORC1) regulates mRNA translation and ribosome biogenesis, mTORC2 plays an important role in the phosphorylation and subsequent activation of Akt. Interestingly, mTORC1 negatively regulates Akt activation, but whether mTORC1 signaling directly targets mTORC2 remains unknown. Here we show that growth factors promote the phosphorylation of Rictor (rapamycin-insensitive companion of mTOR), an essential subunit of mTORC2. We found that Rictor phosphorylation requires mTORC1 activity and, more specifically, the p70 ribosomal S6 kinase 1 (S6K1). We identified several phosphorylation sites in Rictor and found that Thr1135 is directly phosphorylated by S6K1 in vitro and in vivo, in a rapamycin-sensitive manner. Phosphorylation of Rictor on Thr1135 did not affect mTORC2 assembly, kinase activity, or cellular localization. However, cells expressing a Rictor T1135A mutant were found to have increased mTORC2-dependent phosphorylation of Akt. In addition, phosphorylation of the Akt substrates FoxO1/3a and glycogen synthase kinase 3␣/␤ (GSK3␣/␤) was found to be increased in these cells, indicating that S6K1-mediated phosphorylation of Rictor inhibits mTORC2 and Akt signaling. Together, our results uncover a new regulatory link between the two mTOR complexes, whereby Rictor integrates mTORC1-dependent signaling.

Activity and complexes of mTOR in diffuse large B-cell lymphomas—a tissue microarray study

“…Concerning intracellular localization, nuclear-in addition to cytoplasmicp-mTOR reaction was observed in o5% of the cells in our study and we did not attribute this to mTORC2 activity-especially because Rictor (an element of the mTORC2 complex) was present only in the cytoplasm, as functionally expected and reported also by others. 23 One of the key targets of mTORC2 is Akt. 23 Akt has been reported to show activity in about 30-40% of diffuse large B-cell lymphoma cases, 24,25 which is in the same range as overall Rictor expression in our study.…”

“…23 One of the key targets of mTORC2 is Akt. 23 Akt has been reported to show activity in about 30-40% of diffuse large B-cell lymphoma cases, 24,25 which is in the same range as overall Rictor expression in our study. Rictor expression and Akt activity may explain the inefficiency of rapamycin analog (rapalog) therapy in recent phase I/II studies, where targeting mTORC1 resulted in 28-32% response rates in high-grade lymphomas.…”

mTOR, la cible fonctionnelle de la rapamycine

“…En effet, la phosphorylation des protéines adaptatrices IRS (insulin receptor substrate 1) par mTORC1 et S6K a pour effet de diminuer leur stabilité et d'empêcher le recrutement de PI3K [25]. De plus, des étu-des plus récentes ont démontré que Rictor est phosphorylé par S6K1, ce qui inhibe l'activité de mTORC2 et diminue la phosphorylation et l'activation d'Akt [26][27][28].…”

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].…”