Cross-talk between Sirtuin and Mammalian Target of Rapamycin Complex 1 ( mTORC1) Signaling in the Regulation of S6 Kinase 1 ( S6K1) Phosphorylation

Background: The mTOR complex 1 (mTORC1) phosphorylates ribosomal S6 kinase (S6K1) on Thr-389, leading to S6K1 activation. Results: Inhibition of SIRT1/2, members of the sirtuin family of proteins, induces S6K1 acetylation and inhibits mTORC1-dependent S6K1 phosphorylation. Conclusion: The SIRT1/2 support mTORC1-induced S6K1 activation by inhibiting S6K1 acetylation. Significance: The study provides a novel mechanistic insight into the cross-talk between sirtuins and mTORC1 signaling. p70 ribosomal S6 kinase (S6K1), a major substrate of the mammalian target of rapamycin (mTOR) kinase, regulates diverse cellular processes including protein synthesis, cell growth, and survival. Although it is well known that the activity of S6K1 is tightly coupled to its phosphorylation status, the regulation of S6K1 activity by other post-translational modifications such as acetylation has not been well understood. Here we show that the acetylation of the C-terminal region (CTR) of S6K1 blocks mTORC1-dependent Thr-389 phosphorylation, an essential phosphorylation site for S6K1 activity. The acetylation of the CTR of S6K1 is inhibited by the class III histone deacetylases, SIRT1 and SIRT2. An S6K1 mutant lacking acetylation sites in its CTR shows enhanced Thr-389 phosphorylation and kinase activity, whereas the acetylation-mimetic S6K1 mutant exhibits decreased Thr-389 phosphorylation and kinase activity. Interestingly, relative to the acetylation-mimetic S6K1 mutant, the acetylation-defective mutant displays higher affinity toward Raptor, an essential scaffolding component of mTORC1 that recruits mTORC1 substrates. These observations indicate that sirtuin-mediated regulation of S6K1 acetylation is an additional important regulatory modification that impinges on the mechanisms underlying mTORC1-dependent S6K1 activation.