Abstract 4621: Rapamycin inhibits the phosphorylation of mSin1 by targeting a new mTOR complex

Abstract Current knowledge indicates that mTOR functions as two complexes, mTORC1 and mTORC2, in mammalian cells. mTORC1 comprises mTOR, mLST8 and raptor, and is sensitive to acute rapamycin treatment, whereas mTORC2 consists of mTOR, mLST8, rictor, mSin1 and Protor, and is sensitive to chronic rapamycin treatment in some cases. It is well known that mTORC1 phosphorylates 4E-BP1 and S6K1, while mTORC2 phosphorylates Akt and SGK1. Recently, we and others have observed that rapamycin inhibits the phosphorylation of mSin1. However, little is known about the molecular mechanism by which rapamycin inhibits mSin1 phosphorylation. Here we show that rapamycin inhibits mSin1 phosphorylation in mTOR kinase dependent manner. This is strongly supported by the findings that expression of rapamycin-resistant mTOR (mTOR-T), but not rapamycin-resistant and kinase dead mTOR (mTOR-TE) prevented rapamycin from inhibiting mSin1 phosphorylation; Knockdown of mTOR inhibited mSin1 phosphorylation. However, surprisingly, disruption of either mTORC1 or mTORC2 (by silencing raptor and rictor, respectively) failed to inhibit the phosphorylation of mSin1 as rapamycin did. Similarly, downregulation of S6K1 or Akt did not inhibit mSin1 phosphorylation as rapamycin did either. Of interest, knockdown of mLST8, a component shared by mTORC1/2, inhibited mSin1 phosphorylation. Furthermore, rapamycin treatment reduced the physical interaction of mSin1 with mLST8 and mTOR, but not with rictor. The results suggest that rapamycin inhibits mSin1 phosphorylation, which is not by inhibiting mTORC1 or mTORC2, but via inhibiting a new mTOR complex, which contains at least mTOR and mLST8. (Supported by the FWCC, LSU Health Sciences Center) Citation Format: Shile Huang, Yan Luo, Lei Liu. Rapamycin inhibits the phosphorylation of mSin1 by targeting a new mTOR complex. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4621.