Regulatory mechanisms in the Akt-mTOR signalings axis

Mutations in the TSC1 and TSC2 tumor suppressor genes give rise to the neoplastic disorders tuberous sclerosis complex (TSC) and lymphangioleiomyomatosis (LAM). Their gene products form a complex that is a critical negative regulator of mTOR complex 1 (mTORC1) and cell growth. Downstream of phosphoinositide 3- kinase (PI3K), Akt phosphorylates TSC2 directly on multiple sites. These phosphorylation events relieve the inhibitory effects of the TSC 1-TSC2 complex on mTORC1, thereby activating mTORC1 in response to growth factors. Further, these phosphorylation events on TSC2 regulate mTORC1-mediated effects on cell size, adipocyte differentiation and colony formation on soft agar. The manner by which the second mTOR complex, mTORC2 is regulated is less clear and whether the TSC1-TSC2 complex is involved is not known prior to this study. We find that the TSC1-TSC2 complex promotes the activity of mTOR complex 2 (mTORC2), independent of its inhibitory effects on mTORC 1 and its GTPase-activating protein activity towards Rheb. Together with an mTORC1-mediated feedback mechanism inhibiting activation of phosphoinositide 3-kinase (PI3K), the loss of mTORC2 activity strongly attenuates the growth factor-stimulated phosphorylation of Akt on Ser473 in cells lacking the TSC 1-TSC2 complex. Interestingly, both PI3Kdependent and independent mTORC2 substrates are affected by loss of the TSC1-TSC2 complex in cell culture models and kidney tumors from both Tsc2 +/- mice (i.e., adenoma) and TSC patients (i.e., angiomyolipoma). These mTORC2 targets are all members of the AGC family kinases and include Akt, PKCα, and SGK1, and are important for tumorigenic processes. We also demonstrate that TSC1-TSC2 can physically associate with mTORC2, but not mTORC1 and the interaction between the two complexes is mediated primarily through regions on TSC2 and Rictor. Finally, the TSC1-TSC2 complex can directly stimulate the in vitro kinase activity of mTORC2. Hence, loss of the TSC tumor suppressors results in elevated Rheb-mTORC1 signaling and attenuated mTORC2 signaling. These findings suggest that the TSC1-TSC2 complex might play opposing roles in tumor progression, both blocking and promoting specific oncogenic pathways through its effects on mTORC1 inhibition and mTORC2 activation, respectively.