mTOR Complexes and Signaling: 1 protein, 2 complexes, seemingly infinite impacts

One could be forgiven if, after reading a synopsis of the mTOR complex, one walks away with the feeling that mTOR controls most every aspect of being alive.

mTOR, the mechanistic target of rapamycin, sits at the center of a signaling network that coordinates cell growth based on environmental cues and plays an integral role in both cellular and organismal physiology. mTOR forms two distinct complexes, mTORC1 and mTORC2: mTORC1 – more widely studied due to its known sensitivity to rapamycin – is responsible for protein synthesis and turnover, as well as lipid, glucose, and nucleotide metabolism. mTORC2 – found to be largely insensitive to rapamycin – is less well known but has been shown to have roles in proliferation and survival¹. We will outline the core components of each mTOR complex and provide a brief overview of their activities (as whole book chapters have been written on just one complex alone) and finish our overview off with a discussion of the role of mTOR in disease.

mTORC1 is comprised of three core components: mTOR, Raptor, and mLST8². Raptor is responsible for substrate recruitment and proper subcellular localization of mTORC1³. mTOR is a serine / threonine protein kinase and mLST8 is thought to bind and stabilize the catalytically active subunit of mTOR⁴. mTORC1 is a known nutrient sensor, promoting energy-expending activities like macromolecular synthesis when nutrients are abundantly available or triggering autophagy when resources are scarce¹. mTORC1 can sense cytosolic leucine and arginine through its interaction with the GATOR1 and GATOR2 complexes, resulting in mTORC1 activation⁵. mTORC1 can be inhibited by glucose deprivation as well as through exposure to hypoxic conditions^6,7. Such environments are incompatible with energy expenditure, and therefore a decrease in mTORC1 activity results in activation of autophagy and protein degradation pathways.

mTORC2, likewise, is composed of three core components: mTOR, Rictor, and mLST8⁸. Where mTORC1 plays crucial roles in cell growth, mTORC2 is critical for proliferation and survival, primarily through regulating both AKT and the AGC family of protein kinases (PKA/PKG/PKC)¹. As an activator of AKT, it makes sense that mTORC2 is an effector of the insulin / PI3K pathway⁹. The mTORC2 subunit, mSin1, inhibits mTOR activity in the absence of insulin¹⁰. Interestingly, mTORC1 functions as a negative regulator of mTORC2 as mTORC1 phosphorylates a negative regulator of the insulin / IGF-1 pathway, upstream of mTORC2 activation¹¹. Further complicating the mTORC1/2 story with AKT is that AKT is involved in the phosphorylation and inactivation of a key mTORC1 regulator – TSC – which firmly planted mTORC1 as a proposed druggable target in cancer pathways¹². At the time, given the growing body of knowledge on the role AKT played in tumorigenesis, and the knowledge that AKT activation positively regulates mTORC, oncologists assumed that rapamycin would be a magic bullet treatment for cancer. However, the first wave of rapamycin-based therapies (rapalogs) were far less efficacious than expected, due in part to three things:

1. Rapalogs did not effect all mTORC1 activated pathways equally, meaning some pathways were not impacted by rapalog addition¹³
2. Inhibition of mTORC1 threw the AKT homeostasis maintained by both mTORC1 and 2 out of balance¹⁴
3. Inhibition of mTORC1 results in autophagy, which can improve cancer cell survival in poor microenvironments¹⁵

This setback has caused scientists to develop mTOR inhibitors which binding the active site of mTOR itself, effectively disrupting both complexes. Given the myriad of roles mTORC1 and 2 play, these new drugs are being tested in combination with PI3K inhibitors to further remove both mTOR complexes and AKT from the cancer equation^1.

Given mTORC’s role in cell growth and proliferation, the complexes are also known to be involved in aging, brain function, immune cell function, muscle growth, glucose homeostasis, and more¹. Much more than can be covered in this blog post. Regardless as to your pathway, cell type, or disease of interest, our HAP1 and Cancer cell lines cover a majority of the mTORC1/2 pathways, as assessed by Kegg Pathway Analysis. Check out the table below for your gene of interest.

Gene knockouts available in HAP1 and Cancer Cell Lines
Gene knockouts available in Cancer Cell Lines

References

1. Saxton, R.A., Sabatini, D.M. (2017). mTOR Signaling in Growth, Metabolism, and Disease. Cell. 168, 960-976.
2. Kim, D.H., Sarbassov, D.D., Ali, S.M., King, J.E., Latek, R.R., Erdjument-Bromage, H., Tempst, P., and Sabatini, D.M. (2002). mTOR interacts with raptor to form a nutrient-sensitive complex that signals to the cell growth machinery. Cell 110, 163–175.
3. Nojima, H., Tokunaga, C., Eguchi, S., Oshiro, N., Hidayat, S., Yoshino, K., Hara, K., Tanaka, N., Avruch, J., and Yonezawa, K. (2003). The mammalian target of rapamycin (mTOR) partner, raptor, binds the mTOR substrates p70 S6 kinase and 4E-BP1 through their TOR signaling (TOS) motif. J. Biol. Chem. 278, 15461–15464.
4. Yang, H., Rudge, D.G., Koos, J.D., Vaidialingam, B., Yang, H.J., and Pavletich, N.P. (2013). mTOR kinase structure, mechanism and regulation. Nature 497, 217–223.
5. Bar-Peled, L., Chantranupong, L., Cherniack, A.D., Chen, W.W., Ottina, K.A., Grabiner, B.C., Spear, E.D., Carter, S.L., Meyerson, M., and Sabatini, D.M. (2013). A Tumor suppressor complex with GAP activity for the Rag GTPases that signal amino acid sufficiency to mTORC1. Science 340, 1100–1106.
6. Gwinn, D.M., Shackelford, D.B., Egan, D.F., Mihaylova, M.M., Mery, A., Vasquez, D.S., Turk, B.E., and Shaw, R.J. (2008). AMPK phosphorylation of raptor mediates a metabolic checkpoint. Mol. Cell 30, 214–226.
7. Brugarolas, J., Lei, K., Hurley, R.L., Manning, B.D., Reiling, J.H., Hafen, E., Witters, L.A., Ellisen, L.W., and Kaelin, W.G., Jr. (2004). Regulation of mTOR function in response to hypoxia by REDD1 and the TSC1/TSC2 tumor suppressor complex. Genes Dev. 18, 2893–2904.
8. Sarbassov, D.D., Ali, S.M., Kim, D.H., Guertin, D.A., Latek, R.R., Erdjument- Bromage, H., Tempst, P., and Sabatini, D.M. (2004). Rictor, a novel binding partner of mTOR, defines a rapamycin-insensitive and raptor-independent pathway that regulates the cytoskeleton. Curr. Biol. 14, 1296–1302.
9. Sarbassov, D.D., Guertin, D.A., Ali, S.M., and Sabatini, D.M. (2005). Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex. Science 307, 1098–1101.
10. Liu, P., Gan, W., Chin, Y.R., Ogura, K., Guo, J., Zhang, J., Wang, B., Blenis, J., Cantley, L.C., Toker, A., et al. (2015). PtdIns(3,4,5)P3-Dependent Activation of the mTORC2 Kinase Complex. Cancer Discov. 5, 1194–1209.
11. Hsu, P.P., Kang, S.A., Rameseder, J., Zhang, Y., Ottina, K.A., Lim, D., Peterson, T.R., Choi, Y., Gray, N.S., Yaffe, M.B., et al. (2011). The mTOR-regulated phosphoproteome reveals a mechanism of mTORC1-mediated inhibition of growth factor signaling. Science 332, 1317–1322.
12. Inoki, K., Li, Y., Zhu, T., and Guan, K.L. (2002). TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat Cell Biol. 4, 648–657.
13. Choo, A.Y., Yoon, S.O., Kim, S.G., Roux, P.P., and Blenis, J. (2008). Rapamycin differentially inhibits S6Ks and 4E-BP1 to mediate cell-type-specific repression of mRNA translation. Proc. Natl. Acad. Sci. USA 105, 17414–17419.
14. Tabernero, J., Rojo, F., Calvo, E., Burris, H., Judson, I., Hazell, K., Martinelli, E., Ramon y Cajal, S., Jones, S., Vidal, L., et al. (2008). Dose- and scheduledependent inhibition of themammalian target of rapamycin pathway with everolimus: A phase I tumor pharmacodynamic study in patients with advanced solid tumors. J. Clin. Oncol. 26, 1603–1610.
15. Palm, W., Park, Y., Wright, K., Pavlova, N.N., Tuveson, D.A., and Thompson, C.B. (2015). The Utilization of Extracellular Proteins as Nutrients Is Suppressed by mTORC1. Cell 162, 259–270.