Cell Research (2016) 26:1-2. npg © 2016 IBCB, SIBS, CAS All rights reserved 1001-0602/16 $ 32.00 RESEARCH HIGHLIGHT www.nature.com/cr mTORC2 is a tyrosine Cell Research (2016) 26:1-2. doi:10.1038/cr.2015.134; published online 20 November 2015

The mammalian target of rapamy- remains poorly understood. mTORC2 does not appear to directly cin (mTOR), a protein kinase, is the mTOR complexes play multifaceted phosphorylate IRS1/2. One possible centre of huge attention due to its roles in insulin signaling. For example, way in which mTORC2 increases tyro- importance in intracellular signaling Akt plays key roles in insulin signal- sine phosphorylation of InsR or IGF-IR and in health and disease. In their re- ing, mediating the regulation of various is by stimulating the kinase activity of cent study, Yin et al. show that mTOR proteins involved in the effects of this the receptors which then catalyse the can regulate signaling through the hormone on metabolism, e.g., glucose phosphorylation of the receptors on insulin-like growth factor 1 receptor transport. Akt signaling indirectly acti- tyrosine. The authors ruled this out, by and that it possesses a new enzymatic vates mTORC1. In turn, mTORC1 regu- using kinase-dead versions of the recep- activity — the ability to phosphory- lates key anabolic processes including tors or mTOR. Therefore, mTORC2 late proteins on tyrosine residues. protein, lipid and ribosome synthesis. promotes the tyrosine phosphorylation mTOR is a large, multi-domain pro- However, mTORC1 can, via the S6Ks, of InsR/IGF-1R, which is required tein; its catalytic domain resembles that inhibit insulin signaling. This involves for downstream signaling from these of lipid such as phosphoinosit- the phosphorylation of insulin receptor receptors. While these authors clearly ide 3-kinase (PI 3-kinase), but mTOR substrates 1 or 2 (IRS1/2), a crucial link show that rapamycin causes increased actually has protein kinase activity, between insulin (and related) receptors phosphorylation of the mTORC2 sub- adding phosphate groups to serine or and downstream signalling protein, strate AKT, earlier studies showed that, threonine residues in a growing catalog e.g., Akt. at similar time points of treatment in of substrates, many of which are in- The receptors for insulin (InsR) and the same cell-type, rapamycin inhib- volved in anabolic pathways. insulin-like growth factor I (IGF-IR) ited AKT phosphorylation indicating mTOR binds to several protein part- are ligand-activated tyrosine kinases, interference with mTORC2 function ners in the cell to form two distinct types which undergo autophosphorylation [2]. It is not clear how rapamycin of complexes, termed mTOR complexes allowing them to phosphorylate ad- promotes mTORC2 function under the 1 and 2 (mTORC1/2 [1]). These differ ditional proteins such as IRS1. In turn, conditions used in this study. Another in their protein components, substrate phosphorylated IRS1 binds PI 3-kinase; study [4] found that mTORC2 promotes specificity and regulation. For example, this leads to enhanced production of degradation of IRS1, suggesting, in mTORC1 is activated by amino acids, phosphatidylinositol 3,4,5-trisphos- contrast to the conclusions of Yin et and by hormones and growth factors. phate, PIP3, and to activation of Akt. al., that mTORC2 can promote insulin mTORC1 contains a protein termed Yin et al. [3] found that rapamycin resistance. These and other data suggest Raptor which allows it to phosphorylate led to increased phosphorylation of InsR that the web of interactions between substrates such as the ribosomal protein and IGF-IR at key autophosphorylation these signaling components is indeed S6 kinases (S6Ks), and this effect is sites, reflecting increased kinase activity very complex (Figure 1). blocked by rapamycin. of these receptors. mTOR has previously only been mTORC2 contains Rictor in place of Knockdown of mTOR or Rictor, reported to act on serine or threonine Raptor and therefore phosphorylates a or treatment of cells with an inhibitor residues; the present report shows that distinct set of substrates. These include of mTOR kinase activity, Torin 2, de- mTOR can efficiently phosphorylate regulatory (so-called ‘hydrophobic’) creased the rapamycin-induced phos- tyrosines in vitro using either recombi- sites in a family of protein kinases phorylation of InsR or IGF-IR, while nant InsR or peptides as substrate. These which include Akt, also called protein Raptor knockdown had the converse data reveal that mTORC2 function is a kinase B (PKB). Rapamycin does not effect. This indicates the effect requires ‘dual-specificity’ protein kinase phos- directly inhibit mTORC2 function, but mTORC2; indeed, the authors show phorylating tyrosine as well as serine/ can impair it after longer-term treatment that mTORC2 binds to these recep- threonine sites. Interestingly, mTORC1 [2]. The regulation of mTORC2 activity tors, apparently via IRS1/2. However, was unable to phosphorylate tyrosines. npg 2 as via Grb10, a target for mTORC1 itself [5, 6]) both by the mechanism delineated here and by abrogating the feedback loop from mTORC1 via the S6Ks to IRS1. Second, combining Ins/ IGF-1R receptor inhibitors with mTOR inhibitors may be a more effective anti-cancer treatment than inhibiting the individual pathways. Third, mTORC2 may phosphorylate additional, so far unidentified proteins on tyrosine, add- ing to the growing repertoire of mTOR substrates.

Xuemin Wang1, Christopher G Proud1

1Nutrition and Metabolism Theme, South Austra- Figure 1 Summary of the signalling connections discussed here, including the new lian Health & Medical Research Institute, North link described by Yin et al. [3] between mTORC2 and the insulin/IGF-1 receptors. Terrace, and University of Adelaide, Adelaide, Phosphorylation sites are shown schematically (not all are indicated) as ‘P’ in a Australia yellow background; Y, S and T indicate tyrosine, serine and threonine respectively. Correspondence: Christopher G Proud Green and red arrows show activating and inhibitory phosphorylation events re- E-mail: [email protected] spectively. The gray arrow and ‘?’ indicate potential further tyrosine phosphorylation events catalysed by mTORC2. Solid arrows show direct phosphorylation events; References dashed lines are indirect signalling links. 1 Laplante M, Sabatini DM. Cell 2012; 149:274-293. 2 Sarbassov dD, Ali SM, Sengupta S, et al. Does the mTORC2-stimulated phos- proliferation in IGF-1- or insulin-stimu- Mol Cell 2006; 22:159-168. 3 Yin Y, Hua H, Li M, et al. Cell Res 2016; phorylation of the InsR/IGF-1R play lated conditions. Rictor overexpression 1:46-65. a role in the actions of the ligands for increased proliferation, an effect that 4 Kim SJ, DeStefano MA, Oh WJ, et al. Mol these receptors? To test this, the authors requires the activity of the InsR/IGF-1R. Cell 2012; 48:875-887. examined the Rictor knockdown on What are the main implications of 5 Yu Y, Yoon SO, Poulogiannis G, et al. Sci- ence 2011; 332:1322-1326. HepG2 cell proliferation. While this these data? First, rapamycin may actu- 6 Hsu PP, Kang SA, Rameseder J, et al. Sci- had no effect in the absence of insulin ally promote signaling from the InsR/ ence 2011; 332:1317-1322. or IGF-1, depletion of Rictor did inhibit IGF-1R through mTORC2 (as well

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