Leukemia (2010) 24, 1686–1699 & 2010 Macmillan Publishers Limited All rights reserved 0887-6924/10 www.nature.com/leu REVIEW Perspectives on inhibiting mTOR as a future treatment strategy for hematological malignancies N Chapuis1,2,3, J Tamburini1,2,4, AS Green1,2, L Willems1,2,4, V Bardet1,2,3, S Park1,2,4, C Lacombe1,2,3, P Mayeux1,2 and D Bouscary1,2,4 1De´partement d’Immunologie-He´matologie, Institut Cochin, Universite´ Paris Descartes, CNRS, UMR8104, Paris, France; 2INSERM, U1016, Paris, France; 3Service d’He´matologie Biologique, Hoˆpital Cochin, AP-HP, Paris, France and 4Service de Me´decine Interne-UF d’He´matologie, Hoˆpital Cochin, Paris, France Mammalian target of rapamycin (mTOR) is a protein kinase Structure of mTOR implicated in the regulation of various cellular processes, including those required for tumor development, such as the mTOR is a serine/threonine kinase that belongs to the phospho- initiation of mRNA translation, cell-cycle progression and cellular proliferation. In a wide range of hematological malig- inositide 3-kinase (PI3K)-related kinase family and is ubiqui- nancies, the mTORC1 signaling pathway has been found to be tously expressed in mammalian cells. mTOR was first described deregulated and has been designed as a major target for tumor in yeast as the cellular target of rapamycin, an immunosuppres- therapy. Given that pre-clinical studies have clearly established sive agent first identified as a product of the bacterium the therapeutic value of mTORC1 inhibition, numerous clinical Streptomyces hygroscopicus.1 The mTOR protein comprises trials of rapamycin and its derivates (rapalogs) are ongoing a C-terminal catalytic domain, an FRB domain (FKBP-12- for treatment of these diseases. At this time, although disease stabilization and tumor regression have been observed, rapamycin binding domain) that facilitates the binding of objective responses in some tumor types have been modest. FKBP-12(FK506 binding protein 12)/rapamycin complexes, a Nevertheless, some of the mechanisms underlying cancer-cell C-terminal negative regulatory domain, 20 or more N-terminal resistance to rapamycin have now been described, thereby HEAT repeats (Huntingtin, EF3, A subunit of PP2A and TOR) that leading to the development of new strategy to efficiently target enable protein–protein interactions and FRAP-ATM-TRRAP mTOR signaling in these diseases. In this review, we discuss (FAT) domains that modulate catalytic activity.2 It is now known the rationale for using mTOR inhibitors as novel therapies for a variety of hematological, malignancies with a focus on that mTOR resides in at least two distinctive multi-protein promising new perspectives for these approaches. complexes, mTORC1 and mTORC2, which are distinguished by Leukemia (2010) 24, 1686–1699; doi:10.1038/leu.2010.170; their partner proteins, their substrate specificities and their published online 12 August 2010 differential sensitivity to rapamycin. Keywords: mTORC1; mTORC2; PI3K/Akt; hematological malignancies; rapalogs; TORKinhib mTORC1 Introduction mTORC1 is usually defined as a complex of mTOR and raptor (regulatory-associated protein of mTOR). However, mTORC1 The mammalian target of rapamycin (mTOR) is a key regulator also contains mLST8 (also known as GbL), the proline-rich of various cellular processes needed for growth, cell-cycle 3 Akt/PKB substrate 40kDa (PRAS40) and the recently identified progression and cell metabolism. The mTOR pathway functions dishevelled, egl-10, pleckstrin (DEP)-domain-containing mTOR- as a sensor to ensure an appropriate nutritional state to support 4 interacting protein (deptor). The mTORC1 complex is generally cell development. The blockade of the mTOR signaling can thus inhibited by rapamycin (Sirolimus) and its analogs (CCI-779, prevent cells from responding to extracellular signals transduced RAD001 and AP23573), which are referred to as rapalogs. In from growth factor receptors under conditions of energy and/or most cell types, rapalogs specifically induce the allosteric nutrient deficiency. Conversely, mTOR activation promotes cell repression of mTORC1 activity through their association with growth and proliferation under conditions of energy and nutrient 5 FKBP-12. These FKBP-12/rapamycin complexes bind mTOR in a repletion, through an increase in ribosomal biogenesis and 6 unique region known as the FKBP-rapamycin-binding domain, protein synthesis. As mTOR activity is frequently observed to be 7,8 resulting in the dissociation of mTOR from raptor. The FKBP- deregulated in cancer, including hematological malignancies, 12/rapamycin complexes thus fuction as allosteric inhibitors of this kinase represents a highly attractive target for novel cancer mTORC1 without directly affecting mTOR catalytic activity. therapies. In this review, we discuss the current model for the The exact functions of most mTOR-interacting proteins within mTOR signaling pathway and then focus on the mechanisms the mTORC1 complex remain unclear. Nevertheless, the underlying its deregulation in hematological malignancies. activity of mTORC1 is dependent upon raptor, which functions Finally, we explore the value of targeting mTOR as a future 7,9 as a scaffold for recruiting mTORC1 substrates. The mLST8 anti-cancer therapy. protein lacks kinase activity, but is also required for proper mTOR activity.10 PRAS40 is a negative regulator of mTORC1 Correspondence: Professor D Bouscary, De´partement d’Immunology- and blocks the interaction between mTOR and its substrates. ´ ´ Hematology, Institut Cochin, Universite Paris Descartes, 22 rue Upon phosphorylation by Akt, PRAS40 dissociates from Me´chain, Paris 75014, France. E-mail: [email protected] or [email protected] mTORC1 and binds 14.3.3 proteins, which release mTORC1 11 Received 17 April 2010; revised 7 June 2010; accepted 29 June 2010; activity. Recently a new protein, deptor, has been identified as published online 12 August 2010 an mTOR-interacting protein4 and has been found to negatively Perspectives on inhibiting mTOR N Chapuis et al 1687 regulate mTORC1. Accordingly, the loss of deptor expression (Ras homolog enriched in brain). The active GTP-bound form leads to the activation of mTORC1 signaling.4 of Rheb directly interacts with mTORC1 and stimulates its activity. Accordingly, through its GTPase-activating protein activity TSC1/2 maintains the binding of Rheb to Guanosine mTORC2 diphosphate and suppresses mTORC1 activity (Figure 1). mTORC2 comprises mLST8 and deptor, which are also found Conversely, the activation of Akt downstream of PI3K and in mTORC1, but also specifically contain mSIN1, rictor of the extra-cellular regulated kinase 1/2 (ERK1/2) leads to (rapamycin insensitive companion of mTOR) and protor-1. the inhibition of TSC2 by phosphorylation at its S939/S1462 and Moreover, in contrast to mTORC1, mTORC2 is usually S540/S664 residues, respectively, which leads, therefore, to rapamycin-insensitive,12,13 although some evidence has sug- mTORC1 activation.17–19 gested that prolonged treatment with rapamycin may inhibit its activity towards Akt, depending on the cell type. The induction by rapamycin of an imbalance in the mTORC2 constitution Adenosine mono phosphate-activated protein has been suggested to explain this effect.14,15 Finally, deptor kinase (AMPK) also negatively regulates mTOR activity in mTORC2.4 Energy metabolism has been found to regulate mTOR through the AMPK. AMPK, which is a sensor for cellular energy, responds to the cellular AMP/ATP (adenosine triphosphate) ratio and is Upstream mechanisms leading to mTORC1 activation activated by metabolic stresses that inhibit ATP production (for example, hypoxia and glucose deprivation) or that stimulate ATP Growth factor receptors consumption. Under energy stress conditions, AMPK activates The mTORC1 pathway can be activated either by extracellular TSC2 by phopshorylation on T1227 and S1345,whichleads, signals transduced from growth factor receptors or by modifica- therefore, to the repression of mTORC1 activity.20 tions of the metabolic status of the cell.16 Downstream of growth factor receptors, extracellular signals converge at the tuberous sclerosis complex (TSC1/TSC2/Rheb) axis. TSC1/2 comprises The amino acid pathway hamartin (TSC1) and tuberin (TSC2) and exhibits a GTPase- From a deterministic point of view, it is easy to understand that activating protein function toward the Ras-related GTPase Rheb the rate of protein synthesis is directly dependent upon amino Amino Energy Growth factors ? acids (ATP) TKR PIP2 PIP3 308 P IRS T P S473 PI3K PDK1 P Akt AMPK P P P TSC1 TSC2 ERK1/2 Feedback PRAS40 Protor inhibition mLST8 mSIN1 mTOR Rheb GTP mTOR Deptor mLST8 mTORC1 Raptor Deptor Rictor mTORC2 T389 65 37/46 S T P RhoA P P P SGK P P70S6K 70 4E-BP1 T PKCα P Cytoskelatal Protein eIF4E eIF4G P organization synthesis NDRG1 P Paxillin 4E-BP1 Cap-dependent translation eIF4E Cell survival Figure 1 The mTOR signaling pathway. Red lines indicate the different mechanisms of mTOR activation. Abbreviations: AMPK, AMP-activated kinase; deptor, DEP-domain-containing mTOR interacting protein; 4E-BP1, eIF4E-binding protein 1; eIF, eukaryotic initiation factors; ERK1/2, extra-cellular regulated kinase 1/2; FKBP12, FK506 binding protein 12; IRS, Insulin receptor substrates; mLST8, mammalian lethal with Sec13 protein 8; mTORC, mammalian target of rapamycin complex; NDRG1, N-Myc downstream regulated gene-1; PDK1, phosphoinositide-dependent kinase 1; PI3K, phosphatidylinositol 3-kinase; PIP2, phosphatidylinositol bisphosphate; PIP3,