Keywords

1 Introduction

The PI3K/AKT/mTOR signalling pathway regulates a number of critical cellular processes including growth, proliferation, differentiation, and survival. The serine/threonine kinase mTOR integrates a wide range of extra- and intra-cellular signals to control these processes through the regulation of mRNA translation. mTOR mediates its effects on mRNA translation through phosphorylation of its major downstream targets: the 4E binding proteins (4E-BPs) and the ribosomal protein S6 kinases (S6K1 and S6K2). The 4E-BPs suppress translation initiation and are inhibited via mTOR-mediated phosphorylation, while the S6Ks enhance translation upon activation by mTOR by targeting components of the translational machinery. Inappropriate activation of mTOR signalling can promote tumorigenesis through an increase in the translation of mRNAs encoding growth factors, pro-survival proteins, cell cycle regulators, and angiogenic factors. Consequently, dysregulation of the mTOR pathway is implicated in a number of human cancers. The role of mTOR and its downstream targets in mRNA translation initiation will be the focus of this chapter. In particular, the regulation of the translational machinery via mTOR-mediated phosphorylation will be described and the importance of the downstream targets of mTOR in the development of human cancer will be addressed.

2 Translation Initiation

The process of translation is divided into three stages: initiation, elongation, and termination. Initiation is rate limiting under most circumstances and as a result is a primary target of translational control. Translation initiation is a complex, highly ordered process that culminates in the assembly of the 80S ribosome at the initiation codon of an mRNA. The rate-limiting step is thought to be the formation of the eIF4F (eukaryotic initiation factor 4F) complex, which mediates the recruitment of ribosomal subunits to the mRNA [1]. eIF4F is composed of eIF4E, which binds the 7-methylguanosine “cap” (m7GpppX, where X is any nucleotide and m is a methyl group) found on the 5′-end of all nuclear-transcribed cellular mRNAs, the helicase, eIF4A, and the large scaffolding protein, eIF4G (Fig. 1) [2, 3]. While eIF4E is required for recognition and binding of the 5′-cap, eIF4G bridges the ribosome and the mRNA by interacting with eIF3, which binds the 40S ribosome. eIF4A unwinds any 5′-mRNA secondary structure to facilitate binding of the 40S subunit and scanning of the mRNA 5′-untranslated region (UTR). The ATPase and helicase activities of eIF4A are stimulated by eIF4B, a small RNA-binding protein that is involved in ribosomal recruitment to mRNA [2, 4]. The 40S ribosome with its associated initiation factors scans the 5′-UTR until it encounters an initiation codon (AUG or a cognate thereof). Once the initiation codon is encountered, the 60S ribosome joins to form the active 80S ribosome.

Fig. 1
figure 1

eIF4F complex formation: The eIF4F complex mediates the recruitment of ribosomal subunits to the mRNA and is composed of the cap-binding protein eIF4E, the large scaffolding protein eIF4G, and the helicase eIF4A. eIF4G is involved in circularization of the mRNA through an interaction with PABP and bridges the 40S ribosome and the mRNA by interacting with eIF3. eIF4B is an RNA-binding protein that stimulates the ATPase and helicase activities of eIF4A

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3 TOR Complex Formation

Two mTOR containing complexes exist in mammalian cells, namely mTORC1 and mTORC2. mTORC1 is rapamycin sensitive, regulated by nutrients and growth factors, and composed of mTOR, raptor (regulatory-associated protein of TOR), and mLst8 (also known as GβL) [5, 6]. Raptor functions as an adaptor protein that is responsible for recruiting the mTORC1 substrates 4E-BP1, S6K, and PRAS40 (proline-rich AKT substrate 40 kDa) [7, 8]. Raptor is required for mTOR-mediated phosphorylation of S6K and 4E-BP1 and interacts with mTOR targets through a TOR signalling (TOS) motif. The TOS motif is located in the NH2 terminus of S6K (FDIDL for S6K1 and FDLDL for S6K2) and the COOH terminus of 4E-BP1 (FEMDI) [9, 10]. Mutation of this motif greatly reduces mTOR-mediated phosphorylation of S6K and 4E-BP1 [9–12]. mLst8 interacts with the kinase domain of mTOR and stabilizes the mTOR–raptor interaction [13]. Recently, it was reported that PRAS40 associates with mTORC1 in cells and negatively regulates mTOR activity [14, 15]. The inhibitory effects of PRAS40 on mTORC1 signalling are due to competition with S6K and 4E-BP1 for binding to raptor, as PRAS40 contains a TOS motif (FVMDE) [16–18]. The activity of PRAS40 is regulated by phosphorylation such that AKT-mediated phosphorylation of PRAS40 on Thr246 reduces its inhibitory effects on mTOR signalling [14, 15]. In addition, PRAS40 was also recently identified as an mTORC1 substrate [8, 17].

In contrast, mTORC2 is composed of mTOR, mLst8, rictor (rapamycin-insensitive component of TOR), and mSIN1 (mammalian stress-activated protein kinase (SAPK)-interacting protein). mTORC2 is rapamycin insensitive, regulated by growth factors, and is implicated in cytoskeletal organization. The only substrate of mTORC2 is believed to be AKT (PKB); however, mTORC2 also regulates the phosphorylation of PKC-α (protein kinase C) and potentially other AGC-family kinases [19–21].

4 Regulation of Translation Initiation by mTOR Signalling

mTOR integrates signals from nutrients, growth factors, cellular energy stores, and other cues to control a variety of important cellular processes including growth, proliferation, differentiation, transcription, and mRNA translation [4, 22, 23]. mTOR is a serine/threonine kinase that co-ordinates these processes via phosphorylation of its downstream targets. The major targets of mTORC1 are integral components of the translational machinery, particularly those involved in translation initiation. Consequently, this chapter will focus on mTORC1 signalling, unless otherwise noted. The eIF4F components eIF4G and eIF4E are regulated by mTORC1, with the activity of eIF4E being controlled by mTORC1-mediated phosphorylation and inactivation of its repressors, the 4E-BPs. In addition, S6K1 and S6K2 are direct targets of mTORC1. S6K1 plays an important role in translational control via regulation of the ribosomal protein S6 (rpS6), eukaryotic elongation factor 2 (eEF2) kinase, and eIF4B.

4.1 The 4E-BPs

The assembly of the eIF4F complex is controlled by a family of small translational repressor proteins known as the 4E-BPs. Three 4E-BPs exist in mammals (4E-BP1, 4E-BP2, and 4E-BP3), each encoded by a separate gene, while Drosophila only express one 4E-BP [24–26]. The 4E-BPs negatively regulate eIF4F assembly by competing with eIF4G for binding to a shared site on eIF4E. The binding of the 4E-BPs to eIF4E is controlled by mTORC1-mediated phosphorylation. The majority of studies on 4E-BP regulation have focussed on 4E-BP1. Consequently, 4E-BP1 is the best characterized member of the 4E-BP family. In its hypophosphorylated form, 4E-BP1 binds to eIF4E with high affinity, preventing eIF4F complex formation and suppressing cap-dependent translation. However, upon stimulation by growth factors, nutrients, or hormones, mTORC1 phosphorylates and inhibits 4E-BP1 leading to the release of eIF4E and an increase in translation initiation (Fig. 2) [24, 27]. Seven phosphorylation sites have been identified in 4E-BP1 (Thr 37, Thr 46, Ser 65, Thr 70, Ser 83, Ser 101, and Ser 112) and four of which (Thr 37, Thr 46, Ser 65, and Thr 70) are linked to mTORC1. The importance of these individual phosphorylation sites in the regulation of 4E-BP1–eIF4E binding is not completely clear. However, the four mTORC1-specific sites are known to be involved in the release of eIF4E from 4E-BP1 [4, 27–29]. The phosphorylation of 4E-BP1 is a complex process that proceeds in a hierarchical manner (Fig. 2). Upon stimulation, mTOR phosphorylates 4E-BP1 on Thr 37 and Thr 46. The phosphorylation of these sites is believed to act as a priming event that is required for the subsequent phosphorylation of residues Thr 70 and Ser 65 [27, 28]. Phosphorylation of 4E-BP1 on residues Thr 37 and Thr 46 does not disrupt binding of eIF4E by 4E-BP1, and phosphorylation of Ser 65 and Thr 70 alone is insufficient to block eIF4E binding [27, 28]. Therefore, it is likely that in combination, these phosphorylation events co-operate to cause the dissociation of eIF4E from 4E-BP1.

Fig. 2
figure 2

Regulation of eIF4E-BP1 by phosphorylation: Hypophosphorylated 4E-BP1 binds to eIF4E with high affinity and inhibits cap-dependent translation. Upon stimulation, mTORC1 phosphorylates 4E-BP1 on Thr 37 and Thr 46, which acts as a priming event that is required for the subsequent phosphorylation of residues Ser 65 and Thr 70. The hyperphosphorylated form of 4E-BP1 is released from eIF4E, allowing formation of the active eIF4F complex and an increase in cap-dependent translation

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4.1.1 S6 Kinase

The ribosomal protein S6 kinases are direct targets of mTORC1 and play important roles in the regulation of mRNA translation. Two S6 kinase proteins (S6K1 and S6K2) are expressed in mammalian cells, and both proteins are phosphorylated and activated by mTORC1 [30]. Despite being encoded by separate genes, the phosphorylation sites on S6K1 and S6K2 are conserved. S6K1, the most characterized of the two kinases, is involved in the regulation of cell growth in Drosophila and mammalian cells [31, 32]. Complete activation of S6K1 requires two phosphorylation events. Phosphorylation at Thr 389 is mediated by mTORC1 and is required for subsequent phosphorylation of Thr 229 by PDK1 (phosphoinositide-dependent kinase 1) [33–35]. S6K1 is believed to promote cell growth by increasing mRNA translation via phosphorylation of its downstream targets rpS6, eIF4B, and eEF2 kinase (Fig. 3). The best characterized substrate of S6K1 is rpS6; however, the functional significance of rpS6 phosphorylation is somewhat unclear. In the past, the phosphorylation of rpS6 correlated well with the translational activation of mRNAs that contain a 5′-terminal oligopyrimidine tract (TOP). However, studies in cells lacking S6K1 and S6K2 have demonstrated that rpS6 phosphorylation is not required for translation of 5′-TOP mRNAs [36–38]. These studies indicate that rpS6 may not be the major target through which the S6 kinases mediate their effects on mRNA translation and cell growth. Despite its unknown function, the phosphorylation of rpS6 is routinely used as an indicator of S6K and mTORC1 activity.

Fig. 3
figure 3

S6 kinase 1 targets and their role in mRNA translation. Phosphorylation of S6K1 by mTORC1 enhances its kinase activity towards its downstream targets (ribosomal protein S6; rpS6, eukaryotic initiation factor 4B; eIF4B, programmed cell death protein 4; PDCD4, insulin receptor substrate-1; IRS-1, S6K1 Aly/REF-like target; SKAR, and eukaryotic elongation factor-2 kinase; eEF2K), which are involved directly or indirectly in the regulation of mRNA translation. eIF4B, rpS6, and SKAR are activated by S6K1-mediated phosphorylation, whereas PDCD4, eEF2K, and IRS-1 are inhibited by phosphorylation

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While the physiological relevance of rpS6 phosphorylation remains unclear, S6K1-mediated phosphorylation of eIF4B plays an important role in eIF4F activity and the recruitment of ribosomes to mRNA. eIF4B is an RNA-binding protein that stimulates the ATPase and helicase activities of eIF4A. S6K1 phosphorylates eIF4B on Ser 422, leading to an increase in mRNA translation [39]. This increase in translation is likely due to an enhanced interaction between phosphorylated eIF4B and eukaryotic translation initiation factor 3 (eIF3) [40, 41]. Phosphorylation may also increase the activity of eIF4B towards the helicase eIF4A, leading to more efficient translation of mRNAs containing large amounts of 5′-UTR secondary structure. For example, footprinting assays have demonstrated that eIF4B is required for the binding of ribosomes to an mRNA-containing secondary structure, and knock-down of eIF4B by RNA interference results in reduced translation of highly structured mRNAs [4, 42, 43]. The phosphorylation of eIF4B is increased by a variety of extracellular stimuli that induce cell growth including serum, insulin, and phorbol esters [4, 44]. In addition to S6K1, eIF4B is also phosphorylated on Ser 422 by the p90 ribosomal protein S6 kinase (RSK) in response to stimulation of the ERK1/2 MAPK pathway [40]. S6K1 may also indirectly affect eIF4F activity via regulation of the eIF4A inhibitor PDCD4 (programmed cell death protein 4) [19]. PDCD4 is a tumour suppressor that binds to and inhibits the helicase activity of eIF4A, thus negatively affecting the unwinding of 5′-mRNA secondary structure and translation initiation. Phosphorylation of PDCD4 by S6K1 targets it for ubiquitination and degradation, relieving its inhibitory effect on eIF4A function and protein synthesis [45].

S6K1 also plays an important role in the regulation of translation elongation via phosphorylation of eEF2 kinase. eEF2 kinase is a negative regulator of protein synthesis as it phosphorylates and inactivates eEF2, leading to inhibition of the translocation step in translation elongation [46]. Upon activation, S6K1 phosphorylates eEF2 kinase on Ser 366, which inhibits its kinase activity towards eEF2 and relieves the suppression of translation elongation [47].

An additional target of S6K1 is SKAR (S6K1 Aly/REF-like target). SKAR is phosphorylated on residues Ser 383 and 385 specifically by S6K1, not S6K2 [48]. SKAR is involved in the control of cell growth and, in co-operation with S6K1, is believed to enhance the translation of spliced mRNAs [49].

In addition to being downstream of mTORC1, S6K1 is also involved in the regulation of mTOR activity by controlling insulin signalling. Insulin or IGF (insulin-like growth factor) binds to their receptor, leading to phosphorylation and activation of the insulin receptor substrate-1 (IRS-1). Activated IRS-1 recruits and activates PI3K, which in turn activates AKT and consequently, mTOR. Upon stimulation by mTORC1, S6K1 phosphorylates IRS-1, marking it for degradation and causing a suppression of PI3K/AKT signalling [50–53]. This negative feedback loop is particularly active in cells exhibiting enhanced mTOR activity due to genetic mutations or prolonged nutrient exposure. The regulation of insulin signalling by S6K1 poses a challenge for anti-cancer therapies that target mTORC1 (such as rapamycin and its derivatives) since mTORC1 inhibition reduces S6K1 activity, which results in increased IRS-1 activation and PI3K-AKT signalling that itself plays a role in the control of cell proliferation. As a result, a great deal of research has focussed on the use of mTORC1 inhibitors in combination with inhibitors of insulin signalling for the treatment of cancer [54].

4.1.2 eIF4G

The large scaffolding protein eIF4G is an integral component of the eIF4F translation initiation complex. eIF4G interacts with the other eIF4F components, eIF4E and eIF4A, and is responsible for bridging the 40S ribosomal subunit to the mRNA through an interaction with the ribosome-associated initiation factor eIF3. eIF4G also contains binding sites for PABP (poly-A-binding protein) and the Mnk kinases (mitogen-activated protein kinase signal-integrating kinase; Mnk1, 2) [2]. The interaction between eIF4G and PABP is important for circularization of the mRNA, while the Mnks regulate the phosphorylation of eIF4E on Ser 209 [55–57]. All eukaryotes express two related eIF4G proteins, eIF4GI and eIF4GII, which are encoded by separate genes [2]. Both eIF4GI and eIF4GII are phosphorylated on multiple sites; however, the phosphorylation of eIF4GI is better characterized. eIF4GI is phosphorylated in response to extracellular stimuli that promote cell growth including serum, insulin, and growth factors [58, 59]. The phosphorylation of eIF4GI is mediated by mTORC1 as a number of rapamycin-sensitive phosphorylation sites including Ser 1108, Ser 1148, Ser 1192 have been identified [58]. However, the functional significance of eIF4G phosphorylation remains unclear. Phosphorylation does not affect the activity of eIF4G or its ability to associate with other initiation factors, but may change its structural conformation and alter the translation of specific mRNAs.

4.1.3 Other Targets of mTORC1

No other direct targets of mTORC1 have been identified; however, a number of additional cellular proteins contain putative TOS motifs, indicating that other mTORC1 substrates may exist. For example, HIF1-α, STAT3, and some PKC isoforms contain potential TOS motifs that may target them for raptor recruitment and mTORC1-mediated phosphorylation [60]. In fact, some reports indicate that STAT3 phosphorylation is sensitive to rapamycin treatment, supporting a role for mTORC1 in the regulation of STAT3 activity [61].

4.1.4 mTORC2 Regulation of AKT

While the mTORC1 phosphorylation targets are well characterized, very few mTORC2 substrates have been identified. The mTORC2 complex is rapamycin insensitive and involved in cytoskeletal regulation. Recently, mTORC2 was identified as the kinase responsible for phosphorylating Ser 473 in AKT. AKT is a positive regulator of mTOR signalling that phosphorylates and inhibits the TSC1/2 (tuberous sclerosis complex 1 and 2) complex, which negatively controls mTOR activity. Complete activation of AKT requires phosphorylation on Thr 308 within the activation loop and Ser 473, which lies in the hydrophobic motif [5]. PDK1 is responsible for AKT Thr 308 phosphorylation, and recent studies demonstrate that mTORC2 targets Ser 473. mTORC2 phosphorylates AKT on Ser 473 in vitro and knock-down of rictor in cells reduces Ser 473 phosphorylation, thus establishing mTORC2 as the PDK2 kinase involved in AKT regulation [20, 21]. In addition to AKT, mTORC2 may be involved in the regulation of PKC-α phosphorylation. PKC-α is involved in a number of cellular processes including growth, apoptosis, cellular structure, and motility. Knock-down of rictor reduces PKC-α phosphorylation and stability; however, this effect may not be direct [62].

5 Translation and Cancer

The translation of mRNA is a process that is important for a number of critical cellular processes including growth, survival, proliferation, and differentiation. Therefore, it is not surprising that mRNA translation is implicated in the development of cancer. Increased levels of translation are believed to promote transformation and tumorigenesis through an increase in the expression of proteins involved in growth, proliferation, and survival. Many of these proteins are encoded by a subset of mRNAs that contain long, highly structured 5′-UTRs. For example, the mRNAs encoding survivin, cyclin D1, TGF-β (transforming growth factor), CDK4 (cyclin-dependent kinase), and IGF-II (insulin-like growth factor) contain structured 5′-UTRs [83, 86]. Efficient translation of these mRNAs requires high levels of the eIF4F complex, which through the actions of its helicase eIF4A unwinds the extensive mRNA secondary structure and facilitates translation initiation. Consequently, increased expression or activation of translation initiation factors, particularly components of the eIF4F complex, can promote synthesis of oncogenic proteins and tumorigenesis. As mentioned above, a number of key translation initiation factors, as well as other important regulators of translation, are targets of the mTOR signalling pathway. Increased mTOR signalling can contribute to higher levels of translation and the development of cancer. A number of mechanisms can lead to over-activation of mTOR. Mutational loss or inactivation of tumour suppressors that negatively regulate mTOR can cause hereditary hamartomatous diseases and human cancers. For example, mutation or loss of TSC2, LKB1, and PTEN (phosphatase and tensin homologue deleted on chromosome 10) is associated with cancer-like syndromes in which patients exhibit hamartomatous polyps in multiple organs and an increased risk of cancer development [63–66]. Loss of these tumour suppressors leads to over-active mTOR signalling and an increase in the phosphorylation of S6K and 4E-BP1 [67–70]. In addition, genetic amplification of positive regulators of mTOR, such as AKT and PI3K, has been reported in breast, ovarian, and head and neck cancers [71, 72]. Regardless of the mechanism, enhanced mTOR signalling contributes to tumorigenesis via increased phosphorylation of its downstream targets and elevated mRNA translation.

6 Downstream Targets of mTOR and Their Role in Cancer

6.1 The 4E-BPs

A key mechanism by which mTORC1 signalling contributes to tumorigenesis is increased phosphorylation of the 4E-BPs. The 4E-BPs are believed to act as tumour suppressors by negatively regulating formation of the eIF4F complex that is required for mRNA translation initiation. In support of their role as tumour suppressors, over-expression of 4E-BP1 or 4E-BP2 causes a reversion of the transformed phenotype in cells transformed by eIF4E, ras, or src [73]. As mentioned above, mTORC1-mediated phosphorylation of 4E-BP1 leads to its release from eIF4E and a subsequent increase in eIF4F formation and mRNA translation. Consequently, inactivation of the 4E-BPs by phosphorylation is likely a key step in the development of cancer. For instance, ectopic expression of a non-phosphorylatable 4E-BP1 mutant, which remains capable of binding eIF4E regardless of mTOR activation, suppressed the tumorigenicity of human breast cancer cells [74]. Increased levels of 4E-BP1 phosphorylation have been observed in human breast, prostate, and ovarian tumours, supporting a role for elevated mTORC1 signalling and subsequent eIF4F complex formation in the development of these cancers [75–79]. In fact, 4E-BP1 may be useful as a prognostic indicator in human cancer as increased 4E-BP1 phosphorylation correlated with malignant progression and poor prognosis.

While the 4E-BPs are considered tumour suppressors, their binding partner, eIF4E, acts as an oncoprotein. The cap-binding protein eIF4E is the least abundant component of the eIF4F complex, and its over-expression can lead to oncogenesis. Over-expression of eIF4E transforms rodent fibroblasts and causes the malignant transformation of primary embryo fibroblasts in co-operation with the immortalizing proteins myc and E1A [80, 81]. eIF4E expression also transforms human mammary epithelial cells [74]. Consistent with these data, mice over-expressing eIF4E develop lymphomas, angiosarcomas, lung adenocarcinomas, and hepatocellular adenomas [82, 83]. In humans, elevated eIF4E levels have been reported in colon, breast, bladder, lung, and prostate tumours highlighting its importance in tumour development and progression [84–86]. In fact, eIF4E has emerged as a major target for anti-cancer therapies. Reduction of eIF4E levels by administration of anti-sense oligonucleotides reduced the growth of tumours in mice and use of these anti-sense oligonucleotides is currently under investigation in phase I clinical trials (Eli Lilly Co.) [87, 88].

6.2 S6 Kinase

The S6 kinases are important regulators of protein synthesis as well as cell growth [32]. Phosphorylation of S6K1 by mTORC1 enhances its kinase activity towards its downstream targets, rpS6, eIF4B, eEF2 kinase, PDCD4, and SKAR, leading to increased mRNA translation. The role of S6K1 in oncogenesis is not as well characterized as eIF4E or the 4E-BPs. However, the involvement of S6K1 in the regulation of cell growth strongly suggests that it may be involved in cancer. Over-expression of S6K1 causes increased cell growth and expression of a constitutively active form of S6K1 induces invasive and migratory phenotypes in ovarian cancer cells [89, 90]. Furthermore, it has been reported that S6K1 is important for the epithelial to mesenchymal transition (EMT) of ovarian cancer cells, a key process involved in tumour invasion, metastasis, and progression [91]. S6K1 is believed to promote migration and EMT in ovarian cancer cells via up-regulation of proteins involved in migration and invasion, such as matrix metalloproteinases and Snail [89, 91]. Increased S6K1 activity has been observed in breast cancer cells as well as a number of other cancer cell lines [92–95]. S6K1 is also involved in glial transformation as knock-down of S6K1 by siRNA or shRNA reduces anchorage-independent growth of human astrocytes in culture as well as the growth of intracranial tumours in mice [96]. In humans, S6K1 levels are elevated in 16% of primary breast tumours and the phosphorylation of S6K1 correlates with higher tumour grade in ovarian cancer patients [91, 93, 95, 97–99].

6.3 eIF4G

The functional significance of mTORC1-mediated eIF4G phosphorylation is somewhat unclear; however, due to its importance in eIF4F complex formation and mRNA translation initiation, eIF4G is likely implicated in the development of cancer. Over-expression of eIF4GI causes malignant transformation of rodent fibroblasts and a number of human breast cancer cells contain elevated levels of eIF4G [74, 100]. Recent work by Braunstein and colleagues has demonstrated that together with 4E-BP1, eIF4G plays an important role in causing a hypoxia-activated switch to cap-independent translation, which favours the translation of mRNAs encoding angiogenic factors, and pro-survival proteins [101]. eIF4GI levels are up-regulated in squamous cell lung carcinomas as well as advanced breast tumours [101–103]. Elevated levels of eIF4G likely contribute to tumorigenesis via increased formation of the eIF4F complex and subsequent translation of highly structured mRNAs encoding growth factors and other oncogenic proteins.

6.4 AKT Regulation by mTORC2 in Cancer

mTORC2 is the kinase responsible for phosphorylating and activating AKT. Rapamycin specifically inhibits mTORC1 by disrupting the interaction between mTOR and raptor; however, mTORC2 is not affected by rapamycin. Consequently, mTORC2-AKT signalling represents a significant challenge to the treatment of cancers with rapamycin and its derivatives since AKT is involved in the regulation of cell growth and survival. However, some research indicates that mTORC2 signalling is inhibited by prolonged treatment with rapamycin [104]. Further research is required to elucidate the role of mTORC2 in tumorigenesis, and the development of mTORC2 inhibitors as anti-cancer therapies represents an area of research that requires further examination.

7 Conclusions

Due to its importance in the regulation of cellular growth and proliferation, mTOR has emerged as a key factor in the process of tumorigenesis. Over-active mTOR signalling due to mutation or loss of its negative regulators is implicated in a number of hereditary diseases and human cancers. Consequently, a great deal of research has focussed on the regulation of mTOR and its downstream targets. The majority of mTOR targets are components of the translational machinery, highlighting its importance in the regulation of mRNA translation. The major targets of mTORC1, the 4E-BPs and the S6Ks, have been extensively characterized and their roles in protein synthesis are well described. However, their involvement in the development of cancer is an area of research that requires further attention. In addition, a great deal of work will be required to identify additional substrates of mTORC1 and to elucidate the potential role of the mTORC2 complex in transformation and cancer. Future studies focussing on the function of mTOR and its downstream targets in cancer will not only aid in the understanding of the biochemical regulation and function of mTOR but also provide an opportunity for the identification and development of novel anti-cancer therapies.