Downstream the mTOR: S6 Kinases between Divergence and …researchpub.org/journal/jbpr/number/vol1-no2/vol1-no2-4.pdf · 2014-06-21 · Abstract: The ribosomal protein S6 kinase (S6K)

H. M. S. Ismail / Journal of Biochemical and Pharmacological Research, Vol. 1 (2): 94-105, June 2013

ISSN 2168-8761 print/ISSN 2168-877X online ~ 94 ~ http://www.researchpub.org/journal/jbpr/jbpr.html

Abstract: The ribosomal protein S6 kinase (S6K) family is a key regulator of cell growth and cell size. It acts downstream of the PI3K/mTOR pathway, and has been identified to date to exert crucial functions in many other cellular processes such as metabolism and transcription regulation. S6 kinase 1 (S6K1) was purified in 1988 and a decade later S6 kinase 2 (S6K2) was cloned simultaneously by four different groups. The first impression was that this kinase is a homologue to its close relative S6K1 and may function redundantly in the cell. By the time, more studies emerged and showed distinct as well as similar functions of the two kinases in human and animal tissues. The purpose here is to review the role of S6K enzymes and to describe the many recent developments in our understanding of signalling upstream and downstream of the S6 kinase family.

Keywords: mTOR; S6K1; S6K2; Cancer 1. S6 kinases identification and expression profiles

he ribosomal S6 kinase (S6K) is a serine/threonine kinase which belongs to the family of AGC kinases. The family of

S6K consists of two genes (S6K1 and S6K2) that are mapped to two different chromosomal locations. The S6K1 gene is located on chromosome 17q23.1 while S6K2 is mapped to chromosome 11q13.1 (NCBI/Unigene). Initially, S6 kinases gained their name due to their ability to phosphorylate ribosomal S6 protein (rs6p) within the 40S ribosomal subunit. The S6K activity was originally purified from cultured mammalian cells in George Thomas laboratory [1]. Shortly thereafter, two independent groups have cloned this kinase from rat cDNA libraries [2; 3] and named it p70S6K (the __________________________________________ aCancer Biology Department, National Cancer Institute, Cairo University, Kasr El-Aini st. Cairo 11796, Egypt. Email: [email protected] (Received June 16, 2013; Accepted June 17, 2013; Published online June 24, 2013)

former name for S6K1). Studies concerned with the functional importance of S6K in the cell, have used Knock-out approaches in drosophila and mice. Interestingly, cells derived from mice lacking S6K1 showed persistent rs6p phosphorylation in vivo [4], which suggested the existence of additional mammalian S6K that can substitute for S6K1 and led to the identification and cloning of the highly homologous ribosomal S6 kinase 2 (S6K2). S6K2 was cloned 15 years ago by a number of groups [4-7] based on homology with the close relative S6K1. A recent study by Nadrella et al. [8], reported that S6K1 and S6K2 proteins are differentially expressed in mouse and human tissues. We have also noticed that the gene expression profiles of S6 kinases are different in some tissues as compared using human gene atlas (Figure 1). 2. S6 kinases domain structure and subcellular localisation S6 kinases have five domains: the N-terminal regulatory domain, catalytic domain, linker region, autoinhibitory domain and the C-terminal domain. It is reported that, the catalytic domain of S6K2 shares 82% homology with its virtual in S6K1 sequence and the overall sequence resemblance between the two kinases reaches 70% in total [9-12]. Using EMBL sequence alignment tools such as clustal omega, we compared the sequences of human S6K1 and S6K2. As shown in Figure 2, conserved regions are mainly located in the catalytic and regulatory domains while the C-terminal and N-terminal regions are very dissimilar. The main divergence between S6 kinases resides in their non-catalytic regions, hinting at differential function and/or regulation. Alternative splicing in the S6Ks genes result in two isoforms for each kinase, p70 (αII) and p85 (αI) isoforms of S6K1 and also p54 (ßII) and p60 (ßI) isoforms of S6K2. Both longer isoforms of S6K1 (p85) and S6K2 (p60) have N-terminal extensions of 23- or 13-amino acids respectively [5; 7; 13; 14]. These extensions include nuclear localisation sequences (NLS) resulting in predominant nuclear localisation of the two long isoforms [14;15]. The short isoforms of S6K1 (p70) and S6K2 (p54) are predominantly cytoplasmic, but

Review

Downstream the mTOR: S6 Kinases between Divergence and Redundancy

Heba M. S. Ismaila

T



Figure 1. S6 kinases expression profile in human tissues. The Gene Expression Atlas database was examined for human S6K1 (RPS6KB1) and S6K2 (RPS6KB2) mRNA expression levels. (http://biogps.org/#goto=genereport&id=6198,6199).

translocate to the nucleus in cellular response to growth factors and mitogens. While the two kinases show an overall homology of 70%, the C-terminal of both S6K isoforms exhibit only 20% identity. The S6K1 C-terminus sequence contains a PDZ domain binding motif which might recruit the kinase to the actin

cytoskeleton via binding to neurabin [16]. A number of distinct features reside in the C-terminal region of S6K2, including nuclear localisation sequence (NLS), proline-rich motif, and several regulatory phosphorylation sites. The presence of functional nuclear localisation motif at the C-terminus has been demonstrated by Lee-Fruman et al. [5]. Phosphorylation of S6K2 (p54) by protein kinase C(PKC) at S486 in the



Figure 2. Sequence alignment between human S6K1 and S6K2 proteins using Clustal Omega alignment tools. (http://www.ebi.ac.uk/Tools/msa/clustalo/)

C-terminal region was shown to disable the function of the NLS, resulting in the accumulation of phosphorylated S6K2 in the cytoplasm [17]. Recently, S6K2 but not S6K1 was reported to be localized at centrosomes [18]. However, the physiological relevance of centrosomal localisation of S6K2 remains to be further investigated. The presence of a stretch of proline-rich sequences implicates the C-terminus of S6K2 in mediating protein–protein interactions with SH3 domain-containing proteins [5; 7]. The similarities and differences in the S6 kinases structure and subcellular localisation hint at possible redundant and distinct functional roles played by the S6 kinases isoforms in the cell. 3. Upstream regulation of S6 kinases: common and distinct mechanisms Signalling via S6 kinases is a good example of the coordination of different extracellular signals, such as growth factors, cytokines, nutrients and oncogene products from cell signalling pathways. Two signalling pathways have been shown to positively converge on activation of S6 Kinases: the insulin–PI3K pathway and a nutrient pathway involving the mammalian target of rapamycin (mTOR) (reviewed in [19; 20]). On the other hand, inhibitory mechanisms were described to target S6K pathway through inhibiting its upstream signalling. Inhibitory mechanisms are exerted by a number of tumour suppressor genes such as PTEN, TSC1/2, REDD1 and others. A review of activating pathways and molecules upstream of S6 kinases is demonstrated in this section with discussion of the differences between S6K1 and S6K2.

3.1. The general mechanism of S6 kinases activation The regulation of S6K1 and S6K2 activities is highly conserved [21] and involves sequential phosphorylation at multiple Ser/Thr sites [22-26]. The model is primarily focusing on S6K1 regulation and is applicable for S6K2. It is believed that in the presence of calcium, the interaction between the C-terminal autoinhibitory and N-terminal domains of S6K1 is relieved [23], consequently allowing for the phosphorylation of four sites in the autoinhibitory domain (S/TP residues): S411, S418, T421 and S424 [27]. The phosphorylation of these sites opens the kinase domain, allowing subsequent phosphorylation of critical sites in the kinase and kinase-extension domains. Initially, mTOR kinase phosphorylates T389, which is localised to a hydrophobic motif conserved in other members of the AGC kinase family [16; 25; 27; 28]. This leads to the phosphorylation of T229 in the catalytic domain activation loop by 3-phosphoinositide dependent protein kinase 1 (PDK1). At this stage the kinase reaches its maximal activation which could be attributed to an open conformation of S6K1[29]. Even though PDK1 is a rapamycin resistant kinase, phosphorylation of T229 is abolished by treatment with this inhibitor, indicating a requirement for prior phosphorylation of T389 which perhaps forms a docking site for PDK1 and can thus lead to maximal kinase activation. 3.2. S6K2 shows some regulatory differences from S6K1 The set of phosphorylation residues in S6K1 that are important for its activation are very similar to that in S6K2, but there are some regulatory differences. For example, the



overexpression of PDK1 was shown to be sufficient to induce the kinase basal activity of S6K1, but not S6K2. This may indicate that additional factors may be required in order to achieve basal S6K2 `potentiation' in the absence of mitogenic stimuli [5]. Three of the four proline-directed serines identified in S6K1, are conserved in S6K2. Martin et al. [21] have reported that, proline-directed phosphorylation sites are essential for the activation of S6K2 and also confer greater sensitivity to MAPK inhibition in contrast to S6K1. Mutational studies revealed that phosphorylation of T388 in S6K2 results in an inhibition of the kinase activation upon serum stimulation, which is the case for S6K1. However, phosphorylation-mimicking mutation of T388 to glutamic acid results in the induction of the kinase basal activity that is comparable to that of serum stimulation; moreover, this activity does not change upon serum stimulation [30]. Recently, it has been identified that both S6K1 and 2 are targets of E3 ubiquitin ligases and can be polyubiquitinated in vivo [31]. This has linked S6K to degradation via the proteosomal pathway. S6Ks are also targeted by lysine acetylation, following mitogen stimulation; S6Ks interact with the p300 and p300/CBP-associated factor (PCAF) acetyltransferases. S6Ks can be acetylated by p300 and PCAF in vitro and S6K acetylation is detected in cells expressing p300 [32]. These data suggest that S6K regulation is much more complex and can be mediated by other modifications in addition to phosphorylation. 3.3. Regulation of S6 kinases by growth factors via the PI3K/ mTOR pathway The model that is currently accepted for S6 kinases activation involves a complex signalling cascade and connects a number of critical kinases in S6 kinases activation. Briefly, in response to mitogens or growth factors, activation of extracellular receptors occurs, which then leads to the stimulation of PI3K activity. This in turn converts phosphatidylinositol- 4, 5- bisphosphate (PIP2) to phosphatidylinositol- 3, 4, 5- triphosphate (PIP3) [33]. The increase in PIP3 concentration at the intracellular membrane recruits the serine/threonine kinases AKT and PDK1 through their Pleckstrin homology (PH) domains. PDK1 phosphorylates AKT on T308 within its catalytic domain activation loop [34;35]. For full activation, AKT also requires the phosphorylation of the hydrophobic motif S473 site which has recently been found to be mediated by mTOR complex 2 (mTORC2) [36-38]. Activated AKT is then able to phosphorylate a number of substrates, including the tuberous sclerosis complex 2 (TSC2) protein, which attenuates its interaction with its binding partner, TSC1 [39; 40]. This inhibits TSC2 GTPase activating protein (GAP) functioning on the small GTPase, Ras homolog enriched in brain (Rheb) [41; 42]. As a result, GTP-bound active Rheb can interact with members of the mTOR Complex 1 (mTORC1), composed of mTOR, Raptor, mLST8 and PRAS40, to upregulate its activity in a manner which is still not fully understood [43-45].

The activity of mTORC1 is acutely sensitive to treatment with rapamycin, which stabilizes the mTOR-Raptor interaction and inhibits mTOR kinase activity [46; 47]. The identification of Raptor as a component of the mTORC1 complex provided important insight into the mechanism of S6K1 activation by mTOR. It was found that Raptor binds to S6K1 at its mTOR-signalling (TOS) motif, thus recruiting S6K1 to mTOR and allowing for its efficient phosphorylation [48-50]. Further studies dissecting the role of the TOS motif in S6K signalling showed its importance in suppressing the inhibitory effect of the S6K1 C-terminus, possibly via a phosphatase [51]. This is consistent with previous studies which have found that S6K1 can interact with protein phosphatase 2A (PP2A) and that rapamycin- or amino acid withdrawal-induced dephosphorylation of S6K1 can be blocked by the phosphatase inhibitor, calyculin A [52], thus suggesting a two-tier TOS motif mediated regulation of S6K1 activity. In contrast, the above mentioned mTORC2, which consists of mTOR, Rictor, mLST8, mSIN1 and Protor/PRR5 [53-55] is largely resistant to the inhibitory activity of Rapamycin. However, in under some conditions, prolonged exposure to this drug does inhibit mTORC2 assembly and thus the phosphorylation of AKT [56] . 3.4. Regulation of S6 kinases activity by the levels of nutrients In addition to regulation by growth factors, the activity of S6K1 is dependent on the levels of nutrients, including amino acids and energy in the form of ATP (reviewed in [19; 57]). Recent studies investigating the mechanisms of amino acid signalling to mTORC1 and S6K1 have found a prominent role for the class 3, wortmannin-sensitive PI3K, hVps34 in this signalling [58; 59]. Both studies showed that activation of hVps34 is required for S6K1 activation downstream of amino acid signalling. Moreover, the activation of S6K1 by amino acids was found to be independent of the growth factor-mediated signalling via the PI3K-AKT-TSC1/2-RHEB pathway. It remains unclear what the intermediate mediator(s) of hVps34 signalling to mTORC1 may be. Although it has been previously been proposed that it may require increased intracellular Ca2+ levels, independently of effects on AKT activity [60]. In addition to acting as a mediator of amino acid signalling, it has been proposed that hVps34 may also integrate energy signalling based on the observation that glucose deprivation or treatment with AICAR, an agonist of AMP-activated protein kinase (AMPK), inhibits hVps34 activity [58]. In fact, AMPK signalling has been implicated in the regulation of mTORC1-S6K1 signalling via activation of the TSC1/2 complex [61]. Future studies investigating the mechanisms of hVps34 regulation by AMPK are likely to uncover further complexity in the energy-dependent regulation of the mTOR-S6K1/2 pathway. 3.5. Activation of S6K independently from PI3K/mTOR signaling



Figure 3. S6 kinases binding parteners using STRING database.

In addition to the known mechanism of S6K1 activation via the PI3K-AKT-TSC1/2-Rheb pathway, a number of studies have shown that under certain cellular conditions, S6K1 can be activated independently of this pathway. For instance, growth factor-dependent activation of the small G protein Cdc42 has been shown to activate phospholipase D1 (PLD1) and to increase the cellular concentration of phosphatidic acid (PA). Phosphatidic acid in turn binds to and stimulates mTORC1 activity [62]. The exact mechanism exerted by PLD1 to activate S6K1 was not identified until very recently, when Sun and co-workers identified that PLD1, but not PLD2, is required for Rheb-driven activation of the mTOR pathway. The overexpression of Rheb activates PLD1 in cells in the absence of mitogenic stimulation, and the knockdown of Rheb impairs serum stimulation of PLD activation. Furthermore, the overexpression of TSC2 suppresses PLD1 activation, whereas the knockdown or deletion of TSC2 leads to elevated basal activity of PLD. Rheb binds and activates PLD1 in vitro in a GTP-dependent manner, strongly suggesting that PLD1 is a bona fide effector for Rheb. Hence, their findings reveal an unexpected interaction between two cascades in the mTOR signalling pathways and opens up additional possibilities for targeting this important growth-regulating network for the development of anticancer drugs [63]. More recently, PA was identified to directly interact with and increase S6K1 activity in an mTORC1-independent manner through PLD2-mediated synthesis of PA [64]. Further investigation is needed to address this issue, as the authors used suboptimal methods that did not abolish mTOR signalling (low dose of 1 nM rapamycin or modest knock-down of mTOR expression).

It has also been found that the PI3K-dependent atypical PKC (aPKC) isoforms λ and ζ are able to form a complex with S6K1 and PDK1 which is necessary, but not sufficient, for S6K1 activation [65; 66]. Interestingly, the aPKC isoforms also associate with Cdc42 [67], which poses the question of whether their role in S6K1 activation is mediated through the Cdc42-PLD-PA pathway or via another currently unknown mechanism. The Ras/MEK/ERK pathway has been found to contribute to the regulation of S6K1 activity via its effect on TSC2 phosphorylation [68]. As such, p90RSK, a downstream target of ERK, was found to interact with and phosphorylate TSC2 on S1798, suppressing its GAP activity, resulting in the activation of mTORC1 and S6K1 signalling. 3.6. Differential upstream regulation of S6 Kinase 2 Although most of the regulatory mechanisms outlined above with respect to S6K1 also apply to S6K2, some differences in the regulation of the latter kinase have been observed. Firstly, while it does not stimulate S6K1 activity, a constitutively active, myristoylated form of PKCζ is able to enhance the basal activity of S6K2 to the same level as obtained by insulin treatment [21]. This could be strongly attributed to the ability of PKC to translocate to the nucleus [69]and phosphorylate nuclear S6K2. Furthermore, several PKC isoforms are able to phosphorylate the short form of S6K2 (bII) on S486 which resides in the C-terminus NLS (KKSK). As a consequence of this phosphorylation, S6K2 NLS is impaired and the kinase is translocated to the cytoplasm [17]. A further link between S6K2 and PKC signalling was



Figure 4. S6K1 regulates protein synthesis at multiple levels. S6K1 directly targets proteins involved in the regulation of gene transcription, precursor RNA splicing and export and protein translation (see text for details).

reported by [70]. The authors reported that S6K2, unlike S6K1, is selectively recruited into a signalling complex containing PKC and B-Raf. The multienzyme S6K2/B-Raf/PKC complex forms in response to serum and FGF-2 stimulation of cells and functions to protect the cell from apoptosis through translation of mRNA species involved in controlling cell survival. The MEK pathway also appears to play a greater role in the activation of S6K2 than S6K1, likewise PKC signalling. The deletion of the C-terminal autoinhibitory domain of S6K2 was found to potentiate its kinase activity, but the opposite effect was observed when the activity of S6K1 deletion mutants was examined [21; 26]. This was found to be largely mediated by MEK-regulated phosphorylation of the autoinhibitory domain sites (S410, S417 and S423) in S6K2 [21; 30]. Treatment of cells with MEK inhibitors provided further evidence for their greater inhibitory effect on the activity of S6K2 than S6K1 [21]. In view of the above observations, the activation of S6K1 and 2 requires complex signalling through a number of cellular pathways. Although extensively studied to date, the complexity of these processes is still not fully understood and further studies are required to investigate the mechanisms and

characterize any additional discrepancies in the activation requirements of S6K1 and S6K2. 4. S6 kinases cellular functions and downstream substrates Studies in both animal models as well as in cell culture have demonstrated that S6K1 regulates cell growth and size [4; 71; 72]. Interestingly, the disruption of the S6K1/S6K2 gene orthologue, Cep70 in C. elegans by RNA interference, resulted in minor defects in cell growth [73]. The deletion of the dS6K gene in Drosophila melanogaster was semi-lethal with severe effects on surviving flies’ size and female sterility [74]. The studies in knock-out mice revealed that deletion of S6K1 gene did not affect mice fertility and viability but the newborn mice were 15-20% smaller. Moreover, the phosphorylation status of the S6 protein was not affected in S6K1 deficient mice. These findings led to the discovery of the second isoform of S6K, termed S6K2. Interestingly, S6K2-/- mice did not display a small body phenotype, instead they were comparable in size to WT mice [75]. Furthermore, there was no additional decrease in the size of the S6K1-/-/S6K2-/- double knock-out mice compared to the S6K1-/- mice. This difference



Figure 5. Expression profiles of S6K1 and S6K2 genes in different human cancers using ONCOMINE cancer microarray databases. Oncomine™ (Compendia Bioscience, Ann Arbor, MI) was used for analysis and visualization.

suggests that it is only the S6K1 isoform that is important in the regulation of cell growth. S6K2 may therefore play a different role within the cell. These animal models have also highlighted an importance of S6K in the regulation of other cellular events. Interestingly, complete S6K knock-out mice have drastically reduced viability compared to either S6K1-/- or S6K2-/- mice. This suggests that S6K may be required for normal cell development in mammals. Interestingly, the preferred S6K phosphorylation motif RXRXXS/T (where X is any amino acid) was found in most of

the S6K1 substrates identified to date. On the other hand, unique substrates of S6K2 have yet to be identified. However a number of unique binding partners have been identified for S6K2, functional relevance of many of them needs further investigation. Figure 3 shows some binding partners and known substrates determined by STRING database for S6K1 and S6K2 proteins. S6 kinases have been linked to a number of cellular processes including cell cycle progression, feedback mechanisms, cell survival and mitochondrial metabolism in



addition to cell growth control though protein synthesis. Here, we will focus on the role of S6 kinases in protein synthesis and cell survival. 4.1. S6K regulates protein synthesis at multiple stages: from translation initiation to RNA splicing Given that protein synthesis is a prerequisite for cell growth, a number of the S6K1 substrates identified to date consistently include factors involved in the regulation of translation. Identification of those substrates has supported the hypothesis that S6K1 may regulate protein synthesis at multiple stages from RNA splicing, initiation to elongation (Figure 4). S6K1 was found to phosphorylate five serine residues (S235, S236, S240, S244 and S247) within the C-terminus of rpS6 [76; 77]. S6K1 has been implicated in regulating translation by phosphorylating translation factors involved in the initiation of protein synthesis. Phosphorylation of the initiation factor eIF4B (eukaryotic initiation factor 4B) on S422 is S6K-dependent [78]. Additionally, S6K1, but not S6K2, has been found to contribute further to translation initiation through its involvement in the pre-initiation complex of translation (PIC) [79]. S6K1 has been also shown to play a role in the activation of elongation by regulating the function of eEF2. This occurs through S6K1-mediated phosphorylation of the eEF2 kinase on S366 which results in the inhibition of its kinase activity and the reduction in eEF2 phosphorylation. S6K2 has low activity towards eEF2K in vitro, and eEF2K phosphorylation is reduced in S6K1-/- ES cells. It has been recently shown that the inhibition of S6K1 by AICAR (AICAR is an activator of AMPK) resulted in an elevated phosphorylation of eEF2, consistent with an inhibition of protein translation [80]. In addition to the role in controlling various stages of protein synthesis, S6K1 may also participate in regulation of cap-dependent mRNA splicing. S6K1 was found to phosphorylate a subunit of Cap Binding Complex (CBC), CBP80 in vitro, at sites which are phosphorylated in vivo in response to growth factor stimulation. In agreement with these findings, rapamycin treatment of T-cells was found to affect the transcription and translation of a set of mRNAs corresponding to splicing factors, such as pre-mRNA splicing factor SRp20, hnRNPU, snRPC and snRPD2 proteins [81]. A further evidence for the involvement of S6K signalling in mRNA splicing was provided by the identification of SKAR as a binding partner and substrate for S6K1 but not S6K2. SKAR is a nuclear protein with homology to the family of Aly/REF RNA binding proteins that couple transcription, pre-mRNA splicing and nucleo-cytoplasmic mRNA export [82]. It was demonstrated that S6K1, but not S6K2, phosphorylates SKAR on S383 or S385 in an insulin-stimulated and rapamycin-sensitive manner. Furthermore, siRNA-mediated inhibition of SKAR led to a reduction in cell size to a similar extent as observed with knock-down of S6K1, suggesting that SKAR is at least in part responsible for the regulation of cell size downstream of S6K1 [82]. The mechanism by which S6K1 might regulate RNA splicing via SKAR has been recently revealed by Ma et al. [83]. The authors have

demonstrated that when activated, S6K1 is recruited to the newly synthesized mRNA by SKAR and deposited at the exon junction complex (EJC) and SKAR and S6K1 increase the translation efficiency of spliced mRNA. Together, the findings outlined above indicate that S6K1 regulates the efficiency of protein translation at levels of initiation and elongation and perhaps mRNA processing. Furthermore, these data suggest that the S6K1-mediated regulation of cell size can be, at least in part, mediated through these effectors. 4.2. S6 kinases and cell survival Rapamycin is able to induce apoptosis in a number of different cell lines, which implicates the mTOR pathway in pro-survival signalling. S6K1 has been linked in regulating cell survival through phosphorylation of the pro-apoptotic protein Bad at S136 after IGF-1 treatment [84]. Phosphorylated Bad is unable to interact with BCL-X or BCL-2 to promote cell death. Studies in S6K1-/- ES cells show no IGF-1 dependent Bad phosphorylation, suggesting that S136 phosphorylation is specifically mediated by S6K1 (at least under IGF-1 stimulation). A number of studies have shown that S6K2 may also play a role in regulating apoptosis. Treatment of small cell lung cancer (SCLC) cells with FGF2 prevents the cells from entering apoptosis [85]. This is thought to be due to the upregulation of the anti-apoptotic proteins BCL-XL and BCL-2 at the translational level. Further work showed that this FGF2 pro-survival function is dependent on a complex containing B-Raf, PKCε and S6K2 [86]. The use of RNAi on any of these components causes the FGF2 protection to be lost. Furthermore, overexpression of S6K2 upregulates the anti-apoptotic proteins XIAP and BCL-XL and promotes cell survival. Recently, Sridharan et al., 2011 reported a significant role of S6K2 in inducing breast cancer cell survival via AKT2 [87]. Overall, these data suggest that S6K may have a dual function in promoting cell growth and survival within the cell when protein synthesis and growth pathways are switched on and the apoptotic pathway is suppressed. 5. Deregulation of S6 kinases in human cancers Deregulated signalling via S6K has been linked to various human pathologies, including cancer. Many of the molecules signalling upstream of S6K have been shown to be either mutated or overexpressed in tumours, leading to S6K activation [88; 89]. For example, PI3K and AKT transform cells in vitro [90] suggesting that they act as oncogenes, while molecules such as PTEN, TSC1/2 and LKB1, which restrain the activity of the PI3K/mTOR pathway, act as tumour suppressors [91-93]. Furthermore, a number of studies have shown that the mTOR inhibitor, rapamycin, attenuates the growth of tumours resulting from deregulation of components of the insulin signalling pathway [90; 92; 94]. Clinical trials with rapamycin analogues (CCI-771 and RAD001) have confirmed this



observation in a selection of human tumour types (discussed in [95-97]. However they showed limited success. Using publically available cancer microarray database Oncomine, we compared the expression levels of S6K1 and S6K2 in different tumours (Figure 5). Interestingly, we have noticed differential expression pattern of the two kinases in different tumours. As an example, S6K1 is significantly up-regulated in Cervical and gastric cancers while S6K2 is down-regulated in the same tumours. We recently showed that the gene expression of S6K1 is upregulated in brain tumors and is correlated with upregulation of hypoxia responsive genes. We furthermore showed that this overexpression of S6K1 may predict patients survival [98]. A number of recent studies showed differential role of S6 kinase 2 in cancer. S6K2 amplification was frequently observed in gastric cancer and was related to a poor prognosis [99]. These findings may provide novel insight into the dysregulation of mammalian target of rapamycin signaling by S6K2 amplification in gastric cancer. Furthermore, Sirdharan et al. 2011 [100] have demonstrated that S6K2 but not S6K1 activates breast cancer cell survival via AKT and the loss of S6K2 may sensitize cancer cells to chemotherapeutic agents. Recently, we showed a direct interaction between S6K2 and YY1 in a number of cancer cell lines. This interaction was serum inducible and rapamycin sensitive. However, the physiological relevance of this interaction needs further investigation [101]. 6. Perspectives S6 kinases acting downstream of the mTOR pathway have showed redundant as well as distinct functions in the cell. While much is known about the regulation of S6Ks by phosphorylation and although several functions of S6K1 have been elucidated, distinct S6K2 functions have been reported and the possibility of novel modes of S6K regulation need to be fully addressed. In some cancer types, the two kinases showed opposite effects on cell survival which raises the need to specific inhibitors that differentiate between the S6K1 and S6K2 kinase activities in cancer cells. Conflict of interest: No conflict of interest. References [1] Jeno P, Ballou LM, Novak-Hofer I, and Thomas G,

Identification and characterization of a mitogen-activated S6 kinase. Proc.Natl.Acad.Sci.U.S.A 1988; 85: 406-410.

[2] Banerjee P, Ahmad MF, Grove JR, Kozlosky C, Price DJ, and Avruch J, Molecular structure of a major insulin/mitogen-activated 70-kDa S6 protein kinase. Proc.Natl.Acad.Sci.U.S.A 1990; 87: 8550-8554.

[3] Kozma SC, Ferrari S, Bassand P, Siegmann M, Totty N, and Thomas G, Cloning of the mitogen-activated S6 kinase from rat liver reveals an enzyme of the second messenger subfamily. Proc.Natl.Acad.Sci.U.S.A 1990; 87: 7365-7369.

[4] Shima H, Pende M, Chen Y, Fumagalli S, Thomas G, and Kozma SC, Disruption of the p70(s6k)/p85(s6k) gene reveals a small mouse phenotype and a new functional S6 kinase. EMBO J. 1998; 17: 6649-6659.

[5] Lee-Fruman KK, Kuo CJ, Lippincott J, Terada N, and Blenis J, Characterization of S6K2, a novel kinase homologous to S6K1. Oncogene 1999; 18: 5108-5114.

[6] Koh H, Jee K, Lee B, Kim J, Kim D, Yun YH, Kim JW, Choi HS, and Chung J, Cloning and characterization of a nuclear S6 kinase, S6 kinase-related kinase (SRK); a novel nuclear target of Akt. Oncogene 1999; 18: 5115-5119.

[7] Gout I, Minami T, Hara K, Tsujishita Y, Filonenko V, Waterfield MD, and Yonezawa K, Molecular cloning and characterization of a novel p70 S6 kinase, p70 S6 kinase beta containing a proline-rich region. J.Biol.Chem. 1998; 273: 30061-30064.

[8] Nardella C, Lunardi A, Fedele G, Clohessy JG, Alimonti A, Kozma SC, Thomas G, Loda M, and Pandolfi PP, Differential expression of S6K2 dictates tissue-specific requirement for S6K1 in mediating aberrant mTORC1 signaling and tumorigenesis. Cancer Res. 2011; 71: 3669-3675.

[9] Koh H, Jee K, Lee B, Kim J, Kim D, Yun YH, Kim JW, Choi HS, and Chung J, Cloning and characterization of a nuclear S6 kinase, S6 kinase-related kinase (SRK); a novel nuclear target of Akt. Oncogene 1999; 18: 5115-5119.

[10] Shima H, Pende M, Chen Y, Fumagalli S, Thomas G, and Kozma SC, Disruption of the p70(s6k)/p85(s6k) gene reveals a small mouse phenotype and a new functional S6 kinase. EMBO J. 1998; 17: 6649-6659.

[11] Lee-Fruman KK, Kuo CJ, Lippincott J, Terada N, and Blenis J, Characterization of S6K2, a novel kinase homologous to S6K1. Oncogene 1999; 18: 5108-5114.

[12] Gout I, Minami T, Hara K, Tsujishita Y, Filonenko V, Waterfield MD, and Yonezawa K, Molecular cloning and characterization of a novel p70 S6 kinase, p70 S6 kinase beta containing a proline-rich region. J.Biol.Chem. 1998; 273: 30061-30064.

[13] Coffer PJ and Woodgett JR, Differential subcellular localisation of two isoforms of p70 S6 protein kinase. Biochem.Biophys.Res.Commun. 1994; 198: 780-786.

[14] Reinhard C, Fernandez A, Lamb NJ, and Thomas G, Nuclear localization of p85s6k: functional requirement for entry into S phase. EMBO J. 1994; 13: 1557-1565.

[15] Minami T, Hara K, Oshiro N, Ueoku S, Yoshino K, Tokunaga C, Shirai Y, Saito N, Gout I, and Yonezawa K, Distinct regulatory mechanism for p70 S6 kinase beta from that for p70 S6 kinase alpha. Genes Cells 2001; 6: 1003-1015.

[16] Burnett PE, Blackshaw S, Lai MM, Qureshi IA, Burnett AF, Sabatini DM, and Snyder SH, Neurabin is a synaptic protein linking p70 S6 kinase and the neuronal cytoskeleton. Proc.Natl.Acad.Sci.U.S.A 1998; 95: 8351-8356.

[17] Valovka T, Verdier F, Cramer R, Zhyvoloup A, Fenton T, Rebholz H, Wang ML, Gzhegotsky M, Lutsyk A, Matsuka G, Filonenko V, Wang L, Proud CG, Parker PJ, and Gout IT, Protein kinase C phosphorylates ribosomal protein S6 kinase betaII and regulates its subcellular localization. Mol.Cell Biol. 2003; 23: 852-863.

[18] Rossi R, Pester JM, McDowell M, Soza S, Montecucco A, and Lee-Fruman KK, Identification of S6K2 as a centrosome-located kinase. FEBS Lett. 2007; 581: 4058-4064.

[19] Dann SG, Selvaraj A, and Thomas G, mTOR Complex1-S6K1 signaling: at the crossroads of obesity, diabetes and cancer. Trends Mol.Med. 2007; 13: 252-259.



[20] Fingar DC and Blenis J, Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression. Oncogene 2004; 23: 3151-3171.

[21] Martin KA, Schalm SS, Romanelli A, Keon KL, and Blenis J, Ribosomal S6 kinase 2 inhibition by a potent C-terminal repressor domain is relieved by mitogen-activated protein-extracellular signal-regulated kinase kinase-regulated phosphorylation. J.Biol.Chem. 2001; 276: 7892-7898.

[22] Ferrari S, Pearson RB, Siegmann M, Kozma SC, and Thomas G, The immunosuppressant rapamycin induces inactivation of p70s6k through dephosphorylation of a novel set of sites. J.Biol.Chem. 1993; 268: 16091-16094.

[23] Hannan KM, Thomas G, and Pearson RB, Activation of S6K1 (p70 ribosomal protein S6 kinase 1) requires an initial calcium-dependent priming event involving formation of a high-molecular-mass signalling complex. Biochem.J. 2003; 370: 469-477.

[24] Pearson RB, Dennis PB, Han JW, Williamson NA, Kozma SC, Wettenhall RE, and Thomas G, The principal target of rapamycin-induced p70s6k inactivation is a novel phosphorylation site within a conserved hydrophobic domain. EMBO J. 1995; 14: 5279-5287.

[25] Saitoh M, Pullen N, Brennan P, Cantrell D, Dennis PB, and Thomas G, Regulation of an activated S6 kinase 1 variant reveals a novel mammalian target of rapamycin phosphorylation site. J.Biol.Chem. 2002; 277: 20104-20112.

[26] Weng QP, Andrabi K, Kozlowski MT, Grove JR, and Avruch J, Multiple independent inputs are required for activation of the p70 S6 kinase. Mol.Cell Biol. 1995; 15: 2333-2340.

[27] Isotani S, Hara K, Tokunaga C, Inoue H, Avruch J, and Yonezawa K, Immunopurified mammalian target of rapamycin phosphorylates and activates p70 S6 kinase alpha in vitro. J.Biol.Chem. 1999; 274: 34493-34498.

[28] McMahon LP, Choi KM, Lin TA, Abraham RT, and Lawrence JC, Jr., The rapamycin-binding domain governs substrate selectivity by the mammalian target of rapamycin. Mol.Cell Biol. 2002; 22: 7428-7438.

[29] Pullen N, Dennis PB, Andjelkovic M, Dufner A, Kozma SC, Hemmings BA, and Thomas G, Phosphorylation and activation of p70s6k by PDK1. Science 1998; 279: 707-710.

[30] Phin S, Kupferwasser D, Lam J, and Lee-Fruman KK, Mutational analysis of ribosomal S6 kinase 2 shows differential regulation of its kinase activity from that of ribosomal S6 kinase 1. Biochem.J. 2003; 373: 583-591.

[31] Wang ML, Panasyuk G, Gwalter J, Nemazanyy I, Fenton T, Filonenko V, and Gout I, Regulation of ribosomal protein S6 kinases by ubiquitination. Biochem.Biophys.Res.Commun. 2008; 369: 382-387.

[32] Fenton TR, Gwalter J, Ericsson J, and Gout IT, Histone acetyltransferases interact with and acetylate p70 ribosomal S6 kinases in vitro and in vivo. Int.J.Biochem.Cell Biol. 2010; 42: 359-366.

[33] Ward SG and Cantrell DA, Phosphoinositide 3-kinases in T lymphocyte activation. Curr.Opin.Immunol. 2001; 13: 332-338.

[34] Scheid MP, Marignani PA, and Woodgett JR, Multiple phosphoinositide 3-kinase-dependent steps in activation of protein kinase B. Mol.Cell Biol. 2002; 22: 6247-6260.

[35] Yang H, Shaw G, and Raizada MK, ANG II stimulation of neuritogenesis involves protein kinase B in brain neurons. Am.J.Physiol Regul.Integr.Comp Physiol 2002; 283: R107-R114.

[36] Frias MA, Thoreen CC, Jaffe JD, Schroder W, Sculley T, Carr SA, and Sabatini DM, mSin1 is necessary for Akt/PKB phosphorylation, and its isoforms define three distinct mTORC2s. Curr.Biol. 2006; 16: 1865-1870.

[37] Jacinto E, Facchinetti V, Liu D, Soto N, Wei S, Jung SY, Huang Q, Qin J, and Su B, SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity. Cell 2006; 127: 125-137.

[38] Pearce LR, Huang X, Boudeau J, Pawlowski R, Wullschleger S, Deak M, Ibrahim AF, Gourlay R, Magnuson MA, and Alessi DR, Identification of Protor as a novel Rictor-binding component of mTOR complex-2. Biochem.J. 2007; 405: 513-522.

[39] Inoki K, Li Y, Zhu T, Wu J, and Guan KL, TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat.Cell Biol. 2002; 4: 648-657.

[40] Potter CJ, Pedraza LG, and Xu T, Akt regulates growth by directly phosphorylating Tsc2. Nat.Cell Biol. 2002; 4: 658-665.

[41] Castro AF, Rebhun JF, Clark GJ, and Quilliam LA, Rheb binds tuberous sclerosis complex 2 (TSC2) and promotes S6 kinase activation in a rapamycin- and farnesylation-dependent manner. J.Biol.Chem. 2003; 278: 32493-32496.

[42] Inoki K, Li Y, Xu T, and Guan KL, Rheb GTPase is a direct target of TSC2 GAP activity and regulates mTOR signaling. Genes Dev. 2003; 17: 1829-1834.

[43] Fonseca BD, Smith EM, Lee VH, MacKintosh C, and Proud CG, PRAS40 is a target for mammalian target of rapamycin complex 1 and is required for signaling downstream of this complex. J.Biol.Chem. 2007; 282: 24514-24524.

[44] Vander HE, Lee SI, Bandhakavi S, Griffin TJ, and Kim DH, Insulin signalling to mTOR mediated by the Akt/PKB substrate PRAS40. Nat.Cell Biol. 2007; 9: 316-323.

[45] Oshiro N, Takahashi R, Yoshino K, Tanimura K, Nakashima A, Eguchi S, Miyamoto T, Hara K, Takehana K, Avruch J, Kikkawa U, and Yonezawa K, The proline-rich Akt substrate of 40 kDa (PRAS40) is a physiological substrate of mammalian target of rapamycin complex 1. J.Biol.Chem.2007; 282: 20329-20339.

[46] Kim YB, Shulman GI, and Kahn BB, Fatty acid infusion selectively impairs insulin action on Akt1 and protein kinase C lambda /zeta but not on glycogen synthase kinase-3. J.Biol.Chem. 2002; 277: 32915-32922.

[47] Hara K, Maruki Y, Long X, Yoshino K, Oshiro N, Hidayat S, Tokunaga C, Avruch J, and Yonezawa K, Raptor, a binding partner of target of rapamycin (TOR), mediates TOR action. Cell 2002; 110: 177-189.

[48] Beugnet A, Wang X, and Proud CG, Target of rapamycin (TOR)-signaling and RAIP motifs play distinct roles in the mammalian TOR-dependent phosphorylation of initiation factor 4E-binding protein 1. J.Biol.Chem. 2003; 278: 40717-40722.

[49] Nojima H, Tokunaga C, Eguchi S, Oshiro N, Hidayat S, Yoshino K, Hara K, Tanaka N, Avruch J, and Yonezawa K, 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. 2003; 278: 15461-15464.

[50] Schalm SS, Fingar DC, Sabatini DM, and Blenis J, TOS motif-mediated raptor binding regulates 4E-BP1 multisite phosphorylation and function. Curr.Biol. 2003; 13: 797-806.

[51] Schalm SS, Tee AR, and Blenis J, Characterization of a conserved C-terminal motif (RSPRR) in ribosomal protein S6 kinase 1 required for its mammalian target of



rapamycin-dependent regulation. J.Biol.Chem. 2005; 280: 11101-11106.

[52] Peterson RT, Desai BN, Hardwick JS, and Schreiber SL, Protein phosphatase 2A interacts with the 70-kDa S6 kinase and is activated by inhibition of FKBP12-rapamycinassociated protein. Proc.Natl.Acad.Sci.U.S.A 1999; 96: 4438-4442.

[53] Jacinto E, Facchinetti V, Liu D, Soto N, Wei S, Jung SY, Huang Q, Qin J, and Su B, SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity. Cell 2006; 127: 125-137.

[54] Frias MA, Thoreen CC, Jaffe JD, Schroder W, Sculley T, Carr SA, and Sabatini DM, mSin1 is necessary for Akt/PKB phosphorylation, and its isoforms define three distinct mTORC2s. Curr.Biol. 2006; 16: 1865-1870.

[55] Pearce LR, Huang X, Boudeau J, Pawlowski R, Wullschleger S, Deak M, Ibrahim AF, Gourlay R, Magnuson MA, and Alessi DR, Identification of Protor as a novel Rictor-binding component of mTOR complex-2. Biochem.J. 2007; 405: 513-522.

[56] Sarbassov DD, Ali SM, Sengupta S, Sheen JH, Hsu PP, Bagley AF, Markhard AL, and Sabatini DM, Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB. Mol.Cell 2006; 22: 159-168.

[57] Nobukuni T, Kozma SC, and Thomas G, hvps34, an ancient player, enters a growing game: mTOR Complex1/S6K1 signaling. Curr.Opin.Cell Biol. 2007; 19: 135-141.

[58] Byfield MP, Murray JT, and Backer JM, hVps34 is a nutrient-regulated lipid kinase required for activation of p70 S6 kinase. J.Biol.Chem. 2005; 280: 33076-33082.

[59] Nobukuni T, Joaquin M, Roccio M, Dann SG, Kim SY, Gulati P, Byfield MP, Backer JM, Natt F, Bos JL, Zwartkruis FJ, and Thomas G, Amino acids mediate mTOR/raptor signaling through activation of class 3 phosphatidylinositol 3OH-kinase. Proc.Natl.Acad.Sci.U.S.A 2005; 102: 14238-14243.

[60] Conus NM, Hemmings BA, and Pearson RB, Differential regulation by calcium reveals distinct signaling requirements for the activation of Akt and p70S6k. J.Biol.Chem. 1998; 273: 4776-4782.

[61] Inoki K, Zhu T, and Guan KL, TSC2 mediates cellular energy response to control cell growth and survival. Cell 2003; 115: 577-590.

[62] Fang Y, Park IH, Wu AL, Du G, Huang P, Frohman MA, Walker SJ, Brown HA, and Chen J, PLD1 regulates mTOR signaling and mediates Cdc42 activation of S6K1. Curr.Biol. 2003; 13: 2037-2044.

[63] Sun D, Toan X, Zhang Y, Chen Y, Lu R, Wang X, and Fang J, Mammalian target of rapamycin pathway inhibition enhances the effects of 5-aza-dC on suppressing cell proliferation in human gastric cancer cell lines. Sci.China C.Life Sci. 2008; 51: 640-647.

[64] Lehman N, Ledford B, Di Fulvio M, Frondorf K, McPhail LC, and Gomez-Cambronero J, Phospholipase D2-derived phosphatidic acid binds to and activates ribosomal p70 S6 kinase independently of mTOR. FASEB J. 2007; 21: 1075-1087.

[65] Akimoto K, Nakaya M, Yamanaka T, Tanaka J, Matsuda S, Weng QP, Avruch J, and Ohno S, Atypical protein kinase Clambda binds and regulates p70 S6 kinase. Biochem.J. 1998; 335 ( Pt 2): 417-424.

[66] Romanelli A, Martin KA, Toker A, and Blenis J, p70 S6 kinase is regulated by protein kinase Czeta and participates in a phosphoinositide 3-kinase-regulated signalling complex. Mol.Cell Biol. 1999; 19: 2921-2928.

[67] Coghlan MP, Chou MM, and Carpenter CL, Atypical protein kinases Clambda and -zeta associate with the GTP-binding protein Cdc42 and mediate stress fiber loss. Mol.Cell Biol. 2000; 20: 2880-2889.

[68] Roux PP, Ballif BA, Anjum R, Gygi SP, and Blenis J, Tumor-promoting phorbol esters and activated Ras inactivate the tuberous sclerosis tumor suppressor complex via p90 ribosomal S6 kinase. Proc.Natl.Acad.Sci.U.S.A 2004; 101: 13489-13494.

[69] Mizukami Y, Hirata T, and Yoshida K, Nuclear translocation of PKC zeta during ischemia and its inhibition by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. FEBS Lett. 1997; 401: 247-251.

[70] Pardo OE, Wellbrock C, Khanzada UK, Aubert M, Arozarena I, Davidson S, Bowen F, Parker PJ, Filonenko VV, Gout IT, Sebire N, Marais R, Downward J, and Seckl MJ, FGF-2 protects small cell lung cancer cells from apoptosis through a complex involving PKCepsilon, B-Raf and S6K2. EMBO J. 2006; 25: 3078-3088.

[71] Fingar DC, Richardson CJ, Tee AR, Cheatham L, Tsou C, and Blenis J, mTOR controls cell cycle progression through its cell growth effectors S6K1 and 4E-BP1/eukaryotic translation initiation factor 4E. Mol.Cell Biol. 2004; 24: 200-216.

[72] Montagne J, Stewart MJ, Stocker H, Hafen E, Kozma SC, and Thomas G, Drosophila S6 kinase: a regulator of cell size. Science 1999; 285: 2126-2129.

[73] Long X, Spycher C, Han ZS, Rose AM, Muller F, and Avruch J, TOR deficiency in C. elegans causes developmental arrest and intestinal atrophy by inhibition of mRNA translation. Curr.Biol. 2002; 12: 1448-1461.

[74] Montagne J, Stewart MJ, Stocker H, Hafen E, Kozma SC, and Thomas G, Drosophila S6 kinase: a regulator of cell size. Science 1999; 285: 2126-2129.

[75] Pende M, Um SH, Mieulet V, Sticker M, Goss VL, Mestan J, Mueller M, Fumagalli S, Kozma SC, and Thomas G, S6K1(-/-)/S6K2(-/-) mice exhibit perinatal lethality and rapamycin-sensitive 5'-terminal oligopyrimidine mRNA translation and reveal a mitogen-activated protein kinase-dependent S6 kinase pathway. Mol.Cell Biol. 2004; 24: 3112-3124.

[76] Olivier AR, Ballou LM, and Thomas G, Differential regulation of S6 phosphorylation by insulin and epidermal growth factor in Swiss mouse 3T3 cells: insulin activation of type 1 phosphatase. Proc.Natl.Acad.Sci.U.S.A 1988; 85: 4720-4724.

[77] Ferrari S, Bandi HR, Hofsteenge J, Bussian BM, and Thomas G, Mitogen-activated 70K S6 kinase. Identification of in vitro 40 S ribosomal S6 phosphorylation sites. J.Biol.Chem. 1991; 266: 22770-22775.

[78] Raught B, Peiretti F, Gingras AC, Livingstone M, Shahbazian D, Mayeur GL, Polakiewicz RD, Sonenberg N, and Hershey JW, Phosphorylation of eucaryotic translation initiation factor 4B Ser422 is modulated by S6 kinases. EMBO J. 2004; 23: 1761-1769.

[79] Holz MK, Ballif BA, Gygi SP, and Blenis J, mTOR and S6K1 mediate assembly of the translation preinitiation complex through dynamic protein interchange and ordered phosphorylation events. Cell 2005; 123: 569-580.

[80] Thomson DM, Fick CA, and Gordon SE, AMPK activation attenuates S6K1, 4E-BP1, and eEF2 signaling responses to high-frequency electrically stimulated skeletal muscle contractions. J.Appl.Physiol 2008; 104: 625-632.

[81] Grolleau A, Bowman J, Pradet-Balade B, Puravs E, Hanash S, Garcia-Sanz JA, and Beretta L, Global and specific translational control by rapamycin in T cells uncovered by



microarrays and proteomics. J.Biol.Chem. 2002; 277: 22175-22184.

[82] Richardson CJ, Broenstrup M, Fingar DC, Julich K, Ballif BA, Gygi S, and Blenis J, SKAR is a specific target of S6 kinase 1 in cell growth control. Curr.Biol. 2004; 14: 1540-1549.

[83] Ma XM, Yoon SO, Richardson CJ, Julich K, and Blenis J, SKAR links pre-mRNA splicing to mTOR/S6K1-mediated enhanced translation efficiency of spliced mRNAs. Cell 2008; 133: 303-313.

[84] Harada H, Andersen JS, Mann M, Terada N, and Korsmeyer SJ, p70S6 kinase signals cell survival as well as growth, inactivating the pro-apoptotic molecule BAD. Proc.Natl.Acad.Sci.U.S.A 2001; 98: 9666-9670.

[85] Pardo OE, Arcaro A, Salerno G, Tetley TD, Valovka T, Gout I, and Seckl MJ, Novel cross talk between MEK and S6K2 in FGF-2 induced proliferation of SCLC cells. Oncogene 2001; 20: 7658-7667.

[86] Pardo OE, Wellbrock C, Khanzada UK, Aubert M, Arozarena I, Davidson S, Bowen F, Parker PJ, Filonenko VV, Gout IT, Sebire N, Marais R, Downward J, and Seckl MJ, FGF-2 protects small cell lung cancer cells from apoptosis through a complex involving PKCepsilon, B-Raf and S6K2. EMBO J. 2006; 25: 3078-3088.

[87] Sridharan S and Basu A, S6 kinase 2 promotes breast cancer cell survival via Akt. Cancer Res. 2011; 71: 2590-2599.

[88] Campbell M, Allen WE, Sawyer C, Vanhaesebroeck B, and Trimble ER, Glucose-potentiated chemotaxis in human vascular smooth muscle is dependent on cross-talk between the PI3K and MAPK signaling pathways. Circ.Res. 2004; 95: 380-388.

[89] Carpten JD, Faber AL, Horn C, Donoho GP, Briggs SL, Robbins CM, Hostetter G, Boguslawski S, Moses TY, Savage S, Uhlik M, Lin A, Du J, Qian YW, Zeckner DJ, Tucker-Kellogg G, Touchman J, Patel K, Mousses S, Bittner M, Schevitz R, Lai MH, Blanchard KL, and Thomas JE, A transforming mutation in the pleckstrin homology domain of AKT1 in cancer. Nature 2007; 448: 439-444.

[90] Aoki M, Blazek E, and Vogt PK, A role of the kinase mTOR in cellular transformation induced by the oncoproteins P3k and

Akt. Proc.Natl.Acad.Sci.U.S.A 2001; 98: 136-141. [91] El Hashemite N, Zhang H, Henske EP, and Kwiatkowski DJ,

Mutation in TSC2 and activation of mammalian target of rapamycin signalling pathway in renal angiomyolipoma. Lancet 2003; 361: 1348-1349.

[92] Neshat MS, Mellinghoff IK, Tran C, Stiles B, Thomas G, Petersen R, Frost P, Gibbons JJ, Wu H, and Sawyers CL, Enhanced sensitivity of PTEN-deficient tumors to inhibition of FRAP/mTOR. Proc.Natl.Acad.Sci.U.S.A 2001; 98: 10314-10319.

[93] Shaw RJ, Bardeesy N, Manning BD, Lopez L, Kosmatka M, DePinho RA, and Cantley LC, The LKB1 tumor suppressor negatively regulates mTOR signaling. Cancer Cell 2004; 6: 91-99.

[94 ] Podsypanina K, Lee RT, Politis C, Hennessy I, Crane A, Puc J, Neshat M, Wang H, Yang L, Gibbons J, Frost P, Dreisbach V, Blenis J, Gaciong Z, Fisher P, Sawyers C, Hedrick-Ellenson L, and Parsons R, An inhibitor of mTOR reduces neoplasia and

normalizes p70/S6 kinase activity in Pten+/- mice. Proc.Natl.Acad.Sci.U.S.A 2001; 98: 10320-10325.

[95] Bjornsti MA and Houghton PJ, Lost in translation: dysregulation of cap-dependent translation and cancer. Cancer Cell 2004; 5: 519-523.

[96] Bjornsti MA and Houghton PJ, The TOR pathway: a target for cancer therapy. Nat.Rev.Cancer 2004; 4: 335-348.

[97] Janus A, Robak T, and Smolewski P, The mammalian target of the rapamycin (mTOR) kinase pathway: its role in tumourigenesis and targeted antitumour therapy. Cell Mol.Biol.Lett. 2005; 10: 479-498.

[98] Ismail HM, Overexpression of s6 kinase 1 in brain tumours is associated with induction of hypoxia-responsive genes and predicts patients' survival. J.Oncol. 2012: 416927.

[99] Yoshida S, Matsumoto K, Arao T, Taniguchi H, Goto I, Hanafusa T, Nishio K, and Yamada Y, Gene amplification of ribosomal protein S6 kinase-1 and -2 in gastric cancer. Anticancer Res. 2013; 33: 469-475.

[100] Sridharan S and Basu A, S6 kinase 2 promotes breast cancer cell survival via Akt. Cancer Res. 2011; 71: 2590-2599.

[101] Ismail HM, Myronova O, Tsuchiya Y, Niewiarowski A, Tsaneva I, and Gout I, Identification of the general transcription factor Yin Yang 1 as a novel and specific binding partner for S6 kinase 2. Cell Signal. 2013; 25: 1054-1063.

Documents

Downstream the mTOR: S6 Kinases between Divergence and …researchpub.org/journal/jbpr/number/vol1-no2/vol1-no2-4.pdf · 2014-06-21 · Abstract: The ribosomal protein S6 kinase (S6K)