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PHYSIOLOGY 21: 362–369, 2006; 10.1152/physiol.00024.2006
The mTOR Pathway in the Control of
Protein Synthesis
Xuemin Wang
and Christopher G. Proud
Signaling through mammalian target of rapamycin (mTOR) is activated by amino
Department of Biochemistry & Molecular Biology,
University of British Columbia, Life Sciences Centre,
Vancouver, British Columbia, Canada
[email protected]
acids, insulin, and growth factors, and impaired by nutrient or energy deficiency.
mTOR plays key roles in cell physiology. mTOR regulates numerous components
involved in protein synthesis, including initiation and elongation factors, and the biogenesis of ribosomes themselves.
Like its orthologs in lower eukaryotes, the mammalian
target of rapamycin (mTOR) is a multidomain protein
and a member of the family of phosphoinositide (PI)
3-kinase-related kinases [PIKKs; which also include
ATM and ATR, proteins involved in DNA repair (17)].
Major features of mTOR are depicted in FIGURE 1A.
Genetic knockouts of mTOR have revealed it to be an
essential gene: mice lacking mTOR die in utero shortly after implantation (10, 36). Embryonic development
appears to be arrested at E5.5, with evidence of multiple developmental aberrations.
Although mTOR is related to lipid kinases, it displays protein serine/threonine kinase activity in vitro
(7), in common with other PIKKs. mTOR interacts
with a number of protein partners, including proteins
that probably act as adaptors or scaffolds and others
that regulate mTOR. Immediately NH2 terminal to the
kinase domain of mTOR lies a region that binds the
immunophilin FKBP12 when it is bound to
rapamycin. The rapamycin:FKBP12 complex inhibits
several functions of mTOR, although it is now very
clear that it does not block all of them (see below).
Rapamycin is an immunosuppressant and is used
clinically to prevent kidney graft rejection, as well for
other purposes (e.g., to treat restenosis after angioplasty) (8).
The other binding partners for mTOR include raptor
and rictor, which respectively form complexes with
mTOR termed mTORC1 and mTORC2 (FIGURE 1B).
TORC2 mediates effects that are insensitive to
rapamycin, whereas the best known actions of the
TORC1 complex are sensitive to this drug (49). Raptor,
a component of mTORC1, interacts with proteins that
are regulated by mTOR in a rapamycin-sensitive manner. These mTOR targets contain short amino acid
sequence motifs called TOS (mTOR-signaling) motifs.
The best-characterized TOS motif-containing proteins
are ones involved in regulating the translational
machinery, as described below.
mTOR signaling is activated by hormones and
growth factors. In the case of insulin, for example, this
is believed to be mediated through the PI3-kinase
362
pathway and PKB/Akt. PKB phosphorylates a protein
called TSC2 (for tuberous sclerosis complex 2).
Together with its partner TSC1, TSC2 acts as a GTPaseactivator protein (GAP) for the small G-protein Rheb
(Ras-homolog enriched in brain) (30). Phosphorylation of TSC2 is thought to inhibit its GAP activity,
allowing Rheb to accumulate in its active GTP-bound
form, although direct evidence is lacking for the alteration of the GAP activity of TSC2 in response to phosphorylation. Rheb interacts with mTOR complexes
and Rheb. GTP stimulates the kinase activity of mTOR
[measured in vitro (28)]. Signaling from the classical
MAP kinase pathway also activates mTOR signaling,
although it is unclear whether this results from phosphorylation of TSC2 by Erk itself or by its downstream
effector, ribosomal protein S6 kinase (RSK) (29, 45).
These signaling connections are illustrated schematically in FIGURE 1C.
Many lines of evidence indicate that the hyperactivation of mTOR signaling favors cell and tissue growth
(49). These include data from genetic studies in model
organisms such as Drosophila and mice, and findings
from investigations into human cell growth disorders
such as tuberous sclerosis. This condition is caused by
mutations in the genes for the tuberous sclerosis complex proteins TSC1 or TSC2. Since they are negative
regulators of mTOR, loss of TSC1 or TSC2 leads to
hyperactivation of mTOR signaling and increased cell
growth, as manifested in the benign tumors (hamartomas), which are characterized by very large cell size.
Regulating the rate of protein synthesis is an important
mechanism for controlling cell growth. Disorders that
involve enhanced rates of protein synthesis can lead to
tissue hypertrophy (increased cell growth), as in the
case of cardiac hypertrophy (14). Indeed, cardiac
hypertrophy may be regarded as a useful model for
studying the control of the growth of differentiated
cells.
In addition to the upstream control by hormones
and growth factors, mTOR signaling is also regulated
by amino acids and by cellular energy status. In most
cell types, the effective amino acid is leucine (reviewed
in Ref. 24). Omission of leucine from the medium
causes the rapid inactivation of mTOR signaling, and
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Overview of mTOR Signaling
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mTOR signaling becomes refractory to stimulation by
extracellular factors such as insulin. The effects of
leucine appear not to be a direct effect of the amino
acid itself acting at an intracellular locus. They do not
appear to requite its metabolism or the mediation of a
cell membrane receptor (2). Amino acids appear to
regulate mTOR signaling via mechanisms that are
independent of TSC2, although small changes in
Rheb.GTP binding have been reported (44, 50). Recent
data point to a role for the PI3-kinase-related protein
Vps34 in the control of mTOR by amino acids (5, 37).
The effect of amino acid deprivation on mTOR signaling varies greatly between cell types, perhaps reflecting differences in the availability of amino acids from
intracellular sources, such as protein breakdown by
autophagy or the proteasome.
Depletion of cellular ATP also causes an impairment of mTOR signaling, which involves the phosphorylation (and presumed activation) of TSC2 by the
AMP-activated protein kinase (AMPK) (21), an important sensor of cellular energy status (21). By shutting
down mTOR signaling, this effect likely serves to save
valuable energy for the most essential cell functions,
such as maintaining ionic gradients across the plasma
membrane (e.g., the Na+/H+ exchanger).
The inhibition of mTOR signaling caused by lack of
amino acids or energy clearly makes sense given that a
major function of mTOR is to promote mRNA translation, a process that requires amino acids as precursors
and consumes a high proportion of cellular energy. As
described below, the elongation phase of translation
(where almost all the energy is used) is also subject to
control by an additional AMPK-mediated mechanism.
Overview of mRNA Translation
The process of protein synthesis (mRNA translation)
involves the sequential decoding of the mRNA into
protein, performed on the ribosome, which acts as an
enzyme (a ribozyme) to catalyse the formation of the
peptide bonds linking the amino acids of the new protein. Protein synthesis is conventionally divided into
three main stages: initiation, elongation, and termination. Each involves a number of protein factors that are
extrinsic to the ribosome. Their regulation generally
involves alterations in their phosphorylation. mTOR
controls a number of components involved in the initiation and elongation stages of translation. In a number of cases, the rapid activation of protein synthesis
by insulin, growth factors, or other growth-promoting
agonists is inhibited, at least partially, by rapamycin,
implying that mTOR signaling is involved in stimulating the translational machinery. It is important to note
that mTOR signaling likely contributes both to the
short-term (minutes) activation of translation and to
longer-term (hours) increases in the translational
capacity of the cell through increased levels of ribosomes and other translational components.
FIGURE 1. Features of mTOR
A: schematic depiction of the main features of mTOR. The HEAT repeats
likely mediate interactions between mTOR and some of its partners. The
role of the FRB (FKBP-rapamycin binding) domain is evident from its
name. FAT and FATC domains are found in all PIKKs. B: selected components of the mTOR complexes 1 and 2 (mTORC 1/2). The mechanism by
which rictor links mTOR to control of targets such as PKB/Akt is so far
unclear. C: signaling connections upstream of mTOR. Insulin, through
PI3-kinase and PKB/Akt, induces the phosphorylation of the TSC2 component of the TSC1/2 complex that acts as a GAP for Rheb. This is
believed to inactivate TSC1/2, allowing Rheb to accumulate in its active
GTP-bound form and turn on mTORC1. Through a component of
mTORC1 (raptor), mTOR brings about the phosphorylation of targets
such as 4E-BP1 and the S6 kinases. Certain agents act through ERK signaling rather than PI3-kinase/PKB to elicit phosphorylation of TSC2 (at
different sites). It is not yet established whether this is mediated directly
by ERK or by its downstream effector p90RSK. These phosphorylation
events are also thought to inactivate TSC2. S6Ks can phosphorylate
IRS-1 and thus impair signaling from, e.g., the insulin receptor to PI3-kinase,
leading to a feedback inhibitory effect. Dashed lines indicate signaling connections that remain unclear at the time of writing. These include the mechanism by which amino acids (leucine) control mTOR signaling.
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Control of Translation
Initiation Factors
A: schematic illustration of eukaryotic mRNA showing the 5’-cap (m7GTP) and the start
codon (AUG). Most of the coding region (dashed line) and the 3’-UTR/poly(A) tail are
not shown. B: by binding the 5’ cap, eIF4E allows formation of translation initiation factor complexes at the 5’ end of the mRNA. The scaffold protein eIF4G, which binds
eIF4E, also interacts with several other proteins, including the helicase eIF4A and the
poly(A)-binding protein PABP. The latter interaction results in the circularization of the
mRNA. The diagram is not intended to reflect relative sizes of components; not all
components are shown. C: schematic diagram of 4E-BP1, showing all the known phosphorylation sites (P), the eIF4E binding motif, and the regulatory RAIP and TOS motifs.
The RAIP motif mediates inputs from mTOR that are promoted by amino acids but
insensitive to rapamycin. D: control of eEF2 kinase by mTOR involves its phosphorylation at three or more sites, one of which (Ser366) is a direct target for the S6 kinases.
The kinases acting at Ser78 and Ser359 have still to be identified. Phosphorylation at
each of these sites inhibits eEF2 kinase. In contrast, phosphorylation at Ser398 by
AMPK appears to activate it, serving to slow elongation when ATP levels fall. Other
important features of eEF2 kinase are shown: the calmodulin (CaM) binding site; the
catalytic domain (large shaded block), and the probable substrate (eEF2) binding site
at the extreme COOH terminus.
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FIGURE 2. Eukaryotic mRNA
These proteins are termed eukaryotic initiation factors
(eIFs). They mediate key steps in translation initiation,
such as the recruitment of the mRNA to the small (40S)
ribosome subunit (eIF4 group of factors) (12) and the
recruitment of the initiator methionyl-tRNA (MettRNAi) that recognizes the start codon at the beginning
of the coding region, after a process termed “scanning,” during which the preinitiation complex (including the 40S subunit and Met-tRNAi) inspects the 5’untranslated region (5’-UTR) of the mRNA for a suitable start codon (FIGURE 2A).
eIF4E binds to the 5’-cap structure of the mRNA,
which includes a 7-methyl guanosine triphosphate
(m7GTP) moiety (12). eIF4E also binds protein partners such as eIF4G, a large scaffold protein that interacts with several other proteins (FIGURE 2B). These
partners include the RNA helicase eIF4A and the
poly(A)-binding protein PABP, which interacts with
the tract of adenyl nucleotides at the 3’ end of the
mRNA. By binding eIF4E and PABP, eIF4G in effect circularizes the message. The eIF4A/E/G complex is
often referred to as eIF4F. Such complexes are thought
to be especially important for the translation of
mRNAs whose 5’-UTRs contain secondary structure,
since eIF4A can unwind such features, which otherwise impair the scanning process by which the ribosome is thought to locate the start codon in the mRNA.
The helicase activity of eIF4A is enhanced by a further
factor, eIF4B.
eIF4E also binds small phosphoproteins termed 4E
binding proteins (4E-BPs). There are three 4E-BPs in
mammals, with 4E-BP1 being by far the best characterized (FIGURE 2C). These proteins bind to the same
region of eIF4E as eIF4G does, so binding of 4E-BPs to
eIF4E prevents eIF4E from binding eIF4G and engaging
in active translation initiation complexes (12). The association of 4E-BP1 with eIF4E is regulated by phosphorylation of 4E-BP1, although the situation is complicated.
Although some phosphorylation sites in 4E-BP1 directly impair eIF4E binding, others do not, but these sites
may influence the ability of the regulatory sites to
undergo phosphorylation. For example, phosphorylation at the NH2-terminal sites (Thr37/46 in human 4EBP1) is required for subsequent modification of the sites
adjacent to the eIF4E-binding site (Ser65/Thr70) but
does not in itself affect eIF4E binding (11, 40). It remains
unclear for several reasons (see Ref. 55) whether mTOR
directly phosphorylates all the regulated sites in 4EBP1. Some may be targets for other kinases that are
associated with and/or controlled by mTOR. Although
much attention has focused on the regulation of 4E-BPs
and their role (as targets for mTOR signaling) in the
control of mRNA translation, their importance for the
regulation of overall protein synthesis remains unclear.
For example, knockouts of 4E-BP1 or 4E-BP2 do not
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inhibits eEF2 kinase activity. These regulatory inputs
into eEF2 kinase are summarized in FIGURE 2D.
Stimuli that activate mTOR, such as insulin and
amino acids, can thus activate translation elongation,
and this complements their abilities to enhance the
loading of ribosomes onto mRNAs through their
effects on translation elongation. Most of the large
amount of energy used by protein synthesis is used in
elongation, and the inhibition of mTOR by AMPK
would serve to slow elongation and save energy, while
preserving polysomes. In fact, a second AMPKdependent input also serves to slow elongation: this is
the direct phosphorylation and apparent activation of
eEF2 kinase by AMPK (3, 20) (FIGURE 2D).
Two recent studies have highlighted the importance
of inhibition of mTOR and eEF2 for cell survival during
hypoxia, a condition that is associated with potential
depletion of cellular energy, although other responses
are clearly also involved (25, 27).
Control of Translation Elongation
The RSKs
In mammalian cells, translation elongation requires
two factors: eEF1 and eEF2. eEF2 is regulated by
mTOR signaling. Its role is to promote the translocation step of elongation, in which the ribosome moves
by the equivalent of one codon relative to the mRNA,
and the peptidyl-tRNA migrates from the ribosomal A
site into the P site, following formation of the new peptide bond. eEF2 undergoes phosphorylation at Thr56
in its GTP-binding domain (41). This inhibits the binding of eEF2 to ribosomes and thereby impairs its activity (6). The phosphorylation of eEF2 is catalysed by a
highly specific protein kinase, eEF2 kinase (FIGURE
2D). This calcium/calmodulin (CaM)-dependent
enzyme is an unusual kinase that does not belong to
the main protein kinase superfamily (48) but to another small group of enzymes whose primary sequences
show no similarity to other protein kinases.
Insulin and a number of other agents that activate
protein synthesis (41) induce the rapid dephosphorylation of eEF2. This results in increased eEF2 activity
and accelerated elongation (43). The dephosphorylation of eEF2 appears to be due to the inhibition of eEF2
kinase, which is also brought about by insulin. Both
these effects are blocked by rapamycin (43).
The mTOR-dependent control of eEF2 kinase
involves its phosphorylation at several sites. Ser366 (in
human eEF2 kinase) is phosphorylated by S6K1, and
this leads to inhibition of eEF2 kinase activity, at least
at submaximal Ca2+ ion concentrations (57). This site
is also phosphorylated by the RSKs, which lie downstream of the Erk pathway. A second input is provided
by phosphorylation at Ser78, which is induced by
insulin and tightly controlled by mTOR. This phosphorylation impairs the binding of CaM to eEF2
kinase, thus inhibiting its activity (4). Ser359 is a further mTOR-regulated site. Phosphorylation here
The ribosomal protein (rp) S6 kinases first came to
attention as enzymes that phosphorylate rpS6, a component of the small (40S) ribosomal subunit. rpS6 lies in a
region of the ribosome close to the interface between the
large and small subunits that also contacts mRNA and
tRNA, as well as certain translation factors (reviewed in
Ref. 9). Phosphorylation of S6 increases under a variety
of conditions linked to cell growth and/or proliferation,
such as stimulation of quiescent cells with serum or
mitogens, insulin treatment, in the regenerating liver, or
in vivo on refeeding. Phosphorylation of S6 is strongly
inhibited by rapamycin, implying a role for mTOR. The
activation of the S6 kinases is likewise blocked by
rapamycin. [Other kinases have also been termed or
RSKs (46), but their regulation is not sensitive to
rapamycin, and they likely play, at most, a minor role in
phosphorylating S6 physiologically.]
There are two S6 kinase genes in mammals (S6K1,
S6K2). The control of the S6K1 and S6K2 proteins
appears to be similar. Their substrate specificities are
not identical, and data from knockout mice suggest
that S6K2, rather than S6K1, may be the major physiological kinase for S6 (39). Mice in which S6K1 has been
knocked out show a small cell-size phenotype, but this
is not seen for S6K2 knockouts (39). The knockout of
the single S6K gene in Drosophila also yields a small
cell-size phenotype (35), but the fact that S6K2 seems
to be the main S6 kinase in mammals indicates that it
is not phosphorylation of S6 that is responsible for the
effect on cell size. S6K1–/– mice show decreased circulating insulin levels, smaller pancreatic -cells, and
mild glucose intolerance (38).
S6K1 and S6K2 each exist as two splice variants, one
of which has a nuclear localization signal (FIGURE
3A). The regulation of S6K1 has been the subject of
intensive study: its activity is controlled by amino
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have general phenotypes, with the reported effects
being in specific tissues such as fat and brain, respectively, in which these proteins are expressed at relatively high levels (1, 51). This could, of course, reflect the
fact that many tissues express both proteins, leading to
redundancy in function.
eIF4B also interacts with the eIF4F complex (12). It
contains two RNA-binding domains, interacts with
eIF4A, and stimulates its RNA helicase activity (26).
eIF4B is a substrate for phosphorylation by S6 kinase
(42). It is suggested that the phosphorylation of eIF4B
by S6 kinases (which are regulated by mTOR; see
Control of Translation Elongation) stimulates its function. Indeed, this phosphorylation event favors
recruitment of eIF4B into complexes with eIF3, which
promotes the recruitment of ribosomes to the 5’ end of
the message (19). This likely serves to bring eIF4B to
complexes containing eIF4G/4A and enhance RNA
unwinding during active translation initiation.
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acids and insulin, such that they have a low basal
activity that is maintained by amino acids and can be
elevated further in response to insulin, provided that
amino acids are available (see, e.g., Refs. 15, 56).
Despite many years of study, the physiological function of S6 phosphorylation remains unclear. One idea
was that it might be related to the control of the translation of the mRNAs that encode ribosomal proteins
(see below; Ref. 9). However, data from mice in which
Control of Ribosome Biogenesis
The above mechanisms function to control the activity of components of the translational machinery
acutely over periods of minutes to a few hours. In the
FIGURE 3. S6 kinase 1
A: schematic diagram of S6 kinase 1 showing the catalytic domain and the nuclear localization signal found only
in the longer splice variant. Phosphorylation at several
sites is required for full activation, and these are shown
in cartoon form. As indicated, phosphorylation of a site
COOH terminal to the catalytic domain by mTOR
(mTORC1) allows phosphorylation within the catalytic
domain. This results in kinase activation. B: 5’-TOP
mRNAs contain an inhibitory element (‘TOP’; red) that
acts to repress their translation under unstimulated conditions. Stimulation of cells, e.g., with serum, causes the
5’-TOP mRNAs to more efficiently bind ribosomes and
become translated. This is blocked by rapamycin,
although the mTOR-linked mechanism by which these
mRNAs are controlled is unknown. Although the figure
depicts 5’-TOP mRNAs as bound to a single ribosome in
unstimulated cells, this is purely schematic, and many
may be in non-ribosomal complexes. C: mTOR regulates
by the transcription of rRNA (in the nucleolus) and the
translation of r-protein (5’-TOP) mRNAs in the cytoplasm.
This coordinated control ultimately leads to the assembly
of functional ribosomes in the nucleolus.
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both S6K1 and S6K2 are knocked out have disproved
this (39), as has use of mice in which the phosphorylation sites in rpS6 have been converted to alanines (47).
The role of S6 phosphorylation still remains obscure;
some interesting phenotypes are seen in the mice
lacking phosphorylatable rpS6, such as a selective
decrease in the size of pancreatic -cells and
hypoinsulinemia. Cells (embryonic fibroblasts) from
these animals actually show an enhanced rate of protein synthesis, faster proliferation, and reduced size
(47), but it is unclear how these effects are causally
connected to the phosphorylation of rpS6.
As mentioned already, S6Ks have been shown to phosphorylate other components of the translational machinery such as eIF4B (42) and eEF2 kinase (57). It will be
informative to use cells from S6K1/2 knockout animals to
explore their importance in controlling specific stages in
mRNA translation. Recent data have shown that S6K1
associates with the translation initiation factor eIF3 in a
regulated manner. mTOR also associates with eIF3 (19).
As discussed above, the association of S6K1 with eIF3
likely targets it to its substrate eIF4B. Interestingly, cell
activation results in recruitment of the upstream regulator of S6K1, mTOR/raptor (TORC1) to eIF3, where it can
presumably phosphorylate eIF4B (19).
S6K1 can phosphorylate insulin receptor substrate
(IRS) 1 and impair the activation by insulin of PI3kinase and PKB/Akt (16, 52) (indicated as a dashed
line in FIGURE 1C). This effect would interfere with
the ability of insulin to exert many of its effects, which
are mediated through these signaling connections.
Since S6K is activated in response to nutrients, it has
been suggested that this may contribute to the insulin
resistance observed in obesity.
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Further work is clearly needed to explore the control
of rDNA transcription in systems such as this model
of cell size control.
mTOR also regulates Pol I transcription through
controlling the activity of TIF-IA (FIGURE 3B) (31).
TIF-1A is a regulatory factor that senses nutrient and
growth-factor availability. Treatment of cells with
rapamycin causes the translocation of TIF-IA into the
cytoplasm and consequently inhibits transcriptioninitiation complex formation.
The coordinated control of the synthesis of ribosomal proteins and rRNA is logical (FIGURE 3B), as is
the involvement of mTOR in this, since it coordinates
responses to nutrients and hormones/growth factors.
Identification of the molecular events involved in
these effects is a high priority, as is information on the
control of r-protein mRNA transcription.
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longer term, the cellular capacity for protein synthesis
is also regulated, such that anabolic stimuli increase
the numbers of ribosomes and other translational
components (e.g., elongation factors). mTOR signaling
also seems to play an important role in this.
Mammalian ribosomes are composed of four different ribosomal RNAs (rRNAs) and around 85–90 distinct proteins (r proteins). The mRNAs for r proteins
are subject to translational control (34). In serumstarved cells, they are translated inefficiently and are
found mainly in the non-polysomal region of a sucrose
density gradient. In response to stimulation of the cells
(e.g., addition of fresh serum to serum-deprived cells),
these mRNAs shift markedly into polysomes and are
thus translated more efficiently (23) (FIGURE 3A).
This control is mediated via a tract of pyrimidines at
the extreme 5’ end of these mRNAs (5’-TOP), which
acts to suppress their translation under basal conditions (FIGURE 3B). Importantly, the shift into
polysomes is inhibited, at least partially, by
rapamycin. This finding indicates that mTOR provides
an input into the translational control of the 5’-TOP
mRNAs. This makes excellent “sense” since mTOR signaling is activated by amino acids (precursors for ribosome biogenesis and protein synthesis) and by anabolic and hypertrophic stimuli.
How does mTOR control 5’-TOP mRNA translation?
Earlier data were interpreted as suggesting that this
involved the S6 kinases and were interpreted in terms
of a role for the S6 kinases in promoting 5’-TOP mRNA
translation (22). However, more recent findings have
disproved this idea. For example, rapamycin-sensitive
5’-TOP mRNA translation is still observed in cells
lacking both isoforms of S6 kinase (39). Furthermore,
in cells in which the five phosphorylation sites in S6
have all been altered to alanines, 5’-TOP mRNAs still
undergo mTOR-regulated translation (47). In particular, they are still found in polysomes, which would not
be observed if S6 phosphorylation was required for
their efficient translation. The key issue, therefore, is
still how does mTOR regulate 5’-TOP mRNA translation?
Of course, making a functional ribosome requires
rRNAs as well as proteins. rDNA transcription, which
occurs in the nucleolar compartment and is largely
catalysed by RNA polymerase I (Pol I), is also under
the control of mTOR signaling. mTOR is required for
the activation of rDNA transcription (13) and positively regulates the phosphorylation and function of
UBF (upstream binding factor), an rDNA transcription factor (FIGURE 3C). The data from this study
indicated a key role for S6K1 in linking mTOR to the
control of UBF and rDNA transcription. However,
although UBF seems to play a key role in the hypertrophic growth of cardiomyocytes, data from mice in
which both S6K genes have been knocked out indicate that they are dispensable for pathological or
physiological cardiac hypertrophy in mice (33).
FIGURE 4. Sensitivity of protein synthesis in
selected cell types to inhibition of mTOR signaling by rapamycin
The figure depicts the effects of the phorbol ester TPA
(tetradecanoyl phorbol acetate, 1 M, 45 min), insulin
(100 nM, 45 min in A, 90 min in B), or the hypertrophic
-agonist PE (phenylephrine, 10 M, 90 min) on rates of
protein synthesis in HEK293 cells (A) or in primary adult
rat ventricular cardiomyocytes (B). Protein synthesis was
determined by incorporation of [35S]methionine into total
protein. Cells were treated with rapamycin (typically 50
nM for 30 min) before addition of agonist. These data
are taken from Refs. 18, 53, 54.
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mTOR Signaling and the Control of
Protein Synthesis
Perspectives and Future Directions
There is currently a very high level of interest in mTOR
signaling. In mammalian cells, the best understood
targets for control by mTOR remain the components of
the translational machinery (initiation and elongation
factors) described in this article. Nevertheless, key
questions remain. It is still not clear how mTOR controls several of its targets, such as eEF2 kinase, 4E-BP1,
and the translation of the 5’-TOP mRNAs. Our understanding of the control of ribosome RNA transcription
and its coordination with ribosomal protein production is incomplete. The quantitative contributions of
the regulation of the 4E-BPs, eIF4B, eEF2 kinase, and
eEF2 in cell physiology remain to be established, as do
the physiological functions of the much-studied S6
kinases. Indeed, although several components of the
translational are controlled by the mTOR pathway, it is
still not yet clear what quantitative contributions these
various regulatory events make to the overall activation of protein synthesis, or the control of the translation of specific mRNAs, under physiologically relevant
conditions. The availability of transgenic mice in
which these regulatory components have been delet368
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Rapamycin substantially inhibits the short-term (1–2
h) activation of protein synthesis by a variety of agents
in certain cell types, as shown in FIGURE 4. In the
widely used cell line HEK (human embryonic kidney)
293, the activation of protein synthesis either by phorbol ester (TPA; which activates ERK signaling) or by
insulin (via PI3-kinase/PKB; Ref. 18) was decreased by
50% or more by pretreatment of cells with rapamycin.
In cardiomyocytes, the activation of protein synthesis
induced by either insulin or the hypertrophic agonist
phenylephrine was inhibited >50% by rapamycin (53,
54). In each case, the rapamycin-insensitive component of activation is modest (which amounts to
around 40% of the overall increase in cardiomyocytes)
but statistically significant. This component may be
entirely independent of mTOR (e.g., through control of
regulatory translational components that are not
downstream of mTOR, such as eIF2B), or it may
involve rapamycin-insensitive outputs from mTOR.
Increased rates of protein synthesis are a key feature
of hypertrophy driving myocyte growth. Certain types
of hypertrophy (which involve, e.g., structural and
functional remodeling of the heart) constitute a major
risk factor for heart failure and mortality. It is important to note in this context that in vivo hypertrophy is
prevented, or even reversed, by treatment of rats with
rapamycin (32), implying a key role for mTOR signaling in this process. It is likely that mTOR also plays a
role in other types of physiological and developmental
growth of the myocardium too.
ed or otherwise modified will be crucial to unraveling
how mTOR signaling contributes to the control of protein synthesis and amino acid metabolism in whole
tissues and animals. „
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