Download Mechanical stimuli of skeletal muscle: implications on mTOR/p70s6k

Eur J Appl Physiol
DOI 10.1007/s00421-007-0588-3
INV ITED R EVIEW
Mechanical stimuli of skeletal muscle: implications
on mTOR/p70s6k and protein synthesis
Nelo Eidy Zanchi · Antonio Herbert Lancha Jr
Accepted: 25 September 2007
© Springer-Verlag 2007
Abstract The skeletal muscle is a tissue with adaptive
properties which are essential to the survival of many species. When mechanically stimulated it is liable to undergo
remodeling, namely, changes in its mass/volume resulting
mainly from myoWbrillar protein accumulation. The mTOR
pathway (mammalian target of rapamycin) via its eVector
p70s6k (ribosomal protein kinase S6) has been reported to
be of importance to the control of skeletal muscle mass,
particularly under mechanical stimulation. However, not all
mechanical stimuli are capable of activating this pathway,
and among those who are, there are diVerences in the activation magnitude. Likewise, not all skeletal muscle Wbers
respond to the same extent to mechanical stimulation. Such
evidences suggest speciWc mechanical stimuli through
appropriate cellular signaling to be responsible for the Wnal
physiological response, namely, the accumulation of myoWbrillar protein. Lately, after the mTOR signaling pathway
has been acknowledged as of importance for remodeling,
the interest for the mechanical/chemical mediators capable
of activating it has increased. Apart from the already
known MGF (mechano growth factor), some other mediators such as phosphatidic acid (PA) have been identiWed.
This review article comprises and discusses relevant information on the mechano-chemical transduction of the pathway mTOR, with especial emphasis on the muscle protein
synthesis.
N. E. Zanchi (&) · A. H. Lancha Jr
Laboratory of Applied Nutrition and Metabolism,
Physical Education and Sport School,
University of São Paulo, Av. Prof. Mello Moraes,
65, PO Box 05508-900, São Paulo, SP, Brazil
e-mail: [email protected]
Keywords mTOR · p70s6k · Mechanical stimulation ·
Hypertrophy · Protein synthesis
Introduction
The skeletal muscle is a highly plastic tissue, namely, capable of adapting to adverse conditions. In nature, this intrinsic adaptive feature is crucial and ensures the survival of
many species, once a vast number of environmental impositions (e.g. contractile activity and mechanic overload) can
be overcome by means of adaptive responses from the muscles. There are two main changes promoted by mechanical
stimulation upon the skeletal muscle: (1) remodeling
(namely, adaptation which induces changes in the muscle
mass/volume); (2) alterations in the molecular engine of the
muscle Wber, e.g. phenotypical changes in myosin isoforms
expression, important constituents of the skeletal muscle
(Booth and Baldwin 1966). Taken together, these changes
allow the muscle to adapt its contractile properties so that
both power and strength are incremented.
Until lately, little was known about the way the skeletal
muscle increases its mass in order to respond to mechanical
stimulation. However, important recent studies have demonstrated protein kinase mammalian target of rapamycin
(mTOR) to be a key regulator of the muscle size and that
the remodeling response of the skeletal muscle submitted to
mechanical stimulation depends on mTOR (Bodine et al.
2001; Reynolds et al. 2002; Pallafacchina et al. 2002;
Hornberger et al. 2003). Corroborating its speciWc role in
size control, the mTOR pathway does not seem to control
the skeletal muscle Wber speciWcation (Pallafachina et al.
2002).
Adult skeletal muscle cells are not liable to undergo
division (mitosis). As a consequence, adaptive mecha-
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Eur J Appl Physiol
nisms which induce contractile Wber protein accumulation through either increased protein synthesis or reduced
protein degradation on pre existing Wbers are the most
likely candidates to remodeling control of the skeletal
muscle. The mTOR pathway seems to control mechanisms of protein synthesis at several levels (e.g. translation capacity, translation eYciency), through increases of
translation of speciWc mRNAs, which culminate in skeletal muscle Wber enlargement (Wullschleger et al. 2006;
Nader et al. 2005). Nonetheless, so that this process
occurs, it is essential that beyond mechanical stimuli (e.g.
overload) a biological interpretation (e.g. cell signaling)
happens. Taken together, these events are called mechanotransduction.
It is generally agreed that mTOR is activated via its
upstream signaling PI3K (phosphatidylinositol 3 kinase,
which is considered crucial to Akt activation) (Stokoe et al.
1997), and Akt (phosphatidilinositol regulated protein
kinase), culminating in the activation of its eVector p70s6k
(ribosomal protein kinase S6) under overload conditions
(Bodine et al. 2001). This indicates that mechanical stimuli
are capable of stimulating the muscle anabolic mediators,
via the mTOR pathway (PI3K/Akt/mTOR/p70s6k).
Although it may seem repetitive to mention, these mediators remain not entirely understood.
The autocrine/paracrine factors, which are released by
the muscle in response to mechanical stimuli, such as the
Insulin-like growth factor 1 (IGF1) and the mechanical
growth factor (MGF), deWnitely represent strong hypertrophy stimuli via the PI3K/Akt/mTOR/p70s6k pathway
(Rommel et al. 2001; Ohanna et al. 2005; Nave et al. 1999;
Scott et al. 1998). Nevertheless, these mechanisms, seem
to be responsible for the late activation response of this
pathway (Rennie and Wackerhage 2003; Haddad and
Adams 2002). In dramatic contrast, the mTOR pathway
can be activated almost immediately after the muscle overload stimulation (Bolster et al. 2003). This indicates that
more immediate signaling mechanisms induced by per se
muscle stimulation are also regulatory targets. All of them
activate potential mechanisms that either control or
positively inXuence muscle remodeling via the mTOR
pathway.
Acute mechanical stimulation inducing mTOR activation seems to be more complex than that mediated by
growth factors, once elements inside the mTOR pathway
seem to respond nonlinearly (e.g. not exactly via the PI3K/
Akt/mTOR/p70s6k pathway) to diVerent types of mechanical stimuli. Therefore, the purpose of the present review is
that of presenting and discussing the recent advances concerning mechanotransduction of the mTOR pathway. Apart
from that, questions thus far not yet clariWed will be
brought into discussion, with a permanent focus on the
muscle protein synthesis control mechanisms.
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mTOR, p70s6k and mechanical stimulation
The mTOR is a key signaling pathway in the diVerentiation
process and control of cell size (Schmelzle and Hall 2000)
activated by nutritional, chemical and mechanical stimuli
(Reiling and Sabatini 2006). Studies in yeast have demonstrated that the target of rapamycin (TOR) performs two
essential functions in this organism; one regulates the
moment when a cell grows and the other determines the
place where a cell grows (Wullschleger et al. 2006). Therefore, growth control seems to be a crucial function performed by the TOR/mTOR.
In the skeletal muscle cells, the rapamycin (a potent
inhibitor of the mTOR) attenuates proliferation, survival,
diVerentiation and hypertrophy, thus indicating that the
mTOR participates in the muscle developmental programme at diVerent levels (Ohanna et al. 2005). Corroborating this, the delivery of rapamycin to 2-weak-old rat
pups for 11 days resulted in a 30% reduction in body
weight in both male and female pups (Bodine 2006). Surprisingly, the treatment with rapamycin (1.5 mg/kg) in
young adult rats, produced no change in the muscle mass,
but its concomitant administration under overload conditions almost completely blocked the hypertrophy process
(Bodine 2006; Bodine et al. 2001). This indicates that, at
least in the adult skeletal muscle, increased mechanical
stimuli exert speciWc activation of the mTOR pathway
which is not attained by normal life mechanical stimulation. Thus, it has been proposed that the mechanically
induced tension plays a critical role in the regulation of the
skeletal muscle mass (Hornberger and Chien 2006; Hornberger et al. 2006a). However, the events linking mechanical stimuli to the anabolic signaling pathways remain still
not thoroughly understood.
Increased muscle tension seems to stimulate skeletal
muscle growth in two diVerent manners: either via increased
growth factors production (e.g. IGF-1, MGF) or via contractions per se. While growth factors regulating the mTOR
pathway have been well characterized, the characterization
of the regulation of the mTOR pathway in response to
mechanical stimuli remains still at its very beginning.
It is generally agreed that growth factors stimulate the
mTOR pathway via PI3K/Akt dependent mechanisms
(Rommel et al. 2001; Nave et al. 1999; Ohanna et al. 2005).
However, under acute mechanical stimulation, the importance of PI3K/Akt dependent mechanisms in order to activate mTOR calls for clariWcation. As a matter of fact,
recent studies have suggested mTOR, when under mechanical stimulation, to be liable to be activated via PI3K/Akt
independent mechanisms (Hornberger et al. 2004). Independently of the requirement of mechanical stimuli as a
means to activate Akt or not, contractile stimuli seem to be
capable of activating the mTOR pathway.
Eur J Appl Physiol
Several phosphorylation sites have been proposed as
capable of activating mTOR. For example, phosphorylation
on its serine 2481 residue was suggested to be of importance for the regulation of its eVector p70s6k under a treatment with rapamycin (Peterson et al. 2000). Likewise, the
treatment of several cellular types with IGF-1 seems to activate mTOR on its serine residue 2481, correlating with the
activation of p70s6k (Thiimmaiah et al. 2005). Even though
the mTOR regulation mediated by phosphorylation on serine 2481 seems to be of importance for its activity regulation, a very small number of studies seem to have assessed
it. To our knowledge, only three studies have so far
assessed the eVects of mechanical stimulation on phosphorylation of mTOR (Ser 2481) and no alterations have been
observed by its authors (Eliasson et al. 2006; Mascher et al.
2007; Hornberger and Chien 2006a). Contrariwise, it has
been acknowledged that under mechanical stimuli the
mTOR protein is phosphorylated in its 2448 serine end
(Reynolds et al. 2002). This event activates several eVector
pathways, like the eukaryotic initiation factor 4 (EIF4)binding protein 1 (4E-BP1) and p70s6k, which culminates
in increased protein synthesis.
It has been widely assumed that the activation of mTOR
signaling leads to increased protein synthesis via (1) activation of p70s6k, subsequent phosphorylation of mRNAs
containing a 5⬘- terminal oligopyrimidine tract (5⬘TOP)
(encoding elongation factors and ribosomal proteins) and,
ultimately, an increase in the translation capacity and (2)
phosphorylation of 4E-BP1, increased eukaryotic initiation
factor 4E (eIF4E) availability, and eukaryotic initiation factor 4F (eIF4F) complex assembly, leading to increased rates
of translation initiation (Kubica et al. 2005). Thus, the
p70s6k activation under the eVect of mechanical stimuli
seems to be critical for the hypertrophy response.
It is unquestionable that p70s6k has been one of the most
widely studied of mTOR eVectors. Its phosphorylation in
the 389 threonine end has been considered a mTOR activation mirror (Jacinto and Hall 2003; Chen 2004; Sarbassov
et al. 2005) and a marker of increased protein synthesis
induced by mechanical stimulation (Baar and Esser 1999).
Consequently, a great amount of information on mTOR is
available in the literature, owing to studies conducted with
p70s6k. However, recent studies with human subjects
focusing on mechanical stimulation of the skeletal muscle
have demonstrated that the activation of p70s6k (phosphorylation in its threonine 389 residue) is not always correlated
with the activation of mTOR (phosphorylation on serine
2448) (Eliasson et al. 2006; Fujita et al. 2007). Moreover,
the activation of mTOR seems to be not always capable of
activating p70s6k (Mascher et al. 2007). In fact, recent “in
vitro” studies have questioned the dependence of mTOR on
p70s6k activation (Chiang and Abraham 2005; Holz and
Blenis 2005). The signiWcance of this Wnding remains
unknown given that it is still unclear whether mTOR phosphorylation Ser 2448 is a positive, negative or inconsequential regulatory phosphorylation (Holz and Blenis
2005), at least in the “in-vitro” context. That is why, in this
review article, the mTOR pathway will be analyzed with a
focus not only on mTOR but also on p70s6k. We believe
that this particular aspect of the mTOR signaling review
will allow a closer approach to the mTOR pathway study.
EVects of acute mechanical stimuli on mTOR/p70s6k:
dependence on PI3K/Akt in signaling
Increased mechanical stimulation is a powerful stimulus
toward protein synthesis in the skeletal muscle. This relation has been clearly demonstrated in a number of diVerent
species, using diVerent models of mechanical stimulation
(Timson 1990). In the last years, several studies have demonstrated the involvement of the mTOR pathway in such a
process (Bodine et al. 2001; Reynolds et al. 2002; Pallafacchina et al. 2002; Hornberger et al. 2003). More speciWcally, important previous studies using synergist ablation as
a model of mechanical stimulation have demonstrated
mTOR to be liable of being activated by Akt dependent
mechanisms (Bodine et al. 2001).
Synergist ablation is a very interesting mechanical stimulation model, since it allows the overloaded muscle to
undergo robust hypertrophy. However, it remains unclear,
from this model of mechanical stimulation, whether activation of mTOR via Akt is acutely or chronically accomplished. In other words, under continuous mechanical
stimulation, it remains unclear whether the pathways are
directly activated by either mechanical stimulation or by
circulating factors released by the skeletal muscle, such as
MGF. Likewise, it remains unclear whether diVerent
mechanical stimuli are able to diVerently activate these signaling elements.
In the skeletal muscle, increased protein synthesis is
mediated by a continuum time course initiated by mechanical stimuli, which can be modulated by diVerent anabolic
inputs, such as mechanical and circulating factors. Such
anabolic stimuli can exert distinct contributions to signaling
pathways, although both reXect on protein synthesis. As an
example, growth factors (e.g. IGF, MGF) released by the
skeletal muscle in response to mechanical stimulation are
generally accepted to promote increased protein synthesis
through PI3K/Akt/mTOR pathway (Rommel et al. 2001;
Bodine et al. 2001). Moreover, growth factors leading to
PI3K/Akt/mTOR activation seem to be part of a late
component of protein synthesis signaling, since MGF
mRNA expression increases can be observed around 2.5 h
following a resistance training session or even later (Hameed et al. 2003; Haddad and Adams 2002). Contrariwise,
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Eur J Appl Physiol
acute skeletal muscle stimulation seems to be not always
dependent on PI3K/Akt as a means to activate mTOR
(Hornberger et al. 2004, 2006c). Acute exercise studies
may help to explain the way this element is diVerently regulated. Nonetheless, as stated by Tidball (2005) in a recent
review article tackling the theme, “The general picture that
is emerging from these investigations is that there are multiple processes through which muscle adaptation loading
can occur. However, the current understanding is fragmentary”. Therefore, in face of these comments, a deWnite
answer calls for research.
In relation to dependence of Akt in order to stimulate
mTOR/p70s6k through mechanical stimulation, the Wrst
point to be evaluated is: Is acute mTOR/p70s6k activation
always a result of acute Akt activation? To respond this, a
recent study carried out by Hornberger et al. (2004) has
shown intermittent stretch induced signaling through
mTOR/p70s6k not to be disrupted in muscles from Akt1¡/
¡ mice. Moreover, the same mechanical stimuli performed
in the presence of Wortmannin (an inhibitor of PI3K) were
capable of activating p70s6k through PI3K/Akt independent pathway (Horngerger et al. 2006). In line with this,
recent studies have shown activation of mTOR/p70s6k
even in the absence of Akt activation (Fujita et al. 2007;
Eliasson et al. 2006; Parkington et al. 2003; Blomstrand
et al. 2006; Mascher et al. 2007). Taken together, these
recent Wndings suggest that under acute stimuli mTOR/
p70s6k is activated not always by Akt but via alternative
pathways.
Contrarily, several studies carried out in animals and
human subjects have shown that Akt is activated under
acute contractile stimuli (Sakamoto et al. 2003; Nader and
Esser 2001; Bolster et al. 2003; Dreyer et al. 2006). These
studies have been performed under diVerent contractile
stimuli, such as resistance and endurance types. Thus,
diVerent contraction patterns are capable of activating Akt
in a distinct manner, still not thoroughly understood. In face
of these Wndings, a number of questions still remain. One of
the main concerns related to Akt activation is: Why some
mechanical stimuli are capable of activating Akt whereas
others are not? What are the implications on the activation
of mTOR/p70s6k? Presently, an accurate answer calls for
further research. However, some speculations have taken
place in order to explain these diVerential responses.
One important key to elucidate the dependence of PI3K/
Akt in order to activate mTOR/p70s6k is the investigation
of Akt related to the intensity of mechanical stimulation.
Higher mechanical stimuli intensity have been advocated to
be necessary in order to activate Akt, as observed by
Sakamoto et al. (2002), and conWrmed by Atherton et al.
(2005) and Dreyer et al. (2006). Nonetheless, such assumption is not a consensus. For example, Sakamoto et al.
(2003) carried out studies with diVerent aerobic exercise
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intensities, and observed that Akt activation was not dependent on the exercise intensity. In agreement with these
results, three recently published studies with human subjects have reported no changes in Akt phosphorylation after
high intensity resistance exercise (CoVey et al. 2005; Eliasson et al. 2006; Deshmukh et al. 2006). The reason for the
divergent changes, though, remains unclear. It is likely to
be related to either temporary activation of Akt after exercise (Fujita et al. 2007), to diVerent patterns of contraction
elicited by voluntary or electrical stimulus (Fry 2004), or
even related to diVerent species. An additional possibility is
that diVerent intensity/duration of mechanical stimulation
can exert divergent responses on insulin level, which can,
in turn, work against a contraction induced activation of
Akt, as suggested by Sakamoto et al. (2004). In fact,
Mascher et al. (2007) reported a reduced level of insulin
immediately after cycling exercise in human subjects,
which correlated with Akt inhibition.
Another possibility to explain these discrepancies of Akt
signaling on the mTOR pathway are likely to be related to
the fact that multiple isoforms of Akt are present in the
skeletal muscle (Akt1, Akt2, Akt3), and seem to have distinct physiological roles. Of the three isoforms, Akt 1 is the
most active and is the isoform that seems to respond to
mechanical stimuli (Turinsky and Damrau-Abney 1999).
Apart from the above Wnding, little is known about the
eVect of mechanical stimuli on the other isoforms of Akt.
Furthermore, variables such as Wnal activation of
mTOR/p70s6k (specially related to phosphorylation on residues responsible for the full activation of the enzymes) and
protein synthesis or indirect markers of protein synthesis
(e.g. phosphorylation of ribosomal protein S6, polisome
proWle, etc.) are said to provide detailed information on the
importance of mechanical stimuli on the PI3K/Akt/mTOR/
p70s6k pathway. As an example, a recent study has indicated that acute activation of Akt and ribosomal protein S6
is not necessarily correlated with mTOR activation (Bolster
et al. 2003). Therefore, it is highly advisable that future
studies consider investigating all the key elements related
to this signaling pathway and its consequences on protein
synthesis.
Finally, redundant systems of mTOR activation are in all
likelihood to be activated speciWcally by mechanical stimuli. As an example, in “in vitro” studies, lisophospatidic
acid (LPA) and platelets derived growth factor (PDGF)
seem to activate mTOR via diVerent PI3K/Akt mechanisms
(Kam and Exton 2004).
Mechanotransduction related to mTOR/p70s6k
Skeletal muscle is intrinsically a mechanical stimulation
responsive tissue being, therefore, named “mechanocyte”
Eur J Appl Physiol
(Goldspink and Booth 1992). It suggests that mediators
sensitive to mechanical stimuli are likely to promote signal
transduction processes, mediating remodeling responses
and metabolic adaptive responses in the contractile machinery. The understanding of mechanotransduction (the mechanism by which cells convert mechanical signals into
biological responses) controlled by pathways sensitive to
rapamycin is still beginning (Hornberger and Chien 2006a).
Nevertheless, it is interesting to speculate the way mechanical stimuli acutely activate physical/chemical mediators
that are capable of modulating mTOR pathway.
Phosphatidic acid (PA)
One of the substances capable of activating mTOR is phosphatidic acid (PA), a well-known lipid based second messenger which plays a role in several signaling systems,
whose activation inXuences mitogenesis, secretory processes and cytoskeleton reorganization, among others. PA
activation depends on phospholipase D (PLD) enzyme
activity, which hydrolyses phosphatidilcholine in PA and
choline (Hong et al. 2001). Once synthesized, PA binds the
FRB domain of mTOR and activates p70s6k activity (Fang
et al. 2003). Therefore, PA is unquestionably a strong candidate to mechanotransducer in skeletal muscle. The question which has remained unanswered until recently is: Are
mechanical stimuli enough to increase PA production in
skeletal muscle? To address this question, Hornberger and
Chien (2006) submitted skeletal muscle “in vitro” to
stretching and observed PA and its precursor enzyme PLD
behavior. In this study, stretching is reported to have caused
increments of PA and its precursor enzyme PLD concentration, besides having activated p70s6k. Additionally, treating muscles with a PLD inhibitor, neutralized stretching
eVects over p70s6k in a dose dependent way. These data
provided consistent information that mechanical stimulation acutely activates mTOR pathway through increments
in intracellular concentration of PA. At which level this
activation occurs is a question that remains open to investigation. PI3K activation through PA increments was
observed in a Wbroblast cells culture (Kam and Exton
2004). In the skeletal muscle, however, mechanical stimulation does not seem to be dependent on PI3K activation
(Hornberger and Chien 2006a; Hornberger et al. 2004).
Another relevant question is the understanding of the
processes which link the muscle contraction with PA intracellular increments. Even though no study has thus far
addressed question, some speculations have taken place.
One of them is based on PLD and structural protein -actinin proximity (Hornberger et al. 2006a). It has been stated,
for instance, that -actinin protein and -actin inhibit PLD
activity through physical interaction (Lee et al. 2001). The
observation that PLD is located near this structural proteins
and Z discs, which are essential components to force transmission in skeletal muscle, generates speculation that proximity of these two regulatory elements might constitute one
of the ways of mechanical activation of PA (Park et al.
2000). Additionally, Hornberger and Chien (2006b), speculate that mechanical activation produces a physical dissociation between PLD and -actinin and/or -actin, which
would further reinforce the concept of PA as an acute mechano-chemical mediator. However, this speculation calls
for experimental evidences to conWrm it. Finally, PA concentrations control in skeletal muscle also depends on several synthesis and degradation regulatory mechanisms. It is
possible that mechanical stimulation modulates these
mechanisms leading to increments in PA production. Further details on this issue can be found in the excellent paper
by Hornberger et al. (2006b).
Integrin signaling
Several systems of mechanotransduction use membrane
protein/receptors sensitive to physical stimulation to convert mechanical information into molecular events (Ingber
2006). Thus, it is likely that skeletal muscle have also some
similar control mechanisms. In skeletal muscle, structures
known as costameres play an important role in lateral force
transmission (Bloch and Gonzales-Serratos 2003; Ervasti
2003). SpeciWcally, costameres are composed by several
structural proteins that are overexpressed at regular intervals in skeletal muscle Wber, overcoming Z lines. Given the
central role that extracellular matrix plays on force transmission in tissues, structures connecting extracellular
matrix to the muscle cell membrane (and z discs) have been
regarded as possible mechanoreceptors (Ingber 2006). Not
coincidentally, one of the costameres constitutive proteins
which connect extracellular matrix elements to the cell
membrane are integrins (Grounds et al. 2005), which have
been speculated to participate in the mechanical stimulation
that activates mTOR. Concerning this, Boppart et al.
(2006), showed that exercise carried out on a treadmill with
some decline degree is able to activate mTOR and p70s6k.
Nonetheless, in genetically modiWed mice that over
expressed 71-integrin protein (a membrane glucoprotein
that belongs to a major family of surface adhesion cellular
receptors), mTOR activation did not occur. Interestingly,
overexpression of 71-integrin, which might be induced
by physical exercise, attenuates muscular damage appearance as much as mTOR activation pathway, suggesting that
integrins act as negative regulators of mTOR.
What implications would lie behind integrin regulation?
Apparently, structural proteins protective eVect would be
the Wrst option. It is already known that integrins play an
important role in structural conservation of skeletal muscle
and that several myopathies are related to the absence of
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Eur J Appl Physiol
71-integrins (Mayer 2003). Therefore, it is possible that
its expression increase works as a chronic protective mechanism against damage caused by muscle contraction in
skeletal muscle cell. However, it is interesting to notice that
trained individuals compared to untrained ones show acute
reduction in protein synthesis after strength training (Phillips et al. 2002). Furthermore, it is equally observed that
trained individuals submitted to strength training show less
activation of the mTOR pathway in comparison with
untrained ones (CoVey et al. 2005), a phenomenon which
must be approached with caution, once there is the possibility that mTOR pathway is chronically increased in trained
individuals. Anyway, it is possible that there is some connection between acute decrease in protein synthesis, mTOR
activation decrease and integrin overexpression as chronic
eVects of mechanical stimuli. Despite the former information, it is important integrin as a negative regulator of
mTOR pathway to be assessed with caution. For instance,
Malik and Parsons (1996) observed that p70s6k activation
was positively related to the integrin presence, at least in
cell culture. Additionally, King et al. (1997) observed that
PI3K/Akt activation is dependent on the integrin presence.
Although these cells were not submitted to mechanical
stimuli and were not even skeletal muscle cells, this data
suggest that the integrin presence is likely to act as positive
mTOR regulators.
Capacity of skeletal muscle cell to sense divergent
mechanical stimuli
A Wnal topic on mechanotransduction concerns the kind of
mechanical force to which the muscle responds. This theme
is of utmost importance to other areas of knowledge. For
instance, in endothelial cells, laminar blood Xow imposes
mechanical forces totally diVerent from oscillatory blood
Xow. This results in very diVerent biochemical responses,
even imitating pathological conditions (De Keulenaer et al.
1998). In order to elucidate this question, Hornberger et al.
(2005) observed that uniaxial stretching of myotubes
(C2C12) activates Akt and ERK (extracellular regulated
protein kinase). However, only multiaxial stretching of
myotubes seems to activate p70s6k. That indicates muscle
cells are also capable of identifying and respond in a speciWc manner to diVerent mechanical stimuli. Nevertheless,
its physiological implications and possible correlations
with the diVerent types of muscle actions remain unclear.
mTOR/p70s6k stimulated by diVerent actions/patterns
of muscle activation
According to studies performed with “in vitro” skeletal
muscle culture cells, diVerent mechanical stimuli (e.g.
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actions/patterns) of in-vivo skeletal muscle contractions are
capable of signaling hypertrophy pathways diVerently. A
classical example to illustrate this is the diVerential hypertrophy process which cardiac muscle exhibits when submitted to diVerent overload patterns of activation.
Similarly to cardiac muscle, skeletal muscle also adapts
in a peculiar manner. However, while for the cardiac muscle eccentric stimuli seem to stretch the Wbers sometimes
weakening them, on the skeletal muscle, the same stimuli
promotes a robust hypertrophy process (Russel et al. 2000).
Beyond muscle arrangement diVerences and the type of
mechanical tension to which Wbers are submitted, it is possible that under speciWc conditions of skeletal muscle contraction, mechano sensitive activators respond with
diVerent magnitudes, as observed in MGF (Goldspink
1999; Mckoy et al. 1999). As an example, Eliasson et al.
(2006) conducted a study with human subjects observing
that maximal eccentric contractions are more eYcient in
activating p70s6k than maximal concentric contractions.
Nader and Esser (2001) observed the same in a study carried out with rodents. One of the hypotheses to explain the
stronger activation is based on the type of mechanical activity induced by eccentric contractions in skeletal muscle.
During eccentric contractions, the muscle is submitted to
both, an increase in its tension and in its length, while for
concentric contractions, the stretching stimuli either does
not occur or occurs with small intensity. Thus, the added
response to both stimuli (e.g. stretching and tension) during
eccentric contraction seems to act on p70s6k. In fact, it has
been recently demonstrated that chronic stretching of skeletal muscle is capable of increasing its mass/length through
activation of mTOR pathway (Aoki et al. 2006). It is worth
saying, though, that this response is not a consensus among
scholars. Cuthbertson et al. (2006) observed equal magnitude of responses in p70s6k activation induced either by
dynamic concentric or eccentric contractions, although in
the last study, subjects were supplemented with amino
acids and sucrose, which are likely to have inXuenced the
observed response.
Apart from the type of muscle action performed, other
interactive factors such as intensity/duration and frequency
of the Wber stimulation are also likely to promote speciWc
signaling responses. Atherton et al. (2005), carried out a
study which tested electric long term and low intensity
stimuli, similar to those promoted by aerobic training,
against electric high intensity and short term stimuli, similar to resistance strength training. It was observed that aerobic exercise stimuli promoted speciWc adaptive responses
toward mitochondrial biogenesis besides slow twitch Wbers
phenotype, not activating mTOR and p70s6k. Resistance
strength training stimuli, however, intensely activated
mTOR and p70s6k pathway. Interestingly, Nader and Esser
(2001) found that both high intensity and low intensity
Eur J Appl Physiol
electrical stimuli promoted activation of p70s6k, with a less
prolonged response promoted by the low intensity electrical
stimuli. The diVerent responses related to low intensity
electrical stimulation in the two studies can be related to the
fact that Atherton et al. (2005) carried out his study in “in
vitro” conditions, while Nader and Esser (2001) carried out
their study in in-vivo conditions. Furthermore, Atherton
et al. (2005) submitted skeletal muscles to contraction during 3 h, while in the Nader and Esser study, skeletal muscle
underwent contraction during only 30 min. These diVerences could generate distinct patterns of substrates depletion (e.g. glycogen content) culminating in the diVerential
activation of energetic sensors inside the muscle cell, such
as AMP kinase (AMPK), which has been shown to suppress protein synthesis and downregulate mTOR signaling
(Bolster et al. 2002). Although not measured in the Nader
and Esser’s (2001) study, Atherton et al. (2005) found that
the increased activation of AMPK was strictly related to the
inhibition of mTOR and p70s6k, which would be likely to
explain the null activation of p70s6k.
There is hardly any information in literature on the eVect
of endurance exercise in the activation of signaling pathways controlling protein synthesis. However, several studies showed an acute increase in muscle protein synthesis
after a single session of aerobic exercise (Carraro et al.
1990; SheYeld-Moore et al. 2004). These eVects are probably mediated by the activation of anabolic signaling pathway. In order to verify the involvement of the mTOR
pathway in this response, Mascher et al. (2007) carried out
a study with human subjects, using cycling exercise. They
observed that cycling exercise was capable of acutely phosphorylating the mTOR (Serine 2448) and p70s6k (Serine
424/Threonine 421). Nevertheless, p70s6k was not activated since phosphorylation of threonine 389 did not occur
(Pullen and Thomas 1997). The lack of p70s6k activation
despite an increase in mTOR phosphorylation suggests
another role for mTOR following endurance exercise.
Therefore, as proposed by Mascher et al. (2007), mTOR
could promote increases in protein synthesis mediated not
only by p70s6k but also via deactivation of eukaryotic elongation factor 2 kinase (eEF2k). Inactivating eEF2k causes a
dephosphorilation of eEF2 in threonine 56 and eEF2
becomes activated, thus increasing the protein synthesis
(Redpath et al. 1993). Actually, it was observed in Mascher
et al. (2007) study, that eEF2 becomes activated following
aerobic exercise. In support of this notion, there is the fact
that the muscle protein synthesis increased in association
with a decrease in the phosphorylation of eEF2 after resistance exercise (Dreyer et al. 2006). Thus far, the way these
pathways can be diVerently activated through diVerent
types of exercise (e.g. resistance or aerobic exercise)
remains unknown. Aerobic exercise is not likely to cause
any marked hypertrophy response. Thus, Mascher et al.
(2007) speculated that, at least in humans, the acute activation of mTOR and the activation of GSK-3 (glycogen synthase kinase-3) without the activation of p70s6k could be
related to an increased rate of the mitochondrial protein
synthesis.
The activation of p70s6k (phosphorylation in Threonine
389) was reported to occur only in response to high force
contractions in rodents (Nader and Esser 2001; Haddad and
Addams 2002; Atherton et al. 2005) as well as in human
subjects (Dreyer et al. 2006; Eliasson et al. 2006) and this
event has been associated with increased protein synthesis
after resistance exercise (Hernandez et al. 2000; Cuthbertson et al. 2006). However, a recent study carried out in
human subjects showed a marked increase in p70s6k during
low intensity resistance exercise, combined with a moderate reduction of blood Xow to the working muscle (Fujita
et al. 2007). Furthermore, in this study, increased protein
synthesis was correlated to p70s6k activation, which
occurred only in response to exercise performed with vascular occlusion (e.g. it did not occur in the low intensity
exercise control group). In the light of the present knowledge, these Wndings suggest that speciWc activation of
p70s6k mediated by diVerent combinations of contraction
patterns continues to be a “marker for the acute characterization of growth stimuli”, as proposed by Baar and Esser
(1999). Additionally, it suggests that other factors unrelated
to mechanical stimuli (e.g. hypoxia, metabolic stress) are
capable of activating it. Nonetheless, whether the divergent
responses related to the activation of mTOR without the
activation of p70s6k can be responsible for the diVerential
translation of speciWc mRNA mediated by diVerent modes
of exercise remains, thus far, unclear.
DiVerential activation of mTOR/p70s6k mediated
by diVerent skeletal muscle Wber types
Traditionally, Wber type discrimination and its contractile
and metabolic properties have been one of the main subjects of research in exercise physiology. Biochemical and
physiological information discerning these features are
widely found in books which discuss adaptive aspects of
strength training/overload (Fleck and Kraemer 1997). However, molecular diVerentiations responsible for such features are still under investigation.
An important observation about the remodeling process
is related to the lower susceptibility that the slow twitch
Wber (type I) shows in comparison with the fast twitch Wber
(type II) in the hypertrophy process (Widrick et al. 2002).
Besides the classical concepts involving diVerent motor
recruitment patterns, molecular mechanisms begin to be
elucidated. Among such mechanisms, a decreased activation
of the mTOR pathway in the skeletal muscle containing
123
Eur J Appl Physiol
slow twitch Wbers appears. For example, Baar and Esser
(1999) reported the phosphorylation of p70s6k to be more
dramatic in the muscle groups containing a greater proportion of type II vs. type I muscle Wbers (e.g. tibialis anterior
vs. soleus). More recently, in a study designed to investigate responsiveness of the skeletal muscle group to mTOR
and p70s6k, it has been observed the skeletal muscle
groups predominantly containing fast twitch Wber (tibialis
anterior, plantaris) to be responsible for activating/phosphorylating mTOR in its 2448 serine end (especially type II
Wber), exhibiting similar pattern of activation of p70s6k
(Parkington et al. 2003). Contrariwise, soleus muscle,
known to have predominantly slow twitch Wbers, does not
show activation/phosphorylation of mTOR. In that study,
the authors used electrical stimulation directly upon the sciatic nerve, which recruits all the motor units inside the
stimulated muscle. Hence, the observed response does not
seem to result from diVerential activation of motor units.
Although coherent, the preferential activation of the mTOR
pathway is not always demonstrated. As an example, Koopman et al. (2006) conducted a study with human subjects
submitted to resistance exercise and reported increased
phosphorylation of p70s6k in the type II Wbers when compared to the type I Wbers, measured in threonine 421/serine
424 residue. However, although the authors did not measure phosphorylation of p70s6k (threonine 389) and ribosomal protein S6 in a Wber type speciWc manner,
phosphorylation of S6 was not increased in homogenates of
the vastus lateralis muscle, suggesting that the greater phosphorylation of p70s6k (thr421/ser424) in the fast Wber type
does not account for its activation. Whether maximal
lengthening contractions are capable of diVerentially activating mTOR or p70s6k in human subjects, though,
remains unclear. Therefore, these results demonstrate, at
least in rodents, the reduced susceptibility that the type I
skeletal muscle Wber presents in the mTOR pathway activation, thus partly explaining its reduced hypertrophy capacity induced by mechanical stimulation. Nevertheless, the
fact that the type I skeletal muscle Wber also increases its
size when induced by mechanical stimulation should be
bore in mind. This suggests that diVerential time courses of
activation or diVerential extents of signaling responses
might explain the diVerential responses observed in human
subjects versus those observed in studies with rodents.
Besides, diVerent forms of the skeletal muscle stimulation
were also used in the above-mentioned studies (voluntary
stimulation vs. electrical stimulation) which might have
diVerently aVected the activated muscles.
Another relevant hypothesis which might explain muscle
Wber type-speciWc eVects on hypertrophy response is that
contracting skeletal muscle Wbers are less eVective of
increasing the protein synthesis and activating the mTOR
pathway (Atherton et al. 2005; Rennie 2005; Goldberg
123
1967). Thus, because the muscle groups containing predominantly slow twitch Wbers are mainly postural or tonic, their
continuous mechanical stimulation does not allow contractile proteins accumulation similar to phasic or fast twitch
Wbers, which usually show much more intermittent mechanical stimulation. However, Mittendorfer et al. (2005)
observed in human subjects neither diVerences of anatomical location nor Wber type composition in the protein synthesis. The observed response was under basal conditions, a
situation which neutralizes diVerential activation of tonic or
phasic Wbers (although the authors did not investigate this
response under overload conditions, which is obviously
diYcult). It is possible diVerent responses related to Wber
types to be speciWc under diVerent mechanical stimulation.
mTOR/p70s6k and the protein synthesis
According to the previously discussed information, the pathways activated by mTOR are highly complex. However, this
knowledge is of the utmost importance, once depending on
the cellular target activated by mTOR, diVerent translational
processes will be recruited. For example, activation (phosphorylation) of p70s6k seems to be involved in translation of
the mRNA containing oligopirimidinic terminations (5⬘
TOP mRNA) important to the mRNA translation coding
ribossomal proteins as well as to increases in translational
eYciency (Ruvinsky and Meyuhas 2006), whereas the
4EBP-1 activation seems to be involved in all kinds of eukariotes mRNA (Lawrence and Abraham 1997). Collectively,
the activation of these eVectors seems to determine
increased skeletal muscle protein synthesis induced by
mechanical stimuli. However, skeletal muscle hypertrophy
selectively occurs through the accumulation of myoWbrillar
proteins and not by sarcoplasmic proteins (Balagopal et al.
2001; Hasten et al. 2000; Rasmussen and Phillips 2003). As
an example, a recently published study has demonstrated
that, in basal conditions, the sarcoplasmic protein synthesis
was higher than the myoWbrillar protein synthesis. However,
after resistance exercise, the stimulation of rates of sarcoplasmic protein synthesis was »20% less than that of myoWbrillar protein synthesis (Cuthbertson et al. 2006).
Likewise, during atrophic state mediated by denervation,
negative remodeling occurs predominantly due to contractile
protein losses (Almon and Dubois 1990). This indicates that
the correct understanding of the translational process of speciWc mRNA (e.g. coding myoWbrillar proteins) seems to be a
crucial aspect of mechanical stimulation inducing skeletal
muscle remodeling. Surprisingly, the way the mRNA coding
contractile proteins are recruited by mTOR under mechanical stimuli remains still not thoroughly understood. The current knowledge is that the general muscle protein synthesis
is regulated via increases in translational eYciency (e.g. ini-
Eur J Appl Physiol
tiation, elongation and termination) or capacity, since muscular mRNA abundance has been observed even in atrophy
states (Welle et al. 1999; Meyuhas 2000; Goldspink 2002;
Goldspink 1999). Once again, however, messages coding
myoWbrillar mRNA are the main responsible for the remodeling process induced by mechanical stimulation. Thus, as a
recommendation to future studies, measurements of both
changes in the rates of protein synthesis (or speciWc protein
synthesis) and signaling proteins alike (including mTOR and
p70s6k), as already observed recently (Dreyer et al. 2006;
Cuthbertson et al. 2006), will help gain insight on the speciWc role of the protein signaling in speciWc muscular adaptations induced by diVerent mechanical stimuli.
Final considerations
Day after day, new mechanistic studies unmask important
physiological responses. Thirty years ago it was speculated
whether the skeletal muscle was capable of generating biochemical events induced by contraction that would culminate in the hypertrophy of the skeletal muscle (Goldberg
et al. 1975). However, owing to the lack of knowledge as
well as to the lack of the necessary technological tools,
three decades went by until the discovery of these mechanochemical mediators took place. Mechanical stimulation
is capable of activating several growth factors including
MGF, a potent stimulator of hypertrophy in the skeletal
muscle. Besides, new mediators responsible for the acute
conversion of the mechanical information into molecular
signals have been studied. One of these mediators is PA.
Although several advances have been attained, other questions remain open to investigation, particularly those
related to the pathway mediated by mTOR. For example,
stimulation induced by nutrients seems to activate common
pathways mediated by growth factors, which, in turn, activate the mTOR/p70s6k (Horberger et al. 2006c). Nonetheless, overexpression of growth factors results in a strong
hypertrophic response (Lai et al. 2004; Barton-Davis et al.
1998; Goldspink and Yang 2001), whereas stimulation
induced by nutrients seems to be much more important in
the inhibition of the atrophic process (Almon and Dubois
1988). This response is further supported by the fact that
overfeeding does not induce additional skeletal muscle protein synthesis (Glick et al. 1982). Likewise, although nutrient induced stimulation does not induce expressive increase
in the skeletal muscle size, its utilization during strength
exercise emphasizes the p70s6k activation and increases
the protein synthesis (Karlsson et al. 2004; Wackerhage
and Rennie 2006). This indicates, once again, that parallel
or synergic pathways can be activated through diVerent
stimuli combinations. As a result, the characterization and
additional detailing regarding the magnitude and duration
of these pathways induced by diVerent combinations (e.g.
diVerent mechanical stimuli and/or nutritional stimuli)
might be important aspects to be considered for a better
understanding of the mTOR signaling pathway.
Acknowledgments We thank Dr. Érico Chagas Caperuto for his
critical reading of the manuscript and Maria Cristina Fioratti Florez for
the careful specialized language revision of the present review article.
References
Almon RR, Dubois DC (1988) Adrenalectomy eliminates both Wbertype diVerences and starvation eVects on denervated muscle. Am
J Physiol 225:E850–E856
Almon RR, Dubois DC (1990) Fiber-type discrimination in disuse and
glucocorticoid induced atrophy. Med Sci Sports Exerc 22:304–311
Aoki MS, Myiabara EH, Soares AG, Saito ET, Moriscot AS (2006)
mTOR pathway inhibition attenuates skeletal muscle growth induced by stretching. Cell Tissue Res 324:149–156
Atherton PJ, Babraj J, Smith K, Singh J, Rennie MJ, Wackerhage H
(2005) Selective activation of AMPK-PGC-1alpha or PKBTSC2-mTOR signaling can explain speciWc adaptive responses to
endurance or resistance training-like electrical muscle stimulation. FASEB J 19:786–798
Baar K, Esser K (1999) Phosphorylation of p70(S6k) correlates with
increased skeletal muscle mass following resistance exercise. Am
J Physiol 45:C120-C127
Balagopal P, Schimke JC, Ades PA, Adey DB, Nair KS (2001) Age
eVect on transcript levels and synthesis rate of muscle MHC and
response to resistance exercise. Am J Physiol 280:E203–E208
Barton-Davis E, Shoturma DI, Musaro A, Rosenthal N, Sweeney Hl
(1998) Viral mediated expression of insulin-like growth factor-1
blocks the aging-related loss of skeletal muscle function. Proc
Natl Acad Sci USA 95:15603–15607
Bloch RJ, Gonzalez-Serratos H (2003) Lateral force transmission across
costameres in skeletal muscle. Exerc Sport Sci Rev 31:73–78
Blomstrand E, Eliasson J, Karlsson HK, Kohnke R. (2006) Branched
chain amino acids activate key enzymes in protein synthesis after
physical exercise. J Nutr 136:269S–273S
Bodine SC (2006) mTOR signaling and the molecular adaptation to
resistance exercise. Med Sci Sports Exerc 38:1950–1957
Bodine SC, Stitt TN, Gonzalez M, Kline WO, Stover GL, Bauerlein R,
Zlotchenko E, Scrimgeour A, Lawrence JC, Glass DJ, Yancopoulos GD (2001) Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo.
Nat Cell Biol 3:1014–1019
Bolster DR, Crozier SJ, Kimball SR, JeVerson LS (2002) AMP-activated protein kinase suppresses protein synthesis in rat skeletal
muscle through downregulated mTOR signalling J Biol Chem
277:23977–23980
Bolster DR, Kubica N, Crozier SJ, Williamson DL, Farrell PA, Kimball SR, JeVerson LS (2003) Immediate response of mammalian
target of rapamycin (mTOR)-mediated signalling following acute
resistance exercise in rat skeletal muscle. J Physiol 15:213–220
Booth FW, Baldwin KM (1966) Muscle plasticity: energy demand and
supply processes. In: Rowell LB, Shepard JT (eds) Handbook of
physiology. Oxford University Press, New York, pp 1074–1123
Boppart MD, Burkin DJ, Kaufman SJ (2006) Alpha7beta1-integrin
regulates mechanotransduction and prevents skeletal muscle injury. Am J Physiol 290:C1660–C1665
Carraro F, Stuart CA, Hartl WH, Rosenblatt J, Wolfe RR (1990) EVect
of exercise and recovery on muscle protein synthesis in human
subjects. Am J Physiol 259:E470–E476
123
Eur J Appl Physiol
Chen J (2004) Novel regulatory mechanisms of mTOR signaling. Curr
Top Microbiol Immunol 279:245–257
Chiang GC, Abraham RT (2005) Phosphorylation of mammalian target of rapamycin (mTOR) at Ser-2448 is mediated by p70s6 kinase. J Biol Chem 280:25485–25490
CoVey VG, Zhong Z, Shield A, Canny BJ, Chibalin AV, Zierath JR,
Hawley JA (2005) Early signaling responses to divergent exercise
stimuli in skeletal muscle from well-trained humans. FASEB J
20:190–200
Cuthberson DJ, Babraj J, Smith K, Wilkes E, Fedele MJ, Esser K, Rennie M (2006) Anabolic signaling and protein synthesis in human
skeletal muscle after dynamic shortenin or lengthening exercise.
Am J Physiol 290:E731–E738
De Keulenaer GW, Chappell DC, Ishizaka N, Nerem RM, Alexander
RW, Griendling KK (1998) Oscillatory and steady laminar
shear stress diVerentially aVect human endothelial redox state:
role of a superoxide-producing NADH oxidase. Circ Res
82:1094–101
Deshmukh A, CoVey VG, Zhong Z, Chibalin AV, Hawley JÁ, Zierath
JR (2006) Exercise-induced phosprolylation of the novel Akt substrates AS 160 and Wlamin A in human skeletal muscle. Diabetes
55:1776–1782
Dreyer HC, Fujita S, Cadenas J, Chinkes DL, Volpi E, Rasmussen BB
(2006) Resistance exercise increases AMPK activity and reduces
4E-BP1 phosphorylation and protein synthesis in human skeletal
muscle. J Physiol 576:613–624
Eliasson J, Elfegoun T, Nilsson J, Kohnke R, Ekblom B, Blomstrand E
(2006) Maximal lengthening contractions increase p70s6k kinase
phosphorylation in human skeletal muscle in the absence of nutritional supply. Am J Physiol 291:E1197–E1205
Ervasti JM (2003) Costameres: the Achilles’ heel of Herculean muscle.
J Biol Chem 278: 13591–13594
Fang Y, Park IH, Wu AL, Du G, Huang P, Frohman MA, Huang P,
Frohman MA, Brown HA, Chen J (2003) PLD1 regulates mTOR
signaling and mediates Cdc42 activation of S6K1. Curr Biol
13:2037–2044
Fleck SJ, Kraemer WJ (1997) Muscle physiology. In: Designing resistance training programs. Human kinetics, Ilinois, pp 45–63
Fry AC (2004) The role of resistance exercise intensity on muscle Wbre
adaptations. Sports Med 34:663–679
Fujita S, Abe T, Drummond MJ, Cadenas JG, Dreyer HC, Sato Y, Volpi E, Rasmussen BB (2007) Blood Xow restriction during lowintensity resistance exercise increases S6K1 phosphorylation and
muscle protein synthesis. J Appl Physiol 103:903–910
Glick Z, Mcnurlan MA, Garlick PJ (1982) Protein synthesis rate in liver and muscle of rats following four days of overfeeding. J Nutr
112:391–397
Goldberg AL (1967) Protein synthesis in tonic and phasic skeletal
muscles. Nature 216:1219–1220
Goldberg AL, Etlinger JD, Goldspink DF, Jablecky C (1975) Mechanism of workinduced hypertrophy of skeletal muscle. Med Sci
Sports 7:185–198
Goldspink G (1999) Changes in muscle mass and phenotype and the
expression of autocrine and systemic growth factors by muscle in
response to stretch and overload. J Anat 194:323–334
Goldspink G (2002) Gene expression in skeletal muscle. Biochem Soc
Trans 30:285– 290
Goldspink G, Booth F (1992) General remarks: mechanical signals and
gene expression in muscle. Am J Physiol 262:R327–R328
Goldspink G, Yang SY (2001) Method of treating muscular disorders.
United States Patent. Patent N° US 6,221,842 B1, April 24
Grounds MD, Sorokin L, White J (2005) Strength at the extracellular
matrix–muscle interface. Scand J Med Sci Sports 15:347–358
Haddad F, Adams GR (2002) Selected contribution: acute cellular and
molecular responses to resistance exercise. J Appl Physiol
93:393–403
123
Hameed M, Orrell RW, Cobbold M, Goldspink G, Harridge SDR
(2003) Expression of IGF-1 splice variants in young and old human skeletal muscle after high resistance exercise. J Physiol
547:247–254
Hasten DL, Pak-Loduca KA, Obert KE, Yarasheski KE (2000) Resistance exercise acutely increases MHC and mixed muscle protein
synthesis rates in 78–84 and 23–32 yr olds. Am J Physiol
278:E620–E626
Hernandez JM, Fedele MJ, Farrell PA (2000) Time course evaluation
of protein synthesis and glucose uptake after acute resistance
exercise in rats. J Appl Physiol 88:1142–1149
Holz MK, Blenis J (2005) IdentiWcation of S6 kinase 1 as a novel mammalian target of rapamycin (mTOR)- phosphorylating kinase. J
Biol Chem 280:26089–26093
Hong JH, Oh SO, Lee M, Kim YR, Kim DU, Hur GM, Lee JH, Lim K,
Hwang BD, Park SK (2001) Enhancement of lysophosphatidic
acid-induced ERK phosphorylation by phospholipase D1 via the
formation of phosphatidic acid. Biochem Biophys Res Commun
281:1337–1342
Hornberger TA, Chien S (2006) Mechanical stimuli and nutrients regulate rapamycin-sensitive signaling through distinct mechanisms
in skeletal muscle. J Cell Biochem 97:1207–1216
Hornberger TA, McLoughlin TJ, Leszczynski JK, Armstrong DD,
Jameson RR, Bowen PE, Hwang ES, Hou H, Moustafa ME, Carlson BA, HatWeld DL, Diamond AM, Esser KA (2003) Selenoprotein-deWcient transgenic mice exhibit enhanced exercise-induced
muscle growth. J Nutr 133:3091–3097
Hornberger TA, Armstrong DD, Koh TJ, Burkholder TJ, Esser K
(2005) Intracellular signaling speciWcity in response to uniaxial
vs. multiaxial stretch: implications for mechanotransduction. Am
J Physiol 288:C185–C194
Hornberger TA, Stuppard R, Conley KE, Fedele MJ, Fiorotto ML,
Chin ER, Esser KA (2004) Mechanical stimuli regulate rapamycin sensitive signaling by a phosphoinositide 3-kinase-, protein
kinase B- and growth factor-independent mechanism. Biochem J
380:795–804
Hornberger TA, Mak YW, Hsiung JW, Huang SA, Chien S (2006a)
The role of phospholipase D and phosphatidic acid in the mechanical activation of mTOR signaling in skeletal muscle. Proc Natl
Acad Sci USA 103:4741–4746
Hornberger TA, Sukhija KB, Chien S (2006b) Regulation of mTOR by
mechanically induced signaling events in skeletal muscle. Cell
Cycle 13:1391–1396
Ingber DE (2006) Cellular mechanotransduction: putting all the pieces
together again. FASEB J 20:811–827
Jacinto E, Hall MN (2003) Tor signalling in bugs, brain and brawn. Nat
Rev Mol Cell Biol 4:117–126
Kam Y, Exton JH (2004) Role of phospholipase D1 in the regulation of
mTOR activity by lysophosphatidic acid. FASEB J 18:311–319
Karlsson HK, Nilsson PA, Nilsson J, Chibalin AV, Zierath JR, Blomstrand E (2004). Branched-chain amino acids increase p70s6k
phosphosphorylation in human skeletal muscle after resistance
exercise. Am J Physiol 287:E1–E7
King WG, Mattaliano MD, Chan TO, Tsichlis PN, Brugge JS (1997)
Phosphatidylinositol 3-kinase is required for integrin-stimulated
akt and raf-1/mitogen-activated protein kinase pathway activation. Mol Cell Biol 17:4406–4418
Koopman R, Zorenc AHG, Gransier RJJ, Cameron-Smith D, Van
Loon LJC (2006) Increase in S6K1 phosphorylation in human
skeletal muscle following resistance exercise occurs mainly in
type II muscle Wbers. Am J Physiol 290:E1245–E1252
Kubica N, Bolster D, Farrell PA, Kimball SR, JeVerson L (2005)
Resistance exercise increases muscle protein synthesis and translation of eukaryotic initiation factor 2Be mRNA in a mammalian
target of rapamycin-dependent manner. J Biol Chem 280:7570–
7580
Eur J Appl Physiol
Lai KM, Gonzalez M, Poueymirou WT, Kline WO, Na E, Zlotchenko
E, Stitt TN, Economides AN, Yancopoulos GD, Glass DJ (2004)
Conditional activation of Akt in adult skeletal muscle induces
rapid hypertrophy. Mol Cell Biol 24:9295–9304
Lawrence JC, Abraham RT (1997) PHAS/4E-BPs as regulators of
mRNA translation and cell proliferation. Trends Biochem Sci
22:345–349
Lee S, Park JB, Kim JH, Kim Y, Kim JH, Shin KJ, Lee JS, Ha SH, Suh
PG, Ryu SH (2001) Actin directly interacts with phospholipase D,
inhibiting its activity. J Biol Chem 276:28252–28260
Malik RK, Parsons JT (1996) Integrin-dependent activation of the ribosomal s6 kinase signaling pathway. J Biol Chem 271:29785–29791
Mascher H, Andersson H, Nilsson PA, Ekblom B, Blomstrand E
(2007) Changes in signaling pathways regulating protein synthesis in human muscle in the recovery period after endurance exercise. Acta Physiol 191:67–75
Mayer U (2003) Integrins: redundant or important players in skeletal
muscle? J Biol Chem 278:14587–14590
McKoy G, Ashley W, Mander J, Yang SY, Williams N, Russell B,
Goldspink G (1999) Expression of insulin growth factor-1 splice
variants and structural genes in rabbit skeletal muscle induced by
stretch and stimulation. J Physiol 516:583–592
Meyuhas O (2000) Synthesis of the translational apparatus is regulated
at the translational level. Eur J Biochem 267:6321–6330
Mittendorfer B, Andersen JL, Plomgaard P, Saltin B, Babraj JA, Smith
K, Rennie MJ (2005) Protein synthesis rates in human muscles:
neither anatomical location nor Wbre-type composition are major
determinants. J Physiol 563:203–211
Nader GA, Esser K (2001) Intracellular signaling speciWcity in skeletal
muscle in response to diVerent modes of exercise. J Appl Physiol
90:1936–1942
Nader GA, McLoughlin TJ, Esser KA (2005) mTOR function in skeletal muscle hypertrophy: increased ribosomal RNA via cell cycle
regulators. Am J Physiol 289:C1457–C1465
Nave BT, Ouwens DJ, Withers DR, Shepherd A, Shepherd PR (1999)
Mammalian target of rapamycin is a direct target for protein kinase B: identiWcation of a convergence point for opposing eVects
of insulin and amino-acid deWciency on protein translation. Biochem J 344:427–431
Ohanna M, Sobering AK, Lapointe T, Lorenzo L, Praud C, Petroulakis
E, Sonenberg N, Kelly PA, Sotiropoulos A, Pende M (2005) Atrophy of S6k1 ¡/¡ skeletal muscle cells reveals distinct mTOR
eVectors for cell cycle and size control. Nat Cell Biol 7:286–294
Pallafacchina G, Calabria E, Serrano A, Kalhovde JM, SchiaVino S
(2002) A protein kinase B-dependent and rapamycin sensitive
pathway controls skeletal muscle growth but not Wber type speciWcation. Proc Natl Acad Sci USA 99:9213–9218
Park JB, Kim JH, Kim Y, Há SH, Yoo JS, Du G, Frohman MA, Suh PG,
Ryu SH (2000) Cardiac phospholipase D2 localizes to sarcolemmal
membranes and is inhibited by alpha-actinin in an ADP-ribosylation factor-reversible manner. J Biol Chem 275:21295–21301
Parkington JD, Siebert AP, Lebrasseur NK, Fielding RA (2003) DiVerential activation of mTOR signaling by contractile activity in
skeletal muscle. Am J Physiol 285:R1086–R1090
Peterson RT, Beal PA, Comb MJ, Schreiber SL (2000) FKBP12-Rapamycin-associated protein (FRAP) autophosphorylates at serine
2481 under translationally repressive conditions. J Biol Chem
275:7416–7423
Phillips SM, Parise G, Roy BD, Tipton KD, Wolfe RR, Tarnopolsky
MA (2002) Resistance-training-induced adaptations in skeletal
muscle protein turnover in fed state. Can J Physiol Pharmacol
80:1045–1053
Pullen N, Thomas G (1997) The modular phosphorylation and activation of p70s6k. FEBS Lett 410:78–83
Rasmussen BB, Phillips SM (2003) Contractile and nutritional regulation of human muscle growth. Exerc Sports Sci Rev 31:127–131
Redpath NT, Price NT, Severinov KV, Proud GC (1993) Regulation of
elongation factor-2 by multisite phosphorylation. Eur J Biochem
213:689–699
Reiling JH, Sabatini DM (2006) Stress and mTORture signaling.
Oncogene 25:6373–6383
Rennie MJ (2005) Why muscle stops building when it’s working. J
Physiol 569:3
Rennie MJ, Wackerhage H (2003) Connecting the dots for mechanochemical transduction in muscle. J Physiol 553:1
Reynolds TH, Bodine SC, Lawrence Jr JC (2002) Control of Ser 2448
phosphorylation in the mammalian target of rapamycin by insulin
and skeletal muscle load. J Biol Chem 277:17657–17662
Rommel C, Bodine SC, Clarke BA, Rossman R, Nunez L, Stitt TN,
Yancopoulos GD, Glass DJ (2001) Mediation of IGF-1 induced
skeletal myotube hypertrophy by PI(3)K/Akt/mTOR and PI(3)K/
Akt/GSK3 pathways. Nat Cell Biol 3:1009–1013
Russell B, Motlagh D, Ashley W (2000) Form follows function: how
muscle shape is regulated by work. J Appl Physiol 88:1127–1132
Ruvinsky I, Meyuhas O (2006) Ribossomal protein S6 phosphorylation: from protein synthesis to cell size. Trends Biochem Sci
31:342–348
Sakamoto K, Hirshman MF, Aschenbach WG, Goodyear LJ (2002)
Contraction regulation of Akt in rat skeletal muscle. J Biol Chem
277:11910–11917
Sakamoto K, Aschenbach WG, Hirshman MF, Goodyear LJ (2003)
Akt signaling in skeletal muscle: regulation by exercise and passive stretch. Am J Physiol 285:E1081–E1088
Sakamoto K, Arnolds DE, Ekberg I, Thorell A, Goodyear LJ (2004)
Exercise regulates Akt, glycogen synthase kinase-3 activities in human skeletal muscle Biochem Biophys Res Commun 319:419–425
Sarbassov D, Ali SM, Sabatini DM (2005) Growing roles for the
mTOR pathway. Curr Opin Cell Biol17:596–603
Schmelzle T, Hall MN (2000) TOR, a central controller of cell growth.
Cell 103:253–262
Scott PH, Brunn GJ, Konh AD, Roth RA, Lawrence Jc-JR (1998) Evidence of insulin-stimulated phosphorylation and activation of the
mammalian target of rapamycin mediated by a protein kinase B
signaling pathway. Proc Natl Acad Sci USA 95:7772–7777
SheYeld-Moore M, Yeckel CW, Volpi et al\ (2004) Post exercise protein metabolism in older and younger men following moderate
intensity aerobic exercise. Am J Physiol 287:E513–E522
Stokoe D, Stephens LR, Copeland T, GaVney PR, Reese CB, Painter
GF, Holmes AB, Mccormick F, Hawkins PT (1997) Dual role of
phosphatidylinositol-3,4,5-trisphosphate in the activation of protein kinase B. Science 277:567–701
Thiimmaiah KL, Easton JB, Germain GS, Morton CL, Kamath S, Buolamwini JK, Houghton PJ (2005) IdentiWcation of N10-substituted phenoxazines as potent and speciWc inhibitors of Akt
signaling. J Biol Chem 280:31924–31935
Tidball JG (2005) Mechanical signal transduction in skeletal muscle
growth and adaptation. J Appl Physiol 98:1900–1908
Timson BF (1990) Evaluation of animal models for the study of exercise-induced muscle enlargement. J Appl Physiol 69:1935–1945
Turinsky J, Damrau-Abney A (1999) Akt kinases and 2-deoxyglucose
uptake in rat skeletal muscles in vivo: study with insulin and exercise. Am J Physiol 276:R277–R282
Wackerhage H, Rennie MJ (2006) How nutrition and exercise maintain the human musculoskeletal mass. J Anat 208:451–458
Welle S, Bhatt K, Thornton CA (1999) Stimulation of myoWbrillar synthesis by exercise is mediated by more eYcient translation of mRNA. J Appl Physiol 86:1220–1225
Widrick JJ, Stelzer JE, Shoepe TC, Garner DP (2002) Functional properties of human muscle Wbers after short-term resistance exercise
training. Am J Physiol 283:R408–R416
Wullschleger S, Loewith R, Hall MN (2006) TOR signaling in growth
and metabolism. Cell 124:471–484
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