Download Differential Effects of Rapamycin on Mammalian Target of

[CANCER RESEARCH 63, 8451– 8460, December 1, 2003]
Differential Effects of Rapamycin on Mammalian Target of Rapamycin Signaling
Functions in Mammalian Cells
Aimee L. Edinger,1 Corinne M. Linardic,2 Gary G. Chiang,3 Craig B. Thompson,1 and Robert T. Abraham3
1
Abramson Family Cancer Research Institute, University of Pennsylvania, Philadelphia, Pennsylvania; 2Departments of Pediatrics and Pharmacology and Cancer Biology, Duke
University, Durham, North Carolina; and 3Program in Signal Transduction Research, The Burnham Institute, La Jolla, California
ABSTRACT
Rapamycin and its analogues have shown promising anticancer activities in preclinical and clinical studies. However, the mechanism whereby
rapamycin inhibits signaling through the mammalian target of rapamycin
(mTOR) remains poorly understood. Here, we show that the FKBP12/
rapamycin complex is an essentially irreversible inhibitor of mTOR kinase activity in vitro. However, we observe no suppression of mTOR
catalytic activity after immunoprecipitation from rapamycin-treated cells.
These results suggest either that rapamycin acts as a reversible kinase
inhibitor in intact cells or that the cellular effects of rapamycin are not
mediated through global suppression in mTOR kinase activity. To better
understand the cellular pharmacology of rapamycin, we compared the
individual and combined effects of rapamycin and kinase-inactive mTOR
expression on a panel of mTOR-dependent cellular responses. These
studies identified glycolytic activity, amino acid transporter trafficking,
and Akt kinase activity as novel, mTOR-modulated functions in mammalian cells. Whereas kinase-inactive mTOR did not enhance the decreases
in cell size and glycolysis induced by rapamycin, expression of this mTOR
mutant significantly enhanced the inhibitory effects of rapamycin on cell
proliferation, 4EBP1 phosphorylation, and Akt activity. Unexpectedly,
amino acid transporter trafficking was perturbed by kinase-inactive
mTOR but not by rapamycin, indicating that this process is rapamycin
insensitive. These results indicate that rapamycin exerts variable inhibitory actions on mTOR signaling functions and suggest that direct inhibitors of the mTOR kinase domain will display substantially broader
anticancer activities than rapamycin.
INTRODUCTION
The TOR4 proteins were first identified during a screen for mutations that suppressed the growth-inhibitory effects of rapamycin in
budding yeast (1– 4). TOR homologs were subsequently identified in
flies and mammals and, like the yeast TORs, shown to be the relevant
targets of rapamycin in these organisms (5–10). Based on its exquisite
specificity for TOR as an intracellular target, rapamycin has been
broadly used as a chemical probe to delineate TOR-dependent responses in eukaryotic cells. Indeed, our understanding of the functions
of mTOR stems largely from observations made with rapamycintreated cells.
Genetic analyses in budding yeast offered strong evidence to support the conclusion that rapamycin is actually a pro-drug that is
converted to the proximate inhibitor of TOR via the formation of a
complex with the immunophilin FKBP12 (11). Although it is generally assumed that FKBP12 is a requisite cofactor for rapamycin
Received 6/26/03; revised 8/29/02; accepted 9/3/03.
Grant support: Fellowship from the Helen Hay Whitney Foundation (A. L. E.), NIH
grant CA76193 and grant from Johnson and Johnson (R. T. A.), NCI grant 5T32CA09307
(Research Training and Cancer Chemotherapy; C. M. L.), and in part by a NCI grant
(C. B. T.).
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance with
18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Robert T. Abraham, Program in Signal Transduction Research,
The Burnham Institute, 10901 North Torrey Pines Road, La Jolla, California 92037.
Phone: (858) 646-3182; Fax: (858) 713-6274; E-mail: [email protected].
4
The abbreviations used are: TOR, target of rapamycin; FRB, FKBP12/rapamycin
binding; TORC, TOR-containing complex; PI3k, phosphatidylinositol 3⬘-kinase; PI, propidium iodide; RIPA, radioimmunoprecipitation assay; GST, glutathione S-transferase;
HEK, human embryonic kidney; ␮Ci, microcurie; HA, hemagglutinin.
activity in all eukaryotic cell types, this notion has not been tested
rigorously in systems other than budding yeast. It is particularly
noteworthy in this regard that studies of rapamycin action in the
fission yeast led to the conclusion that the FKBP12/rapamycin complex was not responsible for the suppressive effect of this drug on
sexual development in this organism (12). The latter results raise the
possibility that the pharmacological actions of rapamycin in mammalian cells may not be entirely dependent on the formation of FKBP12/
rapamycin complexes.
A related area of uncertainty surrounds the mechanism whereby
this drug interferes with TOR signaling in eukaryotic cells. Several
reports have documented that exposure of anti-mTOR immunoprecipitates to FKBP12/rapamycin leads to inhibition of mTOR kinase
activity (13–15). However, it is noteworthy that the FRB domain of
mTOR lies outside of the catalytic domain. Thus, although the binding
of FKBP12/rapamycin to the FRB domain may affect the catalytic
activity of mTOR through an allosteric mechanism, we cannot exclude the possibility that the drug perturbs the interactions of mTOR
with critical regulatory proteins and/or its downstream target proteins.
Given the interest in rapamycin as a potential anticancer agent, a
detailed knowledge of the underlying pharmacology is crucial if we
are to understand how this drug affects tumor growth in vivo, and it
may facilitate the creation of second-generation inhibitors of mTOR
as cancer therapeutics.
A compelling body of genetic evidence indicates that TOR is a
central regulator of cell growth in budding yeast and flies (reviewed
in Refs. 16 and 17). In contrast, our understanding of mTOR function
rests almost entirely on studies of rapamycin-treated mammalian cells.
This approach to the study of mTOR has been highly informative;
however, an important caveat is that in budding yeast, the phenotypic
consequences of rapamycin exposure are substantially less severe than
those induced by the depletion of both TOR proteins. TOR2 is an
essential gene in this organism, whereas rapamycin exposure produces
G1 arrest but not cell death (18). The segregation of TOR functions
based on rapamycin sensitivity can now be explained by the existence
of two different TORCs in yeast. TORC1 contains either TOR1 or
TOR2 and is functionally suppressed by rapamycin. On the other
hand, the second complex, TORC2, contains only TOR2 and is not
susceptible to rapamycin (19). In contrast to yeast, mammalian cells
express a single TOR protein, and the available evidence strongly
supports the existence of a TORC1-like complex but not a TORC2like complex in these cells (19 –21). Nonetheless, the potential existence of multiple mTOR complexes in mammalian cells raises the
possibility that the downstream events governed by mTOR may also
show variable sensitivities to rapamycin.
Interest in mTOR as an anticancer drug target has surged recently,
based in part on reports that rapamycin and related compounds exert
selective cytostatic/cytotoxic effects on PTEN ⫺/⫺ tumors in vivo
(22, 23). The PTEN tumor suppressor protein is lost or mutated in
many human cancers, particularly those that have progressed to an
advanced stage (24, 25). Loss of PTEN leads to deregulated signaling
through the PI3k pathway and, in turn, to the generation of a host of
cell growth- and survival-promoting signals. A pivotal target for
PI3k-derived second messengers is the proto-oncogene product Akt.
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mTOR POSSESSES RAPAMYCIN-RESISTANT ACTIVITY
MnCl2, and 1 mM DTT], and kinase reactions were initiated with 1 ␮g of
GST-p70S6k fragment (amino acids 332– 414), 10 ␮M ATP, and 10 ␮Ci of
[␥-32P]ATP (6000 Ci/mmol; DuPont NEN). Reactions were incubated for 20
min at 30°C and terminated with SDS-PAGE sample buffer.
For Akt kinase assays, HEK 293 cells were transfected with a HA-tagged
wild-type Akt expression vector. After 48 h, the transfected cells were harvested in lysis buffer [50 mM Tris-Cl (pH 7.4), 100 mM NaCl, 50 mM
␤-glycerophosphate, 10% glycerol, 1% Triton X-100, 1 mM EDTA, 10 mg/ml
aprotinin, 1 mg/ml pepstatin A, 10 mg/ml leupeptin, 2 mM phenylmethylsulfonyl fluoride, 20 ␮M microcystin-LR, and 25 mM NaF]. Clarified lysates were
immunoprecipitated with anti-HA antibody (12CA5). Immune complexes were
collected on protein A-Sepharose (Sigma) prebound with rabbit antimouse
antibodies (Pierce) and washed three times in lysis buffer and one time in Akt
kinase buffer [25 mM Tris-HCl (pH 7.4), 5 mM ␤-glycerophosphate, 10%
MgCl2, and 1 mM DTT]. Immunoprecipitates were resuspended in Akt kinase
buffer, and kinase reactions were initiated with 1 ␮g of GST-mTOR RD
fragment (amino acids 2405–2517; Ref. 28), 10 ␮M ATP, 10 ␮Ci of
[␥-32P]ATP (6000 Ci/mmol; DuPont NEN). Reactions were incubated for 20
min at 30°C and terminated with SDS-PAGE sample buffer.
GST-FKBP12-Binding Assays. Five hundred ␮g of ammonium sulfatefractionated rat brain extract (9) were incubated with 5 ␮g of purified GSTFKBP12 (9) and either 10 ␮M rapamycin or ethanol vehicle alone. GSTFKBP12 was collected by incubation with glutathione-Sepharose (Amersham
Biosciences), and the resulting precipitates were washed once with precipitation buffer [50 mM Tris-HCl (pH 7.4), 100 mM NaCl, 50 mM ␤-glycerophosphate, 10% glycerol (w/v), 1 mM EDTA, 10 ␮g/ml aprotinin, 1 ␮g/ml pepstatin
A, 10 ␮g/ml leupeptin, 2 mM phenylmethylsulphonyl fluoride, 20 ␮M microcystin-LR,
and 25 mM NaF] containing 0.02% Tween 20. The precipitates
MATERIALS AND METHODS
were washed three times as indicated in precipitation buffer supplied with
Materials. Rapamycin was purchased from Calbiochem, and L-mimosine one of the following detergent mixtures: 0.02% Tween 20; 1% Tween 20;
and BrdUrd were from Sigma. The anti-mTOR, anti-phospho-4EBP (37/45), 1% Triton X-100; 1% NP40; 0.5% 3-[(3-cholamidopropyl)dimethylammonio]anti-4EBP1, anti-phospho-p70S6k (421/424), anti-p70S6k, anti-phospho- 1-propanesulfonic acid; or modified RIPA buffer [50 mM Tris-HCl (pH 7.4),
p44/42 MAP kinase, anti-p44/42 MAP kinase, anti-phospho-S6 (235/236), 150 mM NaCl, 1 mM EDTA, 1% NP40, 1% sodium deoxycholate, and 0.1%
anti-S6, anti-phospho-Akt (Ser473), anti-Akt, and horseradish peroxidase- SDS]. For those samples indicated “high salt,” samples were washed an
conjugated goat antirabbit antibodies were purchased from Cell Signaling additional time with high-salt buffer [100 mM Tris-HCl (pH 7.4) and 500 mM
Technologies. The anti-phospho-Akt (Thr308) antibody was from Affinity LiCl]. Samples were resuspended in SDS-PAGE sample buffer, resolved by
Bioreagents. The anti-4F2hc and the FITC-conjugated anti-BrdUrd antibodies SDS-PAGE, transferred to polyvinylidene difluoride membranes, and immuwere obtained from BD PharMingen. The monoclonal AU-1 and 12CA5 noblotted with anti-mTOR antibodies.
FL5.12 Cell Immunoblotting. Cells were washed with PBS and lysed in
antibodies were from Babco, and the horseradish peroxidase-conjugated goat
antimouse antibody was purchased from Promega. PI, Hoechst 33342, and RIPA buffer [150 mM NaCl, 1% NP40, 0.5% sodium deoxycholate, 0.1% SDS,
Alexa488-conjugated secondary were from Molecular Probes. Tritiated glu- and 50 mM Tris-HCl (pH 8.0)] containing protease and phosphatase inhibitors
cose was from Amersham Biosciences. The bicinchoninic acid protein assay [Complete (Roche) and Phosphatase Inhibitor Set 1 (Calbiochem)]. Fifty to
100 ␮g clarified lysate/lane were loaded onto Tris-glycine SDS-PAGE gels
kit (Pierce) was used to determine total protein in cell lysates.
Cell Culture. HEK 293 cells were maintained in DMEM containing 10% (Invitrogen). Proteins were transferred to nitrocellulose, and membranes were
FCS, antibiotics, and L-glutamine. All mTOR constructs were expressed in blocked with BLOTTO (5% nonfat dry milk and 0.1% Tween 20 in PBS) and
HEK 293 cells in pcDNA3 (Invitrogen) and transfected using FuGene 6 incubated with the indicated antibodies before probing with enhanced chemi(Roche Molecular Biochemicals, Indianapolis, IN). Plasmids were linearized luminescence (Amersham Biosciences).
Measurement of Cellular Growth and Proliferation. The growth rate of
with SalI before transfection. Stable transfectants were selected with 1 mg/ml
G418 (Invitrogen). Clonogenic assays were performed by seeding equal num- HEK 293 cells was determined with a CellTiter 96 Aqueous One Solution Cell
bers of HEK 293 cells into 60-mm dishes. The medium was replaced every 3rd Proliferation kit (Promega). FL5.12 proliferation assays were performed by
day until colonies were visible to the naked eye. Colonies were stained with plating 50,000 cells/ml in the presence or absence of 20 nM rapamycin and
crystal violet (0.1% in 20% methanol). FL5.12 cells were maintained in RPMI measuring cell number at 24-h intervals with a Coulter Z2 particle analyzer.
supplemented with 10% FCS, 8% WEHI-conditioned medium, 10 mM HEPES, For size analyses, live cells were incubated in medium containing 10 ␮g/ml
55 ␮M ␤-mercaptoethanol, antibiotics, and L-glutamine. All experiments were Hoechst 33342 (Molecular Probes) and 10 ␮g/ml PI for 30 min at 37°C and
conducted in medium containing 500 pg/ml recombinant IL-3 (BD PharMin- analyzed with a Becton Dickinson LSR flow cytometer. To confirm that
gen). mTOR constructs were stably expressed in FL5.12 cells using the EF6 L-mimosine blocked DNA synthesis, cells were incubated for 1 h with 10 ␮M
BrdUrd, washed, fixed, and stained with anti-BrdUrd FITC as recommended
vector (Invitrogen) and Blasticidin S (Invitrogen) selection.
Immunoprecipitations and Kinase Assays. For mTOR immunoprecipi- by the manufacturer.
Fluorescence Microscopy. FL5.12 cells were fixed for 10 min at room
tations, cells were solubilized in lysis buffer [50 mM Tris-HCl (pH 7.4), 100
mM NaCl, 50 mM ␤-glycerophosphate, 10% glycerol (w/v), 1% Tween 20, 1 temperature in 1% paraformaldehyde in PBS. Cells were permeabilized with
wash buffer (2% FCS and 0.03% saponin in PBS) and then incubated sequenmM EDTA, 10 ␮g/ml aprotinin, 1 ␮g/ml pepstatin A, 10 ␮g/ml leupeptin, 2
mM phenylmethylsulfonyl fluoride, 20 ␮M microcystin-LR, and 25 mM NaF]. tially with primary and secondary antibodies for 30 min at room temperature
Clarified lysates were immunoprecipitated with anti-mTOR antibodies. Im- in PBS containing 10% FCS and 0.3% saponin. Cells were evaluated on a
mune complexes were collected on protein A-Sepharose (Sigma) and washed Nikon E800 fluorescence microscope equipped with a CCD camera, and
three times in lysis buffer, once in high-salt buffer [100 mM Tris-HCl (pH 7.4) images were analyzed using the Metamorph software package.
Measurement of Glycolytic Rate. One million FL5.12 cells were resusand 500 mM LiCl], and once in mTOR kinase wash buffer [10 mM HEPES (pH
7.4), 50 mM NaCl, 50 mM ␤-glycerophosphate, and 10% glycerol]. Immuno- pended in 0.5 ml of RPMI 1640 that had been pre-equilibrated in a 37°C
precipitates were resuspended in mTOR kinase assay buffer [10 mM HEPES incubator under 5% CO2. Ten ␮Ci of 5-[3H]glucose were added to each well,
(pH 7.4), 50 mM NaCl, 50 mM ␤-glycerophosphate, 10% glycerol, 10 mM and samples were incubated for 1 h at 37°C in a humidified incubator under
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Oncogenic versions of Akt promote cell growth and survival through
a mTOR-dependent mechanism (26). Thus, the rapamycin sensitivity
of PTEN-deficient tumors may stem from an acquired “addiction” to
the PI3k-AKT signaling pathway, which increases the dependency of
such tumors on mTOR signaling functions. Based on encouraging
results in Phase I clinical cancer trials, three rapamycin analogues,
CCI-779 (Wyeth-Ayerst), AP23573 (Ariad Pharmaceuticals), and
RAD001 (Novartis), are in Phase II and III trials in patients with renal
cancer and other tumors (27). These clinical studies have validated
mTOR as an anticancer drug target and have fueled broad interest in
the development of novel compounds that inhibit this protein kinase
through mechanisms distinct from that of rapamycin.
The objectives of the present study were to examine further the
impact of rapamycin on mTOR signaling functions and to determine
the individual and combined effects of rapamycin treatment and
kinase-inactive mTOR expression on a battery of mTOR-dependent
cellular responses. We identified three categories of mTOR-dependent
responses that ranged from fully sensitive to rapamycin to largely
resistant to this mTOR inhibitor. These findings have important implications both for the use of rapamycin as a probe to analyze
mTOR-dependent signaling pathways in mammalian cells and for the
future development of mTOR inhibitors as cancer chemotherapeutic
agents.
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mTOR POSSESSES RAPAMYCIN-RESISTANT ACTIVITY
5% CO2. Reactions were terminated with 0.5 ml of 0.2 N HCl, and 100 ␮l of
the cell/HCl mixture were added to open PCR tubes, which were then placed
upright in 4-ml scintillation vials containing 0.5 ml of H2O. The vials were
capped, sealed with parafilm, and incubated for 2 days at room temperature.
During the incubation, [3H]2O generated by glycolysis diffused from the PCR
tube into the H2O in the scintillation vial through evaporation and condensation. The contents of the PCR tube were transferred to a new scintillation vial
with 0.5 ml of H2O, the PCR tube was discarded, and scintillation fluid was
added to both the original (diffused counts) and second (undiffused counts)
vials. Vials were evaluated for tritium content with a Wallac Microbeta 1450.
The fraction of [3H]2O that diffused in 2 days was determined with a control
PCR tube containing 1 ␮Ci of [3H]2O. The background diffusion ratio was
determined with a cell-free control sample. To calculate the glycolytic rate, the
sample diffusion ratio (diffused counts/undiffused counts) minus the background diffusion ratio was divided by the diffusion fraction from the [3H]2O
control. This number was multiplied by 5500 (the nmols of glucose in 0.5 ml
of RPMI) to obtain the nmol glucose consumed/million cells/h.
RESULTS
Cellular Treatment with Rapamycin Does Not Lead to Irreversible Inhibition of mTOR Kinase Activity. During early attempts to isolate mTOR by affinity purification over a FKBP12/
rapamycin column, we made the empirical observation that once
formed, the ternary complex of FKBP12/rapamycin-mTOR was
poorly reversible under nondenaturing conditions (13). To reexamine
this finding in a more systematic fashion, we precipitated mTOR from
rat brain extract with an immobilized GST-FKBP12 fusion protein
loaded with rapamycin (Fig. 1A). As predicted, precipitation of mTOR
by GST-FKBP12 was completely dependent on the presence of rapamycin. To evaluate the stability of this complex, parallel samples were
washed with buffered solutions containing nonionic detergents
(Tween 20, NP40, and Triton X-100), ionic detergents (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid and RIPA
buffer), or nonionic detergent plus high-salt concentrations (0.5 M
LiCl). The GST-FKBP12/rapamycin-bound mTOR was not dislodged
from the affinity matrix by any of the wash buffers used in this
experiment.
Based on these results, we reasoned that endogenous FKBP12/
rapamycin-mTOR complexes should easily survive cell lysis and
precipitation in 1% Tween 20-containing, isotonic extraction buffer.
Given that the interaction with FKBP12/rapamycin strongly inhibits
mTOR kinase activity (13–15), we would expect that formation of the
ternary complex in rapamycin-treated cells would lead to a clear
reduction in the protein kinase activity present in anti-mTOR immunoprecipitates from these cells. To examine the impact of rapamycin
treatment on mTOR kinase activity, HEK 293 cells were serum
starved for 16 h and incubated for 30 min with 100 nM rapamycin or
10 ␮M wortmannin. Cells were then stimulated for 10 min with serum,
and mTOR activity was determined in immune complex kinase assays
(Fig. 1B). Treatment of the cells with rapamycin had no effect on the
mTOR kinase activity as measured with either GST-p70S6k (Fig. 1B)
or PHAS-I/4E-BP1 (data not shown) as the substrate. Similar results
were obtained after cells were exposed to rapamycin at concentrations
up to 10 ␮M (data not shown). In contrast, treatment of cells with
wortmannin, which binds irreversibly to the mTOR kinase domain
(29, 30), strongly inhibited mTOR-dependent GST-p70S6k phosphorylation. The HEK 293 cells were responsive to rapamycin, because
endogenous p70S6k was extensively dephosphorylated (indicated by
an increase in electrophoretic mobility) in the drug-treated cells (Fig.
1B). Identical results were obtained when the experiment was repeated
with MCF-7 breast carcinoma cells as the test cell line (data not
shown). Collectively, these observations suggest that treatment of
intact cells with rapamycin does not lead to poorly reversible sup-
Fig. 1. Effect of cellular exposure to rapamycin on mTOR kinase activity. A, poorly
reversible binding of mTOR to the FKBP12/rapamycin complex. Rat brain extract was
incubated for 2 h with GST-FKBP12 in the absence or presence of rapamycin. GSTFKBP12 was then captured with glutathione-Sepharose beads, and precipitates were
washed with the indicated buffer solutions. Top panel, anti-mTOR Western blot of
glutathione-Sepharose-bound GST-FKBP12 samples. Bottom panel, amido black-staining
of the same protein blot. B, kinase activity of mTOR isolated from rapamycin-treated
cells. Top panel, Lanes 1– 4, HEK 293 cells were maintained for 16 h in DMEM ⫹ 0.1%
FBS. Cells were treated with 100 nM rapamycin (Rap) or 10 ␮M wortmannin (Wort) as
indicated for 30 min before stimulation with serum for an additional 10 min. Lanes 5 and
6, HEK 293 cells were grown for 16 h in DMEM ⫹ 10% FBS in the absence or presence
of 20 nM rapamycin (Rapⴱ). Immune complex kinase assays were performed with
anti-mTOR immunoprecipitates and a GST-p70S6k fragment (amino acids 332– 414) as
substrate. Middle panel, anti-mTOR immunoprecipitates were immunoblotted with antimTOR antibodies. Bottom panel, whole-cell extracts were immunoblotted with antip70S6k antibodies.
pression of mTOR kinase activity as would be expected if the
FKBP12/rapamycin complex were the predominant effector of mTOR
inhibition in this context (see “Discussion”).
Generation of Stably Transfected HEK 293 Cells Expressing a
Dominant Interfering mTOR Mutant. The biochemical observations described above raise the possibility that rapamycin exerts subtle
and reversible effects on mTOR signaling functions as opposed to
acting solely as a potent inhibitor of mTOR kinase activity in vivo. To
define further the inhibitory effect of rapamycin on mTOR signaling
functions, we transfected HEK 293 cells with a mTOR double mutant
bearing a Ser20353 Ile (SI) substitution in the FKBP12-rapamycinbinding domain and an inactivating Asp23383 Ala (DA) substitution
in the catalytic domain (28). This SIDA mTOR double mutant has a
markedly reduced binding affinity for FKBP12-rapamycin and is
catalytically inactive. Studies in yeast have shown that both the
rapamycin-sensitive and -insensitive functions of the TOR proteins
are contingent on the expression of an intact catalytic domain (18). By
analogy to many other catalytically inactive protein kinases, we predicted that SIDA mTOR would exert dominant inhibitory effects on
mTOR signaling in transfected cells. The SI mutation was incorporated into the kinase-inactive mTOR construct to ensure that the
potential dominant inhibitory activity of the kinase-inactive protein
was not affected by rapamycin and that the kinase-inactive mTOR
mutant was not simply acting as a sink for FKBP12-rapamycin
complexes in drug-treated cells. Our underlying prediction was that
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cells grew at approximately the same rate as control cells. Cells
expressing SIDA mTOR grew more slowly than control cells, consistent with the predicted dominant inhibitory effect of the kinaseinactive mTOR mutant. The growth rates of these cell lines were next
measured in the presence of 20 nM rapamycin, a maximally effective
drug concentration in this assay (Fig. 2C; data not shown). As expected, growth of the control cell lines was suppressed by rapamycin.
In contrast, cells expressing SI mTOR were highly resistant to the
drug, indicating that the growth-inhibitory effect of rapamycin was
due to its effects on mTOR. In contrast, when SIDA mTOR-expressing cells were treated with rapamycin, cell growth was almost completely arrested. The slight increase in cell mass observed after 60 h
in culture may reflect the degradation of rapamycin, which has a finite
half-life (⬃10 h) in aqueous solution (31). Thus, the antiproliferative
activity of rapamycin toward HEK 293 cells is dramatically enhanced
in the presence of SIDA mTOR.
Effect of Rapamycin Treatment and SIDA mTOR Expression
on Clonogenic Activity under Nutrient-Replete and -Restricted
Conditions. To examine further the overall contribution of mTOR to
cell growth, clonogenic assays were performed. Rapamycin treatment
decreased the size of colonies formed by vector control cells, whereas
SI mTOR-expressing cells were relatively resistant to the antiproliferative effect of this drug (Fig. 3). In the absence of rapamycin, the
SIDA mTOR-expressing cells displayed slightly lower plating efficiencies and a modest reduction in average colony size. However,
when SIDA mTOR-expressing cells were treated with rapamycin,
very few colonies emerged after 6 days in culture. These results
indicate that the inhibitory effect of rapamycin on clonogenic activity
was enhanced by SIDA mTOR.
Clonogenic assays were also conducted in nutrient- and growth
factor-deficient medium. Cells were seeded in complete medium and
cultured overnight. The culture medium was then replaced with
DMEM containing 2% FCS and 20% of the normal level of amino
Fig. 2. mTOR stimulation of cell growth includes a rapamycin-insensitive component.
A, HEK 293 lines stably expressing AU-1 tagged SI and SIDA mTOR or empty vector
(VEC) were created and evaluated for transgene expression by Western blot. B, the growth
rates of vector control, SI-, and SIDA-expressing HEK 293 cells were determined with a
3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxy-phenyl)-2-(4-sulfonyl)-2H-tetrazolium tetrazolium assay. C, cellular growth rates were evaluated as in B, except that cells
were incubated in the presence of 20 nM rapamycin. All measurements were performed in
triplicate. Error bars represent SD; one representative of three similar experiments is
shown.
mTOR-dependent cellular responses that are only partially inhibited
by rapamycin in normal cells should be suppressed further in drugtreated cells that express the dominant inhibitory SIDA mTOR protein. To confirm that the measured cellular responses were mTOR
dependent, cells expressing the kinase active mTOR SI mutant were
also generated. In mTOR SI-expressing cells, mTOR signaling should
be largely refractory to rapamycin treatment.
Clonal lines of SI or SIDA mTOR-expressing HEK 293 cells were
derived (Fig. 2A), and their growth rates were compared with that of
control cells expressing vector alone (Fig. 2B). The term “growth” is
used here to refer to the accumulation of total cell mass due to both
cell division and changes in single-cell volume. SI mTOR-expressing
Fig. 3. The clonogenic growth-promoting activity of mTOR includes a rapamycininsensitive component. Equivalent numbers of HEK 293 cells expressing the indicated
mTOR constructs were plated in 60-mm dishes. Twenty-four h later, the medium was
replaced with complete DMEM or with DMEM containing 2% FCS and 20% of the
normal level of amino acids. The indicated samples (RAP) received 20 nM (final concentration) rapamycin. After 6 days of growth, cells were stained with crystal violet and
photographed.
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acids. Vector control and SI mTOR-expressing cells formed readily
detectable colonies under conditions of nutrient-stress, although the
average colony size was reduced (Fig. 3). In contrast, SIDA mTORexpressing cells failed to generate visible colonies under these conditions. Rapamycin treatment resulted in a slight decrease in the
colony size of control cells under nutrient-limiting conditions, but the
magnitude of this effect was quantitatively less than that seen in
complete medium. These results demonstrate that expression of SIDA
mTOR leads to greater sensitivity to growth factor/nutrient limitation
than does treatment of HEK 293 cells with rapamycin.
SIDA mTOR Fails to Enhance the Suppressive Effect of Rapamycin on Cell Size. We have used the term cellular growth to
encompass both cellular proliferation and increases in individual cell
mass. Because mTOR regulates mammalian cell size (20, 32), we
wished to evaluate the sensitivity of cell size control to rapamycin.
However, HEK 293 cells in monolayer culture are ill suited for these
studies due to their intrinsic variation in cell size at various stages of
confluence and to the rapid loss of cell viability after detachment from
the plastic surface. Hematopoietic cells are more amenable to cell
volume measurements, because these cells grow naturally in suspension and exhibit nearly spherical morphology. Consequently, we
generated stable transfectants expressing empty vector, SI mTOR, or
SIDA mTOR in the murine IL-3-dependent hematopoietic cell line,
FL5.12 (Fig. 4A).
To confirm that the regulation of cell growth by mTOR contained
a rapamycin-insensitive component, we compared the proliferative
rates of these FL5.12 cell lines in the presence or absence of rapamycin. Expression of SI mTOR had no effect on the proliferation of
FL5.12 cells, whereas the expression of SIDA mTOR slowed cellular
proliferation, confirming that the kinase-inactive mTOR mutant dominantly interferes with mitogenic signal transduction in FL5.12 cells
(Fig. 4B). As expected, rapamycin treatment limited cellular proliferation in vector control FL5.12 cells, whereas cell lines expressing SI
mTOR were resistant to treatment with the drug (Fig. 4C). Treatment
of SIDA mTOR-expressing FL5.12 cells with rapamycin produced a
more dramatic antiproliferative effect than was observed in control
cells. Importantly, identical results were obtained when these experiments were repeated with a 10-fold higher concentration of rapamycin (data not shown), indicating that the standard drug concentration
(20 nM) used in our studies caused the maximal suppression of mTOR
function that can be obtained with rapamycin alone. These observations were consistent with the results obtained with HEK 293 cells,
i.e., mTOR function is essential for FL5.12 cell proliferation, and the
contribution of mTOR to this process is only partially suppressed by
rapamycin.
We next evaluated whether the regulation of cell size by mTOR
included a rapamycin-insensitive component. Relative cell volumes
were measured by forward light scatter on a flow cytometer. The
confounding effects of cell cycle position on cell size were eliminated
by gating on viable (PI-negative), G1-phase cells. To avoid artifacts
due to sample processing, cells were maintained in complete medium
during the staining and analysis procedures. As expected based on
previous reports (20, 32), rapamycin treatment decreased the size of
control cells, whereas the volume of SI mTOR-expressing cells was
unaffected by the drug (Fig. 5). Interestingly, both SIDA mTORexpressing clones displayed a constitutive reduction in cell size resembling that provoked by rapamycin treatment in the control cells.
Exposure of SIDA mTOR-expressing cells to rapamycin caused no
additional decrease in cell size.
Modulation of Amino Acid Transporter Trafficking by mTOR
Includes a Rapamycin-Insensitive Component. Previous studies
demonstrated that mTOR activity is required for activated forms of
Akt to maintain cell surface expression of the 4F2hc amino acid
Fig. 4. Growth rates of FL5.12 cells in the presence of rapamycin and kinase-inactive
mTOR. A, FL5.12 cells stably expressing the indicated constructs were generated and
screened for transgene expression by Western blot. B, the effect of kinase-inactive mTOR
expression on cell growth rates was determined by plating 5 ⫻ 104 cells/ml and measuring
cell concentration with a Coulter counter at the indicated time points. All measurements
were performed in triplicate. C, growth rates were evaluated as in B, except that 20 nM
rapamycin was added to the culture medium. Error bars represent SD; one of three similar
experiments is shown.
transporter in the absence of growth factors (26). In these earlier
studies, however, rapamycin treatment did not alter 4F2hc localization
in the presence of growth factors; therefore, it remained unclear
whether mTOR regulated transporter trafficking under normal cell
culture conditions. We hypothesized that control of amino acid transporter trafficking by mTOR might be partially rapamycin insensitive
in mammalian cells. If so, expression of SIDA mTOR, either alone or
in combination with rapamycin, might alter amino acid transporter
localization in growth factor-stimulated cells. In control FL5.12 cells,
a surface-staining pattern for 4F2hc was observed, with no staining of
the intracellular compartment (Fig. 6). Consistent with our previous
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mTOR POSSESSES RAPAMYCIN-RESISTANT ACTIVITY
Fig. 5. Cell size regulation by mTOR is fully sensitive to rapamycin. FL5.12 cells
expressing the indicated mTOR constructs were cultured for 24 h in the presence or
absence of 20 nM rapamycin (RAP). Cell size was determined by forward light scatter
(FSC) analysis with a flow cytometer. Error bars represent SD; one of four similar
experiments is shown.
transcripts are more strongly affected than others. In the present study,
we examined the individual and combined effects of rapamycin and
SIDA mTOR on the phosphorylation states of p70S6k and 4EBP1.
In response to mitogenic stimuli, p70S6k undergoes multisite phosphorylation by several upstream kinases, including mTOR (14, 34,
35). Both rapamycin treatment and SIDA mTOR expression decreased the phosphorylation of p70S6k, as indicated by increased
mobility in SDS-PAGE (Fig. 7). However, the protein mobility shift
assay lacked the resolution needed to determine whether the combined
effects of rapamycin and kinase-inactive mTOR were additive. Consequently, we examined the phosphorylation of a known p70S6k
target, the ribosomal protein S6. Consistent with the observed effect
on p70S6k phosphorylation, both rapamycin exposure and SIDA
mTOR expression inhibited S6 phosphorylation. An unexpected finding was that S6 phosphorylation was also suppressed in the SI mTORexpressing clones. This alteration might reflect a compensatory adjustment made to the chronic elevation of mTOR signaling in the SI
mTOR-expressing cells (see “Discussion”). Nonetheless, the observation that rapamycin alone provoked complete S6 dephosphorylation
in the vector control FL5.12 line argues that this mTOR-dependent
response is fully sensitive to the drug.
Earlier studies demonstrated that rapamycin treatment leads to the
dephosphorylation of five Ser and Thr residues in 4EBP1 (reviewed in
Ref. 33). Treatment of control cells with rapamycin produced the
expected reduction in 4EBP1 phosphorylation (Fig. 7). The effect of
rapamycin on 4EBP1 phosphorylation was reversed by expression of
rapamycin-resistant SI mTOR. Surprisingly, cells expressing SIDA
mTOR exhibited greater 4EBP1 dephosphorylation than did control
cells treated with rapamycin. Furthermore, the effects of rapamycin
treatment and the expression of kinase-inactive mTOR on 4EBP1
phosphorylation were additive. These results indicate that the regulation of 4EBP1 phosphorylation by rapamycin includes both rapamycin-sensitive and -insensitive components.
Chronic Inhibition of mTOR Suppresses Akt Kinase Activity.
Earlier studies (23, 28, 36, 37) demonstrated that acute exposure to
rapamycin does not inhibit the activity of AKT, a PI3k-regulated
kinase that may reside upstream of mTOR (28, 38). However, the
impact of longer-term exposure to rapamycin on Akt activity has not
Fig. 6. Amino acid transporter trafficking is affected by kinase-inactive mTOR expression but not by rapamycin treatment. Vector control (VEC) or SIDA mTOR-expressing cells were maintained in the presence or absence of 20 nM rapamycin for 24 h before
fixation and staining for 4F2hc expression. Scale bars denote 10 ␮m; insets are enlarged
to better demonstrate the intracellular staining pattern.
studies (26), little change in this staining pattern was observed after a
24-h treatment with rapamycin. However, when SIDA mTORexpressing cells were examined for 4F2hc localization, many small
4F2hc-positive intracellular vesicles were apparent. This pattern of
4F2hc staining was not altered by rapamycin treatment. The effect of
SIDA mTOR on amino acid transporter localization was attributable
to the mutated catalytic domain, because expression of SI mTOR had
no effect on 4F2hc localization (data not shown). These results suggest that the trafficking of 4F2hc is regulated by mTOR but is largely
insensitive to rapamycin.
Phosphorylation of 4EBP1 Is Partially Rapamycin Insensitive.
In mammalian cells, mTOR regulates translation by promoting the
phosphorylation of p70S6k and 4EBP1. Rapamycin treatment causes
rapid decreases in the phosphorylation of these proteins, which results
in the inactivation of p70S6k and increased binding of 4EBP1 to the
translation initiation factor, eIF-4E (33). The net outcome of these
events is a decrease in mRNA translation rates, although certain
Fig. 7. Effects of rapamycin treatment and kinase-inactive mTOR expression on
phosphorylation of p70S6k and 4EBP1. FL5.12 cells expressing the indicated mTOR
constructs were cultured for 24 h in the presence or absence of 20 nM rapamycin (RAP).
Detergent extracts were prepared and evaluated by Western blotting with the indicated
antibodies.
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been investigated. Therefore, we evaluated whether a more prolonged
(24-h) drug treatment affected Akt phosphorylation at the regulatory
Thr308 and Ser473 residues. When control HEK 293 cells were treated
with rapamycin, AKT phosphorylation was clearly decreased, with no
change in total Akt levels (Fig. 8A, top panel). The dephosphorylation
of Akt induced by long-term rapamycin treatment was blocked by SI
mTOR expression. Similar decreases in Akt phosphorylation were
observed in SIDA mTOR-expressing cells. Moreover, when rapamycin treatment was combined with SIDA mTOR expression, Akt phosphorylation was compromised further. Finally, we demonstrated that
chronic rapamycin exposure had no effect on the phosphorylation of
the p44 and p42 MAP kinase isoforms in HEK 293 cells, indicating
that the drug did not globally suppress the activities of cytoplasmic
protein kinases under these treatment conditions (Fig. 8A, bottom
panel). Similar results were obtained in parallel experiments with
FL5.12 cells (data not shown), indicating that chronic rapamycin
treatment affects Akt phosphorylation in multiple cell types.
To address the possibility that the observed inhibition of Akt
phosphorylation was a secondary consequence of the antiproliferative
activity of rapamycin, we treated HEK 293 cells for 24 h with 500 ␮M
L-mimosine and examined the phosphorylation status of Akt as described above. Consistent with the production of an early S-phase cell
cycle arrest, L-mimosine treatment completely abrogated BrdUrd incorporation (data not shown). Despite the induction of a complete cell
cycle arrest, L-mimosine treatment had no effect on Akt phosphorylation, whereas rapamycin-treated cells displayed the expected decreases in Akt phosphorylation at Ser308 and Thr473 (Fig. 8B).
Immune complex kinase assays were performed to confirm that
chronic rapamycin treatment actually suppressed Akt kinase activity.
HEK 293 cells expressing empty vector, SI mTOR, or SIDA mTOR
were transfected with HA-Akt and then treated with 20 nM rapamycin
for 24 h. Rapamycin treatment alone caused a 50% reduction in Akt
kinase activity present in anti-HA immunoprecipitates from control
cells (Fig. 8C). As expected, Akt activity was not affected by rapamycin in the SI mTOR-expressing cells. In contrast, SIDA mTORexpressing cells displayed a constitutive decrease in Akt activity
similar to rapamycin-treated control cells. Rapamycin treatment of the
SIDA mTOR-expressing clone produced an even greater reduction in
Akt kinase activity, to a level less than 25% of that observed in
untreated controls. These results indicate that long-term inhibition of
mTOR function leads to suppression of Akt kinase activity and that
this mTOR-dependent outcome is partially suppressed by rapamycin.
The Regulation of Glycolysis by mTOR Is Rapamycin Sensitive. Using DNA microarrays, others have shown that treatment of
yeast and mammalian cells with rapamycin decreases the level of
mRNA transcripts encoding glycolytic enzymes and increases the
abundance of mRNAs coding for tricarboxylic acid cycle enzymes
(39, 40). These findings hinted that the TOR proteins serve as rapamycin-sensitive stimulators of glycolytic activity in nutrient-replete
Fig. 8. mTOR regulates Akt kinase activity by a mechanism that is partially inhibited by
rapamycin. A, HEK 293 cells expressing the indicated mTOR constructs were incubated
for 24 h in the presence or absence of 20 nM rapamycin (RAP). The cells were lysed, and
detergent extracts were immunoblotted with the indicated antibodies. B, cellular extracts
were analyzed as in A after a 24-h incubation with 500 ␮M L-mimosine or 20 nM
rapamycin as indicated. C, HEK 293-derived stable transfectants (pCDNA3, mTOR SI 5,
and mTOR SIDA 7) were transiently transfected with an HA-tagged Akt expression
plasmid. Twenty-four h post-transfection, the indicated samples were treated for 24 h with
20 nM rapamycin. Clarified lysates were immunoprecipitated with anti-HA antibodies
(12CA5) and subjected to immune complex kinase assays. Top panel, immune complex
kinase assays were performed with anti-HA immunoprecipitates and GST-mTOR RD
(amino acids 2405–2517) as substrate. Middle panel, the same membrane was immunoblotted with anti-HA antibodies. Bottom panel, phosphorimager analysis of GST-mTOR
RD phosphorylation. Substrate phosphorylation by HA-Akt in each sample was normalized to that obtained with the untreated HEK 293 vector-only cell line.
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Fig. 9. mTOR regulates glycolytic rate through a mechanism that is fully sensitive to
rapamycin. FL5.12 cells expressing the indicated mTOR constructs were incubated in the
presence or absence of 20 nM rapamycin (RAP), and glycolytic rates were determined by
measuring the conversion of [3H]2O from [3H]glucose. Error bars represent SE; a
representative experiment of three similar trials is shown.
cells. In light of these reports, we examined whether mTOR regulates
glycolytic activity in FL5.12 cells cultured in the presence of IL-3. As
shown in Fig. 9, treatment with rapamycin decreased the rate of
glycolysis in control cells but not in cells expressing rapamycinresistant SI mTOR. Interestingly, expression of SIDA mTOR decreased the rate of glycolysis to a level similar to that observed in
rapamycin-treated control cells. The addition of rapamycin to SIDA
mTOR-expressing cells produced no additional decrease in the glycolytic rate. These findings indicate that mTOR is a positive regulator
of glycolysis in mammalian cells and that this activity of mTOR is
fully suppressed by rapamycin.
DISCUSSION
Rapamycin has been used extensively to identify the contributions
of mTOR to various signaling pathways and cellular responses. An
implicit assumption in these experiments is that exposure to maximally effective concentrations of rapamycin translates into a “chemical knockout” of mTOR function in drug-treated cells. However, in
this report, we demonstrate that rapamycin exerts differential inhibitory effects on mTOR-dependent responses in both epithelial and
hematopoietic cells. Certain mTOR-regulated processes, such as glycolytic activity and cell size control, were inhibited to similar extents
by either rapamycin treatment or SIDA mTOR expression. Other
mTOR-dependent outcomes, such as 4EBP1 phosphorylation, Akt
activity, and cellular growth, were only partially suppressed by rapamycin. Finally, amino acid transporter trafficking was disrupted by
SIDA mTOR expression but was not altered by rapamycin treatment,
indicating that some mTOR-dependent functions are rapamycin insensitive. These results strongly suggest that chemical genetic experiments involving rapamycin as the probe will not uncover the full
range of mTOR-dependent responses in mammalian cells.
The complex pharmacology of rapamycin was underscored by the
unexpected finding that mTOR kinase activity was not reduced by
treatment of intact cells with rapamycin. These results were not easily
reconciled with the observation that the FKBP12/rapamycin complex,
the presumed effector of intracellular mTOR inhibition, is a poorly
reversible and highly effective inhibitor of mTOR kinase activity in
vitro (see Fig. 1A; Refs. 13–15). The present findings suggest that if
rapamycin functions primarily as a inhibitor of mTOR kinase activity
in intact cells, it does so in a reversible fashion. An alternative, but
nonexclusive possibility is that the drug interferes with the recognition
of upstream regulatory signals by mTOR, and/or with the phosphorylation of downstream targets for this protein kinase. Moreover,
certain functions of mTOR may be sensitive to rapamycin alone,
whereas others may be inhibited by rapamycin only when the drug is
complexed to FKBP12. If reversible interaction of rapamycin with the
FRB domain has differential effects on signal transmission through
mTOR, then a search for novel ligands for the FRB domain of mTOR
could yield drugs with a different spectrum of immunosuppressive
and anticancer activities than those exhibited by rapamycin.
The presence of two TORCs (TORC1 and TORC2) provides a
rational explanation for the existence of rapamycin-sensitive and
-insensitive TOR functions in budding yeast (19). Although mammalian cells express a rapamycin-sensitive, TORC1-like (raptor-containing) complex (19 –21), the expression of additional mTOR complexes
remains speculative. Our finding that mTOR carries out activities
(e.g., amino acid transporter localization) that are relatively resistant
to rapamycin hints that two (or more) mTOR-containing complexes
may be present in mammalian cells. Alternatively, as discussed above,
rapamycin binding to a single mTOR-containing complex could result
in differential inhibition of the various efferent signaling outputs
emanating from this complex. Clearly, additional studies of mTOR
and its partner proteins in mammalian cells will be needed to distinguish between these alternative models.
The finding that rapamycin exerts variable effects on mTOR functions also has important implications for cancer chemotherapy with
rapamycin and other inhibitors of the mTOR signaling pathway now
under preclinical and clinical development. In our analysis of mTORdependent outcomes, the suppressive effect of SIDA mTOR expression on endogenous mTOR function was typically comparable with or
greater than that of rapamycin. Kinase-inactive proteins exert dominant inhibitory activities through sequestration of associated regulatory proteins and/or substrates of the endogenous protein kinase.
Direct inhibitors of the mTOR kinase domain should result in effects
similar to those induced by forced expression of SIDA mTOR. In fact,
mTOR kinase inhibitors may be even more efficacious inhibitors of
mTOR function, because the dominant-negative protein can be stably
expressed only to a level compatible with continued cell growth. Our
results suggest that small molecule inhibitors of the mTOR kinase
domain will exert considerably broader effects on mTOR function
than does rapamycin. Whether such drugs will show selectivity toward tumor cells remains an open question.
Our results also indicate that mTOR regulates amino acid transporter trafficking in mammalian cells. Rapamycin has been shown
previously to alter amino acid transporter localization in FL5.12 cells
(26). However, this drug effect was only apparent when cells expressing an activated Akt mutant were deprived of growth factors. In the
present report, disruption of mTOR function by SIDA mTOR expression altered transporter localization in the presence of growth factors.
This observation correlated with the finding that SIDA mTOR expression was more effective than rapamycin as a suppressor of the
clonogenic activity of HEK 293 cells cultured under growth factor/
nutrient-limited conditions. Interestingly, TOR-dependent amino acid
transporter trafficking in yeast is sensitive to rapamycin in optimal
growth medium (41, 42). Our findings suggest that multiple upstream
signals converge on mTOR to regulate amino acid transporter expression and that some do so in a rapamycin-sensitive fashion (e.g., the
Akt-dependent signal), whereas others are less affected by drug treatment.
We also observed that mTOR regulates the glycolytic rate in
mammalian cells. Although DNA microarray studies suggested that
TOR controls glucose metabolism in eukaryotic cells (39, 40), direct
measurements of glycolytic rates in rapamycin-treated cells had not
been performed before this study. Because cancer cells characteristi-
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cally display abnormally elevated glycolytic activity (43), it is plausible that the antitumor effects of rapamycin reflect, in part, the
suppression of glycolysis and the consequent depletion of energy
supplies needed for tumor growth.
A surprising observation was that chronic repression of mTOR
signaling caused a significant reduction in Akt phosphorylation and
catalytic activity in the host cells. Previous findings indicated that
short-term (ⱕ2 h) rapamycin treatment had no effect on Akt kinase
activity (23, 28, 36, 37). We found that a minimum rapamycin
exposure time of 8 h was required to induce significant dephosphorylation of Akt. These results suggest that mTOR function contributes
to sustained Akt activation in growth factor-stimulated cells. In contrast, chronic elevations of dTOR/p70S6k activity lead to decreased
Akt activity in Drosophila larva (44, 45). Although the reason for this
apparent discrepancy is unknown, it is possible that long-term mTOR
suppression inhibits Akt via an indirect mechanism that targets a
component(s) of Akt-containing protein complexes in mammalian
cells (46). Regardless, the observation that long-term rapamycin exposure interferes with Akt function may be highly relevant to the
anticancer activities of rapamycin-like compounds in human patients.
As predicted, expression of the catalytically active SI mTOR mutant uniformly rescued rapamycin-sensitive mTOR functions in HEK
293 cells. An unexpected observation was that SI mTOR expression
caused a consistent reduction in the level of phosphorylated S6 in
these cells. These results may be linked to the recent observation that
lethality associated with deletion of Tsc1 in Drosophila was rescued
by manipulations that lowered p70S6k activity (44). Stable expression
of SI mTOR in mammalian cells may resemble disruption of the Tsc1
gene in the fly in that both alterations would lead to chronically
elevated TOR activity (47–51), which would need to be countered by
a reduction in p7056K activity to maintain normal cell growth.
In conclusion, the present findings indicate that rapamycin exerts
surprisingly variable effects on mTOR-dependent signaling in mammalian cells. Our results additionally suggest that a direct inhibitor of
the mTOR kinase domain will display a profile of pharmacological
activities that only partially mimics those associated with rapamycin
treatment. Clinical experience with rapamycin indicates that this drug
possesses a high therapeutic index. As with many conventional anticancer agents, nonspecific toxicity to proliferating tissues may limit
the clinical application of an agent that globally blocks signaling
through mTOR. Nonetheless, our findings indicate that further development of mTOR inhibitors is clearly warranted and could yield a
new class of anticancer drugs that selectively target anabolic metabolism and energy production in developing tumors.
ACKNOWLEDGMENTS
We thank the members of the Thompson and Abraham labs for helpful
discussions and for technical advice.
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Differential Effects of Rapamycin on Mammalian Target of
Rapamycin Signaling Functions in Mammalian Cells
Aimee L. Edinger, Corinne M. Linardic, Gary G. Chiang, et al.
Cancer Res 2003;63:8451-8460.
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