*The Campbell Family
Institute for Breast Cancer
Research, University Health
Network, University of
Toronto, Toronto, Ontario
M5G 2C1, Canada.
‡Current address: Signal
Cancer Research UK London
Research Institute, 44
Lincoln’s Inn Fields, London
WC2A 3PX, UK.
§The Cancer Institute of New
Jersey and the Institute for
Advanced Study, New Jersey,
New Jersey, USA.
Correspondence to T.W.M.
2 February 2006
Beyond PTEN mutations: the PI3K
pathway as an integrator of multiple
inputs during tumorigenesis
Megan Cully*‡, Han You*, Arnold J. Levine§ and Tak W. Mak*
Abstract | The tumour-suppressor phosphatase with tensin homology (PTEN) is the most
important negative regulator of the cell-survival signalling pathway initiated by phosphati-
dylinositol 3-kinase (PI3K). Although PTEN is mutated or deleted in many tumours,
deregulation of the PI3K–PTEN network also occurs through other mechanisms. Crosstalk
between the PI3K pathways and other tumorigenic signalling pathways, such as those that
involve Ras, p53, TOR (target of rapamycin) or DJ1, can contribute to this deregulation. How
does the PI3K pathway integrate signals from numerous sources, and how can this
information be used in the rational design of cancer therapies?
The tumour suppressor PTEN (phosphatase with tensin
homology, which is deleted on chromosome 10) was
originally identified as a gene that is mutated in multiple
sporadic tumour types as well as in patients with can-
cer predisposition syndromes such as Cowden disease.
PTEN is a lipid phosphatase that negatively regulates
the phosphatidylinositol 3-kinase (PI3K) signalling
pathway1. The PI3K pathway is an important driver
of cell proliferation and cell survival, most notably in
cells that are responding to growth-factor–receptor
engagement. By opposing the effects of PI3K activa-
tion, PTEN functions as a tumour suppressor. So, the
PI3K–PTEN signalling network functions as a crucial
regulator of cell survival decisions. When PTEN is
deleted, mutated or otherwise inactivated, activation of
PI3K effectors — particularly the activation of the key
survival kinase protein kinase B (PKB, also known as
AKT) — can occur in the absence of any exogenous
stimulus, and tumorigenesis can be initiated. Numerous
types of tumours, both sporadic and those that arise as a
component of a cancer predisposition syndrome, show
alterations in PTEN2.
PTEN mutations — the tip of the iceberg?
When PTEN was first discovered, it seemed likely that
PTEN mutations would account for most of the cases
of PI3K pathway deregulation observed in tumours3–5.
However, 8 years of analysis have shown some intrigu-
ing discontinuities between PTEN mutations, the
tumour spectrum of Cowden disease4 and the activa-
tion of known downstream PI3K-pathway components
such as PKB. For example, spontaneous forms of breast
cancer rarely show loss of both PTEN alleles, but this
tumour type is commonly observed in patients with
germline PTEN mutations2,4. Immunohistochemical
staining of breast tumour samples has shown that
approximately half contain hyperactive PKB signal-
ling, but as few as 3% contain identifiable PTEN
mutations2,4,6. It has therefore become clear that there
must be mechanisms in addition to direct mutation
or deletion of PTEN by which the PI3K signalling
pathway can become constitutively activated. Within
the complex environment of an organism, cells are
continually integrating a plethora of signals to deter-
mine cell fate, survival and proliferation. It is in this
light that the PI3K–PTEN pathway can be considered
as a central integrator of a tangled web of signalling
networks with direct and indirect effects on each other.
This integratory role casts the PI3K–PTEN pathway as
an important arbiter of cell fate. Recent data supports
the idea that crosstalk among signalling pathways con-
tributes to a deregulation of PI3K–PTEN signalling
that can lead to tumorigenesis.
Biochemistry of the PI3K pathway
Identifying the component elements of the PI3K–PTEN
signalling network and determining how they are
regulated will improve our understanding of cancer
pathogenesis and lead to the rational development
of novel therapeutics. The core PI3K pathway has
been defined through both biochemical and genetic
experiments (FIG. 1).
184 | MARCH 2006 | VOLUME 6
© 2006 Nature Publishing Group
A benign growth.
Loss of heterozygosity
The loss of the remaining
normal allele when one allele is
already lost or mutated.
In its active form, PI3K consists of a regulatory p85
subunit and a catalytic p110 subunit. When activated
by any one of a variety of mechanisms (BOX 1), PI3K
activation results in the generation of the second mes-
senger lipid phosphatidylinositol (3,4,5) triphosphate
(PIP3). PIP3 in turn recruits both phosphatidyli-
nositol-dependent kinase 1 (PDK1) and PKB to the
membrane, where PDK1 phosphorylates and activates
PKB. There are three highly homologous isoforms of
PKB that are transcribed from independent genes
and have overlapping but distinct functions7. In mice,
PKBα (also known as AKT1) mediates signals down-
stream of PI3K activation that promote cell survival
and proliferation. By contrast, PKBβ (also known as
AKT2) activation is associated with insulin-mediated
metabolic processes8,9. Pkbγ–/– (also known as Akt3)
mice have reduced brain size and weight, which might
be attributed to reduced cell size and cell number10.
The net result of the activation of all isoforms of PKB is
protection from apoptosis and increased proliferation
— events that favour tumorigenesis.
Several direct substrates of PKB phosphorylation
have crucial roles in cell-cycle regulation. These sub-
strates include the cell-cycle inhibitor p27 (also known
as KIP1), the forkhead box transcription factors (FOXO),
glycogen synthase kinase 3 (GSK3), serum- and gluco-
corticoid-induced kinase 1 (SGK1) and tuberous sclerosis
complex 2 (TSC2)11–13 (FIG. 1). Phosphorylation of p27 by
PKB results in p27 inactivation and thereby promotes
cell cycle entry. In addition, p27 expression is subject
to another level of regulation, which is mediated by
FOXO3A. When unphosphorylated, FOXO3A functions
as a selective transcription factor in the nucleus, inducing
the transcription of the genes that encode p27, the cell-
cycle-inhibitor cyclin G2 and the pro-apoptotic molecule
BIM14. Phosphorylation of FOXO3A by PKB results in
expulsion of FOXO3A from the nucleus and, therefore,
decreased transcription of the gene that encodes p27. In
addition, nuclear exclusion of FOXO3A increases cyclin
D1 expression, as unphosphorylated FOXO3A functions
as a transcriptional repressor for this gene (among several
others)15. Interestingly, Drosophila melanogaster Foxo and
mammalian FOXO1 have been found to transcription-
ally regulate expression of the insulin receptor when
the nutrient supply is limited. FOXO proteins can act
as insulin sensors that allow the rapid activation of the
insulin signalling pathway during times of low nutrient
levels16. FOXO could therefore mediate a key feedback-
control mechanism that regulates insulin signalling
Another key molecule inactivated by PKB phos-
phorylation is TSC2. When unphosphorylated, TSC2
hetero dimerizes with TSC1 to promote the GTPase
activity of RHEB, a Ras homologue that is highly
expressed in brain tissue17. Active, GTP-bound RHEB
promotes the activity of the kinase TOR (target of
rapamycin). TOR functions as a nutrient sensor that
integrates PI3K-mediated growth-factor signalling,
glucose availability and amino-acid availability18.
PKB activation inhibits the ability of the TSC1–TSC2
complex to act as a RHEB-GTPase activating protein
(RHEB-GAP), which therefore increases the amount of
RHEB-GTP present and consequently activates TOR.
Activated TOR exists in two complexes. The
rapa mycin-sensitive TOR complex contains rap-
tor (regulatory associated protein of TOR) and GβL
(G-protein β-subunit-like), and phosphorylates S6
kinase (S6K) and the EIF4E (eukaryotic transla-
tion-initiation factor 4E)-inhibitory binding protein
4EBP19–22. Phosphorylated S6K is active, and might
affect protein translation and cell size, although the
mechanisms remain controversial. In both mice and
D. melanogaster, deficiency for s6k results in decreased
cell size23,24. The second, rapamycin-insensitive TOR
complex contains rictor (rapamycin-insensitive com-
panion of TOR) and mediates signals to the cytoskel-
eton25–27. The rictor-containing TOR complex can also
phosphorylate and activate PKB28.
PI3K, PTEN and the TSC1–TSC2–TOR axis
The signalling axis that involves the TSC1–TSC2 complex
and TOR (TSC1–TSC2–TOR) has become a focal point
in studies of PI3K-mediated tumorigenesis for two rea-
sons. First of all, mutations in either TSC1 or TSC2 lead
to tuberous sclerosis, a hamartoma syndrome associated
with a predisposition to malignancy17. Notably, tuber-
ous sclerosis hamartomas generally display loss of
heterozygosity LOH at the mutant locus29. Secondly,
there is indirect evidence derived from studies of TOR
inhibition that a decrease in TOR activity prevents tumour
development in both humans and mice. For example,
chemical TOR inhibitors such as the rapamycin deriva-
tives CCI-779, RAD001 and AP23573 seem to have anti-
tumour activity for a wide range of malignancies, including
At a glance
• The phosphatidylinositol 3-kinase (PI3K)–phosphatase with tensin homology (PTEN)
signalling pathway is one of the most commonly altered pathways in human tumours.
However, mutations of the PTEN gene itself account for only a fraction of these
• The PI3K–PTEN pathway promotes cell survival and proliferation, increases in cell size
and chemoresistance. Each of these biological outcomes results from the interaction
of this pathway with other signalling networks.
• Ras and its downstream effectors can activate components of the PI3K–PTEN pathway
through numerous mechanisms. Each mechanism might be restricted to a particular
tumour type, allowing the design of a specific therapy that kills cancer cells but leaves
normal tissue unharmed.
• Crosstalk between the PI3K–PTEN and p53 pathways occurs at multiple nodes in
these pathways. When both PTEN and p53 are inactivated by mutations, malignancy is
promoted in a synergistic manner.
• The Ras, PI3K–PTEN and p53 pathways all converge either directly or indirectly on the
tumour suppressor TSC2, indicating a crucial role for this molecule in the integration
of multiple signals.
• DJ1 is a novel regulator of the PI3K–PTEN pathway and is associated with breast and
• The multiple pathways that influence the PI3K–PTEN signalling network do so through
a variety of mechanisms, providing numerous potential drug targets. Drugs that act
on these targets could be formulated to work either synergistically with agents that
act directly on PI3K or on elements that function downstream of mutated pathway
components. These drugs might offer an attractive additional or alternative approach
to combating PI3K-dependent tumours.
NATURE REVIEWS | CANCER
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Competing interests statement
The authors declare no competing financial interests.
The following terms in this article are linked online to:
ABL | BCR | eIF4E | ERK1 | ERK2 | FOXO3A | GAB1 | GAB2 |
GRB2 | NF1 | p27 | p53 | PDK1 | PKBα | PKBβ | Pkbγ | PTEN |
RHEB | SGK1 | TOR | TSC1 | TSC2
National Cancer Institute: http://www.cancer.gov
breast cancer | lung cancer
Nature signalling gateway:
Science signal transduction knowledge environment:
Biomolecular interaction network database: http://bind.ca/
Access to this interactive links box is free online.
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© 2006 Nature Publishing Group