The Regulation and Activities of the Multifunctional
Serine/Threonine Kinase Akt/PKB
Eugene S. Kandel and Nissim Hay1
Department of Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois 60607
T he serine/threonine kinase Akt, or protein kinase B
(PK B), has recently been a focus of intense research. It
appears that Akt/PK B lies in the crossroads of multi-
ple cellular signaling pathways and acts as a trans-
ducer of many functions initiated by growth factor
receptors that activate phosphatidylinositol 3-kinase
(PI 3-kinase). Akt/PK B is particularly important in me-
diating several metabolic actions of insulin. Another
major activity of Akt/PK B is to mediate cell survival.
In addition, the recent discovery of the tumor suppres-
sor PT E N as an antagonist of PI 3-kinase and Akt/PK B
kinase activity suggests that Akt/PK B is a critical fac-
tor in the genesis of cancer. T hus, elucidation of the
mechanisms of Akt/PK B regulation and its physiolog-
ical functions should be important for the understand-
ing of cellular metabolism, apoptosis, and cancer.
© 1999 Academic Press
INTRODUCTION AND HISTORICAL PERSPECTIVE
In 1991 twoindependent lines of research converged
on the discovery of a cDNA encoding a novel serine/
threonine kinase. One group cloned the cellular homo-
logue of the v-akt oncogene from a transforming retro-
virus (AKT8) in spontaneous thymoma of the AKR
mouseand its product was called c-Akt [1, 2]. Thesame
cDNA was cloned by two other groups searching for
novel members of the protein kinase C (PKC) and
protein kinase A (PKA) superfamily as possible partic-
ipants in signal transduction cascades [3, 4]. Accord-
ingly, the novel kinase was called RAC (related to A
and C kinases) or PKB (protein kinase B)—and in this
review we will refer to it as Akt/PKB. Eight years of
subsequent research has left little doubt that Akt/PKB
plays a prominent role in both growth factor signaling
and oncogenesis. Currently, closeAkt homologues have
been identified in a variety of species, including birds,
insects, nematodes, slime mold, and yeast and at least
some organisms have more than one gene for similar
yet distinct isoforms of this enzyme [5–11].
Interest in Akt/PKB was piqued in 1995 when it was
shown to be a direct downstream effector of phospha-
tidylinositol 3-kinase (PI 3-kinase) [12, 13]. When gly-
cogen synthase kinase 3 (GSK3) was identified as an
Akt/PKB target , it established the current para-
digm for insulin signaling in which Akt/PKB plays the
key role. The finding that PI 3-kinase activates Akt/
PKB led to studies showing that it is a major partici-
pant in growth factor-mediated cell survival [15–18]. It
also became clear that Akt/PKB is capable of linking
growth factor signaling through PI 3-kinase to basic
metabolic functions, such as protein and lipid synthe-
sis, carbohydrate metabolism, and transcription. As
the field evolved, new prospectives on the interplay
between cell growth, survival, and metabolism, under
both normal and pathological conditions, have been
established. These concepts and several emerging
questions will be discussed in this review.
THE AKT/PKB GENE FAMILY
Three major isoforms of Akt/PKB encoded by three
separate genes have been found in mammalian cells.
Akt1 or PKB? was the first isolated isoform; Akt2/
PKB? and Akt3/PKB? were subsequently cloned
through homology screen [19–22]. All three genes have
greater than 85% sequence identity and their protein
products share the same structural organization (Fig.
1). The first amino-terminal 100 amino acids possess a
pleckstrin homology (PH) domain that binds phospho-
lipids. A short glycine-rich region that bridges the PH
domain tothe catalytic domain follows the PH domain.
All Akt/PKB isoforms are assumed tohave identical or
similar substrate specificity but this has never been
overtly tested. The last 70 amino acids of the carboxy-
terminal tail contain a putative regulatory domain. In
v-Akt, a truncated viral group-specific antigen, gag, is
fused in frame to the full-length Akt1 coding region
through a short 5? untranslated region of Akt1 . All
three Akt/PKB isoforms possess conserved threonine
and serine residues (T308 and S473 in Akt1/PKBa)
that together with the PH domain are critical for Akt/
1To whom correspondence and reprint requests should be ad-
dressed at the Department of Molecular Genetics (M/C 669), Univer-
sity of Illinois in Chicago, 900 South Ashland Avenue, Chicago, IL
60607. Fax: (312) 355-2032. E-mail: firstname.lastname@example.org.
Copyright © 1999 by Academic Press
All rights of reproduction in any form reserved.
Experimental Cell Research 253, 210–229 (1999)
Article ID excr.1999.4690, available online at http://www.idealibrary.com on
PKB activation (see below). Equivalent threonine and
serine residues in a similar aminoacid context are also
present in p70 S6 kinase and in all PKC isoforms. It is
noteworthy that the distance between the two phos-
phorylated residues (?160–170 aa) is alsoconserved in
these different protein kinases. Two additional Akt
isoforms have been described and represent minor
splice variants of human Akt2 and rat Akt3 [19, 23].
These isoforms exhibit a carboxy-therminal insertion
of 40 aa and a partial deletion of 25 aa in the carboxy-
terminal regulatory domain (including S473), respec-
tively. The biological significance of these isoforms re-
A close relative of Akt/PKB is serum- and glucocor-
ticoid-induciblekinase(SGK) that was shown tohavea
substrate specificity similar to that of Akt/PKB [24,
25]. SGK has extensive sequence homology toAkt/PKB
in the catalytic domain and possesses residues equiv-
alent toT308 and S473 of Akt1, but lacks a PH domain.
SGK is more similar to the Akt homologues, Ypk1 and
Ypk2/Ykr2, in budding yeast , suggesting that SGK
might be closer to the ancestral prototype of the Akt/
All three Akt/PKB isoforms are ubiquitously ex-
pressed in mammals, although the levels of expression
vary among tissues [19–23, 27]. Akt1/PKB? is the pre-
dominant isoform in most tissues. The highest expres-
sion of Akt2/PKB? was observed in the insulin-respon-
sive tissues: skeletal muscle, heart, liver, and kidney
, suggesting that this isoform is important for in-
sulin signaling. This is further substantiated by the
observation that Akt2/PKB? expression in developing
embryos is also highest in the insulin-responsive tis-
sues, including liver, brown fat, and skeletal muscle
. A peculiar pattern of Akt1/PKB? expression was
detected in brain, where it is markedly increased in
regenerating neurons . Akt1/PKB? is also the pre-
dominant isoform in mouse embryo fibroblasts (W.
Chen and N.H., unpublished). Unlike two other iso-
forms, Akt3/PBK? shows a more restricted pattern of
expression. Higher levels of Akt3/PKB? were detected
in testis and brain and low levels in theadult pancreas,
heart, and kidney [21–23]. The expression pattern of
the three isoforms may not always reflect their activi-
ties. Different levels of kinase activities of the different
isoforms have been observed in certain tissues and
during differentiation, which is not necessarily corre-
lated with their level of expression [30, 31].
THE MECHANISMS OF AKT/PKB ACTIVATION
Activation by PI 3-Kinase
The viral gag domain in v-Akt possesses a myristoyl-
ation signal that mediates its targeting to the plasma
membrane and renders the enzyme constitutively ac-
tive. This suggested that membrane association might
be important in the activation process of c-Akt and
subsequent studies bore out this hypothesis. Indeed,
while c-Akt is localized primarily to the cytoplasm, a
large proportion of v-Akt is localized to the plasma
membrane . Only upon stimulation does a fraction
of c-Akt migrate tothe membrane and attach there via
its PH domain . The presence of a PH domain
together with the observation that the kinase activities
of both Akt1/PKB? and Akt2/PKB? can be rapidly ac-
tivated by PDGF in rodent fibroblasts led to studies
showing that Akt/PKB is a direct target of PI 3-kinase
[14, 13]. PI 3-kinase is activated by growth factor re-
ceptors through binding of its regulatory subunit to
phosphotyrosine residues in the receptor. Upon activa-
tion the catalytic subunit of PI 3-kinase phosphory-
lates phosphoinositides (PI) at the 3-position of the
PI(3,4,5)P3 (see review by B. van Haesebroeke and M.
Waterfield in this issue). PI 3-kinases are classified
F IG. 1.
forms is shown in comparison to virally encoded v-Akt and serum-
and glucocorticoid-inducible kinase SGK. All Akt/PKB variants con-
tain a plecsktrin homology domain (PH), a catalytic domain, and a
putative regulatory fragment at the C-terminus (regul). SGK has a
similar structure and sequence, but lacks a pleckstrin homology
domain. v-Akt is an in-frame fusion of Akt-1 with a portion of
retroviral group-specific antigen (gag). Amino acid positions are
shown for mouse proteins. Threonine and serine residues whose
phosphorylation is required to induce activities of the enzymes are
indicated. See text for details.
Structural organization of the three major Akt/PKB iso-
REGULATION AND ACTIVITIES OF AKT/PKB
into three major groups. In this review we will use the
term PI 3-kinase to refer to the heterodimeric enzyme
composed of p85 regulatory subunit and p110 catalytic
subunit. Two major observations strongly suggested
that the activation of Akt/PKB is dependent on PI
3-kinase. First, activation of Akt/PKB was shown to
depend on tyrosines Y740 and Y751 in the PDGF re-
ceptor that had been identified as the binding sites for
the p85 regulatory subunit of PI 3-kinase . Second,
the PI 3-kinase inhibitors wortmannin and LY94002
could diminish the activation of Akt/PKB by growth
factors [12–14, 34]. These initial observations were
followed by experiments showing that overexpression
of a constitutively activated p110 catalytic subunit of
PI 3-kinase can activate Akt/PKB . In addition, it
was shown that point mutations in thePH domain that
reduce phospholipid binding abrogate the ability of
Akt/PKB to be activated by growth factors, and a mu-
tation that increases phospholipid binding superacti-
vates the enzyme [13, 36]. Further studies showed that
both PI(3,4)P2 and PI(3,4,5)P3 bind with high affinity
to the PH domain of Akt/PKB [37–39]. However, the
relative contribution of each 3-phosphorylated phos-
phoinositide species to Akt/PKB activation in vivo re-
mains unclear. Exposure of cells to synthetic phospho-
lipids showed that PI(3,4)P2 is a better activator in
some experiments [38, 39], whereas other experiments
showed higher binding affinity and better activation by
PI(3,4,5)P3 [37, 40, 41]. The latter experiments were
corroborated by observations that the SH2-containing
PI(3,4,5)P3 to PI(3,4)P2, is a potent inhibitor of Akt/
PKB activity in vivo ([42, 43], and see below).
Upon binding of 3-phosphorylated phosphoinosi-
tides, the PH domain of Akt/PKB facilitates dimeriza-
tion of the enzyme [44, 38]. Experimental evidence
suggests that Akt/PKB exists in vivo as a dimer or a
trimer and this multimerization is required for the
regulation of Akt/PKB activity. The interaction be-
tween monomers within such a complex may well ex-
plain the behavior of at least some dominant-negative
forms of the enzyme (reviewed in ).
Because Akt/PKB activity is dependent on PI 3-ki-
nase, any mechanism that activates PI 3-kinase can
theoretically lead to stimulation of Akt/PKB activity.
Indeed, activation of Akt/PKB through PI 3-kinase is
not restricted togrowth factors. For example, Akt/PKB
is activated by integrins through activation of focal
adhesion kinase, which in turn binds and activates PI
3-kinase and subsequently Akt/PKB [46–48]. Other
cell surface receptors that activate Akt/PKB via PI
3-kinase include CD28 and CD5 in T cells, B cell re-
ceptor (BCR) in B cells, G-protein-coupled receptors,
and the ?-opioid receptor [42, 49–54]. Angiotensin II
and hydrogen peroxide were also reported to activate
Akt/PKB through PI 3-kinase [55–57]. Among viral
proteins that activate Akt/PKB via PI 3-kinase are
polyomavirus middle-T antigen and HIV Tat protein
[58, 59]. In addition, Akt/PKB was shown to be acti-
vated by the oncogenic Ras through PI 3-kinase (re-
viewed in ). The GTP-bound Ras binds and acti-
vates the catalytic subunit p110 of PI 3-kinase and Ras
mutant that is not able to bind p110 could not activate
Akt/PKB . It is not clear, however, whether the
activation of Akt/PKB by activated Ras is as strong as
the activation by growth factors and activated p110,
and whether it is universal or dependent on the cell
Finally, as discussed below, thereis someevidenceof
PI 3-kinase-independent mechanisms of Akt/PKB acti-
Activation by Phosphorylation: The PDKs
Activation of Akt/PKB in vivo by exposure to growth
factors or synthetic phospholipids is preceded by an
increasein serineand threoninephosphorylation of the
kinase itself. Some residues such as serine 124 and
threonine 450 in the mouse Akt1/PKB? are constitu-
tively phosphorylated in a growth factor-independent
manner and were predicted to render the protein re-
sponsive to subsequent activation events . Two
other residues that are rapidly phosphorylated upon
exposure togrowth factors and are the most critical for
full activation of Akt/PKB are threonine 308 (T308)
and serine 473 (S473). T308 resides within the activa-
tion loop of the kinase domain and S473 lies in the
carboxy-terminal tail. When these twoaminoacids are
mutated tononphosphorylatable residues activation of
the kinase is abolished, whereas mutations to acidic
residues render the kinase more active even in the
absenceof growth factors [62, 36]. Thephosphorylation
of T308 and S473 induced by IGF-1 or insulin is sen-
sitive to wortmannin, suggesting that this process is
dependent on PI 3-kinase . Two possible explana-
tions for this phenomenon are that the binding of phos-
pholipids to the PH domain of Akt/PKB is a prerequi-
site for its availability to other kinases or that the
kinases that phosphorylate T308 and S473 are also
dependent on PI 3-kinase. It turns out that both expla-
nations are correct. The enzyme that phosphorylates
T308 was purified and cloned [63, 64, 40, 41]. The
ability of the enzyme to phosphorylate T308 is depen-
dent on the presence of synthetic PI(3,4,5)P3 in vitro
and therefore it was termed 3-phosphoinositide-depen-
dent kinase (PDK1) [64, 41]. PDK1 possesses a PH
domain in its carboxy-terminus and binds with high
affinity to PI(3,4,5)P3 and more weakly to PI(3,4)P2.
Deletion of thePH domain of PDK1 and mutations that
decrease binding to PI(3,4,5)P3 strongly decrease its
ability to activate Akt1/PKB? . However, the ki-
nase activity of PDK1 is tolerant to low concentrations
KANDEL AND HAY
of wortmannin. This is likely to be explained by a
relatively high affinity of the PH domain of PDK1 to
PI(3,4,5)P3 [41, 65]. Although it was reported that
PDK1 can be translocated to the plasma membrane
upon growth factor stimulation , other studies us-
ing immunoelectron microscopy, confocal microscopy,
and a green fluorescent protein–PDK1 chimera clearly
show that it is mostly cytosolic and remains so upon
stimulation . Nevertheless a small portion of PDK1
was always found in the plasma membrane even in
unstimulated cells and this may be alsodue tothe high
affinity of its PH domain to PI(3,4,5)P3. Alternatively
another unknown factor is required for the binding of
PDK1 to the plasma membrane.
It is possible that the membrane-bound PDK1 may
be required for the phosphorylation of Akt/PKB and
other membrane-localized substrates, whereas the cy-
tosolic form is required for the phosphorylation of cy-
tosolic proteins such as p70 S6 kinase. Like Akt/PKB,
PDK1 is evolutionarily conserved [64, 67] and genetic
studies in Caenorhabditis elegans confirm that PDK1
lies upstream of Akt but downstream of PI 3-kinase.
PDK1 gain-of-function mutant bypasses the require-
ment of PI 3-kinase for Akt activation .
In addition to T308, the phosphorylation of S473 is
also required for maximal activation of Akt/PKB [62,
36]. The findings that PDK1 cannot phosphorylate
S473 in vitro or in cotransfection experiments sug-
gested that a distinct kinase activity termed PDK2 is
responsible for this function [63, 40, 41] but the iden-
tity of this kinase has remained elusive. It has been
reported that integrin-linked kinase (ILK-1) is capable
of phosphorylating S473 in vitro and in cotransfection
experiments . However, others failed to reproduce
these results . Recently it was shown that PDK1
interacts specifically in vitro and in vivo with the car-
boxy-terminus region of protein kinase C-related ki-
nase (PRK2) that was termed PDK1-interacting frag-
ment (PIF) . The interaction of PDK1 with PIF
converts it to an enzyme that can phosphorylate both
T308 and S473 residues in Akt/PKB . The possibil-
ity exists therefore that PDK1 can phosphorylate both
residues in vivo depending on postranslational confor-
mational change and/or interaction with another cellu-
lar protein. A related observation may be the finding
that one point mutation in PDK1 of C. elegans is suf-
ficient to bypass the requirement of PI 3-kinase for
Akt/PKB activation . It remains to be determined,
however, whether this constitutively active form of
PDK1 is capable of phosphorylating both T308 and
S473. Interestingly, the minimal functional fragment
of PIF contains the putative PDK2 recognition site
with serine substituted with the negatively charged
aspartate . Thus, PIF may be considered a mimic of
PDK2 substrate. While modulation of PDK1 specificity
by endogenous PRK2 remains tobe demonstrated, it is
tempting to speculate that once S473 or an equivalent
residue in other enzymes is phosphorylated it will
serve as a catalyst that primes PDK2 activity in PDK1.
Another possibility that cannot be completely excluded
at present is that T308 phosphorylation permits auto-
phosphorylation of S473 by Akt/PKB itself. This possi-
bility cannot be ruled out by the observation that a
kinase-deficient mutant of Akt/PKB can still be phos-
phorylated on S473 , because multimerization with
the wild-type Akt/PKB in vivo may enable the phos-
phorylation of S473 of the mutant protein.
Negative Regulation of Akt/PKB Activity
Similar to other protein kinases, Akt/PKB is subject
to negative regulation. The PH domain of Akt/PKB for
example may act both as a negative and as a positive
regulator of the enzyme. Deletion of the PH domain
rendering the enzyme incapable of interaction with
3-phosphoinositides leads to a slightly higher basal
kinase activity than that of wild type, but this activity
can still be normally elevated in a PI 3-kinase-depen-
dent manner [34, 64]. This suggests that in its inactive
form (not bound to 3-phosphoinositides) the PH do-
main may confer a conformation that is not accessible
to PDKs. Another possibility is that the PH domain
interacts with a cellular protein that negatively regu-
lates the kinase and binding to 3-phosphoinositides
relieves this interaction.
The subcellular localization of Akt/PKB is also
tightly regulated and may provide a mechanism for
regulating cytoplasmic Akt/PKB activity. After 2 min
of stimulation with IGF-1, Akt1/PKB? is translocated
to the plasma membrane in a PH domain-dependent
manner but following this Akt/PKB is translocated to
the nucleus by an unknown mechanism . Nuclear
translocation is probably independent of the PH do-
main or kinase activity because both a mutant that
lacks the PH domain and a kinase-deficient mutant of
Akt/PKB are also found in the nucleus . The phys-
iological significance of the nuclear localization is not
clear. It might be required for phosphorylation of nu-
clear proteins. Alternatively, the sequestration in the
nucleus could bea way tolimit theexposureof cytosolic
substrates to the kinase and might serve as a mecha-
nism that indirectly negatively controls the kinase. It
has to be noted that Akt/PKB targeted to the plasma
membrane via a myristoylation signal exhibits consti-
tutively active phenotype in regard to all known func-
tions of activated wild-type enzyme, but fails to trans-
locate to the nucleus .
As described above phosphorylation of Akt/PKB is
required for its activation. It appears that this phos-
phorylation is tightly controlled. The facts that the key
phosphoserine and phosphothreonine residues in Akt/
PKB have a relatively short half-life and that phospha-
REGULATION AND ACTIVITIES OF AKT/PKB
tase inhibitors such as vanadate and okadaic acid were
shown to augment Akt/PKB kinase activity both indi-
cate that the enzyme is negatively regulated by de-
phosphorylation [70, 71]. Phosphatase 2A (PP2A) may
be the key enzyme associated with dephosphorylation
of Akt/PKB in vitro and in vivo [70–72]. Hyperosmotic
shock rapidly inactivates Akt/PKB and this is preceded
by dephosphorylation of T308 and S473. The dephos-
phorylation and the decrease in Akt/PKB activity can
be prevented by calyculin A, a relatively specific inhib-
itor of PP2A .
It was alsoshown that the major Src family tyrosine
kinase in hematopoeitic cells, Lyn, antagonizes the
activation and phosphorylation of Akt/PKB by BCR in
B cells . It is possible that Lyn exerts its effect by
activation of serine/threonine phosphatases or by acti-
vation of a 3-phosphoinositide-preferring phosphatase
that antagonizes PI 3-kinase activity.
A 3-phosphoinositide-specific phosphatase activity
was found toreside in the tumor suppressor PTEN (for
phosphatase and tensin homologue deleted from chro-
mosome 10), which is mutated or deleted in a wide
range of human cancers (reviewed in ). PTEN
shares homology with dual-specificity phosphatases
that can dephosphorylate serine, threonine, and ty-
rosine residues. However, attempts to confirm PTEN
as a protein phosphatase revealed only relatively weak
activity [75, 76], suggesting that PTEN is a specialized
phosphatase for certain proteins and/or it possesses a
different activity. Indeed it was found that PTEN is a
potent lipid phosphatase [77, 78]. Overexpression of
PTEN significantly reduced PI(3,4,5)P3 production in-
duced by insulin, and PTEN-null cells have higher
levels of PI(3,4,5)P3 [79, 77, 80]. A recombinant PTEN
dephosphorylates 3-phosphoinositides specifically at
position 3 of the inositol ring and has highest specific-
ity for PI(3,4,5)P3 . Since PTEN antagonizes the PI
3-kinase activity its role in tumor suppression may
involve Akt/PKB (see discussion below). Experiments
in tumor cell lines with inactive PTEN and in PTEN-
null fibroblasts showed that these cells exhibit high
basal activity of Akt/PKB [78–82]. These results, to-
gether with genetic studies in C. elegans demonstrat-
ing that PTEN lies in the same pathway with PI 3-ki-
nase/Akt and inhibits Akt , established PTEN as a
bona fide negative regulator of Akt/PKB.
Another lipid phosphatase that can negatively regu-
late Akt/PKB activity is SHIP—an inositol 5? phospha-
tase that hydrolyzes PI(3,4,5)P3 to PI(3,4)P2. Overex-
pression of SHIP was shown to inhibit Akt/PKB
activity and SHIP-null cells exhibit prolonged activa-
tion of Akt/PKB upon stimulation [42, 43]. SHIP1 is
mostly found in hematopoeitic cells, while another iso-
form (SHIP2) is widely expressed in nonhematopoietic
cells [84, 85]. The relative roles of PTEN and SHIP in
the regulation of PIP3 levels may well be tissue and
The Emerging Model for the Regulation of Akt/PKB
Activity by Growth Factors
The current model for the activation of Akt/PKB is
based on the studies described above and is schemati-
cally illustrated in Fig. 2. Upon binding of a growth
factor to a tyrosine kinase receptor, the receptor is
activated and phosphorylated on tyrosine residues.
The phosphotyrosine residues can then recruit Src ho-
mology 2 (SH2)-containing proteins (see the review by
J . Pessin in this issue). The regulatory subunit p85 of
PI 3-kinase is recruited to the active receptor through
binding of its SH2 domain tospecified phosphotyrosine
residues in the receptor, and in the case of insulin
receptor this recruitment is largely mediated by the
IRS adapter proteins (see the review by J . Pessin in
this issue). Recruitment of the catalytic subunit, p110,
tothe receptor via the regulatory subunit and the close
proximity to the plasma membrane lead to its activa-
tion. ThePI(3,4,5)P3 produced by theactivated PI 3-ki-
nase then binds to the PH domain of the dormant
cytosolic form of Akt/PKB. Increased production of
PI(3,4,5)P3 also activates PDK1 and PDK2. The iden-
tity of the latter still remains controversial.
Binding of PI(3,4,5)P3 to the PH domain induces a
conformation change and recruitment to the plasma
membrane, which in turn expose Akt/PKB to phos-
phorylation by PDK1 at T308 and subsequent phos-
phorylation of S473 by PDK2. These events promote
full activation of Akt/PKB. Following activation, Akt/
PKB is detached from the plasma membrane and
translocates to the cytosol and the nucleus. The trans-
location to the nucleus may have a dual role, one is to
have access to target proteins in the nucleus and the
other is toregulate its activity in cytoplasm by seques-
tration in the nucleus. The activity of Akt/PKB is also
regulated by PTEN that counteracts PI 3-kinase and
by SHIP that converts PI(3,4,5)P3 to PI(3,4)P2.
The model described above is based mostly on exper-
iments performed with Akt1/PKB?. Several reports
show that the same paradigm of activation is true for
Akt2/PKB? and Akt3/PKB?, although thedifferent iso-
forms can be activated to different extents in different
tissues and in response to different stimuli and it is
possible that they exhibit different affinities for cellu-
lar substrates [71, 28, 86, 87, 30, 21].
Activation of Akt/PKB by PI 3-Kinase-Independent
Several reports have suggested that Akt/PKB could
be activated in a PI 3-kinase-independent manner.
cAMP-elevating agents such as forskolin, chlorophe-
nylthio-cAMP, prostaglandin E1, and 8-bromo-cAMP
KANDEL AND HAY
F IG. 2.
receptor elicits recruitment and activation of PI 3-kinase. Upon activation, PI 3-kinase catalyzes production of phosphoinositides phosphor-
ylated at position 3. PI(3,4,5)P3 then binds with high affinity to the PH domain of PDK1 and Akt/PKB. The action of PI 3-kinase is
counteracted by the lipid phosphatases that reduce the intracellular level of PI(3,4,5)P3. PTEN converts PI(3,4,5)P3 to PI(4,5)P2, and SHIP
converts PI(3,4,5)P3 to PI(3,5)P2 (see text for details). (B) Upon binding to PI(3,4,5)P3, Akt/PKB is anchored to the plasma membrane. This
is accompanied by a conformational change that relieves the inhibitory function of the PH domain and exposes Akt/PKB to subsequent
activation by other kinases. Both PDK1 and PDK2 activities are increased by PI(3,4,5)P3, then they phosphorylate threonine 308 and serine
473 on Akt/PKB to fully activate the kinase. The identity of PDK2 is still questionable, with ILK or PDK1 combined with PIF being the
candidate enzymes. Subsequently, activated Akt/PKB translocates to the cytoplasm and the nucleus (see text for details).
Schematic illustration of the current model of Akt/PKB activation by growth factos. (A) Growth factor binding totyrosine kinase
were shown to activate Akt/PKB through PKA al-
though toa lesser extent than insulin [88, 89]. The PH
domain of Akt/PKB is not required for this activation
and phosphorylation of S473 is not necessary for PKA-
induced activation, although the phosphorylation of
T308 is required . Because PKA does not activate
PI 3-kinase and cAMP-mediated activation of Akt/PKB
is wortmannin resistant it was concluded that the ef-
fect is PI 3-kinaseindependent . Themechanism by
which PKA activates Akt/PKB is not clear but is un-
likely to involve direct phosphorylation as it is not
dependent on the putative PKA site in Akt/PKB .
Ca2?/calmodulin-dependent kinase was reported to ac-
tivate Akt/PKB directly in vitro and in cotransfection
experiments through phosphorylation of T308 . It
was alsoreported that Akt/PKB is activated by cellular
stress and heat shock through association with HSP27
[23, 91] and that the ?-adrenergic agonist, isoprotere-
nol, can activate Akt/PKB in a wortmannin-resistant
manner . In endothelial cells it was shown that
?-catenin in a complex with VE-cadherin acts syner-
gistically with PI 3-kinase to activate Akt/PKB .
The significance of these findings remains to be deter-
WHAT ARE THE PHYSIOLOGICAL FUNCTIONS
Cellular Substrates of Akt/PKB
To understand the physiological functions of Akt/
PKB it is important to identify its natural substrates.
The use of a histone H2B as a substrate to assay
Akt/PKB kinase activity in vitro and the identification
of Akt/PKB phosphorylation sites in GSK-3? and
GSK-3? helped the deduction of the optimal Akt/PKB
phosphorylation site . The minimum consensus se-
quence for efficient phosphorylation by Akt/PKB was
established by Alessi and colleagues as Arg-Xaa-Arg-
Yaa-Zaa-Ser/Thr-Hyd, where Xaa is any amino acid,
Yaa and Zaa are small amino acids other than glycine,
and Hyd is a bulky hydrophobic residue . Protein
database search identifies a large number of proteins
with this consensus site (N.H., unpublished). However,
the specificity of this consensus site is likely dependent
on the context in which this site resides and the con-
formation of the protein. In addition, there are other
protein kinases such as PKA, MAPKAP-K1, p70 S6-
kinase, and p90RSK that can phosphorylate a similar
consensus site. Therefore, identification of a consensus
site in a given protein and demonstration that Akt/
PKB can phosphorylate the protein in vitro does not
necessarily imply that the protein is a physiological
target for Akt/PKB in vivo. One important factor in
predicting a physiological substrate is the conservation
of this site in the cadidate substrate among different
mammalian species and in evolution.
One system in which Akt/PKB plays a physiological
role is the regulation of glycogen synthesis in insulin-
responsive tissues. Serine 21 in GSK3? and serine 9 in
GSK3? are phosphorylated by Akt/PKB  and this
serves to negatively regulate GSK3 activity and to
maintain glycogen synthase in a relatively dephospho-
rylated and active state. Serine 9 in GSK3? was shown
tohave a very high affinity for Akt/PKB in comparison
with other protein kinases [14, 62]. This site is con-
served in vertebrates and in Drosophila, suggesting
that GK3? is a physiological substrate of Akt/PKB. In
addition to GSK3, other targets associated with the
metabolic effects of insulin and glucose metabolism
have been identified. Akt/PKB phosphorylates and in-
duces the activity of the heart 6-phosphofructo-2-ki-
nase (PFK-2), which in turn stimulates glycolysis .
Insulin induces activation of phosphodiesterase 3B
(PDE3B) through a phosphorylation of a serine residue
in PDE3B which in turn regulates intracellular levels
of cAMP. This serine residue of PDE3B was shown to
be phosphorylated by Akt/PKB . Other down-
stream effectors of insulin and growth factors include
the regulator of protein synthesis eIF-4E, p70 S6 ki-
nase, and eIF-2B. The repressor of translation, eIF-4E
binding protein (4E-BP), regulates the elongation/ini-
tiation factor eIF-4E (for review see ). In unstimu-
lated cells eIF-4E and 4E-BP exist as an inactive het-
erodimer. Upon growth factor stimulation 4E-BP is
phosphorylated and dissociates from eIF4E, which
then is free to bind eIF4G and activate mRNA trans-
lation. The phosphorylation of 4E-BP is dependent on
Akt/PKB and the target of rapamycin, mTOR or FRAP
. mTOR/FRAP, which is directly phosphorylated by
Akt/PKB, may then directly phosphorylate 4E-BP [98–
100]. In addition, a parallel pathway that elicits indi-
rect phosphorylation of 4E-BP through Akt/PKB is also
required tofully inactivate4E-BP [97, 100]. Thep70 S6
kinase regulates mRNA translation through the phos-
phorylation of the 40S ribosomal protein S6 .
Phosphorylation of p70 S6 kinase by growth factors
leads to its activation and this phosphorylation is in-
directly dependent on Akt/PKB. However, the issue
remains controversial since Akt dependence of p70 S6
kinase phosphorylation was not confirmed in studies
using dominant-negative Akt/PKB . The guanine
nucleotide exchange factor eIF-2B that is required for
initiation of protein synthesis is inactivated by GSK3
and therefore is considered an indirect target of Akt/
PKB as well .
Thus far, the only target of Akt/PKB that has been
identified through genetic studies in C. elegans is the
forkhead transcription factor DAF-16 . The studies
in C. elegans suggest that insulin and Akt negatively
regulate DAF-16. Three homologues of DAF-16 have
been identified in human and mouse and termed
FKHR, FKHRL, and AFX [104–107]. DAF-16 and its
KANDEL AND HAY
mammalian homologues contain three consensus sites
that can be effectively phosphorylated in vitro by Akt/
PKB and in cotransfection experiments and this mod-
ification serves to retain the transcription factors in
the cytoplasm and to inhibit their activity [108–111].
The transcription of multiple genes may well be in-
fluenced by Akt/PKB-mediated phosphorylation of
FKHR, FKHRL, and AFX. It was shown that FKHR
binds the insulin response sequence, which is present
in multiple genes that are inactivated by insulin. In-
sulin inhibits the activation of these genes by inactiva-
tion of FKHR. The phosphorylation of serine 256 of
FKHR by Akt/PKB was shown to be necessary and
sufficient to mediate this effect .
The ability of Akt/PKB to mediate growth factor-
induced cell survival led toa search for target proteins
that exert this effect. The proapoptotic Bcl-2 family
member Bad has been shown tobe phosphorylated and
caspase-9 that executes apoptosis was shown to be
phosphorylated and inactivated by Akt/PKB . Al-
though these mechanisms may explain some circum-
stances in which Akt/PKB promotes cell survival, they
may not be the major mechanisms by which growth
factors and Akt/PKB mediate cell survival (see discus-
Recently it was shown that Akt/PKB could regulate
the level of nitric oxide (NO) through direct phosphor-
ylation and activation of endothelial NO synthase[116,
117]. It was proposed that Akt/PKB regulates contin-
uous formation of NO that is generated by the “shear
stress” induced by the streaming blood in the endothe-
lial layer of blood vessels. NO then diffuses to neigh-
boring smooth muscles where it is involved in cGMP-
[113, 114]. The human
Cellular Processes That Are Regulated by Akt/PKB
Identification of targets for Akt/PKB activity helped
the deduction of cellular pathways in which Akt/PKB
functions. The Akt/PKB targets described above sug-
gest that Akt/PKB is associated with a wide range of
activities in the cell, some of which are schematically
illustrated in Fig. 3.
Gene transcription.As mentioned above Akt/PKB
mediates regulation of transcription of certain genes
through direct phosphorylation of FKHR, FKHRL, and
AFX (Fig. 3A). The inactivation of these transcription
factors was reported to downregulate transcription of
the IGF binding protein 1 gene through the IRS ele-
ment as well as transcription from the Fas ligand pro-
moter [109, 112]. Other transcription factors that may
be indirect targets for Akt/PKB activity include cAMP-
responsive element binding protein (CREB), E2F, and
NF-?B (Fig. 3A). Akt/PKB has been reported to phos-
phorylate the transcription factor CREB  and to
induce transcription of the Bcl-2 family member Mcl-1
through the cAMP-responsive element . Activated
Akt/PKB was shown to be sufficient to induce E2F
activity . Akt/PKB is abletoactivatetranscription
from the IL-2 promoter through the activation of the
transcription factor NF-?B in T cells . The activa-
tion of NF-?B by Akt/PKB is executed through activa-
tion of NF-?B inhibitor (I?B) kinase (IKK) and degra-
dation of I?B .
The inhibition of GSK3 by Akt/PKB might have an
impact on the activity of various transcription factors
that are regulated by GSK3. For example, GSK3?
phosphorylates ?-catenin associated with the product
of adenomatous polyposis coli gene and axin protein
complex, thereby promoting its degradation by the pro-
teosome. As Akt/PKB phosphorylation of GSK3? inhib-
its the activity of the latter, it may lead tostabilization
of ?-catenin. ?-Catenin can then translocate to the
nucleus, form a complex with the T-cell factor/lym-
phoid enhancer factor (TCF/LEF) family of high-mobil-
ity-group transcription factors (Fig. 3A), and increase
their activity (reviewed in ).
Phosphorylation by GSK3 was also reported to in-
hibit the DNA binding activity of c-J UN, the het-
erodimeric partner of FOS and a component of the
AP-1 transcription complex (Fig. 3A and [122, 123]).
Other genes that were reported to be transcription-
ally regulated by Akt/PKB include the fatty acid syn-
thase promoter , c-fos , and GLUT1 and
GLUT3 [126, 127], which are activated, and the phos-
phoenol pyruvate carboxykinase gene, which is re-
pressed . Akt/PKB may also activate the hypoxia-
inducible factor-1 transcriptional response element
Protein synthesis. As described above, Akt/PKB
modulates the activity of multiple factors that are re-
quired for mRNA translation (Fig. 3B). Akt/PKB can
induce initiation of mRNA translation through phos-
phorylation of 4E-BP and activation of eIF-4E . The
observation that Akt/PKB may regulate the activity of
mTOR/FRAP  opens the possibility that it can lead
to the activation of elongation of translation through
p70 S6 kinase and the elongation factor eEF2 [130,
131]. The inhibition of GSK3 by Akt/PKB could poten-
tially increase mRNA translation through activation of
Glucosemetabolism. Akt/PKB can regulate several
levels of glucose metabolism (Fig. 3C). It enhances
glucose uptake in insulin-responsive tissues by affect-
ing the glucose transporters GLUT1, GLUT3, and
GLUT4. Akt/PKB induces the expression of GLUT1
and GLUT3 [126, 127] and the translocation of GLUT4
tothe plasma membrane [133–135]. Akt/PKB may also
affect glucose transport through a general increase of
endocytosis . It activates glycogen synthesis
REGULATION AND ACTIVITIES OF AKT/PKB
KANDEL AND HAY
through the inactivation of GSK3, which in turn leads
to activation of glycogen synthase . Akt/PKB may
induce glycolysis in multiple ways. Thus far, it was
shown that Akt/PKB can directly activate the cardiac
PFK-2 , which activates the rate-limiting enzyme
in mammalian glycolysis, PFK-1 through generation of
fructose 2,6-bisphosphate (Fig. 3C).
Akt/PKB and the Cell Cycle
Although it remains to be proven that Akt/PKB ac-
tivity is required for cell cycle progression per se, sev-
eral downstream targets of Akt/PKB could potentially
impact cell cycle progression directly. The ability of
Akt/PKB to induce E2F activity , as well as its
ability to elevate cyclin D1 levels by induction of nu-
clear mRNA translocation and mRNA translation 
and by increasing cyclin D1 protein stability , may
enhance cell cycle progression. It was shown that
GSK3? phosphorylates cyclin D1 protein and triggers
its redistribution from the nucleus to the cytoplasm
followed by a rapid degradation . It implies that
inactivation of GSK3? by Akt/PKB elevates the cyclin
D1 level and increases its accumulation in the nucleus.
The ability of activated Akt/PKB to induce transcrip-
tion of c-Myc  can also contribute to induction of
cell cycle progression. In addition, Akt/PKB may regu-
late the levels of the cyclin kinase inhibitor p27
through mTOR/FRAP. The possibility that Akt/PKB
down regulates p27 was substantiated by the observa-
tion that ectopic expression of PTEN elicits growth
arrest through elevation of p27 [81, 140].
Akt/PKB and Cell Survival
In the past 3 years mounting evidence has suggested
that one of the major functions of Akt/PKB is to pro-
mote growth factor-mediated cell survival and to block
programmed cell death or apoptosis. Thus far, Akt/
PKB is perhaps the only protein kinase that consis-
tently and reproducibly demonstrates antiapoptotic ac-
tivity in a wide range of biological systems.
The demonstration that growth factor-mediated cell
survival is dependent on PI 3-kinase [141, 142], and
the failure of the parallel Ras/Raf/MAP-kinase cascade
to consistently promote cell survival [141, 16, 17], in-
spired the idea that Akt/PKB is the downstream effec-
tor of growth factor-mediated cell survival [15–18]. Ini-
tial experimental evidence showed that Akt/PKB is
sufficient and necessary to promote cell survival by
growth factors [15–18]. Ectopic expression of domi-
nant-negative alleles of Akt abrogates the ability of
IGF-1 to promote cell survival of cultured cerebellar
granule cells and the ability of IGF-1 and fetal calf
serum to promote survival of fibroblasts [15, 17]. The
dominant-negative form of Akt/PKB that was used in
these studies has a point mutation in the ATP binding
site that converts lysine 179 to methionine or alanine
and abolishes its kinase activity . Ectopic expres-
sion of a constitutively active form of Akt/PKB that is
targeted to the plasma membrane (gag-Akt/PKB or
myristolated-Akt/PKB) has been shown topromote cell
survival in the absence of IGF-1 or serum [15–18]. The
initial experimental evidence was subsequently vali-
dated in a wide range of studies showing that Akt/PKB
promotes cell survival in various cell types. This in-
cludes several types of hematopoeitic cells [139, 114,
143–147, 54, 43]; several types of neuronal cells, in-
cluding sympathetic neurons, motorneurons, and hyp-
pocampal neuronal cells [148–150]; and epithelial and
endothelial cells [46, 151–154].
Activated Akt/PKB blocks cell death induced by a wide
range of apoptotic stimuli. In addition to growth factor
withdrawal, constitutively activated Akt/PKB defends
cells against overexpression of Myc [16, 17], UV irradia-
tion and DNA damage[18, 155], matrix detachment ,
and anti-Fas antibody and TGF? [156–158, 154]. The
emerging picture is that, like growth factors, Akt/PKB is
a general inhibitor of apoptosis and the activated Akt/
PKB form that is anchored to the plasma membrane
exhibits a more robust antiapoptotic activity than indi-
vidual growth factors because it is overexpressed and
constitutively active. The function of Akt/PKB was fur-
ther substantiated by the finding that PTEN null cells
exhibit higher Akt/PKB kinase activity and are more
resistant to apoptosis (reviewed in ).
F IG. 3.
phosphorylation inactivates FKHR, while inactivation of GSK3? relieves the inhibition of AP-1. The same mechanism prevents degradation
of ?-catenin in complex with APC and allows ?-catenin to potentially transactivate transcription in association with TCF/LEF proteins.
Akt/PKB activates NF-?B through IKK activation that leads todegradation of I?B. Themechanisms through which Akt/PKB activates CREB
and E2F remain unknown. Bold arrows indicate activation or inhibition of transcriptional activity (see text for details). (B) Several functions
of Akt contribute to enhancement of mRNA translation. Inhibition of GSK3 relieves inhibition of eIF2B. Activation of mTOR/FRAP leads to
phosphorylation of 4E-BP, preventing it from sequestering eIF-4E. It also potentiates the function of p70s6k kinase and prevents
eEF2K-mediated inhibition of eEF2. Both 4E-BP and p70s6k may be also activated in an Akt/PKB-dependent mTOR/FRAP-independent
pathway. See text for details. (C) Akt/PKB effects glucose metabolislm via inhibition of GSK3 and increase in glycogen synthesis, via
increased glucose uptake through at least three glucose transporters and through activation of PFK-2 that stimulates the rate-limiting
enzyme of glycolysis (PFK-1). Note that Akt/PKB may be involved in carbohydrate metabolism through other routes as well, and not all of
these pathways may function at the same time in the same cell (see text for details).
Metabolic functions of Akt/PKB. (A) Akt/PKB directly or indirectly controls activity of many transcription factors. Direct
REGULATION AND ACTIVITIES OF AKT/PKB
The function of Akt/PKB as a mediator of cell sur-
vival is conserved in Drosophila. A point mutation that
inactivates Drosophila Akt/PKB causes embryonic le-
thality . The mutant embryos showed extensive
apoptosis and the phenotype could be rescued by ex-
pression of baculovirus protein P35, which inhibits ac-
tivation of caspases, the executioners of apoptosis
. Akt/PKB was also found to be partially required
to prevent apoptosis in the Drosophila eye . On
the other hand, while the regulation of mammalian
Akt/PKB and that of its homologues from C. elegans
are similar, there is no evidence that the latter are
linked to the control of apoptosis (see discussion be-
The major task ahead is elucidation of the mecha-
nisms by which Akt/PKB promotes cell survival. It is
possible that some of the already established functions
of Akt/PKB detailed above, such as gene transcription,
mRNA translation, and glycolysis, may contribute di-
rectly or indirectly toits antiapoptotic effect. However,
the robust antiapoptotic activity of Akt/PKB in the
absence of de novo gene expression has prompted a
search for more direct effects on the apoptotic cascade.
Programmed cell death or apoptosis in mammalian
cells is a multistep process (reviewed in ). In the
current model an early event is the loss of mitochon-
drial integrity followed by cytochrome c release. The
released cytochrome c then binds to the apoptotic pro-
tease-activating factor (Apaf-1) and activates it. Apaf-1
then binds to, cleaves, and activates the cysteine pro-
tease, caspase 9. This initiates a caspase cascade cul-
minating in the activation of the executioner caspases,
caspase 3 and caspase 7 (reviewed in ). Critical
regulators in this pathway are various members of the
Bcl-2 family. These proteins include antiapoptotic ef-
fectors such as Bcl-2 and Bcl-xL and their proapoptotic
counterparts such as Bad, Bid, and Bik (the BH3 sub-
family) and Bax and Bak (the Bax subfamily) (re-
viewed in ). A major question therefore becomes
whether Akt/PKB somehow enhances the activity of
antiapoptotic Bcl-2 family members or reduces the ac-
tivity of one or more proapoptotic congeners. Both hy-
potheses have received some support but they seem
unlikely to explain the potent survival-promoting ef-
fects of Akt across many cell types. For example, an
increase in Bcl-2 expression was reported in activated
Akt-overexpressing hematopoeitic cells, but this phe-
nomenon may be due to an increase in production of
interleukins 2 and 3 and is unlikely to be a general
mechanism. In fact, Akt/PKB does appear to change
the steady-state levels of Bcl-2 family members but
inhibits the processing and activities of caspase 3 and
caspase 7 [17, 155], suggesting that Akt/PKB may in-
tervene in the apoptotic cascade by directly or indi-
rectly phosphorylating key regulators of apoptosis or
through another posttranslational modification.
Some members of the Bcl-2 family, such as Bcl-2 and
Bcl-xL, are predominantly localized to the outer mem-
brane of mitochondria, while others interact with mi-
tochondria indirectly. For example, the proapoptotic
protein Bad does not possess a mitochondrial targeting
sequence but rather localizes the mitochondria, by het-
erodimerization with Bcl-2 family members .
Phosphorylated Bad remains cytosolic due to its inter-
action with the chaperone 14-3-3 proteins, and a criti-
cal phosphorylated residue in Bad is serine 136, which
is phosphorylated by Akt/PKB both in vivoand in vitro
[113, 114]. Indeed, Akt/PKB can block apoptosis in-
duced by overexpression of wild-type Bad but is less
effective against a serine 136 mutant of Bad [113, 114,
155]. Thus, phosphorylation of Bad could be one mech-
anism by which Akt/PKB blocks apoptosis. However,
while Akt/PKB blocks apoptosis induced by other pro-
apoptotic Bcl-2 family members and in response to
diverse stimuli  and promotes survival of cells
from various tissues origins, Bad is expressed at very
low levels and not ubiquitously. This suggests that
phosphorylation of Bad is unlikely to be the major and
general mechanism by which Akt/PKB blocks apopto-
It was recently shown that Akt/PKB, like growth
factors, maintains the integrity of mitochondria and
prevents the release of cytochrome c that initiates the
apoptotic cascade . Although the exact mecha-
nism by which growth factors of Akt/PKB maintain the
integrity of mitochondria is not yet known, it seems
probable that this represents a more general mecha-
nism by which Akt/PKB fulfills its antiapoptotic func-
tion. This may explain why Akt/PKB has not yet been
implicated in apoptosis in C. elegans as there is no
evidence for the involvement of mitochondria and cy-
tochromec in apoptosis in this organism. In Drosophila
in which, like in mammalian cells, Akt/PKB is anti-
apoptotic, the mitochondria and cytochrome c play an
active role in the induction of apoptosis . Because
the integrity of mitochondria is dependent on general
metabolism and glycolysis it is possible that the activ-
ities of Akt/PKB in these processes, as described above,
may alsocontribute toits antiapoptotic function. Inter-
estingly, overexpression of GSK3 elicits apoptosis and
Akt/PKB that phosphorylates and inactivates GSK3
can block it .
Another target of Akt/PKB in the cell death cascade
is caspase 9 . Once again it is unlikely that this is
the major mechanism by which Akt/PKB
apoptosis, because Akt/PKB cannot effectively block
apoptosis after cytochrome c release  and K.
Gottlob and N.H., unpublished results). Moreover, al-
though potential Akt/PKB phosphorylation sites are
present in human caspase 9, it is not clear whether
these sites are conserved in other mammalian species.
The phosphorylation of the Forkhead transcription
KANDEL AND HAY
factors by Akt/PKB was alsoattributed toits antiapop-
totic function . Similar towhat has been observed
with Bad, overexpression of FKHRL1 provoked apopto-
sis that can be prevented via phosphorylation by Akt/
PKB . It was also shown that FKHRL1 transcrip-
tionally activates Fas ligand promoter in transient
transfection experiments and, therefore, suggested
that Akt/PKB promotes cell survival by preventing the
expression of Fas ligand through the phosphorylation
and inactivation of FKHRL1 . It is not clear, how-
ever, whether this is a relevant major function of Akt/
PKB; first, it has not been demonstrated that Akt/PKB
is regulating the steady-state levels of endogenous Fas
ligand; second, as was indicated above Akt/PKB pre-
vents apoptosis downstream of Fas ligand production
[156, 154]; third, Akt/PKB prevents cytochrome c re-
lease after growth factor withdrawal, whereas inhibi-
tion of the downstream effector of Fas, caspase 8, does
not prevent cytochrome c release .
Because growth factors were shown to block apopto-
sis through maintaining mitochondrial metabolism, at
least in part, by retaining the access of ADP to the
mitochondria , it is tempting tospeculatethat this
might be the major mechanism by which Akt/PKB
promotes cell survival. In addition to its direct role in
apoptosis, Akt/PKB may promote long-term survival
through its metabolic functions such as protein synthe-
sis and glycolysis that can contribute to the overall
fitness of thecells. Thecurrent possiblemechanisms by
which Akt/PKB blocks apoptosis are summarized in
Akt/PKB and Cancer
The identification of Akt/PKB as a fusion protein
with retroviral gag in oncogenic retrovirus provided
the first clue that Akt/PKB might be a proto-oncogene.
Further experiments showed that ectopic expression of
activated Akt/PKB elicits cellular transformation in
vitro ([168, 169] and E.S.K. and N.H., unpublished).
The finding of an activated catalytic subunit of PI
3-kinase in avian sarcoma virus , and the observa-
tion that the tumor suppressor PTEN is frequently
mutated or deleted in a largenumber of human cancers
(reviewed in ), put Akt/PKB in a critical position in
the genesis of cancer. The frequency by which the
PTEN/PI 3-kinase/Akt (PKB) pathway is altered in
human tumors either directly or indirectly (e.g., by
activation of Ras) strongly suggests that the activation
of this pathway in human tumors will be as common as
the inactivation of p53 and p16.
The deficiency of PTEN in mice is embryonic lethal.
However, three different groups have reported high
incidence of tumors in colon, testes, thyroid, prostate,
liver, and hematopoeitic cells in heterozygous mice of
different genetic background [170–172].
In some human tumors activation of PI 3-kinase has
been reported . Akt/PKB itself was found overex-
pressed in various human cancers. Akt1 amplification
was reported in primary gastric adenocarcinoma ,
while Akt2 was found amplified and overexpressed in
pancreatic, ovarian, and breast cancer [175–179]. In
fact, Akt2 overexpression has been reported tocoincide
with poor prognosis and reduced survival among pa-
tients with ovarian cancer . Akt3 is up regulated
in estrogen receptor-deficient breast cancer and in an-
drogen-independent prostate cell lines . All tumor
cell lines in which PTEN was inactivated exhibit ele-
vated Akt/PKB kinase activity (reviewed in ). In
addition, Akt/PKB is required for the oncogenic activ-
ity of BCR-ABL and is activated by transforming Ras
mutants [46, 181].
How does activation of Akt/PKB contribute to the
genesis of cancer? In general, cancer is viewed as a
disease of abnormal cell survival and proliferation. Ac-
cordingly, activation of cellular proto-oncogenes may
F IG. 4.
prevents translocation of Bad to mitochondria by facilitating its
association with 14-3-3 proteins. However, Akt/PKB can maintain
the integrity of mitochondria via a Bad-independent and a caspase-
independent mechanism that prevents release of cytochrome c. Ad-
ditionally, direct or indirect effects of Akt/PKB on a number of
transcription factors may modualte expression of apoptosis-related
genes. The effect of Akt/PKB on protein synthesis may also have an
impact on cell survival. PI 3-kinase activates Akt/PKB and therefore
promotes cell survival, whereas PTEN and SHIP that negatively
regulate Akt/PKB enhance cell death. See text for details.
Akt/PKB and cell survival. Phosphorylation by Akt/PKB
REGULATION AND ACTIVITIES OF AKT/PKB
contribute to either one or both processes. The consid-
erable body of evidence described above documents the
antiapoptotic effect of Akt/PKB. This property is suffi-
cient to suggest that activation of Akt/PKB could be
oncogenic by preventing normal programmed elimina-
tion of cells, thereby enabling accumulation of more
oncogenic mutations in these cells. However, as de-
scribed above, it is possible that activation of Akt/PKB
can lead to enhanced cell cycle progression and prolif-
eration. Especially intriguing is the possibility that
activation of Akt/PKB bypasses a specific checkpoint in
cell cycle. This is supported by the observation that,
like p53, overexpression of PTEN can either elicit cell
cycle arrest or accelerate apoptosis and that activated
Akt/PKB can overcome both effects of PTEN [182, 80].
Akt/PKB and Diabetes
As discussed above, Akt/PKB mediates many of the
metabolic actions of insulin. Therefore, it is reasonable
to assume that Akt/PKB kinase activity has implica-
tions for diabetes. It was reported that insulin-stimu-
lated Akt/PKB kinase activity is reduced in skeletal
muscle of non-insulin-dependent diabetes (NIDDM)
patients [183, 184], and diabetes-prone mice exhibit
elevated activity of GSK3 which is negatively regu-
lated by Akt/PKB . However, thus far, no muta-
tions in Akt1 and Akt2 have been found in patients
with NIDDM . The observation that quantitative
rather than qualitative changes in Akt/PKB function
are found in the disease provides a hope that diabetes
may be helped by pharmacological modulation of Akt/
In vitro Akt/PKB was shown to mediate the differ-
entiation of insulin-responsive cell lines such as pre-
adipocytes and myocytes. Ectopic expression of Akt/
PKB appears to bypass the requirement for insulin to
differentiate these cells [133, 126, 187, 188]. The func-
tion of Akt/PKB in cell survival is probably also re-
quired for normal differentiation of these cells because
although mitotic response is halted survival must be
maintained. Thus, similar to the oncogenic Ras ,
an activated Akt/PKB can elicit either cellular trans-
formation or differentiation in vitro depending on the
SUMMARY AND FUTURE PERSPECTIVE
It is evident that Akt/PKB is a mediator of several
signaling pathways and cell survival. Therefore it is
likely that it will continue to be a focus of intense
research in years to come. Although substantial infor-
mation has been accumulated in the past few years,
many issues remain tobe resolved. Major progress has
been achieved in elucidating the mechanism by which
Akt/PKB is activated; yet there are few remained ques-
tions. For example, some upstream regulators of Akt/
PKB have not yet been identified and these include
PDK2 whose activity is required for full activation of
Akt/PKB. The PI 3-kinase-independent mechanisms of
Akt/PKB activation, which arepoorly understood, need
to be elucidated and the regulators of these mecha-
nisms need to be identified. The reason for the trans-
location of Akt/PKB to the nucleus is still a mystery.
Although some substrates of Akt/PKB have been iden-
tified, the most relevant physiological substrates need
to be found and selected.
A major task ahead is to elucidate the mechanisms
by which Akt/PKB executes its multiple functions, in
particular, the mechanisms by which Akt/PKB blocks
apoptosis and its role in cell cycle progression and
tumorigenesis. While Akt/PKB activation may cause
both oncogenic transformation and differentiation, it is
still unknown how either of these outcomes is deter-
mined. The search for targets and cofactors of Akt/PKB
that are differentially expressed among tissues and
tissue culture models is likely to bring an answer to
Finally, Akt/PKB may be an attractive therapeutic
target. Specific inhibitors of the kinase might be used
for cancer therapy, whereas specific activators might
potentially be used for treatment of diabetes and of
degenerative diseases that result from inappropriately
increased cell death.
We thank Clive Palfrey for critically reading the manuscript and
for valuable suggestions. Research in our laboratory is supported by
NIH Grants CA71874 and AG16972 to N.H. and by Quark Biotech.
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