Download Protein Phosphatase 2A as a Potential Target for Anticancer Therapy

38
Anti-Cancer Agents in Medicinal Chemistry, 2011, 11, 38-46
Protein Phosphatase 2A as a Potential Target for Anticancer Therapy
Peter Kalev2 and Anna A. Sablina1,2,*
1
Department for Molecular and Developmental Genetics, Flanders Institute for Biotechnology (VIB); 2Center for Human Genetics,
KULeuven, Belgium
Abstract: The kinase oncogenes are well-characterized drivers of cancer development, and several targeted therapies focused on both
specific and selectively nonselective kinase inhibitors have now been approved for clinical use. In contrast, much less is known about the
role of protein phosphatases, although modulation of their activities might form the foundation for an effective anti-cancer approach. The
serine-threonine protein phosphatase 2A (PP2A) is implicated in the regulation of numerous signaling pathways and may function as a
tumor suppressor. Recently pharmacological modulation of PP2A activity has been showed to have a potent anti-tumor activity in both in
vitro and in vivo cancer models. These studies implicate PP2A as a promising therapeutic target for the treatment of cancer.
Keywords: Protein phosphatase 2A, Drug Development, Cancer, CIP2A, FTY720, Forskolin, Norcantharidin derivatives, Cantharidin.
INTRODUCTION
The aberrant activation of cellular signalling cascades contributes directly to human diseases and plays a causal role in most if
not all cancers. The kinase oncogenes are well-characterized drivers
of cancer development [1, 2], which makes them attractive targets
for drug development. Several kinase inhibitors, including imatinib
(Gleevec) for chronic myelogenous leukemia (CML) and gastrointestinal tumors, and gefitinib (Iressa) for non-small-cell lung cancer, have been recently approved for therapeutic use. Although it is
clear that phosphatases antagonize the action of kinases, much less
is known concerning the role of protein phosphatases in development and progression of human cancers.
The Protein Phosphatase 2A (PP2A), a large family of heterotrimeric serine-threonine phosphatases, regulates numerous cell
signalling cascades mostly by opposing the activity of kinase oncogenes [3]. Initial evidence for the role of PP2A in cell transformation and tumorigenesis came from studies with the sponge toxin
okadaic acid (OA). OA is both a selective inhibitor of PP2A and a
strong tumor promoter [4, 5]. The finding that PP2A is an important
cellular target of several viral oncoproteins further supports the idea
that PP2A contributes to tumor suppression. More recently, a decrease in PP2A activity have been found in different types of human cancers. The loss of PP2A function occurs either through inactivating mutations of PP2A structural subunits, or by upregulation
of the cellular PP2A inhibitors such as Cancerous inhibitor of PP2A
(CIP2A) and SET.
The promise of kinase inhibitors as anticancer agents raised the
question whether protein phosphatases could also represent potential drug targets. Recently published reports have demonstrated that
modulation of PP2A activity could be beneficial for the treatment
of cancer. Furthermore, the involvement of PP2A in regulation of
multiple physiological functions provides us an opportunity to employ several different strategies for PP2A targeting therapies. Here
we will review recent progress in our understanding of PP2A as a
potential drug target for cancer treatment.
THE PP2A FAMILY
PP2A accounts for the majority of serine/threonine phosphatase
activity in mammalian cells [reviewed in [6]]. The term PP2A refers to a diverse family of phosphatases that contains a catalytic C
subunit, whose activity is regulated by a diverse set of regulatory
proteins. The most common forms of PP2A contain a highly active
core dimer composed of a catalytic C subunit, also called PP2AC or
*Address correspondence to this author at the CME Department,
KULeuven, O&N I Herestraat 49, bus 602, Leuven, Belgium 3000,
Tel: 32-16-330790; E-mail: [email protected]
1871-5206/11 $58.00+.00
PPP2C, and a scaffolding A subunit, also known as PR65 or PPP2R1.
The AC dimer recruits an additional third regulatory B subunit,
which is predicted to dictate the substrate specificity and localization of the PP2A heterotrimeric complex. Four unrelated families of
B subunits have identified to date: B/B55/PR55/PPP2R2, B’/B56/
PR61/PPP2R5, B’’/PR72/PPP2R3, and Striatin [7] (Fig. 1).
SET
CIP2A
TIP
PME1
PPP2CA/B
PTPA
LCMT1
Cα/Cβ
PPP2R1A/B
Aα/Aβ
PPP2R2A(α)
PPP2R2B(β)
PPP2R2C(γ) B
PPP2R2D(δ)
PPP2R3A(α)
PPP2R3B(β) B’’
PPP2R3C(λ)
B’’’
Striatin
Striatin-3
PPP2R5A(a)
B’
PPP2R5B(β)
PPP2R5C(γ)
PPP2R5D(δ)
PPP2R5E(ε)
Fig. (1). The PP2A family of serine/threonine heterotrimeric phosphatases. The PP2A holoenzyme represents a heterotrimeric complex composed of a catalytic subunit C and a structural subunit A, and a single regulatory B subunit. In mammals, two distinct genes encode closely related
versions of both the A and C subunits of PP2A, PPP2R1A/A and
PPP2R1B/A, and PPP2CA/C and PPP2CB/C correspondently. Four
unrelated families of PP2A B regulatory subunits (B /B´/B´´/B´´´) have been
identified to date. The assembly and activity of PP2A heterotrimeric complexes is regulated by multiple interacting proteins such as PP2A intracellular inhibitors (e.g. SET, CIP2, and TIP) and PP2A activators (PTPA and
LCMT1).
The mechanism of assembly of heterotrimeric PP2A complexes
is not completely understood. The dynamic exchange of PP2A
regulatory subunits and PP2A activity could be modulated by reversible methylation and phosphorylation of the carboxy-terminal
tail of the C subunit. Methylation at Leu309 by Leucine carboxyl
methyltransferase-1 (LCMT1) enhances the affinity of the PP2A
core dimer for PP2A regulatory subunits while phosphorylation at
Tyr307 might indirectly affect the assembly of PP2A holoenzyme
complexes by preventing methylation of the PP2AC subunit [8-11].
© 2011 Bentham Science Publishers Ltd.
PP2A as a Drug Target
Multiple tyrosine kinases, including c-src, EGFR, Her-2, and JAK
family members, have been shown to transiently inactivate PP2A
activity by phosphorylation at Tyr307 in response to cytokine
stimulation [12-16].
The assembly of PP2A complexes is also regulated by the
PP2A activator (PTPA or PPP2R4) that was initially identified as a
PP2A B regulatory subunit due to its ability to interact with the AC
dimer. PTPA potentially serves as a metal chaperone by activating
the phosphoserine/threonine activity of PP2AC from a poorly active
state [8, 17]. PTPA has demonstrated peptidyl prolyl cis/transisomerase (PPIase) activity in vitro, which acts specifically on
Pro190 of PP2AC [18]. This isomerase activity could potentially
control the folding of PP2AC and its activity. However, the relative
contribution of these PTPA activities to the assembly of specific
PP2A complexes is unknown.
In the absence of these modifications, the C subunit preferentially associates with TAP42/4 protein rather than with the PP2A
structural A subunit. It has been suggested that TAP/4 also acts as
a general scaffold/chaperone protein in maintaining proper PP2A
function and assembly [19].
The ability of the PP2A dimer to associate with numerous regulatory subunits suggests that the full complement of PP2A complexes may exceed more than 100 different multimers. The cellular
localization, substrate specificity and physiological functions of
specific PP2A complexes have comprehensively reviewed in [55,
87]. This remarkable complexity in the assembly of the holoenzyme
trimer provides PP2A with the ability to dephosphorylate a large
repertoire of different substrates and mediate a wide diversity of
physiological functions.
THE ROLE OF PP2A IN TUMOR SUPPRESSION
The first evidence that PP2A acts as a tumor suppressor came
from the longstanding observation that a PP2A inhibitor, OA, at
low concentrations (50-500 pM) promotes tumor growth in mouse
skin [5], stomach, and liver [20] and induces genomic instability
[21, 22]. PP2A is also found to be one of the major cellular binding
partners of several viral oncoproteins such as small and middle t
antigens of polyoma virus, and adenoviral E4orf4 protein (reviewed
in [23]), all of which contribute to cell transformation.
More recently, inactivating alterations of PP2A subunits have
been found to occur in human cancers, further supporting the idea
that PP2A is involved in suppressing cancer development. Spontaneous mutations in both and isoforms of the A structural
subunit have been found to occur in several types of human cancers. Specifically, somatic alterations of the gene encoding the
PP2A A subunit have been identified in 8–15% of colon cancers,
15% of lung cancers, 13% of breast cancers, and 6% of tumor cell
lines [24-29]. Mutations of the more abundant PP2A A subunit
have also been observed in breast and lung tumors, although at a
low frequency [24].
Most of the A cancer-associated mutants are composed of
missense mutations including G8R, P65S, L101P, K343E, D504G,
V545A, V448A, one contains the double mutant L101P/V448A,
and one an in frame deletion E344–E388 [24-29]. All cancerassociated mutations of PP2A structural subunits interrupt interaction between A and C subunits by preventing the formation of the
PP2A heterotrimeric holoenzymes [25, 26, 30]. Further studies
demonstrated that loss of PP2A A contributes to cancer progression through dysregulation of the Ras-like GTPase RalA activity
[30]. On the other hand, haploinsufficiency of PP2A A seems to
induce human cell transformation by activating the PI3K/AKT
signaling pathway [31].
Recent studies identified several cellular inhibitors of PP2A
(Fig. 1). Two endogenous inhibitors of PP2A, I1PP2A and I2PP2A,
were purified from bovine kidney. I1PP2A is also called PHAP-I
(putative class II human histocompatibility leucocyte-associated
Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
39
protein I) [32] while I2PP2A represents a truncated form of myeloidleukemia associated protein SET (Suvar3-9, enhancer of zeste,
thithorax), also termed PHAP-II [33]. SET is a regulator of various
physiological processes including histone acetylation, regulation of
transcription [34], cell growth [35] and transformation [36]. SET
upregulation has been observed in chronic myelogenous leukemia
and Wilm’s tumors [37, 38]. Moreover, SET was found to be a
fusion partner with the CAN/Nup214 gene in an acute undifferentiated leukemia fusion gene [33]. It is still unclear which specific
cancer-related pathways are affected by SET dysregulation.
The recently identified CIP2A was found to inhibit PP2A activity towards c-Myc thereby preventing its proteolytic degradation
[39]. Suppression of CIP2A inhibits anchorage independent cell
growth and tumor growth in vivo while CIP2A overexpression cooperates with Ras and c-Myc oncoproteins to induce tumorigenic
conversion of mouse primary embryo fibroblasts. Expression of
CIP2A was found to be up-regulated in head and neck squamous
cell carcinomas, and in colon cancer [39].
In addition to SET and CIP2A, PP2A activity in cancer cells
could be upregulated due to overexpression of protein phosphatase
methylesterase-1 (PME-1), which mediates PP2A methyl ester
hydrolysis of the PP2AC C-terminal Leu307 and exerts negative
regulation on the preassembled holoenzymes. PME-1-mediated
inhibition of PP2A results in activation of the MAPK pathway. In
malignant gliomas, PME-1 overexpression correlates with both
increased Erk1/2 activity and the disease stage [40]. Another endogenous PP2A inhibitor with a potential link to human cancer is
type 2A-interacting protein (TIP), which was also found to be overexpressed in human cancer cells. TIP directly inhibits PP2A activity
in vitro while TIP depletion results in a significant decrease in the
phosphorylation of a recently identified target protein substrate of
ataxia-telangiectasia mutated (ATM) - and Rad3-related (ATR)
kinases [41].
Taken together, these findings strongly suggest that PP2A plays
a key role in tumor suppression. Moreover, inactivation of specific
PP2A complexes by different mechanisms may alter multiple pathways, each of which may contribute independently to cancer development and progression.
PP2A ACTIVATION AS AN ANTI-CANCER THERAPY
Since PP2A has tumor suppressing activity by regulating cell
signaling cascades in a way opposite to the activity of kinase oncogenes [3, 87], it suggests that pharmacological activation of PP2A
activity would restrain the activity of the kinase oncogenes and
could be beneficial for cancer treatment (Fig. 2A). Consistently
with this idea, the sphingolipid metabolite ceramide (Fig. 3A),
which directly induces PP2A phosphatase activity, has strong antiproliferative and pro-apoptotic activities [67-69].
PP2A Activators
The first example of PP2A activator is FTY720 (2-amino-2-[2(4-octylphenyl) ethyl]-1,3-propanediol hydrochloride) (Fingolimod;
Novartis), a synthetic myriocin analogue structurally similar to
sphingosine (Fig. 3A). It is a known immunomodulator, which is
currently being evaluated in phase III clinical trials in patients with
either multiple sclerosis or undergoing renal transplantation [7073]. FTY720 is a relatively nontoxic drug with high oral bioavailability. Although the precise mechanism underlying induction by
FTY720 of PP2A activation is unclear, evidence suggests that, like
ceramide, FTY720 has a direct effect on the PP2A heterotrimeric
complexes [74]. However, a recent study demonstrates that
ceramide directly binds to the PP2A inhibitor SET and induces
PP2A activity by disturbing interaction between PP2A and the
SET protein [75]. Increased SET levels were also observed in cells
that undergo FTY720-induced apoptosis [76], suggesting that the
effects of FTY720 might be as a result of restraining the inhibitory
function of SET.
40 Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
A
Kalev and Sablina
Activation of PP2AC
PP2Ac
PP2A activators
(e.g. Forskolin or FTY720)
B
Kinase signaling
Tumor Regression
Inhibition of PP2AC
PP2Ac
PP2A inhibitors
(e.g. Fostriecin or Cantharidin)
PP2A inhibitors + Chemotherapy
C
Mitotic Crisis
Mitotic Crisis
Inhibition of Cell Cycle Checkpoints
Impaired DNA repair
Tumor Regression
Disruption of specific PP2A complexes
Inhibitors of specific
protein-protein interactions
Modulation of activity
of specific PP2A targets
Tumor Regression
Fig. (2). PP2A as a potential therapeutic target for cancer treatment. A, Pharmacological induction of PP2A activity downregulates cancer-associated
signaling pathways such as PIK3/Akt, MAPK, and Jak/STAT. PP2A activators (FTY720 or forskolin) are effectively antagonizing leukemogenesis and represent an alternative therapeutic approach in treating this type of cancer. B, Inhibition of PP2A in dividing cells abrogates cell cycle checkpoints and promotes
premature entry into mitosis. Combination of PP2A inhibitors with commonly used chemotherapeutic drugs (e.g. TMZ or DOX) could enhance the effectiveness of cytotoxic treatment. C, Disruption of specific PP2A complexes is an alternative approach to inhibit PP2A activity toward particular substrates and
specifically suppress the growth of cancer cells.
PP2A activity can be also indirectly triggered by forskolin (or
its derivative, 1,9-dideoxyforskolin), a labdane diterpene that is
produced by the Indian Coleus plant (Coleus forskohlii) (Fig. 3A).
Forskolin is known to directly activate the adenylyl cyclase enzyme
that results in an increase of cAMP intracellular levels and upregulation of protein kinase A (PKA). Although most of the forskolinmediated effects depend on augmented PKA and adenylate cyclase
activities [77-79], the ability of forskolin and its derivatives to
upregulate PP2A activity does not require PKA activation or increased cAMP levels [76]. The exact mechanism of PP2A activation by forskolin still has to be elucidated.
PP2A Activators as Potential Drugs for Cancer Therapy
PP2A activators have been recently suggested to use in treatment of different types of human leukemia. The potential efficacy
of FTY720 in treatment of B-cell chronic lymphocytic leukemia (BCLL) has been investigated by [80]. This study demonstrated that
FTY720 mediates toxic effects in cell lines representing different
B-cell malignancies and primary B cells from patients with CLL.
Expose with FTY720 of transformed B-cells results in decreased
levels of phospho-ERK1/2 and Bcl-2 independent apoptosis. Notably, FTY720 treatment prolonged animal survival in a xenograft
SCID mouse model of B cell lymphoma/leukemia [80].
Loss of PP2A enzymatic activity appears to contribute to the
pathophysiology of p210- and p190-BCR/ABL-driven leukemias
[76, 81]. Most of the chronic phase CML patients are effectively
treated with BCR/ABL inhibitor imatinib. However, some of the
patients develop resistance to imatinib and progress to the next
stage of the disease, blast crisis. It was recently reported that PP2A
activity is significantly decreased in blast crisis CML patients due
to overexpression of the SET inhibitor [76, 81].
Restoration of PP2A phosphatase activity in BCR/ABL cell
lines by forskolin or FTY720 promotes BCR/ABL tyrosine dephosphorylation leading to its proteasomal degradation. BCR/ABL proteolysis appears to depend on the PP2A-induced activation of the
tumor suppressor SHP-1 tyrosine phosphatase [76]. Treatment with
either forskolin or FTY720 inhibits cell growth and induces apoptosis by inhibiting dephosphorylation of both Akt and Erk1/2 [74, 76,
81]. FTY720-treated SCID mice injected with p210- and p190BCR/ABL cells were alive and showed no signs of leukemia after
27 weeks, while all of the untreated animals died by the fifth week
after injection [81]. Importantly, long-term in vivo administration of
PP2A activators markedly suppresses the development of
BCR/ABL-induced CML blast crisis without exerting significant
adverse effects on normal hematopoiesis [76].
FTY720 has been proven effective in suppression of c-Kitmediated tumourigenesis of acute myeloid leukemias (AML) [82].
Although the tyrosine kinase inhibitor imatinib has shown remarkable success in treating tumor patients that harbor juxtamembrane
c-KIT mutations, mutations involving the kinase domain are resistant to imatinib inhibition. Clinical trials with the second generation
tyrosine kinase inhibitor dasatinib (SPRYCEL, Bristol-Myers
Squib) have also reported disappointing results [83]. Thus, there is
a clinical need for additional therapies specifically for patients with
c-Kit mutated tumors.
Cancer-associated c-Kit mutations result in inhibition of PP2A
activity in myeloid cells. This effect is associated with the reduced
expression of PP2A structural A subunit and several PP2A regulatory subunits, including B55, B56, B56, and B56. Pharmacologic reactivation of PP2A by FTY720 causes decreased phosphorylation of c-Kit receptor and its downstream signaling targets,
PP2A as a Drug Target
Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
O
A
O
HO
HN
R
O
HO
H2N
OH
OH
OH
O
H
HCl
OH
Ceramide
FTY720
Forskolin
OH
H
B
O
H
O
O
O
OH
OH
H
H
O
OH
O
O
O
H
O
OH
Okadaic acid
O
O
O
+
NaO
O
P
OH
O
OH
O
Cantharidin
OH
Fostriecin
NH2
HN
O
O
OH
OH
R
O
O
OH
NH
O
O
N
H
N
O
N
H
HN
O
N
OH
HN
O
O
O
N
N
O
HO
NH
O
O
P
OH
O
O
O
OH
OH
OH
Calyculin A/C
Nodularin
R = H Calyculin A
R = Me Calyculin C
HN
O
OH
O
N
H
H
N
NH
O
O
H
N
O H
N
N
H
N
O
NH2
O
H
N
O
HO
O
O
Microcystin LR
O
O
OH
O
OH
O
OH
O
O
O
Tautomycetin
Fig. (3). Structures of PP2A modulators. A, PP2A activators. B, PP2A inhibitors.
O
OH
41
42 Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
including Akt, STAT5, and ERK1/2. As a result, addition of
FTY720 blocks proliferation and induces apoptosis of cells harboring c-Kit mutants, while having no effect on cells with wild-type cKit. In addition, in vivo administration of FTY720 delays the
growth of c-Kit harboring tumors, and results in inhibited splenic
and bone marrow infiltration, and prolonged survival [82].
These findings indicate that incorporation of PP2A activating
drugs in current therapeutic protocols for blast crisis CML,
imatinib/dasatinib-resistant CML, and B-ALL patients may offer a
novel attractive therapeutic strategy.
PP2A INHIBITION AS A STRATEGY FOR AN ANTICANCER THERAPY
PP2A is a negative regulator of the cell cycle, and inhibition of
PP2A appears to accelerate cell cycle progression and abrogate cell
cycle checkpoints [55]. This suggests a counterintuitive approach to
treat cancer cells that have hyperactive oncoproteins with PP2A
inhibitors that enhance their action further, in the hope of crossing a
cell death threshold. In fact, treatment with the PP2A inhibitors
such as OA (at concentration higher than 1nM) or calyculin A results in uncontrolled-premature entry into mitosis and death of cancer cells [56]. Further studies supported a strong correlation between inhibition of PP2A activity and increased toxicity especially
to actively dividing cells [57].These observations validate inhibition
of PP2A as a strategy for anticancer therapy (Fig. 2B).
Natural Compounds that Inhibit PP2A
The PP2A catalytic subunit shares 50% identity with the catalytic domains of other members of the serine/threonine phosphatase
family. The structure of the catalytic subunit of PP2A has been
recently solved, even though the protein was purified and cloned 20
years ago [7]. The structural analysis of the PP2A catalytic subunit
reveals that it is similar to the catalytic subunits of PP1,
PP2B/calcineurin, and PP5. As in the catalytic subunits of other
serine-threonine phosphatases, the PP2A enzyme has the N- and Cterminal domains linked together by a central -sandwich, which
comprises the catalytic core. A narrow channel formed by the interface of the -sheets creates the catalytic cleft, which contains two
metal ions positioned by invariant amino acid residues within the
loops of the -sheets [42, 43].
The catalytic subunits of serine/threonine phosphatases are
targets of several natural toxins produced by different microorganisms (Table 1, Fig. 3B). Consistently with the homology between
catalytic subunits, all described compounds have overlapping subTable 1.
Kalev and Sablina
strate selectivity. Although some of these compounds such as microcystin-LR, nodularin, calyculin A, and cantharidin have similar
IC50 values for PP1, PP2A, PP4, and PP5, OA is a more potent
inhibitor of PP2A (Table 1). The greater affinity of PP2A for OA is
due to a “hydrophobic cage” within the binding pocket of PP2A
that is not conserved in other serine-threonine phosphatases. This
cage surrounds the hydrophobic portion of OA and is the structural
feature responsible for the 100-fold higher affinity of OA for the
PP2A enzyme [7]. Structure-activity relationship studies have facilitated the design and synthesis of inhibitor analogs with preferential selectivity for PP2A [44, 45].
The most successful example is the development of the norcantharidin analogue, LB1.2 [46]. Norcantharidin inhibits several serine/threonine phosphatases (Table 1) and demonstrates anticancer
properties [47, 48]. It is currently unknown whether the anti-cancer
effect of this compound is due to the inhibition of one or more
phosphatases. Norcantharidin has several characteristics that make
it ideal for development as an anticancer drug. It is a lipophilic
compound that can easily traverse biological membranes and, importantly, is not a substrate for the drug transporter P-glycoprotein
[49]. Significant effort has been devoted for development of norcantharidin analogues with high selectivity for PP2A [50-52].
Among numerous compounds that were generated during these
studies, LB1.2 was selected for the future investigations as the most
effective inhibitor of PP2A. LB1.2 inhibits PP2A (IC50 0.4 μM)
with more specificity than PP1 (IC50 80 μM). It mimics the effect
of OA, and has less pronounced toxic effect, which makes LB1.2
suitable for use in anti-cancer therapies [53, 54].
PP2A Inhibitors as Potential Drugs for Cancer Therapy
Over other natural PP2A inhibitors such as OA, microcystin-LR
or calyculin A, fostriecin has a clear advantage for development as
an anticancer drug because of its low toxicity and high specificity
profiles [56]. Fostriecin was discovered based on its antitumor
properties without any knowledge of its mechanism of action. At
first, fostriecin's tumor cytotoxic activity was attributed to the inhibition of DNA topoisomerase II, but its effective concentration
(IC50 = 40 μM) and cell cycle effects were subsequently found to be
inconsistent with classic topoisomerase II inhibitors [56, 58].
Preclinical studies revealed that fostriecin suppresses the
growth of cancer cells by triggering premature mitotic entry and
resulting in apoptotic cell death [56, 59, 60]. Fostriecin has completed a phase I clinical trial in 20 patients [61, 62]. Unfortunately,
it was found to be highly susceptible to oxidation and have a short
Natural Inhibitors of Serine/Threonine Phosphatases
IC50 (nM)a
Inhibitors
a
Name
Origin
PP1
PP2A
PP4
PP5
Okadaic acid
Halichondria okadai
Halichondria malanodocia
Dinophysis acuminata
Prorocentrum concavum
10-50
0.02-0.3
0.1
3.5
Fostriecin
Streptomyces pulveraceus
4.5104-5.8104
0.15-5
0.4
3
Cantharidin/Noncantaridin
Lytta vesicatoria
0.5 - 2.0
0.2
50
600
Nodularin-V
Nodularia spumigena
0.5 - 3
00.3 - 1.0
ND
4
Mycrocystin-LR
Microcystis aeruginosa
0.3 – 1
0.04 – 2.0
0.15
1
Tautomycin
Streptomyces verticillatus
0.23 – 22
0.94 - 32
10
ND
Calyculin A/C
Discodermia calyx
0.6
2.8
3.0
5.0104 – 7.0104
The IC50 values are based on published studies conducted using purified catalytic subunits of the indicated phosphatases and phosphoproteins as substrates. Because IC50 values are
influenced by the mode of inhibition, and the amount of both enzyme and substrate employed in an assay the different values shown reflect the variations in the assay conditions used
in different laboratories [90-93].
PP2A as a Drug Target
plasma half-life. No tumor response was observed, presumably due
to drug instability and inadequate purity in the clinical supply [56].
To overcome these issues investigators used complementary strategies to generate analogues of fostriecin, including the development
of several novel syntheses [56] and the manipulation of Streptomyces pulveraceus polyketide synthase gene clusters. The latter strategy has been successfully used to create bacterial strains that produce a single analog of phoslactomycin B, a drug structurally related to fostriecin [63]. Phoslactomycins are potent and selective
inhibitors of PP2A, and phoslactomycin A has been shown to inhibit lung metastasis in mice through activation of natural killer
cells [64]. The ability to produce a pure supply of phoslactomycins
provides us a proof of principle for the production of pure supplies
of fostriecin analogues that can be clinically tested for their anticancer activity [65].
Another natural product known for its antitumor properties is
cantharidin secreted by many species of blister beetles, and most
notably by the Spanish fly, Lytta vesicatoria. Despite its potential
toxicities the Chinese have used cantharidin as a medicinal agent
for over 2000 years and as an anti-cancer drug since 1264 for
treatment of hepatomas and oesophageal carcinomas [66]. Western
medicine has prohibited cantharidin from clinical use because of its
high nephrotoxicity. However, the demethylated analogue of cantharidin, norcantharidin, not only displays less toxicity but induces
haemopoiesis via bone marrow stimulation and retains anticancer
activity. Norcantharidin was shown to suppress the growth of cancer cell lines both in vitro and in vivo [66].
Since 1984 norcantharidin has being used in cancer therapy of
primary hepatomas and upper gastrointestinal carcinomas in China.
It has been reported that norcantharidin increased the mean survival
time of 285 patients with primary hepatoma from 4.7 to 11.1
months, and the 1 year survival rate from 17% to 30%, as compared
to 102 patients treated with conventional chemotherapy (5fluorouracil, hydroxycamptothecin, vincristine, thiophosphoramide,
or mitomycin) [66]. Although further clinical trials are required to
explore the anticancer properties of these compounds, the reported
results look promising.
The Combination of Chemotherapy and PP2A Inhibitors for
Cancer Treatment
A recent study [46] proposes to combine PP2A inhibitors with
classical chemo- and radiotherapies to enhance the effectiveness of
classical cytotoxic treatment (Fig. 2B). In this study, the authors
used the described above synthetic derivative of norcantharidin,
LB1.2. Treatment with LB1.2 in combination with temozolomide
(TMZ), a DNA-methylating chemotherapeutic drug, causes complete regression of glioblastoma multiforme (GBM) xenografts
without recurrence in 50% of animals (up to 28 weeks) and complete inhibition of growth of neuroblastoma xenografts. Treatment
with either drug alone results in only short-term regression with all
xenografts resuming rapid growth. Combined with another widely
used anticancer drug, doxorubicin (DOX) LB1.2 also causes
marked GBM xenograft regression, whereas DOX alone only slows
down the tumor growth.
Exposure of cancer cells to LB1.2 induces increased phosphorylation of both Akt-1 and polo-like kinase 1 (Plk-1) that, in
turn, abolishes G2/M checkpoint and promotes premature entry into
mitosis. Moreover, LB-1.2 treatment leads to a marked decrease of
translationally controlled tumor protein (TCTP), which stabilizes
microtubules before and after mitosis and exerts potent antiapoptotic activity. As result, LB1.2 treatment induces mitotic catastrophe and cell death.
On the other hand, increased Akt-1 activity induced by LB1.2
treatment stabilizes the p53 ubiquitin ligase MDM2 that results in
down-regulation of p53 expression and activity. Consistently,
LB1.2 treatment impairs p53-mediated cell cycle arrest. Therefore,
LB1.2 treatment triggers a chain of alterations in cancer cell signal-
Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
43
ing that accelerates inappropriate entry of cells into mitosis and, at
the same time, impairs cell cycle arrest at G1 and G2/M. In the
presence of DNA damaging agents inhibition of PP2A blocks cell
cycle arrest and increases progression of cell cycle that results in
increased sensitivity of cells to DNA damage, disordered DNA
replication, and increased levels of cell death. The accentuation of
DNA-damage response by PP2A inhibition markedly enhances the
effectiveness of chemotherapeutic drugs such as TMZ and DOX.
These data imply PP2A inhibitors as a general method to improve
the effectiveness of known radio- and chemotherapies [46]. However, inhibition of cell cycle checkpoints by PP2A suppression
could also result in accumulation of genetically altered cells that
potentially may form a tumorigenic subpopulation. Thus, this issue
will require more attention, both in statistical and pharmacological
aspects, to evaluate the effects of prolonged applications of PP2A
inhibitors on cancer incidence.
DISRUPTING OF SPECIFIC PP2A COMPLEXES AS A
NOVEL STRATEGY FOR CANCER THERAPY
Prior work showed that viral oncoproteins such as SV40ST
induce cell transformation by displacing multiple PP2A regulatory
B subunits from the AC core dimer [84-86]. This suggests that
more than one PP2A specific complex is involved in control of cell
transformation. Consistent with this idea, suppression of a single
PP2A B56 subunit only partially substituted for the SV40ST in
cell transformation [84]. Recent studies identified several PP2A
specific complexes that could contribute to cell transformation [87].
These finding challenges the widely accepted view that phosphatases act promiscuously and constitutively, which devaluated the
possible contribution of protein phosphatases to tumor suppression.
Rather, these studies indicates that dysfunction of particular PP2A
complexes regulate specific phosphorylation events necessary for
tumor initiation and/or maintenance.
The disruption of specific phosphatase complexes might help to
design more specific and less toxic cancer drugs (Fig. 2C). Smallmolecule and peptide antagonists have been already identified for
several phosphatase complexes [65]. A recent study exploited cellpermeable peptides that mimic the structure of “phosphataseinteracting motifs” in specific PP2A targets. Based on structural
analysis of PP2A interactions with casein kinase 2 (CK2) from
various species and the SV40 virus encoded SV40ST, the authors
identified a short basic peptide from Ck2 that interacted with the
PP2A A subunit and behaves as a nontoxic penetrating shuttle in
several cultivated human cell lines and chick embryos [88]. Further
this basic stretch of amino acids was fused to short peptides that
contain PP2A B56 interacting sequences deduced from CD28
antigen. The introduction of the fusion peptides into cells in culture
induced cell death, probably by inhibiting the activity of B56containing complexes. Notably, disruption of PP2A B55containing complexes by adenoviral E4orf4 protein also results in
apoptosis of transformed cells [89], suggesting that the generated
peptides mimic signaling generated by E4orf4 and could represent a
new creative approach to counteract resistance of cancer cells
against drug-induced apoptosis.
The targeted disruption of the regulatory subunit from the
holoenzyme could prevent the association of the catalytic subunit
with its substrate and/or result in dissociation of the catalytic
subunit from its substrate. PP2A activity in human cancers could be
also restored by disrupting interaction of PP2A with its intracellular
inhibitors (e.g. SET and CIP2A). However, a systematic view of the
function and activity of PP2A complexes in human cancer is necessary to elucidate inhibition of which specific PP2A complexes may
provide therapeutic benefits.
CONCLUSION
The studies of natural compounds and their derivatives, which
affect PP2A activity, revealed that manipulation of PP2A has po-
44 Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
tential therapeutic benefits in treating cancer. Accumulation of
structural data for phosphatase holoenzymes and development of
novel approaches for drug design will allow us to obtain more
specific and less toxic modulators of PP2A activity.
Furthermore, several lines of evidence suggest that dysfunction
of specific PP2A complexes is likely to be primarily responsible for
the changes that lead to cell transformation. The discovery of small
molecule selectively targeted specific PP2A complexes opens up
new possibilities for disruption of particular phosphatase complexes
to achieve a therapeutic effect. Systematic characterization of
the regulation and function of specific PP2A complexes may lay
the foundation for future studies aimed at developing specific
antagonists for these complexes, which may ultimately prove to be
valuable therapeutic agents for cancer treatment.
Kalev and Sablina
[16]
[17]
[18]
[19]
ACKNOWLEDGEMENTS
We thank Thomas Soin for helpful discussions on the manuscript. This work was supported in part by a VIB startup grant.
[20]
REFERENCES
[21]
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Broach, J. R.; Levine, A. J. Oncogenes and cell proliferation. Curr.
Opin. Genet. Dev., 1997, 7(1), 1-6.
Hanahan, D.; Weinberg, R. A. The hallmarks of cancer. Cell, 2000,
100(1), 57-70.
Millward, T. A.; Zolnierowicz, S.; Hemmings, B. A. Regulation of
protein kinase cascades by protein phosphatase 2A. Trends Biochem. Sci., 1999, 24(5), 186-91.
Bialojan, C.; Takai, A. Inhibitory effect of a marine-sponge toxin,
okadaic acid, on protein phosphatases. Specificity and kinetics.
Biochem. J., 1988, 256(1), 283-90.
Suganuma, M.; Fujiki, H.; Suguri, H.; Yoshizawa, S.; Hirota, M.;
Nakayasu, M.; Ojika, M.; Wakamatsu, K.; Yamada, K.; Sugimura,
T. Okadaic acid: an additional non-phorbol-12-tetradecanoate-13acetate-type tumor promoter. Proc. Natl. Acad. Sci. USA, 1988,
85(6), 1768-71.
Janssens, V.; Goris, J.; Van Hoof, C. PP2A: the expected tumor
suppressor. Curr. Opin. Genet. Dev., 2005, 15(1), 34-41.
Mumby, M. The 3D structure of protein phosphatase 2A: new
insights into a ubiquitous regulator of cell signaling. ACS Chem.
Biol., 2007, 2(2), 99-103.
Longin, S.; Jordens, J.; Martens, E.; Stevens, I.; Janssens, V.; Rondelez, E.; De Baere, I.; Derua, R.; Waelkens, E.; Goris, J.; Van
Hoof, C. An inactive protein phosphatase 2A population is associated with methylesterase and can be re-activated by the phosphotyrosyl phosphatase activator. Biochem. J., 2004, 380, 111-9.
Longin, S.; Zwaenepoel, K.; Louis, J. V.; Dilworth, S.; Goris, J.;
Janssens, V. Selection of protein phosphatase 2A regulatory
subunits is mediated by the C terminus of the catalytic Subunit. J.
Biol. Chem., 2007, 282(37), 26971-80.
Ogris, E.; Gibson, D. M.; Pallas, D. C. Protein phosphatase 2A
subunit assembly: the catalytic subunit carboxy terminus is important for binding cellular B subunit but not polyomavirus middle
tumor antigen. Oncogene, 1997, 15(8), 911-7.
Yu, X. X.; Du, X.; Moreno, C. S.; Green, R. E.; Ogris, E.; Feng,
Q.; Chou, L.; McQuoid, M. J.; Pallas, D. C. Methylation of the protein phosphatase 2A catalytic subunit is essential for association of
Balpha regulatory subunit but not SG2NA, striatin, or polyomavirus middle tumor antigen. Mol. Biol. Cell, 2001, 12(1), 185-99.
Chen, J.; Martin, B. L.; Brautigan, D. L. Regulation of protein
serine-threonine phosphatase type-2A by tyrosine phosphorylation.
Science, 1992, 257(5074), 1261-4.
Chen, J.; Parsons, S.; Brautigan, D. L. Tyrosine phosphorylation of
protein phosphatase 2A in response to growth stimulation and v-src
transformation of fibroblasts. J. Biol. Chem., 1994, 269(11), 795762.
de la Torre, P.; Diaz-Sanjuan, T.; Garcia-Ruiz, I.; Esteban, E.;
Canga, F.; Munoz-Yague, T.; Solis-Herruzo, J. A. Interleukin-6 increases rat metalloproteinase-13 gene expression through Janus
kinase-2-mediated inhibition of serine/threonine phosphatase-2A.
Cell Signal., 2005, 17(4), 427-35.
Wong, L. L.; Chang, C. F.; Koay, E. S.; Zhang, D. Tyrosine phosphorylation of PP2A is regulated by HER-2 signalling and corre-
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
lates with breast cancer progression. Int. J. Oncol., 2009, 34(5),
1291-301.
Yumoto, N.; Yu, X.; Hatakeyama, M. Expression of the ErbB4
receptor causes reversal regulation of PP2A in the Shc signal transduction pathway in human cancer cells. Mol. Cell Biochem., 2006,
285(1-2), 165-71.
Fellner, T.; Lackner, D. H.; Hombauer, H.; Piribauer, P.; Mudrak,
I.; Zaragoza, K.; Juno, C.; Ogris, E. A novel and essential mechanism determining specificity and activity of protein phosphatase 2A
(PP2A) in vivo. Genes Dev., 2003, 17(17), 2138-50.
Jordens, J.; Janssens, V.; Longin, S.; Stevens, I.; Martens, E.;
Bultynck, G.; Engelborghs, Y.; Lescrinier, E.; Waelkens, E.; Goris,
J.; Van Hoof, C. The protein phosphatase 2A phosphatase activator
is a novel peptidyl-prolyl cis/trans-isomerase. J. Biol. Chem., 2006,
281(10), 6349-57.
Kong, M.; Ditsworth, D.; Lindsten, T.; Thompson, C. B. Alpha4 is
an essential regulator of PP2A phosphatase activity. Mol. Cell,
2009, 36(1), 51-60.
Fujiki, H.; Suganuma, M. Carcinogenic aspects of protein phosphatase 1 and 2A inhibitors. Prog. Mol. Subcell. Biol.,2009, 46, 22154.
Nagao, M.; Shima, H.; Nakayasu, M.; Sugimura, T. Protein serine/threonine phosphatases as binding proteins for okadaic acid.
Mutat. Res., 1995, 333(1-2), 173-9.
Tohda, H.; Nagao, M.; Sugimura, T.; Oikawa, A. Okadaic acid, a
protein phosphatase inhibitor, induces sister-chromatid exchanges
depending on the presence of bromodeoxyuridine. Mutat. Res.
1993, 289(2), 275-80.
Schonthal, A. H. Role of serine/threonine protein phosphatase 2A
in cancer. Cancer Lett., 2001, 170(1), 1-13.
Calin, G. A.; di Iasio, M. G.; Caprini, E.; Vorechovsky, I.; Natali,
P. G.; Sozzi, G.; Croce, C. M.; Barbanti-Brodano, G.; Russo, G.;
Negrini, M. Low frequency of alterations of the alpha (PPP2R1A)
and beta (PPP2R1B) isoforms of the subunit A of the serinethreonine phosphatase 2A in human neoplasms. Oncogene 2000,
19(9), 1191-5.
Ruediger, R.; Pham, H. T.; Walter, G. Alterations in protein phosphatase 2A subunit interaction in human carcinomas of the lung
and colon with mutations in the A beta subunit gene. Oncogene,
2001, 20(15), 1892-9.
Ruediger, R.; Pham, H. T.; Walter, G. Disruption of protein phosphatase 2A subunit interaction in human cancers with mutations in
the A alpha subunit gene. Oncogene, 2001, 20(1), 10-5.
Takagi, Y.; Futamura, M.; Yamaguchi, K.; Aoki, S.; Takahashi, T.;
Saji, S. Alterations of the PPP2R1B gene located at 11q23 in human colorectal cancers. Gut, 2000, 47(2), 268-71.
Tamaki, M.; Goi, T.; Hirono, Y.; Katayama, K.; Yamaguchi, A.
PPP2R1B gene alterations inhibit interaction of PP2A-Abeta and
PP2A-C proteins in colorectal cancers. Oncol. Rep., 2004, 11(3),
655-9.
Wang, S. S.; Esplin, E. D.; Li, J. L.; Huang, L.; Gazdar, A.; Minna,
J.; Evans, G. A. Alterations of the PPP2R1B gene in human lung
and colon cancer. Science, 1998, 282(5387), 284-7.
Sablina, A. A.; Chen, W.; Arroyo, J. D.; Corral, L.; Hector, M.;
Bulmer, S. E.; Decaprio, J. A.; Hahn, W. C. The tumor suppressor
PP2A abeta regulates the RalA GTPase. Cell, 2007, 129(5), 969982.
Chen, W.; Arroyo, J. D.; Timmons, J. C.; Possemato, R.; Hahn, W.
C. Cancer-associated PP2A Aalpha subunits induce functional haploinsufficiency and tumorigenicity. Cancer Res., 2005, 65(18),
8183-92.
Tsujio, I.; Zaidi, T.; Xu, J.; Kotula, L.; Grundke-Iqbal, I.; Iqbal, K.
Inhibitors of protein phosphatase-2A from human brain structures,
immunocytological localization and activities towards dephosphorylation of the Alzheimer type hyperphosphorylated tau. FEBS
Lett., 2005, 579(2), 363-72.
von Lindern, M.; van Baal, S.; Wiegant, J.; Raap, A.; Hagemeijer,
A.; Grosveld, G. Can, a putative oncogene associated with myeloid
leukemogenesis, may be activated by fusion of its 3' half to different genes: characterization of the set gene. Mol. Cell Biol., 1992,
12(8), 3346-55.
Seo, S. B.; McNamara, P.; Heo, S.; Turner, A.; Lane, W. S.; Chakravarti, D. Regulation of histone acetylation and transcription by
PP2A as a Drug Target
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
INHAT, a human cellular complex containing the set oncoprotein.
Cell, 2001, 104(1), 119-30.
Canela, N.; Rodriguez-Vilarrupla, A.; Estanyol, J. M.; Diaz, C.;
Pujol, M. J.; Agell, N.; Bachs, O. The SET protein regulates G2/M
transition by modulating cyclin B-cyclin-dependent kinase 1 activity. J. Biol. Chem., 2003, 278(2), 1158-64.
Kumar, R. N.; Radhakrishnan, R.; Ha, J. H.; Dhanasekaran, N.
Proteome analysis of NIH3T3 cells transformed by activated Galpha12: regulation of leukemia-associated protein SET. J. Proteome
Res., 2004, 3(6), 1177-83.
Carlson, S. G.; Eng, E.; Kim, E. G.; Perlman, E. J.; Copeland, T.
D.; Ballermann, B. J. Expression of SET, an inhibitor of protein
phosphatase 2A, in renal development and Wilms' tumor. J. Am.
Soc. Nephrol., 1998, 9(10), 1873-80.
Fornerod, M.; Boer, J.; van Baal, S.; Jaegle, M.; von Lindern, M.;
Murti, K. G.; Davis, D.; Bonten, J.; Buijs, A.; Grosveld, G. Relocation of the carboxyterminal part of CAN from the nuclear envelope
to the nucleus as a result of leukemia-specific chromosome rearrangements. Oncogene, 1995, 10(9), 1739-48.
Junttila, M. R.; Puustinen, P.; Niemela, M.; Ahola, R.; Arnold, H.;
Bottzauw, T.; Ala-aho, R.; Nielsen, C.; Ivaska, J.; Taya, Y.; Lu, S.
L.; Lin, S.; Chan, E. K.; Wang, X. J.; Grenman, R.; Kast, J.; Kallunki, T.; Sears, R.; Kahari, V. M.; Westermarck, J. CIP2A inhibits
PP2A in human malignancies. Cell, 2007, 130(1), 51-62.
Puustinen, P.; Junttila, M. R.; Vanhatupa, S.; Sablina, A. A.; Hector, M. E.; Teittinen, K.; Raheem, O.; Ketola, K.; Lin, S.; Kast, J.;
Haapasalo, H.; Hahn, W. C.; Westermarck, J. PME-1 protects extracellular signal-regulated kinase pathway activity from protein
phosphatase 2A-mediated inactivation in human malignant glioma.
Cancer Res., 2009, 69(7), 2870-7.
McConnell, J. L.; Gomez, R. J.; McCorvey, L. R.; Law, B. K.;
Wadzinski, B. E. Identification of a PP2A-interacting protein that
functions as a negative regulator of phosphatase activity in the
ATM/ATR signaling pathway. Oncogene, 2007, 26(41), 6021-30.
Cho, U. S.; Xu, W. Crystal structure of a protein phosphatase 2A
heterotrimeric holoenzyme. Nature, 2007, 445(7123), 53-7.
Xu, Y.; Xing, Y.; Chen, Y.; Chao, Y.; Lin, Z.; Fan, E.; Yu, J. W.;
Strack, S.; Jeffrey, P. D.; Shi, Y. Structure of the protein phosphatase 2A holoenzyme. Cell, 2006, 127(6), 1239-51.
Aggen, J. B.; Humphrey, J. M.; Gauss, C. M.; Huang, H. B.; Nairn,
A. C.; Chamberlin, A. R. The design, synthesis, and biological
evaluation of analogues of the serine-threonine protein phosphatase
1 and 2A selective inhibitor microcystin LA: rational modifications
imparting PP1 selectivity. Bioorg. Med. Chem., 1999, 7(3), 543-64.
Sakoff, J. A.; McCluskey, A. Protein phosphatase inhibition: structure based design. Towards new therapeutic agents. Curr. Pharm.
Des., 2004, 10(10), 1139-59.
Lu, J.; Kovach, J. S.; Johnson, F.; Chiang, J.; Hodes, R.; Lonser,
R.; Zhuang, Z. Inhibition of serine/threonine phosphatase PP2A
enhances cancer chemotherapy by blocking DNA damage induced
defense mechanisms. Proc. Natl. Acad. Sci. USA, 2009, 106(28),
11697-702.
Hong, C. Y.; Huang, S. C.; Lin, S. K.; Lee, J. J.; Chueh, L. L.; Lee,
C. H.; Lin, J. H.; Hsiao, M. Norcantharidin-induced post-G(2)/M
apoptosis is dependent on wild-type p53 gene. Biochem. Biophys.
Res. Commun., 2000, 276(1), 278-85.
Luan, J.; Duan, H.; Liu, Q.; Yagasaki, K.; Zhang, G. Inhibitory
effects of norcantharidin against human lung cancer cell growth
and migration. Cytotechnology, 2010, 62(4), 349-355.
Sieder, S.; Richter, E.; Becker, K.; Heins, R.; Steinfelder, H. J.
Doxorubicin-resistant LoVo adenocarcinoma cells display resistance to apoptosis induction by some but not all inhibitors of ser/thr
phosphatases 1 and 2A. Toxicology, 1999, 134(2-3), 109-15.
Hill, T. A.; Stewart, S. G.; Gordon, C. P.; Ackland, S. P.; Gilbert,
J.; Sauer, B.; Sakoff, J. A.; McCluskey, A. Norcantharidin analogues: synthesis, anticancer activity and protein phosphatase 1 and
2A inhibition. Chem. Med. Chem., 2008, 3(12), 1878-92.
Hill, T. A.; Stewart, S. G.; Sauer, B.; Gilbert, J.; Ackland, S. P.;
Sakoff, J. A.; McCluskey, A. Heterocyclic substituted cantharidin
and norcantharidin analogues--synthesis, protein phosphatase (1
and 2A) inhibition, and anti-cancer activity. Bioorg. Med. Chem.
Lett., 2007, 17(12), 3392-7.
McCluskey, A.; Bowyer, M. C.; Collins, E.; Sim, A. T. R.; Sakoff,
J. A.; Baldwin, M. L. Anhydride modified cantharidin analogues:
Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
[53]
[54]
[55]
[56.
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
[69]
[70]
[71]
45
synthesis, inhibition of protein phosphatases 1 and 2A and anticancer activity. Bioorg. Med. Chem. Lett., 2000, 10(15), 1687-90.
Bonness, K.; Aragon, I. V.; Rutland, B.; Ofori-Acquah, S.; Dean,
N. M.; Honkanen, R. E. Cantharidin-induced mitotic arrest is associated with the formation of aberrant mitotic spindles and lagging
chromosomes resulting, in part, from the suppression of
PP2Aalpha. Mol. Cancer Ther., 2006, 5(11), 2727-36.
Hart, M. E.; Chamberlin, A. R.; Walkom, C.; Sakoff, J. A.;
McCluskey, A. Modified norcantharidins; synthesis, protein phosphatases 1 and 2A inhibition, and anticancer activity. Bioorg. Med.
Chem. Lett., 2004, 14(8), 1969-73.
Janssens, V.; Goris, J. Protein phosphatase 2A: a highly regulated
family of serine/threonine phosphatases implicated in cell growth
and signalling. Biochem. J., 2001, 353, 417-39.
Lewy, D. S.; Gauss, C. M.; Soenen, D. R.; Boger, D. L. Fostriecin:
chemistry and biology. Curr. Med. Chem., 2002, 9(22), 2005-32.
Buck, S. B.; Hardouin, C.; Ichikawa, S.; Soenen, D. R.; Gauss, C.
M.; Hwang, I.; Swingle, M. R.; Bonness, K. M.; Honkanen, R. E.;
Boger, D. L. Fundamental role of the fostriecin unsaturated lactone
and implications for selective protein phosphatase inhibition. J.
Am. Chem. Soc., 2003, 125(51), 15694-5.
Chen, M.; Beck, W. T. Teniposide-resistant CEM cells, which
express mutant DNA topoisomerase II alpha, when treated with
non-complex-stabilizing inhibitors of the enzyme, display no crossresistance and reveal aberrant functions of the mutant enzyme.
Cancer Res., 1993, 53(24), 5946-53.
Cheng, A.; Balczon, R.; Zuo, Z.; Koons, J. S.; Walsh, A. H.;
Honkanen, R. E. Fostriecin-mediated G2-M-phase growth arrest
correlates with abnormal centrosome replication, the formation of
aberrant mitotic spindles, and the inhibition of serine/threonine protein phosphatase activity. Cancer Res., 1998, 58(16), 3611-9.
Roberge, M.; Tudan, C.; Hung, S. M.; Harder, K. W.; Jirik, F. R.;
Anderson, H. Antitumor drug fostriecin inhibits the mitotic entry
checkpoint and protein phosphatases 1 and 2A. Cancer Res., 1994,
54(23), 6115-21.
de Jong, R. S.; Mulder, N. H.; Uges, D. R.; Sleijfer, D. T.; Hoppener, F. J.; Groen, H. J.; Willemse, P. H.; van der Graaf, W. T.; de
Vries, E. G. Phase I and pharmacokinetic study of the topoisomerase II catalytic inhibitor fostriecin. Br. J. Cancer, 1999, 79(56), 882-7.
de Jong, R. S.; de Vries, E. G.; Mulder, N. H. Fostriecin: a review
of the preclinical data. Anticancer Drugs, 1997, 8(5), 413-8.
Palaniappan, N.; Kim, B. S.; Sekiyama, Y.; Osada, H.; Reynolds,
K. A. Enhancement and selective production of phoslactomycin B,
a protein phosphatase IIa inhibitor, through identification and engineering of the corresponding biosynthetic gene cluster. J. Biol.
Chem., 2003, 278(37), 35552-7.
Kawada, M.; Kawatsu, M.; Masuda, T.; Ohba, S.; Amemiya, M.;
Kohama, T.; Ishizuka, M.; Takeuchi, T. Specific inhibitors of protein phosphatase 2A inhibit tumor metastasis through augmentation
of natural killer cells. Int. Immunopharmacol., 2003, 3(2), 179-88.
McConnell, J. L.; Wadzinski, B. E. Targeting protein serine/threonine phosphatases for drug development. Mol. Pharmacol., 2009, 75(6), 1249-61.
Wang, G. S. Medical uses of mylabris in ancient China and recent
studies. J. Ethnopharmacol., 1989, 26(2), 147-62.
Chalfant, C. E.; Kishikawa, K.; Mumby, M. C.; Kamibayashi, C.;
Bielawska, A.; Hannun, Y. A. Long chain ceramides activate protein phosphatase-1 and protein phosphatase-2A. Activation is
stereospecific and regulated by phosphatidic acid. J. Biol. Chem.,
1999, 274(29), 20313-7.
Chalfant, C. E.; Szulc, Z.; Roddy, P.; Bielawska, A.; Hannun, Y. A.
The structural requirements for ceramide activation of serinethreonine protein phosphatases. J. Lipid Res., 2004, 45(3), 496-506.
Dobrowsky, R. T.; Kamibayashi, C.; Mumby, M. C.; Hannun, Y.
A. Ceramide activates heterotrimeric protein phosphatase 2A. J.
Biol. Chem., 1993, 268(21), 15523-30.
Budde, K.; Schmouder, R. L.; Brunkhorst, R.; Nashan, B.; Lucker,
P. W.; Mayer, T.; Choudhury, S.; Skerjanec, A.; Kraus, G.; Neumayer, H. H. First human trial of FTY720, a novel immunomodulator, in stable renal transplant patients. J. Am. Soc. Nephrol., 2002,
13(4), 1073-83.
Dumont, F. J. Fingolimod. Mitsubishi Pharma/Novartis. IDrugs,
2005, 8(3), 236-53.
46 Anti-Cancer Agents in Medicinal Chemistry, 2011, Vol. 11, No. 1
[72]
[73]
[74]
[75]
[76]
[77]
[78]
[79]
[80]
[81]
Skerjanec, A.; Tedesco, H.; Neumayer, H. H.; Cole, E.; Budde, K.;
Hsu, C. H.; Schmouder, R. FTY720, a novel immunomodulator
in de novo kidney transplant patients: pharmacokinetics and
exposure-response relationship. J. Clin. Pharmacol., 2005, 45(11),
1268-78.
Virley, D. J. Developing therapeutics for the treatment of multiple
sclerosis. NeuroRx, 2005, 2(4), 638-49.
Matsuoka, Y.; Nagahara, Y.; Ikekita, M.; Shinomiya, T. A novel
immunosuppressive agent FTY720 induced Akt dephosphorylation
in leukemia cells. Br. J. Pharmacol., 2003, 138(7), 1303-12.
Mukhopadhyay, A.; Saddoughi, S. A.; Song, P.; Sultan, I.; Ponnusamy, S.; Senkal, C. E.; Snook, C. F.; Arnold, H. K.; Sears, R.
C.; Hannun, Y. A.; Ogretmen, B. Direct interaction between the inhibitor 2 and ceramide via sphingolipid-protein binding is involved
in the regulation of protein phosphatase 2A activity and signaling.
FASEB J., 2009, 23(3), 751-63.
Neviani, P.; Santhanam, R.; Trotta, R.; Notari, M.; Blaser, B. W.;
Liu, S.; Mao, H.; Chang, J. S.; Galietta, A.; Uttam, A.; Roy, D. C.;
Valtieri, M.; Bruner-Klisovic, R.; Caligiuri, M. A.; Bloomfield, C.
D.; Marcucci, G.; Perrotti, D. The tumor suppressor PP2A is functionally inactivated in blast crisis CML through the inhibitory activity of the BCR/ABL-regulated SET protein. Cancer Cell, 2005,
8(5), 355-68.
Ding, X.; Staudinger, J. L. Induction of drug metabolism by
forskolin: the role of the pregnane X receptor and the protein
kinase a signal transduction pathway. J. Pharmacol. Exp. Ther.,
2005, 312(2), 849-56.
Feschenko, M. S.; Stevenson, E.; Nairn, A. C.; Sweadner, K. J. A
novel cAMP-stimulated pathway in protein phosphatase 2A activation. J. Pharmacol. Exp. Ther., 2002, 302(1), 111-8.
Seamon, K.; Daly, J. W. Activation of adenylate cyclase by the
diterpene forskolin does not require the guanine nucleotide regulatory protein. J. Biol. Chem., 1981, 256(19), 9799-801.
Liu, Q.; Zhao, X.; Frissora, F.; Ma, Y.; Santhanam, R.; Jarjoura,
D.; Lehman, A.; Perrotti, D.; Chen, C. S.; Dalton, J. T.;
Muthusamy, N.; Byrd, J. C. FTY720 demonstrates promising preclinical activity for chronic lymphocytic leukemia and lymphoblastic leukemia/lymphoma. Blood, 2008, 111(1), 275-84.
Neviani, P.; Santhanam, R.; Oaks, J. J.; Eiring, A. M.; Notari, M.;
Blaser, B. W.; Liu, S.; Trotta, R.; Muthusamy, N.; GambacortiPasserini, C.; Druker, B. J.; Cortes, J.; Marcucci, G.; Chen, C. S.;
Verrills, N. M.; Roy, D. C.; Caligiuri, M. A.; Bloomfield, C. D.;
Byrd, J. C.; Perrotti, D. FTY720, a new alternative for treating blast
crisis chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphocytic leukemia. J. Clin. Invest., 2007,
117(9), 2408-21.
Received: September 30, 2010
Revised: January 18, 2011
Accepted: January 28, 2011
Kalev and Sablina
[82]
[83]
[84]
[85]
[86]
[87]
[88]
[89]
[90]
[91]
[92]
[93]
Roberts, K. G.; Smith, A. M.; McDougall, F.; Carpenter, H.; Horan, M.; Neviani, P.; Powell, J. A.; Thomas, D.; Guthridge, M. A.;
Perrotti, D.; Sim, A. T.; Ashman, L. K.; Verrills, N. M. Essential
requirement for PP2A inhibition by the oncogenic receptor c-KIT
suggests PP2A reactivation as a strategy to treat c-KIT+ cancers.
Cancer Res., 2010, 70(13), 5438-47.
Verstovsek, S.; Tefferi, A.; Cortes, J.; O'Brien, S.; Garcia-Manero,
G.; Pardanani, A.; Akin, C.; Faderl, S.; Manshouri, T.; Thomas, D.;
Kantarjian, H. Phase II study of dasatinib in Philadelphia chromosome-negative acute and chronic myeloid diseases, including systemic mastocytosis. Clin. Cancer Res., 2008, 14(12), 3906-15.
Chen, W.; Possemato, R.; Campbell, K. T.; Plattner, C. A.; Pallas,
D. C.; Hahn, W. C. Identification of specific PP2A complexes involved in human cell transformation. Cancer Cell, 2004, 5(2), 12736.
Cho, U. S.; Morrone, S.; Sablina, A. A.; Arroyo, J. D.; Hahn, W.
C.; Xu, W. Structural basis of PP2A inhibition by small t antigen.
PLoS Biol., 2007, 5(8), e202.
Sontag, E.; Fedorov, S.; Kamibayashi, C.; Robbins, D.; Cobb, M.;
Mumby, M. The interaction of SV40 small tumor antigen with protein phosphatase 2A stimulates the map kinase pathway and induces cell proliferation. Cell, 1993, 75(5), 887-97.
Eichhorn, P. J.; Creyghton, M. P.; Bernards, R. Protein phosphatase
2A regulatory subunits and cancer. Biochim. Biophys. Acta, 2009,
1795(1), 1-15.
Guergnon, J.; Dessauge, F.; Dominguez, V.; Viallet, J.; Bonnefoy,
S.; Yuste, V. J.; Mercereau-Puijalon, O.; Cayla, X.; Rebollo, A.;
Susin, S. A.; Bost, P. E.; Garcia, A. Use of penetrating peptides interacting with PP1/PP2A proteins as a general approach for a drug
phosphatase technology. Mol. Pharmacol., 2006, 69(4), 1115-24.
Livne, A.; Shtrichman, R.; Kleinberger, T. Caspase activation by
adenovirus e4orf4 protein is cell line specific and Is mediated by
the death receptor pathway. J. Virol., 2001, 75(2), 789-98.
Gauss, C. M.; Sheppeck, J. E., 2nd; Nairn, A. C.; Chamberlin, R. A
molecular modeling analysis of the binding interactions between
the okadaic acid class of natural product inhibitors and the Ser-Thr
phosphatases, PP1 and PP2A. Bioorg. Med. Chem., 1997, 5(9),
1751-73.
Gupta, V.; Ogawa, A. K.; Du, X.; Houk, K. N.; Armstrong, R. W.
A model for binding of structurally diverse natural product inhibitors of protein phosphatases PP1 and PP2A. J. Med. Chem., 1997,
40(20), 3199-206.
Moorhead, G. Protein Phosphatase Protocols. Humana Press:
Totowa, N.J., 2007, p. 387
Sheppeck, J. E., 2nd; Gauss, C. M.; Chamberlin, A. R. Inhibition of
the Ser-Thr phosphatases PP1 and PP2A by naturally occurring
toxins. Bioorg. Med. Chem., 1997, 5(9), 1739-50.