Download Targeting the MDM2-p53 Protein Interaction as a Novel Therapeutic

Targeting the MDM2-p53
Protein Interaction as a Novel
Therapeutic Strategy in Cancer
The MDM2–p53 interaction plays a
key role in the prevention of
malignant transformation
T
he p53 and MDM2 (murine double minute 2) proteins interact
to modulate critical functions involved in regulation of normal
cell growth, and the development and progression of cancer.
p53 is an important tumor suppressor that acts as a genomic
watchdog, providing a surveillance system to maintain cellular
integrity and protect cells from malignant transformation.1–3
p53 functions as a primary anticancer defense mechanism and
is activated by cellular damage or other stressors.1 Activation
of p53 induces prolonged cell-cycle arrest and prevents the
replication of damaged cells or promotes apoptosis. Disruptions
or abnormalities in p53 abrogate the effectiveness of the
molecule to protect the genome, leading to uncontrolled
cell proliferation and cancer tumors.1
Nucleus
p53 and MDM2 form a negative
feedback loop–p53 stimulates the
expression of MDM2; in turn, MDM2
targets p53 for degradation
MDM2 binds to p53,
inactivating p53
function
MDM2
p53
p53-MDM2
Complex
Proteolytic
degradation of p53
In unstressed, normal cells, both p53 and MDM2 move between the
nucleus and the cytosol. MDM2 binds with p53 to form a complex
in the nucleus where MDM2 ubiquitinates p53, causing p53 to exit
the nucleus. Once in the cytosol, p53 degradation is mediated and
completed by MDM2.
While mutations in the p53 gene are present in approximately half of all
cancers, in cancers with unmutated p53, the protective effects of normally
functioning wild-type p53 (p53WT) can be diminished by exuberant negative
regulation by MDM2.3,4 MDM2 tightly controls p53 levels via a negative
feedback loop. MDM2 must maintain a delicate and sustained balance with
p53 in order to prevent proliferation of compromised cells.2,4 As such, the
inhibition of MDM2 may promote restoration of critical p53 functions in
patients with p53WT tumors and may represent a promising strategy for the
development of anticancer drug development.
The p53 protein plays a critical role in
maintaining genome integrity
T
he p53 protein is often referred to as the guardian of the genome and
provides continuous surveillance of host cells.2,5 p53 plays a vital role in the
prevention of malignant transformation. It also works to recognize cells that,
if propagated, might transfer mutations or other harmful alterations to the
organism; by identifying these cells, p53 contributes to genetic stability.
Activation of normally functioning p53, in response to unfavorable cellular
conditions such as ribonucleotide depletion, oncogene activation, oxidative
stressor hypoxia, DNA damage, or telomere erosion, induces cell cycle arrest or
apoptosis, and serves to mitigate the potential damage from cellular insults.2,5
Telomere damage
MDM2
Transcriptional
regulation by p53
p53-MDM2 complex exits the nucleus
into cytosol where p53 is tagged for
proteolytic degradation
Cellular insults activate p53 function
DNA damage
p53
Oncogene activation
Apoptosis
Ribonucleotide depletion
Oxidative stressor hypoxia
Cellular insults trigger phosphorylation of the N-terminal domain of p53.2 Upon phosphorylation,
conformational changes in p53 enable the protein to function as a regulator of transcription.
In addition, the change in conformation acts to prevent p53 degradation, prolong molecular
half-life, and allow the protein to accumulate to high levels.1 In contrast, cells without normal
functioning p53 fail to recognize and address cellular stressors or abnormalities, and replication of
affected cells is not constrained. In this way, loss of functioning p53 promotes the oncogenic
phenotype.3 When p53 is functioning properly, oncogenic mutations are not propagated, as
DNA repair activities are intact and cell cycling is tightly controlled.2
p53 serves as a major mainstay in the
body’s anticancer defense mechanisms
I
nactivation of the p53 pathway in tumors confers a strong selective advantage
in the carcinogenic process.6 In fact, it has been proposed that eliminating
p53 function may be a prerequisite for tumor survival. Preclinical work in mice
has shown that the absence of p53 function is a continuous requirement for the
maintenance of established tumors. When p53 function in these tumors was
restored, tumor regression was observed.7–9
p53 is the most frequently altered gene in human cancer.3 Somatic mutations
in p53 have been seen in several human tumors types—p53 mutations or
deletions are observed in approximately 50% of all solid tumors. Ovarian
cancers, colorectal carcinomas, lung cancers, and head and neck carcinomas
are among the types of solid tumors that most commonly exhibit the mutated
p53 gene.10 Missense mutations in p53 are frequent, and can result in gain
of function changes that enhance metastatic potential and invasiveness.11
In many tumor types, p53 mutations have been associated with diminished
overall survival and more aggressive tumor behavior.7 Thus, mutations in p53
not only abrogate tumor suppressive functions, but may also confer oncogenic
properties, and contribute to a poorer overall prognosis.
Sustaining
proliferative
signaling
Activating
invasion and
metatasis
Resisting
cell death
Inducing
angiogenesis
Tumors have the ability to evade growth suppression, normal surveillance
and immune defenses; they are often not susceptible to p53-mediated
cell death. These qualities are among the hallmarks of cancer that enable
tumors to survive, proliferate, and metastasize.6
Evading growth
suppressors
Enabling
replicative
immortality
It is important to note that p53 is not mutated in up to 50% of solid tumors and
is generally not mutated in hematologic malignancies.4 In these tumors, p53WT
is most likely inactivated; inactivation of p53 plays a critical role in development
and progression of the cancers.12 Approaches that re-activate p53WT, permitting
normal function, may lead to tumor regression. Therefore, with the development
of novel therapies designed to re-establish function of p53WT in tumor cells,
determining the mutational status of p53 in tumors may be important predictor
of response to treatment.
Overexpression of MDM2 promotes
the development of the malignant
phenotype
MDM2 functions as an E3
ligase that marks p53 for
proteolytic degradation
T
he MDM2 protein is an important negative regulator of p53.4,13 MDM2
inhibits p53 activity via three mechanisms: as an E3 ubiquitin ligase, MDM2
targets p53 for destruction in the cytosol, driving it to the proteasome. It also
facilitates transport of p53 from the nucleus to the cytosol, in order for
degradation to occur. Furthermore, MDM2 acts to block the transactivation
domain of p53. These three actions in concert facilitate degradation of p53,
and inhibit its ability to influence transcription of genes to block cell cycle
progression and promote apoptosis, markedly impinging on the tumor
suppressive actions of p53.
MDM2 inhibits
transcriptional
activity of p53
MDM2 transports p53 from
nucleus to cytosol to facilitate
proteolytic degradation
Proteasome
p53-MDM2
Complex
Binding of MDM2 inhibits p53 activity
Some human tumors overexpress MDM2, which abrogates the protective effects of normal
functioning p53.2,13 Consequently, MDM2 overexpression negatively affects prognosis. For
example, solid tumors including breast, lung, esophagus, and stomach, and sarcomas,
glioblastomas, and leukemias, have been associated with high levels of MDM2. High
levels of MDM2 have been shown to correlate with poor prognosis, promotion of the
malignant phenotype, treatment resistance, and metastatic capacity.
Inhibition of MDM2 allows for restoration
of p53 tumor suppressor function
D
evelopmental efforts on cancer therapeutics that act to remove the negative
regulatory effect of excessive MDM2, and allow restoration of normal p53 activity are
ongoing.4 One such approach is via the effective inhibition of MDM2 to diminish p53
transport to the cytosol, allowing for p53 accumulation in the nucleus, and enabling
transactivating functionality—allowing p53 to interrupt propagation of abnormal or
oncogenic cells. Normal functioning p53 would have the capacity to recognize
replicating cells that could prove to be a threat to the organism, and would be able
to promote tumor cell death. MDM2 inhibitors in development have been shown to
activate p53, promoting cell cycle arrest and apoptosis; a number of such agents are
currently in clinical trials.
Apoptosis
Cell-cycle
arrest
Inhibition of MDM2 allows
restoration of p53 tumor
suppressor function
Inhibition of
DNA synthesis
MDM2
p53
Nucleus
Several different mechanisms to inhibit MDM2 have shown activity in tumor
xenograft models.13 These approaches all act to down-regulate the p53-MDM2
loop, thus increasing levels of activated p53 and inhibiting tumor growth. An
alternate mechanism of MDM2 inactivation targets the interaction of MDM2 with
proteins of the proteasome that degrades p53. Disturbance of the ubiquitinproteasome proteolysis pathway has been shown to induce apoptosis and growth
inhibition in several different tumor models. Moreover, preclinical experiments
have shown that molecules that inhibit MDM2-p53 protein-protein interactions
can induce apoptosis, cell cycle arrest, and inhibit DNA synthesis, indicating
restoration of p53 functionality in tumor cells.14
The MDM2-p53 interaction
represents a compelling therapeutic
strategy in cancer
A
ctivated p53 leads to transactivation of a range of genes involved
in metabolic homeostasis, DNA repair, growth arrest and other
critical functions, with the purpose of preventing propagation of cellular
abnormalities.5 p53 exists in a tightly controlled feedback loop with MDM2.
MDM2 also blocks the transactivating function of p53. In some human cancers,
p53 is mutated or deleted, resulting in uncontrolled proliferation.2 In other
cancers, however, p53 is intact but overexpression of MDM2 contributes
markedly to the malignant phenotype, correlating with poor prognosis,
treatment resistance, and metastatic capacity. Inhibition of MDM2 in p53WT
tumor cells may restore p53 function, permitting cell cycle arrest and apoptosis
of cancerous cells. Currently there are several approaches under investigation
to exploit the anti-tumor effects of reactivating p53 by MDM2 inhibition.2,4
p53
MDM2
Inhibition of MDM2 allows
restoration of p53 tumor
suppressor function
Apoptosis
References
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