Download Molecular States Underlying Androgen Receptor

VOLUME
30
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NUMBER
6
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FEBRUARY
20
2012
JOURNAL OF CLINICAL ONCOLOGY
UNDERSTANDING THE PATHWAY
Molecular States Underlying Androgen Receptor
Activation: A Framework for Therapeutics Targeting
Androgen Signaling in Prostate Cancer
Peter S. Nelson, Fred Hutchinson Cancer Research Center, Seattle, WA
See accompanying article on page 637; listen to the podcast by Dr Taplin at www.jco.org/podcasts
“You can observe a lot just by watching.” —Yogi Berra
The androgen receptor (AR) is a resilient foe. Since the landmark
studies of Huggins et al1 demonstrated the sensitivity of prostate
cancer to androgenic hormones, androgen deprivation therapy has
been the most widely used and effective treatment for metastatic
disease. Although initial response rates exceed 90%, the eventual
emergence of castration-resistant prostate cancer (CRPC) is nearly
universal, and it represents a disease state that is usually fatal. However, a remarkable and extremely important aspect of CRPC is the
near-universal reactivation of AR signaling, a finding readily substantiated through measurements of high serum concentrations prostatespecific antigen (PSA), a gene directly and exclusively regulated by the
AR. Several studies have documented that most, and potentially all,
genes known to be under AR transcriptional control in prostate cancer
cells are re-expressed in CRPC tumors.2-4 In this context, the AR may
be the earliest known example of a lineage oncogene—a master regulator to which neoplastic cells derived from prostate epithelium are addicted.5
AR engagement by androgenic ligands such as testosterone and
dihydrotestosterone motivate AR migration from the cytoplasm to
the nucleus, where AR target genes are recognized and activated
through DNA binding to locations specified by nucleotide sequence
and chromatin accessibility. The observation that AR-regulated genes
are active in CRPC— despite undetectable levels of testosterone in
blood— has prompted efforts to identify processes driving AR function in the castrate environment.6,7 Such alternative mechanisms of
AR activation could represent targets for therapeutic inhibition.
A key question that should influence further investment in obstructing the AR program in CRPC is whether the AR is continuing to
provide the growth and survival signals for these tumors, or if the AR
program is simply baggage carried along by other oncogenic drivers.
Compelling evidence indicates the former is likely to be true.
For example, experiments abolishing the AR itself in androgenrefractory prostate cancer cells in vitro effectively suppressed
proliferation.8 CRPC tumors proliferating in castrate mice consistently regress after the targeted elimination of AR expression.9
Finally, clinical studies of secondary and tertiary methods to inhibit AR signaling in CRPC usually result in PSA and clinical
responses, although they are generally of relatively short dura644
© 2011 by American Society of Clinical Oncology
tion.10 Newer agents targeting androgen biosynthesis or blocking
AR activation carry on this tendency.11,12 Together, these laboratory and clinical observations emphasize the continued relevance
of the AR pathway as a key therapeutic node in the vast majority of
patients with advanced CRPC.
Unfortunately, over the past several decades, efforts to target AR
signaling have blown hot and cold, with irrational exuberance followed by neglect. Insightful studies by Geller et al13,14 in the 1970s
demonstrated that androgen levels within prostate cancers far exceeded concentrations found in castrate men, a finding that ushered in
the era of combined androgen blockade (CAB) with steroidal and
nonsteroidal androgen receptor antagonists. Despite many trials of
CAB, the therapeutic advantage remains highly debated. Initial studies
using either surgical or medical castration in conjunction with an
antiandrogen suggested significant improvements in survival compared with historical controls.15 However, the benefits measured in
subsequent randomized trials were not encouraging, although metaanalyses have consistently shown a statistically significant improvement in 5-year survival, on the order of 5%, in favor of CAB.16
Although certainly less impressive than anticipated, the trials of
these early antiandrogens did not refute the hypothesis that AR is
still a key driver in CRPC because of one key observation: Patients
for whom these drugs failed experienced progression with a rising
PSA, indicating the AR pathway remained active. Determining
how the AR remained engaged awaited detailed molecular studies
that identified mechanisms underlying the limited effectiveness of
these AR antagonists and led to the next generation of AR inhibitors, such as MDV3100, with enhanced binding affinities and
lacking agonist properties.17,18
To provide a framework for the clinical application of new agents
designed to suppress androgen signaling, it is useful to consider the
molecular endocrinology that underlies the maintenance of AR activity. On the basis of current knowledge of prostate cancer molecular
biology, at least four discrete cellular states of prostate cancer can be
defined, based solely on the status of AR program activity and the
mechanism by which it is activated. Of clinical relevance, the cellular
states are dynamic and evolve either through adaptation or genomic
events, and each molecular state is also associated with a specific
therapeutic node that generally requires effective inhibition—and
Journal of Clinical Oncology, Vol 30, No 6 (February 20), 2012: pp 644-646
Information downloaded from jco.ascopubs.org and provided by at Danish National Library Authority on March 26, 2014 from
Copyright © 2012 American Society
of Clinical Oncology. All rights reserved.
193.163.235.201
Understanding the Pathway
STATE 1
Endocrine androgen dependent
and AR dependent
STATE 2
STATE 3
STATE 4
Intracrine androgen dependent
and AR dependent
Androgen (ligand) independent
and AR dependent
Androgen (ligand) independent
and AR independent
Others…
IL-6
HER2
Intracrine
testosterone
Prostate cancer cells
AR
AR
DHT
T
DHEA
Endocrine
T testosterone (T)
Testis
T
testis
Others…
Adrenal
ARSV
T DHT
cholesterol
T
testis
DHEA
DHE
A
Others
Oth
ers…
Others…
Adrenal
N-cadherin
Src?
AR
T DHT
Src?
AR
T
T DHT
IL-6
HER2
Neuroendocrine
carcinoma
?
T
testis
DHEA
Othe
Oth
ers…
ers
s
Others…
Adrenal
Fig 1. Molecular states framework for androgen receptor (AR) activation in prostate cancer. Four discrete states of prostate carcinoma can be defined on the
basis of sources of androgenic ligands and the activity of AR. In state 1 (endocrine androgen dependent and AR dependent), prostate carcinoma cells are
dependent on high levels of circulating testosterone (T) synthesized in the testis. In state 2 (intracrine androgen dependent and AR dependent), after the
suppression of circulating T, androgenic ligands can be found in tumor cells through de novo synthesis or the conversion of adrenal precursors such as DHEA.
In state 3 (androgen independent and AR depentent), the AR remains active in the absence of ligands through crosstalk with other signal transduction pathways
that may include HER2/neu, IL-6, Src kinase, and others, or through the expression of ligand-independent AR splice variant (ARSV). In state 4 (androgen
independent and AR independent), AR signaling is abolished, and complete responses are achieved, or tumor progression is driven by other oncogenic programs
yet to be determined. Ablating the AR program should be objective of the successful combinatorial applications of emerging AR therapeutics. DHEA,
dehydroepiandrosterone; DHT, dihydrotestosterone; HER2, human epidermal growth factor receptor 2; IL-6, interleukin 6.
subsequent resistance— before transition. For this discussion, dependence is operationally defined as that factor or component required by
a cell for survival and the engagement of other functions carried out by
neoplastic prostate epithelium such as PSA protein secretion. The
molecular states (Fig 1) are as follows:
State 1: Endocrine androgen dependent. The drivers and therapeutic node promoting the AR pathway in this state are serum
androgens produced in the testis. With few exceptions, current
therapies such as luteinizing hormone–releasing hormone agonists
and antagonists generally produce excellent results in terms of
achieving low serum testosterone levels in most patients. Thus,
further research efforts in this domain are unlikely to contribute
substantially to improved outcomes.
State 2: Intracrine androgen dependent. The drivers of AR signaling in this state are also androgens, but they are produced within
prostate cancer cells either from adrenal precursors or through de
novo metabolism.19 Recent studies, fundamentally rediscovering
the early work of Geller et al13,14 demonstrating high intratumoral
androgen concentrations relative to serum levels, have strongly
supported the existence of this cellular state, although many questions remain concerning how androgens are generated.20 The process of steroid biosynthesis, whether within the adrenal gland or
the prostate, involves a series of metabolic enzymes responsible for
the stepwise conversion of cholesterol to testosterone and dihydrotestosterone, and several represent druggable nodes. Although
the metabolic pathway is complex (as reviewed21), the clinical
responses observed in men with CRPC treated with the 17␣hydroxylase-17,20-lyase inhibitor abiraterone support at least
CYP17 as a relevant target. Although abiraterone treatment extends survival, failure of this treatment is common and also
marked by rising PSA— once again demonstrating that therapy
failure is associated with reactivation of AR signaling.12 Establishing the mechanism by which the AR program is maintained in this
setting is essential and will require direct assessments of tumor
tissue to confirm adequate suppression of CYP17 activity and
www.jco.org
ideally quantitation of intratumor androgen levels. In this issue of
the Journal of Clinical Oncology, Efstathiou et al22 provide an important first step in distinguishing those tumors that may be most
susceptible to abiraterone by directly measuring the expression of
CYP17 in metastatic tumor cells acquired from bone. Intense nuclear
localization AR concomitant with CYP17 expression correlated with
longer time remaining on treatment. Of interest, bone marrow plasma
concentrations of testosterone and DHT were undetectable in men
progressing while receiving abiraterone, indicating either that these
tumors are no longer dependent on AR ligands or that marrow plasma
is not an accurate surrogate for intratumoral measurements.
State 3: Ligand independent, AR dependent. Extensive literature
exists regarding mechanisms with the potential to activate the AR in
the complete absence of androgens. Elegant in vitro studies, performed in laboratory situations in which androgen levels can be rigorously eliminated, have demonstrated that crosstalk with other signal
transduction pathways can activate AR signaling.23,24 However, it has
been challenging to confirm that these networks promote AR effects in
humans. Recently, several AR variants (ARVs) have been identified
that result from alternative splicing of the AR transcript.25-28 A striking
feature of these receptors, if such terminology is still applicable, is the
lack of a C-terminal ligand-binding domain; however, ARVs are functionally able to confer ligand-independent activation of known AR
target genes. ARVs may also regulate a gene repertoire distinct from
the full-length AR. Under the selective pressure of improved antagonists with higher affinities for AR and drugs that further suppress
ligands, ARVs may emerge as more common mediators of ligandindependent AR-dependent tumor progression and will require new
strategies, such as compounds that interfere with N-terminal ARDNA interactions.29
State 4: Ligand independent, AR independent. This state of prostate cancer may synonymously be termed AR pathway–independent
prostate cancer, which is effectively AR null. With the exception of
small-cell or neuroendocrine carcinomas, this tumor cell state is rarely
observed in the clinical management of men with prostate cancer and
© 2011 by American Society of Clinical Oncology
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of Clinical Oncology. All rights reserved.
193.163.235.201
645
Peter S. Nelson
may be simply defined as tumor progression in the setting of undetectable levels of PSA. Only in this setting could the AR program be
considered extinguished.
Future clinical strategies for men with CPRC are obvious: As
several new active drugs are approved or in late stages of development
that inhibit distinct noncrossreactive nodes in the androgen signaling
pathway, they should be evaluated in combinations to determine if
such highly active anti-AR therapy will successfully extinguish the AR
program, a concept analogous to the successful deployment of combinatorial, highly active antiretroviral therapy for the treatment of
HIV.30 The result may produce complete remissions, possibly cures,
or the emergence of prostate cancers that no longer depend on AR
signaling. Until that time, it is important to follow Sutton’s law and go
“where the money is,”30 because in this case, the money is still on the AR.
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AUTHOR’S DISCLOSURES OF POTENTIAL CONFLICTS OF
INTEREST
Although all authors completed the disclosure declaration, the following
author(s) indicated a financial or other interest that is relevant to the subject
matter under consideration in this article. Certain relationships marked with a
“U” are those for which no compensation was received; those relationships
marked with a “C” were compensated. For a detailed description of the
disclosure categories, or for more information about ASCO’s conflict of interest
policy, please refer to the Author Disclosure Declaration and the Disclosures of
Potential Conflicts of Interest section in Information for Contributors.
Employment or Leadership Position: None Consultant or Advisory Role:
Peter S. Nelson, Janssen Biotech (C), Tokai Pharmaceuticals (C) Stock
Ownership: None Honoraria: None Research Funding: None Expert
Testimony: None Other Remuneration: None
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DOI: 10.1200/JCO.2011.39.1300; published
online ahead of print at www.jco.org on
December 19, 2011
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JOURNAL OF CLINICAL ONCOLOGY
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Copyright © 2012 American Society
of Clinical Oncology. All rights reserved.
193.163.235.201