REVIEW
`
`Endocrine-Related Cancer (2004) 11 459–476
`
`Targeting the androgen receptor:
`improving outcomes for castration-
`resistant prostate cancer
`
`Howard I Scher, Grant Buchanan1, William Gerald 2, Lisa M Butler1
`and Wayne D Tilley1
`
`Genitourinary Oncology Service, Division of Solid Tumor Oncology, Memorial Sloan-Kettering Cancer Center,
`Department of Medicine, Joan and Sanford I Weill College of Medicine, New York, New York 10021, USA
`1Dame Roma Mitchell Cancer Research Laboratories, Department of Medicine, University of Adelaide
`& Hanson Institute, Adelaide SA 5000, Australia
`2Department of Pathology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA
`
`(Requests for offprints should be addressed to Howard I Scher, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York,
`New York 10021, USA; Email: scherh@mskcc.org or Wayne Tilley, The Hanson Institute, PO Box 14 Rundle Mall, Adelaide SA 5000, Australia;
`Email: wayne.tilley@adelaide.edu.au)
`
`Abstract
`
`The categorization of prostate cancers that are progressing after castration as ‘hormone-refractory’
`evolved from the clinical observation that surgical or medical castration (i.e. androgen ablation therapy;
`AAT) is not curative and, despite an initial response, virtually all tumors eventually regrow. Successful
`AAT is contingent on the dependence of prostate cancer cells for androgen signaling through an
`intracellular mediator, the androgen receptor (AR) for survival. Current preclinical and clinical data
`imply that the AR is expressed and continues to mediate androgen signaling after failure of AAT. As
`AAT does not
`completely eliminate circulating androgens,
`sufficient
`concentrations of
`dihydrotestosterone may accumulate in tumor cells to maintain AR signaling, especially in the
`context of upregulated receptor levels or increased sensitivity of the AR for activation. In addition,
`ligands of non-testicular origin or ligand-independent activation can contribute to continued AR
`signaling.
`In many cases,
`therefore,
`from the perspective of
`the AR, a ‘hormone-refractory’
`classification after failure of AAT is inappropriate. Classifying prostate tumors that progress after
`AAT as ‘castration-resistant’ may be more relevant. Clinical responses to second- and third-line
`hormonal therapies suggest that the mechanisms of AR activation are in part a function of previously
`administered AAT. Accordingly, the increasing trend to utilize AAT earlier in the course of the clinical
`disease may have a greater influence on the genotype and phenotype of the resistant tumor. In this
`article, we detail strategies to inhibit the growth of prostate cancer cells that specifically target the AR in
`addition to those practiced traditionally that indirectly target the receptor by reducing the amount of
`circulating ligand. We propose that
`treatment regimes combining AAT with direct AR targeting
`strategies may provide a more complete blockade of androgen signaling, thereby preventing or
`significantly delaying the emergence of treatment-resistant disease.
`
`Endocrine-Related Cancer (2004) 11 459–476
`
`Introduction
`
`The demonstration that exogenous estrogens or surgical
`orchidectomy could produce tumor shrinkage in the
`advanced disease setting ushered in the era of hormonal
`management of prostate cancer in 1941 (Huggins &
`Hodges 1941, Huggins et al. 1941). Benefits to the patient
`included palliation of pain and relief of urinary
`
`symptoms, with a concomitant decline in acid phospha-
`tase concentrations, consistent with the clinical findings.
`The success of hormone treatment, or more specifically of
`androgen ablation therapy (AAT), is contingent on the
`dependence of prostate cancer cells on the more potent
`5a-reduced metabolite of testosterone, 5a-dihydrotestos-
`terone (DHT),
`for
`their growth and survival. The
`traditional view of hormones and the prostate is therefore
`
`Endocrine-Related Cancer (2004) 11 459–476
`1351-0088/04/011–459 # 2004 Society for Endocrinology Printed in Great Britain
`
`DOI:10.1677/erc.1.00525
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`Scher et al.: Castration-resistant prostate cancer
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`Figure 1 Endocrine control of prostatic growth. The growth and development of the normal prostate requires a functioning androgen
`signaling pathway, which is regulated by the hypothalamic–pituitary–gonadal axis. Androgens [testosterone (T), androstenedione,
`dehydroepiandrosterone (DHEA)] and other steroids are synthesized in the testes or adrenal glands and released into the circulation
`in response to specific hormonal signals [adrenocorticotropic hormone (ACTH), corticotropin-releasing hormone (CRH), follicle
`stimulating hormone (FSH), gonadotropin releasing hormone (GnRH), luteinizing hormone (LH), luteinizing hormone releasing
`hormone (LHRH)]. Testosterone is transported by steroid hormone binding globulin (SHBG) to the prostate, where it is
`predominantly converted, by 5a-reductase, to its more active metabolite, 5a-dihydrotestosterone (DHT). In the prostate, androgens
`mediate their effects via high-affinity binding to the androgen receptor (AR), a nuclear transcription factor that controls expression of
`genes involved in growth, differentiation, homeostasis and apoptosis. Medical and surgical interventions commonly used to treat
`prostate cancer (i.e. androgen ablation therapy) are indicated by dotted lines at the point in which they disrupt the androgen
`signaling pathway.
`
`focused on the ligand, and in particular on reducing or
`blocking the action of DHT (Fig. 1). DHT binds to and
`activates the androgen receptor (AR), which regulates
`transcription of a diverse range of target genes involved in
`prostate cell homeostasis, angiogenesis, differentiation
`and apoptosis (Buchanan et al. 2001b, Gelmann 2002,
`Tilley et al. 2003).
`In the modern era, the clinical use of AAT has been
`expanded to include medical therapies such as luteinizing
`hormone-releasing hormone (LHRH) agonists/antago-
`nists or estrogens that target the hypothalamic–pitui-
`tary–gonadal axis (Fig. 1). Combined androgen blockade
`strategies utilize LHRH agonists/antagonists and AR
`antagonists to inhibit respectively the production of
`testicular androgens and the binding of residual andro-
`gens to the AR (Labrie et al. 1983). Enzymatic inhibitors
`of adrenal synthetic enzymes are used to block the
`production of adrenal androgens. The common feature
`of these approaches is that they target the receptor
`indirectly, by (i) reducing circulating concentrations of the
`
`native ligand (i.e. testicular androgens) either medically or
`surgically, or (ii) blocking the ability of androgens to bind
`to the AR using receptor antagonists. Although these
`strategies were originally used to treat patients with
`metastatic disease, clinical use of AAT expanded to the
`neoadjuvant
`(before primary treatment) and, more
`commonly, to the adjuvant (during primary treatment)
`setting in combination with surgery or radiation,
`in
`addition to conditions of
`increasing prostate specific
`antigen (PSA) concentrations (no detectable disease on
`an imaging study). However, irrespective of the nature
`and timing of AAT, overall outcomes are similar: an
`initial response, then a period of stability, followed by
`biochemical, radiographic and, ultimately, clinical pro-
`gression. What is not clear, however, are the mechanisms
`contributing to the failure of AAT. In particular, whether
`renewed growth of prostate tumors is the result of
`maintenance of AR signaling in a castrate setting or of
`activation of AR-independent survival pathways,
`is a
`topic of considerable debate. In this review, we develop
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`Endocrine-Related Cancer (2004) 11 459–476
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`Figure 2 An example of a sequential hormonal response. A clinical example of sequential decreases in PSA concentration,
`associated with no progression in other sites such as bone, lymph nodes or viscera, and no new symptoms of disease, in a patient
`treated (i) with 6 months of a GnRH analog and bicalutamide (as indicated), and then (ii) a second course of combined blockade. At
`the time of progression, (iii) bicalutamide was discontinued with no response, following which (iv) nilutamide was added, producing a
`decline in PSA for more than 8 months. Later, an increase in PSA was noted, at which point (v) nilutamide was discontinued, with a
`tertiary response.
`
`the case for the persistence of AR-dependent signaling
`mechanisms after failure of AAT, suggest novel strategies
`using existing hormonal treatments for prostate cancer,
`and discuss new therapies that directly target the AR,
`which may be more effective than conventional androgen
`ablation.
`
`Evidence for maintenance of AR signaling
`after failure of AAT
`
`Clinical studies
`
`Prostate cancers that progress despite castrate concentra-
`tions of testosterone in the blood have been categorized as
`‘hormone-refractory’,
`implying that
`further hormonal
`treatments would be of limited clinical value. That PSA
`concentrations increase in virtually all cases of resistance
`to AAT argues against
`this categorization, because
`signaling is mediated through a specific androgen
`response element in the promoter of the PSA gene (Balk
`et al. 2003). Further evidence against a hormone-
`refractory categorization is the observation that more
`than 20–40% of prostate tumors that progress on AAT
`respond to second- and third-line hormonal treatments
`(Kojima et al. 2004). These therapies include anti-
`
`inhibitors
`androgens, estrogens, progestational agents,
`of adrenal steroid synthesis such as ketoconazole and
`glucocorticoids (Scher et al. 1995, Small 1997). The
`paradoxical responses to the discontinuation of anti-
`androgens, estrogens, glucocorticoids and progestational
`agents (Kelly & Scher 1993, Scher & Kelly 1993, Wirth &
`Froschermaier 1997), and disease flares that occur when
`exogenous androgens are administered (Fowler & Whit-
`more 1981, Manni et al. 1988), are additional illustrations
`of continued hormonal sensitivity despite failure of AAT.
`Anti-androgen withdrawal responses have been docu-
`mented in more than 30% of patients who received
`flutamide as part of a combined androgen blockade
`approach (Scher et al. 1995). Secondary clinical responses
`to bicalutamide observed in patients who have progressed
`on flutamide independent of a withdrawal response (Scher
`& Kolvenbag 1997), and the PSA response to nilutamide
`in patients with a previous anti-androgen withdrawal
`response, provide additional evidence of hormone sensi-
`tivity (Kassouf et al. 2003). A clinical example of
`secondary and tertiary responses to different androgen
`ablations is shown in Fig. 2. This patient was treated
`initially with a 6-month course of a gonadotropin-
`releasing hormone (GnRH) analog and bicalutamide,
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`Scher et al.: Castration-resistant prostate cancer
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`Figure 3 (a) Clinical states: a framework with which to define treatment objectives, to select treatment and to assess outcomes. Based
`on recognizable points in the natural history of prostate cancer, key milestones or clinical states that apply to an individual or to a
`population can be defined. This stratification provides a framework for defining therapeutic objectives, disease management and
`understanding the basis of progression. As more men die with prostate cancer than from it, effective management relies on
`determining which state a patient resides in at a given point of time, and assessment of the probability that a patient will progress to
`castration-resistant metastatic disease before dying of other causes. Androgen ablation traditionally was used to treat metastatic
`disease. More recently, androgen deprivation has been used in a neoadjuvant setting (before primary treatment), but is more
`commonly used in an adjuvant setting (during primary treatment) for clinically localized disease when an increasing PSA is first
`documented. The longer the duration of treatment in an adjuvant setting, the more likely that the specific hormone therapy
`administered will affect the biology of the progressing tumor. (b) Profiling of gene expression in primary and castrate-resistant prostate
`tumors. Microarray and tissue array studies were conducted on clinical prostate cancer samples, which were chosen from the clinical
`states indicated in (a), using procedures approved by the Institutional Review Board of the Memorial Sloan-Kettering Cancer Center.
`Malignant and benign regions were microdissected from (i) 3 non-cancerous prostates (NM) and 49 prostate tumors, the latter
`consisting of (ii) 23 untreated primary tumors, (iii) 17 primary tumors after neoadjuvant AAT, (iv) 6 metastatic tumors and (v) 3 recurrent
`metastatic tumors after androgen ablation. mRNA was extracted from each sample and from the human prostate cancer cell line,
`LNCaP, stimulated with 0.1 nM R1881, a synthetic androgen, for 24 h and either harvested immediately or incubated in steroid-free
`conditions for a further 36 h to simulate androgen withdrawal. Gene expression profiling was performed on mRNA samples with
`Affymetrix U95 human gene arrays using instruments and procedures as described previously (Holzbeierlein et al. 2004). To identify
`differentially expressed genes, comparisons between three tumor groups were performed: Group 1, (ii) versus (iii); Group 2, (ii) versus
`(v); Group 3, (iv) versus (v), as indicated. Similarly, comparisons were made between LNCaP cells cultured with or without androgens.
`Differential expression was defined as a minimum of threefold difference between expression of selected genes.
`
`after which all treatment was discontinued. When the PSA
`increased, combined androgen ablation was reinstituted
`on a continuous basis until the PSA increased again and
`bicalutamide was discontinued. No response to bicaluta-
`mide discontinuation was observed, but the addition of
`nilutamide resulted in a decline in PSA concentrations for
`more than 8 months. Later, an increase in PSA was noted,
`at which time nilutamide was discontinued and the PSA
`declined again.
`Immunohistochemical and other studies of clinical
`tumor
`samples have demonstrated that
`the AR is
`expressed in the majority of AAT-naı¨ ve and -resistant
`tumors, and that the tissue concentrations of PSA and
`other androgen-responsive genes increase in the setting of
`castration-resistant tumor growth (Hobisch et al. 1995,
`
`Bentel & Tilley 1996, Culig et al. 1998). To investigate
`overall changes in gene expression during progression of
`clinical prostate cancer after androgen ablation, we
`undertook microarray gene profiling of both naı¨ ve and
`AAT-treated primary prostate cancers removed by radical
`prostatectomy, and castration-resistant metastatic tumors
`(Fig. 3) (Holzbeierlein et al. 2004). As expected, many of
`the genes with altered expression in primary tumors
`removed 3 months after initiation of androgen depriva-
`tion, and those in LNCaP cells after androgen manipula-
`tion, included known targets of the androgen receptor
`(e.g. KLK3, KLK2; Fig. 4a). Significantly, the gene
`expression profile of
`castration-resistant metastatic
`tumors is similar to that of hormone-naı¨ ve lesions,
`but quite distinct from that of primary tumors after
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`Endocrine-Related Cancer (2004) 11 459–476
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`Figure 4 AR expression is increased in castrate-resistant prostate cancer. (a) Gene expression of AR and the androgen-responsive
`kallikreins, KLK2 and KLK3, determined from Affymetrix array profiling experiments (refer to Fig. 3) for each of the prostate tumor
`groups. (b) Multi-tissue blocks of formalin-fixed, paraffin-embedded tissue corresponding to the same benign and malignant prostate
`samples analysed in (a) were prepared using a tissue arrayer (Beecher Instruments, Silver Spring, MD, USA). The multi-tissue
`blocks included three representative 0.6 mm cores of each sample derived from diagnostic regions. Immunohistochemical staining
`for AR was carried out with standard streptavidin–biotin peroxidase methodology using formalin-fixed, paraffin-embedded tissue and
`microwave antigen retrieval (LaTulippe et al. 2002).
`
`short-term androgen ablation. Approximately 97% of the
`genes with altered expression after neoadjuvant AAT
`were not altered in castration-resistant
`tumors (e.g.
`KLK3, KLK2; Fig. 4a). The median level of AR
`expression was markedly increased (9–11-fold) in castra-
`tion-resistant metastatic disease (Mann-Whitney U test;
`P ¼ 0:028) relative to untreated or neoadjuvant-treated
`primary tumors (Fig. 4a). Immunohistochemical analysis
`confirmed that AR RNA levels were concordant with the
`amount of receptor protein in individual tumor samples
`(Fig. 4b). These data strongly suggest
`that prostate
`tumors evolve mechanisms to reactivate AR expression
`and AR-responsive gene pathways after AAT, and that
`these changes have a key role in the development of
`resistance to hormonal treatment (Amler et al. 2000,
`LaTulippe et al. 2002, Holzbeierlein et al. 2004).
`
`Animal studies
`
`The CWR22 xenograft is an androgen-dependent tumor
`derived from a patient with metastatic prostate cancer
`grown subcutaneously in athymic nude mice (Pretlow et
`
`al. 1993, Wainstein et al. 1994, Nagabhushan et al. 1996,
`Tan et al. 1997). After androgen ablation, these tumors
`show regression, stability and later progression, similar to
`what is seen in the human condition. In most animals,
`castration-resistant CWR22 tumors emerge after 80–200
`days after androgen withdrawal (Nagabhushan et al.
`1996). A marked reduction in the expression of AR and
`markers of cellular proliferation is observed in CWR22
`tumors two days post-castration (Agus et al. 1999).
`However,
`subsequent
`proliferation
`during
`tumor
`regrowth is associated with re-expression of AR and
`androgen-regulated genes to levels comparable to those
`seen in tumors from intact mice (Gregory et al. 1998,
`Agus et al. 1999, Kim et al. 2002). Expression profiling of
`AAT-naı¨ ve and castration-resistant CWR22 tumors
`demonstrated that
`the expression of only a small
`proportion of genes (<5%) was altered in the recurrent
`tumors (Amler et al. 2000). Collectively, these studies
`suggest that restoration of AR signaling pathways is
`associated with renewed growth of CWR22 tumors in a
`castrate environment. More recently, Chen and colleagues
`(2004), who compared the gene expression profiles of
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`isogenic androgen withdrawal-sensitive and -resistant
`xenograft tumors, demonstrated that, from seven human
`prostate cancer xenografts examined, the AR was the only
`gene
`consistently upregulated in castration-resistant
`tumors.
`
`Mechanisms of continued AR signaling
`during progression
`
`Studies of androgen-mediated signaling in animal models
`and human tumor specimens must be interpreted in the
`context of the point in the illness that the tumor sample
`represents. Different results will be obtained depending on
`the stage of disease, whether the sample was from the
`prostate or a particular metastatic site, whether the tumor
`has or has not been exposed to a specific form of
`androgen deprivation and whether it
`is proliferating
`(i.e. the disease is progressing) or non-proliferating (i.e.
`regressing or static). All these states are difficult to
`characterize, because human tumor samples often are not
`obtained in the course of routine medical management
`after diagnosis. Nevertheless, as discussed above, the
`findings of recent studies support the concept that AR
`signaling is maintained or upregulated in tumors that
`regrow after failure of AAT, and that the associated
`activation of androgen-regulated genes is sufficient to
`facilitate tumor survival. The specific alterations
`in
`prostate tumor cells that facilitate increased sensitivity
`of the AR signaling pathway can be considered at the level
`of the ligand and the receptor, the structure and function
`of the AR and its coregulators, or cross-talk with other
`signaling pathways (Fig. 5) (Tilley et al. 1996, Grossmann
`et al. 2001).
`
`Increased bioavailability of ligand
`
`Whereas testosterone and DHT concentrations in the
`blood are low in a patient whose tumor is progressing
`after castration,
`intratumoral androgen concentrations
`may be sufficient to maintain tumor growth (Labrie et al.
`1983, Geller et al. 1984a,b, Mohler et al. 2004). Tumor
`cells may acquire mechanisms to accumulate androgens,
`such as
`sequestration by steroid hormone binding
`globulin, which is synthesized and secreted by prostatic
`epithelial and stromal cells (Hryb et al. 2002), or by
`altered regulation of enzymes involved in the synthesis
`and metabolism of androgens.
`In support of
`this
`hypothesis, the comparative microarray analysis detailed
`above detected increased expression of enzymes in the
`steroid precursor synthesis pathway in castration-resistant
`tumors compared with that in castration-naı¨ ve samples
`(Holzbeierlein et al. 2004). These enzymes included 3-
`hydroxy-3-methylglutaryl-coenzyme A synthase 1 and
`
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`
`squalene epoxidase, which are considered to be rate-
`limiting enzymes in sterol biosynthesis (Chugh et al.
`2003). Genes involved in fatty acid and steroid metabo-
`lism, which potentially could facilitate steroid production,
`were also upregulated. Recently, Mohler and coworkers
`(2004) measured the concentrations of AR and androgens
`in the tissues of locally recurrent prostate cancers after
`AAT. Similar
`concentrations of
`testosterone were
`detected in recurrent tumor samples and in control benign
`prostatic hyperplasia specimens. Moreover, whereas the
`concentrations of DHT, dehydroepiandrosterone and
`androstenedione were lower in recurrent prostate tumor
`tissues than in benign prostatic hyperplasia samples, there
`was a sufficient concentration of ligand to account for the
`expression of the AR-regulated PSA protein. These results
`ague that, despite androgen ablation, prostate tumors
`may never encounter a completely ‘androgen-indepen-
`dent’ environment (Mohler et al. 2004).
`
`AR expression
`
`Immunohistochemical studies demonstrate that the AR is
`expressed in essentially all human prostate cancers,
`including those that regrow after failure of AAT, and
`that the level of AR expression is at least retained, and
`often increased, relative to untreated tumors (e.g. Fig. 4)
`(Sadi & Barrack 1993, Pertschuk et al. 1994, Tilley et al.
`1994, Takeda et al. 1996, Culig et al. 1998, Prins et al. 1998,
`Mohler et al. 2004). One mechanism for increased receptor
`concentrations is amplification of the AR gene, which has
`been reported in 22% of castration-resistant metastatic
`tumors, and in 23–28% of recurrent primary tumors
`(Bubendorf et al. 1999). AR gene amplification is
`associated with increased concentrations of the AR and
`AR-regulated proteins (Koivisto et al. 1996, 1997, Koivisto
`& Helin 1999, Linja et al. 2001). Only eight castration-
`resistant tumors of 28 examined (29%) in our independent
`studies exhibited amplification of the AR gene, whereas 26
`of the 28 (93%) overexpressed the AR protein (Holzbeier-
`lein et al. 2004). Increased concentrations of AR in prostate
`tumors could result from increased AR protein stability, as
`observed in recurrent CWR22 and LNCaP xenograft
`tumors (Gregory et al. 2001a), or from increased activation
`of the AR promoter (Jarrard et al. 1998, Gregory et al.
`2001b, Takahashi et al. 2002). Irrespective of the mechan-
`ism, after castration the concentration of AR protein in
`prostate tumors appears to be sufficient to allow continued
`AR signaling, particularly if tumor tissues retain significant
`concentrations of ligand as discussed above. In support of
`this hypothesis,
`increasing the AR concentration in
`prostate cancer cells using an AR-expressing lentivirus
`reduced the latency period for the development of LNCaP
`and LAPC4 xenograft tumors in castrate mice (Chen et al.
`2004). An additional consequence in those studies was that
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`Figure 5 Signaling in castration-resistant prostate cancer and potential points of therapeutic intervention. After synthesis, the AR
`exists in dynamic equilibrium between an immature state and an active form capable of binding high-affinity androgenic ligands via
`association/dissociation with a complex that includes heat-shock proteins, p23 and a tetratricopeptide (TPR)-containing protein.
`(a) Receptor-dependent, ligand-mediated signaling. Ligand binding results in the dissociation of this complex, receptor dimerization
`and phosphorylation, nuclear transport, DNA binding, the recruitment of components of the transcription machinery and other cofactor
`molecules, such as the p160 coactivators, and ultimately, the activation of particular gene pathways. (b) Receptor-dependent,
`ligand-independent signaling. The AR can also be activated in the absence of ligand by membrane-bound tyrosine kinase receptors
`such as HER2/neu, and by signaling molecules, growth factors and cytokines. Intracellular kinase cascades result in receptor
`activation, transport, binding to androgen response elements and the transactivation of target genes. 1–5. Mechanisms of continued
`androgen signaling implicated in maintaining prostate cancer growth in a castrate environment after androgen ablation therapy. 1.
`Tumor cells may acquire mechanisms to accumulate androgens, such as sequestration by steroid hormone binding globulin (SHBG)
`or altered regulation of enzymes involved in the synthesis and metabolism of androgens. 2. Castrate-resistant clinical prostate
`cancer samples often exhibit increased AR concentrations compared with early-stage tumors or normal prostate cells. This may
`result from amplification or overexpression of the AR gene. 3. AR gene mutations can allow promiscuous activation of the AR by
`alternative ligands, such as glucocorticoids, estrogens, adrenal androgens, progestins and traditional receptor antagonists such as
`hydroxyflutamide. Other mutations may alter the recruitment of cofactors. 4. Cross-talk with other signaling pathways may activate
`the AR in the absence of native ligands. 5. An altered profile of AR coregulators (coactivators and corepressors) may facilitate
`ligand-independent AR signaling, or enhance AR activation by low levels of ligand. (i–vii). Potential points of therapeutic intervention:
`(i) 5a-reductase inhibitors (e.g. finasteride, dutasteride); (ii) antisense AR oligonucleotides; (iii) Hsp90 inhibitors (e.g. 17-
`allylaminogeldanamycin); (iv) AR inhibitors, antibodies, histone acetylase and deacetylase inhibitors (e.g. SAHA); (v) specific
`response element blockers (e.g. polyamides); (vi) growth factor receptor antibodies (e.g. herceptin); (vii) inhibitors of MAPK, the
`JAK-STAT pathway or Akt.
`
`increased expression of AR reversed the antagonist
`function of bicalutamide such that it acted as a weak AR
`agonist (Chen et al. 2004). The precise consequences of
`increased expression of AR are not known, but recruitment
`and inactivation of pro-apoptotic factors by the AR can
`impair cell cycle arrest and apoptosis of prostate cancer
`cells (Li et al. 2003), suggesting that indirect mechanisms
`may, in part, facilitate survival of prostate cancer cells with
`higher concentrations of AR. The direct effects of increased
`AR concentrations probably derive from altered transcrip-
`tion of AR-responsive genes expressing products that are
`
`involved in both steroid biosynthesis and cell cycle control,
`apoptosis and differentiation (Nelson et al. 2002, Holz-
`beierlein et al. 2004).
`
`Structure and activation of the AR
`
`The AR protein has three major functional domains:
`a large amino-terminal domain (NTD) that contains at
`least two activation functions, AF-1 and AF-5; a DNA-
`binding domain (DBD); and a carboxy-terminal ligand
`binding domain (LBD) that contains a highly conserved
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`Figure 6 Collocation of AR gene mutations in clinical prostate cancer. (a) The position and frequency of missense mutations
`identified in the AR LBD in human prostate cancer (hPCa) and the inherited from of androgen insensitivity (AIS) were used to
`calculate a proportional frequency distribution of mutations. Vertical bars represent the percentage of mutations identified in the AR
`LBD in either hPCa or AIS within a 9 amino acid region centered on individual codons. A cut-off of 4% was applied such that bars
`represent at least two mutations from prostate cancer within each 9 amino acid region. Areas of collocation in hPCa (green boxes)
`and AIS (blue boxes) were determined by delineating the actual position of mutations occurring in regions defined by the frequency
`distribution, and required a minimum of four independent mutations. The amino acid residues encompassing these regions are
`indicated. The arrows (hPCa) denote the seven locations of the eight of 57 missense mutations that do not fall within the indicated
`regions of collocation. (b) Schematic representation of the AR NTD showing position of key functional sub-domain structures (AF1,
`activation function 1; AF5, activation function 5; PolyQ, polyglutamine; PolyG, polyglycine), and the position and frequency (vertical
`bars) of somatic mutations identified in clinical prostate cancer. Two putative areas of collocation are indicated.
`
`ligand-dependent transactivation function (AF-2). More
`than 85% of mutations detected in the AR LBD in clinical
`prostate cancer (Gottlieb et al. 1999), in addition to those
`identified in cell lines and animal models, collocate to a
`small number of discrete regions of the receptor (Fig. 6a)
`(Buchanan et al. 2001a,b). In all, 86% of mutations in the
`LBD in prostate cancer and 72% of inactivating mutations
`in the AR identified in the inherited form of androgen
`insensitivity collocate to regions that collectively encom-
`pass only 10% and 11% of the AR coding sequence
`
`respectively (Fig. 6a). The regions of collocation in prostate
`cancer, with the exception of that encompassing amino
`acids 739–755, are distinct
`from those in androgen
`insensitivity and have been implicated in modulating the
`specificity of the ligand binding, cofactor responses and
`transactivation capacity of the receptor (Buchanan et al.
`2000, 2001b). It is hypothesized that, given the appropriate
`hormonal environment, mutations in these regions of
`collocation in prostate tumors facilitate increased AR
`function, resulting in a survival advantage. Although the
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`

`
`Table 1 Mutations detected in activation function 5 of the androgen receptor in clinical prostate cancer
`
`Endocrine-Related Cancer (2004) 11 459–476
`
`Nucleotide change
`CCG! CTG
`CCT! CTT
`CCC! TCC
`AGT! GGT
`ATG! GTG
`GGC! AGC
`GGC! GAC
`TGG! TAG
`GAT! GGT
`CCT! TCT
`ATG! AGG
`ATG! GTG
`
`Amino acid substitutiona
`
`Reference
`
`Pro389Leu
`Pro502Leu
`Pro512Ser
`Ser513Gly
`Met521Val
`Gly522Ser
`Gly522Asp
`Trp524STOP
`Asp526Gly
`Pro531Ser
`Met535Arg
`Met535Val
`
`Taplin et al. 2001
`Tilley et al. unpublished
`Hyytinen et al. 2002
`Tilley et al. unpublished
`Tilley et al. unpublished
`Hyytinen et al. 2002
`Hyytinen et al. 2002
`Hyytinen et al. 2002
`Tilley et al. 1996
`Hyytinen et al. 2002
`Tilley et al. unpublished
`Tilley et al. unpublished
`
`aNumbering according to Tilley et al. 1989.
`
`AR gene mutation collocation data are currently less
`compelling for the AR NTD than for the LBD, only a few
`studies have examined the coding sequence of the AR NTD
`for mutations (Fig. 6b). This is particularly relevant to
`resolving conflicting reports of the frequency of AR gene
`mutations in prostate cancer, as the findings of recent
`animal model and clinical studies suggest that surgical and
`medical castration result in the preferential accumulation
`of mutations in the AR NTD (Han et al. 2001, Hyytinen et
`al. 2002). Studies by Hyytenin and colleagues (2002) and
`our own unpublished work found that more than 50% of
`the AR gene mutations detected in cohorts of patients with
`prostate cancer receiving combined androgen blockade
`were located within a C-terminal 34 amino acid region
`(amino acids 502–535) of the AF-5 activation function in
`the NTD (Fig. 6b, Table 1). AR gene mutations that confer
`enhanced responsiveness to putative AR coregulators have
`also been identified in the AF-1 activation function (Han et
`al. 2001). One of these mutations (Glu231Gly), located in
`the highly conserved AR NTD signature sequence, is of
`particular interest, as enforced expression of the receptor
`variant in the mouse prostate confers rapid development of
`prostatic intraepithelial neoplasia that progresses
`to
`invasive and metastatic disease in 100% of mice (N
`Greenberg, personal
`communication).
`In contrast,
`enforced expression of the wild-type AR has no observable
`effect on the prostate. The findings of that study highlight
`the potential functional significance of mutations in the AR
`NTD, and demonstrate that specific mutations can turn the
`AR into a potent oncogene sufficient to promote metastatic
`prostate cancer.
`
`Level and function of AR coregulators
`
`conformational
`High-