Combined inhibition of MAP kinase and KIT signaling synergistically destabilizes ETV1 and suppresses GIST tumor growth

Gastrointestinal stromal tumor (GIST), originating from the interstitial cells of Cajal (ICC), is characterized by frequent activating mutations of the KIT receptor tyrosine kinase. Despite the clinical success of imatinib, which targets KIT, most patients with advanced GIST develop resistance and eventually die of the disease. The ETS family transcription factor ETV1 is a master regulator of the ICC lineage. Using mouse models of Kit activation and Etv1 ablation, we demonstrate that ETV1 is required for GIST initiation and proliferation in vivo, validating it as a therapeutic target. We further uncover a positive feedback circuit where MAP kinase activation downstream of KIT stabilizes the ETV1 protein, and ETV1 positively regulates KIT expression. Combined targeting of ETV1 stability by imatinib and MEK162 resulted in increased growth suppression in vitro and complete tumor regression in vivo. The combination strategy to target ETV1 may provide an effective therapeutic strategy in GIST clinical management.

SIGNIFICANCE

ETV1 is a lineage-specific oncogenic transcription factor required for the growth and survival of GIST. We describe a novel strategy of targeting ETV1 protein stability by the combination of MEK and KIT inhibitors that synergistically suppress tumor growth. This strategy has the potential to change first-line therapy in GIST clinical management.

Identifying actionable targets through integrative analyses of GEM model and human prostate cancer genomic profiling

Copy-number alterations (CNA) are among the most common molecular events in human prostate cancer genomes and are associated with worse prognosis. Identification of the oncogenic drivers within these CNAs is challenging due to the broad nature of these genomic gains or losses which can include large numbers of genes within a given region. Here, we profiled the genomes of four genetically engineered mouse prostate cancer models that reflect oncogenic events common in human prostate tumors, with the goal of integrating these data with human prostate cancer datasets to identify shared molecular events. Met was amplified in 67% of prostate tumors from Pten p53 prostate conditional null mice and in approximately 30% of metastatic human prostate cancer specimens, often in association with loss of PTEN and TP53. In murine tumors with Met amplification, Met copy-number gain and expression was present in some cells but not others, revealing intratumoral heterogeneity. Forced MET overexpression in non-MET-amplified prostate tumor cells activated PI3K and MAPK signaling and promoted cell proliferation and tumor growth, whereas MET kinase inhibition selectively impaired the growth of tumors with Met amplification. However, the impact of MET inhibitor therapy was compromised by the persistent growth of non-Met-amplified cells within Met-amplified tumors. These findings establish the importance of MET in prostate cancer progression but reveal potential limitations in the clinical use of MET inhibitors in late-stage prostate cancer.

Annotating STEAP1 regulation in prostate cancer with 89Zr immuno-PET

Antibodies and antibody-drug conjugates targeting the cell surface protein 6 transmembrane epithelial antigen of prostate 1 (STEAP1) are in early clinical development for the treatment of castration-resistant prostate cancer (PCa). In general, antigen expression directly affects the bioactivity of therapeutic antibodies, and the biologic regulation of STEAP1 is unusually complicated in PCa. Paradoxically, STEAP1 can be induced or repressed by the androgen receptor (AR) in different human PCa models, while also expressed in AR-null PCa. Consequently, there is an urgent need to translate diagnostic strategies to establish which regulatory mechanism predominates in patients to situate the appropriate therapy within standard of care therapies inhibiting AR.

METHODS

To this end, we prepared and evaluated (89)Zr-labeled MSTP2109A ((89)Zr-2109A), a radiotracer for PET derived from a fully humanized monoclonal antibody to STEAP1 in preclinical PCa models.

RESULTS

(89)Zr-2109A specifically localized to the STEAP1-positive human PCa models CWR22Pc, 22Rv1, and PC3. Moreover, (89)Zr-2109A sensitively measured treatment-induced changes (∼66% decline) in STEAP1 expression in CWR22PC in vitro and in vivo, a model we showed to express STEAP1 in an AR-dependent manner.

CONCLUSION

These findings highlight the ability of immuno-PET with (89)Zr-2109A to detect acute changes in STEAP1 expression and argue for an expansion of ongoing efforts to image PCa patients with (89)Zr-2109A to maximize the clinical benefit associated with antibodies or antibody-drug conjugates to STEAP1.

Abstract 3396: Dual lineage inhibition of ETV1 and KIT disrupts the ETV1-KIT feed forward circuit and potentiates imatinib antitumor effect in GIST oncogenesis

Gastrointestinal stromal tumour (GIST) is one of the most common types of human sarcoma and is primarily defined by activating mutations in the KIT or PDGFRA receptor tyrosine kinases. Despite the initial clinical success of imatinib that specifically target mutant KIT and PDGFRA, imatinib resistance has become the biggest challenge in the management of advanced GIST patient. Novel therapeutics that can improve the efficacy of first line imatinib therapy and/or prevent and overcome imatinib resistance is imperative. The ETS family member, ETV1, has been previously identified as a lineage-specific survival factor that cooperates with mutant KIT by ETV1 protein stabilization by active MAP kinase signaling downstream of KIT signaling in GIST oncogenesis. Here we demonstrate that ETV1 is required for GIST initiation and maintenance in vivo using compound genetically engineered mouse models (GEMM). We further identified that ETV1 forms a feed forward circuit in GIST oncogenesis where the ETV1 protein is stabilized by active MAP kinase signaling downstream of KIT and stabilized ETV1enhances KIT expression through direct binding to the KIT enhancer regions in both GIST mouse models and human GIST cell lines. The dual lineage targeting of KIT by imatinib and ETV1 by MEK162 (an MEK inhibitor) disrupts the ETV1-KIT feed forward circuit and induces more apoptosis than single agent imatinib or MEK162 in human GIST cells. The combination therapy resulted in complete tumor regression whereas single agent imatinib or MEK162 resulted in stabilization of disease in human GIST xenograft studies. Moreover, the combination therapy also induced more tumor fibrosis than single agent imatinib or MEK162 in GIST GEMM. These observations demonstrate the in vivo role of ETV1 in GIST oncogenesis and the feasibility of targeting ETV1 protein stability by inhibiting MAP kinase signaling. They also suggest that the dual lineage targeting of ETV1 and KIT by the combination therapy may provide a more effective therapeutic strategy than imatinib alone in GIST management.

Citation Format: Leili Ran, Inna Sirota, Zhen Cao, Devan Murphy, Shipra Shukla, Ferdinando Rossi, John Wongvipat, William D. Tap, Peter Besmer, Cristina R. Antonescu, Yu Chen, Ping Chi. Dual lineage inhibition of ETV1 and KIT disrupts the ETV1-KIT feed forward circuit and potentiates imatinib antitumor effect in GIST oncogenesis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3396. doi:10.1158/1538-7445.AM2014-3396

Abstract 4774: The antitumor effects of PI3K beta inhibitors in PTEN negative prostate cancer are enhanced by inhibition of reactivated PI3K alpha signaling

The PI3K pathway is dysregulated in many cancers via selective activation of class 1 isoforms. In tumors with deficient PTEN function, signaling is driven by PI3K beta. We show here that a selective inhibitor of PI3K beta inhibits the AKT/mTOR pathway in tumors with defective PTEN function, but is ineffective in those where the pathway is driven by receptor tyrosine kinases. However, inhibition of PI3K signaling by PI3K beta inhibitors is limited by relief of AKT/mTOR dependent feedback and reactivation of IGF1R and other receptors. This results in activation of PI3K alpha and a rebound of PI3K-AKT signaling. This rebound is suppressed and tumor cell inhibition is enhanced with combined inhibition of PI3K alpha and beta. Combined administration of isoform selective PI3K inhibitors may more effectively inhibit the pathway than pan-PI3K inhibitors because of the greater selectivity and decreased off-target toxicity of the former.

In PTEN deficient models of prostate cancer, triple therapy with PI3K alpha and beta selective inhibitors combined with a potent androgen receptor inhibitor suppresses the reciprocal feedback activation of both pathways and results in marked (complete) eradication of tumors in vivo.

Citation Format: Sarit Schwartz, Brett S. Carver, John Wongvipat, Vanessa Rodrik-Outmezguine, Elisa De Stanchina, Cath Trigwell, Simon Barry, Jose Baselga, Sarat Chandarlapaty, Howard I. Scher, Charles L. Sawyers, Neal Rosen. The antitumor effects of PI3K beta inhibitors in PTEN negative prostate cancer are enhanced by inhibition of reactivated PI3K alpha signaling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4774. doi:10.1158/1538-7445.AM2014-4774

Organoid cultures derived from patients with advanced prostate cancer

The lack of in vitro prostate cancer models that recapitulate the diversity of human prostate cancer has hampered progress in understanding disease pathogenesis and therapy response. Using a 3D organoid system, we report success in long-term culture of prostate cancer from biopsy specimens and circulating tumor cells. The first seven fully characterized organoid lines recapitulate the molecular diversity of prostate cancer subtypes, including TMPRSS2-ERG fusion, SPOP mutation, SPINK1 overexpression, and CHD1 loss. Whole-exome sequencing shows a low mutational burden, consistent with genomics studies, but with mutations in FOXA1 and PIK3R1, as well as in DNA repair and chromatin modifier pathways that have been reported in advanced disease. Loss of p53 and RB tumor suppressor pathway function are the most common feature shared across the organoid lines. The methodology described here should enable the generation of a large repertoire of patient-derived prostate cancer lines amenable to genetic and pharmacologic studies.

Identification of multipotent luminal progenitor cells in human prostate organoid cultures

The prostate gland consists of basal and luminal cells arranged as pseudostratified epithelium. In tissue recombination models, only basal cells reconstitute a complete prostate gland, yet murine lineage-tracing experiments show that luminal cells generate basal cells. It has remained challenging to address the molecular details of these transitions and whether they apply to humans, due to the lack of culture conditions that recapitulate prostate gland architecture. Here, we describe a 3D culture system that supports long-term expansion of primary mouse and human prostate organoids, composed of fully differentiated CK5+ basal and CK8+ luminal cells. Organoids are genetically stable, reconstitute prostate glands in recombination assays, and can be experimentally manipulated. Single human luminal and basal cells give rise to organoids, yet luminal-cell-derived organoids more closely resemble prostate glands. These data support a luminal multilineage progenitor cell model for prostate tissue and establish a robust, scalable system for mechanistic studies.

Glucocorticoid receptor confers resistance to antiandrogens by bypassing androgen receptor blockade

The treatment of advanced prostate cancer has been transformed by novel antiandrogen therapies such as enzalutamide. Here, we identify induction of glucocorticoid receptor (GR) expression as a common feature of drug-resistant tumors in a credentialed preclinical model, a finding also confirmed in patient samples. GR substituted for the androgen receptor (AR) to activate a similar but distinguishable set of target genes and was necessary for maintenance of the resistant phenotype. The GR agonist dexamethasone was sufficient to confer enzalutamide resistance, whereas a GR antagonist restored sensitivity. Acute AR inhibition resulted in GR upregulation in a subset of prostate cancer cells due to relief of AR-mediated feedback repression of GR expression. These findings establish a mechanism of escape from AR blockade through expansion of cells primed to drive AR target genes via an alternative nuclear receptor upon drug exposure.

Development of novel metastatic prostate cancer cell lines by “organoid” in vitro culture technology

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Background: The inability to propagate patient-derived prostate cancer cells in vitro is a major impediment in the mechanistic understanding of tumorigenesis and therapeutic response. In order to generate accurate in vitro models that represent the diversity of in situ prostate cancer, we have developed a three-dimensional “organoid” system to culture metastasis samples and integrated it into our precision medicine workflow of attaining and characterizing pre-treatment biopsies. Methods: Biopsy samples of prostate cancer metastases, both soft tissue and bone, acquired at the time of therapeutic or diagnostic interventions following informed consent and institutional review board approval were obtained from two institutions. Samples were digested in Type II Collagenase (Gibco) and re-suspended in growth factor reduced Matrigel (BD), plated on plastic, and overlaid with prostate culture media (PCM). PCM consists of serum free Advanced DMEM/F12 (Gibco) with multiple growth factors optimized to propagate benign primary prostate cells. Cultures were maintained at 37°C in 5% CO2. Results: In the initial 51 samples, 15 continuous organoid cultures (29%) were established from distinct sites (9 of 32 bone, 6 of 19 soft). Tumor content of the biopsy represents a major determinant of organoid growth. Once established, organoids propagate indefinitely with different kinetics (approximately 48 hours to 1 week doubling time), and can be cryopreserved. Histological analysis shows that the organoids recapitulate the structure of the in situ cancer and genomic analysis using array CGH and whole-exome sequencing (WES) shows the presence of typical copy number alterations including TMPRSS2-ERG interstitial deletion, PTEN loss, CHD1 loss, and AR amplification. WES of two organoid/metastasis pairs shows that the growth conditions do not generate additional mutations. Conclusions: This novel tissue culture technique enables the development of new cell lines derived from metastatic deposits. This advance will facilitate research by availing new and varied cell lines, which will hopefully be more closely aligned to the spectrum of behavior of the clinical disease in comparison to the limited and problematic cell line models currently available.

Androgen receptor signaling regulates DNA repair in prostate cancers

We demonstrate that the androgen receptor (AR) regulates a transcriptional program of DNA repair genes that promotes prostate cancer radioresistance, providing a potential mechanism by which androgen deprivation therapy synergizes with ionizing radiation. Using a model of castration-resistant prostate cancer, we show that second-generation antiandrogen therapy results in downregulation of DNA repair genes. Next, we demonstrate that primary prostate cancers display a significant spectrum of AR transcriptional output, which correlates with expression of a set of DNA repair genes. Using RNA-seq and ChIP-seq, we define which of these DNA repair genes are both induced by androgen and represent direct AR targets. We establish that prostate cancer cells treated with ionizing radiation plus androgen demonstrate enhanced DNA repair and decreased DNA damage and furthermore that antiandrogen treatment causes increased DNA damage and decreased clonogenic survival. Finally, we demonstrate that antiandrogen treatment results in decreased classical nonhomologous end-joining.

SIGNIFICANCE

We demonstrate that the AR regulates a network of DNA repair genes, providing a potential mechanism by which androgen deprivation synergizes with radiotherapy for prostate cancer.

Overcoming mutation-based resistance to antiandrogens with rational drug design

The second-generation antiandrogen enzalutamide was recently approved for patients with castration-resistant prostate cancer. Despite its success, the duration of response is often limited. For previous antiandrogens, one mechanism of resistance is mutation of the androgen receptor (AR). To prospectively identify AR mutations that might confer resistance to enzalutamide, we performed a reporter-based mutagenesis screen and identified a novel mutation, F876L, which converted enzalutamide into an AR agonist. Ectopic expression of AR F876L rescued the growth inhibition of enzalutamide treatment. Molecular dynamics simulations performed on antiandrogen–AR complexes suggested a mechanism by which the F876L substitution alleviates antagonism through repositioning of the coactivator recruiting helix 12. This model then provided the rationale for a focused chemical screen which, based on existing antiandrogen scaffolds, identified three novel compounds that effectively antagonized AR F876L (and AR WT) to suppress the growth of prostate cancer cells resistant to enzalutamide.

DOI:http://dx.doi.org/10.7554/eLife.00499.001

Prostate cancer is the most commonly diagnosed cancer in men, and the second most lethal. All stages of prostate cancer depend upon male sex hormones, also known as androgens, to grow because these hormones bind and activate androgen receptors. A class of drugs termed ‘antiandrogens’ can effectively treat prostate cancer because they bind to androgen receptors without activating them, thereby preventing androgens from binding. However, the efficacy of even highly potent antiandrogen drugs, such as enzalutamide is short-lived in many patients, and understanding the biological mechanisms that cause drug resistance is one of the major objectives in translational prostate cancer research.

Resistance can arise through mutations of the androgen receptor that result in the receptor being activated, rather than inhibited, by antiandrogen drugs. However, no such mutations are known yet for enzalutamide, and researchers are keen to understand whether they exist and, if so, to generate new drugs for prostate cancer that overcome them. To identify mutations that may lead to resistance, Balbas et al. designed a new screening method in human prostate cancer cells and showed that androgen receptors with a specific mutation (called F876L) can be activated by enzalutamide. More comprehensive biological studies showed that prostate cancer cells harboring the mutation continued to grow when treated with the drug. Balbas et al. also showed that this mutation can arise spontaneously in human prostate cancer cells treated long term with enzalutamide.

Balbas et al. reasoned that the mutation likely altered the way enzalutamide binds to the androgen receptor, and used computer-guided structural modeling of the complex formed by the receptor and the drug to investigate how this might occur. These studies indicated that the region of the androgen receptor containing the F876L mutation comes into direct contact with the drug, and provided a structural explanation for the loss of inhibition. Because these studies showed how enzalutamide might bind to the androgen receptor, they also suggested ways in which enzalutamide could be chemically modified to restore its inhibitory activity against the mutant receptor.

Balbas et al. then designed and synthesized a set of novel compounds, which the modeling data suggested could act as inhibitors of the mutant receptor. Several of these compounds inhibited the activity of both mutant and wild-type forms of the androgen receptor, and suppressed the growth of both enzalutamide-resistant and nonresistant prostate cancer cells.

The work of Balbas et al. outlines a general screening strategy for the discovery of clinically relevant mutations in cancer genes, and shows how in silico technologies can accelerate drug discovery in the absence of a crystal structure of a protein–drug complex. It also emphasizes how understanding the manner in which a drug binds its target can stimulate rational design of improved drug candidates.

DOI:http://dx.doi.org/10.7554/eLife.00499.002

Annotating MYC status with 89Zr-transferrin imaging

A non-invasive technology that quantitatively measures the activity of oncogenic signaling pathways could broadly impact cancer diagnosis and treatment using targeted therapies. Here we describe the development of ⁸⁹Zr-desferrioxamine transferrin (⁸⁹Zr-Tf), a novel positron emission tomography (PET) radiotracer that binds the transferrin receptor 1 (TFRC, CD71) with high avidity. ⁸⁹Zr-Tf produces high contrast PET images that quantitatively reflect treatment-induced changes in MYC-regulated TFRC expression in a MYC oncogene-driven prostate cancer xenograft model. Moreover, ⁸⁹Zr-Tf imaging can detect the in situ development of prostate cancer in a transgenic MYC prostate cancer model, as well as prostatic intraepithelial neoplasia (PIN) prior to histological or anatomic evidence of invasive cancer. These preclinical data establish ⁸⁹Zr-Tf as a sensitive tool for non-invasive measurement of oncogene-driven TFRC expression in prostate, and potentially other cancers, with prospective near-term clinical application.

Imaging androgen receptor signaling with a radiotracer targeting free prostate-specific antigen

Despite intense efforts to develop radiotracers to detect cancers or monitor treatment response, few are widely used as a result of challenges with demonstrating clear clinical use. We reasoned that a radiotracer targeting a validated clinical biomarker could more clearly assess the advantages of imaging cancer. The virtues and shortcomings of measuring secreted prostate-specific antigen (PSA), an androgen receptor (AR) target gene, in patients with prostate cancer are well documented, making it a logical candidate for assessing whether a radiotracer can reveal new (and useful) information beyond that conferred by serum PSA. Therefore, we developed (89)Zr-labeled 5A10, a novel radiotracer that targets "free" PSA. (89)Zr-5A10 localizes in an AR-dependent manner in vivo to models of castration-resistant prostate cancer, a disease state in which serum PSA may not reflect clinical outcomes. Finally, we demonstrate that (89)Zr-5A10 can detect osseous prostate cancer lesions, a context where bone scans fail to discriminate malignant and nonmalignant signals.

SIGNIFICANCE

This report establishes that AR-dependent changes in PSA expression levels can be quantitatively measured at tumor lesions using a radiotracer that can be rapidly translated for human application and advances a new paradigm for radiotracer development that may more clearly highlight the unique virtues of an imaging biomarker.

Abstract PR3: The snoRNP assembly factor SHQ1 is a novel prostate cancer tumor suppressor gene

Approximately 50% of prostate tumors harbor the TMPRSS2-ERG translocation, but precisely how this mutation contributes to prostate cancer initiation and progression is unclear. In an effort to identify other causative mutations involved in prostate cancer, we previously performed an integrated genomic analysis of < 200 primary and advanced human prostate cancer samples and cell lines, including assessment of genomic copy-number alterations, mRNA expression, and focused exon sequencing. We identified a recurrent genomic loss, a focal region of chromosome 3p14.1-p13, which was significantly associated with TMPRSS2-ERG translocation. Comparison of copy-number and mRNA expression data implicated at least three genes in this region (FOXP1, RYBP, and SHQ1) as potential cooperative tumor suppressors, which may function in concert with TMPRSS2-ERG translocation.

In addition to the genomic loss of SHQ1, we identified point mutations in both SHQ1 and its interacting partner DKC1/dyskerin in primary prostate tumors, leading us to focus on this SHQ1-dyskerin pathway as a potential tumor-suppressive mechanism. SHQ1 is a critical assembly factor for H/ACA-class snoRNA-containing snoRNPs (small nucleolar ribonucleoproteins), of which the core component is the RNA-modifying enzyme DKC1/dyskerin. Downstream targets of dyskerin-containing snoRNPs include the ribosome, splicesome, and telomerase RNPs. DKC1/dyskerin is mutated in the human syndrome dyskeratosis congenita (DC), a disease also caused by mutations in the telomerase complex, which results in bone marrow failure and increased incidence of various neoplasias. We found that, in both human prostate cancer cell lines and mouse fibroblasts in vitro, knockdown of SHQ1 led to increased growth and partial transformation, as evidenced by loss of anchorage-dependence. Additionally, loss of either SHQ1 or dyskerin in vitro led to a global impairment of snoRNA levels, confirming that snoRNA maturation is a major downstream target of the SHQ1-dyskerin pathway in prostate cancer cells. Strikingly, in a mouse model of prostate regeneration by sub-renal capsule implantation, SHQ1-loss in conjunction with ERG expression, but not SHQ1-loss alone, led to development of prostate intraepithelial neoplasia and a low incidence of invasive cancer. Finally, in an in vitro interaction assay, prostate cancer-derived mutations in either SHQ1 or dyskerin impaired their association, to a degree similar to that seen with mutations in dyskerin found in DC. These data, along with the identification of point mutations in both SHQ1 and DKC1/dyskerin in other human cancers, strongly implicate SHQ1 as a novel prostate cancer tumor suppressor gene, potentially acting via disruption of snoRNA maturation.

This abstract is also presented as Poster C60.

Citation Format: Phillip J. Iaquinta, Chee Wai Chua, Rosario Machado-Pinilla, Haley Hieronymus, John Wongvipat, U. Thomas Meier, Michael Shen, Charles L. Sawyers. The snoRNP assembly factor SHQ1 is a novel prostate cancer tumor suppressor gene [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr PR3.

ARN-509: a novel antiandrogen for prostate cancer treatment

Continued reliance on the androgen receptor (AR) is now understood as a core mechanism in castration-resistant prostate cancer (CRPC), the most advanced form of this disease. While established and novel AR pathway-targeting agents display clinical efficacy in metastatic CRPC, dose-limiting side effects remain problematic for all current agents. In this study, we report the discovery and development of ARN-509, a competitive AR inhibitor that is fully antagonistic to AR overexpression, a common and important feature of CRPC. ARN-509 was optimized for inhibition of AR transcriptional activity and prostate cancer cell proliferation, pharmacokinetics, and in vivo efficacy. In contrast to bicalutamide, ARN-509 lacked significant agonist activity in preclinical models of CRPC. Moreover, ARN-509 lacked inducing activity for AR nuclear localization or DNA binding. In a clinically valid murine xenograft model of human CRPC, ARN-509 showed greater efficacy than MDV3100. Maximal therapeutic response in this model was achieved at 30 mg/kg/d of ARN-509, whereas the same response required 100 mg/kg/d of MDV3100 and higher steady-state plasma concentrations. Thus, ARN-509 exhibits characteristics predicting a higher therapeutic index with a greater potential to reach maximally efficacious doses in man than current AR antagonists. Our findings offer preclinical proof of principle for ARN-509 as a promising therapeutic in both castration-sensitive and castration-resistant forms of prostate cancer.

Reciprocal feedback regulation of PI3K and androgen receptor signaling in PTEN-deficient prostate cancer

Prostate cancer is characterized by its dependence on androgen receptor (AR) and frequent activation of PI3K signaling. We find that AR transcriptional output is decreased in human and murine tumors with PTEN deletion and that PI3K pathway inhibition activates AR signaling by relieving feedback inhibition of HER kinases. Similarly, AR inhibition activates AKT signaling by reducing levels of the AKT phosphatase PHLPP. Thus, these two oncogenic pathways cross-regulate each other by reciprocal feedback. Inhibition of one activates the other, thereby maintaining tumor cell survival. However, combined pharmacologic inhibition of PI3K and AR signaling caused near-complete prostate cancer regressions in a Pten-deficient murine prostate cancer model and in human prostate cancer xenografts, indicating that both pathways coordinately support survival.

MYC cooperates with AKT in prostate tumorigenesis and alters sensitivity to mTOR inhibitors

MYC and phosphoinositide 3-kinase (PI3K)-pathway deregulation are common in human prostate cancer. Through examination of 194 human prostate tumors, we observed statistically significant co-occurrence of MYC amplification and PI3K-pathway alteration, raising the possibility that these two lesions cooperate in prostate cancer progression. To investigate this, we generated bigenic mice in which both activated human AKT1 and human MYC are expressed in the prostate (MPAKT/Hi-MYC model). In contrast to mice expressing AKT1 alone (MPAKT model) or MYC alone (Hi-MYC model), the bigenic phenotype demonstrates accelerated progression of mouse prostate intraepithelial neoplasia (mPIN) to microinvasive disease with disruption of basement membrane, significant stromal remodeling and infiltration of macrophages, B- and T-lymphocytes, similar to inflammation observed in human prostate tumors. In contrast to the reversibility of mPIN lesions in young MPAKT mice after treatment with mTOR inhibitors, Hi-MYC and bigenic MPAKT/Hi-MYC mice were resistant. Additionally, older MPAKT mice showed reduced sensitivity to mTOR inhibition, suggesting that additional genetic events may dampen mTOR dependence. Since increased MYC expression is an early feature of many human prostate cancers, these data have implications for treatment of human prostate cancers with PI3K-pathway alterations using mTOR inhibitors.

ETV1 is a lineage survival factor that cooperates with KIT in gastrointestinal stromal tumours

Gastrointestinal stromal tumour (GIST) is the most common human sarcoma and is primarily defined by activating mutations in the KIT or PDGFRA receptor tyrosine kinases1,2. KIT is highly expressed in interstitial cells of Cajal (ICCs)—the presumed cell of origin for GIST—as well as in hematopoietic stem cells, melanocytes, mast cells and germ cells2,3. Yet, families harbouring germline activating KIT mutations and mice with knock-in Kit mutations almost exclusively develop ICC hyperplasia and GIST4–7, suggesting that the cellular context is important for KIT to mediated oncogenesis. Here we show that the ETS family member ETV1 is highly expressed in the subtypes of ICCs sensitive to oncogenic KIT mediated transformation8, and is required for their development. In addition, ETV1 is universally highly expressed in GISTs and is required for growth of imatinib-sensitive and resistant GIST cell lines. Transcriptome profiling and global analyses of ETV1-binding sites suggest that ETV1 is a master regulator of an ICC-GIST-specific transcription network mainly through enhancer binding. The ETV1 transcriptional program is further regulated by activated KIT, which prolongs ETV1 protein stability and cooperates with ETV1 to promote tumourigenesis. We propose that GIST arises from ICCs with high levels of endogenous ETV1 expression that, when coupled with an activating KIT mutation, drives an oncogenic ETS transcription program. This differs from other ETS-dependent tumours such as prostate cancer, melanoma, and Ewing sarcoma where genomic translocation or amplification drives aberrant ETS expression9–11 and represents a novel mechanism of oncogenic transcription factor activation.

Structure-activity relationship for thiohydantoin androgen receptor antagonists for castration-resistant prostate cancer (CRPC)

A structure-activity relationship study was carried out on a series of thiohydantoins and their analogues 14 which led to the discovery of 92 (MDV3100) as the clinical candidate for the treatment of hormone refractory prostate cancer.

Abstract B207: Prostate specific membrane antigen is a diagnostic marker of response to allosteric androgen receptor inhibition

Molecular imaging in oncology aims to complement the growing repertoire of chemotherapies with noninvasive diagnostic tools to measure pharmacodynamics and improve predictions of disease response. In prostate cancer, several chemotherapeutic strategies have been developed to inhibit androgen receptor (AR) function, and currently show promise in treating advanced, castration resistant disease. Among these, competitive inhibitors of steroid agonist/receptor interactions have been effectively staged with fluorine-18-fluoro-5α-dihydrotestosterone, an AR agonist compatible with quantitative PET imaging. However, some allosteric inhibitors of AR do not impact agonist/receptor interaction (or AR stability) and currently cannot be evaluated with a quantitative imaging technology. We address this problem here by presenting evidence that prostate specific membrane antigen is a diagnostic marker of allosteric AR inhibition. In culture or in xenograft models of LNCaP, 22Rv1, LAPC4, and VCaP, PSMA is repressed by androgen treatment. Ablation of AR with siRNA inhibits the hormone-mediated suppression of PSMA, directly implicating AR in the mechanism. We further show that PSMA suppression is antagonized by several competitive and allosteric AR inhibitors. Finally, we conducted a pilot study with 22Rv1 xenografts to demonstrate that 64Cu-DOTA-J591, a monoclonal antibody designed for clinical PET imaging of PSMA, quantitatively detects hormone-mediated PSMA suppression. Collectively, these results promote the use of PSMA imaging reagents in the clinic to stage allosteric AR inhibitors.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B207.

Development of a second-generation antiandrogen for treatment of advanced prostate cancer

Metastatic prostate cancer is treated with drugs that antagonize androgen action, but most patients progress to a more aggressive form of the disease called castration-resistant prostate cancer, driven by elevated expression of the androgen receptor. Here we characterize the diarylthiohydantoins RD162 and MDV3100, two compounds optimized from a screen for nonsteroidal antiandrogens that retain activity in the setting of increased androgen receptor expression. Both compounds bind to the androgen receptor with greater relative affinity than the clinically used antiandrogen bicalutamide, reduce the efficiency of its nuclear translocation, and impair both DNA binding to androgen response elements and recruitment of coactivators. RD162 and MDV3100 are orally available and induce tumor regression in mouse models of castration-resistant human prostate cancer. Of the first 30 patients treated with MDV3100 in a Phase I/II clinical trial, 13 of 30 (43%) showed sustained declines (by >50%) in serum concentrations of prostate-specific antigen, a biomarker of prostate cancer. These compounds thus appear to be promising candidates for treatment of advanced prostate cancer.

Transgenic mouse model for rapid pharmacodynamic evaluation of antiandrogens

Persistent androgen receptor signaling has been implicated as a critical factor in prostate cancer progression even at the hormone-refractory stage and provides strong rationale for developing novel androgen receptor antagonists. Traditional models for in vivo evaluation of antiandrogens are cumbersome because they rely on physiologic end points, such as the size of androgen-dependent tissues. Here, we describe a transgenic mouse (ARR2 Pb-Lux) that expresses luciferase specifically in the prostate in an androgen-dependent fashion. This signal is reduced by castration or by treatment with bicalutamide and can be quantified through noninvasive bioluminescent imaging. ARR2 Pb-Lux mice provide a novel method for rapid pharmacodynamic evaluation of novel pharmacologic compounds designed to inhibit androgen receptor signaling.

Context-dependent hormone-refractory progression revealed through characterization of a novel murine prostate cancer cell line

Insights into the molecular basis of hormone-refractory prostate cancer have principally relied on human prostate cancer cell lines, all of which were derived from patients who had already failed hormonal therapy. Recent progress in developing genetically engineered mouse prostate cancer models provides an opportunity to isolate novel cell lines from animals never exposed to hormone ablation, avoiding any potential bias conferred by the selective pressure of the castrate environment. Here we report the isolation of such a cell line (Myc-CaP) from a c-myc transgenic mouse with prostate cancer. Myc-CaP cells have an amplified androgen receptor gene despite no prior exposure to androgen withdrawal and they retain androgen-dependent transgene expression as well as androgen-dependent growth in soft agar and in mice. Reexpression of c-Myc from a hormone-independent promoter rescues growth in androgen-depleted agar but not in castrated mice, showing a clear distinction between the molecular requirements for hormone-refractory growth in vitro versus in vivo. Myc-CaP cells represent a unique reagent for dissecting discreet steps in hormone-refractory prostate cancer progression and show the general utility of using genetically engineered mouse models for establishing new prostate cancer cell lines.

HER2/neu kinase-dependent modulation of androgen receptor function through effects on DNA binding and stability

Given the role of the EGFR/HER2 family of tyrosine kinases in breast cancer, we dissected the molecular basis of EGFR/HER2 kinase signaling in prostate cancer. Using the small molecule dual EGFR/HER2 inhibitor PKI-166, we show that the biologic effects of EGFR/HER-2 pathway inhibition are caused by reduced AR transcriptional activity. Additional genetic and pharmacologic experiments show that this modulation of AR function is mediated by the HER2/ERBB3 pathway, not by EGFR. This HER2/ERBB3 signal stabilizes AR protein levels and optimizes binding of AR to promoter/enhancer regions of androgen-regulated genes. Surprisingly, the downstream signaling pathway responsible for these effects appears to involve kinases other than Akt. These data suggest that the HER2/ERBB3 pathway is a critical target in hormone-refractory prostate cancer.