Timing of treatment shapes the path to androgen receptor signaling inhibitor resistance in prostate cancer

There is optimism that cancer drug resistance can be addressed through appropriate combination therapy, but success requires understanding the growing complexity of resistance mechanisms, including the evolution and population dynamics of drug-sensitive and drug-resistant clones over time. Using DNA barcoding to trace individual prostate tumor cells in vivo , we find that the evolutionary path to acquired resistance to androgen receptor signaling inhibition (ARSI) is dependent on the timing of treatment. In established tumors, resistance occurs through polyclonal adaptation of drug-sensitive clones, despite the presence of rare subclones with known, pre-existing ARSI resistance. Conversely, in an experimental setting designed to mimic minimal residual disease, resistance occurs through outgrowth of pre-existing resistant clones and not by adaptation. Despite these different evolutionary paths, the underlying mechanisms responsible for resistance are shared across the two evolutionary paths. Furthermore, mixing experiments reveal that the evolutionary path to adaptive resistance requires cooperativity between subclones. Thus, despite the presence of pre-existing ARSI-resistant subclones, acquired resistance in established tumors occurs primarily through cooperative, polyclonal adaptation of drug-sensitive cells. This tumor ecosystem model of resistance has new implications for developing effective combination therapy.

Abstract 3430: The protein phosphatase 4 complex is a tumor suppressor in prostate cancer

Regions of recurring chromosomal deletions have been annotated in clinical prostate cancer (PCa), but for many of them there is an incomplete understanding of their contributions to disease progression. From an in vivo shRNA enrichment screen of the PCa deletome, we identified the serine threonine protein phosphatase 4 (PP4) regulatory subunit 2 (PPP4R2) as a candidate tumor suppressor, which undergoes copy number loss in 16% of primary PCa. Knockdown or CRISPR-Cas9 mediated knockout of PPP4R2 increased the growth rate of PCa xenograft tumors. Along with the catalytic subunit PP4C and an additional regulatory subunit (either PP4R3A or PP4R3B), PP4R2 forms two distinct heterotrimeric phosphatase holoenzymes. PP4R2 was critical for holoenzyme stability, as its deletion resulted in rapid, moderate downregulation of the other PP4 subunit proteins. The increased tumor growth rate and downregulation of PP4C resulting from loss of PP4R2 could be replicated by combined deletion of PPP4R3A and PPP4R3B, or by directly targeting PPP4C itself. Collectively, these findings implicated PP4C as the likely direct mediator of tumor suppression through the PP4 complex. This was confirmed in both PPP4R2 and PPP4R3A/PPP4R3B knockout cells in vivo by restoring PP4C to its normal physiological level through a dox-inducible cDNA, which reversed the accelerated tumor growth rate conferred by the deletion of the regulatory subunits. Phosphatase activity of PP4C was critical, as add-back of a catalytically dead PP4C mutant (D82A) was unable to reverse the growth rate of PPP4C knockout tumors. Interestingly, we found that the androgen receptor (AR), which is the principal therapeutic target for PCa, was co-immunoprecipitated by PP4C and its regulatory subunits. This suggests that AR could be a substrate for PP4C and is potentially present in a hyperphosphorylated and hyperactivated state in PCa that have decreased PP4 phosphatase activity. Our findings highlight a previously unappreciated tumor suppressor role for the PP4 complex in PCa, with the regulatory subunit PP4R2 providing a key function in stabilizing PP4C and thereby preserving its phosphatase activity.

Citation Format: Philip A. Watson, Kayla E. Lawrence, Tejasveeta V. Nadkarni, Taslima F. Ishmael, Eugine Lee, Aura C. Agudelo Rivera, Jillian R. Love, John Wongvipat, Ling Cai, Abbas Nazir, Phillip J. Iaquinta, Kenneth Chang, Yupu Liang, Greg J. Hannon, Charles L. Sawyers. The protein phosphatase 4 complex is a tumor suppressor in prostate cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3430.

Abstract NG06: CHD1-loss confers AR targeted therapy resistance via promoting cancer heterogeneity and lineage plasticity

BACKGROUND: Pharmacological targeting of driver alterations in cancer has resulted in many clinical successes but is limited by concurrent or novel genomic alterations. One potential explanation for this heterogeneity is the presence of additional genomic alterations which modify the degree of dependence on the targeted driver mutation. Metastatic prostate cancer (mPCa) serves as a relevant example, where the molecular target is the androgen receptor (AR) which functions as a lineage survival factor of luminal prostate epithelial cells. Next generation AR targeted therapies such as abiraterone, enzalutamide and apalutamide have significantly improved the survival of men with mPCa and achieved exciting clinical success. However, resistance to these therapies with disease progression is unfortunately inevitable, with intrinsic resistance noted in around 30% patients and acquired resistance in most patients. Therefore, there is an unmet need to understand the mechanism of therapy resistance to AR targeted therapies and identify novel therapeutic approach to prevent or reverse resistance. Previously, we have revealed that the deactivation of two genes, TP53 and RB1, confers AR targeted therapy resistance through a novel mechanism by which tumor cells acquire lineage plasticity and transit to a multi-lineage, progenitor-like state no longer dependent on AR. This lineage plasticity and resistance is enabled by the activation of SOX2 and is completely reversible by knocking down SOX2. This observation not only adds clarity to the mechanism of resistance, but also suggests that appropriate clinical interventions of lineage plasticity may be a potential avenue to overcome resistance. However, there is only 10% mPCa patients carrying homozygous deletions in both TP53 and RB1 loci, thus additional genomic alterations may be responsible for the resistance in other patients.

METHODS: To gain functional insight into the genes impacted by the copy number alterations in mPCa, we screened 4234 short hairpin RNAs (shRNAs) targeting 730 genes often deleted in human prostate cancer (annotated from a survey of six prostate cancer genomic datasets) for hairpins that confer in vivo resistance to the antiandrogen enzalutamide, in a well credentialed enzalutamide-sensitive xenograft model LNCaP/AR. More than 350 resistant tumors emerged by 16 weeks of xenografting and the genomic DNA of these tumors were extracted and sequenced to determine the enrichment of specific shRNAs compared to the starting material. A classic probabilistic model RIGER-E was used to determine the significance of enrichment of each hairpins/genes.

RESULTS: The chromodomain helicase DNA-binding protein 1 (CHD1) emerged as a top candidate, a finding supported by patient data showing that CHD1 expression is inversely correlated with clinical benefit from AR targeted therapy enzalutamide. CRISPR based depletion of CHD1 confers significant resistance to enzalutamide both in vitro and in vivo, supported by similar results from multiple human prostate cancer cell line models. To our surprise, we observed sustained inhibition of the canonical AR target genes, indicating that CHD1 loss might activate transcriptional programs that relieve prostate tumor cells from their dependence on AR by reprogramming away from their luminal lineage, as we have observed in the setting of combined loss of RB1 and TP53. Indeed, CHD1 loss led to global changes in open and closed chromatin, indicative of an altered chromatin state, with associated changes in gene expression. Integrative analysis of ATAC-seq and RNA-seq changes identified 22 transcription factors as candidate drivers of enzalutamide resistance. CRISPR deletion of four of these (NR3C1, BRN2, NR2F1, TBX2) restored in vitro enzalutamide sensitivity in CHD1 deleted cells. Independently derived, enzalutamide-resistant, CHD1-deleted subclones expressed elevated levels of 1 or more of these 4 transcription factors. This pattern suggests a state of chromatin plasticity and enhanced heterogeneity, initiated by CHD1 loss, which enables upregulation of distinct sets of genes in response to selective pressure. This concept is further supported by RNA-seq data from a mCRPC patients cohort, in which we examined the co-association of CHD1 levels with each of these four TFs across 212 tumors. Unsupervised clustering analysis of just these five genes identified five distinct clusters, four of which display relatively higher expression of either CHD1 or one or two of these four resistance TFs. Interestingly, we observed altered expression of many canonical lineage specific genes in the same panel of CHD1-deleted, enzalutamide resistant xenografts that displayed heterogenous upregulation of the four TFs, including consistent downregulation of luminal genes and upregulation of genes specify epithelial to mesenchymal transition (EMT). Furthermore, these upregulation of 4 resistance TFs, as well as the observed lineage switchs, are both rapid and reversible, suggesting a status of plasticity. Collectively, these results indicate that CHD1 loss establishes an altered chromatin landscape which, in the face of stresses such as antiandrogen therapy, enables resistant subclones to emerge through activation of alternative, non-luminal lineage programs that reduce dependence on AR.

CONCLUSIONS: We demonstrated that loss of the chromodomain gene CHD1, a commonly deleted prostate cancer gene (in 15~20% patients), through global effects on chromatin, establishes a state of plasticity that accelerates the development of AR targeted therapy resistance through heterogeneous activation of downstream effectors, which mediated the transition away from luminal lineage identity and AR dependency. This model provides the first demonstration that early genomic lesions of critical epigenetic modulator promotes prostate cancer heterogeneity and lineage plasticity, consequently leading to the resistance to AR targeted therapy. Therefore, appropriate clinical intervention of these heterogenous resistance driver TFs, as well as the chromatin dysregulation, may be potential therapeutic avenues to prevent or reverse AR targeted resistance.

Citation Format: Zeda Zhang, Chuanli Zhou, Xiaoling Li, Spencer Barnes, Su Deng, Elizabeth Hoover, Chi-Chao Chen, Young Sun Lee, Choushi Wang, Carla Tirado, Lauren Metang, Nickolas Johnson, John Wongvipat, Kristina Navrazhina, Zhen Cao, Wassim Abida, Amaia Lujambio, Sheng Li, Vankat Malladi, Charles Sawyers, Ping Mu. CHD1-loss confers AR targeted therapy resistance via promoting cancer heterogeneity and lineage plasticity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr NG06.

Tumor Microenvironment-Derived NRG1 Promotes Antiandrogen Resistance in Prostate Cancer

Despite the development of second-generation antiandrogens, acquired resistance to hormone therapy remains a major challenge in treating advanced prostate cancer. We find that cancer-associated fibroblasts (CAFs) can promote antiandrogen resistance in mouse models and in prostate organoid cultures. We identify neuregulin 1 (NRG1) in CAF supernatant, which promotes resistance in tumor cells through activation of HER3. Pharmacological blockade of the NRG1/HER3 axis using clinical-grade blocking antibodies re-sensitizes tumors to hormone deprivation in vitro and in vivo. Furthermore, patients with castration-resistant prostate cancer with increased tumor NRG1 activity have an inferior response to second-generation antiandrogen therapy. This work reveals a paracrine mechanism of antiandrogen resistance in prostate cancer amenable to clinical testing using available targeted therapies.

Loss of CHD1 Promotes Heterogeneous Mechanisms of Resistance to AR-Targeted Therapy via Chromatin Dysregulation

Metastatic prostate cancer is characterized by recurrent genomic copy number alterations that are presumed to contribute to resistance to hormone therapy. We identified CHD1 loss as a cause of antiandrogen resistance in an in vivo small hairpin RNA (shRNA) screen of 730 genes deleted in prostate cancer. ATAC-seq and RNA-seq analyses showed that CHD1 loss resulted in global changes in open and closed chromatin with associated transcriptomic changes. Integrative analysis of this data, together with CRISPR-based functional screening, identified four transcription factors (NR3C1, POU3F2, NR2F1, and TBX2) that contribute to antiandrogen resistance, with associated activation of non-luminal lineage programs. Thus, CHD1 loss results in chromatin dysregulation, thereby establishing a state of transcriptional plasticity that enables the emergence of antiandrogen resistance through heterogeneous mechanisms.

GREB1 amplifies androgen receptor output in human prostate cancer and contributes to antiandrogen resistance

Genomic amplification of the androgen receptor (AR) is an established mechanism of antiandrogen resistance in prostate cancer. Here, we show that the magnitude of AR signaling output, independent of AR genomic alteration or expression level, also contributes to antiandrogen resistance, through upregulation of the coactivator GREB1. We demonstrate 100-fold heterogeneity in AR output within human prostate cancer cell lines and show that cells with high AR output have reduced sensitivity to enzalutamide. Through transcriptomic and shRNA knockdown studies, together with analysis of clinical datasets, we identify GREB1 as a gene responsible for high AR output. We show that GREB1 is an AR target gene that amplifies AR output by enhancing AR DNA binding and promoting EP300 recruitment. GREB1 knockdown in high AR output cells restores enzalutamide sensitivity in vivo. Thus, GREB1 is a candidate driver of enzalutamide resistance through a novel feed forward mechanism.

Abstract A074: Aberrant activation of a gastrointestinal transcriptional circuit in prostate cancer mediates castration resistance

Prostate cancer exhibits a remarkable lineage-specific dependence on androgen signaling. Lineage-directed therapy using androgen deprivation has been the mainstay of prostate cancer treatment for 70 years. Castration resistance involves reactivation of androgen signaling or activation of alternative lineage programs to bypass androgen requirement. Our studies found that an aberrant gastrointestinal lineage transcriptome is expressed in ~5% of primary prostate cancer that is characterized by abbreviated response to androgen deprivation therapy and in ~30% of castration-resistant prostate cancer. This program is governed by a transcriptional circuit consisting of HNF4G and HNF1A. Cistrome and chromatin analyses revealed that HNF4G is a pioneer factor that generates and maintains enhancer landscape at gastrointestinal lineage genes, independent of AR signaling. In HNF4G/1A-negative prostate cancer, exogenous expression of HNF4G at physiologic levels recapitulates the GI transcriptome, chromatin landscape and leads to relative castration resistance.

Citation Format: Shipra Shukla, Joanna Cyrta, Devan Murphy, Edward Walczak, Leili Ran, Praveen Agrawal, Yuanyuan Xie, Yuedan Chen, Shangqian Wang, Yu Zhan, Wai Pung E. Wong, Andrea Sboner, Himisha Beltran, Juan-Miguel Mosquera, Jessica Sher, Zhen Cao, John Wongvipat, Richard P. Koche, Anuradha Gopalan, Deyou Zheng, Mark Rubin, Howard I. Scher, Ping Chi, Yu Chen. Aberrant activation of a gastrointestinal transcriptional circuit in prostate cancer mediates castration resistance [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A074.

COP1/DET1/ETS axis regulates ERK transcriptome and sensitivity to MAPK inhibitors

Aberrant activation of MAPK signaling leads to the activation of oncogenic transcriptomes. How MAPK signaling is coupled with the transcriptional response in cancer is not fully understood. In 2 MAPK-activated tumor types, gastrointestinal stromal tumor and melanoma, we found that ETV1 and other Pea3-ETS transcription factors are critical nuclear effectors of MAPK signaling that are regulated through protein stability. Expression of stabilized Pea3-ETS factors can partially rescue the MAPK transcriptome and cell viability after MAPK inhibition. To identify the players involved in this process, we performed a pooled genome-wide RNAi screen using a fluorescence-based ETV1 protein stability sensor and identified COP1, DET1, DDB1, UBE3C, PSMD4, and COP9 signalosome members. COP1 or DET1 loss led to decoupling between MAPK signaling and the downstream transcriptional response, where MAPK inhibition failed to destabilize Pea3 factors and fully inhibit the MAPK transcriptome, thus resulting in decreased sensitivity to MAPK pathway inhibitors. We identified multiple COP1 and DET1 mutations in human tumors that were defective in the degradation of Pea3-ETS factors. Two melanoma patients had de novo DET1 mutations arising after vemurafenib treatment. These observations indicate that MAPK signaling-dependent regulation of Pea3-ETS protein stability is a key signaling node in oncogenesis and therapeutic resistance to MAPK pathway inhibition.

Abstract B13: COP1-ETS axis regulates ERK transcriptional output and modulates sensitivity to MAPK inhibitors

Aberrant activation of the mitogen activate kinase (MAPK) pathway is highly prevalent in cancer and therapies targeting the pathway are approved or under active investigation in multiple malignancies. MAPK signaling leads to activation of a transcriptional program that includes general growth promoting genes, negative feedback regulators of the MAPK pathway, and lineage-specific genes. While the mechanisms of upstream signal transduction that leads to MAPK activation have been studied in detail, how MAPK activation is dynamically coupled with downstream nuclear transcriptional response is not fully understood. In gastrointestinal stomal tumor (GIST) and melanoma, two malignancies with aberrant MAPK activation, we find that Pea3 family ETS transcription factors ETV1, ETV4, and ETV5 are critical nuclear effectors of MAPK signaling. We find that the primary mechanism linking MAPK and Pea3 activity is through protein stability via the COP1 E3 ligase. The loss of COP1 leads to decoupling between upstream MAPK signaling and downstream transcription with constitutively stabilized Pea3 protein levels, constitutively high MAPK transcriptome, yet decreased upstream signaling due to Pea3-mediated transcription of negative feedback regulators. This leads to decreased therapeutic sensitivity to MAPK pathway inhibition in vitro and in vivo. These observations indicate that MAPK signaling-dependent regulation of Pea3 ETS protein stability is a crucial pathway that couples downstream transcriptional response to MAPK signaling and can shape the therapeutic sensitivity to MAPK pathway inhibition in cancer.

Citation Format: Yuanyuan Xie, Zhen Cao, Wai Pung Wong, Youxin Guan, Jenny Zhang, Edward Walczak, Devan Murphy, Leili Ran, Inna Sirota, Shangqian Wang, Shipra Shukla, Dong Gao, John Wongvipat, Simon Knott, Kenneth Chang, Cristina Antonescu, Gregory Hannon, Ping Chi, Yu Chen. COP1-ETS axis regulates ERK transcriptional output and modulates sensitivity to MAPK inhibitors [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B13.

Aberrant Activation of a Gastrointestinal Transcriptional Circuit in Prostate Cancer Mediates Castration Resistance

Prostate cancer exhibits a lineage-specific dependence on androgen signaling. Castration resistance involves reactivation of androgen signaling or activation of alternative lineage programs to bypass androgen requirement. We describe an aberrant gastrointestinal-lineage transcriptome expressed in ∼5% of primary prostate cancer that is characterized by abbreviated response to androgen-deprivation therapy and in ∼30% of castration-resistant prostate cancer. This program is governed by a transcriptional circuit consisting of HNF4G and HNF1A. Cistrome and chromatin analyses revealed that HNF4G is a pioneer factor that generates and maintains enhancer landscape at gastrointestinal-lineage genes, independent of androgen-receptor signaling. In HNF4G/HNF1A-double-negative prostate cancer, exogenous expression of HNF4G at physiologic levels recapitulates the gastrointestinal transcriptome, chromatin landscape, and leads to relative castration resistance.

Deletion of 3p13-14 locus spanning FOXP1 to SHQ1 cooperates with PTEN loss in prostate oncogenesis

A multigenic locus at 3p13-14, spanning FOXP1 to SHQ1, is commonly deleted in prostate cancer and lost broadly in a range of cancers but has unknown significance to oncogenesis or prognosis. Here, we report that FOXP1-SHQ1 deletion cooperates with PTEN loss to accelerate prostate oncogenesis and that loss of component genes correlates with prostate, breast, and head and neck cancer recurrence. We demonstrate that Foxp1-Shq1 deletion accelerates prostate tumorigenesis in mice in combination with Pten loss, consistent with the association of FOXP1-SHQ1 and PTEN loss observed in human cancers. Tumors with combined Foxp1-Shq1 and Pten deletion show increased proliferation and anaplastic dedifferentiation, as well as mTORC1 hyperactivation with reduced Akt phosphorylation. Foxp1-Shq1 deletion restores expression of AR target genes repressed in tumors with Pten loss, circumventing PI3K-mediated repression of the androgen axis. Moreover, FOXP1-SHQ1 deletion has prognostic relevance, with cancer recurrence associated with combined loss of PTEN and FOXP1-SHQ1 genes.

Regulation of the glucocorticoid receptor via a BET-dependent enhancer drives antiandrogen resistance in prostate cancer

In prostate cancer, resistance to the antiandrogen enzalutamide (Enz) can occur through bypass of androgen receptor (AR) blockade by the glucocorticoid receptor (GR). In contrast to fixed genomic alterations, here we show that GR-mediated antiandrogen resistance is adaptive and reversible due to regulation of GR expression by a tissue-specific enhancer. GR expression is silenced in prostate cancer by a combination of AR binding and EZH2-mediated repression at the GR locus, but is restored in advanced prostate cancers upon reversion of both repressive signals. Remarkably, BET bromodomain inhibition resensitizes drug-resistant tumors to Enz by selectively impairing the GR signaling axis via this enhancer. In addition to revealing an underlying molecular mechanism of GR-driven drug resistance, these data suggest that inhibitors of broadly active chromatin-readers could have utility in nuanced clinical contexts of acquired drug resistance with a more favorable therapeutic index.

SOX2 promotes lineage plasticity and antiandrogen resistance in TP53- and RB1-deficient prostate cancer

Some cancers evade targeted therapies through a mechanism known as lineage plasticity, whereby tumor cells acquire phenotypic characteristics of a cell lineage whose survival no longer depends on the drug target. We use in vitro and in vivo human prostate cancer models to show that these tumors can develop resistance to the antiandrogen drug enzalutamide by a phenotypic shift from androgen receptor (AR)-dependent luminal epithelial cells to AR-independent basal-like cells. This lineage plasticity is enabled by the loss of TP53 and RB1 function, is mediated by increased expression of the reprogramming transcription factor SOX2, and can be reversed by restoring TP53 and RB1 function or by inhibiting SOX2 expression. Thus, mutations in tumor suppressor genes can create a state of increased cellular plasticity that, when challenged with antiandrogen therapy, promotes resistance through lineage switching.

Abstract 1537: 3p13-14 FOXP1-SHQ1 deletion spanning multiple potential tumor suppressor genes cooperates with PTEN loss in cancer

The recurrent 3p13-14 deletion spanning from FOXP1 to SHQ1 occurs frequently in prostate cancer and is broadly lost in a range of cancer types, but this deletion has unknown tumor suppressive potential. FOXP1-SHQ1 deletion significantly co-occurs with PTEN loss in prostate cancer and other cancers. We find that FOXP1-SHQ1 deletion cooperates with PTEN loss to accelerate prostate cancer development in mice, resulting in tumors with increased proliferation and highly anaplastic dedifferentiation. FOXP1-SHQ1 deletion in these PTEN null tumors results in selective mTORC1 pathway hyperactivation beyond that mediated by PTEN loss alone. FOXP1-SHQ1 deletion also partially rescues AR target gene inhibition caused by PTEN loss, circumventing the repression of the androgen axis seen upon PI3K pathway activation. Clinically, combined FOXP1 and PTEN loss is associated with increased prostate cancer recurrence, and this finding extends to other cancer types, most notably breast cancer.

Citation Format: Haley Hieronymus, Philip J. Iaquinta, John Wongvipat, Anuradha Gopalan, Rajmohan Murali, Ninghui Mao, Brett S. Carver, Charles L. Sawyers. 3p13-14 FOXP1-SHQ1 deletion spanning multiple potential tumor suppressor genes cooperates with PTEN loss in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1537. doi:10.1158/1538-7445.AM2017-1537

Abstract 4165: SOX2 promotes lineage plasticity and antiandrogen resistance in TP53 and RB1 deficient prostate cancer

Some cancers evade targeted therapies through a mechanism known as lineage plasticity, whereby tumor cells acquire phenotypic characteristics of a cell lineage whose survival no longer depends on the drug target. Here we show, using in vitro and in vivo prostate cancer models, that these tumors can develop resistance to the antiandrogen drug enzalutamide by a phenotypic shift from androgen receptor (AR) dependent luminal epithelial cells to AR independent basal-like cells. This lineage plasticity is enabled by loss of TP53 and RB1 function, is mediated by increased expression of the reprogramming transcription factor SOX2 and can be reversed by restoring TP53 and RB1 function or by inhibiting SOX2 expression. Thus, mutations in tumor suppressor genes can create a state of increased cellular plasticity that, when challenged with antiandrogen therapy, promotes resistance through lineage switching.

Citation Format: Ping Mu, Zeda Zhang, Matteo Benelli, Wouter Karthaus, Elizebeth Hoover, Chi-Chao Chen, John Wongvipat, Sheng-Yu Ku, Dong Gao, Zhen Cao, Neel Shah, Elizabeth Adams, Wassim Abida, Philip Watson, Davide Prandi, Chun-Hao Huang, Elisa de Stanchina, Scott Lowe, Leigh Ellis, Himisha Beltran, Mark Rubin, David Goodrich, Francesca Demichelis, Charles L. Sawyers. SOX2 promotes lineage plasticity and antiandrogen resistance in TP53 and RB1 deficient prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4165. doi:10.1158/1538-7445.AM2017-4165

Abstract 3098: Epigenetic therapy to target neuroendocrine prostate cancer using precision medicine models

Background

Neuroendocrine prostate cancer (NEPC) is a highly aggressive subtype of prostate cancer that may either arise de novo or much more commonly after hormonal therapy for prostate adenocarcinoma. Patients diagnosed with NEPC are often treated with platinum chemotherapy able to produce only short duration responses underling the urgent need of identifying novel potential therapeutic targets for this lethal disease.

In the context of our Englander Institute for Precision Medicine we developed patient derived 3D NEPC tumor organoids and patient derived PDXs to test specific inhibitors on molecular targets identified by genomic analysis of native tumors. Emerging data from an integrative molecular analysis of metastatic tumors from a large cohort of castration resistant prostate cancer (CRPC) patients, including NEPC, points to a key role of the Polycomb gene EZH2 and the epigenome in the pathogenesis of NEPC.

Methods

Tumor organoids were developed according to protocols developed by our Englander Institute for Precision Medicine and other Institutes. Briefly the tissue biopsies (liver and bone biopsy) were washed, enzymatically digested and then seeded in a Matrigel (BD) droplet. Organoids were then characterized at both genomic (WES) and protein level (IHC) to confirm the expression of specific markers. Organoids were also subcutaneously injected in NSG mice to generate PDX for drug treatment in vivo.

Results

Based on the significant EZH2 overexpression in NEPC tumors by RNA-Seq and tissue microarray, we checked the expression of EZH2 and H3K273M, the readout of its activity, in NEPC organoids and we found out that both EZH2 and H3K273M were high expressed in NEPC organoids. Therefore we evaluated the effects of the EZH2 inhibitor, GSK343, in NEPC versus CRPC organoids and in the castration resistant line DU145 versus the NEPC cell line NCI-H660. We found out that GSK343 effectively inhibited H3K27me3 and resulted in a significant reduction of NEPC organoids and H660 viability while DU145 as well as CRPC organoids were insensitive to the drug. We extended our studies generating PDXs by subcutaneously injecting NEPC tumor organoids in NSG mouse. The tumor extracted from the PDXs showed a high proliferative phenotype with molecular features characteristic of NEPC as chromogranin A expression and no androgen receptor expression. NEPC PDXs were treated with the EZH2 inhibitor, GSK126, and we observed a significant reduction of tumor size along with the treatment suggesting that EZH2 is a potential therapeutic target for this highly aggressive disease.

Conclusions

In the Englander Institute for Precision Medicine we are generating NEPC patient tumor organoids and PDXs to unveil new targets to facilitate therapeutic decision at this late stage disease. Among the possible hits, EZH2 represents a promising drug target and a potential modulator of the NEPC phenotype.

Citation Format: Loredana Puca, Wouter R. Karthaus, Dong Gao, John Wongvipat, Andrea Sboner, Marcello Gaudiano, Chantal Pauli, Rema A. Rao, Juan Miguel Mosquera, Joanna Cyrta, Theresa Y. MacDonald, Giorgio Ga Inghirami, Yu Chen, Mark A. Rubin, Himisha Beltran. Epigenetic therapy to target neuroendocrine prostate cancer using precision medicine models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3098.

Abstract LB-018: Loss of function mutations of an ETS repressor expand the ETS positive subset of prostate cancer

Recent genomic profiling of both primary and metastatic prostate cancers has revealed novel loss-of-function point mutations and copy number deletions of the gene ERF, an ETS transcriptional repressor with a nearly identical DNA-binding domain to the TMPRSS2-ERG oncogene product. Furthermore, ERF homozygous loss or point mutations occur exclusive of the ERG upregulation that occurs in 50% of prostate cancers following fusion of androgen-regulated upstream genomic elements of TMPRSS2 to the ERG gene.

We characterized the function of ERF, as well as its relationship to ERG using CRISPR and shRNA technology to investigate both normal prostate- and patient tumor-derived organoid cultures, as well as existing TMPRSS2-ERG positive models. In both normal prostate and tumor models lacking upregulated ERG, inhibition of the ERF repressor leads to an expansion of the androgen transcriptome, and recapitulates phenotypic features seen with ERG upregulation. Cistromic analysis reveals that ERF occupies androgen-receptor (AR) associated chromatin, overlapping with potential ERG sites. Accordingly, in the presence of the upregulated ERG oncogene, ERF is now unable to bind any AR associated chromatin, and the AR transcriptome is expanded. CRISPR-mediated loss of ERG in TMPRSS2-ERG positive models leads to a halt of tumor cell growth, but concomitant loss of the ERF repressor partially restores cellular proliferation.

We conclude that ERF is an endogenous repressor of androgen signaling in normal prostatic tissue and a potential tumor suppressor in prostate cancer. It occupies chromatin associated with AR-regulated genes and inhibits their transcriptional regulation. Loss of ERF function, either by mutation or more frequently by competition with the TMPRSS2-ERG gene product, leads to an expansion of the androgen transcriptome. Thereby, the ETS positive subtype of prostate cancer, currently defined by an activating ETS fusion event (e.g. TMPRSS2-ERG) should be expanded to encompass genomic loss of a repressive ETS factor.

Citation Format: Rohit Bose, Wassim Abida, Wouter Karthaus, Joshua Armenia, Phillip Iaquinta, John Wongvipat, Michael Doran, Haley Hieronymus, Philip Watson, Patrick Sullivan, Yupu Liang, Nikolaus Schultz, Charles Sawyers. Loss of function mutations of an ETS repressor expand the ETS positive subset of prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-018.

Abstract 238: Androgen insensitivity syndrome germline loss-of-function mutations in the androgen receptor that acquire somatic gain-of-function in prostate cancer

Genomic alterations in the androgen receptor (AR) commonly occur in patients with advanced prostate cancer resistant to androgen deprivation therapies. While still retaining androgen sensitivity, recurring AR mutations become drivers of antiandrogen resistance by causing alterations in the ligand binding pocket that enables one specific antiandrogen to bind that particular mutant receptor in an agonist conformation, thereby aberrantly activating AR transcriptional activity. We chronically treated antiandrogen-sensitive preclinical prostate cancer models (LNCaP-AR and CWR22Pc) with antiandrogens followed by high-throughput sequencing of the AR exons 2-8 to search for novel mutations that may be associated with antiandrogen resistance. The mutation A597T in the DNA binding domain was found in four LNCaP-AR xenograft tumors from mice receiving enzalutamide or ARN-509, while the ligand binding domain mutation P893S was uncovered in CWR22Pc cells treated in vitro with combined androgen depletion and bicalutamide. Recent whole exome sequencing studies of clinical prostate cancer identified single patients with each of these mutations at high allele frequencies and without concurrent AR amplification (www.cBioPortal.org). A597T occurred in a Gleason 6 primary tumor, whereas P893S was found in a patient with metastatic castration resistant prostate cancer who had received bicalutamide during the course of treatment. Remarkably, both A597T and P893S were earlier reported as germline loss-of-function mutations in patients with androgen insensitivity syndrome. Using an AR reporter assay in AR-null cells coupled with overexpression of these AR mutants, we confirmed that P893S is not activated by the androgen dihydrotestosterone (DHT), but nevertheless is potently stimulated by bicalutamide. In silico modeling showed that in contrast to the wild-type AR, cofactor-recruiting helix 12 of the ligand binding domain of P893S substantially drifted away from an agonist conformation when bound to DHT but was able to adopt the active conformation when simulated with bicalutamide. Importantly, overexpression of AR-P893S in CWR22Pc cells conferred bicalutamide resistance in vivo. These findings highlight an unexpected contextual element to the function of AR mutants, and suggest that rare or private AR mutations may nonetheless act as drivers of clinical progression.

Citation Format: Philip A. Watson, Minna D. Balbas, Zeda Zhang, Taslima F. Ishmael, Kayla E. Lawrence, John Wongvipat, Sophie D. Sawyers, Yi-Mi Wu, Dan Robinson, Yang Shen, Arul M. Chinnaiyan, Charles L. Sawyers. Androgen insensitivity syndrome germline loss-of-function mutations in the androgen receptor that acquire somatic gain-of-function in prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 238.

Applying ⁸⁹Zr-Transferrin To Study the Pharmacology of Inhibitors to BET Bromodomain Containing Proteins

Chromatin modifying proteins are attractive drug targets in oncology, given the fundamental reliance of cancer on altered transcriptional activity. Multiple transcription factors can be impacted downstream of primary target inhibition, thus making it challenging to understand the driving mechanism of action of pharmacologic inhibition of chromatin modifying proteins. This in turn makes it difficult to identify biomarkers predictive of response and pharmacodynamic tools to optimize drug dosing. In this report, we show that ⁸⁹Zr-transferrin, an imaging tool we developed to measure MYC activity in cancer, can be used to identify cancer models that respond to broad spectrum inhibitors of transcription primarily due to MYC inhibition. As a proof of concept, we studied inhibitors of BET bromodomain containing proteins, as they can impart antitumor effects in a MYC dependent or independent fashion. In vitro, we show that transferrin receptor biology is inhibited in multiple MYC positive models of prostate cancer and double hit lymphoma when MYC biology is impacted. Moreover, we show that bromodomain inhibition in one lymphoma model results in transferrin receptor expression changes large enough to be quantified with ⁸⁹Zr-transferrin and positron emission tomography (PET) in vivo. Collectively, these data further underscore the diagnostic utility of the relationship between MYC and transferrin in oncology, and provide the rationale to incorporate transferrin-based PET into early clinical trials with bromodomain inhibitors for the treatment of solid tumors.

Investigation of a frequently mutated transcriptional repressor in prostate cancer, in particular its role in modulating androgen signaling and its effects on TMPRSS2-ERG dependent tumor maintenance

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Background: Recent genomic profiling of primary and metastatic prostate cancers revealed up to 27% of tumors contain putative loss-of-function point mutations or copy number deletions within the gene ERF, an ETS transcriptional repressor with a remarkably similar DNA-binding domain to the TMPRSS2-ERG gene product. Methods: Bioinformatic analysis of 450 patient tumors was obtained from the TCGA and SU2C datasets. Transcriptomic and cistromic analysis was achieved by RNA-seq and ChIP-seq on VCaP cells, as well as patient-derived organoid cultures of both normal and neoplastic prostates. Gene expression was inhibited by CRISPR or shRNA technology. Results: ERF copy number deletions are associated with Gleason 8+ primary disease (p = 0.0369), and ERF mRNA level inversely correlates with androgen-driven transcription (p < 0.0001) in the absence of androgen receptor amplification. Transient inhibition of ERG expression in TMPRSS2-ERG+ cancer models leads to a contraction of the androgen transcriptome. On the other hand, inhibition of ERF leads to an expansion of the androgen transcriptome, and moreover, supersedes the opposing effects of ERG. ERF and the TMPRSS2-ERG gene product have overlapping ETS cistromes that are associated with androgen-receptor binding sites; inhibition of one leads to occupancy at the vacated sites by the other. CRISPR-mediated inhibition of ERG in TMPRSS2-ERG+ models leads to a complete halt of growth, but concomitant loss of ERF partially restores cellular proliferation. Conclusions: ERF is a tumor suppressor with genomic loss-of-function mutations in up to 27% of prostate cancers, thereby expanding the concept of an ETS positive subtype. ERF occupies ETS sites of androgen-regulated genes and is an endogenous dampener of their transcription. Moreover, even in tumors lacking ERF mutations, upregulated ERG now occupies many of these same ETS sites, unmasking their transcriptional control by androgen. Thus, a key oncogenic activity of TMPRSS2-ERG is to antagonize endogenous ERF.

Androgen Receptor Upregulation Mediates Radioresistance after Ionizing Radiation

Clinical trials have established the benefit of androgen deprivation therapy (ADT) combined with radiotherapy in prostate cancer. ADT sensitizes prostate cancer to radiotherapy-induced death at least in part through inhibition of DNA repair machinery, but for unknown reasons, adjuvant ADT provides further survival benefits. Here, we show that androgen receptor (AR) expression and activity are durably upregulated following radiotherapy in multiple human prostate cancer models in vitro and in vivo. Moreover, the degree of AR upregulation correlates with survival in vitro and time to tumor progression in animal models. We also provide evidence of AR pathway upregulation, measured by a rise in serum levels of AR-regulated hK2 protein, in nearly 20% of patients after radiotherapy. Furthermore, these men were three-fold more likely to experience subsequent biochemical failure. Collectively, these data demonstrate that radiotherapy can upregulate AR signaling after therapy to an extent that negatively affects disease progression and/or survival.

Abstract LB-056: TP53 and RB1 alterations promote reprogramming and antiandrogen resistance in advanced prostate cancer

Castration-resistant prostate cancer (CRPC) is one of the most difficult cancers to treat with conventional methods and is responsible for nearly all prostate cancer deaths in the US. The Sawyers laboratory first showed that the primary mechanism of resistance to antiandrogen therapy is elevated androgen receptor (AR) expression. Research based on this finding has led to the development of next-generation antiandrogen: enzalutamide. Despite the exciting clinical success of enzalutamide, about 60% of patients exhibit various degrees of resistance to this agent. Highly variable responses to enzalutamide limit the clinical benefit of this novel antiandrogen, underscoring the importance of understanding the mechanisms of enzalutamide resistance. Most recently, an unbiased SU2C-Prostate Cancer Dream Team metastatic CRPC sequencing project led by Dr. Sawyers and Dr. Chinnaiyan revealed that mutations in the TP53 locus are the most significantly enriched alteration in CRPC tumors when compared to primary prostate cancers. Moreover, deletions and decreased expressions of the TP53 and RB1 loci (co-occurrence and individual occurrence) are more commonly associated with CRPC than with primary tumors. These results established that alteration of the TP53 and RB1 pathways are associated with the development of antiandrogen resistance.

By knockdowning TP53 or/and RB1 in the castration resistant LNCaP/AR model, we demonstrate that the disruption of either TP53 or RB1 alone confers significant resistance to enzalutamide both in vitro and in vivo. Strikingly, the co-inactivation of these pathways confers the most dramatic resistance. Since up-regulation of either AR or AR target genes is not observed in the resistant tumors, loss of TP53 and RB1 function confers enzalutamide resistance likely through an AR independent mechanism. In the clinic, resistance to enzalutamide is increasingly being associated with a transition to a poorly differentiated or neuroendocrine-like histology. Interestingly, we observed significant up-regulations of the basal cell marker Ck5 and the neuroendocrine-like cell marker Synaptophysin in the TP53 and RB1 inactivated cells, as well as down-regulation of the luminal cell marker Ck8. The differences between these markers became even greater after enzalutamide treatment. By using the p53-stabilizing drug Nutlin, level of p53 is rescued and consequently the the decrease of AR protein caused by RB1 and TP53 knockdown is reversed. These results strongly suggest that interference of TP53 and RB1 pathways confers antiandrogen resistance by “priming” prostate cancer cells to reprogramming or transdifferentiation, likely neuroendocrine-like differentiation, in response to treatment. Futher experiments will be performed to assess the molecular mechanism of TP53/RB1 alterations in mediating cell programming and conferring antiandrogen resistance.

Citation Format: Ping Mu, Zhen Cao, Elizabeth Hoover, John Wongvipat, Chun-Hao Huang, Wouter Karthaus, Wassim Abida, Elisa De Stanchina, Charles Sawyers. TP53 and RB1 alterations promote reprogramming and antiandrogen resistance in advanced prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-056. doi:10.1158/1538-7445.AM2015-LB-056

Cabozantinib resolves bone scans in tumor-naïve mice harboring skeletal injuries

The receptor tyrosine kinase inhibitor cabozantinib (XL184, BMS-907351 Cometriq) has displayed impressive clinical activity against several indications, culminating in its recent approval for medullary thyroid cancer. Among malignancies with tropism for the bone (prostate, breast), one striking feature of early clinical reports about this drug has been the rapid and complete resolution of bone scans, a phenomenon almost never observed even among therapies already shown to confer survival benefit. In castration-resistant prostate cancer, not all conventional response indicators change as dramatically posttreatment, raising the possibility that cabozantinib may impair the ability of bone-seeking radionuclides to integrate within the remodeling bone. To test this hypothesis, we surgically induced bone remodeling via physical insult in non-tumor-bearing mice and performed 18F-sodium fluoride (18F-NaF) positron emission tomographic (PET) and technetium 99m-methylene diphosphonate (99mTc-MDP) single-photon emission computed tomographic (SPECT) scans pre- and posttreatment with cabozantinib and related inhibitors. A consistent reduction in the accumulation of either radiotracer at the site of bone remodeling was observed in animals treated with cabozantinib. Given that cabozantinib is known to inhibit several receptor tyrosine kinases, we drugged animals with various permutations of more selective inhibitors to attempt to refine the molecular basis of bone scan resolution. Neither the vascular endothelial growth factor receptor (VEGFR) inhibitor axitinib, the MET inhibitor crizotinib, nor the combination was capable of inhibiting 18F-NaF accumulation at known bioactive doses. In summary, although the mechanism by which cabozantinib suppresses radionuclide incorporation into foci undergoing bone remodeling remains unknown, that this phenomenon occurs in tumor-naïve models indicates that caution should be exercised in interpreting the clinical significance of this event.

Feedback suppression of PI3Kα signaling in PTEN-mutated tumors is relieved by selective inhibition of PI3Kβ

In PTEN-mutated tumors, we show that PI3Kα activity is suppressed and PI3K signaling is driven by PI3Kβ. A selective inhibitor of PI3Kβ inhibits the Akt/mTOR pathway in these tumors but not in those driven by receptor tyrosine kinases. However, inhibition of PI3Kβ only transiently inhibits Akt/mTOR signaling because it relieves feedback inhibition of IGF1R and other receptors and thus causes activation of PI3Kα and a rebound in downstream signaling. This rebound is suppressed and tumor growth inhibition enhanced with combined inhibition of PI3Kα and PI3Kβ. In PTEN-deficient models of prostate cancer, this effective inhibition of PI3K causes marked activation of androgen receptor activity. Combined inhibition of both PI3K isoforms and androgen receptor results in major tumor regressions.

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 &lt; 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.