Lineage-specific canonical and non-canonical activity of EZH2 in advanced prostate cancer subtypes

Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase and emerging therapeutic target that is overexpressed in most castration-resistant prostate cancers and implicated as a driver of disease progression and resistance to hormonal therapies. Here we define the lineage-specific action and differential activity of EZH2 in both prostate adenocarcinoma (PRAD) and neuroendocrine prostate cancer (NEPC) subtypes of advanced prostate cancer to better understand the role of EZH2 in modulating differentiation, lineage plasticity, and to identify mediators of response and resistance to EZH2 inhibitor therapy. Mechanistically, EZH2 modulates bivalent genes that results in upregulation of NEPC-associated transcriptional drivers (e.g., ASCL1) and neuronal gene programs, and leads to forward differentiation after targeting EZH2 in NEPC. Subtype-specific downstream effects of EZH2 inhibition on cell cycle genes support the potential rationale for co-targeting cyclin/CDK to overcome resistance to EZH2 inhibition.

Prostate Cancer Progression Relies on the Mitotic Kinase Citron Kinase

Prostate cancer remains the second leading cause of cancer death in men in Western cultures. A deeper understanding of the mechanisms by which prostate cancer cells divide to support tumor growth could help devise strategies to overcome treatment resistance and improve survival. Here, we identified that the mitotic AGC family protein kinase citron kinase (CIT) is a pivotal regulator of prostate cancer growth that mediates prostate cancer cell interphase progression. Increased CIT expression correlated with prostate cancer growth induction and aggressive prostate cancer progression, and CIT was overexpressed in prostate cancer compared with benign prostate tissue. CIT overexpression was controlled by an E2F2-Skp2-p27 signaling axis and conferred resistance to androgen-targeted treatment strategies. The effects of CIT relied entirely on its kinase activity. Conversely, CIT silencing inhibited the growth of cell lines and xenografts representing different stages of prostate cancer progression and treatment resistance but did not affect benign epithelial prostate cells or nonprostatic normal cells, indicating a potential therapeutic window for CIT inhibition. CIT kinase activity was identified as druggable and was potently inhibited by the multikinase inhibitor OTS-167, which decreased the proliferation of treatment-resistant prostate cancer cells and patient-derived organoids. Isolation of the in vivo CIT substrates identified proteins involved in diverse cellular functions ranging from proliferation to alternative splicing events that are enriched in treatment-resistant prostate cancer. These findings provide insights into the regulation of aggressive prostate cancer cell behavior by CIT and identify CIT as a functionally diverse and druggable driver of prostate cancer progression.

SIGNIFICANCE

The poorly characterized protein kinase citron kinase is a therapeutic target in prostate cancer that drives tumor growth by regulating diverse substrates, which control several hallmarks of aggressive prostate cancer progression. See related commentary by Mishra et al., p. 4008.

Targeting DNA methylation and B7-H3 in RB1-deficient and neuroendocrine prostate cancer

Aberrant DNA methylation has been implicated as a key driver of prostate cancer lineage plasticity and histologic transformation to neuroendocrine prostate cancer (NEPC). DNA methyltransferases (DNMTs) are highly expressed, and global DNA methylation is dysregulated in NEPC. We identified that deletion of DNMT genes decreases expression of neuroendocrine lineage markers and substantially reduced NEPC tumor development and metastasis in vivo. Decitabine, a pan-DNMT inhibitor, attenuated tumor growth in NEPC patient-derived xenograft models, as well as retinoblastoma gene (RB1)-deficient castration-resistant prostate adenocarcinoma (CRPC) models compared with RB1-proficient CRPC. We further found that DNMT inhibition increased expression of B7 homolog 3 (B7-H3), an emerging druggable target, via demethylation of B7-H3. We tested DS-7300a (i-DXd), an antibody-drug conjugate targeting B7-H3, alone and in combination with decitabine in models of advanced prostate cancer. There was potent single-agent antitumor activity of DS-7300a in both CRPC and NEPC bearing high expression of B7-H3. In B7-H3-low models, combination therapy of decitabine plus DS-7300a resulted in enhanced response. DNMT inhibition may therefore be a promising therapeutic target for NEPC and RB1-deficient CRPC and may sensitize B7-H3-low prostate cancer to DS-7300a through increasing target expression. NEPC and RB1-deficient CRPC represent prostate cancer subgroups with poor prognosis, and the development of biomarker-driven therapeutic strategies for these populations may ultimately help improve patient outcomes.

Significance of RB Loss in Unlocking Phenotypic Plasticity in Advanced Cancers

Cancer cells can undergo plasticity in response to environmental stimuli or under selective therapeutic pressures that result in changes in phenotype. This complex phenomenon of phenotypic plasticity is now recognized as a hallmark of cancer. Lineage plasticity is often associated with loss of dependence on the original oncogenic driver and is facilitated, in part, by underlying genomic and epigenetic alterations. Understanding the molecular drivers of cancer plasticity is critical for the development of novel therapeutic strategies. The retinoblastoma gene RB1 (encoding RB) is the first tumor suppressor gene to be discovered and has a well-described role in cell-cycle regulation. RB is also involved in diverse cellular functions beyond cell cycle including differentiation. Here, we describe the emerging role of RB loss in unlocking cancer phenotypic plasticity and driving therapy resistance across cancer types. We highlight parallels in cancer with the noncanonical role of RB that is critical for normal development and lineage specification, and the downstream consequences of RB loss including epigenetic reprogramming and chromatin reorganization that can lead to changes in lineage program. Finally, we discuss potential therapeutic approaches geared toward RB loss cancers undergoing lineage reprogramming.

Landscape of prostate-specific membrane antigen heterogeneity and regulation in AR-positive and AR-negative metastatic prostate cancer

Tumor expression of prostate-specific membrane antigen (PSMA) is lost in 15-20% of men with castration-resistant prostate cancer (CRPC), yet the underlying mechanisms remain poorly defined. In androgen receptor (AR)-positive CRPC, we observed lower PSMA expression in liver lesions versus other sites, suggesting a role of the microenvironment in modulating PSMA. PSMA suppression was associated with promoter histone 3 lysine 27 methylation and higher levels of neutral amino acid transporters, correlating with 18F-fluciclovine uptake on positron emission tomography imaging. While PSMA is regulated by AR, we identified a subset of AR-negative CRPC with high PSMA. HOXB13 and AR co-occupancy at the PSMA enhancer and knockout models point to HOXB13 as an upstream regulator of PSMA in AR-positive and AR-negative prostate cancer. These data demonstrate how PSMA expression is differentially regulated across metastatic lesions and in the context of the AR, which may inform selection for PSMA-targeted therapies and development of complementary biomarkers.

Abstract B022: CHD1 promotes sensitivity to aurora kinase inhibitors by modulating the interaction of AURKA with TPX2

Clinical studies have shown that subsets of cancer patients achieve a significant benefit from Aurora kinase inhibitors, suggesting an urgent need to identify biomarkers for predicting drug response. Chromodomain-helicase-DNA-binding protein 1 (CHD1) is involved in chromatin remodeling, DNA repair, and transcriptional plasticity. Prior studies have demonstrated that CHD1 has distinct expression patterns in cancers with different molecular features, but its impact on drug responsiveness remains understudied. Here, we show that CHD1 promotes the susceptibility of prostate cancer cells to inhibitors targeting Aurora kinases, while depletion of CHD1 impairs their efficacy in vitro and in vivo. Pan-cancer drug sensitivity analyses revealed that high expression of CHD1 was associated with increased sensitivity to Aurora kinase A (AURKA) inhibitors. Mechanistically, KPNA2 served as a direct target of CHD1 and suppressed the interaction of AURKA with the co-activator TPX2, thereby rendering cancer cells more vulnerable to AURKA inhibitors. Consistent with previous research reporting that loss of PTEN elevates CHD1 levels, studies in a genetically engineered mouse model, patient-derived organoids, and patient samples showed that PTEN defects are associated with a better response to AURKA inhibition in advanced prostate cancer. These observations demonstrate that CHD1 plays an important role in modulating Aurora kinases and drug sensitivities, providing new insights into biomarker-driven therapies targeting Aurora kinases for future clinical studies.

Citation Format: Haoyan Li, Yin Wang, Kevin Lin, Varadha Balaji Venkadakrishnan, Martin Bakht, Kaitlyn Tremble, Yue Lu, Himisha Beltran, Di Zhao. CHD1 promotes sensitivity to aurora kinase inhibitors by modulating the interaction of AURKA with TPX2. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr B022.

CHD1 Promotes Sensitivity to Aurora Kinase Inhibitors by Suppressing Interaction of AURKA with Its Coactivator TPX2

Clinical studies have shown that subsets of patients with cancer achieve a significant benefit from Aurora kinase inhibitors, suggesting an urgent need to identify biomarkers for predicting drug response. Chromodomain helicase DNA binding protein 1 (CHD1) is involved in chromatin remodeling, DNA repair, and transcriptional plasticity. Prior studies have demonstrated that CHD1 has distinct expression patterns in cancers with different molecular features, but its impact on drug responsiveness remains understudied. Here, we show that CHD1 promotes the susceptibility of prostate cancer cells to inhibitors targeting Aurora kinases, while depletion of CHD1 impairs their efficacy in vitro and in vivo. Pan-cancer drug sensitivity analyses revealed that high expression of CHD1 was associated with increased sensitivity to Aurora kinase A (AURKA) inhibitors. Mechanistically, KPNA2 served as a direct target of CHD1 and suppressed the interaction of AURKA with the coactivator TPX2, thereby rendering cancer cells more vulnerable to AURKA inhibitors. Consistent with previous research reporting that loss of PTEN elevates CHD1 levels, studies in a genetically engineered mouse model, patient-derived organoids, and patient samples showed that PTEN defects are associated with a better response to AURKA inhibition in advanced prostate cancer. These observations demonstrate that CHD1 plays an important role in modulating Aurora kinases and drug sensitivities, providing new insights into biomarker-driven therapies targeting Aurora kinases for future clinical studies.

SIGNIFICANCE

CHD1 plays a critical role in controlling AURKA activation and promoting Aurora kinase inhibitor sensitivity, providing a potential clinical biomarker to guide cancer treatment.

Genomic alterations impact cell cycle-related genes during prostate cancer progression

The recent genomic characterization of patient specimens has started to reveal the landscape of somatic alterations in clinical prostate cancer (CaP) and its association with disease progression and treatment resistance. The extent to which such alterations impact hallmarks of cancer is still unclear. Here, we interrogate genomic data from thousands of clinical CaP specimens that reflect progression from treatment-naïve, to castration-recurrent, and in some cases, neuroendocrine CaP for alterations in cell cycle-associated and -regulated genes, which are central to cancer initiation and progression. We evaluate gene signatures previously curated to evaluate G1-S and G2-M phase transitions or to represent the cell cycle-dependent proteome. The resulting CaP (stage)-specific overview confirmed the presence of well-known driver alterations impacting, for instance, the genes encoding p53 and MYC, and uncovered novel previously unrecognized mutations that affect others such as the PKMYT1 and MTBP genes. The cancer dependency and drugability of representative genomically altered cell cycle determinants were verified also. Taken together, these analyses on hundreds of often less-characterized cell cycle regulators expand considerably the scope of genomic alterations associated with CaP cell proliferation and cell cycle and isolate such regulatory proteins as putative drivers of CaP treatment resistance and entirely novel therapeutic targets for CaP therapy.

Novel insights in cell cycle dysregulation during prostate cancer progression

Prostate cancer (CaP) remains the second leading cause of cancer deaths in Western men. These deaths occur because metastatic CaP acquires resistance to available treatments. The novel and functionally diverse treatment options that have been introduced in the clinic over the past decade each eventually induce resistance for which the molecular basis is diverse. Both initiation and progression of CaP have been associated with enhanced cell proliferation and cell cycle dysregulation. A better understanding of the specific pro-proliferative molecular shifts that control cell division and proliferation during CaP progression may ultimately overcome treatment resistance. Here, we examine literature for support of this possibility. We start by reviewing recently renewed insights in prostate cell types and their proliferative and oncogenic potential. We then provide an overview of the basic knowledge on the molecular machinery in charge of cell cycle progression and its regulation by well-recognized drivers of CaP progression such as androgen receptor and retinoblastoma protein. In this respect, we pay particular attention to interactions and reciprocal interplay between cell cycle regulators and androgen receptor. Somatic alterations that impact the cell cycle-associated and -regulated genes encoding p53, PTEN and MYC during progression from treatment-naïve, to castration-recurrent, and in some cases, neuroendocrine CaP are discussed. We considered also non-genomic events that impact cell cycle determinants, including transcriptional, epigenetic and micro-environmental switches that occur during CaP progression. Finally, we evaluate the therapeutic potential of cell cycle regulators and address challenges and limitations in the approaches modulating their action for CaP treatment.

Diversity in Androgen Receptor Action Among Treatment-naïve Prostate Cancers Is Reflected in Treatment Response Predictions and Molecular Subtypes

Background

Metastatic prostate cancer (CaP) treatments are evolving rapidly but without evidence-based biomarkers to predict responses, and to maximize remissions and survival.

Objective

To determine the activity of androgen receptor (AR), the target for default first-line systemic treatment, in localized treatment-naïve CaP and its association with clinical risk factors, molecular markers, CaP subtypes, and predictors of treatment response.

Design setting and participants

We examined 452 bona fide AR target genes in clinical-grade expression profiles from 6532 such CaPs collected between 2013 and 2017 by US physicians ordering the Decipher RP test. Results were validated in three independent smaller cohorts (n = 73, 90, and 127) and clinical CaP AR ChIP-Seq data. Association with CaP differentiation and progression was analyzed in independent datasets.

Outcome measurements and statistical analysis

Unsupervised clustering of CaPs based on AR target gene expression was aligned with clinical variables, differentiation scores, molecular subtypes, and predictors of response to hormonal therapy, radiotherapy, and chemotherapy. AR target gene sets were analyzed via Gene Set Enrichment Analysis for differentiation and treatment resistance, Ingenuity Pathway Analysis for associated biology, and Cistrome for genomic AR binding site (ARBS) composition.

Results and limitations

Expression of eight AR target gene subsignatures gave rise to five CaP clusters, which were preferentially associated with CaP molecular subtypes, differentiation, and predictors of treatment response rather than with clinical variables. Subsignatures differed in contribution to CaP progression, luminal/basal differentiation, CaP biology, and ARBS composition. Validation in prospective trials and optimized quantitation are needed for clinical implementation.

Conclusions

Measurement of AR activity patterns in treatment-naïve CaP may serve as a first branch of an evidence-based decision tree to optimize personalized treatment plans.

Patient summary

Treatment options for metastatic prostate cancer are increasing without information needed to choose the right treatment for the right patient. We found variation in the behavior of the target for the default first-line therapy before treatment, which may help optimize treatment plans.

AR-dependent phosphorylation and phospho-proteome targets in prostate cancer

Prostate cancer (CaP) is the second leading cause of cancer-related deaths in Western men. Because androgens drive CaP by activating the androgen receptor (AR), blocking AR's ligand activation, known as androgen deprivation therapy (ADT), is the default treatment for metastatic CaP. Despite an initial remission, CaP eventually develops resistance to ADT and progresses to castration-recurrent CaP (CRPC). CRPC continues to rely on aberrantly activated AR that is no longer inhibited effectively by available therapeutics. Interference with signaling pathways downstream of activated AR that mediate aggressive CRPC behavior may lead to alternative CaP treatments. Developing such therapeutic strategies requires a thorough mechanistic understanding of the most clinically relevant and druggable AR-dependent signaling events. Recent proteomics analyses of CRPC clinical specimens indicate a shift in the phosphoproteome during CaP progression. Kinases and phosphatases represent druggable entities, for which clinically tested inhibitors are available, some of which are incorporated already in treatment plans for other human malignancies. Here, we reviewed the AR-associated transcriptome and translational regulon, and AR interactome involved in CaP phosphorylation events. Novel and for the most part mutually exclusive AR-dependent transcriptional and post-transcriptional control over kinase and phosphatase expression was found, with yet other phospho-regulators interacting with AR. The multiple mechanisms by which AR can shape and fine-tune the CaP phosphoproteome were reflected in diverse aspects of CaP biology such as cell cycle progression and cell migration. Furthermore, we examined the potential, limitations and challenges of interfering with AR-mediated phosphorylation events as alternative strategy to block AR function during CaP progression.

Abstract 390: Prostate cancer progression depends on the activity of the mitotic kinase citron kinase

Androgen receptor (AR) is the major driver of prostate cancer (CaP) progression. Despite initially inducing remissions, androgen deprivation therapy (ADT) does not cure CaP and the majority of CaP that recurs under ADT continues to rely on AR. In a subset of patients, administration of novel, more potent ADT drugs leads to emergence of a neuroendocrine (NE) CaP phenotype that is AR-indifferent and even harder to treat. The molecular mechanisms by which AR regulates cell cycle progression and that can be developed into alternative targets to inhibit CaP growth, overcome acquired resistance to ADT, or prevent NE progression, however, remain largely unknown. Here, we identify citron kinase (CIT), which controls cell division, as a novel drugabble target that acts downstream of AR. In multiple CaP models, CIT protein expression was stimulated selectively by low doses of androgens that promote CaP cell proliferation and decreased by AR silencing or short-term administration of AR-antagonist enzalutamide, confirming its AR dependence. Silencing of CIT significantly reduced CaP cell viability and cell proliferation, delayed cell progression, increased the number of multinucleated cells in ADT-naïve and -recurrent CaP cells, and attenuated CaP growth in xenograft models. Overexpression of CIT, transiently or stably, stimulated CaP cell proliferation in androgen-supplemented conditions and under ADT, and this depended entirely on an intact CIT kinase domain. In 2 independent clinical CaP datasets, CIT mRNA levels increased during CaP progression and higher CIT expression correlated with shorter disease-free and overall survival. Using CaP tissue microarrays that contain more than 200 patient specimens, CIT protein expression was significantly higher in primary CaP when compared to adjacent non-neoplastic prostate tissues and correlated with increasing Gleason scores, validating the relevance of CIT overexpression to CaP aggressiveness. Integrated RNA-Seq, MSigDB and GSEA analyses confirmed that the AR- and CIT-dependent transcriptome preferentially controlled CaP cell cycle progression and proliferation. Mechanistically, the use of inhibitors of gene transcription, mRNA translation and proteasome indicated that the androgen-dependence of CIT expression is regulated at the post-transcriptional level. Specifically, CIT protein expression was controlled by the E2F family of cell cycle regulators, with androgen-responsive E2F2 as the major determinant of CaP CIT expression. Our work, thus, isolated a novel role for the mitotic kinase CIT in AR-dependent CaP cell proliferation and clinical progression and identified CIT’s kinase moiety as a novel target for CaP therapy.

Funding: NIH/NCI, Case Comprehensive Cancer Center Pilot Research Award

Citation Format: Salma Ben-Salem, Salam Bachour, Varadha Balaji Venkadakrishnan, Yixue Su, Eduardo Cortes Gomez, Qiang Hu, Eric Klein, Nicolas Marlo, Cristina Magi-Galluzzi, Liu Song, Hannelore V. Heemers. Prostate cancer progression depends on the activity of the mitotic kinase citron kinase [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 390.

Protein Kinase N1 control of androgen-responsive serum response factor action provides rationale for novel prostate cancer treatment strategy

Sustained reliance on androgen receptor (AR) after failure of AR-targeting androgen deprivation therapy (ADT) prevents effective treatment of castration-recurrent (CR) prostate cancer (CaP). Interfering with the molecular machinery by which AR drives CaP progression may be an alternative therapeutic strategy but its feasibility remains to be tested. Here, we explore targeting the mechanism by which AR, via RhoA, conveys androgen-responsiveness to serum response factor (SRF), which controls aggressive CaP behavior and is maintained in CR-CaP. Following a siRNA screen and candidate gene approach, RNA-Seq studies confirmed that the RhoA effector Protein Kinase N1 (PKN1) transduces androgen-responsiveness to SRF. Androgen treatment induced SRF-PKN1 interaction, and PKN1 knockdown or overexpression severely impaired or stimulated, respectively, androgen regulation of SRF target genes. PKN1 overexpression occurred during clinical CR-CaP progression, and hastened CaP growth and shortened CR-CaP survival in orthotopic CaP xenografts. PKN1's effects on SRF relied on its kinase domain. The multikinase inhibitor lestaurtinib inhibited PKN1 action and preferentially affected androgen regulation of SRF over direct AR target genes. In a CR-CaP patient-derived xenograft, expression of SRF target genes was maintained while AR target gene expression declined and proliferative gene expression increased. PKN1 inhibition decreased viability of CaP cells before and after ADT. In patient-derived CaP explants, lestaurtinib increased AR target gene expression but did not significantly alter SRF target gene or proliferative gene expression. These results provide proof-of-principle for selective forms of ADT that preferentially target different fractions of AR's transcriptional output to inhibit CaP growth.

Abstract LB-211: The cardiac transcription factor Nkx2-5 contributes to SRF-dependent AR action in prostate cancer

Ligand-activated androgen receptor (AR) drives prostate cancer (CaP) growth. Heterogeneity in AR action exists and AR often remains active when therapies targeting its ligand-activation have failed. Understanding the mechanisms by which AR mediates CaP progression may lead to new treatments. Previously, we identified a mechanism in which AR imparts androgen-responsiveness to the transcription factor Serum Response Factor (SRF). SRF-dependent AR action is enriched in CaP, correlates with CaP recurrence and is maintained in late-stage CaP. SRF binds constitutively at its target genes and becomes transcriptionally active via upstream signaling cascades or SRF cofactors. Our prior work showed that RhoA signaling transduces androgen regulation to a significant fraction, but not all, SRF target genes. Here, we isolate the homeodomain protein Nkx2-5 as a novel regulator of androgen-responsive SRF action. Nkx2-5 co-operates with SRF in transcriptional control of heart development. In CaP, Nkx2-5 promoter hypermethylation is a biomarker that distinguishes CaP from benign prostate, yet consequences of differential Nkx2-5 expression on CaP are unknown. In the CaP cell models LNCaP, C4-2 and VCaP, siRNA-mediated loss of Nkx2-5 increased basal and androgen-dependent expression of exogenously and endogenously expressed SRF target genes, and these effects were reversed after silencing of SRF. Conversely, overexpression of Nkx2-5 decreased androgen-responsiveness of SRF target genes. Co-IP and ChIP assays indicated androgen-stimulated SRF-Nkx2-5 interaction in CaP cell nuclei. Loss of Nkx2-5 resulted in distinct morphological changes in CaP cells, which depended entirely on SRF. In rhodamine phalloidin staining assays, loss of Nkx2-5 increased significantly (p<0.01) cell length, width and circumference, and the number of cell protrusions in cultures with and without androgen stimulation. Changes in cell shape were accompanied by modest decreases in CaP cell proliferation and migration, but significant (p<0.05) decreases in CaP cell-matrix adhesion. Nkx2-5 silencing increased the size of the cell nucleus and led to multilobed and grooved nuclei and enlarged nucleoli. These nuclear shape changes were not associated with alterations in DNA ploidy, but were associated with changed heterochromatin organization as reflected in a reduction in heterochromatin protein 1 immunostaining and nuclear content. Such Nkx2-5-dependent changes in nuclear shape and size and heterochromatin organization occurred also in non-malignant prostate epithelial RWPE2 cells. This work uncovered a novel role for Nkx2-5 in the transition from benign prostate to CaP, showing that Nkx2-5 regulates SRF transcriptional activity in CaP cells and demonstrating that differential Nkx2-5 expression alters cell-matrix adhesion and nuclear morphology, both processes that are associated with aggressive CaP progression.

Citation Format: Salma Ben-Salem, Dhirodatta Senapati, Giridhar Mudduluru, Yixue Su, Varadha Balaji Venkadakrishnan, Hannelore V. Heemers. The cardiac transcription factor Nkx2-5 contributes to SRF-dependent AR action in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-211.

A comprehensive analysis of coregulator recruitment, androgen receptor function and gene expression in prostate cancer

Standard treatment for metastatic prostate cancer (CaP) prevents ligand-activation of androgen receptor (AR). Despite initial remission, CaP progresses while relying on AR. AR transcriptional output controls CaP behavior and is an alternative therapeutic target, but its molecular regulation is poorly understood. Here, we show that action of activated AR partitions into fractions that are controlled preferentially by different coregulators. In a 452-AR-target gene panel, each of 18 clinically relevant coregulators mediates androgen-responsiveness of 0–57% genes and acts as a coactivator or corepressor in a gene-specific manner. Selectivity in coregulator-dependent AR action is reflected in differential AR binding site composition and involvement with CaP biology and progression. Isolation of a novel transcriptional mechanism in which WDR77 unites the actions of AR and p53, the major genomic drivers of lethal CaP, to control cell cycle progression provides proof-of-principle for treatment via selective interference with AR action by exploiting AR dependence on coregulators.

Prostate cancer is the second leading cause of cancer deaths in men in the Western world. Almost all of these deaths happen when the main treatment for advanced prostate cancers stops working. The treatment, known as androgen deprivation therapy, targets a protein called the androgen receptor. This receptor is activated when it binds to signaling molecules and, once active, it switches on genes that encourage the cancer cells to grow. Androgen deprivation therapy blocks the androgen receptor from interacting with the signaling molecules; however, this treatment eventually fails because the receptor finds other ways to remain active in prostate cancer.

Increasing the survival of patients with prostate cancer will depend on new treatments that can inhibit androgen receptors that no longer respond to androgen deprivation therapy. The androgen receptor’s ability to switch on genes could be another target for prostate cancer therapy – though not enough was known about the way this ability is regulated and how it controls the progression of prostate cancer. Liu, Kumari et al. set out to better define how this ability drives the growth of prostate cancer.

The androgen receptor needs to interact with other proteins, known as coregulators, to work, and Liu, Kumari et al. developed an assay that examines, all at the same time, how important 18 such coregulators are for more than 400 genes that are regulated by the androgen receptor. This revealed that the coregulators did not all affect the same genes and that each coregulator tended to help activate sets of genes associated with a specific aspect of the biology of prostate cancer cells.

Liu, Kumari et al. also discovered previously unknown interactions between androgen receptors, coregulators and other proteins that were responsible for the specific associations between genes and corregulators. The most important of these new interactions was one between the androgen receptor, the coregulator WDR77, and a protein called p53. These interactions are enriched in prostate cancers, including those that do not respond to androgen deprivation therapy, where they promote cancer growth.

These findings lay the foundation to develop new drugs that interfere with the interactions between the androgen receptor and other proteins that are most important for the progression of advanced prostate cancers. Other researchers have already shown that it is possible to develop such drugs – though further testing is needed before any new treatments begin to help prostate cancer patients who no longer respond to androgen deprivation therapy.

Abstract 1581: Androgen receptor action in prostate cancer partitions into distinct transcriptional codes that differ in clinical relevance

With few exceptions the 30,000 prostate cancer (CaP) deaths annually in the US are due to failure of androgen deprivation therapy (ADT). ADT prevents ligand-activation of androgen receptor (AR). Despite remission, CaP progresses while continuing to rely on AR. AR’s transcriptional output that controls CaP behavior is an alternative therapeutic target, but its molecular regulation is poorly understood. Here, we determined the androgen dependence of hundreds of direct AR target genes on 18 coregulators that are relevant to CaP progression, and uncovered that the AR-dependent transcriptome breaks down in coregulator-dependent gene sets. Pairwise comparison and unsupervised clustering showed limited overlap between the gene subsignatures. Neither ChIP nor qRT-PCR studies revealed differences in the kinetics of coregulator recruitment to Androgen Response Elements (AREs) or of androgen regulation of different gene sets. Rather, Cistrome analyses of AR binding sites demonstrated preferential enrichment in binding motifs for distinct transcription factors (TFs) between coregulator-dependent gene sets. Pathway and GSEA analyses of these signatures indicated associations with select biological processes and differential enrichments between normal prostate and CaP, CaPs of different stages, and CaP and bone marrow microenvironment. These results suggested that coregulators may unite the action of DNA-bound AR and TF(s) to control select aspects of androgen-dependent CaP cell biology. The presence and functionality of such novel predicted AR transcriptional codes, namely AR-WDR77-p53 and AR-STAT3-IRF1 with projected roles in CaP cell proliferation and stemness respectively, was confirmed in Co-IP and gene expression profiling. As AR and p53 are the major drivers of lethal CaP, the AR-WDR77-p53 code was explored further. Mass spectrometry after WDR77 and p53 IP independently identified PGAM5, a recently isolated serine/threonine protein phosphatase that regulates cell death and unknown to be relevant to AR signaling or CaP biology, as part of the AR-WDR77-p53 complex. Co-IP assays verified IP-mass spectrometry results and ChIP studies confirmed recruitment of PGAM5 to WDR77-dependent AREs. Expression profiling demonstrated significant overlap in genes for which androgen regulation was altered after silencing of WDR77, PGAM5 or p53. Co-IP and ChIP studies showed that androgen-dependent recruitment of p53 to ARE-bound AR requires WDR77 and PGAM5. Western blotting and flow cytometry analyses indicated that WDR77 and PGAM5 and their dependent AR target genes control G1/S cell cycle progression, which was maintained in presence of clinically relevant gain-of-function p53 mutants. These novel insights indicate that disrupting select protein-protein and protein-DNA interactions may be a viable strategy to inhibit AR action that drives CaP lethal progression.

Citation Format: Sangeeta Kumari, Varadha Balaji Venkadakrishnan, Dhirodatta Senapati, Qiang Hu, Song Liu, Hannelore V. Heemers. Androgen receptor action in prostate cancer partitions into distinct transcriptional codes that differ in clinical relevance [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 1581. doi:10.1158/1538-7445.AM2017-1581