PARP inhibition and pharmacological ascorbate demonstrate synergy in castration-resistant prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer death for men in the United States. While organ-confined disease has reasonable expectation of cure, metastatic PCa is universally fatal upon recurrence during hormone therapy, a stage termed castration-resistant prostate cancer (CRPC). Until such time as molecularly defined subtypes can be identified and targeted using precision medicine, it is necessary to investigate new therapies that may apply to the CRPC population as a whole. The administration of ascorbate, more commonly known as ascorbic acid or Vitamin C, has proved lethal to and highly selective for a variety of cancer cell types. There are several mechanisms currently under investigation to explain how ascorbate exerts anti-cancer effects. A simplified model depicts ascorbate as a pro-drug for reactive oxygen species (ROS), which accumulate intracellularly and generate DNA damage. It was therefore hypothesized that poly(ADP-ribose) polymerase (PARP) inhibitors, by inhibiting DNA damage repair, would augment the toxicity of ascorbate.

Results

Two distinct CRPC models were found to be sensitive to physiologically relevant doses of ascorbate. Moreover, additional studies indicate that ascorbate inhibits CRPC growth in vitro via multiple mechanisms including disruption of cellular energy dynamics and accumulation of DNA damage. Combination studies were performed in CRPC models with ascorbate in conjunction with escalating doses of three different PARP inhibitors (niraparib, olaparib, and talazoparib). The addition of ascorbate augmented the toxicity of all three PARP inhibitors and proved synergistic with olaparib in both CRPC models. Finally, the combination of olaparib and ascorbate was tested in vivo in both castrated and non-castrated models. In both cohorts, the combination treatment significantly delayed tumor growth compared to monotherapy or untreated control.

Conclusions

These data indicate that pharmacological ascorbate is an effective monotherapy at physiological concentrations and kills CRPC cells. Ascorbate-induced tumor cell death was associated with disruption of cellular energy dynamics and accumulation of DNA damage. The addition of PARP inhibition increased the extent of DNA damage and proved effective at slowing CRPC growth both in vitro and in vivo. These findings nominate ascorbate and PARPi as a novel therapeutic regimen that has the potential to improve CRPC patient outcomes.

A Novel Role for DNA-PK in Metabolism by Regulating Glycolysis in Castration-Resistant Prostate Cancer

PURPOSE

DNA-dependent protein kinase catalytic subunit (DNA-PKcs, herein referred as DNA-PK) is a multifunctional kinase of high cancer relevance. DNA-PK is deregulated in multiple tumor types, including prostate cancer, and is associated with poor outcomes. DNA-PK was previously nominated as a therapeutic target and DNA-PK inhibitors are currently undergoing clinical investigation. Although DNA-PK is well studied in DNA repair and transcriptional regulation, much remains to be understood about the way by which DNA-PK drives aggressive disease phenotypes.

EXPERIMENTAL DESIGN

Here, unbiased proteomic and metabolomic approaches in clinically relevant tumor models uncovered a novel role of DNA-PK in metabolic regulation of cancer progression. DNA-PK regulation of metabolism was interrogated using pharmacologic and genetic perturbation using in vitro cell models, in vivo xenografts, and ex vivo in patient-derived explants (PDE).

RESULTS

Key findings reveal: (i) the first-in-field DNA-PK protein interactome; (ii) numerous DNA-PK novel partners involved in glycolysis; (iii) DNA-PK interacts with, phosphorylates (in vitro), and increases the enzymatic activity of glycolytic enzymes ALDOA and PKM2; (iv) DNA-PK drives synthesis of glucose-derived pyruvate and lactate; (v) DNA-PK regulates glycolysis in vitro, in vivo, and ex vivo; and (vi) combination of DNA-PK inhibitor with glycolytic inhibitor 2-deoxyglucose leads to additive anti-proliferative effects in aggressive disease.

CONCLUSIONS

Findings herein unveil novel DNA-PK partners, substrates, and function in prostate cancer. DNA-PK impacts glycolysis through direct interaction with glycolytic enzymes and modulation of enzymatic activity. These events support energy production that may contribute to generation and/or maintenance of DNA-PK-mediated aggressive disease phenotypes.

Novel Oncogenic Transcription Factor Cooperation in RB-Deficient Cancer

The retinoblastoma tumor suppressor (RB) is a critical regulator of E2F-dependent transcription, controlling a multitude of protumorigenic networks including but not limited to cell-cycle control. Here, genome-wide assessment of E2F1 function after RB loss in isogenic models of prostate cancer revealed unexpected repositioning and cooperation with oncogenic transcription factors, including the major driver of disease progression, the androgen receptor (AR). Further investigation revealed that observed AR/E2F1 cooperation elicited novel transcriptional networks that promote cancer phenotypes, especially as related to evasion of cell death. These observations were reflected in assessment of human disease, indicating the clinical relevance of the AR/E2F1 cooperome in prostate cancer. Together, these studies reveal new mechanisms by which RB loss induces cancer progression and highlight the importance of understanding the targets of E2F1 function. SIGNIFICANCE: This study identifies that RB loss in prostate cancer drives cooperation between AR and E2F1 as coregulators of transcription, which is linked to the progression of advanced disease.

Relevance of pRB Loss in Human Malignancies

The retinoblastoma tumor suppressor protein (pRB) is a known regulator of cell-cycle control; however, recent studies identified critical functions for pRB in regulating cancer-associated gene networks that influence the DNA damage response, apoptosis, and cell metabolism. Understanding the impact of these pRB functions on cancer development and progression in the clinical setting will be essential, given the prevalence of pRB loss of function across disease types. Moreover, the current state of evidence supports the concept that pRB loss results in pleiotropic effects distinct from tumor proliferation. Here, the implications of pRB loss (and resultant pathway deregulation) on disease progression and therapeutic response will be reviewed, based on clinical observation. Developing a better understanding of the pRB-regulated pathways that underpin the aggressive features of pRB-deficient tumors will be essential for further developing pRB as a biomarker of disease progression and for stratifying pRB-deficient tumors into more effective treatment regimens.

Mutant p53 elicits context-dependent pro-tumorigenic phenotypes

The tumor suppressor gene TP53 is the most frequently mutated gene in numerous cancer types, including prostate cancer (PCa). Specifically, missense mutations in TP53 are selectively enriched in PCa, and cluster to particular "hot spots" in the p53 DNA binding domain with mutation at the R273 residue occurring most frequently. While this residue is similarly mutated to R273C-p53 or R273H-p53 in all cancer types examined, in PCa selective enrichment of R273C-p53 is observed. Importantly, examination of clinical datasets indicated that TP53 heterozygosity can either be maintained or loss of heterozygosity (LOH) occurs. Thus, to mimic tumor-associated mutant p53, R273C-p53 and R273H-p53 isogenic PCa models were developed in the presence or absence of wild-type p53. In the absence of wild-type p53, both R273C-p53 and R273H-p53 exhibited similar loss of DNA binding, transcriptional profiles, and loss of canonical tumor suppressor functions associated with wild-type p53. In the presence of wild-type p53 expression, both R273C-p53 and R273H-p53 supported canonical p53 target gene expression yet elicited distinct cistromic and transcriptional profiles when compared to each other. Moreover, heterozygous modeling of R273C-p53 or R273H-p53 expression resulted in distinct phenotypic outcomes in vitro and in vivo. Thus, mutant p53 acts in a context-dependent manner to elicit pro-tumorigenic transcriptional profiles, providing critical insight into mutant p53-mediated prostate cancer progression.

RB/E2F1 as a Master Regulator of Cancer Cell Metabolism in Advanced Disease

Loss of the retinoblastoma (RB) tumor suppressor protein is a critical step in reprogramming biological networks that drive cancer progression, although mechanistic insight has been largely limited to the impact of RB loss on cell-cycle regulation. Here, isogenic modeling of RB loss identified disease stage-specific rewiring of E2F1 function, providing the first-in-field mapping of the E2F1 cistrome and transcriptome after RB loss across disease progression. Biochemical and functional assessment using both in vitro and in vivo models identified an unexpected, prominent role for E2F1 in regulation of redox metabolism after RB loss, driving an increase in the synthesis of the antioxidant glutathione, specific to advanced disease. These E2F1-dependent events resulted in protection from reactive oxygen species in response to therapeutic intervention. On balance, these findings reveal novel pathways through which RB loss promotes cancer progression and highlight potentially new nodes of intervention for treating RB-deficient cancers. SIGNIFICANCE: This study identifies stage-specific consequences of RB loss across cancer progression that have a direct impact on tumor response to clinically utilized therapeutics. The study herein is the first to investigate the effect of RB loss on global metabolic regulation and link RB/E2F1 to redox control in multiple advanced diseases.This article is highlighted in the In This Issue feature, p. 2113.

Differential expression of αVβ3 and αVβ6 integrins in prostate cancer progression

Neuroendocrine prostate cancer (NEPrCa) arises de novo or after accumulation of genomic alterations in pre-existing adenocarcinoma tumors in response to androgen deprivation therapies. We have provided evidence that small extracellular vesicles released by PrCa cells and containing the αVβ3 integrin promote neuroendocrine differentiation of PrCa in vivo and in vitro. Here, we examined αVβ3 integrin expression in three murine models carrying a deletion of PTEN (SKO), PTEN and RB1 (DKO), or PTEN, RB1 and TRP53 (TKO) genes in the prostatic epithelium; of these three models, the DKO and TKO tumors develop NEPrCa with a gene signature comparable to those of human NEPrCa. Immunostaining analysis of SKO, DKO and TKO tumors shows that αVβ3 integrin expression is increased in DKO and TKO primary tumors and metastatic lesions, but absent in SKO primary tumors. On the other hand, SKO tumors show higher levels of a different αV integrin, αVβ6, as compared to DKO and TKO tumors. These results are confirmed by RNA-sequencing analysis. Moreover, TRAMP mice, which carry NEPrCa and adenocarcinoma of the prostate, also have increased levels of αVβ3 in their NEPrCa primary tumors. In contrast, the αVβ6 integrin is only detectable in the adenocarcinoma areas. Finally, analysis of 42 LuCaP patient-derived xenografts and primary adenocarcinoma samples shows a positive correlation between αVβ3, but not αVβ6, and the neuronal marker synaptophysin; it also demonstrates that αVβ3 is absent in prostatic adenocarcinomas. In summary, we demonstrate that αVβ3 integrin is upregulated in NEPrCa primary and metastatic lesions; in contrast, the αVβ6 integrin is confined to adenocarcinoma of the prostate. Our findings suggest that the αVβ3 integrin, but not αVβ6, may promote a shift in lineage plasticity towards a NE phenotype and might serve as an informative biomarker for the early detection of NE differentiation in prostate cancer.

Molecular underpinnings of RB status as a biomarker of poor outcome in advanced prostate cancer

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Background: There is emergent and compelling evidence to support RB status as a biomarker in advanced prostate cancer. RB loss is strongly associated with poor progression-free, disease-specific, and overall survival in prostate cancer (PCa). Preclinical studies in PCa have revealed RB positive tumors are more responsive to CDK4/6 inhibitors. An ongoing randomized Phase IB/II study of enzalutamide with and without ribociclib in patients with metastatic castration-resistant, chemotherapy naïve PCa has become a pioneer trial to include a positive RB status as inclusion criteria in a PCa study (NCT02555189). Beyond CDK4/6 inhibitors, therapeutic agents that target tumor metabolism have been introduced in the clinic. Current data suggests that RB status may be crucial to understand and predict therapeutic response to these agents within tumors. Methods: The biological significance of RB loss was studied utilizing isogenic model systems and human tumor xenografts of castration resistant prostate cancer (CRPC) with and without RB deletion. The mechanism that drives aggressive tumor phenotypes was identified through comprehensive transcriptomic, cistromic, and metabolomic analysis. Novel functions of RB were identified and the response to clinically-relevant therapeutics was examined. Results: Exclusively in CRPC, RB loss results in significant rewiring of cancer cell metabolism. Functional investigation revealed a causative link between RB loss and antioxidant production sufficient to alter responsiveness to genomic insult and selected chemotherapeutics. Observed changes in response to therapeutic intervention were attributed to RB-dependent modulation of intracellular reactive oxygen species. Conclusions: RB loss is strongly associated with poor outcome in advanced PCa. Molecular investigation identified RB-dependent rewiring of cancer cell metabolism as a significant consequence of RB loss, sufficient to alter response in model systems to therapeutic strategies of clinical relevance. These studies significantly advance understanding of the means by which RB loss enhances lethal tumor phenotypes, and are of relevance for development of RB status as a clinically actionable biomarker.

Double Trouble: Concomitant RB1 and BRCA2 Depletion Evokes Aggressive Phenotypes

Coordinate single- or two copy loss of the BRCA2/RB1 tumor suppressor genes, which reside in close chromosomal proximity, were found to be associated with aggressive prostate cancer and therapeutic resistance. Modeling these events and analyses of human cancers suggest that dual depletion of BRCA2/RB1 may represent a distinct subtype of disease.See related article by Chakraborty et al., p. 2047.

USP22 Functions as an Oncogenic Driver in Prostate Cancer by Regulating Cell Proliferation and DNA Repair

Emerging evidence indicates the deubiquitinase USP22 regulates transcriptional activation and modification of target substrates to promote pro-oncogenic phenotypes. Here, in vivo characterization of tumor-associated USP22 upregulation and unbiased interrogation of USP22-regulated functions in vitro demonstrated critical roles for USP22 in prostate cancer. Specifically, clinical datasets validated that USP22 expression is elevated in prostate cancer, and a novel murine model demonstrated a hyperproliferative phenotype with prostate-specific USP22 overexpression. Accordingly, upon overexpression or depletion of USP22, enrichment of cell-cycle and DNA repair pathways was observed in the USP22-sensitive transcriptome and ubiquitylome using prostate cancer models of clinical relevance. Depletion of USP22 sensitized cells to genotoxic insult, and the role of USP22 in response to genotoxic insult was further confirmed using mouse adult fibroblasts from the novel murine model of USP22 expression. As it was hypothesized that USP22 deubiquitylates target substrates to promote protumorigenic phenotypes, analysis of the USP22-sensitive ubiquitylome identified the nucleotide excision repair protein, XPC, as a critical mediator of the USP22-mediated response to genotoxic insult. Thus, XPC undergoes deubiquitylation as a result of USP22 function and promotes USP22-mediated survival to DNA damage. Combined, these findings reveal unexpected functions of USP22 as a driver of protumorigenic phenotypes and have significant implications for the role of USP22 in therapeutic outcomes. SIGNIFICANCE: The studies herein present a novel mouse model of tumor-associated USP22 overexpression and implicate USP22 in modulation of cellular survival and DNA repair, in part through regulation of XPC.

Pleiotropic Impact of DNA-PK in Cancer and Implications for Therapeutic Strategies

PURPOSE

DNA-dependent protein kinase catalytic subunit (DNA-PK) is a pleiotropic kinase involved in DNA repair and transcriptional regulation. DNA-PK is deregulated in selected cancer types and is strongly associated with poor outcome. The underlying mechanisms by which DNA-PK promotes aggressive tumor phenotypes are not well understood. Here, unbiased molecular investigation in clinically relevant tumor models reveals novel functions of DNA-PK in cancer.Experimental Design: DNA-PK function was modulated using both genetic and pharmacologic methods in a series of in vitro models, in vivo xenografts, and patient-derived explants (PDE), and the impact on the downstream signaling and cellular cancer phenotypes was discerned. Data obtained were used to develop novel strategies for combinatorial targeting of DNA-PK and hormone signaling pathways.

RESULTS

Key findings reveal that (i) DNA-PK regulates tumor cell proliferation; (ii) pharmacologic targeting of DNA-PK suppresses tumor growth both in vitro, in vivo, and ex vivo; (iii) DNA-PK transcriptionally regulates the known DNA-PK-mediated functions as well as novel cancer-related pathways that promote tumor growth; (iv) dual targeting of DNA-PK/TOR kinase (TORK) transcriptionally upregulates androgen signaling, which can be mitigated using the androgen receptor (AR) antagonist enzalutamide; (v) cotargeting AR and DNA-PK/TORK leads to the expansion of antitumor effects, uncovering the modulation of novel, highly relevant protumorigenic cancer pathways; and (viii) cotargeting DNA-PK/TORK and AR has cooperative growth inhibitory effects in vitro and in vivo.

CONCLUSIONS

These findings uncovered novel DNA-PK transcriptional regulatory functions and led to the development of a combinatorial therapeutic strategy for patients with advanced prostate cancer, currently being tested in the clinical setting.

Abstract 5217: RB loss reprograms AR and E2F1 signaling in models of prostate cancer progression

Prostatic adenocarcinoma (PCa) is the most frequently diagnosed non-cutaneous malignancy, and the third leading cause of cancer death in males in the United States. A crucial component to the development and progression of PCa is the activity of the androgen receptor (AR). As such, targeting the AR-signaling axis through androgen deprivation therapy (ADT) is the first line of therapy for PCa. However, cells invariably become resistant to this therapy and men relapse with the incurable form of the disease termed, castration-resistant prostate cancer (CRPC). In addition to AR, another principal component aiding in the progression of disease is the retinoblastoma tumor suppressor protein (RB). RB functions to repress tumor development by negatively regulating the activity of the E2F family of transcription factors, preventing cell cycle progression. RB is lost in roughly 30% of CRPC tumors and is sufficient to induce a CRPC phenotype in hormone-sensitive cells under ADT conditions. Analyses into the molecular significance of RB loss on disease progression identified a potential cooperation between AR and the RB-E2F1 signaling axis. Biological assessment performed in isogenic RB knockdown (i.e. hormone-sensitive and CRPC models) identified distinct functional consequences of RB loss depending on AR status and disease state. Transcriptome analysis identified divergently regulated gene signatures between disease stages in the presence of AR activation, which were not seen under ADT conditions, implicating a unique role of AR in transcriptional regulation with the loss of RB. Data to be discussed will also include further comparison of the E2F1 and AR cistromes in the absence of RB to identify the mechanism by which the AR-RB-E2F1 signaling axis function in promoting the progression of disease.

Citation Format: Amy C. Mandigo, Chris McNair, Matthew J. Schiewer, Karen E. Knudsen. RB loss reprograms AR and E2F1 signaling in models of prostate cancer progression [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 5217.

Synergistic effects of the PARP inhibitor olaparib and pharmacological ascorbate in castration-resistant prostate cancer

326

Background: The administration of ascorbate has proved lethal to and highly selective for a variety of cancer cell types; however, despite an increasingly impressive body of evidence, there has not been a robust effort to translate the observed in vitro and in vivo outcomes to the clinic. This is partially due to the fact that the mechanism by which ascorbate exerts its anti-cancer effect is still under investigation. A simplified model depicts ascorbate as a pro-drug for reactive oxygen species (ROS), which accumulate intracellularly and generate DNA damage. It was therefore hypothesized that poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi), by inhibiting DNA damage repair, would augment the toxicity of ascorbate. Methods: In vitro and in vivo models systems queried for anti-tumor effects of PARP inhibitors and ascorbate. Results: Two distinct castration-resistant prostate cancer (CRPC) models were sensitive to ascorbate at physiologically attainable concentrations. These in vitro models were then subjected to treatment with three different PARP inhibitors (olaparib, niraparib, and talazoparib) alone and in combination with ascorbate. The addition of a sub-lethal dose of ascorbate significantly increased cell death across a range of doses for all three PARP inhibitors. A combination index was generated for olaparib and ascorbate in both CRPC models; the results suggest a strongly synergistic relationship between olaparib and ascorbate. Use of a CRPC in vivo model demonstrated that the combination of olaparib and ascorbate significantly increased tumor doubling time compared to vehicle controls and monotherapy. This in vivo efficacy was even more profound in an additional model using castrated mice to mimic the effect of hormone therapy. Additional mechanistic studies are in progress to further investigate the potential for ascorbate and olaparib combination therapy. Conclusions: Ultimately, these data suggest the combination of ascorbate and PARP inhibitors could be an effective treatment for CRPC.

PARP-1 regulation of DNA repair factor availability

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Background: PARP-1 holds major functions on chromatin, DNA damage repair and transcriptional regulation, both of which are relevant in the context of cancer. Previously, it was determined that PARP-1 ins involved in regulation of androgen receptor activity. Methods: Here, unbiased transcriptional profiling revealed the downstream transcriptional profile of PARP-1 enzymatic activity.Results: Further investigation of the PARP-1-regulated transcriptome and secondary strategies for assessing PARP-1 activity in patient tissues revealed that PARP-1 activity was unexpectedly enriched as a function of disease progression and was associated with poor outcome independent of DNA double-strand breaks, suggesting that enhanced PARP-1 activity may promote aggressive phenotypes. Mechanistic investigation revealed that active PARP-1 served to enhance E2F1 transcription factor activity, and specifically promoted E2F1-mediated induction of DNA repair factors involved in homologous recombination (HR). Conversely, PARP-1 inhibition reduced HR factor availability and thus acted to induce or enhance “BRCA-ness”. Conclusions: These observations bring new understanding of PARP-1 function in cancer and have significant ramifications on predicting PARP-1 inhibitor function in the clinical setting.

PARP-1 regulates DNA repair factor availability

PARP-1 holds major functions on chromatin, DNA damage repair and transcriptional regulation, both of which are relevant in the context of cancer. Here, unbiased transcriptional profiling revealed the downstream transcriptional profile of PARP-1 enzymatic activity. Further investigation of the PARP-1-regulated transcriptome and secondary strategies for assessing PARP-1 activity in patient tissues revealed that PARP-1 activity was unexpectedly enriched as a function of disease progression and was associated with poor outcome independent of DNA double-strand breaks, suggesting that enhanced PARP-1 activity may promote aggressive phenotypes. Mechanistic investigation revealed that active PARP-1 served to enhance E2F1 transcription factor activity, and specifically promoted E2F1-mediated induction of DNA repair factors involved in homologous recombination (HR). Conversely, PARP-1 inhibition reduced HR factor availability and thus acted to induce or enhance "BRCA-ness". These observations bring new understanding of PARP-1 function in cancer and have significant ramifications on predicting PARP-1 inhibitor function in the clinical setting.

Abstract B040: Differential impact of RB status on E2F1 reprogramming in human cancer

Recent examination of advanced prostate cancer (PCa) has suggested a major mechanism of progression to castration-resistant disease (CRPC) to be loss of the retinoblastoma (RB) protein. Along with its critical role in controlling cell cycle progression, RB is known to have important tumor-suppressor functions, and has been shown in PCa to be lost exclusively in late-stage disease. Additionally, loss of RB has been shown to correlate with increased E2F1 transcript and protein expression, via E2F-dependent mechanisms. Despite the vital role RB loss has been shown to play in this fatal stage of disease, the molecular underpinnings remain undefined. Thus, in order to elucidate these CRPC specific alterations, the current study utilizes isogenic models of RB loss in combination with genome-wide binding and transcriptional studies. Data presented herein demonstrate that loss of RB is frequent in CRPC, and represents the main mechanism of RB pathways disruption in PCa as detected through analyses of tumor samples and cell-free DNA. However, this phenomenon is not correlated with changes in proliferative indices, suggesting a role for RB loss outside of canonical cell cycle control. Further, RB loss induces significant genome-wide transcriptional alterations, including upregulation in Myc, E2F, and DNA-repair related pathways. Additionally, loss of RB significantly expands E2F1 binding capacity in castrate conditions, while largely maintaining the RB-intact E2F1 cistrome. Strikingly, while the current RB/E2F1 paradigm suggests that E2F1 exclusively occupies promoter regions of DNA in order to regulate transcriptional changes, RB loss induces marked reprogramming of E2F1 occupied regions, with a distinct increase in enhancer-bound E2F1. Further, motif analyses suggest divergence away from canonical E2F1 binding motifs after RB loss, specifically in regions of expanded E2F1 binding, and additionally suggest likely interaction of novel E2F1 cofactors under RB loss conditions. Interestingly, changes in E2F1 binding capacity after RB loss were seen to be distinct from those detected after androgen-induced RB inactivation, suggesting that the molecular alterations underlying RB loss are discrete from those resulting from functional inactivation. With respect to putative mechanism, it is of note that chromatin accessibility was not significantly altered to sufficiently explain the widespread changes in E2F1 cistrome, regardless of RB status, suggesting a mechanism outside simple opportunistic E2F1 binding after RB loss. Finally, interrogation of a CRPC patient tumor cohort showed predictive capacity for an “Expanded E2F1 Signature,” resulting from genes exhibiting gained E2F1 binding and differential expression after RB loss, in predicting loss of RB in patient samples, and indicating a novel E2F1-driven set of targets vital for CRPC transition in human disease. Together, these data present the first insight into E2F1 activity resulting from RB loss, and the role these changes play in progression to CRPC.

Citation Format: Christopher McNair, Kexin Xu, Amy C. Mandigo, Matteo Benelli, Benjamin Leiby, Daniel Rodrigues, Johan Lindberg, Henrik Gronberg, Mateus Crespo, Bram De Laere, Luc Dirix, Tapio Visakorpi, Fugen Li, Felix Y. Feng, Johann de Bono, Francesca Demichelis, Mark A. Rubin, Myles Brown, Karen E. Knudsen. Differential impact of RB status on E2F1 reprogramming in human cancer [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 B040.

Abstract IA03: Differential impact of RB pathway status on E2F1 reprogramming and disease progression in human prostate cancer

The retinoblastoma tumor suppressor (RB) is mechanistically linked to suppression of E2F1-mediated cell cycle regulation. Abrogation of RB function is associated with poor clinical outcome across various tumor types, which frequently elicit a preference for either RB depletion or functional inactivation, yet the basis for selectivity is unknown. Here, examination of RB pathway alterations in advanced prostate cancer revealed that cyclin dependent kinase (CDK)/cyclin/CDKi alterations are infrequent, and identify RB loss as the major mechanism of pathway disruption in human disease. Furthermore, RB status was readily traced through cell-free DNA analyses in human specimens, thus identifying new ways to assign RB status in the clinical setting. Strikingly, RB depletion in human disease was not associated with a higher Ki67 index, indicating a role for the RB/E2F1 pathway in regulating processes distinct from cell cycle control and associated with lethal-stage disease. Subsequent mechanistic investigation utilized isogenic prostate cancer models, wherein RB could be differentially inactivated through depletion or through hormone-induced, CDK-mediated phosphorylation. Unbiased molecular interrogation uncovered a novel E2F1 cistrome and downstream engagement of transcriptional networks exclusively observed after RB loss, with binding specificity divergent from canonically described E2F1 binding patterns. Additionally, E2F1 cistrome alterations elicited by RB depletion were seen to be distinct from those after phosphorylation-induced RB functional inactivation, providing needed insight into the basis of selectivity for RB loss versus CDK-mediated inactivation observed in human disease. Analyses of human CRPC tumor samples further underscored the clinical relevance of RB loss-induced gene expression programs, which were significantly correlated with reprogrammed E2F1 binding identified herein. Taken together, the studies presented are the first to identify the consequences of RB loss, demonstrating molecular distinction from RB inactivation and illustrating the clinical relevance of RB loss-induced E2F rewiring.

Citation Format: Christopher McNair, Kexin Xu, Amy C. Mandigo, Matteo Benelli, Benjamin Leiby, Daniel Rodrigues, Johan Lindberg, Henrik Gronberg, Bram De Laere, Luc Dirix, Tapio Visakorpi, Fugen Li, Felix Y. Feng, Johann de Bono, Francesca Demichelis, Mark A Rubin, Myles Brown, Karen E. Knudsen. Differential impact of RB pathway status on E2F1 reprogramming and disease progression in human prostate cancer [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 IA03.

Differential impact of RB status on E2F1 reprogramming in human cancer

The tumor suppressor protein retinoblastoma (RB) is mechanistically linked to suppression of transcription factor E2F1-mediated cell cycle regulation. For multiple tumor types, loss of RB function is associated with poor clinical outcome. RB action is abrogated either by direct depletion or through inactivation of RB function; however, the basis for this selectivity is unknown. Here, analysis of tumor samples and cell-free DNA from patients with advanced prostate cancer showed that direct RB loss was the preferred pathway of disruption in human disease. While RB loss was associated with lethal disease, RB-deficient tumors had no proliferative advantage and exhibited downstream effects distinct from cell cycle control. Mechanistically, RB loss led to E2F1 cistrome expansion and different binding specificity, alterations distinct from those observed after functional RB inactivation. Additionally, identification of protumorigenic transcriptional networks specific to RB loss that were validated in clinical samples demonstrated the ability of RB loss to differentially reprogram E2F1 in human cancers. Together, these findings not only identify tumor-suppressive functions of RB that are distinct from cell cycle control, but also demonstrate that the molecular consequence of RB loss is distinct from RB inactivation. Thus, these studies provide insight into how RB loss promotes disease progression, and identify new nodes for therapeutic intervention.

Abstract LB-086: PARP-1 controls the DNA damage response by regulating E2F1 transcriptional activity

PARP-1 holds two major functions on chromatin, DNA damage repair and transcriptional regulation, both of which are relevant in the context of cancer. Notably, PARP-1 has been found to be a key modulator of androgen receptor (AR) function and AR-dependent phenotypes, which is a driving factor in prostate cancer (PCa) biology and therapeutic management. Recent studies indicate an unanticipated prevalence of DNA repair alterations in advanced PCa and showed that PARP-1 inhibitors (PARPi) can effectively manage of a subset of these tumors. Despite the functions of PARP-1 in DNA repair having been exploited as a therapeutic target for tumors with BRCA1/2 aberrations, factors beyond DNA repair alterations clearly play a role in the response to PARPi. Notably, in the TO-PARP trial, not all patients with DNA repair aberrations responded to PARPi; conversely, tumors lacking BRCA1/2 or other DNA repair alterations show objective response to PARPi in PCa and other tumor types. These clinical data suggest that the genetic (e.g. BRCA-ness) and pharmacologic interplay is complex in the context of PARPi. Given the preclinical and clinical data, pursuing a deeper understanding of the molecular underpinnings of PARPi action in PCa may yield significant benefit. Genome-wide transcriptional profiling in response to PARPi was performed and the PARP-1-regulated transcriptome was identified. Human tissue microarrays were utilized to quantify PARP-1 levels and activity as a function of PCa progression. Both the PARP-1-regulated transcriptome, as well as PARP-1 enzymatic activity, were found to be elevated as a function of PCa progression. Further interrogation of the PARP-1-regulated transcriptome revealed a major impact on E2F1-regulated genes, and chromatin immunoprecipitation analyses indicated that PARP-1 functions to regulate the chromatin architecture and E2F1 occupancy at E2F1 target gene loci. Most prominent among the E2F1-regulated genes responsive to PARPi were genes associated with DNA damage repair, with a particular enrichment for genes involved in homologous recombination (HR). In sum, these data indicate PARP-1 regulates the function of key oncogenic transcription factors (AR and E2F1) in PCa, and part of the effect of PARPi may be through down-regulation of DNA repair factors.

Citation Format: Matthew J. Schiewer, Amy C. Mandigo, Nicolas Gordon, Fangjin Huang, Sanchaika Gaur, Shuang Zhao, Joseph Evans, Sumin Han, Theodore Parsons, Ruth Birbe, Peter McCue, Tapio Visakorpi, Ganesh Raj, Mark Rubin, Johann de Bono, Costas Lallas, Edouard Trabulsi, Leonard G. Gomella, Adam P. Dicker, Kevin Kelly, Beatrice Knudsen, Felix Feng, Karen E. Knudsen. PARP-1 controls the DNA damage response by regulating E2F1 transcriptional activity [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 LB-086. doi:10.1158/1538-7445.AM2017-LB-086

Abstract LB-087: Stage specific re-calibration of E2F1 function after RB loss

Prostate cancer (PCa) is the most frequently diagnosed non-cutaneous malignancy, and the third leading cause of cancer death in males in the United States. The development of the disease is reliant on the activity of the androgen receptor (AR), which promotes proliferation and is required for cell growth and survival. Deregulation of AR leads to the progression of PCa to the incurable form of disease termed castration-resistant prostate cancer (CRPC). One of the underlying mechanisms of AR deregulation is the loss of function of the retinoblastoma (RB) tumor suppressor protein. RB loss is almost exclusively seen in CRPC and is sufficient to induce a CRPC phenotype in primary PCa models. However, RB function is only lost in roughly 30% of tumors. The remaining 70% of tumors that progress to CRPC do so via other mechanisms and retain RB function. A subset of RB positive CRPC tumors progress to lose RB function even in this post-CRPC transition stage of disease. Biological assessment performed in isogenic RB knockdown, both hormone-sensitive and CRPC models, identified distinct functional consequences of RB loss depending on disease state. Transcriptome analysis identified opposing regulated gene signatures between disease stages. Further investigation of the E2F1 cistrome revealed differential E2F1 binding with RB loss in hormone sensitive and CRPC models, identifying distinct roles of E2F1, in the absence of RB, in different disease states. Data to be discussed reveal the stage-specific molecular pathways driven by E2F1 upon RB loss with the potential to promote disease progression.

Citation Format: Amy C. Mandigo, Christopher McNair, Kexin Xu, Fugen Li, Matthew J. Schiewer, Myles Brown, Karen E. Knudsen. Stage specific re-calibration of E2F1 function after RB loss [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 LB-087. doi:10.1158/1538-7445.AM2017-LB-087

Abstract A08: PARP1-mediated E2F1 regulation of DNA repair capacity

PARP1 holds two major functions on chromatin, DNA damage repair and transcriptional regulation, both of which are relevant in the context of cancer. Notably, PARP1 has been found to be a key modulator of androgen receptor (AR) function and AR-dependent phenotypes, which is a driving factor in prostate cancer (PCa) biology and therapeutic management. Recent studies indicate an unanticipated prevalence of DNA repair alterations in advanced PCa and showed that PARP1 inhibitors (PARPi) can effectively manage of a subset of these tumors. Despite the functions of PARP1 in DNA repair having been exploited as a therapeutic target for tumors with BRCA1/2 aberrations, factors beyond DNA repair alterations clearly play a role in the response to PARPi. Notably, in the TO-PARP trial, not all patients with DNA repair aberrations responded to PARPi; conversely, tumors lacking BRCA1/2 or other DNA repair alterations show objective response to PARPi in PCa and other tumor types. These clinical data suggest that the genetic (e.g. BRCA-ness) and pharmacologic interplay is complex in the context of PARPi. Given the preclinical and clinical data, pursuing a deeper understanding of the molecular underpinnings of PARPi action in PCa may yield significant benefit. Genome-wide transcriptional profiling in response to PARPi was performed and the PARP1-regulated transcriptome was identified. Both the PARP1-regulated transcriptome, as well as PARP1 enzymatic activity were found to be elevated as a function of PCa progression. Further interrogation of the PARP1-regulated transcriptome revealed a major impact on E2F1-regulated genes, and chromatin immunoprecipitation analyses indicated that PARP1 functions to regulate the chromatin architecture and E2F1 occupancy at E2F1 target gene loci. Most prominent among the E2F1-regulated genes responsive to PARPi were genes associated with DNA damage repair, with a particular enrichment for genes involved in homologous recombination (HR). In sum, these data indicate PARP1 regulates function of key oncogenic transcription factors (AR and E2F1) in PCa, and part of the effect of PARPi may be through down-regulation of DNA repair factors.

Citation Format: Matthew J. Schiewer, Amy C. Mandigo, Nicholas Gordon, Sumin Han, Shuang Zhao, Joseph Evans, Theodore Parsons, Ruth Birbe, Peter McCue, Tapio Visakorpi, Ganesh Raj, Mark Rubin, Johann de Bono, Costas Lallas, Edouard Trabulsi, Leonard G. Gomella, Adam P. Dicker, Wm. Kevin Kelly, Felix Y. Feng, Karen E. Knudsen. PARP1-mediated E2F1 regulation of DNA repair capacity [abstract]. In: Proceedings of the AACR Special Conference on DNA Repair: Tumor Development and Therapeutic Response; 2016 Nov 2-5; Montreal, QC, Canada. Philadelphia (PA): AACR; Mol Cancer Res 2017;15(4_Suppl):Abstract nr A08.

Chemical contamination of soils in the New York City area following Hurricane Sandy

This paper presents a unique data set of lead, arsenic, polychlorinated biphenyl (PCB), and polycyclic aromatic hydrocarbon (PAH) concentrations in soil samples collected from the metropolitan New York City area in the aftermath of Hurricane Sandy. Initial samples were collected by citizen scientists recruited via social media, a relatively unusual approach for a sample collection project. Participants in the affected areas collected 63 usable samples from basements, gardens, roads, and beaches. Results indicate high levels of arsenic, lead, PCBs, and PAHs in an area approximately 800 feet south of the United States Environmental Protection Agency (US EPA) Superfund site at Newtown Creek. A location adjacent to the Gowanus Canal, another Superfund site, was found to have high PCB concentrations. Areas of high PAH contamination tended to be near high traffic areas or next to sites of known contamination. While contamination as a direct result of Hurricane Sandy cannot be demonstrated conclusively, the presence of high levels of contamination close to known contamination sites, evidence for co-contamination, and decrease in number of samples containing measureable amounts of semi-volatile compounds from samples collected at similar locations 9 months after the storm suggest that contaminated particles may have migrated to residential areas as a result of flooding.