Single Cell Analysis of Treatment-Resistant Prostate Cancer: Implications of Cell State Changes for Cell Surface Antigen Targeted Therapies

Targeting cell surface molecules using radioligand and antibody-based therapies has yielded considerable success across cancers. However, it remains unclear how the expression of putative lineage markers, particularly cell surface molecules, varies in the process of lineage plasticity, wherein tumor cells alter their identity and acquire new oncogenic properties. A notable example of lineage plasticity is the transformation of prostate adenocarcinoma (PRAD) to neuroendocrine prostate cancer (NEPC)--a growing resistance mechanism that results in the loss of responsiveness to androgen blockade and portends dismal patient survival. To understand how lineage markers vary across the evolution of lineage plasticity in prostate cancer, we applied single cell analyses to 21 human prostate tumor biopsies and two genetically engineered mouse models, together with tissue microarray analysis (TMA) on 131 tumor samples. Not only did we observe a higher degree of phenotypic heterogeneity in castrate-resistant PRAD and NEPC than previously anticipated, but also found that the expression of molecules targeted therapeutically, namely PSMA, STEAP1, STEAP2, TROP2, CEACAM5, and DLL3, varied within a subset of gene-regulatory networks (GRNs). We also noted that NEPC and small cell lung cancer (SCLC) subtypes shared a set of GRNs, indicative of conserved biologic pathways that may be exploited therapeutically across tumor types. While this extreme level of transcriptional heterogeneity, particularly in cell surface marker expression, may mitigate the durability of clinical responses to novel antigen-directed therapies, its delineation may yield signatures for patient selection in clinical trials, potentially across distinct cancer types.

Characterization of HOXB13 expression patterns in localized and metastatic castration-resistant prostate cancer

HOXB13 is a key lineage homeobox transcription factor that plays a critical role in the differentiation of the prostate gland. Several studies have suggested that HOXB13 alterations may be involved in prostate cancer development and progression. Despite its potential biological relevance, little is known about the expression of HOXB13 across the disease spectrum of prostate cancer. To this end, we validated a HOXB13 antibody using genetic controls and investigated HOXB13 protein expression in murine and human developing prostates, localized prostate cancers, and metastatic castration-resistant prostate cancers. We observed that HOXB13 expression increases during later stages of murine prostate development. All localized prostate cancers showed HOXB13 protein expression. Interestingly, lower HOXB13 expression levels were observed in higher-grade tumors, although no significant association between HOXB13 expression and recurrence or disease-specific survival was found. In advanced metastatic prostate cancers, HOXB13 expression was retained in the majority of tumors. While we observed lower levels of HOXB13 protein and mRNA levels in tumors with evidence of lineage plasticity, 84% of androgen receptor-negative castration-resistant prostate cancers and neuroendocrine prostate cancers (NEPCs) retained detectable levels of HOXB13. Notably, the reduced expression observed in NEPCs was associated with a gain of HOXB13 gene body CpG methylation. In comparison to the commonly used prostate lineage marker NKX3.1, HOXB13 showed greater sensitivity in detecting advanced metastatic prostate cancers. Additionally, in a cohort of 837 patients, 383 with prostatic and 454 with non-prostatic tumors, we found that HOXB13 immunohistochemistry had a 97% sensitivity and 99% specificity for prostatic origin. Taken together, our studies provide valuable insight into the expression pattern of HOXB13 during prostate development and cancer progression. Furthermore, our findings support the utility of HOXB13 as a diagnostic biomarker for prostate cancer, particularly to confirm the prostatic origin of advanced metastatic castration-resistant tumors. © 2023 The Pathological Society of Great Britain and Ireland.

PTEN-regulated PI3K-p110 and AKT isoform plasticity controls metastatic prostate cancer progression

PTEN loss, one of the most frequent mutations in prostate cancer (PC), is presumed to drive disease progression through AKT activation. However, two transgenic PC models with Akt activation plus Rb loss exhibited different metastatic development: Pten/RbPE:-/- mice produced systemic metastatic adenocarcinomas with high AKT2 activation, whereas RbPE:-/- mice deficient for the Src-scaffolding protein, Akap12, induced high-grade prostatic intraepithelial neoplasias and indolent lymph node dissemination, correlating with upregulated phosphotyrosyl PI3K-p85α. Using PC cells isogenic for PTEN, we show that PTEN-deficiency correlated with dependence on both p110β and AKT2 for in vitro and in vivo parameters of metastatic growth or motility, and with downregulation of SMAD4, a known PC metastasis suppressor. In contrast, PTEN expression, which dampened these oncogenic behaviors, correlated with greater dependence on p110α plus AKT1. Our data suggest that metastatic PC aggressiveness is controlled by specific PI3K/AKT isoform combinations influenced by divergent Src activation or PTEN-loss pathways.

Abstract 2606: Requirement of MDM2 E3 ligase activity for regulating p53 during normal development, cell cycle regulation and genome integrity

p53 tumor suppressor, the most mutated gene in human cancer, is negatively regulated by MDM2 and MDM4 proteins. Two distinct mechanisms of MDM2/MDM4 regulation of p53 include, one, suppression of p53 transactivation activity by direct interaction of MDM2/MDM4 with p53 protein, thus blocking its transactivation interface and two, targeting p53 to proteasomal degradation by MDM2 E3 ligase activity. However, the importance of these mechanisms in normal p53 regulation in vivo has remained controversial. To genetically separate these two mechanisms of p53 regulation we generated a novel Mdm2 mutant mouse (Mdm2L466A) that lacks E3 ligase activity but retains the ability of Mdm2 to heterodimerize with Mdm4 and inhibit p53 transactivation. Homozygous Mdm2L466A mice are embryonic lethal due to dysregulated p53 activity, thus demonstrating Mdm2 E3 ligase mediated regulation of p53 is essential during embryonic development. Additionally, we uncovered novel p53-independent functions of MDM2 E3 ligase in cell cycle regulation and genome integrity in cells. Cells lacking MDM2 E3 ligase activity have defective G2-M transition during cell cycle and developed elevated levels of aneuploidy regardless of p53 status. This study unequivocally demonstrates the requirement of Mdm2 E3 ligase activity for normal regulation of p53 and uncovers previously unknown p53-independent role of Mdm2 in cell cycle regulation and maintaining genome integrity. A current therapeutic approach involves blocking MDM2-p53 interaction to promote p53 tumor suppression function in cancer cells. Our findings suggest that this approach alone is insufficient because MDM2 has other potential oncogenic mechanisms independent of its role as negative regulator of p53. Hence, targeting MDM2 E3 ligase activity is likely to be more effective cancer therapy as it addresses both p53-dependent and p53-independent oncogenic mechanisms of MDM2.

Citation Format: Meenalakshmi Chinnam, Rati Lama, Chao Xu, Xiaojing Zhang, Carlos Cedeno, Yanqing Wang, Aimee B. Stablewski, David W. Goodrich, Xinjiang Wang. Requirement of MDM2 E3 ligase activity for regulating p53 during normal development, cell cycle regulation and genome integrity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2606.

PTEN Loss Expands the Histopathologic Diversity and Lineage Plasticity of Lung Cancers Initiated by Rb1/Trp53 Deletion

INTRODUCTION

High-grade neuroendocrine tumors of the lung such as SCLC are recalcitrant cancers for which more effective systemic therapies are needed. Despite their histopathologic and molecular heterogeneity, they are generally treated as a single disease entity with similar chemotherapy regimens. Whereas marked clinical responses can be observed, they are short-lived. Inter- and intratumoral heterogeneity is considered a confounding factor in these unsatisfactory clinical outcomes, yet the origin of this heterogeneity and its impact on therapeutic responses is not well understood.

METHODS

New genetically engineered mouse models are used to test the effects of PTEN loss on the development of lung tumors initiated by Rb1 and Trp53 tumor suppressor gene deletion.

RESULTS

Complete PTEN loss drives more rapid tumor development with a greater diversity of tumor histopathology ranging from adenocarcinoma to SCLC. PTEN loss also drives transcriptional heterogeneity as marked lineage plasticity is observed within histopathologic subtypes. Spatial profiling indicates transcriptional heterogeneity exists both within and among tumor foci with transcriptional patterns correlating with spatial position, implying that the growth environment influences gene expression.

CONCLUSIONS

These results identify PTEN loss as a clinically relevant genetic alteration driving the molecular and histopathologic heterogeneity of neuroendocrine lung tumors initiated by Rb1/Trp53 mutations.

The NOGO receptor NgR2, a novel αVβ3 integrin effector, induces neuroendocrine differentiation in prostate cancer

Androgen deprivation therapies aimed to target prostate cancer (PrCa) are only partially successful given the occurrence of neuroendocrine PrCa (NEPrCa), a highly aggressive and highly metastatic form of PrCa, for which there is no effective therapeutic approach. Our group has demonstrated that while absent in prostate adenocarcinoma, the αVβ3 integrin expression is increased during PrCa progression toward NEPrCa. Here, we show a novel pathway activated by αVβ3 that promotes NE differentiation (NED). This novel pathway requires the expression of a GPI-linked surface molecule, NgR2, also known as Nogo-66 receptor homolog 1. We show here that NgR2 is upregulated by αVβ3, to which it associates; we also show that it promotes NED and anchorage-independent growth, as well as a motile phenotype of PrCa cells. Given our observations that high levels of αVβ3 and, as shown here, of NgR2 are detected in human and mouse NEPrCa, our findings appear to be highly relevant to this aggressive and metastatic subtype of PrCa. This study is novel because NgR2 role has only minimally been investigated in cancer and has instead predominantly been analyzed in neurons. These data thus pave new avenues toward a comprehensive mechanistic understanding of integrin-directed signaling during PrCa progression toward a NE phenotype.

Lineage plasticity in prostate cancer depends on JAK/STAT inflammatory signaling

Drug resistance in cancer is often linked to changes in tumor cell state or lineage, but the molecular mechanisms driving this plasticity remain unclear. Using murine organoid and genetically engineered mouse models, we investigated the causes of lineage plasticity in prostate cancer and its relationship to antiandrogen resistance. We found that plasticity initiates in an epithelial population defined by mixed luminal-basal phenotype and that it depends on increased Janus kinase (JAK) and fibroblast growth factor receptor (FGFR) activity. Organoid cultures from patients with castration-resistant disease harboring mixed-lineage cells reproduce the dependency observed in mice by up-regulating luminal gene expression upon JAK and FGFR inhibitor treatment. Single-cell analysis confirms the presence of mixed-lineage cells with increased JAK/STAT (signal transducer and activator of transcription) and FGFR signaling in a subset of patients with metastatic disease, with implications for stratifying patients for clinical trials.

Abstract 1093: BRD9 inhibition overcomes epithelial to mesenchymal transition (EMT)-associated tyrosine kinase inhibitor (TKI) tolerance in epidermal growth factor receptor (EGFR) mutant lung cancer

Advanced lung cancer patients that present with activating mutations in the epidermal growth factor receptor (EGFR) are treated with EGFR tyrosine kinase inhibitors (EGFR TKIs). While initially effective, all patients eventually develop therapeutic resistance and experience disease progression. Strategies to prevent EGFR TKI resistance are needed to improve patient outcomes. We chronically exposed H1975 cells (EGFR-L858R/T790M) to EGFR TKI osimertinib to study emergence of resistance. H1975 cells that were expanded under drug treatment (designated H1975OR) acquired an EMT phenotype and were re-sensitized to EGFR TKI upon prolonged drug withdrawal. These features led us to classify H1975OR as a model of drug tolerance rather than a model of stable drug resistance. Bulk RNA-sequencing revealed significant dysregulation of chromatin modifying genes in H1975OR. Bromodomain containing protein 9 (BRD9) was among the set of significantly upregulated chromatin regulators and was selected for further investigation as a mediator of drug tolerance. Although BRD9 is known to control stemness and EMT, its role in promoting EMT-associated TKI resistance is unknown. To test the contribution of BRD9 to EGFR TKI tolerance, pharmacological inhibition of BRD9 by the selective inhibitor, I-BRD9 significantly increased sensitivity to TKI in models with EMT phenotypes (IC50 reduced 3-4 fold). In contrast, I-BRD9 did not affect TKI sensitivity in two models where EGFR TKI resistance was genetically fixed. To gain mechanistic insights, H1975OR cells were treated with osimertinib +/-I-BRD9 and subjected to RNA-sequencing. Combination of osimertinib with I-BRD9 resulted in a significant decrease in EMT-related genes, including MMP9, Zeb2, PDGFRb and IL6. Further, use of I-BRD9 in these models diminished the mesenchymal phenotype, as measured in cell invasion assays. Genetic knockdown of BRD9 via shRNA phenocopied effects of I-BRD9 treatment, supporting BRD9 as the therapeutic target of I-BRD9 in our cell line models. To further establish a role for BRD9 in the emergence of drug tolerant cells, we exposed treatment naïve H1975 cells to EGFR TKI ± I-BRD9. I-BRD9 significantly decreased the size of the EGFR TKI tolerant population. In time course studies, I-BRD9 also delayed onset of TKI resistance. In conclusion, our data identifies BRD9 as a novel mediator of EMT-associated EGFR-TKI tolerance in EGFR-mutant lung cancer. Our data further implicates BRD9 inhibition as a novel strategy to delay the emergence of drug tolerant cells that eventually give rise to stable drug resistance. This work was supported by the Roswell Park Alliance Foundation and National Cancer Institute (NCI) grant P30CA016056 involving the use of Roswell Park Comprehensive Cancer Center’s Genomics and Bioinformatics Shared Resources.

Citation Format: Hannah Calkins, Tatiana Shaurova, David W. Goodrich, Mukund Seshadri, Candace S. Johnson, Pamela A. Hershberger. BRD9 inhibition overcomes epithelial to mesenchymal transition (EMT)-associated tyrosine kinase inhibitor (TKI) tolerance in epidermal growth factor receptor (EGFR) mutant lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1093.

A mitochondrial unfolded protein response inhibitor suppresses prostate cancer growth in mice via HSP60

Mitochondrial proteostasis, regulated by the mitochondrial unfolded protein response (UPR^mt), is crucial for maintenance of cellular functions and survival. Elevated oxidative and proteotoxic stress in mitochondria must be attenuated by the activation of a ubiquitous UPR^mt to promote prostate cancer (PCa) growth. Here we show that the 2 key components of the UPR^mt, heat shock protein 60 (HSP60, a mitochondrial chaperonin) and caseinolytic protease P (ClpP, a mitochondrial protease), were required for the development of advanced PCa. HSP60 regulated ClpP expression via c-Myc and physically interacted with ClpP to restore mitochondrial functions that promote cancer cell survival. HSP60 maintained the ATP-producing functions of mitochondria, which activated the β-catenin pathway and led to the upregulation of c-Myc. We identified a UPR^mt inhibitor that blocked HSP60’s interaction with ClpP and abrogated survival signaling without altering HSP60’s chaperonin function. Disruption of HSP60-ClpP interaction with the UPR^mt inhibitor triggered metabolic stress and impeded PCa-promoting signaling. Treatment with the UPR^mt inhibitor or genetic ablation of Hsp60 inhibited PCa growth and progression. Together, our findings demonstrate that the HSP60-ClpP–mediated UPR^mt is essential for prostate tumorigenesis and the HSP60-ClpP interaction represents a therapeutic vulnerability in PCa.

Correction: MDM2 E3 ligase activity is essential for p53 regulation and cell cycle integrity

[This corrects the article DOI: 10.1371/journal.pgen.1010171.].

Single-Cell Analyses of a Novel Mouse Urothelial Carcinoma Model Reveal a Role of Tumor-Associated Macrophages in Response to Anti-PD-1 Therapy

Approximately 80% of patients with advanced bladder cancer do not respond to immune checkpoint inhibitor (ICI) immunotherapy. Therefore, there is an urgent unmet need to develop clinically relevant preclinical models so that factors governing immunotherapy responses can be studied in immunocompetent mice. We developed a line of mouse triple knockout (TKO: Trp53, Pten, Rb1) urothelial carcinoma organoids transplanted into immunocompetent mice. These bladder tumors recapitulate the molecular phenotypes and heterogeneous immunotherapy responses observed in human bladder cancers. The TKO organoids were characterized in vivo and in vitro and compared to the widely used MB49 murine bladder cancer model. RNAseq analysis of the TKO tumors demonstrated a basal subtype. The TKO xenografts demonstrated the expression of urothelial markers (CK5, CK7, GATA3, and p63), whereas MB49 subcutaneous xenografts did not express urothelial markers. Anti-PD-1 immunotherapy resulted in a mixed pattern of treatment responses for individual tumors. Eight immune cell types were identified (basophils, B cells, dendritic cells, macrophages, monocytes, neutrophils, NK cells, and T cells) in ICI-treated xenografts. Responder xenografts displayed significantly increased immune cell infiltration (15.3%, 742 immune cells/4861 total cells) compared to the non-responder tumors (10.1%, 452 immune cells/4459 total cells, Fisher Exact Test p < 0.0001). Specifically, there were more T cells (1.0% vs. 0.4%, p = 0.002) and macrophages (8.6% vs. 6.4%, p = 0.0002) in responder xenografts than in non-responder xenografts. In conclusion, we have developed a novel preclinical model that exhibits a mixed pattern of response to anti-PD-1 immunotherapy. The higher percentage of macrophage tumor infiltration in responders suggests a potential role for the innate immune microenvironment in regulating ICI treatment responses.

Ex Vivo Organoid Model of Adenovirus-Cre Mediated Gene Deletions in Mouse Urothelial Cells

Bladder cancer is an understudied area, particularly in genetically engineered mouse models (GEMMs). Inbred GEMMs with tissue-specific Cre and loxP sites have been the gold standards for conditional or inducible gene targeting. To provide faster and more efficient experimental models, an ex vivo organoid culture system is developed using adenovirus Cre and normal urothelial cells carrying multiple loxP alleles of the tumor suppressors Trp53, Pten, and Rb1. Normal urothelial cells are enzymatically disassociated from four bladders of triple floxed mice (Trp53f/f: Ptenf/f: Rb1f/f). The urothelial cells are transduced ex vivo with adenovirus-Cre driven by a CMV promoter (Ad5CMVCre). The transduced bladder organoids are cultured, propagated, and characterized in vitro and in vivo. PCR is used to confirm gene deletions in Trp53, Pten, and Rb1. Immunofluorescence (IF) staining of organoids demonstrates positive expression of urothelial lineage markers (CK5 and p63). The organoids are injected subcutaneously into host mice for tumor expansion and serial passages. The immunohistochemistry (IHC) of xenografts exhibits positive expression of CK7, CK5, and p63 and negative expression of CK8 and Uroplakin 3. In summary, adenovirus-mediated gene deletion from mouse urothelial cells engineered with loxP sites is an efficient method to rapidly test the tumorigenic potential of defined genetic alterations.

A Preclinical Study to Repurpose Spironolactone for Enhancing Chemotherapy Response in Bladder Cancer

Neoadjuvant chemotherapy (NAC) followed by radical cystectomy is the standard-of-care for patients with muscle-invasive bladder cancer (MIBC). Defects in nucleotide excision repair (NER) are associated with improved responses to NAC. Excision Repair Cross-Complementation group 3 (ERCC3) is a key component of NER process. No NER inhibitors are available for treating patients with bladder cancer. We have developed an ex vivo cell-based assay of 6-4 pyrimidine-pyrimidinone (6-4PP) removal as a surrogate measure of NER capacity in human bladder cancer cell lines. The protein expression of ERCC3 was examined in human MIBC specimens and cell lines. Small molecule inhibitors were screened for NER inhibition in bladder cancer cell lines. Spironolactone was identified as a potent NER inhibitor. Combined effects of spironolactone with chemo-drugs were evaluated in vitro and in vivo. The efficacy between platinum and spironolactone on cytotoxicity was determined by combination index. A correlation between NER capacity and cisplatin sensitivity was demonstrated in a series of bladder cancer cell lines. Further, siRNA-mediated knockdown of ERCC3 abrogated NER capacity and enhanced cisplatin cytotoxicity. Spironolactone inhibited ERCC3 protein expression, abrogated NER capacity, and increased platinum-induced cytotoxicity in bladder cancer cells in vivo and in patient-derived organoids. Moreover, spironolactone exhibited the potential synergism effects with other clinical chemotherapy regimens in bladder cancer cell lines. Our data support the notion of repurposing spironolactone for improving the chemotherapy response of NAC in patients with MIBC. Further clinical trials are warranted to determine the safety and efficacy of spironolactone in combination with chemotherapy.

MDM2 E3 ligase activity is essential for p53 regulation and cell cycle integrity

MDM2 and MDM4 are key regulators of p53 and function as oncogenes when aberrantly expressed. MDM2 and MDM4 partner to suppress p53 transcriptional transactivation and polyubiquitinate p53 for degradation. The importance of MDM2 E3-ligase-mediated p53 regulation remains controversial. To resolve this, we generated mice with an Mdm2 L466A mutation that specifically compromises E2 interaction, abolishing MDM2 E3 ligase activity while preserving its ability to bind MDM4 and suppress p53 transactivation. Mdm2^L466A/L466A mice exhibit p53-dependent embryonic lethality, demonstrating MDM2 E3 ligase activity is essential for p53 regulation in vivo. Unexpectedly, cells expressing Mdm2^L466A manifest cell cycle G2-M transition defects and increased aneuploidy even in the absence of p53, suggesting MDM2 E3 ligase plays a p53-independent role in cell cycle regulation and genome integrity. Furthermore, cells bearing the E3-dead MDM2 mutant show aberrant cell cycle regulation in response to DNA damage. This study uncovers an uncharacterized role for MDM2’s E3 ligase activity in cell cycle beyond its essential role in regulating p53’s stability in vivo.

The most frequently mutated protein in human cancer, the p53 tumor suppressor protein, is negatively regulated by the potentially oncogenic proteins MDM2 and MDM4. MDM2/MDM4 regulates p53 through two mechanisms, MDM2 E3 ubiquitin ligase activity marks p53 for degradation while MDM2/MDM4 can bind p53 to inhibit its ability to promote RNA transcription. Whether these mechanisms contribute to normal p53 regulation in vivo remains controversial. Using a newly developed mouse model that genetically separates these two mechanisms, we find that mice expressing MDM2 deficient specifically for E3 ubiquitin ligase activity do not survive embryonic development because unregulated p53 is lethal. In contrast to prior reports, MDM2 E3 ubiquitin ligase activity is thus required for p53 regulation during embryonic development. In addition, cells lacking MDM2 E3 ubiquitin ligase activity have cell cycle defects regardless of p53 status, uncovering a p53-independent function for MDM2 in regulating the cell cycle. Activating p53 by blocking physical interaction with MDM2/MDM4 is one currently pursued approach for cancer therapy, but this approach does not account for cancer-promoting activities of MDM2/MDM4 independent of p53. Findings reported here suggest targeting MDM2 E3 ligase activity directly may be advantageous as it would inhibit both p53-dependent and p53-independent oncogenic mechanisms.

Retinoblastoma Protein Paralogs and Tumor Suppression

The retinoblastoma susceptibility gene (RB1) is the first tumor suppressor gene discovered and a prototype for understanding regulatory networks that function in opposition to oncogenic stimuli. More than 3 decades of research has firmly established a widespread and prominent role for RB1 in human cancer. Yet, this gene encodes but one of three structurally and functionally related proteins that comprise the pocket protein family. A central question in the field is whether the additional genes in this family, RBL1 and RBL2, are important tumor suppressor genes. If so, how does their tumor suppressor activity overlap or differ from RB1. Here we revisit these questions by reviewing relevant data from human cancer genome sequencing studies that have been rapidly accumulating in recent years as well as pertinent functional studies in genetically engineered mice. We conclude that RBL1 and RBL2 do have important tumor suppressor activity in some contexts, but RB1 remains the dominant tumor suppressor in the family. Given their similarities, we speculate on why RB1 tumor suppressor activity is unique.

Binary pan-cancer classes with distinct vulnerabilities defined by pro- or anti-cancer YAP/TEAD activity

Cancer heterogeneity impacts therapeutic response, driving efforts to discover over-arching rules that supersede variability. Here, we define pan-cancer binary classes based on distinct expression of YAP and YAP-responsive adhesion regulators. Combining informatics with in vivo and in vitro gain- and loss-of-function studies across multiple murine and human tumor types, we show that opposite pro- or anti-cancer YAP activity functionally defines binary YAP^on or YAP^off cancer classes that express or silence YAP, respectively. YAP^off solid cancers are neural/neuroendocrine and frequently RB1^-/-, such as retinoblastoma, small cell lung cancer, and neuroendocrine prostate cancer. YAP silencing is intrinsic to the cell of origin, or acquired with lineage switching and drug resistance. The binary cancer groups exhibit distinct YAP-dependent adhesive behavior and pharmaceutical vulnerabilities, underscoring clinical relevance. Mechanistically, distinct YAP/TEAD enhancers in YAP^off or YAP^on cancers deploy anti-cancer integrin or pro-cancer proliferative programs, respectively. YAP is thus pivotal across cancer, but in opposite ways, with therapeutic implications.

Abstract 2418: PTEN regulated PI3K-p110 and AKT isoform plasticity controls metastatic prostate cancer progression

Background: The loss of PTEN function, one of the most frequent mutations in prostate cancer (CaP), is presumed to drive disease progression through AKT activation. However, two transgenic mouse CaP models, Pten/RbPE:-/- and Akap12-/-;RbPE:-/-, exhibited different CaP and metastasis development even though both shared Rb loss plus Akt activation, based on increased relative AKTpoS473 levels.

Experimental Procedures: We analyzed whether PI3K-p110 and/or AKT isoform reliance, using isoform-specific RNAi or small molecule inhibitors, played a role in the different CaP outcomes.

Findings: In comparison to Akap12/Rb-null high-grade prostatic intraepithelial neoplasias, Pten/Rb-null adenocarcinomas had increased protein and activation levels of AKT2 and decreased relative levels of Smad4, a known CaP metastasis suppressor. Comparison of isogenic PTEN +/- pairs of human or mouse CaP cell lines indicated that PTEN-deficiency correlated with dependence on both p110β and AKT2 for survival or oncogenic growth/invasiveness, whereas PTEN expression correlated with greater dependence on AKT1 plus p110α. Inhibition of AKT2 in PTEN-null CaP cells induced higher SMAD4 levels and decreased chemotaxis and Matrigel invasiveness. Given that PTEN likely regulates non-redundant roles of the AKT isoforms during CaP progression, we sought to characterize AKT isoform substrates by phosphoproteomics analysis. Specifically, after endogenous kinases were inactivated in mouse Pten/Rb-null adenocarcinoma cell lysates using 5'-fluorosulfonylbenzoyl-5'-adenosine (FSBA), dialyzed samples were subjected to in vitro kinase reactions using enzymatically-active, full-length baculovirus-expressed AKT1, AKT2 or AKT3, or as a negative control, a heat-inactivated mixture of all three isoforms. Tryptic peptides isolated on TiO2 beads were subjected to LC-MS/MS. Data will be shown on the role of AKT isoform-shared vs. -preferred substrates in specific parameters of CaP progression controlled by PTEN.

Conclusion: Our data suggest that in the context of RB1 loss, PTEN controls a signaling plasticity and drug sensitivity through specific pairings of PI3K-p110 and AKT isoforms.

Citation Format: Karina Miller, Seamus Degan, Yanqing Wang, Sheng-Yu Ku, David W. Goodrich, Irwin H. Gelman. PTEN regulated PI3K-p110 and AKT isoform plasticity controls metastatic prostate cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2418.

Posttranslational regulation of FOXA1 by Polycomb and BUB3/USP7 deubiquitin complex in prostate cancer

Forkhead box protein A1 (FOXA1) is essential for androgen-dependent prostate cancer (PCa) growth. However, how FOXA1 levels are regulated remains elusive and its therapeutic targeting proven challenging. Here, we report FOXA1 as a nonhistone substrate of enhancer of zeste homolog 2 (EZH2), which methylates FOXA1 at lysine-295. This methylation is recognized by WD40 repeat protein BUB3, which subsequently recruits ubiquitin-specific protease 7 (USP7) to remove ubiquitination and enhance FOXA1 protein stability. They functionally converge in regulating cell cycle genes and promoting PCa growth. FOXA1 is a major therapeutic target of the inhibitors of EZH2 methyltransferase activities in PCa. FOXA1-driven PCa growth can be effectively mitigated by EZH2 enzymatic inhibitors, either alone or in combination with USP7 inhibitors. Together, our study reports EZH2-catalyzed methylation as a key mechanism to FOXA1 protein stability, which may be leveraged to enhance therapeutic targeting of PCa using enzymatic EZH2 inhibitors.

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.

Pan-cancer molecular analysis of the RB tumor suppressor pathway

The retinoblastoma tumor suppressor gene (RB1) plays a critical role in coordinating multiple pathways that impact cancer initiation, disease progression, and therapeutic responses. Here we probed molecular features associated with the RB-pathway across 31 tumor-types. While the RB-pathway has been purported to exhibit multiple mutually exclusive genetic events, only RB1 alteration is mutually exclusive with deregulation of CDK4/6 activity. An ER+ breast cancer model with targeted RB1 deletion was used to identify signatures of CDK4/6 activity and RB-dependency (CDK4/6-RB integrated signature). This signature was prognostic in tumor-types with gene expression features indicative of slower growth. Single copy loss on chromosome 13q encompassing the RB1 locus is prevalent in many cancers, yielding reduced expression of multiple genes in cis, and is inversely related to the CDK4/6-RB integrated signature supporting a cause-effect relationship. Genes that are positively and inversely correlated with the CDK4/6-RB integrated signature define new tumor-specific pathways associated with RB-pathway activity.

Understanding Lineage Plasticity as a Path to Targeted Therapy Failure in EGFR-Mutant Non-small Cell Lung Cancer

Somatic alterations in the epidermal growth factor receptor gene (EGFR) result in aberrant activation of kinase signaling and occur in ∼15% of non-small cell lung cancers (NSCLC). Patients diagnosed with EGFR-mutant NSCLC have good initial clinical response to EGFR tyrosine kinase inhibitors (EGFR TKIs), yet tumor recurrence is common and quick to develop. Mechanisms of acquired resistance to EGFR TKIs have been studied extensively over the past decade. Great progress has been made in understanding two major routes of therapeutic failure: additional genomic alterations in the EGFR gene and activation of alternative kinase signaling (so-called “bypass activation”). Several pharmacological agents aimed at overcoming these modes of EGFR TKI resistance are FDA-approved or under clinical development. Phenotypic transformation, a less common and less well understood mechanism of EGFR TKI resistance is yet to be addressed in the clinic. In the context of acquired EGFR TKI resistance, phenotypic transformation encompasses epithelial to mesenchymal transition (EMT), transformation of adenocarcinoma of the lung (LUAD) to squamous cell carcinoma (SCC) or small cell lung cancer (SCLC). SCLC transformation, or neuroendocrine differentiation, has been linked to inactivation of TP53 and RB1 signaling. However, the exact mechanism that permits lineage switching needs further investigation. Recent reports indicate that LUAD and SCLC have a common cell of origin, and that trans-differentiation occurs under the right conditions. Options for therapeutic targeting of EGFR-mutant SCLC are limited currently to conventional genotoxic chemotherapy. Similarly, the basis of EMT-associated resistance is not clear. EMT is a complex process that can be characterized by a spectrum of intermediate states with diverse expression of epithelial and mesenchymal factors. In the context of acquired resistance to EGFR TKIs, EMT frequently co-occurs with bypass activation, making it challenging to determine the exact contribution of EMT to therapeutic failure. Reversibility of EMT-associated resistance points toward its epigenetic origin, with additional adjustments, such as genetic alterations and bypass activation, occurring later during disease progression. This review will discuss the mechanistic basis for EGFR TKI resistance linked to phenotypic transformation, as well as challenges and opportunities in addressing this type of targeted therapy resistance in EGFR-mutant NSCLC.

Vitamin D3 Metabolites Demonstrate Prognostic Value in EGFR-Mutant Lung Adenocarcinoma and Can be Deployed to Oppose Acquired Therapeutic Resistance

EGFR tyrosine kinase inhibitors (EGFR TKIs) are the standard of care treatment for patients with EGFR-mutant lung adenocarcinoma (LUAD). Although initially effective, EGFR TKIs are not curative. Disease inevitably relapses due to acquired drug resistance. We hypothesized that vitamin D metabolites could be used with EGFR TKIs to prevent therapeutic failure. To test this idea, we investigated the link between serum 25-hydroxyvitamin D3 (25(OH)D3) and progression-free survival (PFS) in patients with EGFR-mutant LUAD that received EGFR TKIs (erlotinib n = 20 and afatinib n = 1). Patients who were 25(OH)D3-sufficient experienced significantly longer benefit from EGFR TKI therapy (mean 14.5 months) than those with 25(OH)D3 insufficiency (mean 10.6 months, p = 0.026). In contrast, 25(OH)D3 had no prognostic value in patients with KRAS-mutant LUAD that received cytotoxic chemotherapy. To gain mechanistic insights, we tested 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) activity in vitro. 1,25(OH)2D3 promoted epithelial differentiation and restored EGFR TKI sensitivity in models of EGFR TKI resistance that were associated with epithelial–mesenchymal transition (EMT). 1,25(OH)2D3 was ineffective in a non-EMT model of resistance. We conclude that vitamin D sufficiency portends increased PFS among EGFR-mutant LUAD patients that receive EGFR TKIs, and that vitamin D signaling maintains drug efficacy in this specific patient subset by opposing EMT.

The Role of Lineage Plasticity in Prostate Cancer Therapy Resistance

Lineage plasticity has emerged as an important mechanism of treatment resistance in prostate cancer. Treatment-refractory prostate cancers are increasingly associated with loss of luminal prostate markers, and in many cases induction of developmental programs, stem cell-like phenotypes, and neuroendocrine/neuronal features. Clinically, lineage plasticity may manifest as low PSA progression, resistance to androgen receptor (AR) pathway inhibitors, and sometimes small cell/neuroendocrine pathologic features observed on metastatic biopsy. This mechanism is not restricted to prostate cancer as other malignancies also demonstrate lineage plasticity during resistance to targeted therapies. At present, there is no established therapeutic approach for patients with advanced prostate cancer developing lineage plasticity or small cell neuroendocrine prostate cancer (NEPC) due to knowledge gaps in the underlying biology. Few clinical trials address questions in this space, and the outlook for patients remains poor. To move forward, urgently needed are: (i) a fundamental understanding of how lineage plasticity occurs and how it can best be defined; (ii) the temporal contribution and cooperation of emerging drivers; (iii) preclinical models that recapitulate biology of the disease and the recognized phenotypes; (iv) identification of therapeutic targets; and (v) novel trial designs dedicated to the entity as it is defined. This Perspective represents a consensus arising from the NCI Workshop on Lineage Plasticity and Androgen Receptor-Independent Prostate Cancer. We focus on the critical questions underlying lineage plasticity and AR-independent prostate cancer, outline knowledge and resource gaps, and identify strategies to facilitate future collaborative clinical translational and basic studies in this space.

Generation of Tumor Organoids from Genetically Engineered Mouse Models of Prostate Cancer

Methods based on homologous recombination to modify genes have significantly furthered biological research. Genetically engineered mouse models (GEMMs) are a rigorous method for studying mammalian development and disease. Our laboratory has developed several GEMMs of prostate cancer (PCa) that lack expression of one or multiple tumor suppressor genes using the site-specific Cre-loxP recombinase system and a prostate-specific promoter. In this article, we describe our method for necropsy of these PCa GEMMs, primarily focusing on dissection of mouse prostate tumors. New methods developed over the last decade have facilitated the culture of epithelial-derived cells to model organ systems in vitro in three dimensions. We also detail a 3D cell culture method to generate tumor organoids from mouse PCa GEMMs. Pre-clinical cancer research has been dominated by 2D cell culture and cell line-derived or patient-derived xenograft models. These methods lack tumor microenvironment, a limitation of using these techniques in pre-clinical studies. GEMMs are more physiologically-relevant for understanding tumorigenesis and cancer progression. Tumor organoid culture is an in vitro model system that recapitulates tumor architecture and cell lineage characteristics. In addition, 3D cell culture methods allow for growth of normal cells for comparison to tumor cell cultures, rarely possible using 2D cell culture techniques. In combination, use of GEMMs and 3D cell culture in pre-clinical studies has the potential to improve our understanding of cancer biology.

Cell Cycle and Beyond: Exploiting New RB1 Controlled Mechanisms for Cancer Therapy

Recent studies highlight the importance of the RB1 tumor suppressor as a target for cancer therapy. Canonically, RB1 regulates cell cycle progression and represents the downstream target for cyclin-dependent kinase (CDK) 4/6 inhibitors that are in clinical use. However, newly discovered features of the RB1 pathway suggest new therapeutic strategies to counter resistance and improve precision medicine. These therapeutic strategies include deepening cell cycle exit with CDK4/6 inhibitor combinations, selectively targeting tumors that have lost RB1, and expanding therapeutic index by mitigating therapy-associated adverse effects. In addition, RB1 impacts immunological features of tumors and the microenvironment that can enhance sensitivity to immunotherapy. Lastly, RB1 specifies epigenetically determined cell lineage states that are disrupted during therapy resistance and could be re-installed through the direct use of epigenetic therapies. Thus, new opportunities are emerging to improve cancer therapy by exploiting the RB1 pathway.

Riluzole induces AR degradation via endoplasmic reticulum stress pathway in androgen-dependent and castration-resistant prostate cancer cells

BACKGROUND

Prostate cancer (PCa) is diagnosed at the highest rate of all non-cutaneous male cancers in the United States. The androgen-dependent (AD) transcription factor, androgen receptor (AR), drives PCa-but inhibiting AR or androgen biosynthesis induces remission for only a short time. At which point, patients acquire more aggressive castration-resistant (CR) disease with re-activated AR-dependent signaling. To combat treatment resistance, down-regulating AR protein expression has been considered as a potential treatment strategy for CR-PCa.

METHODS

AD- and CR-PCa cell lines were treated with the well-tolerated FDA-approved oral medicine, riluzole. Expression of full-length or wild-type AR (AR-FL) and constitutively active AR-splice variant 7 (AR-V7) was assessed by immunoblotting. AR-FL/AR-V7 activity was measured using qRT-PCR of AR-target genes. Cytoplasmic [Ca²⁺ ] levels were measured using a fluorescent Ca²⁺ indicator microplate assay. Markers of the endoplasmic reticulum stress (ERS) pathway and autophagy were assessed by immunoblotting. Direct interaction between AR and selective autophagy receptor p62 was demonstrated by co-immunoprecipitation.

RESULTS

We demonstrate that riluzole downregulates AR-FL, mutant ARs, and AR-V7 proteins expression by protein degradation through ERS pathway and selective autophagy. Riluzole also significantly inhibited AR transcription activity by decreasing its target genes expression (PSA, TMPRSS2, and KLK2).

CONCLUSIONS

We provide key mechanistic insights by which riluzole exerts its anti-tumorigenic effects and induces AR protein degradation via ERS pathways. Our findings support the potential utility of riluzole for treatment of PCa.

Mechanisms Behind Resistance to PI3K Inhibitor Treatment Induced by the PIM Kinase

Cancer resistance to PI3K inhibitor therapy can be in part mediated by increases in the PIM1 kinase. However, the exact mechanism by which PIM kinase promotes tumor cell resistance is unknown. Our study unveils the pivotal control of redox signaling by PIM kinases as a driver of this resistance mechanism. PIM1 kinase functions to decrease cellular ROS levels by enhancing nuclear factor erythroid 2-related factor 2 (NRF2)/antioxidant response element activity. PIM prevents cell death induced by PI3K-AKT-inhibitory drugs through a noncanonical mechanism of NRF2 ubiquitination and degradation and translational control of NRF2 protein levels through modulation of eIF4B and mTORC1 activity. Importantly, PIM also controls NAD(P)H production by increasing glucose flux through the pentose phosphate shunt decreasing ROS production, and thereby diminishing the cytotoxicity of PI3K-AKT inhibitors. Treatment with PIM kinase inhibitors reverses this resistance phenotype, making tumors increasingly susceptible to small-molecule therapeutics, which block the PI3K-AKT pathway.

An approach for controlling the timing and order of engineered mutations in mice

Significant advances in our understanding of normal development and disease have been facilitated by engineered mice in which genes can be altered in a spatially, temporally, or cell type restricted manner using site specific recombinase systems like Cre-loxP or Flp-frt. In many circumstances it is important to understand how interactions between multiple genes influence a given phenotype. Robust approaches for precisely controlling multiple genetic alterations independently are limited, however, thus the impact of mutation order and timing on phenotype is generally unknown. Here we describe and validate a novel Gt(ROSA)26Sor targeted transgene allowing precise control over the order and timing of multiple genetic mutations in the mouse. The transgene expresses an optimized, Flp-estrogen receptor fusion protein (Flpo-ERT2) under the control of a loxP-stop-loxP cassette. In this system, genes modified by loxP sites are altered first upon expression of Cre. Cre also eliminates the loxP-stop-loxP cassette, permitting widespread expression of Flpo-ERT2. Because of the estrogen receptor fusion, Flp activity remains inert until administration of tamoxifen, allowing genes modified by frt sites to be modified subsequently with controllable timing. This mouse transgene will be useful in a wide variety of applications where independent control of different mutations in the mouse is desirable.

Non-canonical functions of the RB protein in cancer

The canonical model of RB-mediated tumour suppression developed over the past 30 years is based on the regulation of E2F transcription factors to restrict cell cycle progression. Several additional functions have been proposed for RB, on the basis of which a non-canonical RB pathway can be described. Mechanistically, the non-canonical RB pathway promotes histone modification and regulates chromosome structure in a manner distinct from cell cycle regulation. These functions have implications for chemotherapy response and resistance to targeted anticancer agents. This Opinion offers a framework to guide future studies of RB in basic and clinical research.

Abstract 3016: Ezh2 is a dose-dependent mediator of prostate cancer aggressiveness and lineage transformation

Resistance to therapies for metastatic prostate cancer (PCa) causes patients to rapidly succumb to their disease, especially when resistance is associated with development of aggressive variant PCa. These PCa variants are found in 20-30% of patient autopsies relapsing from androgen deprivation therapy (ADT) and often lack androgen receptor (AR), aberrantly express alternative lineage markers, and have altered histology and clinical course. Of these PCa variants, neuroendocrine prostate cancer (NEPC) has been the most well-studied. Our recently published data show that Rb1 loss promotes NEPC transformation in the mouse and induces pervasive changes in gene expression characteristic of human NEPC. These changes include increased expression of histone methyltransferase Ezh2, a known PCa oncogene. In addition, treatment with Ezh2 inhibitors restores AR expression and ADT sensitivity in Rb1-deficient PCa. We hypothesize that Ezh2 is a driver of NEPC transformation in Rb1-deficient PCa. To test this hypothesis, we generated a new genetically engineered mouse model (GEMM) of Rb1-deficient PCa that also lacks Ezh2. Preliminary data suggest that overall survival of PBCre4:Ptenf/f:Rb1f/f:Ezh2f/+ (DKOE/+) mice with loss of one Ezh2 allele is increased compared to PBCre4:Ptenf/f:Rb1f/f (DKO). In prostate tumors from DKOE/+ mice, individual glands are apparent, characteristic of adenocarcinoma. Immunohistochemistry shows that expression of luminal markers, including AR, CK8, and CK18, are increased in DKOE/+ compared to DKO. NEPC markers synaptophysin and chromagranin A are decreased in DKOE/+ compared to DKO, suggesting that loss of one Ezh2 allele slows neuroendocrine transformation. To our surprise, overall survival of PBCre4:Ptenf/f:Rb1f/f:Ezh2f/f (DKOE/E) was decreased compared to DKO. Histological analysis of end-stage primary tumors from DKOE/E mice indicate they develop high grade lesions containing sheets of cells with minimal gland formation. These mice develop hemorrhagic ascites and metastases to the liver, lung, kidney, bone, thymus, and lymph node. AR expression in DKOE/E is decreased compared to both DKO and DKOE/+. When organoids were established from DKO, DKOE/+, and DKOE/E end-stage tumors, preliminary analysis suggest that DKOE/E have greater organoid renewal capability. Altogether, initial data from our new GEMMs show that loss of one allele of Ezh2 in DKOE/+ inhibits development of lethal NEPC transformation in Rb1-deficient PCa. Whereas, complete loss of Ezh2 in the context of Rb1-deficiency makes disease more aggressive. These data suggest that Ezh2's effect on PCa progression in the absence of Rb1 is dose-dependent. Future work is required to investigate of the exact mechanism of Ezh2's contribution to PCa aggressiveness and NEPC transformation.

Citation Format: Kristine M. Wadosky, Yanqing Wang, Leigh Ellis, David W. Goodrich. Ezh2 is a dose-dependent mediator of prostate cancer aggressiveness and lineage transformation [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 3016.

TOP2A and EZH2 Provide Early Detection of an Aggressive Prostate Cancer Subgroup

Purpose: Current clinical parameters do not stratify indolent from aggressive prostate cancer. Aggressive prostate cancer, defined by the progression from localized disease to metastasis, is responsible for the majority of prostate cancer-associated mortality. Recent gene expression profiling has proven successful in predicting the outcome of prostate cancer patients; however, they have yet to provide targeted therapy approaches that could inhibit a patient's progression to metastatic disease.Experimental Design: We have interrogated a total of seven primary prostate cancer cohorts (n = 1,900), two metastatic castration-resistant prostate cancer datasets (n = 293), and one prospective cohort (n = 1,385) to assess the impact of TOP2A and EZH2 expression on prostate cancer cellular program and patient outcomes. We also performed IHC staining for TOP2A and EZH2 in a cohort of primary prostate cancer patients (n = 89) with known outcome. Finally, we explored the therapeutic potential of a combination therapy targeting both TOP2A and EZH2 using novel prostate cancer-derived murine cell lines.Results: We demonstrate by genome-wide analysis of independent primary and metastatic prostate cancer datasets that concurrent TOP2A and EZH2 mRNA and protein upregulation selected for a subgroup of primary and metastatic patients with more aggressive disease and notable overlap of genes involved in mitotic regulation. Importantly, TOP2A and EZH2 in prostate cancer cells act as key driving oncogenes, a fact highlighted by sensitivity to combination-targeted therapy.Conclusions: Overall, our data support further assessment of TOP2A and EZH2 as biomarkers for early identification of patients with increased metastatic potential that may benefit from adjuvant or neoadjuvant targeted therapy approaches. Clin Cancer Res; 23(22); 7072-83. ©2017 AACR.

Rb1 and Trp53 cooperate to suppress prostate cancer lineage plasticity, metastasis, and antiandrogen resistance

Prostate cancer relapsing from antiandrogen therapies can exhibit variant histology with altered lineage marker expression, suggesting that lineage plasticity facilitates therapeutic resistance. The mechanisms underlying prostate cancer lineage plasticity are incompletely understood. Studying mouse models, we demonstrate that Rb1 loss facilitates lineage plasticity and metastasis of prostate adenocarcinoma initiated by Pten mutation. Additional loss of Trp53 causes resistance to antiandrogen therapy. Gene expression profiling indicates that mouse tumors resemble human prostate cancer neuroendocrine variants; both mouse and human tumors exhibit increased expression of epigenetic reprogramming factors such as Ezh2 and Sox2. Clinically relevant Ezh2 inhibitors restore androgen receptor expression and sensitivity to antiandrogen therapy. These findings uncover genetic mutations that enable prostate cancer progression; identify mouse models for studying prostate cancer lineage plasticity; and suggest an epigenetic approach for extending clinical responses to antiandrogen therapy.

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.

Evasion of targeted cancer therapy through stem-cell-like reprogramming

Prostate cancer variants expressing alternative lineage markers appear at relapse from antiandrogen therapy. We show that loss of the retinoblastoma (RB1) and tumor protein 53 (TP53) genes drives expression of stem cell reprogramming factors, lineage plasticity, and antiandrogen resistance. Epigenetic manipulation restores antiandrogen sensitivity-suggesting an approach for treating lethal prostate cancers.

Generation of a C57BL/6 MYC-Driven Mouse Model and Cell Line of Prostate Cancer

INTRODUCTION

Transgenic mouse modeling is a favorable tool to reflect human prostate tumorigenesis and interactions between prostate cancer and the microenvironment. The use of GEMMs and derived cell lines represent powerful tools to study prostate cancer initiation and progression with an associated tumor microenvironment. Notably, such models provide the capacity for rapid preclinical therapy studies including immune therapies for prostate cancer treatment.

METHODS

Backcrossing FVB Hi-MYC mice with C57BL/6N mice, we established a Hi-MYC transgenic mouse model on a C57BL/6 background (B6MYC). In addition, using a conditional reprogramming method, a novel C57BL/6 MYC driven prostate adenocarcinoma cell line was generated.

RESULTS

Our results demonstrate that disease progression is significantly delayed in B6MYC when compared to their FVB counterparts. Current data also indicates infiltrating immune cells are present in pre-cancer lesions, prostate intraepithelial neoplasia (PIN). Further, immunophenotyping of this immune infiltrate demonstrates the predominant population as myeloid-derived suppressor cells (MDSC). Also, we successfully generated a B6MYC-CaP cell line, and determined that this new PCa cell line express markers of luminal epithelial lineage.

DISCUSSION

This novel model of PCa provides a new platform to understand the cross talk between MYC driven prostate cancer and the microenvironment. Importantly, these models will be an ideal tool to support the clinical development of immunotherapy as well as other novel therapeutic strategies for prostate cancer treatment. Prostate 76:1192-1202, 2016. © 2016 Wiley Periodicals, Inc.

Combination therapy induces unfolded protein response and cytoskeletal rearrangement leading to mitochondrial apoptosis in prostate cancer

Development of therapeutic resistance is responsible for most prostate cancer (PCa) related mortality. Resistance has been attributed to an acquired or selected cancer stem cell phenotype. Here we report the histone deacetylase inhibitor apicidin (APC) or ER stressor thapsigargin (TG) potentiate paclitaxel (TXL)-induced apoptosis in PCa cells and limit accumulation of cancer stem cells. TXL-induced responses were modulated in the presence of TG with increased accumulation of cells at G1-phase, rearrangement of the cytoskeleton, and changes in cytokine release. Cytoskeletal rearrangement was associated with modulation of the cytoplasmic and mitochondrial unfolded protein response leading to mitochondrial dysfunction and release of proapoptotic proteins from mitochondria. TXL in combination with APC or TG enhanced caspase activation. Importantly, TXL in combination with TG induced caspase activation and apoptosis in X-ray resistant LNCaP cells. Increased release of transforming growth factor-beta (TGF-β) was observed while phosphorylated β-catenin level was suppressed with TXL combination treatments. This was accompanied by a decrease in the CD44(+)CD133(+) cancer stem cell-like population, suggesting treatment affects cancer stem cell properties. Taken together, combination treatment with TXL and either APC or TG induces efficient apoptosis in both proliferating and cancer stem cells, suggesting this therapeutic combination may overcome drug resistance and recurrence in PCa.

Multi-Drug Resistance ABC Transporter Inhibition Enhances Murine Ventral Prostate Stem/Progenitor Cell Differentiation

Multi-drug resistance (MDR)-ATP binding cassette (ABC) transporters, ABCB1, ABCC1, and ABCG2 participate in the efflux of steroid hormones, estrogens, and androgens, which regulate prostate development and differentiation. The role of MDR-ABC efflux transporters in prostate epithelial proliferation and differentiation remains unclear. We hypothesized that MDR-ABC transporters regulate prostate differentiation and epithelium regeneration. Prostate epithelial differentiation was studied using histology, sphere formation assay, and prostate regeneration induced by cycles of repeated androgen withdrawal and replacement. Embryonic deletion of Abcg2 resulted in a decreased number of luminal cells in the prostate and increased sphere formation efficiency, indicating an imbalance in the prostate epithelial differentiation pattern. Decreased luminal cell number in the Abcg2 null prostate implies reduced differentiation. Enhanced sphere formation efficiency in Abcg2 null prostate cells implies activation of the stem/progenitor cells. Prostate regeneration was associated with profound activation of the stem/progenitor cells, indicating the role of Abcg2 in maintaining stem/progenitor cell pool. Since embryonic deletion of Abcg2 may result in compensation by other ABC transporters, pharmacological inhibition of MDR-ABC efflux was performed. Pharmacological inhibition of MDR-ABC efflux enhanced prostate epithelial differentiation in sphere culture and during prostate regeneration. In conclusion, Abcg2 deletion leads to activation of the stem/progenitor cells and enhances differentiating divisions; and pharmacological inhibition of MDR-ABC efflux leads to epithelial differentiation. Our study demonstrates for the first time that MDR-ABC efflux transporter inhibition results in enhanced prostate epithelial cell differentiation.

The Thoc1 ribonucleoprotein and prostate cancer progression

BACKGROUND

The majority of newly diagnosed prostate cancers will remain indolent, but distinguishing between aggressive and indolent disease is imprecise. This has led to the important clinical problem of overtreatment. THOC1 encodes a nuclear ribonucleoprotein whose expression is higher in some cancers than in normal tissue. The hypothesis that THOC1 may be a functionally relevant biomarker that can improve the identification of aggressive prostate cancer has not been tested.

METHODS

THOC1 protein immunostaining was evaluated in a retrospective collection of more than 700 human prostate cancer specimens and the results associated with clinical variables and outcome. Thoc1 was conditionally deleted in an autochthonous mouse model (n = 22 or 23 per genotype) to test whether it is required for prostate cancer progression. All statistical tests were two-sided.

RESULTS

THOC1 protein immunostaining increases with higher Gleason score and more advanced Tumor/Node/Metastasis stage. Time to biochemical recurrence is statistically significantly shorter for cancers with high THOC1 protein (log-rank P = .002, and it remains statistically significantly associated with biochemical recurrence after adjusting for Gleason score, clinical stage, and prostate-specific antigen levels (hazard ratio = 1.61, 95% confidence interval = 1.03 to 2.51, P = .04). Thoc1 deletion prevents prostate cancer progression in mice, but has little effect on normal tissue. Prostate cancer cells deprived of Thoc1 show gene expression defects that compromise cell growth.

CONCLUSIONS

Thoc1 is required to support the unique gene expression requirements of aggressive prostate cancer in mice. In humans, high THOC1 protein immunostaining associates with prostate cancer aggressiveness and recurrence. Thus, THOC1 protein is a functionally relevant molecular marker that may improve the identification of aggressive prostate cancers, potentially reducing overtreatment.

The Thoc1 encoded ribonucleoprotein is required for myeloid progenitor cell homeostasis in the adult mouse

Co-transcriptionally assembled ribonucleoprotein (RNP) complexes are critical for RNA processing and nuclear export. RNPs have been hypothesized to contribute to the regulation of coordinated gene expression, and defects in RNP biogenesis contribute to genome instability and disease. Despite the large number of RNPs and the importance of the molecular processes they mediate, the requirements for individual RNP complexes in mammalian development and tissue homeostasis are not well characterized. THO is an evolutionarily conserved, nuclear RNP complex that physically links nascent transcripts with the nuclear export apparatus. THO is essential for early mouse embryonic development, limiting characterization of the requirements for THO in adult tissues. To address this shortcoming, a mouse strain has been generated allowing inducible deletion of the Thoc1 gene which encodes an essential protein subunit of THO. Bone marrow reconstitution was used to generate mice in which Thoc1 deletion could be induced specifically in the hematopoietic system. We find that granulocyte macrophage progenitors have a cell autonomous requirement for Thoc1 to maintain cell growth and viability. Lymphoid lineages are not detectably affected by Thoc1 loss under the homeostatic conditions tested. Myeloid lineages may be more sensitive to Thoc1 loss due to their relatively high rate of proliferation and turnover.

A transgenic mouse model for early prostate metastasis to lymph nodes

The emergence of recurrent, metastatic prostate cancer following the failure of androgen-deprivation therapy represents the lethal phenotype of this disease. However, little is known regarding the genes and pathways that regulate this metastatic process, and moreover, it is unclear whether metastasis is an early or late event. The individual genetic loss of the metastasis suppressor, SSeCKS/Gravin/AKAP12 or Rb, genes that are downregulated or deleted in human prostate cancer, results in prostatic hyperplasia. Here, we show that the combined loss of Akap12 and Rb results in prostatic intraepithelial neoplasia (PIN) that fails to progress to malignancy after 18 months. Strikingly, 83% of mice with PIN lesions exhibited metastases to draining lymph nodes, marked by relatively differentiated tumor cells expressing markers of basal (p63, cytokeratin 14) and luminal (cytokeratin 8 and androgen receptor) epithelial cells, although none expressed the basal marker, cytokeratin 5. The finding that PIN lesions contain increased numbers of p63/AR-positive, cytokeratin 5-negative basal cells compared with WT or Akap12-/- prostate lobes suggests that these transitional cells may be the source of the lymph node metastases. Taken together, these data suggest that in the context of Rb loss, Akap12 suppresses the oncogenic proliferation and early metastatic spread of basal-luminal prostate tumor cells.

Abstract 3863: Transgenic model for early prostate metastasis to the lymph nodes

Background: The emergence of recurrent, metastatic prostate cancer following the failure of androgen-deprivation therapy represents the lethal phenotype of this disease. However, little is known regarding the genes and pathways that regulate this metastatic process, and moreover, it is unclear whether metastasis is an early or late event. The SSeCKS/Gravin/AKAP12 (“SSeCKS”) gene, previously shown to suppress prostate cancer metastasis upon re-expression (1,2), as well as the tumor suppressor gene, Rb, are either transcriptionally downregulated or deleted in human prostate cancers (3,4). Mouse models deficient in either SSeCKS or Rb exhibit prostatic hyperplasia (5,6). Moreover, SSeCKS-null fibroblasts as well as SSeCKS-null prostates display Rb-dependent premature senescence markers (7). Methods: In order to determine whether the combined loss of AKAP12 and Rb in the prostate synergizes to induce oncogenic progression, Akap12-/- mice were crossed with Pb4-Cre;RbloxP/loxP mice to generate Akap12-/-;RbPE-/− progeny, with the Probasin-Cre passed only through males. Results: The combined loss of SSeCKS and Rb results in prostatic intraepithelial neoplasia (PIN) starting at 6 months of age that fails to progress to malignancy (adenocarcinoma) after 18 months. The PIN lesions were marked by increased Ki-67 proliferation of cytokeratin 8 (CK8), p63-negative luminal cells as well as p63-positive basal cells. Interestingly, these lesions also had increased numbers of androgen-receptor (AR)-positive, p63-positive, CK5-negative cells. There was evidence of reactive stroma including mural hyperplasia and inflammatory cell infiltration. Strikingly, 83% of mice with PIN lesions exhibited metastases to draining lymph nodes (LN), marked by well-differentiated tumors cells expressing markers of basal (p63, CK14) and luminal (CK8 and AR) epithelial cells, although none expressed the basal marker, CK5. PCR-based tests for the deleted floxed Rb allele from laser capture microdissected LN lesions confirmed the prostatic origin of these metastatic cells. The LN lesions showed very limited inflammation, based on very few cells staining with the histiocyte marker, CD68. Conclusions: Taken together, these data suggest that in the context of Rb loss, SSeCKS suppresses the oncogenic proliferation and early metastatic spread of transitional, basal-luminal prostate tumor cells.

Citation Format: Hyun-Kyung Ko, Shin Akakura, Jennifer Peresie, David W. Goodrich, Barbara A. Foster, Irwin H. Gelman. Transgenic model for early prostate metastasis to the lymph nodes. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3863. doi:10.1158/1538-7445.AM2013-3863

The Thoc1 encoded ribonucleoprotein is a substrate for the NEDD4-1 E3 ubiquitin protein ligase

Ribonucleoprotein (RNP) complexes form around nascent RNA during transcription to facilitate proper transcriptional elongation, RNA processing, and nuclear export. RNPs are highly heterogeneous, and different types of RNPs tend to package functionally related transcripts. These observations have inspired the hypothesis that RNP mediated mechanisms help specify coordinated gene expression. This hypothesis is supported by the observation that mutations in RNP components can cause defects in specific developmental pathways. How RNP biogenesis itself is regulated, however, is not well understood. The evolutionarily conserved THO RNP complex functions early during transcription to package nascent transcripts and facilitate subsequent RNP biogenesis. THO deficiency compromises transcriptional elongation as well as RNP mediated events like 3′ end formation and nuclear export for some transcripts. Using molecularly manipulated cells and in vitro reconstituted biochemical reactions, we demonstrate that the essential THO protein component encoded by the Thoc1 gene is poly-ubiquitinated by the NEDD4-1 E3 ubiquitin ligase. Poly-ubiquitinated pThoc1 is degraded by the proteasome. These results indicate THO activity is regulated by the ubiquitin-proteasome pathway, and that this regulation is evolutionarily conserved between yeast and mammals. Manipulation of NEDD4-1 levels has modest effects on Thoc1 protein levels under steady state conditions, but destabilization of Thoc1 protein upon treatment with a transcriptional elongation inhibitor is dependent on NEDD4-1. This suggests NEDD4-1 functions in conjunction with other post-translational mechanisms to regulate Thoc1 protein and THO activity.

Abstract 1850: The role of Thoc1 in tissue homeostasis in the adult mouse

Thoc1 encodes a protein (Thoc1) that interacts with the retinoblastoma protein (Rb). Thoc1 functions in an evolutionary conserved protein complex called THO that is involved in nuclear ribonucleoprotein particle (RNP) formation. Loss of THO function compromises the packaging of nascent RNA, which causes defects in transcription, RNA processing and mRNA export. In mice, Thoc1 is required for embryonic development. It is currently unknown whether Thoc1 is required in adult mice for normal tissue homeostasis. We hypothesize that different tissues will vary in their dependence on Thoc1, possibly based on their proliferative potential. To test this hypothesis, we conditionally deleted Thoc1 in adult mice in a wide variety of tissues using the Tamoxifen/CreER recombination system, and assessed the consequences on tissue homeostasis. Thoc1F/F:Rosa26CreER+/− (test) and Thoc1+/+:Rosa26CreER+/− (control) adult mice were treated with 2 mg/day Tamoxifen for 6 days. Slowly proliferating (liver, kidney, colon) and rapidly proliferating (small intestinal (S.I.)) tissue were collected 24 hours after the last treatment. Histological analysis shows Thoc1 loss disrupts the architecture of the small intestine (S.I.) but does not affect the other tissues examined. The functional unit of the S.I. is the crypt-villus axis. S.I. crypts contain rapidly proliferating stem and progenitor cells that continuously feed the differentiated, non-proliferative villus epithelium. The crypts proliferate so rapidly that the entire S.I. epithelium is replaced every three to five days. Interestingly, Thoc1 loss only causes apoptosis in the S.I. crypts, suggesting disruption of villi architecture following Thoc1 loss is a result of altered crypt function, not a direct effect of Thoc1 loss. Supporting this, the crypts of mice in which Thoc1 has been deleted also have a loss of cell proliferation. While the colon and S.I. have similar tissue architecture, the colon's rate of turnover is much slower. Therefore, it is not surprising the colon is not affected by Thoc1 loss. These findings support the hypothesis that tissues with a high cell proliferative potential require Thoc1. To explore this further, we examined how Thoc1 deletion would affect the highly proliferative hematopoietic system using the same test and control mice as previously listed. Further supporting the hypothesis, Thoc1 loss decreases the number of lymphocytes in the blood, but does not affect spleen and thymus (slowly proliferating tissues) histology. The results of this study suggest highly proliferative tissues that turnover rapidly require Thoc1, most likely to support the gene expression and cell growth necessary for rapid cell division. This hypothesis is significant as it predicts cancer cells will be more sensitive to Thoc1 loss than normal cells, a prediction verified by preliminary results in our lab.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1850. doi:1538-7445.AM2012-1850

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.

Abstract LB-143: The role of Thoc1 in cell differentiation of the small intestine

Thoc1 encodes an RNA binding protein (pThoc1) that was identified due to its interaction with the retinoblastoma tumor suppressor protein (Rb). pThoc1 regulates transcriptional elongation and mRNA export as part of the TREX/THO (Transcription/Export) complex. In vivo, mice lacking Thoc1 fail to develop past the late blastocyst stage and mice hypomorphic for Thoc1 have compromised gametogenesis and are sterile. Elevated levels of pThoc1 occur in multiple cancers (breast, lung, colorectal), in cancer cell lines, and in transformed fibroblasts. In vitro, neoplastic cells appear to be more sensitive to pThoc1 loss than normal cells. These observations lead to the hypothesis that cells with extended replicative potential (i.e. stem, progenitor, & cancer cells) require Thoc1. To test this hypothesis, we are examining the role of Thoc1 in vivo utilizing the crypt-villus axis of the murine small intestine as a model system. We have deleted Thoc1 in the murine small intestine using the Rosa26CreER transgene and the Tamoxifen induced CreER mediated recombination system. Thoc1F/F:Rosa26CreER (test) and Thoc1+/+:Rosa26CreER (control) 22 week old mice were treated with intraperitoneal injections of 1mg/day Tamoxifen for 5 days. Small intestinal tissue was collected 24 hours after the last injection. Histological analysis shows Thoc1 deletion disrupts small intestinal crypt architecture, but does not affect villus architecture. Immunohistochemistry illustrates Thoc1 deletion correlates with an increase in cleaved Caspase 3 and decrease in Ki67 in the crypts. No cleaved Caspase 3 is observed in the differentiated villi. These findings suggest intestinal stem cells (ISCs) and progenitor transit amplifying cells of the highly proliferative intestinal crypts require Thoc1 to a greater extent than the differentiated cells of the villi. Since the presence of Thoc1 appears to be required in the crypts, we investigated how Tamoxifen treatment and Thoc1 deletion would affect the crypt-villus axis over time. Thoc1F/F:Rosa26CreER and Thoc1+/+:Rosa26CreER mice were treated as previously described, however small intestinal tissue was collected 3 weeks after the last Tamoxifen injection. Interestingly, no change in body weight, survival, or crypt-villus architecture was observed. These findings lead us to believe ISCs lacking Thoc1 are at a competitive disadvantage and will eventually be replaced by Thoc1 positive ISCs. Consistent with this, while Thoc1 deletion and pThoc1 loss was readily detected 24 hours after Tamoxifen treatment, it was not three weeks after treatment. Future work involves deleting Thoc1 specifically in ISCs using the Lgr5CreER transgene to allow cell differentiation lineage tracing and cell fate studies of ISCs in which Thoc1 has been deleted. Additionally, since Thoc1 is elevated in multiple tumor types, we will delete Thoc1 in transformed ISCs using the Lgr5CreER transgene in ApcMin/+ mice to study Thoc1 in cancer stem cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-143. doi:10.1158/1538-7445.AM2011-LB-143

Construction of a dual affinity tagged allele of the Rb1 tumor suppressor gene in the mouse

Loss of Rb1 tumor suppressor gene function is involved in the genesis of most human cancers. Novel therapies targeting Rb1 have been slow to develop because of our incomplete understanding of its molecular mechanisms of action. Rb1 protein (pRb) binds a host of cellular genes and proteins, and these molecular interactions mediate its various functions. Given the potential complexity of these molecular interactions and the lack of established methods for pRb purification, it has been difficult to systematically identify gene and protein interactions relevant to tumor suppression in different tissues in vivo. To address this limitation, we have generated a dual affinity tagged Rb1 allele in the mouse. The tagged allele functions as wild type and the encoded protein can be purified by tandem affinity chromatography. This allele will facilitate identification and characterization of native pRb molecular interactions in any tissue accessible in the mouse.

Abstract B24: Thoc1 is required for mouse prostate tumorigenesis initiated by loss of Rb and p53

A large percentage of prostate cancers show either mutational inactivation or deregulation of the Rb tumor suppressor gene. Rb mediates its tumor suppressor function through its association with other cellular proteins. Most of the Rb-binding proteins interact with the A/B pocket domain and the C terminal region. Less than 10 proteins are known to interact with Rb N terminal domain, a domain that appears to be required for normal Rb function in vivo. The Thoc1 gene encodes a protein that binds the Rb N terminal domain. We hypothesize that some of pRb functions in vivo could be mediated in part by interaction with pThoc1. Thoc1 protein has been found to be an essential component of the TREX (transcription/export) complex which is important for mRNP biogenesis and physically couples transcription elongation with RNA processing and export. We have previously reported that E1A/Ras transformed MEFs (mouse embryonic fibroblasts) but not normal MEFs were dependent on Thoc1 for their survival. These observations suggest that tumor cells specifically may be dependent on Thoc1 for survival and Thoc1 may play a role in tumorigenesis. To test our hypothesis, we used a mouse model of prostate cancer where prostatespecific deletion of Rb and p53 genes leads to development of metastatic adenocarcinoma. We find that compound loss of Thoc1, Rb and p53 increased the life-span of mice compared to mice with loss of Rb and p53 alone. Histopathological analyses of prostate tissue showed that initiation of tumorigenesis is delayed in the absence of Thoc1. Tumors that do arise in these mice retain expression of Thoc1. These findings indicate that Thoc1 is required for prostate tumorigenesis. Conditional deletion of Thoc1 alone in mouse prostate does not appear to affect normal prostate development. To test whether Thoc1 is relevant to human prostate cancer, we examined expression of Thoc1 protein in matched normal and prostate tumor tissue cores on tissue microarray. Analysis of nearly 600 patient samples reveals that Thoc1 is significantly overexpressed in tumor tissue compared to normal prostate tissue and pThoc1 levels positively correlated with tumor grade and negatively correlated with biochemical recurrence as indicated by elevated PSA (prostate-specific antigen) levels. Taken together the above findings suggest that Thoc1 is synthetic lethal with the genetic and epigenetic alterations in prostate tumor cells, and hence it may be a potential target for prostate cancer therapy.

Citation Information: Cancer Res 2009;69(23 Suppl):B24.

Relationships of hHpr1/p84/Thoc1 expression to clinicopathologic characteristics and prognosis in non-small cell lung cancer

Nuclear matrix proteins (NMPs) are important diagnostic and prognostic markers in various human cancers. The hHpr1/p84/Thoc1 protein, a key NMP, resides in the nuclear matrix and is involved in the human TREX complex, which is required for regulation of transcription elongation, pre-RNA splicing, and mRNA export of a subset of human genes. Depletion of hHpr1/p84/Thoc1 decreases growth rates in multiple cancer cell lines, and the expression levels of hHpr1/p84/Thoc1 are strongly associated with tumor size and aggressiveness of several human cancers. Little is known about the expression of this protein in human non-small cell lung cancer (NSCLC) and its association with patients' clinicopathologic characteristics and prognosis. We evaluated hHpr1/p84/Thoc1 expression in 133 NSCLC patients by immunohistochemistry of tissue microarrays using paraffin-embedded tumor tissue and we confirmed the tissue staining by Western blot analysis. The prognostic significance of hHpr1/p84/Thoc1 expression in tumor tissue was assessed by the Cox proportional hazards regression model. hHpr1/p84/Thoc1 expression was found in 51% of patients, and was more prevalent in males than females (59% vs 43%, p = 0.07) and in blacks than whites (91% vs 48%, p = 0.009). In survival analysis, hHpr1/p84/Thoc1 expression appeared to be weakly associated with elevated risk of death among patients with stage I tumors (RR = 1.53, 95% CI = 0.85-2.77, p = 0.16), squamous cell carcinomas (RR = 1.75, 95% CI = 0.73-4.21, p = 0.21), and family histories of lung cancer (RR = 1.55, 95% CI = 0.81-2.97, p=0.18), although none of these associations was statistically significant. Thus elevated expression of hHpr1/p84/Thoc1 is common in NSCLC and may have prognostic significance in subgroups of patients. Further studies with larger sample size are needed to elucidate the role of this critical nuclear matrix protein in NSCLC prognosis.

Synergy of p53 and Rb deficiency in a conditional mouse model for metastatic prostate cancer

Pathways mediated by p53 and Rb are frequently altered in aggressive human cancers, including prostate carcinoma. To test directly the roles of p53 and Rb in prostate carcinogenesis, we have conditionally inactivated these genes in the prostate epithelium of the mouse. Inactivation of either p53 or Rb leads to prostatic intraepithelial neoplasia developing from the luminal epithelium by 600 days of age. In contrast, inactivation of both genes results in rapidly developing (median survival, 226 days) carcinomas showing both luminal epithelial and neuroendocrine differentiation. The resulting neoplasms are highly metastatic, resistant to androgen depletion from the early stage of development, and marked with multiple gene expression signatures commonly found in human prostate carcinomas. Interestingly, gains at 4qC3 and 4qD2.2 and loss at 14qA2-qD2 have been consistently found by comparative genomic hybridization. These loci contain such human cancer-related genes as Nfib, L-myc, and Nkx3.1, respectively. Our studies show a critical role for p53 and Rb deficiency in prostate carcinogenesis and identify likely secondary genetic alterations. The new genetically defined model should be particularly valuable for providing new molecular insights into the pathogenesis of human prostate cancer.