Inactivation of Apc in the mouse prostate causes prostate carcinoma

Diverse Landscape of Genetically Engineered Mouse Models: Genomic and Molecular Insights into Prostate Cancer

Prostate cancer (PCa) is a significant health concern for men worldwide and is particularly prevalent in the United States. It is a complex disease presenting different molecular subtypes and varying degrees of aggressiveness. Transgenic/genetically engineered mouse models (GEMMs) greatly enhanced our understanding of the intricate molecular processes that underlie PCa progression and advancement and have offered valuable insights into potential therapeutic targets for this disease. The integration of whole-exome and whole-genome sequencing, along with expression profiling, has played a pivotal role in advancing GEMMs by facilitating the identification of genetic alterations driving PCa development. This review focuses on genetically modified mice classified into the first and second generations of PCa models. We summarize whether models created by manipulating the function of specific genes replicate the consequences of genomic alterations observed in human PCa, including early and later disease stages. We discuss cases where GEMMs did not fully exhibit the expected human PCa phenotypes and possible causes of the failure. Here, we summarize the comprehensive understanding, recent advances, and the strengths and limitations of the GEMMs in advancing our insights into PCa, offering genetic and molecular perspectives for developing novel GEMM models.

Microtubule-binding protein MAP1B regulates interstitial axon branching of cortical neurons via the tubulin tyrosination cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial (or collateral) axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs). This method allows for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. These data suggest a cell-autonomous molecular regulation of cortical neuron axon morphology, in which GSK3β can release a MAP1B-mediated brake on interstitial axon branching upstream of the posttranslational tubulin code.

Molecular classifications of prostate cancer: basis for individualized risk stratification and precision therapy

Tumour classifications play a pivotal role in prostate cancer (PCa) management. It can predict the clinical outcomes of PCa as early as the disease is diagnosed and then guide therapeutic schemes, such as active monitoring, standalone surgical intervention, or surgery supplemented with postoperative adjunctive therapy, thereby circumventing disease exacerbation and excessive treatment. Classifications based on clinicopathological features, such as prostate cancer-specific antigen, Gleason score, and TNM stage, are still the main risk stratification strategies and have played an essential role in standardized clinical decision-making. However, mounting evidence indicates that clinicopathological parameters in isolation fail to adequately capture the heterogeneity exhibited among distinct PCa patients, such as those sharing identical Gleason scores yet experiencing divergent prognoses. As a remedy, molecular classifications have been introduced. Currently, molecular studies have revealed the characteristic genomic alterations, epigenetic modulations, and tumour microenvironment associated with different types of PCa, which provide a chance for urologists to refine the PCa classification. In this context, numerous invaluable molecular classifications have been devised, employing disparate statistical methodologies and algorithmic approaches, encompassing self-organizing map clustering, unsupervised cluster analysis, and multifarious algorithms. Interestingly, the classifier PAM50 was used in a phase-2 multicentre open-label trial, NRG-GU-006, for further validation, which hints at the promise of molecular classification for clinical use. Consequently, this review examines the extant molecular classifications, delineates the prevailing panorama of clinically pertinent molecular signatures, and delves into eight emblematic molecular classifications, dissecting their methodological underpinnings and clinical utility.

MAP1B Regulates Cortical Neuron Interstitial Axon Branching Through the Tubulin Tyrosination Cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs), allowing for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. We propose that MAP1B functions as a brake on axon branching that can be released by GSK3β activation, regulating the tubulin code and thereby playing an integral role in sculpting cortical neuron axon morphology.

Modulation of the canonical Wnt activity by androgen signaling in prostate epithelial basal stem cells

Both the canonical Wnt and androgen receptor (AR) signaling pathways are important for prostate organogenesis and homeostasis. How they crosstalk to regulate prostate stem cell behaviors remains unclear. Here, we show in lineage-tracing mouse models that although Wnt is essential for basal stem cell multipotency, ectopic Wnt activity promotes basal cell over-proliferation and squamous phenotypes, which are counteracted by elevated levels of androgen. In prostate basal cell organoids, dihydrotestosterone (DHT) antagonizes R-spondin-stimulated growth in a concentration-dependent manner. DHT down-regulates the expressions of a Wnt reporter and target genes, and RNA sequencing (RNA-seq) analyses identify Wnt signaling as a key altered pathway. Mechanistically, DHT enhances AR and β-catenin protein binding, and CUT&RUN analyses reveal that ectopic AR sequesters β-catenin away from its Wnt-related cistrome. Our results suggest that an intermediate level of Wnt activity in prostate basal stem cells, achieved via AR-β-catenin interaction, is essential for normal prostate homeostasis.

Autocrine Canonical Wnt Signaling Primes Noncanonical Signaling through ROR1 in Metastatic Castration-Resistant Prostate Cancer

Wnt signaling driven by genomic alterations in genes including APC and CTNNB, which encodes β-catenin, have been implicated in prostate cancer development and progression to metastatic castration-resistant prostate cancer (mCRPC). However, nongenomic drivers and downstream effectors of Wnt signaling in prostate cancer and the therapeutic potential of targeting this pathway in prostate cancer have not been fully established. Here we analyzed Wnt/β-catenin signaling in prostate cancer and identified effectors distinct from those found in other tissues, including aryl hydrocarbon receptor and RUNX1, which are linked to stem cell maintenance, and ROR1, a noncanonical Wnt5a coreceptor. Wnt/β-catenin signaling-mediated increases in ROR1 enhanced noncanonical responses to Wnt5a. Regarding upstream drivers, APC genomic loss, but not its epigenetic downregulation commonly observed in prostate cancer, was strongly associated with Wnt/β-catenin pathway activation in clinical samples. Tumor cell upregulation of the Wnt transporter Wntless (WLS) was strongly associated with Wnt/β-catenin pathway activity in primary prostate cancer but also associated with both canonical and noncanonical Wnt signaling in mCRPC. IHC confirmed tumor cell WLS expression in primary prostate cancer and mCRPC, and patient-derived prostate cancer xenografts expressing WLS were responsive to treatment with Wnt synthesis inhibitor ETC-1922159. These findings reveal that Wnt/β-catenin signaling in prostate cancer drives stem cell maintenance and invasion and primes for noncanonical Wnt signaling through ROR1. They further show that autocrine Wnt production is a nongenomic driver of canonical and noncanonical Wnt signaling in prostate cancer, which can be targeted with Wnt synthesis inhibitors to suppress tumor growth.

SIGNIFICANCE

This work provides fundamental insights into Wnt signaling and prostate cancer cell biology and indicates that a subset of prostate cancer driven by autocrine Wnt signaling is sensitive to Wnt synthesis inhibitors.

Exploring the Wnt Pathway as a Therapeutic Target for Prostate Cancer

Aberrant activation of the Wnt pathway is emerging as a frequent event during prostate cancer that can facilitate tumor formation, progression, and therapeutic resistance. Recent discoveries indicate that targeting the Wnt pathway to treat prostate cancer may be efficacious. However, the functional consequence of activating the Wnt pathway during the different stages of prostate cancer progression remains unclear. Preclinical work investigating the efficacy of targeting Wnt signaling for the treatment of prostate cancer, both in primary and metastatic lesions, and improving our molecular understanding of treatment responses is crucial to identifying effective treatment strategies and biomarkers that help guide treatment decisions and improve patient care. In this review, we outline the type of genetic alterations that lead to activated Wnt signaling in prostate cancer, highlight the range of laboratory models used to study the role of Wnt genetic drivers in prostate cancer, and discuss new mechanistic insights into how the Wnt cascade facilitates prostate cancer growth, metastasis, and drug resistance.

An overview of the latest in state-of-the-art murine models for prostate cancer

INTRODUCTION

Prostate cancer (PCa) is a complex, heterogenous and multifocal disease, which is debilitating for patients and often fatal - due to bone metastasis and castration-resistant cancer. The use of murine models that mimic human disease has been crucial in the development of innovative therapies and for better understanding the mechanisms associated with initiation and progression of PCa.

AREAS COVERED

This review presents a critical analysis of murine models for the study of PCa, highlighting their strengths, weaknesses and applications.

EXPERT OPINION

In animal models, disease may not occur exactly as it does in humans, and sometimes the levels of efficacy that certain treatments obtain in animal models cannot be translated into clinical practice. To choose the most appropriate animal model for each research work, it is crucial to understand the anatomical and physiological differences between the mouse and the human prostate, while it is also important to identify biological similarities and differences between murine and human prostate tumors. Although significant progress has already been made, thanks to many years of research and study, the number of new challenges and obstacles to overcome mean there is a long and difficult road still to travel.

The PI3K-AKT-mTOR Pathway and Prostate Cancer: At the Crossroads of AR, MAPK, and WNT Signaling

Oncogenic activation of the phosphatidylinositol-3-kinase (PI3K), protein kinase B (PKB/AKT), and mammalian target of rapamycin (mTOR) pathway is a frequent event in prostate cancer that facilitates tumor formation, disease progression and therapeutic resistance. Recent discoveries indicate that the complex crosstalk between the PI3K-AKT-mTOR pathway and multiple interacting cell signaling cascades can further promote prostate cancer progression and influence the sensitivity of prostate cancer cells to PI3K-AKT-mTOR-targeted therapies being explored in the clinic, as well as standard treatment approaches such as androgen-deprivation therapy (ADT). However, the full extent of the PI3K-AKT-mTOR signaling network during prostate tumorigenesis, invasive progression and disease recurrence remains to be determined. In this review, we outline the emerging diversity of the genetic alterations that lead to activated PI3K-AKT-mTOR signaling in prostate cancer, and discuss new mechanistic insights into the interplay between the PI3K-AKT-mTOR pathway and several key interacting oncogenic signaling cascades that can cooperate to facilitate prostate cancer growth and drug-resistance, specifically the androgen receptor (AR), mitogen-activated protein kinase (MAPK), and WNT signaling cascades. Ultimately, deepening our understanding of the broader PI3K-AKT-mTOR signaling network is crucial to aid patient stratification for PI3K-AKT-mTOR pathway-directed therapies, and to discover new therapeutic approaches for prostate cancer that improve patient outcome.

Effective combinatorial immunotherapy for penile squamous cell carcinoma

Penile squamous cell carcinoma (PSCC) accounts for over 95% of penile malignancies and causes significant mortality and morbidity in developing countries. Molecular mechanisms and therapies of PSCC are understudied, owing to scarcity of laboratory models. Herein, we describe a genetically engineered mouse model of PSCC, by co-deletion of Smad4 and Apc in the androgen-responsive epithelium of the penis. Mouse PSCC fosters an immunosuppressive microenvironment with myeloid-derived suppressor cells (MDSCs) as a dominant population. Preclinical trials in the model demonstrate synergistic efficacy of immune checkpoint blockade with the MDSC-diminishing drugs cabozantinib or celecoxib. A critical clinical problem of PSCC is chemoresistance to cisplatin, which is induced by Pten deficiency on the backdrop of Smad4/Apc co-deletion. Drug screen studies informed by targeted proteomics identify a few potential therapeutic strategies for PSCC. Our studies have established what we believe to be essential resources for studying PSCC biology and developing therapeutic strategies.

The Hippo Pathway in Prostate Cancer

Despite recent efforts, prostate cancer (PCa) remains one of the most common cancers in men. Currently, there is no effective treatment for castration-resistant prostate cancer (CRPC). There is, therefore, an urgent need to identify new therapeutic targets. The Hippo pathway and its downstream effectors—the transcriptional co-activators, Yes-associated protein (YAP) and its paralog, transcriptional co-activator with PDZ-binding motif (TAZ)—are foremost regulators of stem cells and cancer biology. Defective Hippo pathway signaling and YAP/TAZ hyperactivation are common across various cancers. Here, we draw on insights learned from other types of cancers and review the latest advances linking the Hippo pathway and YAP/TAZ to PCa onset and progression. We examine the regulatory interaction between Hippo-YAP/TAZ and the androgen receptor (AR), as main regulators of PCa development, and how uncontrolled expression of YAP/TAZ drives castration resistance by inducing cellular stemness. Finally, we survey the potential therapeutic targeting of the Hippo pathway and YAP/TAZ to overcome PCa.

Genetically Engineered Mouse Models of Prostate Cancer in the Postgenomic Era

Recent genomic sequencing analyses have unveiled the spectrum of genomic alterations that occur in primary and advanced prostate cancer, raising the question of whether the corresponding genes are functionally relevant for prostate tumorigenesis, and whether such functions are associated with particular disease stages. In this review, we describe genetically engineered mouse models (GEMMs) of prostate cancer, focusing on those that model genomic alterations known to occur in human prostate cancer. We consider whether the phenotypes of GEMMs based on gain or loss of function of the relevant genes provide reliable counterparts to study the predicted consequences of the corresponding genomic alterations as occur in human prostate cancer, and we discuss exceptions in which the GEMMs do not fully emulate the expected phenotypes. Last, we highlight future directions for the generation of new GEMMs of prostate cancer and consider how we can use GEMMs most effectively to decipher the biological and molecular mechanisms of disease progression, as well as to tackle clinically relevant questions.

Wnt/Beta-Catenin Signaling and Prostate Cancer Therapy Resistance

Metastatic or locally advanced prostate cancer (PCa) is typically treated with androgen deprivation therapy (ADT). Initially, PCa responds to the treatment and regresses. However, PCa almost always develops resistance to androgen deprivation and progresses to castrate-resistant prostate cancer (CRPCa), a currently incurable form of PCa. Wnt/β-Catenin signaling is frequently activated in late stage PCa and contributes to the development of therapy resistance. Although activating mutations in the Wnt/β-Catenin pathway are not common in primary PCa, this signaling cascade can be activated through other mechanisms in late stage PCa, including cross talk with other signaling pathways, growth factors and cytokines produced by the damaged tumor microenvironment, release of the co-activator β-Catenin from sequestration after inhibition of androgen receptor (AR) signaling, altered expression of Wnt ligands and factors that modulate the Wnt signaling, and therapy-induced cellular senescence. Research from genetically engineered mouse models indicates that activation of Wnt/β-Catenin signaling in the prostate is oncogenic, enables castrate-resistant PCa growth, induces an epithelial-to-mesenchymal transition (EMT), promotes neuroendocrine (NE) differentiation, and confers stem cell-like features to PCa cells. These important roles of Wnt/β-Catenin signaling in PCa progression underscore the need for the development of drugs targeting this pathway to treat therapy-resistant PCa.

Androgen signaling is essential for development of prostate cancer initiated from prostatic basal cells

Emerging evidence has shown that both prostatic basal and luminal cells are able to initiate oncogenic transformation. However, despite the diversity of tumor-initiating cells, most prostate cancer cells express the androgen receptor (AR) and depend on androgens for their growth and expansion, implicating an essential role of androgen signaling in prostate tumorigenesis. Prostatic basal cells express p63 and are able to differentiate into luminal, neuroendocrine, and basal cells. Here, we directly assessed the essential role of androgen signaling in prostatic p63-expressing cell initiated oncogenic transformation and tumor formation. Using novel and relevant mouse models, we demonstrated that, with stabilized β-catenin expression, prostatic p63-expressing cells possess the ability to initiate oncogenic transformation and, in the presence of androgens, they further transdifferentiate into luminal-like tumor cells and develop adenocarcinomas. Castration prior to activating stabilized β-catenin sensitizes p63-expressing cells and increases their sensitivity to androgens, resulting in aggressive and fast growing tumor phenotypes. These findings are consistent with what have been observed in human prostate cancers, demonstrating an essential role for androgen signaling in prostate cancer initiation and progression. This study also provides fresh insight into developing new therapeutic strategies for better treating prostate cancer patients.

TGFβ signaling limits lineage plasticity in prostate cancer

Although treatment options for localized prostate cancer (CaP) are initially effective, the five-year survival for metastatic CaP is below 30%. Mutation or deletion of the PTEN tumor suppressor is a frequent event in metastatic CaP, and inactivation of the transforming growth factor (TGF) ß signaling pathway is associated with more advanced disease. We previously demonstrated that mouse models of CaP based on inactivation of Pten and the TGFß type II receptor (Tgfbr2) rapidly become invasive and metastatic. Here we show that mouse prostate tumors lacking Pten and Tgfbr2 have higher expression of stem cell markers and genes indicative of basal epithelial cells, and that basal cell proliferation is increased compared to Pten mutants. To better model the primarily luminal phenotype of human CaP we mutated Pten and Tgfbr2 specifically in luminal cells, and found that these tumors also progress to invasive and metastatic cancer. Accompanying the transition to invasive cancer we observed de-differentiation of luminal tumor cells to an intermediate cell type with both basal and luminal markers, as well as differentiation to basal cells. Proliferation rates in these de-differentiated cells were lower than in either basal or luminal cells. However, de-differentiated cells account for the majority of cells in micro-metastases consistent with a preferential contribution to metastasis. We suggest that active TGFß signaling limits lineage plasticity in prostate luminal cells, and that de-differentiation of luminal tumor cells can drive progression to metastatic disease.

Calcium and Nuclear Signaling in Prostate Cancer

Recently, there have been a number of developments in the fields of calcium and nuclear signaling that point to new avenues for a more effective diagnosis and treatment of prostate cancer. An example is the discovery of new classes of molecules involved in calcium-regulated nuclear import and nuclear calcium signaling, from the G protein-coupled receptor (GPCR) and myosin families. This review surveys the new state of the calcium and nuclear signaling fields with the aim of identifying the unifying themes that hold out promise in the context of the problems presented by prostate cancer. Genomic perturbations, kinase cascades, developmental pathways, and channels and transporters are covered, with an emphasis on nuclear transport and functions. Special attention is paid to the molecular mechanisms behind prostate cancer progression to the malignant forms and the unfavorable response to anti-androgen treatment. The survey leads to some new hypotheses that connect heretofore disparate results and may present a translational interest.

PTEN loss and activation of K-RAS and β-catenin cooperate to accelerate prostate tumourigenesis

Aberrant phosphoinositide 3-kinase (PI3K), mitogen-activated protein kinase (MAPK) and WNT signalling are emerging as key events in the multistep nature of prostate tumourigenesis and progression. Here, we report a compound prostate cancer murine model in which these signalling pathways cooperate to produce a more aggressive prostate cancer phenotype. Using Cre-LoxP technology and the probasin promoter, we combined the loss of Pten (Ptenfl/fl ), to activate the PI3K signalling pathway, with either dominant stabilized β-catenin [Catnb+/lox(ex3) ] or activated K-RAS (K-Ras+/V12 ) to aberrantly activate WNT and MAPK signalling, respectively. Synchronous activation of all three pathways (triple mutants) significantly reduced survival (median 96 days) as compared with double mutants [median: 140 days for Catnb+/lox(ex3) Ptenfl/fl ; 182 days for Catnb+/lox(ex3) K-Ras+/V12 ; 238 days for Ptenfl/fl K-Ras+/V12 ], and single mutants [median: 383 days for Catnb+/lox(ex3) ; 407 days for Ptenfl/fl ], reflecting the accelerated tumourigenesis. Tumours followed a stepwise progression from mouse prostate intraepithelial neoplasia to invasive adenocarcinoma, similar to that seen in human disease. There was significantly elevated cellular proliferation, tumour growth and percentage of invasive adenocarcinoma in triple mutants as compared with double mutants and single mutants. Triple mutants showed not only activated AKT, extracellular-signal regulated kinase 1/2, and nuclear β-catenin, but also significantly elevated signalling through mechanistic target of rapamycin complex 1 (mTORC1). In summary, we show that combined deregulation of the PI3K, MAPK and WNT signalling pathways drives rapid progression of prostate tumourigenesis, and that deregulation of all three pathways results in tumours showing aberrant mTORC1 signalling. As mTORC1 signalling is emerging as a key driver of androgen deprivation therapy resistance, our findings are important for understanding the biology of therapy-resistant prostate cancer and identifying potential approaches to overcome this. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Type II transmembrane serine proteases as potential targets for cancer therapy

Carcinogenesis is accompanied by increased protein and activity levels of extracellular cell-surface proteases that are capable of modifying the tumor microenvironment by directly cleaving the extracellular matrix, as well as activating growth factors and proinflammatory mediators involved in proliferation and invasion of cancer cells, and recruitment of inflammatory cells. These complex processes ultimately potentiate neoplastic progression leading to local tumor cell invasion, entry into the vasculature, and metastasis to distal sites. Several members of the type II transmembrane serine protease (TTSP) family have been shown to play critical roles in cancer progression. In this review the knowledge collected over the past two decades about the molecular mechanisms underlying the pro-cancerous properties of selected TTSPs will be summarized. Furthermore, we will discuss how these insights may facilitate the translation into clinical settings in the future by specifically targeting TTSPs as part of novel cancer treatment regimens.

Deletion of tumor suppressors adenomatous polyposis coli and Smad4 in murine luminal epithelial cells causes invasive prostate cancer and loss of androgen receptor expression

Prostate cancer is the most diagnosed non-skin cancer in the US and kills approximately 27,000 men per year in the US. Additional genetic mouse models are needed that recapitulate the heterogeneous nature of human prostate cancer. The Wnt/beta-catenin signaling pathway is important for human prostate tumorigenesis and metastasis, and also drives tumorigenesis in mouse models. Loss of Smad4 has also been found in human prostate cancer and drives tumorigenesis and metastasis when coupled with other genetic aberrations in mouse models. In this work, we concurrently deleted Smad4 and the tumor suppressor and endogenous Wnt/beta-catenin inhibitor adenomatous polyposis coli (Apc) in luminal prostate cells in mice. This double conditional knockout model produced invasive castration-resistant prostate carcinoma with no evidence of metastasis. We observed mixed differentiation phenotypes, including basaloid and squamous differentiation. Interestingly, tumor cells in this model commonly lose androgen receptor expression. In addition, tumors disappear in these mice during androgen cycling (castration followed by testosterone reintroduction). These mice model non-metastatic castration resistant prostate cancer and should provide novel information for tumors that have genetic aberrations in the Wnt pathway or Smad4.

TWIST1-WDR5-Hottip Regulates Hoxa9 Chromatin to Facilitate Prostate Cancer Metastasis

TWIST1 is a transcription factor critical for development that can promote prostate cancer metastasis. During embryonic development, TWIST1 and HOXA9 are coexpressed in mouse prostate and then silenced postnatally. Here we report that TWIST1 and HOXA9 coexpression are reactivated in mouse and human primary prostate tumors and are further enriched in human metastases, correlating with survival. TWIST1 formed a complex with WDR5 and the lncRNA Hottip/HOTTIP, members of the MLL/COMPASS-like H3K4 methylases, which regulate chromatin in the Hox/HOX cluster during development. TWIST1 overexpression led to coenrichment of TWIST1 and WDR5 as well as increased H3K4me3 chromatin at the Hoxa9/HOXA9 promoter, which was dependent on WDR5. Expression of WDR5 and Hottip/HOTTIP was also required for TWIST1-induced upregulation of HOXA9 and aggressive cellular phenotypes such as invasion and migration. Pharmacologic inhibition of HOXA9 prevented TWIST1-induced aggressive prostate cancer cellular phenotypes in vitro and metastasis in vivo This study demonstrates a novel mechanism by which TWIST1 regulates chromatin and gene expression by cooperating with the COMPASS-like complex to increase H3K4 trimethylation at target gene promoters. Our findings highlight a TWIST1-HOXA9 embryonic prostate developmental program that is reactivated during prostate cancer metastasis and is therapeutically targetable. Cancer Res; 77(12); 3181-93. ©2017 AACR.

Bone Metastasis of Prostate Cancer Can Be Therapeutically Targeted at the TBX2-WNT Signaling Axis

Identification of factors that mediate visceral and bone metastatic spread and subsequent bone remodeling events is highly relevant to successful therapeutic intervention in advanced human prostate cancer. TBX2, a T-box family transcription factor that negatively regulates cell-cycle inhibitor p21, plays critical roles during embryonic development, and recent studies have highlighted its role in cancer. Here, we report that TBX2 is overexpressed in human prostate cancer specimens and bone metastases from xenograft mouse models of human prostate cancer. Blocking endogenous TBX2 expression in PC3 and ARCaP_M prostate cancer cell models using a dominant-negative construct resulted in decreased tumor cell proliferation, colony formation, and invasion in vitro Blocking endogenous TBX2 in human prostate cancer mouse xenografts decreased invasion and abrogation of bone and soft tissue metastasis. Furthermore, blocking endogenous TBX2 in prostate cancer cells dramatically reduced bone-colonizing capability through reduced tumor cell growth and bone remodeling in an intratibial mouse model. TBX2 acted in trans by promoting transcription of the canonical WNT (WNT3A) promoter. Genetically rescuing WNT3A levels in prostate cancer cells with endogenously blocked TBX2 partially restored the TBX2-induced prostate cancer metastatic capability in mice. Conversely, WNT3A-neutralizing antibodies or WNT antagonist SFRP-2 blocked TBX2-induced invasion. Our findings highlight TBX2 as a novel therapeutic target upstream of WNT3A, where WNT3A antagonists could be novel agents for the treatment of metastasis and for skeletal complications in prostate cancer patients. Cancer Res; 77(6); 1331-44. ©2017 AACR.

Mouse Models in Prostate Cancer Translational Research: From Xenograft to PDX

Despite the advancement of clinical and preclinical research on PCa, which resulted in the last five years in a decrement of disease incidence by 3-4%, it remains the most frequent cancer in men and the second for mortality rate. Based on this evidence we present a brief dissertation on numerous preclinical models, comparing their advantages and disadvantages; among this we report the PDX mouse models that show greater fidelity to the disease, in terms of histopathologic features of implanted tumor, gene and miRNA expression, and metastatic pattern, well describing all tumor progression stages; this characteristic encourages the translation of preclinical results. These models become particularly useful in meeting the need of new treatments identification that eradicate PCa bone metastases growing, clarifying pathway of angiogenesis, identifying castration-resistant stem-like cells, and studying the antiandrogen therapies. Also of considerable interest are the studies of 3D cell cultures derived from PDX, which have the ability to maintain PDX cell viability with continued native androgen receptor expression, also showing a differential sensitivity to drugs. 3D PDX PCa may represent a diagnostic platform for the rapid assessment of drugs and push personalized medicine. Today the development of preclinical models in vitro and in vivo is necessary in order to obtain increasingly reliable answers before reaching phase III of the drug discovery.

Divergent Androgen Receptor and Beta-Catenin Signaling in Prostate Cancer Cells

Despite decades of effort to develop effective therapy and to identify promising new drugs, prostate cancer is lethal once it progresses to castration-resistant disease. Studies show mis-regulation of multiple pathways in castration-resistant prostate cancer (CRPC), reflecting the heterogeneity of the tumors and also hinting that targeting androgen receptor (AR) pathway alone might not be sufficient to treat CRPC. In this study, we present evidence that the Wnt/β-catenin pathway might be activated in prostate cancer cells after androgen-deprivation to promote androgen-independent growth, partly through enhanced interaction of β-catenin with TCF4. Androgen-independent prostate cancer cells were more prone to activate a Wnt-reporter, and inhibition of the Wnt/β-catenin pathway increased sensitivity of these cells to the second-generation antiandrogen, enzalutamide. Combined treatment of enzalutamide and Wnt/β-catenin inhibitor showed increased growth repression in both androgen-dependent and -independent prostate cancer cells, suggesting therapeutic potential for this approach.

Concurrent Hepsin overexpression and adenomatous polyposis coli deletion causes invasive prostate carcinoma in mice

BACKGROUND

A clinical need to better categorize patients with prostate cancer exists. The Wnt/β-catenin signaling pathway plays important roles in human prostate cancer progression. Deletion of the endogenous Wnt antagonist adenomatous polyposis coli (Apc) in mice causes high grade prostate intraepithelial neoplasia, widely thought to be the precursor to prostate cancer. However, no metastasis occurrs in this model. New mouse models are needed to determine molecular causes of tumorigenesis, progression, and metastasis.

METHODS

To determine whether the overexpression of the prostate oncogene Hepsin could cause prostate cancer progression, we crossed a prostate-specific Hepsin overexpression model to a prostate-specific Apc-deletion model and classified the observed phenotype.

RESULTS

When Apc was deleted and Hepsin overexpressed concurrently, mice displayed invasive carcinoma, with loss of membrane characteristics and increase of fibrosis. These tumors had both luminal and basaloid characteristics. Though no metastasis was observed, there was evidence of adenomas and lung necrosis, inflammation, and chronic hemorrhage.

CONCLUSIONS

This work indicates that the Wnt/β-catenin pathway and the Hepsin pathway act in concert to promote prostate cancer progression. Both of these pathways are up-regulated in human prostate cancer and could represent chemotherapeutic targets.

Androgen receptor inhibits epithelial-mesenchymal transition, migration, and invasion of PC-3 prostate cancer cells

Bone metastasis is very common in prostate cancer (PCa) and causes severe pain. PC-3 is an androgen receptor (AR)-negative PCa cell line with high metastatic potential established from PCa bone metastasis. We observed that re-expression of AR, which is located in the cytoplasm in the absence of androgen, suppressed cell motility, migration, and invasion of PC-3 cells as determined by wound healing assay and transwell assay. Micro-Western Array and Western blotting analysis indicated that re-expression of AR increased APC, Akt2, Akt3, PI3K p85, phospho-PI3K p85 Tyr458, PI3K p85, and E-cadherin but decreased GSK-3β, phospho-GSK-3β Ser9, phospho-mTOR Ser2448, Skp2, NF-κB p50, Slug, N-cadherin, β-catenin, vimentin, MMP-9, and Snail. Migration and invasion of PC-3 and PC-3(AR) cells were promoted by EGF or IGF-1 but were suppressed by Casodex. Re-expression of AR reduced the activity of MMP-2 and MMP-9 in PC-3 cells. Our observations suggested that re-expressing AR suppresses migration and invasion of PC-3 cells via regulation of EMT marker proteins and MMP activity.

Wnt/β-Catenin-Responsive Cells in Prostatic Development and Regeneration

The precise role of Wnt/β-catenin signaling during prostatic development and tumorigenesis is unclear. Axin2 is a direct transcriptional target of β-catenin. Recent studies have shown that Axin2-expressing cells have stem/progenitor cell properties in a variety of mouse tissues. Here, we genetically labeled Axin2-expressing cells at various time points and tracked their cellular behavior at different developmental and mature stages. We found that prostatic Axin2-expressing cells mainly express luminal epithelial cell markers and are able to expand luminal cell lineages during prostatic development and maturation. They can also survive androgen withdrawal and regenerate prostatic luminal epithelial cells following androgen replacement. Deletion of β-catenin or expression of stabilized β-catenin in these Axin2-expressing cells results in abnormal development or oncogenic transformation, respectively. Our study uncovers a critical role of Wnt/β-catenin-responsive cells in prostatic development and regeneration, and that dysregulation of Wnt/β-catenin signaling in these cells contributes to prostatic developmental defects and tumorigenesis.

Drug discovery in prostate cancer mouse models

INTRODUCTION

The mouse is an important, though imperfect, organism with which to model human disease and to discover and test novel drugs in a preclinical setting. Many experimental strategies have been used to discover new biological and molecular targets in the mouse, with the hopes of translating these discoveries into novel drugs to treat prostate cancer in humans. Modeling prostate cancer in the mouse, however, has been challenging, and often drugs that work in mice have failed in human trials.

AREAS COVERED

The authors discuss the similarities and differences between mice and men; the types of mouse models that exist to model prostate cancer; practical questions one must ask when using a mouse as a model; and potential reasons that drugs do not often translate to humans. They also discuss the current value in using mouse models for drug discovery to treat prostate cancer and what needs are still unmet in field.

EXPERT OPINION

With proper planning and following practical guidelines by the researcher, the mouse is a powerful experimental tool. The field lacks genetically engineered metastatic models, and xenograft models do not allow for the study of the immune system during the metastatic process. There remain several important limitations to discovering and testing novel drugs in mice for eventual human use, but these can often be overcome. Overall, mouse modeling is an essential part of prostate cancer research and drug discovery. Emerging technologies and better and ever-increasing forms of communication are moving the field in a hopeful direction.

Androgen Receptor-dependent Expression of Low-density Lipoprotein Receptor-related Protein 6 is Necessary for Prostate Cancer Cell Proliferation

Androgen receptor (AR) signaling is important for prostate cancer (PCa) cell proliferation. Here, we showed that proliferation of hormone-sensitive prostate cancer cells such as LNCaP was significantly enhanced by testosterone stimulation whereas hormone-insensitive prostate cancer cells such as PC3 and VCaP did not respond to testosterone stimulation. Blocking of AR using bicalutamide abolished testosterone-induced proliferation of LNCaP cells. In addition, knockdown of AR blocked testosterone-induced proliferation of LNCaP cells. Basal expression of low-density lipoprotein receptor-related protein 6 (LRP6) was elevated in VCaP cells whereas stimulation of testosterone did not affect the expression of LRP6. However, expression of LRP6 in LNCaP cells was increased by testosterone stimulation. In addition, knockdown of LRP6 abrogated testosterone-induced proliferation of LNCaP cells. Given these results, we suggest that androgen-dependent expression of LRP6 plays a crucial role in hormone-sensitive prostate cancer cell proliferation.

Androgen signaling is a confounding factor for β-catenin-mediated prostate tumorigenesis

Emerging evidence has demonstrated the critical roles for both androgen and Wnt pathways in prostate tumorigenesis. A recent integrative genomic analysis of human prostate cancers has revealed a unique enrichment of androgen and Wnt signaling in early onset prostate cancers, implying their clinical significance in the disease. Additionally, interaction between the androgen receptor (AR) and β-catenin has long been detected in prostate cancer cells. However, the consequence of this interaction in prostate tumorigenesis is still unknown. Because mutations in adenomatous polyposis coli (APC), β-catenin, and other components of the destruction-complex are generally rare in prostate cancers, other mechanisms of aberrant Wnt signaling activation have been speculated. To address these critical questions, we developed Ctnnb1^L(ex3)/+/R26hAR^L/+:PB-Cre4 mice, in which transgenic AR and stabilized β-catenin are co-expressed in prostatic epithelial cells. We observed accelerated tumor development, aggressive tumor invasion, and a decreased survival rate in Ctnnb1^L(ex3)/+/R26hAR^L/+:PB-Cre4 compound mice compared to age-matched Ctnnb1^L(ex3)/+:PB-Cre4 littermate controls, which only have stabilized β-catenin expression in the prostate. Castration of the above transgenic mice resulted in significant tumor regression, implying an essential role of androgen signaling in tumor growth and maintenance. Implantation of the prostatic epithelial cells isolated from the transgenic mice regenerated PIN and prostatic adenocarcinoma lesions. Microarray analyses of transcriptional profiles showed more robust enrichment of known tumor and metastasis promoting genes: Spp1, Egr1, c-Myc, Sp5, and Sp6 genes in samples isolated from Ctnnb1^L(ex3)/+/R26hAR^L/+:PB-Cre4 compound mice than those from Ctnnb1^L(ex3)/+:PB-Cre4 and R26hAR^L/+:PB-Cre4 littermate controls. Together, these data demonstrate a confounding role of androgen signaling in β-catenin initiated oncogenic transformation in prostate tumorigenesis.

Mouse models of prostate cancer: picking the best model for the question

When the National Institutes of Health Mouse Models of Human Cancer Consortium initiated the Prostate Steering Committee 15 years ago, there were no genetically engineered mouse (GEM) models of prostate cancer (PCa). Today, a PubMed search for "prostate cancer mouse model" yields 3,200 publications and this list continues to grow. The first generation of GEM utilized the newly discovered and characterized probasin promoter driving viral oncogenes such as Simian virus 40 large T antigen to yield the LADY and TRAMP models. As the PCa research field has matured, the second generation of models has incorporated the single and multiple molecular changes observed in human disease, such as loss of PTEN and overexpression of Myc. Application of these models has revealed that mice are particularly resistant to developing invasive PCa, and once they achieve invasive disease, the PCa rarely resembles human disease. Nevertheless, these models and their application have provided vital information on human PCa progression. The aim of this review is to provide a brief primer on mouse and human prostate histology and pathology, provide descriptions of mouse models, as well as attempt to answer the age old question: Which GEM model of PCa is the best for my research question?

Activation of Wnt/β-catenin signaling in a subpopulation of murine prostate luminal epithelial cells induces high grade prostate intraepithelial neoplasia

BACKGROUND

Wnt/β-catenin signaling is important for prostate development and cancer in humans. Activation of this pathway in differentiated luminal cells of mice induces high-grade prostate intraepithelial neoplasia (HGPIN). Though the cell of origin of prostate cancer has yet to be conclusively identified, a castration-resistant Nkx3.1-expressing cell (CARN) may act as a cell of origin for prostate cancer.

METHODS

To activate Wnt/β-catenin signaling in CARNs, we crossed mice carrying tamoxifen-inducible Nkx3.1-driven Cre to mice containing loxP sites in order to either conditionally knock out adenomatous polyposis coli (Apc) or constitutively activate β-catenin directly. We then castrated and hormonally regenerated these mice to target the CARN population.

RESULTS

Loss of Apc in hormonally normal mice induced HGPIN; however, after one or more rounds of castration and hormonal regeneration, Apc-null CARNs disappeared. Alternatively, when β-catenin was constitutively activated under the same conditions, HGPIN was apparent.

CONCLUSION

Activation of Wnt/β-catenin signaling via Apc deletion is sufficient to produce HGPIN in hormonally normal mice. Loss of Apc may destabilize the CARN population under regeneration conditions. When β-catenin is constitutively activated, HGPIN occurs in hormonally regenerated mice. A second genetic hit is likely required to cause progression to carcinoma and metastasis.

The many faces of neuroendocrine differentiation in prostate cancer progression

In normal prostate, neuroendocrine (NE) cells are rare and interspersed among the epithelium. These cells are believed to provide trophic signals to epithelial cell populations through the secretion of an abundance of neuropeptides that can diffuse to influence surrounding cells. In the setting of prostate cancer (PC), NE cells can also stimulate surrounding prostate adenocarcinoma cell growth, but in some cases adenocarcinoma cells themselves acquire NE characteristics. This epithelial plasticity is associated with decreased androgen receptor (AR) signaling and the accumulation of neuronal and stem cell characteristics. Transformation to an NE phenotype is one proposed mechanism of resistance to contemporary AR-targeted treatments, is associated with poor prognosis, and thought to represent up to 25% of lethal PCs. Importantly, the advent of high-throughput technologies has started to provide clues for understanding the complex molecular profiles of tumors exhibiting NE differentiation. Here, we discuss these recent advances, the multifaceted manner by which an NE-like state may arise during the different stages of disease progression, and the potential benefit of this knowledge for the management of patients with advanced PC.

Prostate cancer induced by loss of Apc is restrained by TGFβ signaling

Recent work with mouse models of prostate cancer (CaP) has shown that inactivation of TGFβ signaling in prostate epithelium can cooperate with deletion of the Pten tumor suppressor to drive locally aggressive cancer and metastatic disease. Here, we show that inactivating the TGFβ pathway by deleting the gene encoding the TGFβ type II receptor (Tgfbr2) in combination with a deletion of the Apc tumor suppressor gene specifically in mouse prostate epithelium, results in the rapid onset of invasive CaP. Micro-metastases were observed in the lymph nodes and lungs of a proportion of the double mutant mice, whereas no metastases were observed in Apc single mutant mice. Prostate-specific Apc;Tgfbr2 mutants had a lower frequency of metastasis and survived significantly longer than Pten;Tgfbr2 double mutants. However, all Apc;Tgfbr2 mutants developed invasive cancer by 30 weeks of age, whereas invasive cancer was rarely observed in Apc single mutant animals, even by one year of age. Further comparison of the Pten and Apc models of CaP revealed additional differences, including adenosquamous carcinoma in the Apc;Tgfbr2 mutants that was not seen in the Pten model, and a lack of robust induction of the TGFβ pathway in Apc null prostate. In addition to causing high-grade prostate intra-epithelial neoplasia (HGPIN), deletion of either Pten or Apc induced senescence in affected prostate ducts, and this restraint was overcome by loss of Tgfbr2. In summary, this work demonstrates that TGFβ signaling restrains the progression of CaP induced by different tumor suppressor mutations, suggesting that TGFβ signaling exerts a general tumor suppressive effect in prostate.

Pathogenic intestinal bacteria enhance prostate cancer development via systemic activation of immune cells in mice

A role for microbes has been suspected in prostate cancer but difficult to confirm in human patients. We show here that a gastrointestinal (GI) tract bacterial infection is sufficient to enhance prostate intraepithelial neoplasia (PIN) and microinvasive carcinoma in a mouse model. We found that animals with a genetic predilection for dysregulation of wnt signaling, Apc^Min/+ mutant mice, were significantly susceptible to prostate cancer in an inflammation-dependent manner following infection with Helicobacter hepaticus. Further, early neoplasia observed in infected Apc^Min/+ mice was transmissible to uninfected mice by intraperitoneal injection of mesenteric lymph node (MLN) cells alone from H. hepaticus-infected mutant mice. Transmissibility of neoplasia was preventable by prior neutralization of inflammation using anti-TNF-α antibody in infected MLN donor mice. Taken together, these data confirm that systemic inflammation triggered by GI tract bacteria plays a pivotal role in tumorigenesis of the prostate gland.

Current mouse and cell models in prostate cancer research

Mouse models of prostate cancer (PCa) are critical for understanding the biology of PCa initiation, progression, and treatment modalities. Here, we summarize recent advances in PCa mouse models that led to new insights into specific gene functions in PCa. For example, the study of transgenic mice with TMPRSS2/ERG, an androgen-regulated fusion protein, revealed its role in developing PCa precursor lesions, prostate intraepithelial neoplasia; however, it is not sufficient for PCa development. Double deficiency of Pten and Smad4 leads to a high incidence of metastatic PCa. Targeted deletion of Pten in castration-resistant Nkx3-1-expressing cells results in rapid carcinoma formation after androgen-mediated regeneration, indicating that progenitor cells with luminal characteristics can play a role in initiation of PCa. Transgenic mice with activated oncogenes, growth factors, and steroid hormone receptors or inactivated tumor suppressors continue to provide insights into disease progression from initiation to metastasis. Further development of new PCa models with spatial and temporal regulation of candidate gene expression will probably enhance our understanding of the complex events that lead to PCa initiation and progression, thereby invoking novel strategies to combat this common disease in men.

Modeling prostate cancer in mice: something old, something new, something premalignant, something metastatic

More than 15 years ago, the first generation of genetically engineered mouse (GEM) models of prostate cancer was introduced. These transgenic models utilized prostate-specific promoters to express SV40 oncogenes specifically in prostate epithelium. Since the description of these initial models, there have been a plethora of GEM models of prostate cancer representing various perturbations of oncogenes or tumor suppressors, either alone or in combination. This review describes these GEM models, focusing on their relevance for human prostate cancer and highlighting their strengths and limitations, as well as opportunities for the future.

Dissecting Major Signaling Pathways throughout the Development of Prostate Cancer

Prostate cancer (PCa) is one of the most common malignancies found in males. The development of PCa involves several mutations in prostate epithelial cells, usually linked to developmental changes, such as enhanced resistance to apoptotic death, constitutive proliferation, and, in some cases, to differentiation into an androgen deprivation-resistant phenotype, leading to the appearance of castration-resistant PCa (CRPCa), which leads to a poor prognosis in patients. In this review, we summarize recent findings concerning the main deregulations into signaling pathways that will lead to the development of PCa and/or CRPCa. Key mutations in some pathway molecules are often linked to a higher prevalence of PCa, by directly affecting the respective cascade and, in some cases, by deregulating a cross-talk node or junction along the pathways. We also discuss the possible environmental and nonenvironmental inducers for these mutations, as well as the potential therapeutic strategies targeting these signaling pathways. A better understanding of how some risk factors induce deregulation of these signaling pathways, as well as how these deregulated pathways affect the development of PCa and CRPCa, will further help in the development of new treatments and prevention strategies for this disease.

Animal models of human prostate cancer: the consensus report of the New York meeting of the Mouse Models of Human Cancers Consortium Prostate Pathology Committee

Animal models, particularly mouse models, play a central role in the study of the etiology, prevention, and treatment of human prostate cancer. While tissue culture models are extremely useful in understanding the biology of prostate cancer, they cannot recapitulate the complex cellular interactions within the tumor microenvironment that play a key role in cancer initiation and progression. The National Cancer Institute (NCI) Mouse Models of Human Cancers Consortium convened a group of human and veterinary pathologists to review the current animal models of prostate cancer and make recommendations about the pathologic analysis of these models. More than 40 different models with 439 samples were reviewed, including genetically engineered mouse models, xenograft, rat, and canine models. Numerous relevant models have been developed over the past 15 years, and each approach has strengths and weaknesses. Analysis of multiple genetically engineered models has shown that reactive stroma formation is present in all the models developing invasive carcinomas. In addition, numerous models with multiple genetic alterations display aggressive phenotypes characterized by sarcomatoid carcinomas and metastases, which is presumably a histologic manifestation of epithelial-mesenchymal transition. The significant progress in development of improved models of prostate cancer has already accelerated our understanding of the complex biology of prostate cancer and promises to enhance development of new approaches to prevention, detection, and treatment of this common malignancy.

Genetically engineered mouse models of prostate cancer

Despite major improvement in treatment of early stage localised prostate cancer, the distinction between indolent tumors and those that will become aggressive, as well as the lack of efficient therapies of advanced prostate cancer, remain major health problems. Genetically engineered mice (GEM) have been extensively used to investigate the molecular and cellular mechanisms underlying prostate tumor initiation and progression, and to evaluate new therapies. Moreover, the recent development of conditional somatic mutagenesis in the mouse prostate offers the possibility to generate new models that more faithfully reproduce the human disease, and thus should contribute to improve diagnosis and treatments. The strengths and weaknesses of various models will be discussed, as well as future opportunities.

Skeletal metastasis: treatments, mouse models, and the Wnt signaling

Skeletal metastases result in significant morbidity and mortality. This is particularly true of cancers with a strong predilection for the bone, such as breast, prostate, and lung cancers. There is currently no reliable cure for skeletal metastasis, and palliative therapy options are limited. The Wnt signaling pathway has been found to play an integral role in the process of skeletal metastasis and may be an important clinical target. Several experimental models of skeletal metastasis have been used to find new biomarkers and test new treatments. In this review, we discuss pathologic process of bone metastasis, the roles of the Wnt signaling, and the available experimental models and treatments.

Use of PB-Cre4 mice for mosaic gene deletion

Transgene expression from short promoters in transgenic animals can lead to unwanted transgene expression patterns, often as a byproduct of random integration of the expression cassette into the host genome. Here I demonstrate that the often used PB-Cre4 line (also referred to as “Probasin-Cre”), although expressing exclusively in the male prostate epithelium when transmitted through male mice, can lead to recombination of loxP-flanked alleles in a large variety of tissues when transmitted through female mice. This aberrant Cre activity due to Cre expression in the oocytes leads to different outcomes for maternally or paternally transmitted loxP-flanked alleles: Maternally inherited loxP-flanked alleles undergo recombination very efficiently, making female PB-Cre4 mice an efficient monoallelic “Cre deleter line”. However, paternally inherited loxP-flanked alleles are inefficiently recombined by maternal PB-Cre4, giving rise to mosaic expression patterns in the offspring. This mosaic recombination is difficult to detect with standard genotyping approaches of many mouse lines and should therefore caution researchers using PB-Cre4 to use additional approaches to exclude the presence of recombined alleles. However, mosaic recombination should also be useful in transgenic “knockout” approaches for mosaic gene deletion experiments.

Wnt signaling though beta-catenin is required for prostate lineage specification

Androgens initiate a complex network of signals within the UGS that trigger prostate lineage commitment and bud formation. Given its contributions to organogenesis in other systems, we investigated a role for canonical Wnt signaling in prostate development. We developed a new method to achieve complete deletion of beta-catenin, the transcriptional coactivator required for canonical Wnt signaling, in early prostate development. Beta-catenin deletion abrogated canonical Wnt signaling and yielded prostate rudiments that exhibited dramatically decreased budding and failed to adopt prostatic identity. This requirement for canonical Wnt signaling was limited to a brief critical period during the initial molecular phase of prostate identity specification. Deletion of beta-catenin in the adult prostate did not significantly affect organ homeostasis. Collectively, these data establish that beta-catenin and Wnt signaling play key roles in prostate lineage specification and bud outgrowth.

Wnt/β-catenin signalling in prostate cancer

The Wnts are secreted cysteine-rich glycoproteins that have important roles in the developing embryo as well as in tissue homeostasis in adults. Dysregulation of Wnt signalling can lead to several types of cancer, including prostate cancer. A hallmark of the signalling pathway is the stabilization of the transcriptional co-activator β-catenin, which not only regulates expression of many genes implicated in cancer but is also an essential component of cadherin cell adhesion complexes. β-catenin regulates gene expression by binding members of the T-cell-specific transcription factor/lymphoid enhancer-binding factor 1 (TCF/LEF-1) family of transcription factors. In addition, β-catenin associates with the androgen receptor, a key regulator of prostate growth that drives prostate cancer progression. Wnt/β-catenin signalling can be controlled by secreted Wnt antagonists, many of which are downregulated in cancer. Activation of the Wnt/β-catenin pathway has effects on prostate cell proliferation, differentiation and the epithelial-mesenchymal transition, which is thought to regulate the invasive behaviour of tumour cells. However, whether targeting Wnt/β-catenin signalling is a good therapeutic option for prostate cancer remains unclear.

Activation of Wnt signaling by chemically induced dimerization of LRP5 disrupts cellular homeostasis

Wnt signaling is crucial for a variety of biological processes, including body axis formation, planar polarity, stem cell maintenance and cellular differentiation. Therefore, targeted manipulation of Wnt signaling in vivo would be extremely useful. By applying chemical inducer of dimerization (CID) technology, we were able to modify the Wnt co-receptor, low-density lipoprotein (LDL)-receptor-related protein 5 (LRP5), to generate the synthetic ligand inducible Wnt switch, iLRP5. We show that iLRP5 oligomerization results in its localization to disheveled-containing punctate structures and sequestration of scaffold protein Axin, leading to robust β-catenin-mediated signaling. Moreover, we identify a novel LRP5 cytoplasmic domain critical for its intracellular localization and casein kinase 1-dependent β-catenin signaling. Finally, by utilizing iLRP5 as a Wnt signaling switch, we generated the Ubiquitous Activator of β-catenin (Ubi-Cat) transgenic mouse line. The Ubi-Cat line allows for nearly ubiquitous expression of iLRP5 under control of the H-2K^b promoter. Activation of iLRP5 in isolated prostate basal epithelial stem cells resulted in expansion of p63⁺ cells and development of hyperplasia in reconstituted murine prostate grafts. Independently, iLRP5 induction in adult prostate stroma enhanced prostate tissue regeneration. Moreover, induction of iLRP5 in male Ubi-Cat mice resulted in prostate tumor progression over several months from prostate hyperplasia to adenocarcinoma. We also investigated iLRP5 activation in Ubi-Cat-derived mammary cells, observing that prolonged activation results in mammary tumor formation. Thus, in two distinct experimental mouse models, activation of iLRP5 results in disruption of tissue homeostasis, demonstrating the utility of iLRP5 as a novel research tool for determining the outcome of Wnt activation in a precise spatially and temporally determined fashion.

Mouse prostate cancer cell lines established from primary and postcastration recurrent tumors

The clinical course of prostate cancer is grouped into two broad phases. The first phase, which is the growth of the androgen-dependent cancer (AD-Ca) responds well to androgen depletion treatment while the second phase, that could be termed as androgen depletion-independent cancer (ADI-Ca) does not. We used two separate prostate tumors, one AD-Ca and one ADI-Ca from the conditional Pten deletion mouse model to generate from each a pair of cell lines. The AD-Ca cell lines (E2 and E4) and the ADI-Ca cell lines (cE1 and cE2) display bi-allelic deletion at the Pten gene locus, an event which is specific for the prostate epithelium for this mouse model, and a fairly similar level of expression of the androgen receptor (AR). The ADI-Ca cell lines (cE series) grow well in the absence of androgen, display increased AR transcription under androgen-deprived environment, and retain the sensitivity to increased proliferation when androgen is supplemented. The AD-Ca cell lines (E series) grow slowly in the absence of androgen, and, unlike cE cells, do not show increased AR expression when maintained in the absence of androgen. The detection of epithelial cell markers, such as CK8, CK14, CK18 and E-cadherin in the cE series is conforming with the polygonal epithelial morphology of these cells in culture. The E cells also present mostly polygonal-shaped morphology with a small percent of cells with fibroblastoid morphology, and produce little or very low levels of cytokeratins, but increased levels of vimentin, Twist and Slug, the markers known to be associated with epithelial-mesenchymal transition. Each of the cell lines, when inoculated subcutaneously into male or female NOD.SCID mice induced tumors within eight weeks with 100% incidence. Histopathological examinations of the tumor sections, however, led to noticeable biological differences. The cE series engenders adenocarcinomas, particularly in male hosts, and the E series induces sarcomatoid carcinomas (positively stained for CK8 and AR as well as vimentin expression) in either male or female hosts. These new cell lines are promising models for the elucidation of the androgen metabolism and their role in prostate cancer.

Cancer stem cells and microenvironment in prostate cancer progression

For a study of interactions between the cancer-associated fibroblasts (CAFs) and the putative prostate cancer stem cells (CSCs), we used a conditional Pten deletion mouse model of prostatic adenocarcinoma to isolate both CAF cultures and CSC-enriched cell fractions from the primary tumors. The CSC subpopulation exhibited a collective phenotype of Lin(-)/SCA-1(hi)/CD49f(hi)/p63(hi)/CK5(hi)/AR(lo)/CK18(lo)/Survivin(hi)/Runx2(hi) and contained cells with the ability to both self-renew and differentiate into basal and luminal cells in vitro. The spheroids generated from the CSC-enriched subpopulation mimicked the glandular structures that could be produced from a similarly isolated cell fraction from the normal mouse prostate. The efficiency of spheroid formation was found to be influenced differentially by the nature of the fibroblasts that were co-cultured in the 3-D system. The growth and differentiation properties of the CSCs were significantly more enhanced by factors released from CAFs relative to normal prostate fibroblasts (NPFs). Additionally, increased commitment to differentiation to the luminal cell lineage was noted when CAFs were present. When CSCs admixed with either CAFs or NPFs were examined for formation of prostatic glandular structures in renal grafts in vivo, the lesions formed were generally more in numbers in the presence of CAFs than NPFs. Furthermore, lesions formed with CAFs often displayed tumor-like complex histopathology and contained increased numbers of proliferating cells. Taken together, the results suggested that the CAFs in the prostate tumor microenvironment can contribute to the biologic properties of the CSCs and by this account may play a major role in prostate tumorigenesis and progression. Thus, it would be important now to identify the paracrine and/or juxtacrine factors that are responsible for the stimulation of the cancer stem cells.

MYC and Prostate Cancer

Prostate cancer, the majority of which is adenocarcinoma, is the most common epithelial cancer affecting a majority of elderly men in Western nations. Its manifestation, however, varies from clinically asymptomatic insidious neoplasms that progress slowly and do not threaten life to one that is highly aggressive with a propensity for metastatic spread and lethality if not treated in time. A number of somatic genetic and epigenetic alterations occur in prostate cancer cells. Some of these changes, such as loss of the tumor suppressors PTEN and p53, are linked to disease progression. Others, such as ETS gene fusions, appear to be linked more with early phases of the disease, such as invasion. Alterations in chromosome 8q24 in the region of MYC have also been linked to disease aggressiveness for many years. However, a number of recent studies in human tissues have indicated that MYC appears to be activated at the earliest phases of prostate cancer (e.g., in tumor-initiating cells) in prostatic intraepithelial neoplasia, a key precursor lesion to invasive prostatic adenocarcinoma. The initiation and early progression of prostate cancer can be recapitulated in genetically engineered mouse models, permitting a richer understanding of the cause and effects of loss of tumor suppressors and activation of MYC. The combination of studies using human tissues and mouse models paints an emerging molecular picture of prostate cancer development and early progression. This picture reveals that MYC contributes to disease initiation and progression by stimulating an embryonic stem cell-like signature characterized by an enrichment of genes involved in ribosome biogenesis and by repressing differentiation. These insights pave the way to potential novel therapeutic concepts based on MYC biology.

The tumor suppressor kinase LKB1: lessons from mouse models

Mutations in the tumor suppressor gene LKB1 are important in hereditary Peutz-Jeghers syndrome, as well as in sporadic cancers including lung and cervical cancer. LKB1 is a kinase-activating kinase, and a number of LKB1-dependent phosphorylation cascades regulate fundamental cellular and organismal processes in at least metabolism, polarity, cytoskeleton organization, and proliferation. Conditional targeting approaches are beginning to demonstrate the relevance and specificity of these signaling pathways in development and homeostasis of multiple organs. More than one of the pathways also appear to contribute to tumor growth following Lkb1 deficiencies based on a number of mouse tumor models. Lkb1-dependent activation of AMPK and subsequent inactivation of mammalian target of rapamycin signaling are implicated in several of the models, and other less well characterized pathways are also involved. Conditional targeting studies of Lkb1 also point an important role of LKB1 in epithelial-mesenchymal interactions, significantly expanding knowledge on the relevance of LKB1 in human disease.

Wnt/β-catenin Signaling in Normal and Cancer Stem Cells

The ability of Wnt ligands to initiate a signaling cascade that results in cytoplasmic stabilization of, and nuclear localization of, β-catenin underlies their ability to regulate progenitor cell differentiation. In this review, we will summarize the current knowledge of the mechanisms underlying Wnt/β-catenin signaling and how the pathway regulates normal differentiation of stem cells in the intestine, mammary gland, and prostate. We will also discuss how dysregulation of the pathway is associated with putative cancer stem cells and the potential therapeutic implications of regulating Wnt signaling.