Mouse models of prostate adenocarcinoma with the capacity to monitor spontaneous carcinogenesis by bioluminescence or fluorescence

Multi-scale in vivo imaging of tumour development using a germline conditional triple-reporter system

Imaging reporter genes are indispensable for visualising biological processes in living subjects, particularly in cancer research where they have been used to observe tumour development, cancer cell dissemination, and treatment response. Engineering reporter genes into the germline frequently involves single imaging modality reporters operating over limited spatial scales. To address these limitations, we developed an inducible triple-reporter mouse model (Rosa26LSL - NRL) that integrates reporters for complementary imaging modalities, flfluorescence, bioluminescence and positron emission tomography (PET), along with inducible Cre-lox functionality for precise spatiotemporal control of reporter expression. We demonstrated robust reporter inducibility across various tissues in the Rosa26LSL - NRL mouse, facilitating effective tracking and characterisation of tumours in liver and lung cancer mouse models. We precisely pinpointed tumour location using multimodal whole-body imaging which guided in situ lung microscopy to visualise cell-cell interactions within the tumour microenvironment. The triple-reporter system establishes a robust new platform technology for multi-scale investigation of biological processes within whole animals, enabling tissue-specific and sensitive cell tracking, spanning from the whole-body to cellular scales.

Variability of fluorescence intensity distribution measured by flow cytometry is influenced by cell size and cell cycle progression

The distribution of fluorescence signals measured with flow cytometry can be influenced by several factors, including qualitative and quantitative properties of the used fluorochromes, optical properties of the detection system, as well as the variability within the analyzed cell population itself. Most of the single cell samples prepared from in vitrocultures or clinical specimens contain a variable cell cycle component. Cell cycle, together with changes in the cell size, are two of the factors that alter the functional properties of analyzed cells and thus affect the interpretation of obtained results. Here, we describe the association between cell cycle status and cell size, and the variability in the distribution of fluorescence intensity as determined with flow cytometry, at population scale. We show that variability in the distribution of background and specific fluorescence signals is related to the cell cycle state of the selected population, with the 10% low fluorescence signal fraction enriched mainly in cells in their G0/G1 cell cycle phase, and the 10% high fraction containing cells mostly in the G2/M phase. Therefore we advise using caution and additional experimental validation when comparing populations defined by fractions at both ends of fluorescence signal distribution to avoid biases caused by the effect of cell cycle and cell size.

Anti-androgen therapy induces transcriptomic reprogramming in metastatic castration-resistant prostate cancer in a murine model

Despite recent development of next-generation androgen receptor (AR) antagonists, metastatic castration-resistant prostate cancer (CRPC) remains incurable and requires deeper understanding through studies in suitable animal models. Prostate-specific deletion of Pten and Smad4 in mice recapitulated the disease progression of human prostate adenocarcinoma, including metastasis to lymph nodes and lung. Moreover, Pten/Smad4 tumors fostered an immunosuppressive microenvironment dominated by myeloid-derived suppressor cells (MDSCs). However, the response of Pten/Smad4 tumors to androgen deprivation and anti-androgen therapies has not been described. Here, we report that the combination of surgical castration and enzalutamide treatment in Pten/Smad4 mice slowed down the tumor growth and prolonged the median survival of the mice for 8 weeks. Treatment-naïve and castration-resistant primary tumors exhibited comparable levels of immune infiltrations with the exception of reduced monocytic MDSCs in CRPC. RNA profiling of treatment-naïve and castration-resistant primary tumors revealed largely preserved transcriptome with modest expressional alterations of collagen-related and immune-related genes, among which CC chemokine receptor type 2 (Ccr2) downregulation and predicted negative activation in CRPC was consistent with reduced monocytic MDSC infiltration. Importantly, significant transcriptomic reprograming was observed in lung metastatic CRPC compared with primary CRPC and enriched for immune-related and coagulation-related pathways. At the individual gene level, we validated the expression changes of some of the most upregulated (Cd36, Bmp5, Bmp6, Etv5, Prex2, Ptprb, Egfl6, Itga8 and Cxcl12) and downregulated genes (Cxcl9 and Adamts5). Together, this study uncovers the inherent activity of Pten/Smad4 tumors to progress to CRPC and highlights potentially targetable transcriptomic signatures associated with CRPC metastasis.

Membrane metalloendopeptidase suppresses prostate carcinogenesis by attenuating effects of gastrin-releasing peptide on stem/progenitor cells

Aberrant neuroendocrine signaling is frequent yet poorly understood feature of prostate cancers. Membrane metalloendopeptidase (MME) is responsible for the catalytic inactivation of neuropeptide substrates, and is downregulated in nearly 50% of prostate cancers. However its role in prostate carcinogenesis, including formation of castration-resistant prostate carcinomas, remains uncertain. Here we report that MME cooperates with PTEN in suppression of carcinogenesis by controlling activities of prostate stem/progenitor cells. Lack of MME and PTEN results in development of adenocarcinomas characterized by propensity for vascular invasion and formation of proliferative neuroendocrine clusters after castration. Effects of MME on prostate stem/progenitor cells depend on its catalytic activity and can be recapitulated by addition of the MME substrate, gastrin-releasing peptide (GRP). Knockdown or inhibition of GRP receptor (GRPR) abrogate effects of MME deficiency and delay growth of human prostate cancer xenografts by reducing the number of cancer-propagating cells. In sum, our study provides a definitive proof of tumor-suppressive role of MME, links GRP/GRPR signaling to the control of prostate stem/progenitor cells, and shows how dysregulation of such signaling may promote formation of castration-resistant prostate carcinomas. It also identifies GRPR as a valuable target for therapies aimed at eradication of cancer-propagating cells in prostate cancers with MME downregulation.

Transgenic Animal Models to Visualize Cancer-Related Cellular Processes by Bioluminescence Imaging

Preclinical animal models are valuable tools to improve treatments of malignant diseases, being an intermediate step of experimentation between cell culture and human clinical trials. Among different animal models frequently used in cancer research are mouse and, more recently, zebrafish models. Indeed, most of the cellular pathways are highly conserved between human, mouse and zebrafish, thus rendering these models very attractive. Recently, several transgenic reporter mice and zebrafishes have been generated in which the luciferase reporter gene are placed under the control of a promoter whose activity is strictly related to specific cancer cellular processes. Other mouse models have been generated by the cDNA luciferase knockin in the locus of a gene whose expression/activity has increased in cancer. Using BioLuminescence Imaging (BLI), we have now the opportunity to spatiotemporal visualize cell behaviors, among which proliferation, apoptosis, migration and immune responses, in any body district in living animal in a time frame process. We provide here a review of the available models to visualized cancer and cancer-associated events in living animals by BLI and as they have been successful in identifying new stages of early tumor progression, new interactions between different tissues and new therapeutic responsiveness.

Loss of MAOA in epithelia inhibits adenocarcinoma development, cell proliferation and cancer stem cells in prostate

Monoamine oxidase A (MAOA) is a mitochondrial enzyme, which degrades monoamine neurotransmitters and dietary amines and produces H₂O₂. Recent studies have shown increased MAOA expression in prostate cancer (PCa), glioma, and classical Hodgkin lymphoma. However, the biological function of MAOA in cancer development remains unknown. In this study, we investigated the role of MAOA in the development of prostate adenocarcinoma by creating a prostate-specific Pten/MAOA knockout (KO) mouse model, in which MAOA-floxP mouse was crossed with the conditional Pten KO PCa mouse that develops invasive PCa. In contrast to Pten KO mice, age-matched Pten/MAOA KO mice exhibited a significant decrease in both prostate size and the incidence of invasive cancer. We observed a significant decline in AKT phosphorylation and Ki67 expression in Pten/MAOA KO mice, which reduced epithelial cell growth and proliferation. As cancer stem cells (CSCs) are required for tumor initiation and growth, we investigated expression of OCT4 and NANOG in the setting of decreased MAOA expression. We found that both OCT4 and NANOG were significantly attenuated in the prostate epithelia of Pten/MAOA KO mice compared to Pten KO mice, which was confirmed with targeted knockdown of MAOA with a short-hairpin(sh) vector targeting MAOA compared to cells transfected with a control vector. Expression of other markers associated with the a stem cell phenotype, including CD44, α2β1, and CD133 as well as HIF-1α⁺CD44⁺ stem cells were all decreased in shMAOA PCa cells compared with empty vector-transfected control cells. We also found spheroid formation ability in PCa cells was decreased when endogenous MAOA was suppressed by siRNA or MAOA inhibitor clorgyline in a colony formation assay. Using the TCGA database, elevated MAOA expression was associated with reduced Pten levels in high Gleason grade in patient samples. Further, we found that Pten-positive PCa cells were more resistant to clorgyline treatments than Pten-null cells in tumorigenicity and stemness. Taken together, these studies suggest that MAOA expression promotes PCa development by increasing cell proliferation and CSCs and highlights the potential use of MAOA inhibitors for the treatment of PCa.

Quantitative volumetric imaging of normal, neoplastic and hyperplastic mouse prostate using ultrasound

Background

Genetically engineered mouse models are essential to the investigation of the molecular mechanisms underlying human prostate pathology and the effects of therapy on the diseased prostate. Serial in vivo volumetric imaging expands the scope and accuracy of experimental investigations of models of normal prostate physiology, benign prostatic hyperplasia and prostate cancer, which are otherwise limited by the anatomy of the mouse prostate. Moreover, accurate imaging of hyperplastic and tumorigenic prostates is now recognized as essential to rigorous pre-clinical trials of new therapies. Bioluminescent imaging has been widely used to determine prostate tumor size, but is semi-quantitative at best. Magnetic resonance imaging can determine prostate volume very accurately, but is expensive and has low throughput. We therefore sought to develop and implement a high throughput, low cost, and accurate serial imaging protocol for the mouse prostate.

Methods

We developed a high frequency ultrasound imaging technique employing 3D reconstruction that allows rapid and precise assessment of mouse prostate volume. Wild-type mouse prostates were examined (n = 4) for reproducible baseline imaging, and treatment effects on volume were compared, and blinded data analyzed for intra- and inter-operator assessments of reproducibility by correlation and for Bland-Altman analysis. Examples of benign prostatic hyperplasia mouse model prostate (n = 2) and mouse prostate implantation of orthotopic human prostate cancer tumor and its growth (n = 6) are also demonstrated.

Results

Serial measurement volume of the mouse prostate revealed that high frequency ultrasound was very precise. Following endocrine manipulation, regression and regrowth of the prostate could be monitored with very low intra- and interobserver variability. This technique was also valuable to monitor the development of prostate growth in a model of benign prostatic hyperplasia. Additionally, we demonstrate accurate ultrasound image-guided implantation of orthotopic tumor xenografts and monitoring of subsequent tumor growth from ~10 to ~750 mm³ volume.

Discussion

High frequency ultrasound imaging allows precise determination of normal, neoplastic and hyperplastic mouse prostate. Low cost and small image size allows incorporation of this imaging modality inside clean animal facilities, and thereby imaging of immunocompromised models. 3D reconstruction for volume determination is easily mastered, and both small and large relative changes in volume are accurately visualized. Ultrasound imaging does not rely on penetration of exogenous imaging agents, and so may therefore better measure poorly vascularized or necrotic diseased tissue, relative to bioluminescent imaging (IVIS).

Conclusions

Our method is precise and reproducible with very low inter- and intra-observer variability. Because it is non-invasive, mouse models of prostatic disease states can be imaged serially, reducing inter-animal variability, and enhancing the power to detect small volume changes following therapeutic intervention.

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?

Overexpression of miR-155 in the liver of transgenic mice alters the expression profiling of hepatic genes associated with lipid metabolism

Hepatic expression profiling has revealed miRNA changes in liver diseases, while hepatic miR-155 expression was increased in murine non-alcoholic fatty liver disease, suggesting that miR-155 might regulate the biological process of lipid metabolism. To illustrate the effects of miR-155 gain of function in transgenic mouse liver on lipid metabolism, transgenic mice (i.e., Rm155LG mice) for the conditional overexpression of mouse miR-155 transgene mediated by Cre/lox P system were firstly generated around the world in this study. Rm155LG mice were further crossed to Alb-Cre mice to realize the liver-specific overexpression of miR-155 transgene in Rm155LG/Alb-Cre double transgenic mice which showed the unaltered body weight, liver weight, epididymal fat pad weight and gross morphology and appearance of liver. Furthermore, liver-specific overexpression of miR-155 transgene resulted in significantly reduced levels of serum total cholesterol, triglycerides (TG) and high-density lipoprotein (HDL), as well as remarkably decreased contents of hepatic lipid, TG, HDL and free fatty acid in Rm155LG/Alb-Cre transgenic mice. More importantly, microarray data revealed a general downward trend in the expression profile of hepatic genes with functions typically associated with fatty acid, cholesterol and triglyceride metabolism, which is likely at least partially responsible for serum cholesterol and triglyceride lowering observed in Rm155LG/Alb-Cre mice. In this study, we demonstrated that hepatic overexpression of miR-155 alleviated nonalcoholic fatty liver induced by a high-fat diet. Additionally, carboxylesterase 3/triacylglycerol hydrolase (Ces3/TGH) was identified as a direct miR-155 target gene that is potentially responsible for the partial liver phenotypes observed in Rm155LG/Alb-Cre mice. Taken together, these data from miR-155 gain of function study suggest, for what we believe is the first time, the altered lipid metabolism and provide new insights into the metabolic state of the liver in Rm155LG/Alb-Cre mice.

Expression and functional role of orphan receptor GPR158 in prostate cancer growth and progression

Prostate cancer (PCa) is the second-leading cause of cancer-related mortality, after lung cancer, in men from developed countries. In its early stages, primary tumor growth is dependent on androgens, thus generally can be controlled by androgen deprivation therapy (ADT). Eventually however, the disease progresses to castration-resistant prostate cancer (CRPC), a lethal form in need of more effective treatments. G-protein coupled receptors (GPCRs) comprise a large clan of cell surface proteins that have been implicated as therapeutic targets in PCa growth and progression. The findings reported here provide intriguing evidence of a role for the newly characterized glutamate family member GPR158 in PCa growth and progression. We found that GPR158 promotes PCa cell proliferation independent of androgen receptor (AR) functionality and that this requires its localization in the nucleus of the cell. This suggests that GPR158 acts by mechanisms different from other GPCRs. GPR158 expression is stimulated by androgens and GPR158 stimulates AR expression, implying a potential to sensitize tumors to low androgen conditions during ADT via a positive feedback loop. Further, we found GPR158 expression correlates with a neuroendocrine (NE) differentiation phenotype and promotes anchorage-independent colony formation implying a role for GPR158 in therapeutic progression and tumor formation. GPR158 expression was increased at the invading front of prostate tumors that formed in the genetically defined conditional Pten knockout mouse model, and co-localized with elevated AR expression in the cell nucleus. Kaplan-Meier analysis on a dataset from the Memorial Sloan Kettering cancer genome portal showed that increased GPR158 expression in tumors is associated with lower disease-free survival. Our findings strongly suggest that pharmaceuticals targeting GPR158 activities could represent a novel and innovative approach to the prevention and management of CRPC.

Permissive expansion and homing of adoptively transferred T cells in tumor-bearing hosts

Activated T cells expressing endogenous or transduced TCRs are two cell types currently used in clinical adoptive T-cell therapy. The ability of these cells to recognize their antigen, expand and traffic to the tumor site are the initial steps necessary for successful therapy. In this study, we used in vivo bioluminescent imaging (BLI) of Renilla luciferase (RLuc) expressing T cells to evaluate the ability of adoptively transferred T cells to survive, expand and home to tumor site in vivo. Using this method, termed RT-Rack (Rluc T cell tracking), we followed T-cell response against tumors in vivo. Expansion and homing of adoptively transferred T cells were antigen dependent, but independent of the host immune status. Moreover, we successfully detected T-cell response to small and large tumors, including autochthonous liver tumors. The adoptively transferred T cells were not ignorant or excluded in a partially tolerant host, which expressed low level of the target in the periphery. Using T cell receptor (TCR)-engineered T cells, we showed the ability of these cells to respond in tumor-bearing hosts by expanding and homing to the tumor site. In all these models, the host immune status, the nature of the tumor or of the antigen, the tumor size and the presence of the targeted antigen in the periphery did not prevent the adoptively transferred T cells from responding by expanding and homing to the tumor. However, T cells had higher expression of the inhibitory receptor PD1 and reduced functional activity when a self-antigen was targeted.

Transgenic mouse model expressing P53(R172H), luciferase, EGFP, and KRAS(G12D) in a single open reading frame for live imaging of tumor

Genetically engineered mouse cancer models allow tumors to be imaged in vivo via co-expression of a reporter gene with a tumor-initiating gene. However, differential transcriptional and translational regulation between the tumor-initiating gene and the reporter gene can result in inconsistency between the actual tumor size and the size indicated by the imaging assay. To overcome this limitation, we developed a transgenic mouse in which two oncogenes, encoding P53^R172H and KRAS^G12D, are expressed together with two reporter genes, encoding enhanced green fluorescent protein (EGFP) and firefly luciferase, in a single open reading frame following Cre-mediated DNA excision. Systemic administration of adenovirus encoding Cre to these mice induced specific transgene expression in the liver. Repeated bioluminescence imaging of the mice revealed a continuous increase in the bioluminescent signal over time. A strong correlation was found between the bioluminescent signal and actual tumor size. Interestingly, all liver tumors induced by P53^R172H and KRAS^G12D in the model were hepatocellular adenomas. The mouse model was also used to trace cell proliferation in the epidermis via live fluorescence imaging. We anticipate that the transgenic mouse model will be useful for imaging tumor development in vivo and for investigating the oncogenic collaboration between P53^R172H and KRAS^G12D.

Non-invasive molecular imaging for preclinical cancer therapeutic development

Molecular and non-invasive imaging are rapidly emerging fields in preclinical cancer drug discovery. This is driven by the need to develop more efficacious and safer treatments, the advent of molecular-targeted therapeutics, and the requirements to reduce and refine current preclinical in vivo models. Such bioimaging strategies include MRI, PET, single positron emission computed tomography, ultrasound, and optical approaches such as bioluminescence and fluorescence imaging. These molecular imaging modalities have several advantages over traditional screening methods, not least the ability to quantitatively monitor pharmacodynamic changes at the cellular and molecular level in living animals non-invasively in real time. This review aims to provide an overview of non-invasive molecular imaging techniques, highlighting the strengths, limitations and versatility of these approaches in preclinical cancer drug discovery and development.

Dependence of castration-resistant prostate cancer (CRPC) stem cells on CRPC-associated fibroblasts

We previously established a role for cancer-associated fibroblasts (CAF) in enhancing the self-renewal and differentiation potentials of putative prostate cancer stem cells (CSC). Our published work focused on androgen-dependent prostate cancer (ADPC) using the conditional Pten deletion mouse model. Employing the same model, we now describe the interaction of CAF and CSC in castration-resistant prostate cancer (CRPC). CAF isolated from ADPC (ADPCAF) and from CRPC (CRPCAF) were compared in terms of their ability to support organoid formation and tumor initiation by CSC from CRPC (CRPCSC) in vitro and in vivo. CRPCSC formed spheroids in vitro and well-differentiated glandular structures under the renal capsules of recipient mice in vivo more effectively in the presence of CRPCAF compared to ADPCAF. Furthermore, whereas CSC with CAF from ADPC formed mostly well-differentiated tumors in our previous study, we now show that CRPCSC, when combined with CRPCAF (but not ADPCAF), can form aggressive, poorly-differentiated tumors. The potential of CRPCAF to support organoid/tumor formation by CRPCSC remained greater even when compared to 10-fold more ADPCAF, suggesting that paracrine factors produced specifically by CRPCAF preferentially potentiate the stemness and tumorigenic properties of the corresponding CSC. This apparently unique property of CRPCAF was notable when the CAF and CSC were grafted in either intact or castrated recipient mice. In both environments, CRPCAF induced in the epithelial compartment higher proliferative activity compared to ADPCAF, indicated by a higher Ki67 index. Factors released by CRPCAF to regulate CRPCSC may be targeted to develop novel therapeutic approaches to manage advanced prostate cancer.

[How do metastases of urological tumors develop?]

This overview mainly focuses on the topic of epithelial tumors (carcinomas) because urological tumors are generally of this type. The importance of the topic is reflected by the fact that patients rarely die of the primary tumor, but the majority die of metastases that cause life-threatening situations. More recent findings show that treatment decisions should be based on the metastasis site and less on the tumor's tissue of origin. Given the progression of clinical oncology toward individualized medicine, a better understanding of the biology of metastases is therefore acute and includes some important challenges.

Bioluminescent imaging of HPV-positive oral tumor growth and its response to image-guided radiotherapy

The treatment paradigms for head and neck squamous cell cancer (HNSCC) are changing due to the emergence of human papillomavirus (HPV)-associated tumors possessing distinct molecular profiles and responses to therapy. Although patients with HNSCCs are often treated with radiotherapy, preclinical models are limited by the ability to deliver precise radiation to orthotopic tumors and to monitor treatment responses accordingly. To better model this clinical scenario, we developed a novel autochthonous HPV-positive oral tumor model to track responses to small molecules and image-guided radiation. We used a tamoxifen-regulated Cre recombinase system to conditionally express the HPV oncogenes E6 and E7 as well as a luciferase reporter (iHPV-Luc) in the epithelial cells of transgenic mice. In the presence of activated Cre recombinase, luciferase activity, and by proxy, HPV oncogenes were induced to 11-fold higher levels. In triple transgenic mice containing the iHPV-Luc, K14-CreER(tam), and LSL-Kras transgenes, tamoxifen treatment resulted in oral tumor development with increased bioluminescent activity within 6 days that reached a maximum of 74.8-fold higher bioluminescence compared with uninduced mice. Oral tumors expressed p16 and MCM7, two biomarkers associated with HPV-positive tumors. After treatment with rapamycin or image-guided radiotherapy, tumors regressed and possessed decreased bioluminescence. Thus, this novel system enables us to rapidly visualize HPV-positive tumor growth to model existing and new interventions using clinically relevant drugs and radiotherapy techniques.

CAF-secreted annexin A1 induces prostate cancer cells to gain stem cell-like features

Annexin A1 (AnxA1), a phospholipid-binding protein and regulator of glucocorticoid-induced inflammatory signaling, has implications in cancer. Here, a role for AnxA1 in prostate adenocarcinoma was determined using primary cultures and a tumor cell line (cE1), all derived from the conditional Pten deletion mouse model of prostate cancer. AnxA1 secretion by prostate-derived cancer-associated fibroblasts (CAF) was significantly higher than by normal prostate fibroblasts (NPF). Prostate tumor cells were sorted to enrich for epithelial subpopulations based on nonhematopoietic lineage, high SCA-1, and high or medium levels of CD49f. Compared with controls, AnxA1 enhanced stem cell-like properties in high- and medium-expression subpopulations of sorted cE1 and primary cells, in vitro, through formation of greater number of spheroids with increased complexity, and in vivo, through generation of more, larger, and histologically complex glandular structures, along with increased expression of p63, a basal/progenitor marker. The differentiated medium-expression subpopulations from cE1 and primary cells were most susceptible to gain stem cell-like properties as shown by increased spheroid and glandular formation. Further supporting this increased plasticity, AnxA1 was shown to regulate epithelial-to-mesenchymal transition in cE1 cells. These results suggest that CAF-secreted AnxA1 contributes to tumor stem cell dynamics via two separate but complementary pathways: induction of a dedifferentiation process leading to generation of stem-like cells from a subpopulation of cancer epithelial cells and stimulation of proliferation and differentiation of the cancer stem-like cells.

IMPLICATIONS

AnxA1 participates in a paradigm in which malignant prostate epithelial cells that are not cancer stem cells are induced to gain cancer stem cell-like properties.

Generation of Rm21LG transgenic mice: a powerful tool to generate conditional overexpression of miR-21 that is involved in oncogenesis

miR-21 is highly expressed in a variety of cancers, suggesting that it might play a role in the process of oncogenesis, as supported by it directly causing pre-B cell lymphomas in transgenic mice. Rm21LG transgenic mice for the conditional co-expression of miR-21 and luciferase (Luc) mediated by Cre/lox P system were generated. The homozygous Rm21LG transgenic mice were visually and readily characterized immediately after birth by whole-body fluorescence imaging. More importantly, miR-21 and Luc were successfully activated in the liver of Rm21LG/Alb-Cre double-transgenic mice, demonstrating that Rm21LG conditional transgenic system could work in a Cre-dependent manner. The combined use of this conditional miR-21 transgenic mouse line, various cell/tissue-specific Cre mouse lines and bioluminescence imaging will be a valuable tool in vivo to uncover the functions of miR-21 as oncomiR in initiating tumors.

Contextual effect of repression of bone morphogenetic protein activity in prostate cancer

Several studies have focused on the effect of bone morphogenetic protein (BMP) on prostate cancer homing and growth at distant metastatic sites, but very little effect at the primary site. Here, we used two cell lines, one (E8) isolated from a primary tumor and the other (cE1) from a recurrent tumor arising at the primary site, both from the conditional Pten deletion mouse model of prostatic adenocarcinoma. Over-expression of the BMP antagonist noggin inhibited proliferation of cE1 cells in vitro while enhancing their ability to migrate. On the other hand, cE1/noggin grafts grown in vivo showed a greater mass and a higher proliferation index than the cE1/control grafts. For suppression of BMP activity in the context of cancer-associated fibroblasts (CAFs), we used noggin-transduced CAFs from the same mouse model to determine their effect on E8- or cE1-induced tumor growth. CAF/noggin led to increased tumor mass and greater de-differentiation of the E8 cell when compared with tumors formed in the presence of CAF/control cells. A trend of increase in the size of the tumor was also noted for cE1 cells when inoculated with CAF/noggin. Together, the results may point to a potential inhibitory role of BMP in the growth or re-growth of prostate tumor at the primary site. Additionally, results for cE1/noggin, and cE1 mixed with CAF/noggin, suggested that suppression of BMP activity in the cancer cells may have a stronger growth-enhancing effect on the tumor than its suppression in the fibroblastic compartment of the tumor microenvironment.

Monoclonal antibody against cell surface GRP78 as a novel agent in suppressing PI3K/AKT signaling, tumor growth, and metastasis

PURPOSE

The ER chaperone GRP78 translocates to the surface of tumor cells and promotes survival, metastasis, and resistance to therapy. An oncogenic function of cell surface GRP78 has been attributed to the activation of the phosphoinositide 3-kinase (PI3K) pathway. We intend to use a novel anti-GRP78 monoclonal antibody (MAb159) to attenuate PI3K signaling and inhibit tumor growth and metastasis.

EXPERIMENTAL DESIGN

MAb159 was characterized biochemically. Antitumor activity was tested in cancer cell culture, tumor xenograft models, tumor metastasis models, and spontaneous tumor models. Cancer cells and tumor tissues were analyzed for PI3K activity. MAb159 was humanized and validated for diagnostic and therapeutic application.

RESULTS

MAb159 specifically recognized surface GRP78, triggered GRP78 endocytosis, and localized to tumors but not to normal organs in vivo. MAb159 inhibited tumor cell proliferation and enhanced tumor cell death both in vitro and in vivo. In MAb159-treated tumors, PI3K signaling was inhibited without compensatory MAPK pathway activation. Furthermore, MAb159 halted or reversed tumor progression in the spontaneous PTEN-loss-driven prostate and leukemia tumor models, and inhibited tumor growth and metastasis in xenograft models. Humanized MAb159, which retains high affinity, tumor specific localization, and the antitumor activity, was nontoxic in mice, and had desirable pharmacokinetics.

CONCLUSIONS

GRP78-specific antibody MAb159 modulates the PI3K pathway and inhibits tumor growth and metastasis. Humanized MAb159 will enter human trials shortly.

Loss of survivin in the prostate epithelium impedes carcinogenesis in a mouse model of prostate adenocarcinoma

The inhibitor of apoptosis protein survivin is expressed in most cancers. Using the conditional PTEN deletion mouse model, we previously reported that survivin levels increase with prostate tumor growth. Here we evaluated the functional role of survivin in prostate tumor growth. First, we demonstrated that mice lacking the survivin gene in prostate epithelium were fertile and had normal prostate growth and development. We then serially, from about 10-56 weeks of age, evaluated histopathologic changes in the prostate of mice with PTEN deletion combined with survivin mono- or bi-allelic gene deletion. While within this time period most of the animals with wild-type or monoallelic survivin deletion developed adenocarcinomas, the most severe lesions in the biallelic survivin deleted mice were high-grade prostatic intra-epithelial neoplasia with distinct histopathology. Many atypical cells contained large hypertrophic cytoplasm and desmoplastic reaction in the prostatic intra-epithelial neoplasia lesions of this group was minimal until the late ages. A reduced proliferation index as well as apoptotic and senescent cells were detected in the lesions of mice with compound PTEN/survivin deficiency throughout the time points examined. Survivin deletion was also associated with reduced tumor expression of another inhibitor of apoptosis member, the X-linked inhibitor of apoptosis. Our findings suggest that survivin participates in the progression of prostatic intraepithelial neoplasia to adenocarcinoma, and that survivin interference at the prostatic intraepithelial neoplasia stages may be a potential therapeutic strategy to halt or delay further progression.

Detection and organ-specific ablation of neuroendocrine cells by synaptophysin locus-based BAC cassette in transgenic mice

The role of cells of the diffuse neuroendocrine system in development and maintenance of individual organs and tissues remains poorly understood. Here we identify a regulatory region sufficient for accurate in vivo expression of synaptophysin (SYP), a common marker of neuroendocrine differentiation, and report generation of Tg(Syp-EGFP(loxP)-DTA)147(Ayn) (SypELDTA) mice suitable for flexible organ-specific ablation of neuroendocrine cells. These mice express EGFP and diphtheria toxin fragment A (DTA) in SYP positive cells before and after Cre-loxP mediated recombination, respectively. As a proof of principle, we have crossed SypELDTA mice with EIIA-Cre and PB-Cre4 mice. EIIA-Cre mice express Cre recombinase in a broad range of tissues, while PB-Cre4 mice specifically express Cre recombinase in the prostate epithelium. Double transgenic EIIA-Cre; SypELDTA embryos exhibited massive cell death in SYP positive cells. At the same time, PB-Cre4; SypELDTA mice showed a substantial decrease in the number of neuroendocrine cells and associated prostate hypotrophy. As no increase in cell death and/or Cre-loxP mediated recombination was observed in non-neuroendocrine epithelium cells, these results suggest that neuroendocrine cells play an important role in prostate development. High cell type specificity of Syp locus-based cassette and versatility of generated mouse model should assure applicability of these resources to studies of neuroendocrine cell functions in various tissues and organs.

Illuminating cancer systems with genetically engineered mouse models and coupled luciferase reporters in vivo

Bioluminescent imaging (BLI) is a powerful noninvasive tool that has dramatically accelerated the in vivo interrogation of cancer systems and longitudinal analysis of mouse models of cancer over the past decade. Various luciferase enzymes have been genetically engineered into mouse models (GEMM) of cancer, which permit investigation of cellular and molecular events associated with oncogenic transcription, posttranslational processing, protein-protein interactions, transformation, and oncogene addiction in live cells and animals. Luciferase-coupled GEMMs ultimately serve as a noninvasive, repetitive, longitudinal, and physiologic means by which cancer systems and therapeutic responses can be investigated accurately within the autochthonous context of a living animal.

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.

Conditionally ablated Pten in prostate basal cells promotes basal-to-luminal differentiation and causes invasive prostate cancer in mice

Prostate glands comprise two major epithelial cell types: luminal and basal. Luminal cells have long been considered the cellular origin of prostate cancer (CaP). However, recent evidence from a prostate regeneration assay suggests that prostate basal cells can also give rise to CaP. Here, we characterize Pten-deficient prostate lesions arising from keratin 5-expressing basal cells in a temporally controlled system in mice. Pten-deficient prostate lesions arising from basal cells exhibited luminal phenotypes with higher invasiveness, and the cell fate of Pten-deficient basal cells was traced to neoplastic luminal cells. After temporally ablating Pten in keratin 8-expressing luminal cells, luminal-derived Pten-deficient prostate tumors exhibited slower disease progression, compared with basal-derived tumors, within 13 weeks after Pten ablation. Cellular proliferation was significantly increased in basal-derived versus luminal-derived Pten-deficient prostate lesions. Increased tumor invasion into the smooth muscle layer and aberrantly regulated aggressive signatures (Smad4 and Spp1) were identified exclusively in basal-derived Pten-deficient lesions. Interestingly, p63-expressing cells, which represent basal stem and progenitor cells of basal-derived Pten-deficient prostate lesions, were significantly increased, relative to cells of the luminal-derived prostate lesion. Furthermore, castration did not suppress cellular proliferation of either basal-derived or luminal-derived Pten-deficient prostate tumors. Taken together, our data suggest that, although prostate malignancy can originate from both basal and luminal populations, these two populations differ in aggressive potential.

Prostate cancer stem cells: are they androgen-responsive?

The prostate gland is highly dependent on androgens for its development, growth and function. Consequently, the prostatic epithelium predominantly consists of androgen-dependent luminal cells, which express the androgen receptor at high levels. In contrast, androgens are not required for the survival of the androgen-responsive, but androgen-independent, basal compartment in which stem cells reside. Basal and luminal cells are linked in a hierarchical pathway, which most probably exists as a continuum with different stages of phenotypic change. Prostate cancer is also characterised by heterogeneity, which is reflected in its response to treatment. The putative androgen receptor negative cancer stem cell (CSC) is likely to form a resistant core after most androgen-based therapies, contributing to the evolution of castration-resistant disease. The development of CSC-targeted therapies is now of crucial importance and identifying the phenotypic differences between CSCs and both their progeny will be key in this process.

JNK and PTEN cooperatively control the development of invasive adenocarcinoma of the prostate

The c-Jun NH(2)-terminal kinase (JNK) signal transduction pathway is implicated in cancer, but the role of JNK in tumorigenesis is poorly understood. Here, we demonstrate that the JNK signaling pathway reduces the development of invasive adenocarcinoma in the phosphatase and tensin homolog (Pten) conditional deletion model of prostate cancer. Mice with JNK deficiency in the prostate epithelium (ΔJnk ΔPten mice) develop androgen-independent metastatic prostate cancer more rapidly than control (ΔPten) mice. Similarly, prevention of JNK activation in the prostate epithelium (ΔMkk4 ΔMkk7 ΔPten mice) causes rapid development of invasive adenocarcinoma. We found that JNK signaling defects cause an androgen-independent expansion of the immature progenitor cell population in the primary tumor. The JNK-deficient progenitor cells display increased proliferation and tumorigenic potential compared with progenitor cells from control prostate tumors. These data demonstrate that the JNK and PTEN signaling pathways can cooperate to regulate the progression of prostate neoplasia to invasive adenocarcinoma.

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.

In vivo optical imaging and dynamic contrast methods for biomedical research

This paper provides an overview of optical imaging methods commonly applied to basic research applications. Optical imaging is well suited for non-clinical use, since it can exploit an enormous range of endogenous and exogenous forms of contrast that provide information about the structure and function of tissues ranging from single cells to entire organisms. An additional benefit of optical imaging that is often under-exploited is its ability to acquire data at high speeds; a feature that enables it to not only observe static distributions of contrast, but to probe and characterize dynamic events related to physiology, disease progression and acute interventions in real time. The benefits and limitations of in vivo optical imaging for biomedical research applications are described, followed by a perspective on future applications of optical imaging for basic research centred on a recently introduced real-time imaging technique called dynamic contrast-enhanced small animal molecular imaging (DyCE).

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.

Slow disease progression in a C57BL/6 pten-deficient mouse model of prostate cancer

Prostate-specific deletion of Pten in mice has been reported to recapitulate histological progression of human prostate cancer. To improve on this model, we introduced the conditional ROSA26 luciferase reporter allele to monitor prostate cancer progression via bioluminescence imaging and extensively backcrossed mice onto the albino C57BL/6 genetic background to address variability in tumor kinetics and to enhance imaging sensitivity. Bioluminescence signal increased rapidly in Pten(p-/-) mice from 3 to 11 weeks, but was much slower from 11 to 52 weeks. Changes in bioluminescence signal were correlated with epithelial proliferation. Magnetic resonance imaging revealed progressive increases in prostate volume, which were attributed to excessive fluid retention in the anterior prostate and to expansion of the stroma. Development of invasive prostate cancer in 52-week-old Pten(p-/-) mice was rare, indicating that disease progression was slowed relative to that in previous reports. Tumors in these mice exhibited a spontaneous inflammatory phenotype and were rapidly infiltrated by myeloid-derived suppressor cells. Although Pten(p-/-) tumors responded to androgen withdrawal, they failed to exhibit relapsed growth for up to 1 year. Taken together, these data identify a mild prostate cancer phenotype in C57BL/6 prostate-specific Pten-deficient mice, reflecting effects of the C57BL/6 genetic background on cancer progression. This model provides a platform for noninvasive assessment of how genetic and environmental risk factors may affect disease progression.

TPL2/COT/MAP3K8 (TPL2) activation promotes androgen depletion-independent (ADI) prostate cancer growth

BACKGROUND

Despite its initial positive response to hormone ablation therapy, prostate cancers invariably recur in more aggressive, treatment resistant forms. The lack of our understanding of underlying genetic alterations for the transition from androgen-dependent (AD) to ADI prostate cancer growth hampers our ability to develop target-driven therapeutic strategies for the efficient treatment of ADI prostate cancer.

METHODOLOGY/PRINCIPAL FINDINGS

By screening a library of activated human kinases, we have identified TPL2, encoding a serine/threonine kinase, as driving ADI prostate cancer growth. TPL2 activation by over-expressing either wild-type or a constitutively activated form of TPL2 induced ADI growth, whereas the suppression of TPL2 expression and its kinase activity in ADI prostate cancer cells inhibited cell proliferation under androgen-depleted conditions. Most importantly, TPL2 is upregulated in ADI prostate cancers of both the Pten deletion mouse model and the clinical prostate cancer specimens.

CONCLUSIONS/SIGNIFICANCE

Together these data suggest that TPL2 kinase plays a critical role in the promotion of ADI prostate cancer progression. Furthermore, the suppression of TPL2 diminishes ADI prostate cancer growth and a high frequency of TPL2 overexpression in human ADI prostate cancer samples validates TPL2 as a target for the treatment of this deadly disease.

A Cre-reporter transgenic mouse expressing the far-red fluorescent protein Katushka

Cre/loxP-dependent expression of fluorescent proteins represents a powerful biological tool for cell lineage, fate-mapping, and genetic analysis. Live tissue imaging has significantly improved with the development of far-red fluorescent proteins, with optimized spectral characteristics for in vivo applications. Here, we report the generation of the first transgenic mouse line expressing the far-red fluorescent protein Katushka, driven by the hybrid CAG promoter upon Cre-mediated recombination. After germ line or tissue-specific Cre-driven reporter activation, Katushka expression is strong and ubiquitous, without toxic effects, allowing fluorescence detection in fresh and fixed samples from all tissues examined. Moreover, fluorescence can be detected by in vivo noninvasive whole-body imaging when Katuhska is expressed exclusively in a specific cell population deep within the animal body such as pancreatic beta cells. Thus, this reporter model enables early, widespread, and sensitive in vivo detection of Cre activity and should provide a versatile tool for a wide spectrum of fluorescence and live-imaging applications.

Molecular genetics of prostate cancer: new prospects for old challenges

Despite much recent progress, prostate cancer continues to represent a major cause of cancer-related mortality and morbidity in men. Since early studies on the role of the androgen receptor that led to the advent of androgen deprivation therapy in the 1940s, there has long been intensive interest in the basic mechanisms underlying prostate cancer initiation and progression, as well as the potential to target these processes for therapeutic intervention. Here, we present an overview of major themes in prostate cancer research, focusing on current knowledge of principal events in cancer initiation and progression. We discuss recent advances, including new insights into the mechanisms of castration resistance, identification of stem cells and tumor-initiating cells, and development of mouse models for preclinical evaluation of novel therapuetics. Overall, we highlight the tremendous research progress made in recent years, and underscore the challenges that lie ahead.

Cancer-associated fibroblasts enhance the gland-forming capability of prostate cancer stem cells

Signals originating from cancer-associated fibroblasts (CAF) may positively regulate proliferation and tumorigenicity in prostate cancer. In this study, we investigated whether CAFs may regulate the biology of prostate cancer stem cells (CSC) by using a conditional Pten deletion mouse model of prostate adenocarcinoma to isolate both CAF cultures and CSC-enriched cell fractions from the tumors. CSCs that were isolated possessed self-renewal, spheroid-forming, and multipotential differentiation activities in tissue culture, segregating with a cell fraction exhibiting a signature expression phenotype, including SCA-1 (high), CD49f (high), CK5 (high), p63 (high), Survivin (high), RUNX2 (high), CD44 (low), CD133 (low), CK18 (low), and Androgen Receptor (low). CSC spheroid-forming efficiency was differentially influenced by the nature of fibroblasts in a coculture system: Compared with mouse urogenital sinus mesenchyme or normal prostate fibroblasts, CAFs enhanced spheroid formation, with the spheroids displaying generally larger sizes and more complex histology. Graft experiments showed that CSCs admixed with CAFs produced prostatic glandular structures with more numerous lesions, high proliferative index, and tumor-like histopathologies, compared with those formed in the presence of normal prostate fibroblasts. Together, our findings underscore a significant role of CAFs in CSC biology.

Hepsin cooperates with MYC in the progression of adenocarcinoma in a prostate cancer mouse model

BACKGROUND

Hepsin is a cell surface protease that is over-expressed in more than 90% of human prostate cancer cases. The previously developed Probasin-hepsin/Large Probasin-T antigen (PB-hepsin/LPB-Tag) bigenic mouse model of prostate cancer demonstrates that hepsin promotes primary tumors that are a mixture of adenocarcinoma and neuroendocrine (NE) lesions, and metastases that are NE in nature. However, since the majority of human prostate tumors are adenocarcinomas, the contribution of hepsin in the progression of adenocarcinoma requires further investigation.

METHODS

We crossed the PB-hepsin mice with PB-Hi-myc transgenic mouse model of prostate adenocarcinoma and characterized the tumor progression in the resulting PB-hepsin/PB-Hi-myc bigenic mice.

RESULTS

We report that PB-hepsin/PB-Hi-myc bigenic mice develop invasive adenocarcinoma at 4.5 months. Further, histological analysis of the 12- to 17-month-old mice revealed that the PB-hepsin/PB-Hi-myc model develops a higher grade adenocarcinoma compared with age-matched tumors expressing only PB-Hi-myc. Consistent with targeting hepsin to the prostate, the PB-hepsin/PB-Hi-myc tumors showed higher hepsin expression as compared to the age-matched myc tumors. Furthermore, endogenous expression of hepsin increased in the PB-Hi-myc mice as the tumors progressed.

CONCLUSIONS

Although we did not detect any metastases from the prostates in either the PB-hepsin/PB-Hi-myc or the PB-Hi-myc mice, our data suggests that hepsin and myc cooperate during the progression to high-grade prostatic adenocarcinoma.

Modeling prostate cancer: a perspective on transgenic mouse models

Despite considerable success in treatment of early stage localized prostate cancer (PC), acute inadequacy of late stage PC treatment and its inherent heterogeneity poses a formidable challenge. Clearly, an improved understanding of PC genesis and progression along with the development of new targeted therapies are warranted. Animal models, especially, transgenic immunocompetent mouse models, have proven to be the best ally in this respect. A series of models have been developed by modulation of expression of genes implicated in cancer-genesis and progression; mainly, modulation of expression of oncogenes, steroid hormone receptors, growth factors and their receptors, cell cycle and apoptosis regulators, and tumor suppressor genes have been used. Such models have contributed significantly to our understanding of the molecular and pathological aspects of PC initiation and progression. In particular, the transgenic mouse models based on multiple genetic alterations can more accurately address the inherent complexity of PC, not only in revealing the mechanisms of tumorigenesis and progression but also for clinically relevant evaluation of new therapies. Further, with advances in conditional knockout technologies, otherwise embryonically lethal gene changes can be incorporated leading to the development of new generation transgenics, thus adding significantly to our existing knowledge base. Different models and their relevance to PC research are discussed.

Bioluminescent imaging: a critical tool in pre-clinical oncology research

Bioluminescent imaging (BLI) is a non-invasive imaging modality widely used in the field of pre-clinical oncology research. Imaging of small animal tumour models using BLI involves the generation of light by luciferase-expressing cells in the animal following administration of substrate. This light may be imaged using an external detector. The technique allows a variety of tumour-associated properties to be visualized dynamically in living models. The increasing use of BLI as a small-animal imaging modality has led to advances in the development of xenogeneic, orthotopic, and genetically engineered animal models expressing luciferase genes. This review aims to provide insight into the principles of BLI and its applications in cancer research. Many studies to assess tumour growth and development, as well as efficacy of candidate therapeutics, have been performed using BLI. More recently, advances have also been made using bioluminescent imaging in studies of protein-protein interactions, genetic screening, cell-cycle regulators, and spontaneous cancer development. Such novel studies highlight the versatility and potential of bioluminescent imaging in future oncological research.

A Cre/LoxP conditional luciferase reporter transgenic mouse for bioluminescence monitoring of tumorigenesis

Genetically engineered, Cre/LoxP-conditional mouse models of cancer are designed to investigate the genetic contributors of tumorigenesis and are well suited to assess therapeutic treatment responses. The capacity to serially visualize tumor burden in a noninvasive fashion would greatly strengthen their applications. We report the generation of a bioluminescent reporter strain that allows monitoring of tumor development in preexisting conditional mouse tumor models. We demonstrate that, in a Cre-dependent glioblastoma multiforme model, tumor initiation and progression is readily monitored over time and that luminescent output is related to tumor volume. Our results show that this reporter strain may be combined with various Cre/loxP mouse tumor models to allow for noninvasive longitudinal monitoring of tumor growth and therapeutic response in vivo.

In vivo imaging of an inducible oncogenic tumor antigen visualizes tumor progression and predicts CTL tolerance

Visualizing oncogene/tumor Ag expression by noninvasive imaging is of great interest for understanding processes of tumor development and therapy. We established transgenic (Tg) mice conditionally expressing a fusion protein of the SV40 large T Ag and luciferase (TagLuc) that allows monitoring of oncogene/tumor Ag expression by bioluminescent imaging upon Cre recombinase-mediated activation. Independent of Cre-mediated recombination, the TagLuc gene was expressed at low levels in different tissues, probably due to the leakiness of the stop cassette. The level of spontaneous TagLuc expression, detected by bioluminescent imaging, varied between the different Tg lines, depended on the nature of the Tg expression cassette, and correlated with Tag-specific CTL tolerance. Following liver-specific Cre-loxP site-mediated excision of the stop cassette that separated the promoter from the TagLuc fusion gene, hepatocellular carcinoma development was visualized. The ubiquitous low level TagLuc expression caused the failure of transferred effector T cells to reject Tag-expressing tumors rather than causing graft-versus-host disease. This model may be useful to study different levels of tolerance, monitor tumor development at an early stage, and rapidly visualize the efficacy of therapeutic intervention versus potential side effects of low-level Ag expression in normal tissues.

Reprogramming murine telomerase rapidly inhibits the growth of mouse cancer cells in vitro and in vivo

Telomerase plays a critical role in cancer, prompting the pursuit of various telomerase-based therapeutic strategies. One such strategy, telomerase interference, exploits the high telomerase activity in cancer cells and reprograms telomerase to encode "toxic" telomeres. To date, telomerase interference has been tested in human cancer cells xenografted into mice, an approach that does not recapitulate spontaneous malignancy and offers few insights about host toxicities, because human telomerase is targeted in a mouse host. To address these limitations, we designed and validated two new gene constructs specifically targeting mouse telomerase: mutant template mouse telomerase RNA (MT-mTer) and small interfering RNA against wild-type mouse telomerase RNA (α-mTer-siRNA). Using lentiviral delivery in mouse prostate cancer cells, we achieved α-mTer-siRNA-mediated knockdown of wild-type mTer (80% depletion) and concurrent overexpression of MT-mTer (50-fold). We showed that the two constructs effectively synergize to reprogram murine telomerase to add mutant instead of wild-type telomeric repeats, resulting in rapid telomeric uncapping (5-fold increase in DNA damage foci). This, in turn, led to rapid and significant apoptosis (>90% of cells) and growth inhibition in vitro (90% reduction in viable cell mass) and in vivo (75% reduction in tumor allograft wet weight). In summary, we have shown that mouse cancer cells are vulnerable to direct telomerase interference using novel murine telomerase-targeting constructs; this approach can now be used to study the true therapeutic potential of telomerase interference in mouse spontaneous cancer models.

Runx2 regulates survivin expression in prostate cancer cells

Previously we described that bone morphogenetic protein-7 (BMP7) could protect prostate cancer C4-2B cells from serum starvation-induced apoptosis via survivin induction. Here, for the first time, we identify Runx2 as a key regulator of survivin transcription. In C4-2B cells grown normally, suppression of Runx2 reduced survivin expression. Using ChIP assays, two regions of the survivin promoter, -1953 to -1812 (I) and -1485 to -1119 (II) encompassing consensus Runx-binding sites were examined. Runx2 was found to be associated with both regions, with a stronger affinity to region-I. In serum-starved cells neither region was occupied, but BMP7 restored association to region-II and not region-I. In reporter assays, transcription activity by BMP7 was significantly reduced when sequences including binding sites of region-II were deleted. Additionally, Runx2 expression was enhanced by BMP7 in these cells. Along with a strong survivin expression, a trend in increased Runx2 expression in human prostate cancer cells and tissues was noted. In the conditional Pten-knockout mouse, Runx2 level increased with growth of prostate tumor. The data define a novel role of Runx2 in regulating survivin expression in malignant epithelial cells and identify it as a critical factor in BMP signaling that protects cancer cells against apoptosis.

Mouse models for cancer stem cell research

The cancer stem cell concept assumes that cancers are mainly sustained by a small pool of neoplastic cells, known as cancer stem cells or tumor initiating cells, which are able to reproduce themselves and produce phenotypically heterogeneous cells with lesser tumorigenic potential. Cancer stem cells represent an appealing target for development of more selective and efficient therapies. However, direct testing of the cancer stem cell concept and assessment of its therapeutic implications in human cancers have been complicated by the use of immunocompromised mice. Genetically defined immunocompetent autochthonous mouse models of human cancer provide a valuable tool to address this problem. Furthermore, they allow for a better understanding of the relevance of mechanisms controlling normal stem cell compartment to carcinogenesis. Advantages and disadvantages of some of the existing mouse models are reviewed, and future challenges in cancer stem cell research are outlined.

Selective expression of CD44, a putative prostate cancer stem cell marker, in neuroendocrine tumor cells of human prostate cancer

BACKGROUND

Hormonal therapy is effective for advanced prostate cancer (PC) but the disease often recurs and becomes hormone-refractory. It is hypothesized that a subpopulation of cancer cells, that is, cancer stem cells (CSCs), survives hormonal therapy and leads to tumor recurrence. CD44 expression was shown to identify tumor cells with CSC features. PC contains secretory type epithelial cells and a minor population of neuroendocrine cells. Neuroendocrine cells do not express androgen receptor and are quiescent, features associated with CSCs. The purpose of the study was to determine the expression of CD44 in human PC and its relationship to neuroendocrine tumor cells.

METHODS

Immunohistochemistry and immunofluorescence were performed to study CD44 expression in PC cell lines, single cells from fresh PC tissue and archival tissue sections of PC. We then determined if CD44+ cells represent neuroendocrine tumor cells.

RESULTS

In human PC cell lines, expression of CD44 is associated with cells of NE phenotype. In human PC tissues, NE tumor cells are virtually all positive for CD44 and CD44+ cells, excluding lymphocytes, are all NE tumor cells.

CONCLUSIONS

Selective expression of the stem cell-associated marker CD44 in NE tumor cells of PC, in combination with their other known features, further supports the significance of such cells in therapy resistance and tumor recurrence.