Transgenic models of tumor development

Genetically engineered mouse models for hereditary cancer syndromes

Advances in molecular diagnostics have led to improved diagnosis and molecular understanding of hereditary cancers in the clinic. Improving the management, treatment, and potential prevention of cancers in carriers of predisposing mutations requires preclinical experimental models that reflect the key pathogenic features of the specific syndrome associated with the mutations. Numerous genetically engineered mouse (GEM) models of hereditary cancer have been developed. In this review, we describe the models of Lynch syndrome and hereditary breast and ovarian cancer syndrome, the two most common hereditary cancer predisposition syndromes. We focus on Lynch syndrome models as illustrative of the potential for using mouse models to devise improved approaches to prevention of cancer in a high-risk population. GEM models are an invaluable tool for hereditary cancer models. Here, we review GEM models for some hereditary cancers and their potential use in cancer prevention studies.

Development of extrahepatic bile ducts and mechanisms of tumorigenesis: Lessons from mouse models

The biliary system is a highly branched tubular network consisting of intrahepatic bile ducts (IHBDs) and extrahepatic bile ducts (EHBDs). IHBDs are derived from hepatic progenitor cells, while EHBDs originate directly from the endoderm through a separate branching morphogenetic process. Traits that are important for cancer are often found to overlap in developmental and other processes. Therefore, it has been suggested that intrahepatic cholangiocarcinomas (iCCAs) and extrahepatic cholangiocarcinomas (eCCAs) have different developmental mechanisms. While much evidence is being gathered on the mechanism of iCCAs, the evidence for eCCA is still very limited. The main reason for this is that there are very few appropriate animal models for eCCA. We can gain important insights from these animal models, particularly genetically engineered mouse models (GEMMs). GEMMs are immunocompetent and mimic human CCA subtypes with a specific mutational pattern, allowing the development of precancerous lesions, that is, biliary intraepithelial neoplasia (BilIN) and intraductal papillary neoplasm of the bile duct (IPNB). This review provides a summary of the pathogenesis and mechanisms of eCCA that can be revealed by GEMMs. Furthermore, we discuss several clinical questions, such as whether BilIN and IPNB really become malignant, whether the peribiliary gland is the origin of eCCAs, and others.

Zebrafish imaging reveals TP53 mutation switching oncogene-induced senescence from suppressor to driver in primary tumorigenesis

Most tumours are thought to arise through oncogenic cell generation followed by additional mutations. How a new oncogenic cell primes tumorigenesis by acquiring additional mutations remains unclear. We show that an additional TP53 mutation stimulates primary tumorigenesis by switching oncogene-induced senescence from a tumour suppressor to a driver. Zebrafish imaging reveals that a newly emerged oncogenic cell with the Ras^G12V mutation becomes senescent and is eliminated from the epithelia, which is prevented by adding a TP53 gain-of-function mutation (TP53^R175H) into Ras^G12V cells. Surviving Ras^G12V-TP53^R175H double-mutant cells senesce and secrete senescence-associated secretory phenotype (SASP)-related inflammatory molecules that convert neighbouring normal cells into SASP factor-secreting senescent cells, generating a heterogeneous tumour-like cell mass. We identify oncogenic cell behaviours that may control the initial human tumorigenesis step. Ras and TP53 mutations and cellular senescence are frequently detected in human tumours; similar switching may occur during the initial step of human tumorigenesis.

It is unclear how a single oncogenic cell primes tumorigenesis. Here the authors visualised this behaviour using a zebrafish larval skin as a model and show that RasG12V oncogenic cell is eliminated through oncogene-senescence while a gain of function mutation in p53 alters this behaviour from tumour suppressive to tumour promoting.

Patient Derived Xenografts for Genome-Driven Therapy of Osteosarcoma

Osteosarcoma (OS) is a rare malignant primary tumor of mesenchymal origin affecting bone. It is characterized by a complex genotype, mainly due to the high frequency of chromothripsis, which leads to multiple somatic copy number alterations and structural rearrangements. Any effort to design genome-driven therapies must therefore consider such high inter- and intra-tumor heterogeneity. Therefore, many laboratories and international networks are developing and sharing OS patient-derived xenografts (OS PDX) to broaden the availability of models that reproduce OS complex clinical heterogeneity. OS PDXs, and new cell lines derived from PDXs, faithfully preserve tumor heterogeneity, genetic, and epigenetic features and are thus valuable tools for predicting drug responses. Here, we review recent achievements concerning OS PDXs, summarizing the methods used to obtain ectopic and orthotopic xenografts and to fully characterize these models. The availability of OS PDXs across the many international PDX platforms and their possible use in PDX clinical trials are also described. We recommend the coupling of next-generation sequencing (NGS) data analysis with functional studies in OS PDXs, as well as the setup of OS PDX clinical trials and co-clinical trials, to enhance the predictive power of experimental evidence and to accelerate the clinical translation of effective genome-guided therapies for this aggressive disease.

Model Systems for the Study of Malignant Melanoma

Since the first resection of melanoma by Hunter in 1787, efforts to treat patients with this deadly malignancy have been ongoing. Initial work to understand melanoma biology for therapeutics development began with the employment of isolated cancer cells grown in cell cultures. However, these models lack in vivo interactions with the tumor microenvironment. Melanoma cell line transplantation into suitable animals such as mice has been informative and useful for testing therapeutics as a preclinical model. Injection of freshly isolated patient melanomas into immunodeficient animals has shown the capacity to retain the genetic heterogeneity of the tumors, which is lost during the long-term culture of melanoma cells. Upon advancement of technology, genetically engineered animals have been generated to study the spontaneous development of melanomas in light of newly discovered genetic aberrations associated with melanoma formation. Culturing melanoma cells in a matrix generate tumor spheroids, providing an in vitro environment that promotes the heterogeneity commonplace with human melanoma and displaces the need for animal care facilities. Advanced 3D cultures have been created simulating the structure and cellularity of human skin to permit in vitro testing of therapeutics on melanomas expressing the same phenotype as demonstrated in vivo. This review will discuss these models and their relevance to the study of melanomagenesis, growth, metastasis, and therapy.

In Vivo Models for Cholangiocarcinoma-What Can We Learn for Human Disease?

Cholangiocarcinoma (CCA) comprises a heterogeneous group of primary liver tumors. They emerge from different hepatic (progenitor) cell populations, typically via sporadic mutations. Chronic biliary inflammation, as seen in primary sclerosing cholangitis (PSC), may trigger CCA development. Although several efforts were made in the last decade to better understand the complex processes of biliary carcinogenesis, it was only recently that new therapeutic advances have been achieved. Animal models are a crucial bridge between in vitro findings on molecular or genetic alterations, pathophysiological understanding, and new therapeutic strategies for the clinic. Nevertheless, it is inherently difficult to recapitulate simultaneously the stromal microenvironment (e.g., immune-competent cells, cholestasis, inflammation, PSC-like changes, fibrosis) and the tumor biology (e.g., mutational burden, local growth, and metastatic spread) in an animal model, so that it would reflect the full clinical reality of CCA. In this review, we highlight available data on animal models for CCA. We discuss if and how these models reflect human disease and whether they can serve as a tool for understanding the pathogenesis, or for predicting a treatment response in patients. In addition, open issues for future developments will be discussed.

Mouse Models for Exploring the Biological Consequences and Clinical Significance of PIK3CA Mutations

The phosphatidylinositol 3-kinase (PI3K) pathway is involved in a myriad of cellular signalling pathways that regulate cell growth, metabolism, proliferation and survival. As a result, alterations in the PI3K pathway are frequently associated with human cancers. Indeed, PIK3CA-the gene encoding the p110α catalytic subunit of PI3K-is one of the most commonly mutated human oncogenes. PIK3CA mutations have also been implicated in non-malignant conditions including congenital overgrowth syndromes and vascular malformations. In order to study the role of PIK3CA mutations in driving tumorigenesis and tissue overgrowth and to test potential therapeutic interventions for these conditions, model systems are essential. In this review we discuss the various mouse models currently available for preclinical studies into the biological consequences and clinical significance of PIK3CA mutations.

Altered B-lymphopoiesis in mice with deregulated thrombopoietin signaling

Thrombopoietin (TPO) is the master cytokine regulator of megakaryopoiesis. In addition to regulation of megakaryocyte and platelet number, TPO is important for maintaining proper hematopoietic stem cell (HSC) function. It was previously shown that a number of lymphoid genes were upregulated in HSCs from Tpo^−/− mice. We investigated if absent or enhanced TPO signaling would influence normal B-lymphopoiesis. Absent TPO signaling in Mpl^−/− mice led to enrichment of a common lymphoid progenitor (CLP) signature in multipotential lineage-negative Sca-1⁺c-Kit⁺ (LSK) cells and an increase in CLP formation. Moreover, Mpl^−/− mice exhibited increased numbers of PreB2 and immature B-cells in bone marrow and spleen, with an increased proportion of B-lymphoid cells in the G1 phase of the cell cycle. Conversely, elevated TPO signaling in Tpo^Tg mice was associated with reduced B-lymphopoiesis. Although at steady state, peripheral blood lymphocyte counts were normal in both models, Mpl^−/− Eµ-myc mice showed an enhanced preneoplastic phase with increased numbers of splenic PreB2 and immature B-cells, a reduced quiescent fraction, and augmented blood lymphocyte counts. Thus, although Mpl is not expressed on lymphoid cells, TPO signaling may indirectly influence B-lymphopoiesis and the preneoplastic state in Myc-driven B-cell lymphomagenesis by lineage priming in multipotential progenitor cells.

Genetically engineered mouse models of melanoma

Melanoma is a complex disease that exhibits highly heterogeneous etiological, histopathological, and genetic features, as well as therapeutic responses. Genetically engineered mouse (GEM) models provide powerful tools to unravel the molecular mechanisms critical for melanoma development and drug resistance. Here, we expound briefly the basis of the mouse modeling design, the available technology for genetic engineering, and the aspects influencing the use of GEMs to model melanoma. Furthermore, we describe in detail the currently available GEM models of melanoma. Cancer 2017;123:2089-103. © 2017 American Cancer Society.

Targeting the KRAS variant for treatment of non-small cell lung cancer: potential therapeutic applications

Lung cancer is the leading cause of cancer deaths worldwide, with non-small cell lung cancer (NSCLC) accounting for 80% of all lung cancers. Kirsten rat sarcoma viral oncogene homolog (KRAS) is one of the deadliest cancer-related proteins and plays a pivotal role in the most aggressive and lethal human cancers, including lung adenocarcinoma where it represents one of the most frequently mutated oncogene. Although therapeutic progresses have made an impact over the last decade, median survival for patients with advanced lung cancer remains disappointing, with a 5-year worldwide survival rate of <15%. For more than 20 years it has been recognized that constitutively active signaling downstream of KRAS is a fundamental driver of lung tumorigenesis. However, years of pursuit have failed to yield a drug that can safely curb KRAS activity; up to now no approved therapies exist for KRAS-mutant NSCLC. The aim of this review is to discuss the current knowledge of KRAS-mutated NSCLC, touching upon KRAS clinical relevance as a prognostic and predictive biomarker, with an emphasis on novel therapeutic approaches for the treatment of KRAS-variant NSCLC.

The application of antitumor drug-targeting models on liver cancer

Hepatocarcinoma animal models, such as the induced tumor model, transplanted tumor model, gene animal model, are significant experimental tools for the evaluation of targeting drug delivery system as well as the pre-clinical studies of liver cancer. The application of antitumor drug-targeting models not only furnishes similar biological characteristics to human liver cancer but also offers guarantee of pharmacokinetic indicators of the liver-targeting preparations. In this article, we have reviewed some kinds of antitumor drug-targeting models of hepatoma and speculated that the research on this field would be capable of attaining a deeper level and expecting a superior achievement in the future.

Genetically engineered knock-in and conditional knock-in mouse models of cancer

Classical transgenic models are useful for quickly gauging the impact of transgene overexpression, but they are limited by the absence of the innate, subtle regulatory elements encoded in introns and other untranslated regions. Moreover, the widespread, high-level expression of oncogenes often leads to tumors that lack the histopathological and acquired genetic features of human cancers. Targeted mutation of endogenous loci, or knock-in (KI) alleles, facilitates more accurate modeling of human tumors by allowing for the expression of mutant alleles under normal physiological regulation. Advanced strategies enable the stochastic activation of such alleles in somatic cells, such that genotypically wild-type cells surround individual mutant cells. More recent technologies, such as site-specific engineered nucleases, have also accelerated the design and implementation of KI strategies. Together, these tools aid in the development of advanced mouse models that better recapitulate the features of human disease.

Tumor models for prostate cancer exemplified by fibroblast growth factor 8-induced tumorigenesis and tumor progression

Prostate cancer is a very common malignancy among Western males. Although most tumors are indolent and grow slowly, some grow and metastasize aggressively. Because prostate cancer growth is usually androgen-dependent, androgen ablation offers a therapeutic option to treat post-resection tumor recurrence or primarily metastasized prostate cancer. However, patients often relapse after the primary response to androgen ablation therapy, and there is no effective cure for cases of castration-resistant prostate cancer (CRPC). The mechanisms of tumor growth in CRPC are poorly understood. Although the androgen receptors (ARs) remain functional in CRPC, other mechanisms are clearly activated (e.g., disturbed growth factor signaling). Results from our laboratory and others have shown that dysregulation of fibroblast growth factor (FGF) signaling, including FGF receptor 1 (FGFR1) activation and FGF8b overexpression, has an important role in prostate cancer growth and progression. Several experimental models have been developed for prostate tumorigenesis and various stages of tumor progression. These models include genetically engineered mice and rats, as well as induced tumors and xenografts in immunodeficient mice. The latter was created using parental and genetically modified cell lines. All of these models greatly helped to elucidate the roles of different genes in prostate carcinogenesis and tumor progression. Recently, patient-derived xenografts have been studied for possible use in testing individual, specific responses of tumor tissue to different treatment options. Feasible and functional CRPC models for drug responsiveness analysis and the development of effective therapies targeting the FGF signaling pathway and other pathways in prostate cancer are being actively investigated.

Transgenic models for prostate cancer research

The ability to introduce novel or specifically altered genes into the germ line of mice and directly perturb gene expression in a specific tissue can facilitate characterization of the molecular mechanisms governing transformation of differentiating tissue within the context of an intact developing animal. Transgenics provide a powerful and remarkably flexible system that can be used to study the cooperation between proto-oncogenes, tumor suppressor genes, and other epigenetic factors in the development of cancer.

Nanoparticle characterization: state of the art, challenges, and emerging technologies

Nanoparticles have received enormous attention as a promising tool to enhance target-specific drug delivery and diagnosis. Various in vitro and in vivo techniques are used to characterize a new system and predict its clinical efficacy. These techniques enable efficient comparison across nanoparticles and facilitate a product optimization process. On the other hand, we recognize their limitations as a prediction tool, due to inadequate applications and overly simplified test conditions. We provide a critical review of in vitro and in vivo techniques currently used for evaluation of nanoparticles and introduce emerging techniques and models that may be used complementarily.

Regulation of NF-E2-related factor 2 signaling for cancer chemoprevention: antioxidant coupled with antiinflammatory

Cancer chemoprevention is a process of using either natural or synthetic compounds to reduce the risk of developing cancer. Observations that NF-E2-related factor 2 (Nrf2)-deficient mice lack response to some chemopreventive agents point to the important role of Nrf2 in chemoprevention. Nrf2 is a member of basic-leucine zipper transcription factor family and has been shown to regulate gene expression by binding to a response element, antioxidant responsive element. It is generally believed that activation of Nrf2 signaling is an adaptive response to the environmental and endogenous stresses. Under homeostatic conditions, Nrf2 is suppressed by association with Kelch-like ECH-associated protein 1 (Keap1), but is stimulated upon exposure to oxidative or electrophilic stress. Once activated, Nrf2 translocates into nuclei and upregulates a group of genes that act in concert to combat oxidative stress. Nrf2 is also shown to have protective function against inflammation, a pathological process that could contribute to carcinogenesis. In this review, we will discuss the current progress in the study of Nrf2 signaling, in particular, the mechanisms of Nrf2 activation by chemopreventive agents. We will also discuss some of the potential caveats of Nrf2 in cancer treatment and future opportunity and challenges on regulation of Nrf2-mediated antioxidant and antiinflammatory signaling in the context of cancer prevention.

Transgenic oncogenes induce oncogene-independent cancers with individual karyotypes and phenotypes

Cancers are clones of autonomous cells defined by individual karyotypes, much like species. Despite such karyotypic evidence for causality, three to six synergistic mutations, termed oncogenes, are generally thought to cause cancer. To test single oncogenes, they are artificially activated with heterologous promoters and spliced into the germ line of mice to initiate cancers with collaborating spontaneous oncogenes. Because such cancers are studied as models for the treatment of natural cancers with related oncogenes, the following must be answered: 1) which oncogenes collaborate with the transgenes in cancers; 2) how do single transgenic oncogenes induce diverse cancers and hyperplasias; 3) what maintains cancers that lose initiating transgenes; 4) why are cancers aneuploid, over- and underexpressing thousands of normal genes? Here we try to answer these questions with the theory that carcinogenesis is a form of speciation. We postulate that transgenic oncogenes initiate carcinogenesis by inducing aneuploidy. Aneuploidy destabilizes the karyotype by unbalancing teams of mitosis genes. This instability thus catalyzes the evolution of new cancer species with individual karyotypes. Depending on their degree of aneuploidy, these cancers then evolve new subspecies. To test this theory, we have analyzed the karyotypes and phenotypes of mammary carcinomas of mice with transgenic SV40 tumor virus- and hepatitis B virus-derived oncogenes. We found that (1) a given transgene induced diverse carcinomas with individual karyotypes and phenotypes; (2) these karyotypes coevolved with newly acquired phenotypes such as drug resistance; (3) 8 of 12 carcinomas were transgene negative. Having found one-to-one correlations between individual karyotypes and phenotypes and consistent coevolutions of karyotypes and phenotypes, we conclude that carcinogenesis is a form of speciation and that individual karyotypes maintain cancers as they maintain species. Because activated oncogenes destabilize karyotypes and are dispensable in cancers, we conclude that they function indirectly, like carcinogens. Such oncogenes would thus not be valid models for the treatment of cancers.

Loss of p73 promotes dissemination of Myc-induced B cell lymphomas in mice

Mice engineered to express c-Myc in B cells (Emu-myc mice) develop lethal lymphomas in which the gene encoding the p53 tumor suppressor is frequently mutated. Whether the p53 homolog p73 also functions as a tumor suppressor in vivo remains controversial. Here we have shown that p73 loss does not substantially affect disease onset and mortality in Emu-myc mice. However, it does alter the phenotype of the disease. Specifically, p73 loss decreased nodal disease and increased widespread extranodal dissemination. We further found that p53 acted as the dominant tumor suppressor during the onset of Emu-myc-driven B cell lymphomagenesis, while p73 modulated tumor dissemination and extranodal growth. Immunophenotyping and expression profiling suggested that p73 loss allowed increased maturation of malignant B cells and deregulated genes involved in lymphocyte homing and dissemination of human lymphomas. Consistent with this, p73 expression was frequently downregulated in a large cohort of human mature aggressive B cell lymphomas, and both the incidence and degree of p73 downregulation in these tumors correlated with their extranodal dissemination status. These data indicate that p73 is a modifier of Myc-driven lymphomas in mice, favoring tumor dissemination, and suggest that p73 could be a biomarker for human B cell lymphoma dissemination, a notion that can now be tested in clinicopathologic correlation studies.

Modeling chromosomal translocations using conditional alleles to recapitulate initiating events in human leukemias

Recurrent reciprocal chromosomal translocations are present in more than 50% of leukemias. A deeper understanding of how they affect cancer initiation is essential for evaluating the origins of cancer and the potential for therapy based on the translocation products. Mouse models of chromosomal translocations are required for this. Here we summarize three methodologies developed in our laboratory to model chromosomal translocations (knock-in, translocator, and invertor methods). We have used these models to study leukemias caused by fusions of the mixed lineage leukemia (MLL) gene and the Ews-ERG fusion gene to evaluate oncogenicity and elucidate some general principles about translocation products. We show that MLL fusions have the capacity to cause hematopoietic tumors only if expressed in permissive cells and that the Mll-Enl fusion can cause lineage reassignment if the chromosomal translocation occurs in lineage noncommitted progenitors. The leukemia-initiating cells generated by Mll fusions or by Ews-ERG fusion can be committed cells within the hematopoietic pathway. Our translocation mimic models are applicable to any human reciprocal chromosomal translocation.

Micromanagement of lymphomas

B cell receptor signaling participates in the genesis of lymphoma and influences the characteristics of the tumor cells.

A roadmap to safe, efficient, and stable lentivirus-mediated gene therapy with hematopoietic cell transplantation

Hematopoietic stem cells comprise a prominent target for gene therapy aimed at treating various genetic and acquired disorders. A number of limitations associated with hematopoietic cell transplantation can be circumvented by the use of cells stably modified by retroviral gene transfer. Oncoretroviral and lentiviral vectors offer means for generating efficient and stable transgene expression. This review summarizes the state of the field today in terms of vector development and clinical experimentation. In particular, concerns with the safety of retroviral vectors intended for clinical gene transfer, applicability of preclinical data in directing clinical trial design, and recent research aimed at resolving some of these issues are addressed. Finally, this review underlines the specific advantages offered by lentiviral gene-transfer vectors for gene therapy in stem cells.

Generation and characterization of islet cell tumor in pTet-on/pTRE-SV40Tag double-transgenic mice model

A line of double-transgenic mice that develop neoplasms arising primarily in the pancreas was established. In these mice, the oncogene SV40 T antigen (Tag) was detected in the pancreas with and without the control of Tet-on system. The transgenic mice that developed pancreatic tumors as early as 20 weeks of age showed hypoglycemia on a blood glucose test. Pathological and immunohistochemical characterizations demonstrated that the tumors belonged to neuroendocrine neoplasms arising from pancreatic islets. A change in IGFs/IGF-1R signaling pathway was detected using real-time PCR analysis. A potential association between the IGFs/IGF-1R system and SV40Tag was studied to further explain the cancerogenesis of the double-transgenic mice by Western blot analysis and immunoprecipitation experiments. The results suggest that a Tag transgenic mice model could be used to study the molecular mechanism of the tumorigenesis of islets.

Engineering skin to study human disease--tissue models for cancer biology and wound repair

Recent advances in the engineering of three-dimensional tissues known as skin equivalents, that have morphologic and phenotypic properties of human skin, have provided new ways to study human disease processes. This chapter will supply an overview of two such applications--investigations of the incipient development of squamous cell cancer, and studies that have characterized the response of human epithelium during wound repair. Using these novel tools to study cancer biology, it has been shown that cell-cell interactions inherent in three-dimensional tissue architecture can suppress early cancer progression by inducing a state of intraepithelial dormancy. This dormant state can be overcome and cancer progression enabled by altering tissue organization in response to tumor promoters or UV irradiation or by modifying the interaction of tumor cells with extracellular matrix proteins or their adjacent epithelia. By adapting skin equivalent models of human skin to study wound reepithelialization, it has been shown that several key responses, including cell proliferation, migration, differentiation, growth-factor responsiveness and protease expression, will mimic the response seen in human skin. In this light, these engineered models of human skin provide powerful new tools for studying disease processes in these tissues as they occur in humans.

The biological sense of cancer: a hypothesis

BACKGROUND

Most theories about cancer proposed during the last century share a common denominator: cancer is believed to be a biological nonsense for the organism in which it originates, since cancer cells are believed to be ones evading the rules that control normal cell proliferation and differentiation. In this essay, we have challenged this interpretation on the basis that, throughout the animal kingdom, cancer seems to arise only in injured organs and tissues that display lost or diminished regenerative ability.

HYPOTHESIS

According to our hypothesis, a tumor cell would be the only one able to respond to the demand to proliferate in the organ of origin. It would be surrounded by "normal" aged cells that cannot respond to that signal. According to this interpretation, cancer would have a profound biological sense: it would be the ultimate way to attempt to restore organ functions and structures that have been lost or altered by aging or noxious environmental agents. In this way, the features commonly associated with tumor cells could be reinterpreted as progressively acquired adaptations for responding to a permanent regenerative signal in the context of tissue injury. Analogously, several embryo developmental stages could be dependent on cellular damage and death, which together disrupt the field topography. However, unlike normal structures, cancer would have no physiological value, because the usually poor or non-functional nature of its cells would make their reparative task unattainable.

CONCLUSION

The hypothesis advanced in this essay might have significant practical implications. All conventional therapies against cancer attempt to kill all cancer cells. However, according to our hypothesis, the problem might not be solved even if all the tumor cells were eradicated. In effect, if the organ failure remained, new tumor cells would emerge and the tumor would reinitiate its progressive growth in response to the permanent regenerative signal of the non-restored organ. Therefore, efficient anti-cancer therapy should combine an attack against the tumor cells themselves with the correction of the organ failure, which, according to this hypothesis, is fundamental to the origin of the cancer.

Mouse Modeling in Oncologic Preclinical and Translational Research

Through scientific and technological advancements, our ability to manipulate the mouse genome has allowed us to evaluate the effect of specific genetic alterations on in vivo tumorigenesis. This has allowed and will allow us to define molecular pathways describing the processes of tumor initiation, invasion, and progression to metastatic disease. Additionally, these models may serve as an excellent platform for the identification of novel molecular targets for therapy as well as to evaluate the efficacy of targeted therapies. Ultimately this will translate from preclinical mouse model trials to the development of clinical trials and protocols for cancer patients. Here we review the usefulness of mouse modeling in oncologic translational research.

Mouse models in oncogenesis and cancer therapy

Animal models have been critical in the study of the molecular mechanisms of cancer and in the development of new antitumor agents; nevertheless, there is still much room for improvement. The relevance of each particular model depends on how close it replicates the histology, physiological effects, biochemical pathways and metastatic pattern observed in the same human tumor type. Metastases are especially important because they are the main determinants of the clinical course of the disease and patient survival, and are the target of systemic therapy. The generation of clinically relevant models using the mouse requires their humanization, since differences exist in transformation and oncogenesis between human and mouse. Although genetically modified (GM) mice have been instrumental in understanding the molecular mechanisms involved in tumor initiation, they have been less successful in replicating advanced cancer. Moreover, a particular genetic alteration frequently leads to different tumor types in human and mouse and to lower metastastatic rates in GM mice than in humans. These findings question the capacity of current GM mouse carcinoma models to predict clinical response to therapy. On the other hand, orthotopic (ORT) xenografts of human tumors, or tumor cell lines, in nude mice reproduce the histology and metastatic pattern of most human tumors at advanced stage. Using ex vivo genetic manipulation of human tumor cells, ORT models can be used to molecularly dissect the metastatic process and to evaluate in vivo tumor response to therapy, using non-invasive procedures. Nevertheless, this approach is not useful in the study of the initial stages of tumorigenesis or the contribution of the immune system in this process. Despite ORT models are more promising than the most commonly used subcutaneous xenografts in preclinical drug development, their capacity to predict clinical response to antitumor agents remains to be studied. Humanizing mouse models of cancer will most likely require the combined use of currently available methodologies.

The mighty mouse: genetically engineered mouse models in cancer drug development

Deficiencies in the standard preclinical methods for evaluating potential anticancer drugs,such as xenograft mouse models, have been highlighted as a key obstacle in the translation of the major advances in basic cancer research into meaningful clinical benefits. In this article, we discuss the established uses and limitations of xenograft mouse models for cancer drug development, and then describe the opportunities and challenges in the application of novel genetically engineered mouse models that more faithfully mimic the genetic and biological evolution of human cancers. Greater use of such models in target validation, assessment of tumour response, investigation of pharmacodynamic markers of drug action, modelling resistance and understanding toxicity has the potential to markedly improve the success of cancer drug development.

Proteomics Analysis Reveals Insight into the Mechanism of H-Ras-Mediated Transformation

We implemented a proteomics approach to the systematical analysis of the alterations in the proteome of NIH3T3 cells transformed by oncogenic H-RasV12. Forty-four proteins associated with Ras-mediated transformation have been identified, and 28 proteins were not previously reported. RT-PCR analysis showed that approximately 44% of target proteins identified showed concomitant changes in mRNA abundance. A principal finding was the up-regulation of gankyrin, which was the first evidence to show that gankyrin pathway was implicated in Ras-activated transformation.

Cell cycle regulator E2F1 modulates angiogenesis via p53-dependent transcriptional control of VEGF

The transcription factor E2F1 is known to regulate cell proliferation and has been thought to modulate tumorigenesis via this mechanism alone. Here we show that mice deficient in E2F1 exhibit enhanced angiogenesis. The proangiogenic phenotype in E2F1 deficiency is the result of overproduction of vascular endothelial growth factor (VEGF) and is prevented by VEGF blockade. Under hypoxic conditions, E2F1 down-regulates the expression of VEGF promoter activity by associating with p53 and specifically down-regulating expression of VEGF but not other hypoxia-inducible genes, suggesting a promoter structure context-dependent regulation mechanism. We found that the minimum VEGF promoter mediating transcriptional repression by E2F1 features an E2F1- binding site with four Sp-1 sites in close proximity. These data disclose an unexpected function of endogenous E2F1: regulation of angiogenic activity via p53-dependent transcriptional control of VEGF expression.

Neoplastic transformation and angiogenesis in the thymus of transgenic mice expressing SV40 T and t antigen under an L-pyruvate kinase promoter (SV12 mice)

Using several techniques, we have assessed morphological characteristics of a malignant thymic tumour in SV12 transgenic (Tg) mice expressing SV40 T and t antigens under control of an L-PK promoter. We describe the development of a carcinoma originating from thymic hyperplasia and followed by the formation of a benign tumour composed chiefly of medullary epithelial cells expressing the transgene and of lymphocytes, a pathology very rarely reported in mice. Our study of the SV12 Tg mice represents the first description of a model of a pure malignant thymic tumour associated with extensive angiogenesis maintained in numerous descendants. The formation of a large tumoral neovascular network, observed here, has never been described in human and/or experimental thymic tumours. Tumoral transformation and angiogenesis are demonstrated by immunolabelling with antibodies against various cytokeratins (CKs) of different molecular weights, vascular endothelial cell markers and VEGF/receptor-2 (Flk-1) present on the neovascular endothelial cells. Different points raised by the originality of this model are discussed. These include the medullary nature of the cells expressing the SV40 transgene and their relationship with the tumoral development. The subset of different molecular weight CK components and their modifications are also considered, as well as the presence of type IV epithelial cells, progenitors of medullary epithelial cells. Finally, the cell signals involved in angiogenesis and the possible action of an angiogenic factor, probably secreted by the tumoral cells themselves, are discussed.

The Myc target gene JPO1/CDCA7 is frequently overexpressed in human tumors and has limited transforming activity in vivo

MYC is frequently overexpressed in human cancers, but the downstream events contributing to tumorigenesis remain incompletely understood. MYC encodes an oncogenic transcription factor, of which target genes presumably contribute to cellular transformation. Although Myc regulates about 15% of genes and combinations of target genes are likely required for tumorigenesis, we studied in depth the expression of the Myc target gene, JPO1/CDCA7, in human cancers and its ability to provoke tumorigenesis in transgenic mice. JPO1/CDCA7 is frequently overexpressed in human cancers, and in particular, its expression is highly elevated in chronic myelogenous leukemia blast crisis as compared with the chronic phase. In murine lymphoid tissues, ectopic human JPO1/CDCA7 expression resulted in a 2-fold increased risk of lymphoid malignancies at 1 year. The transgene, which was driven by the H2-K promoter, exhibited leaky expression in nonlymphoid tissues such as kidney. We observed a significant increased incidence of transgenic animal solid tumors, which were not seen in littermate controls. These observations suggest that JPO1/CDCA7 may contribute to Myc-mediated tumorigenesis.

Bioinformatics approaches for cross-species liver cancer analysis based on microarray gene expression profiling

Background

The completion of the sequencing of human, mouse and rat genomes and knowledge of cross-species gene homologies enables studies of differential gene expression in animal models. These types of studies have the potential to greatly enhance our understanding of diseases such as liver cancer in humans. Genes co-expressed across multiple species are most likely to have conserved functions. We have used various bioinformatics approaches to examine microarray expression profiles from liver neoplasms that arise in albumin-SV40 transgenic rats to elucidate genes, chromosome aberrations and pathways that might be associated with human liver cancer.

Results

In this study, we first identified 2223 differentially expressed genes by comparing gene expression profiles for two control, two adenoma and two carcinoma samples using an F-test. These genes were subsequently mapped to the rat chromosomes using a novel visualization tool, the Chromosome Plot. Using the same plot, we further mapped the significant genes to orthologous chromosomal locations in human and mouse. Many genes expressed in rat 1q that are amplified in rat liver cancer map to the human chromosomes 10, 11 and 19 and to the mouse chromosomes 7, 17 and 19, which have been implicated in studies of human and mouse liver cancer. Using Comparative Genomics Microarray Analysis (CGMA), we identified regions of potential aberrations in human. Lastly, a pathway analysis was conducted to predict altered human pathways based on statistical analysis and extrapolation from the rat data. All of the identified pathways have been known to be important in the etiology of human liver cancer, including cell cycle control, cell growth and differentiation, apoptosis, transcriptional regulation, and protein metabolism.

Conclusion

The study demonstrates that the hepatic gene expression profiles from the albumin-SV40 transgenic rat model revealed genes, pathways and chromosome alterations consistent with experimental and clinical research in human liver cancer. The bioinformatics tools presented in this paper are essential for cross species extrapolation and mapping of microarray data, its analysis and interpretation.

Protein farnesyltransferase inhibitors

Current systemic cytotoxic therapies for cancer are limited by their nonspecific mechanism of action, unwanted toxicities on normal tissues and short-term efficacy due to the emergence of drug resistance. However, identification of the molecular abnormalities in cancer, in particular the key proteins involved in abnormal cell growth, has resulted in various signal transduction inhibitor drugs being developed as new treatment strategies against the disease. Protein farnesyltransferase inhibitors (FTIs) were originally designed to target the Ras signal transduction pathway, although it is now clear that several other intracellular proteins are dependent on post-translational farnesylation (addition of a 15-carbon farnesyl moiety) for their function. Preclinical data revealed that although FTIs inhibit the growth of ras-transformed cells, they are also potent inhibitors of a wide range of cancer cell lines, many of which contain wild type ras. While understanding the mechanism of action of FTIs remains an important research goal, three different FTIs have entered clinical development. Several Phase I trials with each drug have explored different schedules for prolonged administration, and dose-limiting toxicities (DLTs) have varied from myelosuppression, gastrointestinal toxicity and neuropathy. Evidence for anticancer efficacy has come from a number of Phase II studies, not necessarily in tumour types containing ras mutations, which were the initial target for these drugs. Perhaps the most promising use for FTIs will be in combination with conventional cytotoxic drugs, based on preclinical data suggesting synergy, particularly with the taxanes. Clinical combination studies are in progress, and larger Phase II/III clinical trials are planned to see if FTIs can add to the efficacy of conventional therapies.

The cell-type-specificity of inherited predispositions to tumours: review and hypothesis

Most hereditary predispositions to tumours affect only one particular cell type of the body but the genes bearing the relevant germ-line mutation are not cell-type-specific. Some predisposition syndromes include increased risks of lesions (developmental or tumourous) of unrelated cell types, in any individual predisposed to the main lesion (e.g. osteosarcoma in patients predisposed to retinoblastoma). Other predispositions to additional lesions occur only in members of some families with the predisposition to the basic lesion (e.g. Gardner's syndrome in some families suffering familial adenomatous polyposis). In yet other predisposition syndromes, different mutations of the same gene are associated with markedly differing family-specific clinical syndromes. In particular, identical germline mutations (e.g. in APC, RET and PTEN genes), have been found associated with differing clinical syndromes in different families. This paper reviews previously suggested mechanisms of the cell-type specificity of inherited predispositions to tumour. Models of tumour formation in predisposition syndromes are discussed, especially those involving a germline mutation (the first 'hit') of a tumour suppressor gene (TSG) and a second (somatic) hit on the second allele of the same TSG. A modified model is suggested, such that the second hit is a co-mutation of the second allele of the TSG and a regulator which is specific for growth and/or differentiation of the cell type which is susceptible to the tumour predisposition. In some cases of tumour, the second hit may be large enough to be associated with a cytogenetically-demonstrable abnormality of the part of the chromosome carrying the TSG, but in other cases, the co-mutation may be of 'sub-cytogenetic' size (i.e. 10(2)-10(5) bases). For the latter, mutational mechanisms of frameshift and impaired fidelity of replication of DNA by DNA polyerases may sometimes be involved. Candidate cell-type-specific regulators may include microRNAs and perhaps transcription factors. It is suggested that searching the introns within 10(5)-10(6) bases either side of known of exonic mutations of TSGs associated with inherited tumour predisposition might reveal microRNA cell-type-specific regulators. Additional investigations may involve fluorescent in situ hybridisations on interphase tumour nuclei.

Hematopoietic cell fate and the initiation of leukemic properties in primitive primary human cells are influenced by Ras activity and farnesyltransferase inhibition

The Ras pathway transduces divergent signals determining normal cell fate and is frequently activated in hematopoietic malignancies, but the manner in which activation contributes to human leukemia is poorly understood. We report that a high level of activated H-Ras signaling in transduced primary human hematopoietic progenitors reduced their proliferation and enhanced monocyte/macrophage differentiation. However, the exposure of these cells to a farnesyltransferase inhibitor and establishment of a moderate level of Ras activity showed increased proliferation, an elevated frequency of primitive blast-like cells, and progenitors with enhanced self-renewal capacity. These results suggest that the amplitude of Ras pathway signaling is a determinant of myeloid cell fate and that moderate Ras activation in primitive hematopoietic cells can be an early event in leukemogenesis.

Normal human fibroblasts are resistant to RAS-induced senescence

Oncogenic stimuli are thought to induce senescence in normal cells in order to protect against transformation and to induce proliferation in cells with altered p53 and/or retinoblastoma (Rb) pathways. In human fibroblasts, RAS initiates senescence through upregulation of the cyclin-dependent kinase inhibitor p16INK4A. We show here that in contrast to cultured fibroblast strains, freshly isolated normal fibroblasts are resistant to RAS-induced senescence and instead show some characteristics of transformation. RAS did not induce growth arrest or expression of senescence-associated beta-galactosidase, and Rb remained hyperphosphorylated despite elevated levels of p16. Instead, RAS promoted anchorage-independent growth of normal fibroblasts, although expression of hTert with RAS increased colony formation and allowed normal fibroblasts to bypass contact inhibition. To test the hypothesis that p16 levels determine how cells respond to RAS, we expressed RAS in freshly isolated fibroblasts that expressed very low levels of p16, in hTert-immortalized fibroblasts that had accumulated intermediate levels of p16, and in IMR90 fibroblasts with high levels of p16. RAS induced growth arrest in cells with higher p16 levels, and this effect was reversed by p16 knockdown in the hTert-immortalized fibroblasts. These findings indicate that culture-imposed stress sensitizes cells to RAS-induced arrest, whereas early passage cells do not arrest in response to RAS.

Progressive paralysis associated with diffuse astrocyte anaplasia in Delta202 mice homozygous for a transgene encoding the SV40 T antigen*

A convenient transgenic astrocytoma model in delta202 mice, homozygous for a construct encoding the early region of the SV40 virus genome, is described. In the offspring of crosses between delta202 mice heterozygous for the transgene nearly 60% were transgenic; one third of these developed progressive paralysis starting in the hindlimbs at approximately 35 days of age and died at 90 +/- 30 days of age. In affected mice proliferating-non-neuronal cells immunostained with antibodies to the GFAP, an astrocyte marker, whose number increased with age were found in the white matter of the brain, cerebellum and spinal cord, and progressive degeneration and necrosis of spinal motoneurons was observed that-may explain the paralysis. The early onset and reproducible time course of the neurological disease suggest that homozygous delta202 mice, whose proliferating astrocytes appear to damage spinal motoneurons, are a useful model to study astrocyte differentiation, function and tumorigenesis.

Towards the cellular functions of tumour suppressors

Many of the documented changes in cellular DNA that occur during tumour development involve activation of proto-oncogenes, but newer evidence has shown that oncogenesis can involve loss or inactivation of a different group of genes, called tumour suppressor genes (TSGs). Molecular analysis of TSGs is revealing that their protein products are involved in cell adhesion, signal transduction, transcription, translation and cell cycle control. Surprisingly, most of the TSG products had not been previously identified in studies of normal cells, so their analysis is contributing not only to our understanding of oncogenesis, but also to basic cell biology. The 'comment' articles in this issue discuss progress towards understanding the cellular functions of TSG products.

Antitumor activity of bruceantin: an old drug with new promise

Bruceantin was first isolated from Brucea antidysenterica, a tree used in Ethiopia for the treatment of cancer, and activity was observed against B16 melanoma, colon 38, and L1210 and P388 leukemia in mice. Phase I and II clinical trials were then initiated, but no objective tumor regressions were observed and clinical development was terminated. Recently, the activity of bruceantin has been studied with a number of leukemia, lymphoma, and myeloma cell lines. Cell differentiation was induced and c-MYC was down-regulated, suggesting a mechanistic correlation between c-MYC down-regulation and induction of cell differentiation or cell death. Treatment of HL-60 and RPMI 8226 cell lines induced apoptosis, and this involved the caspase and mitochondrial pathways. Moreover, an in vivo study using RPMI 8226 human-SCID xenografts demonstrated that bruceantin induced regression in early as well as advanced tumors, and these significant antitumor responses were facilitated in the absence of overt toxicity. Apoptosis was significantly elevated in tumors derived from animals treated with bruceantin. In sum, bruceantin interferes with the growth of leukemia, lymphoma, and myeloma cells in culture and xenograft models. Responses of this type suggest bruceantin should be reinvestigated for clinical efficacy against hematological malignancies.

Transgenic Mice as an In Vivo Model of Lymphomagenesis

This review covers multiple data obtained on genetically modified mice that help to elucidate various intricate molecular mechanisms of lymphomagenesis in humans. We are in a "golden age" of mouse genetics. The mouse is by far the most accessible mammalian system physiologically similar to humans. Transgenic mouse models have illuminated how different genes contribute to human lymphomagenesis. Multiple experiments with transgenic mice have not only confirmed the data obtained for human lymphomas but also gave additional evidence for the role of some genes and cooperative participation of their products in the development of human lymphomas. Genes and gene networks detected on transgenic mice can successfully serve as molecular targets for tumor therapy. This review demonstrates the extraordinary possibilities of transgenic technology, which is presently one of the readily available, efficient, and accurate tools to solve the problem of cancer.

Pituitary tumorigenesis targeted by the ovine follicle-stimulating hormone beta-subunit gene regulatory region in transgenic mice

Targeted tumorigenesis in transgenic mice has been a powerful tool for the study of gene expression and oncogenesis, as well as for the production of differentiated immortal cell lines from rare cell types. Follicle-stimulating hormone (FSH) is secreted by the gonadotrope cells of the anterior pituitary gland and plays a pivotal role in mammalian reproduction. Here we have used the regulatory region of the ovine FSH beta gene to direct expression of the SV40 T antigen oncogene to gonadotrope cells in the pituitary of transgenic mice. Two of five transgenic mouse lines bearing this fusion gene rapidly developed pituitary tumors, with appearance of adenomatous foci as early as 6 weeks of age, resulting in death by 12 weeks of age in both genders. Histologic examination of tumor development over time revealed that increases in cell proliferation and dysplasia were accompanied by decreases in synthesis of pituitary hormones, indicating dedifferentiation of the pituitary cells. Histological features observed in these tumors were in agreement with this rapid transformation of cell phenotype. Tumors were multifocal in origin, and the most highly transformed cell types observed consisted of giant pale basophilic cells with enormous hyperploid nuclei associated with infiltrating neuronal-like cells, which were very abundant at later stages of tumor development. Mitotic indices were much higher in transgenic than wild-type pituitaries, as expected. Morphologic analysis of the gonads of these transgenic mice showed no major developmental differences, as compared to wild-type littermates, however the length of the seminiferous tubules in transgenic males was greater than age-matched wild-type animals. Despite this phenotype difference, both genders were fertile, with normal sperm development observed in males and normal estrous cycle stages in females. Moreover, while 8 -- 10-week-old transgenic males had much lower blood levels of FSH than littermates, transgenic female FSH levels were the same as those of wild-type females. These animals offer a unique and potentially useful model of organ-specific tumorigenesis, where a multistage pathway of tumor development is evident, both histologically and temporally. Study of such models will advance our knowledge on the physiological and molecular mechanisms involved in gene expression as well as tumor formation.

Loss of the cell cycle inhibitors p21(Cip1) and p27(Kip1) enhances tumorigenesis in knockout mouse models

Events that contribute to tumor formation include mutations in the ras gene and loss or inactivation of cell cycle inhibitors such as p21(Cip1) and p27(Kip1). In our previous publication, we showed that mice expressing the MMTV/v-Ha-ras transgene developed tumors earlier and at higher multiplicities in the absence than in the presence of p21(Cip1). To further evaluate the combinatorial role of genetic alterations and loss of cell cycle inhibitors in tumorigenesis, we performed two companion studies. In the first study, wild type and p21(Cip1)-null mice were exposed to the chemical carcinogen, urethane. Similar to its effects in v-Ha-ras mice, loss of p21(Cip1) accelerated tumor onset and increased tumor multiplicity in urethane-treated mice. Lung tumors were the predominant tumor type in urethane-treated mice regardless of p21(Cip1) status. In the second study, tumor formation was monitored in v-Ha-ras mice expressing or lacking p27(Kip1). Unlike p21(Cip1), the absence of p27(Kip1) had no effect on the timing or multiplicity of tumor formation, which was largely restricted to mammary and salivary glands. However, once tumors appeared, they grew faster in p27(Kip1)-null mice than in p27(Kip1)-wild type mice. Increases in growth rate were particularly striking for salivary tumors in ras/p27(-/-) mice. Loss of p21(Cip1), on the other hand, had no effect on tumor growth rate in v-Ha-ras mice. Collectively, our data suggest that p21(Cip1) suppresses tumor formation elicited by multiple agents and that p21(Cip1) and p27(Kip1) suppress tumor formation in different ways.

Animal models for studying lung cancer and evaluating novel intervention strategies

The pathogenesis of lung cancer progression, invasion and metastasis remains undefined. Clinically relevant laboratory models of the disease could greatly facilitate its clarification. Model systems of lung cancer that accurately reflect different biologic properties and disease stages are necessary to ensure proper experimental design of studies aimed at increasing our understanding of the disease. Such models are also essential tools to accelerate development of new therapies for lung cancer. In this review we summarize the available lung cancer model systems in use today and define both their utility and limitations.

Brusatol-mediated induction of leukemic cell differentiation and G(1) arrest is associated with down-regulation of c-myc

Employing the natural product quassinoid brusatol, we currently report cellular and molecular events leading to cell death or terminal differentiation in a panel of leukemic cells. Brusatol and bruceantin exerted significant cytotoxic effects with several leukemic cell lines, but not with K562 or normal lymphocytic cells. Cell lines that were less sensitive to the cytotoxic effects of brusatol responded primarily through induction of terminal differentiation. The differentiated phenotype in cell lines derived from acute or chronic myeloid leukemias (HL-60, K562, Kasumi-1, NB4, U937, BV173) was characterized for producing superoxide and non-specific esterase, and some with up-regulation of CD13 (cluster of differentiation) and down-regulation of CD15. Chronic myeloid leukemic cell lines, K562 and BV173, and acute lymphoblastic cell lines, SUPB13 and RS4;11, were induced to differentiate along the erythrocytic pathway. Withdrawal studies showed that brusatol treatment for 48 h was sufficient to induce commitment towards terminal differentiation in HL-60, K562 and SUPB13. Reh cells did not undergo maturation. Analysis of c-MYC protein expression revealed that brusatol or bruceantin down-regulated expression to undetectable levels in cell lines that were most sensitive, based on cell death or terminal differentiation. Generally, c-myc RNA was reduced, but to a lower extent than c-MYC protein levels, indicating c-myc expression was regulated by quassinoids at the post-transcriptional level. Thus, regulation of c-myc expression may represent a critical event that leads to terminal differentiation. Since these responses are facilitated at clinically achievable concentrations, quassinoids may be of value for the management of hematological malignancies.

Epithelial and fibroblast cell lines derived from a spontaneous mammary carcinoma in a MMTV/neu transgenic mouse

Female murine mammary tumor virus (MMTV)/neu transgenic mice, expressing a wild-type rat neu oncogene driven by an MMTV promoter, develop focal mammary adenocarcinomas that are pathologically very similar to human breast tumors. Two new cell lines were established from a mammary tumor that arose in a female MMTV/neu transgenic mouse. One of these lines, mammary carcinoma from Neu transgenic mouse A (MCNeuA), has an epithelial morphology, is cytokeratin positive, and expresses high levels of the neu transgene. Karyotyping and comparative genomic hybridization analyses demonstrated genomic alterations in the MCNeuA cell line. The other line, N202Fb3, has a fibroblast morphology, is cytokeratin negative, and expresses the neu transgene at a very low level. This cell line also expresses smooth muscle alpha-actin, suggesting that it is a myofibroblast line. The MCNeuA cell line is tumorigenic when injected into syngeneic MMTV/neu transgenic mice, with an in vivo doubling time of about 14 d. The rationale for establishing this tumor cell line was to provide a tumor transplantation system for rapidly assessing immunotherapeutic interventions before testing in the more cumbersome model of spontaneous tumor development in the MMTV/neu transgenic mice. Mice immunized with a Neu extracellular domain protein vaccine were protected against a subsequent inoculation of MCNeuA cells, indicating that this cell line will be useful for evaluating cancer vaccine strategies. This tumor cell line may also prove useful in studying the biological properties of the neu oncogene and its role in the malignant process. In addition, the tumor-derived fibroblast line may be useful for studying tumor-stromal cell interactions.

P53 is necessary for the apoptotic response mediated by a transient increase of Ras activity

The tumor suppressor p53 eliminates cancer-prone cells via multiple mechanisms, including apoptosis. Ras elicits apoptosis in cells after protein kinase C (PKC) downregulation. However, the role of p53 in Ras-mediated apoptosis has not been fully investigated. Here, we demonstrate that mouse fibroblasts that express wild-type p53 are more susceptible to apoptosis elicited by PKC inhibition if Ras is transiently expressed or upregulated as opposed to stably expressed. In the latter case, p53 is frequently mutated. Transiently increased Ras activity induces Bax, and PKC inhibition augments this induction. Overexpression of E6 inactivates p53 and thereby suppresses both Bax induction and apoptosis. In contrast, Bax is not induced in stable ras transfectants, regardless of PKC inhibition. The data suggest that short- and long-term activation of Ras use a different mechanism(s) to initiate apoptosis. The status of p53 may contribute to such differences.