The c-myc and PyMT oncogenes induce different tumor types in a somatic mouse model for pancreatic cancer

Mice transgenic with SV40-late-promoter-driven Polyomavirus Middle T oncogene exclusively develop hemangiomas

In order to develop a model system of infantile hemangioma, transgenic mice were developed carrying the Polyomavirus Middle T (PyMT) gene driven by the SV40 late promoter. From the 520 fertilized eggs surviving microinjection, there were 25 live births. Three of these showed the hemangioma phenotype and carried and expressed the PyMT gene; the remaining descendants were normal. The tumors showed abnormal vascular proliferation with cavernous hemangioma-like structures in the skin surface, tongue, ear mucosa and gastric mucosal tissue in the transgenic mice with hemangioma phenotype. Immunohistochemical staining for Ki-67 was negative, showing the tumors were hemangiomas rather than angiosarcomas. None of the PyMT transgenic mice survived beyond 4 weeks. Previously reported PyMT transgenic mice under the control of various promoters induce many tumor types including hemangiomas. PyMT driven by the SV40 late promoter is an improved model system because it only induces hemangiomas. However, it is limited by the post-natal lethality. Thus, conditional variants of this model system would be desirable.

Lessons from polyoma middle T antigen on signaling and transformation: A DNA tumor virus contribution to the war on cancer

Middle T antigen (MT) is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles of tyrosine kinase and phosphoinositide 3-kinase (PI3K) signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals. When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Protein phosphatase 2A, protein tyrosine kinases of the src family, PI3K, phospholipase Cgamma1 as well as the Shc/Grb2 adaptors are all assembled on MT. Their activation sets off a series of signaling cascades. Analyses of virus mutants as well as transgenic animals have demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal. There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer.

Progressive metaplastic and dysplastic changes in mouse pancreas induced by cyclooxygenase-2 overexpression

Cyclooxygenase-2 (COX-2) overexpression is an established factor linking chronic inflammation with metaplastic and neoplastic change in various tissues. We generated transgenic mice (BK5.COX-2) in which elevation of COX-2 and its effectors trigger a metaplasia-dysplasia sequence in exocrine pancreas. Histologic evaluation revealed a chronic pancreatitis-like state characterized by acinar-to-ductal metaplasia and a well-vascularized fibroinflammatory stroma that develops by 3 months. By 6 to 8 months, strongly dysplastic features suggestive of pancreatic ductal adenocarcinoma emerge in the metaplastic ducts. Increased proliferation, cellular atypia, and loss of normal cell/tissue organization are typical features in transgenic pancreata. Alterations in biomarkers associated with human inflammatory and neoplastic pancreatic disease were detected using immunohistochemistry. The abnormal pancreatic phenotype can be completely prevented by maintaining mice on a diet containing celecoxib, a well-characterized COX-2 inhibitor. Despite the high degree of atypia, only limited evidence of invasion to adjacent tissues was observed, with no evidence of distant metastases. However, cell lines derived from spontaneous lesions are aggressively tumorigenic when injected into syngeneic or nude mice. The progressive nature of the metaplastic/dysplastic changes observed in this model make it a valuable tool for examining the transition from chronic inflammation to neoplasia.

Lentivirus-mediated oncogene introduction into mammary cells in vivo induces tumors

We recently reported the introduction of oncogene-expressing avian retroviruses into somatic mammary cells in mice susceptible to infection by transgenic expression of tva, encoding the receptor for subgroup A avian leukosis-sarcoma virus (ALSV). Because ALSV-based vectors poorly infect nondividing cells, they are inadequate for studying carcinogenesis initiated from nonproliferative cells (e.g., stem cells). Lentivirus pseudotyped with the envelope protein of ALSV infects nondividing TVA-producing cells in culture but has not previously been tested for introducing genes in vivo. Here, we demonstrate that these vectors infected mammary cells in vivo when injected into the mammary ductal lumen of mice expressing tva under the control of the keratin 19 promoter. Furthermore, intraductal injection of this lentiviral vector carrying the polyoma middle T antigen gene induced atypical ductal hyperplasia and ductal carcinoma in situ-like premalignant lesions in 30 days and palpable invasive tumors at a median latency of 3.3 months. Induced tumors were a mixed epithelial/myoepithelial histologic diagnosis, occasionally displayed squamous metaplasia, and were estrogen receptor-negative. This work demonstrates the first use of a lentiviral vector to introduce oncogenes for modeling cancer in mice, and this vector system may be especially suitable for introducing genetic alterations into quiescent cells in vivo.

A Cre-loxP-based mouse model for conditional somatic gene expression and knockdown in vivo by using avian retroviral vectors

Site- and time-specific somatic gene transfer by using the avian sarcoma-leukosis retrovirus RCAS (replication-competent avian sarcoma-leukosis virus long terminal repeat with splice acceptor) has been shown to be a powerful tool to analyze gene function in vivo. RCAS retroviruses that express the avian subgroup A envelope transduce only mammalian cells genetically engineered to express the avian retroviral receptor, tumor virus A (TVA). Here, we generated a knockin mouse line termed LSL-R26(Tva-lacZ) with concomitant conditional expression of TVA and lacZ by targeting the Rosa26 locus. A loxP-flanked transcriptional stop cassette was used for conditional activation of TVA and LacZ expression in a Cre-recombinase-dependent manner. To demonstrate the ability of this system for conditional somatic gene transfer in vivo, we directed TVA expression to the pancreas. Introduction of an RCAS vector with Bryan-RSV polymerase and subgroup A envelope [RCASBP(A)] carrying oncogenic Kras(G12D) induced focal ductal pancreatic lesions that recapitulate human pancreatic intraepithelial neoplasias that progress to pancreatic ductal adenocarcinomas. TVA-mediated infection of genetically engineered mice with endogenous expression of Kras(G12D) in pancreatic progenitor cells by using RCASBP(A) virus carrying a short hairpin RNA directed against murine TP53, resulted in dramatically enhanced progression to invasive adenocarcinomas. These results show that conditional expression of TVA enables spatiotemporal gene expression and knockdown in a small subset of somatic cells in vivo. Therefore, it closely models carcinogenesis in humans where tumors evolve from somatic gene mutations in developmentally normal cells. Combined with the growing number of Cre expression models, RCAS-TVA-based gene expression and knockdown systems open up promising perspectives for analysis of gene function in a time-controlled and tissue-specific fashion in vitro and in vivo.

Gene targeting to the stroma of the prostate and bone

Stromal-epithelial interactions mediated by paracrine signaling mechanisms dictate prostate development and progression of prostate cancer. The regulatory role of androgens in both the prostate stromal and epithelial compartments set the prostate apart from many other organs and tissues with regard to gene targeting. The identification of androgen-dependent prostate epithelial promoters has allowed successful gene targeting to the prostate epithelial compartment. Currently, there are no transgenic mouse models available to specifically alter gene expression within the prostate stromal compartment. As a primary metastatic site for prostate cancer is bone, the functional dissection of the bone stromal compartment is important for understanding stromal-epithelial interactions associated with metastatic tumor growth. Use of currently available methodologies for the expression or deletion of gene expression in recent research studies has advanced our understanding of the stroma. However, the complexity of stromal heterogeneity within the prostate remains a challenge to obtaining compartment or cell-lineage-specific in vivo models necessary for furthering our understanding of prostatic developmental, benign, tumorigenic, and metastatic growth.

Assessing tumor progression factors by somatic gene transfer into a mouse model: Bcl-xL promotes islet tumor cell invasion

Tumors develop through multiple stages, implicating multiple effectors, but the tools to assess how candidate genes contribute to stepwise tumor progression have been limited. We have developed a novel system in which progression of phenotypes in a mouse model of pancreatic islet cell tumorigenesis can be used to measure the effects of genes introduced by cell-type-specific infection with retroviral vectors. In this system, bitransgenic mice, in which the rat insulin promoter (RIP) drives expression of both the SV40 T antigen (RIP-Tag) and the receptor for subgroup A avian leukosis virus (RIP-tva), are infected with avian viral vectors carrying cDNAs encoding candidate progression factors. Like RIP-Tag mice, RIP-Tag; RIP-tva bitransgenic mice develop isolated carcinomas by ∼14 wk of age, after progression through well-defined stages that are similar to aspects of human tumor progression, including hyperplasia, angiogenesis, adenoma, and invasive carcinoma. When avian retroviral vectors carrying a green fluorescent protein marker were introduced into RIP-Tag; RIP-tva mice by intra-cardiac injection at the hyperplastic or early dysplastic stage of tumorigenesis, approximately 20% of the TVA-positive cells were infected and expressed green fluorescent proteins as measured by flow cytometry. Similar infection with vectors carrying cDNA encoding either of two progression factors, a dominant-negative version of cadherin 1 (dnE-cad) or Bcl-xL, accelerated the formation of islet tumors with invasive properties and pancreatic lymph node metastasis. To begin studying the mechanism by which Bcl-xL, an anti-apoptotic protein, promotes invasion and metastasis, RIP-Tag; RIP-tva pancreatic islet tumor cells were infected in vitro with RCASBP-Bcl-xL. Although no changes were observed in rates of proliferation or apoptosis, Bcl-xL altered cell morphology, remodeled the actin cytoskeleton, and down-regulated cadherin 1; it also induced cell migration and invasion, as evaluated using two-chamber transwell assays. In addition, myosin Va was identified as a novel Bcl-xL-interacting protein that might mediate the effects of Bcl-xL on tumor cell migration and invasion.

Cancer cells accumulate multiple genetic alterations. Some of these contribute to tumor development while others are a mere by-product of genomic instability. To determine whether a candidate gene can promote tumor development, we have developed a novel experimental system using engineered viruses to deliver genes into premalignant lesions. We used genetically engineered mice in which both an oncogene (SV40 T antigen) and a specific docking molecule for the virus are produced in β cells in the pancreatic islets of Langerhans. Tumors form in only a subset of the islets expressing this oncogene, indicating that tumor development requires other events. Since these precancerous cells also express the virus docking molecule, we could deliver candidate progression genes via the virus to see whether they contributed to tumor progression. We show that genes encoding two proteins (a survival factor, Bcl-xL, and an inhibitory form of the cell adhesion molecule cadherin 1) can be delivered into premalignant β cells and thereby enhance tumorigenesis. Both of these proteins were previously implicated in tumor progression, confirming that our technique can identify such progression genes. Moreover, we find that Bcl-xL promotes tumor cell migration and invasion by a mechanism distinct from its known role in cell survival.

A new technique using lentivirus to deliver genes into targeted cell types reveals that the tumor progression geneBcl-xL not only promotes cell survival in premalignant cells but also promotes tumor cell invasion.

Polyomavirus middle T antigen induces the transcription of osteopontin, a gene important for the migration of transformed cells

Middle T antigen (MT) is the principal oncoprotein of murine polyomavirus. Experiments on the acute immediate effects of MT expression on cellular RNA levels showed that expression of osteopontin (OPN) was strongly induced by MT expression. Osteopontin is a protein known to be associated with cancer. It has a role in tumor progression and invasion. Protein analysis confirmed that MT induced the secretion of OPN into the extracellular medium. Expression of antisense OPN RNA had no effect on the growth of MT-transformed cells. However, it had a strong effect on the ability of MT transformants to migrate or to fill a wound. Analysis of MT mutants implicated both the SHC and phosphatidylinositol 3-kinase pathways in OPN induction. Reporter assays showed that MT regulated the OPN promoter through two of its PEA3 (polyoma enhancer activator 3) sites. As critical PEA3 sites are also part of the polyomavirus enhancer, the same signaling important for viral replication also contributes to virally induced metastatic potential.

Trp53 deletion stimulates the formation of metastatic pancreatic tumors

The presence of distant metastases is a common finding on diagnosis of pancreatic cancer; however, the mechanisms underlying the dissemination of this tumor type remain poorly understood. Loss of the p53 tumor suppressor protein has been associated with tumor progression and metastasis in several tumor types including pancreatic ductal adenocarcinoma. Here, we describe the generation of a progressive and metastatic pancreatic cancer mouse model after the somatic and sporadic delivery of avian retroviruses encoding the mouse polyoma virus middle T antigen to elastase-tv-a transgenic mice with a pancreas-specific deletion of the Trp53 tumor suppressor locus. In this model, the tumors metastasize most frequently to the liver, consistent with human pancreatic carcinomas. Analysis of metastatic lesions demonstrated that concomitant loss of the Ink4a/Arf locus was not required for metastasis; however, pancreas-specific deletion of a single Ink4a/Arf allele cooperated with Trp53 deletion in a haploinsufficient manner to accelerate tumor development. Thus, our findings illustrate the potential role of p53 loss of function in pancreatic tumor progression, demonstrate the feasibility of modeling pancreatic cancer metastasis after somatic and sporadic oncogene activation, and indicate that our model may provide a useful experimental system for investigation of the molecular mechanisms underlying pancreatic cancer progression and metastasis.

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.

Loss of p53 and Ink4a/Arf cooperate in a cell autonomous fashion to induce metastasis of hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related death worldwide. HCC patients frequently present with disease that has metastasized to other regions of the liver, the portal vein, lymph nodes, or lungs, leading to poor prognoses. Therefore, model systems that allow exploration of the molecular mechanisms underlying metastasis in this disease are greatly needed. We describe here a metastatic HCC model generated after the somatic introduction of the mouse polyoma virus middle T antigen to mice with liver-specific deletion of the Trp53 tumor suppressor locus and show the cell autonomous effect of p53 loss of function on HCC metastasis. We additionally find that cholangiocarcinoma also develops in these mice, and some tumors display features of both HCC and cholangiocarcinoma, suggestive of origin from liver progenitor cells. Concomitant loss of the Ink4a/Arf tumor suppressor locus accelerates tumor formation and metastasis, suggesting potential roles for the p16 and p19 tumor suppressors in this process. Significantly, tumor cell lines isolated from tumors lacking both Trp53 and Ink4a/Arf display enhanced invasion activity in vitro relative to those lacking Trp53 alone. Thus, our data illustrate a new model system amenable for the analysis of HCC metastasis.

Maximizing mouse cancer models

Animal models of cancer provide an alternative means to determine the causes of and treatments for malignancy, thus representing a resource of immense potential for cancer medicine. The sophistication of modelling cancer in mice has increased to the extent that investigators can both observe and manipulate a complex disease process in a manner impossible to perform in patients. However, owing to limitations in model design and technology development, and the surprising underuse of existing models, only now are we realising the full potential of mouse models of cancer and what new approaches are needed to derive the maximum value for cancer patients from this investment.

Wnt Signaling, Stem Cells, and the Cellular Origin of Breast Cancer

The breast epithelium comprises cells at different stages of differentiation, including stem cells, progenitor cells, and more differentiated epithelial and myoepithelial cells. Wnt signaling plays a critical role in regulating stem/progenitor cells in the mammary gland as well as other tissue compartments. Furthermore, there is strong evidence suggesting that aberrant activation of Wnt signaling induces mammary tumors from stem/progenitor cells, and that Wnt exerts its oncogenic effects through LRP5/6-mediated activation of beta-catenin and mTOR pathways. Recent studies using avian retrovirus-mediated introduction of oncogenes into a small subset of somatic mammary cells suggest that polyoma middle T antigen (PyMT) may also preferentially transform stem/progenitor cells. These observations suggest that stem/progenitor cells in the mammary gland may be especially susceptible to oncogenic transformation. Whether more differentiated cells may also be transformed by particular oncogenes is actively debated; it is presently unclear whether stem cells or differentiated mammary cells are more susceptible to transformation by individual oncogenes. Better stem cell and progenitor cell markers as well as the ability to specifically target oncogenes into different mammary cell types will be needed to determine the spectrum of oncogene transformation for stem cells versus more differentiated cells.

Histological complexities of pancreatic lesions from transgenic mouse models are consistent with biological and morphological heterogeneity of human pancreatic cancer

Although pancreatic cancer is the fourth leading cause of cancer death, it has received much less attention compared to other malignancies. There are several transgenic animal models available for studies of pancreatic carcinogenesis, but most of them do not recapitulate, histologically, human pancreatic cancer. Here we review some detailed molecular complexity of human pancreatic cancer and their reflection in histomorphological complexities of pancreatic lesions developed in various transgenic mouse models with a special concern for studying the effects of chemotherapeutic and chemopreventive agents. These studies usually require a large number of animals that are at the same age and gender and should be either homozygote or heterozygote but not a mixture of both. Only single-transgene models can meet these special requirements, but many currently available models require a mouse to simultaneously bear several transgene alleles. Thus it is imperative to identify new gene promoters or enhancers that are specific for the ductal cells of the pancreas and are highly active in vivo so as to establish new single-transgene models that yield pancreatic ductal adenocarcinomas for chemotherapeutic and chemopreventive studies.

Sonic hedgehog acts at multiple stages during pancreatic tumorigenesis

Activation of sonic hedgehog (Shh) signaling occurs in the majority of pancreatic ductal adenocarcinomas. Here we investigate the mechanisms by which Shh contributes to pancreatic tumorigenesis. We find that Shh expression enhances proliferation of pancreatic duct epithelial cells, potentially through the transcriptional regulation of the cell cycle regulators cyclin D1 and p21. We further show that Shh protects pancreatic duct epithelial cells from apoptosis through the activation of phosphatidylinositol 3-kinase signaling and the stabilization of Bcl-2 and Bcl-X(L). Significantly, Shh also cooperates with activated K-Ras to promote pancreatic tumor development. Finally, Shh signaling enhances K-Ras-induced pancreatic tumorigenesis by reducing the dependence of tumor cells on the sustained activation of the MAPK and phosphatidylinositol 3-kinase/Akt/mTOR signaling pathways. Thus, our data suggest that Shh signaling contributes to tumor initiation in the pancreas through at least two mechanisms and additionally enhances tumor cell resistance to therapeutic intervention. Collectively, our findings demonstrate crucial roles for Shh signaling in multiple stages of pancreatic carcinogenesis.

Transgenic expression of human thymidylate synthase accelerates the development of hyperplasia and tumors in the endocrine pancreas

Thymidylate synthase (TS) is an essential enzyme for DNA synthesis and repair and elevated levels of TS have been identified as an important prognostic biomarker for colorectal cancer and several other common human malignancies. In addition, TS gene expression has been linked with cell-cycle regulation and cell proliferation through the ability of retinoblastoma protein to repress the transcriptional activation of E2F target genes such as TS. Therefore, overproduction of TS could participate in the progression to a neoplastic phenotype. Consistent with this model, a recent study has suggested that ectopic TS expression can induce a transformed phenotype in mammalian cells. To investigate the role of deregulated TS activity in tumor development, we generated transgenic mice that express high levels of catalytically active human TS (hTS) exclusively in the pancreas and low levels of hTS in multiple other tissues. Analyses of pancreatic tissue in TS transgenic mice revealed abnormalities within the endocrine pancreas, ranging from pancreatic islet hyperplasia to the detection of islet cell tumors. Overexpression of hTS in murine islets provides a new model to study genetic alterations associated with the progression from normal cells to hyperplasia to islet cell tumors, and suggests that this mouse model may be useful for regulating TS activity in vivo for development of cancer prevention and new therapies.

Islet hypertrophy following pancreatic disruption of Smad4 signaling

To investigate the role of transforming growth factor (TGF)-beta family signaling in the adult pancreas, a transgenic mouse (E-dnSmad4) was created that expresses a dominant-negative Smad4 protein driven by a fragment of the elastase promoter. Although E-dnSmad4 mice have normal growth, pancreas weight, and pancreatic exocrine and ductal histology, beginning at 4-6 wk of age, E-dnSmad4 mice show an age-dependent increase in the size of islets. In parallel, an expanded population of replicating cells expressing the E-dnSmad4 transgene is found in the stroma between the enlarged islets and pancreatic ducts. Despite the marked enlargement, E-dnSmad4 islets contain normal ratios and spatial organization of endocrine cell subtypes and have normal glucose homeostasis. Replication of cells derived from primary duct cultures of wild-type mice, but not E-dnSmad4 mice, was inhibited by the addition of TGF-beta family proteins, demonstrating a cell-autonomous effect of the transgene. These data show that, in the adult pancreas, TGF-beta family signaling plays a role in islet size by regulating the growth of a pluripotent progenitor cell residing in the periductal stroma of the pancreas.

Introduction of oncogenes into mammary glands in vivo with an avian retroviral vector initiates and promotes carcinogenesis in mouse models

We have adapted the avian leukosis virus RCAS (replication-competent avian sarcoma-leukosis virus LTR splice acceptor)-mediated somatic gene transfer technique to introduce oncogenes into mammary cells in mice transgenic for the avian subgroup A receptor gene, tva, under control of the mouse mammary tumor virus (MMTV) promoter. Intraductal instillation of an RCAS vector carrying the polyoma middle T antigen (PyMT) gene (RCAS-PyMT) induced multiple, oligoclonal tumors within 3 weeks in infected mammary glands of MMTV-tva transgenic mice. The rapid appearance of these tumors from a relatively small pool of infected cells (estimated to be approximately 2 x 10(3) cells per gland by infection with RCAS carrying a GFP gene; RCAS-GFP) was accompanied by a high fraction of cells positive for Ki67, Cyclin D1, and c-Myc, implying strong proliferation competence. Furthermore, the tumors displayed greater cellular heterogeneity than did tumors arising in MMTV-PyMT mice, suggesting that RCAS-PyMT transforms a relatively immature cell type. Infection of mice transgenic for both MMTV-Wnt-1 and MMTV-tva with RCAS virus carrying an activated Neu oncogene dramatically enhanced tumor formation over what is observed in uninfected bitransgenic animals. We conclude that infection of mammary glands with retrovirus vectors is an efficient means to screen candidate oncogenes for their capacity to initiate or promote mammary carcinogenesis in the mouse.

MicroRNA expression abnormalities in pancreatic endocrine and acinar tumors are associated with distinctive pathologic features and clinical behavior

PURPOSE

We investigated the global microRNA expression patterns in normal pancreas, pancreatic endocrine tumors and acinar carcinomas to evaluate their involvement in transformation and malignant progression of these tumor types. MicroRNAs are small noncoding RNAs that regulate gene expression by targeting specific mRNAs for degradation or translation inhibition. Recent evidence indicates that microRNAs can contribute to tumor development and progression and may have diagnostic and prognostic value in several human malignancies.

MATERIALS AND METHODS

Using a custom microarray, we studied the global microRNA expression in 12 nontumor pancreas and 44 pancreatic primary tumors, including 12 insulinomas, 28 nonfunctioning endocrine tumors, and four acinar carcinomas.

RESULTS

Our data showed that a common pattern of microRNA expression distinguishes any tumor type from normal pancreas, suggesting that this set of microRNAs might be involved in pancreatic tumorigenesis; the expression of miR-103 and miR-107, associated with lack of expression of miR-155, discriminates tumors from normal; a set of 10 microRNAs distinguishes endocrine from acinar tumors and is possibly associated with either normal endocrine differentiation or endocrine tumorigenesis; miR-204 is primarily expressed in insulinomas and correlates with immunohistochemical expression of insulin; and the overexpression of miR-21 is strongly associated with both a high Ki67 proliferation index and presence of liver metastasis.

CONCLUSION

These results suggest that alteration in microRNA expression is related to endocrine and acinar neoplastic transformation and progression of malignancy, and might prove useful in distinguishing tumors with different clinical behavior.

Genetics and biology of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer death in the United States with a median survival of <6 mo and a dismal 5-yr survival rate of 3%-5%. The cancer's lethal nature stems from its propensity to rapidly disseminate to the lymphatic system and distant organs. This aggressive biology and resistance to conventional and targeted therapeutic agents leads to a typical clinical presentation of incurable disease at the time of diagnosis. The well-defined serial histopathologic picture and accompanying molecular profiles of PDAC and its precursor lesions have provided the framework for emerging basic and translational research. Recent advances include insights into the cancer's cellular origins, high-resolution genomic profiles pointing to potential new therapeutic targets, and refined mouse models reflecting both the genetics and histopathologic evolution of human PDAC. This confluence of developments offers the opportunity for accelerated discovery and the future promise of improved treatment.

Characterization of pancreatic lesions from MT-tgf alpha, Ela-myc and MT-tgf alpha/Ela-myc single and double transgenic mice

In order to identify good animal models for investigating therapeutic and preventive strategies for pancreatic cancer, we analyzed pancreatic lesions from several transgenic models and made a series of novel findings. Female MT-tgfα mice of the MT100 line developed pancreatic proliferation, acinar-ductal metaplasia, multilocular cystic neoplasms, ductal adenocarcinomas and prominent fibrosis, while the lesions in males were less severe. MT-tgfα-ES transgenic lines of both sexes developed slowly progressing lesions that were similar to what was seen in MT100 males. In both MT100 and MT-tgfα-ES lines, TGFα transgene was expressed mainly in proliferating ductal cells. Ela-myc transgenic mice with a mixed C57BL/6, SJL and FVB genetic background developed pancreatic tumors at 2–7 months of age, and half of the tumors were ductal adenocarcinomas, similar to what was reported originally by Sandgren et al [1]. However, in 20% of the mice, the tumors metastasized to the liver. MT100/Ela-myc and MT-tgfα-ES/Ela-myc double transgenic mice developed not only acinar carcinomas and mixed carcinomas as previously reported but also various ductal-originated lesions, including multilocular cystic neoplasms and ductal adenocarcinomas. The double transgenic tumors were more malignant and metastasized to the liver at a higher frequency (33%) compared with the Ela-myc tumors. Sequencing of the coding region of p16ink4, k-ras and Rb cDNA in small numbers of pancreatic tumors did not identify mutations. The short latency for tumor development, the variety of tumor morphology and the liver metastases seen in Ela-myc and MT-tgfα/Ela-myc mice make these animals good models for investigating new therapeutic and preventive strategies for pancreatic cancer.

Development of the pancreas and pancreatic cancer

The pancreas is specified during embryonic development from the gut endoderm. Among the signaling pathways required for the proper development of the organ are the notch and hedgehog signaling pathways. Both of these pathways are reactivated in pancreatic cancers, and sustained hedgehog signaling is required for the viability of most pancreatic cancer cell lines. Further, mouse models of the disease show activation of these pathways, and expression of pancreas progenitor markers. These findings indicate that developmentally regulated gene expression programs are important in the pathogenesis of pancreatic cancer.

N-cadherin and keratinocyte growth factor receptor mediate the functional interplay between Ki-RASG12V and p53V143A in promoting pancreatic cell migration, invasion, and tissue architecture disruption

The genetic basis of pancreatic ductal adenocarcinoma, which constitutes the most common type of pancreatic malignancy, involves the sequential activation of oncogenes and inactivation of tumor suppressor genes. Among the pivotal genetic alterations are Ki-RAS oncogene activation and p53 tumor suppressor gene inactivation. We explain that the combination of these genetic events facilitates pancreatic carcinogenesis as revealed in novel three-dimensional cell (spheroid cyst) culture and in vivo subcutaneous and orthotopic xenotransplantation models. N-cadherin, a member of the classic cadherins important in the regulation of cell-cell adhesion, is induced in the presence of Ki-RAS mutation but subsequently downregulated with the acquisition of p53 mutation as revealed by gene microarrays and corroborated by reverse transcription-PCR and Western blotting. N-cadherin modulates the capacity of pancreatic ductal cells to migrate and invade, in part via complex formation with keratinocyte growth factor receptor and neural cell adhesion molecule and in part via interaction with p120-catenin. However, modulation of these complexes by Ki-RAS and p53 leads to enhanced cell migration and invasion. This preferentially induces the downstream effector AKT over mitogen-activated protein kinase to execute changes in cellular behavior. Thus, we are able to define molecules that in part are directly affected by Ki-RAS and p53 during pancreatic ductal carcinogenesis, and this provides a platform for potential new molecularly based therapeutic interventions.

Differential sensitivity of mouse epithelial tissues to the polyomavirus middle T oncogene

To determine how different epithelial cell types respond to the same oncogenic stimulation, we have used a modified human keratin 18 gene to conditionally express the polyomavirus middle T antigen (PyMT) oncogene in simple epithelial tissues of transgenic mice. Activation of PyMT expression by transgenic Cre recombinase in mammary epithelial cells resulted in carcinomas in all bitransgenic females. PyMT expression induced by K18-driven Cre in internal epithelial organs resulted in pancreatic acinar metaplasia and ductal dysplasia with remarkable desmoplastic stromal responses in all 25 bitransgenic mice. Hepatoma formation with altered lipid metabolism and gastric adenocarcinoma occurred in 96 and 54% of these mice, respectively. Elevated PyMT RNA expression also correlated with intraepithelial neoplasia in the prostate. Activated Erk2 was found in mammary tumors, pancreatic tissues, and affected livers. Hes1 RNA, a target of Notch signaling that has been implicated downstream of Ras pathway activation, was elevated in pancreatic and liver lesions. The variety of responses of different epithelia to PyMT demonstrates the importance of the differentiated state in interpreting oncogenic signals.

Pathology of genetically engineered mouse models of pancreatic exocrine cancer: consensus report and recommendations

Several diverse genetically engineered mouse models of pancreatic exocrine neoplasia have been developed. These mouse models have a spectrum of pathologic changes; however, until now, there has been no uniform nomenclature to characterize these changes. An international workshop, sponsored by The National Cancer Institute and the University of Pennsylvania, was held from December 1 to 3, 2004 with the goal of establishing an internationally accepted uniform nomenclature for the pathology of genetically engineered mouse models of pancreatic exocrine neoplasia. The pancreatic pathology in 12 existing mouse models of pancreatic neoplasia was reviewed at this workshop, and a standardized nomenclature with definitions and associated images was developed. It is our intention that this nomenclature will standardize the reporting of genetically engineered mouse models of pancreatic exocrine neoplasia, that it will facilitate comparisons between genetically engineered mouse models and human pancreatic disease, and that it will be broad enough to accommodate newly emerging mouse models of pancreatic neoplasia.

In vivo disruption of TGF-beta signaling by Smad7 leads to premalignant ductal lesions in the pancreas

TGF-beta has been postulated to play an important role in the development of pancreatic cancers. More than 50% of human pancreatic cancers bear mutations of Sma- and Mad-related protein (Smad) 4, a critical protein required for TGF-beta signaling. To evaluate the in vivo function of TGF-beta in the development of pancreatic cancers, we generated a transgenic mouse model with pancreas-specific expression of Smad7, a specific inhibitor of TGF-beta signaling. Through the use of elastase I promoter, we directed the tissue specific expression of exogenous Smad7. Consistently, the exogenous Smad7 was detected only in the pancreas in the transgenic mice, and, furthermore, phosphorylation of Smad2 was blocked in the pancreatic tissues. At 6 months of age, most transgenic animals developed premalignant ductal lesions in the pancreas, with characteristics of pancreatic intraepithelial neoplasia (PanIN), a precursor to invasive pancreatic cancers. The premalignant lesions of the pancreas were accompanied by accelerated proliferation of the ductal epithelium and acinar cells, as well as increased fibrosis around the ductal lesions. This study not only demonstrated that in vivo inactivation of TGF-beta signaling is implicated in the development of early stage of pancreatic cancers, but also provided a promising animal model useful for the investigation and intervention of pancreatic cancers in humans.

Historical, current and future perspectives on gastrointestinal and pancreatic endocrine tumors

Gastrointestinal and pancreatic endocrine tumors are neoplasms of which the pathogenesis is not completely understood and of which the clinical behavior is difficult to predict. Originally, Masson suggested that the cell of origin was an endocrine cell derived from the gastrointestinal epithelium. However, Pearse showed that the endocrine cells throughout the body shared various features, among others the amine precursor uptake and decarboxylation (APUD) capacity, and postulated the neural crest as the common origin for all APUD cells, a hypothesis that received support from the scientific community for many years. Now, biologists start to elucidate the various transcription factors that drive gastrointestinal development, and it has become evident that Masson was presumably right. Transcription factors relevant for development may also operate during tumorigenesis, and their expression may determine tumor biology. With other genetic factors, they may play a role in the pathogenesis of gastrointestinal and pancreatic endocrine tumors, and perhaps, their expression will turn out to be of prognostic or therapeutic value. In this review, current knowledge on the development of endocrine cells, hypotheses on the origin of endocrine tumors, genetic alterations, and prognostic factors are discussed. It is suggested that the increasing understanding of the normal development of gastrointestinal and pancreatic endocrine cells, the accumulating data on genetic alterations in endocrine tumors and the reappraisal of the hypotheses on their pathogenesis formulated in the past may help in elucidating their pathogenesis and in more accurately predicting prognosis.

Mutant KRAS in the initiation of pancreatic cancer

Pancreatic ductal adenocarcinoma is the most common pancreatic neoplasm. There are approximately 33,000 new cases of pancreatic ductal adenocarcinoma annually in the United States with approximately the same number of deaths. Surgery represents the only opportunity for cure, but this is restricted to early stage pancreatic cancer. Pancreatic ductal adenocarcinoma evolves from a progressive cascade of cellular, morphological and architectural changes from normal ductal epithelium through preneoplastic lesions termed pancreatic intraepithelial neoplasia (PanIN). These PanIN lesions are in turn associated with somatic alterations in canonical oncogenes and tumor suppressor genes. Most notably, early PanIN lesions and almost all pancreatic ductal adenocarcinomas involve mutations in the K-ras oncogene. Thus, it is believed that activating K-ras mutations are critical for initiation of pancreatic ductal carcinogenesis. This has been proven through elegant genetically engineered mouse models in which a Cre-activated K-Ras(G12D) allele is knocked into the endogenous K-Ras locus and crossed with mice expressing Cre recombinase in pancreatic tissue. As a result, mechanistic insights are now possible into how K-Ras contributes to pancreatic ductal carcinogenesis, what cooperating events are required, and armed with this knowledge, new therapeutic approaches can be pursued and tested.

The absence of p53 promotes metastasis in a novel somatic mouse model for hepatocellular carcinoma

We have generated a mouse model for hepatocellular carcinoma using somatic delivery of oncogene-bearing avian retroviral vectors to the liver cells of mice expressing the viral receptor TVA under the control of the albumin gene promoter (Alb-TVA mice). Viruses encoding mouse polyoma virus middle T antigen (PyMT) induced tumors, which can be visualized with magnetic resonance imaging, in 65% of TVA-positive animals. While these tumors can exceed 10 mm in diameter, they do not invade locally or metastasize to the lungs. Delivery of PyMT-expressing viruses to Alb-TVA mice lacking an intact p53 gene does not increase tumor incidence. However, the resulting tumors are poorly differentiated, invasive, and metastatic to the lungs. Gene expression microarrays identified over 100 genes that are differentially expressed between tumors found in p53 wild-type and p53 null mice. Some of these genes, such as cathepsin E and Igf2, have been previously implicated in tumor cell migration and invasion. Tumors induced in p53 null, TVA transgenic mice by PyMT mutants with changes in specific tyrosine residues fail to form metastases, indicating that metastasis is dependent on both the oncogene and the absence of p53.

Targeted expression of human MYCN selectively causes pancreatic neuroendocrine tumors in transgenic zebrafish

The zebrafish model organism has been used extensively for studies of genetic pathways in development, indicating its potential applicability to cancer. Here we show that targeted expression of MYCN in cells of the pancreatic islet induces neuroendocrine carcinoma. Four transgenic fish developed abdominal tumors between 4 and 6 months of age, and histologic analysis revealed lobulated arrangements of neoplastic cells with expression of the MYCN transgene. The tumors also expressed insulin mRNA, and pancreatic exocrine cells and ducts were identified within the neoplasms, indicating a pancreatic origin for the tumor. Transmission electron microscopy revealed cytoplasmic, endocrine-dense core granules, analogous to those found in human neuroendocrine tumors. Our studies establish a zebrafish transgenic model of pancreatic neuroendocrine carcinoma, setting the stage to evaluate molecular pathways downstream of MYCN in this vertebrate forward genetic model system.

Oncogenic K-ras drives cell cycle progression and phenotypic conversion of primary pancreatic duct epithelial cells

We have established a primary pancreatic duct epithelial cell culture (PDEC) system to investigate the relationship between oncogenic activation of K-ras and pancreatic ductal tumorigenesis. We have found that the acute introduction of physiological levels of oncogenic K-ras (K-rasV12) into quiescent PDECs stimulates S-phase entry and induces a pronounced increase in cell size. Both effects are dependent on the functional integrity of the phosphatidylinositol 3'-kinase (PI3K)/mammalian target of rapamycin (mTOR) signaling pathway. In addition, K-rasV12 promotes the loss of epithelial E-cadherin and the gain of mesenchymal N-cadherin in PDEC. Our observations indicate that the oncogenic activation of K-ras is sufficient to elicit mitogenic and morphogenic responses in pancreatic ductal cells and hence is likely to play an instructive role in the initiation of pancreatic ductal adenocarcinoma.

Mouse models of pancreatic cancer: the fur is finally flying!

Pancreatic cancer remains one of the most lethal of all human malignancies. Until recently, preclinical studies have been hampered by the absence of mouse models faithfully recapitulating critical elements of the human disease. However, recent months have witnessed a flurry of activity with respect to prospective mouse models. This progress now allows the evaluation of novel strategies for early detection, chemoprevention, and therapy and also provides new insights regarding the potential for differentiated and undifferentiated cell types to act as cells of origin for pancreatic ductal adenocarcinoma.