Acute mutation of retinoblastoma gene function is sufficient for cell cycle re-entry

Cancer cells arise from normal cells through the acquisition of a series of mutations in oncogenes and tumour suppressor genes. Mouse models of human cancer often rely on germline alterations that activate or inactivate genes of interest. One limitation of this approach is that germline mutations might have effects other than somatic mutations, owing to developmental compensation. To model sporadic cancers associated with inactivation of the retinoblastoma (RB) tumour suppressor gene in humans, we have produced a conditional allele of the mouse Rb gene. We show here that acute loss of Rb in primary quiescent cells is sufficient for cell cycle entry and has phenotypic consequences different from germline loss of Rb function. This difference is explained in part by functional compensation by the Rb-related gene p107. We also show that acute loss of Rb in senescent cells leads to reversal of the cellular senescence programme. Thus, the use of conditional knockout strategies might refine our understanding of gene function and help to model human cancer more accurately.

Inside and out: the activities of senescence in cancer

The core aspect of the senescent phenotype is a stable state of cell cycle arrest. However, this is a disguise that conceals a highly active metabolic cell state with diverse functionality. Both the cell-autonomous and the non-cell-autonomous activities of senescent cells create spatiotemporally dynamic and context-dependent tissue reactions. For example, the senescence-associated secretory phenotype (SASP) provokes not only tumour-suppressive but also tumour-promoting responses. Senescence is now increasingly considered to be an integrated and widespread component that is potentially important for tumour development, tumour suppression and the response to therapy.

Redistribution of the Lamin B1 genomic binding profile affects rearrangement of heterochromatic domains and SAHF formation during senescence

Senescence is a stress-responsive form of stable cell cycle exit. Senescent cells have a distinct gene expression profile, which is often accompanied by the spatial redistribution of heterochromatin into senescence-associated heterochromatic foci (SAHFs). Studying a key component of the nuclear lamina lamin B1 (LMNB1), we report dynamic alterations in its genomic profile and their implications for SAHF formation and gene regulation during senescence. Genome-wide mapping reveals that LMNB1 is depleted during senescence, preferentially from the central regions of lamina-associated domains (LADs), which are enriched for Lys9 trimethylation on histone H3 (H3K9me3). LMNB1 knockdown facilitates the spatial relocalization of perinuclear H3K9me3-positive heterochromatin, thus promoting SAHF formation, which could be inhibited by ectopic LMNB1 expression. Furthermore, despite the global reduction in LMNB1 protein levels, LMNB1 binding increases during senescence in a small subset of gene-rich regions where H3K27me3 also increases and gene expression becomes repressed. These results suggest that LMNB1 may contribute to senescence in at least two ways due to its uneven genome-wide redistribution: first, through the spatial reorganization of chromatin and, second, through gene repression.

Immortalized mouse mammary fibroblasts lacking dioxin receptor have impaired tumorigenicity in a subcutaneous mouse xenograft model

Although the dioxin receptor, the aryl hydrocarbon receptor (AhR), is considered a major regulator of xenobiotic-induced carcinogenesis, its role in tumor formation in the absence of xenobiotics is still largely unknown. Trying to address this question, we have produced immortalized cell lines from wild-type (T-FGM-AhR+/+) and mutant (T-FGM-AhR-/-) mouse mammary fibroblasts by stable co-transfection with the simian virus 40 (SV-40) large T antigen and proto-oncogenic c-H-Ras. Both cell lines had a myofibroblast phenotype and similar proliferation, doubling time, SV-40 and c-H-Ras expression and activity, and cell cycle distribution. AhR+/+ and AhR-/- cells were also equally able to support growth factor- and anchorage-independent proliferation. However, the ability of T-FGM-AhR-/- to induce subcutaneous tumors (leimyosarcomas) in NOD/SCID-immunodeficient mice was close to 4-fold lower than T-FGM-AhR+/+. In culture, T-FGM-AhR-/- had diminished migration in collagen-I and decreased lamellipodia formation. VEGFR-1/Flt-1, a VEGF receptor that regulates cell migration and blood vessel formation, was also down-regulated in AhR-/- cells. Signaling through the ERK-FAK-PKB/AKT-Rac-1 pathway, which contributes to cell motility and invasion, was also significantly inhibited in T-FGM-AhR-/-. Thus, the lower tumorigenic potential of T-FGM-AhR-/- could result from a compromised adaptability of these cells to the in vivo microenvironment, possibly because of an impaired ability to migrate and to respond to angiogenesis.

The radioresistance biological function of the SCF/kit signaling pathway is mediated by the zinc-finger transcription factor Slug

Radiation-induced destruction of the hematopoietic system is the primary cause of death based on the findings that transfer of normal bone marrow cells prevents death from lethal irradiation. The stem cell factor-c-kit signaling pathway (SCF/c-kit) has been previously implicated in the hematopoietic recovery which prevents death from lethal irradiation, but the molecular mechanisms that mediate this biological effect are unknown. Since mutations on SCF, c-kit and Slug genes have a similar phenotype in mice, we examined if Slug could complement the radiosensitivity of kit-deficient mice. In this report, we show that Slug acts as a radioprotection agent as lack of Slug results in increased radiosensitivity. This effect cannot be recovered by activating SCF/c-kit in lethally irradiated Slug-deficient mice, as SCF-treated mice did not demonstrate stimulation of hematopoietic recovery leading to survival of the Slug-deficient mice. We found that we could complement the hematopoietic failure in lethally irradiated c-kit-deficient mice by transducing them with a TAT-Slug protein. We conclude that the zinc-finger transcription factor Slug is absolutely necessary for survival from lethal irradiation and identify Slug as the molecular target that mediates the radioprotection through SCF/c-kit. These results indicate that Slug may be a molecular component conferring radioresistance to cancer cells.

Liposarcoma initiated by FUS/TLS-CHOP: the FUS/TLS domain plays a critical role in the pathogenesis of liposarcoma

The most common chromosomal translocation in liposarcomas, t(12;16)(q13;p11), creates the FUS/TLS-CHOP fusion gene. We previously developed a mouse model of liposarcoma by expressing FUS-CHOP in murine mesenchymal stem cells. In order to understand how FUS-CHOP can initiate liposarcoma, we have now generated transgenic mice expressing altered forms of the FUS-CHOP protein. Transgenic mice expressing high levels of CHOP, which lacks the FUS domain, do not develop any tumor despite its tumorigenicity in vitro and widespread activity of the EF1alpha promoter. These animals consistently show the accumulation of a glycoprotein material within the terminally differentiated adipocytes, a characteristic figure of liposarcomas associated with FUS-CHOP. On the contrary, transgenic mice expressing the altered form of FUS-CHOP created by the in frame fusion of the FUS domain to the carboxy end of CHOP (CHOP-FUS) developed liposarcomas. No tumors of other tissues were found in these transgenic mice despite widespread activity of the EF1alpha promoter. The characteristics of the liposarcomas arising in the CHOP-FUS mice were very similar to those previously observed in our FUS-CHOP transgenic mice indicating that the FUS domain is required not only for transformation but also influences the phenotype of the tumor cells. These results provide evidence that the FUS domain of FUS-CHOP plays a specific and critical role in the pathogenesis of liposarcoma.

Phenotype specific analyses reveal distinct regulatory mechanism for chronically activated p53

The downstream functions of the DNA binding tumor suppressor p53 vary depending on the cellular context, and persistent p53 activation has recently been implicated in tumor suppression and senescence. However, genome-wide information about p53-target gene regulation has been derived mostly from acute genotoxic conditions. Using ChIP-seq and expression data, we have found distinct p53 binding profiles between acutely activated (through DNA damage) and chronically activated (in senescent or pro-apoptotic conditions) p53. Compared to the classical 'acute' p53 binding profile, 'chronic' p53 peaks were closely associated with CpG-islands. Furthermore, the chronic CpG-island binding of p53 conferred distinct expression patterns between senescent and pro-apoptotic conditions. Using the p53 targets seen in the chronic conditions together with external high-throughput datasets, we have built p53 networks that revealed extensive self-regulatory 'p53 hubs' where p53 and many p53 targets can physically interact with each other. Integrating these results with public clinical datasets identified the cancer-associated lipogenic enzyme, SCD, which we found to be directly repressed by p53 through the CpG-island promoter, providing a mechanistic link between p53 and the 'lipogenic phenotype', a hallmark of cancer. Our data reveal distinct phenotype associations of chronic p53 targets that underlie specific gene regulatory mechanisms.

FUS-DDIT3 prevents the development of adipocytic precursors in liposarcoma by repressing PPARgamma and C/EBPalpha and activating eIF4E

Background

FUS-DDIT3 is a chimeric protein generated by the most common chromosomal translocation t(12;16)(q13;p11) linked to liposarcomas, which are characterized by the accumulation of early adipocytic precursors. Current studies indicate that FUS-DDIT3- liposarcoma develops from uncommitted progenitors. However, the precise mechanism whereby FUS-DDIT3 contributes to the differentiation arrest remains to be elucidated.

Methodology/Principal Findings

Here we have characterized the adipocyte regulatory protein network in liposarcomas of FUS-DITT3 transgenic mice and showed that PPARγ2 and C/EBPα expression was altered. Consistent with in vivo data, FUS-DDIT3 MEFs and human liposarcoma cell lines showed a similar downregulation of both PPARγ2 and C/EBPα expression. Complementation studies with PPARγ but not C/EBPα rescued the differentiation block in committed adipocytic precursors expressing FUS-DDIT3. Our results further show that FUS-DDIT3 interferes with the control of initiation of translation by upregulation of the eukaryotic translation initiation factors eIF2 and eIF4E both in FUS-DDIT3 mice and human liposarcomas cell lines, explaining the shift towards the truncated p30 isoform of C/EBPα in liposarcomas. Suppression of the FUS-DDIT3 transgene did rescue this adipocyte differentiation block. Moreover, eIF4E was also strongly upregulated in normal adipose tissue of FUS-DDIT3 transgenic mice, suggesting that overexpression of eIF4E may be a primary event in the initiation of liposarcomas. Reporter assays showed FUS-DDIT3 is involved in the upregulation of eIF4E in liposarcomas and that both domains of the fusion protein are required for affecting eIF4E expression.

Conclusions/Significance

Taken together, this study provides evidence of the molecular mechanisms involve in the disruption of normal adipocyte differentiation program in liposarcoma harbouring the chimeric gene FUS-DDIT3.

A Novel Scaffold-Based Hybrid Multicellular Model for Pancreatic Ductal Adenocarcinoma-Toward a Better Mimicry of the in vivo Tumor Microenvironment

With a very low survival rate, pancreatic ductal adenocarcinoma (PDAC) is a deadly disease. This has been primarily attributed to (i) its late diagnosis and (ii) its high resistance to current treatment methods. The latter specifically requires the development of robust, realistic in vitro models of PDAC, capable of accurately mimicking the in vivo tumor niche. Advancements in the field of tissue engineering (TE) have helped the development of such models for PDAC. Herein, we report for the first time a novel hybrid, polyurethane (PU) scaffold-based, long-term, multicellular (tri-culture) model of pancreatic cancer involving cancer cells, endothelial cells, and stellate cells. Recognizing the importance of ECM proteins for optimal growth of different cell types, the model consists of two different zones/compartments: an inner tumor compartment consisting of cancer cells [fibronectin (FN)-coated] and a surrounding stromal compartment consisting of stellate and endothelial cells [collagen I (COL)-coated]. Our developed novel hybrid, tri-culture model supports the proliferation of all different cell types for 35 days (5 weeks), which is the longest reported timeframe in vitro. Furthermore, the hybrid model showed extensive COL production by the cells, mimicking desmoplasia, one of PDAC's hallmark features. Fibril alignment of the stellate cells was observed, which attested to their activated state. All three cell types expressed various cell-specific markers within the scaffolds, throughout the culture period and showed cellular migration between the two zones of the hybrid scaffold. Our novel model has great potential as a low-cost tool for in vitro studies of PDAC, as well as for treatment screening.

Understanding mesenchymal cancer: the liposarcoma-associated FUS-DDIT3 fusion gene as a model

Chromosomal translocations entail the generation of gene fusions in mesenchymal tumors. Despite the successful identification of these specific and consistent genetic events, the nature of the intimate association between the gene fusion and the resulting phenotype still remains to be elucidated. Here these studies are reviewed, using FUS-DDIT3 as a model to illustrate how they have contributed to current understanding in unique and unexpected ways. FUS-DDIT3 is a chimeric oncogene generated by the most common chromosomal translocation t(12;16)(q13;p11) associated with liposarcomas. The application of transgenic methods to the study of this sarcoma-associated FUS-DDIT3 gene fusion has provided insights into their functions in vivo, and suggested mechanisms by which lineage selection may be achieved.

Designing a bio-inspired biomimetic in vitro system for the optimization of ex vivo studies of pancreatic cancer

Pancreatic cancer is one of the most aggressive and lethal human malignancies. Drug therapies and radiotherapy are used for treatment as adjuvants to surgery, but outcomes remain disappointing. Advances in tissue engineering suggest that 3D cultures can reflect the in vivo tumor microenvironment and can guarantee a physiological distribution of oxygen, nutrients, and drugs, making them promising low-cost tools for therapy development. Here, we review crucial structural and environmental elements that should be considered for an accurate design of an ex vivo platform for studies of pancreatic cancer. Furthermore, we propose environmental stress response biomarkers as platform readouts for the efficient control and further prediction of the pancreatic cancer response to the environmental and treatment input.

A novel Atg5-shRNA mouse model enables temporal control of Autophagy in vivo

Macroautophagy/autophagy is an evolutionarily conserved catabolic pathway whose modulation has been linked to diverse disease states, including age-associated disorders. Conventional and conditional whole-body knockout mouse models of key autophagy genes display perinatal death and lethal neurotoxicity, respectively, limiting their applications for in vivo studies. Here, we have developed an inducible shRNA mouse model targeting Atg5, allowing us to dynamically inhibit autophagy in vivo, termed ATG5i mice. The lack of brain-associated shRNA expression in this model circumvents the lethal phenotypes associated with complete autophagy knockouts. We show that ATG5i mice recapitulate many of the previously described phenotypes of tissue-specific knockouts. While restoration of autophagy in the liver rescues hepatomegaly and other pathologies associated with autophagy deficiency, this coincides with the development of hepatic fibrosis. These results highlight the need to consider the potential side effects of systemic anti-autophagy therapies.

SLUG (SNAI2) overexpression in embryonic development

The Snail-related zinc-finger transcription factor, SLUG (SNAI2), is critical for the normal development of neural crest-derived cells and loss-of-function SLUG mutations have been proven to cause piebaldism and Waardenburg syndrome type 2 in a dose-dependent fashion. However, little is known about the consequences of SLUG overexpression in embryonic development. We report SLUG duplication in a child with a unique de novo 8q11.2-->q13.3 duplication associated with tetralogy of Fallot, submucous cleft palate, renal anomalies, hypotonia and developmental delay. To investigate the effects of Slug overexpression on development, we analyzed mice carrying a Slug transgene. These mice were morphologically normal at birth, inferring that Slug overexpression is not sufficient to cause overt morphogenetic defects. In the adult mice, there was a 20% incidence of sudden death, cardiomegaly and cardiac failure associated with incipient mesenchymal tumorigenesis. These findings, while not directly implicating Slug in congenital and acquired heart disease, raise the possibility that Slug overexpression may contribute to specific cardiac phenotypes and cancer development.

Chemoradiotherapy screening in a novel biomimetic polymer based pancreatic cancer model

Pancreatic Ductal Adenocarcinoma (PDAC) is a deadly and aggressive disease with a very low survival rate. This is partly due to the resistance of the disease to currently available treatment options. Herein, we report for the first time the use of a novel polyurethane scaffold based PDAC model for screening the short and relatively long term (1 and 17 days post-treatment) responses of chemotherapy, radiotherapy and their combination. We show a dose dependent cell viability reduction and apoptosis induction for both chemotherapy and radiotherapy. Furthermore, we observe a change in the impact of the treatment depending on the time-frame, especially for radiation for which the PDAC scaffolds showed resistance after 1 day but responded more 17 days post-treatment. This is the first study to report a viable PDAC culture in a scaffold for more than 2 months and the first to perform long-term (17 days) post-treatment observations in vitro. This is particularly important as a longer time-frame is much closer to animal studies and to patient treatment regimes, highlighting that our scaffold system has great potential to be used as an animal free model for screening of PDAC.

Poly-urethane scaffold based 3D pancreatic cancer model enables realistic long term chemotherapy and radiotherapy screening. This model can be used for personalised treatment screening.

Use of DNA transposons for functional genetic screens in mouse models of cancer

Cancer is a very heterogeneous disease with complex genetic interactions. In recent years, the systematic sequencing of cancer genomes has provided information to design personalized therapeutic interventions. However, the complexity of cancer genomes commonly makes it difficult to identify specific genes involved in tumour development or therapeutic responsiveness. The generation of mouse models of cancer using transposon-mediated approaches has provided a powerful tool to unveil the role of key genes during cancer development. Here we will discuss how the use of forward and reverse genetic approaches mediated by DNA transposons can support the investigation of cancer pathogenesis, including the identification of cancer promoting mechanisms and potential therapeutic targets.

Selective destruction of tumor cells through specific inhibition of products resulting from chromosomal translocations

A key problem in the effective treatment of patients with cancer (both leukemia and solid tumors) is to distinguish between tumor and normal cells. This problem is the main reason why current treatments for cancer are often ineffective. There have been remarkable advances in our understanding of the molecular biology of cancer that provides new selective tumor destruction mechanisms. The molecular characterization of the tumor-specific chromosomal abnormalities has revealed that fusion proteins are the consequence in the majority of cancers. These fusion proteins result from chimeric genes created by the translocations, which form chimeric mRNA species that contain exons from the genes involved in the translocation. Obviously, these chimeric molecules are attractive therapeutic targets since they are unique to the disease (they only exist in the tumor cells but not in the normal cells of the patient), allowing the design of specific anti-tumor drugs. Inhibition of chimeric gene expression by anti-tumor agents specifically kills leukemic cells without affecting normal cells. As therapeutic agents targeting chimeric genes, zinc-finger proteins, antisense RNAs or hammerhead-based ribozymes have been used. All of these agents have some limitations, indicating that new therapeutic tools are required as gene inactivating agents that should be able to inhibit any chimeric fusion gene product. Recently, we have used the catalytic RNA subunit of RNase P from Escherichia coli, which can be specifically directed to cut any mRNA sequence, to specifically destroy tumor-specific fusion genes created as a result of chromosomal translocations. In this chapter, we will review the advances made to selectively destroy tumor cells through specific inhibition of products resulting from chromosomal translocations.

Mapping Tumor-Stroma-ECM Interactions in Spatially Advanced 3D Models of Pancreatic Cancer

Bioengineering-based in vitro tumor models are increasingly important as tools for studying disease progression and therapy response for many cancers, including the deadly pancreatic ductal adenocarcinoma (PDAC) that exhibits a tumor/tissue microenvironment of high cellular/biochemical complexity. Therefore, it is crucial for in vitro models to capture that complexity and to enable investigation of the interplay between cancer cells and factors such as extracellular matrix (ECM) proteins or stroma cells. Using polyurethane (PU) scaffolds, we performed a systematic study on how different ECM protein scaffold coatings impact the long-term cell evolution in scaffolds containing only cancer or only stroma cells (activated stellate and endothelial cells). To investigate potential further changes in those biomarkers due to cancer-stroma interactions, we mapped their expression in dual/zonal scaffolds consisting of a cancer core and a stroma periphery, spatially mimicking the fibrotic/desmoplastic reaction in PDAC. In our single scaffolds, we observed that the protein coating affected the cancer cell spatial aggregation, matrix deposition, and biomarker upregulation in a cell-line-dependent manner. In single stroma scaffolds, different levels of fibrosis/desmoplasia in terms of ECM composition/quantity were generated depending on the ECM coating. When studying the evolution of cancer and stroma cells in our dual/zonal model, biomarkers linked to cell aggressiveness/invasiveness were further upregulated by both cancer and stroma cells as compared to single scaffold models. Collectively, our study advances the understanding of how different ECM proteins impact the long-term cell evolution in PU scaffolds. Our findings show that within our bioengineered models, we can stimulate the cells of the PDAC microenvironment to develop different levels of aggressiveness/invasiveness, as well as different levels of fibrosis. Furthermore, we highlight the importance of considering spatial complexity to map cell invasion. Our work contributes to the design of in vitro models with variable, yet biomimetic, tissue-like properties for studying the tumor microenvironment's role in cancer progression.

Chemotherapy Assessment in Advanced Multicellular 3D Models of Pancreatic Cancer: Unravelling the Importance of Spatiotemporal Mimicry of the Tumor Microenvironment

Pancreatic ductal adenocarcinoma (PDAC) is a challenge for global health with very low survival rate and high therapeutic resistance. Hence, advanced preclinical models for treatment screening are of paramount importance. Herein, chemotherapeutic (gemcitabine) assessment on novel (polyurethane) scaffold-based spatially advanced 3D multicellular PDAC models is carried out. Through comprehensive image-based analysis at the protein level, and expression analysis at the mRNA level, the importance of stromal cells is confirmed, primarily activated stellate cells in the chemoresistance of PDAC cells within the models. Furthermore, it is demonstrated that, in addition to the presence of activated stellate cells, the spatial architecture of the scaffolds, i.e., segregation/compartmentalization of the cancer and stromal zones, affect the cellular evolution and is necessary for the development of chemoresistance. These results highlight that, further to multicellularity, mapping the tumor structure/architecture and zonal complexity in 3D cancer models is important for better mimicry of the in vivo therapeutic response.