Unit: Oncogenic Viruses - URA1644 du CNRS

Director: YANIV Moshe

Our research unit studies the interplay between transcription factors and chromatin remodelling in activating and repressing gene expression programmes in mammalian cells. Our research examines the importance of several families of transcription factors in controlling cell differentiation and organogenesis, cell growth or apoptosis. We use a combination of approaches including complete or conditional gene inactivation in mice, microarray analysis and bioinformatics. Our work has implications for understanding viral and non-viral malignant cell transformation and for deciphering the mechanisms underlying diseases such as Type II diabetes, kidney polycystic disease or hepatic ductal plates malformation.

HNF1alpha and HNF1beta : development and disease Group leader: Marco Pontoglio

Members of the group: Olivier Bluteau, Antonia Doyen, Evelyne Fischer, Lionel Gresh, Serge Garbay and Andreas Reimann

Organ formation during development is a complex phenomenon whose molecular mechanisms are only partially understood. Hepatocyte Nuclear Factors 1 alpha and beta (HNF1α and HNF1β) are two homologous, atypical homeoproteins that appeared during evolution with the first vertebrates. They are expressed in the polarized epithelia of different organs, including liver, kidney, pancreas and intestine, where they control the expression of numerous tissue-specific genes. To gain insights into the role of these genes during development, we have generated mouse models carrying null mutations in HNF1 genes. Mice lacking HNF1β die in utero at embryonic day 7.5 because of a defect in visceral endoderm differentiation. However, the conditional inactivation of HNF1β has shown that this factor plays also a crucial role, later, during organ development. In the liver, HNF1β is essential for bile duct and hepatic artery formation, whereas in the kidney the lack of HNF1β leads to polycystic kidney disease. This renal phenotype is due to the defective expression of PKD2 and PKHD1, two genes whose mutations causes polycystic kidney disease with dominant and recessive inheritance, respectively. Interestingly, mutations in the human HNF1β gene are associated with renal cysts and Type II diabetes. Inactivation of HNF1α leads to postnatal dysfunctions in liver, kidney and pancreas. HNF1α-deficient mice suffer from phenylketonuria, renal Fanconi syndrome and Type II diabetes. Using DNA oligonucleotide microarray hybridization technology (Affymetrix) we identified several hundred genes whose expression is downregulated in the absence of either of these two transcription factors in the liver. We are currently studying the correlation between the transcriptional effects on these target genes and the enrichment for HNF1 binding sites detected in the nearby genomic regions using an in silico approach. Our aim is to develop approaches to predict genetic programs controlled by specific transcription factors.

Proto-oncogenes Jun and Fos: transcription factors regulating the cellular stress response Group leaders: Fatima Mechta-Grigoriou and Jonathan Weitzman

Members of the group : Maya Ameyar-Zazoua, Damien Gérald, Chaouki Miled, Marta Wisniewska.

The AP-1 transcription factor is central to the cell's ability to integrate multiple extracellular signals and initiate the appropriate genetic programme. AP-1 plays a critical role in regulating the cell cycle and the cellular response to stress. AP-1 is composed of dimers of Jun (c-Jun, JunB or JunD) and Fos (c-Fos, FosB, Fra1 and Fra2) proto-oncoproteins. Our laboratory focuses on unravelling the functions of the different Jun and Fos proteins using genetic and biochemical approaches. We have shown that the balance between different AP-1 dimers (Jun/Fos) controls cell cycle progression and the process of oncogenic transformation. We have taken a genomic approach to dissect the complex functional relationship between AP-1 and p53, in order to identify their convergent transcriptional programmes and their roles in tumorigenesis. We discovered a functional cooperation between oncogenic RAS and specific AP-1 heterodimers (c-Jun/Fra-1) in the regulation of the p14/p19ARF tumour suppressor gene, a key regulator of the p53 pathway. Furthermore, we identified novel p53 transcriptional targets that place p53 upstream of the JNK signal transduction cascade, leading to c-Jun phosphorylation and activation. These genomic studies have highlighted the complex convergence of the p53 and AP-1 pathways and their cooperation in determining tumour progression and apoptosis. In addition, we found that JunD protects cells from p53-dependent senescence and stress-induced apoptosis both in vitro and in vivo. Analysis of mice carrying single or double mutations in the c-jun and junD genes has revealed pleiotropic roles for these genes during development. c-Jun and JunD play critical roles in heart morphogenesis and embryonic angiogenesis. A systematic genomic study using microarrays allowed us to identify novel gene targets of AP-1 implicated in the response to diverse stress stimuli including hypoxic and oxidative stress. Our recent results show that JunD can suppress the expression of genes involved in H2O2 production and can activate genes required for anti-oxidative defence. Thus, JunD appears to be a general inhibitor of cellular damage, preventing oxidative stress and tumour angiogenesis in vivo.

Chromatin and transcription Group leader: Christian Muchardt

Members of the group: Eric Batsché, Brigitte Bourachot, Marc Lavigne, Julien Lefèvre, Bogdan Mateescu.

Opening and closing chromatin is essential for the control of gene expression throughout the life of eukaryotic cell. The SWI/SNF complex is one of the machineries controlling the compaction of chromatin. This complex uses the energy of ATP hydrolysis to increase accessibility of histone-associated DNA. It is required for normal development of the embryo and deregulated expression of some of the SWI/SNF sub-units affects cell cycle progression and will in some cases lead to tumour formation. Recently, we identified a mechanism for negative regulation of the SWI/SNF complex via acetylation of the carboxy-terminal region of the catalytic subunit Brm. This post-translational modification affects interaction of the Brm protein with the product of the Retinoblastoma tumour suppressor gene. In addition, we have unveiled a functional antagonism between the SWI/SNF complex and the heterochromatin protein HP1, a potent inhibitor of transcription. This study showed that transcriptional repression by HP1 is partially caused by the inhibiting effect of this protein on SWI/SNF chromatin remodelling. During this study, HP1-binding to chromatin was found to be dependent on an RNA component that is also required for SWI/SNF activity. Currently, we investigate the role of this RNA component in chromatin compaction. In addition, we are characterising the mechanisms of transcriptional repression by the chromatin components HP1 and Polycomb. Finally, we are using in vitro reconstituted chromatin to address the role of the SWI/SNF complex in the integration of HIV1 into the host genome.

Control of human papillomavirus type 18 carcinogenesis Group leader: Françoise Thierry

Members of the group : Sophie Bellanger, Stéphanie Blachon, Caroline Demeret, Sébastien Teissier.

Our research focuses on the mechanisms of malignant conversion of cervical cells infected with the Human Papillomavirus type 18 (HPV18). Cervical cancer is the second cause of mortality in women from cancer worldwide, after breast cancer. The HPV virus specifically infects the genital tract, where it replicates in the upper layers of the epidermis. The virus encodes two oncogenic proteins, E6 and E7, which alter the normal proliferative control of cells by interfering with two regulatory proteins of the cell cycle, p53 and pRB. Continuous expression of the viral oncogenes is required to induce and maintain the transformed phenotype. Transcription of the two viral oncogenes is strongly activated by a higher-order transcriptional control element called an "enhanceosome" that we have characterised in detail. We are currently searching for proteins involved in its activity and its cell specificity using chromatin immunoprecipitation experiments. We have shown that transcription of the viral oncogenes is repressed by the viral protein E2. We used this property of E2 to compare the transcriptome of HeLa cells expressing the viral protein using microarray analyses. We were able to define a pattern of cellular genes, targets of p53 or E2F, whose expression is specifically modulated in cervical carcinomas, thus representing a genetic signature of cervical carcinogenesis. During carcinogenic progression, the viral genome integrates into the cellular genome with specific disruption of the viral E2 gene. Reintroduction of the viral E2 protein into cervical carcinoma cells induces a strong anti-proliferative effect that is due to at last three independent mechanisms. Besides its repressive effect on E6 and E7 transcription, E2 can interact with many cellular proteins such as caspase 8 to induce apoptosis through the extrinsic pathway, and with the APC ubiquitin ligase, specific to the metaphase/anaphase transition, thereby inducing cell cycle arrest in G2/M. These different functions of E2 all slow down cellular proliferation, indicating that E2 is a strong antagonist of the viral oncogenes in carcinogenic progression. E2 inactivation is a key factor of HPV18-associated carcinogenic progression, offering a potential application as a therapeutic agent.

Legends

Renal-specific inactivation of HNF1β in the tubular epithelium leads to polycystic kidney disease. X-gal staining of histological sections of kidneys from control (A) or HNF1β deficient mice (B). β-galactosidase activity (ROSA26) depicts the expression pattern of the CRE recombinase driven by the KSP promoter (Shao et al., 2002). CRE expression inactivated the HNF1β gene in mice carrying a floxed homozygous allele (B). In controls, the ROSA26R driven β-galactosidase activity was seen in a large portion of medullary tubular cells. In mutants, several cysts lack the typical monolayered epithelial structure (arrows). All cyst-lining epithelial cells are positive for β-galactosidase, demonstrating that they underwent Cre recombination. Scale bars: 200 μm.

Keywords: oncogenes, transcription, chromatin, development, cell cycle, diabetes


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