Mécanismes de régulation transcriptionnelle du gène de l'alpha-foetoprotéine
|Abstract:||The AFP gene is expressed specifically in the fetal liver as well as in some hepatoma cells, which makes this gene an excellent model to study differentiation processes that are deregulated in cancer. Hepatoma cells, for example, are often refractory to the AFP-suppressive as well as anti-proliferative effects of glucocorticoid hormones. This thesis explores transcriptional regulation of the AFP locus with a particular attention given to the identification of hepatic differentiation factors involved in the activation of this locus, the ultimate goal being to enforce tumor differentiation. Using in vivo footprinting experiments, I demonstrate for the first time the occupation of a C/EBP site in the AFP promoter, a site which was generally believed to be occupied by HNF1 proteins. Chromatin immunoprecipitation experiments confirmed the binding of C/EBP proteins to the AFP gene promoter as well as its enhancers. Furthermore, we showed by using transfection and transgenesis experiments that C/EBP proteins activate the AFP gene in tandem with the nuclear receptor FTF and that this cooperativity depends on the precise positionning of the two sites relative to one another. A deletion or modification of the distance between these two factors drastically modifies the expression timing of the AFP gene during hepatic development, suggesting a nucleosome position effect crucial for the onco-fetal activity of the gene. We also observe a differential recruitement of C/EBP or the coregulators CBP/p300 on AFP regulatory regions during hepatic development and growth of a hepatoma cell line. We then identified HNF4 on the enhancer regions of the AFP locus in a rat hepatoma model and in adult rat liver. This suggests that HNF4 negatively influences AFP gene transcription and underscores a possible deregulation of this factor in hepatoma cells. Our work also indicates that the C/EBP complexes present at two enhancer regions of the AFP gene are directly targeted by the glucocorticoid receptor (via its DNA-binding domain), leading to a loss of cofactor recruitement and AFP gene repression. On the whole, this thesis underscores a central role for C/EBP proteins in AFP locus developmental and hormonal regulation. Therefore, C/EBP combined with FTF represent good candidates to enforce tumor cell differentiation.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||16 April 2018|
|Collection:||Thèses et mémoires|
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