Régulations de l'expression des gênes de la prolactine et de l'hormone de croissance dans des cellules hypophysaires en culture

The rat pituitary tumor derived cell lines of the "GH" family offer a fruitful model for studying the expressions of the prolactin (rPRL) and growth hormone (rGH) genes in basic and regulated states. In order to assess the potential role of DNA methylation in the basic expressions of rPRL and rGH genes we have used different cell strains which produce either high level of rPRL (GH3B6 cells) or of rGH (GC cells) and minute amounts of both hormones (GH3CDL cells). The cleavage patterns generated by the methylation sensitive enzymes Hpa II and Msp I indicated an inverse correlation between the extent of gene methylation and the level of expression. However the use of 5-azacytidine which decreases DNA methylation suggested a variable importance of gene methylation in the respective control of rPRL and rGH genes depending on the cell lines. In an other hand we attempted to elucidate some of the mechanisms by which thyroliberin (TRH) enhances rPRL gene transcription in GH3B6 cells. Preliminary results indicated that the persistent occupancy of the TRH receptors was required to sustain at least for the first 5 hours the increased rate of rPRL gene transcription. In addition the possible relationship between the TRH-induced acute rPRL release and the stimulation of rPRL gene transcription was investigated. The results suggested that the activators of the C kinase-mediated pathway which are actually involved in the stimulation of the acute release were not sufficient alone for eliciting the maximum TRH response at the gene level.