Differential implication of deoxyribonucleic acid methylation in rat prolactin and rat growth hormone gene expressions: a comparison between rat pituitary cell strains

In order to assess the potential role of DNA methylation in the expression of rat PRL (rPRL) as compared to rat GH (rGH) gene, the cleavage patterns generated by the isoschizomeric restriction enzymes HpaII and MspI were examined in DNA isolated from rat pituitary cell lines producing either high levels of rPRL (GH3B6) or of rGH (GC) and in a stable variant cell strain which produces minute amounts of both hormones (GH3CDL cells). The rPRL and the rGH genes were found hypomethylated in GH3B6 and GC cells, respectively, whereas in GH3CDL cells both genes were methylated, indicating a correlation between the extent of gene methylation and the level of expression. However the use of 5-azacytidine (5-azaC), which decreases DNA methylation, suggested a variable importance of gene methylation in the control of rPRL and rGH gene expression. 5-AzaC was unable to increase rPRL production to a detectable level in GC cells, whereas the cytidine analog markedly increased rPRL production and rGH production in GH3CDL cells. Further analysis using GH3CDL cells showed that the extent of the 5-azaC-induced rPRL and rGH gene demethylation was consistent with the 5-azaC-induced increase of gene expressions. However, in these cells, the stimulation of rPRL and rGH production unexpectedly increased as a function of time elapsed after drug withdrawal. The maximal stimulation, 30-fold and 7-fold, respectively, was observed 3 weeks after a 60-h exposure to 5-azaC. This pattern suggests that other events are required for the full expression of rPRL and rGH genes in addition to their own demethylation.