Differential roles for signal transducers and activators of transcription 5a and 5b in PRL stimulation of ERα and ERβ transcription

PRL has been shown to stimulate mRNA expression of both ER alpha and ER beta in the rat corpus luteum and decidua of pregnancy. To investigate whether PRL may stimulate ER expression at the level of transcription and which transcription factors may mediate this stimulation, we have cloned the 5'-flanking regions of both rat ER genes. A constitutively active PRL receptor (PRL-R-CA) stimulated both ERa and ERP promoter activity, indicating that PRL is acting to stimulate ER transcription. Putative signal transducer and activator of transcription (Stat)5 response elements were identified at -189 in the ER alpha promoter and at -330 in the ERP promoter. Mutation of these response elements or overexpression of dominant negative Stat5 prevented stimulation of ER alpha and ER beta promoter activity, indicating that PRL regulation of ER expression requires both intact Stat5 binding sites as well as functional Stat5. Interestingly, either Stat5a or Stat5b could stimulate ER alpha transcription while stimulation of ER beta occurred only in the presence of Stat5b. Through mutational analysis, a single nucleotide difference between the ER alpha and ER beta Stat5 response elements was shown to be responsible for the lack of Stat5a-mediated stimulation of ER beta. These findings indicate that PRL stimulation of ER expression occurs at the level of transcription and that PRL regulation of ER alpha can be mediated by either Stat5a or Stat5b, while regulation of ERP appears to be mediated only by Stat5b.