Suppression of Prolactin Expression by Cabergoline Requires Prolactin Regulatory Element-binding Protein (PREB) in GH3 Cells

The prolactin regulatory element-binding protein (PREB) is a transcriptional factor that regulates prolactin (PRL) promoter activity in the anterior pituitary. Prolactinomas are the most common pituitary tumors. Administration of cabergoline, a selective dopamine D2-receptor agonist, has become the initial therapy of choice for most patients with prolactinomas. Although activation of the D2 receptor results in the inhibition of PRL synthesis, the details of the underlying mechanisms remain unknown. Samples of ten prolactinomas and ten nonfunctioning pituitary adenomas were analyzed by immunohistochemistry to detect the expression of PREB. The effect of cabergoline on PREB expression was assessed by western blotting and real-time polymerase chain reaction (PCR) analysis. Reporter gene analysis of PRL was employed to examine the role of PREB on cabergoline-induced suppression of PRL transcription. Immunohistochemical analysis revealed strong positive PREB expression in the prolactinoma tissue, but extremely weak or undetected expression in the nonfunctioning pituitary tumor tissue. Western blots probed with a PREB-specific antiserum revealed that the relative abundance of the PREB protein in the GH3 cells decreased in a dose-dependent manner in response to cabergoline treatment, as did the relative abundance of PREB mRNA. Although cabergoline inhibited the activity of the PRL promoter, mutation of PREB-binding site within the promoter abrogated the ability of cabergoline to inhibit the PRL promoter activity. We have demonstrated that PREB is expressed in prolactinomas and that the suppression of PRL expression by cabergoline requires the transcriptional factor PREB.