ESTRADIOL TRANSCRIPTIONALLY AND POSTTRANSCRIPTIONALLY UP-REGULATES THYROTROPIN-RELEASING-HORMONE RECEPTOR MESSENGER-RIBONUCLEIC-ACID IN RAT PITUITARY-CELLS

17 beta-Estradiol (E(2)) has been shown to up-regulate the binding activity of the TRH receptor in rat pituitary cells. In this report, we investigated whether and how E(2) alters TRH receptor expression at the mRNA level using the oocyte mRNA expression system, Northern blot analysis, and nuclear run-on assay. In oocytes injected with mRNA from the anterior pituitaries or GH(3) cells, a TRH-induced Ca-45(2+) efflux appeared. This efflux was dependent on the amount of TRH and injected RNA, and was inhibited by simultaneous addition of chlordiazepoxide, an antagonist of the TRH receptor. Treatment of GH(3) cells with E(2) increased TRH receptor mRNA activity, as assessed in the oocyte expression system; the E(2) effect became apparent after 3 h of treatment and reached a maximum (3- to and 4-fold) between 6-72 h after addition. Northern blot analysis with a 412-basepair cDNA fragment or 3.7-kilobasepair full-length cDNA of the TRH receptor as a probe showed that E(2) maximally (5-fold) increased the TRH receptor mRNA level of GH(3) cells, with a half-maximal concentration of 0.1 nM after 6 h of treatment. The elevated level of mRNA induced by E(2) was augmented rather than impeded, by cycloheximide, indicating that ongoing protein synthesis was not required for the induction. The rate of transcription of the TRH receptor gene in isolated nuclei taken from GH(3) cells was increased 3-fold by 2 h of treatment with E(2). Furthermore, the half-life of the TRH receptor mRNA in GH(3) cells was prolonged by E(2) (from 2.3 to 4.4 h). These results demonstrate that E(2) up-regulates the TRH receptors of the pituitary cells at the mRNA level by increasing both the transcription rate and stability.