Estradiol transcriptionally and posttranscriptionally up-regulates thyrotropin-releasing hormone receptor messenger ribonucleic acid in rat pituitary cells

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Abstract
17 beta-Estradiol (E2) 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 E2 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 GH3 cells, a TRH-induced 45Ca2+ efflux appeared. This efflux was dependent on the amount of TRH and injected RNA, and was inhibited by simultaneous addition of chloridazepoxide, an antagonist of the TRH receptor. Treatment of GH3 cells with E2 increased TRH receptor mRNA activity, as assessed in the oocyte expression system; the E2 effect became apparent after 3 h of treatment and reached a maximum (3- to 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 E2 maximally (5-fold) increased the TRH receptor mRNA level of GH3 cells, with a half-maximal concentration of 0.1 nM after 6 h of treatment. The elevated level of mRNA induced by E2 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 GH3 cells was increased 3-fold by 2 h of treatment with E2. Furthermore, the half-life of the TRH receptor mRNA in GH3 cells was prolonged by E2 (from 2.3 to 4.4 h). These results demonstrate that E2 up-regulates the TRH receptors of the pituitary cells at the mRNA level by increasing both the transcription rate and stability.

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