Inverse control of prolactin and growth hormone gene expression: effect of thyroliberin on transcription and RNA stabilization.

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Abstract
The hypothalamic tripeptide TRH regulates prolactin (PRL) and growth hormone (GH) synthesis inversely by modulating the levels of their specific mRNA. Changes in mRNA levels could involve both transcriptional and post-transcriptional events. The effect of TRH on the biosynthesis and degradation of PRL, and GH RNA in a rat pituitary tumor cell line was investigated. Newly synthesized PRL and GH RNA sequences were quantified in nuclear and cytoplasmic fractions by hybridization of 3H-labeled RNA to immobilized plasmid DNA containing either PRL or GH c[complementary]DNA sequences. Steady-state levels of specific RNA were estimated by RNA blot hybridization. TRH increases in a rapid but transient manner the transcription of the PRL gene; it evidently does not alter the processing and the transport to the cytoplasm. In contrast, after a lag-time, TRH seems to induce a long-lasting inhibition on GH, as well as on overall gene transcription. An effect of TRH on mRNA stability was observed. TRH significantly increases the half-life of PRL mRNA. The hypothesis that TRH decreases the half-life of GH mRNA is supported. Such post-transcriptional action of TRH amplifies and prolongs the regulations exerted at the transcriptional level.