Mechanism of induction of prolactin synthesis in GH cells.

Prolactin-specific RNA (RNA(PRL)) in total nuclear RNA and in cytoplasmic poly(A)(+)RNA isolated from GH (rat pituitary) cells was selectively hybridized to immobilized cloned cDNA(PRL). Agarose gel electrophoresis of the nuclear RNA(PRL) sequences eluted from the nitrocellulose filters revealed several RNA species of approximately 25-30, 18-19, and 12-13 S. Only the 12-13 S RNA species could be detected in the cytoplasmic poly(A)(+)RNA fraction. Comparative analysis of total nuclear RNA of control and thyrotropin-releasing hormone (thyroliberin)-treated cells by the reverse Southern blot technique demonstrated increased levels of all the nuclear RNA(PRL) species in hormone-treated cells. Nuclear and cytoplasmic RNA(PRL) sequences in control and treated cells were quantitated by molecular hybridization to cloned cDNA(PRL). The 2- to 3-fold stimulation of PRL production by thyrotropin-releasing hormone-treated GH(4)C(1) cells could be correlated to the corresponding increase of nuclear RNA(PRL) sequences. The hybrid strain, which produces 1/5th the amount of PRL that the parent GH(4)C(1) does, had 1/5th the amounts of nuclear RNA(PRL) sequences. Thyrotropin-releasing hormone affected neither prolactin production nor nuclear RNA(PRL) level in 928-9b cells. RNA(PRL) sequences could not be detected either in nuclei or in cytoplasm of prolactin nonproducing F(1)BGH(1)2C(1) cells. However, prolactin production could be induced and RNA(PRL) sequences could be detected in the total nuclear RNA and in cytoplasmic poly(A)(+)RNA fraction after treatment of this GH cell substrain with 5-bromodeoxyuridine. These results demonstrate that differential basal prolactin production and its modulation by thyrotropin-releasing hormone and by 5-bromodeoxyuridine can be correlated to the altered levels of nuclear RNA(PRL) sequences in the three GH cell strains.