Regulated expression of the prolactin gene in rat pituitary tumor cells.

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Abstract
Prolactin (PRL) gene expression in 3 strains of GH cells (rat pituitary tumor cells) was quantitated by measurement of: intracellular and extracellular PRL, cytoplasmic translatable PRL-specific mRNA (mRNAPRL) and molecular hybridization of cytoplasmic poly(A) RNA to cDNAPRL (DNA complementary to mRNAPRL). The cell lines utilized in this investigation were a PRL-producing (PRL+) strain, GH4C1, a PRL nonproducing 5-bromo-deoxyuridine resistant (PRL- BrdUrdr) strain, F1BGH12C1, and a new strain, 928-9b, derived by fusion of PRL+ cells with a nuclear monolayer of the PRL-, BrdUrdr GH cell strain. PRL production is a characteristic of 928-9b cells, but the level of PRL production (2-4 .mu.g/mg protein/24 h) is much lower than that of the PRL+ strain, GH4C1 (15-25 .mu.g/mg protein/24 h). Levels of cytoplasmic translatable mRNAPRL and cytoplasmic PRL-RNA sequences quantitated with a cDNAPRL probe were also much lower in 928-9b as compared to the PRL+ parent. PRL-RNA sequences could not be detected in the PRL- strain. Thyrotropin-releasing hormone (TRH) stimulates PRL synthesis N3-fold and inhibits growth hormone (GH) synthesis 72% in the PRL+ strain. TRH has no effect on the synthesis of PRL or GH in the 928-9b strain, although TRH receptors could be detected in these cells. Stimulation of PRL synthesis in the PRL+ strain by TRH could be correlated with increases in levels of cytoplasmic translatable mRNAPRL and increases in cytoplasmic PRL-RNA sequences. The graded expression of the PRL gene at the basal level, and in the response to TRH, is caused by the regulated production of mRNAPRL in these 3 GH cell strains.