We previously reported that PRL production is significantly enhanced by a PRL-releasing/regulating factor derived from the posterior pituitary (PP). Specifically, the levels of PRL messenger RNA synthesis and release were dramatically increased in cocultures of GH3 and PP cells. The present objectives were to: 1) determine whether PP cells activate PRL gene transcription in a promoter- and cell-specific manner; 2) compare promoter activation by PP cells with that caused by selected substances that regulate the PRL gene; and 3) examine which region of the promoter (proximal and/or distal) mediates the action of the PP. In Exp 1, GH3 cells, transfected either with luciferase reporter plasmids containing a wild type PRL promoter, a GH promoter, or a glycoprotein α-subunit promoter, were cocultured with PP cells. Luciferase activity was used as an index for promoter activation. PP cells induced an 18-fold stimulation of the PRL promoter, as compared with a 2-fold stimulation of the GH promoter and no effect on the glycoprotein α-subunit promoter. In Exp 2, GH3 cells transfected with the wild type PRL promoter were cocultured with PP, anterior pituitary, uterine, or PC12 cells for 24 h. PP cells caused a 20-fold stimulation of the PRL promoter, whereas anterior pituitary cells showed a moderate 5-fold stimulation; uterine and PC12 cells caused minimal (<2-fold) increases in luciferase activity. In Exp 3, GH3 cells were transfected with either the wild type PRL promoter (-2500 PRLluc) or with a truncated promoter (-425PRLluc) containing the proximal region only and were incubated with PP cells, TRH, vasoactive intestinal peptide (VIP), epidermal growth factor (EGF), or estradiol (E2) for 24 h. Compared with the induction of the wild type promoter, PP cells activated the truncated promoter by 30% only. Stimulation of the promoter by relatively high concentrations of TRH, VIP, EGF, or E2, either alone or in combination, was significantly less effective than that caused by PP cells. Conclusions: 1) PP cells stimulate PRL gene transcription in a tissue- and promoter-specific manner; 2) the magnitude of induction caused by PP cells far exceeds that caused by high concentrations of TRH, VIP, EGF, or E2; and 3) the distal enhancer region of the rat PRL gene is necessary for maximal responsiveness of the PRL promoter to PP cells.
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