[Activin inhibits the promoter activity of human growth hormone gene in rat pituitary GH3 cells].

The present study was aimed at investigating the effect of activin on the activity of human growth hormone (hGH) gene promoter in rat pituitary GH3 cells and the underlying molecular mechanism. The method of luciferase reporter gene was used. We firstly established a stable GH3 cell line which contains hGH gene promoter (-484 to 30 bp) and luciferase reporter gene by transfecting pGL3-484-Luc2 luciferase expression plasmid into GH3 cells using Lipofectamine transfection reagent. After treating these cells with activin or activin plus various signaling transduction activators, the concentration of GH in the medium and lysate of GH3 cells and luciferase activities in GH3 cells were measured. The results showed that activin (5 nmol/L, 50 nmol/L) decreased the secretion and synthesis of GH. The amounts of GH content in GH3 lysate and medium treated with 50 nmol/L activin were 82% and 59% of the control, respectively. Furthermore, activin (5, 50 nmol/L) reduced the luciferase expression in stable GH3 cells, with the expression being 77% and 69% of the control (P<0.001). Among the activators of intracellular signaling transduction pathways, mitogen-activated protein kinases kinase (MAPKK/MEK) activators C(6) ceramide (1 micromol/L) abolished completely the inhibitory effect of activin. Western blot analysis further confirmed the inhibition of phosphorylated MEK in GH3 cells. The inhibitory effect of activin was abrogated following the deletion of the fragment from -132 to -66 bp within the hGH gene promoter. These results indicate that activin decreases the activity of hGH gene promoter in rat pituitary GH3 cells. The intracellular MEK dependent signaling pathway and the promoter sequence that spans the -132 to -66 bp fragment of hGH gene are involved in the inhibitory effect of activin.