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Mechanisms of Hormone Action |
Unité de Physiologie de la Reproduction et des Comportements,2 UMR INRA-CNRS, 37380 Nouzilly, France
Medical Research Council Human Reproductive Sciences Unit,3 Center for Reproductive Biology, Edinburgh EH16 4SB, United Kingdom
Centre de Recherches de Jouy,4 Département de Génétique Animale, Institut National de la Recherche Agronomique, 78352 Jouy-en-Josas, France
In the pituitary, activin stimulates the synthesis and release of FSH. However, the activin receptor signaling pathways that mediate these effects are poorly known. We investigated these mechanisms in primary ovine pituitary cells (POP) and in the murine LßT2 gonadotrope cell line. POP cells and LßT2 cells express the different activin receptors (types IA, IB, IIA, and IIB) and the Smad proteins (Smad-2, -3, -4, and -7). In both POP and LßT2 cells, activin activated several signaling pathways: Smad-2, extracellular regulated kinase-1/2 (ERK1/2), p38, and phosphatidylinositol 3'-kinase (PI3K)/Akt. Phosphorylation of ERK1/2 and p38 were stimulated (3- to 6-fold) rapidly in 5 min, whereas activation of both Smad-2 and Akt (3- to 5-fold) occurred later, in 60 min. Activin also increased the association of activin receptor IIB with PI3K. Using specific inhibitors, we demonstrated that the activation of Smad-2 was partially blocked by the inhibition of PI3K but not by the inhibition of ERK1/2 or p38, suggesting a cross-talk between the Smad and PI3K/Akt pathways. In both POP and LßT2 cells, FSH expression and secretion in response to activin were not altered by the inhibition of PI3K/Akt, ERK1/2, or p38 pathways, whereas they were reduced by about 2-fold by expression of a dominant negative of Smad-2 or the natural inhibitory Smad-7 in LßT2 cells. These results indicate that activin activates several signaling pathways with different time courses in both POP and LßT2 cells, but only the Smad-2 pathway appears to be directly implicated in FSH expression and release in LßT2 cells.
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