Activin Signaling Pathways in Ovine Pituitary and L(beta)T2 Gonadotrope Cells

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 L(beta)T2 gonadotrope cell line. POP cells and L(beta)T2 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 L(beta)T2 cells, activin activated several signaling pathways: Smad-2, extracellular regulated kinase-1/2 (ERK1/2), p38, and phosphatidylinositol 3ʹ-kinase (PI3K)/Akt. Phosphorylation of ERK1/2 and p38 were stimulated (3- to 6fold) 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 L(beta)T2 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 L(beta)T2 cells. These results indicate that activin activates several signaling pathways with different time courses in both POP and L(beta)T2 cells, but only the Smad-2 pathway appears to be directly implicated in FSH expression and release in L(beta)T2 cells.