Activation of Follistatin Promoter by GnRH in LβT2 Gonadotroph Cells

Abstract

Abstract. Follistatin (FS) is produced and secreted from gonadotroph cells in pituitary gland as well as granulosa cells in the ovary. In the present study, we found that the FS promoter is activated by GnRH in the gonadotroph cell line, LbT2. Therefore, we examined the signal transduction pathways involved in the mechanism. The activation of the FS promoter by GnRH was inhibited by calphostin C, a protein kinase C inhibitor, and U0126, a MAP kinase kinase (MEK) inhibitor. Phosphorylation by protein kinase C of myristoylated alanine-rich C kinase substrate (MARCKS) in LbT2 cells was observed after 3-min treatment with GnRH and declined after 30 min. The subsequent activation of MAP kinase was also transient, and down-regulation of protein kinase C completely inhibited the MAP kinase activation by GnRH, suggesting that the transient activation of protein kinase C led to the transient activation of MAP kinase. Although phorbol 12-myristate 13-acetate treatment increased phosphorylation of MARCKS and activated MAP kinase, it did not activate the FS promoter. Genistein, a tyrosine kinase inhibitor, completely inhibited the GnRH-induced activation of the FS promoter, while no inhibition of the MAP kinase pathway was observed. These results suggest that the activations of both the protein kinase C and tyrosine kinase pathways are necessary for the activation of the FS promoter in gonadotroph cells. FOLLISTATIN (FS) was first identified from ovarian follicular fluid on the basis of its ability to suppress the secretion of follicle-stimulating hormone (FSH) from pituitary gonadotrophs [1][2][3][4]. FS is a monomeric glycoprotein structurally unrelated to inhibin [3]. FS binds to activin with high affinity [5] and inhibits the activity of activin to stimulate FSH secretion [6][7][8]. In ovary, the granulosa cells are responsible for producing and secreting FS in most species [9][10][11][12]. FS expression in granulosa cells is increased through both cyclic AMP and protein kinase C pathways [13][14][15][16][17]. In cultured rat granulosa cells, FSH and gonadotropin-releasing hormone (GnRH) increased FS protein production through the cyclic AMP and protein kinase C pathways, respectively. The expression of FS is not limited to the ovary, and FS is secreted within the pituitary gland, where it exerts autocrine/paracrine effect [18]. It has been reported that FS expression in gonadotrophs is regulated by GnRH at the steady state mRNA levels [18,19]. GnRH is a hypothalamic peptide and serves as a critical regulator for the synthesis and secretion of FSH [20,21]. FSH is composed of two noncovalently linked subunits, a and b. It is well known that the synthesis of the b-subunit (FSHb) is the rate-limiting step in the production of FSH [20]. Slow-frequency GnRH pulses increase FSHb mRNA through activation of the

Soy Phytoestrogens Do Not Fully Reverse Changes in Rat Pituitary Castration Cells: Unbiased Stereological Study

“…Inhibins act to reduce FSH secretion; activins stimulate FSH production by gonadotropic cells, while follistatins suppress FSH via binding to and inactivation of activins (Winters and Moore, ). By inhibiting tyrosine‐kinase, genistein might have interrupted the pathway of follistatin promoter activation (Kihara et al, ), allowing stimulation of FSH production by activin.…”

The Local Control of the Pituitary by Activin Signaling and Modulation

“…The promoters of the rat or human FST genes have been evaluated in a variety of cell lines and shown to be targets of inputs such as the cAMP/TPA pathway in P19 and F9 embryonic carcinoma cells [184, 185], the Wnt/β-catenin pathway in NCCIT human embryonic carcinoma and F9 teratocarcinoma cells [186, 187] and by PPARα and PPARγ nuclear receptors in LβT2 cells [188]. GnRH activates the Fst promoter in LβT2 cells [189–191]. In non-pituitary cells such as HepG2 or HEK293T cells, the Fst promoter is induced by activin or TGF-β via Smad2/3 [192, 193].…”