GnRH receptor signaling in tilapia pituitary cells: Role of mitogen-activated protein kinase (MAPK)

The role of mitogen-activated protein kinase (MAPK, also known as extracellular signal regulated kinase; ERK) stimulation in gonadotropin-releasing hormone (GnRH) signaling was investigated in cultured pituitary cells of tilapia hybrids (Oreochromis niloticus x O. aureus). Exposure of the cells to salmon GnRH (sGnRH) resulted in a dose- and time-dependent elevation in ERK levels. The PKC activator, 1-O-tetradecanoyl phorbol-13-acetate (TPA) increased kinase levels, while addition of GnRH had no further effect. However, chronic exposure to TPA resulted in reduction of basal and GnRH-induced ERK elevation. When PKC was inhibited by GF109203X, the GnRH-elevated ERK levels were totally abolished. The role of MAPK activation on GPα, FSHβ and LHβ gene expression was determined by administration of MAPK-kinase (MEK) inhibitor (PD98059; PD). This inhibitor completely blocked GnRH-induced increases in ERK activity. Furthermore, it suppressed GPα and LHβ mRNA responses to GnRH, but had no effect on FSHβ transcript levels. PD also decreased basal LHβ mRNA levels. These results indicate that in tilapia pituitary cells, GnRH activates MAPK cascade in a PKC-dependent manner. ERK is involved in GnRH elevation of GPα and LHβ, but not in FSHβ genes transcription.