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 extracell ular signal regulated kinase; ERK) stimulation in gonadotropin-releasing ho rmone (GnRH) signaling was investigated in cultured pituitary cells of tila pia hybrids (Oreochromis niloticus x O, aureus). Exposure of the cells to s almon GnRH (sGnRH) resulted ina dose- and time-dependent elevation in ERK l evels. The PKC activator, 1-O-tetradecanoyl phorbol-13-acetate (TPA) increa sed kinase levels, while addition of GnRH had no further effect. However, c hronic exposure to TPA resulted in reduction of basal and GnRH-induced ERK elevation. When PKC was inhibited by GF109203X, the GnRH-elevated ERK level s were totally abolished. The role of MAPK activation on GP alpha, FSH beta and LH beta gene expression was determined by administration of MAPK-kinas e (MEK) inhibitor (PD98059; PD). This inhibitor completely blocked GnRH-ind uced increases in ERK activity. Furthermore, it suppressed GP alpha and LH beta mRNA responses to GnRH, but had no effect on FSH beta transcript level s. PD also decreased basal LH beta 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 GP alpha and LH beta, but not in FSK beta genes transcription. (C) 2001 Elsevier Science Inc. All rights reserved.