GnRH signaling pathways regulate differentially the tilapia gonadotropin subunit genes

Exposure of tilapia pituitary cells in culture to salmon gonadotropin-releasing hormone (sGnRH; 0.01-100 nM) elevated the phosphorylated extracellular signal-regulated kinase (pERK) levels. sGnRH also elevated the α, FSHβ and LHβ subunit mRNA levels. The phorbol ester, 1-O-tetradecanoyl phorbol-13-acetate (TPA; 12.5 nM) increased pERK levels, whereas protein kinase C (PKC) depletion or inhibition by GF109203X (GF; 0.01-10 μM) suppressed GnRH-activated ERKs. GF too abated the GnRH-induced α and LHβ mRNA levels, but had no effect on those of FSHβ. Forskolin (0.001-100 μM) activated ERK, while inhibition of protein kinase A (PKA) by H89 (0.01-10 μM) suppressed pERK levels and all GnRH-stimulated gonadotropin subunit transcripts. Exposure of cells to the mitogen-activated protein kinase kinase (MAPK kinase; MEK) inhibitor (PD98059; PD 10, 25 and 50 μM) completely blocked GnRH-induced increase in ERKs activation. Furthermore, PD suppressed the α and LHβ mRNA responses to GnRH, but had no effect on FSHβ mRNA levels. It is suggested that in tilapia the differential regulation of gonadotropin subunit gene expression by GnRH results from a divergent recruitment of signal transduction pathways, activated upon GnRH binding; PKC-ERK cascade is involved in elevating α and LHβ mRNAs, whereas induction of FSHβ transcript is ERK-independent and is under direct cAMP-PKA regulation or through other MAPK cascades.