Pituitary Adenylate Cyclase Activating Polypeptide and Neuropeptide Y Regulation of Gonadotropin Subunit Gene Expression in Tilapia: Role of PKC, PKA and ERK

Abstract

There is ample information on the hypophysiotropic function of pituitary adenylate cyclase-activating polypeptide (PACAP) and neuropeptide Y (NPY) in fish as in mammals, although evidence as to their direct effects on gonadotropic cells is scarce. We have previously reported that NPY and PACAP38 augment gonadotropin-releasing hormone (GnRH)-induced expression of glycoprotein α (α) subunit gene in the teleost fish, tilapia. The aim of the present study was to elucidate possible direct effects of these peptides on gonadotropin subunit gene expression in culture of tilapia pituitary cells, as well as the transduction pathways involved. Both NPY and PACAP38 (0.001–10 nM) increased the level of phosphorylated extracellular signal-regulated kinase (pERK) dose-dependently, reaching a peak at 0.1 and 0.01 nM, respectively. Inhibition of protein kinase C (PKC) by GF109203X (GF; 0.01–10 nM) suppressed NPY-stimulated pERK levels and its effect on α and luteinizing hormone (LH) β subunit mRNA levels. However, NPY had no effect on follicle stimulating hormone (FSH) β mRNA levels. NPY-elevated α, LHβ mRNA and pERK levels were also attenuated by inhibition of protein kinase A (PKA) with H89 (0.01–10 nM). Exposure of the cells to the MAPK kinase (MEK) inhibitor (PD98059; PD 10, 25 and 50 µM) completely blocked NPY-induced ERK activity. In addition, this inhibitor abated the α and LHβ mRNA responses to NPY. Similar experiments conducted to elucidate PACAP38 signaling revealed that PACAP38 (0.01 nM) elevated all three-gonadotropin subunit gene expression via both PKC-ERK and PKA-ERK cascades. It is suggested that both NPY and PACAP38 act directly on gonadotropes to elevate gonadotropin subunit gene expression. Whereas the expression of α and LHβ subunit genes is regulated by both NPY and PACAP, the effect on the FSHβ transcript is elicited only by PACAP38. NPY and PACAP38 stimulatory actions are mediated via protein kinase C (PKC) and protein kinase A (PKA), converging at the MEK-ERK cascade. These findings represent one of the fine tuning levels that differentially regulates gonadotropin subunit gene expression.