Regulation of Gonadotropin mRNA Levels in Cultured Rat Pituitary Cells by Gonadotropin-Releasing Hormone (GnRH): Role for Ca²⁺ and Protein Kinase C

Incubation of cultured rat pituitary cells with gonadotropin-releasing hormone (GnRH, 1 nM) resulted in a rapid elevation of gonadotropin subunit steady-state mRNA levels(α, 2.2-fold, LH β 2.1-fold, and FSH β 2.2-fold increases at 30 min). Addition of actinomycin D abolished the stimulatory effect of GnRH upon α and LH β and reduced the effect upon FSH β mRNA levels. The effect of GnRH is biphasic, where the early phase is being observed at 30–60 min, while the late phase is noticed between 12–24 h. A significant decrease in FSH β mRNA levels was found after 6 h of incubation when using a stable GnRH analog. The unique profile of the time response enabled us to attempt to dissect the signal transduction cascade involved in the neurohormone action. Addition of the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), or the Ca²⁺ ionophore, ionomycin, mimicked the profile of GnRH-induced α and LH β mRNA elevation. The two phases of FSH β mRNA elevation induced by GnRH could be mimicked by TPA, while the decrease at 6 h was mimicked by ionomycin. The rapid stimulatory effect of GnRH on gonadotropin subunit mRNA levels was abolished by the PKC inhibitors, staurosporine and GF109203X. Similarly, the rapid stimulatory effect of GnRH on a and LH β, but not FSH β, was abolished in Ca²⁺-free medium. While additivity in LH release is obtained upon the combined addition of TPA and ionomycin for 30 min of incubation, LH β and FSH β gene expression is inhibited. Since GnRH is known to elevate Ca²⁺ and activate PKC, we conclude that whereas simultaneous signaling elicits exocytosis, divergence of signaling by Ca²⁺ and PKC is most likely responsible for gonadotropin gene expression.