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(alpha, 2.2-fold, LH beta, 2.1-fold, and FSH beta 2.2-fold increases at 30 min). Addition of actinomycin D abolished the stimulatory effect of GnRH upon alpha and LH beta and reduced the effect upon FSH beta 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 beta 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 alpha and LH beta mRNA elevation. The two phases of FSH beta 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 GF 109203X. Similarly, the rapid stimulatory effect of GnRH on alpha and LH beta, but not FSH beta, was abolished in Ca(2+)-free medium. While additivity in LH release is obtained upon the combined addition of TPA and ionomycin for 30 min of incubation, LH beta and FSH beta gene expression is inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)