Follistatin gene expression by gonadotropin-releasing hormone: A role for cyclic AMP and mitogen-activated protein kinase signaling pathways in clonal gonadotroph LβT2 cells

Abstract

The purpose of the present study was to examine the signal transduction pathways involved in follistatin gene expression induced by GnRH in the LβT2 cell line. The LHβ-subunit was predominantly increased by high frequency GnRH pulses (30min interval); whereas low frequency pulses (120min) increased FSHβ. In a static culture, follistatin expression was significantly increased at 12h (2.35±0.80-fold) after the addition of GnRH. Following pulsatile stimulation, follistatin mRNA was increased by high frequency GnRH pulses, but not by low frequency pulses. In a static culture, GnRH maximally activated extracellular signal-regulated kinase (ERK) 10min (3.2±0.55-fold) after treatment. In addition, intracellular cAMP accumulated up to 2.1±0.76-fold. Follistatin promoter activity was significantly increased following transfection with either a constitutively active cAMP dependent protein kinase (PKA) or a constitutively active MEK kinase (MEKK). The induction of follistatin gene expression by GnRH was completely inhibited by H89, a protein kinase A inhibitor, and U0126, a MEK inhibitor. Follistatin gene expression was also activated by both PACAP and CPT-cAMP under static culture conditions. Maximal ERK activation levels were nearly identical regardless of GnRH pulse frequency; however, high frequency GnRH pulses elevated both the intracellular cAMP level as well as cAMP-response element (Cre) promoter activity. These results suggest that both the PKA and ERK pathways are necessary for the induction of the follistatin promoter. Furthermore, the intracellular cAMP level, but not ERK activity, determined whether follistatin was induced following high frequency GnRH pulses.