Elucidation of Mechanisms of the Reciprocal Cross Talk between Gonadotropin-Releasing Hormone and Prostaglandin Receptors

Abstract

We recently described a novel GnRH receptor signaling pathway mediated by the prostaglandins (PGs) F(2alpha) and PGI(2), which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and inhibit LH but not FSH release. Here we further explore the cross talk between GnRH and the PG receptors. GnRH stimulates arachidonic acid (AA) release from LbetaT2 gonadotrope cells via the Ca(2+)-independent phospholipase A(2) (iPLA(2)) and not via the more common Ca(2+)-dependent cytosolic phospholipase A(2)alpha (cPLA(2)alpha). AA release was followed by a marked induction of cyclooxygenase (COX)-1 and COX-2 by GnRH via the protein kinase C/c-Src/phosphatidylinositol 3-kinase/MAPK pathway. COX-2 transcription by GnRH is mediated by the two nuclear factor-kappaB sites and the CCAAT/enhancer-binding protein site within its promoter. Indeed, GnRH stimulates p65/RelA phosphorylation (22-fold) in LbetaT2 cells and the two nuclear factor-kappaB sites apparently act as a composite response element. Although GnRH stimulates cAMP formation in LbetaT2 cells, we found no role for cAMP acting via the cAMP response element site in the COX-2 promoter. PGF(2alpha), PGI(2), or PGE(2) had no effect on GnRH-stimulated ERK, c-Jun N-terminal kinase, and p38MAPK activation or on GnRH- and high K(+)-stimulated intracellular Ca(2+) elevation in LbetaT2 and gonadotropes in primary culture. Although, PGF(2alpha), PGI(2), and PGE(2) reduced GnRH-stimulated cAMP formation, we could not correlate it to the inhibition of GnRH receptor expression, which is exerted only by PGF(2alpha) and PGI(2.) Hence, the inhibition by PGF(2alpha) and PGI(2) of the autoregulation of GnRH receptor expression is most likely mediated via inhibition of GnRH-stimulated phosphoinositide turnover and not by inhibition of Ca(2+) elevation and MAPK activation.

Characterization of Gonadotrope Secretoproteome Identifies Neurosecretory Protein VGF-derived Peptide Suppression of Follicle-stimulating Hormone Gene Expression

“…Another study revealed that inducible cAMP early repressor was preferentially induced by high frequency GnRH stimulation and that it abrogated the GnRH pulse frequency-dependent effects on Fshb gene transcription (58). Secreted prostaglandins have been reported to affect GnRH regulation of Lhb promoter constructs (76,77). Of interest, VGF may regulate prostaglandin secretion as VGF-derived peptide TLQP-21 also modulates pancreatic exocrine secretion and gastric contractility via stimulation of prostaglandin release (78,79).…”

Ion Channels of Pituitary Gonadotrophs and Their Roles in Signaling and Secretion

“…Figure 2A illustrated GnRH-induced Ca 2+ oscillations. In contrast, αT3-1 (Figure 2B) and LβT2 gonadotrophs (not shown) release Ca 2+ in a non-oscillatory manner when stimulated with GnRH (81, 82). GnRH-induced calcium signaling is also non-oscillatory in fish pituitary cells (83) as well as in rat Leydig cells (Figure 2B) (84).…”

Outside the box signaling: Secreted factors modulate GnRH receptor-mediated gonadotropin regulation

“…In turn, the released PGs inhibited GnRHR gene expression and selectively reduced GnRH-induced LHβ gene expression in an autocrine/paracrine fashion (Fig. 3B; (Naor et al, 2007,Naidich et al, 2010). The transcriptional regulation of GnRHR and gonadotropin subunit genes was analyzed by transfecting immortalized gonadotropes with the corresponding promoter constructs.…”