A novel inside-out prostanoid signaling pathway that mediates GnRH receptor auto-regulation and LH release
Zvi Naor, Tel Aviv University
The asynchronous phased secretion of luteinising hormone (LH) and follicle stimulating
hormone (FSH) from the gonadotrope in response to the single hypothalamic
gonadotropin-releasing hormone (GnRH) during the female reproductive cycle is a
central dogma of reproduction, the mechanism of which remains unresolved.
GnRH regulates its own receptors to enhance gonadotrope responsiveness during the
reproductive cycle in preparation for the LH surge and ovulation, but the mechanism
of the ensuing LH refractoriness during continued FSH secretion is not clarified.
We now demonstrate that GnRH stimulates arachidonic acid (AA) release from the
gonadotrope LbetaT2 cells via the Ca2+-independent cytosolic phospholipase A2 (iPLA2)
and not via the more common cPLA2alpha. AA release was followed by a marked induction
of COX-1 and COX-2 by GnRH, which was mediated by the PKC/c-Src/PI3K/MAPK pathway and
iPLA2 but not via cPLA2alpha, or transactivation of the EGF receptor. COX1/2 act on AA
to produce prostaglandins (PGs) and GnRH stimulates PGE2, PGI2 and PGF2alpha production.
We considered that these PGs may act in an autocrine manner to regulate gonadotrope
function and demonstrate that rat pituitary gonadotropes express the prostanoids
receptors EP1, EP2, FP and IP, while EP3 and EP4 were localized to the prolactin and
growth hormone producing cells, respectively. PGF2alpha and PGI2, but not PGE2, inhibit
GnRH receptor expression through FP and IP receptors. The inhibitory effect of PGF2alpha
and PGI2 seems to be mediated by inhibition of GnRH-stimulated phosphoinositide turnover.
PGF2alpha but not PGE2 or PGI2, reduced GnRH-induction of LHbeta, but like PGE2 and PGI2
had no effect on the induction of common alpha, or FSHbeta. PGF2alpha, or the COX1/2
inhibitor, indomethacin, inhibited and enhanced GnRH-induced LH secretion, respectively
from rat pituitaries, but both had no effect on FSH secretion. PGE2 and PGI2 had no effect
on LH and FSH secretion induced by GnRH. Therefore, reduction of both GnRH receptor and
LHbeta expression are required to observe the differential inhibition of LH release.
Hence, a novel inside-out signaling pathway mediated by PGF2alpha-FP and PGI2-IP,
acting in an autocrine/paracrine loop, limits GnRH regulation of the GnRH receptor,
while PGF2alpha inhibits also GnRH stimulation of LH but not FSH release. This mechanism
may provide a means for the cyclical responsiveness of pituitary gonadotropes and the
asynchronous LH and FSH release during the female reproductive cycle.