Regulation of Cyclic Adenosine 3′,5′-Monophosphate Signaling and Pulsatile Neurosecretion by G_i-coupled Plasma Membrane Estrogen Receptors in Immortalized Gonadotropin-Releasing Hormone Neurons

Abstract

Immortalized GnRH neurons (GT1-7) express receptors for estrogen [estrogen receptor-alpha and -beta(ERalpha and ERbeta)] and progesterone (progesterone receptor A) and exhibit positive immunostaining for both intracellular and plasma membrane ERs. Exposure of GT1-7 cells to picomolar estradiol concentrations for 5-60 min caused rapid, sustained, and dose-dependent inhibition of cAMP production. In contrast, treatment with nanomolar estradiol concentrations for 60 min increased cAMP production. The inhibitory and stimulatory actions of estradiol on cAMP formation were abolished by the ER antagonist, ICI 182,780. The estradiol-induced inhibition of cAMP production was prevented by treatment with pertussis toxin, consistent with coupling of the plasma membrane ER to an inhibitory G protein. Coimmunoprecipitation studies demonstrated an estradiol-regulated stimulatory interaction between ERalpha and Galphai3 that was prevented by the ER antagonist, ICI 182,780. Exposure of perifused GT1-7 cells and hypothalamic neurons to picomolar estradiol levels increased the GnRH peak interval, shortened peak duration, and increased peak amplitude. These findings indicate that occupancy of the plasma membrane-associated ERs expressed in GT1-7 neurons by physiological estradiol levels causes activation of a Gi protein and modulates cAMP signaling and neuropeptide secretion.

GPR30: A G protein-coupled receptor for estrogen

“…In many reports, the estrogen-responsive receptor is proposed to be ER itself (either α or β), or a modified form of the protein (Acconcia et al, 2004;Li et al, 2003). Complexes between the classical ERs and G proteins (Navarro et al, 2003) as well as with PI3 kinase (Simoncini et al, 2003) have been described. Recently, ER associations with plasma membrane Gi proteins have been reported to mediate NO production (Wyckoff et al, 2001) and cAMP inhibition (Navarro et al, 2003).…”

“…Complexes between the classical ERs and G proteins (Navarro et al, 2003) as well as with PI3 kinase (Simoncini et al, 2003) have been described. Recently, ER associations with plasma membrane Gi proteins have been reported to mediate NO production (Wyckoff et al, 2001) and cAMP inhibition (Navarro et al, 2003). From these examples, it is clear that estrogen can mediate a multitude of complex rapid cellular activation events.…”

Membrane-initiated estrogen signaling in hypothalamic neurons

“…These findings, therefore, suggest a direct hyperpolarizing action of estrogen on GnRH neurons via a Gα i,o -coupled receptor. Indeed, in GT1-7 cells, an immortalized GnRH neuronal cell line, estrogen inhibits adenylyl cyclase activity (cAMP production) via a pertussis toxin (Gα i,o coupling) mechanism (Navarro et al, 2003). The electrophysiological effects are much too rapid to involve transcription through classical ERs, but an estrogen -Gα i,o -coupled receptor has not been identified.…”

Rapid signaling mechanisms of estrogens in the developing cerebellum

“…In some neuronal cells and some breast cancer cells, heterotrimeric G-protein coupled mechanisms appear involved in rapid estradiol-induced ERK-activation Filardo et al, 2000;Filardo, 2002;Navarro et al, 2003;Razandi et al, 2003;Thomas et al, 2005). In MCF-7 breast cancer cells, conflicting results indicate that either ERα alone, or an orphan GPCR (GPR30), but not ERα or ERβ, is required for estradiol to rapidly activate ERK1/2 via the same G-protein dependent mechanism of epidermal growth factor receptor (EGFR) transactivation ( Fig.…”

“…In immortalized gonadotropin-releasing hormone (GnRH) cells, estradiol rapidly modulates cAMP levels via ERα-dependent activation of G αi3 . (Navarro et al, 2003). In midbrain dopaminergic neurons, the IP3 kinase/AKT signaling pathway is rapidly estradiol-responsive ).…”