A Nongenomic Action of Estradiol as the Mechanism Underlying the Acute Suppression of Secretion of Luteinizing Hormone in Ovariectomized Ewes1

Abstract

The objective of the present study was to determine how rapidly estradiol (E2) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E2 (E2 conjugated to BSA [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethyloxime:BSA [E2-BSA] and a novel conjugate, E2 conjugated to a small peptide [E2-PEP]) to mimic the actions of E2 on secretion of LH and FSH. Animals (n = 5-6 per group) were given infusions for 4 h of 50 microg of E2 or equimolar concentrations of E2-BSA or E2-PEP. Treatments with E2, E2-BSA, and E2-PEP each induced an acute suppression of LH secretion (<20 min, P < 0.01). In contrast, E2, but not E2-BSA or E2-PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E2 in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E2 mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E2 may be mediated via the plasma membrane.

New insights into the classical and non-classical actions of estrogen: Evidence from estrogen receptor knock-out and knock-in mice

“…Evidence from ovariectomized ewes (Nett et al, 1984), monkeys (Pau et al, 1990), and guinea pigs (Condon et al, 1988) demonstrates that E 2 can rapidly decrease LH secretion, indicating non-genotropic mechanisms of E 2 action. Furthermore, the fact that conjugated forms of E 2 can rapidly prevent GnRH-induced LH secretion in ovine pituitary cells suggests that E 2 's suppressive effects on LH release are mediated by a plasma membrane-associated E 2 -binding protein (Arreguin-Arevalo and Nett, 2006). It is therefore possible that EREindependent estrogen negative feedback is mediated by genotropic mechanisms, nongenotropic mechanisms, or both.…”

“…Interestingly, GnRH surges can occur even after estrogen withdrawal, suggesting that the neuronal network supporting the surge process is sufficiently activated by previous E 2 exposure and does not require short-term E 2 action (Evans et al, 1997). Furthermore, acute treatments of E 2 are not sufficient to induce LH surges in the ewe (Arreguin-Arevalo and Nett, 2006). Our findings extend this idea of genotropic-dependent LH surges to include a specific requirement for ERα-DNA binding, and therefore suggest that downstream genes involved in estrogen regulation of GnRH/LH surges likely contain EREs.…”

Does a Nonclassical Signaling Mechanism Underlie an Increase of Estradiol-Mediated Gonadotropin-Releasing Hormone Receptor Binding in Ovine Pituitary Cells?1

“…Herein we have explored the possibility that this effect of E2 may reflect a mechanism mediated via an ER on the plasma membrane through the nonclassical signaling pathway. Our interest in this possibility was stimulated by previous work examining the impact of membrane-impermeable E2 conjugates on LH secretion in sheep [16]. In that study, E2, E2-PEP, and E2-BSA led to a rapid suppression of LH secretion that persisted for approximately 6 h. The rapidity of this response (within 30 min) is not consistent with an exclusively genomic mechanism.…”

“…Based on previous extractions, approximately 20 pg/ml of free E2 remained within the E2-BSA conjugate after seven extractions (Arreguin-Arevalo and Nett, unpublished). As described previously [17], 6-keto-17b-estradiol-6-carboxymethyl oxime (E2-6-CMO; Steraloids Inc.) was conjugated to a 15-amino acid sequence (Nterminus-SGGEVVVDQPMERLY-C-terminus) on the amino group of the serine reside. The conjugation reaction was added to a Sephadex LH20 column to separate E2-6-CMO from the conjugated form (E2-PEP).…”

“…This is troubling because E2 is considered the primary regulator of gene expression and subsequent number of GnRHR on gonadotrophs. Based on the ability of membrane-impermeable E2 conjugates to elicit an acute decrease in LH secretion independent of a hypothalamic site of action [16,17], we speculated that E2 activation of the GNRHR gene may be initiated at a plasma membrane site of action and increase the number of GnRHR, as determined by GnRH binding, in a nonclassical signaling manner. Herein, we show that E2 conjugated to BSA (E2-BSA) mimics the effects of free E2 on enhancing GnRHR binding in ovine pituitary cells.…”