Tamoxifen induces gonadotropin-releasing hormone self-priming through an estrogen-dependent progesterone receptor expression in the gonadotrope of the rat

Estrogen Receptors in Nonfunctioning Pituitary Neuroendocrine Tumors: Review on Expression and Gonadotroph Functions

Estrogen (17β-estradiol or E2) is a crucial regulator of the synthesis and secretion of pituitary reproductive hormones luteinizing hormone, follicle-stimulating hormone, and prolactin. In this review, we summarize the role of estrogen receptors in nonfunctioning pituitary neuroendocrine tumors (NF-Pitnets), focusing on immunoexpression and gonadotroph cell proliferation and apoptosis. Gonadotroph tumors are the most common subtype of NF-Pitnets. Two major estrogen receptor (ER) isoforms expressed in the pituitary are estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). Overall, estrogen actions are mostly exerted through the ERα isoform on the pituitary. The G protein-coupled estrogen receptor (GPER) located at the plasma membrane may contribute to nongenomic effects of estrogen. Nuclear immunoreactivity for ERα and ERβ was highest among gonadotroph and null cell tumors. Silent corticotroph tumors are the least immunoreactive for both receptors. A significantly elevated ERα expression was observed in macroadenomas compared with microadenomas. ERα and ERβ may act in opposite directions to regulate the Slug-E-cadherin pathway and to affect invasiveness of NF-Pitnets. In the cellular pathway, ERs regulate estrogen-induced proliferation and differentiation and impact several signaling pathways including the MAPK and PI3K/Akt pathway. Estrogen was the first-discovered inducer of pituitary tumor transforming gene 1 that was abundantly expressed in NF-Pitnets. ERα can be a potential biomarker for predicting tumor size and invasiveness as well as therapeutic target for NF-Pitnets. Selective estrogen receptor modulators or antiestrogen may represent as an alternative choice for the treatment of NF-Pitnets.

Hypothalamic Effects of Tamoxifen on Oestrogen Regulation of Luteinising Hormone and Prolactin Secretion in Female Rats

Oestradiol (E2) acts in the hypothalamus to regulate luteinising hormone (LH) and prolactin (PRL) secretion. Tamoxifen (TX) has been extensively used as a selective oestrogen receptor modulator, although its neuroendocrine effects remain poorly understood. In the present study, we investigated the hypothalamic effects of TX in rats under low or high circulating E2 levels. Ovariectomised (OVX) rats treated with oil, E2 or TX, or E2 plus TX, were evaluated for hormonal secretion and immunohistochemical analyses in hypothalamic areas. Both E2 and TX reduced LH levels, whereas TX blocked the E2 -induced surges of LH and PRL. TX prevented the E2 -induced expression of progesterone receptor (PR) in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC), although it did not alter PR expression in OVX rats. TX blocked the E2 induction of c-Fos in AVPV neurones, consistent with the suppression of LH surge. However, TX failed to prevent E2 inhibition of kisspeptin expression in the ARC. In association with the blockade of PRL surge, TX increased the phosphorylation of tyrosine hydroxylase (TH) in the median eminence of OVX, E2 -treated rats. TX also precluded the E2 -induced increase in TH expression in the ARC. In all immunohistochemical analyses, TX treatment in OVX rats caused no measurable effect on the hypothalamus. Thus, TX is able to prevent the positive- but not negative-feedback effect of E2 on the hypothalamus. TX also blocks the effects of E2 on tuberoinfundibular dopaminergic neurones and PRL secretion. These findings further characterise the anti-oestrogenic actions of TX in the hypothalamus and provide new information on the oestrogenic regulation of LH and PRL.

Estrogen, but not progesterone, induces the activity of nitric oxide synthase within the medial preoptic area in female rats

The control of gonadotropin-releasing hormone (GnRH) secretion depends on the action of ovarian steroids and several substances, including nitric oxide (NO). NO in the medial preoptic area (MPOA) stimulates the proestrus surge of luteinizing hormone (LH). We studied the effect of estrogen (Tamoxifen-TMX) and progesterone (RU-486) antagonists on mRNA and protein expression of NO synthase (NOS), the enzyme that produces NO, as well as its activity within MPOA. Female rats received s.c. injections of TMX (3mg/animal) on first and second days of the estrous cycle (9 am), RU-486 (2mg/animal) on first, second, (8 am and 5 pm) and third days of the estrous cycle (8 am) or oil (controls) and were killed on the third day (5 pm). Real time-PCR and western blotting were performed to study NOS mRNA and protein expressions. The NOS activity was indirectly assessed by measuring the conversion from [(14)C]-L-arginine into [(14)C]-L-citrulline. TMX significantly decreased neuronal NOS (nNOS) mRNA expression (90%), and the activity of NOS, but did not alter nNOS protein expression. Also, TMX significantly decreased LH, FSH, estrogen and progesterone plasma levels. RU-486 nor affected NOS mRNA and protein expressions neither the NOS activity in the MPOA, but reduced FSH levels. The nitrergic system in the MPOA can be stimulated by estrogen whereas TMX decreased NOS activity and mRNA expression. In conclusion, the involvement of the nitrergic system in the MPOA to induce the surge of LH on proestrus depends on the estrogen action to stimulate the mRNA-nNOS expression and the activity of nNOS but it does not seem to depend on progesterone action.

Testosterone reduces cumulative burying in female Wistar rats with minimal participation of estradiol

Testosterone exerts anxiolytic effects, but the participation of its aromatase metabolic product estradiol is controversial. Therefore, we used the defensive burying paradigm in female Wistar rats to explore testosterone's (1.0 mg/rat, s.c.) interactions with picrotoxin (a noncompetitive gamma-aminobutyric acid-A receptor [GABA(A)] antagonist; 1.0 mg/kg, i.p.), formestane (an aromatase inhibitor; 3.0 mg/rat, s.c.), and tamoxifen (an estrogen receptor-beta antagonist; 1.0 mg/kg, s.c.). Serum levels of testosterone, estradiol, and progesterone were determined in the same rats. Burying latency and locomotion did not significantly change. Systemic testosterone administration enhanced serum testosterone and estradiol levels and reduced defensive burying. This reduction in total burying was blocked by pretreatment with picrotoxin and tamoxifen, but not formestane. We conclude that testosterone produced anxiolytic-like effects in female rats that were mediated by actions at the GABA(A) receptor, with participation of the estradiol receptor-beta, rather than estradiol aromatization.

Follicle-stimulating hormone responses to kisspeptin in the female rat at the preovulatory period: modulation by estrogen and progesterone receptors

Ovulation is triggered by the preovulatory surge of gonadotropins that, in rodents, is defined by the concomitant rise in circulating LH and FSH at the afternoon of proestrus (primary surge), followed by persistently elevated FSH levels at early estrus (secondary surge). In recent years, kisspeptins, products of the KiSS-1 gene that act via G protein-coupled receptor 54, have emerged as an essential hypothalamic conduit for the generation of the preovulatory LH surge by conveying positive feedback effects of estradiol onto GnRH neurons, an event that involves not only estradiol-induced transcription of the KiSS-1 gene at the anteroventral periventricular nucleus but also its ability to modulate GnRH/LH responses to kisspeptin. However, little is known about the potential modulation of FSH responsiveness to kisspeptin by sex steroids in the cyclic female. We report herein analyses on the consequences of selective blockade of estrogen receptors (ER)-alpha and -beta, as well as progesterone receptor (PR), on the ovulatory surges of FSH and their modulation by kisspeptin. Antagonism of ERalpha or PR equally blunted the primary and secondary surges of FSH and nullified FSH responses to kisspeptin at the preovulatory period. Conversely, selective blockade of ERbeta failed to induce major changes in terms of endogenous FSH surges, yet it decreased FSH responses to exogenous kisspeptin. In contrast, FSH responses to GnRH were fully conserved after ERbeta blockade and partially preserved after inhibition of ERalpha and PR signaling. Finally, secondary FSH secretion was rescued by kisspeptin in females with selective blockade of ERalpha but not PR. In sum, our results substantiate a concurrent, indispensable role of ERalpha and PR in the generation of FSH surges and the stimulation of FSH responses to kisspeptin at the ovulatory period. In addition, our data suggest that ERbeta might operate as a subtle, positive modulator of the preovulatory FSH responses to kisspeptin, a role that is opposite to its putative inhibitory action on kisspeptin-induced LH secretion and might contribute to the dissociation of gonadotropin secretion at the ovulatory phase in the cyclic female rat.

Estradiol and its membrane-impermeable conjugate estradiol-BSA inhibit tamoxifen-stimulated prolactin secretion in incubated rat pituitaries

In the absence of estrogen (E), the selective E receptor modulator tamoxifen (TX) has two agonist effects in the rat pituitary: induction of progesterone receptor (PR)-dependent GnRH self-priming in the gonadotrope, and stimulation of prolactin (PRL) secretion in the lactotrope. TX-induced gonadotropin (GnRH) self-priming is absent when 10(-8) M estradiol-17beta (E2) is added to the incubation medium of pituitaries from TX-treated rats. The present experiments investigated whether PR-independent PRL release into the incubation medium of pituitaries from TX-treated ovariectomized (OVX) rats was affected by E2, and the effect of different ER ligands (ICI182780, TX, estradiol-17alpha, E2 -BSA) on TX-stimulated PRL secretion. Moreover, the effect of E2 on TRH-stimulated PRL secretion in pituitaries collected from estradiol benzoate- and TX-treated OVX rats was studied. It was found that: i) incubation with E2 supressed the PRL releasing effect of injected TX; ii) whereas coincubation with the pure anti-E type II ICI182780 antagonized the inhibitory effect of E2, coincubation with the anti-E type I TX did not; iii) estradiol-17alpha lacked inhibitory action, whereas a dose-dependent inhibitory effect of both E2 and E2 -BSA was noticed; and iv) TRH stimulatory effect on PRL release in pituitaries from TX-treated rats was blocked by addition of E2 to the medium. Taken together, these data argue in favor of the presence of specific membrane recognition sites for E in the lactotrope involved in steroid-specific E2 inhibition of TX-stimulated PRL secretion.