Tamoxifen Induces Gonadotropin-Releasing Hormone Self-Priming through an Estrogen-Dependent Progesterone Receptor Expression in the Gonadotrope of the Rat

Abstract

Tamoxifen (TX) is an antiestrogen with varying levels of antagonist/agonist activity on the reproductive axis of the rat. It has been reported that TX, in contrast to other selective estrogen receptor modulators (SERMs), increases the content of cytosolic estrogen receptors (ER) in the gonadotrope and induces gonadotropin releasing hormone (GnRH) self-priming in the absence of E. GnRH priming is believed to be a consequence of E-dependent progesterone receptor (PR) activation. The purpose of this study was to determine whether TX induces PR expression in the gonadotrope in an E-dependent manner, and whether the blockade of PR activation affects TX-dependent GnRH self-priming in ovariectomized (OVX) rats. Chronic OVX rats were injected (sc) over 3 days with 25 µg estradiol benzoate (EB), 3 mg TX, 0.5 mg RU58668, a ‘pure’ anti-E (aE), 2 mg RU38486, an anti-P at the receptor (aP), TX+aE and TX+aP. Controls were given 0.2 ml oil. While EB and TX increased mRNA for both PR A+B and PR B expression and the number and intensity of nuclei immunoreactive (IR) for PR in the gonadotrope, the aE and aP given alone had no effect on either PR mRNA levels or nuclear PR-IR. The aE reduced the effect of TX on PR expression (mRNA and nuclear IR) while the aP slightly reduced nuclear PR-IR only. In addition, pituitaries from each of the seven groups were incubated with: 10^–8M E₂, 10^–7M TX, 10^–8M aE, 10^–8M aP, TX+aE, TX+aP or medium alone, respectively. Pituitaries were tested for GnRH self-priming (two pulses of 15 min 1 h apart) and the secretion of LH and PRL determined by specific RIAs. Pituitaries from rats treated with EB and incubated with E₂ had increased basal and GnRH-stimulated luteinizing hormone (LH) and prolactin (PRL) secretion and GnRH self-priming. TX reduced basal and stimulated LH secretion, increased PRL secretion and induced a robust GnRH self-priming. All these effects of TX were blocked by the aE, while the aP blocked GnRH self-priming only. In conclusion, tamoxifen induced PR expression (mRNA and nuclear IR) in the gonadotrope in an E-dependent manner, while activation of these PR through intracellular signaling of GnRH induced GnRH self-priming.

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

“…At 0900 h on the first day after treatment, rats from all three experiments were decapitated, the neural lobe discarded and anterior pituitary glands dissected out, divided in halves, and incubated. Doses employed of EB and TX came from previous publications by this laboratory (Bellido et al 2003, Sánchez-Criado et al 2002, 2004.…”

“…Incubation of pituitaries was carried out as previously described (Bellido et al 2003, Sánchez-Criado et al 2004.…”

“…Incubated pituitaries from two-week OVX rats injected daily over three days with 3 mg TX exhibit increased PRL release into the medium, as do pituitaries from OVX rats injected with the cognate ligand (Bellido et al 2003). In this model, TX has been found to induce progesterone receptor (PR) expression in gonadotropes (Bellido et al 2003, Sánchez-Criado et al 2004. Activation of these PR by GnRH-increased intracellular cAMP levels, in a P-independent manner (Waring & Turgeon 1992, Turgeon & Waring 1994, elicits GnRH selfpriming (Bellido et al 2003).…”

“…Whereas in the presence of the cognate ligand, TX antagonizes E action on luteinizing hormone (LH) (Tebar et al 1994, Sánchez-Criado et al 2002 and PRL (Lieberman et al 1983, Spritzer et al 1996 secretion, in the absence of the cognate ligand, TX induces GnRH selfpriming without affecting basal or GnRH-stimulated LH release in incubated pituitaries (Sánchez-Criado et al 2002), and stimulates PRL secretion (González et al 2000, Bellido et al 2003. Incubated pituitaries from two-week OVX rats injected daily over three days with 3 mg TX exhibit increased PRL release into the medium, as do pituitaries from OVX rats injected with the cognate ligand (Bellido et al 2003). In this model, TX has been found to induce progesterone receptor (PR) expression in gonadotropes (Bellido et al 2003, Sánchez-Criado et al 2004.…”

“…In this model, TX has been found to induce progesterone receptor (PR) expression in gonadotropes (Bellido et al 2003, Sánchez-Criado et al 2004. Activation of these PR by GnRH-increased intracellular cAMP levels, in a P-independent manner (Waring & Turgeon 1992, Turgeon & Waring 1994, elicits GnRH selfpriming (Bellido et al 2003). This agonistic effect of TX is silenced by the addition of 10 28 M estradiol-17b (E 2 ) or the membrane-impermeable conjugate E 2 -BSA to the medium, suggesting the mediation of a surface E receptor (ER) .…”