Selective Estrogen Receptor Modulators. Antonio Cano

Chapter 5. The Hypothalamus-Pituitary-Ovarian Axis as a Model System for the Study of SERM Effects: An Overview of Experimental and Clinical Studies

• R. Alonso

• F. Marin

• M. Gonzalez

• P. Guelmes

• C. Bellido

• G. Hernandez

• R. Marin

• M. Di'az

• J.E. Sanchez-Criado

5.1

Introduction

Female reproductive function depends on the coordinated activity of different brain areas and peripheral organs, finally leading to pregnancy and delivery. These include the hypothalamus, the anterior pituitary, the ovaries, and the uterus. During reproductive age, oocytes mature and are released from the ovaries in a cyclic manner in response to a neural signal. Central in the control of female reproduction is the feedback action of the ovarian hormones estradiol and progesterone, which act at different levels of the reproductive system through specific receptors and modulate the activity of a wide variety of cell types. Therefore, although the primary control of female reproductive cycles arises from the brain, it is actually the ovary that controls its own function by cyclically secreting estradiol and progesterone, which in turn feed back at different levels of the system (Freeman 1994; Hotchkiss and Knobill 1994).

In all sex-steroid-responding tissues, the magnitude and type of response are determined by the relative population of specific steroid hormone receptors and their space-temporal expression pattern (Conneely 2001). In the classical model of steroid hormone action, cell response to a particular hormone depends upon different receptor subtypes and the presence of a constellation of proteins that act in complexes as coactivators, corepressors, and coregulators and whose interactions after hormone-receptor binding induce either an increment or an inhibition of gene transcription (Tsai and O’Malley 1994; McKenna et al. 1999; Glass and Rosenfeld 2000; Smith and O’Malley 2004). In the case of estrogen receptors (ERs), two main subtypes, ERa and ERβ, encoded by two different genes have been identified (Kuiper et al. 1996; Couse and Korach 1999).

During the last decade, the development of animal models with selective ablation of specific genes has allowed the identification of physiological responses associated with each receptor subtype, as well as their complementarity in the regulation of reproductive function (Lubahn et al. 1993; Krege et al. 1998; Dupont et al. 2000). In addition, a large number of recent reports have shown that in a variety of tissue and cell types sex steroids are also able to interact with specific binding sites at the plasma membrane to activate different, highly coordinated signaling pathways (Nadal et al. 2001; Morales et al. 2003; Guerra et al. 2004; Marin et al. 2005). To add more complexity, ERs can be activated either by the cognate ligand or, in some tissues, in a ligand-independent manner (Demay et al. 2001; Blaustein 2004). These findings explain the wide spectrum of estrogen physiological actions, as well as the diverse repertoire of responses to either endogenously or exogenously administered steroid molecules in a given organism (Conneely 2001; McDonnell 2003). Furthermore, the variety of estrogen actions and the potential beneficial effects of this sex hormone on a number of tissues, in addition to the reproductive system, has opened a fascinating field of pharmacological development (McDonnell 2003). As described in other parts of this book, selective estrogen receptor modulators (SERMs) are compounds that can interact with ERs and act either as estrogen agonists or antagonists, depending on the tissue and the cellular environment (McDonnell 1999; McDonnell et al. 2002; Smith and O’Malley 2004). Due to the development of SERMs, pharmacology is now growing up in the context of modern hormone therapy (Turgeon et al. 2004), and there is a need to analyze the potential effects of these compounds on different levels of the female reproductive system. In this chapter we attempt to discuss results from our and other laboratories regarding the effects of SERMs on the hypothalamus-pituitary system of female rats or tissue explants. In addition, we have reviewed most relevant clinical findings in women treated with different SERMs. Because of the complexity of this subject and to avoid undesirable confusion, we have focused on the functional unit formed by gonadotropin releasing hormone (GnRH) secreted by hypothalamic neurons and gonadotropins secreted by the anterior pituitary.

5.2

Hypothalamus-Pituitary-Ovarian Axis of Mammals

5.2.1

General Aspects

During follicular development and while circulating levels of ovarian hormones are reduced, basal secretion of gonadotropins, follicle stimulating hormone (FSH), and luteinizing hormone (LH) determine both maturation of granulose cells and production of steroid hormones. A gradual rise in ovarian hormone levels exerts several coordinated actions at different levels of the reproductive axis (Fink 1988). On one hand, estrogen exerts a negative feedback effect on gonadotropin secretion by acting both at the hypothalamus and the anterior pituitary, which partially prevents the development of additional follicles during a cycle. At the same time, estrogen stimulates its own secretion by granulose cells, which allows continuous follicular steroid hormone secretion even at a low gonadotropin secretory rate. When circulating estrogen reaches a critical concentration, it switches to a positive feedback mode of action, at both the hypothalamic and pituitary levels (Fink 1995, 2000). This is potentiated by rising levels of progesterone released from granulose cells of dominant follicles. These synergistic influences induce a dramatic increase in the synthesis and secretion of GnRH from a subset of hypothalamic neurons (Kimura and Fumabashi 1998) as well as an enhancement of pituitary responsiveness to this peptide (de Koning et al. 2001).

GnRH has the capacity to enhance pituitary responsiveness to itself, a unique phenomenon known as GnRH self-priming (Fink 1995, 2000; de Koning et al. 2001). In addition, the stimulatory action of GnRH is facilitated by a decreased bioactivity of the putative ovarian protein gonadotropin surge- attenuating/inhibiting factor (GnSAF/IF, attenuin) (Fowler and Templeton 1996), a 60-66 Kda protein that is presently being isolated and characterized (Fowler et al. 2002,2003; Fowler and Spears 2004). The secretion of GnSAF/IF is dependent on FSH action on granulose cells, and it has been suggested that its inhibitory action on pituitary sensitivity to GnRH might be exerted through interactions with estrogen-dependent PR activation (Byrne et al. 1996; Tebar et al. 1998). The overall consequence of all these convergent inputs is a surge of LH - the preovulatory peak - and, to a lesser extent, of FSH.

5.2.2

Functional Organization

In the majority of mammals, functional relationships between the hypothalamus and the anterior pituitary are mediated by similar mechanisms (Levine et al. 1985; Moenter et al. 1991; Freeman 1994; Hotchkiss and Knobil 1994). Since most information on SERM effects in experimental animals comes from the female rat, we will refer to this model in the following description. As mentioned above, gonadotropin secretion exhibits two patterns: a tonic pattern, which is responsible for follicular growth, and a phasic pattern, which is characterized by the preovulatory surge of LH and FSH (Fink 1988,2000). Both secretory patterns are under the control of GnRH, which is released episodi cally from nerve terminals at the median eminence into the hypophyseal portal system (Levine et al. 1991; Terasawa 2001). GnRH secretion also shows two patterns of secretory activity, one characterized by pulses of low amplitude and high frequency and one characterized by pulses of high amplitude and low frequency (Levine et al. 1995). A neuronal subset of GnRH neurons localized at the arcuate and ventromedial nucleus of the mediobasal hypothalamus (MBH) seems to be responsible for the tonic pattern of GnRH secretion (“pulse generator”), while another group of neurons at the preoptic area (POA) is responsible for the GnRH surge (“surge generator”) (Kimura and Funabashi 1998).

As described below, GnRH neurons receive several synaptic afferents from both hypothalamic and extrahypothalamic regions (for a review see Herbi- son 1998; Herbison and Pape 2001). Some of these influences are excitatory in nature and probably mediated by excitatory aminoacids (Lopez et al. 1990; van den Pol et al. 1990; Ping et al. 1994), while others are inhibitory and exerted through a variety of interneurons that use y-aminobutyric acid (GABA) (Jarry et al. 1991; Herbison et al. 1991; Mitsushima et al. 1997) or opioid peptides (Weisner et al. 1984; Lustig et al. 1988; Mallory and Gallo 1990). In addition, many other synaptic contacts, including different monoamin- ergic and peptidergic terminals, may modulate the activity of GnRH neurons (Herbison 1998; Herbison and Pape 2001). Thus, the activity of different subsets of GnRH neurons may be the consequence of a complex interplay between their intrinsic oscillatory activity and the overall synaptic input (Suter et al. 2000; Nunemaker et al. 2002). The combination of cyclic fluctuations in the secretory pattern of GnRH neurons with changes in the synthetic and secretory capacity of pituitary gonadotropes generates the dramatic changes in gonadotropin secretory profiles observed during the ovarian cycle in all mammals (Freeman 1994; Hotchkiss and Knobil 1994).

5.2.3

Estrogen Feedback Regulation of Hypothalamus-Pituitary Axis

In both males and females during the luteal phase of the ovarian cycle estrogen restrains LH secretion through what has been called its “negative feedback”. This effect is due to a combined inhibitory action on GnRH secretion by hypothalamic GnRH neurons (Sarkar and Fink 1980; Chongthammakun and Teresawa 1993; Evans et al. 1994) and, although less documented, on pituitary gonadotropes (Shupnik 1996). In addition, in the female of most mammalian species, estrogen also exerts a “positive feedback” action on GnRH neurons (Sarkar et al. 1976; Moenter et al. 1990; Rosie et al. 1990; Xia et al. 1992) and sensitizes anterior pituitary cells to GnRH (Speight et al. 1981). However, specific cellular mechanisms responsible for these estrogen actions remain partially understood.

5.2.3.1

Estrogen Receptors in GnRH Neurons

Up until the last few years, it was thought that hypothalamic GnRH neurons did not contain ERs, as they were not able to either concentrate estradiol within the nucleus (Shivers et al. 1983) or present immunoreactivity corresponding to the classical ER (Watson et al. 1992; Herbison et al. 1993; Sullivan et al. 1995).

However, the finding of a second subtype of ER (Kuiper et al. 1996), ERβ, completely changed this point of view. With the use of highly sensitive techniques of in situ hybridization and immunocytochemistry, it has been recently shown that certain populations of GnRH neurons from rats and mice express both the mRNA encoding ERβ (Skynner et al. 1999; Hrabovsky et al. 2000; Sharifi et al. 2002) and the protein (Hrabovsky et al. 2001; Kallo et al. 2001; Legan and Tsai 2003) (Table 5.1). On the other hand, studies from immortalized cell lines producing GnRH (Mellon et al. 1990) have allowed the combination of immunocytochemical identification of ERs with the characterization of signaling pathways. Thus, GnRH-releasing GT1-7 cells appear to express ERa and ERβ transcripts and proteins (Butler et al. 1999; Roy et al. 1999; Martinez-Morales et al. 2001; Navarro et al. 2003), as well as plasma membrane estrogen binding sites. Nevertheless, while GT1-7 cells clearly express ERa, with the exception of one study in rats treated with colchicine (Butler et al. 1999), this protein has not been detected in vivo. Since GT1-7 cells may represent immature GnRH neurons, developmental studies must be performed in different species in order to clarify this point.

Table 5.1. Detection of ERa and ERβ protein and/or mRNA in hypothalamic GnRH neurons of rats and mice

Hypothalamus

ERa

ERβ

Technique

Reference

Low levels

 

Dual immunolabeling

Butler et al. 1999

High levels

Low levels

RT-PCR

Skynner et al. 1999

Not detected

High levels

Dual-label in situ hybridization

Hrabovszky et al. 2000

Not detected

High levels

Dual immunolabeling

Hrabovszky et al. 2001

Not detected

High levels

Dual immunolabeling

Kallo et al. 2001

Not detected

High levels

Immunohistochemistry

Legan and Tsai 2003

5.2.3.2

Estrogen Receptors in Anterior Pituitary Cells

In rats and mice both ER transcripts and proteins have been identified in 6070% of anterior pituitary cells (Kuiper et al. 1997; Wilson et al. 1998; Wilson et al. 1998; Nishihara et al. 2000; Pelletier et al. 2000). While ERa is the predominant subtype in adult rats, in anterior pituitaries from fetal and prepubertal animals ERβ appears to be expressed in greater abundance (Nishihara et al. 2000). In addition, differential developmental and estrogen-dependent expression of pituitary ERs has been reported (Pasqualini et al. 1999). Approximately 8-10% of anterior pituitary cells express both ER subtypes, suggesting that interaction between them through heterodimers may have functional significance (Mitchner et al. 1998). With respect to cell types expressing each ER subtype, ERa is expressed at high levels in lactotropes and, to a lesser extent, in gonadotropes, while ERβ is expressed at low levels in all anterior pituitary cells (Mitchner et al. 1998; Nishihara e al. 2000; Childs et al. 2001; Sanchez- Criado et al. 2004) (Fig. 5.1; Table 5.2). Two isoforms of truncated estrogen receptor products (TERP), TERP1 and TERP2, are also expressed in the rat pituitary and are capable of forming heterodimers with ERa and ERβ (Schrei- hofer et al. 2002; Vaillant et al. 2002). In addition, several reports have indicated the presence of ERa associated with the plasma membrane of rat pituitary cell lines (Pappas et al. 1994; Norfleet et al. 1999), which might be related to rapid estrogen actions on prolactin (PRL) secretion (Christian and Morris 2002).

Table 5.2. Detection of ERa and/or ERβ in different cell types of rat anterior pituitary

Anterior pituitary gland

ERα

ERβ

Technique

Reference

Lactotropes, foli- cullostellate cells, corticotropes, and gonadotropes

Lactotropes, foli- cullostellate cells, corticotropes, and gonadotropes

Combined in situ hybridization and immunohisto- chemistry

Mitchner et al. 1998

 

Lactotropes and gonadotropes

Immunohisto-

chemistry

Nishihara et al. 2000

Gonadotropes

Gonadotropes

Dual immuno- labeling

Childs et al. 2001

Gonadotropes

Gonadotropes

Dual immuno- labeling

Sanchez-Criado et al. 2005

Lactotropes, somatotropes, thyrotropes, and gonadotropes

Lactotropes, somatotropes, and gonadotropes

Dual immuno- labeling

Gonzalez et al. unpublished

5.2.3.3

Estrogen Negative Feedback on GnRH Neurons

Estradiol represses GnRH gene expression in immortalized GT1 cells (Roy et al. 1999; Bowe et al. 2003) and may exert either stimulatory or inhibitory effects in transfected nonneural cells through ERa (Wierman et al. 1992; Dong et al. 1996; Chen et al. 2001). In addition, estrogen inhibitory effects on GnRH gene expression have been found after in vivo treatment in experimental animals (El Majdoubi et al. 1998; Pelletier et al. 2001) as well as in brain tissue from menopausal women (Rance and Uswandi 1996). On the other hand, in mice lacking ERa, but not ERβ, the inhibitory effects of estrogen on either hypothalamic GnRH mRNA levels or LH secretion are absent (Wersinger et al. 1999; Dorling et al. 2003). However, direct estrogen actions on GnRH gene expression ofhypothalamic GnRH neurons have not been demonstrated. Therefore, since these cells do not express ERa in large amounts, it is reasonable to think that classical ER-mediated negative feedback is not exerted on GnRH neurons directly but rather through modulation of excitatory or inhibitory interneurons.

Fig. 5.1. Comparison between ERa and ERβ immunoreactivities in lactotropes (PRL-ir) and gonadotropes (FSH-ir) of the female rat anterior pituitary. The left panel shows anterior pituitary sections of double-label immunostaining for ERa and PRL (A, B) or FSH (E, F). The right panel shows anterior pituitary sections double-label immunostaining for ERβ and PRL (C, D) or FSH (G, H). Numerous ERa-positive nuclei are seen in many lactotrope (A) and several gonadotrope (E) cells, whereas scarce ERβ-positive cytoplasms are detected in isolated lactotrope (C) and gonadotrope (G) cells. Cellular details with double labels are depicted in the upper-right sections (B, D, F, H). Arrows indicate double-labeled cells. Magnifications are 270x (C, G), 320x (E), 340x (A), 430x (D, H), and 650x (B, F). Briefly, anterior pituitaries were fixed with 4% paraformaldehyde in phosphate buffer saline (PBS, 0.1 M, pH 7.4) and frozen. The indirect immunocytochemical procedure was carried out by incubating pituitary sections with MC20 rabbit antimouse ERa (1/250, Santa Cruz Biotechnology) or with Y-19 goat antimouse ERβ (1/250, Santa Cruz Biotechnology). This first immunostaining was revealed by the streptavidin-biotin-peroxidase method (1:1000 and 1:1500, respectively) using diethyl-carbazol (red product). The same pituitary sections were also incubated in rabbit anti-PRL (1:1000, Chemicon International) or in rabbit anti- FSH (1:1000, Chemicon International), and 4-chloro-1-naftol was used for the subsequent labeling (blue product)

Results from experimental animals have shown that estrogen administration to ovariectomized mammals reduces GnRH (Sharkar and Fink 1980) and LH levels (Condon et al. 1988) in portal blood and peripheral plasma within minutes. Since it suggests a rapid, nongenomic, estrogen direct action on either GnRH hypothalamic neurons or pituitary gonadotropes, a number of studies have addressed this issue by the use of different experimental models. Studies in hypothalamic slices from ovariectomized guinea pigs have shown that estradiol hyperpolarizes within seconds GnRH neurons and reduces the potency of μ-opioid and GABAB receptor agonists (Lagrange et al. 1995) through modulation of Ca2+-activated K+-channels (Kelly et al. 2002). On the other hand, in GT1-7 cells estradiol acutely reduces ACh-induced Ca2+ signals, an effect that is apparently mediated by a specific membrane receptor and not blocked by the antiestrogen ICI 182,780 (Morales et al. 2003). In addition, estradiol could exert part of its negative feedback effect indirectly by modulating the activity of some presynaptic inputs. Thus, in hypothalamic slices from guinea pigs and mice, estradiol increases the inhibitory tone of both GABAergic and β-endorphin neurons by interaction with specific membrane receptors coupled to Gaq-protein and activation of PKC (Qiu et al. 2003). Therefore, although the identity of receptor proteins and the second messenger cascades that are activated remain to be clarified, all these findings indicate that estradiol may exert a critical part of its negative feedback effects on GnRH neurons through specific membrane sites, either directly or transynaptically.

5.2.3.4

Estrogen Positive Feedback on GnRH Neurons

Clear experimental evidence supporting direct estrogen action on GnRH neurons in its positive feedback mode is lacking at the present time. Nevertheless, recent studies have shown that estradiol can exert rapid stimulatory effects (less than 30 min) on mice GnRH neurons, both in vivo (Abraham et al. 2003) and in nasal explants (Temple et al. 2004). Apparently, these estrogen effects are exerted through intracellular ERβ and may be related to phosphorylation of cAMP response element binding protein (CREB) (Abraham et al. 2003) and changes in transcriptional activity (Temple et al. 2004). However, neither the downstream signals activated by estradiol nor their relation to GnRH synthesis or secretion is presently known. On the other hand, in GT1-7 cells expressing both ERa and ERβbiphasic, dose-dependent effects of estradiol on cAMP signaling have been demonstrated; these effects are apparently exerted through membrane receptors and coupled to changes in pulsatile GnRH secretion (Navarro et al. 2003). Unfortunately, given the difficulty of generalizing results from immortalized cell lines, the interpretation of these findings should await further experimentation.

In contrast, most available evidence suggests that estrogen positive feedback effects on GnRH neurons are exerted via indirect, transynaptic mechanisms (Herbison and Pape 2001). Thus, ERa and ERβ containing neurons at the anteroventral periventricular region of rats (AVPV) appear to be critical for estrogen positive feedback since they are coactivated with GnRH neurons at the time of LH surge (Le et al. 1999). In addition, electrolytic lesions of this region impair the LH surge (Wiegand et al. 1982), and antiestrogen microimplants block both the estrogen-induced LH surge and the phasic increase in GnRH gene expression (Petersen et al. 1995). Even though the identity of these neuronal inputs on GnRH neurons remains to be clarified, most experimental evidence indicates that they are mainly GABAergic and glutamatergic (for review see Herbison and Pape 2001; Petersen et al. 2003). Thus, estrogen-sensitive neurons projecting to the location of GnRH neurons contain and release GABA (Ondo et al. 1982; Flugge et al. 1986) or glutamate (Ping et al. 1994; Jarry et al. 1995), and receptors for these neurotransmitters have been found in GnRH neurons (Eyigor and Jennes 1997; Spergel et al. 1999). In addition, estrogen- dependent progesterone receptor (PR) on AVPV neurons (Chappel and Levine 2000) as well as several paracrine factors released from glial cells (Prevot et al. 2000) also appear to be critical components of events leading to the preovulatory GnRH surge.

5.2.3.5

Estrogen Feedback Actions on Anterior Pituitary

It has been known for years that estrogen regulation of the hypothalamus- pituitary axis in females is the result of a combined action on hypothalamic neurons releasing GnRH (as described above) and on the responsiveness of anterior pituitary to GnRH (Fink 1995,2000; de Koning et al. 2001). Since both gonadotropes and lactotropes contain ERs, anterior pituitary cells are potential targets for estrogen action in the regulation of the female reproductive cycle. However, even though estrogen seems to contribute to the negative feedback on LH secretion by direct actions on gonadotropes (Henderson et al. 1977), most evidence indicates that the major site for estrogen action is the hypothalamus rather than the pituitary (Leipheimer et al. 1983). By contrast, in the case of the positive feedback, there is little doubt that estrogen acts directly on gonadotropes to enhance their responsiveness to GnRH (Dronin and Labrie 1981). In addition, estradiol elicits GnRH self-priming by inducing pituitary PR receptor expression in the gonadotrope (Szabo et al. 2000; Scott et al. 2002), a phenomenon that is determinant in the expression of the preovulatory LH surge (Levine 1997; Fink 2000). Although the intracellular signals and downstream cascades activated by estradiol remain to be clarified, most positive estrogen effects on gonadotropin and PRL synthesis and secretion at the level of anterior pituitary cells seem to be dependent on ERa (Scully et al. 1997; Sanchez-Criado et al. 2004). Studies from pituitary cell lines and rat pituitary cells have demonstrated that estrogen differentially regulates gene expression of different ER subtypes, a mechanism that may serve to modulate estrogen responsiveness (Mitchner et al. 1998; Schreihofer et al. 2000). Also, ERs in anterior pituitary cells can be activated in a ligand-independent manner through several signal cascades that include PKA, PKC, or MAPK activation (Schreihofer et al. 2001). Furthermore, in gonadotrope a-T3-1 cells, GnRH triggers signaling pathways that result in estrogen-independent transactivation ofERa and potentiate estrogen-dependent ERa transactivation (Demay et al. 2001).

5.2.4

Overview of Hypothalamic-Pituitary Function at Time of Gonadotropin Surge

In summary, even though the evidence concerning GnRH neurons is still fragmentary, regulatory estrogen effects on the activity of the GnRH neuronal system and the secretion of GnRH to the hypophyseal-portal system have been demonstrated in most species. In addition, estrogen modulates the expression of its own receptors in anterior pituitary cells and thereby regulates their responsiveness to GnRH. Some estrogen effects may be exerted directly on GnRH neurons through intracellular estrogen receptors (probably ERβ) and/or putative membrane receptors. Also, a variety of estrogen actions on GnRH neurons are exerted transynaptically through interactions on different estrogen-sensitive interneurons that send a complex synaptic input to GnRH neurons.

Although the precise cellular mechanisms underlying estrogen feedback actions on the GnRH neuronal system and, as a consequence, GnRH secretion remain to be clarified, a rather speculative working model can be provided (Fig. 5.2). In rats and other mammals, low estrogen concentrations during most of the cycle exert a direct negative feedback action on a population of GnRH neurons at MBH (“pulse generator”) through ERβ and, perhaps, several potential estrogen-sensitive membrane sites. This low activity of GnRH neuronsisprobablyreinforcedormaintainedbyavarietyofinhibitoryafferents from different interneurons. A rise in estrogen levels prior to ovulation would reclute several estrogen-sensitive presynaptic neurons through ERa and ERβ  which in turn modulate the activity of another subgroup of GnRH neurons at POA (“surge generator”). The overall predominance of afferent excitatory inputs and/or the reduction of inhibitory inputs would allow GnRH neurons to increase their firing rate and the magnitude of GnRH secretion in the hypophyseal-portal system. In addition, the concurrent action of estrogen and GnRH on gonadotrope cells in the anterior pituitary elicits GnRH self-priming and increases gonadotrope responsiveness, which is further increased by progesterone. Thus, estrogen appears to coordinate the surge of GnRH and the gonadotrope responsiveness to it in such a way that both events reach a peak at the same time and provoke the preovulatory surge of LH.

Fig. 5.2. Schematic representation of estrogen feedback actions on GnRH-gonadotrope system in mammals. As described in the text, estrogen may modulate the activity of hypothalamic GnRH neurons either directly or transynaptically. In rats and mice, direct estrogen interactions with GnRH neurons seem to be exerted via genomic mechanisms through nuclear ERβ In addition, acute estrogen effects on GnRH neurons may also be exerted directly through potential membrane binding sites, which remain to be characterized. On the other hand, estrogen also interacts with several interneurons and glial cells, which in turn exert either excitatory or inhibitory actions on GnRH neurons. Thus, low estrogen concentrations during most of the ovarian cycle exert a direct negative feedback action on GnRH neurons and thereby maintain the tonic pattern of GnRH secretion. This low rate of GnRH secretion is probably reinforced by a variety of inhibitory afferents from different interneurons. A rise in estrogen levels prior to ovulation would reclute several estrogen-sensitive presynaptic neurons through ERa and ERβ to further modulate, transynaptically, the activity of other subgroups of GnRH neurons. The overall predominance of afferent excitatory inputs and/or the reduction of inhibitory inputs would allow GnRH neurons to increase their firing rate and the magnitude of GnRH secretion in the hypophyseal-portal system. In addition, the concurrent action of estrogen and GnRH on gonadotrope cells in the anterior pituitary elicits GnRH self-priming and increases gonadotrope responsiveness, which is further increased by progesterone

5.3

Effects of SERMs on Hypothalamus-Pituitary-Ovarian Axis

Since, as described above, estrogen exerts a complex constellation of effects on the female reproductive axis, differential effects of SERMs would be expected. Thus, the administration of different SERMs to experimental animals shows a variety of results depending on dose, method of treatment, experimental model, and tissue-specific response. While the use of cycling rats allows the observation of the hypothalamus-pituitary-ovarian axis as a whole, ovariec- tomized animals permit the modification of estrogen status as an experimental controlled variable (Bellido et al. 2003; Hernandez et al. 2003; Sanchez-Criado et al. 2002). In addition, studies on tissue explants, dispersed pituitary cells, or immortalized cell lines give the possibility of detailed analysis of signaling pathways involved in a particular estrogen response. Nevertheless, all this information must be integrated in order to understand the relative effect of a particular compound on different levels of the female reproductive axis.

In the case of clinical studies, although pharmacological effects of SERMs have been extensively analyzed in several reproductive and nonreproductive tissues of menopausal women, the influence of these compounds upon regulation of the hypothalamus-pituitary-ovarian axis has not been fully clarified, and conflicting results are frequently published. Whereas high physiological concentrations of estradiol in postmenopausal women inhibit GnRH secretion and reduce plasma LH and FSH levels, exogenous pharmacological estrogen concentrations sensitize the anterior pituitary to GnRH and lead to increased gonadotropin levels. As a consequence, the net estrogenic or antiestrogenic activity of a given SERM will depend upon the balance between these opposite actions (Ravdin et al. 1988; Jordan et al. 1991; Szamel et al. 1998). On the other hand, the effect ofSERMs on the human hypothalamus-pituitary-gonadal axis may also depend on a combination of clinical variables. Thus, diverse and conflicting results have been reported depending on gender, ovarian function status (pre- or postmenopausal), dose and duration of treatment, coexistence of diseases, and use of concomitant medications that can alter the hypothalamus- pituitary axis, like the frequent case of adjuvant chemotherapy in advanced breast cancer patients (Jordan et al. 1987a,b; Kostoglou-Athanassiou et al. 1997; Ellmen et al. 2003). Moreover, several of the published reports included small sample sizes or heterogeneous patient populations which may account for nonsignificant results.

In the following sections, we will first review recent experimental findings from our and other laboratories on the effects of different SERMs on the hypothalamus-pituitary-ovarian axis in the cyclic female rat, as well as from anterior pituitary explants from ovariectomized rats subjected to different estrogen environments. We will next review most of the relevant published data on the effects of SERMs on the hypothalamus-pituitary-ovarian axis in humans, with special emphasis to gonadotropin and sex hormones in postmenopausal women. Given the extensive current use of these drugs in both the prevention and treatment of breast cancer and postmenopausal osteoporosis, as well as the continuous flow of new SERMs in late-phase clinical development, it is interesting to know whether these compounds differ in terms of their effects on gonadotropin secretion. Moreover, the analysis of pharmacological effects of SERMs on the hypothalamus-pituitary-ovarian axis of both humans and experimental animals may help to understand the complex mechanisms that control the regulation of reproductive function.

5.3.1

Experimental Studies in the Rat

5.3.1.1

Trifenilethylene Derivatives

Clomiphen is a racemic mixture of two molecules with different estrogen agonist and antagonist activity, which induces ovulation in rats, probably acting at all levels of the hypothalamus-pituitary axis (Adashi 1984). In perifusion experiments of MBH and pituitary, this compound induces GnRH and LH release, both effects being potentiated by estrogen in the incubation medium (Miyake et al. 1983). However, when implanted into the MPA of ovariectomized rats, clomiphen has been shown to inhibit both negative and positive estrogen feedback actions on LH secretion (Docke et al. 1989, 1990). At the level of the anterior pituitary, clomiphen blocks nuclear translocation of ERs (Ter- akawa et al. 1985) and inhibits estrogen-induced PR in ovariectomized rats (Terakawa et al. 1986). However, both in cyclic (Kilic-Okman et al. 2003) and ovariectomized rats (Schuiling et al. 1985) clomiphen stimulates gonadotropin release by enhancing pituitary responsiveness to GnRH (Adashi et al. 1981; Engel et al. 2002), probably through an increase in the number of pituitary GnRH receptors (Shimizu et al. 1986).

Tamoxifen was the first SERM described and has been used for the treatment of breast cancer for decades (Jordan et al. 1987, 1991; Jordan and Morrow 1999). It exhibits either estrogen agonist or antagonist activities on several reproductive parameters in the female rat. Tamoxifen inhibits ovulation both in adult (Donath and Nisshino 1998) and in prepubertal rats given exogenous gonadotropins to induce follicular development (Gao et al. 2002), and it reduces estrogen and progesterone levels at proestrus (Donath and Nishino 1998). These effects are mainly due to an impairment of the preovulatory surges of LH and FSH since the anovulatory action was reversed by treatment with human chorionic gonadotropin (Gao et al. 2002). Furthermore, tamoxifen treatment of cyclic rats at proestrus reduces both basal and GnRH-stimulated LH secretion, either in vivo or in vitro (Sanchez-Criado et al. 2002). In addition, tamoxifen reverses estrogen facilitation of high K+- induced GnRH release from rat hypothalamic explants (Drouva et al. 1988) and antagonizes the stimulatory effect of estradiol and E-BSA on nitric oxide (NO) release from the rat median eminence (Prevot et al. 1999). With respect to other brain areas that are involved in reproductive behavior, chronic tamoxifen treatment increases oxytocin receptor binding and ERβ gene expression either in the ventromedial nucleus (VMN) (Pautisaul et al. 2003) or in the total hypothalamus (Zhou et al. 2002) by itself, without antagonizing the effects of estradiol on this parameter, and inhibits estrogen-dependent PR gene expression and PR immunoreactivity in the medial preoptic nucleus (MPN) and the VMN (Shugrue et al. 1997; Yin et al. 2002; Patisaul et al. 2003).

While the above-mentioned results indicate that this compound may act mainly as an overall estrogen antagonist on the estrogen positive feedback, their effects on gonadotropin secretion suggest a more complex behavior. Thus, tamoxifen elevates GnRH-induced LH release and PRL release in anterior pituitaries from proestrous rats (Gonzalez et al. 2000) and increases GnRH self-priming (Sanchez-Criado et al. 2002). Interestingly, while tamoxifen induces GnRH self-priming by itself, it reduces the estrogen-sensitizing effect on GnRH-stimulated LH secretion and abolishes estrogen-dependent GnRH self-priming (see discussion below). On the other hand, treatment of ovariectomized rats with tamoxifen enhances PR-B mRNA levels in a similar extent to that of estradiol and increases the number of anterior pituitary cells expressing immunoreactive PR (Sanchez-Criado et al. 2003). Moreover, pretreatment with the “pure” antiestrogen RU58668 reduces tamoxifen-induced PR expression and GnRH self-priming, while pretreatment with the antiprogestin RU38486 blocks tamoxifen-induced GnRH self-priming (Sanchez-Criado et al. 2003). In addition, treatment of ovariectomized rats with tamoxifen increases the number of LH-positive cells expressing ERα to an extent similar to that of of cycling proestrous rats (Sanchez-Criado et al. 2005a). Therefore, at the rat gonadotrope level, tamoxifen behaves either as an estrogen agonist or antagonist, its estrogen agonistic activity being related to a direct induction of PR expression in the gonadotrope through ERa (Tena-Sempere et al. 2004). With respect to PRL secretion, tamoxifen shows also a mixed agonist/antagonist activity depending on the estrogen status of the animal. Thus, this compound increases PRL levels in both ovariectomized (Gonzalez et al. 2000) and prepubertal rats (Toney and Katzenellenbogen 1986), whereas it inhibits estrogen- induced PRL elevations (Donath and Nishino 1998; Toney and Katzenellenbogen 1986).

5.3.1.2

Benzotiophene Derivatives

In the female rat, raloxifene acts as a complete antiestrogen on the hypothalamus-pituitary-gonadal axis and displays clear anovulatory effects under chronic treatment (Long et al. 2001). Although there are few studies at hypothalamic level, this compound apparently lacks estrogen agonist activity on the expression of ERs or PR in all brain areas (Zhou et al. 2002), and reduces estrogen-induced PR expression in the MPN (Shughrue et al. 1997). With respect to the anterior pituitary, raloxifene inhibits the expression of ERa in cyclic rats to the same extent of “pure” antiestrogens and completely abolishes nuclear localization of ERa in gonadotropes at proestrus (Sanchez-Criado et al. 2002). In addition, in ovariectomized rats, raloxifene exhibits rather deleterious effects on pituitary ERs since it decreases the proportion of LH-positive cells staining for ERa and shows no evidence of estrogen agonist activity on ERβ or PR expression (Sanchez-Criado et al. 2004). These effects are consistent with an overall antiestrogenic activity on basal and GnRH-stimulated LH release or estrogen-induced GnRH self-priming (Gonzalez et al. 2000; Sanchez-Criado et al. 2002). On the other hand, raloxifene treatment of ovariectomized rats reduces estrogen-induced PRL release (Buelke-Sam et al. 1998), whereas it has either no effect (Gonzalez et al. 2000) or a stimulatory action (Pinilla et al. 2001) on PRL secretion.

We have used a pyrroliding analog of raloxifene, LY117018, to study SERM effects at different levels of the reproductive system of normal cycling rats. In the anterior pituitary, LY117018 inhibits the expression of ERa and blocks the proestrus-induced nuclear localization of this protein to the same extent as raloxifene (Sanchez-Criado et al. 2002). In addition, treatment with LY117018 inhibits ovulation in a dose-dependent manner and reduces both negative and positive estrogen feedback actions on gonadotropin secretion in ovariectomized rats (Hernandez et al. 2003), without significantly affecting the release of GnRH into the hypophyseal system at proestrus, and reduces estrogen-induced GnRH self-priming (Sanchez-Criado et al. 2002). Therefore, LY117018 anovulatory actions seem to be due mainly to the inhibitory effect of this compound on the preovulatory surge of LH, probably by reducing the pituitary responsiveness to GnRH (Fig. 5.3) (Guelmes et al. 2003; Hernandez et al. 2003).

Fig. 5.3. Effect of benzothiophen derivative LY117018 on various reproductive parameters in female rats (modified from Hernandez et al. Reproduction 125:597-606, 2003). In this and other similar figures, values are mean ± SEM of the corresponding variable. (A) Different doses of LY117018 were administered s.c. to female rats at diestrus, and ovulation was assessed at estrus by inspection of the ampullary region. (A) • represents the percentage of ovulating rats after treatment with high doses of LY117018 plus human chorionic gonadotropin (hCG). *P < 0.01 by two-tailed Fisher’s exact probability test versus controls (bar). (B) Effect of administration of LY117018 (16mg/Kg) to cyclic female rats at proestrus on GnRH secretion rate from hypophyseal portal system. (C) Time course of effect of LY117018 (16mg/Kg) on pituitary sensitivity to GnRH (50nG/100g) in cyclic female rats. Blood samples were collected during proestrus at the time of expected endogenous surge of LH. *P < 0.01 versus controls. (D) Effect of estradiol (40 |μg/day, 2 d) and LY117018 (16 mg/Kg) on GnRH-induced (10 nM during 40 min) phospholipase activity in hemipituitaries from ovariectomized rats. *P < 0.05 versus controls

5.3.2

Lessons in SERM Behavior from Effects of Tamoxifen on Rat Pituitary Function

In 1994, we found that endogenous estradiol decreased serum LH concentrations in tamoxifen-treated cyclic rats, and we explained this paradoxical effect as an “inappropriate feedback of endogenous estradiol” (Tebar et al. 1994). More recently, our laboratories have routinely used anterior pituitary glands harvested from ovariectomized rats treated with different ER ligands. Thereafter, these pituitaries were incubated with the corresponding ligand and LH secretion was measured (Gonzalez et al. 2000; Sanchez-Criado et al. 2002; Bellido et al. 2003; Sanchez Criado et al. 2004, 2005a,b ). More by accident than on purpose, it was observed that 2-h incubation with 10-8 M estradiol of pituitaries from ovariectomized rats treated with 3 mg tamoxifen blocks the agonist effect of this SERM on GnRH self-priming (Fig. 5.4). GnRH selfpriming is a phenomenon different from the GnRH-releasing action in which, under endogenous estrogen levels, the magnitude of LH release after a first exposure to GnRH (“umprimed response”) sensitizes pituitary gonadotropes to a second GnRH pulse given 60 min later (“primed response”) (Fink 1995; 2000; de Koning et al. 2001). Therefore, as described above, although tamoxifen treatment has no effect on LH secretion in ovariectomized rats, it induces PR expression in the gonadotrope (Sanchez-Criado et al. 2004), as well as a robust GnRH self-priming (Bellido et al. 2003) that is abolished by incubation with estradiol (Fig. 5.5). Moreover, whereas coincubation with ICI 182,780, a “pure” type-II antiestrogen, reverses the inhibitory effect of estradiol on tamoxifen- induced GnRH self-priming, tamoxifen by itself (a type-I antiestrogen) has no effect (Sanchez-Criado et al. 2005b). These findings indicate that the inhibitory effect of estradiol on tamoxifen-induced GnRH self-priming is probably exerted at the level of an unknown ER with extremely low affinity for tamoxifen, which is different from classical ERs. Additional preliminary data have shown that both the ERαagonist PPT (Sanchez-Criado et al. 2004) and the membrane impermeant estradiol analog E-BSA also inhibit tamoxifen-induced GnRH selfpriming (Sanchez-Criado et al. 2005b). Taken together, these findings lead us to the conclusion that tamoxifen may evoke GnRH self-priming in the anterior pituitary of ovariectomized rats by acting on ERa. This interpretation would imply that, under physiological conditions, a cross-talk between nuclear and membrane ERs may exist in the gonadotrope to modulate estrogen action on GnRH self-priming and, hence, on LH release.

Fig. 5.4. LH secretion from hemipituitaries from ovariectomized rats treated during 3 d with oil (0.2 ml), estradiol benzoate (EB, 25 μg), or tamoxifen (TX, 3 mg) and incubated for 3 h with medium alone, 17β-estradiol (E2, 10-8 M), or TX (10-7 M), in response to two consecutive GnRH challenges (10-8 M, for 15 min) at indicated time periods. Values of LH secretion from hemipituitaries of oil- and EB-injected ovariectomized rats incubated with medium alone, E2, or TX (24 hemipituitaries), and from hemipituitaries of TX-injected rats incubated with medium alone or TX (16 hemipituitaries) are represented together, as no effect of the incubation conditions was observed. Values of LH secretion from hemipitu- itaries of TX-injected ovariectomized rats incubated with E2 are the mean of 8 half glands. *P < 0.01 versus non-EB-treated rats (modified from Sanchez-Criado et al. J Endocrinol 186:43-49,2005)

Furthermore, using the new selective ER agonists PPT and DPN (Sun et al. 1999; Stauffer et al. 2000; Meyers et al. 2001), we have found that ERa, which is predominant in the anterior pituitary (Scully et al. 1997; Nishihara et al. 2000), mediates all actions of estradiol on the gonadotrope (Sanchez-Criado et al. 2004; Tena-Sempere et al. 2004). However, in the absence of ERa activation, the ER,β isoform can replace the effect of ERa on the synthesis, but not on the release, of gonadotropins (Sanchez-Criado et al. 2004). In addition, our recent results indicate that, whereas selective ERa activation by PPT restores the estrogen negative feedback on LH secretion, sensitizes the gonadotrope to GnRH, and induces PR expression and GnRH self-priming (Sanchez-Criado et al. 2004), selective ERβ activation by DPN stimulates all steps leading to LH secretion except exocytosis (Sanchez-Criado et al. 2005b). Therefore, this suggests that, in the gonadotrope, ERa and ERβ are not mere components of a redundant regulatory system but rather complementary players involved in the regulation of gonadotropin synthesis in a “ying-yang” relationship.

Fig. 5.5. Percentage of GnRH self-priming in hemipituitaries from ovariectomized rats treated for 3 d with oil, B, or TX and incubated for 3 h with medium alone, E2, or TX. GnRH self-priming was calculated as the peak response to the second GnRH pulse x 100/peak response to the first GnRH pulse. A value of 100% or less indicates absence of GnRH selfpriming. *P < 0.05 versus oil (modified from Sanchez-Criado et al. J Endocrinol 186:43-49, 2005)

We are now tempted to speculate that estrogen action on gonadotropin secretion, which includes both synthesis and release, not only involves the two ER isoforms identified so far, but also plasma membrane ERs working in an integrated manner (Fig. 5.6). Whereas the predominant action of nuclear ERa on gonadotropin synthesis, but not release, is probably modulated by nuclear ERβ, plasma membrane ER may inhibit gonadotropin release elicited by nuclear ERa by acting specifically on PR-dependent GnRH self-priming (Waring and Turgeon 1992; Sanchez-Criado et al. 2004). Whether these as yet unidentified plasma membrane ERs in the anterior pituitary are structurally related to ERa, as has been suggested for other cellular functions (Marin et al. 2003), and how signaling pathways activated after estrogen binding to membrane ERs are coordinated with those dependent on nuclear ER activation is just starting to be understood (Valverde and Parker 2002; Azuma et al. 2004; Razandi et al. 2004; Marin et al. 2005). The finding that particular SERMs, like tamoxifen, may differentially interact with different ER-dependent pathways within the same cell and modulate a particular cell function, as occurs in the rat gonadotrope, may be extremely useful in both cell biology and estrogen pharmacology (McDonnell 2003). On one hand, the discovery of the action mechanisms of different SERMs is a key factor for their proper clinical use. On the other hand, the combination of SERMs with ERa and ERβ selective analogs will surely provide investigators with tools needed to dissect the biology of classical nuclear ERa and ERβ isoforms and novel, membrane-related ERs. Most probably, a “new” estrogen biology will be discovered in the near future using these compounds to isolate ER isoforms and plasma membrane estrogen actions that had been masked by the simultaneous activation of the complete orchestra of ERs by the cognate ligand.

Fig. 5.6. Schematic diagram of estrogen (E2) actions on LH secretion in rat gonadotropes. Activation of nuclear ERa stimulates LH secretion (synthesis + release), whereas activation of nuclear ERβ modulates the effect of ERa on the synthesis, but not the release, of LH in a sort of ying-yang relationship. In addition, activation of putative membrane ERa by the cognate ligand would blunt the releasing effects of nuclear ERa activation by a potential, and as yet uncharacterized, cross-talk interaction

5.3.3

Clinical Studies

5.3.3.1

Trifenilethylene Derivatives

Tamoxifen has been used for over 30 years, and the clinical experience from over 10 million women per year is a proof of its beneficial effect in the treatment of disseminated breast cancer and as an adjuvant drug in primary prevention in women at high risk of developing this disease. Other members of this chemical group are clomiphene, a classical antiestrogen used for initiation of ovulation in anovulatory women of reproductive age, toremifene, droloxifene, idoxifene, miproxifene, ospemifene, and fispemifene (Chap. 2, Pharmacology of SERMs, Marin and Barbancho). Among all these newer triphenylethylenes, only toremifene has been commercialized for the treatment of disseminated hormone-responsive breast cancer, and available data on the neuroendocrinological effects in humans of this class of SERMs are limited to tamoxifen, toremifene, and droloxifene. It should be noted that many of these results originated in women with advanced breast carcinoma who were receiving adjuvant chemotherapy, which likely influenced the hormonal results, mainly in premenopausal women (Jordan et al. 1987a; Ravdin et al. 1988; Jordan et al. 1991; Ellmen et al. 2003).

Administration of tamoxifen to postmenopausal women reduces plasma gonadotropin levels, probably due to a partial estrogen agonist activity at both the hypothalamus and the anterior pituitary, and increases plasma levels of sex hormone binding globulin (SHBG) by an estrogen agonist action on the liver (Kostoglou-Athanassiou et al. 1997). In a recent series of 32 postmenopausal women with breast cancer, plasma FSH and LH fell by a mean of 45% and 48%, respectively, after approximately 12 months of treatment, with increases in SHBG of 65% and slight decreases in plasma estradiol and testosterone (L0nning et al. 1995). This magnitude of gonadotropin suppression is similar to previous reports in postmenopausal women receiving tamoxifen and adjuvant chemotherapy (Jordan et al. 1987b) or to the suppression of basal and GnRH-induced gonadotropin secretion observed in estrogen-deprived postmenopausal women receiving clomiphene, the first SERM used in humans (Messinis and Templeton 1990; Garas et al. 2004). The modest decrease in plasma estradiol observed by L0nning et al. (1995) may be due to the drop in plasma testosterone, as aromatization indexes are not influenced by tamoxifen. Overall, the moderate but significant reduction in plasma estradiol concentrations during tamoxifen therapy, combined with an increase in plasma SHBG, indicates a reduced plasma level of its free fraction. The influence of these effects on estradiol delivery to breast cancer tumor cells is unknown. However, it should be noted that the effects of tamoxifen on serum estradiol levels are not a uniform finding in postmenopausal women, as some reports show either no significant changes in this sex hormone during treatment (Ellmen et al. 2003) or a slight increase (Kostoglou-Athanassiou et al. 1997).

Data on hypothalamic-pituitary and ovarian hormonal parameters in premenopausal women treated with tamoxifen are rather scarce and limited to short series of subjects receiving this drug as adjuvant therapy after mastectomy (Jordan et al. 1991). These women continue having menstrual cycles, unaffected SHBG levels, and normal circulating levels of FSH and LH, including LH surges and subsequent increases in progesterone, which indicates that ovulation has occurred and patients remain at risk of pregnancy. Overall, the results in women with circulating estrogen levels in the normal premenopausal range indicate that the activity of tamoxifen at the hypothalamus-pituitary axis is negligible when used at therapeutic doses. These results are in contrast with the estrogen antagonistic effects of tamoxifen and clomiphene in anovulatory or oligo-ovulatory women at reproductive age, where both drugs initiate or augment ovulation by blocking endogenous estrogen negative feedback and promote FSH and LH release (Messini and Nillius 1982; Adashi 1984).

In contrast with the partial estrogen agonist effect of tamoxifen on gonadotropins, several studies have consistently shown that PRL levels are suppressed by 30-50% in pre- and postmenopausal women taking the drug (Jordan et al. 1987b, 1991; Kostoglou-Athanassiou et al. 1997), which indicates a partial estrogen antagonistic effect on lactotropes in humans. This antiestrogenic action has also been observed with toremifene (chlorotamoxifen) on both basal and thyrotropin-releasing hormone (TRH)-induced PRL release (Szamel et al. 1994). In addition, an increase in SHBG and a reduction in serum estradiol concentrations have also been shown, thereby confirming similar effects of toremifene and tamoxifen on both the anterior pituitary and the liver. The effects of toremifene (60 and 200 mg daily) and tamoxifen (20 mg daily) on FSH and LH levels were also studied in another recent phase II trial in a large series of postmenopausal women with advanced breast cancer (Ellmen et al. 2003). Serum FSH and LH declined during the 10 months of treatment with both drugs, reaching premenopausal values after 8 weeks (Fig. 5.7), whereas SHBG increased in all treated groups by approximately 100%. Similar results have been reported in postmenopausal women with breast cancer in shorter phase I trials with droloxifene (3-hydroxy-tamoxifen) and idoxifene (4-iodo- pirrolidine-tamoxifen). After 3 months of droloxifene therapy, plasma levels of SHBG increased in a dose-dependent manner by 17-74%, while FSH and LH levels decreased by approximately 16-20%, which suggests a less potent estrogen agonistic effect of this drug on the liver and on the hypothalamus- pituitary axis as compared with tamoxifen (Geisler et al. 1995a,b). In addition, two weeks of idoxifene treatment at several doses was also associated with significant reductions in FSH and LH levels, with no differences in serum estradiol or SHBG concentrations in postmenopausal women with advanced breast cancer (Coombes et al. 1995).

Fig. 5.7. Effect of daily treatment with 20 mg tamoxifen (solid line), 60 mg toremifene (dotted line), or 200 mg toremifene (dashed line) on mean LH and FSH serum concentrations of postmenopausal breast cancer patients for at least 8 weeks (modified from Ellmen et al. Breast Cancer Res Treat 82:103-111, 2003)

5.3.3.2

Benzotiophene Derivatives

The effects of raloxifene on the human hypothalamus-pituitary-gonadal axis have been studied primarily in postmenopausal women (Lasco et al. 2002; Reindollar et al. 2002; Cheng et al. 2004; Garas et al. 2004), although results in normal premenopausal women (Baker et al. 1998) and healthy men (Blum et al. 2000; Doran et al. 2001; Ueberlhart et al. 2004) have also been reported. The analysis of results from these studies reveals clear differences based on gender, hormonal milieu, and ovarian functional status that are incompletely understood at the present time and await further investigations. Regarding the gonadal axis, the reported results on the effects of raloxifene on basal gonadotropin levels in postmenopausal women are not uniform. While treatment with 60 mg daily of raloxifene, the approved dose in humans, induced a significant decrease in FSH levels after 3 months (Reindollar et al. 2002), and of FSH and LH after 12 months of therapy (Cheng et al. 2004), other studies have reported no changes (Lasco et al. 2002; Garas et al. 2004). Although the reasons for these discrepancies are unclear, they may be due to the small sample size and the variety of doses used. Nevertheless, the reported decreases in gonadotropin levels after raloxifene treatment are of smaller magnitude than those described for tamoxifen, which suggests a less potent estrogen agonist effect of the former compound on the hypothalamus-pituitary-gonadotrope axis.

Like tamoxifen and toremifene, raloxifene induces a decrease in both baseline and TRH-induced PRL levels (Lasco et al. 2002; Cheng et al. 2004), which may indicate a direct antiestrogenic effect on the lactotrope or, alternatively, an increase in β-endorphin (Florio et al. 2001; Genazzani et al. 2003). The effects of raloxifene on sex hormones and SHBG in healthy or osteoporotic postmenopausal women are also similar to those of tamoxifen or toremifene. Thus, raloxifene increases SHBG by 21% (Coombes et al. 1995), without significant changes (Geisler et al. 1995a) or even producing small decreases (Doran et al. 2001) in serum estradiol levels.

The effects of raloxifene in premenopausal women have been analyzed in subjects with normal ovarian function treated with high doses (100 to 400 mg daily) at either different time points of their menstrual cycle or continuously for 4 weeks (Baker et al. 1998). Raloxifene did not prevent ovulation, nor did it alter the length of the menstrual cycle or the day of the LH surge. However, it did stimulate FSH secretion, increase serum estradiol levels, and decrease serum PRL. These results are also similar to those reported for premenopausal women taking tamoxifen (Jordan et al. 1991) and are indicative of some antiestrogenic action at either the hypothalamic and/or pituitary level.

The effects of arzoxifene, a third-generation SERM similar to raloxifene but with improved estrogen antagonistic activity in the breast and the uterus, has been recently investigated in two short-term phase I studies in pre- and postmenopausal women (Fabian et al. 2004). As observed for other SERMs, ar- zoxifene increased SHBG levels after 2 weeks of treatment and induced a slight reduction in serum LH levels, without affecting estradiol, estrone, or FSH serum concentrations. This probably indicates certain estrogenic properties on the gonadotrope axis and the liver similar to other SERMs clinically tested.

Finally, although it is beyond the scope of this review, the effects of raloxifene on the hypothalamus-pituitary axis of human males have been analyzed in few clinical trials. Even though different doses and treatment duration have been used, in contrast with the findings in postmenopausal women, raloxifene appears to increase serum gonadotropin levels in adult eugonadal men (Doran et al. 2001; Uebelhart et al. 2004) and either elevate or not affect serum estradiol and testosterone levels (Blum et al. 2000; Doran et al. 2001; Uebelhart et al. 2004). Since those subjects with low baseline estrogen levels displayed a higher response to both gonadotropin and sex hormones, it is possible that sex hormonal status may influence overall SERM actions in men. Further studies must be conducted before a clear relationship between gender and hormonal status could be established for the differential effects of SERMs in humans.

5.4

Summary and Conclusions

Ovarian hormones control female reproductive function by acting at different levels of the hypothalamus-pituitary-gonadal axis throughout the synergistic activation of several receptor subtypes. In most mammalian species, including the human, the activity of this highly coordinated system is aimed at maintaining reproductive cycles, leading to eventual pregnancy, and at promoting body adaptations to maternal metabolism. In the case of women, given the beneficial effects of sex steroid hormones on a variety of reproductive and nonreproductive tissues, the development of specific molecules capable of reproducing steroid hormone action after cessation of ovarian function with aging has been a main objective of modern pharmacology. As described in different parts of this book, SERMs are compounds that may act either as estrogen agonists or antagonists in a tissue- or cell-specific manner and therefore have the capacity to influence hypothalamus-pituitary function in a complex manner. In this chapter, we have analyzed the available evidence from both experimental and clinical studies in order to understand the impact of SERM treatment on gonadotropin secretion. Recent findings indicate that particular SERMs may interact with several ER-dependent pathways within the same cell, thereby inducing a variety of responses that are highly dependent on estrogen and progesterone status. This fact partially explains why treatment with different SERMs in human subjects frequently gives conflicting results depending on dose, age, gender, reproductive status, duration of treatment, and coadjuvant medication. Therefore, the use of newer SERMs in whole animal and cell studies not only will provide basic investigators with tools to dissect the biology of classical and alternative ERs but also will surely help to design specific and selective approaches in hormone therapy. In addition, the analysis of clinical trials with steroid hormone analogs from the consideration of integrated estrogen and progesterone molecular interactions will provide critical insight for drug development in this promising field.

Acknowledgments

Work from the authors’ laboratories has been supported by Grants BFI2002- 00485,1FD97-1065-C03-01/03, SAF2001-3614-C03-01/02, and SAF2004-08316- C02/01 (Ministerio de Ciencia y Tecnologi'a, Spain), PI2003/98 and PI2003/154 (Consejeri'a de Educacion, Gobierno de Canarias), and PI042640 (ISCIII, Min- isterio de Sanidad y Consumo, Spain). The authors also thank Lilly S.A. and AstraZeneca for financial support. M.G. holds a fellowship from the Spanish Network of Neurological Research (CIEN, ISCIII-C03/06), and R.M. is a fellow of the Ramon y Cajal Programme (Ministerio de Ciencia y Tecnologi'a, Spain).

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