Role of gonadotropin-releasing hormone (GnRH) in ovarian cancer

Determination of the binding mode of thienopyrimidinedione antagonists to the human gonadotropin releasing hormone receptor using structure-activity relationships, site-directed mutagenesis, and homology modeling

We have investigated the specific interactions of a series thienopyrimidinediones with the gonadotropin-releasing hormone receptor (GnRH-R). Competitive radioligand binding assays were used to determine the effect of several mutants on nonpeptide binding. Distinct interactions were observed in two separate regions: the N-terminal end of TM7 and the C-terminal end of TM6. The effects of mutants at D302((7.32)) and H306((7.36)) suggest that these residues are part of a hydrogen-bond network important for anchoring the nonpeptides. Structure-activity relationships indicated urea substituents on the 6-(4-aminophenyl) group with a trans conformational preference bind with high affinity and are sensitive to D302((7.32)) mutations. Another interaction area was found between the N-benzyl-N-methylamino substituent and L300((6.68)) and Y290((6.58)). These interaction sites facilitated the derivation of a model in which a representative member of the series was docked into GnRH-R. The model is consistent with known SAR and illuminates inconsistencies with previous hypotheses regarding how this series interacts with the receptor.

Allosteric and Orthosteric Binding Modes of Two Nonpeptide Human Gonadotropin-Releasing Hormone Receptor Antagonists

Nonpeptide antagonists of the human gonadotropin-releasing hormone receptor (GnRH-R) have been the subject of considerable interest because of their potential as a new class of oral therapeutics for the treatment of sex hormone-dependent diseases and infertility. While many classes of competitive GnRH-R antagonists have been described, we present here the first characterization of an allosteric nonpeptide GnRH-R antagonist. Previously, 5-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-ylmethyl)furan-2-carboxylic acid (2,4,6-trimethoxyphenyl)amide (here called Furan-1) had been demonstrated to be a potent GnRH-R antagonist both in vitro and in vivo. Using mutagenesis, the binding sites for Furan-1 and another potent nonpeptide antagonist (NBI-42902) have been mapped and are shown to be adjacent but nonoverlapping. Furan-1 is shown to affect the binding kinetics of radiolabeled peptide agonists as well as radiolabeled NBI-42902, and the kinetic data fit the allosteric ternary complex model. Furan-1 is considerably negatively cooperative with the nonpeptide antagonist and extremely negatively cooperative with the peptide agonist [125I-His5,d-Tyr6]GnRH so that it is nearly indistinguishable from an orthosteric competitive compound. Taken together, these data were used to develop a model of the nonpeptides bound to the GnRH-R binding site consistent with the current data.

Activin modulates the transcriptional response of LbetaT2 cells to gonadotropin-releasing hormone and alters cellular proliferation

Both GnRH and activin are crucial for the correct function of pituitary gonadotrope cells. GnRH regulates LH and FSH synthesis and secretion and gonadotrope proliferation, whereas activin is essential for expression of FSH. Little is known, however, about the interplay of signaling downstream of these two hormones. In this study, we undertook expression profiling to determine how activin pretreatment alters the transcriptional response of LbetaT2 gonadotrope cells to GnRH stimulation. Activin treatment alone altered the transcriptional profile of 303 genes including inducing that of the 17beta-hydroxysteroid dehydrogenase B1 gene that converts estrone to 17beta-estradiol, altering the sensitivity of the cells to estrone. Furthermore, activin had a dramatic effect on the response of LbetaT2 cells to GnRH. Hierarchical clustering of 2453 GnRH-responsive genes identified groups of genes the response of which to GnRH was either enhanced or blunted after activin treatment. Mapping of these genes to gene ontology classifications or signaling pathways highlighted significant differences in the classes of altered genes. In the presence of activin, GnRH regulates genes in pathways controlling cell energetics, cytoskeletal rearrangements, organelle organization, and mitosis in the absence of activin, but genes controlling protein processing, cell differentiation, and secretion. Therefore, we demonstrated that activin enhanced GnRH induction of p38MAPK activity, caused GnRH-dependent phosphorylation of p53, and reduced the ability of GnRH to cause G1 arrest. Thus, although activin alone changes a modest number of transcripts, activin pretreatment dramatically alters the response to GnRH from an antiproliferative response to a more differentiated, synthetic response appropriate for a secretory cell.

Gonadotropin-releasing hormone in apoptosis of prostate cancer cells

GnRH and its analogs (GnRH-a) are used extensively for the treatment of prostate cancer and other hormone-dependent diseases via the desensitization of pituitary gonadotropes, which consequently leads to the inhibition of gonadotropins, gonadal steroids and tumor growth. The actions of GnRH-a are mediated by the GnRH receptor (GnRHR) that is expressed in both the pituitary and extrapituitary sites, including normal tissues and tumors. Several studies have provided evidence that besides its pituitary effects, GnRH-a may exert direct anti-proliferative and apoptotic effects in tumor cells. These effects are mediated by the GnRHRs via signal transduction mechanisms that are distinct from the classical pituitary mechanisms. Here we describe the direct effects of GnRH-a on prostate cancer and other types of cancer. Interestingly, androgen ablation by GnRH-a is the main treatment for hormone-dependent prostate cancer. However, most of these tumors become eventually hormone-refractory, and are no longer sensitive to the GnRH-a-mediated reduction in androgen levels. Hence, the ability of GnRH-a to induce direct effects such as apoptosis may have large implications regarding the clinical use of GnRH-a. Therefore, an understanding of the cellular mechanisms involved in GnRH-a action may lead to better therapeutic modalities for the treatment of advanced prostate cancer and other malignancies.

Role of gonadotropin-releasing hormone II in the mammalian nervous system

Gonadotropin-releasing hormone (GnRH) is a small neuropeptide of which there are multiple structural variants. The first variant identified in mammals, GnRH I, controls the release of pituitary gonadotropins. More recently, a second isoform, GnRH II, first isolated in the bird, was identified in the mammalian brain and periphery. Although it is unlikely to be a primary regulator of gonadotropin release, GnRH II appears to have a wide array of physiological and behavioral functions. GnRH II-containing fibers are present in several nuclei known to regulate reproduction and/or feeding, and its concentration in several of these areas fluctuates in response to changes in food availability, and thus energetic status. In musk shrews, GnRH II acts as a permissive regulator of female reproductive behavior based on energy status, as well as an inhibitor of short-term food intake. In this regard, GnRH II is similar to leptin, neuropeptide Y and several other neurotransmitters that regulate both feeding and reproduction. At least two GnRH receptors are present in the mammalian brain, and increasing evidence suggests that the behavioral effects of GnRH II are mediated by receptor subtypes distinct from the type-1 GnRH receptor (which mediates GnRH I action); the most probable candidate is the type-2 GnRH receptor. GnRH II also regulates the density and/or activity of calcium and potassium channels in the nervous systems of amphibians and fish, a function that may also exist in mammalian neurons. It is likely that the highly conserved GnRH II system has been co-opted over evolutionary time to possess multiple regulatory functions in a broad range of neurobiological aspects.

Overlapping, Nonidentical Binding Sites of Different Classes of Nonpeptide Antagonists for the Human Gonadotropin-Releasing Hormone Receptor

Peptide agonists and antagonists of the human gonadotropin-releasing hormone receptor (GnRH-R) are widely used to treat a range of reproductive hormone related diseases. Recently, nonpeptide, orally available GnRH-R antagonists have emerged from several chemical classes. To understand how a relatively large peptide-binding pocket can recognize numerous nonpeptide ligands, we undertook a systematic mapping of GnRH-R residues involved in the binding of three nonpeptide antagonists. A region composed of the extracellular portions of transmembrane helices 6 and 7, extracellular loop 3, and the N-terminal domain significantly contributed to nonpeptide antagonist binding. However, each molecule was affected by a different subset of residues in these regions, indicating that each appears to occupy distinct, partially overlapping subregions within the more extensive peptide-binding pocket. Moreover, the resulting receptor interaction maps provide a basis to begin to reconcile structure-activity relationships between various nonpeptide and peptide series and facilitate the design of improved therapeutic agents.

Regulation of expression of mammalian gonadotrophin-releasing hormone receptor genes

Gonadotrophin-releasing hormone (GnRH), acting via its cognate GnRH receptor (GnRHR), is the primary regulator of mammalian reproductive function, and hence GnRH analogues are extensively used in the treatment of hormone-dependent diseases, as well as for assisted reproductive techniques. In addition to its established endocrine role in gonadotrophin regulation in the pituitary, evidence is rapidly accumulating to support the expression and functional roles for two forms of GnRHR (GnRHR I and GnRHR II) in multiple and diverse extra-pituitary mammalian tissues and cells. These findings, together with findings indicating that mutations of the GnRHR are linked to the disease hypogonadotrophic hypogonadism and that GnRHRs play a direct role in neuronal migration and reproductive cancers, have presented new therapeutic targets and intensified research into the structure, function and mechanisms of regulation of expression of GnRHR genes. The present review focuses on the current knowledge on tissue-specific and hormonal regulation of transcription of mammalian GnRH receptor genes. Emerging insights, such as the discovery of diverse regulatory mechanisms in pituitary and extra-pituitary cell types, nonclassical mechanisms of steroid regulation, the use of composite elements for cell-specific expression, the increasing profile of hormones involved in regulation, the complexity of kinase pathways that target the GnRHR I gene, as well as species-differences, are highlighted. Although further research is necessary to understand the mechanisms of regulation of expression of GnRHR I and GnRHR II genes, the GnRHR is emerging as a potential target gene for facilitating cross-talk between neuroendocrine, immune and stress-response systems in multiple tissues via autocrine, paracrine and endocrine signalling.

Differential gonadotropin releasing hormone (GnRH) expression, autoregulation and effects in two models of rat luteinized ovarian cells

GnRH has been suggested to participate in corpus luteum function. Here we studied the expression of GnRH mRNA and peptide in two models of rat luteinized tissues: ovarian cells from PMSG-hCG treated prepubertal rats (SPO) and from intrasplenic ovarian tumors (Luteoma). A GnRH autoregulatory effect was evaluated as well as its action on cell proliferation and apoptosis. GnRH mRNA was present in SPO, isolated corpora lutea from SPO and Luteoma from 1 week to 7 months of development. In vitro cultures of Luteoma cells expressed 2-fold higher GnRH mRNA and 10-fold higher GnRH peptide than SPO cells. Buserelin (GnRH analog) increased GnRH mRNA and peptide expression in SPO but not in Luteoma cells. While basal proliferation was very low in Luteoma cells, SPO cells showed a significant increase in cell number by both the thymidine and the MTS methods after 72 h in culture. Buserelin induced a decrease in cell number in both cell types to a similar degree. Although basal apoptosis levels were higher in SPO than in Luteoma cells, Buserelin-induced apoptosis was only detected in Luteoma cells after 48 h treatment. These results show that the two types of rat, luteinized tissues, Luteoma and SPO, markedly differed in some intrinsic properties and in their local GnRH systems. Luteoma cells proliferate very weakly, express and secrete high amounts of GnRH, do not show an autoregulatory effect and respond to the decapeptide with apoptosis stimulation. In contrast SPO cells proliferate significantly, secrete low levels of GnRH but possess a positive, autoregulatory mechanism and respond to GnRH stimulation with impairment of proliferation.

Differential effects of gonadotropin-releasing hormone (GnRH)-I and GnRH-II on prostate cancer cell signaling and death

CONTEXT

GnRH is known to directly regulate prostate cancer cell proliferation, but the precise mechanism of action of the peptide is still under investigation.

OBJECTIVE

This study demonstrates differential effects of GnRH-I and GnRH-II on androgen-independent human prostate cancer cells.

RESULTS

Both GnRH-I and GnRH-II increased the intracellular Ca(2+) concentration ([Ca(2+)](i)) either through Ca(2+) influx from external Ca(2+) source or via mobilization of Ca(2+) from internal Ca(2+) stores. Interestingly, the [Ca(2+)](i) increase was mediated by activation of the ryanodine receptor but not the inositol trisphosphate receptor. Trptorelix-1, a novel GnRH-II antagonist but not cetrorelix, a classical GnRH-I antagonist, completely inhibited the GnRH-II-induced [Ca(2+)](i) increase. Concurrently at high concentrations, trptorelix-1 and cetrorelix inhibited GnRH-I-induced [Ca(2+)](i) increase, whereas at low concentrations they exerted an agonistic action, inducing Ca(2+) influx. High concentrations of trptorelix-1 but not cetrorelix-induced prostate cancer cell death, probably through an apoptotic process. Using photoaffinity labeling with (125)I-[azidobenzoyl-D-Lys(6)]GnRH-II, we observed that an 80-kDa protein specifically bound to GnRH-II.

CONCLUSIONS

This study suggests the existence of a novel GnRH-II binding protein, in addition to a conventional GnRH-I receptor, in prostate cancer cells. These data may facilitate the development of innovatory therapeutic drugs for the treatment of prostate cancer.

Carcinogenic potential of ovulatory genotoxicity

Ovulation is a rate-limiting event for the perpetuation of a species; unfortunately, it imparts a cancer risk. Reactive oxidants generated during the mechanics of ovulatory follicular rupture damage the DNA of ovarian surface epithelial cells that are located within a limited diffusion radius. Those cells that survive the trauma of ovulation, along the margins of a ruptured follicle, proliferate and migrate to reconcile the discontinuity within the ovarian epithelium created at the site of oocyte release. It is conceivable that clonal expansion of an ovarian surface epithelial cell with unrepaired DNA, but not committed to death, could be an initiating factor in the etiology of common ovarian cancer. In fact, the majority of cancers of the ovary are derived from the surface epithelium; and circumstances that avert ovulation (oral contraceptive use, pregnancy/lactation) protect against ovarian adenocarcinoma. Not surprisingly, the genotoxic potential of ovulation is exacerbated by malfunctions in tumor suppressor/cell-cycle arrest and base-excision repair mechanisms. Recent experimental evidence indicates that vitamin E and progesterone protect against ovarian metaplasia by negating the oxidative stress of ovulation and by enhancing the repair capacity (genomic integrity) of the surface epithelium, respectively. Ovarian cancer of surface epithelial origin is a deadly insidious disease because it characteristically remains asymptomatic until it has metastasized throughout the abdominal cavity; therefore, prevention is a high priority.

Inhibition of human type i gonadotropin-releasing hormone receptor (GnRHR) function by expression of a human type II GnRHR gene fragment

Humans possess only one functional GnRH receptor, the type I GnRH receptor (GnRHR-I). A type II GnRH receptor (GnRHR-II) gene homolog exists, but it is disrupted by a frame shift and premature stop codon, suggesting that a conventional receptor is not translated from this gene. However, the gene remains transcriptionally active and displays alternative splicing. We identified a putative translational start site 117 bp downstream of the premature stop codon. Use of this start codon encodes a protein (designated as the GnRHR-II-reliquum) corresponding to the domains from the cytoplasmic end of transmembrane domain-5 to the carboxyl terminus of the putative full-length receptor. Immunocytochemistry revealed that GnRHR-II-reliquum expression appeared to be localized throughout the cytoplasm. Transient cotransfection of GnRHR-I and GnRHR-II-reliquum constructs into COS-7 cells resulted in reduced expression of the GnRHR-I at the cell surface and impaired signaling via the GnRHR-I as revealed by reduction of GnRH-induced inositol phosphate accumulation. This inhibitory effect was specific and dependent on the degree of GnRHR-II-reliquum coexpressed. Immunoblot analysis revealed that the total cell GnRHR-I complement, i.e. both cell-surface and nascent intracellular receptors, was markedly reduced by coexpression of the GnRHR-II-reliquum. Treatments with cell-permeable agents that blocked either de novo protein synthesis (cycloheximide) or proteinase-mediated degradation (leupeptin and phenylmethylsulfonyl fluoride) failed to alter the inhibitory effect of GnRHR-II-reliquum coexpression, suggesting that the inhibitory effect is exerted at the nucleus/endoplasmic reticulum or Golgi apparatus level, possibly by perturbing normal processing of GnRHR-I from these sites. We suggest that the GnRHR-II-reliquum plays a modulatory role in GnRHR-I expression.

Gonadotropin-releasing hormone antagonist cetrorelix down-regulates proliferating cell nuclear antigen and epidermal growth factor expression and up-regulates apoptosis in association with enhanced poly(adenosine 5'-diphosphate-ribose) polymerase expression in cultured human leiomyoma cells

The objective of this study was to elucidate the effects of GnRH antagonist Cetrorelix on proliferation and apoptosis in human leiomyoma cells cultured in vitro. Isolated leiomyoma cells were subcultured in phenol red-free DMEM supplemented with 10% fetal bovine serum for 120 h and then stepped down to serum-free conditions in the presence or absence of graded concentrations of Cetrorelix (10(-5) to 10(-8) mol/liter) for 6 d. Cultured leiomyoma cells were used for semiquantitative RT-PCR, immunocytochemistry, Western blot analysis, and terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate nick-end labeling assay. RT-PCR analysis revealed the presence of mRNAs encoding for GnRH receptor and epidermal growth factor (EGF) in cultured leiomyoma cells. The number of viable cultured leiomyoma cells was significantly (P < 0.01) decreased by treatment with Cetrorelix compared with untreated control cultures. Immunocytochemical examination demonstrated that treatment with Cetrorelix attenuated the expression of proliferating cell nuclear antigen (PCNA) and EGF in cultured leiomyoma cells. Western blot analysis revealed that treatment with 10(-5) mol/liter Cetrorelix significantly (P < 0.01) decreased PCNA expression. In addition, treatment with 10(-5) mol/liter Cetrorelix remarkably increased the terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate nick-end labeling-positive rate and poly(ADP-ribose) polymerase expression at 24 h of treatment compared with untreated control cultures (P < 0.01). Furthermore, treatment with 10(-5) mol/liter Cetrorelix decreased immunoreactive EGF protein and EGF mRNA expression in cultured leiomyoma cells at 4 d of treatment. GnRH antagonist Cetrorelix may directly inhibit leiomyoma cell growth by down-regulating proliferation in association with a decrease in EGF mRNA expression and by up-regulating apoptosis in those cells.

Newly recognized GnRH receptors: function and relative role

Hypothalamic gonadotropin releasing hormone (GnRH I) and its pituitary receptor are responsible for the CNS regulation of reproduction. However, a second GnRH (GnRH II) is also expressed in humans and a gene that resembles the GnRH II receptor in fish has been identified in humans and monkeys. The amino-acid sequence of this newly identified, seven-transmembrane, G-protein-coupled receptor in monkeys differs from the human GnRH I receptor by having a C-terminal, cytoplasmic tail. GnRH II is approximately 400-fold more potent at GnRH II receptors than GnRH I receptors. GnRH I directly inhibits proliferation of human tumor cells, and GnRH II and its receptor might have a similar role. Limited progress has been made, however, because of difficulty translating the mRNA that encodes the human GnRH II receptor. Nevertheless, such receptors are likely to exist in humans because GnRH II is more inhibitory to tumor cell replication than GnRH I, and GnRH I and GnRH II have reciprocal effects on human decidual stromal cells in culture. The focus of this review is the identity of a possible translatable, functional GnRH II receptor in humans. The two possibilities considered are either that GnRH II receptor mRNA is expressed that encodes either 5 or 7 transmembrane domains or that a GnRH II-responsive complex is formed by the GnRH I receptor and fragments derived from the GnRH II receptor.

A critical role for the evolutionarily conserved gonadotropin-releasing hormone II: mediation of energy status and female sexual behavior

GnRH is an evolutionarily conserved neuropeptide, of which there are multiple structural variants; the function of the most widespread variant, GnRH-II, remains undefined. GnRH-II may affect reproductive behavior; GnRH-II administration to female musk shrews reinstates mating behavior previously inhibited by food restriction. To determine whether this action of GnRH-II is universal, we conducted the following studies in mice. Ovariectomized mice were primed with estradiol benzoate and progesterone once a week and tested for sexual behavior. Females showing a lordosis quotient (LQ) of 50 or higher on the fourth trial underwent food deprivation (FD) for either 24 or 48 h before an additional behavior test. FD for 48 h significantly reduced LQ compared with ad libitum-fed females. Next, females were FD for 48 h or maintained on ad libitum feeding and retested for sexual behavior after an intracerebroventricular infusion of either GnRH-I, GnRH-II, or saline. GnRH-II, but not GnRH-I, significantly increased LQ in FD females compared with FD females treated with saline. Lordosis was unaffected by GnRH-II in females maintained on ad libitum feeding. To assess whether the GnRH-I receptor mediates GnRH-II's behavioral effects, underfed females were pretreated with the type 1 GnRH receptor antagonist Antide and retested for sexual behavior. Antide pretreatment did not prevent GnRH-II from promoting mating behavior, suggesting that GnRH-II's behavioral actions are mediated through the type 2 GnRH receptor. We speculate that GnRH-II acts via its own receptor as a regulatory signal in mammals to ensure that reproduction is synchronized with energetically favorable conditions.

Expression of gonadotropin-releasing hormone receptor and effect of gonadotropin-releasing hormone analogue on proliferation of cultured gastric smooth muscle cells of rats

AIM: To investigate the expression of gonadotropin-releasing hormone (GnRH) receptor and the effects of GnRH analog (alarelin) on proliferation of cultured gastric smooth muscle cells (GSMC) of rats.

METHODS: Immunohistochemical ABC methods and in situ hybridization methods were used to dectect protein and mRNA expression of GnRH receptor in GSMC, respectively. Techniques of cell culture, OD value of MTT test, measure of ³H-TdR incorporation, average fluorescent values of proliferating cell nuclear antigen (PCNA) and flow cytometric DNA analysis were used in the experiment.

RESULTS: The cultured GSMC of rats showed immunoreactivity for GnRH receptor; positive staining was located in cytoplasm. GnRH receptor mRNA hybridized signals were also detected in cytoplasm. When alarelin (10^-9, 10^-7, 10^-5 mol/L) was administered into the medium and incubated for 24 h, OD value of MTT, ³H-TdR incorporation and average fluorescent values of PCNA all decreased significantly as compared with the control group (P < 0.05). The maximum inhibitory effect on cell proliferation was achieved a concentration of 10^-5 mol/L and it acted in a dose-dependent manner. Flow cytometric DNA analysis revealed that alarelin could significantly enhance ratio of G₁ phase and decrease ratio of S phase of GSMC of rats (P < 0.05).The maximum inhibitory effect on ratio of S phase was at the concentration of 10^-5 mol/L and also acted in a dose-dependent manner.

CONCLUSION: Our data suggest that GnRH receptor can be expressed by GSMC of rats. GnRH analogue can directly inhibit proliferation and DNA synthesis of rat GSMC through GnRH receptors.

Evidence for different gonadotropin-releasing hormone response sites in rat ovarian and pituitary cells

The participation of type I GnRH receptor (GnRH-R) on GnRH-II-induced gonadotropin secretion in rat pituitary cells was investigated. Furthermore, we extended the study of GnRH-II action to ovarian cells. The GnRH-II was able to mobilize inositol triphosphate (IP(3)) and to induce LH and FSH release in a dose-dependent manner in pituitary cells and in a GnRH-I-like manner. The GnRH-analog 135-18 (agonist for type II GnRH-R and antagonist for type I GnRH-R) was unable to elicit any cellular response tested in these pituitary cells. The GnRH-II responses were blocked by the type I GnRH-R-antagonists CRX or 135-18, suggesting that these effects were mediated by the type I GnRH-R. In contrast to pituitary cells, GnRH-I, but not GnRH-II, elicited an IP(3) response in superovulated ovarian cells; 135-18 also had no effect. However, GnRH-II as well as GnRH-I presented antiproliferative effects on these cells. Surprisingly, 135-18 had stronger antiproliferative effects than either GnRH peptide. The 135-18 analog, but not GnRH-I or GnRH-II, increased progesterone secretion in superovulated ovarian cells. These results strongly suggest that GnRH-II is able to stimulate rat pituitary cells through the type I GnRH-R, with no evidence for the presence of type II GnRH-R. On the other hand, our results indicate a putative GnRH-R in superovulated ovarian cells with response characteristics that differ from those of the GnRH-R in the pituitary.