Evidence that prostate gonadotropin-releasing hormone receptors mediate an anti-tumourigenic response to analogue therapy in hormone refractory prostate cancer

Dissecting the Hormonal Signaling Landscape in Castration-Resistant Prostate Cancer

Understanding the molecular mechanisms underlying prostate cancer (PCa) progression towards its most aggressive, castration-resistant (CRPC) stage is urgently needed to improve the therapeutic options for this almost incurable pathology. Interestingly, CRPC is known to be characterized by a peculiar hormonal landscape. It is now well established that the androgen/androgen receptor (AR) axis is still active in CRPC cells. The persistent activity of this axis in PCa progression has been shown to be related to different mechanisms, such as intratumoral androgen synthesis, AR amplification and mutations, AR mRNA alternative splicing, increased expression/activity of AR-related transcription factors and coregulators. The hypothalamic gonadotropin-releasing hormone (GnRH), by binding to its specific receptors (GnRH-Rs) at the pituitary level, plays a pivotal role in the regulation of the reproductive functions. GnRH and GnRH-R are also expressed in different types of tumors, including PCa. Specifically, it has been demonstrated that, in CRPC cells, the activation of GnRH-Rs is associated with a significant antiproliferative/proapoptotic, antimetastatic and antiangiogenic activity. This antitumor activity is mainly mediated by the GnRH-R-associated Gαi/cAMP signaling pathway. In this review, we dissect the molecular mechanisms underlying the role of the androgen/AR and GnRH/GnRH-R axes in CRPC progression and the possible therapeutic implications.

Gonadotropin-Releasing Hormone Receptors in Prostate Cancer: Molecular Aspects and Biological Functions

Pituitary Gonadotropin-Releasing Hormone receptors (GnRH-R) mediate the activity of the hypothalamic decapeptide GnRH, thus playing a key role in the regulation of the reproductive axis. Early-stage prostate cancer (PCa) is dependent on serum androgen levels, and androgen-deprivation therapy (ADT), based on GnRH agonists and antagonists, represents the standard therapeutic approach for PCa patients. Unfortunately, the tumor often progresses towards the more aggressive castration-resistant prostate cancer (CRPC) stage. GnRH receptors are also expressed in CRPC tissues, where their binding to both GnRH agonists and antagonists is associated with significant antiproliferative/proapoptotic, antimetastatic and antiangiogenic effects, mediated by the Gαi/cAMP signaling cascade. GnRH agonists and antagonists are now considered as an effective therapeutic strategy for CRPC patients with many clinical trials demonstrating that the combined use of these drugs with standard therapies (i.e., docetaxel, enzalutamide, abiraterone) significantly improves disease-free survival. In this context, GnRH-based bioconjugates (cytotoxic drugs covalently linked to a GnRH-based decapeptide) have been recently developed. The rationale of this treatment is that the GnRH peptide selectively binds to its receptors, delivering the cytotoxic drug to CRPC cells while sparing nontumor cells. Some of these compounds have already entered clinical trials.

Stress alters the expression of cancer-related genes in the prostate

Background

Prostate cancer is a major contributor to mortality worldwide, and significant efforts are being undertaken to decipher specific cellular and molecular pathways underlying the disease. Chronic stress is known to suppress reproductive function and promote tumor progression in several cancer models, but our understanding of the mechanisms through which stress contributes to cancer development and progression is incomplete. We therefore examined the relationship between stress, modulation of the gonadotropin-releasing hormone (GnRH) system, and changes in the expression of cancer-related genes in the rat prostate.

Methods

Adult male rats were acutely or repeatedly exposed to restraint stress, and compared to unstressed controls and groups that were allowed 14 days of recovery from the stress. Prostate tissue was collected and frozen for gene expression analyses by PCR array before the rats were transcardially perfused; and brain tissues harvested and immunohistochemically stained for Fos to determine neuronal activation.

Results

Acute stress elevated Fos expression in the paraventricular nucleus of the hypothalamus (PVH), an effect that habituated with repeated stress exposure. Data from the PCR arrays showed that repeated stress significantly increases the transcript levels of several genes associated with cellular proliferation, including proto-oncogenes. Data from another array platform showed that both acute and repeated stress can induce significant changes in metastatic gene expression. The functional diversity of genes with altered expression, which includes transcription factors, growth factor receptors, apoptotic genes, and extracellular matrix components, suggests that stress is able to induce aberrant changes in pathways that are deregulated in prostate cancer.

Conclusions

Our findings further support the notion that stress can affect cancer outcomes, perhaps by interfering with neuroendocrine mechanisms involved in the control of reproduction.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3635-4) contains supplementary material, which is available to authorized users.

The gonadotropin-releasing hormone system: Perspectives from reproduction to cancer (Review)

Recently, an increasing amount of evidence indicates that human gonadotropin-releasing hormone (hGnRH) and its receptor (hGnRHR) are important regulatory components not only to the reproduction process but also in the regulation of some cancer cell functions such as cell proliferation, in both hormone-dependent and -independent types of tumors. The hGnRHR is a naturally misfolded protein that is retained mostly in the endoplasmic reticulum; however, this mechanism can be overcome by treatment with several pharmacoperones, therefore, increasing the amount of receptors in the cell membrane. In addition, several reports indicate that the expression level of hGnRHR in tumor cells is even lower than in pituitary or gonadotrope cells. The signal transduction pathways activated by hGnRH in both gonadotrope and different cancer cell types are described in the present review. We also discuss how the rescue of misfolded receptors in tumor cells could be a promising strategy for cancer therapy.

Oxime bond-linked daunorubicin-GnRH-III bioconjugates exert antitumor activity in castration-resistant prostate cancer cells via the type I GnRH receptor

It is well established that gonadotropin-releasing hormone receptors (GnRH-R) are expressed in different types of cancers, including castration-resistant prostate cancer (CRPC) and mediate the antiproliferative effect of GnRH analogs. Thus, these compounds are employed as targeting moieties to selectively deliver chemotherapeutic agents to cancer cells. GnRH-III, the decapeptide isolated from the sea lamprey brain, has lower potency than GnRH in stimulating gonadotropin secretion, but it exerts antiproliferative effects on many tumors expressing the GnRH-R. GnRH-III-based peptides are considered promising targeting moieties for the preparation of anticancer drug delivery systems. These studies were aimed at i) evaluating the antitumor activity of two cytotoxic oxime bond-linked daunorubicin (Dau)-GnRH-III derivative bioconjugates (Dau-GnRH-III, in which daunorubicin was coupled to the 8Lys in the native form of GnRH-III, and Dau-[4Lys(Ac)]-GnRH-III, in which daunorubicin was attached to the 8Lys of a GnRH-III derivative where 4Ser was replaced by an acetylated lysine) on CRPC cells; and ii) to elucidate the involvement of the classical GnRH-R (type I GnRH-R) in this antitumor activity. Our results demonstrated that both Dau-GnRH-III and Dau-[4Lys(Ac)]-GnRH-III were rapidly internalized into DU145 prostate cancer cells and exerted a significant cytostatic effect. Both bioconjugates increased the levels of the active form of caspase-3, indicating the involvement of apoptosis in their antitumor activity. The antiproliferative effect of both Dau-GnRH-III and Dau-[4Lys(Ac)]-GnRH-III was counteracted by the simultaneous treatment of the cells with Antide, an antagonist of the GnRH-R. Moreover, after silencing the type I GnRH-R the antitumor activity of both bioconjugates was completely abolished. These data demonstrate that in CRPC cells, daunorubicin-GnRH-III derivative bioconjugates: i) inhibit tumor cell proliferation, by triggering the apoptosis process; ii) exert their antitumor effect through the activation of the type I GnRH-R expressed on these cells. Cytotoxic-GnRH-III derivative may represent promising targeted chemotherapeutics for the treatment of CRPC patients.

Gonadotropin-releasing hormone agonists sensitize, and resensitize, prostate cancer cells to docetaxel in a p53-dependent manner

Gonadotropin-releasing hormone (GnRH) receptors are expressed in prostate cancer, specifically in the most aggressive stage of the tumor (castration-resistant prostate cancer, CRPC) for which the standard treatment, docetaxel-based chemotherapy, can only improve the median survival time by few months. We previously showed that GnRH agonists exert an antitumor activity in CRPC cells; however, a link between GnRH receptors and the apoptotic machinery remains to be defined. Aim of this study was to evaluate whether, in CRPC cells, GnRH agonists might affect the expression/activity of apoptosis-related proteins and might sensitize, or resensitize, cancer cells to chemotherapeutics. We demonstrated that, in p53-positive DU145 cells, GnRH agonists: a) increase the expression of the proapoptotic protein Bax; this effect is mediated by the phosphorylation (activation) of p53, triggered by the p38 MAPK; b) potentiate the antiproliferative/proapoptotic activity of docetaxel; c) resensitize docetaxel-resistant cells to the antitumor activity of the cytotoxic drug. These data indicate that GnRH agonists sensitize and, more importantly, resensitize DU145 CRPC cells to chemotherapy in a p53-dependent manner. To confirm the crucial role of p53 in the activity of GnRH agonists, experiments were performed in p53-null PC3 cells. We found that GnRH agonists fail to increase Bax expression and do not potentiate the cytotoxic activity of docetaxel. These results may provide a rationale for novel combination treatment strategies, especially for docetaxel-resistant CRPC patients expressing a functional p53 protein.

Treatment of high risk Sertoli-Leydig cell tumors of the ovary using a gonadotropin releasing hormone (GnRH) analog

Sertoli-Leydig cell tumors are rare ovarian neoplasms. We report two unusual cases with bilateral SLCTs suggesting evidence of genetic predisposition and at high risk of recurrence. To reduce this risk, we exploited the use of GnRH analog to lower gondadotropin and potentially directly inhibit the tumors through expressed GnRH receptors. We used it as maintenance antitumor therapy for 2 years after completion of chemotherapy, to cover the period of risk for recurrence. Both patients remain in complete remission at >2 years after completing leuprorelin therapy. Of note, both patients carry DICER1 mutations, frequently found in pleuropulmonary blastoma syndrome.

GnRH receptors in cancer: from cell biology to novel targeted therapeutic strategies

The crucial role of pituitary GnRH receptors (GnRH-R) in the control of reproductive functions is well established. These receptors are the target of GnRH agonists (through receptor desensitization) and antagonists (through receptor blockade) for the treatment of steroid-dependent pathologies, including hormone-dependent tumors. It has also become increasingly clear that GnRH-R are expressed in cancer tissues, either related (i.e. prostate, breast, endometrial, and ovarian cancers) or unrelated (i.e. melanoma, glioblastoma, lung, and pancreatic cancers) to the reproductive system. In hormone-related tumors, GnRH-R appear to be expressed even when the tumor has escaped steroid dependence (such as castration-resistant prostate cancer). These receptors are coupled to a G(αi)-mediated intracellular signaling pathway. Activation of tumor GnRH-R by means of GnRH agonists elicits a strong antiproliferative, antimetastatic, and antiangiogenic (more recently demonstrated) activity. Interestingly, GnRH antagonists have also been shown to elicit a direct antitumor effect; thus, these compounds behave as antagonists of GnRH-R at the pituitary level and as agonists of the same receptors expressed in tumors. According to the ligand-induced selective-signaling theory, GnRH-R might assume various conformations, endowed with different activities for GnRH analogs and with different intracellular signaling pathways, according to the cell context. Based on these consistent experimental observations, tumor GnRH-R are now considered a very interesting candidate for novel molecular, GnRH analog-based, targeted strategies for the treatment of tumors expressing these receptors. These agents include GnRH agonists and antagonists, GnRH analog-based cytotoxic (i.e. doxorubicin) or nutraceutic (i.e. curcumin) hybrids, and GnRH-R-targeted nanoparticles delivering anticancer compounds.

New hormonal therapies for castration-resistant prostate cancer

Continued activation of the androgen receptor (AR) axis despite castration remains a critical force in the development of castration-resistant prostate cancer (CRPC). Therapeutic strategies designed to more effectively ablate tumoral androgen activity are required to improve clinical efficacy and prevent disease progression. Tumor-based alterations in expression and activity of the AR and in steroidogenic pathways mediating ligand generation facilitate the development of CRPC. This article reviews AR and ligand-dependent mechanisms underlying CRPC progression and the status of novel hormonal therapies targeting the AR axis that are currently in clinical and preclinical development.

Agonists and antagonists of GnRH-I and -II reduce metastasis formation by triple-negative human breast cancer cells in vivo

Metastasis to bone is a frequent problem of advanced breast cancer. Particularly breast cancers, which do not express estrogen and progesterone receptors and which have no overexpression/amplification of the HER2-neu gene, so called triple-negative breast cancers, are considered as very aggressive and possess a bad prognosis. About 60% of all human breast cancers and about 74% of triple-negative breast cancers express receptors for gonadotropin-releasing hormone (GnRH), which might be used as a therapeutic target. Recently, we could show that bone-directed invasion of human breast cancer cells in vitro is time- and dose-dependently reduced by GnRH analogs. In the present study, we have analyzed whether GnRH analogs are able to reduce metastases of triple-negative breast cancers in vivo. In addition, we have evaluated the effects of GnRH analogs on tumor growth. To quantify formation of metastasis by triple-negative MDA-MB-435 and MDA-MB-231 human breast cancers, we used a real-time PCR method based on detection of human-specific alu sequences measuring accurately the amount of human tumor DNA in athymic mouse organs. To analyze tumor growth, the volumes of breast cancer xenotransplants into nude mice were measured. We could demonstrate that GnRH analogs significantly reduced metastasis formation by triple-negative breast cancer in vivo. In addition, we could show that GnRH analogs significantly inhibited the growth of breast cancer into nude mice. Side effects were not detectable. In conclusion, GnRH analogs seem to be suitable drugs for an efficacious therapy for triple-negative, GnRH receptor-positive human breast cancers to prevent metastasis formation.

Dual targeting of tumor and endothelial cells by gonadotropin-releasing hormone agonists to reduce melanoma angiogenesis

We showed previously that GnRH receptors are expressed in melanoma cells; their activation reduces cell growth and metastatic behavior. Here, we investigated whether GnRH agonists might affect the expression of genes involved in melanoma progression. By genome-wide transcriptomic and real-time PCR analysis, we first observed that GnRH agonists decrease the expression of the pro-angiogenic factor vascular endothelial growth factor (VEGF) (all isoforms) in BLM melanoma cells. Then, we demonstrated that GnRH agonists specifically decrease the expression of the VEGF165 isoform as well as its secretion from BLM cells. These data suggested that activation of GnRH receptors might reduce the pro-angiogenic behavior of melanoma cells. To verify this hypothesis, we treated BLM cells with a GnRH agonist; the conditioned medium from these cells was tested to assess its capability to stimulate human umbilical vein endothelial cell (HUVEC) motility. The migration of HUVECs towards the conditioned medium of GnRH agonist-treated BLM cells was significantly lower than the migration of HUVECs toward the conditioned medium of untreated cells. Thus, GnRH agonists reduce the pro-angiogenic behavior of melanoma cells through a decreased production of bioactive VEGF. We then found that GnRH receptors are also expressed on HUVECs and that GnRH agonists reduce their ability to proliferate and to form capillary-like tubes when stimulated by VEGF. These findings suggest that GnRH agonists exert an anti-angiogenic activity indirectly by decreasing VEGF secretion from tumor cells and directly by counteracting the pro-angiogenic activity of the growth factor. These data might lead to the development of novel targeted approaches for melanoma.

Growth inhibition of tumor cells in vitro by using monoclonal antibodies against gonadotropin-releasing hormone receptor

As the continuation of a previous study, synthetic peptides corresponding to the extracellular domains of human gonadotropin-releasing hormone (GnRH) receptor were used to generate additional monoclonal antibodies which were further characterized biochemically and immunologically. Among those identified to recognize GnRH receptor, monoclonal antibodies designated as GHR-103, GHR-106 and GHR-114 were found to exhibit high affinity (Kd < or = 1 x 10(-8) M) and specificity to GnRH receptor as judged by the whole cell binding immunoassay and Western blot assay. Both anti-GnRH receptor monoclonal antibodies and GnRH were shown to compete for the same binding site of GnRH receptor on the surface of cultured cancer cells. Growth inhibitions of cancer cells cultured in vitro were demonstrated by cellular apoptosis experiments (TUNEL and MTT assays) under different conditions of treatment with GHR-106 monoclonal antibody or GnRH analogs. It was generally observed that both GnRH I and GHR-106 effectively induce the apoptosis of cultured cancer cells as determined by TUNEL and MTT assays. Consistently, suppressions of gene expressions at mRNA levels were demonstrated with several ribosomal proteins (P0, P1, P2 and L37), when cancer cells were incubated with GnRH or GHR-106. The widespread expressions of GnRH receptor in almost all of the studied human cancer cell lines were also demonstrated by RT-PCR and Western blot assay, as well as indirect immunofluorescence assay with either of these monoclonal antibodies as the primary antibody. In view of the longer half life of antibodies as compared to that of GnRH or its analogs, anti-GnRH receptor monoclonal antibodies in humanized forms could function as GnRH analogs and serve as an ideal candidate of anti-cancer drugs for therapeutic treatments of various cancers in humans as well as for fertility regulations.

Is the metalloendopeptidase EC 3.4.24.15 (EP24.15), the enzyme that cleaves luteinizing hormone-releasing hormone (LHRH), an activating enzyme?

LHRH (GNRH) was first isolated in the mammalian hypothalamus and shown to be the primary regulator of the reproductive neuroendocrine axis comprising of the hypothalamus, pituitary and gonads. LHRH acts centrally through its initiation of pituitary gonadotrophin release. Since its discovery, this form of LHRH (LHRH-I) has been shown to be one of over 20 structural variants with a variety of roles in both the brain and peripheral tissues. LHRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15) that cleaves the hormone at the fifth and sixth bond of the decapeptide (Tyr5-Gly6) to form LHRH-(1–5). We have previously reported that the auto-regulation of LHRH-I (GNRH1) gene expression and secretion can also be mediated by itself and its processed peptide, LHRH-(1–5), centrally and in peripheral tissues. In this review, we present the evidence that EP24.15 is the main enzyme of LHRH metabolism. Following this, we look at the metabolism of other neuropeptides where an active peptide fragments is formed during degradation and use this as a platform to postulate that EP24.15 may also produce an active peptide fragment in the process of breaking down LHRH. We close this review by the role EP24.15 may have in regulation of the complex LHRH system.

GnRH receptor expression in human prostate cancer cells is affected by hormones and growth factors

GnRH receptors (GnRH-R) have been found in various malignancies, including prostate cancer (PCa). They mediate the direct antitumor effects of GnRH analogs. Nevertheless, few reports concern drug-induced modulation of GnRH-R levels. In this study, we investigated GnRH-R expression in androgen-sensitive (LNCaP) and -insensitive (PC-3) PCa cells treated for 4 and 6 days with a GnRH agonist (Leuprorelin acetate, LA, 10(-11) or 10(-6) M), Dihydrotestosterone (DHT, 10(-9) M), Cyproterone acetate (CA, 10(-7) M), and Epidermal growth factor (EGF, 10 ng/ml), either alone or combined. The RT-PCR analysis showed no variation in GnRH-R mRNA levels of both treated LNCaP and PC-3 cells. On the contrary, immunoblotting indicated that in LNCaP and PC-3 cells, LA upregulated membrane GnRH-R expression (up to 92%). In androgen-sensitive cells, DHT induced a GnRH-R increase (up to 119%) always comparable to that occurring in the presence of CA. GnRH-R upregulation by LA/DHT or CA/DHT association was similar to that promoted by the single agents. In PC-3 cells, EGF upregulated GnRH-R (up to 110%). A prolonged treatment (for 12 days) determined a greater EGF-induced increase in GnRH-R levels (142%). Lower (or no) receptor enhancement occurred when LA and EGF were associated. Our findings indicate that LA post-transcriptionally upregulates its own membrane receptor in androgen-sensitive and -insensitive PCa cells, counteracting the receptor enhancement produced by DHT and EGF. The effects, obtained with a relatively long and continuous treatment, may have implications in the choice of therapy modality with GnRH analogs and may render the receptor a novel therapeutic target, particularly in hormone-refractory PCa.

SRC family kinase activity is up-regulated in hormone-refractory prostate cancer

PURPOSE

Although Src family kinase (SFK) inhibitors are now in clinical trials for the treatment of androgen-independent prostate cancer (AIPC), there are no studies relating SFK activation to patient survival. This study was designed to determine if SFK activation was up-regulated with the development of AIPC and if patients could be selected who were more likely to respond to therapy.

EXPERIMENTAL DESIGN

A unique cohort of matched prostate tumor samples, taken before hormone deprivation therapy and following hormone relapse, was used to determine by immunohistochemistry on an individual patient basis if SFK activity changed with progression to AIPC and whether this related to patient outcome measures. Using matched, hormone-sensitive and hormone-refractory cell lines, we determined if hormone status affected the way prostate cancer cells respond to suppression of SFK activity by the small-molecule inhibitor dasatinib.

RESULTS

In the current study, 28% of patients with AIPC exhibited an increase in SFK activity in prostate cancer tissue, these patients had significantly shorter overall survival (P<0.0001), and activated SFK expression correlated with the presence of distant metastases. Dasatinib inhibited phosphorylation of Src and Lyn and the downstream substrate FAK in hormone-sensitive and hormone-refractory cell lines. Although migration was reduced by dasatinib in both cell lines, proliferation of hormone-refractory cells only was inhibited.

CONCLUSION

Appropriate patient selection may allow better targeting of prostate cancer patients who are likely to respond to the treatment with SFK inhibitors at the same time improving the outcome of clinical trials.

Activin receptor signaling regulates prostatic epithelial cell adhesion and viability

Mutational changes coupled with endocrine, paracrine, and/or autocrine signals regulate cell division during carcinogenesis. The hormone signals remain undefined, although the absolute requirement in vitro for fetal serum indicates the necessity for a fetal serum factor(s) in cell proliferation. Using prostatic cancer cell (PCC) lines as a model of cancer cell proliferation, we have identified the fetal serum component activin A and its signaling through the activin receptor type II (ActRII), as necessary, although not sufficient, for PCC proliferation. Activin A induced Smad2 phosphorylation and PCC proliferation, but only in the presence of fetal bovine serum (FBS). Conversely, activin A antibodies and inhibin A suppressed FBS-induced PCC proliferation confirming activin A as one of multiple serum components required for PCC proliferation. Basic fibroblast growth factor was subsequently shown to synergize activin A-induced PCC proliferation. Inhibition of ActRII signaling using a blocking antibody or antisense-P decreased mature ActRII expression, Smad2 phosphorylation, and the apparent viability of PCCs and neuroblastoma cells grown in FBS. Suppression of ActRII signaling in PCC and neuroblastoma cells did not induce apoptosis as indicated by the ratio of active/inactive caspase 3 but did correlate with increased cell detachment and ADAM-15 expression, a disintegrin whose expression is strongly correlated with prostatic metastasis. These findings indicate that ActRII signaling is required for PCC and neuroblastoma cell viability, with ActRII mediating cell fate via the regulation of cell adhesion. That ActRII signaling governs both cell viability and cell adhesion has important implications for developing therapeutic strategies to regulate cancer growth and metastasis.

Type I gonadotropin-releasing hormone receptor mediates the antiproliferative effects of GnRH-II on prostate cancer cells

BACKGROUND

GnRH-II has been shown to exert a strong antiproliferative action on tumors of the female reproductive system. The data so far reported on the effects of GnRH-II on prostate cancer growth are controversial. Moreover, it is still unclear through which receptor [type I or type II GnRH-receptor (GnRH-R)] GnRH-II might modulate cancer cell proliferation.

OBJECTIVE

The objective of this work was to investigate whether GnRH-II might affect the proliferation of prostate cancer cells and to identify the GnRH-R through which the peptide might exert its activity.

DESIGN

We investigated the effects of GnRH-II on prostate cancer cell proliferation. We then transfected PC3 cells with a small interfering RNA targeted to type I GnRH-R. After receptor silencing we evaluated the effects of GnRH-II on cell proliferation and on forskolin-induced intracellular cAMP accumulation. Similar experiments were performed by silencing type II GnRH-R.

RESULTS

GnRH-II exerted an antiproliferative activity on prostate cancer cells. Transfection of PC3 cells with a type I GnRH-R small interfering RNA resulted in a significant decrease of the expression of this receptor. After type I GnRH-R silencing: 1) the antiproliferative effect of GnRH-II was completely abrogated; and 2) GnRH-II lost its capacity to counteract the forskolin-induced cAMP accumulation. On the contrary, type II GnRH-R silencing did not counteract the antiproliferative effect of GnRH-II.

CONCLUSIONS

GnRH-II exerts a specific and significant antiproliferative action on prostate cancer cells. This antitumor effect is mediated by the activation of type I (but not of type II) GnRH-R and by its coupled cAMP intracellular signaling pathway.

G protein-coupled receptor expression in the adult and fetal adrenal glands

Hormonal regulation of adrenal function occurs primarily through G protein-coupled receptors (GPCR), which may play different roles in fetal vs. adult adrenal glands. In this study, we compared the transcript levels of GPCR between fetal and adult adrenal and found that gonadotropin-releasing hormone receptor (GnRHR), latrophilin 3 receptor, G protein-coupled receptor 37, angiotensin II receptor type 2, latrophilin 2 receptor and melanocortin receptor were expressed at significantly higher levels in fetal adrenal. High GnRHR protein expression was also detected in fetal adrenal using immunohistochemical analysis. To define potential ligand sources for fetal adrenal GnRHR, we demonstrated that GnRH1 mRNA was expressed at high levels in the placenta, while fetal adrenal had high expression of GnRH2. In summary, certain GPCR particularly GnRHR were highly expressed in fetal adrenal and the expression of GnRH mRNA in the placenta and the fetal adrenal raises the possibility of endocrine and/or paracrine/autocrine influences on fetal adrenal function. However, the exact function of GnRHR in fetal adrenal remains to be determined.

Gonadotropin-releasing hormone: GnRH receptor signaling in extrapituitary tissues

Gonadotropin-releasing hormone (GnRH) has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in locally produced, extrapituitary GnRH. GnRH receptor (GnRHR) was found to be expressed in normal human reproductive tissues (e.g. breast, endometrium, ovary, and prostate) and tumors derived from these tissues. Numerous studies have provided evidence for a role of GnRH in cell proliferation. More recently, we and others have reported a novel role for GnRH in other aspects of tumor progression, such as metastasis and angiogenesis. The multiple actions of GnRH could be linked to the divergence of signaling pathways that are activated by GnRHR. Recent observations also demonstrate cross-talk between GnRHR and growth factor receptors. Intriguingly, the classical G(alphaq)-11-phospholipase C signal transduction pathway, known to function in pituitary gonadotropes, is not involved in GnRH actions at nonpituitary targets. Herein, we review the key findings on the role of GnRH in the control of tumor growth, progression, and dissemination. The emerging role of GnRHR in actin cytoskeleton remodeling (small Rho GTPases), expression and/or activity of adhesion molecules (integrins), proteolytic enzymes (matrix metalloproteinases) and angiogenic factors is explored. The signal transduction mechanisms of GnRHR in mediating these activities is described. Finally, we discuss how a common GnRHR may mediate different, even opposite, responses to GnRH in the same tissue/cell type and whether an additional receptor(s) for GnRH exists.

A crucial role for Galphaq/11, but not Galphai/o or Galphas, in gonadotropin-releasing hormone receptor-mediated cell growth inhibition

GnRH acts on its cognate receptor in pituitary gonadotropes to regulate the biosynthesis and secretion of gonadotropins. It may also have direct extrapituitary actions, including inhibition of cell growth in reproductive malignancies, in which GnRH activation of the MAPK cascades is thought to play a pivotal role. In extrapituitary tissues, GnRH receptor signaling has been postulated to involve coupling of the receptor to different G proteins. We examined the ability of the GnRH receptor to couple directly to Galpha(q/11), Galpha(i/o), and Galpha(s), their roles in the activation of the MAPK cascades, and the subsequent cellular effects. We show that in Galpha(q/11)-negative cells stably expressing the GnRH receptor, GnRH did not induce activation of ERK, jun-N-terminal kinase, or P38 MAPK. In contrast to Galpha(i) or chimeric Galpha(qi5), transfection of Galpha(q) cDNA enabled GnRH to induce phosphorylation of ERK, jun-N-terminal kinase, and P38. Furthermore, no GnRH-mediated cAMP response or inhibition of isoproterenol-induced cAMP accumulation was observed. In another cellular background, [35S]GTPgammaS binding assays confirmed that the GnRH receptor was unable to directly couple to Galpha(i) but could directly interact with Galpha(q/11). Interestingly, GnRH stimulated a marked reduction in cell growth only in cells expressing Galpha(q), and this inhibition could be significantly rescued by blocking ERK activation. We therefore provide direct evidence, in multiple cellular backgrounds, that coupling of the GnRH receptor to Galpha(q/11), but not to Galpha(i/o) or Galpha(s), and consequent activation of ERK plays a crucial role in GnRH-mediated cell death.

The basic biochemistry and molecular events of hormone therapy

Data regarding the molecular response of prostate cancer to hormone therapy continue to emerge, identifying a complex network of autocrine and paracrine signaling events mediating the tumor response to androgen suppression. Emerging data provide insight into cellular pathways important in the apoptotic response to therapy, including the transforming growth factor-beta, insulin-like growth factor-1, and vascular endothelial growth factor signaling axes. They also reveal mechanisms of direct antitumor cytotoxicity mediated by various hormonal agents and highlight the importance of developing antiandrogens capable of irreversibly inhibiting the androgen receptor. Accumulated data emphasize the presence of residual androgens and persistent activation of androgen receptor signaling in advanced prostate tumors despite castration. These factors suggest that a multitargeted treatment approach designed to ablate all contributions to the androgen receptor signaling axis within the prostate tumor microenvironment will be required in order for hormonal therapy to achieve optimal antitumor efficacy.

GnRH analogs reduce invasiveness of human breast cancer cells

OBJECTIVE

Bone, besides lung and liver, is one of the most preferential metastatic target sites for breast cancers. Although the precise molecular mechanisms underlying this preference need to be elucidated, it appears that bone microenvironments possess unique biological features that enable circulating cancer cells to home, survive and proliferate, and destroy bone. The majority of human breast cancers and in addition most breast cancer cell lines express GnRH receptors. Their proliferation is time- and dose-dependently reduced by GnRH-I and GnRH-II agonists by counteracting of the mitogenic signal transduction.

METHODS

We have established a coculture system of different breast cancer cell lines stable transfected with red fluorescence (DS-Red) and human primary osteoblasts (hOB) or MG63 human osteosarcoma cells to analyze tumor cell invasion to bone.

RESULTS

We could show that breast cancer cell invasion was increased when cocultured with hOB or MG63. Treatment with GnRH-I and GnRH-II analogs reduced the ability to invade a reconstituted basement membrane (Matrigel) and to migrate in response to the cellular stimulus. Searching for the molecular mechanisms we found that GnRH treatment reduces expression of the osteoblast derived chemokine SDF-1 by hOB or MG63 cells cocultured with breast cancer cells.

CONCLUSION

These data represent the first report that the activation of tumor GnRH receptors reduces the metastatic potential of breast cancer cells. The crosstalk between metastatic breast cancer cells and bone is critical to the development and progression of bone metastases. Disruption of this interaction will allow us to design mechanism-based effective and specific therapeutic interventions for bone metastases.

Gonadotropin-releasing hormone functionally antagonizes testosterone activation of the human androgen receptor in prostate cells through focal adhesion complexes involving Hic-5

Gonadotropin-releasing hormone (GnRH) analogs constitute the most widely employed medical treatment for prostatic cancer. The predominant mechanism of action is presumed to be via the inhibition of gonadotropins and resultant decrease in androgen. However, GnRH analogs have also been shown to directly inhibit prostate cancer cells both in vitro and in vivo through antiproliferative cell cycle arrest and stimulation of apoptosis. Since the GnRH receptor has been shown to affect sex steroid hormone receptor function, we considered that part of GnRH analog actions on prostate cells may be mediated through modulation of the human androgen receptor. Using a model HEK293 cell line expressing the GnRH receptor, we demonstrated a novel signalling pathway of the GnRH receptor that induces nuclear translocation of the androgen receptor that renders it transcriptionally inactive. This mechanism involves the calcium-dependent tyrosine kinase Pyk2, the non-receptor tyrosine kinase c-Src and the focal adhesion protein/steroid receptor co-factor, Hic-5. In this setting there is a GnRH-induced association and nuclear translocation of the androgen receptor with Hic-5. GnRH-induced Pyk2 activation opposed the association of Hic-5 with androgen receptor as overexpression of a dominant negative Pyk2 enhanced the GnRH-induced nuclear translocation of a green fluorescent protein-tagged human androgen receptor. GnRH-induced c-Src activation resulted in the phosphorylation of expressed Hic-5 and promoted its association with the human androgen receptor. In contrast to testosterone, GnRH-induced nuclear translocation did not transcriptionally activate the androgen receptor. We then demonstrated that GnRH can also stimulate androgen receptor mobilization in human prostate PC3, BPH-1 and LNCaP cells, and in cultured rat ventral prostate cells through the same mechanism. To determine if GnRH could antagonize androgen effects in normal tissue, we examined the effect of GnRH on rat ventral prostate organ cultures and demonstrated that GnRH can functionally antagonize the actions of testosterone on prostate cell proliferation and tissue growth. This antagonism of testosterone action by GnRH may underlie in part the capacity of GnRH receptor activation to inhibit prostate tumor growth.