Immunology, Endocrine & Metabolic Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Immunology, Endocrine and Metabolic Agents) - Volume 11, Issue 4, 2011

Estrogens regulate numerous physiological functions through two well-characterized members of the nuclear receptor superfamily, the estrogen receptors (ER)α and ERβ. In recent years, numerous studies have shown that the G protein-coupled receptor 30 (GPR30, also named GPER) mediates rapid responses to estrogens and even ER antagonists in diverse normal and cancer cell types. The GPER-activated signaling is clearly distinct from that of the classical nuclear ERs, however the transduction pathways triggered by GPER and ERs interact in some cases and cooperatively regulate relevant biological functions. In addition to various investigations which focused on GPER-mediated action in cancer, several reports have also been performed in order to uncover the role exerted by GPER in the reproductive organs, the bone, the cardiovascular, nervous and immunological systems. In this respect, new pharmacological tools as agonist/antagonist compounds have been identified and used to dissect and differentiate the peculiar GPER-dependent signaling. Overall, an increasing number of reports have provided novel insight toward a better understanding of the estrogen action elicited through many receptors which engage in an intricate transduction network involved in both physiological and pathological cell responses. Certainly, the discovery of GPER has opened a remarkable debate in the scientific community encouraging new efforts in order to further assess the multifaceted estrogen routes. This review highlights the ability of GPER in mediating the estrogen signals in diverse cell types and tissues, hence contributing to the current questions regarding its function as an alternate estrogen receptor.

The action exerted by estrogens in normal and neoplastic tissues is mainly mediated by the classical estrogen receptors (ER)α and ERβ. However, recent studies have also indicated that GPR30/GPER mediates rapid effects induced by estrogens through ERK activation and gene transcription. Moreover, GPER overexpression was associated with an aggressive phenotype of estrogen-sensitive tumors, suggesting that the regulation of GPER may be involved in cancer progression. In this context, an EGFR-dependent up-regulation of GPER expression was shown in both ERα-negative and positive cancer cells, providing additional evidence that estrogen and growth factor signaling cooperate in the amplification of mitogenic stimuli in different cell lines. In addition, the physical interaction between either GPER and EGFR or GPER and ERα generated a multi-protein complex, further supporting the functional cross-talk elicited by these receptors in the progression of estrogen-sensitive tumors. Numerous studies have evidenced that the EGFR-mediated transduction pathway contributes to the tamoxifen resistance in patients with breast cancer. As the 4-hydroxytamoxifen binds to and activates GPER, the EGFR-dependent GPER induction observed in tamoxifen-resistant breast cancer cells might be included among the mechanisms involved in the tamoxifen failure in breast malignancy. Surprisingly, an important hallmark of cancer growth, resistance to chemotherapy and decreased survival of patients, such as hypoxia, was demonstrated to regulate GPER expression in breast tumor cells and even in cardiomyocytes. Hence, GPER regulation may be included in the mechanisms leading to the adaptation to hypoxia in cancer and in the cardiovascular system.

Comparative clinical studies indicate that blockade of estrogen biosynthesis by the use of aromatase inhibitors may have benefit over estrogen receptor (ER) antagonism as a strategy for treating breast cancer. One plausible explanation for this idea is that more than one type of estrogen receptor may promote the biological effects of estrogen. Recent findings that G-protein-coupled receptor-30, (GPR30/GPER) promotes specific estrogen binding and manifests plasma membrane-initiated signaling events suggests that this previously unappreciated receptor may have importance in breast cancer, a concept further supported by the observation that GPER expression is linked to disease progression in cancers that arise from breast, ovary and endometrium. In breast cancer cells, estrogen action via GPER coordinates two cellular activities that have important implications for disease progression, integrin α5β1-dependent fibrillogenesis and release of plasma membrane tethered HB-EGF. GPER has further potential significance in breast cancer in that it acts independently of nuclear steroid hormone receptors, ERα and ERβ, and ER antagonists function as GPER agonists. In support of the idea that GPER may have independent influence from ER in breast cancer, these receptors have been reported to exhibit distinct patterns of association with clinicopathological parameters of disease progression. This review article evaluates this concept and outlines a model by which GPER and ER may act independently to drive breast cancer progression.

Natural estrogens such as 17β-estradiol are endogenous vasodilators and have been implicated in the gender differences of hypertension. These hormones activate estrogen receptors ERα and ERβ, which mediate part of estrogendependent vasodilation. In addition, a novel G protein-coupled estrogen-binding receptor termed GPER/GPR30 has been identified that is expressed in the cardiovascular system. Using knock-out animals or drugs selectively targeting GPER/GPR30, a significant role for this receptor as a mediator of acute estrogen-dependent vasodilation involving nitric oxide (NO) and blood pressure-lowering activity has been demonstrated. The accumulating evidence that GPER/GPR30 is responsible for control of vascular tone indicates that this receptor may represent a novel drug target for pharmacologic treatment of hypertension in postmenopausal women and possibly also men.

A major focus of our laboratory has been an in-depth evaluation as to how estrogens exert a pronounced protective effect on clinical and histological disease in the animal model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE). An important issue regarding their therapeutic application has been the undesirable estrogenic side effects thought to be mediated primarily through 17β-estradiol (E2) binding to intracellular estrogen receptor alpha (ERα). With the discovery and characterization of GPR30 as the putative membrane estrogen receptor, we sought to study whether signaling through GPR30 was sufficient to mediate protection against EAE without engagement of ERα. Treatment of EAE in WT mice with G-1, a selective GPR30 agonist, retained estradiol's ability to protect against clinical and histological EAE without estrogenic side effects. G-1 treatment deviated cytokine profiles and enhanced suppressive activity of CD4+Foxp3+ Treg cells through a GPR30- and programmed death 1 (PD-1)-dependent mechanism. This novel finding was indicative of the protective effect of GPR30 activation in EAE and provides a strong foundation for the clinical application of GPR30 agonists such as G-1 in MS. However, future studies are needed to elucidate cross-signaling and evaluate possible additive effects of combined signaling through both GPR30 and ER-α. Deciphering the possible mechanism of involvement of GPR30 in estrogenmediated protection against EAE may result in lowering treatment doses of E2 and GPR30 agonists that could minimize risks and maximize immunoregulation and therapeutic effects in MS. Alternatively, one might envision using E2 derivatives with reduced estrogenic activity alone or in combination with GPR30 agonists as therapies for both male and female MS patients.

Estrogens are known to regulate bone growth, mass, and turnover. The actions of estrogens on the skeleton are thought to occur mainly through the action of the nuclear receptors ERα and ERβ. Although, estrogens are commonly thought of as more important in females, estrogen regulation of the male skeleton has been clearly demonstrated in patients with aromatase deficiency or a mutation in the ERα gene or in clinical studies in men using pharmacologic interventions to inhibit estrogen synthesis or estrogen repletion. Recently, in vitro and in vivo studies have shown that the G protein- coupled receptor, GPR30, is a functional estrogen receptor (ER) or modulates tissue response to estradiol. As such it has been renamed GPR30/GPER1. In the absence of any reported human cases of GPR30/GPER1 deficiency, genetically defined mouse models have been used to study the in vivo function of this protein. Female GPR30/GPER1 null mice display reduced body size and bone mass. Ovariectomized females given estradiol replacement display reduced longitudinal skeletal growth, as measured by femur length, and decreased growth plate height in WT but not GPR30KO/GPER1KO mice. In contrast to female mice, male mice with GPR30/GPER1 deficiency have increased body size and bone mass accompanied by increased growth plate proliferation. Additionally, studies of male stromal bone marrow progenitors deficient in GPR30/GPER1 and differentiated in osteogenic medium displayed decreased proliferation and increased mineralized nodule formation. These data demonstrate that GPR30/GPER1 action is an important regulator of skeletal growth and metabolism. The balance between classical ER signaling and GPR30/GPER1 signaling is an important issue to be addressed in order to progress our understanding of bone biology and the treatment of abnormal bone mass and metabolism.

A role for estrogens in regulating blood glucose homeostasis is no longer in question. Glycemia is a process finely controlled by the interplay of several tissues: brain, skeletal muscle, liver, adipose tissue and endocrine pancreas. Failure to regulate blood glucose levels leads to the development of type 1 and type 2 diabetes. Type 2 diabetes is the combination of insulin resistance and pancreatic β-cell failure. Evidence both in humans and in animals show that lack of estrogens alters blood glucose homeostasis. Additionally, exogenous estrogens may also affect glycemia. The estrogen receptor alpha, ERα, seems to be the main receptor responsible for most of the effects of estrogens in glycemia. ERβ involvement is still under study, although it is suggested to play a negative role. GPR30/GPER participation in the estrogenic effects is a matter of controversy, mainly due to the different metabolic phenotype of the four GPR30-deficient mice produced. The existence of other receptors for estrogens has been suggested in other systems. However, the role they may be playing in the effect of estrogens in blood glucose levels is still understudied. In this review we present a summary of the complex system responsible for controlling blood glucose homeostasis, the regulation by estrogens and the participation of the different receptors molecularly characterized. However, more research is required to unveil the specific role of each receptor in the modulation of blood glucose homeostasis by the hormone.

The role of estrogens in male reproductive tract physiology has for a long time been a subject of debate. A substantial advance in our understanding of the estrogen signaling in testicular cells occurred in the last years. The testis produces significant amounts of estrogenic hormones, via aromatase, and estrogen receptors (ER) α and β are selectively expressed in testicular cells. ERα and ERβ belong to the nuclear receptor family of transcription factors. In addition to the well established transcriptional effects, estrogens can mediate rapid signaling, triggered within seconds or minutes. These rapid effects can be mediated by ERs or by the recently discovered G protein-coupled estrogen receptor GPR30, involved in testicular cell proliferation, differentiation and apoptosis. This review summarizes the current knowledge on rodent spermatogenesis concerning the functions regulated by estrogens and mediated via both genomic and rapid mechanisms of action in testicular cells.

Aim of the study: B-Lymphocyte Stimulator (BLyS), a key regulator of B-cell homeostasis, was recently involved in the regulation of malignant cell survival in both hematological and non-hematological cancers. In this study we analyzed the possible role of BLyS in neuroendocrine tumors (NET). Methods: Sixty-two consecutive unselected patients with a diagnosis of NET were enrolled in the study. According to the clinical course, patients were classified in 3 subgroups: patients with evidence of persistent but stable disease (n = 19), patients in remission (n = 13) and patients with evidence of recurrent disease (progressive patients, n = 30). Patients were compared to 77 sex-matched blood donors (HBDs). BLyS and Chromogranin A (CgA) serum levels were analyzed by ELISA. Results: Overall, NET patients presented more elevated BLyS levels than HBDs (1195± 568 pg/ml vs 666± 240 pg/ml; p <0.0001) and BLyS levels correlated with tumor differentiation. Patients with stable disease or in remission presented with significant lower BLyS levels than patients with disease progression (906±273 pg/ml versus 1503±637 pg/ml; p <0.0001). Patients with metastases displayed higher BLyS levels than patients without metastases (1391±724 pg/ml versus 1079±422 pg/ml; p=0.022). Of note, BLyS levels during the follow-up (after 6.6±2.8 months) demonstrated a significant increase in progressive patients (from 1576±927 pg/ml to 2003±1268 pg/ml; p=0.0107), while remained substantially unchanged in stable/remission cases (from 1103±427 pg/ml to 1060±400 pg/ml; p=0.52). In contrast, CgA in the same series showed contradictory change Conclusions: Elevated BLyS levels characterized patients with NET and BLyS appears as a new potential marker in the management of these neoplastic diseases.