EGF activates highly selective estrogen-responsive reporter plasmids by an ER-independent pathway

Insulin-like growth factor and epidermal growth factor signaling in breast cancer cell growth: focus on endocrine resistant disease

Breast cancer is the most common type of cancer for women worldwide with a lifetime risk amounting to a staggering total of 10%. It is well established that the endogenous synthesis of insulin-like growth factor (IGF) and epidermal growth factor (EGF) polypeptide growth factors are closely correlated to malignant transformation and all the steps of the breast cancer metastatic cascade. Numerous studies have demonstrated that both estrogens and growth factors stimulate the proliferation of steroid-dependent tumor cells, and that the interaction between these signaling pathways occurs at several levels. Importantly, the majority of breast cancer cases are estrogen receptor- (ER-) positive which have a more favorable prognosis and pattern of recurrence with endocrine therapy being the backbone of treatment. Unfortunately, the majority of patients progress to endocrine therapy resistant disease (acquired resistance) whereas a proportion of patients may fail to respond to initial therapy (de novo resistance). The IGF-I and EGF downstream signaling pathways are closely involved in the process of progression to therapy resistant disease. Modifications in the bioavailability of these growth factors contribute critically to disease progression. In the present review therefore, we will discuss in depth how IGF and EGF signaling participate in breast cancer pathogenesis and progression to endocrine resistant disease.

Minireview: Tipping the balance: ligand-independent activation of steroid receptors

Steroid receptors are prototypical ligand-dependent transcription factors and a textbook example for allosteric regulation. According to this canonical model, binding of cognate steroid is an absolute requirement for transcriptional activation. Remarkably, the simple one ligand-one receptor model could not be farther from the truth. Steroid receptors, notably the sex steroid receptors, can receive multiple inputs. Activation of steroid receptors by other signals, working through their own signaling pathways, in the absence of the cognate steroids, represents the most extreme form of signaling cross talk. Compared with cognate steroids, ligand-independent activation pathways produce similar but not identical outputs. Here we review the phenomena and discuss what is known about the underlying molecular mechanisms and the biological significance. We hypothesize that steroid receptors may have evolved to be trigger happy. In addition to their cognate steroids, many posttranslational modifications and interactors, modulated by other signals, may be able to tip the balance.

Activation of Estrogen Receptor by Bavachin from Psoralea corylifolia

In this study, we examined the estrogenic activity of bavachin, a component of Psoralea corylifolia that has been used as a traditional medicine in Asia. Bavachin was purified from ethanolic extract of Psoralea corylifolia and characterized its estrogenic activity by ligand binding, reporter gene activation, and endogenous estrogen receptor (ER) target gene regulation. Bavachin showed ER ligand binding activity in competitive displacement of [(3)H] E2 from recombinant ER. The estrogenic activity of bavachin was characterized in a transient transfection system using ERα or ERβ and estrogen-responsive luciferase plasmids in CV-1 cells with an EC50 of 320 nM and 680 nM, respectively. Bavachin increased the mRNA levels of estrogen-responsive genes such as pS2 and PR, and decreased the protein level of ERα by proteasomal pathway. However, bavachin failed to activate the androgen receptor in CV-1 cells transiently transfected with the corresponding receptor and hormone responsive reporter plasmid. These data indicate that bavachin acts as a weak phytoestrogen by binding and activating the ER.

Estrogen receptor is activated by korean red ginseng in vitro but not in vivo

Ginseng has been used as a traditional medicine for treatment of many diseases and for general health maintenance in people of all ages. Ginseng is also used to ameliorate menopausal systems. We investigated the estrogenic activity of Korean red ginseng (KRG) in a transient transfection system, using estrogen receptor (ER) and estrogen-responsive luciferase plasmids in MCF-7 cells. The extract activated both ERα and ERβ. KRG modulated the mRNA levels of estrogen-responsive genes such as pS2 and ESR1 and decreased the protein level of ERα. In order to examine in vivo estrogenic activity of KRG, sixteen female Sprague-Dawley rats separated into four groups were studied for nine weeks: non-ovariectomized (OVX) rats treated with olive oil, OVX rats treated with olive oil, OVX rats treated with 17-β-estradiol (E2) in olive oil, and OVX rats treated with KRG extract in olive oil. The experiments were repeated for three times and the data of twelve rats were combined. Body weight of OVX rats was greater than that of sham-operated control rats and was decreased by E2 treatment. Uterine weight increased after E2 treatment compared to OVX rats. However, no difference in body or uterine weight was observed with KRG intake. KRG induced reductions in total cholesterol, low density lipoprotein cholesterol/total cholesterol, high density lipoprotein cholesterol/total cholesterol, and low density lipoprotein cholesterol/high density lipoprotein cholesterol, but not to the same degree as did E2 intake. These results show that KRG does contain estrogenic activity as manifested by in vitro study but the activity is not strong enough to elicit physiological responses.

Estradiol protects dermal hyaluronan/versican matrix during photoaging by release of epidermal growth factor from keratinocytes

Hyaluronan (HA) and versican are key components of the dermis and are responsive to ultraviolet (UV)B-induced remodeling. The aim of this study was to explore the molecular mechanisms mediating the effects of estrogen (E(2)) on HA-rich extracellular matrix during photoaging. Hairless skh-1 mice were irradiated with UVB (three times, 1 minimal erythema dose (80 mJ/cm(2)), weekly) for 10 weeks, and endogenous sex hormone production was abrogated by ovariectomy. Subcutaneous substitution of E(2) by means of controlled-release pellets caused a strong increase in the dermal HA content in both irradiated and nonirradiated skin. The increase in dermal HA correlated with induction of HA synthase HAS3 by E(2). Expression of splice variant 2 of the HA-binding proteoglycan versican was also increased by E(2). In search of candidate mediators of these effects, it was found that E(2) strongly induced the expression of epidermal growth factor (EGF) in UVB-irradiated epidermis in vivo and in keratinocytes in vitro. EGF in turn up-regulated the expression of HAS3 and versican V2 in dermal fibroblasts. HAS3 knockdown by shRNA caused inhibition of fibroblast proliferation. Furthermore, HAS3 and versican V2 induction by E(2) correlated positively with proliferation in vivo. In addition, the accumulation of inflammatory macrophages, expression of inducible cyclooxygenase 2, as well as proinflammatory monocyte chemotactic protein 1 were decreased in response to E(2) in the dermis. Collectively, these data suggest that E(2) treatment increases the amount of dermal HA and versican V2 via paracrine release of EGF, which may be implicated in the pro-proliferative and anti-inflammatory effects of E(2) during photoaging.

Absence of ligand-independent transcriptional activation of the estrogen receptor via the estrogen response element in pituitary lactotrophs in primary culture

The estrogen receptor (ER) is a ligand-activated transcription factor that enhances gene expression by binding to specific regulatory DNA sequences called estrogen response elements (EREs). In some cell lines, the ER is also activated in a ligand-independent manner by multiple signaling pathways. In this study, we developed a novel adenovirus-mediated assay for promoter activation, termed LASETA, which we then used to examine whether ligand-independent activation of the ER occurred in normal pituitary lactotrophs in primary culture. In the LASETA adenovirus vector, the loxP-flanked stop sequence was deleted by prolactin (PRL) promoter-regulated expression of Cre recombinase. This led to lactotroph-specific expression of a reporter gene driven by an ERE-containing promoter. Estrogen-induced expression of the reporter protein luciferase in LASETA was specific for lactotrophs and was ER-dependent. LASETA was shown to be reliable even with varying Cre recombinase expression levels, which were caused by changes in PRL promoter activity. Using LASETA, we observed no change in ERE-mediated ER activity in the absence of estrogen after treatment of normal lactotrophs with agents such as insulin-like growth factor-1, epidermal growth factor, the adenylate cyclase activator forskolin, the extracellular signal-regulated kinase kinase inhibitor U0126, and the protein kinase A inhibitor H89. The ERE-mediated ligand-independent ER activity was induced by the growth factors and forskolin in the somatolactotroph tumor cell line GH4C1 cells. These results suggest that ERE-mediated ligand-independent activation of ER does not occur in normal lactotrophs in primary culture, and is a phenomenon likely restricted to transformed cells.

Positive correlation of steroid hormones and EGF in canine mammary cancer

There are no published studies focused on the potential crosstalk between steroid hormones and EGF in canine mammary tumourigenesis. The objective was to investigate the role of EGF in canine mammary tumours (CMT) and the relationship with steroid hormones. Sixty-three CMT (39 malignant including 10 inflammatory mammary carcinomas (IMC); 19 benign and 5 dysplasias), and 13 normal mammary glands from dogs without history of neoplastic disease were analysed. Levels of EGF and steroid hormones [progesterone (P4); 17beta-estradiol (E2); androstenedione (A4) and dehydroepiandrosterone (DHEA)], were analysed by EIA in CMT homogenates. Levels of EGF were significantly higher in malignant compared with benign tumours, dysplasias and normal mammary glands (p<0.001). IMC presented the highest EGF levels, with statistical significant difference between IMC and non-IMC cases (p<0.05). Steroid hormone levels were also significantly higher in malignant tumours compared with benign tumours, dysplasias and normal mammary glands (p<0.001). In malignant tumours (non-IMC and IMC), a strong correlation was observed between EGF and: P4 (r=0.452; p=0.003); E2 (r=0.624; p=0.023); A4 (r=0.496; p=0.038); DHEA (r=0.431; p=0.005). These results suggest that EGF is implicated in canine mammary tumourigenesis. The positive correlation observed, opens an interesting perspective of interaction that should be further investigated.

Serum free BG-1 cell proliferation assay: A sensitive method for determining organochlorine pesticide estrogen receptor activation at the nanomolar range

Most xenobiotic estrogeniety assay methods rely on direct agonist action on the estrogen receptor (ER) to approximate activation potential. Such methods do have drawbacks since some ER activating pesticides are weak or non-agonistic in ligand-binding assays. This study discusses a method that detects pesticide estrogenic actions regardless of ER ligand binding ability. Using a serum-free BG-1 ovarian cell culture model, we investigated the ability of several organochlorine (OC) pesticides to stimulate known estrogenic actions. We observed concentration dependent ER mediated cell proliferation in BG-1 cells using heptachlor epoxide (HE), beta-hexachlorohexane (beta-HCH), and endosulfan (Endo). In addition, we observed upregulation of the ERE-dependent proteins progesterone receptor and PS2. Gel-shift/EMSA studies for ERE binding further supported these OC's ERE activating abilities. All of these effects were abolished using ICI 164,384(ICI). Using the same culture conditions, we tested the blocking action of growth factor antibodies for erbB2(9G6) and insulin-like growth factor (IGF-Ab) receptors and discovered they inhibited BG-1 proliferation (9G6: HE and beta-HCH/ IGF-Ab: Endo.) This experiment confirms the existence of a possible cross-talk between ER and growth factor receptors in OC ligand-dependent activation and also validates this sensitive method for determining both ligand-dependent and independent estrogenic activity of selected pesticides.

Estrogenic effects of resveratrol in breast cancer cells expressing mutant and wild-type estrogen receptors: role of AF-1 and AF-2

Resveratrol, a hydroxystilbene found in grapes and wine, has previously been shown to be a non-flavonoid phytoestrogen, and to act as an estrogen receptor (ER) superagonist in MCF-7 cells transiently transfected with estrogen-responsive reporter constructs. Several additional hydroxystilbenes, including diethylstilbestrol (DES) and piceatannol, were tested, and all showed ER agonism or partial agonism, but superagonism was specific to resveratrol. Moreover, superagonism was observed in cells carrying a stably integrated reporter gene, indicating that this phenomenon is not a result of transient transfection. To examine the role of the transcriptional activation function (AF) domains of ERalpha in resveratrol agonism, we compared the effects of resveratrol and estradiol (E2) on expression of exogenous reporter genes and an endogenous estrogen-regulated gene (TGFalpha) in MDA-MB-231 cells stably transfected with wild-type (wt) ERalpha or mutants with deleted or mutated AF domains. In reporter gene assays, cells expressing wtERalpha showed a superagonistic response to resveratrol. Deletion of AF-1 or mutation of AF-2 attenuated the effect of resveratrol disproportionately compared to that of E2, while deletion of AF-2 abrogated the response to both ligands. In TGFalpha expression assays, resveratrol acted as a full agonist in cells expressing wtERalpha. Deletion of AF-1 attenuated stimulation by E2 more severely than that by resveratrol, as did deletion of AF-2. In contrast, mutation of AF-2 left both ligands with a limited ability to induced TGFalpha expression. In summary, the effect of modifying or deleting AF domains depends strongly on the ligand and the target gene.

Extracts from Schizandra chinensis Fruit Activate Estrogen Receptors: A Possible Clue to Its Effects on Nitric Oxide-Mediated Vasorelaxation

Schizandra chinensis fruit has long been used for the treatment of cardiovascular symptoms associated especially with menopausal symptoms in Korea. To provide a scientific rationale for such uses, we have investigated the vasorelaxant effects of Schizandra chinensis fruit on the vasomotor tone of the rat thoracic aorta in an organ bath. The crude extracts of Schizandra chinensis fruit (SC-Ex) elicited a transient relaxing response in the endothelium-intact rat aorta contracted with norepinephrine. This relaxant effect was abolished by removal of the endothelium, and also by pretreatment with nitric oxide synthase inhibitor. We then examined whether this vasodilatory effect occurs through estrogen receptor by reporter assays. SC-Ex activated the estrogen-responsive luciferase gene in COS cells transiently transfected with estrogen receptor and reporter plasmids. The activation was maintained in the butanol-soluble fraction and further increased in the successively fractionated C(18) cartridge-adsorbed fraction (SC-ADF). Reporter gene activation by SC-ADF was inhibited by the specific estrogen receptor antagonist ICI 182,780, indicating that the effect is estrogen receptor dependent. However, SC-ADF failed to activate the androgen receptor in COS cells transfected with the corresponding receptor and reporter plasmids. These data show that extracts of Schizandra chinensis fruit act as a weak phytoestrogen.

Evaluation of an estrogen receptor-beta agonist in animal models of human disease

The discovery of a second estrogen receptor (ER), called ERbeta, in 1996 sparked intense interest within the scientific community to discover its role in mediating estrogen action. However, despite more than 6 yr of research into the function of this receptor, its physiological role in mediating estrogen action remains unclear and controversial. We have developed a series of highly selective agonists for ERbeta and have characterized their activity in several clinically relevant rodent models of human disease. The activity of one such compound, ERB-041, is reported here. We conclude from these studies that ERbeta does not mediate the bone-sparing activity of estrogen on the rat skeleton and that it does not affect ovulation or ovariectomy-induced weight gain. In addition, these compounds are nonuterotrophic and nonmammotrophic. However, ERB-041 has a dramatic beneficial effect in the HLA-B27 transgenic rat model of inflammatory bowel disease and the Lewis rat adjuvant-induced arthritis model. Daily oral doses as low as 1 mg/kg reverse the chronic diarrhea of HLA-B27 transgenic rats and dramatically improve histological disease scores in the colon. The same dosing regimen in the therapeutic adjuvant-induced arthritis model reduces joint scores from 12 (maximal inflammation) to 1 over a period of 10 d. Synovitis and Mankin (articular cartilage) histological scores are also significantly lowered (50-75%). These data suggest that one function of ERbeta may be to modulate the immune response, and that ERbeta-selective ligands may be therapeutically useful agents to treat chronic intestinal and joint inflammation.

H-ras dependent estrogenic effects of epidermal growth factor in the estrogen-independent breast cancer cell line MDA-MB-231

A crosstalk between cellular estrogen response and receptor tyrosine kinase signaling has been shown in a variety of estrogen receptor (ER)-positive cell lines. We intended to examine the presence of estrogenic growth factor effects in an ER alpha-negative breast cancer cell line. By means of reporter gene assays, we investigated the activation of estrogen response elements (EREs) by epidermal growth factor (EGF) in the estrogen-unresponsive cell line MDA-MB-231. Our results demonstrate the H-ras-dependent activation of EREs after EGF treatment in this estrogen-unresponsive cell line, an effect which was not observed in the ERalpha/beta-positive breast cancer cell line MCF-7. In MDA-MB-231 cells, the transcriptional activity of an ERE-containing promotor was enhanced dose dependently by all tested EGF concentrations. This effect could be blocked by co-treatment with the epidermal growth factor receptor (EGFR) inhibitors AG1478 and ZD1839, as well as by co-transfection with a vector coding for a dominant negative H-ras mutant, but not by co-treatment with the pure antiestrogen ICI182,780. Furthermore, expression of constitutively active H-ras was shown to be sufficient to activate EREs in MDA-MB-231 cells. Our results suggest alternative utilization of ERE-mediated gene regulation in an estradiol-unresponsive breast cancer cell line in response to an EGF stimulus. This mechanism was shown to be dependent on EGFR and H-ras activity, but independent of the presence of functional ERalpha.

A ginsenoside-Rh1, a component of ginseng saponin, activates estrogen receptor in human breast carcinoma MCF-7 cells

We have examined the possibility that a component of Panax ginseng, ginsenoside-Rh1, acts by binding to steroid hormone receptors such as receptors for estrogen, glucocorticoid, androgen, and retinoic acid. Ginsenoside-Rh1 activated the transcription of the estrogen-responsive luciferase reporter gene in MCF-7 breast cancer cells at a concentration of 50 microM. Activation was inhibited by the specific estrogen receptor antagonist ICI 182,780, indicating that the estrogenic effect of ginsenoside-Rh1 is estrogen receptor dependent. Ginsenoside-Rh1 induction of luciferase activity was dose-dependent in CV-1 cells transiently transfected with estrogen receptor and reporter plasmids. Next, we evaluated the ability of ginsenoside-Rh1 to induce the estrogen-responsive genes in MCF-7 cells. Ginsenoside-Rh1 increased c-fos and pS2 at the mRNA levels at 24h after treatment, although the effects were not as prominent as 17beta-estradiol. Western blot analysis showed that progesterone receptor protein was induced at 24h of treatment of ginsenoside-Rh1. However, ginsenoside-Rh1 failed to activate the glucocorticoid receptor, the androgen receptor, or the retinoic acid receptor in CV-1 cells transiently transfected with the corresponding steroid hormone receptors and hormone responsive reporter plasmids. These data support our hypothesis that ginsenoside-Rh1 acts as a weak phytoestrogen, presumably by binding and activating the estrogen receptor.

Ginsenoside-Rb1 acts as a weak phytoestrogen in MCF-7 human breast cancer cells

Ginseng has been recommended to alleviate the menopausal symptoms, which indicates that components of ginseng very likely contain estrogenic activity. We have examined the possibility that a component of Panax ginseng, ginsenoside-Rb1, acts by binding to estrogen receptor. We have investigated the estrogenic activity of ginsenoside-Rb1 in a transient transfection system using estrogen-responsive luciferase plasmids in MCF-7 cells. Ginsenoside-Rb1 activated the transcription of the estrogen-responsive luciferase reporter gene in MCF-7 breast cancer cells at a concentration of 50 microM. Activation was inhibited by the specific estrogen receptor antagonist ICI 182,780, indicating that the estrogenic effect of ginsenoside-Rb1 is estrogen receptor dependent. Next, we evaluated the ability of ginsenoside-Rb1 to induce the estrogen-responsive gene c-fos by semi-quantitative RT-PCR assays and Western analyses. Ginsenoside-Rb1 increased c-fos both at mRNA and protein levels. However, ginsenoside-Rb1 failed to activate the glucocorticoid receptor, the retinoic acid receptor, or the androgen receptor in CV-1 cells transiently transfected with the corresponding steroid hormone receptors and hormone responsive reporter plasmids. These data support our hypothesis that ginsenoside-Rb1 acts a weak phytoestrogen, presumably by binding and activating the estrogen receptor.

Potential action of IGF-1 and EGF on androgen receptor nuclear transfer and transactivation in normal and cancer human prostate cell lines

This work was designed to determine whether IGF-1 and EGF modulate nuclear transfer and transactivation of the androgen receptor (AR) in human prostate cell lines (PNT1A and DU-145). We first characterized the IGF-1 and EGF receptors by ligand-binding assays with [125I] IGF-1 and [125I] EGF in a normal human prostate epithelial cell line, PNT1A. We then evaluated the effects of these growth factors on AR nuclear transfer and transcriptional activation in this cell line and in DU-145, a human prostate tumor cell line. The cell lines were cotransfected with an AR expression vector and an androgen-responsive luciferase gene driven by the mouse mammary tumor virus (MMTV-luciferase) promoter. Neither IGF-1 nor EGF could activate reporter gene in the absence of androgens. Conversely, both enhanced the magnitude of the AR response in the presence of low levels of androgen (10(-11)-10(-9) M) and this response, increased by twofold, was inhibited by hydroxyflutamide. No effect of IGF-1 and EGF was observed on the intracellular localization of the fusion protein EGFP-AR in either cell line. The fluorescence stayed cytoplasmic even after 24 h of IGF-1 or EGF treatment. Taken together, these data indicate that growth factors are unable to initiate the nuclear translocation of AR in the absence of androgens or to induce ligand-independent transcriptional activity. We observed only cross-talk in the presence of androgens and IGF-1 or EGF, leading to an over-activated AR. In conclusion, the cross-talk between AR and growth factor signaling pathways may sensitize AR to suboptimal stimulation by low levels of androgens.

Conditional over-expression of estrogen receptor alpha in a transgenic mouse model

Attempts to delineate the mechanisms of estrogen action have promoted the creation of several estrogen receptor alpha (ERalpha) mouse models in the past decade. These traditional models are limited by the fact that the receptors are either absent or present throughout all stages of development. The purpose of this work was to develop a conditional transgenic model that would provide an in vivo method of controlling the spatial and temporal regulation of ERalpha expression. The tetracycline responsive system was utilized. Three lines of transgenic mice carrying a transgene composed of the coding sequence for murine ERalpha placed under the regulatory control of a tet operator promoter (tet-op) were generated. These three lines of tet-op-mERa mice were each mated to an established line of transgenic mice expressing a tetracycline-dependent transactivator protein (tTA) from the mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Double transgenic MMTV-tTA/tet-op-mERalpha mice were produced. All three lines demonstrated dominant gain of ERalpha shown by RT-PCR, immunoprecipitation, and immunohistochemistry. Transgene-specific ERalpha was expressed in numerous tissues including the mammary gland, salivary gland, testis, seminal vesicle, and epididymis. Expression was silenced by administration of doxycycline in the drinking water. This model can be utilized to evaluate the consequences of ERalpha dominant gain in targeted tissues at specific times during development. In this study dominant gain of ERalpha was associated with a reduction in epididymal/vas deferens and seminal vesicle weights consistent with the proposed action of ERalpha on fluid transport in the male reproductive tract. Combining this model with other dominant gain and gene knockout mouse models will be useful for testing effects of ERalpha action in combination with specific gene products and to evaluate if developmental and stage-specific expression of ERalpha can rescue identified phenotypes in gene knockout mice.

Ligand-independent activation of steroid receptors

In several transformed cell lines, the growth factors IGF-I and epidermal growth factor (EGF) activate second messenger systems that cause the phosphorylation of the estrogen receptor (ER). One kinase catalysing receptor phosphorylation is mitogen activated protein (MAP) kinase, and the result of phosphorylation is an increase in receptor transactivation function. EGF and IGF-I, secreted locally and systemically, are involved in uterine-conceptus interactions in early pregnancy, and therefore it is of interest to determine whether these growth factors affect ER function in the uterus. An estrogen response element, chloramphenicol acetyl transferase reporter gene construct (CATERE) was transfected into bovine endometrial epithelial and stromal cells in vitro, and CAT measured during transient expression. Growth factors were added at various times following transfection, and MAP kinase phosphorylation was monitored by western blotting of p42 and p44. The MEK inhibitor U 0126 was used to determine whether the effect of IGF-I on CATERE expression was mediated through MAP kinase, and the anti-estrogen ICI 182780 was used to identify effects involving the ER. In stromal cells, reporter gene activity was increased in a dose dependent manner by IGF-I or hEGF in the presence or absence of estradiol-17beta. In the absence of estradiol the effect of IGF-I was not inhibited by ICI 182780. The effect of IGF-I occurred within an hour, before any detectable increase in cell proliferation, and the activation of CAT expression in response to IGF-I or EGF was blocked by U 0126. In contrast to their effects in stromal cells, neither IGF-I nor EGF affected CAT expression in bovine endometrial epithelial cells. Measurement of phosphorylated MAP kinases p42/p44 by western blotting showed that EGF but not IGF-I activated MAP kinase phosphorylation in both epithelial and stromal cells. In stromal cells, the fact that U 0126 blocked the CAT responses to IGF-I and EGF indicates the involvement of a MAP kinase. But since IGF-I did not activate p42/p44, a different MAP kinase, not detected by the antibody used here, is implicated. As the response was not blocked by ICI 182780, we conclude this effect is independent of ER activation. Therefore in bovine uterine cells in culture effects on MAP kinases p42/p44 can be dissociated from those on ERE-dependent gene expression, and reporter gene expression may be independent of ER activation.

Dominant negative ER induces apoptosis in GH(4) pituitary lactotrope cells and inhibits tumor growth in nude mice

The ER plays an important role in the proliferation and differentiation of lactotrope tumor cells. GH(4) cells were infected with adenoviral vectors (AdL540Q and Ad1-536) to investigate the ability of dominant negative ER mutants to affect the regulation of gene expression and cell growth by endogenous ER. The dominant negative mutants suppressed estradiol stimulation of an estrogen-responsive reporter gene and the PRL promoter in these cells. AdL540Q or Ad1--536 infection also inhibited GH(4) cell growth and induced apoptosis, increasing the expression of the proapoptotic Bax protein and decreasing the expression of antiapoptotic Bcl-2. AdwtER-infected cells also showed decreased Bcl-2 protein. E2-induced activation of p38 MAPK, an enzyme that may participate in apoptosis, was observed in cells infected with AdwtER, AdL540Q, and Ad1--536. Consistent with the apoptotic effects in vitro, infection of GH(4) cells with AdL540Q or Ad1--536 inhibited the ability of the cells to form tumors in nude mice. These results indicate that dominant negative ER mutants induce apoptosis of GH(4) cells and suppress tumor formation and development. The delivery of dominant negative ERs by adenoviral vectors may provide an alternative modality for the targeted therapy of pituitary lactotrope adenomas and other estrogen-responsive tumors.

Effects of estrogen on leptin gene promoter activation in MCF-7 breast cancer and JEG-3 choriocarcinoma cells: selective regulation via estrogen receptors alpha and beta

Leptin is a potential regulator of conceptus development. We have previously suggested that in primate pregnancy, leptin biosynthesis is regulated by estrogen in a tissue-specific manner. Therefore, the objective of the current study was to determine the mechanism of estrogen action on LEP promoter activation in divergent cell types. The effects of estrogen were investigated in estrogen receptor (ER)-positive MCF-7 breast cancer cells and in ER-negative JEG-3 choriocarcinoma cells. Cells were transfected with a leptin-luciferase or an estrogen responsive element (ERE)-luciferase reporter construct, in conjunction with ERalpha, ERbeta, or empty vector expression plasmids. Cells were treated with estradiol and/or the specific estrogen antagonists, ICI-182,780 or 4-hydroxytamoxifen. In MCF-7 cells, estradiol stimulated (P<0.05) ERE-luciferase activity and was inhibited by ICI-182,780, but did not stimulate leptin-luciferase activity. However, leptin-luciferase was stimulated by estradiol (P<0.05) and inhibited by antiestrogens in JEG-3 cells that were co-transfected with ERalpha. Both antiestrogens stimulated leptin-luciferase activity (P<0.05) in JEG-3 cells co-transfected with ERbeta. Results suggested that LEP promoter activation may depend upon co-activators present in leptin-producing cells and may be inhibited by repressors present in non-leptin producing cells. Divergent effects of estrogen may be owed to differences in the type of ER (alpha or beta) expressed in target tissues.

Direct effects of estrogen on the vessel wall

The understanding of the biological effects of estrogen on the vessel wall has improved dramatically since the discovery of estrogen receptors (ERs). Most, but not all estrogen-mediated effects in blood vessels are thought to be mediated by ERs. Two major ER subclasses have been characterized so far: the ERalpha and the more recently described ERbeta. This review will primarily focus on a new perspective that highlights ERs as essential mediators of the vascular effects of estrogen. In view of the rising research interest in this area, it can be also expected that tissue- and ER subclass-selective agonists and antagonists will be developed over the next few years, thus providing invaluable tools for pharmacological and clinical applications.