Signal transduction by steroid hormones: nuclear localization is differentially regulated in estrogen and glucocorticoid receptors

Estrogen Receptor α Isoforms Generated by Alternative Use of Cryptic Exons

Estrogen receptor α (ERα) regulates several physiological functions. In pathophysiological conditions, ERα is involved in the development and progression of estrogen-sensitive tumors. The ERα gene contains multiple 5'-untranslated exons and eight conventional coding exons and presents multiple isoforms generated by alternative promoter usage and alternative splicing. This gene also possesses non-conventional exons in the 3'- and intronic regions, and alternative use of cryptic exons contributes to further diversity of ERα mRNAs and proteins. Recently, the genomic organization of ERα genes and the splicing profiles of their transcripts were comparatively analyzed in humans, mice, and rats, and multiple ERα isoforms with distinct structures and functions were identified. These transcripts contain cryptic sequences that encode insertion-containing or truncated ERα proteins. In particular, alternative cryptic exons with in-frame stop codons yield transcripts encoding C-terminally-truncated ERα proteins. The C-terminally-truncated ERα isoforms lack part or all of the ligand-binding domain but possess an isoform-specific sequence. Some of these isoforms exhibit constitutive transactivation and resistance to estrogen receptor antagonists. Although numerous studies have reported conflicting results regarding their functions, the critical determinant for their gain-of-function has been identified structurally. Here we review recent progress in ERα variant research concerning the genomic organization of ERα genes, splicing profiles of ERα transcripts, and transactivation properties of ERα isoforms.

Fine-tuned nitric oxide and hormone interface in plant root development and regeneration

Plant root growth and developmental capacities reside in a few stem cells of the root apical meristem (RAM). Maintenance of these stem cells requires regenerative divisions of the initial stem cell niche (SCN) cells, self-maintenance, and proliferative divisions of the daughter cells. This ensures sufficient cell diversity to guarantee the development of complex root tissues in the plant. Damage in the root during growth involves the formation of a new post-embryonic root, a process known as regeneration. Post-embryonic root development and organogenesis processes include primary root development and SCN maintenance, plant regeneration, and the development of adventitious and lateral roots. These developmental processes require a fine-tuned balance between cell proliferation and maintenance. An important regulator during root development and regeneration is the gasotransmitter nitric oxide (NO). In this review we have sought to compile how NO regulates cell rate proliferation, cell differentiation, and quiescence of SCNs, usually through interaction with phytohormones, or other molecular mechanisms involved in cellular redox homeostasis. NO exerts a role on molecular components of the auxin and cytokinin signaling pathways in primary roots that affects cell proliferation and maintenance of the RAM. During root regeneration, a peak of auxin and cytokinin triggers specific molecular programs. Moreover, NO participates in adventitious root formation through its interaction with players of the brassinosteroid and cytokinin signaling cascade. Lately, NO has been implicated in root regeneration under hypoxia conditions by regulating stem cell specification through phytoglobins.

Immunohistochemical Detection of Estrogen Receptor-Beta (ERβ) with PPZ0506 Antibody in Murine Tissue: From Pitfalls to Optimization

The estrogen receptor beta (ERβ) is physiologically essential for reproductive biology and is implicated in various diseases. However, despite more than 20 years of intensive research on ERβ, there are still uncertainties about its distribution in tissues and cellular expression. Several studies show contrasts between mRNA and protein levels, and the use of knockout strategies revealed that many commercially available antibodies gave false-positive expression results. Recently, a specific monoclonal antibody against human ERβ (PPZ0506) showed cross-reactivity with rodents and was optimized for the detection of rat ERβ. Herein, we established an immunohistochemical detection protocol for ERβ protein in mouse tissue. Staining was optimized on murine ovaries, as granulosa cells are known to strongly express ERβ. The staining results were confirmed by western blot analysis and RT-PCR. To obtain accurate and reliable staining results, different staining conditions were tested in paraffin-embedded tissues. Different pitfalls were encountered in immunohistochemical detection. Strong heat-induced epitope retrieval (HIER) and appropriate antibody dilution were required to visualize specific nuclear expression of ERβ. Finally, the specificity of the antibody was confirmed by using ovaries from Esr2-depleted mice. However, in some animals, strong (non-specific) background staining appeared. These signals could not be significantly alleviated with commercially available additional blocking solutions and are most likely due to estrus-dependent expression of endogenous immunoglobulins. In summary, our study showed that the antibody PPZ0506, originally directed against human ERβ, is also suitable for reliable detection of murine ERβ. An established staining protocol mitigated ambiguities regarding the expression and distribution of ERβ in different tissues and will contribute to an improved understanding of its role and functions in murine tissues in the future.

An overview on Estrogen receptors signaling and its ligands in breast cancer

Estrogen governs the regulations of various pathological and physiological actions throughout the body in both males and females. Generally, 17β-estradiol an endogenous estrogen is responsible for different health problems in pre and postmenopausal women. The major activities of endogenous estrogen are executed by nuclear estrogen receptors (ERs) ERα and ERβ while non-genomic cytoplasmic pathways also govern cell growth and apoptosis. Estrogen accomplished a fundamental role in the formation and progression of breast cancer. In this review, we have hyphenated different studies regarding ERs and a thorough and detailed study of estrogen receptors is presented. This review highlights different aspects of estrogens ranging from receptor types, their isoforms, structures, signaling pathways of ERα, ERβ and GPER along with their crystal structures, pathological roles of ER, ER ligands, and therapeutic strategies to overcome the resistance.

Importins involved in the nuclear transportation of steroid hormone receptors: In silico and in vitro data

The nuclear receptor superfamily (NRS) consists of 48 receptors for lipophilic substances and is divided into 7 different subfamilies, with subfamily 3 comprising steroid hormone receptors. Several nuclear receptors usually bind their cognate ligands in the cytosol and the complex (mono- or dimerized) is transported to the nucleus, where it acts as a transcription initiating factor for a number of genes. The general structure of nuclear receptors consists of an N-terminal activating domain (A/B), important for the binding of activating or inhibitory co-factors, the DNA-binding domain (C), responsible for the association of the receptor-ligand-co-factor complex to the nucleus, the ligand-AF2 domain (E/F), where ligand binding occurs as well as that of ligand-dependent activating/inhibiting factors, and a flexible/non-structured domain (D), linking the DBD and LBD, called hinge region, on which a significant number of post-translational modifications occur. This hinge domain, for the sub-class of steroid receptors, is a non-structured domain and was reported as mainly responsible for the nuclear transport of steroid receptors, since it contains a specific amino acid sequence (Nuclear Localization Signal-NLS), recognized by importin α. In addition to the importin α/β complex, a number of other importins have been discovered and reported to be responsible for the nuclear transport of a number of significant proteins; however, the corresponding recognition sequences for these importins have not been identified. Recently, we have reported the identification of the NLS sequences for importins 4, 5 and 7. In this work, we provide in silico data, followed by experimental in vitro validation, showing that these alternative importins are responsible for the nuclear transportation of steroid hormone receptors such as ERα, AR and PR, and therefore they may consist of alternative targets for the pharmacological manipulation of steroid hormone actions. Moreover, we provide additional in silico data for the hinge region of steroid hormone receptors which is highly enriched with NLS sequences for importins 4, 5 and 7, in addition to the recognition NLS for importin α/β.

Rapid Screening of Glucocorticoid Receptor (GR) Effectors Using Cortisol-Detecting Sensor Cells

Cortisol, a stress hormone, plays key roles in mediating stress and anti-inflammatory responses. As abnormal cortisol levels can induce various adverse effects, screening cortisol and cortisol analogues is important for monitoring stress levels and for identifying drug candidates. A novel cell-based sensing system was adopted for rapid screening of cortisol and its functional analogues under complex cellular regulation. We used glucocorticoid receptor (GR) fused to a split intein which reconstituted with the counterpart to trigger conditional protein splicing (CPS) in the presence of targets. CPS generates functional signal peptides which promptly translocate the fluorescent cargo. The sensor cells exhibited exceptional performance in discriminating between the functional and structural analogues of cortisol with improved sensitivity. Essential oil extracts with stress relief activity were screened using the sensor cells to identify GR effectors. The sensor cells responded to peppermint oil, and L-limonene and L-menthol were identified as potential GR effectors from the major components of peppermint oil. Further analysis indicated L-limonene as a selective GR agonist (SEGRA) which is a potential anti-inflammatory agent as it attenuates proinflammatory responses without causing notable adverse effects of GR agonists.

Optimization of immunohistochemical detection of rat ESR2 proteins with well-validated monoclonal antibody PPZ0506

Although there are few well-validated antibodies against ESR2 proteins, a recent validation assessment identified a specific monoclonal antibody against human ESR2 proteins (PPZ0506). Furthermore, our previous study confirmed its cross-reactivity and specificity against rodent ESR2 proteins, enabling the determination of true ESR2 distribution profiles in rodents. Therefore, we aimed to determine optimal conditions for ESR2 detection by PPZ0506 immunostaining and analyze ESR2 distribution in rats. We evaluated several staining conditions using paraffin-embedded and frozen ovary sections. Immunohistochemical staining with PPZ0506 antibody required strong antigen retrieval and appropriate antibody dilution. Subsequent immunohistochemical analysis in multiple tissues under optimized conditions revealed that rat ESR2 proteins are expressed in a more localized manner than previously assumed. Our results suggest that previous immunohistochemical studies using inadequately validated antibodies against ESR2 proteins overestimated their distribution profiles. We expect that optimized immunohistochemical detection with PPZ0506 antibody can help researchers solve several conflicting problems in ESR2 research.

Transportin-2 plays a critical role in nucleocytoplasmic shuttling of oestrogen receptor-α

Oestrogen receptor-α (ERα) shuttles continuously between the nucleus and the cytoplasm, and functions as an oestrogen-dependent transcription factor in the nucleus and as an active mediator of signalling pathways, such as phosphatidylinositol 3-kinase (PI3K)/AKT, in the cytoplasm. However, little is known regarding the mechanism of ERα nucleocytoplasmic shuttling. In this study, we found that ERα is transported into the nucleus by importin-α/β1. Furthermore, we found that Transportin-2 (TNPO2) is involved in 17β-oestradiol (E2)-dependent cytoplasmic localisation of ERα. Interestingly, it was found that TNPO2 does not mediate nuclear export, but rather is involved in the cytoplasmic retention of ERα via the proline/tyrosine (PY) motifs. Moreover, we found that TNPO2 competitively binds to the basic nuclear localisation signal (NLS) of ERα with importin-α to inhibit importin-α/β-dependent ERα nuclear import. Finally, we confirmed that TNPO2 knockdown enhances the nuclear localisation of wild-type ERα and reduces PI3K/AKT phosphorylation in the presence of E2. These results reveal that TNPO2 regulates nucleocytoplasmic shuttling and cytoplasmic retention of ERα, so that ERα has precise functions depending on the stimulation.

Tributyltin enhanced anxiety of adult male zebrafish through elevating cortisol level and disruption in serotonin, dopamine and gamma-aminobutyric acid neurotransmitter pathways

Tributyltin (TBT), a widely and persistently distributed organontin, has been well documented to disrupt reproduction and behaviors in animals due to its anti-aromatase activity. TBT has been also reported to enhance anxiety in several fish species, whereas the mechanism underlying remains largely unknown. To investigate the disruption of TBT on fish anxiety and the mechanisms possibly involved, adult male zebrafish (Danio rerio) were treated with TBT (100 and 500 ng/L) for 28 days and anxiety behavior was further investigated using a novel tank dive test. Result showed that TBT treatment significantly enhanced the total time of the fish spent in the lower half, delayed the onset time to the higher half of the tank and increased the total duration of freezing of the fish, indicating an enhanced anxiety in TBT-treated fish. Accordingly, TBT sharply elevated the cortisol levels in plasma in a concentration-dependent manner, suggesting that the elevated cortisol level might be involved in the enhanced anxiety. Although the expression of crha was significantly increased and crhbp was significantly decreased in the brain of TBT-treated fish which is consistent to the elevated cortisol level, the expressions of actha and acthb were sharply down-regulated. In contrast, the expressions of genes responsible for the synthesis and action of serotonin (5-HT) (pet1, thp2 and htr1aa), dopamine (DA) (th1, slc6a3, drd2a and drd2b) and gamma-aminobutyric acid (GABA) (gad2 and gabrg2) were all significantly inhibited. The down-regulation of these pivotal genes acting in 5-HT, DA and GABA neurotransmitter systems in response to TBT corresponded well with the TBT-enhanced anxiety in fish. It was thus strongly suggested that these neurotransmitters might be also involved in TBT-enhanced anxiety in adult male zebrafish. The present study extended our understanding of the neurotoxicity of TBT on the anxiety control and behavioral modulation in fish.

R269C variant of ESR1: high prevalence and differential function in a subset of pancreatic cancers

Background

Estrogen receptor α (ESR1) plays a critical role in promoting growth of various cancers. Yet, its role in the development of pancreatic cancer is not well-defined. A less studied region of ESR1 is the hinge region, connecting the ligand binding and DNA domains. rs142712646 is a rare SNP in ESR1, which leads to a substitution of arginine to cysteine at amino acid 269 (R269C). The mutation is positioned in the hinge region of ESR1, hence may affect the receptor structure and function. We aimed to characterize the activity of R269C-ESR1 and study its role in the development of pancreatic cancer.

Methods

Transcriptional activity was evaluated by E2-response element (ERE) and AP1 –luciferase reporter assays and qRT-PCR. Proliferation and migration were assessed using MTT and wound healing assays. Gene-expression analysis was performed using RNAseq.

Results

We examined the presence of this SNP in various malignancies, using the entire database of FoundationOne and noted enrichment of it in a subset of pancreatic non-ductal adenocarcinoma (n = 2800) compared to pancreatic ductal adenocarcinoma (PDAC) as well as other tumor types (0.53% vs 0.29%, p = 0.02). Studies in breast and pancreatic cancer cells indicated cell type-dependent activity of ESR1 harboring R269C. Thus, expression of R269C-ESR1 enhanced proliferation and migration of PANC-1 and COLO-357 pancreatic cancer cells but not of MCF-7 breast cancer cells. Moreover, R269C-ESR1 enhanced E2-response elements (ERE) and AP1-dependent transcriptional activity and increased mRNA levels of ERE and AP1-regulated genes in pancreatic cancer cell lines, but had a modest effect on MCF-7 breast cancer cells. Accordingly, whole transcriptome analysis indicated alterations of genes associated with tumorigenicity in pancreatic cancer cells and upregulation of genes associated with cell metabolism and hormone biosynthesis in breast cancer cells.

Conclusions

Our study shed new light on the role of the hinge region in regulating transcriptional activity of the ER and indicates cell-type specific activity, namely increased activity in pancreatic cancer cells but reduced activity in breast cancer cells. While rare, the presence of rs142712646 may serve as a novel genetic risk factor, and a possible target for therapy in a subset of non-ductal pancreatic cancers.

Identification of a novel C-terminally truncated estrogen receptor α variant (ERαi34) with constitutive transactivation and estrogen receptor antagonist resistance

Constitutively active estrogen receptor α (ERα) variants with C-terminal truncation are candidate molecules for gain of both endocrine- and chemotherapy-resistance in estrogen-sensitive tumors. Our previous reports using artificially truncated ERα constructs demonstrated that ERα variants encoded in 1-2-3-cryptic exon- and 1-2-3-4-cryptic exon-types of transcripts have potentials to display constitutive transactivation of an estrogen response element reporter. However, naturally occurring 1-2-3-cryptic exon-type ERα variants have not been cloned yet. Therefore, the present study was designed to identify naturally occurring ERα variants encoded in 1-2-3-cryptic exon-type ERα transcripts. We cloned a novel C-terminally truncated ERα variant (ERαi34) encoded in a 1-2-3-i34 transcript from MCF-7 cells and characterized its constitutive and ER antagonist-resistant transactivation in transfected cells. Stable transfection of the variant into MCF-7 cells augmented basal cell proliferation insensitive to fulvestrant. Collectively, we validated the structure-based mechanisms underlying constitutive and ER antagonist-resistant transactivation by C-terminally truncated ERα variants.

Endocrine Resistance in Hormone Receptor Positive Breast Cancer-From Mechanism to Therapy

The importance and role of the estrogen receptor (ER) pathway has been well-documented in both breast cancer (BC) development and progression. The treatment of choice in women with metastatic breast cancer (MBC) is classically divided into a variety of endocrine therapies, 3 of the most common being: selective estrogen receptor modulators (SERM), aromatase inhibitors (AI) and selective estrogen receptor down-regulators (SERD). In a proportion of patients, resistance develops to endocrine therapy due to a sophisticated and at times redundant interference, at the molecular level between the ER and growth factor. The progression to endocrine resistance is considered to be a gradual, step-wise process. Several mechanisms have been proposed but thus far none of them can be defined as the complete explanation behind the phenomenon of endocrine resistance. Although multiple cellular, molecular and immune mechanisms have been and are being extensively studied, their individual roles are often poorly understood. In this review, we summarize current progress in our understanding of ER biology and the molecular mechanisms that predispose and determine endocrine resistance in breast cancer patients.

Over-expressed, N-terminally truncated BRAF is detected in the nucleus of cells with nuclear phosphorylated MEK and ERK

BRAF is a cytoplasmic protein kinase, which activates the MEK-ERK signalling pathway. Deregulation of the pathway is associated with the presence of BRAF mutations in human cancer, the most common being V600E BRAF, although structural rearrangements, which remove N-terminal regulatory sequences, have also been reported. RAF-MEK-ERK signalling is normally thought to occur in the cytoplasm of the cell. However, in an investigation of BRAF localisation using fluorescence microscopy combined with subcellular fractionation of Green Fluorescent Protein (GFP)-tagged proteins expressed in NIH3T3 cells, surprisingly, we detected N-terminally truncated BRAF (ΔBRAF) in both nuclear and cytoplasmic compartments. In contrast, ΔCRAF and full-length, wild-type BRAF (WTBRAF) were detected at lower levels in the nucleus while full-length V600EBRAF was virtually excluded from this compartment. Similar results were obtained using ΔBRAF tagged with the hormone-binding domain of the oestrogen receptor (hbER) and with the KIAA1549-ΔBRAF translocation mutant found in human pilocytic astrocytomas. Here we show that GFP-ΔBRAF nuclear translocation does not involve a canonical Nuclear Localisation Signal (NLS), but is suppressed by N-terminal sequences. Nuclear GFP-ΔBRAF retains MEK/ERK activating potential and is associated with the accumulation of phosphorylated MEK and ERK in the nucleus. In contrast, full-length GFP-WTBRAF and GFP-V600EBRAF are associated with the accumulation of phosphorylated ERK but not phosphorylated MEK in the nucleus. These data have implications for cancers bearing single nucleotide variants or N-terminal deleted structural variants of BRAF.

Cell-Based Biosensors Based on Intein-Mediated Protein Engineering for Detection of Biologically Active Signaling Molecules

Live-cell-based biosensors have emerged as a useful tool for biotechnology and chemical biology. Genetically encoded sensor cells often use bimolecular fluorescence complementation or fluorescence resonance energy transfer to build a reporter unit that suffers from nonspecific signal activation at high concentrations. Here, we designed genetically encoded sensor cells that can report the presence of biologically active molecules via fluorescence-translocation based on split intein-mediated conditional protein trans-splicing (PTS) and conditional protein trans-cleavage (PTC) reactions. In this work, the target molecules or the external stimuli activated intein-mediated reactions, which resulted in activation of the fluorophore-conjugated signal peptide. This approach fully valued the bond-making and bond-breaking features of intein-mediated reactions in sensor construction and thus eliminated the interference of false-positive signals resulting from the mere binding of fragmented reporters. We could also avoid the necessity of designing split reporters to refold into active structures upon reconstitution. These live-cell-based sensors were able to detect biologically active signaling molecules, such as Ca2+ and cortisol, as well as relevant biological stimuli, such as histamine-induced Ca2+ stimuli and the glucocorticoid receptor agonist, dexamethasone. These live-cell-based sensing systems hold large potential for applications such as drug screening and toxicology studies, which require functional information about targets.

Enhancement of the bioavailability of a novel anticancer compound (acetyltanshinone IIA) by encapsulation within mPEG-PLGA nanoparticles: a study of formulation optimization, toxicity, and pharmacokinetics

The Poly (ethylene glycol) methyl ether-block-poly (lactide-co-glycolide) (mPEG-PLGA) nanoparticles carrying acetyltanshinone IIA (ATA), a novel anti-breast cancer agent, were prepared by ultrasonic emulsion method to enhance the bioavailability and reduce the toxicity. Systematic optimization of encapsulation process was achieved using an orthogonal design. Drug efficacy analysis showed that ATA nanoparticles were as effective as free ATA against estrogen receptor positive breast cancer cells, but much less toxic towards human endothelial cells. Furthermore, in zebrafish, ATA nanoparticles displayed much lower toxicity than free ATA. More importantly, the blood concentration of ATA nanoparticles indicated by 24 hour-area under the curve (AUC_0-24h) was 10 times higher than free ATA. These results indicated the potential of ATA-loaded mPEG-PLGA nanoparticles for the delivery of ATA in a clinical formulation, and their potential for use in tumor therapy in the future.

ESRRB plays a crucial role in the promotion of porcine cell reprograming

The estrogen-related receptor b (ESRRB) is an orphan nuclear receptor and targets many genes involved in self-renewal and pluripotency. In mouse ES cells, overexpression of ESRRB can maintain LIF-independent self-renewal in the absence of Nanog. However, the fundamental features of porcine ESRRB remain elusive. In this study, we revealed the expression profiles of ESRRB in both porcine pluripotent stem cells and early stage embryos and dissected the functional domains of ESRRB protein to prove that ESRRB is a key transcription factor that enhanced porcine pluripotent gene activation. Addition of ESRRB into the cocktail of core pluripotent factors Oct4, Sox2, Klf4, and c-Myc (OSKM + E) could significantly enhance the reprograming efficiency and the formation of alkaline phosphatase positive colonies. Conversely, knockdown of ESRRB in piPSCs significantly reduced the expression level of pluripotent genes, minimized the alkaline phosphatase activity, and initiated the porcine induced pluripotent stem cell differentiation. Therefore, porcine ESRRB is a crucial transcription factor to improve the self-renewal of piPSCs.

Characterization of rodent constitutively active estrogen receptor α variants and their constitutive transactivation mechanisms

Estrogen receptor α (ERα) mRNAs exhibit remarkable heterogeneity owing to complicated alternative splicing. Some encode C-terminally-truncated ERα proteins, which display ligand-independent transactivation or dominant-negative activity. We previously characterized C-terminally-truncated ERα mRNA variants with cryptic sequences in humans and mice, and demonstrated that helices in the ligand-binding domains (LBDs) of ERα variants contribute to ligand-independent transcriptional activity. However, existence of non-conventional coding exons and generation of constitutively active ERα variants have remained to be examined in rats. To comparatively analyze modular organization and splicing profiles of the ERα genes, the range of C-terminally-truncated ERα variants was explored in rats and mice using rapid amplification of cDNA ends and RT-PCR cloning. Furthermore, their functions were determined in transiently transfected cells using expression constructs and luciferase reporter assays. Multiple cryptic exons and C-terminally-truncated ERα variant mRNAs were identified in rats and mice. Naturally occurring and artificially truncated variants/constructs lacking helix 5 potentially exhibited gain-of-function in transfected cells. Although cryptic exons and splicing profiles were poorly conserved among humans, mice, and rats, constitutively active variants were generated from the ERα genes. Moreover, the primary mechanism of ligand-independent activation by C-terminally-truncated ERα variants is C-terminus to helix 5 deletion in the LBD. This comparative study documented the complexity of ERα genes, mRNAs, and proteins, and further determined the underlying structural basis of ligand-independent activation by C-terminally-truncated ERα variants.

ER-α36 Interactions With Cytosolic Molecular Network in Acquired Tamoxifen Resistance

According to the World Health Organization (WHO) published data in 2015; breast cancer is the most prevalent and the second leading cause of cancer death among females. As approximately 70% of breast cancer tumor cells are estrogen receptor (ER) positive, primary therapeutic agents such as Anti-estrogens were produced mostly in a way to target this receptor. Anti-estrogen therapies mostly target Estrogen receptor and block its underlying signaling pathways. Nevertheless, resistance to these agents made the condition more complicated. Recently the role of one molecule in the resistance development has been studied in some cases: ER-α36 is a 36 kDa variant of estrogen receptor molecule which is mostly absent in normal breast cells. Its interactions with epidermal growth factor receptors and ER-α66 leads in over-activation and/or over-expression of estrogen-independent pathways and suppression of estrogen-dependent pathways; they all in turn, will maintain tumor cell's growth even in the presence of tamoxifen. In this mini-review, we mainly surveyed different pathways which ER-α36 could lead to tamoxifen resistance. We also briefly mentioned how ER-α36 could switch the growth cascades from estrogen dependent into independent and make this resistance network become even more complicated.

Human C-terminally truncated ERα variants resulting from the use of alternative exons in the ligand-binding domain

The nuclear receptor genes contain alternative internal and terminal exons, with alternative exon incorporation yielding mRNA variants that encode various receptor types, including some with C-terminal truncation that exhibit constitutive activation or dominant-negative transcriptional transactivation. However, C-terminally truncated estrogen receptor α (ERα) variants with alternative sequences have rarely been reported in humans. Therefore, we assessed human ERα genomic organization and alternative splicing profiles, and identified both alternative exons and C-terminally truncated ERα variants. These naturally occurring C-terminally truncated ERα proteins were localized in the nuclei of transfected cells. In addition, ERαi45c and ERαΔ5 variants exhibited constitutive transactivation of an estrogen responsive element-driven promoter in transfected cells. We manufactured expression vectors encoding artificially truncated ERα constructs and evaluated their transactivation abilities to establish mechanisms determining the constitutive activity and dominant-negative properties of truncated variants. Lack of the region encoded in exon 8 eliminated basal and ligand-induced transcriptional transactivation. The C-terminally truncated ERα variants/constructs containing the helices 5 in their ligand-binding domains did not exhibit constitutive transactivation. Furthermore, we demonstrated that truncation from C-termini to helices 5 in the variant ligand-binding domains was required for constitutive activation and found that the remnant regions of the ligand-binding domains and variant-specific sequences influenced transcriptional transactivation efficiency. In conclusion, we elucidated the structural and functional features of novel C-terminally truncated ERα variants and revealed the mechanisms underlying constitutive transactivation by C-terminally truncated nuclear receptor variants.

The estrogen receptor alpha nuclear localization sequence is critical for fulvestrant-induced degradation of the receptor

Fulvestrant, a selective estrogen receptor down-regulator (SERD) is a pure competitive antagonist of estrogen receptor alpha (ERα). Fulvestrant binds ERα and reduces the receptor's half-life by increasing protein turnover, however, its mechanism of action is not fully understood. In this study, we show that removal of the ERα nuclear localization sequence (ERΔNLS) resulted in a predominantly cytoplasmic ERα that was degraded in response to 17-β-estradiol (E2) but was resistant to degradation by fulvestrant. ERΔNLS bound the ligands and exhibited receptor interaction similar to ERα, indicating that the lack of degradation was not due to disruption of these processes. Forcing ERΔNLS into the nucleus with a heterologous SV40-NLS did not restore degradation, suggesting that the NLS domain itself, and not merely receptor localization, is critical for fulvestrant-induced ERα degradation. Indeed, cloning of the endogenous ERα NLS onto the N-terminus of ERΔNLS significantly restored both its nuclear localization and turnover in response to fulvestrant. Moreover, mutation of the sumoylation targets K266 and K268 within the NLS impaired fulvestrant-induced ERα degradation. In conclusion, our study provides evidence for the unique role of the ERα NLS in fulvestrant-induced degradation of the receptor.

Estrogen biosynthesis and action in ovarian cancer

Ovarian cancer is still the deadliest of all gynecologic malignancies in women worldwide. This is attributed to two main features of these tumors, namely, (i) a diagnosis at an advanced tumor stage, and, (ii) the rapid onset of resistance to standard chemotherapy after an initial successful therapy with platin- and taxol-derivatives. Therefore, novel targets for an early diagnosis and better treatment options for these tumors are urgently needed. Epidemiological data show that induction and biology of ovarian cancer is related to life-time estrogen exposure. Also experimental data reveal that ovarian cancer cells share a number of estrogen regulated pathways with other hormone-dependent cancers, e.g., breast and endometrial cancer. However, ovarian cancer is a heterogeneous disease and the subtypes are quite different with respect to mutations, origins, behaviors, markers, and prognosis and respond differently to standard chemotherapy. Therefore, a characterization of ovarian cancer subtypes may lead to better treatment options for the various subtypes and in particular for the most frequently observed high-grade serous ovarian carcinoma. For this intention, further studies on estrogen-related pathways and estrogen formation in ovarian cancer cells are warranted. The review gives an overview on ovarian cancer subtypes and explains the role of estrogen in ovarian cancer. Furthermore, enzymes active to synthesize and metabolize estrogens are described and strategies to target these pathways are discussed.

A novel anti-cancer agent, acetyltanshinone IIA, inhibits oestrogen receptor positive breast cancer cell growth by down-regulating the oestrogen receptor

In this paper we show that acetyltanshinone IIA (ATA), a novel anti-cancer agent, preferentially inhibits cell growth of oestrogen receptor positive (ER+) breast cancer cells and that it is more potent than the commonly used anti-breast cancer agent, tamoxifen. The metabolic product of ATA, hydroquinone tanshinone IIA (HTA) binds to the ERα and causes its degradation mainly in the nucleus via an ubiquitin-mediated proteasome-dependent pathway. In addition, ATA also reduced the mRNA levels of the ERα encoding gene, ESR1, distinguishing ATA from another anti-breast cancer drug, fulvestrant. Finally, ATA reduced the transcription of an ER-responsive gene, GREB1.

Genomic and non-genomic effects of glucocorticoids on allergic rhinitis model in mice

Glucocorticoids (GCs) are well known for their anti-inflammatory effects, which are elicited through a transcriptional mechanism via a cytosolic glucocorticoid receptor (cGR)-mediated genomic effect. However, recent in vitro studies report that GCs can act as a membrane glucocorticoid receptor (mGR). This study aimed to examine whether mometasone furoate (MF) influences the nasal symptoms induced by histamine, substance P, ATP. Furthermore, the influences of various compounds on MF action were studied in vivo. The mice were intranasally administered with nasal symptom-inciting agents, and the occurrences of sneezing and nasal rubbing were counted. MF repressed the nasal symptoms caused when it was administered 10, 30 and 60min before the induction of nasal symptoms. The repressive effect observed 10min after the administration of MF was inhibited by RU486, a GR antagonist, but not by actinomycin D, a transcriptional inhibitor. In contrast, the repressive effect observed 60min after the administration of MF was inhibited by RU486 and actinomycin D. Therefore, the effects observed 10 and 60min after the MF administration were classified as non-genomic and genomic effects, respectively. The non-genomic effect suppressed the nasal symptoms induced by m-3M3FBS, a phospholipase C (PLC) activator, and was inhibited by U-73122, a PLC inhibitor. The genomic effect was inhibited by N-(p-amylcinnamoyl) anthranilic acid, a phospholipase A2 (PLA2) inhibitor. These results indicate that MF has a non-genomic effect through repression of the activation of PLC via the mGR, and MF has also a genomic effect that was influenced by the inhibition of PLA2 through transcriptional regulation via cGR.

Mechanisms of resistance to endocrine therapy in breast cancer: focus on signaling pathways, miRNAs and genetically based resistance

Breast cancer is the most frequent malignancy diagnosed in women. Approximately 70% of breast tumors express the estrogen receptor (ER). Tamoxifen and aromatase inhibitors (AIs) are the most common and effective therapies for patients with ERα-positive breast cancer. Alone or combined with chemotherapy, tamoxifen significantly reduces disease progression and is associated with more favorable impact on survival in patients. Unfortunately, endocrine resistance occurs, either de novo or acquired during the course of the treatment. The mechanisms that contribute to hormonal resistance include loss or modification in the ERα expression, regulation of signal transduction pathways, altered expression of specific microRNAs, balance of co-regulatory proteins, and genetic polymorphisms involved in tamoxifen metabolic activity. Because of the clinical consequences of endocrine resistance, new treatment strategies are arising to make the cells sensitive to tamoxifen. Here, we will review the current knowledge on mechanisms of endocrine resistance in breast cancer cells. In addition, we will discuss novel therapeutic strategies to overcome such resistance. Undoubtedly, circumventing endocrine resistance should help to improve therapy for the benefit of breast cancer patients.

Identification of androgen receptor variants in testis from humans and other vertebrates

The androgen receptor (AR) is a ligand-activated transcription factor member of the nuclear receptor superfamily. The existence of alternatively spliced variants is well recognised for several members of this superfamily, most of them having functional importance. For example, several testicular oestrogen receptor variants have been suggested to play a role in the regulation of spermatogenesis. However, information on AR variants is mostly related to cancer and androgen insensitivity syndrome (AIS) cases. The objective of this study was to investigate the expression of AR variants in the testis from humans and other vertebrates. Four AR variants [ARΔ2(Stop) , ARΔ2(23Stop) , ARΔ3 and ARΔ4(120)] were identified in human testis. ARΔ2(Stop) and ARΔ3, with exon 2 or 3 deleted, respectively, were also expressed in human liver, lung, kidney and heart. In addition, ARΔ2(Stop) was expressed in rat and gilthead seabream testis, while an ARΔ3 was detected in African clawed frog testis. This is the first report revealing the existence of AR variants in the testis of evolutionarily distant vertebrate species and in nonpathological tissues. These data suggest the functional importance of these novel AR forms and demonstrate a complexity in AR signalling that is not exclusive of pathological conditions.

Development, characterization, and applications of a novel estrogen receptor beta monoclonal antibody

The role of estrogen receptor alpha (ERα) in breast cancer has been studied extensively, and its protein expression is prognostic and a primary determinant of endocrine sensitivity. However, much less is known about the role of ERβ and its relevance remains unclear due to the publication of conflicting reports. Here, we provide evidence that much of this controversy may be explained by variability in antibody sensitivity and specificity and describe the development, characterization, and potential applications of a novel monoclonal antibody targeting full-length human ERβ and its splice variant forms. Specifically, we demonstrate that a number of commercially available ERβ antibodies are insensitive for ERβ and exhibit significant cross-reaction with ERα. However, our newly developed MC10 ERβ antibody is shown to be highly specific and sensitive for detection of full-length ERβ and its variant forms. Strong and variable staining patterns for endogenous levels of ERβ protein were detected in normal human tissues and breast tumors using the MC10 antibody. Importantly, ERβ was shown to be expressed in a limited cohort of both ERα positive and ERα negative breast tumors. Taken together, these data demonstrate that the use of poorly validated ERβ antibodies is likely to explain much of the controversy in the field with regard to the biological relevance of ERβ in breast cancer. The use of the MC10 antibody, in combination with highly specific antibodies targeting only full-length ERβ, is likely to provide additional discriminatory features in breast cancers that may be useful in predicting response to therapy.

Glucocorticoid-independent repression of tumor necrosis factor (TNF) alpha-stimulated interleukin (IL)-6 expression by the glucocorticoid receptor: a potential mechanism for protection against an excessive inflammatory response

TNFα signaling and cytokine levels play a crucial role in cervical immunity and the host response to infections. We investigated the role of liganded and unliganded glucocorticoid receptor (GR) in IL-6 and IL-8 gene regulation in response to TNFα in the End1/E6E7 immortalized human endocervical epithelial cell line. In the absence of glucocorticoids, both decreasing GR protein levels by an siRNA strategy and results with the GR antagonist RU486 suggest a role for the unliganded GR in reduction of TNFα-induced IL-6 and IL-8 mRNA levels in End1/E6E7 cells. Moreover, GR-dependent repression of endogenous IL-6 mRNA as well as a minimal IL-6 promoter-reporter gene is also demonstrated in COS-1 cells in the absence of GR ligand, suggesting a transcriptional mechanism that is not cell-specific. TNFα induced recruitment of both the unliganded GR and GR-interacting protein type 1 (GRIP-1) to the IL-6 promoter. This, together with GRIP-1 overexpression studies, suggests a function for GRIP-1 as a GR co-repressor in this context. TNFα was shown to induce phosphorylation of the unliganded human GR at Ser-226 but not Ser-211, unlike dexamethasone, which induced hyperphosphorylation at both serine residues. Ser-226 is shown to be required for the ligand-independent GR-mediated repression of IL-6 in response to TNFα. Taken together, these results support a model whereby the unliganded GR attenuates TNFα-stimulated IL-6 transcription by a mechanism involving selective phosphorylation and recruitment of the unliganded GR and GRIP-1 to the IL-6 promoter. These findings suggest the presence of a novel autoregulatory mechanism that may prevent overproduction of IL-6 in the endocervix, possibly protecting against negative effects of excessive inflammation.

Selective mutations in estrogen receptor alpha D-domain alters nuclear translocation and non-estrogen response element gene regulatory mechanisms

The three main mechanisms of ERα action are: 1) nuclear, genomic, direct DNA binding, 2) nuclear, genomic, "tethered"-mediated, protein-protein interactions, and 3) non-nuclear, non-genomic, rapid action responses. Reports suggest the D-domain or hinge region of ERα plays an important role in mechanisms 1 and 2 above. Studies demonstrating the functionality of the ERα hinge region have resected the full D-domain; therefore, site directed mutations were made to attribute precise sequence functionality to this domain. This study focuses on the characterization and properties of three novel site directed ERα- D-domain mutants. The Hinge 1 (H1) ERα mutant has disrupted nuclear localization, can no longer perform tethered mediated responses and has lost interaction with c-Jun, but retains estrogen response element (ERE)-mediated functions as demonstrated by confocal microscopy, reporter assays, endogenous gene expression and co-immunoprecipitation. The H2 ERα mutant is non-nuclear, but translocates to the nucleus with estradiol (E2) treatment and maintains ERE-mediated functionality. The H2+NES ERα mutant does not maintain nuclear translocation with hormone binding, no longer activates ERE-target genes, functions in ERE- or tethered-mediated luciferase assays, but does retain the non-genomic, non-nuclear, rapid action response. These studies reveal the sequence(s) in the ERα hinge region that are involved in tethered-mediated actions as well as nuclear localization and attribute important functionality to this region of the receptor. In addition, the properties of these ERα mutants will allow future studies to further dissect and characterize the three main ERα mechanisms of action and determine the mechanistic role each action has in estrogen hormone regulation.

Identification of C-terminally and N-terminally truncated estrogen receptor α variants in the mouse

We re-examined mouse ERα mRNA variants using rapid amplification of cDNA ends (RACE) and RT-PCR. Our analysis showed the presence of several mRNA variants containing unique 5'- or 3'-nucleotide sequences. We mapped the cDNA sequences on the mouse genome, and identified four novel 3'-terminal and 5'-leader exons in the intronic region between exons 4 and 5. RT-PCR analysis revealed that the expression patterns of the C-terminally truncated ERα products (CTERPs) were similar to that of Wild-type ERα and that the N-terminally truncated ERα products (NTERPs) appeared to have different expression profiles. Moreover, we constructed expression vectors and analyzed the subcellular localization and the transcriptional activation abilities of the variant proteins in transfected HEK293 cells using immunocytochemistry and luciferase reporter assay. The CTERP variants localized in the nuclei and constitutively activated estrogen response element (ERE)-driven promoters, while the NTERP variant was located in the extra-nuclear regions and had no ability to activate the ERE promoters in the presence or absence of 10 nM estradiol. Our results indicate that the mouse ERα gene is more complex than previously thought in terms of genomic organization and that alternative splicing and alternative usage of intronic promoters contribute to the remarkable diversity of ERα mRNAs and proteins.

Identification and functional analysis of NOL7 nuclear and nucleolar localization signals

BACKGROUND

NOL7 is a candidate tumor suppressor that localizes to a chromosomal region 6p23. This locus is frequently lost in a number of malignancies, and consistent loss of NOL7 through loss of heterozygosity and decreased mRNA and protein expression has been observed in tumors and cell lines. Reintroduction of NOL7 into cells resulted in significant suppression of in vivo tumor growth and modulation of the angiogenic phenotype. Further, NOL7 was observed to localize to the nucleus and nucleolus of cells. However, the mechanisms regulating its subcellular localization have not been elucidated.

RESULTS

An in vitro import assay demonstrated that NOL7 requires cytosolic machinery for active nuclear transport. Using sequence homology and prediction algorithms, four putative nuclear localization signals (NLSs) were identified. NOL7 deletion constructs and cytoplasmic pyruvate kinase (PK) fusion proteins confirmed the functionality of three of these NLSs. Site-directed mutagenesis of PK fusions and full-length NOL7 defined the minimal functional regions within each NLS. Further characterization revealed that NLS2 and NLS3 were critical for both the rate and efficiency of nuclear targeting. In addition, four basic clusters within NLS2 and NLS3 were independently capable of nucleolar targeting. The nucleolar occupancy of NOL7 revealed a complex balance of rapid nucleoplasmic shuttling but low nucleolar mobility, suggesting NOL7 may play functional roles in both compartments. In support, targeting to the nucleolar compartment was dependent on the presence of RNA, as depletion of total RNA or rRNA resulted in a nucleoplasmic shift of NOL7.

CONCLUSIONS

These results identify the minimal sequences required for the active targeting of NOL7 to the nucleus and nucleolus. Further, this work characterizes the relative contribution of each sequence to NOL7 nuclear and nucleolar dynamics, the subnuclear constituents that participate in this targeting, and suggests a functional role for NOL7 in both compartments. Taken together, these results identify the requisite protein domains for NOL7 localization, the kinetics that drive this targeting, and suggest NOL7 may function in both the nucleus and nucleolus.

Hormone binding and co-regulator binding to the glucocorticoid receptor are allosterically coupled

The glucocorticoid receptor initiates the cellular response to glucocorticoid steroid hormones in vertebrates. Co-regulator proteins dock to the receptor in response to hormone binding and potentiate the transcriptional activity of the receptor by modifying DNA and recruiting essential transcription factors like RNA polymerase II. Hormones and co-regulators bind at distinct sites in the ligand binding domain yet function cooperatively to mediate transcriptional control. This study reveals and quantifies energetic coupling between two binding sites using purified components. Using a library of peptides taken from co-regulator proteins, we determine the pattern of co-regulator binding to the glucocorticoid receptor ligand binding domain. We show that peptides from co-regulators differ in their effects on hormone binding and kinetics. Peptides from DAX1 and SRC1 bind with similar affinity, but DAX1 binding is coupled to hormone binding, and SRC1 is not. Mechanistic details of co-regulator binding and coupling to the hormone binding pocket are uncovered by analysis of properties endowed by mutation of a key residue in the allosteric network connecting the sites.

Molecular chaperones, essential partners of steroid hormone receptors for activity and mobility

Steroid hormone receptors (SHRs) are notorious intracellular travellers, transiting among different cellular compartments as they mature, are subjected to regulation and exert their biological functions. Understanding the processes governing the intracellular traffic of SHRs is important, since their unbalanced or erroneous localization could lead to the development of diseases. In this review, we not only explore the functions of the heat-shock protein 90 (Hsp90) molecular chaperone machine for the intracellular transport of SHRs, but also for the regulation of their nuclear mobility, for their recycling and for the regulation of their transcriptional output.

Control of glucocorticoid and progesterone receptor subcellular localization by the ligand-binding domain is mediated by distinct interactions with tetratricopeptide repeat proteins

The TPR proteins FKBP52, FKBP51, Cyp40, and PP5 are found in steroid receptor (SR) complexes, but their receptor-specific preferences and roles remain unresolved. We have undertaken a systematic approach to this problem by examining the contribution of all four TPRs to the localization properties of glucocorticoid (GR) and progesterone (PR) receptors. The GR of L929 cells was found in the cytoplasm in a complex containing PP5 and FKBP51, while the GR of WCL2 cells was nuclear and contained PP5 and FKBP52. Cyp40 did not interact with the GR in either cell line. To test whether FKBP interaction determined localization, we overexpressed Flag-tagged FKBP51 in WCL2 cells and Flag-FKBP52 in L929 cells. In WCL2 cells, the GR exhibited a shift to greater cytoplasmic localization that correlated with recruitment of Flag-FKBP51. In contrast, Flag-FKBP52 was not recruited to the GR of L929 cells, and no change in localization was observed, suggesting that both cell-type-specific mechanisms and TPR abundance contribute to the SR-TPR interaction. As a further test, GR-GFP and PR-GFP constructs were expressed in COS cells. The GR-GFP construct localized to the cytoplasm, while the PR-GFP construct was predominantly nuclear. Similar to L929 cells, the GR in COS interacted with PP5 and FKBP51, while PR interacted with FKBP52. Analysis of GR-PR chimeric constructs revealed that the ligand-binding domain of each receptor determines both TPR specificity and localization. Lastly, we analyzed GR and PR localization in cells completely lacking TPR. PR in FKBP52 KO cells showed a complete shift to the cytoplasm, while GR in FKBP51 KO and PP5 KO cells showed a moderate shift to the nucleus, indicating that both TPRs contribute to GR localization. Our results demonstrate that SRs have distinct preferences for TPR proteins, a property that resides in the LBD and which can now explain long-standing differences in receptor subcellular localization.

The A/B domain of the teleost glucocorticoid receptors influences partial nuclear localization in the absence of hormone

The glucocorticoid (GR) and mineralocorticoid receptor (MR) of extant jawed vertebrates emerged after duplication of an ancestral corticosteroid receptor. The ancestral corticosteroid receptor resembled extant MRs in hormone selectivity, and the different ligand specificity of extant GRs is a secondary derived characteristic. An additional characteristic that distinguishes the mammalian GR from the MR is the cellular distribution pattern in the absence of hormone: the naïve GR resides in the cytoplasm, whereas the naïve MR is found in both the nucleus and cytoplasm. Our results show, by the use of green fluorescent protein-tagged fusion proteins, that the GRs [rainbow trout (rt) GR1 and rtGR2] from a lower vertebrate, the teleost fish, rainbow trout (Oncorhynchus mykiss) resemble mammalian MR rather than GR in their subcellular localization pattern. The addition of cortisol caused the remaining cytoplasmic rtGR1 and rtGR2 to migrate to the nucleus. The speed of nuclear localization was cortisol concentration dependent, with rtGR2 being more sensitive than rtGR1, mimicking the transactivational properties of the receptors in which the cortisol EC50 value is an order of magnitude lower for rtGR2. By the use of chimera constructs between the trout GRs and the rat GR C656G, we show that the E domain of the trout receptors are not involved in the nucleocytoplasmic localization of naïve trout GRs, but the A/B domain, especially if linked to the corresponding trout CD region, plays a pivotal role in the cellular distribution pattern. This is unrelated to the difference in the trout GRs transactivation sensitivity, which is determined by the receptor's E-domains.

Molecular mechanisms of endocrine resistance and their implication in the therapy of breast cancer

The use of endocrine agents is a safe and effective treatment in the management of hormone-sensitive breast cancer. Unfortunately, sooner or later, tumor cells develop resistance to endocrine manipulation making useless this approach. During the last decade, new molecules and intracellular signaling pathways involved in endocrine resistance have been identified. Several studies have documented that estrogen receptor signaling may maintain a pivotal role in the tumor growth despite the failure of a previous hormonal treatment. In this review we will discuss the general principles for optimizing the choice of endocrine therapy based on an understanding of the molecular mechanisms responsible for resistance to the different anti-hormonal agents.

Estrogen receptor beta isoform-specific induction of transforming growth factor beta-inducible early gene-1 in human osteoblast cells: an essential role for the activation function 1 domain

The estrogen receptors (ER) alpha and beta are important ligand-mediated transcription factors known to play significant biological roles in numerous tissues including bone. Despite the high homology shared by these receptors, recent studies have suggested that their function is largely unique. Although these receptors have been studied in detail for more than a decade, little data exist concerning the mechanisms by which these two proteins regulate distinct sets of genes. Using the TGFbeta-inducible early gene-1 (TIEG) as a model, we demonstrate that TIEG is rapidly induced in response to estrogen in osteoblasts by ERbeta, but not ERalpha. We have identified the regulatory elements utilized by ERbeta and have demonstrated that ERbeta recruits steroid receptor coactivator (SRC)1 and SRC2 to this regulatory region. Additionally, deletion of the ERbeta-activation function 1 (AF1) domain drastically decreases the estrogen induction of TIEG. Through the use of chimeric receptors, we have demonstrated that the AF1 domain of ERbeta is responsible for recruiting SRC1 and SRC2 and inducing the expression of TIEG in osteoblasts. Finally, SRC1, but not SRC2, is essential for TIEG induction by ERbeta. Overall, these data demonstrate that the estrogen induction of TIEG is ERbeta specific and that the AF1 domain of ERbeta confers this specificity. Finally, a novel and important role for ERbeta's AF1 is implicated in the recruitment of specific coactivators, suggesting that the AF1 may play a significant role in conferring the differences in regulation of gene expression by these two receptors.

Regulatory function of the P295-T311 motif of the estrogen receptor alpha - does proteasomal degradation of the receptor induce emergence of peptides implicated in estrogenic responses?

The way in which estrogen receptor α (ERα) mediates gene transcription and hormone-dependent cancer cell proliferation is now being largely reconsidered in view of several recent discoveries. ERα-mediated transcription appears to be a cyclic and transient process where the proteasome - and thus receptor degradation - plays a pivotal role. In view of our recent investigations, which demonstrate the estrogenic activity of a synthetic peptide corresponding to a regulatory motif of the receptor (ERα17p), we propose that ERα proteasomal degradation could induce the emergence of regulatory peptide(s). The latter would function as a signal and contribute to the ERα activation process, amplifying the initial hormonal stimulation and giving rise to sustained estrogenic response.

Nucleocytoplasmic transport of proteins

In eukaryotic cells, the movement of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (NPC)--a large protein complex spanning the nuclear envelope. The nuclear transport of proteins is usually mediated by a family of transport receptors known as karyopherins. Karyopherins bind to their cargoes via recognition of nuclear localization signal (NLS) for nuclear import or nuclear export signal (NES) for export to form a transport complex. Its transport through NPC is facilitated by transient interactions between the karyopherins and NPC components. The interactions of karyopherins with their cargoes are regulated by GTPase Ran. In the current review, we describe the NPC structure, NLS, and NES, as well as the model of classic Ran-dependent transport, with special emphasis on existing alternative mechanisms; we also propose a classification of the basic mechanisms of protein transport regulation.

Trophic effect in MCF-7 cells of ERalpha17p, a peptide corresponding to a platform regulatory motif of the estrogen receptor alpha--underlying mechanisms

As yet, estrogen receptor alpha (ERalpha) inhibitors used in clinical practice target a unique site, i.e. the hormone-binding pocket. With the aim of discovering other potential therapeutic targets in the receptor, we studied its AF-2a domain, a site that proves to be critical for ligand-independent ERalpha activity. Previous studies from our laboratory highlighted an auto-inhibitory action associated with a site included in this domain, i.e. the P295-T311 sequence. Accordingly, a deletion of this sequence produces a constitutively activated receptor mutant. More interestingly, a synthetic peptide with the P295-T311 sequence (ERalpha17p) elicits in breast cancer cell lines estrogenic responses that may be ascribed to a competitive mechanism towards the P295-T311-associated auto-inhibition of ERalpha. In the present study, we show that ERalpha17p sustains MCF-7 cell growth in estrogen-depleted culture medium by inducing molecular events promoting G1/S phase transition. We demonstrate, moreover, that this proliferative activity is associated with receptor down regulation (acceleration of ERalpha degradation and repression of ESR1 gene transcription), similar to that induced by estrogen agonists. Complementary studies suggest that our observations may be, at least in part, relevant to a competitive inhibition affecting ERalpha-Hsp70 association. Hence, the design of drugs able to stabilize ERalpha-Hsp70 complexes - where the receptor is in an inactive conformation - may be of therapeutic value.

Estrogen receptor beta: an overview and update

The discovery of a second estrogen receptor (ER), designated ERbeta (NR3A2), has redefined our knowledge about the mechanisms underlying cellular signaling by estrogens and has broad implications for our understanding of regulation of estrogen-responsive tissues. Highly variable and even contrasting effects of estrogens in different tissues seem to be at least partially explained by different estrogen signaling pathways, involving ERalpha (NR3A1) and/or ERbeta. To date, two key conclusions can be drawn from the significant body of work carried out on the specific roles of the two receptor subtypes in diverse estrogen target tissues. First, ERalpha and ERbeta have different biological functions, as indicated by their specific expression patterns and the distinct phenotypes observed in ERalpha and ERbeta knockout (alphaERKO and betaERKO) mice. Second, ERalpha and ERbeta appear to have overlapping but also unique sets of downstream target genes, as judged from a set of microarray experiments. Thus, ERalpha and ERbeta have different transcriptional activities in certain ligand, cell-type, and promoter contexts, which may help to explain some of the major differences in their tissue-specific biological actions. The phenotypes observed for betaERKO mice have suggested certain therapeutic areas to be further explored. The development of ERbeta-selective ligands active in animal disease models indicates new avenues for clinical exploration. ERbeta agonists are being explored and validated as drugs for a growing number of indications. Hopefully, some ERbeta targeted drugs will prove to be efficient in enhancing human health.

Influence of hormone on intracellular localization of the Drosophila melanogaster ecdysteroid receptor (EcR)

In the absence of hormone the ecdysteroid receptor (EcR) is distributed between the cytoplasm and the nucleus. Addition of the hormone muristerone A increases nuclear localization of wild type EcR within 5-10 min. Mutation of M504 to alanine, an amino acid, which is essential for ligand binding and which is situated in helix 5 of the ligand binding domain, abolishes hormone binding but still allows nuclear localization at only slightly reduced levels in the absence of hormone, whereas nuclear localization of EcR(M504R) is nearly abolished. Cotransfection with ultraspiracle (USP), the invertebrate ortholog of RXR, leads to exclusively nuclear localization of wild type EcR and EcR(M504A) indicating that basal heterodimerization in the absence of hormone is still possible. In the presence of Usp, EcR(M504R) is only partially localized in the nucleus. EMSA experiments show that the ligand muristerone A enhances binding of wild type EcR, but only slightly of mutated EcRs, to the canonical hsp 27 ecdysone response element. This is confirmed by transactivation studies. The results indicate that the architecture of the E-domain of EcR is important for nuclear localization even in the absence of a ligand.

ERbeta shifts from mitochondria to nucleus during estrogen-induced neoplastic transformation of human breast epithelial cells and is involved in estrogen-induced synthesis of mitochondrial respiratory chain proteins

Both estrogen receptors (ER) alpha (ERalpha) and beta (ERbeta) are localized in the nucleus, plasma membrane, and mitochondria, where they mediate the different physiological effects of estrogens. It has been observed that the relative subcellular localization of ERs is altered in several cancer cells. We have demonstrated that MCF-10F cells, the immortal and non-tumorigenic human breast epithelial cells (HBEC) that are ERalpha-negative and ERbeta-positive, are transformed in vitro by 17beta-estradiol (E(2)), generating highly invasive cells that are tumorigenic in severe combined immunodeficient mice. E(2)-transformed MCF-10F (trMCF) cells exhibit progressive loss of ductulogenesis, invasive (bsMCF) and tumorigenic (caMCF) phenotypes. Immunolocalization of ERbeta by confocal fluorescent microscopy and electron microscopy revealed that ERbeta is predominantly localized in mitochondria of MCF-10F and trMCF cells. Silencing ERbeta expression with ERbeta-specific small interference RNA (siRNA-ERbeta) markedly diminishes both nuclear and mitochondrial ERbeta in MCF-10F cells. The ERbeta shifts from its predominant localization in the mitochondria of MCF-10F and trMCF cells to the nucleus of bsMCF cells, becoming predominantly nuclear in caMCF cells. Furthermore, we demonstrated that the mitochondrial ERbeta in MCF-10F cells is involved in E(2)-induced expression of mitochondrial DNA (mtDNA)-encoded respiratory chain (MRC) proteins. This is the first report of an association of changes in the subcellular localization of ERbeta with various stages of E(2)-induced transformation of HBEC and a functional role of mitochondrial ERbeta in mediating E(2)-induced MRC protein synthesis. Our findings provide a new insight into one of the potential roles of ERbeta in human breast cancer.