17β-Estradiol stimulates the translocation of endogenous estrogen receptor α at the plasma membrane of normal anterior pituitary cells

Perinatal DEHP exposure modulates pituitary estrogen receptor α and β expression altering lactotroph and somatotroph cell growth in prepuberal and adult male rats

Phthalates are synthetic chemicals widely used to make polyvinylchloride (PVC) soft and flexible. Of these, Di-(2-ethylhexyl) phthalate (DEHP) is the most commonly used, with high human exposure occurring as early as the fetal developmental stage and affecting the endocrine system. We focused on the perinatal DEHP effects on pituitary estrogen receptor (ER) expression in male rats, explored their impact on lactotroph and somatotroph cell growth, and evaluated the direct effects of this phthalate on pituitary cell cultures. Our results showed that DEHP perinatal exposure was unable to modify the ERα+ pituitary cell number from prepuberal rats, but increased ERβ+ cells. In adulthood, the pituitary ERα+ cells underwent a slight decrease with ERβ showing the greatest changes, and with a significant increase observed in somatotroph cells. Also, in vitro, DEHP reduced the ERα+ cells, increased the percentage of ERβ+ pituitary cells and modified the Ki67 index, as well as decreasing the lactotrophs and increasing the somatotroph cells. In conclusion, the present study showed that DEHP induced ER expression changes in normal pituitary glands from male rats in in vivo and in vitro conditions, suggesting that DEHP could differentially modulate lactotroph and somatotroph cell growth, possibly as a consequence of ER imbalance.

The phthalate DEHP modulates the estrogen receptors α and β increasing lactotroph cell population in female pituitary glands

Humans are exposed to numerous endocrine disruptors on a daily basis, which may interfere with endogenous estrogens, with Di-(2-ethylhexyl) phthalate (DEHP) being one of the most employed. The anterior pituitary gland is a target of 17β-estradiol (E2) through the specific estrogen receptors (ERs) α and β, whose expression levels fluctuate in the gland under different contexts, and the ERα/β index is responsible for the final E2 effect. The aim of the present study was to evaluate in vivo and in vitro the DEHP effects on ERα and β expression in the pituitary cell population, and also its impact on lactotroph and somatotroph cell growth. Our results revealed that perinatal exposure to DEHP altered the ERα and β expression pattern in pituitary glands from prepubertal and adult female rats and increased the percentage of lactotroph cells in adulthood. In the in vitro system, DEHP down-regulated ERα and β expression, and as a result increased the ERα/β ratio and decreased the percentages of lactotrophs and somatotrophs expressing ERα and β. In addition, DEHP increased the S + G2M phases, Ki67 index and cyclin D1 in vitro, leading to a rise in the lactotroph and somatotroph cell populations. These results showed that DEHP modified the pituitary ERα and β expression in lactotrophs and somatotrophs from female rats and had an impact on the pituitary cell growth. These changes in ER expression may be a mechanism underlying DEHP exposure in the pituitary gland, leading to cell growth deregulation.

The ERα membrane pool modulates the proliferation of pituitary tumours

The molecular mechanisms underlying the ERα nuclear/cytoplasmic pool that modulates pituitary cell proliferation have been widely described, but it is still not clear how ERα is targeted to the plasma membrane. The aim of this study was to analyse ERα palmitoylation and the plasma membrane ERα (mERα) pool, and their participation in E2-triggered membrane-initiated signalling in normal and pituitary tumour cell growth. Cell cultures were prepared from anterior pituitaries of female Wistar rats and tumour GH3 cells, and treated with 10 nM of oestradiol (E2). The basal expression of ERα was higher in tumour GH3 than in normal pituitary cells. Full-length palmitoylated ERα was observed in normal and pituitary tumour cells, demonstrating that E2 stimulation increased both, ERα in plasma membrane and ERα and caveolin-1 interaction after short-term treatment. In addition, the Dhhc7 and Dhhc21 palmitoylases were negatively regulated after sustained stimulation of E2 for 3 h. Although the uptake of BrdU into the nucleus in normal pituitary cells was not modified by E2, a significant increase in the GH3 tumoural cell, as well as ERK1/2 activation, with this effect being mimicked by PPT, a selective antagonist of ERα. These proliferative effects were blocked by ICI 182780 and the global inhibitor of palmitoylation. These findings indicate that ERα palmitoylation modulated the mERα pool and consequently the ERK1/2 pathway, thereby contributing to pituitary tumour cell proliferation. These results suggest that the plasma membrane ERα pool might be related to the proliferative behaviour of prolactinoma and may be a marker of pituitary tumour growth.

Estradiol-induced regulation of GLUT4 in 3T3-L1 cells: involvement of ESR1 and AKT activation

Impaired insulin-stimulated glucose uptake involves reduced expression of the GLUT4 (solute carrier family 2 facilitated glucose transporter member 4, SLC2A4 gene). 17β-estradiol (E₂) modulates SLC2A4/GLUT4 expression, but the involved mechanisms are unclear. Although E₂ exerts biological effects by binding to estrogen receptors 1/2 (ESR1/2), which are nuclear transcriptional factors; extranuclear effects have also been proposed. We hypothesize that E₂ regulates GLUT4 through an extranuclear ESR1 mechanism. Thus, we investigated the effects of E₂ upon (1) subcellular distribution of ESRs and the proto-oncogene tyrosine-protein kinases (SRC) involvement; (2) serine/threonine-protein kinase (AKT) activation; (3) Slc2a4/GLUT4 expression and (4) GLUT4 subcellular distribution and glucose uptake in 3T3-L1 adipocytes. Differentiated 3T3-L1 adipocytes were cultivated or not with E₂ for 24 h, and additionally treated or not with ESR1-selective agonist (PPT), ESR1-selective antagonist (MPP) or selective SRC inhibitor (PP2). Subcellular distribution of ESR1, ESR2 and GLUT4 was analyzed by immunocytochemistry; Slc2a4 mRNA and GLUT4 were quantified by qPCR and Western blotting, respectively; plasma membrane GLUT4 translocation and glucose uptake were analyzed under insulin stimulus for 20 min or not. E₂ induced (1) translocation of ESR1, but not of ESR2, from nucleus to plasma membrane and AKT phosphorylation, effects mimicked by PPT and blocked by MPP and PP2; (2) increased Slc2a4/GLUT4 expression and (3) increased insulin-stimulated GLUT4 translocation and glucose uptake. In conclusion, E₂ treatment promoted a SRC-mediated nucleus-plasma membrane shuttle of ESR1, and increased AKT phosphorylation, Slc2a4/GLUT4 expression and plasma membrane GLUT4 translocation; consequently, improving insulin-stimulated glucose uptake. These results unravel mechanisms through which estrogen improves insulin sensitivity.

Estrogen receptor β regulates the tumoral suppressor PTEN to modulate pituitary cell growth

In this study, we focused on ERβ regulation in the adenohypophysis under different estrogenic milieu, by analyzing whether ER modulates the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression and its subcellular localization on anterior pituitary glands from Wistar rats and GH3 lactosomatotroph cells that over-expressed ERβ. ERβ was regulated in a cyclic manner, and underwent dynamic changes throughout the estrous cycle, with decreased ERβ+ cells in estrus and under E2 treatment, but increased in ovariectomized rats. In addition, the ERα/β ratio increased in estrus and under E2 stimulation, but decreased in ovariectomized rats. Double immunofluorescence revealed that lactotroph and somatotroph ERβ+ were significantly decreased in estrus. Also, variations in the PTEN expression was observed, which was diminished with high E2 conditions but augmented with low E2 milieu. The subcellular localization of this phosphatase was cell cycle-dependent, with remarkable changes in the immunostaining pattern: nuclear in arrested pituitary cells but cytoplasmic in stimulated cells, and responding differently to ER agonists, with only DPN being able to increase PTEN expression and retaining it in the nucleus. Finally, ERβ over-expression increased PTEN with a noticeable subcellular redistribution, and with a significant nuclear signal increase in correlation with an increase of cells in G0/G1 phase. These results showed that E2 is able to inhibit ERβ expression and suggests that the tumoral suppressor PTEN might be one of the signaling proteins by which E2, through ERβ, acts to modulate pituitary cell proliferation, thereby adapting endocrine populations in relation with hormonal necessities.

Activation of ClC-3 chloride channel by 17β-estradiol relies on the estrogen receptor α expression in breast cancer

Although extensively studied, the mechanisms by which estrogen promotes breast cancer growth remain to be fully elucidated. Tamoxifen, an antiestrogen agent to treat ERα+ breast cancer, is also a high-affinity blocker of the chloride channels. In this study, we explored the involvement of the chloride channels in the action of estrogen in breast cancer. We found that 17β-estradiol (17β-E2) concentration-dependently activated the chloride currents in ERα+ breast cancer MCF-7 cells. Extracellular hypertonic challenge and chloride channel blockers, NPPB and DIDS inhibited the 17β-E2-activated chloride currents. Decreased the ClC-3 protein expression caused the depletion of the 17β-E2-activated chloride currents. 17β-E2-activated chloride currents which relied on the ERα expression were demonstrated by the following evidences. Firstly, 17β-E2-activated chloride currents could not be observed in ERα- breast cancer MDA-MB-231 cells. Secondly, ER antagonists, tamoxifen and ICI 182,780, and downregulation of ERα expression inhibited or abolished the 17β-E2-activated chloride currents. Thirdly, ERα expression was induced in MDA-MB-231 cells by ESR1 gene transfection, and then 17β-E2-activated chloride currents could be observed. In MCF-7 cells, ERα and ClC-3 mainly located in nucleus and translocated to cell plasma and membrane with respect to co-localization following treatment of 17β-E2. Downregulation of ERα expression could decrease the expression of ClC-3 protein. Conversely, downregulation of ClC-3 expression did not influence the ERα expression. Taken together, our findings demonstrated that ClC-3 is a potential target of 17β-E2 and is modulated by the ERα in breast cancer cell. Pharmacological modulation of ClC-3 may provide a deep understanding in antiestrogen treatment of breast cancer patients.

ERα36--Another piece of the estrogen puzzle

Although the nuclear action of estrogen receptors (ER) is a well-known fact, evidence supporting membrane estrogen receptors is steadily accumulating. New ER variants of unrecognized function have been discovered. ERα is a product of the ESR1 gene. It serves not only as a template for the full-length 66kDa protein, but also for smaller isoforms which exist as independent receptors. The recently discovered ERα36 (36kDa), consisting of 310 amino acids of total 595 ERα66 protein residues, is an example of that group. The transcription initiation site is identified in the first intron of the ESR1 gene. C-Terminal 27 amino acids are encoded by previously unknown exon 9. The presence of this unique C-terminal sequence creates an opportunity for the production of selective antibodies. ERα36 has been shown to have a high affinity to the cell membrane and as much as 90% of the protein can be bound with it. Post-translational palmitoylation is suspected to play a crucial role in ERα36 anchoring to the cell membrane. In silico analysis suggests the existence of a potential transmembrane domain in ERα36. ERα36 was found in most cells of animals at various ages, but its exact physiological function remains to be fully elucidated. It seems that cells traditionally considered as being deprived of ER are able to respond to hormonal stimulation via the ERα36 receptor. Moreover, ERα36 displays unique pharmacological properties and its action may be behind antiestrogen resistance. The use of ERα36 in cancer diagnosis gives rise to great expectations.

Differential ERα-mediated rapid estrogenic actions of ginsenoside Rg1 and estren in human breast cancer MCF-7 cells

Recent studies indicated that both estren and Rg1 appear to be able to activate mitogen-activated protein kinase (MAPK) pathway in estrogen responsive cells. Rg1 could lead to MAPK activation through ligand-independent activation of estrogen receptor (ER), while estren could activate the Src-MAPK pathway in an ERE-independent manner. Thus, it is important to understand the mechanistic insights on the difference in transcriptional activation between estren and Rg1. The present study also addressed the differential abilities of Rg1 and estren in terms of the ability to activate ER and the ability to induce ER translocation in MCF-7 cells. Our data indicated that Rg1 could increase pS2 gene expression, and could recruit the co-activator steroid receptor co-activator-1 (SRC-1) to the pS2 promoter. Rg1 could also induce ERα nuclear translocation as well as ERα phosphorylation at Ser118 principally in the cytoplasm in MCF-7 cells. We deduced that estren induced ERE-dependent transcriptional activity and activated ERα at Ser118 occurred in the nucleus of MCF-7 cells. However, it was found to decrease pS2 gene expression and failed to induce the recruitment of SRC-1 to the pS2 promoter in MCF-7 cells. Our results suggest that the abilities of Rg1 and estren to regulate pS2 gene expression, to recruit co-activators as well as to induce sub-cellular distribution of ERα are dramatically different.

Cooperative effect of E₂ and FGF2 on lactotroph proliferation triggered by signaling initiated at the plasma membrane

In the present work, we investigated the effect of 17β-estradiol (E₂) and basic fibroblast growth factor 2 (FGF2) on the lactotroph cell-proliferative response and the related membrane-initiated signaling pathway. Anterior pituitary mixed-cell cultures of random, cycling 3-mo-old female rats were treated with 10 nM E₂, E₂ membrane-impermeable conjugated BSA (E₂-BSA), PPT (ERα agonist), and DPN (ERβ agonist) alone or combined with FGF2 (10 ng/ml) for 30 min or 4 h. Although our results showed that the uptake of BrdU into the nucleus of lactotrophs was not modified by E₂ or FGF2 alone, a significant increase in the lactotroph uptake of BrdU was observed after E₂/FGF2 coincubation, with this effect being mimicked by PPT/FGF2. These proliferative effects were blocked by ICI 182,780 or PD-98059. The involvement of membrane ER in the proliferative response of prolactin cells induced by the steroid and FGF2 coincubation was confirmed using E₂-BSA, and the association between ERα and FGF receptor was observed after E₂/FGF2 treatment by immunoprecipitation. A significant increase in the ERK1/2 expression was noted after E₂, E₂-BSA, PPT, and FGF2 alone, which was more noticeable after E₂-BSA/FGF2, E₂/FGF2, or PPT/FGF2 treatments. This study provides evidence that E₂ and FGF2 exert a cooperative effect on the lactotroph proliferation principally by signaling initiated at the plasma membrane triggering a genomic effect mediated by MEK/ERK1/2, a common signaling pathway, that finally regulates the lactotroph population, thus contributing to pituitary plasticity.

Membrane-initiated estradiol signaling in immortalized hypothalamic N-38 neurons

Regulation of sexual reproduction by estradiol involves the activation of estrogen receptors (ERs) in the hypothalamus. Of the two classical ERs involved in reproduction, ERα appears to be the critical isoform. The role of ERα in reproduction has been found to involve a nuclear ERα that induces a genomic mechanism of action. More recently, a plasma membrane ERα has been shown to trigger signaling pathways involved in reproduction. Mechanisms underlying membrane-initiated estradiol signaling are emerging, including evidence that activation of plasma membrane ERα involves receptor trafficking. The present study examined the insertion of ERα into the plasma membrane of N-38 neurons, an immortalized murine hypothalamic cell line. We identified, using western blotting and PCR that N-38 neurons express full-length 66kDa ERα and a 52kDa ERα spliced variant missing the fourth exon - ERαΔ4. Using surface biotinylation, we observed that treatment of N-38 neurons with estradiol or with a membrane impermeant estradiol elevated plasma membrane ERα protein levels, indicating that membrane signaling increased receptor insertion into the cell membrane. Insertion of ERα was blocked by the ER antagonist ICI 182,780 or with the protein kinase C (PKC) pathway inhibitor bisindolylmaleimide (BIS). Downstream membrane-initiated signaling was confirmed by estradiol activation of PKC-theta (PKCθ) and the release of intracellular calcium. These results indicate that membrane ERα levels in N-38 neurons are dynamically autoregulated by estradiol.

Extranuclear ERα is associated with regression of T47D PKCα-overexpressing, tamoxifen-resistant breast cancer

Background

Prior to the introduction of tamoxifen, high dose estradiol was used to treat breast cancer patients with similar efficacy as tamoxifen, albeit with some undesirable side effects. There is renewed interest to utilize estradiol to treat endocrine resistant breast cancers, especially since findings from several preclinical models and clinical trials indicate that estradiol may be a rational second-line therapy in patients exhibiting resistance to tamoxifen and/or aromatase inhibitors. We and others reported that breast cancer patients bearing protein kinase C alpha (PKCα)- expressing tumors exhibit endocrine resistance and tumor aggressiveness. Our T47D:A18/PKCα preclinical model is tamoxifen-resistant, hormone-independent, yet is inhibited by 17β-estradiol (E2) in vivo. We previously reported that E2-induced T47D:A18/PKCα tumor regression requires extranuclear ERα and interaction with the extracellular matrix.

Methods

T47D:A18/PKCα cells were grown in vitro using two-dimensional (2D) cell culture, three-dimensional (3D) Matrigel and in vivo by establishing xenografts in athymic mice. Immunofluoresence confocal microscopy and co-localization were applied to determine estrogen receptor alpha (ERα) subcellular localization. Co-immunoprecipitation and western blot were used to examine interaction of ERα with caveolin-1.

Results

We report that although T47D:A18/PKCα cells are cross-resistant to raloxifene in cell culture and in Matrigel, raloxifene induces regression of tamoxifen-resistant tumors. ERα rapidly translocates to extranuclear sites during T47D:A18/PKCα tumor regression in response to both raloxifene and E2, whereas ERα is primarily localized in the nucleus in proliferating tumors. E2 treatment induced complete tumor regression whereas cessation of raloxifene treatment resulted in tumor regrowth accompanied by re-localization of ERα to the nucleus. T47D:A18/neo tumors that do not overexpress PKCα maintain ERα in the nucleus during tamoxifen-mediated regression. An association between ERα and caveolin-1 increases in tumors regressing in response to E2.

Conclusions

Extranuclear ERα plays a role in the regression of PKCα-overexpressing tamoxifen-resistant tumors. These studies underline the unique role of extranuclear ERα in E2- and raloxifene-induced tumor regression that may have implications for treatment of endocrine-resistant PKCα-expressing tumors encountered in the clinic.

Estrogens induce expression of membrane-associated estrogen receptor α isoforms in lactotropes

Estrogens are key to anterior pituitary function, stimulating hormone release and controlling cell fate to achieve pituitary dynamic adaptation to changing physiological conditions. In addition to their classical mechanism of action through intracellular estrogen receptors (ERs), estrogens exert rapid actions via cell membrane-localized ERs (mERs). We previously showed that E2 exerts a rapid pro-apoptotic action in anterior pituitary cells, especially in lactotropes and somatotropes, through activation of mERs. In the present study, we examined the involvement of mERα in the rapid pro-apoptotic action of estradiol by TUNEL in primary cultures of anterior pituitary cells from ovariectomized rats using a cell-impermeable E2 conjugate (E2-BSA) and an ERα selective antagonist (MPP dihydrochloride). We studied mERα expression during the estrous cycle and its regulation by gonadal steroids in vivo by flow cytometry. We identified ERα variants in the plasma membrane of anterior pituitary cells during the estrous cycle and studied E2 regulation of these mERα variants in vitro by surface biotinylation and Western Blot. E2-BSA-induced apoptosis was abrogated by MPP in total anterior pituitary cells and lactotropes. In cycling rats, we detected a higher number of lactotropes and a lower number of somatotropes expressing mERα at proestrus than at diestrus. Acute E2 treatment increased the percentage of mERα-expressing lactotropes whereas it decreased the percentage of mERα-expressing somatotropes. We detected three mERα isoforms of 66, 39 and 22 kDa. Expression of mERα66 and mERα39 was higher at proestrus than at diestrus, and short-term E2 incubation increased expression of these two mERα variants. Our results indicate that the rapid apoptotic action exerted by E2 in lactotropes depends on mERα, probably full-length ERα and/or a 39 kDa ERα variant. Expression and activation of mERα variants in lactotropes could be one of the mechanisms through which E2 participates in anterior pituitary cell renewal during the estrous cycle.

17β-Estradiol modulates the prolactin secretion induced by TRH through membrane estrogen receptors via PI3K/Akt in female rat anterior pituitary cell culture

Considering that estradiol is a major modulator of prolactin (PRL) secretion, the aim of the present study was to analyze the role of membrane estradiol receptor-α (mERα) in the regulatory effect of this hormone on the PRL secretion induced by thyrotropin-releasing hormone (TRH) by focusing on the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway activation. Anterior pituitary cell cultures from female rats were treated with 17β-estradiol (E(2), 10 nM) and its membrane-impermeable conjugated estradiol (E(2)-BSA, 10 nM) alone or coincubated with TRH (10 nM) for 30 min, with PRL levels being determined by RIA. Although E(2), E(2)-BSA, TRH, and E(2)/TRH differentially increased the PRL secretion, the highest levels were achieved with E(2)-BSA/TRH. ICI-182,780 did not modify the TRH-induced PRL release but significantly inhibited the PRL secretion promoted by E(2) or E(2)-BSA alone or in coincubation with TRH. The PI3K inhibitors LY-294002 and wortmannin partially inhibited the PRL release induced by E(2)-BSA, TRH, and E(2)/TRH and totally inhibited the PRL levels stimulated by E(2)-BSA/TRH, suggesting that the mER mediated the cooperative effect of E(2) on TRH-induced PRL release through the PI3K pathway. Also, the involvement of this kinase was supported by the translocation of its regulatory subunit p85α from the cytoplasm to the plasma membrane in the lactotroph cells treated with E(2)-BSA and TRH alone or in coincubation. A significant increase of phosphorylated Akt was induced by E(2)-BSA/TRH. Finally, the changes of ERα expression in the plasmalemma of pituitary cells were examined by confocal microscopy and flow cytometry, which revealed that the mobilization of intracellular ERα to the plasma membrane of lactotroph cells was only induced by E(2). These finding showed that E(2) may act as a modulator of the secretory response of lactotrophs induced by TRH through mER, with the contribution by PI3K/Akt pathway activation providing a new insight into the mechanisms underlying the nongenomic action of E(2) in the pituitary.