Estrogen-induced mitogenesis of MCF-7 cells does not require the induction of mitogen-activated protein kinase activity

A Modified Formulation of Sutaehwan Ameliorates Menopausal Anxiety, Depression and Heart Hypertrophy in the VCD-Induced Menopausal Mouse Model

Sutaehwan (STH) has been used in Korean medicine for the treatment of abortus habitualis such as fetal restlessness in the uterus. Previously, we reported that a modified formulation of STH, Sutaehwan-Gami, has phytoestrogen-like properties in an ovariectomized menopausal rat model. However, the therapeutic effects of STH and the precise mechanisms by which STH affects various menopausal symptoms remain poorly understood. The current study was designed to explore the effects of a modified form of STH on menopausal anxiety, depression and heart hypertrophy and its mechanisms in 4-vinylcyclohexene diepoxide (VCD)-induced menopausal mouse models. VCD-induced menopausal model mice were fed a modified form of STH, which contained water extract of 3 herbs (called STH_KP17001) at a dose of 100 or 300 mg/kg/d or as a positive control, estradiol at a dose of 0.2 mg/kg/d with standard mouse pellets for 13 weeks. The results show that STH_KP17001 significantly restored the VCD-induced weight reduction of uterine and ovary through the phosphorylation of extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) in the uterus and ovary. Moreover, STH_KP17001 showed slight proliferative effects and estrogen receptor α phosphorylation in MCF-7 cells. Treatment with STH_KP17001 reversed VCD-induced anxiety and depression through AMP-activated protein kinase (AMPK) activation and brain-derived neurotrophic factor (BDNF) expression in the cerebral cortex, while improving heart hypertrophy through inactivation of inhibitor of kappaB α (IκBα) in the heart. The results indicate that STH_KP17001 improves menopause-induced anxiety, depression and heart hypertrophy, implying its protective role for the management of menopausal symptoms.

Estradiol-mediated regulation of hepatic iNOS in obese rats: Impact of Src, ERK1/2, AMPKα, and miR-221

PURPOSE

This study aimed to investigate in vivo effects of estradiol on the regulation of hepatic inducible nitric oxide synthase (iNOS) expression in the high fat (HF) diet-induced obesity. Also, we aimed to investigate whether activation of the extracellular signal-regulated kinase (ERK1/2), adenosine monophosphate-activated protein kinase (AMPK), Src kinase, and miR-221 is involved in estradiol-mediated regulation of iNOS in the liver of obese male Wistar rats. Male Wistar rats were fed a standard laboratory diet or a HF diet for 10 weeks. Half of HF rats were treated with estradiol intraperitoneally (40 μg/kg), whereas the other half were placebo-treated 24 H before euthanasia. Results show that estradiol treatment of HF rats decreased hepatic iNOS mRNA (P < 0.05) and protein expression (P < 0.01), the protein levels of p65 subunit of nuclear factor κB (P < 0.05) and ERα (P < 0.05), ERK1/2 phosphorylation (P < 0.001), and ERα/Src kinase association (P < 0.05). By contrast, hepatic Src protein level (P < 0.05), AMPKα phosphorylation (P < 0.05), and miR-221 expression (P < 0.05) were increased in HF rats after estradiol treatment. Our results indicate that estradiol in vivo regulates hepatic iNOS expression in obese rats via molecular mechanisms involving ERK1/2, AMPK, Src, and miR-221 signaling.

Assessment of cell line competence for studies of pharmacological GPR30 modulation

CONTEXT/OBJECTIVE

Cell lines used to study the role of the G protein-coupled receptor 30 (GPR30) or G protein-coupled estrogen receptor (GPER) as a mediator of estrogen responses have yielded conflicting results. This work identified a simple assay to predict cell line competence for pharmacological studies of GPR30.

MATERIALS AND METHODS

The phosphorylation or expression levels of ERK1/2, Akt, c-Fos and eNOS were evaluated to assess GPR30 activation in response to known agonists (17β-estradiol and G-1) in MCF-7 and T-47D breast cancer cell lines and in bovine aortic endothelial cells. GPR30 expression was analyzed by qRT-PCR and Western blot with two distinct antibodies directed at its carboxy and amino terminals.

RESULTS

None of the agonists, at any of the concentrations tested, activated any of those target proteins. Additional experiments excluded the disruption of the signaling pathway, interference of phenol red in the culture medium and constitutive proteasome degradation of GPR30 as possible causes for the lack of response of the three cell lines. Analysis of receptor expression showed the absence of clearly detectable GPR30 species of 44 and 50-55 kDa previously identified in cell lines that respond to 17β-estradiol and G-1.

DISCUSSION AND CONCLUSION

Cells that do not express the 44 and 50-55 kDa species do not respond to GPR30 agonists. Thus, the presence or absence of these GPR30 species is a simple and rapid manner to determine whether a given cell line is suitable for pharmacological or molecular studies of GPR30 modulation.

Requirement of ERα and basal activities of EGFR and Src kinase in Cd-induced activation of MAPK/ERK pathway in human breast cancer MCF-7 cells

Cadmium (Cd) is a common environmental toxicant and an established carcinogen. Epidemiological studies implicate Cd with human breast cancer. Low micromolar concentrations of Cd promote proliferation of human breast cancer cells in vitro. The growth promotion of breast cancer cells is associated with the activation of MAPK/ERK pathway. This study explores the mechanism of Cd-induced activation of MAPK/ERK pathway. Specifically, the role of cell surface receptors ERα, EGFR, and Src kinase was evaluated in human breast cancer MCF-7 cells treated with 1-3μM Cd. The activation of ERK was studied using a serum response element (SRE) luciferase reporter assay. Receptor phosphorylation was detected by Western blot analyses. Cd treatment increased both the SRE reporter activity and ERK1/2 phosphorylation in a concentration-dependent manner. Cd treatment had no effect on reactive oxygen species (ROS) generation. Also, blocking the entry of Cd into the cells with manganese did not diminish Cd-induced activation of MAPK/ERK. These results suggest that the effect of Cd was likely not caused by intracellular ROS generation, but through interaction with the membrane receptors. While Cd did not appear to activate either EGFR or Src kinase, their inhibition completely blocked the Cd-induced activation of ERK as well as cell proliferation. Similarly, silencing ERα with siRNA or use of ERα antagonist blocked the effects of Cd. Based on these results, it is concluded that not only ERα, but also basal activities of EGFR and Src kinase are essential for Cd-induced signal transduction and activation of MAPK/ERK pathway for breast cancer cell proliferation.

Estrogen activation of the mitogen-activated protein kinase is mediated by ER-α36 in ER-positive breast cancer cells

It is well known that there are two estrogen-signaling pathways, genomic estrogen signaling and non-genomic or rapid estrogen signaling. Although both ER-α and ER-β have been suggested to mediate both genomic and non-genomic estrogen signaling, rapid estrogen signaling such as activation of the MAPK/ERK signaling in ER-positive breast cancer MCF7 cells has been controversial. Previously, our laboratory cloned a 36kDa variant of ER-α, ER-α36, that is mainly localized at the plasma membrane and is able to mediate rapid estrogen signaling. In this study, we investigated the function and the underlying mechanisms of ER-α36 in rapid estrogen signaling of ER-positive breast cancer cells. ER-positive breast cancer cells MCF7, T47D and H3396 as well as their variants with different levels of ER-α and ER-α36 expression were used to examine estrogen induction of the MAPK/ERK1/2 signaling. The underlying mechanisms were also studied in these cells with the neutralizing antibodies and chemical inhibitors against different growth factors and their receptors. We found that ER-α36 mediated estrogen induction of the MAPK/ERK phosphorylation in ER-positive breast cancer cells while the full-length ER-α failed to do so. The rapid estrogen signaling mediated by ER-α36 involved a orchestrated action of matrix metalloproteinases (MMPs), heparin-binding epidermal growth factor (HB-EGF), amphiregulin, insulin-like growth factor 1 receptor (IGF-1R), epidermal growth factor receptor (EGFR), HER2/Neu and Src. Our results thus indicated that ER-α36 is the estrogen receptor that mediates estrogen induction of the MAPK/ERK signaling in ER-positive breast cancer cells.

Responses of PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) in MCF-7 cells are culture condition dependent

To compare the effects of the food toxin 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) and estradiol in hormone-responsive MCF-7 cells, the cells were exposed to different concentrations of either PhIP or estradiol. The effect of various culture conditions (e.g. phenol red, FBS, vehicle (DMSO/EtOH) and seeding density) on responses was studied. Cells were continuously grown with steroid-containing or -deprived medium, or switched from steroid-containing to -deprived medium for the experiments to minimize the effect of background estrogenicity. Effects of PhIP and estradiol on cell viability and proliferation were determined by ATP analysis and Ki-67 immunocytochemistry. Expression of estrogen receptor alpha, cell stress markers (p53 and ERK) and estrogen responsive proteins (c-myc and ERK) were immunoblotted. All concentrations of estradiol induced cell proliferation, viability and changes in protein expression, typical for estrogenic responses. PhIP, however, increased viability only at low concentrations and depending on culture conditions. No changes in protein expressions by PhIP were noted, not even when switching cells from steroid-containing to -deprived medium which down-regulated the expression of proteins at basal level. Vehicle affected significantly viability, especially after exposure to PhIP, but not protein expression while medium changes affected both. In conclusion, the effects of PhIP and estradiol in MCF-7 cells are dependent on culture conditions. The detected PhIP-induced changes are weaker compared to those induced by estradiol.

Genetic screening reveals an essential role of p27kip1 in restriction of breast cancer progression

The genetic changes and mechanisms underlying the progression of estrogen-dependent breast cancers to estrogen-independent, antiestrogen-resistant, and metastatic breast cancers are unclear despite being a major problem in endocrine therapy. To identify genes responsible for this progression, we carried out a genetic screening by an enhanced retroviral mutagen (ERM)-mediated random mutagenesis in the estrogen-dependent T47D breast cancer cells. We found that T47D cells contain only one p27kip1 (p27) allele coding for the p27 cyclin-dependent kinase (CDK) inhibitor. An ERM insertion into the p27 locus of T47D cells disrupted the p27 gene and created estrogen-independent and antiestrogen-resistant breast cancer cells that still maintained functional estrogen receptors. Disruption of p27 in T47D cells resulted in several changes, and most of these changes could be rescued by p27 restoration. First, CDK2 activity was increased in the absence of estrogen or in the presence of estrogen antagonists tamoxifen or ICI 182780; second, amplified in breast cancer 1 (AIB1), a cancer overexpressed transcriptional coactivator, was hyperphosphorylated, which made AIB1 a better coactivator for E2F1; and third, growth factor receptor binding protein 2-associated binder 2 (Gab2) and Akt activity were increased following E2F1 overactivation, leading to a significant enhancement of cell migration and invasion. Furthermore, the p27-deficient cells, but not T47D control cells, developed lung metastasis in an ovarian hormone-independent manner when they were i.v. injected into nude mice. In sum, loss of p27 activated AIB1, E2F1, Gab2, and Akt; increased cell migration and invasion; caused antiestrogen insensitivity; and promoted metastasis of breast cancer cells. These findings suggest that p27 plays an essential role in restriction of breast cancer progression.

Cyclin D1 is necessary for tamoxifen-induced cell cycle progression in human breast cancer cells

Despite the success of tamoxifen in treating hormone-responsive breast cancer, its use is limited by the development of resistance to the drug. Understanding the pathways involved in the growth of tamoxifen-resistant cells may lead to new ways to treat tamoxifen-resistant breast cancer. Here, we investigate the role of cyclin D1, a mediator of estrogen-dependent proliferation, in growth of tamoxifen-resistant cells using a cell culture model of acquired resistance to tamoxifen. We show that tamoxifen and 4-hydroxytamoxifen (OHT) promoted cell cycle progression of tamoxifen-resistant cells after growth-arrest mediated by the estrogen receptor down-regulator ICI 182,780. Down-regulation of cyclin D1 with small interfering RNA blocked basal cell growth of tamoxifen-resistant cells and induction of cell proliferation by OHT. In addition, pharmacologic inhibition of phosphatidylinositol 3-kinase/Akt or mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 pathways decreased basal cyclin D1 expression and impaired OHT-mediated cyclin D1 induction and cell cycle progression. These findings indicate that cyclin D1 expression is necessary for proliferation of tamoxifen-resistant cells and for tamoxifen-induced cell cycle progression. These results suggest that therapeutic strategies to block cyclin D1 expression or function may inhibit development and growth of tamoxifen-resistant tumors.

Therapeutic intervention of experimental breast cancer bone metastasis by indole-3-carbinol in SCID-human mouse model

Several lines of experimental evidence have suggested that chemokine receptor CXCR4, a metastasis-promoting molecule, may play important roles in breast cancer bone metastasis. There is emerging evidence linking CXCR4 to matrix metalloproteinases (MMP) as well as their regulator nuclear factor-kappaB (NF-kappaB), a key transcription factor, which is known to activate metastasis-promoting molecules for many types of malignancies, including breast cancer. A recent study also showed that promoter region of CXCR4 has several NF-kappaB-binding sites, suggesting that there may be a cross-talk between CXCR4 and NF-kappaB. We have shown previously that indole-3-carbinol (I3C), a natural compound present in vegetables of the genus Brassica, can inhibit NF-kappaB in breast cancer cells. However, there are no reports in the literature showing any effect of I3C on CXCR4 expression in vitro and in vivo. We therefore examined whether I3C could inhibit bone metastasis of breast cancer by inhibiting CXCR4 and MMP-9 expression mediated via the inhibition of the NF-kappaB signaling pathway. Here, we have modified the severe combined immunodeficient (SCID)-human mouse model of experimental bone metastasis for use with the MDA-MB-231 breast cancer cell line. In this animal model, we found that I3C significantly inhibited MDA-MB-231 bone tumor growth, and our results were correlated with the down-regulation of NF-kappaB. Moreover, we found that I3C significantly inhibited the expression of multiple genes involved in the control of metastasis and invasion in vitro and in vivo, especially the expression of CXCR4 and MMP-9 along with pro-MMP-9, with concomitant decrease in Bcl-2 and increase in the proapoptotic protein Bax. From these results, we conclude that the CXCR4/NF-kappaB pathway is critical during I3C-induced inhibition of experimental breast cancer bone metastasis. These results also suggest that I3C could be a promising agent for the prevention and/or treatment of breast cancer bone metastasis in the future.

Rapamycin inhibits proliferation of estrogen-receptor-positive breast cancer cells

BACKGROUND

Estrogen-receptor (ER)-positive breast cancers comprise the majority of sporadic breast cancers. Although 50% respond to antihormonal treatment, both primary and acquired resistance limit the utility of this therapy, and other agents are needed. Rapamycin, an inhibitor of the mammalian Target of Rapamycin (mTOR), possesses antitumor activity against many tumors including breast tumors, and particularly against ER-positive breast cancer cell lines. The sensitivity of these cells to rapamycin has been attributed to activation of the PI3K/Akt/mTOR pathway by nongenomic ER signaling. The purpose of this study was to evaluate the efficacy of rapamycin against ER-positive breast cancer, particularly under 17beta-estradiol (E2)-dependent conditions, and to investigate mechanisms of rapamycin-sensitivity in ER-positive cells.

MATERIALS AND METHODS

Breast cancer cell lines were tested for sensitivity to rapamycin. Antiproliferative effects of rapamycin, alone and in combination with tamoxifen, were assessed under E2-dependent conditions. Western blot analysis was used to detect activation of mTOR by nongenomic ER signaling.

RESULTS

Rapamycin effectively inhibits proliferation of the ER-positive MCF-7 cell line. In our system, this sensitivity is probably not due to nongenomic ER activation of the PI3K/Akt/mTOR pathway; rapid stimulation of mTOR occurred nonspecifically after medium replacement, and addition of E2 stimulated mTOR only after 1 h. Combining rapamycin and tamoxifen under E2-dependent conditions yielded additive/synergistic effects at effective concentrations.

CONCLUSIONS

These results suggest that rapamycin may be an effective treatment for ER-positive breast cancer, either alone or in combination with tamoxifen, and also may be a potential therapy for tamoxifen-resistant cancers.

Elevated expression of mitogen-activated protein kinase phosphatase 3 in breast tumors: a mechanism of tamoxifen resistance

Antiestrogen resistance is a major clinical problem in the treatment of breast cancer. Altered growth factor signaling with estrogen receptor (ER)-alpha is associated with the development of resistance. Gene expression profiling was used to identify mitogen-activated protein kinase (MAPK) phosphatase 3 (MKP3) whose expression was correlated with response to the antiestrogen tamoxifen in both patients and in vitro-derived cell line models. Overexpression of MKP3 rendered ER-alpha-positive breast cancer cells resistant to the growth-inhibitory effects of tamoxifen and enhanced tamoxifen agonist activity in endometrial cells. MKP3 overexpression was associated with lower levels of activated extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in the presence of estrogen but that estrogen deprivation and tamoxifen treatment decreased MKP3 phosphatase activity, leading to an up-regulation of pERK1/2 MAPK, phosphorylated Ser(118)-ER-alpha, and cyclin D1. The MAPK/ERK kinase inhibitor PD98059 blocked tamoxifen-resistant growth. Accumulation of reactive oxygen species was observed with tamoxifen treatment of MKP3-overexpressing cells, and antioxidant treatment increased MKP3 phosphatase activity, thereby blocking resistance. Furthermore, PD98059 increased the levels of phosphorylated c-Jun NH(2)-terminal kinase (JNK) in tamoxifen-treated MKP3-overexpressing cells, suggesting an interaction between MKP3 levels, activation of ERK1/2 MAPK, and JNK signaling in human breast cancer cells. MKP3 represents a novel mechanism of resistance, which may be a potential biomarker for the use of ERK1/2 and/or JNK inhibitors in combination with tamoxifen treatment.

Estradiol-17β stimulates proliferation of mouse embryonic stem cells: involvement of MAPKs and CDKs as well as protooncogenes

Although the importance of estradiol-17β (E2) in many physiological processes has been reported, to date no researchers have investigated the effects of E2 on embryonic stem (ES) cell proliferation. Therefore, in the present study, we have examined the effect of E2 on the DNA synthesis of murine ES (ES-E14TG2a) cells and its related signaling pathways. The results of this study show that E2 (10−9 M) significantly increased [3H]thymidine incorporation at >4 h and that E2 (>10−12 M) induced an increase of [3H]thymidine incorporation after 8-h incubation. Moreover, E2 (>10−12 M) also increased 5′-bromo-2′-deoxyuridine (BrdU) incorporation and cell number. Indeed, E2 stimulated estrogen receptor (ER)-α and -β protein levels and increased mRNA expression levels of protooncogenes (c- fos, c- jun, and c- myc). Tamoxifen (antiestrogen) completely inhibited E2-induced increases in [3H]thymidine incorporation. In addition, estradiol-6- O-carboxymethyl oxime-BSA (E2-BSA; 10−9 M) increased [3H]thymidine incorporation at >1 h, and E2-BSA (>10−12 M) increased [3H]thymidine incorporation after 1-h incubation. E2-BSA-induced increase in BrdU incorporation also occurred in a dose-dependent manner. Tamoxifen had no effect on E2-BSA-induced increase of [3H]thymidine incorporation. Also, E2 and E2-BSA displayed maximal phosphorylation of p44/42 MAPKs at 10 and 5 min, respectively. E2 increased cyclins D1 and E as well as cyclin-dependent kinase (CDK)2 and CDK4. In contrast, E2 decreased the levels of p21cip1 and p27kip1 (CDK-inhibitory proteins). Increases of these cell cycle regulators were blocked by 10−5 M PD-98059 (MEK inhibitor). Moreover, E2-induced increase of [3H]thymidine incorporation was inhibited by PD-98059 or butyrolactone I (CDK2 inhibitor). In conclusion, estradiol-17β stimulates the proliferation of murine ES cells, and this action is mediated by MAPKs, CDKs, or protooncogenes.

Difficulty demonstrating estradiol-mediated Erk1/2 phosphorylation in MCF-7 cells

While some studies report that estradiol (E2) activates extracellular-signal regulated kinase (Erk1/2) in MCF-7 breast cancer cells, others report E2 does not activate this signaling pathway. This study attempted to resolve the conflicting reports by investigating experimental variables that could impact Erk1/2 activation using a high through-put assay that quantitatively assessed Erk1/2 phosphorylation. Variables tested included: cell staging and dosing regimes with and without charcoal-stripped serum, different MCF-7 cell sublines and culture densities and several E2 formulations and solvents. Levels of phosphorylated Erk1/2 were normalized to cellular protein rather than to total Erk1/2 protein because an antibody purported to recognize total Erk1/2 preferentially reacted with non-phosphorylated Erk1/2, potentially exaggerating the apparent level of Erk1/2 activation. Dosing MCF-7 cells with E2 containing small amounts of stripped serum induced Erk1/2 phosphorylation; however, this induction was largely attributed to serum factors. E2 administered in serum-free medium did not significantly alter Erk1/2 phosphorylation under any condition tested; immunocytochemistry corroborated this conclusion. While phosphatase inhibitors generally increased Erk1/2 phosphorylation, they did not impact E2-altered Erk1/2 phosphorylation. It remains important to resolve the basis of conflicting reports regarding E2-induced Erk1/2 activation due to the potential importance of this pathway on breast cancer and other processes.

Thyroid hormone can increase estrogen-mediated transcription from a consensus estrogen response element in neuroblastoma cells

Thyroid hormones (T) and estrogens (E) are nuclear receptor ligands with at least two molecular mechanisms of action: (i) relatively slow genomic effects, such as the regulation of transcription by cognate T receptors (TR) and E receptors (ER); and (ii) relatively rapid nongenomic effects, such as kinase activation and calcium release initiated at the membrane by putative membrane receptors. Genomic and nongenomic effects were thought to be disparate and independent. However, in a previous study using a two-pulse paradigm in neuroblastoma cells, we showed that E acting at the membrane could potentiate transcription from an E-driven reporter gene in the nucleus. Because both T and E can have important effects on mood and cognition, it is possible that the two hormones can act synergistically. In this study, we demonstrate that early actions of T via TRalpha1 and TRbeta1 can potentiate E-mediated transcription (genomic effects) from a consensus E response element (ERE)-driven reporter gene in transiently transfected neuroblastoma cells. Such potentiation was reduced by inhibition of mitogen-activated protein kinase. Using phosphomutants of ERalpha, we also show that probable mitogen-activated protein kinase phosphorylation sites on the ERalpha, the serines at position 167 and 118, are important in TRbeta1-mediated potentiation of ERalpha-induced transactivation. We suggest that crosstalk between T and E includes potential interactions through both nuclear and membrane-initiated molecular mechanisms of hormone signaling.

Kinase-mediated transcription, activators of nongenotropic estrogen-like signaling (ANGELS), and osteoporosis: a different perspective on the HRT dilemma

Studies in bone, as well as other nonreproductive target tissues of sex steroid, like the cardiovascular and the central nervous system (CNS), have elucidated a previously unappreciated mechanism of sex steroid action involving the rapid activation of mitogen-activated protein kinases and/or phosphatidyl inositol 3 kinase, and consequent potent regulatory affects on the transcription of a set of genes that is distinct from that regulated through classic (genotropic) control of transcription. These actions stem from an unexpected function of the classic nuclear receptors outside the nucleus, most probably from receptor interactions within distinct signal transduction pathways in preassembled scaffolds. Importantly, these nongenotropic actions are mediated by the ligand-binding domain of the receptor and can be functionally dissociated from classic transcriptional activation with synthetic ligands, termed activators of nongenotropic estrogen-like signaling (ANGELS). We highlight this evidence and discuss its pharmacotherapeutic implications vis a vis the dilemmas posed by the recently appreciated shortfalls of postmenopausal hormone replacement therapy.

Identification of the First Specific Inhibitor of p90 Ribosomal S6 Kinase (RSK) Reveals an Unexpected Role for RSK in Cancer Cell Proliferation

p90 ribosomal S6 kinase (RSK) is an important downstream effector of mitogen-activated protein kinase, but its biological functions are not well understood. We have now identified the first small-molecule, RSK-specific inhibitor, which we isolated from the tropical plant Forsteronia refracta. We have named this novel inhibitor SL0101. SL0101 shows remarkable specificity for RSK. The major determinant of SL0101-binding specificity is the unique ATP-interacting sequence in the amino-terminal kinase domain of RSK. SL0101 inhibits proliferation of the human breast cancer cell line MCF-7, producing a cell cycle block in G1 phase with an efficacy paralleling its ability to inhibit RSK in intact cells. RNA interference of RSK expression confirmed that RSK regulates MCF-7 proliferation. Interestingly, SL0101 does not alter proliferation of a normal human breast cell line MCF-10A, although SL0101 inhibits RSK in these cells. We show that RSK is overexpressed in ∼50% of human breast cancer tissue samples, suggesting that regulation of RSK has been compromised. Thus, we show that RSK has an unexpected role in proliferation of transformed cells and may be a useful new target for chemotherapeutic agents. SL0101 will provide a powerful new tool to dissect the molecular functions of RSK in cancer cells.

Membrane estrogen receptor-alpha levels predict estrogen-induced ERK1/2 activation in MCF-7 cells

INTRODUCTION

We examined the participation of a membrane form of estrogen receptor (mER)-alpha in the activation of mitogen-activated protein kinases (extracellular signal-regulated kinase [ERK]1 and ERK2) related to cell growth responses in MCF-7 cells.

METHODS

We immunopanned and subsequently separated MCF-7 cells (using fluorescence-activated cell sorting) into mER-alpha-enriched (mERhigh) and mER-alpha-depleted (mERlow) populations. We then measured the expression levels of mER-alpha on the surface of these separated cell populations by immunocytochemical analysis and by a quantitative 96-well plate immunoassay that distinguished between mER-alpha and intracellular ER-alpha. Western analysis was used to determine colocalized estrogen receptor (ER)-alpha and caveolins in membrane subfractions. The levels of activated ERK1 and ERK2 were determined using a fixed cell-based enzyme-linked immunosorbent assay developed in our laboratory.

RESULTS

Immunocytochemical studies revealed punctate ER-alpha antibody staining of the surface of nonpermeabilized mERhigh cells, whereas the majority of mERlow cells exhibited little or no staining. Western analysis demonstrated that mERhigh cells expressed caveolin-1 and caveolin-2, and that ER-alpha was contained in the same gradient-separated membrane fractions. The quantitative immunoassay for ER-alpha detected a significant difference in mER-alpha levels between mERhigh and mERlow cells when cells were grown at a sufficiently low cell density, but equivalent levels of total ER-alpha (membrane plus intracellular receptors). These two separated cell subpopulations also exhibited different kinetics of ERK1/2 activation with 1 pmol/l 17beta-estradiol (E2), as well as different patterns of E2 dose-dependent responsiveness. The maximal kinase activation was achieved after 10 min versus 6 min in mERhigh versus mERlow cells, respectively. After a decline in the level of phosphorylated ERKs, a reactivation was seen at 60 min in mERhigh cells but not in mERlow cells. Both 1A and 2B protein phosphatases participated in dephosphorylation of ERKs, as demonstrated by efficient reversal of ERK1/2 inactivation with okadaic acid and cyclosporin A.

CONCLUSION

Our results suggest that the levels of mER-alpha play a role in the temporal coordination of phosphorylation/dephosphorylation events for the ERKs in breast cancer cells, and that these signaling differences can be correlated to previously demonstrated differences in E2-induced cell proliferation outcomes in these cell types.

Prolonged extracellular signal-regulated kinase 1/2 activation during fibroblast growth factor 1- or heregulin beta1-induced antiestrogen-resistant growth of breast cancer cells is resistant to mitogen-activated protein/extracellular regulated kinase kinase inhibitors

Increased growth factor receptor signaling is implicated in antiestrogen-resistant breast tumors suggesting that abrogation of such signaling could restore or prolong sensitivity to antihormonal agents. Activation of the mitogen-activated protein/extracellular regulated kinase kinase (MEK)-extracellular regulated kinase (ERK)1/2 cascade is a common component of such pathways. We investigated the ability of the MEK activation inhibitor U0126 to block the increased growth of estrogen receptor-positive MCF-7 breast cancer cells caused by fibroblast growth factor 1 (FGF-1), heregulin beta1 (HRGbeta1), and epidermal growth factor (EGF) in the presence of the pure antiestrogen ICI 182780 (Faslodex; fulvestrant). We found that either FGF-1 or HRGbeta1 but not EGF substantially reduced the inhibitory effects of U0126 on growth and ERK1/2 activation, including the combined inhibitory effects of U0126 and ICI 182780. FGF-1 and HRGbeta1 also reduced the inhibition of ERK1/2 phosphorylation by the MEK inhibitors PD98059 and PD184161. Interestingly, a transiently transfected dominant-negative MEK1 completely abrogated activation of a coexpressed green fluorescent protein-ERK2 reporter by all three of the factors. Despite a short-lived activation of Ras and Raf-1 by all three of the growth factors, both FGF-1 and HRGbeta1, unlike EGF, induced a prolonged activation of MEK and ERK1/2 in these cells. Thus, activation of FGF-1- and HRGbeta1-specific signaling causes MEK-dependent prolonged activation of ERK1/2, which is incompletely susceptible to known MEK inhibitors. We also demonstrate that the cytosolic phospholipase A2 inhibitor arachidonyl trifluoro methyl ketone and the pan PKC inhibitor bisindolymaleimide abrogated U0126-resistant phosphorylation of ERK1/2 induced by HRGbeta1 but not by FGF-1. Phosphorylation of ERK5 by all three of the factors was also resistant to U0126 suggesting that its activation is not sufficient to overturn growth inhibition due to diminished ERK1/2 activation. Therefore, therapy combining antiestrogens and MEK inhibitors may be ineffective in some antiestrogen-resistant estrogen receptor-positive breast cancers.

Mitogenic activity of estrogens in human breast cancer cells does not rely on direct induction of mitogen-activated protein kinase/extracellularly regulated kinase or phosphatidylinositol 3-kinase

We have addressed the question of rapid, nongenomic mechanisms that may be involved in the mitogenic action of estrogens in hormone-dependent breast cancer cells. In quiescent, estrogen-deprived MCF-7 cells, estradiol did not induce a rapid activation of either the MAPK/ERK or phosphatidylinositol-3 kinase (PI-3K)/Akt pathway, whereas the entry into the cell cycle was documented by the successive inductions of cyclin D1 expression, hyperphosphorylation of the retinoblastoma protein (Rb), activity of the promoter of the cyclin A gene, and DNA synthesis. However, pharmacological inhibitors of the src family kinases, 4-amino-5-(4-methylphenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP1) or of the PI-3K (LY294002) did prevent the entry of the cells into the cell cycle and inhibited the late G1 phase progression, whereas the inhibitor of MAPK/ERK activation (U0126) had only a partial inhibitory effect in the early G1 phase. In agreement with these results, small interfering RNA targeting Akt strongly inhibited the estradiolinduced cell cycle progression monitored by the activation of the promoter of the cyclin A gene. The expression of small interfering RNA targeting MAPK 1 and 2 also had a clear inhibitory effect on the estradiol-induced activation of the cyclin A promoter and also antagonized the estradiol-induced transcription directed by the estrogen response element. Finally, transfection of the estrogen receptor into NIH3T3 fibroblasts did not confer to the cells sensitivity to a mitogenic action of estradiol. We conclude that the induction of the cell cycle by estradiol does not require a direct activation of MAPK/ERK or PI-3K signaling protein kinase cascades, but that these kinases appear to have a permissive role in the cell cycle progression.

Thyroid hormone causes mitogen-activated protein kinase-dependent phosphorylation of the nuclear estrogen receptor

Activated by thyroid hormone, the MAPK (ERK1/2) signaling pathway causes serine phosphorylation by MAPK of several nucleoproteins, including the nuclear thyroid hormone receptor beta1. Because estrogen can activate MAPK and cause MAPK-dependent serine phosphorylation of nuclear estrogen receptor (ER)alpha, we studied whether thyroid hormone also promoted MAPK-mediated ERalpha phosphorylation. Human breast cancer (MCF-7) cells were incubated with physiological concentrations of l-T(4) or 17beta-estradiol (E(2)) for 15 min to 24 h, and nuclear ERalpha and serine-118-phosphorylated ERalpha were identified by Western blotting. Serine-118-phosphorylated ERalpha was recovered at 15 min in nuclei of MCF-7 cells exposed to either T(4) or E(2). The T(4) effect was apparent at 15 min and peaked at 2 h, whereas the E(2) effect was maximal at 4-6 h. T(4)-agarose was as effective as T(4) in causing phosphorylation of ERalpha. T(4) action on ERalpha was inhibited by PD 98059, an inhibitor of ERK1/2 phosphorylation, and by tetraiodothyroacetic acid, a T(4) analog that blocks cell surface-initiated actions of T(4) but is not itself an agonist. Electrophoretic mobility shift assay of nuclear extracts from T(4)-treated and E(2)-treated cells showed similar specific protein-DNA-binding. Indexed by [(3)H]thymidine incorporation and nuclear proliferating cell nuclear antigen, MCF-7 cell proliferation was stimulated by T(4) and T(4)-agarose to an extent comparable with the effect of E(2). This T(4) effect was blocked by either PD 98059 or ICI 182,780, an ER antagonist. Thus, T(4), like E(2), causes phosphorylation by MAPK of nuclear ERalpha at serine-118 in MCF-7 cells and promotes cell proliferation through the ER by a MAPK-dependent pathway.

Enhanced estrogen receptor (ER) alpha, ERBB2, and MAPK signal transduction pathways operate during the adaptation of MCF-7 cells to long term estrogen deprivation

The mechanisms involved in resistance to estrogen deprivation are of major importance for optimal patient therapy and the development of new drugs. Long term culture of MCF-7 cells in estrogen (E2)-depleted medium (long term estrogen deprivation; LTED) results in hypersensitivity to E2 coinciding with elevated levels of estrogen receptor (ER) alpha phosphorylated on Ser118 and MAPK, together with several of its downstream targets associated previously with ERalpha phosphorylation. Our data suggest elevated MAPK activity results from enhanced ERBB2 expression in the LTED cells versus the wild-type (wt), and treatment with the tyrosine kinase inhibitor ZD1839 revealed increased sensitivity in both transcription and proliferation assays. Similarly the MEK inhibitor U0126 decreased transcription and proliferation in the LTED cells and reduced their sensitivity to the proliferative effects of E2, while having no effect on the wt. However, the complete suppression of MAPK activity in the LTED cells did not inhibit ERalpha Ser118 phosphorylation suggesting that ER activity remained ligand-dependant. The LTED cells also expressed elevated levels of insulin-like growth factor-1R, and inhibition of phosphatidylinositol 3-kinase activity with LY294002 reduced basal ERalpha transactivation by 70% in the LTED cells compared with the wt. However, LY294002 had no effect on ERalpha Ser118 phosphorylation. These data suggest that although elevated levels of MAPK occur during LTED and influence the phenotype, this is unlikely to be the sole pathway operating to achieve adaptation.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.

Steroid hormone mimics: molecular mechanisms of cell growth and apoptosis in normal and malignant mammary epithelial cells

Anti-estrogen (anti-E2) therapy with E2 receptor antagonists has a significant benefit in women with breast cancer, but it may also increase the risk for developing hormone-independent breast cancer for which there is no therapy similar to that used in hormone-dependent breast cancer. Therefore, there is a significant interest in the development of compounds that may provide therapeutic benefit for hormone-independent breast cancer without untoward risks and adverse effects. The estrogen receptor (ER) modulators with both agonistic as well as antagonistic properties may, thus, be exploited for the development of the next generation of compounds for the prevention and/or treatment of breast cancer. In this article, we have discussed the clinical indications, risks, benefits and mechanisms of action of ER modulators and related compounds, particularly indole-3-carbinol (I3C), which may open new avenues for the prevention and/or treatment of breast cancer.