Down-regulation of Bcl-2 enhances estrogen apoptotic action in long-term estradiol-depleted ER+ breast cancer cells

Revisiting Estrogen for the Treatment of Endocrine-Resistant Breast Cancer: Novel Therapeutic Approaches

Estrogen receptor (ER)-positive breast cancer is the most common subtype, representing 70-75% of all breast cancers. Several ER-targeted drugs commonly used include the selective estrogen receptor modulator (SERM), tamoxifen (TAM), aromatase inhibitors (AIs) and selective estrogen receptor degraders (SERDs). Through different mechanisms of action, all three drug classes reduce estrogen receptor signaling. Inevitably, resistance occurs, resulting in disease progression. The counterintuitive action of estrogen to inhibit ER-positive breast cancer was first observed over 80 years ago. High-dose estrogen and diethylstilbestrol (DES) were used to treat metastatic breast cancer accompanied by harsh side effects until the approval of TAM in the 1970s. After the development of TAM, randomized trials comparing TAM to estrogen found similar or slightly inferior efficacy but much better tolerability. After decades of research, it was learned that estrogen induces tumor regression only after a period of long-term estrogen deprivation, and the mechanisms of tumor regression were described. Despite the long history of breast cancer treatment with estrogen, this therapeutic modality is now revitalized due to the development of novel estrogenic compounds with improved side effect profiles, newly discovered predictive biomarkers, the development of non-estrogen small molecules and new combination therapeutic approaches.

Modeling breast cancer proliferation, drug synergies, and alternating therapies

Estrogen receptor positive (ER+) breast cancer is responsive to a number of targeted therapies used clinically. Unfortunately, the continuous application of targeted therapy often results in resistance, driving the consideration of combination and alternating therapies. Toward this end, we developed a mathematical model that can simulate various mono, combination, and alternating therapies for ER + breast cancer cells at different doses over long time scales. The model is used to look for optimal drug combinations and predicts a significant synergism between Cdk4/6 inhibitors in combination with the anti-estrogen fulvestrant, which may help explain the clinical success of adding Cdk4/6 inhibitors to anti-estrogen therapy. Furthermore, the model is used to optimize an alternating treatment protocol so it works as well as monotherapy while using less total drug dose.

BCL2 G quadruplex-binding small molecules: Current status and prospects for the development of next-generation anticancer therapeutics

B cell lymphoma 2 (BCL2) overexpression in a range of human tumors is often related to chemotherapy resistance and poor prognosis. GC-rich regions upstream of the P1 promoter in human BCL2 can form G-quadruplex (G4) structures through the stacking of four Hoogsteen-paired guanine bases. Stabilizing the G4 fold implies the inhibition of BCL2 expression and, thus, small molecules that selectively bind to the G4 are promising anticancer candidates. In this review, we discuss the structural aspects, binding affinity, selectivity, and biological activity of well-characterized BCL2 G4 binding ligands in vitro and in vivo. We also explore future directions in the research and development of G4-based anticancer therapeutics.

Mechanistic Effects of Estrogens on Breast Cancer

PURPOSE

Current concepts regarding estrogen and its mechanistic effects on breast cancer in women are evolving. This article reviews studies that address estrogen-mediated breast cancer development, the prevalence of occult tumors at autopsy, and the natural history of breast cancer as predicted by a newly developed tumor kinetic model.

METHODS

This article reviews previously published studies from the authors and articles pertinent to the data presented.

RESULTS

We discuss the concepts of adaptive hypersensitivity that develops in response to long-term deprivation of estrogen and results in both increased cell proliferation and apoptosis. The effects of menopausal hormonal therapy on breast cancer in postmenopausal women are interpreted based on the tumor kinetic model. Studies of the administration of a tissue selective estrogen complex in vitro, in vivo, and in patients are described. We review the various clinical studies of breast cancer prevention with selective estrogen receptor modulators and aromatase inhibitors. Finally, the effects of the underlying risk of breast cancer on the effects of menopausal hormone therapy are outlined.

DISCUSSION

The overall intent of this review is to present data supporting recent concepts, discuss pertinent literature, and critically examine areas of controversy.

TGFα-EGFR pathway in breast carcinogenesis, association with WWOX expression and estrogen activation

WWOX is a tumor-suppressive steroid dehydrogenase, which relationship with hormone receptors was shown both in animal models and breast cancer patients. Herein, through nAnT-iCAGE high-throughput gene expression profiling, we studied the interplay of estrogen receptors and the WWOX in breast cancer cell lines (MCF7, T47D, MDA-MB-231, BT20) under estrogen stimulation and either introduction of the WWOX gene by retroviral transfection (MDA-MB-231, T47D) or silenced with shRNA (MCF7, BT20). Additionally, we evaluated the consequent biological characteristics by proliferation, apoptosis, invasion, and adhesion assays. TGFα-EGFR signaling was found to be significantly affected in all examined breast cancer cell lines in response to estrogen and strongly associated with the level of WWOX expression, especially in ER-positive MCF7 cells. Under the influence of 17β-estradiol presence, biological characteristics of the cell lines were also delineated. The study revealed modulation of adhesion, invasion, and apoptosis. The obtained results point at a complex role of the WWOX gene in the carcinogenesis of the breast tissue, which seems to be closely related to the presence of estrogen α and/or β receptors.

Estrogen Receptors-Mediated Apoptosis in Hormone-Dependent Cancers

It is known that estrogen stimulates growth and inhibits apoptosis through estrogen receptor(ER)-mediated mechanisms in many cancer cell types. Interestingly, there is strong evidence that estrogens can also induce apoptosis, activating different ER isoforms in cancer cells. It has been observed that E2/ERα complex activates multiple pathways involved in both cell cycle progression and apoptotic cascade prevention, while E2/ERβ complex in many cases directs the cells to apoptosis. However, the exact mechanism of estrogen-induced tumor regression is not completely known. Nevertheless, ERs expression levels of specific splice variants and their cellular localization differentially affect outcome of estrogen-dependent tumors. The goal of this review is to provide a general overview of current knowledge on ERs-mediated apoptosis that occurs in main hormone dependent-cancers. Understanding the molecular mechanisms underlying the induction of ER-mediated cell death will be useful for the development of specific ligands capable of triggering apoptosis to counteract estrogen-dependent tumor growth.

The Eleanor ncRNAs activate the topological domain of the ESR1 locus to balance against apoptosis

MCF7 cells acquire estrogen-independent proliferation after long-term estrogen deprivation (LTED), which recapitulates endocrine therapy resistance. LTED cells can become primed for apoptosis, but the underlying mechanism is largely unknown. We previously reported that Eleanor non-coding RNAs (ncRNAs) upregulate the ESR1 gene in LTED cells. Here, we show that Eleanors delineate the topologically associating domain (TAD) of the ESR1 locus in the active nuclear compartment of LTED cells. The TAD interacts with another transcriptionally active TAD, which is 42.9 Mb away from ESR1 and contains a gene encoding the apoptotic transcription factor FOXO3. Inhibition of a promoter-associated Eleanor suppresses all genes inside the Eleanor TAD and the long-range interaction between the two TADs, but keeps FOXO3 active to facilitate apoptosis in LTED cells. These data indicate a role of ncRNAs in chromatin domain regulation, which may underlie the apoptosis-prone nature of therapy-resistant breast cancer cells and could be good therapeutic targets.

Long term estrogen deprivation can result in apoptosis in breast cancer cells. Here, the authors show that this apoptosis is induced by the long-range chromatin interaction of loci containing the ESR1 and FOXO3 genes, resulting in FOXO3-mediated apoptosis.

Cause or prevention of breast cancer with estrogens: analysis from tumor biologic data, growth kinetic model and Women's Health Initiative study

The existing medical literature suggests that estrogens may cause breast cancer but, paradoxically, can also prevent this neoplasm under specific circumstances. Appropriate interpretation of this complex data requires an understanding of emerging concepts of tumor biology. A substantial body of data, including animal models and epidemiologic studies, suggests that estrogens contribute to the development of breast cancer. Additionally, pre-clinical experiments indicate that the responsible mechanisms include both estrogen receptor α-dependent and -independent effects (ERα-dependent and ERα-independent effects). We recently developed two models to describe the growth kinetics of occult breast tumors, one based on autopsy studies and tumor doubling time and the other, computer-based. Validation of the models involved comparison of the predicted incidence of breast cancer with the actual incidence in population-based studies. Utilization of these models allowed us to determine that 16 years on average are required for tumors to undergo the 30 doubling times necessary for the occult tumors to reach the threshold for clinical detection. These models suggest that menopausal hormone therapy with estrogen plus a progestogen in the Women's Health Initiative (WHI) study accelerated the doubling time of occult, pre-existing tumors from 200 to 150 days and thus, increased the rate of tumor diagnosis. Based on estrogen-induced apoptosis data, the model accurately predicted the prevention of diagnosed breast cancer in the estrogen-alone arm of the WHI. Notably, pre-clinical studies demonstrated that conjugated equine estrogen, as used in the WHI, has unique, pro-apoptotic properties compared to the anti-apoptotic effects of estradiol, a finding providing an explanation for the reduction in breast cancer with conjugated equine estrogen.

Endocrine therapy-resistant breast cancer model cells are inhibited by soybean glyceollin I through Eleanor non-coding RNA

Long-term estrogen deprivation (LTED) of an estrogen receptor (ER) α-positive breast cancer cell line recapitulates cancer cells that have acquired estrogen-independent cell proliferation and endocrine therapy resistance. Previously, we have shown that a cluster of non-coding RNAs, Eleanors (ESR1 locus enhancing and activating non-coding RNAs) formed RNA cloud and upregulated the ESR1 gene in the nuclei of LTED cells. Eleanors were inhibited by resveratrol through ER. Here we prepared another polyphenol, glyceollin I from stressed soybeans, and identified it as a major inhibitor of the Eleanor RNA cloud and ESR1 mRNA transcription. The inhibition was independent of ER, unlike one by resveratrol. This was consistent with a distinct tertiary structure of glyceollin I for ER binding. Glyceollin I preferentially inhibited the growth of LTED cells and induced apoptosis. Our results suggest that glyceollin I has a novel role in LTED cell inhibition through Eleanors. In other words, LTED cells or endocrine therapy-resistant breast cancer cells may be ready for apoptosis, which can be triggered with polyphenols both in ER-dependent and ER-independent manners.

Estrogen can restore Tamoxifen sensitivity in breast cancer cells amidst the complex network of resistance

Breast cancer-related deaths have been on the decline ever since the application of systemic therapies. Chiefly, endocrine therapy, such as Tamoxifen, enhances the survival of estrogen receptor (ER)-positive patients. More than a decade has passed since the introduction of Tamoxifen, however, drug resistance, particularly to Tamoxifen, still remains a major challenge. It has been shown that not only does chronic Tamoxifen exposures induce resistance, but estrogen deprivation can as well. There are two Tamoxifen resistant cell lines, long term estrogen deprived (LTED) cells and cells that have acquired resistance due to long-term exposure to Tamoxifen (Tam-R). Despite having similar cytosolic pathways over-activated in Tam-R and LTED-R cells during the development of resistance, the administration of receptor tyrosine kinases (RTKs) inhibitors fail to restore Tamoxifen sensitivity in LTED-Rs. This alludes to existing differences in the underlying molecular mechanisms of resistance. Surprisingly, despite estrogen being recognized as a breast cancer stimulator; it has recently been introduced as an apoptotic inducer in unresponsive cells. Furthermore, the addition of estrogen to the media of LTED and Tam-R cells triggers cell death, perhaps is functioning as an anti-proliferative agent. In this review, we outline the molecular pathways potentially facilitating estrogen-induced apoptosis in resistant cells.

GPER/ERK&AKT/NF-κB pathway is involved in cadmium-induced proliferation, invasion and migration of GPER-positive thyroid cancer cells

The higher incidence of thyroid cancer in women during reproductive years compared with men and the increased risk associated with the therapeutic use of estrogen have strongly suggested that estrogen may be involved in the occurrence and development of thyroid cancer. Cadmium (Cd) is a potent metalloestrogen that disrupts the endocrine system by mimicking the effects of 17β-estradiol (E2). In the present study, we demonstrate that similar to E2 and G1, a specific agonist for G protein-coupled estrogen receptor (GPER), Cd induces the proliferation, invasion and migration of human WRO and FRO thyroid cancer cells that have endogenous GPER. Moreover, like E2 and G1, Cd leads to a rapid activation of ERK/AKT, and then nuclear translocation of NF-κB, increased expression of cyclin A and D1, and secretion of IL-8, all of which are significantly attenuated by GPER blockage or knock-down in both WRO and FRO cells. Furthermore, the Cd-induced proliferation, invasion and migration are suppressed either by specific inhibitors for GPER, ERK, AKT and NF-κB, or by knock-down of GPER. These results suggest that GPER/ERK&AKT/NF-κB signaling pathway is involved in the Cd-induced proliferation, invasion and migration of GPER-positive thyroid cancer cells.

Positron emission tomography of tumour [(18)F]fluoroestradiol uptake in patients with acquired hormone-resistant metastatic breast cancer prior to oestradiol therapy

PURPOSE

Whereas anti-oestrogen therapy is widely applied to treat oestrogen receptor (ER) positive breast cancer, paradoxically, oestrogens can also induce tumour regression. Up-regulation of ER expression is a marker for oestrogen hypersensitivity. We, therefore, performed an exploratory study to evaluate positron emission tomography (PET) with the tracer 16α-[(18)F]fluoro-17β-oestradiol ((18)F-FES) as potential marker to select breast cancer patients for oestradiol therapy.

METHODS

Eligible patients had acquired endocrine-resistant metastatic breast cancer that progressed after ≥2 lines of endocrine therapy. All patients had prior ER-positive histology. Treatment consisted of oestradiol 2 mg, three times daily, orally. Patients underwent (18)F-FES-PET/CT imaging at baseline. Tumour (18)F-FES-uptake was quantified for a maximum of 20 lesions and expressed as maximum standardised uptake value (SUVmax). CT-scan was repeated every 3 months to evaluate treatment response. Clinical benefit was defined as time to radiologic or clinical progression ≥24 weeks.

RESULTS

(18)F-FES uptake, quantified for 255 lesions in 19 patients, varied greatly between lesions (median 2.8; range 0.6-24.3) and between patients (median 2.5; range 1.1-15.5). Seven (37%) patients experienced clinical benefit of oestrogen therapy, eight progressed (PD), and four were non-evaluable due to side effects. The positive and negative predictive value (PPV/NPV) of (18)F-FES-PET for response to treatment were 60% (95% CI: 31-83%) and 80% (95% CI: 38-96%), respectively, using SUVmax >1.5.

CONCLUSION

(18)F-FES-PET may aid identification of patients with acquired antihormone resistant breast cancer that are unlikely to benefit from oestradiol therapy.

Supplementation with Selenium yeast on the prooxidant-antioxidant activities and anti-tumor effects in breast tumor xenograft-bearing mice

Selenium (Se) is essential for antioxidant activity involved in immune function and anti-carcinogenic action, whereas at higher concentrations, Se may have pro-oxidant properties. The present study was aimed at determining the effects of Se supplementation, as Se yeast, on oxidative stress in non-tumor/tumor tissues, as well as regulation of the apoptotic process, and immune responses in mice-bearing breast tumor xenografts. Female BALB/cByJNarl mice were divided into control (CNL and CNL-con), Se-supplemented control (CNL-HS, given as a single oral dose of 912 ng Se daily), breast tumor-bearing (TB and TB-con), TB-LS (228 ng Se), TB-MS (456 ng Se) and TB-HS (912 ng Se) groups. All mice were treated with/without Se for 14 days. A number of variables were further measured. Compared with the TB groups, tumor bearing mice with Se supplement had increased plasma Se concentrations, reduced erythrocyte Se-dependent glutathione peroxidase (GPx) activity and malondialdehyde (MDA) products and inhibited tumor growth. They have also higher Se concentrations in non-tumor and tumor tissues. Significantly elevated concentrations of MDA and reduced GPx activities, as well as increased anti-apoptotic bcl-2 and tumor suppressor p53 concentrations in tumor tissues were observed as Se accumulated in tumor, whereas lower MDA products were found in various non-tumor tissues than did the corresponding values. Further, there were elevated concentrations of Th1-derived cytokines and decreased Th2-type interleukin (IL)-4 in tumor-bearing mice with the treatment of Se. In conclusion, accumulation of Se in tumors may induce oxidative stress and p53-dependent pro-oxidative apoptosis, thus inhibiting the growth of breast tumor.

The new biology of estrogen-induced apoptosis applied to treat and prevent breast cancer

The successful use of high-dose synthetic estrogens to treat postmenopausal metastatic breast cancer is the first effective 'chemical therapy' proven in clinical trial to treat any cancer. This review documents the clinical use of estrogen for breast cancer treatment or estrogen replacement therapy (ERT) in postmenopausal hysterectomized women, which can either result in breast cancer cell growth or breast cancer regression. This has remained a paradox since the 1950s until the discovery of the new biology of estrogen-induced apoptosis at the end of the 20th century. The key to triggering apoptosis with estrogen is the selection of breast cancer cell populations that are resistant to long-term estrogen deprivation. However, estrogen-independent growth occurs through trial and error. At the cellular level, estrogen-induced apoptosis is dependent upon the presence of the estrogen receptor (ER), which can be blocked by nonsteroidal or steroidal antiestrogens. The shape of an estrogenic ligand programs the conformation of the ER complex, which, in turn, can modulate estrogen-induced apoptosis: class I planar estrogens (e.g., estradiol) trigger apoptosis after 24 h, whereas class II angular estrogens (e.g., bisphenol triphenylethylene) delay the process until after 72 h. This contrasts with paclitaxel, which causes G2 blockade with immediate apoptosis. The process is complete within 24 h. Estrogen-induced apoptosis is modulated by glucocorticoids and cSrc inhibitors, but the target mechanism for estrogen action is genomic and not through a nongenomic pathway. The process is stepwise through the creation of endoplasmic reticulum stress and inflammatory responses, which then initiate an unfolded protein response. This, in turn, initiates apoptosis through the intrinsic pathway (mitochondrial) with the subsequent recruitment of the extrinsic pathway (death receptor) to complete the process. The symmetry of the clinical and laboratory studies now permits the creation of rules for the future clinical application of ERT or phytoestrogen supplements: a 5-year gap is necessary after menopause to permit the selection of estrogen-deprived breast cancer cell populations to cause them to become vulnerable to apoptotic cell death. Earlier treatment with estrogen around menopause encourages growth of ER-positive tumor cells, as the cells are still dependent on estrogen to maintain replication within the expanding population. An awareness of the evidence that the molecular events associated with estrogen-induced apoptosis can be orchestrated in the laboratory in estrogen-deprived breast cancers now supports the clinical findings regarding the treatment of metastatic breast cancer following estrogen deprivation, decreases in mortality following long-term antihormonal adjuvant therapy, and the results of treatment with ERT and ERT plus progestin in the Women's Health Initiative for women over the age of 60. Principles have emerged for understanding and applying physiological estrogen therapy appropriately by targeting the correct patient populations.

Molecular mechanisms of hormone resistance of breast cancer

More than 70% malignant mammary tumors contain steroid hormone receptors; this suggests the possibility of hormone therapy in the majority of patients with breast cancer (BC). The main cause of inefficiency of hormone therapy in BC is hormone resistance (tumor resistance to hormonal cytostatics). Here we discuss the main mechanisms of hormone resistance of BC and the mechanisms underlying the formation of hormone resistance of the tumors are analyzed at the molecular level. The data on the signal pathways of estrogen receptors (ER), the key regulators of BC cell proliferation, are presented. The most important factors of BC hormone resistance are: high activity/expression of receptor tyrosine kinases; high activity of proteins regulating cell defense mechanisms (Akt PI3K, mTOR); changes in the activities of cell cycle regulator proteins (Myc, c-Fos, Cyclin D1). Our experiments have demonstrated that estrogen-independent BC cell growth is supported by VEGF/VEGFR2 and EGF/EGFR mitogenic signal pathways. Our data indicate that NF-kappaB transcription factor is directly involved in the regulation of hormone-resistant BC cell growth and survival, while NF-kappaB suppression determines cell sensitivity to apoptotic activity of antitumor compounds. On the whole, the results indicate good prospects of using EGFR, HER-2/neu, mTOR, VEGFR, PI3K/Akt molecular pathways as targets for BC therapy, including therapy for BC resistant forms.

Menopausal hormone therapy and breast cancer

Observational and randomized controlled trial data have extensively examined the relationship between menopausal hormone therapy (MHT) and risk of developing breast cancer. A highly influential study from the Women's Health Initiative (WHI) in 2002 reported that a MHT regimen of conjugated equine estrogens and medroxyprogesterone acetate increased the risk of breast cancer by 26%. Later reports from the WHI indicated that a MHT regimen with conjugated equine estrogens alone decreased the risk of breast cancer by 23%. Critical re-examination of the WHI study noted that the average participant age was 63, that few women had symptoms, and that the WHI results might not apply to younger, symptomatic women shortly after menopause. Since the original publications, several post hoc analyses and observational studies have stimulated reconsideration of the WHI findings. Emphasis has been directed toward risks in younger women just entering the menopause, the subgroup who are most likely to be considering MHT use. The goal of this treatise is to integrate available mechanistic and clinical information related to the use of estrogen alone or estrogen plus a progestogen for five years or less. These data suggest that estrogen alone neither decreases nor increases risk in younger women initiating therapy close to the time of menopause but decreases risk in older women. Both younger and older women experience an excess risk with estrogen plus a progestogen. The attributable risk in younger women is less in those with a low underlying Gail Model risk score. Effects of MHT on risk largely reflect actions on pre-existing, occult, undiagnosed breast cancers. Tumor kinetic models suggest that the pro-proliferative effects of estrogen plus a progestogen on occult tumors provide a mechanistic explanation for the increased risk with this therapy. Pro-apoptotic effects of estrogen alone may explain the reduction of breast cancer in women starting this therapy at an average age of 63 as reported in the WHI study. This article is part of a Special Issue entitled 'Menopause'.

Breast cancer: current and future endocrine therapies

Endocrine therapy forms a central modality in the treatment of estrogen receptor positive breast cancer. The routine use of 5 years of adjuvant tamoxifen has improved survival rates for early breast cancer, and more recently has evolved in the postmenopausal setting to include aromatase inhibitors. The optimal duration of adjuvant endocrine therapy remains an active area of clinical study with recent data supporting 10 years rather than 5 years of adjuvant tamoxifen. However, endocrine therapy is limited by the development of resistance, this can occur by a number of possible mechanisms and numerous studies have been performed which combine endocrine therapy with agents that modulate these mechanisms with the aim of preventing or delaying the emergence of resistance. Recent trial data regarding the combination of the mammalian target of rapamycin (mTOR) inhibitor, everolimus with endocrine therapy have resulted in a redefinition of the clinical treatment pathway in the metastatic setting. This review details the current endocrine therapy utilized in both early and advanced disease, as well as exploring potential new targets which modulate pathways of resistance, as well as agents which aim to modulate adrenal derived steroidogenic hormones.

Targeting BCL2-Proteins for the Treatment of Solid Tumours

Due to their central role in the regulation of apoptosis, the antiapoptotic BCL2-proteins are highly promising targets for the development of novel anticancer treatments. To this end, several strategies have been developed to inhibit BCL2, BCL-XL, BCL-w, and MCL1. While early clinical trials in haematological malignancies demonstrated exciting single-agent activity of BCL2-inhibitors, the response in solid tumours was limited, indicating that, in solid tumours, different strategies have to be developed in order to successfully treat patients with BCL2-inhibitors. In this review, the function of the different antiapoptotic BCL2-proteins and their role in solid tumours will be discussed. In addition, a comprehensive analysis of current small molecules targeting these antiapoptotic BCL2-proteins (e.g., ABT-737, ABT-263, ABT-199, TW-37, sabutoclax, obatoclax, and MIM1) will be provided including a discussion of the results of any clinical trials. This analysis will summarise the potential of BCL2-inhibitors for the treatment of solid tumours and will unravel novel approaches to utilise these inhibitors in clinical applications.

Occult breast tumor reservoir: biological properties and clinical significance

Small, occult, undiagnosed breast cancers are found at autopsy in up to 15.6 % of women dying from unrelated causes with an average of 7 % from eight separate studies. The mammographic detection threshold of breast tumors ranges from 0.88 to 1.66 cm in diameter based on the patient's age. Tumor growth rates, expressed as "effective doubling times," vary from 10 to >700 days. We previously reported two models, based on iterative analysis of these parameters, to describe the biologic behavior of undiagnosed, occult breast tumors. Our models facilitate interpretation of the Women's Health Initiative (WHI) and antiestrogen breast cancer prevention studies. A nude mouse xenograft model was used to validate our assumption that breast tumors grow in a log-linear fashion. We then used our previously reported occult tumor growth (OTG) and computer-simulated tumor growth models to analyze various clinical trial data. Parameters used in the OTG model included a 200-day effective doubling time, 7 % prevalence of occult tumors, and 1.16 cm detection threshold. These models had been validated by comparing predicted with observed incidence of breast cancer in eight different populations of women. Our model suggests that menopausal hormone therapy with estrogens plus a progestogen (E + P) in the WHI trial primarily promoted the growth of pre-existing, occult lesions and minimally initiated de novo tumors. We provide a potential explanation for the lack of an increase in breast cancer incidence in the subgroup of women in the WHI who had not received E + P prior to randomization. This result may have reflected a leftward skew in the distribution of occult tumor doublings and insufficient time for stimulated tumors to reach the detection threshold. Our model predicted that estrogen alone reduced the incidence of breast cancer as a result of apoptosis. Understanding of the biology of occult tumors suggests that breast cancer "prevention" with antiestrogens or aromatase inhibitors represents early treatment rather than a reduction in de novo tumor formation. Our models suggest that occult, undiagnosed tumors are prevalent, grow slowly, and are the biologic targets of a hormone therapy in menopausal women and of antiestrogen therapy for prevention.

Hormone replacement therapy after breast cancer: 10 year follow up of the Stockholm randomised trial

BACKGROUND

The management of hormonal deficiency symptoms in breast cancer survivors is an unsolved problem. While hormone replacement therapy (HRT) may increase the risk of breast cancer in healthy women, its effects on recurrence is unclear. Observational studies have suggested decreased recurrence rates from HRT. The few clinical trials in this field have all been closed preterm.

METHODS

The Stockholm trial was started in 1997 and designed to minimise the dose of progestogen in the HRT arm. Disease-free women with a history of breast cancer were randomised to HRT (n=188) or no HRT (n=190). The trial was stopped in 2003 when another Swedish study (HABITS, the Hormonal Replacement After Breast Cancer - Is it Safe?) reported increased recurrence. However the Stockholm material showed no excess risk after 4 years of follow-up. A long term follow-up has now been performed.

FINDINGS

After 10.8 years of follow-up, there was no difference in new breast cancer events: 60 in the HRT group versus 48 among controls (hazard ratio (HR)=1.3; 95% confidence interval (CI)=0.9-1.9). Among women on HRT, 11 had local recurrence and 12 distant metastases versus 15 and 12 for the controls. There were 14 contra-lateral breast cancers in the HRT group and four in the control group (HR=3.6; 95% CI=1.2-10.9; p=0.013). No differences in mortality or new primary malignancies were found.

INTERPRETATION

The number of new events did not differ significantly between groups, in contrast to previous reports. The increased recurrence in HABITS has been attributed to higher progestogen exposure. As both trials were prematurely closed, data do not allow firm conclusions. Both studies found no increased mortality from breast cancer or other causes from HRT. Current guidelines typically consider HRT contraindicated in breast cancer survivors. Findings suggest that, in some women symptom relief may outweigh the potential risks of HRT.

Modeling of the growth kinetics of occult breast tumors: role in interpretation of studies of prevention and menopausal hormone therapy

BACKGROUND

Autopsy studies report a reservoir of small, occult, undiagnosed breast cancers in up to 15.6% of women dying from unrelated causes. The effective doubling times (EDT) of these occult neoplasms range from 70 to 350 days and mammographic detection threshold diameters from 0.88 to 1.66 cm. Modeling of the biologic behavior of these occult tumors facilitates interpretation of tamoxifen breast cancer prevention and menopausal hormone therapy studies.

METHODS

We used iterative and mathematical techniques to develop a model of occult tumor growth (OTG) whose parameters included prevalence, EDT, and detection threshold. The model was validated by comparing predicted with observed incidence of breast cancer in several populations.

RESULTS

Iterative analysis identified a 200-day EDT, 7% prevalence and 1.16 cm detection threshold as optimal parameters for an OTG model as judged by comparison with Surveillance Epidemiology and End Results (SEER) population incidence rates in the United States. We validated the model by comparing predicted incidence rates with those observed in five separate population databases, in three long-term contralateral breast cancer detection studies, and with data from a computer-simulated tumor growth (CSTG) model. Our model strongly suggests that breast cancer prevention with anti-estrogens or aromatase inhibitors represents early treatment not prevention. In addition, menopausal hormone therapy does not primarily induce de novo tumors but promotes the growth of occult lesions.

CONCLUSIONS

Our OGTG model suggests that occult, undiagnosed tumors are prevalent, grow slowly, and are the biologic targets of anti-estrogen therapy for prevention and hormone therapy for menopausal women.

Models and Mechanisms of Acquired Antihormone Resistance in Breast Cancer: Significant Clinical Progress Despite Limitations

Translational research for the treatment and prevention of breast cancer depends upon the four Ms: models, molecules, and mechanisms in order to create medicines. The process, to target the estrogen receptor (ER) in estrogen-dependent breast cancer, has yielded significant advances in patient survivorship and the first approved medicines (tamoxifen and raloxifene) to reduce the incidence of any cancer in high- or low-risk women. This review focuses on the critical role of the few ER-positive cell lines (MCF-7, T47D, BT474, ZR-75) that continue to advance our understanding of the estrogen-regulated biology of breast cancer. More importantly, the model cell lines have provided an opportunity to document the development and evolution of acquired antihormone resistance. The description of this evolutionary process that occurs in micrometastatic disease during up to a decade of adjuvant therapy would not be possible in the patient. The use of the MCF-7 breast cancer cell line in particular has been instrumental in discovering a vulnerability of ER-positive breast cancer exhaustively treated with antihormone therapy. Physiologic estradiol acts as an apoptotic trigger to cause tumor regression. These unanticipated findings in the laboratory have translated to clinical advances in our knowledge of the paradoxical role of estrogen in the life and death of breast cancer.

Delineating the receptor mechanisms underlying the rapid vascular contractile effects of aldosterone and estradiol

It is increasingly appreciated that steroid hormones such as aldosterone and estradiol can mediate important cardiovascular effects. Many of these effects occur over a time course not consistent with the genomic actions of these hormones acting through classical nuclear receptors / transcription factors. Further, multiple receptors have been implicated in mediating these rapid effects of both aldosterone and estradiol, including a newly appreciated G-protein-coupled receptor, GPR30. In previous studies we demonstrated that both aldosterone and estradiol mediate contraction in vascular smooth muscle cells, as assessed in single cell assays. However, the receptor mechanisms underlying these effects remained unclear. Therefore, we studied the actions of estradiol and aldosterone on rat aortic vascular smooth muscle cells. Both aldosterone and estradiol mediated a concentration-dependent increase in contraction, as assessed in substrate deformation assays with EC50s in the range of nanomoles per litre. These effects paralleled increased myosin light chain phosphorylation. The effects of aldosterone were inhibited by the mineralocorticoid selective antagonist eplerenone. Further, aldosterone’s contractile effects were enhanced by increased expression of the mineralocorticoid receptor. The contractile effects of estradiol were inhibited by estrogen receptor (ER)-selective antagonists, tamoxifen, and ICI 182780, as well as eplerenone. Further, estradiol’s effects were enhanced by the increased expression of both ERα and the mineralocorticoid receptor (MR). To assess the potential role of GPR30 in mediating the effects of aldosterone and estradiol, GPR30 was re-introduced, since these cells lose endogenous GPR30 expression in culture. Re-expression of GPR30 enhanced both estradiol- and aldosterone-mediated contraction. These studies demonstrate that in rat aortic vascular smooth muscle cells, both aldosterone and estradiol mediate vascular smooth muscle contraction and that these effects can be mediated by MR, ERα, and by GPR30.

Transient over-expression of estrogen receptor-α in breast cancer cells promotes cell survival and estrogen-independent growth

Estrogen receptor-α (ERα) positive breast cancer frequently responds to inhibitors of ERα activity, such as tamoxifen, and/or to aromatase inhibitors that block estrogen biosynthesis. However, many patients become resistant to these agents through mechanisms that remain unclear. Previous studies have shown that expression of ERα in ERα-negative breast cancer cell lines frequently inhibits their growth. In order to determine the consequence of ERα over-expression in ERα-positive breast cancer cells, we over-expressed ERα in the MCF-7 breast cancer cell line using adenovirus gene transduction. ERα over-expression led to ligand-independent expression of the estrogen-regulated genes pS2 and PR and growth in the absence of estrogen. Interestingly, prolonged culturing of these cells in estrogen-free conditions led to the outgrowth of cells capable of growth in cultures from ERα transduced, but not in control cultures. From these cultures a line, MLET5, was established which remained ERα-positive, but grew in an estrogen-independent manner. Moreover, MLET5 cells were inhibited by anti-estrogens showing that ERα remains important for their growth. Gene expression microarray analysis comparing MCF-7 cells with MLET5 highlighted apoptosis as a major functional grouping that is altered in MLET5 cells, such that cell survival would be favoured. This conclusion was further substantiated by the demonstration that MLET5 show resistance to etoposide-induced apoptosis. As the gene expression microarray analysis also shows that the apoptosis gene set differentially expressed in MLET5 is enriched for estrogen-regulated genes, our findings suggest that transient over-expression of ERα could lead to increased cell survival and the development of estrogen-independent growth, thereby contributing to resistance to endocrine therapies in breast cancer patients.

Oestrogen treatment enhances the sensitivity of hormone-resistant breast cancer cells to doxorubicin

Recently, it was shown that the resistance of breast cancer cells to growth-stimulating oestrogen action may be accompanied with the paradoxical tumour sensitization to oestrogen apoptotic action. In the present paper, we studied the influence of oestrogens on the sensitivity of resistant breast tumours to cytostatic drugs, and to evaluate the role of NF-κB (nuclear factor κB) signalling in the regulation of the apoptotic response of the resistant cells. The experiments were carried out on the oestrogen-dependent MCF-7 breast cancer cells and resistant MCF-7/LS subline generated through long-term cultivation of the parental cells in the absence of oestrogen. The cell treatment with the combination of oestradiol and Dox (doxorubicin) was found to enhance the apoptotic action of Dox in MCF-7/LS cells but not in the parent cells. MCF-7/LS cells were characterized by the increased level of ROS (reactive oxygen species) and decreased NF-κB activity. Oestradiol in combination with Dox leads to significant NF-κB stimulation and its accumulation in the nucleus of MCF-7/LS cells. The knockdown of NF-κB with siRNA (small interfering RNA) increased the apoptotic response of the MCF-7/LS cells to both Dox and oestradiol demonstrating the important role of NF-κB in the protection of the MCF-7/LS cells against apoptosis. In general, the results obtained show that: (i) oestradiol enhances the apoptotic action of Dox in the resistant breast cancer cells; and (ii) suppression of NF-κB signalling amplifies the apoptotic response of the resistant cells to both oestrogen and Dox, demonstrating that NF-κB may serve as a potential target in the therapy of the resistant breast cancer.

Raloxifene-stimulated experimental breast cancer with the paradoxical actions of estrogen to promote or prevent tumor growth: a unifying concept in anti-hormone resistance

We have previously demonstrated that prolonged treatments with raloxifene (RAL) in vitro will result in phase II RAL resistance and RAL-induced tumor growth. Clinical interest prompted us to re-examine RAL resistance in vivo, particularly the effects of long-term treatments (a decade or more) on the evolution of RAL resistance. In this study, we have addressed the question of this being a reproducible phenomenon in wild-type estrogen receptor (ER)-positive human breast cell line MCF-7. MCF-7 cells cultured under estrogen-deprived conditions in the presence of 1 microM RAL for more than a year develop RAL resistance resulting in an independent cell line, MCF7-RAL. The MCF7-RAL cells grow in response to both estradiol E2 and RAL. Fulvestrant (FUL) blocks RAL and E2-mediated growth. Transplantation of MCF7-RAL cells into athymic ovariectomized mice and treatment with physiologic doses of E2 causes early E2-stimulated tumor growth. In contrast, continuous treatment of implanted animals with daily oral RAL (1.5 mg daily) causes growth of small tumors within 15 weeks. Continuous re-transplantation of the tumors growing in RAL-treated mice indicated that RAL stimulated tumor growth. Tumors in the untreated mice did not grow. Bi-transplantation of MCF7-E2 and MCF7-RAL tumors into the opposing mammary fat pads of the same ovariectomized animal demonstrated that MCF7-E2 grew with E2 stimulation and not with RAL. Conversely, MCF7-RAL tumors grew with RAL and not E2, a characteristic of phase II resistance. Established phase II resistance of MCF7-RAL tumors was confirmed following up to 7 years of serial transplantation in RAL-treated athymic mice. The ERalpha was retained in these tumors. The cyclical nature of RAL resistance was confirmed and extended during a 2-year evolution of the resistant phases of the MCF7-RAL tumors. The MCF7-RAL tumors that initially were inhibited by E2 grew in the presence of E2 and subsequently grew with either RAL or E2. RAL remained the major grow stimulus and RAL enhanced E2-stimulated growth. Subsequent transplantation of E2 stimulated tumors and evaluations of the actions of RAL, demonstrated robust E2-stimulated growth that was blocked by RAL. These are the characteristics of the anti-estrogenic actions of RAL on E2-stimulated breast cancer growth with a minor component of phase I RAL resistance. Continuous transplantation of the phase I RAL-stimulated tumors for >8 months causes reversion to phase II resistance. These data and literature reports of the cyclical nature of anti-androgen/androgen responsiveness of prostate cancer growth, illustrate the generality of the evolution of anti-hormonal resistance in sex steroid-sensitive target tissues.

Inhibition of mammary tumor growth by estrogens: is there a specific role for estrogen receptors alpha and beta?

To evaluate the extent to which each estrogen receptor (ER) subtype contributes to the stimulation or to the inhibition of mammary tumor growth, we evaluated the effects of specific agonists in MC4-L2 cells, which are stimulated by 17β-estradiol (E(2)), and in mammary carcinomas of the MPA mouse breast cancer model, which are inhibited by E(2). Both express ERα and ERβ. In MC4-L2 cells, 4,4',4"-(4-propyl-(1H)-pyrazole-1,3,5-triyl)trisphenol (PPT; ERα agonist) and (4-hydroxy-phenyl)-propionitrile (DPN; ERβ agonist) stimulated cell proliferation, whereas the opposite occurred in C4-HI primary cultures. The inhibitory effect was associated with a decrease in ERα and cyclin D1 expression and an increase in progesterone receptor (PR) expression as well as in the Bax/Bcl-xl ratio. In vivo, mice carrying C4-HI or 32-2-HI tumors were treated with E(2), PPT or DPN (3 mg/kg/day) or with vehicle. PPT and DPN inhibited tumor size, as did E(2), during the first 72 h. After a few days, DPN-treated tumors started to grow again, while PPT-treated tumors remained quiescent for a longer period of time. A pronounced decrease in the mitotic index and an increase in the apoptotic index was associated with tumor regression. All treated tumors showed: (a) an increase in integrin α6 and Bax expression, (b) an increased stromal laminin redistribution, and (c) a decrease in ERα, Bcl-xl and Bcl-2 expression (P < 0.001). Apoptosis-inducing factor (Aif) expression was increased in DPN-treated tumors, while active caspase 9 was up-regulated in PPT-treated mice, demonstrating the involvement of the intrinsic apoptotic pathway in estrogen-induced regression in this model. In conclusion, our data indicate that although there may be some preferences for activation pathways by the different agonists, the stimulatory or inhibitory effects triggered by estrogens are cell-context dependent rather than ER isoform dependent.

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

Estrogen regulation of apoptosis: how can one hormone stimulate and inhibit?

The link between estrogen and the development and proliferation of breast cancer is well documented. Estrogen stimulates growth and inhibits apoptosis through estrogen receptor-mediated mechanisms in many cell types. Interestingly, there is strong evidence that estrogen induces apoptosis in breast cancer and other cell types. Forty years ago, before the development of tamoxifen, high-dose estrogen was used to induce tumor regression of hormone-dependent breast cancer in post-menopausal women. While the mechanisms by which estrogen induces apoptosis were not completely known, recent evidence from our laboratory and others demonstrates the involvement of the extrinsic (Fas/FasL) and the intrinsic (mitochondria) pathways in this process. We discuss the different apoptotic signaling pathways involved in E2 (17beta-estradiol)-induced apoptosis, including the intrinsic and extrinsic apoptosis pathways, the NF-kappaB (nuclear factor-kappa-B)-mediated survival pathway as well as the PI3K (phosphoinositide 3-kinase)/Akt signaling pathway. Breast cancer cells can also be sensitized to estrogen-induced apoptosis through suppression of glutathione by BSO (L-buthionine sulfoximine). This finding has implications for the control of breast cancer with low-dose estrogen and other targeted therapeutic drugs.

The Paradox of Oestradiol-Induced Breast Cancer Cell Growth and Apoptosis

High dose oestrogen therapy was used as a treatment for postmenopausal patients with breast cancer from the 1950s until the introduction of the safer antioestrogen, tamoxifen in the 1970s. The anti-tumour mechanism of high dose oestrogen therapy remained unknown. There was no enthusiasm to study these signal transduction pathways as oestrogen therapy has almost completely been eliminated from the treatment paradigm. Current use of tamoxifen and the aromatase inhibitors seek to create oestrogen deprivation that prevents the growth of oestrogen stimulated oestrogen receptor (ER) positive breast cancer cells. However, acquired resistance to antihormonal therapy does occur, but it is through investigation of laboratory models that a vulnerability of the cancer cell has been discovered and is being investigated to provide new opportunities in therapy with the potential for discovering new cancer-specific apoptotic drugs. Laboratory models of resistance to raloxifene and tamoxifen, the selective oestrogen receptor modulators (SERMs) and aromatase inhibitors demonstrate an evolution of drug resistance so that after many years of oestrogen deprivation, the ER positive cancer cell reconfigures the survival signal transduction pathways so oestrogen now becomes an apoptotic trigger rather than a survival signal. Current efforts are evaluating the mechanisms of oestrogen-induced apoptosis and how this new biology of oestrogen action can be amplified and enhanced, thereby increasing the value of this therapeutic opportunity for the treatment of breast cancer. Several synergistic approaches to therapeutic enhancement are being advanced which involve drug combinations to impair survival signaling with the use of specific agents and to impair bcl-2 that protects the cancer cell from apoptosis. We highlight the historical understanding of oestrogen's role in cell survival and death and specifically illustrate the progress that has been made in the last five years to understand the mechanisms of oestrogen-induced apoptosis. There are opportunities to harness knowledge from this new signal transduction pathway to discover the precise mechanism of this oestrogen-induced apoptotic trigger. Indeed, the new biology of oestrogen action also has significance for understanding the physiology of bone remodeling. Thus, the pathway has a broad appeal in both physiology and cancer research.

NF-kappaB suppression provokes the sensitization of hormone-resistant breast cancer cells to estrogen apoptosis

The progression of breast cancer cells to estrogen-independent growth may be accompanied with the paradoxical cell sensitization to estrogen apoptotic action; however, the mechanism of this phenomenon is still unclear. In the present study, we have shown that the sensitization of hormone-resistant breast cancer cells to estrogen apoptotic action is accompanied with the gradual NF-kappaB suppression. Using the chemical inhibitors of NF-kappaB as well as the dominant-negative NF-kappaB constructs, we have proved the sufficiency of NF-kappaB inhibition for the sensitization of the resistant cells to estrogen apoptosis. Estradiol treatment results in the additional suppression of NF-kappaB, demonstrating the possible NF-kappaB involvement in the regulation of cell response to estrogens. Totally, the results presented suggest that the constitutive NF-kappaB suppression in the estrogen-independent cells may be considered as one of the factors resulting in a imbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.

The 38th David A. Karnofsky lecture: the paradoxical actions of estrogen in breast cancer--survival or death?

During the first David A. Karnofsky Award lecture entitled "Thoughts on Chemical Therapy" in 1970, Sir Alexander Haddow commented about the dramatic regressions observed with estrogen in some breast cancers in postmenopausal women, but regrettably the mechanism was unknown. He was concerned that a cancer-specific target would remain elusive, without tests to predict response to therapy. At that time, I was conducting research for my PhD on an obscure group of estrogen derivatives called nonsteroidal antiestrogens. Antiestrogens had failed to fulfill their promise as postcoital contraceptives and were unlikely to be developed further by the pharmaceutical industry. In 1972, that perspective started to change and ICI 46,474 was subsequently reinvented as the first targeted therapy for breast cancer. The scientific strategy of targeting the estrogen receptor (ER) in the tumor, treating patients with long-term adjuvant therapy, examining active metabolites, and considering chemoprevention all translated through clinical trials to clinical practice during the next 35 years. Hundreds of thousands of women now have enhanced survivorship after their diagnosis of ER-positive breast cancer. However, it was the recognition of selective ER modulation (SERM) that created a new dimension in therapeutics. Nonsteroidal antiestrogens selectively turn on or turn off estrogen target tissues throughout the body. Patient care was immediately affected by the recognition in the laboratory that tamoxifen would potentially increase the growth of endometrial cancer during long-term adjuvant therapy. At that time, a failed breast cancer drug, keoxifene, was found to maintain bone density of rats (estrogenic action) while simultaneously preventing mammary carcinogenesis (antiestrogenic action). Perhaps a SERM used to prevent osteoporosis could simultaneously prevent breast cancer? Keoxifene was renamed raloxifene and became the first SERM for the treatment and prevention of osteoporosis as well as the prevention of breast cancer, but without an increase in endometrial cancer. There the story might have ended had the study of antihormone resistance not revealed a vulnerability of cancer cells that could be exploited in the clinic. The evolution of antihormone resistance over years of therapy reconfigures the survival mechanism of the breast cancer cell, so estrogen no longer is a survival signal but a death signal. Remarkably, remaining tumor tissue is again responsive to continuing antihormone therapy. This new discovery is currently being evaluated in clinical trials but it also solves the mystery mechanism of chemical therapy with estrogen noted by Haddow in the first Karnofsky lecture.

Selective Estrogen-Receptor Modulators and Antihormonal Resistance in Breast Cancer

Selective estrogen-receptor (ER) modulators (SERMs) are synthetic nonsteroidal compounds that switch on and switch off target sites throughout the body. Tamoxifen, the pioneering SERM, blocks estrogen action by binding to the ER in breast cancers. Tamoxifen has been used ubiquitously in clinical practice during the last 30 years for the treatment of breast cancer and is currently available to reduce the risk of breast cancer in high-risk women. Raloxifene maintains bone density (estrogen-like effect) in postmenopausal osteoporotic women, but at the same time reduces the incidence of breast cancer in both high- and low-risk (osteoporotic) postmenopausal women. Unlike tamoxifen, raloxifene does not increase the incidence of endometrial cancer. Clearly, the simple ER model of estrogen action can no longer be used to explain SERM action at different sites around the body. Instead, a new model has evolved on the basis of the discovery of protein partners that modulate estrogen action at distinct target sites. Coactivators are the principal players that assemble a complex of functional proteins around the ligand ER complex to initiate transcription of a target gene at its promoter site. A promiscuous SERM ER complex creates a stimulatory signal in growth factor receptor–rich breast or endometrial cancer cells. These events cause drug-resistant, SERM-stimulated growth. The sometimes surprising pharmacology of SERMs has resulted in a growing interest in the development of new selective medicines for other members of the nuclear receptor superfamily. This will allow the precise treatment of diseases that was previously considered impossible.

Mechanism of estrogen-induced apoptosis in breast cancer cells: role of the NF-kappaB signaling pathway

The ability of sex steroid hormones to up-regulate the apoptotic signaling proteins is well documented; however, the apoptotic potential of sex hormones is not remarkable and fully compensated by their growth stimulatory action to target cells. In the present study using the long-term cultivation of estrogen-dependent MCF-7 breast cancer cells in steroid-free medium, we have established a cell subline, designed as MCF-7/LS, which was characterized by the resistance to growth stimulatory estradiol action and hypersensitivity to estrogen-induced apoptosis. We have demonstrated that estrogen treatment of the cells does not influence on the level of TNF-R1 or Fas, but dramatically decreases the transcriptional activity of NF-kappaB. Importantly, the MCF-7/LS cells, which are insensitive to growth stimulatory estrogen action, retain the ability to decrease in the NF-kappaB activity in response to estrogen stimulus. Furthermore, the significant increase in the basal (in the absence of ligand) estrogen receptor (ER)-dependent transcriptional activity in the MCF-7/LS cells was revealed and reciprocal transcriptional antagonism between ER and NF-kappaB was demonstrated. Finally, we proved the possible involvement of phosphatidylinositol-3 kinase (PI3K) in the ligand-independent ER activation. In general, the results presented suggest that long-term growth of MCF-7 breast cancer cells in steroid-free medium is accompanied with the increase in the basal ER-dependent transcriptional activity as well as the maintenance of the negative regulatory loop ER-NF-kappaB. The latter may be considered as one of the factors resulting in a disbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.

Mitochondrial estrogen receptors--new insights into specific functions

Estrogen receptor (ER) alpha and ERbeta are present in mitochondria but little is known about their contribution to the overall constellation of cellular responses to 17beta-estradiol, which inhibits the early stages of apoptosis through membrane and mitochondrial, but not nuclear, ER signaling pathways. Whereas membrane ER signaling leading to inhibition of apoptosis involves the activation of protein kinase pathways, mitochondrial ER-mediated inhibition involves the direct and/or indirect activation of manganese superoxide dismutase by the mitochondrial ER.

Activation of mitogenic pathways and sensitization to estrogen-induced apoptosis: two independent characteristics of tamoxifen-resistant breast cancer cells?

Paradoxical induction of apoptosis by estrogen has been described previously for estrogen-deprived and antiestrogen-resistant breast cancer cells. In this study we analyzed the possible interrelations between cell sensitization to estrogen apoptotic action and cell ability to (anti)estrogen-independent growth. Using tamoxifen-resistant sublines derived from the parent MCF-7 breast cancer cells by long-term tamoxifen treatment we demonstrated that resistant cells are characterized by increased level of EGF receptor and unexpected increase of VEGF receptor 2 (Flk-1/KDR) and its specific ligand, VEGF-A. The importance of the VEGF signaling in the autocrine regulation of cell growth was indicated by the ability of VEGF inhibitor, soluble fragment of Flt-1/Fc chimera, to suppress the phosphorylation of MAP kinases as well as to inhibit the estrogen-independent growth of MCF-7 cells. Sensitization of tamoxifen-resistant cells to estrogen-induced apoptosis required the additional continuous cultivation in steroid-depleted medium and did not depend on the activity of both EGF and VEGF pathways. Finally, we showed that treatment of the cells with 17beta-estradiol (10(-9) M) resulted in a marked increase in p53 level both in the resistant cells undergoing apoptosis and in the parent MCF-7 cells insensitive to apoptotic estrogen action. These data provide an important support for the existence of a disbalance between pro- and anti-apoptotic machinery in the resistant breast cancer cells that forms independently of the acquired ability to estrogen-independent growth.

Sensitization of MCF-7 breast cancer cells to the apoptotic effect of estradiol

A new substrain of hormone-resistant MCF-7/T breast cancer cells was selected after long-term culturing of estrogen-dependent MCF-7 cells in the presence of tamoxifen. These cells were resistant to the growth-stimulating and cytostatic effects of estradiol and tamoxifen, respectively. MCF-7/T cells gained paradoxical sensitivity to the apoptotic effect of estradiol. Estradiol stimulated p53 expression and decreased DNA-binding activity of NF-kappaB. Our findings provide indirect evidence that these proteins are involved in the regulation of estrogen-induced apoptosis. These results indicate that tamoxifen-resistant breast cancer cells can be sensitized to the apoptotic effect of estradiol. The data form a basis for the development of new methods of endocrine therapy for breast cancer patients.