- Research article
- Open Access
- Open Peer Review
Everolimus downregulates estrogen receptor and induces autophagy in aromatase inhibitor-resistant breast cancer cells
© The Author(s). 2016
- Received: 24 February 2016
- Accepted: 30 June 2016
- Published: 16 July 2016
mTOR inhibition of aromatase inhibitor (AI)-resistant breast cancer is currently under evaluation in the clinic. Everolimus/RAD001 (Afinitor®) has had limited efficacy as a solo agent but is projected to become part of combination therapy for AI-resistant breast cancer. This study was conducted to investigate the anti-proliferative and resistance mechanisms of everolimus in AI-resistant breast cancer cells.
In this study we utilized two AI-resistant breast cancer cell lines, MCF-7:5C and MCF-7:2A, which were clonally derived from estrogen receptor positive (ER+) MCF-7 breast cancer cells following long-term estrogen deprivation. Cell viability assay, colony formation assay, cell cycle analysis and soft agar anchorage-independent growth assay were used to determine the efficacy of everolimus in inhibiting the proliferation and tumor forming potential of MCF-7, MCF-7:5C, MCF-7:2A and MCF10A cells. Confocal microscopy and transmission electron microscopy were used to evaluate LC3-II production and autophagosome formation, while ERE-luciferase reporter, Western blot, and RT-PCR analyses were used to assess ER expression and transcriptional activity.
Everolimus inhibited the proliferation of MCF-7:5C and MCF-7:2A cells with relatively equal efficiency to parental MCF-7 breast cancer cells. The inhibitory effect of everolimus was due to G1 arrest as a result of downregulation of cyclin D1 and p21. Everolimus also dramatically reduced estrogen receptor (ER) expression (mRNA and protein) and transcriptional activity in addition to the ER chaperone, heat shock protein 90 protein (HSP90). Everolimus restored 4-hydroxy-tamoxifen (4OHT) sensitivity in MCF-7:5C cells and enhanced 4OHT sensitivity in MCF-7 and MCF-7:2A cells. Notably, we found that autophagy is one method of everolimus insensitivity in MCF-7 breast cancer cell lines.
This study provides additional insight into the mechanism(s) of action of everolimus that can be used to enhance the utility of mTOR inhibitors as part of combination therapy for AI-resistant breast cancer.
- Breast cancer
- Aromatase inhibitor
- Estrogen receptor
Estrogen deprivation using aromatase inhibitors (AIs) is currently the standard of care for patients with estrogen receptor-positive (ER+) breast cancer. Unfortunately, ~30 % of breast cancer patients develop resistance to AIs following long-term treatment . The mechanism by which AI resistance develops is still not completely understood, however, several contributing factors have been identified including; alterations in ER signaling, enhanced growth factor signaling, and imbalance in the phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway [2, 3]. The activation of the PI3K/Akt/mTOR pathway is considered clinically relevant for tumor escape from hormone dependence in breast cancer, promoting the survival of breast cancer cells in estrogen-deprived conditions . Additionally, upregulation of the PI3K/Akt/mTOR pathway is associated with poor outcome for breast cancer patients and has been observed in AI-resistant breast cancer models [5, 6]. As a result, a variety of PI3K/Akt/mTOR pathway inhibitors have been under study, including everolimus/RAD001 (Afinitor®).
Everolimus is a rapamycin analog that is currently approved for treatment of metastatic breast cancer. It inhibits the PI3K/Akt/mTOR signaling pathway by preventing the phosphorylation of mTORC1, which interrupts the signaling cascade and results in inhibition of cell proliferation and growth . Everolimus treatment has shown promising anti-cancer effects in preclinical studies; however, when used as a single agent, it does not significantly decrease tumor size . As a result, recent clinical trials have focused instead on simultaneous targeting of the PI3K/AKT/mTOR and ER pathways in ER+ breast cancer [9–11]. The results from these trials have been very encouraging due to significant improvements in response rate and progression free survival for both AI-sensitive and AI-resistant patients [12–14]. Subsequent laboratory studies have focused on comparison of everolimus in combination with endocrine therapies [15, 16] as well as other PI3K/Akt/mTOR inhibitors in a variety of breast cancer cell lines [17, 18] and these studies have reported synergy between tamoxifen or AI therapy and everolimus. However, these studies have not investigated the anti-cancer mechanisms of everolimus alone in AI-resistant breast cancer cells.
Everolimus and other PI3K/Akt/mTOR inhibitors are known to induce autophagy in both solid and blood tumors [19, 20]; however, to our knowledge, the phenomenon has not been reported in breast cancer. Autophagy allows cells to degrade dysfunctional organelles and proteins, and recycle their components. Autophagy can support tumor survival during treatment, making it a possible mechanism for AI-resistance [21, 22]. This process is dependent upon the cleavage of microtubule associated light chain 3 (LC3) to LC3-I and subsequent lipidation to LC3-II which allows for final formation of the autophagosome membrane . A group of small proteins, called heat shock proteins (HSPs), promote cell survival during stress reactions by promoting the refolding of denatured proteins and directly regulating autophagy. Specifically, HSP70 is thought to be required for the induction of autophagy [24, 25] in response to inhibition of the PI3K/Akt/mTOR pathway by either starvation or rapamycin treatment [26, 27]. Another heat shock protein, HSP27, allows cells to survive a variety of cytotoxic stimuli [28, 29] and is thought to be degraded during starvation and rapamycin-induced autophagy . Due to the link between drug resistance and autophagy, we hypothesized that the induction of autophagy may contribute to everolimus insensitivity in MCF-7 breast cancer cell lines.
In this study, we investigated the effects of everolimus, as a single agent, or in combination with 4-hydroxy tamoxifen (4-OHT) or chloroquine on cell proliferation, anchorage-independent growth, PI3K/Akt/mTOR signaling, ER expression and transcriptional activity, LC3 turnover, and autophagosome induction in AI-sensitive MCF-7 and AI-resistant MCF-7:5C and MCF-7:2A breast cancer cells. We report that everolimus exerts similar anti-proliferative effects in both the AI-sensitive and AI-resistant breast cancer cell lines and that its inhibitory activity is associated with G1 arrest and down regulation of ERα expression. Everolimus also reverses and enhances 4OHT sensitivity during long-term co-treatment of the AI-resistant cell lines. Lastly, we report that autophagy is a mechanism of everolimus insensitivity in MCF-7, MCF-7:5C and MCF-7:2A cells, possibly explaining the equal response of these cell lines to treatment. The information from this study may enhance future selection of everolimus containing combination therapies for AI-resistant breast cancer.
Cell lines and culture conditions
The MCF-7 cell line [31, 32] was obtained from Dr. V. Craig Jordan (University of Texas MD Anderson Cancer Center, Houston) and maintained in RPMI-1640 medium supplemented with 10 % fetal bovine serum, 2 mM glutamine, Antibiotic/Antimitotic mix, MEM Non-Essential Amino Acids (Invitrogen, Waltham, MA), and bovine insulin at 6 ng/mL (Sigma Aldrich, St. Louis, MO). The long-term estrogen deprived human breast cancer cell lines; MCF-7:5C [31, 33] and MCF-7:2A [32, 34] were cloned from parental MCF-7 cells following long term (>12 months) culture in estrogen-free medium composed of phenol red-free RPMI-1640, 10 % fetal bovine serum treated three times with dextran-coated charcoal (SFS), 2 mM glutamine, bovine insulin at 6 ng/mL, Antibiotic/Antimitotic mix, and MEM Non-Essential Amino Acids (Invitrogen). The MCF10A cell line was purchased from the American Type Tissue Culture Collection. They are maintained in Dulbecco's Modified Eagle Medium: Nutrient Mixture F-12 (DMEM/F12) in a 1:1 mixture and supplemented with 5 % horse serum, Antibiotic/Antimitotic mix (100 IU/mL penicillin, 100 μg/mL streptomycin, 25 μg/mL of Fungizone® from Invitrogen, Grand Island, NY), 20 ng/ml EGF (Millipore), 0.5 mg/ml hydrocortisone, 100 ng/ml cholera toxin (Sigma Aldrich). All cell lines were cultured at 37 °C under 5 % CO2. After overnight acclimatization period, cells were cultured with 20 nM everolimus alone or in combination with 1 μM 4-hydroxytamoxifen (Sigma Aldrich) or 50 μM Chloroquine (InvivoGen, San Diego, CA), in their normal culture medium.
Cells were assayed for viability in 24-well plates using the Cell-Titer Blue Assay Kit (Promega, Madison, WI) per the manufacturer’s instructions. Assay plates were kept at 37 °C in 5 % CO2 for 3 h and read at 560–590 nM on a BioTek Synergy 4 microplate reader using the Gen 5 data analysis software (BioTek Instruments, Winooski, VT).
Cells were seeded in 6-well plates, collected using a cell scraper and suspended in RIPA buffer (Thermo Scientific, Pittsburgh, PA) supplemented with protease inhibitor cocktail and phosphatase inhibitor (Sigma Aldrich). Cells were homogenized over ice by sonication. After purification of the sample by centrifugation, protein concentration was determined by protein assay (Bio-Rad, Hercules, CA). The proteins were separated by 4-12 % SDS–polyacrylamide gel electrophoresis (SDS–PAGE) and electrically transferred to a polyvinylidene difluoride membrane (Santa Cruz Biotechnology). After blocking the membrane using 5 % non-fat milk, target proteins were detected using either Anti-mTOR, anti-phospho-mTOR, anti- LC3A/B (Cell Signaling, Beverly, MA), anti-p70S6K, anti-phospho-p70S6K, anti-AKT, anti-phospho-AKT (S473) or anti-ERα (Santa Cruz Biotechnology) antibodies. Membranes were stripped and re-probed for β-actin (Cell Signaling) or β-tubulin (Sigma Aldrich). The appropriate horseradish peroxidase (HRP)-conjugated secondary antibody was applied and the positive bands were detected using Amersham ECL Plus Western blotting detection reagents (GE Health care, Piscataway, NJ). In the case of LC3 analysis, cells were treated with 50 μM chloroquine (CQ) for 24 or 48 h to allow for LC3-II accumulation. Immunoreactivity was detected using anti-mouse or anti-rabbit IgG conjugated to Dylight 680 or 800 fluorochromes (Thermo Scientific, Waltham, MA), respectively. Blots were visualized on Odyssey imager (LiCor, Lincoln, NE). Quantitation of immunoreactive signals was done by densitometry using ImageJ 1.46r software (NIH, Bethesda, MA). The ratio of protein expression to β-tubulin in each lane was calculated and presented relative to the respective controls within each experiment.
Cell cycle analysis
Cells were incubated in the appropriate cell culture media with and without drug treatment. Cells were harvested at the indicated time points by trypsinization. They were washed once with cold PBS and stained with 50 μg/mL Propidium Iodide and 100 μg/mL RNase A in PBS (Invitrogen). Samples were analyzed using a BD FACSAria™ II Flow Cytometer (BD, Franklin Lakes, NJ) and the data were analyzed with FlowJo software (Ashland, OR).
Clonogenic proliferation assay
Cells were seeded at 1,000 cells per well in 6-well plate in singe cell suspension. After 24 h acclimatization, they were treated every three days and allowed to proliferate and establish colonies for 9 days. Cells were stained with 0.5 % crystal violet in 1:7 acetic acid: methanol and imaged at 1X in a Bio-Rad ChemiDoc™ XRS+ System with Image Lab™ Software (Bio-Rad Laboratories Inc., Hercules, CA). Colonies were counted and measured using Image J software (The National Institute of Health, Bethesda, MD).
Soft agar anchorage-independent growth assay
6-well plates were coated with 1 mL of 0.8 % agarose in the appropriate culture media. Cells were then suspended in 0.48 % agarose and immediately overlaid on the pre-coated plates. Once the agarose was solid, 1 mL of culture medium with or without 20 nM everolimus was added and replaced every 4 days for 15 days. Cultures were then stained with 0.005 % crystal violet in PBS, washed with PBS until the background was clear and imaged microscopically at 10X for measurement of colony diameter. Plates were also imaged at 1X in a Bio-Rad ChemiDoc™ XRS+ System with Image Lab™ Software (Bio-Rad). Colonies were counted and measured using Image J software (NIH).
Real time PCR
Cells were seeded in 6-well plates and allowed to acclimatize overnight. Following 72 h treatment with 20 nM everolimus, cells were harvested by cell scraping in RLT lysis buffer and total RNA was isolated using the Qiagen RNeasy kit (Venlo, Limburg). First strand cDNA synthesis was performed from 3 μg total RNA using MulV Reverse Transcriptase (Applied Biosystems, Carlsbad, CA) on a Bio Rad MyCycler™. RT-PCR was conducted using the ViiA™ 7 Real-Time PCR system (Applied Biosystems) and SYBR Green Reagent (Life Technologies, Carlsbad, CA) with primers specific for ERα and the housekeeping gene PUM1. Primers for ERα: Forward 5’–AAGAGGGTGCCAGGCTTTGT–3’, Reverse 5’–CAGGATCTCTAG CCAGGCACAT –3’. Primers for PUM1: Forward-TCACCGAGGCCCCTCTGAACCCTA Reverse- GGCAGTAATCTCCTTCTGCATCCT (Integrated DNA Technologies, Coralville, Iowa). Relative ERα mRNA expression level was determined as the ratio of the signal intensity to that of PUM1 using the formula: 2-ΔCT. When treated with everolimus, fold change in ERα expression was normalized to PUM1 and then compared to the untreated value for that cell line using the formula: 2-ΔΔCT.
Luciferase reporter assay
Cells were seeded in 12-well tissue culture plates overnight for attachment before transfection. The cells were transfected using Lipofectamine 2000™ Transfection Reagent (Invitrogen, San Diego) according to the manufacturer's recommendations. Briefly, 4 μL of Lipofectamine 2000, 0.8 μg of ERE Luciferase plasmid DNA and 0.01 μg of the pRL CMV Renilla (Promega) were diluted individually in 250-μl aliquots of OptiMEM Reduced-Serum Medium (Invitrogen). Cells were incubated for 24 h after transfection, and then treated with 20 nM EVE or vehicle in complete media for 24 h. The Luciferase and Renilla activities were measured using the dual luciferase assay kit (Promega) according to the manufacturer's instructions. To confirm the specificity of the ERE Luciferase construct, EVE treated cells were also compared to those treated with 1nM 17β-estradiol and 1nM Fulvestrant, a pure anti-estrogen (Sigma). Relative Fluorescence Units (RFUs) were calculated as a ratio of Luciferase to Renilla signal intensity. The ERE Luciferase reporter construct was a kind gift from Dr. Clodia Osipo (Loyola University, Chicago, IL).
Cells grown on glass coverslips were washed in PBS and fixed with 100 % ice cold methanol for 10 min. After permeabilization by 0.1 % Triton X-100 in PBS for 10 min, cells were incubated with 5 % normal horse serum/PBS for 30 min, followed by incubation with ERα or LC3B antibody, 2 μg/mL in 0.01 % Triton X-100 /PBS overnight (Santa Cruz Biotechnology). Cells were stained with fluorescein isothiocyanate (FITC)-conjugated labeled goat anti-rabbit IgG (Cell Signaling), 4 μg/mL in PBS for 1 h, followed by coverslip mounting with the ProLong® Gold anti-fade reagent with DAPI (Life Technologies). Samples were imaged on a Leica TCS SPE confocal microscope in the Confocal Imaging Core at The University of Kansas Medical Center. Images were collected and analyzed using the Leica LAS AF Lite software (Leica Biosystems, Nussloch, Germany). For quantification, mean fluorescent intensity was determined using Image J software on green (FITC) channel images.
1 x106 cells were seeded in 6-well plates and treated after an overnight acclimatization. After 72 h of treatment cells were harvested by scraping and fixed for 24 h at 4 °C in 0.1 M cacodylate buffer supplemented with 2 % glutaraldehyde. Samples were then processed in The Electron Microscopy Research Laboratory at KU Medical Center as follows. Briefly, cell pellets were washed in 0.1 M cacodylate buffer for 10 min, and resuspended. Cell pellets were post fixed in 1 % osmium tetroxide buffered in 0.1 M cacodylate, rinsed in distilled water 3X’s and then dehydrated in a graded series of ethanol as follows: 50 %, 70 %, 80 %, 95 %, 100 %, 100 % 10 min each step. Cells were placed into propylene oxide for 20 min, then into a 50:50 mixture of propylene oxide and Embed 812 resin medium overnight at room temperature. Samples were cured overnight in beem capsules at 60 degrees and then sectioned with a diamond knife on a Leica UC-7. Sections were cut at 80 nm and contrasted with uranyl acetate and lead citrate and imaged on a JEOL 100CX II Transmission Electron Microscope (Tokyo, Japan).
At least three separate experiments were performed for each measurement unless otherwise indicated. All quantitative data were expressed as means with error bars representing 1 standard deviation (mean ± 1SD). Comparisons between two treatments were analyzed using a two-way student t-test with P-value of < 0.05 considered to be statistically significant. *p < 0.05, **p < 0.01, ***p < 0.001 unless otherwise indicated.
Everolimus inhibits proliferation through induction of G1 arrest
Everolimus reduces the anchorage-independent growth
Effects of everolimus on the PI3K/Akt/mTOR pathway
Everolimus reduces estrogen receptor (ER) expression and transcriptional activity
Everolimus reverses 4-OH tamoxifen resistance
Everolimus induces autophagy in breast cancer cells, which mediates insensitivity
This study was conducted to provide mechanistic insights into the anti-proliferative effects of everolimus in AI-resistant MCF-7:5C and MCF-7:2A breast cancer cells, and AI-sensitive MCF-7 cells. We found that everolimus was equally effective against all three breast cancer cell lines. The anti-proliferative mechanisms included downregulation of ER expression and transcriptional activity, possibly through the suppression of HSP90. We also demonstrated that everolimus enhanced 4OHT sensitivity in all three cell lines. Everolimus treatment significantly induced autophagy, which was associated with downregulation of HSP70 and HSP27 expression. Additionally, we confirmed that everolimus inhibits the activation of the PI3K/Akt/mTOR pathway, resulting in the downregulation of cyclin D1 and p21 expression, which induced G1 arrest.
MCF-7 cells and their derivatives are more resistant to everolimus as compared to other luminal A breast cancer cell lines [37, 38]. Our study is consistent with this observation, as total inhibition of the MCF-7, MCF-7:5C and MCF-7:2A cells did not exceed 60 %, making them suitable to model a patient population that is not highly sensitive to everolimus. Additionally, our IC50 values were consistent with the frequent use of 20 nM everolimus when studying MCF-7 cell lines [17, 39, 40]. The enhanced insensitivity of the MCF-7:5C cells may be due to increased expression of c-myc, which is thought to confer some resistance in ER+ breast cancer [41, 42]. The slightly enhanced sensitivity of the MCF-7:2A cells to everolimus may be related to comparatively lower levels of PTEN . It should be noted that the MCF-7 and MCF-7:2A cells are progesterone receptor-positive (PR+), while the MCF-7:5C cells are PR-negative (PR-) and they overexpresses interferon stimulated genes . These differences do not seem to mediate any variances in everolimus sensitivity, suggesting that the MCF-7 background of these cell lines is the most prominent determinant of response.
Elevated ER expression and signaling has been observed in both endocrine resistance cells and endocrine resistant tumors [2, 3, 44, 45]. The ability of everolimus to reduce ER transcriptional activity and phospho-ER (p-ER) expression in MCF-7/LTED cells has been previously reported but was not assessed in wild type MCF-7 cells . In our study, we found that the inhibition of ER transcriptional activity was due to profound downregulation of total ER expression in all cell models. MCF-7:5C and MCF-7:2A cells are selected clones maintained in estrogen-free media that have retained ER transcriptional activity by upregulation of ER expression and ligand-independent ER signaling. In contrast, the MCF-7/LTED cells are a mixed population of cells that have developed hypersensitivity to estrogen . Additionally, the studies by Martin and colleagues were conducted after acute insulin deprivation, which probably contributed to the enhanced sensitivity to everolimus and to the impact on both p-ER and p-Akt expression in their study. The clinical efficacy of everolimus as a solo agent was thought to be limited by a compensatory increase in Akt phosphorylation through mTORC2 . We did not observe this reflexive increase in Akt phosphorylation in our cells lines. Our results suggest that everolimus as a single agent has the ability to function in a manner similar to combination therapy by inhibiting both growth factor and ER signaling simultaneously in some systems. Downregulation of total ER expression by everolimus has not, to our knowledge, been previously reported.
The downregulation of ER expression was likely due to reduced expression of HSP90, a well-known ER chaperone. HSP expression in general is controlled by the PI3K/Akt/mTOR pathway through phosphorylation of the transcription factor HSF1 and is consistent with the loss of HSP90, HSP27 and HSP70 expression observed in this study . Everolimus induced loss of HSP90 mRNA expression has been observed in other cancers  and a loss of HSP90 expression has been linked to autophagic cell death . Two HSP90 inhibitors, NVP-AUY922 and STA-9090, are currently in clinical trials for the treatment of breast cancer [49, 50]. HSP90 inhibitor therapy has been limited by reflexive increase in HSP signaling, especially enhanced HSP27 expression [51, 52]. Here, we report that everolimus inhibits HSP27 in our cell lines, and so may potentiate HSP90 inhibitor treatment. These results suggest that everolimus may be combined with HSP90 inhibitors or drugs that target HSP90 clients for the treatment of AI-resistant breast cancer.
AI-resistant tumors are known to retain dependence on ER signaling for growth and survival. Given that everolimus reduces ER expression, our observation that it also enhances 4OHT sensitivity during long-term co-treatment is very interesting and warrants further investigation. It should be noted, however, that everolimus has previously been shown to inhibit ER phosphorylation on serine 167 despite documented synergy between everolimus and tamoxifen in AI-resistant models and patients [10, 15, 17]. It is likely that these two drugs exhibit synergy by targeting ER signaling through separate but complementary mechanisms. Our results indicate that everolimus reduces ER expression through inhibition of ER mRNA transcription, while tamoxifen targets the ER protein. Combining everolimus and tamoxifen ensures that ER signaling is inhibited continuously over time in all cells within a heterogeneous breast tumor. The improved efficacy of everolimus in combination with 4OHT in our study is consistent with results from the TAMRAD clinical trial, . The data from our study suggest that everolimus could benefit AI-resistant patients with ligand-independent ER activity by targeting ER expression and signaling.
We have demonstrated that everolimus dramatically induces autophagy, and is associated with significant downregulation of HSP90, HSP70 and HSP27. Loss of HSP70 and HSP27 is associated with starvation-induced autophagy and rapamycin treatment, both of which target the PI3K/Akt/mTOR pathway [26, 30]. Autophagy is a mechanism of drug insensitivity in cancer because it allows tumors to recycle cellular components and survive treatment [21, 22]. Inhibition of autophagy significantly improved the anti-proliferative effects of everolimus in MCF-7, MCF-7:5C and MCF-7:2A cells. This is consistent with a previous study reporting enhanced inhibition of MCF-7 cell proliferation when combining chloroquine and everolimus . Everolimus and other PI3K/Akt/mTOR inhibitors are known to induce autophagy in both solid and blood tumors [19, 20]; however, to our knowledge, this phenomenon has not been reported in breast cancer cells. We conclude that the induction of autophagy is likely a mechanism of everolimus insensitivity in MCF-7 [37, 38], MCF-7:5C and MCF-7:2A cells.
Although everolimus treatment induced PARP cleavage in all three cell lines, we did not observe apoptotic cell death normally associated with PARP cleavage. PARP cleavage is thought to mediate autophagy rather than apoptosis in response to certain stimuli . Since HSP70 is known to stabilize PARP , loss of HSP70-mediated stability offers an explanation for the everolimus induced PARP cleavage seen in this study. While there have been reports of autophagic and apoptotic cell death in leukemia  and nasopharyngeal carcinoma cells , everolimus is not known to induce cell death in breast cancer cells on its own [19, 56]. To our knowledge, everolimus induced cell death in breast cancer has only been observed in aromatase expressing MCF-7/Aro cells when combined with letrozole . We have shown that induction of autophagy limits the anti-proliferative response of everolimus treatment, hence a combination of everolimus with the autophagy inhibitors chloroquine or hydroxychloroquine, which are currently in clinical trials as part of combination therapy , may be beneficial in the treatment of AI-resistant breast cancer.
Overall, this study demonstrated that everolimus inhibits the proliferation of AI-resistant breast cancer cells through down regulation of ER expression and also that induction of autophagy is a method of everolimus insensitivity. We found that everolimus had similar effect on the proliferation of both our AI-sensitive (MCF-7) and AI-resistant (MCF-7:5C and MCF-7:2A) models, suggesting that the MCF-7 background of these cell lines overrides any other differences that might impact everolimus sensitivity. The inhibition of proliferation was seen regardless of PR status, PTEN expression, type 1interferon signaling, and 4OHT sensitivity, supporting a conclusion that everolimus holds promise as part of combination therapy for a wide variety of AI-resistant patients, for whom AI treatment is not an option.
AI, aromatase inhibitor; CQ, chloroquine; EGF, epidermal growth factor; ER, estrogen receptor; EVE, everolimus; FUL, fulvestrant; HER2, human epidermal growth factor receptor 2; HIF-1α, hypoxia inducible factor 1 alpha subunit; HSP27, Heat shock protein 27; HSP70, heat shock protein 70; HSP90, heat shock protein 90; IC50, drug concentration that provides 50 % maximal growth inhibition; LC3B, Microtubule associated light chain 3; mTOR, mammalian target of rapamycin; PARP, Poly ADP ribose polymerase; PI3K, phosphoinositide 3-kinase; PR, Progesterone receptor; PTEN, phosphatase and tensin homolog; P21, cyclin dependent kinas inhibitor 1; p70S6K, p70 S6 ribosomal protein kinase; TSC, tuberous sclerosis complex; 4OHT, 4- hydroxytamoxifenv; 4EBP1, Eukaryotic translation initiation factor 4E-bidning proteinv.
We would like to thank the Flow Cytometry Core Laboratory at the University of Kansas Medical Center, which is sponsored, in part, by the NIH/NIGMS COBRE grant P30 GM103326. We also wish to acknowledge the University of Kansas Medical Center Electron Microscopy Research Lab (EMRL) facility for assistance with the electron microscopy. The EMRL is supported in part, by NIH COBRE grant 9P20GM104936. The JEOL JEM-1400 TEM used in the study was purchased with funds from NIH grant S10RR027564.
This study was supported, in part, by grants from the Department of Defense (W81XWH-12-1-0139 to JLW), the National Cancer Institute (K01CA120051 to JLW), start-up funds from the University of Kansas Medical Center (to JLW), the KUMC Biomedical Research Training Program (BRTP to AL), the National Cancer Institute (1F30CA203160-01 to AL), and the University of Kansas Cancer Center (CCSG grants P30 CA168524-0 to ST and JLW).
Availability of data and materials
All data supporting the findings in this study are included within the manuscript and the two supplementary figures.
AL designed and executed this study and wrote the manuscript. JN assisted with the experiments concerning analysis of autophagy and contributed to the manuscript. JO edited the manuscript and contributed intellectually to the study. ST oversaw the autophagy analysis and edited the manuscript. JLW, conceived of the study and edited the manuscript. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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