Long-term Estrogen-deprived Cells Research Articles

Adenosine monophosphate activated protein kinase (AMPK), a mediator of estradiol-induced apoptosis in long-term estrogen deprived breast cancer cells.

Estrogens stimulate growth of hormone-dependent breast cancer but paradoxically induce tumor regress under certain circumstances. We have shown that long-term estrogen deprivation (LTED) enhances the sensitivity of hormone dependent breast cancer cells to estradiol (E2) so that physiological concentrations of estradiol induce apoptosis in these cells. E2-induced apoptosis involve both intrinsic and extrinsic pathways but precise mechanisms remain unclear. We found that exposure of LTED MCF-7 cells to E2 activated AMP activated protein kinase (AMPK). In contrast, E2 inhibited AMPK activation in wild type MCF-7 cells where E2 prevents apoptosis. As a result of AMPK activation, the transcriptional activity of FoxO3, a downstream factor of AMPK, was up-regulated in E2 treatment of LTED. Increased activity of FoxO3 was demonstrated by up-regulation of three FoxO3 target genes, Bim, Fas ligand (FasL), and Gadd45α. Among them, Bim and FasL mediate intrinsic and extrinsic apoptosis respectively and Gadd45α causes cell cycle arrest at the G2/M phase. To further confirm the role of AMPK in apoptosis, we used AMPK activator AICAR in wild type MCF-7 cells and examined apoptosis, proliferation and expression of Bim, FasL, and Gadd45α. The effects of AICAR on these parameters recapitulated those observed in E2-treated LTED cells. Activation of AMPK by AICAR also increased expression of Bax in MCF-7 cells and its localization to mitochondria, which is a required process for apoptosis. These results reveal that AMPK is an important factor mediating E2-induced apoptosis in LTED cells, which is implicative of therapeutic potential for relapsing breast cancer after hormone therapy.

Kinome-wide functional screen identifies role of PLK1 in hormone-independent, ER-positive breast cancer.

Estrogen receptor (ER) α-positive breast cancers initially respond to antiestrogens but eventually become estrogen independent and recur. ER(+) breast cancer cells resistant to long-term estrogen deprivation (LTED) exhibit hormone-independent ER transcriptional activity and growth. A kinome-wide siRNA screen using a library targeting 720 kinases identified Polo-like kinase 1 (PLK1) as one of the top genes whose downregulation resulted in inhibition of estrogen-independent ER transcriptional activity and growth of LTED cells. High PLK1 mRNA and protein correlated with a high Ki-67 score in primary ER(+) breast cancers after treatment with the aromatase inhibitor letrozole. RNAi-mediated knockdown of PLK1 inhibited ER expression, estrogen-independent growth, and ER transcription in MCF7 and HCC1428 LTED cells. Pharmacologic inhibition of PLK1 with volasertib, a small-molecule ATP-competitive PLK1 inhibitor, decreased LTED cell growth, ER transcriptional activity, and ER expression. Volasertib in combination with the ER antagonist, fulvestrant, decreased MCF7 xenograft growth in ovariectomized mice more potently than each drug alone. JUNB, a component of the AP-1 complex, was expressed 16-fold higher in MCF7/LTED compared with parental MCF7 cells. Furthermore, JUNB and BCL2L1 (which encodes antiapoptotic BCL-xL) mRNA levels were markedly reduced upon volasertib treatment in MCF7/LTED cells, while they were increased in parental MCF7 cells. Finally, JUNB knockdown decreased ER expression and transcriptional activity in MCF7/LTED cells, suggesting that PLK1 drives ER expression and estrogen-independent growth via JUNB. These data support a critical role of PLK1 in acquired hormone-independent growth of ER(+) human breast cancer and is therefore a promising target in tumors that have escaped estrogen deprivation therapy.

Targeting tyrosine-kinases and estrogen receptor abrogates resistance to endocrine therapy in breast cancer.

Despite numerous therapies that effectively inhibit estrogen signaling in breast cancer, a significant proportion of patients with estrogen receptor (ER)-positive malignancy will succumb to their disease. Herein we demonstrate that long-term estrogen deprivation (LTED) therapy among ER-positive breast cancer cells results in the adaptive increase in ER expression and subsequent activation of multiple tyrosine kinases. Combination therapy with the ER down-regulator fulvestrant and dasatinib, a broad kinase inhibitor, exhibits synergistic activity against LTED cells, by reduction of cell proliferation, cell survival, cell invasion and mammary acinar formation. Screening kinase phosphorylation using protein arrays and functional proteomic analysis demonstrates that the combination of fulvestrant and dasatinib inhibits multiple tyrosine kinases and cancer-related pathways that are constitutively activated in LTED cells. Because LTED cells display increased insulin receptor (InsR)/insulin-like growth factor 1 receptor (IGF-1R) signaling, we added an ant-IGF-1 antibody to the combination with fulvestrant and dasatinib in an effort to further increase the inhibition. However, adding MK0646 only modestly increased the inhibition of cell growth in monolayer culture, but neither suppressed acinar formation nor inhibited cell migration in vitro and invasion in vivo. Therefore, combinations of fulvestrant and dasatinib, but not MK0646, may benefit patients with tyrosine-kinase-activated, endocrine therapy-resistant breast cancer.

Inhibition of polo-like kinase 1 (PLK1) in endocrine-resistant ER+ breast cancer.

515 Background: Estrogen receptor (ER)-positive breast cancers initially respond to antiestrogens but eventually become hormone-independent and recur. ER+ breast cancer cells resistant to long-term estrogen deprivation (LTED) exhibit estrogen-independent ER transcriptional activity and growth. Mechanisms of endocrine resistance remain to be fully characterized. Methods: A siRNA screen was used to identify kinases required for growth of MCF7/LTED cells. PLK1 RNAi oligonucleotides and the small molecule inhibitor volasertib were tested against ER+ LTED cells. Estrogen independent transcription and target genes were assessed with ER reporter assays and qRT-PCR, respectively. Volasertib and fulvestrant were used in ovariectomized athymic mice bearing MCF7 xenografts. Preclinical findings were correlated with RPPA and RNA-seq data from tumor biopsies of patients with ER+/HER2– breast cancer who received letrozole prior to surgery (NCT00651976). Results: A siRNA kinome screen identified Polo-like kinase 1 (PLK1...

Abstract P5-09-08: A comparison of the non-genomic effects of endoxifen and tamoxifen in aromatase inhibitor resistant breast cancer: Differential effects on the estrogen receptor co-regulator SRC3 (AIB1) and identification of PKC and PI3K as endoxifen substrates

Abstract Background: Endoxifen (Endx) is being developed as a drug for patients with estrogen receptor (ER)+ breast cancer (BC). We previously demonstrated that Endx was superior to TAM in both a letrozole (L) sensitive and resistant MCF7/AC1 model. Tamoxifen (TAM) and its most commonly studied metabolite, 4-hydroxy TAM (4HT), are known to activate critical ER co-regulators such as SRC3, which are oncogenic, and contribute to acquired and de novo endocrine resistance. While both Endx and 4HT are more potent inhibitors of genomic ERα functions compared to TAM, the effects of Endx on non-genomic signaling have not been characterized and its efficacy as an anti-cancer agent compared to RAD001 in AI resistant BC is unknown. Methods: We evaluated the 1 hour in vitro effects of TAM, 4HT, Endx, and L, in terms of activation of multiple signaling cascades using an MCF7/AC1 L-resistant cells and long term estrogen deprived (LTED) cells (MDA 361 and ZR75). Protein Docking studies (shape screening, pharmacophore modeling and ligand-protein docking) for oncogenic kinases, and their associated IC50's were determined for TAM and Endx. The in vivo activity of Endx (50 mg/kg, oral) was determined in MCF/AC1 L-resistant xenograft tumors and compared with L control (10 ug/day, sc), Exemestane (E) (250 ug/d, sc) and E+RAD001 (2.5 mg/kg/d, oral). Results: In MCF7/AC1 L-resistant cells, 1 hr treatment with either TAM (100 nM or 5 uM) or 4HT (100 nM or 5 uM) phosphorylated SRC3 (AIB1) and resulted in downstream activation (phosphorylation) of IGF1R, PDK1, AKT, and p70S6K. In contrast, Endx (100 nM and 5 uM) did not phosphorylate SRC3 nor any downstream kinases but resulted in a dose dependent decrease in AKT phosphorylation. Similar findings were observed in LTED cells MDA 361 and ZR75. Docking studies identified a significant decrease in the binding energy (>2 kcal/mol) of Endx compared to TAM for PKC and PI3K and subsequent kinase profiling demonstrated that Endx more potently inhibits PKCβ1 (IC50 357nM) and PI3KΔ (2.5uM) compared to TAM (IC50 4.8μM and 6.2μM respectively). Therefore, we evaluated the in vivo anti-tumor activity of Endx with E and E+RAD001 in MCF7 L-resistant xenograft tumors. After 9 weeks, the% change in tumor volume relative to baseline was significantly lower for both Endx (median -34%, p = 0.0002) and E+RAD001 (median +1.5%, p = 0.0002) as compared to L controls (median +301%). In contrast, E alone was non-significantly lower (median +172%, (p = 0.09). No significant differences were observed comparing Endx and E+RAD001 (p = 0.56). Conclusions: Endx inhibits PKCβ1 and PI3KΔ at concentrations that are pharmacologically relevant, and unlike TAM and 4HT, does not activate SRC3 nor induce oncogenic signaling pathways in AI resistant cells. Importantly, Endx elicits substantial anti-tumor effects in AI resistant models that are similar to the current standard of care of E+RAD001. These data suggest that Endx may be superior to TAM for the treatment of ER+ BC and may result in clinical benefit for patients with AI resistant disease. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-08.

Autocrine IGF-I/insulin receptor axis compensates for inhibition of AKT in ER-positive breast cancer cells with resistance to estrogen deprivation

IntroductionEstrogen receptor α-positive (ER+) breast cancers adapt to hormone deprivation and acquire resistance to antiestrogen therapies. Upon acquisition of hormone independence, ER+ breast cancer cells increase their dependence on the phosphatidylinositol-3 kinase (PI3K)/AKT pathway. We examined the effects of AKT inhibition and its compensatory upregulation of insulin-like growth factor (IGF)-I/InsR signaling in ER+ breast cancer cells with acquired resistance to estrogen deprivation.MethodsInhibition of AKT using the catalytic inhibitor AZD5363 was examined in four ER+ breast cancer cell lines resistant to long-term estrogen deprivation (LTED) by western blotting and proliferation assays. Feedback upregulation and activation of receptor tyrosine kinases (RTKs) was examined by western blotting, real-time qPCR, ELISAs, membrane localization of AKT PH-GFP by immunofluorescence and phospho-RTK arrays. For studies in vivo, athymic mice with MCF-7 xenografts were treated with AZD5363 and fulvestrant with either the ATP-competitive IGF-IR/InsR inhibitor AZD9362 or the fibroblast growth factor receptor (FGFR) inhibitor AZD4547.ResultsTreatment with AZD5363 reduced phosphorylation of the AKT/mTOR substrates PRAS40, GSK3α/β and S6K while inducing hyperphosphorylation of AKT at T308 and S473. Inhibition of AKT with AZD5363 suppressed growth of three of four ER+ LTED lines and prevented emergence of hormone-independent MCF-7, ZR75-1 and MDA-361 cells. AZD5363 suppressed growth of MCF-7 xenografts in ovariectomized mice and a patient-derived luminal B xenograft unresponsive to tamoxifen or fulvestrant. Combined treatment with AZD5363 and fulvestrant suppressed MCF-7 xenograft growth better than either drug alone. Inhibition of AKT with AZD5363 resulted in upregulation and activation of RTKs, including IGF-IR and InsR, upregulation of FoxO3a and ERα mRNAs as well as FoxO- and ER-dependent transcription of IGF-I and IGF-II ligands. Inhibition of IGF-IR/InsR or PI3K abrogated AKT PH-GFP membrane localization and T308 P-AKT following treatment with AZD5363. Treatment with IGFBP-3 blocked AZD5363-induced P-IGF-IR/InsR and T308 P-AKT, suggesting that receptor phosphorylation was dependent on increased autocrine ligands. Finally, treatment with the dual IGF-IR/InsR inhibitor AZD9362 enhanced the anti-tumor effect of AZD5363 in MCF-7/LTED cells and MCF-7 xenografts in ovariectomized mice devoid of estrogen supplementation.ConclusionsThese data suggest combinations of AKT and IGF-IR/InsR inhibitors would be an effective treatment strategy against hormone-independent ER+ breast cancer.

Abstract P4-07-03: Rapamycin and Dalotuzumab in combination inhibit parental and endocrine resistant breast cancer cells.

Abstract The PI3K/Akt/mTOR pathway is involved in breast cancer resistance to endocrine therapy. Inhibitors of mTOR have been shown to have benefit for patients with ER positive tumors that have developed endocrine therapy resistance. Inhibition of mTOR triggers a negative feedback loop resulting in enhanced phosphorylation of Akt and subsequent breast cancer cell proliferation via the IGF signaling pathway. Therefore, blockade of both IGF-1R and mTOR may be necessary to completely suppress this pathway. Our laboratory studied the effect of dual inhibition of mTOR and IGF-1R with rapamycin and dalotuzumab respectively on parental and endocrine resistant MCF-7 breast cancer cell tumorigenesis. Immunoblotting demonstrates parental MCF-7L cells treated with rapamycin alone demonstrate increased phosphorylation of Akt. Inhibition of phosphorylation of Akt can be achieved with combined treatment with rapamycin and dalotuzumab at concentrations of 5 nM and 2 µg/mL respectively. In addition to inhibition of pAkt, combined treatment inhibits phosphorylation of S6K1 and the translational repressor protein, 4eBP1. Furthermore, MTT proliferation assay and soft agar assay demonstrate inhibition of parental cell proliferation and colonization respectively with combined rapamycin and dalotuzumab treatment. In order to examine the effect of combination therapy in endocrine resistant cell lines, we established tamoxifen resistant (TamR) and long term estrogen deprived (LTED) cell lines. TamR cells were generated by culturing MCF-7L cells in the presence of tamoxifen for more than 1 year. LTED cells were generated by culturing MCF-7L cells in the absence of estrogen for more than 6 months. Proliferation of TamR and LTED cells treated with rapamycin at a concentration of 5nM and dalotuzumab at a concentration of 2µg/mL in combination was significantly inhibited. We conclude that combination of IGF1R and mTOR inhibition might be necessary to inhibit endocrine resistant breast cancers. Ongoing clinical trials will test this combination of drugs. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-07-03.

Effectiveness and molecular interactions of the clinically active mTORC1 inhibitor everolimus in combination with tamoxifen or letrozole in vitro and in vivo

IntroductionStrategies to improve the efficacy of endocrine agents in breast cancer (BC) therapy and to delay the onset of resistance include concomitant targeting of the estrogen receptor alpha (ER) and the mammalian target of rapamycin complex 1 (mTORC1), which regulate cell-cycle progression and are supported by recent clinical results.MethodsBC cell lines expressing aromatase (AROM) and modeling endocrine-sensitive (MCF7-AROM1) and human epidermal growth factor receptor 2 (HER2)-dependent de novo resistant disease (BT474-AROM3) and long-term estrogen-deprived (LTED) MCF7 cells that had acquired resistance associated with HER2 overexpression were treated in vitro and as subcutaneous xenografts with everolimus (RAD001-mTORC1 inhibitor), in combination with tamoxifen or letrozole. End points included proliferation, cell-cycle arrest, cell signaling, and effects on ER-mediated transactivation.ResultsEverolimus caused a concentration-dependent decrease in proliferation in all cell lines, which was associated with reductions in S6 phosphorylation. Everolimus plus letrozole or tamoxifen enhanced the antiproliferative effect and G1-accumulation compared with monotherapy, as well as increased phosphorylation (Ser10) and nuclear accumulation of p27 and pronounced dephosphorylation of Rb. Sensitivity was greatest to everolimus in the LTED cells but was reduced by added estrogen. Increased pAKT occurred in all circumstances with everolimus and, in the BT474 and LTED cells, was associated with increased pHER3. Decreased ER transactivation suggested that the effectiveness of everolimus might be partly related to interrupting cross-talk between growth-factor signaling and ER. In MCF7-AROM1 xenografts, letrozole plus everolimus showed a trend toward enhanced tumor regression, versus the single agents. In BT474-AROM3 xenografts, everolimus alone was equally effective at reducing tumor volume as were the combination therapies.ConclusionsThe results provide mechanistic support for recent positive clinical data on the combination of everolimus and endocrine therapy, as well as data on potential routes of escape via enhanced HER2/3 signaling. This merits investigation for further improvements in treatment efficacy.

Preclinical and clinical studies of estrogen deprivation support the PDGF/Abl pathway as a novel therapeutic target for overcoming endocrine resistance in breast cancer

IntroductionThe majority of breast tumors at primary diagnosis are estrogen receptor positive (ER+). Estrogen (E) mediates its effects by binding to the ER. Therapies targeting the estrogenic stimulation of tumor growth reduce mortality from ER+ breast cancer. However, resistance remains a major clinical problem.MethodsTo identify molecular mechanisms associated with resistance to E-deprivation, we assessed the temporal changes in global gene expression during adaptation to long-term culture of MCF7 human breast cancer cells in the absence of estradiol (E2), long term estrogen deprived (LTED), that leads to recovery of proliferative status and models resistance to an aromatase inhibitor (AI). The expression levels of proteins were determined by western blotting. Proliferation assays were carried out using the dual platelet derived growth factor receptor (PDGFR)/Abelson tyrosine kinase (Abl) inhibitor nilotinib. Luciferase reporter assays were used to determine effects on ER-mediated transactivation. Changes in recruitment of cofactors to the gene regulated by estrogen in breast cancer 1 (GREB1) promoter were determined by chromatin immunoprecipitation (ChIP). Gene expression data were derived from 81 postmenopausal women with ER+ BC pre-treatment and at two-weeks post-treatment with single agent anastrozole in a neoadjuvant trial.ResultsThe PDGF/Abl canonical pathway was significantly elevated as early as one week post E-deprivation (P = 1.94 E-04) and this became the top adaptive pathway at the point of proliferative recovery (P = 1.15 E-07). Both PDGFRβ and Abl protein levels were elevated in the LTED cells compared to wild type (wt)-MCF7 cells. The PDGF/Abl tyrosine kinase inhibitor nilotinib, suppressed proliferation in LTED cells in the presence or absence of E. Nilotinib also suppressed ER-mediated transcription by destabilizing the ER and reducing recruitment of amplified in breast cancer-1 (AIB1) and the CREB binding protein (CBP) to the promoter of the E-responsive gene GREB1. High PDGFRβ in primary ER+ breast cancer of 81 patients prior to neoadjuvant treatment with an AI was associated with poorer antiproliferative response. Additionally PDGFRβ expression increased after two weeks of AI therapy (1.25 fold, P = 0.003).ConclusionsThese preclinical and clinical data indicate that the PDGF/Abl signaling pathway merits clinical evaluation as a therapeutic target with endocrine therapy in ER+ breast cancer.

Abstract 2702: Effects of Src and IGF1R inhibition on acquired endocrine therapy resistant breast cancer

Abstract Effective standard therapy for hormone receptor-positive (HR+) breast cancer targets the estrogen receptor (ER). However, a significant proportion of women with HR+ breast cancer relapse. Furthermore, the majority of women with metastatic HR+ breast cancer develop resistance to endocrine therapy. Adaptive cross-talk between ER and growth factor receptor pathways including IGF1R and Src is involved in acquired endocrine therapy resistance. To determine effects of adding dasatinib (D), Src inhibitor, and/or anti-IGF1R Ab MK0646 (M) to the ER antagonist fulvestrant (F) in an acquired endocrine therapy resistant breast cancer, we used long-term estrogen-deprived (LTED) derivative of ER+ breast cancer cell lines, MCF-7/LTED and HCC-1428/LTED. Parental and LTED cells were treated with variable combinations of F, D and M, followed by determinations of drug effects on cell growth, migration, and invasion in vitro and tumor growth in vivo. We demonstrated that LTED cells were resistant to F alone, comparing parental cells (P<0.05) in cell growth, migration and invasion. Addition of D to F appeared to be synergistic in all systems including cell growth, cell migration and invasion. However, the addition of M to F or to F+D appeared inhibition on cell growth in monolayer with low-serum medium, but not to significantly add inhibitory effects on cell growth in 3D and cell migration and invasion. The effects of IGF-1R antibody, MK0646, on cells growth in monolayer showed not consistent with 3D, which might be due to growth factors in the matrigel, although we used growth factors decreased matrigel. To verify the effects of the drugs on tumor growth in vivo, we developed xenograft models using MCF-7/LTED cells and treated the mice with F, D, and M along or variable combinations. The study demonstrated that all the combinations of F+D+M, F+D, and F+M (from P< 0.05 to <0.001 in viable time points) show inhibited the tumor growth comparing vehicle or F alone. The inhibitory effect of F+D+M is higher than F+D and F+M. In summary, the addition of dasatinib to fulvestrant appeared to be synergistic in all systems including cell growth, cell migration and invasion and tumor growth in vivo. The addition of MK0646 to fulvestrant mildly inhibited MCF-7/LTED cell growth and xenografts growth, but not cell migration and cell invasion. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2702. doi:1538-7445.AM2012-2702

Abstract 786: In-vivo and in-vitro breast cancer profiling to delineate adaptive endocrine resistance: A pilot study

Abstract A substantial fraction of estrogen receptor (ER) positive breast cancers fail to respond to endocrine treatment, i.e. tamoxifen and aromatase inhibitors. While tamoxifen failure has been partially attributed to primary resistance, for example CYP2D6 deficiency (Schroth et al JAMA 2009), adaptive resistance during ER blockade/estrogen deprivation plays a prominent role. Little is known about shared molecular patterns driving both resistance types. Long-term estrogen deprived (LTED) breast cancer can be induced to undergo apoptosis by pulse estrogen (Song et al JNCI 2001; Jordan JCO 2008). We hypothesize that it should be possible to identify predictive markers for adaptive endocrine resistance and sensitivity to apoptosis to recognize endocrine refractory tumors. To study a putative interface of primary and acquired resistance we performed gene expression profiling with low-density arrays (Fluidigm BioMark system) in tumors of 12 breast cancer patients treated with tamoxifen. They were selected to model CYP2D6 independent primary resistance: half of the patients had functional CYP2D6 but were tamoxifen resistant due to relapse (median time-to-event 3.8 years). The other half of patients were CYP2D6 deficient but tamoxifen sensitive due to the absence of relapse (median follow-up 13.5 years). Expression profiles have been compared to LTED MCF-7 cells (3 and 6 months estrogen deprived) and MCF-7 cells grown in the presence of estrogen thereby modeling adaptive resistance. Gene targets were selected based on literature i) involved in endocrine resistance (N=62), ii) playing a role in transport and estrogen supply (N=30), and iii) being p53 targets for apoptosis and cell-cycle check points (N=46). We observed both shared and specific expression phenotypes for adaptive and primary resistance. The proliferation genes Cyclin B1 and STK15 were downregulated in LTED MCF-7 cells in line with lowered proliferation rates and, unexpectedly, Cyclin B1 expression was also decreased in tamoxifen insensitive tumors suggesting an involvement of G2/M cell cycle transition. The chemokine receptor CXCR4 was upregulated in both LTED cells and resistant tumors, confirming its prominent role in breast cancer progression. The 25-hydroxyvitamin D3 hydroxylase CYP27B1 was upregulated in LTED, but not in resistant tumors suggesting a different role between the two models. Other potential markers involve differentiation, inflammatory response, cell cycle/apoptosis, and estrogen disposition pathways. Our preliminary data demonstrate the feasibility to discern between short-term adaptive versus primary resistance patterns. Expression profiling of LTED cells following estrogen-induced apoptosis is currently underway. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 786. doi:1538-7445.AM2012-786

Abstract 4825: Inhibition of AKT abrogates resistance to endocrine therapy in estrogen receptor positive breast cancer

Abstract Estrogen receptor α-positive (ER+) breast cancers eventually adapt to hormone deprivation and acquire resistance to aromatase inhibitors. Upon acquisition of hormone independence, ER+ breast cancer cells increase their dependence on the phosphatidylinositol-3 kinase (PI3K)/AKT signaling pathway. Herein we investigated the effects of AKT inhibition on the hormone-independent growth of four ER+ breast cancer cell lines (MCF-7, ZR75-1, MDA-MB-361 and HCC-1428) with acquired resistance to estrogen deprivation (termed long-term estrogen-deprived cells, LTED). Treatment with the ATP-competitive AKT inhibitor AZD5363 inhibited phosphorylation of the AKT/mTOR substrates PRAS40, GSK3β/α, and S6K while inducing hyperphosphorylation of AKT at S473 and T308. AZD5363 suppressed the hormone-independent growth of 3/4 LTED lines. AZD5363 prevented the emergence of hormone-independent MCF-7, ZR75-1, and MDA-361 cells. Further, treatment of ovariectomized athymic mice with AZD5363 suppressed the hormone-independent growth of ER+/PI3K mutant MCF-7 xenografts. Combined treatment with AZD5363 and the ER downregulator fulvestrant more effectively suppressed xenograft growth than either drug alone. AZD5363 also caused dose-dependent inhibition of an ER+ breast cancer explant model unresponsive to fulvestrant or tamoxifen. We next examined whether AKT inhibition results in feedback upregulation of mitogenic signaling pathways. Treatment with AZD5363 resulted in upregulation and phosphorylation of the HER3 receptor tyrosine kinase (RTK), as well as upregulation of insulin receptor (InsR) protein levels and phosphorylation of Src at Y416. A Src family kinase inhibitor suppressed AZD5363-induced upregulation of P-HER3 in MCF-7/LTED cells. Src inhibition or siRNA-mediated knockdown of InsR or IGF-IR significantly enhanced the growth inhibitory effects of AZD5363. Following treatment with AZD5363, phospho-RTK array analysis revealed increased phosphorylation of multiple RTKs in MCF-7/LTED and/or ZR75-1/LTED cells, including InsR, IGF-IR, EGFR, HER2, HER3, HER4, Dtk, and FGFR3. Treatment with AZD5363 also upregulated RTK mRNA levels. Further, treatment of MCF-7/LTED cells with AZD5363 resulted in marked translocation of AKT PH-GFP to the membrane, reflective of increased PIP3 production and, thus, AKT phosphorylation at T308. Pre-treatment with the PI3K inhibitor BKM120 or the IGF-IR/InsR TKI AEW541 prevented AZD5363-induced membrane localization of PH-GFP. These results suggest that 1) AKT signaling is critical for hormone-independent growth of ER+ breast cancer cells in vitro and in vivo; and 2) AKT inhibition in these cells induces feedback upregulation of RTK expression and activity. Thus, inhibitors of AKT merit evaluation as a potential treatment for endocrine-resistant breast cancer. The combination of these agents with inhibitors of upstream RTKs are under evaluation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4825. doi:1538-7445.AM2012-4825

A Kinome-Wide Screen Identifies the Insulin/IGF-I Receptor Pathway as a Mechanism of Escape from Hormone Dependence in Breast Cancer

Estrogen receptor α (ER)-positive breast cancers adapt to hormone deprivation and become resistant to antiestrogens. In this study, we sought to identify kinases essential for growth of ER(+) breast cancer cells resistant to long-term estrogen deprivation (LTED). A kinome-wide siRNA screen showed that the insulin receptor (InsR) is required for growth of MCF-7/LTED cells. Knockdown of InsR and/or insulin-like growth factor-I receptor (IGF-IR) inhibited growth of 3 of 4 LTED cell lines. Inhibition of InsR and IGF-IR with the dual tyrosine kinase inhibitor OSI-906 prevented the emergence of hormone-independent cells and tumors in vivo, inhibited parental and LTED cell growth and PI3K/AKT signaling, and suppressed growth of established MCF-7 xenografts in ovariectomized mice, whereas treatment with the neutralizing IGF-IR monoclonal antibody MAB391 was ineffective. Combined treatment with OSI-906 and the ER downregulator fulvestrant more effectively suppressed hormone-independent tumor growth than either drug alone. Finally, an insulin/IGF-I gene expression signature predicted recurrence-free survival in patients with ER(+) breast cancer treated with the antiestrogen tamoxifen. We conclude that therapeutic targeting of both InsR and IGF-IR should be more effective than targeting IGF-IR alone in abrogating resistance to endocrine therapy in breast cancer.

Abstract A31: The Myb transcription factor links the ERα and EGFR pathways in breast cancer cells

Abstract Drug resistance in breast cancer remains a big challenge, with many patients relapsing on endocrine therapies. To investigate growth-dependent signaling pathways in cancer we have generated a novel long-term estrogen-deprived cell line in serum-free conditions called DH as a model for antiestrogen treatment. DH cells are sensitive to ICI 182780, showing they are estrogen receptor (ERα)-dependent, they upregulate estrogen (E2)-dependent genes, indicating they are E2 responsive and are highly tamoxifen sensitive. This led us to hypothesize that in the total absence of E2 these cells might evolve to become highly E2 responsive and be capable of activating other growth-promoting pathways. We find that unliganded ERα is a negative regulator of EGFR expression but E2 stimulation triggers a slow upregulation of EGFR and a rapid reduction in ERα levels. After initial stimulation, EGFR expression is maintained despite E2 withdrawal, indicating a stable epigenetic modification at the EGFR promoter. E2 activates the growth-promoting MAPK pathway in an EGFR-dependent manner and we show a 5-day E2 pulse substantially increases the resistance of DH, MCF7, and ZR75-1 cells to tamoxifen and is accompanied by EGFR upregulation and phosphorylation. The cells remain sensitive to AG1478, an EGFR inhibitor, demonstrating a surprising functional switch between ERα and EGFR survival pathways, triggered by E2. To further investigate the relationship between ERα and EGFR, we used expression microarrays to identify MYB, a transcription factor, which is upregulated in response to E2. Upon E2 treatment, ERα binds to MYB intron 1, promoting MYB expression and consequently MYB is transiently located upstream of the EGFR promoter. MYB depletion by siRNA abrogates E2-dependent EGFR expression showing a direct link between these two pathways. These findings maybe significant for understanding acquired drug resistance on endocrine therapies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr A31.

ERα-dependent E2F transcription can mediate resistance to estrogen deprivation in human breast cancer.

Most estrogen receptor α (ER)-positive breast cancers initially respond to antiestrogens, but many eventually become estrogen-independent and recur. We identified an estrogen-independent role for ER and the CDK4/Rb/E2F transcriptional axis in the hormone-independent growth of breast cancer cells. ER downregulation with fulvestrant or small interfering RNA (siRNA) inhibited estrogen-independent growth. Chromatin immunoprecipitation identified ER genomic binding activity in estrogen-deprived cells and primary breast tumors treated with aromatase inhibitors. Gene expression profiling revealed an estrogen-independent, ER/E2F-directed transcriptional program. An E2F activation gene signature correlated with a lesser response to aromatase inhibitors in patients' tumors. siRNA screening showed that CDK4, an activator of E2F, is required for estrogen-independent cell growth. Long-term estrogen-deprived cells hyperactivate phosphatidylinositol 3-kinase (PI3K) independently of ER/E2F. Fulvestrant combined with the pan-PI3K inhibitor BKM120 induced regression of ER(+) xenografts. These data support further development of ER downregulators and CDK4 inhibitors, and their combination with PI3K inhibitors for treatment of antiestrogen-resistant breast cancers.

Abstract 5478: Combination of fulvestrant with the allosteric Akt inhibitor MK-2206 promotes anti-tumor activity in estrogen deprivation-resistant estrogen receptor positive breast cancer

Abstract Introduction: Activation of the phosphatidylinositol-3-kinase (PI3K)/Akt pathway through gene mutation, most frequently in PIK3CA, is common in estrogen receptor-positive (ER+) breast cancer (BC) and deregulation of PI3K signaling contributes to endocrine resistance. These data suggest that proteins within the PI3K/Akt cascade are important therapeutic targets for patients who experience relapse despite adjuvant endocrine treatment. We have demonstrated previously that simultaneous inhibition of PI3K catalytic subunits and ER (through estrogen deprivation) activates cell death in estrogen-dependent ER+ BC cells (Cancer Res 69:3955, 2009). In order to develop rationales for combination therapies in the advanced disease setting we tested a clinical grade Akt inhibitor, MK-2206 in both estrogen-dependent BC cells and in BC cells that had lost the requirement for estrogen due to long-term estrogen deprivation (LTED). Methods: The effects of MK-2206 on cell signaling and apoptosis were determined in a panel of growth-arrested ER+ BC cells following short-term estrogen deprivation (STED) and in estrogen-independent MCF7 and T47D cells established following LTED. The impact of estrogen deprivation and treatment with the ER down-regulator fulvestrant on MK-2206 treatment-induced apoptosis was assessed in STED and LTED cells. We also determined the activity of MK-2206 with and without fulvestrant in vivo in MCF7 LTED xenografts, a model for aromatase inhibitor-resistant ER+ breast cancer. Results: MK-2206-induced apoptosis was most marked in STED cells with PIK3CA mutation and PTEN loss, indicating that ER+ BC with PI3K pathway mutations will be most sensitive to MK-2206 treatment. Estradiol antagonized MK2206-induced apoptosis following STED, emphasizing a role for estrogen-deprivation therapy in promoting Akt inhibitor activity. MCF7 LTED cells exhibited increased ER expression and resistance to MK-2206 which was reversed by treatment with fulvestrant. In contrast, T47D LTED cells exhibited ER loss and resistance to MK-2206 that was not overcome by fulvestrant. In MCF7 LTED xenografts the combination MK-2206 with fulvestrant showed a synergistic effect on inhibiting tumor cell proliferation and tumor growth. Conclusions: In estrogen-dependent ER+ BC cells with PIK3CA mutation or PTEN loss, acute estrogen deprivation increases the apoptotic effects of MK-2206 providing a rationale for Akt/aromatase inhibitor combinations in the first line setting. In ER+ BC cells that have lost the requirement for estrogen following LTED, fulvestrant antagonizes ER function and promotes Akt inhibitor activity suggesting that this combination would be appropriate for testing as a second line approach. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5478. doi:10.1158/1538-7445.AM2011-5478

A Gene Expression Signature from Human Breast Cancer Cells with Acquired Hormone Independence Identifies MYC as a Mediator of Antiestrogen Resistance

Although most patients with estrogen receptor α (ER)-positive breast cancer initially respond to endocrine therapy, many ultimately develop resistance to antiestrogens. However, mechanisms of antiestrogen resistance and biomarkers predictive of such resistance are underdeveloped. We adapted four ER(+) human breast cancer cell lines to grow in an estrogen-depleted medium. A gene signature of estrogen independence was developed by comparing expression profiles of long-term estrogen-deprived (LTED) cells to their parental counterparts. We evaluated the ability of the LTED signature to predict tumor response to neoadjuvant therapy with an aromatase inhibitor and disease outcome following adjuvant tamoxifen. We utilized Gene Set Analysis (GSA) of LTED cell gene expression profiles and a loss-of-function approach to identify pathways causally associated with resistance to endocrine therapy. The LTED gene expression signature was predictive of high tumor cell proliferation following neoadjuvant therapy with anastrozole and letrozole, each in different patient cohorts. This signature was also predictive of poor recurrence-free survival in two studies of patients treated with adjuvant tamoxifen. Bioinformatic interrogation of expression profiles in LTED cells revealed a signature of MYC activation. The MYC activation signature and high MYC protein levels were both predictive of poor outcome following tamoxifen therapy. Finally, knockdown of MYC inhibited LTED cell growth. A gene expression signature derived from ER(+) breast cancer cells with acquired hormone independence predicted tumor response to aromatase inhibitors and associated with clinical markers of resistance to tamoxifen. Activation of the MYC pathway was associated with this resistance.

Preclinical modeling of combined phosphatidylinositol-3-kinase inhibition with endocrine therapy for estrogen receptor-positive breast cancer

IntroductionInhibition of phosphatidylinositol-3-kinase (PI3K) induces apoptosis when combined with estrogen deprivation in estrogen receptor (ER)-positive breast cancer. The aims of the present study were to identify effective PI3K pathway inhibitor and endocrine therapy combinations, to evaluate the effect of PI3K pathway mutations and estrogen dependency on tumor response, and to determine the relevance of PIK3CA mutation in recurrent disease.MethodsThe PI3K catalytic subunit inhibitor BKM120, the mammalian target of rapamycin (mTOR) inhibitor RAD001 and the dual PI3K/mTOR inhibitor BGT226 were tested against ER-positive breast cancer cell lines before and after long-term estrogen deprivation (LTED). The impact of estradiol deprivation and the ER downregulator fulvestrant on PI3K pathway inhibitor-induced apoptosis was assessed. PIK3CA hotspot mutation analysis was performed in 51 recurrent or metastatic breast cancers and correlated with ER status and survival.ResultsDrug-induced apoptosis was most marked in short-term estrogen-deprived cells with PIK3CA mutation and phosphatase and tensin homolog loss. Apoptosis was most highly induced by BGT226, followed by BKM120, and then RAD001. Estradiol antagonized PI3K inhibitor-induced apoptosis following short-term estrogen deprivation, emphasizing a role for estrogen-deprivation therapy in promoting PI3K inhibitor activity in the first-line setting. ER-positive MCF7 LTED cells exhibited relative resistance to PI3K pathway inhibition that was reversed by fulvestrant. In contrast, T47D LTED cells exhibited ER loss and ER-independent PI3K agent sensitivity. PIK3CA mutation was prevalent in relapsed ER-positive disease (48%) and was associated with persistent ER positivity and a late relapse pattern.ConclusionsEstrogen deprivation increased the apoptotic effects of PI3K and dual PI3K/mTOR inhibitors in ER-positive disease, providing a rationale for PI3K/aromatase inhibitor combinations as first-line therapy. In LTED cells, differential effects on ER expression may be a relevant consideration. When ER was persistently expressed, fulvestrant strongly promoted PI3K drug activity. When ER was lost, PI3K inhibitor monotherapy was sufficient to induce high-level apoptosis. Although tumors with PIK3CA mutation had a late recurrence pattern, these mutations were common in metastatic disease and were most often associated with persistent ER expression. Targeting PIK3CA mutant tumors with a PI3K pathway inhibitor and fulvestrant is therefore a feasible strategy for aromatase-inhibitor-resistant ER-positive relapsed breast cancer.

Abstract PD05-05: Hormone-Independent Breast Cancer Cells Maintain an ERα — Dependent, E2F-Directed Transcriptional Program

Abstract In order to discover mechanisms of escape from hormone dependence, we established four ERα-positive human breast cancer cell lines after long-term estrogen-deprivation (LTED). Growth of MCF-7/LTED and HCC-1428/LTED cells was inhibited by the ER downregulator fulvestrant and by ER siRNA, suggesting continued dependence upon estrogen-independent ER signaling. Fulvestrant also prevented the emergence of hormone-independent MCF-7, HCC-1428, and BT-474 cells, but not ZR75- 1, MDA-361, and T47D cells. Gene expression analysis revealed that ER modulates a transcriptional program directed by the E2F proteins, a family of transcription factors derepressed by CDK4/6-mediated phosphorylation of Rb. A set of 1,003 genes with expression altered ≥1.5-fold (P<0.05) by fulvestrant in MCF-7/LTED and HCC-1428/LTED cells was used to query the TRANSFAC database. Five of the top ten sets of genes which share a transcription factor binding motif ± 2 kb of transcription start sites contained E2F motifs (all p<10-12). From these, we derived an expression signature of 24 genes with no cell cycle association. This signature was predictive of response of 48 ER+ primary tumors to 2 weeks of neoadjuvant letrozole as assessed by Ki67 IHC. In a separate set of experiments, we screened a siRNA library targeting 779 kinases and found that CDK4 is required for MCF-7/LTED cell growth. Inhibition of CDK4/6 using the small molecule PD-0332991 or CDK4 siRNA phenocopied the inhibitory effect of fulvestrant on MCF-7/LTED and HCC-1428/LTED cells, implying that the primary growth-promoting effect of ligand-independent ER is activation of the Rb/E2F pathway. CDK4 inhibition also suppressed the hormone-independent growth of 4 other ER+ breast cancer lines. We previously demonstrated that phosphatidylinositol-3 kinase (PI3K) signaling is required for the escape of ER+ breast cancer cells from hormone dependence. Thus, we postulated that treatment with the PI3K inhibitor BKM120 (Novartis) would synergize with fulvestrant to suppress hormone-independent cell growth. Indeed, these drugs exerted a synergistic inhibitory effect in 6/8 ER+ breast cancer cell lines. Finally, we injected MCF-7 cells and 14-day-release 17P-estradiol pellets into female athymic mice. After 19-29 days, mice bearing tumors ≥100 mm3 were randomized to treatment with vehicle, fulvestrant, BKM120, or the combination. While BKM120 and fulvestrant individually slowed tumor growth, the combination induced near-complete tumor regression. Fulvestrant downregulated ER and PR levels, while BKM120 decreased P-AKT-S473, P-AKT-T308, P-S6-S240/244 (by immunoblotting), and [18F]fluorodeoxyglucose (FDG) tumor uptake as measured by PET, indicative of suppressed ER and PI3K signaling, respectively. These findings collectively suggest that 1) a fraction of ER+ breast cancer cells engage ERα in a ligand-independent manner to drive cell growth, 2) pharmacological downregulation of ER may be effective in a fraction of ER+ tumor cells that escape estrogen deprivation, 3) CDK4/6 inhibition suppresses the hormone-independent growth of fulvestrant-sensitive and-insensitive ER+ breast cancer cells, and 4) resistance to endocrine therapy in breast cancer may be abrogated by combination therapies targeting both the ER and PI3K pathways. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD05-05.

Abstract P2-19-01: Modulating Response to Phosphoinositol-3-Kinase/mTOR Inhibition in ER Positive Breast Cancer Cell Lines after Acquired Resistance to Estrogen Deprivation

Abstract Background: We have recently shown that acquired resistance to aromatase inhibitors (AI) can, in some instances, be reversed low-dose estradiol treatment until progression followed by repeat treatment with an aromatase inhibitor (Ellis, M. JAMA 302:774, 2009). We have also demonstrated that simultaneous inhibition of phosphoinositol-3-kinase (PI3K) and ER (though estrogen deprivation) activates cell death through apoptosis, which is reversed by estradiol, revealing a link between ER activation and the efficacy of PI3 kinase inhibition (Crowder, R. Cancer Res 69:3955, 2009). Consistent with this concept we have also shown that long term estrogen deprived (LTED) cells are co-resistant to both classes of agent (SABCS 2009 Abs#3131). We therefore studied ways to overcome resistance to PI3 kinase inhibitors in LTED cell lines using second line endocrine therapy strategies that could be offered to patients with AI resistant advanced disease. Methods: ER+ MCF7 and T47D (PIK3CA mutant) cells were cultured in phenol-red free media containing charcoal-stripped serum for 12 months to create long term estrogen deprivation (LTED) variants that mimicked clinical endocrine therapy resistance. A variant of the MCF7 LTED line that regained estrogen-dependent proliferation was created by re-exposure to estradiol for 4 months (MCF7 LTED-R). The effects of the PI3K isoform inhibitor BKM120, the mTOR inhibitor RAD001 and the dual PI3K/mTOR inhibitor BGT226 in MCF7 LTED and T47D LTED were assessed in combination with estrogen deprivation and fulvestrant. MCF7 LTED-R cells were treated with PI3 kinase pathway inhibitors after acute estradiol withdrawal. Apoptosis was measured using TUNEL flow cytometry. Results: MCF7 LTED were resistant to all three PI3 kinase inhibitors, but combination treatment with fulvestrant restored apoptosis to levels seen with parental MCF7 cells after acute estrogen-deprivation. Acute estrogen deprivation in MCF7 LTED-R cells was also effective in restoring an apoptotic response to BGT226. T47D LTED cells, which, unlike MCF7 cells, have markedly down-regulated ER expression, become more sensitive to PI3 kinase monotherapy than the parental T47D cell line, particularly in response to BKM120, and fulvestrant did not increase the apoptotic response. Conclusions: The MCF7 cell results suggest that PI3 kinase inhibitors will be effective in endocrine therapy resistant advanced breast cancer if these agents are administered with appropriate second line endocrine strategies. Potentially efficacious approaches include fulvestrant combinations and second-line aromatase inhibition, but only after restoration of AI sensitivity with estradiol. The T47D experiments suggest that in the setting of ER+ tumors that lose ER upon disease progression PI3 kinase inhibitors may be very effective. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P2-19-01.