Emily M. Fox

Hyperactivation of phosphatidylinositol-3 kinase promotes escape from hormone dependence in estrogen receptor–positive human breast cancer

Many breast cancers exhibit a degree of dependence on estrogen for tumor growth. Although several therapies have been developed to treat individuals with estrogen-dependent breast cancers, some tumors show de novo or acquired resistance, rendering them particularly elusive to current therapeutic strategies. Understanding the mechanisms by which these cancers develop resistance would enable the development of new and effective therapeutics. In order to determine mechanisms of escape from hormone dependence in estrogen receptor-positive (ER-positive) breast cancer, we established 4 human breast cancer cell lines after long-term estrogen deprivation (LTED). LTED cells showed variable changes in ER levels and sensitivity to 17beta-estradiol. Proteomic profiling of LTED cells revealed increased phosphorylation of the mammalian target of rapamycin (mTOR) substrates p70S6 kinase and p85S6 kinase as well as the PI3K substrate AKT. Inhibition of PI3K and mTOR induced LTED cell apoptosis and prevented the emergence of hormone-independent cells. Using reverse-phase protein microarrays, we identified a breast tumor protein signature of PI3K pathway activation that predicted poor outcome after adjuvant endocrine therapy in patients. Our data suggest that upon adaptation to hormone deprivation, breast cancer cells rely heavily on PI3K signaling. Our findings also imply that acquired resistance to endocrine therapy in breast cancer may be abrogated by combination therapies targeting both ER and PI3K pathways.

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.

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.

Abrogating endocrine resistance by targeting ERα and PI3K in breast cancer

Antiestrogen therapies targeting estrogen receptor α (ER) signaling are a mainstay for patients with ER+ breast cancer. While many cancers exhibit resistance to antiestrogen therapies, a large body of clinical and experimental evidence indicates that hyperactivation of the phosphatidylinositol 3-kinase (PI3K) pathway promotes antiestrogen resistance. In addition, continued ligand-independent ER signaling in the setting of estrogen deprivation may contribute to resistance to endocrine therapy. PI3K activates several proteins which promote cell cycle progression and survival. In ER+ breast cancer cells, PI3K promotes ligand-dependent and -independent ER transcriptional activity. Models of antiestrogen-resistant breast cancer often remain sensitive to estrogen stimulation and PI3K inhibition, suggesting that clinical trials with combinations of drugs targeting both the PI3K and ER pathways are warranted. Herein, we review recent findings on the roles of PI3K and ER in antiestrogen resistance, and clinical trials testing drug combinations which target both pathways. We also discuss the need for clinical investigation of ER downregulators in combination with PI3K inhibitors.

LYN-activating mutations mediate antiestrogen resistance in estrogen receptor–positive breast cancer

Estrogen receptor-positive (ER(+)) breast cancers adapt to hormone deprivation and become resistant to antiestrogen therapy. Here, we performed deep sequencing on ER(+) tumors that remained highly proliferative after treatment with the aromatase inhibitor letrozole and identified a D189Y mutation in the inhibitory SH2 domain of the SRC family kinase (SFK) LYN. Evaluation of 463 breast tumors in The Cancer Genome Atlas revealed four LYN mutations, two of which affected the SH2 domain. In addition, LYN was upregulated in multiple ER(+) breast cancer lines resistant to long-term estrogen deprivation (LTED). An RNAi-based kinome screen revealed that LYN is required for growth of ER(+) LTED breast cancer cells. Kinase assays and immunoblot analyses of SRC substrates in transfected cells indicated that LYN(D189Y) has higher catalytic activity than WT protein. Further, LYN(D189Y) exhibited reduced phosphorylation at the inhibitory Y507 site compared with LYN(WT). Other SH2 domain LYN mutants, E159K and K209N, also exhibited higher catalytic activity and reduced inhibitory site phosphorylation. LYN(D189Y) overexpression abrogated growth inhibition by fulvestrant and/or the PI3K inhibitor BKM120 in 3 ER(+) breast cancer cell lines. The SFK inhibitor dasatinib enhanced the antitumor effect of BKM120 and fulvestrant against estrogen-deprived ER(+) xenografts but not LYN(D189Y)-expressing xenografts. These results suggest that LYN mutations mediate escape from antiestrogens in a subset of ER(+) breast cancers.

Resistance to Endocrine Therapy in Estrogen Receptor-Positive (ER+) Breast Cancer Is Dependent upon Phosphatidylinositol-3 Kinase (PI3K) Signaling.

ER+ breast cancers typically respond to treatment with aromatase inhibitors (AIs), but a significant fraction exhibit de novo or acquired resistance. To model AI resistance, we cultured four ER+, hormone-dependent human breast cancer cell lines under hormone-depleted conditions for several months until hormone-independent populations emerged (termed long-term estrogen-deprived, LTED). LTED cells outgrew parental counterparts under hormone-depleted conditions. While two LTED lines showed increased ER levels and response to estradiol, two LTED lines showed the opposite. Therefore, ER+ breast cancer cells did not consistently overcome hormone deprivation by increasing sensitivity to low estrogen levels.We analyzed protein lysate arrays with 625 antibodies against signaling proteins to discover common mechanisms of hormone-independent growth. All LTED lines showed increased phosphorylation of p70S6K, an mTOR substrate, compared to parental controls. An siRNA library screen targeting 779 kinases revealed that downregulation of kinases linked to the phosphatidylinositol-3 kinase (PI3K) signaling pathway (serum/glucocorticoid-regulated kinase 1; insulin receptor; p110α/PI3K) inhibited the growth of MCF-7/LTED but not parental MCF-7 cells under hormone-depleted conditions. Immunoblot analysis for P-AKT and P-p70S6K showed that PI3K/AKT/mTOR signaling was increased in all LTED lines. Pathway analysis of gene expression microarray data showed significant alteration of genes involved in insulin-like growth factor-I (IGF-I) signaling in 3/4 LTED lines. Receptor tyrosine kinase array analysis revealed increased activation of receptors for IGF-I (IGF-IR) and/or insulin (InsR) in 3/4 LTED lines. These receptors transduce signals to potently activate PI3K. Treatment with the IGF-IR/InsR kinase inhibitor AEW541 decreased P-AKT in 3/4 LTED lines. Treatment with inhibitors of IGF-IR/InsR (AEW541), PI3K/mTOR (BEZ235), or mTOR (RAD001) suppressed the monolayer and anchorage-independent growth of 3/4, 4/4, and 4/4 LTED lines, respectively, and prevented the emergence of hormone-independent cells. Treatment with PI3K and mTOR inhibitors induced apoptosis in 3/4 LTED lines. Finally, we analyzed levels of PI3K pathway markers (P-AKT, P-S6, P-GSK3α/β) in primary tumor lysates from 65 ER+ breast cancer patients using reverse-phase protein arrays. Hierarchical clustering yielded two groups of tumors with high or low PI3K activation. Following adjuvant anastrozole therapy, PI3K-high patients had significantly shorter disease-free survival compared to PI3K-low patients. With a median follow-up of 9.4 months, recurrence rates were 16% and 0%, respectively ( p Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 403.

Abstract PD01-04: Deep kinome sequencing identifies a novel D189Y mutation in the Src family kinase LYN as a possible mediator of antiestrogen resistance in ER+ breast cancer

Estrogen receptor-positive (ER+) breast tumors adapt to hormone deprivation and acquire resistance to aromatase inhibitors (AIs). The proliferative rate of tumor cells measured by Ki67 after short-term treatment with an AI has been proposed as a surrogate endpoint of long term outcome. The purpose of this study was to identify kinase mutations associated with resistance to endocrine therapy as identified by a high Ki67 score after two weeks of pre-surgical therapy with letrozole. We performed deep-kinome sequencing on 4 ER+/HER2– breast tumors that retained high Ki67 scores (14.8 to 24.5%) following short-term letrozole treatment. Genomic DNA from the surgically excised tumors was deep sequenced using a capture approach with 120-bp biotinylated oligonucleotides hybridizing to 612 genes, including 517 kinases with ≥300x coverage. All 4 tumors contained a ‘hot spot’ mutation in PIK3CA , the gene encoding the p110α catalytic subunit of PI3K. One tumor also contained a novel D189Y somatic mutation in LYN, a member of the Src family kinases (SFKs; variant frequency of 8%). D189Y (565G>T) is located in the LYN SH2 domain. Reverse-phase protein array (RPPA) analysis available in 10 tumors in this study revealed a significant correlation (p = 0.006) between Y416 P-Src (which detects all SFKs) and a high post-letrozole Ki67 score. A siRNA screen targeting 779 kinases identified LYN as one of the top hits whose knockdown significantly reduced growth of MCF-7 breast cancer cells with acquired resistance to estrogen deprivation. We next analyzed the Cancer Genome Atlas (TCGA) SNP data to detect copy number changes for 444 tumors where corresponding gene expression data were available. Copy number increases (>0.8 log2 ratio over normal matched DNA) in LYN were present in approximately 10% of breast cancers, with the highest copy number gains observed in luminal B tumors. In contrast, other SFKs showed less frequent copy number increases (YES1 Finally, we investigated the role of D189Y LYN in endocrine-resistant breast cancer using ER+ MCF-7 cells transduced with GFP (control), wild-type (WT) LYN, or D189Y LYN vectors. Overexpression of WT or mutant LYN resulted in increased phosphorylation of Src (at Y416), IGF-IR, EGFR, STAT3, AKT, and MAPK. D189Y LYN overexpression also induced phosphorylation of IRS-1. Although stable transduction of either WT or D189Y LYN accelerated MCF-7 cell growth in estrogen-depleted medium, the mutant was more potent than WT LYN at inducing this effect. Further, D189Y LYN overexpression rendered treatment with the ER downregulator fulvestrant or the PI3K inhibitor BKM120 less effective. These results suggest, first, that LYN may play a role in escape from estrogen deprivation in a subset of ER+ breast cancers. Second, ER+ breast cancers harbor multiple molecular alterations capable of mediating hormone-independent growth. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD01-04.

Cooperative Interactions Between c-Src, Estrogen Receptors and Receptor Tyrosine Kinases in Breast Cancer

Estrogen predominantly mediates its effects through the estrogen receptors, ER-α and ER-β, which directly modulate gene expression and participate in rapid cytoplasmic signaling. Within minutes of estrogen stimulation, the ER interacts with and signals through the non-receptor tyrosine kinase, c-Src, and HER family of receptor tyrosine kinases to promote biological outcomes. In addition, these proteins have been shown to cooperate with one another to facilitate growth factor and progesterone signaling in the absence of estrogen. Because of their involvement in estrogen-dependent and independent signaling, cancer progression, and resistance to hormonal and cytotoxic therapies, the c-Src and HER family proteins have been identified as targets for the treatment of breast cancer, as well as other malignancies. Several small-molecule inhibitors and monoclonal antibodies are currently being tested or used in the clinic for the treatment of these tumors.

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

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL 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

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

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 Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD05-05.