Additive antitumour effect of the epidermal growth factor receptor tyrosine kinase inhibitor gefitinib (Iressa, ZD1839) and the antioestrogen fulvestrant (Faslodex, ICI 182,780) in breast cancer cells

A high expression level of epidermal growth factor receptor (EGFR)/HER1 has been suggested to lead to a shorter survival time and resistance to endocrine therapy in patients with breast cancer. To test the hypothesis that inhibition of the EGFR signalling pathway affects the antitumour effect of endocrine therapy, an EGFR tyrosine kinase inhibitor (EGFR-TKI), gefitinib, and an oestrogen receptor (ER) antagonist, fulvestrant, were administered to human breast cancer cells. A total of five human breast cancer cell lines were used. The effects of single or combined treatments with gefitinib and/or fulvestrant on cell growth, cell cycle progression and apoptosis were analysed. Changes in the expression levels of cyclin-dependent kinase inhibitors, p21 and p27, an antiapoptotic factor, Bcl-2, and a proapoptotic factor, Bax, were also investigated. All cell lines tested were sensitive to gefitinib (50% growth inhibitory concentration, 10-28.5 microM). Breast cancer cell lines with a high expression level of HER1 or HER2 were more sensitive to gefitinib than the others. Gefitinib induced a significant G1-S blockade in ER-positive KPL-3C cells. Gefitinib induced significant apoptosis in HER1-overexpressing MDA-MB-231 cells. Gefitinib additively increased the antitumour effect of fulvestrant in all three ER-positive cell lines in a medium supplemented with 17beta-oestradiol. The combined treatment promoted cell cycle retardation in KPL-3C cells, which is associated with an upregulation of p21 by fulvestrant and gefitinib, respectively. Apoptosis was associated with downregulation of Bcl-2 by gefitinib in MDA-MB-231 cells. These results suggest an additive interaction between the EGFR-TKI gefitinib and the antioestrogen fulvestrant in ER-positive breast cancer cells.     

Mechanisms of tumor regression and resistance to estrogen deprivation and fulvestrant in a model of estrogen receptor-positive, HER-2/neu-positive breast cancer

HER-2/neu in breast cancer is associated with tamoxifen resistance, but little data exist on its interaction with estrogen deprivation or fulvestrant. Here, we used an in vivo xenograft model of estrogen receptor (ER)-positive breast cancer with HER-2/neu overexpression (MCF7/HER-2/neu-18) to investigate mechanisms of growth inhibition and treatment resistance. MCF7/HER-2/neu-18 tumors were growth inhibited by estrogen deprivation and with fulvestrant, but resistance developed in 2 to 3 months. Inhibited tumors had reductions in ER, insulin-like growth factor-I receptor (IGF-IR), phosphorylated HER-2/neu (p-HER-2/neu), and phosphorylated p42/44 mitogen-activated protein kinase (p-MAPK). p27 was increased especially in tumors sensitive to estrogen deprivation. Tumors with acquired resistance to these therapies had complete loss of ER, increased p-HER-2/neu, increased p-MAPK, and reduced p27. In contrast, IGF-IR and phosphorylated AKT (p-AKT) levels were markedly reduced in these resistant tumors. The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor gefitinib, which can block EGFR/HER-2/neu signaling, significantly delayed the emergence of resistance to both estrogen deprivation and fulvestrant. Levels of p-MAPK and p-AKT decreased with gefitinib, whereas high ER levels were restored. Eventually, however, tumors progressed in mice treated with gefitinib combined with estrogen deprivation or fulvestrant accompanied again by loss of ER and IGF-IR, increased p-HER-2/neu, high p-MAPK, and now increased p-AKT. Thus, estrogen deprivation and fulvestrant can effectively inhibit HER-2/neu-overexpressing tumors but resistance develops quickly. EGFR/HER-2/neu inhibitors can delay resistance, but reactivation of HER-2/neu and signaling through AKT leads to tumor regrowth. Combining endocrine therapy with EGFR/HER-2/neu inhibitors should be tested in clinical breast cancer, but a more complete blockade of EGFR/HER-2/neu may be optimal.     

A randomized trial of combination anastrozole plus gefitinib and of combination fulvestrant plus gefitinib in the treatment of postmenopausal women with hormone receptor positive metastatic breast cancer

EGFR signalling pathways appear involved in endocrine therapy resistance in breast cancer. This trial estimates the antitumor efficacy and toxicity of the EGFR tyrosine kinase inhibitor gefitinib in combination with anastrozole or fulvestrant in postmenopausal hormone receptor positive breast cancer. Subjects with estrogen receptor and/or PgR positive, metastatic breast cancer were randomized into this phase II study of gefitinib (initial dose was 500 mg orally daily, due to high rate of diarrhea, starting dose was reduced to 250 mg orally daily) with either anastrozole 1 mg daily or fulvestrant 250 mg every 4 weeks. The primary endpoint was clinical benefit (complete responses plus partial responses plus stable disease for 6 months or longer). 141 eligible subjects were enrolled, 72 in the anastrozole plus gefitinib arm, and 69 in the fulvestrant plus gefitinib arm. Anastrozole plus gefitinib had a clinical benefit rate of 44% [95% confidence interval (CI) 33-57%] and fulvestrant plus gefitinib 41% (95% CI 29-53%). Median progression-free survival was 5.3 months (95% CI 3.1-10.4) versus 5.2 months (95% CI 2.9-8.2) for anastrozole plus gefitinib versus fulvestrant plus gefitinib, respectively. Median survival was 30.3 months (95% CI 21.2-38.9+) versus 23.9 months (95% CI 15.4-33.5) for anastrozole plus gefitinib versus fulvestrant plus gefitinib, respectively. In general, the toxicity is greater than expected for single agent endocrine therapy alone. Anastrozole plus gefitinib and fulvestrant plus gefitinib have similar clinical benefit rates in the treatment of estrogen and/or PgR positive metastatic breast cancer, and the rates of response are not clearly superior to gefitinib or endocrine therapy alone. Further studies of EGFR inhibition plus endocrine therapy do not appear warranted, but if performed should include attempts to identify biomarkers predictive of antitumor activity.     

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.     

Combined targeting of the estrogen receptor and the epidermal growth factor receptor in non-small cell lung cancer shows enhanced antiproliferative effects

Identifying new effective therapeutic treatments for lung cancer is critical to improving overall patient survival. We have targeted both the estrogen receptor (ER) and the epidermal growth factor receptor (EGFR) pathways using an ER antagonist, fulvestrant ("Faslodex"), and the selective EGFR tyrosine kinase inhibitor, gefitinib ("Iressa"), in non-small cell lung cancer (NSCLC) cells. Rapid activation of phospho-EGFR and phospho-p44/p42 mitogen-activated protein kinase by estrogen was observed, indicating nonnuclear ER transactivation of EGFR. Additionally, EGFR protein expression was down-regulated in response to estrogen and up-regulated in response to fulvestrant in vitro, suggesting that the EGFR pathway is activated when estrogen is depleted in NSCLC cells. Cell growth and apoptosis were examined in several NSCLC lines that express varying amounts of ERbeta, EGFR, and Neu but no full-length ERalpha. One cell line contained an EGFR mutation. Cells were exposed to 10 nmol/L estrogen and 10 ng/mL EGF and either 1 mumol/L fulvestrant or 1 mumol/L gefitinib alone or in combination. In all cell lines, the drug combination decreased cell proliferation up to 90% and increased apoptosis 2-fold. The relative responses to gefitinib and fulvestrant were similar regardless of ER and EGFR expression and mutation status. In an in vivo lung tumor xenograft model, the drug combination decreased tumor volume in severe combined immunodeficient mice by approximately 60% compared with 49% and 32% for gefitinib and fulvestrant treatment alone, respectively. Antitumor effects of the combination therapy were accompanied by biochemical and histologic evidence of increased apoptosis, decreased phospho-p44/p42 mitogen-activated protein kinase expression, and increased Ki-67 expression compared with individual treatment. These studies provide evidence of a functional interaction between the ER and the EGFR pathways in NSCLC.     

Alteration of Y-box binding protein-1 expression modifies the response to endocrine therapy in estrogen receptor-positive breast cancer

Y-box binding protein-1 (YB-1) plays an important role in tumor progression and drug resistance. This study examined whether YB-1 is involved in the alteration of response to endocrine therapy in estrogen receptor (ER)-positive breast cancer cells. MCF7 cells that stably expressed YB-1 (MCF7-YB-1) and vector control cells (MCF7-vector) were established. These cells were used to analyze the expression of the factors related to ER and growth factor receptor signaling pathways and responses to antiestrogens (tamoxifen and fulvestrant) and estrogen responsive element (ERE) activity. The effect of knocking down endogenous YB-1 expression was tested in wild-type MCF7 cells. In addition, the expression of YB-1 and the factors related to ER and growth factor receptor signaling pathways were evaluated in clinical breast cancers treated with preoperative chemotherapy. The expression of HER2, AIB1, p-Erk, and c-Myc was increased in MCF7-YB-1 cells. In contrast, knocking down of YB-1 decreased the expression of these factors but increased the expression of ERα in wild-type MCF7 cells. Furthermore, sensitivity to antiestrogens was decreased in the MCF7-YB-1 in comparison to that in MCF7-vector cells. The introduction of YB-1 into MCF7 cells inhibited apoptosis and cell cycle arrest at G1 phase induced by antiestrogens. In MCF7-YB-1 cells, the expression levels of p-Erk and c-Myc were continuously upregulated when cells were treated with either tamoxifen or fulvestrant. The ERE activity was reduced in MCF7-YB-1 cells in comparison to MCF7-vector cells, and the ERE activity in MCF7-YB-1 cells was inhibited by fulvestrant at a lower concentration than that which inhibited the ERE activity in MCF7-vector cells. In ER-positive clinical breast cancers treated with preoperative chemotherapy, significantly more number of specimens that showed increased or positive YB-1 expression after chemotherapy was positive for HER2 expression. These data suggest that alteration of YB-1 may modify the crosstalk between the ER pathway and HER2 pathway in ER-positive breast cancer cells, and consequently, may alter the response to endocrine therapy in ER-positive breast cancer cells.     

Cooperative effect of gefitinib and fumitremorgin c on cell growth and chemosensitivity in estrogen receptor alpha negative fulvestrant-resistant MCF-7 cells

The selective ER downregulator, fulvestrant, is currently approved as a second line endocrine therapy after onset of resistance to prior antiestrogen therapy in postmenopausal breast cancer patients. Resistance to antihormonal therapies is common and, therefore, we anticipate that fulvestrant-resistance will occur as well. The current study was undertaken to investigate the underlying molecular changes after fulvestrant-resistance and find new therapeutic targets and agents for fulvestrant-resistant breast cancer cells. We developed a unique fulvestrant-resistant cell line (MCF-7/F), derived from MCF-7 estrogen receptor alpha (ERalpha)-positive human breast cancer cells, by culturing them in 1 microM fulvestrant containing medium for approximately 18 months. MCF-7/F cells became irreversibly ERalpha negative as withdrawal of fulvestrant did not alter the ERalpha-negative phenotype, determined by real-time PCR, Western blot analysis, and ERE-luciferase transfection assays. MCF-7/F cells grew in a hormone-independent manner. Interestingly, MCF-7/F cells overexpressed both epidermal growth factor receptor (EGFR) and breast cancer resistant protein (BCRP). Gefitinib, a specific EGFR tyrosine kinase inhibitor, preferentially inhibited the growth of MCF-7/F cells relative to MCF-7 cells by inhibiting both MAPK44/42 and Akt phosphorylation. MCF-7/F cells became less sensitive to chemotherapeutic agents such as mitoxantrone. Moreover, fumitremorgin C, a specific BCRP inhibitor, significantly increased the efficacy of mitoxantrone in MCF-7/F cells. Gefitinib increased the inhibitory effect of mitoxantrone on cell growth. Similarly, fumitremorgin C increased the inhibitory effect of gefitinib on cell growth, suggesting that there is a bidirectional crosstalk between EGFR and BCRP. More importantly, these results provide a molecular basis for using gefitinib, BCRP inhibitors, and chemotherapeutic agents as combination therapy approaches in fulvestrant-resistant breast cancer.     

乳腺癌内分泌治疗耐药机制研究进展

内分泌治疗(ednocrine therapy,ET)是乳腺癌综合治疗中的重要措施之一,选择性雌激素受体调节剂(selectiveestrogen receptor modulators,SERMs)、第三代芳香化酶抑制剂(aromatase inhibi-tors,AIs)和氟维司群是重要的内分泌治疗药物。但目前内分泌疗法面临的最大难题是原、继发耐药,其确切机制尚不清楚,近年来的研究发现,多种因素与耐药相关,如ER信号传导途径中的共激活与共抑制因子的比例,HER-2生长因子受体的高表达,各生长因子和激酶的串话调节,雌激素受体基因关键氨基酸位点的突变和多态、芳香化酶基因的突变与多态以及他莫昔芬(tamoxifen,TAM)代谢异常等。     

Fulvestrant regulates epidermal growth factor (EGF) family ligands to activate EGF receptor (EGFR) signaling in breast cancer cells

Estrogen receptor-α (ER) targeted therapies are routinely used to treat breast cancer. However, patient responses are limited by resistance to endocrine therapy. Breast cancer cells resistant to the pure steroidal ER antagonist fulvestrant (fulv) demonstrate increased activation of epidermal growth factor receptor (EGFR) family members and downstream ERK signaling. In this study, we investigated the effects of fulv on EGFR signaling and ligand regulation in several breast cancer cell lines. EGFR/HER2/HER3 phosphorylation and ERK1,2 activation were seen after 24–48 h after fulvestrant treatment in ER-positive breast cancer cell lines. 4-Hydroxy-tamoxifen and estradiol did not cause EGFR activation. Fulvestrant did not affect EGFR expression. Cycloheximide abolished the ability of fulv to activate EGFR suggesting the autocrine production of EGFR ligands might be responsible for fulvestrant induced EGFR signaling. qRT-PCR results showed fulv differentially regulated EGFR ligands; HB-EGF mRNA was increased, while amphiregulin and epiregulin mRNAs were decreased. Fulvestrant induced EGFR activation and upregulation of EGFR ligands were ER dependent since fulv treatment in C4-12, an ER-negative cell line derivative of MCF-7 cells, did not result in EGFR activation or change in ligand mRNA levels. ER downregulation by siRNA induced similar EGFR activation and regulation of EGFR ligands as fulvestrant. Neutralizing HB-EGF antibody blocked fulv-induced EGFR activation. Combination of fulv and EGFR family tyrosine kinase inhibitors (erlotinib and lapatinib) significantly decreased EGFR signaling and cell survival. In conclusion, fulvestrant-activated EGFR family members accompanied by ER dependent upregulation of HB-EGF within 48 h. EGF receptor or ligand inhibition might enhance or prolong the therapeutic effects of targeting ER by fulvestrant in breast cancer.     

Estrogen receptor α is the major driving factor for growth in tamoxifen-resistant breast cancer and supported by HER/ERK signaling

Resistance to tamoxifen is a major clinical challenge in the treatment of breast cancer; however, it is still unclear which signaling pathways are the major drivers of tamoxifen-resistant growth. To characterize resistance mechanisms, we have generated different tamoxifen-resistant breast cancer cell lines from MCF-7. In this model, we investigated whether signaling from human epidermal growth factor receptors (HERs), their downstream kinases, or from the estrogen receptor α (ERα) was driving tamoxifen-resistant cell growth. Increased expression of EGFR and increased phosphorylation of HER3 were observed upon acquisition of tamoxifen resistance, and the extracellular activated kinase (ERK) signaling pathway was highly activated in the resistant cells. The EGFR inhibitor gefitinib and the ERK pathway inhibitor U0126 resulted in partial and preferential growth inhibition of tamoxifen-resistant cells. All the tamoxifen-resistant cell lines retained ERα expression but at a lower level compared to that in MCF-7. Importantly, we showed via ERα knockdown that the tamoxifen-resistant cells were dependent on functional ERα for growth and we observed a clear growth stimulation of resistant cell lines with clinically relevant concentrations of tamoxifen and 4-OH-tamoxifen, indicating that tamoxifen-resistant cells utilize agonistic ERα stimulation by tamoxifen for growth. The tamoxifen-resistant cells displayed high phosphorylation of ERα at Ser118 in the presence of tamoxifen; however, treatment with U0126 neither affected the level of Ser118 phosphorylation nor expression of the ERα target Bcl-2, suggesting that ERK contributes to cell growth independently of ERα in our cell model. In support of this, combined treatment against ERα and ERK signaling in resistant cells was superior to single-agent treatment and as effective as fulvestrant treatment of MCF-7 cells. Together, these findings demonstrate that ERα is a major driver of growth in tamoxifen-resistant cells supported by HER/ERK growth signaling, implying that combined targeting of these pathways may have a clinical potential for overcoming tamoxifen resistance.     

Enhancing the Efficacy of Hormonal Agents with Selected Targeted Agents

Several selected targeted agents are being investigated in combination with endocrine therapy for patients with breast cancer in an attempt to overcome or prevent endocrine resistance. The role of type I growth factor receptors epidermal growth factor receptor (EGFR) and HER2 in cross-talk with estrogen receptor (ER) signaling has been confirmed in preclinical studies in which various inhibitors have yielded additive or synergistic effects when combined with endocrine agents. Recently, several results from clinical trials investigating this concept have been reported. In ER-positive/HER-positive advanced breast cancer, the addition of trastuzumab to the aromatase inhibitor anastrozole, or the tyrosine kinase inhibitor (TKI) lapatinib to letrozole, both have significantly improved progression-free survival (PFS). The EGFR TKI gefitinib combined with tamoxifen as first-line therapy for ER-positive metastatic disease improved PFS (but not objective response rate) for patients with no previous endocrine therapy or completion of previous adjuvant therapy. A second study in a similar setting showed significant improvement in PFS for gefitinib plus anastrozole. Although it is encouraging that this approach could delay resistance, only a small proportion of patients benefit. Attempts to identify likely responders have been made in the neoadjuvant setting, with pre- and post-treatment biopsies being used to study biomarker changes. A recent preoperative study of letrozole with or without the mammalian target of rapamycin (mTOR) inhibitor everolimus reported greater tumor shrinkage for the combination, with changes in proliferation being predictive for response together with strong expression of protein S6 kinase, a downstream marker of activated mTOR. Key aspects that need to be addressed in future trials include understanding the mechanisms of action for each novel agent, designing the best trial and endpoints to demonstrate added benefit, and ensuring appropriately stratified populations based on previous endocrine exposure and/or sensitivity.     

mTOR inhibition reverses acquired endocrine therapy resistance of breast cancer cells at the cell proliferation and gene‐expression levels

Activation of the Akt/mammalian target of rapamycin (mTOR) pathway has been shown to be associated with resistance to endocrine therapy in estrogen receptor alpha (ERα)‐positive breast cancer patients. Utmost importance is attached to strategies aimed at overcoming treatment resistance. In this context, this work aimed to investigate whether, in breast cancer cells, the use of an mTOR inhibitor would be sufficient to reverse the resistance acquired after exposure to endocrine therapy. The ERα‐positive human breast adenocarcinoma derived‐MCF‐7 cells used in this study have acquired both cross‐resistance to hydroxy‐tamoxifen (OH‐Tam) and to fulvestrant and strong activation of the Akt/mTOR pathway. Cell proliferation tests in control cells demonstrated that the mTOR inhibitor rapamycin enhanced cell sensitivity to endocrine therapy when combined to OH‐Tam or to fulvestrant. In resistant cells, rapamycin used alone greatly inhibited cell proliferation and reversed resistance to endocrine therapy by blocking the agonist‐like activity of OH‐Tam on cell proliferation and bypassing fulvestrant resistance. Reversion of resistance by rapamycin was associated with increased ERα protein expression levels and modification of the balance of phospho‐ser167 ERα/total ERα ratio. Pangenomic DNA array experiments demonstrated that the cotreatment of resistant cells with fulvestrant and rapamycin allowed the restoration of 40% of the fulvestrant gene‐expression signature. Taken together, data presented herein strongly support the idea that mTOR inhibitor might be one of the promising therapeutic approaches for patients with ERα‐positive endocrine therapy‐resistant breast cancers. (Cancer Sci 2008; 99: 1992–2003)     

Mechanisms and therapeutic advances in the management of endocrine-resistant breast cancer

The estrogen receptor (ER) pathway plays a critical role in breast cancer development and progression. Endocrine therapy targeting estrogen action is the most important systemic therapy for ER positive breast cancer. However its efficacy is limited by intrinsic and acquired resistance. Mechanisms responsible for endocrine resistance include deregulation of the ER pathway itself, including loss of ER expression, post-translational modification of ER, deregulation of ER co-activators; increased receptor tyrosine kinase signaling leading to activation of various intracellular pathways involved in signal transduction, proliferation and cell survival, including growth factor receptor tyrosine kinases human epidermal growth factor receptor-2, epidermal growth factor receptor, PI3K/AKT/mammalian target of rapamycin (mTOR), Mitogen activated kinase (MAPK)/ERK, fibroblast growth factor receptor, insulin-like growth factor-1 receptor; alterations in cell cycle and apoptotic machinery; Epigenetic modification including dysregulation of DNA methylation, histone modification, and nucleosome remodeling; and altered expression of specific microRNAs. Functional genomics has helped us identify a catalog of genetic and epigenetic alterations that may be exploited as potential therapeutic targets and biomarkers of response. New treatment combinations targeting ER and such oncogenic signaling pathways which block the crosstalk between these pathways have been proven effective in preclinical models. Results of recent clinical studies suggest that subsets of patients benefit from the combination of inhibitor targeting certain oncogenic signaling pathway with endocrine therapy. Especially, inhibition of the mTOR signaling pathway, a key component implicated in mediating multiple signaling cascades, offers a promising approach to restore sensitivity to endocrine therapy in breast cancer. We systematically reviewed important publications cited in PubMed, recent abstracts from ASCO annual meetings and San Antonio Breast Cancer Symposium, and relevant trials registered at ClinicalTrials.gov. We present the molecular mechanisms contributing to endocrine resistance, in particular focusing on the biological rationale for the clinical development of novel targeted agents in endocrine resistant breast cancer. We summarize clinical trials utilizing novel strategies to overcome therapeutic resistance, highlighting the need to better identify the appropriate patients whose diseases are most likely to benefit from these specific strategies.     

Overcoming resistance to fulvestrant (ICI182,780) by downregulating the c-ABL proto-oncogene in breast cancer

Inhibiting estrogen receptor (ER) function with specific estrogen receptor modulators (SERM) can achieve a primary response in cancer patients; however, intrinsic or subsequently acquired resistance to the therapy remains a major obstacle in treatment. The pure anti-estrogen fulvestrant has been shown to be a promising antagonist of ERα in treating advanced breast cancer. However, our knowledge of the mechanisms governing cellular responsiveness to this agent is limited. Here we show that down-regulation of the nonreceptor tyrosine kinase c-ABL enhanced sensitization to fulvestrant in breast cancer cells. Blocking c-ABL kinase activity with the inhibitor imatinib further increased ERα downregulation induced by fulvestrant, decreased the number of proliferating cells entering the cell cycle, and increased cellular sensitivity to fulvestrant treatment. Conversely, introducing kinase-activated c-ABL can rescue fulvestrant-induced ERα downregulation. Consistent with the effects of imatinib, the silencing of endogenous c-ABL increased the sensitivity of breast cancer cells to fulvestrant treatment. These results demonstrate a role for c-ABL in mediating resistance to the pure anti-estrogen fulvestrant.     

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.