Herceptin-resistant Breast Cancer Cells Research Articles

Targeting erbB Pathways in Breast Cancer: Dual Kinase Inhibition for Brain Metastasis and Prevention of p185HER2/Neu Tumor Development.

Breast cancer predominantly affects women and poses challenges in the treatment of both local and advanced diseases. In a previous study, we reported the effectiveness of ER121, a structurally resolved small compound specifically designed to target human cancers expressing or overexpressing mutant EGFR and HER2. The objective of this study is to assess the efficacy and toxicity of ER121 in metastatic and triple negative breast cancer (TNBC, HER2+) cells and tumor models. The Herceptin-resistant breast cancer cell line JIMT-1 was used in an in vivo tumor model, and MMTV-erbB2 (Fo5) transgenic mice models were used to evaluate the efficacy and safety of ER121 as neoadjuvant. ER121 treatment focusing on experimental brain metastasis in TNBC, HER2+ model, was quantified by total flux employing the In Vivo Imaging System (IVIS). We also compared the brain tissue from the treated and the controls groups. Additionally, ER121 was evaluated in JIMT-1, a Herceptin-resistant breast cancer cell line, both in vitro and in vivo tumor model. We also administered ER121 orally in neoadjuvant model with the MMTV-erbB2 (Fo5) transgenic mice, the survival rates were compared with the control group. Tumor-free survival of multiple treated groups were analyzed by Kaplan-Meier analysis employing the log-rank test with the Bonferroni correction using R Statistical Software. In this study, we present findings indicating that ER121 treatment significantly attenuated breast tumor growth using a TNBC, HER2+ model, focusing on experimental brain metastasis, as quantified by total flux employing IVIS. These observations were further corroborated by analysis of brain tissue from the treatment group compared to controls. Data is presented as Mean ± S.D. statistical significance was calculated using Student t test (*p < 0.05). Additionally, ER121 significantly inhibited JIMT-1, a Herceptin-resistant breast cancer cell line was used in vivo xenograft model. Additionally, we used a neoadjuvant model with the MMTV-erbB2 (Fo5) transgenics and the tumor-free survival rates exhibited a remarkable difference between the control and treated groups when ER121 was administered orally. We found statistically significant p values of 0.048 employing log-rank test with Bonferroni Correction for comparing ER121high, ER121 Low, Herceptin and PBS groups. All analyses were performed using R Statistical Software. ER121 is a non-toxic small-molecule erbB kinase inhibitor and holds promise as an oral and systemic therapeutic agent for treating progressive erbB-driven tumors in therapeutic settings. Moreover, ER121 shows potential as a preventive therapy in neoadjuvant settings for erbB2-associated tumors and when administered systemically can dramatically limit erbB2 brain metastases in animal models.

Abstract 1790: Herceptin-resistant breast cancer cells exhibits distinct sensitivity to lapatinib in in vivo tumor xenograft models

Abstract Introduction: HER2-targeted therapies, including the monoclonal antibody (Ab) herceptin and the tyrosine kinase inhibitor (TKI) lapatinib, have been commonly used to treat HER2-positive breast cancer (BC). However, resistance to herceptin and/or lapatinib frequently occurs and currently represents a significant clinical problem. We previously demonstrated that both HER3- and IGF-1 receptor (IGF-1R)-initiated signaling played crucial roles in the development of herceptin resistance. Further studies showed that our herceptin-resistant SKBR3-pool2 and BT474-HR20 sublines, as compared their parental SKBR3 and BT474 cells, respectively, also exhibited refractoriness to lapatinib in vitro. However, the in vivo efficacy of lapatinib against the herceptin-resistant BC models (SKBR3-pool2 and BT474-HR20) remains unclear. Methods: Tumor xenograft models were established via subcutaneous inoculation of SKBR3-pool2 or BT474-HR20 cells into athymic nude mice. Tumor formation was assessed by palpation and measured with fine calipers twice a week. The tumor-bearing mice were treated with vehicle (DMSO) or lapatinib (80 mg/kg) via intraperitoneal injection daily for three weeks. Immunohistochemistry (IHC) assays were performed to assess the expression and activation of proteins. Two individuals independently read and quantified all IHC slides by considering both the intensity of immunostaining and the percentage of positive-staining tumor cells. Results: Treatment with lapatinib profoundly inhibited growth of the tumors established with SKBR3-pool2 cells. In contrast, lapatinib slightly, but not significantly, reduced tumor growth in BT474-HR20 cells-derived xenograft models. IHC analyses of the tumors obtained from both SKBR3-pool2 and BT474-HR20 xenografts showed that lapatinib treatment had little effect on the expression of HER3 and the protein phosphatase PPP3CB. Lapatinib also did not alter the levels of phosphorylated Akt (p-Akt) and FOXO3a (p-FOXO3a) in vivo. Interestingly, in BT474-HR20-derived tumors, lapatinib treatment dramatically enhanced expression of insulin receptor substrate-1 (IRS1) and increased the levels of phosphorylated HER3 (p-HER3). However, such phenomena were not observed in the tumors established with SKBR3-pool2 cells. Conclusion: While the tumors derived from BT474-HR20 cells retain their resistant phenotype to lapatinib, SKBR3-pool2-tumors are highly sensitive to lapatinib treatment. It seemed that lapatinib-induced upregulation of IRS1 and activation of HER3, evidenced by increased p-HER3, contributed to the inefficacy of lapatinib against BT474-HR20-tumors in vivo. Keywords: HER3, IRS1, treatment resistance, HER2-targeted therapy, breast cancer Citation Format: Sanbao Ruan, Hao Liu, Hui Lyu, Congcong Tan, Bolin Liu. Herceptin-resistant breast cancer cells exhibits distinct sensitivity to lapatinib in in vivo tumor xenograft models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1790.

Kinetic Modeling of DUSP Regulation in Herceptin-Resistant HER2-Positive Breast Cancer.

Background: HER2 (human epidermal growth factor 2)-positive breast cancer is an aggressive type of breast cancer characterized by the overexpression of the receptor-type protein tyrosine kinase HER2 or amplification of the HER2 gene. It is commonly treated by the drug trastuzumab (Herceptin), but resistance to its action frequently develops and limits its therapeutic benefit. Dual-specificity phosphatases (DUSPs) were previously highlighted as central regulators of HER2 signaling; therefore, understanding their role is crucial to designing new strategies to improve the efficacy of Herceptin treatment. We investigated whether inhibiting certain DUSPs re-sensitized Herceptin-resistant breast cancer cells to the drug. We built a series of kinetic models incorporating the key players of HER2 signaling pathways and simulating a range of inhibition intensities. The simulation results were compared to live tumor cells in culture, and showed good agreement with the experimental analyses. In particular, we observed that Herceptin-resistant DUSP16-silenced breast cancer cells became more responsive to the drug when treated for 72 h with Herceptin, showing a decrease in resistance, in agreement with the model predictions. Overall, we showed that the kinetic modeling of signaling pathways is able to generate predictions that assist experimental research in the identification of potential targets for cancer treatment.

Modelling the role of dual specificity phosphatases in herceptin resistant breast cancer cell lines

BackgroundBreast cancer remains the most lethal type of cancer for women. A significant proportion of breast cancer cases are characterised by overexpression of the human epidermal growth factor receptor 2 protein (HER2). These cancers are commonly treated by Herceptin (Trastuzumab), but resistance to drug treatment frequently develops in tumour cells. Dual-specificity phosphatases (DUSPs) are thought to play a role in the mechanism of resistance, since some of them were reported to be overexpressed in tumours resistant to Herceptin. ResultsWe used a systems biology approach to investigate how DUSP overexpression could favour cell proliferation and to predict how this mechanism could be reversed by targeted inhibition of selected DUSPs. We measured the expression of 20 DUSP genes in two breast cancer cell lines following long-term (6 months) exposure to Herceptin, after confirming that these cells had become resistant to the drug. We constructed several Boolean models including specific substrates of each DUSP, and showed that our models correctly account for resistance when overexpressed DUSPs were kept activated. We then simulated inhibition of both individual and combinations of DUSPs, and determined conditions under which the resistance could be reversed. ConclusionsThese results show how a combination of experimental analysis and modelling help to understand cell survival mechanisms in breast cancer tumours, and crucially enable us to generate testable predictions potentially leading to new treatments of resistant tumours.

Abstract 5888: Disruption of FOXO3a-miRNAs mediated feedback inhibition of IGF2-IRS1 signaling contributes to Herceptin resistance in HER2 positive breast cancer cells

Abstract Resistance to HER2-target therapy remains a clinical challenge for HER2+ breast cancer subtype. The molecular mechanisms for Herceptin resistance remain elusive. Here, we demonstrated that the IGF2 and IRS1 protein levels were increased in Herceptin resistant breast cancer cells and that the IGF2/IGF1R/IRS1/Akt/mTOR signaling was involved in Herceptin resistance. In Herceptin sensitive HER2+ breast cancer cells, IRS1 protein level resulted from mTOR2 mediated FOXO3a status determined the feedback regulation of IGF2. Upon high dose rhIGF2 treatment, the mTOR1 translationally upregulated PPP3CB protein level that recovered the function of FOXO3a inactivated by mTOR2. FOXO3a positively transcriptionally regulated miR-128-3p and miR-30-5p expression that targeted IRS1 mRNA to control the feedback regulation of IGF2 signaling. In agreement, in Herceptin resistant cells, the decrease of miR-128-3p and miR-30-5p resulted from constitutively inactivation of FOXO3a by mTOR2, led to increase of IRS1 protein level, even in response to high endogenous IGF2 protein level in culture medium. High IGF2 protein level was resulted from decrease of miR-193a-5p that was also regulated by FOXO3a. Unexpectedly, in Herceptin resistant cells, Src/p-STAT5 signaling cooperating with HDAC1 transcriptionally repressed PPP3CB expression resulting into FOXO3a inactivation. Moreover, we confirmed that IRS1 knockdown reversed the Herceptin resistance in vivo using a xenograft mouse model. Serums form patients with poor response to Herceptin, had higher IGF2 protein level. Tissues from patients with poor response had higher IRS1, p-Akts473, p-FOXO3a and p-Src levels, but had lower PPP3CB, miR-128-3p, miR-30-5p and miR-193a-5p levels. In conclusions, disruption of PPP3CB-FOXO3a-miRNAs mediated feedback inhibition of IGF2-IRS1 signaling plays pivotal role in Herceptin resistance of HER2+ breast cancer. Related molecules in this signaling may represent potential biomarkers and therapeutic targets for HER2+ breast cancer treatment. Keywords: Breast cancer, Herceptin resistance, IGF2, IRS1, FOXO3a This study was supported by grants from the National Natural Science Foundation of China: No.81672616, No.81402196 and No.81272450, supported by grants from Guangdong Natural Science Funds for Distinguished Young Scholars No.2016A030306003 (G Zheng). Citation Format: Zhimin He. Disruption of FOXO3a-miRNAs mediated feedback inhibition of IGF2-IRS1 signaling contributes to Herceptin resistance in HER2 positive breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5888.

The function and mechanism of ErbB3 and IGF1R in breast cancer with Herceptin resistance

Objective To investigate the role of epidermal growth factor receptor 3 (ErbB3) and insulin-like growth factor-1 receptor (IGF1R) in enhancing the resistance of Herceptin in human breast cancer. Methods HRG (Heregulin, the ligand of ErbB3) or IGF2 (insulin-like growth factor2, the ligand of IGF1R) was correspondingly added into breast cancer cells SKBR3 and BT474, and then 3-(4, 5-dimenthylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay and were performed in these cells to evaluate the sensitivity of these cells to Herceptin. Furthermore, we used HRG or IGF2 antibodies to inhibit their joint receptors in Herceptin-resistant breast cancer cells SKBR3/POOL2 and BT474/HR20. Finally, the sensitivity of these treated cells to Herceptin was detected via MTS assay. HRG or IGF2 was added into breast cancer cell BT474, and co-IP assay was used to detect the expressions of ErbB3 and IGF1R which combined with ErbB2. Results The treatment groups used HRG or IGF2 enhanced the resistance of Herceptin in Herceptin-sensitive breast cancer cells. On the other hand, we used antibodies of HRG and IGF2 to block their combining with their receptors in Herceptin-resistant breast cancer cells, the cells became more sensitive to Herceptin. BT474 cell was treated with HRG or IGF2. The expressions of ErbB3 and IGF1R which combined with ErbB2 were increased. Conclusions The formation of heterodimers ErbB2/ErbB3 and ErbB2/IGF1R might enhance the resistance of Herceptin in ErbB2-overexpression human breast cancers. Key words: Receptor, epidermal growth factor; Receptor, IGF type 1; Breast neoplasms/DT/ME; Drug tolerance; Antibodies, monoclonal/TU

Abstract 347: Developing a novel immunotoxin that targets cells overexpressing ErbB2

Abstract Immunotoxins are chimeric proteins comprising a specific cellular targeting domain linked to a cytotoxic factor. Here we describe the design and use of a novel, peptide-based immunotoxin that can initiate selective cytotoxicity on ErbB2-positive cells. ErbB2 is a receptor tyrosine kinase that is overexpressed in the tumor cells of approximately 30% of breast cancer patients. Immunotoxin candidates were designed to incorporate a targeting ligand with affinity for ErbB2 along with a membrane lysin-based toxin domain. The use of a membrane lysin as the toxin domain distinguishes our immunotoxins from numerous others in that internalization of the toxin is not required to induce cell death. One particular peptide candidate, NL1.1-PSA, demonstrated selective cytotoxicity towards ErbB2-overexpressing cell lines. We utilized a bioengineering strategy to show that recombinant NL1.1-PSA immunotoxin expression by Escherichia coli also conferred selective cytotoxicity towards ErbB2-overexpressing cells. Our findings hold significant promise for the use of effective immunotoxins in cancer therapeutics. While Herceptin treatment has been effective in patients with ErbB2 positive breast cancer, our novel immunotoxin may ultimately prove to be a novel and efficacious addition to the arsenal of approaches used to eliminate Herceptin-resistant breast cancer cells. Citation Format: Luqun Shen, Kelsey Weigel, Clayton Thomas, Lauren Drapalik, Zachary T. Schafer, Shaun Lee. Developing a novel immunotoxin that targets cells overexpressing ErbB2. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 347.

Abstract 1677: Annexin A2 antibody inhibits the progression of triple-negative and herceptin-resistant breast cancer by blocking epidermal growth factor receptor functions

Abstract Short Description: Annexin A2 (AnxA2), a calcium-dependent phospholipid binding protein, is abundantly present at the surface of triple-negative and Herceptin-resistant breast cancer cells. Interactions between cell-surface AnxA2 and tyrosine kinase receptors have an important role in the tumour microenvironment and act together to enhance tumour growth. The mechanism supporting this role is still unknown. Methods: Triple-negative (MDA-MB-231) and Herceptin-resistant (JIMT-1) breast cancer cell lines were grown in complete DMEM and DMEM/F12 medium respectively, in a humidified incubator at 37°C with 5% CO2. The membrane function of AnxA2 was blocked by incubating cells with anti-AnxA2 antibodies. Western blotting, immunoprecipitation, immunofluorescence, 1-(4,5-Dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT), flow cytometry, Clonogenic, and wound-healing assays were performed in this study Results: We demonstrate that in triple-negative and Herceptin-resistant breast cancer cells a large amount of AnxA2 is specifically translocated to the outer membrane domain. In addition, AnxA2 interacts with extracellular domain of epidermal growth factor receptor (EGFR) at the cell surface and has an important role in cancer cell proliferation and migration by modulating EGFR functions. Blocking AnxA2 function at the cell surface by anti-AnxA2 antibody suppressed the EGF-induced EGFR tyrosine phosphorylation and internalization by blocking its homodimerization. Furthermore, addition of AnxA2 antibody significantly inhibited the EGFR-dependent PI3K-AKT and Raf-MEK-ERK downstream pathways under both EGF-induced and basal growth conditions, resulting in lower cell proliferation and migration. Conclusions: These studies indicate that association of AnxA2 with EGFR in the membrane domain might play a positive regulatory role in keeping EGFR signaling events in an activated state in triple negative and Herceptin-resistant breast cancer thus making AnxA2 an important therapeutic target. Citation Format: Pankaj Chaudhary, Lee Daniel Gibbs, Jamboor K. Vishwanatha. Annexin A2 antibody inhibits the progression of triple-negative and herceptin-resistant breast cancer by blocking epidermal growth factor receptor functions. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1677. doi:10.1158/1538-7445.AM2015-1677

Inhibition of triple-negative and Herceptin-resistant breast cancer cell proliferation and migration by Annexin A2 antibodies.

Background:Annexin A2 (AnxA2), a calcium-dependent phospholipid binding protein, is abundantly present at the surface of triple-negative and Herceptin-resistant breast cancer cells. Interactions between cell-surface AnxA2 and tyrosine kinase receptors have an important role in the tumour microenvironment and act together to enhance tumour growth. The mechanism supporting this role is still unknown.Methods:The membrane function of AnxA2 was blocked by incubating cells with anti-AnxA2 antibodies. Western blotting, immunoprecipitation, immunofluorescence, 1-(4,5-Dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT), flow cytometry, Clonogenic, and wound-healing assays were performed in this study.Results:We demonstrate that AnxA2 interacts with epidermal growth factor receptor (EGFR) at the cell surface and has an important role in cancer cell proliferation and migration by modulating EGFR functions. Blocking AnxA2 function at the cell surface by anti-AnxA2 antibody suppressed the EGF-induced EGFR tyrosine phosphorylation and internalisation by blocking its homodimerisation. Furthermore, addition of AnxA2 antibody significantly inhibited the EGFR-dependent PI3K-AKT and Raf-MEK-ERK downstream pathways under both EGF-induced and basal growth conditions, resulting in lower cell proliferation and migration.Conclusions:These findings suggest that cell-surface AnxA2 has an important regulatory role in EGFR-mediated oncogenic processes by keeping EGFR signalling events in an activated state. Therefore, AnxA2 could potentially be used as a therapeutic target in triple-negative and Herceptin-resistant breast cancers.

Targeting Bcl-2 in Herceptin-Resistant Breast Cancer Cell Lines

Acquired resistance to Herceptin is a major clinical problem in the treatment of HER2-overexpressing breast cancer. Understanding the molecular mechanisms leading to resistance will allow identification of novel therapeutic targets and predictors of therapeutic response. To this end, up-regulation of anti-apoptotic proteins has been associated with resistance to the HER2-targeted drug lapatinib, but has not yet been linked to Herceptin resistance. The aim of the current study was to determine if the Bcl-2 anti-apoptotic protein is a potential therapeutic target in cells with acquired Herceptin resistance. The BT474 HER2-overexpressing breast cancer cell line and BT474-derived acquired Herceptin-resistant clones were used as models in this study. Bcl-2 and Bax expression were assessed by Western blotting. Proliferation assays were performed on cells treated with the Bcl-2 inhibitor ABT-737 in the absence or presence of Herceptin. Finally, the effect of PI3K inhibition or IKK inhibition on Bcl-2 expression and Herceptin sensitivity was examined by Western blotting and established proliferation assays. We show that cells with acquired resistance to Herceptin have an increased Bcl-2:Bax ratio. Resistant cells have increased sensitivity to ABT-737. Further, pharmacologic inhibition of Bcl-2 improved sensitivity to Herceptin in acquired resistant cells. Finally, PI3K and IKK inhibition down-regulated Bcl-2 expression and increased sensitivity to Herceptin in resistant cells. Taken together, these new observations support further study of Bcl-2-targeted therapies in Herceptin-resistant breast cancers, and importantly, future investigation of Bcl-2 expression as a potential predictor of Herceptin response in patients with HER2-overexpressing breast cancer.

Abstract 1759: Design optimization and characterization of novel herceptin-based immunotoxins for cancer therapy. Enhanced antitumor activity in vitro and in vivo

Abstract The human epidermal growth factor receptor (Her) family plays an important role in cell survival and proliferation, and is implicated in oncogenesis. Overexpression of Her2/neu is associated with aggressive disease and poor prognosis. Herceptin, the only anti-Her2/neu, humanized monoclonal antibody approved by FDA for therapy of mammary carcinoma is currently and successfully prescribed for treatment of Her2/neu positive cancer. However, in 40-60% of all patients with Her2/neu-overexpressing tumors, Herceptin has little or no effect. The goal of this study was to demonstrate the construction and characterization of various Herceptin-based immunotoxins, loaded with a highly cytotoxic recombinant gelonin (rGel) molecule. Utilizing a humanized anti-Her2/neu scFv (4D5), we generated 2 orientation constructs: 4D5/rGel and rGel/4D5. We also generated a conventional chemical conjugate with full-length Herceptin (Herceptin/rGel conjugate), and we generated a unique fusion of anti-Her2/neu IgG containing 2 rGel molecules (Aglyco-Herceptin/rGel). This is the first report of the preparation of active full-length antibody-toxin construct from E.coli. In vitro studies showed that all immunotoxins retained the affinity and specificity of the original antibody, as well as the biological activity of rGel toxin. Against the Her2/neu overexpressing cancer cell lines, all immunotoxins demonstrated specific cytotoxic effects, even against Herceptin-resistant breast cancer cells. Against ovarian (SKOV3) cells, the Herceptin/rGel conjugate, Aglyco-Herceptin/rGel and rGel/4D5 fusion constructs demonstrated the best in vitro efficacy (IC50 of 0.28nM, 12.19nM and 1.57nM for each construct respectively vs 70.86 nM for 4D5/rGel). rGel/4D5 outperformed 4D5/rGel demonstrating improved internalization efficiency and increased intracellular delivery of rGel. All the immunotoxins displayed similar minimal exposure times to target cells for optimal cytotoxic activity. All constructs demonstrated a distinct cell-killing efficiency time course. Functional stability studies of the constructs showed that all remained biologically active after incubation in human plasma. Efficacy studies of the various constructs against SKOV3 tumor xenografts are ongoing and will be reported. Based on our in vitro results, the bivalent full-length antibody-toxin immunotoxins were superior to the corresponding recombinant monovalent constructs. Among monovalent constructs, the rGel/4D5 fusion was superior in vitro overall to 4D5/rGel. Different orientations of antibody and toxin can promote optimal cell internalization and toxicity of the immunotoxin, and may be valuable considerations for further construct optimization. Research conducted, in part, by the Clayton Foundation for Research. 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 1759. doi:10.1158/1538-7445.AM2011-1759

Improvement of sensitivity to tamoxifen in estrogen receptor-positive and Herceptin-resistant breast cancer cells

ERBB2 overexpression in estrogen receptor (ER)-positive breast cancer cells such as BT474 (BT) cells has been found to confer resistance to tamoxifen, and suppression of ERBB2 improves the antiproliferative effects of tamoxifen. In this study, the responsiveness to tamoxifen in the BT/HerR, Herceptin-resistant BT cell lines established through constant Herceptin exposure, was evaluated. Compared with BT cells, improvement of sensitivity to tamoxifen in BT/HerR was demonstrated by ER functional analysis and cell proliferation assay. Tamoxifen in the resistant cell line was found to inhibit 17beta-estradiol-stimulating estrogen-responsive gene pS2 expression more effectively than in BT cells in real-time PCR assay. Western blot analysis showed that cross-phosphorylation between ER and downstream components of ERBB2 was attenuated in BT/HerR cells. ER redistribution from cytoplasm to nucleus could be found in these cells through immunofluorescence and confocal studies, and importantly, chromatin immunoprecipitation studies demonstrated that tamoxifen induced occupancy of the pS2 promoter by ER and nuclear receptor corepressor (NCOR1) instead of coactivator NCOA3 in these cells. Finally, combination of tamoxifen and Herceptin was found to improve the sensitivity of BT/HerR cells to Herceptin. Our results suggest that the ER genomic pathway in the ER-positive and Herceptin-resistant breast cancer cells may be reactivated, allowing tamoxifen therapy to be effective again, and a combination of tamoxifen and Herceptin can be a potential therapeutic strategy for ER-positive and Herceptin-resistant human breast cancer.