Mechanisms of acquired resistance to endocrine therapy in hormone-dependent breast cancer cells

Neo-adjuvant therapies for ER positive/HER2 negative breast cancers: from chemotherapy to hormonal therapy, CDK inhibitors, and beyond

INTRODUCTION

Chemotherapy has been traditionally used as neo-adjuvant therapy in breast cancer for down-staging of locally advanced disease in all sub-types. In the adjuvant setting, genomic assays have shown that a significant proportion of ER positive/HER2 negative patients do not derive benefit from the addition of chemotherapy to adjuvant endocrine therapy. An interest in hormonal treatments as neo-adjuvant therapies in ER positive/HER2 negative cancers has been borne by their documented success in the adjuvant setting. Moreover, cytotoxic chemotherapy is less effective in ER positive/HER2 negative disease compared with other breast cancer subtypes in obtaining pathologic complete responses.

AREAS COVERED

Neo-adjuvant therapies for ER positive/HER2 negative breast cancers and associated biomarkers are reviewed, using a Medline survey. A focus of discussion is the prediction of patients that are unlikely to derive extra benefit from chemotherapy and have the highest probabilities of benefiting from hormonal and other targeted therapies.

EXPERT OPINION

Predictive biomarkers of response to neo-adjuvant chemotherapy and hormonal therapies are instrumental for selecting ER positive/HER2 negative breast cancer patients for each treatment. Chemotherapy remains the standard of care for many of those patients requiring neo-adjuvant treatment, but other neo-adjuvant therapies are increasingly used.

Targeting Breast Cancer: The Familiar, the Emerging, and the Uncharted Territories

Breast cancer became the most diagnosed cancer in the world in 2020. Chemotherapy is still the leading clinical strategy in breast cancer treatment, followed by hormone therapy (mostly used in hormone receptor-positive types). However, with our ever-expanding knowledge of signaling pathways in cancer biology, new molecular targets are identified for potential novel molecularly targeted drugs in breast cancer treatment. While this has resulted in the approval of a few molecularly targeted drugs by the FDA (including drugs targeting immune checkpoints), a wide array of signaling pathways seem to be still underexplored. Also, while combinatorial treatments have become common practice in clinics, the majority of these approaches seem to combine molecularly targeted drugs with chemotherapeutic agents. In this manuscript, we start by analyzing the list of FDA-approved molecularly targeted drugs for breast cancer to evaluate where molecular targeting stands in breast cancer treatment today. We will then provide an overview of other options currently under clinical trial or being investigated in pre-clinical studies.

Adrenal Steroids and Resistance to Hormonal Blockade of Prostate and Breast Cancer

Prostate cancer and breast cancer are sex-steroid-dependent diseases that are driven in major part by gonadal sex steroids. Testosterone (T) is converted to 5α-dihydrotestosterone, both of which stimulate the androgen receptor (AR) and prostate cancer progression. Estradiol is the major stimulus for estrogen receptor-α (ERα) and proliferation of ERα-expressing breast cancer. However, the human adrenal provides an alternative source for sex steroids. A number of different androgens are produced by the adrenals, the most abundant of which is dehydroepiandrosterone (DHEA) and DHEA sulfate. These precursor steroids are subject to metabolism by peripherally expressed enzymes that are responsible for the synthesis of potent androgens and estrogens. In the case of prostate cancer, the regulation of one of these enzymatic steps occurs at least in part by way of a germline-encoded missense in 3β-hydroxysteroid dehydrogenase-1 (3βHSD1), which regulates potent androgen biosynthesis and clinical outcomes in men with advanced prostate cancer treated with gonadal T deprivation. The sex steroids that drive prostate cancer and breast cancer require a common set of enzymes for their generation. However, the pathways diverge once 3-keto, Δ4-androgens are generated and these steroids are either turned into potent androgens by steroid-5α-reductase, or into estrogens by aromatase. Alternative steroid receptors have also emerged as disease- and treatment-resistance modifiers, including a role for AR in breast cancer and glucocorticoid receptor both in breast and prostate cancer. In this review, we integrate the commonalities of adrenal steroid physiology that regulate both prostate and breast cancer while recognizing the clear distinctions between these diseases.

miR-489 Confines Uncontrolled Estrogen Signaling through a Negative Feedback Mechanism and Regulates Tamoxifen Resistance in Breast Cancer

Approximately 75% of diagnosed breast cancer tumors are estrogen-receptor-positive tumors and are associated with a better prognosis due to response to hormonal therapies. However, around 40% of patients relapse after hormonal therapies. Genomic analysis of gene expression profiles in primary breast cancers and tamoxifen-resistant cell lines suggested the potential role of miR-489 in the regulation of estrogen signaling and development of tamoxifen resistance. Our in vitro analysis showed that loss of miR-489 expression promoted tamoxifen resistance, while overexpression of miR-489 in tamoxifen-resistant cells restored tamoxifen sensitivity. Mechanistically, we found that miR-489 is an estrogen-regulated miRNA that negatively regulates estrogen receptor signaling by using at least the following two mechanisms: (i) modulation of the ER phosphorylation status by inhibiting MAPK and AKT kinase activities; (ii) regulation of nuclear-to-cytosol translocation of estrogen receptor α (ERα) by decreasing p38 expression and consequently ER phosphorylation. In addition, miR-489 can break the positive feed-forward loop between the estrogen-Erα axis and p38 MAPK in breast cancer cells, which is necessary for its function as a transcription factor. Overall, our study unveiled the underlying molecular mechanism by which miR-489 regulates an estrogen signaling pathway through a negative feedback loop and uncovered its role in both the development of and overcoming of tamoxifen resistance in breast cancers.

Endocrine Therapy-Resistant Breast Cancer Cells Are More Sensitive to Ceramide Kinase Inhibition and Elevated Ceramide Levels Than Therapy-Sensitive Breast Cancer Cells

Simple Summary

Endocrine therapy (ET) resistance is a major problem in estrogen receptor-positive breast cancer patients. Since there have been few lipidomic studies in ET resistance and sphingolipids are heavily implicated in multidrug-resistant and chemotherapy-resistant cancers, we aimed to investigate the sphingolipidome of tamoxifen-resistant breast cancer cells in search of a unique sphingolipid profile that can potentially be exploited therapeutically. We found that ET-resistant breast cancer cells maintain a lower level of ceramides for their survival. In order to achieve this, they are dependent on ceramide kinase (CERK), the activity of which helps maintain low endogenous ceramide levels, therefore promoting tamoxifen-resistant cell survival. Targeting CERK can therefore represent an opportunity to target therapy-resistant breast tumors and improve the patient outcome for women with ET-resistant disease.

Abstract

ET resistance is a critical problem for estrogen receptor-positive (ER+) breast cancer. In this study, we have investigated how alterations in sphingolipids promote cell survival in ET-resistant breast cancer. We have performed LC-MS-based targeted sphingolipidomics of tamoxifen-sensitive and -resistant MCF-7 breast cancer cell lines. Follow-up studies included treatments of cell lines and patient-derived xenograft organoids (PDxO) with small molecule inhibitors; cytometric analyses to measure cell death, proliferation, and apoptosis; siRNA-mediated knockdown; RT-qPCR and Western blot for gene and protein expression; targeted lipid analysis; and lipid addback experiments. We found that tamoxifen-resistant cells have lower levels of ceramides and hexosylceramides compared to their tamoxifen-sensitive counterpart. Upon perturbing the sphingolipid pathway with small molecule inhibitors of key enzymes, we identified that CERK is essential for tamoxifen-resistant breast cancer cell survival, as well as a fulvestrant-resistant PDxO. CERK inhibition induces ceramide-mediated cell death in tamoxifen-resistant cells. Ceramide-1-phosphate (C1P) partially reverses CERK inhibition-induced cell death in tamoxifen-resistant cells, likely through lowering endogenous ceramide levels. Our findings suggest that ET-resistant breast cancer cells maintain lower ceramide levels as an essential pro-survival mechanism. Consequently, ET-resistant breast cancer models have a unique dependence on CERK as its activity can inhibit de novo ceramide production.

Mechanistic Effects of Estrogens on Breast Cancer

PURPOSE

Current concepts regarding estrogen and its mechanistic effects on breast cancer in women are evolving. This article reviews studies that address estrogen-mediated breast cancer development, the prevalence of occult tumors at autopsy, and the natural history of breast cancer as predicted by a newly developed tumor kinetic model.

METHODS

This article reviews previously published studies from the authors and articles pertinent to the data presented.

RESULTS

We discuss the concepts of adaptive hypersensitivity that develops in response to long-term deprivation of estrogen and results in both increased cell proliferation and apoptosis. The effects of menopausal hormonal therapy on breast cancer in postmenopausal women are interpreted based on the tumor kinetic model. Studies of the administration of a tissue selective estrogen complex in vitro, in vivo, and in patients are described. We review the various clinical studies of breast cancer prevention with selective estrogen receptor modulators and aromatase inhibitors. Finally, the effects of the underlying risk of breast cancer on the effects of menopausal hormone therapy are outlined.

DISCUSSION

The overall intent of this review is to present data supporting recent concepts, discuss pertinent literature, and critically examine areas of controversy.

Global expression response of genes in sex-undifferentiated Nile tilapia gonads after exposure to trace letrozole

The aromatase inhibitor letrozole can be found in rivers, effluents, and even drinking water. Studies have demonstrated that letrozole affects various metabolic pathways and may cause reproductive toxicity, especially in fish exposed during development. However, studies on the effect of a low concentration of letrozole at the whole-gonad transcriptomic level in the early stage of fish sexual development have not been investigated. The aim of our study was to explore the potential effects of a low concentration of letrozole on the gonad transcriptome of Nile tilapia at an early stage of sexual development. In this study, 9 dpf (days postfertilization) Nile tilapia were exposed to trace letrozole for 12 days. Letrozole exposure from 9 dpf to 21 dpf persistently altered phenotypic sex development and induced the male-biased sex ratio. The transcriptome results showed that 1173 differentially expressed genes (DEGs) were present in the female control vs 1.5 μg/L letrozole-treated female comparison group and that 1576 DEGs were present in the 1.5 μg/L letrozole-treated female vs male control comparison group. Differentially expressed gene enrichment analysis revealed several crucial pathways, including the drug metabolism-cytochrome P450 pathway, the ErbB-PI3K/Akt/mTOR pathway, and the calcium signalling pathway. Further analysis of these identified DEGs indicated that some key genes correlated with metabolism and epigenetic regulation were significantly affected by letrozole, such as UDP-glucuronosyltransferase (Ugt), glutathione S-transferase omega-1 (Gsto1), lysine-specific demethylase 6bb (Kdm6bb, original name is Kdm6a), jumonji and AT-rich interaction domain containing 2 (Jarid2b, original name is Jarid2), growth arrest and DNA damage inducible gamma (Gadd45g), and chromobox protein 7 (Cbx7). The qRT-PCR validation results for twelve DEGs showed that the Pearson's correlation of the log₁₀^{fold change} values between the qPCR and RNA-Seq results was 0.90, indicating the accuracy and reliability of the RNA-Seq results. Our study is the first to report the effect of letrozole on the transcriptome of gonads from fish during early-stage sexual development. These findings will be useful for understanding the toxic effects and molecular mechanisms of letrozole exposure at the early stage of gonad development on the sexual development of aquatic organisms.

Sapanisertib Plus Exemestane or Fulvestrant in Women with Hormone Receptor-Positive/HER2-Negative Advanced or Metastatic Breast Cancer

PURPOSE

This open-label, multicenter, phase IB/II study evaluated sapanisertib, a dual inhibitor of mTOR kinase complexes 1/2, plus exemestane or fulvestrant in postmenopausal women with hormone receptor-positive (HR⁺)/HER2-negative (HER2^-) advanced/metastatic breast cancer.

PATIENTS AND METHODS

Eligible patients had previously progressed on everolimus with exemestane/fulvestrant and received ≤3 (phase IB) or ≤1 (phase II) prior chemotherapy regimens. Patients received sapanisertib 3 to 5 mg every day (phase IB), or 4 mg every day (phase II) with exemestane 25 mg every day or fulvestrant 500 mg monthly in 28-day cycles. Phase II enrolled parallel cohorts based on prior response to everolimus. The primary objective of phase II was to evaluate antitumor activity by clinical benefit rate at 16 weeks (CBR-16).

RESULTS

Overall, 118 patients enrolled in phase IB (n = 24) and II (n = 94). Five patients in phase IB experienced dose-limiting toxicities, at sapanisertib doses of 5 mg every day (n = 4) and 4 mg every day (n = 1); sapanisertib 4 mg every day was the MTD in combination with exemestane or fulvestrant. In phase II, in everolimus-sensitive versus everolimus-resistant cohorts, CBR-16 was 45% versus 23%, and overall response rate was 8% versus 2%, respectively. The most common adverse events were nausea (52%), fatigue (47%), diarrhea (37%), and hyperglycemia (33%); rash occurred in 17% of patients. Molecular analysis suggested positive association between AKT1 mutation status and best treatment response (complete + partial response; P = 0.0262).

CONCLUSIONS

Sapanisertib plus exemestane or fulvestrant was well tolerated and exhibited clinical benefit in postmenopausal women with pretreated everolimus-sensitive or everolimus-resistant breast cancer.

Resuming Sensitivity of Tamoxifen-Resistant Breast Cancer Cells to Tamoxifen by Tetrandrine

Background:

Tamoxifen is one of the medicines for adjuvant endocrine therapy of hormone-dependent breast cancer. However, development of resistance to tamoxifen occurs inevitably during treatment. This study aimed to determine whether sensitivity of tamoxifen-resistant breast cancer cells (TAM-R) could be reinstated by tetrandrine (Tet).

Methods:

All experiments were conducted in TAM-R cells derived from the MCF-7 breast cancer cell line by long-term tamoxifen exposure. Cell growth, apoptosis, and autophagy were end-points that evaluated the effect of Tet (0.9 μg/ml, 1.8 μg/ml, and 3.75 μg/ml) alone or in combination with TAM (1 μM). Cell apoptosis was determined by an ELISA assay and autophagy was determined by fluorescent staining using the Enzo autophagy detection kit. Immunoblotting was used to evaluate markers for apoptosis, autophagy, and related signal pathway molecules.

Results:

Growth of TAM-R cells was significantly inhibited by Tet. Combination of Tet with tamoxifen induced a greater inhibition on cell growth than tamoxifen alone, which was predominantly due to enhancement of pro-apoptotic effect of TAM by Tet. Autophagy was significantly inhibited in TAM-R cells treated with Tet plus TAM as shown by increased autophagosomes and the levels of LC3-II and p62. At 0.9 μg/ml, Tet increased the levels of both apoptosis and autophagy markers. Among them increase in p53 levels was more dramatic.

Conclusions:

Tet as a monotherapy inhibits TAM-R cells. Tet potentiates the pro-apoptotic effect of TAM via inhibition of autophagy.

The Evolving Complexity of Treating Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor-2 (HER2)-Negative Breast Cancer: Special Considerations in Older Breast Cancer Patients-Part II: Metastatic Disease

Breast cancer is a disease of aging, and the incidence of breast cancer is projected to increase dramatically as the global population ages. The majority of breast cancers that occur in older adults are hormone-receptor positive, human epidermal growth factor receptor-2 (HER2)-negative phenotypes, with favorable tumor biology; yet, because of underrepresentation in clinical trials, less evidence is available to guide the complex care for this population. Providing care for older patients with metastatic breast cancer, with coexisting medical conditions, increased risk of treatment toxicity, and frailty, remains a clinical challenge in oncology. In this review, we provide an overview of the current evidence from clinical trials and subanalyses of older adults with hormone receptor-positive, HER2-negative metastatic breast cancer, highlighting data on the safety and efficacy of oral therapies, including endocrine therapy alone or in combination with cyclin-dependent kinase (CDK) 4/6 inhibitors, phosphatidylinositol 3-kinase (PI3K) inhibitors, and mammalian target of rapamycin (mTOR) inhibitors. In addition, we note the significant underrepresentation of older and frail adults in these studies. Current and future directions in research for this special population, in order to address significant knowledge gaps, include the need to improve long-term adherence to hormonal and targeted therapy, prospective clinical trials that capture clinical and biological aging endpoints, and the need for a multidisciplinary approach with integration of geriatric and oncology principles.

Methylome Variation Predicts Exemestane Resistance in Advanced ER+ Breast Cancer

Background:

More than 30% of estrogen receptor-positive breast cancers are resistant to primary hormone therapy, and about 40% that initially respond to hormone therapy eventually acquire resistance. Although the mechanisms of hormone therapy resistance remain unclear, aberrant DNA methylation has been implicated in oncogenesis and drug resistance.

Purpose:

We investigated the relationship between methylome variations in circulating tumor DNA and exemestane resistance, to track hormone therapy efficacy.

Methods:

We prospectively recruited 16 patients who were receiving first-line therapy in our center. All patients received exemestane-based hormone therapy after enrollment. We collected blood samples at baseline, first follow-up (after 2 therapeutic cycles) and at detection of disease progression. Disease that progressed within 6 months under exemestane treatment was considered exemestane resistance but was considered relatively exemestane-sensitive otherwise. We obtained circulating tumor DNA-derived methylomes using the whole-genome bisulfide sequencing method. Methylation calling was done by BISMARK software; differentially methylated regions for exemestane resistance were calculated afterward.

Results:

Median follow-up for the 16 patients was 19.0 months. We found 7 exemestane resistance-related differentially methylated regions, located in different chromosomes, with both significantly different methylation density and methylation ratio. Baseline methylation density and methylation ratio of chromosome 6 [32400000-32599999] were both high in exemestane resistance. High baseline methylation ratios of chromosome 3 [67800000-67999999] (P = .013), chromosome 3 [140200000-140399999] (P = .037), and chromosome 12 [101200000-101399999] (P = .026) could also predict exemestane resistance. During exemestane treatment, synchronized changes in methylation density and methylation ratio in chromosome 6 [32400000-32599999] could accurately stratify patients in terms of progression-free survival (P = .000033). Cutoff values of methylation density and methylation ratio for chromosome 6 [149600000-149799999] were 0.066 and 0.076, respectively.

Conclusion:

Methylation change in chromosome 6 [149600000-149799999] is an ideal predictor of exemestane resistance with great clinical potential.

Estrogen Receptor α (ERα)-targeting Compounds and Derivatives: Recent Advances in Structural Modification and Bioactivity

Breast cancer is the most common cancer suffered by female, and the second highest cause of cancer-related death among women worldwide. At present, hormone therapy is still the main treatment route and can be divided into three main categories: selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), and aromatase inhibitors (AIs). However, breast cancer is difficult to cure even after several rounds of anti-estrogen therapy and most drugs have serious side-effects. Here, we review the literature published over the past five years regarding the isolation and synthesis of analogs and their derivatives.

Exploring Biomarkers of Phosphoinositide 3-Kinase Pathway Activation in the Treatment of Hormone Receptor Positive, Human Epidermal Growth Receptor 2 Negative Advanced Breast Cancer

Resistance to endocrine therapy (ET) is common in patients with hormone receptor positive (HR+) advanced breast cancer (ABC). Consequently, new targeted treatment options are needed in the post-ET setting, with validated biomarkers to inform treatment decisions. Hyperactivation of the phosphoinositide 3-kinase (PI3K) signaling pathway is common in ABC and is implicated in resistance to ET. The most frequent mechanism of PI3K pathway activation is activating mutations or amplification of PIK3CA, which encodes the α-isoform of the catalytic subunit of PI3K. Combining buparlisib, a pan-PI3K-targeted agent, with ET demonstrated modest clinical benefits in patients with aromatase inhibitor-resistant, HR+, human epidermal growth receptor 2 negative (HER2-) ABC in two phase III trials. Importantly, greater efficacy gains were observed in individuals with PIK3CA-mutated disease versus PIK3CA-wild-type tumors. Although the challenging safety profile did not support widespread use of this treatment combination, isoform-selective PI3K inhibitors may improve tolerability. In early clinical trials, promising disease control benefits were demonstrated with the PI3K isoform-selective inhibitors alpelisib and taselisib in patients with PIK3CA-mutated HR+, HER2- ABC. Ongoing biomarker-guided phase II/III studies may provide further opportunities to identify patients most likely to benefit from treatment with PI3K inhibitors and provide insight into optimizing the therapeutic index of PI3K inhibitors. Challenges facing the implementation of routine PIK3CA mutation testing must be addressed promptly so robust and reproducible genotyping can be obtained with liquid and tumor biopsies in a timely and cost-effective manner. IMPLICATIONS FOR PRACTICE: The development of phosphoinositide 3-kinase (PI3K) inhibitors, especially those that selectively target isoforms, may be an effective strategy for overcoming endocrine therapy resistance in hormone receptor positive, human epidermal growth receptor 2 negative advanced breast cancer. Early-phase studies have confirmed that patients with PIK3CA mutations respond best to PI3Kα-isoform inhibition. Ongoing phase III trials will provide further data regarding the efficacy and safety of PI3K inhibitors in patients with different biomarker profiles.

Treatment of advanced HR+/HER2- breast cancer with new targeted agents in combination with endocrine therapy: a review of efficacy and tolerability based on available randomized trials on everolimus, ribociclib, palbociclib and abemaciclib

INTRODUCTION

Recently, new targeted agents have been developed, which can prolong the effect of endocrine treatment (ET) by targeting resistance pathways in HR+/HER2- advanced breast cancer. This review examines available studies of everolimus, an mTOR inhibitor, and the CDK 4/6 inhibitors ribociclib, palbociclib and abemaciclib in terms of efficacy, tolerability and safety.

MATERIAL AND METHODS

A systematic literature search was performed in Pubmed. Evaluation of the quality of the identified studies was based on selected elements from the GRADE guidelines.

RESULTS

The literature search yielded eight randomized trials that all presented a significant increase in the progression free survival (PFS)/time to progression (TTP) for the targeted agents plus ET vs ET only. The improvement was evident as first-line therapy with an increase in PFS of 10-11 months when adding a CDK4/6 inhibitor to ET, as well as in patients previously treated for metastatic disease, with an increase of 5-6 months. The common adverse events (AEs) of the CDK 4/6 inhibitors were due to myelosuppression. In addition, abemaciclib was associated with liver toxicity and diarrhea, and ribociclib with liver toxicity and QTcF prolongation. The most common grade 3/4 AE of everolimus was stomatitis. The majority (five) of the trials had no serious limitations, and thus the quality of evidence was high.

DISCUSSION

The new targeted agents are all associated with an improvement of the PFS with an acceptable tolerability, and they should be offered to women with advanced HR+/HER2- breast cancer both as first-line therapy as well as among patients previously treated in metastatic regimens. However, further data regarding the impact on overall survival are required to evaluate the full benefit for patients. Price and differences in AEs could become substantial arguments for the choice of therapy for the individual patient.

Isobavachalcone sensitizes cells to E2-induced paclitaxel resistance by down-regulating CD44 expression in ER+ breast cancer cells

Oestrogen receptor (ER) is expressed in approximately 60%‐70% of human breast cancer. Clinical trials and retrospective analyses have shown that ER‐positive (ER+) tumours are more tolerant to chemotherapeutic drug resistance than ER‐negative (ER−) tumours. In addition, isobavachalcone (IBC) is known as a kind of phytoestrogen with antitumour effect. However, the underlying mechanism of IBC in ER+ breast cancer needs to be elucidated further. Our in vitro experiments showed that IBC could attenuate 17β‐estradiol (E₂)‐induced paclitaxel resistance and that E₂ could stimulate CD44 expression in ER+ breast cancer cells but not in ER− cells. Meanwhile, E₂ could promote ERα expression to render ER+ breast cancer cells resistant to paclitaxel. Furthermore, we established paclitaxel‐resistant breast cancer cell lines and determined the function of ERα in the enhancement of paclitaxel resistance via the regulation of CD44 transcription. IBC down‐regulated ERα and CD44 expression and thus inhibited tumour growth in paclitaxel‐resistant xenograft models. Overall, our data demonstrated for the first time that IBC could decrease CD44 expression level via the ERα pathway and make ER+ breast cancer cells sensitive to paclitaxel treatment.

Emerging data on improving response to hormone therapy: the role of novel targeted agents

INTRODUCTION

Hormone receptor positive (HR+) breast cancer represents the most common subtype of breast cancer. Metastatic HR+ breast cancer may develop resistance to standard hormone therapies, arising from genomic alterations in the estrogen receptor and/or upregulation of other signal transduction pathways. Areas covered: In this review, we discuss hormone resistance and strategies to overcome it, from the pre-clinical and clinical perspectives. This review includes a discussion of inhibition of the PI3K/AKT/mTOR, CDK 4/6, histone deacetylation, fibroblast growth factor receptor, and immune pathways, based on review of relevant literature. Expert commentary: Several emerging novel therapies to improve the response to hormone therapy are approved or are in development. The most promising agents at present are inhibitors of CDK 4/6 and mTOR, which have already been incorporated into treatment in the advanced stage setting and are under study for early stage disease.

Treatment for the endocrine resistant breast cancer: Current options and future perspectives

Endocrine resistance remains a challenge and an unmet need for managing hormone receptor-positive breast cancer. The mechanisms of endocrine resistance are multifaceted and are likely to evolve over time following various single or combination therapies. The purpose of this review article is to provide general understanding of molecular basis of endocrine resistance of breast cancer and to offer comprehensive review on current treatment options and potential new treatment strategies for endocrine resistant breast cancers. Last but not the least, we discuss current challenges and future directions for management of endocrine resistant breast cancers.

Treatment with aromatase inhibitors stimulates the expression of epidermal growth factor receptor-1 and neuregulin 1 in ER positive/HER-2/neu non-amplified primary breast cancers

While estrogens have been shown to modulate EGFR/HER-1 and HER-2/neu expression in experimental systems, the effects of estrogen deprivation on expression levels of the HER-receptors and the neuregulin (NRG)1 ligand in breast cancers remain unknown. Here, we measured EGFR/HER-1-4 and NRG1 mRNA in ER positive tumors from 85 postmenopausal breast cancer patients before and after two weeks (n=64) and three months (n=85) of primary treatment with an aromatase inhibitor (AI). In tumors lacking HER-2/neu amplification, quantitative real-time PCR analyses revealed EGFR/HER-1 and NRG1 to vary significantly between the three time points (before therapy, after 2 weeks and after 3 months on treatment; P≤0.001 for both). Pair-wise comparison revealed a significant increase in EGFR/HER-1 already during the first two weeks of treatment (P=0.049) with a further increase for both EGFR/HER-1 and NRG1 after 3 months on treatment (P≤0.001 and P=0.001 for both comparing values at 3 months to values at baseline and 2 weeks respectively). No difference between tumors responding versus non-responders was recorded. Further, no significant change in any parameter was observed among HER-2/neu amplified tumors. Analyzing components of the HER-2/neu PI3K/Akt downstream pathway, the PIK3CA H1047R mutation was associated with treatment response (P=0.035); however no association between either AKT phosphorylation status or PIK3CA gene mutations and EGFR/HER-1 or NRG1 expression levels were observed. Our results indicate primary AI treatment to modulate expression of HER-family members and the growth factor NRG1 in HER-2/neu non-amplified breast cancers in vivo. Potential implications to long term sensitivity warrants further investigations.

Re-adopting classical nuclear receptors by cholesterol metabolites

Since the first cloning of the human estrogen receptor (ER) α in 1986 and the subsequent cloning of human ERβ, there has been extensive investigation of the role of estrogen/ER. Estrogens/ER play important roles not only in sexual development and reproduction but also in a variety of other functions in multiple tissues. Selective Estrogen Receptor Modulators (SERMs) are ER lignds that act as agonists or antagonists depending on the target genes and tissues, and until recently, only synthetic SERMs have been recognized. However, the discovery of the first endogenous SERM, 27-hydroxycholesterol (27HC), opened a new dimension of ER action in health and disease. In addition to the identification of 27HC as a SERM, oxysterols have been recently demonstrated as indirect modulators of ER through interaction with the nuclear receptor Liver X Receptor (LXR) β. In this review, the recent progress on these novel roles of oxysterols in ER modulation is summarized.

Interaction of Salicylates and the Other Nonsteroidal Anti-Inflammatory Agents With Breast Cancer Endocrine Treatment: Systematic Review

Despite advances in breast cancer treatment, mortality from breast cancer is still high. Undoubtedly, novel treatment strategies are needed for chemoprevention of high-risk women and for the treatment of receptor-negative breast cancer. An appealing strategy would be the combination of breast endocrine treatment with salicylates and the other nonsteroidal anti-inflammatory agents (NSAIDs). This systematic review aimed to synthesize evidence on the possible synergistic antitumor effect of breast cancer endocrine treatment with salicylates and the other NSAIDs. Electronic databases were searched with the appropriate search terms. Most of the identified studies investigated the possible synergistic effect of exemestane with celecoxib in different clinical settings including metastatic treatment, adjuvant treatment, ductal carcinoma in situ. The possible synergistic effect of tamoxifen with celecoxib was investigated in one experimental study and the possible synergistic effect of exemestane with aspirin was investigated in another experimental study. Synergistic effect was detected in the majority of the studies. In conclusion, existing limited evidence suggests synergistic interaction of salicylates and the other NSAIDs in the treatment of estrogen responsive breast cancer with clinical implications in the reversal of acquired resistance to breast cancer endocrine treatment and in chemoprophylaxis.

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.

The best of both worlds - managing the cancer, saving the bone

In the context of breast cancer, the importance of the skeleton in the regulation of primary tumour development and as a site for subsequent metastasis is well characterized. Our understanding of the contributions made by the host bone and bone marrow cells increasingly demonstrates the extent of the interaction between tumour cells and normal host cells. As a result, the need to develop and utilize therapies that can impede the growth and/or function of tumour cells while sparing normal host bone and bone marrow cells is immense and expanding. The need for these new treatments is, however, superimposed on the orthopaedic management of patients' quality of life, where pain control and continued locomotion are paramount. Indeed, the majority of the anticancer therapies used to date often result in direct or indirect damage to bone. Thus, although the bone microenvironment regulates tumour cell growth in bone, cells within the bone marrow niche also mediate many of the orthopaedic consequences of tumour progression as well as resistance to the antitumour effects of existing therapies. In this Review, we highlight the effects of existing cancer treatments on bone and the bone marrow microenvironment as well as the mechanisms mediating these effects and the current utility of modern orthopaedic interventions.

Efficacy of SERD/SERM Hybrid-CDK4/6 Inhibitor Combinations in Models of Endocrine Therapy-Resistant Breast Cancer

PURPOSE

Endocrine therapy, using tamoxifen or an aromatase inhibitor, remains first-line therapy for the management of estrogen receptor (ESR1)-positive breast cancer. However, ESR1 mutations or other ligand-independent ESR1 activation mechanisms limit the duration of response. The clinical efficacy of fulvestrant, a selective estrogen receptor downregulator (SERD) that competitively inhibits agonist binding to ESR1 and triggers receptor downregulation, has confirmed that ESR1 frequently remains engaged in endocrine therapy-resistant cancers. We evaluated the activity of a new class of selective estrogen receptor modulators (SERM)/SERD hybrids (SSH) that downregulate ESR1 in relevant models of endocrine-resistant breast cancer. Building on the observation that concurrent inhibition of ESR1 and the cyclin-dependent kinases 4 and 6 (CDK4/6) significantly increased progression-free survival in advanced patients, we explored the activity of different SERD- or SSH-CDK4/6 inhibitor combinations in models of endocrine therapy-resistant ESR1(+) breast cancer.

EXPERIMENTAL DESIGN

SERDs, SSHs, and the CDK4/6 inhibitor palbociclib were evaluated as single agents or in combination in established cellular and animal models of endocrine therapy-resistant ESR1(+) breast cancer.

RESULTS

The combination of palbociclib with a SERD or an SSH was shown to effectively inhibit the growth of MCF7 cell or ESR1-mutant patient-derived tumor xenografts. In tamoxifen-resistant MCF7 xenografts, the palbociclib/SERD or SSH combination resulted in an increased duration of response as compared with either drug alone.

CONCLUSIONS

A SERD- or SSH-palbociclib combination has therapeutic potential in breast tumors resistant to endocrine therapies or those expressing ESR1 mutations. See related commentary by DeMichele and Chodosh, p. 4999.

A Case of Disease Improvement after Treatment with Everolimus plus Exemestane in a Patient with Hormone Receptor-Positive Metastatic Breast Cancer with Bone Metastases

Breast cancer is one of the most frequently diagnosed cancers and a leading cause of death in women worldwide. Despite significant advances in the treatment of hormone receptor-positive breast cancer, tumor metastasis occurs frequently and is associated with poor long-term prognosis. The mammalian target of rapamycin (mTOR) pathway plays a central role in cancer cell growth, proliferation, and resistance to endocrine therapies. Therefore, mTOR inhibitors such as everolimus in combination with nonsteroidal aromatase inhibitors might reverse endocrine resistance and improve clinical outcomes in patients. Here, we report on a case of infiltrating lobular carcinoma of the breast with metastases to the bone. Histopathologic analysis showed that the patient was estrogen and progesterone receptor positive and human epidermal growth factor-2 negative. This case represents the clinical spectrum of complications caused by metastasis: the patient experienced a considerable amount of skeletal-related complications, had previously received chemotherapy, and experienced disease progression while taking nonsteroidal aromatase inhibitors. After treatment with oral everolimus 10 mg daily plus oral exemestane 25 mg daily, the patient's disease was ameliorated. Combination therapy was well tolerated, with minimal adverse effects that were manageable with concomitant medications. Although further analyses in larger populations are necessary, the addition of everolimus to exemestane might provide an effective new treatment option for patients with bone metastasis.

USP9X downregulation renders breast cancer cells resistant to tamoxifen

Tamoxifen is one of the most widely used endocrine agents for the treatment of estrogen receptor α (ERα)-positive breast cancer. Although effective in most patients, resistance to tamoxifen is a clinically significant problem and the mechanisms responsible remain elusive. To address this problem, we performed a large scale loss-of-function genetic screen in ZR-75-1 luminal breast cancer cells to identify candidate resistance genes. In this manner, we found that loss of function in the deubiquitinase USP9X prevented proliferation arrest by tamoxifen, but not by the ER downregulator fulvestrant. RNAi-mediated attenuation of USP9X was sufficient to stabilize ERα on chromatin in the presence of tamoxifen, causing a global tamoxifen-driven activation of ERα-responsive genes. Using a gene signature defined by their differential expression after USP9X attenuation in the presence of tamoxifen, we were able to define patients with ERα-positive breast cancer experiencing a poor outcome after adjuvant treatment with tamoxifen. The signature was specific in its lack of correlation with survival in patients with breast cancer who did not receive endocrine therapy. Overall, our findings identify a gene signature as a candidate biomarker of response to tamoxifen in breast cancer.

Everolimus in postmenopausal, hormone receptor-positive advanced breast cancer: summary and results of an austrian expert panel discussion

In patients with hormone receptor-positive advanced breast cancer, response to endocrine therapy is frequently limited by endocrine resistance. One important mechanism of resistance is related to mammalian target of rapamycin (mTOR), a molecule involved in the activation of alternative signaling pathways. Preclinically, resensitization of endocrine resistance can be achieved by the addition of the mTOR inhibitor everolimus to endocrine therapy. Recent results of clinical trials confirmed the clinical activity of combining everolimus and endocrine therapy in neoadjuvant and advanced breast cancer. The BOLERO-2 trial demonstrated significant progression-free survival benefits for the addition of everolimus to exemestane. These data were the basis for the recent approval of everolimus in combination with exemestane for the treatment of advanced hormone r eceptor-positive breast cancer. In clinical practice, the following 3 patient groups are particularly suitable for this treatment: those with progression on aromatase inhibitor therapy, those who respond well to chemotherapy and might benefit from subsequent endocrine therapy, and those with non-aggressive tumor biology. Everolimus treatment requires careful clinical monitoring due to the potentially serious side effects, e.g. stomatitis and pneumonitis. It is also important to educate patients and physicians in order to increase their awareness of side effects. At present, everolimus is investigated in clinical trials.

VAV3 mediates resistance to breast cancer endocrine therapy

Introduction

Endocrine therapies targeting cell proliferation and survival mediated by estrogen receptor α (ERα) are among the most effective systemic treatments for ERα-positive breast cancer. However, most tumors initially responsive to these therapies acquire resistance through mechanisms that involve ERα transcriptional regulatory plasticity. Herein we identify VAV3 as a critical component in this process.

Methods

A cell-based chemical compound screen was carried out to identify therapeutic strategies against resistance to endocrine therapy. Binding to ERα was evaluated by molecular docking analyses, an agonist fluoligand assay and short hairpin (sh)RNA–mediated protein depletion. Microarray analyses were performed to identify altered gene expression. Western blot analysis of signaling and proliferation markers, and shRNA-mediated protein depletion in viability and clonogenic assays, were performed to delineate the role of VAV3. Genetic variation in VAV3 was assessed for association with the response to tamoxifen. Immunohistochemical analyses of VAV3 were carried out to determine its association with therapeutic response and different tumor markers. An analysis of gene expression association with drug sensitivity was carried out to identify a potential therapeutic approach based on differential VAV3 expression.

Results

The compound YC-1 was found to comparatively reduce the viability of cell models of acquired resistance. This effect was probably not due to activation of its canonical target (soluble guanylyl cyclase), but instead was likely a result of binding to ERα. VAV3 was selectively reduced upon exposure to YC-1 or ERα depletion, and, accordingly, VAV3 depletion comparatively reduced the viability of cell models of acquired resistance. In the clinical scenario, germline variation in VAV3 was associated with the response to tamoxifen in Japanese breast cancer patients (rs10494071 combined P value = 8.4 × 10⁻⁴). The allele association combined with gene expression analyses indicated that low VAV3 expression predicts better clinical outcome. Conversely, high nuclear VAV3 expression in tumor cells was associated with poorer endocrine therapy response. Based on VAV3 expression levels and the response to erlotinib in cancer cell lines, targeting EGFR signaling may be a promising therapeutic strategy.

Conclusions

This study proposes VAV3 as a biomarker and a rationale for its use as a signaling target to prevent and/or overcome resistance to endocrine therapy in breast cancer.

The phosphoinositide-3-kinase-Akt-mTOR pathway as a therapeutic target in breast cancer

The phosphoinositide-3-kinase (PI3-kinase)-Akt-mTOR pathway is a central signal transduction pathway that regulates many critical aspects of normal and cancer physiology, including cell proliferation, apoptosis, cell morphology and migration, protein synthesis, and integration of metabolism. In breast cancer, somatic mutations that activate the pathway occur in more than 50% of tumors, underscoring the potentially broad impact of targeting the pathway for therapy. A vast body of preclinical data demonstrates the efficacy of pathway inhibition on tumor growth, and evidence also shows that activation of the pathway occurs in models of acquired resistance to hormonal therapy. This preclinical work led to the investigation of allosteric mTOR inhibitors, everolimus and temsirolimus, in metastatic hormone receptor-positive breast cancer. The recent BOLERO-2 trial comparing everolimus plus exemestane versus placebo plus exemestane in women with resistance to nonsteroidal aromatase inhibitors demonstrated a 6-month improvement in progression-free survival and led to FDA approval of everolimus for this indication in the United States. This landmark trial is the first demonstration of significant clinical benefit using drugs targeting this pathway in breast cancer. Many questions remain about the role of everolimus and other pathway-targeting drugs in clinical development in breast cancer treatment. This article reviews the role of the PI3-kinase-Akt-mTOR pathway in breast cancer biology and the clinical trial evidence available to date.

GPR30 as an initiator of tamoxifen resistance in hormone-dependent breast cancer

Introduction

Tamoxifen is widely used to treat hormone-dependent breast cancer, but its therapeutic benefit is limited by the development of drug resistance. Here, we investigated the role of estrogen G-protein coupled receptor 30 (GPR30) on Tamoxifen resistance in breast cancer.

Methods

Primary tumors (PTs) of breast cancer and corresponding metastases (MTs) were used to evaluate the expression of GPR30 and epidermal growth factor receptor (EGFR) immunohistochemically. Tamoxifen-resistant (TAM-R) subclones derived from parent MCF-7 cells were used to investigate the role of GPR30 in the development of tamoxifen resistance, using MTT assay, western blot, RT-PCR, immunofluorescence, ELISA and flow cytometry. TAM-R xenografts were established to assess anti-tumor effects of combination therapy with GPR30 antagonist G15 plus 4-hydroxytamoxifen (Tam), using tumor volume measurement and Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL).

Results

In 53 human breast cancer specimens, GPR30 expression in MTs increased compared to matched PTs; in MTs, the expression patterns of GPR30 and EGFR were closely related. Compared to parent MCF-7 cells, TAM-R cells had greater growth responses to 17β-estradiol (E2), GPR30 agonist G1 and Tam, and significantly higher activation of Mitogen-activated protein (MAP) kinases; but this increased activity was abolished by G15 or AG1478. In TAM-R cells, GPR30 cell-surface translocation facilitated crosstalk with EGFR, and reduced cAMP generation, attenuating inhibition of EGFR signaling. Combination therapy both promoted apoptosis in TAM-R cells and decreased drug-resistant tumor progression.

Conclusions

Long-term endocrine treatment facilitates the translocation of GPR30 to cell surfaces, which interferes with the EGFR signaling pathway; GPR30 also attenuates the inhibition of MAP kinases. These factors contribute to tamoxifen resistance development in breast cancer. Combination therapy with GPR30 inhibitors and tamoxifen may provide a new therapeutic option for drug-resistant breast cancer.

Clinical instability of breast cancer markers is reflected in long-term in vitro estrogen deprivation studies

BACKGROUND

Long-term estrogen deprivation models are widely employed in an in vitro setting to recapitulate the hormonal milieu of breast cancer patients treated with endocrine therapy. Despite the wealth information we have garnered from these models thus far, a comprehensive time-course analysis of the estrogen (ER), progesterone (PR), and human epidermal growth factor 2 (HER-2/neu) receptors on the gene and protein level, coupled with expression array data is currently lacking. We aimed to address this knowledge gap in order to enhance our understanding of endocrine therapy resistance in breast cancer patients.

METHODS

ER positive MCF7 and BT474 breast cancer cells were grown in estrogen depleted medium for 10 months with the ER negative MDA-MB-231 cell line employed as control. ER, PR and HER-2/neu expression were analysed at defined short and long-term time points by immunocytochemistry (ICC), and quantitative real-time RT-PCR (qRT-PCR). Microarray analysis was performed on representative samples.

RESULTS

MCF7 cells cultured in estrogen depleted medium displayed decreasing expression of ER up to 8 weeks, which was then re-expressed at 10 months. PR was also down-regulated at early time points and remained so for the duration of the study. BT474 cells generally displayed no changes in ER during the first 8 weeks of deprivation, however its expression was significantly decreased at 10 months. PR expression was also down-regulated early in BT474 samples and was absent at later time points. Finally, microarray data revealed that genes and cell processes down-regulated in both cell lines at 6 weeks overlapped with those down-regulated in aromatase inhibitor treated breast cancer patients.

CONCLUSIONS

Our data demonstrate that expression of ER, PR, and cell metabolic/proliferative processes are unstable in response to long-term estrogen deprivation in breast cancer cell lines. These results mirror recent clinical findings and again emphasize the utility of LTED models in translational research.

Overcoming endocrine resistance in breast cancer: importance of mTOR inhibition

Approaches to treatment for many patients with advanced breast cancer are based on the expression of specific receptors. Treatments targeting the hormone receptor (typically the estrogen receptor) are used to reduce signaling through these receptors and thereby inhibit proliferation of breast cancer cells expressing these receptors. Although these treatments are effective for many patients, resistance to treatment is common. Recent clinical trials suggest that using multiple agents targeting the same pathway is not sufficient to overcome resistance. New treatment approaches are needed for these patients. Inhibition of the mTOR signaling pathway, a key point of confluence for multiple signaling cascades, offers a promising approach to restoring sensitivity to endocrine therapy in breast cancer. This article reviews the current data from studies of mTOR inhibitors everolimus and temsirolimus in combination with endocrine therapies to overcome treatment resistance in patients with advanced breast cancer.

mTOR inhibitors in advanced breast cancer: ready for prime time?

Current therapeutic approaches for advanced breast cancer frequently target receptors mediating cell survival and proliferation, such as the estrogen receptor and/or progesterone receptor and human epidermal growth factor receptor-2. Although these approaches are effective for many patients, treatment resistance is common. Therefore, new treatment approaches are needed for patients with advanced breast cancer. Mammalian target of rapamycin is a highly conserved serine-threonine kinase that acts as a major signaling hub that integrates and synergizes with cellular proliferation, survival, and/or motility signals mediated by estrogen receptor, human epidermal growth factor receptor-2, and other receptor tyrosine kinases. Dysregulation of mammalian target of rapamycin signaling occurs in various tumor types, including breast cancer, and has been associated with cancer pathogenesis, disease progression, and treatment resistance. Recent clinical trials show that combined inhibition of mammalian target of rapamycin and estrogen receptor represents an effective strategy for treating hormone receptor-positive advanced breast cancer progressing on nonsteroidal aromatase inhibitor therapy, and data from ongoing trials combining mammalian target of rapamycin inhibition with human epidermal growth factor receptor-2-targeted therapy are awaited. This review focuses on the molecular rationale underlying strategies to enhance sensitivity to treatment in hormone receptor-positive and human epidermal growth factor receptor-2-positive advanced breast cancer, the clinical efficacy of such approaches, and future perspectives.

Effect of everolimus on bone marker levels and progressive disease in bone in BOLERO-2

BACKGROUND

Breast Cancer Trials of Oral Everolimus 2 (BOLERO-2), a phase III study in postmenopausal women with estrogen receptor-positive breast cancer progressing despite nonsteroidal aromatase inhibitor therapy, showed statistically significant benefits with adding everolimus to exemestane. Moreover, in preclinical studies, mammalian target of rapamycin inhibition was associated with decreased osteoclast survival and activity. Exploratory analyses in BOLERO-2 evaluated the effect of everolimus on bone marker levels and progressive disease in bone.

METHODS

Patients were treated with exemestane (25mg/day) and randomized (2:1) to everolimus (10mg/day; combination) or placebo (exemestane only). Exploratory endpoints included changes in bone turnover marker levels vs baseline and progressive disease in bone, defined as unequivocal progression of a preexisting bone lesion or the appearance of a new bone lesion.

RESULTS

Baseline disease characteristics were well balanced between arms (N = 724); baseline bisphosphonate use was not (43.9% combination vs 54.0% exemestane only). At a median of 18 months of follow-up, median progression-free survival (primary endpoint) was statistically significantly longer with the combination vs exemestane only (Cox proportional hazard ratio = 0.45, 95% confidence interval = 0.38 to 0.54; log-rank, 1-sided P < .0001). Bone marker levels at 6 and 12 weeks increased with exemestane only, as expected, but decreased with the combination. The cumulative incidence rate of progressive disease in bone was lower in the combination arm. Bone-related adverse events occurred with similar frequency in both arms (3.3% combination vs 4.2% exemestane only).

CONCLUSION

These exploratory analyses suggest that everolimus has beneficial effects on bone turnover and progressive disease in bone in patients receiving exemestane for hormone receptor-positive breast cancer progressing during/after nonsteroidal aromatase inhibitor therapy.

New and emerging treatments for estrogen receptor-positive breast cancer: focus on everolimus

Management of patients with metastatic hormone receptor-positive breast cancer poses a challenge due to the inevitable development of endocrine resistance. Hormone resistance is associated with a complex interaction of the estrogen receptor with growth factors, transmembrane receptors, and intracellular growth cascades. The PI3K/Akt/mTOR pathway plays a major role in hormone resistance and proliferation of breast cancer. Preclinical and clinical data indicate that inhibitors of human epidermal growth factor receptor-2, epidermal growth factor receptor, insulin-like growth factor-1 receptor, and the mammalian target of rapamycin pathway may act synergistically with hormone therapy to circumvent endocrine resistance. Everolimus is currently approved for combination with exemestane in postmenopausal women with advanced hormone receptor-positive breast cancer. However, we still need to unfold the full potential of targeted agents in the hormone-refractory setting and to identify the subsets of patients who will benefit from combination hormonal therapy using targeted agents.

Everolimus in the treatment of hormone receptor-positive breast cancer

INTRODUCTION

The phosphoinositide triphosphate kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) is a central regulatory pathway involved in cell proliferation, growth, differentiation, metabolism and survival. Deregulation of this pathway is well described in breast cancer and is associated to the development of endocrine resistance among hormone receptor (HR)-positive tumors. Everolimus , an mTOR-inhibitor has clinical activity against breast cancer and has shown to restore sensitivity to endocrine therapy.

AREAS COVERED

We review the clinical data and the results of the recently published clinical trials evaluating the use of everolimus in HR-positive breast cancer patients in combination with endocrine therapy. We discuss the data regarding efficacy but also describe in detail the side effect profile of this drug.

EXPERT OPINION

Everolimus represents a new therapeutic alternative for the treatment of HR-positive metastatic breast cancer. Everolimus is in general a well-tolerated drug, however, stomatitis, fatigue and hematological abnormalities are common. It is still unclear if there are specific subgroups of patients that receive greater benefit from everolimus and whether there is a relationship between the presence of PIK3CA mutations and efficacy. The results of biomarker studies will hopefully provide information that will help us determine which patients are most likely to benefit from this treatment.

Endocrine resistance in breast cancer: molecular pathways and rational development of targeted therapies

Endocrine resistance presents a major challenge in the management of estrogen receptor (ER)-positive breast cancer and is an area under intense investigation. Although the underlying mechanism is still poorly understood, many studies point towards the 'cross-talk' between ER and growth factor receptor signaling pathways as the key in the development of estrogen-independent growth in breast cancer. This review aims to provide the reader our current understanding of various molecular pathways that mediate endocrine resistance and that are being evaluated as therapeutic targets for ER-positive breast cancer. While most of the agents that target these pathways have only been tested in Phase I or small Phase II trials, some have shown encouraging results. A critical issue that remains is the development of research strategies and clinical trials that take into account the molecular heterogeneity of ER-positive breast cancer.

Sirtinol, a class III HDAC inhibitor, induces apoptotic and autophagic cell death in MCF-7 human breast cancer cells

Sirtuins (SIRTs), NAD+-dependent class III histone deacetylases (HDACs), play an important role in the regulation of cell division, survival and senescence. Although a number of effective SIRT inhibitors have been developed, little is known about the specific mechanisms of their anticancer activity. In this study, we investigated the anticancer effects of sirtinol, a SIRT inhibitor, on MCF-7 human breast cancer cells. Apoptotic and autophagic cell death were measured. Sirtinol significantly inhibited the proliferation of MCF-7 cells in a concentration-dependent manner. The IC50 values of sirtinol were 48.6 µM (24 h) and 43.5 µM (48 h) in MCF-7 cells. As expected, sirtinol significantly increased the acetylation of p53, which has been reported to be a target of SIRT1/2. Flow cyto-metry analysis revealed that sirtinol significantly increased the G1 phase of the cell cycle. The upregulation of Bax, downregulation of Bcl-2 and cytochrome c release into the cytoplasm, which are considered as mechanisms of apoptotic cell death, were observed in the MCF-7 cells treated with sirtinol. The annexin V-FITC assay was used to confirm sirtinol-induced apoptotic cell death. Furthermore, the expression of LC3-II, an autophagy-related molecule, was significantly increased in MCF-7 cells after sirtinol treatment. Autophagic cell death was confirmed by acridine orange and monodansylcadaverine (MDC) staining. Of note, pre-treatment with 3-methyladenine (3-MA) increased the sirtinol-induced MCF-7 cell cytotoxicity, which is associated with blocking autophagic cell death and increasing apoptotic cell death. Based on our results, the downregulation of SIRT1/2 expression may play an important role in the regulation of breast cancer cell death; thus, SIRT1/2 may be a novel molecular target for cancer therapy and these findings may provide a molecular basis for targeting SIRT1/2 in future cancer therapy.

Randomized phase II trial of everolimus in combination with tamoxifen in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer with prior exposure to aromatase inhibitors: a GINECO study

PURPOSE

Cross-talk between signal transduction pathways likely contributes to hormone resistance in metastatic breast cancer (mBC). Everolimus, an oral inhibitor of the mammalian target of rapamycin, has restored sensitivity in endocrine-resistance models and shown anticancer activity in early-phase mBC clinical trials. This analysis evaluated efficacy and safety of everolimus in combination with tamoxifen in patients with mBC resistant to aromatase inhibitors (AIs).

PATIENTS AND METHODS

This open-label, phase II study randomly assigned postmenopausal women with hormone receptor-positive, human epidermal growth factor receptor 2-negative, AI-resistant mBC to tamoxifen 20 mg/d plus everolimus 10 mg/d (n = 54) or tamoxifen 20 mg/d alone (n = 57). Randomization was stratified by primary and secondary hormone resistance. Primary end point was clinical benefit rate (CBR), defined as the percentage of all patients with a complete or partial response or stable disease at 6 months. No formal statistical comparison between groups was planned.

RESULTS

The 6-month CBR was 61% (95% CI, 47 to 74) with tamoxifen plus everolimus and 42% (95% CI, 29 to 56) with tamoxifen alone. Time to progression (TTP) increased from 4.5 months with tamoxifen alone to 8.6 months with tamoxifen plus everolimus, corresponding to a 46% reduction in risk of progression with the combination (hazard ratio [HR], 0.54; 95% CI, 0.36 to 0.81). Risk of death was reduced by 55% with tamoxifen plus everolimus versus tamoxifen alone (HR, 0.45; 95% CI, 0.24 to 0.81). The main toxicities associated with tamoxifen plus everolimus were fatigue (72% v 53% with tamoxifen alone), stomatitis (56% v 7%), rash (44% v 7%), anorexia (43% v 18%), and diarrhea (39% v 11%).

CONCLUSION

This study suggests that tamoxifen plus everolimus increased CBR, TTP, and overall survival compared with tamoxifen alone in postmenopausal women with AI-resistant mBC.

A phase II trial of capecitabine in combination with the farnesyltransferase inhibitor tipifarnib in patients with anthracycline-treated and taxane-resistant metastatic breast cancer: an Eastern Cooperative Oncology Group Study (E1103)

Capecitabine produces an objective response rate of up to 25% in anthracycline-treated, taxane-resistant metastatic breast cancer (MBC). The farnesyltransferase inhibitor tipifarnib inhibits Ras signaling and has clinical activity when used alone in MBC. The objective of this study was to determine the efficacy and safety of tipifarnib-capecitabine combination in MBC patients who were previously treated with an anthracycline and progressed on taxane therapy. Eligible patients received oral capecitabine 1,000 mg/m2 twice daily plus oral tipifarnib 300 mg twice daily on days 1-14 every 21 days. The primary endpoint was ORR. The trial was powered to detect an improvement in response rate from 25 to 40%. Among 63 eligible, partial response occurred in six patients (9.5%; 90% CI 4.2-17.9%), median progression-free survival was 2.6 months (95% CI 2.1-4.4), and median overall survival was 11.4 months (95% CI 7.7-14.0). Dose modifications were required for 43 patients (68%) for either tipifarnib and/or capecitabine. Grades 3 and 4 toxicities were seen in 30 patients (44%; 90% CI 44.4-67.0%) and 11 patients (16%; 90% CI 10.8-29.0%), respectively. The most common grade 3 toxicities included neutropenia, nausea, and vomiting; and the most common grade 4 toxicity was neutropenia (8 out of 11 cases). The tipifarnib-capecitabine combination is not more effective than capecitabine alone in MBC patients who were previously treated with an anthracycline and taxane therapy.

Targeted agents to reverse resistance to endocrine therapy in metastatic breast cancer: where are we now and where are we going?

Endocrine therapy is the most important systemic therapy for hormone receptor positive breast cancer; however, some patients with ER+ breast cancer show intrinsic resistance to endocrine therapy, whereas others develop acquired resistance. Preclinical models have shown that endocrine resistance is associated with enhanced expression of membrane growth factor pathways or activation of various intracellular pathways involved in signal transduction and cell survival. Despite encouraging preclinical data, clinical trials investigating the combination of endocrine therapy with trastuzumab or the TKIs gefitinib, erlotinib and lapatinib have yielded varied results. This may be related to some limitations in the studies conducted so far: lack of appropriate patient selection and stratification based on previous endocrine exposure and/or sensitivity; lack of identification of a molecular biomarker; lack of appropriate clinical endpoints in the trial design. More promising results come from clinical studies which have focused on novel agents such as the mTOR inhibitor everolimus. The two randomized trials (BOLERO-2 and TAMRAD) evaluating everolimus±endocrine therapy in a selected subgroup of HR-positive metastatic breast cancer patients have demonstrated a significant improvement in progression free survival for the combination compared to the endocrine therapy alone. The data reported so far show that the combination of target agents with endocrine therapy is effective in overcoming acquired resistance in patients with hormone receptor positive metastatic breast cancer. However, this therapeutic strategy is not yet a standard treatment for this patients. Application of more rigorous trial design, tumor and patient selection criteria will be important to better understand the complexity of endocrine resistance.

Targeting of the adaptor protein Tab2 as a novel approach to revert tamoxifen resistance in breast cancer cells

Pharmacological resistance is a serious threat to the clinical success of hormone therapy for breast cancer. The antiproliferative response to antagonistic drugs such as tamoxifen (Tam) critically depends on the recruitment of NCoR/SMRT corepressors to estrogen receptor alpha (ERα) bound to estrogen target genes. Under certain circumstances, as demonstrated in the case of interleukin-1β (IL-1β) treatment, the protein Tab2 interacts with ERα/NCoR and causes dismissal of NCoR from these genes, leading to loss of the antiproliferative response. In Tam-resistant (TamR) ER-positive breast cancer cells, we observed that Tab2 presents a shift in mobility on sodium dodecyl sulfate--PAGE (SDS-PAGE) similar to that seen in MCF7 wt upon stimulation with IL-1β, suggesting constitutive activation. Accordingly, TamR treatment with Tab2-specific short interfering RNA, restored the antiproliferative response to Tam in these cells. As Tab2 is known to directly interact with the N-terminal domain of ERα, we synthesized a peptide composed of a 14-aa motif of this domain, which effectively competes with ERα/Tab2 interaction in pull-down and co-immunoprecipitation experiments, fused to the carrier TAT peptide to allow internalization. Treatment of TamR cells with this peptide resulted in partial recovery of the antiproliferative response to Tam, suggesting a strategy to revert pharmacological resistance in breast cancer. Silencing of Tab2 in TamR cells by siRNA caused modulation of a gene set related to the control of cell cycle and extensively connected to BRCA1 in a functional network. These genes were able to discern two groups of patients, from a published data set of Tam-treated breast cancer profiles, with significantly different disease-free survival. Altogether, our data implicate Tab2 as a mediator of resistance to endocrine therapy and as a potential new target to reverse pharmacological resistance and potentiate antiestrogen action.

Molecular mechanism of SAHA on regulation of autophagic cell death in tamoxifen-resistant MCF-7 breast cancer cells

OBJECTIVE

Tamoxifen is currently used for the treatment of estrogen receptor-positive breast cancer patients, but acquired resistance to tamoxifen is a critical problem in breast cancer therapy. Suberoylanilide hydroxamic acid (SAHA) is a prototype of the newly developed HDAC inhibitor. The aim of this study is to investigate the anticancer effects of SAHA in tamoxifen-resistant MCF-7 (TAMR/MCF-7) cells.

METHODS

Cytotoxicity, apoptosis and autophagic cell death induced by SAHA were studied. A TAMR/MCF-7 cells xenograft model was established to investigate the inhibitory effect of SAHA on tumor growth in vivo.

RESULTS

SAHA inhibited the proliferation of TAMR/MCF-7 cells in a dose-dependent manner. SAHA significantly reduced the expression of HDAC1, 2, 3, 4 and 7 and increased acetylated histone H3 and H4. Although SAHA induced G2/M phase arrest of cell cycle, apoptotic cell death was very low, which is correlated with the slight change in the activation of caspases and PARP cleavage. Interestingly, expression of the autophagic cell death markers, LC3-II and beclin-1, was significantly increased in TAMR/MCF-7 cells treated with SAHA. Autophagic cell death induced by SAHA was confirmed by acridine orange staining and transmission electron microscopy (TEM) in TAMR/MCF-7 cells. In mice bearing the TAMR/MCF-7 cell xenografts, SAHA significantly reduced the tumor growth and weight, without apparent side effects.

CONCLUSION

These results suggest that SAHA can induce caspase-independent autophagic cell death rather than apoptotic cell death in TAMR/MCF-7 cells. SAHA-mediated autophagic cell death is a promising new strategy to treatment of tamoxifen-resistant human breast cancer.

Progesterone receptor inhibits proliferation of human breast cancer cells via induction of MAPK phosphatase 1 (MKP-1/DUSP1)

The roles of progesterone (P(4)) and of progesterone receptor (PR) in development and pathogenesis of breast cancer remain unclear. In this study, we observed that treatment of T47D breast cancer cells with progestin antagonized effects of fetal bovine serum (FBS) to stimulate cell proliferation, whereas siRNA-mediated knockdown of endogenous PR abrogated progestin-mediated anti-proliferative effects. To begin to define mechanisms for the anti-proliferative action of P(4)/PR, we considered the role of MAPK phosphatase 1 (MKP-1/DUSP1), which catalyzes dephosphorylation and inactivation of MAPKs. Progestin treatment of T47D cells rapidly induced MKP-1 expression in a PR-dependent manner. Importantly, P(4) induction of MKP-1 was associated with reduced levels of phosphorylated ERK1/2, whereas siRNA knockdown of MKP-1 blocked progestin-mediated ERK1/2 dephosphorylation and repression of FBS-induced cell proliferation. The importance of PR in MKP-1 expression was supported by findings that MKP-1 and PR mRNA levels were significantly correlated in 30 human breast cancer cell lines. By contrast, no correlation was observed with the glucocorticoid receptor, a known regulator of MKP-1 in other cell types. ChIP and luciferase reporter assay findings suggest that PR acts in a ligand-dependent manner through binding to two progesterone response elements downstream of the MKP-1 transcription start site to up-regulate MKP-1 promoter activity. PR also interacts with two Sp1 sites just downstream of the transcription start site to increase MKP-1 expression. Collectively, these findings suggest that MKP-1 is a critical mediator of anti-proliferative and anti-inflammatory actions of PR in the breast.