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

Aberrations in translational regulation are associated with poor prognosis in hormone receptor-positive breast cancer

Introduction

Translation initiation is activated in cancer through increase in eukaryotic initiation factor 4E (eIF4E), eIF4G, phosphorylated eIF4E-binding protein (p4E-BP1) and phosphorylated ribosomal protein S6 (pS6), and decreased programmed cell death protein 4 (pdcd4), a translational inhibitor. Further, translation elongation is deregulated though alterations in eukaryotic elongation factor 2 (eEF2) and eEF2 kinase (eEF2K). We sought to determine the association of these translational aberrations with clinical-pathologic factors and survival outcomes in hormone receptor-positive breast cancer.

Methods

Primary tumors were collected from 190 patients with Stage I to III hormone receptor-positive breast cancer. Expression of eIF4E, eIF4G, 4E-BP1, p4E-BP1 T37/46, p4E-BP1 S65, p4E-BP1 T70, S6, pS6 S235/236, pS6 S240/244, pdcd4, eEF2 and eEF2K was assessed by reverse phase protein arrays. Univariable and multivariable analyses for recurrence-free survival (RFS) and overall survival (OS) were performed.

Results

High eEF2, S6, pS6 S240/244, p4E-BP1 T70, and low pdcd4 were significantly associated with node positivity. Median follow-up for living patients was 96 months.

High p4E-BP1 T36/47, p4E-BP1 S65, p4E-BP1 T70 and 4E-BP1 were associated with worse RFS. High p4E-BP1 T70 and pS6 S235/236, and low pdcd4, were associated with worse OS. In multivariable analysis, in addition to positive nodes, p4E-BP1 S65 remained a significant predictor of RFS (HR = 1.62, 95% CI = 1.13-2.31; P = 0.008). In addition to age, pS6 S235/236 (HR = 1.73, 95% CI = 1.03-2.90, P = 0.039), eEF2K (HR = 2.19, 95% CI = 1.35-3.56, P = 0.002) and pdcd4 (HR = 0.42, 95% CI = 0.25-0.70, P = 0.001) were associated with OS.

Conclusions

Increased pS6, p4E-BP1, eEF2K and decreased pdcd4 are associated with poor prognosis in hormone receptor-positive breast cancer, suggesting their role as prognostic markers and therapeutic targets.

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

INTRODUCTION

Strategies to improve the efficacy of endocrine agents in breast cancer (BC) therapy and to delay the onset of resistance include concomitant targeting of the estrogen receptor alpha (ER) and the mammalian target of rapamycin complex 1 (mTORC1), which regulate cell-cycle progression and are supported by recent clinical results.

METHODS

BC cell lines expressing aromatase (AROM) and modeling endocrine-sensitive (MCF7-AROM1) and human epidermal growth factor receptor 2 (HER2)-dependent de novo resistant disease (BT474-AROM3) and long-term estrogen-deprived (LTED) MCF7 cells that had acquired resistance associated with HER2 overexpression were treated in vitro and as subcutaneous xenografts with everolimus (RAD001-mTORC1 inhibitor), in combination with tamoxifen or letrozole. End points included proliferation, cell-cycle arrest, cell signaling, and effects on ER-mediated transactivation.

RESULTS

Everolimus caused a concentration-dependent decrease in proliferation in all cell lines, which was associated with reductions in S6 phosphorylation. Everolimus plus letrozole or tamoxifen enhanced the antiproliferative effect and G1-accumulation compared with monotherapy, as well as increased phosphorylation (Ser10) and nuclear accumulation of p27 and pronounced dephosphorylation of Rb. Sensitivity was greatest to everolimus in the LTED cells but was reduced by added estrogen. Increased pAKT occurred in all circumstances with everolimus and, in the BT474 and LTED cells, was associated with increased pHER3. Decreased ER transactivation suggested that the effectiveness of everolimus might be partly related to interrupting cross-talk between growth-factor signaling and ER. In MCF7-AROM1 xenografts, letrozole plus everolimus showed a trend toward enhanced tumor regression, versus the single agents. In BT474-AROM3 xenografts, everolimus alone was equally effective at reducing tumor volume as were the combination therapies.

CONCLUSIONS

The results provide mechanistic support for recent positive clinical data on the combination of everolimus and endocrine therapy, as well as data on potential routes of escape via enhanced HER2/3 signaling. This merits investigation for further improvements in treatment efficacy.

Abrogating endocrine resistance by targeting ERα and PI3K in breast cancer

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

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.

Interactions of PI3K/Akt/mTOR and estrogen receptor signaling in breast cancer

SUMMARY Endocrine therapies are used to treat estrogen receptor-positive (ER+) breast cancer; however, patients develop resistance in some cases due to hormone-independent activation of ER signaling. Dysregulation of mTOR, a central hub for various signaling pathways regulated by hormones and growth factors, is a mechanism of endocrine therapy resistance. Activation of kinases in these pathways can cause ligand-independent ER signaling. Phosphorylation of ER regulates activity and predicts clinical outcome in ER+ breast cancer. PI3K/Akt/mTOR pathway activation in breast cancer is common and considered a therapeutic target. PI3K/Akt/mTOR signaling is complex and interacts with ER signaling. mTOR’s downstream target p70S6K negatively regulates Akt on one hand and can phosphorylate ER. Moreover, overexpressed p70S6K activates ER in breast cancer cells. An overall understanding of signaling events, especially those governed by mTOR, is important in deciding treatment protocols for ER+ breast cancers.

The development, application and limitations of breast cancer cell lines to study tamoxifen and aromatase inhibitor resistance

Estrogen plays important roles in hormone receptor-positive breast cancer. Endocrine therapies, such as the antiestrogen tamoxifen, antagonize the binding of estrogen to estrogen receptor (ER), whereas aromatase inhibitors (AIs) directly inhibit the production of estrogen. Understanding the mechanisms of endocrine resistance and the ways in which we may better treat these types of resistance has been aided by the development of cellular models for resistant breast cancers. In this review, we will discuss what is known thus far regarding both de novo and acquired resistance to tamoxifen or AIs. Our laboratory has generated a collection of AI- and tamoxifen-resistant cell lines in order to comprehensively study the individual types of resistance mechanisms. Through the use of microarray analysis, we have determined that our cell lines resistant to a particular AI (anastrozole, letrozole, or exemestane) or tamoxifen are distinct from each other, indicating that these mechanisms can be quite complex. Furthermore, we will describe two novel de novo AI-resistant cell lines that were generated from our laboratory. Initial characterization of these cells reveals that they are distinct from our acquired AI-resistant cell models. In addition, we will review potential therapies which may be useful for overcoming resistant breast cancers through studies using endocrine resistant cell lines. Finally, we will discuss the benefits and shortcomings of cell models. Together, the information presented in this review will provide us a better understanding of acquired and de novo resistance to tamoxifen and AI therapies, the use of appropriate cell models to better study these types of breast cancer, which are valuable for identifying novel treatments and strategies for overcoming both tamoxifen and AI-resistant breast cancers.

Biomarkers of response to Akt inhibitor MK-2206 in breast cancer

PURPOSE

We tested the hypothesis that allosteric Akt inhibitor MK-2206 inhibits tumor growth, and that PTEN/PIK3CA mutations confer MK-2206 sensitivity.

EXPERIMENTAL DESIGN

MK-2206 effects on cell signaling were assessed in vitro and in vivo. Its antitumor efficacy was assessed in vitro in a panel of cancer cell lines with differing PIK3CA and PTEN status. Its in vivo efficacy was tested as a single agent and in combination with paclitaxel.

RESULTS

MK-2206 inhibited Akt signaling and cell-cycle progression, and increased apoptosis in a dose-dependent manner in breast cancer cell lines. Cell lines with PTEN or PIK3CA mutations were significantly more sensitive to MK-2206; however, several lines with PTEN/PIK3CA mutations were MK-2206 resistant. siRNA knockdown of PTEN in breast cancer cells increased Akt phosphorylation concordant with increased MK-2206 sensitivity. Stable transfection of PIK3CA E545K or H1047R mutant plasmids into normal-like MCF10A breast cells enhanced MK-2206 sensitivity. Cell lines that were less sensitive to MK-2206 had lower ratios of Akt1/Akt2 and had less growth inhibition with Akt siRNA knockdown. In PTEN-mutant ZR75-1 breast cancer xenografts, MK-2206 treatment inhibited Akt signaling, cell proliferation, and tumor growth. In vitro, MK-2206 showed a synergistic interaction with paclitaxel in MK-2206-sensitive cell lines, and this combination had significantly greater antitumor efficacy than either agent alone in vivo.

CONCLUSIONS

MK-2206 has antitumor activity alone and in combination with chemotherapy. This activity may be greater in tumors with PTEN loss or PIK3CA mutation, providing a strategy for patient enrichment in clinical trials.

The molecular profile of luminal B breast cancer

Molecular profiling studies have found that estrogen receptor-positive (ER+) human breast cancers are comprised of at least two distinct diseases with differing biologies. With the advent of DNA microarrays, global gene expression patterns were used to define the luminal A and luminal B subtypes of ER+ breast cancer, with luminal B cancers showing a more aggressive phenotype including substantially worse outcomes in patients. The luminal B subtype designation could be considered a surrogate for those ER+ tumors having low progesterone receptors, high proliferation, high grade, and predicted poor response to hormone therapy. While they express estrogen receptors, luminal B cancers do not show a corresponding expression of estrogen-regulated genes, and may therefore rely upon alternative pathways for growth. At the molecular level, luminal B cancers appear dramatically distinct from luminal A cancers, at the levels of gene expression, gene copy, somatic mutation, and DNA methylation; luminal B cancers are also genetically and genomically altered to a greater extent than luminal A cancers. While, in the clinical setting, luminal B is typically regarded as an ER+, hormone-sensitive disease, more research is needed into how to better treat it. Comprehensive profiling initiatives, such as The Cancer Genome Atlas, have recently provided us a catalog of mutated or copy altered genes, from which new therapeutic targets could potentially be mined. Candidate pathways that might be targeted in luminal B include those involving growth factor receptors, including HER2 and EGFR, as well as PI3K/Akt/mTor.

The role of S6K1 in ER-positive breast cancer

The 40S ribosomal S6 kinase 1 (S6K1) is a conserved serine/threonine protein kinase that belongs to the AGC family of protein kinases, which also includes Akt and many others. S6K1 is the principal kinase effector downstream of the mammalian target of rapamycin complex 1 (mTORC1). S6K1 is sensitive to a wide range of signaling inputs, including growth factors, amino acids, energy levels and hypoxia. S6K1 relays these signals to regulate a growing list of substrates and interacting proteins in control of oncogenic processes, such as cell growth and proliferation, cell survival and apoptosis and cell migration and invasion. Several lines of evidence suggest an important role for S6K1 in estrogen receptor (ER)-positive breast cancer. S6K1 directly phosphorylates and activates ERα. Furthermore, S6K1 expression is estrogenically regulated. Therefore, hyperactivation of mTORC1/S6K1 signaling may be closely related to ER-positive status in breast cancer and may be utilized as a marker for prognosis and a therapeutic target.

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.

Membrane-initiated estrogen signaling: oncogenic or impotent?

A recent study by Kampa et al. provided further evidence of nongenomic, membrane-initiated estrogen signaling. Using a cell-impermeable estradiol-bovine serum albumin conjugate, the authors identified genes and kinases that were modulated by a membrane-associated estrogen receptor, which they have termed 'ERx.' They describe how estradiol-induced ERx function is insensitive to antiestrogens tamoxifen and fulvestrant. The ERx components upregulated kinase signaling pathways and genes implicated in cancer progression, suggesting that ERx may promote antiestrogen resistance. However, the existence of membrane-initiated estrogen signaling and its contribution to antiestrogen resistance in vivo await confirmation.

The evolving landscape of protein kinases in breast cancer: clinical implications

Dysfunction of protein kinases has been associated with the development of the various molecular subtypes of breast cancer. The best example is the known role of HER2 in the tumorigenesis of a proportion of breast tumors. In this article, we review the state of the art knowledge on protein kinases involved in breast cancer. Special attention is given to those that are potentially druggable and those for which targeted agents are currently under clinical evaluation. Options for targeted drug combinations will be discussed, as well as the optimal way to integrate new kinase inhibitors in the clinical armamentarium to fight breast cancer. We will review recent results from clinical studies with agents targeting different kinases involved in the pathophysiology of breast cancer. In addition, we will evaluate the clinical development of kinase inhibitors to identify areas of knowledge that could be explored in future preclinical and clinical studies.

Caveolin-1 and accelerated host aging in the breast tumor microenvironment: chemoprevention with rapamycin, an mTOR inhibitor and anti-aging drug

Increasing chronological age is the most significant risk factor for human cancer development. To examine the effects of host aging on mammary tumor growth, we used caveolin (Cav)-1 knockout mice as a bona fide model of accelerated host aging. Mammary tumor cells were orthotopically implanted into these distinct microenvironments (Cav-1(+/+) versus Cav-1(-/-) age-matched young female mice). Mammary tumors grown in a Cav-1-deficient tumor microenvironment have an increased stromal content, with vimentin-positive myofibroblasts (a marker associated with oxidative stress) that are also positive for S6-kinase activation (a marker associated with aging). Mammary tumors grown in a Cav-1-deficient tumor microenvironment were more than fivefold larger than tumors grown in a wild-type microenvironment. Thus, a Cav-1-deficient tumor microenvironment provides a fertile soil for breast cancer tumor growth. Interestingly, the mammary tumor-promoting effects of a Cav-1-deficient microenvironment were estrogen and progesterone independent. In this context, chemoprevention was achieved by using the mammalian target of rapamycin (mTOR) inhibitor and anti-aging drug, rapamycin. Systemic rapamycin treatment of mammary tumors grown in a Cav-1-deficient microenvironment significantly inhibited their tumor growth, decreased their stromal content, and reduced the levels of both vimentin and phospho-S6 in Cav-1-deficient cancer-associated fibroblasts. Since stromal loss of Cav-1 is a marker of a lethal tumor microenvironment in breast tumors, these high-risk patients might benefit from treatment with mTOR inhibitors, such as rapamycin or other rapamycin-related compounds (rapalogues).

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

Introduction

The majority of breast tumors at primary diagnosis are estrogen receptor positive (ER+). Estrogen (E) mediates its effects by binding to the ER. Therapies targeting the estrogenic stimulation of tumor growth reduce mortality from ER+ breast cancer. However, resistance remains a major clinical problem.

Methods

To identify molecular mechanisms associated with resistance to E-deprivation, we assessed the temporal changes in global gene expression during adaptation to long-term culture of MCF7 human breast cancer cells in the absence of estradiol (E2), long term estrogen deprived (LTED), that leads to recovery of proliferative status and models resistance to an aromatase inhibitor (AI). The expression levels of proteins were determined by western blotting. Proliferation assays were carried out using the dual platelet derived growth factor receptor (PDGFR)/Abelson tyrosine kinase (Abl) inhibitor nilotinib. Luciferase reporter assays were used to determine effects on ER-mediated transactivation. Changes in recruitment of cofactors to the gene regulated by estrogen in breast cancer 1 (GREB1) promoter were determined by chromatin immunoprecipitation (ChIP). Gene expression data were derived from 81 postmenopausal women with ER+ BC pre-treatment and at two-weeks post-treatment with single agent anastrozole in a neoadjuvant trial.

Results

The PDGF/Abl canonical pathway was significantly elevated as early as one week post E-deprivation (P = 1.94 E-04) and this became the top adaptive pathway at the point of proliferative recovery (P = 1.15 E-07). Both PDGFRβ and Abl protein levels were elevated in the LTED cells compared to wild type (wt)-MCF7 cells. The PDGF/Abl tyrosine kinase inhibitor nilotinib, suppressed proliferation in LTED cells in the presence or absence of E. Nilotinib also suppressed ER-mediated transcription by destabilizing the ER and reducing recruitment of amplified in breast cancer-1 (AIB1) and the CREB binding protein (CBP) to the promoter of the E-responsive gene GREB1. High PDGFRβ in primary ER+ breast cancer of 81 patients prior to neoadjuvant treatment with an AI was associated with poorer antiproliferative response. Additionally PDGFRβ expression increased after two weeks of AI therapy (1.25 fold, P = 0.003).

Conclusions

These preclinical and clinical data indicate that the PDGF/Abl signaling pathway merits clinical evaluation as a therapeutic target with endocrine therapy in ER+ breast cancer.

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.

AIB1:ERα transcriptional activity is selectively enhanced in aromatase inhibitor-resistant breast cancer cells

PURPOSE

The use of aromatase inhibitors (AI) in the treatment of estrogen receptor (ER)-positive, postmenopausal breast cancer has proven efficacy. However, inappropriate activation of ER target genes has been implicated in the development of resistant tumors. The ER coactivator protein AIB1 has previously been associated with initiation of breast cancer and resistance to endocrine therapy.

EXPERIMENTAL DESIGN

Here, we investigated the role of AIB1 in the deregulation of ER target genes occurring as a consequence of AI resistance using tissue microarrays of patients with breast cancer and cell line models of resistance to the AI letrozole.

RESULTS

Expression of AIB1 associated with disease recurrence (P = 0.025) and reduced disease-free survival time (P = 0.0471) in patients treated with an AI as first-line therapy. In a cell line model of resistance to letrozole (LetR), we found ERα/AIB1 promoter recruitment and subsequent expression of the classic ER target genes pS2 and Myc to be constitutively upregulated in the presence of both androstenedione and letrozole. In contrast, the recruitment of the ERα/AIB1 transcriptional complex to the nonclassic ER target cyclin D1 and its subsequent expression remained sensitive to steroid treatment and could be inhibited by treatment with letrozole. Molecular studies revealed that this may be due in part to direct steroid regulation of c-jun-NH(2)-kinase (JNK), signaling to Jun and Fos at the cyclin D1 promoter.

CONCLUSION

This study establishes a role for AIB1 in AI-resistant breast cancer and describes a new mechanism of ERα/AIB1 gene regulation which could contribute to the development of an aggressive tumor phenotype.

mTOR as a target in breast cancer: the emerging role of everolimus

SUMMARY The majority of patients with breast cancer are estrogen- and progesterone-receptor positive, and have benefited from the development of anti-estrogen therapies, such as tamoxifen and aromatase inhibitors. Unfortunately, metastatic patients will eventually develop resistance to these agents. Inhibitors of the mTOR, particularly everolimus, show promising activity in this group of patients. mTOR inhibition appears to reverse resistance to anti-estrogen therapy in the estrogen-receptor-/progesterone-receptor-positive subset. Additionally, they may have a similar effect by reversing anti-HER2 resistance in patients who overexpress HER2. This article reviews the mechanism of action of mTOR inhibitors and summarizes the available clinical data of their use in breast cancer.

G1P3, an interferon- and estrogen-induced survival protein contributes to hyperplasia, tamoxifen resistance and poor outcomes in breast cancer

Hormonally regulated survival factors can have an important role in breast cancer. Here we elucidate G1P3, a survival protein induced by interferons (IFNs), as a target of estrogen signaling and a contributor to poor outcomes in estrogen receptor-positive (ER(+)) breast cancer. Compared with normal breast tissue, G1P3 was upregulated in the malignant epithelium (50 × higher) and was induced by estrogen ex vivo. In accord with its overexpression in early stages of breast cancer (hyperplasia and ductal carcinoma in situ), in morphogenesis assays G1P3 enhanced the survival of MCF10A acinar luminal cells causing hyperplasia by suppressing detachment-induced loss of mitochondrial potential and apoptosis (anoikis). In cells undergoing anoikis, G1P3 attenuated the induction of Bim protein, a proapoptotic member of the Bcl-2 family and reversed the downmodulation of Bcl-2 protein. Downregulation of G1P3 induced spontaneous apoptosis in BT-549 breast cancer cells and significantly reduced the growth of ER(+) breast cancer cell MCF7 (P≤0.01), further suggesting its prosurvival activity. In agreement with its induction by estrogen, G1P3 antagonized tamoxifen, an inhibitor of ER in MCF7 cells. More importantly, elevated expression of G1P3 was significantly associated with decreased relapse-free and overall survival in ER(+) breast cancer patients (P≤0.01). Our studies suggest that elevated expression of G1P3 may perturb canonical tumor-suppressing activity of IFNs partly by affecting the balance of pro- and antiapoptotic members of Bcl-2 family proteins, leading to breast cancer development and resistance to therapies.

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.

Mechanisms of estrogen-independent breast cancer growth driven by low estrogen concentrations are unique versus complete estrogen deprivation

Despite the success of the aromatase inhibitors (AIs) in treating estrogen receptor positive breast cancer, 15-20 % of patients receiving adjuvant AIs will relapse within 5-10 years of treatment initiation. Long-term estrogen deprivation (LTED) of breast cancer cells in culture mimics AI-induced estrogen depletion to dissect mechanisms of AI resistance. However, we hypothesized that a subset of patients receiving AI therapy may maintain low circulating concentrations of estrogens that influence the development of endocrine resistance. We expanded established LTED models to account for incomplete suppression of estrogen synthesis during AI therapy. MCF-7 cells were grown in medium with charcoal-stripped serum supplemented with defined concentrations of 17β-estradiol (E2) or the estrogenic androgen metabolite 5α-androstane-3β,17β-diol (3βAdiol), an endogenous selective estrogen receptor modulator. Cells were selected in concentrations of E2 or 3βAdiol that induce 10 or 90 percent of maximal proliferation (EC(10) and EC(90), respectively), or estrogen deprived. Estrogen independence was evaluated during selection by assessing cell growth in the absence or presence of E2 or 3βAdiol. Following >7 months of selection, estrogen independence developed in estrogen-deprived cells and EC(10)-selected cells. Functional analyses demonstrated that estrogen-deprived and EC(10)-selected cells developed estrogen independence via unique mechanisms, ERα-independent and dependent, respectively. Estrogen-independent proliferation in EC(10)-selected cells could be blocked by kinase inhibitors. However, these cells were resistant to kinase inhibition in the presence of low steroid concentrations. These data demonstrate that further understanding of the total estrogen environment in patients on AI therapy who experience recurrence is necessary to effectively treat endocrine-resistant disease.

Insulin-like growth factor 1 attenuates antiestrogen- and antiprogestin-induced apoptosis in ER+ breast cancer cells by MEK1 regulation of the BH3-only pro-apoptotic protein Bim

Introduction

In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). Our goal was to identify new molecular targets to improve the efficacy of hormonal therapy in breast cancer patients that have a poor response to hormonal therapy, in part, due to high circulating levels of unbound insulinIGF-1.

Methods

IGF-1-mediated effects on cytostasis and apoptotic cell death were determined with cell counts conducted in the presence and absence of trypan blue; enzyme-linked immunosorbent assays to determine the intracellular levels of cleaved cytokeratin 18, a marker of epithelial cancer cell apoptosis; and immunoblot analysis to determine the levels of cleaved poly-ADP ribose polymerase (PARP) and lamin A that result from caspase-dependent apoptosis. Cytotoxicity was further characterized by determination of the levels of reactive oxygen species (ROS) and the percent of mitochondrial membrane depolarization in cell populations treated with the different hormones in the presence and absence of IGF-1. Small molecule inhibitors of the dual-specificity protein kinase MEK1, MEK1 siRNA, Bim siRNA, and vectors overexpressing MEK1 wild type and mutant, dominant negative cDNA were used to identify key IGF-1 downstream prosurvival effectors.

Results

IGF-1, at physiologically relevant levels, blocked the cytotoxic action(s) of the antiestrogens 4-hydroxytamoxifen (4-OHT) and tamoxifen (TAM) when used as single agents or in combination with the antiprogestin MIF. The antiapoptotic action of IGF-1 was mediated primarily through the action of MEK1. MEK1 expression reduced the levels of ROS and mitochondrial membrane depolarization induced by the hormonal treatments via a mechanism that involved the phosphorylation and proteasomal turnover of the proapoptotic BH3-only Bcl-2 family member Bim. Importantly, small-molecule inhibitors of MEK1 circumvented the prosurvival action of IGF-1 by restoring Bim to levels that more effectively mediated apoptosis in ER⁺ breast cancer cells.

Conclusion

his study provides strong support for the use of MEK1 inhibitors in combination with hormonal therapy to effectively affect cytostasis and activate a Bim-dependent apoptotic pathway in ER⁺ breast cancer cells. We discuss that MEK1 blockade may be a particularly effective treatment for women with high circulating levels of IGF-1, which have been correlated to a poor prognosis.

Dual mTORC1/2 and HER2 blockade results in antitumor activity in preclinical models of breast cancer resistant to anti-HER2 therapy

PURPOSE

The PI3K/Akt/mTOR pathway is an attractive target in HER2-positive breast cancer that is refractory to anti-HER2 therapy. The hypothesis is that the suppression of this pathway results in sensitization to anti-HER2 agents. However, this combinatorial strategy has not been comprehensively tested in models of trastuzumab and lapatinib resistance.

EXPERIMENTAL DESIGN

We analyzed in vitro cell viability and induction of apoptosis in five different cell lines resistant to trastuzumab and lapatinib. Inhibition of HER2/HER3 phosphorylation, PI3K/Akt/mTOR, and extracellular signal-regulated kinase (ERK) signaling pathways was evaluated by Western blotting. Tumor growth inhibition after treatment with lapatinib, INK-128, or the combination of both agents was evaluated in three different animal models: two cell-based xenograft models refractory to both trastuzumab and lapatinib and a xenograft derived from a patient who relapsed on trastuzumab-based therapy.

RESULTS

The addition of lapatinib to INK-128 prevented both HER2 and HER3 phosphorylation induced by INK-128, resulting in inhibition of both PI3K/Akt/mTOR and ERK pathways. This dual blockade produced synergistic induction of cell death in five different HER2-positive cell lines resistant to trastuzumab and lapatinib. In vivo, both cell line-based and patient-derived xenografts showed exquisite sensitivity to the antitumor activity of the combination of lapatinib and INK-128, which resulted in durable tumor shrinkage and exhibited no signs of toxicity in these models.

CONCLUSIONS

The simultaneous blockade of both PI3K/Akt/mTOR and ERK pathways obtained by combining lapatinib with INK-128 acts synergistically in inducing cell death and tumor regression in breast cancer models refractory to anti-HER2 therapy.

Preclinical pharmacology of AZD5363, an inhibitor of AKT: pharmacodynamics, antitumor activity, and correlation of monotherapy activity with genetic background

AKT is a key node in the most frequently deregulated signaling network in human cancer. AZD5363, a novel pyrrolopyrimidine-derived compound, inhibited all AKT isoforms with a potency of 10 nmol/L or less and inhibited phosphorylation of AKT substrates in cells with a potency of approximately 0.3 to 0.8 μmol/L. AZD5363 monotherapy inhibited the proliferation of 41 of 182 solid and hematologic tumor cell lines with a potency of 3 μmol/L or less. Cell lines derived from breast cancers showed the highest frequency of sensitivity. There was a significant relationship between the presence of PIK3CA and/or PTEN mutations and sensitivity to AZD5363 and between RAS mutations and resistance. Oral dosing of AZD5363 to nude mice caused dose- and time-dependent reduction of PRAS40, GSK3β, and S6 phosphorylation in BT474c xenografts (PRAS40 phosphorylation EC(50) ~ 0.1 μmol/L total plasma exposure), reversible increases in blood glucose concentrations, and dose-dependent decreases in 2[18F]fluoro-2-deoxy-D-glucose ((18)F-FDG) uptake in U87-MG xenografts. Chronic oral dosing of AZD5363 caused dose-dependent growth inhibition of xenografts derived from various tumor types, including HER2(+) breast cancer models that are resistant to trastuzumab. AZD5363 also significantly enhanced the antitumor activity of docetaxel, lapatinib, and trastuzumab in breast cancer xenografts. It is concluded that AZD5363 is a potent inhibitor of AKT with pharmacodynamic activity in vivo, has potential to treat a range of solid and hematologic tumors as monotherapy or a combinatorial agent, and has potential for personalized medicine based on the genetic status of PIK3CA, PTEN, and RAS. AZD5363 is currently in phase I clinical trials.

PI3K/AKT pathway regulates phosphorylation of steroid receptors, hormone independence and tumor differentiation in breast cancer

Using a model of medroxyprogesterone acetate (MPA)-induced mouse mammary tumors that transit through different stages of hormone dependence, we previously reported that the activation of the phosphatidylinositol 3-kinase (PI3K)/AKT (protein kinase B) pathway is critical for the growth of hormone-independent (HI) mammary carcinomas but not for the growth of hormone-dependent (HD) mammary carcinomas. The objective of this work was to explore whether the activation of the PI3K/AKT pathway is responsible for the changes in tumor phenotype and for the transition to autonomous growth. We found that the inhibition of the PI3K/AKT/mTOR (mammalian target of rapamycin) pathway suppresses HI tumor growth. In addition, we were able to induce mammary tumors in mice in the absence of MPA by inoculating HD tumor cells expressing a constitutively active form of AKT1, myristoylated AKT1 (myrAKT1). These tumors were highly differentiated and displayed a ductal phenotype with laminin-1 and cytokeratin 8 expression patterns typical of HI tumors. Furthermore, myrAKT1 increased the tumor growth of IBH-6 and IBH-7 human breast cancer cell lines. In the estrogen-dependent IBH-7 cell line, myrAKT1 induced estrogen-independent growth accompanied by the expression of the adhesion markers focal adhesion kinase and E-cadherin. Finally, we found that cells expressing myrAKT1 exhibited increased phosphorylation of the progesterone receptor at Ser190 and Ser294 and of the estrogen receptor α at Ser118 and Ser167, independently of exogenous MPA or estrogen supply. Our results indicate that the activation of the PI3K/AKT/mTOR pathway promotes tissue architecture remodeling and the activation of steroid receptors.

Phosphatidylinositol 3-kinase and antiestrogen resistance in breast cancer

Although antiestrogen therapies targeting estrogen receptor (ER) α signaling prevent disease recurrence in the majority of patients with hormone-dependent breast cancer, a significant fraction of patients exhibit de novo or acquired resistance. Currently, the only accepted mechanism linked with endocrine resistance is amplification or overexpression of the ERBB2 (human epidermal growth factor receptor 2 [HER2]) proto-oncogene. Experimental and clinical evidence suggests that hyperactivation of the phosphatidylinositol 3-kinase (PI3K) pathway, the most frequently mutated pathway in breast cancer, promotes antiestrogen resistance. PI3K is a major signaling hub downstream of HER2 and other receptor tyrosine kinases. PI3K activates several molecules involved in cell-cycle progression and survival, and in ER-positive breast cancer cells, it promotes estrogen-dependent and -independent ER transcriptional activity. Preclinical tumor models of antiestrogen-resistant breast cancer often remain sensitive to estrogens and PI3K inhibition, suggesting that simultaneous targeting of the PI3K and ER pathways may be most effective. Herein, we review alterations in the PI3K pathway associated with resistance to endocrine therapy, the state of clinical development of PI3K inhibitors, and strategies for the clinical investigation of such drugs in hormone receptor-positive breast cancer.

Anti-estrogen resistance in breast cancer is induced by the tumor microenvironment and can be overcome by inhibiting mitochondrial function in epithelial cancer cells

Here, we show that tamoxifen resistance is induced by cancer-associated fibroblasts (CAFs). Coculture of estrogen receptor positive (ER+) MCF7 cells with fibroblasts induces tamoxifen and fulvestrant resistance with 4.4 and 2.5-fold reductions, respectively, in apoptosis compared with homotypic MCF7 cell cultures. Treatment of MCF7 cells cultured alone with high-energy mitochondrial "fuels" (L-lactate or ketone bodies) is sufficient to confer tamoxifen resistance, mimicking the effects of coculture with fibroblasts. To further demonstrate that epithelial cancer cell mitochondrial activity is the origin of tamoxifen resistance, we employed complementary pharmacological and genetic approaches. First, we studied the effects of two mitochondrial "poisons," namely metformin and arsenic trioxide (ATO), on fibroblast-induced tamoxifen resistance. We show here that treatment with metformin or ATO overcomes fibroblast-induced tamoxifen resistance in MCF7 cells. Treatment with the combination of tamoxifen plus metformin or ATO leads to increases in glucose uptake in MCF7 cells, reflecting metabolic uncoupling between epithelial cancer cells and fibroblasts. In coculture, tamoxifen induces the upregulation of TIGAR (TP53-induced glycolysis and apoptosis regulator), a p53 regulated gene that simultaneously inhibits glycolysis, autophagy and apoptosis and reduces ROS generation, thereby promoting oxidative mitochondrial metabolism. To genetically mimic the effects of coculture, we next recombinantly overexpressed TIGAR in MCF7 cells. Remarkably, TIGAR overexpression protects epithelial cancer cells from tamoxifen-induced apoptosis, providing genetic evidence that increased mitochondrial function confers tamoxifen resistance. Finally, CAFs also protect MCF7 cells against apoptosis induced by other anticancer agents, such as the topoisomerase inhibitor doxorubicin (adriamycin) and the PARP-1 inhibitor ABT-888. These results suggest that the tumor microenvironment may be a general mechanism for conferring drug resistance. In summary, we have discovered that mitochondrial activity in epithelial cancer cells drives tamoxifen resistance in breast cancer and that mitochondrial "poisons" are able to re-sensitize these cancer cells to tamoxifen. In this context, TIGAR may be a key "druggable" target for preventing drug resistance in cancer cells, as it protects cancer cells against the onset of stress-induced mitochondrial dys-function and aerobic glycolysis.

Mutations in the phosphatidylinositol 3-kinase pathway: role in tumor progression and therapeutic implications in breast cancer

Mutations in genes that constitute the phosphatidylinositol 3-kinase (PI3K) pathway occur in >70% of breast cancers. Clinical and experimental evidence suggest that PI3K pathway activation promotes resistance to some of the current breast cancer therapies. PI3K is a major signaling hub downstream of human epidermal growth factor receptor (HER)2 and other receptor tyrosine kinases. PI3K activates AKT, serum/glucocorticoid regulated kinase (SGK), phosphoinositide-dependent kinase 1 (PDK1), mammalian target of rapamycin (mTOR), and several other molecules involved in cell cycle progression and survival. In estrogen receptor (ER)+ breast cancer cells, PI3K activation promotes estrogen-dependent and -independent ER transcriptional activity, which, in turn, may contribute to anti-estrogen resistance. Activation of this pathway also confers resistance to HER2-targeted therapies. In experimental models of resistance to anti-estrogens and HER2 inhibitors, pharmacological inhibition of PI3K/AKT/mTOR has been shown to overcome drug resistance. Early clinical data suggest that combined inhibition of either HER2 or ER plus inhibition of the PI3K pathway might be an effective strategy for treatment of respective HER2+ and ER+ breast cancers resistant to standard therapies. Here, we review alterations in the PI3K pathway in breast cancer, their association with therapeutic resistance, and the state of clinical development of PI3K pathway inhibitors.

A kinome-wide screen identifies the insulin/IGF-I receptor pathway as a mechanism of escape from hormone dependence in breast cancer

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

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

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

An in vitro model showing adaptation to long-term oestrogen deprivation highlights the clinical potential for targeting kinase pathways in combination with aromatase inhibition

Aromatase inhibitors (AI) have improved the treatment of oestrogen receptor positive (ER+) breast cancer. Despite the efficacy of these agents over 40% of patients relapse with endocrine resistant disease. Here we describe an in vitro model of acquired resistance to long-term oestrogen deprivation (LTED). The LTED cells retain expression of the ER and appear hypersensitive to oestrogen as a result of altered kinase activity. Furthermore analysis of temporal changes in gene expression during the acquisition of resistance highlight growth factor receptor pathways as key mediators of this adaptive process.

Importance of PI3-kinase pathway in response/resistance to aromatase inhibitors

Endocrine therapy has been the most effective treatment modality for hormone receptor positive breast cancer. However, its efficacy has been limited by either de novo or acquired resistance. Recent data indicates that activation of the phosphatidylinositol 3-kinase (PI3K) signaling is associated with the poor outcome luminal B subtype of breast cancer and accompanied by the development of endocrine therapy resistance. Importantly, inhibition of PI3K pathway signaling in endocrine resistant breast cancer cell lines reduces cell survival and improves treatment response to endocrine agents. Interestingly, mutations in PIK3CA, the alpha catalytic subunit of the class IA PI3K, which renders cells dependent on PI3K pathway signaling, is the most common genetic abnormality identified in hormone receptor positive breast cancer. The synthetic lethality observed between estrogen deprivation and PI3K pathway inhibition in estrogen receptor positive (ER+) breast cancer cell lines provides further scientific rational to target both estrogen receptor and the PI3K pathway in order to improve the outcome of ER+ breast cancer.

Effects of S-equol and natural S-equol supplement (SE5-OH) on the growth of MCF-7 in vitro and as tumors implanted into ovariectomized athymic mice

Hormone replacement therapy (HRT) for treatment of menopausal symptoms is controversial because of reported breast cancer resulting from estrogen treatment and consequent estrogenic stimulation. S-equol, a natural metabolite of the soy isoflavone daidzein produced by intestinal bacteria, has been shown to ameliorate menopausal symptoms, with relatively low concomitant estrogenic receptor stimulation. Although synthesis of equol produces the racemate, the S-isomer may be produced in commercial amounts by bacterial fermentation of soy germ, during the production of the supplement SE5-OH. This study aims to investigate the effects of S-equol and SE5-OH on the growth of MCF-7 in vitro and in vivo. In vitro, purified S-equol, and the isoflavonoid mixture present in SE5-OH stimulated estrogenic transcriptional activity and proliferation of MCF-7-E10 cells, similar to that observed for genistein (another soy isoflavone), but at concentrations from 10(4)-fold to 10(6)-fold higher than seen with 17β-estradiol (E2). Ovariectomized (OVX) mice implanted with MCF-7-E10 cells were fed diets containing 250 or 500 ppm of purified S-equol, isoflavonoid mixture, or genistein. There were no significant differences in tumor growth between the treatment groups and control group. These results suggest that S-equol and natural S-equol in the supplement (SE5-OH), do not promote the progression of breast cancer.

GP88 (PC-Cell Derived Growth Factor, progranulin) stimulates proliferation and confers letrozole resistance to aromatase overexpressing breast cancer cells

Background

Aromatase inhibitors (AI) that inhibit breast cancer cell growth by blocking estrogen synthesis have become the treatment of choice for post-menopausal women with estrogen receptor positive (ER⁺) breast cancer. However, some patients display de novo or acquired resistance to AI. Interactions between estrogen and growth factor signaling pathways have been identified in estrogen-responsive cells as one possible reason for acquisition of resistance. Our laboratory has characterized an autocrine growth factor overexpressed in invasive ductal carcinoma named PC-Cell Derived Growth Factor (GP88), also known as progranulin. In the present study, we investigated the role GP88 on the acquisition of resistance to letrozole in ER⁺ breast cancer cells

Methods

We used two aromatase overexpressing human breast cancer cell lines MCF-7-CA cells and AC1 cells and their letrozole resistant counterparts as study models. Effect of stimulating or inhibiting GP88 expression on proliferation, anchorage-independent growth, survival and letrozole responsiveness was examined.

Results

GP88 induced cell proliferation and conferred letrozole resistance in a time- and dose-dependent fashion. Conversely, naturally letrozole resistant breast cancer cells displayed a 10-fold increase in GP88 expression when compared to letrozole sensitive cells. GP88 overexpression, or exogenous addition blocked the inhibitory effect of letrozole on proliferation, and stimulated survival and soft agar colony formation. In letrozole resistant cells, silencing GP88 by siRNA inhibited cell proliferation and restored their sensitivity to letrozole.

Conclusion

Our findings provide information on the role of an alternate growth and survival factor on the acquisition of aromatase inhibitor resistance in ER⁺ breast cancer.

Comparison of the effects of the PI3K/mTOR inhibitors NVP-BEZ235 and GSK2126458 on tamoxifen-resistant breast cancer cells

BACKGROUND

Treatment with anti-estrogens or aromatase inhibitors is commonly used for patients with estrogen receptor-positive (ER+) breast cancers; however resistant disease develops almost inevitably, requiring a choice of secondary therapy. One possibility is to use inhibitors of the PI3K/mTOR pathway and several candidate drugs are in development. We examined the in vitro effects of two inhibitors of the PI3K/mTOR pathway on resistant MCF-7 cells.

METHODS

We cultured MCF-7 cells for prolonged periods either in the presence of the anti-estrogen tamoxifen (3 sub-lines) or in estrogen free medium (2 sub-lines) to mimic the effects of clinical treatment. We then analyzed the effects of two dual PI3K/mTOR phosphoinositide-3-kinase inhibitors, NVP-BEZ235 and GSK2126458, on the growth and signaling pathways of these MCF-7 sub-lines. The functional status of the PI3K, mTOR and ERK pathways was analyzed by measuring phosphorylation of AKT, p70S6K, rpS6 and ERK.

RESULTS

The derived sub-lines showed increased resistance to tamoxifen but none exhibited concomitantly increased sensitivity to the PI3K inhibitors. NVP-BEZ235 and GSK2126458 acted mainly by induction of cell cycle arrest, particularly in G1-phase, rather than by induction of apoptosis. The lines varied considerably in their utilization of the AKT, p70S6K and ERK pathways. NVP-BEZ235 and GSK2126458 inhibited AKT signaling but NVP-BEZ235 showed greater effects than GSK2126458 on p70S6K and rpS6 signaling with effects resembling those of rapamycin.

CONCLUSION

Increased resistance to tamoxifen in these MCF-7 sub-lines is not associated with hypersensitivity to PI3K inhibitors. While both drugs inhibited AKT signaling, NVP-BEZ235 resembled rapamycin in inhibiting the mTOR pathway.

Small molecule inhibitors of the IGF-1R/IR axis for the treatment of cancer

INTRODUCTION

The IGF-1 receptor (IGF-1R) is a receptor tyrosine kinase and is well established as a key regulator of tumor cell growth and survival. There is also a growing body of data to support a role for the structurally and functionally related insulin receptor (IR) in human cancer. Bidirectional crosstalk between IGF-1R and IR is observed, where specific inhibition of either receptor confers a compensatory increase in the activity for the reciprocal receptor, therefore dual inhibition of both IGF-1R and IR may be important for optimal efficacy. The importance of IGF-1R and IR as targets in cancer is further underscored by their contribution to resistance against both cytotoxic and molecularly targeted anti-cancer therapeutics. Currently, both IGF-1R-neutralizing antibodies and small-molecule tyrosine kinase inhibitors of IGF-1R/IR are in clinical development.

AREAS COVERED

The importance of IGF-1R and IR as cancer targets and how IGF-1R/IR inhibitors may sensitize tumor cells to the anti-proliferative and pro-apoptotic effects of other anti-tumor agents. The potential advantages of small molecule IGF-1R/IR inhibitors compared with IGF-1R-specific neutralizing antibodies, and the characteristics of small-molecule IGF-1R inhibitors that have entered clinical development.

EXPERT OPINION

Because of compensatory crosstalk between IGF-1R and IR, dual IGF-1R and IR tyrosine kinase inhibitors may have superior anti-tumor activity compared to anti-IGF-1R specific antibodies. The clinical success for IGF-1R/IR inhibitors may ultimately be dependent upon our ability to correctly administer these agents to the right niche patient subpopulation using single agent therapy, when appropriate, or using the right combination therapy.

PI3K-based molecular signatures link high PI3K pathway activity with low ER levels in ER+ breast cancer

Resistance to endocrine therapy is a major clinical problem in estrogen receptor (ER)(+) breast cancer. Endocrine resistance can be caused by multiple mechanisms but unfortunately is not fully understood. The work reported by Creighton et al. investigated whether high PI3K pathway activity decreases ER levels, causing endocrine resistance in ER(+) breast cancer. They developed two PI3K-based molecular signatures--proteomic and transcriptomic--and discovered that the signatures were associated with low ER levels in a set of 429 ER(+) breast cancer tumors. Signature-based scoring in tumor samples and functional studies with cancer cell lines suggested that blocking both PI3K and estrogen pathways together could be a good therapeutic approach for ER(+) breast cancer patients with high growth-factor receptor (GFR) signaling. The results presented in the paper could offer new alternatives for the therapeutic treatment of endocrine-resistant breast cancer.

A gene expression signature from human breast cancer cells with acquired hormone independence identifies MYC as a mediator of antiestrogen resistance

PURPOSE

Although most patients with estrogen receptor α (ER)-positive breast cancer initially respond to endocrine therapy, many ultimately develop resistance to antiestrogens. However, mechanisms of antiestrogen resistance and biomarkers predictive of such resistance are underdeveloped.

EXPERIMENTAL DESIGN

We adapted four ER(+) human breast cancer cell lines to grow in an estrogen-depleted medium. A gene signature of estrogen independence was developed by comparing expression profiles of long-term estrogen-deprived (LTED) cells to their parental counterparts. We evaluated the ability of the LTED signature to predict tumor response to neoadjuvant therapy with an aromatase inhibitor and disease outcome following adjuvant tamoxifen. We utilized Gene Set Analysis (GSA) of LTED cell gene expression profiles and a loss-of-function approach to identify pathways causally associated with resistance to endocrine therapy.

RESULTS

The LTED gene expression signature was predictive of high tumor cell proliferation following neoadjuvant therapy with anastrozole and letrozole, each in different patient cohorts. This signature was also predictive of poor recurrence-free survival in two studies of patients treated with adjuvant tamoxifen. Bioinformatic interrogation of expression profiles in LTED cells revealed a signature of MYC activation. The MYC activation signature and high MYC protein levels were both predictive of poor outcome following tamoxifen therapy. Finally, knockdown of MYC inhibited LTED cell growth.

CONCLUSIONS

A gene expression signature derived from ER(+) breast cancer cells with acquired hormone independence predicted tumor response to aromatase inhibitors and associated with clinical markers of resistance to tamoxifen. Activation of the MYC pathway was associated with this resistance.

The phosphatidyl inositol 3-kinase/AKT signaling pathway in breast cancer

The phosphatidyl inositol 3-kinase (PI3K)/Akt pathway mediates the effects of a variety of extracellular signals in a number of cellular processes including cell growth, proliferation, and survival. The alteration of integrants of this pathway through mutation of its coding genes increases the activation status of the signaling and can thus lead to cellular transformation. The frequent dysregulation of the PI3K/Akt pathway in breast cancer (BC) and the mediation of this pathway in different processes characteristically implicated in tumorigenesis have attracted the interest of this pathway in BC; however, a more comprehensive understanding of the signaling intricacies is necessary to develop clinical applications of the modulation of this pathway in this pathology. We review a series of experiments examining the contribution of alteration of integrants of this signaling network to human BC and we make an update of the information about the effect of the modulation of this pathway in this cancer.

Can phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibition ERase them all?

Seventy percent of breast tumors are estrogen receptor (ER) positive. Although endocrine therapy is successful for the majority of patients with ER-positive tumors, approximately 30% show de novo or acquired resistance and the underlying molecular mechanisms and biomarkers that predict such resistance remain elusive. Two recent papers report that hyperactivation of the phosphatidylinositol 3-kinase (PI3K) pathway produces resistance to tamoxifen. This raises the possibility that combining endocrine therapy and PI3K inhibition may be more effective than monotherapy for treating ER-positive breast tumors, either as first-line therapy for tumors with high PI3K activity or after the development of resistance to endocrine therapy.