Tamoxifen resistance in breast tumors is driven by growth factor receptor signaling with repression of classic estrogen receptor genomic function

The broad-spectrum metalloproteinase inhibitor BB-94 inhibits growth, HER3 and Erk activation in fulvestrant-resistant breast cancer cell lines

Breast cancer cells can switch from estrogen receptor α (ER)- to human epidermal growth factor receptor (HER)-driven cell growth upon acquiring antiestrogen resistance. HER ligands are cleaved by metalloproteinases leading to release of active HER ligands, activation of HER receptors and consequently increased cell growth. In this study, we investigated the importance of HER receptors, in particular HER3, and HER ligand shedding for growth and signaling in human MCF-7 breast cancer cells and MCF-7-derived sublines resistant to the antiestrogen fulvestrant. The HER3/HER4 ligand heregulin 1β induced phosphorylation of HER3, Akt and Erk, and partly rescued fulvestrant-inhibited growth of MCF-7 cells. HER3 ligands were found to be produced and shed from the fulvestrant-resistant cells as conditioned medium from fulvestrant-resistant MCF-7 cells induced phosphorylation of HER3 and Akt in MCF-7 cells. This was prevented by treatment of resistant cells with the metalloproteinase inhibitor TAPI-2. Only the broad-spectrum metalloproteinase inhibitor BB-94, and not the more selective inhibitors GM6001 or TAPI-2, which inhibited shedding of the HER ligands produced by the fulvestrant-resistant cells, was able to inhibit growth and activation of HER3 and Erk in resistant cells. Compared to MCF-7, fulvestrant-resistant cells have increased HER3 phosphorylation, but knockdown of HER3 had no inhibitory effect on resistant cell growth. The EGFR inhibitor gefitinib exhibited only a minor growth inhibition, whereas the pan-HER inhibitor CI-1033 exerted growth arrest. Thus, neither HER3 nor EGFR alone are the main driver of fulvestrant-resistant cell growth and treatment should target both receptors. Ligand shedding is not a treatment target, as receptor activation occurred, independent of release of ligands. Only the broad-spectrum metalloproteinase inhibitor BB-94 could abrogate HER3 and Erk activation in the resistant cells, which stresses the complexity of the resistance mechanisms and the requirement of targeting signaling from HER receptors by multiple strategies.

Epidermal growth factor receptor signalling in human breast cancer cells operates parallel to estrogen receptor α signalling and results in tamoxifen insensitive proliferation

Background

Tamoxifen resistance is a major problem in the treatment of estrogen receptor (ER) α -positive breast cancer patients. Although the mechanisms behind tamoxifen resistance are still not completely understood, clinical data suggests that increased expression of receptor tyrosine kinases is involved. Here, we studied the estrogen and anti-estrogen sensitivity of human breast cancer MCF7 cells that have a moderate, retroviral-mediated, ectopic expression of epidermal growth factor receptor (MCF7-EGFR).

Methods

Proliferation of MCF7-EGFR and parental cells was induced by 17β-estradiol (E2), epidermal growth factor (EGF) or a combination of these. Inhibition of proliferation under these conditions was investigated with 4-hydroxy-tamoxifen (TAM) or fulvestrant at 10^-12 to 10^-6 M. Cells were lysed at different time points to determine the phosphorylation status of EGFR, MAPK_1/3, AKT and the expression of ERα. Knockdown of target genes was established using smartpool siRNAs. Transcriptomics analysis was done 6 hr after stimulation with growth factors using Affymetrix HG-U133 PM array plates.

Results

While proliferation of parental MCF7 cells could only be induced by E2, proliferation of MCF7-EGFR cells could be induced by either E2 or EGF. Treatment with TAM or fulvestrant did significantly inhibit proliferation of MCF7-EGFR cells stimulated with E2 alone. EGF treatment of E2/TAM treated cells led to a marked cell proliferation thereby overruling the anti-estrogen-mediated inhibition of cell proliferation. Under these conditions, TAM however did still inhibit ERα- mediated transcription. While siRNA-mediated knock-down of EGFR inhibited the EGF- driven proliferation under TAM/E2/EGF condition, knock down of ERα did not. The TAM resistant cell proliferation mediated by the conditional EGFR-signaling may be dependent on the PI3K/Akt pathway but not the MEK/MAPK pathway, since a MEK inhibitor (U0126), did not block the proliferation. Transcriptomic analysis under the various E2/TAM/EGF conditions revealed that E2 and EGF dependent transcription have little overlap and rather operate in a parallel fashion.

Conclusions

Our data indicate that enhanced EGFR-driven signalling is sufficient to overrule the TAM- mediated inhibition of E2-driven cell proliferation. This may have profound implications for the anti-estrogen treatment of ER-positive breast cancers that have increased levels of EGFR.

Therapeutic potential of the dual EGFR/HER2 inhibitor AZD8931 in circumventing endocrine resistance

Modest up-regulation of either HER-ligands or receptors has been implicated in acquired endocrine resistance. AZD8931, a dual tyrosine kinase inhibitor (TKI) of epithelial growth factor receptor (EGFR)/HER2, has been shown to more effectively block ligand-dependent HER signaling than the HER TKIs lapatinib or gefitinib. We therefore examined the effect of AZD8931 in ER-positive/HER2-negative breast cancer cells with acquired resistance to tamoxifen, where there is ligand up-regulation associated with HER pathway activation. RNA-seq ligand profiling and levels of HER receptors and signaling by western blotting were conducted in ER+ MCF7 and T47D parental cells and their Tam-resistant derivatives (TamRes). In vitro cell growth and apoptosis and HER ligand-stimulated signaling were measured in response to endocrine and HER TKIs. For studies in vivo, transplantable MCF7/TamRes xenografts were treated with tamoxifen or fulvestrant, either alone or in combination with AZD8931. AZD8931 only minimally enhanced endocrine sensitivity in MCF7 parental cells, but showed a greater effect in the T47D parental model. AZD8931 combined with either tamoxifen or fulvestrant inhibited cell growth more than lapatinib in T47D TamRes cells, and was also significantly, though modestly, more potent in MCF7 TamRes cells. In both TamRes models, AZD8931 significantly inhibited cell proliferation and induced apoptosis. Under ligand-stimulated conditions, AZD8931 more potently inhibited HER signaling than lapatinib or gefitinib. AZD8931 also significantly delayed the growth of MCF7 TamRes xenografts in the presence of tamoxifen or fulvestrant. The strongest inhibition was achieved with a fulvestrant and AZD8931 combination, though no tumor regression was observed. This study provides evidence that AZD8931 has greater inhibitory efficacy in tamoxifen-resistant settings than in an endocrine therapy naïve setting. The absence of tumor regression, however, suggests that additional escape pathways contribute to resistant growth and will need to be targeted to fully circumvent tamoxifen resistance.

Biomarker changes associated with the development of resistance to aromatase inhibitors (AIs) in estrogen receptor-positive breast cancer

BACKGROUND

The purpose of this study was to identify any differences in key biomarkers associated with estrogen action between biopsies taken at diagnosis and at recurrence or progression during treatment with an aromatase inhibitor (AI).

PATIENTS AND METHODS

Patients were retrospectively identified from a clinical database as having relapsed or progressed during AI treatment. Immunohistochemistry was carried out against estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor receptor 2 (HER2), insulin-like growth factor type-1 receptor (IGF1R), insulin receptor substrate-1 (IRS-1), stathmin, phosphatase and tensin homolog and Ki67.

RESULTS

Fifty-five pairs of samples were identified with ER- and/or PgR-positive diseases. Four (7%) patients were ER-negative at progression. Overall, PgR levels were lower in the recurrence sample, but 35% of cases remained positive. IGF1R levels decreased significantly. There were no substantial changes in HER2, IRS-1 or stathmin levels to indicate a role in resistance. Higher Ki67 levels at resistance indicate more proliferative disease.

CONCLUSIONS

The phenotype of AI-recurrent lesions shows high between-tumour heterogeneity. There is evidence of an increase in Ki67, a reduction in IGF1R and a loss of ER expression in some individuals and some activation of growth factor signalling pathways that may explain resistance in individuals and merit treatment targeted to those pathways. Biopsy at recurrence will be necessary to identify the relevant target for individuals.

Functional analysis of MKP-1 and MKP-2 in breast cancer tamoxifen sensitivity

Increased activation of ERK signaling has been reported in breast cancer models of acquired tamoxifen resistance. Here, we examined the expression of Mitogen-Activated Protein Kinase Phosphatases (MKPs) 1 and 2 following tamoxifen treatment and the effects of MKP-1/MKP-2 overexpression on tamoxifen sensitivity. Treatment of MCF7 breast cancer cells with tamoxifen increased MKP-2, but not MKP-1, protein levels. Overexpression of MKP-1 or MKP-2 inhibited estrogen-induced MCF7 cell proliferation compared to vector controls. MCF7-MKP-2 cells displayed significantly increased sensitivity to tamoxifen as compared to vector control or MCF7-MKP-1 cells. MKP-1 or MKP-2 overexpression eliminated ERK1/2 phosphorylation, suggesting that decreases in estrogen-induced proliferation of MKP-1 and MKP-2 overexpressing cells are due to ERK1/2 dephosphorylation. JNK1/2 activation was not detectable in any of these cells. These data suggest that tamoxifen-induced death of these cells is not dependent upon JNK signaling, but rather that ERK is the major MAPK driving their proliferation. MCF7-TAMR cells express higher levels of MKP-2 mRNA and protein than MCF7 cells. MKP-2 and phospho-ERK1/2 proteins are constitutively expressed in MCF7-TAMR cells, and activated JNK1/2 is not detectable. These data suggest that MKP-2 rather than MKP-1 is tamoxifen-regulated and that the elevated expression of MKP-2 in MCF7-TAMR cells potentially functions to restore tamoxifen sensitivity.

Insulin-like growth factor receptor polymorphism defines clinical outcome in estrogen receptor-positive breast cancer patients treated with tamoxifen

Compelling evidence points to a key role for insulin-like growth factor 1 (IGF1) signaling in breast cancer development and progression. In addition, IGF1 receptor (IGF1R) expression has been correlated and functionally linked with estrogen receptor (ER) signaling. Recent translational studies support a cross talk between IGF1R and ERα at different levels and data suggest enhanced IGF1R signaling as a causative mechanism of tamoxifen (TAM) resistance. We tested whether functional germline variations in the IGF pathway are associated with clinical outcome in ER-positive primary invasive breast cancer patients, who were treated with surgery and adjuvant TAM. Tissue samples of 222 patients with ER+ primary invasive breast cancer, who had undergone surgery at Charing Cross Hospital, London, UK between 1981 and 2003, were analyzed. Genomic DNA was extracted from formalin-fixed, paraffin-embedded tissue samples and six functional IGF1 pathway polymorphisms were analyzed using direct DNA sequencing and PCR-restriction fragment length polymorphism. In multivariable analysis, patients with primary invasive breast cancer carrying IGF1R_rs2016347 G allele had a significantly increased risk of early tumor progression (hazard ratio (HR) 2.01; adjusted P=0.004) and death (HR 1.84; adjusted P=0.023) compared with patients carrying G/T or T/T, independent of established clinicopathological determinants. This association remained significant after adjusting for multiple testing. In addition, we were able to demonstrate that IRS1_rs1801123 and IGFBP3_rs2854744 were significantly associated with lymph node involvement and tumor size, respectively. We provide the first evidence for IGF1R_rs2016347 as an independent prognostic marker for ER+ breast cancer patients treated with TAM and support a rational for combined treatment strategies.

Growth factor receptor/steroid receptor cross talk in trastuzumab-treated breast cancer

Treatment with tyrosine kinase inhibitors (TKIs) including trastuzumab has revolutionized the management of HER2-positive breast cancer. Recent evaluation of clinical trial data suggests that a subset of HER2/ER double-positive cancers may not receive significant benefit from the TKI therapy. Here we investigate the cross talk between HER2 and ER in breast cancer and monitor the effect of trastuzumab on the tyrosine kinase effector transcription factor Myc. In HER2-positive breast cancer patients treated with neoadjuvant trastuzumab, steroid receptor-negative status (ER and PR negative) of pre-treatment biopsies predicted pathological complete response (pCR) (n=31 patients, P=0.0486), whereas elevated Myc protein inversely associated with pCR (P=0.0446). Liquid chromatography mass spectrometry identified the corepressor SMRT as a novel Myc-interacting protein. Trastuzumab treatment enhanced Myc-SMRT interactions in HER2-overexpressing breast cancer cells (LCC1) and inhibited expression of the Myc target gene survivin. In HER2-low, ER-positive steroid-dominant cells (MCF7), trastuzumab therapy repressed Myc-SMRT interactions and upregulated survivin expression. Trastuzumab treatment induced ER-CBP interactions, enhanced ER transcriptional activity and upregulated expression of the ER target gene pS2. The absence of pS2 expression in pre-treatment biopsies predicted pCR to neoadjuvant trastuzumab in breast cancer patients (n=25, P=0.0089) and pS2 expression associated with residual cancer burden (P=0.0196). Furthermore, metastatic tissues from patients who had failed trastuzumab therapy were pS2 positive. In HER2-overexpressing cells, trastuzumab treatment can repress Myc transcriptional activity and clinical response is favorable. However, with co-expression of the steroid pathway, this inhibition is lost and response to treatment is often poor.

Progesterone receptor-B enhances estrogen responsiveness of breast cancer cells via scaffolding PELP1- and estrogen receptor-containing transcription complexes

Progesterone and estrogen are important drivers of breast cancer proliferation. Herein, we probed ER-alpha and PR cross-talk in breast cancer models. Stable expression of PR-B in PR-low/ER+ MCF7 cells increased cellular sensitivity to estradiol and IGF1, as measured in growth assays performed in the absence of exogenous progestin; similar results were obtained in PR-null/ER+ T47D cells stably expressing PR-B. Genome-wide microarray analyses revealed that unliganded PR-B induced robust expression of a subset of estradiol-responsive ER-target genes, including CathepsinD (CTSD). Estradiol-treated MCF7 cells stably expressing PR-B exhibited enhanced ER Ser167 phosphorylation and recruitment of ER, PR, and the proline, glutamate and leucine rich protein 1 (PELP1) to an estrogen response element (ERE) in the CTSD distal promoter; this complex co-immunoprecipitated with IGF1R in whole cell lysates. Importantly, ER/PR/PELP1 complexes were also detected in human breast cancer samples. Inhibition of IGF1R or PI3K blocked PR-B-dependent CTSD mRNA upregulation in response to estradiol. Similarly, inhibition of IGF1R or PR significantly reduced ER recruitment to the CTSD promoter. Stable knockdown of endogenous PR or onapristone treatment of multiple unmodified breast cancer cell lines blocked estradiol-mediated CTSD induction, inhibited growth in soft agar, and partially restored tamoxifen-sensitivity of resistant cells. Further, combination treatment of breast cancer cells with both onapristone and IGF1R tyrosine kinase inhibitor AEW541 was more effective than either agent alone. In summary, unliganded PR-B enhanced proliferative responses to estradiol and IGF1 via scaffolding of ERalpha/PELP1/IGF1R-containing complexes. Our data provide a strong rationale for targeting PR in combination with ER and IGF1R in patients with luminal breast cancer.

Breast cancer: current and future endocrine therapies

Endocrine therapy forms a central modality in the treatment of estrogen receptor positive breast cancer. The routine use of 5 years of adjuvant tamoxifen has improved survival rates for early breast cancer, and more recently has evolved in the postmenopausal setting to include aromatase inhibitors. The optimal duration of adjuvant endocrine therapy remains an active area of clinical study with recent data supporting 10 years rather than 5 years of adjuvant tamoxifen. However, endocrine therapy is limited by the development of resistance, this can occur by a number of possible mechanisms and numerous studies have been performed which combine endocrine therapy with agents that modulate these mechanisms with the aim of preventing or delaying the emergence of resistance. Recent trial data regarding the combination of the mammalian target of rapamycin (mTOR) inhibitor, everolimus with endocrine therapy have resulted in a redefinition of the clinical treatment pathway in the metastatic setting. This review details the current endocrine therapy utilized in both early and advanced disease, as well as exploring potential new targets which modulate pathways of resistance, as well as agents which aim to modulate adrenal derived steroidogenic hormones.

PIK3CA mutations in breast cancer: reconciling findings from preclinical and clinical data

PIK3CA mutations represent one of the most common genetic aberrations in breast cancer. They have been reported to be present in over one-third of cases, with enrichment in the luminal and in human epidermal growth factor receptor 2-positive subtypes. Substantial preclinical data on the oncogenic properties of these mutations have been reported. However, whilst the preclinical data have clearly shown an association with robust activation of the pathway and resistance to common therapies used in breast cancer, the clinical data reported up to now do not support that the PIK3CA mutated genotype is associated with high levels of pathway activation or with a poor prognosis. We speculate that this may be due to the minimal use of transgenic mice models thus far. In this review, we discuss both the preclinical and clinical data associated with PIK3CA mutations and their potential implications. Prospective clinical trials stratifying by PIK3CA genotype will be necessary to determine if the mutation also predicts for increased sensitivity to agents targeting the phosphoinositide 3-kinase pathway.

Estrogen response element-GFP (ERE-GFP) introduced MCF-7 cells demonstrated the coexistence of multiple estrogen-deprivation resistant mechanisms

The acquisition of estrogen-deprivation resistance and estrogen receptor (ER) signal-independence in ER-positive breast cancer is one of the crucial steps in advancing the aggressiveness of breast cancer; however, this has not yet been elucidated in detail. To address this issue, we established several estrogen-deprivation-resistant (EDR) breast cancer cell lines from our unique MCF-7 cells, which had been stably transfected with an ERE-GFP reporter plasmid. Three cell lines with high ER activity and another 3 cell lines with no ER activity were established from cell cloning by monitoring GFP expression in living cells. The former three ERE-GFP-positive EDR cell lines showed the overexpression of ER and high expression of several ER-target genes. Further analysis of intracellular signaling factors revealed a marked change in the phosphorylation status of ERα on Ser167 and Akt on Thr308 by similar mechanisms reported previously; however, we could not find any changes in MAP-kinase factors. Comprehensive phospho-proteomic analysis also indicated the possible contribution of the Akt pathway to the phosphorylation of ERα. On the other hand, constitutive activation of c-Jun N-terminal kinase (JNK) was observed in ERE-GFP-negative EDR cells, and the growth of these cells was inhibited by a JNK inhibitor. An IGF1R-specific inhibitor diminished the phosphorylation of JNK, which suggested that a novel signaling pathway, IGF1R-JNK, may be important for the proliferation of ER-independent MCF-7 cells. These results indicate that ER-positive breast cancer cells can acquire resistance by more than two mechanisms at a time, which suggests that multiple mechanisms may occur simultaneously. This finding also implies that breast cancers with different resistance mechanisms can concomitantly occur and mingle in an individual patient, and may be a cause of the recurrence of cancer.

Overview of diagnostic/targeted treatment combinations in personalized medicine for breast cancer patients

Breast cancer includes a body of molecularly distinct subgroups, characterized by different presentation, prognosis, and sensitivity to treatments. Significant advances in our understanding of the complex architecture of this pathology have been achieved in the last few decades, thanks to new biotechnologies that have recently come into the research field and the clinical practice, giving oncologists new instruments that are based on biomarkers and allowing them to set up a personalized approach for each individual patient. Here we review the main treatments available or in preclinical development, the biomolecular diagnostic and prognostic approaches that changed our perspective about breast cancer, giving an overview of targeted therapies that represent the current standard of care for these patients. Finally, we report some examples of how new technologies in clinical practice can set in motion the development of new drugs.

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.

Cancer cells resistant to therapy promote cell surface relocalization of GRP78 which complexes with PI3K and enhances PI(3,4,5)P3 production

Traditionally, GRP78 has been regarded as an endoplasmic reticulum (ER) lumenal protein due to its carboxyl KDEL retention motif. Recently, a subfraction of GRP78 is found to localize to the surface of specific cell types, serving as co-receptors and regulating signaling. However, the physiological relevance of cell surface GRP78 (sGRP78) expression in cancer and its functional interactions at the cell surface are just emerging. In this report, we combined biochemical, imaging and mutational approaches to address these issues. For detection of sGRP78, we utilized a mouse monoclonal antibody highly potent and specific for GRP78 or epitope-tagged GRP78, coupled with imaging and biochemical techniques that allowed detection of sGRP78 but not intracellular GRP78. Our studies revealed that breast and prostate cancer cells resistant to hormonal therapy actively promote GRP78 to the cell surface, which can be further elevated by a variety of ER stress-inducing conditions. We showed that sGRP78 forms complex with PI3K, and overexpression of sGRP78 promotes PIP3 formation, indicative of PI3K activation. We further discovered that an insertion mutant of GRP78 at its N-terminus domain, while retaining stable expression and the ability to translocate to the cell surface as the wild-type protein, exhibited reduced complex formation with p85 and production of PIP3. Thus, our studies provide a mechanistic explanation for the regulation of the PI3K/AKT signaling by sGRP78. Our findings suggest that targeting sGRP78 may suppress therapeutic resistance in cancer cells and offer a novel strategy to suppress PI3K activity.

Quantitative ER and PgR assessment as predictors of benefit from lapatinib in postmenopausal women with hormone receptor-positive, HER2-negative metastatic breast cancer

PURPOSE

Lapatinib, a dual epidermal growth factor receptor (EGFR) and HER2 inhibitor, remains unproven in non-HER2-amplified metastatic breast cancer (MBC). EGF30008, a phase III trial of letrozole and lapatinib versus letrozole and placebo, demonstrated that lapatinib significantly improves outcome for postmenopausal women with HER2-amplified, but not HER2-negative, MBC. The hypothesis that low hormone receptor status is associated with benefit in this HER2-negative cohort was tested.

EXPERIMENTAL DESIGN

A blinded retrospective biomarker evaluation used immunohistochemistry (IHC) to semiquantify estrogen receptor (ER) and progesterone receptor (PgR) expression (n = 821/952). HER2 status was determined by IHC and confirmed by FISH (n = 326). Effects of these biomarkers on progression-free survival (PFS) were examined in patients with available tissue.

RESULTS

In HER2-negative, ER-positive MBC, median PFS was analyzed by ER and PgR expression (H-score) by quartile (Q). There was significant improvement in patients with low ER expression (Q1, H-score <160) with lapatinib and letrozole (13.6 vs. 6.7 months; P = 0.01). No benefit was associated with stronger ER expression (Q2/3, H-score ≥ 160 and <250; 13.6 vs. 14.2 months; Q4, H-score ≥ 250; 11.2 vs. 14.2 months). There was no association between PgR H-score and benefit from lapatinib.

CONCLUSION

In postmenopausal patients with advanced hormone receptor-positive disease, weak ER expression is associated with worse outcome with letrozole treatment compared with the combination. The addition of lapatinib significantly improved PFS for this patient subgroup and augments data supporting interaction between steroid hormone and peptide hormone signaling. A prospective study validating this hypothesis is required.

Targeting receptor tyrosine kinases in HER2-negative breast cancer

PURPOSE OF REVIEW

The targeting of receptor tyrosine kinases (RTKs) has been a major area for breast cancer therapy, exemplified by the targeting of HER2-amplified breast cancer.

RECENT FINDINGS

We review the data on the activation of RTKs in HER2-negative breast cancer, and discuss the clinical translational challenge of identifying cancers that are reliant on a specific kinase for growth and survival. Substantial evidence suggests that subsets of breast cancer may be reliant on specific kinases, and that this could be exploited therapeutically. The heterogeneity of breast cancer, however, and the potential for adaptive switching between RTKs after inhibition of a single RTK, present challenges to targeting individual RTKs in the clinic

SUMMARY

Targeting of RTKs in HER2-negative breast cancer presents a major therapeutic opportunity in breast cancer, although robust selection strategies will be required to identify cancers with activation of specific RTKs if this potential is to be realized.

Do type 1 receptor tyrosine kinases inform treatment choice? A prospectively planned analysis of the TEAM trial

BACKGROUND

Epidermal growth factor receptors contribute to breast cancer relapse during endocrine therapy. Substitution of aromatase inhibitors (AIs) may improve outcomes in HER-positive cancers.

METHODS

Tissue microarrays were constructed. Quantitative analysis of HER1, HER2, and HER3 was performed. Data were analysed relative to disease-free survival and treatment using outcomes at 2.75 and 6.5 years.

RESULTS

Among 4541 eligible samples, 4225 (93%) had complete HER1-3 data. Overall, 5% were HER1-positive, 13% HER2-positive, and 21% HER3-positive; 32% (n=1351) overexpressed at least one HER receptor. In the HER1-3-negative subgroup, the hazard ratio (HR) for upfront exemestane vs tamoxifen at 2.75 years was 0.67 (95% confidence interval (CI), 0.52-0.87), in the HER1-3-positive subgroup, the HR was 1.15 (95% CI, 0.85-1.56). A prospectively planned treatment-by-marker analysis demonstrated a significant interaction between HER1-3 and treatment at 2.75 years (HR=0.58; 95% CI, 0.39-0.87; P=0.008), as confirmed by multivariate regression analysis adjusting for prognostic factors (HR=0.55; 95% CI, 0.36-0.85; P=0.005). This effect was time dependent.

CONCLUSION

In the 2.75 years prior to switching patients initially treated with tamoxifen to exemestane, a significant treatment-by-marker effect exists between AI/tamoxifen treatment and HER1-3 expression, suggesting HER expression could be used to select appropriate endocrine treatment at diagnosis to prevent or delay early relapses.

Biology and therapeutic potential of PI3K signaling in ER+/HER2-negative breast cancer

PI3K is a central node mediating growth factor receptor signaling. With its downstream effectors such as AKT and mTOR, and its crosstalk with the RAS/RAF/MEK/MAPK pathway, it plays a vital role in cancer cell proliferation, metabolism, and survival. Recent breast cancer (BC) molecular portraits delineate PI3K as the most frequently altered pathway, with recurrent PIK3CA mutations mostly found in the luminal subtypes of BC. The transcriptomic and proteomic signatures of PI3K pathway activation associate with reduced estrogen receptor α (ER) levels and activity, and with the luminal B subtype of BC that has a relatively poor outcome. However, oncogenic transforming PIK3CA mutations have been shown to predict a better outcome in ER+/HER2-negative BC treated with endocrine therapy. In this review, we summarize the recent findings in the cause-and-effect of PI3K pathway aberration and endocrine sensitivity, especially the crosstalk with the ER pathway. Potential therapeutic approaches based on these findings are also discussed.

Cross-talk between GPER and growth factor signaling

G protein-coupled receptors (GPCRs) and growth factor receptors mediate multiple physio-pathological responses to a diverse array of extracellular stimuli. In this regard, it has been largely demonstrated that GPCRs and growth factor receptors generate a multifaceted signaling network, which triggers relevant biological effects in normal and cancer cells. For instance, some GPCRs transactivate the epidermal growth factor receptor (EGFR), which stimulates diverse transduction pathways leading to gene expression changes, cell migration, survival and proliferation. Moreover, it has been reported that a functional interaction between growth factor receptors and steroid hormones like estrogens is involved in the growth of many types of tumors as well as in the resistance to endocrine therapy. This review highlights recent findings on the cross-talk between a member of the GPCR family, the G protein-coupled estrogen receptor 1 (GPER, formerly known as GPR30) and two main growth factor receptors like EGFR and insulin-like growth factor-I receptor (IGF-IR). The biological implications of the functional interaction between these important mediators of cell responses particularly in cancer are discussed. This article is part of a Special Issue entitled 'CSR 2013'.

Growth hormone potentiates 17β-estradiol-dependent breast cancer cell proliferation independently of IGF-I receptor signaling

Estrogen action in mammary gland development and breast cancer progression is tightly linked to the GH/IGF-I axis. Although many of the effects of GH on mammary gland growth and development require IGF-I, the extent to which GH action in breast cancer depends on IGF-I is not known. We examined GH action in a panel of estrogen receptor-positive breast cancer cell lines and found that T47D cells express significant levels of GH receptor and that GH significantly enhances 17β-estradiol (E2)-stimulated proliferation in these cells. GH action in the T47D cells was independent of changes in IGF-I and IGF-I receptor (IGF-IR) expression and IGF-IR signaling, suggesting that GH can exert direct effects on breast cancer cells. Although E2-dependent proliferation required IGF-IR signaling, the combination of GH+E2 overcame inhibition of IGF-IR activity to restore proliferation. In contrast, GH required both Janus kinase 2 and epidermal growth factor receptor signaling for subsequent ERK activation and potentiation of E2-dependent proliferation. Downstream of these pathways, we identified a number of immediate early-response genes associated with proliferation that are rapidly and robustly up-regulated by GH. These findings demonstrate that GH can have important effects in breast cancer cells that are distinct from IGF-IR activity, suggesting that novel drugs or improved combination therapies targeting estrogen receptor and the GH/IGF axis may be beneficial for breast cancer patients.

Post-transcriptional controls by ribonucleoprotein complexes in the acquisition of drug resistance

Acquisition of drug resistance leads to failure of anti-cancer treatments and therapies. Although several successive chemotherapies are available, along with efforts towards clinical applications of new anti-cancer drugs, it is generally realized that there is a long way to go to treat cancers. Resistance to anti-cancer drugs results from various factors, including genetic as well as epigenetic differences in tumors. Determining the molecular and cellular mechanisms responsible for the acquisition of drug resistance may be a helpful approach for the development of new therapeutic strategies to overcome treatment failure. Several studies have shown that the acquisition of drug resistance is tightly regulated by post-transcriptional regulators such as RNA binding proteins (RBPs) and microRNAs (miRNAs), which change the stability and translation of mRNAs encoding factors involved in cell survival, proliferation, epithelial-mesenchymal transition, and drug metabolism. Here, we review our current understanding of ribonucleoprotein complexes, including RBPs and miRNAs, which play critical roles in the acquisition of drug resistance and have potential clinical implications for cancer.

Global characterization of signalling networks associated with tamoxifen resistance in breast cancer

Acquired resistance to the anti-estrogen tamoxifen remains a significant challenge in breast cancer management. In this study, we used an integrative approach to characterize global protein expression and tyrosine phosphorylation events in tamoxifen-resistant MCF7 breast cancer cells (TamR) compared with parental controls. Quantitative mass spectrometry and computational approaches were combined to identify perturbed signalling networks, and candidate regulatory proteins were functionally interrogated by siRNA-mediated knockdown. Network analysis revealed that cellular metabolism was perturbed in TamR cells, together with pathways enriched for proteins associated with growth factor, cell-cell and cell matrix-initiated signalling. Consistent with known roles for Ras/MAPK and PI3-kinase signalling in tamoxifen resistance, tyrosine-phosphorylated MAPK1, SHC1 and PIK3R2 were elevated in TamR cells. Phosphorylation of the tyrosine kinase Yes and expression of the actin-binding protein myristoylated alanine-rich C-kinase substrate (MARCKS) were increased two- and eightfold in TamR cells respectively, and these proteins were selected for further analysis. Knockdown of either protein in TamR cells had no effect on anti-estrogen sensitivity, but significantly decreased cell motility. MARCKS expression was significantly higher in breast cancer cell lines than normal mammary epithelial cells and in ER-negative versus ER-positive breast cancer cell lines. In primary breast cancers, cytoplasmic MARCKS staining was significantly higher in basal-like and HER2 cancers than in luminal cancers, and was independently predictive of poor survival in multivariate analyses of the whole cohort (P < 0.0001) and in ER-positive patients (P = 0.0005). These findings provide network-level insights into the molecular alterations associated with the tamoxifen-resistant phenotype, and identify MARCKS as a potential biomarker of therapeutic responsiveness that may assist in stratification of patients for optimal therapy.

Expression and activation of P38 MAP kinase in invasive ductal breast cancers: correlation with expression of the estrogen receptor, HER2 and downstream signaling phosphorylated proteins

MAP kinase signaling proteins have major implications in the molecular oncogenesis of breast cancers and have been extensively investigated as putative targets for therapy. This study reports the investigation of the expression of P38 MAPK and its phosphorylated form (p-P38 MAPK) in clinical specimens of invasive breast carcinomas and their correlation with estrogen receptor (ER) and HER2 expression, as well as MAPK and PI3 kinase-AKT pathway signaling phosphorylated proteins. Expression levels of P38 MAPK and p-P38 MAPK as well as p-AKT, p-GSK3β, p-S6 kinase, p-MEK1 and p-ERK1/2 were quantitatively assessed using multiplex bead immunoassay in frozen specimens from 45 invasive ductal breast cancers. Twenty-nine specimens were ER+, 15 were HER2+ and 10 were triple‑negative breast cancers (TNBCs). P38 MAPK was found to be expressed in all tumor specimens and was significantly (P=0.002) overexpressed in ER+ tumors. P38 MAPK expression was lower in TNBCs than in all of the other tumors. The median expression of p-P38 MAPK was also higher in ER+ tumors while lower in the TNBCs. HER2 status had no effect on P38 MAPK and p-P38 MAPK expression. No variation in the phosphorylation rate of P38 MAPK was observed in relation with ER, HER2 or TNBC status. Significantly higher (P=0.0048) expression of p-AKT was observed in HER2+ tumors. No significant difference in p-MEK1, p-GSK3β and p-S6K expression was found in any other comparisons based on ER and HER2 expression subtypes. Investigation of the expression of multiple phosphorylated signaling proteins can be used for personalized targeted therapy. In invasive breast cancer, the overexpression of P38 MAPK may serve as a biomarker for the evaluation of P38 MAPK inhibitors.

c-Src modulates estrogen-induced stress and apoptosis in estrogen-deprived breast cancer cells

The emergence of anti-estrogen resistance in breast cancer is an important clinical phenomenon affecting long-term survival in this disease. Identifying factors that convey cell survival in this setting may guide improvements in treatment. Estrogen (E2) can induce apoptosis in breast cancer cells that have been selected for survival after E2 deprivation for long periods (MCF-7:5C cells), but the mechanisms underlying E2-induced stress in this setting have not been elucidated. Here, we report that the c-Src kinase functions as a key adapter protein for the estrogen receptor (ER, ESR1) in its activation of stress responses induced by E2 in MCF-7:5C cells. E2 elevated phosphorylation of c-Src, which was blocked by 4-hydroxytamoxifen (4-OHT), suggesting that E2 activated c-Src through the ER. We found that E2 activated the sensors of the unfolded protein response (UPR), IRE1α (ERN1) and PERK kinase (EIF2AK3), the latter of which phosphorylates eukaryotic translation initiation factor-2α (eIF2α). E2 also dramatically increased reactive oxygen species production and upregulated expression of heme oxygenase HO-1 (HMOX1), an indicator of oxidative stress, along with the central energy sensor kinase AMPK (PRKAA2). Pharmacologic or RNA interference-mediated inhibition of c-Src abolished the phosphorylation of eIF2α and AMPK, blocked E2-induced ROS production, and inhibited E2-induced apoptosis. Together, our results establish that c-Src kinase mediates stresses generated by E2 in long-term E2-deprived cells that trigger apoptosis. This work offers a mechanistic rationale for a new approach in the treatment of endocrine-resistant breast cancer.

Sex steroid hormones regulate constitutive expression of Cyp2e1 in female mouse liver

CYP2E1 is of paramount toxicological significance because it metabolically activates a large number of low-molecular-weight toxicants and carcinogens. In this context, factors that interfere with Cyp2e1 regulation may critically affect xenobiotic toxicity and carcinogenicity. The aim of this study was to investigate the role of female steroid hormones in the regulation of CYP2E1, as estrogens and progesterone are the bases of contraceptives and hormonal replacement therapy in menopausal women. Interestingly, a fluctuation in the hepatic expression pattern of Cyp2e1 was revealed in the different phases of the estrous cycle of female mice, with higher Cyp2e1 expression at estrus (E) and lower at methestrus (ME), highly correlated with that in plasma gonadal hormone levels. Depletion of sex steroids by ovariectomy repressed Cyp2e1 expression to levels similar to those detected in males and cyclic females at ME. Hormonal supplementation brought Cyp2e1 expression back to levels detected at E. The role of progesterone appeared to be more prominent than that of 17β-estradiol. Progesterone-induced Cyp2e1 upregulation could be attributed to inactivation of the insulin/PI3K/Akt/FOXO1 signaling pathway. Tamoxifen, an anti-estrogen, repressed Cyp2e1 expression potentially via activation of the PI3K/Akt/FOXO1 and GH/STAT5b-linked pathways. The sex steroid hormone-related changes in hepatic Cyp2e1 expression were highly correlated with those observed in Hnf-1α, β-catenin, and Srebp-1c. In conclusion, female steroid hormones are clearly involved in the regulation of CYP2E1, thus affecting the metabolism of a plethora of toxicants and carcinogenic agents, conditions that may trigger several pathologies or exacerbate the outcomes of various pathophysiological states.

Reduced expression of miR-200 family members contributes to antiestrogen resistance in LY2 human breast cancer cells

INTRODUCTION

The role of miRNAs in acquired endocrine-resistant breast cancer is not fully understood. One hallmark of tumor progression is epithelial-to-mesenchymal transition (EMT), characterized by a loss of cell adhesion resulting from reduced E-cadherin and increased cell mobility. miR-200 family members regulate EMT by suppressing expression of transcriptional repressors ZEB1/2. Previously we reported that the expression of miR-200a, miR-200b, and miR-200c was lower in LY2 endocrine-resistant, mesenchymal breast cancer cells compared to parental, endocrine sensitive, epithelial MCF-7 breast cancer cells. Here we investigated the regulation of miR-200 family members and their role in endocrine-sensitivity in breast cancer cells.

RESULTS

miR-200 family expression was progressively reduced in a breast cancer cell line model of advancing endocrine/tamoxifen (TAM) resistance. Concomitant with miR-200 decrease, there was an increase in ZEB1 mRNA expression. Overexpression of miR-200b or miR-200c in LY2 cells altered cell morphology to a more epithelial appearance and inhibited cell migration. Further, miR-200b and miR-200c overexpression sensitized LY2 cells to growth inhibition by estrogen receptor (ER) antagonists TAM and fulvestrant. Knockdown of ZEB1 in LY2 cells recapitulated the effect of miR-200b and miR-200c overexpression resulting in inhibition of LY2 cell proliferation by TAM and fulvestrant, but not the aromatase inhibitor exemestane. Demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) in combination with histone deacetylase inhibitor trichostatin A (TSA) increased miR-200b and miR-200c in LY2 cells. Concomitant with the increase in miR-200b and miR-200c, ZEB1 expression was decreased and cells appeared more epithelial in morphology and were sensitized to TAM and fulvestrant inhibition. Likewise, knockdown of ZEB1 increased antiestrogen sensitivity of LY2 cells resulting in inhibition of cell proliferation.

CONCLUSIONS

Our data indicate that reduced miRNA-200b and miR-200c expression contributes to endocrine resistance in breast cancer cells and that the reduced expression of these miR-200 family members in endocrine-resistant cells can be reversed by 5-aza-dC+TSA.

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.

Tamoxifen resistance: from bench to bedside

Although tamoxifen is a classical example of a targeted drug, a substantial proportion of estrogen receptor alpha positive breast cancer patients does not benefit from the drug. Over the last few decades, many potential biomarkers have been discovered in cell biological studies that may aid in the prediction of tamoxifen sensitivity and guide in treatment selection. Nonetheless, the transition of such a biomarker from the scientific community towards a diagnostic test that can be used in daily clinical practice has been far from ideal, and such markers seldom face clinical introduction. From a large number of potential predictive biomarkers as described in cell biological literature, the clinical (translational) scientist has to make a decision which of these biomarkers should be tested in clinical material to determine their clinical validity. This problem is not trivial, since patient samples with clinical follow-up are a valuable asset that should therefore be cherished. In this review, we will describe a number of 'cell biological biomarkers' for tamoxifen resistance and their possible clinical implications. This may guide the clinical scientist in choosing what potential biomarkers to test on tumour samples, which may catalyse the translation of scientific discoveries into daily clinical practice of breast cancer medicine.

Phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin pathway inhibition: a breakthrough in the management of luminal (ER+/HER2-) breast cancers?

PURPOSE OF REVIEW

Recent data from clinical trials evaluating mammalian target of rapamycin (mTOR) inhibitors in the setting of endocrine resistance in luminal (estrogen receptor-positive, human epidermal growth factor receptor 2-negative) breast cancers have validated this pathway as a bona-fide therapeutic target in this setting. There are currently many agents under clinical investigation that inhibit the phosphatidylinositol 3-kinase (PI3K) pathway. We review these findings in the context of the preclinical data and the current status of biomarker development in this field.

RECENT FINDINGS

Clinical trials in the neoadjuvant (RAD2222) and metastatic setting (TAMRAD, BOLERO-2) have reported improved clinical outcome of patients with unselected luminal breast cancer through the addition of mTOR inhibitors to standard endocrine treatment. PI3K molecular aberrations are frequently found in luminal breast cancer, yet the role of these in defining patients' prognosis and response to PI3K/AKT/mTOR inhibitors remains to be determined.

SUMMARY

Therapeutic targeting of the PI3K pathway promises improved clinical outcome for patients with luminal breast cancer. Correspondingly, agents that target this pathway are entering the clinic at an unprecedented rate. Future clinical trials that incorporate correlative translational research will help us decipher important information critical for successful development of these agents in breast cancer: which part of the pathway should be targeted and in which clinical scenario; and which patients are more likely to benefit from these drugs, particularly in the adjuvant setting.

Bazedoxifene exhibits antiestrogenic activity in animal models of tamoxifen-resistant breast cancer: implications for treatment of advanced disease

PURPOSE

There is compelling evidence to suggest that drugs that function as pure estrogen receptor (ER-α) antagonists, or that downregulate the expression of ER-α, would have clinical use in the treatment of advanced tamoxifen- and aromatase-resistant breast cancer. Although such compounds are currently in development, we reasoned, based on our understanding of ER-α pharmacology, that there may already exist among the most recently developed selective estrogen receptor modulators (SERM) compounds that would have usage as breast cancer therapeutics. Thus, our objective was to identify among available SERMs those with unique pharmacologic activities and to evaluate their potential clinical use with predictive models of advanced breast cancer.

EXPERIMENTAL DESIGN

A validated molecular profiling technology was used to classify clinically relevant SERMs based on their impact on ER-α conformation. The functional consequences of these observed mechanistic differences on (i) gene expression, (ii) receptor stability, and (iii) activity in cellular and animal models of advanced endocrine-resistant breast cancer were assessed.

RESULTS

The high-affinity SERM bazedoxifene was shown to function as a pure ER-α antagonist in cellular models of breast cancer and effectively inhibited the growth of both tamoxifen-sensitive and -resistant breast tumor xenografts. Interestingly, bazedoxifene induced a unique conformational change in ER-α that resulted in its proteasomal degradation, although the latter activity was dispensable for its antagonist efficacy.

CONCLUSION

Bazedoxifene was recently approved for use in the European Union for the treatment of osteoporosis and thus may represent a near-term therapeutic option for patients with advanced breast cancer.

Estrogen receptor-α36 is involved in development of acquired tamoxifen resistance via regulating the growth status switch in breast cancer cells

Acquired tamoxifen (TAM) resistance limits the therapeutic benefit of TAM in patients with hormone-dependent breast cancer. The switch from estrogen-dependent to growth factor-dependent growth is a critical step in this process. However, the molecular mechanisms underlying this switch remain poorly understood. In this study, we established a TAM resistant cell sub line (MCF-7/TAM) from estrogen receptor-α (ER-α66) positive breast cancer MCF-7 cells by culturing ER-α66-positive MCF-7 cells in medium plus 1 μM TAM over 6 months. MCF-7/TAM cells were then found to exhibit accelerated proliferation rate together with enhanced in vitro migratory and invasive ability. And the estrogen receptor-α36 (ER-α36), a novel 36-kDa variant of ER-α66, was dramatically overexpressed in this in vitro model, compared to the parental MCF-7 cells. Meanwhile, the expression of epidermal growth factor receptor (EGFR) in MCF-7/TAM cells was significantly up-regulated both in mRNA level and protein level, and the expression of ER-α66 was greatly down-regulated oppositely. In the subsequent studies, we overexpressed ER-α36 in MCF-7 cells by stable transfection and found that ER-α36 transfected MCF-7 cells (MCF-7/ER-α36) similarly exhibited decreased sensitivity to TAM, accelerated proliferative rate and enhanced in vitro migratory and invasive ability, compared to empty vector transfected MCF-7 cells (MCF-7/V). Real-time qPCR and Western blotting analysis revealed that MCF-7/ER-α36 cells possessed increased EGFR expression but decreased ER-α66 expression both in mRNA level and protein level, compared to MCF-7/V cells. This change in MCF-7/ER-α36 cells could be reversed by neutralizing anti-ER-α36 antibody treatment. Furthermore, knock-down of ER-α36 expression in MCF-7/TAM cells resulted in reduced proliferation rate together with decreased in vitro migratory and invasive ability. Decreased EGFR mRNA and protein expression as well as increased ER-α66 mRNA expression were also observed in MCF-7/TAM cells with down-regulated ER-α36 expression. In addition, blocking EGFR/ERK signaling in MCF-7/ER-α36 cells could restore the expression of ER-α66 partly, suggesting a regulatory function of EGFR/ERK signaling in down-regulation of ER-α66 expression. In conclusion, our results indicated for the first time a regulatory role of ER-α36 in up-regulation of EGFR expression and down-regulation of ER-α66 expression, which could be an underlying mechanism for the growth status switch in breast tumors that contribute to the generation of acquired TAM resistance. And ER-α36 could be considered a potential new therapeutic target in breast tumors which have acquired resistance to TAM.

Inhibition of p38 mitogen-activated protein kinase alters microRNA expression and reverses epithelial-to-mesenchymal transition

Acquired chemoresistance and epithelial-to-mesenchymal transition (EMT) are hallmarks of cancer progression and of increasing clinical relevance. We investigated the role of miRNA and p38 mitogen-activated protein kinase (MAPK) signaling in the progression of breast cancer to a drug-resistant and mesenchymal phenotype. We demonstrate that acquired death receptor resistance results in increased hormone-independent tumorigenesis compared to hormone-sensitive parental cells. Utilizing global miRNA gene expression profiling, we identified miRNA alterations associated with the development of death receptor resistance and EMT progression. We further investigated the role of p38 MAPK in this process, showing dose-dependent inactivation of p38 by its inhibitor RWJ67657 and decreased downstream ATF and NF-κB signaling. Pharmacological inhibition of p38 also decreased chemoresistant cancer tumor growth in xenograft animal models. Interestingly, inhibition of p38 partially reversed the EMT changes found in this cell system, as illustrated by decreased gene expression of the EMT markers Twist, Snail, Slug and ZEB and protein and mRNA levels of Twist, a known EMT promoter, concomitant with decreased N-cadherin protein. RWJ67657 treatment also altered the expression of several miRNAs known to promote therapeutic resistance, including miR-200, miR-303, miR-302, miR-199 and miR-328. Taken together, our results demonstrate the roles of multiple microRNAs and p38 signaling in the progression of cancer and demonstrate the therapeutic potential of targeting the p38 MAPK pathway for reversing EMT in an advanced tumor phenotype.

Overcoming acquired resistance to anticancer therapy: focus on the PI3K/AKT/mTOR pathway

BACKGROUND

Most targeted anticancer therapies, as well as cytotoxic and radiation therapies, are encumbered by the development of secondary resistance by cancer cells. Resistance is a complex phenomenon involving multiple mechanisms, including activation of signaling pathways such as phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR). Novel strategies to overcome resistance by targeting these signaling pathways are being evaluated.

METHODS

PubMed and key cancer congress abstracts were searched until July 2012 for preclinical and clinical data relating to the PI3K/AKT/mTOR pathway and anticancer treatment resistance, and use of PI3K/AKT/mTOR inhibitors in resistant cancer cell lines and patient populations.

RESULTS

Activation of the PI3K/AKT/mTOR pathway is frequently implicated in resistance to anticancer therapies, including biologics, tyrosine kinase inhibitors, radiation, and cytotoxics. As such, inhibitors of the PI3K/AKT/mTOR pathway are being rapidly evaluated in preclinical models and in clinical studies to determine whether they can restore therapeutic sensitivity when given in combination. In breast cancer, non-small-cell lung cancer, and glioblastoma, we find compelling preclinical evidence to show that inhibitors of PI3K or mTOR can restore sensitivity in resistant cells. Although clinical evidence is less mature, a recent Phase III study with the mTORC1 inhibitor everolimus in patients with advanced breast cancer resistant to aromatase inhibition and several Phase I/II studies with PI3K inhibitors demonstrate proof-of-concept, warranting future clinical evaluation.

CONCLUSION

Current preclinical and clinical evidence suggest that inhibitors of the PI3K/AKT/mTOR pathway could have utility in combination with other anticancer therapies to circumvent resistance by cancer cells. Multiple clinical studies are ongoing.

Multiple breast cancer cell-lines derived from a single tumor differ in their molecular characteristics and tumorigenic potential

BACKGROUND

Breast cancer cell lines are widely used tools to investigate breast cancer biology and to develop new therapies. Breast cancer tissue contains molecularly heterogeneous cell populations. Thus, it is important to understand which cell lines best represent the primary tumor and have similarly diverse phenotype. Here, we describe the development of five breast cancer cell lines from a single patient's breast cancer tissue. We characterize the molecular profiles, tumorigenicity and metastatic ability in vivo of all five cell lines and compare their responsiveness to 4-hydroxytamoxifen (4-OHT) treatment.

METHODS

Five breast cancer cell lines were derived from a single patient's primary breast cancer tissue. Expression of different antigens including HER2, estrogen receptor (ER), CK8/18, CD44 and CD24 was determined by flow cytometry, western blotting and immunohistochemistry (IHC). In addition, a Fluorescent In Situ Hybridization (FISH) assay for HER2 gene amplification and p53 genotyping was performed on all cell lines. A xenograft model in nude mice was utilized to assess the tumorigenic and metastatic abilities of the breast cancer cells.

RESULTS

We have isolated, cloned and established five new breast cancer cell lines with different tumorigenicity and metastatic abilities from a single primary breast cancer. Although all the cell lines expressed low levels of ER, their growth was estrogen-independent and all had high-levels of expression of mutated non-functional p53. The HER2 gene was rearranged in all cell lines. Low doses of 4-OHT induced proliferation of these breast cancer cell lines.

CONCLUSIONS

All five breast cancer cell lines have different antigenic expression profiles, tumorigenicity and organ specific metastatic abilities although they derive from a single tumor. None of the studied markers correlated with tumorigenic potential. These new cell lines could serve as a model for detailed genomic and proteomic analyses to identify mechanisms of organ-specific metastasis of breast cancer.

Genome-wide mapping of FOXM1 binding reveals co-binding with estrogen receptor alpha in breast cancer cells

BACKGROUND

The forkhead transcription factor FOXM1 is a key regulator of the cell cycle. It is frequently over-expressed in cancer and is emerging as an important therapeutic target. In breast cancer FOXM1 expression is linked with estrogen receptor (ERα) activity and resistance to endocrine therapies, with high levels correlated with poor prognosis. However, the precise role of FOXM1 in ER positive breast cancer is not yet fully understood.

RESULTS

The study utilizes chromatin immunoprecipitation followed by high-throughput sequencing to map FOXM1 binding in both ERα-positive and -negative breast cancer cell lines. The comparison between binding site distributions in the two cell lines uncovered a previously undescribed relationship between binding of FOXM1 and ERα. Further molecular analyses demonstrated that these two factors can bind simultaneously at genomic sites and furthermore that FOXM1 regulates the transcriptional activity of ERα via interaction with the coactivator CARM1. Inhibition of FOXM1 activity using the natural product thiostrepton revealed down-regulation of a set of FOXM1-regulated genes that are correlated with patient outcome in clinical breast cancer samples.

CONCLUSIONS

These findings reveal a novel role for FOXM1 in ERα transcriptional activity in breast cancer and uncover a FOXM1-regulated gene signature associated with ER-positive breast cancer patient prognosis.

Endocrine resistance: what do we know?

Adjuvant therapy with antiestrogens targeting estrogen receptor α (ER) signaling prevents disease recurrence in many patients with early-stage ER+ breast cancer. However, a significant number of cases exhibit de novo or acquired endocrine resistance. While other clinical subtypes of breast cancer (HER2+, triple-negative) have disproportionately higher rates of mortality, ER+ breast cancer is responsible for at least as many deaths because it is the most common subtype. Therefore, identifying mechanisms that drive endocrine resistance is a high clinical priority. A large body of experimental evidence indicates that oncogenic signaling pathways underlie endocrine resistance, including growth factor receptor tyrosine kinases (HER2, epidermal growth factor receptor [EGFR], fibroblast growth factor receptor 1/2 [FGFR], insulin-like growth factor-1 receptor [IGF-1R]/ insulin receptor [InsR]), PI3K/AKT/ mTOR, MAPK/ERK, Src, CDK4/CDK6, and ER itself. Combined targeting of ER and such pathways may be the most effective means to combat antiestrogen resistance, and clinical trials testing such strategies show promising results. Herein, we discuss pathways associated with endocrine resistance, biomarkers that may be useful to predict response to targeted agents, and avenues for further exploration to identify strategies for the treatment of patients with endocrine-resistant disease.

Treatment algorithms for hormone receptor-positive advanced breast cancer: going forward in endocrine therapy—overcoming resistance and introducing new agents

Overcoming de novo or acquired endocrine resistance remains critical to further enhancing the benefit of existing endocrine therapies. Recent progress has been made in understanding the molecular biology associated with acquired endocrine resistance, including adaptive "cross-talk" between ER and various growth factor receptor and cell-signaling pathways. Strategies that combine endocrine therapy with targeted inhibitors of growth factor receptors or cell-survival pathways to further enhance first-line response have largely been disappointing, suggesting that any attempts to prevent endocrine resistance by blocking specific pathways from the outset will be futile. In contrast, success has been seen by selecting patients with acquired endocrine resistance and enhancing response to further endocrine therapy by the addition of mTOR antagonists. Numerous other therapeutics are being evaluated in combination with endocrine therapies based on varying levels of preclinical science to support their use, including inhibitors of PI3K, HDAC, Src, IGFR-1, and CDK4/6. Enriching trial recruitment by molecular profiling of different ER+ subtypes will become increasingly important to maximize any additional benefit that these new agents may bring to current endocrine therapies for breast cancer.

Modulation of estrogen receptor alpha activity and expression during breast cancer progression

Seventy percent of breast tumors express the estrogen receptor (ER), which is generally considered to predict a better outcome relative to ER-negative tumors, as they often respond to antiestrogen therapies. During cancer progression, mammary tumors can escape from estrogen control, resulting in the acquisition of invasive properties and resistance to treatment. ER expression is a dynamic phenomenon and is finely regulated at numerous levels, including the gene, mRNA, and protein levels. As a consequence, many molecular mechanisms have been implicated in modulating ER activity and estrogen signaling in mammary cancer. In fact, one-third of ER-positive breast cancer cells do not respond to first-line endocrine therapies, and a large subset of relapsing tumors retain ER expression. Increased knowledge of these mechanisms has led to the development of better prognostic methods and targeted therapies for patients; however, additional research is still needed to improve patient survival. In this chapter, we focus on the signaling pathways leading to changes in or loss of ER activity in breast cancer progression.

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.

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.